Joint force control of parallel robot for ankle rehabilitation

Impedance control is a common interaction control strategy used in rehabilitation robots. A joint force controller has been developed for a redundantly actuated parallel robot to facilitate force based impedance control. The proposed controller utilizes only position/orientation dependent information and therefore does not require accurate dynamic modeling and measurement of velocity quantities. The proposed controller uses a disturbance observer based approach and is designed for situations where the external forces applied on the robot are dominant compared to the inertial terms in the robot dynamics. Since the robot used is redundantly actuated, control of forces along the null space of the manipulator Jacobian transpose was also considered. Rationale behind the structure of the controller is developed throughout the paper and experimental results are also presented.