Applications of Piezo Actuators for Space Instrument Optical Alignment

Transcription

1 Year 4 University of Birmingham Presentation Applications of Piezo Actuators for Space Instrument Optical Alignment Michelle Louise Antonik Supervisor: Prof. B. Swinyard

2 Outline of Presentation Introduction Background Method Developed Results Gained Future Possibilities Conclusions Questions

3 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

4 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.

5 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

6 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

7 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

8 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

9 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 nonferroelectric

10 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.

11 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.

12 Piezoelectric Crystals Equations for crystal s polarisation and elastic strain both contain the stress the crystal is under and the electric field affecting it

13 How the Piezo Actuator Works Cont. a Piezoelectric crystal, b Sliding block, c guiding rod, d fixed frame Courtesy of attocube systems User Manual Inertial XYZ Positioner ANPxyz100.

14 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

15 Rough Calibration of the Piezo LVDT placed against the piezo actuator Piezo actuator moved outwards by 50 steps at a time Mean Step Size (mm) Step Number

16 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

17 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

18 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

19 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 750 from the webcam 650 One standard deviation was found to be 2 pixels 550 Gave resolution of 20μm Height of light source (pixels) Distance from lens (mm)

20 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

21 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 Average Centroid (pixel) Position (mm)

22 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

23 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

24 Development of Method Dark rings are from destructive interference pd = mλ/2 Bright rings are from constructive interference pd = mλ Movement of piezo causes rings to disappear into centre Courtesy of

25 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

26 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

27 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

28 Results Finding the Zero Path Difference Area Maximum Gray Scale Value Step Number

29 Results Close-up of the Zero Path Difference Area Gray Scale Value Step Number

30 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

31 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 I = * E p ( φ) E ( φ) 1 p2 Light maybe changing polarisation slightly during reflection

32 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

33 Results Background Removed from Image Taken Just Outside the Zero Path Difference Area 10 Gray Scale Value Distance Along CCD

34 Real Fringe Pattern If two mirrors are not aligned exactly a fringe pattern occurs Courtesy of Optics by E. Hecht

35 Results Background Removed from Image Inside the Zero Path Difference Area Gray Scale Value Distance Along CCD

36 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

37 Future Work Cryogenically cool the piezo actuator for recalibration Piezo steps sizes will change as the piezo contracts Replace detector with fibre optic sensors

38 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

39 Questions