WO2021037471A1

WO2021037471A1 - A method for assembly of a monolithically impregnated cylindrical coil assembly

Info

Publication number: WO2021037471A1
Application number: PCT/EP2020/071414
Authority: WO
Inventors: Yunxin GAO; Edward McCARRON
Original assignee: Siemens Healthcare Limited
Priority date: 2019-08-23
Filing date: 2020-07-29
Publication date: 2021-03-04
Also published as: GB2586505B; GB2586505A; GB201912127D0

Abstract

A method for assembly of a monolithically impregnated cylindrical coil assembly comprising axially alternating coils and spacer rings, comprising the steps of arranging spacer rings on a winding tool in predetermined axial locations, said spacer rings defining coil winding cavities between themselves; winding superconducting wire into the coil winding cavities; impregnating the resulting structure to form the monolithically impregnated cylindrical coil assembly comprising axially alternating coils and spacer rings; and removing the winding tool.

Description

A METHOD FOR ASSEMBLY OF A MONOLITHICALLY IMPREGNATED CYLINDRICAL COIL ASSEMBLY

The present invention relates to the manufacture of cylindrical superconducting magnets. It particularly relates to the manufacture of cylindrical superconducting magnets for MRI systems, although it could be applied to the manufacture of cylindrical superconducting magnets for other applications.

Fig. 1 illustrates a conventional arrangement for manufacturing a cylindrical superconducting magnet in which composite spacer rings are provided, coils of superconducting wire are wound into gaps between the composite spacer rings, and the assembly then impregnated to form a monolithically impregnated coil assembly comprising axially alternating coils and composite spacer rings.

Fig. 1 shows a partial axial cross-section of a winding tool support 20. The structure is essentially symmetrical about axis A-A which is schematically represented in the drawing and extends in the axial direction beyond the view in Fig. 1. Directions parallel to axis A-A will herein be referred to as "axial" and directions perpendicular to the axial direction will be referred to as "radial".

Winding tool support 20 may have an inner radius ri of approximately 500-600mm and an axial length of 1000-1500mm. In the illustrated example, winding tool support 20 has a plain cylindrical inner surface 12 and a stepped cylindrical outer surface 14. Other examples may provide features on the inner surface of the winding tool support 20.

Winding tool support 20 is essentially cylindrical, is essentially symmetrical about axis A-A and is shaped with steps 22, 24, 26. The winding tool support 20 may be divided axially into several parts 20a, 20b etc.

Steps 22, 24, 26 etc. serve to locate spacer rings 32, 34, 36 in correct axial locations on the winding tool support 20. As illustrated in Fig. 1, the steps 22, 24, 26 of winding tool support 20 define locations of spacer rings by interaction with complementary steps 42, 44, 46 in the material of the spacer rings 32, 34, 36. Coils 50, 52, 54 are formed by winding superconducting wire onto the winding tool support 20 into coil winding cavities defined by gaps between spacer rings 32, 34, 36. These spacer rings are retained in position by steps 22, 24, 26; 42, 44, 46. The resultant assembly is resin-impregnated and then the winding tool support 20 is withdrawn, to leave a superconducting coil assembly, monolithically impregnated with a thermosetting resin, comprising axially alternating coils and spacer rings. This conventional arrangement has to cope with manufacturing tolerances in the dimensions of various features. In the illustrated example, axial tolerances derive at least from variation in axial length s between steps in the winding tool support 20 and variation in axial length p of spacer rings 34, 36. These axial variations in dimensions will have the effect of causing variation in axial extent of the finished coils 50, 52, 54, and variation in the axial position of the coils, both of which will affect the field homogeneity of the finished magnet assembly.

Similarly, radial tolerances may cause variation in the outer radius rt of the winding tool support 20 and inner radius rs of spacer rings 32, 34, 36. These radial tolerances together may mean that a radial gap 60 of radial extent g is present between winding tool support 20 and any one or more of spacer rings 32, 34, 36.

Conventionally, such radial gaps 60 have been found tolerable as their maximum radial extent g has been significantly less than the radial extent of wire used to wind the coils 50, 52, 54, so that the coil winding has been unaffected by the radial gaps 60. However, recent developments in superconducting wire have produced wire of reduced thickness, and it has been found in some instances that, during winding of a coil, a turn of wire may move into the radial gap 60, causing a defect in size and shape of the final coil.

The present invention addresses the problem of the radial gap 60 being large enough that a turn of wire may fit within the radial gap 60. The present invention also addresses the problem of axial tolerances which lead to variation in axial extent, and axial position, of the superconducting coils.

The above, and further, objects, characteristics and advantages of the present invention will become more apparent from the following description of certain embodiments, given by way of non-limiting examples only, in conjunction with the accompanying drawing, wherein:

Fig. 1 shows a partial radial cross-section of a conventional arrangement of winding tools, spacer rings and coils;

Fig. 2 shows detail of an axial end-view of a spacer ring;

Fig. 3 shows a partial axial cross-section, similar to that shown in Fig. 1, of an arrangement according to the present invention;

Figs. 4A-4C show alternative forms of radial cut, according to embodiments of the invention;

Fig. 5 shows tooling for coil winding;

Fig. 6 illustrates an arrangement of spacer blocks in defining a coil winding cavity; Fig. 7 shows the retention of spacer rings and spacer blocks by luggage straps or ratchet cargo straps; and

Fig. 8 shows the positioning of prepreg tape over a spacer ring to hold it firmly in position.

According to an aspect of the present invention, a spacer ring and a method for assembly are provided which removes the possibility of a turn of wire moving into a gap between a spacer ring and a winding tool support. This is achieved by providing a spacer ring of adjustable diameter, which is constricted onto the winding tool support, to provide a reduced, and preferably zero-thickness gap 60 between spacer ring and winding tool support.

Fig. 2 shows detail of an axial end-view of a spacer ring 100 of adjustable diameter, according to a feature of an embodiment of the present invention. In this example, an annular spacer ring 100 of a composite material such as resin-impregnated glass fibre has a radial cut 102 made at a certain location in its circumference. The cut goes right through the material of the spacer ring, so that the spacer ring is no longer strictly annular, but is in fact discontinuous, and c-shaped.

Fig. 3 shows a partial axial cross-section, similar to that shown in Fig. 1, of winding tool support 20. A discontinuous spacer ring 100 of adjustable diameter is shown applied over the winding tool support 20. In this embodiment, and preferably, winding tool support 20 has a plain cylindrical radially outer surface, without the steps of the prior art embodiment of Fig. 1. As in the conventional arrangement of Fig. 2, spacer ring 100 is slid over the winding tool support 20. If necessary, the c-shaped spacer ring 100 may be slightly flexed outward in order to allow it to pass over the winding tool support 20, in case manufacturing tolerances mean that the internal diameter of the spacer ring 100 is no greater than the outer diameter of the winding tool support 20. Such action is not possible with the conventional arrangement of Fig. 2, which requires that the internal diameter of spacer rings 32, 34, 36 is not less than the outer diameter of the winding tool support 20.

In embodiments where winding tool support 20 has a plain cylindrical outer surface, all spacer rings 100 may have a same inner diameter.

According to a feature of the present invention, once the spacer ring 100 is located over the winding tool support 20, it is compressed into close fit over the winding tool 20. This may be achieved by tensioning luggage straps or ratchet cargo straps or similar over the spacer ring 100 to close the cut 102 a certain extent. Preferably, the spacer ring 100 tightens onto the winding tool support 20 over its entire inner surface, so that there is no gap between the radially outer surface of winding tool support 20 and radially inner surface of the c-shaped discontinuous spacer ring 100. In some instances, due to variations in manufacturing tolerances, this may not be found possible, and the cut 102 may fully close without the spacer ring 100 tightening onto the winding tool support 20. A gap may then still exist between the radially outer surface of the winding tool support 20 and the radially inner surface of the spacer ring 100, but this gap will be reduced as compared to a gap which would have existed if spacer ring 100 did not have an adjustable diameter, and preferably any such gap is too small for a turn of wire to fit within the gap.

In an example embodiment, cut 102 is 1mm wide, and tightening the spacer ring 100 onto the winding tool support until the cut 102 is closed will provide a reduction of about 0.2mm in internal diameter of the spacer ring 100. Should it become apparent that a larger variation in internal diameter of the spacer ring 100 is required, then a wider cut 102 may be provided.

In the example embodiment shown in Fig. 2, the cut 102 is a simple radial cut, which may be formed in a composite spacer ring 100 by sawing, or by other methods such as water jet, laser cutting. However, other types of cut may be employed, as discussed below. The inventors believe that the discontinuous spacer ring of the present invention maintains sufficient mechanical strength during winding of magnet coils, and during impregnation of the coils and spacer ring assembly, a thermosetting resin will fill the cut and restore the spacer ring to approximately the strength of a continuous annular spacer ring of similar dimensions. However, additional strength and reduction of any mechanical discontinuity provided by the cut may be reduced by any of the embodiments illustrated in Fig. 4A-4C, and others as will be apparent to those skilled in the art.

In the embodiment of Fig. 4A, cut 106 is provided through the composite spacer ring 100. The cut 106 is oblique, in that it is planar but does not extend perpendicular to axial end faces 107 of the discontinuous spacer ring 100. An effect of such an embodiment is that the cut surfaces are larger than in the case of a perpendicular cut as shown in Fig. 2, and so the spacer ring may be stronger, once the cut 106 is filled with cured thermosetting resin, than in the case of perpendicular cut 102 of Fig. 2. In an embodiment, tensioning straps may be applied near both axial extremities of the spacer ring 100, and in this embodiment, the cut surfaces may be retained nearer to the outer surface of the winding tool support 20 than in the case of the embodiment of Fig. 2.

In the embodiment of Fig. 4B, the cut 108 is formed in a sawtooth shape, such that the cut surface has a much greater surface area than in the case of the perpendicular cut 102 of Fig. 2, again such that the spacer ring may be stronger, once the cut 108 is filled with cured thermosetting resin, than in the case of perpendicular cut 102 of Fig. 2. Such arrangement also has the advantage that, during assembly, one cut surface is less likely to be pushed out of alignment with the other cut surface, as the sawtooth forms will interact and restrict the freedom of movement. In the embodiment of Fig. 4C, the cut 110 is formed in a castellated shape, such that the cut surface has a much greater surface area than in the case of the perpendicular cut 102 of Fig. 2, again such that the spacer ring may be stronger, once the cut 110 is filled with cured thermosetting resin, than in the case of perpendicular cut 102 of Fig. 2. Such arrangement also has the advantage that, during assembly, one cut surface is less likely to be pushed out of alignment with the other cut surface, as the castellations will interact and restrict the freedom of movement.

In preferred embodiments, the discontinuous spacer rings 100 described above are slid onto the winding tool support 20, luggage straps or ratchet cargo straps or similar are then located over the discontinuous spacer ring 100 and tensioned to compress the spacer ring into contact, or into close proximity, with the winding tool support 20. A permanent binding may then be applied over the spacer ring to retain it in position. For example, a "pre-preg" tape, that is to say a tape of glass fibre cloth or similar reinforcement material, ready-impregnated with uncured thermosetting resin, may be wound over the spacer ring in several layers. The uncured thermosetting resin may be caused, or allowed, to cure, for example by heating, so as to permanently retain the discontinuous spacer ring in its contracted state. Typically, the pre-preg tape will contract on curing, and so will increase the tension on the discontinuous spacer ring 100, retaining it firmly in position against the winding tool support 20.

Figs. 5-8 illustrate another aspect of the present invention. In the prior art of Fig. 1, the axial positioning of coils, and the axial extent of the coils is determined by the location of steps 22, 24, 26 in the radially outer surface of the winding tool support 20. In the presently-described embodiment of the present invention, the radially outer surface of the winding tool support 20 is not provided with such steps, but is of plain cylindrical shape. The spacer rings 100 of the present invention are produced with a strict tolerance in their axial dimension. If it is not possible to mould the spacer rings with a strict enough axial tolerance, the spacer rings may be manufactured over-sized in the axial dimension, and machined to a required axial dimension.

In preferred embodiments of the present invention, and as illustrated in Fig. 5-8, spacer blocks 120 may be used to define winding cavities for superconducting coils. Such spacer blocks are manufactured with strict tolerance in their axial dimension, matching the required dimension of a coil to be formed. As illustrated in Fig. 5, tooling for coil winding may be assembled by locating a first spacer ring 100a over the winding tool support 20, fixing it in place, for example as described above. Spacer blocks 120 are then positioned on the radially outer surface of the winding tool support 20 around its circumference, and held in place temporarily with a luggage strap, ratchet cargo strap or similar. The spacer blocks may be of a composite material such as fibreglass-reinforced thermosetting resin. As shown in Figs. 5-8 a second spacer ring or end tooling flange 100b is then slid on to the winding tool support 20. If a second spacer ring is used, that second spacer ring is preferably also discontinuous, and is tightened onto the winding tool support in the same way as described above. If end tooling flange 100b is used, the end tooling flange may be affixed temporarily in place in a similar manner, having a discontinuous end tooling flange 100b clamped onto the winding tool support, or the end tooling flange 100b may be temporarily affixed in place by any other arrangement as may be apparent to those skilled in the art. The location of the end tooling flange or second spacer ring 100b is determined by the location and axial dimension of the first spacer ring 100a and by the axial dimension of the spacer blocks 120. Since the discontinuous spacer rings 100 of the present invention grip onto the winding tool support 20 themselves, and their axial location is not determined by any other component, use of discontinuous spacer rings according to the present invention enables the axial dimension of the coil winding cavity, and so the axial dimension of the finished coil, to be determined solely by the axial extent of the spacer blocks 120.

As illustrated in Figs. 6 and 7, the spacer blocks 120 may be located intermittently around the outer circumference of the winding tool support 20. They may be temporarily retained in position by luggage straps or ratchet cargo straps 124 as shown in Fig. 7.

Fig. 8 illustrates a step in a procedure in which a "pre-preg" tape 126, that is to say a tape of glass fibre cloth or similar reinforcement material, ready-impregnated with uncured thermosetting resin, is wound over the discontinuous spacer ring 100a in several layers. In the illustrated example, the pre- preg tape 126 occupies only a small part of the axial dimension of the first spacer ring 100a. When the pre-preg tape 126 is cured, straps 124 retaining the discontinuous spacer ring 100a may be released. In other embodiments, the pre-preg tape may extend over a broader axial extent of the first spacer ring 100a.

Once the second spacer ring or end tooling flange 100b is affixed in position, the axial dimension of a coil winding cavity is defined, and the spacer blocks 120 are removed by releasing and removing the associated strap, or other retaining device. A superconducting coil 50, 52, 54 may then be wound into the coil winding cavity in the conventional manner, as will be familiar to those skilled in the art.

The above-described steps may be replicated in axially displaced locations to define other coil winding cavities, such that a required arrangement of coils and spacer rings may be formed in a winding step as will be familiar to those skilled in the art.

The resulting assembly is then subjected to a resin impregnation step, known in itself, to provide a monolithically impregnated coil assembly comprising axially alternating coils and spacer rings. Referring back to the arrangement of Fig. 3, the axial positions of coils 50, 52, 54 are defined by the location of steps 22, 24, 26 in the outer surface of the winding tool support 20, and the dimensions and corresponding step locations 42, 44, 46 in spacer rings 32, 34, 36. Tolerances in the relevant dimensions p, s accumulate to a significant range of dimensions for the coil winding cavity into which coils 50, 52, 54 are wound. As described above, this can lead to inhomogeneities in the magnetic field produced by the completed magnet assembly. In the present invention, as exemplified by the embodiment of Figs. 5-8, the position and extent of the coil winding cavities are defined by the axial dimensions of the spacer rings 100 and the spacer blocks 120. As described above, both the spacer rings 100 and the spacer blocks 120 may be machined to precise axial dimensions, and so the size and location of coil winding cavities formed according to the present invention can be tightly controlled.

The present invention accordingly provides methods and structures for defining coil winding cavities, in which the axial position and axial extent of the coil winding cavities are tightly controlled, due to being defined by a small number of accurately-dimensioned components, and in which the radial position of the spacer rings is adjustable to ensure that the coil wound into the coil cavity cannot extend axially under any of the spacer rings.

The present invention accordingly provides methods and apparatus for precisely defining the radial and axial locations of a coil winding cavity, and so also precisely defining the radial and axial locations of a coil in a completed coil assembly.

According to the present invention, spacer rings 100 are discontinuous, with a cut through the spacer ring at some location. This allows the diameter of the spacer ring to be adjusted somewhat, thereby reducing a radial gap 60 between the spacer ring and the winding tool support 20, reducing the chance that superconducting wire may enter a radial gap 60 between spacer ring 100 and winding tool support 20 during winding of coils into a coil winding cavity at least partly defined by such spacer ring.

The use of spacer blocks 120 as provided by the present invention allows the axial dimension of coil winding cavities to be more accurately controlled, and so provides more accurate control of the axial dimension of coils wound into these cavities. The spacer blocks 120 may, for example, be manufactured with an axial dimension precise to within +/-0.05mm, producing a coil winding cavity with an axial dimension precise to within +/-0.05mm, and providing an axial spacer ring position precise to within +/-0.1mm, significantly more precise than was possible in the prior art. The present invention is believed to reduce the set-up time of tooling prior to commencing the winding of superconducting coils. This enables a greater through-put of magnet assemblies at the winding stage.

By using tooling spacer blocks 120 as described, the axial positional tolerance of the coils is improved. In an embodiment, the axial positional tolerance becomes +/-0.1mm. The turns count of the finished coils becomes more predictable, and horizontal symmetry is improved. It is a simple matter to provide tooling spacer blocks 120 of differing sizes to provide coil winding cavities of differing axial dimensions.

The radial clearance between the winding tool support 20 and the spacer ring 100 may become zero, in that the spacer ring touches the winding tool support.

No steps are required in the surfaces of the winding tool support or the spacer ring. This saves machining time in producing the spacer rings and the winding tool support 20. All spacer rings may have a same inner diameter, making their production simpler and cheaper.

Radial tolerance in the bore diameter of the spacer rings may be relaxed, making the spacer rings 100 cheaper to produce, since their final diameter is determined by the winding tool support 20.

In a preferred embodiment, all spacer rings grip on to the winding tool support 20. In an impregnation step, it is typical for the magnet coil to be placed on end - with its axis vertical. In an arrangement of the present invention, the weight of each coil may be carried by the spacer ring beneath it, gripping onto the winding tool support 20. In conventional arrangements, where spacer rings did not grip onto the winding tool support, the weight of several coils may bear upon the lower-most coil during impregnation, possibly resulting in some mechanical deformation.

Claims

1. A superconducting magnet assembly comprising a monolithically impregnated coil assembly comprising axially alternating coils and spacer rings, wherein at least one of the spacer rings is a discontinuous spacer ring (100a) of composite material, characterised in that a radial cut (102) is provided through the material of the spacer ring to provide a spacer ring of adjustable diameter.

2. A superconducting magnet assembly according to claim 1, wherein the radial cut (102) extends perpendicular to axial end surfaces of the spacer ring.

3. A superconducting magnet assembly according to claim 1, wherein the radial cut (20) extends oblique to axial end surfaces of the spacer ring.

4. A superconducting magnet assembly according to claim 1, wherein the radial cut (106) has a sawtooth shape.

5. A superconducting magnet assembly according to claim 1, wherein the radial cut (110) has a castellated shape.

6. A method for assembly of a monolithically impregnated cylindrical coil assembly comprising axially alternating coils and spacer rings, comprising the steps of: arranging spacer rings on a winding tool in predetermined axial locations, said spacer rings defining coil winding cavities between themselves; winding superconducting wire into the coil winding cavities; impregnating the resulting structure to form the monolithically impregnated cylindrical coil assembly comprising axially alternating coils and spacer rings; and removing the winding tool, characterised in that at least one of the spacer rings is a discontinuous spacer ring (100a) of composite material having a radial cut (102) through the material of the spacer ring to provide a spacer ring of adjustable diameter, and the discontinuous spacer ring is compressed into close fit over the winding tool at the predetermined axial location.

7. A method according to claim 6, wherein the step of arranging spacer rings on the winding tool comprises the step of positioning spacer blocks (120) on a radially outer surface of the winding tool between a first spacer ring (100a) and a second spacer ring or end tooling flange (100b), such that an axial dimension of the spacer blocks defines an axial dimension of a coil winding cavity.

8. A method according to claim 7 wherein the spacer blocks (120) are located intermittently around the outer circumference of the winding tool (20).