Optical Sensor Bridges Gap for Robotic Welders

Daniel C. McCarthy

Two robotic welders at Vauxhall Motors Ltd. complete about 300 wheel arches a day on models such as the Astra. Before the robots' installation, production was slightly faster, but about 40 percent of the parts rolled off the line with holes blown through the side panel to the rear of the outer arch because of the position of the welder.

"Robots offered greater accuracy," said Ray Price, the lead engineer for body assembly at Vauxhall. "But not without some method that would allow them to find the targeted spot between the steel arches and, if necessary, adjust the welding filament."

Price investigated the touch-sensing technique, which inserts an electrical wire to locate the gap and provide feedback on the offset, but it increased cycle time too substantially. Other optical sensors offered two-dimensional information, but Price decided they provided insufficient feedback to position the welder accurately. Finally, he selected the Circular Scanning Sensor from Oxford Sensor Technology Ltd.

With the help of a laser-based three-dimensional sensor, robotic welders reduced the number of reworked wheel arches from 40 percent to about 2 percent. Courtesy of Vauxhall Motors Ltd.

The sensor uses a visible red diode laser to measure both the width and height of the gap distance. "It actually gives you the data in a three-dimensional representation of what it's looking at, and you can take any info that you need from the measurement," said Price.

The sensor helped the automaker reduce the number of reworked wheel arches to about 2 percent. This heightened accuracy costs two or three seconds of cycle time per part, but at 300 parts a day that reduces production by only 15 minutes -- less time than it takes a welder to complete one part.

The application requires positioning accuracy down to 0.2 to 0.3 mm, or the automated welder will burn a hole through one side of the part, said Wolfgang Kölbl, Oxford's CEO. The system collimates the laser light at an angle to the seam the welder will attach. A linear CCD camera captures the reflected light through a revolving lens, which provides a radial spoke measurement. This not only indicates the position of the two parts before they are welded, it also tracks the direction of the seam. Digital signal processing helps to align the welder to the sensor's measurement.

Price speculated that the laser could enable the sensor to be tailored to different applications at the plant. "We thought of all sorts of qualities that it could measure, including height, flux, depth measurements -- all of which are significant in the car industry," he said. "The laser gives you all that back in one junk."