WO2002097742A1

WO2002097742A1 - Magnetic field sensor and method

Info

Publication number: WO2002097742A1
Application number: PCT/GB2002/002163
Authority: WO
Inventors: Michael Potter
Original assignee: De La Rue International Limited
Priority date: 2001-05-25
Filing date: 2002-05-10
Publication date: 2002-12-05
Also published as: GB0112783D0

Abstract

A magnetic sensor comprises a magnetoresistive element and a monitoring system for monitoring the resistivity of the magnetoresistive element. The electrical resistivity of the element varies in accordance with a magnetic field to which the magnetoresistive element is exposed. The monitoring system comprises a beam of charged particles and a particle acceleration system for causing the beam to impinge on the surface of the magnetoresistive element and the resistivity of the element is calculated from the beam current.

Description

MAGNETIC FIELD SENSOR AND METHOD

The invention relates to a sensor and method for measuring magnetic fields, in particular magnetic fields generated by magnetic material present on or in sheet documents .

It is well known that this type of magnetic field may be measured by many types of magnetic sensor, for example electromagnetic inductive sensors and magnetoresistive sensors. For example, typical apparatus for reading banknote security threads comprises a document transport system, a magnetisation station, one or more magnetic sensors and a control and processing system for controlling the document transport system and processing the signals produced by the magnetic heads. If magnetic materials having a very low remanence are to be measured, it is necessary to have a magnetic field at the sensor. Magnetoresistive sensors usually have an integral magnet that supplies such a field. It is possible to pass a bias current through an inductive sensor to produce the bias field.

The documents are carried by the transport system past the magnetisation station (if used) and the magnetic sensor. As the magnetic material, for example constituting a security thread, passes the magnetic sensors, a signal is generated in the magnetic head. The magnitude of the signal generated is a measure of the amount of magnetic material on the document. With a single magnetic sensor, it is only possible to examine a single strip of a document.

Examples of situations when it is desirable to determine the magnetic image of a complete document are given in GB 2098768 and EP 0493438 which describe such apparatus for reading the security thread on banknotes. In both cases, the apparatus comprises an array of magnetic heads and an associated processing system. A typical requirement is to resolve the magnetic elements of a coded magnetic thread, as described in GB2098768.

For a typical element of 1mm to 2mm, the spacing between adjacent sensors must be about 0.5mm to 1mm to allow proper resolution of the individual elements. For a transport system working with documents up to 100mm wide, between one hundred and two hundred sensors are required.

This involves several practical difficulties. The individual sensor elements must have a restricted area of sensitivity and there will be at least two connections to each. There are, therefore, many connecting wires to the processing electronics. Each sensor will have its own preamplifier and associated circuitry, similar to that described in EP0493438. If this circuitry is mounted close to the sensors there will be problems of size and heat dissipation amongst other things.

There is a need to provide a sensor that can perform high speed, high resolution acquisitions of the patterns in which magnetic material is deposited on sheet documents without these practical difficulties.

In accordance with one aspect* of the present invention, a magnetic sensor is provided comprising a magnetoresistive element exposed in use to a magnetic field whereby the electrical resistivity of the element varies in accordance with the magnetic field; and a monitoring system for monitoring the resistivity of the magnetoresistive element, wherein the monitoring system comprises a beam of charged particles, for example electrons, and a particle acceleration system for causing the beam to impinge on the surface of the magnetoresistive element.

Preferably, the monitoring system monitors the resistivity at a number of positions across the magnetoresistive element. The particle acceleration system may comprise an anode and a cathode, the magnetoresistive element being disposed between them. Advantageously, a deflection system may be provided in order to deflect the particle beam so that it impinges on any desired point on the surface of the magnetoresistive element. The same result may, of course, be achieved by moving the magnetoresistive element relative to the particle beam although this is less desirable since it is more difficult to arrange.

Normally, a focussing system is provided in order to focus the particle beam onto the surface of the magnetoresistive element although this is necessary only if the resolution required of the monitoring system demands it.

The deflection system may comprise electrodes for electrostatically deflecting the particle beam or magnetic assemblies for the same purpose. Similarly, the focussing system may comprise electrodes for electrostatically focussing the beam or magnetic assemblies for the same purpose. In addition, the sensor may include a magnetic shielding assembly to prevent magnetic interaction between magnetic focussing or deflection systems, if fitted, and the subject to be monitored by the sensor. Similar focussing and deflection systems are found, for example, in cathode ray tubes used for televisions and Vidicon camera tubes . The sensor may include a layer of a hard, low friction material covering the magnetoresistive element to reduce wear .

Such a sensor can be incorporated into apparatus for reading the magnetic image of or detecting magnetic material in a sheet document, along with a document transport system for moving the document past the magnetic sensor.

Preferably this apparatus would also comprise a mechanism for maintaining contact between the document and the magnetic sensor although this is not always necessary.

This mechanism may, for example, comprise a pinch roller or a spring. In addition, the apparatus may include a magnetisation device or station for magnetising the magnetic material on the sheet document. This is not necessary if the magnetic material on the sheet document is permanently magnetised.

Furthermore, the sensor may include a magnetic bias system for magnetically biassing the magnetoresistive material and the magnetic material on the document at the point of measurement . The magnetisation device and magnetic bias systems may both comprise one or more permanent magnets. They may also comprise one or more electromagnets either in addition to or in place of the permanent magnets.

In accordance with a second aspect of the invention, a method of monitoring a magnetic field is provided, the method comprising exposing a magnetoresistive element to the magnetic field whereby the electrical resistivity of the magnetoresistive element varies in accordance with the magnetic field; and monitoring the electrical resistivity of the magnetoresistive element.

The magnetic field across an extended area may be monitored by monitoring the electrical resistivity of the magnetoresistive element at all points of at least a portion of the magnetoresistive element. This may be achieved by controlling the focussing and/or deflection systems so that a beam of charged particles is scanned along at least a portion of the magnetoresistive element. The invention thus produces an output signal representative of the magnetic field along a line. If the source of the magnetic field is moved past the magnetoresistive element, advantageously but not necessarily, in a direction orthogonal to the direction in which the beam is scanned, it is possible to construct a two-dimensional image representative of the distribution of the magnetic material by making repeated scans. It is also possible to move the magnetoresistive element past the source of the magnetic field, again , advantageously but not necessarily, in a direction orthogonal to the direction in which the beam is scanned.

There is further provided a method in accordance with the second aspect of the invention, wherein the electrical resistivity varies across the magnetoresistive element in accordance with the magnetic field strength variation across magnetic material proximal to the magnetoresistive element, the magnetic field variation representing a magnetic image. The source of this magnetic image may be magnetic material on or in a sheet document. This sheet document may be security document such as a banknote. The magnetic material may be present as inclusions in the substrate, in the inks, in the security thread, or applied in any other fashion.

The sheet document or banknote may contain a magnetic thread, and the magnetic thread may be coded, for example as described in GB2098768.

The magnetic material, unless permanently magnetic, is magnetised prior to the exposure of the magnetoresistive element to the magnetic field generated by the magnetic material .

An example of a magnetic imaging apparatus incorporating a magnetic sensor according to the invention will now be described with reference to the accompanying drawings, in which: -

Figure 1 is a cross-sectional view of the magnetic imaging apparatus;

Figure 2 is a schematic showing some elements of the sensor and processing system; and,

Figure 3 illustrates the variation of resistance with magnetic field of a typical magnetoresistive material.

Figure 1 shows a cross-sectional view of magnetic imaging apparatus incorporating a magnetic field sensor, a document transport system and associated measurement and power supply systems . A magnetic sensor 1 comprises a glass envelope 2 which is evacuated and sealed. At one end of the envelope 2 there is an anode 4 which is non-magnetic and upon this anode is deposited a magnetoresistive element 5. This has a resistivity which varies with applied magnetic field. The dependence of resistivity on magnetic field strength of a typical material is shown in Figure 3.

Covering the anode, there may be a layer of a hard, low friction material 8, for example quartz, polyimide or diamond. In order not to reduce the sensitivity of the apparatus significantly, this layer is relatively thin, typically 20 microns or less.

At the other end of the tube there is a cathode 6 and optionally focussing and deflection electrodes 7. As an alternative to an electrostatic focussing electrode, a magnetic focussing system 16 may be provided instead. Similarly, a magnetic deflection system 16 may be provided in place of electrostatic deflection electrodes. These may comprise one or more electromagnets. Alternatively, they may comprise either in addition or in place of the electromagnets one or more permanent magnets. If a magnetic focussing or deflection system is used then it may be necessary to provide a magnetic screening system 17 in order to prevent any interaction between magnetic focussing or deflection system 16 and the magnetoresistive element 5.

Optionally, a magnetic bias system 19 can be mounted on the tube in close proximity to the magnetoresistive element 5. The purpose of this is to provide a magnetic bias field for the magnetoresistive material and for the magnetic material on the sheet at the point of measurement. The magnetic bias system 19 may comprise one or more permanent magnets when a fixed magnetic bias is desired. However, if electromagnets are used then it is possible to vary the magnitude and polarity of the bias under the control of controlling system 20 via a line 30. Suitable potentials are applied to the anode 4, cathode 6 and, if fitted, the deflection and focussing electrodes 7 by the power supply 12. This is connected to the anode 4 via a resistor 13 and a connecting lead 9. The high potential difference between the anode 4 and the cathode 6 causes a beam of electrons to be emitted from the cathode and to be accelerated towards the anode. The beam current flows from the power supply through the resistor 13 and the connecting lead 9 and returns to the power supply through the cathode 6. As a result of this beam current, a potential difference is developed across the resistor 13, the potential difference being related to the beam current by Ohm's Law. Since the beam 7 impinges on the surface of the magnetoresistive element 5, the beam current is dependent on the resistivity of the magnetoresistive element 5. The potential difference across the resistor 13 hence gives an indication of the local magnetic field to which the magnetoresistive element is exposed and this potential difference can be measured using an electronic measurement, storage and analysis system 15.

A controlling system 20 controls the power supply 12, the electronic measurement, storage and analysis system 15 the magnetic focussing and scanning system 16, and the magnetic bias system 19 if fitted.

A document 11, such as a banknote or other document of value, containing magnetic material to be scanned with the apparatus, is first moved past a magnetising device 18 in order to temporarily magnetise the magnetic material. The document 11 is then moved by the document transport system so that it is in close proximity with the magnetoresistive element 5 of the magnetic sensor 1. It is held in close proximity to the magnetic sensor 1 by a mechanism 10 for maintaining contact between the document and the magnetic sensor. The mechanism 10 may, for example, comprise a pinch roller made from rubber that rotates with a linear speed substantially equal to the transport speed of the document 11. Alternatively, the mechanism 10 may comprise a non-magnetic spring made, for example, from a polymer film. In this case the magnetic bias system 19 may be mounted behind the spring. As the document 11 reaches the magnetic sensor 1, the magnetic field generated by the magnetic material on the document 11 alters the resistivity of the magnetoresistive element 5. The resistivity of the magnetoresistive element 5 will vary across its surface in accordance with the pattern of magnetisation of the magnetic material in the document 11.

Figure 2 shows a schematic of the document 11 when it is in close proximity to the magnetoresistive element 5. In this example, the document 11 has a thread 20 comprising alternating magnetised elements 20a and non-magnetised elements 20b.

The magnetised elements 20a induce changes to the resistivity of adjacent sections 5a of the magnetoresistive element 5. The resistivity of sections 5a may increase or decrease depending on the characteristics of the magnetoresistive material and the magnetic bias system.

The variations of resistivity of the magnetoresistive element 5 is dependent on the magnetic fields generated by elements 5a. This variation will not be identical to the distribution of magnetic material. The storage and analysis system 15 is used to determine the distribution of magnetic material.

The electron beam 7 is then deflected across the entire width of the magnetoresistive element 5 from an initial position 7a at one edge of the element 5 to a final position 7b at the opposite edge.

Hence, as the beam 7 is deflected across the element

5, the beam current is modulated by the varying resistivity of the element 5. This modulation of the beam current produces a corresponding change in the potential difference developed across the resistor 13. The measurement, storage and analysis system 15 measures this changing potential and can hence determine the magnetic image imprinted on the thread 20 of document 11. This image can be subsequently stored and analysed as required.

Claims

1. A magnetic sensor comprising a magnetoresistive element exposed in use to a magnetic field whereby the electrical resistivity of the element varies in accordance with the magnetic field; and a monitoring system for monitoring the resistivity of the magnetoresistive element, wherein the monitoring system comprises a beam of charged particles and a particle acceleration system for causing the beam to impinge on the surface of the magnetoresistive element .

2. A sensor according to claim 1 wherein the monitoring system monitors the resistivity at a number of positions across the magnetoresistive element.

3. A sensor according to either of the preceding claims, wherein the charged particles are electrons.

4. A sensor according to claim 3 wherein the particle acceleration system for causing the electrons to impinge on the surface of the element comprises an anode and a cathode, the magnetoresistive element being disposed between the cathode and the anode .

5. A sensor according to any of the preceding claims, further comprising a deflection system for causing the charged particle beam to be deflected so that it impinges on any desired point on the surface of the magnetoresistive element .

6. A sensor according to claim 5 further comprising at least one deflection electrode for deflecting the charged particle beam to the desired point on the surface of the magnetoresistive element.

7. A sensor according to claim 5 further comprising a magnetic deflection assembly for deflecting the charged particle beam to the desired point on the surface of the magnetoresistive element.

8. A sensor according to any of the preceding claims, further comprising at least one focussing electrode for focussing the charged particle beam on to the surface of the magnetoresistive element.

9. A sensor according to any of the preceding claims, further comprising a magnetic focussing assembly for focussing the charged particle beam on to the surface of the magnetoresistive element.

10. A sensor according to claim 7 or 9 further comprising a magnetic shielding assembly.

11. A sensor according to any of the preceding claims further comprising a layer of a hard, low friction material covering the element .

12. A sensor according to any of the preceding claims, further comprising a magnetic bias system for magnetically biassing the magnetoresistive material .

13. A sensor according to claim 12, wherein the magnetic bias system comprises one or more permanent magnets.

14. A sensor according to claim 12, wherein the magnetic bias system comprises one or more electromagnets.

15. A sensor according to claim 14, further comprising a controller for controlling the or each electromagnet.

16. A magnetic sensor substantially as hereinbefore described with reference to the accompanying drawings.

17. Apparatus for reading a magnetic image on or in sheet documents comprising a magnetic sensor according to any of the preceding claims; and a document transport system for moving the document past the magnetic sensor.

18. Apparatus according to claim 17 further comprising a mechanism for maintaining contact between the document and the magnetic sensor.

19. Apparatus according to claim 18, wherein the mechanism for maintaining contact between the document and the magnetic sensor comprises a pinch roller.

20. Apparatus according to claim 18, wherein the mechanism for maintaining contact between the document and the magnetic sensor comprises a spring.

21. Apparatus according to claims 17 to 20 further comprising a magnetisation device for magnetising the magnetic material on or in the sheet document.

22. Apparatus according to claim 21, wherein the magnetisation device comprises one or more permanent magnets .

23. Apparatus according to claim 21, wherein the magnetisation device comprises one or more electromagnets.

24. A method of monitoring a magnetic field comprising exposing a magnetoresistive element to the magnetic field whereby the electrical resistivity of the magnetoresistive element varies in accordance with the magnetic field; and monitoring the electrical resistivity of the magnetoresistive element by measuring the current due to a beam of charged particles accelerated such that they impinge on the surface of the magnetoresistive element.

25. A method according to claim 24 wherein the electrical resistivity varies across the magnetoresistive element in accordance with the magnetic field strength variation across magnetic material proximal to the magnetoresistive element, the magnetic field variation representing a magnetic image.

26. A method according to claim 25 wherein the source of the magnetic image is magnetic material on or in a sheet document.

27. A method according to claim 26 wherein the sheet document is a bank note.

28. A method according to claim 26 or 27 wherein the document contains a magnetic thread.

29. A method according to claim 28 wherein the magnetic thread is a coded thread.

30. A method according to any of claims 25 to 29 wherein the magnetic material is magnetised prior to or during the exposure of the magnetoresistive element to the magnetic field generated by the magnetic material.

31. A method of monitoring a magnetic field substantially as hereinbefore described with reference to the accompanying drawings .