New measurement and control technology in tunnelling - Tunnel Engineering: Basics and Additional Services for Design and Construction - page 247

There are two types of meridian pointer:
- The tape-suspended meridian gyroscope is normally installed in gyrotheodolites. In-
stead of being mounted in a gimbal, the spinning rotor is suspended on a very thin tape
(Fig. 6-3 left).
- The north-seeking gyroscope is mounted in one gimbal in air bearings, which results
in an elastic fixing to the vertical as with a tape-suspended meridian gyroscope (Fig.
6-3 right).
The spinning rotor oscillates as it seeks north, and this has to be damped. The energy of
oscillation can be removed by delayed operation of the upper tape clamp; this can also be
done manually. The north-seeking gyroscope is normally fitted with electric damping. The
method of damping is also determined by the method of reading the north direction. The
following processes can be used:
- Determination of the location of the settled gyroscope.
- Calculation of the centre point of the oscillations.
- The gyroscope is held in position electrically and the compensating north-seeking mo-
ment is measured. This is proportional to the deviation from north.
All meridian gyroscopes can only determine the north direction if set up in a fixed position
and kept free of vibration. In addition to the drift, which is partly due to imperfect agree-
ment of the rotational axis of the rotor with the axis of inertia of the suspension frame, the
influence of the mass of the gimbals and bearing friction, there are a range of mechanisms
by which vibration could lead to reading deviations. Even slight vibration from machinery,
vehicles or general seismic activity can disturb readings. The cause is that vibration can
produce erroneous torques, which can exceed and disturb the north-seeking moment.
The tape-suspended meridian gyroscope reacts particularly sensitively to horizontal trans-
lations, but rotations about the vertical are not a problem. The north-seeking gyroscope,
on the other hand, does not react much to translations but is very sensitive to rotations.
Figure 6-3 Tape-suspend-
ed meridian gyroscope
(left) and north-seeking
meridian gyroscope with
air bearings [72] (right).
In general, vibration causes more disturbance, the lower its frequency, and this should always
be considered for installation. Disturbance torques normally oscillate at higher frequencies
than the north-seeking moments. In order to avoid disturbance from vibration, north-seeking