Major Areas of Opportunity - Implications of Emerging Micro-and Nanotechnologies - page 69

Sunlight

Cross Section:

Slit (90 microns x 11 mm)

8 mm

Fused Silica
Opaque Coating

Digital
Single -Axis
Sun Sensor: Detector Dice

12 mm

90 o FOV
0.35 o
resolution

Detector Plane:

Comparators and Encoders

Power

12 mm

Data Output

Ground

Detectors

FIGURE 3-10 Micromachined Sun sensor. SOURCE: Helvajian, H., and S.W. Janson.
1999. Microengineering space systems. Pp. 29-72 in Microengineering Space Systems.
H. Helvajian, ed. El Segundo, Calif.: The Aerospace Press. © The Aerospace Corpora-
tion, used by permission.

Inertial Sensors. Spacecraft IMUs use micromachined accelerometers, 113,114 gy-
roscopes, and other angular sensors to monitor their own forces and track their
position and orientation. Gyroscopes are used to monitor spacecraft attitude, with
optical sensors for absolute calibration. These gyroscopes and angular rate sen-
sors are typically based on a rotating mass, a vibrating fork, or the circulation of
light around a closed loop. Micromachined angle rate sensors monitor Coriolis
forces, which are proportional to the angular rotation rate.
Inertial sensors consist primarily of accelerometers and gyroscopes. The
current trend is to make these in silicon using MEMS, which allows dramatic
reductions in weight, size, and power while also improving robustness. MEMS
accelerometers are commonly used as impact sensors in automotive airbag sys-
tems. 115 New silicon micromachined microgyroscopes will soon be available. In
addition, an even newer angle rate sensor design that uses quantum tunneling for
the transduction mechanism is being tested. 116 This design may allow yet smaller
(hence cheaper and lower power) angle rate sensors to be produced. Smaller
angle rate sensors with accuracies similar to fiber-optic gyroscopes would allow
more precise control of smaller vehicles, including arrays of nanosatellites, mis-
siles, and UAVs.
When the IMU loses track of its star or the Sun, it must search rather broadly
to regain lock. The addition of a very fast GPS could reduce the search area
dramatically, providing a much faster lock. However, for reasons of weight and