Patent Publication Number: US-10777031-B2

Title: Coin detection system

Description:
PRIORITY CLAIM TO RELATED APPLICATIONS 
     This application is a U.S. national stage application filed under 35 U.S.C. § 371 from International Application Serial No. PCT/CN2015/081290, which was filed 12 Jun. 2015, and published as WO2015/196932 on 30 Dec. 2015, and which claims priority to Chinese Application No. 201410284349.2, filed 23 Jun. 2014, which applications and publication are incorporated by reference as if reproduced herein and made a part hereof in their entirety, and the benefit of priority of each of which is claimed herein. 
     TECHNICAL FIELD 
     The present invention relates to a coin detection system, and in particular, to a coin detection system that uses magnetoresistive sensors to form a magnetic gradiometer. 
     BACKGROUND ART 
     Coins are an indispensable part of modern society, are a necessary tool for humans to exchange materials, and have a large circulation in our daily life. As the coins are increasingly widely used, traffic, financial, and other institutions increasingly rely on applications that judge denominations and authenticity of the coins and count the coins. At present, there are mainly the following several manners of counting the coins and identifying authenticity. (1) An alternating magnetic field is applied to a coin, then an induced eddy current field thereof is measured to judge the material of the coin, so as to identify the authenticity thereof; such a method measures an axial magnetic field of the coin mainly by using an induction coil or a combination of an induction coil and a Hall sensor, this can only measure one kind of signals that identify features, while for different coins having similar resonance frequencies, amplitudes or phases, such a method evidently cannot judge the authenticity accurately. (2) Multiple magnetoresistive sensors are used to form a sensor unit array to detect magnetic field distribution around the coin, so as to judge the denomination of the coin and the authenticity thereof, for example, the patent application CN103617669A discloses a coin detection device, such a device can also detect signals in only one direction, for coins that have similar diameters and have similar responses in the same direction, accuracy of the judgment result of such a method is not high enough, and the measurement result includes a new signal generated by an applied pulse field, subsequent processing is required to remove the signal, the operation process is relatively complicated, and the resolution may be reduced. (3) The authenticity of the coin is detected by performing variable-frequency input on a transmitting coil and measuring output of a receiver in different frequency points, for example, U.S. Pat. No. 4,086,527 discloses a testing method, although the method can obtain information such as amplitude, phase, and resonance frequency of the output signal, a single-axis sensor is still employed, and it is very difficult to identify some coins that have similar features. In addition, the authenticity may also be tested with methods such as using a pulse field for excitation and then removing the pulse field, and performing phase shifting, but all the methods can only provide one kind of signals that identify features, which cannot identify the coins that have the similar features accurately. As the coin forging technology is becoming increasingly excellent, the existing coin detection device cannot meet high precision requirements for coin detection in the modern institutions such as transportation and financial. 
     SUMMARY OF THE INVENTION 
     An objective of the present invention is to provide a coin detection system with a simple structure, high accuracy, high sensitivity and a wide dynamic linear range, so as to overcome the defects existing in the prior art. 
     In order to achieve the foregoing objective, the present invention adopts the following technical solution: a coin detection system, wherein the coin detection system includes an excitation coil, a radial magnetic gradiometer and an axial magnetic gradiometer; 
     the excitation coil is used for providing an axial excitation magnetic field for a to-be-detected coin, the excitation magnetic field induces eddy currents inside the to-be-detected coin, and the eddy currents generate an induced magnetic field; 
     the radial magnetic gradiometer includes at least two radial magnetoresistive sensors and the axial magnetic gradiometer includes at least two axial magnetoresistive sensors, the radial magnetoresistive sensors and the axial magnetoresistive sensors being symmetrically distributed relative to a central plane or a central point of the excitation coil respectively; the radial magnetic gradiometer is used for detecting a difference of magnetic field components of the induced magnetic field on two corresponding sides of the excitation coil and along a radial direction of the to-be-detected coin, and the axial magnetic gradiometer is used for detecting a difference of magnetic field components of the induced magnetic field on two corresponding sides of the excitation coil and along an axial direction of the to-be-detected coin, the two corresponding sides referring to two opposite sides along an axial direction of the excitation coil; and 
     the excitation coil is positioned such that a surface of the to-be-detected coin is parallel to the central plane of the excitation coil, and a distance between the surface of the to-be-detected coin and the central plane is at least half of the height of the excitation coil. 
     Preferably, the coin detection system further includes: a signal excitation source and a drive circuit that are used for exciting the excitation coil, an analog front-end circuit for amplifying signals generated by the radial magnetic gradiometer and the axial magnetic gradiometer, and a processor for calculating a real component and an imaginary component of an amplified signal output by the analog front-end circuit. 
     Preferably, a signal generated by the signal excitation source includes an AC signal, the AC signal including at least one frequency component; the processor calculates the real component and the imaginary component of the amplified signal corresponding to each frequency component. 
     Preferably, the signal excitation source is further used for applying a DC signal in the duration of the AC signal, and the excitation magnetic field generated by the excitation coil is a superposed field of a DC magnetic field and an AC magnetic field. 
     Preferably, when the to-be-detected coin is made of a ferromagnetic material or the surface of the to-be-detected coin is coated with a ferromagnetic material, an amplitude value of the output signal is reduced after the DC magnetic field is applied; and when the to-be-detected coin is made of a conductor, the DC magnetic field does not affect the amplitude value of the output signal. 
     Preferably, the coin detection system is capable of detecting amplitude values of a real component and an imaginary component corresponding to each type of coins. 
     Preferably, the excitation coil is a single coil or an array formed by superposing multiple coils, and a diameter of a circumference encircled by the excitation coil is greater than or equal to that of the to-be-detected coin. 
     Preferably, the radial magnetic gradiometer is located at an inner edge of the excitation coil and located below an edge of the to-be-detected coin, and the radial magnetoresistive sensors are symmetrical relative to the center of the excitation coil; the axial magnetic gradiometer is located inside the excitation coil and located at or close to a lower side of the center of the to-be-detected coin, and the axial magnetoresistive sensors are symmetrically distributed relative to the center of the excitation coil along the axial direction of the excitation coil. 
     Preferably, the coin detection system further includes a first PCB and a second PCB, the radial magnetoresistive sensors are located on the first PCB and the second PCB respectively, the axial magnetoresistive sensors are located on the first PCB and the second PCB respectively, and the excitation coil is fixed between the first PCB and the second PCB; and the to-be-detected coin is located above the first PCB and the second PCB. 
     Preferably, the radial magnetoresistive sensors are X-axis linear sensors, the axial magnetoresistive sensors are Z-axis linear sensors, sensing directions of the X-axis linear sensors are parallel to the radial direction of the to-be-detected coin, and sensing directions of the Z-axis linear sensors are parallel to the axial direction of the to-be-detected coin. 
     Preferably, the X-axis linear sensors and the Z-axis linear sensors are of a structure of a single resistor, half bridge or full bridge, and the single resistor, bridge arms of the half bridge or bridge arms of the full bridge consist of one or more magnetoresistive elements electrically connected with each other. 
     Preferably, the magnetoresistive elements are Hall or SMRE (semiconductor magnetoresistive element), anisotropic magnetoresistance (AMR), giant magnetoresistance (GMR) or tunnel magnetoresistance (TMR) elements. 
     Preferably, the coin detection system further includes a positioning device for positioning a position where the to-be-detected coin is placed, such that the to-be-detected coin is close to one side of the radial magnetic gradiometer and the axial magnetic gradiometer. 
     Compared with the prior art, the prevent invention has the following technical effects: 
     (1) Radial and axial magnetic gradiometers are used to detect radial and axial magnetic field components of an eddy current magnetic field induced by a to-be-detected coin, which achieves dual-axis measurement and is not affected by an excitation magnetic field, and this can improve accuracy of the measurement greatly. 
     (2) When the to-be-detected coin is not placed, the two magnetic gradiometers may not display any excitation signal, such that the excitation signal will not generate a saturation effect, and the gain can be improved as much as possible, thereby improving the resolution. 
     (3) The radial and axial magnetic gradiometers consist of linear magnetoresistive sensors, for example, TMR sensors, and this can improve sensitivity of the coin detection system and increase the dynamic linear range; in addition, relative to the coil, the magnetoresistive sensor is smaller in size and lower in cost, such that the coin detection system has a more compact structure and can also save the cost. 
     (4) The two magnetic gradiometers in the present invention can implement temperature compensation for system responses and eliminate thermal drift errors. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In order to describe the technical solutions in technologies of embodiments of the present invention more clearly, the accompanying drawings to be used in the description about the technologies of the embodiments are briefly introduced in the following. It is apparent that the accompanying drawings in the following description are only some embodiments of the present invention. Persons of ordinary skill in the art can also obtain other accompanying drawings according to the accompanying drawings without making creative efforts. 
         FIG. 1  is a schematic structural diagram of a coin detection system in the present invention; 
         FIG. 2  is a sectional view of some details of the coin detection system in the present invention; 
         FIG. 3  is a top view of some details of the coin detection system in the present invention; 
         FIGS. 4A-4B  are relational curves of real and imaginary components of a magnetic field around the coil vs. measurement positions when a measurement frequency is 1 KHz; 
         FIGS. 5A-5B  are relational curves of real and imaginary components of a magnetic field around the coil vs. measurement positions when a measurement frequency is 10 KHz; 
         FIGS. 6A-6D  are calculation results of relationships between a real component and an imaginary component of an eddy current field induced by a coin made of a different material and frequencies; 
         FIGS. 7A-7B  are curves of testing results of coins of 1 Yuan and 0.1 Yuan; 
         FIG. 8  is a measurement result of ten types of coins at frequencies of 160 Hz and 9800 Hz; 
         FIGS. 9A-9B  are output curves obtained when an axial magnetic gradiometer and a radial magnetic gradiometer measure two types of coins respectively; and 
         FIG. 10  is a diagram of measurement results of radial and axial magnetic field components of different types of coins at different frequencies. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention is described in detail below with reference to the accompanying drawings and in combination with embodiments. 
     Embodiments 
       FIG. 1  is a schematic structural diagram of a coin detection system in the present invention. The coin detection system includes a signal excitation source  1 , a drive circuit  2 , an excitation coil  3 , a to-be-detected coin  4 , a radial magnetic gradiometer  5 , an axial magnetic gradiometer  6 , an analog front-end circuit  7 , and a processor  8 . During operation, after the excitation coil  3  is excited by the signal excitation source  1  and the drive circuit  2 , the excitation coil  3  generates an excitation magnetic field  10  parallel to the axial direction of the to-be-detected coin  4 , and under the influence of the excitation magnetic field  10 , the to-be-detected coin  4  generates eddy currents in the coin and then induces a magnetic field  11 ; the radial magnetic gradiometer  5  and the axial magnetic gradiometer  6  detect a difference of magnetic field components of the magnetic field  11  on two corresponding sides of the excitation coil  3  in the radial and axial directions of the to-be-detected coin  4  respectively; the corresponding two sides here refer to two opposite sides along an axial direction (as shown by the vertical dotted line in  FIG. 2 ) of the excitation coil, which refer to upper and lower sides in this embodiment; then, the detected signal is transmitted to the analog front-end circuit  7  for amplification; the processor  8  processes the amplified signal transmitted by the analog front-end circuit  7  and then outputs through an output end  9 ; the processor  8  may include an MCU or a DSP, the output signal is a voltage signal which may be converted to a magnetic field signal, and the magnetic field signal includes a real portion and an imaginary portion; the output signal is relevant to the material, size, and design of the coin and the position of the coin relative to the radial magnetic gradiometer  5  and the axial magnetic gradiometer  6 ; in order to avoid influences caused by different positions, a positioning column is used to position the to-be-detected coin. Different coins have standard values, and by comparing and analyzing detection results and the standard values, denominations and authenticity thereof can be judged. In this embodiment, the signal excitation source  1  is a sinusoidal signal, but it may also be an AC signal that includes one or more frequency components. After the AC signal is successfully excited, detection is carried out, and the measurement results are compared and analyzed with the standard values. Also, after the AC signal is successfully excited and an output signal is detected, a DC magnetic field may be applied to the to-be-detected coin  4 , the DC magnetic field may be generated by an external permanent magnet and may also be generated by applying a DC signal to the excitation coil  3  through the signal excitation source  1 , which is the latter in this embodiment, and then the output signal is detected once again. In this case, for coins made of a conductor, the measurement results are not affected, but for coins made of a ferromagnetic material or surface-coated with a ferromagnetic layer (e.g., nickel), the measurement results will change, the amplitude value of the output signal may tend to decrease, and this can further improve the accuracy of identification of authenticity of the coin. 
       FIG. 2  and  FIG. 3  are respectively a sectional view and a top view of details such as the excitation coil, the to-be-detected coin, and the radial and axial magnetic gradiometers in the coin detection system. The radial magnetic gradiometer and the axial magnetic gradiometer are surrounded by the excitation coil, and they include two X-axis linear magnetoresistive sensors  15 ,  15 ′ and two Z-axis linear magnetoresistive sensors  16 ,  16 ′ respectively, wherein the X-axis linear magnetoresistive sensors  15 ,  15 ′ are not only located at an inner edge of the excitation coil  3  and symmetrical relative to the center of the excitation coil  3 , but also symmetrically located below an edge of the to-be-detected coin  4 ; the Z-axis linear magnetoresistive sensors  16 ,  16 ′ are not only symmetrical relative to the center of the excitation coil, but also distributed below the center of the to-be-detected coin  4 , or located near a lower side of the center of the to-be-detected coin  4 . Objectives of symmetrical distribution of the X-axis linear magnetoresistive sensors  15 ,  15 ′ and the Z-axis linear magnetoresistive sensors  16 ,  16 ′ are as follows: (1) in the absence of a to-be-detected coin but in the presence of an excitation magnetic field, output signals of the radial magnetic gradiometer and the axial magnetic gradiometer are both 0; and (2) in the presence of a to-be-detected coin, the radial magnetic gradiometer and the axial magnetic gradiometer can measure corresponding magnetic field gradients. In the present invention, the X-axis linear magnetoresistive sensors  15 ,  15 ′ may also be distributed on the same left side or right side of the excitation coil  3 , and be longitudinally symmetrical. Certainly, the radial magnetic gradiometer and the axial magnetic gradiometer may also be located outside the excitation coil, which is not limited in the present invention. 
     The X-axis linear magnetoresistive sensor  15  and the Z-axis linear magnetoresistive sensor  16  are disposed on a PCB  13  near the to-be-detected coin, the X-axis linear magnetoresistive sensor  15 ′ and the Z-axis linear magnetoresistive sensor  16 ′ are disposed on a PCB  14  away from the to-be-detected coin  4 , and the PCB  13  and the PCB  14  are identical. Sensing directions of the X-axis linear magnetoresistive sensors  15 ,  15 ′ are parallel to a radial direction of the to-be-detected coin  4 , that is, the sensing directions point to edges of the to-be-detected coin  4  from the center thereof, while sensing directions of the Z-axis linear magnetoresistive sensors  16 ,  16 ′ are parallel to an axial direction of the to-be-detected coin  4 , that is, the sensing directions point to the outside from the center of the to-be-detected coin  4 . In  FIG. 2 , as placement directions of the PCB  13  and the PCB  14  are opposite, the sensing directions of the X-axis linear magnetoresistive sensors  15 ,  15 ′ and the Z-axis linear magnetoresistive sensors  16 ,  16 ′ are anti-parallel to each other respectively. In this example, the X-axis linear magnetoresistive sensors  15 ,  15 ′ and the Z-axis linear magnetoresistive sensors  16 ,  16 ′ are of a gradient full bridge structure, whose bridge arm consists of one or more TMR elements electrically connected with each other. In addition, the X-axis linear magnetoresistive sensors  15 ,  15 ′ and the Z-axis linear magnetoresistive sensors  16 ,  16 ′ are a single resistor or gradient half bridge structure, whose bridge arm may also consist of one or more magnetoresistive elements, such as Hall, AMR, or GMR, electrically connected with each other. The excitation coil  3  is located between the two PCBs  13  and  14 , and encircles the X-axis linear magnetoresistive sensors  15 ,  15 ′ and the Z-axis linear magnetoresistive sensors  16 ,  16 ′. The excitation coil  3  is a single coil, but if it is necessary to enhance the signals and cause magnetic fields around the to-be-detected coin  4  generated by the signals to be more uniform, at this point, an array formed by superposing multiple coils may also be used. A diameter of circumference encircled by the excitation coil  3  is greater than or equal to that of the to-be-detected coin  4 . The excitation coil  3  is positioned by the upper and lower PCBs  13  and  14 , such that the to-be-detected coin  4  is located on one side thereof. In this embodiment, the to-be-detected coin  4  is located above the excitation coil  3 . Specifically, the surface of the to-be-detected coin  4  is parallel to a central plane (shown by the horizontal dotted line in  FIG. 2 ) of the excitation coil  3 , and a distance between the surface of the to-be-detected coin  4  and the central plane of the excitation coil  3  is at least half of the height H of the excitation coil. A current direction in the excitation coil  3  is as shown by  17  and  18  in  FIG. 2 , that is, comes in from  17  and goes out of  18 , the current direction is parallel to the central plane of the excitation coil, directions of magnetic fields generated at the X-axis linear magnetoresistive sensors  15 ,  15 ′ are the same, directions of magnetic fields generated at the Z-axis linear magnetoresistive sensors  16 ,  16 ′ are also the same, but their sensing directions are opposite to each other respectively, and thus they may offset each other through operations, which does not affect measurement results. Compared with the X-axis linear magnetoresistive sensor  15 ′ and the Z-axis linear magnetoresistive sensor  16 ′, the X-axis linear magnetoresistive sensor  15  and the Z-axis linear magnetoresistive sensor  16  are closer to the to-be-detected coin  4 , so as to form gradient magnetic field measurement for an eddy current field induced by the to-be-detected coin  4 . The positioning column  12  in  FIG. 2  and  FIG. 3  is used for positioning the to-be-detected coin  4 , so as to avoid influences caused by different positions where the to-be-detected coin  4  is placed, but the placement position of the positioning column  12  is not limited to that shown in the figures, which, for example, may also be placed on an opposite side of the position shown in the figures. 
       FIGS. 4A-4B  are respectively relational curves of a real component and an imaginary component of an eddy current field induced by a coin made of stainless steel and coated with nickel on the surface vs. measurement positions when a measurement frequency is 1 KHz. Position  0  in the figures represents the central point of the coin. Curves  19  and  22  are analog results of the axial magnetic gradiometer, and curves  20  and  21  are analog results of the radial magnetic gradiometer. It can be seen from  FIG. 4A  that axial magnetic field components near the center of the coin are the greatest and uniformly distributed, while radial magnetic field components are the greatest at edges of the coin. It can be found by comparing  FIG. 4A  and  FIG. 4B  that the real component of the eddy current field induced by the coin is more affected by the measurement position. 
       FIGS. 5A-5B  are respectively relational curves of a real component and an imaginary component of a magnetic field around a coin made of stainless steel and coated with nickel on the surface vs. measurement positions when a measurement frequency is 10 KHz. Curves  23  and  26  are analog results of the axial magnetic gradiometer, and curves  24  and  25  are analog results of the radial magnetic gradiometer. A conclusion the same as that in  FIG. 4  may also be derived from  FIG. 5 . 
       FIGS. 6A-6D  are calculation results of relationships between a real component and an imaginary component of an eddy current field induced by a coin made of a different material and frequencies. In  FIG. 6A , the coin is made of pure nickel, in  FIG. 6B , the coin is made of stainless steel and surface-coated with nickel having a thickness of 100 um, in  FIG. 6C , the coin is made of stainless steel and surface-coated with nickel having a thickness of 10 um, and in  FIG. 6D , the coin is made of pure stainless steel; curves  27 ,  31 ,  35 , and  39  are real components measured by the radial magnetic gradiometer, curves  28 ,  32 ,  36 , and  40  are imaginary components measured by the radial magnetic gradiometer, curves  29 ,  33 ,  37 , and  41  are real components measured by the axial magnetic gradiometer, and curves  30 ,  34 ,  38 , and  42  are imaginary components measured by the axial magnetic gradiometer. It can be seen from the figures that measurement results are different for the coins made of different materials, the real component is more sensitive to magnetic conductance materials, while the imaginary component is sensitive to eddy currents. The denomination, material and other information of the coin can be obtained according to real and imaginary components corresponding to each frequency. 
       FIGS. 7A-7B  are respectively curves of testing results of coins of 1 Yuan and 0.1 Yuan. Curves  44  and  45  and curves  48  and  49  are real components and imaginary components measured by the axial magnetic gradiometer respectively; and curves  43  and  46  and curves  47  and  50  are real components and imaginary components measured by the radial magnetic gradiometer respectively. It can be seen by comparing the two figures that output results are different for coins with different denominations. The denomination and authenticity of the coin can be judged by comparing a measurement result with a standard value. Measurement results of some coins at a certain frequency and in a certain direction are the same or very close, resulting in that it is difficult to judge the denomination and authenticity thereof; at this point, it is necessary to make judgment in combination with output results corresponding to multiple frequencies, as shown in  FIG. 10  and  FIG. 8  corresponding to  FIG. 10 . 
     It can be seen from  FIG. 10  and  FIG. 8  that when the coins with the denominations of 1 JPY and 10 JPY are at a frequency of 9800 Hz, measurement results of the axial magnetic gradiometer are the same, and the denominations can be identified only in combination with the measurement results of the radial magnetic gradiometer. In addition, when the coins with the denominations of 0.1 CNY and 0.5 CNY are at a frequency of 9800 Hz, amplitude values of magnetic field components in the radial direction and the axial direction are very close and are not easy to identify, at this point, the denominations of the coins can be identified accurately in combination with the measurement result when the frequency is 160 Hz, and the coins with the denominations of 100 JPY and US5CENT is just opposite to the former. When the frequency is 160 Hz, amplitude values of magnetic field components in the radial direction and the axial direction are very close and can be accurately identified only in combination with the measurement result when the frequency is 9800 Hz. 
     Amplitudes of magnetic field components of some coins in a certain direction are very close, and identification is very difficult when a single-axis magnetic gradiometer is used for measurement. Two coins whose denominations are 100 JPY and 5 US cent are taken as an example, as shown in  FIGS. 9A-9B .  FIG. 9A  is a relational curve of amplitude values of magnetic field components in a Z-axis direction vs. frequencies measured by using an axial magnetic gradiometer, and  FIG. 9B  is a relational curve of amplitude values of magnetic field components in an X-axis direction vs. frequencies measured by using a radial magnetic gradiometer. It can be seen from the two figures that within a frequency range of 0 to 10 KHz, measurement results of the two coins in the axial direction (i.e., the Z-axis direction) are very close, measurement results in the radial direction (i.e., the X-axis direction) vary within a frequency range of 2.5 to 10 KHz, it is very difficult to judge the denominations if magnetic field components in the axial direction are measured only, and the denominations of the coins can be accurately judged only in combination with the measurement results in the X-axis direction. For some coins, the measurement results in the axial direction may be different but the measurement results in the radial direction are very close; it is thus clear that, only when magnetic field components in the radial direction and the axial direction are measured at the same time, can the denominations of the coins be identified more accurately, and then the authenticity thereof can be judged by comparing with the standard result. The coin detection system of the present invention measures magnetic field components in the radial direction and the axial direction at the same time, and thus accuracy of judging the denominations and the authenticity of the coins by using measurement results thereof is higher. 
     The above descriptions are merely preferred embodiments of the present invention, and are not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and changes. Any modification, equivalent replacement, improvement or the like made without departing from the spirit and principle of the present invention shall all fall within the protection scope of the present invention.