Patent Publication Number: US-6340082-B1

Title: Coin discriminating apparatus

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a coin discriminating apparatus wherein eddy current is induced in a coin with the use of a plurality of eddy current induction coils and based on the impedances of the individual coils that vary depending on the induced eddy current, a stamp (strike) pattern on the coin surface, the material (composition) of the coin, etc. are checked to determine the denomination and authenticity of the coin. More particularly, the invention relates to a coin discriminating apparatus wherein information on surface displacement irregularities of the stamp pattern on a surface of a coin is acquired based on the impedances of the individual eddy current induction coils excited at high frequency, and information on the material of the coin is acquired based on the impedances of specified ones of the eddy current induction coils excited at low frequency. 
     2. Description of the Related Art 
     In vending machines, automatic teller machines (ATMs), etc., a coin discriminating apparatus is incorporated to determine the denomination and authenticity of coins, as a process preliminary to the calculation of the sum of inserted coins. This type of coin discriminating apparatus is generally designed to measure the outer diameter, thickness and weight of a coin and compare the measured values with previously obtained outer diameter, thickness and weight of a true coin (one of coins of different denominations to be handled) to determine the denomination of the coin, and nonauthentic coins are rejected. Among a diversity of coins, however, there are coins, for example, coins used in foreign countries, that should not be accepted but yet have features (outer diameter, thickness, weight) resembling those of true coins to be accepted, and such coins can be erroneously judged to be authentic. 
     Attempts have therefore been made to acquire, in the form of an image, information on surface displacement irregularities of the stamp pattern on a coin surface and recognize features of the image, thereby to determine the denomination of the coin. Because of dust or smudge adhering to a coin surface, however, it is sometimes difficult to detect with accuracy the features of the stamp pattern on the coin surface. Further, in cases where the features of an image showing the stamp pattern on a surface of a coin to be discriminated are compared with those of an image showing the stamp pattern of a true coin, it is necessary that a matching process be performed following the rotation of the image for comparison, or that an extra process such as Fourier transform be performed as needed. Thus, a complicated process is required for the discrimination of coins and the process consumes much time. 
     SUMMARY OF THE INVENTION 
     The present invention has been created to solve the above problem and is based on the knowledge that, when a electromagnetic field is applied to a coin by using an eddy current induction coil, eddy current induced in the coin by the electromagnetic field varies depending on the material, thickness, etc. of the coin and that the impedance of the eddy current induction coil varies under the influence of the induced eddy current. 
     An object of the present invention is to provide a coin discriminating apparatus capable of easily determining the denomination and authenticity of a coin with high accuracy. Eddy current induction coils are successively excited to apply high-frequency electromagnetic field to the coin over an entire surface thereof, and impedances of the eddy current induction coils, which vary under the influence of eddy current induced in the coin by the high-frequency electromagnetic field, are measured to obtain information on surface displacement irregularities of a stamp pattern on the coin surface. Also, low-frequency electromagnetic field is applied to the coin by using the eddy current induction coils, and impedances of the individual coils, which vary under the influence of the eddy current induced in the coin by the low-frequency electromagnetic field, are measured to obtain information on the material of the coin. Discrimination of the coin is made based on the information thus obtained. 
     In the coin discriminating apparatus according to the present invention, a plurality of eddy current induction coils arranged two-dimensionally so as to face a surface of a coin are excited at high frequency and at low frequency, and in synchronism with the individual excitations of the coils, the impedances of the coils, which vary under the influence of the eddy current induced in the coin, are detected. The impedances of the eddy current induction coils detected when the coils are excited at low frequency are compared with an impedance obtained beforehand using a true coin, to determine the material of the coin. Also, the impedances of the eddy current induction coils detected when the coils are excited at high frequency are acquired as feature information indicative of information on surface displacement irregularities of the stamp pattern on the coin surface, and the feature information is compared with true coin feature information obtained beforehand, to determine the denomination of the coin. Especially, a histogram showing impedance distribution is created as feature information indicative of the information on surface displacement irregularities of the stamp pattern on the coin surface, and is compared with a histogram of a true coin obtained beforehand, to determine the denomination of the coin. 
     In a preferred embodiment of the present invention, the eddy current induction coils are arranged on a plane in the form rectangular grid or other suitable geometry to constitute a coil array, high-frequency exciting means successively excites all the eddy current induction coils constituting the coil array at high frequency to scan the entire surface of the coin, and low-frequency exciting means excites only specified ones of the eddy current induction coils in the coil array at low frequency. The specified eddy current induction coils in particular, which are excited at low frequency, comprise a predetermined number of eddy current induction coils located approximately in a central portion of the array constituted by the eddy current induction coils. 
     Alternatively, the specified eddy current induction coils excited at low frequency may be constituted by a special eddy current induction coil which is separate from the aforementioned coil array and is arranged side by side or face to face with respect to the coil array. 
     The coin discriminating apparatus of the present invention may further comprise coin diameter measuring means for measuring the diameter of the coin based on the impedances of the eddy current induction coils detected when the coils are excited at high frequency. The apparatus may also comprise coin thickness measuring means for measuring the thickness of the coin based on the impedances of the eddy current induction coils detected when the coils are excited at high frequency. 
     Further, the coin discriminating apparatus of the present invention may additionally comprise image processing means for acquiring the information on surface displacement irregularities of the stamp pattern on the surface of the coin as a two- or three-dimensional image, based on the impedances of the eddy current induction coils detected when the coils are excited at high frequency. 
     Also, in order to induce eddy current in the surface region of the coin, the high-frequency exciting means excites the eddy current induction coils at a frequency of about 1 MHz, for example, to produce a high-frequency electromagnetic field, and in order to induce eddy current in the inner part of the coin, the low-frequency exciting means excites the eddy current induction coils at a frequency of about 100 kHz, for example, to produce a low-frequency electromagnetic field. Specifically, the high-frequency exciting means and the low-frequency exciting means are constituted by a voltage-controlled oscillator whose oscillation frequency is variably controlled by a control voltage externally applied thereto. The control voltage is varied to switch the frequency for exciting the eddy current induction coils, thereby making the voltage-controlled oscillator function as the high-frequency exciting means or the low-frequency exciting means. 
     Further, according to the present invention, the eddy current induction coils are selectively oscillated by being applied with the output from the voltage-controlled oscillator via a multiplexer, and the multiplexer scans the eddy current induction coils for oscillation at high speed. 
     The eddy current induction coils excited at low frequency may be separate from those excited at high frequency. Also, to determine the material of the coin by exciting the eddy current induction coils at low frequency, preferably, the low-frequency exciting means selectively excites the eddy current induction coils at a plurality of different frequencies over a possible range of 100 kHz to 700 kHz or continuously varies the excitation frequency for the coils within a frequency range over a possible range of 100 kHz to 700 kHz, to produce a low-frequency electromagnetic field to be applied to a plurality of portions of the coin. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1A is a schematic diagram showing the arrangement of a coil array incorporated in a coin discriminating apparatus according to one embodiment of the present invention; 
     FIG. 1B is a schematic diagram showing the arrangement of a coil array and a low frequency-excited eddy current induction coil incorporated in a coin discriminating apparatus according to another embodiment of the present invention; 
     FIG. 2 is a diagram showing the construction of a flat coil (eddy current induction coil) constituting the coil array shown in FIG. 1; 
     FIG. 3 is a partly broken front view showing the internal arrangement of a sensing section of the coin discriminating apparatus according to the embodiment; 
     FIG. 4 is a plan view showing the sensing section as viewed from above; 
     FIG. 5 is a side view showing the sensing section as viewed from a moving direction of a coin; 
     FIG. 6A is a schematic diagram showing another example of how eddy current induction coils are arranged with respect to a coin; 
     FIG. 6B is a schematic diagram showing still another example of how eddy current induction coils are arranged with respect to a coin; 
     FIG. 7 is a schematic diagram showing the entire configuration of the coin discriminating apparatus according to the embodiment; 
     FIG. 8 is a diagram schematically showing the relationship between an eddy current induction coil of the coin discriminating apparatus and a coin to which an alternating electromagnetic field is locally applied from the eddy current induction coil; 
     FIG. 9 is a chart schematically showing, by way of example, a procedure of a coin discrimination process executed by a microprocessor; 
     FIG. 10 is a diagram showing an example of a table storing coin information and used in the coin discrimination process; and 
     FIG. 11 is a diagram showing examples of impedance histograms each indicating distribution of surface displacement surface displacement irregularities of a stamp pattern on a coin. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A coin discriminating apparatus according to one embodiment of the present invention will be hereinafter described with reference to the drawings. 
     FIG. 1A schematically illustrates the arrangement of a coil array  1  incorporated in the coin discriminating apparatus according to the embodiment. The coil array  1  comprises a plurality of (m×n) eddy current induction coils  2  arranged on a plane in the form of a rectangular grid of m rows by n columns. Specifically, the coil array  1  is formed as a printed circuit board which includes a predetermined insulating substrate  3  having a size greater than the outer diameters of coins to be handled, for example, a size of approximately 30 mm×50 mm, and a plurality of flat coils (eddy current induction coils)  2  of spiral form with an outer diameter of about 2 mm to approximately 5 mm, as shown in FIG. 2, formed on the insulating substrate  3  at a predetermined pitch P x , P y  (e.g., about 6 mm). Each eddy current induction coil  2  has a pair of lead terminals  2   a  and  2   b , and the corresponding lead terminals of the coils  2  of the individual rows and columns are connected together and are extended as row select lead terminals  4   a  and column select lead terminals  4   b  of the coil array  1 . Thus, by selecting one of the row select lead terminals  4   a  and at the same time one of the column select lead terminals  4   b  and passing electric current between the selected lead terminals  4   a  and  4   b , it is possible to selectively specify and excite one eddy current induction coil  2  in the coil array  1 . 
     The eddy current induction coils  2  constituting the coil array  1  are used to locally apply a high-frequency electromagnetic field to a coin, as described later. Specified ones of the eddy current induction coils  2  arranged in matrix form, for example, four eddy current induction coils  2   x  located approximately in the center of the matrix, are also used to apply a low-frequency electromagnetic field to a coin. 
     Each of the eddy current induction coils  2  ( 2   x ) serves to produce a electromagnetic field (high- or low-frequency electromagnetic field) when supplied with and excited by an alternating current of predetermined frequency, and to locally apply the electromagnetic field (alternating electromagnetic field) to a coin to thereby induce an eddy current in the coin corresponding to the material, thickness, etc. of the coin. The eddy current induced in the coin acts upon (exerts an influence upon) the eddy current induction coil  2  ( 2   x ), as described later, to change the impedance thereof. The eddy current induction coil  2  ( 2   x ) detects such a change in the impedance as a feature of the coin and thus functions as a sensor section. 
     As seen from the schematic arrangement of the sensing section of the coin discriminating apparatus shown in FIGS. 3 to  5 , two coil arrays  1 , each constituted by the eddy current induction coils  2  as described above, are arranged along respective guides  11  defining a passage for a coin  10 . FIG. 3 is a partly broken front view showing the internal arrangement of the sensing section, FIG. 4 is a plan view showing the sensing section as viewed from above, and FIG. 5 is a side view showing the sensing section as viewed from a moving direction of the coin  10 . 
     As illustrated, the sensing section is constituted by the two parallel coil arrays  1  arranged on opposite sides of the guides  11  forming the passage for a coin  10 . The coil arrays  1  are positioned such that their surfaces on which the eddy current induction coils  2  are arrayed respectively face the obverse and reverse sides of the coin  10  sliding along the guides  11 , in a manner substantially parallel thereto. The distance, in particular, between each of the coil arrays  1  and a corresponding one of the obverse and reverse sides of the coin  10  on which irregular stamp patterns are formed is set to a very small value such that the electromagnetic field produced by the eddy current induction coil  2  acts satisfactorily intensely upon the coin  10  and also that the eddy current induced in the coin  10  exerts a sufficiently great influence upon the eddy current induction coil  2 . 
     In the illustrated example, the sensing section is arranged at a passage along which a coin  10  slides, but it may be arranged at a passage along which a coin  10  is slid or at a passage through which a coin  10  falls. The surface of each coil array  1  on which the eddy current induction coils  2  are formed may of course be covered with a protective film such that the coil arrays  1  themselves constitute part of the guides  11  defining the coin passage. 
     The eddy current induction coils for applying low-frequency electromagnetic field to the coin  10  may be constituted by a special eddy current induction coil  2   y  as shown in FIG. 1B, for example, which is separate from the coil array  1  of high frequency-excited eddy current induction coils  2  and is arranged side by side with respect thereto. The special eddy current induction coil  2   y  may alternatively be arranged face to face with respect to the coil array  1 . In this case, the low frequency-excited eddy current induction coil  2   y  should preferably has a large diameter corresponding to the diameter of the coin  10 . These eddy current induction coils  2 ,  2   x ,  2   y  may be arranged along the coin passage so as to face the coin  10 , as shown in FIGS. 6A and 6B. 
     The coin discriminating apparatus, in which the individual eddy current induction coils  2  of the coil arrays  1  are excited to detect features of a coin  10  and thereby identify the denomination thereof, is generally configured as shown in FIG.  7 . In the coin discriminating apparatus, a microprocessor  21  controls the operation of a controller  22 , which in turn excites the individual eddy current induction coils  2  of the coil array  1  and detects features of a coin  10  as variations in impedance of the eddy current induction coils  2 , in the manner described below. In accordance with the detected impedances of the eddy current induction coils  2 , the denomination and authenticity of the coin  10  are determined. 
     Specifically, the controller  22  drives a multiplexer  23  to successively select the eddy current induction coils  2  of the coil array  1  and at the same time causes an alternating current of predetermined frequency, output from a voltage-controlled oscillator (VCO)  24 , to be applied to the then selected eddy current induction coil  2 , thereby exciting the selected eddy current induction coil  2 . In accordance with a clock signal CLK of predetermined frequency generated from the controller  22 , for example, the multiplexer  23  cyclically and successively selects the column select lead terminals  4   b  of the coil array  1 , one at a time, and allows the output (alternating current) of the voltage-controlled oscillator  24  to be applied to the then selected column of eddy current induction coils  2 . Simultaneously, the multiplexer  23  selectively connects one of the row select lead terminals  4   a  of the coil array  1  to the ground in a manner such that each time one cyclic selection of all the column select lead terminals  4   b  is completed, the row select lead terminal  4   a  to be grounded is switched from one to another. Owing to such selection of the rows and columns of the coil array  1  by the multiplexer  23 , the eddy current induction coils  2  arranged in a rectangular grid or other suitable array geometry are successively selected, one at a time, to be excited with current supplied thereto from the voltage-controlled oscillator  24 . Namely, the array of the eddy current induction coils  2  is scanned two-dimensionally for excitation. 
     The terminal voltage (amplitude and/or phase) of the eddy current induction coil  2  selected and excited by the multiplexer  23  is detected through an amplifier  25  as the output (alternating current) of the voltage-controlled oscillator  24  selectively applied to the column select lead terminals  4   b  of the coil array  1 , for example. The amplifier  25  serves to detect a change in impedance of the eddy current induction coil  2  as a change in amplitude and/or phase of the signal (output of the voltage-controlled oscillator  24 ) for exciting the eddy current induction coil  2 . A induction coil impedance amplitude and/or phase detector circuit  26  samples the output of the amplifier  25  in synchronism with the operation timing of the multiplexer  23  operated by the controller  22 , that is, in synchronism with the selection of the eddy current induction coils  2 , and holds the sampled value, which is then collected and stored by the microprocessor  21 . 
     When a coin  10  is introduced to the sensing section, the controller  22  controls the operation of the multiplexer  23  in accordance with a command supplied thereto from the microprocessor  21  so that all of the eddy current induction coils  2  of the coil array  1 , for example, may first be successively excited. In this case, the controller  22  applies a first control voltage to the voltage-controlled oscillator  24  and drives the oscillator  24  to oscillate at a frequency of about 1 MHz. Consequently, all of the eddy current induction coils  2  are successively excited at a high frequency of about 700 kHz or higher. When the high-frequency excitation of all the eddy current induction coils  2  is completed, the controller  22  then controls the operation of the multiplexer  23  so that only the aforementioned specified eddy current induction coils  2   x  may be successively excited. In this case, the controller  22  applies a second control voltage to the voltage-controlled oscillator  24  and drives the same to oscillate at a frequency of about 100 kHz. Consequently, only the specified eddy current induction coils  2   x  are simultaneously excited at a low frequency of about 100 kHz to 700 kHz. Thus, the voltage-controlled oscillator  24  selectively functions as high-frequency exciting means for exciting the eddy current induction coils  2  at high frequency and low-frequency exciting means for exciting the eddy current induction coils  2   x  at low frequency, in cooperation with the controller  22 . 
     Alternatively, when any one of the specified eddy current induction coils  2   x  is selected in the process of successive selection of the eddy current induction coils  2  for high-frequency excitation, the operation of the voltage-controlled oscillator  24  may be controlled synchronously such that the selected eddy current induction coil  2   x  is excited at low frequency. Namely, settings may be made beforehand such that the specified eddy current induction coils  2   x  are excited at low frequency while the other eddy current induction coils  2  are excited at high frequency, in which case scanning of the entire coil array  1  is completed as soon as the eddy current induction coils  2  ( 2   x ) of the coil array  1  are successively excited only once. 
     The oscillation amplitudes of the individual eddy current induction coils  2  ( 2   x ) excited with the excitation condition varied in this manner are successively detected via the amplifier  25  and the amplitude and/or phase detector circuit  26  as information indicative of the impedances of the eddy current induction coils  2  ( 2   x ) that vary according to the coin  10 . Namely, the amplifier  25  is used as impedance measuring means for the eddy current induction coils  2  ( 2   x ). 
     The following explains how the impedances of the eddy current induction coils  2  ( 2   x ) vary according to a coin  10 . 
     FIG. 8 schematically shows the relationship between one eddy current induction coil  2  which is selectively supplied with and excited by the output from the voltage-controlled oscillator  24  under the control of the multiplexer  23 , and a coin  10  to which an alternating electromagnetic field is locally applied from the eddy current induction coil  2 . When the coin  10  is applied with an alternating electromagnetic field φ produced by the eddy current induction coil  2 , an eddy current I c  is induced in a portion of the coin  10  where the alternating electromagnetic field intersects. The magnitude of the induced eddy current I c  varies depending on the material composition and thickness of the coin  10  and the distance of the induction coil from of the coin  10 . Also, the electromagnetic flux produced by the eddy current I c  acts so as to cancel out the alternating electromagnetic flux produced by the eddy current induction coil  2 . Accordingly, the electromagnetic flux produced by the eddy current induction coil  2  is virtually reduced even if the electric current for exciting the eddy current induction coil  2  is constant, so that the inductance, that is, the impedance Z of the eddy current induction coil  2  decreases. In other words, when an alternating electromagnetic field is applied to the coin  10  from the eddy current induction coil  2  to induce eddy current in the coin  10 , the impedance of the eddy current induction coil  2  lowers under the influence of the induced eddy current. Further, the smaller the distance d between the eddy current induction coil  2  and the surface of the coin  10 , the greater influence the electromagnetic flux produced by the eddy current I c  exerts upon the eddy current induction coil  2 , causing a greater drop in the impedance of the coil  2 . 
     By acquiring such a change in the impedance of the eddy current induction coil  2  as a change in the amplitude and/or phase of the signal for exciting the coil  2 , the amplifier  25  detects the impedance of the eddy current induction coil  2 . The impedance of the eddy current induction coil  2  that varies under the influence of the eddy current induced in the coin  10 , in particular, is dependent not only on the material of the coin  10  but also on surface displacement irregularities of the stamp pattern on the coin surface, and consequently on variations in the distance d of the coin  10  from the coil  2 . It is therefore possible to extract features of the coin  10  by detecting the impedances of the eddy current induction coils  2 . 
     As the frequency of the alternating electromagnetic field produced by the eddy current induction coil  2  increases, the eddy current is induced in a region of the coin  10  closer to its surface. Conversely, as the frequency of the alternating electromagnetic field decreases, the electromagnetic field is able to penetrate deeper in the interior of the coin  10  and the eddy current is induced in the inner part. Accordingly, to obtain information on surface displacement irregularities of the stamp pattern on the coin surface, the eddy current induction coil  2  may be excited at a high frequency of, for example, about 700 kHz to 1 MHz so that the eddy current may be induced in the surface of the coin  10  having surface displacement irregularities forming the stamp pattern. By thus making the eddy current I c  induced in the surface of the coin  10 , the eddy current induction coil  2  is greatly influenced by the eddy current I c  in accordance with the distance d of the coil  2  to the coin  10  that varies depending upon the surface displacement irregularities on the coin surface, with the result that the impedance of the coil  2  greatly varies. Consequently, based on variations in the impedance of the eddy current induction coils  2 , it is possible to effectively detect the surface displacement irregularities of the stamp pattern on the surface of the coin  10 . 
     On the other hand, to obtain information on the material (composition) of the coin  10 , the excitation frequency for the eddy current induction coil  2  may be set to a low frequency of, for example, 10 kHz to 100 kHz so that the eddy current I c  may be induced in the inner part of the coin  10  and may greatly vary depending on the material of the coin  10 . By thus making the eddy current I c  induced in the inner part of the coin  10 , the eddy current induction coil  2  is scarcely affected by variations in the distance d to the coin  10  due to the surface displacement irregularities, and is influenced only by the magnitude of the eddy current I c  induced in the inner part of the coin  10 . The magnitude of the eddy current I c  induced in the inner part of the coin  10  greatly varies depending on the material of the coin  10 . Consequently, based on variation in the impedance of the eddy current induction coil  2 , it is possible to obtain information on the material of the coin  10 . 
     The excitation condition (excitation frequency) for the eddy current induction coils  2 , which is set by controlling the operation of the voltage-controlled oscillator  24  as mentioned above, is determined based on the knowledge described above. 
     Referring now to FIG. 9, a coin discrimination process executed by the microprocessor  21  will be described. FIG. 9 schematically illustrates an example of a procedure for the microprocessor  21 . This process is started upon detection of an input (insertion) of a coin  10  by means of various coin sensors (not shown) arranged in the coin passage [Step S 1 ]. On detecting the input of a coin  10  which is an object of discrimination, the microprocessor  21  starts the controller  22 , which then operates the voltage-controlled oscillator (VCO)  24  first at high frequency [Step S 2 ] and controls the operation of the multiplexer  23  to successively excite all eddy current induction coils  2  of the coil array  1  at high frequency [Step S 3 ]. Also, the controller  26  operates the amplitude and/or phase detector circuit  26  in synchronism with the high-frequency excitation of the eddy current induction coils  2 , to successively sample and hold the impedances of the coils  2  measured via the amplifier  25  [Step S 4 ]. The impedances of the individual eddy current induction coils  2  thus measured are successively stored in an internal memory (not shown) of the microprocessor  21  [Step S 5 ], and this completes the process of obtaining information on the surface displacement irregularities of the coin  10  by means of high-frequency excitation of the eddy current induction coils  2 . 
     Subsequently, the microprocessor  21  operates the voltage-controlled oscillator  24  at low frequency [Step S 6 ], and controls the operation of the multiplexer  23  to successively excite only the specified eddy current induction coils  2   x  of the coil array  1  at low frequency [Step S 7 ]. The amplitude and/or phase detector sample-and-hold circuit  26  is operated in synchronism with the low-frequency excitation of these eddy current induction coils  2   x , to successively sample and hold the impedances of the coils  2   x  measured via the amplifier  25  [Step S 8 ]. The thus-measured impedances of the eddy current induction coils  2   x  are also successively stored in the internal memory (not shown) of the microprocessor  21  [Step S 9 ]. This completes the process of obtaining information on the material of the coin  10  by means of low-frequency excitation of the eddy current induction coils  2   x.    
     The microprocessor  21  then starts, as its internal process, a discrimination process for the coin  10  in accordance with the impedances of the eddy current induction coils  2  ( 2   x ) stored in its memory. In this discrimination process, first, the impedances of the eddy current induction coils  2  detected during the high-frequency excitation, for example, are compared with a predetermined threshold level, to sort out those coils  2  which showed no change in impedance and to detect their positions on the coil array  1  [Step S 10 ]. Based on the information on the positions of the eddy current induction coils  2  which showed no change in impedance, those coils  2  which were facing the coin  10  at the time of the impedance measurement are identified to detect the profile (overall size) of the coin  10 , and a maximum diameter is determined as the outer diameter of the coin  10  [Step S 11 ]. Then, a table shown in FIG. 10, for example, which is prepared in the microprocessor  21 , is looked up to select a denomination corresponding to the outer diameter of the coin  10  [Step S 12 ]. 
     Specifically, in the table are described, as reference data, information on the outer diameters and thicknesses of coins (true coins) of different denominations to be handled (to be discriminated), information on the materials of the coins (different impedances of the eddy current induction coil according to the materials), information on surface displacement irregularities of the stamp patterns of the coins (different impedances according to surface displacement irregularities), etc. By looking up the table, a denomination corresponding to the measured profile (outer diameter) of the coin  10  is selected. If no corresponding denomination is found [Step S 13 ], the coin  10  is judged to be a coin which should not be accepted (nonauthentic coin) and is rejected [Step S 14 ]. 
     After a corresponding denomination for the coin  10  is selected, the impedances of the specified eddy current induction coils  2   x  detected during the aforementioned low-frequency excitation are read out from the memory and matched against information (material-dependent impedance of the eddy current induction coil) on the material of the corresponding denomination described in the table [Step S 15 ]. In this case, the impedances of the four specified eddy current induction coils  2   x  are added together or averaged to obtain a measured impedance, in accordance with the method of obtaining the information (impedances of the eddy current induction coil) on the materials of the coins described in the table, and the measured impedance is compared with a corresponding impedance stored in the table. By performing the impedance matching process in this manner, a determination is made as to whether or not the coin  10  to be discriminated, of which the denomination is selected based on the outer diameter as described above, has conformity also in respect of the material [Step S 16 ]. If, as a result of the impedance matching process, no conformity is found, that is, the material of the coin  10  is found to be different from that of the coin to be accepted, the coin  10  is judged a nonauthentic coin and rejected [Step S 14 ]. 
     If conformity of material is found in the matching process, a discrimination process is then performed based on the information on the surface displacement irregularities of the stamp pattern on the surface of the coin  10 . This process is started by first reading out the impedances of the individual eddy current induction coils  2  detected during the high-frequency excitation and creating a histogram of the thus-read impedances [Step S 17 ]. To create such a histogram, the impedances of the eddy current induction coils  2  are classified into a plurality of preset impedance ranges according to their magnitude, and the number of eddy current induction coils  2  which impedance falls within a specified an identical impedance range is counted. In the histogram, the impedance ranges are indicated along the abscissa and the respective numbers of eddy current induction coils  2  are indicated along the ordinate, so that the histogram shows a distribution of the impedances. 
     As mentioned above, the impedances of the individual eddy current induction coils  2  obtained during the high-frequency excitation show variations according to the distance d to the irregular surface of the coin  10 , and the surface displacement irregularities on the surface of the coin  10  represent the stamp pattern. Accordingly, the impedances classified into the respective impedance ranges s described above indicate variations in the distance d, and consequently the degree of surface displacement irregularities on the surface of the coin  10 . The histogram therefore represents a distribution of the surface displacement irregularities formed on the surface of the coin  10  as the stamp pattern. 
     The histogram created in this manner is matched against information on surface displacement irregularities of the stamp pattern of an acceptable coin stored in advance in the table (histogram of impedances dependent on surface displacement irregularities), more particularly, against a histogram for the denomination determined as described above [Step S 19 ], to determine conformity in respect of the stamp pattern of the coin  10 . 
     Even if coins of different denominations have similar stamp patterns, generally there is a great difference in the surface displacement irregularities created by the stamp patterns as well as in the distribution of the surface displacement irregularities over the entire surfaces of the coins. Especially, where a coin  10  is falsified by making a hole to adjust its weight, the stamp pattern itself greatly changes and the distribution of the surface displacement irregularities also greatly varies. Namely, even if two coins, one acceptable and the other unacceptable, have similar outer diameters and similar stamp patterns, they exhibit noticeable differences as shown in FIG. 11; for example, the distribution (histogram B) of surface displacement irregularities of the unacceptable coin clearly differs from that (histogram A) of the acceptable coin in the position of the peak, spread width, deviation, etc. By comparing the histograms showing distributions of surface displacement irregularities with each other, therefore, it is possible to effectively determine the surface displacement irregularities of the stamp pattern on the surface of the coin  10 , that is, the features of the stamp pattern. 
     If, as a result of the above histogram matching process, conformity is found as to the information on the surface displacement irregularities of the stamp pattern, a decision is made that the coin  10  is an authentic coin of the denomination determined as above [Step S 20 ]. If conformity fails to be found in the histogram matching, the stamp pattern is judged improper, that is, it is concluded that the coin  10  differs from the acceptable one, and thus the coin  10  is rejected [Step S 14 ]. 
     The above matching process for the stamp pattern on the surface of the coin  10  by means of impedance histograms is preferably carried out with respect to the stamp patterns of both the obverse and reverse sides of the coin  10  by using two sets of information (impedances) detected by the two coil arrays  1  facing the both sides (obverse and reverse sides) of the coin  10 , respectively. 
     With the above-described coin discriminating apparatus in which the material and outer diameter of a coin  10  as well as information on surface displacement irregularities of the stamp pattern on the coin surface are detected as variations in impedance of the eddy current induction coils  2  ( 2   x ), the denomination and authenticity of the coin  10  can be easily determined with accuracy based on these items of information, without being affected by dust or smudge adhering to the coin surface, unlike the apparatus in which information about the surface of the coin  10  is optically obtained. Further, since the impedances of the eddy current induction coils  2  ( 2   x ) themselves, which vary under the influence of the eddy current induced in the coin  10  by the alternating electromagnetic field applied thereto from the coils  2  ( 2   x ), are detected as information indicative of features of the coin  10 , it is unnecessary to separately provide alternating electromagnetic field production coils and sensing coils, thus simplifying the arrangement of the sensing section. Accordingly, where information on the surface displacement irregularities of the stamp patterns on both the obverse and reverse sides of a coin  10  is to be obtained, two coil arrays  1  have only to be arranged so as to face the opposite sides of the coin  10 , respectively, whereby the sensor arrangement is simplified. 
     Also, the eddy current induction coils  2  are excited at high frequency to induce eddy current in the surface region of a coin  10  and information on surface displacement irregularities of the coin surface is acquired based on variations in impedance of the coils  2 ; on the other hand, the eddy current induction coils  2   x  are excited at low frequency to induce eddy current in the inner part of the coin  10  and information on the material of the coin  10  is acquired based on change in impedance of the coils  2   x . Thus, by only changing the excitation condition for the eddy current induction coils  2  ( 2   x ), for example, it is possible to effectively detect different features of the coin  10 . 
     Further, the surface displacement irregularities of the stamp pattern on the surface of the coin  10  are detected as variations in impedance of the eddy current induction coils  2 , and a histogram showing the impedance distribution is created, wherein the impedance ranges are indicated along the abscissa and the numbers of eddy current induction coils  2  showing impedances falling within the respective impedance ranges are indicated along the ordinate, to thereby acquire features of the stamp pattern created by the surface displacement irregularities on the surface of the coin  10 . The histogram is subjected to matching process; therefore, the coin  10  can be easily discriminated (collated) based on the features of the stamp pattern on the surface of the coin and the discrimination accuracy can be significantly enhanced. Also, the use of the histogram eliminates the need for a complicated process such as rotation of the information indicative of the stamp pattern for alignment, whereby the discrimination process can be greatly simplified and the time required for the processing can be shortened. 
     The present invention is not limited to the embodiment described above. For example, the information on surface displacement irregularities on the surfaces of a coin  10  obtained by exciting the eddy current induction coils  2  at high frequency may be used to obtain average distances d ave1  and d ave2  of the obverse and reverse surfaces of the coin  10  from the respective two coil arrays  1  (eddy current induction coils  2 ) arranged on both sides of the coin  10 , as shown in FIG. 5, and using a distance D between the two coil arrays  1 , a thickness t (=D−d ave1 −d ave2 ) of the coin  10  may be obtained. The thickness t thus obtained may be compared with coin thickness information stored in the table, as an auxiliary step in the discrimination process. 
     In the foregoing embodiment, information on the stamp pattern of a coin  10  is acquired as an impedance histogram showing irregularities and is used in the discrimination process. The information (impedances) on irregularities of the stamp pattern at individual positions of a coin  10  may instead be used as intensity information to obtain a two-dimensional image with the detection positions laid out on a plane coordinate system, so that the two-dimensional image may be used in the discrimination process. Alternatively, the information (impedances) on irregularities of the stamp pattern at individual positions of a coin  10  may be treated as distances (heights) to the respective eddy current induction coils  2 , to obtain three-dimensional data with the detection positions laid out as plane coordinates, so that the obtained data may be used in the discrimination process. 
     Further, when acquiring information about the material of a coin  10  by exciting the eddy current induction coils  2   x  at low frequency, the excitation frequency may be varied in steps or continuously within a predetermined frequency range (e.g., 10 to 700 kHz) to measure impedances at different frequencies, and a pattern of frequency-dependent changes of impedance may be acquired to determine the material of the coin  10 . In this case, when the eddy current induction coils  2   x  or large embedded coil are excited at low frequency, the oscillation frequency of the voltage-controlled oscillator  24  may be varied under the control of the controller  22 . 
     Also, the number of eddy current induction coils  2  incorporated in each coil array  1 , the pitch and pattern of arrangement of the coils  2 , etc. may be determined as needed according to specifications of coins to be handled, and thus the present invention can be modified in various ways without departing from the scope and spirit thereof.