Extended Sliding Mode Observer-Based Admittance Control for Hydraulic Robots

In this letter, an admittance control algorithm has been proposed for hydraulic robots in the presence of model uncertainties, external disturbances, and system nonlinearities without contact force sensors. An extended sliding mode observer (ESMO) is firstly developed based on joint force sensors and pressure signals in chambers of hydraulic actuators to estimate not only the contact force with an environment but also the joint velocities which are robust to unknown friction in hydraulic actuators. Matched uncertainties and disturbances in actuator dynamics are observed by a matched disturbance observer. These techniques are integrated into the backstepping framework to handle high nonlinearities in the hydraulic and mechanical systems and achieve the desired admittance behavior of the robotics system. Simulation results are conducted with a two-degree-of-freedom (2-DOF) hydraulic leg robot with different case studies, which verifies the effectiveness of the proposed control method.