Electricity: measuring and testing – Magnetic – With means to create magnetic field to test material
Patent
1997-06-27
2000-05-02
Strecker, Gerard
Electricity: measuring and testing
Magnetic
With means to create magnetic field to test material
324225, 324242, G01N 2783, G01R 3312
Patent
active
060576847
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to a magnetic sensor for detecting a micro defect portion such as a flaw, or the like in a ferromagnetic subject to be inspected such as various steel plate, pipe, bar materials or the like, by a magnetic leakage method, and a magnetic flaw detection apparatus using such a magnetic sensor.
BACKGROUND ART
A magnetic leakage method is widely used as a method for detecting a defect in a magnetic substance such as a steel belt. The theory thereof is shown in FIG. 1. In FIG. 1, the reference numeral 11 designates a magnetic sensor; 12, a magnetizer; 13, a subject to be inspected such as a steel belt or the like; 14, a defect; and 15, magnetic flux. The subject 13 is magnetized by the magnetizer 12. A large part of the magnetic flux generated by the magnetizer 12 passes through the subject 13 which is small in magnetic reluctance. If there is a defect 14 in the subject 13, the passage of the magnetic flux is, however, impeded by the defect 14 so that a part of the magnetic flux leaks into air. This leakage flux is detected by the magnetic sensor 11 to thereby detect the presence of a defect 14.
A Hall device, a magnetic reluctance device, a magnetic semiconductor device or the like, is used as the magnetic sensor 11. As other examples of the magnetic sensor, a magnetic flaw detection coil constituted by a coil wound on a cylindrical iron core as disclosed in Japanese Patent Unexamined Publication No. Sho-59-160750, or a magnetic flaw detection coil constituted by a coil wound on a ferromagnetic core which coil is supplied with an alternating current to thereby detect the difference between a positive side voltage and a negative side voltage generated across the opposite ends of the flaw detection coil, as disclosed in Japanese Patent Unexamined Publication No. Hei-2-162276 is used.
FIG. 2 is an explanatory view for explaining the operation of a conventional flaw detection coil (search coil). As shown in the drawing, a search coil 21 is constituted by a ferromagnetic core 22, and a coil 23 wound on the core 22. A voltage V which is induced, for example, when an electromagnet 24 is made to approach the search coil 21 so as to make alternating magnetic flux cross the core, is expressed by the following expression (1): ##EQU1## in which .mu..sub.2 is the effective magnetic permeability of the ferromagnetic core 22, H is the intensity of a magnetic field crossing the ferromagnetic core 22, N is the number of turns of the coil 23, S is the sectional area of the ferromagnetic core 22, and .phi. is the magnetic flux crossing the ferromagnetic core 22.
As is obvious from the expression (1), a voltage V proportional to the intensity H of a magnetic field crossing the ferromagnetic core 22 and the change of the intensity of a magnetic field at intervals of a unit time is induced in the coil 23 when the sectional area S of the ferromagnetic core 22, the effective magnetic permeability .mu..sub.2 thereof and the number of turns N of the coil are fixed.
The outline of a voltage V induced in the coil 23 in the conventional search coil 21 at the time when the position of the search coil 21 relatively changes to the electromagnet 24 will be described below.
FIG. 3 is a typical view for explaining a voltage induced in the search coil at the time when the position of the search coil relatively changes to the electromagnet. FIGS. 4A and 4B are characteristic graphs of the detection sensitivity of the search coil at the time when the position of the search coil relatively changes to the electromagnet.
When the electromagnet 24 is moved (in the X-axis direction) so as to intersect the center axis Xc of the ferromagnetic core 22 perpendicularly, the voltage V induced in the coil 23 increases as the electromagnet 24 approaches the center axis Xc so that the voltage V is maximized when the electromagnet 24 crosses the center axis Xc and, contrariwise, the voltage V induced in the coil 23 decreases as the electromagnet 24 goes far from the center axis Xc so that normal distribution charac
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Iwanaga Kenichi
Kato Hiroharu
Maeda Kozo
Murakami Yoshihiro
Nagamune Akio
Lieberstein Eugene
Meller Michael N.
Murakami Yoshihiro
NKK Corporation
Strecker Gerard
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