Article surveillance magnetic marker having an hysteresis loop with large Barkhausen discontinuities
First Claim
1. A marker for use in an article surveillance system in which an alternating magnetic field is established in a surveillance region and an alarm is activated when a predetermined perturbation to said field is detected, said marker comprising a body of magnetic material with retained stress and having a magnetic hysteresis loop with a large Barkhausen discontinuity such that exposure of said body to an external magnetic field, whose field strength in the direction opposing the magnetic polarization of said body exceeds a predetermined threshold value, results in regenerative reversal of said magnetic polarization, and means for securing said body to an article to be maintained under sureveillance.
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Accused Products
Abstract
A marker for an electronic article surveillance system is disclosed comprising a body of magnetic material with retained stress and having a magnetic hysteresis loop with a large Barkhausen discontinuity such that, upon exposure of the marker to an external magnetic field whose field strength in the direction opposing the instantaneous magnetic polarization of the marker exceeds a predetermined threshold value, there results a regenerative reversal of the magnetic polarization of the marker. An electronic article surveillance system and a method utilizing the marker are also disclosed. Exciting the marker with a low frequency and low field strength, so long as the field strength exceeds the low threshold level for the marker, causes a regenerative reversal of magnetic polarity generating a harmonically rich pulse that is readily detected and easily distinguished.
110 Citations
44 Claims
- 1. A marker for use in an article surveillance system in which an alternating magnetic field is established in a surveillance region and an alarm is activated when a predetermined perturbation to said field is detected, said marker comprising a body of magnetic material with retained stress and having a magnetic hysteresis loop with a large Barkhausen discontinuity such that exposure of said body to an external magnetic field, whose field strength in the direction opposing the magnetic polarization of said body exceeds a predetermined threshold value, results in regenerative reversal of said magnetic polarization, and means for securing said body to an article to be maintained under sureveillance.
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34. A marker for use in an article surveillance system in which an alternating magnetic field is established in a surveillance region and an alarm is activated when a predetermined perturbation to said field is detected, said marker comprising a body of magnetic material characterized in that the magnetic polarity thereof commences and completes reversal when the magnitude of strength of said field attains a given value, without need for increase in field strength above said given value.
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35. A method for detection of the presence of an article in an interrogation zone comprising the steps of:
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a. generating an incident alternating low frequency magnetic field within an interrogation zone; b. securing a marker to an article, said marker being selected to comprise a body of magnetic material with retained stress and having a magnetic hysteresis loop with a large Barkausen discontinuity such that upon exposure of said body to an external magnetic field, whose field strength in the direction opposing the magnetic polarization of said body exceeds a given threshold value, there results a regenerative reversal of said magnetic polarization; and c. detecting perturbations of the magnetic field in said interrogation zone having a frequency higher than the 30th harmonic of the incident alternating low frequency magnetic field when said marker is present in said interrogation zone. - View Dependent Claims (36, 37, 38, 39)
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40. An article surveillance system for detection of the presence of an article in an interrogation zone comprising:
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a. low frequency generator means for generating an incident alternating low frequency magnetic field within an interogation zone having a magnetic field intensity throughout said interrogation zone in excess of a predetermined threshold value; b. a marker secured to an article, said marker comprising a body of magnetic material with retained stress and having a magnetic hysteresis loop with a large Barkhausen discontinuity such that upon exposure of said body to an external magnetic field, whose field strength in the direction opposing the instaneous magnetic polarization of said body exceeds said predetermined threshold value, there results a regenerative reversal of said magnetic polarization; c. receiving means for detecting perturbations of the magnetic field in said interrogation zone having a frequency higher than the 30th harmonic of the incident alternating low frequency magnetic field when said marker is present in said interrogation zone. - View Dependent Claims (41, 42, 43, 44)
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Specification