Balanced field theft detection system

A theft detection system for identifying a presence of at least one preselected article at a predetermined location, comprising:

• means for applying simultaneously at least two electro-magnetic fields having a given frequency to said predetermined location to produce a single curved electro-magnetic field;
• means for detecting said curved electro-magnetic field wherein said means for detecting is located relative to said curved electro-magnetic field to provide generally a null output signal in an absence of substantial interaction with said curved electro-magnetic field disturbed by foreign materials;
• means for detecting and measuring an amplitude of the output signal of said detecting means of the given frequency produced by a perturbation of said curved electro-magnetic field;
• means for detecting and measuring a phase shift of the detecting means output signal of said given frequency produced by said perturbation of said electro-magnetic field;
• and
• means for signaling a perturbation of the applied electro-magnetic fields at said predetermined locations when said measured output signal amplitude and phase are within pre-established values.

A method of selectively detecting objects at a predetermined location, said objects having a preselected interaction with electro-magnetic fields, comprising the steps of:

• simultaneously applying at least two periodic electro-magnetic fields of the same frequency to said predetermined location said electro-magnetic fields producing a resultant curved electro-matic field;
• locating an electro-magnetic field detector to provide a substantially null output signal in an absence of interactions with said resultant curved electro-magnetic field, wherein an article having an interaction with said resultant curved electro-magnetic field produces an output signal;
• measuring an amplitude of said output signal produced by the article and said resultant curved electro-magnetic field interaction;
• measuring a phase of said output signal produced by the article and said resultant curved electro-magnetic field;
• and
• providing an indication signal when said measured amplitude and phase produced by at least one of said objects being at said predetermined location are within predetermined limits.

Apparatus for selectively detecting a presence of one of a plurality of objects at a predetermined location, each of said objects having a substantially identical interaction with an electro-magnetic field, comprising:

• means for simultaneously applying at least two electro-magnetic fields with a given frequency at said predetermined location, said electro-magnetic fields producing a resultant curved electro-magnetic field;
• means for detecting an electro-magnetic field of said preselected frequency, wherein said detection means is located to provide a substantially null output signal when interactions with said resultant curved electro-magnetic field are substantially absent;
• means for measuring an amplitude of said detector output signal;
• means for measuring a phase of said detector output signal;
• and
• means for signalling a presence of at least one of said objects at said predetermined location when said output signal amplitude and phase are within preselected values, said preselected values determined by an interaction between said object and said resultant curved electro-magnetic field at said predetermined location.