WO1996005575A1

WO1996005575A1 - Coin validators

Info

Publication number: WO1996005575A1
Application number: PCT/GB1995/001720
Authority: WO
Inventors: James Churchman; Micheal John Evans
Original assignee: Tetrel Limited
Priority date: 1994-08-12
Filing date: 1995-07-20
Publication date: 1996-02-22
Also published as: GB9416351D0; GB2292473A

Abstract

A coin validator which uses inductive and capacitive effects to validate a coin has an inductor coil close to the capacitor for compactness. The capacitor plate is formed so as to obstruct circulating currents which would otherwise be induced in it by the inductor field. Such currents would otherwise lead to detrimental interaction between the capacitor and the inductor. The inductor coil can be used as one plate of the capacitor.

Description

COIN VALIDATORS

This invention relates to coin validators such as for use in pay telephones or vending machines. It has particular application to coin validators which have capacitive and inductive coin sensors.

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

Coin validators of the above-mentioned type are known from, for example, O94/09452 which proposes a system in which a coin alters both the capacitance of a capacitor and the inductance of an inductor in an oscillator circuit, thereby altering the frequency of the oscillator. The frequency change is detected by a microprocessor and used to reference a look-up table containing information which determines whether the frequency change indicates a valid coin, and if so, what the denomination of that coin is.

WO 94/09452 proposes a compact arrangement in which the capacitance and the inductance are at the same physical location on a coin path along which a coin which is to be validated will travel. The inductive sensor coil is located on an inner wall of the coin chute and each capacitor plate is located either side of the inductive sensor coil on the outer walls of the coin chute. However when the capacitive and inductive sensors are placed in such close proximity they interact and the effect of a coin on the inductive sensor is reduced, thereby reducing the sensitivity of the coin validator. In one aspect of the present invention a conductive plate for a capacitor for capacitive sensing is formed in a manner which inhibits the induction of currents in the plate even if the plate is subject to the field of an inductive coil for inductive sensing. This aspect of the invention allows a reduction in detrimental interaction between a conductive plate for a capacitor and an inductive coil even if they are not spaced far apart. Accordingly it assists the design of a compact sensor in which a coin to be validated is required to influence the inductor and the capacitor substantially at the same position. It appears that the detrimental interaction arises at least in part because induced circulating currents in the capacitor plate generate a magnetic field which opposes that of the inductor, so that inhibiting these currents reduces the interaction. The capacitor plate may take various forms, for example non-conductive areas may sub-divide the conductive capacitor plate and/or the plate may be made of material which is less conductive than a metal, such as a printed carbon film.

In another aspect of the present invention, an inductive coil for inductive sensing is used as a conductive plate of a capacitor for capacitive sensing. This allows an inductive and capacitive coin sensor to be provided using just two sensor components rather than three as described above for WO 94/09452.

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 an electrical block diagram of the coin validation system;

Figure 3 is a schematic diagram of a memory in the circuit of Figure 2;

Figure 4a shows an example of a prior art oscillator circuit for use in a coin validation system;

Figure 4b shows an example of a detection circuit for use in a coin validation system.

Figure 5 is a section through a coin guide shown in the prior art; Figure 6 is a section along the line VI-VI of Figure l;

Figure 7 is a side view of one sensor plate in the sensing portion of Figure 1;

Figure 8 is a side view of another sensor plate in the sensing portion of Figure 1;

Figure 9 is a section through a connection point of the plate shown in Figure 8;

Figure 10 shows an example of an oscillator circuit which may be used in the circuit of Figure 2;

Figure 11a is a diagram illustrating a coin moving between the sensor plates of the embodiment of Figure 1; Figure lib is a diagram which illustrates signals produced in the circuitry of Figure 2 as a coin is registered between the sensor plates of Figure 11a;

Figure 12 is a side view of a sensor plate in a second embodiment; Figure 13 is a side view of a sensor plate in a third embodiment;

Figure 14 is a section through a coin guide in a further embodiment; and

Figure 15 is a side view of a sensor plate in another embodiment.

Figures 1 to 3 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 moves down a longitudinally inclined guide 3 which defines a coin path P.

As the coin 1 rolls down the guide 3, it passes through sensor 7 which has both capacitance and inductance. The presence of the coin 1 in the sensor 7 will alter the capacitance and/or the inductance of the sensor, and this alteration is detected by the detection circuit as will be described later. The sensing portion 14 of the coin validation system is enclosed in a casing 13, in which the aperture 15 is provided. At the end of the guide 3 the coin 1 leaves the casing 13 through an exit aperture 17.

Figure 2 shows the electrical circuit of the coin validation system in block form. As shown in Figure 2, the detection circuit 11 comprises an oscillator circuit 23 to which the sensor 7 is connected. The frequency at which the oscillator circuit 23 oscillates depends on the inductance and the capacitance of the sensor 7. The oscillator circuit 23 is tuned to oscillate at a predetermined nominal rest frequency, for example 6MHz, when no coin is present in the sensor 7. The oscillator circuit 23 has an output fed via a buffer 25 to the signal processing portion 12. The signal processing portion 12 of the system of Figures 2 and 3 will be described later. There now follows a description of the sensor and oscillator portions of the known system, and the novel sensor and oscillator of the present embodiment.

In WO94/09452 an oscillator corresponding to the oscillator circuit 23 is implemented as shown in Figure 4a, in which capacitor C represents the capacitance of the sensor (corresponding to sensor 7 of the present embodiment) and inductor LI represents the inductance of the sensor. The path P of the coin 1 is shown both moving past the inductance LI and through the plates of the capacitor C. In Figure 4a, the oscillator circuit is an LC tuned oscillator. An alternative circuit is shown in Figure 4b, which is suitable for implementation using a printed circuit. In Figure 4b, the oscillator circuit 23 also uses an LC tuned oscillator. The capacitor plates of the sensor are connected across terminals JP1 and the inductor of the sensor is connected across terminals JP2. The same reference numerals as in Figure 2 are used in Figure 4b to identify the more important portions of he circuit. The values of the capacitance and inductance in the oscillator circuit will determine the oscillator frequency. The capacitance provided by the sensor can be arranged to be of the order of 2 to 3pF. Similarly the inductance provided by the sensor can be arranged to be of the order of 2 or 3μH. Therefore, the sensor 7 provides a significant portion of the total capacitance and a significant portion of the total inductance in the circuit, so that alterations of these due to the presence of a coin will result in a detectable change in the resonant frequency.

In GB-A-WO94/09452 and as shown in Figure 5 of the present application the guide 55 has a U-shaped section and is inclined laterally to cause the coin to rest against side wall 51 of the guide 55 as well as resting on the floor 53 of the guide 55. Consequently, the radial direction of the coin 49 is maintained parallel to conductive plates 61 and 63 which form the capacitor

C. In addition, an inductive plate 65 is situated on the inner wall of the coin guide 55 opposite the wall along which the coin 49 moves, to provide the inductance LI.

As described in WO94/09452, the conductive plates 61,63 and the inductive plate 65 can be provided in the same position along the guide 55. In this arrangement the oscillating magnetic field from the inductive plate 65 tends to induce circulating currents within the conductive plates 61 and 63. These circulating currents result in a magnetic field which opposes that of the inductive plate 65 and therefore reduces its effective inductance. Therefore, the sensitivity of the inductive plate 65 to a passing coin is reduced as a result of the close proximity of conductive plates 61,63. Figures 6 to 9 show the arrangement of the sensor 7 according to the first embodiment of the present invention. The sensor 7 shown in Figure 1 is provided by two plates 67,71 which are provided on the outside walls of the guide 3, so that the coin 1 does not contact them. This protects the plates 67,71 from mechanical abrasion by the coin 1. Additionally, the guide 3 is made of non-conductive material so as to insulate electrically the plates 67,71 from each other. The guide

3 has two generally parallel walls, about 4 to 5mm apart. It is also inclined laterally as shown in Figure 6. 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 the side wall 2 of the guide 3. In addition, the floor 4 of the guide 3 is inclined towards the side wall 2. Consequently, the radial direction of the coin 1 is maintained parallel to the plates 67,71 and a position of the coin across the width of the gap between the plates 67,71 is determined. Also, the floor

4 of the guide 3 is provided with a ceramic strip running along its length. This is provided so that when a coin is inserted through aperture 15 its initial fall is checked when it strikes the ceramic strip 75. The ceramic strip 75 absorbs most of the coin's kinetic energy before the coin begins to roll along the guide 3 through the sensor 7. This has the effect of producing relatively uniform coin speeds through the sensor 7 even if the speed of insertion of the coin varies. When this feature is combined with the inclination of the floor 4 and the guide 3 this results in all coins following the same coin path P at similar speeds, to enable consistent detection of coins.

In this embodiment, the plates 67,71 are spaced 8 to 9mm apart and may be held in such a position using plastic spacers 77 located on the side of the guide 3. The detection circuit 11 is provided on a circuit board mounted alongside the guide 3, and spaced about 8mm from it, as shown in Figure 2.

As will be understood from the above description with reference to Figures 1 and 6 the sensor 7 comprises the two plates 67,71 and has both capacitance and inductance. Figure 7 shows a plan view of the first plate 67 which carries an inductor coil 69. Figure 8 shows the second plate 71 of the sensor 7 which carries a capacitor plate 73. The capacitor plate 73 is arranged with gaps 79 in its conductive material which radiate outwardly from around the central region of the capacitor plate 73.

The plates 67,71 preferably extend from the bottom of the guide 3 up to a height equal to or greater than the height of the greatest diameter coin intended to be accepted by the validator. The plates 67,71 may be provided by any convenient method, such as plating them on to the guide 3 using printed circuit techniques. When the plates 67,71 are arranged opposite one another as shown in Figure 6, a capacitor is created with the capacitor plate 73 as the first plate and the inductor coil 69 providing the second plate. In this way, the sensor 7 is provided with both capacitance and inductance while only having two plates. When the sensor 7 is in operation, the capacitor plate 73 is situated in the magnetic field of the inductor coil 69. The capacitor plate 73 is arranged with the gaps 79 to create breaks in the paths that the circulating current induced by the magnetic field of the inductor coil 69 would otherwise take. This has the result of reducing the tendency of the capacitor plate 73 to produce a magnetic field to oppose the field from the inductor coil 69. In this way, the effect on the inductance of the inductor coil 69 due to the close proximity of the capacitor plate 73 to the inductor coil 69 is reduced. In addition, or alternatively to providing the gaps 79, the capacitor plate 73 can also be printed from conductive ink which has relatively high electrical resistance, for example, printed carbon conductor. This further reduces circulating currents in the plate capacitor 73 as a result of the magnetic field of the inductor coil 69. Accordingly, the sensitivity of the inductor coil 69 is affected less by the capacitor plate 73 than it would be by a plate of low electrical resistance or unobstructed paths along which larger currents could circulate. As shown in Figures 7 and 8 the inductor coil 69 and the capacitor plate 73 are provided with connection points 81a to 81f and 83 respectively. The connection points 81a to 8If on the inductor coil 69 are provided at stages along the length of its conductive track, thereby allowing different values of inductance to be selected depending on the required inductance sensitivity of the sensor 7.

Figure 9 shows the connection point 83 for the capacitor plate 73 which is provided by a through hole 85 formed in the plate 71. The through hole 85 is plated so as to provide electrical connection between both sides of the plate 71. The capacitor plate 73 can then be formed on one surface of the plate 71 in electrical contact with the connection point 83. The contact points for the inductor coil 69 are formed in the same way. Forming the connection point 81a to 81f and 83 in this manner allows the plates 67,71 to be flush mounted on the sides of the guide 3 as described above.

Mounting the plates 67,71 in this manner reduces the risk of mechanical damage to the plates. The conductor coil 69 and the capacitor plate 73 can then be electrically connected to the detector circuit 11 by the exposed side of their connection points 81a to 81f and 83 respectively.

Figure 10 shows a slightly modified version of the oscillator circuit shown in Figure 4a, suitable for use with the sensor 7 of Figures 6 to 9. The inductor coil 69 is represented by inductance LI and is connected in series with the inductance L2 and the capacitor formed by inductor coil 69 with the capacitor plate 73 is represented by capacitance C and is connected in parallel with capacitance C2. The coin path P is shown with dotted arrows moving between the inductor coil 69 and the capacitor plate 73.

The circuit of Figure 4b can be used with the sensor 7 of Figures 6 to 9 without modification. The inductor coil 69 is connected across terminals JP2 as described above. The capacitor plate 73 is connected to the lower (in the Figure) one of terminals JP1

, to be connected directly to the collector of transistor Q2. The upper (in the Figure) one of terminals JP1 is left disconnected, since it is electrically shorted to one of the terminals JP2, so that one end of the inductor coil 69 (providing the second capacitor plate) is electrically at the upper one of the terminals JP1.

With reference to Figure 2 of the present application, the 6MHz output of the oscillator circuit 23 is buffered in a buffer 25, and is fed to a pulse shaper 29. The pulse shaper 29 squares the waveform of the signal received and provides it to the clock input of a counter 31. In the circuit of Figure 4b, the pulse shaper is not necessary, as the design of the buffer 25 ensures that the oscillator signal triggers the counter 31. The counter 31 is controlled by a microprocessor 35 to count the oscillations of the signal received at its clock input. At the end of a predetermined counting period, for example a 10ms period, the counter 31 is stopped by the microprocessor 35 and the contents of the counter 31 are loaded in parallel into the shift register 33 under the 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 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 oscillator cycles of the oscillator circuit 23 during the counting period. Consequently, this count value gives a measure of the frequency of the signal produced by the oscillator circuit 23.

As shown in Figure 3, a memory 37 for the microprocessor 35 comprises three registers. Store A 41, Store B 43 and difference register 45 and a look-up table 47. Store A 41 contains a reference frequency value of about 60,000 (the number of oscillations of a 6MHz signal in a 10ms counting period).

The precise value stored in store A 41 is updated from time to time by the microprocessor, which counts the number of oscillators in the 10ms counting period when no coin is present. This ensures that the store A value tracks any deviations of the oscillator circuit 23 from its nominal rest frequency (e.g. owing to temperature changes or the aging of components).

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.

Further discussion of this circuit is given in WO94/09452.

It will be appreciated by those skilled in the art that the effect of a coin 1 on the inductance of the sensor 7 will be determined by a combination of qualities of the material from which the coin is made as well as the thickness and diameter of the coin. The electrical conductivity, magnetic permeability and magnetic hysteresis characteristics of the coin material will all determine to some degree the change in effective inductance of the sensor 7. The electrical conductivity of the material from which a coin 1 is made determines the level of the eddy currents created in the coin 1 as a result of it being placed in the magnetic field of the inductive coil 69. The more electrically conductive the coin material the higher the eddy currents and consequently the higher the magnetic field produced by the eddy currents which opposes the magnetic field of the inductive coil 69 and thereby reduces the effective inductance of the coil.

The permeability of the material from which a coin 1 is made affects the flux density through the inductor coil 69. The higher the permeability of the material from which the coin is made, the greater the flux density within the coil when the coin is present and therefore the higher the effective inductance of the coil.

Magnetic hysteresis losses affect the effective inductance by absorbing power from the magnetic field produced by the inductive coil 69 to produce heat within the coin material. The higher the magnetic hysteresis losses of the coin, the more the effective inductance of the inductive coil 69 is reduced. Preferably, the material from which the capacitor plate 73 is made has low magnetic hysteresis losses. However, in practice, it has been found that, compared to the effects of magnetic permeability and electrical conductivity discussed above, the effect of hysteresis losses are insignificant. It will also be appreciated by those skilled in the art that the effect of a coin on the inductance will be a combination of an increase dependent on the electrical conductivity of the coin material and a decrease dependent on the magnetic permeability of the coin material. If the resulting change in inductance is a decrease, this will tend to increase the frequency of the oscillator and vice versa. In addition to the frequency change caused by the effect of the coin on the inductance there is also the effect of the coin on the capacitance which, generally, is to increase the capacitance and thereby lower the frequency to the oscillator 23 (a mathematical discussion of the effect of a coin on capacitance is contained in WO 94/09452).

Accordingly the overall effect of the coin 1 on the frequency of the oscillator 23 will be a combination of an increase in capacitance and an increase or decrease in inductance. For example, if a coin is made of material with relatively high magnetic permeability and relatively low electrical conductivity it will result in a relatively large decrease in frequency of the oscillator circuit 23. If a coin is made out of a material which has relatively low magnetic permeability but is highly electrically conductive this will result in a relatively large increase in the frequency of the oscillator circuit 23. In theory, the increase in inductance as a result of the magnetic permeability of the coin material and the decrease in inductance as a result of the electrical conductivity of the coin material may cancel out to cause a very small or insignificant change in inductance, in which case, the change in frequency of the oscillator circuit 23 would be as a result of the effect of the coin on the capacitance alone. In practice, the effect of electrical conductivity will normally predominate owing to the high oscillation frequency (6MHz) of the magnetic field, and also because coins are not normally made from highly magnetic materials.

In the present embodiment the microprocessor 35 inspects both the contents of store A 41 and store B 43 and determines which is the larger value, subtracts the smaller value from the larger value and places the result in the difference register 45. This ensures that the value in the difference register 45 is an absolute value which represents the difference between the frequency of the oscillator 23 and the 6MHz reference frequency. The coin validator according to the present embodiment also uses an optical diameter detection 21,22 system identical to that described in WO 94/09452 to enable the system to distinguish between different coins which create similar outputs from the sensor 7.

Figures 11a and lib illustrate the difference values stored in the difference register 45 (Figure 5) for successive 10ms counting periods as a typical coin 1 passes through the sensor 7. The difference values stored in the difference register 45 are absolute values and therefore always positive. Figure lib also shows the value of Store A minus Store B where this is negative so as to demonstrate more clearly the decrease or increase in frequency of the oscillator circuit 23 from which difference values result.

The coin 1 of Figure 11a is made of a material which has relatively low magnetic permeability and relatively high electrical conductivity, resulting in a relatively large decrease in the inductance of the oscillator circuit 23. As the coin 1 enters the space between the plates 67,71 (position lb in Figure 11a) the capacitance of the sensor 7 increases causing a frequency drop, and the difference value stored in the difference register by the microprocessor 35 in each counting period rises to a first peak. The first peak value is reached before the coin 1 begins to affect the inductor coil 69 substantially. After this point the frequency rises rapidly as the coin reduces the inductance. The value of Store A minus Store B falls and becomes negative, moving rapidly to a minimum, as shown in a dotted line in Figure lib. The difference value stored in the difference register 45, being an absolute value, switches to the Store B value minus the Store A value, and moves rapidly to a mirror image maximum positive value. This results in a second peak value when the coin 1 is fully between the plates 67,71 of the sensor 7 (e.g. at position la). As the coin 1 begins to move out from between the plates 67,71, (at position lc) the frequency reduces again, the value of Store A minus Store B becomes positive again, and the absolute value of the difference reaches a third peak value where only the capacitance of the sensor 7 is being affected substantially. When the coin has fully left the sensor 7 (at position Id) the difference value returns substantially to zero.

As shown in Figure lib, the capacitance and the inductance of the sensor 7 are arranged so that the second peak of the absolute value, when both the capacitance and the inductance are affected by the coin 1, is the greatest. The actual difference values obtained will depend on the phase of the 10ms counting period relative to the moment that the coin 1 passes through the point of maximum coupling to the inductance of the sensor 7, and so the microprocessor 35 calculates a 2-point moving average of the difference value to compensate for this phase uncertainty. The maximum value for the 2-point moving average as a coin 1 passes through the sensor 7 is taken as a measure of the height of the second peak of the absolute difference value.

The microprocessor 35 uses the maximum value of the 2-point moving average to interrogate the look-up table 47. As in WO 94/09452 the look-up table 47 contains an entry for each possible (2-point moving average) difference value determined by the microprocessor 35 and corresponding coin validation information. The coin validation information for each possible 2-point moving average value is a code specifying whether the coin is invalid or (for a valid coin) identifying its value. The coin validation information is created and stored in the look-up table by inserting samples of valid coins into the validator in a training mode and storing the results, as described in WO94/09452.

Figure 12 shows a modified capacitor plate 73a for use with a second embodiment of the present invention. The capacitor plate 73a is made up from parallel bars 87 of conductive material connected at one end by a smaller conductive strip 89 to a further conductive strip 91 arranged at 90° to the bars 87. The further conductor strip 91 has a connection point 93 which may be formed in the same manner as the connection point 83 as described with reference to Figure 9. The capacitor plate 73a may be printed with conductive ink or made from metal using standard printed circuit board techniques. Alternatively the conductive strips 89,91 may be made of metal while the bars 87 are printed from conductive ink. The arrangement described above has the advantage that the thin conductive strips 89 may be cut during prototyping in order to vary the capacitance of the capacitor formed with the capacitor plate 73a. By choosing which thin conductive strips 89 to cut, it is also possible to vary the response of the capacitor plate 73a to coins of different diameters. As will be appreciated by those skilled in the art the capacitor plate 73a also has similar qualities in reducing the degree to which currents can circulate within the plate, induced by the magnetic field from the inductor.

Figure 13 shows a modified inductor coil 69a for use in a third embodiment of the present invention and which may be used in combination with the capacitor plate 73 of Figure 8 or the capacitor plate 73a described above with reference to Figure 12. The inductor coil 69a has two further plates 95 and 97 connected to either end of a coil 99. Connection points 101,103 are provided on each of the extra plates 95,97 for connection to the oscillator circuit 23. The addition of the extra plates 95,97 increases the capacitance of the inductor coil 69a by increasing the overall area of the plate 67a exposed to the capacitor plate 73 located opposite it. The plates 95,97 may also be constructed using resistive material such as a printed carbon film, and/or with suitable gaps 105, 107 as described above for the previous capacitive plates for reducing the degree to which currents can circulate within the plate. Gap 107 is provided at a location on plate 95 distanced from the connection point 103 so as to split the plate 95 into portions which are substantially electrically equal.

In a further embodiment, instead of using the inductor coil to provide a plate of the capacitor, a three plate arrangement is used with two capacitor plates both separate from the inductor plate. The capacitor plates are formed with gaps and/or of resistive material in the same manner as the plates described with reference to Figures 8 and 12. The plates may be arranged on a coin guide in the same manner as shown in Figure 5, all at a common position along the coin path. The oscillator circuit shown in Figure 4a can be used with this three plate coin guide without any modification. The detection circuit shown in Figure 4b can be used with the present three plate embodiment, with the plates being connected to the terminals JPl and JP2 in the same manner as for the sensor of Figure 5.

With reference to Figure 14, the three plate embodiment described above can be provided in a more compact form to that shown in Figure 5 by providing a circuit board 109 carrying a capacitor plate 73 on one side and the inductor coil 69 on the other side of the same circuit board 109. The circuit board 109 is then attached to one side of the coin guide 3 with the second capacitor plate 73 being provided on a second circuit board 71 attached to the other side. The inductor coil 69 is connected to a detection circuit by a connection point (not shown) provided by a plated through hole to the outer side of the circuit board 109.

With reference to Figure 15, in yet another modification of the three plate arrangement described above, a capacitor plate and the inductor coil are provided on the same side of a circuit board. As shown in Figure 15, the capacitor plate is provided by two sub- plates 111, 113 with connection points 115 and 117 and provided with gaps 119, 121 for reducing the circulating current. The inductor 123 is provided inside the outer plate 111 and surrounding the inner plate 113. The inductor coil has a connection point at either end for its connection to an oscillator circuit. Preferably, the capacitor plates 111, 113 are interconnected by their connection points either by a plated connection across the back of the circuit board or a connection wire. This combined capacitor and inductor plate is placed on one side of a coin guide with a second plate being provided on the opposite side, the second plate being of the type described with reference to Figures 8 or 12. As with the three plate embodiments described above, the present three plate embodiments can be used with the detection circuit shown in Figure 4b without any modification of that circuit. Alternatively, one end of the inductor coil 123 can be connected to the capacitor plates 111, 113 on the circuit board. In this case, the plates can be connected as for the sensor of Figures 6 to 9, with the end of the coil 123 which is connected to the capacitor sub-plates 111,113 being connected to the 3p3 fixed capacitor in Figure 10 or being connected to the upper terminal of JP2 in Figure 4b. Accordingly, it can be seen that the essential difference between the plate of Figure 13 and the plate of Figure 15 is that in Figure 13 the capacitor sub-plates 95,97 are connected to opposite ends of the coil 99 whereas in Figure 15 the capacitor sub-plates 111,113 are connected to the same end of the coil 123.

It will be appreciated from the above description that the effect of the close proximity of the capacitor plate or plates on the inductor coil is reduced by providing obstructions to circulating currents within the capacitor plate. These obstructions are preferably provided in a manner which does not substantially obstruct the flow of the capacitor charging currents across the capacitor plate.

In addition, it is also preferable to minimise the electrical resistance between the connection point or points for a capacitor plate and the areas of the capacitor plate most distant from that point or points in order to reduce the electrical resistance to the charging and discharging currents across the capacitor plate. Therefore, if material of relatively high electrical resistance is used to print the capacitor plate such as printed carbon, then it is preferable to have as short a path length as practicable from between the connection point connecting the plate to the oscillator circuit (or any other point connected with a low resistance to the connection point) and the extremities of the plate. In this case, the obstructions provided across the paths of circulating currents are preferably substantially parallel to the paths of the capacitor charging currents. If it is not practical to provide the obstructions parallel to charging currents, e.g. in the outer capacitor sub-plates 95, 111 in Figures 13 and 15, these parts of the capacitor plate are preferably made of a highly conductive material (e.g. Cu, Al or another suitable metal), or a highly conductive strip is provided to help to distribute the charges to points remote from the connection point, as shown by the broken lines in Figures 13 and 15.

It will be appreciated from the discussion above that the capacitor plate should preferably be designed with low resistance to charging/discharging currents and with high resistance to circulating currents. Ideally, if a material having non-negligible resistance is used to form the capacitor plate, substantially all points of the non-negligible resistance material should have a straight line connection to a point on the capacitor plate having a negligible resistance connection to an electrical connection point for the capacitor plate.

It will be appreciated by those skilled in the art that many different shapes of plates are possible using various combinations of conductive materials with different resistance properties, which will have relatively high resistance to circulating currents while having relatively low resistance to the charging/discharging currents.

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. The contents of WO94/09452 are hereby incorporated by reference. It should be appreciated, however, that the novel sensor arrangements disclosed herein can be applied by the skilled designer in such other types of system as may be desired. In particular, other methods of measuring the change in the resonant frequency of an oscillator may be used, and the sensor may also be used in systems where the effect of a coin on the inductance and the capacitance is detected other than by measuring the change of the resonant frequency of an oscillator.

Claims

1. Apparatus for distinguishing between a plurality of types of coin, comprising: guide means to guide an input coin along a coin path; capacitor means and inductor means located at a common position along the coin path so that the input coin will affect the capacitance of the capacitor means and the inductance of the inductor means; and means for inhibiting the induction of currents within the capacitor means.

2. Apparatus according to claim 1 in which the means for inhibiting the induction of currents includes sub¬ divisions of a conductive area of the capacitor means by non-conductive areas.

3. Apparatus according to claim 1 or 2 in which the means for inhibiting the induction of currents includes a material with high electrical resistance forming a conductive area of the capacitor means.

4. Apparatus according to any preceding claim in which the capacitor means has at least one capacitor plate formed with radially extending conductive portions.

5. Apparatus according to any of claims 1 to 4 in which the capacitor means comprises at least one capacitor plate formed by a plurality of elongate conducting members interconnected with each other at their ends.

6. Apparatus according to claim 4 or 5 in which the capacitance of the capacitor means can be modified by disconnecting or connecting one or more of the radially extending portions, or elongate conducting members, as the case may be.

7. Apparatus according to any preceding claim in which the inductor means comprises a substantially flat coil.

8. Apparatus according to claim 7 in which the coil comprises a flat spiral of printed wiring.

9. Apparatus according to claim 7 or 8, in which the coil of the inductor means also forms a part of the capacitor means.

10. Apparatus according to any preceding claim in which the inductor means is provided with means for adjusting inductance of the inductor means.

11. Apparatus according to any preceding claim further comprising: oscillator means for providing an oscillating output signal the frequency of which is affected by the capacitance of the capacitor means and the inductance of the inductor means and therefore dependent on the type of the input coin.

12. Apparatus according to claim 11 which further comprises means for obtaining a value which represents a parameter of the input coin, independently of its effect on the oscillating frequency.

13. Apparatus according to claim 12 in which the further means comprises a coin diameter detector for detecting whether the diameter of the input coin is more or less than a threshold diameter.

14. Apparatus according to any preceding claim further comprising decision means for receiving the oscillating output signal and making a decision on the basis of a frequency thereof whether the input coin is acceptable and, if so, which of a plurality of acceptable coins it is.

15. Apparatus for distinguishing between a plurality of types of coin, comprising: sensor means comprising first and second members, the first member providing an inductor means, and the first and second members combining to provide a capacitor means; and guide means to guide an input coin along a coin path past the sensor means thereby to affect the capacitance of the capacitor means and the inductance of the inductor means.

16. Apparatus according to claim 15 in which the inductor means comprises a substantially flat coil.

17. Apparatus according to claim 15 or 16 in which the coil comprises a flat spiral of printed wiring.

18. Apparatus according to any of claims 15 to 17 in which the coil of the inductor means also forms a part of the capacitor means, and in which the capacitance of the capacitor means is increased by providing extended areas of conductive material connected to the coil.

19. Apparatus according to any of claims 15 to 18 in which the inductor means is provided with means for adjusting inductance of the inductor means.

20. An apparatus according to any of claims 15 to 19 in which the capacitor means includes means for inhibiting the induction of currents within the capacitor means.

21. Apparatus according to any of claims 16 to 20 in which the means for inhibiting the induction of currents includes sub-dividing a conductive area of the capacitor means with non-conductive areas.

22. Apparatus according to any of claims 16 to 21 in which the means for inhibiting the induction of currents includes a material with high electrical resistance forming a conductive area of the capacitor means.

23. Apparatus according to any of claims 15 to 22 in which the capacitor means has at least one capacitor plate formed with radially extending conductive portions.

24. Apparatus according to any of claims 15 to 23 in which the capacitor means comprises at least one capacitor plate formed by a plurality of elongate conducting members interconnected with each other at their ends.

25. Apparatus according to claim 15 or 24 in which the capacitance of the capacitor means can be modified by disconnecting or connecting one or more of the radially extending portions, or elongate conducting members, as the case may be.

26. Apparatus according to any of claims 15 to 25, further comprising: oscillator means for providing an oscillating output signal the frequency of which is affected by the capacitance of the capacitor means and the inductance of the inductor means.

27. Apparatus according to any of claims 15 to 26 further comprising decision means for receiving the oscillating output signal and making a decision on the basis of a frequency thereof whether the input coin is acceptable and, if so, which of the plurality of acceptable coins it is.

28. Apparatus according to any of claims 15 to 27 which comprises further means for obtaining a value which represents a parameter of the input coin, independently of its effect on the oscillating frequency.

29. Apparatus according to claim 28 in which the further means comprises a coin diameter detector for detecting whether the diameter of the input coin is more or less than a threshold diameter.

30. Apparatus for distinguishing between a plurality of types of coin, comprising: guide means to guide an input coin along a path; capacitor means and inductor means located along the coin path so that there is a position along the coin path at which a coin will simultaneously affect the capacitance of the capacitor means and the inductance of the inductor means, the capacitor means comprising a capacitor plate having a shape and/or composition effective substantially to inhibit the induction by the inductor means of currents within the capacitor plate.

31. Apparatus according to claim 30 in which the capacitor plate is formed from a plurality of conductive portions of generally elongate shape, interconnected with one another.

32. Apparatus according to claim 31 in which the conductive portions are arranged generally perpendicular to the direction of circulating currents induced by the magnetic field of the inductor means.

33. Apparatus according to claim 32 in which the or each interconnection does not provide a current path for substantial circulating currents.

34. Apparatus according to any of claims 31 to 33 in which the conductive portion comprises a material with a non-negligible electrical resistance and the interconnection comprises a material with low or negligible electrical resistance such as a metal.

35. Apparatus according to any one of claims 30 to 33 in which the capacitor plate has portions comprising a material of non-negligible resistance and one or more portions comprising a material of negligible resistance, and a connection point at which the capacitor plate is connected to a circuit of the apparatus, each point on the capacitor plate which has an electrical connection to the connection point with non-negligible resistance is connected by a substantially straight line path to a point having a negligible resistance connection to the connection point.

36. Apparatus for distinguishing between a plurality of types of coin substantially as described herein with reference to the accompanying drawings.