Fractional-Order Prescribed Performance Sliding-Mode Control With Time-Delay Estimation for Wearable Exoskeletons

Abstract

Wearable exoskeletons can help people with spinal cord injuries regain mobility. For the control of wearable exoskeletons, transient performance and steady-state performance are the important indices to be considered. In this paper, a model-free fractional-order prescribed performance sliding mode control scheme is proposed for the joint angle tracking control. Time delay control is developed based on the ultra-local model of the wearable exoskeleton to form the model-free feature. Afterward, a novel exponential-type barrier Lyapunov function is designed to ensure the prescribed transient performance on the tracking errors. Then, a fractional-order sliding mode surface is introduced, based on the exponential-type barrier Lyapunov function and fractional-order sliding mode surface, a fixed-time sliding mode controller is designed to further enhance the control performance. Benefiting from the above methods, the proposed controller is model-free and has fixed-time convergence with prescribed performance. In addition, the adjustable range of the parameters is enlarged by the fractional-order sliding mode surface. Stability is analyzed based on the Lyapunov theory. Finally, experiments are implemented in the wearable exoskeleton experimental platform, experiment results demonstrate the effectiveness of the proposed scheme.