專業(yè)英語(yǔ)第8單元作業(yè)

8ControlSensors

8.1INTRODUCTION

Infeedbackcontrolsystem-plant（設(shè)備）responseismeasuredandcomparedwith

areferenceinputandtheerrorisautomaticallyemployedincontrollingtheplant.Itfollows

thatameasurementsystemisanessentialcomponentinanyfeedbackcontrolsystemand

formsavitallinkbetweentheplantandthecontroller.Measurementsareneededinmany

engineeringapplications.Themeasurementprocesshastobeautomated,however,in

controlsystemsapplications.

Atypicalmeasurementsystemconsistsofoneormoresensor-transducerunitsand

associatedsignal-conditioning（信號(hào)調(diào)理）（andmodification改善）devices（seeFig.8.1）.

Filteringtoremoveunwantednoiseandamplificationtostrengthenaneededsignalare

consideredsignalconditioning.Analog-to-digitalconversion（ADC）,digital-to-analog

conversion（DAC）,modulation（調(diào)制），anddemodulation（角軍調(diào)）aresignalmodification

（信號(hào)增強(qiáng)、信號(hào)改善）methods.Notethatsignalconditioningcanbeconsideredunder

thegeneralheadingofsignalmodification.Eventhoughdatarecordingisanintegral

functioninatypicaldataacquisition（數(shù)據(jù)采集）system,itisnotacrucialfunctionina

feedbackcontrolsystem.Forthisreason,weshallnotgointodetailsofdatarecording

devicesinthisbook.Inamultiplemeasurementenvironment,amultiplexer（多路轉(zhuǎn)換器）

couldbeemployedpriortoorfollowingthesignal-conditioningprocess,inordertopick

onemeasuredsignalatatimefromabankofdatachannelsforsubsequentprocessing.In

thismanner,oneunitofexpensiveprocessinghardwarecanbetime-shared（時(shí)分復(fù)用）

betweenseveralsignals.Sensor-transducer（傳感器-變送器）devicesarepredominantly

（主要的）analogcomponentsthatgenerateanalogsignals,eventhoughdirectdigital

transducersarebecomingincreasinglypopularindigitalcontrolapplications.Whenanalog

transducersareemployed,analog-to-digitalconverters（ADCs）havetobeusedtoconvert

analogsignalsintodigitaldatafordigitalcontrol.Thissignalmodificationprocessrequires

samplingofanalogsignalsatdiscretetimepoints.Onceavalueissampled,itisencoded

intoadigitalrepresentationsuchasstraightbinarycode,agraycode,binary-codedecimal

（BCD）codeorAmericanStandardCodeforInformationInterchange（ASCII）.Thechanges

inananalogsignalduetoitstransient（瞬態(tài)的）natureshouldnotaffectthisprocessof

ADC.Toguaranteethis,asample-and-hold（取樣保持）operationisrequiredduringeach

samplingperiod.Forexample,thevalueofananalogsignalsdetected（sampled）inthe

beginningofeachsamplingperiodandisassumedconstant（held）throughouttheentire

samplingperiod.Thisis,infact,thezero-orderhold（零階保持器）operation.The

operationsofmultiplexing（多路選擇）,sampling,anddigitizinghavetobeproperly

synchronizedunderthecontrolofanaccuratetimingdevice（aclock）forproperoperation

ofthecontrolsystem.ThisprocedureisshownschematicallyinFig.8.2.

Alldevicesthatassistinthemeasurementprocedurecanbeinterpreted（解釋）as

componentsofthemeasurementsystem.Selectionofavailablecomponentsforaparticular

applicationordesignofnewcomponentsshouldrelyheavilyonperformance（性能）

specification（規(guī)范）forthesecomponents.Agreatmajorityofinstrumentratingsprovided

bymanufacturersareintheformofstaticparameters.Incontrolapplications,however,

dynamicperformancespecificationsarealsoveryimportant.

Whentwoormorecomponentsareinterconnected,thebehaviorofindividual

componentsintheoverallsystemcandeviate（偏離）significantlyfromtheirbehavior

wheneachcomponentoperatesindependently.Matchingofcomponentsina

multicomponentsystem,particularlywithrespecttotheirimpedancecharacteristics,should

bedonecarefullyinordertoimprovesystemperformanceandaccuracy.

8.2SENSORSANDTRANSDUCERS

Theoutputvariable（orresponse）thatisbeingmeasuredistermed（）the

measurand（被狽！J量）.Examplesareaccelerationandvelocityifavehicle,temperatureand

pressureifaprocessplant（冶煉廠），andcurrentthroughanelectriccircuit.Ameasuring

devicepassesthroughtwostageswhilemeasuringasignal.First,themeasuredissensed.

Then,themeasuredsignalistransuded（orconverted）intoaformthatisparticularly

suitablefortransmitting,signalconditioning,processing,ordrivingacontrolleroractuator.

Forthisreason,outputofthetransducerstageisoftenanelectricalsignal.Themeasuredis

usuallyananalogsignal,becauseitrepresentstheoutputofadynamicsysteminfeedback

controlapplications.Transduceroutputisdiscreteindirectdigitaltransducers.This

facilitatesthedirectinterfaceofatransducerwithadigitalprocessor.

Thesensorandtransducerstagesofatypicalmeasuringdevicearerepresented

schematically(圖角軍地)inFig.8.3(a).Asanexample,considertheoperationofa

piezoelectric(壓電的)accelerometer(seeFig.8.3(b)).Inthiscase,accelerationisthe

measured.Itisfirstconvertedintoaninertia(慣性的)forcethroughamasselementandis

exertedonapiezoelectriccrystalwithinwhichastrain(張力)(stress應(yīng)力)isgenerated.

Thisisconsideredthesensingstage.Thestressgeneratesachargeinsidethecrystal,which

appearsasanelectricsignalattheoutputoftheaccelerometer.Thisstress-to-charge

conversionorstress-to-voltageconversioncanbeinterpretedasthetransducerstage.

Measuringdevice

Measured

—>

(TypicallySignal一Transducer

一Transmittablesensor

Analogysignal)

Acomplexmeasuringdevicecanhavemorethanonesensingstage.Moreoften,the

measuredgoesthoughseveraltransducerstagesbeforeitisavailableforcontroland

actuatingpurposes.Sensorandtransducerstagesarefunctionalstages,andsometimesitis

noteasyorevenfeasibletoidentifyphysicalelementsassociatedwiththem.Furthermore,

thisseparationisnotveryimportantinusingexistingdevices.Properseparationofsensor

andtransducerstages(physicallyaswellasfunctionally)canbecrucial,however,when

designingnewmeasuringinstruments.

Insomebooks,signal-conditioningdevicessuchaselectronicamplifiersatealso

classifiedastransducers.Sincewearetreatingsignal-conditioningandmodification

devicesseparatelyfrommeasuringdevices,thisunifiedclassificationisavoidedwhenever

possibleinthisbook.Instead,thetermtransducerisusedprimarilyinrelationtomeasuring

instruments.Followingthecommonpractice,however,thetermssensorandtransducerwill

beusedinterchangeablytodenotemeasuringinstruments.

8.3ANALOGSENSORSFORMOTIONMEASUREMENT

8.3.1Introduction

Measurementofplantoutputsisessentialforfeedbackcontrol.Output

measurementsarealsousefulinperformanceevaluation（性能評(píng)價(jià)）ofaprocess.

Furthermore,inlearningsystems（e.g.,teach-repeatoperation（示教操作）ofrobotic

manipulators機(jī)器人操作臂），measurementsaremadeandstoredinthecomputerfor

subsequentuseinoperatingthesystem.Inputmeasurementsareneededinfeedforward

control.Itisevident明顯的，therefore,thatthemeasurementsubsystemisanimportant

partofacontrolsystem.

Themeasurementsubsysteminacontrolsystemcontainssensorsandtransducers

thatdetectmisbrandandconvertthemintoacceptablesignals---typically,voltages.These

voltagesignalsarethenappropriatelymodifiedusingsignal-conditioninghardwaresuchas

filters,amplifiers,demodulators,andanalog-to-digitalconverters.Impedancematching（阻

抗匹配）mightbenecessarytoconnectsensorsandtransducerstosignal-conditioning

hardware.

Accuracyofsensors,transducers,andassociatedsignal-conditioningdevicesis

importantincontrolsystemapplicationsfortwomainreasons.Themeasurementsystemin

afeedbackcontrolsystemissituatedinthefeedbackpathofthecontrolsystem.Even

thoughmeasurementsareusedtocompensateforthepoorperformanceintheopen-loop

system,anyerrorsinmeasurementsthemselveswillenterdirectlyintothesystemand

cannotbecorrectediftheyareunknown.Furthermore,itcanbeshownthatsensitivityofa

controlsystemtoparameterchangesinthemeasurementsystemisdirect.Thissensitivity

cannotbereducedbyincreasingtheloopgain,unlikeinthecaseofsensitivitytothe

open-loopcomponents.Accordingly,thecaseofsensitivitytotheopen-loopcomponents.

Accordingly,thedesignstrategy（策略）forclosed-loop（feedback）controlistomakethe

measurementsveryaccurateandtoemployasuitablecontrollertoreduceothertypesof

errors.

Mostsensor-transducerdevicesusedinfeedbackcontrolapplicationsateanalog

componentsthatgenerateanalogoutputsignals.Thisisthecaseeveninreal-timedirect

digitalcontrolsystems.Whenanalogtransducersareusedindigitalcontrolapplications,

however,sometypeofanalog-to-digitalconversion（ADC）isneededtoobtainadigital

representationofthemeasuredsignal.Theresultingdigitalsignalissubsequently

conditionedandprocessedusingdigitalmeans.

Inthesensorstage,thesignalbeingmeasuredisfeltasthe"'responseofthesensor

element.9,Thisisconvertedbythetransducerintothetransmitted(ormeasured)quantity.

Inthisrespect,theoutputofameasuringdevicecanbeinterpretedasthe"responseofthe

transducer.^^Incontrolsystemapplications,thisoutputistypically(andpreferably)an

electricalsignal.Notethatitissomewhatredundant(重復(fù)的)toconsider

electrical-to-electricalsensors-transducersasmeasuringdevices,particularlyincontrol

systemstudies,becauseelectricalsignalsneedconditioningonlybeforetheyarefedintoa

controllerortoadrivesystem.Inthissense,electrical-to-electricaltransductionshouldbe

consideredauconditioning^^taskratherthana"measuring“function.

8.3.2MotionTransducers

Bymotion,wemeanthefourkinematicsvariables:

?Displacement(includingposition,distance,proximity接近，andsize大〃、or

gage位移/厚度)

?Velocity

?Acceleration

?Jerk振動(dòng)/沖擊/加加速度

Notethateachvariableisthetimederivativeoftheprecedingone.Motion

measurementisextremelyusefulincontrollingmechanicalresponsesandinteractionsin

dynamicsystems.Numerousexamplescanbecitedofsituationsinwhichmotion

measurementsareusedforcontrolpurposes.Therotatingspeedofaworkpieceandthe

feedrateofatoolaremeasuredincontrollingmachiningoperations.Displacementsand

speeds(bothangularandtranslatory)atjoints(revoluteandprismatic)ofrobotic

manipulatorsorkinematicslinkagesareusedincontrollingmanipulatortrajectory.Angular

speedisacrucialmeasurementthatisusedinthecontrolofrotatingmachinery,suchas

turbines,pumps,compressors,motors,andgeneratorsinpower-generatingplants.

Proximitysensors(tomeasuredisplacement)andaccelerometers(tomeasureacceleration)

arethetwomostcommontypesofmeasuringdevicesusedinmachineprotectionsystems

forconditionmonitoring,faultdetection（故障定位），diagnosis,andon-line（often

real-time）controloflargeandcomplexmachinery.Theaccelerometerisoftentheonly

measuringdeviceusedincontrollingdynamictestrigs（設(shè)備）.Displacement

measurementsareusedforvalve（閥）controlinprocessapplications.Platethickness（or

gage）iscontinuouslymonitoredbytheautomaticgagecontrol（AGC自動(dòng)厚度控制）

systeminsteelrollingmills（軋鋼廠）.

以上716完成

Aone-to-onerelationshipmaynotalwaysexistbetweenameasuringdeviceanda

measuredvariable.Forexample,althoughstraingages（張力應(yīng)力計(jì)，應(yīng)變計(jì)）aredevices

thatmeasurestrains（and,hence,stressesandforces）,theycanbeadaptedtomeasure

displacementsbyusingasuitablefront-endauxiliarysensorelement,suchasacantilever

（懸臂）（orspring）.Furthermore,thesamemeasuringdevicemaybeusedtomeasure

differentvariablesthroughappropriatedatainterpretation（信號(hào)解釋）techniques.For

example,piezoelectricaccelerometerswithbuilt-inmicroelectronicintegratedcircuitryare

marketedaspiezoelectricvelocitytransducers.Revolversignalsthatprovideangular

displacementsaredifferentiatedtogetangularvelocities.Pulse-generating（ordigital）

transducers,suchasopticalencodersanddigitaltachometers（轉(zhuǎn)速計(jì)）,canserveasboth

displacementtransducersandvelocitytransducers,dependingonwhethertheabsolute

numberofpulsesgeneratediscountedorthepulserateismeasured.Notethatpulserate

canbemeasuredeitherbycountingthenumberofpulsesduringaunitintervaloftimeor

bygatingahigh-frequencyclocksignalthroughthepulsewidth.Furthermore,inprinciple,

anyforcesensorcanbeusedasanaccelerationsensor,velocitysensor,ordisplacement

sensor,dependingonwhetheraninertiaelement（convertingaccelerationintoforce）,a

damping（阻尼）element（convertingvelocityintoforce）,oraspringelement（converting

displacementintoforce）,respectively,isusedasthefront-end（前端）auxiliarysensor.

Wemightquestiontheneedforseparatetransducerstomeasurethefourkinematics

variables---displacement,velocity,acceleration,andjerk--becauseanyonevariableis

relatedtoanyotherthroughsimpleintegrationordifferentiation.Itshouldbepossible,in

theory,tomeasureonlyoneofthesefourvariablesanduseeitheranalogprocessing

(throughanalogcircuithardware)ordigitalprocessing(throughadedicatedprocessor)to

obtainanyoftheremainingmotionvariables.Thefeasibility(可行性)ofthisapproachis

highlylimited,however,anditdependscruciallyonseveralfactors,includingthe

following:

(1)Thenatureofthemeasuredsignal(e.g.,steady,highlytransient,periodic,

narrow-band窄帶,broad-band寬帶).

(2)Therequiredfrequencycontentoftheprocessedsignal(orthefrequencyrange

ofinterest).

(3)Thesignal-to-noiseratio(SNR)ofthemeasurement.

(4)Availableprocessingcapabilities(e.g.,analogordigitalprocessing,limitations

ofthedigitalprocessor,andinterface,suchasthespeedofprocessing,samplingrate,and

buffersize).

(5)Controllerrequirementsandthenatureoftheplant(e.g.,timeconstants,delays,

hardwarelimitations).

(6)Requiredaccuracyintheendobjective(onwhichprocessingrequirementsand

hardwarecostswilldepend).

Forinstance,differentiationofasignal(inthetimedomain)isoftenunacceptable

fornoisyandhigh-frequencynarrow-bandsignals.Inanyevent,costlysignal-conditioning

hardwaremightbeneededforpreprocessingpriortodifferentiatingasignal.Asarueof

thumb(根據(jù)經(jīng)驗(yàn))，inlow-frequencyapplications(ontheorderof1Hz),displacement

measurementsgenerallyprovidegoodaccuracies.Inintermediate-frequency(中頻的)

applications(lessthan1kHz),velocitymeasurementisusuallyfavored有禾U的.In

measuringhigh-frequencymotionswithhighnoiselevels,accelerationmeasurementis

preferred.Jerkisparticularlyusefulingroundtransit(ridequality適用于乘坐的品質(zhì))，

manufacturing(forging鍛造,rolling軋制,andsimilarimpact-type沖擊型operations),

andshockisolation(delicate精密andsensitive敏感equipment)applications.

Ourdiscussionofmotiontransducerswillbelimitedmainlytothefollowing

typesofdevices:

Potentiometers(電位計(jì))(resistivelycoupled(電耦)devices)

Variable-inductance變電感transducers(electromagneticallycoupleddevices)

Eddycurrenttransducers

Variable-capacitance變?nèi)輙ransducers

Piezoelectrictransducers

8.4DIGITALTRANSDUCERS

8.4.1Introduction

Anytransducerthatpresentsinformationasdiscretesamplesandthatdoesnot

introduceaquantizationerrorwhenthereadingisrepresentedinthedigitalformmaybe

classifiedasadigitaltransducer.Adigitalprocessorplaystheroleofcontrollerinadigital

controlsystem.Thisfacilitates(使利于，促進(jìn))complexprocessingofmeasuredsignals

andotherknownquantitiesinordertoobtaincontrolsignalsfortheactuators(執(zhí)行器、激

勵(lì)器)thatdrivetheplant(機(jī)械設(shè)備)ofthecontrolsystem.Ifthemeasuredsignalsare

inanalogform,ananalog-to-digitalconversion(ADC)stageisnecessarypriortodigital

processing.Thereareseveralothershortcomingsofanalogsignalsincomparisontodigital

measuringdevicesfordigitalcontrolsystems.

Digitalmeasuringdevices(ordigitaltransducers,astheyarecommonlyknown)

generatediscreteoutputsignalssuchaspulsetrains(脈沖序歹!])orencodeddatathatcan

bedirectlyreadbyacontrolprocessor.Nevertheless,thesensorstageofdigitalmeasuring

devicesisusuallyquitesimilartothatoftheiranalogcounterparts.Therearedigital

measuringdevicesthatincorporatemicroprocessorstoperformnumericalmanipulations

andconditioninglocallyandprovideoutputsignalsineitherdigitaloranalogform.These

measuringsystemsareparticularlyusefulwhentherequiredvariableisnotdirectly

measurablebutcouldbecomputedusingoneormoremeasuredoutputs(e.g.,power

=force*speed).Althoughamicroprocessorisanintegralpartofthemeasuringdevicein

thiscase,itperformsnotameasuringtaskbut,rather,aconditiontask.Forourpurposes,

weshallconsiderthetwotasksseparately.

Theobjectiveofthissectionistostudytheoperationandutilizationofseveraltypes

ofdirectdigitaltransducers.Ourdiscussionwillbelimitedtomotiontransducer.Note,

however,thatbyusingasuitableauxiliaryfront-endsensor,othermeasured---suchasforce,

torque,andpressure---maybeconvertedintoamotionandsubsequentlymeasuredusinga

motiontransducer.Forexample,altitude(高度)(orpressure)measurementsinaircraftand

aerospaceapplicationsaremadeusingapressuresensingfrontend,suchasabellows(膜

盒)ordiaphragm(膜片)device,inconjunctionwithanopticalencodertomeasurethe

resultingdisplacement.Motion,asmanifestedinphysicalsystems,istypicallycontinuous

intime.Therefore,wecannotspeakofdigitalmotionsensorsingeneral.Actually,itisthe

transducerstagethatgeneratesthediscreteoutputsignalinadigitalmotionmeasuring

device.Commerciallyavailabledirectdigitaltransducersarenotasnumerousasanalog

sensors,butwhatisavailablehasfoundextensiveapplication.

Whentheoutputofadigitaltransducerisapulsesignal,acounterisusedeitherto

countthepulsesortocountclockcyclesoveronepulseduration.Thecountisfirst

representedasadigitalwordaccordingtosomecode;thenitisreadbyadataacquisition

andcontrolcomputer.If,ontheinacodedform(e.g.,binary,binary-codeddecimal,

ASCII),itcanbedirectlyreadbyacomputer.Inthelattercase,thecodedsignalis

normallygeneratedbyaparallelsetofpulsesignals;theworddependsonthepatternofthe

generatedpulses.

8.4.2ShaftEncoders(軸角編碼器)

Anytransducerthatgeneratesacodedreadingofameasurementcanbetermedan

encoder.Shaftencodersaredigitaltransducersthatareusedformeasuringangular

displacementsandangularvelocities.Applicationofthesedevicesincludemotion

measurementinperformancemonitoringandcontrolofroboticmanipulators,machine

tools,digitaltape-transportmechanisms,servoplottersandprinters,satellitemirror

positioningsystems,androtatingmachinerysuchasmotors,pumps,compressors,turbines,

andgenerators.Highresolution(dependingonthewordsizeoftheencoder),high

accuracy(particularlyduetonoiseimmunityofdigitalsignalsandsuperiorconstruction),

andrelativeeaseofadaptationindigitalcontrolsystemcostandimprovementofsystem

reliability,aresomeoftherelativeadvantagesofdigitaltransducerovertheiranalog

counterparts.

Shaftencoderscanbeclassifiedintotwocategories,dependingonthenatureand

themethodofinterpretation(譯碼)ofthetransduceroutput:(1)incrementalencoders(增

量編碼器)and(2)absoluteencoders(絕對(duì)編碼器).Theoutputofanincrementalencoder

isapulsesignalthatisgeneratedwhenthetransducerdiskrotatesasaresultofthemotion

thatisbeingmeasured.Bycountingthepulsesorbytimingthepulsewidthusingaclock

signal,bothangulardisplacementandangularvelocitycanbedetermined.Displacement,

however,isobtainedwithrespectto關(guān)于somereferencepointonthedisk,asindicatedby

areferencepulse(indexpulse)generatedatthatlocationonthedisk.Theindexpulsecount

determinesthenumberoffullrevolutions.

★Anabsoluteencoder(orwhole-wordencoder整字編碼器)hasmanyofpulse

tracks軌道onitstransducerdisk.Whenthediskofanabsoluteencoderrotates,several

pulsetrains脈沖串---equalinnumbertothetracksonthedisk------aregenerated

simultaneously.Atagiveninstant,themagnitudeofeachpulsesignalwillhaveoneoftwo

signallevels(i.e.,abinarystate),asdeterminedbyaleveldetector.Thissignallevel

correspondstoabinarydigit(0or1).Hence,thesetofpulsetrainsgivesanencodedbinary

numberatanyinstant.Thepulsewindows(孑L)onthetrackscanbeorganizedintosome

pattern(code)sothateachofthesebinarynumberscorrespondstotheangularpositionof

theencoderdiskatthetimewhentheparticularbinarynumberisdetected.Furthermore,

pulsevoltagecanbemadecompatiblewithsomeformofdigitallogic(e.g.,

transistor-to-transistorlogic,orTTL).Consequently,thedirectdigitalreadoutofanangular

positionispossible,therebyexpediting加速,促進(jìn)digitaldataacquisitionandprocessing.

Absoluteencodersarecommonlyusedtomeasurefractionsofarevolution.However,

completerevolutioncanbemeasuredusinganadditionaltrackthatgeneratesanindex

pulse,asinthecaseofincrementalencoder.

以上717完成

Thesamesignalgeneration(andpick-off拾取，截取)mechanismmaybeusedin

bothtypesoftransducers.Fourtechniquesoftransducersignalgenerationcanbeidentified:

(1)Optical(photosensor光學(xué)傳感器)method.

(2)Slidingcontact滑動(dòng)接觸(electricalconducting電導(dǎo))method.

(3)Magneticsaturation飽和(reluctance磁阻)method.

(4)Proximitysensormethod.

Foragiventypeofencoder(incrementalorabsolute),themethodofsignal

interpretationisidenticalforallfourtypesofsignalgeneration.

Inmanycontrolapplications,encodersarebuiltintotheplantitself,ratherthanbeing

externallyfittedontoarotationshaft.Forinstance,inarobotarm,theencodermightbean

integralpartofthejoint關(guān)節(jié)motorandmaybelocatedwithinitshousing夕卜殼/夕卜罩.This

reducescouplingerrors(耦合誤差)(e.g.,errorsduetobacklash間隙，shaftflexibility彈

T生/柔度，andresonances諧振addedbythetransducerandfixtures夾具)，installation

errors(e.g.,eccentricity偏心率),andoverallcost.

8.4.3DigitalTachometers

★Sinceshaftencodersarealsousedformeasuringangularvelocities,theycanbe

consideredtachometers.Inclassicterminology,adigitaltachometerisadevicethat

employsatoothedwheel()tomeasureangularvelocities.Aschematicdiagramofone

suchdeviceisshowninFig.8.4.Thisisamagneticinductiontachometerofthe

variable-reluctance(變磁阻)type.Theteethonthewheelaremadeofferromagnetic

material.Thetwomagneticinduction(andvariable)proximityprobes(探針)areplaced

facingtheteethradially徑向的，aquarter-pitch四分之一齒距apart.Whenthetoothed

wheelrotates,thetwoprobesgenerateoutputsignalsthatare90°outof距離phase.One

signalleadstheotherinonedirectionofrotationandlags滯后theotherintheopposite

directionofrotation.Inthismanner,directionalreadingsareobtained.Thespeedisthe

pulsewidth,asinthecaseofanincrementalencoder

以上718完成

Theadvantagesofthesedigital(pulse)tachometersoveropticalencodersinclude

simplicity,robustness,andlowcost.Thedisadvantagesincludepoorresolution(determined

bythenumberofteeth,thespeedofrotation,andthewordsizeusedfordatatransmission),

andmechanicalerrorsduetoloading,hysteriscs(遲滯),andmanufacturingirregularities.

8.4.4HallEffectSensors

ConsiderasemiconductorelementsubjecttoaDCvoltageUref.Ifamagneticfield

isappliedperpendiculartothedirectionofthisvoltage,avoltageUowillbegeneratedin

thethirdorthogonal［番且同個(gè)番丫中??］垂直的directionwithinthesemiconductorelement.

ThisisknownastheHallEffect霍爾效應(yīng)(observedbyE.H.Hallin1897).Aschematic

representationofaHallEffectsensorisshowninFig.8.5.

AHallEffectsensormaybeusedformotionsensinginmanyways-fbrexample,as

ananalogproximitysensor,adigitallimitswitch,oradigitalshaftencoder.Sincethe

outputvoltageUoincreasesasthedistancefromthemagneticsourcetothesemiconductor

elementdecreases,theoutputsignalUocanbeusedasameasureofproximity.

Alternatively,acertainthresholdlevelofoutputvoltageUocanbeusedtoactivateadigital

switchortocreateadigitaloutput,henceformingadigitallimitswitch.

Amorepracticalarrangementwouldbetohavethesemiconductorfixedrelativeto

oneanotherinasinglepackage.Bymovingaferromagneticmemberintotheairgap

betweenthemagneticsourceandthesemiconductorelement,thefluxlinkagecanbe

altered.ThischangesUo.Thisarrangementissuitablebothasananalogproximitysensor

andasalimitswitch.Furthermore,ifatoothedferromagneticwheelisusedtochangeUo,

wehaveashaftencoderoradigitaltachometer(seeFig.8.6).

以上719完成

Thelongitudinal縱向的arrangementofaproximitysensor,inwhichthemoving

elementapproacheshead-ontowardthesensor,isnotsuitablewhenthereisadangeror

overshootingthetarget,sinceitwilldamagethesensor.Amoredesirableconfigurationis

thelateral側(cè)面的arrangement,inwhichthemovingmemberslidesbythesensingfaceof

thesensor.Thesensitivitywillbelower,however,withthislateralarrangement.

以上女生完成

TherelationshipbetweentheoutputvoltageUoandthedistancexofaHallEffect

sensormeasuredfromthemo