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US 5,703,566 AGranted
Anti-shoplifting security system
OwnerJon Neal Weaverinventor
Life of this patent
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Abstract
This invention relates to detecting the presence of a soft magnetic marker in an interrogation zone. A low frequency electromagnetic field is produced utilizing dual frequency generators causing the magnetic marker to produce a phase shift of the modulation of transmission harmonics. These signals are readily distinguished from ferrous alloys and noise.
Claims23
A system for detecting the presence of a ferromagnetic marker in an interrogation zone, comprising:
- a first generating means for generating a first magnetic field in the interrogation zone at a first frequency;
- a second generating means for generating a second magnetic field in the interrogation zone at a second frequency;
- a detecting means for detecting a phase shift of a signal produced by a ferromagnetic marker present within the interrogation zone;
- wherein said second frequency is about 3 to about 50 hertz lower or higher than said first frequency.
1 more element
A method for detecting the presence of a ferromagnetic marker in an interrogation zone, comprising the steps of:
- generating a first magnetic field in the interrogation zone at a first frequency;
- generating a second magnetic field in the interrogation zone at a second frequency;
- detecting a phase shift of a signal produced by a ferromagnetic marker present within the interrogation zone;
- wherein said second frequency is about 3 to about 50 hertz lower or higher than said first frequency.
1 more element
Dependencies are inferred from the claim text.