LIBRARY ı6ıul 0) - Cranfield University

eceptor. The sensors can also be built in such a way as to perform both the emission
and reception of the ultrasound, resulting in very compact designs.
These kind of sensors can be used for seam tracking by either oscillating them
over the joint and measuring the distances on various crossing positions, or by using
two sensors mounted at 90 degrees, in the case of a fillet joint.
2.6.1.3.4 Through-the-arc sensing
Through-the-arc sensing involves the analysis of the welding current and
voltage signals of processes such as gas metal arc welding (GMAW) and gas tungsten
arc welding (GTAW) for joint tracking. The arc sensing method requires no additional
equipment at the welding head and is applicable to a large number of welding tasks,
particularly single pass fillet welds, heavy-section V butt and narrow-gap welding
situations on steel. This section will only focus on the through-the-arc sensing
techniques
for gas metal arc welding.
Through-the-arc sensing is based on the principle that, for a constant voltage
power source and constant wire feed speed, a change in the torch-to-workpiece
distance's results in a change in the welding current, which can be measured.
According to equation (2.2), the melting rate (wm) depends on the joule
heating in the electrode stick-out and on the heat generated by the arc. The joule
heating depends on the electrode resistance, which in turn depends on the electrode
cross sectional area (A, ), resistivity (r) and length (Le). Since the welding voltage and
the wire feed speed are pre-set welding parameters, it is possible to calculate the
distance from the contact-tip to the workpiece, SO.
Since the process is in steady state, it can be assumed that the melting rate
(wm) is equal to the wire feed speed (WFS) [refs. 128,129]. Hence the wire extension
(Le) could be calculated by substituting wfor WFS in equation (2.2) and arranging its
terms according to equation (2.8).
Lý =
WFS-al (2.8)
ß1Z
In order to 'obtain the contact tip-to-workpiece distance it is also necessary to
calculate the arc length. It is possible to estimate this by using an empirical formula
which establishes a relationship between the arc length (L. ), the welding current (1)
and the arc voltage (Va). The empirical formula is shown in equation (2.9). It was
obtained experimentally for the GTAW process but can also be applied in the case of
GMAW [refs. 128,129].
V. =K, La+K2+K31+ 14
(2.9)
18 This can be measured either by the stand-off (distance between the gas nozzle and the workpiece)
or by the contact tip-to-workpiece distance. The difference between both is just a constant.
33