thin optic constraint - mitweb.mit.edu/makilian/www/2.75/2.75 final presentation.pdfmxf fl m l eiei...
TRANSCRIPT
Thin Optic Constraint
Mireille AkilianAmir Torkaman
Space Nanotechnology LaboratoryDecember 10, 2003
Outline
• Problem Statement• Functional Requirements• Strategies• Concepts developed• Detail design• Results and Conclusions
Problem StatementForces that lead to optic surface warp:•Gravity (weight sag at angle θ, effect linear with θ; θ < 70 arcsec)
•Friction (2-3 µm error)
•Thermal expansion mismatch between optic and device (100’s of microns)
Optics flatness on one side < 0.5 µm peak to valley
Functional Requirements• Reduced effects of thermal expansion and friction• Gravity pitch accuracy 36 arcsec• Placement repeatability:
Pitch: 72 arcsecLateral: 1.5 mm
• Ease of inserting optic into device
• Optic front surface clear during metrology
Three constraint points on surface of rectangular and circular optics
Optic
StrategiesAir pressure
optic
Vacuum
optic
Vacuum preloaded air
bearings
Double-Sided air bearings
optic
Double-Sided flexures
Concepts DevelopedFunctional Requirements Design Parameters
Double-Sided Air Bearings
Double-Sided Flexures
Air gap variation < 3 µm
Air temperature drop < 0.5°C
Acceptable stiffness
Inclinometer & tilt stageHold optic vertically
Back surface actuationOptic front surface clear
Small bearing OD
Metal bearings
Optimum feeding parameter
Flexure lengthConstrain up to 1.6 mm thick optic
Monolithic flexuresMinimum centerline misalignment
Flexure geometryAccount for 1°C temperature change
Double-Sided Air BearingsDesign Analysis
Y-translationconstraintsOpposed, inherently
compensated bearings(x6)
Vacuum preloadedbearings
Optic
Set Set AAoo: constrained by geometry: constrained by geometry
Choose hChoose hChoose AChoose Aii//AAoo
0.6 <Λξoptimum < 1.1
Choose Ps
Calculate load capacityand stiffness
khAo di Aido WPsΛξmm mm2 NN/mm2 N/µmmm mm2 µm
38.5 0.13 10 1.5 3.17 0.2 0.376 0.172
Double Side Air BearingsExperiment, Results, and Conclusions
Load vs. air gap
0
1
2
3
4
0 5 10 15 20Air gap ( m)
Load
(N)
Operating region
Theoretical load capacity
µ
No friction and thermal mismatch between optic and deviceNo contact deformationAcceptable load capacity and stiffness
Design and assembly complexity
Moderate to High cost
Double-Sided Flexure Assembly
Double-sided flexures
Bottom and side flexures
Reference Block
Flexure tilt stage
Double-Sided FlexuresDesign Analysis
Wire EDM-ed, Monolithic flexures
42 mm
ϕ = 2 mm ruby balls
0.8 mm
6.75 mm0.6 mm
21 mm
2 mm
1. Vertical flexures
Allow for optic insertion/removal
Provide preload
(klateral = 2.45E-4 N/µm)
2. Horizontal flexures
Accommodate for thermal expansion
(klateral = 0.024 N/µm)
Material Aluminum 6061 T651
Double-Sided FlexuresFlexure Deflection and Stress
Lateral displacement due to optic thickness
After optic placement:
Horizontal displacement 283 µm
Vertical displacement 6.6 µm
Vertical parasitic motion
Bottom/Side Flexure Design2
3 2
3
2.47
3 2
3.52
buckle z
z z
x zFx
axial
xlateral
M
nflx
EIF FL
M X F
F L M LEI EIEAkLFk
EIm L
δ
δ
ω
= >
= ∆ ×
= +
=
=
=
Outer Diameter (mm) 0.635
Inner Diameter (mm) 0.508
Length (mm) 50
Buckling Force (N) 0.93
Axial Stiffness (N/µm) 0.456
Lateral Stiffness (N/µm) 2.22E-05
Vertical Reference Flat• Front surface flatness: 0.1um
– Optically polished Nickel coated Aluminum block
– 90 deg Angle +/- 1 arcsec• Base has tilt adjustment
– Resolution: 2 µrad• Inclinometer resolution: 14 arcsec
Flexure Tilt Stage Design
• Allows for pitch / yaw adjustments (2 ± 0.0005°)
• Actuation Mechanism:– Fine-thread (#¼-100) screws
• Preload Mechanism:– Springs or Belleville washers
System Assembly
Preliminary ExperimentsAutocollimator Experiments:
• Flexure tilt stage: achieves desired range & accuracy (2 µrad)
• RepeatabilityPitch: 1.2 µradYaw: 11 µrad
Surface Metrology ResultsShack-Hartman Surface Metrology Results:
• No local deformations at flexure/optic interface
• Placement repeatability 55 nm
Conclusion• Identified and analyzed optic deformation forces
– Gravity– Friction– Thermal expansion
• Theoretically and experimentally proved the ideal concept of using air bearings
• Designed and built an optic holding device using flexures
• Future Improvements:– Reference flat must be polished– Systematic and repeatable optic placement procedure– Further surface metrology experiments