Control With Wireless Hart

8/3/2019 Control With Wireless Hart

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ControlwithWirelessHART


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Copyright2009 HARTCommunicationFoundationThisdocumentcontainscopyrightedmaterialandmaynotbereproducedinanyfashionwithoutthewritten

permissionoftheHARTCommunicationFoundation.

TrademarkInformationHARTisaregisteredtrademarkoftheHARTCommunicationFoundation,Austin,Texas,USA.

AnyuseofthetermHARThereafterinthisdocument,orinanydocumentreferencedbythisdocument,implies

theregisteredtrademark.WirelessHARTisatrademarkoftheHARTCommunicationFoundation.Allother

trademarks

used

in

this

or

referenced

documents

are

trademarks

of

their

respective

companies.

For

more

informationcontacttheHCFStaffattheaddressbelow.

Attention:FoundationDirector

HARTCommunicationFoundation

9390ResearchBoulevard

SuiteI350

Austin,TX78759,USA

Voice:(512)7940369

FAX:(512)7943904

http://www.hartcomm.org


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Synopsys:TheWirelessHARTstandardisthefirstopenwirelesscommunicationstandardfor

measurementandcontrolintheprocessindustries.Ituseswirelessmesh

networkingbetweenfielddevices,aswellasotherinnovations,toprovidesecure,

reliabledigitalcommunicationsthatcanmeetthestringentrequirementsof

industrialapplications.

Thisisoneofaseriesofpapershelpingusersrecognizethebenefitsof

WirelessHART,aswellasaddressingspecificquestionsaboutWirelessHART.


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Fornearlytwodecades,HARTCommunicationhasbeentheprocessindustrystandardforsecure,

simple,andreliableoperations.NewcapabilitiesandwirelesscommunicationintroducedwithHART

version7,complimentestablishedpracticeandexpandtheuseofHARTCommunicationintonewareas

andapplications.

WirelessHARTwasdesignedspecificallytosupportthewiderangeofprocessindustryusecasesfrom

simplemonitoringtoclosedloopcontrol.Testingandfieldtrialswithwirelessdeviceshave

demonstratedthatthecommunicationaccuracy,stability,totalperformance,andreliabilitycanmeet

thedemandsofindustrialprocessmonitoringandcontrolapplications.

Controlapplicationrequirementsforsamplingintervals,jitter,andlatencywerespecificallyaddressed

anddesignedintotheWirelessHARTtechnology.Infact,controlperformancewithWirelessHARTcanbe

comparabletothatofawiredsystemusingtraditionalfieldbuses.Let'slookatsomeofthefactorsthat

cancomeupinconsideringWirelessHARTforcontrolapplications.Let'slookatsomeofthefactorsthatcancomeupinconsideringWirelessHARTforcontrolapplications.

SamplingintervalsWirelessHARTallowssamplingintervalsthatmeettherequirementsofmostcontrolloopswhileatthe

sametimeminimizingtheimpactonfielddevicesthatmaybepoweredbyabattery.

Thetypicalruleofthumbisthatfeedbackcontrolshouldbeexecuted410timesfasterthantheprocess

responsetime,whereresponsetimeequalstheprocesstimeconstantplusdeadtime.

Becausemeasurementsystemsareoftenunsynchronizedwiththecontrolsystem,measurementvalues

aretypicallysampledasmuchas210timesfasterthantheprocesscanrespond(Figure1below).


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Withwirelesssystems,however,it'sdesirabletoreducethefrequencythatmeasurementsaretaken

andcommunicatedinordertoextendmeasurementdevicebatterylife.

TheabilitytoschedulecommunicationswithWirelessHARTmakesthiseasytodowithoutcompromising

controlreliability.Figure2addsthesetwomethodstothepreviousdiagram:

Synchronized.Measurementsaretakenandtransmittedonly(andexactly)whenthey'reneededforcontrolexecution.

Synchronizedwithexceptionreporting.Measurementsaretakenatscheduledintervalsforexample,410timesfasterthantheprocessresponsetimebuttransmittedonlyifthe

measurementhaschangedbyaspecifiedamountorifthetimesincethelastcommunication

exceedsaspecifiedinterval.

Figure2

Morefrequentcommunicationofmeasuredvaluesiscertainlypossibleandinthecaseoflinepowered

devicescanworksimilartowirednetworks.Inthecaseofbatterypoweredordeviceswhereenergy

conservationisimportant,WirelessHARToffersuserstheopportunitytofindanapplication'soptimum

balancebetweencommunicationintervalsandbatterylife.

LatencyandJitterEffectivecontrolrequirestimelyaccesstomeasurementandcontrolinformation.Asystem'sabilityto

meetitscontrolperformancerequirementscanbeaffectedbothbydelays(latency)andvariation(jitter)

inwhentheinformationisavailable.

Insomesystems,latencyandjittercanstartwiththetimingofthemeasurementsthemselves.But

WirelessHARTisatimesynchronizedprotocol,witheverydevicehavingacommonsenseoftime

accurateto1millisecondacrosstheentirenetworkacapabilitynotavailableinmanyotherprotocols.

Themeasurementsoftwareandcircuitryusethissenseoftimeinschedulingmeasurements,allbut

eliminatingdelaysandvariationinmeasurementtiming.


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Latencyandjittercanalsobeintroducedwhendataiscommunicatedforexample,fromatransmitter

toagateway.Inthiscase,latencyisthetimeittakesforacommunicationpackettomakeitswayfrom

thesourcetothedestination,whilejitterisvariationinlatencybetweendifferentcommunicationcycles

(Figure3).Excessivelatency(whicheffectivelyaddsdeadtimetotheprocess)andjitter(whichadds

errorinto

the

control

calculations)

can

lead

to

significant

degradation

in

control

performance.

Figure3

In

direct

communications,

WirelessHART

has

a

transmission

rate

that

is

faster

than

some

traditional

wiredfieldbustechnology.Forexample,ifthecommunicationrateis31.25kilobits/second,the

communicationsdelaywillbe32microseconds/bit.WirelessHARThasamuchfastercommunications

rate250kilobits/second sothedelayintroducedbythecommunicationsrateisonly4

microseconds/bit.

SinceatypicalWirelessHARTmessageis128bytes,thetimeforcompletemessagetransmissionis4

milliseconds.Eachtransmissionanditscorrespondingacknowledgementoccurwithina10millisecond

"timeslot"inaperiodiccommunicationSuperframeormacrocycle(Figure4).

Figure4.


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However,inmanyscenarioscommunicationsrequiremorethanonetimeslotforamessagetotravel

fromthesourcetothedestination.Let'slookatonesuchscenario.

Ifacommunicationcan'treachitsdestinationdirectly,itcan"hop"fromnodetonodetobridgethegap

orcircumventobstructions.Thisabilitytoroutearoundphysicalobstaclesorinterferenceisacore

featureof

the

WirelessHART

mesh

technology.

Figure

5shows

three

paths

acommunication

might

followfromthedeviceonthelefttothegatewayontheright.

Figure5

Changingtheroutethedatatravelscancontributetovariationincommunicationtime(jitter).Although

eachadditionalhopincreaseslatency,intypicalapplicationstheaveragedelayiswellwithinthe

requirementsforcontrol.Wecanillustratethiswithanexample.

Inmostcases,aWirelessHARTnetworkwillbeabletoretryafailedmessageinthenexttimeslotorthe

onefollowing.Forourexamplewellassumeittakes10millisecondstoprocessamessageandassignit

toanothertimeslot.PathAinFigure5couldthereforeproduceasmuchas50millisecondsoftotal

latency(10ms+[10ms+10ms]+[10ms+10ms]).PathBhasthesamenumberofhopsandthusthe

samecommunications

latency.

But

Path

Chas

an

additional

hop,

bringing

total

communications

latency

to70milliseconds.Thistimingdifferenceintroducesa20millisecondjitterinthecommunications.(In

manycasestheroutingdevicewillbeabletoretryinthenextslot,whichwouldreducethetotal

latenciesto30millisecondsforPathsAandBandto40millisecondsforPathC.)

Experienceinhundredsofwirelessfielddeviceinstallationsshowsthatcommunicationslatencyon

averageismuchlowerthaninthisexample.Inrealplantsettings,typically30%ofthedevices

communicatedirectlywiththegatewayornetworkaccesspoint(10milliseconds)andabout50%are

onehopaway(30milliseconds).Theremaining20%maybe34hops.Usingthesenumbersfromactual

plantinstallations,theaveragelatencytimewillbeabout30milliseconds.


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Existinginstallationsalsoshowthatnetworkreliabilityistypicallygreaterthan99%,sothelatencytime

willnotvarysignificantlybetweencommunicationseffectivelyeliminatingtheeffectofjitter.

Butisitfastenoughforcontrol?Communications

latency

does

not

affect

control

as

long

as

the

delay

is

small

compared

to

the

process

responsetime.Appropriateschedulingoftransmissionsacrossthetimeslotsinamacrocyclecanensure

thedatareachesitsdestinationwhenneeded.

Forgoodcontrolweneedtobeabletoreadthecontrolmeasurement,communicatethemeasurement

toacontroller,executethecontrolfunction,andcommunicatetheoutputbacktothetargetinonehalf

theprocesstimeconstant.Mostcontrolloopsare1secondormore,sofora1secondcontrolloopwe

wouldneedtobeabletoperformallofthesestepswithin500milliseconds.

Let'sseehowthisworkswiththeexamplecontrolloopshowninFigure6.Inthisexample,the

measurementisprocessedinthefielddevice,thecontrolalgorithmrunsinthegateway,andthe

actuationoccurs

in

avalve

that's

the

same

"hop

depth"

from

the

gateway

as

the

measurement

device.

ThetotalspanofthecycleincludestheAI,PID,AO,andcommunicationtimes.

Figure6

Usingthenumbersfromourearlierexample,eachcommunicationfromthemeasurementdeviceto

thegateway,andfromthegatewaytothevalve wouldtake70milliseconds.Ifwefurtherassumethat

thecontrolexecutiontimeinsidethegatewayisverysmall,thenwecanassumethatthecontrolloop

willexecute

in

140

150

milliseconds

well

below

the

required

500

milliseconds.

Atypicalnetworkscheduletosupportthisscenarioisshownbelow.Theindividualcommunications

shownearlierinFigure5aredistributedbothacrossthe50timeslotsineachmacrocycleandacrossthe

15radiofrequencychannelsusedbyWirelessHART.


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Figure7

Asthisexampleshows,500millisecondmacrocyclesareeasilyachievableevenwhenmultiplehopsare

assumedinthecommunication.Thisisfastenoughfortypicalcontrolloops,whichinmostcasesare

muchslower

than

our

example.

(Thediagramalsoshowsthatthereisalmostnoimpacttothebandwidthofthesystem.Infact,less

than12%ofavailableslotsareneededtodo10highspeedcontrolloopsinparallel.)

Inthisexampleweillustratedwhatwouldhappenifthecommunicationstook70milliseconds.As

mentionedearlier,however,actualplantexperienceshowsthataveragelatencytimesareabout30

milliseconds.Using30millisecondsinourcalculationsreducestheloopexecutiontimetolessthan100

millisecondsandreducesthenumberofcommunicationsinthenetwork.Ifitisimportanttoreducethe

delaysintroducedbymultiplehops,additionalnetworkaccesspointscanbeused.

Itis

also

possible

to

further

reduce

communication

latency

and

address

higher

speed

control

applicationsbyusingpeertopeercommunicationsbetweenfielddevices.Runningthecontrol

algorithminafielddeviceeliminatestheneedforwirelesshopsbetweenthatdeviceandagateway

residentalgorithm.Suchanarrangementmayalsouselessbandwidth,allowingformultiplecontrol

loopswithminimalimpacttooverallbandwidth.Ofcourse,usingthisstrategyisdependentonwhether

thereareadditionalinteractionsbetweenthecontrolloops.

Thisexampleusedwirelessnetworklayoutsthatweremorecomplexthanwhatexperiencehasfoundin

actualplantenvironments.Wecouldhavealsoincorporatedmultipleaccesspointstoshorten

communicationpaths,andallocatedadditionalcommunicationresourcestofurtherenhancethe

effectivenessoftheWirelessHARTnetwork.AndbecauseallWirelessHARTmeasurementsincludea

timestamp,wecouldhaveusedthetimestampinthecontrolalgorithmtofurtherreducetheimpactof

anylatency

and

jitter.

ConclusionEvenwithoututilizinganyoftheseadditionalfeatures,theexampleshowsthattheoverallcontrol

performanceofatypicalWirelessHARTnetworkiscomparabletothatoftraditionalwiredfieldbuses.

TheWirelessHARTprotocolallowsforsecure,highlyreliable,lowlatencycontrolwithalmostnoimpact

onthebandwidthandabsolutelynoimpactonprocessperformance.

WirelessHARTissimple,reliable,andsecure.

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Control With Wireless Hart