Introduction to Hardware-in-the

UniversityCollegeofSoutheastNorway
Hardware-in-the-LoopSimulation
Introduction
WhatisHardware-in-the-Loop(HIL)SimulationorWhatisHardware-in-the-Loop(HIL)Test?
TheHardware-in-the-Loopprocesshasexistedfornomorethan15to20years.Itsrootsare
foundintheAviationindustry.ThereasontheuseofaHILprocessisbecomingmore
prevalentinallindustriesisdrivenbytwomajorfactors:timetomarketandcomplexity.
Hardware-in-the-loop(HIL)simulationisatechniquethatisusedinthedevelopmentand
testofcomplexprocesssystems.HILsimulationprovidesaneffectiveplatformbyaddingthe
complexityoftheplantundercontroltothetestplatform.Thecomplexityoftheplantunder
controlisincludedintestanddevelopmentbyaddingamathematicalrepresentationofall
relateddynamicsystems.Thesemathematicalrepresentationsarereferredtoasthe“plant
simulation.”
Hardware-In-the-Loopisaformofreal-timesimulation.Hardware-In-the-Loopdiffersfrom
real-timesimulationbytheadditionofarealcomponentintheloop.Thiscomponentmay
bean“ElectronicControlUnit”(ECU).
ThepurposeofaHardware-In-the-Loopsystemistoprovidealloftheelectricalstimuli
neededtofullyexercisetheECU.Inthiswayyou“fool”theECUintothinkingthatitisindeed
connectedtoarealplant.
TheHILsimulationincludesamathematicalmodeloftheprocessandahardware
device/ECUyouwanttotest,e.g.anindustrialPIDcontrollerwewilluseinourexample.The
hardwaredeviceisnormallyanembeddedsystem.
WhyuseHILsimulation?
Thisquestionisanimportantpartofunderstandingreal-timetechnology.Torestatethe
questionusingacontrolsystemsterm:Whynotconnecttheembeddedsystemundertestto
the“realplant”,thatisthedynamicsystembeingcontrolled,toperformdevelopmentand
testing?Inmanycases,themosteffectivewaytodevelopanembeddedsystemisto
connecttheembeddedsystemtotherealplant,ifsuchaplantexists.Increasinglyhowever,
HILsimulationismoreefficientandorrequired.
ThemainpurposewiththeHILSimulationistotestthehardwaredeviceonasimulator
beforeweimplementitontherealprocess.
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Themetricofdevelopmentandtestefficiencyistypicallyaformulathatincludesthe
followingfactors:
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Cost
Duration
Safety
Youmaywanttotestthedifferentpartofthesystemindividuallytomakesureitworksas
plannedandHILsimulationisimportantindesignandtestingofthedifferentsystems.
Itmaybeveryuseful,e.g.,totestacontrollerfunctionwithasimulatedprocessbeforethe
controllerisappliedtothereal(physical)process.Ifthemathematicalmodelusedinthe
simulatorisanaccuraterepresentationoftherealprocess,youmayeventunethecontroller
parameters(e.g.thePIDparameters)usingthesimulator.
Itisalsoveryusefulfortrainingpurposes,i.e.,theprocessoperatormaylearnhowthe
systemworksandoperatebyusingthehardware-in-the-loopsimulation.
AnotherbenefitofHardware-In-the-Loopisthattestingcanbedonewithoutdamaging
equipmentorendangeringlives.Forinstance,potentiallydamagingconditionsinanengine,
suchasover-temperature,canbesimulatedtotestiftheECUcandetectandreportit.
Anotherinstancewouldbeananti-lockbraking(ABS)simulationatperformanceextremes.If
simulated,theperformanceoftheABSsystemcanbeevaluatedwithoutrisktothevehicle
oroperator.
HILshouldbeanintegratedpartofthedesignandtestingcycle.
Asthecomplexityofthehardwarebeingcontrolledincreases,sotoodoesthecomplexityof
theembeddedsystemthatisdesignedtocontrolthehardware.Hardware-in-the-Loop(HIL)
simulationisatechniquethatisusedincreasinglyinthedevelopmentandtestofcomplex
real-timeembeddedsystems.
ThepurposeofHILsimulationistoprovideaneffectiveplatformfordevelopingandtesting
real-timeembeddedsystems,oftenincloseparallelwiththedevelopmentofthehardware.
Softwaredevelopmentnolongerneedstowaitforaphysicalplantinordertowriteandtest
code.
HILsimulationprovidesaneffectiveplatformbyaddingthecomplexityoftheplantunder
controltothedevelopmentandtestplatform.Thecomplexityoftheplantundercontrolis
includedintestanddevelopmentbyaddingamathematicalrepresentation(model)ofall
relateddynamicsystems.Thesemathematicalrepresentationsarereferredtoasthe“plant
simulation.”
Challenges
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Whentesting,wehavelotsofchallenges:
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Costtotest
Costoffailure
Availability
Systemvariation
Repeatability
Inthesesituations,isHILsimulationapowerfultechnique.WithHILTestingwewillreduce
costandrisk.
WithHILTestingcostandriskwillbereduced:
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Increasedreliabilityandquality
Moreefficientdevelopment
Lowercosttoinnovate
Applications
EmbeddedControlSystems
HILsimulationiswidelyusedindevelopingEmbeddedControlSystems,suchas:
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MedicalDevices
Industrialmachines
PowerGeneration
Systems
WhiteGoods
Aerospace
Automotive
ProcessControl
Procedure
ThemainstepsinHILSimulationareasfollows:
1. Developamathematicalmodel.Createamathematicalmodelofthereal
environmentwherethehardwaredeviceismeanttobeused.
2. HILSimulation(Software+Hardware).Testyourdeviceonasimulatedprocess
(mathematicalmodel).
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3. ImplementyourhardwareontheRealProcess(Hardwareonly).IfeverythingisOK,
youmaywanttoimplementyourhardwaredeviceintherealenvironmentwhereit
meanttobeused.
ThesetasksfollowthemainideawithaHILsimulation.Firststepistosimulateyoursystem
insoftware.Nextistotestyourhardwareonthesimulatedprocess.Finallyyouimplement
yourhardwareontherealsystem.
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PracticalExample
Introduction
Itmaybeveryusefultotestacontrollerfunctionwithasimulatedprocessbeforethe
controllerisappliedtothereal(physical)process.Ifthemathematicalmodelusedinthe
simulatorisanaccuraterepresentationoftherealprocess,youmayeventunethecontroller
parameters(e.g.thePIDparameters)usingthesimulator.
Ifthecontrollertobetestedisimplementedinthecontrollerhardware,oftendenotedthe
electroniccontrolunit(ECU),andthesimulatorhastoruninrealtime,i.e.thesimulation
timedevelopsasrealtime.Thisrealtimesimulationisobtainedbysettingthesimulation
algorithmcycletimeequaltothesimulationtimestep.
Typically,thesimulatorcommunicateswiththeECUviaordinaryI/O(current,voltage,
digital).Suchasystem-wheretherealcontrolleriscontrollingasimulatedprocess-is
denotedHardware-in-the-loop(HIL)simulation.HIL-simulationisusedinmanyindustries,
e.g.automotiveindustryfortestingclutchautomationsystemsandinmarineandaircraft
industrytotestautopilotsofvessels.
TheFigurebelowillustratestheprincipleoftestingacontrolsystembyreplacingthephysical
system(orprocess)tobecontrolledbyasimulatedsystem.Thecontrollerisassumedtobe
aPIDcontroller,butthefigureappliestoanycontrollerfunction.
SimulatedProcess
Inthisexampleamathematicalmodelofthefollowingsmall-scaleprocessisused(“Air
Heater”):
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Themathematicalmodelis:
𝑇"#$ =
1
−𝑇"#$ + 𝐾+ 𝑢 𝑡 − 𝜃. + 𝑇/01 𝜃$
Where:
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𝑢[V]isthecontrolsignaltotheheater.
𝜃$ [s]istime-constant.
𝐾+ [𝑑𝑒𝑔𝐶/𝑉]istheheatergain.
𝜃. [𝑠]isthetime-delayrepresentingairtransportationandsluggishnessinthe
heater.
𝑇/01 istheenvironmental(room)temperature.Itisthetemperatureintheoutletair
oftheairtubewhenthecontrolsignaltotheheaterhasbeensettozerofor
relativelylongtime(someminutes).
Hardware
ThemainpurposewiththeHILSimulationistotestthehardwaredeviceonasimulator
beforeweimplementitontherealprocess.
InthisweuseanordinaryindustrialPIDcontroller,suchasFujiPGX5.
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WewilltesttheFujiPGX5PIDcontrolleronamodel,andifeverythingisOKwewill
implementthecontrollerontherealsystem.
WewilluseLabVIEWinordertoimplementtheHILSimulation.LabVIEWisagraphical
programminglanguagefromNationInstruments,anditiswellsuitedforsuch
implementation.
Procedure
Theprocedureisasfollows:
4. PIDControlandSimulationinLabVIEW(Softwareonly).Simulatethemodeland
implementthebuilt-inPIDcontrollerinLabVIEW.Nohardwareinvolved.
5. ConfiguretheFujiPGX5PIDcontroller(Hardwareonly).Configureandbefamiliar
withtheindustrialFujiPGX5PIDcontroller.
6. HILSimulationinLabVIEW(Software+Hardware).TestyourindustrialFujiPGX5PID
controlleronyoursimulatedprocess.
7. PIDTuning(Software+Hardware).FindproperPIDparameters,etc.forthe
controllerbasedonthemodel.
8. Implementyourhardware,i.e.,theFujiPGX5PIDcontrollerontheRealProcess
(Hardwareonly).NowthatyouhavetestedyourFujiPGX5PIDcontrolleronthe
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simulatedprocess,it’stimetoimplementitontherealprocess.Fine-tunePID
parametersifneccecary.
ThesetasksfollowthemainideawithaHILsimulation.Firststepistosimulateyoursystem
insoftware.Nextistotestyourhardwareonthesimulatedprocess.Finallyyouimplement
yourhardwareontherealsystem.
Belowweseethedifferencebetweenatraditionalprocesssystemusingasoftwareprogram
forimplementingthecontrolsystemandaHILsimulation.
Traditionalprocesssystemusingasoftwareprogramforimplementingthecontrolsystem:
InthiscaseyouneedtoscalethevoltagesignalyougetfromtheprocessandtheDAQtoa
temperaturevalue(1 − 5𝑉 → 20 − 50℃).
HILSimulation:
Inthiscaseyouneedtoscalethetemperaturevalueyougetfromthesimulatedprocess
beforeyousendthevaluetotheFujiPGX5PIDcontroller(20 − 50℃ → 1 − 5𝑉).
HILSimulationinLabVIEW
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BelowweseeanexcerptoftheprogramcreatedinLabVIEW:
Intheexamplewehaveuseda“SimulationLoop”inLabVIEW,butanordinaryWhileLoop
mayalsobeused.ThemodelisimplementedinaSimulationSubsystem.
PXG5PID.vi:
InsidetheSubVI“PXG5PID.vi”istheI/OfromandtothePXG5PIDcontrollerimplemented
usinganordinaryDAQdevice(NIUSB-6008USBDAQdevice),i.e.,thesimulatedprocess
valueneedstobesenttothecontrollerandthemanipulatedvaluefromthecontrollerneed
tobesenttothesimulatedprocess.ScalingisalsoimplementedinthisSubVI.
Belowweseethe“PXG5PID.vi”:
MathematicalModel:
IntheModel–AirHeater.visimulationsubsystemisthemathematicalmodelimplemented
asshownbelow:
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Results:
Thesimulationresultsbecome:
TheSetPoint(SP)issetonthePXG5PIDcontroller(inthiscase30℃attime𝑡 = 2𝑠).The
simulationisbasedonPIDparameterssetonthePXG5PIDcontrollerusingthebuilt-inAutotuningfunctionalitythatthePXG5PIDcontrollerhas.
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References
Thefollowingreferenceshavebeenusedinthisdocument:
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NationalInstruments(2011)-http://www.ni.com/hil/
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OPAL-RTTECHNOLOGIES(2011)-http://www.opal-rt.com/about-hardware-in-theloop-and-hardware-in-the-loop-simulation
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PrecisionMBA(2011)-http://www.precisionmba.com/hardware_in_the_loop.htm
•
Wikipedia(2011)-http://en.wikipedia.org/wiki/Hardware-in-the-loop_simulation
IntroductiontoHardware-in-the-LoopSimulation
Hans-PetterHalvorsen,M.Sc.
E-mail:hans.p.halvorsen@hit.no
Blog:http://home.hit.no/~hansha/
UniversityCollegeofSoutheastNorway
www.usn.no