Tuning Methods When Process Model Is Unknown

The Ziegler and Nichols closed-loop method requires forcing the loop to cycle uniformly under proportional control, by setting the integral time to maximum and derivative time to zero and reducing the proportional band until a constant-amplitude cycle results. The natural period x of the cycle (the proportional controller contributes no phase shift to alter it) is used to set the optimum integral and derivative time constants. The optimum proportions band is set relative to the undamped proportional band P which was found to produce the uniform oscillation. Table 8-4 lists the tuning rules for a lag-dominant process. 

A uniform cycle can also be forced by using on-off control to cycle the manipulated variable between two limits. The period of the cycle will be close to x if the cycle is symmetric the peak-to-peak ampli- 

TABLE 8-4 Tuning Rules Using Proportional Cycle 

The factor Tt/4 compensates for the square wave in the output. Tuning rules are given in Table 8-4. 

8-29 Tuning proportional and integral settings to optimize set-point response degrades load response using a separate set-point gain adjustment allows Doth responses to be optimized. 

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Tuning Methods When Process Model Is Unknown. 8-15... 

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