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March 2007
The piezoelectric SQUIGGLE motor operates continuously from
room temperature to cryogenic temperatures (77 Kelvins and
lower) . This unique feature presents new opportunities to
improve cryogenic sensors, used for applications such as
infrared imaging, thermal imaging and
hyperspectral imaging.
Application overview
Infrared imaging makes use of wavelengths longer than the
visible spectrum. Greater sensitivity is achieved using
cryogenic sensors - detectors that are cooled to temperatures
much lower than the scene to be imaged. Cooling is most
typically achieved using closed-cycle coolers or cryostats, or
dewars filled with liquid nitrogen or liquid helium. The sensor
is mounted on a "cold finger" in the cooler or dewar. A window
allows outside radiation of desired wavelengths to reach the
cooled detector.
Figure 1: Piezoelectric SQUIGGLE motors
position cooled optics in a cryogenic sensor.
Since any room-temperature optics will emit radiation at
wavelengths that are "noise" to the cooled detector, imaging can
be improved by placing optics inside the cooled chamber.
The drawback is the limited ability to align the optics inside
the cooled chamber. The usual process is to align (or rather, to
intentionally "mis-align") the optics at room temperature so
that they will move into alignment as the dewar is cooled to its
final operating temperature. Calculating the mis-alignment -
based on all of the thermal characteristics of the various
materials used in the dewar cold finger, optics and mounts - is
extremely complicated. Therefore the process relies heavily on
trial-and-error.
After initial alignment at room temperature, the sensor is
cooled, the alignment is tested and corrections are noted. Then
the sensor is brought to room temperature, adjustments are made,
and the sensor cooled again. This process is repeated until
proper alignment at operating temperature is achieved.
Since cool-down and warm-up times are measured over many
minutes or even hours, this iterative process is extremely
time-consuming and not practical for many imaging applications.
Lack of options for active optical alignment
Until now, there have been few options for alignment of
cooled optics in cryogenic sensors. A mechanical pass-through
into the cooled chamber creates an unacceptable thermal load.
Few electrical actuators that work at cryogenic temperatures
have sufficient push force and travel distance to move optics.
Conventional motors are not an option: These complex mechanisms
of gears, couplings and shafts require grease to lubricate the
many parts, and grease freezes solid in a cryogenic cooler. Even
if the lubrication requirement could be eliminated, the
challenge of matching the thermal expansion properties of the
complex gear mechanisms would be insurmountable.
SQUIGGLE motor for cryogenic sensors
With its unique ability to operate continuously from room
temperature down to cryogenic temperatures, the piezoelectric
SQUIGGLE motor provides new opportunities to improve cryogenic
sensors for thermal imaging, hyperspectral imaging and similar
applications. The SQUIGGLE motor can be used inside the cryostat
for active alignment of the optics in real time, at their cooled
operating temperature. This eliminates the iterative "cool down
- test - warm up - adjust - repeat" alignment process.
The SQUIGGLE motor is a patented design consisting of
piezoelectric actuators oriented longitudinally along a threaded
tube. A threaded screw is fitted inside the tube. Alternating
electric drive signals to the actuators cause the tube to
vibrate at its ultrasonic resonant frequency. This creates an
orbital vibration which rotates the screw, causing it to move
through the tube. (See
SQUIGGLE motor overview). The linear motion of the screw is
used to push the load; in this case, the optics mount.
In this motor design, thread friction between the tube and
the screw is an essential part of the drive operation. Unlike in
conventional motors, friction is not something to be overcome;
therefore the SQUIGGLE motor needs no grease or lubricants.
Cryogenic versions of the SQUIGGLE motor are assembled using
materials with matched thermal expansion properties. This is
possible due to the motor’s simple construction with very few
parts.
The simple direct-drive mechanism results in highly precise
motion: A step command to the SQUIGGLE motor immediately moves
the threaded shaft, without intervening gears and mechanisms.
The screw mechanism also gives the SQUIGGLE motor a high push
force and long travel, which simple piezo actuators lack. In
addition, the screw is firmly held in its last position when the
power is turned off. This allows users to turn off power to the
motor once the desired alignment is achieved, thereby minimizing
heat load in the cryostat and preserving battery life in
portable sensor applications.
Also noteworthy are the SQUIGGLE motor’s small size and mass:
the motor takes up very little room in the cooler, and does not
add significantly to the cool-down time.
Further reading on cryogenic motors
Researchers B. Sanguinetti and B.T.H. Varcoe at the
University of Sussex have tested the model SQ-110C
SQUIGGLE motor in a cryostat and found it to operate
satisfactorily with a push force of 2 N from 161 K to 9 K (±3
K). Their results were published in the journal
Cryogenics, Volume 46, Issue 9, September 2006, Pages 694-696.
(Free access to Elsevier subscribers, $30 to non-subscribers, or
email us to receive a copy for personal use.)
A custom cryogenic-compatible version of the smaller
SQL-3.4
model SQUIGGLE motor is currently in use for a cryogenic
sensor application. Publication is pending in NASA Tech Briefs’
Motion Control Technology.
Email Fred Haas or call (585) 924-4450 x 112 for more
information.
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