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ENCODER Operation
Illustration - detail of encoder wheel.
Illustration - detail of encoder signals.
8–38
The ENCODER monitors the speed and position of the MAGNET in its rotation to
ensure extreme smoothness and constant velocity. The ENCODER is made up
of:
• ENCODER WHEEL, which is a clear disk that has been printed with 5000
lines leading from the center to the outer edge.
• The 2 OPTICAL SENSORS, which are placed at the edge of the ENCODER
WHEEL so that the beam of each is interrupted by the printed lines as the
WHEEL rotates.
The sensors are placed so that their signals are nominally 90 degrees out of
phase with each other in a "quadrature relationship": One detector looks at the
edge of a line while the other looks in the middle of the line.
The speed of the SLOW SCAN MOTOR is determined by measuring the
frequency of the signals. The direction of the rotation is determined by the
sequence in which the signals change.
The feedback from the 2 sensors is constantly monitored by the A6 SLOW SCAN
CONTROLLER BD, and the current to the COIL BD is reduced or increased to
ensure smooth rotation and constant velocity.
Each rotation of the LEAD SCREW equals 20,000 encoder counts. The system
drives to each of the soft positions (start of scan, erase, and end of scan) until it
reaches the correct number of counts.
12/12/94 – 1234563
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