Sliding-Mode Control Composite With Disturbance Observer for Tracking Control of Mismatched Uncertain nDoF Nonlinear Systems

Due to the absence of a direct control signal in the affected states, overcoming mismatched disturbances and/or uncertainties is a difficult task in control of nonlinear systems. Up to now, several alternatives have been developed for such situations. However, there are still some limitations, such as the existence of steady-state errors and prior knowledge requirement for disturbance bounds, which restrict the application of most of the designs. The main goal of this paper is to design a novel disturbance observer scheme for dealing with both matched and mismatched uncertainties of uncertain n-degree-of-freedom (nDoF) nonlinear systems. Perturbations of the control inputs together with the effects of the input saturation are also taken into account. In our scheme, there is no need to know the constant parameters of the input saturation function, as well as the bounds of the uncertain terms. To solve the tracking control problem of the uncertain nDoF nonlinear system, an adaptive nonsmooth sliding-mode controller is proposed. Rigorous mathematic stability analysis is performed to guarantee the correctness of the theoretical designs. The main contribution of this study is that a novel disturbance observer along with a new adaptive terminal sliding-mode control is introduced to assure that the effects of mismatched uncertainties, external disturbances, and control fluctuations with completely unknown parameters and bounds are successfully canceled without any steady-state errors. Simulation results and comparative studies on tracking control of two multi-input multi-output mechatronic systems demonstrate the suitable performance of the introduced adaptive composite controller.