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Method and apparatus for the promotion of selected harmonic response signals in an article detection system

  • US 4,326,198 A
  • Filed: 08/18/1976
  • Issued: 04/20/1982
  • Est. Priority Date: 08/18/1976
  • Status: Expired due to Term
First Claim
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1. Apparatus for detecting the presence of an article in an interrogation zone, said apparatus comprising a target formed of a strip of a saturable ferromagnetic material and adapted to be carried on said article, an enlarged, flat, planar interrogation antenna coil positioned adjacent to and extending along said interrogation zone, so that targets carried through said zone must pass through magnetic interrogation fields produced by said coil, electrical oscillator and drive means constructed and arranged to produce an interrogation signal at a first frequency, said interrogation antenna coil being electrically coupled to said electrical oscillator and drive means to produce a varying magnetic interrogation field at said first frequency throughout said interrogation zone, said varying magnetic field having a characteristic magnetic field pattern, magnetic bias means constructed and arranged to produce, throughout said interrogation zone a magnetic biasing field having a characteristic magnetic field pattern essentially the same as the characteristic magnetic field pattern of said varying magnetic interrogation field said magnetic bias means including a bias coil of substantially the same configuration as said interrogation antenna coil and positioned adjacent said interrogation antenna coil and means for supplying electrical current to said bias coil, said drive means, said interrogation antenna coil, and said magnetic bias means further being arranged such that the net magnetic fields produced thereby are capable, at each location throughout said interrogation zone, of causing said target to produce target signals in the form of magnetic fields which vary at frequencies harmonically related to said first frequency and receiver means constructed and arranged to detect the presence, in said interrogation zone of said target signals.

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