Hands-on Robotics_Robotic Arm
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This document provides a final report on a project to build a robotic arm capable of drawing on paper. It describes the background and objectives of the project, the team's design process in which they considered several robot arm designs before selecting a Cartesian robot, and details of how they implemented that design. It discusses sources of error they encountered and modifications made to address them. Sections also cover the robot's motion algorithm, expected performance, results from testing, and ideas for future improvements.
![tomeetthedesignconstraintsoftheproject,anadditionalaxisofrotationwouldhavetobe
implementedinordertocontroltheorientationoftheendeffector.
Thedesignwouldallowustoreacheverypointandpenorientationintheworkspace,butat
theexpenseofdifficultinversekinematicsandthemechanicaldesignchallengeofcorrectlybalancing
thehangingservos.
Fig.1:SCARArobotarm[1]
1.3.1.2. R4Robot
Similarly,totheSCARArobot,theR4robotwasanassemblageof4revolutejointsthatcould
reachallofthepointsandorientationintherobotworkspace.Thedesignwasbetterthanthescara
robot,inthattheinversekinematicscalculationsrequiredforthedesignwouldbesimplified,however
thisdesignwouldstillbeveryhardtoimplementfromamechanicalperspective.Weanticipated
issueswitharmsagandtorquerequirementsoftheservomotorsthatwerearesultoftherobotarms
hangingservos.
Fig.2:CADmodelofaR4botcontrolledwith4revolutejoints](https://image.slidesharecdn.com/b472a9e0-8d07-4473-8f75-8edcadc20c19-150111231851-conversion-gate01/85/Hands-on-Robotics_Robotic-Arm-4-320.jpg)
![5. References
[1]http://www.societyofrobots.com/robot_arm_tutorial.shtml#joint_force
additionally,seereferencesinthepresentations
6. Appendix
7.1 SourceCode:https://wiki.eecs.umich.edu/hrb/index.php/File:Red_2_Source_Code.py
7.2. CADModel:
https://drive.google.com/folderview?id=0Bw2WF0CVsfPnNldDblNQUE5FYVk&usp=sharing
7.3 RobotVideo:https://www.youtube.com/watch?v=2r7dksCIGA
7.4 Modelingcode:https://wiki.eecs.umich.edu/hrb/index.php/File:RedArmModel.txt](https://image.slidesharecdn.com/b472a9e0-8d07-4473-8f75-8edcadc20c19-150111231851-conversion-gate01/85/Hands-on-Robotics_Robotic-Arm-19-320.jpg)
Hands-on Robotics_Robotic Arm
- 2. CONTENT 1. Background 1.1. ProjectIntroduction 1.2. EquipmentInvolved 1.2.1. ComputerVisionSystem 1.2.2. DynamixelRX64andEX106ServoMotors 1.3. Designbrainstorming 1.3.1. Hardware 1.3.1.1. SCARARobot 1.3.1.2. R4Robot 1.3.1.3. CartesianRobot 1.3.2. Software 2. Implementation 2.1. InitialConcept 2.2. RobotDesignandConstruction 2.3. DesignConsiderations 2.3.1. SourcesofError 2.3.2. ReevaluationandModifications 2.4. MotionAlgorithm 2.4.1. Robot 2.4.2. ManualController 2.4.3. AutonomousController 3. ExpectedPerformance 3.1. ModellingSoftware 3.2. CalibrationandModification 4. Results 4.1. PDayResults 4.2. Observations 5. Discussion 5.1. DiscussionofOtherTeams 5.2. ReflectiononCurrentDesign 5.2.1. MechanicalDesign 5.2.2. Software 5.3. FutureModifications 5.3.1. MechanicalDesign 5.3.2. Software 6. References 7. Appendix
- 3. 1. Background 1.1. ProjectIntroduction Ourtaskwastobuildarobotarmwhichiscapableofdrawingonapiece ofpaperwithina1ft.³workspace.Therewereseveralpaperpositionsand orientations,whichwerenotknowninadvanceexceptforthefollowing properties: (1)thepaperisastandardsheetofletterpaper,laidonarigidbacking, entirelywithintheworkspace; (2)thepapermustfaceupwardsoratmost5°downwards. (3)thepapermustfacewithin90°ofadesignated“front”direction. Beforeattemptingtodrawoneachorientation,ourteamwasallowedtocalibratetherobotby hand,bymovingittoknownpointsonthepaper.Wewererequiredtodrawafourbyfourinch square,withsidesparalleltotheedgesofthepaper.Inaddition,theinstructorprovidedasequence ofpenstrokesasalistoftwodimensionalpositionsrelativetothecornerofthepage.Therobotarm hadtoalsodrawthesepenstrokesonthepage. 1.2. EquipmentInvolved 1.2.1. ComputerSystem TherobotwascontrolledviaaJoyAppwritteninpythonandrunningonalinuxmachine.The computercommunicatedwiththerobotviaRS485serialconnection. 1.2.2. DynamixelRX64andMX64ServoMotors Servomotorswereprovidedalongwiththepythonlibrary.Thesemotorsallowedbothconstant rotationandjointmodeoperations,whereprescribedtorquebetween1to+1orprescribedangles between10000to10000couldbespecified.Maximumof6servomotors(acombinationofRX64 andMXmotors)wereallowedinanysuitableconfiguration. 1.3. Designbrainstorming 1.3.1.Hardware Groupbrainstormingmeetingsfocusedondesigningarobotwithsimplifiedinversekinematics. Asateamwecameup3designssatisfyingthedesignconstraintsofthisproblem.Theserobotideas areasfollows: 1.3.1.1. Scara(thecompetition)Robot Thescara(SelectiveComplianceAssemblyRobotArm)isanRRRPRstylerobotarmthatis typicallyusedinindustrialrobotics.ThearmisrigidinthezaxisandpliableintheXYaxes.Inorder
- 9. controlledintheydirectionusinga1/4”threadedrodmountedtoaDynamixelMX64motorproviding continuousrotation.A¼”diameterwoodendowelismountedtoa1/4”nutusingmanufacturedrail supportsandthenut/dowelassemblyisscrewedontothethreadedrod.Finallythexaxisorientation iscontrolledbyatwolinkarmwithapassiverotationaljointattachedtoarotationalservothatis mountedtothebase.Thecartesianservosweremountedbelowtheaxisofthetiltservoinorderto counterbalancetheweightoftheframe. Amoredetaileddescriptionoftherobotsconstructionprocess,withstepbystepprocedures andabillofmaterials,canbefoundinourconstructiondocumentation. 2.2. DesignConsiderations 2.2.1. SourcesofError Duringtestingseveralsourcesoflargeerrorswereidentified.Primaryamongthemwasthe generalsloppynessinallofthemanyjointsontherobot.Snaplockjointsbetweenservoshad substantialplay,thejointsconnectingthelinkageandendeffectorwerelooseandinconsistent,and theUframe/servohorncouplingsthemselvesflexedundertheweightoftherobot.Severaldesign factorsnecessitatedtheuseoflinkagearmsthatwerelongerthanstrictlynecessarytoachievethe fullrangeofmotioninthexaxis.Thisinturnmeantthattheinaccuraciesofsettingthepositiononthe servowasamplified,asthelongerarmsresultedinalargerxdisplacementperdegreeturnedonthe servo.Finally,ourinitialconstructionhadthexaxisrodsupportedononlyoneside.Thisledtoa largeamountofbouncingintherod. 2.2.2. ReevaluationandModifications Effortsweremadetoreducetheamountofplayinalloftherobot’sjoints.Snaplockscrews wereadjustedtoprovidetighter,morestableconnections.Thefoamcorelinkagearmswereadjusted tohaveaslittlestressaspossibleindirectionsotherthanthedrivingdirections.Theframeofthe robotwasalsorebuilttoprovidesupporttobothendsofthexaxisrod.Thisgreatlyreducesthe mechanicalinaccuraciesoftherobot. 2.3. ManualController ManualcontrolwasimplementedforeachservousingtheKORGMidiboard.Thetiltand rotateservoswereallowedtobesetupduringourcalibrationstageforeachpaperorientation,sono furtherimplementationwasdone.Topreventjitterintheservomotionfromdestroyingourrobot,the speedofeachservowassetto0.2,tocreateaverysmoothmotion.Thisintroducedsignificantdelay intheresponse,butitcompletelyeliminatedhumanjitterwhenmovingthesliders,whichimproved theeffectofcontrol.
- 10. Manualoverridewasalwaysavailableduringautonomousdrawingtestingforsafetypurposes. TheimplementationofthemanualcontrolcanbefoundinthepythonfilesintheRedteamproject2 wiki. 2.3.1.1. AutonomousController AutonomousalgorithmswereactivatedbypressingbuttonsontheMidicontrollerandthe keyboard.Severalmethodswereimplementedforsquaredrawingandarbitrarypointfollowing.The firstissuethatweworkedtosolvewasabsoluteyposition.SincetheMXservoonlygivesthecurrent angleasareading,thereneededtobeawaytotrackthenumberofrotationsithadcompletedtoget theheightofthepen.Thethreadedrodweusedwasratedat20rotationsperinch,soifwecould countthenumberofrotationsfromanarbitraryzeropoint,wecoulddetermineabsoluteheight.The firstmethodweimplemented,wastosampletheanglereading,andwhenitreachedacertainrange aroundzerotocountthatasarotation.Thenwhentheanglereadingbecameeitherpositiveor negative90degreeswegetthedirectionofrotation,andweresetthe“fullrotation”counterwhenthe anglereadingreachesarangearound180degrees.Theprevailingissueswiththismethod,isthatit requiredconstantsamplingoftheservo.Thislimitedthespeedatwhichtheservocouldrotate,since weneededatleast2sampleseveryrotationtonotlosecount.Ourservoswouldalsofrequentlylose connectionwiththecomputer,causingtheprogramtocrashandtheservostogetstuck.Usingthis methodwefoundthatrapidlyreadingdatarepeatedlyfromtheservoswasnotaveryrobustsolution, sowedidnotstickwithturncountingandtheMX. Insteadofturncounting,wefixedacertaintorquefortheyservo,andmeasuredtheamountof timeittooktocompleteafullrotationatthattorque.Sinceweknowhowmanyrotationsthethreaded rodhasperinch,andwefoundthetimeforonerotation,wecancalculatethetimeneededtomove anarbitraryheight.Theprocessorwascommandedtosleepduringrotationforaspecifiedamountof time.Wefoundthatwhenapplying.3torqueandmeasuringmanually,theendeffectormovedat approximately1.25s/inch.Thisspeedwasusedtocalculatethetimetheprocessorshouldsleep whentryingtomoveforlongerverticaldistances. Ourcodealsohadteachrepeatfunctionality,althoughwedidnotendupusingitduring PDay.Weimplementedafunctionbywhichwewouldsavethestateofeachservo,andthecurrent absoluteheightoftherobot,thenuseafunctiontoplaybackeachstatefrombeginningtoend. 3. ExpectedPerformance 3.1. ModelingSoftware OurmodelingwasdoneinMATLABusingtheopensourceRoboticsToolbox.TheRobotics Toolboxisidealformodelingrobotarms.Itprovidesamethodtoaddanarbitraryamountofeither rotationalorprismaticjoints,connectedbylinks.Italsocalculatestheforwardkinematicsandcanplot thearm,aswellasmotionofthearm.Unfortunatelythereisalimitation,inthatitdoesnotallow branchesoflinks(twolinkscomingfromthesamenode).Thislimitsourabilitytohaveagood representationofourrobotusingthissoftware.
- 11. Inourmodelingcodewefirstdefinetheorientationofthepaper.Weallowfordifferent orientationsbycreatingapapercoordinatesystem,anddefiningamatrixtoconvertfromthepaper coordinatesystemtotherobotcoordinatesystem.Oursimulationisthengivenaseriesofpoints (eitherforasquareorforstrokes)inpapercoordinates,andweuseourinversekinematicsfunction togettheanglesrequiredateachpoint.Wetheniteratebetweentheminsmallincrementsandtrack thepositiontoseewhatshapethearmwouldmake. 3.2. CalibrationandModification Wedidseveralcalibrationprocedureswhilecreatingcodeforthedriverinourmodel.Seethe softwaresection2.Firstwesetthemaximumandminimumanglesofthexpositionservos.Since uponstartup,therobotdoesnotknowahardcodedvaluefortheyposition(sinceittakesmany rotationstomoveupordown),wecouldnotaddsafetylimitsintheydirection.Forthexposition servos,wemovedthepenverticallyinchbyinch,andrecordedthemaximumandminimumx positions(ateachendofthepaperarea).Wecreatedafilewhichthedriverreadwhenmovingthex servotolimittheresponsetobewithintheallowedxrange,dependingontheyposition. AtPDay,wealsofoundthatwhilethepenwasmovingintheydirection,thepressureofthe penonthepapercausedthexservotodrift,sincetheleverarmwasnotveryrigid.Toaccountfor this,wemeasuredtheamountofdriftthathappenedatalowlevelofpressure,andcorrectedthex servoinrealtimeastheservowasmoving.Thismadeourverticallinesevenstraighter. 3.3. SimulationResults Totestoursimulation,weprovidedanarbitrarylistofpapercoordinates betweenwhichthearmhadtomove.Wefocusedoursimulationeffortson drawingsquaressinceitwasthepassingrequirement.Asyoucanseein thefigureabove,oursimulationwascapableofmovingbetween6points inasquare.Themidpointswereaddedbetweenthehorizontallinesto improveaccuracy,asthearmaswemodeleditwasinaccuratewhen movinginthexdirection. Fig.6:Matlabsimulation 3.4. ModelingValidation Therewereseveraldrawbackswithusingthismodelingmethod.Therobotarmcreatedin Matlabwasanaccuraterobotarm,butdidnotmatchourphysicalconstruction.Thismodeling schemewouldhavebeenidealforoneoftheteamswhichweremakinganarm.Wefoundthatthe inaccuraciescreatedwhendrawingoursquarewerelikelynotgoingtobeaproblemforourdesign, andthatthemodelwasnotverygoodattellinguswhereourdesignwouldstray.Forexample,inthe
- 13. Fig.7:RedteamResults(fromtop,clockwise) (a)run1RMS:.131 (b)run3RMS:0.129(~45degreeorientation) (c)run3repeatedRMS:0.164(~45degrees)(d)run5RMS0.0714(verticalorientation) Fig.8:RedTeamFurtherresults:clockwisefromtop(a)RepeatedverticalorientationRMS:.0616 Repeatedrunsflathorizontalorientation(b)RMS:0.0357(c)0.0200
- 17. 4.4. Discussion 4.4.1. MechanicalDesign Oneareawhereourmechanicaldesigncouldbeimproved,isinthexdirectionleverarm.The currentleverarmmadeoutoffoamcorewasonlyconnectedbyasinglebolt,andthereforenotvery rigid.Thejointhadalotofgive,andforagivenxpositionoftheendeffectortherewasarangeofx servoangles(uptoseveraldegrees)thatwouldresultinthesamexposition.Thereforesmall changesintheangleofthexpositionservodidnotresultinanymotionoftheendeffector.This limitedtheprecisionoftheendeffectorinthexdirection.Ifweimprovedthejointbyaddingadditional boltsandaddingmorefoamcorelayerstothejointtoincreasethegriptheboltswouldhave. Additionallythelocationoftheservoitselfcouldbeoptimized.Inthecurrentconfigurationthe linkagearmsaremuchlongerthanstrictlyneededtoprovideafullrangeofmotiononthexaxis.This isduetotheservobeingmountedsolow,relativetothecartesianframeandtopofthepivotservo. Byraisingtheheightoftheservo,thelengthofthatlinkagecouldbegreatlyreduced,thereby reducingtheeffectsofservoerroronxcoordinateposition. Theinnerandouterguidesfortheslidersupportrodwereanadhocconstructionthatcouldbe greatlyimprovedupon.Grossmanufacturingdefectsinthesepiecesledtounevenverticalmotionof theslidersupportrod.Verticalmotioncouldalsobeaidedbyusingascrewwithalowerpitch.The leadscrewweusedrequired20revolutionsperinchofdisplacement.Thismadeapainfullyslow robotandmadeitdifficulttotrackverticalmovement,astheservosneededtoberunatfullspeed. 4.4.2. Software OursoftwarehadfeatureswhichwereunderdevelopedandthusnotusedatPDay.This preventedusfromtryingthestrokechallengeeventhoughwehadwrittensoftwareforit.Our correctioneffortsworkedwellinkeepingourstraightlinesdrawn,butwecouldhavedonemoreto mitigateandpredicttheeffectofappliedpressureonthepenfromasoftwareperspective.Reducing thespeedoftheservomotionhadaverygoodeffectontheresponsetothemidiboard,andno additionalfilteringshouldbenecessary. Thereismuchmoreworkneededtobedoneonthesoftware.Strokefollowingshouldbe bettertested.Theframeworkhasbeenmade,butsincedrawingasquarewasthemainrequirement oftherobot,alotofeffortwasspentinmakingsurethatitwoulddrawasquare,andnotmuchonthe followingofarbitrarypoints.Muchmoretestingshouldbedonetomakesurethattheproposed functioninRedArmDriver.pywillgobetweenpoits.Theinversekinematicsfunctionmustalsobe implementedtoconvertbetweenpapercoordinatesandrobotarmangles.Itwasdeemed unnecessaryforthedrawingofthesquareaswecouldhardcodethefourcornersinthexyrobot planeandorientourrobotaccordingly. ThemoveTogetherfunctioninthedriverfilehasshowngooddiagonalmotionintesting,so fullytestingthatwithaddedpenpressureandcreatinganadditionalfunctiontoreadthevectorof
- 19. 5. References [1]http://www.societyofrobots.com/robot_arm_tutorial.shtml#joint_force additionally,seereferencesinthepresentations 6. Appendix 7.1 SourceCode:https://wiki.eecs.umich.edu/hrb/index.php/File:Red_2_Source_Code.py 7.2. CADModel: https://drive.google.com/folderview?id=0Bw2WF0CVsfPnNldDblNQUE5FYVk&usp=sharing 7.3 RobotVideo:https://www.youtube.com/watch?v=2r7dksCIGA 7.4 Modelingcode:https://wiki.eecs.umich.edu/hrb/index.php/File:RedArmModel.txt