The Xiris Blog

Using a Weld Camera for Monitoring Plasma Arc Welding (PAW)

Posted by Cameron Serles on Monday, April 29, 2013 @ 11:01 AM

Plasma Arc Welding (PAW) is an arc welding process similar to Gas Tungsten Arc Welding (GTAW). The electric arc is formed between a non-consumable electrode (which is usually but not always made of sintered tungsten) and the work piece. The key difference between PAW and GTAW is that in PAW the electrode is positioned within the body of the torch, allowing the plasma arc to be separated from the shielding gas envelope.

The plasma is then forced through a fine-bore copper nozzle, which constricts the arc, and the plasma exits the orifice at high velocities (approaching the speed of sound) and a temperature approaching 20,000 °C.

Plasma Arc Welding can be used to join all metals that can be welded with GTAW (i.e., most commercial metals and alloys), but PAW has a greater energy concentration than GTAW and therefore allows for deeper penetration and deposition rates.

 

 

PAW process

                                                                 (courtesy www.substech.com)

 

How Weld Cameras Can Improve PAW

Weld Cameras allow Plasma Arc Welding to be monitored remotely, providing several key benefits. Operators can:

  • View through the plasma arc to see the weld seam for verifying alignment.
  • Monitor the shape of the keyhole in the center of the weld.
  • Ensure the correct flow of molten material around the keyhole out of the heat-affected zone into the weld pool.
  • Ensure that variations in fit-up of work pieces and weld head are minimized.
  • Get a good view of the plasma weld cone to ensure it is properly aligned with the pieces it is welding.
  • Confirm that the weld seam and pool are properly aligned and formed during the welding process.

To gain these advantages, the Weld Camera needs to be industrially hardened and have High Dynamic Range imaging capability in order to get high-quality, detailed images despite the extreme range in brightness between the plasma arc and the dark weld background.

Implementation Considerations

  • The intensity of the weld arc is a function of the weld power supply current—as the current increases, so does the intensity of the weld arc.
  • A small Field of View is required to properly image the welding detail, as a plasma arc head is quite small.
  • PAW uses a Pilot Arc source when the welding process is not running that provides enough light to illuminate the local welding area for monitoring the welding work pieces and aiding in set-up of the weld process.

 

 Image of plasma welding

Plasma Welding with Gas from Top

 

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Topics: camera selection, weld camera, field of view, PAW

How to Select the Field of View for a Weld Camera

Posted by Cameron Serles on Monday, April 22, 2013 @ 09:45 AM

The Field of View you choose for your Weld Camera will play a critical role in getting the best-quality images for viewing the required details of your automated welding process. 

But how large an FOV should you use for your camera? There are several factors to consider.

Camera Angle

The first question to ask is: What angle will we place the camera relative to the work pieces? The steeper the angle of the camera, the smaller the view of the weld head in the image and the larger the view of the weld bead and work piece. The shallower the angle, the larger the view of the weld head and the smaller the length of the weld bead and work piece in the image.

Depth of focus is an issue if you are looking at the weld using a camera that has been mounted at a low angle. The most important features in the weld should be kept in focus and be placed in the center of the FOV.

Camera Placement

When viewing the weld from the front, a general rule of thumb is to choose an FOV that is 4 x the bead width to be imaged. This FOV should provide you with most details of the weld in good image resolution.

If the camera is mounted from the side or behind the welding process, you need to ask yourself: What is the distance from the center of the weld head to the point in the weld bead where solidification occurs? It’s most useful to the operator if all these features are visible in one image.

Ideally, you want to be able to see the weld bead as long as it is molten, the torch tip/arc, and some of the parent material (including the unwelded seam) all in a single view.

Type of Welding

For Narrow Groove welding, especially for the root pass where detail of the weld and the seam alignment are most critical, a suggested FOV could be 2 x the bead width. This would allow a closer view of the edges of the seam groove and weld pool to verify the best alignment between groove and weld head.

For subsequent passes after the root pass where weave welding occurs or where multi-pass welding is to take place, a larger FOV should be used to see the full movement of the weld head in a groove. In such cases, the FOV should be large enough to see the entire V groove.

In Cladding/Overlay applications, generally speaking, an FOV of 4 x the bead width should provide enough image size to see 1-2 previous passes of weldment, plus the current pass.

When filler material is used in a welding process, such as in GMAW (MIG), you need to select a FOV that will encompass the filler tip as well as all the other features of interest.

In Laser welding, special care must be made in selecting the appropriate FOV. Laser welding typically has very small welds and the Weld Camera may be limited as to how small an FOV can be used before optical distortions create a problem in the weld images. 

Because Laser welding is typically a higher-speed application, it may be useful to choose a larger FOV (e.g., greater than 4 x the weld bead in this case) so as to see the entire molten pool after the weld head, as the tail of the weld pool may be longer relative to its width than what is found on MIG or TIG welding processes.

Conclusion

Selecting the right FOV is crucial in getting the highest-quality images for remote weld monitoring using a Weld Camera. Your decision should be based on your particular welding environment and operational demands.

Topics: weld camera, camera placement, field of view

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