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Powered lower-limb exoskeletons provide assistive torques to coordinate limbmotion during walking in individuals with movement disorders. Advances insensing and actuation have improved the wearability and portability ofstate-of-the-art exoskeletons for walking. Cable-driven exoskeletons offloadthe actuators away from the user, thus rendering light-weight devices tofacilitate locomotion training. However, cable-driven mechanisms experience aslacking behavior if tension is not accurately controlled. Moreover,counteracting forces can arise between the agonist and antagonist motorsyielding undesired joint motion. In this paper, the strategy is to develop twocontrol layers to improve the performance of a cable-driven exoskeleton. First,a joint tracking controller is designed using a high-gain robust approach totrack desired knee and hip trajectories. Second, a motor synchronizationobjective is developed to mitigate the effects of cable slacking for a pair ofelectric motors that actuate each joint. A sliding-mode robust controller isdesigned for the motor synchronization objective. A Lyapunov-based stabilityanalysis is developed to guarantee a uniformly ultimately bounded result forjoint tracking and exponential tracking for the motor synchronizationobjective. Moreover, an average dwell time analysis provides a bound on thenumber of motor switches when allocating the control between motors thatactuate each joint. An experimental result with an able-bodied individualillustrates the feasibility of the developed control methods.

Closed-loop Control Design and Motor Allocation for a Lower-limb Cable-driven Exoskeleton: A Switched Systems Approach