本科毕业生外文文献翻译.docx
《本科毕业生外文文献翻译.docx》由会员分享,可在线阅读,更多相关《本科毕业生外文文献翻译.docx(16页珍藏版)》请在冰豆网上搜索。
本科毕业生外文文献翻译
本科毕业生外文文献翻译
学生姓名:
史衍彬
指导教师:
荣丽红
所在学院:
信息技术学院
专 业:
农业电气化与自动化
中国·大庆
2010年4 月
SCM profile
IntroductionofProgrammablecontrollers
Fromasimple heritage,theseremarkable systems haveevolvedto not onlyreplaceelectromechanicaldevices,buttosolveanever-increasingarray ofcontrolproblemsinboth process andnonprocessindustries.Byallindications,thesemicroprocessorpowered giantswillcontinuetobreaknewground inthe automatedfactoryinto the1990s.
HISTORY
In the 1960s, electromechanicaldevicesweretheorderoftheday assfaras control wasconcerned. These devices, commonlyknownasrelays,werebeingusedby thethousandstocontrolmanysequential-type manufacturingprocessesandstand-alongmachines.Manyof theserelayswere inuseinthetransportationindustry, more specifically,theautomotiveindustry.Theserelaysusedhundredsofwires and theirinterconnections toeffecta controlsolution.Theperformance ofarelaywasbasicallyreliable-at least asasingledevice. Butthe commonapplicationsfor relaypanels calledfor300to500ormore relays,andthe reliabilityandmaintenance issuesassociatedwithsupportingthesepanels became a very great challenge.Costbecameanother issue, forin spiteofthelow costof therelayitself,the installedcostofthepanelcouldbequitehigh.Thetotalcostincludingpurchased parts, wiring,andinstallationlabor,couldrangefrom$30~$50 perrelay. Tomakemattersworse,theconstantly changing needsofa process called forrecurringmodificationsofacontrolpanel. With relays, thiswasacostlyprospect, asitwas accomplished byamajor rewiring effortonthepanel.Inadditionthesechanges weresometimespoorlydocumented,causing asecond-shift maintenancenightmaremonthslater.Inlightofthis,itwasnotuncommontodiscardanentire control panel infavorofanewonewith theappropriatecomponentswired in amannersuited forthenew process.Add tothis theunpredictable,andpotentiallyhigh,costofmaintainingthesesystems ason high-volumemotorvehicle productionlines, anditbecameclearthat somethingwas neededtoimprovethecontrolprocess–tomakeitmorereliable,easiertotroubleshoot, and moreadaptabletochangingcontrolneeds.
Thatsomething,inthelate1960s,wasthefirst programmable controller.Thisfirst‘revolutionary’system wandevelopedas aspecificresponsetotheneedsof the majorautomotivemanufacturersintheUnitedStates.Theseearlycontrollers, orprogrammable logiccontrollers(PLC),representedthefirst systemsthat1couldbe usedonthefactoryfloor,2could havethere‘logic’changedwithoutextensiverewiring orcomponentchanges,and3wereeasytodiagnoseand repairwhenproblems occurred.
Itisinteresting toobserve theprogressthathasbeenmade inthepast15 years intheprogrammablecontrollerarea.The pioneerproducts of thelate 1960smust havebeenconfusingandfrightening toagreatnumberofpeople.For example,whathappenedtothehardwired andelectromechanicaldevicesthatmaintenancepersonnelwereusedto repairingwithhand tools?
Theywerereplacedwith‘computers’ disguisedaselectronicsdesignedto replacerelays.Even theprogrammingtools were designedtoappear asrelayequivalentpresentations.Wehavethe opportunity nowto examinethepromise, inretrospect, thattheprogrammablecontrollerbrought tomanufacturing.
Allprogrammablecontrollersconsistof thebasicfunctional blocksshowninFig.10.1. We’llexamine eachblocktounderstandtherelationshipto thecontrolsystem.Firstwelook atthe center, as itistheheart (orat leastthebrain)ofthesystem. Itconsistsof amicroprocessor,logicmemoryforthe storageof theactualcontrollogic, storageorvariablememoryforusewithdata thatwillordinarilychangeas afunction power fortheprocessorandmemory.NextcomestheI/Oblock.Thisfunction takesthecontrollevelsignals fortheCPU andconverts themtovoltage andcurrentlevels suitableforconnectionwithfactorygradesensors andactuators.TheI/Otypecanrangefromdigital(discrete or on /off),analog(continuously variable),oravarietyofspecialpurpose‘smart’I/Owhicharededicatedtoa certain applicationtask.Theprogrammerisshownhere, but itis normallyused onlyto initially configureandprogram asystemandis notrequiredforthesystemto operate.Itis alsoused introubleshootinga system,and canproveto beavaluabletoolinpinpointingtheexact causeofaproblem.Thefielddevicesshown hererepresentthevarious sensors andactuatorsconnectedtothe I/O. Thesearethe arms, legs,eyes,andears ofthesystem,includingpushbuttons, limitswitches, proximityswitches,photosensors,thermocouples,RTDS, positionsensingdevices,andbarcode readeras input;andpilotlights,displaydevices,motor starters,DC and ACdrives,solenoids, andprintersasoutputs.
Nosingleattemptcould coveritsrapidlychangingscope,butthree basiccharacteristics canbe examinedtogive classifyanindustrialcontroldeviceas aprogrammable controller.
(1)Itsbasicinternaloperationis tosolve logicfromthe beginningofmemorytosome specifiedpoint,suchasendof memoryorendofprogram.Oncetheendisreached,theoperationbeginsagain atthebeginningofmemory.This scanning process continuesfromthetimepowerissuppliedto thetimeitit removed.
(2)The programminglogicis aformofarelayladderdiagram.Normallyopen,normally closed contacts,and relay coilsareused withinaformat utilizingaleftandaright vertical rail.Powerflow(symbolicpositive electronflow) isused todetermine whichcoilor outputsareenergizedordeenergized.
(3)Themachineis designedforthe industrialenvironmentfromitsbasicconcept;thisprotectionisnot addedata laterdate.Theindustrial environmentincludes unreliable ACpower,hightemperatures(0to60 degreeCelsius),extremesof humidity, vibrations,RFnoise,andothersimilarparameters.
Generalapplicationareas
The programmable controllerisused inawidevarietyofcontrol applicationstoday,manyofwhich were noteconomicallypossible justa fewyearsago.Thisis truefortwogeneralreasons:
1there costeffectiveness(that is, thecost perI/Opoint)hasimproveddramaticallywith thefallingpricesofmicroprocessorsandrelatedcomponents,and2theabilityofthe controller tosolve complexcomputationandcommunicationtaskshasmadeit possible to use it where adedicatedcomputer was previouslyused.
Applicationsforprogrammable controllerscanbecategorizedin anumber ofdifferentways,including generaland industrialapplicationcategories.Butitisimportant tounderstand theframeworkin which controllers arepresentlyunderstoodandusedsothatthefullscopeofpresent and future evolutioncanbe examined.It isthroughthepowerofapplicationsthatcontrollers can beseenintheirfull light.Industrialapplicationsincludemany inbothdiscrete manufacturingandprocess industries.Automotive industryapplications,thegenesis ofthe programmablecontroller, continueto provide thelargestbaseof opportunity. Otherindustries,suchasfoodprocessing andutilities,providecurrent developmentopportunities.
There are fivegeneralapplicationareasin whichprogrammablecontrollersareused.Atypicalinstallationwilluse oneormoreof theseintegrated tothe controlsystemproblem.Thefive generalareas areexplained brieflybelow.
Description
The AT89C51is alow-power,high-performance CMOS8-bitmicrocomputerwith4Kbytesof Flashprogrammable anderasableread onlymemory(PEROM).Thedeviceis manufacturedusingAtmel’s high-densitynonvolatilememory technologyandiscompatible with theindustry-standard MCS-51 instructionset andpinout.Theon-chipFlashallowstheprogrammemorytobe reprogrammedin-system orbyaconventionalnonvolatile memoryprogrammer.Bycombininga versatile8-bitCPUwith Flash on amonolithicchip,the AtmelAT89C51isa powerfulmicrocomputer whichprovidesahighly-flexibleandcost-effectivesolutiontomanyembedded controlapplications.
Functioncharacteristic
The AT89C51providesthefollowingstandardfeatures:
4KbytesofFlash,128 bytes ofRAM, 32I/O lines,two16-bittimer/counters,afivevector two-levelinterruptarchitecture, afull duplexserial port, on-chiposcillator andclockcircuitry.Inaddition,the AT89C51isdesigned withstaticlogic for operationdowntozerofrequencyandsupports two softwareselectablepower savingmodes. The IdleModestops theCPUwhile allowingtheRAM,timer/counters,serialportandinterrupt systemtocontinuefunctioning.ThePower-down ModesavestheRAMcontentsbutfreezestheoscillatordisabling allotherchipfunctionsuntilthenexthardwarereset.
Pin Description
VCC:
Supply voltage.
GND:
Ground.
Port0:
Port 0isan8-bitopen-drain bi-directionalI/Oport. Asanoutputport,eachpin cansinkeightTTLinputs.When 1sarewrittentoport0pins,the pins canbe usedashighimpedanceinputs.Port0may alsobeconfiguredtobe themultiplexed loworderaddress/data bus during accesses toexternalprogram anddata memory.InthismodeP0 has internalpullups.Port 0alsoreceives the codebytesduring Flash programming,andoutputsthe codebytesduringprogramverification. Externalpullupsarerequired during programverification.
Port1
Port1isan8-bitbi-directionalI/O portwith internalpullups.ThePort1output bufferscansink/sourcefourTTLinputs.When1sarewrittento Port1pinstheyarepulled highby theinternalpullups andcanbeusedasinputs.Asinputs,Port1pinsthatareexternallybeingpulledlowwillsourcecurrent (IIL) becauseoftheinternalpullups.Port 1 alsoreceivesthelow