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 TM 55-1905-220-14-11
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.
5-1307
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