METHOD OF PRODUCING TAGS COMPRISING RESONANT CIRCUITS WHICH CAN BE ACTIVATED AND DEACTIVATED

Abstract: In order to assure a safe activation and deactivation of resonant circuits on tags used for being placed on or in articles by means of small electromagnetic or electric fields easy to handle, regions are created in the conductive elements forming part of the resonant circuit, especially in the capacitor parts where the insulating material between two conductive elements in a precise manner is made thinner than the material outside these regions. The thinning out of the insulating material is carried out whether by heating and slight pressure or by inserting small pieces of conductive material or non-conductive material with well defined dimensions in the insulating material under the production of the resonant circuit.

Description: Method of producing tags comprising resonant circuits whi can be activated and deactivated ' The invention relates to a method of producing tag which can be activated and deactivated according to th preamble of claim 1. A known and very spread method of securing article against theft - especially in self-service stores - is t employ electronic security systems elaborated, for example with a transmitter for emitting electromagnetic waves and corresponding receiver. The articles are marked with specia tags on which are placed one or more resonant circuit/ built up of a connection between a coil and a capacitor however, several coils and capacitors may be used in th single circuits. Usually, these elements are made of alumi nium on a bearing layer consisting of plastic or simila material having insulating properties, and the conductiv layer may be placed whether on the one side only or on bot sides of the bearing layer, as the bearing layer is actin as a dielectric medium within the used capacitor/ε. Th circuit is supposed to have a high quality factor (Q-value) The transmitter is emitting signals having frequencie which systematically are varied within a specified * rang where the resonant frequency of the resonant circuit i lying, and due to the high Q-value of the circuit, the re ceiver will be able to detect the resonant circuit when th natural frequency of said resonant circuit is emitted. When the articles provided with tags having resonan circuits are passing by the cashes at the exit of the pre mises where the accounts are to be settled, the removal o destruction of the tags has to take place. If this has no happened, the receiver is detecting the attempt to pass th controlled area and actuating an alarm. In order to modify the resonant circuit, it is pos¬ sible whether to bring about specific areas with reduction of the conductive cross-sectional area, said areas can be overbaked more easily by applying a field generating a cur- rent in the circuit than it is the case with other areas in the circuit, or there can be provided regions with an es¬ pecially small distance between, e.g. the capacitor plates so that the field strength necessary to provoke a breakdown and thus a modification of the circuit assumes a limited size. By applying a tag, said tag is exposed to an electro¬ magnetic field in order to detect whether the resonant cir¬ cuit of the tag has a specified natural frequency and ob¬ viously, said field may not cause modifications of the reso- nant .circuit. On the other hand must a field which is applied in order to modify the circuit be able to accomplish that task perfectly at a determined field strength, thus it has to be required that the disctance between the capacitor plates in the area to be destroyed is well defined by a uniform size. It has appeared to be appropriate to apply a thickness upon the dielectric medium located between the capacitor plates in the area to be destroyed of about 3 μm. Minor thicknesses may reduce the Q-value of the circuit. Instead of reduced' thicknesses of the dielectric medium in the capacitor in single areas, the whole plate area of the capacitor could be covered by a dielectric medium of about 3 μ . However, the manufacture of such a dielectric medium is difficult to control with a sufficiently high accuracy. From US Patent Specification No. 4.021.705 is known a method for destroying the coil in the resonant circuit of the tag, as the conductive path in the coil is kept very thin in a chosen area so that by application of a strong field, the path is broken in the region concerned. This way of modifying the resonant circuit practically requires such a strong field that said field, without exten- εive shielding, may affect the surroundings in a disadvan tageous manner and in practice, the method is not applied. US Patent Specification No. 4.498.076 discloses method of producing a* resonant circuit suitable for modifi cation, in as much as a small distance between two capacito plates within the resonant circuit is created by pressin the conductive layer - normally aluminium but other conduc tive materials may be used as capacitor plates as well into the intermediate dielectric medium by means of a pisto so that in a small region, the thickness of the dielectri medium becomes smaller than outside this region and thuε i is possible to generate a breakdown in the capacitor at lower voltage than it is the case outside such a region. This method presents a number of disadvantages, a the compression of the dielectric medium to the size also preferred here, i.e. 3 μ within a limited region requires a very precise angle of 90° between the piston and the plan of the capacitor and a precisely controlled pressure t obtain usable reproducible results. It appears that the compression is performed on a hard rubber base which contributes to another inaccuracy during the production. A more detailed examination of the circuits produced according to the last-mentioned method has shown that a big number of said circuits has already been destroyed during the production phase, thus no well defined thickness of the dielectric medium is procured by means of that method which results in that: - there are big variations in the size of the quality factor, - the energy necessary for the deactivation is not well defined, - the conductive layer is pressed down in such a manner that the thickness of the conductive layer is reduced, which may - as will be explained later on - result in the fact that the capacitor, in spite of sparking breakdown may continue to act as a capacitor (self-healing capacitor) , and the des¬ truction is uncertain, - the area pressed down has to be strongly limited in size as the risk for destroying the circuit during compression increases with the size of the piston, - the base on which the compression is performed is changing nature in the courεe of time, - variations in temperature in the production premises may cause problems when using the tool applied. As thus, no well defined product can be obtained by this method, there is an evident need for another method of reducing the thickness of the insulating layer, whether between two capacitor plates or between other voltage car¬ rying elements in disclosed resonant circuits. The present invention provides a method where by means of heat impact on the insulated materials placed on the tags, regions are established where the thickness of the insulants by a slight pressure is reduced to a predetermined thickness with high precision so that a breakdown can be produced by applying electromagnetic or electric fields with low field strength. The method comprises the heating of one or more smal- ler regions on a capacitor forming part of the resonant cir¬ cuit. Thus, the dielectric medium is softened or possibly melted, and by applying a slight pressure on the heated region, there is with high precision brought ' about a region having the predetermined thickness of the dielectric medium layer. The invention provides a method which is cheap, tech¬ nically simple and easy to control, as the method comprises several parameters, such as temperature, size of the impact region,, time of the impact, ' size of the pressure applied, shape of the impact region, which can be combined in accor¬ dance with the conditions of production. The invention offers a complete liberty of choosi the way of heating as a heating element, laser, ultrasoun microwave or another appropriate process may be used, a finally, it has to be emphasized that the necessary pressu on the dielectric material is substantially lower than t pressures used in known prior art. The production result obtained is well defined wi a minor failure rate, and the thickness of the dielectr layer in the treated regions is procured with a bigger a curacy and uniformity than in the known prior art. Thus, it is also obtained that the field streng necessary for modifying the resonant circuit can be dete mined rather precisely. In connection with the resonant circuits treat according to the invention, the necessary field strength very low, so that a breakdown, for instance, can be produc in the predetermined region with fields which are small than the field of the piezoelectric crystal in a gas ignite An alternative method of providing a region in t dielectric medium where the dielectric medium has the desir thickness, provides a dielectric medium with a hole throug out it. In this hole, a dielectric medium having the desir thickness is placed in such a way that in this area, on one thickness of the dielectric medium is preεent whi corresponds to the size of the desired fields applied, that a breakdown occurs for sure. Instead of the dielectric medium, there can be plac a piece of a conductive material having a suitable thickne and being in conductive electrical connection with the o capacitor plate. The capacitor is finished by co presεive stress. In connection with the above-mentioned method, t hollowing of the dielectric medium could be avoided by pla cing on the one capacitor plate as above a piece of conduc tive material of suitable dimensions in electrical connecti with this capacitor plate, whereupon the dielectric laye under pressure is applied to the capacitor plate and th other capacitor plate whether is applied at the same tim with it or during another operational step. Also these alternative methods are characterized b the high precision in providing regions which are suitabl for breakdown. The invention will be explained in details in th following in conjunction with the accompanying drawing, i which: Fig. 1 is a plane representation of a resonant circui forming part of a tag, Figs. 2 and 2A show a section across a part of capacitor in the circuit of Fig. 1, Fig. 3 shows a section across a number of conductor being part of the winding of the coil in the circuit o Fig. 1, Fig. 4 is a section across the capacitor where th dielectric layer between the capacitor plates has a thickness of 'a proportional size compared with the thickness of the capacitor plates, Fig. 5 is a section across a capacitor where the dielectric layer between the capacitor plates has a rela¬ tively small thickness compared with the thickness of the capacitor plates, Fig. 6 shows a section across a capacitor with a hole in the dielectric medium in which is placed a precisely dimensioned piece of dielectric medium, Fig. 7 shows a section across a capacitor with a hole in the dielectric medium in which is placed a precisely dimensioned piece of conductive material, and Fig. 8 shows a section across a capacitor being as¬ sembled and where a piece of conductive material is pressed into the dielectric medium. The conductive material in the resonant circuit 1 shown in Fig. 1 will preferably be aluminium but other con¬ ductive materials may be used as well. The conductive layer may be produced by acid etching and placed whether on t one side only of a bearing plastic material 2, or on .bo sides of said plastic material. In the latter, the beari material can immediately act as a dielectric medium in t capacitor 3. When the conductive layer is placed only on the o side of the bearing layer 2, a dielectric medium has to placed on the capacitor plate located on the bearing laye whereupon a conductive layer is applied to the dielectr medium. The one end 5 of the coil 4 is directly connect with the one plate in the capacitor 3, while the other e 6 for example is closed by a plate which is connected wi the other plate in the capacitor, for instance as sho with the two dotted lines 4a, by means of leading a condu tive path across the turns of the coil with an insulatin layer between the turns of the coil and the conductive pat or also - in connection with a bearing layer having a con ductive layer on both sides - by means of leading a connec tion through the plastic layer. The conductive layer may also be produced by mean of blank cutting. Fig. 2 shows a cross-section across a part of a capa citor where the capacitor plates 7 formed in a conducti material are separated by a dielectric medium 8. In prede termined areas, or in only one area, this dielectric mediu is subjected to, for instance, a heating element 9, by mean of which a suitable insulating material is inflicted a con sistency which makes it possible to reduce the thickness o the dielectric medium to the intended size by a slight pres sure. The heating may be performed by a heating element bu also other means for energy transfer may be used, such a for instance ultrasound, laser or microwaves. The pressur on the dielectric medium may be performed by the heatin element or by creating vacuum or excess pressure on th surfaces of the capacitor. The impact on the dielectric medium may be exerte after the final assembling of the resonant circuit, as in dicated in Fig. 2, or directly on the one side of the dielec tric medium before the application of the capacitor plate on that side, see Fig. 2a. Fig. 3 shows a part of a section across a coil in a resonant circuit on a tag where ' the single turns in the coil 10 are placed on a dielectric layer 11, and where above a number of turns 10 is located another dielectric layer 12 with the intended small thickness provided by the method explained herein / and on which is placed a conductive layer 13. By means of a suitable field impact, a breakdown will occur and after that, a subsequent short-circuiting of the coil. The plate 13 with the dielectric layer 12 may be placed in any area on the coil, it shall only cover two or more turns 10, or be located above the inlets of the coil. Figs. 4 and 5 show a section through a capacitor for illustrating how such a capacitor can be "self-healing" after a breakdown. Fig. 4 shows a capacitor where the dielectric material 15 is substantially thicker than the plates 16. The holes resulting from sparking breakdown are bigger in the capacitor plates 16 than in the dielectric material 15. Thus, the two capacitor plates 16 continue to be insulated from each other, and therefore, the capacitor continues to act like this and thus, an inεtable short-circuiting is provided. Fig. 5 shows an embodiment of a capacitor which in a stable manner is destroyed by a deactivation signal. Here, the dielectric layer is much thinner than the capacitor plates 16, and the hole in the metal plates 16 after the breakdown is smaller than the hole in the dielectric layer 15, and therefore, a stable short-circuiting is provided. Finally, Figs. 6, 7 and 8 show alternative methods of producing an element in the reεonant circuit for activa¬ tion and deactivation on tags, in as much as uniform parts have the same numeral reference. The dielectric medium 18 in Fig. 6 having a thickness of e.g. 10 μm is provided with a hole 19 wherei is placed a small piece of a dielectr medium 20 with the intended thickness, e.g. 3 μm. The diele tric constant for the air iε εo little that the air in th part of the hole 19 which iε not filled out with the diele trie medium 20 has no importance in comparison with t corresponding size of the dielectric material. Instead of the dielectric medium, a strip of condu tive material 21 in electric connection with the capacit plate 17 could be inserted, as εhown in Fig. 7, in order obtain the intended precise distance between the capacit plates 17. Finally, Fig. 8 εhows a method where a strip of co ductive material 21 being in conductive connection with t plate 17 by pressure is pressed into the dielectric materi 18 in such a manner that again, the intended preciεe distan between the capacitor plates 17 is obtained. Under certain circumstances, it could be desirab to be able to activate the disclosed resonant circuits aft the placing of the tags on or in the articles which sha be secured. This may be done in the same way as it iε do for the deactivation of the reεonant circuit, as the resona circuit during the assembling of the tag has a natural fr quency which iε sheared in relation to the natural frequen after the activation of the circuit. By inflicting a fie having a lower power than the destruction field, if t deactivation of the tag εhall also be carried out, a conne tion'between two ends can be established by a disconnect flow path, so that the missing connection is established. Hereafter, the resonant circuit may again be cut o with a stronger field, as explained before. By activati the resonant circuit, a modification of the natural frequen of the circuit occurs. Even if the circuit is not activated a resonant circuit will be created by scattered capacit Therefore, the deactivation of the tag may be perform with two frequencies in order to detect the state of th tag.

Claims: PATENT CLAIMS. 1. Method of producing tags comprising resonant cir cuits with at least one coil and at least one capacitor said resonant circuits being able to be activated and deac tivated, and where the tags are especially adapted to b placed on or in the articles for which the accounts have t be settled when they are leaving sales premises, the contro of which is performed by a system including a transmitte for electromagnetic waves and an appropriate receiver, wher the transmitter is emitting signals with several frequencies, said frequencies are pasεing the natural frequency of th reεonant circuit and where the receiver iε recording th presence of such a resonant circuit, and where the dielec tric material located between the capacitor plates in one or more areas has a thicknesε which iε lower than the thick¬ ness of the conductive material, characterized in that by heat impact on the dielectric materials (2, 8, 15) placed on the tag are provided regions where the thickness of the heated dielectric material (8) by means of a slight pressure may be reduced so much that a breakdown will be produced by the application of electromagnetic or electric fields with low field strength. 2. Method according to claim 1, characterized in that the regions for activating the resonant circuit (1) are provided in such a manner that by breakdown is created a connection by means of which the coil (4) is connected with the capacitor (3) in such a manner to create a resonant circuit (l) having a natural frequency which differs from the natural frequency said resonant circuit had before ac- tivation. 3. Method according to claim 1, characterized in that for deactivation of the reεonant circuit (1) , regions between the capacitor plates (7,16) in the resonant circuit (1) are created with a thickness of the insulating layer (8, 15) between the plates which iε minor than outside these regions, as the resonant circuit is destroyed by short-cir- cuiting after breakdown of the capacitor plates (7,36). 4. Method according to claim 1, characterized that for deactivation of the reεonant circuit, a plate (13 of conductive material is provided opposite to an insulatin layer (12) with a thickness which is minor than the othe insulating layers (11) across a number of conductors (10 in the coil so that a number of the turns of the coil i short-circuited by breakdown across the layer 12. 5. Method according to claim 1, characterized i that a deactivation region in the resonant circuit (1) i created by letting pass t e lead-in wires to the coil (4 closely to each other. 6. Method of producing tags comprising resonant cir cuits with at least one coil and at least one capacitor said resonant circuits being able to be activated and deac tivated , and where the tags are eεpecially adapted to b placed on or in the articleε for which the accounts have t be settled when they are removed form sales premiseε, th control of which iε performed by a εyεtem including a trans mitter for electromagnetic waves and an appropriate receiver where the transmitter is emitting signals with several fre quencies, said frequencies are pasεing the natural frequenc of the resonant circuit and where the receiver is recordin the presence of such a resonant circuit, and where the di electric material located between the capacitor plates i one or more areas has a thickness which is lower thant th thickness of the conductive material, characterized in tha one or more holes (19) are provided in the dielectric mediu (18) , where iε placed a ε all piece of dielectric mediu (20) having an uniform thickneεε suitable for activatio and deactivation, after which the capacitor plates (17) ar placed on the dielectric medium (18) . 7. Method according to claim 6, characterized i that one or more holes (19) are provided in the dielectri medium (18) where a small piece of conductive materia (21) having a thickness suitable for activation and deacti vation in electrically conductive connection with the one capacitor plate (17) , after which the capacitor plates (17) and the dielectric medium (18) are assembled. 8. Method according to claim 7 , characterized in that one or more pieces of conductive material (21) having a thickness which is suitable for activation and deactivation are placed in an electrically conductive connection with the capacitor plate (17) , after which, with a slight pres¬ sure, they are preεεed into the dielectric material (18) in connection with the aεsembling of the capacitor plates (17) and the insulating material (18) .

Inventors: JOERGENSEN POUL RICHTER, JOERGENSEN, POUL, RICHTER
Applicants: JOERGENSEN POUL RICHTER, JOERGENSEN, POUL, RICHTER
Application year: 1991
Main classification: G08B13
IPC codes: G08B13