Switching control of functional electrical stimulation and motor assist for muscle fatigue compensation

The torque generation capability of muscles often reduces during a functional electrical stimulation (FES) session due to the rapid onset of muscle fatigue. Hybrid rehabilitation systems that use FES and electric motor assist may overcome this issue. The primary control challenge in such a system is how to allocate control inputs between electric motor and FES during muscle fatigue and muscle recovery. One strategy is to switch between FES and the electric motor by using an estimate of the muscle fatigue. This would allow the system to switch from using FES to using the electric motor when the muscle torque output has significantly decreased, then switch back to FES once the muscles have sufficiently recovered. This paper uses a second order sliding mode controller cascaded with a feedback linearization controller for a switched, FES and electric motor, system. The second order sliding mode is achieved through the use of a variable-gain super-twisting algorithm. A Lyapunov stability analysis was used to prove asymptotic stability of the switched control system. Simulations of the developed controller on a hybrid knee extension model illustrate that prolonged knee movements can be elicited through the switched system.