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TRANSCRIPT
Chapter 1
Introduction
Detailed contents
Page Nos.
1.1 General 2
1.2 Advances in Space Domain 2
1.2.1 Formidable Challenges in Space Domain 2
1.2.2 Ultra light structures and materials - An overview at NASA 4
1.2.3 Frontiers in aerospace technologies - Present & Future 8
1.2.4 Present scenario in antenna reflector domain in India & Abroad 11
1.3 Smart Structural Systems 15
1.3.1 Definition of smart structures 16
1.4 Piezoelectric Materials - A New Era 17
1.5 Facilities Used at SAC 18
1.5.1 Software for FE modeling of smart structural systems 18
1.5.2 Test facilities used in the investigation 19
1.6 The Need of Investigation 20
1.7 Broad Domain of Investigation 21
1.8 Aim of the Present Work 23
1.9 Scope of Proposed Investigation 24
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Introduction
1.1 General
Intelligent / adaptive structures are the state-of-the-art technologies being used for
a few ground & some space borne structures and structural systems. Although
intelligent materials systems and structural concepts may be applied to the design
and implementation of buildings, dams, bridges, pipelines and ground based
vehicles but recent efforts have shown the possible applications in potential
domains of advanced aircrafts, launch vehicles, spacecraft antennas and large
space borne systems. Until now, this has remained as an area not fully explored
although, as a matter of fact, it has lot of built-in future potential.
Smart structural systems, have the tendency to get adapted to the new
environment by changing their shapes and sizes respectively by using the concept
of sensing, actuation and control almost, as smart as, human body having the
reflex action using nerves, muscles and brain. Smart materials should be able to
both sense and communicate with outside intelligence in order to meet functional
requirements.
1.2 Advances in Space Domain
Weight and power consumption are at premium in satellites, hence there is always
a requirement of state-of-the-art light weight, high specific stiffness, high specific
strength and low thermal expansion materials for spacecraft reflectors. Size of the
component is also a major consideration in spacecrafts, hence there is a
requirement of small size composite reflectors operating at high frequencies. To
meet the above requirements; the intelligent and adaptive contemporary materials
to some extent come to our rescue. As matter of fact, the applications of smart
materials and smart structural systems in space domain have there own formidable
challenges and a few of them have been enumerated as follows:
1.2.1 Formidable Challenges in Space Domain
In order to meet the communication and broadcasting needs of the twenty-first
century, antenna reflectors are generally required to have shaped surfaces. It is a
well known fact that maintaining precision surface shape for spacecraft antenna
reflector is a challenging task. The surface errors are introduced by manufacturing
2
errors, thermal distortions in orbit [153], moisture, loose structural joints, material
degradation and creep. Lot of R & D is required in the development of advanced
methods for precision control of piezoelectric smart structures with temperature
and hysteresis compensation.
Piezoelectric smart structures have potential aerospace related applications, such
as active shape control of deployable space antenna reflectors, active vibration
control of flexible solar arrays and position actuation of space-board precision
scanners and mirrors among many others. However, piezoelectric materials exhibit
nonlinearities, such as hysteresis, which adversely affect precision control of the
structures activated by piezoelectric actuators. Also variations in temperature affect
the properties of piezoelectric actuators [153]. To design control methods to
compensate for the nonlinearities associated with piezoelectric actuators poses a
challenge for control engineers and researchers. Conventional linear control designs
cannot solve these issues. Therefore, lot of R & D is ongoing to develop advance
control methods, such as the technique using neural networks and sliding-mode
based robust controller to compensate for hysteresis in smart actuators.
A major issues stems from the fact, that once the antenna is deployed on orbit, the
radiation pattern cannot be modified. If the shape of the antenna is allowed to
change, however, this issue can be addressed. A few illustrations of contemporary
ideas like inflatable / umbrella type reflectors, which may overcome these key
issues in future for the communication satellites of 21 st century are shown in Figs
1.1 & 1.2 : (Few are early artist's concepts only).
Fig 1.1 : Spacecraft antennas of 21st century
3
Fig 1.2 : Spacecraft antennas of 21st century
Schematic diagram (not to scale) of the 14-meter Inflatable Antenna Experiment
(IAE) that was flown from the Space Shuttle. This is a joint JPL, NASA/Goddard and
L'Garde program. Ultra Light weight structures and space observatories as( t v
proposed by NASA (National Aeronautics and Space Administration) is gist are as
follows:
1.2.2 Ultra Light Structures and Materials - An overview at NASA
The Ultra-Lightweight Structures and Space Observatories (ULSSO) thrust develops
revolutionary technology in structures, materials, and optical systems to enable
bold new missions of discovery for deep space missions. NASA is studying future
missions requiring very large space observatories. Long-range plans are aimed at
detection and characterization of planets in orbit around nearby stars to search for
the chemical signatures of life. Achieving this, will require arrays of space
telescopes that have lOOOx the light collecting area of the largest ground-based
telescopes in operation today. Technologies are sought that enable very large
telescopes for imaging extra-solar planets, studying the formation of large-scale
structure in the early universe, and continuously monitoring the Earth form distant
vantage points. Technologies are sought that enable large deployable and inflatable
antennas for space-based radio astronomy, high-bandwidth communications from
deep space, and Earth remote sensing with radar and radiometers; solar sails for
low cost propulsion, station keeping in unstable orbits, and precursor interstellar
exploration missions; gossamer technology for kilometer-scale membrane
spacecraft that weigh less per unit area than a sheet of paper.
4
Revolutionary advances in ultra-lightweight structures and materials technology are
needed to enable a broad range of futuristic NASA's missions. Applications include
large aperture telescopes and antennas, solar sails and telescope sunshields, large
solar arrays and solar concentrators, Earth and planetary balloons, planetary entry
vehicles, and spacecraft operating in extreme environments. Technology
breakthroughs in this area will also enable gossamer spacecraft, which are very
large, ultra-lightweight, highly-integrated systems that can packaged into a small
volume for launch. Technologies of specific interest are:
• Large (> 20 m) deployable and inflatable rigidizable booms and trusses.
• Innovative methods for in-space manufacture and self-assembly of lightweight
structural elements and membranes. Membranes that can be made to grow like
a biological system and that can 'self-heal' is a long-term demand.
• Thermal protection for hypersonic vehicles.
• Highly - integrated multifunctional membranes that incorporate electronics,
MEMS devices, sensors, actuators, power sources, or other spacecraft
components in thin-film materials.
• Ultra - lightweight, high- strength membrane materials for solar sails,
sunshields, inflatables, and balloons. Materials should be resistant to ultraviolet
radiation, particle radiation, and extreme temperatures (lifetime > 10 years).
• High surface precision thin-film materials and reflective coatings for membrane
optics.
• Nano-particle (i.e., organoclays, carbon nanotubes, etc.) containing composite
materials with substantially higher strength-to-weight ratio or thermal
conductivity than state-of-the-art composites. Ideas should not be limited to
filling polymers with nano-particles, but should include concepts such as
chemically linking nano-particles together to form molecular 'net-like'
structures. Applications include ultra-lightweight structural elements, electrically
conductive elements, and efficient thermal management devices.
Proposals are sought for the development of adaptive systems applicable to large,
ultra-lightweight structures and apertures. Adaptive systems are needed for
measuring and correcting surface figure and wave front errors for large telescopes
and antennas, for controlling the dynamics of large flexible structures, and for
enabling gossamer spacecraft that can reconfigure themselves in response to
changing environmental conditions or mission phases. Technologies of specific
interest are:
• Smart inflatable structures with embedded actuators and sensors for controlling
structural geometry and dynamics.
• Innovative methods for shape control of large membrane mirrors and antennas
such as non-contact actuators.
5
• Concepts and components for active, adaptive wave front control systems with
correction to < 1 wave length.
• Materials with controllable surface properties that could adapt to changing
environmental conditions or mission needs.
• Novel concepts for gossamer spacecraft that could enable mission that were
previously considered impossible, while keeping cost and risk within acceptable
limits. An example concept is a gossamer spacecraft capable of modifying its
shape or other functional characteristics so that it can adapt to different mission
phases, such as atmospheric entry, descent, landing, and surface exploration.
Large telescopes and structures 10 times the size of the Rose bowl in Pasadena,
California, that can be compacted and deployed in a single small launch vehicle and
then inflated once they are in the orbit, are a major part of the future of earth and
space exploration.
As part of the Gossamer spacecraft initiative, which is chartered with developing
technology for large telescopes and space sails new ways are being explored to put
large structures in space. The results of these investigations eventually would be
breakthroughs in ultra-light, inflatable materials that will substantially reduce
mission costs and enable large, ultra light objects to observe the Earth and far
reaches of the Universe. One of the proposed studies at Jet Propulsion Laboratory
(JPL), NASA with illustrations is shown in Fig 1.3 :
Fig 1.3 Inflatable space stations research as JPL,NASA
6
According to Mr Artur Chmielewski, Manager of JPL Space Inflatables Technology,
"without new technology and new materials, we can't go forward in our missions to
peer deep into the cosmos and look for eanh-like planets and other stars".
NASA's recent studies on Space Solar Power Satellites (SSPS) for generating large
amounts of electricity from large-scale, space based solar power systems are
shown in Figs 1.4 & Fig 1.5. :
Fig 1.4 Morphology of various SSPS concepts
Baselinel.2GWAbacusssatellite Cylindrical SSPS concept
cz^>
Halo
o oCD CDCD CD
Suntower Dual Backbone Suntower with
Sub-Arrays
Solar Parachute
T - Configurations
AbacusRigid Monolithic
Array (1979 Reference)
Spin-Tensioned Monolithic
Array (Solar Disk)
Fig 1.5 Morphology of various SSPS concepts
SSP Exploratory Research and Technology (SERT) of NASA has also proposed the
concept of Integrated Symmetrical Concentrator (Fig 1.6) to harness Solar Energy :
7
Concept sized for 1.2 GW delivered to Grid on Earth
Docking Pot
500m RF Transmi
3562 x 3644 m Clamshells,
36 456m mirrors
6378iMasl
Quantum Dot PV arrays canted 20°
846mod,200mid
Fig 1.6 Symmetrical Concentrator
1.2.3 Frontiers in Aerospace Technologies- Present & Future
The ultra light weight concept of structures presently can be monocoque (from
French mono - 'single' + coque - 'shell'), semi-monocoque, sandwich, corrugated,
gossamer (a filmy substance consisting of cobwebs spun by spiders) type
structures, isogrids / waffle (fine honey comb weaving) type structures. Ultra high
precision reflectors for Ku band with RMS specifications like (0.25 mm) and Ka band
reflectors with RMS specifications like (0.10 mm) would be required in future as
high stability structures with lay up fibers of high specific strength Carbon-Carbon
vanes (99 % of Phenolic Resin is converted into Carbon by infiltration method for
bonding with the M55J / M18 fibers of the prepregs) to the accuracy of 5 microns
accuracy.
Inflatable structures and foldable composites including Ultra long balloons are the
requirements of the 21st century Stratellites (A high altitude futuristic Air ship that
when in place in stratosphere as a stationary platform can be used for transmitting
various types of wireless communications currently transmitted through cell towers
and satellites. For stratellites dry adhesives would be developed and would have
self launching capability at low earth orbit (20 Kms or so) & would not require
launch vehicles to reach stratellites). Space elevators made up of chirals / carbon
nano tubes would be developed by NASA using tethers and would be working using
the concept of Laser torch. A few illustration (Fig 1.7 - Fig 1.10) are as follows
showing the concept of stratellites, space elevators, Gossamer (thin film type)
spacecraft structures, inflatable antennas & aluminized Kapton solar sails getting
developed at NASA Langley Research center :
8
Fig 1.9 Gossamer spacecraft - Solar sails, . Fig 1.10 - 5m dia. Inflatable Antenna
The performance requirements of structures of space domain are as follows:
• structural integrity• Low response to loads / disturbances• Dimensional accuracy / precision• Low inter system coupling• Jitter control• Stiffness• Agility
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• H igh Design Efficiency
• T h e rm a l C haracteris tics (C o n d u ctiv ity & S u rface p ro p erties )
• E lectric / E lectrom agnetic p e rm eab ility properties
• Fast rea liza tion
• F lexib ility to incorporate th e changes
Following a re th e m ethods o f achieving th e above m entioned design req u irem en ts :
• C o n tem p o rary m ate ria ls s im ila r to High m odulus p o lye th y len e (D y n e e m a )
w ith density o f 0 .9 7 gm /c c , v is -a -v is , CFRP A T 1 .8 gm /c c .
• In n o v a tiv e typ es o f construction
• E ffic ient s tructura l fo rm s / jo in ts / fastners
• O p tim u m design m odeling s im ulation
• Load reduction
• A dap tive / S m a rt s tructures
• M u lti-functiona l s tructures
• S tra te g y fo r design / d eve lo p m e n t fabrication
• In fla ta b le
Following is th e o n -o rb it behaviora l req u irem en ts o f th e space s tructures:
• D im ensional S tab ility & accuracy
• Low response to th e d is turbances - in te rna l & Externa l
• High D am ping (P re fe rred is a lw ays passive dam ping as in active dam ping
th e e rro r in feed back control loop can lead to instab ility o f th e space
s tru c tu re )
• V iscoelastic dam ping to reduce settling tim e o f th e appendages.
• High A gility
• Following a re th e real challenges in th en design o f space structures :
• is th e D im ensional s tab ility & accuracy because o f high h e a t dissipation
characteris tics.
Following a re th e s tandard s tren g th o f m a te ria l p a ra m e te rs considered in th e choice
o f high specific s treng th & high specific s tiffness space m ate ria ls :
E, E l, K, M, f, { F } , E /p , 3 V E/p, o/p, Va/p, a, a, a cr, «p, 5, p,
S ta te -o f-th e -a r t , high dam ping a lloy v iz , T h erm o e las tic M artens itic Alloy e .g
PROTEUS™ stands o u t as th e co n tem p o rary m a te ria l w ith good v ib ra tion dam ping
e ffec t in a w ide te m p e ra tu re range due to m a rte n s ite -m a rte n s ite in te rface
m o vem en ts . O th e r m echan ism s which m a y co n tribu te s ign ificantly to th e a m o u n t o f
10
dam ping a re , such as th e m o v e m e n t o f tw in -b o u n d aries in C u -A I-N i and in N I-T i
alloys. A typ ical illustration o f a spacecraft re flec to r under d e ve lo p m e n t and testing
using PROTEUS™ is illustrated in Fig 1 .1 1 .
Fig 1 .1 1 S p acecraft re flector developed using high dam ping a lloy PROTEUS™
Now focusing & narrow ing on applications of s m a rt s tructures and in te lligent
structura l system s re la ted to spacecraft an tenna dom ain , p resently , follow ing is
th e scenario in gist:
1.2.4 Present Scenario in Antenna reflector domain in India & Abroad
Presently , th e com posite reflectors [1 9 0 ] m ade up o f G rap h ite and K evlar being
used in IN S A T / G EOSAT m issions a re Prim e Fed ty p e Parabolic reflectors or
shaped o ffset reflectors w ithou t an y usage o f s m a rt m ateria ls . Even th e next
g eneration Dual gridded shaped com posite reflectors are being used w ithou t any
concept o f re flector skin sm artness o r reconfigurab ility o f th e re flec to r surface .
H o w ever, Fu ture Ind ian space m issions will requ ire s m art space borne
reconfigurab le an ten n a reflectors w orking on K a-B and radio frequencies fo r
te lecom m unications and h igher frequencies fo r earth observation and scientific
applications , which would requ ire m o d em re flec to r shape changing capabilities fo r
catering to d iffe re n t land m asses w ith th e sam e re flector. P resently , w ithou t th e
use of dedicated an tenna pointing m echanism s for rigid body m o v em e n t o f re flector
surfaces, it is a lim itation in th e design o f spacecraft reflectors .
T h e need fo r high precision th in shell reflectors will also crop up fo r fu turis tic
sate llite reconfigurab le reflectors capab le fo r Q /V -B a n d applications which will need
co n tem p o rary shape a d ju s tm e n t techniques.
R ecent studies on s m art s tructu res w orld over, have found potentia l applications in
Large D ep loyab le spacecraft an ten n as fo r m obile in te rn e t / m obile te le
conferencing ty p e applications, Large space m irrors and in large In fla ta b le space
an tennas [1 7 ] fo r deep space missions.
11
M oreover, app lications a re also possible in fu tu ris tic Radio Frequency Filled
A pertures [4 0 ] / A ntennas in space o f v e ry large d ia m e te r. In addition to th is , th e
applications of Meta m ateria ls & m icro m achined s tructures a re also being ta lked
ab o u t in quasi optic m ultip le frequency applications viz m illim e te r w ave an tennas
(3 0 0 G Hz to 1 Tera Hz fre q u e n cy ). Fig 1 .1 2 shows th e concept o f Large D ep loyab le
R eflectors (LD R s) [1 9 1 ] .
Fig 1 .1 3 shows th e concept o f In fla ta b le a n ten n a and th e use Piezo film s to m ain ta in th e requisite profile o f th e an ten n a .
Fig 1 .1 3 : A pplications o f Piezo film s in In fla ta b le s tructures
12
S m a rt m ate ria ls such as PZTs, PVDF Film s and S h ap e M em o ry A lloys (S M A ),
M agnetos tric tive m a te ria ls like T e rfin o l-D rods, E lectro-R heological Fluids (E R F),
M em o ry M etal Fibers (M M F ) e tc , have a ttra c te d m a n y researchers around th e world
fo r ob ta in ing v ib ra tion contro l, shape contro l, th e rm a l contro l in aerospace
struc tu res . A c tive and passive v ib ra tion c o n tro l[1 5 8 ]o f th in flex ib le s truc tu res using
M ag n eto s tric tive p o w d e rs [1 5 ] has been studied by various researchers including
th e concept o f q u ite coats & s m art constrained lay e r dam ping criteria ; w h ere a
viscoelastic laye r is sandw iched b e tw een a p iezoelectric lay e r and th e su b stra te . In
th is p a rticu la r case, th e v ib ra tion en erg y is d am p ed d u e to s h ear d e fo rm atio n in th e
viscoelastic lay e r, w hich derives sm artn ess w hen th e v ib ra tion o f th e s tru c tu re is
fed back to reg u la te th e axial m otion o f th e p iezoelectric layer.
M oreover, fo r p la tes , in th e concept o f enhanced constra ined lay e r d a m p in g , a
viscoelastic lay e r is constrained b e tw een a s m a rt p iezoelectric lay e r and th e base
substra te which is being contro lled . In th is particu la r case, th e s m a rt dam ping
ta k e s place due to cyclic shearing o f th e viscoelastic lay e r and it ge ts enhanced by
activ e com ponent o f th e d am ping which is th ro u g h th e tra n s fe r o f control
m o v em e n ts .
A rev iew on shape contro l is given by H Irsch ik [1 3 8 ] . Andoh Fukashi [1 ] presented
shape control o f singly curved and doubly curved re flec to r w ith a lim ited n u m b e r o f
discrete actu a to rs and op tim ized th e a c tu a to r location. G upta V .K e t. a l. [4 ]
developed fin ite e le m e n t fo rm u la tio n based on d e g e n e ra te shell e le m e n t fo r p iezo
actuation in shell s tru c tu re and also p erfo rm ed th e e x p e rim e n t on doubly curved
shell s tructures.
For a space a n te n n a s tru c tu res , fo r instan ce , th e requ ired surface accuracy depends
on its frequency band o f op eratio n . Because th e freq u en cy used cu rren tly tends to
be h igher and h ig h er, th e d em an d on th e surface accuracy becom es s everer. The
structura l th e rm a l d e fo rm atio n induced by te m p e ra tu re change ranging from -
1 5 0 °C to + 1 5 0 °C on o rb it and th e in -process m e m b e r length e rro rs a re c ited as
th e m ain causes o f d e te rio ra tio n o f surface accuracy . In g e n e ra l, th e precise
m e a s u re m e n t o f a n te n n a configuration is required fo r high precision shape control
o f a n te n n a s truc tu re . H o w ever, since it is d ifficu lt to place sensors and actuato rs to
all s truc tu ra l com ponents , th e techn iques o f h ighly precise m e a s u re m e n t by a sm all
n u m b e r o f sensors becom e m o re and m o re im p o rta n t.
F u rth e rm o re , low frequency v ib ra tions tend to occur easily in th ese space
structu res due to th e rapid te m p e ra tu re change o f surrounding en v iro n m e n t o r th e
a ltitu d e control o f s truc tu res o r d u e to th re e axis m o v em e n ts o f spacecra ft gyros.
13
Since those v ib ra tions do not decrease in th e e n v iro n m e n t o f m icro g ra v ity and high
vac u u m , th e pe rfo rm an ce o f a s a te llite m ain p a rt o r an e lec tric device m a y drop
re m ark ab ly . T h e re fo re , it is crucial to suppress those s tructura l v ib ra tions by som e
active o r passive control techn iques on real tim e .
R ecently , research on th e sta tic shape contro l and th e v ib ra tion contro l fo r
app lications in space s tructu res has been proposed. In s ta tic shape contro l [1 ]
d o m ain , h ighly precise shape control becom es possible only u n d er th e condition o f
a large n u m b er o f sensors and actuato rs . H o w ever, accurate shape estim ation and
shape contro l using a sm all n u m b e r o f sensors and ac tu a to rs a re still d ifficu lt fo r a
de fo rm ed s tru c tu re w ith respect to a rb itra ry d is turbance enacted on it.
For th e realiza tion o f n e x t gen era tio n space s truc tu res , such as a space a n te n n a
w ith h ig h er perfo rm an ce , th e d e ve lo p m e n t o f h ighly precise shape
e s tim a tio n /c o n tro l techn iques only em p lo y ing a lim ited n u m b e r o f sensors and
actu a to rs is indispensable. Second, fo r v ib ra tion contro l [1 6 4 ] , m a n y researches
using th e conception o f m odal sensor and m odal a c tu a to r to contro l th e d o m inan t
lo w -o rd e r m odes h ave been carried out based on th e m odal analysis o f s tructura l
v ib ra tio n . T h ese kinds o f trad itio n a l approaches can only be applied to som e s im ple
struc tu res , like a beam s tru c tu re . A lso, th e y usually need to a tta c h a sensor, such
as a PVDF film ; on th e w hole surface o f a beam and th e com puta tiona l cost is
co m p ara tive ly high. T h e re fo re , th e techn iques fo r realizing h ighly precise v ib ration
m e a s u re m e n t and contro l using lim ited n u m b e r o f sensors and ac tu a to rs h ave not
been developed y e t fo r la rg e -sca le and com plicated s tructures.
A pplications o f s im ila r bu t practica lly feas ib 'e concepts fo r sm all s ize doubly curved '
parabolic an ten n as surfaces, is still an a rea unexp lored fu lly .
R ecently on ly , in addition to v ib ra tion dam ping d o m a in , w ork has been carried o u t
in th e dom ain o f using TH U N D E R (Th in L ayer U N im orph Ferroelectric D rivER )
actu a to rs [1 7 7 ] , Pow er Pack actuato rs & curved S trip actu a to rs fo r investigating
beam shaping in th e dom ain o f M echanically A ctive A n tenna (M A A ) surfaces in
space s eg m en t based on th e concept o f s m a rt a p e rtu re an ten n as . Fig. 1 .1 4 show s a
typ ica l M echanically A ctive A n ten n a u n d e r d e ve lo p m e n t a t O hio S ta te U n ivers ity ,
USA fo r s teering th e a n ten n a beam fro m No"th A m erica to South A m e ric a .[2 ]
La te ly , th is has opened new v is tas fo r in n o vative m a te ria ls to h an d le th e design
challenges posed by th e fu tu ris tic a n ten n as o f h igh rad io frequencies in te rm s o f
u ltra high precision designs.
14
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OHIO5IATE Mechanically Active Antennas
Fig 1 .1 4 A typ ical M echanically A ctive A ntenna under d eve lo p m en t
Fig. 1 .1 5 shows a typical m odel of 0 .3 m dia. doubly curved s m a rt an ten n a a t Ohio
S ta te U n ivers ity , USA using p iezoceram ic strip actuated approach using Four
T h u n d e r PZT actuato rs fo r 1 1 .8 G H z o f frequency o f signal [ 2 ] [ 6 ] .
Fig 1 .1 5 A typ ical m odel o f 0 .3 m d ia. MAA a n ten n a a t O hio S ta te U n ivers ity , USA
1.3 S m a rt S tru c tu ra l sys tem s
Advanced research in m ate ria ls science resulted in m a n -m a d e m ate ria ls , such as
plastics and com posites. Selection o f unusual shapes in th e design o f s tructura l
15
components and ideas of embedding sensors to monitor complex strain fields then
took hold. Furthermore, materials with unusual properties were discovered:
properties by which material behavior can be varied depending upon the phase of
the material (e.g., shape memory alloys, such as NiTiNol, whose phases change at
critical temperatures), the poling direction (as in piezoelectric materials such as
PZT),and the level of electric field (Electrorheological fluids). These discoveries
have opened up the design space to such an extent that possibilities of designing
structures that can not only monitor themselves but also adapt to the environment
are now contemplated by the research community.
This is the background that has ushered in an era of research efforts leading to
"smartness" in structural design. Not unexpectedly, a variety of names, such as •»
smart materials, intelligent materials, and adaptive structures, have been
proposed.
Clearly, the dictionary definition of "smart" (brisk, spirited, mentally alert, bright,
knowledgeable, shrewd, witty, clever, stylish, being a guided missile, operated by
automation) is not quite adequate in this context. The engineering community has
adapted the term smart structures, over nearly a decade now, and the words have
come to mean a certain extraordinary ability of structures or structural components
in performing their design function. Smartness, in this context, implies (a) the
ability of structural members to sense, diagnose and actuate in order to perform
their function (closed-loop smartness) and/or (b) unusual micro or macro-structural
design that enhances structural integrity (open-loop smartness). A closed-loop
smart structure or component is one which has the ability to sense a variable such
as temperature, pressure, strain, and so forth, to diagnose the nature and extent of
any issues, to initiate an appropriate actior to address the identified issues, and to
store the processes in memory and "learn" to use the actions taken as a basis next
time around. The attributes of smartness may thus include the abilities to self-
diagnose, repair, recover, report, and learn.
1.3.1 Definition of smart structures
Smart structures have the capability to sense, measure, process, and diagnose at
critical locations any change in selected variables, and to command appropriate
action to preserve structural integrity and continue to perform the intended
functions. The variables may include deformation, temperature, pressure, and
changes in state and phase, and may be optical, electrical, magnetic, chemical, or
biological. The question of structural integrity arises when defects develop, cracks
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form and propagate, or vibration occurs at resonance or flutter. Some examples are
earthquake response of buildings, cutting tool chatter, rotor critical speeds, and
turbine engine blade flutter.
Efforts are also being directed toward the development of "smart," or responsive,
materials. Representing another attempt to mimic certain characteristics of living
organisms, smart materials, with their built-in sensors and actuators, would react
to their external environment by bringing on a desired response. This would be
done by linking the mechanical, electrical, and magnetic properties of these
materials. For example, piezoelectric materials generate an electrical current when
they are bent; conversely, when an electrical current is passed through these
materials, they stiffen. This property can be used to suppress vibration.
1.4 Piezoelectric Materials - A new era
Certain materials possess a property by which they experience a dimensional
change when an electrical voltage is applied to them. Such materials are known as
piezoelectric because of the converse effect; that is, they generate electricity when
pressure is applied. Perhaps the best-known such material is Lead-Zirconate-
Titanate (PZT); in fact "PZT" is commonly used to refer to piezoelectric materials in
general, including those of other compositicns.
When manufactured, a piezoelectric material has electric dipoles arranged in
random directions. The responses of these dipoles to an externally applied electric
field would tend to cancel one another, producing no gross change in dimensions of
the PZT specimen. In order to obtain a useful macroscopic response, the dipoles
are permanently aligned with one another through a process called poling.
A piezoelectric material has a characteristic Curie temperature. When it is heated
above this temperature, the dipoles can change their orientation in the solid phase
material. In poling, the material is heated above its Curie temperature and a strong
electric field is applied. The direction of this field is the polarization direction, and
the dipoles shift into alignment with it. The material is then cooled below its curie
temperature while the poling field is maintained, with the result that the alignment
of the dipoles is permanently fixed. The material is then said to be poled.
When the poled ceramic is maintained below its Curie temperature and is subjected
to a small electric field (compared to that used in poling), the dipoles respond
collectively to produce a macroscopic expansion along the poling axis and
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contraction perpendicular to it (or vice versa, depending on the sign of the applied
field).
The working temperature of the PZT is usually well below its Curie temperature. If
the material is heated above its Curie temperature when no electric field is applied,
the dipoles will revert to random orientations. Even at lower temperatures, the
application of too strong a field can cause the dipoles to shift out of the preferred
alignment established during poling.
The piezoelectric aspects are dealt with at length in chapter 4.
1.5 Facilities used at SAC
Access to following labs and software have been made during the course of this
work at SAC :
1.5.1 Software for FE Modeling of Smart Materials and Structural
Systems
• ATILA
ATILA (FEA software for the Analysis of Structures based on Active Materials). It is
a finite element software package specifically developed for the analysis of two or
three dimensional structures that contain piezoelectric, magnetostrictive,
electrostrictive or shape memory materials ; because its formulation is organized
around a strong electrical / mechanical coupling . It also has a formulation for
strong fluid / structure coupling.
In Nonlinearities domain, ATILA nonlinear solver takes into account at present
nonlinearities of constitutive material properties of materials such as electrostriction
and the shape memory only.
ATILA software has been used for the present investigation purpose, but because of
limitations in the domain of handling nonlinearities of piezo materials, hysteresis
and other nonlinearities have not been considered in the models.
• NISA
This is the software called - Numerically Integrated Elements for System Analysis
(NISA) for finite element modeling of Structures but without piezo element
modeling capabilities. This software has been used in the present investigation
work for modeling bare specimen made up of composites .
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1.5.2 Test Facilities Used in The Investigation
• CATF
Compact Antenna Test Range Facility (CATF) has been used for Electrical Testing of
the Reflectors (Fig 1.16) developed under the space simulated compact Test
Chamber at Space Applications Center Ahmedabad.
Fig 1.16 CATF at SAC Ahmedabad
• VTF
Vibration test Facility (Figl. 17) at present is using acceleration based approach to
pick up the responses of the payload under testing. For our investigation presently,
acceleration based approach has been used. The facility is getting upgraded to
force based approach which is considered to be more suitable and relevant for
larger payloads. The present acceleration based approach uses contact method of
mounting accelerometers on the payloads to pick up the 'g' levels at vulnerable
points of the payloads. For our investigation at present, contact based approach
has been used. Moreover, Vibration facility at SAC is also getting upgraded to using
non-contact method of picking up responses using Laser Doppler Vibrometry
approach, using Laser technique which will then cover the following full range of
mechanical vibrations:
Fig 1.17 8T Shaker at VTF/SAC
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For the proposed Forced based approach at SAC, following are the basic
requirements :
Please see Appendix A for the details on VBT technique.
1.6 The need of investigation
Thinking on the lines of requirement of developing futuristic ultra high precision,
high frequency (>30 GHz ) thin reconfigurable spacecraft reflector concepts for
INSAT / GEOSAT satellites, following are the observations from design point of
view :
Presently, in space segment domain, all the spacecraft components including
antenna feed chain components, mounting brackets for satellite reflectors, mast
mounted long wave guides / plumb lines, spacecraft reflectors etc, are all opted for
frequency based designs, basically, with a view to decouple the fundamental
system level frequency of the spacecraft w.r.t the sub-system frequencies
respectively.
Adequate stiffness ( frequency > 50 Hz) is provided for all subsystems including the
flimsy composite spacecraft reflectors to cater to launch conditions. All these sub
systems face severe during launch vibration loads (in plane 20 g and 30 g out of
plane ) approx., depending upon the launch vehicle. They are also subjected to
post launch thermal loads coupled with milli g vibrations [132] generated due to
movement of three axis gyros of the spacecraft, altitude correction exercises and
thermal load variations.
In order to handle the design challenges of the futuristic small size, high precision
,radio frequency Satellite Communication reconfigurable antennas of the space
segment for the Indian space research programme in particular, the need was felt
for investigating the applications of smart materials to meet the structural,
mechanical and electrical design specifications in a practical and feasible way;
purely from the realistic applications point of view.
Efforts, have been envisaged in understanding tomorrow's design challenges for
developing a high precision, thin, space qualified futuristic reflector for high Radio
Frequency signals (Q/V bands) [18] with following desirable design specifications :
Frequency,
Velocity,
Displacement,
Acceleration,
v = 50 q m/s to 30 m/s (9 decades)
6 = 2 b m /s to 10 m (13 decades)
A = 10 8 g to 107 g (15 decades)
f = 0 to 30 MHz
Vibrating OBERST beam Apparatus for Testing at elevated temperatures
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• Diameter of Spacecraft reconfigurable reflector, say less than 1.0m,
• first eigen frequency of ~ 50 Hz (pre-launch - stowed condition)
• Preferable total mass of < 3 Kg
• In-orbit stability RMS < 50 |im
• Pointing error < 0.01°
These futuristic mechanically active reflectors may also modify the radiation
pattern by actively changing the shape of the reflector when on-orbit thermal
distortions deform the reflector shape.
As per the above design specifications, t ie geometry of the high precision thin
reconfigurable shells can be something as shown in Fig 1.18 [18] :
circumferential rib
reflecting surface
Fig 1.18 Basic layout of the high precision thin shell
By keeping in view the requirement of high precision thin shell reconfigurable
reflectors for INSAT / GEOSAT satellites it has been envisaged to decide the
following domain and aim of investigation :
1.7 Broad Domain of investigation
Hence, keeping in view the above mentioned points , the domain of the
investigation is set as follows:
The major decision for the present investigation was taken to confine to the
applications of smart materials only to space segment domain in general and in
particular, spacecraft reflectors of small size i.e upto lm diameter only. Large
Deployable Reflectors (LDRs) or Inflatable reflectors have not been considered in
the present study.
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Moreover, from the initial studies done in this domain in section 1.3 above, out of the wide gamut of smart materials available nowadays (preferably indigenously available), the decision was taken to concentrate only on the piezoelectric materials, keeping in view the their possible availability within India. Another criteria in opting for piezoelectric materials was from the practical usage point of view for avoiding real life issues in spacecraft antenna domain like electromagnetic interference & electromagnetic coupling problems.Firstly, as per the domain of investigation, it was kept in view, the requirement of futuristic high radio frequency flexible light weight thin spacecraft reflectors which would have relatively low inherent out of plane stiffness. It was envisaged that futuristic flexible spacecraft reflectors would need to be designed to damp out low frequency vibrations arising due to the raoid temperature change of surrounding space environment or due to the attitude control of spacecraft structures or due to three axis movement of spacecraft gyros. These low frequency perpetual vibrations would need to be damped at reflector level itself, as these may cause single point failures in spacecrafts components due to sometimes dropping of electrical connectors in spacecrafts due to perpetual disturbances continuing in micro gravity conditions due to post launch milli g vibrations along with thermal loads.It is proposed to explore this aspect by investigating the damping behaviour of piezoceramic materials on CFRP & KFRP composites because as on today also the actual micro vibration levels on the satellite CFRP & KFRP reflectors due to above mentioned factors is important as it can attenuate the high frequency signals and is still an unexplored area which needs a revisit.Passive damping approach is envisaged to be the most preferred, practical and feasible way to damp the micro vibrations of present & futuristic spacecraft reflectors made up of thin flexible composite membranes. Active constrained layer damping approach has not been contemplated as suitable option, from the point of view of practical difficulties of deriving high voltage from satellite bus to energize the piezo layers on the entire surface of the reflector along with Viscoelastic layers ; as it can jeopardize others sub-systems of the spacecraft needing electrical energy generated from the solar panels.Secondly, as per the domain of investigation , It has been proposed to understand the issues involved in the futuristic concept of Reconfigurable antennae [59] capable to handle surface distortions due to post launch thermal loads. It is proposed to explore this aspect by carrying out a preliminary investigation of the shape deformation characteristics of the parabolic spacecraft reflector surfaces due to applications of smart materials like piezo electric patches on the light weight reflector skin[ 127].
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A typical illustration [4] showing the futuristic spacecraft antenna's electrical beam
steering within the Indian subcontinent using a reconfigurable concept is shown in
Fig 1.19 :
Fig 1.19 A typical illustration demonstrating the concept of active antenna
1.8 Aim of the Present Work
Many formal / informal interactions the author had with academicians and industry
to generate the knowledge base to realize the aim of the investigation as the title of
the investigation mentioned below is not the routine one but challenging which
required extensive homework to take up the exercise. As the smart materials have
in-built hysteresis, ageing and non-linearities issues, it was decided to use two
commercial FEA software to attempt the highly indeterminate real life practical
problems in a more realistic meaningful way.
The detailed literature review is given in chapter 2 . The feasibility report
mentioned in section 2.6 presents the extensive study conducted by the author that
brings out the areas which require further investigation from the point of view of
applications of smart materials in the domain spacecraft reflectors for INSAT /
GEOSAT satellites. Keeping this view, the aim of the proposed investigation
entitled, "An Experimental & Theoretical Investigation of Potential Futuristic
Applications of Piezoelectric Powder coatings & PZT patches in SATCOM Reflector
Domain" ,is epitomized in the form of two cases as follows :
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CASE 1 :
Investigation w.r.t the micro Vibration damping for the ultra high precision composite reflectors at ambient temperatures at VTF / SAC in the passive vibration domain.Innovative applications are proposed to be explored using thin hybrid piezoceramic powder (SP4 & SP5A) coatings in SATCOM (Satellite Communication) parabolic composite antenna reflector domain for the passive micro vibration damping at ambient temperature. The specimen in the form of parabolic composite reflectors have been proposed to be structurally analyzed, designed fabricated using machined moulds and tested in vibration damping domain at VTF / SAC Ahmedabad. Moreover, these piezo coated composite reflectors are also proposed to be eventually electrically tested for EMI / EMC interference, Gain & efficiency estimation at CATF /SAC.Aiming for the ultimata applications of piezo coatings in spacecraft composite reflectors for INSAT / GEO SAT/ satellites in space domain , the aim of the investigation is further extended to vibration testing of specimen at elevated temperature also. In the absence of varying temperature testing infrastructure for vibration damping at SAC, the cantilever beam specimen made up of space qualified composites developed and Piezo coated at SAC have been proposed to be outsourced for just estimation of Modal Loss Factors in vibration damping domain at varying temperatures as per ASTM standards .
CASE 2:
It is proposed to carry out , the preliminary investigation of shape deformation of parabolic shells using piezoelectric patches; keeping in view the requirement of futuristic Mechanically Active Antennae for INSAT / GEOSAT satellites .Innovative applications of piezoelectric patches (Unimorphs-SP5H & Bimorphs- SP5A) are proposed to be explored by conducting preliminary investigations for shape deformation of parabolic antenna reflectors. The aim is to make a beginning in the direction of developing futuristic reconfigurable antennae capable to handle thermal distortions of the spacecraft reflectors .
1.9 Scope of Proposed Investigation
Keeping above aim in view, the scope of the present investigation has been spelt out particularly from the test material selection point of view as per the feasibility report outcome described in Section 2.6 :
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I IS 0 ^
a) Keeping in view the points mentioned above , for quick implementation of
work, the scope of the present investigation for Case -1 has been focused w.r.t
the work on the materials currently in vogue for developing antenna reflectors
in SAC / ISRO.
It has been observed that presently, reflectors are being made up of composite
materials both for ground and space segment domains. To start with, keeping the
aim in view, investigation has been planned on composite parabolic reflectors
made up of high specific strength & high specific stiffness materials presently being
used viz., GFRP (Glass Fiber Reinforced Plastics), CFRP (Carbon Fiber Reinforced
Plastics) & KFRP (Kevlar Fiber Reinforced Plastics).
Therefore, in scope, the effect of thin hybrid piezoceramic material coatings on
composites mentioned above have been proposed to be studied, particularly w.r.t
their benefits in passive vibration damping domain. I t is proposed to cover the
vibration damping studies at ambient temperature (for parabolic reflectors made up
of CFRP & GFRP) from the practical utility point of view; in a purely passive
constrained layer damping approach.
Although as an option, the Magnetostrictive materials may be used for damping of
flexible structural systems, but practically speaking for microwave high frequency
antennas, they may have the limitations of usage due to typical EMI
(Electromagnetic Interference) / EMC (Electromagnetic Coupling) issues. By dint of
fact mentioned above & to make the proposed idea of using thin piezo coat on
composite reflectors practically viable, the need was also felt to electrically test the
piezoceramic powder coated reflectors. It has been proposed to include also, the
electrical testing of the piezo coated composite reflector for studying EMI / EMC
interference problems (electrical issues) at Compact Antenna Test Range Facility
at SAC.
b) Keeping in view the points mentioned above, the scope of the present work
for Case-2, has been decided w.r.t carrying out a preliminary shape
deformation investigation of parabolic reflectors due to different types and
layouts of piezo patches pasted on reflectors made up of flexible materials.
Therefore, in the scope ,it has been proposed to investigate the complex static
shape displacement issues of doubly curved realistic parabolic antenna reflectors by
using limited number of discrete piezoceramic actuators (Unimorphs & Bimorphs)
mounted on the convex side of the reflector. A few topological configurations of the
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layouts of the piezo patches are proposed to be tried to achieve required displacement of the reflector skin as per the electrical design calculations. Studies have been proposed on 0.45 m and 0.7 m dia. reflectors of different suitable flexible materials as the investigation is envisaged keeping in view the requirement of futuristic reconfigurable antennae for Indian spacecraft reflectors .
The present investigations on smart materials and their wide gamut of applications are linked with the ongoing Technical Development Programme (TDP- 2004-2008 ) activity on the development of Reconfigurable antenna for GEOSAT programme at SAC / ISRO Ahmedabad.
This entire research work of this study i.e, "An Experimental & Theoretical Investigation of Potential Futuristic Applications of Piezoelectric Powder coatings & PZT patches in SATCOM Reflector Domain", has been fully funded by Indian Space Research Organization.
# 2 M C *
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