fiber optic sensor for unattended ground applications

8/22/2019 Fiber Optic Sensor for Unattended Ground applications

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A Fiber Optic Seismic Sensor for Unattended Ground

Sensing Applications

Joseph Dorleus, Ph.D.U.S. Army Program Executive Office for Simulation, Training, and Instrumentation, Orlando, Florida

Yan Zhang, Ph.D., Jing Ning, Ph.D., Thomas Koscica, Ph.D., Hongbin Li, Ph.D., and

H. L. Cui, Ph.D.Stevens Institute of Technology, Hoboken, New Jersey

Fiber optic seismic sensors have been increasingly recognized as promising technologies for many

applications, such as intruder detection and perimeter defense systems. Among these, a military

seismic sensor is especially challenging because it requires a robust, compact, reliable, easily

installable and operated product. This article reports on our recent experimental investigations

of a military fiber optic seismic sensor. In particular, an improved sensor design and new signal

processing techniques are illustrated in this article, which include: (a) a scanning laserwavelength-based demodulation system; (b) a digital lock-in amplifier and field programmable

gate array techniques for weak signal detection and processing; and (c) overall improved seismic

sensitivity based on carbon fiber composite cantilever and fiber-Bragg-grating sensing

technology. The experimental results show that the fiber-Bragg-grating seismic sensor frequency

response band is between 10 and 300 Hz and that it has a higher sensitivity than the

conventional electromagnetic geophone within the frequency response band most important for

battlefield monitoring. The fiber-Bragg-grating seismic sensor is totally immune to

electromagnetic interference and has a working temperature between 240uC and+50uC for

military harsh environments. Comparisons are also presented of the fiber-Bragg-grating seismic

sensor with the commercial REMBASS sensor in a series of field tests.

Key words: Fiber-Bragg-grating seismic sensor; unattended sensor; intruder detection;perimeter defense; battlefield monitoring; sensors; signal detection; electromagnetic

geophone; laser-based optical demodulation.

Fiber optic sensor technology has been andis being increasingly exploited by theresearch community because of its rela-tively simple design, low power con-sumption, low cost, relatively low main-

tenance cost, and the flexibility it offers for bothcommercial and military applications. In particular,

fiber optic seismic sensors have been recognized aspromising technologies for numerous applications,which include intruder detection and perimeter defensesystems for military applications. However, seismicmilitary sensors are required to be robust, reliable,compact, and easy to install and operate to be effectivein the battlefield environment. This article addressesthe challenges presented by the design of militaryseismic sensors and reports on experimental investiga-

tions of fiber optic seismic sensors. Specifically, weaddress three types of sensor technologies that provideimproved design and novel signal processing tech-niques: (a) a wavelength scanning, pulsed-laserbaseddemodulation system, (b) digital lock-in amplifier andfield programmable gate array (techniques) for weaksignal detection and processing, and (c) improved

seismic sensitivity based on carbon fiber optic com-posite cantilever and fiber-Bragg-grating (FBG). Wepresent our experimental results on the FBG seismicsensor frequency response along with comparisons ofthose with the REMBASS II sensor (RemotelyMonitored Battlefield Sensor System II) resultingfrom field tests.

We have developed a new scheme of laser-basedoptical demodulation with excellent results. At 100-Hz

TechNotesITEA Journal 2009; 30: 455460Copyright 2009 by the International Test and Evaluation Association

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scanning speed we were able to demodulate thewavelength down to 1.1 pm. The signal is completelydigital, clear, with excellent signal-to-noise ratio. Atlower scanning speeds we can achieve subpicometerwavelength resolution. The dynamic following range isabout 10 nm with a dynamic range of 120 dB.

Principle and designThe design of the seismic sensor is illustrated in

Figure 1. The detection is implemented by the FBGdynamic strain sensor, which is attached on a spring-mass system (Pastore, 2006). The acceleration ofground motion is transformed into strain variation onthe FBG sensor through this mechanical design and,after the optical demodulation, generates the analogvoltage output proportional to the strain changes. Byadjusting the mechanical parameter of the springmassconfiguration, we can mechanically tune the naturalresponse frequency of the system within a certain range

in adapting to the different frequencies of seismic wavesources (signals of personnel and vehicles). This sensorhead has a compact size and a mass of only a fewgrams. It has the advantage that the sensor can beeasily embedded and hidden in the battlefield withoutany radio frequency emission or thermal signature tothe environment. The optical fiberbased sensor itselfis resistant to corrosion, high temperature, and fatigue,and so is suitable for deployment in the harshenvironment of battlefield. A damping mechanism isincorporated into the design with critical dampingprovided so that the massspring system will return

quickly to its ready state after detecting a signal. Thisdamping mechanism is shown in Figure 1, where aFaraday induction loop and a permanent magnetprovide the damping. The small induced current inthe Faraday loop is properly sealed so that noelectromagnetic signal will go in or out of the sensorhead. By using the carbon fiber composite cantilever,the overall sensor performance is improved, whichleads to a higher sensitivity, better linearity, andsmaller weight on the sensor head.

Considerations for field applicationsFor rough environments, such as the battlefield

environment, two things are of serious concern. First,the sensor needs to be strictly waterproof, dustproof,and adaptable to vast temperature changes in excess of240uC to +60uC. Two of the advantages of a fiberoptic device are that the device is intrinsicallywaterproof and dustproof, but it is also very sensitiveto temperature changes. Our sensor uses a pair ofmatched FBGs; one is the sensing element and theother is the demodulator. A thermostat and a

temperature control make sure that the two FBGsare always kept at the same temperature.

Power consumption is another consideration forunattended system design. We employ a solar cell onthe top cover of the sensor and a high-density batteryinside the sensor. A digital control block permitscycling of wakesleep configuration. Sleep time is setas long as possible based on average current consump-tion, and the ratio of wakesleep is set depending onthe required reliability of detection, alarm level, andthe states of neighboring sensors.

Improvement of signal detectionOne of the most profound challenges in the design

of the seismic sensor is acquiring and separating weakseismic signals from strong background noise (forexample, wind); another is distinguishing signals fromdifferent sources. Basic requirements for a practicalunattended seismic sensor are that it has to be verysensitive to small vibrations but also have a properstrategy to decrease the false alarm rate (FAR). Tosolve these problems, we designed an original algo-rithm. We illustrate the workings of this algorithm byusing the example of detecting the signals of a humanwalking. First, the system records the seismic response

of a person walking, saving the basic digital model onthe sensor. Second, the system compares the sensorssubsequently acquired seismic signal with the basicdigital model using a correlation operation. Third, thesystem estimates the degree of correlation. If thedegree of correlation is higher than a preset level, it iscounted as a signal impulse. Finally, the system sumsthe number of impulses in a given time interval. If thissum matches the rate of previous signals, the sensor

Figure 1. Basic structure of the FBG sensor head.

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initiates an alarm. Both improved signal detectingability and decreasing false alarm rate are achieved withthis algorithm.

We have carried out a preliminary experimentalstudy to investigate the applicability of our algorithm.Both a fiber-optic Bragg-grating geophone and anelectromagnetic geophone were used for this work.Figure 2 shows the experimental setup with a personwalking directly to the geophone from 100 m away.Figure 3 (top) shows the geophone response without

any signal processing, where we could only see a fewhigh-quality signal cycles. Figure 3 (bottom) showsthat more cycles of the walking signal were detected byour processing method of signal correlation eventhough the signal was buried under noise.

Experiment resultsThe sensitivity test of the FBG seismic sensor was

carried out on a vibration stage that is driven by a series

of sinusoidal waves. We use the conventional electro-magnetic geophone for comparison. Figure 4 is theresult of the FBG seismic sensor and the geophoneresponse to the vibration at 30 Hz. Comparison testsat different frequencies showed that the FBG seismicsensor has a higher sensitivity than the conventionalelectromagnetic geophonebetween 10 and 70 Hz.The frequency analysis results showed that the FBGseismic sensor frequency response band is between 10and 300 Hz, which covers the major frequency band of

the military seismic signals.The electromagnetic interference test of the FBGseismic sensor was carried out and compared with theelectromagnetic geophone (Figure 5). Both geophoneswere initially still and placed near an AC currentsupply (power: 80W; frequency: 60 Hz) at equaldistances of 20 cm. The conventional electromagneticgeophone was severely disturbed by the output signal at60 Hz. The FBG seismic sensor did not exhibit

Figure 2. Experimental setup.

Figure 3. (top) Geophone response without any signal processing; (bottom) geophone response with signal processing.

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electromagnetic interference. This demonstrates the

advantage of a FBG unattended seismic sensor in amilitary environment, an attribute that is one of theintrinsic properties of the fiber optic sensor.

The military field test was carried out at one of theU.S. Army shooting ranges. The REMBASS-II S/Asensor was used as a benchmark. Figure 6 is the sampletest result of the FBG seismic sensor response tomilitary vehicle generated signals. The detectiondistance was calculated by an on-board global posi-tioning system receiver on each vehicle, and it can beinterpreted as the maximum distance over which thesensor can respond to the target. In these tests we used

the first impulse of the seismic signal as the triggersignal to discriminate the sensor response instanta-neously. A series of field tests showed that the FBGsensor equaled or outperformed the REMBASS-IIsystem with both personnel signals and detection ofalmost every wheeled vehicle signal.

Estimated delays with time of arrival fortwo pair of sensors

For a pair of signals from two sensors,

S1 t ~a1x t{t1 zn1

S2 t ~a2x t{t2 zn2,

where x(t) is the common signal received at bothsensors, n1 and n2 are additive interferences, and a1 anda2 are the attenuation factors at the two sensors.

The relative time of arrival can be computed as

dt~arg max S1(t{t):S2 t :

We know the delay of time of arrival can be translatedinto distance difference because the transmission speedis constant. That is, if we know the transmission speed

Sp, we have the distance difference, dl, between twosensors

dl~Sp dt:

For a given dl, the object of interest will be on ahyperbolic curve where every point has the samedistance difference to the two sensors.

If we have one more pair of sensors, we have onemore distance difference on another hyperbolic curve.For two hyperbolic curves, the intersection pointindicates the exact object location.

The time difference plot in Figure 7 shows that forboth the sensor pair 2 and 13 and the sensor pair 2 and

14 the time difference gets smaller as time progresses,which corresponds to the one approaching from sensor2 to sensor 4. At sensor 2 the distance difference islargest, and therefore the time difference is also largest.When the person stops at sensor 4, the time differencebetween two sensors, corresponding to the distance

Figure 4. FBG sensor vs. electromagnetic sensor

sensitivity comparison.Figure 5. FBG sensor vs. electromagnetic sensor

electromagnetic interference.

Figure 6. Sample of field test results of FBG response to

military vehicles signals (in comparison with REMBASS

II sensor).

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difference, has the smallest value. Then the personturned around and walked backward. One can see thatthe time difference gap gets larger and larger as sensor2 is approached. Moreover, we find that two pairs ofsensors have two different time difference valuesbecause the distances from the person to the sensorpairs are different. The sensor pair 2 and 14 has a largertime difference value.

SummaryWe have demonstrated and discussed the feasibility

of an unattended FBG seismic sensor in the battlefieldmonitoring and intrusion detection application. Basedon a novel yet practical design, the FBG seismic sensormeets most of the military requirements such as beinglight weight, of compact size, easy to use, reliable, all-weather, and consuming little power, etc. Theperformance of the FBG seismic sensor in laboratorytests and field tests exceeds several commercial seismicsensors. We could expect FBG unattended seismicsensors to have the potential capability of detectingtime-critical targets (personnel and vehicles) in battle-

field applications.More systematic research by our team is currently

focused on combining FBG magnetic sensors with theFBG geophone and improving the algorithm to obtainboth signal enhancement and reduced FAR. Also,further engineering measures of sealing and packagingare underway to ensure that the sensor can be deployedin the harsh military environment for extended periods.C

JOSEPH DORLEUS is currently a lead telecommunicationsystems engineer at PEO STRI, Orlando, Florida. He hasworked and held both technical and managerial positionsin the private sector as well as in the government. He holdsa bachelors of science degree and a masters of science degreein electrical engineering from Polytechnic University(formerly Polytechnic Institute of New York), Brooklyn,New York, and a doctor of philosophy degree in electricalengineering from Stevens Institute of Technology, Hobo-

ken, New Jersey. His research interests include opticalnetworks, all-optical network management and monitor-ing, and modeling and simulation of wireless networks. Heis a member of the International Test and EvaluationAssociation (ITEA), the Institute of Electrical, ElectronicsEngineering (IEEE), the Defense Technical InformationCenter (DTIC), the Army Acquisition Corps (AAC), andthe International Society for Optical Engineering (SPIE).He has authored, coauthored, and presented numeroustechnical papers that are published in technical journals,conferences, and proceedings such as IEEE, SPIE, ITSEC,and ITEA. Dr. Dorleus is the recipient of the ArmyAchievement Medal for Exceptional Civilian Service. Hewas also the Army Materiel Commands nominee for BlackEngineer of Year Award in 2001. E-mail: [email protected]

YAN ZHANG is a senior engineer at L.C. Pegasus. Sheobtained her bachelors of science and masters of sciencedegrees in optical engineering from Beijing Institute of Technology and her doctor of philosophy degree in appliedphysics from Stevens Institute of Technology. Her currentresearch interest includes research and development of fiber

Figure 7. Corresponding location estimates from pair of time delays.

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optic devices and components in optical sensing and opticalcommunication areas. She has several patents andpublished extensively in the areas of fiber optic sensorsand laser spectroscopy.

JING NING received a bachelors of science degree inelectric engineering in 1984 and a doctor of philosophy

degree in geophysics in 2005. Now, his professionalinterests include new sensing technology, which hasapplication to geological disaster warning and mineralexploration.

THOMAS KOSCICA is presently a principal engineer withL.C. Pegasus, where he leads the effort to design andimplement fiber optic and microwave sensors in structuralhealth monitoring, methane gas detection, and distributedtemperature and pressure monitoring in oil wells, as wellas THz sensors for chemical and biological warfare agentdetection. He served as a consulting designer to Smith andNephew for next-generation therapeutic ultrasound prod-ucts that leverage embedded software to maximize product

flexibility and minimize costs during the entire productlifecycle. From 19871996, while working at the U.S.Army Research Laboratory in Ft. Monmouth, New Jersey,Dr. Koscica was involved in advanced electronicsengineering development for high-performance militarysystems; during the last 2 years there he served as teamleader. For his doctoral thesis, completed at RutgersUniversitys Electrical Engineering Department in1997, he explored novel semiconductor device developmentto utilize the physics of real space transfer withina semiconductor to express compound device behavior froma single device. Dr. Koscica holds more than 40 U.S.patents and has numerous publications and presentations.

HONGBIN LI received the bachelors of science andmasters of science degrees in electrical engineering from theUniversity of Electronic Science and Technology of China,Chengdu, in 1991 and 1994, respectively, and a doctor ofphilosophy degree in electrical engineering from theUniversity of Florida, Gainesville, Florida, in 1999.From July 1996 to May 1999, he was a research assistantin the Department of Electrical and Computer Engineer-ing at the University of Florida. He was a summervisiting faculty member at the Air Force ResearchLaboratory, Rome, New York, in the summers of 2003and 2004. Since July 1999, he has been with theDepartment of Electrical and Computer Engineering,Stevens Institute of Technology, Hoboken, New Jersey,where he is an associate professor. His current researchinterests include statistical signal processing, wireless

communications, and radars. Dr. Li is a member of TauBeta Pi and Phi Kappa Phi. He received the Harvey N.Davis Teaching Award in 2003 and the Jess H. DavisMemorial Award for excellence in research in 2001 fromStevens Institute of Technology, and the Sigma XiGraduate Research Award from the University of Florida

in 1999. He is a member of the Sensor Array andMultichannel (SAM) Technical Committee of the IEEESignal Processing Society. He is or has been an editor orassociate editor for the IEEE Transactions on WirelessCommunications, IEEE Signal Processing Letters, andIEEE Transactions on Signal Processing, and he served asa guest editor for EURASIP Journal on Applied SignalProcessing, Special Issue on Distributed Signal ProcessingTechniques for Wireless Sensor Networks. E-mail: [email protected]

HONG-LIANG CUI is a professor of physics at StevensInstitute of Technology, where he directs the AppliedElectronics Laboratory. He received his undergraduate

education in applied physics with a concentration in laseroptics from the Changchun Institute of Optics and FineMechanics in Changchun, China, with a bachelors ofengineering degree. In 1981 he came to the United States forgraduate study as one of the first groups of Chinese physicsstudents in the CUSPEA program, obtaining a doctor ofphilosophy degree in theoretical condensed matter physics in1987, from Stevens Institute of Technology, where he hasbeen on the faculty ever since. His research efforts have beenconcentrated in the areas of solid-state electronics andnanoelectronics, optical communications and sensing, elec-tromagnetic wave propagation and interaction with matterssuch as chemical and bio-agents, and high-performance

computing approach to modeling of physical devices andphenomena.His work has been funded by NSF, ARO, ONR,and DARPA. He has published more than 190 researchpapers in peer-reviewed scientific journals, holds nine U.S.patents, and has guided more than 30 Ph.D. dissertations tocompletion. He holds membership in the American PhysicalSociety, the Institute of Electrical and Electronics Engineers,the Optical Society of America, and Sigma Xi. E-mail:[email protected]

ReferencesRobert Pastore, Jr, John Kosinski, Hong-Liang Cui,

Yan Zhang, Zhifan Yin, and Bingquan Chen. 2006.

Seismic activity monitor based on optical fiber Bragggratings U.S. patent number: 7,122,783, filed October17, 2006.

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fiber optic sensor for unattended ground applications

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