J. Quintern et al., COMPARISON OF SIMULATION AND EXPERIMENTS OF DIFFERENT CLOSED-LOOP STRATEGIES FOR FUNCTIONAL ELECTRICAL-STIMULATION - EXPERIMENTS IN PARAPLEGICS, Artificial organs, 21(3), 1997, pp. 232-235
Open-loop and closed-loop stimulation of the knee extensors for the co
ntrol of the knee joint angle and torque were tested as a potential ba
sis for more complex functional electrical stimulation (FES) systems t
o be used in human locomotion. The output of the biomechanical simulat
ion model described previously was compared with stimulation experimen
ts in patients with complete thoracic spinal cord injury. Good corresp
ondence between simulation and experiments was obtained under both iso
metric conditions and conditions with a freely swinging shank. For clo
sed-loop control, a simple proportional integral derivative (PID) cont
roller yielded sufficient performance only under isometric conditions,
especially if combined with (linear) feedforward. Because of addition
al nonlinearities of musculotendon and body segmental dynamics, more c
omplex strategies have to be applied to the control of unconstrained m
ovements. To compensate for these nonlinearities, an inverse model was
derived from the direct biomechanical model. This inverse model had s
atisfactory agreement between the measured knee angle and the desired
trajectory already under open-loop conditions. A combination of the in
verse model in the feedforward part of the control loop and a PID cont
roller provided robust and precise control of the knee angle. Further
improvement may be achieved by including elements of spasticity into t
he simulation model and by controlling both agonistic and antagonistic
muscles.