This paper presents a control law for the tracking of a cyclic reference trajectory by an under actuated biped robot. The robot studied is a five-link planar robot. The degree of under actuation is one during the single support phase. The control law is defined in such a way that only the geometric evolution of the robot is controlled, not the temporal evolution. To achieve this objective, we consider a time-scaling control. For a given reference path, the temporal evolution during the geometric tracking is completely defined and can be analyzed through the study of the dynamic model. Continue reading →