MIG processes, particularly short circuit MIG, will generate a huge range in brightness during their metal transfer cycle: when the arc is extinguished as the wire makes contact with the parent material prior to expulsion, the image can be quite dark. However, after an explusion occurs and the arc is re-established, the image may be very bright as the arc intensifies to its maximum.Using a camera to acquire images of a MIG weld process in free running mode can be problematic when the amount of light present in the image varies considerably. The variation in light is based on when during the metal transfer process the image exposure takes place: when the arc is extinguished, the image will be dark; when there is a full arc, there will be a bright image. However, if the camera acquisition is triggered by an electrical pulse generated by the camera power supply, the result will be a consistent image of the weld process that is repeatable because it is at the same point of the weld cycle.
(courtesy ESAB Group, Inc.)
A few words about how Short Circuit MIG and certain other kinds of MIG welding function:
- Wire is fed continuously and makes contact with the workpiece to complete the electrical circuit.
- At the point of contact, a short circuit occurs, resulting in a huge spike of current moving through the wire between the torch and the workpiece.
- At point of wire contacting the workpiece, arc gets extinguished.
- Segment of wire rapidly vaporizes under high current and an arc gets re-established.
- Current falls as there is no short circuit.
- Process repeats.
In a constant voltage welding power supply, the current being fed to the torch can rise and fall based on the metal transfer process. When there is a gap between the wire and the workpiece, the conducting current is low, and increases as the wire begins to touch the workpiece and create a short circuit. Then, once the wire tip explodes, the current falls as there is no conducting circuit. The plot of the current levels look something like this:
(courtesy ESAB Group, Inc.)
While capturing the welding process to see certain features, it is sometimes interesting to only take images at a certain point in the metal transfer cycle. Rather than using a weld camera in free running mode where image acquisition is based on the clock cycles inside the camera, an efficient alternative is to use an external trigger that is based on the current levels present in the welding power supply. If a circuit can be designed to generate a trigger signal based on the rising edge of the current level, then the trigger could be used to initiate image acquisition, resulting in video with an increased consistentency in brightness and quality because each frame will be acquired at precisely the same point in the metal transfer process.
Further enhancement to the performance of the imaging process is possible by tweaking exactly when the images are acquired through adding a delay. A delay can be added after the trigger signal is generated so that the exact imaging characteristic can be seen.
For example, imagine wanting to see only images of the metal transfer process after the weld arc is extinguished. To do this, a trigger signal should be generated based on the current pulses coming from the weld power supply. It may not be possible to receive the trigger at the ideal point in the metal transfer process, so a programmable delay can be added to make sure that the image acquisition occurs at exactly the right point.
Successive Snapshots of a MIG Welding Process Triggered from the Rising Edge of Welding Power Supply’s Current Pulse
The above two successive images show a MiG process at roughly the same point of the metal transfer process over different cycles of the metal transfer. In this case, the imaging was tuned to see exactly what the viewer wanted to see: the melt pool fully visible with the welding arc present.
Imaging a MIG welding process can be fairly difficult if using a weld camera in free running mode. However, if a circuit can be designed to clamp on the rising edge of the current pulse, it can provide an excellent trigger to use to acquire consistent images at similar points in the metal transfer cycle. The result is much more uniform images with similar brightness levels, allowing for better analysis and increased efficiency of the welding process.
Better Images. Better Decisions. Better Process Control.
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