Functional electrical stimulation (FES) is an effective rehabilitation tool for gait retraining for individuals suffering from various neurological disorders. Traditionally, FES is only delivered to activate ankle dorsiflexor muscles during the swing phase of the gait to correct "foot drop". Recent research indicates that improved functional ambulation can be achieved by delivering FES to both the plantarflexor and dorsiflexor muscles during gait. Closed-loop electrical stimulation has the potential to yield positive rehabilitative outcomes by enabling accurate and precise limb motions during gait retraining. Naturally, the motion of ankle during gait is an event-driven system combining continuous evolution of the angle between the foot and shank, alternate moving segments of the foot and shank, and alternate activation of the plantarflexor and dorsiflexor muscles. A switched sliding mode based controller is developed to ensure that the ankle tracks a designed or recorded normal trajectory during gait which can be used for gait retraining. Semi-global asymptotic tracking of the hybrid controller is analyzed using multiple Lyapunov functions and the performance is illustrated through simulations.
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