JPH0246002Y2 - - Google Patents

Description

[Detailed explanation of the invention] Industrial field This invention is a permanent magnet used in medical nuclear magnetic resonance tomography (hereinafter referred to as NMR-CT), which can obtain cross-sectional images of objects and depict the properties of tissues. Regarding the magnetic field generator used,
This invention relates to a magnetic field generating device that generates a strong, highly accurate, and uniform static magnetic field within a large air gap.

BACKGROUND ART
In order to obtain the desired tomographic image by inserting the device into an air gap that generates 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 include normal conducting magnets made of conductive wires made of copper or aluminum wrapped around them in a cylindrical shape, and superconducting magnets that use special conducting wires cooled to a temperature close to 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 negative factor depending on the application. On the other hand, the latter type of superconducting magnet has the advantage of consuming less power and being compact and capable of generating a strong magnetic field, but it requires the use of expensive liquid helium or the like as a coolant. There is a problem in that not only the so-called initial cost but also the running cost is extremely high.

The present applicant previously developed a permanent magnet circuit that has a magnetic field strength equal to or higher than that of the above-mentioned normal conduction magnet, consumes less power, and has a small leakage magnetic field. A magnetic field generating device is proposed in which a magnet is magnetically coupled with a yoke to generate a magnetic field in the air gap, and the magnetic field generating device is characterized in that annular protrusions are provided on each of the opposing surfaces of the magnetic pole pieces. 58-196785).

With the above magnetic field generating device, it was possible to significantly improve the uniformity accuracy of the magnetic field generated in the air gap. However, as shown in FIG. 3, the magnetic pole pieces 11 are fixed to one end of each of a pair of permanent magnets 10 and are opposed to each other, and the other ends are connected with a yoke 12, and in the air gap 13 between the magnetic pole pieces 11, This configuration generates a static magnetic field, and in a configuration in which a pair of magnetic pole pieces 11 has an annular protrusion projecting from the periphery of their opposing surfaces with a substantially trapezoidal cross section having a predetermined inner diameter and height, the magnetic flux from the magnetic poles is It is easy to leak outside the gap, and on the vertical line at the center of the gap,
Since the magnetic field strength tends to be higher as it gets closer to the magnetic pole surface, in order to obtain the required high uniform magnetic field in a normally used magnetic field space, for example, a spherical space with a radius of 200 mm or more, it is necessary to increase the distance between the magnetic poles and the magnetic pole area. However, there was a problem in that it required an air gap ten times larger than the magnetic field space used, and the magnetic circuit could not be made smaller.

Purpose of the invention In view of the current situation, this invention has a magnetic circuit that further expands a highly accurate, uniform, and stable magnetic field in the air gap of a magnetic field generator using permanent magnets that can generate a strong magnetic field. The objective is a magnetic field generator that can prevent leakage of magnetic flux from permanent magnets, reduce the amount of permanent magnets, and achieve a reduction in size and weight.

Structure and effect of the invention This invention was developed as a result of various studies aimed at creating a magnetic circuit that can obtain a highly accurate, uniform, and stable magnetic field and has low magnetic flux leakage. By reducing the thickness of the plate-shaped magnetic pole pieces arranged in the direction at the center, leakage magnetic flux is reduced, the magnetic field uniformity within the magnetic circuit air gap is significantly improved, and a highly accurate and uniform magnetic field area can be expanded. This is what we discovered.

That is, in this invention, a pair of permanent magnets magnetized in the same direction are arranged in parallel with an air gap between them, plate-shaped magnetic pole pieces are attached to a pair of end faces of the pair of permanent magnets, and the air gap is opened in a uniaxial direction. The present invention is a magnetic field generating device characterized in that the magnetic circuit is formed into a magnetic circuit surrounded by a permanent magnet and a magnetic pole piece, and the central side of the plate-shaped magnetic pole piece in the axial direction is made thinner than the outer side.

The plate-shaped magnetic pole piece has a thinner wall at the center in the opening direction of the air gap than at the outside, but if the configuration is such that the generated magnetic flux is converged at the center of the air gap, the position and shape of the outer thick part and the thin center wall can be changed. Various shapes can be adopted for the shape of the connection surface with the part, and the shape of the plate-like pole piece should be selected appropriately depending on the magnetic properties and dimensions of the permanent magnet, the shape and dimensions of the yoke, the size of the required air gap, etc. is desirable.

The magnetic field generating device according to this invention is a device that can generate a strong magnetic field of 1 to 10 kg by using the following permanent magnet, and it 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. The above NMR-CT
By applying it to a permanent magnet, it is possible to achieve a smaller device size and obtain excellent performance.Furthermore, the magnetic properties of this permanent magnet can be used especially when cooled to below 0°C, resulting in an extremely high maximum energy product. can be obtained and used effectively.

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 FIG. 1 is a front view and a side explanatory view showing a magnetic field generating device according to this invention. FIG. 2 is an explanatory cross-sectional view showing another embodiment of the plate-shaped magnetic pole piece according to this invention.

The magnetic circuit shown in FIG. 1 consists of a pair of rectangular parallelepiped Fe-B-R permanent magnets 1, 1 arranged in parallel with their longitudinal main surfaces facing each other at a predetermined interval.
The plate-shaped magnetic pole pieces 3, 3 are attached to the upper and lower end surfaces of the Fe-B-R permanent magnets 1, 1, and an opening is provided in the longitudinal direction of the permanent magnet 1, and the air gap 2 is permanently It is surrounded by a magnet 1 and a plate-shaped pole piece 3.

The plate-shaped magnetic pole piece 3 is formed into a thick outer part 4, a thin central part 5, and a thick outer part 4 in the opening direction of the air gap 2, and has an inclined surface from the outer part 4 to the central part 5. 6 is formed. In addition, Fe-BR permanent magnets 1, 1
are magnetized in the same vertical direction.

In the above configuration, the magnetic flux generated from the plate-shaped magnetic pole pieces 3, 3 is converged in the same direction as the magnetization direction of the Fe-BR permanent magnets 1, 1 to form a magnetic field, so that the leakage magnetic flux is reduced and the air gap 2, a static magnetic field with extremely high uniformity can be obtained.

In addition to the shape shown in Fig. 1, the plate-shaped magnetic pole piece has the shape shown in Fig.
As shown in Figure a, the thick outer part 4 may be constructed by forming the central part 5 with a concave curved surface 7. It is also preferable to have the same structure as the magnetic pole piece, with the thick outer portion 4 being placed closer to the center than the end of the magnetic pole piece.

By the way, NMR-CT with the configuration shown in Figure 1
When assembling the device using Fe-B-R permanent magnets with a maximum energy product of 35 MGOe and setting the facing distance of the magnetic pole pieces to 600 mm, the magnetic field strength at the center of the air gap was 4 KG. It was hot. When an NMR-CT apparatus shown in FIG. 3 having a uniform magnetic field range and magnetic field strength comparable to this was fabricated and the weight of the apparatus was compared, it was found that the apparatus of the present invention was able to achieve a 20% weight reduction.

[Brief explanation of the drawing]

FIG. 1 is a front view and a side view showing a magnetic field generating device according to this invention. FIG. 2 is an explanatory cross-sectional view showing another embodiment of the plate-shaped magnetic pole piece according to this invention. FIG. 3 is a longitudinal sectional view of the magnetic field generator. 1... Fe-B-R permanent magnet, 2... Air gap,
3... Plate magnetic pole piece, 4... Outer part, 5... Central part, 6... Inclined surface, 7... Curved surface, 10... Permanent magnet, 11... Magnetic pole piece, 12... Yoke , 13...
void.

Claims (1)

[Scope of utility model registration request] A pair of permanent magnets magnetized in the same direction are arranged in parallel with an air gap between them, plate-shaped magnetic pole pieces are attached to a pair of end faces of the pair of permanent magnets, and the air gap is opened in a uniaxial direction to connect the permanent magnets and the magnetic pole pieces. 1. A magnetic field generating device comprising a magnetic circuit surrounded by a magnetic field, wherein the central side of the plate-shaped magnetic pole piece in the axial direction is thinner than the outer side.