work-piece, and clear images of keyhole are obtained [37,38].
Fig. 18 shows the experimental system for observing the keyhole
image from the backside of the test plates. The CCD camera
(AM1101A) is equipped with a narrow-band pass filter (central
wavelength is 655 nm, bandwidth is 40 nm, and transparency
is 85%) and a neutral filter, and an image grabber (OK-M10A).
Because of the large volume of the PAW torch, it obstructs the view
of observing the keyhole image from the topside of the test-piece.
Thus, the CCD camera aims at the back-side of the test-piece to
observe the keyhole image. If the work-piece is fully penetrated
and an open keyhole is formed, the keyhole image is captured
Fig. 17. Simultaneous imaging of weld pool and keyhole from backside [36].
by the CCD. The captured keyhole images are digitized through
the image grabber. During welding, both the PAW torch and the
CCD camera are stationary, while the work-piece is traveling at
the welding speed, which is controlled by the computer. The
observation is toward to the rear of the weld pool, as shown in
Fig. 18, and the viewing angle is set to 50◦. The distance from
the camera lens to the object plane is around 150 mm. Fig. 19
shows the underside keyhole images under different levels of
welding current. After camera calibration and image processing,
the keyhole shape and size can be determined.
4.6. Control
If the keyhole status (open or blind, and shape and size)
in PAW is reliably detected in real-time, the process control
and automation of the PAW process is able to be realized. For