THE DEVELOPMENT OF A MODEL-REFERENCE ADAPTIVE CONTROLLER TO CONTROL THE KNEE-JOINT OF PARAPLEGICS

Functional neuromuscular stimulation (FNS) is a technique for restoring motor function by directly activating paralyzed muscles. This paper is concerned with the design and development of closed-loop controllers that, when used in a stimulation system, achieve regulated, repeatable control of the lower limb joints. Models describing the dynamic behavior of the unloaded lower limb joints under electrical stimulation were developed. These models consist of a nonlinear part followed by linear dynamics described by deterministic autoregressive moving average (ARMA) models. These models were used in the design of an adaptive controller to control the movement of the leg joints. Design requirements, including a severe constraint on the control rate so as not to excite spastic reflexes, were formulated and a model reference adaptive controller (MRAC) design was modified and implemented. It was found that to overcome the gain nonlinearity in the plant, the parameter estimator had to be continually switched on and off depending on the level of the control signal. The control rate was effectively governed by severely limiting the closed-loop bandwidth through appropriate choices of reference model parameters.