Reaction-curve method

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... 

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. 

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. 

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