Controller design for soft-disability remedy of th

InternationalJournalofAutomotiveTechnology,Vol.10,No.4,pp.497−503(2009)DOI10.1007/s12239−009−0057−6Copyright©2009KSAE1229−9138/2009/047−12497CONTROLLERDESIGNFORSOFT-DISABILITYREMEDYOFTHEELECTRICPOWERSTEERINGSYSTEMX.LI*,X.P.ZHAO,J.CHENandJ.L.MENSchoolofMechanicalEngineering,ShanghaiJiaoTongUniversity,Shanghai,MailBox819,No.800DongchuanRoad,Shanghai200240,China(Received21September2007;Revised3December2008)ABSTRACT−Theelectricpowersteering(EPS)systemisdesignedtoreducetheeffortexertedbydriveronthesteeringwheel.OneofthemostcommonandcriticalfailuresofEPSisthesoft-disabilityofthetorquesensororthelossofitssignal,whichleadstotheinstantshutdownoftheEPSsystemwhileturningandcausesserioustrafficaccidents.Inthispaper,anovelcontrollerbasedontheself-alignmenttorque(SAT)estimationwasdesignedtoremedythesoft-disabilityofEPSsystem.AftertheSATestimationmethodwasverifiedbytheempiricalMagicFormula(MF)tiremodel,theremedycontrolstrategybasedontheSATestimationwasdevelopedandevaluatedbysimulationsunderstepandsinusoidalinputs.Tofurtherevaluatetheperformanceofthecontrolleronarealvehicle,experimentsonarealEPSsystemwereimplementedunderstepandsinusoidalinputs.TheresultsofsimulationandexperimentusingthecontrollerbasedonestimatedSATshowedthiscontrollertobefeasibleandcapableofeliminatingtheabruptreactiontorqueincrementcausedbyshutdownofEPSandofremedyingthesoft-disabilityofEPSsystemundercommoninputsignals.KEYWORDS:Electricpowersteering(EPS),Soft-disabilityremedy,Self-alignmenttorque(SAT)estimation,Matlab/Simulinksimulation,Controlstrategy1.INTRODUCTIONTheelectricpowersteering(EPS)systemisanadvancedsteeringsystemthatappliesanelectricmotortoprovidesteeringassistingtorque.Intherecentyears,theEPSsystemhasbeenwidelyusedastheautomobilepower-steeringequipmentbecauseofitsefficiency,modularity,andtenability(SchoenerandHille,2000;ParmarandHung,2004;ChabaanandWang,2001).ThetypicalcontrolsystemoftheEPSsystemisshownintheblockdiagramofFigure1.Twoprimaryinputs,i.e.,thedrivertorquesignalTsen,detectedbyatorquesensoronthesteeringwheel,andthevehiclespeedsignalV,alongwithothersystemvariables,arecontinuouslyfedintoanelectriccontrolmodule,whichdeterminesthereferencecurrentIrbasedonthetorquemap.Thecontrollercancomputethecontrolsignalu(motorterminalvoltage),whichminimizestheerrorbetweenIrandactualcurrentIadetectedbythemotorcurrentsensor.Thus,theamountandthedirectionofsteeringassistingtorqueTaiscontrolled(KimandSong,2002;KurishigeandKifuku,2001).Inthevehicleturningprocess,oneofthemostcommonandcriticalfailuresoftheEPSsystemisthesoft-disabilityoftorquesensororthelossofitssignal.Theassistingtorqueprovidedbythemotoristhenabruptlycommandedtozero,causingtheshutdownoftheEPSsystemandrevertingtomanualmode.IftheEPSshutdownoccurswhilethemotorprovidesalowmagnitudeofassistingtorque,thedisabilityoftheEPSsystemmaybeimper-ceptibletothedriver.However,iftheEPSshutdownoccurswhilethemotorprovidesamoderateorhighmagni-tudeofassistingtorque,thedrivermayfeelanabruptlyincreasedreactiontorqueinthesteeringwheelduetothesuddendisappearanceofassistingtorque.Thisphenomenonisexplainedasfollows.ThetorqueappliedbythedriverwiththeassistingtorquegeneratedbytheEPSsystemshouldovercometworeactiontorqueswhileturningthevehicle.Oneisself-alignmenttorque(SAT)generatedbythetiresandtheroadsurface.TheotheristhetorquegeneratedbytheCoulombfrictionandViscousfrictionofthesteeringsystem.Whentheassistingtorqueprovidedbythemotorsuddenlydisappears,thereleaseofthestoredenergyinthetiresandsteeringsystemwouldcauseanabruptreactiontorqueincrement.Ifitisfeltbythedriver,hemayinstinctivelyrotatethesteeringwheelwithmore*Correspondingauthor.e-mail:[e-mail],X.P.ZHAO,J.CHENandJ.L.MENforce,causingunwantedsteeringorevendeviationfromtheoriginalpathatahighspeed.Therefore,thedisabilityofEPSisdangerousandmaycauseserioustrafficaccidents.Toremedythedisabilityofthesteeringsystem,onecommonsolutionistodesignthesteeringsystemasafault-tolerantcontrolsystembyusingredundanthardwarecom-ponentstoensurethesystemmorereliable(AnwarandLei,2007;Bingetal.,2005;KimandHwang,2006).However,theadditionalcomponentsduetohardwareredundancymayincreasethetotalmanufacturingcostofthesteeringsystem.Inthispaper,anovelcontrollerbasedontheSATesti-mationwasdesignedtoremedytheabruptsoft-disabilityoftheEPSsystem.SATisestimatedon-linebyinputtingtheexistingsignalsofyawrateγ,lateralaccelerationŸ,steer-ingangleδf,andwheelvelocityV.WhenEPSshutdownoccurs,anewreferencecurrentbasedontheestimatedSATwouldbecalculatedbytheelectroniccontrolunit(ECU)tocontrolthemotor.Therefore,themotorcancontinuallyprovideanassistingtorquetoovercometheabruptlyincreasedtorqueexertedonthesteeringwheeltoensuresafedriving.Theresultsofsimulationandexperimentbyapplyingthiscontrollershowedittobefeasibleandcapableofeliminatingtheabruptreactiontorqueincrementandofremedyingthesoft-disabilityoftheEPSsystem.Thepaperisorganizedasfollows.Section2introducesthesystemmodel,includingthesteeringsystemandvehicledynamics.Section3performsthedesignandvalidat