JP2007208071A - Magnetic shield structure - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optimumly transmissive magnetic shield structure having excellent performance, wherein air and light can transmit it and a plurality of spaced magnetic frames composed of tabular magnetic substance are arranged in the direction of the tabular magnetic substance plates to reduce the weight and the cost of the shield. <P>SOLUTION: In this magnetic shield structure, the magnetic flux density in the magnetic substance constituting the shield has a distribution. The ratio of the maximum flux density value Bm in that distribution to the saturated magnetic flux density Bs is made to satisfy the relation 0.4≤Bm/Bs≤0.9, and the ratio between the magnetic substance width W and the frame spacing d is made to satisfy the relation 0<d/W≤4.5. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a magnetic shield structure using a ferromagnetic material for the purpose of keeping a certain space magnetically clean in the medical field, precision measurement field, VLSI manufacturing field, and the like.

  A magnetic shield technique using a magnetic material having a high magnetic permeability is a general technique. For example, as described in Patent Document 1 and Patent Document 2, the configuration is basically to enclose and enclose a target space with a plate-shaped magnetic body, The shield performance is improved by preventing leakage from the joined portion of the plate with a cover or the like. In these configurations, since the surface of the magnetic body is arranged in parallel with the wall surface of the magnetic shield room and the target space is sealed, there is no air or light permeability. Furthermore, the measures such as the composite of the magnetic material and the leakage prevention cover of the joint have a problem that it is difficult to obtain the shielding performance that is expected from the material characteristics.

  As magnetic shielding methods that allow air and light to pass therethrough, proposals have been made in Patent Documents 3 to 7 in which plate-like magnetic bodies are installed at intervals in the plate thickness direction. However, all of these only give a configuration of one surface, and there is no description about a configuration for magnetically shielding a certain space, and it is insufficient for obtaining an excellent shielding performance. In Patent Document 8, the cross-sectional area Sm of the magnetic material and the cross-sectional area Sa of the air gap are defined as (Sm · μs) / Sa> 1 using the relative magnetic permeability μs of the magnetic material, and the interval between the magnetic materials is substantially reduced. Is stipulated. Considering the case where magnetic materials having a thickness E and a width F are arranged in a bowl shape, the cross-sectional area Sa of the air gap is G · F if the magnetic material interval is G. Therefore, the definition of the cross-sectional area can be (E / G)> (1 / μs). Here, the magnetic permeability μs of the magnetic material varies depending on the material, and also varies depending on the same material or the magnetic flux density. If 60000 described in Patent Document 8 is used as μs (a magnetic material generally used for magnetic shielding is a soft magnetic material, a value of several tens of thousands is reasonable if the maximum relative permeability is). E / G> 1/60000. This indicates that if a 1 mm magnetic material is used, the gap interval can be 60 m, which is insufficient to provide an optimum design guideline for the magnetic shield structure.

  In contrast, as a magnetic shielding method that allows air and light to pass through but has excellent shielding performance, the inventors have arranged a magnetic frame made of a plate-like magnetic body at intervals in the thickness direction of the plate-like magnetic body. Patent Document 9 proposes a structure in which a plurality of structures are arranged.

JP-A-5-327263 JP-A-7-273484 JP-A-8-288688 JP-A-9-287368 Japanese Patent Laid-Open No. 9-273366 JP-A-10-169336 JP 10-46958 A JP 2002-164686 A WO20041086033A1

  As described above, the present inventors have proposed a magnetic shield method in Patent Document 9 that is excellent in shielding performance while allowing air and light to pass therethrough. In this case, by arranging the magnetic frames at intervals, it is possible to ensure free transmission of air and light from the gaps between the magnetic frames. Furthermore, the magnetic bodies forming the magnetic body frame can be joined at the surface to reduce magnetic flux leakage from the joint portion, and a shielding performance equivalent to or better than that of the conventional method can be secured. It is an object of the present invention to provide an optimum shield structure for reducing the weight and cost of the shield for such a transmission type magnetic shield method.

The present invention is based on the optimum determination of the magnetic flux density in the magnetic body constituting the shield body, the width of the magnetic body, and the interval between the frames. Specifically, it is as follows.
(1) A plate-like magnetic body having a width W or a laminated body of these plate-like magnetic bodies are brought into contact with each other at lengthwise ends, and magnetic frames are formed so that the magnetic circuit is closed. In the magnetic shield structure with a gap d in the thickness direction of the magnetic body, the largest value Bm among the magnetic flux density B distributed in the magnetic body frame is saturated as the material characteristic of the plate-like magnetic body. In ratio with magnetic flux density Bs,
0.4 ≤ Bm / Bs ≤ 0.9
And the ratio of the plate-like magnetic body width W to the frame interval d,
0 <d / W ≤ 4.5
Magnetic shield structure characterized by that.
(2) The magnetic shield structure according to (1), wherein the plate-like magnetic body is an electromagnetic steel plate or permalloy.

  The magnetic shield structure using the ferromagnetic material according to the present invention can maintain a specific space in a magnetically clean state in fields such as the medical field, precision measurement field, and VLSI manufacturing field.

  The present invention is described in detail below.

  First, according to the present invention, a plate-like magnetic body having a width W, or a laminate of these plate-like magnetic bodies is brought into contact with each other at end portions in the length direction to form a magnetic frame so as to form a closed magnetic circuit. . The contact at the end is preferably contact between the surfaces of the magnetic body or the magnetic layered body. This is because there is little magnetic flux leakage at the contact site and the magnetic shielding performance is good. In order to facilitate the construction, butt contact may be used as long as some performance deterioration is allowed. Further, it is desirable to keep the distance between the magnetic bodies in the contact portion within 3 mm in order to prevent magnetic flux leakage. As described above, the magnetic frame is formed in accordance with the target space so as to form a closed magnetic circuit. Next, such a magnetic body frame is disposed with a gap d between them to form a magnetic shield structure. Air or light can pass through this gap.

Distribution occurs in the magnetic flux density B in the magnetic frame of the magnetic shield structure. The distribution of B is determined by the magnetic field distribution and strength of the space and the shape of the magnetic shield body. The present inventors have found that controlling the largest magnetic flux density value Bm in the distribution of B is the best policy for efficiently using a magnetic material and obtaining a magnetic shield structure with excellent performance. It was. That is, in the ratio of the maximum value Bm in the distribution of magnetic flux density to the saturation magnetic flux density Bs which is the material property used,
0.4 ≤ Bm / Bs ≤ 0.9
It is to do. The reason for this limitation will be described below based on experimental facts.
<Experiment 1>
A laminated body is made using a directional electrical steel sheet (saturated magnetic flux density Bs = 2.0T) with a thickness of 0.35mm, a width of 33mm, and a length of 300mm with the rolling direction aligned in the longitudinal direction. The surfaces were joined together to form a 300 mm × 300 mm square frame. The magnetic flux density in the frame can be changed by variously changing the number of electromagnetic steel sheets constituting the laminate. Twenty sets of this frame were prepared and arranged as shown in FIG. 1 with a gap of 15 mm to form a magnetic shield. The d / W of this shield body is 15 mm / 33 mm≈0.45. An electromagnet with a coil wound around an iron core with a diameter of 33 mm and a length of 167 mm was installed in the center of the shield body to generate a DC magnetic field. The current of the electromagnet was adjusted so that the strength of the magnetic field was 2000 μT (20 G) and 5000 μT (50 G) without the shield body at the position where the shaft of the electromagnet and the outer wall of the shield body intersected. The leakage magnetic field after shielding was also measured at this position. A magnetic flux density detection coil was wound around a frame having the same height as the shaft of the electromagnet at a pitch of 75 mm as shown in FIG. The DC magnetic flux density of each part was measured using a DC magnetometer, and the largest value was defined as Bm. FIG. 3 shows the relationship between Bm / Bs obtained by normalizing the Bm with Bs and the leakage magnetic field. When the unshielded magnetic field is 2000 μT and 5000 μT, almost the same curve is drawn, and the leakage magnetic field increases as Bm / Bs increases. In particular, when this value is greater than 0.9, the leakage magnetic field increases rapidly. In addition, when the weight of the grain-oriented electrical steel sheet is increased and Bm / Bs is decreased, if it is smaller than 0.6, the magnitude of the leakage magnetic field becomes constant and the shielding performance is saturated.
<Experiment 2>
Next, an experiment was conducted using a PC permalloy which is a Ni—Fe alloy having a saturation magnetic flux density of 1.1 T. Four PC permalloys with a thickness of 1 mm, a width of 33 mm, and a length of 300 mm were joined to each other at the end in the length direction to form a 300 mm × 300 mm square frame. Twenty sets of this frame were prepared and arranged as shown in FIG. 1 with a gap of 15 mm to form a magnetic shield. d / W is 15 mm / 33 mm≈0.45. A Helmholtz coil with a diameter of 900 mm was used to apply a magnetic field to the shield. A shield body was installed at the center of the Helmholtz coil, and the orientation was determined so that the two surfaces of the shield body were perpendicular to the magnetic field direction. The magnetic field generated by the Helmholtz coil was changed, and the magnetic field at the center of the shield body and the magnetic flux density in the permalloy frame were measured. The applied magnetic field was 0 to 100 μT (0 to 10 G) at the center of the coil without the shield. The magnetic flux density in the frame was detected from the voltage of each coil by installing a frame in which the coil of FIG. FIG. 4 shows the relationship between Bm / Bs obtained by normalizing the maximum magnetic flux density Bm in the frame with Bs and the leakage magnetic field. As in the case of <Experiment 1> using a grain-oriented electrical steel sheet, when Bm / Bs exceeds 0.9, the leakage magnetic field increases rapidly. In addition, when the applied magnetic field was decreased and Bm / Bs was decreased, the magnitude of the leakage magnetic field became constant and the shielding performance was saturated when the value was less than 0.4, which is smaller than <Experiment 1>.

  From the above experiment, it was found that the leakage magnetic field suddenly increased when Bm / Bs was greater than 0.9 even if the magnetic materials were different. Therefore, in the present invention, Bm / Bs ≦ 0.9.

  In addition, the lower limit of Bm / Bs at which the shielding performance is saturated was different between grain oriented electrical steel and permalloy. This is considered to reflect that the grain-oriented electrical steel sheet has an easy axis in the rolling direction and strong anisotropy, but permalloy is isotropic. In consideration of the use of an isotropic material, 0.4 ≦ Bm / Bs is set in the present invention.

  Bm / Bs can be controlled by increasing / decreasing the weight of the magnetic materials that make up the frame, but more magnetic materials can be placed where the magnetic field in the space is larger, and fewer magnetic materials can be placed where the magnetic field is small. If devised, the maximum shield performance can be obtained efficiently with the weight of the entire shield body.

Next, in the present invention, the ratio d / W between the magnetic body width W and the frame interval d is 0 <d / W and d / W ≦ 4.5. This is based on the following experimental facts.
<Experiment 3>
Using a directional electrical steel sheet with a thickness of 0.35mm, width of 25mm, 50mm, 100mm, and length of 900mm with the rolling direction aligned in the longitudinal direction, it consists of a square frame of 900mm x 900mm and has various d / W values. A shield body with a height of 900 mm was produced. Details of the configuration are shown in Table 1. A square Helmholtz coil with a coil interval of 1 m and a side of 2.1 m was used to apply a magnetic field to the shield. The orientation was determined so that the two surfaces of the shield body were perpendicular to the magnetic field direction, and the shield body was installed at the center of the Helmholtz coil. The applied magnetic field was 120 μT (1.2 G) at the coil center without the shield body, and the leakage magnetic field was measured at the shield body center position. The configuration of the measuring apparatus is shown in FIG. FIG. 6 shows a change in shield coefficient SE (= magnetic field before shield / magnetic field after shield, shield performance is better as the value is larger) due to d / W. Even if the plate width changes, the shield coefficient SE can be arranged in d / W. In the range of d / W ≦ 4.5, SE increases rapidly and the shielding performance improves. The magnetic flux density was measured at the same time. In all cases, Bm / Bs was in the range of 0.5 to 0.7.

<Experiment 4>
Using a PC permalloy having a thickness of 1.0 mm, a width of 50 mm, a width of 100 mm, and a length of 900 mm, a shield body having a height of 900 mm composed of a square frame of 900 mm × 900 mm was produced. Details of the configuration are shown in Table 2. The d / W was changed. The measurement method and conditions for the leakage magnetic field are the same as in <Experiment 3>. FIG. 7 shows the change of the shield coefficient SE with d / W. Overall, the shielding performance is better than <Experiment 3> using grain-oriented electrical steel sheets. Further, the fact that SE rapidly increases in the range of d / W ≦ 4.5 is the same behavior as in <Experiment 3>. Bm / Bs ranged from 0.7 to 0.8.

<Experiment 5>
Using a directional electromagnetic steel sheet having a thickness of 0.35 mm, a width of 25 mm and 50 mm, and a length of 900 mm, the rolling direction being aligned in the longitudinal direction, a 900 mm high shield body consisting of a square frame of 900 mm × 900 mm was produced. Details of the configuration are shown in Table 3. At the inner center of the magnetic shield body, an electromagnet with a coil wound around an iron core with a diameter of 100mm x length of 500mm is installed to generate a magnetic field. The leakage magnetic field in the vicinity of the wall was examined here, and the distribution of the leakage magnetic field was examined in the vertical direction on the outer wall surface of the shield body perpendicular to the electromagnet axis, with the electromagnet axis as the height reference. The distance of the measuring point from the wall is about 18 mm. The strength of the generated magnetic field was adjusted to 2000 μT (20 G) without the shield body at the position where the outer wall of the shield body intersects the axis of the electromagnet. The configuration of the measuring device is shown in FIG. FIG. 9 shows the magnetic field distribution in the height direction at various d / W. When d / W is 4.5, the magnetic field is attenuated as compared with the case where there is no shield. When d / W is set to 3.0, the leakage magnetic field is suppressed to be small as a whole. When d / W is 1.0 or less, the vibration of the magnetic field is hardly seen and the leakage is very small.

  Furthermore, from <Experiment 3> and <Experiment 4>, good shielding performance is obtained in the range of d / W ≦ 4.5, so d / W ≦ 4.5. Since d / W never becomes 0, 0 <d / W ≦ 4.5. Preferably, d / W ≦ 3.0, and more preferably d / W ≦ 1.0. The reason is that the shield coefficient is drastically improved at d / W ≦ 3.0 from FIG. 6 and FIG. 7, and as shown in Table 2 which is an example in the case where the material is permalloy, the comparative examples of No. 33 and 34 Whereas the shield coefficient is about 11, when d / W ≦ 3.0 of the present invention, the shield coefficient increases rapidly to 17 or more and 50% as in No.28. Furthermore, when d / W ≦ 1.0, the shield coefficient is dramatically improved by a factor of 29 or more as in No.26.

  Further, as can be seen from FIG. 9, leakage from the vicinity of the shield body is significantly suppressed by setting d / W ≦ 3.0, more preferably d / W ≦ 1.0.

  Magnetic materials that can be used in the present invention are so-called soft magnetic materials such as grain-oriented electrical steel sheets, non-oriented electrical steel sheets, Ni, Fe-based PB permalloy, PC permalloy, etc. Iron-based materials, amorphous, microcrystalline soft magnetic materials, and the like can also be used. Depending on the strength of the target magnetic field, required shielding performance, installation environment, cost, etc., materials are used properly.

<Example 1>
A model simulating an MRI magnetic shield room as shown in Fig. 10 was prepared. The inside size of the model room was 2.8m wide x 3.5m long x 1.7m high, and an electromagnet was installed in the room. The electromagnet was excited under the condition that a magnetic field of 1 mT was applied to the inside of the A and B surfaces, 0.5 mT to the C surface, and 5 mT to the inside of the wall D corresponding to the gantry surface. The basic configuration of the shield model room is as follows. A magnetic material frame with an inner dimension of 2.8m x 3.5m was constructed with a 50mm wide grain-oriented electrical steel sheet, and 10 layers were stacked vertically at a pitch of 150mm. The magnetic steel sheets constituting the magnetic frame were laminated in four on the A, B, and D surfaces, and two in the C surface. On the ceiling and floor, 12 grain-oriented electrical steel sheets were pasted on the entire surface with the rolling direction aligned with the D plane.

  In this embodiment, d / W is 3.0. Since the maximum magnetic flux density Bm in the frame when the magnetic field is applied by the electromagnet is 1.5T and Bs is 2.0T, Bm / Bs is 0.75, which indicates that the requirement of the present invention is satisfied.

  A magnetic field was measured along the outer wall of the shield room at a point 1 m above the floor and 0.5 m away from the outer wall of the room. The largest magnetic field was observed on the D plane, and the magnetic field value was 0.3 mT. In other aspects, the magnetic field was 0.1 mT or less. We were able to achieve the regulation that the leakage magnetic field outside the MRI room is 0.5mT or less.

The figure which shows the structure of the shield body used in Experiment 1 and Experiment 2. The figure which shows the mode of the magnetic flux density pick-up coil wound around the magnetic shield frame. The figure which shows the relationship between Bm / Bs and the leakage magnetic field in Experiment 1. The figure which shows the relationship between Bm / Bs and the leakage magnetic field in Experiment 2. The figure which shows the structure of the measuring apparatus in Experiment 3, 4. The figure which shows the relationship between d / W and the shield coefficient SE in the experiment 3. FIG. The figure which shows the relationship between d / W and the shield coefficient SE in the experiment 4. FIG. 6 is a diagram showing a configuration of a measurement apparatus in Experiment 5. The figure which shows the change of the height direction of the magnetic field which leaks from a shield body in Experiment 5. FIG. The top view of the MRI magnetic shield model room used in the Example.

Claims (2)

A plate-like magnetic body having a width W or a laminated body of the plate-like magnetic bodies is brought into contact with each other at the end portions in the length direction to form a magnetic body frame so that the magnetic circuit is closed. In the magnetic shield structure arranged with a gap d in the thickness direction of the body, among the magnetic flux density B distributed in the magnetic body frame, the largest value Bm is the saturation magnetic flux density Bs which is the material characteristic of the plate-like magnetic body In the ratio
0.4 ≤ Bm / Bs ≤ 0.9
And the ratio of the plate-like magnetic body width W to the frame interval d,
0 <d / W ≤ 4.5
Magnetic shield structure characterized by that.   2. The magnetic shield structure according to claim 1, wherein the plate-like magnetic body is an electromagnetic steel plate or permalloy.

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