Terminal Sliding Mode Control of Ankle Movement in Paraplegic Subjects using Functional Electrical Stimulation

Sliding mode control is an effective method for controlling the neuromusculoskeletal systems. A major problem of conventional sliding mode control is exponential convergence of the tracking errors. To solve this problem, researchers proposed a robust control strategy called terminal sliding mode control. The main advantages of the terminal sliding mode control are not only robustness against uncertainties and external disturbances but also finite time convergence of tracking errors.

In this paper, we propose a decentralized control strategy which is based on terminal sliding mode control, for control of the ankle joint in paraplegic subjects using functional electrical stimulation. Agonist-antagonist co-activation is used to control the ankle movement.

The proposed control strategy was employed for control of ankle joint in three paraplegic subjects. The control task was to determine the stimulation pattern in order to converge the ankle movement trajectory to the desired trajectory. The experimental results on three paraplegic subjects showed that the proposed controller provided an excellent tracking control of reference trajectories. It could also generate control signals to compensate the effects of muscle fatigue.

The results of this study showed the proposed control strategy as an effective approach for controlling movements in paraplegic subjects using functional electrical stimulation.