Optimal Adaptive High-Order Super Twisting Sliding Mode Control of a Lower Limb Exoskeleton Robot

External disturbances and internal uncertainties with an unknown range, as well as the connection between the human body and robot, are major problems in control and stability of exoskeleton robots. In order to deal with disturbances and uncertainties with the known range of the system, the sliding mode controller is used as a robust approach. The chattering phenomenon is one of the drawbacks of sliding mode controller, which boundary layer is employed to reduce the effects of this phenomenon. In this case, not only the chattering phenomenon is not completely eliminated, but the robust characteristics of the controller are mitigated. In this paper, in order to cope with the disturbances and uncertainties with unknown range, and guard against chattering as a key ingredient of excessive energy consumption and convergence rate reduction, optimal adaptive high-order super twisting sliding mode control has been applied. The dynamic model of a lower limb exoskeleton robot is extracted using the Lagrange method in which four actuators on the hip and knee joints of the left and right legs are considered. To achieve optimal performance, controller parameters are determined using Harmony Search algorithm by minimizing an objective function consisting of ITAE and control signal rate. The proposed controller performance is compared with optimal adaptive supper twisting sliding mode and optimal sliding mode controllers which shows the superiority of the optimal adaptive high-order sliding mode controller rather than other designed controllers.