文献翻译数字技术在超声波传感器中的应用.docx
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文献翻译数字技术在超声波传感器中的应用
中英文翻译
英语原文
ApplicationofDigitalTechniquesinUltrasonicSensor
1.INTRODUCTION
Itisnecessaryforanumberofapplicationstoperformdistancemeasurements.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),wehaveconcludedthatthosefactorsaffectingvandTfare
thesourcesofthedistancemeasurementuncertainty.Thevariationofvdoesn'tonlydependontherelativehumidityandtemperature,butalsoonsomechemicalandphysicalparametersofthefluid.Fortheseeffects,differenttechniqueshavebeendevelopedsuccessfullytocompensatethemsatisfactorily,sothecriticalpointofthewholemeasurementprocedureistheToFestimation.ManymethodshavebeenproposedtoimprovetheToFmeasurementaccuracy.Somerequirecomplicated,notflexibleandveryexpensivehardwaretomaketheToFmeasurementhigheraccuracy.However,othersusesophisticateddigitaltechniques,thatis,digitalsignalprocessingsoftwaretodothesamething.Fromtheviewpointoftheengineeringapplication,itisnotonlypossible,butalsofeasibletoimproveToFestimationusingdigitaltechniques.
Inthispaper,firstly,thebasictheoriesofthesedigitaltechniquesarepresented,includingsomemathematicalformula.Then,theapplicationsofthesedigitaltechniquesfortheultrasonicdistancemeasurementisdescribed.Finally,aobjectiveassessmentofthemethodsthroughanalyzingboththesystematicandrandomerrorsisgiven
2.SIGNALMODELANDITSBUILDING
Asforthepulse-echomethod,inthemostengineeringapplications,requireonly
onetransducerasboththetransmitterandreceiver.Thatisitoffersasimpleandlowcostsolution.Butatthesametime,itmayresultinlowaccuracyowingtotheerroroftheToFestimation.Howtoimproveitsaccuracybutnotusecomplex,expensiveadditionalhardware?
Infact,thisproblemcanbesolvedbyusingcheapsoftwaresignalprocessing.Therefore,thekeyofthisproblemis.ofcourse,theToFestimation.
Tosolvethisproblem,itisnecessarytosetupasuitablemathematicalmodelofthetransmittedultrasonicpulse.
(2)
whereAs,fsandψsarethesignalamplitude,frequencyandphaserespectively,whilemandnaretheparametersofthecharactersofultrasonictransduceritself.Fortheactualtransducer,bothm>0andu>0isanabsolute
fact.
Atthesametime,asuitablemathematicalmodelofthereflectedultrasonicpulse
mustbeconsideredtobewrittenasfollows:
E(t)=Ar(t-TfTe-(t-Ts)lucos[2xfs(t-Tf)+ψr](3)
whereAr,ψraretheamplitudeandthephaseofthereflectedechosignal.
TfistheToF.
ToperformToFmeasurement,wemustconsiderthemodelofthewholeultrasonicsignal,includingboth.thetransmittedandthereflectedpulse,whichcanberepresentedbytheequation:
(4)
where0>Tand0>Farethesamplingperiodsforthetimeandfrequencydomainrespectively,andkn,areintegers.
AfterSTFThasmappedaone-dimensionalsignaloftimex(t),ontoatwo-dimensionalfunctionoftimeandfrequency),(ftX,whatwedonextistheenvelopeextraction.Thefirststepistochooseasuitablefrequencysf(whenusingthefrequencysf,S/Nisthemaximumandtheinterferenceistheminimum),whichalwaysbetheresonantfrequencyofthesensor.ThesecondstepistofindoutthemagitudefunctionoftheSTFTatthefrequencysfofthesignalx(t),thatistheenvelopeweneed.
3.CORRELATIONMETHODANDToFESTIMATION
Afterrecoveringtheenvelopefromthedistortedandnoisynon-stationarysignal.ThenextworkistocompletetheToFestimation.SeveraltechniquescanbeusedtoaccomplishthetaskoftheToFestimation.Thetraditionalapproachusedinthemostultrasonicsensorsisthethresholdmethod.Athresholdlevelissettled,andtheToFisconsideredtobemeasuredwhenthethresholdisfirstexceededbythe
signaloritsenvelope.
Althoughithasprovedtobesimpleandcheap,thistechniquemaysufferfrompoorresolution,particularlytheechopulsehasbeengreatlyattenuated.
Thecorrelationtechniqueisakindofmoresophisticatedtechniqueforapplicationsrequiringhighaccuracy,whichbasestheToFestimationonthesearchoftherelativemaximuminthecross-correlationfunctionbetweenthetransmittedsignalandtheechosignal.Accordingtotheindexofthemaximumvalueofthecorrelationfunction,thetimedelaybetweenthetwosignalsisobtained.Thecorrelationfunctionisdefinedas:
(5)
whenthevalueof)(hTCisthemaximum,theToFishT.
(6)
Consideringtheprocedureofthesamplingdata,actually,thefTestimationcan
beexpressedintheform:
(7)
wherehistheintegerpartof)5.0/(+TTf,andλisarealnumberinthe
interval)5.0,5.0[?
asuitablemeasurementalgorithmmustbeusedtoevaluateexactlytherealnumberλ.Itis
possibletoestimationthefractionnumberλthroughasuitableinterpolationtechnique.Thesecond-orderapproximateinterpolationdeterminestheparameterλas:
(8)
theequation
(1)canbeexpressedas:
(9)
4.SIMULATIONANDERRORANALYSIS
Inordertovalidatetheproposedmethod,arealisticsimulationmustbesetupto
mirrorthecharactersofanactualultrasonicsensor:
assumingasamplingfrequencyMf=8MHZ,anultrasonicsensorfrequencyfs=550KHZ,anultrasonicwavespeedinthemediumsmv/1390=,thedistanceresolutionis,accordingtotheequation
(1),0.087mm.Theerroranalysiscanbeimplementedonthecomputersimulations.
4.1SystematiccomponentoferrorsinToFestimation
ThefinitesamplingfrequencyduringtheprocedureoftheA/DconversionmaketheToFestimationinaccurate,thatis,thetimeresolutionerrorofsystemisaerrorsourcefortheToFestimation.Thisisasystematicerror,whichcanbethoughtasafunctionofthetimeresolution.Fromtheequation(5),this'ideal'resolutionerror
canbewrittenas:
(10)
thatcorrespondstoamaximumdistanceerrorequaltomm043.0.WhentheToFisamultipleofthesamplingperiod(λ),theerrorwillbezero.
Accordingtotheequation(6),theresultobtainedbycomputersimulationsconfirmthattheinterpolationmethodmakethetimeresolutionerrortoaminimumvalue.Asaresult,thesystematicerrorindistancemeasurementcanbewrittenasfollows:
(11)
4.2RandomcomponentoferrorsinToFestimation
Inthepracticalapplications,theexistenceofnoiseisacauseofToFinaccuracy.Wecanassumethatthenoiseisawhitenoise.Fromtheviewpointofenergy,lowSNRleadstohighnoiseintensity.Forthecorrelationmethod,itislesssensitive
tonoiseduetotheinherentcancellationofzero-meanrandomeffectsitself.
5.CONCLUSIONS
Theuseofdedicateddigitaltechniquesprovedtobeessentialforultrasonicsensors.Asuitablemeasurementprocedurehavebeensetupallowingtheperformanceoftheproposedtechniques.Withregardtoenvelopeextraction,theSTFTisagoodchoice.TheSTFTfilteroutthenoise,andrecovertheoptimizedenvelope.,thecorrelationmethodisbetterthanthethresholdmethodduetobetterperformancebothintermsofsystematicerror(timeresolution)andintermsofrandomerror(noise)forechodetection.
ACKNOWLEDGMENTS
Theauthorswouldliketothankthereviewersfortheirsuggestionsthatimproved
thereadabilityofthispaper.
REFERENCES
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