Hanson Chang MSC.Software Corporation
description
Transcript of Hanson Chang MSC.Software Corporation
Rapid Design Iteration Process for Spacecraft Kinematic Mounts
UsingAutomatic Tet Meshing and Global/Local
Modeling Techniques
Hanson ChangMSC.Software Corporation
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Acknowledgements
• Co-author: Chris Luanglat, TRW Stress analyst
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Presentation Outline
• Spacecraft program and kinematic mounts• Design challenges for kinematic mounts• Rapid design iteration process
– Direct import of CAD solid geometry– Automatic tet meshing with efficient mesh
control and convergence techniques– Global/local modeling techniques
• Conclusions
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Spacecraft Program Overview
• EOS Spacecraft Aqua and Aura
– Mission: To study the Earth and its changing environment by observing the atmosphere, oceans, and land surface.
– Launch dates:
Aqua - 4/2002 Aura - 1/2004
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Spacecraft Overview
• Spacecraft Spec.– Dimensions: 22 ft x 9 ft x 8 ft– Weight: 6,500 lbs– All-composite spacecraft
structures
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Spacecraft FEM – View 1
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Spacecraft FEM – View 2
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FEM – Exploded View
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Spacecraft
Instrument
Load Sharing During Launch
M M
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Load Sharing On Orbit
Spacecraft
Instrument
T
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Load Isolation Concepts
• Statically determinant interface (6 DOF) isolates the instruments from the primary structure load path
• This type of structural interface is called a Kinematic Interface
• The attachment fittings used in this type of structural interface are called Kinematic Mounts
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Releasing a Degree of Freedom
• Sliding Design:– Ball/socket, cup/cone, pin/slot, V block/groove,
etc.– Relies on low and predictable friction
• Flexure Design:– Uses flexibility to isolate loads– Selected for TRW kinematic mount design
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One-Axis Kinematic Mount (KM1)
Notched Column
Stiff Direction
Flexible Direction
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Two-Axis Kinematic Mount (KM2)
Stiff DirectionsFlexible Direction
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Three-Axis Kinematic Mount (KM3)
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Typical KM Arrangement
KM1
KM3
KM2
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Typical Stiffness Matrix
Ideal KM2 Stiffness Matrix
T1 T2 T3 R1 R2 R3
Practical KM2 Stiffness Matrix
T1 T2 T3 R1 R2 R3
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Traditional Solution Space
Strength/Fatigue
StiffnessSize
Solution Space
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Kinematic Mount Solution Space
Strength
StiffnessSize
FlexibilityStability
Fracture/Fatigue
Solution Space
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CAD
Design Iteration Process
Strength SOL 101 Flexibility SOL 101 Stiffness SOL 103 Stability SOL 105 Fracture SOL101/FLAGRO
MSC.Nastran
PRE/POST PROCESSOR
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Rapid Design Iteration Process
• Speeding up the iteration process
– Direct import of CAD solid geometry
– Automatic tet meshing with efficient mesh control and convergence techniques
– Global/local modeling techniques
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Geometry Import - Old Process
• Clean up surfaces (slivers, tee, etc.)
• Create B-rep solid from surfaces
CATIA Solid Geometry
SDRC I-DEASIGES File
Traditional Method 1
CATIA Solid Geometry
SDRC I-DEASDrawing
Traditional Method 2
• Create solid geometry based on drawing
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Geometry Import - New Process
CATIA Solid Geometry
MSC.Patran
CATIA Direct
• Solid geometry directly imported into MSC.Patran as solid geometry with high success rate (95+%)
• Sliver surfaces and short edges (dirty geometry) are best correct in the CAD package
• Conferences held between designers and analysts to discuss how to identify and eliminate problem geometry
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Meshing - Old Process
• Hex element (8-node brick) is the preferred element• Created by manual meshing
– Created by meshing 5 or 6-sided solids (simple solids) or sweeping 2D elements
– Typical part must be broken into simple solids first
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Meshing - Old Process (cont.)
• Hex meshing of above parts is labor intensive• Meshing time for typical KM is several days• Not acceptable the multiple design iteration
environment
Notched Regions
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Meshing - New Process
• Automatic tet meshing using TET10 elements• Can mesh arbitrarily-shaped solids• Meshing time for typical KM is 4 hours• Ideal for the multiple design iteration environment
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Meshing - New Process (cont.)
• Advantage of Tet Meshing• Fast• Quality of TET10 elements (linear strain) is
compatible to HEX8 elements• Disadvantage of Tet Meshing
• Larger model
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Efficient Tet Meshing
• Key to efficient tet meshing is mesh density control
• Hitting the automatic tet mesh button without any mesh control typically results in excessively large modes
• Correct density control puts a lot of elements in the area of interest and coarsens quickly away from this area
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Efficient Tet Meshing (cont.)
• Typical density control techniques– Surface mesh selected solid faces with TRIA6
first to guide subsequent tet meshing– Curvature-based meshing– Break the part into multiple solids – cookie
cutter method
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Cookie Cutter Method
• Break the solid with planes or surfaces
• Critical solid meshed first with a fine mesh
• Sounding solids meshed with a coarse mesh
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Cookie Cutter Method (cont.)
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Cookie Cutter Method (cont.)
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How to Achieve Convergence
• 4 elements thru the thickness?• 8 elements thru the thickness?
• Multi-pass convergence is time consuming• Single-pass convergence is fast but more subjective
– Fringe plot with the “difference” option in MSC.Patran
– Plots the stress jumps (discontinuities)
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How to Achieve Convergence
• Use a combination of both methods
– For each type of notch geometry (circular, square, rectangular, etc.), a multi-pass convergence test is performed to establish the required number of elements thru the thickness
– Each new part is then meshed using this rule of thumb and verified using the single-pass convergence test
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Integrating the Models
Spacecraft Model250,000 DOF
X 20
Kinematic Mount Models100,000 to 750,000 DOF
Each
• Resulting model is unacceptably large
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Global-Local ModelingRBE2
18 x 18
stiffness matrix
• Use Static Reduction (Guyan Reduction) to reduce tet10 model to small stiffness matrix– Use ASET entry to specify boundary DOF
– PARAM,EXTOUT,DMIGPCH to create DMIG entries
– Use K2GG entry to assemble the KM matrices into Spacecraft model
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Global-Local Modeling (cont.)
Coupled with launch vehicle model to perform Coupled loads Analysis
KM boundary node displacements
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Conclusions
• Rapid design iteration process
– Direct import of CAD solid geometry
– Automatic tet meshing with efficient mesh control and convergence techniques
– Global/local modeling techniques
• This process resulted in substantial cycle time reduction for the Aqua and Aura kinematic mounts
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Conclusions (cont.)
• The notched-column kinematic mount design configurations have been incorporated into the TRW Deployables Handbook
Merci beaucoup