Page 206 - MEMS Mechanical Sensors

8.3 Micromachined Gyroscopes 195

z-axis (orthogonal to flat plane of the chip). When the accelerometer system is sub-
jected to accelerations with components parallel to the sensitive axis of the g-cell,
the center plate moves relative to the outer stationary plates, causing two shifts in
capacitance, one for each outer plate, proportional to the magnitude of force
applied. The shifts in capacitance are then processed by the CMOS ASIC, which
determines the acceleration of the system (using switched capacitor techniques),
conditions and filters the signal, and returns a ratiometric high voltage output.
Many companies offer commercial bulk-micromachined accelerometers. For
example, the Swiss company Colibrys produces high-performance sensors suitable
for inertial guidance and navigation. The MS7000 and MS8000 devices (available
from ±1G to ±100G) are their most recent and advanced range. Their devices
excel, having high stability, low noise, low temperature drift, and high shock toler-
ance. The typical long-term stability is less than 0.1% of the full-scale dynamic
range, the bias temperature coefficient is less than 200 mG/°C, and the scale factor
temperature coefficient is less than 200 ppm/°C. They use, contrary to Analog
Devices, a hybrid approach, where the sensing element and the interface electronics
are implemented on separate chips but packaged in a common, standard TO8 or
LCC housing. The sensing element together with the ASIC is shown in Figure 8.20.
Table 8.3 gives an overview of a range of companies producing micromachined
accelerometers with their most important features.

8.3 Micromachined Gyroscopes

8.3.1 Principle of Operation
Virtually all micromachined gyroscopes rely on a mechanical structure that is driven
into resonance and excites a secondary oscillation in either the same structure or in a
second one, due to the Coriolis force. The amplitude of this secondary oscillation is
directly proportional to the angular rate signal to be measured. The Coriolis force is
a virtual force that depends on the inertial frame of the observer. Imagine a person
on a spinning disk, rolling a ball radially away from himself, with a velocity υ . The
r
person in the rotating frame will observe a curved trajectory of the ball. This is due
to the Coriolis acceleration that gives rise to a Coriolis force acting perpendicularly
to the radial component of the velocity vector of the ball. A way of explaining the
origin of this acceleration is to think of the current angular velocity of the ball on its
way from the center of the disk to its edge, as shown in Figure 8.21. The angular

Figure 8.20 Commercial bulk-micromachined accelerometer from Colibrys.