Easy Electronics xLogic Micro PLC - Page 197

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Controlloop
TheprocessvaluexinfluencesthemanipulatedvariableMbymeansoftheregulatingdevice.This
createsaclosedcircuitthatisalsoknownasacontrolloop.
If,intheexampleabove,thewindowisopened,thetemperatureintheroomdrops.Thepersonmust
increasetheheatoutputoftheheater.Iftheheatoutputisincreasedtoomuch,itwillgettoohot.The
personmustthenreducetheheatoutput.
Iftheheatoutputisincreasedorreducedtooquickly,thenthecontrolloopstartstosway.Theroom
temperaturefluctuates.Itiseithertoohotortoocold.Topreventthis,thepersonmustcarefullyand
slowlyreduceorincreasetheheatoutput.
Looperror
Thelooperroristhedifferencebetweenthecommandvariableandtheprocessvalue.Inotherwords:
thedeviationofaprocessvaluefromasetvalue.
e=SP – PV
ThelooperrorebringsaboutachangetothemanipulatedvariableM.
Theexampleaboveillustratesthisverywell:if,withadesiredtemperatureof20 °C(=commandvalue
w),theroomtemperatureis22 °C(=processvaluePV),thisresultsinthelooperror:
e=SP – PV=20 °C-22 °C=-2 °C
Inthiscase,thenegativesignindicatesareversingaction:theheatoutputisreduced.
Inacontrolloop'sstateofequilibrium,thelooperroriszeroorverysmall.Ifthecommandvariable
changesorthereisadisturbance,alooperrorarises.Thelooperroriscorrectedbymeansofthe
manipulatedvariableM.
Controllerbasics
Acontrollercanbesimplyportrayedasfollows:
Thecomparingelementandthecontrollerfunctiondescribetheconductofthecontroller.
Thefollowingdescribesthemostimportanttypesofcontroller.Acontroller'sstepresponsetellsusalot
aboutitsconduct.Thestepresponsedescribeshowacontrollerreactstotheerraticchangeinthe
processvalue.
Thereare3importantbasictypesofcontroller:
Proportional-actioncontroller(Pcontroller)
Integral-actioncontroller(Icontroller)
Differential-actioncontroller(Dcontroller – we'renottouchingonthishere)
Thesearecombinedforarealcontroller.Forinstance,thePIcontroller:
PController