However precise a welding robot is, the seam it welds is not in the same place on every part. Seam tracking is the system that keeps the torch of a robot or welding carriage on the real seam. In this article we explain why seams drift, which tracking methods exist and why torch distance matters as much as seam position.
Why does the weld seam drift?
- Cutting and bending tolerances: Two parts never fit together in exactly the same place.
- Fixture and rail alignment: The fixture or the carriage rail cannot be set exactly parallel to the seam.
- Heat distortion: Welding heat pulls the plate; the seam moves as welding progresses.
- Plate waviness: Long, thin plates wave under their own weight; both the seam and the surface height change.
Seam tracking methods
| Method | How it works | Watch out for |
|---|---|---|
| Touch sensing | The wire or nozzle touches the part before welding to locate the seam. | Done before welding; does not see distortion during welding. |
| Through-arc sensing | Derives the seam centre from changes in welding current while the torch weaves. | Needs weaving and a suitable joint geometry. |
| Laser line scanner | Projects a laser line onto the seam and reads its profile with a camera. | Precise; can be expensive. |
| Camera with image processing | Images the weld zone and finds the seam centre with image processing. | Needs correct positioning and filtering against arc light and surface glare. |
Torch distance: the forgotten second axis
Seam tracking usually brings to mind only the torch moving left and right. Yet the torch’s distance to the part directly affects weld quality: as it changes, wire stick-out, current and penetration change. If the torch moves away, the weld weakens and gas shielding suffers; if it gets too close, the nozzle and contact tip foul quickly. Good seam tracking therefore follows both lateral position and height.
The DKT system: 1 camera and 2 lasers
On DKT welding robots seam tracking has two parts. A camera ahead of the torch scans the weld zone continuously, finds the seam centre with image processing and corrects the torch itself if it drifts. 2 laser distance sensors measure the distance between torch and part continuously and stop the torch from moving away or getting too close. The system is on both our welding carriages and our robot arms; on the robot arm the camera also recognises the welder’s chalk marks with AI.
Making seam tracking work well
- Prepare the joint consistently along its length
- Clean oil, rust and paint from the weld zone
- Protect the camera and sensors from spatter and clean their windows regularly
- Set the rail or fixture as close to the seam line as possible; the system corrects the remaining difference
Summary
- Seams drift because of tolerances, fixture alignment, heat distortion and plate waviness.
- Touch sensing, through-arc sensing, laser scanners and cameras are the main tracking methods.
- Torch distance matters as much as seam position.
- In the DKT system the camera tracks position and 2 laser sensors track distance.
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