Abstract:
A coin processing machine with a coin discriminator operated by a method is provided. The coin discriminator measures how a coin having an iron core covered by a layer of copper, brass, or bronze affects a coil when the coin is subjected to magnetic fields generated by the coil, external to the coin. Eddy currents induced in the coin are detected external of the coin. The discriminator induces a magnetic field in the coil by driving the coil with time varying drive signals having high frequencies. The coin discriminator receives the coin at precise positions in the magnetic field and detects the eddy currents induced in the coin by measuring the eddy currents through the coil. Then, the coin discriminator compares the measured eddy currents with predetermined values for different types of coils, and determines the structure, materials and type of the coin.

Description:
TECHNICAL FIELD  
       [0001]     The present invention relates to a method of identifying a metal coin. The method is used in a coin discriminator measuring how a metal coin, which has an metal core covered by a layer of another metal, affects coil means when the coin reaches magnetic fields generated by the coil means external to the coin. Furthermore, the eddy currents induced in the metal coin are detected by detection means external of the coin.  
         [0002]     The present invention also relates to a coin processing machine including a coin discriminator as above type.  
       DESCRIPTION OF THE PRIOR ART  
       [0003]     Coin discriminators are used for measuring different physical characteristics of a coin in order to determine its type, e.g. its denomination, currency or authenticity. Various dimensional, electric and magnetic characteristics are measured for this purpose, such as the diameter and thickness of the coin, its electric conductivity, its magnetic permeability, and its surface and/or edge pattern, e.g. its edge knurling. Coin discriminators are commonly used in coin handling machines, such as coin counting machines, coin sorting machines, vending machines, gaming machines, etc. Examples of previously known coin handling machines are for instance disclosed in WO97/07485 and WO87/07742.  
         [0004]     Moreover, methods and devices that measure the resistance or conductivity of a coin by exposing it to a magnetic pulse and detecting the decay of eddy currents induced in the coin are generally known in the technical field.  
         [0005]     The way in which such coin discriminators operate is described in e.g. GB-A-2 135 095, in which a coin testing arrangement comprises a transmitter coil, which is pulsed with a rectangular voltage pulse so as to generate a magnetic pulse, which is induced in a passing coin. The eddy currents thus generated in the coin give rise to a magnetic field, which is monitored or detected by a receiver coil. The receiver coil may be a separate coil or may alternatively be constituted by the transmitter coil having two operating modes. By monitoring the decay of the eddy currents induced in the coin, a value representative of the coin conductivity may be obtained, since the rate of decay is a function thereof.  
         [0006]     Coin discriminators in prior art often employ a small coil with a diameter smaller than the diameter of the coin. The coil induces and detects eddy currents in an arbitrary point of the coin, i.e. the actual part of the coin, which is subject to the conductivity measurement above, the eddy currents will vary depending on the orientation, speed, angle, etc., of the coin relative to the coil. This approach is sufficient for a normal homogeneous coin made of a single metal or metal alloy.  
         [0007]     However, in recent years new non-homogeneous coins have been issued in different countries. For example, these coins may contain both bimetallic coins and iron coins covered in copper.  
         [0008]     These new coins are very similar to some existing coins, i.e. they have almost the same physical size and are made from the same or similar materials.  
         [0009]     An iron core or disc forming an iron coin may be plated or clad with one or more layers of copper or brass around either its whole surface or only at both sides leaving its rim freely exposed as an iron rim.  
         [0010]     All of the above-mentioned features make it difficult to discriminate between coins, especially between two iron coins having the same diameter of which one iron coin has a freely exposed iron rim and the other iron coin has an iron rim that is only partly or completely plated with only a thin layer of copper, brass, or bronze.  
         [0011]     One problem occurs when introducing new coins in different countries. This introduction means that coin accepting and counting machines must distinguish between the new coins and the existing national currencies. In most cases, this is not a problem. However, different coins with essentially the same dimensions may have the same “appearence” when measured due to different manufacturing methods of the coins. For example, a type A of a coin is very similar to another coin, a type B coin. The type B and the type A coin are both iron coins. The differences between these iron coins are the following. The type B iron coins are clad in brass in comparison to the type A iron coins, which are either plated or clad with copper. Another difference is that the type B iron coins have the iron exposed on the rim and the type A iron coins have a thin layer of copper over the rim. In theory, there is an average diameter difference of between these two types of A and B iron coins. However, a small sample of the type A iron coins, which diameters were measured with digital callipers, had diameters outside the their specified tolerances. The type B iron coins that have been in use a long time tend to become smaller, especially the diameter. We expect to find type B and type A iron coins with the same size.  
         [0012]     Similarly, a type C and a type D coins are also difficult to discriminate. Both coins are iron coins covered with copper. The type C iron coin has a copper covered rim. The type D iron coin may have a thin smear of copper on one side of the rim. This is due to the manufacturing method of this type D iron coin. This copper smear is created when the die cutter punches out the coin, whereby a thin layer of copper may be smeared over a part of the edge, i.e. the rim, of the iron coin in the punching direction.  
         [0013]     The coin discriminators of the prior art described above fail to provide a sufficiently accurate determination of the type of the above-mentioned iron coins due to a similar effect on resistance for the coils measuring the iron coins conductivity when the measured iron coins passes the coils.  
         [0014]     The coin measurement results obtained vary largely depending on the actual spot of measurement on the coin. If a given coin is measured at a position located in the vicinity of the rim of a coin, which has a thin copper layer around an iron core, a coin with an iron core having an exposed, non-covered, iron rim may be mistakenly “seen” or discriminated as being a coin with an iron core surrounded by a relatively thin copper or brass layer at all sides, i.e. over both the faces and the rim of the iron coin. Furthermore, the prior art solutions have problems in identifying if the layers covering the rims of the iron coins are made of copper, brass or bronze.  
         [0015]     Moreover, iron coins with only a thin smear of copper, brass or bronze partly covering the rim may be difficult to discriminate because they can be “seen” as iron coins with both a non-covered rim or a covered rim.  
       SUMMARY OF THE INVENTION  
       [0016]     The main objects of the present invention are to allow repeatable and accurate determination of coin types, i.e. coins comprising for example an iron core covered completely or partly by a thin layer made of another metal such as copper, brass or bronze and having almost the same physical size, and, in some cases, exactly the same size, by detecting resistance and inductance changes in the coil that measures the coin for determining if the coin has a covered or non-covered rim, and for determining the surface conductivity for the coin.  
         [0017]     These objects are achieved by providing a coin processing machine with a coin discriminator operated by a method according to the invention. The method measures how a coin, which has for example an iron core covered by a layer of another metal such as copper, brass, or bronze, affects coil means when the coin is subjected to magnetic fields generated by the coil means external to the coin. Eddy currents induced in the coin are detected by detection means external of the coin. The coin discriminator induces a magnetic field in the coil means by driving the coil means with time varying drive signals having high frequencies. The coin discriminator receives the iron coin at precise positions in the magnetic field. Then, the coin discriminator detects the eddy currents induced by the magnetic field in the iron coin by measuring the eddy currents through the coil means, and compares the measured eddy currents through the coil means with predetermined values for different types of iron coins. Finally, the coin discriminator determines the structure, materials and type of the measured iron coin using the predetermined values.  
         [0018]     By providing a coin processing machine with a coin discriminator operated by a method according to the invention, the following advantages are obtained. The same coil means is used to make two measurements of the iron coin at different points of time, thereby eliminating the need and cost for an additional coil means and the additional electronics that would have to be operatively connected to the extra coil means. Other advantages are that the construction and maintenance of the coin processing machine are simplified and the associated costs for these measurements are reduced.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]     The invention will now be described in more detail, reference being made to the accompanying drawings, in which:  
         [0020]      FIG. 1  is a schematic sectional view of an iron coin and a coin discriminator according to the invention,  
         [0021]      FIG. 2  is a schematic plan view of the relative positions between an iron coin in two different positions and the coin discriminator during discrimination,  
         [0022]      FIG. 3  is a block diagram of the electronic circuit used in the coin discriminator in  FIGS. 1 and 2 ,  
         [0023]      FIG. 4  is a diagram over readings of frequency changes when three different iron coins pass the coin discriminator,  
         [0024]      FIG. 5  is a diagram over readings of resistances in the coin discriminator when the three iron coins in  FIG. 4  pass it,  
         [0025]      FIG. 6  is a diagram over the three iron coins in  FIGS. 4 and 5  with their iron rim readings highlighted, and  
         [0026]      FIG. 7  is a block diagram of a coin processing machine comprising the coin discriminator in  FIGS. 1 and 2 . 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0027]      FIG. 1  shows a coin discriminator  10  comprising a coil  20  mounted in a housing  30 . The coil  20  is connected to an electrical device (not shown) for supplying current pulses thereto. Voltage pulses may be used instead of current pulses, this is a common knowledge for a skilled person. A coin  40  is shown just as it reaches the generated magnetic field or pulses of the coin discriminator. Furthermore, the coin discriminator comprises detection means (not shown) for detecting changes in resistance and inductance of the coil  20  caused by the iron coin affecting the magnetic pulses generated by the coil means in response to the current pulses supplied from the electrical device.  
         [0028]     In this embodiment, the coin is an iron coin  40  comprising a large electrical conductive core  50  of a first metal or alloy, e.g. iron or steel, in the form of a disc. The coin core  50  is shown with dotted lines inside the coin  40  to the right in  FIG. 2 . The iron core  50  is a little smaller than in reality so that the difference in size between the outer periphery of the iron core and the outer contour of the coin  40  is shown more clearly, i.e. exaggerated. This area between the outer periphery of the iron core  50  and the outer contour of the coin  40  is a thin layer of copper, brass, or bronze or any other metal used as the outer surface on coins, as is readily envisaged by a skilled person.  
         [0029]     The coin discriminator  10  according to the invention makes two measurements, each measurement is done at different parts of each coin, i.e. at the coin core  50  and a coin rim  60 , respectively. This will be explained more in detail below. The detection means (not shown) of the coin discriminator  10  determines the surface conductivity of the coin  40  in one measurement by inducing eddy currents by means of the coil  20  in the surface of the coin core  50 . The other measurement determines whether the coin has a freely exposed iron core  50  at the rim  60  or if the iron core, i.e. the rim, is covered with a thin layer  70  of another metal, e.g. copper, brass or bronze. A bond between the disc shaped core and the layer is labeled  80 . This bond  80  does not exist if the iron core is not covered at the rim  60 , as is understood by a skilled person.  
         [0030]     The coil  20  in  FIGS. 1 and 2  acts as a transmitter coil for exposing the iron coin  40  to a magnetic field. The exposure of the iron coin is done when it moves past the coin discriminator  10  along a coin rail  90  (shown in  FIG. 2 ). The direction of movement for the iron coin is illustrated by a horizontal arrow pointing to the left in  FIG. 1 . Alternatively, the iron coin may move in the other direction, i.e. to the right, in  FIG. 1 , as is envisaged by a skilled person.  
         [0031]     Furthermore, the coil  20  of the coin discriminator  10  shown in  FIGS. 1 and 2  also acts as a receiver coil being operatively connected to suitable electronics, this will be explained in more detail later on in this description, for detecting both the magnetic field variations, and more particularly the inductance and resistance variations in the coil  20  when a measured iron coin  40  passes it, and the surface conductivity of the measured iron coin and converting them into corresponding signals. The signals are supplied to a detector (not shown), which is arranged to measure the decay and change of the signals and in response determine a respective value of the inductance and resistance changes in the coil  20  and the surface conductivity of the coin  40 . The determined surface conductivity values for each measured iron coin  40  and the effect of the iron coin on the inductance and resistance in the coil  20  are subsequently used for identifying the type of the iron coin.  
         [0032]      FIG. 2  shows the relative positions between one of the essential parts of the coin discriminator  10 , i.e. the coil  20  in its housing  30 , and the iron coin  40  during the discrimination. In this embodiment, the coil is mounted about 3 mm above the coin rail  90 , i.e. the lower end of the coil is 3 mm above the rail. This position for the coil  20  depends on which type of iron coins that are going to be measured and the dimension of the iron coins. Smaller or larger iron coins  40  may require other coil positions in order to get accurate readings.  
         [0033]     Each iron coin  40  moves past the coin discriminator  10  on the coin rail  90  during the measurements. The discrimination according to the invention is done at different points of time because the same coil  20  is used for two measurements. One measurement determines if the iron core, i.e. the rim  60  of the iron coin  40  is exposed or covered by a thin layer of copper, brass, or bronze. The other measurement determines if the iron coin core  50  is covered by a copper, brass, or bronze layer  70 . The copper, brass, or bronze layer on the iron coin core is detected by measuring the conductivity of the surface for the iron coin  40 . The bare or covered iron coin/core rim  60  is determined by its magnetic properties, i.e its effect on the resistance and inductance for the coil  20  when the iron coin rim reaches or passes the coil. Depending on the magnetic properties of the rim  60 , the resistance and inductance of the coil  20  will be influenced to different extents.  
         [0034]     When the iron coin  40  moves from left to right in  FIG. 2 , the coin rim  60  first reaches the coil  20 , therefore the rim measurement is done first in this embodiment. The direction of movement for the iron coin is illustrated by a horizontal arrow placed to the left in  FIG. 2  and pointing to the right. The iron coin could of course move in the other direction if desired. When the coin rim  60  is measured the iron coin should leave about 75% of the coil  20  exposed, i.e. the coin rim covers about 25% of the coil. Then, shortly afterwards, the centre of the iron coin, more specifically, the iron coin core  50 , reaches/covers the coil and the second measurement takes place. The rim measurement could of course be done after the surface conductivity measurement is done. This is because the coin rim  60  passes the coil  20  once more before the iron coin  40  is finally transported past the coil, as is readily understood by a skilled person.  
         [0035]     In  FIG. 2 , the iron coin  40  should preferably be placed over the centre of the coil  20  when the surface conductivity measurement takes place, as in the position to the right for the iron coin. When the coin rim  60  is measured the position for the iron coin should be as shown in  FIG. 1  with the magnetic field striking the rim of the iron coin. Additionally, the position for the coil  20  must be decided in relation to the dimensions of the coins that are going to be measured. The position for the coil is chosen so that the rim  60  of the iron coin  40  does not affect this second measuring of the surface conductivity. The distance between the coin rim  60  and the coil  20  should be larger than approximately 1 mm for securing an accurate reading of the surface conductivity.  
         [0036]     The coil  20  of the coin discriminator  10  is small compared to the diameter of the iron coins  40  to be measured. The coil may have a diameter between 5 to 10 mm. Preferably, the ferrite core should have a diameter between 5 to 10 mm, but, preferably, a diameter of 7.3 mm. The ferrite may be between 2 to 6 mm, preferably, 3.7 mm high or thick and be filled with a wire having a diameter between 0.08 to 1 mm, preferably, 0.2-mm. The wire should, preferably, be made of copper. In principle, any small coil could be used in the coin detector or discriminator  10 , as is envisaged by a skilled person. The use of a ferrite pot core to direct the magnetic field makes the detector/discriminator more efficient.  
         [0037]     In a typical coin counting machine (not shown), the position of the iron coin  40  is known from other sensors (not shown), as is envisaged by a skilled person. This information is used to make the two measurements of the coin at different times using the same coil  20 .  
         [0038]     The surface conductivity is measured when the coil  20  is covered by the iron core  50  of the iron coin  40 , as shown in  FIG. 2 . This means that the iron coin has its center essentially aligned with the center of the coil  20  or the coin core  50  at least covers the whole coil. The duration of the current pulses supplied by the electrical device to the coil may be chosen in accordance with the actual application.  
         [0039]     To make the coil  20  work as a part of the coin detector/discriminator  10  an electronic circuit  100  shown in  FIG. 3  put a time varying current through the coil. Changes in the current produce changes in the magnetic field produced by the coil  20 , whereby the changing magnetic field produces an electric current in the iron coin  40 . This current in the iron coin is called an eddy current. The changing eddy current in turn produces a changing magnetic field, which is measured by the coil  20 .  
         [0040]     If the same coil  20  is used for both generating and sensing the eddy currents, the effect of the iron coin  40  is to cause an apparent change in the inductance and resistance of the coil. The electronic circuit  100  measures these changes and uses them to identify the type of the iron coin.  
         [0041]     The electronic circuits used to measure iron coins  40  with a single coil  20  can be divided into two types: 
        1. Continuous wave (CW) techniques that drive the coil  20  with a continuous sine or square wave.     2. Pulse induction (PI) techniques that use a step change in current to produce an exponentially decaying eddy current within the iron coin  40 .        
 
         [0044]     The electronic circuit  100  in  FIG. 3  drives the coin discriminator  10  by using the continuous wave (CW) technique, which drives the coil  20  with a continuous sine or square wave. The CW electronics can be divided into two types: 
        1. Frequency shift     2. Phase shift        
 
         [0047]     The first method, the frequency shift is the simplest and cheapest. With this technique, the coil  20  forms part of the frequency determining elements of an oscillator  110 . A change in the inductance of the coil causes a change in the oscillator frequency. This frequency shift is used to identify the iron coin  40 . The limitation of this simple method is that it does not measure the change in the resistance of the coil  20 , and, thus, it only uses half of the available information.  
         [0048]     The second method, the phase shift method drives the coil  20 , usually at a fixed frequency, and then measures the amplitude and phase of the coil voltage or current. By measuring both amplitude and phase, the change in inductance and resistance for the coil can be calculated.  
         [0049]     To separate a copper covered iron coin  40  from a brass or bronze covered iron coin according to the invention, the coin discriminator  10  according to the invention uses high frequency eddy currents. The skin depth effect will make these currents flow mainly in the copper, brass or bronze layer. The skin depth effect when using AC-power instead of DC-power is a physical effect that is common knowledge for a skilled person.  
         [0050]     In this embodiment, a type A, a type B, and a type D iron coin are used to explain the function of the coin discriminator  10  according to the invention. These coins are quite similar and good examples of reference iron coins  40 . Alternatively, any other type of existing or future iron coin with a large iron core  50 , which is completely or partly covered by a thin layer of copper, brass or bronze may of course be used, as is envisaged by skilled person.  
         [0051]     The type A and the type B coin are made of iron clad in brass. A type E and the type D coins are iron coins clad in copper. A type F, the type C and the type A coins are iron coins either plated or clad in copper. The type E and D coins often have a copper smear on the rim  60 , i.e. the copper smear only partly covers the rim. The brass plating has a specified thickness of 0.068 mm. The skin depth in 25% IACS brass will be this distance at 3.7 MHz. This means that we must use a frequency over 3.7 MHz to “hide” the iron core  50 , i.e. a lower frequency would make the eddy current penetrate further into the coin, thereby “revealing” or reaching the iron core.  
         [0052]     The 25% IACS brass is defined according to the International Annealed Copper Standard (IACS) scale. This scale relates to the conductivity of metals. On this scale, the conductivity of pure annealed copper is taken as 100%, the bronze used in “copper” coins is about 50%, and brass is typically 25%. The gold alloy in some coins is about 16% and the copper-nickel alloy used in “silver” coins is just over 5%. This is readily understood by a skilled person.  
         [0053]     The maximum frequency used in the coin discriminator  10  is also determined by the skin depth effect, which, in this embodiment, is the skin depth in the copper wire of the coil  20 . Because the current only flows on the surface of the wire, the resistance is greater than its resistance when DC power is used. From this point of view, a frequency as low as possible is preferred.  
         [0054]     Based on these Skin depth arguments, the preferred frequencies is in the range of 4 to 10 MHz but, preferably, between 5 to 8 MHz when used in the coin discriminator  10  according to the invention.  
         [0055]     It is possible to design electronics to accurately measure the change in inductance and resistance of a coil  20  at these frequencies using the phase shift method. However, the electronic circuit  100  in  FIG. 3  will not be simple or cheap because of the high frequencies used. The unique feature of the electronic circuit  100  in the coin discriminator  10  according to the invention is the use of a frequency shift method that can accurately measure changes in both inductance and resistance fast and reliable.  
         [0056]     In  FIG. 3 , an electronic circuit  100  for operating the coin discriminator  10  is shown as a block diagram. A voltage controlled oscillator  110  runs at a frequency of eight times the self-resonance frequency for the coil  20 . A divide by 8 circuit  120  generates frequencies at the coil resonance with phases of plus and minus 45°. A controller  130  selects one of these two phases via a selector device  200  and drives the coil  20  via a bi-directional current source  170 . The voltage across the coil  20  will be a sine wave. The output from a comparitor  140  is a logic level square wave with the same zero crossings as the sine wave in the coil  20 . This square wave is compared with a reference phase of 90° from the divide by 8 circuit  120 . The comparison is done via an exclusive OR gate  150  followed by a low pass filter  160  to the left in  FIG. 3 . The low pass filter comprises two main components of which none is explained in more detail, such a low pass filter is a common knowledge for a skilled person. The output voltage from the low pass filter  160  is used to control the frequency of the oscillator  110 . The constant current source  170  is used for driving the coil  20 . A 16-bit counter  180  measures the frequency of the voltage-controlled oscillator  110 . A detector interface  190  is used for connecting the controller  130 , i.e. the electronics driving the coin discriminator  10 , to other electronics (not shown). The other electronics could be any kind of suitable hardware and software, e.g. a PC, used for further processing of the measurement results, e.g. presentation of the result for an operator of the coin counting and sorting machine, or a processor in a coin counting and sorting machine.  
         [0057]     If the current source  170  were driven by a zero degrees phase, the electronic circuit  100  would lock to the resonant frequency of the coil  20 . The electronic circuit would then be recognised as an example of a known type, which is called a phased locked loop and common knowledge for a skilled person.  
         [0058]     By driving the current source  170  with a phase of 45°, the electronic circuit  100  will still lock. However, the frequency will produce a phase shift of 45° between the voltage and current through the coil  20 . From the physics it is known that this 45° phase shift only occurs at the “3 dB points” on the resonance curve. By using phases of both plus and minus 45° the frequencies of the upper and lower “3 dB points” can be measured. The average of these two frequencies is the resonance frequency for the coil  20 . The difference between the frequencies is the width of the resonance frequency.  
         [0059]     The 16 bit counter  180  measures the frequency of the voltage-controlled oscillator  110 . The controller  130  does this by counting how many cycles occur in a fixed period. In this embodiment, a period may be in the range of 50 to 200 μs but a period of 125 μs is preferred, so that the count gives the frequency change in kHz. The controller also interfaces to the rest of the electronics, i.e. the detector interface  190  connected to other components (not shown) of a coin counting and sorting machine  700  shown as a block diagram in  FIG. 7 .  
         [0060]     In  FIG. 4 , readings from the preferred coin discriminator  10  according to the invention are shown. The y-axis on the graph shows the frequency changes in kHz caused by the three iron coins  40  passing the coil  20 . From left to right there are the type B, the type D and the type A coin.  
         [0061]     In  FIG. 4 , the x-axis is simply distance in mm along the coin rail  90 . The solid line is the change in the centre or resonance frequency of the oscillator  110 . The coil  20  used in  FIG. 4  is the coil shown in all of the earlier drawings. With this coil, the no-coin centre or resonance frequency is between 4 to 10 MHz, preferably, 5 MHz. The first iron coin  40 , the brass plated type B, increases this frequency by 600 kHz. This frequency change is range dependent. The above graph was made with a coil to coin range of 0.9 mm.  
         [0062]     The effect from the iron coin  40  on the resistance of the coil  20  depends on the range or distance between the iron coin and the coil. The effect from the iron coin on the resistance for the coil decreases as the distance between the iron coin and the coil increases. The decrease of the coil resistance occurs in proportion to the diameter of the circularly flowing eddy current in the coin. The effect from the coin  40  on the inductance of the coil  20  also depends on the range or distance between the iron coin and the coil. The effect on the inductance for the coil decreases as the distance between the iron coin and the coil increases. The decrease of the coil inductance occurs in proportion to the diameter raised to a second power of the circularly flowing eddy current in the coin, i.e. in proportion to the area covered by the circularly flowing eddy current.  
         [0063]     The dotted line is the width of the resonance frequency, this is the frequency difference between the upper and lower “3 dB points”. The width of the resonance frequency is a direct measurement of resistance in the coil  20 . The wider the resonance frequency, then the higher the resistance. The dotted line demonstrates this effect. The type B iron coin  40  has a resonance frequency width of 570 kHz compared to 530 kHz for the other two iron coins. This is because brass has a higher resistance than copper, i.e. copper is a better conductor than brass. The dotted line shows that without a coin  40 , the resonance frequency width is 430 kHz. This is due to the resistance of the wire in the coil  20 .  
         [0064]     The text book method for finding the resistance of a coil  20  is from its ‘Q’ or quality factor. A high value of Q implies a low coil resistance. The Q and the self-resonant frequency of a coil are given by the equations:  
         [0065]     The self-resonant frequency is  
         f   0     =     1     2   ⁢   π   ⁢     LC             
 
         [0066]     The Q of a coil  20  is  
       Q   =         2   ⁢   π   ⁢           ⁢   fL     R     =     f     Δ   ⁢           ⁢   f             
 
 Where: 
        L is the inductance of the coil  20      C is the total capacitance in parallel with the coil     R is the resistance of the coil at the resonance frequency     f 0  is the self resonant frequency     Δf is the frequency difference between the 3 dB points.        
 
         [0073]     The readings from the three iron coins  40  in  FIG. 4  have been used to produce the graph in  FIG. 5 .  
         [0074]     In  FIG. 5 , the curve shows the Q of the coil  20  dropping each time the measured iron coin  40  passes over it. The graph shows the greater drop for the high resistance brass plated type B iron coin to the left compared to the two copper plated iron coins, i.e. the type D in the middle and the type A to the right.  
         [0075]     The graph in  FIG. 6  shows the same iron coin readings as in  FIG. 4  and  5  but, here, the readings are processed to highlight the readings of the iron rims on each iron coin  40 . As before the type B followed by the type D followed by the type A coin pass the coin discriminator  10  and two measurements for each iron coin are done.  
         [0076]      FIG. 6  simply illustrates the resonance frequency width minus one quarter of the centre frequency shift. The processing must be simple because of the high speed of the iron coins  40  passing through the coin discriminator  10  and the coin counting and/or sorting machine  700  shown in  FIG. 7 .  
         [0077]     The iron rims on the brass plated type B iron coin  40  are shown more clearly than on the copper plated type D iron coin. This is because of two effects pulling in opposite directions. The magnetic properties of the iron are trying to increase the inductance of the coil  20 , whereas the eddy currents in the surface of the iron coin are trying to reduce the coil inductance. The low resistance copper plated type D iron coin allows a greater eddy current and thus hides more of the iron at the rim  60  of the iron coin  40 .  
         [0078]     The copper covered rims  60  of the type A iron coin  40  hide the iron completely at these high frequencies. This means that the inductance of the coil  20  only decreases as this type A iron coin passes over it.  
         [0079]     Referring now to  FIG. 7 , the coin processing machine  700  according to one aspect of the present invention is schematically illustrated. In an exemplifying but not limiting sense, the coin processing machine  700  of  FIG. 7  is selected to be a coin sorter. The mass/es of coins  40 , which are to be sorted by the machine  700 , are deposited into a coin inlet  710 . Here, the coins may be any types of coins not just iron coins covered by a thin layer of copper, brass or bronze, as is readily understood by a skilled person. The coins are fed by a coin feeder  720 , such as a hopper and/or an endless belt, to the coin discriminator  10 , which has been described above with reference to  FIGS. 1, 2 , and  3 . The coin discriminator  10  is operatively connected to a logic device  732  in the form of a CPU, which is operatively connected to a memory  734 , such as a RAM, ROM, EEPROM or flash memory. The memory  734  stores a set of coin reference data, which is used by the logic device  732  to discriminate among the coins  40  received through the coin inlet  710 . More specifically, the coin reference data relates to typical values of conductivity and permeability for all different types of coins and to typical values of the effect from all different types of coins on the resistance and inductance of the coil  20 , that the coin processing machine  700  is capable of processing.  
         [0080]     The logic device  732  is programmed to receive measurement data obtained by the coil  20  and the controller  130 , which is operatively connected to the detector interface  190 , for storing the data relating to the surface conductivity of the coin  40 , and resistance and inductance changes of the coil  20  when the coin passes it. Once these measurement data have been received for a coin, the logic device  732  will read the coin reference data stored in the memory  734 , which also is operatively connected to the detector interface  190 , and search for any matches. If the physical and magnetic properties for the iron coin measured by the coin discriminator  10  correspond to one specific iron coin type defined by the iron coin reference data, then the type of iron coin has been positively identified. Otherwise, the iron coin  40  is of an unknown type and handled by a coin reject device  740 , which preferably will deliver the iron coin through an external opening in the machine  700 , so that the iron coin may be removed by a user. The rejected iron coin  40  may also be re-circulated back into the coin discriminator  10  for another attempt to discriminate it.  
         [0081]     The coin types defined by the coin reference data in the memory  734  may preferably relate to the denomination and currency of each different type of coin  40 , which is to be handled by the coin processing machine  700 .  
         [0082]     Once the type or identity of the coin  40  has been determined by the coin discriminator  10  and the logic device  732 , the coin is passed to a coin sorter  750 , which uses the identified coin type to sort the coin  40  into one specific coin box, etc., in a coin storage  760 . The coin boxes, etc., in the coin storage are preferably externally accessible for the user of the machine  700 .  
         [0083]     A future development of the coin discriminator  10  according to the invention would be to use more than one coil  20  if the coins  40  to be measured would have a larger diameter or thickness than the coins measured in this embodiment. In this case, the coils may have to be placed in different positions in relation to each other to be able to cover coins with different diameters, e.g. higher or lower in relation to the coin rail  90  and the other coil, in order to make accurate measurements of each coin  40 .