Electricity: measuring and testing – Magnetic – With compensation for test variable
Patent
1984-11-13
1987-02-24
Strecker, Gerard R.
Electricity: measuring and testing
Magnetic
With compensation for test variable
324232, G01N 2790, G01R 3312
Patent
active
046460136
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field Of The Invention
A considerable problem in connection with the detection of surface imperfections using, for example, eddy current technique, is the distance dependence (the so-called lift-off dependence) between the transducer and the object under test, which often causes considerable disturbance of the measurements.
2. Prior Art
One method of suppressing the LO dependence is disclosed in Swedish Pat. No. 7507857-6, including the patent of addition.
A similar method is described by Hugo L Libby in "Introduction to Electromagnetic Nondestructive Test Methods". The method of Libby can be regarded as a form of vector transformation technique based on phase discrimination, which means that vectors with different directions are separated with the aid of phase-sensitive/phase-controlled rectifiers.
A greatly limiting factor in Libby's method, especially in the practical case involving superficially located defects, for example surface cracks, is that the directions of the fault vectors in the impedance plane (FIG. 3) substantially coincide with the direction/orientation of the LO vectors (.DELTA.LO) (i.e. .alpha. is small), thus making it very difficult to efficiently separate the fault vectors (F) from the LO vectors by using phase discrimination only, and simultaneously suppress the lift-off dependence.
For, for example, small/shallow surface cracks the angle (.alpha.) between the fault direction and the LO direction may only be 10.degree.-15.degree. in case of a hot (1000.degree.) material.
SUMMARY OF THE INVENTION
The present invention aims to provide a solution to the problems mentioned and other problems associated therewith.
The invention can also be regarded as an optimization method, which may then, for example, complement and improve prior art devices, some of which are previously mentioned patented devices. It is thus important to note that the present invention can be regarded both as a device and/or as a method.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT
In its function as a device, the present invention can be described as follows. The device, which is primarily intended for testing and/or measurement of electrically conducting test objects (2) with respect to changes (3) in/on the test object, comprises at least one transducer (1), which is supplied with signals, for example current (I), having different frequency contents (e.g. .omega..sub.H and .omega..sub.L) so that currents (e.g. eddy currents) of corresponding frequency contents are induced in/on the test objects. The electrical impedance (FIG. 3) of the transducer is then influenced by the test object via the inductive coupling between the transducer and the test object. As a consequence of this, at least two signals, directly or indirectly emanating from the transducer and having different frequency origins (e.g. .omega..sub.H and .omega..sub.L), may be signal processed, for example weighted and combined, in such a way that the result, as a function of the distance of the transducer relative to the test object, is constant (k.sub.1) within a reasonable operating range, when the test object exhibits no change in or near the transducer, and different from the corresponding result (k.sub.2) when there is a change in/on the test object. In this way, the change, for example the surface crack, may be detected and/or be measured as a function of the difference and/or quotient between/of the above-mentioned results (e.g. k.sub.1 --k.sub.2 or k.sub.1 lk.sub.2).
Characteristic of the type of device referred to is that the equivalent current penetration depth of at least one frequency is smaller than the depth (Dj) of the change or changes being detected.
To describe the problems and how to overcome them, the following example may be used, which should then be understood as one of many feasible examples.
BRIEF DESCRIPTION OF THE DRAWINGS
The above objects, advantages and features of the invention are readily apparent when taken in conjunction with the drawings showing a preferred embodiment of carryi
REFERENCES:
patent: 2903642 (1959-09-01), Seigel
patent: 4061968 (1977-12-01), Pigeon
patent: 4303885 (1981-12-01), Davis et al.
patent: 4563644 (1986-01-01), Lenander et al.
Libby, Hugo L., "Basic Principles," Introduction to Electromagnetic Nondestructive Test Methods, Robt. E. Krieger Pub. Co., Huntington, NY, 1979, Chapter 2, pp. 19-58.
Libby, Hugo L., Introduction to Electromagnetic Nondestructive Test Methods, Robt. E. Krieger Pub. Co., Huntington, NY, 1979, pp. 1-8.
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