Coded surveillance system having magnetomechanical marker
AssigneeAllied Corporation
InventorsPhilip M. Anderson, III, James E. Kearney, Gerald R. Bretts
Life of this patent
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Abstract
A magnetic article surveillance system marker is adapted, when armed, to resonate at preselected frequencies provided by an incident magnetic field applied within an interrogation zone. The marker is a plurality of elongated ductile strips of magnetostrictive ferromagnetic material each of the strips being disposed adjacent to a ferromagnetic element which, upon being magnetized, magnetically biases the strips and arms them to resonate at the preselected frequencies. A substantial change in effective magnetic permeability of the marker at the preselected frequencies provides the marker with signal identity.
Claims33
For use in a magnetic article surveillance system, a marker adapted to (i) mechanically resonate at preselected frequencies provided by an incident magnetic field applied within an interrogation zone, and (ii) undergo a substantial change in effective magnetic permeability at each of said preselected frequencies that provides said marker with signal identity, said marker comprising a plurality of strips of magnetostrictive ferromagnetic material, each of said strips being adapted to be magnetically biased and thereby armed to resonate at a single, different one of said preselected frequencies, and having a magnetomechanical coupling factor, k, greater than 0, where k=.sqroot.(1-f.sub.r.sup.2 /f.sub.a.sup.2), f.sub.r and f.sub.a being the resonant and antiresonant frequencies, respectively.
An article surveillance system responsive to the presence of at least one of a plurality of markers 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 plurality of markers associated with an article appointed for passage through said interrogation zone, each of said markers being adapted to undergo a substantial change in its effective magnetic permeability at a different preselected frequency within said frequency band that provides each of said markers 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 k=.sqroot.(1-f.sub.r.sup.2 /f.sub.a.sup.2), f.sub.r and f.sub.a being the resonant and anti-resonant frequencies, respectively;
- and
- d. detecting means for detecting said mechanical resonances of said markers within said interrogation zone at each different preselected frequency.
Dependencies are inferred from the claim text.