New measurement and control technology in tunnelling - Tunnel Engineering: Basics and Additional Services for Design and Construction - page 246

ment of such a gyroscopic device and to a relative deflection, which has to be considered
or corrected. Free gyroscopes are not capable of determining their absolute direction.
Important applications of gyroscopic devices are characterised by the limitation of their
movement. If the mounting only permits rotation about one more axis in addition to the
spinning rotation, then forces act on the gyroscope when it is moved. The gyroscope then
attempts to orient its axis of rotation in the direction of the axis of the external rotation.
Figure 6-1 A free gyro-
scope mounted in gimbals
(left) and the principle of
the mounting of a gyro-
compass (right).
Rate gyro. When a gyroscope is mounted to permit rotation about the horizontal axis, the
force applied by the gyroscope is proportional to the rate of turn of the carrier and can be
measured. Such an instrument is called a rate gyro or turn indicator; it does not display the
magnitude of a turn but the rate of turn, for example of a vehicle.
Gyrocompass. A gyroscope, which also rotates about a horizontal axis, but with a second
degree of freedom about the vertical axis, is subjected to an external torque by the rotation
of the Earth, which forces the axis of the gyroscope to point to true north. This type of de-
vice is called a gyrocompass or meridian pointer (Fig. 6-2 right). A gyrocompass attempts
to orient its axis of rotation as far as possible parallel to the axis of the external rotation,
which is the Earth's axis, so its direction of rotation agrees with that of the Earth (Fig. 6-2).
Figure 6-2 A free gyro-
scope (left) maintains its
location in space; the rota-
tion of the Earth changes
its location relative to the
surface of the earth. The
north-seeking gyrocom-
pass (right) orients itself so
that its axis of rotation is a
parallel as possible to that
of the Earth, and both ro-
tate in the same direction.
After settling, it maintains
its location relative to the
surface of the earth.