Exploiting angular momentum to enhance bipedal center-of-mass control

Abstract

Recent humanoid control investigations have emphasized the importance of controlling whole-body angular momentum throughout a movement task. For typical movement tasks, such as normal walking, such controllers minimize fluctuations in angular momentum about the center of mass (CM). This minimization is consistent with observed behavior of humans for such tasks. However, there are cases where such minimization is not desirable. In this study, we investigate movement tasks where bipedal balance control requires a relaxation of the goal of minimizing whole-body angular moment. We construct a humanoid model having a human-like mass distribution, and a Moment-Exploiting Control algorithm that modulates whole-body angular momentum to enhance CM control. The model only requires reference trajectories for CM position and torso orientation. Joint reference trajectories are not required. While balancing on one leg, we show that the controller is capable of correcting errors in CM state by sacrificing angular postural goals for the swing leg, trunk and head. This prioritization capability provides robustness to significant disturbances, without the need to plan new reference trajectories. We compare the dynamic behavior of our humanoid model to that of human test participants. While standing on one leg, the model, like the human, is shown to reposition its CM just above the stance foot from an initial body state where CM velocity is zero, and the ground CM projection falls outside the foot envelope.