Non-parametric Tuning of PID Controllers: A Modified Relay-Feedback-Test Approach

195 Pages·2013·1.699 MB·English

by  Igor Boiko (auth.)

Advances in Industrial Control
Forfurthervolumes:
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Igor Boiko
Non-parametric
Tuning of PID
Controllers
A Modified Relay-Feedback-Test Approach
IgorBoiko
ElectricalEngineeringDepartment
ThePetroleumInstitute
AbuDhabi
UnitedArabEmirates
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ISSN1430-9491 ISSN2193-1577(electronic)
AdvancesinIndustrialControl
ISBN978-1-4471-4464-9 ISBN978-1-4471-4465-6(eBook)
DOI10.1007/978-1-4471-4465-6
SpringerLondonHeidelbergNewYorkDordrecht
LibraryofCongressControlNumber:2012947218
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To myfamily
Series Editors’ Foreword
The series Advances in Industrial Control aims to report and encourage technol-
ogy transfer in control engineering. The rapid development of control technology
has an impact on all areas of the control discipline. New theory, new controllers,
actuators, sensors, new industrial processes, computer methods, new applications,
new philosophies..., new challenges. Much of this development work resides in
industrialreports,feasibilitystudypapersandthereportsofadvancedcollaborative
projects.Theseriesoffersanopportunityforresearcherstopresentanextendedex-
position of such new work in all aspects of industrial control for wider and rapid
dissemination.
The proportional-integral-derivative (PID) controller continues to be an impor-
tantcontrollerforindustrialapplications.Itismainlyfoundatthelowerlevelofthe
controlhierarchy,attheactuator-sensorlevel,andconfiguredintomultiloopstruc-
tures (for example,cascade loops). Groups of process units may be provided with
optimised set points using advanced techniques like model predictive control, but
the local process loops are likely to be PID controlled. The ease of understanding
of PID controller tuning by maintenance and operational staff and the widespread
availabilityofPIDfunctionalityindistributed-controlsystem/SCADAsoftwareand
programmablelogiccontrollers(PLCs)areimportantfactorsthatsupportthecon-
tinuedindustrialuseofPIDcontrol.
DespitethisfavourablesituationforPIDcontrol,thelargenumbersofPIDcon-
trollersinindustrial-scaleprocessplant,theeconomicpressuretocontainorreduce
maintenance costs and the enabling advances in industrial process computer tech-
nologyledtointerestincreatingtechniquesfortheautomatictuning(termedauto-
tuninginthe1980s)ofPIDcontrolloops.Inthiscontrol-tuningfield,theworkof
ZieglerandNicholsinthe1940sinitiatedaproceduralframeworkfortherulebased
tuning of PID loops. In the 1980s, the work of Åström and Hägglund involving a
relay-test-based procedure, gave the field a new impetus and enabled autonomous
tuningtobecomeanonlinereality.
ThemethodsofPIDcontrollertuninginvolvetwosteps:
Step1AMeasurementExperiment—inwhichsomechosencharacteristicsofthe
processaremeasured.Thesemaybemeasurementstoallowaprocessmodeltobe
vii
viii SeriesEditors’Foreword
identified;theymaybemeasurementsofacharacteristicthatcanbeuseddirectlyin
thecomputationofcontrollerparameters.
Step2ControllerTuningComputation—inwhichcontrolloopperformancere-
quirementsarespecified,thestructureofthecontrollerisdecidedandthecontroller
parametersarecomputedusingthedatacomingfromStep1.
AutomatedPIDcontrollertuningseekstheory,algorithmsandimplementations
that minimise the use of expert input (from, for example, technical, maintenance,
or operational staff) in the tuning process and allows the autonomous tuning of
PIDcontrollersusingcomputersoftwareandhardwareinstalledattheplant.These
techniquescomprisetwocategories:
Parametrictuningmethods,wherethemeasurementsareusedtoidentifyapro-
cessmodelandthecontrollertuningusestheidentifiedprocessmodeldata.
Non-parametrictuningmethods,wherethemeasurementsareofcharacteristics
orparametersthatareuseddirectlyinformulasforthecomputationofthecontroller
parameters.
The “continuous cycling” procedure and its associated tuning rules devised by
Ziegler and Nichols, and the relay test procedure and the associated phase margin
tuning rules are examples of the non-parametric family of tuning methods. These
two contributions have been hugely influential in this particular field of industrial
controlpractice,andtheycontinuetoinspirecurrentdayresearchanddevelopment.
MonographcontributionstothisfieldintheAdvancesinIndustrialControlseries
coverbothPIDcontrolandtheAutomaticPIDcontrollertuningfieldandthosein
thePIDcontrolfieldinclude:
2012 R. Vilanova and A. Visioli (Eds.): PID Control in the Third Millennium:
LessonsLearnedandNewAppoaches,ISBN978-1-4471-2424-5;
2010 A. Visioli and Q.-C. Zhong, Control of Integral Processes with Dead Time,
ISBN978-0-85729-069-4;
2006 A.Visioli,PracticalPIDControl,ISBN978-1-84628-585-1;
1999 A. Datta, M.T. Ho and S.P. Bhattacharyya, Structure and Synthesis of PID
Controllers,ISBN978-1-85233-614-1;and
1999 K.K.Tan,Q.-G.WangandC.C.HangwithT.J.Hägglund,AdvancesinPID
Control,ISBN978-1-85233-138-2;
whereas, monograph contributions to the automatic PID control tuning field in-
clude:
2011 T.LiuandF.Gao,IndustrialProcessIdentificationandControlDesign,ISBN
978-0-85729-976-5;and
1999 C.C. Yu, Autotuning of PID Controllers: Relay Feedback Approach, ISBN
978-3-540-76250-8.
To this activity, it is a pleasure to add this monograph, Non-Parametric Tuning
of PID Controllers: Modified Relay Feedback Test Approach, by Igor Boiko. De-
spiteanextensiveliteratureonthetheoryandapplicationoftherelaytestprocedure
Igor Boiko brings new insights to the field based on his industrial experience by
proposingthatthetuningandtheexperimentalprocedureshouldbematchedtothe
SeriesEditors’Foreword ix
features of specific categories of control loops found in the process industries. In
themonographanalysesofthePIDcontrollerrequirementsforflow,level,pressure,
and temperatureloops are presented.Controller performanceis based on gain and
phase-margin criteria conjoined with time integral (IAE, ITAE, ISE, ITSE) opti-
mality. This novel approach will be especially interesting to the industrial control
practitioner.
Laterchaptersinthemonographdescribeaninvestigationintotheeffectsofpro-
cessnonlinearityonthetuningresultsforflowloops,andpresentmoretheoretical
details of the modified relay test procedure reported in earlier chapters. The clos-
ing chapter of the monograph examines some of the practical issues involved in
performingonlineautomatictuningmethodsanddescribessoftwareforthetuning
task.
ThemixofpracticalexpertiseandthetheoreticalelaborationpresentedbyIgor
BoikointhisAdvancesinIndustrialControlmonographwillattractawidereader-
ship from the control community and the process industries and continues the se-
riestraditionofpublishingexemplaryresearchanddevelopmentinthePIDcontrol
paradigm.
IndustrialControlCentre M.J.Grimble
Glasgow,Scotland,UK M.A.Johnson
Preface
The subject of this book is the method of PID controller tuning based on the con-
tinuouscyclingprinciple.RelayfeedbackfortuningwasproposedbyÅströmand
Hägglund in the 1980s and has been completed since then by numerous modifi-
cations aimed at enhancing some features of the original method. The majority of
these modifications concern parametric methods of tuning that are based on the
identification of certain underlying process models. The method presented herein
isnon-parametric.Itfeaturesaholisticapproachtotestandtuning,orcoordinated
testandtuning,inwhichthetestparametersareselectednotarbitrarilyorapriori
but together with the tuning rule to be applied. As a result, this method provides
exact values of a specified gain or phase margin and does not require any itera-
tiveprocedure.Anothernovelfeatureistheintroductionofprocess-specificoptimal
tuning rules in the non-parametric setup. This allows an engineer to use the flow
loop-optimisedtuningrulesforflowlooptuning,levelloop-optimisedtuningrules
forlevellooptuningandsoon,andobtaininmostcasesabetterresultthangeneric
tuningruleswouldyield.
Wehopereadersmightalsofindthepresentedapproachtoobtainingoptimaltun-
ingrulesaninterestingone.Itinvolvesanontraditionalsolutionoftheoptimisation
problem. We also believe that the recently developed dynamic harmonic balance
principle,whichispresentedinthisbook,mayattractreaderinterestaswell.
Apersonstudyingthesubjectofautomaticcontrolmightbenefitfromthisbook
by learning how linear and nonlinear control theory are brought together to solve
a very important practical control problem—optimal tuning of PID controllers.
A practising control engineer might gain new insights into PID controller tuning.
Thepresentedmethodissimpleinrealisationandefficientintermsofpracticalre-
sults. The reader can use the provided MATLAB code to incorporate the various
componentsofthepresentedtheoryandtuningmethod.
Whenderivingtuningrulesforparticulartypesofindustrialprocesses,focusis
giventothemostcommonprocessindustryapplications:flow,temperature,pressure
andlevelcontrolloops.Itisassumedinmostsituationsthattheprocessismanipu-
latedthroughcontrolvalves(ordampers),andmorespecificallythroughpneumati-
callyactuatedcontrolvalves.Invariousindustriesdifferentfeaturesoflooptuners
xi
xii Preface
are important. In this book we assume that tuning could be carried out on a live
process(i.e.duringplantoperation),sothatsuchfeaturesasminimumdisturbance
totheprocessandshorttimerequiredfortuningareofgreatimportance.
Chapter1isintroductory.Ittracesthehistoryoftuning—andinparticular,non-
parametrictuning.Differencesbetweenparametricandnon-parametricapproaches
arediscussed.Theissueoftheselectionofpropercomplexityoftheprocessmodel
isillustratedbyanexampleofcurvefitting.
Chapter2coversthemethodsofnon-parametrictuningofPIDcontrollers.The
closed-loop Ziegler–Nichols method and Åström–Hägglund’s relay feedback test
arereviewed.Itisshownthattheyallowonetogeneratetestoscillationsatthepoint
◦
corresponding to –180 of the phase characteristic of the process. However, it is
alsoshownthatgenerationofoscillationsinthethirdquadrantwouldbebeneficial.
Anumberofnon-parametrictuningmethodsbasedonvariousmodificationsofthe
relay feedback test (RFT), which are capable of producing test oscillations in the
thirdquadrant,arereviewed.
In Chap. 3, the modified relay feedback test (MRFT) is introduced as a fur-
ther logical development of the closed-loop Ziegler–Nicholsmethod and Åström–
Hägglund’srelayfeedbacktest.Methodsandcriteriaoftuningarepresentedingen-
eralandforeveryconsideredtypicalprocess:flow,level,pressureandtemperature
processes.Withthecriterionofoptimisationselectedandthemechanismofdistur-
bancegenerationanalysedforeachoftheconsideredprocesses,theoptimaltuning
rulesareobtainedbysolvingtheoptimisationproblemonthedomainofparameters
characterisingthesituationalaspectsoftheimpliedprocessmodel.
Chapter4illustratespossiblewaystoimprovetheaccuracyofthetuningrulesvia
anexampleoftheflowloop.Itisshownthattheprecisemodeloftheflowprocessis
nonlineareveniftheinstalledcharacteristicofthevalveislinear.Thisnonlinearity
is revealed as an apparent time constant of the actuator dynamics, which depends
onboththeamplitudeoftherelaytestandtheselectionoftheoperatingpoint.Itis
shownthatwebenefitbyusingtheprecisenonlinearmodeloftheflowloopwhen
findingoptimalPIDtuningrules.
Chapter 5 covers the exact model of oscillations in the system arising from the
modified relay feedback test. The exact model can be used for parametric tuning
thatincludesidentificationoftheprocessmodelparameters.Thedevelopmenthere
is based on the locus of a perturbed relay system (LPRS) method. We present the
LPRSfortheconventionalrelayfeedbacksystem,theMRFTandthetestcontaining
thetwo-relaycontroller.
Chapter 6 provides the model of transient oscillatory motions in a system us-
ing the modified relay feedback test. The treatment is based on the concept of the
dynamicharmonicbalance.
Chapter 7 covers the practical implementation of this book’s tuning method in
software for distributed control systems. The most critical issues encountered in
theimplementationarecovered.Industrialsoftwareusedintheprocessindustryis
described.
Some chapters of the book can be read independently of others. For example,
practising engineers may be more interested in Chaps. 1, 2, 3 and 7. They can
Preface xiii
omitotherchapterswithoutsacrificinganunderstandingofthemainideasofnon-
parametric tuning and the modified relay feedback test in particular. At the same
timeresearchers mayfind thematerialof Chaps.5 and 6 interestingfrom the per-
spectiveoffindingexactmodelsoftheoscillationsandanalysisoftransientoscilla-
torymodes,respectively.
IexpressmyheartfeltgratitudetoProf.M.Johnsonforhiscarefulreadingofthe
manuscriptandnumerouscomments,whichallowedmetosignificantlyimprovethe
book;toProf.M.Grimbleforhisinitiationofthisprojectandencouragementofmy
workonthebook;tomyco-workersandcolleaguesatSyncrudeCanada:D.Brown,
A.Ernyes,W.Oli,andE.Tamayo;tomyformerMScstudentS.Sayedain,whodid
simulations presented in Chap. 4; to A. Breslavskaya, who produced a significant
share of the artwork and LaTeX typesetting for the book; and to K. McKenzie for
correctingandimprovingthequalityoflanguageofthemanuscript.Iamalsograte-
fultomyfamilyfortheirpatienceandthepersonalsacrificeswhichtheyhavegiven
tothismywork.Withouttheirsupportthisundertakingwouldnothavebeenpossi-
ble.
I also gratefully acknowledge the support of RIFP Project No. 12310 of the
PetroleumInstitute,AbuDhabi.
AbuDhabi IgorBoiko