Flexures for Optics

Outline Brief overviews of micro flexures Focus on macro flexures in this tutorial Beam bending Symmetry -> precision Degree of freedom (DOF) Applications

Micro Flexures Comb drive Tip-tilt mirrors discrete vs analog

Optical MEMS devices Analog tip-tilt mirror Resonant frequency of the comb drive depends on the ions hitting the pads

Motivation Need nanometer precision to manipulate light. “Stage” and “driving mechanism”. Sticktion is a problem encountered with screw-type driving mechanisms. Use piezoelectric, capacitive, magnetic, photon,… to drive the “stage”.

Precision Mechanics

Macro Flexures – 1D

Symmetry in 2D In-plane rotation Parasitic motion not di-coupled As soon as the stage moved, Fx developed some “local” y component In-plane rotation minimized Parasitic motion reduced or cancelled Less cross-talk

Parallelogram In-plane rotation constrained Parasitic motion reduced As soon as the stage moved, Fx developed some “local” y component In-plane rotation constrained Parasitic motion further reduced or cancelled Less cross-talk

Deformation Diagram X/Y forces + X/Y moments

5 DOF – Pentaflex Combination of vertical and horizontal blades X/Y/Z translation + X/Y rotation

Highly Symmetric XY Stages Three different anchoring geometries Can be made into XYZ stages by adding the horizontal blades like Pentaflex

Diaphragm Flexures Provide out-of-plane (z, ,  ) motions Constrain the other in-plane (x,y,  ) motions (Voice-coil, pressure sensor, flow control, MEMS devices)

6-axis (nano) Flexures  HexFlex

6-axis Flexures - examples

 Flexures Only allows  DOF, all others conflict.

Tip-tilt Flexures Remove axial misalignment between two parts (shear), but does not remove torque/moment.

 flexure -> 5 DOF

In-plane 1D Flexure Out-of-plane 1D flexure In-plane 1D flexure Symmetric dual 4-bar linkage eliminates  Y errror

Uniform Shaft Loading

XYZ Translation Stage Conflict for all  DOF’s

Bi-stable Flexure Actuation force causes deflection Open/close a valve at some pressure threshold; on/off Have negative stiffness in the unstable region

Non-linear Spring Constant Shape -> deflection -> variable stiffness

Piezoelectric Amplifier

Physik Instrument Piezoelectric drive + capacitive sensor, feedback loop to actively take out platform vibrations

Conclusion Use flexure to avoid sticksion. Use symmetry to cancel/de-couple motions. In-plane vs out-of-plane configurations Flexures for translation, rotation, and any combination of DOF (1-6 DOF). Dynamic range and linearity. Soft flexure -> low resonant frequency, stiff flexure -> high actuation force. References: see FlexureForOptics.doc