Event-Based Force-Sensorless Active Compliance Control with Robust Disturbance Rejection for Hydraulic Quadruped Robots

Achieving high-precision and highly adaptive leg joint control is a formidable challenge for hydraulic quadruped robots, owing to their dynamic uncertainties, nonlinearities, and frequent contact events. This paper proposes an event-based active compliance control strategy for hydraulic quadruped robots, which integrates the extended state observer (ESO) based robust integral of the sign of the error (RISE) inner-loop joint trajectory tracking controller and the event-based admittance outer-loop force controller with no contact force measurement. In the inner loop, ESOs are employed to estimate and compensate for the difficult-to-model dynamic coupling characteristics of the joints, while the RISE controller suppresses the residual disturbance compensation errors, thereby achieving asymptotic tracking performance. In the outer loop, a generalized momentum observer (GMO) compensated by neural networks is adopted to estimate contact forces, avoiding the installation of end-effector force sensors. Meanwhile, a contact detection mechanism is introduced to trigger admittance control upon unintended contact, thereby buffering the contact forces. Demonstrated through simulations, the proposed algorithm enhances joint position tracking accuracy, effectively mitigates impact forces from unintended contact, and improves the stability of robot locomotion.