传感器第二章.pptx

1.INTRODUCTIONThissectiondiscussesthefundamentalconceptsofbridgecircuits.Resistiveelementsaresomeofthemostcommonsensors.Theyareinexpensivetomanufactureandrelativelyeasytointerfacewithsignalconditioningcircuits.Resistiveelementscanbemadesensitivetotemperature,andlight.Usingthesebasicelements,manycomplexphysicalphenomenacanbemeasured.Sensorelementsresistancescanrangefromless100toseveralhundredk.RESISTANCE OF POPULAR SENSORSStrainGages120,3503500Weigh-ScaleloadCells350-3500PressureSensors350-3500RelativeHumidity100k10MResistanceTemperatureDevices(RTDs)100,1000Thermistors100-10MResistive sensors produce small percentage changes inresistanceinresponsetoachangeinaphysicalvariablesuch as temperature or force.Platinum RTDs have atemperaturecoefficientofabout0.385%/oC.Thus,inordertoaccuratelyresolvetemperatureto1oC,themeasurementaccuracymustbemuchbetterthan0.385fora100RTD.Onetechniqueformeasuringresistance is to force aconstantcurrentthroughtheresistivesensorandmeasurethevoltageoutput.VOUT=I(R+R)R+R2.Bridge ConfigurationsBridges offer an attractive alternative for measuringsmallresistancechangesaccurately.ThebasicWheatstonebridge(actuallydevelopedbyS.H.Christiein1833)isshownbelow.Itconsistsoffourresistorsconnected to form a quadrilateral.The detectormeasures the difference between the outputs of twovoltagedividersconnectedacrosstheexcitation.Abridgemeasuresresistanceindirectlybycomparisonwithasimilarresistance.Thetwoprincipleways of operating a bridge are as detector or as adevicethatreadsadifferencedirectlyasvoltageTHE WHEATSTONE BRIDGE V0=VB-VB=ATBALANCE,VO=0IFWhen R1/R4=R2/R3,the resistance bridge is at anull,irrespectiveofthemodeofexcitation(currentorvoltage,AC or DC),the magnitude of excitation,themodeofreadout(currentorvoltage),ortheimpedanceofthedetector.Therefore,iftheratioofR2/R3isfixedat K,a null is achieved when R1=K*R4.If R1 isunknownandR4isanaccuratelydeterminedvariableresistance,the magnitude of R1 can be found byadjusting R4 until null is achieved.Conversely,insensor-type measurements,R4 may be a fixedreference,andanulloccurswhenthemagnitudeoftheexternalvariable(strain,temperature,etc.)issuchthatR1=K*R4.For the majority of sensor applications employingbridges,however,thedeviationofoneormoreresistorsinabridgefromaninitialvalueismeasuredasanindicationofthemagnitude(orachange)inthemeasuredvariable.Inthiscase,theoutputvoltagechangeisanindicationofthe resistance change.Because very small resistancechangesarecommon,theoutputvoltagechangemaybeassmallastensofmillivolts,evenwithVB=10V.Inmanybridgeapplicationstheremaybetwo,orevenfourelementswhichvary.VO:(A)Single-Element(B)Two-Element(1)(C)Two-Element(2)(D)Four-ElementIn each case,the value of the fixed bridge resistor,R,ischosen to be equal to the nominal value of the variableresistor(s).Thedeviationofthevariableresistor(s)aboutthenominal value is proportional to the quantity beingmeasured,such as strain(in the caseof a strain gage)ortemperature(inthecaseofanRTD).The sensitivity of a bridge is the ratio of the maximumexpected change in the output voltage to the excitationvoltage.Forinstance,ifVB=10V,andthefullscalebridgeoutputis10mV,thenthesensitivityis1mV/V.As the equation indicates,the relationship between thebridge output and resistor is not linear.For example,ifR=100andR=0.1(0.1%change in resistance),theoutputofthebridgeis2.49875mVforVB=10V.In some applications,the bridge nonlinearity may beacceptable,but there are various methods available tolinearizebridges.Sincethereisafixedrelationshipbetweenthe bridge resistance change and its output(show in theequations),software can be used to remove the linearityerrorindigitalsystems.Circuittechniquescanalsobeusedto linearize the bridge output directly,and these will bediscussedshortly.Thetwo-elementvaryingbridgeofthetwo-elementvaryingbridge,Case(2),requirestwoidenticalelementsthatvaryinopposite directions.Note that this configuration is linear,andliketwo-elementCase(1),hastwicetheofthesingle-elementconfiguration.Theall-elementvaryingbridgeproducesthemostsignal for a given resistance change and isinherently linear.It is an industry-standardconfigurationforloadcellswhichareconstructedfromfouridenticalstraingages.Bridge may also be driven from constant currentsources.Note also that with constant currentexcitation,all configurations are linear with theexceptionofthesingle-elementvaryingcase.3.Amplifying and Linearizing Bridge OutputsThe output of a single-element varying bridge may beamplified by a single precision op-amp connected in theinvertingmode.Thiscircuit,althoughsimple,haspoorgainaccuracyandalsounbalancesthebridgeduetoloadingfromRFandtheopampbiascurrent.TheRFresistorsmustbecarefully chosen and matched to maximize the commonmoderejection(CMR).AlsoitisdifficulttomaximizetheCMRwhileatthesametimeallowingdifferentgainoptions.Inaddition,theoutputisnonlinear.FORASINGLE-ELEMENTVARYINGBRIDGEVarioustechniquesareavailabletolinearizebridges.LINEARIZINGASINGLE-ELEMENTVARYINGBRIDGEMETHOD1