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1、13IntroductiontoCorrtrolEngineering1X1INTRODUCTIONWheneverenergyistobeusedpurposefully,someformofcontrolisnecessary.Inrecenttimestherehasbeenaconsiderableadvancemadeintheartofautomaticcontrol.Theartis,however,quiteold,stemmingbacktoabout1790whenJamesWattinventedt
2、hecentrifugalgovernor离心调速机tocontrolthespeedofhissteamengines.Hefoundthatwhilstinmanyapplicationsanenginespeedindependentofloadtorquewasnecessary,inpracticewhenaloadwasappliedthespeedfellandwhentheloadwasremovedthespeedincreased.Asimplecentrifugalgovernorisshownin
3、Fig.13.1.Inthissystem,variationsinenginespeedaredetectedandusedtocontrolthepressureofthesteamenteringtheengine.Understeadyconditionsthemomentoftheweightofthemetalspheresbalancesthatduetothecentrifugalforceandthesteamvalveopeningisjustsufficienttomaintaintheengine
4、speedattherequiredlevel・Whenanextraloadtorqueisappliedtotheengine,itsspeedwilltendtofall,thecentrifugalforcewilldecreaseandthemetalsphereswilltendtofallslightly.Theirheightcontrolstheopeningofthesteamvalvewhichnowopensfurthertoallowagreatersteampressureontheengin
5、e.Thespeedthustendstorise,counteracting扌氐消theoriginaltendencyforthespeedtofall.Iftheextraloadisremoved,thereverseprocesstakesplace,themetalspherestendtoriseslightly,sotendingtoclosethesteamvalveandcounteractinganytendencyforthespeedtorise.13IntroductiontoCorrtrol
6、Engineering1X1INTRODUCTIONWheneverenergyistobeusedpurposefully,someformofcontrolisnecessary.Inrecenttimestherehasbeenaconsiderableadvancemadeintheartofautomaticcontrol.Theartis,however,quiteold,stemmingbacktoabout1790whenJamesWattinventedthecentrifugalgovernor离心调
7、速机tocontrolthespeedofhissteamengines.Hefoundthatwhilstinmanyapplicationsanenginespeedindependentofloadtorquewasnecessary,inpracticewhenaloadwasappliedthespeedfellandwhentheloadwasremovedthespeedincreased.AsimplecentrifugalgovernorisshowninFig.13.1.Inthissystem,va
8、riationsinenginespeedaredetectedandusedtocontrolthepressureofthesteamenteringtheengine.Understeadyconditionsthemomentoftheweightofthemetalspheresbalancesthatdu