Dynamic Analyses of Underactuated Virtual Passive Dynamic Walking

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 the problem. It has been empirically known that the effect of convex curve shape of foot, which characterizes passive-dynamic walkers, is important to increase walking speed. This paper then investigates the driving mechanism of compasslike biped robots and the rolling effect of semicircular feet are mainly investigated. We first analyze the mechanism 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 with a flat feet model through linear approximation, and show that the rolling effect is equivalent to its virtual ankle-joint torque. Throughout this paper, we provide novel insights into how ZMP-free robots can generate a dynamic bipedal gait.