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外文翻译--数字技术在超声波传感器中的应用.doc

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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.Ultrasonicsensorsareastateoftheartchoicetoeffectivelysolvethisproblemindifferentengineeringfields,suchasmedicalimaging,robotnavigation,flowvelocity,fluidlevelmeasurementandsoon.Althoughdifferentkindsofmeasuringsystemsandsensorshavebeenusedforthespecificapplicationsrespectively,thereareproblemsrelatedtoechodetectionineverycaseattenuationandbeamspreading,presenceofnoiseandinterference,sensitivitytotemperatureandhumidity,poorresolution.Inmanyapplications,thetimeelapsedbetweenthestimulusultrasonicwaveanditsechoistakenasthebasisofthemeasurement,thatis,themainoperatingprincipleistheestimationofthetimeofflightToF,thetransmittingultrasonictransducertransmitsanultrasonicpulse,thepulsepropagatesthroughthetransmissionmediumair,water,etcandisechoedbyasuitablereflector.Theechopulseisreceivedbythereceivingtransducer,whichmaybethesametransmittingtransducer.Sothedistancefromthetransmittertothereflectorcanbeestimatedindirectlybytheformula1wherevisthepropagationvelocityoftheultrasonicwaveinthemedium.Anotherprincipleisthephaseshiftmethod.Here,thetransmittergeneratesacontinuouswave,whoisreflectedbyanobject,andwhoseechoisdetectedbyaseparatereceiver.Bycomputingthephaseshiftbetweenthetransmittedandthereceivedcontinuouswave,thedistanceisevaluatedtowithinawavelength.Pulseechoandcontinuouswavetechniquesarewidelyknownastherepresentsofthetwomeasuringprinciples.Continuouswavetechniqueismoreaccurate,butmoreexpensiveduetocomplexhardwareusedtomeasurethephaseandduetothedifficultyindeterminingthenumberofintegerwavelengths.Asaresult,thepulseechotechniqueisoftenusedincommercialsystemsforindustrialapplication.Therefore,thispaperputsanemphasisonthepulseechotechnique.Fromtheformula1,wehaveconcludedthatthosefactorsaffectingvandTfarethesourcesofthedistancemeasurementuncertainty.Thevariationofvdoesntonlydependontherelativehumidityandtemperature,butalsoonsomechemicalandphysicalparametersofthefluid.Fortheseeffects,differenttechniqueshavebeendevelopedsuccessfullytocompensatethemsatisfactorily,sothecriticalpointofthewholemeasurementprocedureistheToFestimation.ManymethodshavebeenproposedtoimprovetheToFmeasurementaccuracy.Somerequirecomplicated,notflexibleandveryexpensivehardwaretomaketheToFmeasurementhigheraccuracy.However,othersusesophisticateddigitaltechniques,thatis,digitalsignalprocessingsoftwaretodothesamething.Fromtheviewpointoftheengineeringapplication,itisnotonlypossible,butalsofeasibletoimproveToFestimationusingdigitaltechniques.Inthispaper,firstly,thebasictheoriesofthesedigitaltechniquesarepresented,includingsomemathematicalformula.Then,theapplicationsofthesedigitaltechniquesfortheultrasonicdistancemeasurementisdescribed.Finally,aobjectiveassessmentofthemethodsthroughanalyzingboththesystematicandrandomerrorsisgiven2.SIGNALMODELANDITSBUILDINGAsforthepulseechomethod,inthemostengineeringapplications,requireonlyonetransducerasboththetransmitterandreceiver.Thatisitoffersasimpleandlowcostsolution.Butatthesametime,itmayresultinlowaccuracyowingtotheerroroftheToFestimation.Howtoimproveitsaccuracybutnotusecomplex,expensiveadditionalhardwareInfact,thisproblemcanbesolvedbyusingcheapsoftwaresignalprocessing.Therefore,thekeyofthisproblemis.ofcourse,theToFestimation.Tosolvethisproblem,itisnecessarytosetupasuitablemathematicalmodelofthetransmittedultrasonicpulse.2whereAs,fsandψsarethesignalamplitude,frequencyandphaserespectively,whilemandnaretheparametersofthecharactersofultrasonictransduceritself.Fortheactualtransducer,bothm0andu0isanabsolutefact.Atthesametime,asuitablemathematicalmodelofthereflectedultrasonicpulsemustbeconsideredtobewrittenasfollowsEtArtTfTetTslucos2xfstTfψr3whereAr,ψraretheamplitudeandthephaseofthereflectedechosignal.TfistheToF.ToperformToFmeasurement,wemustconsiderthemodelofthewholeultrasonicsignal,includingboth.thetransmittedandthereflectedpulse,whichcanberepresentedbytheequation(4)where0Tand0Farethesamplingperiodsforthetimeandfrequencydomainrespectively,andkn,areintegers.AfterSTFThasmappedaonedimensionalsignaloftimext,ontoatwodimensionalfunctionoftimeandfrequency,ftX,whatwedonextistheenvelopeextraction.Thefirststepistochooseasuitablefrequencysfwhenusingthefrequencysf,S/Nisthemaximumandtheinterferenceistheminimum,whichalwaysbetheresonantfrequencyofthesensor.ThesecondstepistofindoutthemagitudefunctionoftheSTFTatthefrequencysfofthesignalxt,thatistheenvelopeweneed.3.CORRELATIONMETHODANDToFESTIMATIONAfterrecoveringtheenvelopefromthedistortedandnoisynonstationarysignal.ThenextworkistocompletetheToFestimation.SeveraltechniquescanbeusedtoaccomplishthetaskoftheToFestimation.Thetraditionalapproachusedinthemostultrasonicsensorsisthethresholdmethod.Athresholdlevelissettled,andtheToFisconsideredtobemeasuredwhenthethresholdisfirstexceededbythesignaloritsenvelope.Althoughithasprovedtobesimpleandcheap,thistechniquemaysufferfrompoorresolution,particularlytheechopulsehasbeengreatlyattenuated.Thecorrelationtechniqueisakindofmoresophisticatedtechniqueforapplicationsrequiringhighaccuracy,whichbasestheToFestimationonthesearchoftherelativemaximuminthecrosscorrelationfunctionbetweenthetransmittedsignalandtheechosignal.Accordingtotheindexofthemaximumvalueofthecorrelationfunction,thetimedelaybetweenthetwosignalsisobtained.Thecorrelationfunctionisdefinedas5whenthevalueofhTCisthemaximum,theToFishT.6Consideringtheprocedureofthesamplingdata,actually,thefTestimationcanbeexpressedintheform7wherehistheintegerpartof5.0/TTf,andλisarealnumberintheinterval5.0,5.0,asuitablemeasurementalgorithmmustbeusedtoevaluateexactlytherealnumberλ.Itispossibletoestimationthefractionnumberλthroughasuitableinterpolationtechnique.Thesecondorderapproximateinterpolationdeterminestheparameterλas8theequation1canbeexpressedas94.SIMULATIONANDERRORANALYSISInordertovalidatetheproposedmethod,arealisticsimulationmustbesetuptomirrorthecharactersofanactualultrasonicsensorassumingasamplingfrequencyMf8MHZ,anultrasonicsensorfrequencyfs550KHZ,anultrasonicwavespeedinthemediumsmv/1390,thedistanceresolutionis,accordingtotheequation1,0.087mm.Theerroranalysiscanbeimplementedonthecomputersimulations.4.1SystematiccomponentoferrorsinToFestimationThefinitesamplingfrequencyduringtheprocedureoftheA/DconversionmaketheToFestimationinaccurate,thatis,thetimeresolutionerrorofsystemisaerrorsourcefortheToFestimation.Thisisasystematicerror,whichcanbethoughtasafunctionofthetimeresolution.Fromtheequation5,thisidealresolutionerrorcanbewrittenas10thatcorrespondstoamaximumdistanceerrorequaltomm043.0.WhentheToFisamultipleofthesamplingperiodλ,theerrorwillbezero.Accordingtotheequation6,theresultobtainedbycomputersimulationsconfirmthattheinterpolationmethodmakethetimeresolutionerrortoaminimumvalue.Asaresult,thesystematicerrorindistancemeasurementcanbewrittenasfollows114.2RandomcomponentoferrorsinToFestimationInthepracticalapplications,theexistenceofnoiseisacauseofToFinaccuracy.Wecanassumethatthenoiseisawhitenoise.Fromtheviewpointofenergy,lowSNRleadstohighnoiseintensity.Forthecorrelationmethod,itislesssensitivetonoiseduetotheinherentcancellationofzeromeanrandomeffectsitself.5.CONCLUSIONSTheuseofdedicateddigitaltechniquesprovedtobeessentialforultrasonicsensors.Asuitablemeasurementprocedurehavebeensetupallowingtheperformanceoftheproposedtechniques.Withregardtoenvelopeextraction,theSTFTisagoodchoice.TheSTFTfilteroutthenoise,andrecovertheoptimizedenvelope.,thecorrelationmethodisbetterthanthethresholdmethodduetobetterperformancebothintermsofsystematicerrortimeresolutionandintermsofrandomerrornoiseforechodetection.ACKNOWLEDGMENTS

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