Mechanical Engineering In Orbit
by David Pescovitz
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Professor
David M. Auslander, also the associate dean for research
and student affairs, recently won the 2003 Eckman Award
from the Instrumentation, Systems, and Automation Society.
The award recognizes outstanding contributions to education
and training in science, engineering, and technology
of instrumentation.
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As much as two-thirds of the Universe is made up of energy that's
a complete mystery to scientists. In 1998, researchers at the Lawrence
Berkeley National Laboratory and their colleagues around the world
reported data strongly suggesting that this so-called dark energy
is the cause of the accelerating expansion of the universe. To prove
it, a team of physicists, astronomers, and engineering, including
UC Berkeley mechanical engineering professor David Auslander, are
designing a satellite that will bring scientists closer to the valuable
data hidden in the depths of space.
Based at LBNL, the Supernova/Acceleration Probe (SNAP) project is
a proposed two-meter reflecting telescope that will orbit high above
the Earth. The telescope's eye will repeatedly take digital images
of 20 square degrees of the sky in a quest for a certain type of
supernova, exploding stars that are key to understanding dark energy.
SNAP has the potential to discover and measure the brightness and
redshift, the increase in the light's wavelength, of 2,000 of these
supernovae each year. That's twenty times more supernovae than were
found in a decade of ground-based research.
The purpose of SNAP is to address the most fundamental cosmological
questions: What is the universe made of? Is it infinite? And will
it last forever?
"This is as fundamental as science gets," Auslander says. "It's
only called dark energy because nobody knows what it is."
This
artistic interpretation depicts a rotating SNAP satellite observing a supernova. (courtesy LBNL) Click on image
to link to animated image.
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In October, the Department of Energy and NASA announced a plan for
a Joint Dark Energy Mission (JDEM) to take place nine to eleven years
from now under NASA project management. The SNAP collaboration must
be invited to bid for the project, a process slated to begin a year
from now. However, the SNAP effort is already underway.
Auslander's integral research is focused on the telescope's attitude
control, a system to ensure that the electronic eye remains trained
on the supernovae. Snapping useful images requires keeping the image
steady on the state-of-the art half-billion-pixel digital camera
in development at LBNL. It's a control systems problem, Auslander
explains, with almost no room for error.
The pointing accuracy of the telescope is based on milli arc seconds,
or 1/1000th of an arc second. (An arc-second is a measure of angle,
equivalent to 1/3600 of a degree.) At those small scales, the satellite
is constantly in motion. Vibration is caused by the mechanical components
inside the satellite. Meanwhile, the sensors that track the satellite's
position also bring noise into the system. With the satellite and
instruments still relegated to the drawing board, Auslander's work
is done through computer models that simulate the dynamics of the
system.
"At this early stage, our purpose is to determine whether we
can keep the instruments stable enough to actually do the science," he
says. "Right now, it appears that it's feasible."
The first step for Auslander and his group of graduate and undergraduate
students was to build a mathematical model of the entire vehicle
as a single rigid body.
"SNAP is an excellent project for students because it gets them involved
with a large working group of professionals," Auslander says. "You
can't do the engineering without seeing a significant part of the
whole scientific picture."
A mathematical model of the system enables the researchers to study
the affects of large-scale vibrations and develop control software
that corrects for the errors. The next step will be to model individual
parts to understand how they vibrate with respect to one another.
"You need to know what's causing the noise and vibration and
how much so you can put all of that information together to get a
best possible
estimate of where the satellite is pointing at a given moment," Auslander
says.
Once the noise and vibratory factors are characterized, the researchers
will begin to develop a control algorithm that will tell the satellite
how to adjust itself. Fly wheels on the satellite provide torque
based on their acceleration. Once the fly wheels reach maximum speed,
gas jets kick in to reset them.
Eventually, Auslander's control software will become a guide for
the commercial vendors who are contracted to build SNAP.
"For the science to work, the telescope must point to the right
place for long enough to get the data," he says. "So it's
nice to be right in the middle of this."
Supernova/Acceleration Probe (SNAP) home page
David
Auslander's home page
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© 2004 UC Regents.
Updated 1/01/04.
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