中英文翻译传感器重点.docx
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中英文翻译传感器重点.docx
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中英文翻译传感器重点
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.
ahighfidelity-thetransduceroutputwaveformshapebeafaithful
reproductionofthemeasured;thereshouldbeminimumdistortion.bThereshouldbeminimuminterferencewiththequantitybeingmeasured;thepresenceofthetransducershouldnotalterthemeasuredinanyway.
cSize.Thetransducermustbecapableofbeingplacedexactlywhereitisneeded.
dThereshouldbealinearrelationshipbetweenthemeasuredandthetransducersignal.
eThetransducershouldhaveminimumsensitivitytoexternaleffects,pressuretransducers,forexample,areoftensubjectedtoexternaleffectssuchvibrationandtemperature
fThenaturalfrequencyofthetransducershouldbewellseparatedfromthefrequencyandharmonicsofthemeasurand.
4.ElectricalTransducers
Electricaltransducersexhibitmanyoftheidealcharacteristics.Inadditiontheyofferhighsensitivityaswellaspromotingthepossibleofremoteindicationormesdurement.Electricaltransducerscanbedividedintotwodistinctgroups:
avariable-control-parametertypes,whichinclude:
iresistance
iicapacitance
iiiinductance
ivmutual-inductancetypes
Thesetransducersallrelyonexternalexcitationvoltagefortheiroperation.
bself-generatingtypes,whichinclude
ielectromagnetic
iiphotoemissive
ivpiezo-electrictypes
Theseallthemselvesproduceanoutputvoltageinresponsetothemeasurandinputandtheireffectsarereversible.Forexample,apiezo-electrictransducernormallyproducesanoutputvoltageinresponsetothedeformationofacrystallinematerial;however,ifanalternatingvoltageisappliedacrossthematerial,thetransducerexhibitsthe
reversibleeffectbydeformingorvibratingatthefrequencyofthealternatingvoltage.
5.ResistanceTransducers
Resistancetransducersmaybedividedintotwogroups,asfollows:
iThosewhichexperiencealargeresistancechange,measuredbyusingpotential-dividermethods.Potentiometersareinthisgroup.
iiThosewhichexperienceasmallresistancechange,measuredbybridge-circuitmethods.Examplesofthisgroupincludestraingaugesandresistancethermometers.
5.1Potentiometers
Alinearwire-woundpotentiometerconsistsofanumberofturnsresistancewirewoundaroundanon-conductingformer,togetherwithawipingcontactwhichtravelsoverthebarwires.Theconstructionprinciplesareshowninfigurewhichindicatethatthewiperdisplacementcanberotary,translational,oracombinationofbothtogiveahelical-typemotion.Theexcitationvoltagemaybeeithera.c.ord.c.andtheoutputvoltageisproportionaltotheinputmotion,providedthemeasuringdevicehasaresistancewhichismuchgreaterthanthepotentiometerresistance.Suchpotentiometerssufferfromthelinkedproblemofresolutionandelectricalnoise.
Resolutionisdefinedasthesmallestdetectablechangeininputandisdependentonthecross-sectionalareaofthewindingsandtheareaoftheslidingcontact.Theoutputvoltageisthusaserialsofstepsasthecontactmovesfromonewiretonext.
Electricalnoisemaybegeneratedbyvariationincontactresistance,bymechanicalwentduetocontactfriction,andbycontactvibrationtransmittedfromthesensingelement.Inaddition,themotionbeingmeasuredmayexperiencesignificantmechanicalloadingbytheinertiaandfrictionofthemovingpartsofthepotentiometer.Thewearonthecontactingsurfacelimitsthelifeofapotentiometertoafinitenumberoffullstrokesorrotationsusuallyreferredtointhemanufacture'sspecificationasthe`numberofcyclesoflifeexpectancy',atypicalvaluebeing20M1000000cycles.
TheoutputvoltageVOoftheunloadpotentiometercircuitisdetermined
asfollows.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=(PRt1/2.
5.2ResistanceStrainGauges
Resistancestraingaugesaretransducerswhichexhibitachangeinelectricalresistanceinresponsetomechanicalstrain.Theymaybeofthebondedorunbondedvariety.
abondedstraingauges
Usinganadhesive,thesegaugesarebonded,orcemented,directlyontothesurfaceofthebodyorstructurewhichisbeingexamined.Examplesofbondedgaugesare
ifinewiregaugescementedtopaperbacking
iiphoto-etchedgridsofconductingfoilonanepoxy-resinbackingiiiasinglesemiconductorfilamentmountedonanepoxy-resinbackingwithcopperorniekelleads.
Resistancegaugescanbemadeupassingleelementstomeasuringstraininonedirectiononly,oracombinationofelementssuchasrosetteswillpermitsimultaneousmeasurementsinmorethanonedirection
bunbondedstraingauges
Atypicalunbonded-strain-gaugearrangementshowsfineresistancewiresstretchedaroundsupportsinsuchawaythatthedeflectionofthecantileverspringsystemchangesthetensioninthewiresandthusalterstheresistanceofwire.Suchanarrangementmaybefoundincommerciallyavailableforce,load,orpressuretransducers.
5.3ResistanceTemperatureTransducers
Thematerialsforthesecanbedividedintotwomaingroups:
ametalssuchasplatinum,copper,tungsten,andnickelwhichexhibit
andincreaseinresistanceasthetemperaturerises;theyhaveapositivetemperaturecoefficientofresistance.
bsemiconductors,suchasthermistorswhichuseoxidesofmanganese,
cobalt,chromium,ornickel.Theseexhibitlargenon-linearresistancechangeswithtemperaturevariationandnormallyhaveanegativetemperaturecoefficientofresistance.
ametalresistancetemperaturetransducers
Thesedepend,formanypracticalpurposeandwithinanarrowtemperaturerange,upontherelationshipRI=RO*[1+a*(b1-62]whereacoefficientofresistancein'C-bandROresistanceinohmsatthereferencetemperatureb0=00Catthereferencetemperaturerange0CTheinternationalpracticaltemperaturescaleisbasedontheplatinumresistancethermometer,whichcoversthetemperaturerange-259.350Cto630.5'C.
bthermistorresistancetemperaturetransducers
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,producingacurrentwhichflowsthroughresistorRandresultinginanoutputvoltageVPhotoelectricallygeneratedvoltage
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