Underactuated Virtual Passive Dynamic Walking using Rolling Effect of Semicircular Feet

Realization of an energy-efficient and high-speed dynamic walking has come to be one of the main subjects in the research area of robotic biped locomotion, and passive dynamic walking has been widely attracted as a clue to solve this problem. It has been empirically known that the effect of convex curve shape of feet, which characterizes passive-dynamic walkers, is important to increase walking speed. This research consists of two parts and aims to investigate the rolling effect of semicircular feet on dynamic bipedal walking. In this paper (I), driving mechanisms of compass-like biped robots and the rolling effect of semicircular feet are mainly investigated. We first analyze the mechanisms of a planar fully-actuated compass-like biped model to clarify the importance of ankle-joint torque introducing generalized virtual gravity concept. In the second, a planar underactuated biped model with semicircular feet is introduced and we show that virtual passive dynamic walking by hip-joint torque only can be realized based on the rolling effect. We then compare it with the flat-feet model through linear approximation, and show that the rolling effect is equivalent to its virtual ankle-joint torque. Throughout this paper, we give a novel approach to dynamic bipedal walking as ZMP-free robots.