Brother MFC620CN Service Manual page 98

The outer circumference of the PF roller gear L functions as a 300 dpi (0.084 mm pitch) encoder
disk which the PF encoder sensor uses to generate a signal indicating the gear rotation speed--in
other words, the paper feed roller speed--to the controller for use in controlling paper feed position
and speed. This signal from the PF encoder is a 2-phase one, for a maximum position detection
resolution 300 dpi four times or 1200 dpi.
Paper stop position control
The controller uses the PF encoder signal for proportional, integration, differential (PID) control of
the paper feed motor to produce high-resolution precision paper positioning during printing. This
signal also determines the motor parameters for rapidly and precisely positioning the paper past a
gap.
Fixed-speed paper feed
This operation rotates the paper feed roller to feed the paper at a constant speed regardless of load
fluctuations. The primary application is ejecting paper when printing is complete--in other words,
in situations where precision stop position control is not needed.
Paper feeding amount control
There is variation in the head nozzle pitch of individual head/carriage units, as well as in the shape
of the paper feed and ejection rollers. To make those parts match each other, therefore, updating the
paper feeding correction value and correcting the positioning error of the head/carriage unit are
necessary (see Chapter 6).
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