This can be accomplished with a sensor to monitor
the coarseoutput level and control which error signal
is used. The crossover level is set within 90° or less
of the finespeed null(approximately 3° in a 1:32 sys-
tem) to prevent hang ups at false nulls on the fine
output. Since the fine CT turns n times for one turn
of the coarse CT there will be n points where its out-
put will be at null, therefore, the fine error signal is
only used when it is within 90° of the true null. Or
more correctly when the coarse null is within (90°/n).
If Figure 1.20 is examined it can be seen that for both
even and odd ratios a fine and coarse stable null
exists together only at 0° (or 360°). A stable null is
defined as the point where the error signals pass
through zero in the positive direction. If a servo is set
up to drive towards 0° misalignment it will drive away
from 180° because the phaseof the error signal for a
displacement will be opposite than that at 0°. An
unstable null will exist at 180° but the slightest dis-
turbance will cause it to drive to the correct null.
If the coarse/fine ratio is even, the unstable coarse
nullat 180° will be accompanied by a stable finenull.
If the coarse/fineratio is odd, the coarse and fine sig-
nals each have unstable nulls at 180°, and there is
no danger of the system accidentally remaining
aligned at 180°, but if an even ratio system happened
to be at 180° from the true stable null (as can happen
at power turn on or some forms of switched systems)
then the servo would sense the coarse null (even
though an unstable one) and switch to the fine error
signal. Since the fine error signal is at a stable null
the system would “lock in” at 180° from where it
To enable even-ratio systems to function without the
possibility of nulling at 180°, a stick-off voltage is
added to the coarse control signal.
Figure 1.21 shows a simplified circuit for this pur-
pose. When a fixed AC voltage of the correct phase
is added to the coarse signal, and the stator of the
coarse control transformer (or transmitter) is suitably
repositioned, the coarse system can be made to null
at 180° of fine rotation, which is unstable fine null.
The null at 0° is unaffected.
In detail, the coarse control transformer signal/mis-
alignment curve is shifted obliquely so that it still
passes throughout the same nullat 0° but a different
null at 180°. The stick-off voltage shifts the coarse
null horizontally by 90° of fine rotation, and the
coarse synchro offset shifts the error signal by a fur-
ther 90° of fine rotation. At 0° the two 90° changes
cancel; at 180° they add up to prevent a fine stable
null at 180°.
“Stick-off” voltage can be added to any even-ratio,
two-speed system, regardless of the method of
adding the coarseand fine signals. The voltage must
have the same phase as the normal coarse signal to
avoid introducing quadrature components at null. A
two-speed S/D converter is described in detail on
pages 33 through 36. Three-speed, four-speed, and
even more highly articulated synchros are practical,
although they are rarely used. In all multispeed ser-
vos, the accuracy of the gearing must be high
enough to support the added resolution provided by
the fine synchro.
Where mechanical gearing size and/or errors cannot
be tolerated, electrical two-speed synchros can be
used. Electrical two-speed synchros are devices
with two rotor/stator winding sets. The two-speed
EVEN RATIO n = 8
ODD RATIO n = 5
nX (fine) n=EVEN
nX (fine) n=ODD
Figure 1.20.CT Voltage/Misalignment Curves for
Even and Odd Ratios.