中英文翻译传感器.docx

中英文翻译传感器.docx

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中英文翻译传感器

中英文翻译

传感器的基础知识

Basicknowledgeoftransducers

Atransducerisadevicewhichconvertsthequantitybeingmeasuredintoanoptical,mechanical,or-morecommonly-electricalsignal.Theenergy-conversionprocessthattakesplaceisreferredtoastransduction.

Transducersareclassifiedaccordingtothetransductionprincipleinvolvedandtheformofthemeasured.Thusaresistancetransducerformeasuringdisplacementisclassifiedasaresistancedisplacementtransducer.Otherclassificationexamplesarepressurebellows,forcediaphragm,pressureflapper-nozzle,andsoon.

1.TransducerElements

Althoughthereareexception,mosttransducersconsistofasensingelementandaconversionorcontrolelement.Forexample,diaphragms,bellows,straintubesandrings,bourdontubes,andcantileversaresensingelementswhichrespondtochangesinpressureorforceandconvertthesephysicalquantitiesintoadisplacement.Thisdisplacementmaythenbeusedtochangeanelectricalparametersuchasvoltage,resistance,capacitance,orinductance.Suchcombinationofmechanicalandelectricalelementsformelectromechanicaltransducingdevicesortransducers.Similarcombinationcanbemadeforotherenergyinputsuchasthermal.Photo,magneticandchemical,givingthermoelectric,photoelectric,electromaanetic,andelectrochemicalrespectively.

2.TransducerSensitivity

TherelationshipbetweenthemeasuredandthetransduceroutputsignalisusuallyobtainedbycalibrationtestsandisreferredtoasthetransducersensitivityKl=output-signalincrement/measuredincrement.Inpractice,thetransducersensitivityisusuallyknown,and,bymeasuringtheoutputsignal,theinputquantityisdeterminedfrominput=output-signalincrement/Kl.

3.CharacteristicsofanIdealTransducer

Thehightransducershouldexhibitthefollowingcharacteristics.

a）highfidelity-thetransduceroutputwaveformshapebeafaithfulreproductionofthemeasured;thereshouldbeminimumdistortion.

b）Thereshouldbeminimuminterferencewiththequantitybeingmeasured;thepresenceofthetransducershouldnotalterthemeasuredinanyway.

c）Size.Thetransducermustbecapableofbeingplacedexactlywhereitisneeded.

d）Thereshouldbealinearrelationshipbetweenthemeasuredandthetransducersignal.

e）Thetransducershouldhaveminimumsensitivitytoexternaleffects,pressuretransducers,forexample,areoftensubjectedtoexternaleffectssuchvibrationandtemperature

f）Thenaturalfrequencyofthetransducershouldbewellseparatedfromthefrequencyandharmonicsofthemeasurand.

4.ElectricalTransducers

Electricaltransducersexhibitmanyoftheidealcharacteristics.Inadditiontheyofferhighsensitivityaswellaspromotingthepossibleofremoteindicationormesdurement.Electricaltransducerscanbedividedintotwodistinctgroups:

a）variable-control-parametertypes,whichinclude:

i）resistance

ii）capacitance

iii）inductance

iv）mutual-inductancetypes

Thesetransducersallrelyonexternalexcitationvoltagefortheiroperation.

b）self-generatingtypes,whichinclude

i）electromagnetic

ii）photoemissive

iv）piezo-electrictypes

Theseallthemselvesproduceanoutputvoltageinresponsetothemeasurandinputandtheireffectsarereversible.Forexample,apiezo-electrictransducernormallyproducesanoutputvoltageinresponsetothedeformationofacrystallinematerial;however,ifanalternatingvoltageisappliedacrossthematerial,thetransducerexhibitsthereversibleeffectbydeformingorvibratingatthefrequencyofthealternatingvoltage.

5.ResistanceTransducers

Resistancetransducersmaybedividedintotwogroups,asfollows:

i）Thosewhichexperiencealargeresistancechange,measuredbyusingpotential-dividermethods.Potentiometersareinthisgroup.

ii）Thosewhichexperienceasmallresistancechange,measuredbybridge-circuitmethods.Examplesofthisgroupincludestraingaugesandresistancethermometers.

5.1Potentiometers

Alinearwire-woundpotentiometerconsistsofanumberofturnsresistancewirewoundaroundanon-conductingformer,togetherwithawipingcontactwhichtravelsoverthebarwires.Theconstructionprinciplesareshowninfigurewhichindicatethatthewiperdisplacementcanberotary,translational,oracombinationofbothtogiveahelical-type

motion.Theexcitationvoltagemaybeeithera.c.ord.c.andtheoutputvoltageisproportionaltotheinputmotion,providedthemeasuringdevicehasaresistancewhichismuchgreaterthanthepotentiometerresistance.

Suchpotentiometerssufferfromthelinkedproblemofresolutionandelectricalnoise.

Resolutionisdefinedasthesmallestdetectablechangeininputandisdependentonthecross-sectionalareaofthewindingsandtheareaoftheslidingcontact.Theoutputvoltageisthusaserialsofstepsasthecontactmovesfromonewiretonext.

Electricalnoisemaybegeneratedbyvariationincontactresistance,bymechanicalwentduetocontactfriction,andbycontactvibrationtransmittedfromthesensingelement.Inaddition,themotionbeingmeasuredmayexperiencesignificantmechanicalloadingbytheinertiaandfrictionofthemovingpartsofthepotentiometer.Thewearonthecontactingsurfacelimitsthelifeofapotentiometertoafinitenumberoffullstrokesorrotationsusuallyreferredtointhemanufacture'sspecificationasthe`numberofcyclesoflifeexpectancy',atypicalvalue

being20M1000000cycles.

TheoutputvoltageVOoftheunloadpotentiometercircuitisdeterminedasfollows.LetresistanceRI=xi/xt*Rtwherexi二inputdisplacement,xt=maximumpossibledisplacement,Rttotalresistanceofthepotentiometer.ThenoutputvoltageVO=V

Rl/（Rl+（Rt-Rl））=V*Rl/Rt=V*xi/xt*Rt/Rt=V*xi/xt.Thisshowsthatthereisastraight-linerelationshipbetweenoutputvoltageandinputdisplacementfortheunloadedpotentiometer.ItwouldseenthathighsensitivitycouldbeachievedsimplybyincreasingtheexcitationvoltageV.however,themaximumvalueofVisdeterminedbythemaximumpowerdissipationPofthefinewiresofthepotentiometerwindingandisgivenbyV=（PRt）1/2.

5.2ResistanceStrainGauges

Resistancestraingaugesaretransducerswhichexhibitachangeinelectricalresistanceinresponsetomechanicalstrain.Theymaybeofthebondedorunbondedvariety.

a）bondedstraingauges

Usinganadhesive,thesegaugesarebonded,orcemented,directlyontothesurfaceofthebodyorstructurewhichisbeingexamined.Examplesofbondedgaugesare

i）finewiregaugescementedtopaperbacking

ii）photo-etchedgridsofconductingfoilonanepoxy-resinbacking

iii）asinglesemiconductorfilamentmountedonanepoxy-resinbackingwithcopperorniekelleads.

Resistancegaugescanbemadeupassingleelementstomeasuringstraininonedirectiononly,oracombinationofelementssuchasrosetteswillpermitsimultaneousmeasurementsinmorethanonedirection

b）unbondedstraingauges

Atypicalunbonded-strain-gaugearrangementshowsfineresistancewiresstretchedaroundsupportsinsuchawaythatthedeflectionofthecantileverspringsystemchangesthetensioninthewiresandthusalterstheresistanceofwire.Suchanarrangementmaybefoundincommerciallyavailableforce,load,orpressuretransducers.

5.3ResistanceTemperatureTransducers

Thematerialsforthesecanbedividedintotwomaingroups:

a）metalssuchasplatinum,copper,tungsten,andnickelwhichexhibitandincreaseinresistanceasthetemperaturerises;theyhaveapositivetemperaturecoefficientofresistance.

b）semiconductors,suchasthermistorswhichuseoxidesofmanganese,cobalt,chromium,ornickel.Theseexhibitlargenon-linearresistancechangeswithtemperaturevariationandnormallyhaveanegativetemperaturecoefficientofresistance.

a）metalresistancetemperaturetransducers

Thesedepend,formanypracticalpurposeandwithinanarrowtemperaturerange,upontherelationshipRI=RO*[1+a*（b1-62）]whereacoefficientofresistancein'C-bandROresistanceinohmsatthereferencetemperatureb0=00Catthereferencetemperaturerange0CTheinternationalpracticaltemperaturescaleisbasedontheplatinumresistancethermometer,whichcoversthetemperaturerange-259.350Cto630.5'C.

b）thermistorresistancetemperaturetransducers

Thermistorsaretemperature-sensitiveresistorswhichexhibitlargenon-linerresistancechangeswithtemperaturevariation.Ingeneral,theyhaveanegativetemperaturecoefficient.Forsmalltemperatureincrementsthevariationinresistanceisreasonablylinear;but,iflargetemperaturechangesareexperienced,speciallinearizingtechniquesareusedinthemeasuringcircuitstoproducealinearrelationshipofresistanceagainsttemperature.Thermistorsarenormallymadeintheformofsemiconductordiscsenclosedinglassvitreousenamel.Sincetheycanbemadeassmallaslmm,quiterapidresponsetimesarepossible.

5.4PhotoconductiveCells

Thephotoconductivecell,usesalight-sensitivesemiconductormaterial.Theresistancebetweenthemetalelectrodesdecreaseastheintensityofthelightstrikingthesemiconductorincreases.Commonsemiconductormaterialsusedforphoto-conductivecellsarecadmiumsulphide,leadsulphide,andcopper-dopedgermanium.Theusefulrangeoffrequenciesisdeterminedbymaterialused.Cadmiumsulphideismainlysuitableforvisibletight,whereasleadsulphidehasitspeakresponseintheinfra-redregionandis,therefore,mostsuitableforflame-failuredetectionandtemperaturemeasurement.

5.5PhotoemissiveCells

Whenlightstrikesthecathodeofthephotoemissivecellaregivensufficientenergytoarrivethecathode.Thepositiveanodeattractstheseelectrons,producingacurrentwhichflowsthroughresistorRandresultinginanoutput