Canon imageCLASS MF8450c, Service Manual

Page: 126 / 349

Chapter 7
7-11
7.2.4 Color Displacement Corrective Control
0019-5022
imageCLASS MF9170c / imageCLASS MF9150c / imageCLASS MF8450c
This control corrects color displacement which occurs due to individual difference of the laser scanner unit and toner cartridge.
Since this machine uses a different toner cartridge for each color, a difference may occur in the position or the size of an image depending on the color.
The machine performs correction by calculating the degree of color displacement based on the position of the pattern image for color displacement correction
formed on the ETB and controlling a video signal for each color.
The DC controller executes this control according to the command sent from the main controller.
Corrective control is performed for the following three points.
- Main scanning start position
- Main scanning magnification *
- Sub scanning start position
* This is an image size ratio in the main scanning direction. Since there is individual difference in the toner cartridge for each color, the position of the
photosensitive drum and the laser optical path length vary depending on the color. Since the range of an image in the scanning direction also varies
depending on the color, color displacement occurs at the edge of an image.
The color displacement corrective control is executed at the following timing.
- At power-on after the replacement of the toner cartridge
- When the front cover is closed after the replacement of the toner cartridge
- When a specified number of prints are completed
- When the execution command is received from the main controller
Explanation of operation
1) The DC controller forms a pair of pattern images for color displacement correction (one is on the left side and the other is on the right side) with four colors on
the ETB.
2) The DC controller sends a color displacement/density LED drive signal so that the light emitter (LED) of the color displacement/density sensor emits light.
The color displacement/density sensor is a reflection type sensor located on the upper side of the ETB. It consists of the light emitter (LED) and light receiver
(PD).
3) The light emitted from the emitter is reflected at the ETB and received by the light receiver of the color displacement/density sensor.
The intensity of this reflected light varies depending on whether toner exists or not.
4) The color displacement/density sensor converts the received light into a voltage, and sends it to the DC controller as a color displacement detection signal.
5) The DC controller determines the position of the pattern image based on the timing when the color displacement detection signal changes, and calculates the
degree of color displacement for each color based on the position.
6) The degree of color displacement calculated for each color is sent to the main controller.