Electromagnetic Testing - Eddy Current Testing Applications Chapter 5 & 6

5.2.1.6 Signals from differential probes
The signals produced by a differential probe have a characteristic appearance
as shown in FIG. 5.38. This is because only differences in the conditions sensed
by the two coils are detected. As the probe is pulled along the tube, the first coil
detects the particular condition, and so a signal very similar to that produced by
an absolute probe is produced. However, as the probe is pulled further along,
the second coil starts sensing the condition. The difference between the two
coils is reduced, and so the signal amplitude decreases until, when the condition
is detected equally by the two coils, the spot returns to the operating point. As
the probe is pulled further along, the condition is sensed more by the second
coil than the first coil, so the bridge becomes unbalanced in the opposite
direction, and a signal in the opposite direction is produced.
Charlie Chong/ Fion Zhang