System identification and control of the ankle joint with functional electrical stimulation

The control of the ankle joint induced by functional electrical stimulation (FES) is especially for paraplegic patients of high interest as it allows to partly re-gain the functionality of the lower limb. For this reason, we developed a Simulink model of the ankle joint which describes the dynamic behavior regarding the activation and force development of the three muscle groups of the lower leg. System identification showed that the muscle’s behaviour is highly non linear and strongly differs with the condition of the subject. The overall system behavior (black box modelling) was identified via tests on individual subjects on a mechanical test bench. Two control approaches, a PID and an iterative learning controller (ILC), were designed and evaluated