LIBRARY ı6ıul 0) - Cranfield University
LIBRARY ı6ıul 0) - Cranfield University
LIBRARY ı6ıul 0) - Cranfield University
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The introduction of such a reduced range of variation (rules 2 to 4 in Table<br />
4.4) improved the controller behaviour. However, it still resulted in excessive spatter<br />
for low wire feed speeds and in a slightly oscillatory behaviour. In order to improve<br />
the controller performance further, additional rules based on the other monitoring<br />
indices3 were introduced. The rules were developed from the observation of the<br />
behaviour of these indices during controlled trials carried out with the control rules<br />
shown in Table 4.4. It was observed that the indices would vary with different<br />
sensitivities as a response to instabilities in the process. Based on this observation, - a<br />
set of interdependent rules were generated and logically organised so that the less<br />
sensitive indices would be tested before the most sensitive ones. The rules were<br />
introduced under rule number 3 in Table 4.4 and are shown in Table 4.7. Note that<br />
the amount of voltage correction used in rules 2 and 4 of Table 4.4 was increased.<br />
Table 4.6 shows the linguistic representation of the limits of DCI, TI and TSI used for<br />
fine-tuning the stability in dip transfer.<br />
Table 4.6 - Linguistic representation of limits for DCI, TI and TSI in dip<br />
transfer<br />
DCI-LOW = 0.4 DCI-HIGH - 0.8<br />
TI-LOW = 0.3 TI-HIGH - 0.45<br />
TSI-LOW = 1.6 TSI HIGH -1. Q<br />
Table 4.7 - Control rules based on DCI, TI and TSI<br />
2 If PRLOWDIP 5 PR < minPRdip then process slightly tending to instability<br />
AV = SMALLTRIM<br />
3 IfminPRdi 5 PR :5 maxPRdip then test DCI<br />
3. a<br />
If DCI < DCI-LOW then decrease<br />
the voltage slightly<br />
AV = -MICROTRIM<br />
else if DCI > DCI-HIGH then increase<br />
the voltage slightly<br />
AV = MICROTRIM<br />
else test TI<br />
3. b IfTI > TI-HIGH then increase<br />
the voltage slightly<br />
3. c<br />
AV = MICROTRIM<br />
else if TI < TI-LOW then decrease<br />
the voltage slightly<br />
AV = -MICROTRIM<br />
else test TSI<br />
IfTSI > TSI-HIGH or TSI < TSI-LOW then reduce the voltage slightly<br />
AV = -MICROTRIM<br />
else the process is stable<br />
AV = ZEROTRIM<br />
4 IfmaxPRdip < PR < PRHIGHDIP then process slightly tending to instability<br />
AV = -MICROTRIM<br />
2 Excessive spatter in this context does not imply that the process is unstable. It just implies that<br />
=<br />
better stability state can still be achieved. generating less and finer spatter.<br />
Dip Consistency Index (DCI), Transfer Index (TI) and Transfer Stability Index (TSI)<br />
106
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