WO1993011516A1

WO1993011516A1 - Method of and system for surveillance and detection using magnetic markers

Info

Publication number: WO1993011516A1
Application number: PCT/GB1992/002263
Authority: WO
Inventors: Michael David Crossfield; Dafydd Geraint Davies; Andrew Dames
Original assignee: Esselte Meto International Gmbh
Priority date: 1991-12-04
Filing date: 1992-12-04
Publication date: 1993-06-10
Also published as: GB9125846D0; AU3090392A

Abstract

A method of, and a system for, detecting articles containing or carrying magnetic markers is disclosed. The articles are passed through a surveillance zone in which a high frequency magnetic field is generated and in which the response of said magnetic markers is detected. Means for generating a high frequency field having quadrupole characteristics is provided. Such means generally comprises a transmitting coil and an electrically conductive screen placed behind said coil. This arrangement gives reduced interference from farfield sources, thus improving sensitivity and reliability of detection. It also permits the transmitting coil or part thereof to function as the receiving means.

Description

Method of and System for Surveillance and Detection using Magnetic Markers

This invention relates to the detection of articles containing or carrying magnetic tags or markers; such markers are typically used in electronic article surveillance, theft detection, personnel monitoring and inventory control- Systems for detecting such tags or markers operate by setting up an interrogating electromagnetic field. This field gives rise to a characteristic electromagnetic signal from the markers, which is detected by a receiver system to indicate the presence of the markers. A common drawback with such systems is that the interrogating field often extends outside the region where the markers are to be detected - in particular behind as well as in front of the interrogation system. This can cause the system to interfere with other nearby electronic equipment, and to generate acoustic noise in nearby metal fixings such as supermarket checkouts. A further serious drawback is that the detector may also respond to the presence of markers or other objects having magnetic properties located outside the surveillance or detection zone. This can lead to unwanted detections (false positives) - which can be very embarrassing for store managers - and/or ineffective detection, because the detectors pick up spurious signals from other electronic equipment or objects with magnetic properties. Whatever the cause, far field interference sources limit the effectiveness of magnetic detection systems. These drawbacks are significant barriers to market acceptance of such a system. This invention aims to overcome or ameliorate these drawbacks, and may provide additional benefits.

U.K. Patent GB 2194414 A (Sensormatic) describes the use of a magnetic material as a flux diverting shield to minimise rearward leakage of the interrogation field. The disadvantages of such a shield are that it is bulky, expensive in material cost, and has to be carefully chosen and positioned such that it does not itself produce confusing signals or acoustic noise.

We have now devised a method of, and apparatus for, detecting magnetic markers, which relies on a modification of the normal magnetic field characteristics to provide a system which displays improved rejection of far field interference sources.

According to one aspect of the present invention, there is provided a method of detecting articles containing or carrying magnetic markers by passing the articles through a surveillance zone in which a high frequency magnetic field is generated and in which the response of said magnetic markers is detected, characterised in that the high frequency magnetic field is generated by a transmitting means arranged to generate a field having quadrupole characteristics.

Preferably, the high frequency magnetic field is generated by means of a transmitting coil and the quadrupole field characteristics are generated by the interaction of this transmitting coil with an electrically conductive screen placed behind the coil.

Hitherto the use of a screen of electrically conductive material has been regarded as impracticable because it would substantially short out (destroy) the interrogation and/or detection fields due to eddy currents induced in the screen. We have found, however, that such a screen enables the magnetic field modification of this invention to be achieved satisfactorily.

When magnetic markers are present in the surveillance zone, their response to the magnetic field may be detected using a receiving coil. As with many- conventional systems, the receiving coil may be a separate coil, typically positioned close to the transmitting coil. An alternative arrangement, however, is made possible by the special magnetic field characteristics employed in the present invention: the quadrupole field characteristics mean that the transmitting coil or part thereof can function as the receiving coil. The high frequency magnetic field will generally be in the range 2kHz - 50kHz, and is preferably in the range 4kHz - 10kHz.

Generally, the method of the invention utilises a low frequency magnetic field which, together with the high frequency field, is transmitted into the surveillance zone. The low frequency magnetic field will generally be in the range 1Hz - 400Hz, and is advantageously in the range 4Hz - 40Hz. It is preferred to operate with a ratio between high and low frequencies in the range 1000:1 to 50:1.

According to a second aspect of the present invention, there is provided an article surveillance system in which articles containing or carrying magnetic markers are passed through a surveillance zone for detection purposes, said system comprising: first means for transmitting a high frequency magnetic field into said surveillance zone; second means for transmitting a low frequency magnetic field into said surveillance zone; and means responsive to the magnetic field in said surveillance zone; characterised in that the system further comprises means for generating quadrupole field characteristics within said surveillance zone when said first and second transmitting means are in operation.

Preferably, the means for generating quadrupole field characteristics comprises an electrically conductive screen positioned close to the first and second transmitting means. Such a screen is preferably formed of copper or aluminium.

In a system in accordance with this invention, the dimensions and characteristics of the screen, the first and the second transmitting means, and their mutual disposition, are preferably selected so as influence the high frequency field by generating quadrupole field characteristics within the surveillance zone without significantly affecting the low frequency field generated by the second transmitting means. Advantageously, the spacing between the transmitting means and the screen is very much smaller than the coil dimensions; and the thickness of the screen is selected so that it has no substantial effect on the low frequencies generated by the second transmitting means. The thickness criterion for the screen results in it having a high magnetic impedance with respect to low frequencies; this prevents a 90° phase change which would otherwise interfere with the low frequency field.

The screen effectively decouples the transmitting and receiving means, thereby enhancing receiver performance. It also decouples the first and second transmitting means from one another. Certain embodiments of the invention make particular use of this decoupling effect, in that they employ two or more first transmitting means each with a different frequency. In accordance with a preferred embodiment, the article surveillance system of this invention comprises: first means for transmitting a high frequency magnetic field into said surveillance zone; second means for transmitting a low frequency magnetic field into said surveillance zone; and receiving means responsive to the magnetic field in said surveillance zone; characterised in that the system further comprises an electrically conductive screen, wherein:

(i) said screen is positioned close to said first and second transmitting means; (ii) said screen is located outwardly (with respect to said surveillance zone) of said first and second transmitting means; and (iii) the dimensions of said screen and its disposition with respect to said first and second transmitting means are selected so as to generate a magnetic field having quadrupole field characteristics within said surveillance zone when said first and second transmitting means are in operation.

The transmitting and receiving means will generally be coils; the transmitting means may thus be described as an interrogating coil, while the receiving means may be described as a detection coil. These terms will be used hereinafter, although it should be understood that, as explained earlier herein, the present invention makes it possible to use a single means both for transmitting and receiving. By means of the present invention, the patterns of electromagnetic field generated by the interrogation coil, and the patterns of reception field of the detector or detecting coils, are improved in their near-to-far field behaviour. A further advantage of this invention is that the generation of forward-projecting fields can be sufficiently electrically efficient, whilst permitting effective shielding with respect to the rearward fields.

The screen as used in the present invention is fabricated from a conducting material. This material may be copper sheet, aluminium sheet, or a conductor-loaded insulating material, a composite sheet formed from a number of foils or particulate conducting slurry, or a thick plating of copper or other conductor on a carrier, this list not being exhaustive. The material should be of a thickness which is comparable with the electromagnetic skin depth at the interrogation frequency which is to be screened. The advantage of this type of screen by comparison with magnetic materials is that it is lightweight, cheap, and behaves linearly during operation so that it does not generate frequency harmonics of the type used to detect the tags. In addition, acoustic noise generated through magnetostriction, which is almost impossible to remove from magnetic materials, will be avoided.

Preferably, the spacing between the screen and the interrogator/detector coils is arranged so that they are sufficiently far apart to allow efficient operation of the system, but sufficiently close together to permit the assembly of screen and interrogator/detector coils to be enclosed in a panel having an unobtrusive overall thickness. The effective screening of the rearward interrogation field by means of this invention is expected to allow improved customer acceptance of the product due to reduced acoustic noise, improved detection performance, and reduced installation time and cost. In addition the system does not interfere with other nearby apparatus such as audio systems, telephones, credit card readers, etc.

As well as screening the interrogation field, this invention allows improved detector performance. Firstly, the detector is shielded from rearward sources of interference. Secondly, if a flat dipole coil design of receiver is used, the pattern of receiver detection sensitivity is considerably improved in a number of ways. The screen acts so as to generate additional distributed poles of receiver sensitivity, thereby giving the desired quadrupole characteristics to the magnetic field. These poles cause a rapid far-field fall-off which further reduces sensitivity to external sources. The additional poles, however, are distributed in a manner which does not generate horizontal (or vertical) null planes in the detection zones which might allow markers to go undetected as they pass parallel to the panel. As a result, a single detector channel can be used with comparable performance to multiple channels which have horizontal null-planes. Also, detectors designed in a manner according to this invention have considerably improved signal-to-interference ratios, which further allows the use of a single coil rather than multiple coils.

A further advantage of the use of this type of screen is that it allows the interrogation coil itself to be used as the receiver coil since, as previously described, a single receiver channel used with this type of screen will suffice. Accordingly, in a further aspect, the present invention provides a system for detecting tags or markers which are responsive to an electromagnetic field, which system comprises a single coil which functions both as an interrogation (driver) coil and a receiver coil.

Furthermore, a coil set designed in accordance with this aspect of the invention may have interrogation and receiver patterns which are well matched both in their direction and in fall-off with distance, allowing optimum overall system performance. An important aspect of this type of system is that the interrogation coil must be wound as to give a degree of physical separation between its two halves, in order to take advantage of field cancellation within the receiver.

Some types of security systems use interrogation fields which have multiple frequencies, and which are driven using a single coil. According to another aspect of the present invention, separate interrogation coils which are in separate positions are used to generate fields of different frequencies. The advantage of this is that the driver electronics are considerably simplified since the mutual electrical coupling between the two frequencies is reduced, allowing the system to be manufactured for lower cost despite the additional coil or coils. Use of a screen of the type described in the first aspect of the invention will further decrease the electrical coupling between the coils, allowing the coils to be placed in greater coincidence while still giving the decoupling advantages. This allows greater flexibility in mechanical design. In systems where one of these interrogation frequencies is extremely low (below about 100 Hz, used as a sweeping bias field), it has been found that it is not necessary to screen the low frequency effectively, since it does not induce the same troublesome effects as do higher frequencies.

By way of illustration, two examples of embodiments of this invention will now be given with reference to the drawings.

EXAMPLE 1 Figure 1 is a schematic drawing of apparatus in accordance with the invention, comprising a screen (1) of electrically conductive material placed in close proximity and parallel to a first, high frequency transmitting means in the form of a planar interrogation coil (2). The second transmitting means, in the form of a further coil (20), is omitted from Figure 1 for reasons of clarity but is depicted in Figure 2, which is a section along line II-II of Figure 1. A receiver means in the form of a coil (3) is located in relation to the interrogation coil (2) as shown. The width of the interrogation coil (2) is 310 mm, the width of the receiver coil (2) is 150 mm, and the width of the screen (1) is 600 mm; the distance between the interrogation coil and the screen is 40 mm. The separation of the two runs of the interrogating coil (measured mid-point to mid-point) is 250mm. The electromagnetic field due to the interrogation coil (2) is represented by field lines (4). In this example, the system operates with a 16 Hz bias and a frequency of 7.5 kHz. The thickness of the screen is 0.7 mm, which is appropriate to shielding the kHz field, and it does not need to be of the much greater thickness required to screen the very low bias frequency.

EXAMPLE 2 Figure 3 shows an embodiment of the invention in which the interrogation coil itself is used as the receiver coil. This is achieved through a single, centre-tapped coil (5). The screen (1) is similar to that described above in Example 1. As in Figure 1, the low frequency generating coil is not shown; it is located in a position analogous to that shown in Figure 2.

Figure 4 is a circuit diagram of a system using a single coil (5), with the connection points marked. The left-hand side represents the power electronics, and the right-hand side represents the frame and coil. R_χ represents the receiver.

Claims

CLAIMS :

1. A method of detecting articles containing or carrying magnetic markers by passing the articles through a surveillance zone in which a high frequency magnetic field is generated and in which the response of said magnetic markers is detected, characterised in that the high frequency magnetic field is generated by a transmitting means arranged to generate a field having quadrupole characteristics.

2. A method according to claim 1, characterised in that said high frequency magnetic field is generated by means of a transmitting coil and an electrically conductive screen placed behind said coil.

3. A method according to claim 2, characterised in that the response of said magnetic markers is detected using a receiving coil.

4. A method according to claim 3, wherein said receiving coil is separate from transmitting coil.

5. A method according to claim 4, wherein said transmitting coil or part thereof is arranged to function as said receiving coil.

6. A method according to any preceding claim, characterised in that said high frequency magnetic field is in the range 2kHz - 50kHz.

7. A method according to claim 6, characterised in that said high frequency magnetic field is in the range 4kHz - 10kHz.

8. A method according to any preceding claim, characterised in that a low frequency magnetic field is also transmitted into said surveillance zone.

9. A method according to claim 8, characterised in that said low frequency magnetic field is in the range 1Hz - 400Hz.

10. A method according to claim 9, characterised in that said low frequency magnetic field is in the range 4Hz - 40Hz.

11. A method according to claim 8, 9 or 10, characterised in that the ratio between said high frequency and said low frequency is in the range 1000:1 to 50:1.

12. A method according to any preceding claim, characterised in that said transmitting means is arranged to generate a magnetic field comprising two or more high frequencies.

13. An article surveillance system in which articles containing or carrying magnetic markers are passed through a surveillance zone for detection purposes, said system comprising: first means for transmitting a high frequency magnetic field into said surveillance zone; second means for transmitting a low frequency magnetic field into said surveillance zone; and means responsive to the magnetic field in said surveillance zone; characterised in that the system further comprises means for generating quadrupole field characteristics within said surveillance zone when said first and second transmitting means are in operation.

14. A system as claimed in claim 13, characterised in that said means for generating quadrupole field characteristics comprises an electrically conductive screen positioned close to said first and second transmitting means.

15. A system as claimed in claim 14, characterised in that said electrically conductive screen is formed of copper or aluminium.

16. A system as claimed in claim 14 or 15, characterised in that the dimensions and characteristics of said screen, said first and said second transmitting means, and their mutual disposition, are selected so as influence the high frequency field by generating quadrupole field characteristics within said surveillance zone without significantly affecting the low frequency field generated by said second transmitting means.

17. A system as claimed in claim 13, 14, 15 or 16, characterised in that said first transmitting means comprises two coils arranged to generate a magnetic field at two different high frequencies.

18. An article surveillance system in which articles containing or carrying magnetic markers are passed through a surveillance zone for detection purposes, said system comprising: first means for transmitting a high frequency magnetic field into said surveillance zone; second means for transmitting a low frequency magnetic field into said surveillance zone; and receiving means responsive to the magnetic field in said surveillance zone; characterised in that the system further comprises an electrically conductive screen, wherein:

(i) said screen is positioned close to said first and second transmitting means; (ϋ) said screen is located outwardly (with respect to said surveillance zone) of said first and second transmitting means; and (iii) the dimensions of said screen and its disposition with respect to said first and second transmitting means are selected so as to generate a magnetic field having quadrupole field characteristics within said surveillance zone when said first and second transmitting means are in operation.