外文翻译传感器的基础知识Word文档下载推荐.docx

外文翻译传感器的基础知识Word文档下载推荐.docx

《外文翻译传感器的基础知识Word文档下载推荐.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Word文档下载推荐.docx（7页珍藏版）》请在冰豆网上搜索。

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:

variable-control-parametertypes,whichinclude:

i） 

resistance

ii） 

capacitance

iii） 

inductance

iv） 

mutual-inductancetypes

Thesetransducersallrelyonexternalexcitationvoltagefortheiroperation.

self-generatingtypes,whichinclude

electromagnetic

thermoelectric

photoemissive

piezo-electrictypes

Theseallthemselvesproduceanoutputvoltageinresponsetothemeasurandinputandtheireffectsarereversible.Forexample,apiezo-electrictransducernormallyproducesanoutputvoltageinresponsetothedeformationofacrystallinematerial;

however,ifanalternatingvoltageisappliedacrossthematerial,thetransducerexhibitsthereversibleeffectbydeformingorvibratingatthefrequencyofthealternatingvoltage.

5、ResistanceTransducers

Resistancetransducersmaybedividedintotwogroups,asfollows:

Thosewhichexperiencealargeresistancechange,measuredbyusingpotential-dividermethods.Potentiometersareinthisgroup.

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

finewiregaugescementedtopaperbacking

photo-etchedgridsofconductingfoilonanepoxy-resinbacking

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