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PRINTED PECS GALVO'S
Fig. 5. Schematic circuit of servoamplifiers.
manually controlled attenuators used for setting the operating range of the automatic system and for any compensation for emulsion-to-emulsion differences in the print stock.
The comparison between photocell output and reference voltage is made by means of an electromagnetic chopper. This chopper switches the input to a three-stage amplifier from the photocell output to the reference voltage 480 times per sec. Thus, any unbalance between the photocell output voltage and reference voltage is transformed into a 480-cycle signal. The amplifier output is synchronously rectified by a second chopper.
The servomotors are heavy-duty d'Arsonval galvanometer movements originally designed for graphic recording purposes. The coils are mounted on ball bearings and all restoring torque is removed. The pivot shaft on one end of the coil was extended and the vane mounted directly on this shaft.
Potentiometer P2 is used for balancing out any voltage generated by the photocell dark current. Potentiometer Pa adjusts the gain of the amplifier, i.e., the stiffness of the servoloop. It is
adjusted for good response without overshoot. An Antihunt network is included between the photocell load resistor and the chopper. This materially improves the transient response of the servosystem. Constants are adjusted for best performance of each amplifier.
The coil of the memory relay is connected in parallel with the solenoid which advances the exposure control tape. Thus, the 0.22-juf condenser connected to the relay contact holds its charge and prevents the servosystem from attempting to follow the operation of the attenuator relays while the control tape is being advanced. After the tape has come to rest in its new position, the memory relay closes and the condenser assumes the new voltage established by the relay-controlled attentuator.
In Fig. 6 is shown the circuit diagram of the power supply. The high-voltage supply is of the choke-input type so that a 480-cycle signal can be picked off the high-voltage rectifier by means of a circuit tuned to the fourth harmonic of the ripple frequency. This signal is amplified and used to drive the six choppers. Potentiometers across the 6.3-v heater circuits and across the 12-v, 480
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November 1952 Journal of the SMPTE Vol. 59