Communications: electrical – Selective – Interrogation response
Reexamination Certificate
1997-11-28
2001-02-13
Horabik, Michael (Department: 2735)
Communications: electrical
Selective
Interrogation response
C340S010300
Reexamination Certificate
active
06188310
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to the field of pulsed magnetic electronic article surveillance (EAS) systems, and in particular, to EAS systems in which the natural frequency of magnetic markers within the EAS system's interrogation zone can be measured for enhanced discrimination against both noise and the presence of magnetically deactivated markers.
2. Description of Related Art
The reliability of an EAS system is only as good as the system's ability to discriminate against false alarms. The potential for false alarms comes from three sources. A first source of false alarms is electromagnetic noise, both internal circuit noise and external noise from active electronic sources. A second source of false alarms is metallic objects which produce magnetic signals when stimulated by the system's transmitter field, but which do not have a valid magnetic marker affixed to them. A third source of false alarms is partially deactivated or damaged magnetic markers or labels within the system's interrogation zone. Such partially deactivated or damaged markers are also referred to as wounded markers.
Conventional amplitude modulation (AM) receivers can only determine the amount of energy within the receiver's bandwidth due to sources within the receiver's field at any point in time. Making an alarm decision based on amplitude criteria alone leaves opportunities for any of the above three sources of false alarms to cause an erroneous system output.
Since EAS systems are invariably expected to operate at maximum sensitivity, the receivers are constantly processing information at poor or marginal signal-to-noise ratios. This is where the chance of making an erroneous decision is greatest. For this reason, the detection process is extended for a longer time interval in these cases, before making a decision. In high noise situations, the extra time required sometimes means a magnetic marker is not within the interrogation zone long enough and the system fails to alarm.
In the case of systems like the Ultra*Max technology systems of Sensormatic Corporation, the response of the magnetic markers or labels is unique enough to consider the second source of false alarms, namely metallic objects which produce magnetic signals when stimulated by the system's transmitter, to be virtually nonexistent. Ultra*Max is a trademark of Sensormatic Corporation.
The Ultra*Max systems are based on a proprietary magneto-acoustic technology. The system's transmitter develops a pulsed magnetic field from its antenna which imparts energy to a unique magnetic marker or label. When the transmission stops, the marker continues to “ring down” for a time based on its unique physical and magnetic properties, in a manner analogous to a struck tuning fork. After the transmission burst, the Ultra*Max receiver scans the interrogation zone to detect the decaying magnetic pulse from an active label.
Deactivating a label or marker involves the demagnetization of a strip of high coercivity bias material within the label. This has the effect of reducing the amplitude of the label's response, as well as shifting the natural resonant frequency of the label upward. Since valid labels are manufactured to have a natural frequency distribution centered at the receiver's operating frequency, typically 58.0 KHz, reducing the label's amplitude and shifting the response frequency out of the receiver's bandwidth has the desired effect of dropping the deactivated label's signal down into the system background noise.
As part of the normal validation sequence, some system designs shift the receiver operating frequency momentarily into a range typical of deactivated labels. At this time the detected amplitude is compared to the amplitude from the previous, standard operating frequency. If the detected amplitude at this deactivated frequency is greater than that detected at the standard operating frequency, the signal is presumed to derive from a deactivated label and no alarm is produced.
SUMMARY OF THE INVENTION
The inventive arrangements taught herein represent an improvement over previous designs in that an EAS system can determine the actual ring down frequency of a magnetic marker within the system's interrogation field. The ability of an EAS system to measure the frequency characteristics of detected signals provides a significant advantage over previous designs. This ability provides enhanced immunity to false alarms both due to improperly deactivated magnetic markers and due to electrical noise sources.
An electronic article surveillance system in accordance with the inventive arrangements comprises: an antenna for capturing signals generated in an interrogation zone of the EAS system during successive receive windows of a transmit and receive sequence, the captured signals from the interrogation zone including noise and, when at least one of a plurality of markers is present in the interrogation zone, a ring down signal with a characteristic frequency generated by the at least one marker responsive to a magnetic field transmitted into the interrogation zone; a receiver coupled to the antenna having a first signal processor for extracting amplitude information and frequency information from the captured signals; and, a detector responsive to both the amplitude information and the frequency information for validating when the at least one marker is in the interrogation zone.
The signal processor advantageously determines an average frequency of all markers in the interrogation zone, the detector being responsive to the average frequency
The system signal processor can further determine, advantageously, successive frequency deviations of sequential frequency measurements and magnitudes of the deviations, the detector being responsive to the magnitudes of the deviations, thereby discriminating between the noise and the markers.
The system receiver can comprise: a local oscillator having a first output signal at a first frequency; a first filter for bandpass filtering the signals captured by the antenna, the first filter having a bandpass characteristic centered at the first frequency; a mixer responsive to the local oscillator output signal and to the bandpass filtered signals; an amplifier for generating a second output signal at an intermediate frequency which may be greater than or less than the first frequency, and responsive to the mixer; a second filter for bandpass filtering the second output signal, the second filter having a bandpass characteristic centered at the intermediate frequency; an amplitude modulation detector responsive to the second output signal, generating a DC voltage proportional to in band energy present in the captured and processed signals during each of the receive windows; an analog to digital converter for the DC voltage; a limiter for amplifying the second output signal and converting the amplified second output signal to a square wave having a constant amplitude and edges which vary with the second output signal; and, a second signal processor for determining zero edge crossings of the square wave.
The second signal processor can comprise a microprocessor, the microprocessor comprising: the analog to digital converter; an edge detector responsive to the square wave; a gating circuit responsive to the edge detector; a source of a clock signal; and, an accumulator responsive to the clock signal and to the gating circuit.
The detector initiates frequency sampling sequences of not less than nine half periods of the intermediate frequency.
REFERENCES:
patent: 4031466 (1977-06-01), Krause et al.
patent: 4527152 (1985-07-01), Scarr et al.
patent: 4644286 (1987-02-01), Torre
patent: 4658241 (1987-04-01), Torre
patent: 4675658 (1987-06-01), Anderson et al.
patent: 4963880 (1990-10-01), Torre et al.
patent: 5023600 (1991-06-01), Szklany et al.
patent: 5049857 (1991-09-01), Plonsky et al.
patent: 5353011 (1994-10-01), Wheeler et al.
patent: 5387900 (1995-02-01), Plonsky et al.
patent: 5495229 (1996-02-
Allen John A.
Balch Brent F.
Horabik Michael
Jones Prenell
Quarles & Brady LLP
Sensormatic Electronics Corporation
LandOfFree
Natural frequency measurement of magnetic markers does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Natural frequency measurement of magnetic markers, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Natural frequency measurement of magnetic markers will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2566569