Communications: electrical – Condition responsive indicating system – Specific condition
Reexamination Certificate
1998-11-13
2001-05-01
Hofsass, Jeffery (Department: 2632)
Communications: electrical
Condition responsive indicating system
Specific condition
C340S551000
Reexamination Certificate
active
06225905
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a sensor for remote detection of objects in a surveillance zone, preferably for use in an article surveillance system, which further comprises at least one transmitter means and at least one receiver means for transmitting and receiving, respectively, electromagnetic radio-frequency signals in the surveillance zone, and at least one modulating means for generating a modulation field in the surveillance zone, said sensor being arranged to transmit an electromagnetic reply signal at the reception of electromagnetic energy from said transmitter means, and said reply signal being dependent on said modulation field and being receivable by said receiver means.
DESCRIPTION OF THE PRIOR ART
For many years now a large demand 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 shop antipilferage systems, which are available in many different kinds. A simple and inexpensive protection is obtained by providing articles that are especially liable to be stolen (such as clothes) with an antipilferage cassette, which is attached to the article. The cassette comprises a liquid substance, such as ink, which is arranged to discolour the article to make it useless for normal use, if stolen. In U.S. Pat. No. 5,275,122 an exemplary cassette is shown, comprising a base portion, a mounting pin protruding from the base portion, a pair of liquid-filled glass tubes arranged in the base portion and acting as an ink ampoule, and a disc mounted below the ink ampoule. The mounting pin extends between the two glass tubes and is connected at a first end to the disc. The article to be protected from theft is drawn over the mounting pin in the cassette, for instance through a button hole or directly through the fabric, provided that the article in question is an article of clothing. The cassette is then provided with an upper portion, which is threaded on the mounting pin to secure the article between the base portion and the upper portion. If an unauthorized person tries to remove the cassette upper portion, the disc will impact the fragile ink ampoule through the mounting pin, wherein the former will break or otherwise start leaking. The cassette upper portion may only be separated from the base portion by means of a certain device, without causing damage to the ink ampoule.
Strictly mechanical theft protections according to the above have a disadvantage in that even if a potential shoplifter may be discouraged from attempting to steal an article (since the article will possibly be destroyed, if the theft protection is removed), nothing stops the shoplifter from leaving the theft protection untampered on the article at the actual moment of theft and only later removing the theft protection at an undisturbed location other than the shop premises. This disadvantage may be eliminated by means of electronic article surveillance systems as described below, which may detect the actual shoplifting attempt—i.e. when the shoplifter tries to bring the article out from the shop premises—and in response thereto generate an alarm signal so as to alert the shop personnel about the attempted theft.
According to a common type of electronic article surveillance systems each article is provided with a small label, comprising a thin metal strip with magnetic properties. On either side of the shop exit arc-shaped magnetic field generating means are arranged for generating a magnetic field in between. When an article, which has been provided with an antipilferage label according to the above, is brought in between the arcs, the metal strip is affected by the magnetic field, and a detectable physical change occurs in the metal strip. Frequently, the fact that an alternating magnetic field will periodically switch the magnetic dipole momentum in the metallic strip is used. Alternatively, the metallic strip may be forced into mechanical resonance, provided that the material and dimensions of the strip are chosen accordingly. These physical changes are inductively detected by means of 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, requiring the antipilferage arcs to be arranged close to each other and thereby making 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 semi-conductor 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 as described 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. A drawback of such transponders is that they require a plurality of components as well as a considerable space and a high price per unit.
Normally, for basic antipilferage applications as described above, it is only desired to determine the presence of a transponder or sensor in a surveillance zone, but not its identity or exact position in the zone. Such determination, however, is of interest in an adjacent technical field, e.g. price labelling of articles. A method and a device for this purpose are disclosed in WO93/14478. A label acting as a sensor or transponder is provided with an antenna and at least one electric resonance circuit, comprising inductive as well as capacitive means; a so-called LC-circuit. The resonance circuit is excited to self-oscillation by means of electromagnetic energy transmitted by an excitation means and received through the antenna of the sensor. By providing the label with an amorphous magnetic element and controlling the permeability of this element by means of an external heterogeneous magnetic bias field, also the resonance frequency of the resonance circuit may be controlled, since the change in permeability for the element will affect the inductive properties of the resonance circuit. Due to the factors above, the frequency of the reply signal transmitted from the resonance circuit is dependent on the magnitude and direction of the magnetic bias field in the position, where the sensor happens to be located. As a consequence, simultaneous detections of a plurality of identical sensors present in the surveillance zone are possible, thanks to the reply signals thereof being separated in the frequency domain through their different magnetic bias levels. Alternatively, a calculation “backwards” in three dimensions of the position for the sensor is possible by means of the detected frequency value, on condition that the heterogeneous magnetic bias field is known. By arranging a plurality of labels and/or amorphous magnetic elements in predetermined mutual positions a certain code space may be obtained, wherein the reply signal may for instance represent an article number assigned to the article.
U.S. Pat. No. 5,414,412
Quinones Christian
Tyren Carl
Hofsass Jeffery
Merchant & Gould P.C.
RSO Corporation N.V.
Tweel , Jr. John
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