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Sensitizer for ferromagnetic markers used with electromagnetic article surveillance systems

  • US 5,151,843 A
  • Filed: 12/08/1989
  • Issued: 09/29/1992
  • Est. Priority Date: 12/08/1989
  • Status: Expired due to Fees
First Claim
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1. A resensitizer apparatus adapted for use with an electronic article serveillance system for detecting a resensitizable marker secured to a moving article, in which the marker includes a first, 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 resensitizer apparatus comprising;

    (a) an assembly comprising at least one section of a second ferromagnetic material having two substantially opposed major 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 second ferromagnetic material; and

    (c) an alternating current source capable of driving the assembly at a constant maximum amplitude such that the assembly concentrates external magnetic lines of flux near the gap to produce a sufficient number of field reversals in a spatially decreasing magnetic field outside the gap to resensitize the marker while the marker is moving away from the gap, the current source comprising;

    (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 proportional-integral control circuit which compares the voltage drop to a precision voltage reference and hold the amplitude of the alternating current in the coil at a constant level.

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