HP Color LaserJet CM4730 Service Manual - Page 204

The DC controller controls the following biases to prevent image defects when it detects an environmental change: ● Developing bias ● Primary transfer bias ● Secondary transfer bias If the environment sensor detects a temperature of below -30ºC (22ºF) or over 80ºC (176ºF), the DC controller determines this is an "environment sensor abnormality" and sends an error message to the formatter. Image density calibration control (DMAX) This control stabilizes image density by calibrating the values of the developing bias whenever one of the following events occurs: ● The detected temperature of the sub thermistor is below 50ºC (122ºF) when the MFP is turned on. ● The door is closed after a print cartridge or ETB is replaced. ● A set number of pages is printed. ● The specified value of environmental conditions has changed since the last image density corrective control. ● The formatter sends an operation command. Image halftone calibration control (DHALF) This control measures halftone density output from the formatter and returns the measurements to the formatter so it can perform halftone calibration. Image density calibration must always be performed before image halftone calibration. Image halftone calibration control consists of three steps: 1. The DC controller forms density patterns on the photosensitive drum in each color using the optimum developing bias determined during image density calibration. 2. These density patterns are transferred to the ETB, and the color-registration detection unit measures the image density of these patterns. Image data is returned to the formatter. 3. The formatter uses the image data to perform a halftone calibration to obtain an ideal halftone image. Image density detection Each of the image stabilization controls uses the color registration unit at the top of the ETB to measure image density. The image density detection is performed by the DC controller, which controls the color misregistration unit during the D-half control. The color misregistration unit is placed under the ETB and consists of two detection sensors, each having one light emitter (LED) and one light receiver (PD). Figure 4-33 Image density detection on page 185 illustrates this process. 184 Chapter 4 Theory of operation ENWW