外文翻译--数字技术在超声波传感器中的应用.doc

中英文翻译英语原文ApplicationofDigitalTechniquesinUltrasonicSensor1.INTRODUCTIONItisnecessaryforanumberofapplicationstoperformdistancemeasurements.Ultrasonicsensorsareastate-of=theartchoicetoeffectivelysolvethisproblemindifferentengineeringfields,suchasmedicalimaging,robotnavigation,flowvelocity,fluidlevelmeasurementandsoon.Althoughdifferentkindsofmeasuringsystemsandsensorshavebeenusedforthespecificapplicationsrespectively,thereareproblemsrelatedtoechodetectionineverycase:attenuationandbeamspreading,presenceofnoiseandinterference,sensitivitytotemperatureandhumidity,poorresolution.Inmanyapplications,thetimeelapsedbetweenthestimulusultrasonicwaveanditsechoistakenasthebasisofthemeasurement,thatis,themainoperatingprincipleistheestimationofthetimeofflight(ToF),thetransmittingultrasonictransducertransmitsanultrasonicpulse,thepulsepropagatesthroughthetransmissionmedium(air,water,etc)andisechoedbyasuitablereflector.Theechopulseisreceivedbythereceivingtransducer,whichmaybethesametransmittingtransducer.Sothedistancefromthetransmittertothereflectorcanbeestimatedindirectlybytheformula:(1)wherevisthepropagationvelocityoftheultrasonicwaveinthemedium.Anotherprincipleisthephaseshiftmethod.Here,thetransmittergeneratesacontinuouswave,whoisreflectedbyanobject,andwhoseechoisdetectedbyaseparatereceiver.Bycomputingthephaseshiftbetweenthetransmittedandthereceivedcontinuouswave,thedistanceisevaluatedtowithinawavelength.Pulse-echoandcontinuouswavetechniquesarewidelyknownastherepresentsofthetwomeasuringprinciples.Continuouswavetechniqueismoreaccurate,butmoreexpensiveduetocomplexhardwareusedtomeasurethephaseandduetothedifficultyindeterminingthenumberofintegerwavelengths.Asaresult,thepulse=echotechniqueisoftenusedincommercialsystemsforindustrialapplication.Therefore,thispaperputsanemphasisonthepulse-echotechnique.Fromtheformula(1),wehaveconcludedthatthosefactorsaffectingvandTfarethesourcesofthedistancemeasurementuncertainty.Thevariationofvdoesntonlydependontherelativehumidityandtemperature,butalsoonsomechemicalandphysicalparametersofthefluid.Fortheseeffects,differenttechniqueshavebeendevelopedsuccessfullytocompensatethemsatisfactorily,sothecriticalpointofthewholemeasurementprocedureistheToFestimation.ManymethodshavebeenproposedtoimprovetheToFmeasurementaccuracy.Somerequirecomplicated,notflexibleandveryexpensivehardwaretomaketheToFmeasurementhigheraccuracy.However,othersusesophisticateddigitaltechniques,thatis,digitalsignalprocessingsoftwaretodothesamething.Fromtheviewpointoftheengineeringapplication,itisnotonlypossible,butalsofeasibletoimproveToFestimationusingdigitaltechniques.Inthispaper,firstly,thebasictheoriesofthesedigitaltechniquesarepresented,includingsomemathematicalformula.Then,theapplicationsofthesedigitaltechniquesfortheultrasonicdistancemeasurementisdescribed.Finally,aobjectiveassessmentofthemethodsthroughanalyzingboththesystematicandrandomerrorsisgiven2.SIGNALMODELANDITSBUILDINGAsforthepulse-echomethod,inthemostengineeringapplications,requireonlyonetransducerasboththetransmitterandreceiver.Thatisitoffersasimpleandlowcostsolution.Butatthesametime,itmayresultinlowaccuracyowingtotheerroroftheToFestimation.Howtoimproveitsaccuracybutnotusecomplex,expensiveadditionalhardware?Infact,thisproblemcanbesolvedbyusingcheapsoftwaresignalprocessing.Therefore,thekeyofthisproblemis.ofcourse,theToFestimation.Tosolvethisproblem,itisnecessarytosetupasuitablemathematicalmodelofthetransmittedultrasonicpulse.(2)whereAs,fsandsarethesignalamplitude,frequencyandphaserespectively,whilemandnaretheparametersofthecharactersofultrasonictransduceritself.Fortheactualtransducer,bothm0andu0isanabsolutefact.Atthesametime,asuitablemathematicalmodelofthereflectedultrasonicpulsemustbeconsideredtobewrittenasfollows:E(t)=Ar(t-TfTe-(t-Ts)lucos2xfs(t-Tf)+r(3)whereAr,raretheamplitudeandthephaseofthereflectedechosignal.TfistheToF.ToperformToFmeasurement,wemustconsiderthemodelofthewholeultrasonicsignal,includingboth.thetransmittedandthereflectedpulse,whichcanberepresentedbytheequation:（4）where0Tand0Farethesamplingperiodsforthetimeandfrequencydomainrespectively,andkn,areintegers.AfterSTFThasmappedaone-dimensionalsignaloftimex(t),ontoatwo-dimensionalfunctionoftimeandfrequency),(ftX,whatwedonextistheenvelopeextraction.Thefirststepistochooseasuitablefrequencysf(whenusingthefrequencysf,S/Nisthemaximumandtheinterferenceistheminimum),whichalwaysbetheresonantfrequencyofthesensor.ThesecondstepistofindoutthemagitudefunctionoftheSTFTatthefrequencysfofthesignalx(t),thatistheenvelopeweneed.3.CORRELATIONMETHODANDToFESTIMATIONAfterrecoveringtheenvelopefromthedistortedandnoisynon-stationarysignal.ThenextworkistocompletetheToFestimation.SeveraltechniquescanbeusedtoaccomplishthetaskoftheToFestimation.Thetraditionalapproachusedinthemostultrasonicsensorsisthethresholdmethod.Athresholdlevelissettled,andtheToFisconsideredtobemeasuredwhenthethresholdisfirstexceededbythesignaloritsenvelope.Althoughithasprovedtobesimpleandcheap,thistechniquemaysufferfrompoorresolution,particularlytheechopulsehasbeengreatlyattenuated.Thecorrelationtechniqueisakindofmoresophisticatedtechniqueforapplicationsrequiringhighaccuracy,whichbasestheToFestimationonthesearchoftherelativemaximuminthecross-correlationfunctionbetweenthetransmittedsignalandtheechosignal.Accordingtotheindexofthemaximumvalueofthecorrelationfunction,thetimedelaybetweenthetwosignalsisobtained.Thecorrelationfunctionisdefinedas:(5)whenthevalueof)(hTCisthemaximum,theToFishT.(6)Consideringtheprocedureofthesamplingdata,actually,thefTestimationcanbeexpressedintheform:(7)wherehistheintegerpartof)5.0/(+TTf,andisarealnumberintheinterval)5.0,5.0?,asuitablemeasurementalgorithmmustbeusedtoevaluateexactlytherealnumber.Itispossibletoestimationthefractionnumberthroughasuitableinterpolationtechnique.Thesecond-orderapproximateinterpolationdeterminestheparameteras:(8)theequation(1)canbeexpressedas:(9)4.SIMULATIONANDERRORANALYSISInordertovalidatetheproposedmethod,arealisticsimulationmustbesetuptomirrorthecharactersofanactualultrasonicsensor:assumingasamplingfrequencyMf=8MHZ,anultrasonicsensorfrequencyfs=550KHZ,anultrasonicwavespeedinthemediumsmv/1390=,thedistanceresolutionis,accordingtotheequation(1),0.087mm.Theerroranalysiscanbeimplementedonthecomputersimulations.4.1SystematiccomponentoferrorsinToFestimationThefinitesamplingfrequencyduringtheprocedureoftheA/DconversionmaketheToFestimationinaccurate,thatis,thetimeresolutionerrorofsystemisaerrorsourcefortheToFestimation.Thisisasystematicerror,whichcanbethoughtasafunctionofthetimeresolution.Fromtheequation(5),thisidealresolutionerrorcanbewrittenas:(10)thatcorrespondstoamaximumdistanceerrorequaltomm043.0.WhentheToFisamultipleofthesamplingperiod(),theerrorwillbezero.Accordingtotheequation(6),theresultobtainedbycomputersimulationsconfirmthattheinterpolationmethodmakethetimeresolutionerrortoaminimumvalue.Asaresult,thesystematicerrorindistancemeasurementcanbewrittenasfollows:(11)4.2Rando