EP0431745A2 - Sensibilisateur pour étiquettes ferromagnétiques utilisées avec des systèmes électromagnétiques de surveillance d'articles - Google Patents

Sensibilisateur pour étiquettes ferromagnétiques utilisées avec des systèmes électromagnétiques de surveillance d'articles Download PDF

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Publication number
EP0431745A2
EP0431745A2 EP90311821A EP90311821A EP0431745A2 EP 0431745 A2 EP0431745 A2 EP 0431745A2 EP 90311821 A EP90311821 A EP 90311821A EP 90311821 A EP90311821 A EP 90311821A EP 0431745 A2 EP0431745 A2 EP 0431745A2
Authority
EP
European Patent Office
Prior art keywords
marker
gap
assembly
magnetic
current source
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Withdrawn
Application number
EP90311821A
Other languages
German (de)
English (en)
Other versions
EP0431745A3 (en
Inventor
Samuel C/O Minnesota Mining And Montean
Heinrich C/O Minnesota Mining And Schug
Juergen C/O Minnesota Mining And Sanetra
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
3M Co
Original Assignee
Minnesota Mining and Manufacturing Co
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Minnesota Mining and Manufacturing Co filed Critical Minnesota Mining and Manufacturing Co
Publication of EP0431745A2 publication Critical patent/EP0431745A2/fr
Publication of EP0431745A3 publication Critical patent/EP0431745A3/en
Withdrawn legal-status Critical Current

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    • G—PHYSICS
    • G08—SIGNALLING
    • G08B—SIGNALLING OR CALLING SYSTEMS; ORDER TELEGRAPHS; ALARM SYSTEMS
    • G08B13/00—Burglar, theft or intruder alarms
    • G08B13/22—Electrical actuation
    • G08B13/24—Electrical actuation by interference with electromagnetic field distribution
    • G08B13/2402—Electronic Article Surveillance [EAS], i.e. systems using tags for detecting removal of a tagged item from a secure area, e.g. tags for detecting shoplifting
    • G08B13/2405—Electronic Article Surveillance [EAS], i.e. systems using tags for detecting removal of a tagged item from a secure area, e.g. tags for detecting shoplifting characterised by the tag technology used
    • G08B13/2408—Electronic Article Surveillance [EAS], i.e. systems using tags for detecting removal of a tagged item from a secure area, e.g. tags for detecting shoplifting characterised by the tag technology used using ferromagnetic tags
    • G08B13/2411—Tag deactivation

Definitions

  • This invention relates to electromagnetic article surveillance (EAS) systems of the type in which an alternating magnetic field is applied within an interrogation zone, 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.
  • the marker includes both a high-permeability low-coercive force portion, and at least one magnetizable section having a higher coercive force than the low-coercive force portion. When the higher coercive force section is magnetized, it alters the detectable signal otherwise produced.
  • markers are known as "dual status" markers.
  • An example of a dual status marker is taught in U.S. Patent 4,825,197 (Church and Heltemes).
  • EAS systems of this type are, for example, disclosed and claimed in U.S. Patent 3,665,449 (Elder 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 force section demagnetized) by placing the marker in a large AC field, and gradually withdrawing the marker.
  • German Offenlegungsschrift DE 30 14 667 Al depicts a type of desensitizer employing 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 polarities of the magnetic fields produced alternate.
  • RLC resistive-inductive-capacitive
  • 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).
  • the apparatus of the present invention comprises a ferromagnetic core having two surfaces which face, but do not touch, each other and thereby define a gap.
  • the gap may be formed by surfaces of a pair of pole pieces which concentrate external magnetic flux.
  • 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.
  • an alternating current source is used, in which case a sinusoidal magnetic field is created and the apparatus operates as a resensitizer.
  • the present invention may be used as a desensitizer.
  • Figure 1 is a cross sectional view of 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 embodiment of a circuit portion of the invention.
  • Figures 4A, 4B, and 4C are electronic schematic diagrams of one embodiment of a circuit portion of the invention.
  • 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.
  • an article 16 is moved in the direction shown by arrow 22 so that a resensitizable marker (not shown) which is affixed to the exterior of the article 16 will 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 moved across the horizontal surface.
  • the housing 11 of the apparatus 10 is preferably constructed from non-magnetic materials, e.g., finished hardwood, injection-molded plastic, or non-magnetic metals.
  • the housing 11 may carry appropriate legends, manufacturer identification, instructions, and the like.
  • the cover plate 14 provides a surface over which articles affixed with resensitizable markers may be passed during use of the apparatus.
  • a cover plate 14 may comprise polished stainless steel having a thickness in the range of 0.1mm.
  • the cover plate 14 should be polished metal, as such a surface resists scratching or chipping, and thus remains aesthecially acceptable even over many cycles of use.
  • the marker typically comprises a piece of a high-permeability, low-coercive force ferromagnetic material such as permalloy, certain amorphous alloys, or the like.
  • the 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, "ARNOKROME” (a tradename of the Arnold Engineering Company) or the like, having a coercive force in the range of 0.25 to 3.0 Ampere/meter (A/m).
  • ARNOKROME a tradename of the Arnold Engineering Company
  • A/m Ampere/meter
  • 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.
  • the article 16 includes an outer enclosure 26, and a prerecorded audio cassette 28.
  • the cassette 28 includes a reel of magnetic tape 30 having one portion 32 passing along a tape path in the vicinity of the assembly 13.
  • the configuration of the article 16 thus presents a worst case: a portion of 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.
  • the assembly 13 comprises a high-permeability core 40 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 10.
  • 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 assembly 13 further comprises a conductor 43 wound around the core 40.
  • the conductor 43 is many turns of wire, but for clarity in Figure 1, only a single winding is shown.
  • the conductor 43 is electromagnetically coupled to the core 40 and is electrically connected to an electrical current source (not shown).
  • an electrical current source not shown.
  • 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. However, 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.
  • 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 1 and the prerecorded tape will never be closer than approximately 6mm from the gap 44 as shown in Figure 1.
  • the high-coercive force sections of the marker will typically be separated from the assembly 13 only by the thickness of the cover plate 14 (i.e., about 0.1 mm) and will thus typically be exposed to a much greater field intensity.
  • magnetic recording media typically have a coercive force of 3.75-8.75 A/m. Therefore, the magnetic fields required to resensitize the marker can leave 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.
  • the external field intensity extending beyond a short distance from the gap is insufficient to alter a magnetic state which may exist within an article to which the marker is secured.
  • alternating current is used and the apparatus operates as a resensitizer of previously desensitized markers.
  • the conductor may be treated as an inductive coil.
  • 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.
  • 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.
  • 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 effective 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 rate" of no more than about 25% of the previous cycle of the AC field. The drop rate can be halved by doubling the frequency.
  • the change in inductance of the circuit which occurs as the marker is passed over the gap should be taken into account.
  • a suitable core 40 in the configuration of 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 A/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.
  • FIG. 2A 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 46 has a preferred gap length of 1 mm, but other lengths are possible by adjusting the size and/or positioning of pole pieces 46.
  • assembly 13''' comprises an "E-shaped" core 40''' which has two gaps 44'''.
  • the conductor 43''' is wound within the two interior regions 41'''.
  • optional pole pieces 46' define gaps 44'''.
  • Assemblies constructed according to the designs of Figures 2A, 2B, and 2C may be assembled from commercially available ferrite cores, as opposed to custom-made assemblies.
  • an assembly 13 constructed according to the embodiment of Figure 1 is 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 2A.
  • the former value was approximately 6% as opposed to approximately 20% for the latter.
  • FIG 3 is a block diagram of a suitable circuit for use with the embodiment of Figure 2A.
  • 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. If 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 58 compensates for the power loss of the resonanting circuit. This circuit shows very good independence of resonant frequency with changes in ambient temperature over the range of 20 to 60@C, and relatively good independence of gap field intensity with changes in ambient temperature over the range of 20 to 40@C.
  • Figure 4A shows an example 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.
  • Figure 4B a circuit corresponding to precision voltage reference 57 of Figure 3 is shown, including potentiometers P1 and P2 and jumper JP1, 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.
  • a variety of embodiments and alternative configurations of the apparatus of the present invention are possible, including the use of a variety of wire types, number of turns, and the like; a variety of 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.
  • an apparatus according to the present invention may be constructed having variable length gaps, or varying width gaps.
  • the core need not have parallel faces forming the gap as shown in the figures, but may have beveled or tapered faces to focus magnetic flux, as is known in the art.

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  • Physics & Mathematics (AREA)
  • Engineering & Computer Science (AREA)
  • Automation & Control Theory (AREA)
  • Computer Security & Cryptography (AREA)
  • Electromagnetism (AREA)
  • General Physics & Mathematics (AREA)
  • Burglar Alarm Systems (AREA)
  • Geophysics And Detection Of Objects (AREA)
EP19900311821 1989-12-08 1990-10-29 Sensitizer for ferromagnetic markers used with electromagnetic article surveillance systems Withdrawn EP0431745A3 (en)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
US44768489A 1989-12-08 1989-12-08
US447684 1989-12-08

Publications (2)

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EP0431745A2 true EP0431745A2 (fr) 1991-06-12
EP0431745A3 EP0431745A3 (en) 1991-11-21

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EP19900311821 Withdrawn EP0431745A3 (en) 1989-12-08 1990-10-29 Sensitizer for ferromagnetic markers used with electromagnetic article surveillance systems

Country Status (3)

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EP (1) EP0431745A3 (fr)
JP (1) JPH03189794A (fr)
CA (1) CA2029245A1 (fr)

Cited By (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
GB2263848A (en) * 1992-01-13 1993-08-04 Ramesh Chandra Patel Security tag activating/deactivating apparatus
FR2720538A1 (fr) * 1994-05-25 1995-12-01 Jacques Lewiner Dispositif pour désactiver des éléments antivols magnétiques.
EP0696785A1 (fr) * 1994-08-09 1996-02-14 Sensormatic Electronics Corporation Appareil pour désactiver les étiquettes de surveillance électronique d'articles
EP0715284A3 (fr) * 1994-11-28 1996-06-12 Sensormatic Electronics Corporation Désactivateur d'étiquettes antivol
WO1997025695A1 (fr) * 1996-01-08 1997-07-17 Minnesota Mining And Manufacturing Company Appareil destine a modifier l'etat de marqueurs magnetiques dans un systeme de surveillance electronique d'articles
EP0951000A2 (fr) * 1998-04-17 1999-10-20 Meto International GmbH Dispositif pour désactiver un élément de sécurité pour la protection électronique d'articles

Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3665449A (en) * 1969-07-11 1972-05-23 Minnesota Mining & Mfg Method and apparatus for detecting at a distance the status and identity of objects
DE3014667A1 (de) * 1980-04-16 1981-10-29 Werner A. 4300 Essen Reiter Verfahren zum deaktivieren eines sicherungsstreifen s sowie deaktivator und sicherungsstreifen
EP0129335A1 (fr) * 1983-05-20 1984-12-27 Minnesota Mining And Manufacturing Company Désensibilisation d'un marqueur ferromagnétique dans un système de surveillance électromagnétique pour articles
EP0202033A1 (fr) * 1985-05-13 1986-11-20 Magnetic Peripherals Inc. Appareil de démagnétisation

Patent Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3665449A (en) * 1969-07-11 1972-05-23 Minnesota Mining & Mfg Method and apparatus for detecting at a distance the status and identity of objects
DE3014667A1 (de) * 1980-04-16 1981-10-29 Werner A. 4300 Essen Reiter Verfahren zum deaktivieren eines sicherungsstreifen s sowie deaktivator und sicherungsstreifen
EP0129335A1 (fr) * 1983-05-20 1984-12-27 Minnesota Mining And Manufacturing Company Désensibilisation d'un marqueur ferromagnétique dans un système de surveillance électromagnétique pour articles
EP0202033A1 (fr) * 1985-05-13 1986-11-20 Magnetic Peripherals Inc. Appareil de démagnétisation

Cited By (9)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
GB2263848A (en) * 1992-01-13 1993-08-04 Ramesh Chandra Patel Security tag activating/deactivating apparatus
GB2263848B (en) * 1992-01-13 1995-05-31 Ramesh Chandra Patel Security apparatus
FR2720538A1 (fr) * 1994-05-25 1995-12-01 Jacques Lewiner Dispositif pour désactiver des éléments antivols magnétiques.
EP0696785A1 (fr) * 1994-08-09 1996-02-14 Sensormatic Electronics Corporation Appareil pour désactiver les étiquettes de surveillance électronique d'articles
EP0715284A3 (fr) * 1994-11-28 1996-06-12 Sensormatic Electronics Corporation Désactivateur d'étiquettes antivol
WO1997025695A1 (fr) * 1996-01-08 1997-07-17 Minnesota Mining And Manufacturing Company Appareil destine a modifier l'etat de marqueurs magnetiques dans un systeme de surveillance electronique d'articles
AU704121B2 (en) * 1996-01-08 1999-04-15 Minnesota Mining And Manufacturing Company Apparatus for changing the status of magnetic markers in an electronic article surveillance system
EP0951000A2 (fr) * 1998-04-17 1999-10-20 Meto International GmbH Dispositif pour désactiver un élément de sécurité pour la protection électronique d'articles
EP0951000A3 (fr) * 1998-04-17 2000-06-21 Meto International GmbH Dispositif pour désactiver un élément de sécurité pour la protection électronique d'articles

Also Published As

Publication number Publication date
CA2029245A1 (fr) 1991-06-09
EP0431745A3 (en) 1991-11-21
JPH03189794A (ja) 1991-08-19

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