Self-tuning PID controller for lower limb FES with nonlinear compensation

An adaptive control system with nonlinear compensation of the muscle recruitment curve is presented. A linear forgetting algorithm is implemented and tuning is switched according to changes in the control output to avoid wrong parameter estimations in absence of suitable excitation. Experimental results for the control of the free swinging knee joint of able bodied persons are presented, showing that combining the nonlinear correction with a tuned adaptation allows to obtain clearly better results than using linear fixed or adaptive schemes.