system and circuit design for a capacitive mems gyroscope - Aaltodoc

1.2 Angular Velocity Sensors 3
1.2 Angular Velocity Sensors
Angular velocity sensors are devices which are used to measure the rotation rate of
a body with respect to an inertial frame of reference. Their application areas include
navigation, automotive stability control systems and safety systems, platform stabiliza-
tion, including picture stabilization in camcorders and cameras, robotics, and various
input devices.
Traditionally, angular velocity has been measured with a rotating wheel gyroscope,
which is based on the conservation of angular momentum. This has been replaced by
fiber optic and ring laser gyroscopes in precision applications 2 . Optical gyroscopes are
the most accurate angular velocity sensors available at the moment, and are used, for
instance, in inertial navigation systems. However, they are too expensive and too bulky
for most of the aforementioned applications. [7]
The automotive applications of angular velocity sensors include chassis stability
control systems such as ESC (Electronic Stability Control), and safety systems such as
roll-over detection. In particular, the stability control systems have created a rapidly
growing market for low- to medium-priced, medium-accuracy angular velocity sensors
which can be realized with the MEMS technology [8, 9].
As described earlier, a very rapidly growing MEMS angular velocity sensor appli-
cation area is currently that of consumer applications. These include picture stabiliza-
tion in cameras and camcorders, GPS (Global Positioning System)-assisted navigation
and dead reckoning, and various input devices. An example of the latter application
is game controllers, which are equipped with angular velocity sensors. This makes
it possible to control a computer game by just turning the game controller in various
directions. Another possible example is a mobile handset in which the user interface
can be controlled by rotating the device.
The MEMS technology offers the possibility of miniaturizing the angular velocity
sensors. At the same time, as a mass fabrication technology, it makes it possible to
reduce the price of a single unit, provided that the manufacturing quantities are large
enough. Both of these properties can be considered very advantageous, especially
when the requirements of consumer electronics are considered.
Almost all the micromechanical angular velocity sensing elements reported in the
literature have been vibratory gyroscopes [7]. The reason for this is that, unlike rotating
wheel gyroscopes, they do not require bearings, thus providing much simpler design
and greater long-term reliability. A vibratory gyroscope is composed of two ideally
decoupled resonators. When the gyroscope rotates about its sensitive axis, the Coriolis
2 Strictly speaking, the term gyroscope refers to the traditional rotating wheel device, whereas the term
angular velocity sensor refers to angular rate measurement devices in general. However, they are (mis)used
interchangeably in the literature, and the convention will be followed here as well.