Abstract
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.
Highlights
UBTECH and Tsinghua University jointly developed WALKER-1, a humanoid robot for home services, and its joint actuator selects a position control scheme
Humanoid robots can be regarded as floating-based robotic arms with a bipedal walking design, suitable for various human living environments, making home services the highest-demand application scenario
For the robot to be stable enough to serve in home scenarios, the gait stability problem of walking on uneven terrain needs to be overcome
Summary
2) For the design of the disturbance state observer, the onestep-ahead prediction technique and the algebraic operation of the error dynamics are combined, so the designed observer is called a synchronized error predictive observer. 3) This study compares the performance of different terrain estimation methods in the Webot simulation and validates the proposed synchronized error predictive observer on a WALKER-1 prototype. A dynamic relationship exists between the reference and actual positions of the robot leg; the leg is modeled as an impedance system (Fig. 3(b)), where f is the output state of the impedance system, imp and are the parameters of the actual robot system that can be estimated by experimental measurements, and ΔĤf is the input position of the impedance system (i.e., the observation state that needs to be reconstructed).
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