Patent Description:
Turbomolecular pumps are often employed as a component of the vacuum system used to evacuate devices such as scanning electron microscopes (SEMs) and lithography devices.

It is common for turbomolecular pumps to comprise an oil free, passive permanent magnetic bearing arrangement, located in the high vacuum end of the pump, to provide a substantially friction free, dry bearing arrangement free of lubricating materials that might otherwise cause contamination in the evacuated volume.

As described in <CIT>, known arrangements of passive permanent magnetic bearings employ a plurality of individual axially stacked ring magnets. The bearing arrangement comprises a series of three individual permanent magnet rings fixed to the pump housing surrounded concentrically by a series of three individual permanent magnet rings which are fixed to, and rotate with, the rotor arrangement about the axis. In another bearing arrangement, an array of four outer rotating permanent magnet rings and an array of four inner nonrotating permanent magnetic rings are arranged such that the outer, rotating, array surrounds the inner, static, array in a concentric manner. The magnets are all formed of rare earth magnetic material, such as samarium-cobalt. The outer array is attached to the rotor of a turbomolecular pump with the static array attached to the stator of the pump. For reasons of mechanical strength and practical construction, it is normal for the outer array of rings to form the rotating part of the bearing arrangement and the inner rings to form the stationary part.

In this example the magnetisation (that is, the polarization) of the magnetic rings in each array respectively is substantially aligned with the axis of rotation of the pump rotor.

The magnets are arranged within each array such that they are in mutual repulsion with each other; that is proximate magnets in an array meet their nearest neighbouring magnet in the same array with the same pole (e.g. magnets meet each other with their south pole and the outer magnetic rings in each array have their north poles facing outermost) and therefore create an almost frictionless bearing.

A great many other configurations are possible, using different numbers of rings, with axial or radial magnetisation, and arranged for either repulsive or attractive forces between rotor and stator. Although a variety of configurations are possible, they all perform optimally when the direction of magnetisation in the rings is perfectly symmetrical with respect to their rotational axis.

However, producing such magnets is problematic.

<CIT> describes a monolithic permanent magnet, in particular for a permanent magnet bearing of a vacuum pump. The monolithic permanent magnet has a longitudinal axis and a plurality of sections magnetised at least approximately axially, the magnetisations of which sections are each aligned alternately opposite one another.

It is desired to provide an improved technique for producing such magnets.

According to a first aspect, there is provided a magnetiser apparatus, comprising: a first plurality of rings, each ring having a longitudinal axis, the first plurality of rings being axially-aligned and stacked along the longitudinal axis, each ring having a first face shaped to fit with an adjacent first face of a ring of magnetic material; and a current source operable to provide a current to each ring to generate a magnetic field to magnetise corresponding portions of the ring of magnetic material. The first aspect recognizes that reliably producing magnets for use in a magnetic bearing which has reduced stray transverse fields can be problematic. Typically these are formed by stacking individual magnets in a way which attempts to reduce the stray magnetic field, but this is complex and time consuming. Accordingly, an apparatus may be provided. The apparatus may be a magnetising apparatus. The magnetising apparatus may comprise two or more rings, loops or hoops. Each ring may have a longitudinal axis extending in an axial direction. The rings may be axially or concentrically aligned and stacked or assembled along the longitudinal axis. Each ring may have a face or surface which is shaped to fit or match with an adjacent face of a ring, loop or hoop of magnetic material. The apparatus may comprise a current source or supply which may provide a current to each ring of the apparatus which generates a magnetic field emanating from that ring of the apparatus which magnetises a portion of the ring of magnetic material through which that magnetic field passes. In this way, a ring of magnetic material can be magnetised using different magnetic fields from the different rings of the apparatus in order to create a ring of magnetic material with different axial portions of that ring of magnetic material magnetised in different ways. This helps to provide a more reliable ring magnet having differently magnetised axial portions, which reduces the stray field and ameliorates the need to construct such a magnet from separate ring portions.

In one embodiment, each ring has a pair of axial end-faces and each ring is orientated to position adjacent axial end-faces parallel with respect to each other. Accordingly, the rings may be stacked such that the axial or annular faces are parallel to each other in order to better align the respective magnetic fields.

In one embodiment, each ring has a pair of axial end-faces and each ring is orientated to position adjacent axial end-faces normal with respect to the longitudinal axis. Accordingly, the axial or annular faces of each ring may be located on a plane which extends transversely to the longitudinal axis.

In one embodiment, each ring is stacked to share a common longitudinal axis. Accordingly, a centreline of each ring may be aligned along the same longitudinal axis so that each ring is concentric or coaxial.

In one embodiment, each ring is spaced apart from an adjacent ring along the longitudinal axis to prevent touching. It will be appreciated that an insulator may be provided between each axial or annular end face to help prevent touching or contact between the rings.

In one embodiment, the current source is operable to provide the current to each ring to generate the magnetic field to create a dipole orientated along the longitudinal axis in each corresponding portion of the ring of magnetic material. Accordingly, the magnetic fields generated by each ring may be aligned to generate a corresponding dipole in a corresponding portion of the ring of magnetic material which extends in the axial direction.

In one embodiment, the current source is operable to provide the current which flows circumferentially around each ring. Accordingly, the current may flow around each ring.

In one embodiment, the current source is operable to provide the current to each ring to generate the magnetic field to create opposing dipoles in adjacent portions of the ring of magnetic material. Accordingly, adjacent rings of the apparatus may be arranged to support a current flow in opposing directions in order to generate a magnetic field which produces opposing dipoles in adjacent portions of the ring of magnetic material.

In one embodiment, the current source is coupled with the first plurality of rings to provide the current which flows in opposition directions circumferentially around adjacent rings.

In one embodiment, adjacent faces of the first plurality of rings are parallel.

In one embodiment, at least one of each ring and the ring of magnetic material is cylindrical.

In one embodiment, each ring is a slotted cylinder.

In one embodiment, each ring comprises an incomplete annulus defining a single turn.

In one embodiment, the incomplete annulus is C-shaped, having facing terminating ends.

In one embodiment, the facing terminating ends are spaced apart to prevent touching.

At least one of the rings has a length along the longitudinal axis which differs from other of the first plurality of rings. By changing the length or height of the rings, the dimension of the corresponding portion and the length or height of the dipole in the ring of magnetic material can be adjusted in the longitudinal dimension.

In one embodiment, the first plurality of rings have a pair of axial end rings.

In one embodiment, the axial end rings have a length along the longitudinal axis which differs from other of the first plurality of rings.

In one embodiment, the axial end rings have a greater length along the longitudinal axis than other of the first plurality of rings.

In one embodiment, the current source is coupled with a respective terminating end each of the pair of axially end rings.

In one embodiment, the current source comprises a plurality of current sources coupled with a respective one of the first plurality of rings. Accordingly, each of the rings of the apparatus may be coupled with its own current source.

In one embodiment, the first plurality of rings are arranged electrically in series with the current source. Accordingly, the rings may connected together in series.

In one embodiment, the apparatus comprises inter-ring conductors coupled between terminating ends of adjacent rings and operable to supply current between the adjacent rings. Accordingly, the ends of adjacent rings may be connected together using conductors which connect between terminating ends of those rings.

In one embodiment, the inter-ring conductors are located to extend from other than the first face.

In one embodiment, the inter-ring conductors have an axial portion extending between adjacent rings.

In one embodiment, the inter-ring conductors have radial portions extending from other than the first face of adjacent rings to the an axial portion extending between adjacent rings.

In one embodiment, the termination ends of each ring are circumferentially-aligned.

In one embodiment, the apparatus comprises a second plurality of rings located concentrically with respect to the first plurality of rings to define a void therebetween shaped to receive the ring of magnetic material.

In one embodiment, each ring of the second plurality of rings has a second face shaped to fit with an adjacent second face of the ring of magnetic material.

In one embodiment, the current source is coupled with the second plurality of rings to provide the current which flows in opposition directions circumferentially around adjacent rings.

In one embodiment, the current source is coupled with the second plurality of rings to provide the current which flows in opposition directions to current flow in adjacent rings of the first plurality of rings.

In one embodiment, adjacent faces of the second plurality of rings are parallel.

In one embodiment, at least one of the rings has a length along the longitudinal axis which differs from other of the second plurality of rings.

In one embodiment, the second plurality of rings have a pair of axial end rings.

In one embodiment, the axial end rings have a length along the longitudinal axis which differs from other of the second plurality of rings.

In one embodiment, the axial end rings have a greater length along the longitudinal axis than other of the second plurality of rings.

In one embodiment, the current source comprises a plurality of current sources coupled with a respective one of the second plurality of rings.

In one embodiment, the second plurality of rings are arranged electrically in series with the current source.

In one embodiment, the apparatus comprises inter-ring conductors coupled between terminating ends of adjacent rings and operable to supply current between the adjacent rings.

In one embodiment, the inter-ring conductors are located to extend from other than the second face.

In one embodiment, the inter-ring conductors have radial portions extending from other than the second face of adjacent rings to the an axial portion extending between adjacent rings.

In one embodiment, the termination ends of each ring of the first plurality of rings and the termination ends of the second plurality of rings are circumferentially-aligned.

According to a second aspect, there is provided a method, comprising: locating a ring of magnetic material with the magnetiser as claimed in any preceding claim and magnetising the ring of magnetic material by operating the current source.

In one embodiment, the method comprises rotating the ring of magnetic material circumferentially about the longitudinal axis and remagnetising the ring magnet by operating the current source.

Before discussing the embodiments in detail, first an overview will be provided. Embodiments provide an apparatus which operates to produce a magnet from a ring, cylinder or annulus of magnetic material. A set of rings are stacked or arranged on top of each other. The magnetic material is placed proximate the rings. A current is generated to flow in each of the rings, which creates a magnetic field which acts to magnetise the magnetic material. An additional set of rings may be provided which are positioned proximate the magnetic material, typically concentrically with respect to the set of rings. These may also receive a current which generates a magnetic field to assist magnetising the magnetic material. Each of the rings is typically physically separated from each other in the vicinity of the magnetic material but may be electrically coupled distal from the magnetic material in order to provide a current path from one ring to another. The axial height of the rings is typically set to create a magnetic field in a corresponding axial portion of the magnetic material. The axially end-most rings are typically of a greater axial height than axially-inner rings. Typically, adjacent rings are configured to support current flow in opposing directions in order to create opposing magnetic fields to create opposing dipoles in the magnetic material.

As can be seen in <FIG>, the magnetic material in this embodiment is shaped as a cylinder <NUM>. The cylinder has a height H1 in the along the longitudinal axis A. The cylinder <NUM> has an inner surface <NUM> defined by an inner radius r1. The cylinder <NUM> has an outer surface <NUM> defined by an outer radius R1.

<FIG> is an isometric view, <FIG> show orthogonal views, <FIG> is a sectional view along D-D, <FIG> is a sectional view along B-B, <FIG> is a sectional view along C-C and <FIG> is a sectional view along A-A illustrating a magnetiser apparatus <NUM> according to one embodiment. The magnetiser apparatus <NUM> comprises a radially outermost set of rings <NUM> and a radially innermost set of rings <NUM>. Each ring is generally shaped as a slotted cylinder and is positioned along a common longitudinal axis A. As can be seen in <FIG>, four rings are provided in the outer set of rings <NUM> and, as can be seen in <FIG>, four rings are provided in the inner set of rings <NUM>. A pair of outer, axially end rings 20A, 20B receive a pair of outer, axially inner rings 20C, 20D. Each of the rings is separated by a non-conductive gap <NUM>. The gap <NUM> may be filled with a suitable insulating material to provide adequate electrical and mechanical separation between each of the rings. Likewise, as can be seen in <FIG>, a pair of inner, axially end rings 30A, 30B receive a pair of inner, axially inner rings 30C, 30D. Each of the rings is separated by a non-conductive gap <NUM>. The gap <NUM> may be filled with a suitable insulating material to provide adequate electrical and mechanical separation between each of the rings. The outer set of rings <NUM> are concentric and coaxially aligned with respect to the inner set of rings <NUM> to share the common longitudinal axis A.

A magnetising void <NUM> is an annular space which extends radially and longitudinally between the inner set of rings <NUM> and the outer set of rings <NUM>. The shape and dimensions of the magnetising void <NUM> are set based on the dimensions of the cylinder <NUM>. In particular, the radially outer surface of each of the inner set of rings <NUM> is defined by a radius r, where r is less than r1. Also, the radially inner surface <NUM> of each of the outer set of rings <NUM> is defined by a radius R, where R is greater than R1. Furthermore, the overall height of the inner set of rings <NUM> and the outer set of rings <NUM> along the longitudinal axis A is dimensioned to be H, where H is greater than H1. This enables the cylinder <NUM> to be placed into and removed from the magnetising void <NUM>.

Each of the outer set of rings <NUM> is slotted which defines a pair of opposing faces 90A, 90B which also extend radially and which defines a gap <NUM> also extending radially and along the longitudinal axis A. Hence, the outer set of rings <NUM> is incomplete. Likewise, each of the inner set of rings <NUM> is slotted which defines a pair of opposing faces which also extend radially and which defines a gap also extending radially and along the longitudinal axis A. Hence, the outer set of rings <NUM> is incomplete.

The outer set of rings <NUM> each have stubs <NUM> which extend radially from a radially outer surface <NUM> of the outer set of rings <NUM>. Each stub <NUM> terminates with a coupling structure which is configured to provide current flow in the required direction in each of the rings, as will now be explained in more detail. In the following description a light arrow indicates the direction of flow of current, ⊙ indicates a current flowing towards the surface, Ⓧ indicates a current flowing away from the surface and a dark arrow indicates the direction of a magnetic field. The stub 110A associated with the axially end ring 20A has a coupling 120A which only connects with that stub 110A. A current source is connected to the coupling 120A, which provides a current flow along the stub 110A and clockwise through the axially end ring 20A to the stub 110B associated with the axially end ring 20A. The coupling 120B couples the stubs 110B of the axially end ring 20A and the axially inner ring 20C. Accordingly, current flows through the coupling 120B along the longitudinal axis A from the axially end ring 20A to the axially inner ring 20C. Current then flows along the stub 110B into the axially inner ring 20C and flows in the anticlockwise direction to its stub 110A. Current then flows radially along the stub 110A into the coupling 120C which couples the stubs 110A of the axially inner rings 120C and 120D. Accordingly, current flows along the longitudinal axis A within the coupling 120C from the stub 110A of the axially inner ring 20C to the stub 110A of the axially inner ring 20D. Current then flows radially inwards along the stub 110A of the axially inner ring 20D and current then flows clockwise around the axially inner ring 20D to the stub 110B of the axially inner ring 20D. The current is then received by the coupling 120D which couples stubs 110B of the axially inner ring 20D with the axially end ring 20B. Current then flows radially inwards along the stub 110B of the axially end ring 20B and anticlockwise around the axially end ring 20B to the stub 110A of the axially end ring 20B. The current then flows to a coupling 120E, which is connected to the other terminal of the current source.

The inner set of rings <NUM> each have stubs <NUM> which extend radially from a radially inner surface of the inner set of rings <NUM>. Each stub <NUM> terminates with a coupling structure which is configured to provide current flow in the required direction in each of the rings, as will now be explained in more detail. The stub 115A associated with the axially end ring 30A has a coupling 125A which only connects with that stub 115A. A current source is connected to the coupling 125A, which provides a current flow along the stub 115A and anticlockwise through the axially end ring 30A to the stub 115B associated with the axially end ring 30A. The coupling 125B couples the stubs 115B of the axially end ring 30A and the axially inner ring 30C. Accordingly, current flows through the coupling 125B along the longitudinal axis A from the axially end ring 30A to the axially inner ring 30C. Current then flows along the stub into the axially inner ring 30C and flows in the clockwise direction to its other stub. Current then flows radially along that stub into the coupling 125C which couples the stubs of the axially inner rings 30C and 30D. Accordingly, current flows along the longitudinal axis A within the coupling 125C from the stub of the axially inner ring 30C to the stub of the axially inner ring 30D. Current then flows radially inwards along the stub of the axially inner ring 30D and current then flows anticlockwise around the axially inner ring 30D to the other stub of the axially inner ring 30D. The current is then received by the coupling 125D which couples stubs of the axially inner ring 30D with the axially end ring 30B. Current then flows radially inwards along the stub of the axially end ring 30B and clockwise around the axially end ring 30B to the other stub of the axially end ring 30B. The current then flows to a coupling 125E, which is connected to the other terminal of the current source.

The current flow generates opposing magnetic fields F1 to F4 in portions of the magnetic void <NUM> between the inner set of rings <NUM> and the outer set of rings <NUM>. This magnetises the corresponding portions of the cylinder <NUM> and creates opposing dipoles D1 to D4.

The magnetic field in the vicinity of the gaps <NUM> will be weaker than elsewhere. Accordingly, after performing an initial magnetisation by supplying current to the inner and outer set of rings <NUM>, <NUM>, the cylinder <NUM> is rotated within the magnetic void <NUM> and current is supplied again to perform a further magnetisation.

Accordingly, an embodiment provides a method for polarising multiple axially opposing poles into a single hollow cylinder of magnetic material. A single conductor is arranged in an array of alternating coil directions for the purposes of magnetising all poles simultaneously. The single conductor can be arranged either as a thick conductor with a single turn per magnetic pole or as a thinner conductor with multiple turns per magnetic pole. An embodiment magnetises axially opposing poles along the length of a single piece of material, solving the problems of dipole alignment, transverse stray field and material handling during assembly into the turbo-pump rotor. An embodiment provides for: minimal axial gap between coils; top and bottom coils thicker than middle coils; and terminations allowing current to pass through the coils in a manner which minimises corruption to the polarising fields. The two coils are arranged in series, with appropriate terminations, allowing current to pass through the coils in a manner which minimises corruption to the polarising fields. The polarising field may be applied multiple times with different relative rotational orientations of the coils and magnetic material (to overcome field asymmetry). A second variant of the coils would use multi-turn rather than single turn coils but in other respects would have a similar general form to the single turn variant.

Claim 1:
A magnetiser apparatus (<NUM>), comprising:
a first plurality of rings (<NUM>), each ring (20A, 20B, 20C, 20D) having a longitudinal axis (A), said first plurality of rings being axially-aligned and stacked along said longitudinal axis, each ring of the first plurality of rings having a first face (<NUM>) shaped to fit with an adjacent first face (<NUM>) of a ring of magnetic material (<NUM>) and wherein at least one of said rings of the first plurality of rings has a length along said longitudinal axis which differs from other of said first plurality of rings; and
a current source operable to provide a current to each ring of the first plurality of rings to generate a magnetic field to magnetise corresponding portions of said ring of magnetic material.