Communications: electrical – Condition responsive indicating system – Specific condition
Patent
1996-11-15
1999-03-16
Hofsass, Jeffery A.
Communications: electrical
Condition responsive indicating system
Specific condition
340552, 340511, G08B 1314
Patent
active
058835741
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The present invention relates to an arrangement for preventing disturbances in electronic alarm systems of the kind used in merchandise security systems.
DESCRIPTION OF THE RELATED ART
Many different types of merchandise security alarm systems are described in the patent literature. These arrangements are intended to prevent merchandise from being removed from shops and stores without having been paid for. Normally, such merchandise security arrangements, or theft prevention systems, include some type of alarm element which is secured to the merchandise to be protected, and a detector device which is installed in the vicinity of the store exit and which delivers an alarm through the medium of remote detection control when an alarm element is brought into the vicinity of the store exit.
Remote detection of the alarm element is normally achieved by transmitting a magnetic alternating field, wherewith the presence of an alarm element can be detected through a change in the alternating field characteristic of the alarm element.
The alarm element may have the form of an elongated, narrow thin strip of highly permeable material whose characteristic property is to transmit high order harmonics when subjected to the effect of a magnetic alternating field. This known basic principle enables small and inexpensive alarm elements to be detected with the aid of complicated and relatively expensive detection systems. This type of merchandise alarm is particularly well-suited for protecting everyday commodities or merchandise and is described, inter alia, in European Patent Specification EP 0,153,286.
Also known to the art are alarm elements comprised of a simple electric resonance circuit. In this case, the detection system can be made simple and inexpensive when the coil in the resonance circuit is relatively large, so that a good Q-value can be readily obtained at the same time as the coupling to external fields will be large. In this case, the coil is mounted in an alarm plate which is secured to the merchandise to be protected with the aid of some suitable fastener means. As before mentioned, the detection systems which operate with this type of alarm can be made relatively uncomplicated at a relatively low cost, although it is difficult to avoid the problem of triggering false alarms, because the store environment will often include loops of conductive material which give rise to resonances similar to the resonances obtained from the alarm elements.
Another problem resides in coupling of current in the receiver and transmitter coils and current on the alarm system cables that extend between the transmitter and receiver equipment and the detection system. By current on conductors is meant that a net current flows in a pair of conductors between which an alternating voltage prevails, i.e. the current in one conductor of the conductor pair is greater than the current in the other conductor of said pair. Cable current means that the sum of the currents in all conductors of the cable differ from zero, when taking into account the direction of the current in each conductor.
By pairs of conductors is meant above and below, and also in the Claims, a pair which consists in two simple conductors and a cable including two or more conductors, of which one or more conductors can be comprised of a shield around one or more other conductors.
When the transmitter gives rise to a current on conductors other than those conductors which form the actual coil, an alarm can be triggered by an alarm element that is in the close vicinity of a conductor but far away from the coil itself. This is most undesirable.
Current on conductors other than those which form the receiver coil can give rise to a signal in the receiver as a result of the receiver taking in disturbances that originate from electromagnetic fields that couple to the conductor concerned, for instance as a result of the conductor passing an electrical apparatus.
If a transmitter coil or a receiver coil is touched by a person, and in particular
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Hofsass Jeffery A.
Mangels Alfred J.
Tweel , Jr. John
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