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

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 article, 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 ultralocal model of the wearable exoskeleton to form the model-free feature. Afterward, a novel exponential-type barrier Lyapunov function is de ed 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 abovementioned methods, the proposed controller is model-free and has fixed-time convergence with the 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.