Control of a 7-DOF Exoskeleton for Upper-Limb Rehabilitation using a Fast Terminal Super-Twisting

This paper tackles both the singularity problem in fast terminal variable structure control and the problem of accurate tracking for an exoskeleton robotic system suffering from uncertainties. Indeed, this work presents a fast terminal super-twisting with new stability conditions. Firstly, a new power based on the sign of the tracking error variable is used in the fast terminal switching surface to bypass the problem of singularity. Secondly, for the problem of high accuracy tracking, the controller based on standard super-twisting will be designed with a new stability condition that ensures a finite-time stability of the proposed nonlinear switching function to zero. Besides the fact that the proposed method allows the rejection of the un-certain dynamics and perturbations, it also allows the reduction of the chattering. The proposed control technique is applied in simulation and in real-time on a three-link ANAT robot and on a redundant 7-DOF upper-limb wearable robot, respectively. The results of the simulations and the practical implementation are exposed to reveal the efficiency of the developed method.