EP0789339A2 - Securing element for the electronic protection of articles - Google Patents

Abstract

The security element (1) is made in the form of a ring (5) with inner radius and outer radius R, and is of soft magnetic material (3). There is a selected region that has no soft magnetic material. For this purpose the element can be manufactured without this region or the region can be removed mechanically or it can be treated chemically. XV indicates the direction of the preferred axis of external field H ext. The soft magnetic material has excellent properties, high permeability and low coercivity.

Description

The invention relates to a security element for electronic article security, consisting of an anisotropic magnetic material characterized by a preferred axis of high permeability and low coercive force (soft magnetic material), which has the shape of a simply connected surface and which generates a characteristic signal when an external magnetic alternating field is applied , the signal being maximum when the preferred axis of the securing element is aligned parallel to the external alternating magnetic field.

Elements of the type described above are preferably used in the area of electronic goods security in department stores and warehouses. A particularly advantageous embodiment of such an element has become known from DE 42 42 992 A1. This publication describes so-called “thin film” labels for securing CDs, preferably. They consist of a thin layer of soft magnetic material in the µm range. The layer is e.g. applied to a carrier substrate by means of a physical deposition process under vacuum conditions.

"Thin film" labels show an anisotropic structure. Anisotropic means that the soft magnetic layer is made of of which the "thin film" labels are formed has a preferred axis. The anisotropic structure is noticeable in practice in that the characteristic signal remitted by the "thin film" label in response to a query field is at a maximum if the query field and preferred axis are aligned parallel to one another; the signal disappears, however, if the preferred axis and the query field are perpendicular to each other.

(n, m = 0, 1, 2,...) an. Das niederfrequente Abfragefeld bewirkt, daß das Sicherungselement im Takt dieses Feldes von der Sättigung in eine Richtung zur Sättigung in die andere Richtung getrieben wird. Das charakteristische Signal tritt daher periodisch mit der Frequenz des niederfrequenten Feldes auf.
Als alternative Lösung ist auch bekannt geworden, lediglich ein im kHz-Bereich liegendes Abfragefeld zur Erregung des Sicherungselementes zu verwenden, wobei das charakteristische Signal des Sicherungselementes wiederum im Takte eines niederfrequenten Feldes, das das weichmagnetische Material zwischen den beiden Sättigungen hin- und hertreibt, auftritt.A large number of different methods have become known for detecting security elements in an interrogation zone. EP 123 586 B proposes, in addition to two query fields with the frequencies f1 and f2 in the kHz range, to send a field with a frequency in the Hz range to the query zone. The two interrogation fields with the frequencies f1 and f2 excite a security element located in the interrogation zone for transmitting a characteristic signal with the intermodulation frequencies $\text{nf1 ± mf2}$

(n, m = 0, 1, 2, ...). The low-frequency interrogation field causes the securing element to be driven from saturation in one direction to saturation in the other direction in time with this field. The characteristic signal therefore occurs periodically with the frequency of the low-frequency field.
As an alternative solution, it has also become known to use only a query field in the kHz range to excite the fuse element, the characteristic signal of the fuse element again occurring in the cycle of a low-frequency field, which drives the soft magnetic material back and forth between the two saturations .

For the purpose of evaluation, the shape of the characteristic signal is then compared with a predetermined signal shape. If the two agree, this is interpreted as an illegal stay of a secured article in the surveillance zone; an alarm indicates the theft to the operating personnel.

It has been shown in practice that in the case of security elements which do not consist of a simply coherent surface, the detection rate does not meet the high demands placed on them. An example of such a securing element is the CD ring described in DE 42 42 992 A1. This is - as will be explained in more detail in connection with FIGS. 1a to 1d - that different areas, to which quasi different demagnetization factors can be assigned, provide individual contributions to the characteristic signal in such securing elements. As a result of the different demagnetization factors, the signals of the individual areas occur at different times from one another. Since the evaluation device recognizes only a characteristic signal with a pronounced peak as originating from a securing element, e.g. an article provided with an annular securing element can pass through the surveillance zone unhindered.

The invention has for its object to provide a security element consisting of a non-simply contiguous area, the detection rate of which is comparable to the detection rate of a security element consisting of a simply contiguous area.

The object is achieved in that the securing element has at least one selected area in which the demagnetization factor of the soft magnetic material is changed such that the characteristic signal has a predetermined signal shape. By the way, the demagnetization factor is defined as the inverse slope of the magnetization curve.

According to an advantageous development of the security element according to the invention, the change is made in such a way that the characteristic signal as well as the predetermined one characteristic signal the shape of a pronounced peak or has two symmetrical peaks.

According to one embodiment, which is particularly well suited for securing CDs, the soft magnetic material has the shape of a ring with an outer radius (R) and an inner radius (r). If a securing element designed in this way is placed around the central opening of the CD, there are no imbalances which have a negative effect on the playback result. The soft magnetic material is advantageously produced by means of a deposition process under vacuum conditions. Of course, it is also possible to produce the material layers by punching them out.

The configuration that the selected areas lie symmetrically with respect to the preferred axis of the soft magnetic material is to be regarded as particularly advantageous. In particular, the selected areas have approximately the shape of an isosceles triangle, the base of which is formed by a section of the inner ring radius (r) and the tip of which lies opposite the base on the outer radius (R).
If, as preferred alternatives suggest, the soft magnetic material is completely or at least partially removed from these areas, the characteristic signal is reduced to a pronounced peak (or to two symmetrically lying peaks). As an embodiment, it is proposed in particular to remove the soft magnetic material along the legs of a triangular region and in strip-shaped sections which are arranged parallel to a tangent placed on the inner radius and forming the base of the triangle. The soft magnetic material can be removed from the desired areas, for example by mechanical processing or by steaming away (for example using a laser).

As already described above, the characteristic signal is at a maximum when the exciting magnetic field and the preferred axis are parallel to each other; it disappears in a vertical position. In order to generally increase the detection rate within the monitoring zone, it is provided according to an advantageous further development that the securing element consists of at least two layers of the soft magnetic material, the preferred axes of the layers preferably being rotated relative to one another by an angle of 360 ° / (2 * n) are; n denotes the number of layers in the formula. As a concrete example, two layers of soft magnetic material are mentioned, which are perpendicular to each other with their preferred axes.

As soon as a customer has properly paid for the goods provided with a security element, the detector devices should of course no longer trigger an alarm subsequently. For this purpose, a semi-hard or hard magnetic material is usually arranged on or in the vicinity of the soft magnetic material. This so-called deactivator material is characterized by a high coercive force and a relatively low permeability. If the deactivator material is exposed to such a strong magnetic field that it is driven into saturation, it subsequently prevents the reaction of the soft magnetic material to the magnetic field in the interrogation zone.
In order to enable deactivation of the securing element according to the invention, a layer of semi-hard or hard magnetic material is provided at least between two of the layers of the soft magnetic material.

• Fig. 1a: ein ringförmiges elektronisches Sicherungselement,
• Fig. 1b: einen Ausschnitt des in Fig. 1a gezeigten Sicherungselementes,
• Fig. 1c: ein Diagramm, in dem der Entmagnetisierungsfaktor über dem Radius ρ des in Fig. 1b dargestellten Ringausschnitts aufgetragen ist,
• Fig. 2a: eine vereinfachte Darstellung eines weichmagnetischen Materials mit unterschiedlichen Entmagnetisierungsfaktoren in verschiedenen Volumenelementen,
• Fig. 2b: die Hysteresekurven des in Fig. 2a dargestellten Materials,
• Fig. 2c: eine Darstellung der beiden Signale eines ringförmigen Sicherungselementes,
• Fig. 2d: eine Darstellung des charakteristischen Signals, das sich aus den in Fig. 2c gezeigten Einzelsignalen zusammensetzt,
• Fig. 3a: ein Ausschnitt aus einer Ausführungsform des erfindungsgemäßen Sicherungselementes,
• Fig. 3b: eine Diagramm, das den Entmagnetisierungsfaktor über dem Radius ρ des in Fig. 3a dargestellten Ringausschnittes zeigt,
• Fig. 4: eine Diagrammdarstellung des Entmagnetisierungsfaktors gegen die magnetische Feldstärke für ein übliches (1) und für die in Fig. 3a dargestellte Ausführungsform (2) des erfindungsgemäßen Sicherungselementes,
• Fig. 5a: eine weitere Ausführungsform des erfindungsgemäßen Sicherungselementes,
• Fig. 5b: eine zur Fig. 4 analoge Diagrammdarstellung für die weitere Ausführungsform des erfindungsgemäßen Sicherungselementes,
• Fig. 5c: eine Darstellung des charakteristischen Signals der in Fig. 5a gezeigten Ausführungsform des erfindungsgemäßen Sicherungselementes und
• Fig. 6: eine günstige Ausführungsform des erfindungsgemäßen Sicherungselementes.

The invention is explained in more detail with reference to the following figures. It shows:

• 1a: an annular electronic fuse element,
• 1b: a detail of the securing element shown in Fig. 1a,
• 1c: a diagram in which the demagnetization factor is plotted against the radius ρ of the ring section shown in FIG. 1b,
• 2a: a simplified representation of a soft magnetic material with different demagnetization factors in different volume elements,
• 2b: the hysteresis curves of the material shown in Fig. 2a,
• 2c: a representation of the two signals of an annular fuse element,
• 2d: a representation of the characteristic signal, which is composed of the individual signals shown in FIG. 2c,
• 3a: a detail from an embodiment of the securing element according to the invention,
• 3b: a diagram showing the demagnetization factor over the radius ρ of the ring section shown in FIG. 3a,
• 4 shows a diagram of the demagnetization factor versus the magnetic field strength for a conventional (1) and for the embodiment (2) of the securing element according to the invention shown in FIG. 3a,
• 5a: a further embodiment of the securing element according to the invention,
• 5b: a diagram representation analogous to FIG. 4 for the further embodiment of the securing element according to the invention,
• 5c: a representation of the characteristic signal of the embodiment of the securing element according to the invention and shown in FIG. 5a
• 6: an inexpensive embodiment of the securing element according to the invention.

1 a shows an electronic security element 1, which has the shape of a ring 5. Such a securing element 1 is already known from DE 42 52 992 A1. The soft magnetic material 3 of the securing element 1 has an anisotropic structure, which is characterized by a preferred axis XV. For reasons of symmetry, it is sufficient to consider the ring section of 90 ° shown in FIG. 1b. The ring 5 has an inner radius r and an outer radius R.

The diagram shown in FIG. 1c shows the reciprocal demagnetization factor 1 / N over the radius ρ of the ring section shown in FIG. 1b. The reciprocal demagnetization factor N is in a first approximation proportional to the length of the securing element 1 in the direction of the external magnetic field Hext. The magnetization curve shows a strongly pronounced plateau P, which can be interpreted to mean that a relatively large volume fraction V of the ring 5 has an almost constant demagnetization factor N. This means that a relatively large volume fraction V of the ring 5 is demagnetized with almost the same strength of the external magnetic field hext. The reaction of the ring-shaped securing element 1 to an external magnetic field Hext can be seen from FIGS. 2a to 2d.

2a shows a simplified illustration of a soft magnetic material 3 with different demagnetizing factors N1, N2 in different volume ranges V1, V2. The arrow indicates the preferred axis XV of the soft magnetic material 3.
The hysteresis curves assigned to the volume regions V1, V2 with different demagnetization factors N1, N2 can be seen in FIG. 2b. As is generally known, the securing element 1 is excited in the non-linear regions of the hysteresis curves (-> transition between the linear and the saturated region) to emit a signal A (t). If the hysteresis curves are traversed at a constant speed, the two volume ranges V1, V2 contribute their contributions to the characteristic signal A (t) at different times. A breakdown according to individual signals can be seen in FIG. 2c. The characteristic signal A (t) of the annular securing element 1 resulting from the superimposition of the two signals is shown in FIG. 2d. Due to the shape of the characteristic signal which deviates from the usual signal shape - the signal has a pronounced plateau P - the securing element 1 with a surface which is not simply connected is not recognized as being associated with an electronically secured article in the detector devices which are usually used.

3a shows a section of an embodiment of the securing element 1 according to the invention. Again, the securing element 1 has the shape of a ring 5 with the inner radius r and the outer radius R. In the selected area 2, no soft magnetic material 3 can be found: either the securing elements 1 according to the invention have already been produced with the interrupted ring shape, or the soft magnetic material 2 is subsequently removed by mechanical or chemical treatment of the corresponding areas 2.

The diagram shown in FIG. 3b shows the course of the reciprocal demagnetization factor 1 / N over the radius ρ of the ring section shown in FIG. 3a. As a clear difference to the analog diagram in FIG. 1c, the curve shows no plateau here. The different behavior of the known and the annular securing element 1 according to the invention can be seen particularly well in the diagram which is plotted in FIG. 4. Here, the magnetized volume fraction of the securing element 1 shown in FIG. 1b and in FIG. 3a is plotted as a function of the external magnetic field hext. While the known securing element 1 has a pronounced, unfavorable plateau, the corresponding embodiment of the securing element 1 according to the invention shows a smooth course. As a result of this, the characteristic signal A (t) of the security element 1 according to the invention - only one such is assigned by the detector device to an article which is located in the query zone without authorization - shows a pronounced peak 4 (or two symmetrical peaks 4 which are symmetrical of the two) lying non-linear areas of the hysteresis curve).

5a shows a further embodiment of the securing element 1 according to the invention. Here, the soft magnetic material 3 has not been removed entirely, but only from certain sections of the selected areas 2. A triangular shape of the selected area 2 has proven to be particularly favorable, the triangle being arranged symmetrically with respect to the preferred axis XV. The soft magnetic material 3 is removed in the region of the legs of the triangular region 2 and in strip-shaped sections which run parallel to a tangent placed on the inner circle, the tangent forming the base of the triangle.

A diagram similar to FIG. 4 for this embodiment of the securing element 1 according to the invention can be seen in Fig. 5b. Here too, the securing element 1 according to the invention shows the desired smooth course without a plateau. In the characteristic signal A (t) of the fuse element 1, only the two symmetrically lying peaks occur (FIG. 5c).

It has already been described above that the characteristic signal A (t) of an anisotropically applied soft magnetic material 3 is maximum when its preferred axis XV is parallel to the external magnetic field Hext. In order to increase the detection rate within the monitoring zone, two layers 6 of the soft magnetic material 3 with their preferred axes XV are approximately crossed one above the other according to an advantageous development of the security element 1 according to the invention. This superimposition can be extended to any number of layers 6, the optimal relative angular position of the layers 6 to one another being able to be determined using the formula 360 ° / 2 * n (n denotes the number of layers 6). In the illustrated embodiment, a layer 6 made of semi-hard or hard magnetic material 7 is further provided between the two layers 6 made of soft magnetic material 3. If this material 7 is driven into saturation, the securing element 1 is deactivated.

Furthermore, the layers 6 of the soft magnetic material 3 each have a scratch 9 of length Rr in the two areas perpendicular to the selected areas 2. It has been found that the strength of the characteristic signal A (t) of a ring 5 made of soft magnetic material 3 decreases of a certain size decreases. This negative effect can be eliminated by mechanical or chemical removal of the soft magnetic material 3 (for example by a scratch 9) in the area of the greatest length extension of the securing element 1.

Reference list

1

Securing element

2nd

selected area

3rd

soft magnetic material

4th

Peak

5

ring

6

location

7

semi-hard or hard magnetic material

8th

strip-shaped section

9

scratch

At)

characteristic signal

ρ

radius

r

inner radius

R

outer radius

N

Demagnetization factor

XV

Preferred axis

Witch

external magnetic field

V

Volume element

P

plateau

Claims (11)

Securing element for electronic article surveillance, consisting of an anisotropic magnetic material characterized by a preferred axis, high permeability and low coercive force (soft magnetic material), which has the shape of a simply connected surface and which generates a characteristic signal when an external magnetic alternating field is applied, the signal is maximum if the preferred axis of the securing element is aligned parallel to the external magnetic alternating field.
characterized,
that the securing element (1) has at least one selected area (2) in which the demagnetization factor (E) of the soft magnetic material (3) is changed such that the characteristic signal (A (t)) has a predetermined signal shape. Securing element according to claim 1,
characterized,
that the predetermined characteristic signal (A (t)) preferably has a pronounced peak (4). Securing element according to claim 1 or 2,
characterized,
that the soft magnetic material (3) has the shape of a ring (5) with an outer radius (R) and an inner radius (r). Securing element according to claim 1 and 3,
characterized,
that the selected areas (2) lie symmetrically with respect to the preferred axis (XV) of the soft magnetic material (3). Securing element according to claim 4,
characterized,
that a selected area (2) has approximately the shape of an isosceles triangle, the base of which is formed by a section of the inner ring radius (r) and the tip of which lies opposite the base on the outer radius (R). Securing element according to claim 1 and 5,
characterized,
that no soft magnetic material (3) is present in the selected areas (2). Securing element according to claim 1 and 5,
characterized,
that the demagnetization factor (N) in the selected areas (2) is specifically changed by removing soft magnetic material (3). Securing element according to claim 7,
characterized,
that the soft magnetic material (3) is preferably removed along the legs of the triangular region (2) and in strip-shaped sections (8) which are arranged parallel to a tangent to the inner radius (r) and which forms the base of the triangle. Securing element according to claim 1, 7 or 8,
characterized,
that the securing element (1) consists of at least two layers (6) of the soft magnetic material (3), the preferred axes (XV) of the layers (6) preferably being rotated relative to one another by an angle of 360 ° / (2 * n), where n denotes the number of layers (6). Securing element according to claim 9,
characterized,
that a layer (6) made of semi-hard or hard magnetic material (7) is provided at least between two of the layers (6) of the soft magnetic material (3). Securing element according to claim 1,
characterized,
that the soft magnetic material (3) is a so-called thin-film material.

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