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