Antistickoff

Servo Amplifier

diodes, connected back to back, act as nonlinear

resistances. When a high voltage appears across the

The servo amplifier is a 10-watt plug-in amplifier

diodes, it appears as a low resistance or a short circuit.

with a push-pull output stage that feeds the semomotor

When a low voltage appears across the diodes, it appears

control winding. The servo amplifier drives the servo-

as a high resistance or an open circuit.

motor, which, in turn, repositions the control

When control transformer error voltages are small,

transformer rotors to null the error voltage to the servo

diodes CR12A, 13A, 14, and 15 act as a high resistance

amplifier. The amplifier has an internal power supply

and block the low speed (1X) signal from the servo

operating from 115 volts ac. It provides 12 volts dc and

amplifier. Diodes CR 16 and 17 act as a high resistance

unfiltered 40 volts dc for the amplifier stages. In

and allow the lightspeed (36X) signal to pass to the

addition, the power supply power transformer supplies

servo amplifier.

reference voltage for the servomotor and antistickoff

voltage.

When the error voltages are high, diodes CR12A,

13A, 14, and 15 act as a low resistance and pass the

The amplifiers for 60- and 400-Hz units are similar

low-speed signal to the servo amplifier. Diodes CR16

except for the power transformers and capacitors.

and 17 act as a low resistance and short the high-speed

Gear train oscillation, or hunting, is caused by over-

signal before it reaches the servo amplifier. Resistors

shoot as the servo reaches its null. To prevent this,

R12, 13, and 14 are current-limiting resistors.

clamping circuits introduce a stabilizing voltage at the

amplifier input. This stabilizing voltage is proportional

Antistickoff

to acceleration or deceleration of the unit.

Alarm Circuit

The low-speed control transformer output winding

connects in series with a 2.7-volt winding of the power

The alarm circuits in the synchro signal amplifier

transformer. This small, constant voltage (called the

monitor the 60- and 400-Hz output excitation, servo

antistickoff voltage) is added to the output voltage of the

excitation, and follow-up error. With all power sources

low-speed control transformer. It, in effect, shifts the

present and a follow-up error of less than 2 1/2, the four

angular position of the control transformer null, or

indicator lights on the access door will light. If one of

position of zero output. The antistickoff voltage is either

these conditions fails, the appropriate light will go out,

in phase or 180 out of phase with the low-speed control

indicating the problem area, and an alarm will sound.

transformer output.

With the equipment normally energized and the

If the high- and low-speed control transformers

alarm switch in the ON position, the alarm circuit will

were set to electrical zero at the same position, there

be open. A loss of any of the three power sources, a

would be a point at 0 and 180 where the error voltage

follow-up error of more than 2 1/2, or putting the alarm

would equal zero. Within 2 1/2 of the 180 point, the

switch in the OFF position will close the alarm circuit,

36-speed error signal would drive the servomotor to

causing an alarm to sound.

synchronize the control transformers at the 180 point.

The control transformers would also synchronize at the

180 point if the synchro signal amplifier were

Gear Train

energized when the control transformers were within

2 1/2 of the 180 point.

The gear train consists of a series of tine pitch,

precision, spur gears. They link together the rotors of the

To remove the chance of synchronization of the

two control transformers, four output synchros, and the

control transformers at the 180 point, the low-speed

servomotor.

control transformer is rotated 2 1/2 from corres-

pondence with the high-speed control transformer null,

MAINTENANCE OF SYNCHRO

or zero, position. An antistickoff voltage of constant

SIGNAL AMPLIFIERS

magnitude and phase is added to the single-speed

control transformer output. The resultant voltage is now

The synchro signal amplifier should require little

zero at the 185 point instead of the 180 point. At either

attention in service, there being few parts inside the

side of the 185 point, both the 36-speed and

amplifier unit or the synchros that need lubrication or

single-speed voltage tend to drive the synchro

replacement under normal operating conditions.

transmitters toward true zero.

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