Radar System Engineering

SEC. 13.5] ELECTROMECHANICAL REPEATERS 491
either by a system of cams and microswitches which disables the repeater
when it is in any but the correct 36° sector, or in more elaborate ways.
Standard-design low-impedance synchros are used. Transmitter
and repeater are identical except for the inclusion of a mechanical damper
on the shaft of the repeater. The maximum speed of rotation of the
synchros is about 400 rpm, restricting a 10-speed system to 40 rpm.
The principal disadvantage of using synchros to provide torque is
that power must be transmitted by the primary s.vnchro. This limits
the number of repeaters which can be used. Further, in systems with
more than one repeater, an error in any tends to throw the system off
balance and to affect the accuracy of other repeaters.
The ,%rvomechanism.-Scanner position can be repeated at a distance
with an angular error as small as desired by means of a servomechanism.
>
Antenna
Compass
u
Repeater shaft
~ output
Amplifier
l!mq y~ -
Transmitter
Repeater I
Differential
Gears ---
mq
Motor
AC llne -
FIG, 139.-Servo system including compass-dri~-en differential,
This term refers to a device that is arranged to reduce to zero an “error
signal” present ~vhen misalignment between transmitter and repeater
exists. Synchros can be used to generate such an error signal. The
stators of the synchros are connected in parallel as before, with, perhaps,
a differential synchro inserted bet!veen them (Fig. 13.9) to allow compass
or other corrections. Only the transmitter rotor is excited from the line.
If the two rotors are oriented at right angles to one another, no voltage
is induced on the receiver rotor. .&ny departure from this orientation
results in an error signal ~vhose internal phase is opposite for errors of
opposite sign. This error signal is amplified by vacuum-tube circuits
and used to drive the motor that turns the load. Proper phasing is determined
by reference to the line voltage, usually by using the latter to
provide field current for the motor.
As in the double-synchro direct-drive system, ‘ [gearing up” is often
used. Here, ho!~ever, the proper rotational phase is usuall~- selected by