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 characteristic is obtained (see FIG. 4A). On the other hand, when the electromagnet 24 is moved (in the Y-axis direction) toward a line Yc perpendicular to the center axis Xc, the voltage V induced in the coil 23 increases before the voltage V reaches a certain point and the voltage V decreases after the voltage V exhibits its maximum so that the voltage V becomes 0 V when the electromagnet 4 crosses the line Yc (see FIG. 4B).
Incidentally, the defect heretofore required to be detected in the thin steel belt is a relatively large defect which is called gouge. With the enlargement of application of such a thin steel belt in the industrial field, the detection of a smaller inclusion has been, however, required recently. For example, an inclusion having a volume not larger than 10.sup.-3 (mm.sup.3) has become a subject to be detected. To detect such a micro defect, the magnetic sensor including the aforementioned search coil has the following problems.
(1) A Hall device or a magnetic diode can measure the intensity of a static magnetic field, but cannot be applied to high-accurate magnetic leakage flaw detection because its characteristic varies and the output voltage or the like changes depending on the temperature change.
(2) Further, the conventional search coil is good in temperature characteristic, but an induced voltage V corresponding to the intensity of an external magnetic field is generated in the coil 23 in the up/down direction with respect to the longitudinal direction of the ferromagnetic core 22, and in the outer circumferential direction of the coil 23. Accordingly, a noise voltage due to unnecessary disturbance magnetic flux is induced simultaneously to lower the performance of flaw detection when the search coil is used for magnetic leakage flaw detection as shown in FIGS. 4A and 4B.
(3) Further, in any one of the conventional techniques, the distance (liftoff) between the steel belt and the magnetic sensor is required to be reduced in order to detect a micro defect. As a measure, there is used a method for floating the magnetic sensor in air to keep the liftoff in a small value of about 0.1 (mm) as disclosed in Utility Model Unexamined Publication No. 61-119759. This method, however, has an operational problem since it increases the ppossibility of the magnetic sensor contacting the steel belt, or the like.
(4) If the liftoff is reduced in order to detect a micro defect, the magnetic sensor is easily influenced by disturbance such as vibration of the steel belt or the like, and easily receives background noise (noise due to magnetic distortion, surface roughness, stress distortion, etc. of the steel plate), or the like, caused by the magnetic irregularity of the steel belt. Accordingly, it is difficult to obtain sufficient S/N.
(5) The most part of the frequency component of a defect detection signal overlaps the frequency component of the background noise, so that improvement of S/N cannot be performed sufficiently by means of a filter or the like.
(6) In order to detect a smaller defect, the steel belt is required to be magnetized more intensively so that leakage flux due to the defect is generated efficiently. Floating magnetic flux (magnetic flux reaching a magnetic pole of a magnetizer from another magnetic pole through air) generated in the neighborhood of the steel belt is, however, also increased so that the magnetic sensor may be saturated to bring the lowering of detection sensitivity.