OPC Technology.docx

OPC Technology.docx

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OPCTechnology

Contents

∙HistoryofOPC

∙WhydoweneedOPC?

itsBenefits

∙OPCConcepts&Terminology

∙COMInterfaces&ComponentCategories

∙OPCCompliance

∙OPCSpecifications

∙Resources

SchematicofOPCServer.

Figure1.

etc）-->

Introduction

COM/OLEapplicationsarewidelyusedinIndustrialAutomationandProcessControldomains.Sincenotmanyprogrammersareawareofthesetechnologies,IthoughttosharemyexperienceinOPCandmostdeveloperswouldlovetoexpandtheirknowledgeaboutCOM/OLEapplicationareas.

ThisismyfirsteverarticleonCodeProject,soifyoufeelanythingneedstobeimproved,dropmealine.Yoursuggestionsandcommentsaremostwelcome.

Thegoalofarticle:

Give simple&understandable overviewofOPC.MoredetailedinformationandspecificationscanbedownloadedfromOPCFoundationandwebsitesmentionedinResourcessection.

Sofirst,abouttheabovediagram... 

TheoryaboutFigure1:

Onleft-bottomisahardwaredevice.Itcouldbea PLC （programmablelogiccontroller）,atemperaturesensor,rollerbeltcontroller,orjustaboutanyindustrialdevice.CentralpartistheOPCserver.Rightsideistheclientthatwantsto operatethedevice.TheOPCServergivesoutstandardCOMinterfacessothatanyclientawareofthese OPCInterfaces caninterfacewithserverandoperatethedevice-withoutprogrammingdevicedriverforthathardware.HardwarecanbeasvariedasOPCservercanhandleandclientscanstillreadandwritefromthishardware.

Nowback toourOPCcourse.ThisarticlegivesanoverviewofOPCandcoversfollowingtopics.Sourcecodeisnotavailable,sincemostofservercodeisspecifictohardware.Andclientwouldstillneedaservertoconnectto.Butyes,ifyouwanttoexperimentwithOPC,checkouttheResourcessectionbelow.

HistoryofOPC

In1996,agroupofcompaniesRockwellSoftware,Intellution,et.al.cametogethertoformaconsortiumthatdefinedastandardforprocesscontrol.SCADAandsimilarstandardizationeffortswerebeingmadeatthattimeinprocesscontrolandautomationindustry.ThisgroupcreatedaninitialdraftthatwasbasedonMicrosoft'sObjectLinkingandEmbedding（OLE）.Itwascalled OLEfor Process Control（OPC）. OPCFoundation publishesspecificationsandmanageseverythingthatgoesintoOPCtechnology.

Todaymorethan300companiesarepartofOPCFoundation.Microsoft,GE,Siemens,Rockwell,ABB,Ford,HoneywellandmorearemembersthatactivelycontributeandconsumeOPCtechnology.

In2005,OPCFoundationreleasedOPCUnifiedArchitecturedraft.Therearepredecessordraftsreleasedbefore-OPCHDA,OPCDX,OPCAE,OPCCommands,OPCSecurity.Wewillbrieflystudytheminthisarticle.

Bynow,youmusthaveaquestioninyourmind.Letmeansweritbelow...

WhydoweneedOPC?

itsBenefits

IwasalsoafirsttimertoOPC（comingfromvariedexperienceinDevTools,eLearning,CAD/PLM,Multimedia）,andhadnoclueastowhydoweneedit.Ifoundthatlearningitthroughacasestudywasveryhelpful,soIamgoingtodothesameforyou.Tounderstandtheneedforstandardizationinprocesscontrolindustry,letmegiveyouanactualcasestudyperformedbyagiantorganizationinPetrochemicalsbusiness.

Refertofigure2.Itshowsthedevicesandsoftwareapplicationthatwereinactiononthisparticularpetrochemicalplant.

Figure2.

PLC,Vibrationsensor,andCalculationengineworkondifferentcommunicationprotocols-TSAA,Modbus,andDDE.SothesoftwareapplicationsthatneeddatafromthesedevicesalsoneedtobeawareofTSAA,Modbus,andDDEprotocol.Additionallyeachdeviceneedstoserveallthethreeapplications.Theprocesshistorian,GUIapplicationprovidinghumanmachineinterfaceandthemachineconditionmonitorallcommunicatingwithalldevicesresultsinacomplexity.Thisdeploymenttookthecompanyaround10days.AnditscostwasapproximatelyUS$50,000.

Now,someonelikeme:

）cametothemandsaid,"heyguys,whydon'twethinksmartandimplementasinFigure3?

".Observethereducedcomplexity.NowallsoftwareapplicationstalkonlytotheOPCServers.AndalldevicesalsorespondonlytotheOPCserver.OPCServerprovidesthe BlackBox forsoftwareapplicationsaroundhardware.

Figure3.

Thenewimplementationtookjust2daystodeployandcostincurredwasapproximatelyUS$10,000.Theycouldhaveeliminated2OPCserversaswellandimplementeddevicedriversrightinsideasingleOPCserver.ThisisatradeoffandofftheshelfOPCserversmayhavestandardprotocolssupportlikeDDE,ModbusandTSAA.

Asyouobservedcorrectly,thebenefitsare:

∙Reducedloadondevice.

∙Scalabilityofsystemhasincreased.

∙OPCserverprovidescachingofdata.

∙Clientapplicationsneednotknowhardwareprotocoldetails.

∙Increasedlifefordevice（itneednotservemultipleclientsanymore）.

∙Interoperability（Unix/LinuxandWindows-bothplatformsaresupportedbyOPC）

∙Standardization

NowthatyouhavesomeinsightintoOPCbenefitsandhistory,let'stakeonestepaheadtounderstandOPCterminologyandconcepts.

OPCConcepts&Terminology

OPCspecificationdefinesaconsistentterminologytoidentifyvariousentitiesinvolved.ThebestanalogytoOPCwayofdatarepresentationisthatofafilesystemhierarchy.OPCcallsitasOPCAddressSpace.Withinthisaddressspaceeveryentitycanbeuniquelyidentifiedbyits"full-path".Forexample,refertofigure4.

Figure4.

OPCAddressSpace ishierarchical.Itstartswitharoot.Thentherearelogical folders and sub-folders.Leafnodesare OPCItemsorTags.Generallyyoursensorsandactuatorsformtheleafnodes.Thisisanimportantpointtounderstandandwewillspendsometimeonit.Forexample,youhaveadevicethatcansensetemperature,Pressure,andHumidity.Thentheseattributesaregenerallymappedasleafnodesundersomefolder-subfolderhierarchy.Wewillseewhatthathierarchycanbeinamoment.Ifyouhaveanactuator（say,Turn_ON_Boiler）,thenthatalsobecomesaleafnode.Youcanwriteto Turn_ON_Boiler valuesof1or0forexample,andnecessaryactionwillbetaken-ofcoursewejustassumedthatyourhardwarecontrolmechanismbehavesthisway.Youcanreadfrom Temperature toknowsensedvalue.

Hopeweareallcleartillthispoint.Nowlet'sunderstandthis"logical"folderstuff.Thelogicalfoldersandsub-foldersareusedtorepresentstateofyourIndustrialAutomationsystem.Thisconfigurationcanactuallymaptosomephysicalconfigurationorevengeographicalsetup.Let'sspendsometimetounderstandthisoneaswell.Forexample,inthisdiagramwehaveanethernetcommunicationchanneltowhichawirelessmeterisconnected.Andwithinwirelessmeter,therearetwoclasses"Average"and"Channel1".Thesefoldersandsub-foldersareveryeffectivemeansofgroupingyourprocesscontrolparametersandvalues.

Thisaddressspaceis"published"toallclientsofOPCserver.TheOPCclientscanbesimplespreadsheetslikeExcel,processhistoriansthatkeeprecordofdatacomingfromvariousindustrialplantentities.Everyclientcancreatealogicalentitycalledas group insideOPCserver.Each OPCGroup cancontainasetofOPCItems.

Synchronous&AsynchronousOPCItemreading

OPCspecificationsupportssynchronousaswellasasynchronousreadingandwritingonhardwaredevices.Sincenotallhardwaredeviceoperationsarefast-enough,theasynchronousmechanismofreadingandwritingtoadevicecomesveryhandy.

COMInterfaces&ComponentCategories

Well,sonowsupposeyouwishtowriteyourfirstOPCclient.Ifyouweredevelopingaclientthatrunsonlocalmachineasthatofserver,andifyouwereawareofallOPCServerProgIDs,thenwewouldnothaveanyworries.Butsincethatisnotpossible,weneedanenumerationmechanism.Followingisanenumerationmechanism.

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OPCComponentCategories

EnumerationissueisaddressedbyrequiringOPCserverstoimplement

componentcategories.Forexample,allserversthatareOPCDA1.0

（moreaboutthisinlatersections）compliantimplementCOMcategory

withCatID{63D5F430-CFE4-11d1-B2C8-0060083BA1FB}.Similarlythe

serverscompliantwithOPCDataAccess2.0implementcategorywith

CatID{63D5F432-CFE4-11d1-B2C8-0060083BA1FB}.

SoOPCFoundationdistributesaWindowsServicecalledasOPCEnum.

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EnumeratingOPCServers

OPCEnum.exerunsasaWindowsServiceandprovidesenumeration

functionality.UsingOPCEnum,theclientapplicationscanquery

availableOPCServersonlocalaswellasremotenetworkedmachines.

Nowlet'slookatservercomponentinterfaces.

Figure5.

TherearevariousCOMinterfacesthatOPCServermustsupportaccordingtheOPCSpecificationcompliancerequired.Thesebasicinterfaceshavespecificfunctionality.

IOPCBrowse ThisinterfacehasmethodsthatallowbrowsingOPCAddressSpace.Clientcanquerylevelbylevelfoldersandsubfolderstilltheleafnodes.Thehierarchylookslikeshownin figure4.

IOPCItemIO InterfaceisusedforperformingOPCItemreadandwriteoperations.

Nowlet'sunderstandhowOPCClientinteractswiththeOPCServer.Importantstepsinvolvedaresummarizedinfigure6,below.

Figure6.

Iknowyoumuststillbewonderingaboutthis OPCGroup term.Let'sspendsometimeunderstandingit.EveryclientwhenopensasessionwithOPCServer,needstocreatealogicalgroupofitemsthatitwantstoreadorwrite.Clientcanmaintainmultiplegroups.Eachgroupisgivenauniquenameandhasfollowingattributes:

∙Updaterate[milliseconds]atwhichthevaluesofitemsareread.

∙ActiveorIn-activeFlag[Boolean]onlyifgroupisactivethattheitemsareprocessed.

∙Deadband[%]variationsinsidethisbandareconsideredNULL.

∙IOMode[S