Study On Several Control Methods For Time Delay Systems

The phenomenon of time delay is ubiquitous in the area of process control, and the study of control for time delay systems has been a hot and challenging topic for researchers from control community. PID control has been most widely applied in the field of process control due to its simple structure, good robustness and ease of operation. PID control for time delay systems has been developing towards further development and improvement. Many advanced control schemes can be incorporated into the PID structure, leading to a system with superior system performance in combination with PID structure for ease of application, among which IMC-PID is a typically representative scheme.The ability to reject disturbance for closed-loop system is an important index to measure its performance in process control. Disturbance observer can effectively compensate for adverse effects on closed-loop system caused by unmeasurable disturbances and unmodeled dynamics.Owing to its structural constraints, conventional PID controller cannot be used in some area, where better performance and robustness are demanded for the system. Fractional order PID controller, however, is undoubtedly an alternative control scheme, which is a generalization to conventional PID controller. Fractional order PID controller has more freedom than conventional PID controller and thus can further improve the performance and robustness of closed-loop system.The main works of this article are as follows:An overview about PID control in recent years is given in Chapter 1. Chapter 2 focuses on IMC-PID controller based on mismatched model. The IMC controller is designed based on mismatched models which are obtained through relay identification. The designed internal model controller always has simple structure regardless of the order of processes. And the corresponding analytical expressions of IMC-PID are also deduced through the use of Pade and Taylor formula for ease of application.In Chapter 3, a modified disturbance observer is proposed for non-minimum and dead-time processes. The adverse effects of time delay on systems can be compensated for effectively by utilizing a phase lead compensator.Tuning methods of two kinds of fractional order I?I? controllers are proposed for the first order plus time delay (FOPTD) system in Chapter 4. Robust performance indices in frequency domain are utilized to design controllers, and a graphical method is used to solve nonlinear equations to obtain tuning parameters, which effectively avoids the problem of complex nonlinear numerical optimization. The designed controllers in this paper have one more parameter than conventional PI controller, which can be utilized to improve the robustness to parameter variation of the system.In Chapter 5, a tuning method of fractional order PI? controller is proposed for the first order plus long time delay system. The specification of phase margin is utilized to design the controller, and an optimization is conducted to choose a proper parameter ? to enhance the robustness of the system to the variation of the plant's parameters. Finally, the proposed method is compared with conventional PI controller and Smith predictor. And the conclusions are conducted in Chapter 6, together with some remarks about future work.