GB2310496A

GB2310496A - Method and apparatus for authenticating documents

Info

Publication number: GB2310496A
Application number: GB9603695A
Authority: GB
Inventors: Robin Arthur Brown; Bryan James Christophersen; Ernest Alfred Munn
Original assignee: De la Rue Systems Ltd
Current assignee: De la Rue Systems Ltd
Priority date: 1996-02-21
Filing date: 1996-02-21
Publication date: 1997-08-27
Also published as: GB9603695D0

Abstract

Apparatus for authenticating documents such as banknotes comprises a magnetic head 3 for monitoring magnetic field variations caused by passage of a document 1 adjacent to the head assembly. Signals from the head are repeatedly integrated (9) or differentiated (12) and the resulting signals compared with threshold levels in comparators 10,13 before being passed to a microprocessor 14. The number of pulses produced by the comparators during the passage of a document under test is counted in the microprocessor and compared with preset values to determine the authenticity of the document.

Description

METHOD AND APPARATUS FOR MONITORING A VARYING MAGNETIC FIELD The invention relates to methods and apparatus for monitoring a varying magnetic field as found, for example, on articles such as documents of value, for example banknotes.

Documents of value such as banknotes are often provided with a magnetic property to alloçr the document to be authenticated. Conventional document processing apparatus such as sorters and counters incorporate a magnetic head which detects any magnetic property on or in a document as it passes the head to enable that magnetic property to be analyzed so as to determine the authenticity of the document. There is a need to improve the accuracy of such detection methods and for example overcome problems which can arise due to the document impacting on the magnetic head.

In accordance with a first aspect of the present invention, a method of monitoring a varying magnetic field comprises: i) sensing the magnitude of the magnetic field and generating a corresponding electrical signal; ii) repeatedly integrating the electrical signal and generating a count signal each time the value of the integrated signal reaches a certain value; and iii) determining the number of count signals over a monitoring period.

In accordance with a second aspect of the present invention, apparatus for monitoring a varying magnetic field comprises a magnetic field sensor for sensing the magnitude of the magnetic field and for generating a corresponding electrical signal; an integrator for repeatedly integrating the electrical signal; a comparator for comparing the integrated electrical signal with a certain value and for generating a count signal each time the value of the integrated signal reaches the certain value; and means for determining the number of count signals over a monitoring period.

This aspect of the invention is based on the concept of integrating the electrical signal representing the magnetic field magnitude and using the rate at which the integrated signal reaches the (typically predetermined) value. The factors that determine whether and/or when the integrated signal reaches the certain value include the duration of the magnetic signal, the continuity of the magnetic signal, and the amplitude of the magnetic signal.

Using this method, a high magnetic signal level will produce more integration cycles and hence more count signals than a low signal for the same duration. This differs from other magnetic detectors which use a fixed period of integration, for which the maximum number of magnetic presence counts is limited to the number of fixed periods across the region being monitored, such as a banknote, that occur with magnetic presence, regardless of any increase in magnetic signal amplitude. The method of the invention operates on bulk magnetic signal level as opposed to the changes in the waveform and is more effective at higher rates of variation of a magnetic field due for example to higher processing speeds, where the magnetic signal level increases. Furthermore, the method according to the invention is less sensitive to impact noise glitches on the magnetic signal as compared with known processes.

Preferably, step (ii) is only carried out when the value of the electrical signal exceeds a threshold. This use of a (typically predetermined) threshold or bias level reduces the risk of noise signals being integrated.

Conveniently, the electrical signal is rectified before being integrated. This simplifies the processing circuitry and consequently preferably the apparatus further comprises a rectifier to which the electrical signal is fed, the output of the rectifier being fed to the integrator.

Conveniently, the method further comprises resetting the integrated signal to zero after the generation of each count signal. This is not essential and the integrator could continue integrating without being reset but this would need the generation of a number of separate values for use in generating count signals whereas by resetting the integrated signal to zero, the same value can be used on each repeat of step (ii).

In accordance with a third aspect of the present invention, a method of monitoring a varying magnetic field comprises: i) sensing the magnitude of the magnetic field and generating a corresponding electrical signal; ii) monitoring the rate of change of the electrical signal and generating a count signal each time the monitored rate of change exceeds a threshold; and iii) determining the number of count signals over a monitoring period.

In accordance with a fourth aspect of the present invention, apparatus for monitoring a varying magnetic field comprises a magnetic field sensor for sensing the magnitude of the magnetic field and for generating a corresponding electrical signal; a comparator connected to the output of the monitoring means for comparing the monitored rate of change with a threshold and for generating a count signal each time the monitored rate of change exceeds the threshold; and means for determining the number of count signals over a monitoring period.

The third and fourth aspects of the present invention provide an alternative approach to monitoring a varying magnetic field. This approach operates on the changes in the magnetic signal waveform as opposed to the magnetic signal level and is more effective at lower rates of variation of the magnetic field, corresponding for example to lower processing speeds, where the magnetic signal level is reduced and the waveform is well defined.

Typically, the (usually predetermined) threshold corresponds to a change of sign in the monitored rate of change.

In summary, the "Integral Process" is dependent on the magnitude and time duration of the magnetic signal level, which means that there will be an increase in the number of magnetic presence counts as the average magnetic signal level increases. In the case of monitoring articles, the magnetic signal level also increases when an article is moved passed the magnetic sensor at a faster rate. This means that there is an increase in the number of magnetic presence counts as the note counting speed is increased using the "Integral Process".

The "Differential Process" is dependent on the magnetic signal waveform content as opposed to its magnitude, and responds to changes in the waveform slope direction, which means that there will be an increase in the number of magnetic presence counts as the number of changes in the waveform increases. The changes in magnetic signal waveform slope direction correspond to changes in density of the magnetic ink pattern, for which there is an upward shift in the waveform frequency content as the note is moved past the magnetic head at a faster rate. This upward shift in the waveform frequency content reduces the measured number of slope changes because of the bandwidth limiting on the magnetic amplifier circuit. This means, in the case of monitoring articles, that there is a decrease in the number of magnetic presence counts as the article counting speed is increased using the "Differential Process".

As has been mentioned above, the inventive aspects are particularly suited for use in a method of authenticating an article which generates a magnetic field characteristic of the article, the method comprising causing relative movement between the article and a magnetic sensing head which generates an electrical signal corresponding to the magnitude of the sensed magnetic field; carrying out a method according to the first and/or third aspect of the invention; and determining whether the number of count signals obtained satisfies predetermined conditions corresponding to a genuine article.

Furthermore, the invention applies to apparatus for authenticating an article which generates a magnetic field characteristic of the article, the apparatus comprising monitoring apparatus according to the second and/or fourth aspects of the invention; and means for causing relative movement between the magnetic sensing head and the article.

One or both methods may be used once or more than once to determine, for example, if an article (e.g. banknote) is genuine and/or to sort out poor condition articles. It is particularly advantageous to use both methods and/or both forms of apparatus in which case a common sensing head can be used. The use of both processes to analyze the magnetic signal enables documents to be detected which are in a poor condition and thus provide a weak magnetic signal while at the same time reducing the effect of noise glitches on the magnetic signal produced by the documents impacting on the magnetic sensing head. This also gives an improvement in performance over a wider document processing speed range because of the increase in magnetic presence count produced by the integrator process at upper document feed speeds and the increase in magnetic presence count produced by the differential process at lower document feed speeds.

Typically, where both processes are used, the two count signals are added together and compared with a threshold or count range defining the predetermined conditions. In practice, the count signals due to the two processes tend to balance out in the case of banknotes such as USss notes when they are added together at different counting speeds so that the magnetic presence count obtained is consistent over the note counting speed range.

This allows a higher magnetic presence threshold to be used which increases the noise margin.

The US dollar patterns have areas printed with magnetic and non-magnetic ink which can be measured using the magnetic detector and checked for the correct presence or non-presence of a magnetic signal. The relative magnitude of the magnetic signal measured in certain areas on the magnetic ink pattern can also be checked using the number of magnetic presence counts obtained using the "Integral Process" in each area. The variations in the magnetic ink pattern across the note can also be checked from the number of magnetic presence counts obtained using the 'Differential Process".

The monitoring periods may be continuous or spaced apart in time so that where appropriate there can be regions where the magnetic signal is ignored, checked against a minimum threshold value, or checked against a maximum threshold value. The monitoring periods could be different for the two processes where they are carried out together and in particular for example the differential process could be used only after a predetermined time window to avoid the effects of impact noise due to the leading end of a document.

The invention can be used in a variety of fields for monitoring varying magnetic fields but is particularly suited for monitoring documents of value such as banknotes, for example to test for genuineness. The invention can be used in document sorting and/or counting apparatus of a conventional form.

An example of a method and apparatus according to the present invention will now be described with reference to the accompanying drawing which is a schematic block diagram of the apparatus.

The apparatus shown in the drawing is for use with a banknote counter such as the 2600 counter manufactured by De La Rue Systems Limited. In this counter, a banknote 1 is fed by means not shown past a magnetic head assembly 2 which includes a magnetic sensing head 3 and an amplifier 4. Banknotes may be fed at speeds in the range 600-1800 notes per minute, with notes passing the head 3 at 4.2 metres/sec at the 1800 notes per minute rate. The magnetic head 3 senses a magnetic field generated by the banknote 1 and converts the sensed field to an electrical signal whose magnitude corresponds to the magnitude of the sensed field.

This electrical signal is amplified by the amplifier 4, and the amplified signal is then fed to a further preamplifier 5 mounted on an interface board 6. The preamplifier 5 is a bandwidth limited preamplifier circuit which increases the electrical signal to a suitable working level for subsequent processing. The preamplifier bandwidth is optimised to reduce the high frequency noise level and glitches picked up by the magnetic head 2 from banknotes or foreign bodies impacting on the magnetic head but with sufficient bandwidth to retain the magnetic signal waveform features obtained from the banknotes, such as USS, that are needed for further processing to determine whether the signal has the characteristics of a genuine note.

The board 6 carries two processing circuits connected in parallel to the preamplifier 5. An integrator process circuit 7 comprises a precision rectifier circuit 8 whose output is connected to an integrator circuit 9, the output from the integrator circuit 9 being connected to one input of a comparator 10.

A differential process circuit 11 is also connected to the output of the preamplifier 5 and comprises a differential slope detector 12 whose output is connected to an input of a comparator 13. Each of the comparators 10,13 has an input connected to a respective threshold level, the outputs of the comparators 10,13 being fed to a microcontroller 14.

When the arrival of a banknote 1 at the magnetic head 2 is sensed (by a sensor not shown) the micro-controller 14 resets all counters to zero. The integrator and differential circuits 7,11 then process the detected magnetic field signal in the following way.

Integrator Process Circuit 7 The signal from the preamplifier 5 is fed to the precision rectifier circuit 8 which rectifies the signal, the rectified signal being fed to the integrator 9. The integrator 9 is biassed so that integration does not occur when only noise or offset drift is received. Furthermore, the micro-controller 14 initialises the integrator 9 by issuing a reset control signal on a line 15.

When the rectified signal level exceeds the bias level on the integrator, the integrator output will start to ramp down towards the comparator threshold level (as fed to the comparator 10) at a rate which depends on the amplitude of the rectified signal level. When the integrator 9 ramps below the comparator threshold, the comparator output changes from a low to high state, and the transition is detected by the micro-controller 14 which then increments the magnetic presence count Ni and resets the integrator to re-initialise the integration cycle. The factors that determine whether the integrator output will ramp down to the comparator threshold level are: the duration of the magnetic signal, the continuity of the signal, and its amplitude. Using this method a high magnetic signal level will produce more integration cycles (magnetic presence counts) than a low signal for the signal duration.

The integrator input bias setting, the comparator threshold, and time constants used by the integrator can all be adjusted in order to optimise the processed signal.

Differential Process Circuit 11 The output from the preamplifier 5 is also fed to the differential slope detector 12. This detector 12 generates an output signal representative of the rate of change of the input signal or its slope and this is fed to the comparator 13. The comparator 13 changes state when there is a change in direction or sign of the slope signal from the detector 12, a positive pulse being produced on the output of the comparator 13 when the differential voltage change on the comparator input exceeds a threshold level.

A micro-controller 14 counts the numbers of changes of state or pulses to generate a second count value Nd.

As with the integrator process circuit 7, the threshold applied to the comparator 13 and time constants used by the differential slope detector 12 can be adjusted in order to optimise the processed signal.

The gain and bandwidth of the preamplifier 5 can also be adjusted to optimise the preamplified signal which is processed by the circuits 7,11.

In both cases, the received magnetic signal can be segmented into time windows in which the counts Ni, Nd are either ignored, checked against a minimum threshold value, or checked against a maximum threshold value by the microcontroller 14. These time windows may be different for the two circuits 7,11 and in particular for the circuit 11, counts may be ignored during a time window corresponding to the arrival of the document edge at the magnetic head 2.

Once the count values Ni and Nd have been obtained, the micro-controller 14 will add them together and then determine whether or not they satisfy predetermined conditions. In a simple case, these may include a comparison of the total number of counts with a minimum threshold which is exceeded by a genuine note or there may be a more complex process in which the counts are obtained for each time window and compared with respective thresholds and the note indicated as genuine only if all or a majority of the counts satisfy predetermined conditions.

It will be understood that by combining the counts from the two processes, the magnetic presence threshold can be set higher than that required for a single process which increases the noise margin and improves the tolerance to impact noise on the magnetic signal.

In the example described, a single magnetic head 2 is shown connected to the processing electronics. In more complex examples, more than one magnetic head may be provided for sensing magnetic fields in different parts of the note, each magnetic head being connected to respective processing electronics of the form shown in the drawing.

The outputs from the micro-controllers will then be fed to a host processor which can carry out further processing on the different counts received to determine overall whether or not the note is authentic.

Claims

1. A method of monitoring a varying magnetic field, the method comprising: i) sensing the magnitude of the magnetic field and generating a corresponding electrical signal; ii) repeatedly integrating the electrical signal and generating a count signal each time the value of the integrated signal reaches a certain value; and iii) determining the number of count signals over a monitoring period.

2. A method according to claim 1, wherein step (ii) is only carried out when the value of the electrical signal exceeds a threshold.

3. A method according to claim 1 or claim 2, wherein the electrical signal is rectified before being integrated.

4. A method according to any of the preceding claims, the method further comprising resetting the integrated signal to zero after the generation of each count signal.

5. A method of monitoring a varying magnetic field, the method comprising: i) sensing the magnitude of the magnetic field and generating a corresponding electrical signal; ii) monitoring the rate of change of the electrical signal and generating a count signal each time the monitored rate of change exceeds a threshold; and iii) determining the number of count signals over a monitoring period.

6. A method according to claim 5, wherein the threshold corresponds to a change of sign in the monitored rate of change.

7. A method of monitoring a varying magnetic field substantially as hereinbefore described with reference to the accompanying drawing.

8. A method of authenticating an article which generates a magnetic field characteristic of the article, the method comprising causing relative movement between the article and a magnetic sensing head which generates an electrical signal corresponding to the magnitude of the sensed magnetic field; carrying out a method according to any of the preceding claims; and determining whether the number of count signals obtained satisfies predetermined conditions corresponding to a genuine article.

9. A method according to claim 8, wherein the electrical signal from the magnetic sensing head is processed in accordance with at least claim 1 and claim 5.

10. A method according to claim 9, wherein the predetermined conditions include a total count value threshold which the sum of the counts from the two methods must exceed.

11. A method according to any of claims 8 to 10, wherein the article comprises a document of value, such as a banknote.

12. Apparatus for monitoring a varying magnetic field, the apparatus comprising a magnetic field sensor for sensing the magnitude of the magnetic field and for generating a corresponding electrical signal; an integrator for repeatedly integrating the electrical signal; a comparator for comparing the integrated electrical signal with a certain value and for generating a count signal each time the value of the integrated signal reaches the certain value; and means for determining the number of count signals over a monitoring period.

13. Apparatus according to claim 12, wherein the integrator only operates on an electrical signal which exceeds a threshold.

14. Apparatus according to claim 12 or claim 13, further comprising a rectifier to which the electrical signal is fed, the output of the rectifier being fed to the integrator.

15. Apparatus for monitoring a varying magnetic field, the apparatus comprising a magnetic field sensor for sensing the magnitude of the magnetic field and for generating a corresponding electrical signal; a comparator connected to the output of the monitoring means for comparing the monitored rate of change with a threshold and for generating a count signal each time the monitored rate of change exceeds the threshold; and means for determining the number of count signals over a monitoring period.

16. Apparatus for authenticating an article which generates a magnetic field characteristic of the article, the apparatus comprising monitoring apparatus according to any of claims 12 to 15; and means for causing relative movement between the magnetic sensing head and the article.

17. Apparatus according to any of claims 12 to 16 for carrying out methods according to any of claims 1 to 11.

18. Apparatus for authenticating an article which generates a magnetic field characteristic of the article substantially as hereinbefore described with reference to the accompanying drawing.