JP2009002681A - Magnetic measuring device provided with permanent magnet which performs periodic motion and oscillating coil - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a small and light high-sensitivity magnetic measuring device which uses induced current with respect to an applied AC magnetic field of a constant frequency, one example of which is an eddy current defect detector, and can detect a defect in a deep part of a conductor sample, without deteriorating the sensitivity of the whole of its measurement system even if the applied AC magnetic field has an extremely low frequency of several Hz or less. <P>SOLUTION: The magnetic measuring device is provided with a magnet for applying the AC magnetic field, and a coil for detecting a magnetic signal. The magnet for applying the AC magnetic field is a permanent magnet which performs a periodic motion with the constant frequency mechanically, and the coil for detecting the magnetic signal is a coil which mechanically vibrates with a constant frequency, and moreover in the magnetic measuring device the two of the frequency of the periodic motion of the permanent magnet and the vibration frequency of the vibration detecting coil are different from each other. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a magnetic measurement apparatus using an induced current generated in a conductor.

FIG. 2 is a schematic diagram for explaining a conventional magnetic measuring apparatus using an induced current. In FIG. 2, 5 represents a magnet for applying an alternating magnetic field to the conductor sample 3, and 6 represents a magnetic sensor for detecting a magnetic signal. In many cases, the magnetic sensor 6 is an induction coil wound with a normal metal wire, but may be another type of magnetic sensor. Here, for convenience, a metal wire induction coil is considered. When the magnetic sensor 6 is a metal wire induction coil, the magnetic sensor 6 is referred to as a detection coil 6.

In FIG. 2, the magnet 5 is usually an electromagnet. However, as will be described later, an induced current can be induced inside the conductor sample 3 by mechanically moving the permanent magnet. When the magnet 5 is an electromagnet, one coil may serve as the function of the magnet 5 and the detection coil 6. Here, for convenience, an apparatus including two coils as shown in FIG. 2 is considered. When the magnet 5 is an electromagnet, the magnet 5 is referred to as an application coil 5.

In general, when an AC magnetic field is applied using a magnet 5 in a magnetic measuring device using induced current, an induced current (eddy current) is generated inside the conductor sample 3, and the magnetic field generated by the eddy current is generated by the magnetic sensor 6. To detect. Such a measuring device is, for example, an eddy current flaw detector that detects defects such as metal parts. Usually, the magnetic sensor 6 is scanned in the vicinity of the surface of the conductor sample 3 and the fluctuation of the output of the magnetic sensor 6 is monitored to detect a defective portion inside the conductor sample 3.

Eddy current flaw detectors detect the presence of defects by detecting changes in eddy currents in a conductor sample. In eddy current flaw detectors, in order to detect defects in the deep part, a sufficiently large eddy current is generated to the deep part of the conductor sample and high sensitivity is obtained even if the eddy current disturbance caused by the defect is deep and the magnetic signal is small. It must be detectable.

Here, in order to generate a large eddy current in the deep part of the conductor sample 3, the frequency of the applied AC magnetic field must be low because of the skin effect of the conductor sample 3. From the viewpoint of electromagnetics, when an AC magnetic field is applied in parallel to the surface of a semi-infinite conductor sample, the skin depth, which is a measure of how long the AC magnetic field can penetrate inside the conductor sample, is the square root of the frequency of the AC magnetic field. It is known to be inversely proportional to The skin effect is described in Non-Patent Document 1 below, for example. By definition, the magnetic field lines of an alternating magnetic field must be closed without breaking in the middle, so even when an alternating magnetic field is applied perpendicularly to the surface of a conductor sample of a finite size by a finite size magnet, the skin effect is almost the same. Similarly, therefore, when the size of the conductor sample is sufficiently larger than that of the magnet 5, the skin depth is substantially equal when an AC magnetic field is applied parallel to the surface of the conductor sample, and the skin depth is equal to the AC magnetic field. It can be considered that it is almost inversely proportional to the square root of the frequency. Therefore, even when an AC magnetic field is applied perpendicularly to the surface of the conductor sample, in order to generate a large eddy current in the deep part of the conductor sample 3, the frequency of the applied AC magnetic field must be low. However, as the frequency of the alternating magnetic field decreases, the density of eddy current generated inside the conductor sample 3 also decreases proportionally due to the law of electromagnetic induction and the law of Ohm.

Therefore, in order to generate a large eddy current in the deep part, it is necessary to apply a low-frequency alternating magnetic field having a large amplitude. In order to generate an alternating magnetic field with a large amplitude using a normal electromagnet, it is necessary to pass an alternating current with a large amplitude through the application coil 5, and the application coil 5 is not only large, but also a large alternating current. A current source is also required. Increasing the size of the application coil and the AC current source leads to an increase in the size of the entire device, greatly limiting the applications that can be used as an eddy current flaw detector.

It has been known for a long time that a small and powerful low-frequency alternating magnetic field can be solved by making the magnet 5 a strong permanent magnet and periodically vibrating the permanent magnet 5 in the vicinity of the conductor sample 3. Yes.

However, when a permanent magnet is used as the magnet 5, the types of usable magnetic sensors 6 are limited. That is, when a sensor that uses the physical properties of a sensor material, such as a Hall sensor or a fluxgate magnetometer, is selected as the magnetic sensor 6, a sensor that can be used in a strong magnetic field is not highly sensitive. In many cases, the output is saturated when used in a strong magnetic field. Therefore, an air core induction coil wound with a normal metal wire is preferably used as the detection coil 6 for use in a strong magnetic field.

Therefore, according to the conventional technique, the small and powerful permanent magnet 5 described in 0008 is mechanically moved at a low frequency to generate eddy current in the conductor sample 3, and the change in the eddy current is placed in the vicinity of the sample. It is detected by the air core detecting coil 6 described in 0009. In a general eddy current flaw detector, the detection coil 6 is scanned in the vicinity of the surface of the conductor sample 3. According to the conventional technique, the signal voltage generated in the detection coil 6 has the same frequency as the applied AC magnetic field, That is, a component that changes over time at the same frequency as the frequency of the periodic motion of the permanent magnet 5 is detected as a signal voltage. When lock-in detection of the signal voltage generated in the detection coil 6 is performed, the frequency of the reference signal is the frequency of the periodic motion of the permanent magnet 5.

According to the prior art, in order to detect a defect in a deep part, it is necessary to set the frequency for periodically moving the permanent magnet 5 to a low frequency. However, as described in 0006, the magnitude of the eddy current is reduced. In addition, the induced electromotive force generated in the detection coil 6 is also reduced. This is because the induced electromotive force generated in the detection coil 6 is proportional to the time change rate (frequency) of the magnetic flux linked to the detection coil 6. In general, high-sensitivity measurement of a low-frequency minute signal is difficult, and when the frequency of the applied magnetic field is extremely small and becomes an ultra-low frequency region of several Hz, the sensitivity of the measurement system is degraded due to various causes.

Even when lock-in detection is performed using an applied AC magnetic field as a reference signal, if the reference signal frequency is in an extremely low frequency region of several Hz, the signal-to-noise ratio (SN) is caused by drift, temperature characteristics, etc. of the electronics 4 itself including the lock-in amplifier. Ratio) becomes difficult, and an expensive lock-in amplifier is required, making the electronics 4 complicated.

After all, according to the conventional technique, it is difficult to manufacture a small and lightweight eddy current flaw detector capable of detecting a defect in a deep part of a conductor sample. Due to such problems, there have been no reports of successful examples of eddy current flaw detectors capable of applying an alternating magnetic field of several Hz or less for detecting defects in the depth of a conductor sample.
Published by Shigenobu Sunagawa, Theoretical Electromagnetism, 2nd edition, Kinokuniya, September 30, 1973.

For example, an eddy current flaw detector, which uses an induced current for an applied AC magnetic field with a constant frequency, even if the frequency of the applied AC magnetic field is very low (several Hz or less), the sensitivity of the entire measurement system does not deteriorate and deep defects A small and lightweight high-sensitivity magnetic measurement device capable of detecting the above is realized.

  The solution to the above problem is a magnetometer for applying an alternating magnetic field to a conductor sample and a coil for detecting a magnetic signal, wherein the magnet for applying an alternating magnetic field is constant. A permanent magnet that mechanically moves periodically at a frequency, and the coil that performs magnetic signal detection is a coil that mechanically vibrates at a constant frequency, and the frequency of periodic motion of the permanent magnet and the vibration This is achieved by a magnetic measuring device characterized in that two frequencies of the vibration frequency of the detection coil are different.

  In the prior art, when the applied AC magnetic field frequency is in the ultra-low frequency region, the root cause of problems such as sensitivity deterioration is because the detection coil is mechanically fixed, and the present invention provides the root cause. By removing the, it becomes possible to perform lock-in detection at the vibration frequency of the vibration detection coil regardless of the frequency of the applied AC magnetic field, and to have a magnetic field gradient detection sensitivity determined by the vibration frequency of the vibration detection coil. become. In the present invention, a magnet that applies an alternating magnetic field is a permanent magnet that periodically moves at an ultra-low frequency, so that a strong ultra-low-frequency alternating magnetic field can be applied, and a small size and light weight can be provided. As a result, it is possible to realize a small and practical magnetic measuring apparatus capable of detecting a defect in the deep part of the conductor sample with high sensitivity.

  FIG. 1 shows a magnetometer for applying an AC magnetic field to a conductor sample and a coil for detecting a magnetic signal according to the present invention, the magnet for applying an AC magnetic field. Is a permanent magnet 1 that periodically moves mechanically at a constant frequency, the coil that performs magnetic signal detection is a coil 2 that vibrates mechanically at a constant frequency, and the periodic motion of the permanent magnet 1 is It is a schematic diagram explaining the magnetic measurement apparatus characterized by two frequencies, a frequency and the frequency of the vibration of the said vibration detection coil 2 differing. Here, the form of the periodic motion of the permanent magnet 1 and the vibration direction of the vibration detection coil 2 in FIG. 1 are arbitrary. Further, the electronics 4 in FIG. 1 may be anything as long as it can amplify a minute signal induced in the vibration detection coil 2 with high sensitivity.

The characteristic that the detection coil is mechanically fixed, which is the root cause of problems such as sensitivity degradation when the frequency of the applied AC magnetic field is in the ultra-low frequency region, has been removed in the conventional technology. Regardless of the frequency of the magnetic field, lock-in detection can be performed at the vibration frequency of the vibration detection coil 2, and the magnetic field gradient detection sensitivity determined by the vibration frequency of the vibration detection coil 2 is provided.

When a magnetic field having a local spatial distribution is linked to the vibration detection coil 2 by mechanical vibration, an induced electromotive force modulated at a vibration frequency is generated in the vibration detection coil 2. That is, the output of the vibration detection coil 2 includes a component proportional to the local magnetic field gradient. This magnetic field gradient signal can be lock-in detected at the vibration frequency of the vibration detection coil 2. Since the magnetic field gradient detection sensitivity is determined by the vibration frequency of the vibration detection coil 2, the frequency of the applied AC magnetic field can be arbitrarily reduced. That is, the frequency of the periodic motion of the permanent magnet 1 described in 0017 can be arbitrarily lowered. If the vibration frequency of the vibration detection coil 2 is sufficiently high, even if the frequency of the applied AC magnetic field is in the very low frequency region, no significant deterioration in sensitivity occurs, and defects in the deep part of the conductor sample 3 are highly sensitive. Can be detected.

In order to generate an induced current as deep as possible inside the conductor sample 3, the permanent magnet 1 according to 0017 that performs periodic motion for applying an alternating magnetic field, and the distance from the surface of the conductor sample 3, It is desirable to make it as small as possible, and in order to detect a change in the induced current with as high sensitivity as possible, the distance (lift-off) from the vibration detection coil 2 and the surface of the conductor sample 3 is made as small as possible. It is desirable to do.

Furthermore, since the region where the magnitude of the induced current generated inside the conductor sample 3 is the largest is near the area immediately below the permanent magnet 1, in order to detect the change in the induced current as highly sensitive as possible, It is desirable that the vibration detection coil 2 be arranged so as to be closest to the region of maximum induced current.

FIG. 3 shows that the permanent magnet 1 according to the present invention is a permanent magnet that vibrates mechanically, and the coil 2 according to the present invention that mechanically vibrates for magnetic signal detection. A magnetic measuring device that is an oscillating air-core coil, in which the oscillating axis of the oscillating air-core coil 2 is collinear with the oscillating axis of the permanent magnet 1 oscillating. It is a schematic diagram.

  As is clear from FIG. 3, the vibration detection coil 2 is a concentric air-core coil with respect to the vibration permanent magnet 1, and the vibration center point of the vibration permanent magnet 1 and the vibration of the vibration detection coil 2 are detected. The distance from the surface of the conductor sample 3 to the center point of the conductor sample 3 can be the same, that is, minimized. As described in 0020, this is an advantageous arrangement for generating as large an eddy current as possible inside the conductor sample 3, and is advantageous for detecting a change in eddy current as highly as possible. It is an arrangement. Further, as is apparent from FIG. 3, the vibration detection coil 2 is directly above the region where the magnitude of the induced current generated inside the conductor sample 3 is the largest, and as described in 0021, further induction The arrangement is such that a change in current can be detected with high sensitivity.

FIG. 4 shows a permanent magnet 1 according to the present invention described in 0017 that mechanically moves periodically and rotates so that polarities (N pole and S pole) are alternately reversed with respect to the conductor sample 3. In addition, the mechanically vibrating coil 2 for detecting a magnetic signal is an oscillating air-core coil, and the oscillating axis of the oscillating air-core coil 2 rotates the permanent magnet 1. It is a schematic diagram explaining the magnetic measuring apparatus characterized by crossing on the axis of rotation of this, and the vibration axis of the said air-core coil 2. FIG.

  As is clear from FIG. 4, in this case as well, the distance from the surface of the conductor sample 3 between the rotating shaft of the rotating permanent magnet 1 and the center of vibration of the vibration detecting coil 2 is the same as in FIG. Therefore, as described above, the arrangement is advantageous in order to generate as large an eddy current as possible in the conductor sample 3 and to detect a change in eddy current as highly sensitive as possible. Further, as is clear from FIG. 4, in this case as well, the vibration detection coil 2 is placed immediately above the region where the magnitude of the induced current generated inside the conductor sample 3 is the largest. This is an advantageous arrangement for detecting a change in the position with high sensitivity.

FIG. 5 shows a magnetism having a vibration permanent magnet 1 for applying an alternating magnetic field and a vibration detection coil 2 for detecting a magnetic signal according to the present invention, wherein two vibration frequencies are different. One of the magnetic measuring devices, wherein the vibration detecting coil 2 is an air-core coil, and its vibration axis is collinear with the vibration axis of the vibration permanent magnet 1. It is a schematic diagram explaining an Example.

As apparent from FIG. 5, the vibrating permanent magnet 1 is mechanically vibrated by the vibrator 7 fixed to the gantry. As shown in FIG. 5, the vibration detection coil 2 is arranged on a concentric axis with respect to the vibration permanent magnet 1 and mechanically vibrates by a vibrator 8 fixed to a gantry. That is, the fixing tool that fixes the “lever” to the gantry serves as a fulcrum of the “lever”, the contact point between the other end of the “lever” and the vibrator 8 serves as a power point, and the vibration detection coil 2 is disposed. Is the point of action. The structure shown in FIG. 5 is merely an example, and as will be apparent from the following second and third embodiments, a method for fixing the vibration permanent magnet 1 and the vibration detection coil 2 to a gantry and machine The structure for realizing the dynamic vibration can take various structures depending on the application. The main point of the first embodiment of the present invention is that the vibration detection coil 2 is an air-core coil, and its vibration axis is concentric with the vibration axis of the vibration permanent magnet 1.

As apparent from FIG. 5, the vibration detection coil 2 is arranged on the concentric axis with respect to the vibration permanent magnet 1, so that the distance from the surface of the conductor sample 3 at the center point of both vibrations is the same. I can do it. Therefore, in order to detect a defect in a deep part in the conductor sample 3, it is possible to bring them as close to the surface of the conductor sample 3 as possible. Further, since the vibration detection coil 2 is placed immediately above a region where the magnitude of the induced current generated inside the conductor sample 3 is the largest, it is advantageous for detecting a change in the induced current with high sensitivity. It is an arrangement.

FIG. 6 is a schematic diagram for explaining a second embodiment of the present invention.

As is apparent from FIG. 6, the vibration detection coil 2 vibrates by a vibrator 9 formed in a hollow cylindrical shape. The feature that the vibration detection coil 2 is arranged on a concentric axis with respect to the vibration permanent magnet 1 is the same as in the first embodiment.

FIG. 7 is a schematic diagram for explaining a third embodiment of the present invention.

In the third embodiment, as is apparent from FIG. 7, the vibration detection coil 2 vibrates by the vibrator 9 formed in a hollow cylindrical shape. The permanent magnet 1 is fixed to the gantry so as to rotate, and the rotation of the rotating device 10 fixed to the gantry is transmitted to the permanent magnet 1 by the rotation transmitting device 11 to rotate the permanent magnet 1. Also in this case, the method for fixing the rotating permanent magnet 1 and the vibration detection coil 2 to the gantry and the structure for realizing mechanical rotation or vibration can take various structures depending on the application.

Also in the third embodiment, the distance from the surface of the conductor sample 3 between the rotation axis of the rotating permanent magnet 1 and the center of vibration of the vibration detecting coil 2 can be made the same, and the vibration The detection coil 2 can be placed immediately above a region where the magnitude of the induced current generated inside the conductor sample 3 is the largest, and is advantageous for highly sensitive detection of deep defects.

As described above in detail, according to the present invention, in the conventional technique, the detection coil is a mechanical cause that causes a problem such as sensitivity degradation when the frequency of the applied AC magnetic field is in the very low frequency region. It is possible to remove the feature of being fixed to the surface, and it is possible to apply a strong ultra-low frequency alternating magnetic field by a permanent magnet without using a large coil or power source, and thus, it is small, light and deep. It is possible to realize a highly practical and highly sensitive eddy current flaw detector having features such as defect detection.

The present invention provides a eddy current flaw detector that is small, lightweight, and capable of detecting deep defects with high sensitivity.

It is a schematic diagram explaining the magnetic measuring apparatus provided with the permanent magnet and the coil which vibrates mechanically of the present invention, and having two different frequencies. It is a schematic diagram explaining the conventional magnetic measuring apparatus using an induced current. FIG. 3 is a schematic diagram illustrating a magnetic measurement apparatus according to the present invention, wherein the vibration detection coil is an air-core coil, and the vibration axis thereof is collinear with the vibration axis of a permanent magnet that vibrates. A schematic diagram illustrating a magnetic measurement apparatus according to the present invention, wherein the vibration detection coil is an air-core coil, and the vibration axis intersects the rotation axis of the rotating permanent magnet and the vibration axis of the vibration detection coil. FIG. It is a schematic diagram explaining one Example of this invention. Example 1 It is a schematic diagram explaining the 2nd Example of this invention. (Example 2) It is a schematic diagram explaining the 3rd Example of this invention. (Example 3)

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Permanently moving permanent magnet 2 Vibration detection coil 3 Conductor sample 4 Electronics 5 Magnet 6 Magnetic sensor 7 Vibrator 8 Vibrator 9 Hollow cylindrical vibrator 10 Rotating device 11 Rotating transmission device

Claims (3)

A measurement apparatus using an induced current, comprising: a magnet for applying an alternating magnetic field to a conductor sample; and a coil for detecting a magnetic signal, wherein the magnetic measurement apparatus is configured to apply an alternating magnetic field. The magnet is a permanent magnet that periodically moves mechanically at a constant frequency, and the coil that performs magnetic signal detection is a coil that mechanically vibrates at a constant frequency, and the frequency of the periodic motion of the permanent magnet is The magnetic measurement apparatus is characterized in that two frequencies of vibration frequencies of the vibration coil are different.   The permanent magnet according to claim 1, which is mechanically moved periodically, is a mechanically vibrated permanent magnet, and the mechanically vibrated coil for magnetic signal detection is mechanically vibrated. The magnetic measuring apparatus according to claim 1, wherein the vibration axis of the air core coil is collinear with the vibration axis of the permanent magnet that vibrates.   2. The permanent magnet according to claim 1, wherein the permanent magnet moves mechanically periodically. The permanent magnet rotates so that the polarity is alternately reversed with respect to the conductor sample, and mechanically vibrates to detect a magnetic signal. The coil according to claim 1 is an air-core coil that vibrates mechanically, and a vibration axis of the air-core coil that vibrates is on a rotation axis of the rotating permanent magnet and a vibration axis of the air-core coil. The magnetic measurement apparatus according to claim 1, wherein

Images