Antitheft system
AssigneeShin International, Inc.
InventorEdward R. Fearon
Life of this patent
Estimating the term…
Abstract
The present invention provides an antitheft system comprising a pair of spaced coils which are excited to produce an alternating magnetic field within a surveillance zone between the coils. Thus, a magnetic marker having a permeability of less than 150,000 and a coercive force of substantially 2.5 MA/CM which is carried through the surveillance zone cuts or links lines of magnetic flux within the zone regardless of the orientation of the marker. Electronic circuitry detects the electromagnetic field within the zone at harmonic frequencies greater than the frequency of the oscillating magnetic field and, through coherent correlation, determines the presence of the marker within the surveillance zone as well as distinguishing and differentiating the marker from other ferromagnetic materials.
Claims12
An antitheft system comprising:
- a magnetic marker,
- means for generating an oscillating electromagnetic field within a surveillance zone so that said marker, when positioned within said zone, cuts or links lines of magnetic flux regardless of the orientation of said marker,
- means for receiving an electromagnetic signal from said surveillance zone and for generating an output signal representative thereof,
- means for processing said output signal to determine the presence of said marker within said surveillance zone,
- means responsive to said processing means for indicating the presence of the marker within the surveillance zone
- wherein said generating means generates an oscillating field within the surveillance zone at a substantially constant frequency f.sub.1, said marker being constructed of a material which is exerted by an oscillating electromagnetic field having a frequency of f.sub.1 and which generates a wide range of harmonics, and wherein said processing means comprises:
- first means for determining the electromagnetic field in the surveillance zone between frequencies f.sub.2 and f.sub.3,
- means responsive to said determining means for projecting the electromagnetic field between frequencies f.sub.4 and f.sub.5 ;
- second means for determining the electromagnetic field in the surveillance zone between frequencies f.sub.4 and f.sub.5 ;
- and
- means providing an output signal to said indicating means when the output from said second determining means exceeds the output from the projecting means;
- where f.sub.1 <f.sub.2 <f.sub.3 <f.sub.4 <f.sub.5.
A marker element for use in an oscillating magnetic field comprising a magnetic material having a permeability of less than 150,000 and a coercive force of substantially 2.5 MA/CM.
Dependencies are inferred from the claim text.