Comparison of simulation and experiments of different closed-loop strategies for functional electrical stimulation:...

Open-loop and closed-loop stimulation of the knee extensors for the control of the knee joint angle and torque were tested as a potential basis for more complex functional electrical stimulation (FES) systems to be used in human locomotion. The output of the biomechanical simulation model described previously was compared with stimulation experiments in patients with complete thoracic spinal cord injury. Good correspondence between simulation and experiments was obtained under both isometric conditions and conditions with a freely swinging shank. For closed-loop control, a simple proportional integral derivative (PID) controller yielded sufficient performance only under isometric conditions, especially if combined with (linear) feedforward. Because of additional nonlinearities of musculotendon and body segmental dynamics, more complex strategies have to be applied to the control of unconstrained movements. To compensate for these nonlinearities, an inverse model was derived from the direct biomechanical model. This inverse model had satisfactory agreement between the measured knee angle and the desired trajectory already under open-loop conditions. A combination of the inverse model in the feedforward part of the control loop and a PID controller provided robust and precise control of the knee angle. Further improvement may be achieved by including elements of spasticity into the simulation model and by controlling both agonistic and antagonistic muscles.