畢業(yè)設(shè)計(jì)資料 計(jì)算機(jī)輔助制造英文原文

IntegratedComputerAidedManufacturing

1.INTRODUCTION

Today’sindustrycompetesinatrulyinternationalmarketplace.Efficienttransportationnetworkshavecreateda“worldmarket”inwhichweparticipateonadailybasis.Foranyindustrialcountrytocompeteinthismarket,itmusthavecompaniesthatprovideeconomichigh-qualityproductstotheircustomersinatimelymanner.Theimportanceofintegratingproductdesignandprocessdesigntoachieveadesignforproductionsystemcannotbeoveremphasized.However,evenonceadesignisfinalized,manufacturingindustriesmustbewillingtoaccommodatetheircustomersbyallowinglast-minuteengineering-designchangeswithoutaffectingshippingschedulesoralteringproductquality.

MostU.S.-basedmanufacturingcompanieslooktowardCAD/CAMandCIMtoprovidethisflexibilityintheirmanufacturingsystem.Today,theuseofcomputersinmanufacturingiscommon.Manufacturingsystemarebeingdesignedthatnotonlyprocesspartsautomatically,butalsomovethepartsfrommachinetomachineandsequencetheorderingofoperationsinthesystem.（Figure1）containsaplotoftheeconomicregionsofmanufacturing.ItshouldbenotedthatmanualhandcraftedgoodswillalwayshaveamarketintheUnitedStatesaswellasabroad.Thisisalsotrueofindustrialproducts—therewillcontinuetobeaneedforspecialone-of-a-kinditems.Thespectrumofone-of-a-kindgoodsthroughhigh-volumegoodsdictatesthatavarietyofmanufacturingmethodsbeusedtomeetourvariousindustrialneeds.Someofthesesystemswilllooklikethefactoriesthatourgrandparentslaboredin,whereasotherswilltakeonafuturisticlook.Inthefollowingsections,adiscussionofflexiblemanufacturingsystemsispresented.

Figure1Volumeversusvarietyregionsforeconomicmanufacturing

2.FLEXIBLEMANUFACTURINGSYSTEMS

Aflexiblemanufacturingsystem,orFMSastheyaremorecommonlyknown,isareprogram-ablemanufacturingsystemcapableofproducingavarietyofproductsautomatically.SinceHenryFordfirstintroducedandmodernizedthetransferline,wehavebeenabletoperformavarietyofmanufacturingoperationsautomatically.However,alteringthesesystemstoaccommodateevenminorchangesintheproducthasbeenquitetaxing.Wholemachinesmighthavetobeintroducedtothesystemwhileothermachinesorcomponentsaremodifiedorretiredtoaccommodatesmallchangesinaproduct.Intoday’scompetitivemarketplace,itisnecessarytoaccommodatecustomerchangesorthecustomerwillfindsomeoneelsewhowillaccommodatethechanges.Conventionalmanufacturingsystemshavebeenmarkedbyoneoftwodistinctfeatures:

Jobshoptypesystemswerecapableofproducingavarietyofproduct,butatahighcost.

Transferlinescouldproducelargevolumesofaproductatareasonablecost,butwerelimitedtotheproductionofone,two,orveryfewdifferentparts.

Theadventofnumericalcontrol(NC)androboticshasprovideduswithreprogrammingcapabilitiesatthemachinelevelwithminimumsetuptime.NCmachinesandrobotsprovidethebasicphysicalbuildingblocksforre-programmablemanufacturingsystems.

2.1.FMSEquipment

2.1.1Machines

InordertomeettherequirementsofthedefinitionofanFMS,thebasicprocessinginthesystemmustbeautomated.Becauseautomationmustbeprogrammableinordertoaccommodateavarietyofproduct-processingrequirements,easilyalterableaswellasversatilemachinesmustperformthebasicprocessing.

Forthisreason,CNCturningcenters,CNCmachiningcenters,androboticworkstationscomprisethemajorityofequipmentinthesesystems.Thesemachinesarenotonlycapableofbeingeasilyreprogrammed,butarealsocapableofaccommodatingavarietyoftoolingviaatoolchangerandtool-storagesystem.ItisnotunusualforaCNCmachiningcentertocontainto12ormoretools(right-handturningtools,left-handturningtools,boringbars,drills,andsoon).TheautomatictoolchangerandstoragecapabilitiesofNCmachinesmakethemnaturalchoicesformaterial-processingequipment.

Partsmustalsobemovedbetweenprocessingstationsautomatically.Severaldifferenttypesofmaterial-handlingsystemsareemployedtomovethesepartsfromstationtostation.Theselectionofthetypeofmaterial-handlingsystemisafunctionofseveralsystemfeatures.Thematerial-handlingsystem,first,mustbeabletoaccommodatetheloadandbulkofthepartandperhapsthepartfixture.Large,heavypartsrequirelarge,powerfulhandlingsystemssuchasrollerconveyorsguidedvehiclesortrack-drivenvehiclesystems.Thenumberofmachinestobeincludedinthesystemandthelayoutofthemachinesalsopresentanotherdesignconsideration.Ifsinglematerialhandlermustbeatleastaslargeasthephysicalsystem.Arobotisnormallyonlycapableofaddressingoneortwomachinesandload-and-unloadstation.Aconveyororautomaticguidevehicle(AGV)systemcanbeexpandedtoincludemilesoffactoryfloor.Thematerial-handlingsystemmustalsobecapableofmovingpartsfromonemachinetoanotherinatimelymanner.Machinesinthesystemwillbeunproductiveiftheyspendmuchoftheirtimewaitingforpartstobedeliveredbythematerialhandler.Ifmanypartsareincludedinthesystemandtheyrequirefrequentvisitstomachines,thenthematerial-handlingsystemmustbecapableofsupportingtheseactivities.Thisusuallycanbeaccommodatedbyusingeitheraveryfasthandlingdeviceofbyusingseveraldevicesinparallel,forexample,insteadofusingasinglerobottomovepartstoallthemachinesinthesystem,arobotwouldonlysupportasinglemachine.

2.1.2Toolingandfixtures.

VersatilityisthekeytomostFMSs,andassuchthetoolingusedinthesystemmustbecapableofsupportingavarietyofproductsorparts.TheuseofspecialformingtoolsinanFMSisnottypicalinpractice.Thecontoursobtainedbyusingformingtoolscanusuallybeobtainedthroughacontour-controlNCsystemandastandardmill.Thestandardmillthencanbeusedforavarietyofpartsratherthantoproduceasinglespecialcontour.Aneconomicofthecostandbenefitsofanyspecialtoolingisnecessarytodeterminethebesttoolingcombination.However,becauseNCmachineshavealimitedoftoolsthatareaccessible,veryspecialtoolsshouldbeincluded.

OneofthecommonlyneglectedaspectsofanFMSisthefixturingused.Becausefixturesarepartofthetoolingofthesystem,onecouldarguethattheyshouldalsobestandardforthesystem.Workoncreating“flexiblefixtures”thatcouldbeusedtosupportavarietyofcomponentshasonlyrecentlybegun.SeeChapter5.OneuniqueaspectofmanyFMSsisthatthepartisalsomovedaboutthesysteminthefixture(orpalletfixture).Fixturesaremadetothesamedimensionssothatthematerial-handlingsystemcanbespecializedtohandleasinglegeometry.Partsarelocatedpreciselyonthefixtureandmovedfromonestationtoanotheronthefixture.Fixturesofthistypeareusuallycalledpalletfixtures,orpallets.Manyofthepalletfixturesemployedtodayhavestandard“T-slots”cutinthem,andusestandardfixturekitstocreatethepart-locatingand-holdingenvironmentneedformachining.

3.COMPUTERCONTROLOFFLEXIBLEMANUFACTURINGSYSTEMS

3.1FMSArchitecture

AnFMSisacomplexnetworkofequipmentandprocessesthatmustbecontrolledviaacomputerornetworkofcomputers.InordertomakethetaskofcontrollinganFMSmoretractable,thesystemisusuallydividedintoatask-basedhierarchy.OneofthestandardhierarchiesthathaveevolvedistheNationalInstituteofStandardsandTechnology(NIST)factory-controlhierarchy.(NISTwasformerlytheNationalBureauofstandards.NBS.)ThishierarchyconsistsoffivelevelsandisillustratedinFigures2andFigures3Thesystemconsistsofphysicalmachiningequipmentatthelowestlevelofthesystem.Workstationequipmentresidesjustabovetheprocesslevelandprovidesintegrationandinterfacefunctionsfortheequipment.Forinstancepalletfixturesandprogrammingelementsarepartoftheworkstation.Theworkstationtypicallyprovidesbothman-machineinterfaceaswellasmachine-partinterface.Off-lineprogrammingsuchasAPTforNCorAMLforrobotresidesattheworkstationlevel.

Thecellistheunitinthehierarchywhereinteractionbetweenmachinesbecomespartofthesystem.Thecellcontrollerprovidestheinterfacebetweenthemachinesandmaterial-handlingsystem.Assuch,thecellcontrollerisresponsibleforsequencingandschedulingpartsthroughthesystem.Attheshoplevelintegrationofmultiplecellsoccursaswellastheplanningandmanagementofinventory.The

Fig2

Figure3Therelationshipbetweenthedata-administration(DAS)intheNISTarchitecture:(1)thetopologiesoftheIntegratedManufacturingDataAdministrationSystem(IMDAS)data-administrationsystem;(2)thenetworkdata-communicationnetwork;(3)thehierarchicalsystemofdata-drivencontrol:datapreparationisimpliedin(4)thefacilitylevelofcontrolfacilitylevelistheplaceinthehierarchywherethemasterproductionscheduleisconstructedandmanufacturingresourceplanningisconducted.Orderingmaterialsplanninginventoriesandanalyzingbusinessplansarepartoftheactivitiesthataffecttheproductionsystem.Poorbusinessandmanufacturingplanswillincapacitatethemanufacturingsystemjustassurlytheunavailabilityofamachine.

3.2FMSSchedulingandcontrol

Flexiblemanufacturingsystems,likeothermanufacturingsystemcandiffersignificantlycomplexity.Thiscomplexityisnotonlydeterminedbythenumberofmachinesandthenumberofpartsresidentinthesystem,butalsobythecomplexityofpartsandcontrolrequirementsofthespecificequipment.SomeFMSsrequireonlyasimpleprogrammablecontrollertoregulatetheflowofpartsthough