Year 4 University of Birmingham Presentation

Applications of Piezo Actuators for Space Instrument Optical Alignment

Michelle Louise Antonik

520689

Supervisor: Prof. B. Swinyard

Outline of Presentation

• Introduction• Background• Method Developed• Results Gained• Future Possibilities• Conclusions• Questions

Introduction

• This project aims to create an alignment system that can see movement of 1μm

Why?

• The James Webb Space Telescope will carry several instruments that operate at cryogenic temperatures

• RAL is tasked with observing structural changes in one of the instruments, MIRI, at 7K

• The alignment device that is to be used currently only has a accuracy of 6μm

Fibre Optic Sensors

• Fibre optic sensors are to be used to detect the change in shape

• All electronics are kept out of the cryostat making it a passive system • Work by sending a light signal down the optical fibre which is reflected off a surface back into the fibre.

Purpose of Alignment System

• Need to increase accuracy of the fibre optic system

• Create an alignment system that designed to detect small movements

• Piezo actuator used to simulate the movements of MIRI

• Piezo actuator’s movement is unknown• Calibrate it sufficiently becomes predictable and

repeatable

Purpose of Alignment System

• Once piezo actuator is calibrated place in MIRI’s position

• If small movements by the piezo actuator produce repeatable changes in the fibre optic sensor’s output then the sensors can be calibrated to a higher accuracy

Alignment Systems

What are alignment systems?

• Alignment systems allow components to be placed in the correct relative positions to each other, essential for high accuracy work

• Many different types• Interested in passive, optical systems• These are cheaper and less noisy than other

systems

Alignment Systems

• Passive optical alignment systems are either photogrammetry based or laser based

• Photogrammetry based systems have maximum accuracies of 150μm

• Laser based systems using interferometry have accuracies of less than 1nm

How the Piezo Actuator Works

What are piezo actuators?

• Is based around a piezoelectric crystal• These crystals expand or contract when a

potential difference is placed across them• Two types of crystal: ferroelectric and non-

ferroelectric

Ferroelectric Crystals

• Ferroelectric crystals have two or more stable orientations in which the atoms can be arranged

• By applying a mechanical stress across the crystal the atoms are forced into more compact arrangement

• Change of ion’s position changes the polarisation of the crystal

Courtesy of C. Kittel, Introduction to Solid State Physics, 5th ed., 1976, John Wiley & Sons Inc.

Non-Ferroelectric Crystals

• Non-ferroelectric crystals have three equal dipole moments that have a sum at the vertex of zero

• A mechanical stress compresses the crystal which distorts the dipoles

• When the sum at the vertex is not zero, there is a polarisation across the crystal

Courtesy of C. Kittel, Introduction to Solid State Physics, 5th ed., 1976, John Wiley & Sons Inc.

Piezoelectric Crystals

• Equations for crystal’s polarisation and elastic strain both contain the stress the crystal is under and the electric field affecting it

How the Piezo Actuator Works Cont.

Courtesy of attocube systems’ User Manual Inertial XYZ Positioner ANPxyz100.

a – Piezoelectric crystal, b – Sliding block, c – guiding rod, d – fixed frame

Rough Calibration of the Piezo

• Initially a rough calibration of the piezo was required to understand it’s movement

• This was done by using a linear voltage displacement transducer (LVDT)

• An LVDT probe has a central core that is pushed into three wire coils

Rough Calibration of the Piezo

• LVDT placed against the piezo actuator• Piezo actuator moved outwards by 50 steps at a

time

0.000

0.005

0.010

0.015

0.020

0.025

0.030

0 2000 4000 6000 8000 10000 12000 14000

Step Number

Me

an

Ste

p S

ize

(m

m)

Original Method

Fine calibration of piezo actuator

• Original idea was a basic phogrammetry technique with simple geometry

• As the mirror moved the laser beam travelled further

• Angles translated this to movement across the webcam

Limitations Imposed

• Limitations on the sensitivity of the alignment system are imposed by the equipment used

• The main limitations are– Resolution of the webcam’s CCD– Fish-eye lens has low sensitivity for small movements– Angle of laser beam on the mirror

Limitations Imposed

Resolution• The resolution is the smallest possible distance

between two points that the camera can see• Is given by the Rayleigh Criterion:

Θ = 1.22λ/D

where λ is the wavelength of light and Θ and D are given as below

Limitations Imposed

With the fish-eye lens• Fish-eye lens allows large viewing area for a

small detector– Small movements near the optical axis become hidden

• Resolution was found by moving a large light source away from the webcam

• The height of the source was plotted against distance from the webcam

• One standard deviation was found to be 2 pixels• Gave resolution of 20μm

250

350

450

550

650

750

35 40 45 50 55 60 65 70 75 80 85

Distance from lens (mm)

Heig

ht

of

lig

ht

so

urc

e (

pix

els

)

Limitations Imposed

Without the fish-eye lens • Resolution measured by

moving webcam perpendicular to laser beam

• Linear relationship between distance moved by webcam and laser beam across CCD

• At regular intervals images were taken and brightness measured

Limitations Imposed

• One standard deviation for the points from the line allows an accuracy of the position to be taken to 0.5 pixels

• The resolution was 1.5μm

510

515

520

525

530

535

540

545

550

555

0.00 0.02 0.04 0.06 0.08 0.10 0.12

Position (mm)

Av

era

ge

Ce

ntr

oid

(p

ixe

l)

Limitations Imposed

Angle of the laser beam• The angle at which the laser beam hits the mirror

is the angle at which it is reflected• A larger incident angle gives greater

magnification of the mirror’s movement• Large angle means larger cross-section, gives

less precision

• Angles less than 20° needed

• Even at maximum resolution is still more than 1μm

Development of Method

• As it was not possible to use photogrammetry techniques, interferometry techniques were tried instead.

• A Michelson interferometer was created

• Have accuracies of λ/2

• Any noise will be known to come from cryostat rather than alignment system

Development of Method

• Dark rings are from destructive interference

– pd = mλ/2

• Bright rings are from constructive interference

– pd = mλ

Courtesy of www.search.com/reference/Interference

• Movement of piezo causes rings to disappear into centre

Development of Method

• Michelson set-up:

a – laser, b – beam expander, c – polariser, d – iris, e – half silvered mirror, f – full silvered mirror, g – piezo actuator with full silvered mirror mounted on top, h – lens, i – webcam, j – optical axis

Development of Method

• Laser light too coherent

– Small defects in the set-up obscure the results

• A less perfect light source is needed

– Gives more complicated pattern

• Use a white filtered source

Final Method

• Adapted Michelson:

b – beam expander, d – iris, e – half silvered mirror, f – full silvered mirror, g – piezo actuator with full silvered mirror mounted on top, h – lens, i – webcam, j – optical axis, k – white light source with red filter, l – second iris

Results

146

147

148

149

150

151

152

153

154

155

156

0 1000 2000 3000 4000 5000 6000 7000

Step Number

Max

imu

m G

ray

Sca

le V

alu

eFinding the Zero Path Difference Area

Results

Close-up of the Zero Path Difference Area

118

120

122

124

126

128

130

3900 4100 4300 4500 4700 4900 5100 5300 5500

Step Number

Gra

y S

ca

le V

alu

e

Results

• Do not see the expected pattern

• Get zero intensity where a peak is expected

• Data still useable as distance between null points is the same as distance between peaks

Results

Why do you not see the expected pattern?

• Webcams are designed to view large images• Software maybe reducing fringes as they are

small fluctuations

• The intensity at a given point due to polarisation is

• Light maybe changing polarisation slightly during reflection

)()( *

21 pp EEI

Results

• Rough surfaces can generate a consistent phase change

• Path difference through the half silvered mirror is not equal

• Need to remove background from results

Results

-20

-15

-10

-5

0

5

10

0 50 100 150 200 250 300

Distance Along CCD

Gra

y S

ca

le V

alu

eBackground Removed from Image Taken Just Outside the Zero Path

Difference Area

Real Fringe Pattern

• If two mirrors are not aligned exactly a fringe pattern occurs

Courtesy of ‘Optics’ by E. Hecht

Results

-60

-40

-20

0

20

40

60

0 50 100 150 200 250 300

Distance Along CCD

Gra

y S

cale

Val

ue

Background Removed from Image Inside the Zero Path Difference Area

Future Work

Refine results

• Replace webcam with photo diode

• Step size of the piezo actuator– Initial calibration found the step size to be approximately

400nm– Wavelength of light is approx 600nm

• Can only see to 2λ/3

Future Work

• Cryogenically cool the piezo actuator for recalibration

• Piezo steps sizes will change as the piezo contracts

• Replace detector with fibre optic sensors

Conclusion

• The Michelson interferometer gives highly accurate results ensuring that noise detected will come from the cryostat

• Several adaptations needed before final calibration– Replacing the webcam– Adding in a compensator plate for the optical path

difference– Reducing piezo actuator’s step size

Questions