Communications: electrical – Condition responsive indicating system – Specific condition
Patent
1996-07-19
1998-05-12
Hofsass, Jeffery
Communications: electrical
Condition responsive indicating system
Specific condition
340551, 34082554, G06B 1324
Patent
active
057512132
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an alarm element which forms part of an alarm system.
2. Description of the Related Art
Such an alarm element is preferably used in goods-guarding systems, although it may also be used in other contexts in which goods or articles are provided with a marking which can be subsequently read-off electronically and used in one way or another.
Many different types of alarm systems for the protection or safeguarding of goods are described in the patent literature. These systems are intended to prevent goods from being taken from shops and stores without having been paid for, and will normally include some form of alarm element which is attached to the goods or articles to be protected, and a sensor arrangement which is installed permanently in the proximity of the store exit, this sensor arrangement generating an alarm with the aid of some form of remote sensing device when an alarm element is brought into the proximity of the store exit.
Remote sensing 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 characteristic of the alarm element.
The alarm element may be a narrow, elongated and thin strip of highly permeable material whose characteristic feature resides in the transmission of high order harmonics when it is 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 sensing or detecting devices. This type of goods-protection or theft-prevention alarm is particularly suited for shops and stores which deal on a daily basis and is found described in European Patent Specification EP 0 153 286, among other publications.
Known alarm elements may also be comprised of a simple electric resonance circuit. In this case, a simple and inexpensive sensing device can be used when the coil in the resonance circuit is relatively large, thereby enabling a good Q-value to be readily obtained at the same time as connection with external fields is large. In this regard, the coil is included in an alarm plate or tag which is fastened to the goods to be protected, by means of some suitable looking device. As before mentioned, the sensing or detection devices for this type of alarm system may be relatively uncomplicated and inexpensive, although the problem of false alarms is difficult to avoid, since store environments often contain loops of electrically conductive material which give rise to resonances similar to those obtained from the alarm elements.
One method of avoiding false alarms is to give the alarm element the form of a frequency divider. U.S. Pat. No. Specification 4,670,740 describes an alarm element in the form of a frequency divider. Such a frequency divider can be readily constructed with solely one coil and one capacitance diode.
In this case, the magnetic field transmitted by a transmission coil must be relatively powerful, since the energy is absorbed in the alarm element at a frequency which is far from its resonance frequency. An inexpensive and simple alarm element of this kind therefore has a low degree of responsiveness or sensitivity.
European Patent Specification EP 0 469 769 defines a method of increasing responsiveness, or sensitivity, when two mutually connected magnetic resonance circuits are present. The one circuit receives a first magnetic field having a first frequency. The energy received is transferred to the other resonance circuit, which transmits a field having half the frequency. This thus also concerns a frequency divider. Even though responsiveness is increased in comparison with the use of only one resonance circuit, such an alarm element is both expensive and complicated.
SUMMARY OF THE INVENTION
The present invention relates to an alarm element which solves the aforesaid problems. The alarm element is of very simple and inexpensive const
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Hofsass Jeffery
Lefkowitz Edward
Mangels Alfred J.
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