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elatively low cost but can be affected by arc glare. In order to avoid this, devices
with specific wavelengths have been used in conjunction with the suitable light
sources and filters, which give improved sensitivity.
The linear sensors work with the reflected light that comes from the
illuminated area. They consist of a photodiode array or a line transducer and can be
disposed over the joint in two different ways: (a) longitudinally or (b) transversely to
the joint . In the first case, a semiconductor laser or other point light irradiates the
base whereas the linear sensor captures the reflected light. Since the sensor is
mounted with a defined angle relative to the base, it is possible to measure its distance
from the workpiece through the reflection angle (triangulation). By oscillating the
sensing system across the joint its profile can be mapped and used for seam tracking.
The 3D information obtained with this kind of sensor is normally referred to as laser
range data. Some examples of this kind of system have been reported by Oomen and
Verbeek [ref. 138], Björkelund [ref. 139], Sicard and Levine [ref. 140], Bamba [ref.
141] and Back and Franzen [ref. 142].
When the second approach (linear sensor transversely disposed) is used,
however, the stationary sensor behaves as if it were a point sensor oscillating across
the joint. It captures the reflected light and the seam profile can be obtained through
the analysis of the levels of brightness of the acquired data. In this case, abrupt
changes in brightness means that there is a gap or some area which does not reflect
the light. Care must be taken in the analysis, however, since scratches can be
misinterpreted as a joint line.
The area sensors, on the other hand, use a camera (e. g. CCD20 cameras) to
capture the image that comes from the joint. Two approaches can be used for
determining the position of the joint: (a) the analysis of a structured light strip image
or (b) the analysis of the direct image of the objective area.
In the first one, the camera is used to capture the image of a line of structured
light which is projected over the weld seam in a transverse direction. The line image is
then processed in a computer and the seam position can be calculated. With this
technique, not only the joint position can be detected but the joint geometry and the
gap size as well as misalignments of edges; angle of joint preparation and tack welds
can also be extracted from the image. Most of the systems that use this technique
sense the seam at a small distance ahead of the welding torch. Examples of these
systems have been reported by Drews and Starke [ref. 143], Davey et al. [ref. 144],
Niepold and Brummer [ref. 145], Nakata and Jie [ref. 146] and Sameda [ref. 147].
The laser stripe can be generated by optical techniques [ref. 144] or by using a
mirror to oscillate the laser across the seam [ref. 146]. The joint profile is reproduced
in the video image and this information may be digitised. The difference between the
real and a reference image may be used to generate an error signal which is used by
the control system to correct the lateral position of the torch. To avoid interference
from the arc, a band-pass filter corresponding to the wavelength of the light source is
used; in most cases this will be at the red or infrared part of the spectrum, where arc
radiation levels are low. Light sources other than laser can also be used but a different
20 Charge-Coupled-Device. These cameras are normally preferred for these applications due to their
compact size, robustness and the susceptibility of other kinds of cameras to damage from intense
light sources such as welding arcs.
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