potting optical elements in cells eric booen december 8 th, 2008 opti 521
TRANSCRIPT
Potting Optical Elements in Cells
Eric BooenDecember 8th, 2008
OPTI 521
2
Objective
• Introduce Concept of Potting Optical Components
• Present Adhesive Material Properties• Describe Failure Modes of Adhesives• Suggest Design Criteria and Assembly
Processes
3
Potted Optical Assemblies
• Glass Optic rests on machined surfaces
• Metal cell or mount• Adhesive Creates
Bond• Can reduces part
count, weight, cost, and ease of assembly when compared to mechanical mounts
4
Applications
• Applications with small temperature ranges
• Optics with low LOS sensitivity (mrad/mrad)
• Tolerance Required• Additional testing
should be completed to verify assumption in tolerance analysis
5
Adhesive Materials
• Epoxies– High to moderate
stiffness and strength– Rule of thumb: E = 150
ksi, Shear strength = 2000 psi
• Urethanes– Not recommended for use
at high temperature– Commonly used for
staking• Acrylics
– Quick bonds, high strength, UV cure on occasion
• Silicone Elastomers– Flexible, RTVs, high CTE
• For more information see References 2 and 3
6
Bond Failure
• Adhesive Failure– Critical damage across
the bond interface1
– Decreased by surface preparation, use of primer, proper curing process
• Cohesive Failure– Critical damage to
bulk material of bond1
– How you design your bonds to fail
7
Design
• CTE mismatch causes shear stress – See Reference 1
alternate stress calc.
• Bond size must also prevent shear due to shock loading– F = mgashock
=F/A
• Vibration environment may drive bond stiffness requirements
8
Assembly
• Hole size tolerances should be determined based on gap requirement for adhesive material and optic tolerance– Gap tolerances
recommended by manufacture or based on experience
• Maintain gap through cure temperature– Compliant shims– Spacer Balls added to
adhesive• Cure adhesive as
recommended then temperature cycle through operating temperature range
• Cell is ready for system integration
9
References
1. Blain Olbert, “Adhesive Selection and Characterization – What you don’t know can kill you,” USAO Engineering Seminar (Aug 2004)
2. Daniel Vukobratovich, “Introduction to Opto-Mechanical Design,” Notes, (2008).
3. Paul R. Yoder, Opto-Mechanical Systems Design, 3rd, CRC Press (2006).