先进制造技术 第六单元.docx
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先进制造技术 第六单元.docx
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先进制造技术第六单元
6FlexibleManufacturing
Asanintroductiontothesubsequentdiscussionsofproductionsystems
andadvancedmanufacturingtechnologiesitisusefultopresentadefinitionof
thetermmanufacturingsystem.Amanufacturingsystemcanbedefinedasa
seriesofvalue-addingmanufacturingprocessesconvertingtherawmaterials
intomoreusefulformsandeventuallyfinishedproducts.)11
Inthemodemmanufacturingsetting,flexibilityisanimportantcharacteristic.
Itmeansthatamanufacturingsystemisversatileandadaptable,whilealso
capableofhandlingrelativelyhighproductionruns.Aflexiblemanufacturing
systemisversatileinthatitcanproduceavarietyofparts.Itisadaptablebecause
itcanbequicklymodifiedtoproduceacompletelydifferentlineofparts.
Aflexiblemanufacturingsystem(FMS)isanindividualmachineorgroup
ofmachinesservedbyanautomatedmaterialshandlingsystemthatis
computercontrolledandhasatoolhandlingcapability.Becauseofitstool
handlingcapabilityandcomputercontrol,suchasystemcanbecontinually
reconfiguredtomanufactureawidevarietyofparts.Thisiswhyitiscalleda
flexiblemanufacturingsystem.
Flexiblemanufacturingrepresentsamajorsteptowardthegoaloffully
integratedmanufacturing.Itinvolvesintegrationofautomatedproduction
processes.Inflexiblemanufacturing,theautomatedmanufacturingmachine
(i.e.,lathe,mill,drill)andtheautomatedmaterialshandlingsystemshare
instantaneouscommunicationviaacomputernetwork.Fig.6.1isanexampleof
aflexiblemanufacturingsystem.
Flexiblemanufacturingtakesamajorsteptowardthegoaloffullyintegrated
manufacturingbyintegratingseveralautomatedmanufacturingconcepts:
(1)Computernumericalcontrol(CNC)ofindividualmachinetools.
(2)Distributednumericalcontrol(DNC)ofmanufacturingsystems.
(3)Automatedmaterialshandlingsystems.
(4)Grouptechnology(familiesofparts).
Whentheseautomatedprocesses,machines,andconceptsarebrought
togetherinoneintegratedsystem,anFMSistheresult.Humansandcomputers
playmajorrolesinanFMS.Theamountofhumanlaborismuchlessthanwith
amanuallyoperatedmanufacturingsystem,ofcourse.However,humansstill
playavitalroleintheoperationofanFMS.Humantasksincludethefollowing:
(1)Equipmenttroubleshooting,maintenance,andrepair.
(2)Toolchangingandsetup.
(3)Loadingandunloadingthesystem.
(4)Datainput.
(5)Changingofpartsprograms.
(6)Developmentofprograms.
Flexiblemanufacturingsystemequipment,likeallmanufacturingequipment,
mustbemonitoredforbugs,malfunctions,andbreakdowns.Whenaproblemis
discovered,ahumantroubleshootermustidentifyitssourceandprescribe
correctivemeasures.Evenwhenallsystemsareproperlyfunctioning,periodic
maintenanceisnecessary.
Humanoperatorsalsosetupmachines,changetools,andreconfigure
systemsasnecessary.ThetoolhandlingcapabilityofanFMSincreases,but
doesnoteliminatehumaninvolvementintoolchangingandsetup.Thesameis
trueofloadingandunloadingtheFMS.Oncerawmaterialhasbeenloaded
ontotheautomatedmaterialshandlingsystem,itismovedthroughthesystem
intheprescribedmanner.However,theoriginalloadingontothematerials
handlingsystemisstillusuallydonebyhumanoperators,asistheunloadingof
finishedproducts.
Humansarcalsoneededforinteractionwiththecomputer.Humans
developpartprogramsthatcontroltheFMSviacomputers.Theyalsochange
theprogramsasnecessarywhenreconfiguringtheFMStoproduceanother
typeofpartorparts.Humansplaylesslabor-intensiverolesinanFMS,butthe
rolesarcstillcritical.
ControlatalllevelsinanFMSisprovidedbycomputers.Individual
machinetoolswithinanFMSarecontrolledbyCNC.Theoverallsystemis
controlledbyDNC.Theautomatedmaterialshandlingsystemiscomputer
controlled,asareotherfunctionsincludingdatacollection,systemmonitoring,
toolcontrol,andtrafficcontrol.Human/computerinteractionisthekeytothe
flexibilityofanFMS.
6.1HistoricalDevelopmentofFlexibleManufacturing
Flexiblemanufacturingwasborninthemid-1960swhentheBritishfirm
Molins,Ltd.developeditsSystem24.System24wasarealFMS.However,it
wasdoomedfromtheoutsetbecauseautomation,integration,andcomputer
controltechnologyhadnotyetbeendevelopedtothepointwheretheycould
properlysupportthesystem.ThefirstFMSwasadevelopmentthatwasahead
ofitstime.Assuch,itwaseventuallydiscardedasunworkable.
Flexiblemanufacturingremainedanacademicconcept
remainderofthe1960sand1970s.However,withthe
through
emergence
sophisticatedcomputercontroltechnologyinthelate1970sandearly1980s,
flexiblemanufacturingbecameaviableconcept.Thefirstmajorusersofflexible
manufacturingintheUnitedStatesweremanufacturersofautomobiles,trucks,
andtractors.
6.2RationaleforFlexibleManufacturing
Inmanufacturingtherehavealwaysbeentradeoffsbetweenproduction
ratesandflexibility.Atoneendoftheissuearetransferlinescapableofhigh
productionrates,butlowflexibility.Attheotherendoftheissueare
independentCNCmachinesthatoffermaximumflexibility,butarecapable
onlyoflowproductionrates.Flexiblemanufacturingfallsinthemiddle.There
hasalwaysbeenaneedinmanufacturingforasystemthatcouldproduce
highervolumeandproductionrunsthancouldindependentmachines,while
stillmaintainingflexibility.
Transferlinesarecapableofproducinglargevolumesofpartsathigh
productionrates.Thelinetakesagreatdealofsetup,butcanturnoutidentical
partsinlargequantities.Itschiefshortcomingisthatevenminordesign
changesinapartcancausetheentirelinetobeshutdownandreconfigured.
Thisisacriticalweaknessbecauseitmeansthattransferlinescannotproduce
differentparts,evenpartsfromwithinthesamefamily,withoutcostlyand
time-consumingshutdownandreconfiguration.
Traditionally,CNCmachineshavebeenusedtoproducesmallvolumesof
partsthatdifferslightlyindesign.Suchmachinesareidealforthispurpose
becausetheycanbequicklyreprogrammedtoaccommodateminororeven
majordesignchanges.However,asindependentmachinestheycannotproduce
partsinlargevolumesorathighproductionrates.
AnFMScanhandlehighervolumesandproductionratesthanindependent
CNCmachines.Itcannotquitematchsuchmachinesforflexibility,butthey
comeclose.Whatisparticularlysignificantaboutthemiddlegroundcapabilities
offlexiblemanufacturingisthatmostmanutactunngsituationsrequiremedium
productionratestoproducemediumvolumeswithenoughflexibilitytoquickly
reconfiguretoproduceanotherpartorproduct.Flexiblemanufacturingfills
thislong-standingvoidinmanufacturing.12'
Flexiblemanufacturing,withitsgroundcapabilities,offersanumberof
advantagesformanufacturers:
(1)Flexibilitywithinafamilyofparts.
(2)Randomfeedingofparts
(3)Simultaneousproductionofdifferentparts
(4)Decreasedsetuptimeandleadtime.
(5)Moreefficientmachineusage.
(6)Decreaseddirectandindirectlaborcosts.
(7)Abilitytohandledifferentmaterials
(8)Abilitytocontinuesomeproductionifonemachinebreaksdown.
6.3FlexibleManufacturingSystemComponents
AFMShasthreemajorcomponents:
(一)Machinetools.
(2)Controlsystem.
(3)Materialshandlingsystem.
6.3.1MachineTools
Aflexiblemanufacturingsystemusesthesametypesofmachinetoolsas
anyothermanufacturingsystem,beitautomatedormanuallyoperated.These
includelathes,mills,drills,saws,andsoon.Thetypeofmachinetoolsactually
includedinanFMSdependsonthesettinginwhichthemachinewillbeused.
SomeFMSesaredesignedtomeetaspecific,well-definedneed.Inthesecases
themachinetoolsincludedinthesystemwillbeonlythosenecessaryforthe
plannedoperations.Suchasystemwouldbeknownasadedicatedsystem.
Inajob-shopsetting,oranyothersettinginwhichtheactualapplicationis
notknownaheadoftimeormustnecessarilyincludeawiderangeof
possibilities,machinescapableofperformingatleastthestandardmanufacturing
operationswouldbeincluded.Suchsystemsareknownasgeneralpurpose
systems.
6.3.2ControlSystem
ThecontrolsystemforanFMSservesanumberofdifferentcontrol
functionsforsystem:
(1)Storageanddistributionofpartsprograms.
(2)Workflowcontrolandmonitoring.
(3)Productioncontrol
(4)System/toolcontrol/monitoring.
ThecontrolareawiththecomputerrunningtheFMScontrolsystemisthe
centerfromwhichallactivitiesintheFMSarecontrolledandmonitored.The
FMScontrolsoftwareisrathercomplicatedandsophisticatedsinceithasto
carryoutmanydifferenttasksimultaneously.Despitetheconsiderableresearch
thathasbeencarriedoutinthisarea,thereisnogeneralanswertodesigning
thefunctionsandarchitectureofFMSsoftware
SometypicalfunctionsofaFMScontrolsystemareillustratedinFig.6.2.
Theschedulerfunctioninvolvesplanninghowtoproducethecurrent
volumeofordersintheFMS,consideringthecurrentstatusofmachinetools,
work-in-pocess,tooling,fixtures,andsoon.131Theschedulingcanbedone
automaticallyorcanbeassistedbyanoperator.MostFMScontrolsystems
combineautomaticandmanualscheduling;thesystemgeneratesaninitial
schedulethatcanbechangedmanuallybytheoperator.Thedispatcher
functioninvolvescarryingoutthescheduleandcoordinatingtheactivitieson
theshopfloor,thatis,decidingwhenandwheretotransportapallet,whento
startapr
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