1. Field of the Invention
The present invention relates to a laser beam welding apparatus having welding robots with scan heads for welding a workpiece at a plurality of locations thereon highly efficiently with laser beams.
2. Description of the Related Art
Resistance spot welding robots have heretofore been used to weld inner and outer panels of automobile bodies, for example. The resistance spot welding robots have a welding gun movable to various welding locations on an automobile body that has been delivered to a predetermined welding station.
Since the welding gun of the resistance spot welding robots is relatively heavy, it takes a considerable period of time to move the welding gun through the welding locations. Therefore, workpieces cannot efficiently be welded by the resistance spot welding robots. The time required to weld a workpiece may be shortened by increasing the number of welding robots used.
However, simply increasing the number of welding robots used would result in an increase in investments for the welding facility and also an increase in the length of the welding line because of installation spaces required by the welding robots. The welding guns of the resistance spot welding robots weld workpieces with welding tips thereof which are held in contact with the workpieces. Consequently, the welding tips tend to stick to the workpiece and are subject to wear. It is thus necessary to monitor the welding tips at all times for their sticking to the workpiece, and to shut down the welding line and take necessary measures when the welding tips stick to the workpiece. Furthermore, the welding tips need to be dressed periodically for welding workpieces under optimum welding conditions.
To eliminate the shortcomings of the resistance spot welding robots, a welding robot for welding an automobile body with a laser beam has been proposed as disclosed in Japanese laid-open patent publication No. 4-220187. The disclosed welding robot has a laser beam applying head mounted on a tip end thereof and movable to a position near a welding location of the automobile body for effecting a desired welding operation on the automobile body. The disclosed welding robot is free from the problem of sticking and wear of a welding tip.
However, inasmuch as the laser beam applying head is brought into a given position near a welding location of the automobile body upon movement of the welding robot, it is impossible to move the laser beam applying head at a relatively high speed. There is no guarantee that the automobile body to be welded will accurately be positioned with respect to the welding robot and hence that the automobile body will accurately be welded.
Another conventional welding apparatus disclosed in Japanese patent publication No. 4-36792 has a plurality of welding heads that can be positioned in the vicinity of a welding location on a workpiece. A desired one of the welding heads is selected by adjusting the angle of a plane mirror, and a laser beam is applied from the selected welding head to the welding location on the workpiece. The disclosed welding apparatus is capable of welding a plurality of locations on the workpiece within a relatively short period of time.
However, because the welding heads are fixed relatively to the workpiece, they cannot effect continuous welding, i.e., seam welding, on the workpiece though they can spot-weld the workpiece at given locations thereon. Furthermore, inasmuch as the welding heads are required for each of welding locations on the workpiece, the welding apparatus is highly expensive. In addition, the positions and number of the welding heads have to be changed depending on the type of the workpiece to be welded, and such a changing process is tedious and time-consuming. The welding heads that are positioned near welding locations on the workpiece tend to be contaminated due to sputtering in a welding process, resulting in a reduction in the welding accuracy.