HP Color LaserJet CM4730 Service Manual - Page 201

The DC controller controls the color misregistration unit under the following conditions: ● At power on or door close after cartridge replacement ● After job completion in every specified number of printings ● At a command from the formatter The following is the operational sequence of the color misregistration corrective control: 1. The DC controller forms two pattern images for color misregistration detection with four colors on the ETB when the color misregistration corrective control starts. The two pattern images are the same pattern, one formed on the left side and the other on the right side. 2. The DC controller measures the position of each pattern image and calculates the misregistration amount of each color by controlling the color misregistration unit. 3. The calculated misregistration information of each color is sent to the formatter. 4. The formatter controls each color's video signals according to this information and optimizes the main-scanning start position, the main-scanning scale, and the sub-scanning start position. The color misregistration in sub-scanning direction is corrected at printer engine side by the DC controller controlling the speed of the scanner motor. NOTE: Horizontal magnification is the size of the image in the horizontal direction. Every color has its own photosensitive drum in each cartridge in this MFP. The positions of each drum differ and the laser beam lengths vary because of the variations among the cartridges, so the horizontal scanning area depends on the color. This causes a color misregistration at the edge of image. Color misregistration detection In this MFP, the positions of each color's misregistration pattern images formed on the ETB are detected to measure the misregistration amount. The DC controller controls the color misregistration unit during the foregoing color misregistration corrective control. The color misregistration unit is placed under the ETB and consists of two detection sensors, each one having a light emitter (LED) and a light receiver (PD). The following is the operational sequence of the color misregistration detection. 1. The DC controller allows the LED to emit light by sending the LED DRIVE signal when the color misregistration detection starts. 2. The ETB reflects the light from the LED and the PD in the color misregistration sensor receives the light. This reflected light intensity differs depending on whether the light is reflected from the area of toner. 3. The light receiver converts the received light intensity into voltage and sends it in the form of the color misregistration signal to the DC controller. 4. The DC controller detects the positions of the pattern images according to the timing of the color misregistration signal. Figure 4-31 Color misregistration detection on page 182 illustrates this process. ENWW Image formation system 181