Communications: electrical – Condition responsive indicating system – Specific condition
Patent
1998-10-21
2000-10-24
Wu, Daniel J.
Communications: electrical
Condition responsive indicating system
Specific condition
3405722, 3405725, 3405726, G08B 1314
Patent
active
061374113
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to an article surveillance system for detecting the presence of articles in a detection zone, comprising at least one transmitter means and at least one receiver means for transmitting and receiving, respectively, electromagnetic radio-frequency signals within the detection zone or in a proximity thereof, each article being provided with a sensor operating as a transponder for transmitting an electromagnetic radio-frequency reply signal to said receiver means when receiving a signal from said transmitter means.
DESCRIPTION OF THE PRIOR ART
For many years now a substantial need for simple and still reliable surveillance systems for monitoring objects or articles within a given area has been noticed in various business and industrial applications. A common example is the electronic shop antipilferage systems, which are available in many different kinds.
According to a common kind of electronic article surveillance systems each article is provided with a small label or marker, which comprises a thin metal strip with magnetic properties. At both sides of the shop exit arc-shaped magnetic field generating means are arranged for generating a magnetic field there between. When an article, which has been provided with an antipilferage label according to the above, is carried in between the arcs, the metallic strip is affected by the magnetic field and a detectable physical change occurs in the metal strip. It is common to make use of the fact that an alternating magnetic field will periodically switch the magnetic moment of dipole in the metallic strip. Alternatively, the metallic strip may be forced into mechanical resonance, provided that the material and dimenions of the strip are chosen accordingly. These physical changes are inductively detected by the arcs, wherein an attempted theft may be registered. Since the detection is made by inductive means, antipilferage systems of this kind suffer from a short detection range of a few meters only, which means that the antipilferage arcs must be arranged close to each other and which in turn makes the shop exit narrow and "unfriendly" for the customers.
In addition various antipilferage systems of a more advanced type are previously known. For instance, U.S. Pat. No. 5,030,940 discloses an electronic article surveillance system. Electronic labels are used for marking and theft-protecting the desired articles. Such an electronic label is of a radio-frequency transponder type and comprises for instance an antenna, a power source, such as a battery, and a non-linear circuit, for instance some kind of semiconductor diode. Through its antenna the transponder may receive a first electromagnetic signal of a high frequency, which has been transmitted by a transmitter in the surveillance zone, as well as a second signal of a substantially lower frequency, by means of which an electrostatic field is generated in the surveillance zone. By varying the electrostatic field, certain properties of the non-linear circuit are influenced, the most important of which being the electric reactance. These variations in reactance are amplified by the power source. The antenna is connected to the non-linear circuit, and hence a reply signal may be transmitted, which according to the above is composed by the two signals received. When a modified reply signal is detected according to the above by a receiver in the surveillance zone, the system may determine the presence of an article within the surveillance zone and provide a suitable alarm signal as a consequence thereof. A drawback of such a system is the relatively high price per unit due to the complexity of the sensor, thus making the system less suitable for instance for mass surveillance of articles in a shop. Furthermore, the electrostatic field may quite easily be shielded (so as to avoid detection), for instance by placing the article and the sensor inside a metallic housing, such as a bag covered with a metallic foil on its inside. In addition, a major portion of the electrostatic f
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RSO Corporation N.V.
Tweel , Jr. John
Wu Daniel J.
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