Patent Publication Number: US-2004047106-A1

Title: Device and method for identifying metal body

Description:
TECHNICAL FIELD  
       [0001] This invention relates to a metal object identification apparatus and metal object identification method for identifying metal objects, which in particular are used to identify coins.  
       BACKGROUND ART  
       [0002] In the past, metal object identification apparatuses to identify metal objects and in particular coins have been known; one such example is described in Japanese Patent Laid-open No. 6-84040. The metal object identification apparatus described in the above publication has an oscillation circuit comprising a coil; when a coin (metal object) moves relative to the coil, the changes in impedance and inductance of the coil are detected as changes in the oscillation frequency and amplitude of the oscillation circuit, and the material of the coin is detected from this frequency change, while the outer shape and cross-sectional area of the coin are detected from the amplitude change.  
       DISCLOSURE OF THE INVENTION  
       [0003] However, among foreign coins there exist coins of ferrous metal with a copper plating, and there is scattering in the thickness of the copper plating. Due to this scattering in the copper film thickness, there are large changes in the conductivity, which is due in part to the plating thickness which is a material characteristic of the coin, so that overlaps occur between the parameter values of the coin to be identified and the parameter values of other coins and, and consequently there is the problem that coins cannot be identified correctly.  
       [0004] In the case of Japanese coins, the above-described scattering is comparatively small; however, there are cases in which it is difficult to discriminate true coins (authentic coins) from altered coins which have substantially the same external shape as a result of machining a foreign coin. In other words, in order to discriminate such altered coins from authentic coins, the range of parameters within which a coin is judged to be authentic must be made narrow, but then there is the problem that due to scattering that occurs because of difference in the state of conveyance, material, wear, the parameters of an authentic coin may not fall within this range of parameters, so that the rate of acceptance of authentic coins is reduced.  
       [0005] An objective of the present invention is to provide a metal object identification apparatus and metal object identification method which solves the above-described problems, and can identify a metal object even when there is scattering in the thickness of a metal surface film. A further objective is to provide a metal object identification apparatus and metal object identification method capable of discriminating metal objects even when the external shapes are substantially equal.  
       [0006] A metal object identification apparatus of this invention is characterized in comprising an oscillation circuit incorporating a coil; an extraction portion which, when a metal object to be identified moves with respect to the coil, extracts the output from the oscillation circuit as at least two parameters; a correlation storage portion, which stores in advance correlations of parameters for the metal object based on measured results for a plurality of samples; and, a metal object identification portion, which identifies metal objects by judging whether the parameters extracted by the extraction portion are applicable to the correlation stored in the correlation storage portion.  
       [0007] The metal object identification apparatus of this invention stores the correlations of parameters for metal objects measured in advance, and judges whether at least two parameters output from the oscillation circuit are applicable to the stored correlation. By this means, even when overlap exists in the ranges in which parameters exist for different metal objects, so that it may not be possible to correctly identify metal objects when identifying metal objects using one parameter, metal objects can be identified using the correlation of a plurality of parameters. Here a “correlation” is a relation between parameters determined by the characteristics of a metal object. This correlation is determined based on the parameters of metal objects of the same type obtained from a plurality of samples.  
       [0008] The above-described metal object identification apparatus may be characterized in that the correlation storage portion stores in advance the correlations of the parameters for a plurality of types of metal objects, and the metal object identification portion identifies the type of metal object by judging which of the correlation stored in the correlation storage portion the parameters extracted by the extraction portion are applicable to.  
       [0009] In this way, if correlations for parameters are stored in advance in the correlation storage portion for a plurality of types of metal objects, the metal object type can be identified by the metal object identification portion. The above-described metal object identification apparatus may be characterized in that the parameters extracted by the extraction portion are a parameter relating to the change in oscillation amplitude and a parameter relating to the change in oscillation frequency; correlations for each of the metal objects between the change in oscillation amplitude and the change in oscillation frequency are stored in the correlation storage portion; and the metal object identification portion identifies the type of metal object by judging which of the correlation stored in the correlation storage portion the parameters extracted by the extraction portion are applicable to.  
       [0010] The parameter related to oscillation frequency changes is a parameter which depends on the conductivity of the metal object, which changes mainly due to the material of the metal object; the parameter related to oscillation amplitude changes is a parameter which changes mainly depending on the cross-sectional area of the metal object. The inventors discovered that there is a correlation, unique to each metal object, between oscillation frequency changes and oscillation amplitude changes; utilizing this correlation, an apparatus for identification of metal objects having scattering in the thickness of a metal surface film was invented.  
       [0011] The above-described metal object identification apparatus maybe characterized in that the correlation storage portion stores distributions of the change in oscillation frequency with respect to the change in oscillation amplitude, and the metal object identification portion identifies the type of metal object by judging which of the distribution for the metal object stored in the correlation storage portion the parameters extracted by the extraction portion are included in.  
       [0012] When oscillation frequency changes are plotted against oscillation amplitude changes, the range of the distribution of parameters is determined for each metal object type. Using this correlation, a metal object type can be identified by judging which of the distribution for the metal object stored in the correlation storage portion the parameters of a metal object to be identified are contained in.  
       [0013] The above-described metal object identification apparatus maybe characterized in that the correlation storage portion stores functions which are approximated to the change in oscillation frequency with respect to the change in oscillation amplitude for each of the metal objects, and the metal object identification portion identifies the type of metal object by judging which of the function for the metal object stored in the correlation storage portion the parameters extracted by the extraction portion exist in the vicinity of.  
       [0014] When oscillation frequency changes are plotted against oscillation amplitude changes, the oscillation frequency changes with respect to oscillation amplitude changes can be approximated by functions for each metal object type. Utilizing this correlation, metal object types can be identified by judging which of the stored correlation function for the metal object the parameters of a metal object to be identified are near. Whether parameters exist in the vicinity of a certain function can be judged, for example, by determining the distance in the y-axis direction of the parameters from the function, or the distance in the x-axis direction of parameters from the function and comparing the distance with a prescribed threshold value.  
       [0015] The above-described metal object identification apparatus may be characterized in further comprising correlation parameter calculation means which calculates the average rate of increase of the oscillation frequency with respect to the oscillation amplitude based on the functions stored in the correlation storage portion, and calculates, as a correlation parameter, the value of the oscillation frequency at a prescribed oscillation amplitude based on the calculated average rate of increase and on the parameters extracted by the extraction portion; wherein the metal object identification portion judges which of the function for the metal object stored in the correlation storage portion the parameters extracted by the extraction portion exist in the vicinity of, by judging which of the threshold values set in advance centered on the values of oscillation frequencies at the prescribed oscillation amplitude for each of the functions the correlation parameter calculated by the correlation parameter calculation means is contained within.  
       [0016] The inventors discovered that there are no large differences for metal objects in the rate of increase of oscillation frequency changes with respect to oscillation amplitude changes. Utilizing this property, correlation parameters representing correlations between extracted parameters are newly provided. That is, the functions passing through the point representing extracted parameters, and having the same rates of increase as the respective correlation average increase rates, are determined. Here “increase rate” includes negative increase rates, and cases in which the oscillation frequency decreases with increasing oscillation amplitude are also included in the scope of this invention. The values of oscillation frequencies at prescribed oscillation amplitudes for each calculated function are calculated as correlation parameters. Threshold values set in advance are then used to judge which of the oscillation frequency for the correlation function at the prescribed oscillation amplitude the calculated correlation parameters are close to, and a judgment is made as to which of the correlation function the extracted parameters exist in the vicinity of. By using correlation parameters in this way, it can be easily judged which of the function extracted parameters exist in the vicinity of. Threshold values set in advance are set such that there is no overlap in ranges.  
       [0017] In the above-described metal object identification apparatus, it is preferable that the metal objects be coins. The above-described metal object identification apparatus maybe characterized in that the correlation storage portion stores a correlation between oscillation amplitude changes and oscillation frequency changes measured for a plurality of coins with the conductivity changed within the range of allowable conductivities for authentic coins; and the metal object identification portion judges the authenticity or inauthenticity of a coin based on the correlation stored in the correlation storage portion.  
       [0018] The inventors discovered that when the conductivity of the same coin, that is, of coins with equal outer shape, is changed, there exists a correlation between the oscillation frequency and the oscillation amplitude. By using this correlation metal objects can be identified based on slight differences in conductivity, and authentic coins can be accurately discriminated from forged coins with the same outer shape. A metal object identification method of this invention is characterized in comprising a correlation storage step, in which a correlation between parameters for a metal object is stored in advance in a correlation storage portion, based on measured results for a plurality of samples; an extraction step, in which, when a metal object to be identified moves with respect to a coil constituting a part of an oscillation circuit, the output from the oscillation circuit is extracted as at least two parameters; and, a metal object identification step, in which the metal object is identified by judging whether the parameters extracted in the extraction step are applicable to the correlation stored in the correlation storage portion.  
       [0019] In the correlation storage step of the metal object identification method of this invention, correlations between parameters for metal objects are stored in advance, and a judgment is made as to whether at least two parameters output from the oscillation circuit are applicable to the stored correlation. By this means, even when overlap exists in the ranges in which parameters exist for different metal objects, so that it may not be possible to correctly identify metal objects when identifying metal objects using one parameter, metal objects can be identified using the correlation of a plurality of parameters. Here a “correlation” is a relation between parameters determined by the characteristics of a metal object. This correlation is determined based on the parameters of metal objects of the same type obtained from a plurality of samples. The above-described metal object identification method may be characterized in that in the correlation storage step, correlations of the parameters are stored in advance for a plurality of types of metal objects; and, in the metal object identification step, the metal object type is identified by judging which of the correlation stored in the correlation storage portion the parameters extracted in the extraction step are applicable to.  
       [0020] Thus if in the correlation storage step correlations of parameters are stored in advance in the correlation storage portion for a plurality of types of metal objects, the metal object type can be identified by the metal object identification portion.  
       [0021] The above metal object identification method may be characterized in that the parameters extracted in the extraction step are a parameter relating to the change in oscillation amplitude and a parameter relating to the change in oscillation frequency; in the correlation storage step, correlations for each of the metal objects between the change in oscillation amplitude and the change in oscillation frequency are stored in the correlation storage portion; and in the metal object identification step, the metal object type is identified by judging which of the correlation stored in the correlation storage portion the parameters extracted in the extraction step are applicable to.  
       [0022] The parameter related to oscillation frequency changes is a parameter which depends on the conductivity of the metal object, which changes mainly due to the material of the metal object; the parameter related to oscillation amplitude changes is a parameter which changes mainly depending on the cross-sectional area of the metal object. The inventors discovered that there is a correlation, unique to each metal object, between oscillation frequency changes and oscillation amplitude changes; utilizing this correlation, a method of identification of metal objects having scattering in the thickness of a metal surface film was invented. The above-described metal object identification method may be characterized in that in said correlation storage step, the distributions of the oscillation frequency change with respect to the oscillation amplitude change are stored in said correlation storage portion; and in said metal object identification step, the metal object type is identified by judging which of the distribution for the metal object stored in said correlation storage portion the parameters extracted in said extraction step are included in.  
       [0023] When oscillation frequency changes are plotted against oscillation amplitude changes, the range of the distribution of parameters is determined for each metal object type. Using this correlation, a metal object type can be identified by judging which of the distribution of the correlation for the metal object stored in the correlation storage portion the parameters of a metal object to be identified are contained in.  
       [0024] The above-described metal object identification method may be characterized in that in the correlation storage step, functions which are approximated to the oscillation frequency change with respect to the oscillation amplitude change for each of the metal objects are stored in the correlation storage portion; and in the metal object identification step, the metal object type is identified by judging which of the function for the metal object stored in the correlation storage portion the parameters extracted in the extraction step exist in the vicinity of.  
       [0025] When oscillation frequency changes are plotted against oscillation amplitude changes, the oscillation frequency changes with respect to oscillation amplitude changes can be approximated by functions for each metal object type. Utilizing this correlation, metal object types can be identified by judging which of the stored correlation function for the metal object the parameters of a metal object to be identified are near. Whether parameters exist in the vicinity of a certain function can be judged, for example, by determining the distance in the y-axis direction of the parameters from the function, or the distance in the x-axis direction of parameters from the function and comparing the distance with a prescribed threshold value.  
       [0026] The above-described metal object identification method may be characterized in further comprising a correlation parameter calculation step in which the average rate of increase of the oscillation frequency with respect to the oscillation amplitude is calculated based on the functions stored in the correlation storage portion, and the value of the oscillation frequency at a prescribed oscillation amplitude is calculated, as a correlation parameter, based on the calculated average rate of increase and the parameters extracted in the extraction step; and characterized in that: in the metal object identification step, a judgment is made as to which of the function for the metal object stored in the correlation storage portion the parameters extracted in the extraction step exist in the vicinity of, by judging which of the threshold values set in advance centered on the values of oscillation frequencies at the prescribed oscillation amplitude for each of the functions the correlation parameter calculated in the correlation parameter calculation step is contained within.  
       [0027] The inventors discovered that there are no large differences for metal objects in the rate of increase of oscillation frequency changes with respect to oscillation amplitude changes. Utilizing this property, correlation parameters representing correlations between extracted parameters are newly provided. That is, the functions passing through the point representing extracted parameters, and having the same rates of increase as the respective correlation average increase rates, are determined. Here “increase rate” includes negative increase rates, and cases in which the oscillation frequency decreases with increasing oscillation amplitude are also included in the scope of this invention. The values of oscillation frequencies at prescribed oscillation amplitudes for each calculated function are calculated as correlation parameters. Threshold values set in advance are then used to judge which of the oscillation frequency of the correlation at the prescribed oscillation amplitude the calculated correlation parameters are close to, and a judgment is made as to which of the correlation function the extracted parameters exist in the vicinity of. By using correlation parameters in this way, it can be easily judged which of the function the extracted parameters exist in the vicinity of. Threshold values set in advance a reset such that there is no overlap in ranges.  
       [0028] In the above-described metal object identification method, it is preferable that the metal objects be coins.  
       [0029] The above-described metal object identification method may be characterized in that in the correlation storage step, the correlation between oscillation amplitude changes and oscillation frequency changes measured for a plurality of coins with the conductivity changed within the range of allowable conductivities for authentic coins is stored in the correlation storage portion; and in the metal object identification step, the authenticity or inauthenticity of a coin is judged based on the correlation stored in the correlation storage portion.  
       [0030] The inventors discovered that when the conductivity of the same coin, that is, of coins with equal outer shape, is changed, there exists a correlation between the oscillation frequency and the oscillation amplitude. By using this correlation metal objects can be identified based on slight differences in conductivity, and authentic coins can be accurately discriminated from forged coins with the same outer shape. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0031]FIG. 1 is a block diagram showing the configuration of a coin identification apparatus;  
     [0032]FIG. 2 is a perspective view which explains coin guiding means;  
     [0033]FIG. 3A is a drawing showing the manner in which a coin passes through a coil;  
     [0034]FIG. 3B is a drawing showing a signal S 1  generated by the oscillation circuit  20 ;  
     [0035]FIG. 3C is a drawing showing a signal SL output from an output terminal;  
     [0036]FIG. 3D is a drawing showing a signal Df output from an output terminal;  
     [0037]FIG. 4A is a graph of the distribution of a first parameter for coin A and coin B;  
     [0038]FIG. 4B is a graph of the distribution of a second parameter for coin A and coin B;  
     [0039]FIG. 4C plots the values of the second parameter against the first parameter;  
     [0040]FIG. 5 is a graph showing the allowable ranges of correlation parameters for identification of each coin type;  
     [0041]FIG. 6 is a flowchart showing the operation of a coin identification apparatus;  
     [0042]FIG. 7A is a graph of the distribution of a first parameter for coins the conductivity of which are slightly different;  
     [0043]FIG. 7B is a graph of the distribution of a second parameter for coins the conductivity of which are slightly different;  
     [0044]FIG. 7C is a graph showing the correlations of the first parameter and second parameter; and,  
     [0045]FIG. 7D is a graph showing allowable ranges of correlation parameters for identification. 
    
    
     BEST MODES FOR CARRYING OUT THE INVENTION  
     [0046] Below, preferred aspects of a metal object identification apparatus of this invention are explained in detail, together with the drawings. In the aspects explained below, the example of a coin identification apparatus which identifies coins (metal objects) is explained. In explanations of the drawings, the same elements are assigned the same symbols, and redundant explanations are omitted.  
     [0047]FIG. 1 is a block diagram showing the configuration of a coin identification apparatus  10  of this aspect. The coin identification apparatus  10  has an oscillation circuit  20 , comprising a coil  21 ; an extraction portion  30  which extracts two parameters from the output of the oscillation circuit  20 ; a correlation storage portion  40  which stores, for each coin, the correlation between the parameters based on measurement results for parameters measured in advance; and a coin identification portion  50 , which identifies the type of coin based on the parameters extracted by the extraction portion  30 , referring to the correlation storage portion  40 . The coin identification apparatus  10  further has correlation parameter calculation means  51  which calculates correlation parameters based on two parameters extracted for each coin to be identified (hereinafter called a “coin for identification”).  
     [0048] In addition to the coil  21 , the oscillation circuit  20  has a circuit  22  comprising a capacitor, resistor, and similar. The coin identification apparatus  10  may also have coin guiding means, which guides the trajectory of the coin for identification so that the coin for identification passes through inside the coil  21 . One example of coin guiding means is explained, referring to FIG. 2. The coin guiding means  61  shown in FIG. 2 has a hollow portion  61   a  through which the coin passes, and is a cylindrical body molded from plastic or similar. The coin guiding means  61  may have a pair of flanges  62  on the outside of the cylinder body, formed integrally and substantially parallel, separated by a prescribed interval. The coil  21  is formed by winding comparatively thin insulated copper wire around the outer walls of the cylinder body, with both ends of the copper wire extending outside. The shape of the hollow portion  61   a  is designed so as to be similar to, but somewhat larger than, the cross-section AR (the shaded lines in FIG. 2) in the radial direction of the coin for identification. Returning to FIG. 1, the extraction portion  30  comprises a frequency detection circuit  32  connected to the oscillation circuit  20  and a waveform detection circuit  31 , and extracts the output from the oscillation circuit  20  as two parameters. The waveform detection circuit  31  extracts as a parameter (hereinafter called the “first parameter”) the change in oscillation amplitude when a coin moves relative to the coil  21 , and the frequency detection circuit  32  extracts as a parameter (hereinafter called the “second parameter”) the change in frequency when a coin moves relative to the coil  21 .  
     [0049] The first parameter is obtained by detecting the envelope of the signal appearing at the terminal, and extracting as a parameter the change in amplitude of the envelope of the signal. The second parameter is extracted in the following manner. First the oscillation frequency of the signal appearing at the terminal is detected, and the period, which is the reciprocal of the oscillation frequency, is measured using a CPU counter or other rapid signal. The maximum value is then extracted as the parameter. Here the measured maximum value is taken to be the second parameter, but the minimum value may be used instead of the second parameter. The signal appearing at the terminal and the relation of the parameters are explained referring to FIG. 3A through FIG. 3D. FIGS. 3A to  3 D show that the signal S 1  appearing in the oscillation circuit  20  (FIG. 3B), the signal SL Df appearing at the output terminal (FIG. 3D) change when, as shown in FIG. 3A, a coin passes along the arrow A through the hollow center of the coil  21  in the coin identification apparatus  10 .  
     [0050] When the coin is distant from the coil  21 , such as at times prior to time t1, there is no effect of lines of magnetic force, and in this state there is no change in the inductance or impedance of the coil  21 , and a signal S 1  with constant frequency and amplitude is generated. Hence the signal SL output from the waveform detection circuit  31  remains fixed amplitude, and similarly, the output signal of the frequency detection circuit  32  also appears as a rectangular signal of fixed frequency.  
     [0051] Then when, as shown at time t2, the leading portion of the coin enters the hollow portion  61   a  of the coil  21 , an eddy current is generated in the leading portion by the effect of the lines of magnetic force, and simultaneously the inductance and impedance of the coil  21  change, and the frequency and amplitude of the signal change. In particular, the change in frequency is affected by the conductivity of the metal object, and the amplitude is affected by the cross-sectional area of the overlapping portion of the leading portion of the coin with the coil  21 .  
     [0052] When the coin  21  proceeds into the hollow portion  61   a , the eddy current generation gradually increases. The amplitude of the output signal decreases in accordance with this change in the signal, and the output signal frequency also changes. And when as shown from time t3 to time t5 the coin moves away from the coil  21 , the signal frequency and amplitude both gradually return to their original values, and when the coin has moved completely away from the coil  21 , the signal is restored to the original frequency and amplitude (for example, the frequency and amplitude at time t1). The correlation storage portion  40  stores, as a correlation equation  41 , the correlation between the above-described parameters for each coin, based on the results of measurements made in advance of a plurality of samples of each coin type. Here, the correlation equation  41  is explained referring to FIG. 4A through FIG. 4C. FIG. 4A is a graph of the distribution of the first parameter for a coin A and coin B; FIG. 4B is a graph of the distribution of the second parameter for coin A and coin B; and, FIG. 4C plots the values of the second parameter against the first parameter, and shows the correlation between the first parameter and the second parameter. The correlation equation  41  is an equation which is derived based on the graph shown in FIG. 4C; a first-order equation is approximated to the distributions of the parameters to determine the correlation equation  41 . Here the correlation equation  41  is determined by approximating the first order equation to the parameter distribution; but an approximation using a polynomial equation of second or higher order may be used.  
     [0053] The coin identification portion  50  has a function for identifying the type of coin based on the two parameters extracted by the extraction portion  30  and correlations stored in the correlation storage portion  40 . Here the correlation parameter calculation means  51 , which supports the identification of a coin by the coin identification portion  50  based on correlation equations  41  stored in the correlation storage portion  40 , is explained.  
     [0054] The correlation parameter calculation means  51  has a function for determining correlation parameters from the first parameter and second parameter, in order to judge whether the parameters of the coin for identification are applicable to any of the correlations stored in the correlation storage portion  40 . The correlation parameter calculation means  51  utilizes the fact that the correlations of the parameters for each coin have substantially the same slope (the rate of increase of the second parameter with respect to the first parameter is substantially the same), as shown in FIG. 4C, to calculate correlation parameters. Specifically, first the average slope of the correlation equation is calculated from each of the correlation equations. This average slope is a slope which represents each of the correlation equations. Then, from the parameters for the coin for identification and the average slope, the first-order equation which passes through the parameters of the coin for identification and has the average slope is determined, and the intercept of the first-order equation with the second parameter axis is calculated as a correlation parameter. The correlation parameter calculation means  51  also sets the allowable identification range of correlation parameters in order to judge the type of each coin. Specifically, the intercepts with the second parameter axis of the correlation equations for each coin are calculated, and prescribed ranges centered on these values are set as the allowable identification ranges. As shown in FIG. 5, the allowable identification ranges for each coin are set such that there is no overlap among coins, so that accurate identification of each coin type is possible. Here the correlation parameter calculation means  51  which calculates correlation parameters exists separately from the coin identification portion  50 ; however, the coin identification portion  50  may be provided with a function for calculating correlation parameters.  
     [0055] The coin identification portion  50  has a function for identifying a coin by judging whether a correlation parameter calculated by the correlation parameter calculation means  51  is contained within the correlation parameter range for any coin type. In this aspect, the correlation parameter calculation means  51  calculates the correlation parameter and the coin identification portion  50  judges whether the parameter of the coin for identification is included within the parameter range for any coin type; however, the coin identification portion  50  may identify a coin using another method such as, for example, calculating the distance between the parameter of the coin for identification and a correlation equation  41 .  
     [0056] Next, the operation of the coin identification apparatus  10  of this aspect is explained, and in addition a coin (metal object) identification method of this invention is explained. FIG. 6 is a flowchart showing the operation of the coin identification apparatus  10 .  
     [0057] First, the correlation storage portion  40  of the coin identification apparatus  10  is made to store correlations of the first parameter and second parameter for each coin type (S 10 ). Measurements of the first parameter and second parameter are performed for each coin type, and numerous samples are acquired. Based on these samples, the correlation between the first parameter and the second parameter is calculated as a first-order correlation equation  41 , and is stored in the correlation storage portion  40 . This step is in preparation to register information for each coin type in the coin identification apparatus  10  in order to perform coin identification. Also, in this step the ranges of correlation parameters to discriminate different coin types are set. Specifically, the second parameter axis intercept value of each correlation equation  41  is calculated, and the allowable identification range is set with a prescribed width and including the intercept value.  
     [0058] Next, the coin for identification is made to pass through the hollow portion of the coil  21 , and the first parameter and second parameter are extracted from the change in oscillation amplitude and the change in oscillation frequency (S 12 ).  
     [0059] Next, the correlation parameter calculation means  51  calculates the correlation parameter based on the extracted first parameter and second parameter (S 14 ). Specifically, first the average slope of the correlation equation  41  for each coin type is calculated. Then, a first-order equation which passes through the point determined by the first parameter and the second parameter, and which has the average slope, is calculated. Then the intercept with the second parameter axis of this first-order equation is calculated as the correlation parameter.  
     [0060] Next, the coin type is identified by judging which allowable identification range of a coin type the correlation parameter is contained within (S 16 ).  
     [0061] In the coin identification apparatus  10  of this aspect, the oscillation amplitude change and the oscillation frequency change occurring in the oscillation circuit  20  when a coin passes through the coil  21  are extracted as the first parameter and the second parameter respectively, and by identifying the coin based on the correlation between these parameters, accurate coin identification can be performed. Referring to FIG. 4A and FIG. 4B, individually, the first parameter and the second parameter overlap in the ranges of parameters of coins, but by identifying the coin using the correlation of the two parameters, such inconvenience can be avoided (cf. FIG. 5).  
     [0062] Further, the coin identification apparatus  10  of this aspect has correlation parameter calculation means  51 , and the correlation parameter is calculated based on the first parameter and the second parameter, so that the coin identification portion  50  can easily judge, from the correlation parameter, which correlation equation  41  two parameters of the coin for identification exist in the vicinity of.  
     [0063] The coin identification method of this aspect can, similarly to the above-described coin identification apparatus  10 , accurately identify coins based on the correlation of two parameters.  
     [0064] Next, a second aspect of this invention is explained. The coin identification apparatus of the second aspect has a configuration which is essentially the same as the coin identification apparatus  10  of the first aspect, but the data stored in the correlation storage portion is different. The data stored in the correlation storage portion of the coin identification apparatus of the second aspect is the correlation of two parameters based on the results of measurements performed while varying the conductivity within the range of conductivities allowable for a coin to be judged as an authentic coin of one type. The conductivity of a coin changes depending on the character of the coin material, but while forged coins obtained by altering foreign coins, for example, have different conductivities, it is sometimes difficult to make a judgement based only on differences in conductivity. FIG. 7A is a graph of the distribution of the first parameter, FIG. 7B is a graph of the distribution of the second parameter, FIG. 7C is a graph of the correlation, and FIG. 7D shows allowable ranges of correlation parameters for identification, each for coins the conductivity of which is changed slightly. As shown in FIG. 7A and FIG. 7B, there is a portion of overlap between the distributions of the first parameter and the second parameter, and these parameters independently cannot be used to accurately judge the authenticity of a coin. As shown in FIG. 7C, from the correlation between the first parameter and the second parameter it is seen that there is a correlation different from that of the coin identification apparatus of the first aspect. Using this correlation, authentic and inauthentic coins can be discriminated. In FIG. 7A through FIG. 7D, data for a coin C and coin E with different conductivities is shown as examples for comparison in addition to coin D which is an authentic coin; however, the correlation data stored in the correlation storage portion of the coin identification apparatus of the second aspect is data for the range of correlation parameter to identify coin D as an authentic coin.  
     [0065] The operation of the coin identification apparatus of the second aspect is in essence the same as the operation of the coin identification apparatus of the first aspect. The coin identification apparatus of the second aspect stores correlations of parameters measured for coins with the same outer shape but slightly different conductivities in the correlation storage portion, and identifies coins (in particular authentic and inauthentic coins) based on these correlations. By thus storing correlations for coins with the same outer shape, changes in correlation accompanying changes in the conductivity of coins can be handled more precisely, and new correlations can be ascertained. As a result, authentic and inauthentic coins with slight differences in material can be accurately discriminated. In the above, aspects of this invention have been explained, but the present invention is not limited to the above-described aspects.  
     [0066] In the above aspects, examples of a coin identification apparatus were explained; but configuration as a metal object identification apparatus which identifies metal objects other than coins is possible.  
     [0067] In the above aspects, by using correlation equations, a judgment was made as to whether the parameters of the coin to be identified exist in the vicinity of the correlation equations for any of the coin types; however, correlation equations need not be used, and judgments may be made as to whether parameters of a coin to be identified are contained in a distribution of correlations of coins measured in advance.  
     [0068] In the above aspects, changes in the oscillation amplitude and oscillation frequency occurring in an oscillation circuit when a coin moves relative to a coil were extracted as a first parameter and a second parameter respectively; however, other elements may be used as parameters instead. For example, the diameters of coins can be measured and used as one parameter.  
     INDUSTRIAL APPLICABILITY  
     [0069] In this invention, by storing correlations of parameters for a plurality of metal objects measured in advance, and judging whether at least two parameters output from an oscillation circuit are applicable to any of the stored correlations, type of the metal object can be identified from the correlation of a plurality of parameters, even if where identifying the metal object using a single parameter is difficult for the reason that the overlap occurs in the ranges of existence of the parameter for different metal objects, and the metal object cannot be accurately identified.