GB2393840A

GB2393840A - Coin validation by measuring reflected magnetic field non-homogeneity

Info

Publication number: GB2393840A
Application number: GB0222724A
Authority: GB
Inventors: Thapa Kagendra; Peter Dunlop; Ross Jones
Original assignee: Innovative Technology Ltd
Current assignee: Innovative Technology Ltd
Priority date: 2002-10-01
Filing date: 2002-10-01
Publication date: 2004-04-07
Also published as: GB0222724D0

Abstract

A method of validating a coin by measuring the non-homogeneity of a reflected magnetic field from the coin. The method comprises the steps of: generating a magnetic field in a transmission coil by way of an alternating drive current; measuring the size of the drive current in amplitude and time to determine the strength of the generated magnetic field; measuring the size of an electrical signal in a reception sensor representative of the size of the reflected magnetic field from the coin by measuring the induced voltage in the reception sensor over time, whilst the coin is adjacent both the transmission coil and the reflection sensor; and comparing the measured amplitude and phase of the induced electrical signal relative to the transmission coil's drive current to determine a plot of received magnetic flux with frequency, the plot indicating the level of non-homogeneity of the reflected magnetic field of the coin.

Description

1 2393840

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 non 5 homogeneity of a coin in order to determine the validity of that coin. Such a method is used in an improved coin validation apparatus.

The Problem: Measurement of coin non-homogeneity.

For many years the existing designers of coin validation equipment have been using 10 inductive effects. They have been measuring the effect a coin has on the magnetic fields of a coil(s). The coils are connected to create an oscillator circuit. The coin is

usually rolling between two coils that are usually connected in series with each other.

These coils may be connected so that their fields either oppose or attract each other

(see for example patent applications from 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.

20 Many oscillator types have been used, sometimes the coil(s) are arranged to operate at or near resonance to give the maximum output changes/sensitivity in the presence of coins. (see for example patent applications from Coin Controls et al.) However all the embodiments produce a measurement that is dependent or partially 25 dependent 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 30 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 measurement is dependent upon three or possibly four parameters 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.

5 Another problem occurs with coins that are made from laminates of different alloys -

for example the 1 euro and 2euro coins. The centre section is composed of three layers of metal alloys. With typical existing methods it is theoretically possible to produce a homogenous metal alloy that has the same thickness, conductivity and permeability as a laminated nonhomogenous coin. This may be readily modelled using simple two-

10 dimensional finite element analysis techniques.

The Invention: Referring now to Figure 1, a coin is placed adjacent to two coils, in such a way that the coin is larger than the two coils. One coil is used as a transmitter and the other coil 15 is used as a receiver. An alternating electrical current is supplied to the transmitter coil. This creates a magnetic field that passes through the coin and to a limited extent

bounces of the surfaces and or internal layer boundaries inside the coin, back towards the transmission source. The amount of flux reflected back can be measured in amplitude and phase in relation to transmission source.

The receiver may be a coil of wire, a Hall effect sensor or a magneto effect sensor.

The flux "bounces back" because of the changes in impedance of the air to metal and metal to metal boundaries inside the coin. This is the same effect as reflections seen in a window by a person looking out. If they focus on the glass they see a partial 25 reflection of themselves as well as the view outside.

Transmission 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 very important, as the magnetic field generated is only proportional to the coils' current

30 and the number of turns of wire. The frequency of the current is also required to be varied (see Mars 1971). The transmitted field is now known, regardless of any objects

in the vicinity of the magnetic flux.

Receive coil on the same side as the transmission coil: The receive coil is connected to an high input impedance amplifier and the output voltage measured in amplitude and phase relative to the transmission coils drive current. As the amplifier is very high impedance, the output voltage is directly proportional to the received magnetic flux.

5 Note the receive coil may be substituted with a different type of magnetic flux sensor, for example a Hall effect device or a magneto resistive device.

With no coin or object adjacent to the transmitter and receiver coils, no attenuation or phase shift is created (assuming relative permeability of air is one, and conductivity is 10 zero).

If a coin is now adjacent to these two coils, and the transmission current and phase measured, the received voltage and phase measured, it is possible to plot (see Figure 2) a Nyquist chart reflection by the coin of the magnetic flux with frequency.

As frequency increases the amount of flux that is reflected changes. This is a consequence of the skin affect of the metal(s) and their relative susceptibility.

The shape of the locus reveals changes in homogeneity within the coin. As a result non-homogeneous coins have a different reflected flux locus path and can be uniquely 20 identified. This is true even if the coin thickness and outer layers are the same/similar to a homogeneous coin.

In this invention several ideas are already known: 25 À Skin effect:- a measure of how deep inside a conductive material currents will flow and its relationship with frequency À Measurement of frequency change - very common idea Measurement of phase shift has sometimes been discussed.

30 The important part of this invention is the identification of the separate transmission current and the separate received voltage and the concept of looking at the reflected flux path, caused by air to metal and metal to metal impedance changes, and the amplitude and phase relationship between the two. In this regard, the present

invention can be consider at its broadest aspect to provide a means to determine coin non-homogeneity using a transmission coil in which the current is measured in amplitude and time and a receive coil (sensor) in which the voltage and relative time is measured, whilst the coin is adjacent to the two coils.

Claims

Claims:

1. A method of validating a coin by measuring the non-homogeneity of a reflected magnetic field from the coin, the method comprising:

generating a magnetic field in a transmission coil by way of an alternating

5 drive current, measuring the size of the drive current in amplitude and time to determine the strength of the generated magnetic field;

measuring the size of an electrical signal in a reception sensor representative of the size of the reflected magnetic field from the coin by measuring the induced

10 voltage in the reception sensor over time, whilst the coin is adjacent both the transmission coil and the reflection sensor, and comparing the measured amplitude and phase of the induced electrical signal relative to the transmission coil's drive current to determine a plot of received magnetic flux with frequency, the plot indicating the level of nonhomogeneity of the 15 reflected magnetic field of the coin.

2. A method according to Claim 1, wherein the reception sensor comprises an inductive coil.

20

3. A method according to Claim 1, wherein the reception sensor comprises a Hall sensor or a magneto restrictive effect sensor.

4. A method according to any preceding claim, wherein only the frequency and amplitude of the induced electrical signal are measured and compared with the drive 25 current.

5. A method according to any preceding claim, wherein the drive current measuring step comprises measuring the phase of the drive current.

30

6. A method according to any preceding claim, wherein the comparing step comprises comparing the measured amplitude and phase of the induced electrical signal relative to the transmission coil's drive current to determine a Nyquist plot of received magnetic flux with frequency.

7. A coin validation apparatus for measuring the non-homogeneity of a reflected magnetic field from a coin, the apparatus comprising:

a transmission coil for transmitting a generated magnetic field to the coin

when the same is provided at a sensing location; 5 a reception sensor for receiving a reflected magnetic field from the coin in the

sensing location; a magnetic field generator for generating the magnetic field in the transmission

coil by use of an alternating drive current; first measuring means for measuring the size of the drive current in amplitude 10 and time to determine the strength of the generated magnetic field;

second measuring means for measuring the size of an electrical signal in the reception sensor representative of the size of the reflected magnetic field from the

coin at the sensing location by measuring the induced voltage in the reception sensor over time, whilst the coin is adjacent both the transmission coil and the reception 15 sensor, and comparing means for comparing the measured amplitude and phase of the electrical signal relative to the drive current such that a plot of received magnetic flux with frequency can be determined, the plot indicating the level of nonhomogeneity of the reflected magnetic field of the coin.

8. An apparatus according to Claim 7 wherein the second measuring means comprises a high impedance amplifier.

9. An apparatus according to Claim 7 or 8, wherein the reception sensor 25 comprises a Hall sensor or a magneto restrictive effect sensor.

10. An apparatus according to Claim 7 or 8, wherein the reception sensor comprises an inductive coil.

30

11. An apparatus according to any of Claims 7 to 10, wherein the second measuring means is arranged to measure only the frequency and amplitude of the electrical signal and to compared this with the measured amount of the drive current.

12. A method or apparatus substantially as described herein with reference to the accompanying drawings.