专业外语教学资料专业英语第13单元

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1、13IntroductiontoControlEngineering13・1INTRODUCTIONWheneverenergyistobeusedpurposefully,someformofcontrolisnecessary.Inrecenttimestherehasbeenaconsiderableadvancemadeintheartofautomaticcontrol.Theartis,however,quiteold,stemmingbacktoabout1790whenJamesWattinventedthecentrifugalgovernor离心

2、调速机tocontrolthespeedofhissteamengines・Hefoundthatwhilstinmanyapplicationsanenginespeedindependentofloadtorquewasnecessary,inpracticewhenaloadwasappliedthespeedfellandwhentheloadwasremovedthespeedincreased.AsimplecentrifugalgovernorisshowninFig.13.Linthissystem,variationsinenginespeedar

3、edetectedandusedtocontrolthepressureofthesteamenteringtheengine.Understeadyconditionsthemomentoftheweightofthemetalspheresbalancesthatduetothecentrifugalforceandthesteamvalveopeningisjustsufficienttomaintaintheenginespeedattherequiredlevel・Whenanextraloadtorqueisappliedtotheengine,itss

4、peedwilltendtofall,thecentrifugalforcewilldecreaseandthemetalsphereswilltendtofallsligh什y.Theirheightcontrolstheopeningofthesteamvalvewhichnowopensfurthertoallowagreatersteampressureontheengine.Thespeedthustendstorise,counteracting扌氐消theoriginaltendencyforthespeedtofall.Iftheextraloadi

5、sremoved,thereverseprocesstakesplace,themetalspherestendtoriseslightly,sotendingtoclosethesteamvalveandcounteractinganytendencyforthespeedtorise.Pivot&PivotStcaintoengineMetalsphereMetalsphereThrottlevalveMechanical(linkageCoupledtostcainengineASteampipe-5*SteamfromboilerFig.13.1Simple

6、centrifugalgovernorFig.13.1SimplecentrifugalgovernorTypicalresponsesofthissystemtoloadincreasewithandwithoutthegovernorareshowninFig.13.2.Itisseenthatwithoutthegovernorthespeedwouldfallconsiderablyonload.However,inacorrectlydesignedsystemwithagovernorthefallinspeedwouldbeverymuchless.A

7、nundesirablefeaturewhichaccompaniesasystemwhichhasbeendesignedtobeverysensitivetospeedchangesisthetendencyto'hunt'摆动oroscillate振动aboutthefinalspeed.Therealprobleminthesynthesis综合ofallsystemsofthistypeistopreventexcessiveoscillationbutatthesametimeproducegood'regulation9调速率.applicatio