Cohen-Coon tuning

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 ... 

Table 2 Process reaction curve (Cohen-Coon) tuning parameters...

Compute the settings of a PID controller using the Cohen-Coon tuning methodology. 

IV.44 Consider the feedback loop shown in Figure PIV.3a. Select the settings of the PI controller using the Cohen-Coon tuning technique. In a graph paper display the actual process reaction curve and its first-order plus dead time approximation. 

Figure 9.8 Process reaction curve for Cohen-Coon tuning technique. K = T = Ay/j.

MATLAB calculation details and plots can be found on our Web Support. You should observe that Cohen-Coon and Ziegler-Nichols tuning relations lead to roughly 74% and 64% overshoot, respectively, which are more significant than what we expect with a quarter decay ratio criterion. 

The Cohen-Coon or Ziegler-Nichois methodologies for tuning feedback controllers described in Sections 16.5 and 18.3... 

Why can you use the classical Cohen-Coon or Ziegler-Nichols techniques for tuning a digital PI or PID controller What is the additional tuning parameter introduced by the discrete nature of a process control computer ... 

Example 18.4 Controller Tuning by the Ziegler-Nichols and Cohen-Coon Methods... 

Tci = dfci. In this simplified form, the tuning rules will be on a comparable level with the Ziegler-Nichols and Cohen-Coon rules in terms of ease of use, with the added benefit of having several different performance levels for the user to choose from along with their respective stability margins. 

The most well-known tuning guidelines that make use of these concepts are the Ziegler Nichols method and the Cohen-Coon method. There are also other controller tuning guidelines, such as dead-beat tuning and Internal Model Control tuning. It is beyond the scope of this chapter to discuss the last two methods. 

Using the first order with dead time function, we can go ahead and determine the controller settings with empirical tuning relations. The most common ones are the Ziegler-Nichols relations. In process unit operation applications, we can also use the Cohen and Coon or the Ciancone and Marlin relations. These relations are listed in the Table of Tuning Relations (Table 6.1). 

All tuning relations provide different results. Generally, the Cohen and Coon relation has the largest proportional gain and the dynamic response tends to be the most underdamped. The Ciancone-Marlin relation provides the most conservative setting, and it uses a very small derivative time constant and a relatively large integral time constant. In a way, their correlation reflects a common industrial preference for PI controllers. 

While the calculations in the last example may appear as simple plug-and-chug, we should take a closer look at the tuning relations. The Cohen and Coon equations for the proportional gain taken from Table 6.1 are ... 

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. 

In this example we examine how the dynamics of various typical processes influence the tuning results recommended by Cohen and Coon. 

Two early controller tuning relations were pubhshed by Ziegler and Nichols (1942) and Cohen and Coon (1953). These well-known tuning relations were developed to provide closed-loop responses that have a 1/4 decay ratio (see Section 5.4). Because a response with a 1/4 decay ratio is considered to be excessively oscillatory for most process control applications, these tuning relations are not recommended. 

In 1953, Cohen and Coon [2] developed a set of controller tuning recommendations that correct for one deficiency in the Ziegler-Nichols open-loop rules. This deficiency is the sluggish closed-loop response given by the Ziegler-Nichols rules on the relatively rare occasion when process dead time is large relative to the dominant open-loop time constant. 

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