Coded surveillance system having magnetomechanical marker

Claim 2

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=√(1-f r 2 /f a 2 ), f r and f a being the resonant and antiresonant frequencies, respectively.

Claim 3

A marker as recited in claim 1, wherein said material is at least 50 percent amorphous.

Claim 4

A marker as recited in claim 2, wherein said material has a composition consisting essentially of the formula M a N b O c X d Y e Z f , where M is at least one of iron and cobalt, N is nickel, O is at least one of chromium and molybdenum, X is at least one of boron and phosphorous, Y is silicon, Z is carbon, "a"-"f" are in atom percent, "a" ranges from about 35-85, "b" ranges from about 0-45, "c" ranges from about 0-7, "d" ranges from about 5-22, "e" ranges from about 0-15 and "f" ranges from about 0-2, and the sum of d+e+f ranges from about 15-25.

Claim 5

A marker as recited in claim 1, wherein said material is at least 80 percent amorphous.

Claim 6

A marker as recited in claim 1, wherein each of said strips is sandwiched between the faces of two pieces of fabric, each of said pieces of fabric having planar dimensions greater than the corresponding aggregate dimensions of said strips, said pieces of fabric being joined together at the edges thereof to form fabric sandwiched strips.

Claim 7

A marker as recited in claim 5, wherein said fabric sandwich strips are disposed within an airtight casing of polymeric film.

Claim 8

A marker as recited in claim 1, wherein said incident magnetic field is swept to provide said frequencies.

Claim 9

A marker as recited in claim 1, wherein each of said frequencies is provided in the form of a pulse.

Claim 10

A marker as recited in claim 8, wherein each of said pulses has a width equal to 1/(2f r ), where f r is the resonant frequency of said strip.

Claim 11

A marker as recited in claim 1, wherein said change in effective magnetic permeability is of the order of 1200%.

Claim 12

A marker as recited in claim 1, wherein said strip is magnetically biased by a magnetic bias field ranging from about 0.1 to 20 Oe.

Claim 13

A marker as recited in claim 11, wherein said magnetic bias field has a magnitude which yields a maximum k.

Claim 14

A marker as recited in claim 1, wherein said strips are annealed in a saturating magnetic field applied thereto in a direction perpendicular to the length thereof.

Claim 15

A marker as recited in claim 13, wherein said strips are annealed at a temperature ranging from about 300° to 450° C. for an annealing time ranging from about 7 to 120 min.

Claim 16

A marker as recited in claim 1, wherein said marker further comprises at least one ferromagnetic element disposed adjacent to said strips and adapted to magnetically bias said strips and arm them to resonate at said preselected frequencies.

Claim 17

A marker as recited in claim 15 wherein said ferromagnetic element has coercivity higher than that of any of the magnetostrictive material of the strips.

Claim 18

A marker as recited in claim 15, wherein said ferromagnetic element is adapted to further magnetically bias said strips to decrease resonances and thereby dearm the marker.

Claim 19

A marker as recited in claim 15, wherein said material is at least 50 percent amorphous.

Claim 20

A marker as recited in claim 18, wherein said ferromagnetic element has coercivity higher than said amorphous material.

Claim 21

A marker as recited in claim 15, wherein said material is at least 80 percent amorphous.

Claim 22

A marker as recited in claim 20, wherein each of said strips is comprised of amorphous material and said ferromagnetic element comprises a crystalline region of the amorphous material of each strip.

Claim 23

A marker as recited in claim 16, wherein said ferromagnetic element comprises a plurality of pieces of high magnetic coercivity material, each of said strips having a different one of said pieces disposed adjacent thereto.

Claim 24

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=√(1-f r 2 /f a 2 ), f r and f 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.

Claim 25

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=√(1-f r 2 /f a 2 ), f r and f 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.

Claim 26

An article surveillance system as recited in claim 23, wherein said generating means includes frequency sweeping means adapted to sweep through each different preselected frequency of said markers.

Claim 27

An article surveillance system as recited in claim 23, wherein said generating means includes energizing means adapted to provide said interrogating coil with a burst of sine wave frequencies that includes each said different preselected frequency.

Claim 28

An article surveillance system as recited in claim 23, wherein said generating means includes energizing means adapted to provide said interrogating coil with pulses, wherein the width of each pulse is equal to 1/(2f r ) where f r is the preselected frequency.

Claim 29

An article surveillance system as recited in claim 23, wherein said generating means includes energizing means adapted to provide said interrogating coil with a burst of noise.

Claim 30

An article surveillance system as recited in claim 23, wherein said generating means includes energizing means adapted to provide said interrogating coil with a burst of sweeping sine wave frequency.

Claim 31

An article surveillance system as recited in claim 23 wherein said generating means includes energizing means for providing said interrogating coil with an energizing signal, said detecting means includes receiving means for distinguishing a resonant frequency for each of said markers detected by said receiving coil from other frequencies induced therein and said system further includes synchronizing means associated with said energizing means and said receiving means for sequentially activating and deactivating each of said energizing means and said receiving means.

Claim 32

An article surveillance system as recited in claim 29, wherein said synchronizing means is adapted to prevent activation of said energizing means for substantially the entire period of time that said receiving means is activated.

Claim 33

An article surveillance system as recited in claim 30, wherein said synchronizing means is adapted to prevent activation of said receiving means for substantially the entire period of time that said energizing means is activated.

Claim 34

A marker as recited in claim 6, wherein said casing is seated with air contained therein to form a pillow-like shape.

Claim 35

An article surveillance system as recited in claim 29, wherein each of said markers comprises a plurality of strips of magnetostrictive, ferromagnetic material and said receiving means is adapted to distinguish all resonant frequencies for each of said markers detected by said receiving coil from other frequencies induced therein.