DISTURBANCE OBSERVER-BASED CHATTERING-FREE TERMINAL SLIDING MODE CONTROL FOR NONLINEAR SYSTEMS SUBJECT TO MATCHED AND MISMATCHED DISTURBANCES

Abstract

In a class of nonlinear systems that is susceptible to both matched and mismatched disturbances, the working performance can be greatly influenced and deteriorated if these disturbances are not considered in controller design. While matched disturbances enter the system in the same channel as the control input, mismatched disturbances act in a different channel and are more difficult to deal with. This work proposes a finite-time disturbance observer-based terminal sliding mode control for a class of nonlinear systems subject to both matched and mismatched disturbances. The proposed control algorithm is first developed to stabilizing a class of second-order nonlinear systems subject to both matched and mismatched disturbances. This scheme employs a nonlinear finite-time disturbance observer to estimate mismatched disturbances fast and accurately. The result obtained from the observer is then utilized in the controller design to suppress the influence of the mismatched disturbances. To overcome the nonlinear characteristics, unmodeled dynamics, parameter uncertainties and matched disturbances, a terminal sliding mode control scheme is applied for its finite-time convergence and robustness against disturbances. The structure of a chattering-free full-order terminal sliding mode control is used to design the control algorithm to alleviate chattering and singularity phenomena in the control signal. Later in this dissertation, the proposed control algorithm is extended to the nth-order nonlinear systems. Also, besides stabilizing, the control algorithm is modified and then utilized to address the problem of tracking control. Stability analyses are provided after the mathematical formulation to support the controller design. Four different study cases are carried out to verify the effectiveness of the proposed control algorithm. The first numerical simulation is given to assess and discuss the performance of the proposed controller theoretically. The second simulation study on controlling an electro hydrostatic actuator system partly reflects the practicality of the proposed controller. In the first two simulation studies, the proposed control algorithm is compared with a conventional terminal sliding mode control and an extended-state-observer-based sliding mode control for better performance evaluation. The third study is a task of controlling a bidirectional DC-DC converter, in which the proposed controller is used to suppress the influence of disturbances and produce a desired output voltage value. The final study is carried out in an experimental setup, where the proposed control algorithm was implemented to solve the problem of displacement tracking control for an electro hydrostatic actuator system. PID controller is also employed in this case study for comparison. Throughout the studies, the proposed control algorithm has proved its effectiveness in stabilizing and tracking control for different systems that exhibit both matched and mismatched disturbances. However, more rigorous experiments should be conducted to validate the superiority over other control algorithms.