Zero-Torque Control of an Exoskeleton Joint Actuator Using a Disturbance Observer

Variable impedance actuators are widely used in tasks with an interaction between human and machine. In this paper, a mechanical-rotary variable impedance actuator (MeRIA), designed for knee or hip joint of a lower limb exoskeleton, is investigated. In the torque control loop of the MeRIA, a zero-torque controller is designed to achieve a low output impedance. This control strategy is based on a PI-controller and extended by a disturbance observer (DO). To obtain a LTI transfer function for the DO, the parameter-varying model of the actuator is set to a fixed operating point in terms of the stiffness. The purpose of designing the DO is to enhance the performance of the low-output impedance. Experiments show that the interaction torque between machine and human can significantly be reduced when applying the DO.