JPH0345886B2 - - Google Patents

Description

[Detailed Description of the Invention] Industrial Application Field The present invention is applicable to a medical nuclear magnetic resonance tomography device (hereinafter referred to as NMR-CT) that can obtain a cross-sectional image of an object and depict the properties of the tissue. The present invention relates to a magnetic field generating device that uses a permanent magnet as a magnetic field generating source, and relates to a magnetic field generating device that generates a strong, highly accurate, and uniform static magnetic field in a large space.

Conventional technology
In order to obtain the desired tomographic image by inserting the device into the air, which forms a strong magnetic field of 10 kG, this magnetic field must be strong, uniform, and stable with an accuracy of 10 ^-4 or less. Known magnetic field generators for use include normal conducting magnets made of conductive wire made of copper or aluminum wrapped around a cylindrical shape, or superconducting magnets that use a special conductive wire cooled to a temperature near absolute zero. .

The former is structurally inexpensive, but it requires a huge amount of electricity and cooling water to generate a sufficiently strong magnetic field, resulting in high running costs, and the leakage magnetic field created by the coil can be a factor in adverse effects depending on the application. On the other hand, the latter type of superconducting magnet has the advantage of consuming less power, being compact, and generating a strong magnetic field, but it requires the use of expensive liquid helium as a coolant, and the so-called There is a problem in that not only the initial cost but also the running cost is extremely high.

The present applicant previously developed a magnetic field generator using a permanent magnet with magnetic field strength equal to or higher than that of the above-mentioned normal conducting magnet, no power consumption, and less leakage magnetic field. and at least one permanent magnet component by a yoke to generate a magnetic field in the air, characterized in that an annular protrusion is provided on each opposing surface of the magnetic pole piece. Proposed magnetic field generator (patent application 196785-1985)
No.).

By using the above magnetic field generator, it was possible to significantly improve the uniformity of the magnetic field generated in the air.

Problems to be Solved by the Invention However, in the magnetic field generating device described above, magnetic pole pieces are fixed to one end of each of a pair of permanent magnet components to face each other, the other ends are connected with a yoke, and the space between the magnetic pole pieces is The pair of magnetic pole pieces has a configuration in which a static magnetic field is generated within the magnetic field, and an annular protrusion with a substantially trapezoidal cross section having a predetermined inner diameter and height is protruded from the periphery of the opposing surfaces of the pair of magnetic pole pieces. The magnetic flux generated from the permanent magnet structure tends to leak out from the outer periphery of the magnetic pole piece, and on a line perpendicular to the center of the air, the magnetic field strength tends to increase as it approaches the magnetic pole surface.

Therefore, in order to obtain the required highly uniform magnetic field in the used magnetic field space, it is necessary to increase the distance between the magnetic poles and the magnetic pole area, which requires a space several times larger than the used magnetic field space, making it impossible to miniaturize the magnetic circuit. I had a problem.

In view of the current situation, this invention has a magnetic circuit designed to further expand a highly accurate, uniform, and stable magnetic field in the space of a magnetic field generator using a permanent magnet structure capable of generating a strong magnetic field. The object of the present invention is to provide a magnetic field generating device that can effectively converge magnetic flux from a permanent magnet component into the air, reduce the weight of the permanent magnet component, and achieve a reduction in size and weight.

Means for Solving the Problems As a result of various studies aimed at creating a compact and lightweight magnetic circuit that can obtain a highly accurate, uniform, and stable magnetic field, the present invention has been made based on the results of various studies aimed at creating a compact and lightweight magnetic circuit that can provide a highly accurate, uniform, and stable magnetic field. It is composed of a plurality of permanent magnets that form a concave curved surface, and by making the magnetization direction of these permanent magnets perpendicular to the magnetic pole surface, the magnetic flux generated from the permanent magnets is directed in the direction (magnetization direction). It was discovered that by significantly reducing the magnetic flux that converges and leaks out of the air, the magnetic field uniformity and magnetic field strength within the magnetic circuit air can be significantly improved, and the highly accurate and uniform magnetic field area can be expanded. It is.

That is, the present invention provides a magnetic field generating device that magnetically couples a pair of opposing permanent magnet structures with a yoke to form an air space, and generates a magnetic field in the air space. A plurality of permanent magnets are arranged in parallel so that the magnetic pole surface forms a concave curved surface with respect to the sky, and each of the plurality of permanent magnets is magnetized in a direction perpendicular to the magnetic pole surface. It is a magnetic field generator.

Function Any configuration can be used for the magnetic circuit as long as a pair of opposing permanent magnet structures are magnetically coupled with a yoke to form an air gap, and the magnetic properties of each permanent magnet that make up the permanent magnet structure, It is desirable to appropriately select the arrangement of each permanent magnet depending on the shape and dimensions, the shape and size of the yoke, the required space size, etc.

Furthermore, it is also possible to provide magnetic pole pieces on the magnetic pole faces of the pair of permanent magnet structures as necessary to improve the uniformity of the magnetic field in the air.

In this invention, the pair of permanent magnet structures are
In addition to arranging a plurality of plate-shaped and columnar permanent magnets in parallel so that the opposite magnetic pole surfaces of each pair of permanent magnets form a concave curved surface, the number of magnets is increased to It is also good to configure the opposing magnetic pole surfaces to form a smoother concave curved surface.
Furthermore, it is also possible to use a plurality of permanent magnets arranged in an arcuate cross-section.

Furthermore, it is not necessary that all of the permanent magnets constituting each pair of permanent magnet structures be magnetically coupled to yokes of various shapes, such as rectangular tubes and cylindrical shapes, as in the embodiments described below. It may also be placed on the yoke via a non-magnetic material.

The magnetic field generating device according to the present invention can generate a strong magnetic field of 1 to 5 kG by using the following permanent magnet, and weighs less than 10 tons.
100ton of magnetic field generator using ferrite magnet
Compared to the above, a significant reduction in size and weight can be achieved.

Permanent magnet used in the invention The permanent magnet used in the magnetic field generator of this invention is
Ferrite magnets, alnico magnets, and rare earth cobalt magnets can be used, but the applicant has previously proposed a new high-performance permanent magnet that does not contain expensive Sm or Co. At least one kind of rare earth element) Permanent magnet (Special application)
57-145072) not only has a large maximum energy product, but also has a temperature coefficient of residual magnetic flux density (Br) of _- 0.07%/°C to _- 0.15%/°C. By applying the above NMR-CT, the device can be made smaller.
Excellent performance can be obtained, and furthermore, the magnetic properties of this permanent magnet, especially when used after being cooled to below 0° C., can be effectively used to obtain a significantly high maximum energy product.

The above Fe-BR-based permanent magnet has R (where R is at least one kind of rare earth elements including Y) 8 at% to 30 at%, B2 at% to 28 at%, Fe42 at% to 90 at%. It is a permanent magnet whose main phase is a tetragonal phase, and R is a light rare earth that is abundant in resources, mainly Nd and Pr.
It is an excellent permanent magnet that has extremely high energy product of over 25MGOe as its main component.

Disclosure of the Invention Based on Drawings FIGS. 1 to 3 are longitudinal sectional front views of a magnetic circuit used in a magnetic field generating device according to the present invention.

The magnetic circuit shown in Figure 1 consists of a pair of Fe-B-R
The system permanent magnet structures 1, 1 are opposed to each other via the air 3, and the other end of the permanent magnet structures 1, 1 is a yoke 2 which is made of a rectangular cylinder and has both ends opened in the axial direction.
The pair of permanent magnet components 1, 1 are connected to a plate-shaped main magnet 1a having a length in the axial direction of the yoke. and two plate-shaped auxiliary magnets 1b sandwiching it, each Fe-B-R magnet 1a, 1
b is magnetized in the direction perpendicular to the magnetic pole surface on the 3 side.

In addition, the yoke is constructed here as an octagonal cylinder, and by installing the main magnet 1a and the auxiliary magnet 1b in parallel on each side of the inner surface of the rectangular cylinder, each yoke is formed of three magnets. The opposing magnetic pole surfaces of the pair of permanent magnet structures 1, 1 are configured to form a concave curved surface.

The magnetic circuit shown in FIG. 2 consists of a pair of Fe-BR permanent magnet structures 1, 1, similar to the configuration shown in FIG.
are opposed to each other via the air 3, and the permanent magnet structures 1,
1 is connected to the other end of the yoke 2, which is made of a rectangular cylinder and has both ends opened in the axial direction, to generate a static magnetic field in the air 3, and the yoke is formed into an approximately dodecagonal prism. By installing the main magnet 1a and two auxiliary magnets 1b on each side of the main magnet 1a in parallel on each side of the inner surface of the rectangular cylinder, each pair of five magnets is The opposing magnetic pole surfaces of the permanent magnet structures 1, 1 form concave curved surfaces. Furthermore, a plate-shaped magnetic pole piece 4 is provided on this magnetic pole surface.

The magnetic circuit shown in FIG. 3 uses permanent magnet structures 1, 1 having the same configuration as in FIG.
The system permanent magnet structures 1, 1 are opposed to each other through the air 3, and the other end of the permanent magnet structures 1, 1 is connected to a yoke 2 having a shape of a square cylinder with both axial ends open. The configuration is such that a static magnetic field is generated in the air 3, with one auxiliary magnet 1 on each side of the main magnet 1a.
By arranging the substantially triangular prism-shaped non-magnetic material 5 in parallel through the corners of the yoke 2, the opposite magnetic pole surfaces of each pair of permanent magnet structures 1, 1 each composed of three magnets are is configured to form a concave curved surface. Further, a plate-like yoke 6 is provided between the auxiliary magnet 1b and the non-magnetic material 5.

In the above configuration, the magnetic flux generated from the permanent magnet components 1, 1 is focused in the same direction as the magnetization direction of the Fe-BR permanent magnets 1a, 1b to form a magnetic field, so that leakage magnetic flux is reduced. , a static magnetic field with extremely high uniformity can be obtained in the air 3.

Example By the way, NMR-CT with the configuration shown in Figure 1
has a maximum energy product of 35MGOe.
When assembling was carried out using Fe-BR-based permanent magnets and setting the facing distance of the main magnets to 600 mm, the magnetic field strength at the center of the sky was 3 kG.

The above-mentioned pair of permanent magnets are arranged facing each other with an air gap in between, and a magnetic pole piece having an annular protrusion is provided on the opposing magnetic pole surface. When the weights of the devices were compared, it was found that the device of the present invention was 15% lighter than the comparative device.

Effects of the Invention This invention comprises a plurality of permanent magnets in which the magnetic pole faces of each pair of permanent magnet members form a concave curved surface with respect to the sky, and the magnetization direction of the permanent magnets is set relative to the magnetic pole face. By setting it in the vertical direction,
By focusing the magnetic flux generated from the permanent magnet in the air direction (magnetization direction) and significantly reducing the magnetic flux leaking outside the air, the magnetic field uniformity and magnetic field strength within the magnetic circuit air are significantly improved. , it is possible to expand the uniform magnetic field area with high precision.

[Brief explanation of drawings]

1, 2, and 3 are longitudinal sectional front views of a magnetic circuit used in a magnetic field generating device according to the present invention. 1...Fe-BR-based permanent magnet structure, 2...
Yoke, 3...Empty, 4...Magnetic pole piece, 5...Nonmagnetic material, 6...Plate yoke.

Claims (1)

[Claims] 1. In a magnetic field generating device that magnetically couples a pair of opposing permanent magnet structures forming an air space with a yoke and generates a magnetic field in the air space, each pair of permanent magnet structures A plurality of permanent magnets are arranged in parallel so that the magnetic pole faces form a concave curved surface with respect to the sky, and each of the plurality of permanent magnets is magnetized in a direction perpendicular to the magnetic pole faces. magnetic field generator.