Image Processing and Control of Weld pool in Switch Back Welding without backing Plate

摘要

The formation of stable back beads in the first layer weld during one side multilayer welding is important to achieve high quality welded metal joints. The authors thus employed the switch back welding method for V groove joints without backing plates. In this method, the power source, wire feed motor, and robot manipulators were controlled by the computer based cooperative control. The torch is moved backward and forward like switch back. First, the torch is moved forward with high speed. The arc is discharged to the each root edge of the groove. The droplet is deposited to each root edge, too. After that, the torch is moved backward with the same high speed. During the backward motion, the deposited metal makes the bridge between the both edges due to the surface tension. Next, the torch is moved forward with slow speed to form the bead. The mentioned torch motion is repeated. When constant welding conditions are used, the back bead becomes unstable by affection of the disturbances such as gap variations due to the welding distortion. In this method, it is important to measure the gap. The authors discuss a method to detect the gap and to get the stable back bead by using a visual sensor, the CCD camera. During the slow forward torch motion, the weld pool can be observed. The shape in the front part of the weld pool depends on the gap. It can be detected by processing the weld pool images. The authors investigate the method to take the gap from the weld pool image. The use of feed-forward control of back beads to compensate for variations of root gaps was also tested. In order to verify the performance of the feed forward controller, the gap is changing from 4.6 mm to 2.5 mm without the tack welding.