JPH08338864A - Magnetic sensor - Google Patents

Abstract

PURPOSE: To attain magnetic balance easily by disposing compensation and detection coils at the opposite ends of an excitation coil, shifting any one coil through the exciting coil in the axial direction and fixing the coil. CONSTITUTION: An exciting coil 1 is wound around a circular coil bobbin 10 and bobbins 10a, 10b applied with a compensation coil 7 and a detection coil 3 are disposed on the inside thereof. The surface of bobbin 10a and the inner wall of bobbin 10 are threaded 11 so that they can be shifted as the coil 7 is turned. The coils 7, 3 are disposed at the opposite ends of the coil 1 where the coil 7 can be shifted through the coil in the axial direction and secured at an arbitrary position. The coils 7, 3 are turned reversely with same number of turns and the components of exciting field can be balanced by simply shifting the coil 7 slightly. Since the field components are balanced perfectly, even a faint variation of flux density produced from a magnetic body 4 to be measured disposed closely to the coil 3 can be detected with high sensitivity.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a magnetic sensor used to measure the magnetic properties of magnetic materials.

[0002]

2. Description of the Related Art Conventionally, there are a static magnetic field method and an alternating magnetic field method as methods for measuring the magnetic characteristics (mainly BH characteristics) of magnetic materials. In the static magnetic field method, as shown in FIG. 5 (a), a pair of strong magnets 21a obtained by passing a direct current from a direct current power source 22 to an exciting coil 21 are brought close to each other and a magnetic field to be measured is generated in a magnetic field generated in a gap 21b. Place the body 24, this magnetic body 2
A magnetic flux change caused by vibrating 4 is detected and amplified by the detection coil 23 and the amplifier 25, and a detection signal is taken out (VSM system). The drawback of this method is that the apparatus is large-scaled, and therefore AC magnetic characteristics cannot be easily obtained. Next, in the AC magnetic field method, as shown in FIG. 5B, an AC current is supplied from the AC power supply 26 to the exciting coil 21, the magnetic substance 24 to be measured is placed in the magnetic field generated by the exciting current, and the exciting current changes. The magnetic flux density corresponding to the change in the generated magnetic field strength is detected and amplified by the detection coil 23 and the amplifier 25. The apparatus is relatively simple compared to the previous method, but the one having a small magnetization strength such as a thin film is measured. Requires a special detection coil.

This AC magnetic field system includes a system without a compensation coil and a system with a compensation coil. Figure 5 (b) for the method without compensation coil
It is used to measure a relatively large magnetic material such as the core shown in FIG. 1, and the exciting coil 21 is wound around the magnetic material 24 to be measured, or the detection coil 23 is similarly measured.
The magnetic properties of the magnetic material are measured by wrapping it around No. 4. With this method, the magnetic characteristics can be measured relatively easily, but on the other hand, there is a drawback in that it takes time to wind the coil and the measured magnetic body 24 needs a large amount of mass.

As shown in FIG. 6, the method with a compensation coil has a pair of air-core coils B in the excitation coil 21, one of which serves as a compensation coil 27 for canceling the excitation component, and the other. One is used as the detection coil 23 in which the magnetic material 24 to be measured is inserted. The drawback of this method with the compensation coil is that it is difficult to obtain a strong magnetic field because the air-core coil B is used as the excitation means, and it is difficult to measure a magnetic thin film or magnetic ink having a small magnetization. Further, since the magnetic material to be measured 24 is inserted into the magnetic field, the magnetic material embedded in the medium such as paper or plastic card or sheet or attached to the surface of the medium is used as the medium. It is difficult to measure with it attached.

[0005]

As described above, in the conventional magnetic characteristic measuring device, it is difficult to easily measure the magnetic characteristic of a magnetic material having a small magnetization. That is, in the AC magnetic field method, the magnetic characteristics can be measured relatively easily, but since it is necessary to wind the excitation coil and the detection coil around the object to be measured, the magnetic properties of the film-shaped or powder-shaped magnetic material to be measured can be measured. There is a drawback that the characteristics cannot be measured. Also,
In the AC magnetic field method having the compensation coil, it is necessary to insert the magnetic material to be measured inside the detection coil, and therefore it is necessary to cut out the magnetic material sample into a size that can be put inside the detection coil. In order to prevent this drawback, it is conceivable to bring the magnetic material to be measured close to the periphery of the detection coil for measurement. However, in this method, the exciting magnetic field is weak and the detection sensitivity is also low, so it can be used for things such as a magnetic detector that detects the magnetic substance itself, but a magnetic characteristic detector that detects the characteristics of the magnetic substance. However, there is a drawback that it is not practical.

An object of the present invention is to provide a magnetic structure that has a simple structure and high detection sensitivity, and can measure the magnetic characteristics of a film or powdery magnetic substance to be measured without cutting out the magnetic substance to be measured. To provide a sensor.

[0007]

To achieve this object, in a magnetic sensor according to the present invention, a compensation coil and a detection coil are located at both ends of an excitation coil, and a compensation coil or a detection coil is placed inside the excitation coil. It has a structure that can move along the axial direction and can be fixed at any position within the range of movement.

[0008]

In this magnetic sensor, if the compensation coil and the detection coil have the same number of turns and the winding directions are opposite, the electromotive force excited by the exciting coil outputs the output of the detecting coil when there is no magnetic substance to be measured. It can be almost offset on the side. Moreover, since the magnetic field near the end face of the exciting coil changes monotonously due to the magnetic field distribution characteristics of the air-core exciting coil, it is necessary to make a perfect balance of the components of the exciting magnetic field by moving the compensation coil or the detection coil slightly. You can With such a magnetic sensor that is almost perfectly balanced, even a slight change in the magnetic flux density due to the magnetic material to be measured placed in the vicinity of the detection coil can be detected with high sensitivity.

[0009]

[Principle] In order to facilitate understanding of the present invention, the principle of a magnetic sensor with a compensation coil will be described with reference to FIGS. The magnetization characteristic of a ferromagnetic material is expressed as follows.

## EQU1 ## B = μH (1) B: magnetic flux density, H: magnetic field strength, μ: magnetic permeability However, μ is generally not linear but has a non-linear characteristic peculiar to a magnetic material.

This characteristic is obtained by measuring the magnetic flux density while changing the strength of the magnetic field. When measuring the magnetic properties of extremely thin (thickness of 1 μm or less) magnetic tape, magnetic tape thinly coated with magnetic material, magnetic ink, etc., it is difficult to measure because the magnetization of the magnetic material in this case is extremely small. is there. Expression (1) can be rewritten as Expression (2) below.

[Equation 2] B = μ ₀ H + J (2) H: strength of magnetic field, J: strength of change of magnetic substance, μ ₀ :
Vacuum permeability Here, since μ ₀ is extremely small, B≈J can be set.

The voltage e detected by the detection coil 3 is

## EQU00003 ## e = -d.phi. / Dt (3) .phi .: magnetic flux penetrating the detection coil.

Since φ = ∫Jdv (4), it can be seen that the detection voltage from the magnetic substance having a minute volume dv is extremely small.

On the other hand, the voltage V _e generated in the detection coil 3 having the circuit configuration of FIG. 1 to which the compensation coil 7 is connected is expressed as follows.

(5) V _e = V _b −V _d (5) V _b : voltage of detection coil, V _d : voltage of compensation coil. If the magnetic body 4 to be measured is not present, V _b = V _d. Since it is adjusted, V _e = 0. If there is a magnetic substance 4,

V _e = N _b · S _b · dB / dt (6) N _b : number of turns of the sensor coil, S _b : cross-sectional area of the sensor coil, B: magnetic flux density inside the coil

Here, as shown in the equation (2), B = μ ₀
H has been canceled

## EQU7 ## V _e = N _b S _b dJ / dt (7), and the magnetic characteristic J due to the magnetic material 4 to be measured is obtained.

[0014]

Next, the structure of the present invention in which the compensation coil 7 is movable and its operation will be described. FIG. 1 shows an example of the configuration of a magnetic sensor circuit according to the present invention, which is shown in FIGS.
Is a concrete example of the structure. FIG. 2A shows the compensation coil 7
A bobbin 10 in which an exciting coil 1 is wound around a circular coil bobbin 10 made of a non-metallic material such as plastic or bakelite, and a compensation coil 7 and a detection coil 3 are wound around the magnetic sensor.
It has a structure in which a and 10b are arranged inside the circular coil bobbin 10 of the exciting coil 1. The surface of the coil bobbin 10a of the compensation coil 7 and the coil bobbin 10 of the exciting coil 1
There is a screw 11 between the inner wall of the
By rotating the compensating coil 7, the compensating coil 7 can be slightly moved along the inside of the exciting coil 1.

FIG. 2B shows another configuration example for moving the compensation coil 7. By adjusting the rotation of the screwed bolt 12, the position of the coil bobbin 10c of the compensation coil 7 with respect to the coil bobbin 10 of the exciting coil 1 can be finely adjusted. Unlike the configuration (a), there is an advantage that the compensation coil 7 does not rotate with the movement of the compensation coil 7. All of these are for zero point adjustment, a current is passed through the exciting coil 1, the output voltage of the detecting coil 3 and the compensating coil 7 is monitored by a circuit as shown in FIG. 1, and there is no magnetic substance to be measured. Adjust to the position where the voltage is minimum. In this example, the coil is wound around the circular bobbins 10a, 10b, 10c, but a rectangular bobbin for measuring a laterally long bobbin or a left and right symmetric bobbin can be deformed into a desired shape. Also, instead of the detection coil 3 and the compensation coil 7,
The same effect can be obtained by using a magnetic sensing element such as a Hall element or a magnetoresistive element having uniform characteristics. That is, in order to measure the magnetic characteristics of the magnetic body 4 on the end face a of the air-core coil A with high sensitivity, a magnetic field generated by the air-core coil A is efficiently applied to the magnetic body 4 to be measured and magnetized by this magnetic field. It suffices if only the magnetic flux density of the magnetic body 4 to be measured can be detected.

The magnetic field strength H _r produced by the air-core coil A is expressed by the following equation (8) by the distance r from the end face a.

(Equation 8) N: number of turns, I: current, L: length, a: radius

From the equation (8), the magnetic field strength of the end face a is about half of the central portion, and decreases with distance. The magnetic body 4 to be measured outside the air-core coil A is given a stronger magnetic field as it approaches the end face a of the exciting coil 1. Further, since the detection coil 3 can also detect the magnetic flux change more efficiently as it is closer to the end face a, it can be seen that the configuration shown in FIGS. 1 and 2 is the most efficient. Further, in order to enhance the magnetic detection sensitivity in this method, it is necessary to balance the electromotive force between the detection coil 3 and the compensation coil 7. According to the above formula (8), the magnetic field strength of the end face a of the exciting coil 1 becomes smaller as it goes away from the face a, so that the compensation coil 7 can be moved to achieve zero balance easily. be able to.

That is, in the magnetic sensor according to the present invention, as shown in FIG. 2, the compensation coil 7 and the detection coil 3 are located at both ends of the excitation coil 1, and the compensation coil 7 or the detection coil 3 is inside the excitation coil 1. Has a structure that can move along the axial direction and can be fixed at any position in the moving range. This magnetic sensor 7 has a compensation coil,
If the number of turns of the detection coil 3 is the same and the winding directions are opposite, the electromotive force excited by the excitation coil 1 can be almost canceled at the output side of the detection coil 3 when the magnetic body 4 to be measured does not exist. it can. Moreover, since the magnetic field near the end face of the exciting coil changes monotonously due to the magnetic field distribution characteristic of the air-core exciting coil A, the compensating coil 7 or the detecting coil 3 is slightly moved to achieve a perfect balance of the exciting magnetic field component close to 0. Can be taken. According to such a magnetic sensor that is almost perfectly balanced, even a slight change in the magnetic flux density due to the magnetic body 4 to be measured placed near the detection coil 3 can be detected with high sensitivity.

Further, the magnetic field generated by the exciting coil 1 can ignore the excitation noise which is a problem at the time of measurement, a strong magnetic field can be applied to the magnetic material 4 to be measured, and even a small magnetization can be measured with high sensitivity. It has the advantage of being possible.

Next, the operation of the zero point compensation band will be described. FIG. 3 in this case is a partial structural view showing an embodiment of the present invention, and is a magnetic sensor having a compensating band 13. Even if the structure of FIG. 2 is zero-balanced, if a magnetic material such as iron is nearby due to the purpose of use, the balance may be lost when it is attached to the device.

The configuration of the present invention for easily repairing this imbalance will be examined. When the balance is lost,
The output V _e0 of the detection coil in the absence of the magnetic body 4 to be measured is

V _e0 = ΔN · d (μ ₀ H) / dt (9) ΔN: Balance change amount, and a voltage proportional to the exciting magnetic field strength H is detected.
This voltage is a factor that deteriorates the S / N ratio regardless of the magnetic material 4 to be measured.

The compensating magnetic band 13 having magnetic permeability μ ₁ is
The compensation coil 7 is added to have the following relationship.

[ _Equation 10] V _e0 = ΔN · d (μ ₀ H) / dt−ΔV · d (μ ₁ H) / dt (10) ΔV: The magnetic flux variation μ ₁ and ΔV of the magnetic band are appropriately selected and the excitation space is reduced. The core coil 1 can be adjusted to zero by adjusting the position of the magnetic band 13 so that V _e0 = 0 by utilizing the fact that H is different depending on the position.

A specific structure is a magnetic sensor having a compensation band 13. The compensation band 13 is a plate-shaped band made of a ferromagnetic material. Compensation band 13
It is possible to adjust the zero balance, although it is slight, and it can be easily adjusted.For example, after mounting a magnetic sensor on a certain device, use it to remove the influence of the earth's magnetism and the magnetic field from the periphery of the device. It is convenient. In this embodiment, the band 13 has a structure capable of moving along a moving groove 14 formed in a spiral shape on the groove tube 15.
It can be adjusted more easily. Also the thickness of the band,
The degree of adjustment can be changed by changing the magnetization intensity (magnetic permeability) of the magnetic body. Therefore, the zero-point compensating magnetic band 3 having a variable mounting position has an advantage that it is possible to easily balance the magnetic field with the exciting magnetic field.

The operation of the structure in which the air-core coil A is filled with a ferromagnetic material (magnetic mandrel) 17 having a gap 16 will be described. FIG. 4 is a partial structural view showing an embodiment of the present invention, which is an example of a magnetic sensor containing a magnetic mandrel 17.
When measuring characteristics of a magnetic material, a material having a large B _m (saturation flux density) and B _r (retention flux density) excitation field strength H
Need to be strengthened. Air core coil A alone

H = N · I (11) N: number of turns, I: current From the relationship, it is necessary to increase N and I in order to have a large magnetic field strength H. In order to increase H while N and I are kept as they are, it is possible to place a ferromagnetic material having a large magnetic permeability μ in a magnetic field and use the magnetic field near the end face a ′ of the ferromagnetic material. From B = μH shown in the equation (1), the internal magnetic flux density B of a material having a large magnetic permeability μ can be increased. When materials having a large magnetic flux density B are in contact with each other in the air, the magnetic field strength H of the end face a ′ becomes equal to B near the end face, and a large magnetic field strength is obtained.

In this case, the ferromagnetic mandrel 1 continuously penetrates the magnetic sensor body 20 including the compensation coil and the detection coil.
If the value is 7, the change in the magnetic flux density B in the vicinity of the detection coil, which changes depending on the magnetic material 4 to be measured, directly changes the magnetic flux density B in the vicinity of the compensation coil. Therefore, the characteristics of the magnetic material cannot be detected. In order to eliminate this effect, a gap 16 is provided at the center of the mandrel 17.

R _m = R ₀ + R ₁ + R ₂ (12) R _m : magnetic resistance of the entire mandrel, R ₀ : magnetic resistance of air gap, R
₁ = R _2: magnetic resistance between the right and left of the mandrel, however,

Since R ₀ >> R ₁ and R ₂ (13), the amount of change in the magnetic flux density on the measurement side is transmitted to the compensation coil side is extremely small.

To describe the specific structure, the magnetic mandrel 17 inserted into the air core of the exciting coil 1 can produce a very strong magnetic field at its end face. Therefore, it is convenient when measuring the characteristics of a magnetic material having a large holding magnetic field H _c and saturation magnetic field B _m . However, the air gap 1 is sufficient to ignore magnetic interference between the compensation coil and the detection coil.
It is desirable to provide 6 at the center of the mandrel 17. In this way, the one with the ferromagnetic material with the air gap inserted in the center of the air-core part of the excitation coil can apply the excitation time to the DUT more efficiently, and the characteristics can be measured over a wider magnetization range. can do. As described above, if a magnetic sensor that can easily measure high-sensitivity AC magnetic characteristics can be realized, it is possible to detect the magnetic characteristics of minute magnetic materials that have already been coated or inserted, and the magnetic material that is in the manufacturing process can be detected. Can be measured at any time or continuously without interrupting the manufacturing process. In addition, it can be used as a detection sensor for a security tag using a special magnetic material.

[0027]

As described in detail above, in the magnetic sensor according to the present invention, it is easy to balance the exciting magnetic field component on the output side of the detecting coil, and the magnetic sensor moves on the exciting coil made of a magnetic material. A magnetic sensor with possible bands is easy to balance against an external magnetic field, and has a very high magnetic sensing sensitivity. Further, when a ferromagnetic mandrel having an air gap is provided in the air-core part of the exciting coil, a strong magnetic field can be applied to the magnetic material to be measured, and magnetic characteristics can be measured in a wider measurement range. .

[Brief description of drawings]

FIG. 1 is a circuit diagram for explaining the principle of the present invention.

FIG. 2 is a vertical sectional view showing a structural example of a magnetic sensor according to the present invention.

FIG. 3 is a schematic perspective view illustrating an embodiment of the present invention using a compensation band.

FIG. 4 is a schematic vertical sectional view showing a structural example of a magnetic sensor with a magnetic core according to the present invention.

FIG. 5 is a connection diagram for explaining a magnetic sensor according to a conventional static magnetic field method (a) and an alternating magnetic field method (b).

FIG. 6 is a schematic perspective view showing a conventional magnetic sensor having a compensation coil.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Excitation coil 2 DC power supply 3 Detection coil 4 Magnetic material to be measured 5 Amplifier 6 AC power supply 7 Compensation coil 8 Excitation power supply 9 Differential amplifier 10, 10a, 10b, 10c Coil bobbin 11 Screw 12 Volt 13 Compensation band 14 Moving groove 15 Groove cylinder 16 Air gap 17 Magnetic mandrel 20 Magnetic sensor body 21 Excitation coil 21a Magnet 21b Air gap 22 DC power supply 23 Detection coil 24 Magnetic material to be measured 25 Amplifier 26 AC power supply 27 Compensation coil

Claims (3)

[Claims] 1. The magnetic flux detecting coil and the compensating coil are integrally formed with the exciting coil inside the exciting coil in a state where the detecting coil and the compensating coil are concentrically located at both ends of the air-core exciting coil. A magnetic sensor having a structure in which the compensation coil can move inside the air core of the exciting coil and be fixed at an arbitrary position within the moving range. 2. The magnetic sensor according to claim 1, further comprising a zero-point compensating magnetic band that can be attached at an arbitrary position in a concentric relationship with the exciting coil near the outside of the exciting coil. . 3. The magnetic sensor according to claim 1, wherein a ferromagnetic mandrel having an air gap at a central position is further inserted and arranged in an air-core portion of the exciting coil.