The Magnavox Company

Robert Auger

Edwin C Purinton

Carl S Holzinger

Apparatus and method for identifying the presence in a pre-established location of an object having preselected electrical or magnetic properties. By applying two periodically varying electro-magnetic fields of the same frequency to the pre-established location, the presence of the object is determined by unique perturbations of the electro-magnetic fields produced by the object. The applied electro-magnetic field is generally nonuniform in the pre-established location to maximize the interaction between the object and the electro-magnetic field. A pick-up coil is utilized to sense the resulting field perturbations. A magnetometer can be optionally incorporated in a sensing system to inhibit erroneous indications produced by magnetic objects.

Balanced field theft detection system

Apparatus and method for detecting the presence, in an inspection area, of a marker tag having preselected properties. Transfer of articles from a protected area can be monitored by coupling a marker tag with each article and by including an inspection area in all transfer paths. In a theft detection system, the coupled marker tag can be removed or made inoperative in some manner for authorized withdrawal of an article from the protected area. The marker tag is comprised of a small, thin strip of magnetic material. A time-varying magnetic field is provided for interacting with a marker tag located in the inspection area. The magnetic field, which includes components having frequencies f.sub.1 and f.sub.2, has a time-dependent spatial orientation which permits interaction with a marker tag having an arbitrary orientation in the inspection area. The magnetic field components of frequency f.sub.2 produce changes in the magnetic properties of the marker tag, resulting in the generation of a signal having components of frequencies f.sub.1 .+-.2f.sub.2. When the magnitude and phase of the signal having the component f.sub.1 + 2f.sub.2 falls within a range of values of magnitude and phase peculiar to the marker tag, an alarm is activated by signal detection apparatus. The restricted nature of the signals producing an alarm effectively distinguishes a marker tag from signals produced by other objects in the inspection area.

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Lansdale, PA, United States of America

Robert Auger