Electricity: measuring and testing – Magnetic – With means to create magnetic field to test material
Reexamination Certificate
2000-05-24
2001-10-30
Fuller, Benjamin R. (Department: 2862)
Electricity: measuring and testing
Magnetic
With means to create magnetic field to test material
C324S242000, C324S243000, C324S225000
Reexamination Certificate
active
06310476
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an eddy current flaw detecting probe used for an eddy current test to determine an internal flaw non-destructively.
2. Description of the Prior Art
An eddy current test is now widely practiced for tests in manufacturing iron and steel material and non-ferrous metal material, and maintenance inspection tests in various plants including small diameter tubes for heat exchangers and the like. An eddy current flaw detecting probe is one of the important factors which decide performance of a flaw detector.
One example of a prior art eddy current flaw detecting probe is shown in FIG.
11
. Numeral
1
designates a test object, numeral
50
designates an exciting and detecting coil, numeral
51
designates an exciting coil and numeral
52
designates a detecting coil. In the prior art eddy current flaw detecting probe, there are used a bobbin coil, a pancake coil and the like as the exciting detecting coil. The flaw detecting probe is divided into an absolute type [FIGS.
11
(
a
) and (
b
)] for testing for the presence of a flaw by impedance change in the respective coils
50
,
52
and a differential type [FIGS.
11
(
c
), (
d
) and (
e
)] for the presence of a flaw by differential component in the two coils
50
,
52
.
Also, the flaw detecting probe is divided into a self-induction type [FIGS.
11
(
a
), (
c
) and (
e
)], in which the same coil
50
carries out both excitation for inducing eddy current and detection of magnetic field by the eddy current, and a mutual induction type [FIGS.
11
(
b
) and (
d
)] which comprises two kinds of coil including the exciting coil
51
for excitation and the detecting coil
52
for detection.
The differential type, especially, has an advantage (as compared with the absolute type) in dealing with noises caused by horizontal lift-offs in which a distance between the coil and the test object changes. In the prior art eddy current flaw detecting probe (especially of the absolute type), lift-off signals due to lift-offs occur so that a flaw signal may be buried, which results in problems such as a reduction in detecting power.
Further, even in the differential type which is during generally good for lift-offs, during an inclined lift-off (
FIG. 12
) in which the probe inclines relative to the test object, there occurs a difference in distance
1
1
, and
1
2
from the two coils to the test object
1
. This difference causes inclined lift-off signals, which result in problems such as a reduction in the flaw detecting power. It is to be noted that numeral
51
designates an exciting coil.
Also, in the eddy current flaw detecting probe shown in FIG.
11
(
e
), there is less reduction in the detecting power against the inclined lift-off. This probe is constructed such that two coils, arranged so as to cross each other, are mutually in a differential connection. This probe has, however, a directivity in the detecting power and there is a shortcoming that it has especially less flaw detecting power in the angle of 45° to a scanning direction.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide an eddy current flaw detecting probe in which detecting power does not lower during lift-offs, and which has improved directivity.
In order to attain this objection, the present invention provides an eddy current flaw detecting probe constructed as follows.
(1) An eddy current flaw detector of the present invention includes an eddy current flaw detecting probe having coils for detecting displacement in a magnetic field due to eddy current induced in a test object by alternating current drive. The eddy current flaw detecting probe comprises two coils, having mutually different diameters, which are arranged coaxially and are in a differential connection with each other.
In this case, the construction is preferably a mutual induction type in which the two coils are detecting coils and in which an exciting coil is provided coaxially with the two detecting coils.
(2) Another eddy current flaw detector of the present invention includes an eddy current flaw detecting probe having coils for detecting displacement in a magnetic field due to eddy current induced in a test object by alternating current drive. The eddy current flaw detecting probe is constructed such that each one of the coils is arranged so as to have its center at each apex portion of a lozenge. A pair of the coils arranged on a diagonal of the lozenge are in a common mode connection with each other, and two sets of the pair of coils in the common mode connection are in a differential connection with each other.
(3) Another eddy current flaw detector of the present invention is a self-induction type including an eddy current flaw detecting probe having coils for measuring displacement in a magnetic field due to eddy current induced in a test object by alternating current drive. The eddy current flaw detecting probe is constructed such that each one of the coils having the same characteristics is arranged so as have its center at each apex portion of a lozenge. A pair of the coils arranged at the apex portion on a diagonal of the lozenge are in a reverse mode connection with each other, and two sets of the pair of coils in the reverse mode connection are in a differential connection with each other.
(4) Another eddy current flaw detector of the present invention is similar to those described above. However, the eddy current flaw detecting probe is formed and arranged in a plural number, and two adjacent eddy current flaw detecting probes thereof commonly own one coil out of four coils. Each one of the coils has its center at each apex portion of the lozenge, and there is a means for switching the eddy current flaw detecting probes.
In this case, there are arranged a plurality of eddy current flaw detecting probes on a side face of a column-like base substance.
In each of the above-mentioned constructions, the coils for detecting a magnetic field are adjusted so that interlinkage magnetic fluxes become equal unless there is a flaw in the test object.
According to the eddy current flaw detecting probes of the present invention constructed as described above, the following functions and effects can be obtained.
In the eddy current flaw detector of (1) above, it is preferable to adjust a signal ratio electrically so that signals from the two detecting coils are cancelled when there is no flaw, or to adjust a winding number ratio of the two coils so that interlinkage magnetic fluxes of the two coils become equal when there is no flaw.
If the above probes are constructed in the self-induction type, because there are differences in diameters and winding numbers of the two coils, there occurs a phase difference in the exciting signals generated at the respective coils to cause an irregularity in the distribution of the alternating magnetic field. But if they are constructed in the mutual induction type, the excitation does not relate to the state of the coil and there occurs no irregularity in the distribution of the magnetic field.
Also, if the flaw detecting probe scans a portion where there is a flaw, the eddy current in the vicinity of the flaw in the test object changes and irregularity occurs in the alternating magnetic field generated by the eddy current. The irregularity in the magnetic field is detected first in the coil having a larger diameter. Thus, there occurs a difference in the interlinkage magnetic fluxes of the two coils and the flaw is detected.
In case there is a change in the horizontal lift-off, the distance between the flaw detecting probe and the test object changes. However, because the two coils are arranged on a concentric circle, the interlinkage magnetic fluxes become equal so that a difference therein becomes zero and no lift-off signal is detected. Also, if there is a change in the inclined lift-off as shown in
FIG. 13
, the distance
1
from the centers of the two coils
2
(
2
a
,
2
b
) to the test object
1
are equal as
Kamimura Takeo
Kawanami Seiichi
Kurokawa Masaaki
Andersen Henry S.
Fuller Benjamin R.
Mitsubishi Heavy Industries Ltd.
Wenderoth , Lind & Ponack, L.L.P.
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