EP0762354A1 - Elongated bodies as security label for electromagnetic theft security systems - Google Patents

Abstract

In the heat treatment, the amorphous strip or tape is supplied from the spool (1) to the tensioning roller (2), going between the roller pair (3) connected by the line (4) with a current source (5). The strip (6) then passes into the oven (7), but is surrounded by a screening tube (8) of conducting or soft-magnetic material shielding the strip from external fields. Inside the tube there is a coil (9) connected to a voltage source (10), producing a longitudinal field on the strip. After the oven, the strip passes through a second roller pair (11) that returns current to source of maintains tension and then is wound round another spool (12) for cutting into short pieces.

Description

The invention relates to an elongated body for electromagnetic anti-theft or identification systems with a strip made of amorphous material, which is magnetized in a removal zone with an alternating magnetic field by Barkhausen jumps when magnetization is reversed when certain threshold values of the magnetic field are reached and thereby triggers characteristic voltage pulses in an interrogation coil.

A ferromagnetic wire is already known from DE-A 29 33 337, which contains two layers braced against one another and which experiences a sudden change in magnetism due to a Barkhausen jump in an alternating field when certain threshold values are exceeded or undershot. Among other things, this wire can be used as a security strip for alarm systems. The Barkhausensprung results in a characteristic signal that can be recognized, for example, by evaluating harmonics in a query coil and that cannot be confused with signals from other magnetic parts. However, this known ferromagnetic wire requires relatively high field strengths, for whose generation relatively high alternating fields are necessary, for example, in an interrogation zone at the exit of a shop. Here, however, efforts are made to use the lowest possible fields, on the one hand to be able to make the query zone sufficiently wide and on the other hand to keep health risks for people passing through the query zone as low as possible.

By using certain soft and hard magnetic materials that are braced against each other, a composite body has become known from DE-C-38 24 075, which can be used as a strip for theft protection or identification systems and manages with a low amplitude of an interrogating alternating field. The hard magnetic component of this composite body with impulse behavior can be used to deactivate the theft protection strip by magnetizing and thus saturating the soft magnetic part. The deactivated strip can then be transported through the interrogation zone without triggering an alarm.

Since a strip for anti-theft systems should also be suitable for low-priced goods, it is necessary to provide a strip that is as simple as possible and is therefore relatively cheap. Such a strip has become known, for example, from US 4,298,268. It is proposed here to provide a strip of amorphous material, since the amorphous material has an extraordinarily high permeability and there is therefore also only a low risk of confusion with other soft magnetic objects. In addition, it has been proposed there to create regions with higher coercivity by attaching crystalline regions within the amorphous band, which regions can again contribute to deactivation of the strip when magnetized. This has the advantage that a hard magnetic material does not have to be additionally worked into the strip for deactivation. However, it has been found that it is difficult in practice to set crystalline areas with sufficient coercive field strength and that relatively long strips are required in order to ensure a reasonably reliable response of the monitoring system.

Furthermore, the amorphous strips were heat-treated in a longitudinal field to increase the permeability. A very steep magnetic reversal curve is achieved (induction depending on the field strength), but not the particularly steep impulses that can be achieved with a pulse wire that is suddenly magnetized by Barkhausen jumps and regardless of the rate of field change.

It has furthermore become known from US Pat. No. 4,660,025 to use a strip of an amorphous tape for anti-theft systems which has not undergone any heat treatment and which - due to the manufacturing process - has internal stresses due to rapid quenching from the molten state. The internal stresses in the wire or strip in turn cause Barkhausen jumps during the remagnetization, so that the same effect as with the pulse wire results. In addition, there is the advantage that strips can be produced at low cost, which also require only a low field strength of the interrogation field. The disadvantage of the last-mentioned arrangement, however, is that the strips are very sensitive to voltage and even slight deformations change the internal voltages and thus the Barkhausen jumps that occur during magnetic reversal, so that the monitoring system for detecting the strip must either be set to be insensitive, which gives false alarms by other magnetic materials, or that if the monitoring device is sensitive, not all strips used for theft protection trigger an alarm.

From Journal of Magnetism and Magn. Mat. 133 (1994) pp. 86-89, it has now become known to specifically generate a magnetic reversal behavior in an amorphous band that includes Barkhausen jumps. This even applies to amorphous Materials that have a magnetostriction close to zero, as is the case, for example, with amorphous tapes containing cobalt. These magnetostriction-free amorphous tapes have the advantage over the tapes subject to magnetostriction that they retain their magnetic properties to a large extent even when they are bent and also in the bent state, so that the strip does not necessarily have to maintain an elongated straight shape and better adhere to the item to be protected or identified Can customize goods shape.

The present invention solves the problem of providing a strip for anti-theft or identification systems that manages with low switching field strengths, has a defined impulse behavior due to sudden magnetic reversal as a result of jumps in Barkhausen and is inexpensive to produce and also generates a sufficiently high, characteristic signal even with relatively short strips .

The solution is that a strip is used which consists of an amorphous material with a cobalt content of at least 20 at% and receives its property for pulsed magnetic reversal through a heat treatment to adjust the magnetic anisotropy in the case of current-carrying strips, and that Current through the strip in connection with the temperature and the duration of the heat treatment is adjusted so that there is a ratio of remanent induction to saturation induction between 0.2 and 0.9.

It has been found that an amorphous strip made of an alloy based on cobalt and which has been heat-treated according to the invention, especially when adhering to certain values of remanent induction for saturation induction, triggers particularly high pulse voltages in the interrogation coil, which are triggered due to the periodic magnetic reversal of the strip and the Barkhausen jumps triggered with it. According to the invention, it was recognized that the use of such amorphous strips allows relatively short theft protection strips (less than 50 mm) and nevertheless results in sufficiently high pulse voltages, which in turn trigger characteristic evaluable harmonics in the interrogation coil.

The behavior of the strip according to the invention can be improved if the strip is not only produced from the amorphous band, but the strip consists of this amorphous band and an associated soft-magnetic material which is continuously magnetized.

The result is a mode of operation similar to that described in EP-B 309 679 for a pulse wire made of two materials clamped together. In contrast to the known pulse wire, the amorphous strip according to the invention has a much smaller coercive field strength. It has been found that a particularly effective increase in the pulse height can be achieved if a soft magnetic material is provided whose coercive field strength is below 30 mA / cm and if the cross section multiplied by the saturation induction is higher than the remanent flow of the strip with pulse behavior. This can be achieved if one uses an amorphous or nanocrystalline alloy and provides a sufficient cross section for the soft magnetic strip. It is particularly advantageous if the length of the soft magnetic strip is chosen to be greater than that of the strip with impulse behavior.

As in the case of conventional pulse wires, a permanent magnet connected to the strip according to the invention can also be used to achieve an asymmetrical signal, that is to say a sudden magnetic reversal in the case of different ones Threshold values of the magnetic field, depending on the direction of magnetization, is triggered. This is explained in more detail for pulse wires in EP-B 156 016.

It is particularly advantageous if the material for the strip consists of an alloy _which satisfies the formula Co _a Ni _b (Fe, Mn) _c (Si, B, X) _d , where in at%:

einschließlich üblicher Verunreinigungen gleich 100 und X eines oder mehrere der Übergangsmetalle der Gruppen IIIB-VIB, insbesondere Nb, Mo, Ta, W, V, und/oder eines oder mehrere Elemente der Hauptgruppen IIIA-VA, insbesondere C, P, Ge bezeichnet. Insbesondere sind Legierungen der Zusammensetzung in at.%:

für die Verwendung als Diebstahlsicherungsstreifen nach der Erfindung geeignet.a = 20-85; b = 0-50; c = 0-15 and d = 15-30, where $\text{a + b + d + c}$

including common impurities equal to 100 and X denotes one or more of the transition metals of groups IIIB-VIB, in particular Nb, Mo, Ta, W, V, and / or one or more elements of main groups IIIA-VA, in particular C, P, Ge. In particular, alloys of the composition in at.% Are:

suitable for use as anti-theft strips according to the invention.

For example, an amorphous band from the alloy composition mentioned under 1) has been used. This strip had the dimensions 1.0 × 0.023 mm, a Curie temperature of Tc = 485 ° C. and a saturation induction of 1.0 T. Such a strip with a length of 40 mm was used with a maximum field strength of H = 1.2 A. / cm and the impulses generated were determined in a query coil with 200 turns. The ratio of the remanent induction Jr to the saturation induction Js was measured on 150 mm long strips in order to exclude the influence of the demagnetizing effect. The following values resulted: Result Process parameters HLF {(A / cm} Tensile stress {MPa} U {mV} Jr / Js T {° C} t {s} I {mA} 31 0.41 300 25th 450 0.5 45 20th 0.58 300 25th 200 0.5 45 4th 0.14 300 25th 525 0.5 45 12th 0.65 300 25th 450 5 45

If relatively short lengths of less than 50 mm are used, a correspondingly smaller cross section must be used to reduce the demagnetizing effect of the strip, so that a sufficient signal level is nevertheless achieved. To produce the tape, an amorphous tape (or also a wire) is first produced as usual by rapid quenching from the melt.

An example of a heat treatment according to the invention is provided in FIG. 1. From a supply reel 1 with the amorphous ribbon, the latter arrives via a tensor roller 2 to a first pair of rollers 3, which is connected to a power source 5 via a line 4. After passing through the first pair of rollers, the amorphous band 6 enters an oven 7, in which it is surrounded by a shielding tube 8 made of electrically conductive or magnetically soft material, in order to keep external field influences away.

Inside the shielding tube 8 there is a coil 9, which is connected to a voltage source 10 and generates a longitudinal field acting on the amorphous band 6. The first pair of rollers 3 and a second pair of rollers 11 not only serve to supply the current from the current source 5, but can also be used by appropriate drive to set a certain tension in the amorphous band 6.

The current supplied from the current source 5 to the amorphous band 6 can also be used to heat the band 6, but primarily serves to generate a magnetic part which surrounds the amorphous band inside. After the tape has left the furnace 7, it passes through the second pair of rollers 11 and then passes onto a take-up reel 12. The tape now has the properties required for use as strips for anti-theft and identification systems, so that the strips according to the invention are cut from them can be produced.

It is also possible to completely or partially forego shielding from external fields and, for example, to use the existing earth field as a longitudinal field. For certain materials, it may also be sufficient if during the heat treatment only the circular field generated by the current flow acts on the strip or the wire from which the strips are then produced. In the case of alloys with positive magnetostriction in particular, the effect caused by the longitudinal field can also be produced by tensioning the strip during the heat treatment. Of course, you can also use a longitudinal field and a tension at the same time.

While an amorphous tape is sufficient for use in anti-theft systems, tapes or wires with different reactions have to be arranged in a strip for the purpose of identifying goods, or several strips have to be connected to the identified goods.

2 shows the pulse height U in mV as a function of the current I in mA flowing through the amorphous band 6. To achieve the highest possible pulse height in an interrogation coil, it is necessary to set certain values for the longitudinal field, which, however, depend on the current from the current source 5 and on the cross section of the amorphous band 6.

Fig. 3 shows in the event that a current I = 450 mA flows through the amorphous band 6, that the amorphous band is in the furnace 7 for 25 seconds and that the temperature in the furnace is T = 300 ° C, the Height of the measured pulse (voltage U in mV) compared to the field strength H (LF) of the longitudinal field in A / cm.

The shape of the magnetization curve, which is defined, for example, by the remanence ratio, defined by the quotient from the remanence induction Jr to the saturation induction Js (each measured in Tesla) is essential for the pulse height when using the amorphous wire with a Barkhausensprung effect for strips of theft protection or identification systems. , can be described. It has surprisingly been found that neither flat loops nor rectangular loops with a correspondingly higher remanence ratio are advantageous for the pulse formation in this application. Although the optimum pulse height also depends to a small extent on the material used and the dimensions of the strip, the parameters (longitudinal field, current through the strip and strip tension) must be set during the heat treatment so that a remanence ratio between 0.2 and 0 , 9, preferably between 0.3 and 0.7. For the exemplary embodiment according to FIG. 3, various heat treatments were carried out, which resulted in different remanence ratios.

The result is shown in Fig. 4. It shows that an optimum of 30 mV with a remanence ratio of about 0.4 was found in this strip examined.

In order to influence the remanence ratio, one has to consider the ratio of transverse field which is characterized by heat treatment results in the current in the band, vary with the longitudinal field applied. The transverse field acting on the strip due to the current takes the value zero in the middle of the strip and then increases linearly up to the maximum at the strip surface.

In order to achieve the particularly advantageous remanence ratio between 0.3 and 0.7, the ratio of maximum transverse field to longitudinal field in the range from 1 to 10 must be observed during the heat treatment.

In Fig. 5, for comparison of the strip according to the invention with a strip whose pulse behavior is determined by internal stresses (US 4,660,025), the pulse voltage U and the field strength H are plotted over time t in seconds if the field strength H is corresponding Curve H1 continuously increased. Curve U1 shows the tension that results when using an amorphous wire that has a length of 90 mm and a diameter of 0.13 mm, compared to the voltage curve corresponding to curve U2 when using an amorphous tape according to the invention with the Dimensions: width: 2 mm, thickness: 23 µm and the same length of 90 mm. It can be seen that the peak voltage of the pulse occurs in the amorphous band according to the invention at a higher field strength, but there is a substantially higher voltage pulse and a steep edge when the voltage rises. The measurements show that the voltage pulse in the amorphous band according to the invention is approximately 120 mV, while a maximum voltage amplitude of 30 mV could be achieved with the amorphous wire.

Particularly advantageous alloys for the intended application result when a cobalt content between 60 and 85 at .-% is provided and when the iron / manganese content, which determines the magnetostriction constant, is in the range from 1 to 10 at .-% is chosen that a magnetostriction as low as possible, preferably below ± 4 x 10 ^-6 .

ist. Dabei bezeichnet X entweder eines oder mehrere der Übergangsmetalle der Gruppen IIIB-VIB wie z. B. Nd, Mo, Ta, W, V etc. und/oder eines oder mehrere Elemente der Hauptgruppen IIIA-VA, wie z. B. C, P, Ge.To determine advantageous alloys for the present application, alloys must be selected that satisfy the following formula: Co _a Ni _b (Fe, Mn) _c (Si, B, X) _d with in at .-%:
a = 20-85; b = 0-50; c = 0-15 and d = 15-30, where $\text{a + b + d + c = 100}$

is. X denotes either one or more of the transition metals of groups IIIB-VIB such as. B. Nd, Mo, Ta, W, V etc. and / or one or more elements of the main groups IIIA-VA, such as. B. C, P, Ge.

Permanent magnets can not only change the response field strength depending on the magnetic reversal direction, but - as with known soft magnetic strips - it is also possible to saturate the strips with a slightly stronger permanent magnet and thus switch off the pulse behavior. In this way, a deactivatable security strip can be obtained.

Advantageous dimensions for the amorphous tape, which is contained in the strip according to the invention either alone or together with other materials, result in a length of up to 100 mm if a width of up to 5 mm and a thickness of max. 50 microns for the tape or the diameter of the wire is provided. However, shorter strips are also possible with a sufficient pulse height. Here, the advantageous dimensions with a length of up to 60 mm are that a width of up to 3 mm and a strip thickness of up to 40 µm are used.

With these dimensions, strips with lengths of less than 40 mm can also be produced. Advantageously, the shorter the stripe, the higher the switching field strength. You can, for example, with a strip up to 40 mm maximum 1.5 A / cm, with a strip of up to 60 mm maximum 1.0 A / cm and with a strip up to 100 mm maximum 0.75 A / cm.

If a wire is used instead of a band, this can be reduced after the production by rapid solidification by mechanical deformation in cross section and also changed by z. B. a flat rolled wire with a rectangular or elliptical cross section is used.

In a further development of the present invention, the signal level can be increased by arranging longitudinal strips made of a soft magnetic material at the ends of short tempered strips, ie for strip lengths between 20 and 40 mm. This increases the signal level by a factor of 10. In the case of unannealed bands, the signal level is increased by approximately 1 to 2 times.

The distance between the strips should not be less than 10mm. The maximum pulse height, that is to say the optimal position, depends in particular on the strip length of the amorphous strip and the dimensions of the soft magnetic strips.

Good direct contact between the amorphous tapes and the soft magnetic strips is essential, with external pressure from an adhesive strip being sufficient.

A significant signal increase is also achieved by arranging two soft magnetic strips at the respective ends of the amorphous band above and below.

Claims (24)

Elongated body for electromagnetic anti-theft or identification systems with a strip of amorphous material, the specific in magnetization reversal in an interrogation zone an alternating magnetic field by Barkhausen jumps when reaching threshold values of the magnetic field is suddenly re-magnetized and thereby triggers in a Abtragespule characteristic voltage pulses, characterized in that a Strip is used, which consists of an amorphous material with a cobalt content of at least 20 at% and receives its property for pulsed magnetic reversal through a heat treatment to adjust the magnetic anisotropy in the current-carrying strip, and that the current through the strip in connection with the temperature and the duration of the heat treatment is adjusted so that a ratio of remanent induction to saturation induction results between 0.2 and 0.9. A method of producing a strip according to claim 1, characterized in that an amorphous strip (6) produced by rapid solidification is heat-treated in an oven (7) with a longitudinal field with a current flowing in the longitudinal direction through the strip (6). A process for producing a strip according to claim 1 or 2, characterized in that an amorphous strip (6) produced by rapid solidification is heat-treated in an oven (7) under tensile stress with a current flowing in the longitudinal direction through the strip (6). Process according to Claims 2 to 3, characterized in that the strip is cut into individual strips only after the heat treatment. Elongated body according to Claim 1, characterized in that a strip is used whose ratio of remanent induction to saturation induction is in the range from 0.3 to 0.7. Elongated body according to claim 1, characterized in that the material for the strip consists of an alloy _which satisfies the formula Co _a Ni _b (Fe, Mn) _c (Si, B, X) _d , in at%:
a = 20-85; b = 0-50; c = 0-15 and d = 15-30, where $\text{a + b + d + c}$

including common impurities equal to 100 and X denotes one or more of the transition metals of groups IIIB-VIB, in particular Nb, Mo, Ta, W, V and / or one or more elements of main groups IIIA-VA, in particular C, P, Ge. Elongated body according to claim 6, characterized in that the cobalt content of the alloy used for the strip is greater than 40 at%. Elongated body according to claim 6, characterized in that the cobalt content of the alloy used for the strip is greater than 60 at%. Elongated body according to claim 6, characterized in that the alloy for the strip contains iron and / or manganese in the range from 1 to 10 at%. Elongated body according to claim 1, characterized in that it consists of a strip of amorphous material with a pulsed magnetic reversal behavior and one or more second soft magnetic strips, the direction of magnetization of which is reversed continuously when reversed. Elongated body according to claim 10, characterized in that a soft magnetic material is used to increase the pulse height, the coercive field strength of which is below 30 mA / cm and that the cross section multiplied by the saturation induction is higher than the remanent flow of the strip with pulse behavior. Elongated body according to claim 10 or 11, characterized in that the length of the soft magnetic strip is greater than that of the strip with pulse behavior and that the soft magnetic strip is arranged so that it projects beyond the strip with pulse behavior at both ends. Elongated body according to claim 1, characterized in that it consists of one or more strips which consist of alloys with a magnetostriction below ± 4 x 10 ^-6 , so that the magnetic reversal behavior remains when it is exposed to mechanical stresses. Elongated body according to claim 3, characterized in that it contains an amorphous strip with impulse behavior, which consists of an alloy with a positive magnetostriction. Elongated body according to claim 1, characterized in that it consists of an amorphous strip with a pulsed magnetic reversal behavior and a hard magnetic body for premagnetization connected to the strip, the magnetic field of which is dimensioned such that the alternating field in the interrogation zone depends on the direction of magnetization of the strip set different threshold values (switching field strengths) for the pulsed magnetic reversal. Elongated body according to claim 1, 10 or 15, characterized in that it additionally contains at least one permanent magnet which, in the magnetized state, causes deactivation by saturation of the amorphous band piece. Elongated body according to claim 1, characterized in that the amorphous strip used has a length of up to 100 mm, a width <5 mm and a thickness of at most 50 µm. Elongated body according to claim 1, characterized in that the amorphous strip used has a length of up to 60 mm, a width of up to 3 mm and a band thickness of at most 40 µm. Elongated body according to claim 1, characterized in that the amorphous strip used has a length of up to 40 mm, a width of up to 3 mm and a strip thickness of up to 40 µm. Elongated body according to claim 17, characterized in that the switching field strength is below 0.75 A / cm. Elongated body according to claim 18, characterized in that the switching field strength is less than 1.0 A / cm. Elongated body according to claim 19, characterized in that the switching field strength is less than 1.5 A / cm. Elongated body according to claim 1, characterized in that a wire with a round or elliptical cross section is used as the amorphous strip. A process for producing a strip for an elongated body according to claim 1 to 3, characterized in that the current through the strip during heat treatment is adjusted in relation to the longitudinal field acting so that the ratio of maximum transverse field to longitudinal field is in the range from 1 to 10 lies.

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