EP0807912A1

EP0807912A1 - Flexible detection label for an electronic detection system and method for manufacturing such detection label

Info

Publication number: EP0807912A1
Application number: EP97201455A
Authority: EP
Inventors: Johannes Harm Lukas Hogen Esch
Original assignee: Nederlandsche Apparatenfabriek NEDAP NV
Current assignee: Nederlandsche Apparatenfabriek NEDAP NV
Priority date: 1996-05-14
Filing date: 1997-05-14
Publication date: 1997-11-19
Also published as: NL1003100C2

Abstract

The invention relates to a flexible security label for the prevention of shoplifting. The labels are provided with an electronic resonance circuit, in which both the coil and the capacitor are built up of more than two metal foil layers. These metal foil layers are provided with a heat-activatable adhesive system and are arranged on each other by means of the so-called hot-stamping technique. The activated adhesive serves as an insulator and as a dielectric for the capacitor. The coils and the capacitor parts are so arranged on each other that no electric connections through the adhesive system are necessary. When deactivating or activating the security label, a shift of the resonance frequency occurs, so that in the activated and in the activated state the label remains remotely identifiable.

Description

The invention relates to a flexible detection label for an electronic detection system, comprising a thin, plate-shaped carrier material provided, at least on one side thereof, with flexible conductor tracks which in combination form an electric resonance circuit composed of at least one coil and at least two capacitor plates.
The invention also relates to a method for manufacturing such a detection label.
Flexible detection labels are typically used in supermarkets and shop chains for the electronic security against shoplifting. These detection labels are remotely detected by means of a radiofrequent (RF) transmission-reception system. The operation is based on the energy absorption of a tuned LC-circuit or on specific signals formed by the resonance of this electric resonance circuit.
A detection label of the type described in the opening paragraph is known from, inter alia, European patent application 0 280 361. The known detection label consists of a plastic carrier foil having both sides thereof provided with a single layer of aluminum foil. Via a mask printing and etching process, flexible conductor tracks are formed on either side of the label, which tracks form, in combination, a resonance circuit comprising at least one coil and at least one capacitor.
A drawback of the known detection label is that the sensibility of the label is usually not sufficient. This means that the detection label does not respond when it is disposed in a radiofrequent field that is too weak.
A drawback of the method for manufacturing the known detection label is that for environmental reasons, waste products from this etching process have to be regenerated.
This has a negative effect on the cost price of the detection labels.
The object of the invention is to overcome the above drawbacks. The method according to the invention is characterized in that conductor tracks are provided through hot-stamping of flexible metal foil having at least one side thereof provided with a heat-activatable adhesive. In this manner, no waste matter that is harmful to the environment is released.
A flexible detection label according to the invention has as a characteristic feature that the resonance circuit is built up of at least two stacked metal foil layers, which are mechanically interconnected by an electrically insulating adhesive, each metal foil layer forming a part of the conductor tracks which in combination form the resonance circuit.
If aluminum foil is thin, it is possible that the coil formed by the conductor tracks does not have a sufficiently low electric resistance. For this purpose, in accordance with a particular variant of the invention, the resonance circuit is built up of at least three stacked metal foil layers interconnected by an electrically insulating adhesive, each metal foil layer forming a part of the conductor tracks which, in combination, form a resonance circuit. Thus, a capacitor can be formed that is built up of more than three layer and that hence forms a greater capacity per unit area. Moreover, the coils formed in each layer by the conductor tracks can each comprise the same number of windings, so that these coils carry the same voltage on their ends. When each end is connected to a capacitor plate of the capacitor mentioned, the capacitor plates, separated from each other by a metal foil layer, can be assumed to be connected in parallel. This means that the coils of these layers can likewise be assumed to be connected in parallel. Thus, the drawback that the coil formed by a conductor track of a metal foil layer has too high a resistance is overcome.
Hereinafter, the invention will be specified with reference to the accompanying drawings, wherein:

Fig. 1 shows a resonance circuit of a detection label having one capacitor;
Fig. 2 shows the resonance circuit of a detection label having two capacitors;
Fig. 3 shows a detection label operating according to the principle of a transmission line with distributed capacity;
Fig. 4 is a schematic representation of a flexible detection label according to the invention;
Fig. 5 shows a schematic cross section of a flexible detection label according to the invention;
Fig. 6 is a top plan view of each of the metal foil layers of the detection label according to Fig. 5; and
Fig. 7 is a schematic representation of the resonance circuit of the detection label according to Fig. 5.

Detection labels known per se typically comprise a thin, plate-shaped carrier material provided, on at least one side thereof, with flexible conductor tracks. The flexible conductor tracks in combination form a resonance circuit as is for instance schematically shown in Fig. 1. The resonance circuit comprises a coil 2 and a capacitor 4, connected in series therewith. The flexible conductor tracks are for instance manufactured from an aluminum foil.
Because in a usual detection label, the plastic carrier foil has a thickness of about 15 microns and the tuning capacity required for obtaining a resonance circuit resonating at for instance 8.2 Mhz is for instance 150 pf, the surface area of the plates of the capacitor 4 of the circuit according to Fig. 1 becomes fairly large. As a result, the effectively available area for the coil of Fig. 1 becomes considerably smaller. Hence, for a detection label, it is favorable to have a smallest possible capacitor area.
Fig. 2 shows an alternative embodiment of a resonance circuit used for known detection labels. The resonance circuit comprises two series-connected coils 2, 6 and two series-connected capacitors 4, 8. In this example, the position of the carrier material 10 is schematically indicated by a dotted line. This shows, inter alia, that the coils 2, 6 have been provided on either side of the carrier material. It also shows that the capacitor plates of each capacitor 4, 8 have been provided on either side of the carrier material. As far as the capacitor area is concerned, the circuit according to Fig. 2 is more unfavorable than the circuit according to Fig. 1. After all, the capacitor area of Fig. 2 is larger in that two series-connected capacitors 4, 8 are used. Nevertheless, this circuit is actually used in practice, because no through-connections are required through the carrier material 10. As it is, the capacitor plates of the capacitors 4, 8 provide an electric coupling. Here, too, it applies that the coils 2, 6 and the capacitor plates are manufactured from an aluminum foil in a manner known per se. For a further description of the circuit according to Fig. 2, reference is made to European patent application 0 280 361.
In the circuit of Fig. 3, a capacitor is distributed over the coils 2, 6. The coils 2, 6 are again manufactured from aluminum foil. Also, in this example, the coils 2, 6 are provided on either side of the carrier material 10. This has as a result that an electric through-connection between the ends 12 of the coils 2, 6 and the ends 14 of the coils 2, 6 through the carrier material is necessary. As a matter of fact, something similar would also be necessary in the case of the circuit according to Fig. 1 when the coil 2 and the capacitor 4 were provided on either side of the carrier material 10.
Apart from the coil area, a second parameter that is important for the electronic performance of detection labels is the conductivity of the aluminum foil and, accordingly, in particular the electric resistance of the coils 2, 6 formed by the aluminum foil. For this reason, the aluminum foil, used on one of the sides of the carrier material 10 and forming a coil and a capacitor plate, is typically aluminum foil having a thickness of about 50 microns. The capacitor plate on the other side of the carrier material is manufactured from aluminum foil having a thickness of about 15 microns. If both sides of the carrier material 10 were provided with an aluminum foil having a thickness of about 50 microns, this would be unfavorable in respect of the required flexibility of the detection label. Moreover, the detection labels discussed hereinabove are generally manufactured according to an etching process known per se. For environmental reasons, the waste products of the etching process have to be regenerated. This has of course an adverse effect on the cost price of the detection label.
The object of the present invention is to provide a flexible detection label that can be produced according to an environmentally friendly production technique. Moreover, the object of the invention is to provide a flexible detection label having a minimum capacitor area at the required frequencies of, for instance, 8.2 Mhz as mentioned hereinabove. In addition, the object of the invention is to provide coils having a sufficiently small electric resistance. In order to meet these three requirements, the etching process is abandoned and, instead, use is made of the so-called hot-stamping of aluminum foil. In this process, aluminum is applied in thin layers by activating a layer of adhesive via a heated stamp. This layer of adhesive can for instance be applied to the above-mentioned aluminum foil in advance. When the hot stamp is removed, the aluminum foil, locally activated through the form of the stamp, is left behind on the product. The remainder of the aluminum foil is left behind on a plastic carrier foil to which the aluminum foil had been applied in advance. This carrier foil is usually a polyester foil. If the aluminum foil is sufficiently thin, it tears at the boundary of the activated and non-activated regions when the carrier foil is removed again.
The conductivity of the extremely thin foils is sometimes insufficient. In accordance with the invention, more than two layers can then be employed in order to obtain a coil having a sufficiently low electric resistance. To realize this, in the flexible detection label according to the invention, a capacitor built up of more than two layers is preferably used, which, as a result, forms a greater capacity per unit area. The electronic circuit of Fig. 4 schematically shows such a detection label.
The detection label is built up of six aluminum foil layers 20-25. In this example, the aluminum foil layer 20 is located at the bottom, while the aluminum foil layer 25 is located on top. The layers are numbered so that for instance the aluminum foil layer 23 is located between the aluminum foil layer 22 and the aluminum foil layer 24. The aluminum foil layer 25 forms a coil 6.25 provided on either side with capacitor plates. Something similar applies to the coils 6.23 and 6.21 formed by the layers 23 and 21. In addition, it applies that the layers 20, 22 and 24 respectively each form a coil 2.20, 2.22 and 2.24, each having both sides provided with capacitor plates. As schematically shown in Fig. 4, the resonance circuit according to Fig. 4 comprises two capacitors, each comprising six capacitor plates lying one above the other. These capacitor plates are formed by the relevant layers. The layers 20-25 are electrically insulated from each other by the heat-activated adhesive present between these layers. If the position of the coils 6.21-6.25; 2.20-2.24 is chosen so that no voltage is present between parts of the coils covering each other, as indicated in European patent application 0 280 361, no parasitic losses will occur in the coils. Moreover, the adhesive activated through hot-stamping forms an insulation between the aluminum foil in the different layers and also forms a dielectric for the capacitors formed by the capacitor plates. As stated, these capacitors are each built up of six plates. When the coils 6.21, 6.23 and 6.25 each have the same number of windings and are accurately provided one upon the other, they carry the same voltages on the ends. In other words, the capacitor plates connected to the ends of these coils can be presumed to be connected in parallel. When it moreover applies that the coils 2.20, 2.22 and 2.24 each have the same number of windings and are accurately provided on upon the other, it applies to these coils, too, that they carry the same voltages on their ends. Hence, also to the capacitor plates connected to the ends of the coils 2.20, 2.22 and 2.24, it applies that they can be presumed to be connected in parallel. As a consequence, in the case of six layers, the capacity per surface area will hence increase by a factor five. The coils 2.20-2.24 are then likewise connected in parallel and correspond to the coil 2 of Fig. 2. On the other hand, the coils 6.21-6.25 are connected in parallel and correspond to coil 6 of Fig. 2. The capacitor plate of capacitor 4, connected to the coil 6, then corresponds to the stacked capacitor plates of the stacked coils 6.21, 6.22 and 6.25. Likewise, it applies that the capacitor plate of the capacitor 4 according to Fig. 2, connected to the coil 2 of Fig. 2, corresponds to the capacitor plates that are stacked and associated with the coils 2.20, 2.22 and 2.24. The same comparison holds for the capacitor 8 of Fig. 2.
A further elaborated example of a detection label according to the invention will presently be discussed with reference to Figs. 5-7.
The detection label according to Figs. 5-7 comprises a thin-walled, plate-shaped carrier material 10. The detection label further comprises a resonance circuit built up of four stacked metal foil layers 20, 21, 22 and 23. The metal foil layers 20-23 are interconnected by an electrically insulating adhesive 30. This adhesive 30 consists of a heat-activated adhesive. Each metal foil layer forms conductor tracks, as shown most clearly in Fig. 6. The conductor tracks of the metal foil layer 20 form a coil 6.20 provided on either side with capacitor plates 30.20 and 32.20. Likewise, the layer 22 forms a coil 6.22 having at the ends of the coil 6.22, capacitor plates 30.22 and 32.22. The layer 21 forms a coil part 2.21 having at the ends thereof capacitor plates 34.21 and 36.21. Accordingly, the layer 23 forms a coil 2.23 having at the ends thereof capacitor plates 34.24 and 36.23.
The capacitor plates 30.20, 30.22, 34.21 and 34.23 are stacked one above the other and in fact form a capacitor having four plates. Likewise, the capacitor plates 32.20, 32.22, 36.21 and 36.23 are stacked one above the other and form a capacitor consisting of four plates. The coils 6.20 and 6.22 are provided so that in use, the capacitor plates 30.20 and 30.22 on the one hand and the capacitor plates 32.20 and 32.22 on the other will carry the same voltage. This means that the capacitor plates 30.20 and 30.22 can be presumed to be connected in parallel. Likewise, the capacitor plates 32.20 and 32.22 can be presumed to be connected in parallel.
It also applies that the coils 2.21 and 2.23 are provided so that, in use, the ends of these coils will carry the same voltage. This means that the capacitor plates 34.21 and 34.23, in use, will have the same voltage and can hence also be presumed to be connected in parallel. On the other hand, the capacitor plates 36.21 and 36.23, in use, will carry the same voltage and can hence also be presumed to be connected in parallel. Thus, a resonance circuit is built up whose diagram is shown in Fig. 7.
Fig. 7 directly shows a number of advantages of the invention. The coils 2.21 and 2.23 are connected in parallel. This means that, effectively, one coil is formed with a relatively low resistance. The same applies to the coils 6.20 and 6.22, connected in parallel. Further, it applies that to each of the plates of the capacitor 4, it applies that they are built up of two parallel-connected capacitor plates originating from different layers. Thus, the effective surface area and, accordingly, the capacity of each of the capacitors 4, 8 is increased.
The example of Fig. 6 shows that the conductor tracks formed by n (in this example n = 4, however, other values for n, with n being an integer greater than or equal to 2 and preferably greater than or equal to 3, are possible) stacked metal film layers, in combination, form at least one capacitor with n capacitor plates. In this example, there are even formed two capacitors which are each provided with n capacitor plates. In this example, the capacitor plates of each layer are interconnected via a coil formed by the conductor tracks of this layer. This example also shows that the conductor tracks of each metal foil layer form at least one coil and at least two capacitor plates, with the two capacitor plates of each layer being located opposite the two capacitor plates of an adjacent layer. It also applies that each coil of a layer comprises two ends which are connected with respectively the two capacitor plates of the relevant layer. This involves that the resonance circuit which, in the example of Fig. 6, is built up of four stacked metal foil layers, each of which metal foil layers forming part of the conductor tracks which, in combination, form the resonance circuit, comprises a series connection of at least two capacitors and/or two coils.
Preferably, the coils 2.21, 2.23, 6.20 and 6.22 and capacitor plates 30.20, 30.22, 32.20, 32.22, 34.21, 36.21, 34.23 and 36.23, formed by the conductor tracks, have the same electric properties.
In this example, the conductor tracks of each pair of adjacent layers are so arranged with respect to each other that opposite parts of the coils formed by these conductor tracks, in use, do not carry voltage differences with respect to each other. Thus, opposite parts of the coils 6.20 and 2.21 will not carry voltage differences. As a result, no parasitic losses will occur in these coils. The same applies to other adjacent coils, such as the coils 2.21 and 6.22 and the coils 6.22 and 2.23.
In the example of Fig. 6 it additionally applies that the flexible conductor tracks are so arranged that the capacitor plates, formed by the conductor tracks, of two metal foil layers which are separated from each other by a third metal foil layer, in use, carry the same voltage. Other variants, however, are also conceivable.
Preferably, the conductor tracks of each layer, at least substantially, extend according to a spiral with a decreasing radius. This is also the case with the layers shown in Fig. 6. Here, it additionally applies that one of the two capacitor plates which are electrically connected with the spiral is outside the spiral, while the other of the two capacitor plates is inside the spiral. In the example of the layer 20 it applies, for instance, that the capacitor plate 30.20 is outside the spiral, while the capacitor plate 32.20 is inside the spiral. Something similar applies to the other layers 21-23. The layers are then so arranged with respect to each other that opposite capacitor plates of adjacent metal foil layers are respectively inside and outside the spirals. Thus, the capacitor plate 30.20 is outside the spiral, while the capacitor plate 34.21 is inside the spiral.
Preferably, the detection labels according to the invention are of the deactivatable or the activatable type. To achieve this, different techniques are used. These techniques are based on the fact that, in the known manner, a short-circuit is effected between the capacitor plates. It is also possible that, in the known manner, an interruption is provided in a conductor track. This last method is described in, for instance, applicants' German "Offenlegungsschrift" 195 18 106.
In this example, the layer 23 comprises a conductor track part 40 with a free end 42. Near the free end 42 there is further provided a pyrotechnical material 44 to enable interruption of a part 46 of the conductor tracks. After interruption, the coil part 2.23 will no longer carry current. This has the result that the resonance frequency of the resonance circuit, as shown in Fig. 7, will change. It is also possible that, in the known manner, a short-circuit will be effected between two of the capacitor plates. In that case, the resonance frequency of the detection label will not disappear but shift to a higher frequency. The label is thus remotely identifiable both in the active and in the deactivated state. This is also the case when an interruption is provided in a part of the conductor tracks, as described above. The interruption can then be provided in, for instance, the coil parts themselves or in another position. In the same manner, a detection label according to the invention could also be brought from a non-active into an active state and thus, so to speak, be activated when placing a detection label in, for instance, a sales area.
Such variants are each deemed to fall within the scope of the invention.
As stated above, in a method for manufacturing a detection label according to the invention, conductor tracks are arranged by the hot-stamping of a flexible metal foil which is provided on at least one side with a heat-activatable adhesive. More in particular, it applies that by the successive hot-stamping of at least two sheets of flexible metal foil, there are obtained at least two stacked metal foil layers which are interconnected with an electrically insulating adhesive, with each metal foil layer comprising a part of the conductor tracks which, in combination, form the resonance circuit.
Preferably, it applies that by the successive hot-stamping of at least three sheets of flexible metal foil, there are obtained at least three stacked metal foil layers which are interconnected with an electrically insulating adhesive, with each metal foil layer comprising a part of the conductor tracks which, in combination, form the resonance circuit.
More in particular, it applies that the method comprises at least the following steps:

on the at least one side of the plate-shaped carrier material there is arranged a first metal foil with a heat-activatable adhesive interposed between the plate-shaped carrier material and the first metal foil;
parts of the first metal foil which are intended for forming a flexible conductor track are heated with a hot stamp for locally activating the adhesive and for adhering the conductor tracks to be formed to the carrier material by means of the heated adhesive;
the non-adhered parts of the first metal foil are removed to obtain a first metal foil layer which comprises a part of the conductor tracks;
on the first metal foil layer there is arranged a second metal foil with a heat-activatable adhesive interposed between the first metal foil layer and the second metal foil;
parts of the second metal foil which are intended for forming a flexible conductor track are heated with a hot stamp for locally activating the adhesive; and
the non-adhered parts of the second metal foil are removed to obtain a second metal foil layer comprising a part of the conductor tracks.

Here it further applies that, subsequently, following metal foil layers are arranged on already arranged metal foil layers in the same manner as in which the second metal foil layer is arranged on the first metal foil layer.
It will be clear that the activatable adhesive is preferably arranged on a metal foil to be placed. It is also possible, however, that the adhesive is applied to a metal foil layer already arranged.
Such variants are each deemed to fall within the scope of the invention.

Claims

A flexible detection label for an electronic detection system, comprising a thin plate-shaped carrier material which is provided on at least one side with flexible conductor tracks which, in combination, form an electric resonance circuit composed of at least one coil and at least two capacitor plates, characterized in that the resonance circuit is built up of at least two stacked metal foil layers which are mechanically interconnected with an electrically insulating adhesive, each metal foil layer forming part of the conductor tracks which, in combination, form the resonance circuit.
A flexible detection label according to claim 1, characterized in that the resonance circuit is built up of at least n (n = 2, 3, 4, ...) stacked metal foil layers which are mechanically interconnected with an electrically insulating adhesive, each metal foil layer forming part of the conductor tracks which, in combination, form the resonance circuit.
A flexible detection label according to claim 2, characterized in that the conductor tracks of the n (n = 3, 4, ...) stacked metal foil layers, in combination, form at least one capacitor with n capacitor plates.
A flexible detection label according to claim 2, characterized in that the conductor tracks of the n (n = 3, 4, ...) stacked metal foil layers, in combination, form at least two capacitors with each n capacitor plates.
A flexible detection label according to claim 4, characterized in that the capacitor plates of each layer are interconnected via at least one coil formed by the conductor tracks of said layer.
A flexible detection label according to any of the preceding claims, characterized in that the adhesive is a heat-activated adhesive.
A flexible detection label according to any of the preceding claims, characterized in that the metal foil layers are electrically insulated from each other by the adhesive.
A flexible detection label according to any of the preceding claims, characterized in that the conductor tracks of each metal foil layer form at least one coil and at least two capacitor plates, said two capacitor plates of each layer being located opposite the two capacitor plates of an adjacent layer.
A flexible detection label according to claim 8, characterized in that each coil of a layer comprises two ends which are connected with respectively the two capacitor plates of the relevant layer.
A flexible detection label according to any of the preceding claims, characterized in that the resonance circuit comprises an electrical circuit diagram with a series connection of at least two capacitors and/or two coils.
A flexible detection label according to any of the preceding claims, characterized in that coils and capacitor plates, formed by the conductor tracks, of each pair of adjacent metal foil layers have the same electric properties.
A flexible detection label according to any of the preceding claims, characterized in that the conductor tracks of each pair of adjacent layers are so arranged with respect to each other that opposite parts of the coils formed by said conductor tracks, in use, do not carry voltage differences with respect to each other.
A flexible detection label according to any of the preceding claims, characterized in that the conductor tracks are so arranged that the capacitor plates, formed by the conductor tracks, of two metal foil layers which are separated from each other by a third metal foil layer, in use, carry the same voltage.
A flexible detection label according to claim 8, characterized in that the conductor tracks of each layer, at least substantially, extend according to a spiral with a decreasing radius, one of said two capacitor plates, which is electrically connected with the spiral, is outside the spiral, while the other of the two capacitor plates is inside the spiral.
A flexible detection label according to claim 14, characterized in that opposite capacitor plates of adjacent metal foil layers are respectively inside and outside the spirals.
A method for manufacturing a flexible detection label for an electronic detection system, comprising a thin plate-shaped carrier material which is provided on at least one side with flexible conductor tracks which, in combination, form an electric resonance circuit, characterized in that conductor tracks are arranged by the hot-stamping of flexible metal foil which is provided on at least one side with a heat-activatable adhesive.
A method according to claim 16, characterized in that by the successive hot-stamping of at least two sheets of flexible metal foil there are obtained at least two stacked metal foil layers which are interconnected with an electrically insulating adhesive, each metal foil layer comprising a part of the conductor tracks which, in combination, form the resonance circuit.
A method according to claim 16 or 17, characterized in that by the successive hot-stamping of at least three sheets of flexible metal foil there are obtained at least three stacked metal foil layers which are interconnected with an electrically insulating adhesive, each metal foil layer comprising a part of the conductor tracks which, in combination, form the resonance circuit.
A method for manufacturing a flexible detection label for an electronic detection system, comprising a thin plate-shaped carrier material which is provided on at least one side with flexible conductor tracks which, in combination, form an electric resonance circuit, characterized in that the method comprises at least the following steps:
- on the at least one side of the plate-shaped carrier material there is arranged a first metal foil with a heat-activatable adhesive interposed between the plate-shaped carrier material and the first metal foil;

- parts of the first metal foil which are intended for forming a flexible conductor track are heated with a hot stamp for locally activating the adhesive and for adhering the conductor tracks to be formed to the carrier material by means of the heated adhesive;

- the non-adhered parts of the first metal foil are removed to obtain a first metal foil layer comprising a part of the conductor tracks;

- on the first metal foil layer there is arranged a second metal foil with a heat-activatable adhesive interposed between the first metal foil and the second metal foil;

- parts of the second metal foil which are intended for forming a flexible conductor track are heated with a hot stamp for locally activating the adhesive; and

- the non-adhered parts of the second metal foil are removed to obtain a second metal foil layer comprising a part of the conductor tracks.
A method according to claim 19, characterized in that, subsequently, following metal foil layers are arranged on already arranged metal foil layers in the same manner as in which the second metal foil layer is arranged on the first metal foil layer.