GB2530806A

GB2530806A - Serially bonded magnet assembly method

Info

Publication number: GB2530806A
Application number: GB1417542.6A
Authority: GB
Inventors: Neil Charles Tigwell
Original assignee: Siemens Healthcare Ltd
Current assignee: Siemens Healthcare Ltd
Priority date: 2014-10-03
Filing date: 2014-10-03
Publication date: 2016-04-06
Anticipated expiration: 2034-10-03
Also published as: GB201417542D0; GB2530806B

Abstract

A method for assembling a coil assembly, comprising a plurality of annular coils (10), comprises mounting each of the coils (10) in a respective jig (12), selecting one of the coils for alignment, energising the selected coil, and detecting alignment of a magnetic field produced by the selected coil with a nominal magnet axis (A). The selected coil is then moved to a new position by tilting and/or rotating and/or displacing it to improve alignment between the magnetic field produced by the selected coil and the nominal magnet axis. A hardening filler material (24) may be caused or permitted to harden between the selected coil and an adjacent coil to define a separation between the coils. A corresponding apparatus comprises a jig apparatus comprising a plurality of jigs defining a nominal magnet axis (A), actuators to rotate and translate the jigs, sensors for detecting the magnetic field generated by a coil and control means for driving the actuators.

Description

SERIALLY BONDED MAGNET ASSEMBLY METHOD

Superconducting magnets such as used in Magnetic Resonance Imaging (MRI) systems typically comprise annular coils of superconducting wire impregnated with a thermoset resin. The coils are axially aligned and produce a strong, homogeneous background magnetic field for MRI imaging. Each coil may have a radially outer layer of resin-impregnated glassfibre, or glass beads, or resistive wire, for various mechanical or electrical advantages. A superconducting magnet may consist of inner "drive" coils, which produce the required high-strength magnetic field, and outer "shield" coils, whose primary function is to limit stray field emitted by the drive coils to an acceptable level. The shield coils typically produce a magnetic field in the opposite direction to that produced by the drive coils, and the field produced by the shield coils must be taken into account when calculating the overall magnetic field of the superconducting magnet.

A serially bonded" superconducting magnet consists of several annular drive coils of superconducting wire, each typically impregnated with a thermosetting resin. No mechanical former is required for this type of construction, as the coils are bonded to each other. This may be achieved by spacing the coils apart with spacers such as resin-impregnated glassfibre blocks or annuli, the whole structure then being bonded together with high strength epoxy resin.

As is well known to those skilled in the art, drive coils in superconducting magnets for MRI systems must be very accurately aligned to produce a background magnetic field in the imaging region which is homogeneous to within a few parts-per-million. Despite diligent manufacturing processing, it is often found that the magnetic centre and/or magnetic axis of an annular superconducting coil does not align precisely with its geometrical centre and axis. If such coils are assembled together to produce a superconducting MRI magnet, the resultant field in the imaging region is likely to be inhomogeneous and reqilire significant shimming or other correction arrangements.

The present invention aims to provide methods and apparatus for assertling serially bonded magnets from annular superconducting coils, such that the resultant serially bonded magnets provide a background magnetic field with a high dcgrcc of homogcncity in an imaging rcgion dcspitc misalignment of the magnetic centre from a geometric centre; and/or misalignment of a magnetic axis from a geometric axis in the case of at least one of the annular coils.

The present invention accordingly provides methods and apparatus as defined in the appended claims. In particular, the present invention does not seek to correct any misalignment between the magnetic axis and the geometric axis or between the magnetic centre and the geometric centre of each coil, but rather enables the coils to be mounted geometrically off centre and/or geometrically inclined to one another in order to align their respective magnetic fields.

The above, and further, objects, characteristics and advantages of the present invention will become more apparent from the following description of certain embodiments thereof, in conjunction with the accompanying drawings, wherein: Fig. 1 schematically represents a superconducting coil retained within an adjustable jig, according to a step in a method of the present invention; Fig. 2 schematically represents a number of coils each retained in a jig apparatus similar to the representation of Fig. 1; Fig. 3 schematically represents the positioning of spacers between coils according to a step in a method according to the present invention; Fig. 4 schematically represents injection of a filler material according to a step in a method of the present invention; and Fig. 5 represents a completed coil assembly according to an embodiment of the present invention.

Fig. 1 illustrates a single annular superconductive coil 10 held within a jig 12. Retaining surfaces 14 hold The coil in position, while the jig 12 may be rotated about an axis B-B and translated in orthogonal directions X-Y. Coil 10 is geometrically symmetrical about geometric axis Ag. The jig 12 may also be rotatable about a second axis C, perpendicular to axis B and geometric axis Ag of coil 10 and may also be translatable in direction Z, orthogonal to directions X, Y. In operation, coil 10 generates an axial magnetic field which is symmetrical about its magnetic axis Am, which is not represented in the drawing and which may not coincide with geometric axis Ag.

In the present description, references to an "axial" direction indicate any direction parallel to the relevant axis Ag, Am, A, including along the axis itself, while references to a "radial" direction indicate directions perpendicular to the relevant axis, extending in a plane which contains the axis.

Coil 10 may include a coating of impregnated glass fibre, glass beads or resistive wire on its radially outer surface.

Fig. 2 illustrates a jig apparatus 20 which includes a number of jigs 12 as described with reference to Fig. 1. Each jig 12 may be independently rotated about axis B-B and/or axis C-C and translated in orthogonal directions X-Y and possibly also in direction Z, orthogonal to directions X, Y. A nominal magnet axis A is shown and defined by the geometry of the jig apparatus 20. According to an aspect of the present invention, each coil 10 is tilted with its jig about axes B and/or C and/or translated in X-Y directions to align its magnetic axis Am with the nominal magnet axis A, regardless of the coil's geometric axis Ag.

A method of the present invention may proceed as follows.

Coils 10 are mounted and retained in respective jigs 12 in jig apparatus 20. The jigs 12 may be set to align the geometric axes Ag of the coils with the nominal magnet axis A to provide a known starting point, although that is not necessary. With the coils 16 at room temperature, a coil may be onorgiscd by passing a relatively small amount of current through it. As the coil is at a temperature above the transition temperature of superconducting wire, the current will flow through the cladding material -typically copper or aluminium -rather than superconducting filaments. However, the generated magnetic field will closely resemble the magnetic field which will be produced by the superconducting coil in use. The magnetic field produced by the energised coil is plotted, by use of devices and methods familiar to those skilled in the art. The jig 12 holding the energised coil 10 is then rotated about its axis B and/or axis C and/or translated in directions X, Y to align the magnetic axis Pun of the coil as closely as possible with the nominal magnet axis A. In other embodiments, jig 12 may not be rotatable in axis B and/or axis C at all, but may allow the coil to be loosened within retaining surfaces 14 and tilted or rotated as necessary to improve alignment of the magnetic axis lAm of the coil with the nominal magnet axis A. In a more advanced embodiment, an automated system may be provided, which plots the magnetic field of a coil, and actuates jig 12 to rotate about axes B and C and to translate in directions X, Y to align the magnetic axis Am of the coil with the nominal magnet axis A. Such automated system may comprise jig apparatus 20 comprising a plurality of jigs 12 defining nominal magnet axis A, each jig being for retaining one coil or subset of coils; actuators to rotate each jig about at least one axis B, C and to translate the jig in at least two orthogonal directions X,Y; sensors for detecting a magnetic field generated by a coil; and control means for driving the actuators to tilt and translate a coil whereby to improve the alignment between the magnetic field generated by the coil and the nominal magnet axis A. Preferably, some kind of locking mechanism is provided to retain each jig 12 in place once alignment of the associated coil is complete, to prevent unintentional disruption to the alignment of the associated coil.

The preceding steps of actuating a coil, plotting its magnetic field and adjusting the coil's position by rotation and translation to align its magnetic axis Am with the nominal magnet axis A, is repeated for all other coils in the jig apparatus 20.

It may be preferred to energise only the coil which is being aligned, so that the plotted magnetic field is only the magnetic field attributable to that particular coil.

Alternatively, previously-aligned coils may remain energised so that the plotted magnetic field is a total field generated by all coils already aligned plus the coil presently being aligned.

Each of the coils 12 may be aligned, one-by-one as described above, or alternatively more than one may be aligned at a time.

Rather than plotting the magnetic field of each coil and then correcting it by tilting and translation, suitable sensors may be provided to detect the symmetry of the magnetic field generated by each coil, and the coil rotated about axes B, C and translated in directions X, Y in an empirical, iterative process to find the alignment which generates the magnetic field with the best symmetry about the nominal magnet axis A. As shown in Fig. 3, at the end of this stage in the method, all coils 10 have their magnetic axes Am aligned with the nominal magnet axis A. Spacers 22 are then placed in spaces between the coils. The spacers should be a close fit, but not a tight fit. It may be preferred to provide a selection of spacers 22 of differing axial extents for use depending on respective gaps.

The spacers 22 may be discrete blocks, as illustrated, or may be annuli. Intermediate solutions, where each spacer extends around art arc of each adjacent coil, are also possible.

Discrete spacers 22 may be incorporated into ring-like structuros for oasc of handling.

In the arrangement shown in Fig. 3, each coil is retained in its position by respective jig 12. Spacers 22 are placed between coils and retained in position by some appropriate temporary means -for example, clamps forming part of jig apparatus 20, adhesive tape, resilient interference blocks and so on. A hardening filler material 24 is then introduced between spacers 22 and the adjacent coils 10. This may be a thermosetting or thermoplastic material: any hardening material which is capable of withstanding the temperature cycles and mechanical vibration which is experienced by the superconducting coil assembly in use. The filler material 24 should be viscous, able to hold its shape for the time required to harden. Examples include thermosettinq resin mixed with chopped glass fibre or glass beads.

Once the filler material has hardened, it preferably serves the dual purposes of holding the coils together by a bonding function, and holding them at the required relative positions simply by virtue of its solid shape. In some arrangements, the filler material 24 does not bond to the coils, but simply serves to provide spacers of correct dimensions. In embodiments where the filler material does not bond to the coils, some other mechanical retaining structure, such as an external frame, may be provided to retain the coils in position. In some embodiments, the spacers 22 are dispensed with altogether, and the coils are bonded together and retained in their required relative positions by hardening filler material which traverses a gap between adjacent coils at least in certain radial positions.

Fig. 4 shows detail of a preferred arrangement for introduction of the filler material 24. In this arrangement, each spacer 22 is provided with internal channels 26 leading from an inlet port 28 to outlets 30 on axial end-faces of the spacer. Viscous filler material 24 is introduced through a nozzle 32 through inlet port 28 into channels 26. The filler material flows through channels 26 and out of the spacer at outlets 30, into contact with adjacent coils 10. Such arrangement ensures good penetration by filler material 24 between coils 10 and spacer 22 over the full radial extent of the spacer 22. Such arrangements may be applied to spacers which are in the form of discrete blocks, or annular spacers.

The spacers 22 may be resin-impregnated glassfibre blocks, or may be porous structures which are impregnated with resin from filler material 24 as it is introduced into the spacers.

The filler material is then caused or allowed to harden.

Mechanical retaining structures are applied if necessary, in embodiments where the filler material does not bond to the coils, and the resulting coil structure is removed from the jig.

Fig. 5 shows an example of a resulting structure. Coils 10 are bonded to one another by filler material 24 and spacers 22 to form a self-supporting "serially bonded" magnet structure.

Fig. 6A illustrates an alternative embodiment of the present invention. Rather than using a spacer of a fixed dimension and filling in gaps between spacer and coil with a viscous filler material, as described above, a resilient retaining structure 36 is provided between adjacent coils and is filled with hardening material from nozzle 32 through inlet port 28.

Once filled, the hardening filler material is caused or allowed to harden. In some arrangements, it also bonds to the coils 10 to provide a bonded self-supporting structure.

The resilient retaining structure 36 may be an elastomer cylinder, as illustrated in Fig. 6A or may be a sprung telescopic arrangement as illustrated in Fig. EB, or equivalent.

In some embodiments, the resilient retaining structure may be a closed container such as a bag, closed other than for the inlet port 28, as illustrated in Fig. 7A. Such container is filled with hardening filler material 24 until it bears on the adjacent coils, as shown in Fig. 73, and then the filler material 24 is caused or allowed to harden. In such embodiments, the coils will not be bonded to the hardening material and an external mechanical arrangement must be provided to retain the coils in position.

A further advantage of the present invention lies in that manufacturing tolerance of the outer dimensions of the coils, particularly the axial end-faces is not critical as any variation is taken up by distribution of the viscous hardening filler material 24.

The present invention provides a method for assembling a superconducting magnet structure in which coils are retained in fixed relative positions defined by spacers formed at least partially from a hardening filler material, enabling assembly of a magnet structure with coils correctly positioned and bonded together without any machining operations taking place. The methods of the present invention allow a faster production process at reduced cost and provide a more homogenous field in the finished magnet structure as each coil has been individually adjusted to ensure that its magnetic axis aligns to the nominal axis of the magnet.

While the present invention has been particularly described in relation to the assembly of several coils such as typically used for inner (drive) coils, similar methods can be used for retaining outer (shield) coils. The methods and apparatus of the present invention may be employed to assemble serially bonded magnets of resistive coils, as well as the superconducting coils specifically described above.

The coils assembled by the present invention may be held within a carrier, such as an individual radially cuter mechanical retaining structure. In some embodiments, the spacers may be placed between the mechanical retaining structures, rather than between the coils themselves. Fig. 8 schematically represents such an embodiment, in which radially outer mechanical retaining structures 40, in this casc annular journals arc separated and bondcd by spacers and hardening filler material 24 by a method of the present invention.

In some embodiments of the invention, two or more coils may be attached together before alignment according to the present invention. Tn such embodiments, one or more coil subsets, each comprising two or more coils bonded together in some way may be mounted in respective jig(s) 12 and aligned as provided by the present invention, within the limits imposed by the inability to adjust the alignment between the coils of the subset(s).

While the methods have particularly described the step of moving a coil by tilting it about axis B or C, or translating it in directions X and Y, the method may include movement of the coil by rotation about its geometric axis, while supported by the respective jig and displacement in an axial direction if reguired.

Some improvement may be found according to the present invention even if only one coil 10 has its position adjusted to improve alignment between its magnetic axis Am and the nominal magnet axis A. Therefore, the present invention in its broadest sense encompasses methods in which only one coil has its magnetic field measured and is moved to improve alignment between its magnetic field and the nominal magnet axis A.

Claims

CLAIMS1. A method for assembling a coil assembly comprising a plurality of annular coils (10), the method comprising: (a)-mounting each of the coils (10) in a respective jig (12); (b)-selecting one of the coils for alignment; (0)-energising the selected coil; (d)-detecting alignment of a magnetic field produced by the selected coil with a nominal magnet axis (A) (e)-moving the selected coil to a new position by tilting and/or rotating and/or displacing it to improve alignment between the magnetic field produced by the selected coil and the nominal magnet axis; (f)-retaining the selected coil in its new position; (g)-introducing a hardening filler material (24) into spaces between the selected coil and an adjacent coil; (h)-causing or permitting the hardening filler material to harden between the selected coil and an adjacent coil, thereby defining a corresponding separation between the selected coil and an adjacent coil; and (j)-removing the resultant coil assembly from the jigs (12).
2. A method according to claim 1 wherein steps (b) -(g) are repeated for the remaining coils (10) , each being selected in turn for measurement of magnetic field alignment and coil movement.
3. A method according to claim 1 or claim 2 wherein the hardening filler material contacts respective surfaces of the respective coils and, once hardened, serves to bond the respective coils together.
4. A method according to any preceding claim wherein spacers (22) are positioned between coils prior to introduction of the hardening filler material, and the hardening filler material extends between respective spacers and adjacent coils.
5. A method according to cfaim 4 wherein the spacers are each provided with channels (26) extending from an inlet port (28) to outlets (30) on axial end-faces of the spacer, and wherein the hardening filler material is introduced through a nozzle (32) into the inlet port (28) to flow through the channels (26) and out of the outlets into contact with adjacent coils (10)
6. A method according to any of claims 1-3 wherein the hardening filler material is introduced into a resilient retaining structure (36)
7. A method according to of claims 1-2 wherein the hardening filler material is introduced into a resilient retaining structure (36) which is closed other than for an inlet port (28) for the introduction of the hardening filler material (24)
8. A method according to claim 1, further comprising, between the steps (h) and (j), the step of providing a mechanical retaining strllcture which retains coils (10) and hardened filler material (24) in their relative positions.
9. A method for assembling a coil assembly comprising a plurality of subsets of annular coils (10) , at least one subset of coils comprising at least two coils assembled together, the method comprising: (a)-mounting each of the subsets of coils (10) in a respective jig (12); (b)-selecting one of the subsets of coils for alignment; (c)-energising the coil(s) of the selected subset; (d)-detecting alignment of a magnetic field produced by the selected subset of coils with a nominal magnet axis (A) (e)-moving the selected subset of coils to a new position by tilting and/or rotating and/or displacing it to improve alignment between the magnetic field produced by the selected subset of coils and the nominal magnet axis; (f)-retaining the selected subset of coils in its new position; (g)-introdilcing a hardening filler material (24) into spaces between the selected subset of coils and an adjacent subset of coils; (h)-causing or permitting the hardening filler material to harden between the selected subset of coils and an adjacent subset of coils, thereby defining a corresponding separation between the selected subset of coils and the adjacent subset of coils; and (j)-removing the resultant coil assembly from the jigs (12).
10. A method according to claim 9 wherein steps (b) -(g) are repeated for the remaining subsets of coils, each being selected in turn for measurement of magnetic field alignment and movement of the coil subset.
11. A method according to any preceding claim wherein each coil may be tilted by rotation of the associated jig.
12. A method according to claim 11 wherein each coil may be tilted by rotation of the associated jig two orthogonal directions.
13. A method according to any of claims 1-10 wherein each coil may be tilted by loosening the jig and tilting the coil within the jig.
14. A method according to any preceding claim wherein the steps of detecting the alignment of the magnetic field and moving the selected coil or subset of coils are performed by: plotting the magnetic field of the selected coil or subset of coils, and calculating a corbination of tilting and translation which would improve the alignment between the magnetic field produced by the selected coil and the nominal magnet axis.
15. A method according to any preceding claim wherein the steps of detecting the alignment of the magnetic field and moving the selected coil or subset of coils are performed by: sensors provided to detect symmetry of the magnetic field of the selected coil or subset of coils, followed by an empirical, iterative process of tilting and translating the selected coil or subset of coils to determine a combination of tilting and translation which improves the alignment between the magnetic field produced by the selected coil and the nominal magnet axis.
16. A method according to any preceding claim wherein the coils are silperconducting coils.
17. A method according to any preceding claim, wherein each coil or subset of coils is held within an individual radially outer mechanical retaining structure (40) , and the hardening filler naterial (24) is placed between the mechanical retaining structures, rather than between the coils themselves.
18. Mi automated apparatus for assembling a coil assembly comprising a plurality of annular coils (10), the apparatus comprising: -a jig apparatus (20) comprising a plurality of jigs (12) defining a nominal magnet axis (A), each jig being for retaining one coil or subset of coils; -actilators to rotate each jig about at least one axis (B, C) and to translate the jig in at least two orthogonal directions (X, Y) -sensors for detecting a magnetic field generated by a coil; and -control means for driving the actilators to tilt and translate a coil whereby to improve the alignment between the magnetic field generated by the coil and the nominal magnet axis (A)