Page 225 - MEMS and Microstructures in Aerospace Applications

Osiander / MEMS and microstructures in Aerospace applications DK3181_c010 Final Proof page 216 1.9.2005 12:13pm

216 MEMS and Microstructures in Aerospace Applications

Several MEMS inertial sensor technology developments specifically targeted
for space mission are underway at multiple organizations. MEMS sensors in the
space environment have also undergone limited testing and evaluation. 32 These
developments obviously build upon the solid MEMS technology foundation already
formed within industry for defense and commercial applications. MEMS micro-
systems are currently at a point where their inherent robustness, miniature size, and
low-power and low-mass attributes make them extremely attractive to spacecraft
GN&C designers. Several key issues, however, remain to be resolved before
MEMS inertial sensors will displace the current family of flight-proven gyro and
accelerometer technologies. When one considers the demanding GN&C require-
ments for most space missions it becomes apparent that a MEMS gyro, with
performance and reliability characteristics suitable for guiding the relatively
short-duration flight of a PGM, may not be a realistic alternative. In general,
significant improvements in the standard performance metrics (drift, scale factor,
etc.) of the current generation of MEMS inertial sensors must be accomplished in
tandem with the ability to rigorously demonstrate the reliability specifications for
space flight.
It is encouraging to observe that the majority of industrial inertial system
vendors are either currently offering or actively developing MEMS-based
IMUs. 33 Based upon this trend, and if current R&D investment remains stable or
increases, robust and reliable higher performing space qualified MEMS-based
inertial systems will be commonly available as COTS products within the next 5
to 10 years.

10.4.1 MEMS GYROSCOPES
Gyro inertial sensors are perhaps the most fundamental component of a spacecraft
GN&C system. Gyroscopes or angular rate sensors are used to measure the rotation
angles and rates between the axis system of a moving-body and a fixed body.
Gyroscopes are stabilized by their spin and resultant angular momentum. If applied
torque is zero, then angular momentum is conserved. This means that an undis-
turbed gyro will point in the same direction in inertial space. Hence, a stable
platform is available to reference attitude. It is rare to see a spacecraft GN&C
system that does not include some form of gyro instrument used to provide attitude
and rate measurements for vehicle stabilization and orientation.
MEMS inertial sensors have certainly found a niche in the commercial sector;
solid state silicon gyros are currently being incorporated into automotive antirol-
lover and side airbag deployment systems, used for low-cost attitude heading
reference system (AHRS) avionics for general aviation airplanes, and used for the
stabilization of such platforms as the Segway 1 Human Transporter (HT) (Segway
LLC, Bedford, NH). All signs point to a continued growth in the innovative
application of MEMS inertial sensors for these nonspace product lines.
Inertial sensors have traditionally been classified or grouped as a function of
their performance metrics. The accuracy of a gyro is largely determined by its bias
stability or drift rate, its angle random walk (ARW), and its scale factor stability.

© 2006 by Taylor & Francis Group, LLC