Accurate Terrain Estimation for Humanoid Robot Based on Disturbance State Observer

This study proposes a novel reactive terrain estimation system for humanoid robots from the perspective of a state observer. The design process is as follows: the underfoot force disturbance is converted into an underfoot position disturbance by using an admittance system, the disturbance state observation is designed with a closed-loop observation method, and finally, the observation state is switched by using a gait planning-based state machine. This study combines a one-step-ahead prediction technique with the algebraic operation of error dynamics, and the designed observer is called a synchronized error predictive observer. The observation error dynamics are analyzed by using the robustness theory to prove that the proposed method can reduce the ultimate bounded range of the observation error and the error in terrain estimation. This study has been validated through simulation and experiment using the UBTECH–Tsinghua WALKER-1 Prototype, which can accurately estimate the terrain height difference and orientation within 0.02 m-height of ground obstacles. The designed observer can effectively improve the accuracy and further reduce the instability that the gait control system may have to withstand.