Feasibility Study to Actively Compensate Deformations of Composite Structure in a Space Environment

Copyright ⓒ The Korean Society for Aeronautical & Space Sciences Received: June 6, 2012 Accepted: June 29, 2012

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http://ijass.org pISSN: 2093-274x eISSN: 2093-2480

Technical Paper

Int’l J. of Aeronautical & Space Sci. 13(2), 221–228 (2012) DOI:10.5139/IJASS.2012.13.2.221

Feasibility Study to Actively Compensate Deformations of Composite Structure in a Space Environment

Ciro Farinelli*

Department of aerospace engineering, Politecnico di Milano, Italy

Hong-Il Kim**

Department of aerospace engineering, KAIST, South Korea

Jae-Hung Han***

Department of aerospace engineering, KAIST, South Korea

Abstract

An active compensation method for the deformation of composite structures using additional controllable metal parts is proposed, and its feasibility is experimentally investigated in a simulated space environment. Composite specimens are tested in a vacuum chamber, which is able to maintain pressure on the order of 10-3 torr and interior temperature in the range of ±30

˚C. The displacement-measuring interferometer system, which consists of a heterodyne HeNe laser and an interferometer, is used to measure the displacement of the whole structure. Meanwhile, the strain of the composite part and temperature of both parts are measured by fiber Bragg grating sensors and thermistors, respectively. The displacement of the composite structure is maintained within a tolerance of ±1 μm by controlling the elongation of the metal part, which is bonded to the end of the composite part. Also, the possibility of fiber Bragg grating sensors as control input sensors is successfully demonstrated using a proper corrective factor based on the specimen temperature gradient data.

Key words: composite structure, dimensional stability, coefficient of thermal expansion (CTE), fiber Bragg grating (FBG) sensors, displacement measuring interferometer (DMI), space environment.

1. Introduction

In the past years, the dimensional stability problem has been one of the big issues for optomechanical space structures. Thanks to various innovations in materials and precise analysis and control, the limits of the structure’s stability have gradually increased [1].

High-dimensional-stability devices show significant performance degradation due to misalignment phenomena.

In optical devices or high-accuracy measurement systems,

the relative position and alignment between two optical components like mirrors and sensors should be maintained.

There are three representative types of misalignment between two optical components: 1) the tilt when the relative orientation is different, 2) the decenter when the axis of one of the components presents an offset from those of the other elements, and 3) the despace when the distance between the two components changes. It is crucial to reduce these effects by a determined tolerance [2]. The most substantial destabilizing factors in the space optical devices are vibrations, ambient

This is an Open Access article distributed under the terms of the Creative Com- mons Attribution Non-Commercial License (http://creativecommons.org/licenses/by- nc/3.0/) which permits unrestricted non-commercial use, distribution, and reproduc- tion in any medium, provided the original work is properly cited.

*** Graduate student, E-mail : ciro.farinelli@mail.polimi.it

*** Researcher, Ph. D.

*** Professor, Corresponding author E-mail : jaehunghan@kaist.ac.kr

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Int’l J. of Aeronautical & Space Sci. 13(2), 221–228 (2012)

microgravity, ionizing radiation of the Earth’s natural radiation, and variations in temperature [3].

In this study, the effects of temperature variation on dimensional changes are considered, and an active compensation method for the deformation of composite structures using an additional controllable metal part is proposed. The feasibility of the method is experimentally investigated in a simulated space environment.

1.1 State of art

Hull [4] utilized semi-active focus and thermal compensation of a centrally obscured reflective telescope using a system of heating tapes and thermal sensors on a telescope. The principle is to maintain the temperature of the system within a range where the dimensional stability is defined. Piezoelectric (PZT) actuators were also used to compensate for composite beam deformations [5], and the range of displacement control was on the order of 1 mm.

Dano and Julliere [6] used a Macro-Fiber Composite (MFC)

actuator to compensate the thermal distortion in composite structures made of four layers: one layer made by PZT fibers and epoxy, one layer by epoxy, one layer by copper electrodes and epoxy and one layer by Kapton. In the study by Savitskii [7], material with a negative coefficient of thermal expansion (CTE) was associated to material with positive CTE in order to passively compensate the deformations caused by the temperature. Unfortunately, this solution is only valid only within a relative small temperature range because the CTE is not constant at different temperatures. Cordero et al.

[8] focused on characterizing dimensionally ultra-stable materials. The instrument used for this purpose is a high- precision dilatometer. Its accuracy is at best ±0.1 × 10-6 ℃⁻¹ for data obtained over 100 ℃. They used Peltier elements to change the temperature of the system and reached a dimensional stability tolerance of ±1 nm. Giesen and Folgering [9] developed optical delay lines and combined piezo effects with structures of different CTE materials.

Moreover their paper described principles to minimize thermal disturbance of optical performance.

1.2 Proposed solution

Though there have been various ideas proposed for precise dimensional controls, a space-realizable sensor system needs to be implemented. The proposed solution provides a dimensionally stable structure system that actively compensates for dimensional changes in a space environment. The system consists of a composite part, a metal part, FBG sensors, and a heater. When the dimensions of the composite are changed by temperature variation, FBG sensors define deformation of the whole system by the punctual strain. Then the heater provides thermal expansion of the metal part, which compensates for deformation of the composite. A displacement-measuring interferometer (DMI)

alon was was mea

wh bon meth to en diffe com mat both chos spec cond the veri the

Fig and bon axi

Fig p b

ng which its employed 23.6x10

^-6

K asured follow

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1

- The me ded togethe hod, using nable enoug ference of te mposite par

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end of the

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^-1

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� � 1

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_�

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Δ� �

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and ΔT=

tal part and er by the d epoxy glue gh stiffness emperature rts, and to much as pos epoxy glue a finite el es of finite redict the te

dimensiona ctural integ

metal part

and dimensio site part were d, using Epox

ement model. T od configurati

-0.81x10

^-6

al part, and i

alues define ethod of Ki

� �Δ� �

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� Δ� � Δ

=T

1

-T

2

. d the compo double lap l e. This choi to bear stre

and CTEs o thermally ssible. The e was 0.5 m lement ana

element a emperature al controllab

rity. In the was fixed.

ons of the spec e bonded by xy glue. X is

Temperature o ion (right).

K

^-1

. Al 707 its CTE valu ed in (1) we im et al. [12

�

Δ� (

site part we layer bondin

ice was ma ess due to th

of metal an y isolate t

thickness mm. This w alysis on th analyses we

distribution bility, and

FE analyse Fig. 2 show

cimen. The m double lap la the longitud

of the whole s 3 75 ue ere 2].

(1)

ere ng de he the nd was of he ere ns, es, to ws

th co te w of re th m 1. of

2.

di co er of la M fi la Tw th le th

w th w le pa is T Δ sh fo

metal ayer inal

system (left), d

he steady-st orrespondin emperature was imposed

f 21 C. W eached the t he FE anal maximum V .9 MPa, far f 36 MPa.

.2 DMI syst Interf isplacement ontrolling rror sources f a dual-m aser light at MHz, with h ber optic pi aser consists

wo beams t he interferom ength differ he phase dif

∆ where M is t

hrough the wavelength o

engths of t ath, respect The d s obtained b

he relation φ to the fr hift) measur ollows, by th

detail of Von M

tate temper ng stress d

on the heate d, with an e Within 1 h temperature lysis in Fi Von Mises s below the

tem ferometers ts without the enviro s [13]. The mode laser h

t a 633 nm highly stable ickups and s s of two bea travel along meter and t rence betwe fference Δφ

∆� � ���

the number e interferom

of the laser he referenc ively.

displacemen by measurin n that corre

frequency s red by the D

he time diff

Mises stress o

rature distri distribution er surface o environmen hour, the e of 31 C.

ig. 2 also stress of the

yield comp

are able contacting onmental an

DMI syste head that g wavelength e interferom supporting e ams orthogo g the refere the measure een the two as shown in

���

�

� �

�

�/

r of passes meter (M=

r A, and d

A

ce path an nt of the ta ng the phase elates the p

hift f (Dop DMI system ferentiation

of the epoxy g

ibution and for a 50 of the metal ntal tempera

composite The resul show that e epoxy glu pressive stre

e to mea the specim nd geomet em is comp generates H h at 3.4 and meters, reflec electronics.

onally polar nce path w ement path.

o paths ind n the follow /�

�

the laser m

=2), λ

A

is

A

and d

B

are d measurem arget (x=d

A

e difference phase differ ppler frequ m is expresse

of Δφ:

lue in the dou

d the 0 C l part ature part ts of t the ue is ength

asure mens trical osed HeNe d 4.0 ctors,

The rized.

within duces The wing:

(2)

makes e the the ment

A

−d

B

) e Δφ.

rence ency ed as

uble la

Fig. 1. Concept and dimensions of the specimen. The metal and the composite part were bonded by double lap layer bonding method, using Epoxy glue. X is the longitudinal axis.

alon was was mea

wh bon meth to en diffe com mat both chos spec cond the veri the

Figand bonaxi

Figp b

ng which its employed 23.6x10

^-6

K asured follow

���

here ΔL=L

1

- The me ded togethe hod, using nable enoug ference of te mposite par

erials as m h layers of

sen after a cimen.

A serie ducted to pr

degree of ify the struc

end of the

g. 1. Concept d the compos nding method is.

g. 1. Finite ele bonding metho

s CTE was as the meta K

^-1

. CTE va wing the m

� � 1

�

_�

Δ�

Δ� � -L

2

and ΔT=

tal part and er by the d epoxy glue gh stiffness emperature rts, and to much as pos

epoxy glue a finite el es of finite redict the te dimensiona ctural integ

metal part

and dimensio site part were d, using Epox

ement model. T od configurati

-0.81x10

^-6

al part, and i

alues define ethod of Ki

� �Δ� �

_�

� Δ� � Δ

=T

1

-T

2

. d the compo double lap l e. This choi to bear stre

and CTEs o thermally ssible. The e was 0.5 m lement ana

element a emperature al controllab

rity. In the was fixed.

ons of the spec e bonded by xy glue. X is

Temperature o ion (right).

K

^-1

. Al 707 its CTE valu ed in (1) we im et al. [12

�

Δ�

(

site part we layer bondin

ice was ma ess due to th

of metal an y isolate t

thickness mm. This w alysis on th analyses we

distribution bility, and

FE analyse Fig. 2 show

cimen. The m double lap la the longitud

of the whole s

3 75 ue ere 2].

(1)

ere ng de he the nd was of he ere ns, es, to ws

th co te w of re th m 1. of

2.

di co er of la M fi la Tw th le th

w th w le pa is T Δ sh fo

metal ayer inal

system (left), d

he steady-st orrespondin emperature was imposed

f 21 C. W eached the t he FE anal maximum V .9 MPa, far f 36 MPa.

.2 DMI syst Interf isplacement ontrolling rror sources f a dual-m aser light at MHz, with h ber optic pi aser consists

wo beams t he interferom ength differ he phase dif

∆ where M is t

hrough the wavelength o

engths of t ath, respect The d s obtained b

he relation φ to the fr hift) measur ollows, by th

detail of Von M

tate temper ng stress d on the heate d, with an e Within 1 h temperature lysis in Fi Von Mises s below the

tem ferometers ts without the enviro s [13]. The mode laser h

t a 633 nm highly stable ickups and s s of two bea travel along meter and t rence betwe fference Δφ

∆� � ���

the number e interferom

of the laser he referenc ively.

displacemen by measurin n that corre

frequency s red by the D he time diff

Mises stress o

rature distri distribution er surface o environmen hour, the e of 31 C.

ig. 2 also stress of the

yield comp

are able contacting onmental an

DMI syste head that g wavelength e interferom supporting e ams orthogo g the refere the measure een the two

as shown in

���

�

� �

�

�/

r of passes meter (M=

r A, and d

A

ce path an nt of the ta ng the phase elates the p

hift f (Dop DMI system ferentiation

of the epoxy g

ibution and for a 50 of the metal ntal tempera

composite The resul show that e epoxy glu pressive stre

e to mea the specim nd geomet em is comp generates H h at 3.4 and meters, reflec electronics.

onally polar nce path w ement path.

o paths ind n the follow /�

�

the laser m

=2), λ

A

is

A

and d

B

are d measurem arget (x=d

A

e difference phase differ

ppler frequ m is expresse

of Δφ:

lue in the dou

d the 0 C l part ature part ts of t the ue is ength

asure mens trical osed HeNe d 4.0 ctors,

The rized.

within duces The wing:

(2)

makes e the the ment

A

−d

B

) e Δφ.

rence ency ed as

uble la Fig. 2. Finite element model. Temperature of the whole system (left), detail of Von Mises stress of the epoxy glue in the double lap bonding meth-

od configuration (right).

7)(221-228)12-022.indd 222 2012-07-25 오후 3:50:23

223

Ciro Farinelli Feasibility Study to Actively Compensate Deformations of Composite Structure in a Space Environment

http://ijass.org system that can measure the entire displacement of the

specimen is used to validate the feasibility of the proposed solution.

2. Experimental

2.1 Description of the specimen

The specimen dimensions and the allowable tolerance in displacements were chosen by referring to the references concerning real space structures and space telescopes [7, 10, 11]. The dimensional stability tolerance was chosen to be ±3 μm for an ambient temperature variation of ±8℃. Note that the balance between the controllable thermal deformation and the mechanical stiffness and strength should also be considered.

Fig. 1 shows the dimensions and the shape of the whole specimen chosen in this study, respecting constraints of the vacuum chamber used in this study. Concerning the shape, it was decided to design an oblong specimen to focus the behaviour along one axis, and to facilitate the installation of the specimen on the support of the vacuum chamber. The z-direction (gravity direction) dimension was imposed on 30 mm to have enough stiffness along this direction to avoid eventual deformations caused by the gravity field.

The composite part used in this study was [0˚]16 Gr/Ep, unidirectional along the longitudinal axis, along which its CTE was −0.81 × 10^-6 K^-1. Al 7075 was employed as the metal part, and its CTE value was 23.6 × 10^-6 K^-1. CTE values defined in (1) were measured following the method of Kim et al.

[12].

(1)

where ΔL = L1 − L2 and ΔT = T1 − T2.

The metal part and the composite part were bonded together by the double lap layer bonding method, using epoxy glue. This choice was made to enable enough stiffness to bear stress due to the difference of temperature and CTEs of metal and composite parts, and to thermally isolate the materials as much as possible. The thickness of both layers of epoxy glue was 0.5 mm. This was chosen after a finite element analysis on the specimen.

A series of finite element analyses were conducted to predict the temperature distributions, the degree of dimensional controllability, and to verify the structural integrity. In the FE analyses, the end of the metal part was fixed. Fig. 2 shows the steady-state temperature

distribution and the corresponding stress distribution for a 50 ℃ temperature on the heater surface of the metal part was imposed, with an environmental temperature of 21 ℃.

Within 1 hour, the composite part reached the temperature of 31 ℃. The results of the FE analysis in Fig. 2 also show that the maximum Von Mises stress of the epoxy glue is 1.9 MPa, far below the yield compressive strength of 36 MPa.

2.2 DMI system

Interferometers are able to measure displacements without contacting the specimens controlling the environmental and geometrical error sources [13]. The DMI system is composed of a dual-mode laser head that generates HeNe laser light at a 633 nm wavelength at 3.4 and 4.0 MHz, with highly stable interferometers, reflectors, fiber optic pickups and supporting electronics. The laser consists of two beams orthogonally polarized. Two beams travel along the reference path within the interferometer and the measurement path.

The length difference between the two paths induces the phase difference Δφ as shown in the following:

(2)

where M is the number of passes the laser makes through the interferometer (M=2), λA is the wavelength of the laser A, and dA and dB are the lengths of the reference path and measurement path, respectively.

The displacement of the target (x=dA−dB) is obtained by measuring the phase difference Δφ. The relation that correlates the phase difference Δφ to the frequency shift f (Doppler frequency shift) measured by the DMI system is expressed as follows, by the time differentiation of Δφ:

(3)

where υ is the target velocity. In order to calculate the frequency shift f, the initial frequencies from the laser head and the changed frequencies from the interferometer are collected in the receiving electronics consisting of a phase meter, an integrator and so on. The target velocity υ is calculated using the frequency shift data f from the phase meter. Then, the displacement is obtained by summing υ.

2.3 FBG

The FBG sensor is a type of fiber optic sensor, its use in space structures is being actively studied. The working principle of the FBG is based on a periodic change of the refractive index in the optical fiber. Each FBG reflects a specific wavelength (Bragg wavelength λB). The optical fiber

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used for the FBG sensor has its own CTE αf and thermo- optic coefficient ξf so that the temperature effects should be compensated to measure the pure specimen strain, as in the following description based on the work of Lo and Chuang [14]. When the FBG sensor is attached to the specimen with a temperature change ΔT, the Bragg wavelength’s shift of the attached FBG sensor can be expressed as the sum of the mechanical strain resulting from specimen strain εs, and the temperature variation ΔT in the FBG sensor, as it is expressed as follows:

(4)

where the term pe is the photo-elastic constant of the optical fiber. p11 and p12 are the components of the strain- optic tensor, and ne is the effective refractive index of the Bragg grating.

The specimen strain εs consists of a mechanical strain εm

and a thermal strain expressed as follows:

(5)

where αs is the coefficient of thermal expansion of the specimen. Substituting the value of εs in (4), we obtain:

(6)

For the free FBG sensor (floating on the specimen), no specimen strain εs is transmitted to the FBG sensor. Therefore, the Bragg wavelength’s shift in the free FBG sensor can be expressed erasing the first terms:

(7)

The compensation for the temperature effect on the FBG sensor is achieved by subtracting the two relations above:

(8)

If there are no external loads, the mechanical strain εm

becomes zero. To obtain the value of the strain from the differentiation of the wavelength of the FBG sensors, it is mandatory to define the constant pe. The relation to have this constant is as expressed follows:

(9)

where

(10)

This depends exclusively on the temperature behaviour of the specimen because no mechanical stress is imposed.

The value of ΔL is defined by the DMI system, and L₀ is the initial length of the system. The reference frequencies of wavelengths come from the average of values obtained for 20 minutes at the sample frequency of 0.1 Hz in the thermal and pressure static conditions inside of the vacuum chamber. In this research, pe is 0.305, and the reference frequencies of the attached FBG (λrefG) and floating FBG (λrefF) are 1553.2080 Hz and 1547.1556 Hz, respectively.

2.4 Specimen setup

Concerning the heating tape, the resistance per length is 28 Ω/ft, and the maximum allowable power per length is 70 W/ft. It was attached on the metal part surface in one row, on both faces. The heating tape contained 4 resistive wires, embedded in 11-mm-width tape, heating the surface quasi- isotropically. Fig. 3 explains the connection configuration of the heating tape. The parts, where the tape deflects from up to down, were covered with isolating tape. The little square in the centre was cut out in order to facilitate attaching the thermistor.

FBG sensor arrays can be easily prepared by connecting several Bragg gratings written at different wavelengths in a line along the length of a single fiber and addressing each sensor individually using wavelength division multiplexing (WDM) technology. In this case, an array of two Bragg gratings was created. As Fig. 4 shows, the first Bragg grating (FBG_glued) was completely glued at the surface of the composite. The second Bragg grating (FBG_floating) was attached just in one point in order to be affected just by temperature variations. Then, the fibers were covered by a

wav obta of 0 cond rese of t (λ

ref

F resp 2.4 S

per pow met heat in 1 isotr conf the with was ther

by diffe a s indi mul arra show com The attac

Fig

velengths c ained for 20

0.1 Hz in ditions insi earch, p

e

is 0

the attache

f

F) are 15 pectively.

Specimen s Concern length is 28 wer per leng

al part surf ting tape co 11-mm-widt

ropically.

figuration o tape deflec h isolating t cut out in rmistor.

FBG se connecting ferent wavel single fibe ividually ltiplexing (W ay of two Br

ws, the firs mpletely glu e second B ched just in

g. 3. Heating t

come from 0 minutes a

the therm de of the v 0.305, and t ed FBG (λ

r

53.2080 H

etup

ning the hea 8 Ω/ft, and gth is 70 W/

face in one ontained 4 re

th tape, he Fig. 3 ex of the heatin cts from up tape. The li n order to

ensor arrays several Br lengths in a er and ad

using WDM) tech ragg grating st Bragg gr ued at the su Bragg gratin n one point

tape configura

the averag at the samp mal and pr

vacuum cha the referenc

ref

G) and f Hz and 15

ating tape, t the maximu ft. It was at

row, on bo esistive wir ating the s xplains the ng tape. The to down, w ittle square

facilitate

s can be ea ragg gratin a line along

ddressing wavelength hnology. In gs was creat rating (FBG urface of th ng (FBG_f t in order t

ation on metal

ge of valu ple frequen ressure stat amber. In th ce frequenci

floating FB 47.1556 H

the resistan um allowab ttached on t oth faces. T es, embedd urface quas e connectio e parts, whe

were cover in the cent attaching t

asily prepar gs written the length each sens h divisio

this case, ted. As Fig.

G_glued) w he composit floating) w

o be affect

l part.

5 ues cy

his tic BG ies Hz,

nce ble he he si- ed ere on red tre he

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ju w T al m fr S ca ac

±0 ra fi pa an de T he B sp us 2.

(A (A in m dy N m si pr of th ba fl fo re (o sp co T op

ust by temp were covere

he orientat long the l material.

The t rom the

emiconduct alibration ccuracies o

0.75 C ov ange. They

rst one at 2 art, the seco nd the third epicted in F 2 and T3, r eating tape Between the

pecimen, a sed.

.5 Experime The D Agilent 551 Agilent 10 nterferomete mirror, a r ynamic rece N1231B). T maintained

imulating t ressure (on f temperatu he specimen ase was sup oat with mi

Fused or the spe eference mi on the ord pecimen wa ompensate

he heating ptical feed-t

perature va ed by a Mu

tion of the longitudinal thermistors LM35 se tor. It does

or trimm f ±0.25 C ver a full - were attach 20 mm from

ond one at 3 d one in the Fig. 5. These

respectively e with a e thermistor

layer of the

ent setup DMI system

7D), two pl 0706B hig er), a specim

receiver (A eiver) and a The entire

inside o the space

the order o ure (±10 C

n was laid pported by a inimal fricti d silica wa ecimen ba irror becau der of 10

⁻⁷

as attached for the sp g tape was

through to t

ariations. T ulti-Layer

attached F l axis of

are TO-92 eries by s not requir

ing to p C at room t -55 ~ 150 hed on spec m the end o 30 mm from

center of th e thermistor y. T3 was is

layer of rs and the ermal condu

m consists lane mirror gh-stability

men base w Agilent E a PCI laser experimen of a vac

environmen of 10

^-3

torr) C in about

on the spec a roller to l ion.

as selected se, specim use of its v

7

C

⁻¹

). O to the refe pecimen ba s connecte the power su

Then, the fi Isolator (M FBG should the comp Plastic Pack

the Nati re any exte provide typ emperature

C tempera ified points f the comp m the metal he metal par

rs are called solated from isolating t surface of uctive foam

of a laser h interferom

plane m with a refer 1708A rem r board (Ag ntal setup

uum cham nt in term ) and variat 2 hours). S cimen base

et the speci as the mat men roller

very small ne end of erence mirro

se’s expans ed through

upply regul fibers MLI). d be osite kage ional ernal pical ature and s: the osite part rt, as d T1, m the tape. f the m was

head eters mirror rence mote gilent mber was ms of tions Since , the imen terial CTE and f the or to sion. lator. the

Fig. 3. Heating tape configuration on metal part.

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225

Ciro Farinelli Feasibility Study to Actively Compensate Deformations of Composite Structure in a Space Environment

http://ijass.org Multi-Layer Isolator (MLI). The orientation of the attached

FBG should be along the longitudinal axis of the composite material.

The thermistors are TO-92 Plastic Package from the LM35 series by the National Semiconductor. It does not require any external calibration or trimming to provide typical accuracies of ±0.25 ℃ at room temperature and ±0.75 ℃ over a full -55

~ 150 ℃ temperature range. They were attached on specified points: the first one at 20 mm from the end of the composite part, the second one at 30 mm from the metal part and the third one in the center of the metal part, as depicted in Fig. 5.

These thermistors are called T1, T2 and T3, respectively. T3 was isolated from the heating tape with a layer of isolating tape. Between the thermistors and the surface of the specimen, a layer of thermal conductive foam was used.

2.5 Experiment setup

The DMI system consists of a laser head (Agilent 5517D), two plane mirror interferometers (Agilent 10706B high- stability plane mirror interferometer), a specimen base with a reference mirror, a receiver (Agilent E1708A remote dynamic receiver) and a PCI laser board (Agilent N1231B).

The entire experimental setup was maintained inside of a vacuum chamber simulating the space environment in terms of pressure (on the order of 10-3 torr) and variations of temperature (±10 ℃ in about 2 hours). Since the specimen was laid on the specimen base, the base was supported by a roller to let the specimen float with minimal friction.

Fused silica was selected as the material for the specimen base, specimen roller and reference mirror because of its very small CTE (on the order of 10−7 ℃⁻¹). One end of the specimen was attached to the reference mirror to compensate for the specimen base’s expansion. The heating tape was connected through the optical feed-through to the power supply regulator.

The temperature of the metal part was detected with thermistor T3, and the temperature of the composite part was detected by an average of T1 and T2. All data from the DMI system, FBG sensors and thermistors were collected using a LabVIEW computer (NI PXI 8186). Fig. 6 shows the overall experimental setup scheme. The interferometers and the specimen base were placed inside the vacuum chamber. The laser head and receiver were located outside the chamber.

The quartz window was installed on the vacuum chamber

dete of th of T sens Lab the inter insid rece quar cham tunn spec tran

3. R

dim perf mod

Fig. one

Fig. betw from

The tem ected with t

he composi T1 and T2. A

sors and th bVIEW com overall e rferometers de the vacu eiver were

rtz window mber to let nel, which cimen base nsfer heat th Results

In ord mensional

formed in a de. The me

4. Front side point, the FB

5. Opposite s ween the senso m the metal pa

mperature thermistor T ite part was All data from hermistors mputer (NI P

experimenta s and the spe

uum chamb located ou w was ins the laser lig h surround

e, was coa rough radia

der to fa controls, always-posit etal part wa

of the specim G_glued is co

side of the spe ors and the sp art, T3 at 25 m

of the me T3, and the s detected b m the DMI were colle PXI 8186).

al setup s ecimen base ber. The la utside the c

stalled on ght pass thr ed the sp ated in bla ation.

acilitate th the expe tive heat po as heated by

men. FBG attac ompletely glue

ecimen. Therm ecimen. The t mm from the ri

etal part w e temperatu by an avera

system, FB ected using

Fig. 6 show scheme. T

e were plac aser head an

chamber. T the vacuu rough. A he pecimen an ack to easi

he lab-sca eriment w ower delive y the heatin

chment config ed to surface.

mistor attachm thermistors att

ight side.

6 was ure

BG ge ws a he ed nd he um eat ily nd

was ale ery ng

ta pe te th en hi en sy th te va 30 ch th 47 m te di he to 0~ th co

guration. The Positive and n

ment configura tachment poin

ape; and th erformed emperature han the amb

nough to ke igher than t ntire experi ystem trans he cooling emperature

acuum cha 0~46 C. T hamber and he initial tem

7 C. As sh metal part w

emperature isplacement eaters durin o control th

~6 V. The t he control ontrol input

FBG_floating negative conn

ation. A layer o nts are: T1 at 2

he cooling c by radi of the meta bient tempe eep the tem the compos iment, allow sferring out

phase. Fi time cour amber temp The initial te

d composite mperature o hown in Fig was set to

to increase t by adjus ng the contr he displacem

temperature input of t was the d

g is attached to nections of hea

of thermo-con 20 mm from th

control of iation. T al part was eratures. Th mperature of site tempera wing the he

tward by r ig. 7 show se of the perature ran

emperature e part was s

of the metal g. 7, the tem

9 C highe e the contro

sting the v rol. The vol ment of the e of the com the heating displacemen

o the composi ating tape.

nductive foam he left side, T

both parts Therefore,

set to be hi he gap shal f the metal ature during eat power o radiation du

ws the typ specimen.

nge was se of the vac et at 38 C, l part was s mperature o er than amb

ollability of voltage of ltage range

metal part mposite part

g system.

nt measured

ite part just in

m was created T2 at 30 mm

was the igher ll be part g the f the uring pical et to The cuum , and set at f the bient f the f the used t was was d by The

Fig. 4. Front side of the specimen. FBG attachment configuration. The FBG_floating is attached to the composite part just in one point, the FBG_

glued is completely glued to surface. Positive and negative connections of heating tape.

dete of th of T sens Lab the inter insid rece quar cham tunn spec tran

3. R

dim perf mod

Fig. one

Fig. betw from

The tem ected with t

he composi T1 and T2. A

sors and th bVIEW com overall e rferometers de the vacu eiver were

rtz window mber to let nel, which cimen base nsfer heat th Results

In ord mensional

formed in a de. The me

4. Front side point, the FB

5. Opposite s ween the senso m the metal pa

mperature thermistor T ite part was All data from hermistors mputer (NI P

experimenta s and the spe

uum chamb located ou w was ins the laser lig h surround

e, was coa rough radia

der to fa controls, always-posit etal part wa

of the specim G_glued is co

side of the spe ors and the sp art, T3 at 25 m

of the me T3, and the s detected b m the DMI were colle PXI 8186).

al setup s ecimen base ber. The la utside the c

stalled on ght pass thr ed the sp ated in bla ation.

acilitate th the expe tive heat po as heated by

men. FBG attac ompletely glue

ecimen. Therm ecimen. The t mm from the ri

etal part w e temperatu by an avera

system, FB ected using

Fig. 6 show scheme. T

e were plac aser head an

chamber. T the vacuu rough. A he pecimen an ack to easi

he lab-sca eriment w ower delive y the heatin

chment config ed to surface.

mistor attachm thermistors att

ight side.

6 was ure

BG ge ws a he ed nd he um eat ily nd

was ale ery ng

ta pe te th en hi en sy th te va 30 ch th 47 m te di he to 0~ th co

guration. The Positive and n

ment configura tachment poin

ape; and th erformed emperature han the amb

nough to ke igher than t ntire experi ystem trans he cooling emperature

acuum cha 0~46 C. T hamber and he initial tem

7 C. As sh metal part w

emperature isplacement eaters durin o control th

~6 V. The t he control ontrol input

FBG_floating negative conn

ation. A layer o nts are: T1 at 2

he cooling c by radi of the meta bient tempe eep the tem the compos iment, allow sferring out

phase. Fi time cour amber temp The initial te

d composite mperature o hown in Fig was set to

to increase t by adjus ng the contr he displacem

temperature input of t was the d

g is attached to nections of hea

of thermo-con 20 mm from th

control of iation. T al part was eratures. Th mperature of site tempera wing the he

tward by r ig. 7 show se of the perature ran

emperature e part was s

of the metal g. 7, the tem

9 C highe e the contro

sting the v rol. The vol ment of the e of the com the heating displacemen

o the composi ating tape.

nductive foam he left side, T

both parts Therefore,

set to be hi he gap shal f the metal ature during eat power o radiation du

ws the typ specimen.

nge was se of the vac et at 38 C, l part was s mperature o er than amb

ollability of voltage of ltage range

metal part mposite part

g system.

nt measured

ite part just in

m was created T2 at 30 mm

was the igher ll be part g the f the uring pical et to The cuum , and set at f the bient f the f the used t was was d by The

Fig. 5. Opposite side of the specimen. Thermistor attachment configuration. A layer of thermo-conductive foam was created between the sensors and the specimen. The thermistors attachment points are: T1 at 20 mm from the left side, T2 at 30 mm from the metal part, T3 at 25 mm from the right side.

7)(221-228)12-022.indd 225 2012-07-25 오후 3:50:24

DOI:10.5139/IJASS.2012.13.2.221

226

to let the laser light pass through. A heat tunnel, which surrounded the specimen and specimen base, was coated in black to easily transfer heat through radiation.

3. Results

In order to facilitate the lab-scale dimensional controls, the experiment was performed in always-positive heat power delivery mode. The metal part was heated by the heating tape; and the cooling control of both parts was performed by radiation. Therefore, the temperature of the metal part was set to be higher than the ambient temperatures. The gap shall be enough to keep the temperature of the metal part higher than the composite temperature during the entire experiment, allowing the heat power of the system transferring outward by radiation during the cooling phase. Fig. 7 shows the typical temperature time course of the specimen. The vacuum chamber temperature range was set to 30~46 ℃. The initial temperature of the vacuum chamber and composite part was set at 38 ℃, and the initial temperature of the metal part was set at 47 ℃. As shown in Fig. 7, the temperature of the metal part was set to 9 ℃ higher than ambient temperature to increase the controllability of the displacement by adjusting the voltage of the heaters during the control. The voltage range used to control the

displacement of the metal part was 0~6 V. The temperature of the composite part was the control input of the heating system. The control input was the displacement measured by the DMI system shown in Fig. 6, defined as the variation of the initial elongation of the whole specimen and the elongation during the experiment. The control output was the voltage, manipulated by the power supply in steps of 0.05 V. Control time sampling was at 1 sample per 1 minute.

The minimum variation of voltage between two subsequent values was 0.1 V.

Fig. 8 clearly shows that the deformation measured by the DMI system was maintained within the tolerance of ±1 μm, which satisfies the goal of dimensional stability tolerance of

±3 μm.

Because the bonding was not thermally isolated between both parts, a temperature gradient existed along the longitudinal direction of the specimen as shown on the left side of Fig. 2. Thus, the direct integration of strain measured by FBG was not the whole displacement of the specimen which could be measured by the DMI system. Therefore, a corrective factor with respect to the temperature difference between the composite part T_comp and the initial temperature Tref was introduced to calibrate the specimen displacement with the strains by FBG sensors. The corrective factor is approximated as in (11).

the vari spec expe man V. C min betw

mea with goal isol grad the Thu FBG spec syst to t com T

ref

disp

Figspe delwe

DMI system iation of th cimen and

eriment. Th nipulated by Control tim nute. The

ween two su Fig. 8 c asured by hin the toler

l of dimensi Becaus lated betw dient existed

specimen a us, the direc G was not cimen whic tem. Theref the tempe mposite part was intro placement w

g. 6. Experim ecimen were liver for heati ere connected

m shown in he initial e

d the el he control y the power me sampling minimum ubsequent v clearly show

the DMI s rance of ±1 ional stabili e the bond ween both d along the as shown on ct integratio t the whol ch could be

fore, a corre erature dif t T

comp

and

duced to c with the stra

ment setup sc located in the ing tape were using a feed-t

n Fig. 6, d elongation o

longation output was r supply in s g was at 1

variation values was 0 ws that the system wa 1 μm, which

ity tolerance ding was n

parts, a t longitudina n the left si on of strain e displace e measured ective factor fference b d the initial calibrate th ains by FBG

cheme with D e vacuum cham e outside. Las

through.

defined as t of the who

during t s the voltag

steps of 0.0 sample per

of volta 0.1 V.

e deformatio s maintain h satisfies t e of ±3 μm not thermal temperatu al direction ide of Fig.

measured b ement of th d by the DM r with respe between th

l temperatu he specime G sensors. T

DMI system a mber while th er light passe

7 ole he ge, he r 1 05 ge on ed m. he

ure lly of 2.

by he MI ect ure he en he

co

w co

by th co w to sm on fa or th

and FBG sen he laser heat a ed through the

orrective f ܥ

_௙

ൌ ܣ where A=-1

onstants of t This y the FBG he correctiv

ontrolled di with the co olerance of mall displac nly FBG s actor should

rder to use he control in

nsors interrog and electronic e quartz windo

factor is a ܣ ȉ ൫ܶ

_௖௢௠௣

െ .3x10

^-5

K

^-1

the approxi function sh sensor. Fig ve factor.

isplacement orrective fa

±1 μm. Wi cement was sensors. Th d be applie the strain nput.

gation system cs for the DM ows and the F

approximat

െ ܶ

_௥௘௙

൯ ൅ ܤ

1

and B=-2 imating func

all be appli . 9 shows th

Fig. 10 s t based on th actor remain ithout corre s hardly mea hus, the pr

ed to the F data from F

m. The interfe I and FBG se FBG sensors

ted as in ( ܤ

2.5x10

^-6

are ction.

ed to the str he behaviou shows that

he FBG sen ned within ctive factor asured by u roper correc FBG sensor FBG sensor

erometer and ensors and pow

and heating t

(11).

(11) e the

rains ur of nsors the n the r, the using ctive rs in rs as

wer the tape Fig. 6. Experiment setup scheme with DMI system and FBG sensors interrogation system. The interferometer and the specimen were located in

the vacuum chamber while the laser heat and electronics for the DMI and FBG sensors and power deliver for heating tape were outside.

Laser light passed through the quartz windows and the FBG sensors and heating tape were connected using a feed-through.

7)(221-228)12-022.indd 226 2012-07-25 오후 3:50:25

227

Ciro Farinelli Feasibility Study to Actively Compensate Deformations of Composite Structure in a Space Environment

http://ijass.org (11)

where A=−1.3 × 10⁻⁵ K⁻¹ and B=−2.5 × 10⁻⁶ are the constants of the approximating function.

This function shall be applied to the strains by the FBG sensor. Fig. 9 shows the behaviour of the corrective factor.

Fig. 10 shows that the controlled displacement based on the FBG sensors with the corrective factor remained within the tolerance of ±1 μm. Without corrective factor, the small displacement was hardly measured by using only FBG sensors. Thus, the proper corrective factor should be applied to the FBG sensors in order to use the strain data from FBG sensors as the control input.

4. Conclusions

An active dimension compensation concept for a composite structure using an additional controllable metal part was demonstrated.

From the FEM analysis, proper specimen configuration was designed. The dimensional changes of the specimen were engaged in a vacuum chamber in order to simulate space conditions, and these dimensional changes were

compensated by proper control of the temperature of the metal part. From the test, it has been shown that dimensional stability of ±1 μm was successfully achieved. In addition, the corrective factor calculated from the temperature data showed the possibility of using the FBGs as a monitoring sensor for dimensional stability.

References

[1] Wolff E. G., Introduction to dimensional stability of composite materials, Destech Publications, Lancaster, PA, 2004.

[2] URL: http://www.telescope-optics.net [cited 17 October 2011]

[3] Danilov V. A., Lysenko A. I., Malamed E. R. and Sokol’skii M. N., “Service systems of space telescopes”, Opticheskii Zhurnal, Vol. 69, 2002, pp. 36-44.

[4] David A. H., “Semi-active focus and thermal compensation of centrally obscured reflective telescope”, United States Patent, US 6404547, 2002.

[5] Song G., Zhou X. and Binienda W., “Thermal deformation compensation of a composite beam using

8

Fig. 7. Specimen temperature

Fig. 8. Controlled displacement based on the DMI data.

4. Conclusions

An active dimension compensation concept for a composite structure using an additional controllable metal part was demonstrated.

From the FEM analysis, proper specimen configuration was designed. The dimensional changes of the specimen were engaged in a vacuum chamber in order to simulate space conditions, and these dimensional changes were compensated by proper control of the temperature of the metal part. From the test, it has been shown that dimensional stability of ±1 μm was successfully achieved. In addition, the corrective factor calculated from the temperature data showed the possibility of using the FBGs as a monitoring sensor for dimensional stability.

Fig. 9. Corrective factor

Fig. 10. Controlled displacement based on the FBG data with corrective factor.

References

[1] Wolff E. G., Introduction to dimensional stability of composite materials, Destech Publications, Lancaster, PA, 2004.

[2] URL: http://www.telescope-optics.net [cited 17 October 2011]

[3] Danilov V. A., Lysenko A. I., Malamed E. R.

and Sokol’skii M. N., “Service systems of space telescopes”, Opticheskii Zhurnal, Vol. 69, 2002, pp. 36-44.

[4] David A. H., “Semi-active focus and thermal compensation of centrally obscured reflective telescope”, United States Patent, US 6404547, 2002.

[5] Song G., Zhou X. and Binienda W., “Thermal deformation compensation of a composite beam using piezoelectric actuators”, Smart materials and structures, Vol. 13, No. 1, 2004, pp. 30-37.

doi:10.1088/0964-1726/13/1/004

Fig. 7. Specimen temperature

8

Fig. 7. Specimen temperature

Fig. 8. Controlled displacement based on the DMI data.

4. Conclusions

An active dimension compensation concept for a composite structure using an additional controllable metal part was demonstrated.

From the FEM analysis, proper specimen configuration was designed. The dimensional changes of the specimen were engaged in a vacuum chamber in order to simulate space conditions, and these dimensional changes were compensated by proper control of the temperature of the metal part. From the test, it has been shown that dimensional stability of ±1 μm was successfully achieved. In addition, the corrective factor calculated from the temperature data showed the possibility of using the FBGs as a monitoring sensor for dimensional stability.

Fig. 9. Corrective factor

Fig. 10. Controlled displacement based on the FBG data with corrective factor.

References

[1] Wolff E. G., Introduction to dimensional stability of composite materials, Destech Publications, Lancaster, PA, 2004.

[2] URL: http://www.telescope-optics.net [cited 17 October 2011]

[3] Danilov V. A., Lysenko A. I., Malamed E. R.

and Sokol’skii M. N., “Service systems of space telescopes”, Opticheskii Zhurnal, Vol. 69, 2002, pp. 36-44.

[4] David A. H., “Semi-active focus and thermal compensation of centrally obscured reflective telescope”, United States Patent, US 6404547, 2002.

[5] Song G., Zhou X. and Binienda W., “Thermal deformation compensation of a composite beam using piezoelectric actuators”, Smart materials and structures, Vol. 13, No. 1, 2004, pp. 30-37.

doi:10.1088/0964-1726/13/1/004

Fig. 9. Corrective factor

8

Fig. 7. Specimen temperature

Fig. 8. Controlled displacement based on the DMI data.

4. Conclusions

An active dimension compensation concept for a composite structure using an additional controllable metal part was demonstrated.

From the FEM analysis, proper specimen configuration was designed. The dimensional changes of the specimen were engaged in a vacuum chamber in order to simulate space conditions, and these dimensional changes were compensated by proper control of the temperature of the metal part. From the test, it has been shown that dimensional stability of ±1 μm was successfully achieved. In addition, the corrective factor calculated from the temperature data showed the possibility of using the FBGs as a monitoring sensor for dimensional stability.

Fig. 9. Corrective factor

Fig. 10. Controlled displacement based on the FBG data with corrective factor.

References

[1] Wolff E. G., Introduction to dimensional stability of composite materials, Destech Publications, Lancaster, PA, 2004.

[2] URL: http://www.telescope-optics.net [cited 17 October 2011]

[3] Danilov V. A., Lysenko A. I., Malamed E. R.

and Sokol’skii M. N., “Service systems of space telescopes”, Opticheskii Zhurnal, Vol. 69, 2002, pp. 36-44.

[4] David A. H., “Semi-active focus and thermal compensation of centrally obscured reflective telescope”, United States Patent, US 6404547, 2002.

[5] Song G., Zhou X. and Binienda W., “Thermal deformation compensation of a composite beam using piezoelectric actuators”, Smart materials and structures, Vol. 13, No. 1, 2004, pp. 30-37.

doi:10.1088/0964-1726/13/1/004

Fig. 8. Controlled displacement based on the DMI data.

8

Fig. 7. Specimen temperature

Fig. 8. Controlled displacement based on the DMI data.

4. Conclusions

An active dimension compensation concept for a composite structure using an additional controllable metal part was demonstrated.

From the FEM analysis, proper specimen configuration was designed. The dimensional changes of the specimen were engaged in a vacuum chamber in order to simulate space conditions, and these dimensional changes were compensated by proper control of the temperature of the metal part. From the test, it has been shown that dimensional stability of ±1 μm was successfully achieved. In addition, the corrective factor calculated from the temperature data showed the possibility of using the FBGs as a monitoring sensor for dimensional stability.

Fig. 9. Corrective factor

Fig. 10. Controlled displacement based on the FBG data with corrective factor.

References

[1] Wolff E. G., Introduction to dimensional stability of composite materials, Destech Publications, Lancaster, PA, 2004.

[2] URL: http://www.telescope-optics.net [cited 17 October 2011]

[3] Danilov V. A., Lysenko A. I., Malamed E. R.

and Sokol’skii M. N., “Service systems of space telescopes”, Opticheskii Zhurnal, Vol. 69, 2002, pp. 36-44.

[4] David A. H., “Semi-active focus and thermal compensation of centrally obscured reflective telescope”, United States Patent, US 6404547, 2002.

[5] Song G., Zhou X. and Binienda W., “Thermal deformation compensation of a composite beam using piezoelectric actuators”, Smart materials and structures, Vol. 13, No. 1, 2004, pp. 30-37.

doi:10.1088/0964-1726/13/1/004

Fig. 10. Controlled displacement based on the FBG data with correc- tive factor.

7)(221-228)12-022.indd 227 2012-07-25 오후 3:50:25

DOI:10.5139/IJASS.2012.13.2.221

228

piezoelectric actuators”, Smart materials and structures, Vol.

13, No. 1, 2004, pp. 30-37.

doi:10.1088/0964-1726/13/1/004

[6] Dano M. L. and Julliere B., “Active control of thermally induced distortion in composite structures using Macro fiber composite actuators”, Smart materials and structures, Vol. 16, No. 6, 2007, pp. 2315-2322.

doi:10.1088/0964-1726/16/6/035

[7] Savitskii A. M., “How the thermal regime affects the structural characteristics of a space telescope”, Journal of Optical Technology, Vol. 76, No. 10, 2009, pp. 662-665.

http://dx.doi.org/10.1364/JOT.76.000662

[8] Cordero J., Heinrich T., Shuldt T., Gohlke, M, Lucarelli S., Weise D., Johann U. and Braxmaier C., “Interferometry based high-precision dilatometry for dimensional characterization of high stable materials”, Measurement Science and Technology, Vol. 20, No. 9, 2009, pp. 1-10.

doi:10.1088/0957-0233/20/9/095301

[9] Giesen, P. and Folgering, E., “Design guidelines for thermal stability in optomechanical instruments”, Proceedings of SPIE Optomechanics, Vol. 5176, 2003, pp. 126-134.

[10] Yoshinori S., Kiyoshi I., Yukio K., Saku T., and Toshifumi S., “Instrument design and on-orbit performance of the solar optical telescope aboard hinode (Solar-B)”, Bulletin of the American Astronomical Society, Vol. 39, 2008, pp. 197-220.

[11] Krim M. H., “Design of highly stable optical support structure”, Optical engineering, Vol. 14, No. 6, 1975, pp. 552- 558.

[12] Kim H.-I., Yoon J.-S., Kim H.-B. and Han J.-H.,

“Measurement of the thermal expansion of space structure using fiber Bragg grating sensors and displacement measuring interferometers”, Smart materials and structures, Vol. 21, No. 8, 2010, pp. 1-8.

doi:10.1088/0957-0233/21/8/085704

[13] Laser and Optics User’s Manual: Standard Optics and Assemblies, Agilent Technology, Santa Clara, CA, 2002

[14] Lo Y.-L. and Chuang H. S., “Measurement of thermal expansion coefficients using an in-fibre Bragg-grating sensor”, Measurement Science and technology, Vol. 9, No. 9, 1998, pp. 1543-1547.

doi:10.1088/0957-0233/9/9/025

7)(221-228)12-022.indd 228 2012-07-25 오후 3:50:25