Communications: electrical – Condition responsive indicating system – Specific condition
Patent
1997-04-09
1998-07-07
Swann, Glen
Communications: electrical
Condition responsive indicating system
Specific condition
340551, G08B 13187
Patent
active
057775559
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an electronic alarm system of the kind used in commodity surveillance systems.
2. Description of the Related Art
Many different types of commodity surveillance alarm systems are described in the patent literature. These alarm systems are intended to prevent commodities from being removed from stores and the like without being paid for. The commodity surveillance alarm systems normally include some type of alarm element that is attached to the commodities to be protected, and a detection arrangement which is installed permanently at a store exit and which with the aid of some form of remote sensing device is able to produce an alarm when an alarm element is brought into the vicinity of the store exit.
This remote sensing of a commodity is normally effected by transmitting a magnetic alternating field, wherein the presence of an alarm element can be detected as a result of a change in the alternating field that is characteristic to the alarm element concerned.
Alarm elements may have the form of long, narrow thin strips of highly permeable material capable of transmitting high order harmonics when exposed to a magnetic alternating field. Using this basic principle, it is possible to detect the presence of small and inexpensive alarm elements with the aid of complicated and relatively expensive detector arrangements. This type of commodity surveillance alarm system is particularly well-suited for use in stores that sell everyday commodities, utilities or convenience goods and is described, inter alia, in European Patent Specification EP 0 153 286.
Also known to the art are alarm elements which consist of a simple electric resonance circuit. In this case, the detector arrangement can be made simple and inexpensive if the coil in the resonance circuit is made relatively large, wherewith a good Q-value can be readily obtained at the same time as coupling to external fields becomes large. In this case, the coil is placed in an alarm plate which is secured to the commodity to be protected with the aid of some kind of fastener means. As before mentioned, the detector arrangements for this type of alarm may be made relatively uncomplicated and inexpensive. However, it is difficult to avoid a false alarm, because the store environment will often include loops of conductive material which give rise to resonances similar to the resonances obtained with the alarm elements.
The detector arrangement of such alarm systems will often include two mutually spaced alarm arches which delimit the investigation zone. The alarm arches will normally include separate transmitter and receiver coils.
There is a strong desire to construct alarm systems that are both simpler and less expensive than existing systems. It is also desirable to lower the cost of installing an alarm system, among other things by simplifying the manner in which cables are drawn between the alarm arches and the electronic unit of the alarm system.
The present invention fulfils these desiderata and enables one and the same coil to be used both as a transmitter coil and as a receiver coil, and requires only one coaxial cable to be drawn up to respective alarm arches.
SUMMARY OF THE INVENTION
The present invention thus relates to an electronic alarm system of the kind used in a commodity surveillance system. The system includes an alarm element which functions to receive a magnetic alternating field transmitted from a transmitter by means of a transmitter coil and which also functions to re-transmit a magnetic alternating field. A receiver includes a receiver coil and is adapted to receive and detect the re-transmitted alternating field. The system is characterized in that the transmitter coil and the receiver coil consists of one and the same coil, wherein a transformer and a resistance between the transmitter and the receiver on one side and the transmitter-receiver coil on the other side are so arranged that when transmitting a first part of the transmitter power is appli
REFERENCES:
patent: 4212002 (1980-07-01), Williamson
patent: 5353011 (1994-10-01), Wheeler et al.
Mangels Alfred J.
Swann Glen
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