外文翻译--控制技术.doc

1CONTROLTECHNOLOGY1INTRODUCTIONTOCONTROLENGINEERINGWHENEVERENERGYISTOBEUSEDPURPOSEFULLY,SOMEFORMOFCONTROLISNECESSARY,INRECENTTIMESTHEREHASBEENACONSIDERABLEADVANCEMADEINTHEARTOFAUTOMATICCONTROLTHEARTIS,HOWEVER,QUITEOLD,STEMMINGBACKTOABOUT1790WHENJAMESWATTINVENTEDTHECENTRIFUGALGOVERNORTOCONTROLTHESPEEDOFHISSTEAMENGINESHEFOUNDTHATWHILEINMANYAPPLICATIONSANENGINESPEEDINDEPENDENTOFLOADWASREMOVEDTHESPEEDINCREASEDINASIMPLECENTRIFUGALGOVERNORSYSTEM,VARIATIONSINENGINESPEEDAREDETECTEDANDUSEDTOCONTROLTHEPRESSUREOFTHESTEAMENTERINGTHEENGINEUNDERSTEADYCONDITIONSTHEMOMENTOFTHEWEIGHTOFTHEMETALSPHERESBALANCESTHATDUETOTHECENTRIFUGALFORCEANDTHESTEAMVALVEOPENINGISJUSTSUFFICIENTTOMAINTAINTHEENGINESPEEDATTHEREQUIREDLEVELWHENANEXTRALOADTORQUEISAPPLIEDTOTHEENGINE,ITSSPEEDWILLTENDTOFALL,THECENTRIFUGALFORCEWILLDECREASEANDTHEMETALSPHERESWILLTENDTOFALLSLIGHTLYTHEIRHEIGHTCONTROLSTHEOPENINGOFTHESTEAMVALVEWHICHNOWOPENSFURTHERTOALLOWAGREATERSTEAMPRESSUREONTHEENGINETHESPEEDTHUSTENDSTORISE,COUNTERACTINGTHEORIGINALTENDENCYFORTHESPEEDTOFALLIFTHEEXTRALOADISREMOVED,THEREVERSEPROCESSTAKESPLACE,THEMETALSPHERESTENDTORISESLIGHTLY,SOTENDINGTOCLOSETHESTEAMVALVEANDCOUNTERACTINGANYTENDENCYFORTHESPEEDTORISEITISOBVIOUSLYTHATWITHOUTTHEGOVERNORTHESPEEDWOULDFALLCONSIDERABLYONLANDHOWEVER,INACORRECTLYDESIGNEDSYSTEMWITHAGOVERNORTHEFALLINSPEEDWOULDBEVERYMUCHLESSANUNDESIRABLEFEATUREWHICHACCOMPANIESASYSTEMWHICHHASBEENDESIGNEDTOBEVERYSENSITIVETOSPEEDCHANGESISTHETENDENCYTO“HUNT”OROSCILLATEABOUTTHEFINALSPEEDTHEREALPROBLEMINTHESYNTHESISOFALLSYSTEMSOFTHISTYPEISTOPREVENTEXCESSIVEOSCILLATIONBUTATTHESAMETIMEPRODUCEGOOD“REGULATION”REGULATIONISDEFINEDASTHEPERCENTAGECHANGEINCONTROLLEDQUANTITYONLOADRELATIVETOTHEVALUEOFTHECONTROLLEDUNDERCONDITIONOFZEROLOADREGULATORSFORMANIMPORTANTCLASSOFCONTROLSYSTEM,THEIROBJECTGENERALLYBEINGTOKEEPSOMEPHYSICALQUANTITYCONSTANTEGSPEED,VOLTAGE,LIQUIDLEVEL,HUMIDITY,ETCREGARDLESSOFLOADVARIATIONAGOODREGULATORHASONLYVERYSMALLREGULATIONTHEAUTOMATICCONTROLOFVARIOUSLARGESCALEINDUSTRIALPROCESSES,ASENCOUNTEREDINTHEMANUFACTUREANDTREATMENTOFCHEMICALS,FOODANDMETALS,HASEMERGEDURING2THELASTTHIRTYYEARSASANEXTREMELYIMPORTANTPARTOFTHEGENERALFIELDOFCONTROLENGINEERINGINTHEINITIALSTAGESOFDEVELOPMENTITWASSCARCELYREALIZEDTHATTHETHEORYOFPROCESSCONTROLWASINTIMATELYRELATEDTOTHETHEORYOFSERVOMECHANISMSANDREGULATORSEVENNOWADAYSCOMPLETEACADEMICDESIGNOFPROCESSCONTROLSYSTEMSISVIRTUALLYIMPOSSIBLEOWINGTOOURPOORUNDERSTANDINGOFTHEDYNAMICSOFPROCESSESINMUCHOFTHETHEORYINTRODUCEDINTHISBOOK,SERVOMECHANISMSANDREGULATORSAREUSEDASEXAMPLETOILLUSTRATETHEMETHODSOFANALYSISTHESEMETHODSARE,HOWEVER,OFTENAPPLICABLETOPROCESSCONTROLSYSTEMS,WHICHWILLBETHEMSELVESINTRODUCEDSEPARATELY2PROGRAMMABLECONTROLLERSINTHE1960S,ELECTROMECHANICALDEVICESWERETHEORDEROFTHEDAYASFARASFARASCONTROLWASCONCERNEDTHESEDEVICES,COMMONLYKNOWNASRELAYS,WEREBEINGUSEDBYTHETHOUSANDSTOCONTROLMANYSEQUENTIALTYPEMANUFACTURINGPROCESSESANDSTANDALONEMACHINESMANYOFTHESERELAYSWEREINUSEINTHETRANSPORTATIONINDUSTRY,MORESPECIFICALLY,THEAUTOMOTIVEINDUSTRYTHESERELAYSUSEDHUNDREDSOFWIRESANDTHEIRINTERCONNECTIONSTOAFFECTACONTROLSOLUTIONTHEPERFORMANCEOFARELAYPANELSCALLEDFOR300TO500ORMORERELAYS,ANDTHERELIABILITYANDMAINTENANCEISSUESASSOCIATEDWITHSUPPORTINGTHESEPANELSBECAMEAVERYGREATCHALLENGECOSTBECAMEANOTHERISSUE,FORINSPITEOFTHELOWCOSTOFTHERELAYITSELF,THEINSTALLEDCOSTOFTHEPANELCOULDBEQUITEHIGHTHETOTALCOSTINCLUDINGPURCHASEDPARTS,WIRING,ANDINSTALLATIONLABOR,COULDRANGEFROM＄30～＄50PERRELAYTOMAKEMATTERSWORSE,THECONSTANTLYCHANGINGNEEDSOFAPROCESSCALLEDFORRECURRINGMODIFICATIONSOFACONTROLPANELWITHRELAYS,THISWASACOSTLYPROSPECT,ASITWASACCOMPLISHEDBYAMAJORREWIRINGEFFORTONTHEPANELINADDITION,THESECHANGESWERESOMETIMESPOORLYDOCUMENTED,CAUSINGASECONDSHIFTMAINTENANCENIGHTMAREMONTHSLATERINLIGHTOFTHIS,ITWASNOTUNCOMMONTODISCARDANENTIRECONTROLPANELINFAVOROFANEWONEWITHTHEAPPROPRIATECOMPONENTSWIREDINAMANNERSUITEDFORTHENEWPROCESSADDTOTHISTHEUNPREDICTABLE,ANDPOTENTIALLYHIGH,COSTOFMAINTAININGTHESESYSTEMSASONHIGHVOLUMEMOTORVEHICLEPRODUCTIONLINES,ANDITBECAMECLEARTHATSOMETHINGWASNEEDEDTOIMPROVETHECONTROLPROCESSTOMAKEITMORERELIABLE,EASIERTOTROUBLESHOOT,ANDMOREADAPTABLETOCHANGINGCONTROLNEEDSTHATSOMETHING,INTHELATE1960S,WASTHEFIRSTPROGRAMMABLECONTROLLERTHISFIRST“REVOLUTIONARY”SYSTEMWASDEVELOPEDASASPECIFICRESPONSETOTHENEEDSOFTHEMAJORAUTOMOTIVEMANUFACTURERSINTHEUNITEDSTATESTHESEEARLYCONTROLLERS,ORPROGRAMMABLELOGICCONTROLLERSPLC,REPRESENTEDTHEFIRSTSYSTEMSTHAT1COULDBE3USEDONTHEFACTORYFLOOR,2COULDHAVETHERE“LOGIC”CHANGEWITHOUTEXTENSIVEREWIRINGORCOMPONENTCHANGES,AND3WEREEASYTODIAGNOSEANDREPAIRWHENPROBLEMSOCCURREDITISINTERESTINGTOOBSERVETHEPROGRESSTHATHASBEENMADEINTHEPAST15YEARSINTHEPROGRAMMABLECONTROLLERAREATHEPIONEERPRODUCTSOFTHELATE1960SMUSTHAVEBEENCONFUSINGANDFRIGHTENINGTOAGREATNUMBEROFPEOPLEFOREXAMPLE,WHATHAPPENEDTOTHEHARDWIREDANDELECTROMECHANICALDEVICESTHATMAINTENANCEPERSONNELWEREUSEDTOREPAIRINGWITHHANDTOOLSTHEYWEREREPLACEDWITH“COMPUTERS”DISGUISEDASELECTRONICSDESIGNEDTOREPLACERELAYSEVENTHEPROGRAMMINGTOOLSWEREDESIGNEDTOAPPEARASRELAYEQUIVALENTPRESENTATIONSWEHAVETHEOPPORTUNITYNOWTOEXAMINETHEPROMISE,INRETROSPECT,WHATTHEPROGRAMMABLECONTROLLERBROUGHTMANUFACTURINGFIGURE101ALLPROGRAMMABLECONTROLLERSCONSISTOFTHEBASICFUNCTIONALBLOCKSSHOWNINFIGURE101WEWILLEXAMINEEACHBLOCKTOUNDERSTANDTHERELATIONSHIPTOTHECONTROLSYSTEMFIRSTWELOOKEDATTHECENTER,ASITISTHEHEARTOFTHESYSTEMITCONSISTSOFAMICROPROCESSOR,LOGICMEMORYFORTHESTORAGEOFTHEACTUALCONTROLLOGIC,STORAGEORVARIABLEMEMORYFORUSEWITHDATATHATWILLORDINARILYCHANGEASAFUNCTIONOFTHECONTROLPROGRAMEXECUTION,ANDAPOWERSUPPLYTOPROVIDEELECTRICALPOWERFORTHEPROCESSORANDMEMORYNEXTCOMESTHEI/OBLOCKTHISFUNCTIONTAKESTHECONTROLLEVELSIGNALSFORTHECPUANDCONVERTSTHEMTOVOLTAGEANDCURRENTLEVELSSUITABLEFORCONNECTIONWITHFACTORYGRADESENSORSANDACTUATORSTHEI/OTYPECANRANGEFROMDIGITAL,ANALOG,ORAVARIETYOFSPECIALPURPOSE“SMART”I/OWHICHAREDEDICATEDTOACERTAINAPPLICATIONTASKTHEPROGRAMMERISNORMALLYUSEDONLYTOINITIALLYCONFIGUREANDPROGRAMASYSTEMANDISNOTREQUIREDFORTHESYSTEMTOOPERATEITISALSOUSEDINTROUBLESHOOTINGASYSTEM,ANDCANPROVETOBEAVALUABLETOOLINPINPOINTINGTHEEXACT4CAUSEOFAPROBLEMTHEFIELDDEVICESSHOWNHEREREPRESENTTHEVARIOUSSENSORSANDACTUATORSCONNECTEDTOTHEI/OTHESEARETHEARMS,LEGS,EYES,ANDEARSOFTHESYSTEM,INCLUDINGPUSHBUTTONS,LIMITSWITCHES,PROXIMITYSWITCHES,PHOTOSENSORS,THERMOCOUPLES,POSITIONSENSINGDEVICES,ANDBARCODEREADERASINPUT；ANDPILOTLIGHT,DISPLAYDEVICES,MOTORSTARTERS,DCANDACDRIVERS,SOLENOIDS,ANDPRINTERSASOUTPUTS