ModernSensors

Lecture7X.WuAReviewofLecture6DiaphragmdeformationequationsLinearlimit:centerdeflection<5XthicknessCorrugateddiaphragm~10XthicknessCapacitivesensororstraingaugeSomesiliconbasedexamplecalculationscarriedout,Analternativetechnology:Kavlicosensorandlearnasmuchaswecanfromthewayitisdesigned,built,packaged,andpriced.BasicIntentOverviewbasictechniquesforsensingtemperatureStudysomeproductexamplesSometechniquesforthemeasurementofflowwillbebrieflyhighlightedAnalogsynchronousmodulation/demodulationtechniqueForexample:LaserrangefinderEmittingsignal:Esin(2ft);ReceivedsignalEsin(2ft+),with=2f*2d/cBymixingr(t)byphas(t)=sin(2ft)(Inphase)andbyquad(t)=cos(2ft)(inQuadrature)weget:Vi(t)=r(t).phas(t)=(1/2)E{cos()-cos(2ft+)}Vq(t)=r(t).quud(t)=(1/2)E{sin()+sin(2ft+)}Afterlow-passfilter:I=(1/2)Ecos(),

Q=(1/2)Esin()Ortan()=Q/ILock-inAmplifierOperationofalock-inamplifierreliesontheorthogonalityofsinusoidalfunctions:integrateovermanyperiods

Signalnotthesamefreq.->zeroOut-of-phasesignal->zeroThermometers-traditionaltechniquesThereareanumberofwell-knownhistoricaltechnologiesforthemeasurementoftemperature.mercurythermometer,inwhichareservoirofmercuryissealedinaglasscontainerundervacuum.Whenthereservoirisheated,themercuryexpands,risingthroughalongthincolumn,uponwhichagradedrulerhasbeenetched.Whatsortofsensitivitycanbeexpectedforsuchasystem?SensitivityandResolutionConcernsthermalexpansioncoefficient(CTE)ofmercury~30PPM/KIfweassumethatdimensionsofthecontainerdonotchangeappreciablyThethermalexpansioncoeffiecientoffusedquartzisabout150xsmallerthanthatofmercury,sothisapproximationisroughlyvalid.themercuryinthecolumnexpandslinearlywithtemperature.

Ifwewant1mm/Katroomtemperature,andwehaveareservoirvolumeof0.1cm3,weneeda660mcolumn:thesensitivitydependsverystronglyonthediameterofthecolumn.OthertraditionaltechniquesBimetalbasedonthermalexpansion

verypopulareventoday.switchesarefairlyinexpensive,canoperatereliablyformanycycles,maystillbethecorrectchoicefortemperaturesensingapplications.Shortcomingsnotaccurateenough,notallowoperationoverabroadtemperaturerange.OthertraditionaltechniquesThermocoupleAmetalwire~avesselcontainselectronsHeatingoneendofthewire:theeffectofheatistoincreasetheaveragevelocityoftheelectronsontheheatedendofthewirenon-uniformdistributionofelectronsavoltageacrosstwoendsDifficultyinmeasuringthevoltageusingonewireHeatingthejointsoftwowiresofdifferentmaterialsthermalcoupleThermocouple–goodforhightempmeasurementThevoltagesgeneratedbysucheffectsarefairlysmall.KTypethermocouple:-100℃~1300℃JTypethermocouple:-100℃~760℃Agoodthermocoupleexhibitsavoltagesignalofonly10V/Kelvin.accuratemeasurementsofsmalltempchangesrequireverywell-designedelectronics.Formeasurementswhichrequireaccuracyof+/-10K,andneedtobecarriedoutattemperaturesnear1000K,thermocouplesaredefinitelythewaytogo.ResistanceThermometryPlatinumwiresarecommonlyusedforresistancethermometry.thoughexpensive,itisfavoredfortheseapplicationsforseveralverygoodreasons:reasonablylargetemperaturecoefficient,notaffectedbymostchemicals,mechanicallystablewithstandveryhightemperatures,fewothermetalsoffersuchafavorablecollectionoflong-termstabilityperformanceadvantages.Fornarrowertemperatureranges,whatcanweuse?Thermistorsand

theSteinhart-HartEquationNTCthermisterdepositingasmallquantityofsemi-conductorpasteontocloselyspacedparallelplatinumalloywiressinteredatahightemperatureatwhichtimethematerialformsatightbondbetweenthetwowires.Steinhart-HartequationforthermistorsGlossaryofterms-ThermisterZero-PowerResistance(Ro):Thedcresistancevalueofathermistorataspecifiedtemperaturewithnegligibleelectricalpowertoavoidselfheating.ZeroPowerTemperatureCoefficientofResistance(Alpha):Theratioataspecifiedtemperature(T),oftherateofchangeofzeropowerresistancewithtemperaturetothezeropowerresistanceResistanceTemperatureCharacteristic:

Therelationshipbetweenthezeropowerresistanceofathermistoranditsbodytemperature.Temperature-WattageCharacteristic:Therelationshipataspecifiedambienttemperaturebetweenthethermistortemperatureandtheappliedsteadystatewattage.ProductsbysomemanufacturersResolutionLimitResolutionlimitationimposedbynoiseR1:thermister,R2:loadresistor

ifRl>>R1IfthetempofR1changesby1K,theresistancechangesbyR1,thevoltagechangesby(VinR1/RL).Noise:ImprovetheResolutionbyreducingthetemperature,thebandwidth,andtheloadresistor,byincreasing

thetemperaturecoefficient

thebiasvoltage,Vin.Ofalloftheseparameters,itmaybeeasiesttoincreasethebiasvoltage.Self-heatingProblemThebiascausespowertobedissipatedinthesenseresistor.assumethatthesenseresistorisattachedtotheobjectofinterestwithafinitethermalconductanceG.Therewillbeatemperaturedifferencebetweenthesenseresistorandtheobjectofinterestgivenby:Thermalconductancebetweenthecoreofthethermistorsandthesurfacearegenerallyoforder10-2to1W/Kpowerdissipation1mWT=10-3-0.1degreeCSelf-heatingwillcancausesubstantialerrors.Measurementofchangingtemperature

withcontacttempsensorsAthermometerisattachedtoanobjectwithathermalconductanceofG(W/K).AssumethatThethermometerhasaheatcapacityC(J/K).

somepower,P,isbeingappliedtothethermometer(biascurrents)Fromenergybalance,theenergyintothethermometerequalsthechangeinenergyofthethermometer:

ObjectSensorStatic:Oscillating:Measurementofchangingtemperature

withcontacttempsensorsAfinitetemperatureoffsetduetobiaspower(P/G),oscillationamplitudewhichvarieswithfrequency.Atlowfrequency,Ts2

T2,Athigherfrequencies,thethermaltimeconstantassociatedwiththeheatcapacityofthethermometercancauseareduceoscillationandaphaselag.Theseissuesareimportanttokeepinmindformeasurementsoftime-varyingtemperatures.ThermistorApplicationaresistancemeasuringcircuit:avoltagedividerwithaloadresistor(RL)inserieswiththethermistor(Rs):SensitivityThermistorApplicationThelinearityisimproved(byreducingit)bytakingRL>>Ro,bykeepingeitherTorsmall.IfweneedacertainTandacertainlinearity,wecanselectthethermometerandtheloadresistanceRLtomeetourneeds.Whenusingathermistortomeasuresmalltemperaturechanges,noisecanimposealimitation.IfRL>>RoFlowSensorsTherearethreebasicapproachestothemeasurementofflowthermaleffectstomeasurefluidmotion.Ingeneral,thisapproachusesaheatsourcetodepositheatintothefluid,andathermometertomeasurethetemperatureofthefluid.Iftheheatsourceisupstreamofthesensor,flowincreasesheattransportandcausesthesensortemperaturetoincrease.Anotherpossiblearrangementistoheatathermistorwithafixedpower,andmeasureitstemperature.Inthiscase,fluidflowactstocoolthethermometer.ACommercialProductFlowSensorsAslightlymorecomplicatedapproachreliesonBernoulli'sEquation,whichis:Thisprincipleisappliedbymeasuringpressureatapairofpointsinafluid.Whenwaterflowsthroughapipewithavaryingdiameter,thetotalflowrateineachregionisaconstant,therefore,changesintubediameterarecompensatedforbychanges

influidvelocity.Bymeasuringthepressureinregionswithdifferentdiameter,itispossibletomeasurefluidvelocity.Onlyworkiftheflowisnotturbulent

Optical+MicrofluidicsAnExample:PressureDifferenceBasedFlowmeterMeasurethepressuredifferenceisbetweenthewideandnarrowregionsPressuredroptechniquesonlyworkiftheflowisnotturbulent(dissipative).ModernMFC(massflowcontroller)DopplereffectbasedflowsensorThelasttechniqueforflowmeasurementisbasedonmeasurementofDopplereffectsinsoundtransport.Sincesoundiscarriedbypressurewavesinamedium(thefluid),itstransportlaterallyacrossachannelisaffectedbythemotionofthefluid.Itispossibletomeasurethechangeinsoundfrequencyduetofluidmotion(directDopplereffect,orlistenforchangesinthetraveltimefromtransmittertoreceiver.Highsensitivitytechniquesgenerallymeasurefrequencyshifts,sinceexcellentaccuracymaybeobtainedbyuseofana