Underactuated Four-Fingered Hand with Five Electro Hydrostatic Actuators in Cluster

Abstract

Hands of robots such as humanoid robots commonly suffer from fragility, insufficient force or dexterity, and large weight or size. Though there are a lot of works on robot hands and prostheses, there still exists intrinsic fragility in the gear reducing mechanism. For heavy duty which are needed in field or rough terrain, we developed a hydrostatically actuated anthropomorphic hand. The hand is specifically designed for the humanoid robot HYDRA, whose 30 joints (excluding the hand) are driven by 32 backdrivable electro-hydrostatic actuators (EHA). The hand has four fingers with a total of five dof. Each finger has three joints underactuated by one tendon. Opposition/reposition of the thumb joint is also driven by one tendon. The five tendons are pulled by a miniature linear cluster EHA mounted in the forearm. Its 300 N nominal tendon force generates 1.5 Nm joint torque on each of the finger joints. The cluster EHA consists of a light weight tie-rod cylinder cluster with five pistons, and five low friction trochoid pumps with a crescent separator. Design of the cylinder and pump, with results of evaluation experiments is shown in this paper. Forearm structure with a mechanism to measure the tendon tension, low friction tendon routing in the forearm, low friction wire guiding link for the wrist, and parallel link wrist driving mechanism with two EHAs are also described.