Communications: electrical – Condition responsive indicating system – Specific condition
Reexamination Certificate
1999-11-04
2001-05-15
Trieu, Van T. (Department: 2736)
Communications: electrical
Condition responsive indicating system
Specific condition
C340S572100, C340S572200, C340S568100
Reexamination Certificate
active
06232879
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a sensor for remote detection of objects, preferably for use in an article surveillance system, comprising a wireshaped element, which is arranged to be excited and detected electromagnetically. Furthermore, the present invention relates to a method of detecting the presence of an object in a surveillance zone.
DESCRIPTION OF THE PRIOR ART
There is an increasing demand within business and industry for simple but still reliable systems for contactless surveillance of objects or articles within a given area. A common example is electronic article surveillance systems for e.g. shops, where each monitored object is provided with an article surveillance sensor and where the system is arranged to detect the presence of this sensor as well as to generate an appropriate alarm signal, when an object is about to be removed from e.g. the shop premises without permission.
For a common type of such electronic article surveillance systems each article is provided with a small label, which comprises a thin metallic strip or wire with magnetic properties. At either sides of the shop exit arc-shaped, magnetic field generating devices are located for generating a magnetic field between themselves. If an article, which has been provided with such an article surveillance label, is moved in between the arc-shaped devices, the metallic element is affected by the magnetic field, wherein a detectable physical change occurs for the element. Use is often made of the fact that an alternating magnetic field causes a periodical switch in the magnetic dipole momentum of the metallic element—known as Barkhausenjumps. Alternatively, the metallic element may, if designed appropriately, be made to oscillate mechanically. These physical changes are detected inductively in the arcs, wherein the presence of the article may be determined between the arcs.
Sensors of this kind are for instance disclosed in the U.S. patent publication U.S. Pat. No. 5,496,611, the European Patent publication EP-A-0 710 923 or the European Patent publication EP-A-0 716 393. All these sensors comprise a soft-magnetic metallic wire with a diameter of at least 60-115 &mgr;m and a length of between 5 and 10 cm. During the detection the sensor is exposed to an alternating magnetic field of low frequency, wherein said field generates the Barkhausenjumps described above in the magnetic metal wire. These Barhausenjumps, i.e. periodical switches of the magnetizing direction, are detected inductively.
U.S. Pat. No. 5,406,262 relates to an electronic article surveillance system as described above, the system furthermore being provided with a coil for generating a magnetic bias field in the surveillance zone. Sensors of the types described above require, as a matter of fact, a certain degree of magnetic bias in order to be active, i.e. detectable. By driving the coil in such a way, that the magnetic bias field sweeps, in terms of fieldstrength, within a given range, there is an improved opportunity that sensors present in different parts of the surveillance zone will be exposed to a bias field strength, which lies within the active range of the sensor and therefore prevents the sensor from escaping detection.
U.S. Pat. No. 5,003,291 relates to the use of thin ferromagnetic fibers in electronic article surveillance systems. These fibers, which may have a diameter of between 3 and 80 &mgr;m, are arranged to be excited and detected in an essentially inductive way, in similarity to the above, by means of an alternating magnetic field with a frequency within the kHz-range.
Sensors of the type described above have a disadvantage in that the detection must take place quite close to the sensor, since the detection is carried out inductively. In order to obtain sufficiently large Barkhausenjumps, the magnetic metal wire must furthermore be sufficiently thick, which in reality, for the previously known systems, often has implied a diameter of at least the order of 100 &mgr;m. Such a thick wire must be attached to a carrier, label, etc, for facilitating the mounting of the sensor on the respective article and for avoiding damages to objects or people.
Another problem with conventional article surveillance systems is the risk of false alarms caused by the fact that other magnetic objects than the sensors may become excited and detected.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a sensor for e.g. an article surveillance system, which may be detected at a longer distance than previously known sensors.
Another object is to considerably reduce the risk of false alarms according to the above.
Additionally, the present invention aims at providing a sensor of a very simple design, which may be manufactured at an extremely low cost and which also may be easily mounted to the monitored object, even at such positions, which are hidden or inaccessible to a normal customer and the shop personnel as well as to a potential thief.
The objects above are achieved by a sensor for remote detection of objects, preferably for use in an article surveillance system, comprising a wireshaped element, which is arranged to be excited and detected electromagnetically. The wireshaped element has a diameter less than 30 &mgr;m, and the element is arranged to transmit an amplitude-modulated electromagnetic reply signal in response to an externally applied and time-varying magnetic field. The objects are furthermore achieved by a method of detecting the presence of an object in a surveillance zone, the method comprising the following steps:
the object is provided with a wireshaped element of amorphous or nanocrystalline metal alloy with magnetic properties and with a diameter less than 30 &mgr;m,
electromagnetic signals are generated in the surveillance zone for exciting the wireshaped element,
a time-varying magnetic modulating field is generated in the surveillance zone,
electromagnetic signals generated by the wireshaped element are received and
the received electromagnetic signals are analyzed with respect to a modulation in amplitude caused by the magnetic modulating field for determining the presence of the element (and consequently the object) in the surveillance zone.
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Mechanism of Matteicci Effect Using Armorphous Magnetic Wires, IEEE, Japan, vol. 8, No. 5, May 1993.*
Mechanism of Matteicci Effect Using Armorphous Magnetic Wires, IEEE, Japan, vol. 8, No. 5, May 1993.
Finnegan Henderson Farabow Garrett & Dunner L.L.P.
RSO Corporation N.V.
Trieu Van T.
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