The Ackerman Steering Principle

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Ackerman steering principles : 4 bar linage design and various parameters affecting it.

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    AckermanSteeringPrincipleThisarticleispartofasectionoftheRcTeksitedevotedtoradiocontrolledmodelcarhandling.Ascarhandlingisan

    extremelycomplexsubject,itwillbequitesometimebeforeitisfinished.ThisarticledealswiththeAckermanSteeringPrinciple,whichissometimesreferredtoastoeoutonturn.

    AckermanSteeringPrinciple

    TheAckermanSteeringPrincipledefinesthegeometrythatisappliedtoallvehicles(twoorfourwheeldrive)toenablethecorrectturningangleofthesteeringwheelstobegeneratedwhennegotiatingacorneroracurve.Beforethisprinciplewasdevelopedthevehiclesofthetime(horsedrawn)werefittedwithparallelsteeringarmsandsufferedfrompoorsteeringperformance.AMrRudolfAckermaniscreditedwithworkingoutthatusingangledsteeringarmswouldcurethesevehiclesofsuchsteeringproblems.

    Why&How?

    Theanimationontherightdepictsacartravellingaroundacorner(inthiscase,acontinousone!).Theredlinesrepresentthepaththatthewheelsfollow.IfyouPlayityouwillnoticethattheinsidewheelsofthecararefollowingasmallerdiametercirclethantheoutsidewheels.Ifboththewheelswereturnedbythesameamount,theinsidewheelwouldscrub(effectivelyslidingsideways)andlessentheeffectivenessofthesteering.Thistyrescrubbing,whichalsocreatesunwantedheatandwearinthetyre,canbeeliminatedbyturningtheinsidewheelatagreateranglethantheoutsideone.YoumayStoptheanimationifrequired.

    Thedifferenceintheanglesoftheinsideandoutsidewheelsmaybebetterunderstoodbystudyingthediagramtotheright,wherewehavemarkedtheinsideandoutsideradiusthateachofthetyrespassesthrough.TheInsideRadius(Ri)andtheOutsideRadius(Ro)aredependantonanumberoffactorsincludingthecarwidthandthetightnessofthecornerthecarisintendedtopassthrough.Measurementsforanglesofthearmsarethereforenotderivedfromtheselines,theyarederivedfromthedistancesshowninMore,Less&TrueAckermanSteeringsectionbelow.

    Aligningbothwheelsintheproperdirectionoftravelcreatesconsistentsteeringwithoutunduewearandheatbeinggeneratedineitherofthetyres.Obviouslywithturningonewheelmorethantheotheryouaremisaligningthewheelsandyouneedtodothiswhilstallowingbothwheelstobepointingstraightforwardwhenthecarisnotturning.Toenablethistohappen,themisalignmentneedstoprogressfromzero(wheelspointingstraightahead)toapointwherethereisasufficientlydifferentanglebetweenbothwheelstocreatethealignmentofbothwheelswhentheyarebothfullyturned.

    SteeringArmAngles

    Creatingmisalignmentofthewheelsis,asmentionedintheintroduction,achievedbyacombinationoftheangleandthelengthofthesteeringarms.BelowwehaveafewdiagramsthatgiveexamplesusingparallelandangledsteeringarmstodemonstratewhythereisaneedforusingtheAckerman

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    SteeringPrinciple.Pleaseignorethesizedifferenceoftheyellowcirclesinthediagramsbelow.TheyarearesultofthewaythesteeringarmsweredrawnforsimplicityandarenotmeanttoaddconfusiontothedescriptionoftheAckermanPrinciplewearedescribinginthisarticle.

    ParallelSteeringArmsThesteeringarmsinthediagramtotheleftarestraightandparalleltothesidesofthevehicle,whichwouldcreateasituationwhereequalmovementofthesteeringservowouldproduceequalangularmovementofthewheels.Whythisequalangularmovementoccurscanbeseenbystudyingtheanimationofa

    wheeltotheright,wherearedcirclehasbeendrawntoshowhowthesidewaysmovementofthesteeringarmsisconvertedinacircularone.Asthesteeringarmpivotpoint(A)isverticallyalignedwiththekingpinpivotpoint(B)whenthewheelispointingstraightahead,thesameamountofmovementtotheLeftortotheRightmovesthesteeringarmpivotpointthesameverticaldistanceforwardofit'sstartingpoint.YoucanresettheanimationtoitsStartingpositionifrequired.AmorecompleteexplanationoftheissuesinvolvedisavailableinourTheCirclearticle.Ifthissteeringgeometrywasappliedtoaremotecontrolledmodelcar,theneitherorbothofthefrontwheelswouldnotbeatthecorrectsteeringangleandwouldresultinunpredictablesteering.

    AngledSteeringArmsThesteeringarmsintheimagetotheleftareangledinwardstocreateameansforthewheelanglestochangeatadifferentrate.ThisisthebasisoftheAckermanSteeringPrincipleandcreatesthisunequalangularmovementofthewheels.Whythisunequalangularmovementoccursisshownintheanimatedimagetotherightand

    happensbecauseoftherelativepositionofthesteeringarmpivotpoint(A)aroundthecircumferenceoftheredcirclethathasbeendrawnintoshowhowthesteeringarmpivotpointmovesaroundthekingpinpivotpoint(B).Asthesteeringarmsareangled,thepivotpoint(A)isnotverticallyalignedandis,inastraightaheadposition,partwayroundthecircle.Becauseofthis,aRightmovementofthesteeringarmwillcausethepivotpointtomoveagreaterdistanceintheforwarddirectionthanaLeftmovementofthesteeringarm.YoucanresettheanimationtoitsStartingpositionifrequired.AmorecompleteexplanationoftheissuesinvolvedisavailableinourTheCirclearticle.

    Animportantpointworthnotingisthatthisunequalangularmovementisexponential,thatis,themoreyouturnthewheelthegreatertheangulardifferencebetweenthewheelsotherwiseboththewheelswouldneverpointforwardwhenthecarisnotturning.Thedeliberatelyemphasisedexampleabovewouldresultinawheelangledifferencesomewhereintheregionofthefiguresgivenintheimagetotheleft,whereasthe

    parallelsteeringarmexamplewouldhaveresultedinthesamewheelanglesbeinggeneratedoneachside.

    More,Less&TrueAckermanSteering

    Theseareoftenheardtermsinmodelcarracingandrefertotheamountofinequalityoftheanglesof

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    thewheelsrelativetotrueAckermansteeringgeometry.

    TrueAckermanAngleZeroToeOnTurnInTrueAckermansteeringgeometryisshownintheimagetotheright.Thisisdefinedbyanglingthesteeringarmssothatalinedrawnbetweenboththekingpinandsteeringarmpivotpointsintersectswiththecentrelineoftherearaxle.AsthisgivestrueAckermansteeringgeometry,thereisnoToeAnglechangeontheinsidewheel(thewheelisalignedwiththecircumferenceofthe

    circle),whichcanbeseenintheimageaboveleft.

    MoreAckermanAngleToeOutOnTurnInMoreAckermananglecanbeaddedtoasteeringsetup,whichinvolvesadjustingtheangleofthepivotpointsonthesteeringarmssothatthepointofintersectionisforwardofthecentrelineoftherearaxle.Pleaserefertotheimageontheright.Thissteeringgeometryachievesgreaterangularinequalityoftheturnedwheels,whichresultsintheinsidewheeltryingtofollowasmallerdiameter

    circlethanitactuallydoes.ThiseffectcanbeseenintheimageaboveleftandgeneratesToeOutonthefrontinsidewheel.

    LessAckermanAngleToeInOnTurnInLessAckermananglecanbesetonasteeringsetup,whichinvolvesadjustingtheangleofthepivotpointsonthesteeringarmssothatthepointofintersectionisbehindthecentrelineoftherearaxle.Pleaserefertotheimageontheright.Thissteeringgeometryachievesareducedamountofangularinequalityoftheturnedwheels,whichresultsintheinsidewheeltryingtofollowalarger

    diametercirclethanitactuallydoes.ThiseffectcanbeseenintheimageaboveleftandgeneratesToeInonthefrontinsidewheel.

    SteeringArmLength

    Asthesteeringarmsarelevers,theirlengthismoreorlessafreearea,butisrestrictedbytheclearanceandavailablespaceonthemodelcar.Theamountofmovementthatcanbegeneratedbytheservo/steeringlinkagearrangementisalsoaprimaryconsiderationasyoumustthinkaboutthetorquerequirementsofleverswithdifferentlengths.

    Summary

    ThisarticleisonlyintendedtointroducewhattheAckermanSteeringPrincipleis,wehaveleftoutadescriptionoftheeffectsofMoreorLessAckermananglewhichwillbecoveredinarelatedarticle.WealsohavearelatedarticleaboutHowToeAngleAffectsAckermanSteeringAnglewhichisofrelevancetothisparticularareaofmodelcarhandling.Moreimportantlythough,thereisanotherelementincarhandlingthatisparticularlyimportanttothemodelcarowner.Thiselementiscalledslipangleandwillhaveafuturearticledevotedtoit.

    RelatedInformation

    HowToeAngleAffectsAckermanSteeringAnglesToeIn&ToeOutTheCircle

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