CA2108157C - Permanent magnet assembly - Google Patents
Permanent magnet assembly Download PDFInfo
- Publication number
- CA2108157C CA2108157C CA002108157A CA2108157A CA2108157C CA 2108157 C CA2108157 C CA 2108157C CA 002108157 A CA002108157 A CA 002108157A CA 2108157 A CA2108157 A CA 2108157A CA 2108157 C CA2108157 C CA 2108157C
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- CA
- Canada
- Prior art keywords
- housing
- magnet
- magnet assembly
- pole pieces
- spacer means
- Prior art date
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Classifications
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01F—MAGNETS; INDUCTANCES; TRANSFORMERS; SELECTION OF MATERIALS FOR THEIR MAGNETIC PROPERTIES
- H01F7/00—Magnets
- H01F7/02—Permanent magnets [PM]
- H01F7/0273—Magnetic circuits with PM for magnetic field generation
- H01F7/0278—Magnetic circuits with PM for magnetic field generation for generating uniform fields, focusing, deflecting electrically charged particles
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
- G01R33/00—Arrangements or instruments for measuring magnetic variables
- G01R33/20—Arrangements or instruments for measuring magnetic variables involving magnetic resonance
- G01R33/28—Details of apparatus provided for in groups G01R33/44 - G01R33/64
- G01R33/38—Systems for generation, homogenisation or stabilisation of the main or gradient magnetic field
- G01R33/383—Systems for generation, homogenisation or stabilisation of the main or gradient magnetic field using permanent magnets
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- Physics & Mathematics (AREA)
- Electromagnetism (AREA)
- Engineering & Computer Science (AREA)
- Power Engineering (AREA)
- Condensed Matter Physics & Semiconductors (AREA)
- General Physics & Mathematics (AREA)
- Magnetic Resonance Imaging Apparatus (AREA)
Abstract
A permanent magnet assembly particularly useful in magnetic resonance studies includes a central spacer having opposite shallow recesses therein receiving end portions of pole pieces for aligning same. Main permanent magnets abut the pole pieces on the opposite sides thereof from the central spacer. The main permanent magnets outwardly overlap a peripheral interface between the pole pieces and a plurality of side permanent magnets for minimizing flux leakage at such interface.
Description
~i~~~_~~~
Permanent Magnet Assembly This invention re7.ates to a permanent magnet assembly, and is especially useful in the field of magnetic resonance imaging and spectroscopy. Optimum performance of magnetic resonance devices requires precise alignment of pole pieces of opposite magnetic polaxity.
It is also important to minimize flux leakage at the interface between certain components. ' Us-A-4,998,976 discloses a permanent magnet assembly used for creating a uniform magnetic field across a portion of an air gap. Thie includes a principal magnet, a pole piece and a set of auxiliary magnets extending about the periphery of the pole piece. Two such magnet grpupings are positioned within a test chamber, separated by spacers.
It would ba daairable to have an arrangement for accurately aligning pale pieces and ror minimizing flux leakage at certain interfaces.
It ig a principal object of the invention to provides a permanent magnet assembly having an improved arrangement far aligning opposite pole pieces on opposite sides of~a central spacer.
It is another object of the invention to provide a permanent magnet assQmbly having an improved arrangement for minimizing flux leakage at a peripheral interface between a pole piece and side permanent magnets.
It ie a further objoct of the invention to provide an improved arrangement for directing lead wires on electra-magnetic shunt Coils to the exterior of a housing.
It is also all object of the invention to provide an improved arrangement for guiding a central spacer into a housing for angularly aligning a transverse hole in the spacer with a tranev~rae hole in the housing.
According to the invention, there is provided a permanent magnet assembly for providing a magnetic field across a gap and including opposed magnet members having opposite magnetic poles facing one another across a gap, and opacer means of non-ferromagnetic material 2~~8~ 5'~
positioned between said magnet members for maintaining same in spaced-apart relationship, said magnet members having facing end portions adjacent said spacer means.
characterized by cooperating alignment means between said magnet members and said spacer means for aligning said magnet members along a substantially common longitudinal axis: wherein said alignment means comprises opposite shallow recesses in said spacer means~closely receiving said facing end portions of said magnet members.
In a feature of the invention, main permanent magnets abut the pole pieces or magnet members on the opposite sides thereof from the central epaaer means. The main permanent magnets sire slightly larger than the pole pieces, and outwardly overlap a peripheral interface between the pole pieces and side permanent magnets assembled thereaxound. This overlap minimizes flux leakage at the peripheral interface between a pole piece and side magnets.
tn another feature electro-magnetic shunt coils on the central spacer means have lead wires extending through tubes of fQrromagnetio material that are slidably received in longitudinal grooves in a ferromagnetic housing.
In another featurer pm7~ 91'ide ~ on the .central spacer means is closely received in a longitudinal groove in the,housing tor~~angularly aligning a transverse hole in the central spacer means with a transverse hole in the housing.
A permanent magnet assembly in accordance with the present application pre~erably~has a one-piece housing, not oounting the and asps. However, the housing can be longitudinally or transversely split. In the preferred arrangement, the housing is vylindrical. However, certain features of tha~present application are also advantageous in housings of other shapes, Much as a housing having a square cross-sectional shape.
An embodimen~.of. the invention will now be degcribec~
by way of exdmple only and with reference to the accompanying drawings. ' Figure 1 is an axplodod p~rspective illustration of a perrrianent magnet assembly constructed in accordance with the present application ~1~8~57 Figure 2 is a cross-sectional elevational view of the assembly of Figure 1;
Figure 3 is a plan viQw of a central spacer used in the assembly;
Figure h is a partial tap elevational view of one end portion of a ferromagnetic housing showing a longitudinal groove therein;
Figure 5 is a side elevatianal view of a ferromagnetic tube receivable in the groovs of Figure 4;
Figure 6 is a cross-sectional elevational view ~howing the tube o! Figure 5 attached to a projection an the central spacer of Figure 3, and with lead wires from electro-magnetic shunt coils extending from the spacer through the tube and 1S , Figure 7 is a detail view o~ the central part of Fig. 2 taken on the line VII-VII o~ Fig. 3.
Referxfng new to the drawir~gs~ wherein the showings are for purposes of illustrating certain preferred embodiments of the invention only and not for purposes of .limiting same, Figure 1 is an exploded perspective illustration of Zg a permanent magnet assembly that includes s hollow cylindrical housing 8 of fsrramagnetic material and hawing a longitudinal axis 10. The peripheral wall of housing B
has a pair of holes therethrough located 180 apart, and only one such hale is indicated at 12 in Figure.l. The pair of holes have centers lying on a common axis extending perpendicular to and intersecting the longitudinal axis of housing B. The centers of the holes are also located centrally between the opposite ends of housing B.
A cylindrical spacer disc C of nonmagnetic material is a close sliding fit within housing B. and has a hole thexethrough of the same diameter as hole iz. The axis of hole 14 in spacer disc C extends perpendicular to and intersects longitudinal axis 10 of disc C. Spacer disc C
has sub$tantially plane and parallel opposite faces 7.6, 2108.5 and a cylindrical outer pQrfphery 20. The axis of hole 14 is centrally looated between faces 16, 18.
Each of a pair of permanent magnet subassemblies v, E
ir~aludes a pole piece 30, 40 of =erromagnatfo material, and a plurality of side permanent magnets 32a-h, 42a-h.
pole piece 30 has opposite faces 34, 36 arid a cylindrical outer periphery 38. Pale pieces 40 hats opposite faces 44, 4fi and a cylindrical outer periphery 48. At least faces .
34, 44 are substantially flat and lie in planes extending substantially perpendicular to longitudinal axis 10.
Side permanent magnet means ist shown in the form of eight arouate side permanent magnets 3za-h, each extending over an arc o! approximately 45°, so they extend substantially continuously around the periphery of pole piece 30., The inner surfaeng of side magnets 32a-h era curved to lie on the periphery or a common cylinder with outer peripheral surface 38 of pole piece 30. , Thus, when side magnets 32a-h are positioned against outer periphery 38 of pole piece 30, such side magnets extend comp7.etely around pole piece 30 in gurroundfng relation- , ship thereto. In addition, the outer peripheral surfaces of side magnets 32a-h then lie on the periphery of a cylinder and have a very close sliding fit within housing B. The fit is such that the outer peripheral surfaces of the side mztgnets actually engage the inner peripheral surface of housing B.
Each side magnet 32a-h has a longitudinal thickness such that when the axially inwardly facing surface thereof is positioned against face 16 of spacer disc C, the opposite outwardly facing s~race thereof is flush with puter face 34 of pole piece 30. There are also eight arcuate~side permanent magnets 42a-h, and the relationship between them and pole piece 40 is the same as described with respect to the relationship between side magnets 32a-h and pole piece 210815' 30. Thus, when the magnet is assembled, the outwardly facing surfaces of side magnate 3za-h. are substantially flush with outer face 34 of pole piece 30, and the ' outwardly facing surfaces of side magnate 42a-h era substantially flush with outer face 44 of pole piece 40.
The side permanent magnQts arQ magnetized generally radially of the longitudinal axis of their respective pole p~,eces. All of the aide pdrmanent magnets in one parmanont magnet subassembly D or E arw magnetized to have north magnetic poles on their inner peripheral surfaces and south magnetic poles on their outer peripheral surfaces,~whila all of the side permanent magnets in the ether 'permanent magnet subassembly are magnetized to have south magnetic poles on their inner peripheral suxfaces 15 and north magnetic poles on their outer peripheral surfaces. Therefore, one of pole pieces 30 or 40 is a north magnetic pole, while the other pole piece. is a south magnetic pole, so that the pole pieces axe of opposite magnetic polarity.
20 The side permanent magnet means can take many other forms. For example, the number of individual side magnets can be greater or smaller than eight, including one.
Cylindrical, main permanent magnets F, G have a diameter that is slightly larger than the diameter of po7.e l 25 and are axially concentric therewith. Main pieces 30, 40 permanent magnet F has one flat end 50 positionable in abutting relationship with face 34 of polo piece 30, and an opposite flat end attached to a cylindrical housing cover 52 of ferroma~netio material. Cylindrical main ' 3o permanent magnQt G has a flat and 60 positionable against faces 44 of pole piece 40, and an opposite flat face ' attached to cylindrical cover 62 of ferromagnetic material. Coverss 52, 62 are a close sliding fit within housing B. Ends 50, 60 of main magnets F, G also overlie 21~~~.~
Permanent Magnet Assembly This invention re7.ates to a permanent magnet assembly, and is especially useful in the field of magnetic resonance imaging and spectroscopy. Optimum performance of magnetic resonance devices requires precise alignment of pole pieces of opposite magnetic polaxity.
It is also important to minimize flux leakage at the interface between certain components. ' Us-A-4,998,976 discloses a permanent magnet assembly used for creating a uniform magnetic field across a portion of an air gap. Thie includes a principal magnet, a pole piece and a set of auxiliary magnets extending about the periphery of the pole piece. Two such magnet grpupings are positioned within a test chamber, separated by spacers.
It would ba daairable to have an arrangement for accurately aligning pale pieces and ror minimizing flux leakage at certain interfaces.
It ig a principal object of the invention to provides a permanent magnet assembly having an improved arrangement far aligning opposite pole pieces on opposite sides of~a central spacer.
It is another object of the invention to provide a permanent magnet assQmbly having an improved arrangement for minimizing flux leakage at a peripheral interface between a pole piece and side permanent magnets.
It ie a further objoct of the invention to provide an improved arrangement for directing lead wires on electra-magnetic shunt Coils to the exterior of a housing.
It is also all object of the invention to provide an improved arrangement for guiding a central spacer into a housing for angularly aligning a transverse hole in the spacer with a tranev~rae hole in the housing.
According to the invention, there is provided a permanent magnet assembly for providing a magnetic field across a gap and including opposed magnet members having opposite magnetic poles facing one another across a gap, and opacer means of non-ferromagnetic material 2~~8~ 5'~
positioned between said magnet members for maintaining same in spaced-apart relationship, said magnet members having facing end portions adjacent said spacer means.
characterized by cooperating alignment means between said magnet members and said spacer means for aligning said magnet members along a substantially common longitudinal axis: wherein said alignment means comprises opposite shallow recesses in said spacer means~closely receiving said facing end portions of said magnet members.
In a feature of the invention, main permanent magnets abut the pole pieces or magnet members on the opposite sides thereof from the central epaaer means. The main permanent magnets sire slightly larger than the pole pieces, and outwardly overlap a peripheral interface between the pole pieces and side permanent magnets assembled thereaxound. This overlap minimizes flux leakage at the peripheral interface between a pole piece and side magnets.
tn another feature electro-magnetic shunt coils on the central spacer means have lead wires extending through tubes of fQrromagnetio material that are slidably received in longitudinal grooves in a ferromagnetic housing.
In another featurer pm7~ 91'ide ~ on the .central spacer means is closely received in a longitudinal groove in the,housing tor~~angularly aligning a transverse hole in the central spacer means with a transverse hole in the housing.
A permanent magnet assembly in accordance with the present application pre~erably~has a one-piece housing, not oounting the and asps. However, the housing can be longitudinally or transversely split. In the preferred arrangement, the housing is vylindrical. However, certain features of tha~present application are also advantageous in housings of other shapes, Much as a housing having a square cross-sectional shape.
An embodimen~.of. the invention will now be degcribec~
by way of exdmple only and with reference to the accompanying drawings. ' Figure 1 is an axplodod p~rspective illustration of a perrrianent magnet assembly constructed in accordance with the present application ~1~8~57 Figure 2 is a cross-sectional elevational view of the assembly of Figure 1;
Figure 3 is a plan viQw of a central spacer used in the assembly;
Figure h is a partial tap elevational view of one end portion of a ferromagnetic housing showing a longitudinal groove therein;
Figure 5 is a side elevatianal view of a ferromagnetic tube receivable in the groovs of Figure 4;
Figure 6 is a cross-sectional elevational view ~howing the tube o! Figure 5 attached to a projection an the central spacer of Figure 3, and with lead wires from electro-magnetic shunt coils extending from the spacer through the tube and 1S , Figure 7 is a detail view o~ the central part of Fig. 2 taken on the line VII-VII o~ Fig. 3.
Referxfng new to the drawir~gs~ wherein the showings are for purposes of illustrating certain preferred embodiments of the invention only and not for purposes of .limiting same, Figure 1 is an exploded perspective illustration of Zg a permanent magnet assembly that includes s hollow cylindrical housing 8 of fsrramagnetic material and hawing a longitudinal axis 10. The peripheral wall of housing B
has a pair of holes therethrough located 180 apart, and only one such hale is indicated at 12 in Figure.l. The pair of holes have centers lying on a common axis extending perpendicular to and intersecting the longitudinal axis of housing B. The centers of the holes are also located centrally between the opposite ends of housing B.
A cylindrical spacer disc C of nonmagnetic material is a close sliding fit within housing B. and has a hole thexethrough of the same diameter as hole iz. The axis of hole 14 in spacer disc C extends perpendicular to and intersects longitudinal axis 10 of disc C. Spacer disc C
has sub$tantially plane and parallel opposite faces 7.6, 2108.5 and a cylindrical outer pQrfphery 20. The axis of hole 14 is centrally looated between faces 16, 18.
Each of a pair of permanent magnet subassemblies v, E
ir~aludes a pole piece 30, 40 of =erromagnatfo material, and a plurality of side permanent magnets 32a-h, 42a-h.
pole piece 30 has opposite faces 34, 36 arid a cylindrical outer periphery 38. Pale pieces 40 hats opposite faces 44, 4fi and a cylindrical outer periphery 48. At least faces .
34, 44 are substantially flat and lie in planes extending substantially perpendicular to longitudinal axis 10.
Side permanent magnet means ist shown in the form of eight arouate side permanent magnets 3za-h, each extending over an arc o! approximately 45°, so they extend substantially continuously around the periphery of pole piece 30., The inner surfaeng of side magnets 32a-h era curved to lie on the periphery or a common cylinder with outer peripheral surface 38 of pole piece 30. , Thus, when side magnets 32a-h are positioned against outer periphery 38 of pole piece 30, such side magnets extend comp7.etely around pole piece 30 in gurroundfng relation- , ship thereto. In addition, the outer peripheral surfaces of side magnets 32a-h then lie on the periphery of a cylinder and have a very close sliding fit within housing B. The fit is such that the outer peripheral surfaces of the side mztgnets actually engage the inner peripheral surface of housing B.
Each side magnet 32a-h has a longitudinal thickness such that when the axially inwardly facing surface thereof is positioned against face 16 of spacer disc C, the opposite outwardly facing s~race thereof is flush with puter face 34 of pole piece 30. There are also eight arcuate~side permanent magnets 42a-h, and the relationship between them and pole piece 40 is the same as described with respect to the relationship between side magnets 32a-h and pole piece 210815' 30. Thus, when the magnet is assembled, the outwardly facing surfaces of side magnate 3za-h. are substantially flush with outer face 34 of pole piece 30, and the ' outwardly facing surfaces of side magnate 42a-h era substantially flush with outer face 44 of pole piece 40.
The side permanent magnQts arQ magnetized generally radially of the longitudinal axis of their respective pole p~,eces. All of the aide pdrmanent magnets in one parmanont magnet subassembly D or E arw magnetized to have north magnetic poles on their inner peripheral surfaces and south magnetic poles on their outer peripheral surfaces,~whila all of the side permanent magnets in the ether 'permanent magnet subassembly are magnetized to have south magnetic poles on their inner peripheral suxfaces 15 and north magnetic poles on their outer peripheral surfaces. Therefore, one of pole pieces 30 or 40 is a north magnetic pole, while the other pole piece. is a south magnetic pole, so that the pole pieces axe of opposite magnetic polarity.
20 The side permanent magnet means can take many other forms. For example, the number of individual side magnets can be greater or smaller than eight, including one.
Cylindrical, main permanent magnets F, G have a diameter that is slightly larger than the diameter of po7.e l 25 and are axially concentric therewith. Main pieces 30, 40 permanent magnet F has one flat end 50 positionable in abutting relationship with face 34 of polo piece 30, and an opposite flat end attached to a cylindrical housing cover 52 of ferroma~netio material. Cylindrical main ' 3o permanent magnQt G has a flat and 60 positionable against faces 44 of pole piece 40, and an opposite flat face ' attached to cylindrical cover 62 of ferromagnetic material. Coverss 52, 62 are a close sliding fit within housing B. Ends 50, 60 of main magnets F, G also overlie 21~~~.~
-6~
and abut portions of the outwardly facing surfaces of $ide magnets 32a-h and 42a-h that are located closely adjacent faces 34,~ 44 of pale pieces 30, 40.
Covara 52, 62 havo a plurality of circumferentially-spaced threaded bores 53, 63 alignable with radial holes 416 in housing B. Fasteners are receivable through holes 416 within bores 53, 63 for securing the covers to the housing.
Spool-likes sleeve members H, I of nonmagnetir~ material to have axial cylindrical holes therathrough for receiving permanent magnets F, G with a close sliding fit. The axial length of each sleevQ member H, x is substantially the same as the axial length of a main pQrmanent magnet F, G. Sleeve member H has cylindrical flanges '72, 74 that are a close sliding fit within housing B. Sleeve member I
has cylindrical flanges 76, 78 that are a close sliding fit within housing B. When all of the components are poaitionQd in end-to--end abutting relationship within housing 8, the outer surfaces of covers 52, 62. are 2o substantially flush with the end surfaces of housing B.
Figure 2 shows the components of Figure 1 in assembled relationship. The magnetic polarity of the magnetized components Xs indicated by N and 8. Spacer disc C
includes an outer ring portion 90 of nonmagnetic material z5 having a central circular hole therethrough receiving a central disc 92 of dl~electric material. Hole 14 extends through both ring 9o and disc 92, and that portion of the hole located,in the arQa of longitudinal axis 1o defines a test zone in which materials to be tested are positioned.
3o Recesses in the opposite faces of dielectric central disc, 92 receive windings 94 of electro-magnetic shunt coils for selectively adjusting the magnetic field in the test zone within hoJ.e l4 between poles 30, 4~~. Cooperating align-ment means is provided between spacer C and pole pieces 2108~.~7 30, 40 for aligning such pole pieces along a common longitudinal axis. The alignment means may comprise suitably shaped cooperating projections and recesses.
a most preferred form of which ig shown in the drawing g wherein central circular recesses 96, 98 on the opposite surfaces of ring portion 90 receive end portions of pole pieces 30, 40 with a close sliding fit to center same with respect to spacer disc C and to one another. The opposite outer surfaces of disc 92 are substantially flush with the bottoms of recesses 96, 98.
Aa shown in Figure 2, the axial thickness of each pole piece 30j 40 is greater than such thickness of its associated side magnets 32a-h, 42a-h by an amount equal to the depth of a recess 96 ox 98 in ring portion 90.
Recesses 96, 98 are preferably very shallow so that the axial thickness of the side magnets can be maximized.
The depth of recesses 96, 98 is preferably just sufficient to receive end portions of pole pieces 30, 40 therein for alignment purposes. The raoaBaes help to align the pole pieces along a common longitudinal axis coincidental With longitudinal axis 10, and also help tv ensure~that flat facing surfacQS 36, 46 are parallel.
Main magnets F, G extend outwardly slightly beyond and overlap the circular abutting lines between the outer peripheries 38, 48 of pole pieces 30, 40 and the inner peripheral surfaces of side magnets 32a-h, 42 a-h in order to minimize outward flux leakage. If the main magnets have a diameter that is the Same or slightly smaller than the polo pieces, outward flux leakage is possible at the interface between wthe outer periphery of a pole piece and the inner periphery of thg side magnets. Overlapping such interface with the main magnets inhibits such flux leakage because the force fields of the main magnets oppose such leakage.
~1~8~.~'~
_8_ Faces 36, 46 of pole pieces 30, 40 have centrally located curved depressions 36a, 46a therein symmetrical about longitudinal axis 10. Although depressions could extend. all the way to outer peripheries 38, 48 of pole 5 pieces 30, 40, they are preferably spaced inwardly .
therQfrom to provide flat bearing areas engaging the bottoms of recesses 96, 98 and portions of the outer surfaces of disc 9a. Each depression 36a, 46a preferably occupies substantially mere than one~half the total area of each face 36, 46~on pole pieces 30, 40.
In the most pxe~ferrad arrangement, the entire surface area of depressions 36a, 46a is everywhere curved and non-perpendicular to longitudinal axis 10. In general, the curvature of the depressions will vary from the outer 15 periphery thereof to the conter thereof. That is, the curvature is such that it is not a regular geometric surface, such as the surface of a sphere or parabola, or a surface that can be generated by rotating a regular geometric curve, such as a parabola, about its major or 20 minor axis. The best curvature can be determined by computer modeling, by trial and error, or empirically.
Magnetic flux enters and leaves magnet surfaces perpnndiaular to such surfacos. When surfaces 36, 46 of pole pieces 30, 40 are completely flat and perpendicular 25 to longitudinal, axis 10, the magnetic flux enters and leaves such surfaces perpendicular thereto and parallel to axis 10. However, i.n the space between the pale pieces, the flux tends to bow outwardly away from axis l0, and this reduces the strength and uniformity of the magnetic 30 field in the~test zone.
Curved depressivn~S 36a, 46a are designed to zmpart an inward bowing effect on the flux lines in the space ' between the pole pieces. The curvature is preferably such that the inward bowing effect thereof is precisely matched 35 by the outward bowing forces so that the flux 7.ines in the test zone are actually substantially parallel to 21~8~.~'~
_g..
longitudinal axis 10. This enhances the strength and uniformity of the magnetic field in the test zone.
Housing B has a pain of longitudinal grooves 100, 101 in the inner surface thereof located..180° apart from one another, and spaced 90° from the central axis of housing hole 12 of Figure 1. Pins 102, 103 fixed tv spacer disc C
extend radially outwardly therefrom at locations spaced 90~ from the center of holQ 14. Pins 102, 103 are receivablQ in grooves 7.00, 101 for aligning hole 14 with hole 12 when spacer 3issc C is moved into housing B. Ring portion 90 of spacer iliac C has radially extending grooves 106109 through which wires 110--113 from the electro-magnetic shunt coils extend. Each of grooves -106-109 has a depth greater than the depth of recesses 96, 98,so that the wires will bQ below the bottoms of recesses 96, 98. Central disc 92 also has transverse grooves in the opposit3 surfaces thereof aligned with grooves io6-109 for accommodating they lead wires for cv3.ls 94. Shunt coil lead wires 110, 111 extend through an elongated tube 119 of ferromagnetic material that is closely received in groove 100. Shunt aoil lead wires 112, 113 extend through an elongated tube 115 of ferromagnetic material,that is closely received in groove 7.01.
Figure 3 is a plan view of spacer means C with the elactro-magnetic shunt coils shown very schematically, because the primary purpose of the figure is to show grooves 106, 108 ~.n ring member 90 and grooves 116 in central disc 9x for accommodating the lead wires for the coils. Figure 3 also shows the guide projections defined by pine 102, 103 as being centrally aligned with grooves 106, 108, and angularly-spaced 90° from hole 14.
Figure 4 shows longitudinal circular groove l01 in housing H as opening at a narrow guide portion 117 along the intexior surface of housing B. '~he width of narrow gu~.de portion 117 of the groove is r.:lated to the diameter of a guide pin 103 such that the pin is closely received therein in close sliding relationship thereto.
2~~8' ~'~
-lo-Tube 115 of ferromagnetic material is a close sliding tit in circular groove 101. As shown in Figures 5 and 6, tube 115 has a longitudinal rBCesg 13.8 in a bottom end portion thereof, and a small transverBe hole 119 aligned 5 with recess 118. Pin 103 i.s closely receivable in hole 119. Groove 10o in housing B, and tube 114 receivable therein, have the same relationship to one another and to pin 102 as described for groove 101, tube 115 and pin 103.
To assemble spacer means C within housing B, the lead wires 110, 111 and 112, 113 axe fed through tubes 114, 115, and. pins 102, 103 are inserted in the transverse tube holes as shown for pin 1.03 and hole 119 in Figure 6. The assembled spacer means and tubes ars than slidably guided into housing B and grooves 100, 101 until holes 12, 14 are 15 in alignment. A retaining assembly pin can then be inserted into the aligned holes for maintaining spacer means C in position while thQ other compvnents,are assembled within housing B. With grooves 100,,101 located 90° fz~om the axis of housing holQ 12, and pins 102, 103 located 90° from the axis of hole 14, the holes are automatically in angular alignment when the spacer means and tubes are slidably guided into the housing and housing grooves.
Heating. elements 120, 122 are positioned around 25 sleeves H, I, and are controlled by thermi$tors for maintaining a desired temperature of the magnet assembly.
Flange 72 on sleeve H has an axial groove 124 therein aligned with a hole 126 in cover 52 for extending lead 12s of heating element 120 to the exterior of the housing.
w 3o Likewise, flange 76 on sleeve I has a groove 130 aligned with a hole 132 in cover 62 for extending lead 134 of heating element 122 to the exterior of the housing.
In the present application, materials having a coercivity substantially the same as air, such as 21~~~.5~
aluminAum. brass or plastic, are referred to as non-magnetic or non-ferromagnetic materials for convenience of description. Ferromagnetic materials are those having a very high coercivity, such as iron.
The improvements of the present invention have been shown and described with reference to a cylindrical magnet assembly having a circular cross-sectional shape.
However. it will be recognized that the improvements of the present invention can also be used with magnet to assemblies having other shapes including, but not necessarily limited to, those having generally square or polygonal cross-sectional shapes.
Although the invention has bean shown and described with respect to certain preferred embodiments. it is obvious that equivalent alterations and modifications will occur to others skilled in the art upon the reading and unciarstanding of this speoificatian. The present invention includes all such equivalent alterations and modifications, and is limited only by the scope of the claims.
w
and abut portions of the outwardly facing surfaces of $ide magnets 32a-h and 42a-h that are located closely adjacent faces 34,~ 44 of pale pieces 30, 40.
Covara 52, 62 havo a plurality of circumferentially-spaced threaded bores 53, 63 alignable with radial holes 416 in housing B. Fasteners are receivable through holes 416 within bores 53, 63 for securing the covers to the housing.
Spool-likes sleeve members H, I of nonmagnetir~ material to have axial cylindrical holes therathrough for receiving permanent magnets F, G with a close sliding fit. The axial length of each sleevQ member H, x is substantially the same as the axial length of a main pQrmanent magnet F, G. Sleeve member H has cylindrical flanges '72, 74 that are a close sliding fit within housing B. Sleeve member I
has cylindrical flanges 76, 78 that are a close sliding fit within housing B. When all of the components are poaitionQd in end-to--end abutting relationship within housing 8, the outer surfaces of covers 52, 62. are 2o substantially flush with the end surfaces of housing B.
Figure 2 shows the components of Figure 1 in assembled relationship. The magnetic polarity of the magnetized components Xs indicated by N and 8. Spacer disc C
includes an outer ring portion 90 of nonmagnetic material z5 having a central circular hole therethrough receiving a central disc 92 of dl~electric material. Hole 14 extends through both ring 9o and disc 92, and that portion of the hole located,in the arQa of longitudinal axis 1o defines a test zone in which materials to be tested are positioned.
3o Recesses in the opposite faces of dielectric central disc, 92 receive windings 94 of electro-magnetic shunt coils for selectively adjusting the magnetic field in the test zone within hoJ.e l4 between poles 30, 4~~. Cooperating align-ment means is provided between spacer C and pole pieces 2108~.~7 30, 40 for aligning such pole pieces along a common longitudinal axis. The alignment means may comprise suitably shaped cooperating projections and recesses.
a most preferred form of which ig shown in the drawing g wherein central circular recesses 96, 98 on the opposite surfaces of ring portion 90 receive end portions of pole pieces 30, 40 with a close sliding fit to center same with respect to spacer disc C and to one another. The opposite outer surfaces of disc 92 are substantially flush with the bottoms of recesses 96, 98.
Aa shown in Figure 2, the axial thickness of each pole piece 30j 40 is greater than such thickness of its associated side magnets 32a-h, 42a-h by an amount equal to the depth of a recess 96 ox 98 in ring portion 90.
Recesses 96, 98 are preferably very shallow so that the axial thickness of the side magnets can be maximized.
The depth of recesses 96, 98 is preferably just sufficient to receive end portions of pole pieces 30, 40 therein for alignment purposes. The raoaBaes help to align the pole pieces along a common longitudinal axis coincidental With longitudinal axis 10, and also help tv ensure~that flat facing surfacQS 36, 46 are parallel.
Main magnets F, G extend outwardly slightly beyond and overlap the circular abutting lines between the outer peripheries 38, 48 of pole pieces 30, 40 and the inner peripheral surfaces of side magnets 32a-h, 42 a-h in order to minimize outward flux leakage. If the main magnets have a diameter that is the Same or slightly smaller than the polo pieces, outward flux leakage is possible at the interface between wthe outer periphery of a pole piece and the inner periphery of thg side magnets. Overlapping such interface with the main magnets inhibits such flux leakage because the force fields of the main magnets oppose such leakage.
~1~8~.~'~
_8_ Faces 36, 46 of pole pieces 30, 40 have centrally located curved depressions 36a, 46a therein symmetrical about longitudinal axis 10. Although depressions could extend. all the way to outer peripheries 38, 48 of pole 5 pieces 30, 40, they are preferably spaced inwardly .
therQfrom to provide flat bearing areas engaging the bottoms of recesses 96, 98 and portions of the outer surfaces of disc 9a. Each depression 36a, 46a preferably occupies substantially mere than one~half the total area of each face 36, 46~on pole pieces 30, 40.
In the most pxe~ferrad arrangement, the entire surface area of depressions 36a, 46a is everywhere curved and non-perpendicular to longitudinal axis 10. In general, the curvature of the depressions will vary from the outer 15 periphery thereof to the conter thereof. That is, the curvature is such that it is not a regular geometric surface, such as the surface of a sphere or parabola, or a surface that can be generated by rotating a regular geometric curve, such as a parabola, about its major or 20 minor axis. The best curvature can be determined by computer modeling, by trial and error, or empirically.
Magnetic flux enters and leaves magnet surfaces perpnndiaular to such surfacos. When surfaces 36, 46 of pole pieces 30, 40 are completely flat and perpendicular 25 to longitudinal, axis 10, the magnetic flux enters and leaves such surfaces perpendicular thereto and parallel to axis 10. However, i.n the space between the pale pieces, the flux tends to bow outwardly away from axis l0, and this reduces the strength and uniformity of the magnetic 30 field in the~test zone.
Curved depressivn~S 36a, 46a are designed to zmpart an inward bowing effect on the flux lines in the space ' between the pole pieces. The curvature is preferably such that the inward bowing effect thereof is precisely matched 35 by the outward bowing forces so that the flux 7.ines in the test zone are actually substantially parallel to 21~8~.~'~
_g..
longitudinal axis 10. This enhances the strength and uniformity of the magnetic field in the test zone.
Housing B has a pain of longitudinal grooves 100, 101 in the inner surface thereof located..180° apart from one another, and spaced 90° from the central axis of housing hole 12 of Figure 1. Pins 102, 103 fixed tv spacer disc C
extend radially outwardly therefrom at locations spaced 90~ from the center of holQ 14. Pins 102, 103 are receivablQ in grooves 7.00, 101 for aligning hole 14 with hole 12 when spacer 3issc C is moved into housing B. Ring portion 90 of spacer iliac C has radially extending grooves 106109 through which wires 110--113 from the electro-magnetic shunt coils extend. Each of grooves -106-109 has a depth greater than the depth of recesses 96, 98,so that the wires will bQ below the bottoms of recesses 96, 98. Central disc 92 also has transverse grooves in the opposit3 surfaces thereof aligned with grooves io6-109 for accommodating they lead wires for cv3.ls 94. Shunt coil lead wires 110, 111 extend through an elongated tube 119 of ferromagnetic material that is closely received in groove 100. Shunt aoil lead wires 112, 113 extend through an elongated tube 115 of ferromagnetic material,that is closely received in groove 7.01.
Figure 3 is a plan view of spacer means C with the elactro-magnetic shunt coils shown very schematically, because the primary purpose of the figure is to show grooves 106, 108 ~.n ring member 90 and grooves 116 in central disc 9x for accommodating the lead wires for the coils. Figure 3 also shows the guide projections defined by pine 102, 103 as being centrally aligned with grooves 106, 108, and angularly-spaced 90° from hole 14.
Figure 4 shows longitudinal circular groove l01 in housing H as opening at a narrow guide portion 117 along the intexior surface of housing B. '~he width of narrow gu~.de portion 117 of the groove is r.:lated to the diameter of a guide pin 103 such that the pin is closely received therein in close sliding relationship thereto.
2~~8' ~'~
-lo-Tube 115 of ferromagnetic material is a close sliding tit in circular groove 101. As shown in Figures 5 and 6, tube 115 has a longitudinal rBCesg 13.8 in a bottom end portion thereof, and a small transverBe hole 119 aligned 5 with recess 118. Pin 103 i.s closely receivable in hole 119. Groove 10o in housing B, and tube 114 receivable therein, have the same relationship to one another and to pin 102 as described for groove 101, tube 115 and pin 103.
To assemble spacer means C within housing B, the lead wires 110, 111 and 112, 113 axe fed through tubes 114, 115, and. pins 102, 103 are inserted in the transverse tube holes as shown for pin 1.03 and hole 119 in Figure 6. The assembled spacer means and tubes ars than slidably guided into housing B and grooves 100, 101 until holes 12, 14 are 15 in alignment. A retaining assembly pin can then be inserted into the aligned holes for maintaining spacer means C in position while thQ other compvnents,are assembled within housing B. With grooves 100,,101 located 90° fz~om the axis of housing holQ 12, and pins 102, 103 located 90° from the axis of hole 14, the holes are automatically in angular alignment when the spacer means and tubes are slidably guided into the housing and housing grooves.
Heating. elements 120, 122 are positioned around 25 sleeves H, I, and are controlled by thermi$tors for maintaining a desired temperature of the magnet assembly.
Flange 72 on sleeve H has an axial groove 124 therein aligned with a hole 126 in cover 52 for extending lead 12s of heating element 120 to the exterior of the housing.
w 3o Likewise, flange 76 on sleeve I has a groove 130 aligned with a hole 132 in cover 62 for extending lead 134 of heating element 122 to the exterior of the housing.
In the present application, materials having a coercivity substantially the same as air, such as 21~~~.5~
aluminAum. brass or plastic, are referred to as non-magnetic or non-ferromagnetic materials for convenience of description. Ferromagnetic materials are those having a very high coercivity, such as iron.
The improvements of the present invention have been shown and described with reference to a cylindrical magnet assembly having a circular cross-sectional shape.
However. it will be recognized that the improvements of the present invention can also be used with magnet to assemblies having other shapes including, but not necessarily limited to, those having generally square or polygonal cross-sectional shapes.
Although the invention has bean shown and described with respect to certain preferred embodiments. it is obvious that equivalent alterations and modifications will occur to others skilled in the art upon the reading and unciarstanding of this speoificatian. The present invention includes all such equivalent alterations and modifications, and is limited only by the scope of the claims.
w
Claims (15)
1. A permanent magnet assembly for providing a magnetic-field across a gap and including opposed magnet members having opposite magnetic poles facing one another across a gap, and spacer means of non-ferromagnetic material positioned between said magnet members for maintaining same in spaced-apart relationship, said magnet members having facing end portions adjacent said spacer means, and cooperating alignment means between said magnet members and said spacer means for aligning said magnet members along a substantially common longitudinal axis.
2. The magnet assembly of claim 1 wherein said alignment means comprises opposite shallow recesses in said spacer means closely receiving said facing end portions of said magnet members.
3. The magnet assembly of claim 2 including side magnet means positioned around said magnet members against said spacer means outwardly of said recesses therein.
4. The magnet assembly of claim 2 or 3 Wherein said end portions of said magnet members and said recesses are substantially cylindrical.
5. The magnet assembly of claim 2, 3 or 4 wherein said magnet members comprise pole pieces having flat ends opposite from said facing end portions, and side magnet means positioned around said pole pieces and against said spacer means outwardly of said recesses therein, said side magnet means having first surfaces positioned against said spacer means and having opposite surfaces that are substantially flush with said flat ends.
6. The magnet assembly of claim 5 including principal magnets having plane ends positioned against said flat ends of said pole pieces, said plane ends having substantially the same shape and a slightly larger size than said flat ends so that said plane ends overlap a
5. The magnet assembly of claim 2, 3 or 4 wherein said magnet members comprise pole pieces having flat ends opposite from said facing end portions, and side magnet means positioned around said pole pieces and against said spacer means outwardly of said recesses therein, said side magnet means having first surfaces positioned against said spacer means and having opposite surfaces that are substantially flush with said flat ends.
6. The magnet assembly of claim 5 including principal magnets having plane ends positioned against said flat ends of said pole pieces, said plane ends having substantially the same shape and a slightly larger size than said flat ends so that said plane ends overlap a
5. The magnet assembly of claim 4 including principal magnets (F, G) having plane ends positioned against said flat ends (34,44) of said pole pieces (30,40). said plane ends having substantially the same shape and slightly larger size than said flat ends (34,44) so that said plane ends overlap a peripheral interface between said pole pieces (30,40) and said side magnets (32a-h,42a-h).
6. A permanent magnet assembly fox providing a magnetic field across a gap and including a pair of spaced-apart pole pieces (30,40) having opposed facing end portions (36,46) of opposite magnetic polarity on opposite sides of a gap, said pole pieces having flat ends (34,44) opposite from said facing end portions (36,46), side magnet means (32a-h;42a-h) positioned around said pole pieces (30,40) substantially flush with said flat ends (34,44) of said pole pieces (30.40), said side magnet means (32a-h;42a-h) engaging said pole pieces at a peripheral interface, a pair of main magnets (F, G) having plane ends abutting said flat ends (34,44) of said pole pieces (30,40), said plane ends having the same shape as said flat ends but a slightly larger size such that said plane ends outwardly overlap said peripheral interface, characterised in that each said pole piece (30,40) has a longitudinal axis (10) and an outer periphery and a predetermined thickness adjacent said outer periphery measured parallel to said longitudinal axis (10), said side magnet means (32a-h,42a-h) associated with each said pole piece (30,40) having a thickness measured parallel to said longitudinal axis (10) that is less than said predetermined thickness of its associated pole piece (30,40).
7. A permanent magnet assembly including a ferromagnetic housing (B) having a peripheral wall, opposite ends and an inner peripheral surface, spacer means (C) in said housing for maintaining magnet assemblies, (D, E) of opposite magnetic polarity spaced from one another, said spacer means (C) including shunt coils (94) having lead wires (110-113), characterised by at least one longitudinal groove (100,101) in said housing peripheral wall opening outwardly at at least one of said housing ends and being open along the length thereof at said inner peripheral surface, and said lead Wires (110-113) extending through said groove (100,101), further including a tube (115) of ferromagnetic material received in said groove (100,101) with a close sliding fit, said lead wires (110,113) extending through said tube (115).
8. The magnet assembly of claim 7 wherein said groove (100,101) opens through said inner peripheral surface along a narrow entrance opening, a guide projection (102,103) extending outwardly from said spacer means (C) for close guiding reception in said narrow entrance opening.
9. The magnet assembly of claim 8 including connecting means for connecting said guide projection (102,103) and said tube (115).
10. The magnet assembly of any of claims 7 to 9 wherein said housing (B) has a pair of said longitudinal grooves (100,101) therein located opposite one another.
11, The magnet assembly of any of claims 7 to 10 wherein said housing (B), said spacer means (C) and said groove (100,101) are all cylindrical.
12. A permanent magnet assembly including a ferromagnetic housing (H) having a peripheral wall, opposite ends and an inner peripheral surface, a lateral housing hole (12) through said peripheral wall, spacer means (C) slidably received in said housing for maintaining magnet assemblies (D,E) of opposite magnetic polarity spaced from one another, said spacer means (C) having a lateral spacer hole (14) therein aligned with said lateral housing hole (12), a longitudinal groove (100,101) in said inner peripheral surface of said housing (B) angularly spaced from said housing hole (12) a predetermined angular degree, and a guide projection (102,103) on said spacer means (C) receivable in close guiding relationship in said groove (100,101) and being angularly spaced from said spacer hole (14) substantially the same as said predetermined angular degree, whereby said lateral housing and spacer holes (12,14) are alignable by sliding said spacer means (C) into said housing (B) with said guide projection (102,103) received in said groove (100,101).
13. The magnet assembly of claim 12 wherein said groove (100,101) has a narrow portion intersecting said inner peripheral surface and has an enlarged portion within said housing periphal wall, said spacer means (C) including an electro-magnetic coil (94) having lead wires (110,113) extending through said enlarged portion of said groove (100,101) to the exterior of said housing (B).
14. The magnet assembly of claim 12, or 13 including a tube (115) of ferromagnetic material closely received in said enlarged portion of said groove (100,101). said lead wires (110-113) extending through said tube (115).
15. The magnet assembly of claim 12, 13 or 14 including connecting means for connecting said guide projection (102,103) with said tube (115).
Priority Applications (4)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| DE69116172T DE69116172T2 (en) | 1991-04-12 | 1991-04-12 | PERMANENT MAGNET ARRANGEMENT |
| EP91907658A EP0579598B1 (en) | 1991-04-12 | 1991-04-12 | Permanent magnet assembly |
| CA002108157A CA2108157C (en) | 1991-04-12 | 1991-04-12 | Permanent magnet assembly |
| PCT/EP1991/000716 WO1992018993A1 (en) | 1991-04-12 | 1991-04-12 | Permanent magnet assembly |
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA002108157A CA2108157C (en) | 1991-04-12 | 1991-04-12 | Permanent magnet assembly |
| PCT/EP1991/000716 WO1992018993A1 (en) | 1991-04-12 | 1991-04-12 | Permanent magnet assembly |
Publications (2)
| Publication Number | Publication Date |
|---|---|
| CA2108157A1 CA2108157A1 (en) | 1992-10-13 |
| CA2108157C true CA2108157C (en) | 2000-07-18 |
Family
ID=25676726
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA002108157A Expired - Fee Related CA2108157C (en) | 1991-04-12 | 1991-04-12 | Permanent magnet assembly |
Country Status (2)
| Country | Link |
|---|---|
| CA (1) | CA2108157C (en) |
| WO (1) | WO1992018993A1 (en) |
Families Citing this family (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| FI93060C (en) * | 1992-07-03 | 1995-02-10 | Picker Nordstar Oy | Apparatus and method for magnetic resonance imaging |
| US5978694A (en) * | 1997-02-27 | 1999-11-02 | Uri Rapoport | Method and apparatus for detecting a magnetically responsive substance |
Family Cites Families (5)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US3611223A (en) * | 1968-09-11 | 1971-10-05 | Hitachi Ltd | Magnetic apparatus for producing homogeneous field |
| US4707663A (en) * | 1985-08-15 | 1987-11-17 | Fonar Corporation | Nuclear magnetic resonance apparatus using low energy magnetic elements |
| US4675609A (en) * | 1985-09-18 | 1987-06-23 | Fonar Corporation | Nuclear magnetic resonance apparatus including permanent magnet configuration |
| US4692732A (en) * | 1986-05-30 | 1987-09-08 | The United States Of America As Represented By The Secretary Of The Army | Remanence varying in a leakage free permanent magnet field source |
| US4998976A (en) * | 1987-10-07 | 1991-03-12 | Uri Rapoport | Permanent magnet arrangement |
-
1991
- 1991-04-12 WO PCT/EP1991/000716 patent/WO1992018993A1/en active IP Right Grant
- 1991-04-12 CA CA002108157A patent/CA2108157C/en not_active Expired - Fee Related
Also Published As
| Publication number | Publication date |
|---|---|
| CA2108157A1 (en) | 1992-10-13 |
| WO1992018993A1 (en) | 1992-10-29 |
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| EEER | Examination request | ||
| MKLA | Lapsed |