Optomechanical design - ku · •Introduction to FEM. Outline •Introduction to optomechanical...

Optomechanical designMarco Riva

(INAF Astronomical Observatory of Milan)

Outline

• Introduction to optomechanical engineering

• First order optic

• Image motion

• Technical drawings

• Mechanical fabrication and measurements

• Equilibrium, stress and strain

• Thermal distortions

• Kinematic constraints

• Materials

• Mounting of optical components

• Introduction to FEM

What is optomechanicalengineering?

• Optimize entirely for performance. Fabrication limitations dominate. Optical designer must work closely with mechanical and fabrication engineers to take advantage of state of the art technologies.

• Optimize entirely for cost. Production limitations dominate. Optics cannot be ignored, but the emphasis is on good mechanical engineering and setting up an efficient production process.

Design process

1. Know the requirements:2. Choose design form to constrain all degrees of

freedom3. Design for operational environment4. Design for survival• General rules of design:

• Start with the simplest concept, use rules of thumb and hand calculations.

• Identify “independent parameters” in the design and make sure that you know the impact of these

• Systematically evaluate important degrees of freedom• Perform initial evaluation of manufacture and assembly

before investing in detailed models or calculations

Outline



• Image motion






• Materials



1° order optics

1° order optics

• Plane parallel plate

1° order optics

• Plane mirror

Lens motion I

Axial magnification

See movies

Outline



• Image motion






• Materials



Orthographic view

Section views

Application of dimensions

Datum and tolerancing

Outline



• Image motion






• Materials



Manufacturing and metrology of mechanical partsMost of the small (<1 m) parts for optics are made by cutting from oversized stock on a few common machines. These can be driven by a skilled operator, or by numerical control:

• Milling machine (aka “mill” or “Bridgeport”)

• Lathe

• Drill press

Other processes are used as needed:

• Near net shape forming (Rolling, casting, extruding, stamping)

• Surfacing (bead blasting, grinding, lapping)

• Welding, brazing

• EDM (Electrical discharge machining)

• Precision cutting (Laser, abrasive water jet)

Different materials have very different limitations – Get to know the guys in the shop

Standard tolerances

• ± 1 mm for coarse dimensions that are not important (0.040 inches or “forty thousandths”)

• ± 0.25 mm for typical machining without difficulty (0.010 inches or “ten thousandths””)

• ± 0.025 mm precision machining, readily accessible (0.001” inches or “one thousandths” or “1 mil”)

• < ± 0.002 mm high-precision, requires special tooling (0.0001” or “one ten-thousandths” or “one tenth” or “one hundred millionths”)

Machininig

Measuring• Plastic ruler: Good for quick, rough measurements. Most practical measurements in the

lab will be made with the ruler. Be careful -- the end of the ruler usually does not coincide with the 0 mark

• Steel rule: Allows much more accuracy, costs more.

• Tape measure: Good for quick measurements over wide distance variations. High quality surveying tapes can be used for measuring over dozens of meters to sub-millimeter accuracy.

• Calipers: These are common, inexpensive, fairly accurate, and versatile. Use them for measuring outside dimensions, inside dimensions, and depth.

• Height gage: Usually used on a flat granite table. Measures height from ~1 to 30 inches

• Micrometer: This is a fine pitch screw with accurate marks. Use the vernier for highest accuracy.

• Outside micrometers: A frame holding a micrometer for measuring outside dimensions. These can be purchased for measuring up to about 10 inches.

• Inside micrometers: Holds a micrometer for measuring inside dimensions. These are made with extensions that can be put together for measuring up to 20 feet.

• Gage blocks: Highly accurate for defining length standards for 0.1 – 4 inches. Special length standards can be purchased for much longer distances.

• Indicator: Can be digital or dial. Often used for measuring motion, such as runout on a spindle

• Depth gage: Uses a micrometer or indicator to measure depth.

• Telescoping gages: Measures small gaps, calibrate with outside micrometer

Measuring

Outline



• Image motion






• Materials



Equilibrium

When forces acting on an object which is at rest are balanced, then the object is in a state of static equilibrium.

• No translations

• No rotations

Forces are defined as F = m a

Momentum are defined as

Moment and couples

Stress and strain

Stress strain curve

Materials

Stress Birefringence:

Outline



• Image motion






• Materials



Thermal dilatation

Thermal induced stress

Outline



• Image motion






• Materials



Kinematic

For holding a body (rigid thing) with the highest precision, we require:• Full 6 DoF constraint

• If 6 DoFs not fully constrained, then one is loose.

• No overconstraint• Any overconstraint can cause problems:

• constraints can push against each other, resulting in stress and deformation.

• constraints pushing against each other will “lurch” when forces exceed threshold

Kinematic constraint : All DoFs are constrained, and very strictly, none are overconstrainedSemi-Kinematic : Slight overconstraint is allowed

Constraints Vs spring

Balls/cylynder contact

• Nominally, the contact area is zero for a point or line

• Really, the contact area comes from deformations and depends on the geometry and material properties.

• More force causes more deformation which increases the contact area.

• Non-point contact = not purely kinematic

• Stiffness = Force required for displacement is very low for the unloaded case. and very nonlinear. Preload is required.

• Increased preloading makes stiffer, more stable interface in normal direction

• But:• Stress = Force/Area is very high and can damage the materials• Tangential effects due to friction can be large

Hertzian contact

• Contact Area 𝐴 = 𝜋3 3𝑊𝑟

4𝐸′

2

• Mean Contact pressure: 𝑝𝑚 = 𝑊 𝐴

• Max contact pressure: 𝑝0 = 1.5 ∙ 𝑊 𝐴

Semikinematic

Use kinematic concepts, but allow small amount of overconstraint

• Replace point contacts with “small” contacts

• Replace idealized constraints with flexures, that use compliance to

minimize forces and moments in directions other than the intended constraint

Outline



• Image motion






• Materials



Important properties

• Physical• CTE • Young’s modulus• Shear Modulus (or

Poisson ratio) • Density• Yield strength• Ultimate tensile strength• Hardness• Thermal conductivity• Specific heat• Corrosion resistance

• Processing limitations• Machining• Welding• Cold working• Hot forming• Casting • Annealing• Heat treating

Typical materials

Outline



• Image motion






• Materials



Design process

1. Know the requirements:2. Choose design form to constrain all degrees of

freedom3. Design for operational environment4. Design for survival• General rules of design:

• Start with the simplest concept, use rules of thumb and hand calculations.

• Identify “independent parameters” in the design and make sure that you know the impact of these

• Systematically evaluate important degrees of freedom• Perform initial evaluation of manufacture and assembly

before investing in detailed models or calculations

Clamping Vs Bonding

Clamping

• Apply force in line with constraint to limit stress, possible distortion, and instability

• Analyze shock load: • Force at constraint = Preload Force +/- mass * acceleration

• Stress = Force/Area• Make sure that applied stress is not so large that it breaks the optic• Rule of thumb for glass, limit short term compressive stress to

345MPa and tensile of 7MPa• Special calculations to determine stress for point and line contacts

• Set preload to maintain contact between the optic and the constraint• Otherwise it can rattle, which causes very high local stress, possible

fracture• Position may not be stable if the contact is not maintained

Bonding• The primary issues with bonded joints are the strength, stiffness, and sensitivity to

thermal effects.

• Material properties vary widely for adhesives. Important properties are:• Shear modulus: Low modulus gives good isolation from thermal effects, high modulus provides

good stiffness. • Strength: Properly prepared bonds will achieve either the bulk strength of the adhesive or the

strength of the substrate• Optical transparency for some applications• Viscosity: Viscous (thick) adhesives limit the bond thickness• Sensitivity to environment: adhesives loose their strength in corrosive environments. Also, they

swell in the presence of moisture• Volume change upon curing: The materials generally shrink as they cure, which can cause stress

and deformation• Temperature range for survival: Adhesives soften and let go at high temperatures, they become

stiff and brittle at low temperatures• Outgassing: very important for vacuum environments, but even at ambient pressures some

adhesive release volatiles that contaminate optical surfaces, especially for UV applications

• Common adhesive types used in optomechanics• Elastomeric adhesive: i.e.RTV, very compliant, good sealant, poor strength• Epoxy: Good stiffness, strength• UV curing: Achieve low stress, low CTE

Bonding

• Adhesives are weakest for peal-type loads. The geometry should never allow these loads.

• The strength of the bonded joint is estimated by the shear strength of the adhesive times the bond area. • Typical epoxy has 13 to 20 Mpa shear strength. A bond with 1cm2

area should handle XXX load.

• Loads can come from thermal effects and from acceleration, coupled with the mass.

• Bond thickness is important:• Optimal thickness for strength 100 um for epoxy• Thicker bonds reduce thermal stresses, slightly reduce strength• Set bond thickness with fixtures, shims, or glass beads

• Many adhesives creep under shear load. This mitigates thermal stresses. Analysis uses shear modulus which is a function of time.

Windows

Filters

Prisms

Lenses

Flexure ring

Snap ring

Threaded ring

Radial flexures

Pot in place

Alignment of elements

Lens barrel

Lens assembly

Small mirrors

Thermal mismatch

Flexure mounts

Large mirrors and selfweight

Whiffletree

References

• Yoder, P.R., Opto-Mechanical Systems Design, 3rd

Ed., CRC Press, 2006

• Ahmad, Anees. Handbook of OptomechanicalEngineering. Boca Raton, Fla.: CRC, 1997.

• K. Schwertz, Useful Estimations and Rules of Thumb in Optomechanics, MS Report, University of Arizona, 2010)

• Finite element Analysis

Design tools

FEA what is this?

FEA pitfals

Basic theory

Hooke law

System stiffness matrix : 1D example

How to build the stiffness matrix

Boundary condition

Elements

Element matrix

FEA procedure

1. Identify the problem, sketch the structure and loads.

2. Create the geometry with the FE package solid modeler or a CAD system.

3. Apply material properties.

4. Mesh the model.

5. Apply boundary conditions (constraints and loads) on the model.

6. Solve numerical equations.

7. Evaluate the results.

Integrated modeling

Performance prediction and Optimiz.

Optomechanical design - ku · •Introduction to FEM. Outline •Introduction to optomechanical...

Documents

Transcript of Optomechanical design - ku · •Introduction to FEM. Outline •Introduction to optomechanical...