In this paper, a quadrupedal quasi-passive dynamic walking robot driven by a rocking motion has been investigated. Experimental results show that the robot can walk on the level ground and that a walking speed is related to a period and a phase difference of the rocking motion. Through investigation of a shape of soles, we revealed Duke has a nonholonomic constraint that is comparable to that of a kinematic model of “two-wheeled mobile robot” on its sole. As a result of analyses based on the nonlinear control theory, we conclude that the sole shape contributes to a transition of walking speed accompanying with change of the phase difference.
