共7頁第1頁P(yáng)roductionAutomationIntroductiontoproductionAutomationAutomationisawidelyusedterminmanufacturing.Inthiscontext,automationcanbedefinedastechnologyconcernedwiththeapplicationofmechanical,electronic,andcomputer-basedsystemstooperateandcontrolproduction.Examplesofthistechnologyinclude:Automaticmachinetoolstoprocessparts.AutomatedtransferlinesandsimilarsequentialproductionsystemsAutomaticassemblymachinesIndustrialrobotsAutomaticmaterialhandingandstoragesystemsAutomatedinspectionsystemsforqualitycontrol.Feedbackcontrolandcomputerprocesscontrol.Computersystemsthatautomateproceduresforplanning,datacollection,anddecisionmakingtosupportmanufacturingactivities.Automatedproductionsystemscanbeclassifiedintotwobasiccategories:fixedautomationandprogrammableautomation.FixedAutomationFixedautomationiswhatHarderwasreferringtowhenhecoinedthewordautomation.Fixedautomationreferstoproductionsystemsinwhichthesequenceofprocessingorassemblyoperationsisfixedbytheequipmentconfigurationandcannotbereadilychangedwithoutalteringtheequipment.Althougheachoperationinthesequenceisusuallysimple,theintegrationandcomplex.Typicalfeaturesoffixedautomationinclude1.highinitialinvestmentforcustom-engineeredequipment,2.highproductionrates,3.applicationtoproductsinwhichhighquantitiesaretobeproduced,and4.relativeinflexibilityinaccommodatingproductchanges.Fixedautomationiseconomicallyjustifiableforproductswithhighdemandrates.Thehighinitialinvestmentintheequipmentcanbedividedoveralargenumberofunits,perhapsmillions,thusmakingtheunitcostlowcomparedwithalternativemethodsofproduction.Examplesoffixedautomationincludetransferlinesformachining,dialindexingmachines,andautomatedassemblymachines.Muchofthe共7頁第2頁technologyinfixedautomationwasdevelopedintheautomobileindustry；thetransferline(datingtoabout1920)isanexample.ProgrammableAutomationForprogrammableautomation,theequipmentisdesignedinsuchawaythatthesequenceofproductionoperationsiscontrolledbyaprogram,i.e.,asetofcodedinstructionsthatcanbereadandinterpretedbythesystem.Thustheoperationsequencecanbereadilychangedtopermitdifferentproductconfigurationstobeproducedonthesameequipment.Someofthefeaturesthatcharacterizeprogrammableautomationinclude1.highinvestmentingeneral-purposeprogrammableequipment,2.lowerproductionratesthanfixedautomation,3.flexibilitytodealwithchangesinproductconfiguration,and4.suitedtolowand/ormediumproductionofsimilarproductsorparts(e.g.partfamilies).Examplesofprogrammableautomationincludenumericallycontrolledmachinetools,industrialrobots,andprogrammablelogiccontrollers.Programmableproductionsystemsareoftenusedtoproducepartsorproductsinbatches.Theyareespeciallyappropriatewhenrepeatordersforbatchesofthesameproductareexpected.Toproduceeachbatchofanewproduct,thesystemmustbeprogrammedwiththesetofmachineinstructionsthatcorrespondtothatproduct.Thephysicalsetupoftheequipmentmustalsobechanged；specialfixturesmustbeattachedtothemachine,andtheappropriatetoolsmustbeloaded.Thischangeoverprocedurecanbetime-consuming.Asaresult,theusualproductioncycleforagivenbatchincludes1.aperiodduringwhichthesetupandreprogrammingisaccomplishedand2.aperiodinwhichthebatchisprocessed.Thesetup-reprogrammingperiodconstitutesnonproductivetimeoftheautomatedsystem.Theeconomicsofprogrammableautomationrequirethatasthesetup-reprogrammingtimeincrease,theproductionbatchsizemustbemadelargersoastospreadthecostoflostproductiontimeoveralargernumberofunits.Conversely,ifsetupandreprogrammingtimecanbereducedtozero,thebatchsizecanbereducedtoone.Thisisthetheoreticalbasisforflexibleautomation,anextensionofprogrammableautomation.Aflexibleautomatedsystemisonethatiscapableofproducingavarietyofproducts(orparts)withminimallosttimeforchangeoversfromoneproducttothenext.Thetimetoreprogramthesystemandalterthephysicalsetupisminimalandresultsinvirtuallynolostproductiontime.Consequently,thesystemiscapableofproducingvariouscombinationsandschedules共7頁第3頁ofproductsinacontinuousflow,ratherthanbatchproductionwithinterruptionsbetweenbatches.Thefeaturesofflexibleautomationare1.highinvestmentforacustom-engineeredsystem,2.continuousproductionofmixturesofproducts,3.abilitytochangeproductmixtoaccommodatechangesindemandratesforthedifferentproductsmade,4.mediumproductionrates,and5.flexibilitytodealwithproductdesignvariations.Flexibleautomatedproductionsystemsoperateinpracticebyoneormoreofthefollowingapproaches:1.usingpartfamilyconcepts,bywhichthepartsmadeonthesystemarelimitedinvariety；2.reprogrammingthesysteminadvanceand/oroff-line,sothatreprogrammingdoesnotinterruptproduction；3.downloadingexistingprogramstothesystemtoproducepreviouslymadepartsforwhichprogramarealreadyprepared；4.usingquick-changefixturessothatphysicalsetuptimeisminimized；5.usingafamilyoffixturesthathavebeendesignedforalimitednumberofpartstyles；and6.equippingthesystemwithalargenumberofquick-changetoolsthatincludethevarietyofprocessingoperationsneededtoproducethepartfamily.Fortheseapproachestobesuccessful,thevariationinthepartstylesproducedonaflexibleautomatedproductionsystemisusuallymorelimitedthatabatch-typeprogrammableautomationsystem.Examplesofflexibleautomationaretheflexiblemanufacturingsystemsforperformingmachiningoperationsthatdatebacktolate1960s.NumericalControlNumericalcontrol(oftenabbreviatedNC)canbedefinedasaformofprogrammableautomationinwhichtheprocessiscontrolledbynumbers,letters,andsymbols.InNC,thenumbersformaprogramofinstructionsdesignedforaparticularworkpartorjob.Whenthejobchanges,theprogramofinstructionsischanged.ThiscapabilitytochangetheprogramforeachnewjobiswhatgivesNCitsflexibility.Itismucheasiertowritenewprogramsthantomakemajorchangesintheproductionequipment.NCequipmentisusedinallareasofmetalpartsfabricationandcomprisesroughly15%ofthemodernmachinetoolsinindustrytoday.Sincenumericallycontrolledmachinesareconsiderablymoreexpensivethantheirconventionalcounterparts,theassetvalueofindustrialNCmachinetoolsisproportionallymuchlargerthantheirnumbers.Equipmentutilizingnumericalcontrolhasbeendesigned共7頁第4頁toperformsuchdiverseoperationsasdrilling,milling,turning,grinding,sheetmetalpressworkingspotwelding,arewelding,riveting,assembly,drafting,inspection,andpartshandling.Andthisisbynomeansacompletelist.Numericalcontrolshouldbeconsideredasapossiblemodeofcontrollingtheoperationforanyproductionsituationpossessingthefollowingcharacteristics:1.Similarworkpartsintermsofrawmaterial(e.g.,metalstockformachining).2.Theworkpartsareproducedinvarioussizesandgeometries.3.Theworkpartsareproducedinbatchesofsmalltomedium-sizedquantities.4.Asequenceofsimilarprocessingstepsisrequiredtocompletetheoperationoneachworkpiece.Manymachiningjobsmeettheseconditions.Themachinedworkpartsaremetal,theyarespecifiedinmanydifferentsizesandshapes,andmostmachinedpartsproducedinindustrytodayaremadeinsmalltomedium-sizelotsizes.Toproduceeachpart,asequenceofdrillingoperationsmayberequired,oraseriesofturningormillingoperations.ThesuitabilityofNCforthesekindsofjobsisthereasonforthetremendousgrowthofnumericalcontrolinthemetalworkingindustryoverthelast25yea