Passive Wireless Sensors Application for Extreme Conditions.

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 Passive Wireless Wireless Sensors Application for NASA.

 D e p a r t m e n t o f E C E .

1) INTROD INTRODUCT UCTION ION The enviro environme nment nt of aeronau aeronautica ticall vehicl vehicles es is typica typically lly harsh, harsh, with with tempera temperatur turee extrem extremes es ranging ranging from cryogenic cryogenic to above 1,500 °C. Future Future hypersonic hypersonic vehicles, for example, example, will reuire reuire high!temper high!temperature ature sensors mounte" on the structure, as well as cryogenic cryogenic sensors for  monitoring fuel tan#s. tan#s. $ensors are typically locate" in internal structures with limite" access, ma#i ma#ing ng the the peri perio" o"ic ic chan changi ging ng of batt batteri eries es proh prohib ibiti itive vely ly costl costly y an" an" time time cons consum umin ing. g. Furthe Furthermo rmore, re, batteries batteries "o not wor# well at tempera temperatur turee extrem extremes. es. %n contrast contrast to curren currentt wireless systems, passive wireless sensor systems "o not reuire batteries. &ne of the main challenges challenges for wireless wireless sensors sensors is power. &ften, &ften, batteries batteries cannot be use" "ue to inaccessible inaccessible locations or exposure to large large temperature extremes. 'nergy! harvesting systems that rely on  batteries for energy storage are eliminate" for the same reasons. Thus, passive wireless sensing systems that "o not inclu"e batteries shoul" be "evelope". $everal passive wireless technologies may be a"apte" to meet the nee"s of the extreme environment community. community. ('($, )F%*, $+ $+, an" bac#scatter techniues have all shown  potential for passive wireless an" extreme environment operation. +s a result, -+$+ is investigatin investigating g the use of passive passive wireless technology technology for aeronautical aeronautical application applications. s. From groun" tests to the operation of high!altitu"e long ! "uration aircraft, many applications coul"  benefit from small, passive, wireless sensors that can operate in extremely harsh environments.

1 P

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College of Engineering, Chengannur.

 Passive Wireless Wireless Sensors Application for NASA.

 D e p a r t m e n t o f E C E .

2) GENERAL GENERAL REQUIREME REQUIREMENTS NTS +ircraft sensing euipment must operate in harsh environments that inclu"e temperatures ranging from cryogenic to extremely high temperatures greater than 1,500 °C/. &ften, the tempe temperat ratur uree extre extreme mess precl preclu" u"ee the the use use of batt batteri eries es for for senso sensorr appl applica icatio tions ns.. esi" esi"es es temperature extremes, issues of vibration, humi"ity, an" even ioniing ra"iation must be a""ress a""resse". e". %n a""itio a""ition n to the liste" liste" enviro environme nmenta ntall challen challenges ges,, the ra"io ra"io freue freuency ncy )F/ )F/ environment can also pose a challenge when wireless )F sensors are place" within enclose" metallic structures, such as the interior of wings.

2 P

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College of Engineering, Chengannur.

 Passive Wireless Wireless Sensors Application for NASA.

 D e p a r t m e n t o f E C E .

3) GROUND TESTING APPLICAT APPLICATIONS IONS  -+$+ aeronautical researchers perform tests on components an" systems on the groun" in con2un con2uncti ction on with flight flight testing testing.. These These tests reuire reuire the placem placement ent of a large large number number of  sensors on a test article. To "ate, very few of the sensors have been connecte" connecte" wirelessly wirelessly.. Freuently, these tests are performe" on mo"els an" test articles in -+$+3s win" tunnels. (any of the win" tunnels emulate extreme environments. The -ational Transonic Facility win" tunnel uses nitrogen as the gas an" operates at temperatures from !154 °C to !101 °C, an" an" at pres pressu sures res from 10 #6a to 789 789 #6a #6a . +noth nother er trans transon onic ic tunn tunnel, el, the 0. 0. (ete (eter  r  Cryogenic Tunnel, Tunnel, operates operates "own to !185 °C. The :lenn )esearch Center operates operates an icing tunnel. tunnel. The temperature temperaturess only get "own to !;0 °C, but the liui" liui" water content content can range from 0.<= g>m, g>m, an" the "roplets "roplets can vary from 15 to 50 ?m. The :lenn )esearch )esearch Center  Center  also operates the Cryogenic Test Complex CTC/, which houses a 4.9 m "iameter test chamber that can accommo"ate a col" wall that can reach !<5< !<5< °C. The facility can han"le  both liui" an" gaseous storage of hy"rogen, hy"rogen, oxygen, nitrogen, nitrogen, an" helium. The The @ype @ypers rson onic ic Tunnel nnel Faci Facili lity ty @TF @TF// achi achiev eves es (ach (ach 5, 9 an" an" 4, whil whilee reac reachi hing ng temperatures temperatures of 1,78 °C an" pressures pressures of 7,45 #6a. The 7!ft high temperature temperature tunnel tunnel is a hypersonic hypersonic tunnel tunnel that can achieve spee"s of (ach , ;, 5, an" 4. The temperatures temperatures range range from ;7< °C to 1,8<4 °C, while the pressure pressure ranges from ;5 #6a to <4,548 #6a. #6a. The +rc +rc @eate" $cram2et Facility can reach temperatures of <,919 °C while achieving spee"s up to (ach 7, an" the <0A (ach 9 in" in" Tunnel Tunnel can operate at pressures up to 1,470 1,470 #6a. These tunn tunnels els use use air, air, nitro nitroge gen, n, CF;, CF;, hy"r hy"rog ogen en!co !comb mbus uste te" " air air with with oxyg oxygen en,, comb combus uste" te" methane>liui" oxygen, an" an" )!1;a as the gaseous me"ium. me"ium. in" in" tunnel tests that routinely operate with extreme temperatures an" pressures coul" benefit from wireless sensors. The "evelopment of materials for hypersonic aircraft reuires extreme environments li#e those foun" in spacecraft re!entry. re!entry. +t -+$+3 -+$+3s +mes +mes )esearch )esearch Center, the +rc Bet facility uses an electric arc to accelerate heate" gas from (ach ; to (ach 1< at temperatures up to 1,4<4 °C. The +rc +rc Bet facility was use" to "evelop high!temperature materials for hypersonic aircraft, such as the -ational +erospace 6lane -+$6/. Future aircraft will fly at higher altitu"es an" velocities an" therefore will experience more extreme extreme environment environmentss than those encountere" encountere" by to"ay3s aircraft. aircraft. To a""ress a""ress these nee"s, new new passi passive ve wirel wireless ess senso sensorr syste systems ms will will have have to be "eve "evelo lope pe" " that that can can opera operate te in correspon"ing environments. %n a""ition to aeronautics, other "isciplines coul" coul" benefit from passive wireless sensors. +t the enne"y enne"y $pace Center Center $C/, $C/, wireles wirelesss sensor sensor networ networ#s #s have have been been "evelo "evelope" pe" for  monitoring cryogenic lines an" for centering an" aligning the space shuttle external tan#. &ngoing wor# in the Trans"ucers group at $C integrates wireless communications with sensors an" trans"ucers. 3 P

age|

College of Engineering, Chengannur.

 Passive Wireless Sensors Application for NASA.

 D e p a r t m e n t o f E C E .

+t the Dangley )esearch Center, researchers have "evelope" an" teste" a wireless flui" level system that wor#e" while immerse" in liui" nitrogen. This "evice can wor# in a variety of  harsh environments while "etecting the level of numerous Eflui"sA such as liui" nitrogen, transmission flui", sugar, an" even groun" corn. hile the "evices 2ust mentione" "o not employ passive wireless technology, they "o "emonstrate the current tren" towar" wireless sensing for groun" testing.

3.a) NASA Glenn Research Center :lenn )esearch Center has six uniue worl"!class win" tunnels with varying capabilities. The 1!by 1! Foot $upersonic in" Tunnel specialies in con"ucting fun"amental research in supersonic an" hypersonic flui" mechanics, supersonic!vehicle!focuse" research, an" "etaile"  benchmar# uality experiments for computational flui" "ynamics. This facility is an excellent low!cost testing tool for small!scale research. The 7! by 9!Foot $upersonic in" Tunnel is a worl"!class test facility that provi"es researchers the opportunity to explore the subsonic, transonic, an" supersonic spee" range. The facility tests a"vance" aircraft concepts an" components, engines for high!spee" aircraft, an" launch vehicle concepts. %t is -+$+3s only transonic 6ropulsion win" tunnel, operating from (ach 0.<5 to <.0 an" at very low spee"s from 0 to (ach 0.1. This facility is euippe" for aero"ynamic an" propulsion scale mo"els. The 8! by 15!Foot Dow!$pee" in" Tunnel is the most utilie" low!spee" propulsion acoustic facility in the worl" specialiing in evaluating aero"ynamic performance an" acoustic characteristics fans, noles, inlets, propellers, an" hot gas reingestion of a"vance" $hort Ta#e!off ertical Dan"ing $T&D/ systems. %t is the only national facility that can simulate ta#e!off, approach, an" lan"ing in a continuous flow win" tunnel environment. The 10! by 10!Foot $upersonic in" Tunnel is the largest win" tunnel at -+$+ :lenn specifically "esigne" to test supersonic propulsion components such as inlets an" noles,  propulsion system integration, an" full!scale 2et an" roc#et engines. This "ual!cycle win" tunnel can operate as a close"!loop aero"ynamic cycle/ or open!loop system propulsion cycle/ an" is euippe" for large!scale aero"ynamic mo"els as well as full!scale engine an" aircraft components. This facility operates at test section spee"s of (ach <.0 to .5 an" subsonically from 0 to (ach 0.9. The %cing )esearch Tunnel %)T/ is one of the worl"3s largest refrigerate" win" tunnels "e"icate" to the stu"y of aircraft icing. %n this facility, natural icing con"itions are "uplicate" to test the effects of in!flight icing on actual aircraft components an" mo"els of aircraft, inclu"ing helicopters.

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College of Engineering, Chengannur.

 Passive Wireless Sensors Application for NASA.

 D e p a r t m e n t o f E C E .

The @ypersonic Tunnel Facility @TF/ tests large!scale hypersonic air!breathing propulsion systems. The @TF is a hypersonic (ach 5, 9, an" 4/ blow"own an" nonvitiate" clean air/ win" tunnel capable of testing large!scale propulsion systems at true enthalpy flight con"itions.

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6rovi"es next!generation ice protection systems for military an" commercial aircraft

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College of Engineering, Chengannur.

 Passive Wireless Sensors Application for NASA.

 D e p a r t m e n t o f E C E .

Pr%(ra&s an Pr%*ects S+''%rte

G @igh!$pee" Civil Transport G -ational +erospace 6lane -+$6/ G $pace $huttle G Boint $tri#e Fighter B$F/ G +viation $ociety 6rogram G %ntegrate" system test on an air!breathing roc#et %$T+)/ "irect connect combustion rig test.

FigH .a.1/ Tunnel *etails

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College of Engineering, Chengannur.

 Passive Wireless Sensors Application for NASA.

 D e p a r t m e n t o f E C E .

3., ) -!n T+nnels

in" tunnels are large tubes with air moving insi"e. The tunnels are use" to copy the actions of an ob2ect in flight. )esearchers use win" tunnels to learn more about how an aircraft will fly. -+$+ uses win" tunnels to test scale mo"els of aircraft an" spacecraft. $ome win" tunnels are big enough to hol" full!sie versions of vehicles. The win" tunnel moves air  aroun" an ob2ect, ma#ing it seem li#e the ob2ect is really flying. (ost of the time, powerful fans move air through the tube. The ob2ect to be teste" is fastene" in the tunnel so that it will not move. The ob2ect can be a small mo"el of a vehicle. %t can be  2ust a piece of a vehicle. %t can be a full!sie aircraft or spacecraft. %t can even be a common ob2ect li#e a tennis ball. The air moving aroun" the still ob2ect shows what woul" happen if  the ob2ect were moving through the air. @ow the air moves can be stu"ie" in "ifferent ways. $mo#e or "ye can be place" in the air an" can be seen as it moves. Threa"s can be attache" to the ob2ect to show how the air is moving. $pecial instruments are often use" to measure the force of the air on the ob2ect.  -+$+ has more win" tunnels than any other group. The agency uses the win" tunnels in a lot of ways. &ne of the main ways -+$+ uses win" tunnels is to learn more about airplanes an" how things move through the air. &ne of -+$+Is 2obs is to improve air transportation. in" tunnels help -+$+ test i"eas for ways to ma#e aircraft better an" safer. 'ngineers can test new materials or shapes for airplane parts. Then, before flying a new airplane, -+$+ will test it in a win" tunnel to ma#e sure it will fly as it shoul".  -+$+ also wor#s with others that nee" to use win" tunnels. That way, companies that are  buil"ing new airplanes can test how the planes will fly. y letting these companies use the win" tunnels, -+$+ helps to ma#e air travel safer.  -+$+ also uses win" tunnels to test spacecraft an" roc#ets. These vehicles are ma"e to operate in space. $pace has no atmosphere. $pacecraft an" roc#ets have to travel through the atmosphere to get to space. ehicles that ta#e humans into space also must come bac# through the atmosphere to 'arth. in" tunnels are important in ma#ing the new +res roc#ets an" &rion spacecraft. +res an" &rion are new vehicles that will ta#e astronauts into space. -+$+ engineers teste" i"eas for  the "esign of +res in win" tunnels. They nee"e" to see how well +res woul" fly. 'ngineers teste" &rion mo"els.

Dong after the first "esign wor# is finishe", -+$+ can still use win" tunnels. in" tunnel tests have helpe" -+$+ change the space shuttle to ma#e it safer. in" tunnels will #eep helping ma#e all spacecraft an" roc#ets better.

7 P

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College of Engineering, Chengannur.

 Passive Wireless Sensors Application for NASA.

 D e p a r t m e n t o f E C E .

in" tunnels can even help engineers "esign spacecraft to wor# on other worl"s. (ars has a thin atmosphere. %t is important to #now what the (artian atmosphere will "o to vehicles that are lan"ing there. $pacecraft "esigns an" parachutes are teste" in win" tunnels set up to be li#e the (artian atmosphere.  -+$+ has many "ifferent types of win" tunnels. They are locate" at -+$+ centers all aroun" the country. The win" tunnels come in a lot of sies. $ome are only a few inches suare, an" some are large enough to test a full!sie airplane. $ome win" tunnels test aircraft at very slow spee"s. ut some win" tunnels are ma"e to test at hypersonic spee"s. That is more than ;,000 miles per hourJ +ir is blown or suc#e" through a "uct euippe" with a viewing port an" instrumentation where mo"els or geometrical shapes are mounte" for stu"y. Typically the air is move" through the tunnel using a series of fans. For very large win" tunnels several meters in "iameter, a single large fan is not practical, an" so instea" an array of multiple fans are use" in parallel to provi"e sufficient airflow. *ue to the sheer volume an" spee" of air movement reuire", the fans may be powere" by stationary turbofan engines rather than electric motors. The airflow create" by the fans that is entering the tunnel is itself highly turbulent "ue to the fan bla"e motion when the fan is blowing air into the test section K when it is suc#ing air out of the test section "ownstream, the fan!bla"e turbulence is not a factor/, an" so is not "irectly useful for accurate measurements. The air moving through the tunnel nee"s to be relatively turbulence!free an" laminar . To correct this problem, closely space" vertical an" horiontal air vanes are use" to smooth out the turbulent airflow before reaching the sub2ect of the testing. *ue to the effects of viscosity, the cross!section of a win" tunnel is typically circular rather  than suare, because there will be greater flow constriction in the corners of a suare tunnel that can ma#e the flow turbulent. + circular tunnel provi"es a smoother flow. The insi"e facing of the tunnel is typically as smooth as possible, to re"uce surface "rag an" turbulence that coul" impact the accuracy of the testing. 'ven smooth walls in"uce some "rag into the airflow, an" so the ob2ect being teste" is usually #ept near the center of the tunnel, with an empty buffer one between the ob2ect an" the tunnel walls. There are correction factors to relate win" tunnel test results to open!air results. The lighting is usually embe""e" into the circular walls of the tunnel an" shines in through win"ows. %f the light were mounte" on the insi"e surface of the tunnel in a conventional manner, the light bulb woul" generate turbulence as the air blows aroun" it. $imilarly, observation is usually "one through transparent portholes into the tunnel. )ather than simply  being flat "iscs, these lighting an" observation win"ows may be curve" to match the cross! section of the tunnel an" further re"uce turbulence aroun" the win"ow.

8 P

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College of Engineering, Chengannur.

 Passive Wireless Sensors Application for NASA.

 D e p a r t m e n t o f E C E .

arious techniues are use" to stu"y the actual airflow aroun" the geometry an" compare it with theoretical results, which must also ta#e into account the )eynol"s number  an" (ach number  for the regime of operation.

Figure .b.1 ing Tunnel

FigH .b.< loc# in" Tunnel

Meas+re&ents

9 P

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College of Engineering, Chengannur.

 Passive Wireless Sensors Application for NASA.

 D e p a r t m e n t o f E C E .

3.,.1) Press+re &eas+re&ents 6ressure across the surfaces of the mo"el can be measure" if the mo"el inclu"es pressure taps. This can be useful for pressure!"ominate" phenomena, but this only accounts for normal forces on the bo"y.

3.,.2) %rce an &%&ent &eas+re&ents

Figure .b./ (easurements

+ typical lift coefficient versus angle of attac#  curve. ith the mo"el mounte" on a force balance, one can measure lift, "rag, lateral forces, yaw, roll, an" pitching moments over a range of angle of attac# .  This allows one to pro"uce common curves such as lift coefficient versus angle of attac# shown/.  -ote that the force balance itself creates "rag an" potential turbulence that will affect the mo"el an" intro"uce errors into the measurements. The supporting structures are therefore typically smoothly shape" to minimie turbulence .

Qualitative methods •

$mo#e



Tufts

Tufts are applie" to a mo"el an" remain attache" "uring testing. Tufts can be use" to gauge air flow patterns an" flow separation.

10 P

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College of Engineering, Chengannur.

 Passive Wireless Sensors Application for NASA.

 D e p a r t m e n t o f E C E .

Fluorescent mini!tufts attache" to a wing in the irsten in" Tunnel showing air flow "irection an" separation. +ngle of attac# = 1< "egrees, spee" =1<0 (ph. Figure .b.;

11 P

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College of Engineering, Chengannur.

 Passive Wireless Sensors Application for NASA.

 D e p a r t m e n t o f E C E .

) AIRCRAT PROPULSION APPLICATIONS +ctive wireless sensor systems have been "evelope" for monitoring the health of aircraft engines for commercial, military, an" -+$+ aircraft, but all of these systems reuire  batteries. -+$+ woul" prefer that future sensors to be passive. The +rmy, +ir Force, an"  -+$+ reuire high! temperature propulsion sensors that can operate in environments of up to 1,57 °C aroun" the engine an" insi"e the gas path. oth wiring an" batteries become an issue in these applicationsL therefore, high!temperature!resistant, passive wireless sensors, such as the passive engine!bearing sensor, are nee"e".  -+$+3s :lenn )esearch Center is researching new propulsion technologies in their  +"vance" $ubsonic Combustion )ig +$C)/. The +$C) simulate" combustor inlet can test con"itions of pressures up to 9,00 #6a an" temperatures up to 1,741 °C. 'nvironments with temperature this high coul" benefit from wireless sensors. Commercial airlines use turbofan engines  -+$+3s :lenn )esearch Center an" is combustion station temperature of =1,119 operate for long perio"s are reuire" to efficiency.

for routine flights. This schematic comes from available online. These engines run with a °C. @igh!temperature wireless sensors that can optimie the engine parameters for better fuel

@igh!temperature materials are currently being researche" for wireless sensor applications. +luminium nitri"e is being investigate" for the high!temperature 700 °C/ operation of  temperature!compensating sensors. :allium phosphate :a6&/ has been use" as a substrate in the "evelopment of a temperature sensor. The sensor is wireless, operates at ; (@, an" withstan"s temperatures of 900 °C for 18< hours. $ensors ma"e with exotic materials such as Dangasite, langatite, an" langatate have been characterie" from !100 °C to 800 °C. These sensors reuire metal con"uctors. Thin films, however, "o not behave the same as bul#  materials. Therefore, research is ongoing on thin film metal characteriation for passive wireless sensors. These new materials may enable wireless passive sensors to operate at temperatures higher than 1,000 °C.   %n a""ition to sensory materials research, ra"io freuency )F/ transpon"ers are being "evelope" for passive wireless sensor use on turbine bla"es M1;N. These "evices have been characterie" for operation up to 1,100 °C.

12 P

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College of Engineering, Chengannur.

 Passive Wireless Sensors Application for NASA.

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 D e p a r t m e n t o f E C E .

College of Engineering, Chengannur.

 Passive Wireless Sensors Application for NASA.

 D e p a r t m e n t o f E C E .

/) AIRCRAT STRUCTURAL APPLICATIONS  -+$+ envisions the a""ition of structural health monitoring $@(/ sensors to existing aircraftL however, installing wiring for the sensors a""s cost an" weight to the aircraft. %n a""ition, wires are prone to "amage such as nic#s, brea#s, an" "egra"ation "ue to wear, excessive heating, an" arcing. iring problems have le" to ma2or aircraft acci"ents an" "elays of space vehicle launches. %n contrast, wireless systems present a "esirable option for  retrofitting sensors onto existing aircraft for structural health monitoring. @igh spee"s mean high temperatures from s#in friction heating. $ensors for hypersonic greater than (ach 5/ aircraft may experience aero"ynamic heating above 1,000 °C. @ypersonic aircraft base" on  -+$+3s @yperO O!; "esign will fly at (ach 10 an" therefore reuire sensors that can withstan" temperatures up to 1,<7< °C. Thus, hypersonic vehicles will nee" high! temperature wireless sensors li#e those nee"e" for propulsion applications. The O!51 averi"er is another vehicle that reuire" high temperature sensors. :roun" tests of the O!51 engine were con"ucte" in the 7!ft high!temperature tunnel at -+$+3s Dangley )esearch Center. The O!51+ averi"er set a hypersonic flight recor" when it flew at (ach 5 for <00 secon"s, beating the O!; recor" of 1< secon"s. &n (ay 1, <01, the O!51+ bro#e another worl" recor" when it flew for six minutes. *uring this final flight, the aircraft achieve" (ach 5.1. The nose of the prototype O!51 was expecte" to reach 1,;70 °C "uring flight "ue to s#in friction heating. @ypersonic aircraft often contain cryogenic flui"s in composite overwrappe" pressure vessels C&6/ . @y"rogen, liui" oxygen D&O/, #erosene, an" other fuels are often #ept at cryogenic temperatures. %n a""ition, C&6s are also use" to store gaseous helium , xenon , an" 80P hy"rogen peroxi"e for propulsion applications. The tan#s are constructe" from metal liners that are wrappe" in composite fibers usually carbon or evlar/, which are then cure" in an epoxy matrix to #eep them from bursting at high pressures. The operating temperature of a typical C&6 is aroun" !185 °C to !17; °C, an" the maximum operating pressure is close to 1.1 (pa. 'valuation of the structural health of the C&6 tan#s is necessary "ue to the thin liners being use", the high pressures, an" the ris# of impacts causing lower burst pressures. For applications where sensors nee" to be installe" within a C&6, the sensors must be able to withstan" both high pressure an" cryogenic temperatures. $ome passive wireless sensing technologies, such as $+ "evices, have alrea"y "emonstrate" operation in cryogenic liui" environments. )F%* sensors are also being investigate" for operation "own to !189 °C. ireless sensors utiliing energy harvesting in the form of solar power have also been propose" for monitoring the structural health of C&6 tan#s.

14 P

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College of Engineering, Chengannur.

 Passive Wireless Sensors Application for NASA.

 D e p a r t m e n t o f E C E .

0) CEMICAL SENSING (any of the aerospace research vehicles, such as the $pace $huttle, @yperO, an" @elios, all containe" hy"rogen tan#s. These vehicles coul" have benefite" from -+$+3s high! temperature chemical sensors that can "etect hy"rogen. + surface acoustic wave $+/ hy"rogen sensor base" on a Dangasite substrate that utilies palla"ium as the sensing me"ium has been shown to operate at <50 °C. $ince $+ technology can be use" for hy"rogen sensing, all that is nee"e" is the a""ition of passive wireless capability. $+ "evices can be use" to "etect other chemicals as well. The integrity of wires on boar" aeronautical vehicles coul" be "etermine" by monitoring the effluents given off by the wire3s insulation. The effluents are generate" "uring aging, over!currents, arcing, an" high!temperature con"itions. $+ chemical sensors coul" be use" to "etect effluents an" give an in"ication of wire integrity. $+ technology is very promising for passive wireless operation but is not the only technology being investigate". 6rime 6hotonics has "evelope" a passive wireless temperature sensor that can operate at 19;8 °C base" on )F%* technology. &ther  technologies inclu"e resonant circuits for chemical sensing that can operate at 945 °C. + high!temperature 700 °C/ wireless temperature sensor has been "evelope" for aircraft engine applications. This sensor is the first step in "eveloping a high!temperature wireless C&< sensor. 6assive wireless sensors, inclu"ing chemical sensors, have been propose" for   -+$+3s *evelopment Flight %nstrumentation *F%/ Technology )oa"map. The roa"map calls for passive wireless sensors as part of low power electronics, which is one of six enabling technologies. $ingle!walle" carbon nanotubes $-Ts/ have been use" to "evelop a chemical sensor with high sensitivity to nitrogen "ioxi"e, acetone, benene, nitrotoluene, chlorine, an" ammonia in the concentration range of ppm to ppb. %n an effort to "evelop a  portable wireless airborne ammonia, chlorine gas, an" methane sensing system, researchers at  -+$+3s +mes )esearch Center have "evelope" a chemical sensor that plugs into an i6hone. The "evice utilies 19 high!spee" nano sensors that sniff the air an" communicate wirelessly using the cell phone. )esearchers at -+$+3s :lenn $pace Flight Center are investigating small wireless chemical sensors for space an" aeronautical applications.

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College of Engineering, Chengannur.

 Passive Wireless Sensors Application for NASA.

 D e p a r t m e n t o f E C E .

) SOC 4 IMPACT TESTING  -+$+3s :lenn )esearch Center has three specialty guns for ballistic impact testing. The various rigs in this facility are use" to stu"y the "ynamics of high!spee" pro2ectiles an" the impact "amage they cause. These tests help to "evelop new a"vance" materials an" structures that are lighter, stronger, an" more impact!resistant. Failure an" "eformation characteristics of both structures an" materials are evaluate" un"er ballistic impact loa"ing con"itions. These guns are use" in aeronautics research for assessing the impact resistance of  age" composite fan containment materials an" structures an" for the "evelopment of  lightweight an" multifunctional fan containment structures. %n a""ition, this laboratory  performs the "evelopment an" vali"ation of impact "amage mo"els. For this application, wireless sensors that can "etect the "amage cause" by impacts are sought.

&ne of the lan"mar#s at -+$+3s Dangley )esearch Center is the Dunar Dan"ing )esearch Facility. The <00!ft, 91!m high tower was built to help train the original +pollo astronauts how to lan" on the moon. Currently, the tower is use" for research on lan"ing an" impacts for both aircraft an" spacecraft such as the &rion space capsule. &n +ug. <7, <01, a -avy C@!;9 $ea night helicopter fuselage was "roppe" from 1.4 m to simulate a crash. )ugge" wireless sensing that can withstan" the shoc# loa"s experience" from aircraft crashes woul"  be useful for this application.

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College of Engineering, Chengannur.

 Passive Wireless Sensors Application for NASA.

 D e p a r t m e n t o f E C E .

5) IONI6ING RADIATION @igh!altitu"e sensing applications reuire sensors that exhibit tolerance to ioniing ra"iation. Tests con"ucte" on $+ "evices have "emonstrate" an inherent ra"iation tolerance of up to 10 (ra". The constant sie re"uction of commercial electronics has le" to less ra"iation tolerance an" therefore to more soft errors in stan"ar" commercial electronics. %n 188, static )+( memories were ina"vertently foun" to have neutron!in"uce" bit errors or single!event upsets $'Q/ when flown in aircraft at 10 #m. %n a""ition, in 188, a series of tests on static )+(s "emonstrate" that they have $'Qs "ue to ra"iation at 7.7 #m an" at 18.7 #m. The author use" the "ata from these flights to pre"ict that error correcting co"ing 'CC/ will  be necessary for avionic systems. The author was correct in his pre"ictionH not only are 'CCs use" in avionics now but they are also available for memory use" in consumer  computers. The nee" for ra"iation!tolerant avionics has not "iminishe", as the autopilot memory in a mo"ern commercial airliner has been foun" to have one upset every <00 hours. The amount of ra"iation receive" varies by altitu"e, longitu"e, latitu"e, an" the natural solar  cycles of the sun. For example, four "osage cases. The ra"iation "osage versus altitu"e is  plotte" for the solar minimum 10>79/ an" the solar maximum 4>78/ for 80 "egrees west longitu"e an" for both 5° an" 40° north latitu"e. )a"iation "osages rise with altitu"e, ma#ing spacecraft an" high!altitu"e aircraft more susceptible to ra"iation effects.

(icro!electro!mechanical systems ('($/ are also inherently ra"iation tolerant an" may be use" to "evelop sensors for extreme environments. )a"iation!tolerant electronics are very expensive compare" to commercial electronics. Therefore, inherently ra"iation!tolerant technologies are better can"i"ates for sensors in high!altitu"e an" space applications than "evices ma"e from conventional electronics.

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College of Engineering, Chengannur.

 Passive Wireless Sensors Application for NASA.

 D e p a r t m e n t o f E C E .

7) PASSI8E -IRELESS SENSORS &ne of the main challenges for wireless sensors is power. &ften, batteries cannot be use" "ue to inaccessible locations or exposure to large temperature extremes. 'nergy! harvesting systems that rely on batteries for energy storage are eliminate" for the same reasons. Thus,  passive wireless sensing systems that "o not inclu"e batteries shoul" be "evelope". $everal  passive wireless technologies may be a"apte" to meet the nee"s of the extreme environment community. ('($, )F%*, $+, an" bac#scatter techniues have all shown potential for   passive wireless an" extreme environment operation.

6assive wireless )F%* chips have been "evelope" that can use )F energy to power the electronic circuitry that comprises the wireless sensor no"e. + bac#scatter temperature sensor  that can operate to at least 00 °C has been "evelope". The "evice uses resonators to mo"ulate the freuency of the bac#scattere" ra"ar cross section to ma#e measurements. $+ "evices have been propose" as passive wireless "evices that can operate in high! temperature environments. + wireless temperature sensor on Dangasite D:$/ has been "emonstrate" to operate at up to 750 °C. The company 'vironetix has capitalie" on this technology an" now has a $+ D:$ pro"uct that can measure temperatures up to 810 °C. + wireless oxygen sensor that can operate at up to 950 °C has been "evelope" on a D:$>Rn& substrate. $+ pressure sensors on D:$ that operate at up to 500 °C have also been "evelope". (any other $+ high!temperature sensors are un"er "evelopment. ibration, ioniing ra"iation, )F issues, an" certifications are 2ust a few of the issues that must be a""resse" when "eveloping new sensor systems. There are many challenges in "eveloping wireless sensors for extreme environments. 6ressure variations from vacuum to high pressures shoul" not be an issue for soli"!state "evices such as ('($, %Cs, or $+ "evices. For most cases, corona "ischarge an" arcing at low pressures shoul" not pose a  problem when the voltages are low. @owever, an issue may arise when "evices are miniaturie" an" the spacing between charge" components is re"uce", allowing corona "ischarge an" arcing to occur at lower voltages. *evices must be "esigne" with 6aschen3s law in min" to avoi" arcing an" corona "ischarge when the pressure "rops. ibration is an issue for all aircraft. Component failures from the high levels of shoc# an" vibration "uring operation are not uncommon. (onitoring of "ynamic loa"ing "uring flight is important for $@( an" fatigue life analysis. $tructural health monitoring using conventional sensors such as strain gauges/ has been "ifficult "uring flight "ue to the "ynamics of aircraft loa"ing an" the ran"om vibrations that are generate". $train is often measure" on research aircraft li#e those foun" at -+$+3s *ry"en )esearch Center. Typical small aircraft can experience vibrational noise in the range of S90 U pea# to pea#, while the aircraft experiences up to 1,000 U loa"s "uring flight. These strain measurements are similar in magnitu"e to those ta#en on other research aircraft M;4, ;7N. +ircraft such as the 6 experience S0.9 g of vibrational noise "uring flight M;8N. The "ata in Fig. 4 are from 18 P

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 Passive Wireless Sensors Application for NASA.

 D e p a r t m e n t o f E C E .

three strain sensors mounte" on the wing lea"ing e"ge "uring ta#e!off. The raw sensor "ata were filtere" with a two!point moving average. The moving average was subtracte" from the original "ata, leaving only the vibrational noise. The structural noise, which is mostly "ue to vibrations of the aircraft, is !90 U to V75 U. +lthough $+ sensors can filter out this noise from strain measurements, many other types of sensors, such as conventional strain gauges, cannot. +nother concern for avionics is exposure to corrosive environments. The enne"y $pace Center operates accelerate" corrosive chambers that use salt fog an" aci"s to investigate corrosive environments an" their effects on aerospace vehicles. They also operate the out"oor $C eachsi"e Corrosion Daboratory, where material "egra"ation an" harsh corrosive environments are stu"ie". The :lenn )esearch Center operates an extreme environment chamber that measures 0.8 m x 1.< m. The chamber temperature can reach 57 °C, with a pressure of 10.1 (6a an" an atmosphere of corrosive chemicals such as hy"rogen fluori"e an" hy"rogen chlori"e. The chamber is use" primarily for simulating the con"itions foun" on the planet enus. +lthough this seems to be a space application instea" of an aeronautical application, it has been suggeste" that -+$+ fly an aircraft in the atmosphere of enus. )F communication issues pose a significant challenge to the successful implementation of  $+ systems. (o"ulation metho"s must be chosen to allow large numbers of "evices to communicate without interference. The ban"wi"th must be utilie" carefully to enable high "ata rates while a"hering to FCC limitations. 'nco"ing schemes must be "evelope" to allow for efficient operation in noisy environments. The "evices must be small, which means higher freuencies an" therefore smaller antenna sies. @igher freuencies enable higher "ata ratesL however, the range will begin to "ecrease when the freuency reaches 10 :@. @igher! freuency "evices may also mean that the fabrication feature sies must shrin#, which may lea" to manufacturing issues. %n a""ition, the freuency of operation must follow FCC gui"elines. + 90 :@ $@( system is being "evelope" for aircraft applications. The system exploits an unlicense" ban" in the communication spectrum for 54K9; :@. The "evices use Qltra i"e an" %mpulse )a"io Q!%)/ technology an" very low power. The wireless communicating no"es are passive an" fabricate" on a flexible apton substrate that can be attache" to curve" surfaces within the aircraft. (any of -+$+3s test environments are close" metallic structures with metal test articles. *irect wireless lin#s may not be possible in enclose" metallic spaces of aircraft, so multi!hop or mesh networ# architectures may be reuire". 'lectromagnetic interference poses a  problem for all wireless systems. +ll wireless electronics must be "esigne" to pass tests for   both electromagnetic compatibility an" interference. The certification of wireless sensor networ#s for flight is another issue that must be a""resse". This inclu"es the allocation of freuencies for wireless sensing on aircraft, along with the "etermination of )F power levels an" F++ acceptance for aircraft use. There is a 19 P

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College of Engineering, Chengannur.

 Passive Wireless Sensors Application for NASA.

 D e p a r t m e n t o f E C E .

concern that wireless "evices within the cabin may interfere with aircraft antennas locate" outsi"e the cabin. 6ersonal electronic "evices an" cell phones have been teste" to "etermine the effects that wireless "evices may have on aircraft avionics. +ctive )F%* tags have been teste" for compliance with )TC+>*&190 aircraft emission limits. The tags excee"e" the threshol"s, but further investigations are nee"e" before the effects of the interference can be un"erstoo". The interference from passive tags was not consi"ere" a concern, however,  because the stu"y focuse" on )F%* tags containe" in cargo pallets without an interrogator. Qnfortunately, passive $+ sensors woul" reuire an active interrogator within the aircraft structure. ecause $+ sensors will share the same freuency ban"s with )F%* tags an" some of the same )F mo"ulation techniues, they may excee" the emission threshol"s as well. efore any wireless sensor system can be certifie" for use on aircraft, it will have to be teste".

5.a) S+r$ace Ac%+st!c -a9es $+ filters offer high performance at freuencies up to 5 :@ range. They are small an" suitable to low!cost mass manufacturingL Therefore, they are wi"ely use" in mobile communications applications. $+ )F%* tag is an emerging technology that hol"s promises for long!range i"entification in various applications, with rea"ing "istance of several meters an" completely passive tag "evices. e have "evelope" several low!loss $+ filter "esigns that are base" on longitu"inally couple" resonator structures. The "esigns aim at minimiing the effect of the principal loss mechanisms "ue to the filter structure, such as resistive losses an"  bul#!wave ra"iation losses. &peration of the filters is optimie" using software "evelope" here an" base" on the coupling!of!mo"es C&(/ mo"el. -ovel $+ tag topologies are stu"ie" an" "evelope" using Finite 'lement (etho" F'(/ software.

Figure 8.a.1H $chematic picture of a simple $+ "evice. The acoustic wave propagating on a pieoelectric substrate is generate" an" receive" using inter"igital trans"ucers %*Ts/.

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 Passive Wireless Sensors Application for NASA.

 D e p a r t m e n t o f E C E .

$urface acoustic wave sensors are a class of micro electro mechanical systems ('($/ which rely on the mo"ulation of surface acoustic waves to sense a physical phenomenon. The sensor trans"uces an input electrical signal into a mechanical wave which, unli#e an electrical signal, can be easily influence" by physical phenomena. The "evice then trans"uces this wave bac# into an electrical signal. Changes in amplitu"e, phase, freuency, or time!"elay between the input an" output electrical signals can be use" to measure the  presence of the "esire" phenomenon.

De9!ce La"%+t

FigH 8.a.</ $urface +coustic ave $ensor %nter"igitate" Trans"ucer *iagram

The basic surface acoustic wave "evice consists of a pieoelectric substrate, an input inter"igitate" trans"ucer %*T/ on one si"e of the surface of the substrate, an" a secon", output inter"igitate" trans"ucer on the other si"e of the substrate. The space  between the %*Ts, across which the surface acoustic wave will propagate, is #nown as the "elay!line. This region is calle" the "elay line because the signal, which is a mechanical wave at this point, moves much slower than its electromagnetic form, thus causing an appreciable "elay.

De9!ce O'erat!%n $urface acoustic wave technology ta#es a"vantage of the pieoelectric effect in its operation. (ost mo"ern surface acoustic wave sensors use an input inter"igitate" trans"ucer  %*T/ to convert an electrical signal into an acoustic wave. The sinusoi"al electrical input signal creates alternating polarity between the fingers of the inter"igitate" trans"ucer. etween two a"2acent sets of fingers, polarity of the fingers will be switche" e.g. V ! V/. +s a result, the "irection of the electric fiel"  between two fingers will alternate between a"2acent sets of fingers. This creates alternating regions of tensile an" compressive strain between fingers of the electro"e  by the pieoelectric effect, pro"ucing a mechanical wave at the surface #nown as a surface acoustic wave. +s fingers on the same si"e of the "evice will be at the same level of compression or tension, the space between them!!!#nown as the pitch!!!is the wavelength of the mechanical wave. e can express the synchronous freuency f 0 of  the "evice with phase velocity v p an" pitch p asH

21 P

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 Passive Wireless Sensors Application for NASA.

 D e p a r t m e n t o f E C E .

The synchronous freuency is the natural freuency as which mechanical waves shoul" propagate. %"eally, the input electric signal shoul" be at the synchronous freuency to minimie insertion loss. +s the mechanical wave will propagate in both "irections from the input %*T, half  of the energy of the waveform will propagate across the "elay line in the "irection of the output %*T. %n some "evices, a mechanical absorber or reflector is a""e"  between the %*Ts an" the e"ges of the substrate to prevent interference patterns or  re"uce insertion losses respectively. The acoustic wave travels across the surface of the "evice substrate to the other  inter"igitate" trans"ucer, converting the wave bac# into an electric signal by the  pieoelectric effect. +ny changes that were ma"e to the mechanical wave will be reflecte" in the output electric signal. +s the characteristics of the surface acoustic wave can be mo"ifie" by changes in the surface properties of the "evice substrate, sensors can be "esigne" to uantify any phenomenon which alters these  properties. Typically, this is accomplishe" by the a""ition of mass to the surface or changing the length of the substrate an" the spacing between the fingers. The structure of the basic surface acoustic wave sensor allows for the phenomena of pressure, strain, torue, temperature, an" mass to be sense". The mechanisms for this are "iscusse" belowH Press+re: Stra!n: T%r;+e: Te&'erat+re

The phenomena of pressure, strain, torue, temperature, an" mass can be sense"  by the basic "evice, consisting of two %*Ts separate" by some "istance on the surface of a pieoelectric substrate. These phenomena can all cause a change in length along the surface of the "evice. + change in length will affect both the spacing between the inter"igitate" electro"es!!!altering the pitch!!!an" the spacing  between %*Ts!!!altering the "elay. This can be sense" as a phase!shift, freuency! shift, or time!"elay in the output electrical signal. hen a "iaphragm is place" between the environment at a variable pressure an" a reference cavity at a fixe" pressure, the "iaphragm will ben" in response to a  pressure "ifferential. +s the "iaphragm ben"s, the "istance along the surface in compression will increase. + surface acoustic wave pressure sensor simply replaces the "iaphragm with a pieoelectric substrate patterne" with inter"igitate" electro"es. $train an" torue wor# in a similar manner, as application to the sensor will cause a "eformation of the pieoelectric substrate. + surface acoustic wave temperature sensor can be fashione" from a pieoelectric substrate with a relatively high coefficient of thermal expansion in the "irection of the length of  the "evice.

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 Passive Wireless Sensors Application for NASA.

 D e p a r t m e n t o f E C E .

Mass

The accumulation of mass on the surface of an acoustic wave sensor will affect the surface acoustic wave as it travels across the "elay line. The velocity v of a wave traveling through a soli" is proportional to the suare root of pro"uct of the Woung3s mo"ulus ' an" the "ensity of the material.

Therefore, the wave velocity will "ecrease with a""e" mass. This change can be measure" by a change in time!"elay or phase!shift between input an" output signals. $ignal attenuation coul" be measure" as well, as the coupling with the a""itional surface mass will re"uce the wave energy. %n the case of mass!sensing, as the change in the signal will always be "ue to an increase in mass from a reference signal of ero a""itional mass, signal attenuation can be effectively use". The inherent functionality of a surface acoustic wave sensor can be exten"e" by the "eposition of a thin film of material across the "elay line which is sensitive to the  physical phenomena of interest. %f a physical phenomenon causes a change in length or mass in the "eposite" thin film, the surface acoustic wave will be affecte" by the mechanisms mentione" above. $ome exten"e" functionality examples are liste"  belowH Che&!cal 8a'%+rs

Chemical vapour sensors use the application of a thin film polymer across the "elay line which selectively absorbs the gas or gases of interest. +n array of such sensors with "ifferent polymeric coatings can be use" to sense a large range of gases on a single sensor with resolution "own to parts per trillion, allowing for the creation of a sensitive Xlab on a chip.X #!%l%(!cal Matter

+ biologically!active layer can be place" between the inter"igitate" electro"es which contains immobilie" antibo"ies. %f the correspon"ing antigen is present in a sample, the antigen will bin" to the antibo"ies, causing a mass!loa"ing on the "evice. These sensors can be use" to "etect bacteria an" viruses in samples, as well as to uantify the presence of certain m)-+ an" proteins. 23 P

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 Passive Wireless Sensors Application for NASA.

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+&!!t"

$urface acoustic wave humi"ity sensors reuire a thermoelectric cooler in a""ition to a surface acoustic wave "evice. The thermoelectric cooler is place" below the surface acoustic wave "evice. oth are house" in a cavity with an inlet an" outlet for gases. y cooling the "evice, water vapor will ten" to con"ense on the surface of the "evice, causing a mass!loa"ing. Ultra9!%let Ra!at!%n

$urface acoustic wave "evices can be ma"e sensitive to optical wavelengths through the phenomena #nown as acoustic charge transport +CT/, which involves the interaction between a surface acoustic wave an" photo generate" charge carriers from a photo con"ucting layer. Qltraviolet ra"iation sensors employ the use of a thin film layer of inc oxi"e across the "elay line. hen expose" to ultraviolet ra"iation, inc oxi"e generates charge carriers which interact with the electric fiel"s pro"uce" in the  pieoelectric substrate by the traveling surface acoustic wave.M1N This interaction "ecreases the velocity an" the amplitu"e of the signal. Ma(net!c !els

Ferromagnetic materials, such as iron, nic#el, an" cobalt, exhibit a characteristic calle" magnetostriction, where the WoungIs mo"ulus of the material is "epen"ent on magnetic fiel" strength. %f a constant stress is maintaine" on such a material, the strain will change with a changing WoungIs mo"ulus. %f such a material is "eposite" in the "elay line of a surface acoustic wave sensor, a change in length of the "eposite" film will stress the un"erlying substrate. This stress will result in a strain on the surface of the substrate, affecting the phase velocity, phase!shift, an" time!"elay of  the signal. 8!sc%s!t"

$urface acoustic wave "evices can be use" to measure changes in viscosity of a liui"  place" upon it. +s the liui" becomes more viscous the resonant freuency of the "evice will change in correspon"ence. + networ# analyser is nee"e" to view the resonant freuency.

5.,) Ra!% re;+enc" Ient!$!cat!%n Ener(" ar9est!n( )F energy is currently broa"caste" from billions of ra"io transmitters aroun" the worl", inclu"ing mobile telephones, han"hel" ra"ios, mobile base stations, an" television> ra"io broa"cast stations. The ability to harvest )F energy, from ambient or  "e"icate" sources, enables wireless charging of low!power "evices an" has resulting 24 P

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 Passive Wireless Sensors Application for NASA.

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 benefits to pro"uct "esign, usability, an" reliability. attery!base" systems can be tric#le" charge" to eliminate battery replacement or exten" the operating life of  systems using "isposable batteries. attery!free "evices can be "esigne" to operate upon "eman" or when sufficient charge is accumulate". %n both cases, these "evices can be free of connectors, cables, an" battery access panels, an" have free"om of   placement an" mobility "uring charging an" usage. Ener(" S%+rces

FigH 8.b.1/ )F%* 'nergy @arvesting

The obvious appeal of harvesting ambient )F energy is that it is essentially EfreeA energy. The number of ra"io transmitters, especially for mobile base stations an" han"sets, continues to increase. +% )esearch an" i$upply estimate the number of  mobile phone subscriptions has recently surpasse" 5 billion, an" the %TQ estimates there are over 1 billion subscriptions for mobile broa"ban". (obile phones represent a large source of transmitters from which to harvest )F energy, an" will potentially enable users to provi"e power!on!"eman" for a variety of close range sensing applications. +lso, consi"er the number of iFi routers an" wireless en" "evices such as laptops. %n some urban environments, it is possible to literally "etect hun"re"s of  iFi access points from a single location. +t short range, such as within the same room, it is possible to harvest a tiny amount of energy from a typical iFi router  transmitting at a power level of 50 to 100 m. For longer!range operation, larger  antennas with higher gain are nee"e" for practical harvesting of )F energy from mobile base stations an" broa"cast ra"io towers. %n <005, 6owercast "emonstrate" ambient )F energy harvesting at 1.5 miles =<.; #m/ from a small, 5!# +( ra"io station. )F energy can be broa"caste" in unlicense" ban"s such as 797(@, 815(@, <.;:@, an" 5.7:@ when more power or more pre"ictable energy is nee"e" than what is available from ambient sources. +t 815(@, government regulations limit the output power of ra"ios using unlicense" freuency ban"s to ; effective isotropic ra"iate" power '%)6/, as in the case of ra"io!freuency! i"entification )F%*/ interrogators. +s a comparison, earlier generations of mobile phones base" on analog technology ha" maximum transmission power of .9, an" 6owercast3s TO81501 transmitter that sen"s power an" "ata is . R ar9est!n( Rece!9ers

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)F energy harvesting "evices, such as 6owercast3s 6owerharvesterY receivers, convert )F energy into *C power. These components are easily a""e" to circuit boar" "esigns an" wor# with stan"ar" or custom 50!ohm antennas. ith current )F sensitivity of the 6<110 6owerharvester receiver at !11"m, powering "evices or  charging batteries at "istances of ;0!;5 feet from a  transmitter is easily achieve" an" can be verifie" with 6owercast3s "evelopment #its. %mproving the )F sensitivity allows for )F!to!*C power conversion at greater "istances from an )F energy source. @owever, as the range increases the available power an" rate of charge "ecreases. +n important performance aspect of an )F energy harvester is the ability to maintain )F!to!*C conversion efficiency over a wi"e range of operating con"itions, inclu"ing variations of input power an" output loa" resistance. For example, 6owercast3s )F energy!harvesting components "o not reuire a""itional energy!consuming circuitry for maximum power point trac#ing (66T/ as is reuire" with other energy! harvesting technologies. 6owercast3s components maintain high )F!to!*C conversion efficiency over a wi"e operating range that enables scalability across applications an" "evices. )F energy!harvesting circuits that can accommo"ate multi!ban" or wi"eban" freuency ranges, an" automatic freuency tuning, will further increase the power  output, potentially expan" mobility options, an" simplify ins tallation.

T"'!cal A''l!cat!%ns

)F energy can be use" to charge or operate a wi"e range of low!power "evices. +t close range to a low!power transmitter, this energy can be use" to tric#le charge a number of "evices inclu"ing :6$ or )DT$ trac#ing tags, wearable me"ical sensors, an" consumer electronics such as e!boo# rea"ers an" hea"sets. +t longer range the  power can be use" for battery!base" or battery!free remote sensors for @+C control an" buil"ing automation, structural monitoring, an" in"ustrial control. *epen"ing on the power reuirements an" system operation, power can be sent continuously, on a sche"ule" basis, or on!"eman". %n large!scale sensors "eployments significant labor  cost avoi"ance is possible by eliminating the future maintenance efforts to replace  batteries.

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1<) CALLENGES AND ISSUES O -IRELESS SENSING ibration, ioniing ra"iation, )F issues, an" certifications are 2ust a few of the issues that must be a""resse" when "eveloping new sensor systems. There are many challenges in "eveloping wireless sensors for extreme environments. 6ressure variations from vacuum to high pressures shoul" not be an issue for soli"!state "evices such as ('($, %Cs, or $+ "evices. For most cases, corona "ischarge an" arcing at low pressures shoul" not pose a  problem when the voltages are low. @owever, an issue may arise when "evices are miniaturie" an" the spacing between charge" components is re"uce", allowing corona "ischarge an" arcing to occur at lower voltages. *evices must be "esigne" with 6aschen3s law in min" to avoi" arcing an" corona "ischarge when the pressure "rops. ibration is an issue for all aircraft. Component failures from the high levels of shoc# an" vibration "uring operation are not uncommon. (onitoring of "ynamic loa"ing "uring flight is important for  $@( an" fatigue life analysis. $tructural health monitoring using conventional sensors such as strain gauges/ has been "ifficult "uring flight "ue to the "ynamics of aircraft loa"ing an" the ran"om vibrations that are generate". $train is often measure" on research aircraft li#e those foun" at -+$+3s *ry"en )esearch Center  ypical small aircraft can experience vibrational noise in the range of S90 U pea# to pea#, while the aircraft experiences up to 1,000 U loa"s "uring flight. These strain measurements are similar in magnitu"e to those ta#en on other research aircraft. +ircraft such as the 6 experience S0.9 g of vibrational noise "uring flight. The raw sensor "ata were filtere" with a two!point moving average.

The moving average was subtracte" from the original "ata,

leaving only the vibrational noise. The structural noise, which is mostly "ue to vibrations of  the aircraft, is !90 U to V75 U. +lthough $+ sensors can filter out this noise from strain measurements, many other types of sensors, such as conventional strain gauges, cannot. +nother concern for avionics is exposure to corrosive environments. The enne"y $pace Center operates accelerate" corrosive chambers that use salt fog an" aci"s to investigate 27 P

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corrosive environments an" their effects on aerospace vehicles.

They also operate the

out"oor $C eachsi"e Corrosion Daboratory, where material "egra"ation an" harsh corrosive environments are stu"ie".

The :lenn )esearch Center operates an extreme

environment chamber that measures 0.8 m x 1.< m. The chamber temperature can reach 57 °C, with a pressure of 10.1 (6a an" an atmosphere of corrosive chemicals such as hy"rogen fluori"e an" hy"rogen chlori"e. The chamber is use" primarily for simulating the con"itions foun" on the planet enus. +lthough this seems to be a space application instea" of an aeronautical application, it has been suggeste" that -+$+ fly an aircraft in the atmosphere of  enus.

)F communication issues pose a significant challenge to the successful

implementation of $+ systems.

(o"ulation metho"s must be chosen to allow large

numbers of "evices to communicate without interference. The ban"wi"th must be utilie" carefully to enable high "ata rates while a"hering to FCC limitations. 'nco"ing schemes must be "evelope" to allow for efficient operation in noisy environments. The "evices must be small, which means higher freuencies an" therefore smaller antenna sies. @igher freuencies enable higher "ata ratesL however, the range will begin to "ecrease when the freuency reaches 10 :@. @igher! freuency "evices may also mean that the fabrication feature sies must shrin#, which may lea" to manufacturing issues. %n a""ition, the freuency of operation must follow FCC gui"elines. "evelope" for aircraft applications.

+ 90 :@ $@( system is being

The system exploits an unlicense" ban" in the

communication spectrum for 54K9; :@. The "evices use Qltra i"e an" %mpulse )a"io Q!%)/ technology an" very low power. The wireless communicating no"es are passive an" fabricate" on a flexible apton substrate that can be attache" to curve" surfaces within the aircraft. (any of -+$+3s test environments are close" metallic structures with metal test articles. *irect wireless lin#s may not be possible in enclose" metallic spaces of aircraft, so multi!hop or mesh networ# architectures may be reuire". 'lectromagnetic interference poses a  problem for all wireless systems. +ll wireless electronics must be "esigne" to pass tests for   both electromagnetic compatibility an" interference.

The certification of wireless sensor 

networ#s for flight is another issue that must be a""resse". This inclu"es the allocation of  freuencies for wireless sensing on aircraft, along with the "etermination of )F power levels an" F++ acceptance for aircraft use. There is a concern that wireless "evices within the cabin may interfere with aircraft antennas locate" outsi"e the cabin. 6ersonal electronic "evices an" cell phones have been teste" to "etermine the effects that wireless "evices may 28 P

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have on aircraft avionics.

 D e p a r t m e n t o f E C E .

+ctive )F%* tags have been teste" for compliance with

)TC+>*&190 aircraft emission limits. The tags excee"e" the threshol"s, but further  investigations are nee"e" before the effects of the interference can be un"erstoo".

The

interference from passive tags was not consi"ere" a concern, however because the stu"y focuse" on )F%* tags containe" in cargo pallets without an interrogator. Qnfortunately,  passive $+ sensors woul" reuire an active interrogator within the aircraft structure. ecause $+ sensors will share the same freuency ban"s with )F%* tags an" some of the same )F mo"ulation techniues, they may excee" the emission threshol"s as well. efore any wireless sensor system can be certifie" for use on aircraft, it will have to be teste".

11) CONCLUSION From the groun" up to the e"ge of space, -+$+ has applications that reuire pass ive wireless sensor technologies in extreme aeronautical environments. -+$+ sensor systems inclu"ing acceleration, temperature, pressure, strain, shape, chemical, acoustic emission, ultrasonics, imaging, e""y current, thermography, an" terahert waves/ coul" benefit from becoming  passive an" wireless. @owever, each of these applications has its own reuirements an" issues. 'xtreme environments offer many challenges that must be a""resse", such as temperature, pressure, vibration, ioniing ra"iation, an" certifications. @owever, "espite the issues an" challenges, new technologies such as ('($, $+, bac#scatter, an" )F%* are lea"ing to the "evelopment of robust passive wireless sensor systems that may one "ay operate in extreme environments. The nee" for passive wireless sensors has been presente", but -+$+ "oes not possess all of  the necessary resources to "evelop them. Thus, -+$+ encourages partnerships, inclu"ing those with universities an" in"ustry, to ai" in the "evelopment of passive wireless sensors for  the extreme environments foun" in aeronautical applications.

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12) REERENCES M1N :. +. Dan"is, C. Da(arre, et al., Xenus +tmospheric 'xploration by $olar +ircraft,X +cta +stronautica, vol. 59, 7/, pp. 450!455, +pril <005. M<N B. (. 6erotti an" +. B. 'c#hoff, XDatest *evelopment in +"vance" $ensors at enne"y $pace Center $C/,X in $ensors, <00<. 6rocee"ings of %''', <00<, pp. 14<7!14. MN $. '. oo"ar" an" . *. Taylor, X+ ireless Flui"!Devel (easurement Techniue,X $ensors an" +ctuators +H 6hysical, vol. 14,  </, pp. <97!<47, +pril 7, <004. M;N . @aowei, (. +tiuaman, et al., Xireless $ensor -etwor# for +ircraft @ealth (onitoring,X in roa"ban" -etwor#s, roa"-etsI0;, 6rocee"ings, First Conference on, <00;,  pp. 4;7!450. M5N . -ic#erson an" ). Dally, X*evelopment of a $mart ireless -etwor#able $ensor for  +ircraft 'ngine @ealth (anagement,X in +erospace Conference, %''' 6rocee"ings, <001,  pp. <55!<9<. M9N (. )ei", . :raubar", et al., Xireless '""y Current 6robe for 'ngine @ealth (onitoring,X in Zuantitative -on"estructive 'valuation, :reen ay, %, <00;, pp. ;1;!;<0. M4N *. D. $imon, :. . @unter, et al., X$ensor -ee"s for Control an" @ealth (anagement of  %ntelligent +ircraft 'ngines,X -+$+>T([<00;! <1<0<, -+$+>T([<00;!<1<0</, p. 14, +ugust <00;. M7N +. ovacs, *. 6eroulis, et al., X'arly!arning ireless Telemeter for @arsh!'nvironment earings,X in $ensors, %''', +tlanta, :+, &ctober <7!1, <004, pp. 8;9!8;8. M8N T. enson, X'ngine$im eta 1.4b, Qn"ergra"uate ersion,X httpH>>www.grc.nasa.gov>>#!1<>airplane>ngnsim.html, -+$+ :lenn )esearch Center, &ct. 1< <01<, M10N @. Trang, ). 6atrice, et al., XTemperature!Compensate" $tructure for $aw 6ressure $ensor in ery @igh Temperature,X in Freuency Control $ymposium, Boint with the <1st 'uropean Freuency an" Time Forum, %''', <004, pp. ;0!;;. 30 P

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 Passive Wireless Sensors Application for NASA.

 D e p a r t m e n t o f E C E .

M;8N ]. +n"reassen, C. '. asberg, et al., X$tu"ies of +ero"ynamically %n"uce" ibrations on the 6!C (aritime $urveillance +ircraft an" 6ropose" ibration )e"ucing (easures,X  -orwegian *efence )esearch 'stablishment, FF%!rapport <01>00<;5, 7501, Ban. 1 <01,  p. 5<. M50N +. Coloa an" :. +. Dan"is, XDong *uration $olar Flight on enus,X in +%++ %nfotech^ +erospace, +%++ <005!4159, +rlington, a, $ept. <9!<8, <005, p. 1<. M51N *. *ragomirescu, (. raemer, et al., X90:@ ireless -ano!$ensors -etwor# for  $tructure @ealth (onitoring as 'nabler for $afer, :reener +ircrafts,X in +"vance" Topics in &ptoelectronics, (icroelectronics, an" -anotechnologies , Constanta, )omania +ug. <9!<8, <010, pp. 47<105 1!10. M5<N '. +))')+, (. )Q%R, et al., X$tructural @ealth (onitoring -etwor# $ystem with ireless Communications %nsi"e Close" +erospace $tructures,X in 9th 'uropean or#shop on $tructural @ealth (onitoring, *res"en, :ermany, Buly !9, p. 8. M5N T. la"imirova, C. 6. ri"ges, et al., XCharacterising ireless $ensor (otes for $pace +pplications,X in +"aptive @ar"ware an" $ystems, $econ" -+$+>'$+ Conference on, <004,  pp. ;!50. M5;N T. O. -guyen, $. . oppen, et al., X$mall +ircraft )F %nterference 6ath Doss (easurements,X -+$+ Dangley )esearch Center, @ampton, +, -+$+>T6!<004!<1;781, +ugust <004, p. 1<1. M55N T. O. -guyen, B. B. 'ly, et al., X)F%* Transpon"ersI )a"io Freuency 'missions in +ircraft Communication an" -avigation )a"io an"s,X -+$+ Dangley )esearch Center, @ampton, +, -+$+>T6!<009! <1;<85, (arch <009, p. 75.

34 P

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College of Engineering, Chengannur.

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