Advanced Welding Processes: Technologies and Process Control

Monitoring and control of welding processes 207
Whilst these systems are inherently simple, tracking errors may result if
the surface condition of the plate is poor or in multi-run situations, where the
probe must track a convex weld deposit. These problems may be minimized
by using special probe tips (e.g. to cope with tack welds).
Pre-weld sensing/joint location. In fully automated and robotic welding
cells, the seam may be located before welding using tactile or non-contact
sensors. A common technique is to utilize the end of the GMAW electrode
as a contact probe. [239] In robot applications the torch moves to a taught
point in the vicinity of the joint (Fig. 10.13), the head is then moved from the
taught point along the z axis until it comes into contact with the workpiece,
(contact is detected by a high-voltage, low-current DC signal). The contact
point is confirmed by a short slow sweep along the x axis away from the
contact point and back. The torch then travels along the x axis until it contacts
the other plate and again a short, slow-speed traverse away from and back to
the second contact point is performed. From the measured data the joint
intersection point is calculated and the torch is moved to an appropriate start
point.
Similar systems have been used for applications involving portable robots
in shipbuilding. In this case the robot uses a tactile search routine to check
its position within the welding cell and adjusts the program datum point to
correct any errors. [240]
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10.13 Tactile sensing – seam location.
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