GB2271875A - Coin validator - Google Patents

Abstract

In a coin validation system a coin 1 passes between conductive plates 7, 9 which form a capacitor, which provides part of the frequency controlling capacitance of an LC tuned oscillator circuit 23. The presence of a coin 1 between the conductive plates 7, 9 alters the capacitance, and consequently the output frequency of the circuit 23. The oscillator output is supplied via a frequency divider 27 counter 31 and shift register 33 to a microprocessor 35 which subtracts the count value from a pre-stored reference value, and supplies the difference to a look-up table stored in a memory 37. The output of the look-up table 47 determines whether a coin is valid, and if so, what its denomination is. Preferably, while no coin is present the microprocessor 39 monitors the count values obtained from the counter 31, and updates the stored reference value so that it tracks drift in the frequency of the oscillator 23. The validator may also include optical sensors determining coin diameter. <IMAGE>

Description

COIN VALIDATORS This invention relates to coin validators such as for use in pay telephones or vending machines.

It should be understood that the word "coin" as used herein is not limited to money in general circulation, but may cover a token or slug of any form regardless of whether it has any monetary value and the term "coin validation" is intended to cover the validation such of tokens or slugs.

There have been proposed, e.g. in GB-A-2062327 and US-A-4184366, systems for detecting whether a coin exceeds a threshold diameter, by providing a first capacitor plate spaced from a second capacitor plate.

As a coin passes along a coin chute, it will overlap both capacitor plates at the same time, thereby coupling a signal from one plate to the other, if the coin is large enough to bridge the space between the plates. Therefore the presence or absence of the signal indicates whether the coin exceeds the threshold diameter. Such a system cannot measure the coin diameter, but merely decides whether it exceeds the threshold. US-A-4184366 provides a plurality of second capacitor plates to provide a plurality of diameter thresholds.

GB-A-1464371 and WO 86/06246 propose a capacitor the capacitance of which is altered by a passing coin.

In GB-A-1464371 a signal at a preset frequency is applied to the capacitor and the amplitude of the current flow through the capacitor is detected. In WO 86/06246 the capacitor is provided in an RC circuit to which a signal at a preset frequency is applied, and the amplitude of the current in the RC circuit is detected. In each case, the amplitude is a measure of a property of the coin, and accordingly it can be applied to threshold detectors to determine whether the property of the coin exceeds corresponding thresholds. GB-A-2174227 proposes a system in which a voltage change is caused by a coin passing between capacitor plates and the size of the change is digitised and supplied to a microprocessor.

GB-A-994736 proposes a system in which a coin alters the capacitance of a capacitor in a resonant circuit, thereby altering the Q value of the resonant circuit. The resonant circuit is provided in an oscillator feedback loop so that the oscillator either will or will not oscillate depending on the Q value.

Accordingly, the presence or absence of an oscillator output while a coin is present provides a threshold detection of a property of the coin.

GB-A-2174227 and US-A-4184366, referred to above, also both propose that the coin is used to affect the inductance of a coil. In GB-A-2174227, the inductance change is detected by detecting the change in resonant amplitude of a resonant circuit comprising the coil, as described in GB-A-2169429. In US-A-4184336, the inductance change is detected by detecting the change in the frequency of an oscillator controlled by a tuned circuit comprising the coil.

In one aspect, the present invention provides a system in which a coin affects a capacitance so as to alter the frequency of an oscillator, and the altered oscillator frequency is used to determine which of a plurality of acceptable coins has been input.

In another aspect, the invention provides a coin validation system comprising a detection circuit including a coin sensor and a guide means arranged to guide a coin to be validated between conductive plates to cause a change in the frequency of a signal in the detection circuit which change is indicative of the denomination and validity of the coin.

Embodiments of the invention, given by way of non-limiting example, will now be described with reference to the accompanying drawings, in which: Figure 1 is a side view of the coin sensing portion of a coin validation system according to an embodiment of the invention; Figure 2 is a section along the line II-II of Figure 1; Figure 3 is an electrical block diagram of the coin validator of Figure 1 and Figure 2; Figure 4 is a schematic diagram of a memory in the circuit of Figure 3; Figure 5a is a diagram illustrating a coin moving between the sensor plates of the embodiment of Figure 1; Figure 5b is a diagram which illustrates signals produced in the circuitry of Figure 3 as a coin moves between the sensor plates of Figure 5a; Figure 6 shows an example of an oscillator circuit which may be used in the circuit of Figure 3;; Figures 7a and 7b are diagrams similar to Figures 5a and 5b but showing a second embodiment; Figure 8a and 8b are diagrams similar to Figures 5a and 5b but showing a third embodiment; Figures 9a and 9b are diagrams similar to Figures 5a and 5b but showing a fourth embodiment; Figure 10 shows a section similar to Figure 2 but showing a fifth embodiment of the invention; Figures ila and llb are similar to Figures 5a and 5b but showing the fifth embodiment of the invention; Figure 12 is an example of an oscillator circuit for use in the embodiment of Figures 10 and 11, which circuit is a modification of the circuit of Figure 6; and Figure 13 is an electrical block diagram showing an alternative circuit to that shown in Figure 3.

Figures 1 to 4 show a coin validation system, which is for receiving and discriminating between valid and invalid coins and determining the denomination of valid coins. The system comprises a coin sensing portion 14, shown in schematic side view in Figure 1.

In Figure 1, a coin 1 enters the coin sensing portion 14 through an aperture 15, and rolls down a longitudinally inclined guide 3 which defines a coin path P.

As the coin 1 rolls down the guide 3, it passes between conductive plates 7, 9, which form a capacitor. The presence of the coin 1 between the conductive plates 7, 9 will alter the capacitance of the capacitor, and this alteration is detected by a detection circuit 11 as will be described later. As can be seen in Figure 2, the conductive plates 7, 9 are provided on the outside of walls of the guide 3, so that .the coin 1 does not contact them. This protects the conductive plates 7, 9 from mechanical abrasion by the coin 1. Additionally, the guide 3 is made of non-conductive material so as to insulate electrically from the conductive plates 7, 9 from each other.

The guide 3 has a U-shaped section, with a wall-to-wall separation of about 4mm. It is also inclined laterally as shown in Figure 2. The lateral inclination is not shown in Figure 1 for clarity. The lateral inclination of the guide 3 causes the coin 1 to rest against side wall 2 of the guide 3 as well as resting on the floor 4 of the guide 3. Consequently the radial direction of the coin 1 is maintained parallel to the conductive plates 7, 9 and the position of the coin across the width of the gap between the conductive plates 7, 9 is determined.

This causes all coins to follow the same coin path P, to enable consistent detection of coins.

The conductive plates 7, 9 preferably extend from the bottom of the guide 3 up to a height equal to or slightly greater than the height of the greatest diameter coin intended to be accepted by the validator.

The conductive plates 7, 9 may be provided by any convenient method, such as plating them onto the guide 3 using printed circuit techniques, printing them with a conductive ink, or by adhering pieces of metal (e.g. copper or copper alloy) foil to the guide 3.

The detection circuit 11 is provided on a circuit board mounted alongside the guide 3, and spaced about 10mm to 15mm from it, as shown in Figure 2.

The sensing portion 14 of the coin validation system is enclosed in an RF-shielding box 13 in which aperture 15 is provided. At the end of the guide 3 the coin 1 leaves the RF-shielding box 13 through an exit aperture 17.

Figure 3 shows the electrical circuit of the coin validation system in block form. The detection circuit 11, for detecting alteration in the capacitance of the capacitor formed by the conductive plates 7, 9, is provided inside the RF-shielding box 13. It is connected to a signal processing portion 12, which is outside the RF-shielding box 13, by a coaxial cable 19.

As shown in Figure 3, the detection circuit 11 comprises an oscillator circuit 23 to which the conductive plates 7, 9 are connected. The frequency at which the oscillator circuit 23 oscillates depends on the capacitance of the capacitor formed by the conductive plates 7, 9. The oscillator circuit 23 is tuned to oscillate at a predetermined nominal frequency, for example 192 NHz when no coin is present between conductive plates 7, 9. The oscillator circuit 23 has an output fed via a buffer 25 to a frequency divider 27. The frequency divider 27 divides the frequency of its input by, for example, 32 to produce a nominal output frequency of, for example, 6 MHz when no coin is present between conductive plates 7, 9.

The oscillator circuit 23 may be implemented as shown in Figure 6, in which capacitor C represents the capacitance of the conductive plates 7, 9 and the path P of the coin 1 is shown passing between the conductive plates 7,9. In Figure 6, the oscillator circuit 23 is an LC tuned oscillator. The values of the capacitance and inductance in the circuit will determine the oscillator frequency. The capacitance provided by the conductive plates 7, 9 can be arranged to be of the order of 2 to 3 pF. This should provide a significant proportion of the total capacitance in the circuit, so that alterations of this capacitance due to the presence of a coin will result in a detectable change in the resonant frequency.

In the circuit of Figure 6, the collector of the transistor in the oscillator circuit 23 has a low impedance connection to ground for a.c. signals at the resonant frequency whereas the connection between the capacitors and the inductor has a high impedance connection to ground for a.c. signals at the resonant frequency. Therefore the conductive plate connected to the collector of the transistor has a low impedance connection to ground via the 2k ohm collector resistor and the conductive plate connected to the inductor has a high impedance connection to ground. The conductive plate with the high impedance connection is more sensitive to unwanted external signals, and therefore circuit operation is improved if it is given additional shielding.In the construction shown in Figure 2, this is conveniently provided by arranging the circuit board carrying the detection circuit 11 so that the conductive plate with the high impedance connection is sandwiched between the conductive plate with the low impedance connection and the circuit board. In this way shielding is provided by the conductive plate with the low impedance connection and by the ground plane of the circuit board.

As shown in Figure 6, the buffer 25 may be provided by an emitter follower stage, which prevents the input of the frequency divider 27 loading the oscillator circuit 23 excessively.

Returning to Figure 3, the 6 MHz output of the frequency divider 27 is fed via the co-axial cable 19 to a pulse shaper 29 of the signal processing portion 12. The pulse shaper 29 squares the waveform of the signal received over the co-axial cable 19 and provides it to the clock input of a counter 31. The counter 31 is controlled by a microprocessor 35 to count the oscillations of the signal received at its clock input from the detection circuit 11. At the end of a predetermined counting period, for example a 10 ms period, the counter 31 is stopped by the microprocessor 35 and the contents of the counter 31 are loaded in parallel into a shift register 33 under control of the microprocessor 35. When the contents of the counter 31 have been loaded into the shift register 33, the counter 31 is reset and starts counting for the next counting period.The contents of the shift register 33 are then serially loaded into the microprocessor 35.

Therefore, at the end of each counting period the microprocessor 35 receives, via shift register 33, the count value of the counter 31. This count value equals the number of output cycles of the frequency divider 27 of the detection circuit 11 during the counting period. Consequently, this count value gives a measure of the frequency of the signal produced by the oscillator circuit 23.

The presence of a coin between conductive plates 7, 9 will alter the oscillation frequency of the oscillator circuit 23, and consequently it will alter the count value received by the microprocessor 35.

Different coins will tend to alter the oscillation frequency by different amounts, and accordingly the microprocessor 35 can distinguish between coins on the basis of the count value. In order to enable the microprocessor 35 to do this, a look-up table is provided in a memory 37. The look-up table stores coin denomination information with reference to count value.

The degree to which the presence of a coin between the conductive plates 7, 9 alters the oscillation frequency of the oscillator circuit 23 will depend on the thickness and diameter of the coin 1, and possibly on its composition and construction. Accordingly, it is possible for differences in these factors to cancel out and for different coins of different diameters to have substantially the same effect on the oscillation frequency. In order to enable the system to distinguish between such coins, an optical diameter detection system is provided. This comprises an LED 20 and an optical sensor 21 positioned opposite each other on the guide 3 of Figure 1. The LED 20 and optical sensor 21 are spaced at a predetermined height above the floor 4 of the guide 3.A coin of greater diameter than the predetermined height will intercept the light beam from the LED 20 to the optical sensor 21, and accordingly it can be distinguished from a coin of lesser diameter than the predetermined height.

The predetermined height is chosen so as to distinguish between pairs of coins which have similar effects on the oscillation frequency of the oscillator circuit 23.

The LED 20 is powered by an optical sensor control circuit 22, which also receives the output signal from the optical sensor 21. The optical sensor control circuit 22 outputs an optical sensing signal to the microprocessor 35.

As shown in Figure 4, the memory 37 comprises three registers, Store A 41, Store B 43 and Difference register 45 and the look-up table 47. Store A 41 contains a reference frequency value of 60000 (the number of oscillation of a 6MHz signal in lOms counting period).

In each count period, the count value from the shift register 33 is loaded into Store B 43. The microprocessor 35 then calculates the difference between the count value in Store B 43 and the reference frequency value in Store A 41. The difference is stored in the Difference register 45.

It will be appreciated by those skilled in the art that a coin 1 passing between conductive plates 7, 9 will increase the capacitance provided by the conductive plates, and therefore will reduce the frequency of the signal from the oscillator circuit 23. Therefore, the maximum frequency of the signal will be the 192 MHz frequency output when no coin is present between the conductive plates 7, 9. Thus the number of pulses supplied to the counter 31 in a 10 ms counting period will not exceed 60000, which is well within the counting range of a 16-bit binary counter.

Preferably, therefore the counter 31 and the shift register 33 are both 16-bit binary devices, and Store A 41 and Store B 43 are 16-bit registers.

A relatively large coin, such as the British R1 and 50p coins, may alter the capacitance of the conductive plates 7, 9 by about 0.7pF, and the corresponding change in the frequency of the oscillator circuit 23 will result in a difference between the value stored in Store A 41 and the value stored in Store B 43 which can be represented as a 12-bit binary number. Slight instabilities in the oscillator circuit 23 may cause slight variations in the precise 12-bit value, but these can be accommodated by discarding the bottom 4 bits, and storing only the top 8 bits in the Difference register 45. Consequently, the Difference register 45 can be implemented by an 8-bit register.

Figures 5a and 5b illustrate the difference values stored in the Difference register 45 for successive lOms counting periods as a coin 1 passes between the conductive plates 7, 9. As a coin 1 enters the space between the conductive plates 7, 9 (position 1b in Figure 5a) the difference value stored in the Difference register 45 by the microprocessor 35 in each counting period will increase rapidly to a maximum as shown in Figure 5b. The maximum value is maintained while the coin 1 is fully between the conductive plates 7, 9 (e.g. at position la) and then decreases sharply as the coin 1 leaves the conductive plates 7, 9 (at position lc).When the coin has fully left the conductive plates 7, 9 (at position ld) the difference value returns substantially to zero. The microprocessor 35 determines the maximum frequency difference and uses it to interrogate the look-up table 47.

The look-up table 47 contains an entry for each possible difference value determined by the microprocessor 35 and corresponding coin validation information. For each possible difference value, the microprocessor 35 receives information enabling it to determine whether the coin is valid or invalid, and also to determine the denomination of a valid coin.

The optical sensing signal from the optical sensor control circuit 22 is also input to the look-up table 47. Table 1 gives an example of the contents of the look-up table 47. Different systems will have different values for each valid coin, and the values given are just an example.

TABLE 1

Optical Sensing Difference Signal 0 1 Value 1 to 44 Invalid Invalid 45, 46 5p Invalid 47 to 67 Invalid Invalid 68 to 70 20p Invalid 71 to 84 Invalid Invalid 85 to 89 new 10p Invalid 90 new 10p 2p 91 to 93 Invalid 2p 94 to 154 Invalid Invalid 155 to 158 Invalid lOp 159 to 193 Invalid Invalid 194 çl Invalid 195 fl 50p 196 upwards Invalid Invalid As can be seen from Table 1, a difference value of 90 can be either the highest acceptable difference value for a new (1992) ten pence piece or the lowest acceptable difference value for a two pence piece.

Similarly a difference value of 195 indicates either a one pound coin or a fifty pence coin. The height of the LED 20 and the optical sensor 21 above the floor 4 of the guide 3 is chosen so as to enable both of these ambiguities to be resolved by the optical sensing signal. The output of the optical sensor control circuit 22 will indicate '1' for a two pence coin and a fifty pence coin and '0' for a one pound coin and new ten pence coin.

If the difference value from the look-up table 47 corresponds to a valid coin, the microprocessor 35 indicates to a control circuit 39 that the coin 1 is a valid coin of the denomination indicated by the look-up table 47. In response to this coin validation information the control circuit 39 will control the operation of, for example, the coin operated telephone or vending machine. If the difference value received by the microprocessor 35 corresponds in the look-up table 47 to an invalid coin, the microprocessor 35 will inform the control circuit 39 of this, and the control circuit 39 may e.g. reject the coin 1.

In Figure 3, the control circuit 39 is shown separately from the microprocessor 35. In practice it may be a separate piece of hardware or alternatively its function may be implemented by a program run in the coin validation microprocessor 35.

The circuit of Figure 3 is advantageous because it can be constructed to operate with a power consumption of about lOmA with a 4.5V or 5V supply, especially if the control function of the control circuit 39 is provided by software within the microprocessor 35.

This power consumption is sufficiently low that the circuit can act as a coin validator in a payphone powered only by the power available from the telephone line connection. In this way, the need for electric power cells or a mains electricity connection can be avoided. The most significant power consumption in the circuit is typically in the frequency divider 27.

If this is provided by an emitter coupled logic high speed chip such as chip type SP 8797 of Plessey Semiconductors, it will draw about 7mA.

In a modification, the microprocessor 35 varies the reference frequency value stored in Store A 41 in response to variations in the count value obtained in the absence of a coin. Such variations may occur, for example, owing to changes in the oscillation frequency of the oscillator circuit 23 with temperature. In this modification a count value is supplied to the microprocessor 35 from 16-bit counter 31 in each counting period and is stored in Store B 43 of memory 37. Then the difference is calculated between the values stored in Store A 41 and Store B 43. If the value in Store A 41 is greater than the value in Store B 43, Store A 41 is incremented by 1 and if the difference is the other way round, Store A 41 is decremented by 1.Thus, whilst no coin is present in the sensing portion 14 of the coin validator, a value which follows the frequency of the oscillator signal is maintained in Store A 41 of memory 37, and the system is automatically compensated for frequency drift in the oscillator circuit 23.

Depending on the circuit parameters, such drift compensation may be important. For example, in the circuit described above a frequency drift of 0.1% in the oscillator circuit will change the count value in Store B 43 by 60. If the value in Store A 41 is not altered correspondingly, the difference values will also change by 60 and a look-up table in accordance with Table 1 would cease to provide the correct output.

The microprocessor 35 can be programmed to identify the presence of a coin 1 from a large difference value, e.g. a value in excess of 20 in the case of the Table 1 difference values, and may suspend its function for updating the contents of Store A 41 under these circumstances. This prevents the updating function from artificially reducing the difference values generated by the coin. However, if the microprocessor 35 is programmed to use the largest difference value obtained from a coin, and the contents of the look-up table 47 are prepared appropriately, it may not be necessary to turn off the updating function. In this case, any difference value which has been significantly reduced by the effect of the updating function will not be the largest value, and accordingly it will not be used for coin validation.Since the updating function only changes the value of Store A 41 by 1 in each counting period, however great the difference value is, the value of Store A 41 is only altered slightly by the updating function during the time a coin passes between the conductive plates 7, 9, and the updating function will return Store A 41 to the correct value before the next coin arrives.

In order to enter difference values for valid coins into the look-up table 47, the microprocessor 35 may be set into a training mode. When the microprocessor 35 is in the training mode a number of valid coins may be passed through the coin validator and the microprocessor 35 will store in the look-up table 47 a range of frequency differences and optical sensor pair input values which represent each of the valid coins. The training exercise above will normally be carried out for each coin validation system separately although in some cases it may be possible for training to be carried out centrally and an updated look-up table reproduced and provided to other suitable coin validation systems by exchanging memory chips.

Suitable values for the inductance and the capacitance in the circuit of Figure 6 can provide a resonant frequency of around 200 MHz (e.g. 192 MHz as previously stated).

Provided that the frequency divider 27 can operate at higher input frequencies, the resonant frequency of the oscillator circuit 23 can be increased above 200 MHz by replacing the 3.3pF capacitor in parallel with the conductive plates 7, 9 by a lower value capacitor, or removing it altogether. This will tend to increase the effect of a coin 1 on the resonant frequency.

Reducing the value of the inductor will also increase the resonant frequency of the circuit, but the value of the inductor should be maintained large in comparison with the inherent inductance of the circuit wiring and other components to ensure that the circuit operates in a predictable manner. In practice it will tend to be difficult to provide a circuit having a resonant frequency above about 0.5 GHz.

Problems can arise if a much lower frequency is selected. If the total circuit capacitance is increased to lower the frequency, the effect of the coin 1 on the frequency will tend to reduce, making it harder to detect the presence of a coin 1 and to distinguish between different coins. If the total circuit capacitance is maintained unchanged, and the resonant frequency is lowered solely by increasing the inductance in the circuit, the effects of the inherent resistance and inherent capacitance of the inductor become more significant, causing unsuitable circuit operation. The analysis circuit of Figure 3 throws away the bottom 4 bits of the difference between the counter value stored in Store B 43 and the reference value stored in Store A 41. These bits are treated as noise due to frequency instability in the oscillator circuit 23.Consequently, the smallest detectable frequency change is one which leads to a change of at least 16 in the value counted by the counter 31, which is a change of about 0.027%. Under these circumstances, it is difficult in practice to provide a usable oscillator circuit with a resonant frequency below 10 MHz, and a resonant frequency above 20 MHz will normally be necessary. Preferably the resonant frequency is at least 50 MHz, more preferably at least 100 MHz.

However, if the oscillator circuit 23 is sufficiently stable, some or all of the lowest 4 bits of the calculated difference can be relied on as a measure of coin characteristics, instead of being ignored as noise. In this case, a smaller percentage change in oscillator frequency is measurable, provided that the lowest 4 bits of the calculated difference between the values in Store A 41 and Store B 43 are not thrown away before the difference value is stored in the Difference register 45. The ability to measure a smaller percentage frequency change allows the capacitance in the oscillator circuit to be increased.

This in turn allows the operating frequency of the oscillator circuit 23 to be reduced. If the circuit of Figure 3 is modified in this way, it may be possible to increase the capacitance in parallel with the conductive plates 7,9 to 10 to 15 pF, and to select the inductance to bring the nominal operating frequency of the circuit to 12 MHz.

In this modification, the circuit of Figure 3 is further modified by removing the frequency divider 27.

The pulse shaper 29 now receives a signal at 12 MHz instead of 6 MHz. The counter 31 is operated as before, but in lOms it will overflow once so that its output will be in effect the bottom 16 bits of a 17 bit count. The value in Store A 41, representing the count value for 12 MHz, will nominally be 54464 (the excess of 120000 counts over the overflow value of the counter 31, which is 65536), but it can be updated to track frequency drift as discussed above. The Difference register 45 may store the full 12 bits of the calculated difference, or it may store an 8-bit difference value by choosing the appropriate 8 bits to provide reliable coin identification (e.g. with a particular set of valid coins the top 1 bit may be discarded as unchanging and the bottom 3 bits may be discarded as noise, leaving 8 bits as the difference value).Otherwise, the system works as previously described. This modification allows the frequency divider 27 to be omitted, thereby reducing the overall power consumption. This eases the power consumption constraints on other circuit components, even if the total power consumption is limited to 5mA at 4.5 or 5V.

With this modification, the lowest practical oscillator frequency for the oscillator circuit 23 can be reduced below 10 MHz, to 5MHz or even to 1 MHz.

If a coin is strongly electrically conductive, its effect on the capacitance between the conductive plates 7,9 will largely be a function of its diameter and its thickness. While the coin is between the conductive plates 7, 9, its electrically conductive substance will replace part of the air in the gap between the conductive plates 7, 9, and accordingly it will reduce the effective thickness of the dielectric for part of the capacitor formed by the conductive plates 7, 9. The part of the capacitor which is affected in this matter will be the part where the coin is present, that is to say, the part defined by projecting the outline of the coin onto the conductive plates 7,9. Therefore the larger the diameter of the coin is, the greater is the part of the capacitor which is affected.The degree to which the capacitance of the affected part of the capacitor is altered depends on the thickness of the coin. The greater the thickness of the coin is, the more it will reduce the effective thickness of the dielectric of the affected part of the capacitor.

Accordingly, a thin coin of large diameter will have a small effect over a large part of the capacitor and a thick coin of small diameter will have a large effect over a small part of the capacitor, and it is possible that the overall effect on the capacitor will be the same in each case. If the width between the conductive plates 7, 9 is altered without changing the size of the conductive plates 7, 9, the effect of the diameter of a coin on the capacitance is unchanged but the effect of coin width on the capacitance is altered. Therefore, where a pair of coins, one thin and large diameter and the other thick and small diameter, have similar effects on the capacitance and are hard to distinguish, use of a different separation between the conductive plates 7, 9 will render them distinguishable.However, a different pair of coins which were previously distinguishable may become hard to distinguish. For any given set of coins, it may be possible to find a convenient separation between the conductive plates 7, 9 which allows all the coins to be distinguished by their effects on the capacitance, or it may be necessary to provide other means such as the LED 20 and optical sensor 21 to distinguish between certain coins. In effect, the other means is provided so that two detection values are obtained for each coin, and coins which are difficult to distinguish on the basis of one of the detection values are distinguished on the basis of the other detection value. This also improves the performance of the system in detecting invalid coins. Alternative ways of obtaining more than one detection value will now be described.

In Figure 7a, the conductive plates 51 are divided into a first portion 53 and a second portion 55. In the first portion 53 the conductive plates 51 do not extend down to the bottom of the guide 3, whereas in the second portion 55 the conductive plates 51 do extend down to the bottom of the guide 3. Figure 7a shows a large diameter thin coin 1' and a small diameter thick coin 1" passing between the conductive plates 51, and Figure 7b shows the difference values which will be stored in difference register 45 for each counting period as the coins 1', 1" pass between the conductive plates 51. The difference values for the large diameter coin 1' are shown by circles in Figure 7b and the difference values for the small diameter coin 1" are shown by crosses in Figure 7b.

The difference values obtained for the coins 1', 1" when they are at positions l'b, l"b wholly within the second portion 55 are the same as each other, as shown in Figure 7b. When the small diameter coin 1" is at position l"a, wholly within the first portion 53, a substantial proportion of the coin area is below the bottom of the conductive plates 51, and its effect on the capacitance of the conductive plates 51 is much reduced. Consequently, the difference value obtained at this time is much lower. When the large diameter coin 1' is at position l'a, wholly within the first portion 53, the part of the coin area below the bottom of the conductive plates 51 is a small proportion of the total area, and the difference value obtained is not much lower than the difference value obtained when the coin is within the second portion 55. Thus coins 1, 1" which are difficult to distinguish on the basis of their effects when within one of the portions 53, 55 can be distinguished easily on the basis of their effects when within the other one of the portions 53, 55. In Figure 7a, optical sensor pairs 57 and 59 indicate respectively that the coin 1 is fully within the first portion 53 and the second portion 55 of the conductive plates 51.

Another embodiment of the invention as shown in Figure 8a has a first plate 61 which is planar and a second plate 63 which is stepped, to form a capacitor with a first portion 65 and a second portion 67. The plates 61, 63 have a smaller separation in the first portion 65 than in the second portion 67.

Consequently the capacitance of the first portion 65 is greater than the capacitance of the second portion 67. As shown in Figure 8b when a coin 1 passes between the first and second plates 61 and 63 the detection circuit 11 will produce two distinct difference values. The effect of changing the separation between the conductive plates is discussed above. Since different coins which are confusible at one separation can be distinguished at another, the two distinct difference values of Figure 8b allow such coins to be distinguished.

A further embodiment of the invention is shown in Figure 9a where a capacitor is formed by two plates 69, the bottom edges of which slope over the distance travelled by a coin 1 between the plates 69 along coin path P. This embodiment operates in substantially the same manner as the embodiment of Figure 7a, except that the difference values increase steadily with position along the plates 69 as shown in Figure 9b, instead of changing in a step fashion between two levels as shown in Figure 7b.

In each of Figures 7a, 8a, and 9a, optical sensor pairs 57, 59 are shown which enable the microprocessor 35 to determine when a coin 1 reaches predetermined positions between the conductive plates.

However, it may be possible to program the microprocessor to identify the position of a coin 1 from the shape of the curve of successive difference values, in which case the optical sensor pairs 57, 59 may not be needed.

In a further embodiment of the invention as shown in Figure 10, an inductive sensor 71 is situated on the inner wall of the guide 3 opposite the wall along which a coin 1 moves, in conjunction with capacative plates 73 and 75. The inductive plate 71 is connected in series with the inductance of the oscillator circuit 23 so that as coin 1 moves between the plates 71, 73 and 75 along path P both the capacitance and the inductance of the oscillator circuit 23 are affected and therefore the resonant frequency of the oscillator circuit 23 is changed, as shown in Figure llb. Provided that the coins 1 are electrically conductive, the composition of the coin has relatively little effect on the capacitance of the conductive plates.However, the inductive plate 71 will be affected by the magnetic properties of the coin, and accordingly it enables the system to distinguish between a non-magnetic coin and a ferro-magnetic mild steel blank of the same diameter and thickness.

Figure 12 shows a suitable oscillator circuit 23 for use with the above embodiment, where the inductive plate 71 is represented by inductance L1 is connected in series with the inductance L2. The coin path P is shown with dotted arrows moving between the capacitive plates 73 and 75 and past the inductive plate 71.

The inductive plate 71 can be provided at the same position along the guide 3 as the conductive plates 73, 75 so that a single composite difference value is obtained for each coin. Alternatively, the inductive plate 71 may overlap only part of the conductive plates 73, 75 or not overlap them at all, so that each coin produces two distinct difference values.

Since the inductive plate 71 is used to distinguish between ferro-magnetic and non-magnetic coins, and is not used for detailed size detection, it is not necessary to provide an expensive wound coil or a ferrite core. For the same reason, it does not matter that at the high operating frequencies discussed above for the oscillator circuit 23, the magnetic field of the inductive plate 71 will typically not penetrate a coin. Under these circumstances, the effect of a coin on the inductive plate 71 will largely be due to its effect of concentrating or dispersing magnetic flux, not due to eddy currents in the coin, and this effect will be different for ferro-magnetic and non-magnetic coins.

Figure 13 shows an alternative circuit to that in Figure 3 where the output of the oscillator 77 is fed into a frequency divider 79 via a buffer 81, similarly to the circuit of Figure 3, but the output of the frequency divider 79 is fed via a co-axial cable 83 to a mixer 85 instead of to the pulse shaper 29 of Figure 3. In the mixer 85 the signal is mixed with a reference signal of known frequency produced by a reference oscillator 87. The resulting signal has a lowest frequency component which represents the difference between the reference frequency and the detection signal frequency from the frequency divider 79.This mixed frequency signal is passed to a low-pass filter 89 to obtain the frequency difference signal which is passed to a pulse shaper 91, then to a frequency divider 93 and then to a microprocessor 95 where the frequency difference is compared to the range of frequency differences for known valid coins which are stored in memory 97. If the frequency difference matches that of a known valid coin, the microprocessor 95 indicates to the control circuit 99 that the coin is valid and of a particular denomination and if the frequency difference is not matched against that of known valid coin, the microprocessor 95 sends a signal to the control circuit 99 to indicate that the coin 1 is invalid and should be rejected.

The microprocessor 95 determines the difference frequency by counting pulses received from the frequency divider 93 in a preset period. The frequency divider 93 is used to scale the difference frequency so that the number of pulses counted by the microprocessor does not overflow its internal registers.

As can be seen, the illustrated embodiments provide a coin validator of simple construction, and in which the structure of the validator does not wholly determine which coins can be detected and accepted. Modification of the validator to alter which coins are acceptable can be carried out easily since it will often only be necessary to change the contents of the look-up table.

Claims (16)

CLAIMS:

1. Apparatus for determining whether an input coin is acceptable and for distinguishing between a plurality of acceptable coins, comprising: capacitor means; guide means to guide an input coin along a coin path past the capacitor means thereby to affect its capacitance; oscillator means for providing an oscillating output signal the frequency of which is affected by the capacitance of the capacitor means; and decision means for receiving the oscillating output signal and makes a decision on the basis of the frequency thereof whether the input coin is acceptable and, if so, which of a plurality of acceptable coins it is.

2. Apparatus according to claim 1 in which the decision means derives a difference value representing the difference between the frequency of the oscillating output signal and a reference frequency, and makes the said decision on the basis of the difference value and a pre-stored correlation of possible difference values and input coin identification data.

3. Apparatus according to claim 2 in which the reference frequency is substantially equal to the frequency of the oscillating output signal when no coin is input.

4. Apparatus according to claim 2 or claim 3 in which the reference frequency is updated to take account of changes over time in the frequency of the oscillating output signal.

5. Apparatus according to any one of claims 2 to 4 in which the difference value is obtained by comparing a value representing the frequency of the oscillating output signal with a pre-stored value representing the reference frequency.

6. Apparatus according to claim 5 when dependent on claim 4 in which the reference frequency is updated by updating the pre-stored value.

7. Apparatus according to claim 6 in which the reference frequency is updated by comparing the pre-stored value with a value representing the frequency of the oscillating output signal at a time when no input coin is detected, and updating the pre-stored value in response to the result of the comparison.

8. Apparatus according to any one of the preceding claims in which the capacitor means has different coin area/width trade-off groups at different positions along the coin path, a coin area/width trade-off group being a group of possible coin dimensions the members of which have different coin widths and different coin areas but the same effect on the capacitance of the capacitor means when a coin the coin dimensions is at the position along the coin path.

9. Apparatus according to claim 8 in which the said different coin area/width trade-off groups are provided by providing different separations between conductive plates of the capacitor means at different positions along the coin path.

10. Apparatus according to claim 8 in which the said different. coin area/width trade-off groups are provided by providing conductive plates of the capacitor means with different extents or positions in a direction transverse to the coin path at different positions along the coin path.

11. Apparatus according to any one of the preceding claims in which the coin path passes between conductive plates of the capacitor means.

12. Apparatus according to any one of the preceding claims further comprising inductor means, the guide means guiding an input coin past the inductor means and the frequency of the oscillating output signal being affected by the inductance of the inductor means.

13. Apparatus according to any one of the preceding claims further comprising further means for distinguishing between input coins on the basis of a physical dimension, the decision means making the said decision also on the basis of the output of the further means at least in some circumstances.

14. Apparatus according to claim 13 in which the physical dimension comprises coin diameter.

15. Apparatus according to any one of the preceding claims in which the oscillator means comprises a capacitance/inductance tuned oscillator.

16. A method of determining whether an input coin is acceptable and for distinguishing between a plurality of acceptable coins, in which an input coin is guided past capacitor means so as to affect the capacitance thereof, an oscillator means generates an oscillating output signal the frequency of which is affected by the capacitance of the capacitor means, and the input coin is rejected or it is decided which of a plurality of acceptable coins the input coin is on the basis of the frequency of the oscillating output signal.