ModernSensors

Lecture2X.WuLecture2BasicIntentOverviewthebasicsensorterminologyinproductdatasheets,andinthetechnicalliterature.Casestudy:anoff-the-shelfaccelerometer,ADXL150.Then,asummaryofthebasicelectroniccircuitswewillbecoveringinthecourseispresented.WheredoSensorsComeFrom?Wearegoingtocoverasignificantfractionofmodernsensors,somesortofinventionInterestingtothinkhowsuchthingscometobeManyresearchersouttherecompetingtoinvent,design,buildandsellsensorswhicharegoingtonextbigwaveinindustryThiscompetitionhasbeengoingonfordecadesWhatarethebasicfactsatwork?Basicfacts:R&DofmodernsensorsSomeofthepeopleworkinginthisbusinesshavegreatresourcesattheirdisposal.Researchersatnationallabsorbigindustriallabs,whereaccesstostateoftheartmaterialsandequipmentiskeytofindingthenextreallyimportantsensor.Accesstoresources(people,equipment,…)isabigadvantage.MarketDrive-animportantdrivingpowerIftheAutoIndustrysaysitisinterestedinbuying20milliongyroscopesatapriceof$10eacheveryyear,youcanbesurethathundredsofpeoplearetryingtomeetthatchallenge.So,therearesomeindustrieswhichgettosetthechallengeswhicharethefocusofinventorinterest.IntroductiontosensorterminologyAsensorisadevicewhichconvertsaphysicalphenomenaintoanelectricalsignal.Assuch,sensorsrepresentpartoftheinterfacebetweenthephysicalworldandtheworldofelectricaldevices,suchascomputers.TheotherpartofthisinterfaceisrepresentedbyActuators,whichconvertelectricalsignalsintophysicalphenomena.Foravirtualrealitygameglove,itisbettertohaveboth.IntroductiontosensorterminologyWhydowecaresomuchaboutthisinterface?Inrecentyears,enormouscapabilityforinformationprocessinghasbeendevelopedwithintheelectronicsindustry.theavailabilityofinexpensivemicroprocessorsishavingatremendousimpactonthedesignofproductsrangingfromautomobilestomicrowaveovenstotoys.Inrecentyears,versionsoftheseproductswhichutilizemicroprocessorsforcontroloffunctionalityarebecomingwidelyavailable.IntroductiontosensorterminologyCharacterizationanddatasheetsincetheoutputofthesensorisanelectricalsignal,wetendtocharacterizesensorsinthesamewaywecharacterizeelectronicdevices.Thedatasheetsformanysensorsareformattedjustlikeelectronicproductdatasheets.

However,therearemanyformatsoutthere,andnothingatalllikeaninternationalstandardforsensorspecifications.Wewillencounteravarietyofinterpretationsofsensorperformanceparameters,andsometimesalotofconfusionwillemerge.Itisimportantforyoutorealizethatthisconfusionisnotduetoourinabilitytoexplainthemeaningoftheterms-itisaresultofthefactthatdifferentpartsofthesensorcommunityhavegottencomfortableusingthesetermsdifferently.DataSheet:PurposeandFormatThedatasheetisprimarilyamarketingdocument.Itwillbedesignedtohighlightthepositiveattributesofthesensor,emphasizingsomeofthepotentialusesofthesensor,mightneglecttocommentonsomeofthenegativecharacteristicsofthesensor.Inmanycases,thesensorhasbeendesignedtomeetaparticularperformancespecificationforaspecificcustomer,andthedatasheetwillconcentrateontheperformanceparametersofgreatestinteresttothiscustomer.Inthiscase,thevendorandcustomermighthavegrownaccustomedtounusualdefinitionsforcertainsensorperformanceparameters.Asapotentialnewuserofsuchasensor,itisinitiallyyourproblemtorecognizethissituation,andinterpretthingsreasonably.So,expectthatyouwillencounterodddefinitionshereandthere,andexpectthatyouwillfindthatmostsensordatasheetsaremissingsomeinformationthatyoumightbemostinterestedin.SensorClassificationPassivesensordoesnotneedanyadditionalenergysourceanddirectlygeneratesanelectricsignalinresponsetoanexternalstimulus;athermocouple,aphotodiode,piezoelectricsensor.ActivesensorTheactivesensorssometimesarecalledparametricbecausetheirownpropertieschangeinresponsetoanexternaleffectandthesepropertiescanbesubsequentlyconvertedintoelectricsignals.Forexample,athermistorisatemperature-sensitiveresistor.Thesevariations(presentedinohms)directlyrelatetotemperaturethroughaknownfunction.aresistivestraingaugeinwhichelectricalresistancerelatestoastrain.SensorPerformanceCharacteristicsDefinitionsTransferFunction:ThisfunctionestablishesdependencebetweentheelectricalsignalSproducedbythesensorandthestimuluss.S=f(s),s:thestimulus;S:electricalsignaloutpute.g.S=a+bsLinear,b:sensitivity,a:offsetUsually,thisrelationshipisrepresentedasagraphshowingtherelationshipbetweentheinputandoutputsignal,andthedetailsofthisrelationshipmayconstituteacompletedescriptionofthesensorcharacteristics.Forexpensivesensorswhichareindividuallycalibrated,thismighttaketheformofthecertifiedcalibrationcurve.SensorPerformanceCharacteristicsDefinitionsSpanorDynamicRange:Adynamicrangeofstimuliwhichmaybeconvertedbyasensoriscalledaspanoraninputfullscale(FS).Itrepresentsthehighestpossibleinputvaluethatcanbeappliedtothesensorwithoutcausinganunacceptablylargeinaccuracy.Signalsoutsideofthisrangeareexpectedtocauseunacceptablylargeinaccuracy.Thisspanordynamicrangeisusuallyspecifiedbythesensorsupplierastherange,overwhichotherperformancecharacteristicsdescribedinthedatasheetsareexpectedtoapply.

SensorPerformanceCharacteristicsDefinitionsAccuracy:Generallydefinedasthelargestexpectederrorbetweenactualandidealoutputsignals.Sometimesthisisquotedasafractionofthefullscaleoutput.Forexample,athermometermightbeguaranteedaccuratetowithin5%ofFSO(FullScaleOutput)Hysteresis:Somesensorsdonotreturntothesameoutputvaluewhentheinputstimulusiscycledupordown.Thewidthoftheexpectederrorintermsofthemeasuredquantityisdefinedasthehysteresis.Typicalunits:Kelvinor%ofFSO

SensorPerformanceCharacteristicsDefinitions:SpanandErrorHighestoutputv.s.lowestinputCalibrationSensorPerformanceCharacteristicsDefinitionsNonlinearity(oftencalledLinearity):Themaximumdeviationfromalineartransferfunctionoverthespecifieddynamicrange.Thereareseveralmeasuresofthiserror.Themostcommoncomparestheactualtransferfunctionwiththe`beststraightline',whichliesmidwaybetweenthetwoparallellineswhichencompassestheentiretransferfunctionoverthespecifieddynamicrangeofthedevice.

SensorPerformanceCharacteristicsDefinitionsNoise:Allsensorsproducesomeoutputnoiseinadditiontotheoutputsignal.Ifthenoiseofthesensorislessthanthenoiseofthenextelementintheelectronics,orlessthanthefluctuationsinthephysicalsignal,inwhichcaseitisnotimportant.Manyothercasesexistinwhichthenoiseofthesensorlimitstheperformanceofthesystembasedonthesensor.Noiseisgenerallydistributedacrossthefrequencyspectrum.Manycommonnoisesourcesproduceawhitenoisedistribution,whichistosaythatthespectralnoisedensityisthesameatallfrequencies.Johnsonnoiseinaresistorisagoodexampleofsuchanoisedistribution.Forwhitenoise,

thespectralnoisedensityVolts/Root(Hz).e.g.using10Hzfilter:SensorPerformanceCharacteristicsDefinitionsResolution:Theresolutionofasensorisdefinedastheminimumdetectablesignalfluctuation.datasheetsgenerallyquoteresolutioninunitsofsignal/Root(Hz)ortheygiveaminimumdetectablesignalforaspecificmeasurement.Iftheshapeofthenoisedistributionisalsospecified,itispossibletogeneralizetheseresultstoanymeasurementResolutionResolutiondescribesthesmallestincrementsofstimuluswhichcanbesensed.Whenastimuluscontinuouslyvariesovertherange,theoutputsignalsofsomesensorswillnotbeperfectlysmooth,evenundertheno-noiseconditions.Theoutputmaychangeinsmallsteps.typicalforpotentiometrictransducers,occupancyinfrareddetectorswithgridmasks,andothersensorswheretheoutputsignalchangeisenabledonlyuponacertaindegreeofstimulusvariation.SensorPerformanceCharacteristicsDefinitions

Sincefluctuationsaretemporalphenomena,thereissomerelationshipbetweenthetimescaleforthefluctuationandtheminimumdetectableamplitude.Therefore,thedefinitionofresolutionmustincludesomeinformationaboutthenatureofthemeasurementbeingcarriedout.Manysensorsarelimitedbynoisewithawhitespectraldistribution.Resolution=Theactualresolution=Resolutionx(measurementbandwidth)PhysicalSignalTimeSensorPerformanceCharacteristicsDefinitionsBandwidth:Allsensorshavefiniteresponsetimestoaninstantaneouschangeinphysicalsignal.manysensorshavedecaytimes,whichwouldrepresentthetimeafterastepchangeinphysicalsignalforthesensoroutputtodecaytoitsoriginalvalue.Thereciprocalofthesetimescorrespondtotheupperandlowercutofffrequencies,respectively.Thebandwidthofasensoristhefrequencyrangebetweenthesetwofrequencies.Humanear,e.g.20-20KHzorresponsetime50ms-50s

ThesedefinitionsareadaptedfromthoseinJ.Fraden,AIPHandbookofModernSensors,Physics,Designs,andApplications,AmericanInstituteofPhysics1993AccelerometerADXL150fromAnalogDevicesGeneralDescription50gSin