A Floating-Piston Hydrostatic Linear Actuator and Remote-Direct-Drive 2-DOF Gripper

Dexterous, serial-chain motor-driven robotic arms have high moving mass, since most of the actuators must be located in the arm itself. This necessitates high gear ratios, sacrificing passive compliance, backdrivability, and the capacity for delicate motion. We introduce the concept of a remote direct-drive (RDD) manipulator, in which every motor is located in the base, connected to remote joints via a low-friction hydrostatic transmission. We have designed a new hydrostatic linear actuator with a fully-floating piston; the piston floats within the cylinder using a pair of soft fiber-elastomer rolling-diaphragm seals. This eliminates static friction from seal rubbing and piston/rod misalignment. Actuators were developed with a 20mm bore, weighing 55 grams each with a 400:1 bidirectional strength-to-weight ratio $( + /-230\mathrm {N}$), which drive a 2-DOF manipulator (wrist pitch/finger pinch; 120-degree range-of-motion; 6.6 Nm max grip strength). The gripper is hydrostatically coupled to remotely-located direct-drive/backdrivable brushless electric motors. System hysteresis and friction are 1 percent of full-range force. This low-mass low-friction configuration is of great interest for powered prosthetic hand design, and passively-safe high dynamic range robot arms.