Antipilferage system and marker therefor

• US 3,938,125 A
• Filed: 02/20/1974
• Issued: 02/10/1976
• Est. Priority Date: 02/20/1974
• Status: Expired due to Term

• Current Assignee: 3M Company
• Original Assignee: 3M Company
• Inventors: Benassi, Dominic A.
• Primary Examiner(s): Trafton, David L.
• Application Number:
  US05/443,954
• Time in Patent Office: 720 Days
• Field of Search: 340/258 R, 340/258 C, 340/280, 343/787, 343/6.5 SS, 343/6.8 R, 324/3, 324/41, 29/197, 29/199, 29/194
• US Class Current: 340/572.3
• CPC Class Codes: G08B 13/2408 using ferromagnetic tags
• CPC Class Codes: G08B 13/2442 Tag materials and material properties thereof, e.g. magnetic material details
• CPC Class Codes: Y10S 428/90 Magnetic feature
• CPC Class Codes: Y10T 428/12396 Discontinuous surface component
• CPC Class Codes: Y10T 428/1275 Next to Group VIII or IB metal-base component
• CPC Class Codes: Y10T 428/12757 Fe
• CPC Class Codes: Y10T 428/12986 Adjacent functionally defined components
• CPC Class Codes: Y10T 428/23957 Particular shape or structure of pile

Abstract

An antipilferage system utilizing markers comprising a sheet having both ferromagnetic and electrically conductive characteristics, which markers are detected upon passage through an interrogation zone within which are sequentially generated magnetic fields orthogonally disposed with respect to each other. The marker sheet is preferably a laminate of a ferromagnetic layer and a conductive metal layer, each of which layers exhibits a maximum sensitivity to fields perpendicular to each other. Interrogation of the markers by fields in three dimensions ensures the production of signal components associated with both characteristics of the marker regardless of the orientation of the marker upon passage through the zone. Accordingly, a highly reliable and false alarm free system is provided.

Claims

1. A system for detecting the presence of an object within an interrogation zone comprising:

a. a marker carried by said object, which marker comprises a sheet including a laminate of a ferromagnetic layer and an electrically conductive layer wherein said ferromagnetic layer is characterized by an initial relative permeability in excess of 20,000, a maximum relative permeability in excess of 100,000 and a coercivity less than 0.3 σ

e such that said sheet is capable of responding to a magnetic field having a major field component in one direction in an interrogation zone, which field varies at a predetermined rate of not less than 1 KHz, to cyclically enhance said field in the zone and wherein said electrically conductive layer is characterized by a resistivity of not greater than about 3.0 microhm-cm such that said sheet is capable of responding to another cyclical magnetic field having a major field component in a direction substantially normal to said one direction to diminish said another field in the zone;

b. means defining an interrogation zone;

c. means for sequentially producing in said zone at least three magnetic fields, the intensity of each field periodically varying at a predetermined rate of not less than 1 KHz, a major field component of each field within said zone being orthogonally disposed with respect to a major field component of the other two fields; and

d. means for detecting in the vicinity of the interrogation zone a change in the magnetic field condition due to the presence of a marker in the zone resulting in a signal corresponding to said enhancement and diminishment occurring during at least two of said three produced sequential fields irrespective of the orientation of said marker within said zone.

2. A system according to claim 1, wherein said sheet comprises a ferromagnetic layer having at least two stable magnetic states, which layer is capable of being magnetically switched to any of said states to enhance said field to one degree in one stable state and to enhance said field to another degree when in another of said states, with the difference in the degree of enhancement of said field for said states being sufficient to be detected by said magnetic field sensing means.

3. A system according to claim 2, wherein one of said states corresponds to a desensitized state, said system further including means for desensitizing said marker to cause a said marker when placed within said zone to differently enhance said field.

4. A system according to claim 1, wherein said sequential field producing means sequentially produces three substantially uniaxial mutually orthogonal fields in the zone and wherein

said detecting means comprises at least three magnetic field sensing means each of which is electrically balanced with respect to a corresponding sequentially produced field such that when no marker is present in the zone, the field from a given field producing means is nulled out, resulting in virtually no signal being produced in the cprresponding field sensing means.

5. A system according to claim 4, wherein said detecting means comprises means synchronized to the sequential field producing means for gating the field sensing means to enable the production of a signal from a given field sensing means only during a period when electrical energy is applied to a said corresponding field producing means.

6. A system according to claim 5, wherein the means for sequentially producing at least three magnetic fields comprises a periodically varying signal generator and means coupled thereto for sequentially switching the output of the generator to the field producing means, and wherein

the field detecting means comprises a pulse-decoding means coupled to receive signals passed through said gating means and synchronized to the signal generator for sensing and distinguishing between such signals as are passed through the gating means, the peak intensity of which signals occur substantially in phase with the peak intensity of the corresponding periodic field variations, and between those signals such as are passed through the gating means, the peak intensity of which signals are substantially shifted in phase from the peak intensity of the corresponding periodic field variations, and

an alarm logic means coupled to the pulse decoding means for producing an alarm signal in response to the occurrence of at least one repetitive signal sequence characterized by one signal component produced in response to one of said sequentially applied fields wherein the one component is substantially in phase with the phase of the applied field followed by two successive signal components produced in response to the other two sequentially applied fields wherein the two successive components are substantially shifted in phase with respect to the phase of the applied fields.

7. A method for detecting the presence of an object within an interrogation zone comprising:

a. providing a marker adapted to be carried by a said object, said marker comprising a sheet including a laminate of a ferromagnetic layer and an electrically conductive layer wherein said ferromagnetic layer is characterized by an initial relative permeability in excess of 20,000, a maximum relative permeability in excess of 100,000 and a coercivity less than 0.3 σ

e such that said sheet is capable of responding to a magnetic field having a major field component in one direction in an interrogation zone, which field varies at a predetermined rate of not less than 1 KHz, to cyclically enhance said field in the zone and wherein said electrically conductive layer is characterized by a resistivity of not greater than about 3.0 microhm-cm such that said sheet is capable of responding to another cyclical magnetic field having a major field component in a direction substantially normal to said one direction to diminish said another field in the zone;

b. sequentially producing in an interrogation zone at least three magnetic fields, the intensity of each field periodically varying at a predetermined rate of not less than 1 KHz, a major field component of each field within said zone being orthogonally disposed with respect to a major field component of the other two fields; and

c. detecting in the vicinity of the interrogation zone a change in the magnetic field condition due to the presence of a marker in the zone resulting in a signal corresponding to said enhancement and diminishment occuring during at least two of said three produced sequential fields irrespective of the orientation of said marker within said zone.

8. In a system for detecting the presence of an object within an interrogation zone comprising:

a. a marker carried by said object, which marker is responsive to a cyclical uniaxial magnetic field in an interrogation zone varying at a predetermined rate of not less than 1 KHz applied to cyclically change the field in the zone;

b. means defining an interrogation zone;

c. means for sequentially producing in said zone at least three magnetic fields, the intensity of each field periodically varying at a predetermined rate of not less than 1 KHz, a major field component of each field within said zone being orthogonally disposed with respect to a major field component of the other two fields; and

d. means for detecting in the vicinity of the interrogation zone the presence of cyclical changes in the field due to said marker,

the improvement wherein the marker comprises a sheet having a laminate of a ferromagnetic layer and an electrically conductive layer wherein said ferromagnetic layer is characterized by an initial relative permeability in excess of 20,000, a maximum relative permeability in excess of 100,000 and a coercivity less than 0.3 σ

e such that said sheet is capable of responding to a magnetic field having a major field component in one direction in an interrogation zone, which field varies at a predetermined rate of not less than 1 KHz, to cyclically enhance said field in the zone and wherein said electrically conductive layer is characterized by a resistivity of not greater than about 3.0 microhm-cm such that said sheet is capable of responding to another cyclical magnetic field having a major field component in a direction substantially normal to said one direction to diminish said another field in the zone.