Surveillance system having magnetomechanical marker
AssigneeAllied Corporation
InventorsPhilip M. Anderson, III, Gerald R. Bretts, James E. Kearney
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Abstract
A magnetic article surveillance system marker is adapted, when armed, to resonate at a frequency provided by an incident magnetic field applied within an interrogation zone. The marker is an elongated ductile strip of magnetostrictive ferromagnetic material disposed adjacent to a ferromagnetic element which, upon being magnetized, magnetically biases the strip and arms it to resonate at said frequency. A substantial change in effective magnetic permeability of the marker at the resonant frequency provides the marker with signal identity.
Claims32
For use in a magnetic article surveillance system, a marker adapted to (i) mechanically resonate at a single frequency provided by an incident magnetic field applied within an interrogation zone, and (ii) undergo a substantial change in effective magnetic permeability at said frequency that provides said marker with signal identity, said marker comprising a strip of magnetostrictive ferromagnetic material adapted to be magnetically biased and thereby armed to resonate at said frequency, said strip having a magnetomechanical coupling factor, k, greater than 0, where ##EQU4## f.sub.r and f.sub.a being the resonant and anti-resonant frequencies, respectively.
An article surveillance system responsive to the presence of a marker within an interrogation zone, comprising:
- a. means for defining an interrogation zone;
- b. generating means for generating a magnetic field having a frequency band within said interrogation zone said generating means including an interrogating coil;
- c. a marker secured to an article appointed for passage through said interrogation zone, said marker being adapted to undergo a substantial change in its effective magnetic permeability within said frequency band that provides said marker with signal identity, and comprising a strip of magnetostrictive, ferromagnetic material adapted to be magnetically biased and thereby armed to mechanically resonate at a single frequency within the frequency band of said magnetic field, said strip having a magnetomechanical coupling factor, k, greater than 0, where ##EQU5## f.sub.r and f.sub.a being the resonant and anti-resonant frequencies, respectively;
- and
- d. detecting means for detecting said mechanical resonance of said marker, at said resonant frequency.
Dependencies are inferred from the claim text.