传感器 第六章.ppt

CHAPTER6POSITIONANDMOTIONSENSORS Modernlinearanddigitalintegratedcircuittechnologyisusedthroughoutthefieldofpositionandmotionsensing Fullyintegratedsolutionswhichcombinelinearanddigitalfunctionshaveresultedincosteffectivesolutionstoproblemswhichinthepasthavebeensolvedusingexpensiveelectro mechanicaltechniques Thesesystemsareusedinmanyapplicationsincludingrobotics computer aidedmanufacturing andfactoryautomation POSITIONANDMOTIONSENSORS LinearPosition LinearVariableDifferentialTransformers LVDT HallEffectSensors ProximityDetectors LinearOutput MagneticFieldStrength Acceleration Accelerometers 1 LINEARVARIABLEDIFFERENTIALTRANSFORMERS LVDTs Thelinearvariabledifferentialtransformer LVDT isanaccurateandreliablemethodformeasuringlineardistance LVDTsfindusesinmodernmachine tool robotics avionics andcomputerizedmanufacturing BytheendofWorldWarII theLVDThadgainedacceptanceasasensorelementintheprocesscontrolindustrylargelyasaresultofitsuseinaircraft torpedo andweaponssystems ThepublicationofTheLinearVariableDifferentialTransformerbyHermanSchaevitzin1946madetheusercommunityatlargeawareoftheapplicationsandfeaturesoftheLVDT TheLVDT seeFigurebelow isaposition to electricalsensorwhoseoutputisproportionaltothepositionofamovablemagneticcore Thecoremoveslinearlyinsideatransformerconsistingofacenterprimarycoilandtwooutersecondarycoilswoundonacylindricalform TheprimarywindingisexcitedwithanACvoltagesource typicallyseveralkHz inducingsecondaryvoltageswhichvarywiththepositionofthemagneticcorewithintheassembly Thecoreisusuallythreadedinordertofacilitateattachmenttoanonferromagneticrodwhichinturninattachedtotheobjectwhosemovementordisplacementisbeingmeasured Thesecondarywindingsarewoundoutofphasewitheachother andwhenthecoreiscenteredthevoltagesinthetwosecondarywindingsopposeeachother andthenetoutputvoltageiszero Whenthecoreismovedoffcenter thevoltageinthesecondarytowardwhichthecoreismovedincreases whiletheoppositevoltagedecreases Theresultisadifferentialvoltageoutputwhichvarieslinearlywiththecore sposition Linearityisexcellentoverthedesignrangeofmovement typically0 5 orbetter TheLVDToffersgoodaccuracy linearity sensitivity infiniteresolution AwidevarietyofmeasurementrangesareavailableindifferentLVDTs typicallyfrom 100 mto 25cm Typicalexcitationvoltagesrangefrom1Vto24VRMS withfrequenciesfrom50Hzto20kHz KeyspecificationsfortheSchaevitzEl00LVDTaregiveninFigurebelow NominalLinearRange 0 1inches 2 54mm InputVoltage 3VRMS OperatingFrequency 50Hzto10kHz 2 5kHznominal Linearity 0 5 Fullscale Sensitivity 2 4mVOutput 0 001in VoltExcitation PrimaryImpedance 660 SecondaryImpedance 960 Notethatatruenulldoesnotoccurwhenthecoreisincenterpositionbecauseofmismatchesbetweenthetwosecondarywindingsandleakageinductance Also simplymeasuringtheoutputvoltageVOUTwillnottellonwhichsideofthenullpositionthecoreresides AsignalconditioningcircuitwhichremovesthesedifficultiesisshowninFigurebelowwheretheabsolutevaluesofthetwooutputvoltagesaresubtracted Usingthistechnique bothpositiveandnegativevariationsaboutthecenterpositioncanbemeasured ACSOURCE T Whileadiode capacitor typerectifiercouldbeusedastheabsolutevaluecircuit theprecisionrectifiershowninFigurebelowismoreaccurateandlinear INPUT TheinputisappliedtoaV Iconverterwhichinturndrivesananalogmultiplier ThesignofthedifferentialinputisdetectedbythecomparatorwhoseoutputswitchesthesignoftheV Ioutputviatheanalogmultiplier Thefinaloutputisaprecisionreplicaoftheabsolutevalueoftheinput ThesecircuitsarewellunderstoodbyICdesignersandareeasytoimplementonmodernbipolarprocesses Theindustry standardAD598LVDTsignalconditionershowninFigurebelow simplifiedform performsallrequiredLVDTsignalprocessing Theon chipexcitationfrequencyoscillatorcanbesetfrom20Hzto20kHzwithasingleexternalcapacitor TwoabsolutevaluecircuitsfollowedbytwofiltersareusedtodetecttheamplitudeoftheAandBchannelinputs Analogcircuitsarethenusedtogeneratetheratiometricfunction A B A B Notethatthisfunctionisindependentoftheamplitudeoftheprimarywindingexcitationvoltage assumingthesumoftheLVDToutputvoltageamplitudesremainsconstantovertheoperatingrange ThisisusuallythecaseformostLVDTs buttheusershouldalwayscheckwiththemanufacturerifitisnotspecifiedontheLVDTdatasheet AsingleexternalresistorsetstheAD598excitationvoltagefromapproximately1VRMSto24VRMS Drivecapabilityis30mARMS TheAD598candriveanLVDTattheendof300feetofcable sincethecircuitisnotaffectedbyphaseshiftsorabsolutesignalmagnitudes ThepositionoutputrangeofVOUTis11Vfora6mAloadanditcandriveupto1000feetofcable TheVAandVBinputscanbeaslowas100mVRMS 2 HALLEFFECTMAGNETICSENSORSIfacurrentflowsinaconductor orsemiconductor andthereisamagneticfieldpresentwhichisperpendiculartothecurrentflow thenthecombinationofcurrentandmagneticfieldwillgenerateavoltageperpendiculartoboth seeFigurebelow ThisphenomenoniscalledtheHallEffect wasdiscoveredbyE H Hallin1879 Thevoltage VH isknownastheHallVoltage VHisafunctionofthecurrentdensity themagneticfield andthechargedensityandcarriermobilityoftheconductor MAGNETICFIELDT THICKNESSVH HALLVOLTAGE TheHalleffectmaybeusedtomeasuremagneticfields andhenceincontact freecurrentmeasurement butitscommonestapplicationisinmotionsensorswhereafixedHallsensorandasmallmagnetattachedtoamovingpartcanreplaceacamandcontactswithagreatimprovementinreliability SinceVHisproportionaltomagneticfieldandnottorateofchangeofmagneticfieldlikeaninductivesensor theHallEffectprovidesamorereliablelowspeedsensorthananinductivepickup AlthoughseveralmaterialscanbeusedforHalleffectsensors siliconhastheadvantagethatsignalconditioningcircuitscanbeintegratedonthesamechipasthesensor CMOSprocessesarecommonforthisapplication AsimplerotationalspeeddetectorcanbemadewithaHallsensor againstage andacomparatorasshowninFigurebelow Thecircuitisdesignedtodetectrotationspeedasinautomotiveapplications Itrespondstosmallchangesinfield andthecomparatorhasbuilt inhysteresistopreventoscillation SeveralcompaniesmanufacturesuchHallswitches andtheirusageiswidespread VTHRESHOLD ROTATION MAGNETS Therearemanyotherapplications particularlyinautomotivethrottle pedal andvalvepositionsensing wherealinearrepresentationofthemagneticfieldisdesired TheAD22151isalinearmagneticfieldsensorwhoseoutputvoltageisproportionaltoamagneticfieldappliedperpendicularlytothepackagetopsurface seeFigurebelow TheAD22151combinesHallcelltechnologyandconditioningcircuitrytominimizetemperaturerelateddriftsassociatedwithsiliconHallcellcharacteristics Thearchitecturemaximizestheadvantagesofamonolithicimplementationwhileallowingsufficientversatilitytomeetvariedapplicationrequirementswithaminimumnumberofexternalcomponents Principalfeaturesincludedynamicoffsetdriftcancellationusingachopper typeopampandabuilt intemperaturesensor Designedforsingle 5Vsupplyoperation lowoffsetandgaindriftallowsoperationovera 400Cto 1500Crange Temperaturecompensation setexternallywitharesistorR1 canaccommodateanumberofmagneticmaterialscommonlyutilizedinpositionsensors Outputvoltagerangeandgaincanbeeasilysetwithexternalresistors Typicalgainrangeisusuallysetfrom2mV Gaussto6mV Gauss 3 OPTICALENCODERSOpticalencoderisthepopularpositionmeasuringsensors Anincrementalopticalencoder left handdiagraminFigurebelow isadiscdividedintosectorsthatarealternatelytransparentandopaque Alightsourceispositionedononesideofthedisc andalightsensorontheotherside Asthediscrotates theoutputfromthedetectorswitchesalternatelyonandoff dependingonwhetherthesectorappearingbetweenthelightsourceandthedetectoristransparentoropaque Thus theencoderproducesastreamofsquarewavepulseswhich whencounted indicatetheangularpositionoftheshaft Availableencoderresolutions thenumberofopaqueandtransparentsectorsperdisc rangefrom100to65 000 Mostincrementalencodersfeatureasecondlightsourceandsensoratanangletothemainsourceandsensor toindicatethedirectionofrotation Withoutsomeformofrevolutionmarker absoluteanglesaredifficulttodetermine Apotentiallyseriousdisadvantageisthatincrementalencodersrequireexternalcounterstodetermineabsoluteangleswithinagivenrotation Ifthepowerismomentarilyshutoff oriftheencodermissesapulseduetonoiseoradirtydisc theresultingangularinformationwillbeinerror Theabsoluteopticalencoder right handdiagraminFigureup overcomesthesedisadvantagesbutismoreexpensive Anabsoluteopticalencoder sdiscisdividedupintoNsectors N 5forexampleshown andeachsectorisfurtherdividedradiallyalongitslengthintoopaqueandtransparentsections formingauniqueN bitdigitalwordwithamaximumcountof2N 1 Thedigitalwordformedradiallybyeachsectorincrementsinvaluefromonesectortothenext usuallyemployingGraycode Binarycodingcouldbeused butcanproducelargeerrorsifasinglebitisincorrectlyinterpretedbythesensors Graycodeovercomesthisdefect themaximumerrorproducedbyanerrorinanysinglebitoftheGraycodeisonly1LSBaftertheGraycodeisconvertedintobinarycode AsetofNlightsensorsrespondstotheN bitdigitalwordwhichcorrespondstothedisc sabsoluteangularposition Industrialopticalencodersachieveupto16 bitresolution withabsoluteaccuraciesthatapproachtheresolution 20arcseconds Bothabsoluteandincrementalopticalencoders however maysufferdamageinharshindustrialenvironments 4 RESOLVERSANDSYNCHROSMachine toolandroboticsmanufacturershaveincreasinglyturnedtoresolversandsynchrostoprovideaccurateangularandrotationalinformation Thesedevicesexcelindemandingfactoryapplicationsrequiringsmallsize long termreliability absolutepositionmeasurement highaccuracy andlow noiseoperation AdiagramofatypicalsynchroandresolverisshowninFigurebelow Bothsycnchrosandresolversemploysingle windingrotorsthatrevolveinsidefixedstators Inthecaseofasimplesynchro thestatorhasthreewindingsoriented120oapartandelectricallyconnectedinaY connection Resolversdifferfromsynchrosinthattheirstatorshaveonlytwowindingsorientedat90o ROTOR STAROR R1 R2 S2 S4 S2 S1TOS3 Vsin tsin S3TOS2 Vsin tsin 120o S2TOS1 Vsin tsin 240o RESOLVER S1TOS3 Vsin tsin S4TOS2 Vsin tsin 90o Vsin tcos SYNCHRO Becausesynchroshavethreestatorcoilsina120oorientation theyaremoredifficultthanresolverstomanufactureandarethereforemorecostly Today synchrosfinddecreasinguse exceptincertainmilitaryapplications Modernresolvers incontrast areavailableinabrushlessformthatemployatransformertocoupletherotorsignalsfromthestatortotherotor Theprimarywindingofthistransformerresidesonthestator andthesecondaryontherotor Otherresolversusemoretraditionalbrushestocouplethesignalintotherotorwinding Brushlessresolversaremoreruggedthansynchrosbecausetherearenobrushestobreakordislodge andthelifeofabrushlessresolverislimitedonlybyitsbearings Mostresolversarespecifiedtoworkover2Vto40VRMSandatfrequenciesfrom400Hzto10kHz Angularaccuraciesrangefrom5arc minutesto0 5arc minutes Thereare60arc minutesinonedegree and60arc secondsinonearc minute Hence onearc minuteisequalto0 0167degrees Inoperation synchrosandresolversresemblerotatingtransformers TherotorwindingisexcitedbyanACreferencevoltage atfrequenciesuptoafewkHz Themagnitudeofthevoltageinducedinanystatorwindingisproportionaltothesineoftheangle betweentherotorcoilaxisandthestatorcoilaxis Inthecaseofasynchro thevoltageinducedacrossanypairofstatorterminalswillbethevectorsumofthevoltagesacrossthetwoconnectedcoils Forexample iftherotorofasynchroisexcitedwithareferencevoltage Vsin t acrossitsterminalsR1andR2 thenthestator sterminalwillseevoltagesintheform S1toS3 Vsin tsin S3toS2 Vsin tsin 120o S2toS1 Vsin tsin 240o where istheshaftangle Inthecaseofaresolver witharotorACreferencevoltageofVsin t thestator sterminalvoltageswillbe S1toS3 Vsin tsin S4toS2 Vsin tsin 90o Vsin tcos Itshouldbenotedthatthe3 wiresynchrooutputcanbeeasilyconvertedintotheresolver equivalentformatusingaScott Ttransformer Therefore thefollowingsignalprocessingexampledescribesonlytheresolverconfiguration Atypicalresolver to digitalconverter RDC isshownfunctionallyinFigurebelow Thetwooutputsoftheresolverareappliedtocosineandsinemultipliers Thesemultipliersincorporatesineandcosinelookuptablesandfunctionasmultiplyingdigital to analogconverters Beginbyassumingthatthecurrentstateoftheup downcounterisadigitalnumberrepresentingatrialangle Theconverterseekstoadjustthedigitalangle continuouslytobecomeequalto andtotrack theanaloganglebeingmeasured Theresolver sstatoroutputvoltagesarewrittenas Vsin t V1 Vsin tsin V2 Vsin tcos where istheangleoftheresolver srotor Thedigitalangleisappliedtothecosinemultiplier anditscosineismultipliedbyV1toproducetheterm Vsin tsin cos ThedigitalangleisalsoappliedtothesinemultiplierandmultipliedbyV2toproducttheterm Vsin tcos sin ThesetwosignalsaresubtractedfromeachotherbytheerroramplifiertoyieldanACerrorsignaloftheform Vsin t sin cos cos sin Usingasimpletrigonometricidentity thisreducesto Vsin t sin ThedetectorsynchronouslydemodulatesthisACerrorsignal usingtheresolver srotorvoltageasareference ThisresultsinaDCerrorsignalproportionaltosin TheDCerrorsignalfeedsanintegrator theoutputofwhichdrivesavoltage controlled oscillator VCO TheVCO inturn causestheup downcountertocountintheproperdirectiontocause sin 0Whenthisisachieved 0 andtherefore towithinonecount Hence thecounter sdigitaloutput representstheangle Thelatchesenablethisdatatobetransferredexternallywithoutinterruptingtheloop stracking Thiscircuitisequivalenttoaso calledtype 2servoloop becauseithas ineffect twointegrators Oneisthecounter whichaccumulatespulses theotheristheintegratorattheoutputofthedetector Inatype 2servoloopwithaconstantrotationalvelocityinput theoutputdigitalwordcontinuouslyfollows ortrackstheinput withoutneedingexternallyderivedconvertcommands andwithnosteadystatephaselagbetweenthedigitaloutputwordandactualshaftangle Anerrorsignalappearsonlyduringperiodsofaccelerationordeceleration Asanaddedbonus thetrackingRDCprovides