GB2092798A - Coin discriminator - Google Patents

Abstract

A coin discriminator for a coin operable machine, which comprises a coil-type sensor 19 having a transmit coil 12 and a receive coil 14, these coils 12, 14 being arranged with their magnetic flux axes at right angles so that there is normally a minimum magnetic flux coupling between the coils. A coin feed path 10 extends through the coils 12, 14 to pass the coin 11 under test centrally through the coils, at 45 degrees to the planes of the respective coils for maximum sensitivity, so that the relative amplitude and phase of the signal induced in the receive coil 14 is dependent at least primarily only on eddy currents produced in the coin. <IMAGE>

Description

SPECIFICATION Coin discriminator This invention relates to devices for discriminating between coins in a coin acceptor, which may be incorporated in any type of machine requiring coins for its operation. Such devices are intended to determine whether a coin presented to it is of an expected denomination and are designed to reject other coins or fake coins.

A large number of techniques have previously been proposed for measuring the characteristics of coins. In general the relevant parameters are: Coin thickness and diameter; Magnetic properties; Electrical characteristics.

Many discriminators subject the coin under test to a magnetic field which is generated by a transmit coil. A receive coil is then used to sense the field and to detect the effect of introducing a coin.

Previous discriminators using a transmit coil and a receive coil have positioned these coils in such a way that the field produced by one coil is coupled into the other at all times. The coin therefore only modifies the received signal, often by reducing the coupling between the coils, and therefore causes only a small percentage change in the measured parameters.

It is an object of the present invention to provide an improved coin discriminator in which a much larger percentage change in the received signal occurs when a coin is introduced, thereby to enable better discrimination.

According to one aspect of the present invention, there is provided a coin discriminator which comprises a coil-type sensor having an a.c.

powered transmit coil and a receive coil so positioned relative to the transmit coil in the magnetic flux field of the latter that there is normally a minimal coupling between the two coils, the coil-type sensor being adapted for positioning relative to a coin feed path so that a coin on said path will cause a substantial coupling between the coils substantially solely due to the generation of induced eddy currents in the coin.

Thus, the invention provides a sensor in which the receive coil is so positioned relative to the transmit coil that the flux field of the latter is balanced with respect to the receiving coil and either tends to introduce opposed self-cancelling currents in the receive coil or in effect said flux field does not cut the plane of the receiving coil. In both cases there is said herein to be a minimal coupling between the coils. A major feature of the invention is concerned with the latter case.

Thus, according to another aspect of the present invention, a coin discriminator incorporates a transmit coil and a receive coil which are arranged with their magnetic axes at right angles so that normally there is very little coupling between them. In such an arrangement a signal introduced into the transmit coil will produce only a very small signal in the receive coil.

If a coin is located between the coils, the eddy currents induced therein will disturb the magnetic flux field so that it now cuts the receive coil, whereby the amplitude of the received signal is substantially altered. Means is provided to sense this change and determine if it is such as a true coin of the correct size would produce.

It is found that different coin materials will cause different changes in the phase of the received signal with respect to that of the transmitted signal, and according to a preferred feature of the invention means is also provided for measuring the phase angle between the two signals and further means is provided for determining whether this is such as a true coin of the correct material would produce.

A maximum discriminating effect can be achieved if the sensor is adapted for positioning with the planes of the respective coils intersecting at right angles on a line coplanar with the path of passage of a coin on the coin feed path. The change in output of the receive coil can then be up to five times the change which a similar coin produces when passed between spaced, parallel, opposed transmit and receive coils.

Means may be provided, responsive to two determinations (amplitude change and phase change of the received signal) to generate an output signal of one value if the correct changes have been detected (so as to accept the coin) or of another value in the alternative (so as to reject the coin).

The transfer characteristics between the coils may be used in a number of ways to determine which type of coin has been introduced.

In one arrangement a fixed frequency signal, for example produced by an oscillator in the form of a ceramic oscillator, is introduced into the transmit coil and the amplitude and phase of the received signal are measured independently, preferably with reference to the transmit coil current (this being more stable with respect to temperature variations than the transmit coil voltage), and compared with preset limits.

In another embodiment the two coils may be incorporated into the feedback circuit of an oscillator and a detector threshold adjusted so that the oscillator will only begin to oscillate when a certain type of coin is introduced.

In a third embodiment the coils may be introduced into the feedback circuit of an oscillator whose frequency and output amplitude are monitored and compared with preset values to produce output signals.

The discriminator in accordance with the invention will now be described with reference to the accompanying drawings, by way of example.

In the drawings: Figures 1 and 2 show how two coils of a coiltype sensor may be arranged with their magnetic axes at right angles; Figure 3 is a block circuit diagram of one embodiment of the invention; Figure 4 is a block diagram of an alternative embodiment; Figure 5 shows a modification; and Figure 6 is a block diagram of a preferred arrangement.

In the coil-type sensor diagrammatically illustrated in Figure 1, a first coil 1 2 is arranged so that its magnetic axis 13, which is generally perpendicular to its plane in the case of a flat coil, is orthogonal to the magnetic axis 1 5 of a second coil 14. One coil serves as a transmit coil and the other is a receive coil. As shown the two magnetic axes intersect, but this need not necessarily be the case. Normally, only a minimal magnetic coupling exists between such two coils 12, 14.

Figure 2 shows a coin 11 approaching a particularly sensitive region between the two coils 12, 14, in which eddy currents induced in the coin substantially change the magnetic coupling to produce a substantial signal in the receive coil. In the preferred situation illustrated, the two flat coils 12, 14 lie in mutually perpendicular planes intersecting on a line which is aligned with the coin feed path 10 so that the coin 11 will pass through said line at an equal angle (45 degrees) to both coils.

The diagram in Figure 3 shows one embodiment of the invention. A stable oscillator 17 provides a sinusoidal, fixed frequency output, of a known amplitude, which is fed to the transmit coil 12. The signal from the receive coil 14 is fed to an amplitude measuring circuit and to a phase measuring circuit.

Thus, the amplitude is measured by means of an amplifier 16, peak detector 1 8 and two threshold circuits 20 and 22 which may be adjusted to detect the voltage level expected from the genuine coin.

The phase is measured by two zero crossing detectors 24 and 26 from oscillator 10 and the amplifier 1 6 respectively. The outputs from the zero crossing detectors are combined in an exclusive-OR gate 28, and the mean level of its output is generated in an averaging circuit 30.

This feeds two threshold circuits 32 and 34 which are adjusted to detect the voltage level expected from a genuine coin.

The outputs from all the threshold detectors are gated at 35 together to give a true output 36 only if both the amplitude and phase are within the preset limits.

An alternative and preferred circuit is shown in block diagram form in Figure 4. Here, the oscillator 1 7 again energises the transmit coil 12 of two orthogonally arranged coils 12, 14 of a sensor 19 through which a coin feed path extends in the manner illustrated in Figure 2.

The respective signals in the coils 12, 14 are compared for correct relative amplitude in a comparator 37 and for correct relative phase in a comparator 38, and the two comparator outputs are gated at 40 to control generation of a coin accept signal 36.

In the modification shown in Figure 5, the coils 12, 14 of the sensor 1 9 are connected in the feedback of a variable phase decay oscillator 42.

The phase delay circuit 43 is adjusted so that only a genuine coin of correct value will cause the circuit to oscillate at a given frequency, which is detected by a frequency validator 44.

A preferred pratical form of the Figure 4 circuit is illustrated in block diagram form in Figure 6.

Here the oscillator 17, such as a high stability ceramic resonator provides an output through a frequency divider 46 to power the transmit coil 12. A crystal oscillator or an R-C oscillator could be employed instead of a ceramic resonator. A frequency drive of between 5 kHz and 1 5 kHz, in particular a frequency of 12.5 kHz obtained from a 400 kHz resonator, is preferably employed.

Reference 48 denotes a current limiting resistor.

More important, however, is the resistor 50 connected in series with the transmit coil 12; this enables the transmit coil current (as distinct from voltage) to be used for measurement purposes, thus giving improved temperature stability.

The signal outputs of the coils 12, 14, which are orthogonally arranged in the sensor on the coin feed path as heretofore described, are fed to a validator 52 containing window comparators.

The output 53 of the validator 52 (true coin signal) passes to a control logic circuit 54 which supervises a timer 56 driven by a second output of the divider 46.

The control logic circuit 54 will only provide an accept signal not only if a true coin signal is received from the validator 52 butalso if the coin passes in the correct manner along the coin feed path, e.g. as sensed by one or more further sensing devices.

In all the described arrangements, it is the orthogonality of the coils 12, 14, or equivalent normal flux-balanced condition, which enables a highly discriminative validation to be executed.

When a coin passes through the coils, preferably on a feed path on which the coils are arranged as shown in Figure 2, the coupling between the coils is changed substantially only due to the production of eddy currents in the coin, i.e. not due to interruption by the coin of a normal flux coupling. The resulting amplitude and phase changes in the receive coil signal are thus substantial, in percentage terms, and can be accurately detected and measured.

Winding of the coils on plastics formers is preferred, and it is important that the coils 12, 14 should be shielded, as against influence of external flux sources. For the latter purpose, a material having a small magnetic penetration skin depth, such as mild steel, nickel/iron alloy, e.g. m metal or radio-metal, is preferred.

The preferred circuit of Figure 6 is readily adaptable to the testing of more than one specific true coin using a plurality of window comparators in the validator. On the other hand, when the device is mass produced for testing any selected one of a plurality of possible coins, means may be incorporated for adjusting the comparator windows and for adjusting the frequency of the transmit coil drive for maximum sensitivity.

It skill be appreciated that the above-described embodiments may be modified in various ways within the scope of the invention as hereinbefore defined.

Claims (14)

1. A coin discriminator which comprises a coiltype sensor having an a.c. powered transmit coil and a receive coil so positioned relative to the transmit coil in the magnetic flux field of the latter that there is normally a minimal coupling between the two coils, the coil-type sensor being adapted for positioning relative to a coin feed path so that a coin on said path will cause a substantial coupling between the coils substantially solely due to the generation of induced eddy currents in the coin.

2. A coin discriminator according to claim 1, wherein the transmit coil and the receive coil are relatively positioned with their magnetic axes at right angles.

3. A coin discriminator according to claim 2, wherein the transmit coil and the receive coil are relatively positioned so that their axes intersect.

4. A coin discriminator according to claim 2 or claim 3, wherein the sensor is adapted for positioning with the magnetic axes of both coils substantially at 45 degrees to the direction of the coin feed path.

5. A coin discriminator according to claim 4 when appendant to claim 3, wherein the sensor is adapted for positioning with the planes of the respective coils intersecting at right angles on a line coplanar with the path of passage of a coin on the coin feed path.

6. A coin discriminator according to any of claims 1 to 5, wherein the transmit coil is a.c.

powered at a frequency in the range from several hundred Hz up to several hundred kHz, preferably between 5 and 15kHz.

7. A coin discriminator according to any of claims 1 to 6, wherein detectors are provided for detecting both change in amplitude and change in phase of the output of the receive coil due to passage of a coin.

8. A coin discriminator according to claim 7, wherein said detectors include means for comparing the transmit coil output with the receive coil output.

9. A coin discriminator according to claim 8, wherein the transmit coil output used for the comparison is the transmit coil current.

10. A coin discriminator according to claim 8 or claim 9, wherein the compared outputs are measured both in respect of amplitude change and phase change at a defined coin position relative to the sensor and a gate is provided for producing a true coin signal only if both measurements are correct.

11. A coin discriminator according to any of claims 1 to 10, wherein the coils are shielded with a material having a small magnetic field penetration skin depth.

12. A coin discriminator according to claim 11, wherein the shielding material is mild steel or a nickel/iron alloy.

13. A coin discriminator according to any of claims 1 to 12, wherein the transmit coil is powered by an oscillator in the form of a ceramic resonator or a crystal oscillator or an R-C oscillator.

14. A coin discriminator substantially as hereinbefore described with reference to the accompanying drawings.