Comment on S. Ahmed, H. Wang, and Y. Tian, “Robust adaptive fractional‐order terminal sliding mode control for lower‐limb exoskeleton,” Asian J. Control, vol. 21, no. 1, pp. 1–10 (2019)

Abstract

In this comment, it is shown that there are some non‐negligible big mistakes in the analyses and stability proof of the proposed controller in the quoted paper, which makes the main results of this paper to be incorrect. The main unavoidable mistakes in the stability analysis of the main theorem (Theorem 1) are stated and some remarks are also mentioned to fix some of them.

A New Fractional-Order Adaptive Sliding-Mode Approach for Fast Finite-Time Control of Human Knee Joint Orthosis with Unknown Dynamic

“…Additionally, a robust adaptive TSMC employing fractional-order surfaces was proposed for trajectory tracking in lower limb exoskeleton systems under perturbations [40]. However, a recent commentary [41] has highlighted significant errors in the stability proof of finite-time stability when perturbations are present. One notable mistake pertains to the inability to prove finite-time stability for the error in the sliding mode of the system (when s=0) given the inherently non-linear KJO system model, uncertainties encompassing human and orthosis parameters, coupled with the challenge of identifying these varying parameters and establishing an accurate dynamic model for the system [41].…”