Model-free high-order terminal sliding mode controller for Lipschitz nonlinear systems. Implemented on Exoped® exoskeleton robot

Abstract

In this paper, a model-free high-order terminal sliding mode controller (TSMC) is developed for single input–single output Lipschitz nonlinear systems in presence of external disturbances. The proposed method is data-driven, i.e. it is based on online input and output information. In another word, the method is not model-based and can be applied to systems with unknown dynamics. The proposed method employs a high-order switching surface to mitigate the chattering problem. The disturbance estimation technique is also applied to overcome external disturbances. Rigorous theoretical analysis is carried out to verify convergence of states to the switching surface. To demonstrate the effectiveness and applicability of the proposed approach in different situations, several simulations are provided in addition to experimental studies. Experimental tests are implemented on an exoskeleton robot which follows the human gait trajectory in normal and uncertain conditions. In all studies, the results of the proposed method are compared with model free Linear Sliding Mode Control. It is shown that the proposed method has a better performance.