外文翻译具有2或3个自由度的对应机械手.docx
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外文翻译具有2或3个自由度的对应机械手.docx
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外文翻译具有2或3个自由度的对应机械手
ParallelMechanismswithTwoorThreeDegreesofFreedom
ChristiaanJ.J.Paredis,H.BenjaminBrown,PradeepK.Khosla
Abstract:
ParallelmanipulatorsforthemachinetoolIndustryhavebeenstudiedextensivelyforvariousindustrialapplications.However,limitedusefulworkspaceareas,thepoormobility,anddesigndifficultiesofmorecomplexparallelmanipulatorshaveledtomareinterestinparallelmanipulatorswithlessthansixdegreesoffreedom(DoFs).Severalparallelmechanismswithvariousnumbersandtypesofdegreesoffreedomaredescribedinthispaper,whichcanbeusedinparallelkinematicsmachines,motionsimulators,andindustrialrobots.
Keywords:
parallelmanipulator;parallelkinematicmachine;degreeoffreedom;robot
Introduction
Mechanicalsystemsthatallowarigidbodytomovewithrespecttoafixedbaseplayaveryimportantroleinnumerousapplications.Arigidbodycanmoveinvarioustranslationalorrotationaldirectionswhicharecalleddegreesoffreedom(DoFs).Thetotalnumberofdegreesoffreedomforarigidbodycannotexceedsix,forexample,threeaxes,Arobotincludesasystemtocontrolseveraldegreesoffreedomofanend–effector.
Thelastfewyearshavewitnessedimportantdevelopmentsintheuseofindustrialrobots,mainlyduetotheirflexibility.However,themechanicalarchitectureofthemostcommonrobotsisnotwelladaptedtocertaintasks.Othertypesofarchitectureshave,therefore,recentlybeendevelopedforindustrialuse,includingparallelmanipulators.
Aparallelmanipulator,whichisaclosed-loopmechanism,typicallyconsistsofamovingplatformthatisconnectedtoafixedbasebyseverallimbsorlegs.Typically,thenumberoflimbsisequaltothenumberofdegreesoffreedomsuchthateverylimbiscontrolledbyoneactuatorandalltheactuatorscanbemountedatornearthefixedbase.Forthisreason,parallelmanipulators.Becausetheexternalloadcanbesharedbytheactuators,parallelmanipulatorstendtohavealargeload-carryingcapacity.Parallelmanipulatorsarealwayspresentedashavingverygoodperformanceintermsofaccuracy,rigidityandtheabilitytomanipulatelargeloads.Theyhavebeenusedinalargenumberofapplicationsrangingfromastronomytoflightsimulators,andarebecomingincreasinglypopularinthemachine-toolindustry,
Theconceptualdesignofparallelmanipulatorscanbedatedbackto1947,whenGoughestablishedthebasicprinciplesofamechanismwithaclosed-loopkinematicstructuretocontrolthepositionandorientationofamovingplatformtotesttirewearanddamage.Hebuiltaprototypein1955(Fig,1a)wherethemovingelementwasahexagonalplatformwhoseverticeswereallconnectedtolinksbyball-and-socketjoints.Theotherendofthelinkwasattachedtothebasebyauniversaljoint.Sixlinearactuatorsmodifiedthetotallinklength.Stewartdesignedaplatformmanipulatorasanaircraftsimulatorin1965(Fig,1b),inwhichthemovingelementwasatriangularplatformwhoseverticeswereallconnectedbyball-and-socketjointstosupportmechanismseachconstitutingoftwojacks,alsoplacedinatriangle.In1978,Huntmadeasystematicstudyofkinematicstructureofparallelmanipulators,withtheplanarthree-RPSparallelmanipulatorastypicalexample.Sincethen,parallelmanipulatorshavebeenstudiedextensivelybynumerousresearchers.
Mostofthesix-DoFsparallelmanipulatorsstudiedtodatehaveincludedsixextendiblelimbs.
Theseparallelmanipulatorspossesstheadvantagesofhighstiffness,lowinertia,andlargepayloadcapacity.However,theysuffertheproblemsofrelativelysmallusefulworkspaceanddesigndifficulties.Furthermore,theirdirectkinematicsisverydifficulttoanalyze.Therefore,parallelmanipulatorswithlessthansix-DoFshaveincreasinglyattractedattentionforindustryapplications.
Thispaperintroducesparallelmanipulatorsandtheclassificationofparallelmanipulators.Threetypesofnewparallelmanipulatorsareintroduced:
aspatialthree-DoFsparallelmanipulator,atwo-DoFsparallelmanipulator,andaplanarthree-DoFsserial-parallelmanipulator.
1DefinitionofparallelManipulator
Aparallelmanipulatorismadeofanend-effectorwithndegreesoffreedomwithafixedbaselinkedtogetherbyatleasttwoindependentkinematiclinkages.Actuationtakesplacethroughn-simpleactuators.
Thesemechanismshavethefollowingcharacteristics.
·Atleasttwolinkagessupporttheend-,effector.Eachofthoselinkagescontainsatleastonesimpleactuator.
·Thenumberofactuatorsisthesameasthenumberofdegreesoffreedomoftheend-effecto.
·Themobilityofthemanipulatoriszerowhentheactuatorsarelocked.
Parallelmechanismsareofinterestforthefollowingreasons:
·Theloadcanbedistributedonthemultiplelinkages.
·Fewactuatorsareneeded.
·Whentheactuatorsarelocked,themanipulatorremainsinposition,whichisanimportantsafetyconcernforcertainapplications.
Parallelmanipulatorsforwhichthenumberoflinkagesisstrictlyequaltothenumberofdegreesoffreedomoftheend-effectorarecalledfullyparallelmanipulators.
2DegreesofFreedomofaMechanism
Thedegreesoffreedomofamechanismarethenumberofindependentparametersorinputsneededtocompletelyspecifytheconfigurationofthemechanism.However,ageneralmobilitycriterioncannotbeeasilydefinedforclosed-loopkinematiclinkages,asHuntandLerbetalreadynoted.Classicalmobilityformulaecanindeedneglectsomedegreesoffreedom.Grubler'sformulaeisneverthelessgenerallyused,whichmaybewrittenas
M=d(n-g-1)+
(1)whereMissystemmobility(degreesoffreedom);
disscrewsystemorder(d=3forplanarandsphericalmotion,d=6forspatialmotion);nisnumberoflinksincludingtheframe;gisnumberofjoints;and关aredegreesoffreedomassociatedwiththei-thjoint.
3ClassificationofParallelManipulators
Thetotalnumberofdegreesoffreedomofarigidbodycannotexceed6;therefore,thenumberofDoFsofaparallelmanipulatorwillbebetween2and6.Sincethefirstparallelmechanismdesign,manymechanicaldesignshavebeenproposedforparallelmanipulatorswith2to6DoFs.Asurveyof87actuatorsproposedintheliteratureshowedthat40%hassixDoFs,3.5%fiveDoFs,6%fourDoFs,40%threeDoFs,andtheremainingtwoDoFs.
3.1Two-DoFsparallelmanipulators
Mostexistingtwo-DoFsparallelmanipulatorsareplanarmanipulatorswithtwo-translationalDoFs.Suchdesignsuseonlyprismaticandrevolutejoints.McCloyshowedthatthereare20differentcombinations.Thisnumberisreducedto6asshowninFig.2iftheactuatorsareassumedtobeattachedtotheground.Thereisnopassiveprismaticjointandnoactuatorissupportingtheweightofanotheractuator.
3.2Three-DoFsparallelmanipulators
Therearemanythree-I?
oFsparallelmanipulators,soonlytheclassicaldesignswillbepresentedhere.Oneexampleistheplanarthree-RRR(Rstandsforrevolvingjoint)parallelmanipulatorasshowninFig.3a.ThemovingplatformhasthreeplanarDoFs,whicharetwotranslationsalongthexandyaxesandonerotationaroundtheaxisperpendiculartotheO-xyplane.Anotherexampleisthesphericalthree-RRRparallelmanipulatorasshowninFig.3b,inwhichallthejointaxesintersectatacommonvertex.Themotionofanypointinthemechanismisrotationaboutthevertex.ThemovingplatformhasonlyrotationalDoFswithrespecttothebase.Huntpresentedthethree-RPSparallelmanipulatorshowninFig.3c,whichhascomplexDoFs,whichcannotbestrictlydefined.ThemostfamousrobotwiththreetranslationsistheDELTA(Fig.3d),proposedbyClavelandmarketedbytheDemaurexCompanyandABBunderthenameIRB340FpexPicker.DELTAhasbeenwidelyusedinindustry.Anothertypeofthree-DoFsparallelmanipulatorhasthemovingplatformconnectedtothebasethroughfourlegs,wherethefourthlegispassiveandisalsotheleadingleg,whichmeansthatthelegdeterminesthemotionofthemovingplatform,forexample,inthesphericalcoordinateparallelmanipulatorshowninFig.3e.ThisparallelmanipulatorisusedforthemachinetooldesignbyIFWoftheUniversityofHannover.
3.3Four-DoFsparallelmanipulators.
Afour-DoFsfullyparallelmanipulatorhasd-=6,n=10,g=I2,andM=4.SubstitutingthesecoefficientsintoEq.
(1)getsF=22/4whichisthedegreesoffreedomforeachleg.Therefore,thereaxe,actually,nofour-DoFsfullyparallelmanipulators.Theearlymechanismswithfour-DoFswerenotfullyparallelmanipulators,i,e.,manipulatorswithtwoactuatorsperlinkageorwithpassiveconstraints.
3.4Five-DoFsparallelmanipulators
Five-DoFsfullyparallelmanipulatorsmusthaveF=29/5,sotherearenofive-DoFsfullyparallelmanipulators.Afive-DoFsparallelmanipulatorproposedbyAustadconsistsoftwoparallelmanipulators.
3.5Six-DoFsparallelmanipulators
Six-DoFsparallelmanipulatorsarethemostpopularmanipulatorssotheyhavebeenstudiedextensively.ThearchitectureshowninFig.4aisaclassicalsix-DoFsparallelmanipulator.Mostsix-DoFsparallelmanipulatorshavesixextendiblelimbs.Theseparallelmanipulatorspossesstheadvantagesofhighstiffness,lowinertia,andlargepayloadcapacity.However,theysuffertheproblemsofrelativelysmallusefulworkspaceanddesigndifficulties.Furthermore,analysisoftheirdirectkinematicsisverydifficult.Therearealso
Someexoticchainmanipulatorsinwhichthemanipulatorisactuatedbyaplanarmechanism,suchasafour-barmechanism,orafive-barmechanism,orwhichhavetwoactuatorsperlegandwhichusuallyhavethreelegs
4EvolutionofParallelManipulators
AfterGoughestablishedthebasicprinciplesofmechanis
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