JP2015200524A - magnetic field sensor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To detect a micro magnetic substance without being affected by movement and vibration of a strong magnetic substance in a near distance.SOLUTION: A magnetic field sensor includes: a first sensor head 11a around which a first detection coil 13a is wound on a first magnetic core 12a for magnetization; a second sensor head 11b electrified with a current which is common with the current electrified in the first magnetic core and wound around the second magnetic core 12b arranged in parallel with or coaxially with the first magnetic core in such a manner that the first sensor head cancels the detection signal by the external magnetic field, and having a second detection coli 13b connected in series to the first detection coil; a sensor circuit for outputting a gradient magnetic field based on detection voltages from the first sensor head and the second sensor head; and a magnetic shield plate placed in proximity to the end part position of the first sensor head or the second sensor head. The sensor circuit is in the vicinity of the first sensor head or the second sensor head, and outputs a gradient magnetic field by a micro magnetic substance that passes through a region opposite to a position of the magnetic shield plate for the first sensor head or the second sensor head.

Description

  The present invention relates to a magnetic field sensor that detects a gradient magnetic field while shielding an external magnetic field.

  For example, the mixing of iron powder in the battery manufacturing process significantly reduces the reliability of the battery, so a technique for accurately detecting a fine magnetic material is desired.

  Magnetic fine particles mainly composed of iron oxide are expected to be used as useful tracers in the body. For example, the sentinel lymph nodes that take up cancer cells and suppress the spread first are the lymph nodes in the vicinity of breast cancer lesions. In particular, magnetic particles that are MRI contrast agents are placed in the body. The magnetic field sensor is used to find out where it is injected and where it has accumulated. Furthermore, magnetic fine particles are also used to detect whether or not binding of a specific protein has occurred, called a magnetic assay. In these techniques, it is necessary to detect a weak magnetic field (from a point source).

  The above problems can be solved by using a magnetic shield room, but the magnetic shield room is expensive and heavy, and the entrance is also narrow. In addition, since the work space is considerably limited, workability is extremely poor.

  Further, as a technique for measuring a weak magnetic field with high sensitivity, for example, techniques disclosed in Non-Patent Documents 1 and 2 are disclosed by the inventors. In the technologies shown in Non-Patent Documents 1 and 2, two sensor heads are arranged on the same coaxial axis as the two sensor heads, and the common external magnetic field input to each sensor head cancels each other. Only the gradient magnetic field in the vicinity of the head is detected and output.

Harada Shomu, Ichiro Hamada, “Improvement of anti-noise performance of fluxgate magnetic field sensor and application to cardiac magnetic field measurement”, The Institute of Electrical Engineers of Japan, Magnetics Study Group, IEEJ Study Group Material, October 25, 2013, p7-11 Harada Shoyumu, Ichiro Hamada, “Characteristics of Basic Waveform Fluxgate Gradiometer”, The Institute of Electrical Engineers of Japan, Magnetics, Joint Medical and Biotechnology Study Group, The Institute of Electrical Engineers of Japan, November 29, 2013, p11- 14

  The techniques shown in Non-Patent Documents 1 and 2 are high-sensitivity gradiometers that can measure a localized magnetic field with high sensitivity, but depending on the working environment, it is necessary to use a magnetic shield together. For example, when the driver is moved at a distance of about 1 m from the gradiometer shown in Non-Patent Documents 1 and 2, or the steel chair is moved, the output of the gradiometer may fluctuate.

  The present invention can detect a very small magnetic substance that is localized without being affected by the movement of a ferromagnetic instrument at a short distance (for example, about 1 m to several m) even if it is used with vibration. Provided is a magnetic field sensor capable of

  A magnetic field sensor according to the present invention includes a first sensor head in which a first detection coil is wound around a first magnetic core through which an alternating current and direct current for excitation are passed, and a first magnetic core. A current common to the AC current and DC current to be energized is energized, and the second sensor core is arranged in parallel or coaxially with the first magnetic core. A second sensor head having a second detection coil wound in such a manner as to cancel a detection signal output to the magnetic field and connected in series with the first detection coil; the first sensor head; and the second sensor head A sensor circuit that outputs a gradient magnetic field based on a detection voltage output from the sensor head, and at least the first sensor head and the second sensor head are arranged close to each other in parallel to the sensitivity axis of the sensor of each sensor head. And the sensor circuit is in the vicinity of the end of the first sensor head or the second sensor head and viewed from the position of the magnetic shield plate, the first sensor head or the first sensor head. The gradient magnetic field is output by a magnetic material that is a detection target that passes through a region on the same side as the side on which the two-sensor head is disposed.

  As described above, in the magnetic field sensor according to the present invention, the magnetic shield plate is provided in the vicinity of the highly sensitive gradiometer, and the sensor is located near the end of the sensor head of the gradiometer and viewed from the position of the magnetic shield plate. In order to output a gradient magnetic field due to the magnetic material to be measured that passes the side on which the head is disposed, an external magnetic field is absorbed by the magnetic shield plate to form a sparse region in the external magnetic field, and the sparse region The gradiometer is arranged to eliminate the influence of the external magnetic field, and it is possible to detect the minute gradient magnetic field with high sensitivity.

  In addition, when the magnetic body passes through the region opposite to the position of the magnetic shield plate when viewed from the sensor head, the magnetic field is detected without detecting the gradient magnetic field without being affected by the magnetic shield plate. There is an effect that a minute gradient magnetic field can be detected with high sensitivity.

  In the magnetic field sensor according to the present invention, before the magnetic body passes through the vicinity of the end of the first sensor head or the second sensor head, the magnetization direction of the magnetic body is changed with respect to the plate surface of the magnetic shield plate. And magnetization control means for magnetizing in the vertical direction.

  Thus, in the magnetic field sensor according to the present invention, before the magnetic material passes near the end of the first sensor head or the second sensor head, the magnetization direction of the magnetic material is changed to the plate surface of the magnetic shield plate. On the other hand, since it is magnetized in the vertical direction, it is possible to efficiently link the magnetic flux of the magnetic material to the sensor head while minimizing the influence of the magnetic shield plate, and to increase the detection sensitivity.

  In the magnetic field sensor according to the present invention, the magnetic shield plate is a circular, oval or rectangular plate, and the inner diameter or the length of one side of the magnetic shield plate is the first sensor head and the second sensor head. And the first sensor head and the second sensor head are disposed close to a substantially central portion of the magnetic shield plate.

  Thus, in the magnetic field sensor according to the present invention, the magnetic shield plate is a circular, oval or rectangular plate, and the inner diameter or the length of one side of the magnetic shield plate is the first sensor head and the second sensor head. More than twice the length of the sensor head in the longitudinal direction, and the first sensor head and the second sensor head are disposed close to the substantially central portion of the magnetic shield plate. On the other hand, the magnetic shield plate acts as a sufficiently wide magnetic plate to exhibit a shielding effect, and the gradiometer can be stably operated even with a very small magnetic material.

  In the magnetic field sensor according to the present invention, the distance between the first sensor head and the second sensor head and the magnetic shield plate is such that the center position of the first sensor head and the center position of the second sensor head are the same. The base line length which is the distance between them is 1.5 times or less.

  Thus, in the magnetic field sensor according to the present invention, the distance between the first sensor head and the second sensor head and the magnetic shield plate is such that the center position of the first sensor head and the center position of the second sensor head. Is less than 1.5 times the baseline length, which is the distance between and the magnetic shield plate, so that the shield effect by the magnetic shield plate can be sufficiently obtained, and a minute gradient magnetic field can be detected with high sensitivity. Play.

  In the magnetic field sensor according to the present invention, the magnetic shield plate is a circular, elliptical or rectangular plate-like body, and the distance between the first sensor head and the second sensor head and the magnetic shield plate is The inner diameter of the magnetic shield plate or the length of one side is 1/30 or less.

  Thus, in the magnetic field sensor according to the present invention, the distance between the first sensor head and the second sensor head and the magnetic shield plate is 1/30 or less of the inner diameter or the length of one side of the magnetic shield plate. Therefore, it is possible to sufficiently obtain a shielding effect by the magnetic shield plate, and to produce an effect that a minute gradient magnetic field can be detected with high sensitivity.

  In the magnetic field sensor according to the present invention, a distance between the center position of the first sensor head and the center position of the second sensor head is a baseline length, and the magnetic body is the first sensor head or the second sensor head. It passes through the region within the baseline length in the direction in which the first sensor head or the second sensor head is separated from the end of the sensor head.

  Thus, in the magnetic field sensor according to the present invention, the distance between the center position of the first sensor head and the center position of the second sensor head is the baseline length, and the magnetic material is the first sensor head or the Since it passes through the region within the baseline length in the direction in which the first sensor head or the second sensor head is separated from the end portion of the second sensor head, it is possible to detect the magnetic material to be measured with high sensitivity. There is an effect that can be done.

  In the magnetic field sensor according to the present invention, a distance between the center position of the first sensor head and the center position of the second sensor head is a baseline length, and the magnetic body is the first sensor head or the second sensor head. The sensor head passes through a region of 1/10 or less of the length of the first sensor head or the second sensor head in the center direction of the first sensor head or the second sensor head from the end of the sensor head.

  Thus, in the magnetic field sensor according to the present invention, the distance between the center position of the first sensor head and the center position of the second sensor head is the baseline length, and the magnetic material is the first sensor head or the Measurement is performed since the end of the second sensor head passes through an area of 1/10 or less of the length of the first sensor head or the second sensor head in the center direction of the first sensor head or the second sensor head. There is an effect that the target magnetic substance can be detected with high sensitivity.

It is a figure which shows the structure of the gradiometer in the magnetic field sensor which concerns on 1st Embodiment. It is a figure which shows the coordinate system of the magnetic field sensor which concerns on 1st Embodiment. It is a figure which shows magnetic field distribution near a magnetic board. It is a figure for demonstrating the expression of a magnetic shielding effect. It is a figure which shows the structure of the magnetic field sensor which concerns on 1st Embodiment. It is a figure which shows a mode that a magnetic body is detected in the magnetic field sensor which concerns on 1st Embodiment. It is a figure which shows magnetic flux distribution in the magnetic core in the magnetic field sensor which concerns on 1st Embodiment. It is a figure which shows the effect of the magnetic field sensor which concerns on 1st Embodiment. It is a 1st figure which shows the application example of the magnetic field sensor which concerns on 1st Embodiment. It is a 2nd figure which shows the application example of the magnetic field sensor which concerns on 1st Embodiment. It is a 3rd figure which shows the application example of the magnetic field sensor which concerns on 1st Embodiment. It is a 4th figure which shows the application example of the magnetic field sensor which concerns on 1st Embodiment.

  Embodiments of the present invention will be described below. Also, the same reference numerals are given to the same elements throughout the present embodiment.

(First embodiment of the present invention)
The magnetic field sensor according to the present embodiment will be described with reference to FIGS. FIG. 1 is a diagram showing a configuration of a gravimeter in the magnetic field sensor according to the present embodiment. In FIG. 1, the gradiometer 1 in the present embodiment uses two fundamental wave type orthogonal fluxgate sensor heads 11 (first sensor head 11 a and second sensor head 11 b). The first sensor head 11a and the second sensor head 11b are arranged in parallel or coaxially, and are detected by a magnetic core 12 (first magnetic core 12a, second magnetic core 12b) made of, for example, a U-shaped Co-based amorphous wire. A coil 13 (first detection coil 13a, second detection coil 13b) is wound. The first detection coil 13a and the second detection coil 13b are wound in the reverse direction and connected in series.

  Excitation is provided by a series circuit of an AC power supply 14 and a DC power supply 15 having a value larger than its amplitude, and one of the detection coils 13 is input to the ground and the other is input to a sensor circuit having a negative feedback configuration. As shown in FIG. 1, the output of the detection coil 13 is sent to the error amplifier 20 through the synchronous detection circuit 18 and the smoothing circuit 19. Thereafter, the feedback current if flows to the detection coil 13 through the feedback resistor 21 so that the input to the error amplifier 20 becomes zero. At this time, the voltage drop appearing at the feedback resistor 21 corresponds to the sensor output.

  Since the first detection coil 13a and the second detection coil 13b are wound in the opposite directions and connected in series, the output obtained by taking the difference between the outputs detected by the respective sensor heads 11 appears. That is, uniform magnetic noise arriving from a distance is similarly picked up by both the first sensor head 11a and the second sensor head 11b and does not appear as an output. However, a local magnetic field that causes a magnetic field gradient at the sensor head is picked up by only one sensor head 11 or a magnetic field of a different magnitude, so that it can be observed as a sensor output. . Moreover, since the difference is taken before the signal is amplified by the electronic circuit, it is possible to detect with a high S / N ratio.

  FIG. 2 is a diagram illustrating a coordinate system of the magnetic field sensor according to the present embodiment. As shown in FIG. 2, the magnetic field sensor 10 according to the present embodiment is configured by combining a circular magnetic shield plate 22 with the sensor head 11 of the gradiometer 1 arranged in parallel. The minute magnetic body 23 to be detected passes in the y-axis direction through an upper region near the end of the sensor head 11 while approaching the sensor head 11 from the direction of the arrow a in FIG. The first sensor head 11a and the second sensor head 11b detect the magnetic field of the minute magnetic body 23 when the minute magnetic body 23 passes through the end portion. As an example, a waveform as described below in FIG. 2 is detected. That is, when the minute magnetic body 23 passes in the vicinity of the sensor head 11, a minute flux linkage generated by the minute magnetic body 23 is picked up by the detection coil 13, so that a local gradient magnetic field can be detected. In the present embodiment, these sensor heads 11 are arranged in a predetermined area on the magnetic shield plate 22.

  In general, in the region close to the surface of the magnetic plate (magnetic shield plate 22), the magnetic flux from the outside is sucked into the magnetic plate, so the magnetic field becomes weaker. The closer to the magnetic plate, the closer to the center of the magnetic plate This increases the magnetic shielding effect. FIG. 3 is a diagram showing the magnetic field distribution in the vicinity of the magnetic plate. It can be seen that the magnetic field is weak in the vicinity of the magnetic plate (the white region at the center in FIG. 3 and its vicinity).

  About the shielding effect, the result calculated quantitatively below is shown. FIG. 4 is a diagram illustrating a calculation system and an analysis result of the shield effect. In FIG. 4A, it is assumed that a magnetic field Hx is applied from a horizontal direction to a thin magnetic disk placed horizontally. If there is no magnetic disk, many magnetic fields that would have passed through the surrounding space are sucked into the magnetic disk. As a result, there is a weak magnetic field in the vicinity of both sides of the magnetic disk. The results of numerical analysis of this are shown in FIGS. 4 (B) and 4 (C). In the analysis, the relative permeability of the magnetic plate was changed to 20,000, the thickness was 1 mm, and the diameter D was changed to 6, 8, 10, 12, 14 cm. FIG. 4B shows the magnetic shield effect on the central axis of the disc against a magnetic field parallel to the disc surface, and FIG. 4C shows the disc surface on the axis at a point displaced by 2 cm from the central point of the disc. The magnetic shielding effect with respect to a parallel magnetic field is shown. In both cases, the horizontal axis is the distance in the z direction, and z = 0 is the disk surface.

  The magnetic shield ratio on the vertical axis in FIGS. 4B and 4C is calculated by (externally applied magnetic field Hx) / (magnetic field Hx (z) above the center of the magnetic disk). Each line of the graph corresponds to D = 6 cm, 8 cm, 10 cm, 12 cm, and 14 cm from the bottom at the position of z = 0.5. The graphs intersect around z = 0.3 mm. This is because the shield ratio is higher when D is very close to the surface of the magnetic disk. However, as z increases, the magnetic field penetrates from the periphery of the disk, and the magnetic field gradient is relaxed (density averaging). This is to move forward. Since the spatial spread of this relaxation depends on the size of the disk, the relaxation of the magnetic field gradient becomes smoother as the disk becomes larger. That is, the region with a high shield is maintained up to a relatively large z. The relaxation of the magnetic field can be understood qualitatively considering that the distribution is governed by the Laplace equation in a space where no current is present. This is because when the Laplace equation is approximated by a difference equation, the magnetic field at a certain point is given by the average of the magnetic fields at the difference approximation point (the point on the spatial grid for calculating the difference) connected to that point.

  It can also be seen from the following explanation that the shielding effect is higher when D is closer to the disk. For example, assuming that an external magnetic field H is applied parallel to the plane of the magnetic plate, the magnetic flux density B of the magnetic plate is the effective permeability of the magnetic plate (the magnetic plate has an end and a demagnetizing field is generated. If μr_eff is smaller than the original permeability of the material (= demagnetizing field effect), then B = μr_eff · H, and the magnetic field Ht in the magnetic plate is given by H−Hd (Hd is demagnetizing field). Assuming that the original magnetic permeability of the magnetic plate (the intrinsic permeability when there is no demagnetizing field effect) is μr, Ht = B / μr, and the shield effect of H / Ht = μr / μr_eff is closest to the side of the magnetic plate. Is expressed. This is because μr_eff becomes smaller as D becomes smaller, and μr / μr_eff becomes larger as D becomes smaller. When the magnetic plate length in the magnetic field direction is short, the magnetically shielded region becomes smaller both in the distance from the side surface of the magnetic plate and in the length direction. On the other hand, if the magnetic plate becomes too long to absorb magnetic flux and become magnetically saturated, the magnetic shielding effect disappears. The magnetic plate selectively shields only the magnetic flux component parallel to the surface. In this embodiment, such a selection action is used.

  It is necessary to set the range of the shield space from the magnetic plate surface as necessary. From the manifestation of the shield described with reference to FIG. 4, if the sensor head 11 has a sensitive part within 0.3 mm of the magnetic shield plate 22, even a small magnetic shield plate 22 has a high shield ratio. Can detect the magnetic field. In the case of a sensor head having a diameter of 1 to 3 mm, such as a fundamental wave type orthogonal flux gate, a disk size that provides a high shield ratio at z = 1 to 3 mm is suitable. For example, the shield ratio at z = 3 mm above the center of the disk is 23 at D = 14 cm, 20.7 at D = 12 cm, and the shield ratio at z = 3 mm above the point displaced 2 cm from the center of the disk is D = 14 cm. It is 17.9 at 20.8 and D = 12 cm. The results are summarized in the table below.

  From these results, z (that is, the distance (cm) from the magnetic shield plate 22 to the sensor head 11) is within D / 30 (one third of the diameter (cm) of the magnetic shield plate 22). desirable. Moreover, when z exceeds 3 mm, it is desirable to use the magnetic shield plate 22 larger than D = 10 cm.

  FIG. 5 is a diagram showing the structure of the magnetic field sensor according to the present embodiment. FIG. 5A is a top view of the magnetic field sensor 10 and FIG. 5B is a side view of the magnetic field sensor 10. Here, components other than the magnetic shield plate 22 and the sensor head 11 are omitted.

  In FIG. 5, the first sensor head 11 a and the second sensor head 11 b are arranged in parallel along the surface of the circular magnetic shield plate 22. When the length of the sensor head 11 is L, the diameter of the magnetic shield plate 22 is preferably 2L, more preferably about 3L. Since the gradiometer exhibits sensitivity only in a space close to the core tip of the sensor head 11, at least the end region of the sensor head 11 needs to be arranged so as to obtain a shielding effect by the magnetic shield plate 22. A sufficient shielding effect can be exhibited by the shield plate 22 acting as a sufficiently wide magnetic plate. That is, as shown in FIG. 5A, it is desirable that the sensor head 11 having a length L is disposed at the central portion of the magnetic shield plate 22 having a diameter of 2L, preferably about 3L. If the baseline length, which is the distance between the center of the first sensor head 11a and the center of the second sensor head 11b, is BL, the distance between the magnetic shield plate 22 and each sensor head 11 is 1.5 of BL. It is desirable that it is not more than double, more preferably about BL or less. For example, when the diameter of the magnetic shield plate 22 is 12 cm (three times the sensor head length) and the distance from the magnetic shield plate 22 is about 1.04 cm, a result of a shield ratio of 5.00 is obtained. However, a sufficient shielding effect is obtained.

  Furthermore, in order to increase the detection sensitivity of the gradiometer, it is preferable to set the region through which the minute magnetic body 23 to be measured passes as shown in FIG. That is, it is an area on the same side as the side where the sensor head 11 is disposed with respect to the position where the magnetic shield plate 22 is disposed (in the case of FIG. 5B, above the magnetic shield plate 22). 11 from the end of the sensor head 11 to the outside of the base line BL (in the direction away from the sensor head 11) and from the end of the sensor head 11 by a distance L / 10 (from the end of the sensor head 11 to the center) By setting so that the minute magnetic body 23 to be measured passes between the positions, the minute magnetic body 23 can be detected with high sensitivity.

  The minute magnetic body 23 is on the same side as the side where the sensor head 11 is disposed with respect to the position where the magnetic shield plate 22 is disposed, and the magnetic shield plate 22 is disposed with the sensor head 11 interposed therebetween. By disposing at a position opposite to the position (above the sensor head 11 in the case of FIG. 5B), the minute magnetic body 23 is not affected by the magnetic flux absorption of the magnetic shield plate 22 and is highly sensitive. A minute gradient magnetic field can be detected.

  Although details will be described later, as a control means for controlling the magnetization direction of the minute magnetic body 23 before the minute magnetic body 23 to be measured passes through the vicinity of the sensor head 11, permanent as shown in FIG. A magnet 24 is provided. Here, for the sake of easy understanding, the permanent magnet 24 is shown below the magnetic shield plate 24 as an example, but the arrangement position of the permanent magnet 24 is a distance that does not affect the gradometer 1 and is very small. Any position where the magnetic body 23 can be magnetized before passing through the sensor head 11 may be used.

  FIG. 6 is a diagram illustrating a state in which a magnetic body is detected in the magnetic field sensor according to the present embodiment. The magnetic field sensor 10 according to the present embodiment is magnetized by a permanent magnet or the like before the measurement by the radiometer 1. As shown in FIG. 6A, the magnetization direction is a direction perpendicular to the magnetic shield plate 22 (in the case of FIG. 6, it is an upward direction or a downward direction, and is shown in the upward direction here). By doing so, the magnetic flux of the minute magnetic body 23 is linked to the magnetic shield plate 22 in the vertical direction, so that the influence of the magnetic shield plate 22 can be minimized.

  FIG. 6B shows the magnetic flux when the minute magnetic body 23 passes just around the end of the sensor head 11, and FIG. 6C shows the inner direction of the minute magnetic body 23 from the end of the sensor head 11. The magnetic flux in the case of passing through the position entered (in the center direction of the sensor head 11) is shown. FIG. 7A shows the magnetic flux distribution in the magnetic core 12 in FIG. 6B, and FIG. 7B shows the magnetic flux distribution in the magnetic core 12 in FIG. 6C.

  As shown in FIG. 6B, when the minute magnetic body 23 magnetized upward or downward passes just around the end of the sensor head 11, the flux linkage of the minute magnetic body 23 is efficiently detected. Therefore, as shown in FIG. 7A, the minute magnetic body 23 can be measured with high sensitivity. Further, as described above with reference to FIG. 6A, the magnetic flux of the minute magnetic body 23 is linked to the magnetic shield plate 22 in the vertical direction, so that it is not affected by the magnetic shield plate 22. On the other hand, as shown in FIG. 6C, when the minute magnetic body 23 magnetized in the upward or downward direction passes through the inside of the sensor head 11, a magnetic flux having a component opposite to the magnetic flux that is originally linked is generated. As shown in FIG. 7B, the sensitivity is deteriorated. When passing through the center of the sensor head 11, the total magnetic flux becomes equal to positive and negative and becomes zero.

  Thus, the sensitivity of the sensor changes depending on the relative positional relationship between the sensor head 11 and the minute magnetic body 23. In order to detect the minute magnetic body 23 with high sensitivity, the value of g in FIG. 6 is about 0 to L / 10, that is, the minute magnetic body 23 passes within a distance of L / 10 from the end of the sensor head 11. It is desirable to set so as to. Also, the opposite outer direction is desirably set so that the minute magnetic body 23 passes within the distance of the base line BL from the end of the sensor head 11 as described above.

  FIG. 8 is a diagram illustrating the effect of the magnetic field sensor according to the present embodiment. As a comparison object of the magnetic field sensor 10 according to the present embodiment, the magnetometer and the gradiometer 1 that does not have the magnetic shield plate 22 were used. The magnetic field source was at a distance of 1 m, and a gradient magnetic field of 3.77 μT / m was applied to each sensor. The horizontal axis represents the angle θ formed by the disturbance magnetic field direction with respect to the longitudinal direction of the sensor head 11.

  As shown in FIG. 8, the output of the magnetometer is proportional to cos θ, and since there is no difference between the magnetic field to be detected and the noise magnetic field, there is no noise reduction effect. Since the gradiometer 1 completely cancels the input in the 0 ° direction, the output becomes 0, and until θ reaches about 50 °, the difference in magnetic field applied to each core increases and the output increases, but toward θ = 90 ° It gets smaller again. And when it was set as the structure provided with the magnetic shield board 22 like this embodiment, the noise reduction effect was further improved 15 times.

  Thus, the noise magnetic field having a gradient coming from a direction parallel to the surface on which the sensor head 11 is present can be reduced according to the magnitude of the gradient. The farther away the noise magnetic field source is, the smaller the gradient becomes, so the reduction effect increases. In the magnetic field sensor according to the present embodiment, an iron tool or the like is often used in a normal work place, and even when a noise magnetic field source exists at a distance of about 1 m, it can be effectively used. .

  9 to 12 show application examples of the magnetic field sensor according to this embodiment. In FIG. 9, the magnetic shield plate 22 is elongated in an elliptical shape, and a plurality of the gradiometers 1 are arranged in parallel in the direction in which the magnetic shield plate 22 is elongated. By doing so, the number of channels for searching for the minute magnetic body 23 can be increased.

  In FIG. 10, a plurality of inspection channels are also provided in this case. However, the sensor heads 11 are sequentially displaced in the longitudinal direction of the sensor heads 11 with respect to the entry direction of the minute magnetic body 23. ing. That is, the region having the sensitivity of the sensor head 11 is arranged so as to line up with respect to the minute magnetic body 23 carried into the inspection space. By doing so, the presence, position and magnetic field strength of the minute magnetic body 23 can be detected based on the respective output results of each sensor head 11, and a wide inspection range can be covered.

  In FIG. 11, a plurality of sensor heads 11 are arranged as in FIG. 10, and the first sensor head 11 a and the second sensor head 11 b in each sensor head 11 are displaced in the longitudinal direction of the sensor head 11. It is arranged in a state. By doing so, it is possible to detect the presence, position, and magnetic field strength of the minute magnetic body 23 based on the respective output results of the sensor heads 11.

  FIGS. 12A to 12C show an example of use of the magnetic field sensor according to the present embodiment, in which a measurement object including a minute magnetic body 23 that is a measurement object moves as the roller rotates. Then, the magnetic field of the measurement object being moved is measured by the magnetic field sensor 10 disposed on the upper part. When the magnetic field sensor 10 detects the magnetic field, it becomes clear that the measurement object includes the minute magnetic body 23, so that the quality of the product can be improved. Furthermore, as shown in FIG. 12C, by arranging the magnetic shield plate 22 so as to cover the sensor head 11, an external noise magnetic field can be shielded more reliably.

DESCRIPTION OF SYMBOLS 1 Gradiometer 11 Sensor head 11a 1st sensor head 11b 2nd sensor head 12 Magnetic core 12a 1st magnetic core 12b 2nd magnetic core 13 Detection coil 13a 1st detection coil 13b 2nd detection coil 14 AC power supply 15 DC power supply 18 Synchronization Detection circuit 19 Smoothing circuit 20 Error amplifier 21 Feedback resistor 22 Magnetic shield plate 23 Magnetic body 24 Permanent magnet

Claims (7)

A first sensor head in which a first detection coil is wound around a first magnetic core that is energized with an alternating current and a direct current for excitation;
A current common to the alternating current and direct current supplied to the first magnetic core is supplied, and the first sensor is connected to a second magnetic core arranged parallel to or coaxially with the first magnetic core. A second sensor head having a second detection coil wound around the head to cancel a detection signal output with respect to a common magnetic field from the outside and connected in series with the first detection coil;
A sensor circuit that outputs a gradient magnetic field based on a detection voltage output by the first sensor head and the second sensor head;
A magnetic shield plate disposed close to and parallel to the sensitivity axis of the sensor of each sensor head with respect to at least the first sensor head and the second sensor head;
The sensor circuit is in the vicinity of an end of the first sensor head or the second sensor head, and the first sensor head or the second sensor head is disposed when viewed from the position where the magnetic shield plate is disposed. A magnetic field sensor that outputs the gradient magnetic field generated by a magnetic material that is a detection target that passes through a region on the same side as the current side. The magnetic field sensor according to claim 1.
Magnetization control that magnetizes the magnetization direction of the magnetic body in a direction perpendicular to the plate surface of the magnetic shield plate before the magnetic body passes near the end of the first sensor head or the second sensor head. A magnetic field sensor comprising means. The magnetic field sensor according to claim 1 or 2,
The magnetic shield plate is a circular, oval or rectangular plate-like body, and the inner diameter or the length of one side of the magnetic shield plate is 2 which is the length in the longitudinal direction of the first sensor head and the second sensor head. The magnetic field sensor, wherein the first sensor head and the second sensor head are disposed close to a substantially central portion of the magnetic shield plate. The magnetic field sensor according to any one of claims 1 to 3,
Baseline length in which the distance between the first sensor head and the second sensor head and the magnetic shield plate is the distance between the center position of the first sensor head and the center position of the second sensor head The magnetic field sensor characterized by being 1.5 times or less. The magnetic field sensor according to any one of claims 1 to 4,
The magnetic shield plate is a circular or elliptical or rectangular plate-like body,
A magnetic field sensor, wherein a distance between the first sensor head and the second sensor head and the magnetic shield plate is 1/30 or less of an inner diameter or a side length of the magnetic shield plate. The magnetic field sensor according to any one of claims 1 to 5,
The distance between the center position of the first sensor head and the center position of the second sensor head is a base line length, and the magnetic body is moved from the end of the first sensor head or the second sensor head. A magnetic field sensor that passes through a region within the baseline length in a direction in which one sensor head or the second sensor head is separated. The magnetic field sensor according to any one of claims 1 to 6,
The distance between the center position of the first sensor head and the center position of the second sensor head is a base line length, and the magnetic body is moved from the end of the first sensor head or the second sensor head. A magnetic field sensor that passes through an area of 1/10 or less of the length of the first sensor head or the second sensor head in the center direction of one sensor head or the second sensor head.