Frequently asked questions
Due to the relatively low tolerance of laser welding to joint position, joint misalignment and/or joint gaps, joint tracking systems offer the potential for improved weld quality assurance by allowing real-time on-line adjustment of the laser welding head position with respect to the work piece during welding to compensate for small variations in joint position and fit-up.
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 positioning system to send trajectory corrections, maintaining the tool centre point at the optimum position in the joint. Depending on the equipment and the approach, this correction operation can be done either in two steps (‘first track, then weld’) or in a single step (‘simultaneously track and weld’). The high degree of accuracy achieved while welding can not only improve weld quality, but also 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 different requirements for joint tracking system specifications. For laser welding, fast data handling is very important, due to the higher welding speeds achieved by lasers. Especially when tracking non-linear joint geometries, this results in the need for more frequent positional update information signals to the control system, if an overall faster system response is to be achieved. 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. As a consequence, laser welding heads have to be more accurately aligned with the joint (to within a few tenths of a millimetre or less, typically) to avoid the occurrence of weld imperfections, such as porosity or lack of sidewall fusion.
Vision-based sensors are currently used for the majority of joint tracking systems with 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 lower accuracy and signal update rate.
For laser welding applications, laser vision-based sensors often come in the form of low-power infra-red or visible lasers whose beams are either focused to a spot on the workpiece surface or 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 determine both the joint position and fit up.
Seam tracking systems suitable for laser welding are now commercially available. Examples include their use with robots for welding car frames, for pipe welding with automated machines, shipbuilding and welding railcars
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
- 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
- Sharp profiles such as small radius shapes requiring abrupt changes in direction whilst welding at high speeds can be difficult to track with the precision required for laser welding.
For further information see Joining Technologies.