For the past five years, PennState’s Applied Research Labo-ratory has been building a net-work of relationships with law

enforcement and correctional agenciesthroughout the United States. Thesepartnerships stem from the increasinginterest and use of “smart” technolo-gies, minimal-force approaches andnonlethal devices by these agencies.One example is the Institute for Emerg-ing Defense Technologies’ (IEDT), sensor fence — a novel, low-cost, low-maintenance, tensioned wire systemthat may be used on new or existingfences that results in an improvedcapability to detect, locate and classifyintruders. The sensor fence can beused reliably in large, secure areassuch as correctional institutions, air-ports, military bases, power genera-tion facilities, ports, reservoirs orother large areas typically protectedby fences.

IInnvveennttiioonn aanndd PPrroottoottyyppeeThe concept for the sensor fence

was developed in 1999 through discus-sions at IEDT, a unit of the university’sApplied Research Laboratory. TheIEDT team surveyed existing high-techfences, including microwave, fiberoptic and taut wire (strain gauge) sys-tems, and found that these approachescould cost as much as $165 per foot.Such systems would be prohibitivelyexpensive for any large-perimeter facil-

ity such as a correctional institution,military base, airport or nuclear powerplant.

The team decided to develop itsown approach based on the AppliedResearch Laboratory’s expertise inacoustics and signal processing. TheIEDT approach uses an inconspicuous,ordinary, tensioned steel wire as anextended sensor. The wire can beattached to any new or existing chain-link or wooden fence. Geophones —inexpensive, rugged, off-the-shelfvibration sensors — are attached tothe tensioned wire at about 1,000-footintervals. The entire system is con-nected to a dedicated computer that isequipped with software developed atthe research laboratory. Essentially,the software analyzes the fence’svibrations, pinpoints disturbanceswithin 50 feet and then determineswhether the vibration pattern signals ahuman intruder as opposed to, say,wind or rain.

With the exception of the computerequipment and a few corner bracketsfabricated from 1-inch angle irons, thecomponents — springs, pulleys,clamps and wire — were all availableat a local hardware store (see Figure1). The prototype system was installedcovering roughly 1,000 feet of chain-link fence by one of the authors inabout six hours and the installationcost approximately $2 per foot, plusabout $5,000 for the computer equip-ment.

During one year of continuous oper-ation, there have been no maintenancerequirements on the mechanical orelectronic system. However, if the wirein the sensor fence were to be cut ordamaged, the system would detectand locate the point of signal disrup-tion as it went “deaf.” Repair wouldconsist of simply resplicing the ordi-nary steel wire.

HHooww IItt WWoorrkkssThe sensor fence works by convert-

ing the entire fence into a detector,similar to a spider’s web. A spiderpoised on a web can feel vibrationsconducted by the tensioned silk fromany point in the network and so canthe sensor fence. Vibrations at anypoint along the fence are transmittedvia the tensioned wire to the comput-er, where they are detected and analyzed by the software. The systemsoftware then locates the site of intru-sion by monitoring the vibrations inthe fence and precisely detecting thetime of arrival of signals from two ormore locations. Measured differencesbetween the signal’s arrival times indi-cate the location where the intrusionoccurred.

The sensor software is basically apassive detector designed to rejectenvironmental noises from wind andrain and detect the types of vibrationsproduced by someone climbing over,cutting through or otherwise trying to

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Technology Update

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defeat the fence. The software is pro-tected by encryption and can operateon common Windows-type PCs, allow-ing detection information to be auto-matically routed to desktops usingsecure communication protocol.

The system identifies differentintruders by monitoring changes inloading (weight applied to an area ofthe fence). For example, even carefulclimbing by an intruder will change theloading on the fence and signal that ahuman is present rather than a squir-rel. This approach addresses a prob-lem common with typical sensorfences by minimizing false alarms. Thesensor fence can distinguish betweenthe vibrations caused by wind or smallanimals and man-made vibrations fromclimbing, cutting and digging. Howev-er, tree branches and foliage should becleared away from the fence to avoidimpacts with the fence in strongwinds, which will likely be detected asintrusion signals. The sensor fence isalso being designed into a commercialproduct that steers cameras to cap-ture images of the detected intrusionarea for absolute confirmation.

AApppplliiccaattiioonnssWhile the sensor fence’s low cost,

low maintenance and low false-alarmrate are obvious advantages, thepotential for customization offered bythe approach promises even morevalue. Some advanced concepts envi-sioned by the team include addingsound, motion, imagery or heat detec-

tors to the fence and using data fusionand fuzzy logic techniques to fuse theoutput from the multiple detectorsinto a coherent report.

For example, the team is workingwith a commercial partner to integratevideo cameras into a sensor fence sys-tem. Vibrations at any point in thefence system would cue a camera toturn toward that point and record theintrusion. Currently, many pharmaceu-tical companies monitor their fencedperimeters with video cameras, whichrecord continuously, creating largevolumes of tape. Adding sensor fencecapability to already existing camerasystems will increase sensitivity whilegreatly reducing the volume of tapethat would need to be monitored.

Other advanced concepts IEDT isexploring for integration into sensorfence systems for automated securityinclude using computer-controllednonlethal weapons such as sting balls,pepper spray and even nets to deter orapprehend an intruder without harm.These systems can be installed at fixedlocations or on autonomous groundvehicles for rapid deployment.

Through IEDT, Penn State is work-ing with the Los Angeles Sheriff’sDepartment, the Pennsylvania StatePolice, the New York City PoliceDepartment, the National Institute ofJustice and police agencies in the Unit-ed Kingdom to support law enforce-ment’s need for new approaches andtechnologies to improve its ability tomaintain public order and public safe-ty.

For example, IEDT and the LosAngeles Sheriff’s Department conduct-ed the first assessment of less-than-lethal munitions, such as rubberbullets, and found that these projec-tiles do not approach the accuracydemanded of their lethal counterparts.The report is posted at www.arl.psu.edu/areas/defensetech.html.

Other projects being conducted atIEDT range widely from noise reduc-tion in military armored vehicles tobetter sensors for detecting toxicchemicals or biological agents poten-tially used in terrorist attacks.

As for the sensor approach, itenables great efficiencies over the longterm for maintaining a robust securitypresence over large areas. A key rolefor IEDT is to integrate the sensorfence into appropriate security solu-tions that can be dynamically con-trolled by the user to meet specific,unique or complex security needs, andgive an instant virtual presence at anypoints of intrusion.

Andrew F. Mazzara is director of PennState’s Institute for Emerging DefenseTechnologies in State College, Pa., anda specialist in security technologies.David C. Swanson, Ph.D., is associateprofessor of acoustics and seniorresearch associate at Penn State’sApplied Research Laboratory. He is aco-inventor of the sensor fence. NicholasC. Nicholas, Ph.D., is an AppliedResearch Laboratory senior researchassociate and co-inventor of the sensorfence.

Reprinted from the February 2003 issue of Corrections Today, Vol. 65, No. 1 with permission from:

The American Correctional Association, 4380 Forbes Blvd., Lanham, Maryland 20706