ELECTRICAL WINDING

Abstract: A solenoid, more especially for use in nuclear magnetic resonance spectroscopy where the coil windings need to be very accurately located, has a multiplicity of elongate elements spaced around and accurately positioned on the outside of a cylindrical former, the elements having locating means, con­ veniently in the form of slots, which together define a helix and these serve to locate a respective coil winding. The elongate elements support a further layer of elements, also having locat­ ing means which serve to locate a further winding, and these elements support further elements and so on to provide re­ quired number of winding layers. Means are also described for maintaining the solenoid at a substantially uniform tempera­ ture.

Description: The present invention relates to the construction of electrical windings and in particular it relates to the construction of large accurately wound solenoids of the type used in nuclear magnetic resonance spectroscopy. More particularly, the invention relates to the design and construction of non-superconducting field coils for use in the whole-body imaging nuclear magnetic resonance spectrometers which have recently been developed for use in medical diagnosis and to such spectrometers incorporating the coils. Such spectrometers are commonly known as medical N.M.R. spectrometers and will hereinafter be referred to as such. Medical N.M.R. spectrometers are commonly provided with a set of three or four coaxial field coils disposed symmetrically along a horizontal axis in the Helmholtz or similar configurations capable of providing the required volume of uniform magnetic flux in a efficient manner. The central coil or coils typically have a mean diameter of approximately 1.1 m and the two outer coils typically have a mean diameter of approximately 1.2 m. The distance between the two outer coils is typically about 2 m. In use, the patient lies within the coils approximately along their common axis. The flux density produced by a set of non-superconducting coils in this configuration is generally of the order of 0.2 Tesla. In order to obtain a clear undistorted image of proton density (p) distribution or spin-lattice (T1) relaxation time distribution within the human body it is desirable to provide a magnetic field between the central coils which is spatially uniform to within plus and minus ten parts per million. Such a requirement imposes a strict tolerance on the location and dimensions of individual turns and groups of turns within the coil windings. We have found that in a coil 1.4 m long and having a mean diameter of 1.1 m the turns must be located within approximately 0.1 mm on average of their required positions, in order to produce a magnetic field of this degree of uniformity and in certain dimensions a much greater degree of accuracy is required. Accordingly diameters of the turns and, if conventional helical windings are used, the helix pitch and angle of each layer of turns must be closely controlled. Hitherto, field coils for use in medical N.M.R. spectrometers have generally been spirally wound from insulated aluminium foil, each coil thus having one turn per layer. While such coils possess good dimensional accuracy when cold, they cannot easily be cooled and therefore tend to expand and distort when heated by the heavy currents used in operation of the spectrometer. Furthermore coils of this type tend to reach an equilibrium temperature distribution relatively slowly, and since they are necessarily wound fairly tightly, they tend to expand discontinuously and produce sudden local fluctuations in the magnetic field for some time after the spectrometer has been switched on. An object of the present invention is to provide solenoid suitable for use as a field coil for an N.M.R. spectrometer in which these disadvantages are substantially eliminated. However the invention is also applicable to solenoids utilised for other purposes. According to one aspect of the present invention, a solenoid comprises a generally cylindrical former, a multiplicity of elongate elements aligned substantially parallel to the former axis regularly circumferentially spaced about and rigidly supported from the former surface, each said element being provided with a set of locating means regularly spaced along its length, adjacent ones of said sets being regularly and successively axially displaced so that said locating means define a helix, and a generally helical electrically conductive winding rigidly located on said elements by said means. Each locating means may simply comprise a slot dimensioned so as to grip the winding and thereby prevent axial movement of the part of the winding which it accommodates. Preferably the elongate elements are adapted to rigidly support a further layer of similar elongate elements incorporating similar sets of locating means on which a further layer of turns of the winding may be located. Preferably said further layer of turns is wound in the opposite sense to the first layer of turns. Preferably each elongate element in said further layer is located directly above and is attached to an elongate element of the supporting layer, for example by means of a free-flowing adhesive. The elongate elements may be of aluminium, provided the winding is electrically insulated therefrom, although they are preferably in the form of glass-filled polyester resin sticks, and are axially located by flanges at the ends of the former. The flanges may be integral with or attached to the former. Each stick is conveniently formed from an even number of mouldings disposed about the central plane, with corresponding mouldings dn opposite sides of the plane formed from the same or identical moulds and turned through 1800 about a radial axis, so that they are mirror images of one another. This assists in the provision of a uniform magnetic field as it ensures an identical spacing and location of the winding turns on opposite sides of the central plane. The moulding sections, of which there are conveniently four in each stick, are preferably secured, end to end by an adhesive, and they are accurately positioned with respect to each other whilst the adhesive is setting by means of a jig engaging the winding locating means. The former may be made of aluminium, a metal which has a temperature coefficient of expansion very similar to that of certain types of glass-filled polyester resin. It will be appreciated that a winding formed in the manner described above will not be exactly helical although it will approach a helical configuration if a large number of elongate elements are used in each layer. Using a winding of approximately 1 m in diameter, we have found that an acceptably uniform magnetic field (i.e. varying by no more than plus and minus 10 parts per million) can be obtained by using 48 polyester resin sticks in each layer. However slight imperfections or deformations in the material or the structure of the coil may be found to affect the uniformity of the magnetic field to some extent, and in some cases this can be corrected by the provision of resistive current shunt connected across a part or the whole of the said winding. Different parts of the winding may have shunts of different resistances connected across them, suitable tappings on the winding being provided for this purpose. Where the winding of a solenoid in accordance with the invention comprises a plurality of layers of turns two or more of the layers may be provided with tappings for the connection of resistive current shunts. A winding may consist of a plurality of conducting elements. These can be bare and in electrical contact so that they constitute, in effect, a single conductor. However it may in some cases be desirable for the elements of a multiple element winding to be connected in series, and in such a case they must, of course, be electrically insulated from each other, as by the provision of insulating coatings. The winding itself is preferably of copper rather than aluminium, since although a copper winding has a lower coefficient of expansion than the supporting polyester sticks and former, it has a lower resistivity than aluminium and therefore requires less cooling than an aluminium winding of comparable size. The winding construction described above, has a very open structure and can be cooled by a simple arrangement, for example by enclosing the former and endcheeks in a cylindrical outer casing and pumping cooling fluid through the resulting enclosure in a uniform flow parallel to the former axis. Slight distortions of segments of the winding between adjacent polyester resin sticks will accommodate any thermal strains without significantly distorting the magnetic field produced by the solenoid, provided that these distortions are smaller than the "distortion" from circularity initially introduced by providing the polygonal supporting structure. However the flow rate of cooling fluid is preferably kept sufficiently high to ensure that the temperatures of all parts of the winding remain uniform to within 100 of the mean winding temperature. The temperature differential between the parts of the winding adjacent the coolingfluid inlet and the cooling-fluid outlet is thereby kept below 20C and distortions of the magnetic field kept within acceptable limits. The cooling fluid may be any suitable oil. Preferably, in addition to the main fluid cooling circuit, the solenoid is connected to an auxiliary fluid cooling circuit in parallel with the main cooling circuit, the auxiliary cooling circuit being provided with means such as a variable output pump or fan for controlling its rate of heat removal from the solenoid, so that the temperature of the winding as a whole may be kept constant irrespective of changes in the ambient temperature. In some cases a temperature sensor may be used to monitor changes in the ambient temperature, and its output connected to a microprocessor which is arranged to control means for varying the rate of heat removal in an appropriate sense to maintain the winding temperature at a constant value. In accordance with another aspect of the invention a solenoid for use as a field coil in an N.M.R. spectrometer associated with means for cooling the solenoid, is also associated with heating means, arranged to be operative when the spectrometer is not in use in order thereby to reduce the risk of appreciable magnetic field variations due to temperature changes when the spectrometer is brought into operation. Thus in an arrangement in which the solenoid is connected to a main cooling circuit, and an auxiliary cooling circuit which is provided with means for controlling the rate of heat removal from the solenoid, there is also provided means for heating the fluid in the auxiliary cooling circuit. Then when the spectrometer is not in use, the heating means is arranged to be operative, when the solenoid is not in use, to cause heated fluid at a first temperature T1 to be circulated through the solenoid casing, and when the solenoid is in use the cooling circuit is arranged to maintain the mean temperature of the fluid within the solenoid casing at a second temperature T2, at which temperature the mean temperature of the winding is approximately T1. By this means the temperature changes resulting from energisation of the solenoid can be kept to an absolute minimum, thereby minimising fluctuations in the magnetic field. Moreover the keeping of the mean temperature of the winding at or near its operating temperature in this way, when not being used, permits the N.M.R. spectrometer to be instantly available for use, i.e with no warm up period, whilst at the same time keeping the electrical consumption to a minimum. The construction of a solenoid in accordance with the invention will now be described by reference to Figures 1 to 5 of the accompanying diagrammatic drawings, of which Figure 1 is a sketch perspective view, partially cut away, of a partially formed solenoid in accordance with the invention; Figure 2 illustrates the construction of a winding locating stick utilised iff the solenoid illustrated in Figure 1; Figure 3 is an axial cross section of the solenoid taken on the line II-II of Figure 1; Figure 4 is a representation of the cooling system of a solenoid arrangement in accordance with the invention for use in an N.M.R. spectrometer, and Figure 5 illustrates a circuit diagram of the solenoid and end coil windings of an N.M.R. spectrometer embodying the invention. Referring to Figure 1, in the manufacture of the solenoid a rigid aluminium former 1 provided with end-cheeks 2 and 3 is mounted for rotation (by means not shown) about its axis. Accurately moulded glass-filled polyester resin sticks 4, aligned parallel to the former axis are accurately located in the axial direction by the end-cheeks 2 and 3. The sticks are rectangular in cross section and are regularly circumferentially spaced about the surface of the former 1 by cylindrical pins 9 (only two of which are shown) which fit into accurately drilled holes in the former 1 and engage appropriately positioned holes in the under surfaces of the sticks. Each stick is provided with a similar set of regularly spaced slots 10 in its outwardly facing surface. Adjacent sets of slots are regularly and successively axially displaced in the direction of the former axis, so that the slots lie in a helical configuration. Each of the sticks 4 is conveniently formed from four separate sections I4a, 4b, 4c, 4d joined together end to end as illustrated in Figure 2. The sections of each stick are formed by moulding, the two inner sections 4b, 4c being formed in the same or identical moulds, as are the two outer sections 4a, 4d. The sections on one side of the central plane are turned longitudinally through 1800 with respect to those on the other side so that the two halves of the stick 4 are mirror images of one another. The stick sections are conveniently secured to each other by an adhesive, as at 5, the sections being aligned and located longitudinally with respect to each other whilst the adhesive setting by a jig (not shown) having locating pegs which engage at least some of the slots 10 of the sections being joined. In this way it is ensured that the slots of adjacent sections are accurately'located with respect to each other whilst allowing for slight tolerances in the overall lengths of the sections. Conductors 11 and 12 are wound under constant tension onto the helical arrangement of slots from a mandrel 8. Multiple conductors are used in order to ensure flexibility during winding. Any suitable number of conductors may be used, but only two are shown in Figure 1 for the sake of clarity. The conductors are preferably rectangular in cross-section, and fit tightly in the slots 10. Circumferential bands (not shown) may initially be used to hold the sticks down on the former surface. In order to prevent random wandering of the current between adjacent conductors in use of the completed solenoid, the conductors may be individually insulated by thin tape (not shown) (e.g. 0.01 mm thick) although this is not always necessary. Sets of spacers (not shown) temporarily inserted between the sticks ensure that the sticks do not bend under the winding strain, and help to provide a greater degree of accuracy in the wound coil. When the first layer of the winding has been completed the spacers are removed, the tension in the conductors 11 and 12 is maintained at a constant value, and a second layer of sticks such as 6 (Figure 3) may be fixed directly on top of the sticks of the first layer by means of a free-flowing adhesive. The outwardly facing surfaces of the first layer of sticks serve to accurately radially locate the sticks of the second layer of the winding, the second layer also being located circumferentially by the provision of pips (not shown) the upper surface of the first layer of sticks which fit into appropriately positioned holes in the sticks of the second layer. The sticks of the second layer are slotted at 10' in a precisely similar manner to those of the first, except that the slots form a helix of the opposite sense to the helix in the first layer. A second layer of turns (11', 12') is then wound in this helical configuration of slots and the procedure is repeated to form the requisite number of layers (which should be an even number) and is preferably six, the sticks of each subsequent layer being accurately positioned with respect to the sticks immediately beneath them in the same manner as the sticks of the second layer. The winding is then coated with a thin layer of adhesive and encased in a casing 16 (Figure 3). Figure 3 also shows two of a set of regularly circumferentially spaced oil ducts 17a and 17b pierced in the casing 16. In use, oil is pumped in via the ducts 17a to the spaces 18 between the stacks of polyester resin sticks and out via the ducts 17b. Figure It shows in more detail preferred cooling arrangement for the central solenoid 7 of a set of field coils for a medical N.M.R. machine. A high-velocity primary cooling circuit 19 connected between an inlet duct 17a and an outlet duct 17b of the solenoid casing 16 is operated by a pump P1 only when the solenoid 7 is energised. A low-velocity secondary cooling circuit 20 connected in series with the main cooling circuit 19 is continuously operated by the pump P2, even when the solenoid is switched off and passes through a heat exchanger E. The resulting flow of cooling fluid is indicated by the solid arrows. A heater H is associated with the secondary cooling circuit, and is arranged to be energised only when the solenoid is not operating. The cooling circuits are inter-connected on the inlet sides of the pumps P1, P2 as shown by a common non-return valve V. When the solenoid is not energised heater H is controlled by temperature probe TP in the inlet duct 17a so as to maintain the temperature of the fluid at the inlet 17a at a temperature T1. When the solenoid 7 is energised the heater H is switched off, and the pump P1 is energised to force cooling fluid at a greater velocity through the solenoid, some of the fluid from outlet duct 17b flowing through the valve V (as indicated by the dashed arrows) and the remainder flowing through heat exchanger E and back into cooling circuit 19 (as indicated by the solid arrows). The temperature of fluid in the inlet duct 17a of the solenoid is monitored by the probe TP which controls the effective rate of operation of a further pump P3 which feeds a heat exchange fluid through the heat exchanger E and thereby controls the rate at which heat is removed from the cooling fluid by the heat exchanger so as to maintain the temperature of the cooling fluid at the inlet 17a at a temperature such that when the solenoid is energised, the mean temperature of the winding is also T1. In an alternative arrangement, the probe TP may be located at the outlet duct 17b. In a modification of the solenoid described, the aluminium former 1 may be replaced by a relatively thinwalled cylinder of a synthetic plastics material, which provides a base for the sticks 4, 6. In such a case the cylinder with integral endcheeks, also of plastics material, is arranged to be supported internally during the winding on 6f the conductors 11, 12. Then, after the enclosure of the winding by an outer cylinder, also of plastics material, a solidifiable insulating material bondable to the inner and outer cylinders is introduced into the space between them to impregnate and enclose the winding. After the solidifiable insulating material has been cured, the internal support can be removed, as the assembly is then in the form of a rigid structure. This has the advantage that the inner winding can be disposed closer to the axis of the solenoid than could be achieved with the use of a rigid aluminium former. Cooling may be achieved, either by making the sticks 4, 6 over which the conductors are wound hollow and connecting their ends to manifolds for the passage of the cooling fluid, or alternatively introducing additional tubular members between the sticks for such a purpose. In some cases also the winding conductors can themselves be of tubular form and arranged to carry the cooling fluid. In an N.M.R. spectrometer the central solenoid 7 will be located coaxially between two end coils 22 (Figure 5). In order to provide a degree of adjustment of the magnetic field produced within the solenoid 7 each of the end coils 22 is supported by a mounting which permits the coil axis to be moved transversely and tilted in any direction to a limited extent, as well as enabling the spacing between the coil and the adjacent end of the solenoid 7 to be varied. Means are provided for securing a coil in any set position. Preferably the parts of the current leads 23 adjoining the solenoid 7 and the two end coils 22 are disposed substantially in the same axial plane and at the corresponding ends of the three members. Further adjustment of the magnetic field may be achieved by the connection of one or more shimming resistors 24 between tappings as at 25 on one or more of the winding layers of the solenoid. A shimming resistance can be connected across the whole or only part of a winding layer as may be required. In some cases a temperature sensitive element may be located so as to be responsive to changes in the temperature of the solenoid, and arranged to control the solenoid current in the sense which maintains the field strength of the solenoid approximately constant despite such changes, at least over a predetermined temperature range. For example an increase in the temperature of the solenoid will lead to an expansion thereof with a consequent reduction in the field strength. In N.M.R. spectrometers as constructed hitherto the solenoid current is normally provided by a rectifier circuit controlled by a resistor arranged to be maintained at a constant temperature However if the rectifier control circuit resistor has a suitable negative temperature coefficient of resistance and is located so as to be responsive to the temperature of the solenoid cooling fluid either within, or as it leaves, the solenoid, it can be arranged to increase the solenoid current as the temperature rises and reduce it as the temperature decreases so as to compensate for expansion or contraction of the solenoid due to such temperature changes, and thereby maintain the field strength substantially constant. Alternatively it would be possible to have a resistor with zero temperature coefficient of resistance supplying the rectifier control circuit with a first voltage signal and a second resistor having either a positive or a negative temperature coefficient of resistance arranged to supply the rectifier circuit with another signal which could then be added or subtracted from the first signal to provide a control signal which keeps the rectifier current constant when the solenoid temperature is constant, but increases or decreases the rectifier current when the solenoid temperature increases or decreases.

Claims: CLAIMS 1. A solenoid comprising a generally cylindrical former (1), a multiplicity of elongate elements (4) aligned substantially parallel to the former axis regularly circumferentially spaced about and rigidly supported from the former surface, each said element being provided with a set of locating means (10) regularly spaced along its length, adjacent ones of said sets being regularly and successively axially displaced so that said locating means define a helix, and a generally helical electrically conductive winding (11, 12) rigidly located on said elements by said means. 2. A solenoid according to Claim 1 in which each locating means comprises a slot (10) dimensioned so as to prevent axial movement of the part of the winding which it accommodates. 3. A solenoid according to Claim 1 or Claim. 2 in which the elongate elements (4) are adapted to rigidly support a further layer of similar elongate elements (6) incorporating similar sets of locating means (10') on which a further layer of turns (11', 12') of the winding is located, the further layer of turns being wound in the opposite sense to the first layer of turns. 4. A solenoid according to Claim 3 wherein each elongate element (6) in said further layer is located directly above and is attached to an elongate element of the supporting layer by an adhesive. 5. A solenoid according to any preceding Claim in which the elongate elements are formed of aluminium or of glass-filled polyester resin, and are axially located by means of flanges (2) at the ends of the former. 6. A solenoid according to Claim 5 in which the elongate elements (4) are in the form of glass-filled polyester resin sticks, in which each stick is formed from an even number of mouldings (4a --- 4d) disposed about the central plane, with corresponding mouldings on opposite sides of the plane formed from the same or identical moulds and turned through 1800 about a radial axis, so that they are mirror images of one another. 7. A solenoid according to Claim 6 wherein the mouldings (4a --- 4d) of each stick are secured end to end by an adhesive (5), and are accurately positioned with respect to each other whilst the adhesive s setting by means of a jig engaging the winding locating means. 8. A solenoid according to any preceding Claim including at least one resistive current shunt (24) connected across a part or the whole of the winding. 9. A solenoid according to any preceding Claim in which the former (1), together with the elongated elements and winding, are enclosed within an outer casing (16) and means (19, P1) are provided for circulating fluid through the casing. 10. A solenoid according to Claim 9 having associated with it a first cooling circuit (19) for circulating a cooling fluid through the casing, and an auxiliary cooling circuit (20) for circulating cooling fluid through the casing, and means (TP, P3) controlling the flow of fluid through the auxiliary cooling circuit so as to control the rate of heat removal from the solenoid 11. A solenoid according to Claim 9 incorporating a temperature sensitive resistor responsive to changes in the temperature of the cooling fluid within or as it leaves the solenoid, means for supplying current to the solenoid winding controlled by the temperature sensitive resistor in the sense which causes the solenoid current to increase as the temperature rises and to decrease as the temperature falls so as to compensate for expansion or contraction of the solenoid and thereby maintain the field strength of the solenoid substantially constant.

Inventors: BENNETT, PETER CHARLES, GRANT, ALASTAIR NEISH
Applicants: THE GENERAL ELECTRIC COMPANY, P.L.C.
Application year: 1984
Main classification: H01F5
IPC codes: H01F5, G01R33, H01F7, A61B10, H01F27, A61B5