Compact Electrohydraulic Actuators Generating over 90 N

Soft electrohydraulic actuators are electrostatically-driven transducers in which a high voltage is used to zip together two electrodes patterned on thin polymer films, thus displacing a dielectric fluid that can perform mechanical work. Peano-HASELs are the best-known implementation of this class of actuators. Electro-ribbon actuators use dielectrophoretic liquid zipping to pull together two beams, directly obtaining mechanical work from beam motion, with the dielectric fluid serving to allow operation at higher electric fields than would be possible in air. Increasing the specific energy of both these electrohydraulic configurations can in principle be achieved by making arrays of scaled-down devices. This increase in performance with downscaling has been predicted analytically, but has not been demonstrated experimentally to date.<br/>We report here a monolithic electrostatic actuator that incorporates an array of 240 miniaturized Electro-ribbon actuators in a honeycomb-like structure, operating in a low-viscosity silicone oil bath. At 7.5 kV, the actuator generates over 60 N of force with a 3 mm stroke, and 90 N force with a 0.5 mm stroke, with an actuator volume of only 5 cm x 2 cm x 0.3 cm.<br/>The 240 zipping actuators, configured in 20 rows and 12 columns, are on an 8 mm pitch in the beam direction and a 2 mm pitch in the zipping direction. The beams consist of 50 µm thick aluminized polyimide films bonded with 25 µm thick adhesive. Given the very high forces the device creates, a limiting element is the strength of the adhesive that holds the 40 polyimide layers together.<br/>Packing miniature zipping actuators in dense arrays is a promising path to very high energy and power densities and to high forces. We will discuss the scaling of these high-force electrostatic actuators, address some limitations and present use cases in wearable robotics.<br/><br/>For readers who do not appreciate kV actuators, the second topic we cover in this talk is lowering the actuation voltage in peano-HASELs to below 500 V, by using 14 µm thick dielectric layer of PVDF-TrFE-CTFE. Operating near or below 500 V is appealing as it allows making very low-mass power supplies that the robot can carry. This enabled us to create untethered autonomous swimming robots, only a few mm thick, with a total mass of about 6g, battery included. The locomotion module consists of two independently controlled HASEL actuators, that generate a travelling wave along the 4.5 cm long elastomer fin on each side of the robot. The 30 Hz undulating motion of the fins allows for efficient motion, and very fast rotation. We achieve fast swimming of the untethered robot (5 cm/s forward motion and 195 °/s rotation).<br/>The onboard PCB includes optical sensors (for infrared and visible light), battery, power conversion outputting two bipolar 500 V channels at 100 Hz, and a microcontroller enabling autonomous navigation or decision making. The undulating swimming robot navigates around obstacles, swims through narrow spaces, and can detect moving or stationary light sources and navigate towards them.<br/><br/>Both devices reported here (high force array and low-voltage device) share common challenges in materials and processing. They both also show promise in making soft robots more widely used in wearable and autonomous scenarios.