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Harmonic-based verifier device for a magnetic security thread having linear and non-linear ferromagnetic characteristics

  • US 5,614,824 A
  • Filed: 05/15/1995
  • Issued: 03/25/1997
  • Est. Priority Date: 05/15/1995
  • Status: Expired due to Fees
First Claim
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1. A device for verifying the authenticity of a document having a security thread associated therewith, the security thread including one or more regions of magnetic material, each region of magnetic material having one or more predetermined linear and non-linear ferromagnetic characteristics including a coercivity of no greater than 5000 amperes per meter and a relative permeability of between 200 and 10,000, the device comprising:

  • a. drive means for providing an applied magnetic field as an alternating current magnetic field at a predetermined fundamental frequency within a predetermined spatial region through which the document is passed, the drive means comprises means for providing the applied magnetic field to saturate at least one of the one or more regions of magnetic material;

    b. receive means for sensing a response magnetic field within the predetermined spatial region through which the document is passed at a distance of no greater than ten millimeters from a magnetic field sensing portion of the receive means, and for providing one or more sensed signals indicative of a corresponding one or more characteristics of the response magnetic field, wherein the response magnetic field is an alternating current magnetic field, and wherein in the presence of the security thread within the predetermined spatial region the response magnetic field is at the predetermined fundamental frequency of the applied magnetic field and at one or more harmonic frequencies of the predetermined fundamental frequency; and

    c. signal processing means, responsive to the sensed signals for determining at least one of the one or more predetermined linear and non-linear ferromagnetic characteristics of each region of magnetic material to verify the authenticity of the document.

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