Adaptive terminal sliding mode control of ankle movement using functional electrical stimulation of agonist-antagonist muscles

This paper presents a robust control strategy which is based on synergistic combination of an adaptive controller with terminal sliding mode control (TSMC) for online control of ankle movement using functional electrical stimulation (FES) of dorsiflexor and plantar flexor muscles in paraplegic subjects. The major advantage of TSMC derives from the property of robustness to system uncertainties and external disturbances with fast convergence without imposing strong control force. To implement TSMC, a model of neuromusculoskeletal system should be presented in standard canonical form. In this work, we design an adaptive updating law to estimate the parameters of the model during online control without requiring offline learning phase. The experimental results on two paraplegic subjects show that the TSMC provides excellent tracking control for different reference trajectories and could generate control signals to compensate the effects of muscle fatigue and external disturbance.