Balance control analysis of humanoid robot based on ZMP feedback control

Balance control analysis of humanoid robot based on Zero Moment Point (ZMP) feedback control is presented. ZMP is mostly used as standard evaluation of stability of humanoid robot, and balance control is conducted by controlling ZMP position so that it is always in convex hull of the foot-support area. To simplify the design of controller, it is mainly used as one mass inverted pendulum model which represents lower body of humanoid robot model. However this model causes system to become non-minimum phase and performance limitation of the system occurrs, because of the presence of Waterbed effect in frequency domain and unavoidable undershoot in time domain. This paper proposes ZMP feedback control using two masses inverted pendulum model which represents lower and upper body of humanoid robot and creates minimum phase system. The design of the controller based on the proposed model using linear quadratic by considering output is described and confirmed using simulation.