pic part-c student version

8/3/2019 PIC Part-C Student Version

1/18

2010 PICLecture2010 Prof.T.K.Ghoshal andProf.Smita Sadhu 1

Part-C


Cascade control


2/18


Cascade Control

Cascadecontrolisemployedforcontrollinglargeandcomplexprocesses,whereperformanceofasinglecontrollerisnotacceptabledueto(i)slowclosedloopresponseand/or(ii)highsensitivitytoplantgainchangesandload/disturbances.

InordertoemployCascadeControl,thereshouldbeatleastonemeasurableintermediate(secondary)variablewhichaffecttheprimaryprocessvariable.

Suchprocessesarecalledcascadedecomposable Theprocessequationy=f(u,d)

Whereu=input,y=primaryPVanddisdisturbance

Su y

d

Undecomposedprocess

u y2 y3 y

d3d2d1

S1 S2S3

decomposedprocess


Cascade Control-IITheprocessequationy=f(u,d)

Where u=input,y=primaryPVanddisdisturbance

Maybedecomposableasfollows y=f3(y3,d3)=f3{f2(y2,d2),d3}

=f3[f2 {f1 (u, d1),d2},d3]y2andy3 areintermediate variableswhich,ina

cascadeform,affectsy,theprimaryPV,

u y2 y3 y

d3d2d1

S1 S2S3


3/18


Cascade Control-III

Cascadecontrolcansubstantiallyimprovetheperformanceofthecontrolledsystemoversingle-loopcontrolwhenever (1)Disturbancesaffecttheintermediateorsecondaryprocess

or(2)thegainofthesecondaryprocess,includingtheactuator,isnonlinear.

Inthefirstcase,acascadecontrolsystemcanlimittheeffectofthedisturbancesenteringthesecondaryvariableontheprimaryoutput.

Inthesecondcase,acascadecontrolsystemcanlimittheeffectofgainvariationsintheactuatororthesecondaryprocessontheclosedloopperformance. Suchgainvariationsusuallyarisefromchangesinoperating

pointduetosetpointchangesorsustaineddisturbances.


Cascade control-IVThestructureofa(twoloop)cascadecontrolled

systemwithtwocontrollersisshownbelow

Effectreducedbyinner

loop(2rycontroller)

Effectofgainvariationreducedbyinnerloop

(2rycontroller)


4/18


Cascade control-V

Wenotethefollowing: Theprocessisdecomposedintwopartssometimescalledthe

main/primaryprocessandtheinner/secondaryprocess

Therearetwofeedbackloops,twocontrollersandtwo

transmitters

Theprimarycontrollersuppliesthesetpointtothesecondarycontroller- andareincascade

Morethantwocontrollersmaysimilarlybecascaded


Cascade control-VIOfficialdefinition:

Acascadecontrolsystemisamultiple-loopsystemwheretheprimaryvariableiscontrolledbyadjustingthesetpoint ofarelatedsecondaryvariablecontroller.

Theprimaryobjectiveincascadecontrolis: todivideanotherwisedifficulttocontrolprocessinto

twoportions, wherebyasecondarycontrolloopisformedarounda

majordisturbances thusleavingonlyminordisturbancestobecontrolled

bytheprimarycontroller.Themethodcanalsobeextendedtothreeor

moreloops


5/18


Cascade control-VII

Onlyonecontroller,onecontrolloop,onetransmitter

Thesolecontrollerhastotakecareofthelagandnonlinearityofthevalveactuatorandthatoftheheatexchanger

SETPOINT


Cascade control-VIIISETPOINT


6/18


Cascade control-IX

Thesecondaryloopcontrolssteamflowbydirectly

manipulatingthevalve

Theprimaryloopcontrolsthetemperatureoftheprocess

fluidbydemandingappropriateflowofsteamfromthesecondarycontroller

Oneflowtransmitter,one

temperaturetransmitter,onetemperaturecontrolleranda

flowcontrollerarerequired

SETPOINT


Cascade control-X

TheinputtotheFCEcanbelimited,eitherattheinputorattheoutputofthe2rycontroller.

Bymovingthelimitertotheinputofthe2rycontroller,thesecondaryPVcanalsobelimited,ifdesired.


7/18


Advantages of Cascade control

DisturbancesarisingwithinthesecondarylooparecorrectedbythesecondarycontrollerandtheprimaryPVisnotaffected.

Phaselagexistinginthesecondarypartoftheprocessisreducedsubstantiallybythesecondaryloop. Thisimprovesthespeedofresponseoftheprimary

loop.

Effectofgainvariationsinthesecondarypartoftheprocessarecontainedwithinitsownloop.

Thesecondarylooppermitsanexactmanipulationoftheflowofmassorenergybytheprimarycontroller.


Advantages-II Withpropertuningofparameterscascadecontrolgenerallyprovides

fasterandmorerobustperformance,withoutusingcomplexcompensators

Byapplyingstrong(withadequateproportionalgain)innerloops theouterloopbecomes lessdependentontheplantparametersandnonlinearities

Notundulyaffectedbydisturbancesoftheinnerloop

Withtheuseoflimiters,theintermediatePVsmaybekeptwithinsafelimits

Cascadecontrolusuallyprovidesmoremeaningfulinterpretationofthesetpointvariableandthecontrolleroutputs. Thishelpsinunderstandingaswellasapplyingfeedforward

compensation. E.g.inthelevelcontrolsystem,theoutputcanbefedforwardtothe

inflowcontroller

Theinnerloopsprovideeasierwayofcontrollertuning


8/18


Advantages revisited

Bettercontroloftheprimaryvariable Primaryvariablelessaffectedbydisturbances

Fasterrecoveryfromdisturbances

Reductionofrisetime,increasedspeedofresponseoftheCLsystem

Easiertuningofdynamicperformance

Providelimitsonthesecondaryvariable


Disadvantages of cascade controlMoresensorsarerequired

Eithermultiplecontrollerorspecialpurposecontrollershavetobeused

Tuningofcascadecontrollersrequiresmoreskill

Notallplantsareamenabletocascade

control


9/18


Plants where cascade control is possible

Theplantcanbemodelled asasequenceoftwoormoresubsystemseachprovidingtheexcitationforthenext

Theexcitationvariableshouldbemeasurable

Eachsubsystemshouldhaveonlyoneexcitationandpossibledisturbances


Feed Forward Control


10/18


Feed Forward Control

Traditionallyfeedforwardcontrolusedtheset-pointpointsignaltodirectlycontrolthevalve,bypassingthecontroller. Hencethename ActedasaSPproportionalbias Rarelyusednowadays

Controller PLANT

ValveActuator

SP

PV

FeedForwardPath

PV


Another Classical FF Example


11/18


Feed Forward-II

NowfeedforwardmostlyusesmeasurableloadanddisturbancesignalstoactuatetheFCE

VerycommoninindustrialcontrolUsedoftenwithcascadecontrol


Feed Forward Control-III

TheFFcontrollerreceivesthedisturbancemeasurement,computescontrolactionstocounterits

impendingimpactonthemeasuredprocessvariable,andtransmitstheresulttotheFCE.

FeedforwardisusefulwhenloadandmeasurabledisturbancescausesubstantialchangeinthePV


12/18


Feed Forward Control-IV

Advantages Makesresponsetoload/disturbancechangefaster Possibletoreducesteadystateerrordueto

persistentloadchanges

Disadvantages Loadmustbemeasurable Moretransmittersrequired CannotbeusedasstandaloneCLAWexceptfor

trivialsystems FFgainneedstobechangedwithchangeinplantparameter

Alwaysusedwithfeedback


FF Control Example: liquid level control

Configuration-1 Measuresoutlet

flowQ2

andadjustsinletvalvebyafeedforwardcontroller

Inanattempttomatchthetwoflows

FF H

Q1,P1

Q2

Objective:Tokeeptheliquid

LevelHatadesiredvalue

FlowXmitteromittedforclarity


13/18


FF Control Example: liquid level II

Shortcomings: Valvesettingtoinletflow

maynotbeknownaccurately,dependson

inletpressureP1,

fluiddensity

Flowcoefficientvariationduetoageing,wear,fouling

FFgaincannotbesettoensureconstantlevel

FF H

Q1,P1

Q2


FF Control Example: liquid level III Configuration-II:

Flowlooptoensuretheinletflowmatchesthedemandedflow=theoutletflow(FFgain=1)

Withperfectmatchingofflows,thelevelremainsconstant.

Limitation thoughthelevelwouldremain

constant,itsexactvalueisundetermined.

Thematchingcannotbeperfectbecauseof

Measurementerrors Theremaybetransient

mismatchduetovalvepositioningloopandflowloop.

H

Q2

FCFT

FF

Q1


14/18


FF Control Example: liquid level IV

Configuration-III: Cascadecontrolwithfeed

forward

TheFFattemptstomatchtheinputandoutletflows,thelevelremainsapproximatelyconstant.

Theouterfeedbackloophastotakecareofmatchingimperfections.

Evenalowproportionalgainmaybeadequate.

Q1,P1

H

Q2

FCFT

FF

LC

SP

SP2


Ratio ControlRatiocontrolisusedtoensurethattwoormore

flowsarekeptatthesameratioeveniftheflowsarechanging.

Applicationsofratiocontrol: Maintainingcorrectairandfuelmixtureto

combustion.

Blendingtwoormoreflowstoproduceamixturewithspecifiedcompositionforchemicalreaction(feed).

Blendingtwoormoreflowstoproduceamixturewithspecifiedphysicalproperties.e.g.inlubricant

Blendingtwoormoreflowstoproduceamixturewithspecifiedtestorflavour


15/18


Ratio Control-II

nnumberofinputflowsEachassumedtobemeasurableOneinputflowcannotbecontrolledbytheratio

controlsystemOneoutflow

MixingOr

Reaction

Inflows:

Q1

Q3

Qn

Q(n+1)


Ratio Control-IIISeveralcasesmayariseforratiocontrol,for

example: Thecompositionofthemixturemaynotbe

measurablewithreasonablycostlytransmitter. Onehastousefeedforwardcontrol

Oneoftheflowscannotbecontrolledbecausetheflowisdictatedbyotherconditions(wildflow).

Thisoccursinneutralizingapplication,wherethequantityoftheacidicflowisdictatedbyproductionvolume.

Thewildflowisusuallynotconstantandsoitmustbemeasured.

Onlytheother(n-1)flowsneedtobecontrolledonthebasisofmeasurements


16/18


Ratio Control-V

FlowfollowingconfigurationThewildflowismeasuredandthisinformationis

usedtoprovidetheset-pointoftheotherflowthroughaflowcontroller.

Requiresmanualsettingoftheratio


Ratio Control-VI

Configurationwithfeedbackandcascadecontrol.

AlsocalledRatioRelayControlorautomaticratioadjustment Theoutputcompositionismeasuredandfromthisthesuitableratio

isinferredbythefeedforwardcontroller. Fromthedesiredratioandquantityofwildflow,thesetpointforthe

otherflowisdetermined.


17/18


Air-Fuel Ratio Control



18/18



To ConcludeWhendesigningacontrolsystem,beawareof

thecontrolobjective Alsoforeseeanypossibleconflicts

Usingacombinationoffeedback,feedforward,ratio,andcascadecontrol,onecandesignflexiblecontrolsystems

Morecomplexcontrolsystemsarehardertotuneandmodel,

Complexloopstructuresarejustifiedwhere Performance isimportant.

Thecomplexloopoutperformssimplerarchitectures

pic part-c student version

Documents