Tuning Procedure When Process Model Is Unknown

3 Tuning Procedure When Process Model Is Unknown 

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 controller is placed on manual control and a step change AS is applied. With a recorder, note the size of the step change and time of application. The reaction curve should be approximately as shown in Fig. 4.44. 

Determine the rate of change per unit step change in the manipulated variable, 1 gmax/ S. The recommended controller parameter settings, based on the quarter-decay performance criterion, are shown in Table 4.5. 

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