A Switched Systems Approach for Closed-loop Control of a Lower-Limb Cable-Driven Exoskeleton

Cable-driven exoskeletons are light-weight devices that can provide joint torques to assist or augment human function during locomotion. However, cable mechanisms actuating joints can experience a slacking behavior, slow response, and counteracting forces, if the electric motors are not accurately controlled. To prevent undesired joint motion, feedforward control strategies have been developed to provide cable tension and release. In this paper, the control design of a pair of electric motors is segregated into a joint-level control loop and a low-level loop to adjust cable tensions and apply torque about the knee joint. A robust sliding-mode controller is designed to track the desired knee joint kinematic trajectory, which is assigned to the lead motor to achieve leg flexion or extension. Concurrently, the low-level control objective is designed to adjust the tension of the other motor, called the follower motor, in proportion to the angular position of the lead motor. To achieve leg extension and flexion, the electric motors switch their roles between lead and follower motor. A Lyapunov-based stability analysis is developed to ensure exponential tracking for both control objectives. Moreover, an average dwell time analysis computes an upper bound on the number of motor switches to preserve exponential tracking.