Zero-torque control of an exoskeleton joint actuator using a disturbance observer

Abstract

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.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.