Linear and Nonlinear Approaches to Control of Single Joint Motion by Functional Electrical Stimulation

One of the problems facing systems which restore function to paralyzed muscles by artificial electrical stimulation is control of the stimulated muscle. This paper reviews experimental and simulation work in control of a single joint acted on by electrically stimulated muscle. Results for experimental work in control of the free-swinging human shank are presented for open-loop and PD closed-loop controllers as well as for a novel controller which makes use of a controllable friction brake which acted on the joint in combination with the electrical stimulation. Simulation results are presented for computed torque, sliding mode, and adaptive sliding mode controllers. The simulations used a muscle model with a time-varying muscle gain to a approximate effects of muscle fatigue. The paper concludes that open-loop and fixed-parameter closed-loop controllers are inadequate for precision control of FES systems.