A ZMP control of a powered passive dynamic walking robot

This paper presents a method for controlling the zero moment point (ZMP) of walking robots by using spring-mass systems mounted on the robot body. A pair of oscillating spring-mass systems makes a swing motion of the ZMP via periodic change of the spring forces. In contrast to usual methods, where ZMP is controlled through motion planning in terms of joint trajectories, the proposed method is not a motion planning based method. The ZMP is controlled without being associated with position change of the center of mass (COM). The advantage of our method is that a periodic ZMP motion in lateral direction is generated at low cost, without a complicated motion planning, nor position control via accurate joint angle control. Oscillating the spring-mass systems at the resonance frequency, the mechanical energy is effectively stored in the system. As a result, power of the motor can be highly reduced compared with usual joint servo actuation robots. The proposed mechanism is applied to a powered passive walking robot and several experimental walking results of ZMP and COM trajectories are demonstrated.