JP2014060270A - Magnetic shield device and magnetic shield method - Google Patents

Abstract

A magnetic shield device that further reduces the influence of an external magnetic field is provided. A magnetic shield device (1) includes a passive shield (11) having an internal space (110) and at least one opening (111), and a magnetic field in the vicinity of the opening (111). An external coil 12 that corrects the magnetic field, an internal coil 13 that corrects the magnetic field in the internal space 110, a drive circuit 15A that drives the external coil 12, and a drive circuit 15B that drives the internal coil 13. In the magnetic shield device 1, the magnetic field corrected by the external coil 12 is reduced by the passive shield 11 and further reduced by the internal coil 13.
[Selection] Figure 1

Description

  The present invention relates to a magnetic shield device and a magnetic shield method.

  2. Description of the Related Art Magnetic shields for reducing the influence of an external magnetic field in an apparatus affected by an external magnetic field (disturbance magnetic field) are known. Patent Document 1 discloses a magnetic shield device using a plurality of magnetic field correction coils. Patent Document 2 discloses a technique for reducing the disturbing magnetic field entering the detection coil based on the magnetic field outside the shield room. Patent Document 3 discloses a magnetic shield room having a coil inside. Patent Document 4 discloses a magnetic shield system having a compensation coil and a magnetic detection coil.

JP 2003-273565 A JP 2005-16960 A JP 2007-129049 A JP 2010-93224 A

The techniques described in Patent Documents 1 and 4 are not related to a magnetic shield device using a passive shield. Even with the techniques described in Patent Documents 2 and 3, the influence of an external magnetic field cannot be sufficiently reduced for an apparatus that is affected by a minute magnetic field.
In contrast, the present invention provides a magnetic shield device that further reduces the influence of an external magnetic field.

  The present invention includes a passive shield having an internal space and at least one opening, a first coil for correcting a magnetic field in the vicinity of the opening, a second coil for correcting a magnetic field in the internal space, and the first coil. Provided is a magnetic shield device having a first drive circuit for driving and a second drive circuit for driving the second coil. According to this magnetic shield device, the influence of the external magnetic field can be reduced as compared with the case where only one of the first coil and the second coil is used.

  In a preferred aspect, the first drive circuit supplies the first coil with a current that makes the magnetic field at the end face of the opening zero. According to this magnetic shield device, when no current is supplied to the first coil, or when a current that makes the magnetic field at the central portion of the magnetic shield device generally used is zero, geomagnetism, etc. The influence of the external magnetic field can be best reduced as compared with a case where a current that generates a magnetic field in the opposite direction to the external magnetic field is supplied.

  In another preferable aspect, the magnetic sensor includes a magnetic sensor provided on an end face of the opening, and the first driving circuit supplies a current that makes a magnetic field measured by the magnetic sensor zero to the first coil. It is characterized by that. According to this magnetic shield device, the magnetic field on the end face of the opening can be brought close to zero.

In another preferred embodiment, the magnetic sensor is provided at the center of the end face of the opening. According to this magnetic shield device, the magnetic field at the center of the end face of the opening can be brought close to zero.

  In still another preferred aspect, the second drive circuit supplies a current that makes the magnetic field in the internal space zero to the second coil. According to this magnetic shield device, the influence of the external magnetic field can be reduced as compared with the case where the current that makes the magnetic field in the internal space zero is not supplied to the second coil.

  In still another preferred aspect, the magnetic sensor is provided in the internal space, and the second driving circuit supplies the second coil with a current that makes a magnetic field measured by the magnetic sensor zero. Features. According to this magnetic shield device, the magnetic field in the internal space can be brought close to zero.

  In still another preferred aspect, the dynamic range of the magnetic field generated by the first coil is larger than the dynamic range of the magnetic field generated by the second coil. According to this magnetic shield device, the influence of the external magnetic field can be reduced as compared with the case where the dynamic range of the magnetic field generated by the first coil is smaller than the dynamic range of the magnetic field generated by the second coil.

  In still another preferred aspect, the resolution of the magnetic field generated by the second coil is higher than the resolution of the magnetic field generated by the first coil. According to this magnetic shield device, the influence of the external magnetic field can be reduced as compared with the case where the resolution of the magnetic field generated by the second coil is lower than the resolution of the magnetic field generated by the first coil.

  In still another preferred embodiment, the passive shield has two openings, the magnetic shield device has two first coils, and each of the two first coils has the two openings. It is characterized by correcting the magnetic field in the vicinity. According to this magnetic shield device, when the passive shield has two openings, the influence of the external magnetic field can be reduced as compared with the case where the number of the first coils is one.

  In still another preferred aspect, the first drive circuit supplies a current having the same current value to the two first coils. According to this magnetic shield device, the influence of the external magnetic field can be reduced as compared with the case where currents having different current values are supplied to the two first coils.

  According to another aspect of the present invention, in a passive shield having an internal space and at least one opening, supplying a current to a first coil provided near the opening in order to correct a magnetic field near the opening; Supplying a current to a second coil provided in the internal space in order to correct the magnetic field in the internal space. According to this magnetic shielding method, the influence of the external magnetic field can be reduced as compared with the case where only one of the first coil and the second coil is used.

Schematic of a magnetic shield device. The figure which shows the external appearance of a magnetic shielding apparatus. The figure which illustrates arrangement | positioning of a magnetic sensor. The flowchart which shows operation | movement of a magnetic shielding apparatus. Schematic of the magnetic shielding apparatus which concerns on a modification. The flowchart which shows operation | movement of the magnetic shielding apparatus which concerns on a modification.

  FIG. 1 is a schematic view of a magnetic shield device 1 according to an embodiment of the present invention. The magnetic shield device 1 includes a passive shield 11, an external coil 12, an internal coil 13, a magnetic sensor 14, a drive circuit 15, and a control device 16. The magnetic shield device 1 is a device that combines a passive shield 11 and an active shield (external coil 12 and internal coil 13). In the magnetic shield device 1, the magnetic field corrected by the external coil 12 is reduced by the passive shield 11 and further reduced by the internal coil 13.

  The passive shield 11 is formed of a ferromagnetic material (for example, permalloy, ferrite, or iron, chromium, or cobalt-based amorphous). The passive shield 11 has an internal space 110 and at least one opening 111. In this example, the passive shield 11 has a cylindrical shape with both ends opened. The shape of the cross section perpendicular to the long axis of the passive shield 11 is a quadrangle. The internal space 110 of the passive shield 11 is large enough to accommodate a magnetic measuring device such as a magnetocardiograph and its subject (for example, a human). The external coil 12 is a coil for correcting (cancelling) the magnetic field in the vicinity of the opening 111. The internal coil 13 is a coil for correcting the magnetic field in the internal space 110. As the external coil 12 and the internal coil 13, for example, a Helmholtz coil constituted by two coils (12A and 12B or 13A and 13B) is used. The magnetic sensor 14 measures a magnetic field. The magnetic sensor 14 is provided individually corresponding to each of the external coil 12 and the internal coil 13. The magnetic sensor 14 </ b> A is a sensor corresponding to the external coil 12, and the magnetic sensor 14 </ b> B is a sensor corresponding to the internal coil 13.

  FIG. 2 is a diagram showing an overview of the magnetic shield device 1. For the following description, a three-dimensional orthogonal coordinate system is defined. In this coordinate system, the x axis represents the major axis direction of the passive shield 11, the y axis represents the width direction of the passive shield 11, and the z axis represents the height direction of the passive shield 11. The passive shield 11 has two openings 111A and 111B at both ends in the x-axis direction. The external coil 12 is wound around the side surface of the passive shield 11 such that one coil 12A is near the opening 111A and the other coil 12B is near the opening 111B. In FIG. 2, the external coil 12 has an end on the opening 111A side located on the same plane as the end surface 111S of the opening 111A, and an end on the opening 111B side is in contact with the end surface 111S of the opening 111B. It is wound so that it lies on the same plane. The internal coil 13 is disposed in the internal space 110 so as to sandwich a region where the magnetic field is to be corrected (in this example, the center of the internal space 110) between the coil 13A and the coil 13B.

  FIG. 3 is a diagram illustrating the arrangement of the magnetic sensor 14. FIG. 3A is a diagram of the magnetic shield device 1 viewed from the x-axis direction. FIG. 3B is a diagram of the magnetic shield device 1 viewed from the z-axis direction. The magnetic sensor 14 </ b> A is disposed, for example, at the center of gravity (center) of the opening 111. The magnetic sensor 14 </ b> A measures the direction (positive / negative) and magnitude of the x-axis component of the magnetic field near the opening 111. The magnetic sensor 14B is disposed in a region where it is desired to correct the magnetic field. The magnetic sensor 14B measures the direction (positive / negative) and magnitude of the x-axis component of the magnetic field in the region where the magnetic field is to be corrected. In this example, the resolution of the magnetic sensor 14B is higher than the resolution of the magnetic sensor 14A.

  Refer to FIG. 1 again. The drive circuit 15 </ b> A is a circuit that drives the external coil 12. The drive circuit 15B is a circuit that drives the internal coil 13. The control device 16 is a device that controls the drive circuit 15 in accordance with the magnetic field measured by the magnetic sensor 14. The control device 16 is, for example, a computer device having a CPU (Central Processing Unit), a ROM (Read Only Memory), and a RAM (Random Access Memory).

The control device 16 acquires signals indicating the direction and magnitude of the magnetic field from the magnetic sensors 14A and 14B, respectively. Based on the direction and magnitude of the magnetic field measured by the magnetic sensor 14A, the control device 16 controls the drive circuit 15A so that the magnetic field at the end surface 111S becomes zero. The control device 16 also controls the drive circuit 15B so that the magnetic field in the internal space 110 becomes zero based on the direction and magnitude of the magnetic field measured by the magnetic sensor 14B.

  The drive circuit 15A supplies the external coil 12 with a current that makes the magnetic field at the end face 111S zero. Here, “zero” is a predetermined range including zero, and the predetermined range is determined by the resolution of the magnetic sensor 14A. Usually, it is 1PT (picotesla) or less. The drive circuit 15A supplies currents of the same direction and the same current value to the coils 12A and 12B. When current having the same current value flows in the coils 12A and 12B in the same direction, a magnetic field is generated in the vicinity of the openings 111A and 111B, and the strength of the magnetic field in the x-axis direction of the internal space 110 is When no current is supplied, when a current that makes the magnetic field of the central portion of the magnetic shield device 1 zero is supplied, and when a current that generates a magnetic field in the opposite direction and the same level as an external magnetic field such as geomagnetism is supplied It is reduced in comparison with.

  The drive circuit 15 </ b> B supplies a current that makes the magnetic field in the internal space 110 zero to the internal coil 13. Here, “zero” is also a predetermined range including zero, and the predetermined range is determined by the resolution of the magnetic sensor 14B. Usually, it is 1PT (picotesla) or less. The drive circuit 15B supplies currents having the same current value in the same direction to the coils 13A and 13B. When currents of the same direction and the same current value flow through the coils 13A and 13B, the internal coil 13 is used as the magnetic field strength in the x-axis direction of the small space sandwiched between the coils 13A and 13B of the internal space 110. It is further reduced compared to the case where there is no.

  In this example, the dynamic range of the magnetic field generated by the external coil 12 is larger than the dynamic range of the magnetic field generated by the internal coil 13. Further, the resolution of the magnetic field generated by the internal coil 13 is higher than the resolution of the magnetic field generated by the external coil 12.

  FIG. 4 is a flowchart showing the operation of the magnetic shield device 1. In the magnetic shield device 1, processing from step S1 to step S8 is performed. The following process is started when the user activates the magnetic shield device 1 by operating an input unit (not shown), for example.

In step S1, the control device 16 measures the direction and magnitude of the magnetic field on the end surface 111S. Specifically, the control device 16 acquires a signal indicating the direction and magnitude of the magnetic field on the x axis from the magnetic sensor 14A. In step S <b> 2, the control device 16 controls the drive circuit 15 </ b> A to supply current to the external coil 12. Specifically, the control device 16 outputs a signal for causing the external coil 12 to supply a current that makes the magnetic field measured by the magnetic sensor 14A zero, to the drive circuit 15A. The drive circuit 15 </ b> A supplies current to the external coil 12 based on the signal output from the control device 16. The direction of the current supplied to the external coil 12 is a direction that generates a magnetic field opposite to the magnetic field measured by the magnetic sensor 14A. The magnitude of the current supplied to the external coil 12 is the magnitude of the current generated in the external coil 12 by the same magnitude as the magnetic field measured by the magnetic sensor 14A. The correspondence between the magnitude of the current supplied to the external coil 12 and the magnetic field generated inside the external coil 12 is stored in advance in the ROM. In step S3, the control device 16 again measures the direction and magnitude of the magnetic field on the end surface 111S. In step S4, the control device 16 determines whether or not the magnitude of the magnetic field at the end face 111S is zero. When it is determined that the magnitude of the magnetic field at the end face 111S is zero (step S4: YES), the control device 16 proceeds to step S5. When it is determined that the magnitude of the magnetic field at the end face 111S is not zero (step S4: NO), the control device 16 proceeds to step S2, and from step S2 until the magnitude of the magnetic field at the end face 111S becomes zero. The process of step S4 is repeated.

  In step S <b> 5, the control device 16 measures the direction and magnitude of the magnetic field in the internal space 110. Specifically, the control device 16 acquires a signal indicating the direction and magnitude of the magnetic field from the magnetic sensor 14B. In step S <b> 6, the control device 16 controls the drive circuit 15 </ b> B to supply current to the internal coil 13. Specifically, the control device 16 outputs a signal for causing the internal coil 13 to supply a current that makes the magnetic field measured by the magnetic sensor 14B zero, to the drive circuit 15B. The drive circuit 15B supplies current to the internal coil 13 based on the signal output from the control device 16. The direction of the current supplied to the internal coil 13 is a direction in which a magnetic field opposite to the magnetic field measured by the magnetic sensor 14B is generated. The magnitude of the current supplied to the internal coil 13 is the magnitude of a current that generates a magnetic field having the same magnitude as the magnetic field measured by the magnetic sensor 14B. The correspondence between the magnitude of the current supplied to the internal coil 13 and the magnetic field generated in the internal coil 13 is stored in advance in the ROM. In step S <b> 7, the control device 16 again measures the direction and magnitude of the magnetic field in the internal space 110. In step S8, the control device 16 determines whether or not the magnitude of the magnetic field in the internal space 110 is zero. When it is determined that the magnitude of the magnetic field in the internal space 110 is zero (step S8: YES), the control device 16 ends the process. If it is determined that the magnitude of the magnetic field in the internal space 110 is not zero (step S8: NO), the control device 16 proceeds to step S6 and steps until the magnitude of the magnetic field in the internal space 110 becomes zero. The process from S6 to step S8 is repeated.

  When the processing from step S1 to step S4 is performed, the magnetic field in the internal space 110 approaches zero in the order of the resolution of the external coil 12. When the processing from step S5 to step S8 is performed in this state, the magnetic field in the internal space 110 approaches zero in the order of the resolution of the internal coil 13. According to this embodiment, the influence of the external magnetic field can be further reduced as compared with the case where the magnetic field is canceled only by the external coil 12 or only by the internal coil 13.

  The present invention is not limited to the above-described embodiments, and various modifications can be made. Hereinafter, some modifications will be described. Two or more of the modifications described below may be used in combination.

  The coil included in the magnetic shield device is not limited to one that makes the magnetic field zero in a predetermined region of the magnetic shield device. The magnetic shield device may further include a coil that corrects the gradient of the magnetic field.

  FIG. 5 is a schematic view of a magnetic shield device 2 according to a modification. The magnetic shield device 2 is different from the magnetic shield device 1 shown in FIG. 1 in that it has a gradient coil 17, a magnetic sensor 14C, and a drive circuit 15C. The gradient coil 17 is a coil for correcting the gradient of the magnetic field in the internal space 110. As the gradient coil 17, for example, two coils (17A and 17B) are used. The magnetic sensor 14 </ b> C is a sensor corresponding to the gradient coil 17. The magnetic sensor 14C is disposed on the surface on which the gradient coil 17 is disposed, for example. The magnetic sensor 14C measures the direction (positive / negative) and magnitude of the x-axis component of the magnetic field on the surface on which the gradient coil 17 is disposed. The drive circuit 15 </ b> C is a circuit that drives the gradient coil 17.

In the modification, the control device 16 acquires signals indicating the direction and magnitude of the magnetic field by the magnetic sensor 14C. The control device 16 calculates the gradient of the magnetic field based on the magnitude of the magnetic field measured by the magnetic sensor 14C. Based on the calculated magnetic field gradient, the control device 16 controls the drive circuit 15C so that the magnetic field gradient in the internal space 110 becomes zero. The drive circuit 15 </ b> C supplies a current that makes the gradient of the magnetic field in the internal space 110 zero to the gradient coil 17. Here, “zero” is a predetermined range including zero, and the predetermined range is a range in which the magnetic field gradient is not affected (the magnetic field is uniform). The drive circuit 15C supplies current in the reverse direction to the coil 17A and the coil 17B. When a current in the reverse direction flows through the coil 17A and the coil 17B, the gradient of the magnetic field in the x-axis direction of the small space sandwiched between the coil 17A and the coil 17B in the internal space 110 is larger than that when the gradient coil 17 is not used. Reduced.

  FIG. 6 is a flowchart showing the operation of the magnetic shield device 2. In the magnetic shield device 2, in addition to the processing from step S1 to step S8 shown in FIG. 4, the processing from step S9 to step S12 is performed.

In step S <b> 9, the control device 16 calculates a magnetic field gradient in the internal space 110. Specifically, the control device 16 acquires a signal indicating the direction and magnitude of the magnetic field on the x-axis from the magnetic sensor 14C, and calculates the magnetic field gradient A by the following equation (1).
A = Hmax−Hmin (1)
Hmax and Hmin indicate the larger and smaller ones of the magnetic fields measured by the magnetic sensor 14C, respectively.

In step S10, the control device 16 controls the drive circuit 15C so that the measured magnetic field satisfies the following equation (2) while measuring the magnetic fields H1 and H2 generated by the gradient coil 17A and the gradient coil 17B. A current is supplied to the gradient coil 17.
α = | H1-H2 | − (Hmax−Hmin) = 0 (2)
Specifically, the control device 16 outputs a signal for supplying the gradient coil 17 with a current that makes the calculated magnetic field gradient zero, to the drive circuit 15C. The drive circuit 15 </ b> C supplies a current to the gradient coil 17 based on the signal output from the control device 16. In step S <b> 11, the control device 16 calculates the magnetic field gradient in the internal space 110 again. In step S12, the control device 16 determines whether or not the gradient of the magnetic field in the internal space 110 is zero. When it is determined that the gradient of the magnetic field in the internal space 110 is zero (step S12: YES), the control device 16 ends the process. When it is determined that the magnetic field gradient in the internal space 110 is not zero (step S12: NO), the control device 16 proceeds to step S10, and from step S10 until the magnetic field gradient in the internal space 110 becomes zero. The process of step S12 is repeated. When the processing from step S9 to step S12 is performed, the gradient of the magnetic field in the internal space 110 approaches zero.

  Note that when the gradient coil 17 is used, the magnetic sensor 14C may not be used. In this case, the control device 16 may control the gradient coil 17 using a signal from the magnetic sensor 14B.

  The cross section of the passive shield 11 viewed from the axial direction is not limited to a square shape. For example, the cross section of the passive shield 11 viewed from the axial direction may be a polygon other than a square, a circle, or an ellipse. The passive shield 11 is not limited to the case where the openings 111 are provided at both ends in the axial direction. The passive shield 11 may have an opening 111 on one side in the axial direction. In this case, the other side in the axial direction that does not have the opening 111 is covered with a lid. The lid is formed integrally with the passive shield 11, for example. Furthermore, the passive shield 11 is not limited to a single layer structure, and may have a multilayer structure.

  The external coil 12, the internal coil 13, and the gradient coil 17 are not limited to being wound in a square shape. The outer coil 12, the inner coil 13, and the gradient coil 17 may be wound in a polygon other than a square, a circle, or an ellipse.

  The external coil 12 and the internal coil 13 are not limited to being Helmholtz coils. One coil may be used for each of the external coil 12 and the internal coil 13.

  In the embodiment, the example in which the internal coil 13 is provided only in one axial direction (x-axis direction) has been described. However, the magnetic shield device 1 may have two or more sets of internal coils 13 and cancel components of two or more axes of the magnetic field. For example, the magnetic shield device 1 may have the internal coil 13 for each of the three axes of the x, y, and z axes and cancel the magnetic field of each component in the three axis directions.

  The arrangement and number of the magnetic sensors 14 are not limited to those described in the embodiment. For example, the magnetic sensor 14A may be arranged at the center of gravity of one of the openings 111A and 111B.

  The control method for each coil is not limited to that described in the embodiment. In the embodiment, an example in which each coil is feedback-controlled has been described, but each coil may be controlled by a method other than the feedback control.

  DESCRIPTION OF SYMBOLS 1,2 ... Magnetic shield apparatus, 11 ... Passive shield, 110 ... Internal space, 111 ... Opening part, 12 ... External coil, 13 ... Internal coil, 14 ... Magnetic sensor, 15 ... Drive circuit, 16 ... Control apparatus, 17 ... Gradient coil

Claims (11)

A passive shield having an internal space and at least one opening;
A first coil for correcting the magnetic field in the vicinity of the opening;
A second coil for correcting a magnetic field in the internal space;
A first drive circuit for driving the first coil;
And a second drive circuit for driving the second coil. 2. The magnetic shield device according to claim 1, wherein the first drive circuit supplies a current that makes a magnetic field at an end face of the opening to zero to the first coil. A magnetic sensor provided on an end face of the opening;
3. The magnetic shield device according to claim 2, wherein the first drive circuit supplies a current that makes a magnetic field measured by the magnetic sensor zero to the first coil. The magnetic shield device according to claim 3, wherein the magnetic sensor is provided at a center of an end face of the opening. 5. The magnetic shield device according to claim 1, wherein the second drive circuit supplies a current that makes the magnetic field in the internal space zero to the second coil. 6. A magnetic sensor provided in the internal space;
6. The magnetic shield device according to claim 5, wherein the second drive circuit supplies a current that makes a magnetic field measured by the magnetic sensor zero to the second coil. The magnetic shield device according to any one of claims 1 to 6, wherein a dynamic range of a magnetic field generated by the first coil is larger than a dynamic range of a magnetic field generated by the second coil. 8. The magnetic shield device according to claim 1, wherein the resolution of the magnetic field generated by the second coil is higher than the resolution of the magnetic field generated by the first coil. The passive shield has two openings,
The magnetic shield device has two first coils,
The magnetic shield device according to claim 1, wherein each of the two first coils corrects a magnetic field in the vicinity of the two openings. The magnetic shield device according to claim 9, wherein the first drive circuit supplies a current having the same current value to the two first coils. Supplying a current to a first coil provided near the opening in order to correct a magnetic field near the opening in a passive shield having an internal space and at least one opening;
Supplying a current to a second coil provided in the internal space to correct a magnetic field in the internal space.

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