METHOD OF DEACTIVATING AN ARTICLE SURVEILLANCE SENSOR

Abstract: An article surveillance sensor (21) comprises a non-crystalline, metallic sensor element (22), which in an active state is arranged to be magnetically or electromagnetically excited and detected, respectively, in an article surveillance system (10-18). The sensor is deactivated by the provision of thermal energy to the sensor element, so that the temperature of the sensor material exceeds its crystallization temperature and at least parts of the sensor element are crystallized. The sensor (21) is arranged together with a monitored article (20) in a deactivating volume (30). Essentially the entire deactivating volume is exposed to a magnetic or electromagnetic field, which is generated by external means (40; 50, 52, 54) and which varies in time, for contactless generation of thermal energy in the sensor element (22).

Description: METHOD OF DEACTIVATING AN ARTICLE SURVEILLANCE SENSOR Technical Field The present invention relates to a method of deactivating an article surveillance sensor, comprising a non- crystalline, metallic sensor element, which in an active state is arranged to be excited and detected, respectively, in an article surveillance system by magnetic or electromagnetic means, wherein thermal energy is supplied to the sensor element at the deactivation thereof, so that the temperature of the sensor material exceeds the crystalliza- tion temperature and at least parts of the sensor element crystallize . Description of the Prior Art There is still an increasing need within business and industry for simple and yet reliable systems for remote article surveillance within a given area. A common example is electronic article surveillance systems for e.g. shops, where the monitored articles or objects are provided with a respective article surveillance sensor and where the system is arranged to detect the presence of this sensor and to provide an appropriate alarm signal, when an object is removed, without authorization, from for instance the shop premises . According to a common type of such electronic article surveillance systems, each article is provided with a small label, comprising a thin metal strip or metal wire with magnetic properties. At either sides of the shop exit arc- shaped means are placed, which generate an intermediate alternating magnetic field. When an object, which has been provided with an article surveillance label according to the above, is carried in between the arc-shaped means, the metal element is affected by the magnetic field, wherein a detectable physical change occurs in the former. Use is generally made of the fact that the alternating magnetic field causes a periodical switch of the magnetic dipole momentum in the metal element - known as Barkhausen jumps. An article surveillance system of this kind is shown in US- A-4 660 025, where a sensor or marker comprises a sensor element of an amorphous magnetic metal alloy, which exhibits a large Barkhausen discontinuity. Another common type of article surveillance sensor has a sensor element of an amorphous metal alloy with magnetic properties and high magnetostriction. The magnetostriction or magnetoelasticity of the material gives rise to a mechanical oscillation in the longitudinal direction of the element, when excited by an alternating magnetic field in a surveillance zone. The mechanical resonance is detected inductively in the surveillance arcs, and the presence of the article between the arcs may be determined. Examples of such sensors are shown in EP-B-0 096 182 and WO88/01427. Yet another type of article surveillance sensor is provided with an electric resonant circuit, comprising an inductive element and a capacitive element, together forming a LC-circuit. Proximate to the inductive element an element of an amorphous magnetic metal alloy is arranged. The material of the magnetic element has a permeability, which depends on the magnitude of an applied magnetic field, and by varying the field strength of this magnetic field, the inductance of the inductive element may be controlled and thereby also the value of the resonant frequency of the LC-circuit. The sensor is excited to oscillation by excitation means arranged in the surveillance zone, and the resonant frequency of the circuit is detected inductively or electromagnetically. An article surveillance sensor of this type is shown in W093/14478. Recently a more advanced article surveillance system has been developed by us. This type of system operates by electromagnetic communication at radio frequencies by means of sensors, comprising an amorphous wireshaped element of for instance a cobalt-rich metal alloy. The wireshaped element may advantageously be located in an ink- filled and, from other aspects, conventional antipilferage ampoule made of glass or similar material. The sensor is excited by a radio signal at a frequency adjusted to the length of the element, and the radio-frequency reply signal transmitted by the sensor is modulated in amplitude by a magnetic modulating field generated in the surveillance zone (for instance the shop exit) . By detecting this modulated radio signal the presence of the sensor (and consequently the monitored object) may be determined in the surveillance zone. Alternatively, the sensor may simply comprise a portion of a very thin amorphous metal wire (also known as microwire) , which thanks to its small diameter (of the order of 10 μm) may be easily located at an arbitrary position in or at the monitored object. If for instance the object is an article of clothing, such as a shirt, the sensor may be sewn into a collar tip or a cuff, or may alternatively be woven into the cloth of the shirt, attached to a brand label in the shirt, etc. All sensor types described above use a sensor element, comprising an amorphous, or in some cases nano- crystalline, metal alloy. For natural reasons it is necessary to be able to deactivate such sensors, for instance once a customer has paid for the article. Some of the sensor types described above are not intended to be removed from the monitored object. This is the case particularly for such applications, where the sensor is comprised by an adhesive label or a thin microwire according to the above. In these situations it is all the more important to be able to easily and accurately deactivate the sensor, so that an honest customer will not be embarrassed, at a later stage, by an unjustified alarm at a later return to the same shop. US-A-4 686 516 relates to a method of deactivating article surveillance sensors according to the above. The deactivation is effectuated by supplying thermal energy to the atomically disorganized, for instance amorphous, sensor element to such an extent, that the temperature of the sensor element exceeds the crystallization temperature of the material . According to a preferred embodiment the thermal energy is supplied by connecting an electric current source at a specific point of the sensor element. By conducting an electric current through the sensor element, the element material is crystallized at this point. As a consequence, the sensor element will lose its signal properties, i.e. it will become deactivated. In an alternative embodiment the sensor element is irradiated with laser light at a specific point, wherein the electric resistivity of the sensor material is locally increased. When an electric current is subsequently conducted through the sensor, the heat generation will be larger at this point. Yet another alternative is to locally expose the specific point of the sensor element to radiation of heat from a heating gun. This embodiment is useful for such sensors, which has a built-in mechanical bias. Since the heat is applied locally, the mechanical bias is released, wherein the magnetic reply characteristics of the sensor element are considerably modified, i.e. the sensor is deactivated. The aforesaid patent publication briefly mentions that deactivation might occur by supplying "radiant energy" without subsequent application of electric current, but no further teachings are given. The drawback of a deactivation procedure according to US-A-4 686 516 is, in particular, that the crystallization is concentrated to a specific position of the sensor, thereby requiring that the deactivating equipment and the sensor are located proximate to each other. Furthermore, the method appears less useful in such cases, where the sensor element is arranged inside a sensor body, which isolates the sensor element from the environment . The method is particularly unuseful in those situations, where the exact position of the sensor within the object is not known, which for instance is the case for the microwires described above, when located in articles of clothing. Summary of the Invention The object of the present invention is to provide a more effective and yet simpler deactivation of sensors in an article surveillance system as described above. The present invention is particularly aimed at providing deactivation without requiring an immediate proximity between the sensor and the deactivating equipment, and without requiring an exact knowledge of the location of the sensor element in relation to the monitored object. A further object of some embodiments of the invention is to allow deactivation without any risk of damages to the monitored object due to the increase of temperature in the sensor element . The objects above are achieved by a method of deactivating an article surveillance sensor, comprising a non- crystalline, metallic sensor element, which in an active state is arranged to be excited and detected, respectively, by magnetic or electromagnetic means in an article surveillance system, wherein the sensor element is deactivated by supplying thermal energy to the element, so that the temperature of the sensor material exceeds its crystalliza- tion temperature and at least portions of the sensor element are crystallized. The sensor is placed together with the monitored article in a deactivating volume, wherein essentially the whole of the deactivating volume is exposed to an alternating magnetic or electromagnetic field for contactless generation of thermal energy in the sensor element . Preferred alternatives to the method according to the present invention are defined by the appended patent claims . Brief Description of the Drawings The present invention will now be described in more detail, reference being made to the accompanying drawings, in which: FIG 1 is a schematic block diagram of an article surveillance system using sensors that may be deactivated by the method according to the present invention, FIGs 2 and 3 are exemplary views of articles of clothing, which have been provided with article surveillance sensors, FIG 4 illustrates the deactivating approach according to one embodiment of the invention and FIG 5 illustrates another deactivating embodiment according to the invention. Detailed Disclosure of the Invention An exemplary article surveillance system, which uses amorphous or nano-crystalline sensor elements, will now be described with reference to FIGs 1-3. The deactivating method according to the present invention will thereafter be described with reference to FIGs 4 and 5. Even though the disclosure below is made with reference to an article surveillance system of the type shown in FIG 1, the deactivating method according to the present invention is applicable also to other types of article surveillance systems, particularly the types that have been summarized above in the section Description of the Prior Art. FIG 1 illustrates an article surveillance system for detecting the presence of an object in a surveillance zone 10. Each object to be monitored is provided with an article surveillance sensor 21, which comprises an amorphous or nano-crystalline sensor element 22. The sensor 21 may for instance comprise an essentially conventional ink-filled glass ampoule, in which a wireshaped sensor element 22 has been arranged. Alternatively, the sensor 21 may be of any other commercially available type of sensor. A transmitter antenna 11 and a receiver antenna 12 are connected to a controller 14 through driving stages 13 and 15, respectively. The driving stage 13 is arranged to supply a high- frequency electric current to the transmitter antenna 11, wherein a high-frequency electromagnetic field is generated around the antenna with propagation in essentially the entire surveillance zone 10. The electromagnetic field is used for exciting a sensor 21 present in the surveillance zone 10, so that the sensor will receive electromagnetic energy from the transmitter antenna 11 and in response transmit an electromagnetic reply signal, which is received by the receiver antenna 12. A reply signal received by the receiver antenna 12 is amplified and signal processed in the driving stage 15, and the signal is then supplied to the controller 14. Furthermore, the surveillance zone 10 is provided with a magnetic field generating means 16, preferably in the form of a coil arrangement . The coil arrangement is advantageously (e.g. for aesthetical reasons) arranged immediately below the ceiling, or between ceiling and roof, in applications where the system is used for antipilferage protection of a shop exit. The means 16 preferably comprises an electric conductor, which is wound in one turn or more than one turn and which essentially has the shape of a rectangle with dimensions of about a few meters in the longitudinal direction and some meter in the transversal direction. The coil arrangement 16 is connected to the controller 14 through a driving stage 17. The driving stage 17 generates a modulating current i mod , which is fed through the coil, wherein a magnetic modulating field H mod is generated around the coil with propagation in essentially the entire surveillance zone 10. This modulating current is given a known variation in amplitude as a function of time and corresponds, in its simplest form, to a pure sine-wave of frequency f mod but which alternatively may be given a more complex dependence. A correspondingly varying magnetic field is generated by the varying modulating current. The sensor 21, which according to the above is attached to a monitored object, such as an article of clothing, comprises a wireshaped element of an amorphous or nano-crystalline metal alloy with magnetic properties. Preferably, the wireshaped element 21 is a "microwire" with a diameter less than 30 μm. The composition of the metal alloy is such that the permeability of the material depends on the magnitude of the magnetic field generated by the coil arrangement 16 in the longitudinal direction of the element. Furthermore, use is made of the "Giant Magneto- impedance" effect in such wireshaped elements, when they are exposed to the high-frequency electromagnetic excitation field. The length of the element 22 is adjusted to the frequency of the electromagnetic excitation field, so that an electric current is induced in the wireshaped element, when the sensor 21 is present in the surveillance zone 10. The induced current flows back and forth through the wireshaped element 22, wherein an electromagnetic field is generated around the element, which propagates through the zone 10 and eventually reaches the receiver antenna 12 as a reply signal. Hence, the element 22 has a concurrent function as receiver as well as transmitter of electromag- netic signals. The permeability of the material of the wireshaped element is controlled by the magnetic modulating field. More specifically, the amplitude of the reply signal is controlled through the "Giant Magnetoimpedance" effect (or the "Skin-Depth" effect) in the element material. The impedance Z of the wireshaped element 22 is a function of the square root of the permeability μ, the frequency f and the resistivity p of the element material, and the amplitude of the current through the conductor will change as a function of permeability due to the dependence between impedance and permeability. Consequently, the amplitude of the current, which is induced in the sensor element 22, will vary according to the variations of the magnetic modulating field. This also means that the electromagnetic reply signal transmitted from the element 22 will be a signal, the amplitude of which is modulated by the frequency of the modulating field and the carrier frequency of which corresponds to the frequency of the electromagnetic excitation signal. In other words the sensor element operates as a mixer of a high-frequency electromagnetic signal and a low-frequency magnetic signal. Presence of such a modulation in amplitude in the received reply signal will consequently indicate presence of a sensor element 22 (and accordingly also the presence of the monitored object) in the surveillance zone 10. Thanks to the extremely thin element 22 (as a comparison, a normal human hair grown on the head has a thickness of 100-300 μm, i.e. about 10-30 times thicker than the sensor element 22) , the sensor element may be arranged at a virtually unlimited number of different positions on or in the monitored object. In particular, the sensor element may be arranged at such positions, which are unavailable or hidden from the human eye. Obviously, it is also possible to provide the object with several sensor elements 22, which are located at different positions for improving the accuracy of the detection. FIGs 2 and 3 illustrate a monitored article of clothing 20 in the form of a shirt. According to FIG 2 the sensor element 22 may be sewn into for instance a collar tip 26, a cuff, a clothing label or the like. Alternatively, the sensor element 22 may be sewn or woven into the cloth 28 of the shirt 20, as illustrated in FIG 3. The deactivation of the sensor element 22 is done in the following way according to the present invention (see FIGs 4 and 5) . The monitored object is placed in a deactivating volume 30, in which magnetic or electromagnetic energy is generated by external means, for contactless generation of thermal energy in the sensor element. In the example of FIG 4 the deactivating volume 30 is arranged as a recess, essentially formed as a parallelepiped, in for instance a cash desk 24. A means 40 for generating microwaves 32 is arranged proximate to the deactivating volume 30. The means 40 may be any commercially available device, such as a common magnetron of the type used in household microwave ovens. The means 40 is arranged to generate microwaves 32, which will be given an essentially homogeneous propagation in the whole of the deactivating volume 30 through reflections against the walls of the deactiva- ting volume 30 (cf. the operation of a normal microwave oven) . The object 20, which is illustrated in FIG 4 as a pair of shorts, is as an example provided with two sensor elements 22a and 22b, respectively, which are located at different positions in the article of clothing. When a shop customer has decided to buy the article of clothing 20, he or she will approach the cash desk 24 and pay for the article. A shop assistant will then place the article of clothing 20 in the deactivating volume 30, wherein the means 40 will be activated. The means 40 may for instance be activated by the shop assistant by actua- ting a current switch, or by using photo detectors etc. Furthermore, the deactivating volume may be provided with a type of cover, which is shut above the article of clothing 20, so as to form a completely enclosed deactivating volume . The microwaves 32 will induce electric currents in the wireshaped elements 22a and 22b. In response, these currents will cause a rapid increase of temperature, and provided that the generation of microwaves is continued during a certain period of time, the temperature of the sensor material will eventually exceed its crystallization temperature, wherein the material is crystallized and deactivated. Since the sensor elements 22a and 22b are preferably realized as extremely thin microwires of a very small mass, the elements will be heated to crystallization very quickly. In this way damages are avoided for surrounding materials, for instance the cloth of the article of clothing 20. The method has an additional advantage in that an exact knowledge of the position of the sensor element in the article of clothing 20 is not required, since all parts of the article of clothing are only exposed to harmless microwaves . The method according to the present invention is particularly well suited to be used for such sensors, which comprise a very thin core of amorphous metal alloy (micro- wires) , which is surrounded by a coating of dielectrical material, such as glass. More specifically, the dielectric coating isolates the sensor element thermally, thereby further reducing the risk of heating damages to surrounding material. Since the thermal losses are substantially reduced, the sensor element will furthermore be heated to crystallization temperature more rapidly. An alternative embodiment of the invention is shown in FIG 5. The same reference numerals have been used for parts which are identical in FIGs 4 and 5, and the description thereof is not repeated now. The difference for FIG 5 is that a magnetic field generating means 50 is arranged in the deactivating volume 30, preferably at the bottom there- of. The magnetic field generating means 50 is for instance an electric conductor, which is wound in one turn or more than one turn and is connected to a controller 54 through a driving stage 52. The driving stage 52 supplies an electric alternating current through the means 50, thereby genera- ting an alternating magnetic field 42, which only has been illustrated at one corner of the deactivating volume for reasons of clarity, but which naturally is present in essentially the entire deactivating volume 30. The magnetic field 42 generates eddy currents in the sensor elements 22a and 22b, wherein thermal energy is supplied to the elements and wherein the elements eventually will reach a temperature exceeding the crystallization temperature and therefore become deactivated, in a way corresponding to that of FIG 4. The principle may be compared to the procedure used for inductive heating in a conventional inductive oven. According to yet another alternative embodiment of the invention, electromagnetic transmitters and receivers are arranged proximate to the deactivating volume 30. These are arranged to excite and to detect, respectively, the sensor elements 22a and 22b, and the receiver is connected to a controller, which is operatively connected to the microwave generating means 40 or the magnetic field generating means 50, respectively. The controller is arranged to continuously monitor the signal strength of the electromag- netic reply signal received from the sensor elements 22a-b. As soon as this signal strength is below a predetermined limit, the controller will generate control signals to the means 40 or 50, respectively, wherein the generation of deactivating energy stops. According to this alternative method, the time required for completely deactivating the elements 22a and 22b is reduced, thereby further minimizing the risk of damages to the object 20 in response to locally increased temperatures around the sensor elements. The present invention has been described above by way of a few embodiment examples. However, it is to be emphasized that the invention is applicable also for other embodiments not shown herein. Consequently, the invention is only limited by the scope of invention as defined by the appended independent patent claim.

Claims: CLAIMS 1. A method of deactivating an article surveillance sensor (21) , comprising a non-crystalline, metallic sensor element (22) , which in an active state is arranged to be magnetically or electromagnetically excited and detected, respectively, by an article surveillance system (10-18) , the sensor element being deactivated by the provision of thermal energy, so that the temperature of the sensor material exceeds the crystallization temperature and at least parts of the sensor element are crystallized, c h a r a c t e r i z e d by the steps of arranging the sensor (21) together with a monitored article (20) in a deactivating volume (30) and exposing essentially the entire deactivating volume to a magnetic or electromagnetic field, which is generated by external means (40; 50, 52, 54) and which varies in time, for contactless generation of thermal energy in the sensor element (22) . 2. A method according to claim 1, c h a r a c t e r i z e d in that the deactivating volume (30) is irradiated with microwaves (32) . 3. A method according to any preceding claim, c h a r a c t e r i z e d in that the sensor (22) is excited, and the reply signal thereof is detected, and the deactivating step according to preceding claims is ended, when the magnitude of the detected reply signal is less than a predetermined limit. 4. A method according to any preceding claim, c h a r a c t e r i z e d in that the deactivation is made for a sensor element (22) comprising an amorphous metal alloy. 5. A method according to any of claims 1-3, c h a r a c t e r i z e d in that the deactivation is made for a sensor element (22) comprising a nano-crystalline metal alloy. 6. A method according to claim 4 or 5 , c h a r a c t e r i z e d in that the sensor element (22) is of a microwire type, i.e. has a diameter of the order of 10 μm. 7. A method according to claim 4, 5 or 6 , c h a r a c t e r i z e d in that the sensor element (22) is surrounded by a dielectric coating of glass or similar material. 8. A method according to any preceding claim , c h a r a c t e r i z e d in that the monitored article (20) is an article of clothing. 9. A method according to any preceding claim , c h a r a c t e r i z e d in that the deactivating volume (30) is located proximate to a cash desk (24) in a shop.

Inventors: TYREN CARL, TYREN, CARL
Applicants: RSO CORP, TYREN CARL, RSO CORPORATION N.V., TYREN, CARL
Application year: 1998
Main classification: G08B13
IPC codes: G08B13