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 compass-like 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.