Tutorial: Data Acquisition in LabVIEW

Transcription

UniversityCollegeofSoutheastNorway
DataAcquisitioninLabVIEW
Hans-PetterHalvorsen,2016.10.28
http://home.hit.no/~hansha
Preface
ThistutorialexplainsthebasicconceptsofaDataAcquisitioninLabVIEW.
Youfindthisdocumentandadditionalresourcesonlineat:
http://home.hit.no/~hansha/?tutorial=daq
YoushouldhavesomebasicknowledgeaboutLabVIEW,e.g.,the“IntroductiontoLabVIEW”
tutorial.Thisdocumentisavailablefordownloadathttp://home.hit.no/~hansha/.
FormoreinformationaboutLabVIEW,visit:http://home.hit.no/~hansha/?page=labview.
InadditiontoLabVIEW,youwillneedtoinstallthedriverNI-DAQmx.
[Figureontitlepage:www.ni.com]
2
TableofContents
Preface......................................................................................................................................2
TableofContents.....................................................................................................................iii
1 IntroductiontoLabVIEW.....................................................................................................1
1.1 Dataflowprogramming................................................................................................1
1.2 Graphicalprogramming................................................................................................1
1.3 Benefits.........................................................................................................................2
2 IntroductiontoDataAcquisition.........................................................................................3
2.1 IntroductiontoDAQ-DataAcquisition........................................................................3
2.1.1 Physicalinput/outputsignals.................................................................................4
2.1.2 DAQdevice/hardware...........................................................................................4
2.1.3 Driversoftware......................................................................................................5
2.1.4 Yoursoftwareapplication(Applicationsoftware).................................................6
2.2 MAX–MeasurementandAutomationExplorer..........................................................6
2.3 NI-DAQmx.....................................................................................................................7
2.3.1 DAQAssistant........................................................................................................8
2.4 NIUSB-6008..................................................................................................................8
3 Physicalinput/outputsignals............................................................................................10
3.1 Transducers................................................................................................................10
3.2 Signals.........................................................................................................................11
3.2.1 AnalogSignals......................................................................................................11
3.3 DigitalSignals..............................................................................................................13
4 MAX...................................................................................................................................15
iii
iv
TableofContents
5 NI-DAQmx.........................................................................................................................17
5.1.1 DAQAssistant......................................................................................................18
5.2 SimulatingaDAQDevice............................................................................................18
6 DAQDevices......................................................................................................................20
6.1 PerformingAnalog-to-DigitalConversion...................................................................21
6.2 PerformingDigital-to-AnalogConversion...................................................................21
6.3 UsingCounters...........................................................................................................21
6.4 UsingDigitalI/O..........................................................................................................22
7 NIUSB-6008......................................................................................................................23
7.1 ConnectNIUSB-6008tothePC..................................................................................23
7.1.1 TestingtheUSB-6008inMAX..............................................................................24
7.2 UsingNIUSB-6008inLabVIEW...................................................................................30
7.3 DAQAssistant.............................................................................................................31
7.3.1 AnalogInput........................................................................................................31
7.3.2 AnalogOutput......................................................................................................38
8 LoggingtoFile...................................................................................................................43
8.1 WritingtoMeasurementFile.....................................................................................43
8.2 ReadfromMeasurementFile.....................................................................................46
Tutorial:
Data Acquisition in LabVIEW
1 IntroductiontoLabVIEW
LabVIEW(shortforLaboratoryVirtualInstrumentationEngineeringWorkbench)isa
platformanddevelopmentenvironmentforavisualprogramminglanguagefromNational
Instruments.Thegraphicallanguageisnamed"G".OriginallyreleasedfortheApple
Macintoshin1986,LabVIEWiscommonlyusedfordataacquisition,instrumentcontrol,and
industrialautomationonavarietyofplatformsincludingMicrosoftWindows,variousflavors
ofUNIX,Linux,andMacOSX.VisitNationalInstrumentsatwww.ni.com.
Thecodefileshavetheextension“.vi”,whichisanabbreviationfor“VirtualInstrument”.
LabVIEWofferslotsofadditionalAdd-OnsandToolkits.
1.1 Dataflowprogramming
TheprogramminglanguageusedinLabVIEW,alsoreferredtoasG,isadataflow
programminglanguage.Executionisdeterminedbythestructureofagraphicalblock
diagram(theLV-sourcecode)onwhichtheprogrammerconnectsdifferentfunction-nodes
bydrawingwires.Thesewirespropagatevariablesandanynodecanexecuteassoonasall
itsinputdatabecomeavailable.Sincethismightbethecaseformultiplenodes
simultaneously,Gisinherentlycapableofparallelexecution.Multi-processingandmultithreadinghardwareisautomaticallyexploitedbythebuilt-inscheduler,whichmultiplexes
multipleOSthreadsoverthenodesreadyforexecution.
1.2 Graphicalprogramming
LabVIEWtiesthecreationofuserinterfaces(calledfrontpanels)intothedevelopmentcycle.
LabVIEWprograms/subroutinesarecalledvirtualinstruments(VIs).EachVIhasthree
components:ablockdiagram,afrontpanel,andaconnectorpanel.Thelastisusedto
representtheVIintheblockdiagramsofother,callingVIs.Controlsandindicatorsonthe
frontpanelallowanoperatortoinputdataintoorextractdatafromarunningvirtual
instrument.However,thefrontpanelcanalsoserveasaprogrammaticinterface.Thusa
virtualinstrumentcaneitherberunasaprogram,withthefrontpanelservingasauser
interface,or,whendroppedasanodeontotheblockdiagram,thefrontpaneldefinesthe
inputsandoutputsforthegivennodethroughtheconnectorpane.ThisimplieseachVIcan
beeasilytestedbeforebeingembeddedasasubroutineintoalargerprogram.
1
2
IntroductiontoLabVIEW
Thegraphicalapproachalsoallowsnon-programmerstobuildprogramssimplybydragging
anddroppingvirtualrepresentationsoflabequipmentwithwhichtheyarealreadyfamiliar.
TheLabVIEWprogrammingenvironment,withtheincludedexamplesandthe
documentation,makesitsimpletocreatesmallapplications.Thisisabenefitononeside,
butthereisalsoacertaindangerofunderestimatingtheexpertiseneededforgoodquality
"G"programming.Forcomplexalgorithmsorlarge-scalecode,itisimportantthatthe
programmerpossessanextensiveknowledgeofthespecialLabVIEWsyntaxandthe
topologyofitsmemorymanagement.ThemostadvancedLabVIEWdevelopmentsystems
offerthepossibilityofbuildingstand-aloneapplications.Furthermore,itispossibletocreate
distributedapplications,whichcommunicatebyaclient/serverscheme,andaretherefore
easiertoimplementduetotheinherentlyparallelnatureofG-code.
1.3 Benefits
OnebenefitofLabVIEWoverotherdevelopmentenvironmentsistheextensivesupportfor
accessinginstrumentationhardware.Driversandabstractionlayersformanydifferenttypes
ofinstrumentsandbusesareincludedorareavailableforinclusion.Thesepresent
themselvesasgraphicalnodes.Theabstractionlayersofferstandardsoftwareinterfacesto
communicatewithhardwaredevices.Theprovideddriverinterfacessaveprogram
developmenttime.ThesalespitchofNationalInstrumentsis,therefore,thatevenpeople
withlimitedcodingexperiencecanwriteprogramsanddeploytestsolutionsinareduced
timeframewhencomparedtomoreconventionalorcompetingsystems.Anewhardware
drivertopology(DAQmxBase),whichconsistsmainlyofG-codedcomponentswithonlya
fewregistercallsthroughNIMeasurementHardwareDDK(DriverDevelopmentKit)
functions,providesplatformindependenthardwareaccesstonumerousdataacquisition
andinstrumentationdevices.TheDAQmxBasedriverisavailableforLabVIEWonWindows,
MacOSXandLinuxplatforms.
Tutorial:
2 IntroductiontoData
Acquisition
ThischapterexplainsthebasicconceptsofusingDAQinLabVIEW.
Topics:
•
•
•
IntroductiontoDAQ-DataAcquisition
MAX–MeasurementandAutomationExplorer
NI-DAQmx
LabVIEWisverypowerfulwhenitcomestocreatingDAQapplications.LabVIEWincludesa
setofVIsthatletyouconfigure,acquiredatafrom,andsenddatatoDAQdevices.Often,
onedevicecanperformavarietyoffunctions,suchasanalog-to-digital(A/D)conversion,
digital-to-analog(D/A)conversion,digitalI/O,andcounter/timeroperations.Eachdevice
supportsdifferentDAQandsignalgenerationspeeds.Also,eachDAQdeviceisdesignedfor
specifichardware,platformsandoperatingsystems.
NationalInstruments,theinventorofLabVIEW,alsomakeDAQdevices,sotheintegration
withtheDAQdevicesfromNIandtheLabVIEWsoftwareisseamlessandmakesiteasytodo
I/OoperationsfromtheLabVIEWenvironment.
2.1 IntroductiontoDAQ-DataAcquisition
Thepurposeofdataacquisitionistomeasureanelectricalorphysicalphenomenonsuchas
voltage,current,temperature,pressure,orsound.PC-baseddataacquisitionusesa
combinationofmodularhardware,applicationsoftware,andacomputertotake
measurements.Whileeachdataacquisitionsystemisdefinedbyitsapplication
requirements,everysystemsharesacommongoalofacquiring,analyzing,andpresenting
information.Dataacquisitionsystemsincorporatesignals,sensors,actuators,signal
conditioning,dataacquisitiondevices,andapplicationsoftware. Sosummingup,DataAcquisitionistheprocessof:
•
•
Acquiringsignalsfromreal-worldphenomena
Digitizingthesignals
3
4
•
IntroductiontoDataAcquisition Analyzing,presentingandsavingthedata
TheDAQsystemhasthefollowingpartsinvolved,seeFigure:
Thepartsare:
•
•
•
•
Physicalinput/outputsignals
DAQdevice/hardware
Driversoftware
Yoursoftwareapplication(Applicationsoftware)
ForanIntroductiontoDataAcquisition,seethiswebcast:
http://zone.ni.com/wv/app/doc/p/id/wv-169
2.1.1
Physicalinput/outputsignals
Aphysicalinput/outputsignalistypicallyavoltageorcurrentsignal.
Avoltagesignalcantypicallybea0-5Vsignal,whileacurrentsignalcantypicallybea420mAsignal.
2.1.2
DAQdevice/hardware
DAQhardwareactsastheinterfacebetweenthecomputerandtheoutsideworld.It
primarilyfunctionsasadevicethatdigitizesincominganalogsignalssothatthecomputer
caninterpretthem
ADAQdevice(DataAcquisitionHardware)usuallyhasthesefunctions:
•
•
Analoginput
Analogoutput
Tutorial:
5
•
•
IntroductiontoDataAcquisition DigitalI/O
Counter/timers
WehavedifferentDAQdevices,suchas:
•
•
•
“DesktopDAQdevices”whereyouneedtoplugaPCIDAQboardintoyour
computer.Thesoftwareisrunningonacomputer.
“PortableDAQdevices”forconnectiontotheUSBport,Wi-Ficonnections,etc.The
softwareisrunningonacomputer
“DistributedDAQdevices”wherethesoftwareisdevelopedonyourcomputerand
thenlaterdownloadedtothedistributedDAQdevice.
[www.ni.com]
2.1.3
Driversoftware
Driversoftwareisthelayerofsoftwareforeasilycommunicatingwiththehardware.Itforms
themiddlelayerbetweentheapplicationsoftwareandthehardware.Driversoftwarealso
preventsaprogrammerfromhavingtodoregister-levelprogrammingorcomplicated
commandsinordertoaccessthehardwarefunctions.
DriversoftwarefromNationalInstruments:
•
NI-DAQmx
Tutorial:
6
•
IntroductiontoDataAcquisition NI-DAQmxBase
TheDAQAssistant,includedwithNI-DAQmx,isagraphical,interactiveguideforconfiguring,
testing,andacquiringmeasurementdata.Withasingleclick,youcanevengeneratecode
basedonyourconfiguration,makingiteasierandfastertodevelopcomplexoperations.
BecauseDAQAssistantiscompletelymenu-driven,youwillmakefewerprogrammingerrors
anddrasticallydecreasethetimefromsettingupyourDAQsystemtotakingyourfirst
measurement.
NI-DAQmxBaseoffersasubsetofNI-DAQmxfunctionalityonWindowsandLinux,MacOSX,
WindowsMobileandWindowsCE.
2.1.4
Yoursoftwareapplication(Application
software)
Applicationsoftwareaddsanalysisandpresentationcapabilitiestothedriversoftware.Your
softwareapplicationnormallydoessuchtasksas:
•
•
•
•
•
Real-timemonitoring
Dataanalysis
Datalogging
Controlalgorithms
Humanmachineinterface(HMI)
InordertocreateyourDAQapplicationyouneedaprogrammingdevelopmenttool,suchas
LabVIEW.
2.2 MAX–MeasurementandAutomation
Explorer
Measurement&AutomationExplorer(MAX)providesaccesstoyourNationalInstruments
devicesandsystems.
WithMAX,youcan:
•
•
•
•
ConfigureyourNationalInstrumentshardwareandsoftware Createandeditchannels,tasks,interfaces,scales,andvirtualinstruments Executesystemdiagnostics Viewdevicesandinstrumentsconnectedtoyoursystem Tutorial:
7
•
IntroductiontoDataAcquisition UpdateyourNationalInstrumentssoftware Inadditiontothestandardtools,MAXcanexposeitem-specifictoolsyoucanuseto
configure,diagnose,ortestyoursystem,dependingonwhichNIproductsyouinstall.Asyou
navigatethroughMAX,thecontentsoftheapplicationmenuandtoolbarchangetoreflect
thesenewtools.
2.3 NI-DAQmx
TheNI-DAQmxDriversoftwareisthelayerofsoftwareforeasilycommunicatingwiththe
hardware.Itformsthemiddlelayerbetweentheapplicationsoftwareandthehardware.
Tutorial:
8
IntroductiontoDataAcquisition Driversoftwarealsopreventsaprogrammerfromhavingtodoregister-levelprogramming
orcomplicatedcommandsinordertoaccessthehardwarefunctions.
TheDAQmxpaletteinLabVIEW:
The“DAQAssistant”isaneasywaytostartusingtheDAQfeaturesinLabVIEW.Wewill
learnmoreaboutthe“DAQAssistant”inalaterchapter.
2.3.1
DAQAssistant
measurement.
2.4 NIUSB-6008
NIUSB-6008isasimpleandlow-costmultifunctionI/OdevicefromNationalInstruments.
Tutorial:
9
IntroductiontoDataAcquisition Thedevicehasthefollowingspecifications:
•
•
•
•
•
•
8analoginputs(12-bit,10kS/s)
2analogoutputs(12-bit,150S/s)
12digitalI/O
USBconnection,Noextrapower-supplyneeeded
CompatiblewithLabVIEW,LabWindows/CVI,andMeasurementStudioforVisual
Studio.NET
NI-DAQmxdriversoftware TheNIUSB-6008iswellsuitedforeducationpurposesduetoitssmallsizeandeasyUSB
connection.
Tutorial:
3 Physicalinput/output
signals
Dataacquisitioninvolvesgatheringsignalsfrommeasurementsourcesanddigitizingthe
signalforstorage,analysis,andpresentationonaPC.Dataacquisition(DAQ)systemscome
inmanydifferentPCtechnologyformsforgreatflexibilitywhenchoosingyoursystem.
ScientistsandengineerscanchoosefromPCI,PXI,PCIExpress,PXIExpress,PCMCIA,USB,
WirelessandEthernetdataacquisitionfortest,measurement,andautomationapplications.
TherearefivecomponentstobeconsideredwhenbuildingabasicDAQsystem
•
•
•
•
•
Transducersandsensors
Signals
Signalconditioning
DAQhardware
Driverandapplicationsoftware
InthischapterwefocusonTransducers,sensorsandSignals.
3.1 Transducers
Dataacquisitionbeginswiththephysicalphenomenontobemeasured.Thisphysical
phenomenoncouldbehetemperatureofaroom,theintensityofalightsource,the
pressureinsideachamber,theforceappliedtoanobject,ormanyotherthings.Aneffective
DAQsystemcanmeasureallofthesedifferentphenomena.
Atransducerisadevicethatconvertsaphysicalphenomenonintoameasurableelectrical
signal,suchasvoltageorcurrent.TheabilityofaDAQsystemtomeasuredifferent
phenomenadependsonthetransducerstoconvertthephysicalphenomenaintosignals
measurablebytheDAQhardware.TransducersaresynonymouswithsensorsinDAQ
systems.Therearespecifictransducersformanydifferentapplications,suchasmeasuring
temperature,pressure,orfluidflow.Belowweseesomecommonphenomenaandthe
transducersusedtomeasurethem.
Phenomenon
Transducer
10
11
Physicalinput/outputsignals Temperature
Thermocouple,RTD,Thermistor
Light
PhotoSensor
Sound
Microphone
ForceandPressure
StrainGage,PiezoelectricTransducer
Positionand
Displacement
Potentiometer,LVDT,OpticalEncoder
Acceleration
Accelerometer
pH
pHElectrode
Differenttransducershavedifferentrequirementsforconvertingphenomenaintoa
measurablesignal.Sometransducersmayrequireexcitationintheformofvoltageor
current.Othertransducersmayrequireadditionalcomponentsandevenresistivenetworks
toproduceasignal. Refertoni.com/sensorsformoreinformationontransducers.
3.2 Signals
Theappropriatetransducersconvertphysicalphenomenaintomeasurablesignals.However,
differentsignalsneedtobemeasuredindifferentways.Forthisreason,itisimportantto
understandthedifferenttypesofsignalsandtheircorrespondingattributes.Signalscanbe
categorizedintotwogroups:
•
•
Analog
Digital
3.2.1
AnalogSignals
Analoginputistheprocessofmeasuringananalogsignalandtransferringthemeasurement
toacomputerforanalysis,display,orstorage.Ananalogsignalisasignalthatvaries
continuously.Analoginputismostcommonlyusedtomeasurevoltageorcurrent.Youcan
usemanytypesofdevicestoperformanaloginput,suchasmultifunctionDAQ(MIO)
devices,high-speeddigitizers,digitalmultimeters,andDynamicSignalAcquisition(DSA)
devices.
Tutorial:
12
Physicalinput/outputsignals [ww.ni.com]
Ananalogsignalcanbeatanyvaluewithrespecttotime.Afewexamplesofanalogsignals
includevoltage,temperature,pressure,sound,andload.Thethreeprimarycharacteristicsof
ananalogsignalis:
•
•
•
Level
Shape
Frequency
Level
Becauseanalogsignalscantakeonanyvalue,thelevelgivesvitalinformationaboutthe
measuredanalogsignal.Theintensityofalightsource,thetemperatureinaroom,andthe
pressureinsideachamberareallexamplesthatdemonstratetheimportanceofthelevelof
asignal.Whenmeasuringthelevelofasignal,thesignalgenerallydoesnotchangequickly
Tutorial:
13
Physicalinput/outputsignals withrespecttotime.Theaccuracyofthemeasurement,however,isveryimportant.ADAQ
systemthatyieldsmaximumaccuracyshouldbechosentoaidinanaloglevel
measurements.
Shape
Somesignalsarenamedaftertheirspecificshape-sine,square,sawtooth,andtriangle.The
shapeofananalogsignalcanbeasimportantasthelevel,becausebymeasuringtheshape
ofananalogsignal,youcanfurtheranalyzethesignal,includingpeakvalues,DCvalues,and
slope.Signalswhereshapeisofinterestgenerallychangerapidlywithrespecttotime,but
systemaccuracyisstillimportant.Theanalysisofheartbeats,videosignals,sounds,
vibrations,andcircuitresponsesaresomeapplicationsinvolvingshapemeasurements.
Frequency
Allanalogsignalscanbecategorizedbytheirfrequency.Unlikethelevelorshapeofthe
signal,frequencycannotbedirectlymeasured.Thesignalmustbeanalyzedusingsoftware
todeterminethefrequencyinformation.Thisanalysisisusuallydoneusinganalgorithm
knownastheFouriertransform.
Whenfrequencyisthemostimportantpieceofinformation,itisimportanttoconsider
includingbothaccuracyandacquisitionspeed.Althoughtheacquisitionspeedforacquiring
thefrequencyofasignalislessthanthespeedrequiredforobtainingtheshapeofasignal,
thesignalmuststillbeacquiredfastenoughthatthepertinentinformationisnotlostwhile
theanalogsignalisbeingacquired.Theconditionthatstipulatesthisspeedisknownasthe
NyquistSamplingTheorem.Speechanalysis,telecommunication,andearthquakeanalysis
aresomeexamplesofcommonapplicationswherethefrequencyofthesignalmustbe
known.
3.3 DigitalSignals
Adigitalsignalcannottakeonanyvaluewithrespecttotime.Instead,adigitalsignalhas
twopossiblelevels:highandlow.Digitalsignalsgenerallyconformtocertainspecifications
thatdefinecharacteristicsofthesignal.Digitalsignalsarecommonlyreferredtoas
transistor-to-transistorlogic(TTL).TTLspecificationsindicateadigitalsignaltobelowwhen
thelevelfallswithin0to0.8V,andthesignalishighbetween2to5V.Theuseful
informationthatcanbemeasuredfromadigitalsignalincludesthestateandtherate.
Tutorial:
14
Physicalinput/outputsignals State
Digitalsignalscannottakeonanyvaluewithrespecttotime.Thestateofadigitalsignalis
essentiallythelevelofthesignal-onoroff,highorlow.Monitoringthestateofaswitch-
openorclosed-isacommonapplicationshowingtheimportanceofknowingthestateofa
digitalsignal.
Rate
Therateofadigitalsignaldefineshowthedigitalsignalchangesstatewithrespecttotime.
Anexampleofmeasuringtherateofadigitalsignalincludesdetermininghowfastamotor
shaftspins.Unlikefrequency,therateofadigitalsignalmeasureshowoftenaportionofa
signaloccurs.Asoftwarealgorithmisnotrequiredtodeterminetherateofasignal
Tutorial:
4 MAX
Measurement&AutomationExplorer(MAX)providesaccesstoyourNationalInstruments
devicesandsystems.
WithMAX,youcan:
•
•
•
•
•
ConfigureyourNationalInstrumentshardwareandsoftware Createandeditchannels,tasks,interfaces,scales,andvirtualinstruments Executesystemdiagnostics Viewdevicesandinstrumentsconnectedtoyoursystem UpdateyourNationalInstrumentssoftware Inadditiontothestandardtools,MAXcanexposeitem-specifictoolsyoucanuseto
configure,diagnose,ortestyoursystem,dependingonwhichNIproductsyouinstall.Asyou
navigatethroughMAX,thecontentsoftheapplicationmenuandtoolbarchangetoreflect
thesenewtools.
LabVIEWinstallsMAXtoestablishalldevicesandchannelconfigurationparameters.MAX
readstheinformationtheDeviceManagerrecordsintheWindowsRegistryandassignsa
logicaldevicenumbertoeachDAQdevice.
15
16
MAX YouusethedevicenumbertorefertothedeviceinLabVIEW.YoucanaccessMAXby
selectingTools»Measurement&AutomationExplorerinLabVIEW.Thisdisplaystheprimary
MAXwindow.
Beforeusingadataacquisitionboard,youmustconfirmthatthesoftwarecancommunicate
withtheboardbyconfiguringthedevices.ForWindows,theWindowsConfiguration
Managerkeepstrackofallthehardwareinstalledinthecomputer,includingNational
InstrumentsDAQdevices.TheWindowsConfigurationManagerautomaticallydetectsand
configuresPlug&Play(PnP)devices.
WindowsConfigurationManager
IfyouhaveaPnPdevice,suchasanESeriesMIOdevice,theWindowsConfiguration
Managerautomaticallydetectsandconfiguresthedevice.Ifyouhaveanon-PnPdevice,or
legacydevice,youmustconfigurethedevicemanuallyusingtheAddNewHardwareoption
intheControlPanel.YoucanverifytheWindowsConfigurationbyaccessingtheDevice
Manager.
Tutorial:
5 NI-DAQmx
Driversoftwareisthelayerofsoftwareforeasilycommunicatingwiththehardware.Itforms
themiddlelayerbetweentheapplicationsoftwareandthehardware.Driversoftwarealso
preventsaprogrammerfromhavingtodoregister-levelprogrammingorcomplicated
commandsinordertoaccessthehardwarefunctions.
DriversoftwarefromNationalInstruments:
•
•
NI-DAQmx
NI-DAQmxBase
measurement.
NI-DAQmxBaseoffersasubsetofNI-DAQmxfunctionalityonWindowsandLinux,MacOSX,
WindowsMobileandWindowsCE.
NationalInstrumentsDAQboardshaveadriverenginethatcommunicatesbetweenthe
boardandtheapplicationsoftware.Therearetwodriverengines,NI-DAQmxandTraditional
NI-DAQ.YoucanalsousetheDAQAssistant,anExpressVIthatcommunicateswithNIDAQmx,inLabVIEWtocommunicatewiththeDAQboard.Inaddition,NationalInstruments
providesMeasurement&AutomationExplorer(MAX)forconfiguringDAQboards.
TheNI-DAQmxDriversoftwareisthelayerofsoftwareforeasilycommunicatingwiththe
hardware.Itformsthemiddlelayerbetweentheapplicationsoftwareandthehardware.
Driversoftwarealsopreventsaprogrammerfromhavingtodoregister-levelprogramming
orcomplicatedcommandsinordertoaccessthehardwarefunctions.
TheDAQmxpaletteinLabVIEW:
17
18
NI-DAQmx 5.1.1
DAQAssistant
measurement.
Scales
YoucanconfigurecustomscalesforyourmeasurementsusingMAX.Thisisveryusefulwhen
workingwithsensors.Itallowsyoutobringascaledvalueintoyourapplicationwithout
havingtoworkdirectlywiththerawvalues.Forexample,youcanuseatemperaturesensor
thatrepresentstemperaturewithavoltage.Theconversionequationforthetemperatureis,
Voltagex100=Celsius.Afterascaleisset,youcanuseitinyourapplicationprogram,
providingthetemperaturevalue,ratherthanthevoltage.
Whenperforminganaloginput,thetaskcanbetimedto:
•
•
•
Acquire1Sample
AcquirenSamples
AcquireContinuously
5.2 SimulatingaDAQDevice
Tutorial:
19
NI-DAQmx YoucancreateNI-DAQmxsimulateddevicesinNI-DAQmx7.4orlater.UsingNI-DAQmx
simulateddevices:
YoucantryNIproductsinyourapplicationwithoutthehardware.
Later,whenyouacquirethehardware,youcanimporttheNI-DAQmxsimulateddevice
configurationtothephysicaldeviceusingtheMAXPortableConfigurationWizard.
Youcanworkonyourapplicationsonaportablesystemanduponreturningtotheoriginal
system,youcaneasilyimportyourapplicationwork.
CreatingNI-DAQmxSimulatedDevices
TocreateanNI-DAQmxsimulateddevice,right-clickDevicesandInterfacesandselectCreate
New.TheCreateNewdialogboxpromptsyoutoselectadevicetoadd.SelectNI-DAQmx
SimulatedDeviceandclickFinish.IntheChooseDevicedialogbox,selectthefamilyof
devicesforthedeviceyouwanttosimulate.SelectthedeviceandclickOK.IfyouselectaPXI
device,youarepromptedtoselectachassisnumberandPXIslotnumber.Ifyouselectan
SCXIchassis,theSCXIconfigurationpanelsopen.
Tutorial:
6 DAQDevices
DAQhardwareactsastheinterfacebetweenthecomputerandtheoutsideworld.It
primarilyfunctionsasadevicethatdigitizesincominganalogsignalssothatthecomputer
caninterpretthem
ADAQdevice(DataAcquisitionHardware)usuallyhasthesefunctions:
•
•
•
•
Analoginput
Analogoutput
DigitalI/O
Counter/timers
WehavedifferentDAQdevices,suchas:
•
•
•
“DesktopDAQdevices”whereyouneedtoplugaPCIDAQboardintoyour
computer.Thesoftwareisrunningonacomputer.
“PortableDAQdevices”forconnectiontotheUSBport,Wi-Ficonnections,etc.The
softwareisrunningonacomputer
“DistributedDAQdevices”wherethesoftwareisdevelopedonyourcomputerand
thenlaterdownloadedtothedistributedDAQdevice.
[www.ni.com]
20
21
DAQDevices MostDAQdeviceshavefourstandardelements:analoginput,analogoutput,digitalI/O,and
counters.TheDAQdevicetransfersthemeasuredsignalstoacomputerthroughdifferent
busstructures.Forexample,youcanplugaDAQdeviceintothePCIbusortheUSBportofa
computerorthePersonalComputerMemoryCardInternationalAssociation(PCMCIA)
socketofalaptop.YoualsocanusePXI/CompactPCItocreateaportable,versatile,and
ruggedmeasurementsystem.
6.1 PerformingAnalog-to-DigitalConversion
Analog-to-digitalconversionisaprocessofacquiringandtranslatingsignalsintodigitaldata
sothatacomputercanprocessit.Analog-to-digitalconverters(ADCs)arecircuit
componentsthatconvertavoltagelevelintoaseriesofonesandzeroes.ADCssamplethe
analogsignaloneachrisingorfallingedgeofasampleclock.Ineachcycle,theADCtakesa
snapshotoftheanalogsignal,measuresandconvertsitintoadigitalvalue.TheADCobtains
andapproximatesthesignalwithfixedprecisionandconvertsitintoaseriesofdigital
values.
[www.ni.com]
6.2 PerformingDigital-to-AnalogConversion
Digital-to-analogconversionistheoppositeofanalog-to-digitalconversion.Indigital-toanalogconversion,thecomputergeneratesthedata.
6.3 UsingCounters
Acounterisadigitaltimingdevice.Youtypicallyusecountersforeventcounting,frequency
measurement,periodmeasurement,positionmeasurement,andpulsegeneration.
Tutorial:
22
DAQDevices 6.4 UsingDigitalI/O
Digitalsignalsareelectricalsignalsthattransferdigitaldataoverawire.Thesesignals
typicallyhaveonlytwostates:onandoff,alsoknownashighandlow,or1and0.When
sendingadigitalsignalacrossawire,thesenderappliesavoltagetothewireandthe
receiverusesthevoltageleveltodeterminethevaluebeingsent.Thevoltagerangesfor
eachdigitalvaluedependonthevoltagelevelstandardbeingused.
Digitalsignalshavemanyusers:
•
DigitalsignalscontrolormeasuredigitaldevicessuchasswitchesorLEDs
Tutorial:
7 NIUSB-6008
NIUSB-6008isasimpleandlow-costmultifunctionI/OdevicefromNationalInstruments.
Thedevicehasthefollowingspecifications:
•
•
•
•
•
•
8analoginputs(12-bit,10kS/s)
2analogoutputs(12-bit,150S/s)
12digitalI/O
USBconnection,Noextrapower-supplyneeded
CompatiblewithLabVIEW,LabWindows/CVI,andMeasurementStudioforVisual
Studio.NET
NI-DAQmxdriversoftware TheNIUSB-6008iswellsuitedforeducationpurposesduetoitssmallsizeandeasyUSB
connection.
7.1 ConnectNIUSB-6008tothePC
Configuringandtesting:USB-6008canbeconfiguredandtestedusingMAX(Measurement
andAutomationExplorer),whichisinstalledwiththeNI-DAQmxDriverSoftware. ThefirsttimeyouconnecttheUSB-6008tothePC,theWindowsHardwareInstallerWizard
willopen.
23
24
NIUSB-6008 ThewizardsearchesthePCforthenecessarydriversoftwarefortheUSB-6008.Thisdriver
softwarewasinstalledalongwiththeinstallationoftheNI-DAQsoftware.Whenthewizard
hasfinishedtheinstallationofthedriversoftware,theUSB-6008isreadyforuse.
7.1.1
TestingtheUSB-6008inMAX
BeforeyoustarttousetheUSB-6008inanapplication,youshouldtestthedeviceinthe
MeasurementandAutomationExplorer(MAX).
IntheMAXwindow,expandthe“DevicesandInterfaces”nodeandthen“NIDAQmx
Devices”.Right-clickontheNIUSB-6008deviceandselect“Self-Test”.
Tutorial:
25
NIUSB-6008 Hopefullytheself-testpasseswithouterrors.Then,youshouldtesttheindividualchannels
oftheUSB-6008tocheckthattheinputsignalsaredetectedcorrectlybytheUSB-6008,and
thattheoutputsignalsgeneratedbytheUSB-6009havecorrectvalues.ThisI/Ocanbe
testedinseveralways,dependingonwhichchannelsyouactuallywanttotest. Wewillperformasimpleloopbacktest:
Here,letustestanalogoutputchannel0(AO0)andtheanaloginputchannel0(AI0)toseeif
theyworkcorrectly.Wewillperformaverysimpletest,whichissufficientifwearetocheck
thatbothAO0andAI0workcorrectly.Thetestprocedure,whichisdenotedloopback,isto
connecttheAI0channeltotheAO0channel.ThenwegeneratesomelegalvoltageatAO0.If
AI0detectsthesamevoltage,weknowthatbothAO0andAI0work.(Wemaythenrepeat
thisprocedureforotherchannels.)IfforsomereasonAI0detectssomeothervoltagethan
thevaluewesetforAO0,thenthereisanerrorineithertheAI0channelorinthetheAO0
channel,andfurtherinvestigationsarenecessary.
Topreparefortheloopbacktest,wewiretogetherAI0andAO0.Toseetheterminalsofthe
USB-6000,select“DevicePinouts”fromtheright-clickmenu.
Tutorial:
26
NIUSB-6008 Toactuallyperformtheloopbacktest,right-clickontheNIUSB-6008deviceinMAX,and
thenselect“TestPanels..”inordertoopentheTestPanels.IntheTestPanelswindow,
selecttheAnalogOutputtab.
Tutorial:
27
NIUSB-6008 IntheAnalogOutputtab,selectanyvoltagebetween0Vand5V. Next,clicktheAnalogInputtabintheTestPanelswindow.
Tutorial:
28
NIUSB-6008 TheAnalogInputtabshouldindicatethesame(oralmostthesame)voltageasissetouton
AO0.Theremaybeasmalldifferencebetweenthevaluesduetothelimitedresolutionin
theDA-converter(digital-to-analog)andintheAD-converter(analog-to-digital).
Tutorial:
29
NIUSB-6008 Youshouldalsoalwaysuseamulti-metertocheckifthevoltagelevelsontheoutputand
inputchannelsarecorrectaccordingtoyoursettings. Tutorial:
30
NIUSB-6008 7.2 UsingNIUSB-6008inLabVIEW
InordertousetheNIUSB-6008inLabVIEWyouneedtousetheDAQmxfunctions,see
Figurebelow.
DAQmx–DataAcquisitionpalette:
Tutorial:
31
NIUSB-6008 7.3 DAQAssistant
TheeasiestwaysistousetheDAQAssistant.
7.3.1
AnalogInput
WhenyoudragtheDAQAssistanticononyourBlockDiagram,thefollowingwindow
appears:
Tutorial:
32
NIUSB-6008 Inthiswindowyouneedtoselecteither“AcquireSignals”(i.e.,InputSignals)or“Generate
Signals”(i.e.,OutputSignals).
SelectAcquireSignals→AnalogInput→Voltage.
Tutorial:
33
NIUSB-6008 InthenextwindowyouselectwhichAnalogInputyouwanttouse.Selectai0(AnalogInput
channel0)andclickFinish.
Thefollowingwindowappears:
Tutorial:
34
NIUSB-6008 IntheTimingSettingsSelect“1Sample(OnDemand)”.
Tutorial:
35
NIUSB-6008 ThenextstepistoselecttheSignalInputRange.Acommonsignalis0-5V.
Youmayalsorenamethenameofthechannel(right-clickonthename):
Tutorial:
36
NIUSB-6008 Youarenowfinishedwiththeconfiguration.ClickOKintheDAQAssistantwindowTheDAQ
AssistanticonappearsontheBlockDiagram:
Tutorial:
37
NIUSB-6008 Example:
Wirethedataoutputtoanumericindicatorlikethis(andhittheRunbutton): Thennumericindicatorwillshow,e.g.,thefollowingvalue: [EndofExample]
Example:
Ifyouwanta“continuous”acquisition,putaWhilelooparoundtheDAQAssistantlikethis:
WecanalsocommunicatewiththeDAQdevicewithoutusingtheDAQAssistant:
Tutorial:
38
NIUSB-6008 Formoreadvancedapplicationsthisapproachisrecommended.
[EndofExample]
7.3.2
AnalogOutput
AnalogOutputissimilar.
Tutorial:
39
NIUSB-6008 Tutorial:
40
NIUSB-6008 Example:
Orinsidealoopfor“continuous”writingtotheDAQdevice:
Tutorial:
41
NIUSB-6008 HoweveryoushouldnotusetheDAQAssistantinsidealoopbecauseofthelackof
performance.Thefollowingisthereforebetter:
InthisexamplewehaveusedsomeoftheotherVisintheDAQmxpaletteraswell.
TheFrontPanelmayloooklikethis.
Tutorial:
42
NIUSB-6008 Inthisexampleweassumethatweconnectwiresfor“AnalogOut”and“AnalogIn”together
likethis(aso-calledloopbacktest).
WecanalsocommunicatewiththeDAQdevicewithoutusingtheDAQAssistant:
Formoreadvancedapplicationsthisapproachisrecommended.
[EndofExample]
Tutorial:
8 LoggingtoFile
InmanycasesyouwanttowriteyourdatayougetfromtheDAQdevicetoatextfilefor
lateruse.InthischapterwewilllearnhowtowritetoameasurementfileinLabVIEW.We
willalsolearnhowtoreadthesamefile.
Youcanusethe“WritetoMeasurementFile”functionontheFileI/OpaletteinLabVIEWfor
writingdatatotextfilesYoucansaveyourdatainatabseparatedtext(LVMdatafile
format)orasabinary(TDMSfileformat)file.
IfweusetheLVM,itiseasytoopenandviewthedatainNotepad.
8.1 WritingtoMeasurementFile
Wewillusethe“WritetoMeasurementFile”functionintheFileI/OpaletteinLabVIEWfor
writingdatatotextfiles.WewillalsofocusontheLVMdatafileformat,nottheTDMSfile
formatwhichgivebinaryfiles.
43
44
LoggingtoFile Whenyoudraginthe“WritetoMeasurementFile”,aconfigurationdialogwindowwill
automaticallypopup.
Recommendedsettingsforthe“WriteToMeasurementFile”isasfollows:
Example:
ExampleofLabVIEWProgramthatwritedatatoaMeasurementFile:
FrontPanel:
Tutorial:
45
LoggingtoFile Inthisapplicationwelogdatafromaprocessbasedonamanualcontrolsignal.Boththe
inputsignaluandtheoutputsignal(temperature)issavedtoaMeasurementFile.
BlockDiagram:
TheLVMfilemaylooksomethinglikethis:
Tutorial:
46
LoggingtoFile Thefirstcolumnisthetime,thesecondcolumnistheinputsignal,andthethirdcolumnis
theoutputsignal.
[EndofExample]
8.2 ReadfromMeasurementFile
Whenyoudraginthe“ReadfromMeasurementFile”,aconfigurationdialogwindowwill
automaticallypopup.
Recommendedsettingsforthe“ReadFromMeasurementFile”:
Tutorial:
47
LoggingtoFile Example:
ExampleofLabVIEWProgramthatreaddatafromaMeasurementFile:
Thisapplicationreadsthedataandplotsitin2differentgraphs.
Tutorial:
48
LoggingtoFile BlockDiagram:
[EndofExample]
Tutorial:
Hans-PetterHalvorsen,M.Sc.
E-mail:[email protected]
Blog:http://home.hit.no/~hansha/
UniversityCollegeofSoutheastNorway
www.usn.no

Tutorial: Data Acquisition in LabVIEW

Transcription

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