JP2017152573A - Magnetic shield device and active compensation method of magnetic shield - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a magnetic shield device capable of performing compensation using a compensation magnetic field more efficiently, and to provide an active compensation method of magnetic shield.SOLUTION: A magnetic shield device 1 includes a shield part 10 formed to include a magnetic material, and defining a magnetic shield space S, a compensation coil 20 attached to the shield part, a magnetic field sensor 40 installed at a zero magnetic field point occurring on the outside of the shield space S when the magnetic field at a predetermined position in the shield space S is brought to zero by feeding a compensation current to the compensation coil, and a control section 30 configured to be able to change the magnitude of the compensation current based on the magnetic field detected by the magnetic field sensor.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a magnetic shield device and an active compensation method for a magnetic shield.

  For example, a magnetic field generated from a human body such as the heart is important real-time biological information and includes a lot of information. For example, if the cardiac magnetic field is detected by a magnetocardiograph, for example, 64 channels of a SQUID magnetometer, the electrophysiological function of the heart can be two-dimensionally mapped. In addition, it is possible to obtain overwhelmingly accurate and diverse diagnostic information such as temporal and spatial information of the current vector flowing through the stimulation conduction system, as compared with the method based on electrocardiogram waveform analysis.

  Thus, there is a demand for the development of a high-performance magnetic shield device that can be used reasonably for measurement of biomagnetism such as cardiac magnetic fields emitted from the body, not only for healthy subjects but also for bedridden patients.

In the patent document 1, the present inventors have proposed a separation type magnetic shield device that is more compact than a conventional room type magnetic shield device. Compensation coils made of a conductor wound around the peripheral surface are installed on the outer peripheral wall members of the first magnetic shield main body and the second magnetic shield main body that define the shield space. When a current is passed through the compensation coil, the magnetic flux in the rotationally symmetric axis direction of the first magnetic shield body and the second magnetic shield body is deflected by the compensation magnetic field generated in the compensation coil, thereby preventing the magnetic flux from flowing into the shield space.
This separation type magnetic shield device has high accessibility to the magnetic shield space and can effectively shield magnetic field components in all directions, so that the magnetic shield is achieved very effectively.

JP 2014-8155 A

  The inventors of the present invention have continuously studied to improve the magnetic shield device, have obtained knowledge for more effectively performing the compensation by the compensation magnetic field described above, and completed the present invention.

  It is an object of the present invention to provide a magnetic shield device and an active compensation method for a magnetic shield that can more effectively perform compensation using a compensation magnetic field.

  According to a first aspect of the present invention, there is provided a shield part that includes a magnetic body and defines a magnetic shield space; a compensation coil attached to the shield part; and a compensation current that flows through the compensation coil to provide the shield space. A magnetic field sensor installed at a zero magnetic field point generated outside the shield space when the magnetic field at a predetermined position in the inside is zero, and the magnitude of the compensation current can be changed based on the magnetic field detected by the magnetic field sensor And a control unit.

  The shield part has a first shield member and a second shield member, and the compensation coil has a first compensation coil and a second compensation coil wound around the first shield member and the second shield member. The first shield member and the second shield member may be configured to be relatively movable.

According to a second aspect of the present invention, there is provided an active compensation method for a magnetic shield in a magnetic shield device provided with a compensation coil, wherein an initial compensation current is passed through the compensation coil and a predetermined position in a shield space in the magnetic shield device. A magnetic field sensor is installed at a zero magnetic field point generated outside the shield space when the magnetic field at the predetermined position is zero, and the magnitude of the compensation current is determined based on the magnetic field detected by the magnetic field sensor. This is an active compensation method for the magnetic shield to be set.
At this time, the predetermined position may be the center of the shield space.

  According to the magnetic shield apparatus and the magnetic shield active compensation method of the present invention, compensation using a compensation magnetic field can be performed more effectively.

It is a schematic diagram which shows the structure of the magnetic shield apparatus of this embodiment. It is a perspective view which shows the shield part in the magnetic shielding apparatus. It is a top view of the shield part. It is a figure which shows the example of a zero magnetic field point. It is an example of the graph which shows the relationship of the x direction distance of a 1st compensation coil and a shield space center, and the distance of a shield space center and a zero magnetic field point.

An embodiment of the present invention will be described with reference to FIGS.
FIG. 1 is a schematic diagram showing the configuration of the magnetic shield device of the present embodiment. The magnetic shield device 1 includes a shield unit 10 that defines a shield space S, a compensation coil 20 attached to the shield unit 10, a control unit 30 connected to the compensation coil 20, and a magnetic field sensor 40 connected to the control unit 30. And.

FIG. 2 is a perspective view of the shield part 10. FIG. 3 is a plan view showing a state where the shield part 10 is viewed from above.
As shown in FIGS. 2 and 3, the shield portion 10 includes a first shield member 11 disposed on the upper side and a second shield member 12 disposed on the lower side. The first shield member 11 includes a semi-cylindrical main body portion 11a and flat surface portions 11b provided at both end portions in the circumferential direction of the main body portion 11a. The two flat portions 11a are formed so as to be located on the same plane. Similarly, the second shield member 12 has a main body portion 12a and a flat surface portion 12b.

  The first shield member 11 and the second shield member 12 include a magnetic body and are formed so as to satisfy desired magnetic shield characteristics. As the magnetic material, an amorphous magnetic material, permalloy, or the like can be used. As the structure of the first shield member 11 and the second shield member 12, a single layer or multilayer body of these magnetic bodies, a structure in which a resin or the like is laminated on both surfaces of the magnetic body, and the like can be adopted.

  The first shield member 11 and the second shield member 12 have a semi-cylindrical internal space facing each other, and the plane portion 11b and the plane portion 12b are separated by a predetermined distance (for example, about 3 to 5 centimeters (cm)). Are arranged to be. By arranging the first shield member 11 and the second shield member 12 to face each other, a substantially cylindrical shield space S is defined between the first shield member 11 and the second shield member 12.

  The compensation coil 20 includes a first compensation coil 21 for canceling the geomagnetism and a second compensation coil 22 for canceling the fluctuating magnetic field. A direct current is passed through the first compensation coil 21, and an alternating current generated by the control unit 30 is passed through the second compensation coil 22. The first compensation coil 21 and the second compensation coil 22 are both formed of a conductor, and form a loop that surrounds at least a part of the main body portions 11 a and 12 a of the shield members 11 and 12 in the plan view of the shield portion 10. So that it is wound. In each of the compensation coils 21 and 22, the size of the loop, the positional relationship between the coil and the center of the shield space (described later), the number of turns, and the like are appropriately set in consideration of various conditions described later.

The control unit 30 includes a known gain adjuster, integrator, proportional amplifier, and the like, and is connected to the second compensation coil 22.
As the magnetic field sensor 40, for example, a fundamental wave type orthogonal fluxgate sensor can be used.
The control unit 30 is configured to be able to set and adjust the current flowing through the second compensation coil 22 by performing predetermined control such as PI control based on the value of the magnetic field detected by the magnetic field sensor 40. .

The operation at the time of use of the magnetic shield device 1 of the present embodiment configured as described above will be described.
In the magnetic shield device 1, the shield portion 10 includes a first shield member 11 and a second shield member 12. Therefore, access to the shield space S can be facilitated, for example, by configuring the two so that they can be moved relative to each other. Further, it can be much lighter than the room-type magnetic shield device, and there is an advantage that the relocation is easy.

  On the other hand, since the 1st shield member 11 and the 2nd shield member 12 are spaced apart, it is necessary to pay attention to the point that a shield effect is inferior compared with a magnetic shield device of closed structure. In the present invention, by passing a current through the compensation coil 20, the external magnetic field is preferably canceled by the compensation magnetic field generated by the compensation coil, and the shield performance in the shield space S is stabilized.

In this embodiment, since the geomagnetism, which is the DC component of the external magnetic field, is constant, DC compensation is performed by passing a DC constant current through the first compensation coil 21. On the other hand, since the AC component of the external magnetic field is not always constant, the control unit 30 performs PI control based on the magnetic field detected by the magnetic field sensor 40 and sequentially updates the current value of the current flowing through the second compensation coil 22 by feedback control. To do. Thereby, active compensation of the magnetic shield generated by the magnetic shield device 1 is performed.
The details of the magnetic shield active compensation method in this embodiment will be described below.

  First, a magnetic field sensor is installed at the center (predetermined position) Sc of the shield space S. The center Sc is a position indicated as the origin of the xyz coordinate system shown in FIG. The z axis in FIG. 2 is the same as the axis of the substantially cylindrical shield space S. The y axis in FIG. 2 is an axis that extends in the vertical direction through the z axis at the midpoint in the axial direction of the shield space S. The x axis in FIG. 2 is an axis orthogonal to the y axis and the z axis at the midpoint in the axial direction of the shield space S, and extends in parallel (that is, horizontally) to the radial direction of the substantially cylindrical shield space S.

  Next, a current is passed through the first compensation coil 21 to generate a uniform magnetic field in the y direction, and the current value is set so that the detected value of the DC component of the magnetic field sensor installed at the center Sc of the shield space S becomes zero. Adjust. Then, depending on the distance in the x-axis direction between the first compensation coil 21 and the center Sc (hereinafter referred to as “x-direction distance”) in the x-direction section passing through the center Sc, the signs Zm1 and Zm2 shown in FIG. As described above, the point where the DC component of the magnetic field becomes zero also appears on the y-axis even outside the shield space S. In the present invention, this is referred to as a “zero magnetic field point”.

  FIG. 5 shows an example of a graph showing the relationship between the x-direction distance between the first compensation coil 21 and the center Sc and the distance between the center Sc and the zero magnetic field point. As shown in FIG. 5, since there is a fixed relationship between the distance in the x direction between the first compensation coil 21 and the center Sc and the distance between the center Sc and the zero magnetic field point, the magnetic field sensor 40 is placed on the y axis. The position of the zero magnetic field point can be specified by measuring the DC component of the magnetic field while moving it by a predetermined distance (for example, 1 cm) and searching for a point where the DC component is zero. In specifying the position of the zero magnetic field point, the above procedure may be performed after estimating the approximate position of the zero magnetic field point using, for example, finite element method-based physical simulation software.

  Next, the magnetic field sensor 40 is installed at the zero magnetic field point specified by the above procedure. Although FIG. 1 shows an example in which the magnetic field sensor 40 is installed above the shield part 10, zero magnetic field points Zm1 and Zm2 appear above and below the center Sc of the shield space S as shown in FIG. If the magnetic field sensor 40 is installed, the magnetic field sensor 40 may be installed in any of them. For example, if the shield unit 10 is installed at a sufficient height from the floor surface, the magnetic field sensor 40 may be installed at the zero magnetic field point Zm2 below the shield unit 10.

After the magnetic field sensor 40 is installed at the zero magnetic field point, a constant DC current is passed through the first compensation coil 21 and an alternating compensation current is passed through the second compensation coil 22 to generate magnetism generated in the magnetic shield device 1. Active compensation of the shield can be performed.
That is, when the AC component of the external magnetic field changes, the magnetic field at the center Sc of the shield space S changes and at the same time, the magnetic field at the zero magnetic field point also changes, and the change is detected by the magnetic field sensor 40. The control unit 30 performs PI control based on the magnetic field detected by the magnetic field sensor 40, and the compensation current value that is passed through the second compensation coil 22 is set again and updated. Thereby, in the shield space S, the state of zero magnetic field is suitably maintained.

As described above, according to the magnetic shield device 1 and the magnetic shield active compensation method of the present embodiment, the magnetic field sensor 40 is installed at the zero magnetic field point located outside the shield space S defined by the shield unit 10. Based on the magnetic field detected by the magnetic field sensor 40, the control unit 30 sets a current value that flows through the second compensation coil 22, and changes the current value as needed.
Thereby, a compensation magnetic field can be generated suitably corresponding to a change in the external magnetic field, and suitable active compensation can be performed. As a result, the shielding performance of the shield space S is stably exhibited.

  Since the purpose of the active compensation of the magnetic shield is to stabilize the shielding property in the shield space, it is natural that the magnetic field sensor that detects the magnetic field change that is the starting point of the PI control is installed in the shield space S. is there. However, it has been confirmed by the inventors that active compensation may not work well when the magnetic field sensor is installed in the shield space.

  There are various reasons why active compensation does not work well. One of them is the possibility that the value of the magnetic field detected in the shield space is too small. This time, the inventors have found that when a magnetic field becomes zero at a predetermined position in the shield space, a zero magnetic field point at which the magnetic field becomes zero may be generated outside the shield space at the same time. Furthermore, since the zero magnetic field point is located outside the shield space, it has been found that the magnitude of the magnetic field detected when the AC component of the external magnetic field changes is larger than the magnetic field detected in the shield space. Therefore, the magnetic field used for active compensation is detected by a magnetic field sensor installed at the zero magnetic field point, and by using this to perform active compensation, the magnetic field is detected in the shield space and active compensation is performed. We have succeeded in performing active compensation much better.

The inventors have performed active compensation by installing a magnetic field sensor in the shield space in a substantially cylindrical shield space having a diameter of 65 cm and a height of 240 cm, and active compensation by installing the magnetic field sensor at a zero magnetic field point. The internal noise in the shielded space was compared with the case where the test was performed.
As a result, it was confirmed that noise at a commercial frequency of 60 hertz (Hz) and noise at 1 Hz as low frequency band noise can be greatly reduced when the magnetic field sensor is installed at the zero magnetic field point. Yes.

In the present embodiment, the control mode in the control unit 30 can be set as appropriate. An example of the idea is shown below.
First, parameters related to control are defined as follows.
Sensitivity to magnetic flux density of magnetic field sensor 40 K _FG [V / T]
Gain k ₁ of gain adjuster in control unit 30
Transfer function k ₂ / (sτ + 1) of the integrator in the control unit 30 (where τ is an integrator time constant)
Amplification factor of proportional amplifier in control unit k ₃
Constant k ₄ determined by the shape and number of turns of the second compensation coil 22
Impedance r + sL of second compensation coil 22
Compensation current i flowing through the second compensation coil 22
Compensation magnetic field B _comp generated by the second compensation coil 22
Disturbance magnetic field B _ex
Output voltage vc of control unit 30
Shield characteristics for uniform magnetic field of shield 10 S _sh
Shielding characteristic S ' _sh against compensation magnetic field of shield 10
Magnetic field _in shield space S B _in = B _ex / S _sh + B _comp / S ' _sh
Magnetic field at zero magnetic field point a (B _ex / S _sh + B _comp / S ' _sh ) (where a is a constant greater than or equal to 1)
Then, the following equations (1) to (4) are established.

  From the equations (1) to (4), the following equation (5) is established.

  From the above, the following equation (6) is established.

In Equation (6),
K ₁ = k ₁ × k ₂ × k ₄ , K ₂ = k ₁ × k ₃ × k ₄ , b = S _sh / S ' _sh
It is.
As described above, the magnetic field Bin _{in the} shield space S can be reduced by increasing one or both of K ₁ and K ₂ .

  The embodiments of the present invention have been described above. However, the technical scope of the present invention is not limited to the above-described embodiments, and combinations of components or components may be changed without departing from the spirit of the present invention. It is possible to make various changes to or delete them.

  For example, in the present invention, the specific configuration of the shield portion is not limited to that described above. Therefore, a shield member having a different shape may be used, or the shield part may be a non-separable closed structure formed by a single shield member. The shape of the shield space defined by the shield part may be other than a cylindrical shape, for example, a polygonal column shape, an elliptical column shape, or the like.

  In the above description, when the zero magnetic field point is specified, an example is shown in which the magnetic field sensor is installed at the center of the shield space. However, the zero is detected in a state where the magnetic field sensor is installed at a position in the shield space that is not the center. A magnetic field point may be identified. In this case, the zero magnetic field point may not necessarily exist on the y-axis, but even in this case, a certain active compensation effect can be obtained by installing the magnetic field sensor at the identified zero magnetic field point.

In the present invention, “the magnetic field is zero” includes a case where the magnitude of the magnetic field is not zero but is a value very close to zero (hereinafter, referred to as “near zero value”).
Therefore, a position where the magnetic field is zero when the magnetic field at a predetermined position in the shield space is a value near zero may be specified as the zero magnetic field point. Further, when the magnetic field at a predetermined position in the shield space is zero, a position where the magnetic field has a value near zero outside the shield space may be specified as the zero magnetic field point. Furthermore, when the magnetic field at a predetermined position in the shield space has a value near zero, a position where the magnetic field has a value near zero outside the shield space may be specified as the zero magnetic field point.
Also in these aspects, active compensation of the magnetic shield can be performed while exhibiting a certain effect. However, it is natural that the aspect described in the above embodiment can perform active compensation with the highest accuracy.

DESCRIPTION OF SYMBOLS 1 Magnetic shield apparatus 10 Shield part 11 First shield member 12 Second shield member 20 Compensation coil 21 First compensation coil 22 Second compensation coil 30 Control part 40 Magnetic field sensor S Shield space Sc (Shield space) center (predetermined position)
Zm1, Zm2 Zero magnetic field point

Claims (4)

A shield part that includes a magnetic material and defines a magnetic shield space;
A compensation coil attached to the shield part;
A magnetic field sensor installed at a zero magnetic field point generated outside the shield space when a compensation current is passed through the compensation coil to set the magnetic field at a predetermined position in the shield space to zero; and
A control unit configured to be able to change the magnitude of the compensation current based on the magnetic field detected by the magnetic field sensor;
Comprising
Magnetic shield device. The shield part has a first shield member and a second shield member,
The compensation coil has a first compensation coil and a second compensation coil wound around the first shield member and the second shield member,
The first shield member and the second shield member are configured to be relatively movable,
The magnetic shield device according to claim 1. An active compensation method of a magnetic shield in a magnetic shield device provided with a compensation coil, comprising:
An initial compensation current is passed through the compensation coil to set the magnetic field at a predetermined position in a shield space in the magnetic shield device to zero,
A magnetic field sensor is installed at a zero magnetic field point generated outside the shield space when the magnetic field at the predetermined position is zero,
Setting the magnitude of the compensation current based on the magnetic field detected by the magnetic field sensor;
Magnetic shield active compensation method. The predetermined position is a center of the shield space;
The method for active compensation of a magnetic shield according to claim 3.

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