Electricity: magnetically operated switches – magnets – and electr – Magnets and electromagnets – Magnet structure or material
Patent
1988-11-01
1990-06-26
Harris, George
Electricity: magnetically operated switches, magnets, and electr
Magnets and electromagnets
Magnet structure or material
335301, 335306, 324318, H01F 702
Patent
active
049375458
DESCRIPTION:
BRIEF SUMMARY
DESCRIPTION
The present invention relates to a system of permanent magnets for producing an intense magnetic field, e.g. necessary in imaging by magnetic resonance or tomography.
Several types of magnets have already been used for this purpose. Resistive magnets suffer from the disadvantage of giving off large amounts of heat (several dozen kilowatts for fields of a few tenths of a Tesla) and are consequently virtually not used for this reason. The widely used superconductor magnets require costly equipment consuming large amounts of liquid helium.
In designs involving the use of magnetic circuits with permanent magnets, the main problem relates to the intensity of the field. European patent E-A1-0 161 782 more particularly proposes combining two main magnets on either side of an air gap and whereof the facing faces have opposite polarities, two pole pieces made from magnetic material on these faces and a magnetic yoke surrounding the assembly. The magnetic field lines pass through the two main magnets, traverse the air gap and close on passing through the yoke, which thus prevents the loss of magnetic energy in the external medium.
However, this solution requires a large amount of special, costly materials. Moreover, part of the field lines passing through the periphery of the pole pieces lead directly to zones adjacent to the yoke and are prejudicial to the homogeneity of the magnetic field in the air gap, which it is important to produce. Therefore the patent indicates several means for straightening the field lines, but this in turn makes it necessary to increase again the ferromagnetic material quantity.
European patent EP-A1-0 170 318 refers to the use of additional lateral magnets enclosing the air gap and whose magnetization directions are opposite to those of the main permanent magnets. This arrangement makes it possible to increase the intensity and homogeneity of the magnetic field in the air gap, but the said patent only describes configurations for which the main permanent magnets and the lateral additional magnets are in contact by their edges or are even slightly spaced. Therefore the homogeneity and intensity of the magnetic field produced are still not completely satisfactory.
The present invention makes it possible to satisfy the requirement of an intense magnetic field with a magnetic circuit having a moderate quantity of material and also makes it possible to obtain a good homogeneity of the magnetic field within the air gap.
Thus, it is proposed that the additional permanent magnets on either side of the air gap extend at least partly facing the main permanent magnets and the pole pieces covering them.
It is also very interesting for the additional magnets to extend beyond the pole pieces towards the outside of the air gap and the same applies regarding the main magnets.
The homogeneity of the magnetic field in the air gap can be increased if it is possible to regulate the position of the additional magnets, so as to modify their spacing or their distance from the pole pieces.
More specifically, the present invention relates to a system of permanent magnets having a permeable magnetic yoke formed by two opposite support walls and connected by two connecting walls, two main permanent magnets of the same magnetization direction disposed on the support walls within the yoke and whereof the facing surfaces carry opposite poles, two permeable magnetic plates on the facing surfaces and which define an air gap and two lateral permanent magnets whose magnetization direction is opposite to that of the main permanent magnets and which also define the air gap, being positioned facing the conncting walls and within the yoke, characterized in that the lateral permanent magnets extend laterally between the permeable magnetic plates, so as to be positioned facing the lateral edges of the main permanent magnets and the permeable magnetic plates.
The invention is described in greater detail hereinafter relative to non-limitative embodiments and the attached drawings, wherein show:
FIG. 1 A front view
REFERENCES:
patent: 3437963 (1969-04-01), Gang et al.
patent: 4777464 (1988-10-01), Takabatashi et al.
Chaillout Jean-Jacques
Jeandey Christian
Tournier Edmond
Commissariat a l''Energie Atomique
Harris George
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