ModernMechanicalEngineering,2011,1,47-55doi:10.4236/mme.2011.12007PublishedOnlineNovember2011(http:/www.SciRP.org/journal/mme)Copyright2011SciRes.MMEDesignandDevelopmentofaCompetitiveLow-CostRobotArmwithFourDegreesofFreedomAshrafElfasakhany1,2,EduardoYanez2,KarenBaylon2,RicardoSalgado21DepartmentofMechanicalEngineering,FacultyofEngineering,TaifUniversity,Al-Haweiah,SaudiArabia2TecnolgicodeMonterrey,CampusCiudadJurez,CiudadJuarez,MexicoE-mail:ReceivedOctober19,2011；revisedNovember7,2011；acceptedNovember15,2011AbstractThemainfocusofthisworkwastodesign,developandimplementationofcompetitivelyrobotarmwithen-hancedcontrolandstumpycost.Therobotarmwasdesignedwithfourdegreesoffreedomandtalentedtoaccomplishaccuratelysimpletasks,suchaslightmaterialhandling,whichwillbeintegratedintoamobileplatformthatservesasanassistantforindustrialworkforce.Therobotarmisequippedwithseveralservomotorswhichdolinksbetweenarmsandperformarmmovements.Theservomotorsincludeencodersothatnocontrollerwasimplemented.TocontroltherobotweusedLabview,whichperformsinversekinematiccalculationsandcommunicatestheproperanglesseriallytoamicrocontrollerthatdrivestheservomotorswiththecapabilityofmodifyingposition,speedandacceleration.Testingandvalidationoftherobotarmwascarriedoutandresultsshowsthatitworkproperly.Keywords:RobotArm,Low-Cost,Design,Validation,FourDegreesofFreedom,ServoMotors,ArduinoRobotControl,LabviewRobotControl1.IntroductionThetermroboticsispracticallydefinedasthestudy,designanduseofrobotsystemsformanufacturing1.Robotsaregenerallyusedtoperformunsafe,hazardous,highlyrepetitive,andunpleasanttasks.Theyhavemanydifferentfunctionssuchasmaterialhandling,assembly,arcwelding,resistancewelding,machinetoolloadandunloadfunctions,painting,spraying,etc.Therearemainlytwodifferentkindsofrobots:aser-vicerobotandanindustrialrobotic.Servicerobotisaro-botthatoperatessemiorfullyautonomouslytoperformservicesusefultothewell-beingofhumansandequipment,excludingmanufacturingoperations2.Industrialrobot,ontheotherhand,isofficiallydefinedbyISOasanauto-maticallycontrolledandmultipurposemanipulatorpro-grammableinthreeormoreaxis1.Industrialrobotsaredesignedtomovematerial,parts,tools,orspecializedde-vicesthroughvariableprogrammedmotionstoperformavarietyoftasks.Anindustrialrobotsystemincludesnotonlyindustrialrobotsbutalsoanydevicesand/orsensorsrequiredfortherobottoperformitstasksaswellasse-quencingormonitoringcommunicationinterfaces.In2007theworldmarketgrewby3%withapproxi-mately114,000newinstalledindustrialrobots.Attheendof2007therewerearoundonemillionindustrialro-botsinuse,comparedwithanestimated50,000servicerobotsforindustrialuse3.Duetoincreaseusingofindustrialrobotarms,anevo-lutiontothattopicbegantryingtoimitatehumanmove-mentsinadetailmode.ForexampleagroupofstudentsinKoreamadeadesignofinnovationsthatroboticarmtakeaccountofdancinghand,weightlifting,Chinesecal-ligraphywritingandcolorclassification4.AnothergroupofengineersatUSAdevelopeightdegreesoffreedomrobotarm.Thisrobotisabletograspmanyobjectswithalotofshapesfromapentoaballandsimulatingalsothehandofhumanbeing5.Inspace,theSpaceShuttleRemoteManipulatorSystem,knownasSSRMSorCana-darm,anditssuccessorisexampleofmultidegreeoffreedomrobotarmsthathavebeenusedtoperformava-rietyoftaskssuchasinspectionsofthespaceshuttleusingaspeciallydeployedboomwithcamerasandsen-sorsattachedattheendeffectorandsatellitedeploymentandretrievalmanoeuvresfromthecargobayofthespaceshuttle6.InMexico,Scientistsareontracktodesignandde-velopmanyrobotarms,andtheMexicangovernmentA.ELFASAKHANYETAL.48estimatesthatinMexicothereareabout11,000roboticarmsusedindifferentindustrialapplications.However,theexpertsthinkthattheapogeeoftherobotarmsisnotonlyofhigherquality,butalsoaccurately,repeatability,andstumpycost.Mostrobotsaresetupforanoperationbytheteach-and-repeattechnique.Inthismode,atrainedoperator(pro-grammer)typicallyusesaportablecontroldevice(ateachpendant)toteacharobotitstaskmanually.Robotspeedsduringtheseprogrammingsessionsareslow.Thepresentworkispartofatwo-phaseproject,whichrequiresamobilerobottobeabletotransportthetoolsfromthestorageroomtotheindustrialcell.Inthisphaseintheproject,whichcarriedoutatMonterreyUniversityofTechnology,Mexico,themainfocuswastodesign,developmentandimplementationofanindustrialroboticarmwithstumpycost,accurateandsuperiorcontrol.Thisrobotarmwasdesignedwithfourdegreesoffreedomandtalentedtoaccomplishsimpletasks,suchaslightmate-rialhandling,whichwillbeintegratedintoamobileplat-formthatservesasanassistantforindustrialworkforce.2.MechanicalDesignThemechanicaldesignoftherobotarmisbasedonarobotmanipulatorwithsimilarfunctionstoahumanarm6-8.Thelinksofsuchamanipulatorareconnectedbyjointsallowingrotationalmotionandthelinksofthema-nipulatorisconsideredtoformakinematicchain.Thebusinessendofthekinematicchainofthemanipulatoriscalledtheendeffectororend-of-arm-toolinganditisanalogoustothehumanhand.Figure1showstheFreeBodyDiagramformechanicaldesignoftheroboticarm.Asshown,theendeffectorisnotincludedinthedesignbecauseacommerciallyavailablegripperisused.ThisisbecausethattheendeffectorisoneofthemostcomplexFigure1.Freebodydiagramoftherobotarm.partsofthesystemand,inturn,itismucheasierandeconomicaltouseacommercialonethanbuildit.Figure2showstheworkregionoftheroboticarm.Thisisthetypicalworkspaceofarobotarmwithfourdegreeoffreedom(4DOF).Themechanicaldesignwaslimitedto4DOFmainlybecausethatsuchadesignal-lowsmostofthenecessarymovementsandkeepsthecostsandthecomplexityoftherobotcompetitively.Ac-cordingly,rotationalmotionofthejointsisrestrictedwhererotationisdonearoundtwoaxisintheshoulderandaroundonlyoneintheelbowandthewrist,seeFigure1.Therobotarmjointsaretypicallyactuatedbyelectri-calmotors.Theservomotorswerechosen,sincetheyin-cludeencoderswhichautomaticallyprovidefeedbacktothemotorsandadjustthepositionaccordingly.However,thedisadvantageofthesemotorsisthatrotationrangeislessthan180span,whichgreatlydecreasestheregionreachedbythearmandthepossiblepositions9.Thequalificationsofservomotorswereselectedbasedonthemaximumtorquerequiredbythestructureandpossibleloads.Inthecurrentstudy,thematerialusedforthestruc-turewasacrylic.Figure3showstheforcediagramusedforloadcalcu-lations.Thecalculationswerecarriedoutonlyforthejointsthathavethelargestloads,sincetheotherjointswouldhavethesamemotor,i.e.themotorcanmovethelinkswithoutproblems.Thecalculationsconsideredtheweightofthemotors,about50grams,exceptfortheweightofmotoratjointB,sinceitiscarriedoutbylinkBA.Fig-ure4showstheforcediagramonlinkCB,whichcon-tainsthejoints(BandC)withthehighestload(carrythelinksDCandED)andthecalculationsarecarriedoutasfollows.Figure2.Workregionoftheroboticarm.Copyright2011SciRes.MME49A.ELFASAKHANYETAL.Figure3.Forcediagramofrobotarm.Figure4.ForcediagramoflinkCB.Thevaluesusedforthetorquecalculations:Wd=0.011kg(weightoflinkDE)Wc=0.030kg(weightoflinkCD)Wb=0.030kg(weightoflinkCB)L=1kg(load)Cm=Dm=0.050kg(weightofmotor)LBC=0.14m(lengthoflinkBC)LCD=0.14m(lengthoflinkCD)LDE=0.05m(lengthoflinkDE)PerformingthesumofforcesintheYaxis,usingtheloadsasshowninFigure4,andsolvingforCYandCB,seeEquations(1)-(4).Similarly,performingthesumofmomentsaroundpointC,Equation(5),andpointB,Equa-tion(6),toobtainthetorqueinCandB,Equations(7)and(8),respectively.gydmcmYFLWDWCC0(1)21.141kg9.8ms11.18NYC(2)0ydmcmBBFLWDWCWgC(3)21.171kg9.8ms11.4758NBC(4)220cCDDEcDCDCDDEmCDcWLLMWLLLLDLM(5)2202DEBBCCDDEDBCCDCDmBCCDcBCBCmBCBBLMLLLLWLLLDLLWLLCLWM(6)1.968Nm278.6ozincM(7)3.554Nm503.38ozinBM(8)Theservomotorthatwasselected,basedonthecal-culations,istheHextronikHX12K,whichhasatorqueof280oz/in.Thismotorwasrecommendedbecauseitismuchcheaperthananyothermotorwithsamespecifica-tions.SinceweneedmoretorqueatjointB,seeEquation(8),weusedtwomotorsatpointBtocomplywiththetorquerequirements；however,onemotorisenoughfortheotherjoints.UsingtwomotorsatjointBismuchcheaperthanusingonebigmotorwith560oz/in.Otherrelevantcharacteristicsofthemotors,whichcanbeshowninFigure5,arethattheycanturn60degreesin130mil-lisecondsandtheyhaveaweightof47.9gramseach.Oncetheinitialdimensionsfortherobotarmandthemotorweredefined,thedesignwerecarriedoutusingtheSolidWorksplatform；designshouldcarefullytakeintoaccountthethicknessoftheacrylicsheetandthewaythatthepieceswouldbeattachedtoeachother.Theacrylicsheetusedtomaketherobotis1/8thicknessandFigure5.Servomotor.Copyright2011SciRes.MMEA.ELFASAKHANYETAL.50thatthinsheetwaschosenbecauseiteasierformachiningandlessweightwithagoodresistance.Duringdesign,wefacedsomedifficultiesduetothewayofjoiningthinacrylicpartsstrongly.Itwasneededtoolstoburnandjointheacrylicpartsandthatwerentavail-ableandtheteamconsideredthatamechanicaljunctionbasedonscrewsandnutswouldbemuchstrongthanotheralternatives,suchasglueforexample.Inordertoaccom-plishthis,asmallfeaturewasdesignedwhichallowedtofastentheboltswiththenutswithouthavingtoscrewinthethinacryliclayer.TheresultofthisprocesswasthetridimensionaldesignshowninFigure6.Byendofdesign,eachpartwasprintedinfullscaleincardboardpaperandthenweverifiedallthedimensionsandtheinterfacesoftheassembly.Inturn,webuiltthefirstprototypeoftherobotarm.Next,partsoftherobotarmweremachinedfromtheacrylicsheetusingacircu-larsawandDermaltools.Thedetailingonthepartswasdoneinaprofessionalworkshopsincethepartsofrobotarmweretoosmallanditisnotaneasyforaccomplish-ingsuchsmallandaccuratecuts.Duringassemblingtherobotpartswiththemotors,fewproblemspopup.Therewerecriticalpointsthatdidnotresistthefasteningand,inturn,maybreakdown；henc