ABSTRACT
This chapter addresses the direct control of the Foot Rotation Indicator (FRI) point in the context of the tools associated with the hybrid zero dynamics framework. In particular, control of the FRI point is achieved along with a guarantee of the existence and exponential stability of a periodic walking motion. In order for the supporting foot to remain flat on the ground, the FRI point must never reach the limits of the convex hull of the foot support region. Direct control of the position of the FRI point is a way to prevent this from occurring. When controlling the position of the FRI point during the fully actuated phase, it is straightforward to use the angular momentum about the FRI point in order to study the zero dynamics. The evolution of the FRI point throughout the step affects the existence and stability of the periodic motion obtained with the proposed control law.
