They found that the thermal map for each weld could be characterised in a simple way. First, the maximum temperatures of each profile are averaged over the length of the weld to produce an average temperature for the whole weld. Second, the variation of the maximum temperature from profile to profile, defined by the standard deviation, is calculated. A weld is deemed acceptable if its average temperature and the standard deviation of the temperature fall within specified limits. The computer keeps a record of the last 2,000 welds and alerts the operator if there is any drift in weld temperature which may indicate that the process is deteriorating.
Appropriate maintenance can then take place before the quality becomes unacceptable.
Although the system worked well, there were several practical problems that needed to be overcome. The camera was bulky and cumbersome and needed mirrors to view the weld without obstructing access for maintenance. The scanning mechanism used rotating prisms which required regular attention, and the optical window became contaminated with smoke from the welding process and needed frequent cleaning. Being so close to the welding point, the camera suffered from electrical interference. It turned out to be an unnecessarily complicated and very expensive solution to the problem.