Reactive stepping to prevent falling for humanoids

This paper proposes a reactive motion controller for a humanoid robot to maintain balance against a large disturbance, by relatively stepping. A reactive step is performed by the robot, so that it reduces the disturbance force. Several problems are addressed: first the motion is designed to ensure the respect of stepping constraints such as a dynamical stability, motion feasibility of the swing leg and so on. Moreover the stepping has to be generated in real-time and to be updated as quick as possible after the disturbance. To overcome these problems, we extend simultaneous the center of gravity (COG) and the zero-moment point (ZMP) planning based on a generic analytical solution of the linear inverted pendulum [8]. The ZMP fluctuation and the modification of foot placement are determined by numerical optimization according to the position and velocity error of the COG due to the disturbance. All these computations are performed at low cost. The proposed method is validated through several simulations.