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Robotsgraspandmanipulateobjectswiththeaidofagripper.UsuallytheobjectispresentedCompliancecanbeintroducedtotherobotbyusingacompliantendeffectororgripper.Thiscanbedoneindifferentways.Inliteratureavarietyofsubjectsongrippercompliancecanbefound1,3,4,thesearemainlyfocussedoncontroltheoriesforuniversalgrippersandfineMechanismandMachineTheory3820031509–1522Correspondingauthor.Tel.31152783130fax31152781397.atapredefinedpickuplocationwheretherobotcangraspandmoveittoanotherpredefinedlocation.Difficultiesarisewhenthepickuplocationanddestinationarepartofaheavyrigidbodythatcanmoveduetoexternaldisturbances.Iftherobotcannotadapttothismovementduringpickuporrelease,thefullforceofthemovementwillbetransferredintotherobotC213scomponents,whichcanresultindamage.Thereforetherobotshouldbeflexibleorcompliantwheretheenvironmentisstiff3.ComplianceeffectsinaparalleljawgripperA.J.G.Nuttall,A.J.KleinBretelerFacultyofDesign,ConstructionandProduction,DepartmentofTransportationTechnology,UniversityofTechnologyDelft,Mekelweg2,2628CDDelft,TheNetherlandsReceived17April2002receivedinrevisedform24April2003accepted30June2003AbstractThispaperdiscussesmechanicalcomplianceeffectsinagripperwithparalleljaws.Initacasestudyofadedicatedgripperdesignispresentedtoanalysetwodifferentdesignelementsinfluencingthecompliantbehaviourtheflexibilityintroducedbypreloadedspringsandtheresistancecausedbyfriction.Thegrippermanipulatessemiautomatictwistlocksusedforsecuringseagoingcargocontainers.Thecomplianceeffectsareeffectivetoreducemisalignmentandoverloadofthegripper.C2112003ElsevierLtd.Allrightsreserved.KeywordsMechanicalcomplianceTwistlockmanipulatorPreloadedspringsFrictionforce1.Introductionwww.elsevier.com/locate/mechmtEmailaddressa.j.kleinbretelerwbmt.tudelft.nlA.J.KleinBreteler.0094114X/seefrontmatterC2112003ElsevierLtd.Allrightsreserved.doi10.1016/S0094114X03001009manipulation.In5,6suchacomplianceisinvestigatedusingstiffnessmodelsandin7aremotecompliancecentreisintroduced.Theseinvestigationsintegratecomplianceintothecontrolsystem,withtheaidofspecialsensorsandactuatorsmakingreliableforceandpositioncontrolpossible.Withthisformofelectroniccontroltheuniversalgrippercanmanagemanydifferenttasksandobjects.Thisisincontrasttothespecialpurposeendeffectorthatistobedesignedforaspecifictaskandobject.Bymakinguseofsimplesensorsandactuatorscombinedwithamechanicalformofcomplianceaneffective,reliableandrobustgrippercanresult,whichwillalsobeabletoadaptallbeitinalimitedmannertoamovingpickuppoint.Theadaptationofthisformofcomplianceforgripperconfigurationshasprovenhardtofindinliterature.Thispapergivesaninsightintotheeffectsofmechanicalcomplianceinagripper.Acasestudyofagripperdesignwillaidasexampletodiscusstwodifferentmodesofmechanicalcompliance.Thisexamplecaseconsistsofaparalleljawgripperconfigurationintendedforthemanipulationofsemiautomatictwistlocks.operation.Ifitispulledtheshaftrotatesthatconnectstheconestogether.1510A.J.G.Nuttall,A.J.KleinBreteler/MechanismandMachineTheory3820031509–15222.BackgroundtothetwistlockmanipulatorAmanipulatorwasrequiredtoautomaticallyconnectandremovesemiautomatictwistlockstoandfromacontainerC213sbottomcornercastings.InFig.1asemiautomatictwistlockisshownontheleft.Thistypeoftwistlockisalashingdevicethatisusedtosecureseagoingcargocontainerstothedeckofaship.Itconsistsofabody,anupperandlowerrotatingconeandahandleformanualoperationoftheconepositions.TheupperconecanbeinsertedintothebottomcornercastingdepictedontherightsideofFig.1byunlockingitthroughrotatingthelowercone.Thetopcollarfixesintotheholeofthecornercasting,becauseitmatchestheshapeofthehole.Whentheconesarerotatedbacktotheiroriginalposition,thetwistlockissecuredtothebottomcornercasting.ThehandleisintendedformanualFig.1.Asemiautomatictwistlockandacornercastingofacontainer.Forautomationofthissecuringprocedureandthereverseoperationagripperhadtobedesignedthatcanholddifferenttypesoftwistlocksbytheircollarswithsufficientgraspingforce10.Thejawsalsohavetoopenfarenough,topreventcollisionswiththeconeswhilethemanipulatorispositioningoverthetwistlockwithopenjaws.Thecontainercanmoveduringthepickuporreleaseoperationduetoexternaldisturbances,becauseitwillbehoistedupintheairbycablesorrestingonarollingchassiswithpneumatictyres.Thewindisanexampleofadisturbancethatcangeneratefluctuatingforcesonthesideofthecontainer,whichcanresultinanoscillatingmovement.Duetothepossiblemovementofthelargecontainermass30tonandtherobustconstructionofthetwistlockthegripperwillhavetobecomplianttopreventdamagetoitselforothercomponentsoftherobot.Mechanicalcompliancewillalsohelptackletheproblemofthemovingpickuppointonthecontainerandkeeptherequiredcontrolsystemsimple.collarsizeshadtobetakenintoaccountforareliableoperationofthegripper.IngeneratedA.J.G.Nuttall,A.J.KleinBreteler/MechanismandMachineTheory3820031509–15221511namicforcescreatedonthetwistlock.ThetotalforceFtotthathastobecompensatedinadirectionparalleltothecollarsurfaceis200N.Thiswascalculatedbydeterminingthedynamicforcescausedbymovementofthemanipulatorandthestaticgravitationalforce.Withafrictionalcoefficientlof0.125thegraspingforceexertedbyeachjawFjawcanbecalculatedasfollowsFtot¼2C1lFjawFjaw¼Ftot2l¼800NThisistheminimalforcethathastobeguaranteedduringthemanipulationofatwistlock,forallcollarsizes.Fig.2bshowstheopenandclosedpositionofthejaws.Itshowshowfarthejawshavetoopen,duringthepositioningoftheopengripper.TherehastobeenoughclearancebetweentheconeFig.2atheforcesappliedbythejawsduringagrasparepresented.ThefrictionalforcesonthecollarsideswillhavetobesufficientlylargetocompensatethestaticanddyThecollarwaschosenascontactsurfaceforthegripper,becauseitisthecommonelementindifferenttwistlockdesigns.Ithastofitintothestandardisedholeofthecornercasting,sotheshapeandsizewillberoughlythesame.Althoughthewidthoftheholeisonlyallowedatoleranceof1.5mmthecollarwidthsfoundinpracticecanvarybetween57and62mm.This5mmrangeinFig.2.Graspingforcesontwistlockandtherequiredjawtravel.andjawtopreventacollision,becausetheconediameterislargerthenthecollarwidth.Togetaclearanceof15mmthedisplacementofajawhastobe40mm.3.FindingasuitablegripperconfigurationAnexistinggripperthatiscapableofproducingaratherlargeclampingforceandlargedisplacementisillustratedinFig.3a2.Itconsistsoftwoparalleljaws,actuatedbyadoubleactingpneumaticcylinder.AttachedtothecylinderC213spistonrodisadualrackgear,whichdrivestwopartialsectorsofpiniongears.Twopairsofthesymmetricalarrangedparallelclosinglinkagesaremounteddirectlyonthepartialsectorsofthepinionsandprovidetheclampingforce.Thisdesignonlyfeaturescompliantbehaviourwithrespecttothewidthofthegraspedobject.Ifthegraspedobjectislargerthenthedistancebetweentheclosedjaws,theywillcomeincontactwiththeobjectbeforetheyarefullyclosed.Thereforethepistonwillnottraveltoitsendpositionduringthisclosingoperation.Thismakesitpossibletograspdifferentsizedobjects.Itwillhoweverbemoredifficulttosensetheclosedpositionofthejaws.Aspecialsensingmethodlikeforcedetectionwillberequiredtomeasuretheclosedposition.1512A.J.G.Nuttall,A.J.KleinBreteler/MechanismandMachineTheory3820031509–1522ThegripperconfigurationofFig.3acanbegivenadditionalcomplianceasshowninFig.3b.Preloadedspringshavebeenaddedtothejaws,togetcompliantbehaviourinthehorizontaldirection.Preloadingthespringsgivestwoadvantages.Firstofallthestrokerequiredtobuildupsufficientgraspingforcecanbeshort.Ifthepreloadissettotheminimalrequiredgraspingforce,aftercontactwiththeobjectthespringshardlyneedanytravelforasecuregrip.Secondlytheminimalrequiredgraspingforcecanbeguaranteedwiththeaidofaproximitysensorthatcandetecttheendpositionofthepneumaticcylinder.Ifthecylinderreachestheendoftheclosingstrokewithanobjectbetweenthejaws,thespringswillhavebeenpressedinandthegraspingforcewouldatleasthavetobeequaltothesetpreload.Fig.3.Parallelgripperconfigurationswithcompliance.
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