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电气毕业论文设计英语文献原文+翻译doc
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Programmabledesignedforelectro-pneumaticsystemscontroller
JohnF.Wakerly
Thisprojectdealswiththestudyofelectro-pneumaticsystemsandtheprogrammablecontrollerthatprovidesaneffectiveandeasywaytocontrolthesequenceofthepneumaticactuatorsmovementandthestatesofpneumaticsystem.Theprojectofaspecificcontrollerforpneumaticapplicationsjointhestudyofautomationdesignandthecontrolprocessingofpneumaticsystemswiththeelectronicdesignbasedonmicrocontrollerstoimplementtheresourcesofthecontroller.
1.Introduction
Theautomationsystemsthatuseelectro-pneumatictechnologyareformedmainlybythreekindsofelements:
actuatorsormotors,sensorsorbuttonsandcontrolelementslikevalves.Nowadays,mostofthecontrolelementsusedtoexecutethelogicofthesystemweresubstitutedbytheProgrammableLogicController(PLC).Sensorsandswitchesarepluggedasinputsandthedirectcontrolvalvesfortheactuatorsarepluggedasoutputs.Aninternalprogramexecutesallthelogicnecessarytothesequenceofthemovements,simulatesothercomponentslikecounter,timerandcontrolthestatusofthesystem.
WiththeuseofthePLC,theprojectwinsagility,becauseitispossibletocreateandsimulatethesystemasmanytimesasneeded.Therefore,timecanbesaved,riskofmistakesreducedandcomplexitycanbeincreasedusingthesameelements.
AconventionalPLC,thatispossibletofindonthemarketfrommanycompanies,offersmanyresourcestocontrolnotonlypneumaticsystems,butallkindsofsystemthatuseselectricalcomponents.ThePLCcanbeveryversatileandrobusttobeappliedinmanykindsofapplicationintheindustryorevensecuritysystemandautomationofbuildings.
Becauseofthosecharacteristics,insomeapplicationsthePLCofferstomuchresourcesthatarenotevenusedtocontrolthesystem,electro-pneumaticsystemisoneofthiskindofapplication.TheuseofPLC,especiallyforsmallsizesystems,canbeveryexpensivefortheautomationproject.
Analternativeinthiscaseistocreateaspecificcontrollerthatcanoffertheexactlysizeandresourcesthattheprojectneeds[3,4].Thiscanbemadeusingmicrocontrollersasthebaseofthiscontroller.
Thecontroller,basedonmicrocontroller,canbeveryspecificandadaptedtoonlyonekindofmachineoritcanworkasagenericcontrollerthatcanbeprogrammedasausualPLCandworkwithlogicthatcanbechanged.Allthesecharacteristicsdependonwhatisneededandhowmuchexperiencethedesignerhaswithdevelopinganelectroniccircuitandfirmwareformicrocontroller.Butthemainadvantageofdesignthecontrollerwiththemicrocontrolleristhatthedesignerhasthetotalknowledgeofhiscontroller,whichmakesitpossibletocontrolthesizeofthecontroller,changethecomplexityandtheapplicationofit.Itmeansthattheprojectgetsmoreindependencefromothercompanies,butatthesametimetheresponsibilityofthecontrolofthesystemstaysatthedesignerhands
2.Electro-pneumaticsystem
Onautomationsystemonecanfindthreebasiccomponentsmentionedbefore,plusalogiccircuitthatcontrolsthesystem.Anadequatetechniqueisneededtoprojectthelogiccircuitandintegrateallthenecessarycomponentstoexecutethesequenceofmovementsproperly.
Forasimpledirectsequenceofmovementanintuitivemethodcanbeused[1,5],butforindirectormorecomplexsequencestheintuitioncangenerateaverycomplicatedcircuitandsignalmistakes.Itisnecessarytouseanothermethodthatcansavetimeoftheproject,makeacleancircuit,caneliminateoccasionalsignaloverlappingandredundantcircuits.Thepresentedmethodiscalledstep-by-steporalgorithmic[1,5],itisvalidforpneumaticandelectro-pneumaticsystemsanditwasusedasabaseinthiswork.
Themethodconsistsofdesigningthesystemsbasedonstandardcircuitsmadeforeachchangeonthestateoftheactuators,thesechangesarecalledsteps.
Thefirstpartistodesignthosekindsofstandardcircuitsforeachstep,thenexttaskistolinkthestandardcircuitsandthelastpartistoconnectthecontrolelementsthatreceivesignalsfromsensors,switchesandthepreviousmovements,andgivetheairorelectricitytothesupplylinesofeachstep.InFigs.1and2thestandardcircuitsaredrawnforpneumaticandelectro-pneumaticsystem[8].Itispossibletoseetherelationswiththepreviousandthenextsteps.
3.Themethodappliedinsidethecontroller
Theresultofthemethodpresentedbeforeisasequenceofmovementsoftheactuatorthatiswelldefinedbysteps.Itmeansthateachchangeonthepositionoftheactuatorsisanewstateofthesystemandthetransitionbetweenstatesiscalledstep.
Thestandardcircuitdescribedbeforehelpsthedesignertodefinethestatesofthesystemsandtodefinetheconditiontoeachchangebetweenthestates.Intheendofthedesign,thesystemisdefinedbyasequencethatneverchancesandstatesthathavetheinputsandtheoutputswelldefined.Theinputsaretheconditionforthetransitionandtheoutputsaretheresultofthetransition.
Alltheconfigurationofthosestepsstaysinsideofthemicrocontrollerandisexecutedthesamewayitwasdesigned.Thesequencesofstringsareprogrammedinsidethecontrollerwith5bytes;eachstringhastheconfigurationofonestepoftheprocess.Therearetwobytesfortheinputs,onebytefortheoutputsandtwomorefortheotherconfigurationsandauxiliaryfunctionsofthestep.Afterprogramming,thissequenceofstringsissavedinsideofanon-volatilememoryofthemicrocontroller,sotheycanbereadandexecuted.
ThecontrollertaskisnottoworkinthesamewayasaconventionalPLC,butthepurposeofitistobeanexampleofaversatilecontrollerthatisdesignforanspecificarea.AconventionalPLCprocessthecontrolofthesystemusingacyclewhereitmakesanimageoftheinputs,executealltheconditionsdefinedbytheconfigurationprogrammedinside,andthenupdatethestateoftheoutputs.Thiscontrollerworksinadifferentway,whereitreadtheconfigurationofthestep,waittheconditionofinputstobesatisfied,thenupdatethestateortheoutputsandafterthatjumptothenextstepandstarttheprocessagain.
Itcangeneratesomelimitations,asthefactthatthiscontrollercannotexecute,insidetheprogram,movementsthatmustberepeatedforsometime,butthisproblemcanbesolvedwithsomeexternallogiccomponents.Anotherlimitationisthatthecontrollercannotbeappliedonsystemsthathavenosequence.Theselimitationsareacharacteristicofthesystemthatmustbeanalyzedforeachapplication.
4.Characteristicsofthecontroller
ThecontrollerisbasedontheMICROCHIPmicrocontrollerPIC16F877[6,7]with40pins,andithasalltheresourcesneededforthisproject.Ithasenoughpinsforallthecomponents,serialcommunicationimplementedincircuit,EEPROMmemorytosavealltheconfigurationofthesystemandthesequenceofsteps.Fortheexecutionofthemainprogram,itofferscompleteresourcesastimersandinterruptions.
Thelistofresourcesofthecontrollerwascreatedtoexploreallthecapacityofthemicrocontrollertomakeitascompleteaspossible.Duringthestep,theprogramchooseshowtousetheresourcesreadingtheconfigurationstringofthestep.Thisstringhastwobytesfordigitalinputs,oneusedasamaskandtheotheroneusedasavalueexpected.Onebyteisusedtoconfiguretheoutputsvalue.Onebytesmoreisusedfortheinternaltimer,theanaloginputortime-out.TheEEPROMmemoryinsideis256byteslengththatisenoughtosavethestringofthesteps,withthischaracteristicitispossibletosavebetween48steps(Table1).
Thecontroller(Fig.3)hasalsoadisplayandsomebuttonsthatareusedwithaninteractivemenutoprogramthesequenceofstepsandotherconfigurations.
4.1.Interactioncomponents
Fortherealapplicationthecontrollermusthavesomeelementstointeractwiththefinaluserandtoofferacompletemonitoringofthesystemresourcesthatareavailabletothedesignerwhilecreatingthelogiccontrolofthepneumaticsystem(Fig.3):
•Interactivemodeofwork;functionavailableonthemainprogramfordidacticpurposes,theusergivesthesignaltoexecutethestep.
•LCDdisplay,whichshowsthestatusofthesystem,valuesofinputs,outputs,timerandstatisticsofthesequenceexecution.
•Beeptogiveimportantalerts,stop,startandemergency.
•Ledstoshowpoweronandotherstoshowthestateofinputsandoutputs.
4.2.Security
Tomakethefinalapplicationworksproperty,acorrectconfigurationtoexecutethestepsintherightwayisneeded,butmorethenthatitmustoffersolutionsincaseofbadfunctioningorproblemsintheexecutionofthesequence.Thecontrolleroffersthepossibilitytoconfiguretwointernalvirtualcircuitsthatworkinparalleltotheprincipal.Thesetwocircuitscanbeusedasemergencyorresetbuttonsandcanreturnthesystemtoacertainstateatanytime[2].Therearetwoinputsthatworkwithinterruptiontogetanimmediateaccesstothesefunctions.Itispossibletoconfiguretheposition,thebuttonsandthevalueoftime-outofthesystem.
4.3.Userinterface
Thesequenceofstringscanbeprogrammedusingtheinterfaceelementsofthecontroller.AComputerinterfacecanalsobeusedtogeneratetheuserprogrameasily.Withagooddocumentationthefinalusercanusetheinterfacetoconfigurethestringsofbytesthatdefinethestepsofthesequence.Butitispossibletocreateaprogramwithvisualresourcesthatwork