文献翻译数字技术在超声波传感器中的应用Word格式.docx

文献翻译数字技术在超声波传感器中的应用Word格式.docx

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（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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