Disturbance Rejection For Nonlinear Time-Delay Systems Affected By Persistent Disturbances

Almost all practical control systems are often affected by various external disturbances, for example, wind or wave forces to ship steering control and offshore structures vibration control systems, load disturbances to electric power systems, wind shear to aircraft flight control, vibration in industrial mechanism control systems, and so on. The external disturbances not only can make excursion of the system working point, but also can deteriorate the performance of the control system. Therefore it is significant to design controller to eliminate or attenuate the external disturbances.With the development of manufacturing and science technology, the control performance and precision are required to be improved correspondingly in modern industrial control systems. The conventional linear feedback control no longer can satisfy the requirements of the practical control system because most of the control systems are nonlinear. Moreover it is noticeable that the nonlinearity is complicated and various. So it is impossible to find a uniform approach to solve the nonlinear control problems. Therefore this kind of problem has been a research hotspot in the control field.Time-delay phenomena exist extensively in industrial processes control, biologic systems, network control systems, etc. The inherent time-delay of the research plant can deteriorate the control performance even can cause the systems to be unstable, which brings great difficulty to the system analysis and synthesis. For nonlinear time-delay systems subjected to persistent disturbances, few results are obtained because of the complexity of the system and the design of controller. Therefore the disturbance rejection for nonlinear time-delay systems with persistent disturbances is of significant both in theory and application.The dissertation first reviews the history of the development in disturbance rejection for nonlinear time-delay systems. The latest research tendency and the main methods are also introduced. The major results are shown as follows.1. The conception of zero steady-state error control (ZSSEC) for control systems with disturbances is introduced. And the essence of classical ZSSEC and internal model principle (IMP) is analyzed in frequency field.2. Based on the IMP and sensitivity approach, disturbance rejection problem for time-delay systems with persistent disturbances is considered. According to the dynamics of the disturbance, a disturbance servo compensator is constructed based on IMP. An augmented system is formed by the disturbance servo compensator in series with the original system. This compensator is independent of the control system model and only relates to the unstable model of the disturbance. And the disturbance needn't to satisfy the order matching condition. Then, a disturbance rejection controller is obtained by designing an optimal control law for the augmented system without disturbances. By introducing a sensitivity parameterεand extending the variables of the systems to Maclaurin series atε= 0, the two-point boundary value (TPBV) problem including time-delay, time-advance and coupled terms caused by the optimal control problem is reduced to a series of discoupled TPBV problems without delay or advance terms. By solving the TPBV problem sequence recursively, the optimal control law for the augmented system is obtained. Simulation examples show that the presented algorithm is effective to reject the disturbance.3. Disturbance rejection problem for time-delay large-scale systems is studied. Based on IMP and sensitivity approach, the design of the disturbance servo compensator, the approximate solution of the coupled TPBV problem including time-delay terms and time-advance terms, and the design procedure of the controller, are discussed respectively.4. Disturbance rejection problem for a class of nonlinear systems affected by disturbances is discussed. Based on the IMP, the disturbance internal model is suitably replicated in the feedback path of the closed-loop system to completely eliminate the influence of the disturbances. Then by using the sensitivity approach, the nonlinear TPBV problem caused by the optimal control problem for nonlinear system with quadratic performance index is reduced to a series of linear TPBV problems. A suboptimal control law is obtained by solving the linear TPBV problem sequence recursively. The algorithm only requires the iteration of vector equations. The account of computation and the memory required is low. The suboptimal control law obtained is composed of accurate linear state feedback term, internal model compensation term and nonlinear compensation term which is a series sum of the adjoint vectors. Where the state feedback term can stabilize the system, the internal model compensation term eliminates the influence of the disturbances, and the last one compensates the influence of the nonlinear. Finally, a simulation example shows the effectiveness of the presented algorithm. 5 Disturbance rejection problem for a class of nonlinear time-delay systems affected by disturbances is considered. A disturbance rejection control law is obtained by extending the above approach to a class of nonlinear time-delay systems. By introducing a compensate term for time-delay and nonlinear terms which are considered as the perturbations of the system, a control law for the series system is obtained. The obtained control law acts as an optimal control law when the system is not affected by disturbances. It can eliminate the fluence of the disturbance when there exists disturbances. A simulation shows the effectiveness of the algorithm and the influence of the time-delay.6. Ship steering control affected by disturbances due to wind and wave load is investigated by using the presented algorithm of this dissertation.7. The main work of this dissertation is summarized and the research work in future is prospected.