外文翻译传感器的基础知识.docx

外文翻译传感器的基础知识.docx

《外文翻译传感器的基础知识.docx》由会员分享，可在线阅读，更多相关《外文翻译传感器的基础知识.docx（7页珍藏版）》请在冰豆网上搜索。

外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,andelectrochemicaltransducersrespectively.

2、TransducerSensitivity

TherelationshipbetweenthemeasuredandthetransduceroutputsignalisusuallyobtainedbycalibrationtestsandisreferredtoasthetransducersensitivityK1=output-signalincrement/measuredincrement.Inpractice,thetransducersensitivityisusuallyknown,and,bymeasuringtheoutputsignal,theinputquantityisdeterminedfrominput=output-signalincrement/K1.

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）    thermoelectric

iii）    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-typemotion.Theexcitationvoltagemaybeeithera.c.or  d.c.andtheoutputvoltageisproportionaltotheinputmotion,providedthemeasuringdevicehasaresistancewhichismuchgreaterthanthepotentiometerresistance.

Suchpotentiometerssufferfromthelinkedproblemofresolutionandelectricalnoise.Resolutionisdefinedasthesmallestdetectablechangeininputandisdependentonthecross-sectionalareaofthewindingsandtheareaoftheslidingcontact.Theoutputvoltageisthusaserialsofstepsasthecontactmovesfromonewiretonext.

Electricalnoisemaybegeneratedbyvariationincontactresistance,bymechanicalwearduetocontactfriction,andbycontactvibrationtransmittedfromthesensingelement.Inaddition,themotionbeingmeasuredmayexperiencesignificantmechanicalloadingbytheinertiaandfrictionofthemovingpartsofthepotentiometer.Thewearonthecontactingsurfacelimitsthelifeofapotentiometertoafinitenumberoffullstrokesorrotationsusuallyreferredtointhemanufacture’sspecificationasthe‘numberofcyclesoflifeexpectancy’,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.

5.2ResistanceStrainGauges

Resistancestraingaugesaretransducerswhichexhibitachangeinelectricalresistanceinresponsetomechanicalstrain.Theymaybeofthebondedorunbondedvariety.

a）bondedstraingauges

Usinganadhesive,thesegaugesarebonded,orcemented,directlyontothesurfaceofthebodyorstructurewhichisbeingexamined.

Examplesofbondedgaugesare

i）    finewiregaugescementedtopaperbacking

ii）    photo-etchedgridsofconductingfoilonanepoxy-resinbacking

iii）    asinglesemiconductorfilamentmountedonanepoxy-resinbackingwithcopperornickelleads.

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,upontherelationshipR1=R0*[1+a*（b1-b2）]whereacoefficientofresistancein℃-1,andR0resistanceinohmsatthereferencetemperatureb0=0℃atthereferencetemperaturerange℃.

Theinternationalpracticaltemperaturescaleisbasedontheplatinumresistancethermometer,whichcoversthetemperaturerange-259.35℃to630.5℃.

b）thermistorresistancetemperaturetransducers

Thermistorsaretemperature-sensitiveresistorswhichexhibitlargenon-linerresistancechangeswithtemperaturevariation.Ingeneral,theyhaveanegativetemperaturecoefficient.

Forsmalltemperatureincrementsthevariationinresistanceisreasonablylinear;but,iflargetemperaturechangesareexperienced,speciallinearizingtechniquesareusedinthemeasuringcircuitstoproducealinearrelationshipofresistance  againsttemperature.

Thermistorsarenormallymadeintheformofsemiconductordiscsenclosedinglassvitreousenamel.Sincetheycanbemadeassmallas1mm,quiterapidresponsetimesarepossible.

5.4PhotoconductiveCells

Thephotoconductivecell,usesalight-sensitivesemiconductormaterial.Theresistancebetweenthemetalelectrodesdecreaseastheintensityofthelightstrikingthesemiconductorincreases.Commonsemiconductormaterialsusedforphoto-conductivecellsarecadmiumsulphide,lead  sulphide,andcopper-dopedgermanium.

Theusefulrangeoffrequenciesisdeterminedbymaterialused.Cadmiumsulphideismainlysuitableforvisiblelight,whereasleadsulphidehasitspeakresponseintheinfra-redregionandis,therefore,mostsuitableforflame-failuredetectionandtemperaturemeasurement.

5.5PhotoemissiveCells

Whenlightstrikesthecathodeofthepho