《現(xiàn)代傳感器》課件：Lecture6

ModernSensors

Lecture6X.WuAReviewofLecture5Whycannotpiezoelectricsensorbeusedforthestaticsignal?Doesthechargeproducedbyapiezoelectricelementdependonitssize?Whatarethemeasuresthathavebeentakentogreatlyimprovethereliabilityofmodernaccelerometers?Whatisthetypicalresonantfreqandmaxresponsefreqofacommoncommercialaccelerometer?PressureSensorsAsidefromsomeexoticapproaches,pressuresensorsalloperateonthebasisofthesameprinciple:thedetectionofaphysicalforcewhicharisesduetopressuredifference:Force=(P1-P2)(Diaphragmarea)

measurethedeflectionofthediaphragmwithadisplacementtransducercapacitivetransducer,embeddedstraingaugesondiaphragm.toouradvantagetohaveathindiaphragminordertomaximizethedeflectionthatweplantomeasure.MaterialsfortheDiaphragmThethicknessofthediaphragmlimitedbythetechnologyusedtomanufactureit.metalfoildiaphragmsStandardtechnologyformetalfoilfabrication:~mm

bondingofsuchfoilstotheremainderofthesensorstructurecanbedifficult.CeramicsandglassesCeramicscastingtechniques:~5mmReliabilityathightemperature,andtheirmechanicalandchemicalstability.silicondiaphragmshavebecomepopularThepossibilityforthicknessbelow1mmUseofimplantedsiliconstraingauges,Integrationwithelectronics.excellentmechanicalproperties,theabsenceofplasticdeformation.Mostmodernpressuresensorsutilizethinsiliconorceramicdiaphragmsmountedwithoutinitialtension.DeformationEquationfor

circulardiaphragmP=pressuredifferenceacrossthediaphragmR=radiusE=Young'sModulusT=diaphragmthicknessv=Poisson'sratioNonlinearinYo,andthereforecannotbesolvedforYo.Thefirsttermrepresentsthestiffnessassociatedwiththebendingofthediaphragm;Thesecondtermrepresentsthestiffnessassociatedwiththestretchingofthediaphragm.ForYo<TAnexampleConsideraSiliconDiaphragmE=1.9x1011v=0.25T=100mR=1cm

Assumethatthereisapressuredifferenceof1atmosphereacrossthediaphragm:P=101kPa=101000N/m2

Q:Whatisthecenterdeflection?A:assumingthatthedeflectionissmallenoughtousethelinearexpression:Y0=9.3T,smalldeflectionsisinvalid.->nonlineareq.:Yo=2.5T=250m

DetailsWhentheassumptionoflinearityisvalid,themembranedeflectionatanarbitrarypositionisexpressedas:atx=0,thisreducestotheearlierexpression,atx=R,thisexpressionfallstozero,aswewouldexpect,sincethedeflectionattheperimeterhastobezero.Usingtheseexpressionstocalculatetheresponseofapressuresensorbasedonadisplacementtransducer.Forexample,ifapressuresensorusedanopticaldisplacementtransducer,theaboveexpressionscouldbeusedtocalculatehowmuchareflectiveelementattachedtothecenterofthediaphragmwouldmoveforagivenpressure.xCapacitivePressureSensorThecapacitancebetweentheseelectrodescanbecalculatedbyanintegration:

ExampleCalculationa1cmradiussilicondiaphragmwithathicknessof20umandagapof50um.Theinitialcapacitancebetweentheseelectrodesisgivenby:Forapressuredifferenceof2.5kPa,thecapacitancechangeis:

Capacitancechangesby1%inthiscase.Ameasurablecapacitancechange;largerchangecouldgetclosetotheedgeofthelinearlimit.*thecapacitancechangeexpressionisbasedonthesmalldeflectionassumption.Tocheckthevalidityofthissolution,weshouldcalculatethedeflectionofthecenterofthediaphragmandcompareitwiththediaphragmthickness.LinearityConcernsIfasensordesignedtobelinearupto2.5kPaofpressuredifference,thislinearityissuewouldbeofseriousconcern.Inarealdesign,wewouldincreasethediaphragmstiffness(smallerRorlargerT)tolimitthedeflectiontosmallervalues.Historically,thelinearityissueswereacorrugateddiaphragm,suchastheonesusedinaneroidbarometersallowlargeamplitudedeflectionswithoutrequiringthemembranetobestretched,sincethecorrugationsmaybestraightened.Generally,mechanicaldesignswithflatdiaphragmswhichproducedeflectionsupto10timesthediaphragmthicknessmaybelinearizedbyintroductionofasimplecorrugationstructure.Aneroid(liquid-free)barometersBarometerwithacorrugateddiaphragmRecorderToday’sStructuresPiezoresistivesiliconCapacitiveceramicPerformance

Basicceramicmaterial(Al2O399.9%)

-extremelyhighoverloadlimit

-absolutelyresistanttowear

-hightemperaturestability

-highlong-termstability

-nohysteresis

-corrosion-resistantCapacitiveceramicpressuresensorTechnicaldata

-Supplyvoltage:5VDC

-Range:gauge-25mbar25mbar/-1bar70bar

-absolute0200mbar/070bar

-Risetime:approx.1ms

-Outputsignal:0.5V

-Processtemperaturerange:-40-125

-Compensatedtemperature:-20–80C

-Long-termstability:max.annualerror0.1%ofrangeDimensions(Φ*H):32.4*5.257.12mm(dependingonmeasuringrange)Challengetopic:real-timeex-vivo(non-invasive)bloodpressuresensing

Systolicv.s.diastolicPressureduetoheartpumpingpulseBloodvesselpressureStraingaugebasedpressuresensorAnotherapproachtothemeasurementofforcesonthediaphragmisbasedonmeasurementofstraininthediaphragm.Radialstressinducedontheuppersurfacebyanaxialpressureinathindiaphragmisgivenby:Thesignofthestresschangesfromtheedge(positive-tensile)tothecenter(negative-compressive),Zero-stresscircle,notaffectedbypressureappliedtothediaphragm.Finally,thestressisgreatestattheedgeofthediaphragm,sotheedgesarethebestlocationsforthestraingaugestobeapplied.CaseStudy:aNovasensorproductAstraingaugepressuresensorsoldbyNovasensor,specifiedforapressurerangeof0-2.5kPa,withamaximumpressureof25kPa.Assumethatsilicondiaphragm2mm,thicknessof20m.Q:Wouldsuchadevicegiveameasurablesignal?A:0.25%changeintheresistancevalue,whichissmall,butmeasurable.Canthediaphragmbethinner?Canthediaphragmbethinner?Weneedtostaybelowthefailurelimitforthesilicondiaphragm.thedevicemustsurvivepressuresignalsupto25kPa,whichis10timeslargerthanthecasewecalculated.Byscaling,suchasignalwouldproduceastrainofonly0.025%,whichisverywellbelowtheyieldlimitinsilicon(3%).

Then,howmuchthinnercouldthediaphragmbe?Howclosetoyieldlimit?Aorderofmagnitude->0.3%Givenpmax,R,=0.2,findTmin=?HomeworkStraingaugepressuresensorsareverycommoninindustrythesedays,primarilybecausethesiliconmicromachiningtechnologynecessarytomanufacturedecentsensorshasbeenavailableatverylowcost(<$50)forseveralyears.Atleastadozensmallcompanieshavebeeninthismarketforseveralyears.Today’sPressureGaugesSilicon-basedThin-filmOpticalCaseStudy:KavlicoPressureSensorAlternativetechnologiesavailabletocompetewiththesiliconfabricationsWhatcustomerneedsismostimportant!aparticularpressuresensorbuiltbyKavlico,Inc.,inMoorpark,CAforautomotiveapplicationsAclassicalcaseforstudyoffullyuseofownresourcesoralternativetechnologiestocompetewithbigplayersMaterials:CeramicandConductiveInk!KavlicoSensor:BackgroundInfo1Theintendedapplication:measurementoftheabsolutepressureintheintakemanifold,combinedwithameasurementoftheoxygenintheexhauststreamtoregulatetheintakeofairandfuelintothecylinder.Becauseofthechemistryinvolvedinthecombustion,anabsolute(notrelative)pressuremeasurementisrequired.Absolutepressurecanbemeasuredbymeasuringrelativepressurewithrespecttoavacuumsealedononesideofthediaphragm.Suchameasurementisalwaysadifficultthingtoaccomplish,becausevacuumleaksoroutgassingcanleadtosignificantoffsets.KavlicoSensor:BackgroundInfo2Theenvironmentofthemeasurement:Temperatureandhumiditycanvaryoverawiderangeduringengineoperation,soaccuratecontrolandcompensationoftheseeffectsisrequired.engineisanelectricallynoisyplace.Pulsesoflargecurrentflowtothesparkplug,andthereareelectromagneticdisturbancesassociatedwiththegeneratorsandmotorsthroughouttheenginecompartment.Therefore,itisnecessarytoprotectthesensorfromelectromagneticinterference(EMI).

KavlicoSensor:BackgroundInfo3Lifetimeautomotivecomponentsarenowrequiredtofeatureverylongperformancelifetimeswithverylowriskoffailure.Forexample,Chryslerisnowrequiringcomponentlifetimesof10yearswithfailureratesoflessthan1in10,000.Sothesensordesignandconstructionmustprovideforstable,reliableuseovertheselifecycles.Costthecostofautomotivecomponentsisalwaysundercompetitivepressure.Asageneralrule,automotivesensorsshouldcostabout$5each,fullypackaged,calibrated,andtested.KavlicoInc.-BackgroundKavlico

inthebusinessofmakingautomotivepressuresensorsforalmost20yearsnow.hasoptimizedarobust,low-costceramicsmanufacturingprocessbasedonceramictapecasting,thisprocessformsflexiblesheetsofceramictapewhichareeasilycutandformed.Aftercutting,theceramicpartsarefiredathightemperature,whichresultsinabout30%shrinkage,andtremendousincreasesintheelasticconstantsofthematerial.Theresultingpartsmaybemetallizedbyscreenprintingofconductiveinksbondedinaglasssealingprocess,andmountedwithcircuitsformeasurementapplications.StructureandPerformanceCapacitivesensor2-5cmdiameter,0.2-0.4mmthickceramicdiaphragmMetalsubstrate,metalelectrodesMetallicbondingProvidesanamplifiedvoltageoutputwhiletypicallyoperatingon5Vdc.Operatingtemperaturespanis-40°Cto+125°C.Cost$4:ceramicdiaphragm$1,circuit$1.5,package$1,Trimming$0.5KeyTechnologiesAppropriateforcapacitivepressuresensing.Inthiscase,thediaphragmissealedwithavacuumreservoirononeside,andapairofmetalelectrodepatternsaredepositedonfacingsurfaceswithinthevacuumcavity.Theelectrodeconfigurationfeaturesacommonelectrodeonthediaphragmacenter'senseelectrode'surroundedbyaannular'referenceelectrode'onthebottomofthereservoir.Thisconfigurationallowseasymeasurementofacapacitancedifference,asisusefulforlinearizationandcancellationoftemperatureeffects.KeyTechnologiesThickFilmCircuit,0.1-1.0ummetallayers,screenprintedtracesofaconductiveinkDiscreteelectronicparts,suchasresistorsandcapacitorsaresurface-mountedontothethickfilmcircuitAsingleapplicationspecificintegratedcircuit(ASIC)isrequiredtocarryoutthecapacitancemeasurement.'QuadDiode'circuit,whichrequiresthefabricationofasetof4well-matcheddiodesandasine-waveoscillator.Howdoesthecircuitwork?homeworkThiscircuitryisavailabletoKavlicofromanoutsidevendorasamm-sizedsiliconchip,whichisalsosolder-bumpmountedonthethickfilmcircuit.AnExampleThesensordimensions:diaphragmradius=1.5cm,thickness=250um,gap=50m,pressuresignal=2.5kPa,E(ceramic)=4.5x1011N/m2,v=0.2.SotheKavlicosensorismeasuringcapacitancechangesofupto2%.1%relativeaccuracycanbeexpectedfromthiscircuitpackagedasathickfilmhybrid.Whynotuseastraingauge?Oneproblemisrelatedtothetemperaturesensitivityofsiliconstraingauges,whichwouldneedtobecompensatedveryaccurately.Infact,MotorolaisintroducingadeviceforthisapplicationwhichusesaHC6805microprocessortocompensatefortemperatureandothernonlinearitiesbyusingalook-uptablewhichisstoredonEEPROMmemoryonthemicroprocessor.SuchanapproachisonlyreasonableforMotorola.KavlicotodayFoundedi