DESIGNANDDEVELOPMENTOFANLED-BASEDOPTICALCOMMUNICATION SYSTEM By MohammedAl-rubaiai ATHESIS Submittedto MichiganStateUniversity inpartialentoftherequirements forthedegreeof ElectricalEngineering-MasterofScience 2015 ABSTRACT DESIGNANDDEVELOPMENTOFANLED-BASEDOPTICAL COMMUNICATIONSYSTEM By MohammedAl-rubaiai Strongattenuationofmostelectromagneticsignalsinwateristhemainreasonwhy underwaterexploration,surveillance,andotherapplicationscurrentlyrelyonsub-seacables andtetherstosenddatabacktotheuser.Whileacousticmodemshavelongbeenthe defaultwirelesscommunicationmethodforunderwaterapplications,theyincurhighcost andcanonlydeliverlowdatarates.Inthiswork,anLED-basedopticalcommunication systemispresentedforapplicationsrequiringhighdatarates,lowpower,lowcomplexity, andlow-to-mediancommunicationrange. AnLED-basedcommunicationsystem,consistingofatransmitterwithasuper-bright blueLEDandareceiverbasedonablue-enhancedphotodiode,hasbeendevelopedand testedwiththegoaloftransmittingdataathighratesoverdistancesofatleast20meters. Testresultsinaswimmingpoolshowthesuccessfultransmissionoflargedataoveradistance of23metersandattransmissionratesof115200kbps.Thissystemreliesonaline-of-sight connectionforcommunication.Sincethechangeoftherelativepositionandorientation betweenthetransmitterandthereceiverwillthequalityofcommunicationlink,a mechanismisdesignedsothatthetransmitter/receivermodulescanberotatedtomaintain alignmentbetweenthecommunicatingparties,despitethemovementofunderlyingplatforms (e.g.,robots).Asignalstrength-basedfeedbackcontrolalgorithmisdevelopedtomaintain alignment.Experimentalresultsinvolvingastationarytransmitterandamobilereceiver mountedonaterrestrialrobotarepresentedtoevaluatetheperformanceofthesystem. ACKNOWLEDGMENTS IwouldliketoexpressmydeepestgratitudetomyadvisorProfessorXiaoboTanatMichigan StateUniversityforhisguidance,caring,andpatience,andforprovidingmewithanexcellent atmosphereforthisthesiswork.Heconsistentlysteeredmeintherightdirectionwheneverhe thoughtIneededit.IwouldliketothankProfessorsHassanKhalilandNelsonSepulvedafor servingonmythesiscommittee.Also,IwanttothankJohnThonforhishelpinproviding theaccesstotheswimmingpoolfortesting,andMontasserSharifforhelpingmeinthe swimmingpoolexperiments.Iwouldneverhavebeenabletomythesiswithoutthe guidanceofProfessorTan,thehelpfromfriends,andthesupportfrommyfamily.Iwould alsoliketoacknowledgethesupportofmyresearchfromtheHigherCommittee forEducationDevelopmentinIraq(HCED)andfromtheUSNationalScienceFoundation (IIS1319602). Finally,andmostimportantly,Iwouldliketoexpressmymostprofoundgratitudeto myparentsfortheirendlesslove,consistentsupport,andencouragementthroughoutthis research. iii TABLEOFCONTENTS LISTOFTABLES .................................... vii LISTOFFIGURES ................................... viii Chapter1Introduction ................................ 1 1.1TraditionalUnderwaterWirelessCommunicationMethods..........2 1.1.1AcousticsCommunication........................2 1.1.2ElectromagneticWaves..........................3 1.2UnderwaterOpticalCommunication......................4 1.2.1OverviewofContributions........................7 Chapter2SystemDesign,Implementation,andCharacterization ..... 9 2.1DesignConstraints................................10 2.2Transmitter....................................10 2.2.1LightSource................................10 2.2.2LEDSelection...............................14 2.2.3LEDDriver................................16 2.3Receiver......................................21 2.3.1LightDetector..............................21 2.3.2OpticalSensorSelection.........................25 2.3.3ReceiverCircuitDesign..........................27 2.4RotatingBasefortheTransmitter/Receiver..................34 2.4.1UpdatedPrintedCircuitBoardandSlipRing.............34 2.4.2ActuatorandController.........................35 2.5CharacterizationofOpticalCommunicationLinkPerformanceUnderwater.37 Chapter3OpticalCommunicationSignalStrengthModel .......... 48 3.1MathematicalModel...............................48 3.1.1TransmitterOpticalPower........................49 3.1.2LightDetectorSensitivity........................50 3.2ExperimentalModelIdenandValidation...............53 3.2.1ModelIden...........................53 3.2.2ModelValidation.............................56 Chapter4ActiveAlignmentControlSystem .................. 62 4.1TrackingAlgorithm................................62 4.2ExperimentalResults...............................63 iv Chapter5ConclusionandFutureWork ..................... 72 5.1Conclusion.....................................72 5.2FutureWork....................................73 BIBLIOGRAPHY ................................... 74 v LISTOFTABLES Table1.1Comparisonofcommercialacousticmodems[1]............3 Table1.2Comparisonofunderwateropticalcommunicationsystems......7 Table2.1LEDvs.laserdiodeforwirelessopticalcommunication........13 Table2.2ComparisonofcommerciallyavailablehighbrightnessblueLEDs[2].14 Table2.3Comparisonoflightdetectors[19],[24],[26]..............26 Table4.1Datalostduetorobotmovement....................64 vi LISTOFFIGURES Figure1.1Attenuationcotsofwaterandseawater[3]...........4 Figure2.1CommonexampleofLEDs[4]......................11 Figure2.2AnexampleofLaserDiode[5]......................12 Figure2.3CreeXR-ELEDattachedtoaheatsink[6]..............15 Figure2.4Relativevs.currentrelationforCreeXR-ELED(blackcurve forbluelightLED)[7].........................17 Figure2.5ElectricalCharacteristicsforCreeXR-ELED[7]...........17 Figure2.6MOSFETSchematicSymbols[8]....................19 Figure2.7Transmittercircuitschematic......................20 Figure2.8Anexampleofphotoresistor[9].....................22 Figure2.9Anexampleofaphototransistor[10]..................23 Figure2.10Anexampleofaphotodiode[11]....................24 Figure2.11Anexampleofanavalanchphotodiode[12]...............24 Figure2.12Anexampleofaphotomultiplier[13]..................25 Figure2.13Pictureofblue-enhancedphotodetector(PDBV107)[14].......27 Figure2.14Schematicofatransimpedanceforamplifyingthephotodi- odeoutput.................................28 Figure2.15Schematicofthereceivercircuit.Thevaluesofthecircuitcom- ponentsare:RF=100CF=0.5pF,RF1=RF2=RF3=1 andRF4=10.........................31 Figure2.16Modulesofthetransmittercircuit....................32 Figure2.17Modulesofthereceivercircuit......................32 vii Figure2.18Theassembledreceivercircuitandtransmittercircuit,withallmod- ulesverticallystacked...........................33 Figure2.19RePCBdesigntoachievemorecompactintegrationoftrans- mitter/receivercircuits..........................34 Figure2.20Retransmitter/receivercircuitsassembledtogether.......35 Figure2.21Theslipringusedinthisproject....................36 Figure2.22TheDCmotormountedona3D-printedframe............37 Figure2.23ThecontrolanddrivercircuitsfortheDCmotor...........38 Figure2.24Theassembleddevicewithoutthecover..............39 Figure2.25Theassembleddevicewiththecover...............40 Figure2.26Schematicofthesetupforswimmingpoolexperiments........41 Figure2.27Themeasuredsignalstrengthversusthetransmitter-receiverdistance inswimmingpoolexperimentsforthecaseof60-degreelensforthe transmitter................................41 Figure2.28Themeasuredsignalstrengthversusthetransmitter-receiverdistance inswimmingpoolexperimentsforthecaseof40-degreelensforthe transmitter................................42 Figure2.29Themeasuredsignalstrengthversusthetransmitter-receiverdistance inswimmingpoolexperimentsforthecaseof15-degreelensforthe transmitter................................43 Figure2.30Themeasuredsignalstrengthversusthetransmitter-receiverdistance inswimmingpoolexperimentsforthecaseof5-degreelensforthe transmitter................................44 Figure2.31Measuredsignalwaveformwith10kHzfrequencyfora5-degreeview- ingangleLED,measuredatadistanceof8meters..........45 Figure2.32Measuredsignalwaveformwith100kHzfrequencyfora5-degree viewingangleLED,measuredatadistanceof8meters........46 Figure2.33Thesignalwaveformcapturedduringthecommunicationbiterror testsatabaudrate=115200bps....................47 viii Figure3.1Illustrationoftherelativepositionandorientationbetweenthetrans- mitterandthereceiver..........................49 Figure3.2RelativeintensityofLED[7](red)andGaussian(green).The x-axisshowstheangulardisplacementindegrees,they-axisshows therelativeintensityinpercent.....................50 Figure3.3Frequencyresponseofthetransimpedance.........52 Figure3.4Experimentalsetupfortheidenandvalidationofthesignal strengthmodelinair...........................54 Figure3.5Modeledsignalstrengthandexperimentaldata.Thegreenlineshows theoutputofthemodel.Theexperimentaldataisplottedinredand ( = ˚ =0 )...............................55 Figure3.6Experimentalvalidationofthesignalstrengthmodelasthereceiver anglevaries.Herethedistanceisat20cm...........56 Figure3.7Experimentalvalidationofthesignalstrengthmodelasthereceiver anglevaries.Herethedistanceisat20cm............57 Figure3.8Experimentalvalidationofthesignalstrengthmodelwithtransmis- sionangle =20 andreceptionangle ˚ =0 ............58 Figure3.9Experimentalvalidationofthesignalstrengthmodelwithtransmis- sionangle =40 andreceptionangle ˚ =0 ............59 Figure3.10Experimentalvalidationofthesignalstrengthmodelwithtransmis- sionangle =0 andreceptionangle ˚ =20 ............60 Figure3.11Experimentalvalidationofthesignalstrengthmodelwithtransmis- sionangle =0 andreceptionangle ˚ =40 ............61 Figure4.1Illustrationoftheactivealignmentcontrolalgorithm.........63 Figure4.2Experimentalsetupfortestingtheactivealignmentcontrolalgorithm.65 Figure4.3Measuredrotationangleforthereceiverbaseinresponsetothero- tationangleoftherobotplatform,whenthereceiverisat1meter distancefromthetransmitter......................66 Figure4.4Measuredsignalstrengthswhenthealignmentcontrolisonand respectively,whenthereceiverisatadistanceof1mfromthetrans- mitter...................................67 ix Figure4.5Measuredrotationangleforthereceiverbaseinresponsetothero- tationangleoftherobotplatform,whenthereceiverisat2meter distancefromthetransmitter......................68 Figure4.6Measuredsignalstrengthswhenthealignmentcontrolisonand respectively,whenthereceiverisatadistanceof2mfromthetrans- mitter...................................69 Figure4.7Measuredrotationangleforthereceiverbaseinresponsetothero- tationangleoftherobotplatform,whenthereceiverisat3meter distancefromthetransmitter......................70 Figure4.8Measuredsignalstrengthswhenthealignmentcontrolisonand respectively,whenthereceiverisatadistanceof3mfromthetrans- mitter...................................71 x Chapter1 Introduction Morethan70percentoftheEarth'ssurfaceiscoveredbytheoceansandcontains97percent oftheplanet'swater,yetmorethan95percentofthesubmergedworldstaysunexplored. TheseaandlakesassumeanessentialpartinanumberoftheEarth'sframeworksincluding atmosphereandclimate[15].Itisbroadlyacceptedthatthesubmergedworldholdsthoughts andassetsthatwillfuelagreatpartoftheupandcomingeraofscienceandbusiness.This isbytheconsiderableinvestmentinsupportingtheimprovementofsubmerged observatoriesandunderwaterrobots[16].Oneofthemostimportanttasksthatthesesystems anddevicesneedtobeabletoperformistocommunicatewitheachotherandthesurface inordertomonitorprogressortransmitdatacollectedfromsensors.Theeasiestapproach tocommunicateisthroughaphysicalconnection,forexample,acoppercable.Inspiteof thefactthatthistakesintoconsiderationveandrapidcommunication,acableresults innumerousoperationalconstrainingthereachandmobility. Wirelesscommunicationisaconsiderablymorefeasibleanswer.Applicationsofunder- waterwirelesscommunicationrangefromoilindustrytoaquaculture,andincludepollution instrumentmonitoring,climatechangedatarecording,studyofmarinelife,andsearchand surveymissions.Underwaterwirelesssensornetworks,forexample,areanetworkofsen- sorsorunderwaterobservatoriesthatcollectoceanographicdataoveranintervaloftime andsendthedatatotheoutsideworldperiodicallyforprocessing.Suchsensorsusually transmittamountsofdatawhichrequireahigh-bandwidthlinktotheoutside 1 world.Highbandwidthopticalwirelesslinksareonepossiblesolution,especiallyifcoupled toautonomousunderwatervehicles(AUVs).Nowadaysthereareanumberofunderwater vehiclesforadiversesetofapplications,suchascollectingenvironmentaldata,detecting underwatermines,surveyingunderwaterpipelines,andassistinginharborsecurity.Most AUVsarelimitedinsize,andsincetheyaretypicallybattery-powered,haveveryconstrained powerbudgets.Opticalcommunicationsarepotentiallyverypowert.Inaddition, theuseofhigh-bandwidthopticalsourcesalsohassystem-leveladvantages,byreducingthe amountoftimethatanAUVhastostaynearasensortocollectdata. 1.1TraditionalUnderwaterWirelessCommunication Methods 1.1.1AcousticsCommunication Acousticsignalsarethemostpreferredcarrierinmanyapplications,owingtotheirlow absorptioncharacteristicforunderwatercommunication.Eventhoughthedatatransmission isslowercomparedtoothercarriersignals,thelow-absorptioncharacteristicenablesthe carriertotravelatlongerranges[17]. Nowadays,commerciallyavailableunderwatermodemsareabletotransmitupto30kbps overdistancesrangingfromahundredmeterstoafewkilometers[18].Afewcompaniesare developingacoustics-modemsforunderwatercommunication.Table1.1listssomeexamples ofcommercialacousticmodems.FromTable1.1,itcanbeseenthatingeneral,whenthe distanceincreases,thedataratedecreases,whiletheconsumedpowerincreases. Underwateracousticcommunicationsaregenerallybywatertemperature,pres- 2 Table1.1Comparisonofcommercialacousticmodems[1]. Company Model BitRate (bps) Distance (km) Transmitter Power(W) Receiver Power(W) EvoLogics S2CR48/78 31,200 1 18 1.1 LinkQuest UWM2000 17,800 1.5 2 0.8 Teledyne Benthos ATM9XX(PSK) 2,400 6 20 0.7 DesertStar Systems SAM-1 154 1 32 0.168 sure,salinityofthewater,pathloss,andvariablepropagationdelay.Allthesefactors determinethetemporalandspatialvariabilityoftheacousticchannel,andmaketheavail- ablebandwidthoftheunderwateracousticchannellimitedanddramaticallydependenton bothcommunicationrangeandfrequency,whichleadtolowbitrates[18]. 1.1.2ElectromagneticWaves Electromagneticwavesareaformofradiantenergyreleasedbyelectromagneticprocesses. Visiblelightisonetypeofelectromagneticradiation,andotherfamiliarformsareinvisible electromagneticradiationssuchasX-raysandradiowaves.Thesewavestravelatthespeed oflight,whichismuchfasterthanthesoundspeedinwater. Electromagneticwavesarehighlyattenuatedinwater,whichresultsinshorttraveldis- tanceforthesignalbecauseofabsorptionandscattering.Figure1.1showstheattenuation cotsofntwavelengths.Itisclearthatattenuationisminimuminthewave lengthbetween400-500 m.Theexactwavelengththatpenetratesthefurthestthrough seawaterdependsonthecharacteristicsofthespwater,sinceabsorptionandscatter- ingarebythechemicalandbiologicalmakeupofthewater,butingeneralwave lengthsinthe470nmrangeareattenuatedtheleast[3]. 3 Figure1.1Attenuationcoientsofwaterandseawater[3]. 1.2UnderwaterOpticalCommunication Underwateropticalcommunicationsystemshavegrownmorepopularinrecentyearswith thecreationofreliable,low-costlightsources,likelight-emittingdiodes(LEDs)andlasers. ItwasinuseforcommunicationsthroughairasreportedinSmythetal.[19].Chenetal. useda532nmyttriumaluminumgarnet(YAG)lasertogenerate10nswide,200mJpulses [20].Thiscorrespondedtoa2MWlaserandasaresulttheycouldonlygeneratepulses at50Hz.Theysuccessfullytransmiteddatathrougha50mlongwatertank.In[21],Cox useda405nmlaserfortransmissionandaminiatureHamamatsuR7400Uphotomultiplierin conjunctionwithanAD8015variablegain(VGA)onthereceivingside.Hissystem wastestedusinga3.66mwatertankwithreliablecommunicationat500Kbps.Hansonet al.demonstratedalaser-basedsystemcapableofdataratesof1Gbpsovera2mpathin atankwith36dBextinction(absorbentswereaddedtothewater)[22].Theperformance 4 ofallthesesystemsareimpressive,butitisbasedonabulky,fragile,andpowt lasersetupthatisnotpracticalinthecontextofasmallunderwaterrobot.TheuseofLEDs allowsforsmallerandcheaperdevices,increaseeye-safety,andreducesaimingrequirements. Hagemetal.implementedalow-costandshort-rangeunidirectionalopticalmodem[23]. Theyusefrequencyshiftkeying(FSK)atadatarateof2.4Kbpswithmodulationfrequencies of10and12KHz.Theirimplementationconsistedofa520nm,1.5mWLEDwitha70 degreeeldofviewandanopticaldetectorwithasensitivityof0.3A/W.Theyachieved error-freecommunicationatdistancesupto1.1munderwaterwithoutbubblesandupto 0.7mwithbubbles.Luetal.usedanRL5-G13008Super-GreenLED,withawavelengthof 520nmandamaximumpowerof120mW,andaSilonexSLD-BG2Aphotodiodetogether withaband-gaptorejectinfraredwavelengths[24].Thetransmitterhadaof viewof90degrees,andthereceiverhadaofviewof120degrees.Theytestedtheir systeminapoolatnighttoavoidambientlight.Thesystemdetected80percentofthe transmittedpacketsatacommunicationdistanceof10m.Over95percentofthepackets weredetectedat6mandbelow.Brundageimplementedaunidirectionalcommunication systemusinga465nm,4.8WTitanbluelightingengine(NT-52B1-0469),andaPC10- 6BphotodiodemanufacturedbyPSiliconSensor[2].Thelightsourcehadaof viewof22degree,andthesystemneededtobeaimed.Thesystemachievederror-free communicationat1Mbpsatdistancesupto13m.Schilletal.designedasmalloptical communicationtransceiverforaswarmofsubmersiblerobots[25].TheyusedanIrDA physicallayerbasedontheMCP2120IrDAencoder/decoderchip.Theirreceiveruseda SLD-70BG2AphotodiodetogetherwithaMAX3120forationandThey examinedthreetLuxeonIIILEDsfortransmission:blue(460nm,733mW),cyan (490nm,560mW),andgreen(520nm,165mW).Theyusedabyte-orientedUARTserial 5 interfaceat57600bps.Theytestedtheirsysteminairandachievedreliablecommunication at1.71mfor460nm,2.02mfor490nm,and1.49mfor520nm.Allthreewavelengths establishalinkwhentestedinapoolatrangesupto1.7m.Doniecetal.designeda bidirectionalunderwaterwirelesscommunicationsystemcalledAquaOpticalII[26],which useed18LuxeonRebelLEDsandanavalanchephotodiode(APD).TheLEDstransmitted atawavelengthof470nmwithupto1.126Wofradianteach,resultinginatotal transmitpowerof20W.Thesystemalsoutilizedanavalanchephoto-diodebecauseofits increaseddynamicrangeandquantumency.Itoperatesoverdistancesof50mata datarateof4Mbps. Anguitaetal.implementedatransmitterthatused12LEDsarrangedonacircleto transmitomnidirectionallyintheplane[27],[28].Intwotimplementationstheyused tLEDs.ThewasaLedmanLL1503PLBL1-301withopticalpower15mWand a30-degreeofview.ThesecondisanEvergreenElectronicDLE-038-045withoptical power20mWanda30degreeofview.Theyproposedan18LEDomnidirectional transmitterdesignintheshapeofanoctahedron.TheirreceiverconsistedofaHamamatsu S5971APDwithanareaof1mm 2 andasensitivityof0.2A/Wat470nm.Theoutput signalwasfedthrougha10KHzto20MHzbandpassanwithautomatic gaincontrol(AGC)basedonLT1006,andacomparatortodigitizetheincomingsignalusing thresholding.Theytestedtheirsystemwithasquarewaveatdistancesofupto2mand reportedthevoltageswingaftertheAGC.TheyshowedthattheirAGCallowedforpulse detectionfromdistancesofafewcmto10minair. InTable1.2acomparisonofstate-of-the-artunderwateropticalcommunicationsystems ispresented.Theinformationprovidedintpaperswasnotstandardized.Therefore, wecomparethesesystemsonthemaindeviceelements(lightsourceandlightdetector), 6 rangeofcommunication,datarate,andwhethertheyrequireline-of-sight.FromTable 1.2,itcanbeseenthatthedataratespeedishigherwhenavalanchephotodiodesorpho- tomultipliersinsteadofphotodiodesareused,whichisaccompaniedwithhighercostand powerconsumption.Asimilarobservationappliestothelightsource;forexample,alonger communicationrangetypicallyrequireshigherpowerconsumption. Table1.2Comparisonofunderwateropticalcommunicationsystems. Lightsource Lightdetector Range (m) Baudrate (bps) Lineofsight Hansonetal.[22] Laserdiode Avalanche photodiode 2 1G Required Cox[21] Laser Photomultiplier 3.66 500K Required Brundage[2] High-power LED Photodiode 13 1M Required Doniecetal.[26] 18High-power LEDs Avalanche photodiode 50 4M Required Anguitaetal.[27] 12LEDs arrangedon acircle Avalanche photodiode 2 100K Notrequired Rustetal.[29] High-powerLED 8Photodiodes 23 112.5K Notrequired 1.2.1OverviewofContributions Thecontributionsofthisresearchrincludethedevelopmentandcharacterizationofalow- power,e,highdata-rateopticalcommunicationsystemwithactivealignment maintenance.Thealignmentmaintenancecapabilitygreatlyalleviatestherequirementof line-of-sightforcommunicationandfacilitatestheuseofthedevelopedsystemformobile robots.Amoredetailedaccountofthecontributionsfollows. First,onthehardwareside,wedescribethedesign,development,andcharacterizationof anopticalcommunicationsystem,whichallowsforrelativelyhighdatarate(115200Kbps) underwatercommunicationover23meters. 7 Second,wepresentasignalstrengthmodelforthedevelopedopticalcommunication system.Suchamodelwillbehelpfulindevelopingtheestimationandcontrolalgorithmsin alignmentmaintenanceaswellasinguidingtheoptimaldesignofthesystem. Finally,wepresentanactivecontrolalgorithmformaintainingthealignmentofthe receiverwiththetransmitterforLED-basedcommunicationsystems.Thealgorithmuses thereceivedsignalstrengthasfeedbackforadjustingthereceiverangles.Wedemonstrate theenessofthisalgorithmviaexperimentsinvolvingamobilerobot. 8 Chapter2 SystemDesign,Implementation,and Characterization Theunderwateropticalcommunicationsystemmainlyconsistsoftwoparts,thetransmitter andthereceiver.Thetransmitterconvertstheelectricalsignalintoanopticalsignal.That signalpassesthroughthemediumandispickedupbythereceiver.Thereceiverdetectsthe opticalsignal,andconvertsitbackintoanelectricalsignalfordataprocessing.Therearea varietyoflightsourcesanddetectorstoconsiderforthissystem.Lasersandphotomultiplier tubeshighperformance,andareexpectedtobeusedinmanyunderwateroptical communicationsystems.However,thesecomponentsarerelativelyexpensiveandcanhave largeformfactors.AnalternativesolutionwouldbetousethemuchcheaperLEDsas transmittersandphotodiodesasreceivers.Inadditiontothetransmitterandthereceiver, thesystemalsoincludesamechanismforrotatingthetransmitter/receiver,tomaintain communicationdespitethemovementunderlyingplatform(forexample,arobot).Inthis chapterwediscussthedetailsonthehardwaredesign,andpresentsomecharacterization resultsonthesystem'sunderwatercommunicationperformance. 9 2.1DesignConstraints Themainmotivationforthisthesisisthatitcouldprovideafeasiblehigh-speedcommuni- cationbetweenunderwaterrobotsthatcollaborativelyperformasptask.Inorderto dothat,wewillneedtoconsidersomedesignconstrains.Inparticular,thesystemneedsto havesmallfootprintandconsumelowpowerforintegrationintounderwaterrobots.Italso needstoworkwithoutperfectalignmentbetweenthetransmitterandthereceiver,sinceit willbeforarobottoholdthepositionandtheorientation,becauseofboththena- tureoftheunderwaterenvironmentandthemobilitynatureofarobot.Itisalsopreferable thatthesystemwillrequirelittletonomaintenanceforlongperiodsoftime.Finally,itis desiredtohaverelativelylongcommunicationrangeandrelativelyhighdatarate. 2.2Transmitter Theroleoftheopticaltransmitteristoconverttheelectricalsignalintolightpulses,and launchtheresultingopticalsignalintothetransmissionchannel.Itconsistsoftheinput signal,theopticaldriversystem,thelightsource,andlight-beamconditioningoptics,such asandlenses. 2.2.1LightSource Thestepwhendesigningthetransmitteristodecidewhichlightsourcetouse,since therestofthetransmitterisdesignedtosupportthechosensource.Theoreticallyspeaking, anylightsourcecouldbeused,fromanincandescentlamptoalaser,butthesize,powerand switchingspeedconstraintsplacedonthesystemnarrowdowntheselectionstotwofeasible choices:lightemittingdiodes(LEDs)andlaserdiodes(LDs). 10 AnLED,examplesofwhichareshowninFigure2.1,isasemiconductordevicethat emitsvisiblelightwhenanelectriccurrentpassesthroughit.Thelightisnotparticularly bright,butinmostLEDsitismonochromatic,occurringatasinglewavelength.Theoutput fromanLEDcanrangefromred(atawavelengthofapproximately700nm)toblue-violet (about400nm).SomeLEDsemitinfrared(IR)(830nmorlonger);suchadeviceisknown asaninfrared-emittingdiode(IRED)[30].AnLEDconsistsoftwoelementsofprocessed material,calledP-typesemiconductorandN-typesemiconductor.Thesetwoelementsare placedindirectcontact,formingaregioncalledtheP-Njunction.Inthisrespect,theLED resemblesmostothertypesofdiodes,butthereareimportantAnLEDhasa transparentpackage,allowingvisibleorIRenergytopassthrough[31]. Figure2.1CommonexampleofLEDs[4]. ofLEDsinclude:lowpowerrequirementsincemosttypescanoperatedwith batterypowersupplies,andgoodesincemostofthepowersuppliedtoanLED isconvertedintoradiationinthedesiredform,withminimalheatloss.Whenproperly installed,anLEDcanfunctionfordecades.Becauseoftheirsimplicityandreliability,LEDs arerelativelycheapandwidelyavailable[30]. Laserdiodes(LDs)arealaterinnovationwhichhasevolvedfromfundamentalLED 11 manufacturingmethods[32].LDs(seeFigure2.2foranexample)stillrelyonthetran- sitionofcarriersoverthebandgaptocreateradiantphotons;however,alterationstothe devicestructurepermitsuchdevicestoelydelivercoherentlightoveranarrowopti- calbandwidth[30].Laserdiodesformasubsetofthelargerofsemiconductor p-njunctiondiodes.Forwardelectricalbiasacrossthelaserdiodecausesthetwospeciesof chargecarrier,holesandelectrons,tobeinjectedfromoppositesidesofthep-njunction intothedepletionregion.Holesareinjectedfromthep-dopedregion,andelectronsfrom then-dopedregion.Duetotheuseofchargeinjectioninpoweringmostdiodelasers,this classoflasersissometimestermed\injectionlasers"or\injectionlaserdiode"(ILD). Figure2.2AnexampleofLaserDiode[5]. Asdiodelasersaresemiconductordevices,theymayalsobeassemiconductor lasers.Eitherdesignationdistinguishesdiodelasersfromsolid-statelasers[33].Stimulated emissiononlyhappensafterathresholdcurrenthasbeenreached.Ithasaverysmall recombinationtimeconstant,whichmeansthatlaserdiodeshaveamuchfasterrisetime, allowingformodulationbandwidthsintheGHzrangeandhighopticalpoweremissionsina verynarrowopticalspectrumwidth,around1nm[30].Unfortunately,stimulatedemission ischallengingtomaintain.Laserdiodesareverysensitivetochangesintemperatureand current-thewavelengthofthelaserdiodecanchangebyabout0.1nm/ [34]andthe andthresholdcurrentcanchangesigtlyduetoagingandtemperature[20]. 12 Thermal,opticalandelectricalfeedbackisnecessarytosustainoperationandpreventthe laserdiodefromfailing.Thisaddscomplexityandcosttothesystem.Thesensitivityof laserdiodesresultsinshorterlifetimesandlowerreliability.Table2.1presentsacomparison ofthefeaturesofLDsandLEDsforwirelessopticalapplicationsbasedon[30],[32]. Table2.1LEDvs.laserdiodeforwirelessopticalcommunication. Characteristics LED LaserDiode Opticalspectralwidth 25-100nm 0.1to5nm Modulationbandwidth TensofkHztohunderedsofMHz TensofkHztotensofGHz Specialcircuitrequired None Thresholdandtemperature compensation circuitry Eyesafety Consideredeye-safe Mustberenderedeye-safe Reliability High Moderate Cost Low Moderatetohigh FrompreviousdiscussionsitisclearthatbothLEDsandlaserdiodeshavestrengthsand weaknesseswhenitcomestowirelessopticalcommunications.Laserdiodescanswitchfaster andhavehigheropticalpoweroutput,butLEDsystemsarecheaper,simpler,andmorereli- able.Whilethecoherenceandminimaldivergenceoflaserdiodesareoptimalforeroptic communicationsystems,theydonotplayasbigaroleinwirelessopticalcommunication. Additionally,althoughlaserdiodescanswitchfaster,LEDscanswitchfastenoughformany applications.Sincethegoalistohaveasmall,cheap,reliablecommunicationsystem,LEDs areselectedasthelightsourcefortheopticaltransmitterinthiswork. 13 2.2.2LEDSelection InordertochoosetherightLED,weneedtocheckthecommerciallyavailablehighbrightness blueLEDs.Sinceweneedtoachieverelativelylongrangeofcommunication(over20m) withlowpowerconsumption,themaximumamountofopticalpowerinthe470nmrange isneeded.Thiscanbeachievedbychoosingsuper-brightLEDsorbyusingmultipleLEDs. tLEDmanufacturerswereresearchedtothebrightestblueLEDonthemarket. Table2.2showsacomparisonofvarioushigh-powerLEDsonthemarket(alldatatakenfrom theirrespectivedatasheets).Itisclearthathigh-powerLEDscangivealuminous of30-60lumens.Thisiscomparedtothelessthan1lumenthatstandardLEDsproduce. WhileothercoloredLEDs,suchasAmber,havebeenaroundforalongtimeandareable toproduceupwardsof130lumensperLED,blueLEDtechnologyisnotasadvancedand thereforehasalowerlumenoutput. Table2.2ComparisonofcommerciallyavailablehighbrightnessblueLEDs[2]. Manufacturer Model WaveLength(nm) LuminousFlux(Im) Atlas NT-42C1-0484 460-470 63 AOP PU-5WAS 455-475 54 Lumileds LXML-PBO1-0023 460-490 48 Kingbright AAD1-9090QB11ZC/3 460 35.7 Ligitek LGLB-313E 460-475 30.6 Toshiba TL12B01(T30) 460 6 Lumex SML-LX1610USBC 470 5 Cree XREBLU-L1 465-485 30.6 Typical(nothigh-power) LED 470 < 1 Theelectricpowerrequirementforsuper-brightLEDsisgenerally1-5Wattsanddraws 700-1000mAcurrent,whichmeansthattheyrequireaforwardvoltageof2-5volts.In 14 ordertomaximizelightoutput,moreLEDsaredesirable,butthismustbebalancedwith thesystempowerlimitations.Usually,anunderwaterrobotthatwillbeequippedwiththe transmitterwillcarrya12-24voltbattery,soitmakessensetolimitthenumberofLEDs. Additionally,asmoreLEDsareadded,opticalandthermalpropertiesmustbetakeninto account.SomeoftheelectricityinanLEDisconvertstoheatratherthanlight.Ifthis heatisnotremoved,theLEDsrunathighertemperatures,whichnotonlylowerstheir ,butalsomakestheLEDslessreliable.Therefore,thermalmanagementofhigh powerLEDsisveryimportant.Itisnecessarytokeepthejunctiontemperaturebelow120 CtoruntheLED'sformaximumlifetime[35].EventhoughLEDsarenotassensitiveto hightemperaturesaslaserdiodesanddonotrequirethermalmonitoring,high-powerLEDs muststillbefullythermallycoupledtoheatsinks,whicharepassiveheatexchangersthat cooladevicebydissipatingheatintothesurroundingmedium. Figure2.3CreeXR-ELEDattachedtoaheatsink[6]. Forsimplicityofthesystemandtoensureproperoperation,itisdecidedtolookfor 15 anLED.Designingspecializedopticsandheatsinksisnotinthescopeofthis study.AlthoughmanyLEDmanufacturersmakesingleLEDchips,veryfewintegratethem withheatsinksandlenses.Oneofthesecompanies,SuperBrightLEDs,doesjustthat,and hasverygoodbrightblueLEDsforthepurposeofthisproject.TheyuseCreeXR-ESeries LEDblueLightingSystem(seeFigure2.3)fromCreeCompany,whichprovides30.6lumens at1Aandrequire3.3volts[7].Itcomesassembledwithaheatsink. 2.2.3LEDDriver InordertomodulatetheLED(turnitonand)incorrelationwithbinarydata,acircuit mustbedesignedtopowerit.ThecircuitneedstoprovidetcurrenttolighttheLED attherequiredintensity,butmustlimitthecurrenttopreventdamagingtheLED.LEDs arecurrent-controlleddevices(Figure2.4),whichmeansthattheintensityoftheoutput lightislinearlyrelatedtothesupplycurrent.AndsincethevoltagedropacrossanLEDis approximatelyconstantoverawiderangeofoperatingcurrent,asmallincreaseinapplied voltagegreatlyincreasesthecurrent(Figure2.5)[7]. Commerciallyavailableconstant-currentLEDdriversareavailable.Oneexampleisthe BuckPuckLEDPowerModules,whichisacurrent-regulateddriverforpoweringLEDs.It isusedforpoweringalltypesofhigh-brightnessandhigh-powerLEDpackagesandLED arrays.Italsoexhibitshighandrequiresnoexternalcurrent-limitingresistors oradditionalheatsinkingforoperation.ItcandimconnectedLEDsusingtheinternal5 VDCvoltagereferencebyvaryingthecontrolvoltage[36].Unfortunately,itisdesignedfor lowerspeedswitchingapplications,suchasLEDdimming,andtherefore,itisnotoptimized forthehigh-speedsopticalcommunicationrequirementsinthisproject.Anotherwayof controllingLEDsistoprovideaconstantvoltageandlimitthecurrentpassingthroughthe 16 Figure2.4Relativevs.currentrelationforCreeXR-ELED(blackcurveforbluelight LED)[7] Figure2.5ElectricalCharacteristicsforCreeXR-ELED[7]. 17 LEDsusingresistors.Inthissituation,anelectronicallycontrolledswitchlikeatransistor canbeusedtostartandstopthewofcurrentthroughtheLEDs,turningthemon andAMOSFET(metaloxidesemiconductortransistor)isusedintheLED drivercircuit.MOSFETsareatypeoftransistorsusedforamplifyingorswitchingelectronic signals.AMOSFETconsistsofadrain,agate,andasource.Whenthereisnovoltage betweenthegateandthesource,thereisaveryhighresistancebetweenthedrain andthesourcesominimalcurrentwsfromthedraintothesource-theMOSFETis '.If,however,thegateissuppliedwithavoltage,thedrain-sourcechannelbecomesless resistiveandtheMOSFETis`on'[37].Oncethegatethresholdvoltagehasbeenreached, theMOSFETisfullyonandtheonlyresistancebetweenthedrainandthesourceisthe drain-to-source-onresistance. TheMOSFETisbyfarthemostcommontransistorinbothdigitalandanalogcircuits, althoughthebipolarjunctiontransistorwasatonetimemuchmorecommon.Themain advantageofaMOSFETtransistoroveraregulartransistoristhatitrequiresverylittle currenttobeturnedon(lessthan1mA),whiledeliveringamuchhighercurrenttoaload (10to50Aormore).However,theMOSFETrequiresahighergatevoltage(3-4V)tobe turnedon.MOSFETsarebettersuitedforthisapplicationcomparedtobipolartransistors becausetheyhaveamuchlowerdrain-to-source-onresistance[37].Thismeansthatthey willproduceasmallervoltagedropandcanhandlelargercurrents.Additionally,MOSFETs arebetterthanjunctiontransistors(JFETs)becausetheyhavemuchlargergate- inputimpedance.Thismeansthattheydrawverylittlegatecurrent.Thisisadvantageous sincethecomputerortheintegratedcircuit(IC)sendingthedatasignaltotheMOSFET typicallycannotsourcealotofcurrent.Ifthegateimpedancewerelower,morecurrent wouldbeneededtochangethegatevoltage.Eventhoughthegateimpedanceishigh,there 18 isalsoagatecapacitance.Thelargerthegatecapacitance,themorecurrentthatisneeded tochargethegatebeforethevoltagewillchange. MOSFETsareavailableintwobasicforms:NchannelandPchannel(Figure2.6).For anNchannelMOSFET,thesourceisconnectedtoground.Wecaneasilyraisethevoltage onthegate,toenablethecurrenttow.Ifnocurrentistow,thegatepinshouldbe grounded.ForaPchannelMOSFET,thesourceisconnectedtothepowerrail.Inorderto allowthecurrenttow,thegateneedstobepulledtoground.Tostopthecurrentw, thegateneedstobepulledtopowerrail[37]. Figure2.6MOSFETSchematicSymbols[8]. TherearettypesofLEDdrivingcircuits.Inthisdesignatotempole circuitisadopted.Suchacoismainlyusedtoamplifysquarewavesintostronger squarewavesfordrivingothertypesofsemiconductorsfaster.Atotempoleconsistsofa PchannelandanNchannelMOSFETarrangeddraintodrain,withtheirgatescoupled together.Theoutputsignalistakenfromthepointwherethetwodrainsmeet. TherearemanycommerciallyavailableMOSFETsinapush-pullion.For thisapplication,aMOSFETthatcouldquicklyswitchmorethan5Vat2Aisneeded. Thismeansthatithastohaveadrain-to-sourcebreakdownvoltageofatleast10Vand 19 aminimalriseandfalltime.Additionally,ithastobeabletooperatefromTTLlogic levels,whichmeansthatthegatethresholdvoltagehastobebelow5V.Aftercomparing variousMOSFETs,weselectIRF7307PbF.Withadrain-to-sourcebreakdownvoltageof20 Vandamaximumdraincurrentof5A,itcanadequatelyhandlethepowerrequirements oftheLEDs.Additionally,ithasaveryshortrisetimeof42ns[38].Figure2.7showsthe transmittercircuitschematic. Figure2.7Transmittercircuitschematic. TheTTLdatasignal(0-5V)goesintothegatesofthetwoMOSFETs,whichwhenon, allowthecurrenttowthroughtheLEDsandcurrent-limitingresistors,whicharepowered bythe5Vpowerrail.Aresistorof2limitsthecurrentthatwsthroughtheLEDsto 1A. 20 2.3Receiver Thedetectorisadevicethatconvertstheopticalsignalreceivedfromthetransmitterintoan electricalsignal.Likethetransmitter,thedetectorofthereceiverwilldeterminesthedesign oftherestofthesystem,whichwillconditionthedetectoroutputintoausableelectrical form.Accordingly,thedetectormustbeselectedbeforetherestofthereceiverelements. 2.3.1LightDetector Lightsensingapplicationsvarywidelyfromspecializedscieninstrumentationthatneeds todetectindividuallight-photonstosystemsthatcontrolhigh-speedweldingandcutting lasersthatproducekilowattsofopticalpower.Fortunately,therearesensorsforalmost anyapplicationimaginable:fromaphotomultipliertubewhichgivesalargevoltagepulsefor everyphotonitdetects[37],tocooledthermo-pilesthatabsorbkilowattsofpowerproviding athermocouplevoltageproportionaltotheopticalpowerabsorbed[30].Inourapplication, adetectorneedstoquicklyrespondtoallincidentphotonssentbythetransmitterwithout introducingadditionalnoise.Additionally,itneedstobesmall,robust,cheap,andpower- t.Therearemanytypesofphotodetectorswhichmaybeappropriateinaparticular case. Theoneisthephotoresistor(seeFigure2.8foranexample),alsoknownaslight- dependentresistors(LDR).AnLDRisatypeofphotoconductor,meaningthatitsconduc- tivitychangeswhenexposedtoelectromagneticradiationsuchasvisiblelight[37].AnLDR isusuallyusedtoindicatethepresenceorabsenceoflight,ortomeasurethelightintensity. Inthedark,itsresistanceisveryhigh,sometimesupto1Mohms,butwhentheLDRsensor isexposedtolight,theresistancedropsdramatically,evendowntoafewohms,depending 21 onthelightintensity.LDRshaveasensitivitythatvarieswiththewavelengthofthelight appliedandarenonlineardevices.Duetotheirslowresponse[30],usuallyittakestimein millisecondsforaphoto-resistortofullyrespondtothepresenceoflight,whichmakesitnot feasibleforhigh-speedopticalcommunication. Figure2.8Anexampleofphotoresistor[9]. Anothertypeofphotodetectoristhephototransistor(seeanexampleinFigure2.9), whichisadevicethatconvertslightenergyintoelectricenergy.Theyproducebothcurrent andvoltage.Thisisbecauseaphototransistorismadeofabipolarsemiconductorand focusestheenergythatispassedthroughit.Phototransistorsaregenerallyencasedin aclearcontainerinordertoenhancelightasittravelsthrough[39].Aphototransistor generallyhasanexposedbasethatthelightthatitcomesincontactwith.This causesarelativelyhighcurrenttopassthroughthephototransistor.Asthecurrentspreads fromthebasetotheemitter,thecurrentisconcentratedandconvertedintoavoltage[30]. Phototransistorsaremoderatelyfastandarerelativelyrobustwithregardtonoise. Photodiodesarewidelyuseddetectorsforhigh-speedapplications.SeeFigure2.10for anexample.Therearettypesofphotodiodesandtheyworkinslightlyt ways,butthebasicoperationsremainthesameforallofthem[39].PINphotodiodeisa 22 Figure2.9Anexampleofaphototransistor[10]. photodiodewithanintrinsic(undoped)regioninbetweenthen-andp-dopedregions,and itisasemiconductordevicethatconvertslightintocurrent.Thecurrentisgeneratedwhen photonsareabsorbedinthephotodiode.Whenaphotonoftenergystrikesthediode, itcreatesanelectron-holepair.Thismechanismisalsoknownastheinnerphotoelectric Iftheabsorptionoccursinthejunction'sdepletionregion(aninsulatingregionwithin aconductiveregion),oronelengthawayfromit,thesecarriersaresweptfromthe junctionbythebuilt-inelectricofthedepletionregion.Thusholesmovetowardthe anode,andelectronstowardthecathode,andaphotocurrentisproduced.Thetotalcurrent throughthephotodiodeisthesumofthedarkcurrent(currentthatexistsintheabsence oflight)andthephotocurrent,sothedarkcurrentmustbeminimizedtomaximizethe sensitivityofthedevice[14]. PINphotodiodesdonothaveanyinternalgain,sinceeachphotoncanonlyexciteone electronatmost,butthephotocurrentislinearlyproportionaltotheilluminance.Addi- tionally,sincephotonscangothroughadiodewithoutexcitinganyelectrons,photodiodes donothavethesensitivity(quantumofphotomultipliertubes,whichcandetect singlephotons.Photodiodescanhavearesponsetimeinnanosecondsandarerelatively robusttonoise[30].Theyaresmall,robust,andrdable. 23 Figure2.10Anexampleofaphotodiode[11]. Anavalanchephotodiode(APD)isahighlysensitivesemiconductordevicethatuses thephotoelectrictoconvertlighttoelectricity.SeeFigure2.11foranexample.Itis similartoPINphotodiodes,exceptthattheycangeneratemultipleelectron-holepairsasa resultofabsorbingasinglephotonbytheavalancheprocessthroughapplyingahighreverse biasvoltage(typically100-200Vinsilicon)[14].Asaresult,theavalanchephotodiodeis farmoresensitive.However,itisfoundthatitisnotnearlyaslinear,andadditionallythe avalancheprocessmeansthattheresultantsignalisfarnoisierthanonefromaPINdiode [37]. Figure2.11Anexampleofanavalanchphotodiode[12]. 24 Themostsensitivelightdetectoristhephotomultiplier(seeFigure2.12foranexample), whichisopticallyconnectedtothesodiumiodidecrystal.Itspurposeistoconvertthelight energyfromthecrystaltoelectricalenergyandamplifytheresultantpulseofelectricity. Photomultipliersaretypicallyconstructedwithanevacuatedglasshousing,containinga photocathode,severaldynodes,andananode.Incidentphotonsstrikethephotocathode material,whichisusuallyathinvapor-depositedconductinglayerontheinsideoftheentry windowofthedevice[30].Theycanhavearesponsetimeoflessthanananosecondbut theyarealsoexpensiveandfragile,comeinbigsize,andconsumelargeamountofpower [14],whichmakesthemabadchoiceforapplicationofunderwateropticalcommunication. Figure2.12Anexampleofaphotomultiplier[13]. 2.3.2OpticalSensorSelection Eachopticalsensorhasadvantagesanddisadvantagesasdescribedintheprevioussubsec- tions,andtheselectionprocesswilldependonmanyfactors(Table2.3).Forourapplication, asensorthatisfast,small,cheapandlow-powerisrequired,thereforitisclearthatpho- 25 todiodestherequirementsnecessaryforwirelessunderwateropticalcommunications.A PINphotodiodeischosenovertheavalanchephotodiodebecauseofthehighbiasvoltage requiredtooperateavalanchephotodiodes,aswellastheirsensitivitytonoiseandtheir highercomplexityofcontrolcircuitry. Table2.3Comparisonoflightdetectors[19],[24],[26]. Size Response Time Photocurrent Gain Linearity Cost Photoresistor Small Slow < 1Hz Little Over smallregion Cheap PIN photodiode Small Fast TensofMHzto tensofGHz Unity Exellent Cheap Avalanche photodiode Small Fast HundredsofMHzto tensofGHz 100-100000 NotLinear Expensive Phototransistor Small Moderate < 250KHz 100-1500 Good Cheap Photomultiplyer Large Fastest > 1GHz > 1Million Good Expensive SinceabluelightLEDisused,itmakessensetochooseaphotodiodethatismoresensitive tobluelight.Unfortunatelymostphotodiodesaresensitivetotheredorinfraredspectrum. Butthereareaseriesofphotodiodescalledblue-enhanced,whichhavetsensitivity tobluelight.Therearemanymanufacturersofphotodiodes,butfewproduceblue-enhanced photodiodes.AdvancedPhotonixphotodiodeswithhighquantumat410nm, lowdarkcurrent,andfastrisetime.TheproductwithpartnumberPDB-V107(Figure2.13) isselected.Ithasarelativelylargeactivearea(20.6mm 2 ).Abiggeractiveareameans moresensitivityandshorterrisetime(20ns),andthushigherbandwidth.A12Vreverse 26 biasacrossthephotodiodeisusedtoincreasethebandwidthandquantum[14]. Figure2.13Pictureofblue-enhancedphotodetector(PDBV107)[14]. 2.3.3ReceiverCircuitDesign Afterthephotodiodehasbeenselectedasalightdetector,thedesignoftherequiredelec- troniccircuitrytoprocessthephotodiodescurrentsignalisnext.Amaincomponentneeded isatransimpedancetoconvertthephotodiodecurrentsignalintoavoltagesignal, whichthengoesthroughatoreducethenoise.Afterthatitwillbeand madecompatiblewithTTLvoltagelevels. Transimpedancers(TIAs)areusedtoconvertthecurrentoutputofsensorslike aphotodiodetovoltagesignals,becausemanycircuitsandinstrumentscanonlyaccept avoltageinput.Anoperationalwithafeedbackresistorfromtheoutputtothe invertinginputisthemoststraightforwardimplementationofsuchaTIA.However,eventhis simpleTIAcircuitrequirescarefulamongnoisegain,voltage,bandwidth, andstability.ClearlystabilityinaTIAisessentialforgood,reliableperformance. Figure2.14showstheschematicofatransimpedanceThevalueoftheresistor, R f ,determinesthegainofthetransimpedanceThefeedbackcapacitor, C f ,is 27 Figure2.14Schematicofatransimpedanceforamplifyingthephotodiodeoutput. usedtostabilizethecircuitandreduceovershoot.Althoughthereareequationstohelp determinetheidealvalueof C f ,theprecisevaluewillneedtobetunedexperimentallyto accountforthespofeachlayout. Forthisapplication,highspeedandhighgainoftheTIAareimportant.TexasInstru- ments'THS4631op-ampischosenbecauseithasaveryhighgainbandwidthproductof 210MHzandcanrunthe+12/-12Voltagerails[40].Itiswiredwithoutthefeedback capacitorinitially.Ifthefeedbackresistoristoolow,thegainisverysmallandthesystem canhavetoomuchringingandnoise,whereasifthefeedbackresistorischosentobetoo large,thegainistoohigh,saturatingthesignalandreducingthebandwidth.Forthese reasonsafeedbackresistorvalueof100chosenasagoodcompromisebetweenhighgain andhighbandwidth.Afterthefeedbackresistorhasbeendetermined,testingisdoneto determinethebest C f forthecircuit.If C f ischosentobetoolarge,thetimeconstantof thecircuitwillbetoolongandtheswitchingspeedwillbelow.Whenthecircuitisbuilt onaprintedcircuitboard(PCB),thefeedbackcapacitorvalueischosen,sincethereisless straycapacitanceinthePCB. AftertheTIAstage,thesignalgoesthroughanelectronicTheisanana- logcircuitthatperformssignalprocessingfunctions,foranexample,removingunwanted 28 frequenciesfromthesignal,enhancingwantedones,orboth.Inthisapplicationanac- tiveanaloglow-passisusedtoremovetheunwantednoise.TheLinearTechnology LTC1560-1ICisused.Its5th-order,continuous-time,low-passellipticoperateswith 5Vpowersupply,hasapower-savingmode,doesnotrequireanyexternalcomponents,and isavailableinanSO-8surfacemountpackage. Afterthesignalisprocessedwiththelow-passaninvertingvoltageis usedtoconvertthesignalfromanegativevoltagetoapositivevoltageandamplifyitso thatevenverysmallsignals,receivedwhenthetransmitterisfarawayfromthereceiver, canbedetected. Bythesamewaythatweselectthetransimpedancecomponent,thevoltage mustbeanop-ampthatiscapableofhighgainsathighspeeds.Theresponsetime oftheop-ampmustalsobetakenintoaccount.ThechoiceistouseanAnalogDevicesop-42 IC.Itisahigh-speed,fast-settling(1msresponsetime),precisionoperationand itcomesinanSO-8surfacemountpackage. ThestageinthereceivercircuitistoconvertthedatasignalintoTTLcompatible levels(0and5Volt).Sincethepreviousstepisanrunninga 10Voltpowerrail, thesignalcomingfromthecanbeanythingbetween0and+10Volts,depending ontheamplitudeofthesignalgoingintothefromtheTIA.Acomparatortakes twosignals,comparesthem,andoutputsadigitalsignalindicatingwhichislarger.Ifthe inputsignalislargerthanthereferencesignal,thecomparatoroutputgoeshigh,andvice versa.Thehighandlowoutputvoltagesofthecomparatoraresetbythecircuitdesign,and inthiscase,a`high'outputof+5Voltsanda`low'outputof0Voltsaredesired. Anop-ampisusedasacomparator.Thereferencevoltageisconnectedtotheinverting inputandthesignalisconnectedtonon-invertinginput.TheproductAD790isselectedas 29 thecomparatorbecauseithasashortdelaytime(45ns),canoperatethealreadyexisting +12Voltagerail,andcanhandleinputsignalsupto+12Volts.Apotentiometerisusedto createavoltagedividertosupplythereferencevoltage.Thisallowsthereferencevoltageto beeasilytunedduringtesting.Alowerreferencevoltagemeansthatanysmallsignalcould triggerthecomparator,possiblymakingthecircuitreacttonoise,butalsoallowingitto respondtoverylowinputsignals. Oncethetestshavebeenconducted,acircuitlayoutisdetermined.Figure 2.15showstheschematicofthereceivercircuit.Theprintedcircuitboards(PCBs)and theboardsassembledwithcomponentsforthetransmitterandthereceiverareshownin Figures2.16and2.17,respectively.Thetransmitterandthereceivercircuitsaredivided intothreeandfourmodules,respectively.Forthetransmitter,themodulesholdsthe LEDandthehigh-powerresistor,thesecondmodulecontainstheLEDdrivercircuit,and thethirdmoduleholdsthevoltageregulator.Forthereceiver,themodulecontains thetransimpedancecircuitwiththephotodiode,thesecondoneisthelowpass ,thethirdisthestageforthesignal,andthelastmoduleisthe comparatorcircuit. Eachmoduleisoneinchindiametertomakethedevicefootprintassmallaspossible. Thesemodulesareconnectedwitheeachotherbyusingheaderpins,whichwillserveas electricalandmechanicalconnections.Thiswillalsoallowustochangeparts ofthecircuitsformaintenanceorupgrade.Thefullyassembledtransmitterandreceiverare showninFigure2.18. 30 Figure2.15Schematicofthereceivercircuit.Thevaluesofthecircuitcomponents are:RF=100CF=0.5pF,RF1=RF2=RF3=1andRF4=10 31 Figure2.16Modulesofthetransmittercircuit. Figure2.17Modulesofthereceivercircuit. 32 Figure2.18Theassembledreceivercircuitandtransmittercircuit,withallmodulesvertically stacked. 33 2.4RotatingBasefortheTransmitter/Receiver Sincetheopticalcommunicationsystemisdesignedforuseinrobotsandrequireslineof sight,itiscriticalthattheorientationofthetransmitterandreceivercanbeadjusted,to maintaincommunicationdespitethemovementoftheunderlyingroboticplatform.Forthis purpose,arotatingbaseforthetransmitter/receiverhasbeendesignedanddeveloped. 2.4.1UpdatedPrintedCircuitBoardandSlipRing Inordertominimizesystemcomplexityandfootprint,anewPCBwasdesigned(seeFigure 2.19)toholdthecomponentsofthereceiverandtransmittercircuitsinoneplace.Itcontains twomainbards. Figure2.19PCBdesigntoachievemorecompactintegrationoftransmitter/receiver circuits. ThePCBboardis2inchesindiameterandhastwoholesinthemiddletoattachset screwhubsforconnectingtoamotorshaft.ThesecondPCBboardhasarectangularshape withsizeof1inch 2inch,whichholdstheLEDandthephotodiode,anditismounted 34 perpendicularlytothecircularboardbyusingfour90-degreeheaderpins.Theactual boardsassembledwithcomponentsareshowninFigure2.20. Figure2.20nedtransmitter/receivercircuitsassembledtogether. Thereare8pinsinthePCBcircuitsinvolvingthepowersupply,thetransmittedsignal, andthereceivedsignal.Thesepinsareconnectedbywires,whichwouldbetwistedwhen thePCBsarerotated.Toaddressthisproblem,aslipring,anelectromechanicaldevice thatallowsthetransmissionofpowerandelectricalsignalsfromastationarytoarotating structure,isadopted.AMOFLONMT007seriesproduct(seeFigure2.21)ischosen.This devicehascolor-codedleadwiresonboththestatorandtherotorforelectrical connections.Itprovidesa12.7mmthrough-boreforroutingshaftinstallation,andhasa compactouterdiameterof56mm,featuringlonglifeanderbrushcontact[41]. 2.4.2ActuatorandController Torotatethedevice,arotationalactuatorisneeded.Theactuatormustbesmall,fastand easytocontrol.Forthistask,aPrecisionMicroDrivesminiDCmotorisused.Itisa12 mmferritemotorwithaspurgearopengearbox,steelgeartrain,sinteredbronzebearings 35 Figure2.21Theslipringusedinthisproject. andextralong65mmdoublefacetedshaft.Itisequippedwithashaftencoder,which isinstrumentalforfeedbackcontroloftherotationalbaseforthetransmitter/receiver.See Figures2.22and2.23forthemotor/encoder. TocontroltheDCmotorrotationandspeed,anArduinomicrocontrollerconnectedtoa DCmotordrivercircuitisused.Themicrocontrollertakescommandsfromtheunderlying robottorotatetheopticaltransceivertospanglesforcommunication.Allthesecom- ponentsareintegratedtogetherwitha3D-printedframe.Figures2.24and2.25showthe assemblyofthedevicecomponentsandthewater-proofpackage,respectively. 36 Figure2.22TheDCmotormountedona3D-printedframe. 2.5CharacterizationofOpticalCommunicationLink PerformanceUnderwater Inthissectionwepresentresultsfromexperimentsperformedinaswimmingpooltoexamine theperformanceoftheopticalwirelesscommunicationlinkunderwater.Thepoollengthis 23meter.Thetransmitterandthereceiverareencapsulatedin10cm-diameterPVCtubes (withtransparentwindowatfront)towaterproofthemduringtheunderwaterexperiments. EachofthesetubesisthenattachedtoanotherPVCtubethatwillthedepthunderthe waterto25cm.TheassemblyisinthesamepositionwiththehelpofStyrofoamheld byapersonhelp.Withthisthetransmitterandthereceiverareunderwater 37 Figure2.23ThecontrolanddrivercircuitsfortheDCmotor. atthesamedepthandfacingeachother,asillustratedin2.26. ThelightemittedfromanLEDspreadsoverahemisphereshape.Conditioningoptics arerequiredtoredistributetheLEDlightforsprequirements.Lensesareusedfor thatpurposeandcurrently,themostcommonusedLEDlensdesignisthetotalinternal retractor(TIR)lens[42].Inourexperimentsweusecommerciallyavailablelenssp fortheLED.Theselensescomeintviewingangles:(5,15,40,60)degrees.The receiverisalsoequippedwitha5-degreelenswhichwillhelpincreasetheareatoreceive morephotonstoobtainstrongersignal.Foreachsettingandeachdistance,experimentsare performedetimestomitigatetheimpactofexperimentalerrors.Figures2.27-30show thatthesignalstrengthdeclineswhenthedistancebetweenthereceiverandthetransmitter 38 Figure2.24Theassembleddevicewithoutthecover. increases,fortlensviewinganglesforthetransmitter;however,thereisadequate signalstrength(greaterthan1V)throughoutthefulllengthofthepoolforthedesigned transmitter/receiver,forthe5degreecase.Herewehaveusedsquarewavesoffrequency 10kHzforthetransmittedsignal.Figures2.31and2.32showtheoutputsignalafter thestageforthedistanceof8metersforthefrequencyof10kHzand100 kHz,respectively.Fromthesewecanseethatthesignallargelymaintainsthe shapeofasquarewaveatleastuptothefrequencyof100kHz.Suchminimalwaveform distortionisinstrumentalinreducingthebiterrorrateincommunication.Thenaldesign thetransmitterandthereceiverwereequippedwith5degreelenses. Inadditiontothesignalstrengthmeasurements,wehavealsoexaminedthedatatrans- 39 Figure2.25Theassembleddevicewiththecover. missionperformanceofthesystemunderwater.Intheexperiments,weusetwocomputers toemulatetwounderwaterrobotsandweconnectthemtothetransmitterandthereceiver respectively,fromtheUSBportusingtheFT232RUSB-TTLlevelserialconvertercable. ThecableusesanFT232RQchip,housedintheUSBconnector,toconvertUSBdatainto asynchronousserialdataatTTLlevels.Wesendover270,000bytesetimesatadistance of22mandreceivethosebytesonanothercomputer.Wethatthesystemwill transmitandreceiveataspeedof115.2Kbpswithabiterrorrate(BER)ofzero.Figure 2.33showsanexampleofthereceivedsignalwaveform. 40 Figure2.26Schematicofthesetupforswimmingpoolexperiments. Figure2.27Themeasuredsignalstrengthversusthetransmitter-receiverdistanceinswim- mingpoolexperimentsforthecaseof60-degreelensforthetransmitter. 41 Figure2.28Themeasuredsignalstrengthversusthetransmitter-receiverdistanceinswim- mingpoolexperimentsforthecaseof40-degreelensforthetransmitter. 42 Figure2.29Themeasuredsignalstrengthversusthetransmitter-receiverdistanceinswim- mingpoolexperimentsforthecaseof15-degreelensforthetransmitter. 43 Figure2.30Themeasuredsignalstrengthversusthetransmitter-receiverdistanceinswim- mingpoolexperimentsforthecaseof5-degreelensforthetransmitter. 44 Figure2.31Measuredsignalwaveformwith10kHzfrequencyfora5-degreeviewingangle LED,measuredatadistanceof8meters. 45 Figure2.32Measuredsignalwaveformwith100kHzfrequencyfora5-degreeviewingangle LED,measuredatadistanceof8meters. 46 Figure2.33Thesignalwaveformcapturedduringthecommunicationbiterrortestsata baudrate=115200bps. 47 Chapter3 OpticalCommunicationSignal StrengthModel Inthischapteramathematicalmodelispresentedforcapturingthesignalstrengthina wirelessopticalcommunicationsystem.Suchamodelwillbeinstrumentalindeveloping theestimationandcontrolalgorithmsinalignmentmaintenanceofopticalcommunication systemswhentheunderlyingplatformsaremobile.Suchamodelcanalsoprovideinsight intodesignconsiderationsforimprovingthecommunicationperformance. 3.1MathematicalModel Themodelusedisderivedfrom[43]withsometweakingtoourdevice.Themodelallows ustocomputethepowerorsignalstrengtharrivingfromthelightsourcetotheoptical detector.Themodelincludesthefollowingstagesofthetransmitterandreceivercircuit, includingLED,photodiode,andtransimpedanceAndnoextralensareusedin thismodeldescription.Themodelmainlydescribestheoftherelativepositionand orientationbetweenthetransmitterandthereceiveronthesignalstrength.RefertoFigure 3.1.Forthechosenhardwaren,themodelcomputesthereceivedpowerasa functionoftransmissionangle ,transmissiondistance d ,andreceptionangle ˚ . 48 Figure3.1Illustrationoftherelativepositionandorientationbetweenthetransmitterand thereceiver. 3.1.1TransmitterOpticalPower ThetransmitterlightsourceusedisCreeXR-ESeriesBlueLightingLED.TheLEDdatasheet providesuswiththespatialintensitydistributionoftheLED.Basicallyitisthemeasure- mentofthelightintensityatvaryinganglesfromtheLED'snormaldirection.Thisangular intensitydistributionisrotationallysymmetricabouttheLED'snormal.Ifweknowthe intensityoftheLEDalongthenormaltotheLED( =0),wecandeterminetheintensity atvariousangularpointsbytheuseofthisspatialintensitycurve.Thetypicalrelative intensitygraphforthisLEDisshowninFigure3.2.WeuseasummationofthreeGaussian functionstothedatafromthedatasheet f ( )= a 1 e ( b 1 ) 2 2 ˙ 2 1 + a 2 e ( b 2 ) 2 2 ˙ 2 2 + a 3 e ( b 3 ) 2 2 ˙ 2 3 (3.1) Theparameters( a 1 ;a 2 ;a 3 ;b 1 ;b 2 ;b 3 ;˙ 1 ;˙ 2 ;˙ 3 )arefoundbyusingacurvngtoolbox inMatlabsoftwarewhichusesthemethodofleastsquares.Toexpresstheradiantintensity I ,foragivenangulardisplacement ,weuseEq.(3.2),where f ( ) 2 [0 ; 1]andthemaximum 49 intensity I =0.038W/sr[43]. I = If ( ) (3.2) Figure3.2RelativeintensityofLED[7](red)andGaussian(green).Thex-axisshows theangulardisplacementindegrees,they-axisshowstherelativeintensityinpercent. 3.1.2LightDetectorSensitivity Asphericalspreadingmodelwithexponentialdecayisconsideredinthiswork.Inclearwater, wherescatteringdoesnotplayacrucialrole,thismodelworkswell.First,weconsiderpower lossduetosphericalspreading.Let d bethedistancein[m]atwhichwemeasurethesignal intensity.Usingtheintensity I fromEquation(3.2),wegetirradiance E whichisthe 50 radiantreceivedbyasurfaceperunitareaatdistance d ,measuredinW = m 2 : E ( d )= I =d 2 (3.3) TodescribetheextinctionofthelightsignalwewilluseBeer'sLaw[44],whichisusedin understandingtheattenuationinphysicaloptics.Let c betheattenuationcotforthe mediuminwhichthelighttransmits.Weassumethatthecotisuniformacrossthe entirelengthoftransmission.Beer'slawgivesthesignaldegradationatdistance d caused byabsorptionas A = e cd (3.4) Bycombingtheofsphericalspreadingwithexponentialdecay,wegetairra- dianceequationof E ( d )= I e cd =d 2 (3.5) Theincidentpowercanbecomputedfromthesignalirradiancearrivingatthedetector Eq.(3.5),theincidentangle ˚ ,andthearea A 0 (assumedofthedetector: P in = E ( d ) A 0 cos( ˚ ) (3.6) Foragivensensor,therelationshipbetweentheincidentpowerandthecurrentoutput, calledresponsivity,canbecapturedwithalineargain R d .Sothephotodiodeoutputcurrent isexpressedas I d = R d P in (3.7) 51 Thecurrentproducedbythephotodiodemustbeandconvertedintoavoltage beforeitcanbeprocessedbytheanalogtodigitalconverter.Thisistypicallydoneusing atransimpedance(TIA).Thegainoftheissetbythefeedbackresistor andatlowfrequenciesthefeedbackcapacitorhaslittleontheresponseas showninFigure3.3. Figure3.3Frequencyresponseofthetransimpedance Theresponseisclosetotheideal,andthecurrentisconvertedintoavoltage as 52 V d = R f I d (3.8) Thefullsignalstrengthmodelcanthenbesummarizedas V d = CI e cd cos( ˚ ) =d 2 (3.9) where C = A 0 R d R f isascalingconstant. 3.2ExperimentalModelIdenandValidation Threeexperimentsarepresentedinthissection.Thestexperimentisusedtothe parametersformodel(3.9)whenonlythedistance d ischanged.Thesecondoneisfora communicationdistancewhilethereceiverangle ˚ ischanged,andthelastoneisfor acommunicationdistancewhilethereceiverangle ischanged.Notethatallthese experimentsareconductedinair. Intheexperiments,aone-inch80/20frameandsomesteelplatesareusedtoconstruct aslidingmechanismwithatraveldistanceof2m;seeFigure3.4.Thetransmitterandthe receivercircuitsareinsideaPVCtubtofacilitateattachingthemtothesteelplates withclamps.Themountinganglesofthereceiverandthetransmitterarevariableinthe horizontalplane,toallowanarbitrarychoiceoftherotationanglesforboth. 3.2.1ModelIden Theonlyunknownparametersinthemodel(3.9)are C and c .Inordertoestimatethese parameters,boththetransmitterandthereceiveraresettoaimdirectlyateachother 53 ( = ˚ =0 ).Thenthetransmitterismovedtoaddistance,whereweacquirethe signalstrengthvalue(thevoltageoutputatthereceiverside).Therangeofdistancesused fortheseexperimentsis10cmto150cminincrementof10cm.Themeasurementsandthe modelwithtdisplacement d areshowninFigures3.5.Thedatafromthecase, where( = ˚ =0 )areusedtotheparametersin(3.9).Usingthecurvetoolbox inMatlabsoftware, C isfoundtobe2.099and c is 0 : 8665. Figure3.4Experimentalsetupfortheidenandvalidationofthesignalstrength modelinair. 54 Figure3.5Modeledsignalstrengthandexperimentaldata.Thegreenlineshowstheoutput ofthemodel.Theexperimentaldataisplottedinredand( = ˚ =0 ) 55 3.2.2ModelValidation Firstwevalidatethemodelisabletopredictthesignalstrengthattreceiverangles. Theemitterangleisat =0 andthedistanceisat20cm.Therangeofrotation forthereceiverisfrom0 to70 .Themeasurementsandthemodelpredictionswitht receiverangle ˚ areshowninFigures3.6.Itcanbeseenthatthemodeltheexperimental datawell;inparticular,thecosinefunctionisabletocapturethedependenceofthesignal strengthonthereceiverangle. Figure3.6Experimentalvalidationofthesignalstrengthmodelasthereceiveranglevaries. Herethedistanceisat20cm. Nextweverifythemodelisabletocapturethedependenceofthesignalstrengthonthe 56 emitterangle.Inparticular,wewouldliketovalidateEq.(3.1).Thereceiverangleis at( ˚ =0 )andthedistanceisat20cm.Therangeoftheemitterrotationisfrom0 to70 .Goodmatchisachievedbetweenthesignalstrengthmeasurementsandthemodel predictions,asshowninFigure3.7. Figure3.7Experimentalvalidationofthesignalstrengthmodelasthereceiveranglevaries. Herethedistanceisat20cm. Finally,tovalidatethecompletemodeldescribedin(3.9),multipleexperimentsare conductedforvariabletramitterandreceiveranglesalongwiththevariationofdistance. ComparisonsshowninFigures3.8-3.11showconsistencybetweenthemodelpredictionsand experimentalmeasurements. 57 Figure3.8Experimentalvalidationofthesignalstrengthmodelwithtransmissionangle =20 andreceptionangle ˚ =0 . 58 Figure3.9Experimentalvalidationofthesignalstrengthmodelwithtransmissionangle =40 andreceptionangle ˚ =0 . 59 Figure3.10Experimentalvalidationofthesignalstrengthmodelwithtransmissionangle =0 andreceptionangle ˚ =20 . 60 Figure3.11Experimentalvalidationofthesignalstrengthmodelwithtransmissionangle =0 andreceptionangle ˚ =40 . 61 Chapter4 ActiveAlignmentControlSystem Inthischapter,wepresentanactivecontrolalgorithmformaintainingthealignmentof thereceiverwiththetransmitterforLED-basedcommunicationsystems,inthepresence oftheunderlyingmovementoftheroboticcarriers.Thealgorithmusesthereceivedsignal strengthasfeedbackforadjustingthereceiverangles.Theenessofthisapproach hasbeendemonstratedinexperimentsinvolvingamobilerobot(receiver)andastationary transmitter. 4.1TrackingAlgorithm TheproposedalgorithmalignstherotatingbaseonthereceiverendtotheLEDlight.In particular,itusesthesignalstrengthtoguidethemovementoftherotatingbase,sothat tsignalstrengthismaintaineddespitethemovementofthetransmitter.Thedetails ofthealgorithmsareasfollows: 1.Whenthetrackingisinitiated,theanalogsignalfromthereceivercircuitismeasured. 2.Therotatingbasekeepsrotatinguntilitsensesavoltagesignalhigherthanthethresh- oldvoltage. 3.Afterthecommunicationhasstarted,thealgorithmkeepscheckingthevoltagesignal, andwhenthevaluedropsunderthethreshold(1Volt),thebasestartsasearchfunction. 62 4.Thesearchfunction(seeFigure4.1)willperformaclockwiserotationandthena counter-clockwiserotationforanangle fromtheoriginallocation,anditwilltake thevoltagemeasurementsateachstep( V 1 ;V 2 ;V 3 ),where V 1 , V 2 , V 3 representthe voltagesattheclockwiserotation,counter-clockwiserotation,andoriginallocation ( =0),respectively. 5.Thenewturningangle ˚ p oftherotatingbaseiscalculatedbytakingaweighted averageofthesesignalsatthreesteps: ˚ p = V 1 V 2 V 1 + V 2 + V 3 (4.1) Figure4.1Illustrationoftheactivealignmentcontrolalgorithm. 4.2ExperimentalResults Amobilerobotisusedtothefeasibilityofthealgorithm.Tosetuptheexperiment (seeFigure4.2),thetransmitterisinapositionwhilethereceiverwiththerotating baseismountedonamobilerobot(3WD48mmOmniWheelArduinoMobileRobotKit 63 fromNEXUSrobot),whichisprogrammedtorotate(-60,60)degreesarounditscenteraxis continuously.Theprobingangularrotationfortherotatingis =5 .Thepurposeofthe experimentsistomeasuretherotatingbaserotationangleandthereceivedsignalstrength whenthecontrolisonandatthreentdistances(1m,2m,3m).Figures4.3,4.5, and4.7showthattherotatingbaseisabletocounteracttherotationalmovementofthe robot,tokeepthereceiveralignedapproximatelywiththelightcomingfromthetransmitter, forallthreedistancestested.Figures4.4,4.6,4.8showthevoltagesignalsreceivedatthe receiverintwocases,whenthealgorithmisenabledanddisabled,respectively.Itcanbe seenthat,whenthealgorithmisturnedon,thesignalstrengthisabovethethresholdvalue of1Vformostofthetime;ontheotherhand,thesignalstrengthislowerthanthethreshold valueformuchofthedurationwhenthealgorithmisturnedandthesituationworsens asthedistanceofcommunicationincreases.Finally,adata-transmittingtestisconducted, wherethebaudrateis115200kbpsandtheamountofdatasentis83500bytes.During theseexperimentstheBER(biterrorrate)isfoundtobezeroforallconditions,butsome dataarelostduetothemovementoftherobot.Table4.1comparetheamountofdatalost whentheactivealignmentcontrolisturnedonandrespectively.Theadvantageofthe proposedalignmentcontrolschemeisevident. Table4.1Datalostduetorobotmovement. Distance(m) Datalost (controlon) Datalost (control) 1 3132bytes 9392bytes 2 5218bytes 31312bytes 3 8300bytes 46968bytes 64 Figure4.2Experimentalsetupfortestingtheactivealignmentcontrolalgorithm. 65 Figure4.3Measuredrotationangleforthereceiverbaseinresponsetotherotationangleof therobotplatform,whenthereceiverisat1meterdistancefromthetransmitter. 66 Figure4.4Measuredsignalstrengthswhenthealignmentcontrolisonandrespectively, whenthereceiverisatadistanceof1mfromthetransmitter. 67 Figure4.5Measuredrotationangleforthereceiverbaseinresponsetotherotationangleof therobotplatform,whenthereceiverisat2meterdistancefromthetransmitter. 68 Figure4.6Measuredsignalstrengthswhenthealignmentcontrolisonandrespectively, whenthereceiverisatadistanceof2mfromthetransmitter. 69 Figure4.7Measuredrotationangleforthereceiverbaseinresponsetotherotationangleof therobotplatform,whenthereceiverisat3meterdistancefromthetransmitter. 70 Figure4.8Measuredsignalstrengthswhenthealignmentcontrolisonandrespectively, whenthereceiverisatadistanceof3mfromthetransmitter. 71 Chapter5 ConclusionandFutureWork 5.1Conclusion Inthisthesisawirelessopticalcommunicationsystemhasbeenbuiltusingbluelight.This deviceisabletosenddatawitharateupto115200kbpsatdistancesofupto23min anunderwatersetting,showingitspromiseinunderwatermobilesensingandnavigation applications. Amodelforestimatingtheopticalsignalstrengthhasbeendevelopedandvalidated throughaseriesofexperimentsinair.Thismodelallowsforpredictionstobemadeabout theexpectedsignalstrength,whichcanbeusedtoestimatethelinkqualityandtoestimate therelativedistanceandposebetweenthetransmitterandthereceiver.Thelatterwillbe instrumentalinthedesignofactivealignmentcontrolalgorithms.Duetoconstraintson experimentalconditions,themodelhasonlybevalidatedinair.However,weexpectitto bevalidfortheunderwaterenvironment,butwithtsystemparameters. Anactivecontrolalgorithmhasbeenpresentedformaintainingthealignmentofthe receiverwiththetransmitterforLED-basedcommunicationsystems.Thealgorithmuses thereceivedsignalstrengthasfeedbackforadjustingthereceiverangles.Amobilerobot hasbeenusedtovalidatethisalgorithm. 72 5.2FutureWork Thesystemsandalgorithmspresentedinthisthesislaythegroundworkforpursuingfurther researchonthetandapplicationofLED-basedopticalcommunicationsystems, especiallyfortheunderwatersetting.Animmediateextensionwillbetoexploretheper- formanceoftheactivealignmentcontrolalgorithmunderwater,wherethetransceiversare mountedontwounderwaterrobots. Onthealgorithmside,itwillbeofinteresttouseextendedKalmantoestimate therelativedistanceandposebetweenthetransmitterandthereceiver,bothofwhichcould beonmobilerobots,andtodevelopmoreadvancedalignmentcontrolschemes. Whilethewirelessopticalsystemcreatesthenecessarybackboneforlow-latencyand high-rateratelinks,thecurrentdesignispronetofeedthroughinterference;namely,the receiverreceivesaspurioussignalwhenthetransmitteronthereceiversideistransmitting. Therefore,thecurrentsystemdoesnotallowduplex-modecommunication.Improvementof thesystemdesigntoalleviatethisproblemisanotherdirectionoffuturework.Inparticular, extendingcommunicationsintofullduplexmode. Finally,thecurrentrotatingbasecanonlyrespondaboutasingleaxis.Consequently, itrequirestwocommunicatingrobotstostayinthesameplane.Thislimitationcanbe overcomeifanactivepan-tiltbaseforthetransmitter/receivercircuitsisadopted.This ideaandthecorrespondingalignmentcontrolproblemrepresentanotherdirectionworthyof exploration. 73 BIBLIOGRAPHY 74 BIBLIOGRAPHY [1]S.Climent,A.Sanchez,J.V.Capella,N.Meratnia,andJ.J.Serrano,\Underwater acousticwirelesssensornetworks:Advancesandfuturetrendsinphysical,mac androutinglayers," Sensors ,vol.14,no.1,p.795,2014.[Online].Available: http://www.mdpi.com/1424-8220/14/1/795 [2]H.Brundage.,\Designingawirelessunderwateropticalcommunicationsystem,"Mas- ter'sthesis,MassachusettsInstituteofTechnology,2010. [3]S.N.White,A.D.Chave,andG.T.Reynolds,\Investigationsofambientlight emissionatdeep-seahydrothermalvents," JournalofGeophysicalResearch:Solid Earth ,vol.107,no.B1,pp.EPM1{1{EPM1{13,2002.[Online].Available: http://dx.doi.org/10.1029/2000JB000015 [4]Wikipedia,\Light-emittingdiode,"March2012,https://en.wikipedia.org/wiki/Light- emitting-diode. [5]||,\Laserdiode,"June2015,https://en.wikipedia.org/wiki/Laser-diode. [6]Superbrightleds,\Cree-xre-series-1-watt-blue-led,"March2014, https://www.superbrightleds.com/moreinfo/high-powered/cree-xre-series-1-watt- blue-led-b4k/933/1875/. [7]CREE,\Creexlampxr-eled,"January2006,http://www.cree.com//me- onents20and20modules/xlamp/xlamp7090xre.pdf. [8]Electrotechservices,\Mosfettypes,"February2015, http://www.electrotechservices.com/electronics/metaloxidesemiconductorfets. [9]Wikipedia,\Photoresistor,"March2015,https://en.wikipedia.org/wiki/Photoresistor. [10]||,\Phototransistor,"March2015,https://wikimedia.org/wiki/phototransistor.jpg. [11]||,\Photodiode,"March2015,https://en.wikipedia.org/wiki/Photodiode. [12]||,\Avalanchephotodiode,"March2015,https://en.wikipedia.org/wiki/Avalanche- photodiode. 75 [13]||,\Photomultiplier,"March2015,https://en.wikipedia.org/wiki/Photomultiplier. [14]S.B.Alexander, OpticalCommunicationReceiverDesign .SPIE,1997. [15]NOAA,\Ocean,"August2012,http://www.noaa.gov/ocean.html. [16]WHOI,\Commonwealthawards$5mR&Dgrantforcenterformarinerobotics,"Febru- ray2014,http://www.whoi.edu/news-release/center-for-marine-robotics. [17]J.Catipovic,\Performancelimitationsinunderwateracoustictelemetry," OceanicEn- gineering,IEEEJournalof ,vol.15,no.3,pp.205{216,Jul1990. [18]SounLink,\Underwateracousticmodem,"April2006,http://www.link- quest.com/html/intro1.htm. [19]P.P.Smyth,P.L.Eardley,K.T.Dalton,D.R.Wisely,P.McKee,andD.Wood, \Opticalwireless:aprognosis,"vol.2601,1995,pp.212{225.[Online].Available: http://dx.doi.org/10.1117/12.228143 [20]M.Chen,S.Zhou,andT.Li,\Theimplementationofppminunderwaterlasercom- municationsystem,"in Communications,CircuitsandSystemsProceedings,2006In- ternationalConferenceon ,vol.3,June2006,pp.1901{1903. [21]W.Cox,\1mbpsunderwatercommunicationsystemusinga405nmlaserdiodeand photomultipliertube,"Master'sthesis,NorthCarolinaStateUniversity,2008. [22]F.HansonandS.Radic,\Highbandwidthunderwateropticalcommunication," Appl.Opt. ,vol.47,no.2,pp.277{283,Jan2008.[Online].Available: http://ao.osa.org/abstract.cfm?URI=ao-47-2-277 [23]R.Hagem,D.V.Thiel,S.O'Keefe,A.Wixted,andT.Fickenscher,\Low-costshort -rangewirelessopticalfskmodemforswimmersfeedback,"in Sensors,2011IEEE ,Oct 2011,pp.258{261. [24]F.Lu,S.Lee,J.Mounzer,andC.Schurgers,\Low-costmedium-rangeoptical underwatermodem:Shortpaper,"in ProceedingsoftheFourthACMInternational WorkshoponUnderWaterNetworks ,ser.WUWNet'09.NewYork,NY,USA:ACM, 2009,pp.11:1{11:4.[Online].Available:http://doi.acm.org/10.1145/1654130.1654141 76 [25]F.Schill,U.R.Zimmer,andJ.Trumpf,\Visiblespectrumopticalcommunicationand distancesensingforunderwaterapplications,"in In:Proc.ofAustralasianConference onRoboticsandAutomation ,2004. [26]M.DoniecandD.Rus,\BidirectionalopticalcommunicationwithAquaOpticalII,"in CommunicationSystems(ICCS),2010IEEEInternationalConferenceon ,Nov2010, pp.390{394. [27]D.Anguita,D.Brizzolara,andG.Parodi,\Buildinganunderwaterwirelesssensor networkbasedonoptical:Communication:Researchchallengesandcurrentresults,"in SensorTechnologiesandApplications,2009.SENSORCOMM'09.ThirdInternational Conferenceon .IEEE,2009,pp.476{479. [28]||,\Opticalwirelesscommunicationforunderwaterwirelesssensornetworks:Hard- waremodulesandcircuitsdesignandimplementation,"in OCEANS2010 .IEEE,2010, pp.1{8. [29]I.RustandH.Asada,\Adual-usevisiblelightapproachtointegratedcommunication andlocalizationofunderwaterrobotswithapplicationtonon-destructivenuclearreactor inspection," RoboticsandAutomation(ICRA),2012IEEEInternationalConferenceon , pp.2445{2450,May2012. [30]S.Hranilovic, WirelessOpticalCommunicationSystems .NewYork,NY,USA: Springer-VerlagNewYork,Inc.,2009. [31]E.F.Schubert,T.Gessmann,andJ.K.Kim, LightEmit- tingDiodes .JohnWileySons,Inc.,2000.[Online].Available: http://dx.doi.org/10.1002/0471238961.1209070811091908.a01.pub2 [32]J.M.KahnandJ.R.Barry,\Wirelessinfraredcommunications," Proceedingsofthe IEEE ,vol.85,no.2,pp.265{298,1997. [33]L.A.Coldren,S.W.Corzine,andM.L.Mashanovitch, Diodelasersandphotonic integratedcircuits .JohnWiley&Sons,2012,vol.218. [34]G.Keiser, Opticalercommunications ,ser.McGraw-Hillseriesinelectricaland computerengineering:Communicationsandsignalprocessing.McGraw-Hill,2000. [Online].Available:http://books.google.com/books?id=lANTAAAAMAAJ [35]L.Yuan,S.Liu,M.Chen,andX.Luo,\Thermalanalysisofhighpowerledarray packagingwithmicrochannelcooler,"in ElectronicPackagingTechnology,2006.ICEPT '06.7thInternationalConferenceon ,Aug2006,pp.1{5. 77 [36]LUXDRIVE,\3021/3023buckpuck,"January2007, http://www.digikey.com/catalog/en/partgroup/buckpuck30213023series/28982. [37]P.Scherz, PracticalElectronicsforInventors ,2nded.NewYork,NY,USA:McGraw- Hill,Inc.,2007. [38]I.\Irf7307pbf,"January2013,http://www.digikey.com/IRF7307PBFCT- ND/812597. [39]N.V.Tkachenko, Opticalspectroscopy:methodsandinstrumentations .Elsevier,2006. [40]T.Instruments,\Highvoltagehighslewratewidebandfet-inputoperation March2006,http://www.ti.com/product/ths4631. [41]MOFLON,\Mt007slipring,"January2008,h.com/mt007.html. [42]J.Jiang,S.To,W.Lee,andB.Cheung,\Opticaldesignofafreeform tirlensforledstreetlight," OptikInternationalJournalforLightandElec- tronOptics ,vol.121,no.19,pp.1761{1765,2010.[Online].Available: http://www.sciencedirect.com/science/article/pii/S0030402609002083 [43]M.Doniec,M.Angermann,andD.Rus,\Anend-to-endsignalstrengthmodelfor underwateropticalcommunications," OceanicEngineering,IEEEJournalof ,vol.38, no.4,pp.743{757,Oct2013. [44]F.Miller,A.Vandome,andJ.McBrewster, Beer-LambertLaw .VDMPublishing, 2009.[Online].Available:https://books.google.com/books?id=XO-DQgAACAAJ 78