EP0318229A2

EP0318229A2 - Coin validation apparatus

Info

Publication number: EP0318229A2
Application number: EP88310998A
Authority: EP
Inventors: Roger William Whatmore; Paul Christopher Osbond; Clive Lewis Chitty
Original assignee: Telent Technologies Services Ltd; Plessey Telecommunications Ltd
Current assignee: Telent Technologies Services Ltd
Priority date: 1987-11-24
Filing date: 1988-11-21
Publication date: 1989-05-31
Also published as: DK655688D0; CN1036470A; JPH01244593A; GB2215505A; AU2583588A; ZA888742B; PT89058A; BR8806169A; DK655688A; EP0318229A3; GB8827161D0; GB8727526D0

Abstract

Coin validation apparatus comprising a coin chute 2 arranged for directing a coin 1 entering the apparatus such that the coin will be brought into contact with a hard striking surface 3, a microphone 4 positioned to detect acoustic vibrations of the coin 1 after striking said surface, an electronic circuit capable of comparing data from said coin with stored data representative of a set of standard coins and indicating which value of coin corresponds to that having entered said apparatus, in which the part of the apparatus before the electronic circuit includes a coin detector arranged to actuate said circuit at a time when a coin 1 has been detected as entering the apparatus. The detector may be the microphone 4.

Description

This invention relates to coin validation apparatus. It relates particularly to an apparatus which is applicable to detecting the values of coins dropped into a slot, and therefore it may be used in a vending machine, a telephone coin box, a coin sorting machine or other suitable device where there is a need to check the values of incoming coins inserted by a potential customer or user.
According to the invention, there is provided coin validation apparatus comprising a coin chute arranged for directing a coin entering the apparatus such that the coin will be brought into contact with a hard striking surface, a microphone positioned to detect acoustic vibrations of the coin after striking said surface, an electronic circuit capable of comparing data from said coin with stored data representative of a set of standard coins and indicating which value of coin corresponds to that having entered said apparatus, in which the part of the apparatus before the electronic circuit includes a coin detector arranged to actuate said circuit at a time when a coin has been detected as entering the apparatus.
In one embodiment, the coin detector may comprise a pressure sensor attached to said striking surface. The pressure sensor may be a body of a piezoelectric or piezoresistive material. Alternatively, the coin detector may rely upon the coin causing a change in capacitance, electromagnetic induction or transmission of a light beam.
In a further embodiment, the coin detector may rely upon an electrical output from the microphone to actuate said circuit.
By way of example, some particular embodiments of the invention will now be described with reference to the accompanying drawings, in which:

Figure 1 shows one form of coin validation apparatus for obtaining the acoustic spectrum emitted by a coin,
Figure 2 is a block diagram of an electronic circuit for recognizing the value of the entering coin,
Figure 3 is a similar diagram of a narrow bandpass filter of the electronic circuit,
Figures 4 and 5 are graphs showing the signals obtained when particularly coins are detected in the circuit of Figure 2,
Figure 6 shows graphs depicting the results of a Fast Fourier Transform on one coin,
Figure 7 shows an alternative form of coin validation apparatus in side and plan views,
Figure 8 shows the voltage signal obtained from the piezoelectric sensor in the apparatus of Figure 7 for different values of coin,
Figure 9 is a graph showing the peak-to-peak voltages obtained from different values of coin, and,
Figure 10 shows a further form of coin validation apparatus which makes use of an inductive sensing method.

When a coin is struck against a hard object, it will vibrate with a characteristic set of modes, determined by the metal from which it is made and the dimensions (thickness, diameter if the coin is circular and any other dimensional features, such as the presence of facets, holes or regions or differing composition). The sound emitted by the coin will contain information about these resonant modes, whose relative amplitudes will change with time after the coin has been struck. Figure 1 shows an apparatus which can be used to obtain the acoustic spectrum emitted by a coin. The coin 1 is allowed to drop down a chute 2 to strike a resilient plate 3 where it emits sound. The sound emitted is detected by a pressure microphone 4.
The best placing for the microphone is adjacent the face of the coin, rather than the edge, as the signal will be largest in this location. However, any convenient position close to the coin will provide a usable signal. The signal detected by the microphone is fed into a circuit as shown in Figure 2.
Figure 2 is a block diagram of an electronic circuit for recognising the value of the entering coin. As depicted in the diagram, the microphone 4 is connected to an amplifier 6 and from there to a bandpass filter 7. An output from the filter 7 is applied to an RMS detector 8, which is phase sensitive, and then to a Schmitt trigger device 9.
In the circuit of Figure 2, after the amplification stage, the filter 7 is a "notch" filter whose frequency is set to the fundamental resonant mode frequency of the coin to be recognised. One form of construction of such a filter is shown in Figure 3, and this will act as a narrow bandpass filter. Figure 3 shows an AF oscillator 11 which is connected to a reference unit 12 which provides square wave signals to a phase sensitive detector 13 at the same frequency and in phase with the signal from the AF oscillator 11. The signal input on a line 14 is also applied to the detector 13 and this provides an output on the line 16.
In Figure 3, the reference unit 12 is a Brookdeal Model 9422 and the phase sensitive detector 13, a Brookdeal Model 9412A. The frequency of the resulting filter is set by the AF oscillator while output from the phase sensitive detector 13 is inversely proportional to the frequency difference between that set by the AF oscillator 11 and the signal input frequency. The signal output on the line 16 is an AC signal with a frequency equal to the frequency difference between the input signal on line 14 and that set by the AF oscillator 11. It is important that the bandwidth of the filter to set wide enough so that the resonances of all the coins of a particular denomination will be detected, while being narrow enough to avoid false detection of coins of different denominations. A bandwidth of 5% should do this adequately.
After the filter stage, the RMS detector 8 is used which will give an output proportional to the root mean square of the AC signal passed through the filter (see Figure 2). This part of the circuit is followed by the Schmitt trigger 9 and a monostable 17. The Schmitt trigger 9 will give an output which is zero volts (logic zero) when the output from the RMS circuit is below a predetermined level and an output equal to that of the positive voltage supply (logic one) when the output from the RMS circuit is above that level. The monostable 17 will trigger on the logic one and hold it for a predetermined time so that following circuitry and mechanical devices for dealing with the coin can act upon the information that the coin is of a particular denomination. Figures 4 and 5 show the signals obtained when coins are used in the apparatus.
Figure 4 is a graph which shows the output levels from a filter of the type depicted in Figure 3 when a particular coin is used in the apparatus of Figure 1. The vertical axis shows output amplitude which is the peak-to-peak potential in volts of the signal as measured at the output of the RMS detector 8. A scale from zero to 4.0 volts is marked. A voltage level corresponding to a frequency of 9.987 kilohertz is given by the horizontal line A. The horizontal axis of the graph shows the different amplitudes produced by the coins of ten, five, two and one penny in value. Only the ten pence coin is observed to produce amplitudes that exceed the given discrimination level of the line A.
Figure 5 is a similar graph in which the vertical axis covers the range from zero to 1.0 volt and a discrimination level at a frequency level of 8.460 kilohertz has been marked by the line B. The same four coins were tested and this time only the two pence coin was observed to produce an amplitude exceeding the discrimination level marked by the line B.
It can be seen that in both cases a signal level can be set which will discriminate in favour of one particular coin against virtually all other coins.
There is an instance in which the device described here will provide a false trigger reaction. This could occur with a coin which differs in dimensions and metal type from the intended one but coincidentally has a similar resonant frequency to the intended coin. Such a coin can be sorted by having a device for measuring the diameter and, possibly, the thickness of the coin delivered to the system. A simple logic circuit will then allow sorting of the coins unambiguously. Suitable methods for diameter measurement include inductive techniques and optical techniques. An alternative to measuring the diameter of the coin would be to use a particular resonant mode structure which will be described later.
A system which will sort a number of different coins can be constructed by placing any number of the circuits shown if Figure 2 in parallel with different filter frequencies selected which are to cover the individual coins to be recognised. Each circuit could be linked to a diameter measuring device to give further protection against incorrect selection or sorting.
A number of modifications can be made to the simple system described above which would give improved sorting operation.
In the first embodiment, it is recognised that a coin struck on a metal plate will vibrate in more than one mode and that while the fundamental flexural mode will dominate, other modes such as the higher-order flexural modes, radial modes and thickness modes of vibration will all contribute to the mode structure, and hence the sound emitted by the coin. Furthermore, the temporal shape of each mode will be different for each coin, depending on the inter-mode coupling which occurs and the degree to which each mode is damped. These factors will in turn depend on the material from which the coin is made, its physical dimensions and (from the fact that not all coins are simple discs) its shape. It is therefore considered that coin recognition could be accomplished by analysing the frequency/time structure of the sound emitted by the coin. This could be accomplished in one of the following ways:
a) Digitising and recording the sound emitted and performing a Fast Fourier Transform (FFT) on the resulting signal. The FFT function contains all the frequency information required to do the coin recognition task, and this could be accomplished using digital correlation of the FFT function for the coin with those of a number of library functions for different coins.
The use of this idea has been demonstrated using a Wavetek 5820A FFT Analyser, using a Bruel and Kjaer 4135 Microphone to detect the acoustic signal. Figure 6 gives an example of a recorded spectrum for a single coin.
Figure 6 shows two graphs, an upper one of which has a vertical scale showing the voltage recorded from the sensing microphone with values from -4 to +4. The horizontal axis indicates Time (seconds) from 0 to 5.8 milliseconds. In the lower graph, the vertical axis shows the acoustic power in dB in a 1Hz bandwidth on a scale from -90 to +10. The horizontal axis shows frequency (measured in hertz) on a scale from 0 to 50,000.
The multiple peaks due to the different resonant modes are clearly visible.
The peaks in the acoustic spectra are characteristic of the coin denomination and Table 1 lists the frequency bands up to 50KHz in which major and minor resonant peaks of these spectra occur for the United Kingdom coin set.
Close inspection of the spectra reveals that there are very significant differences between most coins, so that it should be possible to obtain good discrimination between most of these coin denominations on the basis of the acoustic signature alone.
b) Passing the sound signal through a filter bank (each filter corresponding to a band in Table 1) and recording the temporal variation of the signal passing through each filter. These temporal variations would then be compared with values stored in a library to obtain the coin recognition. The filter bank would be implemented by either low frequency acoustic filters or by modulating a radio frequency carrier with the sound produced by the coin. The modulated carrier would then be discriminated using discrete RF filters, which could be implemented as surface acoustic wave or ceramic filters, looking at the modulated side bands. This modulated RF approach would reduce the size and cost of the filters required.
In a second embodiment, it is recognised that a system such as that described in Figure 1, it would be beneficial to provide some means by which the presence of a coin could be detected and used to switch-on the acoustic recognition circuit. A number of possible methods are envisaged by which this could be done:
a) The resilient plate 3 which is struck by the coin could be provided with a touch-sensitive portion which would give an electrical signal to allow the acoustic circuits to be triggered. Such a touch sensitive portion could be provided as a mechanical switch or as an electrical sensor. One suitable electric sensor would be a piece of PZT piezoelectric ceramic, lithium niobate piezoelectric single crystal or PVDF piezoelectric plastics material, bonded to the resilient plate 3. A suitable apparatus for doing this is shown in Figure 7. In this case, the piezoelectric material 18 is bonded to the back of the plate 3, so that when the coin 1 strikes the plate 3 it will bend the plate, producing an electrical output on a line 19 by means of the piezoelectric effect. An advantage of this method is that it will give an output proportional to the mass of the coin 1, which could be used by the electronics of the system in conjunction with the other information to give yet another method for recognising the coin. A demonstration of this is described as follows. A PZT-5 disc constituting the piezoelectric material 18 was soldered to a plate 3 formed by a phosphor/bronze strip with the dimensions: 0.8 x 12 x 51 millimetres. Coins 1 were allowed to run down a chute constituted by a ramp 21 from a known, fixed distance to strike the plate 3 and the outputs on the line 19 from the PZT disc were taken through an amplifier and fed into an oscilloscope. Figure 8 shows the forms of the outputs obtained from several different coins, these coins being, from the upper graph, values of fifty, ten, five, two and one penny, respectively. In these graphs, the vertical axis has a scale of five volts per unit square whilst the horizontal axis measures 0.5 milliseconds per unit square.
Figure 9 shows how the peak-to-peak voltages recorded depend on the different coins used. The vertical axis of the graph shows voltages from 4 to 24 volts. It can be seen that the coins are characterised by the voltage values which can give useful information for the sorting process. The forms of the voltage outputs are complex and again characteristic for the given coin type. The shapes of these waveforms can also be used for coin sorting by use of correlation with a library of known waveforms for different coins. As an alternative to the use of the piezoelectric d₃₁ coefficient, coupled to the bending of the resilient plate 3, a body of piezoelectric material placed on the front of the plate and operating in its d₃₃ mode would give positive indication of the presence of a coin. Other electrical sensors which could be used for the touch sensitive portion include a piezoresistive polymer, such as a conductive carbon loaded rubber, which would give a change in resistance when impacted by a coin. Alternatively a plastic/metal sandwich forming a parallel plate capacitor could be used if the coin impact was required to give a change in capacitance.
b) An inductive method could alternatively be used to give a signal of the presence of a coin. Apparatus for using this method could take the form of two coaxial coils, one of which carries an AC current. These coils might be arranged as shown in Figure 10 where a first coil 22 is positioned to surround a second coil 23. The change in the mutual inductance of the coils caused by the presence of the coin 1 changes the signal induced in the second coil 23 due to the current flowing in the first and this in turn can be used to indicate the presence of the coin.
c) A simple optical beam sensor can be used to give an indication of the presence of a coin. If an optical source is used to provide a beam will give indication of the presence of the coin and it can be used to trigger the electronic sensing circuit.
The foregoing description of embodiments of the invention has been given by way of example only and a number of modifications may be made without departing from the scope of the invention as defined in the appended claims. For instance, the pressure microphone indicated in Figure 1 can be replaced by a directional pressure gradient microphone. A pressure microphone may be subject to interference from external sources of noise which could interfere with the operation of the system. Whilst this may be largely mitigated by ensuring that the microphone is well insulated acoustically from the environment, it may be advantageous under certain circumstances, such as operation in very noisy environments, to use a pressure gradient microphone. Such microphones do not respond to changes in ambient sound pressure such as are caused by distant sources of noise, but are sensitive to the pressure gradients caused by sound sources close to the microphone. Hence, such a microphone would respond well to the sound from the vibrating coin if it were close to it but it would be insensitive to ambient noise. Such microphones are also highly directional and hence would, if correctly positioned, provide a further means for discriminating for the signal from the vibrating coin and against the background noise.

Claims

1. Coin validation apparatus comprising a coin chute arranged for directing a coin entering the apparatus such that the coin will be brought into contact with a hard striking surface, a microphone positioned to detect acoustic vibrations of the coin after striking said surface, an electronic circuit capable of comparing data from said coin with stored data representative of a set of standard coins and indicating which value of coin corresponds to that having entered said apparatus, in which the part of the apparatus before the electronic circuit includes a coin detector arranged to actuate said circuit at a time when a coin has been detected as entering the apparatus.

2. Apparatus as claimed in Claim 1, in which the coin detector comprises a pressure sensor attached to said striking surface.

3. Apparatus as claimed in Claim 2, in which the said pressure sensor is a body of a piezoelectric material.

4. Apparatus as claimed in Claim 2, in which the said pressure sensor is a body of a piezoresistive material.

5. Apparatus as claimed in Claim 1, in which the said coin detector relies upon the coin causing a change in capacitance, electromagnetic induction or transmission of a light beam.

6. Apparatus as claimed in Claim 1, in which the said coin detector relies upon an electrical output from the microphone to actuate said electronic circuit.

7. Coin validation apparatus substantially as hereinbefore described with reference to any one of the accompanying drawings.