The Research On Multivariable IMC And IMC-PID Transformation

In the actual chemical process, the multi-variable system is widespread. The traditional PID control is hard to be applicable because of its multi-coupled multi-delay characteristics. With the advantages of simple to design, excellent robustness, and good prediction ability, the internal model control (IMC) shows the strong vigor to solve the control problems of the multivariable systems. This paper selected typical multivariable chemical process, proposing an IMC-PID filter parameter optimization method based on performance function, and studied the normative approach of decoupling, proposing IMC decoupling controller based on P-norm and V-norm. Meanwhile, this paper proposed a compensation method to deal with time delay. Details as follows:1. This paper designed IMC cotroller for the time-delay process, based on the approximate time delay approach (Taylor approximation, the first order Pade approximation and the all-pole approximation) derived its IMC-PID transformation methods, and the simulations proved that the all-pole approximation has high degree of approximation.2. There isn't a common selection criterion for the filter parameter of IMC controller. This paper proposed an IMC-PID filter parameter optimization method based on performance function. The simulation results showed that this method is effective. The method avoids the trial and error process. It makes the purpose of adjusting more clearly and the result more accurately. So this method has very important practical value.3. Based on P-norm decoupling, this paper proposed a new method to design IMC controller for multi-variable system. The IMC can decouple and control the system at the same time. Comparing to the previous method that the IMC is designed by the model inverse, we use the inverse of the diagonal elements of the system open-loop transfer function to design the main-loop controller, and then design the vice-loop controller by the decoupling conditions. This method uses the concept of inverse model creatively, and it avoids the matrix inversion process, so the design of the IMC gets easier. Simulation results show this method is more excellent.4. Previous muti-variable IMC controllers were designed based on P-norm. This paper proposed a new IMC controller which is designed based on V-norm. By avoiding the complex matrix inversion, its design method and parameter tuning is more simplified compared to P-IMC, especially during the IMC-PID transformation, greatly reducing the computational cost. The advantage is obvious. Simulation results show that the proposed V-IMC decoupling control method is effective and conducive to practical application.5. For some of the control object, when one of its main loop delays is greater than the coupling loop delays, it will lead to the IMC controller cantains advance which is can't be realized. The previous way that compensate to the controller directly would lead to decoupling effect decline. This paper proposes a compensation method to deal with time delay, and gives a simple and practical delay compensation structure. It makes the IMC controller can be applied to most kinds of delay distribution systems, and avoids the complex process of delay analysis and the situation that the decoupling effect gets worse when the time-delay constant values of each loop have big difference.