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 TM 55-1905-220-14-11
5-117. SHIP'S COURSE INDICATOR-MAINTENANCE INSTRUCTIONS (Continued).
would again both equal zero (figure A). Thus, the coarse synchro system provides two null points in
a complete cycle. Regarding the coarse control transformer (1XCT), its null at the 1800 point is an
unstable null, because if the shaft were on either side of that point, by an infinitesimal angle, the
servo would drive toward the correct null 180away. The fine synchro has 72 null positions, or 36
times as many as the coarse synchro system. If only the fine control transformer (36XCT) were
connected in the system, there would be 36 positions of the transmitter shaft that would produce a
stable null error voltage. Only one of these 36 positions is desired; that position being the point
where the 1XCT also provides a stable null.
3
The mixing network switches into the servo amplifier the fine error signal when the error is small
(output of the coarse synchro is small), and introduces to the amplifier the coarse error signal when
the error is large (output of the coarse synchro is large). The coarse error signal can be small enough
at the 180point to result in the fine error signal being fed into the servo, through the action of the
mixing network. If only the 1X error voltage were applied at the 180point, the servo would drive
away from this false null. But, because the 36X voltage has control, it drives the servo toward this
180null point. The 36X error voltage negative between 175and 180tends to drive the servo to
an increased angle (180; where positive between 180and 185it tends to drive the servo to a
)
decrease angle (180 - the same point. In other words, if this condition were tolerated, the servo
)
would lock in at a false null.
FIGURE A
5-1309
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