Biomedical Engineering Reference
In-Depth Information
FIGURE 2-44
The
Coriolis effect.
If
V
(m/s) is the instantaneous velocity of the vibrating structure of mass
m
(kg),
which is rotating at an angular rate
ω
(rad/s), then the Coriolis force is given by
F
=
2
m
ω
V
(2.33)
As the velocity of the vibrating element is sinusoidal, the Coriolis force will introduce
a lateral vibration at the same frequency. This can be converted to an electrical signal using
a piezoelectric stress/bending mechanism or a variable capacitance.
One implementation of a MEMS tuning fork gyro is shown in Figure 2-45. A proof
mass attached to springs is forced to oscillate in the horizontal plane.
A voltage is applied to a sensing electrode (sense plate) below the proof mass, creating
an electrical field. The Coriolis force imparted by angular rotation causes the proof mass
to oscillate vertically, which in turn changes the gap between the proof mass and the
sense plate, as shown in Figure 2-46. This motion generates an AC current with amplitude
proportional to the rotation rate.
Unfortunately, the bias voltage also generates an electrostatic force between the proof
mass and the sense plate. This force acts to pull the proof mass toward the sense plate.
In addition, since the gyro is a mechanical device, it is sensitive to external stimuli in-
cluding vibration, acoustic excitation, and acceleration due to mechanical shock. In some
applications the acceleration levels at the gyro can be as large as 500 g. These stimuli will
FIGURE 2-45
MEMS rate gyro with
capacitive coupling.
