JP2013175625A - Magnetic shield device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce a level of a magnetic field gradient in a radial direction as well as in an axial direction in a magnetic shield device in which a magnetic field generating coil is installed in the axial direction only of a magnetic shield.SOLUTION: A magnetic shield device includes: a cylindrical shield portion constituted of a magnetic body; and a first coil provided on the same plane as one opening surface of the shield portion or at a position opposite to the opening surface. In an excited state, the first coil reduces a level of a magnetic field gradient in a radial direction inside the shield portion while also reducing a level of a magnetic field gradient in an axial direction inside the shield portion.

Description

  The present invention relates to a magnetic shield device that blocks an external magnetic field.

  When measuring a very weak magnetic field of several fT (femtotesla) or less with a magnetoencephalograph or magnetocardiograph, cancel an environmental magnetic field (such as geomagnetism) that is much larger than the measurement target and use only the measurement target magnetic field. It is necessary to take it out. Therefore, conventionally, as a means for canceling the environmental magnetic field, a passive magnetic shield that forms a shield room with a ferromagnetic material typified by permalloy has been widely used.

  A magnetic sensor is generally used for measuring the magnetoencephalogram and magnetocardiogram, but there are cases where it is necessary to perform measurement with a plurality of sensors. However, a magnetic field gradient is generated inside the magnetic shield according to the state of residual magnetization of the magnetic shield and the ability to block the environmental magnetic field. Therefore, the area where a plurality of sensors can be arranged is very limited. In some cases, the sensor cannot be arranged.

  Thus, for example, Non-Patent Document 1 proposes a system in which a magnetic shield is arranged at the center of a three-axis magnetic field generating coil and an environmental magnetic field is canceled using the coil. According to the system, since the magnetic field generating coils are arranged in the three-axis (x, y, z-axis) directions with respect to the magnetic shield, the level of the magnetic field gradient in each axis direction is reduced.

Nobuhito Matsuoka, 4 others, “Examination of Magnetic Shield with Opening Combined with Coil”, Journal of AEM Society of Japan, AEM Society of Japan, September 2010, Vol. 18, No. 3, p. 215-220

  However, the configuration described in Non-Patent Document 1 has a configuration in which the triaxial magnetic field generating coil wraps around the magnetic shield, which increases the size of the entire system. Further, in order to secure a space for accessing the inside of the magnetic shield, it is necessary to secure a sufficient space between the coils, and as a result, the entire system may be further increased in size. In addition, a coil driving circuit and a power source are also required for three axes, which increases costs and installation space.

  An object of the present invention is to reduce the level of the magnetic field gradient not only in the axial direction but also in the radial direction in a magnetic shield device in which a magnetic field generating coil is installed only in the axial direction of the magnetic shield.

  The present invention includes a cylindrical shield portion made of a magnetic material, and a first coil provided on the same plane as one opening surface of the shield portion or at a position facing the opening surface. The first coil reduces the level of the axial magnetic field gradient in the shield part and also reduces the level of the radial magnetic field gradient in the shield part in the excited state. Providing the device. According to this configuration, the magnetic field gradient level in the radial direction as well as the axial direction can be reduced.

  In the above-described magnetic shield device, the first coil may further include a demagnetizing unit that demagnetizes the shield unit when the first coil is in an excited state. According to this configuration, it is possible to optimize the demagnetization effect by demagnetizing the shield portion in the excited state of the first coil.

  In the magnetic shield device, the first coil may be formed along one opening edge of the shield part. According to this configuration, the space occupied by the entire apparatus can be further reduced.

  The magnetic shield device may further include a plurality of magnetic measurement devices arranged in the shield part. According to this configuration, it is possible to perform magnetism measurement using a plurality of measuring devices in the shield part.

  Moreover, in the above-described magnetic shield device, one opening of the shield part may be covered with a lid. According to this configuration, the shielding rate of the external magnetic field can be further improved.

  Further, in the above magnetic shield device, the second coil having an opening surface parallel to the outer surface of the shield part, wherein the magnetic field in the radial direction in the shield part is in an excited state of the second coil. You may make it further have the 2nd coil which reduces the level of gradient. According to this configuration, the level of the radial magnetic field gradient can be further reduced.

It is a figure showing composition of magnetic shield device 10 concerning one embodiment of the present invention. 2 is a diagram illustrating an example of a configuration of a control circuit 5. FIG. It is an overhead view of the shield part 1 and cancellation coil 3A, 3B. 2 is a perspective view of a shield part 1. FIG. 4 is a perspective view of a measurement region R. FIG. It is a figure which shows the gradient of the magnetic field (x component) in the measurement area | region R. FIG. It is a figure which shows the gradient of the magnetic field (z component) in the measurement area | region R. FIG. It is a figure which shows the gradient of the magnetic field (x component) in the measurement area | region R. FIG. It is an overhead view of 1 A of shield parts. It is a figure which shows the gradient of the magnetic field (z component) in the measurement area | region R. FIG. It is a figure which shows the cancellation coil which concerns on one modification. It is a figure which shows the cancellation coil which concerns on one modification.

(1) Embodiment FIG. 1 is a diagram showing a configuration of a magnetic shield device 10 according to an embodiment of the present invention. As shown in FIG. 1, the magnetic shield device 10 includes a shield unit 1, a degaussing coil 2, cancellation coils 3 </ b> A and 3 </ b> B, drive circuits 4 </ b> A to 4 </ b> C, and a control circuit 5. Here, the cancel coils 3A and 3B are examples of the “first coil” according to the present invention, and the demagnetizing coil 2 and the drive circuit 4A are examples of the “demagnetizing unit” according to the present invention.

  The shield part 1 is a cylindrical magnetic shield made of a ferromagnetic material. The shield unit 1 stores one or more magnetic sensors such as a magnetoencephalograph and a magnetocardiograph inside. The material of the shield part 1 is a magnetic sheet.

  The degaussing coil 2 is wound around the shield part 1 and demagnetizes the shield part 1 by generating an alternating magnetic field. The cancel coils 3A and 3B are Helmholtz coils wound in a square shape. Both coils are arranged in parallel and coaxially by a distance of a radius r. When currents in the same direction are passed through both coils, a uniform magnetic field is generated near the center of the space between the coils.

  The shield part 1 is installed in a space between the cancel coils 3A and 3B. In that case, the shield part 1 is arrange | positioned so that cancellation coil 3A, 3B may be located in the axial direction. That is, the shield part 1 is disposed so that the opening part thereof faces the opening surfaces of the cancel coils 3A and 3B.

  The drive circuit 4A is connected to the degaussing coil 2 and causes an alternating current to flow from the power source (not shown) to the coil. The drive circuit 4B is connected to the cancel coil 3A and allows an alternating current to flow from the power source (not shown) to the coil. The drive circuit 4C is connected to the cancel coil 3B and allows an alternating current to flow from the power source (not shown) to the coil. The strength of the magnetic field generated by the cancel coils 3A and 3B is set in advance so that the strength of the environmental magnetic field (such as geomagnetism) at the location where the center of gravity of the shield portion 1 is planned before the shield portion 1 is set to zero. Adjusted. The control circuit 5 is connected to the drive circuits 4A to 4C and controls them.

  FIG. 2 is a diagram illustrating an example of the configuration of the control circuit 5. As shown in the figure, the control circuit 5 includes a CPU (Central Processing Unit) 51, a ROM (Read Only Memory) 52, a RAM (Random Access Memory) 53, an input unit 54, and an output unit 55. Have. Each component is connected to each other via a bus.

  The CPU 51 controls the drive circuits 4A to 4C by reading and executing the control program stored in the ROM 52. The RAM 53 provides a work area when the CPU 51 executes the control program. The input unit 54 receives an operation signal instructing demagnetization of the shield unit 1 and an operation signal instructing generation of a magnetic field by the cancel coils 3A and 3B via an operation unit (for example, a button) (not shown). The output unit 55 outputs a drive signal to the drive circuits 4A to 4C.

  When the operation signal instructing the generation of the magnetic field by the cancel coils 3 </ b> A and 3 </ b> B is received by the input unit 54, the CPU 51 outputs a drive signal to the drive circuits 4 </ b> B and 4 </ b> C via the output unit 55. On the other hand, when the operation signal instructing demagnetization of the shield unit 1 is received by the input unit 54, the CPU 51 outputs a drive signal to the drive circuits 4 </ b> A to 4 </ b> C via the output unit 55. That is, the CPU 51 optimizes the demagnetization effect by demagnetizing the shield unit 1 when the cancel coils 3A and 3B are in the excited state.

Next, the dimensions of the shield part 1 and the cancel coils 3A and 3B will be described.
FIG. 3 is an overhead view of the shield unit 1 and the cancel coils 3A and 3B. The z-axis direction in the figure corresponds to the axial direction of the shield part 1. The y-axis direction corresponds to a direction perpendicular to the installation surface of the shield part 1. The x-axis direction corresponds to a direction perpendicular to the z-axis direction and the y-axis direction. In addition, the symbol which drawn the two line segments which cross | intersect in a white circle shown in the same figure has shown the arrow which goes to a near side from the paper surface back side.

  As shown in the figure, the shield part 1 has a diameter of 20 cm and an axial length of 40 cm. Moreover, although illustration is abbreviate | omitted, the thickness is 0.18 mm. On the other hand, each of the cancel coils 3A and 3B has a length of 200 cm. A region R surrounded by a two-dot chain line in FIG. 3 will be described later.

  Next, the effect obtained by the magnetic shield device 10 having the above configuration will be described. More specifically, the gradient of the magnetic field generated in the shield unit 1 when the magnetic shield device 10 is operated will be described. In the following description, it is assumed that the magnetic shield device 10 is arranged so that the axial direction of the shield part 1 faces east and west. Further, the strength of the geomagnetism is 26 μT (micro Tesla) in the x-axis direction and minus 2 μT in the z-axis direction, and the strength of the magnetic field generated by the cancel coils 3A and 3B is 2 μT in order to cancel the geomagnetism in the z-axis direction. Suppose that

  In the following description, the region where the magnetic field is measured is the measurement region R shown in FIG. This measurement region R is a rectangular region located so that its center of gravity overlaps the center of gravity G of the shield part 1. As shown in FIG. 5, the measurement region R has a length in the x-axis direction of 9 cm and a length in the z-axis direction of 5 cm.

  FIG. 6 is a diagram showing the gradient of the magnetic field (x component) in the measurement region R. As shown in FIG. FIG. 6A is a diagram illustrating a case where magnetic field generation is not performed by the cancel coils 3A and 3B, and FIG. 6B is a diagram illustrating a case where magnetic field generation is performed by the cancel coils 3A and 3B. . In addition, the distance in the axial direction (z-axis direction) and the radial direction (x-axis direction) in FIG. For example, the position 2.5 cm in the axial direction is a position advanced 2.5 cm in the −x direction from the center of gravity G, and the position 4.5 cm in the radial direction is a position advanced 4.5 cm in the + z direction from the center of gravity G. . The same applies to FIGS. 8 and 10 described later.

In FIG. 6, as the gradient of the magnetic flux density in the axial direction, for example, the gradient between the points P _{1 and} P ₂ in FIG. 6A and the gradient between the points Q _{1 and} Q _{2 in} FIG. 6B are compared. Then, the direction of FIG. 6B is clearly gentler. That is, the x-component magnetic field gradient in the axial direction is gentler when compared with the case where the magnetic field is not generated by the cancel coils 3A and 3B. As for the gradient of the x-component magnetic flux density in the radial direction, for example, the gradient between the points P _{2 and} P ₃ in FIG. 6A and the gradient between the points Q _{2 and} Q _{3 in} FIG. In comparison, FIG. 6B is more gradual. That is, compared with the case where the magnetic field generation by the cancel coils 3A and 3B is not performed, the x-component magnetic field gradient in the radial direction is gentler when the magnetic field is generated.

  The above is a description of the gradient of the magnetic field (x component) in the measurement region R. FIG. 7 is a diagram showing the gradient of the magnetic field (z component) in the measurement region R. FIG. 7A is a diagram showing a case where magnetic fields are not generated by the cancel coils 3A and 3B, and FIG. 7B is a diagram showing a case where magnetic fields are generated by the cancel coils 3A and 3B. .

In the figure, as the gradient of the magnetic flux density in the axial direction, for example, the gradient between points P _{4 and} P ₅ in FIG. 7A is compared with the gradient between points Q _{4 and} Q _{5 in} FIG. Then, the direction of FIG.7 (b) becomes looser. That is, the z-component magnetic field gradient in the axial direction is gentler when the magnetic field is not generated by the cancel coils 3A and 3B. As for the gradient of the z-component magnetic flux density in the radial direction, for example, the gradient between the points P _{5 and} P ₆ in FIG. 7A and the gradient between the points Q _{5 and} Q _{6 in} FIG. In comparison, FIG. 7B is more gradual. That is, compared with the case where the magnetic fields are not generated by the cancel coils 3A and 3B, the radial direction z-component magnetic field gradient is gentler in the case where the cancellation coils 3A and 3B are not generated.

  As described above, the magnetic shield device 10 according to the present embodiment that generates the magnetic field by the cancel coils 3A and 3B has a level of the magnetic field gradient in the axial direction as compared with the configuration that does not generate the magnetic field by the cancel coils 3A and 3B. Can be reduced. Moreover, it can be seen from the above measurement data that the magnetic shield device 10 according to the present embodiment can also reduce the level of the radial magnetic field gradient by the cancel coils 3A and 3B arranged in the axial direction.

  Therefore, according to the magnetic shield device 10 according to the present embodiment, the magnetic field gradient level is reduced not only in the axial direction but also in the radial direction, so that the area where the magnetic sensor can be installed is widened. In addition, according to the magnetic shield device 10 according to the present embodiment, since it is sufficient to install a cancel coil only in the axial direction, compared to a configuration in which cancel coils for three axes are installed as in the prior art, the overall system Size and cost are reduced. Further, access to the shield part 1 is facilitated.

  Further, as described below, according to the magnetic shield device 10 according to the present embodiment, the axial length of the shield portion 1 can be shortened. Normally, when the axial length of the shield in the magnetic shield is shortened, the magnetic field gradient level in the axial direction and the radial direction is adversely affected. However, according to the magnetic shield device 10 according to the present embodiment, This is because the level of the magnetic field gradient in the axial direction and the radial direction can be reduced as described above.

  FIG. 8 is a diagram showing the gradient of the magnetic field (x component) in the measurement region R described above. However, in FIG. 8A, the length of the shield part 1 in the axial direction in the above configuration is 80 cm (hereinafter, the shield part 1 having this structure is referred to as “shield part 1A”), and the cancel coils 3A and 3B. It is a figure which shows the case where the magnetic field generation by is not performed. FIG. 9 is an overhead view of the shield part 1A. On the other hand, FIG.8 (b) is a figure which shows the case where the magnetic field generation | occurrence | production by the cancellation coils 3A and 3B was performed in the magnetic shield apparatus 10 which has said structure. That is, it is the same as FIG. 6B described above.

  FIG. 10 is a diagram showing the gradient of the magnetic field (z component) in the measurement region R. As shown in FIG. However, also in this case, FIG. 10A is a diagram showing a case where the shield portion 1A is adopted instead of the shield portion 1 and the magnetic field is not generated by the cancel coils 3A and 3B. FIG. 10B is a diagram showing a case where the magnetic field is generated by the cancel coils 3A and 3B in the magnetic shield device 10 having the above configuration. That is, it is the same as FIG. 7B described above.

  As shown in FIGS. 8 and 10, the shield part 1 according to the present embodiment has only a half length in the axial direction as compared with the shield part 1A, but has cancel coils 3A and 3B. Thus, it can be seen that the same magnetic field gradient level can be ensured as compared with the configuration having the shield portion 1A. That is, according to the magnetic shield device 10 according to the present embodiment, it can be seen that the axial length of the shield portion 1 can be shortened.

(2) Modifications The above embodiment may be modified as follows. The following modifications may be combined with each other.

(2-1) Modification 1
The shield section 1 according to the above embodiment has a circular cross section in the radial direction, but this shape may be an ellipse or a polygon. Moreover, although the both ends are opening the shield part 1 which concerns on said embodiment, the opening part of one (one side) may be covered with the lid | cover. In addition, the shield part 1 and a lid | cover may be shape | molded integrally. Moreover, although the shield part 1 which concerns on said embodiment becomes a single layer structure, a multilayer structure may be sufficient.

  Moreover, the shield part 1 which concerns on said embodiment may be comprised by permalloy, iron, chromium, or cobalt-type amorphous, and a ferrite sintered compact. In the case of a magnetic material having a weak coercive force such as permalloy, the demagnetizing coil 2 (and the drive circuit 4A) may be omitted. Moreover, the dimension of the shield part 1 which concerns on said embodiment is an example to the last, You may employ | adopt another dimension.

(2-2) Modification 2
Although the cancellation coils 3A and 3B according to the above-described embodiment are wound in a square shape, they may be wound in a circular shape, an elliptical shape, or another polygonal shape. Further, the cancel coils 3A and 3B may be formed along the opening edge of the shield portion 1 as shown in FIG. In addition, the symbol which drawn the two line segments which cross | intersect in a white circle shown in FIG. 11 has shown the arrow which goes to a back | inner side from the paper front side.

  The cancel coils 3A and 3B according to the above embodiment are Helmholtz coils, but other configurations may be adopted. For example, as shown in FIG. 12, Simple box-9 (a configuration in which nine square coils are arranged at equal intervals) may be employed. In this case, the outer two coils are arranged such that their opening surfaces are located on the same plane as the opening surface of the shield part 1. In addition, the dimensions of the cancel coils 3A and 3B according to the above embodiment are merely examples, and other dimensions may be adopted.

  In the above embodiment, the cancel coil is installed only in the axial direction. However, the cancel coil may be installed in the radial direction. Specifically, a cancellation coil having an opening surface parallel to the outer surface of the shield part 1, and in the excited state, the cancellation coil reduces the level of the radial magnetic field gradient in the shield part 1. May be further installed. The cancel coil is an example of the “second coil” according to the present invention.

DESCRIPTION OF SYMBOLS 1,1A ... Shield part, 2 ... Demagnetizing coil, 3A, 3B ... Cancel coil, 4A, 4B, 4C ... Drive circuit, 5 ... Control circuit, 51 ... CPU, 52 ... ROM, 53 ... RAM, 54 ... Input part, 55 ... Output section

Claims (6)

A cylindrical shield made of a magnetic material;
A first coil provided on the same plane as the one opening surface of the shield part or at a position facing the opening surface;
The first coil reduces the level of the magnetic field gradient in the axial direction in the shield part and also reduces the level of the magnetic field gradient in the radial direction in the shield part in an excited state. .   The magnetic shield device according to claim 1, further comprising a demagnetizing unit that demagnetizes the shield unit when the first coil is in an excited state.   The magnetic shield device according to claim 2, wherein the first coil is formed along one opening edge of the shield portion.   The magnetic shield device according to claim 3, further comprising a plurality of magnetic measurement devices arranged in the shield part.   The magnetic shield device according to claim 4, wherein one opening portion of the shield portion is covered with a lid.   A second coil having an opening surface parallel to the outer surface of the shield part, wherein the second coil reduces the level of the radial magnetic field gradient in the shield part in the excited state of the second coil. The magnetic shield device according to claim 5, further comprising a coil.