Next to good brakes, considerate motorists, and meatloaf, a properly welded rear axle is probably one of the things truckers appreciate most. To make sure they have it, Scania AB (Lulea, Sweden) recently installed two arc welding Laser SeamFinder systems from Selcom AB (Partille, Sweden).

The two SeamFinders significantly reduced Scania's trimming and fixturing requirements, increased welded seam quality, reduced scrap and rework, and even enhanced workplace safety. The company's former welding system had sensors that located seams using the arc only as a guide. As a result, acceptable tolerances were often not achieved, and the robotic welder wasn't capable of repeatedly locating the correct welding position.

"The previous system didn't give us the consistency and quality we needed in welding thin sheets for rear axle housings," says Bertil Gustavsson, Scania's project manager. "Correct alignment is critical to high-quality welds, so we needed to find an alternative that would give us the accuracy required."

At the suggestion of Sweden-based system integrator Lofquist Engineering, Scania eventually equiped its robots with Selcom's Laser SeamFinder. The system includes a noncontact laser sensor and software that evaluates and transmits the sensor's data to the robotic controller. The sensor's measurements aren't affected by surface characteristics including the presence of oil or paint.

Martin Sanden, Selcom AB's business area manager for robotic guidance systems, says SeamFinder usually needs just 1 sec to locate fillet, butt, and edge/overlap joints and define them in two dimensions. It can also perform three-dimensional measurements. After locating a welding seam, SeamFinder measures the gap, precisely positions the robot's welding torch, and adjusts the welding parameters, adapting to various gaps and weld width measurements.

SeamFinder's sensor uses laser triangulation to project a fixed beam onto the weld at 32,000/sec. The beam hits the surface, scatters, and is collected by the sensor, which focuses a light spot image on the position-sensitive detector in the sensor's head. Consequently, as the distance between sensor and welding surface changes, the image's position on the detector also changes, which makes it possible to control the robot's adaptive movements and weld accurately and repeatedly.

Seam position and gap data are indicated by SeamFinder's output signals. Results are interpreted as deviations from nominal values, which allows on-line modifications and variations in the robot's path and welding parameters. These programmed parameters include plate thickness and joint type. During the search process, one or more points along the joint are defined in two or three dimensions. All searches are, of course, performed without the arc being actuated.

Mr. Sanden adds that SeamFinder can be integrated with almost any arc welding system with a standard digital I/O interface. The sensors can also be mounted on most welding guns.

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