Communications: electrical – Condition responsive indicating system – Specific condition
Patent
1997-04-21
1998-09-01
Hofsass, Jeffrey A.
Communications: electrical
Condition responsive indicating system
Specific condition
340551, G08B 1314
Patent
active
058016317
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 transmitted from the transmission antenna and undergoes resonance, so that the resonance circuit reradiates resonance electromagnetic waves, and the reception antenna receives the resonance electromagnetic waves, thereby detecting passage of the tagged article.
BACKGROUND ART
The theft checking system uses a short wave band around, for example, 8.2 MHz. Such band is subject to intrusion as noises of communication signals, electromagnetic waves from fluorescent lamps or office automation appliances, or thermal noises, and the electromagnetic waves radiated from the transmission antenna is weak. Thus, the signal detection conditions of the resonance electromagnetic waves are extremely bad.
For a reduction of the influence of noises, increasing the electromagnetic wave transmission power, increasing the resonance circuit in size, or reducing the detection area can be considered. 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 offends the eye and is unfavorable.
A reduction of the detection area is difficult to realize because it disagrees with the demand for enlarged detection areas.
Accordingly, conventional technologies are proposed, as described in, for example, Japanese Patent Laid-Open No. Sho 61-118897, wherein the threshold level setting is variable corresponding to the noise level, and only waveforms exceeding the threshold level are regarded as valid detection signals, or Japanese Patent Publication No. Sho 63-1216094, 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 former technology is not only incapable of detection if the level of resonance electromagnetic waves is low, but also mistakenly detects as a resonance signal a noise having a pulse-like sharp amplitude change when the power is thrown in. Thus, the detection precision cannot be said to be high. Although the latter technology achieves a high detection precision since it detects only clear re-radiated electromagnetic waves while ignoring eruptive noises that occur in a period, the tag doubles in size and becomes bulky, thus standing out in appearance.
Moreover, although either technology passes received electromagnetic waves through a band-pass filter corresponding to the frequency of the resonance signal to emphasize the waveform, both technology lack a function of distinguishing resonance signals from noises. Therefore, with short-wave broadcasting or CW jamming waves as for example, either technology is unable to determine whether a waveform is of a resonance signal or of a noise, regardless whether the waveform is a waveform as received (FIG. 7a) or a waveform obtained by passing the received waveform through a band-pass filter to remove unneeded electromagnetic waves FIG. 7b). Thus, the cause for errors still remain.
The present invention is a checking system that improves detection precision and reduces the possibility of errors to a minimum level. The system features a construction comprising analysis means for causing a transmission antenna to output electromagnetic waves of a constant amplitude in which periodical sweep is made between predetermined frequencies, and for extracting a signal that substantially agrees with the period of a model waveform of re-radiated electromagnetic waves pre-registered as a reference signal, from the signals received by a reception antenna, and for comp
REFERENCES:
patent: 4859991 (1989-08-01), Watkins et al.
patent: 5264829 (1993-11-01), Paul et al.
patent: 5353011 (1994-10-01), Wheeler et al.
patent: 5414410 (1995-05-01), Davies et al.
Hofsass Jeffrey A.
Maspro Denkoh Co., Ltd.
Tweel , Jr. John
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