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Page 1: Introduction to Hardware-in-the-Loop Simulation

UniversityCollegeofSoutheastNorway

http://home.hit.no/~hansha

Hardware-in-the-LoopSimulation

Introduction

WhatisHardware-in-the-Loop(HIL)SimulationorWhatisHardware-in-the-Loop(HIL)Test?

TheHardware-in-the-Loopprocesshasexistedfornomorethan15to20years.ItsrootsarefoundintheAviationindustry.ThereasontheuseofaHILprocessisbecomingmoreprevalentinallindustriesisdrivenbytwomajorfactors:timetomarketandcomplexity.

Hardware-in-the-loop(HIL)simulationisatechniquethatisusedinthedevelopmentandtestofcomplexprocesssystems.HILsimulationprovidesaneffectiveplatformbyaddingthecomplexityoftheplantundercontroltothetestplatform.Thecomplexityoftheplantundercontrolisincludedintestanddevelopmentbyaddingamathematicalrepresentationofallrelateddynamicsystems.Thesemathematicalrepresentationsarereferredtoasthe“plantsimulation.”

Hardware-In-the-Loopisaformofreal-timesimulation.Hardware-In-the-Loopdiffersfromreal-timesimulationbytheadditionofarealcomponentintheloop.Thiscomponentmaybean“ElectronicControlUnit”(ECU).

ThepurposeofaHardware-In-the-LoopsystemistoprovidealloftheelectricalstimulineededtofullyexercisetheECU.Inthiswayyou“fool”theECUintothinkingthatitisindeedconnectedtoarealplant.

TheHILsimulationincludesamathematicalmodeloftheprocessandahardwaredevice/ECUyouwanttotest,e.g.anindustrialPIDcontrollerwewilluseinourexample.Thehardwaredeviceisnormallyanembeddedsystem.

WhyuseHILsimulation?

Thisquestionisanimportantpartofunderstandingreal-timetechnology.Torestatethequestionusingacontrolsystemsterm:Whynotconnecttheembeddedsystemundertesttothe“realplant”,thatisthedynamicsystembeingcontrolled,toperformdevelopmentandtesting?Inmanycases,themosteffectivewaytodevelopanembeddedsystemistoconnecttheembeddedsystemtotherealplant,ifsuchaplantexists.Increasinglyhowever,HILsimulationismoreefficientandorrequired.

ThemainpurposewiththeHILSimulationistotestthehardwaredeviceonasimulatorbeforeweimplementitontherealprocess.

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Themetricofdevelopmentandtestefficiencyistypicallyaformulathatincludesthefollowingfactors:

• Cost• Duration• Safety

YoumaywanttotestthedifferentpartofthesystemindividuallytomakesureitworksasplannedandHILsimulationisimportantindesignandtestingofthedifferentsystems.

Itmaybeveryuseful,e.g.,totestacontrollerfunctionwithasimulatedprocessbeforethecontrollerisappliedtothereal(physical)process.Ifthemathematicalmodelusedinthesimulatorisanaccuraterepresentationoftherealprocess,youmayeventunethecontrollerparameters(e.g.thePIDparameters)usingthesimulator.

Itisalsoveryusefulfortrainingpurposes,i.e.,theprocessoperatormaylearnhowthesystemworksandoperatebyusingthehardware-in-the-loopsimulation.

AnotherbenefitofHardware-In-the-Loopisthattestingcanbedonewithoutdamagingequipmentorendangeringlives.Forinstance,potentiallydamagingconditionsinanengine,suchasover-temperature,canbesimulatedtotestiftheECUcandetectandreportit.Anotherinstancewouldbeananti-lockbraking(ABS)simulationatperformanceextremes.Ifsimulated,theperformanceoftheABSsystemcanbeevaluatedwithoutrisktothevehicleoroperator.

HILshouldbeanintegratedpartofthedesignandtestingcycle.

Asthecomplexityofthehardwarebeingcontrolledincreases,sotoodoesthecomplexityoftheembeddedsystemthatisdesignedtocontrolthehardware.Hardware-in-the-Loop(HIL)simulationisatechniquethatisusedincreasinglyinthedevelopmentandtestofcomplexreal-timeembeddedsystems.

ThepurposeofHILsimulationistoprovideaneffectiveplatformfordevelopingandtestingreal-timeembeddedsystems,oftenincloseparallelwiththedevelopmentofthehardware.Softwaredevelopmentnolongerneedstowaitforaphysicalplantinordertowriteandtestcode.

HILsimulationprovidesaneffectiveplatformbyaddingthecomplexityoftheplantundercontroltothedevelopmentandtestplatform.Thecomplexityoftheplantundercontrolisincludedintestanddevelopmentbyaddingamathematicalrepresentation(model)ofallrelateddynamicsystems.Thesemathematicalrepresentationsarereferredtoasthe“plantsimulation.”

Challenges

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Whentesting,wehavelotsofchallenges:

• Costtotest• Costoffailure• Availability• Systemvariation• Repeatability

Inthesesituations,isHILsimulationapowerfultechnique.WithHILTestingwewillreducecostandrisk.

WithHILTestingcostandriskwillbereduced:

• Increasedreliabilityandquality• Moreefficientdevelopment• Lowercosttoinnovate

Applications

EmbeddedControlSystems

HILsimulationiswidelyusedindevelopingEmbeddedControlSystems,suchas:

• MedicalDevices• Industrialmachines• PowerGeneration

Systems• WhiteGoods• Aerospace• Automotive• ProcessControl

Procedure

ThemainstepsinHILSimulationareasfollows:

1. Developamathematicalmodel.Createamathematicalmodeloftherealenvironmentwherethehardwaredeviceismeanttobeused.

2. HILSimulation(Software+Hardware).Testyourdeviceonasimulatedprocess(mathematicalmodel).

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3. ImplementyourhardwareontheRealProcess(Hardwareonly).IfeverythingisOK,youmaywanttoimplementyourhardwaredeviceintherealenvironmentwhereitmeanttobeused.

ThesetasksfollowthemainideawithaHILsimulation.Firststepistosimulateyoursysteminsoftware.Nextistotestyourhardwareonthesimulatedprocess.Finallyyouimplementyourhardwareontherealsystem.

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PracticalExample

Introduction

Itmaybeveryusefultotestacontrollerfunctionwithasimulatedprocessbeforethecontrollerisappliedtothereal(physical)process.Ifthemathematicalmodelusedinthesimulatorisanaccuraterepresentationoftherealprocess,youmayeventunethecontrollerparameters(e.g.thePIDparameters)usingthesimulator.

Ifthecontrollertobetestedisimplementedinthecontrollerhardware,oftendenotedtheelectroniccontrolunit(ECU),andthesimulatorhastoruninrealtime,i.e.thesimulationtimedevelopsasrealtime.Thisrealtimesimulationisobtainedbysettingthesimulationalgorithmcycletimeequaltothesimulationtimestep.

Typically,thesimulatorcommunicateswiththeECUviaordinaryI/O(current,voltage,digital).Suchasystem-wheretherealcontrolleriscontrollingasimulatedprocess-isdenotedHardware-in-the-loop(HIL)simulation.HIL-simulationisusedinmanyindustries,e.g.automotiveindustryfortestingclutchautomationsystemsandinmarineandaircraftindustrytotestautopilotsofvessels.

TheFigurebelowillustratestheprincipleoftestingacontrolsystembyreplacingthephysicalsystem(orprocess)tobecontrolledbyasimulatedsystem.ThecontrollerisassumedtobeaPIDcontroller,butthefigureappliestoanycontrollerfunction.

SimulatedProcess

Inthisexampleamathematicalmodelofthefollowingsmall-scaleprocessisused(“AirHeater”):

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Themathematicalmodelis:

𝑇"#$ =1𝜃$

−𝑇"#$ + 𝐾+𝑢 𝑡 − 𝜃. + 𝑇/01

Where:

• 𝑢[V]isthecontrolsignaltotheheater.• 𝜃$[s]istime-constant.• 𝐾+[𝑑𝑒𝑔𝐶/𝑉]istheheatergain.• 𝜃.[𝑠]isthetime-delayrepresentingairtransportationandsluggishnessinthe

heater.• 𝑇/01istheenvironmental(room)temperature.Itisthetemperatureintheoutletair

oftheairtubewhenthecontrolsignaltotheheaterhasbeensettozeroforrelativelylongtime(someminutes).

Hardware

ThemainpurposewiththeHILSimulationistotestthehardwaredeviceonasimulatorbeforeweimplementitontherealprocess.

InthisweuseanordinaryindustrialPIDcontroller,suchasFujiPGX5.

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WewilltesttheFujiPGX5PIDcontrolleronamodel,andifeverythingisOKwewillimplementthecontrollerontherealsystem.

WewilluseLabVIEWinordertoimplementtheHILSimulation.LabVIEWisagraphicalprogramminglanguagefromNationInstruments,anditiswellsuitedforsuchimplementation.

Procedure

Theprocedureisasfollows:

4. PIDControlandSimulationinLabVIEW(Softwareonly).Simulatethemodelandimplementthebuilt-inPIDcontrollerinLabVIEW.Nohardwareinvolved.

5. ConfiguretheFujiPGX5PIDcontroller(Hardwareonly).ConfigureandbefamiliarwiththeindustrialFujiPGX5PIDcontroller.

6. HILSimulationinLabVIEW(Software+Hardware).TestyourindustrialFujiPGX5PIDcontrolleronyoursimulatedprocess.

7. PIDTuning(Software+Hardware).FindproperPIDparameters,etc.forthecontrollerbasedonthemodel.

8. Implementyourhardware,i.e.,theFujiPGX5PIDcontrollerontheRealProcess(Hardwareonly).NowthatyouhavetestedyourFujiPGX5PIDcontrolleronthe

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simulatedprocess,it’stimetoimplementitontherealprocess.Fine-tunePIDparametersifneccecary.

ThesetasksfollowthemainideawithaHILsimulation.Firststepistosimulateyoursysteminsoftware.Nextistotestyourhardwareonthesimulatedprocess.Finallyyouimplementyourhardwareontherealsystem.

BelowweseethedifferencebetweenatraditionalprocesssystemusingasoftwareprogramforimplementingthecontrolsystemandaHILsimulation.

Traditionalprocesssystemusingasoftwareprogramforimplementingthecontrolsystem:

InthiscaseyouneedtoscalethevoltagesignalyougetfromtheprocessandtheDAQtoatemperaturevalue(1 − 5𝑉 → 20 − 50℃).

HILSimulation:

InthiscaseyouneedtoscalethetemperaturevalueyougetfromthesimulatedprocessbeforeyousendthevaluetotheFujiPGX5PIDcontroller(20 − 50℃ → 1 − 5𝑉).

HILSimulationinLabVIEW

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BelowweseeanexcerptoftheprogramcreatedinLabVIEW:

Intheexamplewehaveuseda“SimulationLoop”inLabVIEW,butanordinaryWhileLoopmayalsobeused.ThemodelisimplementedinaSimulationSubsystem.

PXG5PID.vi:

InsidetheSubVI“PXG5PID.vi”istheI/OfromandtothePXG5PIDcontrollerimplementedusinganordinaryDAQdevice(NIUSB-6008USBDAQdevice),i.e.,thesimulatedprocessvalueneedstobesenttothecontrollerandthemanipulatedvaluefromthecontrollerneedtobesenttothesimulatedprocess.ScalingisalsoimplementedinthisSubVI.

Belowweseethe“PXG5PID.vi”:

MathematicalModel:

IntheModel–AirHeater.visimulationsubsystemisthemathematicalmodelimplementedasshownbelow:

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Results:

Thesimulationresultsbecome:

TheSetPoint(SP)issetonthePXG5PIDcontroller(inthiscase30℃attime𝑡 = 2𝑠).ThesimulationisbasedonPIDparameterssetonthePXG5PIDcontrollerusingthebuilt-inAuto-tuningfunctionalitythatthePXG5PIDcontrollerhas.

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ReferencesThefollowingreferenceshavebeenusedinthisdocument:

• NationalInstruments(2011)-http://www.ni.com/hil/

• OPAL-RTTECHNOLOGIES(2011)-http://www.opal-rt.com/about-hardware-in-the-loop-and-hardware-in-the-loop-simulation

• PrecisionMBA(2011)-http://www.precisionmba.com/hardware_in_the_loop.htm

• Wikipedia(2011)-http://en.wikipedia.org/wiki/Hardware-in-the-loop_simulation

Page 12: Introduction to Hardware-in-the-Loop Simulation

Hans-PetterHalvorsen,M.Sc.

E-mail:[email protected]

Blog:http://home.hit.no/~hansha/

UniversityCollegeofSoutheastNorway

www.usn.no


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