Contact-Robust Whole-Body Control for Torque-Based Humanoid Robots

Abstract

Bipedal humanoid locomotion offers great opportunities for the movement of robots in human environments and interaction with them. It is a challenging control problem for which many different solution approaches have been developed in research in recent years. This work tackles the problem of dynamic modelling errors in the form of constraint violations for inverse dynamics control approaches. A possible constraint violation is contact loss of the foot to the ground, but also other unforeseen disturbances may occur. The introduced methods aim to increase robustness and the probability of a recovery. This work is based on the idea of controlling relative instead of absolute positions, specifically the relative position of the center of mass to the foot (RCF). This solves the problem of uncontrollable degrees of freedom during contact loss when tracking the absolute position is impossible resulting in potentially dangerous controller behavior. Instead, the developed relative controller keeps the posture of the robot in a desirable configuration during contact loss without compromising on the absolute position tracking under nominal foot-ground contact. A passivity based control task is derived for the RCF coordinate. This task is evaluated with different free-floating robot models and embedded into the whole-body control framework of the humanoid robot Toro. Two different additional task formulations are derived for correct tracking of the absolute position reference based on the constrained foot acceleration and the center of mass behavior with foot-ground contact. An approach for implicit orientation control via multiple contact point RCF tasks is introduced. Different passivation approaches are evaluated to suppress short instabilities in the system. The derived controller is analysed in detail in simulation for the humanoid robot and then verified in experiments.