LIBRARY ı6ıul 0) - Cranfield University

Philpott [refs. 21,131], based on the observations of Lesnevich [ref. 132],
considered that the average temperature of the electrode and its distribution are not
affected by changes in the electrode extension. He therefore concluded that the
contact tip-to-workpiece distance is directly proportional to the short circuit
resistance (or dip resistance). This is represented by the equation (2.18), in which the
temperature, 0, is considered as the average temperature of the electrode, 0,.
Ushio et al. [ref. 130] studied the dynamic characteristics of the arc sensor in
the short circuiting mode of metal transfer in GMA welding. The authors found that
characteristic features extracted from the welding current waveform, such as the
welding current at the instant immediately before the short circuit and the short
circuiting frequency provide a good indication of variation in contact tip-to-workpiece
distance. The authors also found that the best response using these parameters occur
for oscillation frequencies around 3Hz.
Another approach for modelling the torch-to-workpiece distance was
proposed by Kim and Na [ref. 133]; multiple regression analysis was used to model
the welding current as a function of the welding variables (welding voltage, wire-feed
speed and tip-to-workpiece distance). The regression model took into account the
effect of individual factors and two factor combinations, as shown in equation (2.19):
I=K, +K2V+K3WFS+K4SO+K3V"WFS+
+K6WFS"SO+K7V"SO
(2.19)
where K. , n=1.. 7, are regression constants, V is the welding voltage, SO is the
contact tip-to-workpiece distance and WFS is the wire feed speed.
Rearranging the terms of equation (2.19), the relationship between welding
current and tip-to-workpiece distance was obtained as follows:
I= (K, + K2V + K3WFS
+ KsV " WFS) + (K4 + K6WFS + K7V)SO
(2.20)
This relationship was claimed to be successfully applied for seam tracking of V-
groove butt joints by oscillating the welding torch across the joint [ref. 133].
The same technique was adopted by Ogunbiyi [ref. 51] for monitoring and
controlling the GMAW process. The model devoloped by the author [ref. 51 ] is of the
form:
I=K, +K2WFS+K3SO"V+K4SO"WFS
(2.21)
where K. , ti-1.. 4 , are regression constants, WFS is the wire feed speed, V is the
welding voltage and SO is the contact tip-to-workpiece distance.
Considering that changes in tip-to-workpiece distance can be estimated from
the deviation of the welding current from a reference value and that when the process
is controlled at regular intervals a new reference value must be determined from
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