压电薄膜换能器2-eTouch

机器人皮肤、面板360度定位、无损检测等应用领域的超声波换能器中,PVDF压电薄膜利用其柔性、易成型的特点占据一定优势,目前已经在40kHz和80kHz在电子白板上取得了巨大的成功。较之PVDF压电薄膜,eTouch压电薄膜提供信噪比更高、价格更低。这一点已经被Ana Jimenez、Fernando Seco等几位科学家证明。

ieeetransactionsonultrasonics,ferroelectrics,andfrequencycontrol,vol.55,no.4,april2008919

BroadbandEMFi-BasedTransducersfor

UltrasonicAirApplications

JoaoL.Ealo,FernandoSeco,andAntonioR.Jimenez

Abstract—Inthiswork,weexplorethepossibilitiesofelectromechanical lm(EMFi)asanewmaterialforde-velopingbroadbandtransducersforultrasonicairapplica-tions.TheadvantagesoftheEMFi lmareitswideusablefrequencyrangeandeasinesstouse,makingithighlysuit-ableforself-made,customizableultrasonicsensors.Thispa-perpresentstheoreticalandexperimentalinformationfo-cusedontheneedsofthesensor’send-user,namely,fre-quencyresponse,actualdynamicmassandYoung’smod-ulus,bandwidth,sensitivity,electromechanicaldynamicalmodel,acousticresponse,anddirectivity.Itisfoundem-piricallythatthebehaviorofthe lmasanalmostidealpiston-likeacousticsourcepermitsaccuratepredictionofthecharacteristicsoftransducersbuiltonadevelopablesurface.Theresultsobtainedrepresentthe rststeptomorecomplexgeometries,and,ultimately,tocompletelycustomizable eldultrasonictransducers.

I.Introduction

U

ltrasonicsignalsarewidelyusedinroboticappli-cationssuchassoundecholocation,identi cationofobjects,localpositioningsystems,distanceorproximitysensing,processinspection,qualitycontrol,etc.Amongthem,rangingisperhapsthemostappliedtechniqueinrobotics,duetoitslowcostimplementation[1].Ingeneral,mostultrasonicapplicationsinairusethefrequencyrangebetween20and300kHz.However,someofthemrequireamaximumfrequencynotgreaterthan120kHzinordertominimizethesignalattenuationcausedbyabsorbtion,e.g.,robotnavigationandlocalizationatlongranges.Nowadays,mostroboticsystemsmakeuseofeitherpiezoceramicorcapacitivetransducers.Theformerareres-onantdeviceswithahighqualityfactorandcorrespond-inglylowbandwidth(typicallylessthan1/10relativetotheircentralfrequency);theyareveryruggedandinex-pensive.Asaconsequenceoftheapplieddampingmethod,theycanachievemorebandwidthbuttheirhighsensitiv-ityisreduced.Inageneralsense,thelatterhaveabetterperformancethanpiezoelectriconesbecausetheyexhibitthehighestbandwidth(about30kHzforsomePolaroidmodels).Besides,theyarethepreferredchoicewhenusingcodedsignals[2],[3].However,theyareexpensiveandnotveryrugged.

ManuscriptreceivedAugust17,2007;acceptedDecember10,2007.TheauthorsarewiththeInstitutodeAutom´aticaIndustrial,ConsejoSuperiordeInvestigacionesCienti cas,ArgandadelRey,Madrid,Spain(e-mail:jealo@iai.csic.es).

J.L.EaloisalsowiththeSchoolofMechanicalEngineering,Uni-versidaddelValle,Cali,Colombia.

DigitalObjectIdenti er10.1109/TUFFC.2008.727

Piezopolymers,mostnotablypolyvinylidene uoride(PVDF),havealsobeenappliedasultrasonictransducers.Yet,theirtransmittingsensitivityisanorderofmagnitudelowerthanthatofpiezoceramics.PVDF lmtransducersareinexpensive,aremoreruggedthancapacitivetransduc-ers,andhaveabandwidthofabout8–10kHzforacentralfrequencyof40kHz.Theyareparticularlyattractiveforaircoupledapplicationsbecauseoftheirlowimpedance.AirrangingdeviceshavebeendesignedbyToda[4],[5]andFiorillo[6].However,theirmechanicalsetupandprin-cipleofoperationbasedonlengthvariationsmakethemnotsuitablewhencustomizableacoustic eldsapplicationsareintended[7].

Inparticular,broadbandwidthisfundamentalforap-plicationsthatrequiresignalmodulationandcodi cationinordertoachievemultipletransduceroperation(CDMA)andexploittheprocessinggaininherenttothesignalstruc-ture.Nevertheless,mostoftheimplementedsystemsarebasedonpiezoelectricnarrow-bandceramicsforrangingmeasurements,whichbringsaboutthreemaindi culties:single-useraccess,lackofidenti cationencoding,andul-trasonicnoisesensitivity[8].Inviewofthis,therestillremainsacurrentdemandtodevelopultrasonictransduc-ersthatsatisfythetrade-o amongdirectivity,bandwidth,sensitivity,andcost.Inthiswork,theuseoftheelectrome-chanical lm(EMFi)isputforwardtoful lthisrequire-ment.

A.TheElectromechanicalFilm

Recently,cellularelectretpolymershavegeneratedgreatinterestduetotheirgoodstoringchargecapabilitiesand exibility.TheEMFiisamongthisnewtypeofma-terials.Basically,thislow-costthin lmisamicroporouspolypropylenefoamwithhighresistivityandpermanentchargeafterbeingpolarizedbythecoronamethod.Theresultantinnerairvoidsactasdipoleswhichmakeitpar-ticularlysensitivetoforcesnormaltoitssurface.Whengluedonarigidsubstrateandexcitedbyanexternalvolt-age,EMFicanbeusedasanactuator,operatingalwaysinthicknessmodewithoutin uenceofthesubstrategeome-try[7].Thesensormodeofoperationisthereciprocaltotheactuatorone.ItsverylowYoung’smodulusresultsinaverylowacousticimpedancesothatgoodadaptationtoairisachieved.Inviewofthis,EMFihasbeenusedtobuildupacoustictransducerssuchasloudspeakersandmicro-phones[9],physiologicalsensors[10],[11],keyboards,forcepositionsensors[12],etc.TheusablefrequencyrangeoftheEMFi lmforairapplicationsbeginsataudiblefrequen-

c2008IEEE0885–3010/$25.00

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