Transcript Mechanical Design Improvement in the Optical Coherence

Mechanical Design
Improvement in the Optical
Coherence Tomography
Scanner
Joshua Rudawitz
Mentor: Professor Kerr-Jia Lu
Overview
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Introduction to Optical Coherence
Tomography
The current limitations of existing
technology
Mechanical Solutions
• Laser vector analysis
• MEMS and Compliant Mechanisms
• Topology I and II
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Pseudo Rigid Body Model
Summary of current results
Optical Coherence Tomography
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This technology uses optical imaging to
produce high-resolution (10µm or less)
cross sectional images of tissue.
Optical Coherence Tomography (OCT) is
similar to ultrasound in that pulses of
infrared light are sent out and the echo is
translated into an image based on
interferometry.
These images are able to show differences
in tissue, such as cancers, and therefore
offers an alternative to conventional
cancer detection (biopsy).
Optical Coherence Tomography
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Current technology uses
the natural frequency of
the structure to provide
the amplification of the
mirror, with the use of a
bimorph actuator.
Due to the scalability of
this technology it is being
used in endoscopes for
non-invasive cancer
detection procedures.
Optical Coherence Tomography
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Limitations
• Large scan angles have been achieved,
however they still provide a limited
range of sight.
• There needed to focus the scan angle to
achieve both a front and side view to be
able to see more of the desired area.
• With regards to the bladder, without the
side view it is difficult to scan areas
close to the entrance of the endoscope.
Mechanical Solution
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Goals:
• To design a mechanism that will allow
for both front and side views.
• Include MEMs technology and compliant
mechanisms.
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Manufacturing
Mechanical Amplification
Achieving Front and Side Viewing
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Constructing a
model to predict
the scanning angle
as a function of the
mirror rotation.
Required additional
analysis to ensure
close to straight
line, or a curve
with a large radius
Reflected Light
Mirror
Optics
Optics
Base
Compliant
Mechanism
Gold Plated
Mirror
Vector Analysis of Laser
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Results
• It was found that with just 45 degrees of actuation a
viewing angle of close to 90 degrees can be achieved.
• This range would be from the tip of the endoscope to the
side of the endoscope.
• By rotating the endoscope the entire area could be
scanned.
Curved Path
of Laser
Viewing
Angle
Tip of Endoscope
Tip of Endoscope
Design of Mirror Structure
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Due to a need for a more focused
location of the scan angle the current
mirror configuration was not
sufficient.
A two torsion hinge design was
created.
Optics
Base
Compliant
Mechanism
Torsion Hinges
Gold Plated
Mirror
Topology I
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The initial design was first analyzed using basic force and
geometry equations.
Using ANSYS for non-linear analysis, it proved difficult to
converge the model.
In addition, it showed that due to the lack of stiffness in the
structure, the hinges did not act expectedly.
The structure was acting like a cantilever beam
Mirror
Torsion
Hinges
Torsion Hinges
Topology II
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This topology is based on compliant
mechanisms theory of a Pseudo-RigidBody Model
This relates a cantilever beam model to a
rigid link with a torsional spring
Mirror
Pseudo Rigid Hinge
Pseudo Rigid Body Model
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This allows for a good design approximation of
large non-linear deflection.
The approximation is based on modeling the
beam (hinge) as a pin joint with a torsion spring.
The strain energy stored in a bending beam can
be simulated using a torsion spring.
P
Torsion Spring
P
Pseudo Rigid Body Model
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Assuming the loading case of Force and
Moment in the Same Direction
It can be modeled as an initially curved
beam, with a moment that correlates to
the initial radius of curvature.
Pseudo Rigid Body Model Analysis
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By using the pseudo rigid body
model, it is then possible to predict
the path of the end point of the
hinge and the tangent angle.
The tangent angle is important
because that will define the
orientation of the mirror after the
beam has been bent.
Pseudo Rigid Body Model Analysis
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Results for two positions of the hinge were
found by using MATLAB.
• Where hinge geometry was the design
variable.
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These two positions correspond to the
required motion of the mirror to achieve
the viewing angle of 90°.
Using the included stress equations it was
determined that the material would hold
up to this range of motion and further
positions can easily be modeled through
the use of the MATLAB files created.
Current Result
Mirror
Pseudo Rigid Hinge
Summary
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A suitable viewing angle was found to achieve the desired
goal of front and side viewing, done using vector analysis.
The vector analysis of the optics showed that with 45° of
rotation a 90° viewing angle is achieved, close to a straight
line.
A pseudo rigid body model was created to approximate
non-linear structural response.
Pseudo rigid body results show that the hinge can go
through the required motion without structural failure.
The model developed can be used to design the remaining
hinges to provide the desired mirror motion.
Continuing research on:
• Actuators to provide the needed range of motion.
• Compatible frequencies with current data acquisition
programs.
• Building scale models for testing.
Acknowledgments
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Professor Kerr-Jia Lu
Professor Jason M. Zara
The George Washington UniversityInstitute for Biomedical Engineering
References
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5.
P. Patterson, P.M. Mills, J. Zara. “Amplified Bimorph
Scanning Mirror For Optical Coherence Tomography.”
S. Kota. “Design of Compliant Mechanisms: Applications
to MEMs.” SPIE Vol. 3673, March 1999
J. Zara, S. Yazdanfar, K.D. Rao, J.A. Izatt, and S.W.
Smith. “Electrostatic Micromachine Scanning Mirror for
Optical Coherence Tomography.” Optics Letters. Vol. 28,
No. 8, April, 15, 2003.
G. Tearney, M. Brezinski, B. Bouma, S. Boppart, C. Pitris,
J.F. Southern, and J.G. Fujimoto. “In vivo Endoscopic
Optical Biopsy with Optical Coherence Tomography.”
Science, New Series, Vol. 276, No. 5321 (Jun. 27, 1997)
Howell, Larry. Compliant Mechanisms. Canada: John
Wiley & Sons, Inc., 2001
Vector Analysis of Laser
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The goals of this step:
• To ensure that by shifting the laser to
the side an acceptable viewing angle is
achieved.
• To confirm that the path was acceptable
for medical purposes.