Humanoid Balance Control Using
Contact and Non-Contact Limbs

The ability to balance in single support (while standing on one leg) is an important requirement for walking and other bipedal locomotory tasks. We have developed a control algorithm that provides enhanced flexibility and robustness in the control of balancing while standing on one leg by coordinating the exertion of stance-leg ankle torques with movement of non-contact limbs. Current approaches to balance control generally assume the presence of explicitly specified joint reference trajectories or desired virtual forces and calculations based on static body configurations to derive the necessary actuator torques. The former approach has limited robustness whereas the latter does not account for, or take advantage of, forces that could be produced by body motions independent of ground contact. The controller presented here improves on these limitations through the following key architectural features: a two-stage model-based plant linearization is used to simplify control of abstract variables such as the center of mass location; a quadratic programming formulation is used to determine motion of contact and non-contact limbs useful for achieving control targets while satisfying dynamic balance constraints; and a sliding control framework provides robustness to modeling error. We have tested the controller with a morphologically realistic, 3-dimensional, 18 degree-of-freedom human model serving as the plant. The controller can use less detailed control targets and reject stronger disturbances than previously implemented controllers based on desired virtual forces and static body calculations.