Virtual Unmodeled Dynamics Compensator Based PI Switching Control For High-order System With Time Delay And Experiments On Water-Tank System

In the industrial process control, the PID controller is a kind of widely used low-order linear controller with ideal control effects. For most model objects, a PID controller can always achieve perfect control effect by adjusting the parameters.However, as for the controlled objects which have strong nonlinear characteristics and large time delay, a PI controller cannot achieve satisfying control effect. Especially, for a kind of high-order single input single output(SISO) linear time invariant(LTI) system with time delay, a PID controller's parameters own a controllable range.The Water-Tank Level System is a classical controlled plant. Many applied systems in industry can be abstracted into a Water-Tank level system in whole or as a part. Thus,the study of level control on such system bares great research value and wide application background. However, for a kind of Water-Tank Level System which is a high-order single input single output (SISO) nonlinear system with time delay, a traditional PID controller cannot achieve ideal control effect. Therefore, it becomes a critical issue to design an effective controller against the object nonlinear characteristics and practical features which can achieve satisfactory results on the water-tank level system.With the support of State Key Laboratory of Synthetical Automation for Process Industries (Northeastern University), this paper proposes a virtual unmodeled dynamics compensator based PI switching control algorithm and relevant control experiments are done for a kind of universal model and for a water-tank level system. The main work is as following:1.The structure of high-order system with time dealy is described, and the controllable range of a PI controller is given;2.The virtual unmodeled dynamics compensator is estimated using two different algorithms based on former data. According to the characteristics of the high-order system with time delay, reform the mathematical model as the combination of a low-level linear model and a high order virtual unmodeled dynamics compensator term. Then, design a nonlinear switching controller around the operating point which is comprised of a linear PI controller, a virtual unmodeled dynamics compensator based PI controller and switching mechanism and relevant simulation are done to testify the correctness of the proposed algorithm;3.The mechanism model of the water-tank level system is established under dynamic mass balance condition and acquire controller design model by process control modeling algorithms according to the characteristics of the water-tank level system. Also, the identification method is introduced. On top of that, a Virtual unmodeled dynamics compensator based PI switching controller is designed using a low-order model and the relevant simulation research were made on the MATLAB/Simulink Platform to verify the effectiveness of such controller by comparing the simulation results with those acquired by a regular linear PI controller, a virtual unmodeled dynamics compensator based PI controller and a PI controller with nonlinear term compensation of last moment;4. The parameters for the controller design model is determined through parameters identification experiments on a real Water-tank level experimental system. Then, calculate the controllable range of PI parameters based on the low-order model. Adopt virtual unmodeled dynamics compensator based PI switching control on the system and testify that it can obtain satisfactory performance for level control around the preset operating point. Then compare with the results of a regular linear PI controller and a PI controller with nonlinear term compensation of last moment, and analyze the achieved results.The experiment results on the water-tank level system indicate that the switching controller can achieve high control accuracy, and thus is of significant value in application.