Impedance Model-based Control for an Electrohydraulic Stewart Platform

In this paper, a novel model-based impedance controller for a 6-degree-of-freedom (dof) electrohydraulic Stewart platform mechanism is developed. Rigid body and experimentally developed electrohydraulic models, including servovalve, friction and leakage models are employed and described by a set of integrated system equations. Despite the fact that an electrohydraulic servoactuator is not a source of force, as is the case with electric actuators, an impedance controller is developed for the electrohydraulic platform. This controller consists of a feedback and a model-based feedforward loop that compute servovalve currents. An impedance filter modifies the desired trajectory according to a specified behavior. This trajectory is fed to a system model in the controller aiming at a reduction of the effects of the nonlinear hydraulic dynamics. Simulations results compare system response of the developed and of a proportional-derivative (PD) controller for the electrohydraulic platform. Results during interactions with the environment show that the impedance controller is superior to available PD controllers, and that its response is smooth.