Feedforward-feedback P-PID control of elbow joint angle for functional electrical stimulation: A simulation study

Functional electrical stimulation (FES) is the most commonly used system for restoring functions after spinal cord injury (SCI). In this study, we used a combined feedforward-feedback PID controller for regulating the elbow joint angle. Feedforward controller is unable to make corrections if the actual angle deviates from the desired angle. Feedback controller monitors the system output, so it can correct the error. So, a combination of feedforward and feedback control is the prefered control method in FES system designs. The controller was tuned using a model contains one joint, two links with one degree of freedom and two muscles in the sagital plane which simulated in MATLAB using SimMechanics and Simulink Toolboxes. The muscle model is based on Zajac musculotendon model. At First, the P-PID coefficients were tuned with error and effort, and then a genetic algorithm (GA) was used to optimize them. The controller optimized by GA is much better in terms of the rise time and settling time and has no overshoot and oscillation but the classical tuned controller response has a big overshoot and oscillations before getting the desired angle and has much less rise time and settling time. A comparison between the results of suggested controlling method and the results of genetic-PID feedback controller has been done. The results show that however the feedforward-feedback controller is better in terms of the rise time and the settling time, the feedback controller has less overshoot.