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Summary What is a flexure? Compare to other mounts Basic types of Flexure Some examples
Citation preview
Flexure Mounts For High Resolution Optical Elements
Mir SalekOptomechanics
Fall 2008Review of: Vukobratovich D, Richard R
M, Proc of SPIE Vol. 0959, Jan 1988
Summary 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
Plunging to the Idea
Lens
Mount
Plunging to the Idea (idea from Yoder’s book)
120º
The Lens
120º
Inward
CT
CR
CT
CT
CR
CR
Points 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 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
Free of slick-slip and friction effects of semi-kinematic 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
Flexure Material Should provide required
compliance within length limitation Should have high dimensional
stability for repeated use in time
Flexure Material Should have high fracture
toughness Thermal properties to maintain
operation with temperature change
Compliance For a given length:
Higher RTS ->maximum compliance Reduced tensile strength is the ratio
of yield strength to modulus of elasticity.
Dimensional Stability 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 It can be used to guide both
translation and rotation The strain is a function of axial
preload
In the table 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; .
Strain versus Axial Stressconstant force
Basic Flexures: Cross-Strip Rotational Hinge 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 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: Force-Displacement Relations
If the force is not applied at the midpoint, the flexure would tilt as it translates
Basic Flexures: Cruciform Flexure Provides limited rotation in very
confined spaces
Basic Flexures: and Tapered Uniform-Stress Cantilever Flexure 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