Communications: electrical – Condition responsive indicating system – Specific condition
Patent
1992-08-13
1995-11-28
Peng, John K.
Communications: electrical
Condition responsive indicating system
Specific condition
455 83, G08B 1314
Patent
active
054711967
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The invention relates to a security system for surveilling the presence in defined zones such as, e.g. exit areas in shops or stores, of objects, for example commodities. The system comprises an antenna with at least one tuning capacitor in the monitored zone and receiver/transmitter devices functioning as tuned resonance circuits on the individual objects monitored, the antenna emitting at short intervals an electrical signal that makes the receiver/transmitter device of a monitored object, which is present in the monitored zone, oscillate, which oscillations in the intermissions between antenna transmissions can be received by the antenna--or by a separate antenna--and used to detect the presence of the monitored object in the monitored zone.
Such security systems are most typically used in shops and stores as a precaution against shoplifting, but may also be used in libraries to protect the books against theft. When an article has been sold or a book loaned, the receiver/transmitter device is removed from the article or book, respectively.
A problem inherent in such systems is that the antenna, which is a tuned circuit, unless special measures are taken, will continue oscillating for a short while into the intermission interval. It is then extremely difficult to distinguish between the residual signal of the antenna and a signal emitted by a monitored object.
In known systems of this kind attempts to solve the problem by shortcircuiting the antenna by the end of transmissions. Thereby, the energy stored in the antenna circuit is lost, but this requires a certain lapse of time, during which the antenna cannot receive possible reply signals from the receiver/transmitter device of a monitored object. During this lapse of time a very important part of the reply signal, which decreases exponentially, is lost, which has an unfortunate effect on the reliability of the monitoring process.
SUMMARY OF THE INVENTION
The object of the invention is to enhance the reliability of the monitoring process by minimizing, preferably eliminating said lapse of time during which the antenna is inactive.
This can be achieved according to the invention by means of a switch which at commencement of antenna intermissions electrically disconnects the functioning tuning capacitor from the antenna circuit to untune the antenna and thereby prevent it from storing energy.
Such a switch, which is electronic, may be a disconnector switch for disconnecting the electrical connection between the antenna and the tuning capacitor, for example by opening the circuit of the tuning capacitor to the antenna in the antenna circuit immediately by the end of transmissions or at commencement of same so that the antenna is untuned, either during intermissions or during transmissions. An untuned antenna cannot store energy. In both cases the detection of a reply signal can commence immediately at the start of intermissions.
From JP-A-171233 an antenna circuit for a radio telephone system is known, in which by means of an electronic switch an additional capacitor is connected in parallel with the tuning capacitor of the antenna to lower the tuning frequency of the antenna when the receiver is turned on.
The purpose of this circuit is to provide a shift of the antenna resonance frequency between the reception and transmission mode of the system.
This well-known form of changing the tuning of the antenna is not suitable, however, for security systems of the above-mentioned kind, such as anti-shoplifting systems, since in such systems a shift between two different tuning frequencies for the transmission and reception modes, respectively, would make the antenna insensitive to reception of reply signals produced by the passive receiver/transmitter devices and oscillating at the transmission frequency of the antenna.
BRIEF DESCRIPTION OF THE DRAWING
The invention is further explained in the following, by means of an exemplified embodiment with reference to the drawing, in which
FIGS. 1 and 2 show signal diagrams for a known
REFERENCES:
patent: 3584301 (1971-06-01), Tomaszewski
patent: 4797659 (1989-01-01), Larsen
patent: 4963880 (1990-10-01), Torre et al.
patent: 5036308 (1991-07-01), Fockens
Form PCT/IPEA/408 dated Dec. 4, 1991, relating to Pat. Application PCT/DK91/00048.
Patent Abstracts of Japan, vol. 10, No. 379, Abstract of JP 61-171233, Aug. 1986.
Mullen Jr. Thomas J.
Peng John K.
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