Compliant Biped Locomotion of Hydra, an Electro-Hydrostatically Driven Humanoid

Abstract

The backdrivability of joints is a critical requirement for the robots that perform tasks in uncertain environments. While series elastic actuators are intrinsically backdrivable, their control bandwidth is limited by the low resonant frequency of the elastic component. To simultaneously realize both of the backdrivability and high control bandwidth, Electro-Hydrostatic Actuator (EHA) is a solution. Based on this idea, we developed the fully electro-hydrostatically driven humanoid robot Hydra, while its evaluation was limited to the joint level one. In this paper, we present evaluations of its whole-body control performance, including locomotion. This is the first time to report a bipedal locomotion by an EHA driven humanoid. We first confirm that Hydra can realize a position feedback control with enough stiffness to realize a position control based locomotion. Secondly, we show that the joint backdrivability can suppress the effect of a disturbance applied to the distal part of the robot on the whole-body motion. As the result, we realized a torque control based locomotion with both a proper COM stabilization and nulls pace compliance.