NL1003100C2 - Security label with increased sensitivity. - Google Patents

Description

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Security label with increased sensitivity

The invention relates to a flexible security label, such as is used at supermarkets and chain stores for electronic security against shoplifting. These security labels are detected remotely using a radio-frequency (RF) transceiver system. The action is based on the energy absorption of a tuned LC circuit, or on specific signals that are generated by the thrown out of this electric resonant circuit.

The usual flexible security labels consist of a plastic carrier foil, which is provided on two sides with an aluminum foil. Through a mask printing and an etching process, a coil and a capacitor are formed in the aluminum foil. For environmental reasons, the waste products of this etching process must be regenerated, which has an adverse effect on the cost price of the aforementioned security labels.

The present invention aims to provide a solution to this problem, and will be explained in more detail below with reference to a few figures.

Figure 1 shows the electronic circuit of a single capacitor protection label.

Figure 2 shows the electronic circuit of a protection label with two capacitors.

Figure 3 shows a security label operating on the principle of a distributed capacity transmission line.

Figure 4 shows a security label according to the invention, constructed from more than two layers of aluminum.

10 0 3 1 0 0 -2-

Since the plastic backing film is about 15 microns thick at a conventional security label and the tuning capacity required to obtain a resonant circuit resonating at about 8.2 Mhz is about 150 pf 5, the area of the capacitor plates (2 ) of the circuit shown in Figure 1 is quite large, as a result of which the effective coil surface of coil (1) becomes considerably smaller. It is advantageous to have the smallest possible capacitor area in a label. The circuit shown in Figure 2 is thus much less favorable in this respect, because with two capacitors (2) and (4) connected in series, the capacitor surface becomes considerably larger. Nevertheless, this circuit is used, because it is not necessary to make a through-connection by means of the plastic carrier foil. The coils (1) and (3) and the capacitor surfaces of the capacitors (2) and (4) are located on either side of the foil.

The circuit of Figure 3, in which the tuning capacitor is distributed over the coil parts of coil 20 (1), is also used. As in the case of the circuit of Figure 1, a connection via the foil is required between two ends of the coil parts of coil (1).

A second parameter, which is important for the electronic performance of the security label, is the conductivity of the aluminum foil and thus the electrical resistance of the coil material. For this reason, the coil part and one capacitor plate of such a security label is usually made of aluminum foil of about 50 micron thickness and the capacitor plate on the other side of the label in aluminum foil of about 15 micron thickness. This is so as not to adversely affect the flexibility of the label and to avoid etching away too much material. The present invention aims to make a flexible security label with a minimum capacitor surface at the required frequency of about 8.2 Mhz, which can be produced using an environmentally friendly production technique. To this end, the etching process is abandoned and the so-called hot stamping of aluminum foil is used. In this process, aluminum is applied in 5 thin layers, by activating an adhesive layer via a heated stamp, which is applied to the above-mentioned aluminum foil. When the stamp is removed, the aluminum which is locally activated by the shape of the stamp remains on the product and the rest remains on a plastic carrier foil, on which the aluminum foil has been pre-applied. This carrier film is usually a polyester film. If the aluminum foil is sufficiently thin, it tears at the boundary of the activated and non-activated areas.

The conductivity of these ultra-thin films is so poor that more than two layers are required to obtain a coil with a sufficiently low electrical resistance. This disadvantageous side effect is used in the security label according to the invention by forming a capacitor which is built up of more than two layers and therefore forms a larger capacity per unit area.

The electronic circuit of figure 4 schematically shows a security label composed of 6 layers of aluminum, whereby no connections are required between the different layers. If the position of the coil parts of coil (1) and coil (3) is chosen such that no voltage is present across the coil parts covering each other, as indicated in the European patent 280361 of 30 applicant, no parasitic losses will occur in the coil parts. The hot-stamped adhesive forms an insulation between the aluminum foil in the different layers and thus also forms the dielectric for the capacitors (2) and (4), which are made up of 6 plates each. Thus, the dielectric loss factor of this material must be low enough to form a good quality capacitor. Because the coil parts of the 10 0 3 1 0 0 fishing reels (1) and (3) each have the same number of turns and are accurately arranged on each other, they carry the same voltages at the ends, so that the capacitor plates can alternately be assumed to be connected in parallel turn into.

Therefore, the capacity per surface at 6 layers will increase by a factor of five, apart from the larger capacity that arises, because the distance between the capacitor plates is smaller because of the thinner dielectric.

Because the circuit is composed of several layers of ultra-thin material, the flexibility of the security label according to the invention will be considerably greater than the flexibility of existing products.

Security labels as described above are generally deactivatable. To accomplish this, 15 different techniques are employed, based on either shorting the capacitor plates or interrupting a conductor or coil section. The latter method is described, for example, in applicant's Dutch patent application 9400810.

The security label according to the invention also has additional options with regard to deactivation. Namely, if a short circuit is established in the known manner between two of the 25 capacitor plates, the resonant frequency of the label will not disappear, but will shift to a higher frequency. Thus, the label can be recognized remotely in both the active and deactivated states. This is also the case if an interruption is made in a conductor or in one of the coil parts.

Likewise, a security label according to the invention could also be brought from an inactive to an active state and thus be activated, as it were, when the carrier of the label is placed in the sales area.

1003100

Claims (5)

1. A flexible security label, provided with an electric resonant circuit, characterized in that both the coil (s) and the capacitor (s) are made up of more than two layers of metal foil, provided with a heat-activated adhesive system, which is applied by means of the so-called hot-stamp technique are applied to each other and this activated glue also serves as an insulator and dielectric for the capacitor. A flexible security label according to the preceding claim, characterized in that at least two series-connected coils and / or capacitors are used, so that no electrical connections through the adhesive system are necessary. A flexible security label according to one or both of the preceding claim (s), characterized in that the different layers, from which the coil (s) is (are) built up, are applied to each other in such a way that the mutual layers of these coil parts relative to Do not supply any voltage to each other and thus do not cause any signal loss via the parasitic capacitance between these coil parts. 4. A flexible security label according to one or more of the preceding claims, characterized in that the resonance frequency of the label shifts to a higher value upon deactivation. A flexible security label according to one or more of the preceding claims, characterized in that in the same manner in which the label is deactivated, activation can take place from a lower resonant frequency to the operating frequency of, for example, 8.2 MHz. 1003100