Inertial Sensors and Their Applications - Signal Processing Systems - page 69

1.2 Gyroscopes

Gyroscope (or gyro for short) is a device for measuring or maintaining angular
orientation. It can measure turn rates caused by changes in attitude with respect
to inertial space. Historically the first sensors of this kind were mechanical gyros.
They exploit the inertial properties of a wheel spinning at high speed, which tends
to keep the direction of its spin axis due to the principles of conservation of
angular momentum. Although the axle orientation does not remain fixed, it changes
in response to an external torque much less and in a different direction than it
would without the large angular momentum associated with the disc's high rate
of spin and moment of inertia. Since external torque is minimized by mounting the
device in gimbals, its orientation remains nearly fixed, regardless of any motion
of the platform on which it is mounted. There are several designs for mechanical
gyros including: dynamically tuned gyroscope (DTG), flex gyro, and dual-axis rate
transducer (DART) which is suitable only for low accuracy applications [ 29 ] .
Following the development of spinning mass gyros, other kinds of angular rate
sensors, such as optical and vibrating gyros, were developed [ 3 ] . These sensors are
based on different physical principles than the conservation of angular momentum.
Optical gyros are based on the Sagnac effect which causes a phase shift between
two waves counter-propagating in a ring interferometer that is rotating; the shift is
proportional to the rate of rotation. Vibrating gyros are based on Coriolis effect that
induces a coupling between two resonant modes of a mechanical resonator. Optical
gyros can be effectively implemented using different integrated optics technologies
that generally fall into two categories: (a) ring laser gyroscopes (RLG) and (b)
fiber optics gyroscopes (FOG). RLGs can be made very accurate to meet the
requirements for navigation grade, but on the other hand, they are expensive, their
size increases with performance, and they are high-voltage devices. FOGs are less
accurate compared to RLGs, but they meet the requirements of medium accuracy
(tactical grade), medium cost gyroscopes.
Vibrating gyros are usually manufactured using MEMS technology [ 8 , Chap.
2.06]. From the accuracy point of view, MEMS gyros are of low to medium accuracy
with their performance approaching FOG. They have low manufacturing costs,
small physical size, and low power consumption; moreover, they can survive severe
shocks and temperature changes. Therefore, MEMS technology is ideally suited for
mass production.

1.3 Areas of Application

Due to the universal presence of motion, vibration, and shocks, inertial sensors can
be applied almost everywhere, from aircraft and space navigation to underground
drilling, from hard disk fall protection to airbags in vehicles, and from video
games to performance improvement of athletes. The large variety of applications