Robust sliding-mode control of nine-link biped robot walking

A nine-link planar biped robot model is considered which, in addition to the main links (i.e., legs, thighs and trunk), includes a two-segment foot. First, a continuous walking pattern of the biped on a Rat terrain is synthesized, and the corresponding desired trajectories of the robot joints are calculated. Next, the kinematic and dynamic equations that describe its locomotion during the various walking phases are briefly presented. Finally, a nonlinear robust control approach is followed, motivated by the fact that the control which has to guarantee the stability of the biped robot must take into account its exact nonlinear dynamics. However, an accurate model of the biped robot is not available in practice, due to the existence of uncertainties of various kinds such as unmodeled dynamics and parameter inaccuracies. Therefore, under the assumption that the estimation error on the unknown (probably time-varying) parameters is bounded by a given function, a sliding-mode controller is applied, which provides a successful way to preserve stability and achieve good performance, despite the presence of strong modeling imprecisions or uncertainties. The paper includes a set of representative simulation results that demonstrate the very good behavior of the sliding-mode robust biped controller.