Position-based impedance control of an industrial hydraulic manipulator

This article addresses the problem of impedance control in hydraulic manipulators. Whereas most impedance and hybrid force/position control formulations have focused on electrically driven robots with controllable actuator torques, torque control of hydraulic actuators is a difficult task. A position-based impedance controller (PBIC) is proposed and demonstrated on an existing industrial hydraulic robot (a Unimate MKII-2000). A nonlinear proportional-integral (NPI) controller is first developed to meet the accurate positioning requirements of this impedance control formulation. The NPI controller is shown to make the manipulator match a range of second-order target impedances. Various experiments in free space and in environmental contact, including a simple impedance modulation experiment, demonstrate the feasibility and the promise of the technique. Finally, explanation of an experimentally observed behaviour is offered, suggesting a basic limitation to the implementation of impedance control.