GB2313693A - Coin validation system - Google Patents

Abstract

A coin validation system includes a plurality of coils (21,22) in an oscillating circuit operable to create a substantially homogenous three dimensional electromagnetic field. The oscillating signal is converted into a square wave signal from which the maximum oscillating frequency on passage of a coin between the coils (21,22) is determined to obtain a measure of coin diameter. A peak detector is provided and an indication of coin composition is obtained from the lowest measured peak. The system can enable detection of coins in free fall through the coils.

Description

COIN VALIDATION SYSTEM This description relates to a system and method of electronic recognition of corns.

The coin recognition system may stand alone or be part of a wider system, e.g.

payphone, vending apparatus, or any other pay-as-you-proceed system.

The invention relates only to the recognition of coins, not to the subsequent coin handling or coin processing system.

The coin validation system comprises an electromagnetic field formed by two spaced coils, preferably ferrite based. The field is affected by the passage of a coin between the coils in two ways: A change in the combined inductance of the two coils providing the field; and A change in the Q factor of the two coils as a result of "eddy currents" induced in the coin.

The electromagnetic field is formed by two coils spaced apart, which form part of an oscillator circuit which is tuned with a single capacitor such that: Oscillation frequency equency =

wherein = the combined self and mutual inductance of two coils = (Ll + L2 + M12).

C = the value of a tuning capacitance.

Several conventional coin validation methods already exist to carry out the above measurements by either: 1 Stopping the coin and taking the measurement with the coin stationary; 2 Slowing down the coin and taking the measurement as the coin rolls along a runway wall with the floor of the runway as a fixed mechanical reference.

The main disadvantage of system 1 is that coin handling rate is relatively slow and the system requires a moving mechanical part drastically reducing the reliability of an otherwise entirely electronic design. Also because of coin dynamics, these systems can require substantial time to make the measurement e.g. > 250ms, increasing the possibility of fraud and damage to the apparatus.

Finally the system has generally to be modified mechanically for different coin sets making it difficult to export and market worldwide.

The main disadvantage of system 2 is that coin dynamics usually dictate that the coin runway moulding has to be highly engineered and manufactured with a Glass Filled Noryl making the unit expensive to design and produce. Also the apparatus has to be specified to operate with a tight mechanical attitude (e.g.

within 5 degrees of a physical reference).

The systems are unable to successfully validate poor grade coinage that may be used in third world countries. Neither 1 nor 2 operate with ease in a harsh mechanical environment (e.g. bus or train).

Examples of earlier systems are shown in GB-A-1461404 and GB-A-2207270.

The invention proposes a new system and method in which a coin is recognised in free fall under gravity, something not previously achieved.

Coin Diameter Measurement When a conductive coin passes between the coils, the mutual inductance (M12) is screened, reducing L and hence increasing the oscillation frequency in accordance with equation (1). The amount of screening (and hence measurement) is largely a function of the coin diameter. If the oscillation signal is converted into a square wave and continually measured by a microprocessor, the maximum frequency may be recorded and compared against a pre-set signature in memory to establish the coin diameter type.

By arranging the electromagnetic field to be homogenous and physically larger than the maximum coin size then the "X" or eylt position of the coin is not critical and the coin can be measured in free fall.

By arranging the microprocessor to clock internally at a much higher frequency than the oscillator then the maximum reading may be trapped even with "fast" coin dynamics, i.e. in free fall. Because of component drift/tolerance in the circuitry, it is prudent to subtract the "no coin" frequency from the maximum frequency to make the measurement.

Coin Material/thickness Measurement As the conductive coin passes the coils, eddy currents are induced in the coin, causing the amplitude of the oscillating signal to be reduced. The induced eddy current loss is a function of the bulk resistivity of the coin (i.e. diameter x thickness x resistance). As the diameter measurement has already been taken this second measurement may be used to establish thickness/material.

If the oscillating signal is passed on to a peak detector then we have a DC signal which may be measured by an A to D converter, the output of which is fed to the microprocessor. If the microprocessor continually monitors the A to D output then the minimum signal may be recorded as the coin passes the coils.

This is compared against a pre-set location in memory to establish coin thickness/ material type.

As with the diameter measurement, the homogenous nature and size of the field allows measurements to be independent of "X" and "Y" coin position. Hence measurements can be taken in free fall.

If both diameter and thickness/material measurements match then the system recognises a "VALID" coin.

The free running frequency of the oscillating circuit is preferably set to 100kHz as a compromise to optimise sensitivity of the electromagnetic field to both coin properties.

The following description concerns an example of the hardware and software design of a coin validation system according to the invention. The design is the first low cost system to operate without stopping the coin or requiring mechanical reference for measurement. The design is very low powered and suitable for use in communications or battery powered equipment.

In the accompanying drawings: Fig. 1 shows a block diagram of a validator system; Fig. 2 shows a circuit diagram of the system in Fig. 1; Figs. 3 & 4 show the physical arrangement of the coin chute; and Figs. 5 & 6 show graphs of the oscillator frequency and amplitude as a coin passes through the system.

The device comprises two parallel PCBs 4mm apart which form a coin chute and an inductor by virtue of an etched 10 turn coil on each outside face of the two PCBs. The coils themselves are thus about 7-iOmm apart. The two PCB based coils are connected in series and form part of an LC tuned circuit oscillator (see Figure 2). The tuned circuit oscillator is connected to a microprocessor which is able to read both the frequency and amplitude of the oscillating signal.

The microprocessor monitors both the frequency and amplitude of the signal and the maximum value and profile of change are used to form a signature for the coin. Both these effects increase to a maximum as the overlap area of the coin and etched coils increase to a maximum. The values in any instance are compared against values held in firmware to establish if the coin is valid.

The oscillator is formed by the longtail pair of transistors T3 and T4. The tuned circuit comprises capacitor C2 and the inductance of the sensor coils L1, L2 and oscillates at a frequency set by equation 1.

The amplitude of the oscillating signal is set by resistor R6 (which sets the collector current) and by losses associated with the tuned circuit. (Eddy current losses in the coils L1, L2; Dielectric losses in C2). C2 is selected to be a low loss device with high temperature stability and R6 is selected to ensure a no-coin oscillation amplitude of 500mV RMS app.

The circuit will operate over a wide supply variance (3.5 v to 6v) but has limited supply noise immunity (i.e. supply should be stabilized to +/- 100mv for best performance). The oscillating frequency varies little with supply but the oscillating amplitude varies significantly.

The circuit will operate over a temperature range of 0 - 50 degrees C. Transistor T2 will null out variations in performance relating to changes in the Vbe drop of transistors T3 and T4. C2 is selected to have high immunity to temperature changes.

MAKING MEASUREMENTS Oscillation Signal The oscillating signal is monitored on transistor T4 and converted to a square wave via transistor T5. The signal is fed to the input of a pulse accumulator on the CPU and the counter is read every 1 to 5ms, preferably every 3ms. These readings are then stored in a memory buffer (100 locations).

Amplitude Signal The oscillation signal is rectified via transistor Tl and fed to the input of an operational amplifier OPA-1. The amplifier is designed to detect changes in amplitude and provide some amplification of the signal before it is input to an 8 bit A to D. (This may be integral to the CPU). The A to D reading is also measured every 3ms and stored in a 100 location memory buffer. When a coin is dropped through the chute both readings vary in accordance with Figures 5 & 6 and the CPU processes the data to obtain a signature for the coin.

The chute is so designed to ensure that the coin signature is consistent in the X and Y planes (see Figures 3 & 4). To achieve this, the etched coils should be far enough away from the coin so that it is always approx. mid-distance from both coils irrespective of the coin position in the X plane The etched coil should also be large enough in the Y plane to ensure coin overlap area is independent of coin position in the Y plane. The etched coil should be large enough in the Z plane to allow the CPU to trap the reading irrespective of the coin dynamics.

The coins are validated in free fall to simplify the mechanical construction of the validator, optimize coin throughput and make the unit robust to withstand mechanical movement. (Many existing systems fail when the unit is more than 10 degrees from the normal operating angle).

To enable this to occur the system must be designed such that the microprocessor is able to make valid readings irrespective of coin dynamics, and such that the coin position in two planes does not affect the readings (see Figures 3 & 4). The only uncontrolled variable therefore is coin velocity and the design must ensure that "fast" coins will not be missed.

This is achieved as follows.

1 The physical dimensions of the coil ensures a finite time of maximum reading for the full range of coins to be measured.

2 The software performs a digital 'sample and hold on the coin reading as it passes all the way through the coil and then analyses the data in retrospect. (This allows any reading noise to be removed and a true maximum to be found).

3 The sample time for the frequency reading is arranged to be 3ms and the 'no coin' frequency arranged to be 1 megahz so that reasonable discrimination is obtained.

4 The main software is able to sample the background reading and detect the presence of a coin via a slight shift in this reading.

5 The main software is able to perform a secondary timing function to establish that the coin has passed through the coil within an acceptable time frame to minimise the fraud opportunity and detect failure of the system.

This system which allows the coin to be validated in free fall by creating a largely homogenous three dimensional electromagnetic field offers the following main advantages: Maximum coin throughput - coin validation in under 50ms (throughput in excess of 10 coins per second); High immunity to coin fraud - any attempt to interfere with the passage of the coin through the electromagnetic field is detected by a drastic reduction in coin velocity; Immune to a harsh mechanical environment - may be operated at +/- 45 degree to a mechanical norm and is immune to vibration; Has a low cost solution in that the coin chute may be ABS and of low grade mechanical design. (High tolerances etc.); May be quickly adaptede for different coin sets without the need to modify tooling to support different coin geometries; Is capable of validating coinage with poor mechanical design (which is too light to run down a sloping coin chute or where the circumference is not round (4 - 12 sided); Requires no mechanical critical or moving parts in the design; Requires no secondary coin monitoring system (optical/magnetic) to either complete the measurement or prevent coin fraud; Can be implemented using low electrical power components making it compatible with line powered telecoms; Can be marketed worldwide without changes to the mechanical configuration; Can be manufactured without the need to make electrical/mechanical adjustments or perform calibration on the production line.

Claims (15)

1. A coin validation system including an oscillating circuit including a plurality of conductive coils and driving means operable to provide an oscillating current in the coils such as to create a substantially homogenous three dimensional electromagnetic field; and means for validating a coin on the basis of field changes occurring during passage of a coin between the coils.

2. A coin validation system according to claim 1, including a mutual inductance, wherein in use the mutual inductance is screened to increase oscillation frequency.

3. A coin validation system according to claim 1 or 2, including converting means operable to convert an oscillating signal from the coils into a square wave; the validating means including processing means operative to determine the maximum oscillating frequency on passage of a coin between the coils for comparison with a stored coin signature.

4. A coin validation system according to claim 2, including a processing clock operable to drive the processing means at a frequency higher than the maximum oscillating frequency of the system.

5. A coin validation system according to claim 3 or 4, wherein the processing means is operable to determine the maximum coin oscillating frequency as the difference between the maximum oscillating frequency during the passage of a coin between the coils and the oscillating frequency when no coin is passing between the coils.

6. A coin validation system according to any preceding claim, wherein the coils are arranged to generate an electromagnetic field larger than the maximum coin size validatable by the system.

7. A coin validation system according to any preceding claim, including a peak detector coupled to the coils for measuring the maximum oscillating frequency, the or a processing means being operable to compare the lowest peak detector signal with predetermined values stored in memory.

8. A coin validating system according to any preceding claim, wherein the oscillating circuit has a free running frequency set to 100 KHz.

9. A coin validating system according to any preceding claim, wherein the validating means is operable to validate a coin after complete passage of the coin between the coils.

10. A coin validating system according to any preceding claim, including means for timing the passage of a coin between the coils, the or a processing means being operable to reject a validation if the time measured exceeds a predetermined time.

11. A method of validating a coin including the steps of providing a substantially homogeneous three dimensional electromagnetic field; passing a coin to be validated through the electromagnetic field; determining characteristics of the coin on the basis of changes in the electromagnetic field during the coin's passage therethrough; and comparing the determined characteristics with predetermined characteristics.

12. A method according to claim 1, including the step of converting an oscillating signal derived from the electromagnetic field into a square wave; the coin characteristics being derived from the maximum oscillating frequency as measured from the square wave on passage of the coin through the electromagnetic field.

13. A method according to claim 11 or 12, including the step of passing the or an oscillating signal derived from the electromagnetic field through a peak detector; obtaining the lowest peak signal from the peak detector; and comparing the lowest peak signal with a predetermined value.

14. A coin validation system substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.

15. A method of validating a coin substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.