超声波srf04参考书

SRF04-Ultra-SonicRangerTechnicalSpecificationThisprojectstartedafterIlookedatthePolaroidUltrasonicRangingmodule.Ithasanumberofdisadvantagesforuseinsmallrobotsetc.1.Themaximumrangeof10.7metreisfarmorethanisnormallyrequired,andasaresult2.Thecurrentconsumption,at2.5Ampsduringthesonicburstistrulyhorrendous.3.The150mAquiescentcurrentisalsofartoohigh.4.Theminimumrangeof26cmisuseless.1-2cmismorelikeit.5.Themoduleisquitelargetofitintosmallsystems,and6.It’sEXPENSIVE.TheSRF04wasdesignedtobejustaseasytouseasthePolaroidsonar,requiringashorttriggerpulseandprovidinganechopulse.Yourcontrolleronlyhastotimethelengthofthispulsetofindtherange.TheconnectionstotheSRF04areshownbelow:TheSRF04Timingdiagramisshownbelow.Youonlyneedtosupplyashort10uSpulsetothetriggerinputtostarttheranging.TheSRF04willsendoutan8cycleburstofultrasoundat40khzandraiseitsecholinehigh.Itthenlistensforanecho,andassoonasitdetectsoneitlowerstheecholineagain.Theecholineisthereforeapulsewhosewidthisproportionaltothedistancetotheobject.Bytimingthepulseitispossibletocalculatetherangeininches/centimetersoranythingelse.IfnothingisdetectedthentheSRF04willloweritsecholineanywayafterabout36mS.Hereistheschematic,Youcandownloadabetterqualitypdf(161k)versionsrf1.pdfThecircuitisdesignedtobelowcost.ItusesaPIC12C508toperformthecontrolfunctionsandstandard40khzpiezotransducers.ThedrivetothetransmittingtransducercouldbesimplestdrivendirectlyfromthePIC.The5vdrivecangiveausefulrangeforlargeobjects,butcanbeproblematicdetectingsmallerobjects.Thetransducercanhandle20vofdrive,soIdecidedtogetupclosetothislevel.AMAX232IC,usuallyusedforRS232communicationmakesandidealdriver,providingabout16vofdrive.Thereceiverisaclassictwostageop-ampcircuit.TheinputcapacitorC8blockssomeresidualDCwhichalwaysseemstobepresent.Eachgainstageissetto24foratotalgainof576-ish.Thisisclosethe25maximumgainavailableusingtheLM1458.ThegainbandwidthproductfortheLM1458is1Mhz.Themaximumgainat40khzis1000000/40000=25.TheoutputoftheamplifierisfedintoanLM311comparator.Asmallamountofpositivefeedbackprovidessomehysterisistogiveacleanstableoutput.Theproblemofgettingoperationdownto1-2cmisthatthereceiverwillpickupdirectcouplingfromthetransmitter,whichisrightnexttoit.Tomakemattersworsethepiezotransducerisamechanicalobjectthatkeepsresonatingsometimeafterthedrivehasbeenremoved.Upto1mSdependingonwhenyoudecideithasstopped.Itismuchhardertotellthedifferencebetweenthisdirectcoupledringingandareturningecho,whichiswhymanydesigns,includingthePolaroidmodule,simplyblankoutthisperiod.Lookingatthereturningechoonanoscilloscopeshowsthatitismuchlargerinmagnitudeatclosequartersthanthecross-coupledsignal.Ithereforeadjustthedetectionthresholdduringthistimesothatonlytheechoisdetectable.The100ncapacitorC10ischargedtoabout–6vduringtheburst.Thisdischargesquitequicklythroughthe10kresistorR6torestoresensitivityformoredistantecho’s.Aconvenientnegativevoltagefortheop-ampandcomparatorisgeneratedbytheMAX232.Unfortunately,thisalsogeneratesquiteabitofhighfrequencynoise.Ithereforeshutitdownwhilstlisteningfortheecho.The10uFcapacitorC9holdsthenegativerailjustlongenoughtodothis.Inoperation,theprocessorwaitsforanactivelowtriggerpulsetocomein.Itthengeneratesjusteightcyclesof40khz.Theecholineisthenraisedtosignalthehostprocessortostarttiming.TheraisingoftheecholinealsoshutsoftheMAX232.Afterawhile–nomorethan10-12mSnormally,thereturningechowillbedetectedandthePICwilllowertheecholine.Thewidthofthispulserepresentstheflighttimeofthesonicburst.Ifnoechoisdetectedthenitwillautomaticallytimeoutafterabout30mS(ItstwotimestheWDTperiodofthePIC).BecausetheMAX232isshutdownduringechodetection,youmustwaitatleast10mSbetweenmeasurementcyclesforthe+/-10vtorecharge.Performanceofthisdesignis,Ithink,quitegood.Itwillreliablymeasuredownto3cmandwillcontinuedetectingdownto1cmorlessbutafter2-3cmthepulsewidthdoesn’tgetanysmaller.Maximumrangeisalittleover3m.Asandexampleofthesensitivityofthisdesign,itwilldetecta1inchthickplasticbroomhandleat2.4m.Averagecurrentconsumptionisreasonableatlessthan50mAandtypicallyabout30mA.Downloadthesourcecodeandareadyassembledhexfile.CalculatingtheDistanceTheSRF04providesanechopulseproportionaltodistance.IfthewidthofthepulseismeasuredinuS,thendividingby58willgiveyouthedistanceincm,ordividingby148willgivethedistanceininches.uS/58=cmoruS/148=inches.ChangingbeampatternandbeamwidthYoucan't!Thisisaquestionwhichcropsupregularly,howeverthereisnoeasywaytoreduceorchangethebeamwidththatI'mawareof.ThebeampatternoftheSRF04isconicalwiththewidthofthebeambeingafunctionofthesurfaceareaofthetransducersandisfixed.ThebeampatternofthetransducersusedontheSRF04,takenfromthemanufacturersdatasheet,isshownbelow.Thereismoreinformationinthesonarfaq.Update-May2003SincetheoriginaldesignoftheSRF04waspublished,therehavebeenincrementalimprovementstoimproveperformanceandmanufacturingreliability.Theop-ampisnowanLMC6032andthecomparatorisanLP311.The10uFcapacitorisnow22uFandafewresistorvalueshavebeentweaked.Thesechangeshavehappenedoveraperiodoftime.AllSRF04'smanufacturedafterMay2003havenewsoftwareimplementi