A method for implementing the ankle and hip balance control strategies, well known from studies on human balance control, is suggested. The moment of the acting disturbance force is evaluated continuously in real time via the difference between the ZMP and the ground projection of the center of mass. Compliant response to the continuous disturbance is ensured by attaching a virtual spring-damper in an appropriate way for each strategy. Further on, whenever the limit of the ankle strategy is reached, a smooth transition toward the hip strategy is initialized and compliance is ensured in a continuous way and in agreement with the disturbance. After the disturbance is removed, the humanoid switches first back to the ankle strategy, and then returns to the initial equilibrium (erect) posture. Experimental data taken with a small humanoid robot (HOAP-2) are presented to validate the method. See also the accompanying video clip.

The self motion of a kinematically redundant robotic limb with single degree of redundancy is analyzed, focusing thereby on the nonlinear self-motion component. The role of this component has been completely ignored in past studies on self motion. It is shown that Jacobian pseudoinverse-based resolution - the usual resolution method for nonlinear self-motion - yields poor dynamic performance. A special type of self motion is identified based on the natural metric on the self-motion manifold and the energy conservation principle. This type of self motion is shown to have superior dynamics in terms of torque requirement and to avoid abrupt fluctuations in acceleration in the vicinity of singular points.