IL31466A - An intruder detection system - Google Patents

Description

An Intruder detection system AND CO The present invention relates to a passive line for an intruder detection system and to the system function of an intruder detector system is to detect the entry of an intruder into a defined area whilst carrying a ferromagnetic such for house breaking small arms or the Various intruder systems have been proposed but in most cases the effectiveness of these known systems is limited by the fact that they are either complicated and therefore liable to fall or to be put out of action by the intruder or can lead to the production of false or that they are relatively difficult to According to the present invention there is provided a passive line for a intruder detection system operable in the magnetic comprisin a plurality of elongated arranged in lengthwise contact to fine a flexible magnetic core unmagnetized except for the effect thereon of the magnetic much the of the wires being than the of the and a coil wound on the core having at least one layer of insulated conductor wound on the core in the form of many successive lengthwise serially conneoted tions of respectively the sections being of substantially equal length and homogeneously The passive system in accordance with the present invention is based on the observed phenomenon that the sence of a ferromagnetic object is associated with geneous magnetic which arises out of the magnetic moment Induced in the object by the magnetic field and out of magnitude of the induced magnetic moment of the ferromagnetic object is related to the and shape of the its magnetic permeability and On the other the magnitude of the magnetic field is a more or less constant factor in these considerations having an intensity of about gauss in a direction which mainly depends on the latitude of the location If now the ferromagnetic object its associated field will also move and in consequence a fluctuation in the magnetic flux density will be observable during the movement at any point along the path of In the case of the present this inhomogeneous magnetic field associated with the object will be greatly enhanced and concentrated by the high permeability core and there will be in the various sections of the electromotive forces which are cumulative along the length of the line and which result the production of a detectable voltage aoross the coil On the other hand any change in the homogeneous field fluctuations the magnetic will induce equal but opposite voltages in all the sections resulting in a substantially zero voltage across the coil Preferably the coil sections are of substantially equal length and numbers of the section length being of the order of one meter in a low impedance line is in order to amplify the voltage signal appearing across the which is of the order of nano the line is coupled to an amplifier via a suitable transformer which is matched to the low impedance of the The output of the amplifier fed to an appropriate discriminator circuit such as for example a threshold detector by means of which unwanted signals are The output of the discriminator can then be coupled to an priate alarm system which can be visual or For a better understanding of the present invention to show how the same can be carried out in practice reference will now be made to the accompanying drawings in which 1 and 2 are schematic representations of three forms of lines in accordance with the present inventio 3 is a schematic circuit block diagram of the Intruder detection system in accordance with the and 4 is a schematic cross sectional representation of a line in accordance with the present 1 shows schematically a line 1 consisting of a ferromagnetic core 2 along the length of which is wound a coil The coil 3 is wound in the form of sections of respectively opposite winding In the embodiment shown in of the drawings seeing that both ends of are to emerge from the same end of the line one return lead of the extends over the whole wound length said one terminal lead can be made extend through tie In the embodiment shown in 2 of the drawings the coil is rewound on itself in such a way that two ends of the coil emerge from one end of the In this way a multilayer coil can be The constituent layers of each section can be successive right and left handed in which case the successive layere are connected in Where however successive sections are to be connected in parallel then the successive constituent layers of each section form the handedness alternating from section to As seen schematically in the output from the line l is fed to a transformer which to impedance match the line 1 with a subsequent amplifier For this purpose a step up transformer 5 is Since ths signal frequency to be detected is in the cycle per second range between 3 a magnetically shielded transformer having a step up ratio of 1 to 50 and a primary winding of 100 Henry is The output of the former 5 fed to the amplifier which has an input impedance of several megohms in the above signal frequency range and has a noise characteristic of less than one micro output of the amplifier 6 is fed to a discriminator 7 whose purpose is to discriminate between signals whose voltage lies above or a certain minimum The output of the discriminator 7 fed to a suitable alarm system which can be visual or With an arrangement as described above an extreme situation can envisaged wherein the ferromagnetic behaving as a magnetic moves in a plane which is always parallel to itself and normal to the In such extreme situation the signal strength arising out of the passage of a ferromagnetic object in the vicinity of the line has a value when this object passes between two ive sections and a theoretically zero value the object passes the mid point of any specific In order to ensure that a detectable signal is obtained even under these extreme circumstances when the object passes the mid point of a an arrangement can be employed wherein two essentially separate independent coils are employed wound one on the other each coil being similar to that described with reference to the two coils being shifted with respect to each other by half a section and being connected respectively via separate transformers and amplifiers to separate With such an arrangement and even under extreme circumstances referred to above when an object passes in the mid point of any specific section a detectable signal will be obtained at least in one of the In one particular example of a flexible line shown schematically in of the the core 1 consists of a considerable number of mumetal mm A double layer of enamel insulated copper wire of mm diameter is wound on the the form of sections of successively opposite winding Each section is 700 mm in length and in consequence each section includes about 1400 With such a line having a length of 100 metres the overall resistance is about 400 In order ensure good electrical insulation between core and the winding and between the two layers The windings are encased a copper braiding shield 11 which in its turn located within an outer plastic envelope 12 and in this way moisture is prevented from entering the It will appreciated that other materials can be for the magnetic core and conductive windings and other dimensions can be whils in the arrangement shown in of the drawings a second winding is between successive sections of a first winding this second winding can be superimposed on the first winding but shifted with respect An intruder detection system including a line in accordance with the present invention the distinct advantage that the line which can be of small diameter can be rapidly and simply buried in a single narrow trench even in difficult Such a single line can be effectively protective against by rodents by enclosing it in a an aluminum irrigation same trenoh in which the is laid can also serve to accommodate the to and from the plurality of successive being about 200 m in can be coupled by means of these to respective electronic units which can be located in a single control insufficientOCRQuality

Claims (1)

1. CLAIMS 1. A passive line for an intruder detection system operable in the earth's magnetic field, comprising a plurality of elongated high-permeability wires arranged in lengthwise contact to define a flexible high-permea* bility magnetic core unmagnetized except for the effect thereon of the earth's magnetic field, the cross-section of the wires being very much smaller than the cross-sectio of the core; and a coil wound on the core havin at least one layer of insulated conduotor wound on the oore in the form of many successive lengthwise serially connected sections of respectively opposite-winding handedness* the sections being of substantially equal length and homogene-ously wound· 2* A line according to Claim 1, including a shield of braided metal encasing the coil and core to ef ect flexing of the line and an outer plastic envelope encasing the shield for waterproofing the line* 3. A line according to either of Claims 1 or 2 wherein the coil oomprlses a plurality of layers of conductors, overlying sections in different layers having the same winding handedness* 4·. A line according to any of Claims 1 to 3 including a second coil wound on the core, the second coil comprising at least one layer of Insulated conduotor wound on the first mentioned coil in the f^rm of successive lengthwise sections of respectively opposite handedness, the sections of one coll being shifted lengthwise by about half a section with respect to the sections of the other coll* 31466/2 1 through 4 comprising a high-gain, low-noise amplifier, ,and an impedance matching transformer coupling the line to the amplifier for, detecting transient inhomogeneoue magnetic fields in the vicinity of the line* 6. A passive line substantially as described above by way of example and with reference to the accompanying drawings* 7· An intruder detection system substantially as described above by way of example and with reference to the accompanying drawings* For the Applicants DR. EXNHOLD OOHN AND PARTNERS B s