SECURITY FENCE

Abstract: A security fence comprising a multiplicity of rigid bar element disposed in a fence configuration, at least one of the rigid bar elements comprising apparatus for providing predetermined bending of an optical fiber in response to bending of the rigid bar element by at least a given amount.

Description: FIELD OF THE INVENTION The invention relates to a security fence comprising: (a) a multiplicity of rigid elements; (b) an optical fiber associated with at least one of said rigid elements; (c) means for securing the optical fiber such that bending of the at least one element causes bending of the optical fiber; and (d) means for sensing the bending of the optical fiber. Such a security fence has an intrusion sensing capability. BACKGROUND OF THE INVENTION Security fences of various types are known in the patent literature and available on the market. These include taut-wire fence systems which employ tensioned wires mounted onto motion sensors, whereby an attempt to cut or spread apart the wires results in an alarm indication of the approximate location of the attempted intrusion. There are also known a variety of security barriers which employ a fiber optics sensing apparatus. GB-A-2 098 770 describes a security barrier structure comprising a lattice of hollow tubular members through which fiber optic cable is threaded. An attempt to break through the barrier breaks or distorts the fiber by overtensioning same, thus causing a sensible attentuation of an optical signal transmitted through the cable. GB-A-2 038 060, GB-A-2 046 971, GB-A-2 062 321, US-A-4 292 628 and US-A-4 399 430 all show security applications, wherein an alarm indication is provided by breakage of an optical fiber. GB-A-2 077 471 shows a security application wherein a pressure sensitive fiber optic composite cable is provided. A security fence of the type mentioned in the beginning is known from GB-A-2 039 683 showing a rigid element which is a bendable or breakable member having an optical fiber secured thereto which fiber is bent or broken when the member is broken. The bending or breaking of this member is detected causing an alarm. However, in the security fence according to GB-A-2 039 683 the means for securing the optical fiber is simply a curved portion of the rigid element. Therefore, it is characteristic for this construction that the optical fiber is bent only by an amount which is equal to the local bending of the rigid element. Thus, to obtain a higher bending of the optical fiber GB-A-2 039 683 proposes to make the rigid element more bendable or breakable that the other rigid elements by providing holes and/or recesses in the rigid element. However, such holes and/or recesses disadvantageously weaken the fence per se. Such weakening of the fence per se will become even more disadavantageous when, as proposed in GB-A-2 039 683, all the rigid elements are formed in the same way i.e. provided with weakening portions. Finally, FR-A-2 418 506 discloses a system for detecting the presence of a railway vehicle along a railroad by means of an optical cable comprising an optical fiber which is arranged in a flexible conduit. Such optical cable is buried within the ground so that the weight of the railway vehicle deforms the flexible conduit under a tie by such an amount that the walls of the conduit contact and bend the optical fiber. This bending is then detected. However, the bending of the optical fiber is less than the bending of the conduit, since the internal walls of the conduit are arranged in a certain distance from the optical fiber when the conduit is not deformed. In addition, the bending of the optical fiber is not sharp, causing a weak indication of bending. SUMMARY OF THE INVENTION The present invention seeks to provide a reliable security fence of the parallel bar type which includes a fiber optics intrusion detection capability. There is thus provided in accordance with the present invention a security fence of the type mentioned in the beginning, which is characterized in that said means for securing the optical fiber comprises (1) first and second optical fiber securing elements for securing said optical fiber, which are arranged not to be displaced when a portion of the at least one element is displaced by an amount less than a given amount; and (2) a third optical fiber securing element arranged to be displaced when said portion of the at least one rigid element is displaced less that the given amount; whereby bending of the element causes displacement of the optical fiber element relative to the first and second elements, producing sensible bending of the optical fiber. Thus, a preferred embodiment of a security fence according to the present invention comprises a multiplicity of rigid bar elements disposed in a fence configuration, at least one of the rigid bar elements comprises means of the above type for providing predetermined bending of an optical fiber in response to bending of the ridig bar element by at least a given amount. For the purpose of this description and claims, "bending" includes microbending. Further in accordance with an embodiment of the present invention, the security fence comprises optical fibers disposed in a plurality of the rigid bar elements, means for transmitting an optical signal through the optical fibers and means for detecting the existence and location of bending in the optical fibers by means of sensed attenuation, phase change, signal reflection, polarization, wave length change, or any combination of the foregoing in the transmitted optical signal. Additionally in accordance with a preferred embodiment of the invention, the means for providing predetermined bending is operative to provide microbending of the optical fiber. BRIEF DESCRIPTION OF THE DRAWINGS The present invention will be understood and appreciated more fully from the following detailed description, taken in conjunction with the drawings in which: Fig. 1 is a pictorial illustration of a security fence system constructed and operative in accordance with a preferred embodiment of the present invention; Fig. 2 is an illustration of the arrangement of optical fibers in a portion of the security fence of Fig. 1; and Figs. 3A and 3B are illustrations of a pair of rigid bars and enlarged sections thereof under conditions of no bending and bending respectively. DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT Reference is now made to Figs. 1 and 2, which illustrate a security fence system constructed and operative in accordance with a preferred embodiment of the present invention. The security fence system typically comprises a conventional type of security barrier, such as a parallel bar type fence 10, as shown, which comprises a plurality of generally upstanding spaced bars 12. According to a preferred embodiment of the present invention, bars 12 are formed of suitably thick steel, plastic or any other material having a desired cross section and are hollow. According to the illustrated embodiment, an optical fiber 14 is threaded serially up and down through a plurality of adjacent bars 12, as shown in Fig. 2. Alternatively any other desired configuration of optical fiber 14 may be employed. A signal transceiver 18, such as a TEK fiber optic TDR cable tester, manufactured by Tektronix, of Portland, Oregon, U.S.A., hereinafter referred to as "OTDR", is connected to an end of the fiber 14 for providing a suitable optical signal for passage therethrough and receiving the reflected signal therefrom. Alternatively, in place of the OTDR, a spectrum analyzer having a built in transceiver, such as a TEK portable spectrum analyzer 490 series, also available from Tektronix, may be employed. Apparatus of this type may be used to provide output indications of the existence and approximate location of bending of the optical fiber and resultant attenuation, phase change and signal reflections, or any one or more of the foregoing. The output of transceiver 18 may be provided to threshold and signal processing circuitry 20 for automatic determination of whether a alarm indication exists based on predetermined thresholds or other criteria. Alternatively, an operator may monitor the transceiver 18 in order to perceive a alarm indication. The output of transceiver and of processing circuitry 20 may be supplied to alarm indication circuitry 22, which provides a suitable alarm output indication of the existence and approximate location of the attempted intrusion. The signal processing circuitry 20 may include means for classifying alarm indications based on the time rate of change of analog bending parameters sensed by the means for detecting. Reference is now made to Figs. 3A and 3B which illustrate an apparatus for providing bending of the optical signals passing through optical fiber 14 in response to bending of bars 12. Disposed interiorly of many or all of bars 12 and in generally parallel spaced relationship thereto is a rigid support rod 24, which is arranged so as not to be bent in response to bending of bar 12, which is less than a predetermined amount, such as 25 degrees. Fixedly mounted on support rod 24 are a plurality of spaced optical fiber securing elements 26 having apertures 28 which receive the optical fiber 14, with relatively little transverse play. Typically, securing elements 26 are separated along the length of the optical fiber by one centimenter and extend over part or most of the length of bar 12. Intermediate each pair of securing elements 26 there is provided a intermediate element 30, which is configured and arranged to peripherally engage the interior surface of bar 12, so as to be displaced when bar 12 is bent. Intermediate element 30 is formed with an optical fiber engaging aperture 32, which normally, i.e. when the bar 12 is unbent, is arranged in registration with apertures 28 such that the optical fiber 14 extends therethrough in a generally straight line, as shown in Fig. 3A. Intermediate element 30 is also provided with a large aperture 34 through which extend support rod 24 and the looped back portion of optical fiber 14. The arrangement of aperture 34 is such that under bending of bar 12 within a predetermined range for which intrusion detection is required, support rod 24 does not contact intermediate element 30 and thus does not restrict its movement. Referring now to Fig. 3B, it is seen that when a bar 12 is bent, intermediate element 30 is displaced laterally, with respect to the axis of optical fiber 14, while securing elements 26, which are mounted on support rod 24, are not bent. As a result apertures 28 and 32 are no longer in registration, and cause bending of the optical fiber 14. The bending of the optical fiber 14 produced by the bending apparatus, an example of which is illustrated in Figs. 3A and 3B, is sensed by receiver 18, spectrum analyzer 20 ad alarm indication circuitry 22 to provide an alarm indication of the existence and general location of the sensed bending. According to a preferred embodiment of the present invention, optical fiber 14 is directly coupled to transceiver 18. Thus, it may be appreciated that a single optical fiber is operative both for detection and for connection. It will be appreciated by persons skilled in the art that the present invention is not limited to what has been particularly shown and described hereinabove. For example, the apparatus shown in Figs. 3A and 3B may be replaced by any other suitable bending responsive apparatus. The scope of the present invention is defined only by the claims which follow:

Claims: A security fence (10) comprising: (a) a multiplicity of rigid elements (12); (b) an optical fiber (14) associated with at least one of said rigid elements (12); (c) means for securing the optical fiber (14) such that bending of the at least one element (12) causes bending of the optical fiber (14); and (d) means (18, 20) for sensing the bending of the optical fiber (14); characterized in that said means for securing the optical fiber (14) comprises (1) first and second optical fiber securing elements (26) for securing said optical fiber (14), which are arranged not to be displaced when a portion of the at least one element (12) is displaced by an amount less than a given amount; and (2) a third optical fiber securing element (30) arranged to be displaced when said portion of the at least one rigid element (12) is displaced less than the given amount; whereby bending of the element (12) causes displacement of the optical fiber (14) relative to the first and second elements (26) producing sensible bending of the optical fiber (14). Apparatus according to claim 1, characterized in that the sensible bending is microbending. Apparatus according to anyone of the preceding claims, characterized by comprising a support rod (24) supporting said first and second optical fiber securing elements (26) and arranged so as to be displaced due to bending of the rigid element (12) up to a given amount. Apparatus according to any one of the preceding claims, characterized in that the means for sensing comprises: (a) means (18) for transmitting an optical signal through the optical fiber (14); and (b) means (18, 20) for detecting the existence and approximate location of the optical fiber (14) by means of sensed changes in the optical signal. Apparatus according to claim 4, characterized in that said means (18) for transmitting and means (18, 20) for detecting are embodied in OTDR apparatus. Apparatus according to claim 4, characterized in that said means (18) for transmitting and means (18, 20) for detecting are embodied in spectrum analyzer apparatus. Apparatus according to claim 4, characterized in that said means (18, 20) for detecting comprise attentuation detection means. Apparatus according to claim 4, characterized in that said means (18, 20) for detecting comprise phase change detection means. Apparatus according to claim 4, characterized in that said means (18, 20) for detecting comprise signal reflection detection means. Apparatus according to claim 4, characterized in that said means (18, 20) for detecting comprise wave length change detection means. Apparatus according to claim 4, characterized in that said means (18, 20) for detecting comprise polarization detection means. Apparatus according to anyone of claims 4 to 11, characterized in that said means (18) for transmitting and said means (18, 20) for detecting are directly coupled to said optical fiber. Apparatus according to anyone of claims 4 to 12, characterized in that said means (18, 20) for detecting is operative to provide an output indication of the existence and approximate location of an intrusion. Apparatus according to anyone of claims 4 to 13, wherein said means (18, 20) for detecting includes means for classifying alarm indications based on the time rate of change of analog bending parameters sensed by the means (18, 20) for detecting.

Inventors: DANK, ZVI
Applicants: MAGAL SECURITY SYSTEMS, LTD.
Application year: 1987
Main classification: G08B13
IPC codes: G08B13, G01V9, E04H17, G02B6, E01F15