JPH06244019A - Magnetic adjuster board for magnetic field generation equipment and preparation thereof - Google Patents

Abstract

PURPOSE:To obtain a magnetic adjuster board which is simple in structure and easy to manufacture by adopting a composite structure which comprises an iron plate having a ring-shaped projection on the outer peripheral part and a resin steel part. CONSTITUTION:This invention relates to a magnetic adjuster board which installs a pair of permanent magnets magnetized in the thickness direction so that they may be opposed to each other and which generates a magnetic field in a space between the magnetic adjuster board on each of the opposed sides of the paired permanent magnets. This magnetic adjuster uses an iron plate 30 having a ring-shaped projection 34 at the outer peripheral part as a board and the opposed sides as a resin steel portion 32, thereby forming composite structure. It is possible to manufacture magnetic adjuster boards by pressed powder-molding resin steel 34 on the iron plate 30 having the ring- shaped projection 34 on the outer peripheral part and thermal setting and integrating the resin steel. This obstruction makes it possible to prepare the magnetic adjuster board with accuracy, and what is more, with simplicity and at low cost and satisfy both the uniformity of magnetic field and the restrain of overcurrent simultaneously.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a magnetic shunt plate for a magnetic field generator and a method for manufacturing the same. The magnetic field generator using the magnetic shunt according to the present invention is particularly suitable for use in a magnetic resonance tomography device for obtaining an image of an object by utilizing magnetic resonance.

[0002]

2. Description of the Related Art A permanent magnet facing type is most mass-produced as a magnetic field generator used in a magnetic resonance tomography apparatus.

FIG. 4 schematically shows a permanent magnet facing type magnetic field generator. Inside the yoke 10, the disk-shaped magnets 12a, 12b
And a magnetic shunt plate 16 having annular projections 14a and 14b on the outer periphery.
a and 16b are provided. As a result, a uniform magnetic field is created in the space between the opposing magnetic shunt plates 16a and 16b. Further, coils for changing the magnetic field strengths in the X-, Y-, and Z-axis directions to form a gradient magnetic field (only the coils 18a and 18b for the Z-axis gradient magnetic field are shown in FIG. 4) are provided. . With the magnetic field generator described above, a tomographic image of a human body or the like can be obtained.

In the permanent magnet facing type magnetic field generator, a pulsed rectangular current is passed through the coil in order to form a gradient magnetic field, which causes an eddy current in the magnetic shunt plate. Due to this eddy current, the rise of the gradient magnetic field is delayed, the uniformity is disturbed, blurring or distortion occurs in the obtained image,
There was a big problem that high-speed sequence could not be performed.

In order to solve the above problems, various ideas have been conventionally considered for the magnetic shunt plate. Among them, a method of producing a magnetic shunt plate from resin iron (a mixture of powder of resin (insulator) and metal magnetic powder of soft iron and then pressure molding) is well known. Since the electric resistance value of resin iron is about three orders of magnitude higher than the electric resistance value of iron, the use of a resin iron magnetic shunt plate is very effective in suppressing eddy currents.

However, although it is desirable that the saturation magnetic flux density of the magnetic shunt is large, the saturation magnetic flux density of resin iron is smaller than that of iron. However, the saturation magnetic flux density of resin iron is also sufficient for use as a magnetic shunt plate for a permanent magnet facing type magnetic field generator that generates only a magnetic field of 0.3 T or less.

Another candidate for the magnetic shunt plate material is, for example, MnZn ferrite. The electric resistance value of MnZn ferrite is sufficiently larger than the electric resistance value of iron, and the relative magnetic permeability is as high as 1000 or more. This point is suitable for a magnetic shunt,
The saturation magnetic flux density is as small as 5000 G at most, and the mechanical strength is small, so it is large (usually,
There is a problem that it is not easy to fabricate a disc type of about m.

Another candidate is a silicon steel sheet. The silicon steel sheet has a high saturation magnetic flux density and magnetic permeability, and has the advantage that the electrical resistance value can be increased by insulating and laminating thin silicon steel sheets to form a magnetic shunt. There is. However, there is a problem in that it is impossible to perform complicated step processing (providing a step on the surface of the magnetic shunt plate to make the magnetic field generated between the magnetic shunt plates more uniform) on the surface of the laminate of silicon steel plates. . In this respect, since resin iron is a massive magnetic material, it is possible to perform step processing.

As can be seen from the above explanation, only resin iron is known at present as a massive magnetic material which has a large electric resistance value and a sufficient saturation magnetic flux density and can be subjected to step processing.

As described above, resin iron is very excellent in terms of magnetic characteristics and processing characteristics as a material for a magnetic shunt plate for a magnetic field generator, and thus has been often used as a material for the magnetic shunt plate. ing.

However, since the mechanical strength of resin iron is reduced to 1/3 or less of that of iron material, it is a safety problem to make the entire magnetic shunt plate from resin iron. This is because an attractive force acts on the magnetic shunt plate in the magnetic field direction, and further, a torque that tends to orient the longitudinal direction of the magnetic shunt plate in the magnetic field direction is also applied. Therefore, as shown in FIG. 5, using the iron plate 20 as a substrate,
A composite structure type magnetic shunt plate having a resin iron portion 22 on the opposite surface side is manufactured. The lower diagram of FIG. 5 is a cross-sectional view of the magnetic shunt plate, and the upper diagram shows the magnetic shunt plate viewed from above.
In this composite structure type magnetic shunt plate, resin iron is fixed to the iron plate by screwing or by bonding with an adhesive.

Further, in the current press machine, it is difficult to integrally form a degaussing plate portion (22 in FIG. 5) having a diameter of 1 m or more with resin iron. Therefore, degaussing is performed by combining a plurality of resin iron plates. I am making a board. Reference numeral 24 shown in FIG.
It is an example of a parting line of (a resin iron portion of) a magnetic shunt plate manufactured by combining a plurality of resin iron plates.

Each of the divided pieces is produced by compressing a resin iron compound (a mixture of iron powder and resin and granulating) in a mold and then taking it out. For this reason, the forming die is tapered so that the formed divided pieces can be taken out without cracking when taken out. Therefore, it is necessary to process all the side surfaces of the divided piece into a final shape and then attach it to the iron plate, or to attach the pre-processed divided piece to the iron plate and then perform the final machining. That is, it is necessary to perform lathe processing and milling on the resin iron. However, since resin iron is brittle, chipping of corners and edges is likely to occur during lathe processing and milling, and there is a problem that extremely careful processing is required.

As described above, resin iron is a very excellent material in terms of magnetic characteristics, but it has many problems in terms of actually producing a magnetic shunt plate.

[0015]

An object of the present invention is to improve a magnetic shunt plate which is a composite structure type of resin iron and iron used in a magnetic field generator of a permanent magnet facing type, and has a simple structure and is easy to manufacture. Is to develop a new magnetic shunt.

[0016]

Means for Solving the Problems 1 magnetized in the thickness direction
A magnetic field generation device for providing a pair of permanent magnets facing each other inside a yoke and for generating a magnetic field in the space between the magnetic shunt plates provided on each of the facing surfaces of the pair of permanent magnets. Provided is a magnetic shunt plate having a composite structure made of an iron plate and resin iron. Further, the magnetic shunting plate is manufactured by a method in which resin iron is integrally powder-molded on an iron plate having an annular protrusion on the outer peripheral portion, and thermosetting is performed to integrate the resin iron.

[0017]

EXAMPLES The present invention proposes a composite structure type magnetic shunt plate and a method for producing the same, which can suppress the processing of resin iron to a necessary minimum.

FIG. 1 shows an example of the composite structure type magnetic shunt plate of the present invention. Although the iron plate 30 is used as the substrate and the opposing surface side is the resin iron portion 32, which is the same as the conventional composite structure type magnetic shunt plate, in the present invention, the iron plate 30 has the annular protrusion 34 on the outer peripheral portion.
Therefore, the diameter of the resin iron portion 32 is smaller than the diameter of the conventional resin iron portion (22 in FIG. 5), and integral molding is possible.

The method for producing a magnetic shunt plate of the present invention will be described with reference to the schematic view shown in FIG. The iron plate 40 serving as a substrate is fixed by the die 42 and the lower punch 44, and the resin iron compound 46 is placed on the iron plate 40. The upper punch 48 is moved in the direction of arrow 50 to compress the resin iron compound 46. That is, resin iron can be integrally compression-molded on the iron plate 40.

By forming a step on the lower surface 52 of the upper punch 48, a step can be formed on the surface of the resin iron during compression molding. That is, it is not necessary to perform step processing again after integrally molding resin iron on an iron plate, which is very efficient.

However, in the above compression molding, there is a phenomenon (called springback) in which the dimension of the molded body changes when the punch is removed and the pressure is removed, and therefore, it is necessary to perform compression molding in consideration of the change in advance. There is.

In the method for producing a magnetic shunt plate of the present invention, since the iron plate serving as the substrate has the annular protrusion, the resin iron portion is completely set inside the iron plate. Therefore, unlike the conventional case, it is not necessary to take out the resin iron molded body from the tapered mold, and the final shape of the magnetic shunt plate can be obtained only by performing integral compression molding, and the manufacturing process becomes very simple.

Further, in the past, since the annular projection portion was made of resin iron, there was a problem that the corners and edges of the projection were easily chipped, but according to the present invention, this problem also disappears.

As shown in FIG. 1, the annular projection on the outer periphery of the magnetic shunt plate may be entirely made of iron material, but since eddy currents also flow near the inner surface of the annular projection, this portion is made of resin. It is preferably composed of iron. FIG. 3 shows an example of a magnetic shunt plate in which the inner surface of the annular protrusion is also made of resin iron. Such a magnetic shunt plate can be manufactured by slightly changing the shape of the outer peripheral portion of the lower surface of the upper punch 48 shown in FIG.

In this case, since the resinous iron portion of the annular projection is small, the problem of chipping of the corners and edges can be suppressed.

The magnetic shunt plate integrally compression-molded as described above is usually heat-cured at a temperature of 80 ° C. or higher for curing the resin component. Since there is almost no dimensional change during thermosetting, subsequent processing is not required if the dimensional accuracy during compression molding is high. Further, when thermosetting, the resin component has an adhesive effect on the iron plate, so that the integration of the resin iron and the iron plate becomes strong, which is preferable. However, since this adhesive effect alone poses a safety problem, it is necessary to mechanically fix it with screws or the like.

According to the method of manufacturing a magnetic shunt plate of the present invention, a composite structure type magnetic shunt plate of resin iron and iron can be manufactured very easily as compared with the conventional method. In addition, the processing after molding of the resin iron part is not necessary except screw processing, the dimensional accuracy between the magnetic compensating plates (between the facing surfaces of the resin iron) is improved, and the corners that have been conventionally generated by machining And edge chipping is greatly reduced. Therefore, the magnetic field homogeneity is higher than in the case of using the resin iron shunt plate by the conventional manufacturing method. Furthermore, there is also an advantage that the manufacturing cost can be reduced because the number of machining processes can be reduced.

[0028]

According to the method of manufacturing a magnetic shunt plate for a magnetic field generator according to the present invention, a composite structure type magnetic shunt plate of resin iron and iron can be manufactured with high accuracy, more easily and at a lower cost than the conventional method. can do. Further, in the magnetic field generator using the magnetic shunt plate of the present invention by the manufacturing method of the present invention, it is possible to satisfy both high magnetic field homogeneity and suppression of eddy current.

[Brief description of drawings]

FIG. 1 is an example of a magnetic shunt plate for a magnetic field generator according to the present invention.

FIG. 2 is a diagram for explaining a method of manufacturing a magnetic shunt plate for a magnetic field generator according to the present invention.

FIG. 3 is another example of the magnetic shunt plate for the magnetic field generator according to the present invention.

FIG. 4 is a schematic diagram of a magnetic field generator.

FIG. 5 shows an example of a magnetic shunt plate for a conventional magnetic field generator.

Continuation of the front page (51) Int.Cl. ⁵ Identification code Office reference number FI technical display location 8203-2G G01R 33/22 E

Claims (2)

[Claims] 1. A pair of permanent magnets magnetized in the thickness direction are provided facing each other inside a yoke, and a magnetic field is generated in the space between the magnetic compensating plates provided on each of the facing surfaces of the pair of permanent magnets. In the magnetic field generator, the magnetic shunt plate has a composite structure made of a resin plate and an iron plate having an annular protrusion on an outer peripheral portion thereof. 2. A magnetic shunt for a magnetic field generator, characterized in that the magnetic shunt is formed by integrally powder-molding resin iron on an iron plate having an annular protrusion on an outer peripheral portion and thermosetting to integrally integrate the resin iron. How to make a plate.