Research On Optimal PID Controller And Its Applications In Control Of Thermal Plants

The PID controllers have simple structure and good robustness, and have been used by far most in the thermal process control field. In the last 40 years, a large amount of PID controller design methods have appeared, but those methods were almost developed based on First-order time-delay models. First-order time-delay models do not always suit high order plants of thermal process, so traditional PID controllers cannot provide satisfactory control. A major obstacle to designing the best PID controller, in any sense whatsoever, has been the difficulty in characterizing the entire set of stabilizing PID controllers. In this thesis, an optimized PID controller is designed of high order system, using the generalized Hermite-Biehler theorem, in stable region of PID controllers. Moreover the robust stability and performance of the PID controllers were evaluated using the Kharitonov theorem, Monte-Carlo stochastic methods and H∞ mixed sensitivity restriction.First, the stable region algorithm of PID controller for high-order plants has been studied, and a common program has been implemented, using generalized Hermite-Biehler theorem. The genetic algorithm was used to tune a PID controller with the optimal integrated time absolute error (ITAE) criterion from this stable region. The robustness stability of the optimal PID controller of interval plants are studied using Kharitonov theorem. When the parameter of plants varied in the interval range, the dynamic performance and robustness of PID control system are analyzed by Monte-Carlo stochastic methods. Second, based on above optimal design methods of PID controller, the stable region method is used to control the common plants in process control field: high order plant, non-minimum phase plant, unstable plant and large inertia plant. The design methods and further improvement of optimal PID controller for these plants are investigated respectively and a lot of simulation experiments are performed. Furthermore, this method is used in typical control systems of thermal process: the reheated temperature control system for boiler, the pressure control system of the circulating fluidized-bed boilers, the superheated temperature cascade control system for boiler, the main steam pressure cascade control system in generating set. Finally, a PID controller design method based on H∞ mixed sensitivity restriction is researched. The algorithm for parameter's region of PID controller, which satisfies H∞ mixed sensitivity restriction, has been studied, and a PID controller with the optimal H∞ mixed sensitivity norm criterion has been designed. All this work has demonstrated that the proposed method can gain effectively control in thermal process control.