Banknote validator

Details Banknote validator Banknote validator

1999-10-12

2002-05-21

Leja, Ronald W. (Department: 2836)

Electricity: electrical systems and devices

Control circuits for electromagnetic devices

Including housing

Reexamination Certificate

active

06392863

ABSTRACT:

The term “banknote” is used herein for convenience and for ease of comprehension. However, it is to be interpreted as including any sheet-like objects having detectable features, for example tickets and vouchers, and fraudulent and counterfeit versions thereof.
It is known that magnetic signatures are printed on many types of banknote and that these signatures are consistent between banknotes of the same type. This property has been used by many manufacturers of banknote validators, in conjunction with optical methods, to determine the value of a banknote and to determine its authenticity.
Several sensor designs have be en used to detect this signature, all of which have disadvantages. A simple type uses an inductive device, similar to those found in tape recorders. These devices are only suitable for use where the banknotes to be validated produce a strong magnetic field. Also, the output of the sensor is dependant on the speed of the banknote. Magneto-resistors have been used in various configurations and have proved not to be sensitive enough.
A derivative of the magneto-resistor is the giant magneto-resistor. These devices are extremely sensitive to small magnetic fields. They are so sensitive that they can detect ferrous materials at considerable distances, making the use of these devices in an unshielded plastic casing impractical. Furthermore, the range of fields that can be measured is very limited and fields from motors and power transformers easily overwhelm the field from a banknote. There are devices that address these problems. However the cost of these devices makes them unsuitable for use in a low cost banknote validator.
According to the present invention, there is provided a magnetic sensor comprising a magnetic circuit and an electronic circuit, the magnetic circuit comprising a yoke and a giant magneto-resistor and the electronic circuit comprising a coil arranged to generate a magnetic field in the yoke and a feedback control loop responsive to the output of the giant magneto-resistor to energise the coil so that the giant magneto-resistor operates in a predetermined region of its characteristic.
Preferably, the frequency response of the control system has a low-pass characteristic. Thus, the bias field applied to the giant magneto-resistor compensates for stationary and relatively slowly changing ambient magnetic fields. In the particular case of a magnetic sensor for a banknote validator, it has been found that a low-pass characteristic with a first order roll-off with a −3 dB point in the range 1 to 5 Hz is desirable. Preferably, however, the −3 dB point is at 2 Hz.
While large stationary or slowly changing ambient magnetic fields can be handled by feedback control of the giant magneto-resistor's magnetic bias, there remains the problem of more rapidly changing magnetic fields.
According to the present invention, there is provided a magnetic sensor comprising two giant magneto-resistors connected by a yoke, and a subtracter configured for subtracting the output of one of the giant magneto-resistors from that of the other, wherein the giant magneto-resistors are arranged such that only one of the giant magneto-resistors is significantly sensitive to magnetic fields generated in a sensing region and both giant magneto-resistors are sensitive to ambient magnetic fields. Consequently, the components of the giant magneto-resistor outputs due to ambient fields cancel and the output from the subtracter is substantially only dependent on the local field detected substantially by only one of the giant magneto-resistors.
The characteristics of the giant magneto-resistors need to be matched. This can be ensured by carefully selecting the giant magneto-resistors to be used together. A preferred alternative is to employ first bias means for applying a constant bias voltage to one of the giant magneto-resistors and second bias means for applying a variable bias voltage to the other giant magneto-resistor, the second bias means being responsive to the output of the subtracter to generate a bias voltage tending to cause the output of the subtracter to be zero. The closed-loop transfer function of the second bias means should be arranged such that desired signals are not significantly attenuated.
Preferably, the yoke comprises two connected arms, one giant magneto-resistor is mounted between free ends of the arms of the yoke, and the other giant magneto-resistor is mounted between the arms of the yoke between their interconnection and said one giant magneto-resistor.
The two techniques for dealing with interfering magnetic fields set out above are preferably combined.
It will be appreciated that applications of magnetic sensors according to the present invention extend far beyond the particular case of sensing magnetic characteristics of banknotes. For instance, such sensors could be used for sensing magnetic characteristics of coins or for reading magnetic recordings.
There are many methods of obtaining a characteristic waveform from a banknote using optical techniques. Typically, a banknote to be validated is illuminated with narrowband light and the amplitude of light reflected and/or transmitted by a banknote measured.
According to the present invention, there is provided a banknote validator including an optical sensor for sensing optical characteristics of a banknote being validated, the sensor comprising a light source, incident light-directing means for directing light from the light source onto a banknote being validated, a photodetector and reflected light-directing means for directing light from the light source, after reflection from a banknote being validated, to the photodetector, characterized in that the light source is a source of broadband light and an optical filter is interposed between reflected light-directing means and the photodetector.
This arrangement takes advantage of all of the light wavelengths that the banknote can reflectively filter. As a result, more distinctive information is yielded. Suitable broadband sources include incandescent bulbs of various types and also broadband light emitting diodes which produce light across substantially the whole of the visible spectrum. The filter responses of the receivers are such that the banknote's properties can be sorted into selected areas of activity to match the banknote designer's chosen wavelength response. When using a narrowband source, a truly distinctive characteristic is only obtained if the wavelength, produced by the narrowband source, is part of the filtering effect of the banknote.
Preferably, a light guide serves as the incident light-directing means and the reflected light-directing means. Conveniently, the light guide is a substantially trapezial, planar solid, the narrow end of which is adjacent the light source and the photodetector and the broad end of which is adjacent a banknote path.
Preferably, the optical sensor comprises a plurality of photodetectors and a plurality of optical filters to which light is directed by the reflected light-directing means, the optical filters having different transmission characteristics and being associated with respective photodetectors.
The filter may be one that passes primarily infrared light or blue-green light. Infrared and blue-green light-passing filters may be arranged in series. Filters having the following 3 dB stopbands have been found to be preferable: 420-720 nm and 480-540 nm together with >820 nm. The filters may be arranged in series.
When reflecting from a specular surface the power of light reflected back in a particular direction is proportional to the degree of specularity and the diffuse behaviour of the surface. Banknotes contain both specular and diffuse surfaces as part of their design, the main surface being predominantly diffuse. Areas of specular reflection are created by using highly reflective devices such as flechetes, plastic holograms, and metalised threads.
The present inventors have discovered that directing light obliquely onto a banknote helps to create highly distinctive wavef

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