Linear and nonlinear approaches to control of single joint motion by functional electrical stimulation

Experimental and simulation work involving the control of a single joint acted on by electrically stimulated muscle is reviewed. 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. Stimulation 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 approximate effects of muscle fatigue. It is concluded that open-loop and fixed-parameter closed-loop controllers are inadequate for precision control of functional electrical stimulation (FES) systems.