数据采集系统文献翻译.docx
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数据采集系统文献翻译
SelectingtheRightDataAcquisitionSystem
Engineersoftenmustmonitorahandfulofsignalsoverextendedperiodsoftime,andthengraphandanalyzetheresultingdata.Theneedtomonitor,recordandanalyzedataarisesinawiderangeofapplications,includingthedesign-verificationstageofproductdevelopment,environmentalchambermonitoring,componentinspection,benchtoptestingandprocesstrouble-shooting.
Thisapplicationnotedescribesthevariousmethodsanddevicesyoucanusetoacquire,recordandanalyzedata,fromthesimplepen-and-papermethodtousingtoday'ssophisticateddataacquisitionsystems.Itdiscussestheadvantagesanddisadvantagesofeachmethodandprovidesalistofquestionsthatwillguideyouinselectingtheapproachthatbestsuitsyourneeds.
Introduction
Ingeotechnicalengineering,wesometimeencountersomedifficultiessuchasmonitoringinstrumentsdistributedinalargearea,dangerousenvironmentofworkingsitethatcausesomedifficultyforeasyaccess.Inthiscase,operatorsmayadoptremotecontrol,bywhichalargeamountofmeasureddatawillbetransmittedtoaobservationroomwherethedataaretobecollected,storedandprocessed.
Theautomaticdataacquisitioncontrolsystemisabletocompletethetasksasregularautomaticdatamonitoring,acquisitionandstore,featuringhighautomation,largedatastorecapacityandreliableperformance.
Thesystemiscomposedofacquisitioncontrolsystemanddisplaysystem,withthefollowingfeatures:
1.No.ofChannels:
32(canbeincreasedordecreasedaccordingtouser'srealneeds.)
2.Scanningduration:
decidedbyuser,fastest32points/second
3.Storecapacity:
20G(maybeincreasedordecreased)
4.Display:
(a)Tableofparameter(b)Historytendency(c)Columngraphics.
5.Function:
realtimemonitoringcontrol,warning
6.Overalldimension:
50cm×50cm×72cm
Dataacquisitionsystems,asthenameimplies,areproductsand/orprocessesusedtocollectinformationtodocumentoranalyzesomephenomenon.Inthesimplestform,atechnicianloggingthetemperatureofanovenonapieceofpaperisperformingdataacquisition.Astechnologyhasprogressed,thistypeofprocesshasbeensimplifiedandmademoreaccurate,versatile,andreliablethroughelectronicequipment.Equipmentrangesfromsimplerecorderstosophisticatedcomputersystems.Dataacquisitionproductsserveasafocalpointinasystem,tyingtogetherawidevarietyofproducts,suchassensorsthatindicatetemperature,flow,level,orpressure.Somecommondataacquistiontermsareshownbelow:
Dataacquisitiontechnologyhastakengiantleapsforwardoverthelast30to40years.Forexample,40yearsago,inatypicalcollegelab,apparatusfortrackingthetemperatureriseinacrucibleofsodiumtungsten-bronzeconsistedofathermocouple,abridge,alookuptable,apadofpaperandapencil.
Today'scollegestudentsaremuchmorelikelytouseanautomatedprocessandanalyzethedataonaPCToday,numerousoptionsareavailableforgatheringdata.Theoptimalchoicedependsonseveralfactors,includingthecomplexityofthetask,thespeedandaccuracyyourequire,andthedocumentationyouwant.Dataacquisitionsystemsrangefromthesimpletothecomplex,witharangeofperformanceandfunctionality.
Pencilandpaper
Theoldpencilandpaperapproachisstillviableforsomesituations,anditisinexpensive,readilyavailable,quickandeasytogetstarted.Allyouneedtodoishookupadigitalmultimeter(DMM)andbeginrecordingdatabyhand.
Unfortunately,thismethodiserror-prone,tendstobeslowandrequiresextensivemanualanalysis.Inaddition,itworksonlyforasinglechannelofdata;whileyoucanusemultipleDMMs,thesystemwillquicklybecomesbulkyandawkward.Accuracyisdependentonthetranscriber'sleveloffastidiousnessandyoumayneedtoscaleinputmanually.Forexample,iftheDMMisnotsetuptohandletemperaturesensors,manualscalingwillberequired.Takingtheselimitationsintoaccount,thisisoftenanacceptablemethodwhenyouneedtoperformaquickexperiment.
Stripchartrecorder
Modernversionsofthevenerablestripchartrecorderallowyoutocapturedatafromseveralinputs.Theyprovideapermanentpaperrecordofthedata,andbecausethisdataisingraphicalformat,theyallowyoutoeasilyspottrends.Oncesetup,mostrecordershavesufficientinternalintelligencetorununattended—withouttheaidofeitheranoperatororacomputer.Drawbacksincludealackofflexibilityandrelativelylowaccuracy,whichisoftenconstrainedtoafewpercentagepoints.Youcantypicallyperceiveonlysmallchangesinthepenplots.Whilerecordersperformwellwhenmonitoringafewchannelsoveralongperiodoftime,theirvaluecanbelimited.Forexample,theyareunabletoturnanotherdeviceonoroff.Otherconcernsincludepenandpapermaintenance,papersupplyanddatastorage,allofwhichtranslateintopaperoveruseandwaste.Still,recordersarefairlyeasytosetupandoperate,andofferapermanentrecordofthedataforquickandsimpleanalysis.
Scanningdigitalmultimeter
SomebenchtopDMMsofferanoptionalscanningcapability.Aslotintherearoftheinstrumentacceptsascannercardthatcanmultiplexbetweenmultipleinputs,with8to10channelsofmuxbeingfairlycommon.DMMaccuracyandthefunctionalityinherentintheinstrument'sfrontpanelareretained.Flexibilityislimitedinthatitisnotpossibletoexpandbeyondthenumberofchannelsavailableintheexpansionslot.AnexternalPCusuallyhandlesdataacquisitionandanalysis.
PCplug-incards
PCplug-incardsaresingle-boardmeasurementsystemsthattakeadvantageoftheISAorPCI-busexpansionslotsinaPC.Theyoftenhavereadingratesashighas100,000readingspersecond.Countsof8to16channelsarecommon,andacquireddataisstoreddirectlyintothecomputer,whereitcanthenbeanalyzed.Becausethecardisessentiallypartofthecomputer,itiseasytosetuptests.PCcardsalsoarerelativelyinexpensive,inpart,becausetheyrelyonthehostPCtoprovidepower,themechanicalenclosureandtheuserinterface.
Dataacquisitionoptions
Inthedownside,PCplug-incardsoftenhaveonly12bitsofresolution,soyoucan'tperceivesmallvariationswiththeinputsignal.Furthermore,theelectricalenvironmentinsideaPCtendstobenoisy,withhigh-speedclocksandbusnoiseradiatedthroughout.Often,thiselectricalinterferencelimitstheaccuracyofthePCplug-incardtothatofahandheldDMM.Thesecardsalsomeasureafairlylimitedrangeofdcvoltage.Tomeasureotherinputsignals,suchasacvoltage,temperatureorresistance,youmayneedsomesortofexternalsignalconditioning.Additionalconcernsincludeproblematiccalibrationandoverallsystemcost,especiallyifyouneedtopurchaseadditionalsignalconditioningaccessoriesoraPCtoaccommodatethecards.Takingthatintoconsideration,PCplug-incardsofferanattractiveapproachtodataacquisitionifyourrequirementsfallwithinthecapabilitiesandlimitationsofthecard.
Dataloggers
Dataloggersaretypicallystand-aloneinstrumentsthat,oncetheyaresetup,canmeasure,recordanddisplaydatawithoutoperatororcomputerintervention.Theycanhandlemultipleinputs,insomeinstancesupto120channels.AccuracyrivalsthatfoundinstandalonebenchDMMs,withperformanceinthe22-bit,0.004-percentaccuracyrange.Somedataloggershavetheabilitytoscalemeasurements,checkresultsagainstuser-definedlimits,andoutputsignalsforcontrol.
Oneadvantageofusingdataloggersistheirbuilt-insignalconditioning.Mostareabletodirectlymeasureanumberofdifferentinputswithouttheneedforadditionalsignalconditioningaccessories.Onechannelcouldbemonitoringathermocouple,anotheraresistivetemperaturedevice(RTD)andstillanothercouldbelookingatvoltage.
Thermocouplereferencecompensationforaccuratetemperaturemeasurementistypicallybuiltintothemultiplexercards.Adatalogger'sbuilt-inintelligencehelpsyousetupthetestroutineandspecifytheparametersofeachchannel.Onceyouhavecompletedthesetup,dataloggerscanrunasstandalonedevices,muchlikearecorder.Theystoredatalocallyininternalmemory,whichcanaccommodate50,000readingsormore.
PCconnectivitymakesiteasytotransferdatatoyourcomputerforin-depthanalysis. Mostdataloggersaredesignedforflexibilityandsimpleconfigurationandoperation,andmanyprovidetheoptionofremotesiteoperationviabatterypacksorothermethods.DependingontheA/Dconvertertechniqueused,certaindataloggerstakereadingsatarelativelyslowrate,especiallycomparedtomanyPCplug-incards.Still,readingspeedsof250readings/secondarenotuncommon.Keepinmindthatmanyofthephenomenabeingmonitoredarephysicalinnature—suchastemperature,pressureandflow—andchangeatafairlyslowrate.Additionally,becauseofadatalogger'ssuperiormeasurementaccuracy,multiplereadingsandaveragingarenotnecessary,astheyoftenareinPCplug-insolutions.
Dataacquisitionfrontends
DataacquisitionfrontendsareoftenmodularandaretypicallyconnectedtoaPCorcontroller.Theyareusedinautomatedtestapplicationsforgatheringdataandforcontrollingandroutingsignalsinotherpartsofthetestsetup.Frontendperformancecanbeveryhigh,withspeedandaccuracyrivalingthebeststandaloneinstruments.Dataacquisitionfrontendsareimplementedinanumberofformats,includingVXIversions,suchastheAgilentE1419AmultifunctionmeasurementandcontrolVXImodule,andproprietarycardcages..Althoughfront-endcosthasbeendecreasing,theses