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 TM 55-1905-219-14-11
5-117. SHIP'S COURSE INDICATOR - MAINTENANCE INSTRUCTIONS (Continued).
2
Inherent inaccuracies in the synchro result in shaft errors. Although small, these errors may be excessive
for some applications and are, therefore, reduced by means of two-speed data system. In a two-speed SCI
one data channel rotates 36 times (36X) while the other channel rotates once (1X) for one complete turn of
the ship. Inside the SCI the two channels are accurately geared together in the ratio of 36:1. The 1X
channel provides coarse dial position to near correspondence with the compass 1X transmitter. The 36X
channel provides fine dial position to accurate correspondence with the compass 36X transmitter. Overall
error is appreciably less in a two-speed indicator than in a one-speed indicator built with similar
components.
3
Two-speed synchro signals feed two control transformers, the outputs of which are mixed and applied to a
servo amplifier as shown below. In other respects a two-speed indicator is similar to a one-speed indicator.
4
So that the dial of a two-speed indicator can be readily read to the full accuracy capability of the servo
system, a mechanically driven vernier dial is provided in the SCI. The vernier dial rotates by a gear drive
from the main dial, the gearing rotating the vernier six times faster than the 1X dial. The vernier dial is
graduated in divisions that correspond to degrees and half degrees with interpolations of tenths of a degree
possible within 0.05 degree.
(b)
Control transformer.
1
The control transformer converts electrical data into rotational position. The stator of a control transformer
receives, from an external synchro transmitter, a voltage that depends on the angular position of the
transmitter rotor. A three-section synchro capacitor connected in parallel with the stator windings provides
power factor correction. When the rotor of the control transformer is in position alignment with the compass
transmitter rotor, no voltage in induced in the control transformer rotor. If the control transformer rotor
differs from this
5-1305
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