超声波测距外文文献加中文翻译毕业设计.docx

超声波测距外文文献加中文翻译毕业设计.docx

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超声波测距外文文献加中文翻译毕业设计

附录A英文原文

ULTASONICRANGINGINAIR

G.E.RudashevskiandA.A.GorbatovUDC534,321.9:

531.71.083.7

Oneofthemostimportantproblemsininstrumentationtechnologyistheremote,contactlessmeasurementofdistancesintheorderof0.2to10minair.Suchaproblemoccurs,forinstance,whenmeasuringtherelativethreedimensionalpositionofseparatemachinemembersorstructuralunits.Interestingpossibilitiesforitssolutionareopenedupbyutilizingultrasonicvibrationsasaninformationcarrier.Thephysicalpropertiesofair,inwhichthemeasurementsaremade,permitvibrationstobeemployedatfrequenciesupto500kHzfordistancesupto0.5mbetweenamemberandthetransducer,orupto60kHzwhenrangingonobstacleslocatedatdistancesupto10m.

Theproblemofmeasuringdistancesinairissomewhatdifferentfromotherproblemsinthea-pplicationofultrasound.Althoughthepossibilityofusingacousticrangingforthispurposehasbeenknownforalongtime,andatfirstglanceappearsverysimple,neverthelessatthepresenttimethereareonlyasmallnumberofdevelopmentsusingthismethodthataresuitableforpracticalpurposes.Themaindifficultyhereisinprovidingareliableacousticthree-dimensionalcontactwiththetestobjectduringseverechangesintheair'scharacteristic.

Practicallyallacousticarrangementspresentlyknownforcheckingdistancesuseamethodofmeasuringthepropagationtimeforcertaininformationsamplesfromtheradiatortothereflectingmemberandback.

Theunmodulatedacoustic（ultrasonic）vibrationsradiatedbyatransducerarenotinthemselvesasourceofinformation.Inordertotransmitsomeinformationalcommunicationthatcanthenbeselectedatthereceivingendafterreflectionfromthetestmember,theradiatedvibrationsmustbemodulated.Inthiscasetheultrasonicvibrationsarethecarrieroftheinformationwhichliesinthemodulationsignal,i.e.,theyarethemeansforestablishingthespatialcontactbetweenthemeasuringinstrumentandtheobjectbeingmeasured.

Thisconclusion,however,doesnotmeanthattheanalysisandselectionofparametersforthecarriervibrationsisofminorimportance.Onthecontrary,thefrequencyofthecarriervibrationsislinkedinaveryclosemannerwiththecodingmethodfortheinformationalcommunication,withthepassbandofthereceivingandradiatingelementsintheapparatus,withthespatialcharacteristicsoftheultrasoniccommunicationchannel,andwiththemeasuringaccuracy.

Letusdwellonthequestionsofgeneralimportanceforultrasonicranginginair,namely:

onthechoiceofacarrierfrequencyandtheamountofacousticpowerreceived.

Ananalysisshowsthatwithconicaldirectivitydiagramsfortheradiatorandreceiver,andassumingthatthedistancebetweenradiatorandreceiverissubstantiallysmallerthanthedistancetotheobstacle,theamountofacousticpowerarrivingatthereceivingareaPrforthecaseofreflectionfromanidealplanesurfacelocatedatrightanglestotheacousticaxisofthetransducercomesto

wherePradistheamountofacousticpowerradiated,Bistheabsorptioncoefficientforaplanewaveinthemedium,Listhedistancebetweentheelectroacoustictransducerandthetestme-mber,disthediameteroftheradiator（receiver）,assumingtheyareequal,andc~istheangleofthedirectivitydiagramfortheelectroacoustictransducerintheradiator.

BothinEq.

（1）andbelow,theabsorptioncoefficientisdependentontheamplitudeandnotontheintensityasinsomeworks[1],andthereforewethinkitnecessarytostressthisdifference.

Inthevariousproblemsofsoundrangingonthetestmembersofmachinesandstructures,therelationshipbetweenthesignalattenuationsduetotheabsorptionofaplanewaveandduetothegeometricalpropertiesofthesoundbeamare,asarule,quitedifferent.Itmustbepointedoutthatthechoiceofthegeometricalparametersforthebeaminspecificpracticalcasesisdictatedbytheshapeofthereflectingsurfaceanditsspatialdistortionrelativetosomeaverageposition.

Letusconsiderinmoredetailtherelationshipbetweenthegeometricandthepowerparametersofacousticbeamsforthemostcommoncasesofrangingonplaneandcylindricalstructuralmembers.

ItiswellknownthatthedirectionalcharacteristicWofacircularpistonvibratinginaninfinitebaffleisafunctionoftheratioofthepiston'sdiametertothewavelengthd/λasfoundfromthefollowingexpression:

（2）

whereJlisaBesselfunctionofthefirstorderandαistheanglebetweenanormaltothepistonandalineprojectedfromthecenterofthepistontothepointofobservation（radiation）.

FromEq.

（2）itisreadilyfoundthatatwo-to-onereductioninthesensitivityofaradiatorwithrespecttosoundpressurewilloccurattheangle

（3）

Foranglesα≤20.Eq.（3）canbesimplifiedto

（4）

wherecisthevelocityofsoundinthemedimaaandfisthefrequencyoftheradiatedvibrations.

ItfollowsfromEq.（4）thatwhenradiatingintoairwherec=330m/sec,thenecessarydiameteroftheradiatorforaspedfiedangleofthedirectivitydiagramatthe0.5levelofpressuretakenwithrespecttotheaxiscanbefoundtobe

（5）

wheredisincm,fisinkHz,andαisindegreesofangle.

CurvesareshowninFig.1plottedfromEq.（5）forsixanglesofaradiator'sdirectivitydiagram.

Thedirectivitydiagrmneededforaradiatorisdictatedbythemaximumdistancetobemeasuredandbythespatialdispositionofthetestmemberrelativetotheotherstructuralmembers.Inordertoavoidtheincidenceofsignalsreflectedfromadjacentmembersontotheacousticreceiver,itisnecessarytoprovideasmallangleofdivergenceforthesoundbeamand,asfaraspossible,asmall-diameterradiator.Thesetworequirementsaremutuallyinconsistentsinceforagivenradiationfrequencyareductionofthebeam'sdivergenceanglerequiresanincreasedradiatordiameter.

Infact,thediameterofthe"sonicated"spotiscontrolledbytwovariables,namely:

thediameteroftheradiatorandthedivergenceangleofthesoundbeam.Inthegeneralcasetheminimumdiameterofthe"sonicated"spotDminonaplanesurfacenormallydisposedtotheradiator'saxisisgivenby

（6）

whereListheleastdistancetothetestsurface.

ThespecifiedvalueofDmincorrespondstoaradiatorwithadiameter

（7）

AsseenfromEqs.（,6）and（7）,theminimumdiameterofthe"sonieated"spotatthemaximumrequireddistancecannotbelessthantworadiatordiameters.Naturally,withshorterdistancestotheobstaclethesizeofthe"sonicated"surfaceisless.

LetusconsiderthecaseofsoundrangingonacylindricallyshapedobjectofradiusR.TheproblemistomeasurethedistancefromtheelectroacoustictransducertothesidesurfaceofthecylinderwithitsvariouspossibledisplacementsalongtheXandYaxes.Thenecessaryangleαoftheradiator'sdirectivitydiagramisgiveninthiscasebytheexpression

（8）

whereαisthevalueoftheangleforthedirectivitydiagram,Ymaxisthemaximumdisplacementofthecylinder'scenterfromtheacousticaxis,andLministheminimumdistancefromthecenteroftheelectroacoustictransducertothereflectingsurfacemeasuredalongthestraightlineconnectingthecenterofthememberwiththecenterofthetransducer.

Itisclearthatwhenmeasuringdistance,the"running"timeoftheinformationsignaliscontrolledbythelengthofthepathinadirectionnormaltothecylinder'ssurface,orinotherwords,themeasuredistanceisalwaystheshortestone.Thisstatementiscorrectforallcasesofspecularreflectionofthevibrationsfromthetestsurface.ThesimultaneoussolutionofEqs.

（2）and（8）whenW=0.5leadstothefollowingexpression:

（9）

Intheparticularcasewherethesoundrangingtakesplaceinairhavingc=330m/sec,andontheasstunptionthatLmin<

（10）

wheredisincmandfisinkHz.

CurvesareshowninFig.2fordeterminingthenecessarydiameteroftheradiatorasafunctionoftheratioofthecylinder'sradiustothemaximumdisplacementfromtheaxisforfourradiationfrequencies.AlsoshowninthisfigureisthedirectivitydiagramangleasafunctionofRandYrnaxforfourratiosofminimumdistancetoradius.

Theultrasonicabsorptioninairisthesecondfactorindeterminingtheresolutionofultrasonicrangingdevicesandtheirrangeofaction.Theresultsofphysicalinvestigationsconcerningthemeasurementofultrasonicvibrationsairaregivenin[1-3].Upuntilnowtherehasbeennounambiguousexplanationofthediscrepancybetweenthetheoreticalandexpe-rimentalabsorptionresultsforultrasonicvibrationsinair.Thus,forfrequenciesintheorderof50to60kHzatatemperatureof+25oCandarelativehumidityof37%theenergyabsorptioncoefficientforaplanewaveisabout2.5dB/mwhilethetheoreticalvalueis0.3dB/m.TheabsorptioncoefficientBasafunctionoffrequencyforatemperatureof+25oCandahumidityof37%accordingtothedatain[2]canbedescribedbyTable1.

Theabsorptioncoefficientdependsontherelativehumidity.Thus,forfrequenciesintheorderof10to20kHzthehighestvalueoftheabsorptioncoefficientoccursat20%humidity[3],andat40%humiditytheabsorptionisreducedbyabouttwotoone.Forfrequenciesintheorderof60kHzthemaximumabsorptionoccursat30.7ohumidity,droppingwhenitisincreasedto98%orloweredto10%byafactorofapproximatelyfourtoone.

Theairtemperaturealsohasanappreciableeffectontheultrasonicabsorption[1].Whenthetemperatureofthemediumisincreasedfrom+10to+30,theabsorptionforfrequenciesbetween