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.Anundesirablefeaturewhichaccompaniesasystemwhichhasbeendesignedtobeverysensitivetospeedchangesisthetendencyto“hunt”oroscillateaboutthefinalspeed.Therealprobleminthesynthesisofallsystemsofthistypeistopreventexcessiveoscillationbutatthesametimeproducegood“regulation”.Regulationisdefinedasthepercentagechangeincontrolledquantityonloadrelativetothevalueofthecontrolledunderconditionofzeroload.Regulatorsformanimportantclassofcontrolsystem,theirobjectgenerallybeingtokeepsomephysicalquantityconstant(e.g.speed,voltage,liquidlevel,humidity,etc.)regardlessofloadvariation.Agoodregulatorhasonlyverysmallregulation.Theautomaticcontrolofvariouslarge-scaleindustrialprocesses,asencounteredinthemanufactureandtreatmentofchemicals,foodandmetals,hasemergeduring2thelastthirtyyearsasanextremelyimportantpartofthegeneralfieldofcontrolengineering.Intheinitialstagesofdevelopmentitwasscarcelyrealizedthatthetheoryofprocesscontrolwasintimatelyrelatedtothetheoryofservomechanismsandregulators.Evennowadayscompleteacademicdesignofprocesscontrolsystemsisvirtuallyimpossibleowingtoourpoorunderstandingofthedynamicsofprocesses.Inmuchofthetheoryintroducedinthisbook,servomechanismsandregulatorsareusedasexampletoillustratethemethodsofanalysis.Thesemethodsare,however,oftenapplicabletoprocesscontrolsystems,whichwillbethemselvesintroducedseparately.2.ProgrammableControllersInthe1960s,electromechanicaldevicesweretheorderofthedayasfarasfarascontrolwasconcerned.Thesedevices,commonlyknownasrelays,werebeingusedbythethousandstocontrolmanysequential-typemanufacturingprocessesandstand-alonemachines.Manyoftheserelayswereinuseinthetransportationindustry,morespecifically,theautomotiveindustry.Theserelaysusedhundredsofwiresandtheirinterconnectionstoaffectacontrolsolution.Theperformanceofarelaypanelscalledfor300to500ormorerelays,andthereliabilityandmaintenanceissuesassociatedwithsupportingthesepanelsbecameaverygreatchallenge.Costbecameanotherissue,forinspiteofthelowcostoftherelayitself,theinstalledcostofthepanelcouldbequitehigh.Thetotalcostincludingpurchasedparts,wiring,andinstallationlabor,couldrangefrom3050perrelay.Tomakemattersworse,theconstantlychangingneedsofaprocesscalledforrecurringmodificationsofacontrolpanel.Withrelays,thiswasacostlyprospect,asitwasaccomplishedbyamajorrewiringeffortonthepanel.Inaddition,thesechangesweresometimespoorlydocumented,causingasecond-shiftmaintenancenightmaremonthslater.Inlightofthis,itwasnotuncommontodiscardanentirecontrolpanelinfavorofanewonewiththeappropriatecomponentswiredinamannersuitedforthenewprocess.Addtothistheunpredictable,andpotentiallyhigh,costofmaintainingthesesystemsasonhigh-volumemotorvehicleproductionlines,anditbecameclearthatsomethingwasneededtoimprovethecontrolprocess-tomakeitmorereliable,easiertotroubleshoot,andmoreadaptabletochangingcontrolneeds.Thatsomething,inthelate1960s,wasthefirstprogrammablecontroller.Thisfirst“revolutionary”systemwasdevelopedasaspecificresponsetotheneedsofthemajorautomotivemanufacturersintheUnitedStates.Theseearlycontrollers,orProgrammableLogicControllers(PLC),representedthefirstsystemsthat(1)couldbe3usedonthefactoryfloor,(2)couldhavethere“logic”changewithoutextensiverewiringorcomponentchanges,and(3)wereeasytodiagnoseandrepairwhenproblemsoccurred.Itisinterestingtoobservetheprogressthathasbeenmadeinthepast15yearsintheprogrammablecontrollerarea.Thepioneerproductsofthelate1960smusthavebeenconfusingandfrighteningtoagreatnumberofpeople.Forexample,whathappenedtothehardwiredandelectromechanicaldevicesthatmaintenancepersonnelwereusedtorepairingwithhandtools?Theywerereplacedwith“computers”disguisedaselectronicsdesignedtoreplacerelays.Eventheprogrammingtoolsweredesignedtoappearasrelayequivalentpresentations.Wehavetheopportunitynowtoexaminethepromise,inretrospect,whattheprogrammablecontrollerbroughtmanufacturing?Figure10.1AllprogrammablecontrollersconsistofthebasicfunctionalblocksshowninFigure10.1.Wewillexamineeachblocktounderstandtherelationshiptothecontrolsystem.Firstwelookedatthecenter,asitistheheartofthesystem.Itconsistsofamicroprocessor,logicmemoryforthestorageoftheactualcontrollogic,storageorvariablememoryforusewithdatathatwillordinarilychangeasafunctionofthecontrolprogramexecution,andapowersupplytoprovideelectricalpowerfortheprocessorandmemory.NextcomestheI/Oblock.ThisfunctiontakesthecontrollevelsignalsfortheCPUandconvertsthemtovoltageandcurrentlevelssuitableforconnectionwithfactorygradesensorsandactuators.TheI/Otypecanrangefromdigi