Prescribed performance model-free adaptive terminal sliding mode control for the pneumatic artificial muscles elbow exoskeleton

This paper focuses on the trajectory tracking issue of the pneumatic artificial muscle (PAM) exoskeleton system. First of all, a new type of the PAM elbow exoskeleton was introduced to assist wearers in elbow flexion/extension movement. Moreover, a model-free adaptive control approach was combined with the prescribed performance control to ensure the tracking errors to be converged to the predefined requirements. Meanwhile, to suffer the influence of the unknown external disturbance on the exoskeleton, a terminal sliding mode control was adopted to reduce the tracking errors. From a theoretical perspective, the stability of the proposed controller can be proved by Lyapunov synthesis. After two sets of experiments, the proposed control method can further improve the tracking accuracy in the PAM elbow exoskeleton, compared with the other three model-free adaptive control methods. Simultaneously, the maximum absolute value of the tracking errors never exceeded the designed boundary.