TWI Frequently asked question
Due to the relatively low tolerance of laser welding to joint misalignment and/or joint gaps, joint tracking systems offer the potential for improved quality assurance by allowing on-line adjustment of the laser welding head position with respect to the workpiece during welding to compensate for small variations in joint position and fit-up.
Real-time joint tracking is performed by a sensor that first detects the position of the joint and then guides an automatic welding machine, such as a robot or a multi-axis tool positioning system, during welding. The sensor communicates with the robot to send trajectory corrections, maintaining the tool centre point at the optimum position in the joint. The high degree of accuracy achieved while welding improves productivity by significantly decreasing the amount of operator monitoring and intervention required, increasing the welding travel speed, and reducing tooling costs.
The generic differences between laser and arc welding result in a requirement for different joint tracking system specifications. Fast data handling is important due to the high welding speed of lasers. This increases the processing speed, especially when using non-linear joint geometries, which results in the need for more frequent positional update information signals to the control system with ultimately faster system response. In addition, seam tracking equipment for laser welding requires more accurate positioning. Laser welds are generally smaller (narrower) than arc welds made in the same material thickness and therefore laser welding heads need to maintain more accurate alignment with the joint to avoid the occurrence of weld imperfections such as lack of side-wall or root fusion.
Vision-based sensors are currently used for the majority of joint tracking systems used for laser welding applications, due to their high accuracy and signal update rate. Tracking systems used for conventional arc welding processes - such as voltage sensors, tactile sensors, ultrasonic sensors, eddy current sensors or plasma sensors - are either not applicable or not suitable for laser welding applications due to their general lack of accuracy and low signal update rate.
For laser welding applications, auxiliary lighting is used with laser vision-based sensors. This often comes in the form of a low-power infra-red or visible laser which is either focused to a spot on the workpiece surface and scanned across or around the joint or focused to a line or series of lines on the workpiece surface. The laser light is reflected to the camera, providing an image that can define both joint position and gap.
Seam tracking systems suitable for laser welding are now commercially available. They have been used with robots for welding car frames and for pipe welding with automated machines, as well as many other applications in both arc and laser welding.
However, it should be noted that some limitations exist:
- welding speed is limited by the rate of acquisition and calculation of data and also by the response of the axes providing the feedback, but can reach speeds of about 10m/min depending on the system
- the fundamental requirement for close fit-up in laser welding may also be a limitation since closely butting edges may not produce sufficient reflected signals to allow the seam to be detected; this effect is worse on highlyreflective materials such as stainless steel or aluminium
- sharp profiles such as small radius shapes requiring abrupt changes in direction whilst welding at high speeds can be difficult to detect and to track with the precision required for laser welding.
