超声波测距外文文献加中文翻译毕业设计Word文件下载.docx
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超声波测距外文文献加中文翻译毕业设计Word文件下载.docx
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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"
spotDminonaplanesurfacenormallydisposedtotheradiator'
saxisisgivenby
(6)
whereListheleastdistancetothetestsurface.
ThespecifiedvalueofDmincorrespondstoaradiatorwithadiameter
(7)
AsseenfromEqs.(,6)and(7),theminimumdiameterofthe"
sonieated"
spotatthemaximumrequireddistancecannotbelessthantworadiatordiameters.Naturally,withshorterdistancestotheobstaclethesizeofthe"
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<
<
R,thenecessarydiameterofaunidirectionalpistonradiatordcanbefoundfromthefomula
(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
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