ANTIPILFERAGE TAGS AND THEIR USE

Abstract: A magnetic antipilferage tag is disclosed which comprises a housing containing means for detecting an external magnetic field; a power supply; a tone generator; and an electric circuit powered by said power supply and arranged to activate said tone generator in response to an output from said magnetic field detector means.

Description: This invention relates to antipilferage tags or markers. Such tags are applied to articles of commerce in order to protect them from theft at the point of sale premises. Typically, the tag is a magnetic medium which is deactivated when a shop assistant carries out the routine procedure at the time of effecting a sale. Such deactivation prevents detection of the magnetic tag when it (and the article to which it is attached) pass through a detection system, typically in the form of a walk-through framework which emits an alternating magnetic interrogation field. This field is designed to interact with a tag prior to deactivation and, in substantially all known prior systems, to cause a warning signal to be emitted in the event that detection of a non-deactivated tag occurs. The present invention relates more particularly to a magnetic antipilferage tag which incorporates 'active' circuitry whereby the tag itself is able to generate an alarm signal when it passes through an interrogating field (e.g. emitted by an interrogating gate) without first having been deactivated at a point of sale by a sales assistant. Thus in contrast to the conventional type of system, in the present invention it is the magnetic tag which generates an alarm in response to an interrogating field, rather than the interrogating gate through which a customer passes at or after leaving a point of sale. For this reason, a magnetic antipilferage tag in accordance with this invention may be termed an "active tag". It is known from GB 2 137 391 A to employ a radio frequency interrogation signal to set off an alarm in a portable electronic article surveillance device. It is also known from US 4,639,716 to provide an alarm packet which releases tear gas or dye when carried through a certain combination of detection fields. However, both these devices have a relatively high power consumption due to the need for a continuous current supply in order to monitor the detection elements within the devices. According to the present invention, there is provided a magnetic antipilferage tag which comprises a housing containing means for detecting an external magnetic field; a power supply; a tone generator; and an electric circuit powered by said power supply and arranged to activate said tone generator in response to an output from said magnetic field detector means characterised in that said means for detecting an external magnetic field comprises a piezoelectric material surrounded about its circumference by a thin layer of magnetostrictive material. The magnetic field detector means advantageously operates by inductive coupling. One or more pick-up coils may be used for this purpose. One form of field sensor provided by this invention comprises a piezoelectric material having disposed about it a magnetostrictive material such that in the presence of a magnetic field the magnetostrictive material imparts compression or tension to the piezoelectric material, thereby generating an electrical output from the piezoelectric material. Since the electrical output of the piezoelectric material is dependent on the stress imparted to it by the magnetostrictive material, and since the dimensional change of a magnetostrictive material is proportional to the magnetic field in the environment in which the magnetostrictive material is located, then the electrical output of the piezoelectric material provides a measure of magnetic field strength. The piezoelectric material will conveniently be provided with electrical connections. The piezoelectric material is advantageously in the form of a cylinder or circular disk with the magnetostrictive material disposed about the circumference thereof. Electrical connections can then be provided on opposite faces of the cylinder or disk. Other configurations may also be adopted if desired. The magnetostrictive material need not completely cover the piezoelectric material or that surface of the piezoelectric material with which it is in contact. Nevertheless, a band of magnetostrictive material surrounding the piezoelectric material is preferred. The magnetostrictive material can be deposited by any suitable technique onto the surface or onto surface regions of the piezoelectric material; for example, the magnetostrictive material can be deposited about the circumference of a cylinder or disk by a vapour deposition process, e.g. sputtering. Preferably, the electric circuit in the tag of this invention is a low-power CMOS integrated circuit. The tone generator is preferably a piezo-electric sounder; suitable devices of this type are available commercially from a number of manufacturers (eg Murata and Toko of Japan), either as unmounted units, or fitted to resonant acoustic enclosures. They provide high audio output and efficiency together with small size and low weight. A typical device can generate a sound pressure at resonance of more than 80dBA at one metre while consuming less than 10mWatts. In one beneficial embodiment, a resonant acoustic enclosure for a piezo-electric tone generator is moulded into the overall casing of the tag. Once activated the active tag will continue to emit an alarm tone until the battery is exhausted or the tag is disabled. It is clearly undesirable to have an easily accessible disabling switch, and in one preferred embodiment the electric circuit within the label is arranged to detect a specially modified form of the interrogation signal in such a way as to reset the device to its untriggered state. An example of a simple "deactivation" signal would be a carrier at the interrogation frequency, amplitude modulated with a fixed mark/space ratio. Clearly many other forms of modulation could be used, complex types giving high security against unauthorised disablement by technically knowledgable thieves. Preferably, the active tag also comprises means allowing removable attachment of the tag to an article of merchandise. In one embodiment the attachment means is able to interact with the circuitry within the tag whereby unauthorised removal of the tag from the item of merchandise activates the tone generator to sound an alarm. Authorised removal would be in the presence of the deactivating signal described above, thereby preventing the alarm being given. The active tag may also be constructed in such a manner that penetration of the body of the tag, crushing the tag or violent shock results in electrical connections being made or broken, these in turn activating the alarm. For a better understanding of the invention, and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which: Figure 1 is a block diagram of one embodiment of an active tag of this invention; Figure 2 is a circuit diagram corresponding to Figure 1; Figure 3 illustrates a typical construction for an inductor forming part of the tag; and Figure 4 is an illustration of one type of magnetic field sensor for use in the invention. Referring now to Figures 1 and 2 of the drawings, the active tag comprises a magnetic field detector 1 in the form of three pick-up coils 1 (of which only one is shown in the drawings). The output from the pick-up coils 1 is fed to a thresholding and modulation detection circuitry 2. Here, the output from pick-up coils 1 is amplified and, when the signal exceeds a predetermined threshold, a rectified output signal is fed to a control logic unit 3 and an alarm driver 4. When activated, alarm driver 4 generates a tone signal which is fed to a piezoelectric loudspeaker 5. In use, the tag is designed to be attached to an article of merchandise by means of an attachment pin 6 which closes contacts 7, thereby rendering the tag operative. Anti-tamper switches 8 and 9 are also included; these function to activate the alarm driver 4 if the tag is damaged or improperly removed from the merchandise which it is protecting. Switch 8 may be located, for example, so that its contacts are opened if the tag is torn from the merchandise; switch 9 is located so that an attempt to crush the tag will close its contacts. The result, in each case, is actuation of alarm driver 4. The power supply within the active tag is preferably a miniature long-life battery 10) see Figure 2). Particularly suitable types are alkaline or lithium button cells, the former having shelf lives of 2 years, the latter 5 years. Using suitable low power electronic design, a cell with a capacity of 50mAh will typically power the untriggered tag for periods in excess of the cell's shelf life. In the event of the tag being triggered this cell will provide many minutes of alarm. Power consumption during emission of an alarm signal can be reduced by incorporating a circuit which causes the tone signal to 'bleep' - this may be done, for example, by interposing a 2Hz oscillator circuit between the control logic and the alarm driver. This further extends the alarm operating time. The basic circuit of Figure 3 comprises invertors I₁, I₂ and I₃; capacitors C₁ - C₇, of which C₁ is the capacitor 11 of Figure 1; resistors R₁ - R₇; diodes D₁ - D₄; D-type flip-flops FF₁ and FF₂; transistor T₁; and piezosounder 5. In addition, a 2Hz oscillator circuit comprises invertors I₄ - I₆; capacitor C₈; resistors R₈ - R₁₀; and diode D₅. Power is supplied by the 3v lithium button battery 10. In use the pick-up coil or coils 1 are arranged to couple inductively with an alternating magnetic field, generated, for example by an interrogating gate (not shown) which includes a coil or loop (typically enclosing an area of several square feet) connected to an alternating current generator. Preferably the alternating current is in the frequency range 1-10kHz. The amplitude of the magnetic field created in this way diminishes very rapidly with distance from the coil or loop thereby giving a well defined interrogation zone, and there is no significant radiation of a propagating electromagnetic signal. Certain designs of pick-up coil are particularly advantageous for this application. In particular a spiral coil manufactured by photolithographic and etching techniques, such as are used in the production of printed circuit boards, is both cheap to manufacture, and convenient from an assembly viewpoint. Another particularly beneficial configuration is illustrated in Figure 3. This uses a high-value, high "Q" ferrite cored inductor, resonated with a suitable tuning capacitor 11 (see Figure 1) at the interrogator frequency. Suitable devices are available commercially from manufacturers such as Toko of Japan. A typical unit has an inductance of 1.5 Henry and a Q of 30 at 5KHz. These units achieve their high inductance largely because the ferrite core material 122 forms a closed loop around the coil windings 13. The effective permeability of the core is thus very high. In theory a closed magnetic core has very low coupling to external fields. However, it has been found that the non-uniform cross-section and form of certain cores causes appreciable external coupling, and a usefully large signal can be developed across the coil, especially at resonance. As an example, one particular 1.5 Henry inductor, resonated at 5kHz, provided an open circuit voltage of 2 volts peak to peak in an alternating 5kHz magnetic field of 20 Amps/metre. To achieve omni-directionality a minimum of three coils is necessary, positioned in mutually orthogonal directions. In this instance it is additionally beneficial to mount the coils in close proximity, and such that the ferrite cores of the different inductors interact in such a way as to further distort the uniformity of the individual magnetic circuits. In this way the received signal amplitude can be further increased. Figure 4 illustrates a simple embodiment of a magnetic field detector in accordance with this invention. This device may be used in the tag in place of the pick-up coils described above. A right circular cylinder 31 is formed of a piezoelectric material and is surrounded about its circumference by a thin layer 32 of a magnetostrictive material. Electrical contacts 33 and 34 are attached to the material 31 to allow the electrical output to be measured, this being proportional to the magnetic field strength prevailing at the time and place of measurement. In another aspect the invention provides an antipilferage system comprising an active tag as defined hereinabove, and an interrogating gate comprising a coil of electrically conductive material and an alternating current generator connected to said coil.

Claims: A magnetic antipilferage tag which comprises a housing containing means for detecting an external magnetic field; a power supply; a tone generator; and an electric circuit powered by said power supply and arranged to activate said tone generator in response to an output from said magnetic field detector means, characterised in that said means for detecting an external magnetic field comprises a piezoelectric material surrounded about its circumference by a thin layer of magnetostrictive material. A tag as claimed in claim 1, wherein said tone generator is a piezoelectric tone generator. A tag as claimed in claim 1 or 2, wherein a resonant acoustic enclosure for said tone generator is provided in the housing. A tag as claimed in any preceding claim, wherein said electric circuit is a low-power CMOS integrated circuit. A tag as claimed in any preceding claim, which further comprises attachment means whereby the tag can be attached to an article of merchandise, and which serves to activate the tone generator in the event of unauthorised removal of the tag. An antipilferage system comprising a tag as claimed in any preceding claim, and an interrogating gate comprising a coil electrically conductive material and an alternating current generator connected to said coil.

Inventors: CROSSFIELD, MICHAEL DAVID
Applicants: ESSELTE METO INTERNATIONAL GMBH
Application year: 1989
Main classification: G08B13
IPC codes: G08B13