Process reaction curve methods

Process Reaction Curve Method (Cohen-Coon Tuning). For some processes, it may be difficult or hazardous to operate with continuous cycling, even for short periods. The process reaction curve method obtains settings based on the open loop response and thereby avoids the potential problem of closed loop instability. The procedure is as follows ... 

In this section we discuss the most popular of the empirical tuning methods, known as the process reaction curve method, developed by Cohen and Coon. 

Unlike the process reaction curve method which uses data from the open-loop response of a system, the Ziegler-Nichols tuning technique is a closed-loop procedure. It goes through the following steps ... 

The controller settings according to the process reaction curve method were found to be ... 

For additional reading on the process reaction curve method and the Cohen-Coon settings, the reader can consult Refs. 8,12,13, and 15. The details on the development of the Cohen-Coon settings can be found in their original work ... 

How can we identify the four process transfer functions In Example 4.13 we saw that a rigorous approach leads to an overwhelming mathematical model. The process reaction curve method, which was discussed in Section 16.5, is a simpler approach and yields the transfer functions between... 

Cohen and Coon Process Reaction Curve Method... 

In the process reaction curve methods a process reaction curve is generated in response to a disturbance. This process curve is then used to calculate the controller gain, integral time and derivative time. These methods are performed in open loop, so no control action occurs and the process response can be isolated. 

Figure 2.34. Process reaction curve for the Ziegler-Nichols Method.

Obtain the process reaction curve for the process with disconnected controller, as explained in Sec. 2.3.3. Analyse this curve to obtain the parameters for the Ziegler-Nichols Method. Use Table 2.2 to obtain the best controller settings for P and PI control. Try these out in a simulation. 

Fig. 7.58. Cohen-Coon method (a) block diagram (b) process reaction curve...

The tuning settings based on the process reaction curves obtained by the open-loop tuning method, in addition to the Ziegler-Nichols method (Table 2.38), can also be selected by other methods. Figure 2.39 compares the load responses and Figure 2.40 compares the set point responses of these methods. 

Figure 9 Process reaction curve (Cohen-Coon) method...

In Section 16.5 we discussed a tuning method based on the process reaction curve. The method is primarily experimental and uses real process data from the system s response. In this section we discuss an alternative method developed by Ziegler and Nichols, which is based on frequency response analysis. 

The tuning technique that is applied will depend on whether or not the process model is known. When the process model is not known, the most widely used tuning techniques incorporate the ultimate-period method, the reaction-curve method,... 

The reaction-curve method is based on the open-loop response of the process to a step input. This response curve can be used to derive the dynamic characteristics of the process. If the process can be described by a first-order lag and dead time, the controller setting can be calculated. 

The two previous tuning techniques require a reasonably detailed control-loop analysis. In practice, many controllers are tuned by trial-and-error methods based on process experience. Both the Ziegler-Nichols method and the reaction-curve method are based on the assumption that the disturbances enter the process at one particular point. These methods, therefore, do not always give satisfactory results. In these cases, the final adjustments must be made by trial-and-error search methods. 

One weU-known technique for determining an empirical model of a process is the process reaction curve (PRC) method. In the PRC technique, the actual process is operated under manual control, and a step change in the controller output (Ap) is carried out. The size of the step is typically 5% of the span, depending on noise levels for the process variables. 

To generate a process reaction curve, the process is allowed to reach steady state or as close to steady state as possible. Then, in open loop, so that there is no control action, a small step disturbance is introduced and the reaction of the process variable is recorded. Figure W5.1 shows a typical process reaction curve for the process variable (PV) generated using the above method for a generic self-regulating process. The term self-regulating refers to a process where the controlled variable eventually returns to a stable value or levels out without external intervention. 

The Ziegler-Nichols process reaction curve tuning method for a PI controller is as... 

It has also been shown that the electrode response of some processes can appear to fit theoretical working curves in which the reaction order in the intermediate differs from the true value (Parker, 1981b). For example, the deprotonation of hexamethylbenzene radical cation studied by derivative cyclic voltammetry gave data which fitted theoretical data for a simple first order decomposition of the intermediate. However, the observed first order rate constants were found to vary significantly with the substrate concentration indicating a higher order reaction. A method was proposed to treat... 

Any solid state reaction, however complex, must resolve itself into interactions between pairs of solid phases, the elementary processes occurring successively or simultaneously to give a variety of intermediate and final products. Because the entropy change is small, all solid state reactions are exothermic. This property forms the basis of the heating curve method for detecting reactivity in solid mixtures. 

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