Abstract:
A method and apparatus employing an array of spaced detectors and a detection circuit to detect disturbance of first and second conductors and provide signals to a computer system to determine both the presence of the intruder and the direction of travel of the intruder. The velocity of the intruder&#39;s travel may also be determined. When used with a destruction device to destroy an intruder such as an explosive shell, the lob time and the killing distance may also be calculated and used to fire the shell and to explode it at a position to destroy the intruder. Often the intruder is an armored vehicle or a tank. The conductors may be spaced in an array that has the appearance of a spider web. The conductors may be light weight conductive elastomeric conductors.

Description:
This application claims the benefit of provisional application entitled “conductive elastomer position detection, U.S. Provisional Patent Application No. 60/349,509, filed Jan. 18, 2002, which is hereby incorporated by reference in its entirety. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to an intrusion detection system and also to a method of destroying the intruder which may be an armed vehicle such as a tank. 
     BACKGROUND OF THE INVENTION 
     A large number of detection systems used sensor wires for detecting the presence of an intruder into a space by the intruder pressing on a sensor wire that may be on or beneath the surface. Various detection systems in prior art patents, U.S. Pat. Nos. 4,527,150; 5,021,766; 4,482,890 and 4,450,434 used optical fibers, coaxial cables, acoustical waves, pressure switches to detect people who may be intruding onto a space. 
     The present invention is directed to not only detecting the presence of an intruder, but also to determining the direction of travel of the intruder particularly if the intruder is a mobile vehicle traveling at a relatively fast velocity. In the illustrated embodiment of the invention, the intruder is a tank having a velocity typically in the range of 7 meters per second (7 m/s) to 22 m/s and it is desired to provide a tripwire system that is reliable, accurate and is low cost so that it can be deployed in various terrains. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, there is provided an intruder detection system having an array of spaced conductors and a detection circuit that detects disturbances of a first conductor and then a spaced second conductor and sends a signal to a computer system to determine not only the presence of the intruder but also the direction of travel of the intruder. Preferably, the computer system also determines the velocity of the intruder and causes the firing of an explosive device such as a shell with a lob time and aim to explode within a killing distance of the intruder vehicle. 
     In the preferred embodiment of the invention, the array of spaced conductors includes radially extending conductors from a central location, extending a predetermined distance, e.g., 20-70 meter length. A trade-off exists between the number of radially extending tripwire conductors and their length for a given coverage where there is needed a two wire contact, i.e., a first hit or disturbance of a first trip and then a second hit or disturbance of a second tripwire. The two wire contact is desired to determine the target&#39;s location, direction of travel and speed of travel so that an explosive shell as from a mortar or artillery piece may be lobbed to intercept the intruding vehicle. 
     In accordance with the preferred embodiment of the invention, the conductors are aligned in a spider web array with circumferentially extending conductors joined to radially extending conductors. If a first radially extending tripwire is tripped as the intruding vehicle rolled over and displaces insulation on a coaxial tripwire and then a second spaced tripwire is similarly tripped then the speed of the intruder is calculated as is the lob time for the shell so that the shell is directed to a spot located on the line of travel and located at the distance the intruder vehicle should have traveled after the second trip. 
     The tripwire array should be light enough so that it can be easily deployed or laid even though the conductors may be quite long, e.g., 20 to 70 meters in length for a radially extending wire. The preferred wire is a coaxial cable having an internal center conductor with an outer tubular-conductor thereabout. When pressure is applied to the outer material by an external force, such as the intruder, the outer conductor is displaced changing an electrical characteristic of the tripwire such as a change in current flow. The resultant current flow can be used to calculate the linear distance down the wire to the impact location or hit by the intruder. Herein, the preferred coaxial conductor comprises a wire with an outer insulator thereon inside of an outer exterior conductive elastomeric tube. The pressure from the intruding vehicle displaces the insulation causing a point of contact between the outer conductive elastomeric and the center conductor wire. 
     In accordance with another embodiment of the invention, the tripwire may be in the form of an optic fiber tripwire which breaks at the point of contact by the intruder and linear distance location of the break may be determined by an optical time domain reflectometer. The preferred fiber optic may be a single mode, glass fiber of about 9 microns. Preferably, the glass fiber is that it is durable enough to withstand deployment of 20-30 meters in a radial direction and yet fragile enough to break when subjected to pressures in the range of 10-15 psi from a tank. In accordance with a further embodiment of the invention, the detection system may deploy a tripwire array of ultrasonic conductors that would have an ultrasonic pulse injected through a hollow tubular conductor. When the tubular conductor is collapsed by the intruder, the time delay between signal generation and reflection signal detection can be measured to locate the intruder&#39;s collapse of the tubular conductor. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a plan diagrammatic view of an intruder detection system having an array of tripwires constructed with and embodying the invention; 
         FIG. 2  is a diagrammatic view of a path of an intruding vehicle impacting the array of tripwires at two trip points and showing the distance traveled and the point of impact; 
         FIG. 3  is a diagrammatic view of a tripwire system constructed in accordance with the preferred embodiment of the invention; 
         FIG. 4  is a cross-sectional view of a coaxial tripwire prior to intrusion; 
         FIG. 4   a  is a cross-sectional view of a tripwire constructed in accordance with another embodiment of the invention; and 
         FIG. 5  illustrates a continuous loop fiber optic tripwire array in accordance with another embodiment of the invention; 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     As shown in the drawings for purposes of illustration, the invention is embodied in an intruder detection system  10  that may also include an explosive destruction device  12  ( FIG. 3 ) in the form of a mine, artillery piece, or mortar. In the case of the mortar and the artillery piece, the intruder detection system is used to determine the location, bearing, i.e., direction of travel, and distance or time to impact the traveling intruder. 
     In accordance with the preferred embodiment of the invention, the system  10  includes an array of conductors or tripwires  20  ( FIG. 1 and 2 ) arranged at spaced locations so that an intruding tank or other vehicle will be detected at a first trip point  22  ( FIG. 2 ) and then detected at a second trip point  24  (FIG.  2 ). The electrical signals from the detection circuit  26  are sent to a computer or calculating device  28  that determines the distance that the intruding vehicle will travel so that the munition may be lobbed to impact the targeted vehicle at the impact point  30  (FIG.  2 ). 
     In a first example, eight tripwires are provided for a tank traveling at 7 m/s and using a lob time of 1 second for the munition, the use of tripwires of 30 meter length provides almost complete coverage with a 19.2 meter kill radius (FIG.  2 ). 
     The speed of the target vehicle and the lob time drastically affects the apparent kill radius of the munition. For example, if the tank speed is 22 m/s rather than 7 m/s and the lob time remains at 1 second, the array would have sixteen tripwires of about 70 meters in length to maintain the same 20 meter coverage. 
     The preferred tripwire  20  is a conductive elastomeric tripwire that is thin and lightweight and yet has sufficient flexibility and strength that it can be deployed automatically by deployment equipment as from a wound or reel configuration. The illustrated tripwire shown in  FIG. 4  has a metal internal conductor or wire  34  surrounded by a highly conductive elastomeric tube  36 . The wire  34  may be a distribution resistance wire, where the resistance per foot is a good measurable value. By use of a single voltage divider in the detection circuit  26 , the distance to the target vehicle applying the pressure can be determined by calculating the overall resistance of the cable. The thickness of the tripwire can range from 10-42 ohms/foot depending on the thickness of wire used. These resistance values can be easily measured to provide distance measurement that are fairly accurate. 
     Preferably, this tripwire is non-conducting until the intruder vehicle places enough pressure on the cable to increase the contact surface between the conductive elastomeric and nickel-chromium wire that will cause current conduction proportional to the position the pressure is applied. If desired, the center conductor may have a thin insulator  38  ( FIG. 4   a ) thereon between it and the center conductor to prevent inadvertent conduction. 
     Turning now in greater detail to the illustrated embodiment of the invention, the detection system  10  may use a set of tripwires  20  extending radially from a mine type of destruction device  12  to a kill/sensing distance of at least 20 meters. By way of example, to achieve one hundred percent (100%) coverage at 20 meters with a Russian T-90 style tank, there should be a minimum of 20 equally spaced 20 meter tripwires (See FIG.  1 ). If two wire contact is necessary, in order to determine target travel direction and speed, longer tripwires will be required to maintain the same kill radius. Due to the length of each tripwire (20 meter, minimum), the tripwire must be as light and thin as possible. The tradeoff exists between the number and length of the tripwires required for given coverage (19.2 meter, minimum kill radius) (See FIG.  2 ). Note that the speed of the target and lob time drastically affects the apparent kill radius of the munition. As an example, if the speed of the target were equal to the max speed of a T72 tank (22 m/s), and lob time remains 1 s, one would need sixteen, 70 meter tripwires for approximately 20 meter coverage. 
     In accordance with an embodiment of the invention, the tripwire may be an optic fiber tripwire. For range finding, a one fiber optic tripwire  20  may be used in accordance with another embodiment of the invention. The fiber optic tripwire will break at the point of contact with a heavy vehicle. One way to determine the location of the break is by using an OTDR (Optical Time Domain Reflectometer). The fiber optic tripwires could be single mode (9 micron) glass fiber. Bare fiber would be the best because it would be more fragile, but the fiber may need to be clad due to possible measurement errors caused by sunlight. Coating the bare fiber with a sprayed on film could reduce optical effects of sunlight and still keep the fiber fragile enough to break if run over. Typical pressures encountered with heavy tanks are in the range of 10-15 psi. The issue may then become deployment. Launching an unclad glass fiber to lengths of 20-30 meters may be difficult due to fiber breakage. The optic fibers have to be durable enough to withstand deployment from the mine to distances of more than 20 meters, yet fragile enough to break when subject to pressures in the range of 10-15 psi. 
     The use of multiple optical fibers presents another problem, multiplexing the OTDR or using multiple OTDR&#39;s. To reduce cost when using the latter, one could use a single fiber  20   a  ( FIG. 5 ) in a continuous loop out and back through the mine. Breakage location could then be determined with a single OTDR and without using a complex multiplexing scheme. A minimum of two OTDR&#39;s operating at different wavelengths may still be necessary if two wire contact is required. This would allow distance measurement from both ends of the single fiber to measure the distance to the two breaks. WDM could be used to multiplex the fibers if the OTDRs have the ability of changing measurement wavelength. This would allow eight individual 30+ meter fibers. 
     The diameter of the clad fiber would be approximately 0.125 inches. With the single fiber out and back approach, this would be a length of approximately 500 meters and a volume of over 240 cubic inches. With eight single strands of fiber, the volume would be about 117 cubic inches of fiber. This would be difficult to coil and launch from the mine. 
     Potential problem areas of using a fiber optic tripwire system  10  are as follows:
         OTDR may be expensive and a high power consumer.   Fiber may be too fragile to deploy reliably due to force required to dispense 20 m.   If fiber does not break, no detection is possible.   Single loop will not work if two target bearings are required unless the second break point occurs between the first and the OTDR or two OTDR&#39;s are used.       

     On the other hand, benefits from using as fiber optic tripwire system  10  are as follows:
         Impervious to corrosion and EMI countermeasures.   If thinly clad, will be nearly invisible on the ground.       

     In accordance with another embodiment of the invention, the range detecting tripwire is a thin, (0.1″ diameter) hollow, highly conductive elastomer with a distributed resistance running through the center. Nickel-chromium wire is one example of a distributed resistance, where the resistance per foot is a good measurable value. By using a simple voltage divider, the mine can determine the distance to the object applying pressure by calculating the overall resistance of the cable. Depending on the thickness of wire used, the resistance of the nickel-chromium wire can range from 10-42 ohms/foot. This easily measurable value could achieve good distance accuracy. 
     The preferred method of operation is to have the tripwire non-conducting until pressure is applied to the elastomer/wire combination. The pressure will increase the contact surface between the conductive elastomer and the nickel-chromium wire that will cause current conduction proportional to the position the pressure is applied. Depending on test results, one may use a center conductor with less resistance if the conductive elastomer has a higher resistance. 
     One potential problem that may be encountered includes corrosion of the center conductor that can reduce accuracy or cause outright failure. In addition, some minimal conduction may occur which would have to be calibrated out on initial deployment or periodically. To overcome this problem, it is preferred to coat the center conductor with a thin insulator to prevent inadvertent conduction. 
     Benefits accruing from the use of the conductive elastomeric tripwire include low cost, durability as far as deployment and breakage are concerned, low power consumption, and fast sample rates with simple A/D converters available on most micro controllers. 
     The diameter of the conductive elastomer would be approximately 0.060 to 0.100 inches. Eight 30 meter tripwires at 0.060″ thickness using 36 or 38 AWG wire internally would amount to a total volume of 27 cubic inches. 
     Potential problem areas of using a conductive elastomeric tripwire system  10  are as follows:
         Susceptibility to corrosion   Variation in conductivity vs. time and temperature   Possible difficulties insulating the center conductor       

     On the other hand, benefits from using a conductive elastomeric tripwire system  10  are as follows:
         Low cost   Low power consumption   Simple design   Durable       

     In accordance with a still further embodiment of the invention, the tripwire may be an ultrasonic tripwire. The ultrasonic tripwire would use a small piezo transducer to inject an ultrasonic pulse through a hollow tube and measure the time delay before a reflection is detected. This tube would require a flat perpendicular end surface to minimize dispersion of the waveform. The end cap would be necessary to prevent dirt or similar material from entering the sound tube and causing a false reading. If necessary, the tube could be filled with a gas or liquid to prevent inadvertent collapsing. 
     Potential problems include the fact that when the sound tube is collapsed, the shape of the reflecting surface will be conical or wedge shaped which will cause dispersion of the reflected ultrasonic pulse. This may make timing more difficult to determine due to reverberations, echos and overall destructive signal attenuation. 
     Another potential problem area associated with using a conductive elastomeric tripwire system  10  are as follows:
         Abnormal shape of collapsed tube attenuates signal   May require high power consumption   Unknown size of available ultrasonic transducers       

     On the other hand, the main benefit from using the ultrasonic tripwire for the detection system  10  is that it is impervious to corrosion. 
     Of the embodiments discussed herein, it appears the easiest and most cost effective system is the conductive elastomer with a center conductor wire. 
     In accordance with a still further embodiment of the invention, discrete sensors, similar to touchpads on laptop computers for mouse control are placed every few feet along a tripwire. This appeared difficult to produce in a small enough footprint, and would result in a rather inaccurate distance measurement, therefore that the idea was not pursued. Other embodiments include a variation on the fabrication of the elastomer tripwire. Two half shells of copper, or other conductive material could be extruded or deposited on the inside of the elastomer. This would replace the wire. The distance could be measured by the resistance of the copper coat when compressed together to complete the circuit.