LIBRARY ı6ıul 0) - Cranfield University

Step 4- Calculate the confidence of bad ignition by applying the moving average
filter of equation (4.7) to the possibility measure of process being unstable, Pr(ign),
using a smoothing factor a=0.5
Conf Bad Ign, _ (1- a) " Pr(ign) +a" Conf Baal lgn, _,
Step 5- Estimate a new stand-off value if and only if Conf Bad Ign < 0.2
(4.11)
The value Conf_Bad_Ign = 0.2 was chosen empirically, based on prior
observations of the stand-off model behaviour for different stability levels.
4.2.4 Reliability of process statistical estimates
The model used for estimating the stand-off was obtained from multiple
regression analysis of average values of calculated resistance (see section 6.2). Due to
the nature of the gas metal arc welding process, the welding current and voltage
signals are generally corrupted with random noise which is passed over to the
calculated resistance and, consequently, to the estimated stand-off. In order to reduce
the oscillation in the stand-off values due to the random noise, a third order moving
average filter was used. The filtered stand-off value was then transferred to the table
controller; whereit ould be compared to the required reference stand-off and an
error would be calculated. Equation (4.12) shows the filter used.
"ýOi. Altned
_'2
2 (4.12)
Iaj
"So, U_A
J=O
Daj
j=0
where
SO, filtered current filtered stand-off estimation;
SO(;
_j), e, t non-filtered stand-off estimates for monitoring cycles i, (i-1) and (i-2);
aj
filter weights, aj = 1.0 for j=0,1,2.
Although the filtered values were transferred to the table controller, the stand-
off error predictions were still oscillating. In order to reduce the response of the table
to random variation, threshold limits were imposed on the stand-off error signal, such
that no correction would occur for errors smaller that 0.5 nun and no correction
greater than 1 mm would be applied in a control cycle. A proportional control scheme
with a unitary gain was adopted for adjusting the stand-ofd After sending an
estimated stand-off value to the table controller, the main controller would reset the
stand-off filter and would only send a new estimated value after three estimation
(monitoring) cycles, that is, after acquiring data for producing a filtered stand-off
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