Communications: electrical – Condition responsive indicating system – Specific condition
Patent
1997-04-28
1999-10-26
Hofsass, Jeffery A.
Communications: electrical
Condition responsive indicating system
Specific condition
340551, G08B 1314
Patent
active
059735970
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to a theft checking system wherein a tag incorporating a resonance circuit is attached to an article and, if the tagged article passes between transmission and reception antennas disposed facing each other, the resonance circuit picks up electromagnetic waves outputted from the transmission antenna and undergoes resonance, and the reception antenna receives the electromagnetic waves re-radiated from the resonance circuit, thereby detecting passage of the tagged article.
BACKGROUND ART
The theft checking system uses a short wave band centered at, for example, 8.2 MHz, and the electromagnetic waves radiated from the transmitter is weak so that it is susceptible to communication signals, electromagnetic waves from fluorescent lamps or office automation appliances, and the like, which intrude as noises.
The influence of noises can be reduced relatively easily by increasing the electromagnetic wave transmission power, increasing the resonance circuit in size, or reducing the detection area. However, an increase in the electromagnetic wave transmission power is only permitted within a limit, and a size increase of the resonance circuit results in a correspondingly increased size of the tag, which is unfavorable.
A reduction in the detection area will by no means be acceptable to consumers when the demand for enlarged detection areas is presently increasing.
Accordingly, a conventional technology is proposed, as described in, for example, Japanese Patent Laid-Open No. Sho 63-126094, wherein two types of resonance circuits having different resonance frequencies are incorporated in a tag, and the electromagnetic waves to be transmitted are subjected to sweeping, and if the re-radiated electromagnetic waves are received four times every period of sweep, validity is established and a detection signal is outputted.
The aforementioned technology achieves a high detection precision since it detects only re-radiated electromagnetic waves while ignoring eruptive noises. However, the tag doubles in size and thus offends the eye.
Moreover, if a signal indistinct from the resonance signal is detected four times within a period of sweep, the signal is detected as the resonance signal.
DISCLOSURE OF THE INVENTION
The present invention is a theft checking system that recognizes a difference between resonance signals and noises based on inter-period relationship of the number of occurrence and occurrence timing of signals detected within each of consecutive cycle periods, thereby reducing the possibility of errors to a minimum level. The system features a construction wherein a transmission antenna outputs electromagnetic waves of a constant amplitude in which periodical sweep is made between predetermined frequencies, and the resonant frequency of the resonance circuit is set so that it is lower than a frequency at the upper limit of the output electromagnetic waves and higher than a frequency corresponding to the lower limit thereof, and analysis means is provided for predicting from the occurrence timing of a signal received in the first half period of a period of the sweep, an occurrence timing of a signal to be received in the second half period, and for comparing a received signal with the predicted timing and, if it is confirmed that the signals are in an opposite phase relationship, recognizing passage of the article.
The sweep frequency of the electromagnetic waves transmitted from the transmission antenna is preferably a frequency such that detection can be made while a person is passing by walking between the transmission and reception antennas.
BRIEF DESCRIPTION OF THE INVENTION
FIG. 1 is a block diagram illustrating a theft checking system according to the present invention.
FIG. 2 is a diagram illustrating the analysis principle.
FIG. 3 is a flowchart illustrating the analysis processing.
BEST MODE FOR CARRYING OUT THE INVENTION
An embodiment of an theft checking apparatus according to the present invention will be described with reference to the drawings.
REFERENCES:
patent: 3990065 (1976-11-01), Purinton et al.
patent: 4704602 (1987-11-01), sbrink
patent: 4727360 (1988-02-01), Ferguson et al.
patent: 4728938 (1988-03-01), Kaltner
patent: 5300922 (1994-04-01), Stoffer
patent: 5387900 (1995-02-01), Plonsky et al.
patent: 5414410 (1995-05-01), Davies et al.
patent: 5541577 (1996-07-01), Cooper et al.
Hofsass Jeffery A.
Maspro Denkoh Co., Ltd.
Tweel , Jr. John
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