Event triggered sliding mode control a new approach to control system design 1st edition bijnan band

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Bijnan Bandyopadhyay

Abhisek K. Behera

Event-Triggered Sliding Mode Control

A New Approach to Control System Design

StudiesinSystems,DecisionandControl

Volume139

Serieseditor

JanuszKacprzyk,PolishAcademyofSciences,Warsaw,Poland e-mail:kacprzyk@ibspan.waw.pl

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Event-TriggeredSliding

ModeControl

ANewApproachtoControlSystemDesign

BijnanBandyopadhyay SystemsandControlEngineering

IndianInstituteofTechnologyBombay

Mumbai,Maharashtra

India

AbhisekK.Behera SystemsandControlEngineering IndianInstituteofTechnologyBombay Mumbai,Maharashtra

India

ISSN2198-4182ISSN2198-4190(electronic) StudiesinSystems,DecisionandControl

ISBN978-3-319-74218-2ISBN978-3-319-74219-9(eBook) https://doi.org/10.1007/978-3-319-74219-9

LibraryofCongressControlNumber:2017963863

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Preface

Variablestructuresystemsusingslidingmodecontrol(SMC)wasoriginatedin USSRinthelate fiftiestostabilizetheuncertaindynamicalsystemswithrelayasa feedbackcontrollaw.ItgainedpopularityoutsideUSSRonlyafterthelateseventiesduetoasurveypaperinEnglishbyProf.VadimI.Utkin.Sincethen,ithas becomenowawell-establishedrobustcontroltechniquetodealwiththeuncertaintiesintheplant,andachievingthesystemstability.Avastnumberofscienti fic publicationsandthepracticalapplicationsofthiscontroltechniquehavemade SMCasanimportantareainthecontrolliteratures.

TheattentionondesignofSMCindiscrete-timedomainwaspaidbymany researcherssoonaftertheimportanceofmicroprocessorandcomputer/processors arerealizedincontrolapplicationsintheearlyeighties.The firstandimportant observationinthediscrete-timedesignisthatnoexactslidingmodeisachievedas inthecontinuous-timecounterpart.Anewnotionofslidingmodeisintroduced whichisknownaquasi-slidingmode(QSM).Thishasledtothedevelopmentof discrete-timeSMCasanimportantareainSMCduetoitspracticalimportance. ManydesignapproacheshavebeenproposedtoimprovetheperformanceofSMC forthesampled-datasystem.

Inthismonograph,anewapproachtodesignSMCispresentedusinganovel implementationstrategy,namelyevent-triggering.Inthisstrategy,thecontrolis updatedwheneveracertainstabilizingconditionisviolated,andhence,thesystem stabilityisalwaysmaintained.Duetotheneed-basedcontrolstrategy,it findsa majorapplicationinspatiallydistributedcontrolsystemstoreducethecommunicationamongdifferentsensorandactuatorends.So,theresourcesofthesystems areoptimallyused.Theevent-triggering-baseddesignofSMCnotonlygivesthe robustperformancebutalsoensuresminimaluseofresourcesinthecontrolsystem. Thismonographpresentstherecentresultsonevent-triggeredSMCforrobust stabilizationofdynamicalsystems.Inthe fi rstpartofthemonograph,thepreliminariesonsampled-datasystemswithanintroductiontoevent-triggeredcontrolare presentedtofamiliarizethereaderstheevent-triggering-baseddesignofcontrol law.Inadditiontothis,abriefintroductiontoSMCanditsdesignarealsodiscussed.Then,thedesignofevent-triggeredSMCforbothlinearandnonlinear

systemsisgiveninChaps. 2 and 3.Theevent-triggeredSMCispresentedforlinear systemsguaranteeingthesemi-globalandglobalstabilityinChap. 2.However, onlylocalstabilizationresultisdiscussedforthenonlinearsystems,whichis presentedindetailinChap. 3.Intheevent-triggeredcontrol,thestatetrajectoryis continuouslymeasuredtoevaluatethetriggeringconditionwhichmaynotbe practicalinallapplications.So,Chaps. 4 and 5 presentfewvariantsof event-triggeredcontrol,namelyself-triggeredanddiscreteevent-triggeredcontrol, respectively.Intheself-triggeringstrategy,thetriggeringstrategyisdeveloped withoutusingthecontinuousstatemeasurements.Ontheotherhand,theperiodic statemeasurementsareusedindiscreteevent-triggeredcontrolandthecontrolis updatedwhenitisviolatedatsomeperiodicinstants.Inrecenttime,therehasbeen aconsiderableamountofinterestinstabilityofquantizedcontrolsystem.The final chapterpresentsthedesignofevent-triggeredSMCwithquantizedstatemeasurements.Itisourbeliefthatthematerialofthemonographwouldserveits purposeandexplorethenewchallengesonthetopic.

Thisworkwouldhavenotbeencompletedwithoutthesupportandencouragementfrommanyofourfriendsandcolleagues.Theauthorswouldliketothank IndianInstituteofTechnologyBombayforprovidingtheconduciveenvironmentto carryouttheresearchreportedinthemonograph.Finally,weextendourgratitude toourfamilyfortheirlove,supportandunderstandingthroughouttheprocessof thisendeavour.

Mumbai,IndiaBijnanBandyopadhyay December2017AbhisekK.Behera

2.4.1Event-TriggeredDesignofSMC

3Event-TriggeredSlidingModeControlforNonlinearSystems

3.1.1DesignofSlidingModeControl

3.3.1DesignofEvent-TriggeringSchemewithConstraints

4.3.1DesignofSelf-TriggeredSlidingModeControl

5DiscreteEvent-TriggeredSlidingModeControlforLinear

5.2.1Bartoszewicz

5.2.2DesignofDiscrete-TimeSlidingModeControl

5.3.1Event-TriggeredBartoszewicz

5.3.2Event-TriggeringRule

5.3.3SimulationResults

5.4DiscreteEvent-TriggeredSlidingModeControl:Output FeedbackApproach

5.4.1MultirateOutputFeedbackTechnique

5.4.2Multirate-BasedEvent-TriggeredDiscrete-Time

5.4.3SimulationResults

6Event-TriggeredSlidingModeControlwithQuantizedState

6.3DesignofEvent-TriggeredSlidingModeControl

6.3.1DesignofEvent-TriggeringRule

Acronyms

A-DAnalog-to-digital

D-ADigital-to-analog

DTSMDiscrete-timeslidingmode

ETCSEvent-triggeredcontrolsystems

FOSFastoutputsampling

ISSInput-to-statestable

LTILineartime-invariant

MIMOMultiple-inputmultiple-output

MROFMultirateoutputfeedback

QSMQuasi-slidingmode

SISOSingle-inputsingle-output

SMCSlidingmodecontrol

VSSVariablestructuresystem

Symbols

R Setofrealnumbers

R 0 Setofnonnegativerealnumbers

Z Setofintegers

Z 0 Setofnonnegativeintegers

Rn n-dimensionalvectorspaceover R

ða bÞðxÞ Compositionofthetwofunctions a and b x y Dotproductofanytwovectors x and y in Rn

P [ ð Þ0Positive(semi)-definitematrix P

Q⊤ Transposeofanymatrix Q

j j Absolutevalueofascalarvariable ‘ ’ : kk1 1-normofa finite-dimensionalvector ‘ ’ kk Euclidean(2-)normofa finite-dimensionalvector ‘ ’ oramatrix ‘ ’ ofappropriatedimension

kmaxðminÞ f g Largest(smallest)eigenvalueofasquarematrix ‘ ’

rf ðxÞ Gradientofareal-valuedfunction f ðxÞ sup(inf)Supremumorleastupperbound(infi mumorgreatestlowerbound) x bc Floorfunctionthatreturnslargestintegerlessthanorequalto x ln Naturallogarithmwithbasee(¼ 2:71828)

K Setofstrictlyincreasingandcontinuousreal-valuedfunctions definedonthenonnegativeintervalwithzeroatzero

K1 Setofunboundedclass-K functions signSignumfunction

FðxÞ Set-valuedmapofthevector field f ðxÞ atthepointofdiscontinuity x inFilippov’sinclusion coConvexclosure

Bd ðxÞ Openballofradius d centredat x lð Þ Lebesguemeasureofaset ‘ ’ v Sensitivityofthequantizer

M Saturationlevelofthequantizer

Ti Inter-eventtime/timeintervalbetweentwoconsecutivetriggering instants

s Constantsamplingperiodforthediscrete-timesystems

N Anintegergreaterthanorequaltotheobservabilityindexofthe system

Chapter1 Introduction

Thischapterbrieflyintroducesthereaderstothepreliminaryideasondesignand analysisofcomputer-controlledsystemsandthenslidingmodecontrol(SMC).In general,computer-controlledsystemsconsistsofbothcontinuousanddiscrete-time systemsthatinteractamongthemselvesthroughthefeedbackchanneltoachievecertainobjectives.Differentavailableclassicaltechniques,namelyemulation,discretetimeandhybridapproaches,aresummarizedherewiththeirownadvantagesand disadvantages.Inalmostallthesetechniques,theperiodicsamplingintervalisoften usedtodesignandanalysethesampled-datasystemsforitssimplicityandeasierin design.Ontheotherhand,aperiodiccontrolimplementationisdesiredinsampleddatasystemstoreducetheperiodiccomputationalburdenandcostassociatedwith theimplementation.However,thisintroducesfewdifficultiesinanalysingclosed loopsystemstability.Anovelsamplingstrategyknownasevent-triggeredcontrol isintroducedherewherethecontrolisupdatedwheneveritisdemanded.Inthis technique,thetimeinstantsforupdatingthecontrolsignalisdeterminedusingsome rulethatensuresthestabilityofthesystem.So,thisstrategymaintainsthesystem stabilitywhilereducingextraburdenonthesystem.

ThedesignofSMCisalsopresentedinthischaptertofamiliarizethereaders.This isarobustcontrollerthatstabilizestheplantinthepresenceofexternaldisturbances. TheslidingmotionandSMCarebrieflyelaboratedtounderstandslidingmode withdiscontinuouscontrolaction.ThisisfollowedbythedesignofSMCforlinear systems.ThediscreterealizationofSMC,unfortunately,doesnotyieldslidingmotion exactlyduetodiscretenatureofcontrolisalsodiscussed.Somecontroldesign techniquesarereviewedfordiscrete-timeslidingmode.

©SpringerInternationalPublishingAG,partofSpringerNature2018 B.BandyopadhyayandA.K.Behera, Event-TriggeredSlidingModeControl,Studies inSystems,DecisionandControl139,https://doi.org/10.1007/978-3-319-74219-9_1

1.1Computer-ControlledSystems

Computer-controlledsystems,ingeneral,arebroadlydefinedasacontrolsystemthat allowsinteractionofbothanalogsystemanddigitalcontrollerthroughcomputer oranyotherdigitalplatform.Suchsystemsareubiquitousinalmostallfieldsof engineeringduetorapidadvancementofdigitaltechnology.Theplantrepresents thecontinuous-timesystem,whilethecontrolsignalisadiscreteinnaturethatis appliedtotheplant.Thecontrolsignalisupdatedatthediscreteinstantsonlyand, however,isheldconstantinbetweentwoconsecutivesamplinginstants.Thisiswhy, itisalsocalledas sampled-datasystem.Theplantdynamicsevolvesinopenloop mannerbetweentwodiscreteinstantsasthereisnocontrolvariable.So,theinterest ismainlyfocusedontheplantbehaviouratthediscreteinstantsonlyleadingto theso-called discrete-timesystems.Inotherwords,thesystemdynamicsevolves atdiscreteinstantswiththecontrolinputappliedattheseinstants.Thisisnotonly easytoanalysebutalsooneofthesimplestwaysofimplementingthecontrollawto achieveacertainobjectivecomparedtoitsanalogcounterparts.

Inactualpractice,computer-controlledsystemshavefacilitatedthecontroldesign problemstoagreatextentbyintroducingthedigitalcontrolsignal.However,itshould notbethoughtofasanidealcontroldesignandimplementationscenarioforaphysicalplant.Rather,itisalimitingapproachoftheanalogcontrolsystem(bothplantand controlevolveincontinuous-time)andalltheanalysesarecarriedoutfortheanalog controlsysteminsampled-datafashion.Thecontrollawisdesignedforanyplant usingsystemdynamicswhichmaybedefinedoncontinuous-and/ordiscrete-time domain.However,inthepresentcaseonly,wefocusoncontinuous-timeplant.So, thecontrolisdesignedfromthecontinuoustimeplantwhichachievesdesiredperformanceofthesystemwhenitisimplementedincontinuousmanner.But,indiscrete implementation,thecontinuous-timeperformancemayalsobeachievedifitisimplementedatafastersamplingrate.So,thecomputer-controlledsystemyetprovidesan alternativetoanalogcontrolimplementationsubjecttosomedesiredperformance. Inspiteofthis,therearenumerouschallengesassociatedwithcomputer-controlled systemsthatmaketheresearcherstorethinkpossiblenewtechniquesforcontrol implementationanddesignphilosophy.

1.1.1BasicArchitecture

Thebasicarchitectureofacomputer-controlledsystemisshowninFig. 1.1.The plant,whichevolvesincontinuous-time,interactswiththecontrolleronlyatdiscrete instants.Theplantstatesaresampledatdiscreteinstantsandconvertedintoadigital signals.Thewholeprocessofconvertinganalog-to-digitalconversionisrepresented bytheanalog-to-digital(A-D)converter.Thesedigitalsignalsarethenprocessed throughacontrolalgorithmtogenerateanewsetofdigitalsignals,knownasdiscrete controlsignals.Weseethatthereissamenumberofdiscretecontrolsignalsasthe

Fig.1.1 Basicarchitecture ofacomputer-controlled systems

ControlLaw/ Algorithm

samplesofplantstates.Now,thediscretesignalisprocessedbydigital-to-analog(DA)convertertogenerateacontinuous-timesignalbetweentwoconsecutivesampling instants.But,thiscontinuoussignalisanapproximationofanalogcontrolsignal.The overallprocessfromA-DtoD-Aiscoordinatedbyaclockthatsynchronizesallthe taskscarriedoutatA-D,controlsynthesisandD-Aconverter.Here,thecontrolsignal maybedesignedinthecontinuous-timeframeworkorindiscretedomaindepending onthedesignrequirements.

Boththeconverters,often,operateinperiodictimeintervalinthesampled-data system.Theinformationissampledattimeinstantsknownas samplinginstant,and theintervalbetweensuccessivesamplinginstantsiscalledas samplingperiod.For everysamplinginstant,thecontrolsignaliscomputedandthesameisappliedtothe plantbyitsapproximateanalogsignalthroughD-Aconverter.Thecontrolsignalis heldconstantineverysamplingintervalmakingthesystemasopenloop,andhence, thesystemevolvesinopenloopmanner.Thisisoneofthedifferencesthatmakes thecomputer-controlledsystemsdifferentfromotherfeedbacksystems.Therehas beenmanytechniquesavailabletoexploretheanalysisanddesignofsuchsystem. But,yetthefullpotentialofthissystemneedstobeinvestigatedforeffectiveuseof computer-controlledsystems.Forinstance,theeffectofsamplingintervalonsystem performance,theabsenceofclockthatsynchronizesbothA-DandD-Aconverters, etc.areneedtobeinvestigated.Ofallthese,samplingintervalvariationisoneof themostimportantandchallengingproblemsincomputer-controlledsystem.Too fastsamplingissometimeunnecessary,whiletheslowsamplingmaydeterioratethe systemperformance.So,itisalwaysdesirabletohaveanoptimalsamplinginterval thatstabilizesthecomputer-controlledsystem.

1.1.2DesignTechniques

Manytechniqueshavebeenusedtoanalyseanddesignofcomputer-controlled systems.Theclosedloopsystemishybridinnatureinvolvingbothcontinuous anddiscretedynamics;however,thestandardavailabletechniquedealswitheither continuous-timedesignordiscrete-timedesign.Thatmeansdesignthecontroleither

Clock

usingcontinuous-timeordiscrete-timemodelandapplyittothecontinuous-time plant.Inthesamemanner,thestabilityanalysisofthesystemiscarriedoutusing eitherofthesemodelsdependingoncontrolsignal.Broadlyspeaking,therearethree methodsavailableforanalysingthecomputer-controllersystem,namelyemulationbasedapproach,discrete-timeapproachandhybridsystemapproach.Also,inmost applicationstheconstantsamplingperiodisusedforimplementingthediscretecontroller.

1.1.2.1Emulation-BasedApproach

Itmayalsobeseenascontinuous-timeapproachdesigntosampled-datasystem.Here, theperformancespecificationsareincontinuous-timedomainsincetheplantand controllerbothareincontinuous-timedomain.ThebasicideaisfirsttoignoretheA-D andD-AconvertersinFig. 1.1 andfollowthedesignstepsofanystabilizingcontroller. Thecontinuous-timecontrolleristhenapproximatedbyreplacingderivativeswith finite-differencesandcontinuous-timesignalswithsampledvaluesatthatinstants. Thoughitisanapproximation,thesatisfactoryperformanceofthesampled-data systemisstillachievedbythisapproximation.Here,thecontrollerdesignedfrom continuous-timeplant emulates thebehaviourofcontinuous-timeplantinspiteof discreteimplementations,andhence,thisisknownas emulation-basedapproach. However,themainissueinthisapproachisthatthesamplingperiodisnottaken intoaccountinthedesignofcontroller.Itisverynaturalthattheemulatedcontroller givesthestabilityofsampled-datasystemforarangeofsamplingperiodonlywhile itdestabilizesforothervaluesofsamplingperiod.

1.1.2.2Discrete-TimeApproach

Thisapproachissimpleandisbasedonthediscrete-timemodeloftheplant.The continuousplantinteractswithdigitalcontrollerthroughA-DandD-Aconverters. Thus,thecontrollerseestheplantasdiscrete-timemodelthroughtheseconverters. Thediscrete-timerepresentationoftheplantisobtainedbycombiningtheplantwith A-DandD-Aconverters.Therearenumerousapproachesavailablefordiscretizing theplantforagivensamplingperiod,andthepopularamongthemareEulerdiscretization,zero-orderhold(ZOH)discretization,etc.Theclosedloopresponseis analysedonlyatdiscreteinstantssinceitignorestheinter-samplingbehaviourof theplant.Ontheotherhand,thestabilityofthesystemisanalysedbyignoringthe dynamicsbetweentwosamplinginstants.Thewideapplicationofthistechniqueis foundinmanyslowprocesseswhereitisenoughtostudythesystembehavioronly atsomeperiodicintervals.However,thewell-knownshortcomingsofthistechnique arethatnointer-samplingdynamicsoftheplantcanbeanalysedandselectionofa suitablesamplingintervalthatcapturestheundesirablephenomenoninthesystem.

1.1.2.3HybridApproach

Asthenamesuggests,inthistechnique,boththecontinuousanddiscretebehaviours areanalysedforthesampled-datasystemwithoutrepresentingthesystembysome approximatedsystemdynamics.Thisiswhy,itisalsoknownasdirectdesign approachtosampled-datasystem.Duetothishybridnatureofthesystem,thedesign andstabilitymethodsarecomplicatedthanearliertwomethods.

Itistobenotedthatvariablesamplingperiodmayalsobeusedfordesigning thecontrollerforsampled-datasystems.However,itinvolvesmanydesignissues foranalysingthestabilityduetorestrictedmathematicaltoolsand/ortime-varying natureofsystemdynamics.Nevertheless,manyattemptshavebeenmadeforstabilizingthesampled-datasystemswithaperiodiccontrolsamplingprocess.Eventtriggeringstrategyisoneofsuchtechniquesthatgeneratesnonuniformsampling instantswhileensuringsystemstability.Inthisbook,onlyevent-triggeredtechnique willbeemphasizedforcomputer-controlledsystems.

1.2Event-TriggeredControl

Event-triggeringstrategyisacontrolimplementationtechniquethatismotivated fromtheLebesguesampling.Inthiscase,samplingperiodisnotconstantbutis determinedbytheevolutionofsystemtrajectorysatisfyingsomestabilitycondition.Tounderstandtheconceptofevent-triggeredcontrol,firstLebesguesampling techniqueisdiscussed.

Inclassicalsampled-datasystem,generallyconstantsamplingperiodischosen andthecontrolisimplementedoncethisconstanttimeperiodiselapsed.Thisis generallyreferredasReimannsamplingasshowninFig. 1.2a.Foranyconstant hR > 0,thesamplingofthecontinuous-timesignal ψ(t ) takesplaceatevery hR intervals oftime.Itdoesnotmonitortheevolutionofsignal ψ(t ).Asaresultofthis,the importantconcerninReimannsamplingistheproperselectionofsamplingperiod tocapturethetransientbehaviouroftheplant.Ontheotherhand,thesignalmay alsobesampledatthetimeinstantswhenstateevolutionfromitsimmediatepast sampledvaluecrossesacertainthresholdvalue(say, hL )asthetimedoesinReimann sampling.ThisisknownasLebesguesamplingwhichisshowninFig. 1.2b.Though boththesamplingtechniquesaredifferent,thesehavesimilarityinthesampling mechanism.Intheformer,thetimeismeasuredwhilestateismonitoredinthelatter case.However,indoingsomanyadvantagesareobtainedinthecaseofLebesgue sampling.Forexample,itisnotnecessarymostofthetimetoupdatethecontrol signalfrequentlyatperiodicinterval.So,Lebesguesamplinggivessamplinginstant wheneveritisneededsubjecttosomesatisfactorysystemperformance.

Incasethefunction ψ(t ) isafinite-dimensionalvectorforanyfixed t ,thesampling instantmaybedecidedbyobservingtheindividualstateevolutionofthevectorvaluedfunction.Then,foranygivenconstants hLi > 0with i = 1, 2,..., n,the individualstateevolution,denotedby hLi (t ),isobservedforgeneratingsampling

Fig.1.2 Comparisonof ReimannandLebesgue samplings

(t )

R

(a)Reimannsampling

(t )

L

(b)Lebesguesampling

instantofthecorrespondingstate.However,itisverycomplicatedanddifficultto analysethestabilityofclosedloopsystem.Anotherschoolofthoughtistosampleall thestatessimultaneouslywhenevercertainconditionisviolated.Thus,thisstrategy doesnotnecessitateindividualsamplingofthestateatdifferenttimeinstants.Dueto this,thelatterismoreconvenientforimplementingpracticallythantheearlierone.

1.2.1PreliminaryIdea

Event-triggeredcontrolisoneofsuchtechniquesthatgeneratesthesamplinginstant (alsocalledastriggeringinstant)forsamplingandupdatingthecontrolsignal.Toprovideapreliminaryideaonevent-triggeredcontrol,weconsideranonlineardynamical system

wherethefunction f ( , ) isLipschitzwithrespecttoboththearguments u ∈ Rm .Let thereexistsacontinuousfeedbackcontrollaw u (x ) = π(x ) suchthatthedynamics ˙ x = f (x ,π(x ))

isasymptoticallystable.Itisassumedthatthecontrolisimplementeddigitallyto theplant.So,thecontrolsignal π(x ) iscomputedforeverysamplinginstantandis appliedtotheplantatthesediscreteinstants.Then,thesystembecomesopenloop betweentwoconsecutivesamplinginstants.However,duetothis,thediscreteerror isintroducedintheplant,definedby e (t ) = x (ti ) x (t ) with e (ti ) = x (ti ) x (ti ) = 0 where t ∈[ti , ti +1 ).Thiserrorappearsintheplantduetodiscreteimplementationof continuous-timecontrol,butitsvalueiszeroifthecontroliscontinuouslyupdated asinanalogimplementation.

Further,weassumethatthesystem(1.1)isinput-to-statestable(ISS)withrespect totheerror e (t ).ThatmeansthereexistsacontinuouslydifferentiableLyapunov function V : Rn → R≥0 suchthat

forsomeclass-K∞ 1 functions a , a , a ,andclass-K function γ .Here,thenotation ‘ ·’denotesinner(scalar)product.Event-triggeringstrategyisdevelopedfordeterminingthesamplinginstantssuchthatdesiredstabilityisachieved.Inthiscase,the asymptoticstabilityofthesystemisdesiredwiththediscreteimplementationofthe controllaw.So,theobviousconditionforwhichthisholdsis γ( e )<σ a ( x ) for some σ ∈ (0, 1).Thiscanbesimplifiedfurther,byassuming a 1 and γ areLipschitz onsomecompacts,as σ x > Le e ,where Le isanappropriateconstant.Thus,the triggeringinstantmaybegeneratedbyexecutingthefollowing,

1 Anyfunction a issaidtobeclass-K ifitiscontinuous,strictlyincreasing,zeroatzero.Again,it issaidtobeclass-K∞ ifitbelongstoclass-K andisunbounded.Clearly,class-K∞ functionsare subsetsofclass-K functions.

Thisisknownastriggeringruleforevent-triggeredcontrol π(x ).Itisseenthat thisensures Le e <σ x whichimpliesthat γ( e )<σ a ( x ) alsoholds.This impliesfrom(1.2)and(1.3)that

where K∞ a := a ◦ a 1 whichisthecompositionoftwofunctions a and a 1 . Thisshowsthatclosedloopsystemisasymptoticallystablewiththecontrolapplied atdiscreteinstants.Moreover,intheevent-triggeringtechniquetheinter-sampling behaviourisconsideredforstabilityoftheclosedloopsystem.

Itisworthmentioningherethattheevent-triggeredcontrolschemenotonlyguaranteessystemstabilitybutalsoensurestheconvergenceofinter-samplingbehaviour. Fromtheabove,itisobservedthattheLyapunovfunctiondecreasescontinuously andgoestozeroasthetimetendstoinfinity.Thisisoneoftheimportantproperties oftheevent-triggeredcontrolsystem(ETCS).

1.2.2StabilityofEvent-TriggeredSystems

Inthistechnique,thetriggeringinstantsaregeneratedbythetriggeringruleatwhich thecontrolsignalisupdatedandthisresultstheclosedloopsystemstability.However, itmighthappenthatthecontrolsignalisnotupdatedatthetriggeringinstantwhen triggeringinstantsaretooclosetoeachother.Thisdemandsfastexecutionofcontrol tasks,oreveninworst-casecontinuous-timelikeexecutionwhichisnotpossible bydigitalprocessor.Itmaybenotedherethatinperiodicexecutionsuchsituation doesnotariseduetoeachsampling/triggeringinstantthatoccursaftereveryconstant samplingperiod.

Let {ti }i ∈Z≥0 besequenceoftriggeringinstantsgeneratedbysomestabilizing triggeringrule.Wedefine Ti = ti +1 ti asthe inter-event/executiontime forany giventriggeringsequence {ti }i ∈Z≥0 .Forstabilityoftheevent-triggeredsystem,the inter-eventtimemustbestrictlygreaterthanzero,i.e. Ti > aT forall i ∈ Z≥0 and somepositiveconstant aT .Thisguaranteesthe Zeno-freeexecutionoftriggering sequence.Thepositiveinter-eventtimeensurescontrolisupdatedaftereveryfinitetimeintervalonly.Thisisessentialfortheprocessortoexecutethecontroltaskand updatethecontrolsignal.Inotherwords,itcanbesaidthat {ti }i ∈Z≥0 isanincreasing sequence,i.e. t0 < t1 < t2 < suchthat ti +1 > ti + aT .Suchatriggeringsequence isfeasibleforimplementingthecontrolpracticallytoensurethestabilityofclosed loopsystem.Thetriggeringinstantsgeneratedbysometriggeringrulethatisnot necessarilysatisfyingtheabovepropertywouldmaketheevent-triggeredsystem unstable.

Example1.1 Considerascalarnonlinearcontrolsystemas

˙ x = x 2 + u

where x ∈{[−c , c ]: c ∈ R>0 } whichisacompactset.Anystabilizingcontrollercan bedesignedfortheabovesystemtoensuretheasymptoticstability.Let u =−x 2 kx beafeedbackcontrolwhichensurestheasymptoticstabilityofthesystemwith k > 0. Thiscontrolisappliedtotheabovesystematdiscreteinstantsonlysuchthatclosed loopsystemisstable.So,thediscrete-timecontrolisgivenas

(t ) =−x 2 (

Itcanbeshownthattheclosedloopsystemwiththeabovediscretecontrolis ISSwithrespecttotheerror.Choose V (x ) = 1 2 x 2 .Then,withsomecalculation,we arriveat

Here,weusethefact |x |≤ c and x (ti ) = e (t ) + x (t ).NowapplyingYoung’s inequality2 tothefirstterm(ε = 2c+k k ),weobtain

where a (r ) and γ(r ) aregivenas a (r ) = k 2 r 2 and γ(r ) = (

Thus,thetriggeringruleisdesignedaccordingto(1.4)whichstabilizesthesystem andisgivenby

2 Young’sinequalityforexponenttwostatesthatforanynonnegativerealnumbers p, q andevery ε> 0,thefollowingholds

forsome σ ∈ (0, 1).Thistriggeringruleensures |e (t )| <σ k 2c+k |x (t )| foralltime andthusimpliesthat ˙ V < 0foralltime.Hence,theclosedloopsystembecomes asymptoticallystableevenifthecontrolisappliedatthediscreteinstantsgenerated bythetriggeringsequence {ti }i ∈Z≥0 .Itcanalsobeestablishedthatthetriggeringrule doesnothaveaZenotriggeringsequence.For x ∈{[−c , c ]: c ∈ R>0 }\{0},onecan write

Now,usingthefact |x |≤ c andthecontrollaw u (t ) =−x 2 (ti ) kx (ti ) forall t ∈[ti , ti +1 ) and i ∈ Z≥0 ,onecaneasilyobtainthat

d t x (t ) = x 2 (t ) x 2 (ti ) kx

Usingthisinthefollowing,itcanbewrittenas

Thesolutiontotheabovedifferentialinequalitycanbeobtainedusingcomparison Lemma.Then,thesolutiontotheabovedifferentialcanbeobtainedas

where μ(t ) satisfiesthedifferentialequation ˙ μ = (2c + k )(1 + μ)2 withtheinitial condition |e (ti )| |x (ti )| = μ(ti ) = 0.Then,correspondingtotriggeringmechanismforthis system,thelowerboundoftheinter-eventtimeisobtainedas

Thisshowsthattheinter-eventtimeislowerboundedbyapositivequantitywhich isstrictlygreaterthanzero.Indeed,itisnecessarytoeliminatetheZenoexecution oftriggeringsequenceandultimatelytoguaranteethesystemstability.

Inthenumericalsimulation,thevaluesof c, k and σ areselectedas5,1and 0 85,respectively.Theinitialconditionistakenas x (0) = 4.Theresponseofthe systemisshowninFig. 1.3.Itisseenthatstatetrajectorygoestozeroastimetends toinfinity.Thevaryingsamplingintervalorinter-eventtimegeneratedbyexecuting thetriggeringruleisshowninFig. 1.4.Itisseenthattheinter-eventtimeislower boundedfromzerowhichisgivenby0 0065anditincreasestoavalueashighas 0 072.TheplotofcontrolsignalisalsoshowninFig. 1.5.Asthesamplinginterval increases,thecontrolsignalalsoremainsconstantuntilthenextsamplinginstantis generated.TheplotofLyapunovfunctionisgiveninFig. 1.6.Itisseenthateventtriggeredcontrolimplementationachievesasymptoticstabilityoftheclosedloop systemwithguaranteedconvergenceofinter-samplingbehaviourofthesystem.

1.2.3NeedofEvent-TriggeredControl

Event-triggeredcontrolisoneoftheaperiodiccontrolimplementationstrategiesin digitalplatformthatensuresclosedloopsystemstability.Unlikeperiodicsampling,

Fig.1.3 Responseofthe system

Fig.1.4 Variationof samplingintervalor inter-eventtime

Fig.1.5 Event-triggered controlsignal

Fig.1.6 Timeevolutionof Lyapunovfunction

heresamplinginstantsaredeterminedwheneveritisdemandedsubjecttosystem stability.Otherwise,nocontrolsignalisupdated.So,ifthereisnotriggeringforlong duration,thenthecontrolisnotupdatedwhilemaintainingthestabilityofthesystem. Suchaneed-basedstrategyismoreusefulinresource-constrainedsystemssuchas networkedcontrolsystem,embeddedcontrolsystems.Inthesesystems,frequent statetransmissioninperiodicintervalissometimesnotdesirableduetobandwidth constraintsofthecommunicationchannel.

Intheevent-triggeredcontrolstrategy,thereislesscomputationalburdenthanthat ofperiodiccontrolimplementationsubjecttosomesatisfactorysystemperformance. Thisismainlybecausethecontrolsignaliscomputedonlywhenaneventistriggered. Thisismoreappealinginmanypracticalapplicationstoavoidcontrolcomputation wheneverthestatesdonotchangerapidly.Moreover,itenablesthedigitalprocessors toworkinparallelschedulingtasksbyminimizingtheperiodicexecution.

Anotherimportantaspectofevent-basedtechniqueistheschedulingofsensorsin practicalsystems.Sensorsaretheintegralpartofthecontrolsystemswhichmeasure thestate/outputtrajectoryandtransmitittothecontrolend.Incasethedataistobe transmittedoverlossychannel,itisdesirabletohaveacontrolpolicythatrequires minimalstatetransmission.So,insuchcases,schedulingofsensorsissodesigned thatitcandecidethetransmissioninstant.Event-triggeringstrategyisverymuch helpfulinschedulingthesesensors.

Apartfromthese,thisstrategyissuccessfullydeployedinmanyotherapplications suchasinsignalprocessing,stateestimation.However,thediscussionofthesetopics isthebeyondthescopeofthisbook,sohereonlythecontrolaspectofevent-triggered techniqueisexploited.

1.3SlidingMode:AnIntroduction

Slidingmodehasitsrootinthevariablestructuresystem(VSS)wherethesystem structurechangesduringtheevolutionofthesystemdynamics.InVSS,thestructure ofthesystemischangedorswitchedsuchthattheclosedloopsystemisasymptoticallystable.However,inSMCthesystemtrajectoryisforcedtoremainona predesignedmanifoldbytheactionofdiscontinuouscontrolsignal.Inthiscase,the vectorfieldsonboththesidesofsliding(wealsoreferitas‘switching’)manifold acttowardsthismanifoldandthusmaintainthetrajectoryalongit.Thiseventually ensuresslidingtakesplaceandiscalled slidingmode.Sincethesystemdynamics switchesbetweentwostructureswhile slidingmode isenforced,SMCisreferredas aspecialclassofVSSwhereswitchingtakesplacealongapredesignedswitching manifold.Duetothis,overallsystembecomesdiscontinuousonthismanifold.Itis notnecessarilyrequiredthatslidingmodeisenforcedinthesystemfromthevery beginning,butitmuststartinsomefinite-time.Otherwise,systemcannotbesaidto bein slidingmode.Incontrolleddynamicalsystem,thecontrollawisoftendesigned thatbrings slidingmode inthesystemandiscalledas slidingmodecontrol.This controlisresponsibleforslidingmode.

Toillustratetheconceptofslidingmode,consideracontrollednonlineardynamicalsystemas

Thevectorfield f : Rn × R → Rn givenin(1.5)isLipschitzwithrespectto itsfirstargument.Lettheslidingmanifoldbegivenforanycontinuousswitching function,denotedby s(x ) whichmaps Rn to R,as

Similarly,wealsodefine S + = {x ∈ Rn : s(x )> 0} and S = {x ∈ Rn : s(x )< 0}.Thecontrollawthatbringsslidingmodeinthesystem(1.5)isgiven by

Thecontrolfunctions u + and u arecontinuous,andalso u + = u .Clearly,this impliesthatthecontrol u isdiscontinuouson s = 0.Thecorrespondingresulting vectorfieldsare f + (x , u + ) and f (x , u ) duetothecontrolsignals u + and u , respectively.So,thevectorfield f ( , ) isalsodiscontinuouson S .Theexistenceof solutionsoftheclosedloopsystem(1.5)withcontrollaw(1.6)cannotbeexplained usingtheclassicalexistencetheorem.Indeed,inthiscase,theclosedloopsystem becomesdiscontinuousandisreferredassystemwithdiscontinuousright-handside. Thoughtherearemanytechniquesavailablefordefiningsolutionsofsuchsystems, inthisbookweunderstoodthesolutionsofthesysteminFilippov’ssense[1].

1.3.1DynamicsDuringSlidingMode

Theexistenceofsolutionsofthedynamicalsystem(1.5)isbrieflydiscussedin Filippov’ssense.Thedynamicalsystemisfirstreplacedbyaset-valuedfunction, calleddifferentialinclusion,onthezero(Lebesgue)measuresetwheresolutionisnot definedinclassicalsense.Wedenoteitby F (x , u ).Thus,thedifferentialinclusion forthesystem(1.5)iswrittenas

x ∈ F (x , u ). (1.7)

Theset-valuedmap F coincideswith f ;i.e., F containsonlyoneelement f , wheneverthelatteriscontinuousonitsrespectivedomain.Inotherwords,thesetvaluedmaprepresentsthevectorfieldsofthedynamicalsystematthepointsof discontinuity.Oneofthemainreasonstoreplacethedynamicalequation(1.5)by

1.3SlidingMode:AnIntroduction15

aninclusion(1.7)isthattocaptureallthevectorfieldsofthesystematthepointof discontinuity.Anotherwayofinterpretationisthattheright-handsideisenlarged suchthatallvectorfieldsinthevicinityofmanifoldarecontainedintheinclusion. Oncethatisaccomplished,themostobviousquestionisunderwhatconditionsthe solutionofthesystem(1.7)exists.Beforethat,webrieflydiscusshowtheset-valued mapcanbeconstructedfromandynamicalsystem(1.5)thathasdiscontinuousrighthandside.

Sincetheinclusion,givenin(1.7),capturesallthevectorfieldsinsomesufficiently small δ -neighbourhoodof s(x ) = 0,theset-valuedmapisconstructedbycollecting theconvexcombinationofthevectorfieldsinthatneighbourhood.Thus,thelimiting vectorfieldsinthesmallneighbourhoodofthedomains S + and S forany x ∈ S areobtainedas

Then,theset-valuedmap F (x , u ) isobtainedbycollectingallthevectorfields pointingthelinesegmentjoiningtheendpointsofthevectorfieldsin S + and S , i.e. f + and f .Notethateverytrajectoryin S + crosses S beforereaching S and viceversa.Thisimpliesthatthelinesegmentjoining f + and f alsointersects S . If Tx S representsthetangentvectorof S atthepoint x ,thenthelinesegmentalso intersects Tx S atsomepoint.Thus,thisintersectionpointgivestheendpointofthe vector f 0 (x , u ) suchthat

holdsforthepoint x ∈ S .Similarly,itcanbedefinedforotherpointson S .This givesthemotionofthesystemtrajectoryontheslidingmanifold.Inotherwords, thefunction x (t ) satisfies(1.8)isasolutionofthesystem(1.5).Thisisbecausethis solutionalsosatisfiestheinclusion(1.7)as f 0 iscontainedintheinclusion.Also, f = f 0 and f + = f 0 ;themotionofthesystemremainstangentto S andiscalled slidingmotion.Thus,duringslidingmode,thesystemtrajectoryisasolutionto(1.8) whichisalsoasolutionof(1.5)dueto(1.7).

Now,wediscusshowtofindthevelocityvector f 0 duringslidingmode.Define f + N and fN astheprojectionsofthevectors f + and f ,respectively,ontothenormal tothesurface S atthepoint x ∈ S .Then,thevectorfield f 0 iscalculatedusing (1.8)as

Thisshowsthevectorfieldduringslidingmodeisaconvexcombinationofthe vectorfields f + and f suchthatthetrajectoryistangenttosurface S .Thevalue of θ givenin(1.9)isobtainedfromtherelation

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N Y Evening Post p30 O 23 ’20 270w

Reviewed by B. R. Redman

R of Rs 62:672 D ’20 160w

REPINGTON, CHARLES À COURT.

First world war, 1914–1918. 2v *$12 Houghton 940.48

20–22246

“Colonel Repington in two stout volumes has recorded his ‘personal experience’ of the great war, and in so doing has given to the public the first of the great books of the war that is not simply military, political or diplomatic, but a combination of each that is focused on the personal activity and relationship of a single individual who was behind the scenes and in touch with almost every phase of the war. These pages of personal experiences during the war are a ‘contribution towards the elucidation of the truth so far as I was able to ascertain it at the time, and will, I hope, enable many to understand better the events of these memorable years, ’ Colonel Repington declares. They are given from his diaries as he scrupulously kept them, recording the most trivial incidents innocently tucked away in some social engagement of chance meeting of soldiers, statesmen, journalists, or comments of the larger events which followed each other with such amazing rapidity.”

Boston Transcript

“Colonel Repington is, in fact, so simple that we cannot take any interest in him. His views on the war, in any important sense, are negligible. The only portions of his diary of any interest are his items of political and military information and the light he throws on prominent personages connected with the war. For the rest, and

except when his professional interests are awakened and he gives lists of troops and ‘wastage’ figures, the whole diary is at the gossip level.”

Ath p436 O 1 ’20 1250w

Booklist 17:149 Ja ’21

Boston Transcript p4 O 20 ’20 1150w

“Colonel Repington moves between a bloodbath and a stale spittoon, and is apparently prouder of dipping his pen in the latter than in the former.” Shane Leslie

Dial 59:64 D ’20 1050w

“The book is a curiosity. We have not been able to find in it the slightest evidence that Col. Repington, viewing the supreme tragedy of secular history, was even remotely aware of its human implications. He could observe a world convulsed, and report upon it without compassion, without gravity, without understanding.”

Lawrence Gilman

Freeman 2:499 F 2 ’21 1800w

“As a diarist he is intimate and unaffected and racy and explicit like Pepys, and he is almost as disconcertingly complete.”

Nation 111:786 D 29 ’20 320w

“The self-assurance of Colonel Repington is to be noted. It is to that self-assurance, plus his vanity, that we owe this monumental

New Repub 24:274 N 10 ’20 3500w

“He has produced an extraordinarily interesting gossip-book which will doubtless be widely read and extensively commented upon. It is apparent from the briefest characterization of this amazing book that it is on the delineation of society in the war that the readers will linger longest. It is one long indiscretion.” W. C. Abbott

N Y Evening Post p2 N 27 ’20 1350w

“To an American reader the chief criticism to be made of all these accounts of luncheons, dinners and concerts in the company of the rich and fashionable is that they are intolerably wearisome. Colonel Repington continually speaks of the play of wit in these high circles, but gives very few examples of it.”

+ − + + book. But if we do not get too weary of his ‘practically no English articles are read and discussed except mine,’ we may find illumination most of it unintentional in his accounts of his work running to and fro between the generals, the politicians and the press. ” F. H.

N Y Times p1 O 24 ’20 2400w

Reviewed by E. L. Pearson

Review 3:376 O 27 ’20 340w

“In short, his tendency to take his hostesses overseriously, apart from some waste of space, does little to impair the value of an

enlightening book. His taste may be a bit at fault but rarely his judgment.”

Review 3:559 D 8 ’20 800w

“This is the best book on the war that has appeared, and we hope it is the last. Everybody is sick of the war, its horrors and its squabbles, and wants to forget it. The excellence of the book consists in its twofold claim on our attention. There is the exhaustive criticism of the conduct of the war by a military expert of European reputation: and there is the picture of manners in that section of society ruled by American women, drawn by one who lived in its favour.”

Sat R 130:260 S 25 ’20 1450w

“Go into a shady part of the garden, or better still, into a damp shrubbery and lift up some big flat stone. Underneath you will find a quantity of crawling creatures, disturbed by the light so suddenly let in upon them.... Such a garden adventure recurs irresistibly to the mind as one reads Colonel Repington’s diary of the war years.... As to the enlightenment which his book should bring in regard to the way in which public affairs are too often handled, as to the advantages of the lessons to be learnt, and finally as to the value of this first step in the reform which comes with knowledge, we have no doubt whatever.”

Spec 125:434 O 2 ’20 2900w REPPLIER, AGNES. Points of friction. *$1.75 (4c) Houghton 814

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