Modeling and Control of Antennas and Telescopes

235 Pages·2008·7.1 MB·English

by  Wodek Gawronski (auth.)

Mechanical Engineering Series
FrederickF.Ling
Editor-in-Chief
Mechanical Engineering Series
WodekGawronski,ModelingandControlofAntennasandTelescopes
Makoto Ohsaki and Kiyohiro Ikeda, Stability and Optimization of Structures: Generalized
SensitivityAnalysis
A.C.Fischer-Cripps,IntroductiontoContactMechanics,2nded.
W.ChengandI.Finnie,ResidualStressMeasurementandtheSlittingMethod
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J.Chakrabarty,AppliedPlasticity
K.K.ChoiandN.H.Kim,StructuralSensitivityAnalysisandOptimization1:LinearSystems
K.K. Choi and N.H. Kim, Structural Sensitivity Analysis and Optimization 2: Nonlinear
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G.Chryssolouris,LaserMachining:TheoryandPractice
V.N.Constantinescu,LaminarViscousFlow
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J.F.Doyle,WavePropagationinStructures:
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A.C.Fischer-Cripps,IntroductiontoContactMechanics
A.C.Fischer-Cripps,Nanoindentation,2nded.
J.Garc´ıadeJalo´nandE.Bayo,KinematicandDynamicSimulationofMultibodySystems:
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(continuedafterindex)
Wodek Gawronski
Modeling and Control
of Antennas and Telescopes
123
WodekGawronski
CaliforniaInstituteofTechnology
JetPropulsionLaboratory
Pasedena,CA,USA
[email protected]
ISSN:0941-5122
ISBN:978-0-387-78792-3 e-ISBN:978-0-387-78793-0
DOI:10.1007/978-0-387-78793-0
LibraryofCongressControlNumber:2008922734
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MechanicalEngineeringSeries
FrederickF.Ling
Editor-in-Chief
TheMechanicalEngineeringSeriesfeaturesgraduatetextsandresearchmonographs
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UniversityofCalifornia,
SantaBarbara
D.Gross
TechnicalUniversityofDarmstadt
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UniversityofCalifornia,
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Series Preface
Mechanical engineering, and engineering discipline born of the needs of the
industrial revolution, is once again asked to do its substantial share in the call
for industrial renewal. The general call is urgent as we face profound issues of
productivityandcompetitivenessthatrequireengineeringsolutions,amongothers.
TheMechanicalEngineeringSeriesisaseriesfeaturinggraduatetextsandresearch
monographsintendedtoaddresstheneedforinformationincontemporaryareasof
mechanicalengineering.
Theseriesisconceivedasacomprehensiveonethatcoversabroadrangeofcon-
centrations important to mechanical engineering graduate education and research.
We are fortunate to have a distinguished roster of series editors, each an expert in
oneoftheareasofconcentration.Thenamesoftheserieseditorsarelistedonpage
viofthisvolume.Theareasofconcentrationareappliedmechanics,biomechanics,
computational mechanics, dynamic systems and control, energetics, mechanics of
materials,processing,thermalscience,andtribology.
Preface
This book is based on my experience with the control systems of antennas and
radiotelescopes. Overwhelmingly, it is based on experience with the NASA Deep
Space Network (DSN) antennas. It includes modeling the antennas, developing
controlalgorithms,fieldtesting,systemidentification,performanceevaluation,and
troubleshooting.Mypreviousbook1 emphasizedthetheoreticalaspectsofantenna
control engineering, while this one describes the application part of the antenna
controlengineering.
Recently, the increased requirements for antenna/telescope pointing accuracy
havebeenimposed.InthecaseofDSNantennas,itwastheshiftfromtheS-band
(4 GHz) and X-band (8 GHz) communication to Ka band (32 GHz). On the other
hand, the Large Millimeter Telescope will operate up to 200 GHz, which requires
extremely accurate pointing. These requirements bring new challenges to the an-
tennacontrolengineers.ClassicalPIcontrollerscannotassuretherequiredaccuracy,
whilemodel-basedcontrollers(LQG,and H∞)increasetheantennaaccuracybya
factorof10.Thesecontrollersarenewforantenna/telescopeengineering.Thisbook
describestheirdevelopmentandapplication.
The book is addressed primarily to the antenna, telescope, and radiotelescope
engineers,engineersinvolvedinmotioncontrol,aswellasstudentsandresearches
inmotioncontrolandmechatronics(antennaisacombinationofmechanical,power,
and electronics subsystems). The book consists of two parts: Modeling (Chapters
2–5)andControl(Chapters6–13).
In the modeling part, Chapter 2 describes the development of the analyti-
cal model of an antenna, including its structure (finite-element model) and drives
(motors,reducers,amplifiers).Thismodelingisusefulinthedesignstageofanan-
tenna.Chapter3describesthedeterminationoftheantennamodelusingfieldtests
and the system identification approach. These models are quite accurate, and are
usedinthedevelopmentofthemodel-basedcontrollers.Becausetheorderofmod-
els(analyticalaswellasfromtheidentification)isoftentoohighfortheanalysisand
1Gawronski W. (2004). Advanced Structural Dynamics, and Active Control of Structures.
Springer,NewYork.
vii
viii Preface
forcontrollerimplementation,itisreducedtoareasonablesizewhilepreservingthe
criticalpropertiesofthefullmodel.Chapter4brieflydescribesthereductiontech-
niquesasappliedtoantennasandtelescopes.Finally,thewinddisturbancemodels
aredevelopedinChapter5.Windisthemajordisturbancesourceforantennasand
telescopes. The steady (or static) wind model is presented, based on wind tunnel
dataandconfirmedbythefielddata.Also,threewindgustsmodelsarepresented.
Thecontrolpartpresentstheperformancecriteriaandshowshowtotransform
the antenna model to be the most suitable for controller tuning. In Chapter 7, the
book presents the development, properties, and limitations of the PI controller. It
showstheimpactoftheproportionalandtheintegralgainsontheantennaclosed-
loop performance. It also analyzes the limits of the PI controller performance.
Chapter 8 describes the tuning process of the LQG controller. It analyzes the per-
formanceoftheLQGcontroller,includingitslimits.Itpresentsthegraphicaluser
interface(GUI)thatallowsustotunetheantennaLQGcontrollerwithoutanalysis,
butbyplayingwiththeGUIslidersandbuttons(“LQGfordummies”).Itisshown
inthischapterthattheperformanceoftheLQGcontrollerdependsonitslocation:
eitheratthepositionlooporatthevelocityloop,oratboth.Itshowsalsothatinthe
caseofthe34-mDSNantennastheservoerrorwithLQGcontrollerdecreasedbya
factorof6.5whencomparedtothePIcontroller.InChapter9, H∞ controllertun-
ingispresented:gaindetermination,closed-loopequations,limitsofperformance.
In the following chapter, the non-linear control issues are addressed. The velocity
and acceleration limits often interfere with antenna dynamics. Two solutions are
proposed:acommandpreprocessor,andtheanti-winduptechnique.Frictionisthe
sourceofadeterioratedpointing.Dithercanbeasolution,aspresentedinthisbook.
Finally,gearboxbacklashisdescribedandthecounter-torquesolutiontominimize
itispresented.
A typical antenna control system consists of two loops: velocity and position
loops.Asingle-loopsolutionisstudiedinChapter11.Theantennacontrolsystem
usestheencodermeasurementstoestimateRFbeamposition,butitisonlyarough
estimate.Theencodermeasurestheantennaangularpositionatitslocation,which
is different from the RF beam location; thus, antenna structural compliance is the
source of the error. Chapter 12 describes two techniques to control the RF beam
position:monopulseandconscan.Finally,inChapter13wedescribeanopen-loop
RFbeamcontrol:thelook-uptablethatcorrectsfortheuneven azimuthtrackthat
impactstheRFbeamposition.
Analysisisaskill.However,evencomplextheoriesandthesupportinganalysis
are,inasense,simplebecausetheyassumecertainpropertiesthatsimplifytheanal-
ysis, or to make it possible. Still, although the theoretical path delivers an answer,
engineershavetoaskiftheanswerisapplicabletotherealenvironment.
Engineeringisanart.Someaspectsofengineeringcanbedescribedrigorously
bytheoreticalanalysis,butnotall.Therearecaseswhereengineering realitydoes
not satisfy analytical assumptions. Hence, engineering solutions are often ad hoc
solutions.ThisengineeringdilemmaissummarizedbyScholnik:“Whocareshow
itworks,justaslongasitgivestherightanswer.”
Preface ix
Theartofengineeringoftenincludesfeaturesthattheoreticallycannotwork.For
example:
(cid:2)
LQGcontrollerscannotbeappliedtoantennas,becausetheantennamodelhave
(cid:2) polesatzero(rigid-bodymode).ButtheLQGcontrollersdocontroltheantennas.
The noise in the tuning of the LQG controller should be the Gaussian noise,
(cid:2) whichisnotthecaseofantennasortelescopes.
Rigidly applied model reduction algorithms do not produce the best reduced
(cid:2) model(bestintermsoftheantennaperformance).
(cid:2) EveryLQGcontrollerisanoptimalcontroller,butnoteveryoneisacceptable.
In the H∞ controller tuning the plant uncertainity should be either additive or
multiplicative.Theantennauncertainity,whichdependsonitselevationposition,
isneitheradditivenormultiplicative.
Despite these difficulties, engineers succeed in developing acceptable control
systems,includingantennasandtelescopes.
Readerswhowouldliketocontactmewithcommentsandquestionsareinvited
[e-mail]@sbc
global.net.
W.Gawronski
Pasedena,California
Acknowledgements
PartoftheworkdescribedinthisbookwascarriedoutattheJetPropulsionLabora-
tory,CaliforniaInstituteofTechnology,undercontractwiththeNationalAeronau-
tics and Space Administration. I thank Alaudin Bhanji, Mark Gatti, Pete Hames,
Wendy Hodgin, Andre Jongeling, Scott Morgan, Jean Patterson, Daniel Rascoe,
Christopher Yung, and Susan Zia, the managers at the Communications Ground
Systems Section, Jet Propulsion Laboratory, for their support of the Deep Space
Networkantennastudy.
I would like to acknowledge the contributions of my colleagues who have had
aninfluenceonthiswork:
(cid:2)
from the Jet Propulsion Laboratory: Harlow Ahlstrom, Mimi Aung, Farrokh
(cid:2) Baher,AbnerBernardo,JohnCucchissi,JefferyMellstrom,MarthaStrain;
from the NASA Goldstone Deep Space Communication Complex (California):
(cid:2) MichaelWindersandJeffreyFrazier;
from the NASA Madrid Deep Space Communication Complex (Spain): Angel
(cid:2) Martin,DavidMunozMochon,andPabloPerez-Zapardiel;
from the NASA Canberra Deep Space Communication Complex (Australia):
(cid:2) PaulRichterandJohnHowell;
visiting students: Jason Brand, Emily Craparo, Brandon Kuczenski, and Erin
Maneriwhoseworkisalsoincludedinthisbook.