Energy and torque efficient ZMP-based bipedal walking with varying center of mass height

Researchers have implemented ZMP equations to realize biped walking. Many of them have introduced constant center of mass during walking, mainly to solve the ZMP equations easily for X and Y center of mass trajectories as the center of mass acceleration in the z-direction becomes zero. This paper discusses about the affect of varying center of mass height in biped walking. There are two cases shown, case 1: where the center of mass height is constant and case 2: where the center of mass height is varying while the pelvis is fixed to be a constant. Different kinematics methods are used for both cases. For simplicity of case 2, the center of mass acceleration in the z-axis is assumed to be zero. A walking simulation of 0.22km/h is used to make comparison between both cases. In the simulation results, case 2 gives less rms torque value at most of the leg joints servo systems. Simulation results also show that case 2 consumed about 20% less energy compared to case 1 total energy at all joints.