Technical Introduction and Overview of EtherCAT.docx

Technical Introduction and Overview of EtherCAT.docx

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TechnicalIntroductionandOverviewofEtherCAT

TechnicalIntroductionandOverview

EtherCAT—EthernetControlAutomationTechnology

Thissectionprovidesanin-depthintroductionintoEtherCAT,theEthernet-basedfieldbussystem.

EtherCATsetsnewperformancestandards.Handlingisstraightforwardandsimilartoafieldbus,thankstoflexibletopologyandsimpleconfiguration.Moreover,sinceEtherCATcanbeimplementedverycost-effectively,thesystemenablesfieldbussestobeusedinapplicationswherefieldbusnetworkingwasnotanoptioninthepast.

1. 

Introduction

1.1 

Ethernetandreal-timecapability

2.

EtherCAToperatingprinciple

3.

EtherCATfeatures

3.1 

Protocol

3.2 

Topology

3.3. 

Distributedclocks

3.4 

Performance

3.5 

Diagnosis

3.6 

Highavailability

3.7 

Safety-over-EtherCAT

3.8 

EtherCATinsteadofPCI

3.9

Deviceprofiles

3.9.1 

CANopenoverEtherCAT（CoE）

3.9.2 

Servodrive-ProfilaccordingtoIEC61491overEtherCAT（SoE）

3.10 

EthernetoverEtherCAT（EoE）

3.11 

FileAccessoverEtherCAT（FoE）

4. 

Infrastructurecosts

5. 

Summary

6. 

Literature

7. 

ImplementationAspects

7.1 

Master

7.1.1 

Masterimplementationservices

7.1.1 

MasterSampleCode

7.2 

Slave

7.3. 

SlaveEvaluationKit

1.Introduction

Fieldbusseshavebecomeanintegratedcomponentofautomationtechnology.Theyhavebeentriedandtestedandarenowwidelyestablished.Itwasfieldbustechnologythatenabledthewide-scaleapplicationofPC-basedcontrolsystems.WhiletheperformanceofcontrollerCPUs-particularlyforIPCs-isincreasingrapidly,conventionalfieldbussystemstendtorepresent"bottlenecks"thatlimittheperformancecontrolsystemscanachieve.Anadditionalfactoristhelayeredcontrolarchitectureconsistingofseveralsubordinate（usuallycyclic）systems:

theactualcontroltask,thefieldbussystemandperhapslocalexpansionbusseswithintheI/Osystemorsimplythelocalfirmwarecycleintheperipheraldevice.Reactiontimesaretypically3-5timeshigherthanthecontrollercycletime-anunsatisfactorysolution（seeFig.1）.

Figure1:

 Responsetimesofconventionalfieldbussystems

Abovethefieldbussystemlevel,i.e.fornetworkingcontrollers,Ethernethasalreadybeenthestateoftheartforsometime.WhatisrelativelynewisitsapplicationatthedriveorI/Olevel,i.e.inareasthatweredominatedbyfieldbussystemsinthepast.Themainrequirementsforthistypeofapplicationarehighreal-timecapability,suitabilityforsmalldataquantities,andnaturallycost-effectiveness.EtherCATmeetstheserequirementsandatthesametimemakesinternettechnologiesavailableattheI/Olevel.

1.1Ethernetandreal-timecapability

Therearemanydifferentapproachesthattryandprovidereal-timecapabilityforEthernet:

forexample,theCSMA/CDmediaaccessprocedureisdisabledviahigherlevelprotocollayersandreplacedbythetimesliceprocedureorpolling;otherpropositionsusespecialswitchesthatdistributeEthernetpacketsinapreciselycontrolledtimelymanner.WhilstthesesolutionsmaybeabletotransportdatapacketsmoreorlessquicklyandaccuratelytotheconnectedEthernetnodes,thetimesrequiredfortheredirectiontotheoutputsordrivecontrollersandforreadingtheinputdatastronglydependontheimplementation.

IfindividualEthernetframesareusedforeachdevice,theusabledatarateisverylowinprinciple:

TheshortestEthernetframeis84byteslong（incl.inter-packetgapIPG）.If,forexample,adrivecyclicallysends4bytesofactualvalueandstatusinformationandaccordinglyreceives4bytesofcommandvalueandcontrolwordinformation,at100%busload（i.e.withinfinitelyshortresponsetimeofthedrive）ausabledatarateofonly4/84=4.8%isachieved.Atanaverageresponsetimeof10µs,theratedropsto1.9%.Theselimitationsapplytoallreal-timeEthernetapproachesthatsendanEthernetframetoeachdevice（orexpectaframefromeachdevice）,irrespectiveoftheprotocolsusedwithintheEthernetframe.

2.EtherCAToperatingprinciple

EtherCATtechnologyovercomestheseinherentlimitationsofotherEthernetsolutions:

theEthernetpacketisnolongerreceived,theninterpretedandprocessdatathencopiedateverydevice.TheEtherCATslavedevicesreadthedataaddressedtothemwhiletheframepassesthroughthenode.Similarly,inputdataisinsertedwhilethetelegrampassesthrough（seeFig.2）.Theframesareonlydelayedbyafewnanoseconds.

Figure2:

 Processdataisinsertedintelegrams

SinceanEthernetframecomprisesthedataofmanydevicesbothinsendandreceivedirection,theusabledatarateincreasestoover90%.Thefull-duplexfeaturesof100BaseTXarefullyutilized,sothateffectivedataratesof>100Mb/s（>90%of2x100Mb/s）canbeachieved（seeFig.3）.

Figure3:

 ComparisonofBandwidthUtilisation

TheEthernetprotocolaccordingtoIEEE802.3remainsintactrightuptotheindividualdevice;nosub-busisrequired.Inordertomeettherequirementsofamodulardevicelikeanelectronicterminalblock,thephysicallayerinthecouplingdevicecanbeconvertedfromtwistedpairoropticalfibretoLVDS（alternativeEthernetphysicallayer,standardizedin[4,5]）.Amodulardevicecanthusbeextendedverycost-efficiently.SubsequentconversionfromthebackplanephysicallayerLVDStothe100BASE-TXphysicallayerispossibleatanytime-asusualwithEthernet.

3.EtherCATfeatures

3.1Protocol

TheEtherCATprotocolisoptimizedforprocessdataandistransporteddirectlywithintheEthernetframethankstoaspecialEthertype.ItmayconsistofseveralEtherCATtelegrams,eachservingaparticularmemoryareaofthelogicalprocessimagesthatcanbeupto4gigabytesinsize.ThedatasequenceisindependentofthephysicalorderoftheEthernetterminalsinthenetwork;addressingcanbeinanyorder.Broadcast,Multicastandcommunicationbetweenslavesarepossible.DirectEthernetframetransferisusedincaseswheremaximumperformanceisrequiredandtheEtherCATcomponentsareoperatedinthesamesubnetasthecontroller.

However,EtherCATapplicationsarenotlimitedtosingleasubnet:

EtherCATUDPpackagestheEtherCATprotocolintoUDP/IPdatagrams（seeFig.4）.ThisenablesanycontrolwithEthernetprotocolstacktoaddressEtherCATsystems.Evencommunicationacrossroutersintoothersubnetsispossible.Inthisvariant,systemperformanceobviouslydependsonthereal-timecharacteristicsofthecontrolanditsEthernetprotocolimplementation.TheresponsetimesoftheEtherCATnetworkitselfarehardlyrestrictedatall:

theUDPdatagramonlyhastobeunpackedinthefirststation.

Figure4:

 EtherCAT:

StandardFramesaccordingtoIEEE802.3[3]

Inadditiontodataexchangeaccordingtothemaster/slaveprinciple,EtherCATisalsoverysuitableforcommunicationbetweencontrollers（master/master）.Freelyaddressablenetworkvariablesforprocessdataandavarietyofservicesforparameterization,diagnosis,programmingandremotecontrolcoverawiderangeofrequirements.Thedatainterfacesformaster/slaveandmaster/mastercommunicationareidentical.

Forslavetoslavecommunication,twomechanismsareavailable.Upstreamdevicescancommunicatetodownstreamdeviceswithinthesamecycle,andthusextremelyfast.Sincethismethodistopologydependent,itisparticularlysuitableforslavetoslavecommunicationrelationshipsgivenbymachinedesign-e.g.inprintingorpackagingapplications.Forfreelyconfigurableslavetoslavecommunication,thesecondmechanismapplies:

thedataisrelayedbythemaster.Heretwocyclesareneeded,butduetotheextraordinaryperformanceofEtherCATthisisstillfasterthananyotherapproach.

EtherCATonlyusesstandardframesaccordingto[3]-theframesarenotshortened.EtherCATframescanthusbesentfromanyEthernetMAC,andstandardtools（e.g.monitor）canbeused.

3.2Topology

Line,treeorstar:

EtherCATsupportsalmostanytopology（seeFig.5）.ThebusorlinestructureknownfromthefieldbussesthusalsobecomesavailableforEthernet,withoutthequantitylimitationsimpliedbycascadedswitchesorhubs.

Figure5:

 FlexibleTopology:

Line,TreeorStar

Particularlyusefulforsystemwiringisthecombinationoflineandbranchesorstubs:

therequiredinterfacesexistonmanydevices（e.g.onI/Omodules）;noadditionalswitchesarerequired.Naturally,theclassicswitch-basedEthernetstartopologycanalsobeused.

Wiringflexibilityisfurthermaximisedthroughthechoiceofdifferentcables.FlexibleandinexpensivestandardEthernetpatchcablestransferthesignalsin100BASE-TXmode.Plasticopticalfibres（POF）complementthesystemforspecialapplications.ThecompletechoiceofEthernetwiring-suchasdifferentopticalfibresandcoppercables-canbeusedincombinationwithswitchesormediaconverters.

TheFastEthernetphysics（100BASE-TX）enablesacablelengthof100mbetweentwodevices.Sinceupto65535devicescanbeconnected,thesizeofthenetworkisalmostunlimited.

3.3.Distributedclocks

Accuratesynchronizationisparticularlyimportantincaseswherespatiallydistributedprocessesrequiresimultaneousactions.Thismaybethecase,forexample,inapplicationswhereseveralservoaxescarryoutcoordinatedmovementssimultaneously.

Themostpowerfulapproachforsynchronizationistheaccuratealignmentofdistributedclocks,asdescribedintheIEEE1588standard[6].Incontrasttofullysynchronouscommunication,wheresynchronizationqualitysuffersimmediatelyintheeventofacommunica