Design-and-Implementation-of-ARCHONs-Coordination-

1DESIGNANDIMPLEMENTATIONOFARCHON’SCOORDINATIONMODULENickR.JenningsandJ.A.PopleDept.ElectronicEngineering,QueenMary&WestfieldCollege,UniversityofLondon,MileEndRoad,LondonE14NS,UK.{n.r.jennings,j.pople}@qmw.ac.ukABSTRACTThispaperdescribesthedesignandimplementationofadomain-independentreusablecoordinationmodule.ThismoduleisattheheartoftheARCHONarchitectureandhasbeenusedinthedevelopmentofcooperatingmulti-agentsystemsinanumberofreal-worldindustrialapplicationsincluding:electricitydistributionmanagement,electricitytransportationmanagement,cementfactorycontrol,flexibleassemblyroboticcellsandparticleacceleratorcontrol.Themoduleisbaseduponthephilosophyofprovidingacor-pusofextensiblegenericknowledgeaboutcooperationandsituationassessment.Specialprominenceisgiventotheproblemofcontrollingthereasoningwithinthecoordinationmodule.1.INTRODUCTIONSystemscomposedofmultiple,interactingcomponents(agents)arebecominganincreasinglypopularmeansofbuildingcomplexindustrialcontrolapplications.Themajorityofthesesystemsarefunctionallydistributedandhavesubcomponentswhichareorderedinsomehierarchicalfashionwithclear,predefinedcommunicationlinks.Althoughthismodularapproachincreasesthemaintainabilityofthesystem,itkeepstheoverallcontrolatacentrallocation(i.e.aglobalcontrollercoordinatestheactivitiesofallthesubcomponents).Thiscentralisationofcontrolhastwoparticulardrawbacksforindustrialcontrolapplications(Jennings&Wittig,1992).Firstlyforlargeapplica-tionswithanumberofdistinctsupervisoryandcontrolsubcomponents,theactivationoftasksinthesub-systemsandthedecisionofwhatdatatoexchangebetweenthemdependsonthestateoftheentireprocess.Inacentrallycontrolledsystemthisassess-mentrequiresthecontrollertotakeintoaccountthedifferentviewsofalltherelevantsub-systemsandcan,therefore,leadtoseveredelayswhiletherelevantinformationisassembledandtheappropriatedecisionsaretaken.Secondlyitisdifficult(sometimesimpossible!)toperformthemodificationsrequiredtointegratethelargenumberofpre-existing(legacy)systemswhichareoftenfoundinindustrialapplicationsintooneunifyingwhole.Toalleviatethedecision-makingbottleneck,increasetheflexibilityofdataexchangeandtaskactivationandfacilitatesoftwarereuse,thenextstageinsystemdesignistodecentralisethecontrolandallowthecomponentstointeractdirectlywithoneanother.Thisapproachnotonlyallocatesmoreresponsibilitytothesub-systems,butalso2requiresthemtocoordinatetheirtasksifthewholesystemistointeractinacoherentmanner.Suchcoordinationcanbehand-craftedforeachandeveryapplicationoritcanbeundertakeninamorestructuredmannerbydevelopingaframeworkwhichcanbere-usedinanumberofdifferentscenarios(theapproachdescribedinthispaper).TheARCHON(ARchitectureforCooperativeHeterogeneousON-linesystems)frame-work(Wittig,1992),whichprovidesthecontextforthiswork,hasbeenusedtobuildcooperative,multipleagentapplicationsinthedomainsof:electricitydistributionmanagement(Vargaetal.,1994),electricitytransportationmanagement(Wittig,1992,ch.8);cementfactorycontrol(StassinopoulosandLembesis,1993);flexibleassemblyroboticcells(Oliveiraetal.,1991)andparticleacceleratorcontrol(Jenningsetal.,1993).WithintheARCHONframeworkeachagentiscomposedofanumberoffunctionalcomponents,oneofwhichisresponsibleforcoordinationinadecentralisedenviron-ment.DuringthedesignanddevelopmentofthisPlanningandCoordinationModule(PCM)anumberofcrucialissuesneededtobeaddressed:(i)whataretherequirementsforcoordinationinlarge,real-worldindustrialapplications?(ii)whattypesoffacilitiesshouldageneral-purposeframeworkprovidetoanapplicationdeveloper?(iii)howcanthereasoningofthecoordinationmodulebecontrolledsothattheagent’sobjectivesaresatisfied?(iv)howcanthecoordinationmodulebedesignedsothatitrespondsrap-idlytoimportanteventsbutalsodealswitheventsinafairmanneravoidingresourcestarvation?(v)howcanagenericcoordinationmodulebetailoredtofitaparticularapplication?(vi)howcansuchacoordinationmodulebeimplementedsothatitmeetstheaforementioneddesiderata?ThispaperdescribeshowtheaboveissuesweretackledandsolvedwithintheARCHONframework.Theseexperiencesandinsightsareimportantforanumberofdifferentreasons.FromtheperspectiveofDistributedArtificialIntelligence(DAI)thisworkrepresentsoneofthefirstseriousattemptstobuildagenericcooperationframe-workforlargescale,real-worldindustrialapplications.Fromtheperspectiveofindustrialcontrolapplications,thisworkhighlightsthefeasibilityofemployingacooperatingsystemsmetaphorandenablestheproblemsassociatedwithbuildingdecentralisedcontrolsystemstobeclearlystatedandevaluated.SectiontwopresentsabriefoverviewoftheARCHONarchitecturesothattheworkonthePCMcanbeplacedincontext.Sectionthreedetailsthephilosophyofre-usablegenericknowledgewhichliesbehindthePCMandsectionfourdescribesitsimple-mentationasanobject-orientedrule-basedsystem.2.STRUCTUREOFANARCHONAGENTARCHONagentshavetwodistinctcomponents;anIntelligentSystem(IS)andanARCHONLayer(seefigure1).Theformermaybepre-existingormaybepurposebuiltandsolvesdomain-levelproblemssuchasdetectingdisturbancesinelectricitynet-worksorcontrollingtheblowerofacementfactorykiln.InthemajorityofAR