Adaptive impedance control for the hydraulic manipulator under the uncertain environment

The contact control of the n-DOF hydraulic manipulators is challenged by the uncertain environment with unknown stiffness and location. Only by a traditional impedance control can obtain satisfying force/position tracking performance. To this end, adaptive impedance control with force sensor-less is presented for the n-DOF hydraulic manipulator. The overall control scheme includes two loops: the inner loop replans a nonlinear model-based controller towards the end-effector position to compensate for the nonlinear dynamic characteristics of the hydraulic manipulator, such that a high-precision trajectory tracking can be achieved; in the outer loop, a force error compensation based on model reference, adaptive control is added to the classical position-based impedance control, such that the impedance control performance can be accommodated to the uncertain environment. The estimation of the end-effector force is designed in terms of the inverse dynamic model of the manipulator, together with the pressure feedback of the hydraulic drive system. The experimental results show that compared with classical impedance control, the proposed approach improves the force tracking accuracy by approximately 50% and has higher stability when the environment stiffness and location change.