LIBRARY ı6ıul 0) - Cranfield University

certain level of process stability was assured, keeping the stand-off previous value
otherwise. The stability level was calculated by using a model based on the Transfer
Index (see equation 2.3) and on the Power Ratio (see equation 2.6). Such a model
was called Confidence of Bad Ignition (Conf_Bad_Ign) and was developed to
indicate the occurrence of an undesirable process instability. Step 1 to Step 5 below
describe the proposed model.
Step 1- Before starting monitoring, set the stability limits for PR and TI based on
the set-up welding parameters and initialise other model variables
Conf_Bad_Ign; 0.0 (Assuming
_1= a good arc start)
Tlifi/tered
ao=0.3
= TTi-Ifiltered = 1.0
a3=0.5
If mode of metal transfer = dip then
PRmin-0.2
TImax=0.5
PR;
f tered
a1=0.5
= PRi-I
filtered = 0.3
else (mode of metal transfer = spray)
PRmin=0.9
77mar-0.2
PRt
filtered = M-1
a1= 0.2
, filtered = 0.95
Step 2- During monitoring, filter PR and TI using equation (4.7) with smoothing
factor ao for filtering PR and al for filtering 77
P1ý.
ý, u, ýd-(1-ao)"PI +ao*PR, -,. f,, d
77,.
fi d- (1-a, )" 77, +a1 ' TI;
-I. fuema
(4.9)
(4.10)
Step 3- Test if the filtered monitoring indices indicate instability and set the
possibility measure of process being unstable, Pr(ign), accordingly
If (PR,
frmered < PRmin) or (Th(te > Tlmax) then
ed
Pr(ign) =1: process unstable
else Pr(ign) =0: process stable
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