GB2422941A

GB2422941A - Coin validation

Info

Publication number: GB2422941A
Application number: GB0501943A
Authority: GB
Inventors: Peter Dunlop; Khagendra Thapa
Original assignee: Innovative Technology Ltd
Current assignee: Innovative Technology Ltd
Priority date: 2005-01-31
Filing date: 2005-01-31
Publication date: 2006-08-09
Also published as: GB0501943D0

Abstract

A method of determining a plurality of physical and material characteristics of a coin (such as coin thickness, conductivity, and permeability) as separate parameters. The method comprising the step of measuring the amplitude and time of a transmission current in a transmission coil used to generate a magnetic field, measuring the amplitude and time of a voltage induced in a receiving sensor by the magnetic field's interaction with a coin when the coin is placed between the transmission coil and the receiving sensor, and analysing the time and amplitude relationship between the measured transmission current and the induced voltage to determine the characteristics as separate parameters.

Description

* 2422941 Improvements Relating to Coin Validation The present invention

concerns improvements relating to coin validation, and more particularly, though not exclusively to an improved method of measuring the material and physical characteristics of a coin in a separable way in order to determine accurately the validity of that coin. Such a method is used in an improved coin validation apparatus.

The Problem: Measurement of coin conductivity, permeability and thickness For many years the existing designers of coin validation equipment have been using inductive effects to measure the physical characteristics (such as coin thickness) and material characteristics (such as permeability and conductivity) of coins. In order to do this they have been measuring the effect a coin has on the magnetic fields of adjacent coils. The coils are connected to create an oscillator circuit. The coin is usually rolling between two coils that are connected electrically in series with each other. These coils may be connected so that their respective generated magnetic fields either oppose or attract each other (see Mars, Coin Controls et al.).

The presence of a coin moving into the coils' magnetic field disturbs the frequency and or amplitude of oscillation. This happens because the coin has an eddy current induced into it. This eddy current in turn produces a magnetic field, which in turn alters the coils' magnetic field. Various mechanical arrangements have been used.

Many oscillator types have been used, sometimes the coils are arranged to operate at or near resonance to give the maximum output changes/sensitivity in the presence of coins (see Coin Controls et al.).

However, all these prior art constructions produce a measurement that is dependant or partially dependant upon the coin thickness, conductivity, permeability and occasionally diameter as well. It is very difficult to create a map or look-up table for a given circuit/coil/coin combination so that these unique properties can be separately measured. Because of this, false coins have been made that approximate the coin diameter, thickness and conductivity and permeability. For example brass discs that are the same diameter, with a dished or thinned out centre have been accepted as real coins, made from a copper nickel alloy. Two foreign coins that may be glued together producing the correct diameter and approximate thickness etc. Because the conventional way of carrying out measurement is dependent upon three or possibly four parameters in combination, it is extremely difficult to predict if other currencies could be confused with each other and also what kinds of false coin could be a problem.

The Invention: Referring now to Figure 1, a coin sensing apparatus embodying the present invention is shown. The apparatus is provided within a coin validator (not shown). In use a coin to be validated by being sensed is placed between two coils, in such a way that the coin is larger than the two coils. One coil is used as a transmitter (TX coil), the other coil is used as a receiver (RX coil). An alternating current is supplied to the transmission coil. This creates a magnetic field (shown as an arrow) that passes through the coin and is detected by the receiver coil. As the coin completely covers the transmission coil, none of the generated magnetic flux passes around the sides of the coin. This prevents confusing edge effects from being created which would, if allowed to occur, introduce errors into the measurements made of the physical and material properties of the coin.

Transmitter side: The current in the transmission coil is measured for amplitude and phase. If required it may be provided by a constant current source. This is important as the magnetic field generated is only proportional to the transmission coil's current and the number of turns of wire in the transmission coil. The frequency of the current is also required to be varied (see for example Mars 1971). The transmitted flux is now known, regardless of any objects in the vicinity of the magnetic flux (note the flux path is not known).

Receiver side: The receiver coil is connected to a high input impedance amplifier (not shown) and the output voltage measured in amplitude and phase relative to the transmission coil's drive current. As the amplifier is very high impedance, the output voltage is directly proportional to the received magnetic flux. In alternative embodiments, the receiver coil may be substituted by a different type of magnetic flux sensor, for example a Hall Effect device or a magneto resistive device.

With no coin or object between the transmission and receiver coils, no attenuation or phase shift is created (assuming relative permeability of air is 1, and conductivity is zero). If a coin is now placed between these two coils, and the transmission current and phase measured, the receive voltage and phase measured, it is possible to plot a Nyquist chart, as shown in Figure 2, describing the absorption by the coin of the magnetic flux with frequency. This is achieved by a microprocessor (not shown) being provided with the apparatus, which receives these measurements as inputs and calculates the required Nyquist plot.

As frequency increases the amount of flux that can pass through the coin reduces and changes phase. This is a consequence of the so-called skin effect' of the conducting metal.

By measuring the speed with which the locus of the absorption characteristic vector changes, it is possible to measure the conductivity of the coin, alone. The shape of the locus describes the magnetic permeability alone, and the point at which the locus crosses the 90degree phase shift line is directly related to the thickness of the coin alone. In this way, the physical and material characteristics of the coin sensed by the receiving coil may be broken down into separately measurable parameters, thickness, conductivity, and permeability.

This apparatus and method of measurement has other advantages. For example, the apparatus can automatically calibrate out differences in mechanical size of the coils and their relationship in space, temperature changes and ageing effects, by simply monitoring its own behaviour with no coin present and removing any offsets or gain changes mathematically. This,, in effect, creates a sensing system with closed loop feedback, which is inherently more stable than an open ioop sensing system used elsewhere.

In the present embodiment, it is necessary that the coin be stationary whilst this multi- frequency measurements are taken. However, in alternative embodiments, which rely on faster possibly more expensive microprocessors, this may not be necessary. A delta function could be applied to the transmission coil as the coin moves between it and the receiver coil. The output signal could then be mathematically reconstructed using Fourier analysis (hence the need for a faster microprocessor) and the same result obtained as a static system.

It is to be appreciated that the present invention utilises several ideas that are already known individually, namely: * The feature of skin effect': a measure of how deep inside a conductive material currents will flow and its relationship with frequency.

* The feature of a coin between two coils which is known and used by many people.

However, the coils are usually connected to each other.

* The feature of measurement of frequency change is known.

* The feature of measurement of phase shift is sometimes discussed in the prior art.

An important part of the present invention is the identification of the separate transmission current and the separate received voltage and the amplitude and phase relationship between the two. This provides the ability to consider the material and physical characteristics of the coin separately as has been discussed above. In this regard, the present invention can be considered at its broadest aspect to provide a means to determine physical and material characteristics of a coin (such as coin thickness, conductivity, and permeability) as separate parameters using a transmission coil in which the current is measured in amplitude and time and a receiving coil in which the voltage and relative time is measured, whilst the coin is between the two coils and completely shading them from each other.

Claims

1. A method of determining a plurality of physical and material characteristics of a coin (such as coin thickness, conductivity, and permeability) as separate parameters; the method comprising: measuring the amplitude and time of a transmission current in a transmission

coil used to generate a magnetic field;

measuring the amplitude and time of a voltage induced in a receiving sensor by the magnetic field's interaction with a coin when the coin is placed between the transmission coil and the receiving sensor; and analysing the time and amplitude relationship between the measured transmission current and the induced voltage to determine the characteristics as separate parameters.

2. A method according to Claim 1, wherein the measuring step comprises measuring the amplitude and time of the voltage induced in the receiving sensor by the magnetic field's interaction with a coin, larger than either the coil or the sensor, when the coin is placed in a position so as to prevent the occurrence of any magnetic

field edge effects occurring.

3. A method according to Claim 1 or 2, wherein the plurality of physical and material characteristics of a coin comprise the coin thickness, the coin material conductivity, and the coin material permeability.

4. A method according to any preceding claim, wherein the analysing step comprises measuring the frequency, amplitude and phase relationship to create a Nyquist chart or equivalent frequency, phase, amplitude relationship (Bode plot).

5. A method according to Claim 4, wherein the analysing step comprises determining each of the different physical and material characteristics of the coin from a predetermined distinct feature of the Nyquist chart or Bode plot.

6. A method according to any of Claim 1 to 3, wherein the measuring steps comprise measuring the frequency and amplitude of the transmission current and induced voltage and the analysing step comprises comparing the frequency and amplitude relationship of the transmission current and the induced voltage.

7. A method according to Claim 6, wherein the receiving sensor comprises a Hall Effect or magneto-resistive effect sensor arid the step of measuring the induced voltage comprises using the Hall Effect sensor or the magneto resistive effect sensor.

8. A method according to any preceding claim, further comprising varying the frequency of the transmission current to generate different magnetic fields.

9. A method according to Claim 8, wherein the varying step is controlled by a microprocessor controller.

10. A method according to any preceding claim, further comprising: measuring the amplitude and time of an ambient voltage induced in a receiving sensor, when no coin is placed between the transmission coil and the receiving sensor; and offsetting the results of the analysis step using the results of the ambient voltage measurement step.

11. An apparatus for determining a plurality of physical and material characteristics of a coin (such as coin thickness, conductivity, and permeability) as separate parameters; the apparatus comprising: a transmission coil for generating a magnetic field; means for measuring the amplitude and time of a transmission current; a receiving sensor for measuring a received magnetic field when a coin, is placed between the transmission coil and the receiving sensor; means for measuring the amplitude and time of a received voltage induced in the receiving sensor by the magnetic field's interaction with the coin; and means for analysing the phase and amplitude relationship between the measured transmission current and the received voltage to determine the characteristics as separate parameters.

12. An apparatus according to Claim 11, wherein the receiving sensor comprises a Hall Effect sensor or a magneto resistive effect sensor.

13. An apparatus according to Claim 11 or 12, wherein the analysing means comprises a microprocessor.

14. An apparatus according to Claim 13, wherein the microprocessor is arranged to vary the frequency of the transmission current to generate varying magnetic fields for the coin validation.

15. A coin validation device comprising an apparatus according to any of Claims lltol4.

16. An apparatus or method substantially as described hereinbefore with reference to the accompanying drawings.