Chapter-1--Sensors-and-Measurement.ppt

ShanghaiMaritimeUniversityElectricalEngineeringDepartment•Chapter1:SensorsandMeasurement•Chapter2:Computer-BasedMeasurementSystems•Chapter3:TemperatureSensors•Chapter4:StressandPressureSensors•Chapter5:Displacementsensors•Chapter6:Electromagnetisminsensing•Chapter7:WirelesssensornetworksContentsChapter1SensorsandMeasurement◆1.1FundamentalsofSensorTechnology◆1.2ApplicationConsiderationsofSensors◆1.3MeasurementCriteria101.1FundamentalsofSensorTechnologyAsensorisadevicethatconvertsaphysicalphenomenonintoanelectricalsignal.sensorsrepresentpartoftheinterfacebetweenthephysicalworldandtheworldofelectricaldevices,suchascomputers.Theotherpartofthisinterfaceisrepresentedbyactuators,whichconvertelectricalsignalsintophysicalphenomena.(1)SensorDataSheetsThedatasheetisprimarilyamarketingdocument.Itistypicallydesignedtohighlightthepositiveattributesofaparticularsensorandemphasizesomeofthepotentialusesofthesensor,andmightneglecttocommentonsomeofthenegativecharacteristicsofthesensor.1.1FundamentalsofSensorTechnology111.1FundamentalsofSensorTechnology(2)SensorPerformanceCharacteristicsDefinitionsTransferFunctionThetransferfunctionshowsthefunctionalrelationshipbetweenphysicalinputsignalandelectricaloutputsignal.Usually,thisrelationshipisrepresentedasagraphshowingtherelationshipbetweentheinputandoutputsignal,andthedetailsofthisrelationshipmayconstituteacompletedescriptionofthesensorcharacteristics.SensitivityThesensitivityisdefinedintermsoftherelationshipbetweeninputphysicalsignalandoutputelectricalsignal.Itisgenerallytheratiobetweenasmallchangeinelectricalsignaltoasmallchangeinphysicalsignal.Assuch,itmaybeexpressedasthederivativeofthetransferfunctionwithrespecttophysicalsignal.121.1FundamentalsofSensorTechnologySpanorDynamicRangeTherangeofinputphysicalsignalsthatmaybeconvertedtoelectricalsignalsbythesensoristhedynamicrangeorspan.Signalsoutsideofthisrangeareexpectedtocauseunacceptablylargeinaccuracy.Thisspanordynamicrangeisusuallyspecifiedbythesensorsupplier.AccuracyorUncertaintyUncertaintyisgenerallydefinedasthelargestexpectederrorbetweenactualandidealoutputsignals.Typicalunitsarekelvin.Sometimesthisisquotedasafractionofthefull-scaleoutputorafractionofthereading.131.1FundamentalsofSensorTechnologyHysteresisSomesensorsdonotreturntothesameoutputvaluewhentheinputstimulusiscycledupordown.Thewidthoftheexpectederrorintermsofthemeasuredquantityisdefinedasthehysteresis.TypicalunitsarekelvinorpercentofFSO.Nonlinearity(oftencalledLinearity)Themaximumdeviationfromalineartransferfunctionoverthespecifieddynamicrange.Thereareseveralmeasuresofthiserror.Themostcommoncomparestheactualtransferfunctionwiththe“beststraightline,”whichliesmidwaybetweenthetwoparallellinesthatencompasstheentiretransferfunctionoverthespecifieddynamicrangeofthedevice.141.1FundamentalsofSensorTechnologyNoiseAllsensorsproducesomeoutputnoiseinadditiontotheoutputsignal.Noiseisgenerallydistributedacrossthefrequencyspectrum.ResolutionTheresolutionofasensorisdefinedastheminimumdetectablesignalfluctuation.BandwidthAllsensorshavefiniteresponsetimestoaninstantaneouschangeinphysicalsignal.Inaddition,manysensorshavedecaytimes,Thereciprocalofthesetimescorrespondtotheupperandlowercutofffrequencies,respectively.Thebandwidthofasensoristhefrequencyrangebetweenthesetwofrequencies.Manysensorsarelimitedbynoisewithawhitespectraldistribution.Inthesecases,theresolutionmaybespecifiedinunitsofphysicalsignal/root(Hz).Thentheactualresolutionforaparticularmeasurementmaybeobtainedbymultiplyingthisquantitybythesquarerootofthemeasurementbandwidth.151.1FundamentalsofSensorTechnology(3)SensorPerformanceCharacteristicsofanExampleDeviceToaddsubstancetothesedefinitions,wewillidentifythenumericalvaluesoftheseparametersforanoff-the-shelfaccelerometer,AnalogDevices’sADXL150.TransferFunctionThefunctionalrelationshipbetweenvoltageandaccelerationisstatedasgmVAccVAccV1675.1)((1.1)SensitivityThesensitivityofthesensorisgivenbythederivativeofthevoltagewithrespecttoaccelerationattheinitialoperatingpoint.Forthisdevice,thesensitivityis167mV/g.161.1FundamentalsofSensorTechnologyDynamicRangeThestateddynamicrangefortheADXL322is±2g.Forsignalsoutsidethisrange,thesignalwillcontinuetoriseorfall,butthesensitivityisnotguaranteedtomatch167mV/gbythemanufacturer.HysteresisThereisnofundamentalsourceofhysteresisinthisdevice.Thereisnomentionofhysteresisinthedatasheets.TemperatureCoefficientThesensitivitychangeswithtemperatureinthissensor,andthischangeisguaranteedtobelessthan0.025%/C.Theoffsetvoltagefornoacceleration(nominally1.5V)alsochangesbyasmuchas2mg/C.Expressedinvoltage,thisoffsetchangeisnolargerthan0.3mV/C.LinearityInthiscase,thelinearityisthedifferencebetweentheactualtransferfunctionandthebeststraightlineoverthespecifiedoperatingrange.Forthisdevice,thisisstatedaslessthan0.2%ofthefull-scaleoutput.171.1FundamentalsofSensorTechnologyNoiseNoiseisexpressedasanoisedensityandisnomorethan300microg/rootHz.Toexpressthisinvoltage,wemultiplybythesensitivity(167mV/g)toget0.5microV/RtHz.Then,ina10Hzlow-pass-filteredapplication