文献翻译——传感器的基础知识

第0页外文文献资料BasicknowledgeoftransducersWiththeadvancementoftechnology,theproductionofelectronicweighingloadcellshavebeenwidelyappliedtoallwalksoflife,therealizationofmaterialfast,accurateweighing,especiallywiththeemergenceofthemicroprocessor,thedegreeofautomationofindustrialprocessestechnologycontinuestoimprove,theloadcellhasbecomeanessentialprocesscontroldevice,notfromthepreviousweighinglargetanks,hopperweightmeasurementandcranescales,truckscalesandothermeasurementcontrol,tomixavarietyofdistributionrawmaterialbatchingsystem,theproductionprocessofautomaticdetectionandcontrolofpowderfeedrate,haveappliedtheloadcell,atpresent,almostappliedtotheloadcellweighingallareas.Aboutquantitativehigh-speeddispensingsystem.Thesystemiscontrolledbythecomputerandloadcellweighing,andoutputsacontrolsignaltoperformvalueweighing,controlexternaltotheoperationofthefeedingsystem,theimplementationofautomaticweighinganddispensingtasksquickly.SystemusesMCS-51microcontrollerandV/Fvoltagefrequencyconverterandotherelectronicdevices,with8031asthecentralprocessor,BCDDIPvalueissetastheinputtray,amaterialinstalledinthehopper,theweightofthesensorelastomeroccursdeformation,andoutputstheelectricsignalproportionaltotheweightofthesensoroutputsignalamplifier,theinputV/Fconverterperformsa/Dconversion,theconversionfrequencysignalisfeddirectlyintothemicroprocessor8031,thedigitalmicrocomputerprocessing.Themicrocomputerontheonehandtheinstantaneousweightwasfedtothedigitaldisplaycircuitdisplaystheinstantaneousweightthereof,ontheotherhandcomparativeweighing,openandclosethechargingport,themiddlerangeofthedischargetankweighingcontrolvalue.Dispensingthepredeterminedvalueinthecontrolsystem,thesensorisacriticalpartoftheweighingscalesaffect第1页measurementaccuracyPieces,useGYL-3straingaugeloadforcesensor.FoursheetresistancestraingaugefullbridgebridgeconstitutedundertheaddedpressureofunchangedbridgesensoroutputsignalisproportionaltotheforceofgravityandpressureforBridgeonthesensor,andthechangeforBridgeofpressuredirectimpactonelectronicscalesmeasuringaccuracy,itrequiresverystablebridgepressure.Millivoltoutputofthesensorisamplifiedintoanoutputvoltagesignalof0-10V,intotheV/FconverterforA/Dconversion,thefrequencysignaloutputterminal8031isappliedtoacountofthetimerinputterminalT1on.Inthemicrocomputerinternaltimercountingbythetimer0,timer0regulartimerequiredbytheA/Dconverterresolutionsettings.Eachloadingboxesorbagsalongtheconveyormovementuntilfittedwithelectronicmaterials,saidthefollowing,theconveyorstopsmoving,twoelectromagneticcoilisenergized,electronichopperflip,sothatallthematerialintoaboxorbag,andwhenfinishedmaterialdowntheconveyorbeltmotorpoweredupagain,thematerialoftheboxorbagfilledoutandprotecttheconveyorbeltcontinuestorununtilthenextemptybagsoremptycontainersoffthelightsourcephotoelectricsensors,meanwhile,saidelectronichopperreset,anelectromagneticcoilisenergized,thefunnelelectronicscalesforautomaticfeeding,weightcontrolbycomputer,electronicscaleswhenthematerialisequaltoagivenvalue,anelectromagneticcoilpower,thespringforcehopperdoor.Thesystemstartsloadingthenextchargecycle.Whentheboxfunnelmaterialontheconveyorbeltforalongtimeandenough,thisprocesscangoonacontinuingbasis.Whennecessary,thestaffcanstoptheconveyortraybypullingenteradifferentcodegivenvalueandthenstart,youcanchangetheweightoftheboxorbag.Usinginfraredraytomeasurethephysicalpropertiesofthesensor.Infraredisalsoknownasinfraredlight,ithasthereflection,refraction,scattering,interference,suchasabsorptionproperties.Anymatteraslongasititselfhasacertaintemperature(aboveabsolutezero)toinfraredradiation.Infraredsensorisnotindirectcontactwiththeobjecttobe第2页testedsothereisnofrictionandahighsensitivity,fastresponse,etc.Infraredsensorsisusedinnon-contacttemperaturemeasurement,gascompositionanalysisandnondestructiveflawdetectionandwidelyusedinareassuchasthemedical,military,spacetechnologyandenvironmentalengineering.Forexample,thehumanbodysurfacetemperaturemeasurementbyinfraredremotesensorhotlikefigurecanbefoundthatthepartofthetemperatureanomalyfordiseasediagnosisandtreatmentintime(seethethermalimager);Bytakingadvantageofsatelliteinfraredsensorstomonitortheearthclouds,whichcanrealizeawiderangeofweatherforecast;Usinginfraredsensorscandetecttheplanerunningengineoverheating,etc.Telescopewithinfraredsensorcanbeusedformilitaryaction,woodlanddetectenemiesintheforest,citydetectwallatthebackoftheenemyinbattle,aboveallareusingtheinfraredsensortomeasurethebodysurfacetemperaturewhichtheenemyislocated.AboutSensorsinthecommercialelectronicscalesCurrently,pricecomputingscalecommercialuseisverypopular,willgraduallyreplacethetraditionalmechanicallever,saidthecasescales.PriceComputingScaleonthescaleplatformstructurehasaremarkablefeature:afairlylargescaleplatform,onlytobearthefullweightofthematerialinthemiddleofthesensordeviceaspeciallydesigned,high-precisionweighingsensorisasensor,youmustpressusingthespecificationsrequired.Iftherequiredspecificationsusednotonlycannotplaytheroleofweighingandeasilydamaged,especiallytheloadmustnotexceedtheabsolutesafetyvalues.Atransducerisadevicewhichconvertsthequantitybeingmeasuredintoanoptical,mechanical,or-morecommonly-electricalsignal.Thenergyconversionprocessthattakesplaceisreferredtoastransduction.Transducersarclassifiedaccordingtothetransductionprincipleinvolvedandtheformofthemeasured.Thusaresistancetransducerformeasuringdisplacementisclassifiedasaresistancedisplacementtransducer.1.TransducerElementsAlthoughthereareexception,mosttransducersconsistofasensing第3页elementandaconversionorcontrolelement.Forexample,diaphragms,bellows,straintubesandrings,bourdontubes,Thisdisplacementmaythenbeusedtochangeanelectricalparametersuchasvoltage,resistance,capacitance,orinductance.Suchcombinationofmechanicalandelectricalelementsformelectromechanicaltransducingdevicesortransducers.Similarcombinationcanbemadeforotherenergyinputsuchasthermal.Photo,andelectrochemicaltransducersrespectively.2.TransducerSensitivityTherelationshipbetweenthemeasuredandthetransduceroutputsignalisusuallyobtainedbycalibrationtestsandisreferredtoasthetransducersensitivityK1=output-signalincrement/measuredincrement.Inpractice,thetransducersensitivityisusuallyknown,and,bymeasuringtheoutputsignal,theinputquantityisdeterminedfrominput=output-signalincrement/K1.3.CharacteristicsofanIdealTransducerThehightransducershouldexhibitthefollowingcharacteristics.a)highfidelity-thetransduceroutputwaveformshapebeafaithfulreproductionofthemeasured;thereshouldbeminimumdistortion.b)Thereshouldbeminimuminterferencewiththequantitybeingmeasured;thepresenceofthetransducershouldnotalterthemeasuredinanyway.c)Size.Thetransducermustbecapableofbeingplacedexactlywhereitisneeded.d)Thereshouldbealinearrelationshipbetweenthemeasuredandthetransducersignal.e)Thetransducershouldhaveminimumsensitivitytoexternaleffects,pressuretransducers,forexample,areoftensubjectedtoexternaleffectssuchvibrationandtemperature.f)Thenaturalfrequencyofthetransducershouldbewellseparatedfromthefrequencyandharmonicsofthemeasurand.4.ElectricalTransducersElectricaltransducersexhibitmanyoftheidealcharacteristics.In第4页additiontheyofferhighsensitivityaswellaspromotingthepossibleofremoteindicationormesdurement.Electricaltransducerscanbedividedintotwodistinctgroups:a)variable-control-parametertypes,whichinclude:i)resistanceii)capacitanceiii)inductanceiv)mutual-inductancetypesThesetransducersallrelyonexternalexcitationvoltagefortheiroperation.b)self-generatingtypes,whichincludei)electromagneticii)thermoelectriciii)photoemissiveiv)piezo-electrictypesTheseallthemselvesproduceanoutputvoltageinresponsetothemeasurandinputandtheireffectsarereversible.Forexample,apiezo-electrictransducernormallyproducesanoutputvoltageinresponsetothedeformationofacrystallinematerial;however,ifanalternatingvoltageisappliedacrossthematerial,thetransducerexhibitsthereversibleeffectbydeformingorvibratingatthefrequencyofthealternatingvoltage.5.ResistanceTransducersResistancetransducersmaybedividedintotwogroups,asfollows:i)Thosewhichexperiencealargeresistancechange,measuredbyusingmethods.Potentiometersareinthisgroup.ii)Thosewhichexperienceasmallresistancechange,measuredbybridge-circuitmethods.Examplesofthisgroupincludestraingaugesandresistancethermometers.5.1PotentiometersAlinearwire-woundpotentiometerconsistsofanumberofturnsresistancewirewoundaroundanon-conductingformer,togetherwithawipingcontactwhichtravelsoverthebarwires.Theconstructionprinciplesareshowninfigure第5页whichindicatethatthewiperdisplacementcanberotary,translational,oracombinationofbothtogiveahelical-typemotion.Theexcitationvoltagemaybeeithera.c.Ord.c.andtheoutputvoltageisproportionaltotheinputmotion,providedthemeasuringdevicehasaresistancewhichismuchgreaterthanthepotentiometerresistance.Suchpotentiometerssufferfromthelinkedproblemofresolutionandelectricalnoise.Resolutionisdefinedasthesmallestdetectablechangeininputandisdependentonthecross-sectionalareaofthewindingsandtheareaoftheslidingcontact.Theoutputvoltageisthusaserialsofstepsasthecontactmovesfromonewiretonext.Electricalnoisemaybegeneratedbyvariationincontactresistance,bymechanicalwearduetocontactfriction,andbycontactvibrationtransmittedfromthesensingelement.Inaddition,themotionbeingmeasuredmayexperiencesignificantmechanicalloadingbytheinertiaandfrictionofthemovingpartsofthepotentiometer.Thewearonthecontactingsurfacelimitsthelifeofapotentiometertoafinitenumberoffullstrokesorrotationsusuallyreferredtointhemanufacturesspecificationasthenumberofcyclesoflifeexpectancy,atypicalvaluebeing20*1000000cycles.TheoutputvoltageV0oftheunloadpotentiometercircuitisdeterminedasfollows.LetresistanceR1=xi/xt*Rtwherexi=inputdisplacement,xt=maximumpossibledisplacement,Rttotalresistanceofthepotentiometer.ThenoutputvoltageV0=V*R1/(R1+(Rt-R1)=V*R1/Rt=V*xi/xt*Rt/Rt=V*xi/xt.Thisshowsthatthereisastraight-linerelationshipbetweenoutputvoltageandinputdisplacementfortheunloadedpotentiometer.ItwouldseenthathighsensitivitycouldbeachievedsimplybyincreasingtheexcitationvoltageV.however,themaximumvalueofVisdeterminedbythemaximumpowerdissipationPofthefinewiresofthepotentiometerwindingandisgivenbyV=(PRt)1/2.5.2ResistanceStrainGaugesResistancestraingaugesaretransducerswhichexhibitachangeinelectricalresistanceinresponsetomechanicalstrain.Theymaybeofthe第6页bondedorunbondedvariety.a)bondedstraingaugesUsinganadhesivethesegaugesarebonded,orcemented,directlyontothesurfaceofthebodyorstructurewhichisbeingexamined.Examplesofbondedgaugesarei)finewiregaugescementedtopaperbackingii)photo-etchedgridsofconductingfoilonanepoxy-resinbackingiii)asinglesemiconductorfilamentmountedonanepoxy-resinbackingwithcopperornickelleads.Resistancegaugescanbemadeupassingleelementstomeasuringstraininonedirectiononly,oracombinationofelementssuchasrosetteswillpermitsimultaneousmeasurementsinmorethanonedirection.b)unbondedstraingaugesAtypicalunbonded-strain-gaugearrangementshowsfineresistancewiresstretchedaroundsupportsinsuchawaythatthedeflectionofthecantileverspringsystemchangesthetensioninthewiresandthusalterstheresistanceofwire.Suchanarrangementmaybefoundincommerciallyavailableforce,load,orpressuretransducers.5.3ResistanceTemperatureTransducersThematerialsforthesecanbedividedintotwomaingroups:a)metalssuchasplatinum,copper,tungsten,andnickelwhichexhibitandincreaseinresistanceasthetemperaturerises;theyhaveapositivetemperaturecoefficientofresistance.b)semiconductors,suchasthermistorswhichuseoxidesofmanganese,cobalt,chromium,ornickel.Theseexhibitlargenon-linearresistancechangeswithtemperaturevariationandnormallyhaveanegativetemperaturecoefficientofresistance.c)metalresistancetemperaturetransducersThesedepend,formanypracticalpurposeandwithinanarrowtemperaturerange,upontherelationshipR1=R0*1+a*(b1-b2)whereacoefficientofresistancein-1,andR0resistanceinohmsatthereferencetemperatureb0=0atthereference第7页temperaturerange.Theinternationalpracticaltemperaturescaleisbasedontheplatinumresistancethermometer,whichcoversthetemperaturerange-259.35to630.5.d)thermistorresistancetemperaturetransducersThermistorsareresistorswhichexhibitlargenon-linerresistancechangeswithtemperaturevariation.Ingeneral,theyhaveanegativetemperaturecoefficient.Forsmalltemperatureincrementsthevariationinresistanceisreasonablylinear;but,iflargetemperaturechangesareexperienced,speciallinearizingtechniquesareusedinthemeasuringcircuitstoproducealinearrelationshipofresistanceagainsttemperature.Thermistorsarenormallymadeintheformofsemiconductordiscsenclosedinglassvitreousenamel.Sincetheycanbemadeassmallas1mm,quiterapidresponsetimesarepossible.5.4PhotoconductiveCellsThephotoconductivecell,usesalight-sensitivesemiconductormaterial.Theresistancebetweenthemetalelectrodesdecreaseastheintensityofthelightstrikingthesemiconductorincreases.Commonsemiconductormaterialsusedforphoto-conductivecellsarecadmiumsulphide,leadsulphide,andcopper-dopedgermanium.Theusefulrangeoffrequenciesisdeterminedbymaterialused.Cadmiumsulphideismainlysuitableforvisiblelight,whereasleadsulphidehasitspeakresponseintheinfra-redregionandis,therefore,mostsuitableforflame-failuredetectionandtemperaturemeasurement.5.5PhotoemissiveCellsWhenlightstrikesthecathodeofthephotoemissivecellaregivensufficientenergytoarrivethecathode.Thepositiveanodeattractstheseelectrons,producingacurrentwhichflowsthroughresistorRandresultinginanoutputvoltageV.PhotoelectricallygeneratedvoltageV=Ip.RlWhereIp=photoelectriccurrent(A),andphotoelectriccurrentIp=Kt.BWhereKt=sensitivity(A/im),andB=illuminationinput(lumen)Althoughtheoutputvoltagedoesgiveagoodindicationofthemagnitudeof第8页illumination,thecellsaremoreoftenusedforcountingorcontrolpurpose,wherethelightstrikingthecathodecanbeinterrupted.6.CapacitiveTransducersThecapacitancecanthusmadetovarybychangingeithertherelativepermittivity,theeffectivearea,orthedistanceseparatingtheplates.Thecharacteristiccurvesindicatethatvariationsofareaandrelativepermittivitygivealinearrelationshiponlyoverasmallrangeofspacings.Thusthesensitivityishighforsmallvaluesofd.Unlikethepotentionmeter,thevariable-distancecapacitivetransducerhasaninfiniteresolutionmakingitmostsuitableformeasuringsmallincrementsofdisplacementorquantitieswhichmaybechangedtoproduceadisplacement.7.InductiveTransducersTheinductancecanthusbemadetovarybychangingthereluctanceoftheinductivecircuit.Measuringtechniquesusedwithcapacitiveandinductivetransducers:a)A.C.excitedbridgesusingdifferentialcapacitorsinductors.b)A.C.potentiometercircuitsfordynamicmeasurements.c)D.C.circuitstogiveavoltageproportionaltovelocityforacapacitor.d)Frequency-modulationmethods,wherethechangeofCorLvariesthefrequencyofanoscillationcircuit.Importantfeaturesofcapacitiveandinductivetransducersareasfollows:i)resolutioninfiniteii)accuracy+-0.1%offullscaleisquotediii)displacementranges25*10-6mto10-3miv)risetimelessthan50uspossibleTypicalmeasurandsaredisplacement,pressure,vibration,sound,andliquidlevel.8.LinearVariable-differentialTtransformerBasedonthepiezoelectriceffectsensors.Isapowergenerationandelectromechanicalsensorsconvertfrom.Itisasensitiveelementmadeofa第9页piezoelectricmaterial.Charge-generatingpiezoelectricmaterialsurfacebytheforce.Thischargebychargeamplifierandimpedancemeasurementcircuitafterbecomingenlargedandtransformedsufferedpoweroutputisproportionaltotheexternalforce.Piezoelectricsensorsareusedtomeasuretheforceandenergyisconvertedintoelectricpowernon-physical.Theadvantageisthatthefrequencybandwidth,highsensitivity,highsignaltonoiseratio,simplestructure,reliableandlightweight.ThedisadvantageisthatsomeDCresponseofpiezoelectricmaterialsrequireproofmeasures,andtheoutputofthedifferentialrequireshighinputimpedancecircuitorachargeamplifiertoovercomethisdefect.9.pyroelectricsensorThepyroelectricsensorisatemperaturevariationisconvertedtochargethedevice.Itistheuseofcertainmaterialpropertieschangewithtemperatureorelementcharacteristictobemeasured.Forexample,atemperaturechangetoaresistancechangeinthermalelectromotiveforce,thermalexpansion,permeability,etc.thenthepurposeofdetectingthetemperaturemeasuredbyasuitablecircuit.Thetemperaturevariationisconvertedtopotentialcalledpyroelectricsensorthermocouple;convertthetemperaturechangeoftheresistancevalueofthepyroelectricsensoriscalledthermalresistance.WorksAthermocoupleisatemperaturesensormadeofapyroelectriceffect.Theso-calledthermoelectriceffectisthetwodifferentconductormaterials(orsemiconductor)formaclosedloop,whenthejunctiontemperatureTandT0oftwophenomenaisnotthesame,thentheloopwillproduceanelectromotiveforce.EMFgeneratedbythethermoelectriceffect,includingthecontactforceandthermoelectricpower.Thecontactforceistheforceduetothedifferentdensityoffreeelectronsinthetwodifferentconductorsformedatthecontact.Itsvaluedependsonthematerialpropertiesandthetemperatureofthepointofcontactoftwodifferentconductors.Anelectromotiveforceisthetemperaturedifferencebetweenthetwoendsofthesameconductoratdifferenttemperaturesanditsgenerated.Mechanismwhichproducesas:high第10页temperaturesidethanthelowendoftheelectronenergyelectronenergy,thenumberofelectronsfromthehighendofthelowendwentfromlowendthanthehigh-temperaturesidetogotomore,theendresultofthelossofelectrontemperatureandpositivelycharged,lowendobtainedbyexcesselectronsarenegativelycharged,thetemperaturedifferenceisformedintheelectromotiveforceacrosstheconductor.Thermalresistanceofthesensoristheuseoftheprincipleconductorresistancechangeswithtemperaturechangesoftemperaturemeasurement.Thermalresistanceiswidelyusedtomeasurethetemperatureintherangeof-200850,andinafewcases,hypothermiameasureto1K,temperaturesupto1000.Accuracystandardplatinumresistancethermometerastemperaturescaleinternationalstandardinstrument.Sensorsvary,evenforthesametypeofmeasuringtheamountoftheworkingprinciplecanbeappliedindifferentsensors,therefore,necessarytoselectthemostsuitablesensors.a)MeasurementconditionsIfyoumistakenlyselectthesensor,itwillreducethereliabilityofthesystem.Forthisreason,fromconsideringtheoverallsystem,thepurposeandtheneedtoadoptexplicituseofthesensor,thesensorisdefinitelynottouseinappropriateandunnecessarysensor.Measurementconditionslistedbelow,i.e.measurementpur