Magnetic article surveillance system and method
Patent 4791412 Issued on December 13, 1988. Estimated Expiration Date: 
January 28, 2008. Estimated Expiration Date is calculated based on simple USPTO term provisions. It does not account for terminal disclaimers, term adjustments, failure to pay maintenance fees, or other factors which might affect the term of a patent.
January 28, 2008. Estimated Expiration Date is calculated based on simple USPTO term provisions. It does not account for terminal disclaimers, term adjustments, failure to pay maintenance fees, or other factors which might affect the term of a patent.Patent ReferencesApparatus and method for theft detection system having different frequencies Magnetic article surveillance system, method and coded marker Patent #: 4622542 InventorAssigneeApplicationNo. 07/149579 filed on 01/28/1988US Classes:340/572.2, Specified relationship between field and detection frequencies (e.g., nth order harmonics)340/551, Disturbance of magnetic field340/572.4, Specified processing arrangement for detected signal342/442Having a phase detectorExaminersPrimary: Goldstein, HerbertAssistant: Tran, Andrew Q. Attorney, Agent or FirmInternational ClassG08B 13/24 (20060101)AbstractA surveillance system based upon generation and detection of phase shifted harmonic signals from markers on articles or persons, responsive to transmission of a reference signal at a fundamental frequency in a detection zone, may be adversely affected by the presence in the detection zone of large metal objects, such as shopping carts. Such objects can alter the phase of the harmonic signals from predetermined values. An alternate signal processing technique for such a surveillance system recognizes that immunity to such disturbances can be achieved by measuring the phase between two or more of the harmonic signals returned from the marker, because the effect of such large metal objects has been found to have the same effect on all of the harmonic signals. Correspondence to a predetermined phase difference is a reliable indicator of marker detection. | |
