Document detecting arrangement

An arrangement for detecting bank notes having magnetic properties, comprising transport means (11) for transporting documents past a magnetic detector (12-14). The object of the invention is to increase the possibility and the reliability of distinguishing bank notes which are not genuine from genuine bank notes and bank notes of a given denomination/kind from bank notes of another denomination/kind. The detector includes a permanent magnet (12) and a magnetic circuit (13) having detector means (14) for detecting changes in the magnetic field through the magnetic circuit (13). The output side of the detecting means (14) is connected to a signal processing means (20) for generating signals in response to the magnetic properties of bank notes transported through the arrangement. The magnetic circuit (13) includes an air gap (131) which is located closely adjacent the bank note transport path (0--0). The magnetic flux of the permanent magnet (12) in the air gap (131) is directed in a plane at right angles to the direction of the transport path (0--0) closely adjacent the air gap.

TECHNICAL FIELD 
The present invention relates to an arrangement for detecting documents, 
such as bank notes, cheques, coupons and like documents, which exhibit 
magnetic properties. The arrangement includes document transporting means 
operative in feeding documents along a document feed path, past a magnetic 
detector. 
BACKGROUND PRIOR ART 
Apparatus intended, inter alia, for checking and controlling means of 
payment, e.g. currency, substitute currency and other credit instruments 
or documents are known to the art. An example of such apparatus is found, 
for instance, in Swedish Lay-Out Print No. 448037, in which the means of 
payment contains magnetic particles and is passed through a primary 
magnetic field. The presence of secondary or introduced magnetic fields 
induced by the presence of the primary magnetic field are detected 
separately, and an error signal is produced when the means of payment 
located in the primary magnetic field fails to introduce secondary 
magnetic fields. 
The drawback with this known apparatus is that the effect of the detection 
of the secondary, introduced magnetic fields is liable to be impaired as a 
result of poor signal/noise relationships and because it is the absolute 
level of the introduced, secondary magnetic field which is to be detected. 
The object of the present invention is, inter alia, to greatly reduce this 
risk and to integrate the primary magnetic field source and the detector 
in one and the same function. 
SUMMARY OF THE INVENTION 
An inventive arrangement comprises a primary magnetic-circuit generating 
device which is positioned in such close relationship with a document feed 
path as to cause a change in the magnetic circuit when a document having 
magnetic properties passes along said path and into the magnetic field. It 
has been found in practice that such an arrangement is particularly 
insensitive to external disturbance, but sensitive to the presence of 
passing documents that have magnetic properties, such as to obtain via 
detector means connected to the magnetic field generating circuit signals 
which are highly dependent on the different magnetic properties of 
mutually different documents. The mutually different "profiles" of these 
signals enable documents which are not genuine to be distinguished 
positively and quickly from genuine documents (counterfeit bank notes 
copied from genuine bank notes) and also one kind of genuine documents 
from another kind of genuine document (Swedish one hundred kronor notes 
from Swedish one thousand kronor notes, etc.).

DESCRIPTION OF PREFERRED EMBODIMENTS 
The arrangement illustrated in FIG. 1 includes a magnetic detector 12-14 
comprising a device 12 for generating a permanent, primary magnetic field 
in a symmetric magnetic circuit 13 of a detector device 14 having the form 
of a coil which is intended to detect any change which might occur in the 
magnetic field. 
The device 12 comprises a round permanent magnet, the upper surface of 
which is the north pole and the bottom surface of which is the south pole. 
Some of the magnetic flux lines in the permanent magnetic field are shown 
in dotted lines and are referenced 121, 122, 123. 
The magnetic circuit 13 has an air gap 131 defined between two identically 
configured arms 132, 133, which are connected together by a yoke, on which 
there is wound a winding 14 which serves as a detecting or sensing means. 
The relative positioning of the permanent magnet 12 in relation to the 
magnetic circuit 13 is such that the flux lines 121, 122, 123 of the 
magnetic field in and closely adjacent to the air gap pass essentially 
through one plane through the air gap, this plane constituting the 
symmetry plane of the magnetic circuit. This plane also forms the plane 
normal to the transport path 0--0 along which bank notes are forced to 
move past the detector for the purpose of detecting whether or not the 
bank notes are genuine and/or the types of bank notes present. 
A bank note is shown in FIG. 1 and is referenced 15. Incorporated in the 
bank note are a plurality of magnetic particles which are placed in a 
specific pattern characteristic to the type of bank note concerned or its 
denomination. These particles are normally invisible to the naked eye, but 
have been heavily emphasized in the Figure in order to facilitate the 
understanding of the description. One such particle, 150, is shown in a 
greatly enlarged view at the bottom of the bank note. An arrow to the 
right of the bank note in the direction 0--0 indicates movement of the 
bank note in the 0--0 direction of the bank note transport path. Bank note 
transporting means 11 for transportation of the bank notes are 
symbolically illustrated in the form of two mutually co-acting rollers. It 
will be understood that a multiple of roller pairs are provided in 
practice. 
FIG. 2 illustrates the bank note 15 as it begins to pass the detector 
12-14. The magnetic particles are herewith influenced by the permanent 
magnetic flux and are magnetized in the opposite direction to the 
permanent magnet 12. This is indicated schematically by the broken flux 
lines and the N-S direction in the greatly enlarged particle 150. 
As a result of movement of the bank note in relation to the detector a 
change occurs in the magnetic circuit 13 and an output signal 
characteristic of the bank note 15 is obtained via the sensing coil 14. 
The signals captured by the coil 14 are fed to a signal processing means, 
which in the FIG. 3 embodiment includes a serially connected input 
amplifier 30 of the direct-current or alternating-current type having an 
amplifying factor of 10.sup.4 ; a band pass filter 31 of the 
intermediate-frequency type for the range 1-10 kHz; a rectifying and 
peak-detector circuit 32; a threshold circuit 33 having a threshold limit 
of about 0.7 to 1.5 volts; and an output amplifier 34. Since these 
components constitute standard solutions the manner in which they are 
constructed or function will not be described in detail here. 
The output side of the output amplifier 34 is connected to registering 
means and to signal means for producing an error signal when a false bank 
note or a bank note of the wrong type is detected. 
The mutually different characters of the signals obtained from two mutually 
different bank notes A and B which have passed the detector are 
illustrated in FIG. 4. As will be seen from FIG. 4, certain respective 
components of the two signals are very different from one another, which 
enables bank notes of varying kind or denomination to be identified with a 
high degree of accuracy, even if a bank note should be heavily dirtied, 
folded, creased, etc. 
Although the illustrated embodiment includes a permanent magnet for 
generating the permanent magnetic flux, it will be understood that this 
flux may, instead, be generated by means of a solenoid. 
The essential feature of the invention is that the magnetic flux is 
directed so as not to disturb or to saturate the magnetic circuit, i.e. 
the flux shall be directed perpendicularly to the longitudinal extension 
of the magnetic circuit on which the winding 14 is mounted. Furthermore, 
the process circuit is provided with non-linear amplification and is 
constructed to suppress disturbance signals and to emphasize the unique 
signal pattern arriving from the detector. 
It will be seen from FIG. 5 that during its passage past the detector 12-14 
the bank note 15 passes close to the magnetic circuit 13 and the permanent 
magnet 12.