Biomedical Engineering Reference
In-Depth Information
in servo systems to supply velocity or damping signals and may be mounted on or in the
same housing as a servo motor.
There are two basic types of tachogenerators—AC and DC—both of which develop
an output voltage proportional to rotational speed. In DC generators the polarity depends
on the direction of rotation, whereas in AC generators the relationship between phases
changes.
Performance specifications for these devices include generated voltage, accuracy,
maximum speed, and ripple. Generated voltage is measured in volts (V) per revolution per
minute (rpm) over a range of speeds. Accuracy or linearity is the deviation of the voltage
output signal expressed as a percentage, and ripple is determined as a percentage of the
output voltage.
2.4.8.3 Rate Gyros
In many applications, angular rate must be measured with reference to an inertial frame
rather than relative to some physical object. Rate gyroscopes, commonly called rate gyros,
use the conservation of angular momentum to keep one or more inertial axes pointed in
one direction as the external frame translates and rotates. An output voltage is produced
that is proportional to the rate of rotation of an axis perpendicular to the axis of the
gyro.
Many high-accuracy rate gyros still consist of a spinning rotor mounted in a single
gimbal, as shown in Figure 2-41. A gyro mounted in this manner has 1 degree of freedom;
that is, it is free to tilt in only one direction. The rotor in a rate gyro is generally restrained
FIGURE 2-41
Rate gyro with a
single degree of
freedom. [Adapted
from Neets,
Electrical
Engineering Training
Series
http://www.tpub.
com/content/
neets/14187/.]
