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Device and a method for detecting a magnetizable marker element

  • US 5,353,010 A
  • Filed: 12/15/1992
  • Issued: 10/04/1994
  • Est. Priority Date: 01/03/1992
  • Status: Expired due to Fees
First Claim
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1. A device for detecting a magnetizable marker element, adapted to be attached to an object to be protected against unauthorized removal, comprising:

  • (a) a transmitter for generating a periodically alternating magnetic excitation field for reversing the magnetic state of the marker element;

    (b) a receiver arranged in said alternating magnetic excitation field for generating an electrical output signal corresponding to a signal received upon a said magnetic state reversal;

    (c) a signal processing means including a band pass filter for filtering said output signal of said receiver, the resultant filtered output signal having a superimposition of signal components, the frequencies of which are integral multiples of the fundamental frequency of the excitation field;

    (d) means for comparing the time interval between two successive maximum signal values of different polarities of the filtered output signal to a predetermined tolerance range associated with half the period duration of the excitation field, and for generating a first output signal when the time interval is within said predetermined tolerance range; and

    (e) an indicator generating a first indication signal upon receiving a first output signal from said comparing means.

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