WO1997025835A1

WO1997025835A1 - Electromagnetic induction heating coil

Info

Publication number: WO1997025835A1
Application number: PCT/GB1997/000003
Authority: WO
Inventors: David Crofts
Original assignee: Raychem Limited
Priority date: 1996-01-05
Filing date: 1997-01-03
Publication date: 1997-07-17
Also published as: GB9600181D0; KR19990077066A; JP2000503160A

Abstract

Open-sided electromagnetic induction heating coil having overlapping coil sections (20, 21, 22, 23, 24, 25) which are electrically connected in parallel to one another enabling relatively large coils to be constructed from conveniently bendable narrow copper tubing while keeping the coil impedance low enough for incorporation in a resonant tank circuit at frequencies watched to a remote high-frequency generator so as to minimise power losses in the leads connecting the tank circuit to the generator. Methods of induction heating using the coil, especially in blocking of wiring harnesses, are also described.

Description

ELECTROMAGNETIC INDUCTION HEATING COIL

This invention relates to an electromagnetic induction coil, which may advantageously be used in apparatus for inductive heating of objects, and to methods of inductive heating using the coil.

It is well known that magnetic materials can be inductively heated by coupling them with high-frequency alternating magnetic fields generated by applying an alternating voltage across a work coil formed by a number of turns of electrical wire or other elongate electrical conductor. During operation, the induction coil acts as the primary winding of a transformer and the workpiece acts as the secondary winding. The material being heated is not part of a closed electrical circuit and the generation of heat is due to the induced electrical current which flows in the workpiece. Heating of the workpiece is the result of internal energy losses, due either to resistance or hysteresis losses in the case of ferromagnetic material, which cause the temperature to rise. Conductive metal tubing may be used as the coil conductor to enable cooling fluid to be passed through the coil in operation. The actual voltage and frequency will depend on several factors, including the size of the power unit, the workpiece and the type of coil. The success of the induction heating process depends to a great degree on the proper design of the work coils which act as inductors. Induction coils which completely surround the workpiece during the heating process are inconvenient to use when the object to be heated is of a size or shape such that it cannot be easily inserted and removed through the ends of the coil along the coil internal axis. One such type of object is an electrical wiring bundle or harness, a short portion of which is to be placed in the coil for inductive heating of materials intended to block interstices in the harness for purposes hereinafter described.

This inconvenience may be alleviated by using an open-sided coil, into and out of which the object to be heated can be moved in a direction lateral to the coil internal axis, instead of along the axis as is unavoidable with a closed coil. Such an open-sided induction coil may, for example, be made by coiling a suitable conductor into a simple flat "pancake" coil of rectangular shape and bending the pancake coil about a centre line running parallel with the shorter rectangular sides, so that the longer sides of the rectangular coil arch around to define a generally U-shaped or C-shaped internal space of the open-sided coil thus formed. However, as the size of such open-sided "bent pancake" coils is increased to accommodate larger inductively-heatable objects, for example the larger electrical wiring harnesses of 20 mm or more diameter now becoming common in the automotive industry, inconveniently large generators using undesirably high voltages and/or currents and/or frequencies are required to generate a sufficiently powerful field in the coils. Very high frequencies (eg 5MHz) have the further disadvantage that unacceptable power losses may occur if the coil is attached to the generator by extended (1 to 4 metre) flexible leads for remote operation.

The present invention addresses this problem by providing an open-sided electromagnetic induction coil comprising at least two coil sections (preferably made of electrically- conductive tubing) electrically connected in parallel to one another, wherein each of the said coil sections comprises a proximal portion arched around the internal coil axis, a distal portion arched around the internal coil axis, and a connecting portion extending substantially in the same direction as the internal coil axis and connecting the proximal portion and the distal portion together so that there is a space between them in a sense lying along the internal coil axis, and wherein a first one of the said coil sections has the proximal portion of at least one other said coil section aligned in the said space between the proximal and distal portions of the first coil section with the distal portion(s) of the said other coil section(s) aligned beyond the distal portion of the first coil section.

By thus constructing the coil in electrically-parallel sections, the present invention ingeniously enables the desired larger coils to be conveniently fabricated from small-bore tubing or relatively narrow-gauge wire, while keeping the total coil inductance low enough to be adequately driven by relatively low and safe voltages (e.g. below 250V), at high frequencies (e.g. 0.5 - 1.5 MHz) obtainable from commercially-available generators, for example the CEIA "Power Cube" (Trade Mark) 120V, 1MHz, 2.5kW generators sometimes used with small coils to heat small metal objects in the jewellery trade. By "proximal" and "distal" portions is meant portions of a given coil section which are respectively nearer to (proximal) and further from (distal) an observer looking along the internal coil axis from one end of the coil. References to these proximal and distal portions being "arched" around the internal coil axis are not intended to limit the arched portions to any specific shape. Arches of angular shape, for example forming three sides of a rectangle, would be conceivable, although more curved shapes may be preferable for generating more uniform fields within the coil. Arches having "legs" on either side of the internal coil axis, which legs extend in a relatively straight line from the side opening and then curve approximately 180° around the coil axis towards the legs on the other side, may be preferred for forming coils of convenient depth and side opening width, capable of receiving relatively large objects wholly within the internal coil space. Substantially circular arches, preferably subtending an angle of at least 180°, preferably 225° or possibly 270°, about their central coil axis, may also be useful.

It will be understood that the proximal (or distal) arched portion of a coil section may be discontinuous if it is desired to form the arch from the two free end regions of the continuous coil section. Preferably the free end regions will be positioned close enough together so that any such discontinuity will have an acceptably small effect on the uniformity of the field generated in the coil in use. Alternatively, the free ends of each coil section may be located in the aforementioned connecting portion, allowing both arched portions to be formed from continuous lengths of the coil section conductor. It is also conceivable that the coil could be discontinuous both at the proximal portions and at the distal portions of each section. Such a coil could be constructed of separate sections on either side of the internal coil axis, each section having a proximal portion and a distal portion, both of which portions arch around the axis toward the corresponding separate coil section on the opposite side of the axis. However this structure would tend to complicate the electrical and cooling fluid connections to an overlapping series of such "left-side and right-side" coil sections. Continuous coil sections which extend along one side of the coil axis and arch around it to return along the other side of the axis are therefore preferred. The indication that the said connecting portion of each coil section extends substantially in the same direction as the internal coil axis is not intended to limit the arrangement to strictly parallel alignments. The connecting portion need not be entirely straight and may slope or deviate towards or away from the internal coil axis to some extent, provided that it achieves the object of adequately spacing the distal portion from the proximal portion along the axis. This longitudinal spacing is preferably sufficient to reduce or nullify the degree of destructive interaction between the opposed fields generated in the respective proximal and distal arched portions of each preferred coil section, which arched portions may be imagined as "travelling" in opposite directions around the internal coil axis as one follows the coil section from one of its free ends to the other.

The present arrangement of the coil sections [with the proximal portion of at least one other said coil section aligned in the said space between the proximal and distal portions of the first coil section and the distal portion(s) of the said other coil section(s) aligned beyond the distal portion of the first coil section] enables all the proximal arched portions "travelling" in one direction (eg. clockwise) around the internal coil axis to be grouped together separately from the corresponding group (further along the coil axis) of distal arched portions "travelling" in the other direction (eg. anti-clockwise). Destructive field interaction thus tends to be restricted to a small central area of the coil between the respective proximal and distal groups. The spacing between immediately adjacent arched portions will preferably be selected to maximise the axial length of the coil while maintaining an acceptably uniform field for the intended purposes in operation. Reference to the proximal or distal portions being "aligned" is intended to convey the sense of the coil sections being arranged to form a recognisable coil structure incorporating the four (or more) arched portions provided by the two (or more) coil sections. Exact alignment is not essential, and some deviation in alignment and/or shape may be tolerable, provided that the field generated by the coil in use has a degree of uniformity suitable for the purpose in question.

In an especially preferred form of the coils according to the present invention, two or more, preferably not more than 5, said other coil sections are aligned with their respective proximal portions in the said space of the first coil section and their respective distal portions beyond that of the said first coil section. In this arrangement, the longitudinal distance between the proximal and distal portion of the said first coil section will be selected to suit the combined widths of the intervening proximal portions of the other coil sections, together with the free space between adjacent proximal portions. It is furthermore preferred that the distal portion of each successive other coil section is aligned beyond the distal portion of the preceding coil section. This advantageously allows the respective coil sections to be made fairly closely resembling one another in size and shape and proximal-to-distal spacing, with only such minor variations as may be necessary for "nesting" the overlapping coil sections within one anotiier to form a recognisable, preferably substantially uniformly aligned, open-sided coil.

The electrically parallel connection of the separate coil sections enables them advantageously to be made from small-bore metal (eg. copper) tubing, for example not more than 5 mm, preferably up to 4 mm, more preferably up to 3.5 mm, especially 2.8 to 3.2 mm, in outside diameter. This in turn enables each coil section to be conveniently made by bending a separate continuous length of the tubing having substantially uniform diameter, whereas tubing of more than 5mm diameter would tend to require cutting and joining, being too wide for bending to the required complex shapes. Each coil section preferably has a total length of the tubing arching around the internal coil axis which exceeds the total length of the connecting sections extending substantially in the same direction as the coil axis. Fluid cooling of the coil in use will usually be advantageous, in which case it is preferred that first ends of the said coil sections are connected in parallel to a shared cooling fluid inlet manifold, and the other ends of the said coil sections are connected in parallel to a shared cooling fluid outlet manifold. The said manifolds may also conveniently connect the respective ends of the coil sections electrically in parallel to one another, the respective manifolds being connected electrically to opposite sides of the high-frequency energising circuit.

Useful coils according to this invention for automotive harness-blocking purposes may preferably have a side opening of width at least 20 mm, preferably at least 25 mm, more preferably at least 30 mm; and a depth of at least 20 mm, preferably at least 25 mm, more preferably at least 30 mm. Axial coil length of at least 45 mm, preferably at least 50mm, measured along the internal coil axis preferably from the first proximal portion to the last distal portion of all the coil sections, may also be preferred.

The coil may be more-or-less rigidly attached to a high-frequency generator incorporating the other known components of a so-called "tank circuit" capable of resonating at the desired frequency, in which case objects to be inductively heated will normally be brought to the coil. However, it may often be more convenient, especially for heating large objects such as wiring harnesses, for the coil to be arranged as part of a tank circuit in an independently-moveable module capable of electrically-inductive coupling with a remote high-frequency generator. Preferably the module is electrically- inductively coupled to a remote high-frequency generator by flexible electrical lead means at least 1 metre, preferably at least 2 metres, more preferably 3 metres, especially 3.8-4.2 metres, in length.

The induction coil is the inductor in this remote tank circuit and capacitance is added to the coil. By matching the capacitance and inductance of this remote tank circuit, a tuned resonant circuit is created, which can be fed with a voltage alternating at the resonant frequency to keep losses in the leads very small. It is preferable to choose the remote tank circuit capacitance and the coil inductance so that the resonant frequency is within the operating range of a commercially-available generator, whose output frequency is preferably self-tuning to match that of the remote tank circuit. Because the losses in this system are low, the power and physical size of the generator may be kept conveniently small. To reduce electrical losses in the leads, the current passing through them is preferably reduced to a minimum by the matching of the frequencies. To achieve this, the coil is preferably connected directly to the tank circuit capacitors in a remote housing attached to the ends of the flexible leads. However, to achieve sufficient current to create a desirably high magnetic flux within the coil, the coil's reactive impedance is preferably as low as possible, preferably less than 5 ohms, more preferably less than 2 ohms, especially below 1 ohm; and to supply desirable power levels, the capacitors in the remote housing are preferably as large as possible, preferably greater than 200nF, more preferably greater than 350nF, especially greater than 500nF. With a frequency of 1MHz, which is very efficient for inductive heating of very small metallic particles, as hereinafter described, a capacitance of 500 nF requires a matching inductance of 0.05 microhenry, which is preferably achieved according to the present invention by increasing the number of parallel-connected coil sections to reduce the coil inductance to the desired value.

The invention includes a method of electromagnetic induction heating, wherein a coil according to any aspect(s) of the present invention is energised by a suitable high- frequency generator and an object capable of being heated by electromagnetic induction is placed within the field generated within the coil and is thereby inductively heated. In one embodiment of this method, the inductively-heatable object is associated with insulated electrical wires, preferably part of a wiring harness, placed so that the wires extend in a direction substantially parallel to the internal coil axis, thereby minimising inductive heating of the wires. The inductive heating of the wires is approximately doubled if the lines of flux within the coil intersect the wires at right angles rather than running substantially along the wires as in this preferred arrangement. On the other hand, when the inductively-heatable object comprises a heat shrinkable tubular sleeve carrying or incorporating inductively-heatable magnetic particles, this is preferably placed so that the tubular axis of the sleeve lies substantially parallel to the coil internal axis (which naturally occurs when the sleeve surrounds the aforementioned part of a wiring harness). This time, the alignment maximises the inductive heating of the sleeve (and preferably causes the sleeve to shrink), since the magnetic flux lines flowing along the direction of the sleeve wall will have a greater chance of interaction with the inductively-heatable particles than would flux lines passing through the sleeve wall at right angles to its surface. Thus, the sleeve can be shrunk by inductive heating while serendipitously minimising the risk of thermal damage to the wire insulation.

In one especially preferred method according to this aspect of the invention, the inductively-heatable object comprises the said part of a wiring harness surrounded by a heat shrinkable sleeve, preferably an inductively-heatable heat-shrinkable sleeve (that is preferably, a sleeve carrying or incorporating inductively-heatable magnetic particles), and the sleeve also encloses a separate body of heat-activatable sealant material, preferably inductively-heatable heat-activatable sealant material (that is preferably incorporating the aforementioned inductively-heatable magnetic particles), which melts and flows to block the interstices within the said part of the harness when the sleeve and/or sealant material and/or the wires of the harness is or are inductively heated by the field within the coil. Inductively-heatable materials and induction heating methods for blocking electrical cables or harnesses are described in US-A-5378879 (MP1474), the disclosure of which is incorporated herein by reference.

The invention will now be further illustrated by way of example with reference to the accompanying drawings, wherein :-

Figure 1 shows for comparison purposes in schematic perspective an example of the aforementioned known "bent pancake" coils;

Figure 2 shows in schematic perspective a coil according to the present invention composed of three parallel-connected coil sections;

Figures 3A and 3B are schematic views from the end and looking into the side opening of a coil similar to that shown in Figure 2;

Figures 4A, 4B and 4C are schematic views of a similar coil according to the present invention having five coil sections instead of the three illustrated in Figures 2 and 3;

Figure 5 shows schematically a possible form of module or housing for the coil and other components of the tank circuit for use with a remotely-coupled generator; and

Figure 6 shows schematically in side view the coil housing of Fig. 5 with a lever- operated flux concentrator moveably positioned in the mouth of the coil to close the flux loop.

The known open-sided coil shown in Figure 1 is made from a single length of copper pipe 10 extending from an inlet end 11 to an outlet end 12, the originally flat pancake coil of roughly rectangular shape having been bent as shown around the inner coil axis indicated by the line A to form proximal and distal arched portions "travelling" in the directions indicated by the arrows on the arches, with connecting portions 13 extending approximately parallel with the internal axis A. Even using the largest bendable diameter (about 5 mm) of copper tubing to minimise the inductance, the maximum tubing length for a coil of this type in a system with a remote tank circuit is about 300 mm, which limits the practical coil dimensions to an internal diameter and depth of at most about 20 mm each and coil length along the internal axis of at most about 40 mm. This limits the diameter of wire bundles which can be placed inside the coil to about 15 mm, which represents a bundle of only up to about 30 wires, which is small for the kinds of wires usually used in modern automotive wiring harnesses.

In contrast with the coil of Figure 1, the coil according to the present invention illustrated in Figure 2 is formed from three separate coil sections respectively having proximal portions 20, 21 , 22 and distal portions 23, 24, 25 arching around the internal axis A and "travelling" in the direction of the arrows on the arches, with respective connecting portions 26, 27, 28 extending roughly parallel to the internal axis A. It may be seen that the arches formed by proximal portions 21 and 22, in the space between proximal portion 20 and its corresponding distal portion 23, are slightly wider than the arches formed by the first proximal portion 20 and the three distal portions 23, 24, 25. This is done in order to accommodate the preferred overlapping arrangement of the three generally similar coil sections with the second distal portion 24 positioned beyond the first distal portion 23 and the third distal portion 25 beyond the second distal portion 24. If desired, the tubing could be bent to allow proximal portions 21 and 22 over most of their length to align more closely with the other arched portions, with the widening taking place only in the region where proximal portions 21 and 22 approach closely to the connecting portions 27 and 28.

The free ends of the coil sections are connected respectively to cooling fluid inlet manifold 29 and outlet manifold 30 made of copper, which effects the necessary electrical connection of the three coil sections in parallel to one another. With this structure, the coil may be made of 3 mm diameter copper tubing with each of the three sections 300 mm long, making a total of 900 mm of easily-bendable copper tubing in the coil, while still achieving a total inductance well within the operating range (for example less than 0.5 micro henries, preferably less than 0.3 micro henries, more preferably 0.08 to 0.12 micro henries) suitable for use in a system with a remote tank circuit. This enables the coil to be made with a much more useful working volume, having an internal width of about 30 mm, a depth of about 32 mm, and a length along the internal axis A of about 50 mm, capable of receiving the increasingly common automotive wire bundles of up to 25 mm diameter incorporating about 60 wires in an average automotive harness.

Figures 3A and 3B respectively show schematic views from an end and looking into the side opening of coils approximately corresponding to that illustrated in Figure 2, with the various parts numbered correspondingly. The aforementioned wider arches of proximal portions 21 and 22 are illustrated in Figure 3 A, and the preferred bending of connecting portions 27 and 28 to bring their respective distal arched portions 24 and 25 into line with the first distal portion 23 is indicated in Figure 3B.

Figure 4A is a view generally similar to that of Figure 3B of a coil comprising five overlapping coil sections with the proximal portions 41, distal portions 42 and connecting portions 43 arranged in a manner generally similar to that of Figure 3B. The inlet and outlet manifolds have been omitted in this view.

Figure 4B shows an end view generally similar to that of Figure 3A with a coil 45 attached to a module 46 housing other components of the aforementioned remote tank circuit. This view also illustrates the aforementioned alternative arrangement where the proximal and distal arched portions are all more-or-less in alignment with widening only in regions 47 near the connecting portion 48 to accommodate the overlapping structure. In all versions, the coil may be protectively potted using appropriate known resin materials and methods, as indicated by the broken lines 49.

Figure 4C shows an idealised side view of the five-section coil having proximal arched sections 50 connected to inlet and outlet manifolds 51, 52, distal arched portions 53, and connecting portions 54, in overlapping arrangement as before.

Figure 5 illustrates schematically a preferred construction for robust industrial use such as on automotive harness production lines, wherein a coil 51 according to this invention is enclosed within a housing 52 having a projecting lip 53 which protects the coil from direct contact contact with an object such as an automotive wiring harness about which the coil is to be positioned in use. This housing 52 may conveniently contain the other components of the remote tank circuit to form an independently- moveable module, which in use will be connected to the required high-frequency generator by the aforementioned flexible electrical leads, preferably incorporating fluid coolant conduits.

In Figure 6, the coil housing of Fig. 5 has the addition of another aspect of the invention in the form of a flux concentrator 70, preferably of ferrite, moveably held by lever 72 (preferably spring-loaded) in the mouth of the coil 61 to close the magnetic flux loop in operation. When handle 74, pivotably mounted on the coil housing 62, is moved in the direction of arrow X, the flux concentrator is moved out of the coil mouth in the direction of arrow Y to permit removal of the coil housing from around a treated wiring bundle followed by positioning of the coil around another wiring bundle indicated by broken lines 76. On release of the handle 74, the preferred spring loading (not shown) ensures that the operator cannot forget to return the ferrite flux concentrator in the opposite direction into the coil mouth to close the flux loop around the new wiring bundle to be inductively heated. An advantageous addition to this arrangement is provided by stand-off members 78, preferably of non-magnetic material, which space the wiring bundle 76 from the flux concentrator 70 and press the bundle 76 into position at or near the coil axis A for optimum inductive effect. The stand-off members 78 are preferably provided at the ends of the flux concentrator 70 outside the ends of the coil, but could alternatively, or in addition, lie within the ends of the coil, provided that the stand-off material does not interfere unacceptably with the working field within the coil. These features of the invention relating to the flux concentrator and stand-off are applicable to any side-entry induction coil for induction heating of elongate objects, for example the aforementioned wiring bundles.

Claims

CI.AIMS :

1. An open-sided electromagnetic induction coil comprising at least two coil sections (preferably made of electrically-conductive tubing) electrically connected in parallel to one another, wherein each of the said coil sections comprises a proximal portion arched around the internal coil axis, a distal portion arched around the internal coil axis, and a connecting portion extending substantially in the same direction as the internal coil axis and connecting the proximal portion and the distal portion together so that there is a space between them in a sense lying along the internal coil axis, and wherein a first one of the said coil sections has the proximal portion of at least one other said coil section aligned in the said space between the proximal and distal portions of the first coil section with the distal portion(s) of the said other coil section(s) aligned beyond the distal portion of the first coil section.

2. A coil according to claim 1, wherein two or more, preferably not more than 5, said other coil sections are aligned with their respective proximal portions in the said space and their respective distal portions beyond that of the said first coil section.

3. A coil according to claim 2, wherein the distal portion of each successive other coil section is aligned beyond the distal portion of the preceding coil section.

4. A coil according to any preceding claim, wherein the respective coil sections substantially resemble one another in size and shape.

5. A coil according to any preceding claim, wherein the tubing of the said coil sections is not more than 5 millimetres, preferably up to 4 mm, more preferably up to 3.5 mm, especially preferably 2.8 to 3.2 mm, in diameter.

6. A coil according to any preceding claim, wherein each said coil section is individually made of a separate continuous length of the said tubing having substantially uniform diameter, which tubing has been bent to form the respective coil section.

7. A coil according to any preceding claim, wherein each said coil section has a total length of the said tubing arching around the coil axis which exceeds the length thereof extending substantially in the same direction as the coil axis.

8. A coil according to any preceding claim, wherein first ends of the said coil sections of tubing are connected in parallel to a shared cooling fluid inlet manifold, and the other ends of the said coil sections are connected in parallel to a shared cooling fluid outlet manifold, the said manifolds preferably also connecting the respective ends of the coil sections electrically in parallel to one another.

9. A coil according to any preceding claim having a side opening at least 20 mm (preferably at least 25 mm, more preferably at least 30 mm) in width, a depth of at least 20 mm (preferably at least 25 mm, more preferably at least 30 mm), and a coil length along the coil axis of at least 40 mm (preferably at least 45 mm, more preferably at least 50 mm).

10. A coil according to any preceding claim, arranged as part of resonant tank circuit in a module capable of electrically-inductive coupling with a remote high frequency generator, having a resonant frequency matched to that of the tank circuit.

11. A coil according to claim 10, wherein the said module is electrically-inductively coupled to a remote high-frequency generator by electrical lead means at least 1 metre, preferably at least 2 metres, more preferably 2.8 to 3.2 metres, in length, the electrical lead means preferably incorporating or being incorporated in cooling fluid conduits capable of conveying cooling fluid to and from the coil.

12. A coil according to claim 10 or 11, wherein the said generator is capable of generating frequencies less than 1 MHz, preferably less than 500 kHz, more preferably 100 to 300 kHz.

13. A method of electromagnetic induction heating, wherein a coil according to any preceding claim is energised by a high-frequency generator and an object capable of being heated by electromagnetic induction is placed within the field generated within the coil and is thereby inductively heated.

14. A method according to claim 13, wherein the said inductively-heatable object is associated with electrical wires, preferably with part of a wiring harness, placed so that the wires extend in a direction substantially parallel to the coil axis, thereby minimising inductive heating of the wires.

15. A method according to claim 13 or 14, wherein the said inductively-heatable object comprises a heat-shrinkable tubular sleeve carrying or incorporating inductively- heatable magnetic particles and is placed so that the tubular axis of the sleeve lies substantially parallel to the coil axis, thereby maximising inductive heating of the sleeve and causing the sleeve to shrink.

16. A method according to claim 14, wherein the said inductively-heatable object comprises the said part of a wiring harness surrounded by a heat-shrinkable sleeve, preferably an inductively-heatable heat-shrinkable sleeve, and the sleeve also encloses a separate body of heat-activatable sealant material, preferably inductively-heatable heat- activatable sealant material, which melts and flows to block the interstices within the said part of the harness when the sleeve and/or the sealant material and/or the wires of the harness is or are inductively heated by the field within the coil.

17. An open-sided electromagnetic induction coil to which is moveably (preferably pivotably) attached a flux concentrator member which can be moveably positioned in the mouth of the coil to substantially close the flux loop during operation of the coil.

18. A coil according to claim 17, wherein the flux concentrator member is biased toward the loop-closing position in the coil mouth.

19. A coil according to claim 17 or 18, wherein stand-off means are provided to bear on an elongate member extending through the coil in use so as to space the elongate member from the flux concentrator and to urge the elongate member into the coil.

20. A coil according to any of claims 17 to 19, which is also as claimed in any of claims 1 to 12.