A SECURITY SYSTEM FOR SURVEILLING THE PASSAGE OF COMMODITIES THROUGH DEFINED ZONES

Abstract: A security system for monitoring the presence in, respectively passage through defined zones such as, e.g. exit areas in shops or stores of objects, for example commodities, which system comprises an antenna (7) with at least one tuning capacitor (1) 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 (7) transmissions can be received by the antenna (7) - or by a separate antenna - and used to detect the presence of the monitored object in the monitored zone. A switch (3) is provided in the electrical circuit between the antenna (7) and the tuning capacitor (1).

Description: A security system for surveilling the passage of com¬ modities through defined zones. The invention relates to a security system for surveilling the presence in, respectively passage through defined zones such as, e.g. exit areas in shops or stores, of objects, for example commodities, which system comprises an antenna with at least one tuning capacitor in the monitored zone and receiver/- trans itter devices functioning as tuned resonance circuits on the individual objects monitored, the an¬ tenna emitting at short intervals an electrical signal that makes the receiver/transmitter device of a monit¬ ored 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 de- tect 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 shoplift¬ ing, 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 re¬ moved from the article or book, respectively. A problem inherent in such systems is that the antenna, which is a tuned circuit, unless special meas- ures are taken, will continue oscillating for a short while into the intermission interval. It is then ex¬ tremely difficult to distinguish between the residual signal of the antenna a * • * nd a signal emitted by a mon- itored object. In known systems of this kind it has been at¬ tempted 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. 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 disconnects the functioning tuning cap¬ acitor from the antenna circuit. Such a switch which is electronic may be a dis¬ connector switch for disconnecting the electrical con¬ nection between the antenna and the tuning capacitor, for example by disconnecting the tuning capacitor of the antenna from the antenna circuit immediately by the end of transmissions or at commencement of same so that the antenna is untuned, either during intermis¬ sions or during transmissions, and an untuned antenna cannot store energy. In both cases the detection of a reply signal can commence immediately at the start of intermissions. The invention can also be realized by designing the contact as a switch for switching between two tun- ing capacitors. The antenna will thus be tuned both during transmissions and during intermissions, but is tuned by different capacitors. The invention is further explained in the fol¬ lowing, by means of an exemplified embodiment with reference to the drawing, in which Figs. 1 and 2 show signal diagrams for a known system and a system according to the invention, re¬ spectively; Fig. 3 is a schema-tic illustration of a known system; Fig. 4 is a corresponding schematic illustrat¬ ion of a system according to the invention. Fig. 1 shows the fraction of an exponentially decreasing reply signal from a monitored object which remains when shortcircuiting of the antenna in a known system ceases and detection of the signal can commen¬ ce. Fig. 2 shows the corresponding condition of the system according to the invention, in which the anten- na is not shortcircuited, but the electrical connect¬ ion between the antenna and the tuning ccpacitor totally cut off. The hatched area signifies the "add¬ itional signal" that is obtained as a result of the elimination of the lapse of time during which the antenna is inactive. Fig. 3 shows a known system, in which a trans¬ mitter 5 actuates the tuned antenna 7 at short intervals, controlled by a control unit 6. In the intervals (T2 in Fig. 1) between transmissions (Tl in Fig. 1) a possible reply signal from a monitored ob¬ ject must be detected, but prior to this, the energy which oscillates to and fro between the antenna 7 and the tuning capacitor 1, must be demolished, which in the known system takes place when the control unit 6 makes the switch 2 conducting, thereby short¬ circuiting the antenna 7. The energy stored in the antenna is thus demolished, but a short while lapses before detection of a reply signal by the receiver 4 can commence, and during this lapse of time an import- ant part of the reply signal is lost as illustrated in Figs. 1 and 2. In the example shown in Fig. 4 of a security system according to the invention, a transmitter 5 likewise activates a tuned antenna 6 at short inter¬ vals, controlled by a control unit 6, but when a transmission period (Tl in Fig. 2) is* terminated, the control unit 6 immediately disconnects the capacitor 1. Thereby, the antenna becomes untuned and thus un¬ able to emit a signal. This means that the entire intermission interval (t2 in Fig. 2) between two trans- missions (Tl in Fig. 2) can be used to receive pos¬ sible reply signals from a monitored object, cf. Fig.2.

Claims: P A T E N C L A I M S 1. A security system for surveilling the pres¬ ence in, respectively passage through defined zones such as, e.g. exit areas in shops or stores, of ob¬ jects, for example commodities, which system comprises an antenna (7) with at least one tuning capacitor (1) in the monitored zone, and receiver/transmitter devices functioning as tuned resonance circuits on the indiv¬ idual objects monitored, the antenna emitting at short intervals (T2) an electrical signal (Tl) that makes the receiver/transmitter device of a monitored object, which is present in the monitored zone, oscillate, which oscillations in the intermissions between anten¬ na (7) transmissions can be received by the antenna (7) - or by a separate antenna - and used to detect the presence of the monitored object in the monitored zone, c h a r a c t e r i z e d in that a switch (3) disconnects the functioning tuning capacitor from the antenna circuit at commencement of said antenna (7) intermissions. 2. A system as claimed in claim 1, c h a r ¬ a c t e r i z e d in that said switch (3) is connect¬ ed in series to the tuning capacitor (1) of the anten¬ na (7). 3. A system as claimed in claim 1, c h a r - a c t e r i z e d in that said switch is designed for switching between two different tuning capacitors in the antenna circuit.

Inventors: PILESTED KARSTEN GYDE, PILESTED, KARSTEN, GYDE
Applicants: PILESTED KARSTEN GYDE, PILESTED, KARSTEN, GYDE
Application year: 1991
Main classification: G01S13
IPC codes: G08B13