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时间:2018-09-27
《河科大 机械制造专业英语 3》由会员上传分享,免费在线阅读,更多相关内容在教育资源-天天文库。
1、Unit16Open-loopandClosed-loopControlOpen-loopControlSystems(NonfeedbackSystems)开环控制系统(无反馈系统)Thewordautomaticimpliesthatthereisacertainamountofsophisticationinthecontrolsystem.Byautomatic,itgenerallymeansthatthesystemisusuallycapableofadaptingtoavarietyofoperatingconditionsandi
2、sabletorespondtoaclassofinputssatisfactorily.自动化一词意味着在控制系统中有一定数量采用先进的技术,它通常意味着系统有普遍适应的能力,对于操作环境的变化能够响应一类令人满意的输入However,notanytypeofcontrolsystemhastheautomaticfeature.Usually,theautomaticfeatureisachievedbyfeedingtheoutputvariablebackandcomparingitwiththecommandsignal.Whenasys
3、temdoesnothavethefeedbackstructure,itiscalledanopen-loopsystem,whichisthesimplestandmosteconomicaltypeofcontrolsystem.然而,不是任何一个控制系统都具有自动化特征。通常,控制特征是通过反馈输出变量和比较控制信号获得的。当一个系统没有反馈结构,这称为开环系统,开环系统是最简单而且最经济的控制系统类型。Unfortunately,open-loopcontrolsystemslackaccuracyandversatilityandcan
4、beusedinnonebutthesimplesttypesofapplications.不幸的是,除了最简单的应用类型,开环控制系统,由于缺乏精确度和多功能不被应用。Consider,forexample,controlofthefurnaceforhomeheating.Letusassumethatthefurnaceisequippedonlywithatimingdevice,whichcontrolstheonandoffperiodsofthefurnace.Toregulatethetemperaturetotheproperle
5、vel,thehumanoperatormustestimatetheamountoftimerequiredforthefurnacetostayonandthensetthetimeraccordingly.Whenthepresettimeisup,thefurnaceisturnedoff.考虑一下,例如,用于家庭取暖的火炉的控制。假设火炉仅配有时间装置,控制火炉周期内的开启和关闭。操作者必须估计火炉达到理想温度所需要的时间,然而设置计时器,当达到预先设计的时间时火炉关闭,However,itisquitelikelythatthehous
6、etemperatureiseitheraboveorbelowthedesiredvalue,owingtoinaccuracyintheestimate.Withoutfurtherdeliberation,itisquiteapparentthatthistypeofcontrolisinaccurateandunreliable.房间的温度也可能高于或者低于期望值,原因就是估计不精确不从长远考虑,很明显这样控制的类型是不精确不可靠的。Onereasonfortheinaccuracyliesinthefactthatonemaynotkno
7、wtheexactcharacteristicsofthefurnace.Theotherfactoristhatonehasnocontrolovertheoutdoortemperature,whichhasadefinitebearingontheindoortemperatureThisalsopointstoan16importantdisadvantageoftheperformanceofanopen-loopcontrolsystem,inthatthesystemisnotcapableofadaptingtovariations
8、inenvironmentalconditionsortoexternaldisturbances.一个列出事实关于精确性
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