外文翻译--控制技术.doc

1ControlTechnology1.IntroductiontoControlEngineeringWheneverenergyistobeusedpurposefully,someformofcontrolisnecessary,inrecenttimestherehasbeenaconsiderableadvancemadeintheartofautomaticcontrol.Theartis,however,quiteold,stemmingbacktoabout1790whenJamesWattinventedthecentrifugalgovernortocontrolthespeedofhissteamengines.Hefoundthatwhileinmanyapplicationsanenginespeedindependentofloadwasremovedthespeedincreased.Inasimplecentrifugalgovernorsystem,variationsinenginespeedaredetectedandusedtocontrolthepressureofthesteamenteringtheengine.Understeadyconditionsthemomentoftheweightofthemetalspheresbalancesthatduetothecentrifugalforceandthesteamvalveopeningisjustsufficienttomaintaintheenginespeedattherequiredlevel.Whenanextraloadtorqueisappliedtotheengine,itsspeedwilltendtofall,thecentrifugalforcewilldecreaseandthemetalsphereswilltendtofallslightly.Theirheightcontrolstheopeningofthesteamvalvewhichnowopensfurthertoallowagreatersteampressureontheengine.Thespeedthustendstorise,counteractingtheoriginaltendencyforthespeedtofall.Iftheextraloadisremoved,thereverseprocesstakesplace,themetalspherestendtoriseslightly,sotendingtoclosethesteamvalveandcounteractinganytendencyforthespeedtorise.Itisobviouslythatwithoutthegovernorthespeedwouldfallconsiderablyonland.However,inacorrectlydesignedsystemwithagovernorthefallinspeedwouldbeverymuchless.Anundesirablefeaturewhichaccompaniesasystemwhichhasbeendesignedtobeverysensitivetospeedchangesisthetendencytohuntoroscillateaboutthefinalspeed.Therealprobleminthesynthesisofallsystemsofthistypeistopreventexcessiveoscillationbutatthesametimeproducegoodregulation.Regulationisdefinedasthepercentagechangeincontrolledquantityonloadrelativetothevalueofthecontrolledunderconditionofzeroload.Regulatorsformanimportantclassofcontrolsystem,theirobjectgenerallybeingtokeepsomephysicalquantityconstante.g.speed,voltage,liquidlevel,humidity,etc.regardlessofloadvariation.Agoodregulatorhasonlyverysmallregulation.Theautomaticcontrolofvariouslargescaleindustrialprocesses,asencounteredinthemanufactureandtreatmentofchemicals,foodandmetals,hasemergeduring2thelastthirtyyearsasanextremelyimportantpartofthegeneralfieldofcontrolengineering.Intheinitialstagesofdevelopmentitwasscarcelyrealizedthatthetheoryofprocesscontrolwasintimatelyrelatedtothetheoryofservomechanismsandregulators.Evennowadayscompleteacademicdesignofprocesscontrolsystemsisvirtuallyimpossibleowingtoourpoorunderstandingofthedynamicsofprocesses.Inmuchofthetheoryintroducedinthisbook,servomechanismsandregulatorsareusedasexampletoillustratethemethodsofanalysis.Thesemethodsare,however,oftenapplicabletoprocesscontrolsystems,whichwillbethemselvesintroducedseparately.2.ProgrammableControllersInthe1960s,electromechanicaldevicesweretheorderofthedayasfarasfarascontrolwasconcerned.Thesedevices,commonlyknownasrelays,werebeingusedbythethousandstocontrolmanysequentialtypemanufacturingprocessesandstandalonemachines.Manyoftheserelayswereinuseinthetransportationindustry,morespecifically,theautomotiveindustry.Theserelaysusedhundredsofwiresandtheirinterconnectionstoaffectacontrolsolution.Theperformanceofarelaypanelscalledfor300to500ormorerelays,andthereliabilityandmaintenanceissuesassociatedwithsupportingthesepanelsbecameaverygreatchallenge.Costbecameanotherissue,forinspiteofthelowcostoftherelayitself,theinstalledcostofthepanelcouldbequitehigh.Thetotalcostincludingpurchasedparts,wiring,andinstallationlabor,couldrangefrom＄30～＄50perrelay.Tomakemattersworse,theconstantlychangingneedsofaprocesscalledforrecurringmodificationsofacontrolpanel.Withrelays,thiswasacostlyprospect,asitwasaccomplishedbyamajorrewiringeffortonthepanel.Inaddition,thesechangesweresometimespoorlydocumented,causingasecondshiftmaintenancenightmaremonthslater.Inlightofthis,itwasnotuncommontodiscardanentirecontrolpanelinfavorofanewonewiththeappropriatecomponentswiredinamannersuitedforthenewprocess.Addtothistheunpredictable,andpotentiallyhigh,costofmaintainingthesesystemsasonhighvolumemotorvehicleproductionlines,anditbecameclearthatsomethingwasneededtoimprovethecontrolprocesstomakeitmorereliable,easiertotroubleshoot,andmoreadaptabletochangingcontrolneeds.Thatsomething,inthelate1960s,wasthefirstprogrammablecontroller.ThisfirstrevolutionarysystemwasdevelopedasaspecificresponsetotheneedsofthemajorautomotivemanufacturersintheUnitedStates.Theseearlycontrollers,orProgrammableLogicControllersPLC,representedthefirstsystemsthat1couldbe3usedonthefactoryfloor,2couldhavetherelogicchangewithoutextensiverewiringorcomponentchanges,and3wereeasytodiagnoseandrepairwhenproblemsoccurred.Itisinterestingtoobservetheprogressthathasbeenmadeinthepast15yearsintheprogrammablecontrollerarea.Thepioneerproductsofthelate1960smusthavebeenconfusingandfrighteningtoagreatnumberofpeople.Forexample,whathappenedtothehardwiredandelectromechanicaldevicesthatmaintenancepersonnelwereusedtorepairingwithhandtoolsTheywerereplacedwithcomputersdisguisedaselectronicsdesignedtoreplacerelays.Eventheprogrammingtoolsweredesignedtoappearasrelayequivalentpresentations.Wehavetheopportunitynowtoexaminethepromise,inretrospect,whattheprogrammablecontrollerbroughtmanufacturingFigure10.1AllprogrammablecontrollersconsistofthebasicfunctionalblocksshowninFigure10.1.Wewillexamineeachblocktounderstandtherelationshiptothecontrolsystem.Firstwelookedatthecenter,asitistheheartofthesystem.Itconsistsofamicroprocessor,logicmemoryforthestorageoftheactualcontrollogic,storageorvariablememoryforusewithdatathatwillordinarilychangeasafunctionofthecontrolprogramexecution,andapowersupplytoprovideelectricalpowerfortheprocessorandmemory.NextcomestheI/Oblock.ThisfunctiontakesthecontrollevelsignalsfortheCPUandconvertsthemtovoltageandcurrentlevelssuitableforconnectionwithfactorygradesensorsandactuators.TheI/Otypecanrangefromdigital,analog,oravarietyofspecialpurposesmartI/Owhicharededicatedtoacertainapplicationtask.Theprogrammerisnormallyusedonlytoinitiallyconfigureandprogramasystemandisnotrequiredforthesystemtooperate.Itisalsousedintroubleshootingasystem,andcanprovetobeavaluabletoolinpinpointingtheexact4causeofaproblem.ThefielddevicesshownhererepresentthevarioussensorsandactuatorsconnectedtotheI/O.Thesearethearms,legs,eyes,andearsofthesystem,includingpushbuttons,limitswitches,proximityswitches,photosensors,thermocouples,positionsensingdevices,andbarcodereaderasinputandpilotlight,displaydevices,motorstarters,DCandACdrivers,solenoids,andprintersasoutputs.