机械外文翻译调直机的反馈方法.docx
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机械外文翻译调直机的反馈方法
AClosedLoopFeedbackMethodforaManualBar
Straightener
RobertJ.Miklosovic,ZhiqiangGao
DepartmentofElectricalandComputerEngineering
ClevelandStateUniversity
Cleveland,Ohio,USA
Abstract—Automationofauniquemanuallycontrolledindustrialbarstraightenerisproposed.Acontinuous-timeclosedloopmodelisconstructedinSimulinkforanevent-drivenprocessthroughtheuseofasynchronoustimers.ThesystemissimulatedwithlinearandnonlinearPDcontrollers.Anonlinearfilter,calledthetrackingdifferentiator,isintroducedasanalternativetoalinearapproximatemeansofprovidingaccuratederivativefeedbackinthepresenceofnoise.Inbothcases,thenonlineartechniquesoutperformedtheirlinearcounterpartswhileretainingtuningsimplicity.
I.BACKGROUND
Precisionstraighteningofacylindricalmetalbarislargelybasedontheabilitytopreciselymeasureitsgeometry.Afewfundamentalmeasurementsandhoweachinfluencesthetolerancespecificationonstraightnessshouldfirstbeunderstood.Methodsformeasuringroundnessandstraightnessarecoveredtolaythegroundworkfortheproblemformulation.ThebasicoperationofthemachineisoutlinedinSectionII,anditsfundamentallimitationsandneedforautomationarediscussed.SectionIIIaddressesthetaskofclosingtheloopthroughblockdiagramsandtheroleofnewhardwareintheprocess.SectionIVcontainsdescriptionsofalloftheblocksthataremodeledinSimulink.ThelinearandnonlinearcontrollerdesignsarediscussedinSectionV,thesystemissimulatedinSectionVI,andconcludingremarksaremadeinSectionVII.
A.MeasuringRoundness
Roundnessisaquantityderivedfromcomparingtheshapeofacross-sectionalareaatonedistinctpointalongacylinder’slengthagainstacircle.Aroundmetalbarthatisarbitrarilylongwithrespecttoitsdiameterhastobecheckedforroundnessinmanylocationslengthwiseandaveragedtoinsureoverallconsistency.RoundnessisapproximatedbyrotatingtheworkpieceonerevolutioninaVeeblockwhilemeasuringthesurfacewithanindicator.Takingthedifferencebetweentheminimumandmaximumindicatorreadingsinthiscaseisreferredtoasthetotalindicatorreading(TIR)[1].
B.MeasuringStraightness
Straightnessisaquantityderivedfromcomparingtheaxialcenterlineofaspecificsectionofacylinder’slengthagainstastraightline.AsimplemethodforapproximatingstraightnessisbyrotatingthebaronerevolutionbetweentwoVeeblocksthatareafixeddistance(d)apart,whilemeasuringinthecenterwithanindicator.Thedistancethattheaxialcenterlineofthepartdeviatesfromatheoreticallystraightcenterlinedirectlybelowtheindicatorequalstheextenttowhichthepartisbowed,orwarped,overlengthd.Themaximumandminimumindicatorreadings(IXandIN)arephysicallyrepresentedinFig.
1.Fromthis,TIRisderivedas:
TIR=IX–IN=(R+|Bow|)-(R–|Bow|)=2*|Bow|
(1)
Deviationsinroundness,outsidediameter(OD)size,andfinishcanadverselyaffectthemeasurement.
Figure1.Max.andmin.indicatorreadingsofabowedpart
C.Straightening
Thestraighteningprocess,whichcanbebrokenintosteps,simplyinvolvescorrectinganyerrorwhilecheckingforstraightness.First,thepartismeasuredforstraightness.Then,itisrotatedsothatthebowisoriented180degreesawayfromtheVeeblockswiththemaximumindicatorreadingfacingupwards.Finally,acounter-bendingforcereplacestheindicatorandstraightenstheworkpieceagainsttheVeeblocks.
II.MACHINEOPERATION
Thestraightenertobeautomatedusesanon-contactultrasonicsensorinplaceoftheindicatorandrollersinplaceoftheVeeblocksinanefforttominimizecontactwear.Thepartslowlyspiralsthroughthemachine.Theindicatorreadingbecomesacontinuoussinusoidatthesensor’soutput,havingapeak-to-peakvalueequaltotheTIReachrevolution.TIRissampledfromthesensoroutputandcalculatedeachrevolution,makingthesampleperiodofonerevolutiontheminimumtimebetweenconsecutivebends(YSP).TIRistheplantoutput(Y)tobecontrolled.Whenthepartisstraightened,themachinestopsrotationwiththebowfacingupwards,butthepartcontinuestofeedlengthwisewhileanaircylindercounterbendsthepartoveraperiodoftime.Thisbendtime(BT)isthecontrolvariable(U).Fig.2illustratesthisoperation.
Figure2.Thestraightenertobeautomated
A.ProcessLimitations
Thereareaspectsoftheprocessthatcanlimitthecontroller’sperformanceandslowitdownbyextendingYSP.Eachisobservedandtakenintoconsiderationwhenproducing
anaccuratesimulationmodel:
1.TheultrasonicsensorintroducesRFInoiseintoits.Theuseofafeedbackfilterisessential.
2.Aroughpartsurfacefinishaddsdistortiontothesensoroutput.
3.Anout-of-roundpartsuperimposesharmonicsonthesensoroutputsinusoid,placingaboundontheminimumsteadystateerrorthatisachievable.
4.Inconsistentmaterialdensityproducesfalsemeasurements.Themeasuredfocallengthofa
transducerisdependentonthedensityofthematerialthatisbeingmeasured[2].Theunitcannotmeasureaccuratelyinthepresenceofatime-variantmaterialdensity(i.e.hardspots).Althoughunavoidable,itcanbedetected,sinceTIRchangesmonotonically.
5.AninconsistentODcausesverticalshiftsinthesensoroutput.AdifferentialTIRmeasurementcancelstheseaffects.
6.Atwistedpartconditionisdetectedwhentheangularpositionofthemaximumindicatorreadingslowlymoveswitheachrevolution.Thisconditioniscreatedwhenthepartisnotstraightenedatthepreciseangularlocationandoccursbecauseofthequantizationaffectofthedigitalreadoutusedbytheoperator.Thenewcontrollerwilluseacontinuoussignalandthepartcanbestraightened30to45degreesaheadofthetwistwhenencountered.
B.TheNeedforAutomation
Replacementoftheoperatorwithelectronichardwareisbeneficialinseveralways.Thecostoftheelectronicsismuchlessthantheongoinghourlywageandscheduleofanoperator.Thelimitationsassociatedwiththedigitalreadoutareeliminated,whichhelpsthemachinetostraightenfasterwithmoreprecision.Theprocesscanbedrasticallyspeduptoproducemore.Thoughtheminimumsampletimeisonerevolution,itdoesnotneedtobeslowenoughforhumancomprehension.AProgrammableLogicController(PLC)canmakeseveralcalculationsandtesttheresultagainstasetofrulesmanytimesfasterthanahumanbeing.
C.ResearchMethodology
Thefocusissplitbetweenmodelingandcontroldesign,sincethisisanewcontrolproblem.Theprocessismanuallycontrolledratherthanbeingstrictlymanualinoperation,meaningthemachineneedsonlyanewcontroller.Thereisnoneedforacompletemechanicaloverhaul,sothebestmethodofstraighteningisnotresearched.Typicalofasmallcompany,timeandmoneyarelimited.GaoandHuang[3]presentedanewerror-basedcontroldesignframeworkincludingsuchinnovationsasanonlineartrackingdifferentiatorandanonlinearproportional-integral-derivative(NPID)controlmethod.Thesemethodsprovetobepowerfulandsimpletotune,whichmakethemidealforuseinanindustrialenvironment.
III.ACLOSEDLOOPSOLUTION
Thetaskofautomationcanbeginoncetheprocessiswelldefined.Astraightforwardsystemblockdiagramisdeveloped,andeachblockismodeledinSimulink.Aspectsofthehardwareconfigurationarecarefullyconsidered.
A.FromOpenLooptoClosedLoop
Theopenloopmulti-input-multi-output(MIMO)blockdiagram,inFig.3,representsthemanuallycontrolledprocess.TheoperatorcalculatesTIRandmonitorstheangularposition(P)ofthebowwitheachrevolution.WhenYapproachesaspecifiedlimit,theoperatorsetsBTproportionaltoYandits
rate,andthenpushesabutton(BP)tostraightenthepart.ThebendtimercreatesapulsetriggeredbyBPthatcounter-bendsthepartforBTminutes.Thiseventforcestheparttohaveaspecificrateforaperiodoftime.Aftertheevent,theparttakesonanewrateandtheprocessperpetuates.Therefore,Y’isa
piece-wisecontinuousfunctionoftime.Theoperator’sinvolvementintheprocessisrepresentedasablockinFig.4.
Figure3.Openloopblockdiagram
Figure4.Operatorblock
Rearrangingtheblocksandbreakingeachfunctiondownintosmallermore-manageableblocksreducestherepresentationtoausableclosedloop,single-input-singleoutput(SISO)form.Fig.5showshowaSISOplantisobtainedbycombiningtheprocesswiththetaskofsamplingtheTIR,sinceitcanbeconsistentlycomputed.
Figure5.SISOplantblock
Theplanthasavariable-widthpulseastheinput(U)andthesampledTIR(Y)astheoutputtobecontrolled.Theincomingchangingpositionoftheworkpieceismodeledasanunknownratedisturbance(D).TheclosedloopSISOblockdiagramisshowninFig.6.
Figure6.Closedloopblockdiagram
B.HardwareConfiguration
DepictedinFig.7,anencoderandaPLCaretheonlyhardwareneededforautomation.Theencoderfeedstheangularpositionofthepartbacktothecontroller,andthePLChandlesallofclosedloopfunctionsoutsideoftheprocess.
Figure7.Hardwareconfiguration
Fig.8depictsthehardwarelayoutforplantdataacquisitionduringmanualoperation.ThePLCisalsousedtocalibrateastripchartrecorder,showninFig.9,whichsimplifiescalibrationfortheoperatorandremovesroomforerrorduringdataacquisition.UsingthePLCforbothdataacquisitionand
controlkeepscostslower.
Figure8.Openloopdataacquisitionconfiguration
Figure9.Chartrecordersettingsfora.006”TIRsignal
IV.MODELING
SimulationmodelinginvolvestheconstructionofSi