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HexapodtutorialInthistutorialwewillbebuildingahexapodwalkingrobot.MakesurethatyouhavereadtheBubbleRobtutorialandthetutorialonimportingandpreparingrigidbodiesbeforestartingwiththisone.TheVREPCADdata,courtesyofLyallRandell,relatedtothistutorial("hexapod.dxf")islocatedinVREP'sinstallationfolder's"cadFiles"folder.AfinishedmodelrelatedtothistutorialcanbefoundinVREP'smodelbrowser.Click[Menubar>File>Import>Mesh...]thenselectthefiletoimport.Alsorefertothesectiononhowtoimport/exportshapes.Adialogpopsopenaskingaboutmeshscalingandmeshorientation.ClickOk.Severalshapeswereimportedandarelocatedinthemiddleofthescene.Theshapesalsoappearinthescenehierarchyonthelefthandsideofthemainwindow.DependingonhowtheoriginalCADdatawasexported,theimportedCADdatacouldbeatadifferentscale,differentlocation,orevengroupedasasingleshape.Theassignedcolorofimportedshapesisrandom.Followingfigureshowstheimportedshapes:

Asyoucansee,thehexapodrobothas6identicallegs.Soinsteadofpreparingeachlegindividually,wewillprepareoneleg,andwhenitisdone,justcopyandpasteittotherightlocations!Selectinthescenetheshapestodelete(usethectrlclickand/ortheshiftclickfortheselectionprocess),thenpressthedeletekey(makesureyoukeptthelegpointingtowardstheworldxaxis):

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Theimportoperationhasleftuswith3shapesperleg,sowewillneedtosubdividethose3shapesfurther,inordertohaveseparateshapesfortheleglinksandthelegservomotors.Selectthe3shapesthenclick[Menubar>Edit>Grouping/Merging>Divideselectedshapes].Youcanseethatthisoperationhasdividedtheshapeclosesttotherobotbodyinto3subshapes,whereweonlyexpected2subshapes.Tocorrectforthis,selecttheleftandrightlinkelementsasshowninfollowingfigure:

Whilethetwoshapesarestillselected,click[Menubar>Edit>Grouping/Merging>Mergeselectedshapes]inordertomergethemintoasingleshape.

Innextstep,wewilladdthejointsrequiredforourlegmechanism.Onewayofdoingthisistoaddthe

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jointsintothescene,thenspecifytheirappropriatepositionandorientation(throughthecoordinateandtransformationdialog).Thisishowevernotpossible,whenyoudon'tknowtheexactjointpositionsasinourcase,andsowewillhavetoextractthemfromtheshapesthatwehave:

Selectthe3shapesthatrepresenttheservomotors,copythem(ctrlcor[Menubar>Edit>Copyselectedobjects]),thencreateanewscenewith[Menubar>File>Newscene],thenpastetheshapes(ctrlvor[Menubar>Edit>PasteBuffer]).Wearenowinanotherscene(scene2),andwhateverwedoherewillnotaffectouroriginalscene.Nowkeepthe3servomotorsselectedandclick[Menubar>Edit>Grouping/Merging>Mergeselectedshapes]tomergethemasasingleshape.Thenenterthetriangleeditmode.Alltrianglescomposingourshapearenowdisplayedandcanbemanipulated.

Switchtopage2andmoveyourcameraclosertotheshapewiththefittoviewtoolbarbutton:

Shiftselectalltrianglescomposingoneoftheservomotors'outputshaftasillustratedinfollowingfigures:

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ThenclicktheExtractshapebuttonintheshapeeditmodedialog.Dothesamewiththetwootherservomotoroutputshafts(forthethirdservomotor,youwillhavetoswitchtopage3tobeabletoshiftselectitsoutputshaft):

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Switchbacktopage1,leavetheeditmode,thendeletetheshaperepresentingthe3servomotors.Nowtheextractedshaftsbecomeclearlyvisible:

Wecanusetheextractedshapestopreciselypositionourjoints.Click[Menubar>Add>Joint>Revolute]toinsertarevolutejointintothescene.Thedefaultpositionisat(000)anditsdefaultorientationisvertical.Whilethejointisstillselected,ctrlselecttheshaperepresentingtheverticaloutputshaft,thenopenthepositionandtranslationdialogandclicktheApplytoselectionbuttonatthebottomofsection2.

Thisjustpositionedthejointattheexactsamecoordinatesastheverticaloutputshaft!Nowrepeatthe

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procedurewithtwoadditionaljointsandthetwohorizontalshafts.Alljointsareinpositionnow,however,onlythefirstaddedjointhasthecorrectorientation.Selectthetwolastaddedjoints,thenenter(90,0,0)fortheAlpha,BetaandGammaitemsinsection2oftheorientationandrotationdialog,thenclicktherelatedApplytoselectionbutton.Alljointshavetherightpositionandorientationnow.Selectall3joints,copythemtothebuffer(ctrlc),switchbacktotheinitialscene(scene1),thenpastethebuffer(ctrlv).Thisiswhatyoushouldhave:

Youcannowadjustthejointsizes(checktheJointlengthandJointdiameteritems)inthejointpropertiesdialog(thatyoucanopenbydoubleclickingajointiconinthescenehierarchy).Additionally,wewantthe2horizontaljointstobelocatedatycoordinate0,andtheverticaljointtobelocatedatazcoordinatesimilartoitsservomotor(usethecoordinateandtransformationdialogfortheseoperations):

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Renameyourjointsto"hexa_joint1","hexa_joint2"and"hexa_joint3"(frombodytofoot).Youcandothisbydoubleclickingthejoint'snamesinthescenehierarchy.Nowlet'ssettheinitialjointvaluessothatwhenthelegishorizontallystretched,alljointvaluesarezero:inthejointdialog,itemPosition,set30and+120for"hexa_joint2"and"hexa_joint3".Noticehowjointsareintorque/forcemode.Inourcasewewanttobeabletocontrolthosejointsininversekinematicsmode,butwealsowanttheinversekinematicsresultstobeappliedasdynamicspositioncontrolvalues.Todothis,wewillsetthejointsininversekinematicsmode(select"jointisininversekinematicsmode"intheJointmodesection).Inadditiontothat,wealsoactivatetheHybridoperationforthatjoint(thistellsVREPthattheinversekinematicscalculationresultswillbeappliedtothejointasdynamictargetvalue).ClickApplytoselectiontoapplythechangesyoujustperformedtotheothertwojointsthatareselected.

Atthispoint,weshouldpreparethevisualappearanceofourshapes,groupthemifrequired,andpreparecorrespondingpureshapesforthedynamicsimulation.Detailsonhowthisisdonewillnotbeexplainedhere,sincetheprocedureisverysimilartowhatisdoneinthetutorialonimportingandpreparingrigidbodies.Onceyoufinishedthisstep,thisiswhatyoushouldhave(visibleandhiddenparts):

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Theabovesceneis"tutorials\Hexapod\hexapodintermediatestep.ttt".Noticefollowing:

theobjectshavebeenappropriatelynamed/renamed.thehexapod'sbodyusedfordynamics("hexa_bodyRespondable")wassimplifiedas3cuboids(the

spaceinbetweenhasbeenfilled).Thetipofthelegisrepresentedasapuresphere,sincepurespheresaremoststable/fastfor

dynamicsimulations.Themasses,inertiaproperties,etc.havebeensetforbestperformance.Italwaysrequiressome(lots

of!)testingtofindgood(i.e.stable)parameters.Consecutivelinkelementswon'tcausecollisionresponses(theRespondablemasksintheshape

dynamicspropertiesdialoghavebeensetaccordingly).

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Next,wewillbelinkingcurrentelementsofthemechanism.Select"hexa_link3Respondable"then"hexa_joint3",andclick[Menubar>Edit>Makelastselectedobjectparent].Dothesamewith"hexa_joint3"and"hexa_link2Respondable","hexa_link2Respondable"and"hexa_joint2","hexa_joint2"and"hexa_link1Respondable","hexa_link1Respondable"and"hexa_joint1",andfinally"hexa_joint1"and"hexapod".Wehaveassembledthefirstleg.Startthesimulationandobservehowrigidobjectsareconstrainedbythejoints.Noticealsothatjointsarenotkeepingafixedposition,butslowlydrifting:thisisbecausewehaven'tyetdefinedaninversekinematicstaskforourkinematicchainrepresentingtheleg.Thisiswhatwewillbedoingnext.

Addadummyobjectwith[Menubar>Add>Dummy].Renameitto"hexa_footTip".Nowpositionthedummyatthetipoftheleg,wherethelegiscontactingthefloor.Youcanachievethisbyusingtheobjectmanipulationtoolbarbuttons,orbyusingthecoordinateandtransformationdialog.Informercase,itcanbeusefultoadjustthepositionstepbystepbyswitchingtoorthogonalprojectionviewsonpage2,page3,page4,page5orpage6.Oncedone,thedummy'sabsolutepositionshouldbecloseto(0.204,0,0).Next,attachthedummyto"hexa_link3"(dragthedummyonto"hexa_link3"inthescenehierarchy).Thencopyandpaste"hexa_footTip"andrenamethecopyto"hexa_footTarget".

Whatwewantatthisstage,istohave"hexa_footTip"follow"hexa_footTarget",whilehavingtheattachedmechanismautomaticallyadjustforthat(i.e.whilehavingthenewjointpositionsautomaticallycalculated).Wehavetodefineaninversekinematictask:

First,let'sinformthetwodummiesthattheyareatiptargetpairforinversekinematicsresolution.Doubleclickthedummyiconof"hexa_footTip"inthescenehierarchy:thisopensthedummypropertiesdialog.IntheDummydummylinkingsection,specify"hexa_footTarget"asLinkeddummy.Noticehowbothdummiesgotlinkedthrougharedstippledlineinthescenehierarchy(thetwodummiesarealsolinkedinthescenethrougharedline,butsincebothdummiesarecoincident,thelinecannotbeseen).Inthesamedialog,Linktypeisalready"IK,tiptarget",whichisthedefaultvalue.Thisiswhatyoushouldhavebynow:

Nowopentheinversekinematicsdialog,andclickAddnewIKgroup.AnewitemappearsintheIKgroupslist:"IK_Group".Whilethatitemisselected,opentheIKelementdialog(clickEditIKelements)andindicate"hexa_footTip"inthedropdownboxrighttoAddnewIKelementwithtip.ThenclickAddnewIKelementwithtip.Closethedialog,andattach"hexa_footTarget"to"hexapod".Ourinversekinematicstaskisreadyforthisleg!Let'stestit.

First,temporarilydisabledynamicsbyopeningthegeneraldynamicspropertiesdialog,thenunchecking

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Dynamicsenabled.Next,startthesimulationandinthescenehierarchselect"hexa_footTarget".Movethe"hexa_footTarget"aroundwiththeobjectmanipulationtoolbarbuttons:thelegshouldfollow.Stopthesimulationandenabledynamicsagain.

Wewillcontrolthehexapodrobotbygeneratingthemovementforoneleg,andapplyingitinadelayedwayforall6legs,whereeachlegwillhaveadifferentdelay.Weimplementthisbyhavingonechildscriptgeneratingthemovementsequenceforoneleg,andhaving6additionalchildscriptsapplyingthatmovementsequenceinadelayedmanner.Select"hexa_joint1"andclick[Menubar>Add>Associatedchildscript>Nonthreaded].Wejustattachedachildscripttothe"hexa_joint1"object.

Nowwewillduplicatetheleg5times.Selectallobjectscomposingtheleg(hiddenobjectsaswell):from"hexa_joint1"to"hexa_footTip".Alsoselect"hexa_footTarget".Makesurethat"hexa_bodyRespondable"and"hexa_body"arenotselected,thencopyandpastetheobjects.Opentheorientationandrotationdialogandinsection3enter"60"fortheAroundZitem(keepingtheAroundXandAroundYitemsat0).MakesurethetransformationwillberelativetoWorld,thenclickRotateselection.Pastetheinitialleganothertime,thenadjusttheAroundZitemto120,thenclickRotateselectionagain.Repeattheprocedurefortheremaining3legs(makesureyouadjusttherotationangleto180,240andfinally300degrees).Thisiswhatyoushouldhave:

Let'sattachalllegstothebody.Select"hexa_joint1#0"through"hexa_joint1#4",then"hexapod",andclick[Menubar>Edit>Makelastselectedobjectparent].Inthescenehierarch,collapsealllegtreesbyclickingtheir""icon.Thenselect"hexa_footTarget#0"through"hexa_footTarget#4",then"hexapod",andclick[Menubar>Edit>Makelastselectedobjectparent].

Opentheinversekinematicsdialog.Noticehowtheinversekinematicstaskswerealsocopied.Thisiswhatyoushouldhave:

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Closetheinversekinematicsdialog.Opentheobjectselectiondialogwith[Menubar>Tools>Selection],orbypressingtheappropriatetoolbarbutton.ClickClearSelection,thenDummies:0/13.Alldummiesinthescenehavebeenselected.Deselect(withctrlclick)thedummythatisnotpartofthehexapodrobot.Wenowhaveour12dummiesonthehexapodselected.Nowopentheobjectcommonproperties.IntheVisibilitylayerssection,disablelayer3andenablelayer11,thenclicktherelatedApplytoselectionbutton.Thisjustsentalldummiestothevisibilitylayer11,effectivelymakingtheminvisible.Havealookatthelayerselectiondialogifyouwishtotemporarilyenable/disablesomelayers.

Next,sendalljointstothevisibilitylayer10byfollowingthesameprocedureasabove.

Thenaddanewdummytothesceneandrenameit"hexa_base".Alsosendittothevisibilitylayer11.Thenmakeitchildof"hexapod"."hexa_base"representsthepositionofourhexapodrobot,thatweselectedtobe(000).

Nowlet'sdefinethehexapodmodel.Asyoumighthavenoticed,whenyouclickanobjectonthehexapodrobot,onlythatobjectgetsselected.Butwhatwenowwantistoprotectindividualobjectsfrommodification,andselectthewholerobotinstead.Foreachobjectcomposingthehexapodrobot,exceptfortheobject"hexapod",enabletheSelectbaseofmodelinsteaditemintheobjectcommonpropertiesdialog.Cleartheselection,thenselect"hexapod".Inthesamedialog,enabletheObjectismodelbaseitem.Nowclickanyobjectonthehexapodrobot:thewholerobotnowgetsselectedinstead:

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Nowopenthepositionandtranslationdialogandinspectitssection1(makesurethat"hexapod"isselected).Youcanseethattranslationof"hexapod"withthemousehappensinanxyplaneoftheabsolutereferenceframe.Youcanchangethedefaultmousetranslationbehaviourfortheselectedobjectbyadjustingthevariousitemsinsection1(ifyoudonotclickRemembersettings,thetranslationbehaviourisjusttemporary,otherwiseitisstoredintheselectedobject).Makesuretoalsoinspectsection1oftheorientationandrotationdialog,whereasimilarmechanismishandlingthemouserotationmode.

Whenyounowtrytoshiftorrotatetherobotinthescene(usingtheobjectmanipulationtoolbarbuttons),itwillalwaysstayonthefloor,andkeepaproperorientation.Tryit!(thenmakesureyouresetittoitsinitialposition/orientationwiththeundotoolbarbutton).Ifyoupressthectrlkeyduringthetranslationoperation,youcanmovetherobotupanddown.

Nowattachanonthreadedchildscriptto"hexapod".Createanotherscene(scene3),andopenthe"hexapod.ttm"modelfile.Doubleclickthechildscripticonofobject"hexapod"toopenthescripteditor.Copythescript,switchbacktotheoriginalscene(scene1),doubleclickthechildscripticonofobject"hexapod"andpastethescript.Repeatthesameprocedureforthechildscriptsassociatedwitheachleg.Noticehowallchildscriptsassociatedwiththehexapod'slegsareexactlythesame.Alastelementisstillmissing:weneedeachlegtoapplythemovementsequencewithadifferenttimedelay.Ifyoutakeacloserlookatthelegs'childscripts,theyareeachreadingadelayvaluefromtheirscriptsimulationparameterswiththeinstruction:

modulePos=simGetScriptSimulationParameter(sim_handle_self,'modulePosition')

Youcanopenthescriptsimulationparameterdialogbydoubleclickingtheiconontherighthandsideofachildscript.Addtheexactsamescriptsimulationparametersasthehexapodintheotherscene.Runthesimulation.Thehexapodrobotshouldnowwalk.

Usingascripttocontrolarobotormodelisonlyonewayofdoing.VREPoffersmanydifferentways(alsocombined),havealookattheexternalcontrollertutorial,orattheplugintutorial.