Soft compliant gripper for safe manipulation of extremely fragile objects

Using our fingers, we can easily pick up fragile or soft objects—such as an egg, a strawberry, or a water balloon—even with our eyes closed. We instinctively use our sense of touch to apply just the right force to grip and manipulate delicate or deformable objects without breaking them. Yet picking up a raw egg, without either crushing or dropping it, is an incredibly hard task for a robotic hand. This is because robots generally lack a good sense of touch, and they must use complex vision and computational schemes. For this reason, typical robotic grippers are highly specialized and designed to grasp one specific object (in contrast to extremely versatile human hands).

Electroadhesion has previously been used by several groups to develop wall-climbing robots^{1, 2} and flexible grippers.³ The adhesion force is electrically controlled and can therefore be modulated by changing the control voltage. In addition, dielectric elastomer actuators (DEAs) consist of two compliant electrodes on either side of a thin elastomer membrane. When an electric field on the order of 100V/μm (corresponding to a voltage of several kilovolts for typical film thicknesses) is applied, the electrostatic force between the electrodes leads to the out-of-plane compression and in-plane expansion of the electrodes.

In our work, we have thus developed a soft elastomer-based gripper (see Figures 1 and 2), in which we combine these two separate grasping technologies.⁴ Our design represents the first use of a DEA to generate a very gentle bending force and the simultaneous use of electroadhesion for the strong grip (high shear force). Specifically, we harness the in-plane expansion of the DEA to obtain a bending motion in dielectric elastomer minimum energy configuration.⁵ We have previously published a detailed description of the fabrication of our DEA.⁶

Figure 1. The compliant versatile soft gripper holding a strawberry.

Figure 2. Left: The 1.5g gripper conforms to the shape of a 61g egg. Right: A schematic cross section of the gripper, showing the fringing electric field lines (for electroadhesion) and the normal field lines (for dielectric elastomer actuation).

A key novel aspect of our design is the electrode configuration, in which we use two sets of offset interdigitated electrodes. This allows simultaneous optimization of both the electroadhesion force (related to the fringing electric field, as shown in Figure 2) and the electrostatic bending force from the DEA (related to the electric field inside the elastomer). We use a single control voltage for both the bending and the electroadhesion.

Our gripper can automatically adapt its shape to that of the object it is grabbing, by delicately wrapping its two fingers around the object. This gripper can therefore safely manipulate extremely fragile objects, but can also hold objects that are up to 80 times heavier than itself. Furthermore, the design of the gripper (which includes the use of passive adaption) means that it can conform to nearly any shape, without requiring any computation power. External control is simply maintained by turning the actuation voltage on or off. In addition, our gripper senses how it is deformed. It thus combines bending actuation, gripping, and position sensing in one simple element.

The compliant gripper we have developed can also readily pick up objects that would be very challenging for conventional robotic manipulators. For example, our device can grasp and release a soft water-filled balloon (35g), a flat sheet of paper (1g), a Teflon block (80g), and a raw egg (61g). Our multifunctional gripper—which can be seen in action, in a short video available online⁷—opens and closes in about 100ms. We achieve this speed with the use of silicone elastomers and silicone-based electrodes that have very low viscoelastic losses.⁸

In summary, we have developed a novel elastomer-based robotic gripper that combines two grasping technologies (dielectric elastomer actuation and electroadhesion). Our highly integrated conformal gripper can be used to manipulate fragile objects and hold items that are many times heavier than its own weight. It will thus find many applications in different areas of soft robotics. We will further develop this soft gripper for safe interactions with humans, for handling food or textiles, and we will combine the gripper with shape memory polymers or phase change materials to achieve even higher holding forces. Given the comparatively low weight of the gripper, it may even be mounted on a drone and thus allow the drone to perch on a branch or hang upside down like a bat.