A ZMP feedback Control for Biped Balance and its Application to In-Place Lateral Stepping Motion

Abstract — Many biped robot control schemes adopt a zero moment point (ZMP) criterion, where the motion is initially planned as the positional trajectories such that ZMP stays within the support polygon, while the feedback control of each joint is later applied to follow the planned reference motion. Although this method is powerful, the ZMP is not always controlled in a feedback manner. Namely, when the environment such as the gradient of the ground varies, the planned motion may cause the tumble and so replanning or modification is sometimes required in order to avoid it. With respect to the environmental variations, the ZMP trajectory is invariant in the lateral plane of the biped robot, in which the ZMP moves from the one side to the other and vice versa. From this point of view, we propose a biped control method for the frontal plane motion based on the ZMP position feedback. It does not required the reference motion of the upper body and the motion replanning or modification of the reference motion are free against environmental variation. This method is applied in the in-place stepping motion and the stability of this method is examined analytically as well as by computer simulations. Finally, the effectiveness of this method is demonstrated by the robot experiment with some improvement points. Index Terms — biped robot, stepping motion, Center of pressure, motion planning, adaptive behavior I