Communications: electrical – Condition responsive indicating system – Specific condition
Patent
1997-04-25
1999-11-30
Mullen, Thomas
Communications: electrical
Condition responsive indicating system
Specific condition
340529, G08B 1314
Patent
active
059950056
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, utilizing the fact that if the tagged article is caused to pass between transmission and reception antennas disposed facing each other the resonance circuit picks up electromagnetic waves outputted from the transmission antenna and undergoes resonance, the reception antenna receives the electromagnetic waves re-radiated from the resonance circuit and passage of the tagged article is detected.
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 transmission antenna are weak so that they are 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 from the two types of resonance circuits a total of four times during a 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 mistaken as a resonance signal and false determination is made.
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 occurrences 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 resonance 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, if a signal is detected twice or more within the period of the sweep, first extracting from the signals a pair of signals that are regarded as resonance signals, and then determining whether a signal substantially agreeing in occurrence timing and pattern with the extracted signals is present in at least one of the periods preceding and following the period during which the extracted signals are detected, and recognizing passage of the tagged article by confirming the presence of such a signal. Preferably, the sweep frequency of the signal outputted from the transmission antenna is a frequency such that detection can be made while a person is passing by or walking between the transmission and reception antennas, and the presence of a signal substantially agreeing in occurrence timing and pattern with extracted signals is determined in connection with each of three consecutive periods o
REFERENCES:
patent: 4429302 (1984-01-01), Vandebult
patent: 4531117 (1985-07-01), Nourse et al.
patent: 4779077 (1988-10-01), Lichtblau
patent: 4812822 (1989-03-01), Feltz et al.
patent: 5353011 (1994-10-01), Wheeler et al.
Maspro Denkoh Company, Ltd.
Mullen Thomas
LandOfFree
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