Taking the nuisance out of intrusion detection.
PREVENTING NUISANCE ALARMS while maintaining perimeter protection has always been a challenge. The obstacles to this perennial are imperfect fences, irregular terrain, electromagnetic interference generators, unpredictable animal activity, wind and other weather conditions. System designers and installers with varying skills, and detection requirements that vary from SWAT team experts to children add to the problem. It would take a truly magical system to leap over this tall mountain of site-specific variables, be free of false and nuisance alarms, and possess the ability to detect intrusions 100 percent of the time. The mountain has not been totally conquered yet, but new fiber optic technology is getting closer to doing so.
Fiber optic sensors that can be fence mounted come in three varieties: those that detect vibration and changes in pressure, those that detect bending of the fiber, and those that detect when the fiber is broken or badly bent. The more versatile type is the one that detects vibration and changes in pressure. It has had three years of commercial market exposure and has gained some of the calluses necessary to be called successful. Following is a review of what makes it work well and some limitations to consider for fence applications.
The fence. It is not the tightness of the fence that matters or the type of material from which it is made. It is whether or not the fence is free of rattles, clangs, knocking noises, or loud squeaks. Chain link is the most common fabric used. To test it, the security manager should walk the perimeter of the fence pulling the fabric hard between each post. When the fabric is released, it will oscillate, coming to rest in its original position. It passes the test if the fabric is secure to the posts and to any existing top, bottom, or center rail or straining wire, so that the fabric quietly settles back into place.
Unit configuration. One of the advantages of fiber optic sensors is that the sensor is a flexible, lightweight cable that may be installed on any type of fence on the most uneven terrain imaginable. The sensor is a classic line sensor. It senses much like the nerve at the end of a finger except that it can feel in a totally uniform manner along its entire length. The sensor can be used in short zone applications of as little as ten feet, but its cost effectiveness begins at 300-foot zones, and it becomes extremely cost-effective in zones over 750 feet. The fiber optic sensor is capable of uniform coverage of zones of more than a mile.
A fiber optic intrusion detection system consists of a fiber optic cable and an electro-optics unit. The fiber optic cable is immune to electromagnetic interference. Telemetry equipment, high voltage transmission lines, switching equipment, radar, or the natural sources of static discharge such as lightning do not affect the sensor in any manner. The electro-optics unit contains the light source, detector, and the alarm processor. As with all electronics, the printed circuit card must be properly shielded and the electrical supply input conditioned to eliminate false alarms due to the sources mentioned earlier.
Animals. The most difficult challenge for the alarm processor is to distinguish animals from human intrusion. Small animals or birds are rarely a problem. The alarm processor converts the signal generated by the sensor into a representation of the average amount of energy imparted to the sensor. Since energy is generated by the mass of the intruder and the speed at which it is moving, the small mass of a rodent or bird is easily ignored. The processor cannot, however, be set to ignore the mass of larger dogs, cattle, horses, deer, and occasional large birds, like owls, interacting with the fence if it is to provide a high probability of detection of a human intruder. Secondary electrical fencing is often used to back livestock away from perimeter fences. Feeding towers have been placed to attract birds away from fences and electrical switching equipment at substations, and poisons are sometimes used to eliminate raccoon and possum intruders.
Video assessment can save personnel response time and is becoming more popular for remote locations where animal-generated nuisance alarms are most prevalent. The increased reliance on remote visual evaluations is due in part to the availability and cost effectiveness of video transmission multiplexers, which can be used on standard telephone lines.
Weather. Harsh weather is less likely to cause nuisance alarms with fiber optic sensors. Fiber is immune to lightning and is insensitive to extreme temperatures. Some intrusion detection technologies do not function well in very hot or cold climates. Fiber optic sensors operate at temperatures from -55 [degrees] C to 85 [degrees] C. Of more importance in the elimination of nuisance alarms due to weather has been the introduction of user-programmable alarm processing that allows the sensor system to be fully tailored to the environment.
If the sensor is placed on a loose chain link fence, the alarm processor can be set to filter out the oscillation of the fence that is generated by wind. The sensitivity and detection threshold can be set from 1 percent (extremely insensitive) to 100 percent (extremely sensitive). On a loose, light-weight fabric it might be set for 10 percent sensitivity, while with a heavy gage, tightly strung fabric it might be set higher at 30 percent sensitivity.
In both cases, the oscillation due to the wind has been filtered out; and at those sensitivities, rain, snow, and light hail have no effect on the system. As with large animals, large hail can sometimes generate enough energy to be detected when the system is calibrated to detect a careful human climber. Gusting winds of over 50 MPH can cause nuisance alarms due to the severity of the motion set into the fence, as can multiple pieces of blown debris hitting the fabric, or gates rattling against supports or locking mechanisms.
Design and installation. Three rules that concern the physical properties of the cable and its connectors must be considered in the design and installation of optical fiber cable. First, when pulling the cable into a conduit, one must pull the cable itself, never the connectors. Second, one must keep the connectors clean--light cannot travel through dirt. Third, the cable is very ragged, it can be pulled, stepped on, and run over with a truck, but bending it in a radius of tighter than 1 inch can cause cracking or snapping of the fiber.
No special training is required to design or install a system. The design phase requires a standard site survey, development of an intrusion scenario, the test criteria to confirm system performance, and a design with installation instructions for the site. Along the open fabric of the fence, only a single run of the sensor is needed. On corners and other reinforced sections, cable should be applied to all fabric sections such as above and below center support rails. At military sites, additional cable is brought up on the outriggers at corner posts to detect sophisticated intruders climbing a corner post. At sites where the intruder is less sophisticated, a single run of cable in the center of the fabric is sufficient. Detection of cutting or lifting of the fabric is often all that is desired.
The first step in installation is to be sure that the fence passes the fitness test described earlier. The fiber optic cable must be installed in a uniform manner either in some type of conduit or directly on the fence. During this process, the rules of handling optical fiber must be observed. Polyvinyl chloride (PVC) conduit is frequently used to help guarantee the uniformity of the installation and to provide some protection against vandalism of the cable.
When the sensing cable is placed in PVC conduit that is then mounted to the fence fabric, the cable lies in the conduit, which couples it to the fabric. When bare cable is attached directly to the fabric, care must be taken to mount it so that it is not loose in sections or pulled too tight around sharp objects or edges. Additionally, the conduit installation requires less maintenance attention to ensure consistent system performance. Wind will often loosen the cable or the cable ties on bare cable and cause nuisance alarms as the fence ages. The conduit is self-supporting to a greater degree even as cable ties age and fall off.
Tests. Once the system is installed, it can be tested in the same manner as any other intrusion system. The criteria of the test should be reasonable for the site. Sites that require high-level security, such as those for the military, are tested by the most sophisticated of intrusion imitators. A minimum acceptable probability of detection is assigned along with a maximum allowable number of nuisance alarms for a given day, week, or month. In a civilian application, the criteria for probability of detection are different because the intruder is assumed to be unsophisticated unless the threat is from terrorism. The maximum allowable number of nuisance alarms is, however, much lower because most civilian sites are unmanned. A combination of alarm processing parameters can be set by the user to meet either objective.
Maintenance. Periodic maintenance and verification of system performance is recommended. Fences degrade with time and become the primary cause of future nuisance alarms. Consequently, the fence should be regularly inspected as recommended earlier. Optical cable is specified to have an operational life of more than twenty years in an exterior environment. Since it is composed of glass, protected by a rugged polyurethane jacket, it will not degrade even in the salt air of a coastal installation.
The alarm processor may be reset at any time to allow the system to adjust to changes in the perimeter. The electronics also allow the maintenance personnel to check the light level of the system along with other key parameters without need of any other equipment.
Should a sensing cable be damaged due to vandalism or accidental damage to the fence, it may be repaired by splicing the cable or placing a connector on each end with a coupler (connectorizing). The ends can then be rejoined with a feed-through optical coupler. Both connectorization and splicing have been made simple with the new field installable connectors and mechanical splices.
In the previous three years, this new technology has been extensively tested and accepted by a wide range of customers, from military to the private sector and from industry to public utilities. The largest single application for fiber optic sensors is for electrical substations. The reason for this use is due in part to its electrical immunity and in part to its cost effectiveness. Fiber optics also have potential as a buried sensor in warmer climates, as a wall sensor for buildings or solid wall perimeters, and for use inside of buildings in drop ceilings and long-term high-value asset storage facilities.
Sandra Reynolds is president of Fiber SenSys, a Corning affiliate. Corning developed fiber sensing technology at Corning Laboratories. Reynolds is an electronics engineer and has worked in pioneering fiber sensors and instrumentation since 1983. She is a member of ASIS.
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|Title Annotation:||fence-mounted fiber optic intrusion detection systems|
|Date:||Aug 1, 1994|
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