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by C. Fisher, D. Braun and J. Kaluzny of Jet Propulsion
Laboratories (JPL) - California Institute of Technology and T.
Haran of New Scale Technologies
Presented at the 2009 IEEE Aerospace Conference 2009,
paper #1185
- download paper
The Wide-Field Multi Object Spectrometer (WFMOS) is a ground
based astronomical instrument that is scheduled to be
commissioned on the Subaru Telescope on Mauna Kea, Hawaii in
2013. The WFMOS, along with corrective optics, will mount in
place of the secondary mirror of the telescope. An array of
fiber positioners that feed light from a 1.5 degree field of
the sky to a visible spectrometer for red shift observations
of 2400 cosmological targets simultaneously. The light is
transferred to the spectrograph using 2400 f/2.4 fibers with
107µm cores. This enables, for the first time, large scale
Galactic Archeology and Dark Energy surveys to help unlock
the secrets of the universe.
The key enabler of this new capability is an array of 2400
Cobra optic fiber positioners made from very small rotary
motors: custom
rotary piezoelectric motors developed specifically for
this purpose by New Scale Technologies.
Overview
Cobra is a two-degree-of-freedom mechanism that can position
an optical fiber in the prime focus of the telescope to a
precision of 5 µm. It is a theta-phi style positioner containing
two rotary piezo SQUIGGLE motors with one offset from the other,
which enables the optic fiber to be placed anywhere in a small
circular patrol region. The patrol diameter of the actuator is
large enough to obtain 100% sky coverage of the close packed hex
array pattern of positioners. The name Cobra was chosen because
the positioner resembles a snake ready to strike.
To achieve the fiber tip positioning goal the motors must:
- Be able to make very small steps.
- Provide adequate torque to overcome the twisting of the
optical fiber and friction within the positioner.
- Have small diameter to enable a close-packed array
pattern.
The overall height of the Cobra positioner is not
constrained, but efforts were made to keep Cobra as short as
possible to reduce the effects of gravity sag.
Custom rotary piezoelectric motors using New Scale
Technologies’ patented SQUIGGLE micro motor technology were
demonstrated as the best solution to meet the requirements of
size, torque, resolution, and cost. A variety of other
positioner concepts were explored but not prototyped, as the
rotary SQUIGGLE motors performed exceptionally well.
Working with JPL’s design requirements, New Scale optimized
the miniature rotary motor for high torque and precision. The
custom rotary motor created for the Cobra features torque >3.0
mN m and step size <0.065°. Each Cobra fiber positioner will
include one 4.4 x 4.4 mm and one 2.5 x 2.5 mm motor in a
two-stage theta-phi configuration. The WFMOS will have 2400
Cobra positioners in a close-packed hex array pattern on 8-mm
centers.
The Cobra positioner was tested in a lab environment in a
manner that simulates its use on the Subaru telescope. The
positioner was controlled in open loop and used a CCD camera to
image its optical fiber to determine its location. Over 100
simulated cosmological targets were tested using the Cobra
positioner and showed that the Cobra positioner can converge on
over 90% of its targets within 5µm in six open loop move
iterations.
The next phase of the project will include a high fidelity
design and test cycle. Characterization of performance versus
lifetime at an environment that represents Mauna Kea will be
conducted.
Learn more
- Download the paper
(4Mb PDF)
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- Contact New Scale
New Scale Technologies’ engineering team works with OEM
customers to integrate SQUIGGLE motor
systems into your next-generation medical systems and devices designs.
Contact us today.
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The Cobra is a theta-phi style positioner containing two rotary
piezo SQUIGGLE micro motors developed for this purpose.
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