A FLEXIBLE DETECTION LABEL

Abstract: A flexible detection label (1) for an electronic detection system, comprising a thin, platelike flexible carrier (2) having on both sides elements made from flexible material, jointly forming at least one resonant circuit. The resonant circuit includes at least two conductor tracks (9, 13) formed as coils (17, 18), said tracks being insulated from one another and disposed on opposite surfaces of the carrier, each track having two ends connected to a conductive region forming a capacitor electrode. The two capacitor electrodes (8, 10) on one surface of the carrier are exactly opposite corresponding capacitor electrodes (11, 12) on the other surface of the carrier, and opposite capacitor electrodes are substantially equally large.

Description: This invention relates to a flexible detection label for an electronic detection system, comprising a thin, platelike flexible carrier having on both sides elements made from flexible material, jointly forming at least one resonant circuit, wherein the resonant circuit includes at least two conductor tracks formed as coils, said tracks being insulated from one another and disposed on opposite surfaces of the carrier, each track having two ends connected to a conductive region forming a capacitor electrode, wherein the two capacitor electrodes on one side of the carrier are located opposite corresponding capacitor electrodes on the other side of the carrier. Such labels may be used e.g. in a shop-lifting detection system, as described e.g. in US patent 4,686,517 (Nedap). The resonant circuit includes at least one coil and at least one capacitive element, jointly forming the operating frequency of the label. The resonant circuit may consist e.g. of a flexible coil of metal foil on a sheet of paper or on a foil of synthetic plastics material, functioning as a carrier. The coil may be made e.g. by etching from aluminum foil or by means of printing techniques or any of the other techniques known therefore. The coil is connected to a capacitor whose one electrode is disposed on the same surface of the carrier as the coil, and is connected to one end of the coil. The other electrode of the capacitor is provided on the other surface of the carrier and is connected to the other end of the coil by means of an interconnection through the carrier or around the edge of the carrier. W.O.-A-83/01697 (Reeb) discloses a flexible identification label consisting of a conductor configuration provided on one of the surfaces of a carrier of paper, synthetic plastics material or cardboard, said configuration extending on both sides of at least one fold line and having a conductor track bridging the fold line. By doubling up the carrier with the conductor configuration thereon along the fold line, there is produced an assembly of two opposite sub-configurations, each forming a coil having a capacitor face on at least one end. In the folded condition, the capacitor faces of the two sub-configurations form one capacitor. The two sub-configurations are electrically interconnected by the conductor track bridging the fold line. A drawback of this known label is that the electrical properties of such a folded label depend strongly on the exact location of the fold line. Consequently, the fold line should be defined as accurately as possible, which is effected by means of perforations or ridges. However, this entails the risk that the conductor track bridging the fold line is interrupted entirely or partly. Furthermore, during the further handling of such a label, a conductor track bridging an outer edge is rather vulnerable. WO-A-86/03052 (MMM) describes various techniques for defining the fold line of a folding label as accurately as possible. However, the above described drawbacks apply just as well to the labels described in that publication. It is observed that both WO-A-83/01697 and WO-A-86/03052 describe embodiments wherein the sub-configurations on opposite sides of the fold line are not connected by a conductor track bridging the fold line. In the folded condition, the two sub-configurations are then coupled exclusively capacitively. In the known configurations, however, this capacitive coupling is defined substantially by the conductor tracks forming the coils. Sometimes, even, the parasitic capacitive coupling of the conductor tracks forming the coils is used only. Needless to say that in that case, even a very small deviation during folding may result in a substantial variation in the resonate frequency of the label. Moreover, the coil and the capacitive element can then no longer be designed independently from one another in an optimal manner, given the desired resonate frequency. WO-A-83/01697 shows an embodiment wherein opposite spiral conductor tracks have inner ends enlarged to form a capacitor electrode. As a result, true, the freedom of design is somewhat increased, but the effective coil surface area is reduced. The major problem of the construction of these flexible labels is to ensure that the effective coil area is as large as possible with slight outer dimensions, as well as to achieve that the quality factor of the circuit is as high as possible. The effective coil area defines the coupling factor with the transmitter/receiver coil of the detection system and hence the detection sensitivity. The loss resistance in the coil, i.e. the resistance of the tracks, defines substantially the quality factor of the resonant circuit. A high quality factor requires a low resistance of the tracks and hence a large track width. Broad tracks, however, result in a smaller effective coil area. It is in an object of the present invention to provide an improved detection label. To that effect, according to the present invention, a flexible detection label of the above described type is characterized in that opposite capacitor electrodes are substantially equally large, and in that the capacitor electrodes and the coil-forming conductor tracks are positioned in such a manner that opposite sections of the conductor tracks on the two carrier surfaces have, in operation, substantially the same electrical potential. The invention will now be described in more detail with reference to the accompanying drawings, in which: Fig. 1 is a diagrammatic cross section of a detection label according to the present invention; Fig. 2 is a diagrammatic top view of an example of a detection label according to the present invention; Fig. 3 is a bottom view of the detection label shown in Fig. 2; and Fig. 4 shows an electrical diagram representing a detection label according to the present invention. Fig. 1 is a diagrammatic, part-cross-sectional view of a flexible detection label 1 according to the present invention. The label 1 comprises a carrier 2 made from an electrically insulating material. Carrier 2 is plate-shaped and has a small thickness. The carrier is made from a material that is flexible, at least at the plate thickness chosen. The carrier may consist of synthetic plastics foil, but may also be made from paper or cardboard. On the two sides of carrier 2 there is formed a track pattern likewise of flexible, conductive material, such as aluminum foil. The respective conductive foil films are indicated in Fig. 1 at 3 and 4. On one side, there may further be provided a paper film or a film of synthetic plastics material, as shown in Fig. 1 at 5, and on the other side, e.g. an adhesive film, as indicated at 6, so that a sticker is produced. Other finishes of the outer surfaces are possible as desired. The adhesive film may be provided e.g. by means of an intermediate film. The adhesive film may be protected from premature adhesion by a detachable cover sheet. Furthermore, printing on film 5 is possible. The track pattern for forming the coils and the capacitors in the metal foil can be made by means of e.g. etching techniques or laser cutting techniques. The capacitors of the resonant circuit are built up from two metal foil portions on either side of the synthetic plastics carrier foil 2. Because, as far as production engineering is concerned, it is difficult and/or expensive to make a reliable connection between the two metal foil films, use is made, according to the present invention, of at least two capacitors and at least two coils connected according to the equivalent diagram shown in Fig. 4. This division has the advantage that the total self-inductance is divided over the two sides of the label, so that the effective coil area can be maximal at the indicated dimensions of the label. Also, this self-inductance is designed so that the stray capacitance normally occurring in opposed windings has practically no effect on the resonant circuit, as will be further explained hereinafter. Fig. 4 shows the electrical diagram of a label according to the present invention. The location of the carrier 2 is indicated in Fig. 4 by a broken line. The carrier is shown to form the dielectric for two capacitors 15,16. The left-hand electrode 15a of capacitor 15, together with a coil 17 and the left-hand electrode 16b of capacitor 16 are situated on one surface of carrier 2. The right-hand electrode 15b of capacitor 15, together with a coil 18, and the right-hand electrode 16a of capacitor 16 are situated on the other surface of carrier 2. Accordingly, the electrical coupling between the two portions consists only of the carrier 2 functioning as the dielectric of the capacitors. In order to optimally use the two surfaces of the carrier, coils 17,18 preferably have the same self-inductance and capacitors 15,16 preferably have the same capacitance. Figs. 2 and 3 show, by way of example, two track patterns according to the present invention provided on either side of a carrier. Fig. 2 may be a top view of a label according to the present invention (without a cover film) and Fig. 3 a bottom view of the same label (likewise without a cover film). Fig. 2 shows an approximately rectangular carrier 2 provided near one of the corners, in this example the left-hand top corner, with an approximately rectangular region 8 of conductive material. Extending spirally from the top corner of region 8 that is located near the top edge of the label, but remote from the corner of the label, and substantially along the circumference of the label, is a right-hand coiled conductor 9. Conductor 9 terminates after some windings, in this example three, in a second approximately rectangular conductive region 10 provided near the right-hand top corner of the label. Conductor 9 does not enclose the first region 8 but it does enclose the second region 10. Similarly, Fig. 3 shows a bottom view of the same label with an approximately rectangular conductive region 11,12 in the top portion, both near the left-hand top corner and near the right-hand top corner. A conductor 13 extends from the top corner of region 11 remote from the corresponding corner of the label, substantially along the circumference of the label in the form of a left-hand coiled spiral. Conductor 13 terminates after three windings in region 12, enclosed by the conductor. Region 11, however, is enclosed only by the outer winding of the spiral conductor 13. Regions 8 and 12 are equally large and together form a first capacitor. Similarly, regions 10 and 11 together form a second, substantially identical capacitor. The spiral conductors 9 and 13 connect the two capacitors and form the desired coils. Due to the configuration chosen, the current, in operation, has the same direction in superposed conductor sections, at least as far as said conductor sections follow the outer edge of the label. For the sake of clearness, Fig. 4 shows a circular current I and Figs. 2 and 3 show the corresponding currents. Moreover, due to the configuration chosen, superposed points of the said conductor sections have substantially the same potential. This means that the stray capacitance prevailing in principle between said conductor sections has no effect on the electrical properties of the label. The stray capacitances 19,20,21 shown in broken lines in Fig. 4, therefore, can be assumed to be zero. Only the conductor sections 14,14′ extending on each side of the carrier between the capacitor electrodes 8,10, and 12,11, respectively, have unequal potentials and hence give rise to undesirable stray capacitances. This effect, however, is minimized through the tracks 14, 14′ being in offset relationship relative to one another, as shown in Fig. 2 and Fig. 3. It is observed that various modifications of the above described embodiment are possible. For instance, a different number of windings can be chosen. Also, the carrier may consist of a plurality of layers. In case the carrier consists of two layers, such a carrier may even be formed by doubling up a carrier having two conductor patterns on one side. Because in that case there are no tracks bridging the fold line, a perforation, if any, is not detrimental. The bringing in register of the two patterns is simplified by the relatively large capacitor surface areas, while deviations have a relatively slight effect due to the corresponding potential variation on both sides of the carrier.

Claims: A flexible detection label for an electronic detection system, comprising a thin, platelike flexible carrier (2) having on both sides (3-6) elements made from flexible material, jointly forming at least one resonant circuit, wherein the resonant circuit includes at least two conductor tracks (9,13) formed as coils, said tracks being insulated from one another and disposed on opposite surfaces of the carrier, each track having two ends connected to a conductive region forming a capacitor electrode (8,10-12), wherein the two capacitor electrodes on one side of the carrier are located opposite corresponding capacitor electrodes on the other side of the carrier, characterized in that opposite capacitor electrodes (8,12; 10,11) are substantially equally large, and in that the capacitor electrodes (8-12) and the coil-forming conductor tracks (9,13) are positioned in such a manner that opposite sections of the conductor tracks on the two carrier surfaces have, in operation, substantially the same electrical potential. A flexible detection label as claimed in claim 1, characterized in that the elements on either side of the carrier (2) have substantially identical electrical properties. A flexible detection label as claimed in claim 1 or 2, characterized in that the coil-forming conductor tracks (9,13) extend substantially along the outer edge of the label, while the conductor tracks on one carrier surface are substantially in register with the tracks on the other carrier surface. A flexible detection label as claimed in any of the preceding claims, characterized by sections (14,14') of conductor tracks (9,13) which are located in opposite regions on different sides of the carrier (2) and, in operation, do not have substantially the same electrical potential, which sections are in offset relationship with each other. A flexible identification label as claimed in any of the preceding claims, characterized in that two adjacent capacitor electrodes (8,10;11,12) are formed on each carrier surface, one electrode (8;11) being connected with a point proximal to an edge of the label to an outer section of a spiral conductor track (9,13) which with a plurality of windings substantially follows the circumferential edge of the label, and which is connected with an inner end to a point of the other capacitor electrode (10;12) distal to the edge of the label, said one capacitor electrode (8;11) being situated substantially beyond the spiral conductor track (9,13), and said other capacitor electrode (10;12) being substantially enclosed by the spiral conductor track (9,13).

Inventors: VAN DER MEIJ, DANIEL AUKE
Applicants: N.V. NEDERLANDSCHE APPARATENFABRIEK NEDAP
Application year: 1988
Main classification: G08B13
IPC codes: G08B13, H01L27