Modeling and Control of Soft Robots Actuated by Dielectric Elastomer

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Over the last decade, soft robots have received tremendous interests for their potential applications. Soft robots mostly refer to robotic systems with soft or compliant structures and muscle-like actuation using soft or highly deformable materials. Unlike conventional robots, they can show high deformation under actuation and change their shapes continuously along their body length, similar to octopus arms, earthworms, and elephant trunks. Recent advances in soft actuation technologies have been favorable to development of soft robotics. Among soft actuation materials, dielectric elastomers (DEs), also known as dielectric electroactive polymers, represent an emerging/innovative class of soft actuation materials which exhibit relatively large deformations when solicited by an electric field. DEs consist of a film of elastic polymeric material covered on both sides by compliant electrodes. When a voltage is applied to the electrodes, the resulting electric field generates a compressive stress that produces a controllable deformation of the film. This deformation can be in some cases one or two orders of magnitude larger than the one obtained by other soft materials. This peculiarity, together with the lightweight, the high energy density, the fast response, and the low costs, makes DEs highly attractive for development of soft robots.On the other hand, robots made wholly or in part from materials that change the shape are more difficult to control than rigid robots. DEs are viscoelastic materials meaning that their stiffness changes with strain rate and frequency. Viscoelasticity complicates the modeling and control of the material response which is often characterized by creep, hysteresis, and stress relaxation phenomena. Owing to these intrinsic nonlinear and time variant characteristics, the precise control of DEs to determine the amount of voltage needed to obtain the deformation for soft robots becomes a non-trivial problem. In particular, hysteresis nonlinearity can significantly degrade actuator performance. However, compensating the hysteresis is a very difficult problem because the vast majority of existing control methods are only applicable to systems that are smooth and differentiable. Stress relaxation is also a significant shortcoming of DEs and can significantly degrade actuator performance over time owing to changes in elastomer thickness. Such time-variant performance characteristics are difficult to model and present challenging control issues. The objective of this research is the development of control theories, engineering design methods and technology for DE actuated soft robots to conform to their surroundings, navigate through unstructured environments, and grasp objects using whole arm manipulation in the presence of model uncertainties.