Incremental Evolution of Robot Controllers for a Highly Integrated Task

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IncrementalEvolutionofRobotControllersforaHighlyIntegratedTask

AndersLyhneChristensenandMarcoDorigoIRIDIA,CoDE,Universit´eLibredeBruxelles,50,Av.Franklin.RooseveltCP194/6,1050Bruxelles,Belgiumalyhne@iridia.ulb.ac.be,mdorigo@ulb.ac.be

Abstract.Inthispaperweapplyincrementalevolutionforautomaticsynthesisofneuralnetworkcontrollersforagroupofphysicallycon-nectedmobilerobotscalleds-bots.Therobotsshouldbeabletosafelyandcooperativelyperformphototaxisinanarenacontainingholes.Weexperimentwithtwoapproachestoincrementalevolution,namelybehav-ioraldecompositionandenvironmentalcomplexityincrease.Ourresultsarecomparedwithresultsobtainedinapreviousstudywhereseveralnon-incrementalevolutionaryalgorithmsweretestedandinwhichtheevolvedcontrollerswereshowntotransfersuccessfullytorealrobots.Surprisingly,noneoftheincrementalevolutionarystrategiesperformsanybetterthanthenon-incrementalapproach.Wediscussthemainrea-sonsforthisandwhyitcanbedifficulttoapplyincrementalevolutionsuccessfullyinhighlyintegratedtasks.

1IntroductionAutomaticsynthesisofrobotcontrollersisaninterestingfield,whichislikelytosomedaycontributesignificantlytotheadvancementandadoptionofrobotsbyindustryandthegeneralpublic.Techniquessuchasartificialevolutionofcontrollersforautonomousrobotscanfreeusfromhavingtounderstandeverydetailrelatedtomappingsensoryinputstoactuatoroutputs.Instead,wecanfocusonmorehigh-levelaspectsinordertoobtainacontrollercapableofsolvingagiventask.Aroboticssetupwhereartificialevolutioncanbeappliedusuallystartsoffwithoneormorerobotsandsometask.Afitnessfunctionisdefined,which,givenabehavior,assignsanumberreflectingthegoodnessofthatbehaviorwithrespecttothetask.Anevolutionaryalgorithmisthenusedtofindanappropriatecontroller.Thecontrollersthemselvesmayconsistofrulesets,decisiontreesorsimilar,butithasbecomecommontouseartificialneuralnetworks(ANNs)duetotheirversatilityandtolerancetonoisysensoryinput.IfthecontrollerisrepresentedasanANN,anevolutionaryalgorithmcanbeemployedtooptimizetheweights,andpossiblythemorphology,ofthenetwork.Solutionsfoundinthiswaycanexploitsubtleenvironmentalfeaturesastheyareperceivedthroughtherobot’ssensors.Therefore,artificialevolutionmightnotonlybeatime-saving

S.Nolfietal.(Eds.):SAB2006,LNAI4095,pp.473–484,2006.c󰀁Springer-VerlagBerlinHeidelberg2006474A.L.ChristensenandM.Dorigoapproachforsynthesizingcontrollers:bettercontrollersthanthosehand-craftedbyhumandeveloperscanbeobtainedinsomecases[1].Thefieldinwhichevolutionarytechniquesareappliedinordertodeveloproboticshardwareand/orsoftwareiscalledevolutionaryrobotics.Onedirectionofstudiesinthisfieldisconcernedwithcognitivescienceandpsychology[2],whileanotherdirectionfocusesontheuseofevolutionarytechniquesasanen-gineeringtool.Ourinterestfallsinthelattercategory.Wefocusonthefeasibil-ityandefficiencyofdifferentapproachestoautomaticsynthesisofcontrollers.Hence,ourobjectiveistofindevolutionarysetupsthatfrequentlyproducecon-trollerscapableofsolvingagiventask.Thetaskweareconcernedwithistheevolutionofcontrollersforanumberofautonomousmobilerobotscalleds-bots[3].Eachs-bothasavarietyofsensorsandactuators.Amongthese,particularlyimportantisthegripper,whichenablesmultiplerobotstophysicallyconnectandformanartifactcalledaswarm-bot.Inswarm-botformationeachs-botmaintainsautonomouscontrol.Ourobjectiveistoobtaincontrollersforagroupofreals-bots,inswarm-botformation,thatallowthemtosafelynavigatethroughanarenacontainingholes.Thetargetlocationisindicatedbyalightsource.Inourpreviousworkwemanagedtoevolvecontrollersforthecombinedpho-totaxisandhole-avoidancetaskinsimulation,andweshowedthatthecontrollerscouldbetransferredsuccessfullytorealrobots[4].Inthatworkwealsocomparedtheperformanceofvariousnon-incrementalevolutionaryalgorithms:genetical-gorithms[5,6],(μ,λ)evolutionarystrategies[7],andcooperativecoevolutionarygeneticalgorithms[8,9].Wefoundthatthe(μ,λ)evolutionarystrategyingen-eralout-performedtheotherevolutionaryalgorithmswithrespecttothenumberandqualityofthesuccessfulsolutionsfound.Furthermore,wetestedanumberofANNstructuresandfoundthatamultilayerperceptronwithahiddenlayeroftwoneuronsissufficienttorepresentsuccessfulsolutionsthatcanbetrans-ferredtorealrobots.Forthestudypresentedinthispaper,weusetheneuralnetworktopology,thefitnessfunctioncomponents,andthe(μ,λ)evolutionarystrategy,whichwepreviouslyfoundbethehighestperformingwhileresultingintransferablecontrollers[4],[10].Inthefollowingsectionwediscusswhatincrementalevolutionisandprovideexamplesofstudiesinwhichthistechniquehasbeenappliedinthefieldofau-tonomousrobots.ThetaskandtherobotichardwareareexplainedinSection3and4.OurapproachandexperimentalsetupisdiscussedinSection5.InSec-tion6,ourresultsarepresented,discussed,andcomparedtoresultsobtainedinourpreviouswork.