HK1022366A - Magnetomechanical electronic article surveillance marker with low-coercivity bias element - Google Patents

Description

Magnetically-actuated electronic article surveillance marker with low coercivity bias element

Technical Field

The present invention relates to magnetic-type markers for use in Electronic Article Surveillance (EAS) systems.

Background

It is known to configure systems for electronic article surveillance to prevent or deter theft of articles in retail stores. In a typical such system, the marker protects the merchandise by interacting with an electromagnetic field disposed at the merchandise outlet. If a marker is brought into the magnetic field or "interrogation zone", it can be detected and an alarm signal emitted. Some of these markers may be removed from the merchandise after payment and passage through the sales counter. Other markers will continue to be attached to the article but are demagnetized using a demagnetization device, thereby changing the magnetic properties of the marker so that it can no longer be detected in the area of interrogation.

One type of EAS system known to date uses a marker that includes an "activatable" magnetostrictive element and a biasing or "control" element formed from a magnet that provides a bias magnetic field. An example of such a marker has been shown in fig. 1 and is designated by reference numeral 10. The marker 10 includes an activatable element 12, a rigid housing 14 and a biasing element 16. The components making up the marker 10 may be suitably assembled so that the magnetostrictive strip 12 fits into the recess 18 in the housing 14 and the biasing element 16 is held within the housing 14 to form a covering for the recess 18. The grooves 18 are of a size comparable to the magnetostrictive strip 12 so that the mechanical resonance generated by such strip 12 when exposed to a constantly changing magnetic field is not mechanically impeded or hindered by the housing 14. And the biasing element 16 should be disposed within the housing 14 so as not to "clamp" the activatable element 12.

As disclosed in U.S. patent 4510489 to Anderson et al, the activatable element 12 should be configured such that when the activatable element is exposed to a magnetic bias field, the activatable element 12 vibrates at a natural resonant frequency, i.e., the mechanical resonant frequency of the activatable element 12 when exposed to an alternating electromagnetic field that varies at the resonant frequency. The bias element 16, when magnetized to saturation, provides a bias magnetic field that varies at the resonant frequency required by the activatable element. In general, the biasing element 16 is constructed of a material having "semi-hardened" magnetic properties. The term "semi-hardened" is defined herein as having a coercivity of about 10-500 oersteds (Oe) and a remanence of about 6 kilogauss (kG) or greater after a DC magnetization field that magnetizes the element to substantial saturation is removed.

In an optimum EAS system constructed in accordance with the teachings of U.S. patent to Anderson et al, the alternating electromagnetic field generated is used as a pulsed interrogation signal at the merchandise outlet. Upon activation by each series of interrogation signals, the activatable element 12 will produce a damped mechanical oscillation at the end of each pulse train. The resulting signal emitted by the activatable element can be detected by a detection circuit which is synchronized with the interrogation circuit and which is activated in a quiet period following the pulse train. Such EAS systems for detecting magnetic markers using pulsed interrogation signals are commercially available from the assignee of the present application and sold under the trademark "ULTRA MAX" and are widely used.

Deactivation of such a magnetic marker is performed by demagnetizing the bias element so that the resonant frequency of the magnetostrictive element is substantially shifted from the frequency of the interrogation signal. After the biasing element is demagnetized, the activatable element will no longer respond to the interrogation signal and will no longer produce a signal of sufficient magnitude to be detected by the detection circuit.

In conventional magnetic EAS markers, the biasing element is constructed of a semi-hardened magnetic material such as "semi vac 90" or the like, which is commercially available from Vacuumschmelze, Hanau, germany. The coercivity of the semi vac90 is approximately 70 to 80 Oe. It is presently believed that the coercivity of the biasing element is at least 60Oe to prevent accidental demagnetization of the biasing magnet (and hence deactivation of the marker) by magnetic fields that may occur during storage, transport or handling of the marker. The semi vac90 material needs to be in a DC magnetic field of 450Oe or higher to reach 99% saturation of magnetization and needs an AC demagnetizing field close to 200Oe to perform 95% demagnetization.

Because of the need to use a relatively high AC deactivation magnetic field, conventional devices that generate an AC deactivation magnetic field (such as those sold by the assignee of the present application under the trade designations "Rapid Pad 2" and "SDeed Station," etc.) must be operated in a pulsed manner to reduce energy losses and meet regulatory limits. However, since the AC magnetic field is generated only in pulses, it is necessary to ensure that the marker is in the vicinity of the device when the deactivation field pulse occurs. Currently known techniques for ensuring that the marker is in the vicinity of the deactivators when a pulse is generated include techniques for generating a pulse in response to manual input from the operator of the device, techniques for locating the marker detection circuit in the demagnetizing device, and so forth. The former technique places this burden on the operator who operates the deactivation device, and both techniques place certain restrictions on the components, which increases the cost of the deactivation device. In addition, when the deactivation magnetic field is generated in a pulsed manner, the coil for generating the magnetic field is also heated, and therefore, it is necessary to make the electronic components in the device have high quality, which further increases the cost. The difficulty in applying a sufficiently strong deactivation field to the marker is also compounded by the ever increasing use of "source tagging," which is the securing of an EAS marker to an item during the packaging of the item at a manufacturing facility or distribution site. Also, in some instances, the marker may be positioned to adhere to an article of commerce in a location that is difficult, if not impossible, to access by conventional demagnetizing devices.

Objects and summary of the invention

It is therefore an object of the present invention to provide a magnetic EAS marker that can be deactivated by applying a deactivation field having a lower strength than conventional magnetic markers.

It is another object of the present invention to provide a magnetic EAS marker that can be deactivated by a magnetic field generated in a continuous manner rather than in pulses.

It is a further object of the present invention to provide a magnetic-type marker that can be deactivated under the condition that the distance between the marker and the deactivation device is greater than the distance between the conventional magnetic-type marker and the conventional deactivation device.

It is a further object of the present invention to provide a magnetic marker that can be deactivated more easily than conventional magnetic markers.

It is still another object of the present invention to provide a magnetic-type marker that can be activated using a DC magnetic field having a lower intensity than that used for activating a conventional magnetic-type marker.

According to a first aspect of the present invention there is provided a marker for use in a magnetic-based electronic article surveillance system, comprising an amorphous magnetostrictive element and a biasing element disposed adjacent the magnetostrictive element, and having a resonant frequency shift characteristic in response to a deactivation magnetic field, the shift gradient being greater than 100 Hz/Oe.

According to a second aspect of the present invention, there is also provided a marker comprising an amorphous magnetostrictive element and a bias element disposed at a nearby position and made of a semi-hardened magnetic material having a coercive force Hc of less than 55 Oe.

According to a third aspect of the present invention, there is also provided a marker comprising an amorphous magnetostrictive element and a bias element disposed at a nearby position and made of a semi-hardened magnetic material having a DC magnetization field characteristic such that a DC magnetic field Ha required for saturation of the bias element is less than 350 Oe.

According to a fourth aspect of the present invention, there is also provided a marker comprising an amorphous magnetostrictive element and a bias element disposed at a nearby position and made of a semi-hardened magnetic material having an AC demagnetizing field characteristic such that when an AC demagnetizing field Hmd having a peak amplitude of less than 150Oe is applied to the bias element in a fully magnetized state, the bias element is demagnetized to a level of not more than 5% of the fully magnetized level.

In accordance with this and other aspects of the present invention, not only is it possible to cause the biasing element to demagnetize at a lower magnetic field level than conventional markers, but it is also possible to cause the biasing element to be substantially free of inadvertent demagnetization when the marker is exposed to a low magnetic field level that may occur during shipping, storage or handling. Thus, a biasing element that produces demagnetization under an AC magnetic field of 150Oe can maintain its stability (i.e., substantially full magnetization) when the marker is exposed to a magnetic field of 0-20 Oe. Such a biasing element may also remain stable when the marker is exposed to a magnetic field of 0-4Oe if it can be demagnetized at an AC field of 30Oe, as is recommended by the present invention.

According to a fifth aspect of the present invention there is also provided a marker comprising an amorphous magnetostrictive element and a biasing element disposed in close proximity and having a target resonant frequency corresponding to an operating frequency of a system for electronic article surveillance, the marker having a resonant frequency shift characteristic associated with a deactivation magnetic field such that the shift in the resonant frequency of the marker relative to the target resonant frequency may be at least 1.5kHz when the marker is exposed to an AC deactivation magnetic field having a peak amplitude of 50Oe or less.

According to a sixth aspect of the present invention there is also provided a tag for use in a magnetic-type electronic article surveillance system such as may transmit a tag interrogation signal in intermittent bursts at a predetermined frequency, the tag comprising an amorphous magnetostrictive element and a biasing element disposed in close proximity thereto, and having an output signal characteristic which varies with a deactivation field such that when the tag is exposed to an AC deactivation field having a peak amplitude below 35Oe, the a1 output signal level produced by the tag will reduce by at least 50% the level of the a1 output signal produced when the tag is exposed to a demagnetization field, wherein the a1 output signal is the signal produced by the tag 1 millisecond after the end of the interrogation signal pulse applied to the tag.

According to a seventh aspect of the present invention, there is also provided a marker comprising an amorphous magnetostrictive element and a bias element disposed in proximity thereto, and the bias element is made of a semi-hardened magnetic material having AC demagnetizing field characteristics such that when the bias element is exposed to an AC magnetic field having a peak amplitude of 15Oe after being fully magnetized and not disposed in the marker, the AC magnetic field causes a significant reduction in the magnetization level of the bias element, and when the bias element is fully magnetized and disposed in the marker in proximity to the magnetostrictive element and the bias element is exposed to an AC magnetic field having a peak amplitude of 15Oe, the magnetostrictive element can deflect magnetic flux away from the bias element such that the magnetization of the bias element is substantially unaffected by the AC magnetic field.

According to an eighth aspect of the present invention, there is also provided a method of activating and deactivating an EAS marker for use in a magnetic EAS system, the method comprising: a step of providing an EAS marker comprising an amorphous magnetostrictive element and a biasing element disposed adjacent the magnetostrictive element; a step of magnetizing the bias element so that the bias element generates a magnetic field that biases the magnetostrictive element to resonate at the operating frequency of the EAS system; the step of demagnetizing the EAS marker is performed by exposing the marker to an AC magnetic field having a peak amplitude below 150 Oe. The step of magnetizing the biasing element may be performed before or after mounting it on the marker, and further, the step of demagnetizing may use a magnetic field having a peak amplitude of less than 100 Oe.

In accordance with the principles provided by the present invention, a magnetic-type marker is constructed using a control element having a relatively low coercivity characteristic, and the resonant frequency of the marker can also drift relatively sharply when a magnetic field of relatively low level is applied. Thus, the present invention can reduce the magnetic field level generated by the marker deactivation device, thereby allowing the generation of a continuous deactivation magnetic field without the need for a pulse-type deactivation magnetic field as in conventional deactivation devices. The invention thus eliminates the need for a marker detection circuit in the deactivation device and the need for manual triggering of the deactivation magnetic field pulse by an operator of the deactivation device when the marker to be deactivated is located in the vicinity of the deactivation device.

Furthermore, since the present invention can use a deactivation magnetic field having a lower level of magnetism, it is possible to manufacture such deactivation devices using lower quality components, which can further reduce costs because they are less expensive than components used in conventional deactivation devices.

Moreover, for such markers constructed in accordance with the principles of the present invention that are more easily deactivatable, deactivation may also be performed when the marker is at a distance, say one foot, from the deactivation device. This makes it more suitable for deactivating markers embedded or concealed on articles of commerce as part of a "source-marking" operation.

The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of preferred embodiments, as illustrated in the accompanying drawings in which like parts and components are designated by like reference numerals throughout the several views.

Brief description of the drawings

Fig. 1 is an isometric view showing the constituent parts of a magnetic-type marker constructed according to the prior art.

Fig. 2 is a schematic graph showing the relationship between the resonant frequency and the amplitude of an output signal of a conventional magnetic-type marker and the intensity of a demagnetized magnetic field applied to the marker.

FIG. 3 is a schematic graph, similar to FIG. 2, showing the variation of the resonant frequency and output signal amplitude of a magnetic marker constructed in accordance with the present invention as a function of demagnetized field strength applied to the marker.

Fig. 4 is a schematic graph showing the relationship between the level of magnetization and the strength of an applied DC magnetizing field, in which case a material constructed in accordance with the present invention is used as a biasing element in a magnetic-type marker.

Fig. 5 is a schematic graph showing the relationship between the magnetization level and the strength of an applied AC deactivating magnetic field applied to a fully magnetized element constructed in accordance with the present invention, which in this example is used as a biasing element in a magnetic-type marker.

Fig. 6 is a schematic graph, similar to fig. 5, showing the relationship between the level of magnetization produced and the strength of an applied AC demagnetizing field applied over a material used as a biasing element in this example and constructed in accordance with the second embodiment of the present invention.

Fig. 7 is a schematic graph, similar to fig. 2 and 3, showing the variation of the resonant frequency and output signal amplitude of a magnetic marker constructed in accordance with a second embodiment of the present invention with respect to the strength of an applied magnetic field.

Fig. 8 is a schematic block diagram illustrating a system for electronic article surveillance equipped with a magnetic-type tag constructed in accordance with the present invention.

Fig. 9 is a schematic graph, similar to fig. 4, showing the relationship between the magnetization level and the strength of the applied DC magnetization field applied to the material used as the biasing element in this example constructed in accordance with the third embodiment of the present invention.

Fig. 10 is a schematic graph, similar to fig. 5 and 6, showing the relationship between the level of magnetization produced and the strength of an applied AC deactivation magnetic field applied to a material used as a biasing element in this example and constructed in accordance with a third embodiment of the present invention.

Fig. 11 is a schematic graph, similar to fig. 2, 3 and 7, showing the variation of the resonant frequency and the amplitude of the output signal of a magnetic marker constructed according to a third embodiment of the present invention with the strength of the applied demagnetizing field.

Description of the preferred embodiments and examples of the invention

The biasing element 16 in a marker constructed in accordance with the present invention, similar to the marker of fig. 1 described above, is fabricated from an alloy material having very low coercivity characteristics, such as the so-called MagnaDur20-4 (which has a coercivity of about 20Oe and is commercially available from Carpenter Technology Corporation, Reading, Pennsylvania, usa), rather than from a material having relatively high coercivity characteristics, such as semi vac 90. In a preferred embodiment of the invention, the activatable element 12 may be made from a strip of amorphous metal alloy material as described above. Such alloys may be, for example, alloys such as2628CoA alloys and the like available from AlliedSignal, AlliedSignal advanced materials, Parsippany, N.J., U.S.A.. Other materials having similar properties may of course be used to form the activatable element 12. 2628CoA alloy is made of Fe₃₂Co₁₈Ni₃₂B₁₃Si₅A compound of (a). The 2628CoA alloy can be continuously annealed by first annealing the material at 360 deg.C for about 7.5 seconds with a DC magnetizing field of 1.2kOe applied in the transverse direction, and then annealing the material at a lower temperature for about 7.5 seconds with substantially the same transverse magnetic field. The two-step annealing process is preferably carried out by passing the continuous ribbon through a processing furnace. The method used may employ methods such as those disclosed by co-pending U.S. patent application serial No. 08/420757, filed 12.4.1995, and the like. This application is assigned to the assignee of the present application. Such an activatable element 12 may be used in a marker manufactured by the assignee of the present application and sold under part number 0630-0687-02.

Fig. 2 shows a schematic characteristic diagram of a known magnetic marker which uses 2628CoA alloy processed as above to make an activatable element and uses SemiVac90 to make a biasing element thereof. Fig. 3 is a schematic graph for comparison showing a schematic characteristic curve of a magnetic-type marker constructed according to the present invention, which uses MagnaDur20-4 instead of semi vac90 to make its bias element.

In fig. 2, reference numeral 20 denotes a characteristic curve of a resonance frequency shift of a conventional marker, which varies with the strength of a demagnetizing field applied to the marker. Such demagnetizing field may be an AC magnetic field or a DC magnetic field applied in a direction opposite to the magnetization direction of the bias element. If this demagnetizing field is an AC field, the represented field level is its peak amplitude. The curve 20 can be understood with reference to the scale (kilohertz) at the left side of fig. 2.

Reference numeral 22 denotes a characteristic curve of the amplitude of the output signal of a conventional marker, which also varies with the strength of the applied demagnetizing field. The curve 22 can be understood with reference to the scale (microvolts) at the right side of fig. 2. The label "a 1" appearing on the right-hand scale of fig. 2 indicates the output signal level produced by the marker at 1 millisecond after the end of the interrogation signal pulse applied to the marker at the resonant frequency indicated by the corresponding vertical point on the curve 20. The resonant frequency of the marker prior to deactivation is 58kHz, which is the standard frequency for interrogation fields of currently known magnetic EAS systems.

The data in fig. 2 also show other key features, such as a demagnetizing field of 50Oe or less, a shift in resonant frequency of less than 1.5kHz for conventional markers, and so on. Moreover, in order to obtain a maximum shift of the resonance frequency with respect to the standard operating frequency, i.e. 58kHz, and to obtain a maximum suppression of the output signal amplitude, it is necessary to apply a demagnetizing field of about 140 to 150 Oe.

In fig. 3, reference numeral 24 denotes a resonance frequency shift characteristic curve of a marker constructed according to the present invention, in which a bias element is made of MagnaDur material, which varies with the applied demagnetizing field. Curve 26 represents the output signal characteristic of a marker constructed in accordance with the present invention, which also varies with the applied demagnetizing field. The output level, as shown by curve 26, varies with the change in the interrogation signal generated at the resonant frequency shown at the corresponding point on curve 24.

An important feature in the characteristic shown in fig. 3 is that a maximum resonance frequency shift of about 60.5kHz can be achieved at applied demagnetizing field strengths as low as about 35 Oe. The abrupt or sharp change in the frequency drift characteristic 24 as shown in fig. 3 is also quite evident: at its mutation point, the gradient of change of curve 24 exceeds 200 Hz/Oe. In contrast, the gradient of curve 20 shown in FIG. 2 at no point exceeds about 60 Hz/Oe. The gradient of the curve 20 is well below 100Hz/Oe at all its points.

Fig. 4 and 5 show a magnetization characteristic curve and a demagnetization characteristic curve, respectively, of a MagnaDur material for manufacturing the bias element in the present invention.

In fig. 4, reference numeral Mra denotes a magnetic level for applying saturation magnetization to a material, and reference numeral Ha denotes a DC magnetic field intensity required for generating saturation magnetization in the material.

As shown in fig. 4, if a DC magnetizing field of about 150Oe is applied to the MagnaDur material in the non-magnetized state, the material will be substantially fully magnetized. In contrast, a DC magnetic field with a magnetic level of 450Oe or more is required to fully magnetize the semi vac90 material.

In fig. 5, reference numeral Mrs denotes the magnetization level at which 95% of the magnetization is saturated, and Hms denotes the level of an AC magnetic field which does not demagnetize a material in a saturated state to 95% or less of the saturation state when applied to the material in the saturated state. Also, reference numeral Mrd denotes a magnetization level when a saturation magnetization of 5% is formed, and reference numeral Hmd denotes a level of an AC magnetic field that will demagnetize a material in a saturated state to 5% or less of the saturation state when applied to the material.

As shown in fig. 5, if a fully magnetized bias element made of MagnaDur material is placed in an AC demagnetizing field of 100Oe, it is demagnetized to less than 5% of the full magnetization. Furthermore, MagnaDur materials have a "stable" region for an applied AC magnetic field of about 20Oe or less, which renders the magnetization of such materials substantially unaffected by an applied AC magnetic field of no more than 20 Oe. Thus, a marker configured with MagnaDur material as a biasing element will not experience relatively large unintentional demagnetization when the ambient magnetic field does not exceed 20 Oe.

A magnetomechanical marker constructed in accordance with the present invention can be deactivated with an AC deactivative magnetic field of much lower strength than would be required under normal conditions, since the biasing element is fabricated from a material having a relatively low coercivity profile, such as MagnaDur or the like. This allows a marker constructed in accordance with the present invention to be deactivatable at a location that is not very close to the deactivators, which is necessary for the prior art. The invention therefore also provides, in effect, a deactivator which can operate at a much lower magnetic level than conventional deactivators. Since the required level of magnetism for deactivation is lower than in the conventional technique, a lower-speed component can be used, and the deactivation magnetic field can be continuously generated, which is necessary in the conventional deactivation apparatus for a pulse-type magnetic field. Since a continuous, rather low deactivation field can be used, it is no longer necessary to provide the deactivation device with a circuit for detecting the presence of the marker and a circuit for the device operator to trigger the deactivation field pulse. This may reduce the costs associated with the deactivation device and may eliminate the effort of an operator to trigger the pulsed deactivation device.

Moreover, a marker constructed in accordance with the present invention and composed of a bias element having a relatively low coercive force characteristic can utilize the original deactivation device and can be more easily deactivated than a marker using a bias element constructed by semi vac 90.

As will be apparent from the description given herein, since the marker can be deactivated with a magnetic field of lower strength, deactivation can be performed at a position where the marker is located further from the deactivation device than in the prior art, and thus it can also be better applied to source-target relative operation. If so, for example, a marker constructed in accordance with the present invention is one that can be deployed at a position one foot away from the coil that generates the deactivation magnetic field.

In a second embodiment constructed in accordance with the present invention, the biasing element 16 is fabricated from a material having a lower coercivity than the MagnaDur material and having no stable region in a magnetic field below 20 Oe. By way of example, a biasing element 16 constructed in accordance with the second embodiment of the present invention may be made from materials such as Metglas 2605SB1, and the like, and these materials are commercially available from AlliedSignal, Inc., as described above. This material may be treated as described below to obtain the desired magnetization characteristics.

In other words, a continuous strip of SB1 material may be cut into rectangular pieces having a length of about 28.6 mm and a width about equal to the width of the activatable element. The cut pieces of material are placed in a lower treatment furnace at room temperature and in a substantially pure nitrogen atmosphere. The material was heated to approximately 485 c and held at this temperature for one hour to prevent dimensional distortion during subsequent processing. The temperature was then increased to about 585 ℃. After one hour at this temperature, outside air was admitted to the treatment furnace to oxidize the material. After oxidation for one hour at a temperature of 585 deg.c, nitrogen was again introduced into the treating furnace to exhaust ambient air, and the oxidation process was terminated. The treatment was carried out for one more hour under a nitrogen atmosphere at a temperature of 585 ℃. Subsequently, the temperature was raised to 710 ℃ and the treatment was continued for one hour under a purified nitrogen atmosphere, followed by cooling to room temperature. After complete cooling, it was exposed to air. (all temperature parameters described above were measured on the samples being processed during all treatments.)

The coercivity of the material after this annealing treatment was about 19Oe, and the demagnetization characteristic curve thereof is shown in fig. 6. As shown in FIG. 6, the annealed SB1 alloy was substantially demagnetized (demagnetized to about 70% of full magnetization) using an AC magnetic field as low as 15 Oe.

Despite the instability of the SB1 material in low-level AC fields, the applicant has found that if the material is provided as a bias element in a magnetic marker in close proximity to an activatable element, the resulting marker will have a relatively high stability when exposed to relatively low AC fields, as can be inferred from the deactivation characteristics of the SB1 material as a material per se.

Fig. 7 shows the resonance frequency shift characteristic curve and the output signal amplitude characteristic curve of the marker when the magnetic bias element is made of the SB1 material after the annealing treatment and the activatable element is made of the 2628CoA alloy material. In fig. 7, a curve 28 shows the resonance frequency shift characteristic of the marker using the SB1 material, which varies with the intensity of the demagnetizing field applied, and a curve 30 shows the amplitude characteristic of the output signal of the marker. Curve 28 can be understood with reference to the scale (kilohertz) on the right side of the figure and curve 30 can be understood with reference to the scale (microvolts) on the left side of the figure.

As shown in fig. 7, when a demagnetizing field of a low level (e.g., 5 to 15Oe) is applied to a marker using SB1 material, the characteristics of the marker, particularly the resonant frequency thereof, are not substantially changed, and thus no demagnetization occurs, although this field may cause significant demagnetization of the biasing element when it is separately provided. It will be appreciated that under the level of the applied deactivation field, the field will couple between the activatable element and the biasing element, thereby causing the activatable element to act as a shunt protecting the SB1 material element from the demagnetization field. When the level of the demagnetizing field applied exceeds about 15Oe, the permeability of the activatable element will decrease sharply, so that the demagnetizing field will cause demagnetization of the biasing element. In general, the frequency drift and output signal characteristics herein can be substantially stabilized at a demagnetizing field level of about 15Oe or less, and substantially sharply changed at a demagnetizing field level of 20 to 30 Oe. Between 20 and 25Oe, the gradient of the resonance frequency shift characteristic will exceed 100 Hz/Oe. It is further noted that when the demagnetizing field strength applied is below 50Oe, a very large shift in resonant frequency (greater than 1.5kHz) will occur and the a1 output signal will be substantially cancelled.

The biasing element can be made of a fairly unstable material that is less costly than not only the conventional semi vac90 material, but also the MagnaDur material due to the shielding provided by the activatable element.

The heat treatment sequence described above may be modified so that the final hour of annealing is performed at 800 c instead of 710 c so that the coercivity of the annealed SB1 material is 11 Oe.

In a third embodiment constructed in accordance with the present invention, the biasing element 16 in the marker 10 is constructed of a so-called Vacozet alloy, and such an alloy is commercially available from Vacuumschmelze GmbH, Gr ü ner Weg37, D-63450, Hanau, Germany. The coercivity of this Vacozet material was 22.7 Oe. [ reference data for Vacozet are used here ]

The magnetization characteristics of the Vacozet material are shown in FIG. 9, and the demagnetization characteristics of this material are shown in FIG. 10. As shown in fig. 9, a DC magnetic field of about 50Oe is sufficient to substantially fully magnetize the material. Fig. 10 shows that if a fully magnetized bias element made of Vacozet material is placed in an AC demagnetizing field of about 30Oe, the element will be demagnetized to less than 5% of full magnetization. As with the SB1 material, this Vacozet material will have some stability when exposed to low magnetic levels of AC magnetic fields, such as those with peak amplitudes of 6 to 15 Oe. But the magnetization of such a material will decrease by less than 5% when exposed to an AC magnetic field having a peak amplitude of 5Oe or less.

FIG. 11 shows the resonant frequency shift and output signal amplitude characteristics of the marker when the biasing element is made of Vacozet material and the activatable element is made of 2628CoA material. In fig. 11, a curve 32 shows a characteristic curve of a shift in resonance frequency of a marker using a Vacozet material, which varies with the intensity of an applied demagnetizing field, and a curve 34 shows a characteristic curve of the amplitude of an output signal of the marker. Curve 32 may be illustrated with reference to the scale (kilohertz) on the right side of the figure and curve 34 may be illustrated with reference to the scale (microvolts) on the left side of the figure.

As shown in fig. 11, the frequency drift and amplitude characteristic curve will remain fairly stable when a demagnetizing field of a certain low level is applied, which can be estimated from the demagnetizing characteristic curve when the bias material is set alone, as shown in fig. 10. In other words, such markers employing Vacozet materials will have some "shielding" effect, as described in the examples relating to the SB1 material. The embodiment using the Vacozet material will have a greater frequency drift when a demagnetizing field of low level is applied, and it will also have a sharper (i.e., more "abrupt") frequency drift characteristic than the embodiment using the SB1 material. By analyzing the region between the 10 and 14Oe points in the frequency shift characteristic 32 shown in fig. 11, the frequency shift will exceed 1.6kHz, and the corresponding gradient will exceed 400 Hz/Oe. Thus, a marker using Vacozet material can be effectively deactivated by applying a demagnetizing field with a magnitude below 20 Oe.

The biasing element 16 configured in the third embodiment constructed in accordance with the present invention may be fabricated by rolling a Vacozet alloy into a thin sheet in crystalline form. The material has a relatively high magnetic flux density due to its relatively low coercive force characteristics, so that the thickness of the material can be made thinner than that of a conventional bias member, and the weight of the material used can be reduced, and the cost corresponding thereto can be saved.

In addition to the MagnaDur alloy, the Vacozet alloy, and the SB1 alloy described above, other materials may be used to form the biasing element 16, including, for example, various materials having the characteristic curves shown in fig. 4, 5, 6, 9, and 10.

In addition to using 2628CoA alloy material that has been subjected to a continuous annealing process, other materials may be used to form the activatable element 12. By way of example, the foundry-type Metglas2628MB material used in conventional magnetic markers for making activatable elements can also be used. The activatable element may also be made from an alloy disclosed in U.S. patent 5469140 that has been cross field annealed. The material disclosed in U.S. patent application serial No. 08/508580 (filed as 1995, 7, 28, and assigned to the present assignee) may also be used to make the activatable element.

Markers constructed in accordance with the present invention have some stability when exposed to relatively low magnetic fields that do not cause the flip effect of conventional markers. It has now been found, however, that markers constructed in accordance with the present invention will not be inadvertently demagnetized due to factors common in the environment of use. The present invention, given by Richard l.copeland, one of the applicants of the present invention, and by Ming r.lian, an employee of dr.copeland, can reduce the risk of accidental deactivation by magnetization treatment of each biasing element in the marker by magnetizing approximately half of the elements to one polarity and the remainder to the other polarity. When a large number of markers are stacked together to form a bundle for shipping and storage, the opposing magnetic properties cancel each other out so that the marker stack in a relatively small volume does not create a relatively large "leakage" magnetic field that tends to demagnetize some of the biasing elements.

Fig. 8 illustrates a pulse interrogation EAS system constructed in accordance with the invention using a magnetic marker with a biasing element made of MagnaDur material or annealed SB1 alloy. The system shown in fig. 8 includes a synchronization circuit 200 for controlling the operation of the excitation circuit 201 and the reception circuit 202. The synchronization circuit 200 sends a synchronization gate pulse signal to the pump circuit 201, and this synchronization gate pulse signal activates the pump circuit 201. The excitation circuit 201, once activated, sends an interrogation signal to the interrogation coil 206 for the duration of the synchronization pulse. The interrogating coil 206 will generate an interrogating magnetic field in response to the interrogating signal, thereby triggering the mechanical response of the tag 10.

Once the pulse interrogation signal is terminated, the synchronization circuit 200 will issue a gate pulse to the receiving circuit 202 and activate the receiving circuit 202 with the gate pulse. During the time period in which the receiving circuit 202 is activated, if a marker is located in the interrogating magnetic field, this marker will generate a signal in the receiving coil 207 whose frequency is the mechanical response frequency of the marker. The receiving circuit 202 may detect this signal and, based on the detected signal, generate a signal that is sent to the indicator 203 to generate an alarm signal or the like. Thus, the receiving circuit 202 will be synchronized with the excitation circuit 201, such that the receiving circuit 202 is only activated in quiet periods between pulses of the pulsed interrogation field.

The system shown in fig. 8 may operate with a single frequency interrogation signal generated by a pulse. However, magnetic EAS systems preferably operate with a sweep or hop frequency type interrogation signal and detect the presence of an activated marker by detecting the insertion of a variable frequency interrogation signal by a magnetic marker. One implementation of such a sweep frequency system is disclosed in U.S. patent 4510489, discussed above.

Markers constructed in accordance with the present invention have a sharply varying resonant frequency drift characteristic, and are therefore particularly well suited for use in magnetic EAS systems that operate by detecting the resonant frequency of the marker, rather than its output signal level.

Other variations of the marker described above and other modifications of the described embodiments may be made by those skilled in the art without departing from the scope of the invention. The preferred embodiments of the present invention are described by way of example only, and not by way of limitation. The scope of the invention is defined by the claims set forth below.

Claims (47)

1. A tag for use in a magnetic-based electronic article surveillance system, comprising:

(a) an amorphous magnetostrictive element;

(b) a bias element disposed adjacent to said magnetostrictive element,

the marker is characterized by having the resonance frequency shift characteristic along with the change of the deactivation magnetic field, and the shift gradient is more than 100 Hz/Oe.

2. A marker according to claim 1, wherein the gradient of the shift characteristic of the resonant frequency of the marker with change in the deactivating magnetic field is greater than 200 Hz/Oe.

3. A marker according to claim 2, wherein the gradient of the shift characteristic of the resonant frequency of the marker with change in the deactivating magnetic field is greater than 400 Hz/Oe.

4. A tag for use in a magnetic-based electronic article surveillance system, comprising:

(a) an amorphous magnetostrictive element;

(b) a bias element disposed adjacent to said magnetostrictive element,

characterized in that said biasing element is made of a semi-hardened magnetic material having a coercivity Hc of less than 55 Oe.

5. A marker according to claim 4, wherein said bias element has an AC demagnetizing field characteristic such that the magnetization level of said bias element is maintained at least 95% or greater of the full magnetization level when said bias element is in the fully magnetized state and exposed to an AC magnetic field Hms having a peak amplitude of 4 Oe.

6. A marker according to claim 4, wherein said biasing element is formed of a semi-hardened magnetic material having a coercivity Hc of less than 40 Oe.

7. A marker according to claim 6, wherein said biasing element is formed of a semi-hardened magnetic material having a coercivity Hc of less than 20 Oe.

8. A marker according to claim 7, wherein said bias element has an AC demagnetizing field characteristic such that the magnetization level of said bias element is maintained at least 95% or greater of the full magnetization level when said bias element is in the fully magnetized state and exposed to an AC magnetic field Hms having a peak amplitude of 4 Oe.

9. A tag for use in a magnetic-based electronic article surveillance system, comprising:

(a) an amorphous magnetostrictive element;

(b) a bias element disposed adjacent to said magnetostrictive element,

the magnetic bias element is made of semi-hardened magnetic material with DC magnetization magnetic field characteristics, and the DC magnetic field Ha required by the magnetic bias element to reach saturation is smaller than 350 Oe.

10. A marker according to claim 9, wherein said bias element has an AC demagnetizing field characteristic such that when said bias element is in a fully magnetized state and exposed to an AC magnetic field Hms having a peak amplitude of 4Oe, the magnetization level of said bias element remains at least 95% or more of the full magnetization level.

11. A marker as claimed in claim 10, wherein said DC magnetization characteristic is such that the DC magnetic field Ha required to saturate said biasing element is less than 200 Oe.

12. A marker as claimed in claim 11, wherein said DC magnetization characteristic is such that the DC magnetic field Ha required to saturate said biasing element is less than 150 Oe.

13. A marker as claimed in claim 12, wherein said DC magnetization characteristic is such that the DC magnetic field Ha required to saturate said biasing element is less than 50 Oe.

14. A tag for use in a magnetic-based electronic article surveillance system, comprising:

(a) an amorphous magnetostrictive element;

(b) a bias element disposed adjacent to said magnetostrictive element,

characterized in that said bias element is made of a semi-hardened magnetic material having an AC demagnetizing field characteristic such that when an AC demagnetizing field Hmd having a peak amplitude of less than 150Oe is applied to the bias element in a fully magnetized state, said bias element is demagnetized to a level of not more than 5% of the fully magnetized level.

15. A marker according to claim 14, wherein said bias element has an AC demagnetizing field characteristic such that when said bias element is in a fully magnetized state and exposed to an AC magnetic field Hms having a peak amplitude of 4Oe, the magnetization level of said bias element remains at least 95% or more of the full magnetization level.

16. A marker according to claim 15, wherein said bias element has an AC demagnetizing field characteristic such that the magnetization level of said bias element is maintained at least greater than 95% of the full magnetization level when said bias element is in the fully magnetized state and exposed to an AC magnetic field Hms having a peak amplitude of 20 Oe.

17. A marker according to claim 15, wherein said bias element has an AC demagnetizing field characteristic such that when an AC demagnetizing field Hmd having a peak amplitude less than 100Oe is applied to said bias element in a fully magnetized state, said bias element is demagnetized to a level no greater than 5% of the fully magnetized level.

18. A marker according to claim 17, wherein said bias element has an AC demagnetizing field characteristic such that a magnetization level of said bias element remains at least 95% of a full magnetization level when said bias element is in a fully magnetized state and exposed to an AC magnetic field Hmd having a peak amplitude of 12 Oe.

19. A marker according to claim 15, wherein said bias element has an AC demagnetizing field characteristic such that when an AC demagnetizing field Hmd having a peak amplitude less than 30Oe is applied to said bias element in a fully magnetized state, said bias element is demagnetized to a level no greater than 5% of the fully magnetized level.

20. A tag for use in a magnetic-based electronic article surveillance system, comprising:

(a) an amorphous magnetostrictive element;

(b) a bias element disposed adjacent to said magnetostrictive element,

characterized in that the target resonance frequency of the marker corresponds to the operating frequency of the system for electronic article surveillance,

the marker has a resonant frequency shift characteristic associated with a deactivating magnetic field such that the resonant frequency of the marker shifts at least 1.5kHz from the target resonant frequency when the marker is exposed to an AC deactivating magnetic field having a peak amplitude of 50Oe or less.

21. A marker according to claim 20, wherein said resonant frequency shift characteristic associated with said deactivation magnetic field is such that when said marker is exposed to an AC deactivation magnetic field having a peak amplitude of 50Oe or less, the resonant frequency of said marker shifts at least 2kHz from said target resonant frequency.

22. A marker according to claim 21, wherein said resonant frequency shift characteristic associated with said deactivation magnetic field is such that when said marker is exposed to an AC deactivation magnetic field having a peak amplitude of 35Oe or less, the resonant frequency of said marker shifts at least 2kHz from said target resonant frequency.

23. A marker according to claim 21, wherein said resonant frequency shift characteristic associated with said deactivation field is such that when said marker is exposed to an AC deactivation field having a peak amplitude of 35Oe or less, the shift in the resonant frequency of said marker relative to said target resonant frequency is at least 11 Hz.

24. A marker according to claim 23, wherein said resonant frequency shift characteristic associated with said deactivation magnetic field is such that when said marker is exposed to an AC deactivation magnetic field having a peak amplitude of less than 20Oe, the resonant frequency of said marker shifts at least 1kHz from said target resonant frequency.

25. A tag for use in a magnetic-type electronic article surveillance system, such as a system that may emit a tag interrogation signal in intermittent bursts at a predetermined frequency, the tag comprising:

(a) an amorphous magnetostrictive element;

(b) a bias element disposed adjacent to said magnetostrictive element,

characterised in that the output signal characteristic of the marker as a function of the deactivating magnetic field is such that when the marker is exposed to an AC deactivating magnetic field having a peak amplitude below 35Oe, the a1 output signal level produced by the marker will reduce the a1 output signal level produced when the marker is exposed to the deactivating magnetic field by at least 50% (where the a1 output signal is the signal produced by the marker 1 millisecond after the end of the interrogation signal pulse applied to the marker).

26. A marker according to claim 25, wherein said bias element has an AC demagnetizing field characteristic such that when said bias element is in a fully magnetized state and exposed to an AC magnetic field having a peak amplitude of 4Oe, the magnetization level of said bias element is maintained at least 95% or more of the full magnetization level.

27. A marker according to claim 26, wherein the output signal characteristic of said marker as a function of said deactivation field is such that when said marker is exposed to an AC deactivation field having a peak amplitude below 25Oe, the level of the a1 output signal produced by said marker will reduce the level of the a1 output signal produced when said marker is exposed to said demagnetization field by at least 50%.

28. A marker according to claim 26, wherein the output signal characteristic of said marker as a function of said deactivation field is such that when said marker is exposed to an AC deactivation field having a peak amplitude below 30Oe, the level of the a1 output signal produced by said marker will reduce the level of the a1 output signal produced when said marker is exposed to said demagnetization field by at least 75%.

29. A marker according to claim 26, wherein the output signal characteristic of said marker as a function of said deactivation field is such that when said marker is exposed to an AC deactivation field having a peak amplitude below 35Oe, the level of the a1 output signal produced by said marker will reduce the level of the a1 output signal produced when said marker is exposed to said demagnetization field by at least 75%.

30. A tag for use in a magnetic-based electronic article surveillance system, comprising:

(a) an amorphous magnetostrictive element;

(b) a bias element disposed adjacent to said magnetostrictive element,

characterized in that said biasing element is made of a semi-hardened magnetic material having an AC demagnetizing field characteristic, whereby when said biasing element is exposed to an AC magnetic field having a peak amplitude when fully magnetized and not disposed in said marker, said AC magnetic field will cause a significant reduction in the magnetization level of said biasing element,

and when the biasing element is fully magnetized and disposed in the marker adjacent the magnetostrictive element and exposed to an AC magnetic field having a peak amplitude, the magnetostrictive element biases magnetic flux away from the biasing element such that the magnetization of the biasing element is substantially unaffected by the AC magnetic field.

31. A marker according to claim 30, wherein said biasing element is formed from Metg1as2605SB1 material.

32. A marker according to claim 31, wherein said biasing element is formed from Metglas2628MB material.

33. A marker according to claim 31, wherein said amorphous magnetostrictive element is formed from Metglas2628CoA material.

34. A marker according to claim 30, wherein said biasing element is formed of Vacozet material.

35. A marker according to claim 34, wherein said amorphous magnetostrictive element is formed from Metglas2628CoA material.

36. A marker according to claim 30, wherein said AC magnetic field has a peak amplitude in a range from about 5Oe to about 15 Oe.

37. A method of activating and deactivating an EAS marker for use in a magnetic EAS system, the method comprising the steps of:

providing an EAS marker comprising an amorphous magnetostrictive element and a biasing magnetic element disposed adjacent said magnetostrictive element;

magnetizing said biasing element such that said biasing element generates a magnetic field that biases said magnetostrictive element to resonate at an operating frequency of said EAS system;

deactivating the EAS marker by exposing the marker to an AC magnetic field having a peak amplitude below 150 Oe.

38. A method according to claim 37, wherein the resonant characteristics of the marker are substantially unchanged when the marker is exposed to an AC magnetic field having a peak amplitude of 4Oe or less.

39. A method according to claim 38, wherein the resonant characteristics of the marker are substantially unchanged when the marker is exposed to an AC magnetic field having a peak amplitude of 20Oe or less.

40. A method as in claim 38 wherein said deactivating step is performed by exposing said marker to an AC magnetic field having a peak amplitude of less than 100 Oe.

41. A method according to claim 40, wherein the resonant characteristics of the marker are substantially unchanged when the marker is exposed to an AC magnetic field having a peak amplitude of 12Oe or less.

42. A method as in claim 37 wherein said deactivating step is performed by exposing said marker to an AC magnetic field having a peak amplitude of less than 30 Oe.

43. A method according to claim 42 wherein the resonant characteristics of the marker are substantially unchanged when the marker is exposed to an AC magnetic field having a peak amplitude of 4Oe or less.

44. A method as in claim 37 wherein said deactivating step is performed by exposing said marker to an AC magnetic field having a peak amplitude of less than 16 Oe.

45. A method according to claim 44 wherein the resonant characteristics of the marker are substantially unchanged when the marker is exposed to an AC magnetic field having a peak amplitude of 6Oe or less.

46. A method as in claim 37 wherein said magnetizing step is performed after said biasing element is installed in said marker.

47. A method as in claim 37 wherein said magnetizing step is performed prior to mounting said biasing element in said marker.