Abstract:
A method of authenticating value documents has the following steps: a) providing a document to be checked for authenticity, b) providing a high-frequency magnetic excitation field, c) providing a trajectory path for checking the document, d) providing a detection coil, e) the detection coil receiving a detection signal when the document follows the trajectory path, f) deriving geometric parameters from the detection signal, g) comparing the geometric parameters with the geometric parameters of a genuine value document.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is a continuation of application no. PCT/EP2006/061829, filed Apr. 26, 2006, which claims the priority of European patent application no. 05105577.0, filed 23 Jun. 2005, and which application no. PCT/EP2006/061829, filed Apr. 26, 2006, claims the priority of European patent application no. 05105578.8, filed 23 Jun. 2005, and each of which is incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention relates to a method for authenticating value documents. The value documents, if genuine, comprise magnetic security particles spread in or on a predetermined or pre-selected location of the value documents.  
       BACKGROUND OF THE INVENTION  
       [0003]     Integrating magnetic security particles in value documents is known. U.S. Pat. No. 5,992,741 discloses integrating soft-magnetic or semi-soft-magnetic fibers of a particular geometry in value documents such as bank notes or credit cards.  
         [0004]     U.S. Pat. No. 5,992,741 also discloses a way of detecting the presence of the magnetic fibers in value documents. The disclosed detection method is based upon an analysis of the harmonics present in the detection signal.  
         [0005]     U.S. Pat. No. 6,707,295 discloses another way of authenticating value documents. This disclosed detection method is based upon an analysis of the dB/dt response signal, which allows deriving magnetic parameters such as the magnetic coercivity or the magnetic saturation.  
         [0006]     As counterfeiting becomes more imminent, protection of value documents has become more sophisticated, e.g. by combination of various different security characteristics.  
         [0007]     WO-A-2005/105902 of applicant mentions the possibility of concentrating the security particles only on predetermined or pre-selected locations of a genuine value document.  
         [0008]     U.S. Pat. No. 5,545,885 discloses a method and apparatus to detect and identify coded patterns on bank notes in the form of magnetic regions. These magnetic regions are small areas printed with ink containing a magnetic pigment.  
         [0009]     U.S. Pat. No. 4,864,238 discloses a device for measuring a weak magnetic field. This device can be used for measuring fields associated with bank notes for identifying the denomination or values of bank notes.  
         [0010]     U.S. Pat. No. 5,808,466 discloses a process for the characterization of magnetic materials for validating documents. The process uses a low-frequency signal emitter. The form of the detection signal is analyzed as to the particular position of the magnetic materials in the documents.  
       OBJECTS AND SUMMARY OF THE INVENTION  
       [0011]     The present invention provides for a method of checking both the genuineness of the security particles and the correct location of the security particles in the value document.  
         [0012]     According to the present invention, there is provided a method of authenticating value documents. The value documents, if genuine, comprise magnetic security particles spread in or on a predetermined location of the value documents.  
         [0013]     The method comprises the following steps: 
        a) providing a document to be checked for authenticity;     b) providing a high-frequency magnetic excitation field;     c) providing a trajectory path for checking the document;     d) providing a detection coil;     e) the detection coil receiving a detection signal when the document follows the trajectory path;     f) deriving geometric parameters from the detection signal;     g) comparing the geometric parameters with the geometric parameters of a genuine value document.        
 
         [0021]     A conclusion to genuineness can be made in case the geometric comparison g) is positive.  
         [0022]     Within the context of the present invention, the terms ‘value documents’ generally refer to bank notes, credit cards, passports, bonds, etc.  
         [0023]     The term ‘magnetic’ refers to magnetic material exhibiting a non-linear BH-curve when being subjected to an alternating excitation field H. Magnetic material preferably refers to soft-magnetic material with magnetic coercivities below 100 A/m (measured at near-DC or low frequencies) and to semi-soft magnetic material with magnetic coercivities higher than 100 A/m (measured at near-DC or low frequencies).  
         [0024]     The term ‘particles’ refers to small elements being able to be integrated in or on the substrate of value documents. The term ‘particles’ also refers to fibers having a diameter ranging from 1 μm to 30 μm and having a length ranging from 1 mm to 20 mm.  
         [0025]     The term ‘high-frequency’ refers to frequencies higher than 1000 Hz, e.g. higher than 3000 Hz. The higher the frequency, the higher the speed of detection.  
         [0026]     In a particular embodiment of the invention one or more detection coils can be provided with a varying concentration of windings along the trajectory path of the document.  
         [0027]     The terms ‘detection coil with a varying concentration of windings along the trajectory path’ refer to a detection coil or a combination of various detection coils where the number of windings per unit of length along the trajectory path varies.  
         [0028]     A simple embodiment of a detection coil with a varying concentration of windings is a detection coil with windings along a part of the trajectory path and without windings along another part of the trajectory path. Another embodiment is realized where the distance between subsequent windings varies, e.g. by varying the thickness of insulation between the windings.  
         [0029]     As to step f), various geometric or physical parameters may be derived from the detection signal.  
         [0030]     As a first possibility, the maximum amplitude of the detection signal may be determined and is a measure for the width of the region where magnetic particles are present.  
         [0031]     As a second possibility, the abscissa of the maximum amplitude in the detection signal may be determined after having detected the edge of the value document. As will be explained hereafter, this abscissa is a measure for the global or average position or location of the location of the magnetic security particles in the document.  
         [0032]     A third possibility is to analyze the form of the detection signal. As will be explained hereafter, the presence or not of sub-maxima and sub-minima and the respective amplitudes or differences in amplitude may be an indication of the width of the location of the magnetic security particles.  
         [0033]     In addition to the derivation of various geometric parameters, various magnetic parameters may be derived from the detection signal.  
         [0034]     A preferable method is to determine the maximum amplitude of the detection signal. This amplitude is a measure for the concentration of the magnetic particles in the value document. The higher the concentration the higher the amplitude. The genuineness of the value document may be based not on the mere presence of the security particles but on the presence of the security particles within a selected range of concentration. Alternatively, or in addition, the excitation current corresponding to the maximum amplitude may be determined. This excitation current is a measure for the magnetic coercivity of the magnetic particles and may be an indication of the genuineness of the value document.  
         [0035]     In the embodiment where both geometric features and magnetic features are derived from the detection signal, a preferable embodiment allows to make a positive conclusion as to genuineness of the document only in case both the magnetic comparison with a genuine document and the geometric comparison with a genuine document are positive. If the magnetic comparison is negative, or if the geometric comparison is negative or if both are negative, a conclusion as to counterfeit may be made.  
         [0036]     The method according to the invention can be used in a bank note sorting machine, a bank note counting machine, an apparatus for distributing bank notes, automatic vending machines, apparatus for authenticating credit cards, etc. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0037]      FIG. 1  shows a BH curve of a magnetic material;  
         [0038]      FIG. 2  shows both a sinusoidal applied magnetic field and a measured magnetic response from a magnetic material;  
         [0039]      FIG. 3  shows a detection apparatus suitable for carrying the detection method according to the invention;  
         [0040]      FIG. 4 ,  FIG. 5 ,  FIG. 6 ,  FIG. 7 ,  FIG. 8 ,  FIG. 9  and  FIG. 10  all show the subsequent response signals captured when a value document is going through a detection apparatus;  
         [0041]      FIG. 11  shows the global response signal;  
         [0042]      FIG. 12  shows various global response signals and the parameters derived from it;  
         [0043]      FIG. 13  shows an alternative embodiment of a detection apparatus;  
         [0044]      FIG. 14  shows another embodiment of a detection apparatus. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0045]      FIG. 1  shows a so-called BH-curve  10  of a magnetic material in the context of the present invention, i.e. a magnetic material with a non-linear hysteresis behavior. The abscissa is the magnetic field H expressed in amperes/meter (A/m) and the ordinate is the magnetic induction B expressed in Tesla (T) or Oersted (Oe). Characteristic magnetic parameters are the coercive field H c , which is the field at which the magnetic response becomes zero, and the saturation value Bs, which is the magnetic induction at the onset of saturation.  
         [0046]     Reference is now made to  FIG. 2 . A sinusoidal magnetic field  12  is applied to the magnetic material particles inside a value document. The measured magnetic response dB/dt (the time derivative of the magnetic induction B) gives two peaks  14 .  
         [0047]      FIG. 3  schematically shows a detection apparatus  16  which is suitable for carrying out a detection method according to the present invention. For the purpose of clarity, only the detection coils  20  and  22  are represented. A document to be checked for authenticity will be guided along a trajectory path in the direction of arrow  24 . Along this trajectory path the concentration of windings  26  is not constant but is—deliberately—changing. Roughly outlined, following regions may be distinguished along the trajectory path: 
        i) in the beginning, absence of windings;     ii) the right windings  26  of the right detection coil  22 ;     iii) absence of windings in the middle of detection coil  22 ;     iv) the left windings  26  of the right detection coil  22 ;     v) the right windings  26  of the left detection coil  20 ;     vi) absence of windings in the middle of detection coil  20 ; and     vii) the left windings  26  of the left detection coil  20 .          
         [0055]      FIG. 4 ,  FIG. 5 ,  FIG. 6 ,  FIG. 7 ,  FIG. 8 ,  FIG. 9 , and  FIG. 10  all illustrate the subsequent stages of a value document  30  passing along the trajectory  24 .  
         [0056]     Document  30  if genuine comprises a predetermined and pre-selected region  32  which does not extend to the whole volume of value document  30  and where magnetic security particles  34  are spread. The document  30  follows the trajectory path  24  from right to left. A high-frequency magnetic field is applied.  
         [0057]      FIG. 4  illustrates the start of document  30  passing along the trajectory  24 . In the very start, there is no detection of dB/dt signals  14  since the presence of security particles  34  cannot yet be noticed by the right detection coil  22 . Document  30  approaching the right windings  26  of right detection coil  22 , a dB/dt signal  14  starts to be detected and an increasing amplitude is noticed since the document  30  is coming closer and since the population of windings  26  becomes denser.  
         [0058]      FIG. 5  illustrates the second stage. The predetermined region  32  of security particles  34  is passing in close neighborhood to the right windings  26  of right detection spool  22 . The amplitude  14  of the dB/dt detection signal is exhibiting a maximum.  
         [0059]      FIG. 6  illustrates the third stage. The predetermined region  32  of security particles  34  is passing in close neighborhood to the center part of the right detection spool  22  where there are no windings. The amplitude  14  of the dB/dt detection signal is exhibiting a minimum.  
         [0060]      FIG. 7  illustrates the fourth stage. The predetermined region  32  of security particles  34  is passing in close neighborhood to the left windings of right detection coil  22  closely followed by the right windings of left detection coil  20 . Both windings represent a very dense and close population of windings. The amplitude  14  of the dB/dt detection signal is exhibiting an absolute maximum.  
         [0061]      FIG. 8  illustrates the fifth stage. The predetermined region  32  of security particles  34  is passing in close neighborhood to the center part of the left detection spool  20  where there are no windings. The amplitude  14  of the dB/dt detection signal is exhibiting a minimum.  
         [0062]      FIG. 9  illustrates the sixth stage. The predetermined region  32  of security particles  34  is passing in close neighborhood to the left windings of the right detection spool  20 . The amplitude  14  of the dB/dt detection signal is exhibiting a local maximum.  
         [0063]      FIG. 10  illustrates the seventh stage. The predetermined region  32  of security particles is leaving the neighborhood of the left windings of the right detection spool  20 . The amplitude  14  of the dB/dt detection signal is decreasing until zero.  
         [0064]      FIG. 11  illustrates the global result of all the various subsequent stages. Curve  40  is a curve enveloping all measured dB/dt amplitudes  14 . Curve  40  will be used to derive both magnetic and geometric parameters from the document to be authenticated.  
         [0065]      FIG. 12  shows various types of enveloping curves  40 ,  40 ′ and 40″ and illustrates various values which can be derived from these curves.  
         [0066]     Curve  40  corresponds to a document with a relatively narrow predetermined region  32  of security particles  34 . As this predetermined region is quite narrow, the absence or presence of detection coils is felt more sharply when this document passes the trajectory  24 .  
         [0067]     Curve  40 ″ corresponds to a document with a relatively large predetermined region  32  of security particles  34 . As this predetermined region is quite large, the absence or presence of detection coils is more spread and curve  40 ″ is more smooth than curve  40 .  
         [0068]     Curve  40 ′ corresponds to a document with an predetermined region  32  of security particles  34  that is larger than the region  32  of the document producing curve  40  and more narrow than the region  32  of the document producing curve  40 ″. Curve  40 ′ holds somewhat the middle between curve  40  and curve  40 ″. Any way, the various curves  40 ,  40 ′ and 40″ show that a detection signal  40 , which is by essence a magnetic detection signal, gives indications about the geometrical width of the predetermined region  32  of security particles  34 .  
         [0069]     Following parameters may be derived from the enveloping curve  40 : 
        the maximum amplitude  42  of curve  40  over the whole trajectory  24 ; this amplitude is an indication of the concentration of security particles  34 ; the higher the concentration the higher the amplitude  42 ;     the abscissa value  44  of the maximum amplitude, this abscissa  44  is an indication of the (average) position of the predetermined region  32  within a value document  30      the lobe-valley difference  46  (or difference between a local maximum and a local minimum); as explained here above with respect to curves  40 ,  40 ′ and 40″, this lobe-valley difference  46  is an indication of the width of the predetermined region  32  with the security particles  34 . The enveloping curve  40  shows two lobe-value differences  46 . Either one of the values can be taken, or, preferably, the average value of the two values can be taken as this is a more robust parameter     the lobe value  48  (or local maximum 48) may be—just as the lobe-valley value  46 —an indication of the width of the predetermined region  32 . The absolute value  48  of the lobe is also dependent upon the magnetic parameters.        
 
         [0074]     Next to these four parameters, other parameters may also be derived. One example is the excitation current which corresponds to the maximum response amplitude  40 . This excitation current is an indication for the coercive field H c .  
         [0075]     The complete course of the enveloping curve  40  may be also be checked and compared with minima and maxima between which a response of a genuine document must fit.  
         [0076]     The detection apparatus  16  is calibrated by passing various genuine documents  30  through it and by determining the maximum values and minimum values for the various magnetic and geometric parameters.  
         [0077]     After this calibration process, the apparatus  16  is ready for authentication. A document passing through it, is considered genuine only if it meets both magnetic and geometric limit ranges.  
         [0078]     In addition to detection apparatus  16  or as alternative for detection apparatus  16 , the alternative embodiment of  FIG. 13  may be used. The alternative embodiment is a printed circuit board (PCB)  50  or a layer of a PCB in addition to other layers. This PCB lodges, for example, five elongated and relatively thin detection coils  51 ,  52 ,  53 ,  54  and  55 . The five detection coils  51 ,  52 ,  53 ,  54  and  55  lie parallel to the trajectory path  24 . Each of the detection coils  51 ,  52 ,  53 ,  54  and  55  gives a response signal in case security particles are detected in the neighborhood of each coil. So this embodiment has the advantage of giving an estimate not of the width of the predetermined zone  32  but of the height of the predetermined zone  32 . The higher the number of thin spools the higher the accuracy is of the height of the predetermined zone  32 .  
         [0079]     Yet another embodiment of a detection apparatus is illustrated in  FIG. 14 . Again, this embodiment may be used as alternative or in addition to the detection apparatus  16 . This other embodiment is a printed circuit board (PCB)  60  or a layer of a PCB. This PCB lodges a very thin detection coil  62  in a direction perpendicular to the trajectory path  24  of the value documents  30 . The thin character of detection coil  62 , which means a small width in the direction of the trajectory path  24 , has the advantage of providing a very sharp signal in case security particles  34  are present in the document to be checked. This sharpness of the signal helps to determine the width of the predetermined zone in a better way.  
         [0080]     While this invention has been described as having a preferred design, it is understood that it is capable of further modifications, and uses and/or adaptations of the invention and following in general the principle of the invention and including such departures from the present disclosure as come within the known or customary practice in the art to which the invention pertains, and as may be applied to the central features hereinbefore set forth, and fall within the scope of the invention or limits of the claims appended hereto.