Stability Analysis of Bipedal Walking with Control or Monitoring of the Center of Pressure.

The objective of this study is to analyze the stability of two control strategies for a planar biped robot. The unexpected rotation of the supporting foot is avoided via the control of the center of pressure or CoP. For the simultaneous control of the joints and of the CoP, the system is under-actuated in the sense that the number of inputs is less than the number of outputs. Thus a control strategy developed for planar robot without actuated ankles can be used in this context. The control law is defined in such a way that only the geometric evolution of the biped configuration is controlled, but not the temporal evolution. The temporal evolution during the geometric tracking is completely defined and can be analyzed through the study of a model with one degree of freedom. Simple conditions, which guarantee the existence of a cyclic motion and the convergence toward this motion, are deduced. These results are illustrated with some simulation results. In the first control strategy, the position of the CoP is tracked precisely, in the second one, only the limits on the CoP position are used to speed-up the convergence to the cyclic motion. [ABSTRACT FROM AUTHOR]