Impedance control of a hydraulic servoactuator joint system is discussed in this paper. Impedance control imparts a desired behavior to a system, rather than it controls position or force individually. Due to nonlinear properties of hydraulic actuators, impedance control is difficult. The control strategy presented here involves a combined feedforward and feedback control. An impedance filter modifies a desired trajectory according to a specified behavior. The modified trajectory is fed to a reduced-order model of the servoactuator hydraulic joint in order to reduce the effects of the nonlinear hydraulic dynamics. Position, velocity and pressure feedback loops compensate for the unmodeled dynamics. Simulation results show the strategy to be promising in providing impedance control to the joint. Special attention is given to the careful choice of impedance and control parameters to ensure smooth transition between contact and non-contact regimes, and to avoid actuator saturation. The developed controller is useful in achieving a desired behavior of hydraulic manipulators in contact tasks. It will provide the basis for a robust impedance control of the SARCOS high-performancehydraulic manipulator.

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