Handbook of Force Transducers: Characteristics and Applications [2] 3030353214, 9783030353216

Citation preview

Dan Mihai Stefanescu

Handbook of Force Transducers

Dan Mihai Ştefănescu

Handbook of Force Transducers Characteristics and Applications

123

Dan Mihai Ştefănescu Romanian Measurement Society Bucharest, Romania

ISBN 978-3-030-35321-6 ISBN 978-3-030-35322-3 https://doi.org/10.1007/978-3-030-35322-3

(eBook)

© Springer Nature Switzerland AG 2020 This work is subject to copyright. All rights are reserved by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publisher, the authors and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, expressed or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publisher remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. This Springer imprint is published by the registered company Springer Nature Switzerland AG The registered company address is: Gewerbestrasse 11, 6330 Cham, Switzerland

Man is the MEASURE of all things. —Protagoras It is not by muscle, speed, or physical dexterity that great things are achieved, but by reflection, FORCE of character, and judgment. —Cicero MEASURE what is measurable, and make measurable what is not so! —Galilei If I have seen further than others, it is by standing upon the shoulders of giants. —Isaac Newton Among the gifts given to us by Mother Nature the FORCE of reasoning is the first one. —Chamfort Skill is the unified FORCE of experience, intellect and passion in their operation. —John Ruskin

Man is the most sensitive INSTRUMENT. —Christof Rohrbach Love is the only FORCE capable of transforming an enemy into a friend. —Martin Luther King, Jr.

I would like to offer my grateful acknowledgements to all those who have generously helped me in this considerable undertaking, especially to my mentor and promoter, Prof. Dr. Eng. Aurel Millea. —Dr. Eng. Dan Mihai Ştefănescu

Foreword

This handbook could best be described as a “potpourri” of ideas and applications in the field of measurement, instrumentation, and transducers. Among them, strain gauges—Wheatstone bridges—force transducers—mechanical testing—electrical measurement—virtual instrumentation are dominating the story, like a red thread, a root of all facets treated by the author. It is addressed mainly to students, technicians, engineers, researchers, and other people desiring to broaden their knowledge area and get new specific information. The book’s table of content is rather balanced, going from simple to complex in 21 chapters. Most delicate subjects, such as force feedback, force control, force vision, smart/intelligent force transducers, virtual measurement, are gradually presented and properly explained. It is worth mentioning the author’s particular care for terminology. An example: distinguish between weighing cells—load cells— force transducers. Then, for measurement units: when expressing force or loading weight in grams (kilograms), the correct unit is grams-force (kilograms-force)! More than 200 illustrations (colorful, complex, suggestive, and informative) and 275 bibliographical references (a quarter from which belonging to the author) bring a plus of value and strength to this spectacular and unique work about various applications with strain gauge force transducers (SGFTs) and their characteristics. A few drawings, originally executed with a pencil, ruler, and compass, have been redesigned with modern programmes like Catia and, where needed, using CorelDraw “insertions.” If, for various subjects, the “historical” SAP IV had been used in the era of punched cards, later they were processed with ANSYS (with the “right-hand rule” for its OXYZ axes system), adding a special application for depicting strain diagrams directly upon the “active” surfaces of elastic elements (EEs). At the beginning of the new millennium, we switched to computer-aided design (CAD), combined with finite element analysis (FEA), which facilitates modeling and parametric optimization of the EEs for SGFTs.

This volume is a continuation of “Handbook of Force Transducers” written by the same author and published at “Springer-Verlag, Berlin and Heidelberg” in 2011. ix

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Foreword

In a multidisciplinary world, with interconnected specialties, the imagistic correlations adopted in this handbook resulted in ingenious solutions, in quite complex applications. It is to be noted the uniform figuring of the strain gauges, connected in Wheatstone bridge after their bonding on the flexible elements of single- or multi-component SGFTs, observing the adopted color convention: red— increasing resistance for (mechanical) tension (T) and blue—decreasing resistance for compression (C). Perhaps one could object to somehow “overloading” certain pictures. An explanation could be the author’s desire to “squeeze” more information into individual images. This could be a great advantage especially for young people, who prefer “concentrated” information and quick learning. Both online and printed editions enjoy and benefit from the “kaleidoscopic” (and often “encyclopedic”) content of the book. Bucharest, Romania June 2019

Prof. Dr. Eng. Aurel Millea

Contents

Part I 1

2

3

Characteristics of Force Transducers

The International System of Units (SI) and the Place of “Force” in Several Graphic Representations . . . . . . . . . . . . . . . . . . . . . . 1.1 Graphic Representations of the SI Units . . . . . . . . . . . . . . . 1.1.1 The “Tree” Representation . . . . . . . . . . . . . . . . . . 1.1.2 The “Planetary” Representation . . . . . . . . . . . . . . . 1.1.3 The “Subway Map” Representation . . . . . . . . . . . . 1.2 Some Thoughts Related to the “Force” as a Physical Quantity . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3 The Place of “Force” within the “New SI” . . . . . . . . . . . . . 1.4 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Metrological Technical Data in the Measurement Process, with Examples in Weighing Cells . . . . . . . . . . . . . . . . . . . . . . . . 2.1 History and Terminology of Weighing . . . . . . . . . . . . . . . . 2.2 Characteristics and Specifications Related to Weighing Cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3 Evolution of Mechanical and Electrical Weighing Systems . 2.4 Modern Achievements in the Fields of Load Cells and Force Transducers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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General Classification of the Electrical Methods and Principles for Measuring Mechanical Quantities . . . . . . . . . . . . . . . . . . . . 3.1 Resistive Force Transducers [10−13 to 108 N] . . . . . . . . . . 3.2 Inductive Force Transducers [10−2 to 105 N] . . . . . . . . . . 3.3 Capacitive Force Transducers [10−9 to 104 N] . . . . . . . . . 3.4 Piezoelectric Force Transducers (PZFT) [10−1 to 109 N] . . 3.5 Electromagnetic Force Transducers [10−14 to 100 N] . . . . .

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Electrodynamic Force Transducers (EMFC) [10−2 to 103 N] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.7 Magnetoelastic Force Transducers [103 to 107 N] . . . . 3.8 Galvanomagnetic Force Transducers (Hall-Effect) [10−12 to 100 N] . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.9 Vibrating-Wire Force Transducers (VWFT) [10−1 to 107 N] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.10 (Micro)Resonator Force Transducers [10−13 to 105 N] . 3.11 Acoustic Force Transducers (SAW) [10−3 to 102 N] . . 3.12 Gyroscopic Force Transducers [101 to 103 N] . . . . . . . 3.13 Comparison of Methods and Principles for Force Measuring . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Application of Electromagnetic and Optical Methods in Small Force Sensing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1 Electromagnetic Methods Classification in Force Sensing . . 4.2 Applications of Electromagnetic Methods in Small Force Sensing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.1 Magnetoresistive Sensing . . . . . . . . . . . . . . . . . . . 4.2.2 Magnetostrictive Sensing . . . . . . . . . . . . . . . . . . . 4.2.3 Galvanomagnetic (Hall-Effect) Sensing . . . . . . . . . 4.2.4 Electrodynamic (EMFC) Sensing . . . . . . . . . . . . . . 4.2.5 Superconducting Quantum Interference Device (SQUID) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.6 Microforce Sensor Based on Floating-Magnetic Principle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.7 Manipulation of Magnetic Skyrmions by Mechanical Force . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3 Perspectives in Small Force Measurements . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Strain Gauges—Resistive and Other Principles . . . 5.1 Resistive Strain Gauges . . . . . . . . . . . . . . . . . 5.1.1 Bonded Metallic Strain Gauges . . . . . 5.1.2 Piezoresistive (Silicon) Strain Gauges 5.1.3 Carbon Nanotubes (CNTs) . . . . . . . . 5.2 Capacitive Strain Gauges . . . . . . . . . . . . . . . . 5.3 Piezoelectric Strain Gauges . . . . . . . . . . . . . . 5.4 Magnetoelastic Strain Gauges . . . . . . . . . . . . 5.5 Acoustic Strain Gauges (SAWs) . . . . . . . . . . 5.6 Optical Strain Gauges . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Wheatstone and Other Bridge-Like Configurations . . . . . . . . 6.1 Resistive Wheatstone Bridge . . . . . . . . . . . . . . . . . . . . . 6.1.1 Wheatstone Bridge History . . . . . . . . . . . . . . . . 6.1.2 Wheatstone Bridge Fundamental Properties . . . . 6.2 Other Bridge-Like Measuring Devices . . . . . . . . . . . . . . 6.2.1 Differential Transformer (LVDT) . . . . . . . . . . . . 6.2.2 Differential Capacitor . . . . . . . . . . . . . . . . . . . . 6.2.3 Magnetoresistive “Bridge” . . . . . . . . . . . . . . . . 6.2.4 Galvanomagnetic Transducer (Hall-Effect) . . . . . 6.2.5 Magnetoelastic (Biparametric R–L Half-Bridge) . 6.2.6 Biparametric (L–C Half-Bridge) . . . . . . . . . . . . 6.3 Instrumentation Applications Based on Wheatstone Bridges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.4 Signal Conditioning for Wheatstone Bridges . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Evolution of Strain Gauge Force Transducers—Design, Fabrication, Testing, Calibration and Databases . . . . . . . . . . . . . 7.1 Design Requirements of Strain Gauge Force Transducers . . 7.2 Finite Elements Optimization of the Force Transducers Elastic Elements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.3 Aspects Concerning Testing and Calibration of Strain Gauge Force Transducers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.4 Scientific and Commercial Databases of Force Transducers . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Part II 8

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Applications with Force Transducers

Testing Equipment to Investigate Elastic Constants of Rocks and Composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.1 Equipment for Determining the Elastic Constants of Rocks . 8.2 Stand for Determining the Elasticity Modulus of Composite Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.3 Other NDT, Static and Dynamic Tests for Composites . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Static and Dynamic Stiffness in Connection with Ball Screws and Reinforced Concrete Components . . . . . . . . . . . . . . . . . . . . 9.1 Stand for Determining the Static Stiffness and the Friction Moment at the Ball Screws . . . . . . . . . . . . . . . . . . . . . . . . 9.2 Measurement of the Dynamic Stiffness of Some Reinforced Concrete Components . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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10 Methods and Means for Measuring Cables Tension . . . . . 10.1 Testing Equipment for Determining the Cables Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.2 Automatic Measurement System for Ground Anchor Proof Testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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11 Measurement of the Axial Loads Transmitted to the Foundation by High Voltage Circuit Breakers When Acting . . . . . . . . . . . . . . . 113 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117 . . . . . .

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13 Robotic and Biomedical Applications Related to Human Hands . 13.1 Human and Robotic Gripping . . . . . . . . . . . . . . . . . . . . . . 13.2 Force Feedback for Human Hands . . . . . . . . . . . . . . . . . . . 13.3 Strain Gauge Devices for Handwriting Analysis . . . . . . . . . 13.4 Force Feedback, Data and Fuzzy Gloves . . . . . . . . . . . . . . 13.5 Vision-Based Force Transducers for Microrobotic Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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14 Multifunctional Transducers for Force and Other Non-electrical Quantities . . . . . . . . . . . . . . . . . . . . 14.1 Multifunctional Force Transducers . . . . . . . . 14.2 Force and Deformation/Elongation . . . . . . . . 14.3 Force and Strain . . . . . . . . . . . . . . . . . . . . . 14.4 Force and Pressure . . . . . . . . . . . . . . . . . . . 14.5 Force and Torque . . . . . . . . . . . . . . . . . . . . 14.6 Force and Hardness . . . . . . . . . . . . . . . . . . . 14.7 Force and Acceleration . . . . . . . . . . . . . . . . 14.8 Force and Mass . . . . . . . . . . . . . . . . . . . . . 14.9 Force, Density and Flow . . . . . . . . . . . . . . . 14.10 Force, Power and Energy . . . . . . . . . . . . . . 14.11 Force and Environment . . . . . . . . . . . . . . . . 14.12 Force and Frequency . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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12 A New Weigh-in-Motion and Traffic Monitoring System 12.1 Methods and Means for WIM . . . . . . . . . . . . . . . . 12.2 WIM with Bending Bridges [8] . . . . . . . . . . . . . . . 12.3 WIM with Shear Bridges [9] . . . . . . . . . . . . . . . . . 12.4 Experimental Results . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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15 Multicomponent Force and Moment Transducers . . . . . . . . . . . 15.1 Classification and Representations of Multicomponent F-M Transducers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15.2 F-M Applications with Two to Six Components . . . . . . . . . 15.2.1 Two Components . . . . . . . . . . . . . . . . . . . . . . . . . 15.2.2 Three Components . . . . . . . . . . . . . . . . . . . . . . . . 15.2.3 Four Components . . . . . . . . . . . . . . . . . . . . . . . . . 15.2.4 Five Components . . . . . . . . . . . . . . . . . . . . . . . . . 15.2.5 Six Components . . . . . . . . . . . . . . . . . . . . . . . . . . 15.3 Dynamic Testing of Multicomponent F-M Transducers . . . . 15.4 Multicomponent F-M Transducers Calibration in the Old and the New SI . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 Equipment for Determining Aerodynamic Forces on Flapping Wings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16.1 Micromechanical Flying Control and Scaling Aspects . 16.2 Equipment for Measuring Aerodynamic Forces . . . . . . 16.3 Experimental Results . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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17 Strain Gauge Balances for Testing Car and Flight Models in Wind Tunnel Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . 17.1 Classifications and Requirements for Strain Gauge Balances in Wind Tunnels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17.2 Strain Gauge Balances for Subsonic Wind Tunnel . . . . . . . 17.2.1 Strain Gauge Balance with Wires . . . . . . . . . . . . . 17.2.2 Half Rigid Strain Gauge Balance . . . . . . . . . . . . . . 17.3 Strain Gauge Balances for Trisonic Wind Tunnel . . . . . . . . 17.3.1 Vertical External Strain Gauge Balance . . . . . . . . . 17.3.2 Strain Gauge Wall Balance for Half Model . . . . . . 17.3.3 Six Component Internal Strain Gauge Balance . . . . 17.4 Calibration of Strain Gauge Balances for Wind Tunnels . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 Recent 18.1 18.2 18.3

Evolution of Smart Force Transducers . . . . . . Smart and/or Intelligent . . . . . . . . . . . . . . . . . . . Smart Circuits . . . . . . . . . . . . . . . . . . . . . . . . . . Smart Force Transducers . . . . . . . . . . . . . . . . . . 18.3.1 Smart Vibrating Wire Force Transducers 18.3.2 Smart Resistive Force Transducers . . . . 18.3.3 Smart Optical Force Transducers . . . . . . 18.4 Smart Imaging . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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19 New Achievements in the Field of Intelligent Force Transducers. Traceability . . . . . . . . . . . . . . . . . . . . . . . . 19.1 Intelligent Design of Force Transducers . . . . . . . . . 19.2 Intelligent Force Transducers . . . . . . . . . . . . . . . . . 19.2.1 Differential Piezoelectric Force Transducer . 19.2.2 Electro-Optical Catheter . . . . . . . . . . . . . . 19.3 Intelligent Force Measurement Channels . . . . . . . . . 19.4 Intelligent Force Sensing Applications . . . . . . . . . . 19.4.1 Intelligent Robots . . . . . . . . . . . . . . . . . . . 19.4.2 Wireless Force Sensing . . . . . . . . . . . . . . . 19.5 Force Measurement Traceability . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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20 Virtual Instrumentation and Force Transducer for Measurements in Dentistry . . . . . . . . . . . . . . . . . . . . . . . . 20.1 Virtual Instrumentation—Components and Characteristics 20.2 Intelligent Design of Force Transducer and Experimental Setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20.3 Virtual Instrumentation for Measurements in Dentistry . . 20.4 Experimental Results . . . . . . . . . . . . . . . . . . . . . . . . . . . 20.5 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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21 A Supplement on Photoelastic and Digital Techniques in Force Measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 229 Annex: Engineering and Art of Illustrated Travel Reports at the IMEKO World Congresses . . . . . . . . . . . . . . . . . . . . . . . . . 233

Part I

Characteristics of Force Transducers

Chapter 1

The International System of Units (SI) and the Place of “Force” in Several Graphic Representations

The International System of Units (SI) is the most widespread system of measurement units and presently it is the official one in almost all countries of the world. Its detailed presentation may be found for example in [1] where all names, symbols, definitions and interrelations of the base and derived units are given. Other international norms and documents are devoted to various recommendations and rules for using, writing, expressing and converting the SI units, especially in scientific texts and also in trade and commercial documents. To a lesser extent are known and used the graphical representations of the International System of Units. Three variants might be considered as most eloquent, and at the same time scientifically correct and explicit. They could be called (a) “tree”, (b) “planetary system” and, respectively, (c) “subway map” representation, in accordance with their specific shape. The purpose of this chapter is to describe the three graphic representations and to highlight the place of the quantity “force” and its measurement unit, the “newton”, in these representations.

1.1 Graphic Representations of the SI Units 1.1.1 The “Tree” Representation The “tree” representation (Fig. 1.1) originates from an OIML (International Organization of Legal Metrology) idea, first appearing in 1984 in the “OIML Bulletin”. The “SI tree” has a trunk whose main branches are the fundamental units, counterclockwise disposed from right to left: meter, kilogram, second, Ampere, Kelvin, mol and candela. Emerging from these fundamental units, placed to form a semicircular base, a number of derived units are connected, in the form of branches of a tree, resulting in an arborescent representation or chart.

© Springer Nature Switzerland AG 2020 D. M. Stef˘ ¸ anescu, Handbook of Force Transducers, https://doi.org/10.1007/978-3-030-35322-3_1

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4

1 The International System of Units (SI) and the Place of “Force” … W J

V Gy Wb

F Adapted after Bulletin OIML No. 95 / 1984

N

Sv

Multiplication

m s2

T

C Hz

H

Bq

S °C

K

A

Division

Pa

s m s

lx

mol lm

m2

Newton’s vision - 1687

kg

cd

m

sr

rad

rad s

Fig. 1.1 European “knowledge tree” representation for SI units (© 2011 by IFSA)

The following “colour code” has been adopted: brown—for the trunk and the thicker branches with the seven base units; green—for the thinner branches and the circles that contain the symbols of the derived units; red—for the “newton apple”. All significant connections are in green: solid lines indicate multiplication, dotted lines indicate division. In this representation, the base units are closer to the roots of the tree, while the derived units are spread throughout the foliage of the tree. It is to be noted that in this representation the radian and the steradian are still regarded as “supplementary” units—in accordance with the SI rules valid up to 1995 [2], when the 20th CGPM abolished the supplementary units and decided to consider them as derived units of dimension 1 (see the next two graphic representations!). As for the position of “force” with its unit, the “newton”, it is located at the top of this tree, here symbolized by a red apple.

1.1.2 The “Planetary” Representation The “planetary” representation (Fig. 1.2) was taken over (with permission) by the author, during his Asian travel, from KRISS (Korea Research Institute of Standards and Science) and Center for Measurement Standards—Industrial Technology Research Institute (Taiwan, R.O.C.) respectively, where it is exposed as large sized posters. However, its real origin remains unclear. In this representation there are seven “planets” along the border of an elliptic field, corresponding to the seven above-mentioned base units (starting counterclockwise with the Length atop, on the left side, in the same succession as before), and a lot of “satellites”, as derived units, “orbiting” inside the ellipse.

Fig. 1.2 Asian “planetary system” representation for SI units (© 2011 by IFSA)

1.1 Graphic Representations of the SI Units 5

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1 The International System of Units (SI) and the Place of “Force” …

Thus, more space is available for representing the derived SI units, their ramification and interconnections are more visible and the whole picture is more intuitive and much richer in information than the “European” SI tree. The base units appear as blue spheres, and the derived units as smaller green circles. Connections are drawn as green lines for multiplication, yellow lines for division and red lines for other conversions (e.g. Kelvin to Celsius degrees). An interesting feature of this representation is that mechanical units are located in the left side of the figure, while the electromagnetic units are situated in the center and others in the right side. Also, the “energetic” quantities (with their units, J, W, W/m2 etc.) are mostly grouped around the center of the ellipse, irrespective of their nature (mechanical, electrical, thermal). “Force”, with its unit “newton”, has been emphasized by an orange circle, in the left side of the diagram. The quantity “moment of force” together with its unit, N·m, is also apparent in this representation, being the 5th connection to the “force” unit.

1.1.3 The “Subway Map” Representation The “subway map” representation (so called by its authors) was posted on internet by Dr. Barry N. Taylor (22 March 2004), then a second variant was obtained by courtesy of Paul Trusten, Director of Public Relations, U.S. Metric Association, Inc. (copyright 2006), and finally published, under a similar form (Fig. 1.3), by NIST (National Institute of Standards and Technology, USA) [3]. This is another kind of “tree”, with multiple interconnecting lines; the representation has the merit of being highly “transparent”, easily “visible” and with a logic grouping of quantities and units. The first column is for the SI base units, the second comprises SI derived units without special names (volume, area, velocity, acceleration) and the third displays 22 derived units with special names. A number of units specific to certain disciplines or chapters of physics are not included (for example, strain, permittivity and permeability, signal level, attenuation, viscosity, thermal conductivity and capacity, entropy, a.o.). Colours have no distinct significance. Conventions for the connecting lines: solid lines indicate multiplication; dotted lines indicate division. The rectangular shape of this representation is optimal from the point of view of space usage, and the “information density”. Moreover, the defining equations of the derived units are explicitly given (e.g.  = V/A). The position of “force” and the associated unit “newton” is a “dominant” one (in yellow), on the first column and first row of the derived units.

1.2 Some Thoughts Related to the “Force” as a Physical Quantity

7

N

Fig. 1.3 American “subway map” representation for SI units (© 2011 by IFSA)

1.2 Some Thoughts Related to the “Force” as a Physical Quantity The global vision of Attila Naszlady defines the Energy, Work and Power from four different angles: mechanical, electrical, chemical and thermal, illustrating the various energetic transformations that can take place in the Universe [4]. As one can see in Fig. 1.4, Force is on the first position in this picture, its electrical measurement being made by appropriate transducers. Force is one of the most complex mechanical quantities. It is a derived quantity in the ISQ (the International System of Quantities, on which the SI is based), an important physical measurand with which many other quantities such as pressure, torque, strain, etc. are related. Five electrical methods of sensing force were categorized by Jacob Fraden in [5] as follows: • weighing the unknown force against the gravitational force of a standard mass; • determining the acceleration of a body with known mass to which the force is applied; • converting the concentrated force to a distributed fluid pressure and measuring that pressure;

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1 The International System of Units (SI) and the Place of “Force” …

Fig. 1.4 Different definitions of Work (Energy) and Power (© 2011 by IFSA)

• balancing the force against an electromagnetically or electrostatically developed force; • measuring the strain produced in an elastic body by the unknown force. We tried to find a unified approach [6] treating the force measurements in a close relation with other mechanical quantities, tightly connected by corresponding physical formulas. The previous “Springer Handbook of Force Transducers” [7] systematizes the knowledge and measuring techniques of force, laying down two basic classifications in this area: • 12 principles and methods of force measurement; • 12 types of elastic elements for force transducers. Part I introduces the basic “Principles and Methods of Force Measurement” according to a classification into a dozen of force transducers types: resistive, inductive, capacitive, piezoelectric, electromagnetic, electrodynamic, magnetoelastic, galvanomagnetic (Hall-effect), vibrating wires, (micro)resonators, acoustic and gyroscopic. Two special chapters refer to force balance techniques and to combined methods in force measurement. Part II discusses the “(Strain Gauge) Force Transducers Components”, evolving from the classical force transducer to the digital/ intelligent one, with the incorporation of three subsystems (sensors, electromechanics and informatics). The elastic element (EE) is the “heart” of the force transducer and basically determines its performance. A 12-type elastic element classification is proposed (stretched/compressed

1.2 Some Thoughts Related to the “Force” as a Physical Quantity

9

column or tube, bending beam, bending and/or torsion shaft, middle bent bar with fixed ends, shear beam, bending ring, yoke or frame, diaphragm, axial-stressed torus, axisymmetrical and voluminous EE), with emphasis on the optimum location of the strain gauges. The main properties of the associated Wheatstone bridge, best suited for the parametrical transducers, are examined, together with the appropriate electronic circuits for SGFTs. The handbook fills a gap in the field of Force Measurement, both experts and newcomers, no matter their particular interest, can find a lot of useful and valuable subjects in the area of Force Transducers; in fact, it is the first specialized monograph in this inter- and multidisciplinary field.

1.3 The Place of “Force” within the “New SI” In “classical” metrology (the old school), Force is a derived quantity and is expressed in terms of three fundamental quantities (Length, Mass and Time) [8], as follows: LMT−2 (Fig. 1.5). Official notice: “On 16 November 2018 a revision of the International System of Units (the SI) was agreed by the General Conference on Weights and Measures. The definitions of the base units were presented in a new format that highlighted the link between each unit and a defined value of an associated constant. The physical concepts underlying the definitions of the kilogram, the ampere, the kelvin and the mole have been changed. The new definition of the kilogram is of particular importance because it eliminated the last definition referring to an artefact. In this way, the new definitions use the rules of nature to create the rules of measurement and tie measurements at the atomic and quantum scales to those at the macroscopic level” [10].

Fig. 1.5 Force as a derived quantity: a the seven SI base quantities; b “burden” of the physical standards for Length, Mass and Time; c Sir Isaac Newton (1642–1727) was an English mathematician, physicist, astronomer, theologian, and author (described in his own way as a “natural philosopher”) who is widely recognized as one of the most influential scientists of all time, and a key figure in the scientific revolution [9]

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1 The International System of Units (SI) and the Place of “Force” …

This major change in vision, expected to take effect in 2019 on World Metrology Day (20 May), is commented by Klaus von Klitzing (Nobel Prize in Physics 1985 “for the discovery of the quantized Hall effect”) and expressed in a “Shakespearean” manner {Measure for Measure} inside a section of “Nature Physics” [11].

The most important application of the “electrical quantum units” is to establish an “electronic kilogram” via the Kibble balance, as a direct connection between the Planck constant and the kilogram, intermediated by other fundamental physical constants. The Planck-Balance (PB), currently under development in a joint project of the Physikalisch-Technische Bundesanstalt and the Technische Universität Ilmenau, funded by the German Federal Ministry of Education and Research, is a new weighing instrument, utilizing the Kibble principle for mass measurements [12]. This principle, recently used for the determination of the Planck constant, provides a path for mass determination via electric measurements, without the need for calibrated weights and is thus a primary method for mass determination. Since user-friendliness is an important design aspect for the use in industrial applications, the balance, presented in Fig. 1.6, should be as compact and lightweight

Fig. 1.6 Main components of the Planck-Balance

1.3 The Place of “Force” within the “New SI”

11

Table 1.1 Main metrological features of two Planck-Balance models PB2

PB1

Mass range

1 mg to 100 g

1 mg to 1 kg

MPE/m(max)a

16 × 10−7 (E2)

5 × 10−7 (E1)

10−7

8.4 × 10−8 (E1)

U rel = um (max)/m(max) (k = 1)

2.7 ×

Measurement environment

Air

High vacuum

Duration of the measurement cycle (s)

10–120

10–120

a MPE

(E2)

is the maximum permissible error of a weight

as possible; it is imagined as a “desktop” device for the mechanical part of the balance, accompanied by a rack for modularized electrical measurement devices. Two models of Planck Balances are presented in Table 1.1. Their names come from the aimed measurement uncertainties corresponding to accuracy levels E2 and E1 for weights, as specified in OIML R-111-1.

1.4 Conclusions We believe that the three suggestive representations of the SI measurement units interconnection—the (European) “tree” representation, the (Asian) “planetary” chart and the (American) “subway map” diagram—could be helpful for a better grasping of the nature and peculiarities of the physical quantity “Force”. These representations have been for the first time presented together in [8], under the copyright of IFSA (International Frequency Sensor Association). Nowadays, as Klaus von Klitzing writes, “with the introduction of a new SI system based on fixed values for constants of nature, Max Planck’s vision will become reality: with the help of fundamental constants we have the possibility of establishing units that necessarily retain their significance for all cultures, even unearthly and nonhuman ones.” This revolutionary step opens a new era in the development of our knowledge and opportunities in the field of measurement instrumentation. For example, linking the “Force” to quantum invariants in electrical units emphasizes the role of this physical quantity and its SI unit (newton), perfectly illustrated by the using of the EMFC (Electro-Magnetic Force Compensation) measuring principle in the Planck-Balance.

References 1. http://www.bipm.org/utils/common/pdf/si_brochure_8_en.pdf. Accessed Feb 2018 2. Millea, A.: In the World of Measurements and Measurements Units (in Romanian). Editura AGIR, Bucure¸sti (2009) 3. http://www.nist.gov/pml/wmd/metric/everyday.cfm. Accessed 13 Sept 2018

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1 The International System of Units (SI) and the Place of “Force” …

4. Naszlady, A.: Universality of measurement in medical sciences. In: Proceedings of the XVIII IMEKO World Congress on Metrology for a Sustainable Development, Paper 654, Rio de Janeiro, Brazil (2006) 5. Fraden, J.: Handbook of Modern Sensors—Physics, Design and Applications, 4th edn. Springer, New York (2010) 6. Stef˘ ¸ anescu, D.M.: Strain gauged elastic elements for force and related quantities measurement. In: Proceedings of the IMEKO International Conference on Cultivating Metrological Knowledge, Paper 22, Merida, Mexico (2007) 7. Stef˘ ¸ anescu, D.M.: Handbook of Force Transducers—Principles and Components. Springer, Berlin and Heidelberg (2011) 8. Stef˘ ¸ anescu, D.M., Millea, A.: The place of “Force” in several graphic representations of the International System of Units (SI). Sens. Transducers J. 131(8), 1–7 (2011). ISSN 1726-5479 9. Pascal, J.B.: Who Was Isaac Newton? Grosset & Dunlap (2013) 10. Stock, M., Davis, R.S., de Mirandes, E., Milton, M.J.T.: The revision of the SI—the result of three decades of progress in metrology. Metrologia (2019). https://doi.org/10.1088/1681-7575/ ab0013 11. von Klitzing, K.: Metrology in 2019. Nat. Phys. 13, 198 (2017) 12. Günther, L., Rothleitner, C., Schleichert, J., Rogge, N., Vasilyan, S., Härtig, F., Fröhlich, T.: The Planck-Balance—primary mass metrology for industrial applications. In: Proceedings of the XXII IMEKO World Congress “Knowledge through Measurement”, Belfast, UK, 3–6 Sept 2018. Open Access—Published under license by IOP Publishing Ltd.

Chapter 2

Metrological Technical Data in the Measurement Process, with Examples in Weighing Cells

2.1 History and Terminology of Weighing The first Conference of IMEKO TC-3 (Measurement of Force and Mass), organised in October 1968 in Braunschweig by Dr. K. Hild (1968), had the historical merit of bringing together the experts of force measurement with those of mass measurement (weighing) in order to establish a common language [1]. As Dr. Paetow (HBM) said, “there is a difference between weighing cell and force transducer” [2]: – For weighing cells, load range is indicated in mass units, having in view that the precise mass determination is made by measuring its force effect (weight) in the gravitational field of the Earth. – From the metrological point of view, the easiest way to determine categories of force transducers (with load range indicated in newtons) is to use accuracy criteria as follows: A. B. C. D.

Universal force transducers, Force measuring devices in material testing machines, Transducers for verification of materials testing machines, Force comparison standards.

Shortly, after Regtien [3], load cell is a force transducer for measuring weight. Dr. Tamás Kemény, founder member of IMEKO TC 3 (1968) and then elected as Secretary General of IMEKO (till 2012), published in 1981 the first Handbook on Weighing, treating along 680 pages of application oriented sections both mechanical and electronic measuring systems [4] (Fig. 2.1).

© Springer Nature Switzerland AG 2020 D. M. Stef˘ ¸ anescu, Handbook of Force Transducers, https://doi.org/10.1007/978-3-030-35322-3_2

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2 Metrological Technical Data in the Measurement Process …

Fig. 2.1 The “Bible” of weighing technology in the years ’80

Fig. 2.2 Sensor versus transducer: strain gauges measure the linear relative elongation (positive or negative strain) caused by the tensile or compressive stress applied on an elastic element of a force transducer (© 2015 Yurish)

One of his conclusions: Almost all electronic weighing systems, excepting the micro-balances, are based on strain gauge load-cells (with Wheatstone bridges). As far as the correct terminology concerns (see VIM), it is worth to mention that Force, being a complex derived quantity, defined through some basic quantities (LMT) or, more recently, in terms of certain electric/quantum constants, cannot be measured with a simple primary sensor, but requires some electro-mechanical structures, smartly instrumented transducer! (Fig. 2.2).

2.2 Characteristics and Specifications Related to Weighing Cells In this domain the leadership belongs to the German VDI/VDE “Electromechanical Weighing” working group, which published a draft proposal VDI/VDE 2637 “Typical

2.2 Characteristics and Specifications Related to Weighing Cells

15

Data of Weighing Cells”. On this basis, several companies assumed yet this document when producing weighing cells. This subchapter has been translated by Prof. Dr. Aurel Millea. In the following some definitions are given. Load(ing): Mass of the object to be weighed. Rated loading: Upper limit of the measurement range. Measurement range: The loading range where the specified error limits are not exceeded. Sensitivity (cell factor): According to Fig. 2.3a, product of the rated loading L n and the slope of the line through points P1 and P2 , by increasing loading. S = (Un −U p )/L n −L p )L n

(2.1)

where: L p is the initial loading and U n and U p are the output signal at rated and initial loadings, respectively. Rated sensitivity: Rated (nominal) value of the cell sensitivity. Sensitivity tolerance: Allowable deviation of the effective sensitivity value from the rated one. Temperature coefficient of sensitivity: Relative variation of the sensitivity by 10 K temperature change, relative to the rated value. The maximum value of the temperature coefficient should be given in the entire rated and operating temperature range.

Fig. 2.3 Sensitivity (a) and combined error (b) diagrams. Adapted by Dr. Millea

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2 Metrological Technical Data in the Measurement Process …

Temperature coefficient of the initial (null-point) sensitivity: Relative variation of the output by 10 K temperature change, related to the rated value. The maximum value of the temperature coefficient should be given in the entire rated and operating temperature range. Combined (composite, resultant) error: According to Fig. 2.3b, this error gives a picture of the linearity error. It represents the half-distance c between the two limits of de tolerance range, which embraces the input-output curves in increasing-decreasing directions. Relative hysteresis (delay, lag or history) error: Maximum deviation u between increasing and decreasing input-output curves (see figure). Repeatability error: Relative dispersion of the cell output at two points of the input-output curve, in 10 repeated measurements, with same load. Between these two points always the same load variation L = L 2 − L I must be produced. The first load L I should always be set in decreasing direction, while L 2 in increasing direction. When determining the repeatability error from the 10 repeated measurements the relative dispersion should be calculated and finally the highest value of the two is to be given. Important: during this measurement the weighing cell should not be entirely unloaded, in order the strain regime not be disturbed. Creep: At rated loading and reference temperature the maximum variation of the output signal during a given time interval. For the measurement, the rated load should quickly be applied (within max. 5 s). Input resistance (at reference temperature): Resistance measured at the input of the connecting cable (delivered together with the cell), measured at reference temperature. Output resistance (at reference temperature): Resistance measured at the output of the connecting cable (delivered together with the cell), measured at reference temperature. Rated range of supply voltage: Supply voltage interval within which the cell normally operates and the specified error limits are fulfilled. Maximum supply voltage: Supply voltage value at which no permanent changes of cell properties arise, but errors may be higher. Rated shape variation: At rated load relative measurable elastic deformation in the measurement direction. Operating limit: Maximum applicable load, upper limit of operation range, within which specified error limits may be exceeded (Fig. 2.4). Maximum loading: Upper limit of the loading range (see figure). Exceeding the maximum loading may permanently damage the cell. Breaking limit: Applicable load in measurement direction beyond which the cell is irremediably impaired.

2.2 Characteristics and Specifications Related to Weighing Cells

17

Fig. 2.4 Specific loading limits

Specific cross-loading: Value of a load applied perpendicularly to the measurement direction, related to the rated load, within which no permanent mechanical or electrical modification occurs. Admissible dynamic load: Maximum amplitude of a variable load, applied in the measurement direction, i.e. difference between the upper and lower limits of that load. Reference temperature: Temperature of environment to which the cell technical data are related. Rated (nominal) temperature range: Environmental temperature range within which the cell may be used and maintains its technical parameters and error limits. Operating temperature range: Environmental temperature range within which the cell may be used without permanent modifications of its technical parameters. Error limits specified may be exceeded. Storage temperature range: Environmental temperature range within which the cell may be deposited (stored) without permanent modifications of its technical parameters. Although the recommendation does not define accuracy classes, certain companies in their specifications—aiming at a better overview—do give an accuracy class, with the following definition: any of the error limits expressed in percents should be smaller than the accuracy class, except the sensitivity tolerance. The basic conception of the norms is that accuracy depends on the divisions’ number. Within the first segment of the measurement range, between 0 and 500 divisions, the maximum allowable error is 1 (one) division. Only approx. 70% of this relatively narrow band may be used by the weighing cell and the mechanical loading system (i.e. scale bridge), the remaining 30% pertains to the electronic data processing (based on quadratic summation of errors).

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2 Metrological Technical Data in the Measurement Process …

A typical example of technical data for commercially available weighing cells is given in Table 2.1. The Hungarian industry manufactured in the 1980s a wide range of products in the area of Weighing Technology, as well as in that of Strain Gauges (Fig. 2.5). Table 2.1 Typical technical characteristics of MOM weighing cells, according to OIML recommendations Data

Unit

Rated load

Accuracy class A

Cell factor

mV/V

B

1.0 ±0.001

Cell factor tolerancea

±0.0025 Nonlinearityb Repeatability error

% %

20 kpc to 2 Mpd

±0.05

±0.1

4–100 Mp

±0.1

±0.25

20 kp to 2 Mp

±0.03

±0.005

4–100 Mp

±0.05

±0.05

Cell factor temp./coeff.

%/10 °C

±0.05

±0.1

Null point temp./coeff.

%/10 °C

±0.05

±0.1

Null point set tolerance

%

±1.0

Measurement length

mm

20–200 kp

0.5–0.9

0.5–2 Mp

0.1–0.2

4–100 Mp Insulation resistance

M

Input resistance



0.04–0.1 500

20 kp to 2 Mp

240 ± 5

4–100 Mp

430 ± 5

20 kp to 2 Mp

200

Output resistance



Overload

%

Same up to breaking

%

20 kp to 2 Mp 4–200 Mp

300

Preferred supply voltage

V

20 kp to 2 Mp

10

4–100 Mp

15

Max. supply voltage

V

20 kp to 2 Mp

15

4–100 Mp

20

4–100 Mp

Working temperature

°C

400 50 500

−20 to +65

Adapted by Aurel Millea from Tamás Kemény a Related to the measure value b Related to the entire range c kP: kilopound (kilogram force) = 9.8 N (gravitational metric unit of force) d MP: megapound (megagram force) = 9800 N

2.2 Characteristics and Specifications Related to Weighing Cells

19

Fig. 2.5 Several resistive strain gauge types manufactured by Kaliber MM, Budapest

Here are a few different types of strain gauges [5]: (a) Shearing strain half Wheatstone bridge for torque transducer; (b) Classical strain gauge for tensile-compressive or bending force transducers; (c) pair of SGs having the transverse sensing network made of two parts placed to the sides of the main longitudinal sensing element; (d) group of four SGs that ensures a “complete measuring point” with two extended and two shortened SGs; placing this quadruple “timbre” at a ±45° angle to the axis of the tested part, it results a full Wheatstone bridge for shearing stress; (e) rosette for measuring three-directional strain; (f) pair of SGs that are differentially stressed—one shortened and one extended— can be placed on top or bottom of an elastic element that bends to an “S” shape.

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2 Metrological Technical Data in the Measurement Process …

2.3 Evolution of Mechanical and Electrical Weighing Systems The electrical weighing systems are with strain gauges, with vibrating strings or gyroscopic. In Chap. 3 will be presented 12 electrical methods and principles used in weighing cells and force transducers, while strain gauges with more details in C5 and Wheatstone bridges in C6 (Table 2.2). Table 2.2 Comparison of various mechanical (7) and electrical (3) weighing systems Structure

Measuring principle

Classical weighing

Sliding weights

Turning added weights

Double dumping

Operating mode

Manual (automation possible)

Mass or force

M

M

M

M

Display

No

Analog

Digital

Analog

Resolution

Over 106

Till 103

Till 105

1000–1500

Duration of measurement (s)

30

20–30

10–20

3–4

Displacement, mm

0

0

0

14–40

Adapting a recorder

No

A simple one

Possible

Possible

Electrical output

No

Difficult

Possible

Possible

Adapting electric control

No

Tough

Tough

Possible

Maintenance

No problems

Cost factor, at l000 d resolution

1

Automatic

By technicians 2

15

10

(continued)

2.3 Evolution of Mechanical and Electrical Weighing Systems

21

Table 2.2 (continued)

Multiple rotations

Projection

Spring

Strain gauges

Wires

Gyroscope

Automatic M

M

F

F

M

F

Analog

Analog

Analog

Digital

Digital

Digital

2000–3000

1000–6000

1000–1500

500–3000

2500–6000

1000–10,000

3–4

2–4

2–3

0.5–1

0.3–1.5

3–10

18–24

20–40

14–80

Nearly 0

Nearly 0

0

No

No

Yes

No

No

No

Difficult

Easy

Easy

Yes

Yes

Yes

30

50

Possible By fine mechanics technicians 8

20

By specialists 10

50

Adapted by Aurel Millea from Tamás Kemény

A “fresco” of graphical representations (Fig. 2.6) from that period (1980s) is owed to: – Klaus Horn (Technical University Braunschweig, Deutschland) [6]: a hybrid version (transducer with hydraulic liquid and a membrane with strain gauges) (a) and a capacitive one, based on a variable inter-electrode distance (b). – Tamás Kemény (Metripond, Budapest, Hungary): the model of a Siemens transducer (c), whereby a “manual” representation of the strain gauges and their connecting wires is noticeable, while the picture is photographic in the last version (see below). – Ural Erdem (Industrial Weighing Division, Negretti Automation, Aylesbury, UK): a Negretti transducer model (d). The author concludes in one of his works (in which the Force-Weight association appears just in the title) that “strain gauge load cell is the most successful force transducer in industrial weighing” [7].

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2 Metrological Technical Data in the Measurement Process …

Fig. 2.6 Graphical representations for load cells and force transducers (1980s)

2.4 Modern Achievements in the Fields of Load Cells and Force Transducers Currently a large variety of RSGFTs exists, in accord with their numerous applications. The diverse loading types of the elastic elements (tensile—compressive, bending, shearing or combined) imposed the optimization through numerical calculation. In this regard, the author presented in 1987 at PTB—Braunschweig a conference entitled “Finite Element Method (FEM) Analysis of Transducer Flexible Elements” (Fig. 2.7).

2.4 Modern Achievements in the Fields of Load Cells …

23

Fig. 2.7 Official notice concerning D. M. Stef˘ ¸ anescu’s conference held at the PTB Centennial Year (1987) in Braunschweig

Table 2.3 presents an extensive classification/systematization of twelve elastic elements types for RSGFT (Resistance Strain Gauge Force Transducers), indicating for each type the way of SG location on the elastic element as well as the mechanical sensitivity (the design formula) and the electrical one (after connecting the SGs in a Wheatstone bridge) [8]. Klaus Bethe (Technical University Braunschweig) considers that force transducers and load cells belong to the same family [9]. Similarities are noticeable within the wide range of products manufactured by the leader of the branch, the Hottinger Baldwin Messtechnik GmbH in Darmstadt, among which the C, U and Z models appear both as Load Cells (Fig. 2.8) and as Force Transducers (Fig. 2.9), differing only by their mounting mode in the specific application. HBM achieves transducers available with nominal (rated) forces between 10 N and 5 MN [10]. A supplementary example is given in Fig. 2.10: a ring torsion transducer (type: C18).

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2 Metrological Technical Data in the Measurement Process …

Table 2.3 Mechanical and electrical sensitivities for twelve types of RSGFTs

(continued)

2.4 Modern Achievements in the Fields of Load Cells … Table 2.3 (continued)

Adapted from Springer Handbook of Force Transducers (2011)

25

26 Fig. 2.8 Load cell types developed by Hottinger Baldwin Messtechnik GmbH

2 Metrological Technical Data in the Measurement Process …

2.4 Modern Achievements in the Fields of Load Cells …

Fig. 2.9 Force transducer types developed by HBM Company

Fig. 2.10 Working function of a force transducer based on strain gauges (HBM)

27

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2 Metrological Technical Data in the Measurement Process …

References 1. Kemény, T.: TC 3—From the birth to worldwide recognition. Report presented at the IMEKO TC 3 International Conference on Force, Mass and Torque Measurements: Theory and Application in Laboratories and Industries, Cairo, Egypt (2005). PDF created on 2005 2. Paetow, J.: Weighing cell and force transducer—there is a difference. Private discussions at Hottinger Baldwin Messtechnik, Darmstadt, Germany, Nov 1987 3. Regtien, P.P.L.: Sensors for Mechatronics. Elsevier Insights Series. Elsevier, Amsterdam (2012) 4. Kemény, T.: Mérlegtechnikai kézikönyv (Handbook of Weighing Technology), 681 p. Technical Publishing House, Budapest (1981) 5. Ballon, I., Zsoldos, B.: Production of strain gauges and special load cells in Kaliber MM Ltd. Co. Kaliber, Budapest, Hungary. PDF created on 12 Oct 2003 6. Horn, K.: Physikalische Prinzipien für elektromechanische Wägezellen—Aufnehmerprinzipien für die Umformung der mechanischen Meßgröße ‘KRAFT’ in elektrisch nutzbare Meßgrößen. wd—wägen & dosieren, Heft 1, S. 5–16, Deutschland (1976) 7. Erdem, U.: Force and weight measurement. J. Phys. E: Sci. Instrum. 15, 857–872 (1982) 8. Stef˘ ¸ anescu, D.M.: Methods for increasing the sensitivity of strain gauge force transducers (in Romanian). Ph.D. dissertation cum laude (160 pages, 26 tables, 86 figures, 336 references), Universitatea “Politehnica” Bucure¸sti, Romania (1999) 9. Bethe, K.: Optimization of a compact force-sensor / load-cell family. Sens. Actuators A Phys. 42(1–3), 362–367 (1994) 10. HBM Test and Measurement—Force Transducers Based on Strain Gauges. PDF created on 10 Nov 2011

Chapter 3

General Classification of the Electrical Methods and Principles for Measuring Mechanical Quantities

For each of the 12 basic principles and electrical methods for electrical measurement of force and related mechanical quantities, mentioned in the introductory chapter, measuring range is indicated in brackets at all subsequent subtitles. Then, relevant figures are shown for each force transducer type, with their main components, construction details and connection modes, some defining formulas and other relevant data. Thus, 12 consistent and coherent graphical representations resulted. Pictures are self-explanatory and therefore only shortly commented; for details, see [1]. The difference between methods/principles are underlined by Millea [2]: Method: generic description of a logical organization of operations used in a measurement (9–12 FT types). Principle: physical phenomenon or effect serving as a basis of a measurement (4–8 FT types); Parametrical ones (1–3 FT types) may be equally seen as belonging to both categories, they using distinct physical principles while implying also complex measuring methods e.g. the differential one.

3.1 Resistive Force Transducers [10−13 to 108 N] Figure 3.1 shows two most wide-spread elastic elements used in force transducers: (a) cantilever beam (type 3 in Table 2.3) and (b) tube (type 2 in Table 2.3). The tube is less sensitive to bending, twisting and buckling than the column (type 1 in Table 2.3) and allows more installation possibilities in the tested equipment. The tube could be a hollow cylinder column (“barrel”) or a thin wall elastic bar (“sleeve”). Figure 3.1c shows the connection mode into a Wheatstone bridge of SGs bonded on the aforementioned elastic elements. Whether using only two SGs placed on a bent lamella (a) and connected in a half-bridge, or four SGs bonded on a tube (b) and connected in a full bridge, the same “colour code” is advisable: © Springer Nature Switzerland AG 2020 D. M. Stef˘ ¸ anescu, Handbook of Force Transducers, https://doi.org/10.1007/978-3-030-35322-3_3

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Fig. 3.1 Strain gauges on cantilever beam (a) or on tube (b), and their connection in Wheatstone bridge

– Red (“warm”) for the stretched SG, corresponding to resistance increase, – Blue (“cold”) for the compressed SG, corresponding to resistance decrease.

3.2 Inductive Force Transducers [10−2 to 105 N] Figure 3.2 shows a practical layout for weighing cells up to 500 kgf, with a nonlinearity of 0.1% and a reproducibility of 0.2% [3]. On the loading rod, joint with the weighing pan, a central core is placed, the “heart” of the LVDT measuring system. At balance, the core is exactly in the center of the primary coil, the assembly being adjusted to zero or null output when the elastic body is unloaded. When a weight is put on the pan, this core moves downward from the upper secondary coil and penetrates the lower secondary coil, producing an output voltage that is proportional to the axial force load. Fig. 3.2 Inductive force transducer type LVDT (linear variable differential transformer)

3.2 Inductive Force Transducers [10−2 to 105 N]

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In case of a force transducer, the bidirectional nature of the displacement characteristic of an LVDT perfectly complements the bidirectional deflection of an elastic element, producing an output voltage that also undergoes a phase or polarity reversal when it changes from tension to compression. Optimum operating characteristics are developed when the linear range of the LVDT corresponds to the deflection of the elastic element.

3.3 Capacitive Force Transducers [10−9 to 104 N] Capacitive transducers are the most precise of all electrical ones and are known for their extremely high sensitivity and resolution, large bandwidth, robustness, stability, and drift-free measurement capability. Their working principle is illustrated in Fig. 3.3: A pair of cylindrical excitation plates are mounted on, but electrically insulated from, the compensating rod, while a sensing ring is coaxial and surrounding the excitation plates [4]. Due to the applied force, the outer ring moves axially with respect to the two inner tubes, changing the differential capacitance C 1 –C 2 , because more or less area of the sensing ring overlaps the respective cylindrical excitation plates. This leads to extremely good linearity in both compression and tension. The differential configuration has several advantages: – nearly insensitive to dimensional variations of the elastic elements, – a “doubled” sensitivity, – partial linearization of the nonlinear dependence of the characteristic displacement versus capacitance.

Fig. 3.3 Capacitive force transducer: components and differential measurement

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Fig. 3.4 Components X–Y–Z separation within a three-axial piezoelectric force transducer and their charge amplifier

3.4 Piezoelectric Force Transducers (PZFT) [10−1 to 109 N] A multicomponent piezoelectric force transducer (Fig. 3.4) measures the forces in three orthogonal axes [3]. Force F is transmitted to each of the three discs with the same magnitude and direction. Each piezoelectric crystal ring (shown “exploded”) has been cut along a specific axis and the orientation of the sensitive axis coincides with the axis of the force component to be measured. Each then produces a charge proportional to the force component specific to that disc and their charge is collected via the electrodes inserted into the stack. The most impressive advantage of piezoelectric force transducers is their “rangeability”, extended from milli- to mega-newtons. They have instantaneous “zoom-in” capability (up to 105 ) and provide higher accuracy and finer resolution measurements of small force changes in the presence of a much larger background force. Finally, piezoelectric transducers sizes are smaller but they cost more than SG sensors of equivalent capacity.

3.5 Electromagnetic Force Transducers [10−14 to 100 N] The basic factor for the magnetoresistivity is the Lorentz force, which causes the electrons to move in curved paths when passing through an electromagnetic field. For small values of the magnetic field, the change in resistance R is proportional to the square of the magnetic field strength H. Typically, four magnetoresistive strips made of permalloy are arranged in a meander pattern to form the arms of a Wheatstone bridge for differential measurement. The magnitude of the bridge imbalance R is used to indicate the magnetic field strength variation as a result of the applied force [5].

3.5 Electromagnetic Force Transducers [10−14 to 100 N]

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Fig. 3.5 Magnetoresistive force transducer with high stiffness

A magnetoresistive force transducer of 1 N is shown in Fig. 3.5. With an appropriate arrangement of the magnetoresistive (MR) field plates and studs into a functionally designed transducer, a linear behaviour and a high resolution of 2 nm is obtainable.

3.6 Electrodynamic Force Transducers (EMFC) [10−2 to 103 N] The weighing cell with compensation is based on a principle similar to that of an electrodynamic loudspeaker. The measuring coil, crossed by the current I, is placed inside a ring-shaped air gap, with a constant magnetic induction B. The coil is joint to the weighing pan, and the amplifier output current produces a gravitational force G, that keeps in balance (“levitates”) the load to be measured. The load overtaking system uses a parallelogram-shaped spring combination [3] (Fig. 3.6). The compensating current depends on the actual position of the contactless sensor, that reacts to any deviation from the initial position, due to the object put on the pan. The error signal is processed and applied to the amplifier, which generates the current needed to restore the balance. The circuit damping is adjusted to ensure a minimal delay of the entire measuring chain (answering time of the order of 80 ms).

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Fig. 3.6 Electrodynamic weighing cell using the EMFC (electromagnetic force compensation) principle

3.7 Magnetoelastic Force Transducers [103 to 107 N] The loading applied on the magnetoelastic element of a force transducer modifies the properties of its magnetic circuit. For example, in Fig. 3.7a a metallic ferromagnetic device changes its permeability μ when subjected to a mechanical stress acting on two spiral-shaped plane coils (primary and secondary), coupled to the amorphous ferromagnetic strips [6]. This is a patented model DE 4309 413 C2 of ME-SG made by ME-Meßsysteme GmbH. A sensitivity factor of 10−8 m/m is obtained, which is two orders of magnitude higher that of resistive strain gauges (1 microstrain = 10−6 m/m). A lot of shapes of elastic elements with adequate magnetic circuits are used in order to process a multitude of mechanical measurands, each of them having specific advantages and disadvantages. Alongside the elastic material and the magnetic circuit shape, the electronic circuitry is a decisive factor in magnetoelastic force transducers.

Fig. 3.7 Magnetoelastic sensors: measuring principle (a) and solutions with four coils, for axial (b) or torsional (shearing) loading (c)

3.7 Magnetoelastic Force Transducers [103 to 107 N]

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Transformer, oscillator and multivibrator are usual solutions in this respect, and analog or binary output signals are obtained, at either frequency (up to 20 kHz).

3.8 Galvanomagnetic Force Transducers (Hall-Effect) [10−12 to 100 N] While at most humanoid robots, force sensing, integrated in the foot, uses the strain gauge principle, here is presented a solution based on the principle of sensing the changes in magnetic flux density (MFD) to measure the deflection of the elastomeric materials and thereby measure the force [7]. As shown in Fig. 3.8, a customized PCB contains five magnetic Hall-effect sensors to the upper plate. A small cylindrical magnet with uniform magnetization M is embedded in the middle of the lower plate. The choice of a small magnet and close proximity between sensors and magnet creates a compact system as well as minimizing possible ferromagnetic disturbances. These five sensors are specially orientated and positioned to possess elevated spatial field sensitivity; S0 being centered at the axis of the magnet is particularly sensitive to Z axis movement of the magnet and the remaining surrounding sensors (S1 and S2 with spacing dX and, respectively, S3 and S4 with spacing dY) are more

Fig. 3.8 Three-directional Hall-effect transducer: galvanomagnetic working principle on three axes (a), top view (b) and side view (c) for magnet and five Hall sensors assembly, and front foot assembly for running quadruped (d)

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responsive to change to lateral position of the magnet. This force transducer, which weighs only 24.5 g, provides a measurement range of 1 kN normal to the ground and up to ±125 N parallel to the ground. The mean force measurement accuracy was found to be within 7% of the applied forces. These Hall devices have the best linearity among all electromagnetic force transducers. Their sensitivity can be increased up to 15 mV/μm by decreasing the gap between the Hall sensors and the associated magnets to the minimum feasible (about 0.1 mm) [1].

3.9 Vibrating-Wire Force Transducers (VWFT) [10−1 to 107 N] The fundamental frequency f 0 of the vibrating wire, expressed in hertz, is given by the equation for simple harmonic motion in the mechanics of oscillation written in Fig. 3.9, where l is the vibrating wire length, σ —its stress (tension) and ρ—the wire density. A transducer of this type is inserted in the feedback loop of an electronic oscillator, in order to generate continuous oscillations on the vibrating-wire resonance frequency. The electronic circuit is a usual arrangement using operational amplifiers. The existence of a variation in electric current does not affect the gauge constant and therefore does not impair the measurement accuracy [3]. The sensitivity of the vibrating-wire transducer, S vw is defined as the ratio between the relative variation of frequency and the relative variation of wire length, which is the specific deformation ε. Experimental results demonstrated that the force measurement sensitivity by means of vibrating wires differentially installed on an elastic element (say a ribbed membrane) is higher than using other parametric transducers (R, L or C). This sensitivity reaches the value of 300 for VWFTs as compared to the 250 in case of the differential capacitor and with the 200 in case of the semiconductor (or only 2 for metallic) strain gauged transducers [8]. Fig. 3.9 Circuit diagram of a vibrating wire measurement system

3.10 (Micro)Resonator Force Transducers [10−13 to 105 N]

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Fig. 3.10 Double-ended tuning fork (metal device used to tune musical instruments) excitation and detection (a) and tubular resonator (like a circular cage) with four equispaced DETFs (b)

3.10 (Micro)Resonator Force Transducers [10−13 to 105 N] The double-ended tuning fork basic structure consists of twin beams which are coupled at their roots (Fig. 3.10a). When one of the tines in this symmetrical structure is excited at its fundamental frequency, the opposite tine resonates in phase. Due to symmetrical vibration of the DETF, the vibration of counterforces F 1 , F 2 and bending moments M 1 , M 2 cancel at the roots, so that at each end of the resonator beyond the roots. This unique property of the DETFs allows them to be clamped beyond their vibrational roots without significant loss of vibrational energy or reduction in their mechanical quality factor Q. The fundamental frequency of a beam is a function of its material properties and dimensions. Its relationship is more complicated than for vibrating wire, but may be used to design a DETF to operate at a particular frequency (usually in the audio range). Mechanical resonators have excellent stability and potentially low hysteresis [9]. Adding the frequency output feature, a considerable practical interest is achieved. Digital transducers have many advantages over conventional analog transducers: high reliability, immunity to electrical interference, good stability with time, etc.

3.11 Acoustic Force Transducers (SAW) [10−3 to 102 N] An acoustic transducer is a device in which an (ultra)sonic signal is used as an intermediate quantity between its input (in this case, force) and output. The measurement principle: When a strain is applied to a substrate containing a SAW (surface acoustic wave) delay line, the propagation path length changes with l, and we obtain for θ (rad), the phase difference between the input and output waves, the expression included in Fig. 3.11a where l—propagation distance (m) between emitter (IDT 1) and receiver (IDT 2), ν—propagation velocity (m/s), and ω—frequency (rad/s).

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Fig. 3.11 Overview of a loaded SAW delay line (a) and two-dimensional SAW transducer “combined” output, with wireless remote sensing system (b)

A two-dimensional strain transducer simultaneously measures strains in two different directions (x and y) using two delay lines that cross each other at right angles [10]. The elastic cantilever (Fig. 3.11b) was vibrated by periodic force with 50 Hz. Measuring the amount of phase change between the input and output of a delay line caused by the mechanical strain, one can determine the magnitude of other related physical quantities, starting with the applied force. When this new SAW transducer is connected directly to an antenna, it is capable of passive remote sensing. The SAW response almost coincides with the results of conventional strain gauge transducers. Being slower than electromagnetic waves by approximately five orders of magnitude, SAWs can be made more compact, furthermore increasing of their frequency, as well as their precision and resolution. They prove outstanding reproducibility and accuracy, as well as long-term stability.

3.12 Gyroscopic Force Transducers [101 to 103 N] A gyroscopic load cell exploits the force sensitive property of a gyroscope mounted in a gimbal system. It incorporates a dynamically balanced heavy rotor on a spindle mounted in the inner frame of a two-gimbal system (Fig. 3.12). This inner frame is then mounted in an outer frame which is suspended between two swivel joints. The arrangement has three axes of rotational freedom mutually at right angles and has their axis origin on the rotor centre of gravity. “The time taken for the outer gimbal to complete one revolution is a measure of the applied load F. This can be measured either by a disc carrying large numbers of lines mounted horizontally on the outer frame and counting these lines, or by monitoring the time taken for each revolution of the outer frame by a single sensor and displaying the inverse of this as a measure of the applied force (e.g. 0.5 s at a maximum load of 25 kg)” [11].

3.13 Comparison of Methods and Principles for Force Measuring

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Fig. 3.12 Gyroscopic load cell (drawing adapted from U. Erdem), a fast responding digital transducer, inherently free of hysteresis and drift

3.13 Comparison of Methods and Principles for Force Measuring Our systematization, logical and intuitive, covers almost all relevant force transducer types and is in accordance with other partial classifications: – Vibrating FTs, containing vibrating wires, tuning-fork resonators and SAW devices (ultrasonic transmitters) [12]. – Digital FTs (with frequency output), containing vibrating wires, tuning-fork resonators and gyroscopic transducers [13]. – Interdigital transducers (IDTs) (like combfingers) can be resonant, acoustic (SAW), as well as capacitive force transducers, and they are applicable for the measurement of a wide range of mechanical quantities [14]. Comparing the ranges of different types of force transducers: – Minimum values for force are indicated for electromagnetic force transducers (see Chap. 4), followed by microresonators and resistive (carbon nanotubes) ones. – Maximum values for force are indicated for piezoelectric force transducers, followed by magnetoelastic and resistive (strain gauges) ones.

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One may notice that resistive force transducers meet the requirements at both “ends”! Strain gauges are presented “in extenso” in C-5, while RSGFTs, used in a wide range of applications, are treated under various aspects in the rest of the chapters.

References 1. Stef˘ ¸ anescu, D.M., Anghel, M.A.: Electrical methods for force measurement—a brief survey. Measurement 46(2), 949–959 (2013) 2. Millea, A.: Electrical Measurements. Principles and Methods (in Romanian). Editura Tehnic˘a, Bucure¸sti, Romania (1980) 3. Kemény, T.: Mérlegtechnikai kézikönyv (Handbook of Weighing Technology), 681 pp. Technical Publishing House, Budapest (1981) 4. Procter, E., Strong, J.T.: Capacitance strain gauges. In: Window, A.L., Holister, G.S. (eds.) Strain Gage Technology, pp. 291–325. Elsevier Applied Science, London and New York (1989) 5. Prinz, R., Charvat, R.: Sensor mit magnetoresistivem System zur Messung extrem kleiner Wegdifferenzen. In: Proceedings of the Sensor 88 Conference, pp. 319–334, Nuremberg, West Germany, 3–5 May 1988 6. St, Keil: Historische Rückschau auf Entstehung und Entwicklung des Dehnungsmessstreifen. CUNEUS, Lippstadt (2006) 7. Ananthanarayanan, A., Foong, S.H., Kim, S.B.: A Compact Two DOF Magneto-elastomeric Force Sensor for a Running Quadruped DARPA M3 Program. Massachusetts Institute of Technology, Cambridge, MA. PDF created on 11 Feb 2012 8. Stef˘ ¸ anescu, D.M.: Handbook of Force Transducers—Principles and Components. Springer, Berlin and Heidelberg (2011) 9. Cheshmehdoost, A., Jones, B.E.: A new cylindrical structure load cell with integral resonators. In: Proceedings of the SENSORS VI: Technology, Systems and Applications, pp. 429–434. IOP Publishing, Bristol, UK (1993) 10. Nomura, T., Kawasaki, K., Saitoh, A.: Wireless passive strain sensor based on surface acoustic wave devices. Sens. Transducers J. 90, 61–71 (2008) 11. Erdem, U.: Force and weight measurement. J. Phys. E: Sci. Instrum. 15, 857–872 (1982) 12. Hunt, A. (coord.): Guide to the Measurement of Force. The Institute of Measurement and Control, London, UK (1998). ISBN 0-904457-28-1 13. Zecchin, P. (chair): Digital Load Cells—A Comparative Review of Performance and Application. The Institute of Measurement and Control, London, UK (2003). Document WP0803 14. Varadan, V.K.: Tutorial Course on Smart Sensors and Materials. In: Proceedings of the 11th Conference on Asia-Pacific Nondestructive Testing, Jeju Island, South Korea, 4 Nov 2003

Chapter 4

Application of Electromagnetic and Optical Methods in Small Force Sensing

4.1 Electromagnetic Methods Classification in Force Sensing As compared with classical strain gauges technique, the electromagnetic measurements feature a high sensitivity in detecting smallest changes, resulting in very high resolution in the range of micro-, nano- and pico-newtons. There are several ways to sense magnetic fields, most of them based on the intimate correlation between magnetic and electric phenomena [1]. Specific differences are: • The magnetoelectric (ME) effect is the appearance of an electric polarization P in a material when a magnetic field H is applied, and conversely, • The electromagnetic (EM) effect is the appearance of a magnetization M in a material when an electric field E is applied. Any electromagnetic system includes at least a coil, an air gap and a magnet made of ferromagnetic materials [2]. The main function of every electromagnet is the conversion of supplied electrical energy into mechanical work, actuating specific tools. This process is reversible (Barry Jones, Brunel University, London: “An actuator is an inverse transducer.”) and a lot of force transducers (FTs) are based on electromagnetic methods or principles. Significant quantities and energetic conversion processes are shown in Fig. 4.1. Among the various works trying to systematically deal with electro-magnetic principles used in measurement of mechanical quantities, the most recent and complete is [3]. It is remarkable that more than half of the dozen selected types of force transducers presented in the above-mentioned book are connected with the electromagnetic measurements (their relative sensitivities are compared in Fig. 4.2): • Magnetoresistive (parametrical): from anisotropic (AMR) to giant (GMR); • Elastomagnetic, based on magnetostrictive effects (Villary—change of magnetic susceptibility due to applied mechanical load, or Matteucci—helical anisotropy and magnetomotive force induced by a torque); • Galvanomagnetic (based on Hall elements); © Springer Nature Switzerland AG 2020 D. M. Stef˘ ¸ anescu, Handbook of Force Transducers, https://doi.org/10.1007/978-3-030-35322-3_4

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Fig. 4.1 Schematic of electro-magneto-mechanical energy conversion for actuators and transducers

Fig. 4.2 Relative sensitivity for various magnetic field measurement techniques. Magnetic induction B is expressed in T (tesla)

• • • •

Electrodynamic, i.e. force balance principle; Resonant transducers: vibrating wire (VW), double-ended tuning fork (DETF); Electromagnetic acoustic transducer (EMAT); Optoelectromagnetic, e.g. Lorentz force magnetic field sensor with optical readout [4].

4.2 Applications of Electromagnetic Methods in Small Force Sensing Here are a few significant applications, shortly presented in my Open Access Paper in Belfast, UK, at the XXII IMEKO World Congress [5].

4.2.1 Magnetoresistive Sensing Sahoo et al. [6] presented a novel MagnetoResistive-sensor-based Scanning Probe Microscopy (MR-SPM) technique (Fig. 4.3). The basic idea is to convert the cantilever motion into a relative displacement between its micromagnet and a multilayered GMR sensor chip (four resistances connected in a Wheatstone bridge). When

4.2 Applications of Electromagnetic Methods in Small Force Sensing

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Fig. 4.3 Scanning probe microscopy (SPM) based on magnetoresistive sensing: approach—retract curve of resistance change (a) and experimental setup (b)

the cantilever experiences a tip-sample force, its deflection is translated into an electrical resistance change as a function of the magnetic field, with 84 pm resolution over 1 MHz bandwidth.

4.2.2 Magnetostrictive Sensing A research group from Auburn University developed a MagnetoStrictive MicroCantilever (MSMC) as a sensing platform actuated by a magnetic field [7]. Due to its magnetic nature, the microcantilever vibration results in an emission of a magnetic signal, which is sensed using a pickup coil (the simplest electromagnetic measuring configuration).

4.2.3 Galvanomagnetic (Hall-Effect) Sensing A multifunctional sensing device with two Hall elements and a magnet (all embedded in silicone rubber) to detect the normal contact force and the temperature is presented in [8]. When a magnetic field is applied at a right angle to the current flow, a small Hall voltage VH appears across the semiconductor plate, following the formula given in Fig. 4.4a. When a finger contacts with a tactile sensor, the magnetic field strength at the Hall element increases, because the silicone rubber is deformed by the normal contact force (Fig. 4.4b). In Sect. 3.8 a three-directional Hall-effect transducer (one magnet and five Hall sensors) has been described.

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Fig. 4.4 Magnetic tactile sensing with two Hall elements: a principle, b experimental device

Fig. 4.5 EMFC principle: 1—weighing pan, 2—load suspension, 3—transmission lever, 4—coupling, 5—parallel lever, 6—bearing, 7—zero position detector, 8—electrodynamic actuator

4.2.4 Electrodynamic (EMFC) Sensing Diethold et al. discuss the determination of spring constants of AFM (atomic force microscopy) cantilevers using force—displacement measuring equipment based on electromagnetic force compensated (EMFC) transducer [9], as shown in Fig. 4.5. This equipment can also be used for flow, angle and other mechanical quantities measurements. Two proposals for improving the EMFC systems are: • Double force compensation using two coils (coarse and fine adjustments) [10]; • Replacing the weighing pan with a Roberval type active elastic element (straingauged double guided cantilever beam) while the feedback force is generated with an electromagnetic force transducer (EMFT) or “forcer” [11].

4.2.5 Superconducting Quantum Interference Device (SQUID) The piconewton force standard system from KRISS utilizes the magnetic flux quantization in a superconducting ring at the position of zero magnetic fields [12]. When a soft cantilever with an anisotropic magnetic sample at its end is placed in a uniform

4.2 Applications of Electromagnetic Methods in Small Force Sensing

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Fig. 4.6 Schematic of quantum-weight generating cantilever device

magnetic field, a varying magnetic torque is exerted on the cantilever, depending on its deflection (Fig. 4.6). The applied magnetic force can be increased or decreased by a step estimated as 0.184 pN for a niobium ring having inner and outer radii 5 µm and 10 µm, respectively, and thickness 50 nm.

4.2.6 Microforce Sensor Based on Floating-Magnetic Principle A special sensor for microbiological applications has a naturally stable six degrees of freedom equilibrium state using the combination of upthrust buoyancy and magnetic force [13]. The sensing element is a triangular platform suspended by three small buoyancy tanks (Fig. 4.7). Each buoyancy tank is formed by two fixed cubic magnets M1 (5 × 5 mm) and a cylindrical moving magnet M2 placed at the corner of the triangular platform, inside a float. The platform mass is supported against gravity by the combined upthrust buoyancy of the three floats. Conclusion of the authors: “In case of an external force applied to the float which tends to push it beyond the point of equilibrium, the activation of coils will produce an opposed electromagnetic force that will maintain the float in its initial position. In this case, the current I in the coils is the new physical value related to the external force.” Final result: measuring ±100 µN with 20 nN resolution (like AFM microcantilevers).

4.2.7 Manipulation of Magnetic Skyrmions by Mechanical Force Skyrmions are tiny nanometer-sized magnetic vortices found at the surface of magnetic materials. They are considered useful for new high-density memory devices because of their small size and relative stability. According to Yoichi Nii of the Emergent Device Research Team (Japan) [14], the force to create and destroy skyrmions

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Fig. 4.7 Floating-magnetic principle for microforce biosensor: Platform configuration with L1 and L2 of the three buoyancy tanks (a) and floating mechanism L1 equipped with two coils (b)

was quite low, less than ten nanonewtons (10 nN) per skyrmion, which is comparable to the pressure exerted by the tip of a conventional pencil when we write in a notebook. A force applied perpendicular to the magnetic field led to the creation of skyrmions, while one parallel to the field turned them off, making it possible to turn them on and off mechanically (Fig. 4.8).

Fig. 4.8 Next generation magnetic memory based on skyrmions (a) and creating and deleting skyrmions using mechanical stimulus (b)

4.3 Perspectives in Small Force Measurements

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4.3 Perspectives in Small Force Measurements The contribution of the National Institute of Standards and Technology (USA) in metrological research for small mass (1 mg and lower) and small force (10 µN and lower) is presented in [15], with emphasis on the implications of redefining the kilogram in terms of Planck’s constant and achieving accurate measurements with quantified uncertainty. Starting from the scale of forces [newton], in the form of a stalactite “descending” from the gravitational field to the electromagnetic and optical ones, that are intertwined, six pictures have been combined into a large sketch (Fig. 4.9), rele-

Fig. 4.9 Specific instrumentation for small force measurement

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Fig. 4.10 PTB Scanning Electron Microscopy (SEM) image (a) and the scanning force microscopy (SFM) equipment (b)

vant for the small force measuring instrumentation, with several key words: atomic force microscopy, optical interferometer and laser. For instance, the force exerted by a laser pointer is only 6.6 pN. And a complex application: a high-sensitivity, high-resolution interferometric micro-opto-electro-mechanical system (MOEMS) for measuring acceleration, force, and pressure of fluids during flow, described in the US patent [16]. Closing this chapter, we mention a recent application from the PTB Newsletter (German Metrology Institute) [17]. When harvesting energy, the mechanical properties (elasticity and hardness) of the flexible columns play an important role, and even more in the nano-range. The geometrical parameters have been determined by means of SFM, while a correction procedure has been developed by PTB using SEM image of a typical GaN pillar after a nanoindentation measurement with a Berkovich indenter (Fig. 4.10a). The best illustration of the SFM principle (Fig. 4.10b) is given by Technical University Leipzig in [18], with the following description: The SFM scans surfaces line by line and assembles topographical images. A laser beams onto a cantilever and reflects from it onto a set of position sensitive photodiodes. While the sharp tip of the cantilever moves over the sample, the cantilever itself bends in consistency with the surface and the photodiodes register the resulting position changes of the laser reflection. Two piezos (a piezoelectric element expands or contracts in direct proportion to an applied electric field) generate the scanning movement of cantilever, laser and photodiodes in x- and y-direction. The signal from the photodiodes goes to a z-piezo, that moves the cantilever up or down to compensate the cantilever deflection. The information of the deflection is used to assemble an image. Since an SFM can image and probe samples in both dry and liquid environments, it is possible to work with living cells under physiological conditions.

4.3 Perspectives in Small Force Measurements

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Finally, comparing the electromagnetic methods for measuring small forces, the magnetoresistors (especially GMR) detect a larger magnetic field than the Hall-effect sensors but the latter satisfy a wider range of applications related to the measurement of mechanical forces. And, these Hall devices have the best linearity among all electromagnetic force transducers. Specialized electromagnetic instrumentation for micro- (EMFC), nano- (AFM and SFM), and pico-force (SQUID) measurements was also presented in this chapter.

References 1. Wang, Y., Li, J., Viehland, D.: Magnetoelectrics for magnetic sensor applications: status, challenges and perspectives. Mater. Today 17, 269–275 (2014) 2. Gadyuchko, A., Kireev, V., Rosenbaum, S.: Potentials of magnetic measurement technology in development and production of electromagnetic actuators. In: Proceedings of IMEKO XXI World Congress, pp. 578–583, Prague, Czech Republic (2015) 3. Stef˘ ¸ anescu, D.M.: Handbook of Force Transducers—Principles and Components. Springer, Berlin and Heidelberg (2011) 4. Keplinger, F., Kvasnica, S., Jachimowicz, A., Kohl, F., Steurer, J., Hauser, H.: Lorentz force based magnetic field sensor with optical readout. Sens. Actuator A Phys. 110, 112–118 (2004) 5. Stef˘ ¸ anescu, D.M.: Application of electromagnetic methods in force sensing, with emphasis on micro, nano and pico ranges. In: Open Access Proceedings of the XXII IMEKO World Congress Knowledge Through Measurement, Paper 407, Belfast, UK, 3–6 Sept 2018. Published under license by IOP Publishing Ltd., Journal of Physics: Conference Series, Vol. 1065, Measurement of Force, Mass and Torque 6. Sahoo, D.R., Sebastian, A., Häberle, W., Pozidis, H., Eleftheriou, E.: Scanning probe microscopy based on magnetoresistive sensing. Nanotechnology 22, Paper 145501 (2011) 7. Fu, S.L., Zhang, K., Chen, I.-H., Petrenko, V.A., Cheng, Z.: Magnetostrictive microcantilever as an advanced transducer for biosensors. Sensors 7, 2929–2941 (2007) 8. Yuji, J., Shiraki, S.: Magnetic tactile sensing method with Hall element for artificial finger. Kumamoto National College of Technology, Yatsushiro, Japan. PDF created on 31 Jul 2013 9. Diethold, C., Kühnel, M., Ivanov, T., Rangelow, I.W., Fröhlich, T.: Determination of AFMcantilever spring constants using the TU Ilmenau force displacement measurement device. In: Proceedings of IMEKO XXI World Congress, pp. 175–180, Prague, Czech Republic (2015) 10. Choi, I.-M., Choi, D.-J., Kim, S.H.: Double force compensation method to enhance the performance of a null balance force sensor. Jpn. Soc. Appl. Phys. 41, 3987–3993 (2002) 11. Izumo, N., Nagane, Y.: Super-hybrid-sensor for new balances. In: Tojo, T., Ohgushi, K. (eds.) ACTA APMF 2000, Tsukuba, Japan (2000) 12. Choi, J.-H., Lee, K.-C., Kim, Y.-W., Kim, M.-S.: Characterization of quantum-weight generating cantilever device. In: CD Proceedings of IMEKO International Conference on Cultivating Metrological Knowledge, Merida, Mexico, Session 1.1 (2007) 13. Cherry, A., Abadie, J., Piat, E.: Microforce sensor for microbiological applications based on a floating-magnetic principle. Laboratoire d’Automatique de Besançon, 21 Mar 2007 14. Nii, Y., Nakajima, T., Kikkawa, A., Yamasaki, Y., Ohishi, K., Suzuki, J., Taguchi, Y., Arima, T., Tokura, Y., Iwasa, Y.: Uniaxial stress control of skyrmion phase. Nat. Commun. RIKEN Press release in Materials Today 13 Oct 2015 https://doi.org/10.1038/NCOMMS9539

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15. Shaw, G.A.: Current state of the art in small mass and force metrology within the international system of units—topical review. Meas. Sci. Technol. 29(7) (2018) 16. Li, C.C.: Interferometric MOEMS sensor. US Patent 7518731, 14 Apr 2009 17. Li, Z., et al.: Mechanical characterization of nanopillars—accurate correction of indentation results from nanoobjects. PTB News (3), 2 (2018) 18. Gerdelmann, J., Brunner, C., Pawlizak, S.: Scanning Force Microscopy—Universität Leipzig. Soft Matter Physics Division (2009)

Chapter 5

Strain Gauges—Resistive and Other Principles

A strain gauge (SG) is, generally speaking, a device used to measure strain on an elastic material (Wikipedia) or (from the instrumentation point of view) a sensor whose resistance varies with applied force (Omega Engineering). Meeting both definitions, it is to be mentioned that strain gauges (American: gages) are bonded on the elastic elements (EE) of the force transducers (FT) or integrated as piezoresistors into the monolithic structure of the FT EEs. Beyond the classical “resistive strain gauge”, we mention that a half of the force measuring methods and principles, presented in Chap. 3, have SG versions as well, demarcation being set by the 2D character (planar, bidimensional), in contrast with the 3D one (volume, tridimensional)—not suitable to miniaturization/flattening. Therefore, in the next sub-chapters the following classes of strain gauges will be presented: resistive, capacitive, piezoelectric, magnetoelastic, acoustic (SAW), and optical.

5.1 Resistive Strain Gauges Without resuming well-known information, but pointing out the main technological development stages of bonded strain gauges, so-called because they are attached to (or integrated into) the elastic element surface, they may be metal foils, thin or thick films (semiconductors), piezoresistive (silicon technology) and solid-state devices. Also resistive, but not bonded on the elastic structure are stretched wires (Fig. 16.1) or the Tekscan “patches” (force sensing resistors) appearing in Fig. 13.4 and in Chap. 18—but these are not strain gauges [1]. Figure 5.1 illustrates the spectacular leap in the carbon technology, from SGs drawn on paper with a pencil to graphene CNTs, which brought the Nobel Prize for Physics to Geim and Novoselov, Manchester University, UK (2004).

© Springer Nature Switzerland AG 2020 D. M. Stef˘ ¸ anescu, Handbook of Force Transducers, https://doi.org/10.1007/978-3-030-35322-3_5

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5 Strain Gauges—Resistive and Other Principles

Fig. 5.1 Strain gauge evolution from graphite resistor (a) to MWCNT (multi-walled carbon nanotube) (b)

5.1.1 Bonded Metallic Strain Gauges A wide selection of classical strain gauge types is shown in Fig. 5.2. Remind that in Fig. 2.5 strain gauge models produced by Kaliber MM Company (Budapest) have been presented, with details concerning their location on various elastic elements of force transducers.

Fig. 5.2 Various bonded metallic strain gauge types made by HBM

5.1 Resistive Strain Gauges

53

5.1.2 Piezoresistive (Silicon) Strain Gauges An elastic structure configured by silicon micromachining and using the piezoresistive effect (change in resistance due to strain) of implanted silicon resistors [2] is shown in Fig. 5.3. A flexible piezoresistive sensor matrix based on a carbon nanotube PDMS composite for dynamic pressure distribution measurement is presented in [3]. A circuit containing both active and passive stress-sensitive elements (a differential amplifier utilizing two n-p-n piezotransistors and, respectively, four p-type piezoresistors) is depicted in Fig. 5.4 [4]. A comparative analysis of a pressure transducer utilizing this circuit with a membrane equipped with four piezoresistors connected in Wheatstone bridge and built on the same mechanical part shows that the “active” silicon membrane has a sensitivity of 0.66 mV/V/kPa, which is 2.2 times greater than those of the “passive” solution.

Fig. 5.3 Piezoresistive strain gauges on microstructures for acceleration measurements: top view (a), cross-sectional view (b), normal loading (c), lateral loading (d), strain gauges in Wheatstone bridge connections and operational amplifiers (e) (with kind permission from Springer International Publishing AG: DMS—Handbook of Force Transducers (2011), p. 453, Fig. 25.7)

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Fig. 5.4 Integral strain gauge (on-chip differential amplifier with bipolar piezotransistors and four piezoresistors) (a) and von Mises stress of the silicon membrane including the piezoresistive elements’ arrangement (b)

5.1.3 Carbon Nanotubes (CNTs) Single wall carbon nanotubes (SWCNTs) are hollow cylinders of graphene composed of a single layer of carbon atoms, densely packed in a honeycomb crystal lattice [5]. The tubes’ length can be several micrometers and their diameters on the order of 1 nm; owing to very high aspect ratios, they ensure gauge factors from −376 (for semi-conducting) up to 856 (for small-gap semiconducting), much better than ±100 (for p- or n-type silicon) or 2 (for constantan alloy). A strain gauge type MWCNT (multi-walled carbon nanotube) is depicted in Fig. 5.1b. A lot of interesting applications with strain gauges based on CNT are indicated in a topical review [6] and in the presentation of the Chair for Measurement and Sensor Technology (Chemnitz University of Technology) lead by Prof. Dr.-Ing. Olfa Kanoun, see a recent application in [3]. CNTs can be connected in Wheatstone bridge, like piezoresistors, but can be also microresonators, piezoelectric or more complex MEMS devices.

5.2 Capacitive Strain Gauges

55

Fig. 5.5 Capacitive force transducer based on variable distance between its plates

5.2 Capacitive Strain Gauges Unlike the capacitive force transducer of differential tubular type, presented in Fig. 3.3, the capacitive transducer in Fig. 5.5 is based on the variation of distance d between plates [7]. Due to its “flattened” structure, it might be enclosed into the “Strain Gauges” category.

5.3 Piezoelectric Strain Gauges Two types of piezoelectric strain gauges with lead attachments and associated dimensions, made by TE Connectivity [8, 9] are presented in Fig. 5.6a–d. Both types are sensing contact forces or impact events. Another model, made by Neue Materialien— Würzburg, is a flexible piezoelectric composite (Fig. 5.6e), a “meander type” just like classical resistive strain gauges [7], which can be used in actuator regime too. Experimental results reveal that the MEMS piezoelectric sensors are able to achieve a better resolution than piezoresistors, while piezoresistors can be built in much smaller areas. Both types of strain sensors are capable of high sensitivity measurements [10].

5.4 Magnetoelastic Strain Gauges Two models of magnetoelastic strain gauges are presented in Fig. 5.7, different from those in Fig. 3.7, mentioning that their sensitivity factor of 10−8 m/m is two orders of magnitude higher that of the resistive strain gauges [7].

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Fig. 5.6 Several types of piezoelectric strain gauges: polymer Mylar (a) with dimensions (b), film PVDF (c) with dimensions (d), and composite (e)

5.5 Acoustic Strain Gauges (SAWs) SAW torque transducer technology, like the piezoresistive and magnetoelastic ones, utilizes the principal tensile and compressive strains, which act at ±45° to the axis, on the surface of a shaft in torsion (Fig. 5.8a). Typically, two SAW devices are mounted directly on the shaft [11] and connected in a half-bridge configuration. A differential measurement of resonant frequency (nominal value: 433 MHz) is performed in order to achieve temperature compensation and eliminate sensitivity to shaft bending. As previously shown in Fig. 3.11c, a SAW force transducer can be connected with a wireless remote sensing system and practical details are given in Fig. 5.8b [12].

5.5 Acoustic Strain Gauges (SAWs)

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Fig. 5.7 Magnetoelastic strain gauges: solutions with two coils, for axial (a) or shearing loading (b)

Fig. 5.8 TorqSense solution with two SAW strain gauges (a) and a simple exploded view of a SAW strain sensor with antenna (b)

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5.6 Optical Strain Gauges Some optical strain gauges (Fig. 5.9a) are based on a Bragg grating which comprises a large number of reflection points written into the fiber at periodic spacing (Fig. 5.9b). The wavelength of the light that is reflected by these reflection points with constructive interference thus depends on the spacing of these reflection points. Therefore, the wavelength of the reflection peak changes when strain is applied. They have a few important advantages [13]: – – – –

insensitive to electromagnetic interferences, application in Ex-areas possible, lower wiring outlay compared to electrical strain gauges, lower mass of glass fiber compared to standard connecting cables.

Geokon fiber optic strain gages (Fig. 5.9d) are designed for use in difficult environments, measuring strains in “older” concrete surfaces, capable of signal transmission over long distances (tunnels, bridges) and suitable for both static and dynamic measurements. They have another operating principle: Geokon gauge comprises a fiber optic cable with a miniature Fabry-Perot strain sensor (interferometer), embedded into a composite carbon fiber laminate, made of uniaxial fibers [14].

Fig. 5.9 Optical strain gauge HBM (a) with Bragg grating (b) and associated instrumentation (c). Fabry-Perot interferometer (f) integrated into an optical fiber Geokon (d) with associated instrumentation (e)

5.6 Optical Strain Gauges

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The best schematic diagrams for both types of optical strain gauges (Fig. 5.9b+f) and for their associated instrumentation (Fig. 5.9c+e) are given in [15]. Since no guideline covering the characteristics of optical strain gages exists as yet, optical strain gauges from HBM have been qualified in compliance with VDI/VDE 2635 “Dehnungsmessstreifen mit elektrischem Messgitter, Kenngrößen und Prüfbedingungen (Strain gauges with electrical measuring grid, characteristics and test conditions)” [16]. When subject to strain, the spacing between the reflection points of Bragg grating increases. The gauge factor for this effect is exactly 1, because only the change in length of the fiber directly effects a change in spacing of the reflection points. Poisson’s ratio may be neglected. Remember, typical gauge factor k for resistive strain gauges is 2. k=

R/R l/l

(5.1)

Here is a complete analogy to the relationship for the resistive strain gauge, where R is replaced by λ: λ =k·ε λ λ λ k ε

Base wavelength of the fiber Bragg grating Wavelength change at the strain impressed in the grid Gauge factor Strain.

Optical strain gauges allow stress tests at high numbers of load cycles (fatigue behavior) even with materials with high strains as well as multiplexing. Several optical strain gauges (up to 13) can be integrated in a single fiber of glass. The optical measurement chain thus adapts to the individual requirements of a complex instrumentation application.

References 1. The difference between Force Measurement Techniques, Machine Design—Tekscan. PDF created on 8 Mar 2018 2. Pavelescu, I.: Acceleration microstructures for industrial applications. Institute for Microtechnology (IMT), Bucharest, Romania, Research Standing (Dec 2002) 3. Ramalingame, R., Hu, Z., Gerlach, C., Rajendran, D., Zubkova, T., Baumann, R., Kanoun, O.: Flexible piezoresistive sensor matrix based on a carbon nanotube PDMS composite for dynamic pressure distribution measurement. J. Sens. Sens. Syst. 8, 1–7 (2019). https://doi.org/ 10.5194/jsss-8-1-2019 4. Basov, M., Prigodskiy, D.M.: Investigation of sensitive element for pressure sensor based on bipolar piezotransistor (Translated from Russian) Nano- i Mikrosistemnaya Tekhnika, Nov 2017

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5. Hierold, C., Stampfer, C., Helbling, T., Jungen, A., Tripp, M., Sarangi, D.: CNT based nano electro mechanical systems (NEMS). In: Proceedings of IEEE International Symposium on MicroNanoMechatronics and Human Science—MHS2005, pp. 1–4, Nagoya University, Japan, 8–9 Nov 2005. ISBN 0-7803-9482-8 6. Obitayo, W., Liu, T.: Carbon nanotube-based piezoresistive strain sensors. J. Sens. Article 652438 (2012) 7. Keil, S.: Historische Rückschau auf Entstehung und Entwicklung des Dehnungsmessstreifen. CUNEUS, Lippstadt (2006) 8. Sensor Solutions. LDT1-028K Piezo Sensor Rev 1, TE Connectivity 7/2017 9. Sensor Solutions. DT Series Rev 1, TE Connectivity 7/2017 10. Kon, S., Oldham, K., Horowitz, R.: Piezoresistive and piezoelectric MEMS strain sensors for vibration detection. In: Proceedings of SPIE, The International Society for Optical Engineering, May 2007 11. Low-cost OEM rotary torque transducers. Special issue of IEN—Europe, PCNE 34, p. 32, Apr 2004 12. Belknap, E.: Mechanical characterization of SAW-based sensors for wireless high temperature strain measurements. Thesis in Mechanical Engineering, The Ohio State University, 2011 13. Optical strain gauges—measure and predict with confidence HBM. Data sheet B2266-6.1 en 14. Fiber Optic Strain Gage Model FP4000 Geokon—Geotechnical and Structural Instrumentation 15. Muhs, J.D.: Fiber Optic Sensors: Providing Cost-Effective Solutions To Industry Needs. Oak Ridge National Laboratory, Nov 2002 16. Kleckers, T., Günther, B.: Optical versus electrical strain gages: a comparison. Published at http://www.hbm.com/custserv/SEURLF/ASP/SFS/ID.802/MM.4,101,180/SFE/ techarticles.htm. ID number: 802_en

Chapter 6

Wheatstone and Other Bridge-Like Configurations

Strain gauges and Wheatstone bridge are indestructibly connected with electrical measurement of force and other mechanical quantities, representing together the “leading thread” of this book dedicated to applications in instrumentation. And, just as in the preceding chapter we also considered SGs other than resistive, here will equally treat other types of bridges than resistive.

6.1 Resistive Wheatstone Bridge 6.1.1 Wheatstone Bridge History The classic configuration associated to the strain gauges is the low-power, Wheatstone bridge as the electrical analog of the mechanical beam balance (Fig. 6.1a). This circuit is usually credited to Charles Wheatstone, although Hunter Christie demonstrated it for the first time in 1833. But Wheatstone had a better public relations agency, namely himself, and his name become a common substantive in this respect [1]. Strain gauges are primary sensors, but, after connecting them in Wheatstone bridge, a series of adjustments are necessary in order to become a transducer instrumented for measurement (Fig. 6.1b) [2]. In the second half of the last century these operations were performed by the experimenters themselves, whilst nowadays they are transferred to the computer. The risk of such an approach is that the transducer turns into a “black box”, making harder the grasping of physical phenomena. Three different and suggestive representations of strain gauges connected in full Wheatstone bridge are given in Fig. 6.2 [3].

© Springer Nature Switzerland AG 2020 D. M. Stef˘ ¸ anescu, Handbook of Force Transducers, https://doi.org/10.1007/978-3-030-35322-3_6

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Fig. 6.1 The Wheatstone bridge circuit in its original form (1843) (a) and the evolution from “sensor” (strain gauge) to “transducer” with Wheatstone bridge (b)

Fig. 6.2 Die-cut strain gauges manufactured by Baldwin-Lima-Hamilton Co. in the 1960s (a), schematic of bridge connections (b) and colour code (c)

6.1 Resistive Wheatstone Bridge

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6.1.2 Wheatstone Bridge Fundamental Properties The simple bridge (Fig. 6.3) is a complete network made of four sides (in which the usual strain gauge values are: 120 , 350  or 1 k), four nodes (A, B, C, D) and two diagonals (U A —supply, U E —signal). Our representation is in accordance with the Romanian tensometric standard and corresponds to the one of the European (German) School. The initials DMS mean DehnungsMeßStreifen, i.e. resistive strain gauges. Considering that the gauge is supplied from a constant voltage source of negligible internal resistance and its load is an amplifier having practically infinite input impedance and applying Kirchhoff’s laws, the following is resulting: R1 R4 UE = − UA R1 + R2 R3 + R4

(6.1)

The initial balance condition is: R1 · R3 = R2 · R4

(6.2)

meaning that the products of the resistances in the opposed arms should be equal and conforming the so-called golden rule of strain measurement by Wheatstone bridge: • the effects from two opposite arms are added, • the effects from two adjacent arms are subtracted. If R1 = R2 = R3 = R4 = R (the usual case in the resistive electrical tensometry) and the resistance variations are considered much lower than the proper resistance values, then one may write the relation [5]:

Fig. 6.3 Wheatstone bridge notations and colour code used in “Handbook of Force Transducers” [4]

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U E =

  U A R1 R2 R3 R4 − + − 4 R1 R2 R3 R4

(6.3)

One may notice that in unloaded state (ΔRi = 0, where i = 1, 2, 3, 4) the balance ΔU E = 0 is found again. In case of a mechanical load (ΔRi = 0) one may write synthetically:    R    U E = U A ·  R 

(6.4)

which, expressed in words, means that: The output voltage variation in the signal diagonal is proportional to the relative resistance variation of the Wheatstone bridge. We define the gauge factor k (also called strain coefficient of resistance) like in Formula (5.1), as k=

R/R l/l

(6.5)

where the relative variation l/l is just the strain ε. As a ratiometric device the Wheatstone bridge is extremely sensitive, its tensometrical sensitivity being expressed, after correlating the relationships (6.4) and (6.5), as S=

U E n = ·k·ε UA 4

(6.6)

where n is the number of active arms, also called the bridge factor: • n = 1 quarter bridge; • n = 2 half bridge; • n = 4 complete (Wheatstone) bridge, made of four strain gauges, all of them being fully sensitive to the applied principal strain. There are cases where the parameter n in Formula (6.6) is considered as an “equivalent number” of the bridge active arms and can have fractionary values. If two of the four strain gauges sense only the transverse effect of the axial loading, being “partially active” (Poisson’s coefficient ν = 0.3), one can consider n = 2.6 (namely: 2 × 1 + 2 × 0.3). A Wheatstone bridge is normally formed by four strain gauges, although it is possible to use only two SGs for a half-bridge or multiples of four on complex-shaped flexible bodies. As the “heart” of transducer, the Wheatstone bridge works in unbalanced (deviation) mode. Due to its perfect symmetry, the input (power supply or excitation) and the output (measuring signal or sensing) diagonals are interchangeable.

6.2 Other Bridge-Like Measuring Devices

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Fig. 6.4 LVDT representations: a RMP Rheinmetall Meß- und Prüftechnik, b RDP Group

6.2 Other Bridge-Like Measuring Devices Stricto sensu: At the beginning, the Wheatstone bridge was a group of four resistors connected in a bridge configuration, with a DC voltage supply in one of the diagonals and a null detector in another. Later on, this concept was extended to include: – Wheatstone bridge supplied from a (constant) current (not voltage) source, – unbalanced Wheatstone bridge (with a voltmeter instead of null detector), – AC Wheatstone bridge. A new concept also appeared, the Wheatstone half-bridge (more correct, “armpair”)—two series connected similar impedances (resistors, inductors or capacitors). All other measuring circuits, involving various effects (inductive, capacitive, magnetoresistive, galvanomagnetic, magnetoelastic, biparametric) are not Wheatstone bridges, even if some of them suggest a bridge-like topology. Actually, they could be called differential circuits/setups/methods. In the same category may be put TorqSense solution too (Fig. 5.8a), having two SAW strain gauges, bonded directly on the torsion shaft and connected in a half-bridge, besides the following six configurations.

6.2.1 Differential Transformer (LVDT) LVDT (linear variable differential transformer) electrical connections are depicted in Fig. 6.4. They can work as inductive half-bridges, inductances being represented by black rectangles (Fig. 6.4a) or by half-spirals (Fig. 6.4b).

6.2.2 Differential Capacitor The main components of the setup and electrical connections are presented in Fig. 6.5

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Fig. 6.5 A section of a differential capacitive force transducer (a) and its wiring scheme (b)

with explanations given in Sect. 3.3. The compensating rod gives the gauge length and also provides intrinsic temperature compensation.

6.2.3 Magnetoresistive “Bridge” A magnetoresistive sensor, made of a permalloy strip positioned on a silicon substrate, is shown in Fig. 6.6a [6]. Each strip is arranged in a meander topology and form an arm of a Wheatstone bridge (Fig. 6.6b). The degree of bridge imbalance depends on the magnetic field strength H under the applied load.

Fig. 6.6 Four magnetoresistors in a Wheatstone bridge configuration (a) and the equivalent circuit (b), and schematic of AMR Wheatstone bridge in an applied external field, H Y (c)

6.2 Other Bridge-Like Measuring Devices

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Fig. 6.7 Force transducer based on galvanometric Hall-effect (a) and the symbolization of Hall devices and their differential connection (b)

A version with AMRs (anisotropic magnetoresistances) is given in Fig. 6.6c. The “barber pole” patternings of each half bridge element are opposite to each other and this doubles the bridge output compared with that of the single magnetoresistive sensor [7].

6.2.4 Galvanomagnetic Transducer (Hall-Effect) A device for measuring displacement and force comprises two bodies of material linked together by a parallel beam linkage which permits displacement of one mass relative to the other in a single direction (Fig. 6.7a). The measurand is applied to one of the bodies in the allowed direction and is sensed by a sensor. In a preferred version [8], this is a Hall-effect sensor, attached to one of the masses (m1 ) and positioned between two magnets attached to the other mass (m2 ). Movement of one mass relative to the other changes the magnetic field around and is sensed by the Hall sensing device. A more sensitive solution is the differential one [9], using two Hall devices (e.g. Siemens KSY-10) connected like in Fig. 6.7b.

6.2.5 Magnetoelastic (Biparametric R–L Half-Bridge) The force transducer prototype (Fig. 6.8a) comprises two coils (excitation and readout). The main part of the set-up is the Terfenol-D rod with a diameter of d = 13 mm and a length of l = 28 mm. A magnetic return path of ferrite material is added to close the magnetic circuit, with reduced eddy current losses. Since Terfenol-D is a rather brittle material, it can easily be destroyed by tensile stress, so only compressive forces were applied [10].

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Fig. 6.8 Magnetoelastic force transducer based on a Terfenol-D rod (a) and block diagram of the electronic circuits (b)

Then the sensor arrangement was minimized by using a single coil to sense the magnetic reluctance Rm of the closed magnetic circuit comprising the sensor. Rm also depends on the permeability of the sensor material and thus results in the force dependent inductance L(F) of the sensing coil (Fig. 6.8b). This coil is part of a quarter AC-bridge with an appropriate impedance (L 1 , R1 ) and two variable resistances (R2 , R3 ), in order to balance the bridge which is driven by a small excitation signal with a frequency of 20 kHz. The bridge signal is demodulated by the linear variable differential transformer (LVDT) chip AD698 (Analog Devices). Following a demodulation and an analog-to-digital conversion step, the signal is further processed in a digital signal processing unit (Blackfin DSP, AD).

6.2.6 Biparametric (L–C Half-Bridge) Figure 6.9a shows the biparametric force transducer block diagram while Fig. 6.9b shows its measurement principle. The two electrodes are ferrite-made tips, outerplated and represent the electrodes of the capacitor C. On the inner side of one of the tips a one-layer plane coil L is printed. Both the coil inductance L and the capacitor capacitance C vary depending on the distance δ between the electrodes. The coil and the capacitor are connected as an LC oscillator circuit [11]. By compressing the elastic bodies 1 and 2 during pumping (Fig. 6.9c), the two electrodes of the internal LC sensitive device come closer, resulting in a frequency variation of the discriminator, proportional to the applied force. Thus, the dynamometer effectively measures the axial stress of the smooth drill pipe. In each elastic body there is such a circuit, and a discriminator is made up using the two LC circuits (Fig. 6.9d). Therefore, a linear zone of the combined characteristics for this biparametric force transducer is achieved, as shown in Fig. 6.9e.

6.2 Other Bridge-Like Measuring Devices

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Fig. 6.9 Biparametric force transducer: transducer block diagram (a), with two LC sensing devices (b), an example with two elastic elements (c), frequency discriminator with passive elements (L and C) (d) and the resulting linear force—frequency characteristic (e) (with kind permission from Springer International Publishing AG: DMS—Handbook of Force Transducers (2011), p. 83, Fig. 4.11)

The LC biparametric force transducer has superior performances such as: • • • •

linear working characteristic (force—frequency), high sensitivity (up to 1 kHz/µm), lack of inertia (measurement possibilities in dynamic conditions), utilization of a digital technique for measurement, display and recording.

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6.3 Instrumentation Applications Based on Wheatstone Bridges Throughout more than four decades of scientific research, I designed and built several FTs based on diverse measuring methods and principles, attempting to systematize the worldwide knowledge in this area [4]. At the IEEE conference on Systems, Signals and Devices held in Tunisia (2011) were presented some applications with resistive SGs placed on well-known EEs (cantilever beams) and connected in Wheatstone bridges [12]. Force calibration of micropipettes (Fig. 6.10) is necessary in application of mechanical stress to isolated ventricular cardiomyocytes which are immobilized on glass substrates. For this purpose a cantilever-type silicon sensor as transferable force standard is used [13]. It enables calibration immediately before and after a cell probing experiment, with a resolution better than 0.1 µN, in a range of (1–50) µN. In order to estimate the minimum measurable force, noise analysis is performed using the setup from Fig. 6.11a. In order to prevent external noise sources, the cantilever and Wheatstone bridge are put together inside a shielding box which is connected to the same ground terminal with an amplifier and fast Fourier transform (FFT) analyzer. After measuring the noise power spectral density and integrating the total noise for a bias voltage of 1.5 V, the minimum measurable fractional change in resistance is found to be 2.8 × 10−6 , and this gives a minimum measurable force value of 6.94 pN.

Fig. 6.10 Force calibration of micropipettes by transferable cantilever-type force standard (a), details of the silicon cantilever with integrated piezoresistive strain gauges and probing tip (b) and Wheatstone bridge connection (c)

6.3 Instrumentation Applications Based on Wheatstone Bridges

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Fig. 6.11 Electronic circuit for establishing the record in force measurement threshold (6.94 pN) (a) and sub-ppm vapors detection by piezoresistive microcantilever array sensors (b)

Piezoresistive microcantilever array sensors (Fig. 6.11b) have the selectivity of discriminating various vapors of volatile organic compounds. In contrast to Atomic Force Microscopy, where the cantilever is the “heart” of a complex measurement system, here cantilevers do not require bulky and expensive instrumentation, having a lot of advantages, such as low cost, simple operation, and miniaturization of the whole system into a matchbox sized device. In Table 6.1 a novel application of technical diagnosis is presented: detecting of interruptions in Wheatstone bridges, useful for practitioners trying to troubleshoot “customized” transducers.

6.4 Signal Conditioning for Wheatstone Bridges For the very sensitive sensors, as strain gauge like, the Wheatstone configuration is the best for the ideal conditioner. The complete bridge (with four equal and “active” resistive arms), in combination with the differential nature of the electronic amplifier configuration, ensures a good rejection of the noise, thermal compensation and a linear transfer characteristic in a wide frequency bandwidth. Strain gauges are typically configured in a Wheatstone bridge to convert the change in resistance into change of voltage. As the change of voltage is often very small, two techniques are frequently employed to retain the signal fidelity before transmission: converting analog signals into frequency domain via voltageto-frequency converter or converting analog signals into digital ones [14]. Several examples are given in the next chapters.

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Table 6.1 Detecting interrupted strain gauges in Wheatstone bridge arms

This chapter concludes with the picture of a universal conditioner (Fig. 6.12). QuantumX (HBM) is a “multi-talented” performer for numerous transducers having different measuring principles. A single MX840 amplifier module has eight universal connectors that suit all common transducer technologies, whatever their combination. The first positions are “reserved” for parametrical transducers (R, L, C) connected in Wheatstone (half)-bridge and, last but not least, options for (acoustic or piezoelectric) frequency measurement and (optical) pulse counting are presented. The QuantumX Assistant sets new standards for functionality and ease of operation having the following features: – – – –

fast and easy setup and parametrization; automatic sensor detection via TEDS (Transducer Electronic Data Sheet); comprehensive sensor database; measurement data visualized as graphs, conveniently integrated into LabVIEW® or other applications and programs via the API (application program interface).

6.4 Signal Conditioning for Wheatstone Bridges

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Fig. 6.12 Universal module that “embeds” a wide range of different techniques for measuring mechanical quantities [15]

References 1. Wheatstone, C.: An account of several new instruments and processes for determining the constants of a voltaic circuit. Philos. Trans. R. Soc. 133, 202–327 (1843) 2. The inside story: Pressure sensors or pressure transducers? Leaflet from Data Instruments— Transducer Products Group, Lexington, MA 3. Keil, S.: Historische Rückschau auf Entstehung und Entwicklung des Dehnungsmessstreifen. CUNEUS, Lippstadt (2006) 4. Stef˘ ¸ anescu, D.M.: Handbook of Force Transducers—Principles and Components. Springer, Berlin and Heidelberg (2011) 5. Pople, J.: Strain Measurement Reference Book. The British Society of Strain Measurement, Newcastle upon Tyne (1979) 6. Fraden, J.: AIP Handbook of Modern Sensors—Physics, Design and Applications. American Institute of Physics, New York (1993) 7. Brown, P., Beek, T., Carr, C., O’Brien, H., Cupido, E., Oddy, T., Horbury, T.S.: Corrigendum: Magnetoresistive magnetometer for space science applications. Meas. Sci. Technol. 23, Paper 059501 (2012) 8. Carignan F.J.: Displacement/force transducers utilizing Hall effect sensors. Data supplied from the worldwide esp@cenet database WO9318380-1993 9. Zabler, E., Heintz, F.: Neue, alternative Lössungen für Drehzahl-sensoren im Kraftfahrzeug auf magnetoresistiver Basis. Artikel 9.8, Sensoren Technologie und Anwendung, Bad Nauheim, Deutschland, 1984 10. Oppermann, K., Zagar, B.G.: A novel magnetoelastic force sensor design based on Terfenol-D. In: Proceedings SENSOR + TEST Conference 2009, Vol. 2, pp. 77–82, Nuremberg, Germany, 26–28 May 2009 11. Racoveanu, N., Dumitrescu, I., Terti¸sco, M.: Dynamometer with digital read-out for deep pumping wells. Autom. Electron. 9(5), 212–217 (1965). (in Romanian)

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12. Stef˘ ¸ anescu, D.M.: Strain gauges and Wheatstone bridges—basic instrumentation and new applications for electrical measurement of non-electrical quantities. In: Proceedings of the 8th International Multi-Conference on Systems, Signals & Devices, pp. 748–752, Sousse, Tunisia, 22–25 Mar 2011. ISBN 978-1-4577-0411-6/11, IEEE Copyright 2011 13. Peiner, E., Doering, L., Brand, U., Christ, A., Isenberg, G., Balke, M.: Force calibration of micro pipettes for single-cell probing. In: CD Proceedings of XVIIIth IMEKO World Congress, Paper 178, Rio de Janeiro, Brazil, 17–22 Sept 2006 14. Koay, K.C., Chan, P.K.A.: Low energy-noise 65 nm CMOS switched-capacitor resistive-bridge sensor interface. IEEE Trans. Circuits Syst. 64(4), 799–809 (2017) 15. Boersch, J.: QuantumX—the new, multi-functional amplifier system. Hotline Hottinger—News World Test Meas. (2), 4–5 (2007)

Chapter 7

Evolution of Strain Gauge Force Transducers—Design, Fabrication, Testing, Calibration and Databases

7.1 Design Requirements of Strain Gauge Force Transducers In principle force transducers and load cells are identical with regard to design and construction and they use the same measurement technique. They vary in the field of applications and the metrological boundary conditions which have to be taken into account [1]. The main component of any force measuring transducer is its elastic element. It is compulsory to produce the elastic element in one piece only, from a good quality and easy to work on material. The monolithic elastic structure ensures the maintenance of repeatable and linear relationship between loads and deformations. The elastic element should have a small displacement in order to reduce the geometric changes and produce minimal interactions between the transducer and its support. There are used specific elastic structures, carefully conceived and produced. According to Rohrbach’s rules, the shape of the elastic element is so chosen that it can present as wide as possible areas, with large and constant specific deformations, measured at enough distance from the point where the load is applied, thus avoiding the “end effect”. In the same time, for the right application of the load over the elastic elements, specific adaptors are necessary, their shape being chosen in such a way as: – to channel the forces to the measuring sections; – to ensure the correct geometrical shape of the deformation field, the symmetric and reproducible distributions of the loadings and temperatures; – to avoid excessive reactions in the supporting structures. EEs deformations analysis is needed in order to establish optimal location of SGs, that convert the mechanical stress into electrical signal. When EEs are complex, analytical computation is not possible and/or design nomograms do not exist, experimental methods of investigation (photoelasticity, holography, Moiré analysis) may be used; however, numerical computation is unavoidable. © Springer Nature Switzerland AG 2020 D. M. Stef˘ ¸ anescu, Handbook of Force Transducers, https://doi.org/10.1007/978-3-030-35322-3_7

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There are certain advantages when using the FEA in the design process [2]: • • • • • •

Decreased design costs; Reduction of manufacturing costs; Material savings; Weaknesses identification; Improvement of the project quality; Components and assembly optimization.

7.2 Finite Elements Optimization of the Force Transducers Elastic Elements Figure 7.1 illustrates evolution steps of design by numerical computation of EEs for SGFT. In the 80’ years the SAP IV programme was widely used, starting from dividing the EE into a number of finite elements, interacting between them in a finite number of nodes having known coordinates, and resulting in their displacements determination by the established nominal load (Fig. 7.1a) [3]. In order to determine the specific deformations of the elastic structure, it was necessary to elaborate a general formula, suitable for a computerized approach using punch cards technique typical in that time (SV-01 Fortran Program), then drawing by hand the ε-diagrams (Fig. 7.1b) and deciding the SGs positioning and their connection in Wheatstone bridge (Fig. 7.1c). Thanks to Dr. Marin Sandu’s fruitful cooperation, Strength of Materials specialist, a series of original EEs has been designed and fabricated; later on, we won an “Aurel Vlaicu prize” of the Romanian Academy in 1991 for the book [4]. At the beginning of the new millennium, we switched to computer aided design (Fig. 7.1d), combined with FEA, which facilitates modeling and parametric optimization [5]. Two suggestive images are shown in Fig. 7.1e (top view) and Fig. 7.1f (bottom view). Achieving representation of specific deformations diagrams directly upon the tensometric active surfaces of axially-symmetric EEs for SGFT will be completed only in the framework of the Brain Pool programme at KRISS (South Korea Research Institute of Standards and Science), through the project “Design Technique Development for Strain Gauge Force Transducers using the Finite Element Method” (July 2003–June 2004) [6]. Numerous personal references illustrate the broad range of domestic and international contributions of the author during a quarter of century [7–11]. A flow chart containing the iterative steps for product development (i.e. EE for SGFTs) is illustrated in Fig. 7.2a (adapted from [12]), with the necessary steps for the determination of the components’ final shape and dimensions by using FEA. Figure 7.2b shows a tubular structure with two fillets, external and internal, determining two tensions, σmin and σmax respectively, and resulting an increased sensitivity as compared with the classical tube (without fillets). The cross section view (2D),

7.2 Finite Elements Optimization of the Force Transducers …

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Fig. 7.1 FEM representation for a profiled membrane (a), strain diagram (b), Wheatstone bridge connection (c) and AutoCAD section (d), with top (e) and bottom (f) views

a particular case of the elastic body 3D view, was achieved on the occasion of an “ANSYS training” at Seoul (August 2003). In Fig. 7.2c is represented an axisymmetric elastic element with rectangular measuring section. The two sides’ ratio may be varied, striving after the best combination of conflicting design criteria: strain, stress (determining the overload) and displacement (determining the stiffness). Applying compressive force F, results in nodal displacements and a stress map inside the deformed structure, indicating the best places for strain gauges’ location: R1 and R3 are increasing while R2 and R4 are

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Fig. 7.2 FEA flow chart (a) and two axisymmetric models: profiled tube (b) and “hat” with rectangular measuring section (c)

decreasing, as they are connected by a Wheatstone bridge. Note the OXYZ axes system used according to the “right-hand screw rule”, i.e. “Z takes X over Y ”. (Here Y “enters” toward the page!) N-shaped elastic elements of strain gauge force transducers (SGFTs), have two cylindrical tubes of different diameters, concentrically telescoping one another, and a conical tube interconnecting their opposite ends (Fig. 7.3a). As reported in Rio de Janeiro in 2006 (Fig. 7.3b), we envisaged the following steps for N-shaped elastic elements design optimization [13], having in view the golden rule of tensometry (Choose the zones with maximum strains and opposite signs!): • mechanical—establishing the optimum measuring range and overall dimensions for universal application of this new one-piece solution (Fig. 7.3c), e.g. force transfer standard (1 MN); • numerical—iterative FEM computation (2D analysis by ANSYS program based on Plane42 finite elements) for choosing the best axisymmetrical profile, to increase the measurement sensitivity; • electrical—designing special circumferential strain gauges and locating R1 and R3 on the lower side of the N-shaped section while R2 and R4 on the upper side of the ‘N’ section.

7.3 Aspects Concerning Testing and Calibration of Strain Gauge …

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Fig. 7.3 N-shaped axisymmetric elastic element: the initial patent (a), presentation in conference (b) and the new model, FEM optimized, with SGs positions and connections in full Wheatstone bridge (c)

7.3 Aspects Concerning Testing and Calibration of Strain Gauge Force Transducers Figure 7.4a shows a testing equipment for three-component SGFT calibration, by means of (dead) weights, applied directly on the transducer, for the vertical component F Z , or through pulleys, for the two transverse F X and F Y [14]. In Fig. 7.4b a tester with lever and spring is depicted, for calibration of a single-component transducer, the axial one. Starting from these simple cases of SGFTs calibration in laboratory conditions, let us have a more extended view on a worldwide scale, where trade imposes harmonization of norms and accreditation, with an example in the area of uniaxial tensilecompression testing machines [15]. In Japan ten thousands of such equipments are operational, involving high expenses for using in each instance (trans)portable instruments, i.e. transfer standards (strain gauged load cells). Therefore, in 2001 an “e-trace” project was launched, remote calibration (Fig. 7.5).

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Fig. 7.4 Testing equipments for SGFT calibration: three-component (a) and single-component (b)

Fig. 7.5 Japanese calibration system of Force Standard Machines at remote locations by Internet

In a complex project the use of a powerful software for experimental data processing allows the researcher to have an overview regarding the phenomena involved [16]. The experiment was designed to obtain strains S in ten and pressures P in three measuring points located on the cylinder-block of an engine (Fig. 7.6a). The sensitive elements are three-element strain gauge rosettes and, respectively, pressure transducers, both types being connected to the same computer (Fig. 7.6b). The influences of the temperature and the dynamic conditions during the experiment, due to the engine in running conditions, must be compensated. The final aim is to obtain a relation between the measured strains and stresses in the measurement points with respect to the crankshaft angle. These values can be used to “calibrate” a fairly accurate numerical model, to check the accuracy of the FEM model of the engine. An original software was written in Visual FoxPro programming language, that offers facilities to readily use of the large databases, having direct connections to Excel and other Microsoft applications.

7.4 Scientific and Commercial Databases of Force Transducers

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Fig. 7.6 Locations of the strain S and pressure P measuring points on an engine cylinder-block (a) and strain gauges in Wheatstone bridges connected to computer for the experimental data processing (b)

7.4 Scientific and Commercial Databases of Force Transducers T-Design developed a program to perform the (analytic) calculation for most popular types of strain gauge-based transducers (Force, Torque and Pressure) [17]. In our case, the user only enters the maximum permitted dimensions, load range and material to be utilized, and the program calculates the main dimensions of the chosen EE for SGFT. Physical properties of some usual spring materials as well as examples for each transducer type are also included in this database. For SGFTs, this software includes the following basic types (Fig. 7.7): – Simple, double or reverse bending beams; – Tension or compression rod; – Shear beam (with strain gauges at ±45°).

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Fig. 7.7 T-Design database: table of contents and example of double bending beam

Following Table 2.3, one can choose from twelve types of EE for SGFT! Four of them have been chosen for the Knowledge-based Intelligent System for the Selection of Industrial Sensors (KISSIS) [18], which has two stages: first, a measurement principle is selected (e.g. strain gauges and Wheatstone bridge) and then one or more transducers are chosen from a supplier’s guide (e.g. bent yoke, as type of elastic element inside the force transducer), using a program called PLib Editor. Figure 7.8a shows a tree reflecting a dictionary with a simplified concept of force transducer. The definitions of the properties of the Yoke-element are shown in Fig. 7.8b. After finishing the description, the user can save it in the form of a PLib-compliant file. Also within the Post Doc NATO grant from Dept. of Measurement and Instrumentation, Twente University of Enschede, The Netherlands (2002), several original EEs have been analyzed, i.e. a Z-shaped one (Fig. 7.9) [19], in which the colour convention (red–blue) is reversed as against that used in this book! As an useful commercial database for SGFT and load cells, please follow the poster of POWER MnC Co. Ltd., Busan, South Korea (Fig. 7.10), having the load expressed in kgf (approx. 9.8 N) or tf, in the mode of tension (T), compression (C) or universal (U, i.e. tension and compression) [20].

7.4 Scientific and Commercial Databases of Force Transducers

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Fig. 7.8 Force transducers “dictionary”: “tree structure” (a) and yoke element description (b)

Fig. 7.9 ‘Z’-shaped elastic element (mirror for ‘S’) with two X rosettes (a), Wheatstone bridge connections (b), KISSIS programme logo (c) and DMS Mechatronics emblem (d)

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Fig. 7.10 Nomenclature code (a), Wheatstone bridge connection (b) and Loadcell Selection Table (c) from Power MnC Company

7.4 Scientific and Commercial Databases of Force Transducers

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Fig. 7.11 HOLI weighing equipment from Changzhou, China

Weighing pads and platforms, as well as different types of scales, are presented in Fig. 7.11, together with Asian Christmas greetings at the age of globalization!

References 1. Mack, O., Mäuselein, S., Schanz, J.: Recommended test procedures for load cells used in legal metrology and unconsidered abnormalities. In: Proceedings of XIX IMEKO World Congress Fundamental and Applied Metrology, Paper 427, Lisbon, Portugal, 6–11 Sept 2009. ISBN 978-963-88410-0-1 2. Cirak, F., Scott, M.J., Antonsson, E.K., Ortiz, M., Schröder, P.: Integrated modeling, finiteelement analysis and engineering design for thin-shell structures using subdivision. Comput. Aided Des. 34, 137–148 (2002) 3. Stef˘ ¸ anescu, D.M.: Optimization of the shape of profiled membranes utilized at strain gauge instrumented transducers by FEM. In: Proceedings of 1st Conference IMEKO TC-15 on Experimental Stress Analysis, pp. 513–518, Plzen, Czechoslovakia, 25–28 May 1987 4. Constantinescu, I.N., Stef˘ ¸ anescu, D.M., Sandu, M.A.: Strain Gauge Measurement of Mechanical Quantities (in Romanian), 264 p. Editura Tehnic˘a, Bucure¸sti, 1989 (Romanian Academic Prize 1991). ISBN 973-31-0127-3

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5. Stef˘ ¸ anescu, D.M., Dolga, L., Marinescu, A.: Parametrical modeling of the strain gauged pressure and/or force transducers. In: CD Proceedings of XVII IMEKO World Congress Metrology in the 3rd Millennium, pp. 1106–1110, Cavtat–Dubrovnik, Croatia, 22–27 Jun 2003 6. Stef˘ ¸ anescu, D.M., Kang, D.-I.: Axisymmetrical elastic elements for very large force transducers. In: CD Proceedings of IMEKO TC-3 19th International Conference Force, Mass and Torque Measurements: Theory and Application in Laboratories and Industries, Article 32, Cairo, Egypt, 19–23 Feb 2005 7. Stef˘ ¸ anescu, D.M.: Die Untersuchung der elastischen Elemente von Aufnehmer mit Dehnungsmeβstreifen durch die Methode der finiten Elemente. In: Konferenz SENSOR’83, Basel, 17–19 Mai 1983 8. Stef˘ ¸ anescu, D.M.: Comparative study of the sensitivity of various measurement techniques on “glasses”-shaped elastic element models analyzed by FEM. In: Wieringa, H. (ed.) Experimental Stress Analysis, pp. 291–300. Martinus Nijhoff Publishers, Dordrecht, The Netherlands (1986). ISBN 90-247-3346-4 9. Stef˘ ¸ anescu, D.M., Stef˘ ¸ anescu, V., Constantinescu, I.N.: Aspects spécifiques de l’application de la méthode des éléments finis à l’analyse des éléments élastiques des capteurs. Rev. Roum. Sci. Techn. Méc. Appl. 32(5), 561–567 (1987). RM-ISSN 0035-4074 10. Stef˘ ¸ anescu, D.M.: Study of S-shaped flexible elements of force transducers instrumented with strain gages using finite element method. In: Proceedings of IMEKO XIth World Congress, Vol. 5: Sensors, pp. 645–652, Houston, Texas, 16–21 Oct 1988 11. Stef˘ ¸ anescu, D.M.: FEM and strain gauge analyses for axisymmetric elastic elements of force transducers. In: Tojo, T., Ohgushi, K. (eds.) ACTA 5th Asia–Pacific Symposium on Measurement of Force, Mass and Torque, pp. 85–90, Tsukuba, Japan, 7–10 Nov 2000 12. McMahon, C.D., Scott, M.L.: Innovative techniques for the finite element analysis and optimization of composite structures. In: Proceedings ICAS 2002 Congress, pp. 1–9, Toronto, ON, Canada, 8–13 Sept 2002 13. Stef˘ ¸ anescu, D.M.: N-shaped axisymmetric elastic elements for strain gauged force transducers. In: CD Proceedings of XVIII IMEKO World Congress Metrology for a Sustainable Development, Paper 141, Rio de Janeiro, Brazil, 17–22 Sept 2006 14. Sandu, M., Sandu, A., G˘avan, M.: Design and calibration of a 3-component force sensor. In: 22-nd Danubia-Adria Symposium on Experimental Methods in Solid Mechanics, Parma, Italy, 28 Sept–1 Oct 2005 15. Iizuka, K.: Remote calibrations of mass, force and pressure standards—present and future. In: Proceedings 7th Asia-Pacific Symposium on Mass, Force and Torque (APMF 2005), pp. 1–5, Jeju Island, Korea, 30 Aug–3 Sept 2005 16. Oan¸ta˘ , E., Nicolescu, B., Boc˘anete, P.: Experimental data processing software. In: Proceedings of ESDA2002: 6th Biennial Conference on Engineering Systems Design and Analysis, Istanbul, Turkey, 8–11 Jul 2002 17. T-Design™—Software to assist design of strain gage-based sensors. Version 1.5.306, Vishay– BLH. Last modified 10 Jan 2007 18. Korsten, M., Stef˘ ¸ anescu, D.M., Regtien, P.P.L.: Sensor specification using the ISA and STEP standards for sensor selection. In: CD Proceedings of XVII IMEKO World Congress Metrology in the 3rd Millennium, pp. 393–396, Cavtat–Dubrovnik, Croatia, 22–27 Jun 2003 19. Stef˘ ¸ anescu, D.M.: Force Transducers Elastic Elements. Poster, Department Measurement and Instrumentation, Twente University of Enschede, The Netherlands, Nov 2002 20. Jeon, J.-Y.: POWER MnC Co. Ltd., Busan, South Korea, Private Communication, 10 Aug 2012

Part II

Applications with Force Transducers

Chapter 8

Testing Equipment to Investigate Elastic Constants of Rocks and Composites

The establishing of the physical-mechanical characteristics of the materials (metal or non-metal), usable in all kinds of technological constructions is performed by means of non- or destructive tests, that are up to or over the elasticity limit of the tested material [1]. In the case of metals there is a clear cut proportion between loads and deformations, expressed by the real elasticity modulus E and conforming to the Hooke’s law. In the case of the non-homogeneous and anisotropic materials (such as wood, rocks, concrete, composite, wires) it is necessary to know the specific diagrams, based on which the apparent elasticity modulus is determined [2].

8.1 Equipment for Determining the Elastic Constants of Rocks The rocks, if considered materials having structure and strain anisotropy of which samples are taken (Fig. 8.1a), have a conventionally attributed elasticity modulus (for example at longitudinal contraction Δl), which is determined by measuring the tangent slope in the initial portion of the specific curve or by assimilating this portion with a line (as in Fig. 2.3). Definition relationships for the stress σ and the strain ε are written on the abscissa and on the ordinate of the curve in Fig. 8.1e. Also made explicit there are the geometrical quantities: l0 and d 0 are constants (the sample initial sizes), also like area A0 . σ = E · εl

(8.1)

Hooke’s law (8.1) is valid only up to the limit of proportionality. Subscripts for σ are the same in English like in Romanian: proportionality, elasticity, creep, and rupture.

© Springer Nature Switzerland AG 2020 D. M. Stef˘ ¸ anescu, Handbook of Force Transducers, https://doi.org/10.1007/978-3-030-35322-3_8

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Fig. 8.1 Equipment for determining the elastic constants of rocks [3]: sample dimensions (a), LVDT transducers for transverse expansion (b), force transducer elastic element as a cylindrical cage with four windows (c), eight strain gauges connected in Wheatstone bridge (d) and typical diagram (e)

8.1 Equipment for Determining the Elastic Constants of Rocks

91

A transverse expansion of the sample will occur at the same time the longitudinal contraction is produced: εtr =

d d0

(8.2)

the link between the two specific deformations being given by the relation εtr = − ν · εl

(8.3)

The measuring of the transverse strain can be made placing four motion transducers on two perpendicular diameters of the centered sample in the test machine (Fig. 8.1b) and using the relation d =

1 (d1 + d2 + d3 + d4 ) 2

(8.4)

In the hypothesis of the linear, homogeneous and isotropic elastic body it is sufficient to determine two independent elastic constants. Taking as basic parameters E (Young’s modulus) and ν (Poisson’s coefficient), it is possible to calculate other pairs of elastic constants, such as G (transverse elasticity modulus) and K (cubic compressibility or elasticity modulus). Operating the adequate replacements in the above formulae, the following relations for finding out the elastic constants will result: E=

l0 · N A0 · l

(8.5)

ν=

l0 · d d0 · l

(8.6)

and the variations of the mechanical outputs N, l and d will be electronically measured. For the force transducer, having the nominal load of 250 kN, a slottedcylinder, multiple-column configuration for high-capacity, low-profile applications was adopted (Fig. 8.1c). The elastic body is made of stainless steel V2A and its annular section ensures the measurement of the axial force with reduced influences of other strains (bending, twisting), avoiding the buckling. One should use resistive strain gauges type 3/120 LB 15 Hottinger, bonded with M-Bond 610 (Vishay) adhesive. Strain gauges are located at a quite long distance from the place the force is applied in a zone of big deformations and reduced strain gradients. The measuring circuit is the Wheatstone bridge (Fig. 8.1d) with eight strain gauges: those with odd indexes are the active elements and those with even indexes have the role of thermal compensation. The strain gauge transducer was calibrated at a Schenck test machine provided with a force transducer class 0.1.

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The transverse expansion of samples (Δd) is measured using inductive devices like in Fig. 8.1b. Some linear motion transducers were conceived having the nominal range of ±5 mm and the accuracy of 1%. In the measuring circuit (Fig. 8.2) there is a three channel amplifier: one for the strain gauge force transducer and another two for the LVDT motion transducers (longitudinal and transverse, respectively). Three identical amplification modules are used together with the indicator module which is part of the electronic multichannel strain gauge type N-2302 made by IEMI Bucharest. The amplifiers ensure large possibilities of recording in voltage or current and the low frequency of the studied phenomena (below 1 Hz) offers the possibility to use any type of recorder. Recording of the current, on multichannel VISICORDER type instruments, using UV sensing paper and proper galvanometric loops (e.g. 1 mA/cm), allows grouping of the three mechanical quantities on the same diagram. Using of two XY voltage recorders (E546G–FEA) or a dual channel storage oscilloscope facilitates visualisation of N/Δl or Δd/Δl parameters dependencies, hence quick determination of elastic constants.

Fig. 8.2 Typical circuit diagram for measuring elasticity modulus: force measured with SGs in a Wheatstone bridge configuration and displacement with a LVDT transducer

8.2 Stand for Determining the Elasticity Modulus …

93

8.2 Stand for Determining the Elasticity Modulus of Composite Materials A relatively new class of materials, of great utility, created by joining several components in order to obtain better mechanical properties—lighter but stronger—is constituted by the composites. For most composites, the specific curve representing the relations between stress σ and specific deformation ε, is non-linear, determining an apparent elasticity modulus in bending, given by the maximum tangent in the increasing portion of the load—deformation curve. The electrical measuring method of the mechanical outputs (e.g. forces and displacements) permits the automatic recording of the specific curves with the possibility of a computerized processing of the test data sets for different types of samples made of various composites. The equipment made by the Testing Materials research laboratory in the “Politehnica” University of Bucharest [4] offers, among others, the following advantages: the non-modification of the upper cross bar of the test machine; the performance of the static bending test at a constant speed loading and which can be adjusted up to 100 N/mm2 /s; a simple construction carried out with own forces and having superior performances as compared to the classical devices. Experimental configuration of three-point bending is more advantageous: the test pieces have much simpler geometry than, for example, the complex-shaped tensile specimen. Strain gauges are bonded on the elastic body of the reusable force transducer and the great deformations of the samples are measured inserting a LDVT transducer. The equipment items, presented in Fig. 8.3 are the following: 1—the upper cross bar of the test machine (Werkstoffprüfmaschinen Leipzig); 2—spherical hinge; 3, 4—studs M 12 and fastening washers, 5—strain gauge force transducer (SGFT) (unique item); 6—intermediary part; 7—punch (contact cylinder having the diameter of 10 mm to avoid the printing inside the sample); 8—fastening bolts, 9—displacement transducer of ±10 mm (manufactured by IAUC Bucharest) the body of which is installed inside item 10—positioning system (vertically and horizontally); 11—parts for sample positioning having median items and an adjustable distance A between supports; 12, 13, 14—screws M 10 for horizontal guiding, Grower washer and nuts; 15—a pad made of welded U profiles, with a ruler graded in centimeters; 16—the lower cross bar of the test machine; 17—a bar-shaped sample with rectangular section b × h; 18—an elastic system for fastening the sensing rod of the motion transducer. The measuring circuit diagram (Fig. 8.4) is similar, but not so complex as that used for elastic modulus determination of rocks (Fig. 8.2). Not only the resistive force transducer, but also the inductive displacement transducer can be connected to the strain gauge bridges with carrier frequency (made by I.E.M.I. Bucharest). The electronic circuits allow the automatic record of the characteristic curves (loaddeformation). The elastic element of force transducer (Fig. 8.5) has been optimized by the finite element method (FEM). The strain gauges instrumented force transducer has the

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Fig. 8.3 Stand for determining the elasticity modulus of composites

following advantages: cheap, solid, compact, one-piece construction, easy to mount, subject both to tension and compression, insensitive to side loads, linear stress-strain characteristic (non-linearity error of 0.1%), signal independent of load position.

8.2 Stand for Determining the Elasticity Modulus …

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Fig. 8.4 Measuring circuit diagram with SGFT, LVDT, two-channel amplifier and XY recorder

Fig. 8.5 Z-shaped (“duck”) force transducer (a), rosette strain gauge (b), Wheatstone bridge connection (c) and Rinstrum digital indicator for smart weighing [5] (d)

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The force transducer of 8 kN was conceived as a Z-shaped elastic element, subject to compression (Fig. 8.5). It comes from a parallelepiped block with two asymmetrical slots, easy to process by milling, and has a minimum deformation. Strain gauges are rosettes of the type 3/120 XA 21, bonded with Z 70 adhesive and protected with AK 22 putty, all made by Hottinger. This type of transducer has been improved later, being used in various applications as well. Using the values of ΔF and Δl from the curve in Fig. 8.4, the elasticity modulus in bending of reinforced polymers and other composites is computed fast and accurate. According to the Romanian standard the apparent elasticity modulus in bending is calculated using formula Ei =

  A3 · F N/mm2 4 b · h 3 · l

(8.7)

being considered as an approximate value of the Young’s modulus. The term modulus is the diminutive of the Latin term modus which means measure.

8.3 Other NDT, Static and Dynamic Tests for Composites A static method for elastic modulus determination of laminated composites in presented in [6]. A four point device is used to produce the bending of the rectangular cross section samples, and a single measurement is performed with a displacement transducer, to determine δ, the maximal beam deflection. Hence, and based on the dimensions involved, the static elasticity modulus can by determined with the formula indicated in Fig. 8.6a. The dynamic elasticity modulus can be determined in the configuration for bending shown in Fig. 8.6b. The setup may be also adapted to longitudinal or torsional vibration modes. In free vibration method the impulse excitation is produced by striking the object with a suitable hammer. As a pickup transducer is used acoustic microphone whose signal is addressed to personal computer with a sound card and processed by signal processing methods (Fourier transform algorithm) in order to identify the values of the natural frequencies of vibration (resonances). Dynamic methods provide an advantage over static methods, allowing a wider variety of specimen shapes and sizes. Another dynamic method, with ultrasound, is illustrated in Fig. 8.7. The ultrasonic system for the measurement of the velocities consisted of two types piezoelectric transducers (with x-cut and y-cut crystals for longitudinal and shear wave generation) operating in through transmission mode and being connected with USB Interface. The ultrasonic frequency used was in the range of 1.5 MHz, so that the wavelength of ultrasonic waves was much larger than the glass fiber diameters.

8.3 Other NDT, Static and Dynamic Tests for Composites

97

Fig. 8.6 Schematic of the setup for static four-point bending test (a) and, respectively, for impulse excitation dynamic technique in a flexural mode (b)

Fig. 8.7 Circuit diagram for ultrasonic measurement with piezoelectric actuator and receiver and frequency analysis by computer

98

8 Testing Equipment to Investigate Elastic Constants …

References 1. Rohrbach, C. (Hrsg.): Handbuch für experimentelle Spannungs-analyse. VDI-Verlag Gmbh Düsseldorf (1989) 2. Stef˘ ¸ anescu, D.M.: Contribution to the upgrading of materials testing machines (in Romanian). Construc¸tia de Ma¸sini 53(1), 54–59 (2001) 3. B˘adescu, Gh, Stef˘ ¸ anescu, D.M.: Equipment for determining the elastic constants of rocks (in Romanian). Studii s¸i cercet˘ari de mecanic˘a aplicat˘a 40(3), 451–466 (1981) 4. Stef˘ ¸ anescu, D.M.: Stand for mechanical testing of composite materials. In: Acta IMEKO XII, Vol. 2: Measurement of Force, Mass, Pressure, Flow and Vibration, pp. 431–435, Beijing, 5–10 Sept 1991. ISBN 7-80003-175-6/TB.18 5. Rinstrum 5000 Digital Indicator Reference Manual, for use with Software Versions 4.6 and above. PDF created on 4 Feb 2004 6. Ivanova, Y., Partalin, T., Georgiev, I.: Comparison of NDT techniques for elastic modulus determination of laminated composites. In: Advanced Computing in Industrial Mathematics, Studies in Computational Intelligence, Vol. 728, pp. 79–89 (2018). © Springer International Publishing AG. https://doi.org/10.1007/978-3-319-65530-7_8

Chapter 9

Static and Dynamic Stiffness in Connection with Ball Screws and Reinforced Concrete Components

9.1 Stand for Determining the Static Stiffness and the Friction Moment at the Ball Screws The stiffness and the friction moment are important technical characteristics of the ball screws. Their measurement, on a special stand achieved at a machine-tool building enterprise in Bucharest, certifies the quality of the obtained production [1]. The stiffness of the ball screw is directly proportional to the interior diameter d of the screw thread and to the longitudinal elastic modulus E of the material and inversely proportional to the screw length. This stiffness is determined in practice by the measurement of the deformation δ of the screw and, because it is necessary a higher precision of measuring, the use of an electronic device is imposed. The friction moment M t is directly proportional to the thread pitch and to the prestressing force applied to the screw. In fact, it was intended that the variation of the friction moment to the rolling in both ways of the ball screws should not exceed by 20% of its nominal value, this being an attesting characteristic. As to the measuring principle and the system of signal transmission, it was chosen the most spread combination (strain gauges + rings and graphitic silver brushes, here AGR 3), also used at the torque transducer T1 (Hottinger), which is successfully replaced by a Romanian product. The schematic diagram in Fig. 9.1 shows the experimental equipment achieved for the determination of the stiffness and moment of friction at the ball screws. The ball screw is fastened between the two ends (left and right), which permit the rotation drive of the screw. In the middle is a slide with fastening device of the nut, with guiding on the stand frame. At the stiffness test the screw is blocked against rotation and the nut is loaded with a known force F, by means of a hydraulic device, while the deformation δ of the nut is being sensed by an electronic transducer (LVDT) and indicated by the first stress analysis bridge. At the test regarding the measuring of the friction moment, the nut travels when the screw is rotating. The measurement of the resistance moment M t , which appears at the screw rotating stimulated by motor, owing to the friction between screw and © Springer Nature Switzerland AG 2020 D. M. Stef˘ ¸ anescu, Handbook of Force Transducers, https://doi.org/10.1007/978-3-030-35322-3_9

99

100

9 Static and Dynamic Stiffness in Connection with Ball Screws …

Fig. 9.1 Stand for determining the static stiffness and the friction moment at the ball screws

nut, is made by means of a torque transducer, inserted between motor and screw, through a coupling device. The corresponding indication is read on the second stress analysis bridge. A special attention is paid to the achievement of the transducers for measuring the two mechanical quantities. For the wide range of produced ball screws (diameters d between 16 mm and 180 mm) the measurement of some displacement of 1…100 μm is necessary. The most adequate is the differential inductive displacement transducer TIC 16.1, made by I.A.U.C. in Bucharest and connected in half-bridge (Fig. 9.2). The transducer has sensitivity of about 50 mV/V/mm, maximum deviation from linearity of 0.14% being able to sense variation of 0.1 μm. At the torque transducer the dimensioning of the shaft was made so that, by a complete bridge connection of the four strain gauges, applied to a section by directions to 45° faced with the shaft axle (Fig. 9.3), should result a certain value of the signal from the bridge for the maximum torsion moment taken into consideration. The strain under a single strain gauge is given by the relation ε1 =

8 Mt π G · d3

(9.1)

where the transverse elasticity modulus is G = E/2(1 + ν). Assigning for the moment M t = 100 N·m the strain indicated by the bridge εread = 3200 μm/m, it results for a single transducer ε1 = 800 μm/m and, from the above stated relation, d = 16 mm. There were used four strain gauges of type 3/120 LB 15, bonded with adhesive Z 70 and protected by putty AK 22 (H.B.M.).

9.1 Stand for Determining the Static Stiffness …

101

Fig. 9.2 LVDT transducer (a) for differential measurement of ball screws deformations (b) and half-bridge connection (c)

Fig. 9.3 Principle of torque measurement (a), with strain gauges (b) connected in Wheatstone bridge (c)

For the constructive variant of torque transducer, the components (all made in Romania, excepting the strain gauges) are indicated in Fig. 9.4. Not only the inductive displacement transducer, but also the resistive torque transducer can be connected to the strain bridges with a carrier frequency of 5 kHz, made at IEMI enterprise in Bucharest. The digital bridges are more accurate, but on the analogical ones the operator can watch more easily the “threshold” value for which is made the attesting.

102

9 Static and Dynamic Stiffness in Connection with Ball Screws …

Fig. 9.4 Romanian torque transducer (a), SGs in Wheatstone bridge (b) and photographic image (c)

The measurement on the most adequate scale is made for each type of ball screw, taking into account the sensitivity coefficient S of 34 μm/m/N·m, established on the basis of the metrological homologation report. Based on data extracted and presented in Table 9.1, they granted the 0.5 accuracy class for the torque transducer.

9.2 Measurement of the Dynamic Stiffness … Table 9.1 Torque measurement data for loading and unloading of a ball screw

103

Torque M (N·m)

Average indication [μm/m] Loaded

Unloaded

0

0

10

349

350

20

686

688

30

1025

1030

40

1366

1372

50

1702

1718

60

2042

2061

70

2385

2395

80

2721

2733

90

3060

3067

100

3400

3400

0

9.2 Measurement of the Dynamic Stiffness of Some Reinforced Concrete Components Dynamic stiffness is defined as the ratio between the amplitudes of the harmonic disturbing force F and of the resulting displacement x in the same point K =

F x

(9.2)

In this application the variation of this ratio with the disturbing force frequency is studied, at constant amplitude, by measuring the corresponding displacement. Based on the determination of this characteristic before and after the cracking of some reinforced concrete structural elements at a real scale, their “force—deformation behaviour” in case of a new dynamic stress may be foreseen, and new global indications are obtained regarding the bearing capacity diminution as a result of cracking [2]. The existing instrumentation at the Materials Strength Chair of “Politehnica” University of Bucharest (the Prodera “cabinet” and various accessories) allows achieving of an experimental set-up (Fig. 9.5) with two excitation variants, depending on the amplitude and frequency of the disturbing force needed, and three variants of measuring the displacement resulted: (a) with an accelerometer, which is a standard equipment for the vibration indicator, using an electronic double integration; (b) with a speed sensor, then the displacement resulting by calculation; (c) with a displacement sensor, using the internal calibration signal of the bridge. The outputs of the two measuring channels (force and deformation) may be connected inside a floating system to a X-Y recorder which permits the real time visualization of the tested material specific curve. Recording is also possible with a thermo sensitive paper recorder, if the frequency does not exceed 100 Hz, or a UV recorder

Fig. 9.5 Complex equipment for vibrations measurements at Strength of Materials Chair, “Politehnica” University of Bucharest, Romania

104 9 Static and Dynamic Stiffness in Connection with Ball Screws …

9.2 Measurement of the Dynamic Stiffness …

105

for higher frequencies. Waveforms may be continuously monitored, they should be perfectly sinusoidal. Variation of dynamic stiffness with frequency is shown in Fig. 9.6a, where a marked diminution of K is noticeable in the vicinity of the resonance frequency f r , place in which the minimum value of K is attained. Figure 9.6b shows that the resonance frequency of the cracked element (f f ) is lower than that of the uncracked element (f n ). Obviously, while the resonance frequency diminishes, the displacement increases at the concrete sample under test. Table 9.2, where the processed vibrograms data are synthesized, shows that the resonance frequency of the cracked element is lower with 26.5% in average for the elastically embedded piles and with 35% for the beams embedded at both ends. (Periods T f are higher in the same ratio.) Dynamic stiffness at the resonance frequency

Fig. 9.6 Variation of the dynamic stiffness K with frequency f (a) and diminution of the resonance frequency from uncracked (f n ) to cracked (f f ) concrete element (b)

Table 9.2 Dynamic parameters (frequency, period, dynamic stiffness) for embedded piles and beams Dynamic parameters

Unit

Embedded piles

Embedded beams

fn ff fn /ff

–

1.35

1.36

1.62

1.50

Tn

ins

58.00

58.17

10.33

9.99

Tf

ms

78.12

79.18

16.74

15.00

Tn /Tf

–

0.74

0.73

0.62

0.67

Kn

N/mm

412

543

6578

11,666

Kf

N/mm

250

382

2788

6490

Kn /Kf

–

1.65

1.42

2.36

1.80

S1 B300 (p = 0.9)

S2 B400 (p = 0.9)

G1 B250 (p = 0.9)

G2 B400 (p = 0.9)

Hz

17.24

17.19

96.81

100.10

Hz

12.80

12.63

59.75

66.66

106

9 Static and Dynamic Stiffness in Connection with Ball Screws …

is 1.42–1.65 times lower at piles and 1.8–2.36 times at cracked beams, as compared with uncracked. In general, much better dynamic stress behaviour has been found at elements made of superior grade concretes. Results obtained contribute to a better evaluation of the dynamic behaviour of various reinforced concrete components, before and after an earthquake, as well as the estimation of stiffness and natural period of buildings, due to local failures.

References 1. Stef˘ ¸ anescu, D.M., Buga, M., Mocanu, D.R.: Stand for determining the stiffness and the friction moment at the ball screws. In: Proceedings of the 2nd Conference on Testing equipment for experimental investigation of mechanical properties of materials and structures, pp. 519–526, Moscow, 9–14 Oct 1989 2. Mih˘ailescu, C., Stef˘ ¸ anescu, D.M.: Dynamic stiffness determination of some reinforced concrete components (in Romanian). In: Al III-lea Simpozion National de Tensometrie, Vol. 3, pp. 131–134, Timisoara, 28 septembrie–1 octombrie 1983

Chapter 10

Methods and Means for Measuring Cables Tension

The measurement of cable extension is a complex problem referring to special mechanical tests and modern measurement means which is particularly important for the safe functioning of some installations in which human values and special materials are involved. The mechanical tests on cables usually refer to the modulus of elasticity and breaking limit establishing.

10.1 Testing Equipment for Determining the Cables Characteristics The complete study of cable extension, necessary for a multitude of applications, requires performing accurate measurements of the traction force T (the cause) and of the lengthening Δl (the effect). The relation between these quantities in the elastic domain, in which Hooke’s law is valid [σ = E · ε], is the following l =

T·l E ·A

(10.1)

where: T —the traction force; —initial wire length; E—longitudinal elasticity module; A—cross-section area; Δl = f −  the lengthening, where f is the final length. One could recognize the mechanical stress σ = T/A as well as the specific deformation or strain: ε = /. At the Laboratory for Research and Testing of Materials from the Polytechnic Institute of Bucharest an experimental rig has been set up permitting the automatic plotting of force versus deformation curves for conductors loaded in tension [1]. Tests are performed on a Werder-type universal testing machine of 1 MN, with horizontal axis, modernized by addition of a hydraulic drive and a chain for the measurement and simultaneous recording of forces and displacements (Fig. 10.1). This “technical jewel” has been appreciated in the history of the Romanian technology being used for verifying the cables utilized during construction of the King © Springer Nature Switzerland AG 2020 D. M. Stef˘ ¸ anescu, Handbook of Force Transducers, https://doi.org/10.1007/978-3-030-35322-3_10

107

108

10 Methods and Means for Measuring Cables Tension

Fig. 10.1 Modernized Werder-type universal testing machine for cables

Carol I Bridge over the Danube (Fig. 10.2a) by the famous engineer Anghel Saligny (Fig. 10.2b). The bridge measures 4037 m in length, with 1662 m over the Danube and 920 m over the Borcea Arm; in 1895 this was the longest bridge in Europe, and the third in the world. In 1980, the Werder machine has been transferred from the old building of Polizu Polytechnical Institute to the new one located in the “Splaiul Independentei” campus. A multidisciplinary team made of engineers specialized in hydraulics, machine building and applied electronics has succeeded in modernizing this installation [2], as shown in Fig. 10.3. (b)

(a) Source: http://art-historia.blogspot.com

Fig. 10.2 King Carol I Bridge over the Danube (a) design by Anghel Saligny (b)

10.1 Testing Equipment for Determining the Cables Characteristics

12

11

13

14

15

P

16

9 8

16

17 y x

l F

10

17

109

18

F

l

5m

5 4 2 3 2 1 6 1 - testing machine base, 2 - clamping jigs, 3 - steel - aluminum conductor, 4 - strain 7 gage force transducer, 5 - mobile framework, 6 - distributor, 7 - pressure control valve, 8 - oil tank, 9 - PS 10 pump, 10 - filter, 11 - double acting cylinder, 12 - piston, 13 - cylinder, 14 - manometer, 15 - displacement inductive transducer, 16 - measuring cables, 17 - carrier frequency amplifier, 18 - recorder.

Fig. 10.3 Schematic view of the new Werder testing machine for cables

The actuation of cable extension installations may be carried out using several methods: – – – –

pneumatic—for reduced forces, electric—which produces large loading in the engine shaft, mechanical—using devices with screws and weights, hydraulic—the most adequate because there are horizontal testing machines, which permit the testing of some big length samples (up to tens of meters), within a large range of loads (60 N to 20 MN), and having the controlled speed variation (100 … 250 mm/min).

The electrical measurement methods have most advantages in point of accuracy, of the possibilities for transmitting at long distances and for recording the variations of the analyzed mechanical quantities. From the usual types of force transducers, those with strain gauges [3] are best suited for most applications, fulfilling the economical and safety requirements at the highest level of the world technique. Force measurement is carried out with a specially constructed transducer (Fig. 10.4a), metrologically attested and having various mounting possibilities in the test rig. The traction force transducer has a tube shaped elastic element, perfectly incorporated in the installation, and is designed for a nominal load of 400 kN. An improved representation was subsequently achieved by my son, in his student days, as a milestone in the spectacular evolution of the computerized design (Fig. 10.4b).

110

10 Methods and Means for Measuring Cables Tension

Fig. 10.4 Tubular force transducer: original drawing (a), Catia design made by Florian Stef˘ ¸ anescu (b), T rosette H.B.M. (c) and Wheatstone bridge connection (d)

In order to sense only the axial load an adequate scheme is used, locating the strain gauges (in this case: two rosettes at 90° made by Hottinger—Fig. 10.4c) according to the known rules from the Strength of Materials. The instrumentation chain contains a carrier frequency amplifier to which both the strain gauged force transducer and the inductive transducer for displacements can be connected. The basic circuit for the electric measurement of non-electric quantities is the Wheatstone bridge (Fig. 10.4d). The own conception tubular force transducer, sized Ø 90 × 270 mm and the high sensitivity of the Wheatstone bridge ensures the monitoring of thrust force F = T, correlated with the displacement variation, both outputs “making up” the specific curve of the tested cable, visualized in real time on the X-Y recorder (Fig. 10.5) or on the associated computer. Determination of the actual characteristics of steelaluminum conductors is necessary for optimizing their design and increasing their safety in operation.

10.2 Automatic Measurement System for Ground Anchor Proof Testing While this complex Romanian stand is frequently utilized in the mechanical devices calibration for checking the extension forces inside the high voltage pile anchors, another interesting application is reported by a team of Portuguese researchers [4].

10.2 Automatic Measurement System for Ground Anchor Proof Testing

111

Fig. 10.5 Romanian team and a characteristic diagram for steel-aluminum cables

The anchor behaviour characterization requires the measurement of a large set of quantities: applied force, pressure installed inside the hydraulic jack used by the pre-stress equipment, displacement of a set of notable points (anchor tendons ends, retaining wall, hydraulic jack rod), and ambient temperature. A complete schematic view of the system installation can be observed in Fig. 10.6a. Conforming to the European norm EN1537:1999 [Execution of Special Geotechnical Work—Ground Anchors], the measurement system designed by the National Laboratory of Civil Engineering in Lisbon has a resolution better than 0.5% of test load. At the beginning of the procedure a residual load (10% of maximum) is applied to the anchor in order to ensure that every element is adjusted during the creep stage and all gaps are closed. Tests are conducted by applying incremental cycles of loading and unloading until the maximum test load is reached (Fig. 10.6b). Anchor load cell (with ring-shaped stainless steel body)—the “core” of the measuring system—is made by SISGEO, having the range of 1 MN. Seven smart-sensors, a Data Collection Unit (DCU) and a battery powered portable console compose the new system. The main component of every smart sensor is a microcontroller with the reference ADuC824BS (supplied by Analog Devices). The smart-sensors are connected to the DCU through a digital data network (field-bus) that provides the physical layer for data communication and power supply within a dedicated intelligent system.

max

112

10 Methods and Means for Measuring Cables Tension

(b)

(a)

Source: National Laboratory of Civil Engineering Lisbon, Portugal

Fig. 10.6 Automatic measurement system for ground anchor proof testing in Portugal (a) and a characteristic loading diagram (b)

References 1. Stef˘ ¸ anescu, D.M.: Contributions to the modernization of the materials testing machines (in Romanian). Construc¸tia de Ma¸sini, Bucure¸sti. 53(1), 54–59 (2001). ISSN 0573-7419 2. Stef˘ ¸ anescu, D.M., G˘avan, M., P˘av˘aluc˘a, C.: Prüfanlage für die Bestimmung der mechanischen Eigenschaften der Freileitungsseilen. Rev. Roum. Sci. Techn. Série de Mécanique Appliquée. 32(6), 671–674 (1987) 3. Stef˘ ¸ anescu, D.M., Stef˘ ¸ anescu, V., M˘anescu, T.: Strain gauge force transducers for dynamic measurements in energetics. In: Proceedings of the XIII IMEKO World Congress, Vol. 3, pp. 2568–2573, Turin, Italy, 5–9 Sept 1994 4. Morais, P.G., Santos, C.A, de Carvalho, M.R.: Automatic measurement system for ground anchor proof testing. In: CD Proceedings of the XVIII IMEKO World Congress on Metrology for a Sustainable Development, Paper 507, Rio de Janeiro, Brazil, 17–22 Sept 2006

Chapter 11

Measurement of the Axial Loads Transmitted to the Foundation by High Voltage Circuit Breakers When Acting

Many applications in Electrical Engineering require the measurement of some mechanical quantities (forces, displacements, accelerations, pressures, etc.) [1]. Compared with other methods, the electrical ones offer some advantages: – – – –

high precision; flexibility and simplicity; possibility to record the time history; transmission at distance of results.

From the usual types of transducers, those with strain gauges are best suited for most applications. They are superior from many points of view: – – – – – – – –

have small size; are sturdy, without moving parts; are relatively stiff, minimizing the energy stored under load; have high sensitivity; present maximum linearity; can be used for both static and dynamic measurements; require common electronic instrumentation; are relatively easy to operate.

High voltage electrical equipment is mounted on foundations by bolts. The dynamic loads (thrust), occurring for example at switching on and off the circuit breakers, are sensibly larger than the static loads. Determination of these loads is a very important factor in establishing the optimal foundation solution. Figure 11.1 shows four measurement methods based on the use of strain gauge transducers where names of parts are shown on the image too! If numbered from up to down, these are: 1—foundation bolt, 2—nut, 3—spherical washer, 4—elastic sleeve, 5—strain gauge transducer, 6—breaker support, (again 4—elastic sleeve), 7—washer for height adjustment, 8—supporting sleeve and 9—concrete foundation. Variants B and C ensure maximum sensitivity and linearity while variant D is easier o achieve. The option for variant D is justified by the superiority of independent transducers over those with strain gauges bonded directly on the structure: © Springer Nature Switzerland AG 2020 D. M. Stef˘ ¸ anescu, Handbook of Force Transducers, https://doi.org/10.1007/978-3-030-35322-3_11

113

Fig. 11.1 Possible alternatives for measurement of the axial loads transmitted to the foundation by high voltage circuit breakers when acting

114 11 Measurement of the Axial Loads Transmitted to the Foundation…

11 Measurement of the Axial Loads Transmitted to the Foundation…

115

Fig. 11.2 Tubular EE shapes used for SGFT structures: classical tube (a), tube with holes (b), horizontal (c) or vertical slots (d)

– – – –

they are manufactured under laboratory conditions; calibrated under known loads; compact and reusable; versatile, can be used either as secondary standards or as force transducers in different other electronic weighing systems.

Measurement of the axial load transmitted to the foundation is possible by means of strain gauge force transducers (SGFTs) mounted on the preloaded bolts of each breaker. Depending on the kind of application, tubular elastic elements (EE) are chosen to “enclose” each fixing bolt in the concrete foundation. Several shapes of elastic tubes are possible (Fig. 11.2), however due to the weathering conditions the most important criterion is stiffness (simple tube) as against sensitivity (tubes with horizontal or vertical slots [2] or various shape holes [3], optimized by the finite elements method). Figure 11.3 shows the layout selected, a classical tube, together with details of the component elements. Measurement of the axial load transmitted to the foundation is done by means of four load cells mounted on the four preloaded bolts of each breaker (Fig. 11.4a). It is possible to use a single measuring channel, summing the four signals from transducers for a global indication (Fig. 11.4b) or to utilize four independent channels in order to analyse the load distribution on foundation (Fig. 11.4c). Details on the design, construction, calibration and use of these “dynamic” load cells can be found in [4]. Their main performance characteristics are the following: – – – – – – – –

nominal load: 20 kN, maximum overload: 50%, input resistance: 122.2 , insulation resistance: 5 G, supply voltage: maximum 5 V, sensitivity: 1 mV/V, global error: ±0.7%, size: Ø50 mm × 50 mm.

116

11 Measurement of the Axial Loads Transmitted to the Foundation…

Fig. 11.3 Customized solution of SGFT for measuring of the axial loads trasmitted to the foundation by high voltage circuit breakers when acting

11 Measurement of the Axial Loads Transmitted to the Foundation…

117

Fig. 11.4 Four SGFTs configuration (a) and single- (b) or multi-channel carrier amplifier (c) for measuring the thrust in foundation

References 1. Stef˘ ¸ anescu, D.M., Stef˘ ¸ anescu, V., M˘anescu, T.: Strain gauge force transducers for dynamic measurements in energetics. In: Proceedings of the XIII IMEKO World Congress, Vol. 3, pp. 2568–2573, Turin, Italy, 5–9 Sept 1994 2. Stef˘ ¸ anescu, D.M.: Untersuchung der Verformungszustsands eines elastischen Rohrelements mit Spalten, anwendbar beim Bau der Kraftaufnehmer mit Dehnungsmeßstreifen. Rev. Roum. Sci. Techn. Série de Mécanique Appliquée. 29(5), 519–533 (1984) 3. Stef˘ ¸ anescu, D.M.: Study of a tubular elastic element with holes, by the finite elements method (in Romanian). In: Al III-lea Simpozion Na¸tional de Tensometrie, Vol. 4, pp. 205–210, Timi¸soara, 28 septembrie–1 octombrie 1983 4. Constantinescu, I.N., Stef˘ ¸ anescu, D.M., Sandu, M.A.: Strain Gauge Measurement of Mechanical Quantities (in Romanian), 264 pp. Editura Tehnic˘a, Bucure¸sti, 1989 (Romanian Academic Prize 1991). ISBN 973-31-0127-3

Chapter 12

A New Weigh-in-Motion and Traffic Monitoring System

Weigh-in-motion transducers are utilized as components of complex measurement instrumentation and traffic monitoring systems. In the standard specification [1], WIM is described as “the process of measuring the dynamic tire forces of a moving vehicle and estimating the corresponding tire loads of the static vehicle”.

12.1 Methods and Means for WIM As Taylor and Bergan (IRD) stated, WIM systems are temporarily installed or permanently imbedded pavement mountings (piezoelectric or capacitive), low profile bending strain gauge devices or weighing scales based on load cells [2]. Weighing in motion, by its dynamic nature, is not as accurate as static weighing. For economic reasons, after a first selection by means of less expensive devices (piezoelectric, Fig. 12.1a), only over-weighted trucks are taken out from traffic and directed to a (static) control point equipped with accurate weighing instrumentation (load cells with strain gauges—Fig. 12.1b) metrologically certified, in order to avoid litigations. The Lineas® (Fig. 12.1a) is a quartz sensor to measure the wheel and axle loads and to determine the vehicle gross weight under rolling traffic conditions [3]. It has the following advantages: • • • • • • •

Excellent long-term stability, Wide measuring range: from slow to high-speed, Very high natural frequency and signal dynamic, Protection from the intrusion of water (degree of protection IP68), It is insensitive to temperature changes, Quick and easy installation, It is adaptive to different pavement characteristics, and safely mounts onto the pavement.

© Springer Nature Switzerland AG 2020 D. M. Stef˘ ¸ anescu, Handbook of Force Transducers, https://doi.org/10.1007/978-3-030-35322-3_12

119

120

12 A New Weigh-in-Motion and Traffic Monitoring System

Fig. 12.1 Dynamic vs static weighing of vehicles: piezoelectric transducer IRD (a) and weighing scale HBM based on strain gauge force transducers (b). Piezoelectric accuracy is twice lower than that of strain gauge devices

There exists a wide range of electronic weighing means [4], the main selection criteria being price and “invasiveness” (depth of intrusion into the pavement). Standard (Fig. 12.1b) or customized versions may equally be used, e.g. those patented by two Romanian authors: – A new wheel scale is a low profile bending plate, not a plane platform, but a profiled one on its backside, inside the “black box” of a Taiwanese patent [5]. – A new type of flat force transducer is mainly based on standard machine parts: two commercial (not expensive) spring discs (35 mm diameter, Company Christian Bauer GmbH), made by special stainless steel (DIN 1.4568), positioned in mirror one to another and joined together by means of electron beam welding [6]. The future development of the WIM transducers implies the following practical applications: – Improvement of the dynamic weighing accuracy at high sample rates; – Improvement of the determination of the European Standard Axle Load (ESAL) factor which will contribute to better pavement design, protection and cost reduction; – Improvement of vehicle detection and classification; – Automated overload detection and enforcement;

12.1 Methods and Means for WIM

121

– Decreasing of the transducer’s cross section, especially its thickness, since the installation costs are proportional to the slot dimensions in the road structure (especially its depth); – The new generation of WIM systems will also be able to measure the footprint dimensions and to detect flat tires. These complex requirements can be fulfilled by using a measuring technology based on strain gauges, designing and achieving special SGFTs. In this chapter two recent accomplishments of our team are compared (both optimized with FEA’s help), namely the “bending bridge” (2007) and the so-called “shear bridge”—the name of the latter being suggested by a term used in civil constructions [7]. Note: “Bridge” has a double meaning: Wheatstone electrical connection and connections in structures with multiple elastic elements.

12.2 WIM with Bending Bridges [8] Figure 12.2 shows an ingenious combination of nine complex bending elastic elements, each of them including seven Wheatstone bridges, for determining the weighing-in-motion load distribution for heavy trucks. In the online version of the book the positively (red) or negatively (blue) stressed SGs are clearly visible in FEA’s representation (Fig. 12.2a). The same colour code is utilized in the case of the Wheatstone bridge connection of SGs (Fig. 12.2b). The modular solution allows increasing the number of slices, as needed (Fig. 12.3a). The number of loaded spring elements gives information about the width of tire footprint (Fig. 12.3b). The calibration tests (diagrams of bridge output versus applied load) prove a satisfactory linearity and are in good agreement with the predictions made using FEA for a load of 2 MPa traveling with 60 km/h speed.

12.3 WIM with Shear Bridges [9] Shear SGs are bonded on ±45° directions upon the central elastic plate (0.5 mm thick), being united with the two outer plates (3.18 mm thick). State of the main stresses (von Mises) in the deformed shear beam module and central elastic plate is shown in Fig. 12.4a. Considering the maximum stress values obtained from the FEA with ABAQUS v6.8, the following modifications have been adopted in order to increase the transducer endurance by reducing stress concentrators: – higher connection radius of the two clearances in the outer plates (to the central axis);

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Fig. 12.2 FEA simulation of normal stress distribution (a) and strain gauges positioning on the WIM elastic element with multiple bending bridges (b)

– the central hole has been relocated by moving it down with a distance of one diameter, in an area with lower stresses (Fig. 12.4b). Strain gauges positioning (shear type rosettes) is shown in Fig. 12.4c, their connection in Wheatstone bridge in Fig. 12.4d and the assembled WIM transducer module for in situ measurements in Fig. 12.4e. The designed shear beam modules have been mounted eight in two rows in a box, together with the associate electronics. Then, these transducer boxes are mounted in series, making up lines of sensitive devices crossing the road. The assemblage has also a data acquisition system, data processing software and a wireless data transmission.

12.3 WIM with Shear Bridges [9]

123

Fig. 12.3 Proposal of a strain gauged WIM structure (“sandwich” type with bending bridges): its module components (a) and how it is embedded in a roadway (b)

Fig. 12.4 Shear beam “sandwich”: initial design (a), improved design (b), strain gauges positioning (c), Wheatstone bridge connection (d) and final WIM assembly (e) developed by ROC Company (Austria) [9]

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12.4 Experimental Results This new weigh-in-motion system mounted in Graz (Austria) has been working with good results for three years and the one mounted on the highway near Rosenheim (Germany) for two years, respectively (during this period of time an approximate number of 70,000 vehicles/week have been recorded). Both the transducer and the entire WIM system have proven their reliability during the in situ working period. Their estimated lifetime is of 6–10 years. In Fig. 12.5 the screen of Rosenheim HS WIM station is presented. High Speed Weigh-in-Motion with its requisite software was developed for semi-automatic overload enforcement (overweight of axles, axle groups or gross weight) and for vehicles classification depending on the number of axles. In Fig. 12.6 the vehicles number per hours and days, recorded by this WIM station during one week, is presented. In Table 12.1 a comparison of the two original WIM transducers models sizes is given: “bending bridge” (2007) and “shear bridge” (2012). One may conclude that the WIM “bending model” (2007) is more robust, fatigueand shock-resistant, whilst the “shear model” (2012) features an output signal independent of the force position (like at balances), higher sensitivity and significantly lower depth [10].

Fig. 12.5 The data display of a WIM station near Rosenheim. The monitor displays the truck’s view, its model, overload and value of the fine

12.4 Experimental Results

125

Fig. 12.6 Processing data acquired by a WIM station (example: 1–7 Oct. 2010)

Table 12.1 Comparison of the two Romanian WIM models

Model

Dimensions [mm] Length

Width

Depth

2007

500

100

100

2012

300

90

24

Remember: The piezoelectric model IRD has 55 mm depth

Modern SGs and sensor matrices production technologies (like sensitive carpets!) tend to allow a more pronounced miniaturization of the WIM devices.

References 1. ASTM Standard E1318-02, Highway Weigh-in-Motion (WIM) systems with user requirements and test method, Annual Book of ASTM Standards 2. Taylor, B, Bergan, A.: The Use of Dual Weighing Elements to Improve the Accuracy of Weigh. In: Motion Systems, and the Effect of Accuracy on Weigh Station Sorting IRD Report, 25 Nov 1993 3. Lineas® Quartz Sensor for Weigh-In-Motion, International Road Dynamics Inc., Saskatchewan, Canada. PDF created on 11 Mar 2009 4. Standardization or customization? How to make monitoring of your filling and batching processes more efficient. Published on hbm.com: www.hbm.com/custserv/SEURLF/ASP/SFS/ID. 801/MM.4,36,34/SFE/techarticles.htm 5. Stef˘ ¸ anescu, D.M.: Wheel Scales, Taiwanese Patent I-273219, 11 Feb 2007 6. Bârs˘anescu, P.D., Cârlescu, P., Stoian, A.: Sensors for in-motion weighing of motor vehicles (in Romanian). Editura Tehnopress, Ia¸si (2009). ISBN 978-973-702-685-9 7. Megally, S.H., Silva, P.F., Seible, F.: Seismic response of sacrificial shear keys in bridge abutments. Final Report Submitted to Caltrans under Contract No. 59A0051, Department of Structural Engineering, University of California, San Diego, May 2002

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8. Bârs˘anescu, P., Cârlescu, P., Stef˘ ¸ anescu, D.M.: A new weigh-in-motion and traffic monitoring system. In: CD Proceedings of the IMEKO International Conference on Cultivating Metrological Knowledge, Paper 5, Merida, Mexico, 27–30 Nov 2007 9. Opitz, R., Goan¸ta˘ , V., Cârlescu, P., Bârs˘anescu, P.D., T˘ ¸ aranu, N., Banu, O.: Use of finite elements analysis for a Weigh-in-Motion sensor design. Sensors. 12, 6978–6994 (2012). https://doi.org/ 10.3390/s120606978 © 2012 by the authors; licensee MDPI, Basel, Switzerland. This Article is an open access article distributed under the terms and conditions of the Creative Commons Attribution license (http://creativecommons.org/licenses/by/3.0/) 10. Bârs˘anescu, P.D.: Private communication, 12 Jul 2018

Chapter 13

Robotic and Biomedical Applications Related to Human Hands

13.1 Human and Robotic Gripping The living creatures come up with a lot of gripping biomechanisms [1]. Their study makes possible the discovery of interesting equivalent mechanisms, which can be used for industrial robots in many fields of activity. It is possible to obtain performing gripping mechanisms starting from the ones of the living creatures in their biological evolution: insects (stag beetle), birds (parrot), marine (crawfish) or terrestrial (elephant) animals, and human beings. It is a good idea to try the synthesizing of a universal gripping mechanism. A series of gripping positions are presented in Fig. 13.1a–k. This universal human function is achieved by two to five fingers. The prehension capacity variation with the fingers number is presented in Table 13.1. Illustrating the position (c) from Fig. 13.1, the grasping power of two fingers (I—thumb and II—index) was assessed with a strain-gauge style force measuring instrument (“square ring” elastic element) having an output signal of 0.9 mV/V for maximum 500 N [2].

Fig. 13.1 Hand in action, using two (a, b, c, d, f, h), three (g), four (k) or five fingers (e, i, j) © Springer Nature Switzerland AG 2020 D. M. Stef˘ ¸ anescu, Handbook of Force Transducers, https://doi.org/10.1007/978-3-030-35322-3_13

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Table 13.1 Gripper capacity reported to the number of fingers Number of fingers Gripper capacity (%)

5

4

3

2

100

99

90

40

Illustrating the position (i) from Fig. 13.1, a cylindrical object instrumented with two strain gauge force transducers (SGFTs) is presented in [3], measuring significant correlations between grip force and supporting force, produced by two hands of one person (the notion of bimanual synergy). Starting from these simple positions a–k, more complex applications can be worked up, such as assessing the push/pull forces exerted in all directions within a construction task [4]. Forces, assessed using a hand-held digital force gauge, were compared to those obtained using a highly accurate measuring frame consisting of six SGFTs (Maywood Instruments Limited U4000). Interesting analogies could be made between the human fingers and the robotic ones, having in view the force measurement possibilities. Comparing technical variants utilized in Robotics (“intelligent connection between perception and action” as Mike Bradley observed), one may say that four fingers ensure a good gripping force while two fingers satisfy only simplified gripping models, schematically representing different human functions. The corresponding structural variants, equipped with strain gauges connected in Wheatstone bridge, are shown in Fig. 13.2. An example is the gripping mechanism of a MERO handling robot for the casting moulds manipulation [5]. It is structured of four elastic fingers made of iron band having properly chosen dimensions in bending. The gripping force is indirectly measured by means of the strains in the manipulator fingers. Because the elastic fingers mode of mounting is not definitely known and the maximum load depends on the material, piece surface quality, fingers shape, operating pressure and motion speed, the gripping force calibration in operating conditions similar to the real ones is more important than the dimensioning.

13.2 Force Feedback for Human Hands Many sports consist of an exhibition of opposing forces competing against one another. Developing minimally-invasive methods to capture these force impacts (e.g. embedding FlexiForce™ touch sensors into athletic equipment) can deliver important insights to address safety concerns and improve the entertainment value of the games we love [6]. A few applications and the advantages of Tekscan devices are illustrated in Fig. 13.3. In the top right corner a data glove is presented for improving grip contact, useful also for normal people. These exercises can be performed at the office, at the kitchen table, and even while stuck in traffic. In the palm of each glove is a custom FSR (force sensitive resistor, having its conductance linearly dependent on the compressive

13.2 Force Feedback for Human Hands

129

Fig. 13.2 Two (a) or four fingers (d) gripping mechanisms in Robotics, their strain gauges positioning (b and e) and connection in Wheatstone bridge (c and f)

load), which sends a signal to an LCD screen on the back of the glove. The screen displays instant results on total time and exertion, also containing a transmitter that sends a signal to a receiver, allowing the user to review his performance. Another application concept is for aging population [7]: FlexiForce sensors designed into hand exercise devices help patients to maintain treatment regimens on their own (Fig. 13.4). You could recognize some typical positions from Fig. 13.1. A method and apparatus for providing force feedback to a user operating a human/computer interface device in conjunction with a graphical user interface (GUI) displayed by a host computer system is presented in [8]. A physical object, such as a joystick or a mouse, controls a graphical object, such as a cursor, within the GUI, which allows the user to interface with operating system functions implemented by the computer system. A signal is output from the host computer to the interface device to apply a force sensation to the physical object using one or more actuators. A combination of the force feedback with the audiovisual one is presented in [9]. The lack or limited sensory feedback becomes a barrier to the widespread use of master-slave, teleoperated robotic systems because the operator cannot be confident about the nature of the manipulated environment. Force feedback information (related to the muscle’s length/velocity and tension respectively) is very important to identify the controlled object properties, so that the human can build an internal model of it.

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Fig. 13.3 Tekscan: how force feedback can make athletics smarter & safer?

The experimental device from Fig. 13.5 consists of a straight beam (600 mm) that serves as a seesaw and a sliding cart that moves over the seesaw. The human operator manipulates this device by handling a knob with a 6-axis force transducer (IFS-50 M Nitta Co.) attached to it and turning it clockwise or counter-clockwise in order to move the sliding cart from one side to another.

13.3 Strain Gauge Devices for Handwriting Analysis The handwriting analysis is a modern method used for diagnosis in psychiatry and in criminology researches. Pressing force together with handwriting speed are dynamic features, which cannot be evaluated by means of classical graphology. There are two possibilities of pressing force measurement in the writing process [10]: – with strain gauges incorporated within the tensometric pen, measuring the resultant of the forces from the paper plane, the pen being in a certain position during the writing process;

13.3 Strain Gauge Devices for Handwriting Analysis

131

Fig. 13.4 Tekscan: force-sensitive devices to support (not only) an aging population

Fig. 13.5 Human audio, visual and force feedback integration in a manipulation task of a haptic device. A haptic interface is a force reflecting device which allows a user to touch, feel, manipulate, create, and/or alter simulated objects in a virtual environment

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13 Robotic and Biomedical Applications Related to Human Hands

– with strain gauge dynamometric table, measuring the normal force, exerted on the paper plane, and the other five components of the load torsor. An original stylus is presented in Fig. 13.6a. It is a tubular elastic element with four equidistant longitudinal slots, having its elastic element shape optimized by means of FEA (COSMOS/M program) (Fig. 13.6b). The fitting rod (Ø = 6 mm) of the

Fig. 13.6 Original stylus equipped with strain gauges for handwriting analysis

13.3 Strain Gauge Devices for Handwriting Analysis

133

Fig. 13.7 Dynamometric table with strain gauges for handwriting analysis

writing mine is considered a three-dimensional bar compressed along the OY axis. The tube shaped elastic item (Øint = 10 mm, Øext = 12.5 mm) can be considered having 17 transversal sections, using 112 bricks, with the seizing knots blocked. Following the stylus direction in hand, the two bending components (F x , F z ) can be measured, as they correspond to the forces in the paper plane or to their resultant. To this purpose four strain gauges, two longitudinal and two transversal, are located on two concentric circles, in section AA, constituting two Wheatstone bridges. The variable ratios between the two components F x and F z detect and indicate with great sensitivity the various modalities of holding the strain gauged pen. The state variations of the same individual can be a measure of different psychic diseases. The performed tensometric stylus has a low weight (85 g), the nut, lid, slotted tube and threaded bushing components being made of dural, the gasket of and the rod of OLC 45 steel. The dynamometric table shown in Fig. 13.7 has as sensitive elements four elastic half-rings, located under the paper holder on which one can write with an ordinary push button pen, leaning the hand on the upper plate. The size of the “window” for writing is: 100 mm × 50 mm. Four strain gauges can be located on the EEs (two on each half-ring in diagonal position). Just like the tensometric stylus, the dynamometric table has a simple and solid structure, and a low cost. The writing table is gravimetrically calibrated by means of standard weights successively laid on the holder and the pen, fixed exactly in the hand taking hold off place, by means of some weights positioned on the top of the pen. The strain—load characteristics are linear in both cases. Figure 13.8 shows three different recordings of the title “Writing Analysis Equipment” translated into Romanian, belonging to different age and sex persons. The differences of the amplitude in the three diagrams can be easily seen, the maximum

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13 Robotic and Biomedical Applications Related to Human Hands

Fig. 13.8 Recordings comparing the amplitude and speed of handwriting for three persons

of the pressure strain being variable, from 1.6 to 2.2 N, according to the characteristic handwriting of the three different considered persons. The writing time is within the range of 12–17 s. For instance, it follows that an intelligent person (say the middle one from these recordings) knows how to economize his own means (a reduced pressing force and a shorter handwriting time), achieving by all these his own “specific mark”. And as neither the printing nor the computer will eliminate the handwriting, these scientific experiments will continue, opening new perspectives even in the field of artificial intelligence and virtual reality.

13.4 Force Feedback, Data and Fuzzy Gloves A force feedback glove is presented in [11], using a magnetorheological fluid (MRF) as smart material with the property of changing its viscosity when exposed to a magnetic field. By placing this fluid into a sealed cylinder with an electromagnet piston as a core, a controllable resistance motion dampener can be created, transmitting these resistive forces to the user’s fingertips. The entire system is lightweight, low power, and easily portable. Data glove is a glove equipped with sensors able to sense movements of the hand and interfacing them with a computer. Data gloves are widely used in virtual reality environments where the user sees an image (in other words: has a vision) of the data glove and can manipulate with it the objects of the virtual environment.

13.4 Force Feedback, Data and Fuzzy Gloves

135

Fig. 13.9 Fuzzy glove: process of recognizing gestures (a) and Cyberglove product (b)

The fuzzy glove is a data glove that has fuzzy sensor functionalities [12]. The first goal is to create a friendly Human Computer Interface (HCI), a computer that understands a user expressing himself with gestures or by speech. There are two gesture recognition families: – Sensor-based systems (usually data gloves); – Vision-based systems. The advent of wireless data gloves makes it possible to imagine an embedded sign recognition system that could be used anywhere. Figure 13.9a illustrates the general structure of the recognition process. The physical to numerical interface is performed by a Cyberglove® from Immersion. It has 15 bending sensors with strain gauges and is integrated with a numerical to linguistic interface: the fuzzy glove. The picture of the cyberglove, names of the sensors and fingers numbering are given in Fig. 13.9b.

13.5 Vision-Based Force Transducers for Microrobotic Applications A new vision-based flexible force transducer concept is presented in [13], as a result of cooperation between Laboratoire de Vision et Robotique (Bourges, France) and Electrotechnical Laboratory MITI (Tsukuba, Japan). Based on the analysis of microhandling experiments, they designed a specific flexible end-effector, integrating a buckling-type force sensor using silicon surface micromachining (Fig. 13.10). The

Fig. 13.10 Set-up of the micro-teleoperation system based on vision control (forces deduced from displacements) providing force feedback capabilities for the human operator

136 13 Robotic and Biomedical Applications Related to Human Hands

13.5 Vision-Based Force Transducers for Microrobotic Applications

137

control of the handling force is made by means of a non-contact measurement system: an optical interferometer combined with a vision system looking at the micromanipulators through an optical microscope. The employed image processing techniques allows getting a resolution better than few 10−3 N. The buckling-type force sensor has good linearity and negligible hysteresis in a given operating zone. A two-dimensional, vision-based force transducer, capable of sensing micronewton level forces for use in microrobotic applications is presented in [14]. It consists of a planar, elastic mechanism with known force-deflection characteristics. A CCD camera attached to an optical microscope is used to track the deformation of the mechanism as it is used to manipulate objects in a microrobotic test-bed. By observing the displacements of selected points in the mechanism, the manipulation forces can be extracted in real-time to achieve force-guided manipulation of microscale objects. At this level, use of strain gauges is no more possible.

References 1. Stef˘ ¸ anescu, D.M., Stef˘ ¸ anescu, F., Luca, L., M˘anescu, T.: Force measurement possibilities for gripping biomechanisms. In: Proceedings 17th International Conference Force, Mass, Torque and Pressure Measurements, pp. 129–133, Istanbul, Turkey, 17–21 Sept 2001. ISBN 975-403221-1 2. Song, H.W., Park, Y.K., Lee, S.J., Woo, S.Y.: Measurements of skin elastic constants for palpation in Oriental Medicine. In: Asia—Pacific Mass and Force Symposium, Taipei, Taiwan (2013) 3. Scholz, J.P., Latash, M.L.: A study of a bimanual synergy associated with holding an object. Hum. Mov. Sci. 17(6), 753–779 (1998) 4. Hoozemans, M.J.M., van der Beek, A.J., Frings-Dresen, M.H.W., van der Molen, H.F.: Evaluation of methods to assess push/pull forces in a construction task. Appl. Ergon. 32(5), 509–516 (2001) 5. Stef˘ ¸ anescu, D.M.: Measurement of the gripping force of a MERO type manipulator robot (in Romanian). In: Al V-lea Simpozion Na¸tional de Robo¸ti Industriali, Vol. 3, pp. 818–824, Bucure¸sti, 17–19 Oct 1985 6. Tekscan.: Sense the Critical Impacts that Change the Game. PDF created after the e-book on 7 Nov 2017 7. Tekscan.: Designing Force-Sensitive Devices to Support an Aging Population. PDF created after the e-book on 23 Jul 2018 8. Rosenberg, L.B., Brave, S.B.: Providing force feedback to a user of an interface device based on interactions of a user-controlled cursor in a graphical user interface. US Patent 7,199,790, 3 Apr 2007 9. Murakami, E.A.Y., Matsui, T.; Analysis of human visual, force and audio sensory feedback integration in manipulation task. In: CD Proceedings XVIII IMEKO World Congress on Metrology for a Sustainable Development, Paper 351, Rio de Janeiro, Brazil, 17–22 Sept 2006 10. Stef˘ ¸ anescu, D.M., Stef˘ ¸ anescu, A.: Strain gauged devices for the handwriting analysis. Preprints of the 4th IFAC Symposium on Modeling and Control in Biomedical Systems, pp. 103–108, Karlsburg, Greifswald (D), 30 Mar–1 Apr 2000. ISBN 0 08 043549 1 11. Winter, S.H., Bouzit, M.: Use of magnetorheological fluid in a force feedback glove. IEEE Trans. Neural Syst. Rehabil. Eng.: Publ. IEEE Eng. Med. Biol. Soc. 15(1), 2–8 (2007)

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12. Allevard, T., Benoit, E., Foulloy, L.: Fuzzy glove for gesture recognition. In: CD Proceedings XVII IMEKO World Congress Metrology in the 3rd Millenium, pp. 2026–2031, Cavtat–Dubrovnik, Croatia, 22–27 Jun 2003 13. Ferreira, A., Fontaine, J-G.: New vision-based flexible force sensor for micro-teleoperation systems. In: XVI IMEKO World Congress, Vol. 11, pp. 101–106, Vienna, Austria, 25–28 Sept 2000 14. Cappelleri, D.J., Piazza, G., Kumar, V.: A two dimensional vision-based force sensor for microrobotic applications. Sens. Actuator A Phys. 171(2), 340–351 (2011)

Chapter 14

Multifunctional Transducers for Force and Other Non-electrical Quantities

14.1 Multifunctional Force Transducers Multi is a prefix that may be associated with a variety of tasks within the area of force and its related quantities measurement. Here are two relevant applications, with multipurpose and multiphysics. UniMeasure/80 [1] is a precision solid state instrument, a patented multipurpose, mechanical-to-electrical transducer, adaptable to an unlimited variety of measurements, including force, torque, pressure, acceleration, weight/mass, displacement/deformation, strain, flow and all the physical parameters derived from these quantities, e.g. Work or Energy, defined as Force times Distance. Mechanically, it is a double-ended stainless steel shaft that is movable along its axis for 2 mm, while electrically, it is a resistor which varies linearly from 100 to 500  as the shaft moves from zero displacement to full scale (Fig. 14.1). The low impedance output may be monitored, recorded or otherwise processed without any special signal conditioning. Its output is pure resistance, readable on any ohmmeter or multimeter. Weight and other force dependent measurements are easily performed by coupling a force adapter kit to either end of this multifunctional device. UniMeasure/80 and its accessories are ideal for test engineers within a variety of continuously changing programs, for process engineers, having the 5 to 1 resistance change compatible with voltage or current recorders/controllers, or for teachers, as a practical, low cost tool for demonstrating a wide range of physical functions in both classroom and laboratory. The secret of its performance is a semiconductor chip whose resistance varies as it is moved with the transducer shaft in a controlled permanent magnet field. It has no contact, no sliding friction and no wear, yet it provides “infinite resolution” (20 millionths of an inch, i.e. 25.4 mm). Multiphysics means multiple physical quantities! One recent research project at the University of Alberta in Canada is aimed at developing embedded MEMS sensors to measure forces and moments in real-time during surgery, thus providing physicians © Springer Nature Switzerland AG 2020 D. M. Stef˘ ¸ anescu, Handbook of Force Transducers, https://doi.org/10.1007/978-3-030-35322-3_14

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14 Multifunctional Transducers for Force …

Fig. 14.1 UniMeasure/80—precision multifunctional instrument for non-electrical quantities, depicted through a variable resistance (a), digital multimeter (b) and a magnified view of semiconductor resistor sensitive to magnetic fields

valuable feedback during the corrective procedure [2]. Two silicon sensor strips were placed at the interface between the rod and the hook/screw heads (Fig. 14.2). Each strip has two sensor pads evenly spaced along the contact line of the corrective rod. Each pad consists of a deformable membrane with four piezoresistive strain gauges that are sensitive to contact forces in shear and normal directions. When combined, electrical outputs from the gauges on both strips indicate the 3-D forces and moments applied during surgery. Multiphysics analysis using ANSYS Mechanical software was used to study simultaneously three aspects of this configuration: – Contact analysis used to predict loads transmitted between the rod and the sensor strips; – Structural analysis used to determine subsequent deformations of the anisotropic silicon membranes; – Electrical analysis used to determine the output voltages from each piezoresistive strain gauge. Apart from these two examples, other idioms containing “multi” may be mentioned: – Multiscale (range from nano- to millimeters) deformation and strain distribution in three-point bending experiment of composite samples [3]; – Multi-station dynamic-culture of cell-populated collagen gel by force monitoring based on a cantilever beam with semiconductor strain gauges [4];

14.1 Multifunctional Force Transducers

141

Fig. 14.2 “ANSYS Advantage” cover (a), diagram of hook/screw heads typically used for scoliosis correction (b) and images before (c) and after surgery correction (d)

– Multivariate sensors [5], comprising several basic sensors, like strain gauges in multicomponent force and moment transducers, more extensively described in Chap. 15; – Multimodal analysis of materials and of vibrations; – Multitasking based upon complex instrumentation. Force and related quantities are strongly connected by their physical formulas [6], by their like measurement principles and methods [7] and—last but not least—by the complex scientific programs of the jointly Technical Committees IMEKO events, like in Mexico in 2007 [8]. Figure 14.3 has suggested twelve other quantities associated with force measurements. In the same work mentioned before, twelve types (apostolic figure!) of elastic elements are illustrated for strain gauge measurement of mechanical measurands (Table 14.1). It can be observed that, on the vertical axis, Force, in a central position among the mechanical quantities written in alphabetical order, can be measured with each of the kinds of usual elastic elements. On the horizontal axis, the cantilever beams (type 3) can be practically used for the measurement of all the physical quantities existing in Table 14.1. In the following subchapters several “combinations” are presented, concerning force and other physical quantities measurement, in a sequence similar to that in [1].

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14 Multifunctional Transducers for Force …

Fig. 14.3 Force and 12 related quantities measurement for car testing

14.2 Force and Deformation/Elongation Strain measurements on materials such as plastic or rubber surpass the capability of classical strain gauges. High elongations are usually expressed as percentages (1% elongation is equal to 10,000 microstrain). Measurements in this range can be made by simple flexure devices [9], designed to reduce the strain level on the gauges (Fig. 14.4a). They are commonly referred to as clip gauges and are “bilateral”, working in tension (T) and/or compression (C) (Fig. 14.4b), and using half—(Fig. 14.4c) or full Wheatstone bridge connection (Fig. 14.4d). Various shapes of clamps, made by the author, are presented in Fig. 14.4e.

14.3 Force and Strain Force and strain constitute the ideal combination in the field of EEs for SGFTs, and so are presented together in Chap. 10 of the “Handbook of Modern Sensors” by Fraden [10]. Strain is elongation related to the initial length. HBM is the best known company producing strain transducers for applications where high forces are to be measured—two models are presented hereunder. The basic DD1 type unit [11] is a displacement transducer which converts the movement of a probe tip into an electrical signal (Fig. 14.5a–b). Conversion is carried out with the aid of strain gages inside the unit, connected in a full bridge circuit on the measuring probe tip (Fig. 14.5c). This basic unit includes a calibration blade and fastening screw. This clamp-on strain transducer can be attached to round or flat specimens made of metal or plastic and the clamping force of the measuring instrument on the specimen can be finely adjusted via the mounting bracket.

Vibration

Torque

x x

x

x x

Pressure

x

Mass

x

x

Force

x

x

3 x

2

Displacement

1

Elastic elements types

Acceleration

Mechanical quantities

x

x

x

4

x

x

x

x

x

5

Table 14.1 Elastic elements used for mechanical quantities measurements

x

x

x

6

x

x

x

x

7

x

x

8

x

x

x

x

x

x

9

x

x

x

10

x

x

11

x

x

x

12

14.3 Force and Strain 143

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14 Multifunctional Transducers for Force …

Fig. 14.4 Rectangular clip for high elongations (a), in compression or in tension (b), half- (c) or full bridge connection (d) and original clamps for testing cables (e)

The SLB 700A type [12] attaches directly to a plane surface with a friction joint and four bolts (Fig. 14.5d). This arrangement enables the strain of the test object to be transferred directly to the strain transducer making it ideal for installations where lack of space, or installation conditions, makes it difficult to use standard force transducers.

14.4 Force and Pressure These quantities are related through the definition of pressure as a force distributed over a surface (usually a membrane). As Jacob Fraden explains: A pressure sensor is combined with a fluid-filled bellows which is subjected to force. The bellows functions as a force-to-pressure converter (transducer) by distributing a localized force at its input over the sensing membrane of a pressure sensor, that, in turn, comprises another displacement transducer (cantilever beam with bonded strain gauges) for converting the membrane motion to an electrical output (Fig. 14.6a).

14.4 Force and Pressure

145

Fig. 14.5 DD1 Strain Transducer: top view (a), side view (b) and Wheatstone bridge connection (c); SLB-700A Strain Transducer (d)

In Fig. 14.6b strain gauges are bonded directly on the elastic membrane, and the transducer can measure both the axial force concentrated on the external nozzle of the membrane and the pressure distributed within the space bordered by the membrane, i.e. the pressure chamber. Two recent writings witness the tight connection between force and pressure: Inkjet- and 3D-printed force/pressure sensing devices (steel or ceramic diaphragms) made by rapid prototyping [13] are cost- and resource-efficient, fast and flexible designs compared to those of solid-state technology. The response of both designs is evaluated by applying force on the diaphragms surface using a moveable load cell. A primary standard for dynamic pressure was developed in VTT Technical Research Centre of Finland Ltd, Centre for Metrology MIKES: A pressure pulse is generated by impact between a dropping weight and a piston of a liquid-filled piston-cylinder assembly [14]. The traceability to SI-units is realized through interferometric measurement of the acceleration of the dropping weight during impact, the effective area of the piston-cylinder assembly and the mass of the weight.

14.5 Force and Torque “Torque and Force Transducers” are often presented together, like in the title of Sect. 7.21 from the huge “Instrument Engineers’ Handbook” (1868 pages) [15],

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Fig. 14.6 Measuring pressure by the same EE frequently used in SGFTs: cantilever (a) and membrane (b)

including a partial list of suppliers for both types of transducers (30 reputed companies all-over-the-world). Typical elastic elements (EEs) for strain gauge torque transducers are presented in [16], six of them are shown in Fig. 14.7. The hollow cruciform EE has been improved after studying by FEM and holographic technique [17], while the hollow circular shaft has been optimized by Bicchi [5] for a six-component force-moment transducer so that the ratio between the maximum and minimum sensitivity is 2.16.

14.6 Force and Hardness

147

Fig. 14.7 Six common configurations of elastic elements for torque transducers

14.6 Force and Hardness We do not refer to known mechanical methods for determining various types of hardness, but describe a modern instrumental achievement [18]. In nanoindentation, an indenter tip, normal to the sample surface, is driven into the sample by applying an increasing load up to some preset value. The load is then gradually decreased until partial or complete relaxation of the material occurs. So, Load versus Depth curve is obtained. A differential solution with two contacts (indenter and reference) is presented in Fig. 14.8a. So, two piezoelectric actuators and two feedback loops become visible, resulting a more accurate force sensing in comparison with the classical L-C equipment (Fig. 14.8b). A capacitive force sensor in the range 3.5–4.5 pF for in situ transmission electron microscope (TEM)-nanoindentation, with simultaneous force (up to 4.5 mN) and current measurement (up to 3 µA), has been developed in [19].

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Fig. 14.8 Nanoindentation for soft materials based on piezoelectric actuation (a) or inductivecapacitive (L-C) version (b)

14.7 Force and Acceleration Force F is mass m multiplied by acceleration a. This dynamic connection is best illustrated by the air bag transducer for drivers’ protection at vehicles impact [20]; two models with vibrating beams are shown in Fig. 14.9: (a) Nwagboso—piezoresistive strain gauges in full Wheatstone bridge, (b) Platil—classical strain gauges in half Wheatstone bridge. In close connection with Accelerometry is Gravimetry, defined as “the measurement of the strength of a gravitational field (…) usually in units of acceleration” (Wikipedia). Its working principles are described in [21]: Kater pendulum and spring-, superconducting- or free falling corner cube gravimeter.

14.8 Force and Mass Force and Mass are inter-related not only through the formula F = m · a, but due to their coexistence in various electronic weighing methods, one of the most usual being the “Electromagnetic Force Compensation”, illustrated under several forms in

14.8 Force and Mass

149

Fig. 14.9 Nwagboso accelerometer arrangement and equivalent circuit diagram (a), and Platil model of strain gauge acceleration transducer (b)

Fig. 1.6 (PTB Planck-Balance), Fig. 3.6 (Dr. Kemény) and Fig. 4.5 (T.U. Ilmenau). An EMFC version from Mettler—Toledo [22] is also shown in Fig. 14.10. A special application is presented in Fig. 14.11 [23], where the external shape is that of a classical “passive” weight, but internally “activated” by using strain gauges! Note: Beginning with the Metrology Day of May 20, 2019 Force and Mass become “electrical quantities”, expressed in terms of universal constants within the New SI! To equip a special test-bed for checking some components produced in series by weighing, the achievement of an original transducer for masses up to 10 kg was of utmost importance [24]. The option was for a transducer having resistive strain gauges (Fig. 14.12a). The design formula for the annular elastic element is: ε = 1.08

F ·R E · b · h2

(14.1)

where ε—the specific deformation (expressed in µm/m), F—the measured force (about 100 N), E—the elasticity modulus (7.5 × 104 N/mm2 for the hard aluminum), b, h, R—dimensions in Fig. 14.12a. The four strain gauges of the type 3/120 LY 13 are bonded with a Z 70 adhesive (HBM) and connected in Wheatstone bridge (Romanian N 2314 or KWS—Hottinger) (Fig. 14.12b).

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Fig. 14.10 Hybrid representation (functional diagram + block layout) of a Mettler-Toledo scale

Fig. 14.11 Modern weighing with strain gauges: profiled diaphragm (a), equivalent bar model and moments diagram (b), FEM analysis (deformed elastic structure and plot of von Mises stress) (c) and legal/metrological weights of classical shape (d)

The calibration of the mass transducer is made gravimetrically using the equipment presented in Fig. 14.12c and reading the electronic indicator directly in mass units (kg). The main technical characteristics of the axially tensioned transducer are the following: – nominal load: 10 kg; – permitted overload: 25%;

14.8 Force and Mass

151

Fig. 14.12 DMS electronic scale: top and side views (a), Wheatstone bridge connections (b) and gravimetrical calibration (c)

– – – –

under load deformation: 0.1 mm; dimensions: 169 mm × 44 mm × 20 mm; sensitivity: 112 µm/m/kg; accuracy: 0.1%.

14.9 Force, Density and Flow This seems to be a surprising title, however the force balance principle may also be applied to a float, the gravity-buoyancy interaction being a measure of the density

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[25] (Fig. 14.13a) or liquid flow [26] (Fig. 14.13b)! Note that Density pertains to the TC-3 of IMEKO, together with Mass, Force and Torque! A new flow sensor (Fig. 14.13c) “consists of a thin film thermally isolated microbridge structure suspended over a cavity in the silicon IC. Because of its small size and excellent thermal resistance, only a few milliwatts of power are required to achieve high air flow sensitivity. The sensor operates on the principle of heat transfer due to mass airflow directed across the surface of the sensing element. Dual sensing elements (upstream and downstream resistors) flanking a central heating element indicate direction as well as rate of flow” [27]. Here, too, the colour code red—hot and blue—cold is preserved.

Fig. 14.13 Densimeter—Archimedes principle (a), flowmeter/rotameter—Platil model (b) and flowmeter by heat transfer, like a “thermal” strain gauge (c)

14.10 Force, Power and Energy

153

Fig. 14.14 Side, top and rear view of the torque sensing system for a wind turbine (WT)

14.10 Force, Power and Energy The mechanical power PWT can be obtained by multiplying the mechanical torque acting on the model wind-turbine rotor T WT and the angular velocity of the rotor ω [28]. Torque is measured through force exerted on a cantilever with two strain gauges connected in half Wheatstone bridge (Fig. 14.14). The nuclear disaster at Fukushima, Japan, triggered a radical turnaround of Germany’s energy policy in March 2011, having in view to improve the energy efficiency of existing power plants and to enhance the quality of new production processes, with emphasis on renewable green energy systems such as wind energy systems (WES). The new created Competence Center for Windenergy (CCW) has three focal points [29]: – calibration of large 3D components of WES drive train components, with focus on the dimensional calibration of large gear standards with diameters of up to 4 m, – portable systems for the measurement of 3D wind speed vectors, enabling the traceable determination of wind profiles up to a height of 250 m (Fig. 14.15), – calibration of torque transducers, allowing the transmission up to 5 MN·m. A novel torque measurement standard (TMS) for the calibration of torque transducers with a maximum moment of 5 MN·m was designed by PTB. In addition to torque, the TMS is also able to induce bending moments up to 3 MN·m, axial forces up to 2 MN and shearing moments up to 1 MN·m to torque transducers under test.

14.11 Force and Environment Force measurements are not directly related to the environmental ones; however there exist measuring means common to them, like strain gauges and Wheatstone bridges. Therefore, they are dominant in this chapter, being also a basic idea of this Force instrumentation book.

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Fig. 14.15 Scheme of the wind tunnel (a) and the PTB’s calibration setup for wind lidar system (b)

The researchers from Mikroelektronik Centret of Lynby, Technical University of Denmark, consider AFM probes suitable for cantilever-based sensors with integrated (polysilicon) piezoresistive read-out [30]. In the probe design they “have placed a full Wheatstone bridge (Fig. 14.16b) symmetrically on the chip, with two resistors placed on cantilevers and two resistors on the substrate (Fig. 14.16a). This design makes it possible to perform differential measurements where the signals from the two cantilevers are subtracted.” Silicon oxide cantilevers are 200 µm long, 90 µm wide and 1.5 µm thick. Fig. 14.16 Schematic drawing of AFM probe with integrated piezoresistive read-out (a) and a “quarter” Wheatstone bridge, only one of the strain gauges being “active” in sensing the environment (b)

14.11 Force and Environment

155

Three examples of environmental applications with this versatile, multifunctional sensing device: – Humidity: One of the two adjacent cantilevers is treated with a water absorbing material, while the other cantilever is coated with gold. The deflection of the first cantilever is registered as the output voltage of the Wheatstone bridge. – Temperature: Only one cantilever is exposed to the temperature to be measured, the other being thermally protected. Due to the symmetrical Wheatstone bridge configuration, the probe records the temperature difference of the two cantilevers (reference and, respectively, measuring). – Concentration of alcohol in water: Only one cantilever is coated with polymer, the other remaining sensitive to the changing concentration of alcohol in water.

14.12 Force and Frequency Force and frequency appear jointly in diagrams in most cases. Here is a relevant example: CPR—a device for training, research and real Cardio-Pulmonary Resuscitation [31]. In addition to the mechanical force indicator, a cantilever beam as SGFT (strain gauge force transducer) was mounted inside the handle of the CardioPump® . A force-sensitive resistor (FSR) element was fitted to the end of the central piston in a way to not restrict the removal or change of its silicon cup. The difference between the forces determined by CardioPump and calibration force transducer is smaller than 2%. Compression frequency was electronically determined by evaluating the force signal (Fig. 14.17): zero crossings were detected by software using the square FSR signals, their intervals were measured and, using the last 1.5 cycles, the mean value was computed. This value was converted into units of beats per minute and displayed on the monitor. It is to emphasize the importance of Force-Frequency characteristics in other activities as well, for example in Force Controllers for haptic devices.

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Fig. 14.17 Force versus time diagrams for SGFT and FSR of CardioPump

References 1. UniMeasure/80—A multipurpose transducer. US Patent 3,842,385. Unimeasure, Inc., Grants Pass, OR (1986) 2. Benfield, D., Moussa, W., Lou, E.: Multiphysics makes spinal surgery safer. ANSYS Advantage 1(4), 6–9 (2007) 3. Tanaka, Y., Naito, K., Kishimoto, S., Kagawa, Y.: Development of a pattern to measure multiscale deformation and strain distribution via in situ FE-SEM observations. Nanotechnology 22, Paper 115704 (5 pp) (2011). https://doi.org/10.1088/0957-4484/22/11/115704 4. Peperzak, K.A., Gilbert, T.W., Wang, J.H.-C.: A multi-station dynamic-culture force monitor system to study cell mechanobiology. Med. Eng. Phys. 26, 355–358 (2004) 5. Bicchi, A., Canepa, G.: Instrument science and technology—optimal design of multivariate sensors. Meas. Sci. Technol. 5, 319–332. © 1994 IOP Publishing Ltd. 6. Environmental Decision Making, Science, and Technology—Science Notes: Measuring Matter, Force, and Energy. PDF created on 10 May 2011 © Copyright 2003 Carnegie Mellon University 7. Stef˘ ¸ anescu, D.M.: Handbook of Force Transducers—Principles and Components. Springer, Berlin and Heidelberg (2011) 8. Stef˘ ¸ anescu, D.M.: Strain gauged elastic elements for force and related quantities measurement. In: CD Proceedings IMEKO International Conference Cultivating Metrological Knowledge, Article 22, Merida, Mexico, 27–30 Nov 2007 9. Micro-Measurements, High-elongation strain measurements. Measurements Group, Vishay Intertechnology, Inc., Tech Tips TT-133 (1976) 10. Fraden, J.: Handbook of Modern Sensors—Physics, Designs, and Applications. Springer International, New York (2014) 11. DD1 Strain Transducer, Data Sheet B0529-1.4, HBM. PDF created on 22 March 2013 12. SLB-700A Strain Transducer, Data Sheet B0165-2.4, HBM. PDF created on 19 May 2009 13. Faller, L.-M., Granig, W., Krivec, M., Abram, A., Zangl, H.: Rapid prototyping of force/pressure sensors using 3D- and inkjet-printing. J. Micromech. Microeng. 28(10) (2018). Special Issue on Modelling, Simulation and Multi-Physical Experimentation of Micro and Nanosystems

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14. Salminen, J., Högström, R., Saxholm, S., Lakka, A., Riski, K., Heinonen, M.: Development of a primary standard for dynamic pressure based on drop weight method covering a range of 10 MPa–400 MPa. Metrologia 55(2). © 2018 BIPM & IOP Publishing Ltd. 15. Brodgesell, A., Lipták, B.G., Silva Girão, P.M.B.: Instrument Engineers’ Handbook: Process Measurement and Analysis, Vol. 1, 4th edn. CRC Press, Boca Raton, CA (2003) 16. Watkins, K.: Torque Sensors. An Overview of their Design and Application. PCB Load & Torque, Inc., Farmington Hills, MI (2010) 17. Joo, J.-W., Kang, D.-I., Kwon, I.-H.: Deformation analysis of low-capacity torque sensors. In: Proceedings Asia—Pacific Symposium Mass, Force and Torque (APMF 2005), pp. 280–285, Jeju Island, South Korea, 30 Aug–3 Sep 2005 18. Randall, N.X.: Nanoindentation testing of some challenging soft materials. In: Webinar Materials Today + CSM Instruments, Needham, MA, USA, 14 Dec 2010 19. Nafari, A., Angenete, J., Svensson, K., Sanz-Velasco, A., Enoksson, P.: MEMS sensor for in situ TEM-nanoindentation with simultaneous force and current measurements. J. Micromech. Microeng. 20, Paper 064017, pp. 1–8 (2010) 20. Nwagboso, C. (ed.): Automotive Sensory Systems, p. 335. Springer Science + Business Media, Dordrecht (1993) 21. De Angelis, M., Bertoldi, A., Cacciapuoti, L., Giorgini, A., Lamporesi, G., Prevedelli, M., Saccorotti, G., Sorrentino, F., Tino, G.M.: Precision gravimetry with atomic sensors. Meas. Sci. Technol. 20, Paper 022001 (2009) 22. Biétry, L., Kochsiek, M.: Mettler Wägelexikon. Praktischer Leitfaden der wägetechnischen Begriffe. Mettler Instrumente AG, Greifensee, Switzerland, ME-720113-84 23. Schlachter, W.: A new smart load and force transmitting system. In: Proceedings 13th International Conference Force and Mass Measurement, pp. 55–58, Helsinki, Finland, 11–14 May 1993 24. Stef˘ ¸ anescu, D.M.: Electro-tensometric resistive transducer for masses up to 10 kg (in Romanian). Instrumenta¸tia (M˘asur˘ari—Automatiz˘ari—Ac¸tion˘ari—Robotic˘a) 6(1), 6 (1996) 25. Lorefice, S.: La misura della densità da Archimede ai giorni nostri. ResearchGate. PDF created by Carlo Ferrero on 7 Dec 2018 26. Platil, A.: Mechatronics Sensors: Flow and Level. PDF created on 19 Apr 2005 27. Bicking, R.E.: Fundamentals of pressure sensor technology—using a flow sensor to measure pressure. Honeywell, PDF created on 9 Feb 2009 28. Kang, H.S., Meneveau, C.: Direct mechanical torque sensor for model wind turbines. Meas. Sci. Technol. 21, Paper 105206 (2010) 29. Härtig, F., Hornig, J., Kahmann, H., Kniel, K., Müller, H., Schlegel, C.: Metrological competence center for windenergy. In: Open Access Proceedings of the XXII IMEKO World Congress Knowledge Through Measurement, Paper 607, Belfast, UK, 3–6 Sept 2018. Published under license by IOP Publishing Ltd, Journal of Physics: Conference Series, Vol. 1065, Measurement of Force, Mass and Torque 30. Boisen, A., Thaysen, J., Jensenius, H., Hansen, O.: Environmental sensors based on micromachined cantilevers with integrated read-out. Ultramicroscopy 82, 11–16 (2000) 31. Baubin, M., Haid, C., Hamm, P., Gilly, H.: Measuring forces and frequency during active compression decompression cardiopulmonary resuscitation: a device for training, research and real CPR. Resuscitation 43(1), 17–24 (1999)

Chapter 15

Multicomponent Force and Moment Transducers

15.1 Classification and Representations of Multicomponent F-M Transducers As Dr. Carlo Ferrero mentioned, the measurement of forces, however oriented in space, and resolvable into the main six components of the force tensor, namely, three perpendicular forces (F x , F y and F z ) and three corresponding moments (M x , M y and M z ), arises from definite scientific, industrial and metrological requirements [1]: (a) development of adaptive-control, machine tools and robots; (b) advanced development, in the aircraft and automobile sectors, of prototype study in wind tunnels with the aid of multicomponent balances; (c) reduction of the uncertainty of force standard deadweight machines. In order to evaluate such complex tasks, Gassmann Theiss Meβtechnik GmbH [2] has developed the Multi Component Analyzer (MCA), a universal tool to measure simultaneously up to six arbitrarily selectable force-, moment- or position (coordinates and angles) components. A more general picture is given in Fig. 15.1. A well designed multicomponent transducer can be considered a force vector resolver (Fig. 15.2a) and the obtained signals should be proportional to those force components which lie on the force transducer sensitive axes [3]. More complex elastic structures are necessary for measuring all the components of the force vector for specific applications in Robotics (Fig. 15.2b), where the usual types of force measuring applications are [4]: – gripping force measurement, correlated with the handled object weight, – robot action force determining in operating process, – internal force measurement of the working gears (reactions in coupling, joggles, joints, etc.), – determining of forces generated by the robot mobile items displacement. In the same domain, a single-block (or integral) dynamometer (designation preferred by Italian metrologists, meaning: device for measuring force and torque) has © Springer Nature Switzerland AG 2020 D. M. Stef˘ ¸ anescu, Handbook of Force Transducers, https://doi.org/10.1007/978-3-030-35322-3_15

159

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15 Multicomponent Force and Moment Transducers

Fig. 15.1 Classification of application areas

Fig. 15.2 The same axes system in two different representations: Brendel—general (a) and DMS for Robotics (b)

certain intrinsic limitations that prevent cross talk effects or interactions from being reduced, but has the advantages of less weight, smaller dimensions and high stiffness (Fig. 15.3). Among various representations of coordinate axes systems and of associate force and moment names, in Fig. 15.4 a version for motor car testing is shown. Note that vertical force in aviation has a name (Lift) differing from the automotive domain (Bounce), where it concerns adherence to the road and certainly not ascension from the ground! There are also other names of forces, depending on their direction, e.g. longitude, latitude, and vertical [5], however a consensus is kept concerning the design requirements. All designs are one piece flexures, machined and gauged to give optimum output, stiffness and durability, and to accurately measure up to six components of load (3 forces and 3 moments), using independent strain gauge Wheatstone bridges.

15.2 F-M Applications with Two to Six Components

161

Fig. 15.3 Multicomponent (3F + 3M) transducer or dynamometer, with dual grid strain gauges [1]

Fig. 15.4 Axes system for vehicles models testing

15.2 F-M Applications with Two to Six Components In German “Drehmoment” means “torque”. In order to avoid confusions, we propose using “moment” when referring to a force tensor component and “torque” for an independent torsion transducer.

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Fig. 15.5 A crawler with eight force transducers (a), each of them measuring two components (b), with strain gauges connected in two Wheatstone bridges, for horizontal F x (c) and vertical F y respectively (d)

15.2.1 Two Components When “climbing” ability is required, the most common solution is to use “spreading systems” [6]. A crawler device with eight “legs” is presented in Fig. 15.5a, having two distinctive elastic elements, a membrane for F x and a beam for F y (Fig. 15.5b), and using two independent Wheatstone bridges, one for F x (Fig. 15.5c) and another for F y (Fig. 15.5d). Knowing the components, horizontal F x (which represents in this case the “adherence” to the tube wall) and vertical F y (the “buoyancy” force, counteracting the gravitational one), and the geometric parameters too, besides calculating the resultant force F, the M z component may also be determined by computation.

15.2.2 Three Components The internal strain gauge balance for three components [7], presented in Fig. 15.6, measures F x and F z by means of two flexible parallelograms located on the vertical and, respectively, on the horizontal sides and M y with a flexible element shaped as a bent bar.

15.2 F-M Applications with Two to Six Components

163

Fig. 15.6 SGFT with three Wheatstone bridges for three components

15.2.3 Four Components Friction stir welding is an autogenous solid-state technique that uses a nonconsumable tool to generate frictional heat [8] and a multiaxial transducer to measure three components of force and the applied torque. Attention: In the measurements with less than six components (n < 6) the remaining secondary components (6 − n) may act as disturbing quantities, implying solutions for their compensation!

15.2.4 Five Components The internal balance for five components [7] has a cage shape with five bars (Fig. 15.7). F y and F z are sensed by the central bar which is bent in the horizontal and, respectively, vertical plane. M x is detected in Section A–A by a pair of twisted extreme bars. M y and M z are measured in Section B–B by the two pairs of extreme bars, bent in the vertical, respectively in the horizontal plane. In all these drawings of tensometric balances an OXYZ axes system is used according to the “right-hand rule”, i.e. “Z takes X over Y”! It is to be noted the uniform figuring of SGs on the EEs in multicomponent FTs, observing the convention adopted in this book: increasing resistance for (mechanical) tension (T) and decreasing resistance for compression (C) applied to each flexible element. Colours chosen by us for the three axes (X—red, Y—green, Z—blue) are the same as those of the ANSYS finite elements program, and their order, RGB, reminds of the colour code in digital imagistics—as you can see, nothing is accidental!

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Fig. 15.7 SGFT with five components and their Wheatstone bridge connections

15.2.5 Six Components An internal balance for six components [7] is presented in Fig. 15.8. The elastic body is made of one piece of highly resistant steel, by making two central asymmetrical slots, necessary to measure the drag. As for the other components the abovementioned beam flexors are used. It is worth remarking that the strain gauges used for the pitch moment M y are alternatively positioned to ensure the symmetry of deformation distribution.

15.2 F-M Applications with Two to Six Components

165

Fig. 15.8 Six-component SGFT and six independent Wheatstone bridges

The interaction reduction and the improvement of the system kinematics can be achieved using an elastic structure with more complicated slots, optimized by FEA and carefully tested before fabrication [9].

15.3 Dynamic Testing of Multicomponent F-M Transducers The first attempt of a dynamic investigation of a three-component force-moment transducer is presented in [10]. The complex elastic structure (a cross inside a square determined by double beams), depicted in Fig. 15.9a, measures forces F x and F y and

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Fig. 15.9 Schematic diagrams: three component F-M transducer (a) with their Wheatstone bridge connections (b) and experimental setup for transverse force components (c)

moment M z using strain gauges connected in three Wheatstone bridges, symbolized in Fig. 15.9b. This 3-component transducer is mounted on a shaker system B&K 4802 + B&K 4818 with a vertical adapter for an external mass of 0.9 kg and three accelerometers connected to charge amplifiers (Fig. 15.9c). The three force/moment signals are applied to a dynamic amplifier HBM MGC, and then all the acceleration and FM signals are passed onto a multi-channel FFT analyzer system HP 3565A. This analyzer controls the shaker through a power amplifier B&K 2708. The experiment reveals that the sensitivity decreases as the frequency increases and that the multicomponent transducer has a 90º functional symmetry due to its geometry. A recent paper presents an improved design of a calibration setup for the dynamic analysis of multicomponent F-M transducers [11]. The design criterion was to move the centre of gravity as close as possible to the origin of the coordinate system of the transducer under test (Fig. 15.10). As the transducer is sensitive to forces and moments in all three spatial directions, a reference force, and equally an acceleration measurement, are required in three directions. An analysis of the dynamic behaviour of these setups was performed using ANSYS FEM software. Different modes were analysed by means of modal analysis and Fig. 15.11 shows a selection of rocking and longitudinal modes for the axial (a–b) and transversal (c–d) versions of the force setup. The main resonances were identified at 1078 Hz and 342 Hz for the axial setup and, respectively, at 360 Hz and 450 Hz for the transversal setup.

15.3 Dynamic Testing of Multicomponent F-M Transducers

167

Fig. 15.10 Dynamic calibration setups for axial (a) and transverse force (b) and for moment components (c)

Fig. 15.11 Results from the modal analysis in rocking (a–c) and longitudinal modes (b–d), for the axial (a–b) and transversal (c–d) versions of the force calibration setup

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15.4 Multicomponent F-M Transducers Calibration in the Old and the New SI Multicomponent force/moment transducers are traditionally calibrated by using forces and torques generated by deadweights and levers, or by comparing measurements with reference transducers. As previously mentioned, multicomponent dynamometers are essentially of two types: single-block (or integral) for Robotics (Fig. 15.3) and composite (assembled or built up) dynamometers for Metrology (Fig. 15.12). The structure of a multicomponent dynamometer has to be very complex if one has to mechanically analyse the different components of the applied force to be measured, taking account of the unavoidable limitations (dimensions, constraint types, etc.) and the different conflicting requirements, such as sensitivity and stiffness, in order to minimize the influence of disturbing quantities and factors (temperature, pressure, vibrations) [12]. Here, the decoupling between the six independent load cells is provided by the use of elastic flexures. At the Italian multicomponent transducer axes are similar with those used in aerodynamics, but their denominations differ, as shown in Fig. 15.12. The axial (vertical) component is the main one, F z , the other 5 components being considered as “parasitic”, and the moment N associated with this axis is called torsion or twisting. Many deadweight force standard machines (FSM) of the main National Institutes of Metrology around the world have been evaluated with the INRiM six-component dynamometer. A recent improvement for the spring testing machines by an integrated hexapodshaped multicomponent force moment transducer is presented in [13]. Fig. 15.12 100 kN six-component dynamometer at INRiM (The Italian Metrology)

15.4 Multicomponent F-M Transducers Calibration in the Old …

169

Fig. 15.13 Levitating element with coils and aperture slits (a) and self-calibrating measuring setup for multicomponent F-M transducers (b)

In the revised version of the SI, the Kibble balance principle is adapted for the traceable force and torque measurement in three orthogonal directions by means of the Planck’s constant h, the speed of light in vacuum c and the hyperfine transition frequency of Cs ν Cs [14]. The main item of the new multicomponent F-M transducer is a levitating element with 12 voice coil actuators (two groups of 6) and 6 position sensors with aperture slits (Fig. 15.13a). This equipment has two operating modes (first—dynamic and second—static): (a) In the velocity mode, the sensing element is controlled by using the coils B and it is moved with known linear and angular velocities in different directions. The voltages induced in the coils A are measured to determine the calibration factors of the multicomponent transducer. (b) During the force/torque mode, the levitating element is controlled by using the coils A, which were calibrated during the velocity mode, and the forces and torques can be measured using the electromagnetic force compensation (EMFC) principle. A self-calibrating measuring system is presented in Fig. 15.13b, based on the same EMFC principle, but with six voice coil actuators only and a load changer mounted above the F-M transducer in order to calibrate force and torque along the vertical direction by using a reference mass with known value [15]. A prototype of the system was designed for measuring forces and torques in a range of 2 N and 0.1 N·m respectively, with a relative combined standard uncertainty (k = 2) of 10−4 .

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References 1. Ferrero, C.: Multicomponent force sensors. ResearchGate: Article in MAPAN—J. Metrol. Soc. India (2005) 2. Allgeier, Th., Gassmann, H., Kolwinski, U., Giesecke, P.: Multi-component measurement technology for forces and moments. In: Joint International Conference IMEKO TC-3/TC5/TC-20 on Force, Mass, Torque, Hardness and Civil Engineering in the age of globalization, VDI-Berichte 1685, pp. 17–26, Celle, Germany, 24–27 Sept 2002. ISBN 3-18-091685-0 3. Brendel, Al.: Application and selection of force sensors. In: Sensors Web Portal—Sensors— Mechanical—Force—Literature, 27 May 2002 4. Stef˘ ¸ anescu, D.M., Marinescu, A., Stef˘ ¸ anescu, Al.: Romanian contributions for evaluating force sensors in Robotics. In: Fachtagung ROBOTIK 2002—Leistungs-stand—Anwendungen—Visionen—Trends, VDI-Berichte 1679, pp. 455–460, Ludwigsburg, Germany, 19–20 Jun 2002 5. Multi-Component Load Cells—Product News, FUTEK Advanced Sensor Technology. Sens. Transducers e-Digest 73(11) (2006). (ISSN 1726-5479) 6. Zagler, A., Pfeiffer, F.: MORITZ—a pipe crawler for tube junctions. In: Proceeding of IEEE International Conference on Robotics and Automation (ICRA’03), pp. 2954–2959, Taipei, Taiwan, Sept 2003 7. Stef˘ ¸ anescu, D.M: Resistive tensometric balances for wind tunnels (in Romanian). Revista Transporturilor s¸i Telecomunica¸tiilor XI(7), 95–99 (1984) 8. Blignault, C., Hattingh, D.G., Kruger, G.H., van Niekerk, T.I., James, M.N.: Friction stir weld process evaluation by multiaxial transducer. Measurement 41, 32–43. ScienceDirect, © Elsevier B.V. (2008) 9. Sandu, M.: Private Communication, 9 Nov 2018 10. Park, Y.-K., Kumme, R., Kang, D.-I.: Dynamic investigation of a three-component forcemoment sensor. Meas. Sci. Technol. 13, 654–659 (2002) 11. Nitsche, J., Bruneniece, S., Kumme, R., Tutsch, R.: Design of a calibration setup for the dynamic analysis of multi-component force and moment sensors. In: Open access proceedings of the XXII IMEKO world congress Knowledge through Measurement, Paper 168, Belfast, UK, 3–6 Sept 2018. Published under license by IOP Publishing Ltd, Journal of Physics: Conference Series, Vol. 1065, Measurement of Force, Mass and Torque 12. Ferrero, C.: Round table on multicomponent measurement to improve the traceability in the force chain (Report on ResearchGate). In: Joint IMEKO TC3, TC5 and TC22 Conferences “Metrology in Modern Context”, Pattaya, Chonburi, Thailand, 22–25 Nov 2010 13. Genta, G., Prato, A., Mazzoleni, F., Germak, A., Galetto, M.: Accurate force and moment measurement in spring testing machines by an integrated hexapod-shaped multicomponent force transducer. Meas. Sci. Technol. 29(9). © 2018 IOP Publishing Ltd. 14. Marangoni, R.R., Schleichert, J., Fröhlich, Th.: Multicomponent force and torque measurement in the new SI. In: Conference on Precision Electromagnetic Measurements, Paris, 8–13 Jul 2018 15. Marangoni, R.R., Schleichert, J., Rahneberg, I., Hilbrunner, F., Fröhlich, Th.: A self-calibrating multicomponent force/torque measuring system. [Precision Measurement and Engineering at the 59th Ilmenau Scientific Colloquium] Meas. Sci. Technol. 29(7) (2018)

Chapter 16

Equipment for Determining Aerodynamic Forces on Flapping Wings

This chapter is devoted to the experimental determination of the non-steady aerodynamic forces acting on the flapping wings of the micro air vehicles (MAVs) [1]. The first attempts to explain the lift generation on the insects’ wings used the so-called “steady-state” aerodynamics, a theory that had been successfully applied in aircraft design. The result was a failure, leading to the conclusion that “a fly cannot fly”! The “steady-state” theory could not explain the high lifting force necessary for an insect to fly! Later, both experimental and theoretical investigations proved that flapping flight uses specific aerodynamic mechanisms being able to increase the lift [2–4]. At the National Institute for Aerospace Research (INCAS), Bucharest, Romania, a multidisciplinary group of scientists performed such research on flapping wings. Some aspects concerning the small force measurement procedure are presented here.

16.1 Micromechanical Flying Control and Scaling Aspects At the first level of MFI (micromechanical flying insect) is the wing control system based on wing and/or thorax mounted force sensitive devices. Traditionally, measuring forces on a flying insect is performed by fixing the insect to a cantilever and measuring its variable position by resistive, capacitive (Fig. 16.1) or optical means. Alternatively, the measurement of direct flight forces involves measuring the moments on the wing using strain gauges mounted directly in the wing spars. The results obtained for 5X scale experiments wing spar sensing are, according to [5]: lift force—32 mN and drag force—25 mN. Decreasing the mechanisms’ size requires serious adjustment of the engineer’s judgment regarding relative dimensions and loads [6]. The attraction forces between (nearly) contacting surfaces are in the macro world usually much lower than the gravity forces while in the micro world, however, forces due to electrostatic charging, Van der Waals forces or surface tension of water films might well dominate gravity. Piezoresistive and capacitive gauges are the commonly used microsensors to evaluate the flying insects (Fig. 16.1), but the problem is the extremely low change of physical © Springer Nature Switzerland AG 2020 D. M. Stef˘ ¸ anescu, Handbook of Force Transducers, https://doi.org/10.1007/978-3-030-35322-3_16

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Fig. 16.1 Fruit fly flight behavior characterization using MEMS force transducers: resistive wire (a) or multifunctional interdigital electrodes (b), for example, capacitive lay-out (c) and associate electronic circuitry (d) (adapted from [3])

quantities like resistance or capacitance on changing the relative position, being difficult to discriminate noise from the useful sensor signal. As Christofer Hierold, from ETH Zurich, Micro and Nanosystems Department, states: While in microelectronics miniaturization and further integration, following Moore’s law, have succeeded in better performing measuring devices (smaller, faster, cheaper), transducers confronting with inertia do not benefit from scaling in general [7]. Three types of sensors have been compared for measuring pressure, acceleration and yaw rate. All of them measure a force as a result of the physical unit applied that acts on a sensing element (resistive, capacitive, electromagnetic etc.) against a spring force.

16.2 Equipment for Measuring Aerodynamic Forces Figure 16.2 shows the experimental setup designed and constructed to perform biomimetic motion of the wings (see the zoom with a Maltese bee!). Within this

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Fig. 16.2 The mechanism that produces beating and flapping motions of the right wing, with force transducer as a “coupling item” of biomimetic wing!

adopted mechanism, driven by an electric motor, the moment of the global forces on the wings is measured using strain gauge transducers. The moment components given by the forces acting in two directions (perpendicular and parallel to the wing plane) are tensometrically measured by strain gauges 0.6/120 LY11 Hottinger (Fig. 16.3) while the flapping and pitching angles are determined using two precision potentiometers, type 601-1045 made by Vishay-Spectrol. All signals are transmitted to computer via a multifunction DAQ National Instruments PCI-6221, Windows compatible (Fig. 16.4). LabVIEW SignalExpress LE together with NI-DAQmx can gather, register, export and visualize experimental data. The analysis includes the extraction of the inertial forces which are the predominant ones, making possible the accurate determination of the aerodynamic forces on the flapping wings.

16.3 Experimental Results Our results are in good agreement with those obtained by B. Singh et al., Department of Aerospace Engineering, University of Maryland at College Park [5], as they are presented in Fig. 16.5.

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Fig. 16.3 Bicomponent force transducer mounted on the wing axle, with customized cantilever beams (a) having strain gauges connected in two independent Wheatstone bridges like in (b) for measuring forces in two perpendicular planes (H—horizontal and V—vertical) as in the side and top views (c)

Fig. 16.4 Functional diagram for computerized measurement of two forces acting in perpendicular directions (F V and F H ) and two rotation angles (pitching and azimuth)

There are a lot of challenges concerning the geometrical, aerodynamic and functional similitude between the real insects and the micro air vehicles. On a smaller scale, a design approach to determine the bending stiffness of the flexible hinge in electromagnetically driven (with applied AC current of 420 mA) artificial flappingwing insects for improved lift force is presented in [8]. The prototype insects are fabricated by laser cutting technique, having a wingspan of 3.6 cm and a total mass

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Fig. 16.5 Thrust measured for flapping motion of the wing

Table 16.1 Artificial insect versus insect simulator Parameter → Equipment Artificial insect Flapping wing for insect simulator

Wingspan (mm) 36 530

Total mass (gram)

Lift force (mN)

Frequency (Hz)

0.084

0.4

65

68

30

14

of 84 mg. They can produce effective wing rotational movements with flapping amplitude of ±50.6° at 65 Hz. Such wing movements can generate a measured average lift force of 384.5 µN to produce a total lift force to weight ratio of 0.47. A comparison between the parameters for the artificial insect (School of Energy and Power Engineering, Beihang University, Beijing) and the flapping wing for insect simulator (INCAS, Bucharest) is given in Table 16.1. Our complex experimental set-up, in course of development in order to improve its metrological characteristics, as well as the original solution concerning the using of a set of large scale wings, has two main advantages [9]: (a) the frequency being small, the inertial forces are not so great; (b) the area being large, the aerodynamic forces could be made large enough to be precisely measured. The value of the lift force (30 mN at 10 Hz) is rather close to the value indicated in C16.1 (32 mN).

References 1. Stef˘ ¸ anescu, D.M., Butoescu, V.: Equipment for determining aerodynamic forces on flapping wings. In: Proceedings of the XIX IMEKO World Congress Fundamental and Applied Metrology, pp. 311–315, Lisbon, Portugal, 6–11 Sept 2009. ISBN 978-963-88410-0-1. www. imeko2009.it.pt/Papers/FP_470.pdf

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2. Perez Goodwyn, P.J., Gorb, S.N.: Attachment forces of the hemelytra-locking mechanisms in aquatic bugs. J. Insect Physiol. 49, 753–764 (2003) 3. Fry, S., Beyler, F., Graetzel, C., Nelson, B.: Fruit fly flight behavior characterization using MEMS force sensors. www.iris.ethz.ch/msrl/research/micro/fly.php, 7 May 2008 4. Singh, B., Ramasamy, M., Chopra, I., Leishman, J.G.: Experimental studies on insect-based flapping wings for micro hovering air vehicles. American Institute of Aeronautics and Astronautics, Report RCL-05 5. Wood, R.J., Fearing, R.S.: Flight force measurements for a micromechanical flying insect. www. robotics.eecs.berkeley.edu, 10 March 2005 6. Soemers, H.M.J.R., Brouwer, D.M.: Mechatronics and micro systems. In: 3rd IFAC Symposium Mechatronic Systems, pp. 609–614, Sydney, Australia, 6–8 Sept 2004, © IFAC Copyright 7. Hierold, Ch.: From micro- to nanosystems: mechanical sensors go nano. J. Micromech. Microeng. 14, S1–S11 (2004) 8. Liu, Z.W., Yan, X.J., Qi, M.J., Zhu, Y.S., Huang, D.W., Zhang, X.Y., Lin, L.W.: Design of flexible hinges in electromagnetically driven artificial flapping-wing insects for improved lift force. J. Micromech. Microeng. 29(1), © 2018 IOP Publishing Ltd. 9. Butoescu, V.: Private communications, 7 Jan and 25 Apr 2019

Chapter 17

Strain Gauge Balances for Testing Car and Flight Models in Wind Tunnel Applications

17.1 Classifications and Requirements for Strain Gauge Balances in Wind Tunnels The study of the transport means behaviour from the aerodynamic point of view is particularly important; that is why tests are performed on mock-ups in wind tunnels both for aircraft and for high speed automobiles and locomotives. The associated balances can simultaneously measure up to six components of the force tensor (three perpendicular forces and three corresponding moments), in a system of three rectangular axes having usually their origin in the mock-up gravity center (Fig. 17.1). Several classifications are possible for strain gauge balances [1]: (A) According to the measuring principle, in the order of technical progress steps, the balances intended for wind tunnels may be: mechanical (with equilibration or dynamometric), pneumatic, hydraulic and electrical. Due to their higher accuracy, the latter got a certain priority and, among them, are the ones with strain gauges connected by Wheatstone bridge. (B) In accordance with the way they measure the components the following can be found: (a) constructions with independent force transducers, each measuring a certain component, just like in the case of half rigid balances, balances with Fig. 17.1 Two representations of the OXYZ axes system using the “right-hand rule” (“Z takes X over Y”) and the components’ names (three forces and three moments) in aerodynamic applications

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wires or platform ones and (b) complex elastic structures which can measure all six components like internal balances. The half rigid and wire balances are recommended for low air speeds, which are for subsonic wind tunnels, while the internal balances are used in supersonic wind tunnels. (C) According to the position of their model, balances may be external or internal. The development of the strain gauge measurement method of forces and moments based on the deformation of some special elastic systems has led to the improvement of the internal balances, where the measuring points are very close to the model and their elastic structure has no moving parts and therefore avoids the friction met with the external mechanical balances. Half models are used too and are obtained by sectioning the mock-ups in the median plane XOZ. Although they were mainly designed for aviation mock-ups, the multicomponent strain balances are successfully applied even outside wind tunnels, such as for measurements of forces and couples existing at the hovercraft vehicles gear boxes or hydrodynamic tests on maritime mock-ups. (D) According to the number of components being measured: Balances specialized for a single [2] or two [3] components are those of platform type, supported on columns or beams subjected to bending. The first measures the thrust produced by a stationary plasma thruster in the range of 11–16 mN with an accuracy of 1 mN and resolution of 0.12 mN, whilst the second is intended for large off-axis loads. Internal balances for 3, 5 and 6 components respectively were presented in C.15.2, and here a personal 6 components application will be shown in detail. Besides these balances, special complex devices are used for the test beds of the turboprop engines, for measuring the hinge moments of the control elements of winglet type or for investigating the strains on the wing surface (Fig. 17.2), but these are beyond of the this chapter’s subject. The strain gauge balances for wind tunnels should meet the following requirements: – to be made of good quality, resistant and light materials with reduced hysteresis (e.g. allied steels); – up-to-date processing technologies and instruments, with high resolution; – solid and reliable constructions insensitive to vibrations and ambient factors, with stable long term performances; – various installation possibilities and protection against overloads; – miniaturization, according to the reduced section of the test room and not to influence the mock-up aerodynamic load (balances with six components having diameter Ø6 are made for aircraft and with five components with diameter Ø4 for rockets); – static tests corresponding to the aircraft travel on a fixed alignment related to the wind direction or dynamic tests simulating various evolutions in flight or on rolling belt carpet; – mechanical and electrical separation of the components, determining the lowest possible coefficients for the inter-influence matrix;

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Fig. 17.2 The control room for the subsonic wind tunnel (a), high lift configuration in a 2.5D layout with asymmetric endplates (b) and wake rake setup (comb with pressure taps, allowing the drag determination by integral computation) (c)

– high sensitivity considering that a tare up to 50% of the usable range is used in operation (Bridges with eight strain gauges are also used, achieving thus a better mechanical-electrical averaging and a “doubled” sensitivity [4]); – highest accuracy, since the same equipment performs measurements within a wide range of values and should estimate the optimum aerodynamic performances for shapes and models with slightest differences between them.

17.2 Strain Gauge Balances for Subsonic Wind Tunnel 17.2.1 Strain Gauge Balance with Wires The SG balance with wires [1] presented in Fig. 17.3 measures three components using five annular force transducers, well-protected against gust. The drag force is sensed by transducer C1 having the nominal load of 450 N, and the lift force and the pitch moment result from the mathematical processing of the indications from transducers C2 …C5 , designed for 1 kN. (C means “Capteur” in French, “Captor” in Romanian.)

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Fig. 17.3 Balance with wires for an aircraft model (a), mock-up on supports (b)

17.2.2 Half Rigid Strain Gauge Balance The half rigid SG balance [1] is installed on the test-bed from Fig. 17.4, where the item denominations are also recorded. To be noted that the steel wire is welded to the balance body and the servomotor ensures the balance verticality, i.e. the correct mockup position. The relatively rigid and light configuration permits the measurement of two components: the Drag with transducer C1 , having the sensitivity of 1.5 μm/m/N, and the Lift with transducers C2 and C3 having the sensitivity of about 0.5 μm/m/N. The strain gauge force transducer C1 with extended beam elastic element is presented in Fig. 17.5.

17.3 Strain Gauge Balances for Trisonic Wind Tunnel Trisonic, the name of the Romanian Wind Tunnel, means: Sub-, Trans- and Supersonic regime.

17.3.1 Vertical External Strain Gauge Balance This vertical external balance [5], which is not subjected to space restrictions, permits the “composition” of a bar made elastic structure such as the four-spoke wheel and the four-column hub. The numerical computation model contains 18 three-dimensional bar elements and 16 nodes. The final version has been imposed by the equivalent stresses existing in the elastic elements and by the possibilities of technological

17.3 Strain Gauge Balances for Trisonic Wind Tunnel

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Fig. 17.4 Half rigid SG balance for INCAS subsonic wind tunnel

Fig. 17.5 Two views of the tensioned lamella for half rigid SG balance

processing. Analyzing the influence of each load on the instrument readings for the other five Wheatstone bridges, one came to the conclusion that the effects are theoretically uncoupled due to the symmetrical or antisymmetrical F–M diagrams. The tensometric sensitivity of Z and N channels was increased using eight strain gauges instead of four, all of them being active (Fig. 17.6).

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Fig. 17.6 Six-component vertical external SG balance for testing vehicle models in wind tunnel

17.3.2 Strain Gauge Wall Balance for Half Model A SG wall balance for half model [6] is presented in Fig. 17.7. This balance measures three components of the “loading torsor”: drag force Fx = 3 kN, lift force Fz = 18 kN and pitching moment My = 2.3 kN·m. A symmetrical structure has been conceived having four bent beam shaped arms with square or rectangular sections (Fig. 17.8). The arms from the horizontal plane (I) and those from the vertical plane (II) are respectively identical. The accurate computation has been performed by the finite element method, using the IMAGES 3D programme. The “spider” elastic structure (ARMCO 17-4 PH) is discretized in 428 finite elements of three-dimensional solid (brick) which interact in 980 nodes. The

17.3 Strain Gauge Balances for Trisonic Wind Tunnel

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Fig. 17.7 Half model (a) mounted on the wall balance (b) and “biomimetic” full model (c)

Fig. 17.8 Three-component SG wall balance for half model (designed by Marin Sandu), with strain gauges (type 3/350 LY 11—HBM) connected in three Wheatstone bridges

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equivalent tensions were calculated according to the von Mises criterion. The components effects are completely decoupled, providing the following indications to the strain gauge bridges: ε(Fx ) = 1397 μm/m, ε(Fz ) = 4990 μm/m and ε(My ) = 1331 μm/m.

17.3.3 Six Component Internal Strain Gauge Balance By that date, this was the first Romanian presence, a multilingual one (English, German, French and Italian), in the Applications Bulletin of HBM, the worldwide leader in the area of transducers for mechanical quantities, starting with Force [7–10]. An original balance for aircraft models being tested in our supersonic wind tunnel is shown in Fig. 17.9, using the “right-hand rule” (“Z takes X over Y”). The axial force is measured in four short lateral arms close to the middle of the balance interior. The other five components are measured in two symmetrical sections each consisting of a casing with three beams. This complicated structure was developed by Finite Element Analysis; the loads from 722 isoparametric elements and the displacements of 1536 nodes were calculated by computer. The elastic tail of the balance was manufactured to the highest possible accuracy by EDM (electrical discharge machining) from a single piece of ARMCO17-4 PH and metallurgically treated to ensure a permissible tensile strength in excess of 400 MPa (N/mm2 ). It is 353 mm long and 50.8 mm in diameter (2 in.). Taking account of two conflicting requirements, the choice went to HBM’s Y series foil SGs with standard resistances as follows: 120  in view of space restrictions; 350  in view of the total power supply (not exceeding 5 V) of the six Wheatstone bridges with four or eight active strain gauges. This integral solution offers the best relation between capacity and volume, since the interference between forces and/or moments is accurately specified by calibration. Maximum operating loads for the individual components are: Forces: Axial X = 2850 N, Side Y = 9650 N and Normal Z = 14,700 N. Moments: Roll l = 320 N·m, Pitch m = 820 N·m and Yaw n = 760 N·m.

17.4 Calibration of Strain Gauge Balances for Wind Tunnels There are two different words in Romanian: “etalonare” that means direct comparison with a standard (traceability) and “calibrare” meaning the adjustment of the sensitivity for the whole measuring system. In the most important languages there

Fig. 17.9 A complex picture of the six component internal SG balance for INCAS Trisonic Wind Tunnel

17.4 Calibration of Strain Gauge Balances for Wind Tunnels 185

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Fig. 17.10 Schematic view of a calibration test-bed for tensometric balances

are unique words: calibration (English), étalonnage (French), Kalibrierung (German) and kalibrovka (Russian). Metrological certification of the multicomponent strain gauge balances requires special benches for calibration and raises numerous force (F), torque (T) and mass (m) metrology problems, highlighting the multidisciplinary character of this application in the new millennium. Beyond the great variety of SG balances, a simplified representation is given in Fig. 17.10, illustrating the Romanian experience in the field of calibration [11]. Two possible versions are to be noticed: – gravimetric: utilizing calibrated dead weights (4); – electro-mechanical: using force transducers (2). The other items in the drawing are: symbolic balance (1), fine adjustment screws (3), positioning devices (5), coupling element (6), release mechanisms (7) and loading system (8). Having in view possible co-operations in this respect, the six-component balance could become a kind of three-directional “weighing machine” intended for forces F or torques T (forces multiplied by distances), respectively. Finally, a few images of the Romanian Trisonic Wind Tunnel, the greatest of this kind in Central and Eastern Europe, are presented in Fig. 17.11.

17.4 Calibration of Strain Gauge Balances for Wind Tunnels

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Fig. 17.11 Trisonic Wind Tunnel: a view of the hall (a) and of the monitors from the control room (c). Dan with the “Ø50 Multicomponent Tensometric Balance” (BTM-50) (b) and the wind tunnel operators, engineers Iva and Dido (c)

References 1. Stef˘ ¸ anescu, D.M.: Resistive tensometric balances for wind tunnels (in Romanian). Rev. Transp. Telecomun. XI(7), 95–99 (1984) 2. Stephen, J.R., Rajanna, K., Dhar, V., Kumar, K.G., Nagabushanam, S.: Thin-film strain gauge sensors for ion thrust measurement. IEEE Sens. J. 4(3), 373–377 (2004) 3. Ostafichuk, P.M., Green, Sh.I.: A low interaction two-axis wind tunnel force balance designed for large off-axis loads. Meas. Sci. Technol. 13, Design Note N73–N76 (2002) 4. Stef˘ ¸ anescu, D.M.: Methods for increasing the sensitivity of strain gauge force transducers (in Romanian). Ph.D. dissertation cum laude (160 pages, 26 tables, 86 figures, 336 references), Universitatea “Politehnica” Bucure¸sti, Romania, 10 Sept 1999

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5. Stef˘ ¸ anescu, D.M., M˘anescu, T.: Multi-component force and torque balances for wind tunnels. In: Joint International Conference on IMEKO TC-3/TC-5/TC-20 on Force, Mass, Torque, Hardness and Civil Engineering in the Age of Globalization, VDI-Berichte 1685, pp. 549–554, Celle, Germany, 24–27 Sept 2002. ISBN 3-18-091685-0 6. Stef˘ ¸ anescu, D.M., M˘anescu, T.: Three-axis strain gauged force transducers. In: Chung, M.S. (ed.) Proceedings of the IMEKO 16th TC3 in Parallel with APMF’98 International Conference Force, Mass and Torque Measurements—Theory and Practice, pp. 118–127, Taejon, Republic of Korea, 14–18 Sept 1998 7. Stef˘ ¸ anescu, D.M.: Strain gauges in a wind-tunnel application. Hotline Hottinger—News from the world of test and measurement, Hottinger Baldwin Messtechnik GmbH, Darmstadt, pp. 12– 13, No. 2 (2001) 8. Stef˘ ¸ anescu, D.M.: Une balance de soufflerie roumaine équipée de jauges d’extensometrie HBM. Hotline—Informations sur les techniques de mesure industrielles, Hottinger, pp. 12–13, No. 2 (2001) 9. Stef˘ ¸ anescu, D.M.: DMS in Windkanal—Rumanische Windkanalwaage mit Dehnungsmeß streifen vom HBM. Hotline Hottinger—Informationen aus der industriellen Messtechnik, pp. 12–13, No. 2 (2001) 10. Stef˘ ¸ anescu, D.M.: Gli estensimetri in un’applicazione nella galleria del vento—HBM tecnica di misura. Hotline Hottinger—online edition (2001) 11. Stef˘ ¸ anescu, D.M.: Metrological check procedure for multi-component internal strain gauged balances. In: Proceedings of 15th IMEKO TC-3 International Conference Accuracy Assurance in Force, Torque and Mass Measurement, pp. 305–310, Madrid, Spain, 7–11 Oct 1996

Chapter 18

Recent Evolution of Smart Force Transducers

18.1 Smart and/or Intelligent As Dr. Sergey Yurish noticed, in some languages “smart” and “intelligent” are translated by the same word that normally means “intelligent”. But in English there is a difference: the first one is more related to technological aspects while the second one is more related to functional aspects [1]. More explicitly, a smart sensor is a combination of a sensor, an analog interface circuit, an analog to digital converter (ADC) and a bus interface in one housing. A smart sensor can be made as integrated (if all elements of a smart sensor are integrated into one chip) or hybrid sensor [2]. What does it make a transducer to be “intelligent”? Very often it means a presence of microcontroller or microprocessor—a necessary but not enough condition. Sometimes a microcontroller is used only for calculation according to predetermined equations, but other intelligent functions mean self-testing, self-validation, self-checking, self-diagnosis, self-calibration, self-compensation, just “selfie” (the word of the year 2013, cf. Oxford English Dictionary). Distinction between a classical/conventional and a smart/intelligent transducer [3] is illustrated in Fig. 18.1, needing no further explanations. Modernizing a transducer involves first of all implementation/integration into the measurement circuit of an ADC or a microprocessor respectively.

18.2 Smart Circuits Let’s follow the development of the “smart” attribute in the area of Wheatstone bridges composed of strain gauges—that being the main subject of this book! The operational amplifier (op-amp), invented by Karl D. Swartzel Jr. and produced first time in 1967, is a DC-coupled high-gain electronic voltage amplifier with a differential input and, usually, a single-ended output, popular as a building block in analog © Springer Nature Switzerland AG 2020 D. M. Stef˘ ¸ anescu, Handbook of Force Transducers, https://doi.org/10.1007/978-3-030-35322-3_18

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Fig. 18.1 Conventional versus “smart” transducers for electrical measurement of mechanical quantities

circuits due to its versatility (Wikipedia). Around the years ’70 we created, using only Romanian components, a strain gauge DC amplifier with integrated circuits [4]. Here are some stages during the evolution of smart transducer technology [5], i.e. the transition from the classical Analog to the intelligent Digital, with their permanent interconnection: – 1983: first intelligent pressure sensor made by Honeywell using DSSP (digital sensor signal processing)

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191

– 1985: a single-chip ASIC (with constant current source) made by Keller using ASSP (analog sensor signal processing); – 1994: hardware independent communication standard for low-cost smart sensors designed by NIST (the National Institute of Standards and Technology in USA). Examples of components widely used in instrumentation: – Analog Devices: AD 522 instrumentation amplifier connected with strain gauge bridges or AD 698 universal signal conditioner for LVDT as traditional transducerbased bridge; – Maxim: MAX4196 high-gain signal conditioning circuit for thin and thick film transducers; – Texas Instruments: TLV 2262 strain gauge transducer for measuring pressure in isothermal environment. The increasing role of the digitization can be followed by examining the signal processing for strain gauge bridge force transducer [6], presented in Fig. 18.2a. Here are the characteristics for the principal module of an AD 101B—DC amplifier for resistive transducers made by HBM: • • • •

recommended both for static and dynamic applications, direct computer connection via RS-232 interface, test certificate for 6000d class III available, high transmission rate and resolution, and memory for users setting.

The measurement amplifier board is designed as a plug-in type board, which can be plugged into the carrier board of the basic device via a 25-Pin D-Type connector (Fig. 18.2b). Quantum MX238B is a multi-functional, “multi-talented” performer for numerous transducers having different measuring principles, but it is “centered” on the strain gauge precision transducers connected in Wheatstone bridge [7]. The bridge can be supplied by DC or better AC, with a frequency f c as “carrier”. The mechanical quantity to be measured changes with the signal frequency f m . An interfering signal (AC or DC) can be separated from the measurement signal of the same frequency by using the carrier frequency method. Precision measurements are performed preferably at f c equal 225 Hz; this frequency has been chosen by many National Metrology Institutes (NMIs), as it is no multiple of the disturbing 50 Hz or 60 Hz power supply. It only allows a signal bandwidth of approx. 45 Hz, and therefore a quasi-static measurement, but on the other hand with very high resolution (Fig. 18.3). First time in HBM history they were using a hermetically sealed resistive divider as bridge standard and decided to implement active temperature compensation. MX238B demands the connexion of the transducers in a six-wire connection circuit (with two additional sensing lines), as for this class of accuracy the resistance of cable lengths in the range of only cm already counts. This new module has an extremely low measurement uncertainty at very good long term stability and illus-

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Fig. 18.2 AED 9101 digital amplifier adapted for strain gauge force transducers: electronic diagram (a) and mechanical construction (b)

trates the substantial progress regarding compact size precision amplifiers for strain gauge based transducers. A lot of advantages for this measurement technology, very smart, indeed!

18.3 Smart Force Transducers Having in view the main types of force transducers (FTs) that could be considered as “smart” and their historical evolution, the following types are considered: vibrating

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Fig. 18.3 The new amplifier module Quantum MX 238B, for any kind of strain gauge transducers, built in compact size (a) and its schematic view (b)

wire, resistive (strain gauges—SGs, piezoresistive films and flexible semiconductive networks) and optical.

18.3.1 Smart Vibrating Wire Force Transducers The K-Tron Smart Force Transducer Weighing Technology [8] is shown in Fig. 18.4a. The applied load causes the vibrating wire to change its resonant frequency (10– 15 kHz measurement range) and the resulting signal is converted to a square wave. In the weighing system SFT2 (Fig. 18.4b) the digital filtration avoids the interference noises in the equipment; there are also thermal compensation circuits and incorporated microprocessors providing the maximum resolution, without the need for in situ recalibration. Other important features of the smart vibrating wire transducer are: – – – –

100% digital design, no calibration required; Resolution of 1:4,000,000 in 80 ms; Weight captured and linearized 112 times per second; On-board microcontroller, custom frequency processor, calibration memory and voltage regulator.

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Fig. 18.4 K-Tron Smart Force Transducer Weighing Technology: vibrating wire assembly (a) and electronic circuitry (b)

18.3.2 Smart Resistive Force Transducers A smart transducer for monitoring railcar braking systems [9] is composed of a wireless transceiver, processor, analog electronics, and a sensing element. An internal battery powers the system during normal operation and is recharged from an external energy harvesting device when the railcar is in motion. A finite element model of the sensing plate was generated in order to efficiently satisfy the design parameters (Fig. 18.5). Strain gauges of 5 k are located at the two thinned sections on the left side of the element and strain relief slots are cut into the element on the right side. Thus, the mechanical strain amplification is about 20:1, making this system more sensitive than a MEMS strain gauge. InControl Solutions Company has achieved a smart force joystick [10] designed for portable and low-power applications with simple input acquisition, stable response, and a small form factor. Conventional joysticks move a game character at a constant rate, no matter how hard the user presses, but adding proportional control would vary this rate based on touch: applying more pressure to the joystick, the character would move faster! In this respect, the Multipath force transducer combines screen print technology with a unique, patented design that places a resistor on one sheet and a silver shunt (or pattern of shunts) on the opposing sheet (Fig. 18.6a). The resistor is a rectangular area of carbon ink printed over two silver ink traces. When the transducer sandwich is lightly pressed, the silver begins to contact the high points on the carbon resistor surface. As more pressure is applied, the number

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Fig. 18.5 Strain-gauge-equipped, self-networking sensing plate on the railcar braking system

Fig. 18.6 The principle of Multipath force sensing (a) and four-sensor array for a smart cursor control (b)

of contact points increases, causing the resistance to decrease and following a forceresistance relationship established for an individual configuration of force transducer. Worth to mention is the Multipath ability to create various sensing element arrays based on adaptive algorithms, from proportional switches to fast directional controls, e.g. the Joydisk from Fig. 18.6b. Flexiforce® sensing devices [11] are presented in Fig. 18.7: principle (a) and technology (b). For example, they are useful for the artificial skin, consisting of two 25 μm thick polyester sheets, which were laminated together with adhesive in the non-sensing area. These sheets carried parallel, thermoplastic, Ag-filled polymer conductive traces covered by a thermoplastic semiconductive ink of resistivity ρ. The two sheets were oriented together so that their traces formed a grid, with the

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Fig. 18.7 Flexiforce® sensing devices made by Tekscan, Inc.: principle (a), technological achievement (b), sensing cells (sensels) for measuring the spatial distribution and magnitude of forces (c), and typical electronic circuit (d)

semiconductive layers facing each other. Each cross section of the grid formed a contact piezoresistive force transducer. The sensitive network resolution, size and shape can be easily tailored to the applications’ requirements, e.g. footprint pressure distribution pattern (Fig. 18.7c). Multiplexing electronics (Fig. 18.7d) is used to scan the array of sensible cells, that can measure the spatial distribution and magnitude of forces perpendicular to the sensing area. The associated hardware system could support an array of 52 × 44 sensels with a scanning frequency of 225 frames per second.

18.3.3 Smart Optical Force Transducers Optical transducers are favourable since they do not suffer from electromagnetic interference and are potentially more sensitive. Therefore, mechanically very flexible optical transducers were developed to measure forces for various medical applications [12]. The transducer operation for the shear forces relies on the changing coupling between a light source and optical detector, separated by a deformable sensing layer (Fig. 18.8a). When the shear force increases, the light intensity and the output current decrease correspondingly. The sensitivity is highest at 5 mA supply current and the response is nearly linear in the largest part of the range (Fig. 18.8b).

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Fig. 18.8 Optical force sensing for smart prostheses: measurement principle (a) and force-tocurrent transduction diagram (b)

A sensorized laparoscopic surgical scissor instrument, using both a fiber Bragg grating (FBG) and a tapered photonic crystal fiber (PCF) as force transducers, is presented in [13]. Experiments show that the PCF transducer has higher strain measurement sensitivity (2 μm/m) than the FBG transducer (1.2 μm/m), these sensitivities being greater than the classical strain gauges sensitivity (1 μm/m). A smart transducer concept for traceable dynamic measurements is presented in [14]. In order to gain maximum control for its development for dynamic measurements, a modular approach with openly accessible hardware and software will be adopted (Fig. 18.9). An independent non-smart sensor module with digital (or even analogue) output will be linked to a network-capable processing unit. The system parts comprising the network interaction and the distribution of the measurement data will be implemented in the environment of a conventional operation system (OS) preferably using an open source kernel. The result would be a device with so-called smart traceability, meaning, a smart transducer in the metrological sense, a real challenge of the current trend towards digitization and Internet of Things (IoT).

Fig. 18.9 Schematic view of the smart transducer concept

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18.4 Smart Imaging Within a wide range of applications, including security inspection systems (SMARTEC—Grancia, Switzerland) and automated image analysis software solutions for scientific and industrial applications (Smart Imaging Technologies Co.— Houston, Texas), we are interested in specific aspects in the area of SGFTs, e.g. Smart Skin [11]. A smart skin embedded in the floor, well hidden so that it is not interfering with people’s behaviour, can be used to identify people by analyzing their footstep force profiles (Fig. 18.10a). A smart skin placed in front of shopping windows, advertising stands or product areas in retail shops would be able to gather valuable marketing information. Hand geometry is one of the physical characteristics of human beings that can be used to recognize or authenticate their identity by Biometrics (Fig. 18.10b). The length, width and surface area of the palm and fingers can be extracted from the pressure image of the hand and then used to create a template for each user during an initial registration process. Furthermore, the pressure distribution of the hand can be used as an additional verification variable in the template or even as an anti-tampering security mechanism against the use of hand molds. The measured pressure data have been used to design and fine-tune the circuit parameters of the data acquisition system. The interface was designed to visualize the complete range of the transducer response starting from 500 k and displays the actual resistance values of the piezoresistive sensor matrix under a load, both numerically and as colour codes for better visualization (see Ref. [3] in Chap 5). Such colour bars are used in a wide range of applications (Fig. 18.11): (a) The relative strength B of the local magnetic field distribution [15]; (b) The relative diffraction intensity for patterns of a suspension in normalized response to the external magnetic field [15];

Fig. 18.10 Pressure distribution image of human feet (a) and right hand (b), with the colour bar scale representing the amount of force in arbitrary units

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Fig. 18.11 Colour bars for magnetic field (a, b), particle interference spectrum (c), piezoelectric potential (d), surface rugosity (e), tomography (f), temperature profile (g), flow speed (h), pressure distribution map for smart skin (i) and—last but not least—stress map for an elastic element of a special SGFT (j)

(c) Contour plots of theoretically computed normalized quasi-particle interference spectrums [16]; (d) The overall distribution when the free charge contribution to the piezoelectric potential (expressed in volts) is considered [17]; (e) Surface profile—rugosity; (f) 3D sensitivity map for tomography; (g) Multiphysics simulation model for a device showing temperature profile at zero strain [18]; (h) The flow speed (in m/s) reconstructed from flow components by acoustic tomography imaging [19]; (i) The pressure distribution map for smart skin from Fig. 18.10b; (j) Stress map (von Mises) for a patented wheel weigher [20]. In a multidisciplinary world, with interconnected specialties, such imagistic correlations may lead to ingenious solutions, in quite complex applications. For instance, the ANSYS program, initially conceived for the structural analysis of mechanical systems, was later extended to thermal, electromagnetic, vibrational and other “multiphysics”, under a unifying colouristic vision (red signifies maximum stress and

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Fig. 18.12 SMART ANSYS, strain diagrams and “smart” placement of SGs on the three sides (upper, lateral and lower) of the FT’s rectangular measuring section

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Fig. 18.13 Conference in New Zealand (a), Maori spectacle at the official banquet, Amora Hotel (b) and the author in Seatoun, near Wellington (c)

blue—minimum)! The colour bars are “self-scaling” according to the stress intensity, and “weight” of various colours is different from one phenomenon to another. SMART ANSYS has adopted a multicolour label (Smart Simulation for Smart Products), as seen in Fig. 18.12, with a suitable personal application [21]. Author’s original contribution consists in the building of strain diagrams and placing them directly on the SGST’s elastic element sides. For this axisymmetric EE a novel placement of SGs was chosen, on three sides of the force measuring active section— one can notice on the EE’s distorted image how its sides extend or contract! In the style of papers published by IEEE [22], this chapter ends on a personal note, with an author’s picture, facing an unconventional winter climate for Europeans, but hot summer-time at a scientific conference in Wellington, New Zealand (Fig. 18.13).

References 1. Yurish, S.Y.: Sensors: Smart vs. Intelligent. Sens. Transducers J. 114(3), I–VI (2010 Mar). ISSN 1726-5479 2. Huijsing, J.H.: Smart sensor systems: Why? Where? How? In: Meijer, G.C.M. (ed.) Smart Sensor Systems. Wiley, Chichester, UK (2008) 3. Bonfig, K.W., Bartz, W.J., Wolff, J. (eds.): Sensoren, Meβaufnehmer: Neue Verfahren und Produkte für die Praxis. Expert Verlag, Grafenau, Deutschland (1988). ISBN 3-8169-0278-2 4. Stef˘ ¸ anescu, D.M.: Strain gauge DC amplifier with integrated circuits (in Romanian). In: Al II-lea Simpozion Na¸tional de Tensometrie, Vol. 1, pp. 423–430, Cluj-Napoca, 11–14 iunie 1980 5. Bryzek, J.: Evolution of smart transducer technology. In: SENSORS’95 Kongreßband, pp. 45– 50, Nürnberg, 9–11 May 1995 6. Digital Transducer—Electronics AED 9101B, Operating manual P1_e_draft_020318.doc. HBM measurement with confidence. PDF created on 17 May 2002 7. Schäfer, A.: High-precision amplifiers for strain gauge based transducers—first time realized in compact size. ACTA IMEKO 6(4), 31–36 (2017 Dec). ISSN: 2221-870X

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8. K-Tron smart force transducer weighing technology for gravimetric feeding, batching and metering. Leaflet F-700012-en, K-Tron International, Inc., Pitman, NJ, May 2009 9. Socie, D., Barkan, C.: Smart sensor system for monitoring railcar braking systems. Final report for High-Speed Rail IDEA Project 51, University of Illinois at Urbana-Champaign, IL, Jun 2008 10. Haverty, Ch., Fildes, G.: Enhancing computer game joysticks with smart force transducers. Sens. Mag. (Sept 1998) 11. Papakostas, T.V., Lima, J., Lowe, M.: A large area force sensor for smart skin applications. Paper 0-7803-7454-1/02, Tekscan, Inc., Boston, MA, ©2002 IEEE 12. Missinne, J., Steenberge, G.V., Vanfleteren, J., Daele, P.V.: Optical force sensors for smart prostheses. Open Access abstract, Centre for Microsystems Technology, University of Gent, Belgium. PDF created on 1 Oct 2010 13. Callaghan, D., Rajan, G., McGrath, M., Coyle, E., Semenova, Y., Farrell, G.: Comparing FBG and PCF force sensors in a laparoscopic smart surgical scissor instrument. In: Joint Workshop on New Technologies for Computer/Robot Assisted Surgery, Graz University of Technology, Austria, 11–13 Jul 2011 14. Bruns, Th., Eichstädt, S.: A smart sensor concept for traceable dynamic measurements. In: Open Access Proceedings of the XXII IMEKO World Congress Knowledge Through Measurement, Paper 372, Belfast, UK, 3–6 Sept 2018. Published under license by IOP Publishing Ltd, Journal of Physics: Conference Series, Vol. 1065, Measurement of Force, Mass and Torque 15. Wang, M.S., He, L., Yin, Y.D.: Magnetic field guided colloidal assembly. Mater. Today 16(4), 110–116 (2013) 16. Davis, S., Morr, D.: Nat. Phys. (2013). https://doi.org/10.1038/nphys2671 17. Wang, X.D.: Piezoelectric nanogenerators—harvesting ambient mechanical energy at the nanometer scale. Nano Energy 1, 13–24 (2012) 18. Alam, M.T., Manoharan, M.P., Haque, M.A., Muratore, C., Voevodin, A.: Influence of strain on thermal conductivity of silicon nitride thin films. J. Micromech. Microeng. 22(4), Paper 045001 (2012). https://doi.org/10.1088/0960-1317/22/4/045001 19. Barth, M., Raabe, A.: Acoustic tomographic imaging of temperature and flow fields in air. Meas. Sci. Technol. 22(3), Paper 035102 (2011). https://doi.org/10.1088/0957-0233/22/3/035102 20. Stef˘ ¸ anescu, D.M.: Wheel Scales, Taiwanese Patent I-273219, 11 Feb 2007 21. Stef˘ ¸ anescu, D.M., Kang, D.-I.: Axisymmetrical elastic elements for very large force transducers. In: CD Proceedings of IMEKO TC-3 19th International Conference on Force, Mass and Torque Measurements: Theory and Application in Laboratories and Industries, Article 32, Cairo, Egypt, 19–23 Feb 2005 22. Stef˘ ¸ anescu, D.M.: Recent evolution of smart force transducers. In: 7th International Conference on Sensing Technology, pp. 330–333, Wellington, New Zealand, 3–5 Dec 2013 © 2013 IEEE

Chapter 19

New Achievements in the Field of Intelligent Force Transducers. Traceability

The first part of this chapter is adapted from the presentation delivered by the author as Keynote Speaker at SEIA’2015 Conference in Dubai [1]. Specific differences between terms smart and intelligent were presented by Yurish in the previous chapter [2]. There is also an attempt to combine both “technological” and “intelligence” aspects of smart sensors in a single definition [3]: Smart sensor is one chip, without external components, including the sensing, interfacing, signal processing and intelligence functions (self-testing, self-identification or self-adaptation). There is an expression found in a technical prospectus that brings together the two “twin” attributes within the same phrase: The Model 4215 of smart indicator is an intelligent microprocessor based instrument for the measurement and control of strain gauge transducer based systems [4].

19.1 Intelligent Design of Force Transducers Currently there are two opposite progress ways in force measurements: – “black box”—providing the transducer with electronic and data processing capabilities, the force transducer having three ports (input, interrogation and output), capable to perform a great part of the intelligent system functions; – “white box”—simplifying the transducer configuration, reduced to a Wheatstone bridge, and using the interfaces (high efficiency and performance data acquisition, conversion and control boards) in order to allow the digital connection to any computer [5]. Intelligence is required in all stages of implementing a high performance force measuring system: – selecting the electric measuring method/principle, for which a useful systematic presentation may be found in [6]; © Springer Nature Switzerland AG 2020 D. M. Stef˘ ¸ anescu, Handbook of Force Transducers, https://doi.org/10.1007/978-3-030-35322-3_19

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– designing the transducer following an iterative process, where the physicomathematical modelling (e.g. finite element method—FEM) is joined with experimental tests, in view of adopting those elastic structures benefiting from symmetry/antisymmetry; – choosing the electronic components of the measuring channel, together with some typical examples of signal conditioners and displays; – signal processing and information technology [7] in a single or multichannel/multi-transducer system, with various facilities such as self-monitoring and self-calibration; – transition to virtual instrumentation will be treated in the next chapter. Brignell [8] has put forward the intelligent principles of refined structural compensation by the symmetry of design in both electrical (strain gauges in Wheatstone bridge) and mechanical (elastic element of transducer) sub-systems, both forming an indestructible monolith, a perfect example of electromechanical integration: – an elastic structure composed of multiple and symmetrical flexible elements minimizes the effect of load eccentricity; – strain gauges connected in full Wheatstone bridge give the maximum differential output with linearity (plus thermal compensation), in contrast to a simple strain gauge. An original multicomponent (force and moment) balance for aircraft models, satisfying these criteria and successfully tested in a supersonic wind tunnel was presented in Fig. 17.9.

19.2 Intelligent Force Transducers 19.2.1 Differential Piezoelectric Force Transducer Amongst all electrical methods for intelligent force measurement we selected here for illustration the piezoelectric one, due to its innovative character [9]. In these Russian patents, beyond the “secretive” description and the exciting term “Intellectual Force Sensor”, it is about piezoelectric crystals placed in a special mount allowing opposed variations of frequencies, then being measured differentially. The working principle of the force transducer is shown, together with clarifying the role of main components, in Fig. 19.1a: Under the action of the measured force (weight) F on the housing (1), in the form of a coupled double beam with three round cavities, there appears a change in frequency of the excitation piezoplates (2), precisely located inside the central cavity, generated by the driving circuits (17). Their frequencies (f 1 and f 2 ) come to the adder (18), the output signal of which is applied to the differential frequency detector (19), its readout being proportional with the applied load. A special converter (Fig. 19.1b) can be provided by the circuit of piezoelectric elements loading in the axial tension-compression, shear or bending modes, but

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Fig. 19.1 Original drawings of two Russian patents on differential piezoelectric force transducers

regardless of this, the vectors of polarization of the piezoelectric elements (2 and 3) must be directed opposite to the vector of polarization of the piezoelectric-insulator (8). The invention describes a differential piezoelectric transducer for measuring forces, pressures or accelerations in low-frequency dynamic processes (in the range up to 50 Hz), in the presence of high-frequency interference (over 1 kHz), such as nuclear power plants under extreme operating conditions. In another application piezoceramic chips are used to measure both normal force and acoustic emission (AE) during grinding process [10]. Small and inexpensive (about 10 × 3 × 2 mm3 ), they are embedded in the aluminum alloy wheel core of 344 mm diameter and 152 mm bore. Their wide frequency response, from a few Hz to several hundred kHz, makes them suitable for measuring both normal force (macroscopic attribute) and acoustic emission (microscopic attribute).

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19.2.2 Electro-Optical Catheter Novel microtactile force transducers have been developed to measure the frictional force and contact force between blood vessel and the side of the catheter [11]. The prototype developed at the Kagawa University, Japan is shown in Fig. 19.2. Pressure piezosensitive rubbers (4.0 × 4.0 × 0.5 mm3 ) are fixed on the side wall of catheter by a linking shape. An optical fiber serves as guide wire to lead the catheter for inserting and rotating and, finally, to measure front end force of the catheter. The FOP-M fiber optic pressure transducer made by FISO Technologies Inc. is based on proven Fabry-Perot interferometer technology. Pressure creates a variation in the length of the cavity and the fiber optic signal conditioners can consistently and accurately measure the cavity length with high accuracy under all adverse conditions of temperature, EMI, humidity and vibration.

19.3 Intelligent Force Measurement Channels A survey of traditional resistance-based transducer conditioning techniques, going from simple analog op amps to sophisticated digital solutions is presented in [12]. The precision measurement equipment (PME) range of HBM products offers various types of modules for determining all measured quantities relevant to processing, such as force and related quantities (Fig. 19.3). DMP series is widely used in high-

Fig. 19.2 Operating difference between a classical catheter (a) and one tooled with frictional and contact sensors (b) and electro-optical catheter component pieces (c)

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Fig. 19.3 MP55DP intelligent module amplifier with scalable analog output and digital interface for measuring mechanical quantities: force, torque, pressure, and so on

precision measurements, as “reference-instrument” [13]. The four main principles used to reach the accuracy class 0.0005 are: – – – –

Wheatstone bridge (for parametric transducers: R, L, C), six wire circuit, carrier frequency, symmetric voltage architecture.

Precision load cells have been developed specifically to be used in dynamic weighing processes by means of fast intelligent transducers (FITs) [14]. For the full measurement channel the intelligent attribute is due to, among others, the metrological self-check feature. This is interpreted as “an automatic check of the metrological health of the measuring system within an operation process, which is carried out by using embedded hardware and software” [15]. A typical representation is given in Fig. 19.4. Some latest achievements for the force measuring chains include the digital display (in engineering units) within the SGFTs, e.g. Rinstrum 5000 (Fig. 8.5). The unit displays real time force data on the LCD screen. The instruments can be set up to record a series of measurement results and to tabulate and plot peak and average values. High and low limit values can be set up for quality control and inspection operators.

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Fig. 19.4 Sensor (or transducer) with built-in self-test or self calibration unit

19.4 Intelligent Force Sensing Applications Once more, regarding terminology: In general, Americans and Asians give preference to “Intelligent Force Sensor” (IFS), while Europeans prefer “Intelligent Force Transducer” (IFT). A unanimously accepted term is Intelligent Force Sensing, giving the specificity of this chapter.

19.4.1 Intelligent Robots Interactive, intelligent robots can be considered both sensitive force transducers and swift actuators [16]. Typical examples are some achievements of top companies such as FANUC (Asia), Adept (North America) and ABB (Europe). The FANUC robot shown in Fig. 19.5a may be seen as a 6-axis force moment (often called torque) transducer, similar to multicomponent tensometric balances (Chap. 17), while notations may be different and the six components are given names specific to the particular application. The generic term Intelligent Force Sensing System is used by Adept (Fig. 19.5b), avoiding the Force Sensor versus Force Transducer “dilemma”. There is a tight integrated hardware and software package that allows controlled robots to react to sensed forces and moments [17]. As a result, it reduces force overshoot and robot stopping time when forces or moments exceed preset thresholds, so the assembling operations can be performed with higher speed and accuracy. Part of hardware, the force transducer is a device that detects forces and moments applied by the end-of-arm gripper by means of strain gauges mounted on internal

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Fig. 19.5 Intelligent force sensing robots: FANUC representation of loading with 3 forces and 3 torques (a) and multicomponent force transducer mounted into Adept smart robot (b)

flexing elements. The electronic readings are transmitted to a Smart Controller EX motion controller and converted into useful force readings using a specific calibration matrix. The software instructions execute a full range of customized functions, including force-guided identification of the geometry of the most common contact configurations and controlling force operating modes. Robots get smart with real-time feedback [18]. Integrated Force Control handles process variations with human sensitivity, improving performance, and reducing programming time. It makes robots more intelligent and able to handle, as a human, delicate items with real-time external inputs. At a microforce scale an example on how to transfer flexible electronic skin to the next generation of humanoid robots at the Institute for Cognitive Systems, Technische Universität München (Germany) is given in [19]. It is about the intelligent integrating (cognitively and mechatronically) of discrete force cells into multi-modal self-organizing robotic skin. Robots can assist people with motor impairments to perform activities of daily living (ADLs) that often involve contact with a person’s face [20]. In a recent application at the Healthcare Robotics Lab, Georgia Institute of Technology, Atlanta, USA, people have the ability to control the forces they apply, and receive feedback from both sides of each contact (i.e. feedback from the hand that is holding the tool and the location on the body where the tool is being used) (Fig. 19.6). So, it is reasonable to assume that able-bodied people tend to apply forces to themselves that they find comfortable, safe, and effective. By emulating these forces, a robot equipped with a six-axis force/torque transducer (ATI Nano 25) could apply such target forces (till 10 N) for various human tasks.

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Fig. 19.6 Assistive robot with models of contact forces from able-bodied face shaving

19.4.2 Wireless Force Sensing Mukhopadhyay [21] links Intelligent Sensing first of all with Wireless Sensors. Varghese et al. [10] have used a DSP (digital signal processing)-based telemetric data acquisition and transmission module from the sensor-integrated “intelligent” grinding wheel to the remote receiver at 900 MHz bandwidth. For bridges in service steel cable forces are an important parameter for their safety assessment [22]. This is monitored by measuring the transverse vibration frequencies of cables by means of a wireless sensing unit model with a powerful computational core (with embedded software) and low power consumption characteristics (Fig. 19.7). The software is embedded in the microcontroller core: at the lowest layer is the real-time operating system (OS) which directly operates the wireless sensor hardware, while software for managing sensors data resides on a second tier, using algorithms exploiting the intelligent characteristics of the wireless sensor.

19.5 Force Measurement Traceability The growing popularity of quality standards, e.g. ISO 9000:2000, has highlighted the importance of measurement traceability. Instrument calibration is one of the most important activities that allow traceability towards primary standards to be obtained. The solutions commonly employed for the instrument calibration (including force transducers) can be classified into two classical and two innovative categories: – In-source—instruments calibrated internally towards reference standards that are maintained in a Metrology laboratory. – Out-source—instruments sent to an external laboratory for periodic calibration.

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Fig. 19.7 Wireless force sensing with three main functional modules: sensor interface, computational core and wireless transceiver (a). The embedded software is structured using the multi-layer approach (b)

– Net-source—based on a travelling standard, which is sent to the client site (without an attendant technician), and on a client-server application over the Internet, allowing the calibration procedure to be remotely exercised [23]. – Virtual calibration by digital twins [24]. Classical traceability scheme is illustrated by images from two National Metrology Institutes, where the author worked as a visiting researcher between July 2003 and August 2005: worldwide measurement traceability in Taiwanese vision (Fig. 19.8) and Force traceability in South Korea (Fig. 19.9). The basic architecture of the remote calibration system, shown in Fig. 19.10, contains a programmable travelling standard (a) and a client-server application on Internet (b). In this scenario, an automatic calibration procedure can be implemented and the operator at the Unit Under Test site is only required to perform the connections between standard and UUT [23]. In the same style, another practical application of the net-source philosophy refers to the calibration of data acquisition (DAQ) boards. In this case, the travelling standard is made up of a micro-controller (µC) based board and a digital multimeter (DMM).

Fig. 19.8 Several most important NMIs in the world (a), traceability “pyramid” and variety of application fields (b), Industrial Technology Research Institute (Hsinchu) by air (c) and two measurement science specialists, together with the metrology twins from a Taiwanese legend, he with a ruler and she with a compass (d). Note Part of the text is bilingual

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Fig. 19.9 Force, pressure and torque traceability, as derived quantities from Mass, with connection relationships (a) and Force traceability at the Korea Research Institute of Standards and Science (Daejeon), in the range from 20 N to 10 MN (b)

Fig. 19.10 Italian six-component force standard (a) and the basic architecture of the remote calibration system (b)

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Fig. 19.11 Digital twin from PTB Virtual Planck-Balance (a) and the BIPM diagram with their official declaration: “In the new SI seven fundamental constants will be determined as defining reference entities. The seven base units – arranged in the outer circle of the diagram – will lose their prominent role.” (b)

As you have read at the end of Chap. 1, the Planck-Balance will be the primary mass metrology standard in the New SI [24] and in Fig. 1.6 you have seen this new weighing instrument for the “electronic kilogram”. PTB has developed a digital calibration certificate, which will be used to store and exchange the calibration data of digital twins (Fig. 19.11a). As a first step towards Virtual Planck-Balance, the digital twin of a weight has been set up, modelling the influences of the weight itself on the measurement result. Effects of air buoyancy, cleaning status of the surface, and height of the center of mass of the weight are considered, as time dependent variables of the measurement uncertainty. Note: The word “twin” is used here in the sense of “duplicate” or “matching”. Remember [25]: Starting with 20 May 2019, all SI units will be based on fix numerical values laid down for seven selected natural constants (Fig. 19.11b). This is especially interesting for metrology institutes, high-tech industry, schools and universities. With this fundamental revision, the SI does away with the old system deficiencies in terms of definitions. One particularly noticeable deficiency was that the mass of the international prototype of the kilogram and its copies has been submitted to variations up to half a microgram per year! The new system has a decisive advantage: natural constants are valid anywhere in the universe and at any time! This is the most intelligent solution, indeed! Note: In our opinion, adopting the “New SI” was mainly a matter of principle. In the old SI the unit of mass was an “unhappy” exception, being the only one defined

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in terms of a prototype, i.e. an “artefact”. Struggle for a new definition, based on natural constants, has been lasting for several decades, because they wanted to reach the same accuracy as with the old one. Finally, while the redefinition will have zero impact on the way fruit and vegetables are weighed at the supermarket, it marks the culmination of decades of work to link the basic units that underpin metrology exclusively to constants of nature. The changes come into effect on 20 May 2019, World Metrology Day. In fact, this change just makes the SI more “elegant”, gaining not much in accuracy, but becoming indeed a system of units “for all times and for all people” [26]!

References 1. Stef˘ ¸ anescu, D.M.: New achievements in the field of intelligent force transducers. In: CD Book of Proceedings of International Conference on Sensors Engineering and Electronics Instrumental Advances (SEIA’ 2015), pp. 28–31, Dubai, UAE, 21–22 November 2015 2. Yurish, S.Y.: Sensors: smart vs. intelligent. Sens. Transducers J. 114(3), I–VI (2010 Mar). ISSN 1726-5479 3. Kirianaki, N.V., Yurish, S.Y., Shpak, N.O., Deynega, V.P.: Data Acquisition and Signal Processing for Smart Sensors. Wiley, Chichester, UK (2001) 4. Specifications: Model 4215 Smart Indicator. Electro Standards Laboratories, Cranston, RI, Pub. 2326-10. PDF created on 22 Sept 2008 5. Zecchin, P.: Digital Load Cells—A Comparative Review of Performance and Application. The Institute of Measurement and Control, London (2003) 6. Stef˘ ¸ anescu, D.M.: Handbook of Force Transducers—Principles and Components. Springer, Berlin and Heidelberg (2011) 7. Tränkler, H.-R., Kanoun, O.: Some contributions to sensor technologies. In: Proceedings of the Conference Sensors and Systems, Saint Petersburg, Russia, 24–27 Jun 2002 8. Brignell, J.B., White, N.M.: Intelligent Sensor Systems. Institute of Physics Publishing, Bristol and Philadelphia, PA (1994) 9. Intelligent sensor power—RussianPatents.com (russianpatents.com/patent/216/2165601.html) 10. Varghese, B., Pathare, S., Gao, R., Guo, C., Malkin, S.: Development of a sensor-integrated “intelligent” grinding wheel for in-process monitoring. Ann. CIRP (The International Academy for Production Engineering) 49(1), 231–234 (2000) 11. Guo, S.X., Guo, J., Xia, N., Tamiya, T.: Robotic catheter operating systems for endovascular neurosurgery. In: Signorelli, F. (ed.) Explicative Cases of Controversial Issues in Neurosurgery, pp. 457–478. InTech (Open Science—Open Minds) (2012) 12. Cheeke, D.: Sensor signal conditioning. Sens. Transducers J. 82(8), 1381–1388 (2007) 13. Schäfer, A.: The ultra-precision instrument DMP 41—first experiences & appropriate filter settings. In: Proceedings of the IMEKO 22nd TC3, 12th TC5 and 3rd TC 22 International Conferences, Paper 127, Cape Town, Republic of South Africa, 3–5 Feb 2014 14. Milz, U.: Soybean oil and baby milk split-second bottling with FIT® fast intelligent transducers. Hotline Hottinger, Issue 1, p. 27 (2002) 15. Taymanov, R., Sapozhnikova, K., Danilova, I., Druzhinin, I.: Multi-channel intelligent measuring systems. In: Proceedings of the XXI IMEKO World Congress Measurement in Research and Industry, pp. 34–39, Prague, Czech Republic, 30 Aug–4 Sept 2015 16. Xiong, G.L., Chen, H.C., Zhang, R.H., Zhang, H., Huang, J.B.: Development of intelligent force sensor system for interactive robot. In: Proceedings of International Conference on Mechanical Engineering and Technology (ICMET 2011), pp. 505–510, London, UK, 24–25 Nov 2011 17. Adept intelligent force sensing system, User’s Guide, 14155-000 Rev. A, Adept Technology, Inc., Pleasanton, CA, USA, Sept 2014

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18. ABB Robotics’ integrated force control, ABB, Zürich, Switzerland. Accessed on 4 Nov 2015. http://foundrymag.com/finishingmro/robots-get-smart-real-time-feedback 19. Mittendorfer, Ph., Cheng, G.: From a multi-modal intelligent cell to a self-organizing robotic skin. Technische Universität München, Germany. PDF created on 11 Mar 2013. See also http:// www.ics.ei.tum.de 20. Hawkins, K.P., King, C.-H., Chen, T.L., Kemp, C.C.: Informing assistive robots with models of contact forces from able-bodied face wiping and shaving, Georgia Institute of Technology, Atlanta, USA. PDF created on 23 Apr 2013 21. Mukhopadhyay, S.C.: Intelligent Sensing, Instrumentation and Measurements. Springer, Berlin and Heidelberg (2013) 22. Lei, Y., Shen, W.A., Song, Y., Wang, Y., Lynch, J.P.: Intelligent sensors with application to the identification of structural modal parameters and steel cable forces. Xiamen University, Fujian, China. PDF created on 9 Oct 2009 23. Carullo, A., Ferraris, F., Parvis, M., Vallan, A.: Internet calibration: an innovative approach for the dissemination of the measurement units. Tutto Misure 2, 165–166 (2003) 24. Günther, L., Rothleitner, C., Schleichert, J., Rogge, N., Vasilyan, S., Härtig, F., Fröhlich, T.: The Planck-Balance—primary mass metrology for industrial applications. In: Proceedings of the XXII IMEKO World Congress “Knowledge through Measurement”, Belfast, UK, 3–6 Sept 2018. Open Access—Published under license by IOP Publishing Ltd. 25. Ullrich, J.: Natural constants as the main protagonists—The General Conference on Weights and Measures (CGPM) adopts revision of the International System of Units. PTB News, Scientific Newsletter, No. 1, pp. 1–2 (2019) 26. Millea, A.: Private communication, 21 May 2019

Chapter 20

Virtual Instrumentation and Force Transducer for Measurements in Dentistry

20.1 Virtual Instrumentation—Components and Characteristics Virtual instrumentation is the use of customizable software and modular measurement hardware to create user-defined measurement systems (Wikipedia). As Tumanski observed, “the virtual world surrounds us in various ways. We can buy various things in virtual shops (although the goods are real). In films, the expensive scenes are imitated by computer created graphics. In many computer games we can take actions as in real life (using) ‘simulators’. (…) So, why not use this power in measurement instrumentation?” [1]. A virtual measurement system for mechanical quantities (force, mass, acceleration, torque, power, etc.), based on high quality equipment and Microsoft Visual Basic language, is intensely used for training students at the “Politehnica” University of Bucharest, Romania, stimulating their creativity [2]. The program is achieved in windows technology, each part corresponding to a component of the measuring chain: • transducer selection (various measurement methods, customized or standard products), e.g. torque and power transducer type T32FN (H.B.M.) for 1 kN·m and 4000 rpm; • conditioning unit, typical for strain gauges connected in Wheatstone bridge, e.g. SCXI-1121, having a lot of facilities, such as offset nulling and shunt calibration with SCXI-1321 (National Instruments); • data acquisition (DAQ) board: multifunctional, 16 inputs (or 8 differential ones), 12 bits resolution, 1 MS/s sample rate, i.e. Model Name 6070E (National Instruments); This chapter is a development of the poster presented by the author at the XX IMEKO World Congress in Busan, Republic of Korea [3]. (©2012, granted by Judit Farago, IMEKO Secretariat in Budapest, Hungary). © Springer Nature Switzerland AG 2020 D. M. Stef˘ ¸ anescu, Handbook of Force Transducers, https://doi.org/10.1007/978-3-030-35322-3_20

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Fig. 20.1 TensoDentar, tested by DMS as creator/director and patient, with supporting arm of SGFT (a) and overall view of this new equipment (virtual instrument + real force transducer) (b)

• programming language: software LabVIEW, the most suitable for data acquisition from analogical and digital I/O. For example, a measurement made by this virtual instrument takes over the torsion moment and the number of revolutions per minute (rpm), and, by multiplication, results the mechanical power. We developed TensoDentar, a complex measurement system of dislodging forces for complete dentures or palatal plates, composed by an original strain gauge force transducer (SGFT) and virtual instrumentation in NI and LabVIEW environment (Fig. 20.1). Figure 20.2 illustrates the “route” followed for creating TensoDentar using NI equipment [4], and Fig. 20.2 shows a “customized” picture of the instrumentation set up by us—an intelligent combination of hard (real) and soft (virtual).

20.2 Intelligent Design of Force Transducer and Experimental Setup The TensoDentar measuring system is mainly intended to evaluate the dislodging forces of complete upper dentures, estimated at F max = 5 N (about 500 grams-force). Having in view the use of this experimental model also for testing detachment of palatal plates, which occurs at only a few tens of grams-force, the wide range of loads requires the creation of a special force transducer, the most appropriate being that based on a flexible beam embedded at one end and subjected to bending at the other end, with two resistive strain gauges as sensing elements [5]. Strain gauges were previously used in measuring this type of forces in connection with analogic measuring equipment: oscilloscope or pen chart recorder, but the involvement of

Fig. 20.2 Transducers (with strain gauges), signal conditioning (Wheatstone bridge), DAQ systems and digital platforms from National Instruments Corporation

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Fig. 20.3 The global picture of the measurement system TensoDentar: DMS strain gauge force transducer (SGFT) integrated in the NI hardware and LabVIEW software

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digital environment substantially increases the measurement precision. The NI hardware and associated LabVIEW software developed on MS Windows platform allow the conversion of analogic inputs into digital signals and their improved analysis by virtual instrumentation. We initially projected the attachment of the tensometric force transducer to a horizontal fixed arm (Fig. 20.1a). But, during the first tests, we found it is very hard to adapt this configuration to the perpendicular dislodging principle in the gravity center of each plate. The initial setup increased the time of preparation which altered the status of the interfacial salivary film, and therefore we abandoned the fixed arm and decided to use this transducer independent of its supporting arm, due to its handle which can be operated by a human operator, after proper training (Fig. 20.1b). The relatively low loads and the limited oral space available determined the choice of a more elastic material than steel or aluminum, namely Plexiglass (polymethyl methacrylate), which has an elasticity modulus E = 2.3–3.3 GPa, depending on the chemical composition and temperature. The elastic lamella, having the “active” length  = 54 mm, breadth b = 20 mm, height h = 4 mm, is equipped with two epoxy strain gauges type 10/120 LB 15, glued with Z 70 bond and protected with SG 250 putty (Hottinger); it constitutes the “heart” of the TensoDentar measurement system. The block diagram of experimental setup includes the following main components (Fig. 20.3): – Force transducer of cantilever beam type with strain gauges (SGs), that cannot be “simulated” by the computer, but should be bought or, even better, built by the experimenter; – Analogic data acquisition board (including the signal conditioner), part of the “architecture” of the NI integrated solutions; – Desktop computer or laptop. Figure 20.4 shows a more detailed image of the TensoDentar instrumentation, together with a thorough description of the measurement procedure, as you cannot usually find in books, however very useful for researchers, students and technicians of various specialisations. The NI-9237 module [6] contains 4 channels with analog inputs in Wheatstone bridges and 24-bit resolution Delta-Sigma type analog-to-digital converters. Only one analogic channel is used (here Ch0) together with its built-in signal conditioner. This module is mounted onto the NI-9172 chassis [7] (See Fig. 20.1b). Its Hi-Speed USB 2.0 interface simplifies the mode in which the users control the peripherals and data transfer, offering the following advantages: “plug-and-play” operation, robustness and easy-to-use. The laptop, which constitutes its “brain”, is Windows-operated and utilizes the LabVIEWTM program, agreed by National Instruments. The entire application, starting with the prototype force transducer DMS and culminating with the dedicated software, is a typical OEM (original equipment manufacturer) one and enjoys all the advantages offered by the virtual instrumentation.

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Fig. 20.4 NI-9237 module (a), connection diagram (b) and SGFT (cantilever beam type) (c) for the TensoDentar computerized measurement system

20.3 Virtual Instrumentation for Measurements in Dentistry The virtual instrument uses the force transducer for sensing the physical quantity to be measured and analog-to-digital conversion module, but beyond these all processing and analysis of measured values, storing and transmission towards the human user are performed by computer [8]. One can notice the sense of loading (traction or compression) and colour conventions for the strain gauges connected in half Wheatstone bridge (Fig. 20.5a), where resistors are symbolized with a zigzag line (American) or a rectangle (European):

Fig. 20.5 SGs wiring connection in Wheatstone bridge (a) and channel settings for STRAIN gauge force transducer (b)

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• The electrical resistance R1 of the strain gauge located on the upper side increases by bending the cantilever and is represented by a “warm” colour: red; • The electrical resistance R2 of the strain gauge located on the lower side decreases by bending the cantilever and is represented by a “cold” colour: blue. The STRAIN configuration was selected for the measurement chain (Fig. 20.5b), which is better fitted with the phenomenon than the alternate setting in mV (Voltage Output). Here are the significances of the text appearing in the graphical “windows”: – Signal Input Range: unipolar signals, corresponding to the bending of strain gauge cantilever in a single direction, with minimum value 0 and maximum one of 5.000 μm/m (written as 5u); – The chosen strain gauges have (in American spelling) Gage Factor = 2, Gage Resistance = 120  and do not require an Initial Voltage; – The internal power source of the DAQ module is utilized, choosing the minimum value (UA = 2.5 V), which ensures sufficient sensitivity of the measurements; – An important adjustment is the Strain Configuration of the Wheatstone bridge, programmed as a type II half-bridge, which means that both strain gauges are “active”, i.e. they are bonded along the axis of the cantilever beam; by its bending under the applied load, strain gauge R1 is extended while strain gauge R2 is shortened in the same ratio. This differential setup ensures a doubled sensibility as compared with using of a single strain gauge (on the upper side of the lamella); – The measurement cable being considered short (2 m), its resistance compensation is not necessary, so the correct setting is Lead Resistance = 0; – Custom Scaling is not necessary; the system works directly in engineering units of specific deformation: microstrain (μm/m). Due to the static character of the application, it is not necessary to set the maximum speed for the DAQ module. The operating mode uses continuous samples and a buffer having the rate of 10 kHz, with 1000 samples to read. Experimental recordings of the strain were done using the NI-9237 data acquisition board, a dedicated hardware for strain measurement, and the associated LabVIEW 2010 software [9], completing the virtual instrument. As any graphical programming environment, our TensoDentar application contains two parts: the block diagram and the front panel. Description of the software and of the NI-DAQmx driver for the carrying out of measurements and control of associated peripherals is given as follows. The block diagram shows a few interconnected blocks/functions which represent the program algorithm (Fig. 20.6). In our application, the block from the step #1 defines the acquisition task and its parameters (bridge type, acquisition mode and sampling rate). The output of this block gives the strain value as measured with the Wheatstone bridge. This value is then multiplied by—1 to have a positive strain for the actual bending sense of the cantilever beam. In step #2 of the block diagram the BIAS value of the strain is removed in order to balance the measurement channel. The next part of the program, step #3, retains in a shift register the maximum value

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Fig. 20.6 The block diagram of logic algorithm of the TensoDentar dedicated LabVIEW application (made by Adrian Toader, National Institute of Aerospace Research, Bucharest, Romania)

of the strain at which the cantilever beam was bent during the experiment. On step #5 this maximum value is recorded in a file, after which the shift register is cleared to zero (RESET), step #4, in order to keep the next experimental value of the maximum strain. The frontal panel is presented in Fig. 20.7, in two situations: at calibration (a) and during a series of measurements of palatal plates dislodging forces (b). The panel contains the BIAS and RESET buttons, as well as the path to the recording file.

20.4 Experimental Results On the calibration diagram (Fig. 20.7a), obtained by removing the 683 g standard weight (average value of determinations made on several digital scales) from the bent lamella hook (Fig. 20.8a), the specific skip can be noticed as a “step” and the value of 3000 μm/m appears on the oscillogram, as Strain Amplitude: ε = 0.003. The exact sensitivity coefficient, determined before each experiment by directly comparing the microstrain indication read with the weight in grams-force, allows relating the maximum indication of each test to the force required for the detachment of palatal plates (Fig. 20.8b) or upper dentures. The clinical testing protocol has to be rigorous, homogenous and reliable [10]. Our objective was to fabricate for each subject two palatal plates which were used to determine the retention force induced only by the salivary film. One set of plates was sandblasted using the dedicated device Point II (Barth, Germany), loaded with

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Fig. 20.7 Part of the frontal panel for signal acquisition and data processing, during calibration (strain amplitude is indicated in m/m and time in seconds) (a). The strain variation in time is represented on a chart (plot), and the maximum value of the strain is simultaneously displayed on a dial gauge as well as on a numeric indicator (readout) (b)

110 μm aluminium oxide particles at a pressure of 2.5 bars. The other set of plates was treated with low pressure plasma in argon medium using a discharge chamber (Bell-Jar configuration). We executed 35 measurements for each plate, excluding the first ten minimum and ten maximum values, before and after surface treatment, and diminishing in this way the measurement errors induced by the operator’s technique. The remaining 15 values were statistically analized using SPSS (Statistical Package for the Social Sciences) for Windows 2010. Then, descriptive statistics were performed and the results obtained for five subjects (A…E) are presented in synoptic tables and in comparative diagrams, like in Fig. 20.9.

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Fig. 20.8 The strain gauge force transducer (SGFT) in two situations: gravimetric calibration with a dead weight, noticing the flexible lamella bending (a), and during the traction force F applied in the geometric center, normal to the palatal plate (b)

Our intention was to evaluate the palatal plates’ performances before and after the specific surface treatment applied. As a conclusion, plasma treatment increases the retention (43.8–76.7%) more than sandblasting (26.1–56.6%) for each subject. This achievement was successfully presented at the IMEKO (International MEasurement COnfederation) world congress (Fig. 20.10).

20.5 Conclusions This instrumentation application had the unprecedented goal to achieve in vivo comparison of two technological procedures applied in Dentistry by our multidisciplinary team. The complex measurement system TensoDentar has been successfully used for the evaluation of dislodging forces for palatal plates, based on an original strain gauge force transducer and on virtual instrumentation in NI and LabVIEW environment [3]. Two surface treatments (sandblasting and, respectively, plasma treatment) on complete denture base materials were applied; their effect was presented and interpreted—a medical application very important for senior scientists! Last but not least, this is also a spectacular illustration of all the keywords underpinning this work on Force instrumentation: Strain gauges—Wheatstone bridge—Force transducers—Mechanical testing—Electrical measurement—Virtual instrumentation.

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Fig. 20.9 Retention of the palatal plates for both treatments, in five cases (A–E), with plasma treatment (on the left side for each patient) producing better results than sandblasting (on the right side in these diagrams) (a). Comparison between the percent values of the retention efficiency for palatal plates after sandblasting and, respectively, plasma treatments (b). Note the same vertical applying of the traction force F (c)

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Fig. 20.10 Cover of the XX IMEKO World Congress program leaflet (a) and Dr. D. M. Stef˘ ¸ anescu presenting the poster TC3-P-8 on 13 September 2012 in Busan, Republic of Korea (b)

References 1. Tumanski, S.: Principles of Electrical Measurement. CRC Press—Taylor & Francis Group, New York, London (2006) 2. Stef˘ ¸ anescu, D.M.: Virtual instrumentation and interactive software for mechanical quantities measurement. In: CD Proceedings of the Third International Conference on Metrology: Trends and Applications in Calibration and Testing Laboratories, Poster 6, Tel Aviv, Israel, 14–16 Nov 2006 3. Stef˘ ¸ anescu, D.M., Farca¸siu, A.-T., Toader, A.: Virtual instrumentation and cantilever beam type transducer with strain gauges for measuring the dislodging forces of removable dentures or palatal plates. In: Proceedings XX IMEKO World Congress Metrology for Green Growth, Poster TC3-P-8, Busan, Republic of Korea, 9–14 Sept 2012 4. Data Acquisition (Benchtop, Industrial, Portable, Embedded), National Instruments Corporation, Brochure 8300-301-101D, p. 1 (2007) 5. Stef˘ ¸ anescu, D.M.: Handbook of Force Transducers—Principles and Components. Springer, Berlin and Heidelberg (2011) 6. Operating Instructions and Specifications for NI 9237: 4-Channel, 24-Bit Half/Full-Bridge Analog Input Module, NI Corporation, Technical Communications 374186E-01 Jun 2009 7. User Guide and Specifications for NI cDAQ-9172, NI Corporation. PDF created on 7 May 2011 8. Savu, T.: Virtual instrumentation: present and future (in Romanian). M˘asur˘ari s¸i Automatiz˘ari (2) (2002) 9. LabVIEW—Developer Zone Community. http://zone.ni.com/dzhp/app/main. Accessed 21 May 2019 10. Farca¸siu, A.-T.: Physical mechanisms of complete denture retention (in Romanian). Ph.D. dissertation, University of Medicine and Pharmacy “Carol Davila” Bucharest—Department of Removable Prosthodontics, 11 Dec 2011

Chapter 21

A Supplement on Photoelastic and Digital Techniques in Force Measurements

A book is like a living organism, being in a perpetual extension, so that its author hardly makes up his mind to bring it to an end. This postscript contains some of my last ideas. After 20 chapters, I came upon a paper of 2015 edited by Electrical Engineering Department, University of Nevada (USA), having a multi-disciplinary topic and authored by a multi-national team (illustrating as well my concern to “multi”, described in Sect. 14.1, the “maximum” being optomagnetopiezoelectricthermoelastic multi-field)!

It seemed an attractive subject and I thought to share it in this supplement, not as a “happy-end” but rather as a never ending research passion! It is a passive wireless strain sensor using a microstrip patch antenna with feed-inset operating in IEEE C-band i.e. 5.8 GHz, designed by computer simulation technology (CST). Its measurement principle: Without strain, the microstrip antenna radiates at its resonance frequency; under the applied strain, changes in the antenna dimensions (i.e. patch length L and width W ) result in a shift of the normalized resonance frequency, a nearly linear dependence. As shown in Fig. 21.1a, the patch antenna consists of a layer of flexible dielectric substrate FR-4 (glass-reinforced epoxy laminate material), having a thickness t of 500 μm and dielectric constant εre of 4.3. On the topside, it has a rectangular patch layer with feed-inset and on the bottom side a ground plane, both made of copper with thickness d of 18 μm. This structure creates an electromagnetic cavity that resonates at a specific resonant frequency f res , with formula given inside the figure, © Springer Nature Switzerland AG 2020 D. M. Stef˘ ¸ anescu, Handbook of Force Transducers, https://doi.org/10.1007/978-3-030-35322-3_21

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Fig. 21.1 3D-views of the passive wireless strain sensor (a) and the radiation pattern of the microstrip patch antenna at 5.8 GHz (b)

where c is the light velocity in free space and ΔL ext —an imaginary line extension that occurs due to the fringing effects influencing the patch edges, which depends linearly on the substrate thickness. It might be said that, this flexible strain sensor based microstrip patch antenna, despite the rather large size (24.8 mm × 31.8 mm), is a sort of “electromagnetic strain gauge”, i.e. a 7-th SGs type in the classification of the Chap. 5. It could also be a “digital strain gauge” (i.e. a double premiere for this sensing device), since measuring strain not by a Wheatstone bridge, but with frequency variation and CST. The shift in f res is more sensitive to strains applied along the length direction (S L = 0.9) than those applied along the width direction (S W = 0.1). The picture from Fig. 21.1b represents a radiation diagram of an antenna, with the maximum direction on the red semicircle and the minimum on the blue semicircle.

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The most important parameter of the antenna is its directivity, given by the maximum radiated field, in our case 6.283 dBi. “Decibels-isotropic” is a logarithmic unit used for expressing the gain of an antenna, having both positive and negative values. They are centered upon a level conventionally chosen as “zero”, all values situated above it being positive and those under it being negative. This would be the 11-th type of colour scale in Fig. 18.11. These wireless sensing devices are useful in some certain “vital” applications i.e. aircraft wings, high buildings and bridges, where a network of distributed strain sensors is needed to obtain a spatial strain distribution, while conventional strain gauges are not favorable due to their high installation and maintenance costs, complexity, need of wiring and supply. This storytelling or case study is a kind of book recap, reviewing a few ideas expressed in Chaps. 5, 14 and 18. Repetitio mater studiorum est! By closing the edition, there is a spectacular “insertion”, bringing together piezoelectric and optical strain gauges, separately described in Sects. 5.3 and 5.6 respectively, through a technological fused and transparent quartz crystal, i.e. a cruciform photoelastic element (PE)!

A more sensitive way of deformation measurement uses the photoelasticity, as a piezo-optical effect due to the polarized light parameters transformation as it passes through the optical components of the above-mentioned transducer. It is an anisotropic change in the refractive index (birefringence) of an initially isotropic medium under an external force. The gauge factor is more than three orders of magnitude higher than of the classical resistive strain gauges. Modern photoelastic instrumentation reveals the spatial stress distribution σ under the applied load and this would be the 12-th type of colour scale in Fig. 18.11, twelve being just the apostolic figure! The piezo-optical instrument transducer, combining compactness, reliability and high sensitivity, opens up new possibilities in deformation measuring and stress analysis applications. For example, the use of only one unified device makes it possible to control all parameters of the elevator movement: acceleration and deceleration, vibration and sound, according to the International standard ISO 18738-1:2012 (E). These “transparent strain gauges” could be also useful for remote monitoring, by mounting them at a certain distance away from the measured deformations zone in high buildings, bridge structures, cars and railway wagons.

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Author graduated from the “Politehnica” University in Bucharest, Romania, a school of long-time tradition, having several names over time: St. Sava College (1818), National School of Bridges and Roads (1881), Bucharest Polytechnics (1920), Polytechnic Institute of Bucharest (1948), “Politehnica” University of Bucharest (1992). During half a century, he gained a ton of experience, both in theory and practice, mainly in the field of electrical measurement of mechanical quantities, largely reflected in this handbook on applications in force measurements.

Bucharest, June 25, 2019

Annex: Engineering and Art of Illustrated Travel Reports at the IMEKO World Congresses

Engineering is art of converting science into useful things. (Jacob Fraden)

Introduction This paper, written in 2017, reflects my nearly three decades of experience participating in various IMEKO conferences, symposia, workshops and mainly world congresses, as an author of works for several technical committees and as Romanian representative in the General Council. A lot of technical and scientific aspects of this activity under the IMEKO patronage are revealed, as well as some “secrets” of preparing illustrated travel reports on the conference-touristic adventures experienced in five continents (Europe, Asia, Africa, North and South America). History of my participations in International Measurement Confederation began in 1988, with the presentation of Romania’s contribution to the Golden Jubilee of strain gauges and load cells in Houston, Texas. 20 years later I set up a first balance, at the IMEKO semi-Centennial celebration in Budapest, Hungary, the birth place of this worldwide organization [1]. This paper illustrates in a “belles-lettres” style, not only scientifically, the complex and various tasks under IMEKO patronage, with audio-visual extension and online presentation. Richly reflecting the triennial IMEKO congresses, we intend to attract the young specialists all over the world to enthusiastically participate in these prestigious scientific events, joining up useful with pleasant. The paper analyzes various facets of composing illustrated travel books, starting from the author’s own experience, reflected in [2]. The book is an educational serial with 16 episodes, reports of conference-touristic journeys performed between 1988 and 2014 in countries outside Europe: Australia, Brazil, China, Egypt, India, Israel,

© Springer Nature Switzerland AG 2020 D. M. Stef˘ ¸ anescu, Handbook of Force Transducers, https://doi.org/10.1007/978-3-030-35322-3

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Fig. A.1 The cover of the book containing illustrated reports of conference-touristic journeys, with the terrestrial globe image focused on the Asia-Pacific zone

Japan, Malaysia, Mexico, New Zealand, South Africa, South Korea, Taiwan, Tunisia, and USA. The new edition, “Beyond Europe”, presented in Fig. A.1, includes a suplementary illustrated report on Dubai (United Arab Emirates).

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Coexistence of Engineering with Art Related to Scientific Traveling Engineering and art appear together in the notable expression of Jacob Fradens’s motto [3]. In many places worldwide the big bridges are considered art works! Equally, engineer proves to be an explorer and processor of ideas and images, wherever he would be! For a creative mind, science and art may form two complementary areas, Leonardo da Vinci being the best “combiner” from everlasting. As the three presidents (US, UK and Chinese) of their Academies of Engineering have underlined, “Globalization has made it easier for people to cross borders and travel great distances. (…) This summit (held in Beijing in 2015) led to international collaborations, friendships, and a renewed sense of what engineering does for people and society” [4]. And, as Robert Socolow, professor emeritus of mechanical and aerospace engineering at Princeton University, remarked, “Air travel brings us access to the extraordinary variety of human cultures and natural settings.” Scientists can also feature a wide cultural horizon, being committed to share it with the younger specialists’ generation in measurement science.

Text Versus Pictures The assumed globetrotter challenge emanates from a digital message exchange (through e-mail) on the optimal proportion between text and illustrations, with Lidia Zaverdeanu-Cosmoiu, who states that: Illustration is welcome, fills the whole, facilitates imagination and visually accredits the text, that has to “stand” independently too… An immense richness of impressions, images, and feelings, generating vivid and interesting pictures, completing and coating a SPHERE (apropos of the Earth globe cover, like that of the “Engineering” review)! Obviously, an illustrated text is more attractive than a simple “photo montage”, a tough task in view of the “confucian” quote that became classic: “A picture is worth a thousand words”. The evolution of this concept may be clearly noticed, by scanning Wikipedia [5], at great thinkers belonging to various peoples, a sui generis think-tank like that of Table A.1—an example of engineering art (concentrates information and optimizes visibility: selection of fonts, positioning of quotes). Table A.1 “Pearls” of the French, Russian, Anglo-American and Chinese wisdom concerning the picture-text balance Napoleon Bonaparte

A good sketch is better than a long speech

Ivan Turgenev

A drawing shows at a glance what might be spread over ten pages

Fred Barnard

One look is worth a thousand words

Chinese people

Seeing something once is better than hearing it a hundred times

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Requirements Concerning Text The four statements from above reflect the variety of audio or video expression means. Facing such a high standard, we begin our analysis following to meet several basic prerequisites: • The “manufacturing secret” resides in the thorough notes taken in “real time”, that keep the authentic experiences, and subsequently correlating them with recollections preserved in the “drawers” of our own memory. The “engraved” style can not be achieved through aerial, naval or terrestrial movements, instead needs a posteriori polish of the travel impressions left over the scientific conferences attended. • Such as global engineering ceased to discriminate between various specialties, this composition has to constantly reflect the mankind’s geographic, scientific and cultural kaleidoscope. It is necessary to masterly join up scientific rigor, literary expressiveness and artistic imaging, so that the information wealth be presented as spectacularly as possible. In this way, the printed or posted on internet page will come alive! • Travel notes combine Da Vinci’s multidisciplinarity (long word!) with Brunel’s communication art (famous English engineer during the Industrial Revolution) and Talleyrand’s expertise in diplomacy—in most diverse hypostases. • Reports are complex, interlacing earliest civilization (some world miracles appear too) with up-to-date know-how, in-flight thoughts with on-place palpable actions. Also, scenical touristic views or specific points are not missing, within an uncommon travel guide, sometimes encyclopedic. • The main effort, exerted by an ingenuous and ingenious engineer, has been focused toward the empathetic and sympathetic rendering the specific of each country, inserted into an ample and spectacular general picture of the world we live in.

Secrets of Image Processing The images exhibit a large density of information, with “inserts” which cover certain “nude” zones and use clever graphic “artifices”, whereupon I fully benefited from the remarkable experience of my mentor, Dr. Aurel Millea. As a “professional deformation”, the artistic fantasy builds up with engineering means. For this, soft and hard happily exist together!

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Fig. A.2 Houston, Texas, 1988: Cover of the jubilee volume “Tensometry 50”, at the IMEKO XI World Congress under the motto “Instrumentation for the 21st Century”. In attach, the organizer P. K. Stein and Romanian delegate D. M. Stef˘ ¸ anescu, with two original contributions of him concerning development of elastic elements for force transducers

Figures A.2, A.3, A.4, A.5, A.6, A.7, A.8, A.9 and A.10 illustrate by various means the complex IMEKO activities: envelopes and invitations to world congresses, personalized maps (subway and/or railway routes, airport marks), covers of work collections or CDs with touristic/advertising role, diverse official documents, group or private photos with friends from all over the world, scientific bulletins edited by IMEKO.

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Fig. A.3 Beijing 1991: Invitation par avion to the IMEKO XII World Congress, under the motto “Unity—Friendship—Progress” with “windows toward world”, “addressed” from the organizers and the Romanian delegate, respectively

Fig. A.4 Japan 1999: Program of the three scientific events under the IMEKO XV aegis at Tokyo (pre-congress), Osaka (congress) and Kyoto (post-congress), and Nippon symbols: engineering achievements (robot and high speed train) or artistic images (Fuji mountain and geisha)

The result is not a mixtum compositum, but a solid construction, made of the best quality materials, a unusual museum that exhibits magnificent pictures from several corners of the world.

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Fig. A.5 Dubrovnik, Croatia, 2003: Certificate of Attendance and CD Proceedings of the IMEKO XVII World Congress under the slogan “Metrology in the 3rd Millennium”

Fig. A.6 Rio de Janeiro, Brazil, 2006: IMEKO XVIII World Congress under the motto “Metrology for a Sustainable Development”. Force measurement experts at a round table discussion: J. Pratt (NIST), M. S. Kim (KRISS) and D. M. Stef˘ ¸ anescu (RMS), with the participant countries’ flags in background

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Fig. A.7 Lisbon, Portugal, 2009: IMEKO XIX World Congress under the slogan “Fundamental and Applied Metrology”. The author with P. P. L. Regtien (IMEKO Vice President for Publications) in the Internet Access Room, FIL Meeting Centre, World Expo 98 Complex

Fig. A.8 Busan Exhibition and Convention Center, South Korea, 2012: “G20 Summit” with IMEKO XX leaders under the motto “Metrology for Green Growth”, a group illustrating IMEKO’s universality, in front of a huge tapestry with Sun and Moon

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Fig. A.9 Historical, last IMEKO Bulletin (No. 50, 2013), containing a technical paper written by D. M. Stef˘ ¸ anescu and A. Millea, actual, and respectively, former Romanian representatives in IMEKO General Council

Periodical tables are presented, summarizing touristic programs/conferences or global inter-country comparisons, enlightening worldwide supremacy like in Table A.2. According to the average of three factors (area, population and GNP) the top three countries are China, USA and Brazil.

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Fig. A.10 Prague, Czech Republic, 2015: Plenary round table “The ‘SI’ Quantities and Units— A Universal Language Bridging over the 24 IMEKO Technical Committees”, within the IMEKO XXI World Congress held under the slogan “Measurement in Research and Industry”, with D. M. Stef˘ ¸ anescu as chairman and keynote speaker Table A.2 Numerical values in the table show positions of states based on area, population and development level (expression of advanced engineering)

Country

Area

Australia Brazil China

Population

GNP

6

50

12

5

5

6

3

1

2

63

15

4

India

7

2

9

Japan

62

10

3

Mexico

14

11

14

Russia

1

9

10

Germany

South Africa

25

24

28

South Korea

109

23

15

4

3

1

USA

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Readers Impressions There is no greater pleasure than going heart and soul into showing people what you have seen worldwide, like an always open book, and then get the recognition of passionate readers. Rodica: Such as it comes out from the description, over there a fairytale world exists. I think you should bring together your scientific travels within a literary work! Daniela: I read “off the reel” the “Korean Journal”! It is so well written that I felt like being there, desiring not to finish! Congratulations for this undertaking, a wonderful opportunity to acquaint a different world, to see it with the eyes of a scientist! Adriana: Seeing so many places and meeting so many people—just apostles had this privilege! With a quite high focusing power, an engineer’s mind and a poet’s spirit, dressed in cloths of normality (…) each report is like a delicious cake, prepared according to an unique and original recipe, a real literary feast! Alexandru: You are like a lightning rod for the information burst we are submitted by the outdoor world, without neglecting your internal “ego”, and make your own way toward fulfilling the supreme project of any scientist-artist genius, namely freedom!

Audio-Visual Presentation Means At the beginning of this millennium recording and archiving audio-video information on magnetic tape (e.g. cassette recorder) was replaced with miniature means (“flash” memory cards, associated with both photo- and video cameras [6]. Technology means “reason” (logos) of “art” (tekhne) and these two facets interlace more and more profoundly, generating spectacular images, easily accesible and utilized by a more and more widespread public. We could ask “to what extent development of technology deepens or smooths the classic vs. modern contradiction?” How can reconcile a solitary scholar from the “Gutenberg Galaxy” with the homo videns of Giovanni Sartori, who feels that he does not exist if not appearing on TV? Can anybody enjoy the intimacy of contemplating the world under the everyday siege of mass-media’s noughts and nays or among people with heads in “digital cloud”, conceived as a universal control tool? Evolution and diversification of communication means is beneficial for mankind’s culture, not only for the superficial consumerism. Exactly like the performant doctors, veritable engineers and artists practice permanent education and “Da Vincian” mastership in order to contribute in a way or another to the well-being of society as a whole.

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A new profession appeared, public videographer [7] or, otherwise expressed, writer with images. People have no more time to read and much less to write, in order to consign their life experience… These “video-memories” (interviews in front of video-cameras) will be transmitted to the next generations. Do not think only to wedding ceremonies in various configurations, but also to audio-visual portraits of certain notorious personages! A videographer is like a jolly-joker: partly director, partly sound man and partly editor. The video-camera merges with man not only as a generic term (cameraman) but as a scientific-fantastic extension ot human senses, used with engineering reason and artistic talent. Not incidentally, professor Christof Rohrbach (Bundesanstalt für Materialprüfung, Berlin) deems that “man is the most sensitive instrument”.

Video-Memoirs About Author and His Illustrated Travel Book Alexandru St˘anescu is a young Romanian film lover, a talented film maker and… my stated fan, who helps me to enlarge the horizon of literary-scientific creation! Alex has compiled my audio-visual CV [8] and the artistic presentation of the book

Fig. A.11 D. M. Stef˘ ¸ anescu presenting his audiovisual CV in which a great part concerns IMEKO activities

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“Out of Europe” [9] on YouTube, two original video-memoirs that accompany the printed book or the digital version for virtual environment. You may appreciate the happy interweaving of his shootings with fixed frames, spectacular montages based on conference slides (DMS), pictures taken during “exotic” journeys and “live” films of documents and of author’s library in February 2016 (Fig. A.11). The presentation has been admirably translated from Romanian to English by Virgil Titeu, facilitating the universal reception of the message, a real cultural-artistic manifesto. Hopefully our science and art will make useful to the worldwide public these testimonies of the “joy of traveling”, in the spirit of Fraden’s quote.

Acknowledgements Thanks are due to sponsors from all around the world, who made possible such dreamy travels, as well as to close collaborators, among them being mentioned in this paper Dr. Aurel Millea, Alex St˘anescu and Virgil Titeu, for their contribution to these printed and/or audio-visual achievements.

References and Links1 1. Stef˘ ¸ anescu, D.M.: My 20 years of IMEKO—History and perspectives, IMEKO Bulletin No. 47, pp. 14–17, Budapest, Oct 2008 2. Stef˘ ¸ anescu, D.M.: Dincolo de Europa (Out of Europe), 268 p. Tipro, Bucure¸sti, Romania (2015) 3. Fraden, J.: Handbook of Modern Sensors, p. 413. Springer International Publishing, Switzerland (2016) 4. Mote Jr., C.D., Dowling, D.A., Zhou, J.: The power of an idea: the international impacts of the grand challenges for engineering. Engineering (The Official Journal of the Chinese Academy of Engineering and Higher Education Press) 2, 4–7 (2016) 5. A picture is worth a thousand words (Chinese proverb), Wikipedia, 18 Jan 2017. https://en.wikipedia.org/wiki/A_picture_is_worth_a_thousand_words 6. St˘anescu, A.: Tehnologie (in Romanian). https://paseist.wordpress.com/2016/07/ 02/tehnologie/

1 For

references [8] and [9], please activate the English captions as follows: Click the nut-shaped button, choose CC—caption / subtitr˘ari—Englez˘a!

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7. Videograf, DEX (Dic¸tionar Explicativ al Limbii Române) online. https:// dexonline.ro/definitie/videograf 8. DMS—CV audiovizual (12:10). https://www.youtube.com/watch?v= nRvczwxsR1A 9. DMS—Dincolo de Europa (8:32). https://www.youtube.com/watch?v= BYVsAHoxEMo