CA2029245A1 - Sensitizer for ferromagnetic markers used with electromagnetic article surveillance systems - Google Patents

Abstract

Abstract of the Disclosure An apparatus for use with magnetically based electronic article surveillance systems employing certain types of markers includes a hollow core having a gap in its perimeter, a coil of wire wrapped around a portion of the core, and appropriate circuitry to drive the combination as an electromagnet. The gap configuration produces an external field of large intensity but limited range, such that the magnetizable portion of a marker is magnetized without affecting magnetic states of the article to which the marker is affixed. Depending on the nature of the circuit, the apparatus may be used as a desensitizer of such markers, or preferably as a resensitizer.
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Description

~ ~ 2 ~ 6~2 CAN BA

SENSITIZER FOR FERROMAGNETIC
MARKERS USED WITH ELECTROMAGNETIC
ARTICLE SURVEILLANCE SYSTEMS

Technical Field This invention relates to electromagnetic article surveillance (EA5) systems of the the type in which an alternating magnetic field is applied within an interrogation æone, and the presence of a high-permeability low coercive force ferromagnetic marker within the zone is detected based on signals produced by the marker in response to the applied field. The present invention is directed to an apparatus for changing the response of such markers.

Background In one type of EAS system, the marker includes both a high-permeability low-coercive force portion, and at least one magnetizable section having a higher cvercive force than the low-coercive force portion. When the higher coercive force section is magnetized, it alters the detectable signal otherwise produced. Such markers are known as l'dual status" markers. An example o~ a dual status marker is taught in U.S. Patent 4,825,1g7 ~Church and Heltemes).
EAS systems o~ this typ~ are, for example, disclosed and claimed in U.S~ Patent 3,665,449 IElder and Wright). As they set forth at column 5, lines 10 to 39, a dual status marker of the type described above may be "sensitized" (i.e., the higher coercive forc~ section demagnetized) by placing the marker in a large AC field, and gradually withdrawing the marker.
German Offenlegungsschrift DE 30 14 667 Al (Reiter) depicts a type of desensitizer employins a resistive-inductive-capacitive (RLC) circuit to produce magnetic fields which steadily alternate in polarity and decrease in magnitude. The magnetic fields are produced by winding the inductive coils around rib-like cores arranged about the desensitization region. The directions of the windings around the coils alternate, and thus the %02(32~

polarities of the magnetic fields produced al~ernate Thus, when the circuit is activated, sharply deEined magnetic zones of alternating polarity arise, through which the article affixed with a marker may be passed.
While such techniques may be useful for the markers affixed to a wide variety of ~rticles, the magnetic fields required for effective resensitization interfere with magnetic states associated with certain articles. For example, the compact size and popularity of prerecorded magnetic audio and video cassettes make such articles frequent targets for shoplifters, and hence likely articles on which EAS markers would be affixed.
However, in a rental situation, when such markers are resensitized upon return from rental, a resensitizer apparatus as described above may unacceptably af$ect the signals prerecorded on the magnetic tapes within the cassettes. Similarly, magnetic disks (flexible or otherwise) or any other magnetic data storage medium may be affected by the resensitizer apparatus.
Commercial embodiments of resensitizers are the Model 950 and 951 resensitizers available from the Minnesota Mining and Manufacturing Company ~3M). Another embodiment is taught in U.S. Patent 4,752,758 (Heltemes).

Di c orure of Invention The apparatus of the present invention compri~s a ferromagnetic co~e having two surfaces which face, but do not touch, each other and thereby deEine a gap.
Optlonally, the gap may be formed by surfaces of a pair of pole pieces which concentrate external magnetic flux.
Furthermore, the core is wrapped with wire, forming an apparatus which may be driven by an electric circuit to produce a magnetic field in the gap. Preferably, an alternating current source is used, in which case a sinusoidal magnetic field is created and the apparatus operates as a resensitizer. However, if a direct current source is used, the present invention may be used as a desensitizer.

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~rie De r~æ ~ thc Drawiny Figure 1 is a cross sectional view o~ an embodiment of the invention;
Figures 2A, 2B, and 2C are cross sectional views of alternative embodiments of a portion of the invention;
Figure 3 is a block diagram of an emhodiment of a circuit portion of the invention; and Figures 4A, 4B, and 4C are electronic schematic diagrams of one embodiment of a circuit portion of the invention Detailed Description As shown in Figure 1, the present invention may be in the form of an apparatus 10 having a housing 11 and a concealed cavity 12. The cavity 12 is covered by a non-magnetic cover plate 14 which both covers and protects an assembly 13 in the cavity 12.
In using the apparatus 10, as shown in Figure 1, an article 16 is moved in the direction shown by arrow ~2 so that a resensitiæable marker (not shown) which is affixed to the exterior of the article 16 wil} pass over the cavity 12, i.e., directly on the cover plate 14. The apparatus 10 may be used with the working surface established by the cover plate 14 in a horizontal position, such that the article 16 may be rnoved ~cros~ the horizontal surface.
The houslng 11 of the apparatus 10 i6 preferably constructed from non--magnetic materials, e.g., ~inished hardwood, in~ection-molded plastic, or non-magnetic metals. rrh~ housing 11 may carry appropriate legends, manuacturer identification, instructions, and the like.
The cover plate 14 provides a surface over which articles af~ixed with resensitizable markers may be passed during use of the apparatus. For example, such a cover plate 14 may co~prise polished stainless steel having a thickness in the range of O.lmm. The cover plate 14 should be polished metal, as such a surface r0sists scratching or chipping, and thus remains aesthecially acceptable even over many cycles of use.

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The marker typically comprises a piece of a high-permeability, low-coercive ~orce feIromagnetic material such as permalloy, certain amorphous alloys, or the like. T}le marker further comprises one or more high-coercive force magnetizable sections in the immediate vicinity of the low-coercive force material. These sections typically are a material such as vicalloy, silicon steel, "ARNQKROME" (a tradename of the Arnold ~ngineering Company) or the like, having a coercive force in the range of 0.25 to 3.0 Ampere/meter (A/m). When such sections are magnetized, the residual ~ields produced magnetically bias the low-coercive force material. This bias substantially alters the signal response produced by the marker in the presence of an interrogating field. To demagnetize the sections, they are brought into close proximity with the assembly 13 within cavity 12, and then mo~ed away.
The assembly 13 is located in the cavity 12.
The cavity 12 is bounded by the housing 11 and the cover plate 14, and open to the latter. The cavity 12 is open to the surface of the apparatus 10, save for the cover plate 14 if one is employed.
For illustrative purposes, the article 16 includes an outer enclosure 26, and a prerecorded audio cassette 28. The cassette 2~ inclu~es a reel of magnetic tape 30 having one portion 32 passing along a tape path in the vicinity of the assembly 13. The conPlguration o~ the article 16 thus presents a worst ca~e: a portion o the tape 32 may be relatively close to the assembly 13, such that the fields which demagnetize the sections could unacceptably affect the magnetic states of the tApe 30, but for the special configuration of the assembly 13.
As shown in Figure 1, the assembly 13 comprises a high-permeability core 4~ which in cross section is substantially continuous around a core interior 41, or "ring shaped," except for a gap 44. The gap 44 is adjacent the surface of the apparatus lOo The length of gap 44, measured from one face to the other, is substantially less than the length of the magnetic circuit around the core interior 41 The ass~mbly 13 further ~2~2~

comprises a conductor 43 wound around the cor~ 40. In practice, the conductor q3 is many turns of wire, but for clarity in Figure 1, only a single winding is ~hown.
The conductor 43 is electromagnetically coupled to the core 40 and is electrically connected to an electrical current source (not shown). When current passes through the conductor 43, a magnetizing field along the magnetic circuit of the core 40 induces magnetic flux throughout the magnetic circuit, and across the gap 44.
The optional bevels 45 in the core 40 concentrate the magnetic flux in the vicinity of the gap 44. ~owever, because the magnetic flux density in the low-permeability gap 44 is substantially less than that in the high-permeability core 40, the magnetic flux "leaks" into regions adjacent the gap 44.
This produces a magnetic field in the direction across the gap which decreases rapidly with perpendicular distance above the gap, and the rate of decrease can be controlled by the selection of gap length. In use, a magnetically sensitive article such as an appropriately boxed prerecorded cassette may be positioned above the working surface of the resensitizer apparatus as shown in Figure l and the prerecorded tape will never be closer than approximately 6mm from the gap 44 as shown in Figure 1. In contrast, the high-coercive force sections o~ the marker will typically be separated from the as~embly 13 only by the thickness of the cover plate 1~ ~i.e., about 0.1 mm) and will thus typically be exposed to a much greater field inten~ity. Also, magnetic ~ecording media typically have a coercive force of 3.75-8.75 A/m.
Therefore, the magnetic fields required to resensitize the marker can lea~e the prerecorded signals on the tape unaffected.
The current source may be direct current, in which case the apparatus operates as a desensitizer of markers. The marker may be moved relative to the gap to expose the section of high coercive force material within the marker to a large magnetic field. As before, the external field intensity extending beyond a short distance from the gap is insufficient to alter a magnetic state ~2~%~.~
~6 which may exist within an article to which the marker is secured. In the preferred embodiment, alternating current is used and the apparatus operates as a resensitizer of previously desensitized markers.
Because the conductor is wound around the magnetic assembly, the conductor may be treated as an inductive coil. Using this concept, a resistor and capacitor can be added in series or in parallel with the conductor to create an RLC circuit with a resonant frequency determined by the appropriate electrical properties of the components.
In general terms, it is preferred ~hat the resensitizer operate effectively when the marker is passed over the gap 44 at a speed of approximately 60 cm/s or less. Non-inventive systems in current use operate effectively at recommended marker speeds of no more than about 8 cm~s. Thus, the preferred resonant frequency of the RLC circuit is 1 KHz or greater, to ensure that a sufficient number of reversals of the field occurs while the marker 18 is being drawn out of the effectlve range of the assembly 13. The actual frequency preferred depends on the speed at which the marker is passed, and the amount of decrease in field strength as a function of distance from the gap. It is preferred that the marker is exposed to a field in which the field strength has a "drop ratel' o~ no more than about 25% o the previous cycle Oe the AC
ield. The drop rate can be halved by doubllng the frequency.
In selecting a frequency, the change in inductance of the circuit which occurs as the marker i5 passed over the gap should be taken into account. This generally means driving the circuit at a refererlce frequency which is slightly less than the calculated resonant frequency, so that the current in the circuit is maximized as the marker is centered over the gap.
Selection of the reference frequency can be done through tests with actual markers being used.
With certain types of markers, it is preferred to shield the assembly and marker from extraneous fields, such as the earth,s magnetic field. Shielding the marker ~7--is often not practical, but shleldincJ the assembly is possible uslng procedures and materials known in the art A suitable core 40 in the configuration o~
Figure 1 was made from 170 laminations of approximately 0.36 mm thick transformer steel, for a total width (i.e., measured perpendicular to the plane of Figure 1~ of approximately 61.2 mm. The gap length was 2.54 mm, and the assembly was wrapped with sixty turns of #23 AWG
enameled wire. Currents on the order of 1.44 to 2.03 amperes were suitable for producing fields in the direction across the gap of about 0.5 1.0 A/m at 6 mm height above the gap, and about 1.5-2.0 ~/m at about 0.25 mm height. This particular embodiment would produce a field of up to the desired 3 A/m if a higher current were used.
An alternative configuration for the core is shown in Figure 2A. The alternative assembly is designated as 13', and portions of it which serve analogous roles to numbered portions of Figure 1 are similarly designated with primed numerals~ The core 40, is essentially "U-shaped" in cross section, and defines core interior 41~. The assembly 13' as shown employs optional pole pieces 46 to define gap 44' and concentrate magnetic flux. The assembly 13' of Figure 2A, including pole pieces 45, has a preferred gap length of 1 mm, but other lengths are possible by ad~usting the size and/or positioning of pole pieces 46.
Other configurations for the core are possible.
For example, as shown in ~igure 2~, two U~shaped cores 41"
may be butted together and sealed at one leg by a sealer 47 to form a gap 44" at the other leg. Then a conductor 43" is wrapp0d around the exterior of the assembly 13" and the core interior 41". As shown in Figure 2C, assembly 13~ comprises an "E-shaped" core 40''' which has two gaps 44'''. In this embodiment the conductor 43''' is wound within the two interior regions 41~. As shown, optional pole pieces 46~,~ define gaps 44~
Assemblies constructed according to the designs of Figures ~, 2B, and 2C may be assembled from commercially available ferrite cores, as opposed to ~292~

custom-made assemblies. ~owever, an assembly 13 constructed according to the embodiment of Figure l i~
preferred because it exhibits less field strength measured at the side of the gap, as a percentage of that measured directly above the gap, than an assembly 13, constructed according to the embodiment of Figure 2~. For representative core assemblies, the former value was approximately 6% as opposed to approximately 20~ for the latter.
Figure 3 is a block diagram of a suitable circuit for use with the embodiment of Figure 2A. In this circuit, a current controlled oscillator holds the current in the coil 50 constant. The coil 50 is in series with a capacitor 51 and a current sense resistor 52. The current in the coil 50 is detected by determining the voltage drop across the sense resistor 52, i.e., the voltage between sense wires 53 and 54. This voltage drop serves as feedback into a control circuit 56 through a rectifier and current sensing amplifier 55. The control circuit 56 is a proportional-integrating regulator which compares the feedback voltage with a precision voltage reference 57.
I these voltages are equal, the circuit resonates at the resonant frequency established by the values of the capacitor 51 and coil 50. A power amplifier $8 compensates for the power loss of the resonanting circuit.
This circuit shows very good independence of rasonant frequency with changes in ambient temperature over the range of 20 to 60@C, and relatively good ind~pendence of gap field intensity with changes in ambient temperature over the range of 20 to 40@C.
Figure 4A shows an e~ample of a circuit built according to the block diagram of Figure 3, suitable for use with the assembly of Figure 2A. The circuit of Figure 4A is powered by the circuit of Figure 4C, which corresponds to the power supply 59 of Figure 3, and which produces suitable positive and negative operating voltages (e.g., ~15 VDC~ and ground level. In Figure 4B, a circuit corresponding to precision voltage reference 57 of Figure 3 - 2 ~
_g is shown, including potentiometers Pl and P2 and ~umper JPl, which allow for adjustment of the reference voltage in the circuit of Figure 4A.
Suitable exemplary components for this circuit are shown in Table I, below, but variations known to those skilled in the art are acceptable. In general, the components are of relatively low tolerance and cost, as the circuit automatically adjusts for the proper resonant frequency despite the component tolerances.

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Table I

Item Component Value or Model, Tolerance, :~atin~

R1 Resistor Buerklin MPC70-OR2~-2W 106 lW
R2 Resistor 6K81 1% 0.25W
R3 Resistor 150K 1% 0.25W
R4 Resistor 33K2 1% 0.25W
R5 Resistor lOOK
R6 Resistor 12K1 1% 0.25W
R7 Resistor 221K 1% 0.25W
R10 Resistor 100K
R12 Resistor 3K32 1% 0.25W
R14 Resistor 2K21 1% 0.25W
P1, P2 Potentiometer Piher PTlOH-lOK (DIN 41450) 10 D1 Diode Valvo/T~ lN4148 D2-5 Diode Valvo/TI lN~OO1 D9 LED Green, 5mm C1 Capacitor Siemens B32650-L3225~1 5% 400V
C2, C3 Capacitor 1 OUF 20% 25V
C4 Capacitor AVX 100N 20% 50V
C5 Capacitor AVX lON 206 50V
C6 Capacitor AVX 3N3 20% 50V
C7 Capacitor 1UE~ 20% 25V
C3, C9 Capacitor Siemens B41010-05103~T lOOOuF 50%

T1 Transistor Valvo BF245A
IC1 Int. Circuit SGS TDA2040V
IC2 Int. Circuit TI T1082P
IC3 Int. Circuit Thomson LM336A
F1, F2 Fuse Buerklin OG (G146, 520) 250V

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A variety of embodiments and alternative con~igurations of the apparatus o~ the present inventlon are possible, including the use of a variety of wire types, number of turns, and the like; a variety oE pole piece configurations; and a variety of driving circuits.
The width of the gap is substantially unlimited, it being limited only by the width of the core and pole pieces ( if used) provided. Thus, an apparatus according to the present invention may be constructed having variable length gaps, or varying width gaps. Furthermore, the core need not have parallel faces forming the gap as ~hown in the figures, bu~ may have beveled or tapered faces to focus magnetic flux, as is known in the art.

Claims (10)

1. An apparatus adapted for changing the status of a resensitizable marker secured to an article used with an electronic article surveillance system, in which the marker includes a low-coercive force, high-permeability ferromagnetic material and at least one section of a remanently magnetizable, relatively higher coercive force material which when magnetized magnetically biases the low coercive force material and thereby alters the detectability of the marker;
the apparatus comprising:

(a) at least one section of ferromagnetic material having two substantially opposed surfaces, the surfaces facing but not touching each other such that a gap exists between the surfaces;
(b) a conductor wound around outer and inner portions of the ferromagnetic material; and (c) a current source capable of driving the assembly such that the ferromagnetic material may be magnetized to present one magnetic polarity at one of the major surfaces and the opposite polarity on the other major surface, and such that the assembly concentrates external magnetic lines of flux near the gap.

2. The apparatus of claim 1, further comprising a housing having a surface adapted to support an article as a marker affixed to the article is moved past the gap, and a cavity within which the assembly is positioned so that the gap of the assembly is adjacent the surface.

3. The apparatus of claim 2, further comprising a thin non-magnetic metallic plate covering the surface.

4. The apparatus of claim 1, in which the current source provides alternating polarity current such that the apparatus operates as a resensitizer.

5. The apparatus of claim 4, in which the current source operates at at least about 1KHz.

6. The apparatus of claim 4, in which the current source operates at a frequency which maximizes current in the coil when a marker is adjacent the gap.

7. The apparatus of claim 4, further comprising shielding which reduces the ambient magnetic flux in the vicinity of the core.

8. The apparatus of claim 4, in which the concentration of magnetic flux is sufficient to resensitize a marker having a higher-coercivity portion of up to 3 ampere/meter in coercivity.

9. The apparatus of claim 4, in which the current source comprises:
(1) a coil in series with a capacitor and a current sense resistor;
(2) means for determining a voltage drop across the sense resistor and using the voltage drop as feedback into a control circuit comprising a proportional-integral regulator with compares the voltage drop with a precision voltage reference such that the current in the coil is held constant.

10. The apparatus of claim 9, in which the current source further comprises means for compensating for power loss of the circuit.