With the automation of arc welding and several other industrial processes, much effort has gone into the development of automatic seam tracking systems. Generally, a robot is programmed using a teach pendant to direct the robot from point to point of the seam. This is a rather precise operation since it is this taught path which a robot will following during processing. As the parts become more complex, this job becomes very tedious and can take several hours. Also, due to part-to-part variations and limited precision in the robot and part fixturing, the robot may not repeatedly follow the seam, resulting in poor quality welds.
A number of methods employing a variety of sensing systems have been applied to seam tracking. Mechanical and electromechanical contacting sensors employing mechanical probes have been used on long, straight, voluminous seams with success. These systems are simple and inexpensive but are intrusive and limited to very simple parts and joints. Eddy current devices and other types of magnetic sensing systems are now being marketed for seam tracking applications having the advantages of low cost high resolution and simplicity. However, these devices have suspect reliability, especially for some joint types, and require perfect fixturing. They are also intrusive and sensitive to electromagnetic interference and weldment temperatures.
Computer vision based seam tracking methods are now becoming more prevalent. These techniques employ a matrix camera to view the seam. A coaxial viewing torch for welding is shown in U.S. Pat. No. 4,595,820 issued to Richard W. Richardson. It uses arc illumination from the torch to highlight the seam. The approach is nonintrusive and has a fairly high resolution but will not work for all joint types and is limited to certain types of welding. The torch is also expensive. Other approaches such as shown in U.S. Pat. No. 4,542,270 issued to Alan W. Case, Jr. et al use structured lighting (usually a plane of laser light) to cast the pattern on the seam. The advantage of this approach is good reliability and the ability to measure three dimensional joint geometry. However, these systems can be very expensive, sometimes lacking the resolution required and can be fairly intrusive.