DISCRIMINATOR FOR BIMETALLIC COINS

Abstract: A coin discriminator has a coin path (3) along which a coin (5) containing a first and a second portion (13a, 13b) made of different metals and/or metal alloys is arranged to pass; coil means (1a, 1b) positioned adjacent to the coin path; electrical means (7) for supplying time varying drive signals to the coil means; and detection means (9) for detecting eddy currents induced in the coin by the coil means. The coil means (1a, 1b) is arranged to induce in the coin (5) an eddy current loop (27), which in a predetermined region (25) of the coin crosses a bond (11) between the first and second portions (13a, 13b) of the coin.

Description: Discriminator for bimetallic coins Technical Field The present invention relates to a coin discriminator, comprising: a coin path along which a coin containing a first and a second portion made of different metals and/or metal alloys is arranged to pass; coil means positioned adjacent to the coin path; electrical means for supplying time varying drive signals to the coil means; and detection means for detecting eddy currents induced in the coin by the coil means. Furthermore, the present invention relates to a method of measuring the conductivity at a bond between the first and second portions of such a coin. Description of the Prior Art Coin discriminators, which are arranged to measure the electric characteristics, e.g. the resistance or conductivity, of a coin by exposing it to a magnetic pulse and detecting the decay of eddy currents induced in the coin, are generally known in the technical field. Such coin discriminators are used in a variety of coin handling machines, such as coin counting machines, coin sorting machines, coin validators for vending and gaming machines, etc. Previously known coin handling devices are for instance disclosed in WO 97/07485 and WO 87/07742. The way in which such coin discriminators operate is described in e.g. GB-A-2 135 095, in which a coin testing arrangement comprises a transmitter coil, which is pulsed with a rectangular voltage pulse so as to generate a magnetic pulse, which is induced in a passing coin. The eddy currents thus generated in the coin give rise to a magnetic field, which is monitored or detected by a receiver coil. The receiver coil may be a separate coil or may alternatively be constituted by the transmitter coil having two operating modes. By monitoring the decay of the eddy currents induced in the coin, a value representative of the coin conductivity may be obtained, since the rate of decay is a function thereof. Prior art coin discriminators often employ a small coil with a diameter smaller than the diameter of the coin. The coil induces and detects eddy currents in an arbitrary point of the coin (the actual part of the coin which is subject to the conductivity measurement above will vary depending on the orientation, speed, angle, etc., of the coin relative to the coil) . This approach is sufficient for a normal homogeneous coin made of a single metal or metal alloy. However, in recent years bimetallic coins have been issued on the market in different countries. A well known example of a bimetallic coin is the French 10 Franc. Furthermore, some of the Euro coins to be issued within the European Community within a near future are planned to be of a bimetallic type. Bimetallic coins are made as follows. Outer rings and central discs are punched from sheets (also known as blanks) of the two metal or metal alloys, of which the bimetallic coin is to be made. The disc is then fitted into the ring, and the coin is minted. Minting consists of pressing the coin between two hardened dies . The dies stamp the head and tail pattern onto the coin and also force the disc and ring together. The joint between the disc and ring is called a bond. If the disc and ring are clean and free from oxide, the bond between the metals will have near zero electrical resistance. Ideally, the resistance of the metals or alloys is much greater than the resistance across the bond. However, if the ring or the disc is covered in an oxide layer before minting, the resistance of the bond will be greater than the resistance of the metals or alloys. Thus, by controlling the handling and storage conditions of the blanks between punching and minting, it is possible to control the bond resistance (or, alternatively, the conductivity, which is basically the inverse of resistance) in the finished bimetallic coin. To control the resistance of the bond in this way may be particularly desired as an anti-fraud measure. At the production coins with too low or too high resistance will not be issued. To make such a controlled production practical, a method of repeatedly measuring the bond resistance of large volumes of coins would be required. The prior art coin discriminators described above fail to provide a sufficiently accurate determination of the bond resistance or conductivity, since the measurement results obtained would vary to a large extent depending on the actual spot of measurement on the coin. In other words, if the conductivity for a given coin would happen to be measured in a spot located in the ring, the measurement results would differ from the results obtained if the measurement would take place in the disc. Furthermore, if the measurement spot would embrace a portion of the bond between the ring and the disc, yet another measurement result would be obtained. A coin discriminator according to the prior art is cited in the introductory part of claim 1. Summary of the Invention It is therefore an object of the present invention to allow repeatable and accurate determination of the bond conductivity or resistance in a coin comprising a first and a second portion made of different metals or metal alloys, e.g. a bimetallic coin. The object is achieved for a coin discriminator, comprising: a coin path along which a coin is arranged to pass; coil means positioned adjacent to the coin path; electrical means for supplying time varying drive signals to the coil means; and detection means for detecting eddy currents induced in the coin by the coil means, by arranging the coil means so that an eddy current loop is induced in the coin in such a way that it crosses, in a predetermined region of the coin, the bond between the first and second portions of the coin. Furthermore, the object above is achieved through a method of measuring the conductivity at the bond between the first and second portions of the coin, wherein the coin is subjected to a magnetic field by coil means external to the coin and wherein eddy currents induced in the coin are detected by detection means external to the coin, the magnetic field being generated such that a loop of eddy currents crosses the bond in a predetermined region of the coin. The solutions described above are defined by the appended independent patent claims. Preferred embodiments of the invention are the subject of dependent claims. Brief Description of the Drawings The invention will now be described in more detail, reference being made to the accompanying drawing, in which: Fig. 1 is a schematic sectional view of a coin discriminator according to a preferred embodiment of the invention, Fig. 2 is a schematic top view of the arrangement in Fig. 1, and Fig. 3 is a schematic illustration of a bimetallic coin and the eddy currents generated therein by the coin discriminator of Figs . 1 and 2. Detailed Description As shown in Fig. 1 the coin discriminator comprises a coil means in the form of two coil portions la and lb, which are connected to an electrical device 7 for supplying voltage pulses thereto. Furthermore, the coin discriminator comprises detection means 9 for detecting eddy currents induced in the coin by the magnetic pulses generated by the coil means in response to the voltage pulses supplied from the electrical means 7. The coil means la, lb acts as a transmitter coil for exposing a bimetallic coin 5, which is moved past the coin discriminator along a 1 mm thick ceramic plate 3 in a direction indicated by an arrow, to a magnetic pulse giving rise to eddy currents in the coin 5, and furthermore the coil means acts as a receiver coil for detecting the magnetic field variations generated by the eddy currents in the coin 5 and converting them into a corresponding voltage signal. As shown in Fig. 3, the coin 5 comprises a ring 13a of a first metal or alloy and a disc 13b of a second metal or alloy. A bond between the disc 13b and the ring 13a is labelled 11. The detection device 9 is arranged to measure the decay of these eddy currents and produce a value of the bond conductivity or resistance in response thereto. As will be described below, the coin discriminator is arranged to carry out the conductivity measurements when the center of the coin 5 is aligned with a center plane 21 of the coin discriminator . As seen in Fig. 2, the coil means la, lb comprises a first and a second coil frame 17a, 17b, which are provided with a respective first and second winding 15a, 15b. The coil frames 17a, 17b have an essentially semi-circular sectional shape and are symmetrically arranged at either sides of the coil center plane 21. The distance between the coil frames 17a and 17b is about 5 to 10 mm, and the radius of each semi-circular section is about 10 to 20 mm. An electrical conductor is wound on the coil in an equal number of turns on each coil frame 17a, 17b. For instance, a polyurethane covered copper wire with an internal diameter of 0.2 mm and an external diameter of about 0.25 mm may be used as the electrical conductor forming the wind- ings 15a, 15b on the coil frames 17a, 17b. Preferably, each winding contains 10 to 100 turns, and furthermore one winding 15a is wound clockwise, while the other winding 15b is wound counter-clockwise, for reasons set out below. The adjacent portions 19a and 19b of the two halves la, lb of the coil contain winding wires, which run essentially parallel to each other and are symmetrically arranged with respect to the coil plane 21. Furthermore, since the windings 15a, 15b are formed by one single contiguous conductor, a common electric current will flow through the entire windings 15a, 15b, when driven by a voltage pulse from the electrical means 7. In response thereto, a pulsed magnetic field will be generated around the windings 15a, 15b. In the central region of the coil, i.e. around the adjacent portions 19a, 19b and the center plane 21, the current will flow in the same direction in both windings 15a, 15b and will hence cooperate in generating a magnetic field. The bond conductivity is measured when the coin is in the middle of the coil, as shown in Fig. 1, i.e. when the diameter 23 (see Fig. 3) of the coin 5 is aligned with the center plane 21 of the coil la, lb. The duration of the voltage pulses supplied by the electrical means 7 to the coil la, lb may be chosen in accordance with the actual application; however, a duration of 10 to 100 microseconds appears appropriate for most situations. Thanks to the arrangement above an eddy current loop 27 is generated in the coin 5 along a path, which approximately corresponds to the wire pattern of the two windings 15a, 15b (i.e. the symmetric double semi-circular shape), as is schematically illustrated in Fig. 3. The exact shape of an eddy current loop generated in a coin is a complex subject, which is difficult to model mathematically. However, tests have indicated that the eddy current loop has a flow approximate to the one described below. 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Claims: CLAIMS 1. A coin discriminator, comprising: a coin path (3) along which a coin (5) containing a first and a second portion (13a, 13b) made of different metals and/or metal alloys is arranged to pass; coil means (la, lb) positioned adjacent to the coin path; electrical means (7) for supplying time varying drive signals to the coil means; and detection means (9) for detecting eddy currents induced in the coin by the coil means , c h a r a c t e r i z e d in that the coil means (la, lb) is arranged to induce in the coin (5) an eddy current loop (27) , which in a predetermined region (25) of the coin crosses a bond (11) between the first and second portions (13a, 13b) of the coin. 2. A coin discriminator according to claim 1, wherein the predetermined region (25) of the coin (5) is proximate to a diameter (23) of the coin. 3. A coin discriminator according to claim 1 or 2 , wherein the coil means (la, lb) comprises a first and a second coil frame (17a, 17b) provided with a first and a second winding (15a, 15b) , respectively, the windings being interconnected and connected to the electrical means (7) in such a way, that the flow of current in the first winding is parallel to and has the same direction as the flow of current in the second winding in adjacent portions (19a, 19b) of the windings. 4. A coin discriminator according to claim 3, wherein the first and second coil frames (17a, 17b) has an essentially semicircular sectional shape. 5. A coin discriminator according to any of claims 2- 4, wherein the first and second coil frames (17a, 17b) are 10 symmetrically arranged with respect to a center plane (21) of the coil means (la, lb) , the adjacent portions (19a, 19b) of the windings (15a, 15b) running essentially parallel to this center plane. 6. A coin discriminator according to any of claims 2- 5, wherein the first and second windings (15a, 15b) comprise an equal number of turns of an electrical conductor, the number of turns preferably being a value between 10 and 100. 7. A coin discriminator according to claim 6, wherein the winding (15a) on the first coil frame (17a) is wound clockwise and the winding (15b) on the second coil frame (17b) is wound counter-clockwise. 8. A method of measuring the conductivity at a bond (11) between a first and a second portion (13a, 13b) of a coin (5) consisting of at least two different metals or metal alloys, wherein the coin is subjected to a magnetic field by coil means (la, lb) external to the coin and wherein eddy currents induced in the coin are detected by detection means (9) external to the coin, c h a r a c t e r i z e d in that the magnetic field is generated such that a loop of eddy currents (27) crosses the bond (11) in a predetermined region (25) of the coin (5) . 9. A method according to claim 8, wherein the loop of eddy currents (27) crosses the bond (11) proximate to a diameter (23) of the coin (5) . 10. A method according to claim 8 or 9, c h a r a c t e r i z e d by the further steps of compa- ring an output of the detection means (9) to a predeter- 11 mined range of conductivity values and determining, based upon a result of the comparison, whether the coin is authentic or false. 11. A method according to claim 8 or 9, c h a r a c t e r i z e d by the further steps of comparing an output of the detection means (9) to a predetermined range of conductivity values and determining, based upon a result of the comparison, whether the conductivity of the coin fulfills preset requirements or not.

Inventors: HOWELLS GEOFFREY, HOWELLS, GEOFFREY
Applicants: HOWELLS GEOFFREY, SCAN COIN AB, HOWELLS, GEOFFREY
Application year: 1999
Main classification: G07D5
IPC codes: G07D5