Xerox 6250N Service Manual - Page 46

Bias control operates as follows: 1. The Temperature/Humidity Sensor sets the target values of the drum charging voltage and biasing DC voltage. 2. Bias control test patches (see the adjacent figure for patch configuration) of each of the four toner colors (yellow, magenta, cyan, and black) are generated and transferred to the Transfer Roller. About 11mm Y About 12mm 3. The CTD (ADC) Sensor compares an area on the Transfer Roller where no toner is present with the test patches to determine toner density for each of the four colors. 4. The density measured in Step 3 is compared with the target value set in Step 1 and the resulting difference is used to adjust the drum charging voltage and the biasing DC voltage for each toner color. About 3mm M About 57mm C K 6250-069 Toner Density Control Toner density must be kept constant to attain a stable print image. To achieve this goal, the rate at which toner is dispensed must be adjusted to exactly match the rate at which it is consumed. There are two systems that work in conjunction to control toner density: the Pixel Count Dispense Control (PCDC) and the Toner Density Control (CTD). (The CTD uses data from the ADC sensor, measured as described in the preceding paragraphs covering bias control.) PCDC: The amount of toner consumed in the developing process is calculated by counting the digital pulses applied to the Laser Unit. The toner motor is then driven for a period determined by the calculated result to supply toner to the developer, equivalent to the amount dispensed. CTD (ADC): The toner test patches of each color (yellow, magenta, cyan, and black), are generated and transferred to the transfer roller as specified in the Bias Control section. The CTD (ADC) sensor measuers the density of each patch. The printer adjusts the toner dispense time, which changes the quantity of toner to be dispensed. This calculation is made separately for each color. The CTD adjustment is made following completion of printing if either of the following conditions is satisfied: ■ Cumulative print count since power on exceeds 16 pages. ■ When a cleaning cycle is executed during continuous printing. If either of the preceding control calculations results in an adjustment to the toner quantity to be dispensed, the result is calculated in terms of number of revolutions of the Toner Motor. For this description, this value is called the dispense count. The dispense count calculated as specified above, is implemented over the next 8 prints. If 16 dispense counts are required as a result of low toner density during the CTD calculation, 2 dispense counts are added during each of the next 8 prints. This is in addition to any counts added or subtracted during each print as a result of PCDC. Theory of Operation 2-7