超声波测距外文文献加中文翻译毕业设计Word文档下载推荐.docx

超声波测距外文文献加中文翻译毕业设计Word文档下载推荐.docx

《超声波测距外文文献加中文翻译毕业设计Word文档下载推荐.docx》由会员分享，可在线阅读，更多相关《超声波测距外文文献加中文翻译毕业设计Word文档下载推荐.docx（11页珍藏版）》请在冰豆网上搜索。

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