|
In this
month's "NDT Solution", the authors try to dissolve some commonly
held misconceptions about the operation of eddy current probes
used for inspecting heat exchanger tubes. With the help of
computer model results, the authors demonstrate that, for
optimized operation, absolute/differential probes should be
operated with coil current flowing in phase with another.
This month's feature should be of interest to researchers
as well as for NDT field personnel.
G.P. Singh
Associate Technical Editor |
Introduction
Modern
instrumentation permits inspection of heat exchanger tubes with eddy
current bobbin coil probes operating in absolute and differential modes
simultaneously. Many believe that differential operation of these probes
requires the current in one coil to be 180 degrees out of phase with
the current in the other. This paper shows that differential operation
of these probes does not depend on the relative sense of the drive current
in the coils but that optimized absolute operation requires that the
drive current in one coil flow parallel to the current in the other.

Figure
1 Schematic of a differential bobbin probe scanning a heat
exchanger tube.
When eddy current probes
were first used for inspecting heat exchanger tubes, they were generally
composed of two identical bobbin coils mounted closely together, operating
in the differential mode. A schematic of such a probe is shown in Figure
1. The recorded signals were the induced voltage in one coil subtracted
from the voltage in the other coil. The advantage of this probe is that
its signal is resistant to various anomalous effects, such as probe
wobble, temperature variations, and gradual variations in the inspected
tube's electrical conductivity, diameter, and ovality. This probe is
very sensitive to abrupt anomalies, such as pitting corrosion and fretting
wear.
Differential
signals are not dependent on whether the current in the coils is flowing
in phase.
Some researchers
had developed computer modeling tools to study the electromagnetic operation
of differential probes and to optimize probe designs for future inspections.
These researchers used finite element modeling (FEM) to generate magnetic
field distribution displays (Palanisamy and Lord, 1980; Palanisamy and
Lord, 1983) and impedance plane signals from anomalies in such items
as support plates (Palanisamy and Lord, 1980; Palanisamy and Lord, 1983;
Ida et al., 1983). They modeled probes with current in one coil flowing
180 degrees out of phase with the other coil.
As reported by Cecco,
Van Drunen, and Sharp (1983), and by many inspectors in the field, the
differential probe was found to be insensitive to gradually varying
wall thinning resulting from corrosive wastage or tube to tube fretting.
To detect such wall thinning, which was often quite severe, absolute
eddy current probes were required. The first absolute probes were operated
with a single bobbin coil, as shown in Figure 2. A second reference
coil, which was used for electronic balancing, was electromagnetically
isolated (shielded) from the inspected tubing. Although this probe was
also sensitive to pitting and fretting wear, it was not as sensitive
to these anomalies as a differential probe.

Figure 2
Schematic of an absolute bobbin probe scanning a heat exchanger
tube.
With modern instrumentation,
bobbin probes are now generally operated simultaneously in absolute
and differential modes. Such probes have optimized sensitivity to abrupt
anomalies, while retaining sensitivity to gradual wall loss. For the
absolute signals, these probes are electronically balanced with identical
probes, which are in isolated reference tubes. It is now common practice
to operate these probes with the current in both coils flowing in phase.
This is contrary to the intuition of most people, including many certified
inspectors, who believe that differential operation of these probes
requires the current in one coil to be 180 degrees out of phase with
the other.
This paper shows that
differential signals are not dependent on whether the current in the
coils is flowing in phase or if the current in one coil is 180 degrees
out of phase with the current in the other. In addition, computer modeling
results are presented, showing that such probes should be operated with
coil current flowing in phase in both coils for optimized absolute mode
operation.
Theory
Eddy current signals are displays of recorded voltage induced in
the probe coils during inspections. In the case of bobbin probes, such
as the one in Figure 3, the voltage across the leads of each of the
probe coils is given by Equations 1 and 2:
| (1) |
V1
= (I *
Z11) ± (I * Z12) |
| (2) |
V2
= (I *
Z22)
± (I * Z21)
|
where I is the current flowing
in the coils, Z11 and Z22 represent
each coil's self impedance in a tube at a given operating frequency,
and Z12 and Z21 are the mutual
impedances between the coils. The ± sign in each of the equations is
dependent on whether the probe's coil current flows in phase or 180
degrees out of phase in the two coils.
The differential signal generated
by the probe is equal to the difference in voltages across the coils.
The law of reciprocity states that Z12 = Z21.
Equation 3 expresses the voltage used for the differential signal:
| (3) |
Vdifferential
= V -
V2
= (I * Z11)
- (I *
Z22) |
Because of the law of
reciprocity, mutual impedance terms and ± signs are eliminated when
subtracting Equation 2 from Equation 1 to obtain the differential voltage
in Equation 3. The differential signal is, therefore, independent of
whether the current in the coils flows in phase or if the current in
one coil is 180 degrees out of phase with the current in the other.

Figure 3
Finite element modeling of an absolute/differential probe with
drive current (a) in one coil 180 degrees out of phase with the
other; (b) in phase in both coils.
Computer Modeling Results
We used the MagNet FEM code (Infolytica, 1995) to generate magnetic
field patterns, as shown in Figure 3. The pattern in Figure 3a is from
a probe with the current in one coil 180 degrees out of phase with the
current in the other and is similar to those produced by Palanisamy
and Lord (1980 and 1983). The pattern in Figure 3b is generated when
the current in both coils is in phase. Although the patterns are significantly
different, the differential probe signal is independent of the relative
sense of the currents.
Figure 4 shows absolute signals
from a true absolute bobbin probe and from a probe operating simultaneously
in absolute and differential modes. Figure 4a shows the signal from
the true absolute probe. Figure 4b shows a calculation of the absolute
signal from an absolute/differential probe with the current in one coil
180 degrees out of phase with the other coil. This signal resembles
a distorted differential signal. Because of such distortion in the signal,
practical usage of such a probe would be extremely difficult even for
trained analysts of eddy current signals. The absolute signal calculated
with in phase coil current is shown in Figure 4c. Although this signal
is not completely identical to the true absolute probe signal, it is
similar, and it possesses the important unipolar qualities required
in an absolute probe. The absolute signal shown in Figure 4c looks much
more like a true absolute signal (shown in Figure 4a) than the absolute
signal shown in Figure 4b. In Figure 4b, the absolute signal is composed
of the self impedance from one coil minus the mutual impedance from
the cross talk between the two coils. In Figure 4c the absolute signal
is composed of the self impedance of the one coil plus the mutual impedance
from the cross talk between the two coils. Because of results such as
these (which had been observed previously by probe manufacturers and
inspectors), such probes are now manufactured so that they operate with
the drive current flowing in phase in both coils.

Figure
4 Computer simulation of absolute anomaly signals from
(a) a true absolute probe; (b) an absolute/differential probe
with drive current in one coil 180 degrees out of phase with the
other, and (c) an absolute/differential probe with drive current
in phase in both coils.
Experimental Verification
Figure 5a shows differential probe signals, measured in a laboratory,
from an absolute/differential probe with the current in one coil 180
degrees out of phase with the current in the other coil. Figure 5b shows
a differential anomaly signal, measured in a laboratory, from an absolute/differential
bobbin probe operating with the drive current flowing in phase in both
coils. The lower traces are strip chart displays of the respective imaginary
(left) and real (right) components of the eddy current probe signals.
The signals in Figures 5a and 5b are virtually identical, which is in
agreement with the prediction of Equation 3.

Figure 5
Laboratory measured differential probe signals with drive current
(a) in one coil 180 degrees out of phase with the other; (b) in
phase in both coils. The lower traces are strip chart displays
of the respective imaginary (left) and real (right) components
of the eddy current probe signals.
Figure 6a shows a signal from a
true absolute probe scanning an anomaly in a heat exchanger tube measured
in a laboratory. The lower traces are strip chart displays of the respective
imaginary (left) and real (right) components of the eddy current probe
signals. Figure 6b shows an absolute signal, measured in a laboratory,
from an absolute/differential probe with the current in one coil 180
degrees out of phase with the current in the other coil. Figure 6c shows
an absolute signal, measured in a laboratory, from an absolute/differential
bobbin probe operating with the drive current flowing in phase in both
coils.

Figure 6
Laboratory measured absolute anomaly signals from (a) a true absolute
probe; (b) an absolute/differential probe with drive current in
one coil 180 degrees out of phase with the other; and (c) in phase
in both coils.
Summary/Conclusions
Absolute/differential probes should be operated with current in the
two bobbin coils flowing in phase with one another to optimize absolute
signals. Differential probe signals are independent of the relative
sense of current flowing in the coils. That is, differential signals
from a probe with the current flowing in the same direction in both
coils are identical to signals from a probe with the drive current in
one coil 180 degrees out of phase with current in the other.
Acknowledgments
The authors are indebted to William Lord, Nathan Ida, and Valentino
Cecco for helpful discussions and comments. This work was supported
by the CANDU Owners Group (COG) Work Package Identification and Release
(WPIR) number 2029.
References
Cecco V.S., G. Van Drunen, and F.L. Sharp, "Eddy Current Manual," Vol.
1, AECL Report, AECL-7523, Chalk River Laboratories, 1983.
Ida, N., R. Palanisamy, and W. Lord,
"Eddy Current Probe Design Using Finite Element Analysis," Materials
Evaluation, Vol. 41, No. 12, November 1983, pp. 1389–1394.
Infolytica, MagNet 5, 2D ToolBox
Reference Manual, Infolytica Corp., March 1995.
Palanisamy, R., and W. Lord, "Prediction
of Eddy Current Probe Signal Trajectories," IEEE Transactions on
Magnetics, Vol. 16, No. 5, September 1980, pp. 1083–1085.
Palanisamy R., and W. Lord, "Prediction
of Eddy Current Signals for Nondestructive Testing of Condenser Tubing,"
IEEE Transactions on Magnetics, Vol. 19, No. 5, September 1983,
pp. 2213–2215.
* Components
& Systems Division, Nondestructive Testing Development Branch,
Chalk River Laboratories, Chalk River, Ontario, Canada K0J 1J0; (613)
584-8811; fax (613) 584-4523; e-mail sullivans@aecl.ca.
Copyright © 2000 by
the American Society for Nondestructive Testing, Inc. All rights reserved.
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