Page 213 - MEMS and Microstructures in Aerospace Applications

Osiander / MEMS and microstructures in Aerospace applications DK3181_c010 Final Proof page 204 1.9.2005 12:13pm

204 MEMS and Microstructures in Aerospace Applications

progress in these critically important systems has been used to measure, guide,
stabilize, and control the trajectory, attitude, and appendages (i.e., steerable antennas,
solar arrays, robotic arms, and pointable sensors) of Earth-orbiting satellites, inter-
planetary spacecraft and probes, space-based robots, planetary rovers, and related
platforms.
A spacecraft’s GN&C system is critical to executing the typical space mission
operational functions such as orbital insertion, Sun acquisition, Earth acquisition,
science target acquisition, pointing and tracking, orbital or trajectory Delta-V
propulsive maneuvers, as well as the articulation of multiple platform appendages.
No matter what the specific mission applications are, all spacecraft GN&C systems
can be deconstructed into the three basic generic functional elements of an auto-
matic feedback control system:

. Sensors
. Processors
. Actuators

Typically, in conventional spacecraft architectures being implemented today, various
individual attitude sensor units (such as star trackers, Sun sensors, Earth sensors,
horizon crossing sensors, magnetometers, rate gyros, accelerometers, etc.) are phys-
ically mounted at discrete locations on the spacecraft structure and electrically
harnessed to the vehicle’s command and data handling system (C&DH). The attitude
measurement data generated by each individual sensor are sampled, at rates ranging
from 1 to 100 Hz typically, by the spacecraft’s on-board digital flight processor in
which attitude determination algorithms compute an updated vehicle state vector.
Control law algorithms, also resident on this on-board processor, will compute the
necessary attitude control torques (and/or forces) required to achieve the desired
attitude, orbit, or trajectory. Command signal outputs from the processor are then
directed to the appropriate attitude control actuators to generate the commanded
torques or forces on the vehicle. This attitude control is cyclically repeated at rates
ranging from 1 to 10 Hz, or possibly faster if the time constants of the fundamental
dynamics of the vehicle to be controlled are very short and high bandwidth control is
required for stabilization.
In the almost 50 years since Sputnik, the global GN&C engineering community
has established and flight-proven multiple methods for determining and controlling
the orientation of spacecraft. 1–3 A GN&C engineer’s choice between such basic
control techniques as gravity gradient stabilization, spin stabilization, and full
three-axis stabilization will depend primarily on the mission-unique drivers of orbit
(or trajectory), payload pointing stability and accuracy requirements, spacecraft
4
attitude and orbital maneuvering requirements and mission life. Multiple opportun-
ities exist to infuse microelectromechanical systems (MEMS) technology in many of
these attitude control and stabilization techniques, particularly in the areas of ad-
vanced attitude control system sensors and actuators. Advanced MEMS-based pro-
cessors for GN&C applications are also a possibility, but that specific area of MEMS
R&D will not be discussed in this chapter.

© 2006 by Taylor & Francis Group, LLC