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

l

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

9

• 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

F ie E at V e w Document Tooh W n d o w H e ir

Open Save a Copy ■ Emal Search J t SetectTesd -

* * d i r t , -

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

16

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

17

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 .

18

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

19

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

20

• 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.

21

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].

22

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 :

23

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 :

24

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

25

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 *

26