Transcript Flexure Mounts For High Resolution Optical Elements

Flexure Mounts For High
Resolution Optical Elements
Mir Salek
Optomechanics
Fall 2008
Review of: Vukobratovich D, Richard R M,
Proc of SPIE Vol. 0959, Jan 1988
Summary
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What is a flexure?
Compare to other mounts
Basic types of Flexure
Some examples
What is a Flexure
By definition, flexure is
an elastic element which
provides controlled motion
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Plunging to the Idea
Lens
Mount
Plunging to the Idea
CR
(idea from Yoder’s book)
Inward
CT
The Lens
120º
CT
120º
CR
CR
CT
Points
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Equal Compliances -> Keeps the lens
centered when temperature changes
The spring forces allow the lens to decenter
during shocks and return afterwards
Minimize stress in optics during shocks
Typically stiff tangentionally and axially and
compliant radially
Uses Kinematic principles to find the location
of flexures
High Performance Lens
Assembly
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Tight tolerance alignment
Maintain alignment under operational
level shock, vibration, pressure,
temperature change
Retain its alignment upon exposure to
survival level of environmental effects
Low stress on optics (particularly
mirrors)
Advantages of Flexure Mounts
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Free of slick-slip and friction effects of semikinematic design
Less hysteresis than rolling or sliding contacts
More robust to adverse environment effects
such as extreme temperatures, vacuum, and
abrasive dust
Needs very little maintenance if any
*
Ideal for space applications
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Flexure Material
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Should provide required compliance
within length limitation
Should have high dimensional stability
for repeated use in time
Flexure Material
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Should have high fracture toughness
Thermal properties to maintain
operation with temperature change
Compliance
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For a given length:
Higher RTS ->maximum compliance
Reduced tensile strength is the ratio of
yield strength to modulus of elasticity.
Dimensional Stability
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Material instability or room temperature creep can
happen at stresses less than micro-yield strength
Andrea’s Beta Law predicts instability with time:
ε = βtm
m ≈ 0.33
Flexure Design
Basic Flexures: Single Strip
Flexure
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It can be used to guide both translation
and rotation
The strain is a function of axial preload
In the table
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L is the flexure length;
E is the elastic modulus;
I is the moment of inertia;
P is the applied axial load;
θ is the end slope of the flexure;
M is the applied torque;
δ is the end displacement of the flexure;
F is the applied force;
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Strain versus Axial Stress
constant force
Basic Flexures: Cross-Strip
Rotational Hinge
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Two single stripped flexures at right angles
provide a rotational hinge
center of rotation shifts as a function of angle
of rotation
Cross-Strip Rotational Hinge:
rotation-torque relations
Basic Flexures: Parallel
Spring Guide Flexure
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A pair of parallel single strip guides provides
linear translation
The range of motion is limited to 1-2mm
also the motion is not purely linear and there
is a height shift as well
Parallel Spring Guide Flexure: ForceDisplacement Relations
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If the force is not applied at the midpoint, the flexure
would tilt as it translates
Basic Flexures: Cruciform
Flexure
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Provides limited rotation in very
confined spaces
Basic Flexures: and Tapered UniformStress Cantilever Flexure
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It is used to provide a small range of
translation motion in very confined space
Flexure Mount Example 1
Flexure Mount Example 2
Flexure Mount Example 3
Bipod Flexure Mount
Happy Finals