Sensors and Actuators: A. Physical All 3D printed ready-to-use flexible electroadhesion pads

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Sensors & Actuators: A. Physical 344 (2022) 113747

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Sensors and Actuators: A. Physical journal homepage: www.journals.elsevier.com/sensors-and-actuators-a-physical

All 3D printed ready-to-use flexible electroadhesion pads Chaoqun Xiang a, b, Yisheng Guan a, *, Haifei Zhu a, Shangcan Lin a, Yaowei Song a a b

School of Electromechanical Engineering, Guangdong University of Technology, Guangzhou, China SoftLab, Bristol Robotics Laboratory, University of Bristol, Bristol, United Kingdom

A R T I C L E I N F O

A B S T R A C T

Keywords: Electroadhesion Flexible robots 3D printing Shape adaptive Pseudo rigid body model

Electroadhesion is a promising adhesion mechanism widely employed in robotics with advantages including enhanced adaptability, gentle/flexible handling, reduced complexity, and ultralow energy consumption. Currently, all electroadhesion pads are manually fabricated, which limits their applications. In contrast, new, easy-to-implement, cost-effective, and entirely 3D printed flexible electroadhesion pads made of both nonconductive polylactide and graphene conductive polylactide are presented in this paper. Moreover, their stat­ ics model for geometric dimensions and flexibility is established via the pseudo rigid body model. Then, normal electroadhesion force measurements and electrostatic simulation of the flexible electroadhesive pads are con­ ducted. Finally, a 3D printed curvature adjustable gripper based on flexible electroadhesive pads that can actively grasp flat, concave, and convex objects is presented. These FEPs are expected to widen the preparation technology and increase the use of electroadhesion in soft robots application.

1. Introduction Electroadhesion (EA) is an electrostatic attraction between two sur­ faces with different electrical potentials that stick to each other[1–3]. EA has been widely employed in a broad range of applications in robotics, such as grasping[4], manipulation robots[5,6], climbing [7], flying [8], mobile robots [9], and automated transportation [10]. Soft and flexible EA can be used in soft robots. Compared to traditionally rigid robots, these intrinsically soft robots are capable of achieving highly compliant and safer performances [10–13]. However, to increase the reliability of this technology, cost-effective mass-producible EA pad fabrication methods and new electrode and dielectric materials consideration, require further efforts [2]. Robotics are essential components in various manufacturing auto­ mation applications. Nowadays, robotics is undergoing a paradigm shift from conventional rigid robots to soft robots [14,15]. Soft and flexible EA pad robots that are used to manipulate fragile objects [16,17] rely on their own advantages including more adaptability and additional safety. However, those projects are not easily achieved on rigid EA, such as soft flat bifilar coil [18], a wearable electroadhesive clutch [19], electro­ adhesion of hydrogels [20], and stretchable suction cup based on elec­ troadhesion [21]. To ensure the applicability of EA in soft robot applications, soft and flexible EA pads are required [22]. Soft and flex­ ible EA pads are attractive due to their promising stretchability and

surface conforming [7,23]. Currently, fabrication methods technologies can be mainly classified into subtractive [24], additive-subtractive so­ lutions [8,25] and additive solutions [2]. The subtractive EA pad manufacturing method is a straightforward, and easiest fabrication method that involves manual preparation. However, the quality of EA pads requires skilled workers and is relatively hard to standardize [26]. Additive-subtractive solutions involve the chemical etching of copper laminates or electroplating of copper, which is not a convenient and straightforward fabrication method [27,28]. Additive EA fabrication methods mainly include inkjet printing [5,29], screen printing, and molding techniques [30]. Inkjet printing EA fabrication method can only print a small subset of materials and is not easily realized. Moreover, screen printing and molding also involve manual preparation. 3D printing technology is a key technology for fabricating soft robots due to its high quality and suitability for mass production [31]. A multi-material 3D printer can be used to print structures with variable stiffness made of polylactide (PLA) and graphene-conductive polylactide (GPLA) [32]. However, no reports on the application of EA pads can be found. Furthermore, objects printed in defined dimensions can generate the desired stiffness. This can be used as a spring and provide adequate compliance. Lastly, it should be mentioned that a single linear actuator 3D printed for interacting with delicate objects was presented in [33]. 3D printed robots in defined dimensions can provide adequate compliance, which in return generates can generate large displacement

* Corresponding author. E-mail address: [email protected] (Y. Guan). https://doi.org/10.1016/j.sna.2022.113747 Received 6 December 2021; Received in revised form 7 July 2022; Accepted 8 July 2022 Available online 11 July 2022 0924-4247/© 2022 Elsevier B.V. All rights reserved.

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The remainder of the paper is organized as follows. In Section 2, the concept design, statics model, and fabrication details of flexible elec­ troadhesive pads are described. Moreover, a customized experimental platform and related experiment procedures for validating the statics model are also presented. Customized electroadhesion force experi­ ments and electrostatic simulations of flexible electroadhesive pads are presented in Section 3. The design and development of a curvature adjustable gripper and an intelligent material handling system are illustrated in Section 4. Discussion is provided in Section 5, while con­ clusions and future work are presented in Section 6. 2. Materials and methods 2.1. Flexible electroadhesive pads (FEPs) concept design and statics model The main concept of a printed flexible electroadhesive pad structure is shown in Fig. 1. A two-material 3D part is designed where the conductive graphene polylactide (GPLA) of 5 mm wide is used as an electrode and polylactide serves as a dielectric material. The space be­ tween electrodes is 5 mm. Black and orange colors represent the GPLA and PLA, respectively. The printed prototype of FEP is shown in Fig. 1 (b). From the side view A-A, the GPLA is evenly arranged in the PLA. The schematic diagram of FEP geometric parameters is presented in Fig. 1 (a), where L, w, and h stand for the length, width, and height of the FEP. To investigate the flexibility of the FEP, the statics model of L, w, and h is established. Here, the deformation of FEP can be regarded as the deformation of the cantilever beam. The stress diagram of the FEP cantilever beam is shown in Fig. 2(a). Here, point A is the fixed end of the cantilever beam, point B is the free end of the cantilever beam, L is the beam length, and F is the vertical load force applied at point B. The deformation of the free end of the cantilever beam can be clas­ sified as large deflection deformation. When the end point B of the cantilever beam is no longer a small deformation, material mechanics cannot be applied to analyze large deflection deformation. Hence, pseudo rigid body model analysis is used. The flexible rod and hinge in the flexible mechanism are replaced with a rigid connecting rod and a torsion spring. In other words, a single free end of the cantilever beam is transformed into a virtual torsion spring connected by two rigid con­ necting rods. The large deformation of the cantilever beam can be analyzed via material mechanics. The pseudo rigid body model is shown in Fig. 2(b), where θ is the angle of rotation of the pseudo rigid bar, r is the characteristic radius coefficient, and rl is the length of the pseudo rigid body bar. A cartesian coordinate system is established at the fixed end of the cantilever beam. The position of the free end is represented by the co­ ordinate value (a, b): { a = (1 − r)L + rL cos θ (1) b = rL sin θ

Fig. 1. Flexible electroadhesive pad: (a) schematic diagram, and (b) prototype.

output under a small load. As a result, these robots are considered to be safer, more adaptable, and compliant than rigid robots [34]. In addition, the statics model of soft robots can guide their design. Typically, soft robots generate large deformation. Ergo, material mechanics does not apply such analyses. Therefore, for large deformations, finite element analysis method, elliptic integration method, and pseudo rigid body model analysis are the most widely employed analysis methods. The finite element method transforms the continuous solution into a discrete solution [35], establishes the system equation by using the stiffness matrix, and obtains the appropriate algorithm to converge to the globally optimal set. Generally, a computer is employed for finite element simulations. However, the errors will accumulate and amplify during the analysis process of each iteration. Therefore, the buckling mode cannot be accurately predicted, i.e., the modeling accuracy of the finite element method is not adequate. The elliptic integration method employs Euler-Bernoulli differential equation [36,37]. The closed solution is solved by numerical method with high precision. However, the elliptic integral method needs to define strict boundary conditions and predetermine the angle of the beam end. In the pseudo rigid body model method, the flexible rod and hinge in the flexible mechanism are replaced by a rigid connecting rod and tor­ sion spring [35]. Then, the cantilever beam with a single free end is transformed into two rigid connecting rods connected by a virtual tor­ sion spring. Therefore, material mechanics can be used to analyze the cantilever beam with large deformation. In this paper, the development of a new, easy-to-implement, and cost-effective flexible electroadhesive pad manufacturing approach manufactured via multi-material 3D printing technology is reported. As a result, the 3D printed flexible EA pads possess the desired flexibility and actively adsorb flat, concave, and convex objects. The proposed 3D printed flexible EA pads may signifi­ cantly promote the application of electroadhesion and soft robot technologies.

Vertical load force F applied at point B is a K-related function [35]. Therefore, the relationship between F and the deflection angle θ of the FEA can be expressed as: F=

Kθ EIθ L2 cos θ

(2)

where Kθ represents stiffness coefficient of the torsion spring at the pseudo hinge point, E = 3.3 × 103 MPa is the elasticity modulus of the polylactic acid beam materials, and I is the moment of inertia of FEP that can be expressed as follows: I=

wh3 , 12

(3)

where w and h stand for the length and height of the FEP, respectively. Thus E, I, and L are constant inherent properties of the beam. According to the geometric relationship shown in Fig. 2(b), the angle 2

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Sensors and Actuators: A. Physical 344 (2022) 113747

Fig. 2. Vertically applied load force effect on the FEP: (a) schematic diagram of bent FEP, (b) schematic diagram of the pseudo rigid body model, (c) deflection angles at different vertical load applied forces, and (d) schematic diagram of the test rig.

of rotation of the pseudo rigid bar can be expressed as follows: θ = arctan

b a − (1 − r)L

%, Wenzhou sundoo instruments co., Ltd, China) was used to measure the vertical load force. According to Eq. (2), the length, width, and thickness of FEPs affect flexibility. However, the thickness of FEPs is investigated in this paper. Therefore, L = 120 mm and w = 50 are chosen for FEPs. FEPs with the width of 0.5 mm, 1 mm, and 1.5 mm were fabricated and tested, and the vertical load applied effect tests were repeated five times. The experimental and corresponding analytical results in similar conditions are shown in Fig. 2(c). The vertical load force applied at point B increases with the deflection angle. The maximum error between the experimental and analytical results is approximately 8.95 %. In addi­ tion, the error increases with the deflection angle. Thus, the relationship model between F and the deflection angle θ of the FEA is adequate for practical use to guide the design of the FEA. The aforementioned error

(4)

To investigate the effect of vertically applied force on FEP, a test rig was set up and the schematic diagram in Fig. 2(d) was used to verify the proposed model. An external force was applied on the end of the FEP utilizing a digital force gauge, and a camera was used to extract the deflection angle of the actuator. The digital force gauge was moved by hand against the edge of FEPs. Therefore, the end face of the digital force gauge was perpendicular to the edge of the FEPs. To investigate the effect of the vertically applied load force at different deflection angles, the deflection angles were set as from 10◦ to 40◦ in intervals of 10◦ . A digital force gauge (SH-10, accuracy of ± 0.5

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2.2. Fabrication All characterized components of FEP were printed by the multimaterial 3D printer (Raise3D, Raised E2, China). Commercial off-theshelf PLA filament (PLA+, Shenzhen esun industrial co., Ltd., China) and conductive graphene PLA filament, GPLA (1.75 mm, Qi hang xu digital products, China) were employed. GPLA had a volume resistivity of 0.7 Ω•cm. The recommended extrusion temperature of PLA and GPLA is 205–225 ◦ C. Three types of thickness test pieces of FEP were fabri­ cated and tested. 3. Results 3.1. Normal force measurement of the flexible electroadhesive pad To test the repeatability of the proposed 3D printed FEP fabrication method, a normal EA force measurement rig was set up, as shown in the inset in Fig. 3(a). The inline digital force gauge was used to measure the normal adhesive force. A linear rail driven by a stepper motor (57BYG5642B, Insunmot, China) was applied to pull a flat PLA substrate with 10 mm thickness away from the EA pad after charging for 30 s at 5 kV using a high voltage power supply (HVA, EMCO E60, XP Power Ltd., USA). A motor driver (DM542, Dongguan Yachida Electrome­ chanical Co., LTD, China) was used to control the step motor. An opensource hardware Arduino Mega2560 (Arduino, Italy) was used to con­ trol the output voltage of the HVA. Then, three FEPs 1 mm wide were fabricated using the same pro­ cedure, geometry, and dimensions. The overall effective electrode area of the FEPs was 120 mm × 40 mm, as shown in Fig. 1(a). During the EA force measurement, normal EA forces were recorded when high voltage was applied to the FEPs. Lastly, five tests were conducted for each FEP. Normal EA repeatability test results can be seen in Fig. 3(a). A maximum relative difference of 9.8 % can be seen in average normal EA forces obtained across the three EA pads. This difference can be further minimized by using more advanced multi-material 3D printers. To test the normal EA force of FEPs at different curvatures, the normal EA force test rig in the inset shown in Fig. 3(a) was employed. One flat and three concave PLA substrates were fabricated for the normal EA force test. Parameters R1 = 345 mm, R2 = 175 mm, and R3 = 120 mm were chosen for three concave substrates radii of curvature, respectively. Moreover, three EA holders with three correspondent radii of curvature were also fabricated. FEP with 1 mm thickness was chosen as the test sample to maintain consistent experimental parameters. The same EA normal force was used for different curvatures, while the environmental conditions for the test procedure were the same as the normal EA repeatability tests. The average value of the results and its standard deviation were reported. The EA normal force under different curvatures test results can be seen in Fig. 3(b). The EA forces decrease with the radius of curvature, while the flat FEP generates the highest EA output force. A probable reason for these results is that the electric field of the FEP decreases with the radius of curvature. The FEP thickness not only affects its flexibility, but also affects the EA forces. Thus, it is necessary to investigate how the thickness affects the EA forces. The test rig in the inset shown in Fig. 3(a) was once again employed. FEPs with the width of 0.5 mm, 1 mm, and 1.5 mm were fabricated, and a concave 3D printed PLA object with a radius of cur­ vature of 175 mm was used as the EA force test substrate. FEPs thickness effect test procedure was the same as the procedure for normal EA repeatability tests. Five tests were conducted for each FEP of different thicknesses. For each experiment, five repetitive tests were conducted. The average value of the results and its standard deviation were re­ ported. The test results can be seen in Fig. 3(c). It can be observed that the EA forces decrease with an increase in the thickness of FEPs. A probable reason for these results is that the electric field of the FEP decreases with the radius of curvature. Another probable reason is that FEPs do not possess good flexibility. Hence, circular uniformity is not

Fig. 3. Normal EA force measurement rig schematic diagram and test results: (a) repeatability test results of the proposed 3D printed FEP fabrication method, where mean and standard deviation values for five tests are shown. Inset shows the schematic diagram of the normal EA force test rig. (b) normal EA force of FEPs at different curvatures test results, and (c) test results of the thickness effects on EA forces.

may be attributed to the preparation of the FEP, the homogeneity, and the compactness of the FEP materials that induce a significant amount of nonlinearity. However, in this research, the error due to the preparation process is assumed as negligible. In addition, the tests conducted were carried out for the relative humidity of 65 ± 1 %, the temperature of 27.3 ± 0.1 ◦ C, and ambient pressure of 1018.5 ± 0.2 hPa. 4

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Fig. 4. Static electric intensity (V/m) and equipotential (V) distribution simulation results of the flat FEP and deformed FEP: (a) five kilovolts was applied to the correspondent GPLA electrodes of the flat FEP, (b) five kilovolts was applied to electrodes was applied to GPLA electrodes of the deformed FEP. Default materials from the FEMM material library were used, including the air (dielectric constant of 1). Customized materials including PLA (dielectric constant of 2) and GPLA (dielectric constant of 3) were designated and defined into the specific areas in the FEPs.

Fig. 6. Curvature-adjustable gripper.

Fig. 5. The magnitude of field density, |E|, along the line P1Q1 and the curve P2Q2 (shown in Fig. 4(a) and Fig. 4(b), respectively).

same size two-dimensional FEP bent at a 345 mm radius of curvature were designed in SolidWorks software and exported as DXF files. Then, the DXF files were imported to the Finite Element Method Magnetics (FEMM) software, and the width, w, was set to 50 mm. Finally, a nu­ merical analysis was conducted to manifest the interactions between PLA and CPLA for 5 kV voltage application strategies. FEPs under different deformation will bring different potential and electric field distributions, as well as PLA and CPLA interactions. Static electric intensity (V/m) and equipotential (V) distribution simulation results of two FEPs are shown in Fig. 4. Flat and deformed FEP static electric intensity and equipotential distribution are shown in Fig. 4(a) and Fig. 4(b), respectively. PLA and CPLA interaction of two FEPs is slightly different. The flat FEP static electric intensity is slightly stronger than the deformed FEP, i.e., higher electric intensities are characterized by a higher EA force. Lines P1Q1 in Fig. 4(a) and P2Q2 in Fig. 4(b) were investigated to check the magnitude of field density for the two FEPs in question. The magnitude of field density for two FEPs is shown in Fig. 5. It can be observed that a slightly uniform magnitude of field density at flat FEP is present. Moreover, a clear potential difference can be observed between the electrode CPLA and PLA.

observed under large bending deformation. Consequently, good contact with substrates with a large radius of curvature is not achieved, which causes a decrease in the EA forces. It should be noted that all tests were conducted in a clean and closed chamber. In addition, all tests were carried out for the temperature of 29.8 ◦ C ± 0.1 ◦ C, relative humidity of 65 % ± 1 % using a weather station, and ambient pressure of 1019.5 ± 0.2 hPa. 3.2. Two-dimensional electrostatic simulation of the flexible electroadhesive pad The electric field affects the EA force. The normal EA force test re­ sults of FEPs decrease with the radius of curvature. Thus, the electric field intensity of FEP under different curvatures was investigated through simulation. Firstly, a flat two-dimensional FEP (L = 120 mm, h = 1 mm) and a 5

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Fig. 7. Grasping of flat, concave, and convex shapes. The left side of the figure shows the schematic diagram of the device adsorption mechanism. The right side shows the physical prototype. The white scale bars on the right side denote 10 mm.

4. Design and development of a curvature-adjustable gripper 4.1. Curvature-adjustable gripper design As introduced in the FEP concept design, FEP can be deformed at different curvatures to generate EA forces. Thus, for practical purposes, a curvature adjustable gripper shown in Fig. 6 was designed. The curvature-adjustable gripper includes a top holder, a fixture, a bottom holder, and a FEP, whose curvature can be adjusted. The entire curvature-adjustable gripper is fabricated in the same 3D printer. By adjusting the distance between the bottom holder and top holder of the gripper, the curvature of FEP can be modified for different grasping tasks. The gripper can grasp concave, flat, and convex objects, which is relatively difficult for other EA grippers, such as the gripper from [38]. An additional pneumatic system has to be added to the system for different shape objects. The capability to grip variously shaped objects that are challenging for conventional grippers is demonstrated in Fig. 7. The left side and right side show the schematic diagram of the device adsorption mechanism and the physical prototype, respectively. The material of grasped objects is commercial A4 paper. The flat object is 30 mm wide and 60 mm long, and the curvature of the concave and convex objects is 8.18 m-1 and 6.94 m-1, respectively. Weights of the flat object, concave object, and convex object are 0.9 g, 1.2 g, and 1.4 g respectively. We explored the capacity to hold items of varied

Fig. 9. The intelligent FEP material handling system: (a) schematic diagram. (b) the system prototype.

sizes, materials, and weights further and report our findings in Fig. 8. This demonstrates that the gripper can securely grasp flexible items made of a variety of materials.

Fig. 8. Demonstration of gripping capability: (a) grasping a PE (polyethylene) bag at 4.5 kV, (b) grasping a textile at 6 kV, and (c) grasping a sand papers at 6 kV. The white scale bars on the right side denote 10 mm.

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on the design of ready-to-use flexible electroadhesion pads. Thus, the solution of FEPs with complex deformation will be conducted in the future. 6. Conclusions In this paper, a new 3D printed, easy-to-implement, cost-effective, and ready-to-use flexible electroadhesion pads made of both nonconductive PLA and graphene conductive PLA were proposed. A pseudo rigid body model was used to establish the statics model of geometric dimensions and flexibility. The maximum error between the experimental and analytical results is approximately 8.95 %. In addi­ tion, this error increases with the deflection angle. Thus, the static model can be utilized for the design of FEP. Normal electroadhesion force measurement was conducted on FEPs with different dimensions and various radii of curvature. It was observed that the flat flexible elec­ troadhesion pad (FEP) generates the highest electroadhesion (EA) output force. A probable reason for these results is that the electric field of the FEP decreases with the radius of curvature. Electrostatic simula­ tion of flexible electroadhesive pad was carried out through the FEMM software. The simulation results demonstrate that the flat FEP static electric intensity is slightly stronger than the one of deformed FEP. Finally, a curvature-adjustable gripper based on FEPs whose entire body is 3D printed and made of non-conductive PLA and graphene conductive PLA was demonstrated. The gripper was able to lift flat, concave, and convex objects due to the combined morphology adaptability and EA application. The 3D printed flexible electroadhesion pad is expected to widen the preparation technology and increase the use of EA in soft robots application. The main contributions of this paper include:

Fig. 10. Movement and control flowchart of the intelligent FEP material handling system.

4.2. Case studies: pick-and-place tasks An intelligent FEP material handling system was developed based on the curvature-adjustable gripper shown in Fig. 9. The schematic diagram of the system is presented in Fig. 9(a). A high voltage power supply (HVA, EMCO E60, XP Power Ltd., USA) was used to energize the FEP. A linear rail driven by a stepper motor (57BYG5642B, Insunmot, China) was used to pull the curvatureadjustable gripper in the vertical direction. An inline miniature SBeam load cell (CHLBS-min, Chuda Technology Co., LTD, China) was integrated with the curvature adjustable gripper to sense the contact force and inform the gripper whether it touches the substrates or not. A motor driver (DM542, Dongguan Yachida Electromechanical Co., LTD, China) was used to control the step motor. A MOSFET switch (Yusong electronic, China) was used to control the HVA. An open-source hard­ ware Arduino Mega2560 (Arduino, Italy) was used to communicate with the MOSFET switch and the step motor driver, as well as to receive the feedback from the S-Beam load cell. The movement and control flowchart of the intelligent curvature adjustable gripper material handling system is presented in Fig. 10. Firstly, the linear rail was lowered to approach the concave object (radius of curvature: 80 mm; weight: 1.5 g) to be grasped and stopped when the force sensor reading was over the threshold value Ft (taken as Ft = 0.2 N in this paper). Then, the EA was turned on and 5 kV high voltage was applied for a period of t (taken as t = 15 s in this paper). Next, the linear rail moved upward to pick up the object and stopped when it reached the top position. Finally, electroadhesion was finally turned off and the curvature adjustable gripper released the object. The demonstration of pick-and-place tasks is recorded and demonstrated in Supplementary Movie S1.

(1) the development of an easy-to-implement and cost-effective flexible electroadhesive pad manufacturing approach, as well as a flexible electroadhesive pad statics model of geometric di­ mensions and flexibility. (2) normal electroadhesion force measurement on FEPs with different dimensions and various radii of curvature, and elec­ trostatic simulation of flexible electroadhesive pad. (3) the curvature-adjustable gripper and material handling system. Future work will include but will not be limited to optimizing the design of the FEPs, automating the design of curvature regulation design, improving the statics model, developing a stiffnesstemperature relationship model, and integrating the FEPs with a soft climb robot. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

5. Discussion According to the grasping of flat, concave, and convex shapes, the curvature-adjustable gripper shows advantages of grasping different shapes objects. The target objects are normalized in the case studies. Nevertheless, FEPs possess quite good morphology adaptability and can generate complex deformation for complex surface objects. This is demonstrated in Fig. 11. According to [32], to achieve complex defor­ mation of FEPs, direct current can be used to heat the GPLA. Then, FEPs are sufficiently soft to achieve complex deformation. This paper focuses

Acknowledgements The work in this paper is supported by the Natural Science Foun­ dation of China, China (Project for Young Scientists: Grant No. 52105010 and Regular Project: Grant No. 51975126), the “Hundred Young Talents Program” scientific research project of Guangdong Uni­ versity of Technology, China (Grant No. QB2112), and the Natural Science Foundation of Guangdong Province, China (Regular Project: Grant No. 2022A1515010327). Furthermore, the authors acknowledge Prof. Jonathan Rossiter, Jianglong Guo, and Rizwan Ur Rehman of the University of Bristol for providing inspiration for the creation of flexible electroadhesion pads.

Fig. 11. Schematic diagram of grasping a complex surface object. 7

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Appendix A. Supporting information

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Chaoqun Xiang obtained his BEng and MSc degree in Me­ chanical Design and Theory from Liaoning University of Technology (P.R. China) in 2009 and 2012 respectively. Dr. Xiang received his Ph.D. degree in Mechatronic Engineering from Northeastern University (P.R. China) in 2017. He is currently a lecture in Guangdong University of Technology. His main research interests include bionic drivers of artificial muscles and intelligent control methods for soft robots.

Yisheng Guan received a Masters degree from Harbin Institute of Technology, Harbin, China, in 1990, and a Ph.D. degree from Beijing University of Aeronautics and Astro­ nautics (BUAA), Beijing, China, in 1998, both in mechanical engineering. He was a postdoctoral fellow in the Department of Computing Science, University of Alberta, Edmonton, AB, Canada, from 1998 to 2000, and a JSPS research fellow in the Intelligent Systems Institute, AIST, Tsukuba, Japan, from 2003 to 2006. In 2007, he joined the School of Mechanical and Automotive Engineering, South China University of Technology, Guangzhou, China as a Professor. Since 2013, he has been the director of the Biomimetic and Intelligent Ro­ botics Lab (BIRL) and a Professor in the School of ElectroMechanical Engineering, Guangdong University of Technology (GDUT), Guangzhou, China. His research interests include biomimetic robotics, intelligent robots, modular ro­ bots and humanoids. Prof. Guan has chaired several IEEE international conferences (PC of ROBIO 2018, Co-PC of ROBIO 2012 and ICIA 2009), has been a PC member of 60 + conferences (AE of IROS since 2012, ROBIO, AIM, ICMA, ICIA, Humanoids, and so on), and a reviewer of about 40 international journals.

Haifei Zhu (M′ 12) received a Bachelors degree from Wuhan University of Technology, Wuhan, China, in 2008, and a Ph. D. degree from South China University of Technology, Guangzhou, China, in 2013, both in mechanical engineering. From 2014–2016, he was a research fellow in the School of Mechanical and Aerospace Engineering, Nanyang Techno­ logical University, Singapore. Since 2016, he has been an Associate Professor in the School of Electro-Mechanical En­ gineering, GDUT, China. His research interests include climbing robots, robotic manipulation and motion/path planning

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C. Xiang et al.

Sensors and Actuators: A. Physical 344 (2022) 113747 Shangcan Lin received his Bachelor’s degree in Mechanical Engineering, from the School of Mechanical Engineering, Wuyi University, China, in 2019. Currently, he is now studying for the master’s degree in the School of ElectroMechanical Engineering, Guangdong University of Technol­ ogy, Guangzhou, China. His research interests mainly include bioinspired robotics,especially on bioinspired design.

Yaowei Song received his Bachelor’s degree in Mechanical Engineering, from the School of Mechanical Engineering, Ningxia University, China, in 2019. Currently, he is now studying for a master’s degree in the School of ElectroMechanical Engineering, Guangdong University of Technol­ ogy, Guangzhou, China. His research interests mainly include biomimetic manipulator, variable stiffness actuator, etc.

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