Ziegler-Nichols Open-Loop Method

As with the Ziegler-Nichols open-loop method recommendations, the Cohen-Coon values should be implemented and tested in closed loop and adjusted accordingly to achieve the QDR. 

Perhaps the most well known and most frequently published method is that by Ziegler and Nichols (Reference 3). They developed an open loop method in addition to the more frequently published closed loop method. The criterion used by both methods is the quarter decay ratio. This is illustrated in Figure 3.21 following a SP change the PV response should be slightly oscillatory with the height of the second peak being one-quarter of that of the first. 

Figure 6.2. Illustration of fitting Eq. (6-2, solid curve) to open-loop step test data representative of self-regulating and multi-capacity processes (dotted curve). The time constant estimation shown here is based on the initial slope and a visual estimation of dead time. The Ziegler-Nichols tuning relation (Table 6.1) also uses the slope through the inflection point of the data (not shown). Alternative estimation methods are provided on our Web Support.

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. 

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 ... 

Ziegler and Nichols presented an open-loop test method in their classic paper on tuning control loops. The method consists of starting with the control loop at steady state, putting the controller in manual output so that there is no feedback response from any change in the process variable, and introducing a step change in the controller output (valve position). Then the results are used to characterize the process response by an apparent... 

In 1942, Ziegler and Nichols [1] changed controller tuning from an art to a science by developing their open-loop step function analysis technique. They also developed a closed-loop technique, which is described in the next section on constant cycling methods. 

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