Page 217 - MEMS Mechanical Sensors

206 Inertial Sensors

of the inner frame. The resulting signal is amplified and demodulated to produce the
rate signal output.
Other commercial gyroscopes are in the final stages of their commercialization
from companies such as Samsung and Sensonor.

8.4 Future Inertial Micromachined Sensors

It is believed that in future years, the major innovation will come from multiaxis sen-
sors, both for linear and angular motion. As described above, three-axis accelerome-
ters using a single proof mass have been presented already as prototypes, but a
commercial version has not yet been implemented. As an ultimate goal, a single sen-
sor capable of measuring linear and angular motion for six degrees of freedom is
envisaged. Such a sensor can be fully integrated with the control and interface elec-
tronics on the same chip.
One interesting approach is to use a mechanical structure similar to the one
shown in Figure 8.16. Watanabe et al. [71] report a five-axis capacitive motion sen-
sor. Linear acceleration is sensed in the same way as described in the paper by
Mineta et al. [44]: Out-of-plane acceleration causes the proof mass to move along
the z-axis, and in-plane acceleration along either the x-or y-axes makes the proof
mass tilt. Additionally, the proof mass is vibrated along the z-axis with electrostatic
forces. Angular motion about the x- or y-axes induces a Coriolis-based tilting oscil-
lation of the proof mass. The oscillatory signals are of much higher frequency (about
2 kHz) as the signals caused by linear acceleration, and hence, they can be separated
easily in the frequency domain using electronic filters. In this way linear acceleration
and angular rate signals can be measured concurrently.
Another very promising approach towards such a sensor is to use a
micromachined disk that is levitated by electrostatic or magnetic forces and spun
about its main axes. This is similar to macroscopic flywheel type gyroscopes; how-
ever, the lack of a good bearing in the microworld has excluded this approach so far
for micromachined gyroscopes. Using a levitated object alleviates this problem. Any
angular motion perpendicular to the spin axis of the disk will cause it to recess, and
this can be detected by a capacitive position measurement to provide a measure of
the angular velocity. Using a levitated object for inertial sensing has several advan-
tages. First, since there is no mechanical connection from the substrate to the disk,
the effective spring constant is solely dependent on the electrostatic forces set up by
voltages or currents applied to surrounding electrodes; hence, the characteristics of
the sensor, such as bandwidth and sensitivity, can be adjusted on-line, according to
the application requirements. Second, when used as a gyroscope, quadrature error is
inherently ruled out. The comparable effect, due to the imbalance of the mass, will
manifest itself at the rotation frequency, whereas the Coriolis force will cause the
disk to recess at the rotational speed of the body of interest. These two frequencies
are several orders of magnitude apart and are easy to separate. Furthermore, there is
no need to tune the drive and sense resonant frequencies since the scale factor does
not depend on the matching of different modal frequencies. Linear acceleration
along the three axes can be measured simultaneously by measuring the displacement
of the disk.