New measurement and control technology in tunnelling - Tunnel Engineering: Basics and Additional Services for Design and Construction - page 248

gyroscopes are fitted with elastic supports, as a short-term outside jolt then only causes a
short-term disturbance moment. On a fixed support, on the other hand, disturbance can take
the form of rotational speeds, which would lead to much more serious errors. The reduction
of errors is a filter problem, which can be approached mechanically through the provision of
a suitable support or electrically through eddy current damping [72].
Ship's gyrocompass. The gyrocompass used in ships is also described as a gyrocompass.
This is indeed a form of meridian gyroscope, but far more elaborate. Since a ship's com-
pass has to cope with vibrations and oscillations with very variable periods, the time taken
to settle is designed to be very long, actually 84.4 min, since this minimises the errors due
to the Earth's rotation. Numerous other features (supported by floating in a fluid, the use
of two acutely angled rotors, suitable matching of all influential parameters), the distur-
bances are sufficiently minimised.
In a ship's gyrocompass are two rotors connected to each other by springs at an angle of about
90° (Fig. 6-4). This arrangement suppresses the errors that occur during the rotation of the
system about the vertical axis (streaming error). The two rotors are installed in the rotor ball,
as shown in Fig. 6-4. The rotor ball floats in an electrically conductive fluid (mixture of water,
glycerine and benzoic acid) in the housing ball, with the floating uplift compensating the effec-
tive weight to a few grams. The remainder of the necessary uplift is provided by the so-called
blowout coil: Alternating current in the bottom of the housing ball induces phase-delayed cur-
rent on the surface of the rotor ball, which achieves a repelling effect. The rotor ball thus floats
without contact in the housing ball. The power supply to the rotors is though the support fluid.
In order to avoid friction when the rotor ball turns, the housing ball is turned with it electromag-
netically when it turns, even through a very small angle. The location of the housing ball is thus
identical with that of the rotor ball and serves to show the direction of north.
Figure 6-4 Arrangement of two rotors in a ship's gyrocompass.
The hinged lever ensures that the rotor caps can only rotate in
opposite directions about the vertical axis relative to the rotor
ball. The direction of the resulting angular momentum, which
is the sum of the angular momentums of the two rotors, is thus
fixed in the rotor ball [72].
The long settling time of a ship's gyrocompass would be a great disadvantage for the
control of machinery. It is however the only north-pointing gyroscope system, which also
works while in motion and its application in tunnelling is practical where continuous slow
movements have to be controlled (pipe jacking, shield tunnelling).