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[Luyben] Process Mod.. - Student subdomain for University of Bath

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604 MULTIVARIABLE PROCESSES<br />

TABLE 17.2<br />

BLT, Ziegler-Nichols, and empirical controller<br />

tuning<br />

Wood and Berry<br />

Ogunnaike and Ray<br />

Empirical<br />

Kc 02-0.04 1.2/-0.15/0.6<br />

TI 4&t/2.61 511014<br />

Z-N<br />

K 0.96/-0.19 3.24/-0.63J5.66<br />

*I 3.2519.2 7.62/8.36/3.08<br />

BLT<br />

L: (dB) +4 +6<br />

F factor 2.55 2.15<br />

KC 0.375/-0.075 1.51/-0.295/2.63<br />

TI 8.29123.6 16.4/18/6.61<br />

If the process transfer functions have greatly differing time constants, the<br />

BLT procedure does not work well. It tends to give a response that is too oscillatory.<br />

The problem can be handled by breaking up the system into fast and slow<br />

sections and applying BLT to each smaller subsection.<br />

The BLT procedure discussed above was applied with PI controllers. The<br />

method can be extended to include derivative action (PID controllers) by using<br />

two detuning factors: F detunes the ZN reset and gain values and F, detunes the<br />

ZN derivative value. The optimum value <strong>of</strong> F, is that which gives the minimum<br />

x,<br />

08<br />

WB<br />

f;‘l--::”<br />

0<br />

0<br />

,y*<br />

0<br />

Time. mm<br />

FIGURE 17.1<br />

Wood and Berry column: BLT and empirical tuning

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