dimensional stability for satellite optical...

위성용 광학 탑재체의 치수안정성 연구

Dimensional stability for satellite optical payload

Smart Systems and Structures Lab Dept. of Aerospace Eng.

Contents Table

ｏ Introduction

ｏ Thermal expansion measurement of composite materials

ｏ Hygroscopic deformation measurement of composite materials

ｏ Optical payload performance simulation

ｏ Conclusion

ｏ Other space related research at SSS lab

Smart Systems and Structures Lab - KAIST 2

INTRODUCTION

• Importance of dimensional stability

• Characteristic of CFRP

• Characteristic of FBG

• Optical performance payload simulation

Introduction

ｏ Dimensional instabilities(static deformation, jitter) in space Static deformation(de-space, tilt, de-center) – radiation, vacuum out-gassing

Dynamic jitter(LOS jitter) – RWA, cooler, flexible body

ｏ Dimensional instabilities causes optical performance degradation[1]

Static deformation – radiation, vacuum –

Dynamic jitter – RWA, Cooler –

Static deformation – tilt, de-center, de-space –

Image quality degradation[1]

Dynamics jitter – LOS jitter –

Introduction

ｏ Materials for telescope structures

• Lightweight and high dimensional stability

High stiffness/density ratio is required

Low thermal expansion is required

• Characteristic of CFRP

High stiffness/density ratio (3times higher than titanium)

Low thermal expansion characteristic can be achieved via “Tailoring”

Hygroscopic deformation characteristic need to be known in advance

Carbon fiber(CME ~ 0, CTE ~ -0.9)

Matrix(cynate/polymer, etc)

Unidirectional carbon fiber Prepreg

Laminate is designed to satisfy desired properties. Low CTE/CME property is useful in space environment for optical structure

Rule of mixture, FEM

α, β, E, vf is known

Desired properties

Carbon Fiber Reinforced Plastics(CFRP) can satisfy these requirements

ｏ Examples of dimensional stability evaluation for CFRP (cont.)

• Precise measurement to evaluate near zero hygro-thermal expansion of CFRP

Introduction

US’s hygroscopic deformation measurement[2] - Simulated space env. - Process for 6 days - Precise weight measurement

Japan’s CTE measurement[3] - Simulated space env. - Precise length measurement - Precise temperature control - 1ppb/℃ resolution

ｏ FBG (Fiber Bragg Grating) sensor • Small, lightweight, high sensitivity, electro-magnetic immunity,

• No hygro-effects and easily installable onto/into host structures.

• Capable of Multiplexing and Temperature measurements

• Real time acquisition

ｏ Working principle

• Bragg wavelength

Grating have sinusoidal refractive index

Reflected wavelength is defined as

• Change of the reflected wavelength is measured

and calculated into physical properties

Introduction

Optical fiber

Bragg grating

Input spectrum

Reflected signal Transmitted signal

L 10mmIndex of

refraction of

fiber core

Δn = 10-5 to 10-3

: effective refractive index

: grating period

Optical fiber

Introduction

ｏ Optical performance simulation for aerospace optical payload[4,5]

• Structure-optical performance tool(SigFit, since late 1980‟s)

Combine CodeV + NASTRAN

Fully numerical, detailed structure and optical surfaces are needed

Image simulation is not supported

• End-to-end image simulation(PICASSO)

MTF/Image performance simulation

Do not consider structural aspect, consider illumination, angle condition etc

Performance calculation from analytic solution

SigFit model PICASSO result

THERMAL EXPANSION MEASUREMENT

• Measurement system construction

• Thermal expansion measurement result

Thermal Expansion Measurement

ｏ Measurement system requirements

• Definition of Coefficient of Thermal Expansion(CTE)

Average CTE

Instantaneous CTE

• Measurement system requirement

Precise length change measurement

Precise temperature change and its measurement

2 1 1*

( ) / 1m

L L L L

T T L T

Average/Instantaneous CTE method

Requirements Detail

Length measurement Measuring ~ 0ppm/℃ Not disturbed by temperature change

Minimize mechanical disturbance

Temperature control Simulate space environment Uniform around specimen

ｏ Optical interferometer and thermal vacuum chamber construction

Requirements Detail Solution

Length measurement - Interferometer

Measuring < 0ppm/℃ Not disturbed by Temperature change

Minimize mechanical disturbance

Resolution < 5nm Quartz base minimize thermal distortion

Temperature control - Thermal vacuum chamber

Simulate space environment Uniform around specimen

Heater and Cooler(3~114℃) Rotary pump and diffuser pump(~ ) 510 torr

Thermal expansion measurement system

ｏ Optical interferometer measuring principle

• Interferometer is appropriate for the measurement of small displacement

Measured material

Light path of interferometer

f horizonal polarized frequency

f vertical polarized frequency

d d specimen length

1 1 2 2

1 2 2 1 2

cos[2 ( ) ]

cos[2 (( ) 2 / 2 / )]

4 ( ) / ,( )

ref A B

mea A B

V A f f t

V B f f t nd nd

Measurement principle

Phase meter measure difference

Measure length change

ｏ Measurement system uncertainty evaluation and measuring process

• Refractive index change in air(~72nm) and uniformity(~68nm) are significant

• Reliable and repeatable measuring process is established

Thermal expansion measuring process

ｏ Thermal expansion measurement composite table

ｏ Thermal expansion measurement result

시편 이름 시편 형상 정보

M55J fiber direction 두께 2mm, 길이 300mm X 3 for 3 times each

M55J transverse direction 두께 2mm, 길이 100mm X 3 for 3 times each

시편 이름 실험 조건 Average CTE

M55J fiber direction 13~23℃ thermal cycle condition -0.92 ppm/℃

M55J transverse direction 13~23℃ thermal cycle condition 38.8 ppm/℃

CFRP specimen Chamber figuration DMI and silica base

ｏ Thermal expansion measurement result

M55J fiber direction

M55J transverse direction

ｏ System configuration

• FBG sensors and Displacement measuring Interferometer

FBG sensors

Axial direction

Radial direction

Free FBG sensor

ｏ Evaluation and monitoring of the thermal deformation

• Measurement of CTE of the Space materials

Invar36® (~1.3ⅹ10-6K-1), Gr/Ep [90]8

Gr/Ep [90]8 Invar 36

HYGROSCOPIC DEFORMATION MEASUREMENT

• Measurement system construction and verification

• Hygroscopic deformation measurement result

Hygroscopic Deformation Measurement

ｏ Measurement system requirements

Requirements

Environment

Constant temperature 28±1℃[1]

Vacuum condition [1]

Precise measurement

Long time duration ~ 3 week

Minimize stress < 1% of displacement

Measurement range 0.1~100㎛

5~10 torr

/ at 58 0%, constant temperature strain L L RH

Precision

Stability

0.1㎛

0.01㎛

1hr 1day 1week 3week

Interferometer w/o stabilizer

Interferometer with stabilizer

Dilatometer

transverse

longitudinal

Thermal Moisture

SSS Lab[1]

Present

EADS[7]

US[6] ESA[8]

ZERODUR

Composite

[6] James F.Gilmore. state of the art CME measurement, SPIE Vol 2542, pp. 121-128, , 1983 [7] A.Poenninger, B.Defoort, Determination of the coefficient of moisture expansion, Proceedings of the 9th International Symposium on Materials in a Space Environment Noordwijk, The Netherlands, 16-20 June 2003 [8] ] D.Bashford, “Low Temperature Cure Cyanate Ester Prepreg Materials for Space Application”, ESA Space, 2001

ｏ Displacement measuring sensor selection

• Long term stable, optical scale sensor[9]

• Sensor module is designed for optical scale sensor measurement

Reader – read deformation

REF – data correction

Scale – Grating is provided

Reader generate A, B phase

Direction determined by A, B phase’s intensity order

Sensor module is designed for measurement

ｏ Basic configuration of measurement system

• Two flexible beams support scale and deliver deformation of specimen

Preload can be given

• External disturbance is minimized

Kinematic coupling is added for mechanical isolation

All structure is made of INVAR – minimize temperature disturbance

Sensor module for ENC(INVAR)

Reference frame (INVAR)

CME measurement system

Measurement direction (+)

Preload direction(+)

Kinematic coupling is added

Preload direction(+)

Read head

Flexible beam

ｏ Sensor module structure design

• FEA is used for design optimization

Flexible enough in the measurement direction (1st mode<100Hz)

Stable enough in the other disturbance directions (2nd mode>10*1st mode)

The displacement of the measurement point and the displacement of the specimen have to be the same. (The present design shows <0.01% error.)

1st 2nd 3rd 4th Error

Sensor Module 80.404(<100) 1397.1(>10*1st) 1416.8 1826.2 <0.01%

1st mode

Measurement direction

Specification

Probe part FEA model

0.5mm 5mm

ｏ Sensor system structure design result

CFRP : 210.5mm ±6.5mm

Ref frame(Invar)

SUS plate

Sensor module(Invar)

Sensor reader

Kinematic coupling - 4 balls, radius 2.5mm

End clamp

1. Ref frame – specimen + sensor module fix 2. Sensor module – measurement using reader& scale 3. SUS plate – isolate external disturbance by kinematic coupling 4. End clamp – scale fix part 5. CFRP specimen – 210.5mm ±6.5mm

ｏ Sensor system verification

• Measurement precision verification

Measurement result was compared with the result of the interferometer

Displacement was given by thermal expansion of steel

Verified displacement range: 1~300㎛

Max speed – 1.5㎛/s

Prestrain – 100㎛ is given

Heat tape – thermal expansion

Interferometer ENC

ENC Interferometer

~1㎛ ~10 ㎛

~100 ㎛ ~300 ㎛

• Long term stability verification and measurement procedure

Long term stability is verified by measuring stainless steel

MEASUREMENT PROCEDURE

• Long term stability verification result

stainless steel keeps 0 in 5 days

Test specimen Acryl results

235㎛(=1119ppm, roughly 1000ppm is expected by acryl reference[12])

System is long term stable and reliable

Steel test Acryl test

Steel: disp~0±0.1㎛ Acryl: disp~235㎛

ｏ Hygroscopic deformation measurement result

• Hygroscopic measurement composite table

• Hygroscopic measurement result

시편 이름 시편 형상 정보

M55J fiber direction 두께 2mm, 길이 210mm X 2

M55J transverse direction 두께 2mm, 길이 210mm X 2

시편 이름 실험조건 Length change

M55J fiber direction 26℃, RH 59%(3 weeks) → 26℃, (3 weeks) 2.71±0.49ppm

M55J transverse direction 26℃, RH 59%(3 weeks) → 26℃, (3 weeks) -77.225±7.9ppm

5~10 torr

Measurement set up

ｏ Hygroscopic deformation measurement result

M55J fiber direction

M55J transverse direction

OPTICAL PAYLOAD PERFORMANCE SIMULATION

• Analytic modeling for diffraction limited system

• Image simulation using MTF

Analytic modeling for diffraction limited system

ｏ Design parameter setting

• GSD, Altitude, CCD size and Q factor can determine specification of telescope

• Determine factor = radius of PM(1.2m, ex), effective focal length(57.7m, ex)

Optional constant = central obstruction ratio

CCD focal plane

Ground

CCD pixel size ~ few ㎛

Effective focal length

Altitude of satellite

GSD – Ground Sampled Distance

~ size of Airy disk(Q=1)

0.1 ~ 0.3,

210 ( / )

AltEFL CCDsize

CCDsize

general case

Rk cycle mm

ｏ MTF calculation of the system w/ central obstruction[2]

• General reflecting telescope has central obstruction

Central obstruction by SM

Focal plane or other auxiliary optical part

( )( )/

i t kRik Yy Zz RA

aperture

eE e ds

Diffraction of light w/ far distance approximation

2( )(sin sin cos cos )/

2( )cos( )/

sin , cos,

sin , cos

2 ( / )

ai t kRik q RA

ai t kR

eE e d d

y zds d d

Y q Z q

eE e d d

eE J k q R d

Calculation for central obstruction(R_sm=ηR_pm)

* 2 41 1 1 1

( ( )) ( )

( / )2 ( )

( / ) ( / ) ( / ) ( / )/ 2 (0) 2

/ / / /

i t kRaA

du J u u J u

e J ukq RE u

R kq R

J kaq R J kaq R J kaq R J kaq RI EE I

kaq R kaq R kaq R kaq R

Light intensity = PSF

1( / )(0)

J kaq RI I

w/o obstruction

ｏ MTF calculation of the system w/ central obstruction[2]

• MTF is Fourier transform of PSF in spatial frequency plane

2 41 1 1 1( / ) ( / ) ( / ) ( / )2

/ / / /

J kaq R J kaq R J kaq R J kaq RMTF Normalized FFT

kaq R kaq R kaq R kaq R

2 2 2 1

cos ( ) 1

{cos ( ) ( ) 1 ( ) }, 0

1 1 1{ sin (1 ) (1 ) tan [ tan ]},

2 1 2 2 2

n n nn

Final form

A B CMTF k

A k k k

k k kB k

Diffraction limited MTF performance of telescope w/ central obstruction

ｏ De-space modeling

• De-space error depends on spherical shape, number of optical surfaces

Only approximation solution can be given

• Reference[3] describes simple spherical aberration

( ) 2 2

1(1 ( ) )

( , ) , 1

4sin( { 1 })

4( ) cos (

ikw x y

f x y e x y

w coefficient of spherical aberration

D kMTF de

a kw k

D k e dxdy

ka y dy

D k ak J aa

1 3 3 5

0 2 2 4 4 6

1 1) sin 2 ( ( ) ( )) sin 4 ( ( ) ( ))

4 1 1sin sin ( ( ) ( )) sin 3 ( ( ) ( )) sin 5 ( ( ) ( ))

1( ) (2 sin 2 )

4, cos

J a J a J a J a

ak J a J a J a J a J a J aa

a w k k

Spherical aberration approximation

MTF change w/ de-space

ｏ Jitter modeling[10]

• Telescope internal jitter – from vibration relative to SM and PM…1

• LOS(Line Of Sight) jitter – from vibration of whole body…2

Pointing error of whole body

• MTF value factor : amplitude/CCD size

movement of CCD plane

, , f LOS error of telescope f effective focal length

0 (2 )

jitter

capture jitter

capture

jitter capture

MTF J k

k spatial frequency cycle mm

F amplitude of vibration

LOS jitter

Internal jitter

Amp/CCD

Image simulation using MTF

ｏ Image resampling

• High quality image degrades due to min GSD(resolution) of system

• Using average filter for times of pixel size(original pixel/system pixel)

ｏ Spatial frequency tuning for image

• FFT of image(in MATLAB) dimension fitting for MTF is needed

• Pixel size of image is same as CCD size(few ㎛)

Resampling image

Resampling by „average filter‟

Original size of pixel(w/ exaggeration)

2 2 1/2( )

1000img

size size

CCD image

Real spatial frequency for FFT of image

Spatial frequency x: i

Spatial frequency

Multiplied by each MTF function

i,j FFT image

ｏ MTF filter for image

• Original system MTF

• De-spaced MTF

• Jittered MTF

• Final system MTF(product of all previous terms)

Spatial frequency x: i

Spatial frequency

Multiplied by each MTF function

i,j FFT image

…are multiplied to filter the original image

Original system

Final system

De-spaced system

Jittered system

ｏ MTF result comparison w/ ZEMAX and image simulation result

• De-spaced MTF and image simulation

10㎛ 50㎛ Original system

CONCLUSION

• Conclusion

Conclusion and Future Plan

ｏ Precise thermal expansion measurement for composite

• Thermal expansion measurement using DMI

• DMI + vacuum chamber system construction

• Thermal expansion of highly stable composites in dimensional is measured

ｏ Precise hygroscopic deformation measurement for composite

• Hygroscopic deformation measurement using optical scale sensor

• Mechanical dilatometer + vacuum chamber system construction

• Precision of dilatometer is verified by DMI comparison test

• Long term stability of measurement system is verified

• Hygroscopic deformation of lamina is measured

ｏ Future plan

• Hygroscopic deformation measurement for various composite

Quasi-isotropic, cross ply specimen

• thermal expansion measurement capability is added to hygroscopic deformation measurement system

Reference

[1] 윤재산, M.S.Thesis, “위성카메라용 복합재료의 흡습변형에 관한 연구”, 2011

[2] James F.Gilmore. "state of the art CME measurement", 1983, SPIE Vol 2542, pp. 121-128

[3] Y.Takeichi et al,High-precision(<1ppb/℃) Optical Heterodyne Interferometeric Dilatometer for Determining Absolute CTE of EUVL Materials, SPIE Vol.6151, 2006

[4] http://www.sigmadyne.com(SigFit)

[5] S.A.Cota et al, PICASSO: an end-to-end image simulation tool for space and airborne imaging systems, Journal of Applied Remote Sensing, Vol.4 (8 June 2010)

[6] James F.Gilmore. state of the art CME measurement, SPIE Vol 2542, pp. 121-128, , 1983

[7] A.Poenninger, B.Defoort, Determination of the coefficient of moisture expansion, Proceedings of the 9th International Symposium on Materials in a Space Environment Noordwijk, The Netherlands, 16-20 June 2003

[8] ] D.Bashford, “Low Temperature Cure Cyanate Ester Prepreg Materials for Space Application”, ESA Space, 2001

[9] H.Kunzmann et al, “Scales vs Laser Interferometers Performance Comparison of Two Measuring Systems”, Annals of the CIRP Vol.42, 1993

[10] Integrated Optomechanical Analysis, K. B. Doyle et al

dimensional stability for satellite optical...

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