Tuning rules

The performance of the interacting controller is almost as good as the noninterac ting controller on most processes, but the tuning rules differ because of the above relationships. With digital PID controllers, the noninteracting version is commonly used. 

TABLE 8-2 Tuning Rules Using Known Process Parameters... 

The Ziegler and Nichols closed-loop method requires forcing the loop to cycle uniformly under proportional control. The natural period of the cycle—the proportional controller contributes no phase shift to alter it—is used to set the optimum integral and derivative time constants. The optimum proportional band is set relative to the undamped proportional band P , which produced the uniform oscillation. Table 8-4 lists the tuning rules for a lag-dominant process. A uniform cycle can also be forced using on/off control to cycle the manipulated variable between two limits. The period of the cycle will be close to if the cycle is symmetrical the peak-to-peak amphtude of the controlled variable divided by the difference between the output limits A, is a measure of process gain at that period and is therefore related to for the proportional cycle ... 

The factor TC/4 compensates for the square wave in the output. Tuning rules are given in Table 8-4. 

TABLE 8-3 Tuning Rules Using Slope and Intercept... 

TABLE 8-4 Tuning Rules Using Proportional Cycle... 

The text is organized as follows. Next section contains a brief review on robust temperature control of complex chemical systems. Alkylation system is presented in third section. Problem statement is discussed in fourth section. Temperature control is analyzed in fifth section. Controller performance and tuning rules are shown in sixth section. The paper is closed with some concluding remarks. 

Figures 6.14 and 6.15 give dynamic responses of the tray temperatures, reboiler heat input, and bottoms product impurity. The temperature loops were tuned using the TL (Tyreus-Luyben) tuning rules after the ultimate gain and ultimate frequency had been determined using a relay-feedback test. Two 0.5-minute first-order lags are used in the temperature loop. Temperature transmitter spans are 100T. The ultimate gain and period for the tray 6 temperature loop are 4.2 and 2.7 minutes, and for the tray 14 loop are 12.7 and 2.5 minutes. These results reflect the fact that the process gain is higher when tray 6 is...

Zafiriou (1991) used a number of examples to demonstrate that the presence of constraints can have a dramatic and often counterintuitive effect on MFC stability properties and can render tuning rules developed for stability or robustness of unconstrained MFC incorrect. The following examples show how the addition of constraints to a robustly stable unconstrained MFC system can lead to instabilities. 

Before discussing issues related to the different classes of models that may be used with MPC, we should stress that the issue of model structure selection and model identification is at the heart of MPC. The greatest contribution of Ziegler and Nichols (1942) is not the famous tuning rule but the cycling identification technique they understood precisely what... 

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