INDUCTOR

Abstract: An inductor (12) comprises a coil (21) wound onto a former (20). The former (20) is made by injection moulding a composition comprising greater than 79 % by weight ferrite and a binder such as polypropylene. The inductor may be used in an electromagnetic sensor such as are used in electronic coin validating apparatus.

Description: Inductor Field of the Invention The present invention relates to an inductor, an inductive sensor and a coin validator. Background to the Invention Inductors, such as are employed in electronic coin validators, are conventionally provided with ferrite magnetic circuit elements, e.g. cores. Ferrites are light and do not corrode but are very hard and brittle. This makes them difficult to work and prone to breakage. Some ferrites are conductive. Consequently, if a coil is to be wound a core of such a ferrite, an insulating layer must be placed between the coil windings and the core. It is an aim of the present invention to overcome these problems. Permanent magnets comprising ferromagnetic powder embedded in a polymer are known for applications such as "fridge magnets". In the following, "magnetically hard" refers to ferromagnetic materials which can be permenantly magnetized and "magnetically soft" refers to ferromagnetic materials which cannot be permenantly magnetized. The present inventors have discovered that, suφrisingly, by substituting the magnetically hard powder by a magnetically soft material, magnetic circuit elements for inductors can be produced and that these elements are acceptable for critical applications such as the inductive sensors of a coin validator. Summary of the Invention According to the present invention, there is provided an inductor including a magnetic circuit element, characterised in that the magnetic circuit element comprises a polymer loaded with a ferromagnetic material in particulate form. The ferromagnetic material will be magnetically soft. The polymer may be thermoplastic or thermosetting. Thermoplastic polymers are preferred as . these allow the use of injection moulding. The magnetic circuit element will be non-conductive and flexibly resilient. The flexibility of the element greatly reduces the risk of damage thereto. io Preferably, the ferromagnetic material comprises manganese-zinc doped ferrite or oxidized iron dust. More preferably, the magnetic circuit element comprises 80% to 93%, advantageously 88% to 91% in the case of manganese- zinc doped ferrite, by weight ferromagnetic material. 1} Nylon 6 or polypropylene may be conveniently used as the polymer. Alternative polymers are polycarbonates, acrylics, ABS and high density styrenes. An advantage of the use of this loaded polymer magnetic material is that is 20 can be readily welded, for instance using ultrasonic welding techniques, and glued. Advantageously, the magnetic circuit element comprises a first portion and a second portion, the first and second portions being glued together. Thus, complex shapes can be easily formed. 2. In an embodiment, the first portion comprises an elongate I- or C-section member and the second portion comprises a C-shaped member having first and second spaced flanges, arranged to cover at least partially respective end faces of the first member, and a strip coupling the flanges and closing the groove in a side of the I- or C-section member. Such elongate forms are very so vulnerable to breakage when formed from solid ferrite material. According to the present invention, there is also provided an inductive sensor including a sensing inductor according to the present invention. Such a sensor may be employed in a coin validator. A coin validator is an apparatus for determining whether a piece offered as a coin or token belongs to a predetermined set of coins or tokens. According to the present invention, there is further provided a method of manufarturing an inductor characterised by injection moulding a composition comprising a polymer loaded with a ferromagnetic material in particulate form to form a structure. Preferably, the composition is heated to a temperature below 220°C for moulding. More preferably, the composition is heated to 150°C for moulding. Conveniently, a coil is assembled in association with the structure by winding directly thereon. The structure may be injection moulded in two parts which are subsequently glued together. Brief Description of the Drawings An embodiment of the present invention will now be described, by way of example, with reference to the accompanying drawings in which: Figure 1 shows a coin validator; Figure 2 is an exploded view of an inductor according to the present invention; and Figure 3 shows an inductor according to the present invention. Detailed Description of an Embodiment of the Invention Referring to Figure 1, a coin validator body 1 defines a rectangular cross- section coin passageway 2. The passageway 2 comprises a straight, vertical upper portion 2a, where various sensor stations 3 are located, and a wider lower portion 2b. An accept gate 4 is arranged for diverting coins along either of two routes. The accept gate 4 normally blocks route one route but is opened if the signals from the sensor stations 3 indicate that a valid coin has been inserted into the validator. The upper portion 2a of the passageway 2 has Λ width greater than the diameter of the largest coin 5 of interest and a depth greater than the thickness of the thickest coin of interest. The entry to the upper portion 2a of the passageway is flared so as to simplify alignment of the validator with a coin insertion slot (not shown). Considering the sensor stations 3 in more detail, an upstream optical sensor station comprises a lensed light emitting diode (LED) 6 mounted in the validator body 1, so as to shine a beam U of light across the width w of the passageway 2 through a slit 7 opening into the passageway 2. The slit 7 extends across the full depth of the upper portion 2a of the passageway. A lensed photosensor 8 aligned to receive the beam from the LED 6 completes the upstream optical sensor station. A downstream optical sensor is similarly constructed from a lensed LED 9, a slit 10 and a lensed photosensor 11 to shine a beam D across the passageway 2, and is located a short distance below the upstream sensor. Two elongate sense coils 12 are located between the upstream and the downstream optical sensor stations. The sense coils 12 are press fitted longitudinally into respective slots extending transversely across the width w of the upper portion 2a of the passageway. Referring to Figures 2 and 3, a coil 12 comprises an elongate, I-section former 20 about which the winding 21 is wound. The former 20 is formed from polypropylene loaded with 90% by weight Mn-Zn ferrite powder. Thus, the former 20, as it is substantially non-conducting, can serve both as a core and as a bobbin onto which the winding 21 is wound directly. An electromagnetic shield 22 comprises an elongate member of the same material as the core, having a flange extending perpendicularly at each end. The shield 22 is arranged to be attached to the former 20 such that the winding 21 is wholly covered along one long side of the former 20 by the elongate member and at least partially covered at the ends of the former 20. The purpose of the shield 22 is to increase the Q of the coil 12 but also reduces both the susceptibility of the coil 12 to electromagnetic interference (EMI) and the electromagnetic energy emanating from the coil, other than into the coin passageway 2 (Figure 1) of the validator. The former 20 and the shield 22 are formed by coating the ferrite particles with a coupling agent such as an alkyl-silane or an amino-silane, and mixing the coated ferrite with polypropylene. The composition thus formed is then heated to about 150°C and injection moulded. After the moulding process, the former 20 and the shield 22 are removed from the sprues to which they are connected. The coil winding 21 is wound directly onto the former 20 and the shield 22 is then glued to the former 20. It will be appreciated that the present invention is applicable to magnetic circuit elements having other geometries, for instance conventional pot cores. Indeed, it is a feature of the present invention that complex geometries can be readily produced by one or more techniques selected from moulding, machining and welding rather than pressing and sintering.

Claims: Claims 1. An inductor (12) including a magnetic circuit element (20,22), characterised in that the magnetic circuit element comprises a polymer loaded with a ferromagnetic material in particulate form. 2. An inductor according to claim 1, wherein the ferromagnetic material comprises manganese-zinc doped ferrite or oxidized iron dust. 3. An inductor according to claim 1 or 2, wherein the magnetic circuit element comprises 80% to 93% by weight ferromagnetic material. 4. An inductor according to claim 3, wherein the magnetic circuit element comprises 88% to 91% by weight ferromagnetic material. 5. An inductor according to any preceding claim, wherein the polymer is nylon 6 or polypropylene. 6. An inductor according to any preceding claim, wherein the magnetic circuit element comprises a first portion (20) and a second portion (22), the first and second portions being glued together. 7. An inductor according to claim 6, wherein the first portion comprises an elongate I- or C-section member (20) and the second portion comprises a C-shaped member (22) having first and second spaced flanges, arranged to cover at least partially respective end faces of the first member, and a strip coupling the flanges and covering the channel in a side of the I- or C-section member. 8. An inductive sensor including a sensing inductor according to any preceding claim. 9. A coin validator including an inductive sensor according to claim 8. 10. A method of manufacturing an inductor characterised by injection moulding a composition comprising a polymer loaded with a ferromagnetic material in particulate form to form a structure (20,22). 11. A method according to claim 10, wherein the composition is heated to a temperature below 220°C for moulding. 12. A method according to claim 11, wherein the composition is heated to 150°C for moulding. 13. A method according to claim 10, 11 or 12, including assembling a coil (21) in association with said structure by winding directly thereon. 14. A method according to any one of claims 10 to 13, wherein the struαure is moulded in a plurality of parts (20,22) which are subsequently joined. 15. A method according to claim 14, wherein said parts are joined by glueing.

Inventors: WOOD DENNIS, WOOD, DENNIS
Applicants: COIN CONTROLS, WOOD DENNIS, COIN CONTROLS LTD., WOOD, DENNIS
Application year: 1996
Main classification: G07D5
IPC codes: G07D5