Eddy Current Probe Design and Matched Filtering for Optimum Flaw Detection

Abstract

Eddy current signals obtained from variations in the probe liftoff are in general much larger in amplitude than the useful flaw signals. Small flaw signals can, however, be detected in the presence of liftoff noise if a large enough phase angle exists between them. Figure 1(a) shows how this phase discrimination can help in liftoff noise suppression. Here, the oscilloscope traces the complex impedance of the probe. The impedance plane has been rotated so that the liftoff noise lies entirely in the horizontal channel. Now if we choose to look only at the signal in the vertical channel of the scope, or the Q channel (in phase quadrature with liftoff), there will be no liftoff noise. This, however, is not a very realistic picture. Figure 1(b) is obtained when we try to detect much smaller flaws (in this case a closed crack of 20 mils in aluminum). We see that the trace of the liftoff noise has a curvature and that there are also fluctuations along the Q channel axis. Both of these effects eventually limit the detectability of small flaws. Since this contribution of liftoff to the Q channel is in practice larger than circuit noise, we define the detection figure of merit for an EC probe as

$$D = \frac{{(\Delta {Z_f})\sin \beta }}{{{{(\Delta {Z_{\ell O}})}_Q}}}.$$

(1)