To show detail in the arc and puddle, one wants to zoom in and shoot a close-up. However, a close-up shows a small area and a moving weld will quickly move off-screen. To address this, and allow longer shots when zoomed-in electronic arc-tracking was added to the WeldWatch camera system.
The system includes two varieties of arc-following. One keeps the arc centered while the second “follows” the arc, reducing the speed of movement, but still allowing it to move from the center of the screen.
The arc tracking systems do not require physical movement of the camera. The compensation is accomplished electronically by dynamically adjusting the camera in real time.
The full sensor contains 1280 x 1024 pixels, while the displayed image uses a smaller 640 x 480 pixel rectangle of sensor pixels: Using a smaller region improves camera performance by reducing bandwidth. The relationship of the displayed image to the full camera detector is shown below.
Normally, the video image (640 x 480 pixels) is taken from the center (sweet-spot) of the full sensor (1280 x 1024 pixels). The figure above shows the display window centered on the array, with pixel column and row coordinates labeled for the detector array origin and two corners of the display window.
The rectangular region-of-interest does not have to be centered in the sensor, under program control, it can be positioned anywhere within the camera sensor array. If it is continuously located over the arc, no matter where the arc moves, this will effectively center the arc on the display screen.
An example of a centered arc image and the total sensor array is shown below:
As the arc moves, the displayed rectangle boundaries are continuously moved to compensate, keeping the arc centered. The figure below shows this display window following the arc as it moves from left to right across the camera sensor. In each case, the arc is in the center of the display area. Onscreen, the arc appears to remain in the center of the video screen.
To provide smooth movement of the display window a PID (proportional-integral-derivative) control loop with an IIR (Infinite Impulse Response) filter was used. This steadies the view when not moving and provides quick movement with a soft landing when it does move. The control loop is tuned so the display window reaches the arc in less than 2 frames and settles in 5 frames (about 1/6th of a second).
A different requirement arises when welding-by-camera using video goggles, e.g. for blind welding behind obstructions. When welding with goggles, auto-centering can be disconcerting as hand and torch movements do not result in onscreen movement – the arc stays still in the center of the screen.
To provide an extended view of the arc and yet maintain movement for eye-hand coordination a 2nd system, “following”, was developed. Following only partially compensates for arc movement, while preserving some visible movement.
Field tests found that a 50% movement (half as much as arc-centering) is a good compromise. This yields twice the range of a fixed view and yet still provides good feedback to the welder.
The position of the display window in Following mode is illustrated below.
Even though the display window tracks the arc, the arc is allowed to move off-center.
In summary, arc-tracking in WeldWatch can be enabled in one of two modes, Centering or Following. When Centering is enabled, the displayed rectangle is continuously centered on the arc in real time, resulting in the arc appearing to stay centered throughout.
In Following the display window tracks the arc, but the arc is allowed to move at half speed. This supports welding with video goggles where the welder needs to see movement for hand-eye coordination.