| The induced draft and forced draft control system of industrial coal-fired boiler is the basic system of the automatic combustion process. Their control quality directly determine the quality of combustion control, and will affect the performance of the boiler operation. The induced draft and force draft of coal-fired boiler is a nonlinear system with the problems of strong interaction 、 time varying and uncertainty.These characters seriously restrict further improvement of the boiler operation. So it has very important significance to design a decoupling controller in order to eliminate the mutual influence between induced draft control system and forced draft control system. By traditional control theory, induced draft control system and forced draft control system are taken as independent system without correlation of each other, ignoring the coupling effect between the control systems. They focus on the theory research of intelligent method and application on independent system, lacking the knowledge of induced and forced draft synthetical system decoupling control application as well as correlative theory direction. And it has became the bottle-neck of application of decoupling in induced and forced draft synthetical system.Induced draft and forced draft system is essentially a multivariable system. Using single loop control scheme to solve multivariable system exists flaws, because this method cannot eliminate the influence between induced draft and forced draft control system.So we should consider it from the view of multivariable system. A method about equivalent transfer functions is proposed for multivariable decoupling control system in order to solve the problem of complex structure, the hard analytical design of the decoupler and the difficult compromise between the control performance and the robustness of the controller. In this way, the system will be decoupled into two single loop without mutual interference. Then designing PID controllers with the method of gain and phase margin to meet the requirement of oxygen and negative pressure of coal-fired boiler.Firstly, this paper introduces the working principle of the fan system of the coal-fired boiler in detail, and analyzes the control requirements and coupling characteristics of the induced and forced draft control system.Secondly, designing the decoupler of induced and forced draft fan system with the methods of feed-forward compensation decoupling and unit matrix decoupling. At thesame time, these methods were analyzed by simulation. Then, designing the decoupler of induced and forced draft fan system with the parametric method based on equivalent transfer function. And the parametric method based on equivalent was better than the other two by simulation.Lastly, designing the decoupling controller of induced and forced fan system with the methods of gain and phase margin and internal model control. Using the MATLAB simulation software to establish simulation model of the system in SIMULINK. Then obtaining the output curves of the decoupling system under parameter perturbation.Through the analysis of simulation curves, the method of gain and phase decoupling control was better than that of Chen method in the sides of dynamic and steady state performance with better control performance and strong robustness. The method is easy for industrial application and has a better practical significance. |
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