Controller Tuning Guidelines

When the phase shift is -180° at co=coco, the crossover frequency, and the amplitude ratio is M, then, according to the Bode criterion, the control system is stable when M 1 and the system is unstable when M 1. The crossover frequency and amplitude ratio Mhave been 

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. 

The procedure for tuning a process according to this rule proceeds as follows (Ziegler and Nichols, 1942)  

The controller settings for various controllers can now be calculated from Table 32.1. 

The Ziegler and Nichols method is difficult to apply in a plant environment, since no process operator would allow a control loop to oscillate. 

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There are mrmerous other tuning guidelines. One that has been found very useful, based on the author s experience, are the following settings for a PI controller, also asstrming a model as in Eqn. (32.29). KcKp = 1 - 0350It, Ti = T. 

In these two texts the reader will find useful practical guidelines in selecting the most appropriate type of feedback controller for a particular application. In addition, one can find alternative tuning techniques employed by the industrial practice. 

Guideline. In general, a well-tuned controller should have a gain margin between 1.7 and 4.0 and a phase margin between 30° and 45°. 

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