Surveillance system having acoustic magnetomechanical marker
First Claim
1. For use in a magnetic article surveillance system, a marker which, when armed, is adapted to resonate at a frequency provided by an incident magnetic field applied within an interrogation zone and to have a substantial change in acoustic output level at said frequency that provides said marker with signal identity, said marker being a strip of magnetostrictive ferromagnetic material disposed adjacent to a hard magnetic element which, upon being magnetized, magnetically biases said strip and arms it to resonate at said frequency, said strip having magnetomechanical coupling factor, k greater than 0, where k=√
- (1-fr2 /fa2), fr and fa being the resonate and anti-resonant frequencies, respectively.
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Accused Products
Abstract
An article surveillance system having an acoustic magnetomechanical marker is adapted, when armed, to resonate at a frequency provided by an incident magnetic field applied within an interrogation zone. The marker is a strip of magnetostrictive ferromagnetic material disposed adjacent to a hard magnetic element which, upon being magnetized, magnetically biases the strip and arms it to mechanically vibrate at said frequency. A substantial change in acoustic output level of the marker at the resonant frequency provides the marker with signal identity.
49 Citations
37 Claims
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1. For use in a magnetic article surveillance system, a marker which, when armed, is adapted to resonate at a frequency provided by an incident magnetic field applied within an interrogation zone and to have a substantial change in acoustic output level at said frequency that provides said marker with signal identity, said marker being a strip of magnetostrictive ferromagnetic material disposed adjacent to a hard magnetic element which, upon being magnetized, magnetically biases said strip and arms it to resonate at said frequency, said strip having magnetomechanical coupling factor, k greater than 0, where k=√
- (1-fr2 /fa2), fr and fa being the resonate and anti-resonant frequencies, respectively.
- View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13)
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14. For use in a magnetic article surveillance system, a marker adapted to resonate at a frequency provided by an incident magnetic field applied within an interrogation zone, and having a substantial change in acoustic output at said frequency that provides said marker with signal identity, said marker being 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 k=√
- (1-fr2 /fa2), fr and fa being the resonant and anti-resonant frequencies, respectively.
- View Dependent Claims (15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28)
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29. An article surveillance system responsive to the presence of a marker within an interrogation zone, comprising:
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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 characterized by a substantial change in its acoustic output level within said frequency band that provides said marker with signal identity, and comprising an elongated ductile strip of magnetostrictive, ferromagnetic material adapted to be magnetically biased and thereby armed to resonate mechanically at a frequency within the frequency band of said magnetic field, said strip having a magnetomechanical coupling factor, k, greater than 0, wherein k=√
(1-fr2 /fa2), fr and fa being the resonant and anti-resonant frequencies, respectively; ande. detecting means for detecting resonance of said marker, within said interrogation zone at said resonant frequency and distinguishing said change in its acoustic output level from changes in acoustic output at other than the resonant frequency of said marker. - View Dependent Claims (30, 31, 32, 33, 34, 35, 36, 37)
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Specification