Hexapod Detector Mounts

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Hexapod Detector Mounts

• B. C. Bigelow, UM Physics
• 3/24/05

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Hexapod Detector Mounts

• Motivations
• Provide a common mount design for Vis and IR
  detectors
• Minimize detector package SS thermal stresses
• Minimize detector package SS temperature
  gradients
• Accommodate various detector package materials
  (Invar, TZM)
• Accommodate various FPA baseplate materials (TZM,
  SiC, ?)
• Accommodate local detector PCBs, connectors,
  heaters, etc.
• Minimize weight, maximize first resonance

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Hexapod Detector Mounts
Detector space frame! but fabrication
unfriendly
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Hexapod Detector Mounts
A fabrication-friendly version
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Hexapod Detector Mounts

• Fabrication options for hexapod
• Fabrication method may depend on hexapod material
  choice
• Powder metallurgy methods (HIP, laser sintering)
• Abrasive water-jet cutting
• Laser cutting
• Plunging and/or wire EDM
• Stress-relieve rough blanks prior to cutting
• Polish blanks flat and parallel prior to cutting
• Final grind/polish mounting pads to spec. after
  cutting
• Other?

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Hexapod Detector Mounts
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Hexapod Detector Mounts
Arbitrary mount height of 12mm can be lower
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Hexapod Detector Mounts
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Hexapod Detector Mounts
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Hexapod Detector Mounts
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Finite Element Analyses

• Quantify performance via FE analyses
• Hexapod flexures are 1mm wide x 3mm high (all
  cases)
• Hexapod material is TZM (Invar another option)
• Static analyses 100g deflections and stresses
• Dynamic analyses first 10 frequencies and mode
  shapes
• Steady-state thermal stress for -150K temp
  excursions
• Steady-state thermal heat flow and temperature
  gradients
• Summary follows individual results

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Focal Plane Material Properties
Room temp. material properties
Material Properties TZM (Moly) Invar 36 SiC (CVD)
E (GPa) 325.0 147.0 466.0
Yield (MPa) 415.0 300.0 470.0
Density (kg/m3) 10160 8050 3210
CTE (PPM/K) 4.90 1.26 2.20
K (W/mK) 138 11.1 300
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FEA - static

• Static FEA
• 100g accelerations, Gx, Gy, Gz
• Det. package base models only, no AlN, MCT,
  epoxy, etc.
• Two material combinations Invar/TZM, and
  TZM/TZM
• Simplified model of hexapod mount (no pads)
• Max deflections 1.5 - 1.9 microns
• Max stresses 20 - 26 MPa (Invar/TZM)
• Invar yield 300 MPa
• TZM yield 860 Mpa
• Low stress in package material - max. 20 Mpa
  (point load)

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FEA - static
Deflections in meters, 1.4 microns max.
Gz, Z-axis deflections 1.4 microns max
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FEA - static
Stress in Pa, 26 MPa max., (point loads)
Gz, Z-axis deflections 1.4 microns max
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FEA - dynamic

• Dynamic FEA
• Det. package base models only, no AlN, Si, MCT,
  epoxy, etc.
• Two material combinations Invar/TZM, and
  TZM/TZM
• Simplified model of TZM hexapod mount
• First resonances
• TZM/invar 3000 Hz
• TZM/TZM 3053 Hz

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FEA - dynamic
Gz, Z-axis deflections 1.4 microns max
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FEA steady state thermal

• Steady-state thermal stress
• Minus 150 K temperature excursion
• Baseplate, hexapod mount, and package base
• Four material combinations for baseplate and
  package
• TZM/Invar, TZM/TZM, SiC/TZM, SiC/Invar
• Simplified model of hexapod mount (no pads)
• Deflections 6.9 8.7 microns (TZM/TZM,
  TZM/Invar)
• Deflections 7.9 - 9.7 microns (SiC/Invar,
  SiC/TZM)
• Pkg stresses 2.3 Mpa (TZM/Invar)
• Pkg stresses 1.1 - 1.7 Mpa (SiC/TZM, SiC/Invar)

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FEA steady state thermal
Elements
Gz, Z-axis deflections 1.4 microns max
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FEA steady state thermal
Stress in Pa, 14.8 MPa max. (point loads)
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FEA steady state thermal

• Steady-state heat flow
• Baseplate, hexapod mount, and package base
• 200 mW heat load imposed on top surface of
  package
• Baseplate back side sunk to a cold source at
  140 K
• Four material combinations for baseplate and
  package
• TZM/Invar, TZM/TZM, SiC/TZM, SiC/Invar
• Simplified model of TZM hexapod mount (no pads)
• Max. temp variation 0.56 K (TZM/Invar)
• Min. temp variation 0.05 K (SiC/TZM and TZM/TZM)
• Min final temp 142.3 K (SiC/TZM)
• Max final temp 144.6 K (TZM/Invar)

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FEA steady state thermal
Boundary cond.
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FEA steady state thermal
Temp variations (K) SiC/TZM
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FEA summary
Materials Materials Pkg, 100g, X,Y,Z Pkg, 100g, X,Y,Z Pkg, 100g, X,Y,Z Pkg, 100g, X,Y,Z Fn -150K hex pkg D T Tf
Base Pkg ux uy uz s,MPa Hz uz s,Mpa s,MPa K K
TZM Inv. 1.9 1.9 1.5 20.8 3000 8.7 20.3 2.3 0.56 144.6
TZM TZM 1.9 1.9 1.4 26.1 3053 6.9 -- -- 0.05 142.4
SiC Inv. -- -- -- -- -- 9.7 28 1.7 0.56 144.5
SiC TZM -- -- -- -- -- 7.9 14.8 1.1 0.05 142.3
deflections, u, in microns
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Detector mount taxonomy
Yale flex
LBL flex
UM flex
UM hexapod
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Detector mount comparison
Pkg thermal stress, -150K Pkg temp gradient First resonance
Design MPa K Hz
Yale flex 41 0.2 1508
LBL flex 31 0.1 988
UM flex 5.6 0.1 3216
UM hexapod 21 0.05 3000
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Hexapod Detector Mounts

• Conclusions
• Hexapod mount kinematically connects detectors to
  focal plane
• Low thermal stress for -150 K temperature change
• Large conduction cross-section minimizes thermal
  gradients
• Common mount design works for both NIR and VIS
  detector packages
• Very low thermal stresses in base plate, mount,
  and packages
• Hexapod provides optimal support for detectors
• Minimum mass, maximum stiffness solution
• Very high first resonance 3000 Hz or higher
• Hexapod mount is readily fabricable by standard
  methods
• Hexapod performance demonstrated via FE analysis