可编程序控制器外文翻译2.docx

可编程序控制器外文翻译2.docx

《可编程序控制器外文翻译2.docx》由会员分享，可在线阅读，更多相关《可编程序控制器外文翻译2.docx（12页珍藏版）》请在冰豆网上搜索。

可编程序控制器外文翻译2

外文原文

ProgrammableLogicControllersversusPersonalComputersforProcessControl

IndustrialEngineeringDepartment

UniversityofPuertoRico

P.O.Box5000

Mayaguez,PuertoRico00681

ABSTRACT

ItisproposedthatPersonalComputers（PCs）canbeusedeffectivelyforthesameindustrialapplicationsasProgrammableLogicControllers（PLCs）.ThebasicconceptsrelatedtotheoperationofaPLCtoemulatethebehaviorofarelaypanelareexplained.ThisisusedtobuildasimplemodelforusingstandardPCsinthesameapplicationsasPLCs.Thisisdemonstratedusingasimpleexampleofanautomatedprocess,Samplecodeinastandardhighlevellanguageispresentedthatcanbeusedasatemplateforfuturedevelopments.Finally,thepotentialadvantagesandenhancementsthatcanresultinusingPCsforprocesscontrolarepresented.

Keywords:

ProgrammableLogicController,PersonalComputer,Automation,LadderLogic,RelayPanels

INTRODUCTION

PersonalComputers（PCs）canbeusedeffectivelyforthesameapplicationsasProgrammableLogicControllers（PLCs）.AmodemPLCisacomputer-baseddevicedesignedtocontrolaprocess.Itrelatesinformationcomingfromsensorsthatmonitorthestateofaprocess,withthestatusofsomeactuatorsthatarecapableofchangingit.ThisrelationshipisestablishedinmostcasesinBooleanlogic.Typicalsensorsusedinindustrialapplicationsincludelimitswitches,proximitysensors,andotherbinarysensors.Actuatorsmayincludesolenoids,motorstarters,andothersimilardevices.Figure1showsasimplifiedmodelofanindustrialapplicationwhereaPLCmightbeused.

Fig.1:

ModelofSimpleProcess

RELAYPANELS

PLCsworedesignedtoreplacerelaypanels.Thesearecustommadecontrollersdedicatedtoaparticularapplication.Theycanbeexpensiveforcomplexsystems,cannotbeeasilyreconfigured,aredifficulttotroubleshoot,consumelotsofenergy,havearelativelymoderatespeedofoperation,andhavelowreliability.Relaypanelsarenotthemostsuitablealternativeforamoderatetocomplexindustrialapplicationwhereflexibility,easeofmaintenanceandtroubleshootingareveryimportant.Ontheotherhand,theyarerelativelylow-techandareeasytounderstandbyelectriciansandnon-engineeringpersonnel.

Theelectricalcontrolcircuitsofrelaypanelsaregenerallydrawnusingso-calledelectricladderdiagrams.Theydifferslightlyfromconventionalwiringdiagramsinthattheydonotshowthephysicalarrangementofthecomponents,butemphasizethefunctionofeachcircuit.Theyareasetofparallelcircuitsthatinessencerepresentahardwiredprogramthatcontrolsthesequenceofoperationsinagivenprocess.Beingelectricalcircuitsinparallelgivestheadvantageofsolvingallthecontrollogicsimultaneouslyandpracticallyinstantaneously.Figure2representsanelectricladderdiagramforarelaypanelthatcanbeusedtocontroltheprocesspresentedinfig.1.

Fig.2:

ElectricLadderDiagram

PLCsaretypicallycomputer-based,solid-state,single-processordevicesthatemulatethebehaviorofanelectricladderdiagram.Sincetheyaresequentialmachines,toemulatetheworkingsofparallelcircuitsthatrespondinstantaneously,PLCsuseaninput/outputimagetableandascanningcycle.

Aninput/outputimagetableisamemorystructurethatstoresalltherelevantinformationaboutthecurrentscanningcycle.Itcanbesubdividedinthreebasicparts:

theinputtable,theoutputtable,andtheinternalrelaytable,Thecurrentstateoftheinputs（sensors）iskeptintheinputtable;thedesiredstateoftheoutputs（actuators）iskeptintheoutputtable;andthestateofthevirtualcontrolrelaysiskeptintheinternalrelaytable.

WhenaprogramisbeingnminaPLCitiscontinuouslyexecutingascanningcycle（fig.3）.Thescanningcyclehastwomajorparts（inanactualPLCithasotherpartsbuttwoarerelevanthereforthesakeofthediscussion）:

theinput/outputscan,andtheprogramscan.Intheinput/outputscanthecurrentstateoftheinputsisreadfromtheinputpointsandstoredintheinputtable,andthedesiredstateofthe

Fig.3:

PLCScanningCycle

outputs（fromtheoutputtable）issenttotheoutputpoints.TheprogramscansolvestheBooleanlogicthatrelatestheinformationintheinputtable,withthatintheoutputandinternalrelaytables.Also,theinformationintheoutputandinternalrelaytablesisupdatedduringtheprogramscan.InaPLCthisBooleanlogicistypicallyrepresentedinagraphicallanguagethatlooksverymuchliketheelectricalcircuitthatitemulates.Thislanguageisknownasladderlogic.Infig.4thereisaladderlogicprogramthatcanbeusedtocontroltheprocessinfig.1.Notethesimilaritieswiththeelectricladderdiagraminfig.2.TheadvantageofthePLCscanningcycleschemeisthatitallowsmultipleprincesses