IES930667A2 - Field adjustable magnetic flux sources - Google Patents

Abstract

A permanent magnet structure comprising multiple cylindrical rod-shaped flux sources magnetized perpendicular to their axes where the axes of the cylinders lie on one or more circles around a central passage, such that each rod can be rotated about its axis in either direction through any desired angle, either independently or in conjunction with other rods. Access to the central passage is along the central axis, and optionally in the transverse direction via gaps between the rods or circumferential grooves in their surfaces. Applications include portable variable-field flux sources providing a substantially uniform magnetic field, multipole field or field gradient, educational demonstrations, magnetometers, powder alignment devices, levitation devices, variable electromagnetic generators, variable torque drives and variable eddy-current brakes.

Description

Field adjustable magnetic flux sources Specification: Technical Field: The present invention relates in general to permanent magnet structures which are used to produce intense magnetic fields in confined spaces. Such magnetic flux sources may be incorporated in laboratory and industrial instrumentation, electromagnetic energy converters, particle beam control devices, force and holding devices, and materials processing and separation.

Background of the invention: Conventional devices for producing magnetic fields have relied on current-carrying coils with many windings which can take the form of normal or superconducting solenoids, or else the coils may be incorporated as the energising coils of an electromagnet with massive pole pieces. In any case the coils are bulky, require large ancillary power supplies to generate currents up to 100 amperes or more, and are dependent on appropriate cooling system. It is very difficult or impossible to generate with such devices intense magnetic field, up to one tesla or more, in a volume comparable to the volume of the device itself. Furthermore, complex spatially-varying field patterns such as high magnetic field gradients are difficult or impossible to realise.

Many of these difficulties can be overcome by using modern permanent magnets, which are magnetically equivalent to compact solenoids with enormous surface current densities, of order 10^ amps/metre. The high anisotropy fields and coercive fields of rare earth permanent magnets allow the superposition of the fields from different flux sources. Permanent magnet structures are now able to generate magnetic fields of order one tesla or more.

One such structure is the cylinder described by K. Halbach in the Proceedings of the Eighth International Workshop on Rare-Earth Cobalt Permanent Magnets” published by the University of Dayton, Ohio, USA, 1985 (pp. 123 - 136). A hollow cylindrical field source made up of polygonal or arc segments magnetised in different directions produces a field near the centre of the cylinder which is substantially uniform in magnitude and direction.

S 9 3 Ο 6 6 7 An extension of the Halbach concept to variable magnetic fields by using two cylinders, one nested inside the other, which can be rotated about their common axis has been described by H. Leupold in US patent 4,862,128 of 29 August 1989. Besides offering the possibility of a field that can be varied in magnitude and direction, the Leupold structures can be stacked to provide periodically-varying fields suitable for use in a wiggler or twister.

The permanent magnet structures described by Halbach and Leupold have some practical disadvantages and limitations: The assembly of the multisegmented cylinders from premagnetized segments is difficult because of the strong repulsive forces between them. The dense oriented magnets which have the high remanence and coercivity needed to produce large magnetic fields must be shaped with several different directions of magnetization relative to the form of the segments, and this necessitates the use of multiple dies or wasteful slicing of large blocks. Furthermore access to the field is restricted to the direction along the axis of the cylinder, which is perpendicular to the field.

The permanent magnet structures of the present invention are substantially different from those of the earlier teaching. They involve only solid cylindrical rods magnetised perpendicular to their axis. The structures are much simpler to manufacture and assemble than the hollow multisegmented cylinders previously described. The present structures are capable of producing high uniform or spatially-varying magnetic fields which may be constant in time or time-varying. Furthermore access to the field can be either parallel to the axes of the cylinders, or in a plane containing the central axis of the structure. This plane may include the direction of the magnetic field, or it may be inclined at any angle to it. . . δί306β7 Description of the invention: A primary object of the invention is to realise an easilymanufactured permanent magnet structure where the magnetic flux density (also known as the magnetic field Bo) can be varied continuously from zero to the maximum value for the configuration, in any direction perpendicular to the central axis.

Another primary object of the invention is to permit substantially unimpeded access to the magnetic field from the exterior, including access in directions parallel or perpendicular to the magnetic field.

Another primary object of the invention is to achieve a variable and spatially-varying magnetic field, such as a magnetic field gradient.

The present invention consists of a plurality of solid cylindrical rod-shaped flux sources being substantially composed of permanent magnet material which is magnetised perpendicular to the axis of the rod. The rods are arranged with their centres on one or more circles. The structure is constructed so that each rod may be rotated clockwise or anticlockwise about its axis through any desired angle. The rods may be rotated independently, or their rotations may be correlated by mechanical or electronic means.

The resulting magnetic field in the space surrounded by the rods is the vector sum of the fields produced by the individual rods. The field may be modified by rotating the rods about their axes.

An advantageous feature of the present invention is that access to the field is possible not exclusively in directions close to the central axis of the structure, but also in perpendicular directions which lie in planes which may be parallel, perpendicular or at any other desired angle to the magnetic field. The access may be achieved through gaps between the cylindrical rods, or by means of circumferential grooves in the surface of the cylinders.

In a preferred embodiment of the present invention, an even number of rotatable cylindrical magnetic rods are symmetrically arranged about a common central axis. The preferred number of rods is 4, 6, 8, 10, 12, 14 or 16.

When the number of rods is 6, 10 or 14, the rotations of the rods may be correlated by a system of interlocking cogs. It is possible S 9 3 ft fi * 7 to mould or form each magnetic rod to incorporate the necessary cogs and axle.

In another preferred embodiment of the invention, two structures of rotatable cylindrical magnetic rods are concentrically arranged about a common central axis, one within the other. Each structure is capable of producing the same maximum field in the central space, and this field can be varied, reduced to zero and reversed by suitable rotations of the rods.

In another preferred embodiment of the invention, two structures of rotatable cylindrical magnetic rods are coaxially arranged about a common central axis, one beside the other. A magnetic field gradient is created in the vicinity of the midpoint of the structure, which may be varied by rotating the rods.

Detailed description and examples: The invention will be appreciated from the following detailed description, with reference to the appended drawings. The scope of the invention will be appreciated from the following examples of its use.

Example 1: One embodiment of the invention is illustrated in figure la). Here six cylindrical rods, 101, each magnetised perpendicular to its axis, are arranged in such a way that opposite pairs of rods, 102, have their magnetisation oriented parallel. The maximum field is obtained in the direction indicated by the grey arrow 103, with the orientations shown figure la. Here the angle θ between the direction of magnetisation of the cylinders and the field is twice the angle φ between the direction of magnetisation of the cylinder and the radius from the centre of the array to the axis of the cylinder.

In figure lb the six cylinders have been rotated to reduce the magnitude of the field 104 while maintaining its direction. In figure lc the six cylinders have been rotated to reduce the magnitude of the field in the centre essentially to zero. Note that opposite pairs are now oriented antiparallel (105).

In figure 2 the angular rotation of the cylinders are correlated by cogs, 201, on the axes of the cylinders. As any cylinder, 202, is rotated through an angle a , its two neighbours 203 and 204 rotate Ε 9 J ft $ ς * Ζ through an angle -a. In this way the field 205 in the centre of the structure is maintained substantially uniform and in the same direction, but its magnitude and sign may be varied at will.

One way of manufacturing the required structure is to form the magnetic cylinders with cogs, 301, and central pins, 302, at each end which locate in holes, 303, in a top and bottom plate (304 and 305), as illustrated on the exploded view of figure 3. By rotating the shaft of one of the magnetic cylinders, using a knob 306 for example, the variable field is achieved.

Example 2 : Another embodiment of the invention which improves the uniformity of the field throughout the central space involves two concentric structures each of which can produce the same maximum field at the centre. The radius of the cylinders should be substantially proportional to the exponential of the radius of the circle on which their axes lie.

Example 3: In figure 4 two similar structures, 401 and 402, as described in example 1, are arranged coaxially so that their respective fields, 403 and 404, produced at the centre of each structure are of opposite direction. This then ensures that a magnetic field gradient exists along the common axis 405, near the mid point 406. The magnitude of this field gradient can be varied by rotating the cylinders as described in example 1.

Example 4 : In order to achieve transverse access to the magnetic field, a gap, 501, may be created by separating the cylindrical rods as shown on figure 5a, or one or several circumferential grooves, 502, may be incorporated, as in figure 5b.

This will allow direct and unobstructed optical or mechanical passage, 503 and 504, into the magnetic field, and the field may be oriented in any desired direction relative to the axis of passage.

It is also possible to arrange for variable multipole fields in the central space by appropriate rotation of the cylinders axes. $930667 We now describe some useful practical applications of the fieldadjustable magnetic flux sources of the invention.

One useful feature is that the usable volume of the magnetic field is comparable to the volume of the whole device. No external power source or cooling system is required to generate or sustain the field, so the flux sources of the invention are well suited to portable applications and use far from civilisation. For example a portable magnetometer which can be used for field measurements of rock or soil magnetisation using the variable magnetic field of the invention. The specimen drops along the axis of the magnet and induces an e.m.f. in pick-up coils proportional to its magnetic moment.

The variable magnetic field generator may also be used for a variety of educational experiments demonstrating magnetic field and magnetic field-gradient dependent phenomena, such as electromagnetic induction, Hall effect, Faraday effect, Nernst effect, magneto-optic Kerr effect, magnetic hysteresis, magnetic susceptibility, magnetostriction, magnetoresistance, magnetic phase transition etc..

Many practical industrial devices exploiting these effects in conjunction with the variable flux sources of the invention can also be conceived: For example, an electromagnetic drive with permanent magnet 25 excitation and electronic commutation can be produced to deliver variable torque by adjusting the excitation field by rotating the cylinders of the flux source of the invention.

As another example, an electromagnetic generator can be realised which will deliver a regulated output by adjusting the excitation field by rotating the cylinders of the flux source of the invention, so as to compensate for a change of speed of the rotation of the generator’s windings. A useful application of this can be to wind driven generators which have hitherto been forced to shut down at excessively high or excessively low wind speeds. With the exciting magnetic field being automatically adjusted in response to the measured wind speed or to the output level, it will be possible to operate these wind generators over a much wider range of wind speeds and therefore improve their efficiency. ··«0ββ7 By rotating the magnetic cylinders to produce different multipole arrangements of magnetic fields, it is possible to generate AC current at different multiple frequencies of the frequency of rotation of the windings.

The possibility of access to a transverse magnetic field along a plane can be used in a novel eddy current brake illustrated in figure 6. Here the variable field, 601, is perpendicular to the plane of a rotating conducting disc, 602, which is then braked by the induced eddy current effect. The innovation in this case is the suppression of the coils and generator hitherto needed to produce the field, thus solving the common problem of overheating. The safety of the device is improved by the simplicity of the variable magnetic flux source.

Having shown and described what are considered at present to be the preferred embodiments of the invention, it should be understood that the same have been shown by way of illustration, and not limitation. All modifications, alterations and changes falling within the spirit of the invention are hereby inferred to be included.

Claims (5)

Claims:

1. A permanent-magnet structure of cylindrical rod-shaped flux sources, each permanently magnetised in a direction perpendicular to the axis of the rod, where the rods are arranged so that their axes lie on one or more concentric circles and each rod may be rotated in either direction through any desired angle about its own axis.

2. A symmetrical arrangement of an even number of permanently magnetised rods as in claim 1 where a substantially uniform magnetic field is generated in part of the central space surrounded by the cylindrical rods by suitable rotation of the cylinders. Access to and passage through the field may be achieved through gaps between the rods, or circumferential grooves on the rods or in directions close to the axis of the structure.

3. An arrangement of permanently magnetised rods as in claim 2 where the substantially uniform magnetic field can be varied in magnitude and/or in direction by rotating the cylindrical permanent magnets about their axes.

4. An arrangement of permanently magnetised rods as in claim 1 where the cylindrical magnetic rods are rotated to create a field of substantially quadrupole or hexadecapole character in part of the space surrounded by the rods.

5. An arrangement of permanently magnetised rods as in claim 1 where two similar sets of rods are coaxially arranged and the rods are rotated in such a way as to create a substantially uniform magnetic field gradient in the vicinity of the midpoint of their common axis.