Online Gain Switching Algorithm for Joint Position Control of a Hydraulic Humanoid Robot

This paper proposes a gain switching algorithm for joint position control of a hydraulic humanoid robot. Accurate position control of the lower body is a one of the basic requirements for robust balance and walking control. In general, it is more difficult to perform joint position control of hydraulic humanoid robots than electric humanoid robots because of a slower actuator time constant and the backdrivability of hydraulic joints. Because of the backdrivability, external forces and torques have a large effect on the position of the joints, and external ground reaction forces prevent a simple PD joint controller from realizing accurate joint position control. We propose a state feedback controller for joint position control of the lower body, define three modes of state feedback gains, and switch the gains effectively according to the Zero Moment Point (ZMP) using linear interpolation. The performance of the algorithm is evaluated with a dynamic simulation of a hydraulic humanoid.