US3223897A

US3223897A - Apparatus for adjusting the configuration of a magnetic field

Info

Publication number: US3223897A
Application number: US310938A
Authority: US
Inventors: Ralph T Sullivan
Original assignee: Varian Associates Inc
Current assignee: Varian Medical Systems Inc
Priority date: 1963-09-23
Filing date: 1963-09-23
Publication date: 1965-12-14
Anticipated expiration: 1982-12-14
Also published as: DE1439896A1; GB1058388A; DE1439896B2

Description

United States Patent 3,223,397 APPARATUS FOR ADJUSTING THE (IUNFIGU- RATIQN OF A MAGNETIC HELD Ralph T. Sullivan, San Mateo, Calif., assignor to Varian Associates, Palo Alto, Caliitl, a corporation of California Filed Sept. 23, 1363, Ser. No. 310,338 11 Claims. (Q1. 317-458) This invention relates to magnets, and in particular to an improved magnet structure having adjustable means for providing a magnetic field with a high degree of homogeneity.

In many instances, it is desirable to utilize magnets that afford high intensity magnetic fields which are homogeneous over a substantially large area. For example, in some types of magnetic resonance apparatus it is necessary that a homogeneous and uniform magnetic field be employed. Also, in some apparatus it may be necessary to introduce flux gradients in an air gap of a magnet. It would be advantageous if the flux pattern associated with a magnet could be controlled and varied by simple adjustment means, whereby the configuration of flux lines can be shaped to accommodate changing conditions.

An object of this invention is to provide a novel and improved adjustable magnet.

Another object of this invention is to provide a magnet having simple and inexpensive means for controllably varying the configuration of a magnetic field.

In accordance with this invention, an adjustable air gap magnet comprises spaced magnetic pole member structures of opposing polarity that encompass a nonmagnetic gap. Each pole member includes a magnetic pole piece and a magnetic pole cap, the pole piece having a recessed portion adjacent to the pole cap so as to form an air gap therebetween. A flexible magnetic shaping disk is located within each of such air gaps formed in the pole members, and each disk is attached to a control rod adapted to move axially relative to the pole member. Movement of the control rod causes the related magnetic disk to bend against fulcrum points, thereby changing the shape of the disk. The change of disk shape effectively alters the magnetic lines of force. Since the control rod is Vernier controlled and the disk shape may be varied by small degrees, the field flux lines may be changed accordingly in minute fashion.

In another embodiment of the invention, the magnetic pole pieces employ a plurality of adjustable magnetic members and rings that may be moved independently and axially relative to the central axis defined by the pole members, so that the flux paths adjacent to the pole caps are varied and realigned to a desired pattern.

The invention will be described in greater detail With reference to the drawing in which:

FIG. 1 is a front view of one type of magnet apparatus that may employ the inventive structure;

FIG. 2 is a fragmentary cross-section of a pole member of the magnet apparatus, incorporating this invention;

FIG. 3 is a fragmentary cross-section of an embodiment of the invention, taken along line 33 of FIG. 1;

FIG. 4 is a plot of characteristic curves illustrating the change in field configuration that may be realized by means of the inventive structure; and

FIG. 5 is a fragmentary cross-section of an alternative embodiment of this invention.

Similar numerals refer to similar parts throughout the drawing.

With reference to FIG. 1, a magnet apparatus 1%) which may be of the electromagnet type includes a magnetic yoke 12 made from highly permeable material. The magnet supports two magnetic pole members 14 spaced in coaxial relation and fastened to the yoke 12 by bolts or screws for example. Electrical coil assemblies 16 are mounted on the pole members 14 whereby the poles are polarized oppositely, in a well known manner. The coil assemblies 16 are secured to side members 18 of the yoke 12 that are rigidly fixed and braced by cross members 20. A mounting platform or stand 22 and support legs 24 ensure physical stability of the magnet apparatus.

A pair of magnetic pole caps 26 at the inner ends of the pole members 14 are joined by threading, for example, to magnetic pole pieces 28 which are disposed coaxially with relation to the caps 26 in the magnet structure 10. The magnet provides a high intensity magnetic field concentrated in a nonmagnetic gap located between the pole caps 26. The high intensity field magnet may be employed in magnetic resonance spectroscopy applications, such as the analysis of a sample, where the sample is contained in a probe assembly that is inserted between the pole caps 26 within the magnetic field. It is highly desirable that the configuration of magnetic flux lines established between the pole caps 26 should be as homogeneous and uniform as possible, especially in the area centrally disposed between the pole caps 26.

In accordance with an embodiment of this invention, a pole member 14 comprises a cylindrical pole cap 26 having a tapered end and a pole piece 28 that is joined to a rear surface of the pole cap 26, as represented in FIG. 2. At the end adjacent to the pole cap 26, the integral pole piece 28 has a recess 30 formed by a first annular portion 32 and a smaller diameter tubular portion 34 coaxially positioned and connected by a shoulder or annular section 36. The recess 30 encloses an air gap that is disposed closely adjacent to the rear planar surface of the pole cap 26.

Within the air gap associated with each pole member 14, there is a flexible magnetic shaping disk 3-8, made from a thin steel diaphragm for example, having an overall diameter greater than that of the tubular portion 34 of the pole piece 28. The shaping disk 38 has a protruding annular section 40 with a central aperture which accommodates one end of a control rod 42 that is joined in fixed relation to the disk 38. The rod 42 may be advanced or retracted through a centrally disposed bore in the pole member 14 by rotation of a plug or nut 44 that engages the other end of the rod 42 by threaded means for example. When the control rod 42 is retracted, the shaping disk 33 is forced against the shoulder 36, which has an end or corner that acts as a fulcrum (point A), causing a bending or deformation of the disk 38. The shape of the disk 38 becomes arcuate or curvilinear, and forms a meniscal magnetic body within the air gap. This cambering effect results in a change of the magnetic flux lines associated with the pole cap 26. By varying the shapes of the disks 38, associated with each pole cap 26, the magnetic field between the pole caps 26 may be controllably varied.

In a specific embodiment of the invention, as illustrated in FIG. 3, a magnet pole member includes a pole piece 46 and associated pole cap 48 joined to the pole piece which has a circular recess at its end, thus forming an air gap therebetween. A relatively thin magnetic disk 50, which may be a steel diaphragm plate about .187 inch thick and approximately 6.980 inches in diameter, is positioned within the air gap. The air gap also contains a multiplicity of ferrules or concentric steel rings 52 of varying diameter seated in a fixed manner at the end of the pole piece 46. Each ring 52 is characterized by a salient or domed portion 54, at which point the disk 50 is deformed when forced into contact with the rings 52. In effect, when the disk 50 is moved away from the pole cap 48 toward the ring 52,'a plurality of fulcrums act to change the shape of the disk 50 at selected spaced points.

The number, shapes and diameters of the rings 52 may be selected to provide various desired changes in the magnetic field. Similarly, the face of the shaping disk 50 may be convex, concave, flat, or any shape that serves to provide a desired field configuration.

To accomplish movement of the disk 50 by extremely small degrees along the common axis defined by the pole piece 46 and pole cap 48, whereby minute changes in the magnetic field configuration may be controllably achieved, a control member 56 constituting a drawbar or rod that is adapted to be advanced or retracted by rotation is provided. The rod 56 is joined to an annular extension 58 of the disk 50 by threaded engagement, and is prevented from rotating relative to the disk by a set screw 60, which locks the rod in place when the rod is bottomed in the disk extension 58. The control rod 56 has another threaded portion 62 which accepts a differential screw 64 that is threaded along both its inner and outer cylindrical surfaces. The inner cylindrical surface, which may be /2 inch in diameter ha thirteen threads per inch, and the outer cylindrical surface which may be 78 inch in diameter has fourteen threads per inch, by way of example. The outer surface of the differential screw 64 engages a threaded inner portion of the pole piece structure 46, and may be rotated relative to the pole piece by means of a pair of slots 66 formed at an end of the differential screw 64. The two slots 66 are shaped to seat a pair of prongs or tines 68 that form part of an adjustment tool 70 extending through an inner hollow passage leading to the exterior of the magnet structure. The adjustment tool '70 comprises a hollow sleeve ,72 and an adjustment knob 74 that projects from the magnet structure, thereby providing easy access during the adjustment process. The sleeve 72 and knob 74 are joined by pins 76 to form an integral structure whereby the sleeve and knob may move in unison. The adjustment tool 70 and its sleeve 72 fit closely within a retaining structure 78, and is mounted for easy rotation for example, on a bearing means (not shown).

During the adjustment process, when it is desired to change the magnetic field configuration to provide relatively flat flux lines in the central area between the pole caps 48, the adjustment knob 74 is rotated while the prongs 68 are engaged with the slots 66, causing the differential screw 64 to turn. Because the threading ratio at the outer surface is greater than that of the inner cylindrical surface of the screw 64, a relatively small longitudinal displacement of the control member 62 occurs. When rotating the adjustment knob in one direction, such as clockwise, the axial displacement of the member 62 retracts the annular extension 58 of the disk 50 so that a concavity is formed at the disk center. Concurrently, concentric areas of the disk 50 are forced against the fulcrum points established by the rings 52 causing a deformation and reshaping of the disk, thereby changing the flux distribution in the vicinity of the pole cap 48. The changes in the flux lines and field configuration may be extremely small since the displacement of the rod 56 and of the central portion of the disk 50 are also minute. For example, with the inventive structure it has been possible to obtain a rod displacement of .0077 inch for a 360 rotation of the adjustment knob 74, with resultant changes in magnetic field strength of substantially less than 0.5 gauss in the area of interest between the pole caps. Also, in the field of gyromagnetic resonance, stability in the order of 2 in 10 parts of the sample being detected has been achieved in accordance with this invention. Basically, the fine vernier mechanical adjustment provided allows the gradients in the selected area of the magnetic field to be flattened or minimized such that a a homogeneous and uniform flux surrounds the sample under analysis.

Furthermore, thermal stability is realized because the control rod 56 is effectively shortened in length by the use of the differential screw 64 linking the rod to the adjustment tool 70, in lieu of utilizing a continuous integral shaft or drawbar for retracting the disk 50. The shorter length when considered with the low coefficient of expansion (about 6.3 10 F.) for the mild steel material used for the control rod 56 afford negligible change in the dimensions of the control rod so that there is no erratic or spurious change in the shape of the disk 50 when the ambient temperature varies. In addition, cooling by fluids and feedback systems may be provided to control temperature effects.

FIG. 4 depicts a plot of curves representing actual measurements taken of the field intensity (H) measured in gauss against the distance (d) in inches along the median plane between the pole caps starting from the center point or the central axis of the pole members, with a nonmagnetic gap about inch wide between the pole caps. A magnetic field of 14,100 gauss was employed with a combination such as exemplified in FIG. 3 and variations in field intensity were noted at discrete intervals from the center for different shapes of the disk. Typical curves are set forth in FIG. 4 to show the effect of disk shaping on the magnetic field.

The broken line curve illustrates the flux distribution with a magnet apparatus that does not utilize the instant invention. With the shaping disk in an optimum retracted position in the air gap behind the pole cap, a flat flux distribution curve 82 was obtained over a relatively large distance. With a maximum retraction of the shaping disk, a curve 84 was obtained having an increased slope in the central region. Thus it is seen that field intensity changesi n milligaus may be effected to provide various field shapes as well as a relatively fiat flux path over a relatively large area, by means of this invention.

In FIG. 5, an alternative embodiment of the invention employs an adjustable pole piece ring 86 and adjustable magnetic body or core 88 which may be moved axially relative to a magnetic pole cap 90 to redistribute the flux associated with such pole cap. The pole cap 90 is joined to an outer ring 92 that serves as a magnetic pole piece, and the adjustable ring or magnetic sleeve 86 is seated against such fixed ring 92 in a sliding relationship. The magnetic core 88 is a solid cylinder that fits closely within the sleeve 86 and is adapted to be moved axially independently of the sleeve 86 by means of a knob (not shown). Alternatively, the sleeve 86 may be in threaded engagement with the fixed ring 92 and the core 88, and may be controllably adjusted by well known means. The outer pole piece ring 92 has an indented portion or lip 94 whereby an air gap is formed between the pole cap 90 and the fixed pole piece 92. The air gap affords adjustments in the vicinity of the rear of the pole cap 90, by moving the adjustable ring 86 and core 88 axially relative to the pole cap. The available adjustments serve to vary the configuration of the air gap behind the pole cap thereby changing the paths of the flux lines associated with such paths of the flux lines associated with such pole cap. It is understood that a multiplicity of adjustable magnetic pieces may be used adjacent to the rear surface of the pole cap to control the flux distribution between the pole caps of a magnet.

It should be noted that variations in component dimen sions of the mechanism, pole cap configuration, field strength and metallurgical characteristics of materials may affect the performance of the mechanism, and therefore the flux density level of the various components should be optimized.

It is to be noted that the scope of the invention is not limited to the particular configurations and values described above. The inventive concept encompasses the use of magnet pole members wherein an air gap is formed between the pole piece and pole cap of each member, and means for varying the flux path distribution to the rear surface of the pole cap effectively varies the flux distribution within the air gap area.

What is claimed is:

1. An adjustable magnet comprising:

a pair of magnetic pole members spaced in opposition and defining a nonmagnetic gap, each member comprising a magnetic pole piece and a magnetic pole cap joined to said pole piece, and a recessed portion formed in said pole piece and forming an air gap between said pole piece and said .pole cap;

a magnetic disk disposed in each such air gap; and

adjustable means coupled to each said disk and disposed within each said member, and means providing a fulcrum within the recessed portion of each pole piece for changing the shape of each such disk to vary the magnetic field between the pole members.

2. An adjustable magnet as in claim 1, wherein the shape changing means comprises a control bar disposed in a central bore of each pole member and coupled to said disk and rotatable means for advancing and retracting each said bar along the axis of its respective pole member.

3. An adjustable magnet as in claim 2, wherein a threaded means engages a threaded portion of each control bar to enable vernier controlled movement of the control bar.

4. An adjustable magnet as in claim 3, wherein said threaded means comprises a differential screw threadedly engaged with said pole piece and with said control bar.

5. An adjustable magnet as in claim 4, wherein said difierential screw has a slotted end; further including a means for engaging said slotted end to rot-ate said screw disposed in said central bore.

6. An adjustable magnet as in claim 2, wherein said shape changing adjustable means includes a plurality of fulcrum points disposed in each such air gap adjacent to each of said pole pieces for changing the shape of each said disk when its respective control bar is moved axially.

7. An adjustable magnet as in claim 2, wherein said shape changing means includes an adjustment tool coupled to each said control bar by a differential screw means for shortening the effective length of the control bar and wherein said bar is formed from a material having a low coefficient of expansion to achieve thermal stability.

8. In a magnet structure, the combination comprising:

opposing pole members, each member having a magnetic pole piece;

a magentic pole cap joined to one end of each said pole piece, each said pole piece having a recess in the surface joined to each said pole cap such that an air gap is formed adjacent to each said pole cap, each said recess having a stepped configuration;

a magnetic body disposed within each said recess; and

means coupled to each said body, and fulcrum providing means within the recessed portion of each pole piece for changing the shape of each such magnetic body whereby the path of the flux lines between the pole caps is varied.

9. A magnet structure as in claim 8 wherein each said magnetic body comprises a flexible disk, and the shape changing means includes a drawbar attached to such disk.

10. A magnet structure as in claim 8 wherein each said recess includes a shoulder section for providing a fulcrum against which each said body may be deformed.

11. In a magnet structure having a magnetic pole member comprising:

a magnetic pole cap;

a hollow cylindrical magnetic pole piece secured to said cap; and

an adjustable magnetic sleeve disposed within the hollow portion of said pole piece for movement along the axis of said pole cap; and a cylindrical magnetic core disposed within said magnetic sleeve, said core adapted to move axially relative to said pole cap and independently of said sleeve.

References Cited by the Examiner UNITED STATES PATENTS 3,017,544 1/1962 Kane et al 317158 3,018,422 1/1962 Seaton 317-158 3,182,231 5/1965 Gang et a1. 317158 ROBERT K. SCHAEFER, Primary Examiner.

JOHN F. BURNS, Examiner.

LARAMIE E. ASKIN, Assistant Examiner.

Claims

1. AN ADJUSTABLE MAGNET COMPRISING: A PAIR OF MAGNETIC POLE MEMBERS SPACED IN OPPOSITION AND DEFINING A NONMAGNETIC GAP, EACH MEMBER COMPRISING A MAGNETIC POLE PIECE AND A MAGNETIC POLE CAP JOINED TO SAID POLE PIECE, AND A RECESSED PORTION FORMED IN SAID POLE PIECE AND FORMING AIR GAP BETWEEN SAID POLE PIECE AND SAID POLE CAP; A MAGNETIC DISK DISPOSED IN EACH SUCH AIR GAP; AND ADJUSTABLE MEANS COUPLED TO EACH SAID DISK AND DISPOSED WITHIN EACH SAID MEMBER, AND MEANS PROVIDING A FULCRUM WITHIN THE RECESSED PORTION OF EACH POLE PIECE FOR CHANIGN THE SHAPE OF EACH SUCH DISK TO VARY THE MAGNETIC FIELD BETWEEN THE POLE MEMBERS.