Mechanics of Soft Actuators

Abstract

This dissertation explores the mechanics of three types of soft actuators experimentally, and theoretically: (i) dielectric elastomer transducers; (ii) fluid-driven actuators; (iii) paper actuators. The dissertation develops new control elements for these actuators, which extend their function, and enable the design of devices with entirely new functions. A stretchable, transparent conductor is introduced, which is based on an ionically conducting hydrogel. This conductor can operate at frequencies above 10 kHz, voltages above 10 kV without electrochemical reactions, and has a superior optical transmittance compared to stretchable, electronic conductors of comparable sheet resistance. A transparent dielectric elastomer loudspeaker is demonstrated, that generates sound across the entire audible frequency range (20 Hz to 20 kHz). The acoustic properties of the transparent loudspeaker are characterized inside an impedance tube for low frequency active noise cancellation. A linear model is developed, which describes sound reflection, transmission, and generation by the transparent loudspeaker. In an active noise cancellation experiment with feedforward control, which is based on the linear model, the sound transmission loss across the membrane improves compared to passive sound absorption by a factor of 3. The durability of dielectric elastomer transducers under electromechanical actuation is investigated, when ionic liquids are used as the conductors. Because ionic liquids diffuse into the dielectric membrane, the capacitance of the transducers changes with time, which can be described with a linear diffusion model. The larger the increase in capacitance, the shorter is the lifetime of the transducers. An entirely soft valve for the autonomous control of pneumatic, soft devices is developed. The valve uses the snap-through instability of an approximately hemispherical membrane to switch air-flow through the valve. In a feedback circuit, the hysteresis of the snap-through instability generates oscillations of pressure using air from a source of constant pressure. Its use for soft robots is demonstrated: (i) in a soft gripper, which autonomously grasps objects upon contact; (ii) in a soft earthworm, which advances autonomously using a source of constant pressure. Finally, actuators are developed, which use the hygro-expansion of paper to generate bending motion. The actuators consist of paper with an integrated electrically conducting channel of PEDTO:PSS, which is bonded to an adhesive tape. When a voltage is applied to the conductive channel, resistive heating causes evaporation of water from the paper. The paper contracts, and the actuator bends. The behavior of these actuators is investigated experimentally and theoretically. The use of the actuators is demonstrated in an optical shutter, and in the manipulation of liquids.