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centre. This is attributed to the more uniform heat input and arc pressure distribution
over the weld pool, provided by the high speed rotation. Effects on the wire bum-off
were also observed in this process. The high speed rotation of the arc induces
centrifugal forces to act on the droplets at the tip of the wire, affecting the droplet
transfer phenomenon [ref. 135]. Dominant forces on the droplets at the wire tip are
magnetic pinch force, detachment force, rotation centrifugal force, and interfacial
force. An increase in rotation speed results in increased centrifugal force, which
induces the droplets to become smaller and the transfer cycle to become shorter.
Correspondingly, the extent of overheating by arc heat reduces and the heat retained
in the droplets (droplet temperature) decreases, resulting in an increased wire bum-off
[refs. 134,135].
rate
The principle by which the rotating arc sensor performs joint tracking is shown
in the Figure 2.20 [ref. 136]. It shows the basic patterns of arc voltage waveform in
relation to the arc rotation positions (Cf, R, C, , L) 19. When the arc rotation axis is
located at the centre of the groove (OX = 0), the arc voltage waveform is shown as a
broken line, with maximum values at Cf and C, , and minimum at R and L. It becomes
symmetrical at Cf (the front of the rotation). When the welding torch deviates, for
example to the right side of the groove (AX # 0), the wave form changes as shown by
the solid line in Figure 2.20. The phase of the waveform advances at point Cf, where
it becomes asymmetrical. By dividing the waveform at Cf into left and right, and by
comparing the integrated values, SL and SR, the desired torch deviation can be
detected. In practice, the voltage signal is likely to be distorted at the C, side, due to
the influence of the weld pool. Therefore, the phase angle of the above integration is
set empirically to a value less than 90 deg.
2.6.1.3.5 Optical sensing
The optical sensing systems normally are constituted by a light source which
illuminates the area of interest, an optical sensor which senses the reflected light, and
a data processing system which processes the data acquired by the optical sensor and
extracts the desired information, through a suitable algorithm.
In the case of welding, special requirements are imposed on these sensors,
since they have to work in a very harsh environment. The intense heat, the arc light
whose spectrum ranges from the ultraviolet to the visible wavelengths, fumes and
spatter constitute sources of noise that must be dealt with.
Several approaches have been used so far and they can be categorised
according to the kind of sensing device [ref. 137]: (a) photoelectronic sensor; (b)
linear sensor; (c) area sensor.
The simplest photoelectronic seam tracking sensors use a photoemitter and
collector which are directed at a well defined joint line. Sometimes, it is necessary to
use paint or to employ a tape laid parallel to the seam, to ensure that a clear signal is
obtained [ref. 3]. This kind of sensing device can also be oscillated over the joint so
that it moves transversely to the seam. The quantity of light received and the sensor
position (co-ordinates) at that time are placed in two dimensions. The gap and the
groove position are detected and tracking is controlled [ref. 137]. These systems are
19 Cf: centre front of rotation, R: extreme right position, C, : back position, L: extreme left position
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