Variable Stiffness Electroadhesion and Compliant Electroadhesive Grippers

Abstract

Soft adhesion is capable of attaching and bonding to rough surfaces and gripping nonplanar materials. It is preferable for material handling applications where safe interactions with external environments and enhanced adaptability to changing conditions are required. Soft electroadhesion (EA) is an emerging controllable adhesion technology that is especially suited to soft adhesion applications, but is prone to contact peeling that causes unwanted de-adhesion and cannot lift heavy objects unless the lifting force is applied parallel to the surface. Variable stiffness electroadhesion (VSEA) can be used to overcome these issues. Here a VSEA solution is developed by integrating electrostatic layer jamming and soft EA into a monolithic electrically controllable structure. The VSEA pad can achieve rapid response (within 1 s) and significant stiffness change (2200%), resist over four times the peeling force under a weight of 70 g, and generate 24.2%, 34.8%, and 49.3% greater adhesive forces on flat, convex, and concave surfaces, respectively. The promising gripping performance of the VSEA gripper was demonstrated by lifting and moving curved and flat objects. The VSEA concept and solution shown in this work may pave the way for the ready integration of EA into soft robotic systems and promote the broad application of EA technologies.

Introduction

As industry becomes more automated, the demand for more ubiquitous, effective, and low-cost controllable adhesion including magnetic, pneumatic, and bio-inspired mechanisms is rapidly increasing.^1,2 Rigid controllable adhesion has been widely used in handling flat objects but cannot be used to lift objects with nonplanar or rough surfaces^3–5 (Fig. 1a). Nevertheless, passive soft or compliant controllable adhesives can overcome this limitation to some degree by conforming to uneven surfaces.

FIG. 1.

Comparison of (a) rigid, (b) soft, and (c) VS adhesion when lifting curved surfaces. Peeling issue occurs in soft adhesion. VS, variable stiffness.

A further advancement is to embed active shape-morphing capabilities into soft adhesion, thereby enabling them to dynamically adjust to a wide range of objects in real time, from rough to convex to concave. Soft controllable adhesion is also highly desirable for the handling of delicate and high-value materials and objects, where soft-and-safe interactions are essential.⁶

Soft adhesives, however, commonly suffer from peeling issues.^7,8 Peeling occurs when the adhesive separates from the surface at one point, and this separation propagates under modest load across the contact interface until adhesion catastrophically fails (Fig. 1b). Peeling in soft adhesion occurs because the stiffness of the material is low compared with that of the substrate and the initial point of separation initiates at a local stress concentration. Once peeling starts, only a relatively small force may be needed for the peeling area to grow.

One solution is to combine passively compliant or active shape-changing structures with soft adhesives. Active variable stiffness (VS) can also be integrated into the soft adhesion to mitigate the issues associated with both rigid and soft adhesion (Fig. 1c). The VS structure can be used in its “soft” state to conform to complex surfaces and can then be transitioned into its “rigid” state to resist peeling and to maintain constant contact between adhesive and substrate.

Soft electroadhesion (EA)^9–12 is a promising and emerging soft controllable adhesion method, which has many attractive features including enhanced adaptability, reduced complexity (through both simple structure and simple control), gentle (or even damage-free) materials handling, and low energy consumption. Soft electroadhesives can be readily fabricated using soft (flexible or stretchable) electrode and dielectric materials and structures. Energizing the electrodes with a high voltage produces electrostatically induced adhesive forces between the EA pad and the substrate material. In this way, objects can be controllably adhered and released. However, current soft EAs, in common with all soft adhesives, suffer from peeling issues, especially when lifting objects with weights above a certain intrinsic threshold.

Here we present the concept of variable stiffness electroadhesion (VSEA) that can overcome the limitations of soft adhesives by reducing the impact of peeling while preserving the controllable adhesion capabilities of electroadhesive systems. A VSEA device requires the integration of three components: (1) a soft body that can conform to complex structures; (2) VS elements that transition the soft body into a rigid structure once it has adapted to the surface; and (3) surface electrode arrays for the generation of electrostatic adhesion forces between the (now rigid) VSEA device and a surface. By transitioning from soft to rigid state, the morphology of the VSEA device is locked to the surface morphology, maintaining maximum contact area for the EA and preventing peeling.

Layer jamming is a widely used VS method that employs a series of interleaved layers that slide against each other when in a “soft” state and which bind tightly when in a “rigid” state.^13,14 The transition from soft to hard state is typically achieved by applying a vacuum, which pulls the layers together by the resulting atmospheric pressure, or by applying an electric potential between alternate layers, thus causing opposite charged layers to be attracted together, or more recently within each layer.¹⁵ Here we present a lightweight, low energy consumption, all-electric control, and cost-effective VSEA solution by combining electrostatic layer jamming with soft EA, which largely overcomes the problem of adhesive peeling.

Materials and Methods

VSEA pad design

One design solution to VSEA is given in Figure 2a. The VSEA pad (a prototype given in Fig. 2d) consists of a VS component based on the electrostatic layer jamming method and EA component based on a coplanar electroadhesive. One can use the intrinsic soft state of the VSEA pad to passively conform to a surface and then use the VSEA state for shape-locking to resist peeling and enhance stiffness when lifting. The EA component was a pair of interdigitated electrodes (overall dimension of 30 × 90 mm, electrode width 3 mm, electrode gap 2 mm; Fig. 2b) sandwiched between a dielectric film (0.025 mm) and an elastomer layer (0.02 mm).

FIG. 2.

VSEA design, geometric dimensions and the VSEA gripper. (a) Schematic diagram of the VSEA solution. (b) The pattern and dimensions of the rectangular electrode (left) and comb electrode (right). (c) Dielectric pattern and dimensions. (d) The VSEA gripper prototype. VSEA, variable stiffness electroadhesion.

One basic unit of the VS component was a rectangular electrode (30 × 90 mm; Fig. 2b) interlayered between dielectric films (40 × 100 mm; Fig. 2c). The VS component of the VSEA pad was a stack of 10 units. The design of the electrodes and the number of VS units were selected based on comparative experiments (Supplementary Data S1 and Supplementary Fig. S1).

VSEA pad fabrication

Fabrication of the VSEA pad consists of four main steps (Fig. 3). Step 1: Electrode screen printing: A polyethylene-terephthalate (PET) film with a thickness of 0.025 mm was cut into 40 × 100 mm pieces as the base of the VS basic unit and EA pad, and then placed on a screen-printing frame. Screens of different electrode patterns were then placed on the PET films. Conductive inks (LN-GCI-2; Leadernano) were printed onto the PET films and cured for 2 h in a vacuum oven at 45°C. Step 2: VS component fabrication: Copper foils were bonded to the VS electrodes for wiring. The VS electrodes were covered with the same PET films using a double-sided tape (30400-white; Deli). A stack of 10 basic VS units was then assembled as the VS component (Fig. 3 (II.iv)). Step 3: EA component fabrication: Copper foils were bonded to the EA electrodes for wiring.

FIG. 3.

Schematic diagram of the VSEA fabrication procedure. Color images are available online.

A degassed dielectric cover (Dragon Skin 10, Dragon SkinTM, Smooth-On) was then evenly coated on the electrodes by an adjustable applicator with a setting height of 0.02 mm. After this, the EA pad was left in the vacuum oven at 70°C for 4 h. It should be noted that the same dielectric material can be used for both the VS and EA component. Dragon Skin 10 was used to provide better EA properties. Step 4: VSEA assembly: The VS component and EA component were bonded using the double-sided tape. Acrylic sheets (40 × 5 × 1 mm) were bonded onto both ends of the VSEA pad with the double-sided tape (as given in Fig. 3IV). Gravity makes the ends of the VSEA pad more prone to bending, so that the entire pad can passively conform to curved surfaces, especially convex surfaces with greater curvatures. The mass of the VSEA pad was 6.6 g.

Results and Discussion

Stiffness change characterization

To investigate the stiffness change characteristics of the VS component, we first define the stiffness of a structure as the ratio between load (F_load, in Newton, N) and displacement (Δx, in meter, m): $S_{t i} = \frac{F_{l o a d}}{Δ x} .$ (1)

The VS component was tested by mounting it between two supports. Weights were added to the center of the beam (Fig. 4a) and the vertical displacement of the center of the beam was measured by an ultrasonic displacement sensor (Risym US-100). The maximum mass the VS component can support (with displacement <4 mm) under different voltages (kV) was first investigated, and results are given in Figure 4c. When applying no voltage (the natural state), the VS component cannot support 0.2 N (gravitational field strength g is 10 N/kg here). At 0.18 N and no voltage, the stiffness of the VS component was 50.99 N/m. The maximum mass increased monotonically with applied voltages from 1 to 6 kV. Owing to the presence of dielectric anisotropy and dielectric relaxation,¹⁶ it was difficult to obtain an analytical relationship between load and voltage.

FIG. 4.

VSEA stiffness change characterization. (a) Schematic diagram of the stiffness measurement setup. (b) Stiffness changes under the natural state under a mass of 18 g (left) and VS state under a mass of 50 g (right). (c) Relationship between applied voltage and load on the VS component. (d) Relationship between the load and defined stiffness. VS, variable stiffness. Color images are available online.

Therefore, based on the experimental results given in Figure 4c, an empirical model (with adjusted R² value of 0.9962) between load weight F_load (N) and applied voltage U (kV) was calculated: $F_{l o a d} = 0.18 + 0.050 U + 0.033 U^{2} - 0.0084 U^{3} + 0.00053 U^{4}$ (2)

By substituting the maximum displacement (4 mm) in Equation (1), we can then obtain the empirical relationship between the stiffness change of the VSEA structure and voltage (the range is 0–6 kV): $S_{t i} = \frac{F_{l o a d}}{Δ x} = 45 + 12.5 U + 8.25 U^{2} - 2.1 U^{3} + 0.1325 U^{4}$ (3)

In practice, Figure 4c indicates that saturation occurs for voltages larger than ∼4 kV. Hence, to simplify control and to reduce the voltage (to prevent breakdown), 4 kV was used for the rest of the tests conducted. Figure 4d shows the calculated stiffness for each load at maximum displacement, for both electrically energized (with VS) and nonenergized (without VS) cases. This shows a maximum stiffness change (at 0.2 N) of 1111.11 N/m, which is approximately a factor of 22 difference (Supplementary Video S1). The charging time of the VS component was observed to be <1 s. Experiments were averaged over three evaluations and mean and standard deviations are given in Figure 4c and d. These results show that the VSEA pad equipped with a VS component can achieve rapid and large stiffness changes.

VSEA peeling resistance performance evaluation

We next verified the improved peeling resistance performance of the VSEA pad through peeling angle and force measurements. Figure 5a shows the customized peeling angle test platform. The VSEA pad was placed on the substrate (glass) with a cotton thread attached to one end and a voltage (4 kV) was applied to either the EA electrodes only or the combined EA and VS components. After 45 s, a load was attached to the other end of the thread and the resulting peeling angle θ was measured (Fig. 5b). With only the EA function active, the peeling angle increased with the increase of load mass and the maximum load was 70 g, above which the VSEA detached entirely from the substrate, as given in Figure 5c. When both EA and VS functions were active, the maximum load increased to 110 g.

FIG. 5.

Peeling test of the VSEA pad with and without VS function. (a) Schematic diagram of the peeling test (glass substrate). (b) Relationship between peeling angle and load mass with and without VS function. (c) Peeling force of the VSEA pad with and without VS function on the glass, ABS, and wood. Color images are available online.

An evaluation factor k was used to express the ratio of the peeling angle without VS function to the peeling angle with VS function: $k = \frac{{θ_{w i t h o u t}_{V S}}_{f u n c t i o n}}{{θ_{w i t h}_{V S}}_{f u n c t i o n}} .$ (4)

At 70 g, k was ∼4.3, which indicates that the peeling resistance performance of the VSEA pad increased over four times by the addition of VS function. Furthermore, the normal (perpendicular) peeling force was measured for different substrates (glass, ABS, and wood). The VSEA was placed on the substrate and energized as mentioned previously. A force gauge was attached to one end of the VSEA and lifted vertically. We recorded the maximum force during this peeling action.

As given in Figure 5c, the VSEA pad requires greater peeling force when it has VS function than without VS function. These results indicate that the VS function can significantly reduce the problem of peeling associated with soft EAs. Note that the peeling forces vary for the three substrates owing to their differing dielectric properties (the dielectric constants of glass, ABS, and pressed wood are 6–7, 2.4–4.1, and 2.0–2.6, respectively) and surface textures.

Normal electroadhesive force measurement

To investigate the benefits of VS and shape locking in soft electroadhesives, a normal force measurement rig was built (Fig. 6a). We characterized the shape-locking adhesive performance of the VSEA pad on a range of surfaces. First, the effect of electrical charge time on the normal adhesive force was investigated. We applied 4 kV to the VSEA pad on a flat glass substrate and found that a charge time of 45 s provided sufficient adhesive force. The increase in normal force >45 s was not significant (Fig. 6b) and therefore 45 s was used for all the subsequent tests.

FIG. 6.

VSEA normal adhesive force characterization. (a) Schematic diagram (above) and photograph (below) of the normal adhesive force measurement setup. (b) Relationship between charge time and normal adhesive force. (c) Normal adhesive force comparison between rigid and VSEA pads on glass, ABS, and wood. (d) Relationship between adhesive force of VSEA with and without VS function and convex surface curvature radius. (e) Relationship between adhesive force of VSEA with and without VS function and concave surface curvature radius. Color images are available online.

We define the relative percentage difference in adhesive force between using and not using the VS function as: ([the normal adhesive force with VS function − the normal adhesive force without VS function]/the normal adhesive force without VS function) × 100%. For instance, at 45 s, there was a relative increase of 24.2% normal force.

We then compared the adhesive forces generated from rigid and VS EA pads on planar glass, ABS, and wood substrates (Fig. 6c). The rigid EA was fabricated by bonding the soft EA pad to a rigid 2 mm thick acrylic sheet. On all flat substrates, the rigid EA pad achieved the highest adhesive forces. However, the rigid EA pad cannot lift nonplanar surfaces, whereas the VSEA pad can. On convex and concave surfaces with a radius of curvature radius R, we quantify the normal adhesive force of VSEA pad with and without VS function. The results, showing the mean of three experiments, are given in Figure 6d and e.

The VSEA pad generated greater adhesive forces for all tests when the VS component was used and shape-locking was achieved. Adhesion was highest for a convex surface with a curvature radius of 50 mm. At this radius, there was a relative increase of 34.8% normal force when using the VS function compared with not using VS function. On concave surfaces, the normal force decreased with increasing curvature radius. At a curvature radius of 40 mm, there was a relative increase of 49.3% normal force when using the VS function. All results demonstrate that the VS function can alleviate the peeling issue and help the VSEA pad achieve greater adhesive forces on flat, concave, and convex surfaces. The shape locking effect was stronger on curved surfaces than on flat surfaces.

Theoretical considerations and discussions

When a soft EA pad adapts to a surface, the EA force generated on the pad will make the pad adhere to the surface. When an applied force pulls the pad vertically upward, it will act against the regional EA force between the pad and the surface. A soft pad will be easily peeled from the surface because of insufficient rigidity. In contrast, when a VSEA pad adapts to a surface, the layers of the VS component slide against each other locally to accommodate the contact with the surface, then the EA force is generated between the pad and surface under an applied voltage.

Meanwhile, EA forces are also generated within the VS component to increase the friction among the layers and prevent sliding, thereby improving the stiffness of the VSEA pad and forming a shape-locking effect, which causes the VSEA pad to “lock” firmly on the surface and resists peeling. A greater applied force is needed to peel the VSEA pad from the surface than the soft pad. For analysis, we assume that the pad is continuous, homogeneous, and isotropic, and only the normal adhesion force changes as the stiffness changes. This change in force, caused by VS, is represented by the correlation factor δ: ${F_{w i t h}_{V S}}_{f u n c t i o n} = δ {F_{w i t h o u t}_{V S}}_{f u n c t i o n} .$ (5)

When a VSEA pad is pulled up, the measured normal force is the sum of the components of all normal EA force F_ea1 and shear EA force F_ea2 on the substrate surface, as given in Figure 7a–c. The shear EA force between the pad and adhered surface mainly occurs when the pad on a nonplanar surface is about to be lifted. To simplify the analysis, we only analyze half of the symmetrical VSEA pad (the other half is the same). Therefore, the normal force of the VSEA pad with VS function can be expressed as follows:

FIG. 7.

The distribution of EA forces on different surfaces. (a–c) Distribution of EA force on flat, convex and concave surfaces, respectively. (d, e) Analysis of shape-locking depth H on convex and concave surfaces. EA, electroadhesion. Color images are available online.

F_{E A n o r m a l} = δ \sum (F_{e a 1} cos β + F_{e a 2} sin β),

(6)

where β is the angle between F_ea1 and the vertical direction. Specifically, on the planar surface, β = 0, the Equation (6) can be expressed as: $F_{E A n o r m a l} = δ \sum (F_{e a 1}) .$ (7)

On a curved surface, the measured normal force follows Equation (6). We can see from Figure 7a–c that the shear EA forces (for EA pads coated with polymer delectrics, the shear EA force is always greater than the normal EA force¹⁷) occur on curved surfaces, which may help the VSEA pad lock onto the surface better after VS, so that the normal adhesion force of the VSEA pad with VS function increases more on curved surfaces than on flat surfaces.

The curvature of curved surfaces have an negligible impact on the layer jamming performance and the resulting EA forces. In general, the larger the shape-locking depth, the more stable the electroadhesive grip. The shape locking depth H can be obtained by: $H = R (1 - cos (α ∕ 2)),$ (8)

where R is curved surface curvature radius, and α is the central angle radians corresponding to the arc L (which is the length of the contact between the VSEA pad and the curved surface). Because the VSEA pad is considered inextensible, L is constant. As given in Figure 7d–e, combined with the arc-length formula L = αR, H can be expressed as: $H = R (1 - cos (L ∕ 2 R)) .$ (9)

It can be seen from Equation (9) that as R increases, H decreases and the effect of shape locking gradually decreases. This is consistent with the trend as given in Figure 6d and e. At a convex radius of 40 mm, there is a slight decrease in the normal force. This is because the VSEA pad prototype is not completely soft and cannot conform perfectly to convex surfaces with small radii.

VSEA gripper lifting performance evaluation

An object handling system was developed by bonding the designed VSEA gripper (the middle of the VSEA pad is fixed by a pad holder to form the gripper) to the end effector of a 4-DoF robotic arm (Dobot Magician; Yuejiang Technology) to evaluate the comprehensive peeling resistance and force maintaining performance, as given in Figure 8. We first placed the VSEA gripper onto a surface, then applied voltages to the EA component (EA mode) or to both the EA component and VS component (VSEA mode). Finally, the comprehensive performance of the VSEA gripper was evaluated by comparing whether the object will fall when moving from one sorting area (the blue border) to another (the red border) in the two modes. Note that the charging time was 45 s and all the lifted objects used the same operating procedure in this section.

FIG. 8.

Comprehensive performance test platform.

In EA-only mode, the VSEA gripper had difficulty in lifting flat and curved objects. However, in VSEA mode, these objects can be lifted successfully and moved steadily from one sorting area to another. For example, a 70.4 g curved object could not be lifted in EA mode. While in VSEA mode, a 220.4 g curved object (curvature radius of 50 mm) weighing ∼33 times as much as the VSEA pad can be lifted and moved stably from the blue sorting area to the red sorting area. Here the object was lifted to a height of >10 cm and moved 30 cm. In addition, it was held for >30 s during this maneuver.

Convex ABS objects with large and small areas (220.4 g/21 g), concave ABS objects with large and small area (23.5 g/18.3 g), and an ABS sphere with radius of 40 mm (45.2 g) could be readily moved when in VSEA mode (Fig. 9 and Supplementary Video S2). Likewise, the VSEA pad can easily lift a 150 × 110 mm acrylic plate (91.2 g) in VSEA mode, but not in EA mode. This is an effective illustration that the VSEA gripper can form shape locking and maintain the adhesion force on the curved surface to ensure the stability of the movement process. While on the flat surface, only the normal force gain effect was evident.

FIG. 9.

Material handling performance evaluated in VSEA mode on different objects. (a) Lifting of an ABS convex object with a large contact area (220.4 g). (b) Lifting of an ABS concave object with a large contact area (23.5 g). (c) Lifting of an ABS sphere with a radius of 40 mm (45.2 g). (d) Lifting of an ABS convex object with a small contact area (21 g). (e) Lifting of ABS concave object with a small contact area (18.3 g). (f) Lifting of an acrylic plate object with a large contact area (91.2 g).

Conclusion and Future Work

Soft EA is a promising method to adhere to curved surfaces and delicate objects owing to its flexibility, conformability, and safe environmental interaction. However, soft EAs suffer from peeling issues and have limited mechanical support. Here we mitigate the peeling issue by changing the structural stiffness of the EA pad, thereby locking its morphology to match that of the surface. We have proposed the VS soft EA concept and developed a VSEA solution through the novel combination of an electrostatic layer jamming mechanism and a soft EA in a monolithic and all-electric controllable structure.

We have demonstrated that: (1)

The VSEA can achieve rapid (within 1 s) stiffness change, and the stiffness in the VS state at 4 kV was ∼22 times that of the natural state. An empirical model was derived to explore the relationship between the stiffness of the VSEA and voltage.

(2)

The VSEA pad with VS function can significantly reduce the impact of peeling. Under a weight of 70 g, the evaluation factor k was 4.3, indicating that the peeling resistance of the pad with VS function has increased over 4 times.

(3)

The VSEA pads with VS function have 24.2%, 34.8%, and 49.3% greater adhesive forces on flat, convex, and concave surfaces, respectively. The increase in normal adhesive force on curved surfaces is owing to the induced stiffness of the electrostatic lamming layer, which ensures that the VSEA pad produces a shape-locking effect on the surface.

(4)

The desirable peeling resistance and force maintaining performances of the VSEA gripper were demonstrated by lifting and moving a range of curved and flat objects in a sorting task.

Future work will focus on further investigating the influence of the number of layers on the effectiveness of the VSEA structure and exploring alternative materials and geometries.

Footnotes

Author Disclosure Statement

No competing financial interests exist.

Funding Information

R.C., Z.Z., and F.L. gratefully acknowledge the financial support from the National Natural Science Foundation of China (Grant No. 52075051), the Program of International S&T Cooperation (Grant No. 2016YFE0113600), and the Foundation for Sci & Tech Research Project of Chongqing Science & Technology Commission (Grant No. cstc2017rgzn-zdyfX0035).

J.G. thanks for the support from the EPSRC grant, EP/M020460/1, while working at the University of Bristol and the support from J.L. whist working at HITSZ. In addition, J.G. appreciates the Scientific Research Foundation for New Faculties at Harbin Institute of Technology (Shenzhen) (Grant No. 20200198), the Scientific Research Foundation for High-level Talents at Shenzhen (Grant No. ZX20210144), and the National Natural Science Foundation of China (Grant No. 12102106). J.R. is supported by EP/M020460/1, EP/R02961X/1, EP/S026096/1, and the Royal Academy of Engineering through the Chair in Emerging Technologies scheme.

Supplementary Material

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