JPH03291997A - Magnetic shielding device - Google Patents

Abstract

PURPOSE:To effectively shield a magnetic flux leakage and to obtain a light weight and inexpensive shielding device by increasing the sectional area of a magnetic material for forming the device at the center of the material in a direction of a shielding magnetic field. CONSTITUTION:A cylindrical shielding vessel is formed of an iron plate having 1500 of maximum magnetic permeability. Its thickness is 2mm at the end and 150mm at the cylindrical part. When the difference in thickness between the center and the end of the material is DELTAt0, a plate thickness difference DELTAt at a distance (x) from the center to the end is decided by an equation DELTAt=DELTAt0(t-(2x/ L)<2>), and the plate is machined. Here, L is the length of the magnetic shielding device.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to devices such as shield containers for shielding magnetism used in connection with electrical equipment, electronic equipment, and the like.

(Conventional technology) With the improvement in performance and rapid expansion of use of electrical and electronic equipment,
As the magnetic fields used by these devices become larger, in order to protect operation rooms, test rooms, and human bodies from magnetic fields, and to prevent interference between electronic devices due to magnetic fields, active shielding is required to shield the magnetic fields emitted from the devices and prevent intrusion into the devices. Passive shielding is used to block the incoming magnetic fields, and magnetic shielding materials are used.

For example, in an imaging apparatus using nuclear magnetic resonance (NMR), a high magnetic field from a superconducting magnet or the like is required, resulting in a large leakage of magnetic flux outside the measurement area. Therefore, magnetic shielding is used to reduce the leakage of magnetic fields into the environment.

In order to avoid such magnetic interference, soft magnetic materials such as iron are used as shielding materials.

For example, magnetic shielding is performed using a thick iron plate using nuclear magnetic resonance (NMR). In addition, when shielding a room, magnetic ribbons such as electromagnetic steel sheets, permalloy, and amorphous have recently come to be used in response to demands for smaller size and lighter weight.

A material with high magnetic permeability is generally used for the magnetic shield. However, since the magnetic permeability actually changes depending on the strength of the magnetic field to be shielded, it is necessary to select an appropriate material. In addition, magnetic shielding collects the magnetic flux of the magnetic field to be shielded, so as the magnetic field increases, it becomes necessary to increase the cross-sectional area of the material or use materials with high magnetic flux density. .

(Problems to be Solved by the Invention) Materials used as materials with high magnetic flux density include pure iron-based electromagnetic thick plates and electromagnetic steel plates. However, even with such materials, as the magnetic field increases, the leakage magnetic field increases, and the cross-sectional area must be increased. In the case of shielding a high magnetic field from a superconducting magnet, the cross-sectional area becomes considerably large, the shielding device alone becomes quite heavy, and the material cost also increases.

Further, materials with high magnetic permeability such as permalloy and amorphous have a saturation magnetic flux density as low as about 1/2 to 1/3 that of electrical steel sheets. Therefore, when the magnetic field to be shielded is large, if the cross-sectional area is increased by laminating layers, the weight and material cost will be enormous. Therefore, there may be problems with construction costs and construction.

SUMMARY OF THE INVENTION Therefore, an object of the present invention is to provide a shielding device that has a small average cross-sectional area, is lightweight, and has a good shielding effect at low cost even when the shielding magnetic field is large.

(Means for Solving the Problems) The present invention provides for changing the cross-sectional shape of the magnetic material for magnetic shielding,
Provided is a shielding device that improves magnetic shielding characteristics and is lightweight and inexpensive.

When performing magnetic shielding, a material with high magnetic permeability is required when selecting a shielding material, but the saturation magnetic flux density of the materials currently available as magnetic materials is low when the magnetic permeability is high. Permalloy PC is a typical material with high magnetic permeability.
Although the maximum magnetic permeability is 100,000 or more, its saturation magnetic flux density is about 7 kG, which is considerably lower than the 20 kG of iron. Permalloy PB exhibits a maximum magnetic permeability of 30,000 or more, but a saturation magnetic flux density as low as 14 kG. When the shielding magnetic field is large, an iron-based material is used, but it has a high saturation magnetic flux density but a low magnetic permeability, so it is necessary to increase the plate thickness.

Therefore, when performing magnetic shielding, if the magnetic material is saturated and magnetic flux leaks in response to the magnetic field to be shielded, this is dealt with by increasing the thickness of the material. However, increasing the thickness of the board naturally increases the weight, and large shield devices are heavier and require more effort to install.In extreme cases, the floor weight installation standards of the building may be exceeded, making it impossible to even install the equipment. In some cases it may be possible. In addition, materials with high magnetic permeability are of high quality, and as the thickness of the plate increases, the cost of shielding increases rapidly.

Therefore, in the present invention, instead of uniformly increasing the cross-sectional area of the magnetic material, only a portion of the cross-sectional area is increased,
The purpose is to provide a shielding device with excellent magnetic shielding properties and workability. In this way, materials with high magnetic permeability can be used effectively, making it possible to reduce weight, improve workability, and reduce shielding costs.

This shielding magnetic material may be any magnetic material, and can be applied to a shielding device composed of permalloy, amorphous, etc. with high magnetic permeability, electromagnetic steel plate, electromagnetic thick plate, etc. with high magnetic flux density.

FIG. 1 is a diagram of magnetic flux leakage into the shield container.

In the case of magnetic shielding, if the shielding magnetic field is small and the soft magnetic material is not saturated, the shielding performance is best at the center of the shielding device - the center of the magnetic material, as shown in Figure 1a. However, it has been found that when the shielding magnetic field is large enough to saturate the magnetic material and the magnetic flux leaks, the leakage of magnetic flux occurs from the center as shown in FIG. 1b, and the shielding performance deteriorates.

Therefore, when the shielding magnetic field is large, it is not necessary to uniformly increase the plate thickness, and it is possible to effectively shield by changing the cross-sectional area of the material.

The cross-sectional area of the magnetic material is changed by making the center part larger and the end parts smaller along the magnetization direction of the material, that is, the direction of the shielding magnetic field. This difference in cross-sectional area can be determined experimentally using the following formula.

Now, assuming that the difference in cross-sectional area between the end of the shield member and the material is ΔS, ΔS is replaced by the plate thickness difference Δt since the material width is constant. Δt may be selected according to the following equation, which is expressed as a function of X with the origin at the center of the plate, where L is the length of the material.

Δt (1-(2x/L)) (
1) However, Δt0 is the thickness of the material at the center and at the ends, and at the ends, x-L/2 is Δt-0.

If the materials are the same, Δt0 is a value determined by L and the shielding magnetic field strength. However, when actually designing a shielded container, if the processing becomes complicated, it is possible to approximate it with a straight line close to equation (1), and if the shielding magnetic field is not that large, it is possible to approximate the distance equivalent to X. It is also possible to set Δ to −Δt0.

(Example 1) A cylindrical shield container with an inner diameter of 100 mm and a length of 300 mm was manufactured using an iron plate with a maximum permeability of 1,500. The plate thickness is 2mm at the end.
5 The thickness was set to 4×1 at the 150 mm position of the cylinder, and the plate thickness was processed in the length direction of the cylinder according to formula (1).

For comparison, a cylinder with a plate thickness of 21 mm, 3 mm, and 411 mm in total length was manufactured using the same material. The shielding property was measured by installing the cylinder in a parallel DC magnetic field so that the length direction was parallel to the magnetic field, and measuring the magnetic field inside the cylinder by changing the DC magnetic field strength.

Table 1 shows the measurement results.

The shielded container 1 according to the present invention exhibited shielding properties equivalent to a container having a total length of 4 mm. On the other hand, the second issue.

In the case of a 3■l plate thickness, as the shielding magnetic field increased, the shielding performance at the center deteriorated.

Table (mountain) Invention comparison 12 Comparison 23 Comparison 34 Hex-5G Hex-20G 12B 7.2 7.8 1.0 16.7 1.1 B2.5 5.2 Sokyo-shield magnetic field strength Hex/shield Rear magnetic field strength Hl (Example 2) The thickness of the center 1/3 of the plate is 2waIss, the thickness of both ends is 1m
The magnetic material II was assembled into a rectangular tube of 100 mm square and 80 hm long, and the same experiment as in Example 1 was conducted.

For comparison, rectangular tubes were made from plates with thicknesses of 1+n and 2m, respectively.

Table 2 shows the results.

It can be seen that it exhibits shielding characteristics equivalent to a square tube with a thickness of 2 mm.

Table 2 (Hex-25G) Shield container　　Shielding property S.

The present invention 9.7 Comparison 12.7 Comparison 2 10.2 (Effects of the invention) The present invention provides a method of magnetic shielding using a magnetic material, in which the cross-sectional area of the magnetic material constituting the shielding device is increased in the direction of the shielding magnetic field. Since the central portion is made large and the end portions are made small, magnetic flux leakage can be effectively shielded, and a lightweight and inexpensive shielding device can be provided.

[Brief explanation of drawings]

FIG. 1 is a diagram of magnetic flux leakage into a shielded container.

Claims (2)

[Claims] 1. A magnetic shielding device characterized in that the cross-sectional area of a central portion of a magnetic material constituting the magnetic shielding device is increased. 2. When the plate thickness difference between the center and the end of the magnetic material is Δt_0, the plate thickness difference Δt at the distance x from the center to the edge is Δt=Δt_0(1-(2x/L)^2) The magnetic shielding device according to claim 1, characterized in that: Here, L: length of the magnetic shielding device