Making a double pendulum walk: from compass- to whip-like gait

The double pendulum is the main mechanical structure on which simple biped robots with compass-like gait are based. Although being very energy efficient, the compass-like gait bears the inherent problems of forcing the swing-leg tip to below-ground position while walking and allowing for almost no terrain irregularities. In this paper, an underactuated biped based on the double pendulum is made to walk via swinging the leg in the direction opposite to that of the compass-like gait. Actuation torque on the stance foot is required to change signs during leg swing in order for the pre- and post-impact states to allow for stable limit-cycle walking. This sign-changing torque is implemented by means of a simple control strategy that generates a whip-like gait and allows the biped to walk over irregular terrain and to admit a broad range of initial conditions. Simulations are given including control tuning guidelines for stable walking.