Eddy current probe with transverse polygonal detecting coil

Details Eddy current probe with transverse polygonal detecting coil Eddy current probe with transverse polygonal detecting coil

2002-10-01

2004-09-14

Patidar, Jay (Department: 2862)

Electricity: measuring and testing

Magnetic

With means to create magnetic field to test material

C324S242000

Reexamination Certificate

active

06791319

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to an eddy current testing probe comprising an exciting coil and a detecting coil, for use in detection of surface flaws of a conductive test material.
An eddy current testing probe is used to detect surface flaws of conductive materials and products such as metals.
FIG. 1
is a schematic view showing schematically the configuration of a conventional general eddy current testing probe. The conventional general eddy current testing probe comprises an exciting coil
1
in the shape of a circular ring, and a detecting coil
2
in the shape of a circular ring having the same diameter as the exciting coil
1
. The exciting coil
1
and the detecting coil
2
are arranged parallel to each other, and a surface of the detecting coil
2
on the side opposite to the exciting coil
1
functions as a flaw detection surface. When using such an eddy current testing probe, the surface of a test material T, such as a conductive material and product, and the flaw detection surface are arranged to face each other with a suitable distance therebetween, the exciting coil
1
and the detecting coil
2
are positioned so that the center axis of the exciting coil
1
is substantially orthogonal to the surface of the test material T, and then an alternating current is caused to flow in the exciting coil
1
. As a result, an AC magnetic field is generated around the exciting coil
1
, and an eddy current is induced on the surface of the test material T by the AC magnetic field.
If there is a flaw on the surface of the test material T, the eddy current flows along the flaw. Therefore, when the eddy current testing probe is moved from a portion where no flaw is present to a portion where a flaw is present, the path of the eddy current changes. Accordingly, the strength and direction of a magnetic field caused by the eddy current vary, and a voltage between the terminals (output voltage) of the detecting coil
2
induced by this magnetic field changes. Since this voltage change is generally detectable as a change in the amplitude and phase of AC voltage, the amplitude and phase of the voltage between the terminals of the detecting coil
2
are measured, and the presence/absence and properties of flaw on the surface of the test material T are detected from the measured results.
Compared to other eddy current testing device such as a through coil that carries out an eddy current test by inserting a test material into a solenoid coil, an eddy current testing probe as mentioned above is applicable to various shapes of test materials and has a simple structure, and therefore it is used in a variety of fields. In such a conventional eddy current testing probe, however, the output of the detecting coil
2
contains a phase component due to the distance between the exciting coil
1
and the surface of the test material T, i.e., so-called lift-off, and a change in lift-off is detected as a noise component. Therefore, there are disadvantages that it is difficult to detect only a flaw and it is hard to adopt a phase analysis used for analyzing the properties of flaw such as the type and depth of the flaw.
The following description will explain an operational principle of the conventional eddy current testing probe. A voltage Vc between the terminals of the detecting coil
2
is expressed by the sum of a voltage Vex induced by a magnetic field generated by an exciting current Iex flowing in the exciting coil
1
and a voltage Vin induced by a magnetic field generated by an eddy current Iin.
Vc=Vex+Vin  (1)
Here, the voltages Vex and Vin can be expressed by equations (2) to (5).
Vex=A
·(
d&phgr;ex/dt
)  (2)
&phgr;
ex=B
·Iex+&PHgr;1(
d
)  (3)
Vin=C·
(
d&phgr;in/dt
)  (4)
&phgr;
in=D·Iin+&PHgr;
2(
d
)  (5)
where A, B, C, D: constants,
&phgr;ex: the strength of the magnetic field generated by the exciting current Iex,
&PHgr;1(d): a varying component of &phgr;ex due to a change in lift-off d,
&phgr;in: the strength of the magnetic field generated by the eddy current Iin, and
&PHgr;2(d): a varying component of &phgr;in due to a change in lift-off d.
Thus, when the lift-off d changes, since the magnetic fields &phgr;ex and &phgr;in vary accordingly, both of the amplitude and phase of the voltage Vc between the terminals of the detecting coil
2
change.
For such a reason, when the lift-off changes or the angle of the exciting coil
1
to the surface of the test material T changes, there occurs a change in the noise component and the phase component due to lift-off, contained in the output of the detecting coil
2
as described above. Therefore, conventionally, there has been used eddy current testing probes having a structure capable of scanning the surface of the test material T while maintaining constant lift-off, or a structure capable of measuring the amount of lift-off and correcting the output of the detecting coil
2
so as to remove the component due to lift-off from the output. Such eddy current testing probes have the problems of complicated structures and high prices.
In order to solve the problems, the following eddy current testing probe was proposed, and reported at p. 131 of the Abstract of Fall Conference, 2000, of the Japanese Society for Non-Destructive Inspection (hereinafter referred to as the “prior art reference”).
FIG. 2
is a schematic view showing schematically the configuration of the eddy current testing probe reported in the prior art reference, and
FIG. 3
is an explanatory view for explaining the operational principle of the eddy current testing probe. As shown in
FIG. 2
, this eddy current testing probe comprises an exciting coil
1
in the shape of a circular ring and a detecting coil
2
in the shape of a quadrangular ring, and the exciting coil
1
and the detecting coil
2
are positioned so that the center axis of the detecting coil
2
is orthogonal to the center axis of the exciting coil
1
in the state where one side of the detecting coil
2
is placed in a diameter direction of the exciting coil
1
, inside the exciting coil
1
.
FIGS. 4A and 4B
are explanatory views for explaining the path of the eddy current generated on the surface of the test material T. As shown in
FIG. 4A
, when there is no flaw on the surface of the test material T, the eddy current on the surface of the test material T flows in a circumferential direction equal to the winding direction of the exciting coil
1
. In this case, almost no magnetic field is generated in a direction crossing the detecting coil
2
by the eddy current, and therefore almost no electromotive force is generated in the detecting coil
2
. Further, in this case, since the output of the detecting coil
2
is substantially zero, even when lift-off changes, the output of the detecting coil
2
contains almost no noise component due to the change in lift-off.
On the other hand, as shown in
FIG. 4B
, when there is a flaw on the surface of the test material T, the eddy current flows along the flaw. When the detecting coil
2
is parallel to the longitudinal direction of the flaw, as shown in
FIG. 3
, a magnetic field is generated in the direction crossing the detecting coil
2
by the eddy current, and an electromotive force is generated in the detecting coil
2
.
For such reasons, according to the eddy current testing probe reported in the prior art reference, since the output of the detecting coil
2
contains almost no noise component, it is possible to significantly improve the flaw detection accuracy.
However, the above-described eddy current testing probe reported in the prior art reference has a problem that the output of the detecting coil
2
still contains a noise component for reasons explained below.
FIG. 5
is a schematic view for explaining the state of a magnetic field generated around the eddy current testing probe reported in the prior art reference. As shown in
FIG. 5
, in the eddy current testing probe, since a solenoid coil having a short length relativ

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