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technique must be employed to prevent arc light interference. For instance, Niepold
and Brummer [ref. 145] developed a special exposure technique, in which the camera
was equipped with a special shutter that could be triggered only when the arc was not
obscuring the scene (during the short-circuit time of a dip-transfer welding). They
have also applied four consecutive laser stripes instead of only one. According to the
authors [ref. 145], the main advantage of this approach lay in the fact that the
measurement started from a section of the seam he and not from one point only. This
is important, because the seam line can be disturbed locally by alterations such as
tacks or spatter.
On the other hand, when structured light is not used, the direct image of the
joint scene must be analysed. With a single camera it is difficult to obtain three-
dimensional information concerning the joint profile but by using optical viewing
systems concentric with the torch, the relative position of the joint may be determined
[refs. 3,148]. By analysing the position of the joint line in a window of the video
image, the lateral error of the torch position can be determined. Although the torch
assembly is rather complex, this approach has the advantage that the arc radiation is
effectively blocked by the electrode.
Suga et al. [ref. 149] employed direct image to develop a weld line tracker for
automatic TIG butt welding of thin aluminium plates. In order to decrease
the effect
of the arc light in the image they employed a special-purpose lighting, which
illuminated the joint laterally from both sides. This reduced the effect of the arc light
in the weld line recognition.
Groom et al. [ref. 1501 developed a V-groove welding seam tracking system
based on the analysis of infrared images of the joint. Through an experimentally
derived image processing technique, the temperature gradients surrounding the
welding torch were extracted and the torch misalignment relative to the joint was
determined, providing the control system with the correction signals necessary to
correctly position the torch.
The optical systems have received special attention during the last years.
According to Nomura et al. [ref. 114] and Boillot et al. [ref. 121], the optical sensors
should prevail in the future. However, there are some drawbacks for these systems,
namely:
1. they are still relatively expensive;
2. maintenance costs are also expensive;
3. they constitute an external equipment to be assembled on the welding
torch. This can cause problems when joints with difficult access must be
welded.
2.6.1.4 Joint recognition
Joint recognition is again a form of adaptive control which recognises the joint
preparation to be welded, monitors the geometry of the joint ahead of the arc and
takes the necessary corrective action to compensate for variations. This may be
carried out in a preliminary scan or in real time [ref. 58].
Two approaches are normally used for joint recognition: this can be performed
either by a vision sensing system or via through-the-arc sensing.
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