Color Registration Control - Xerox 6250DT - Phaser Color Laser Printer Service Manual

depending on the Transfer Roller surface condition or dirty condition of CTD (ADC)
sensor surface. The light density emitted from the LED is controlled so that the
reflected light density meets the specified value.
This control is implemented in two ways:
■
Sets the light density so that the light reflected satisfies a specific value.
■
Adjusts the subsequent light density to be within a specific tolerance value.
1. Light Density Setting
The reflected light density can vary greatly if the transfer roller has been replaced or
the CTD (ADC) sensor has been cleaned. To deal with this variance the light density
is reset when the power is turned on or the front cover is opened and closed.
The intensity of the LED increases gradually and the set value is adjusted when the
output of the CTD (ADC) sensor does not meet the specified value. If the output of
the CTD (ADC) sensor does not reach the specified value, even though the light
density is increased to the maximum limit, the controller signals the sensor is dirty. If
the sensor output is extremely high, the controller will signal that the sensor is faulty.
2. Light Density Adjustment
At the execution of automatic density control, the light density adjustment is made
just before the patches for toner density control are generated.
Light is emitted from the LED using the current light density setting, to check if the
output value of the CTD (ADC) sensor is within the specified range. If the output
value is low, light density is increased. If the output value is high, the light density is
decreased.
If the output value is less than the first lower limit, the controller signals the sensor as
dirty and generates a warning. If the output value is less than the second lower limit,
the controler signals that the sensor is faulty and stops the printing.

Color Registration Control

The printer uses a quad system where drums and developers are used exclusively for
each of the four colors (yellow, magenta, cyan, and black). Images are formed on the
drums, in the respective colors, and then layered to form one image. To avoid a
positional shift between the different color images, the color registration control
calculates how much the registration has shifted by comparing each of the other color
patches in the registration string to the black bar that precedes it. Shift is corrected by
adjusting the laser write timing to compensate.
Color registration control is made depending on the internal temperature and print
count at the execution of process control.
Theory of Operation 2-9