DEVELOPMENT 20 June 2005
6-66
Toner density control
There are two modes for controlling and maintaining constant toner supply: sensor
control (both direct and indirect) and image pixel count control. The mode can be
changed with SP2-208-1 (Toner Supply Mode).
NOTE: The factory setting is sensor control mode; image pixel count mode should
only be used temporarily until a defective TD or ID sensor can be replaced.
Sensor Control Mode
In the sensor control mode, the amount of toner required to print the page is
calculated by the CPU; it adds up the image data value of each pixel and converts
the sum to a value between 0 and 255. (255 would mean a completely black page.)
The machine must vary toner supply for each copy in order to maintain the correct
amount of toner in the developer and to account for changes in drum reflectivity
due to changes in temperature and humidity. The CPU uses data from the TD
sensor and ID sensor to determine whether or not the toner supply motor should be
switched on and to calculate how long it should remain on in order to supply more
toner to the mixture in the development unit.
TD Sensor. When new developer of standard toner concentration is installed,
namely 20 g of toner per 500 g of developer (4.0% by weight), the TD sensor must
be set to its initial setting of 4.0V with SP2-801. This initial setting is used as the
toner supply reference voltage or Vref. For every copy cycle, the TD sensor directly
checks the toner density in the developer mixture, and after 10 copies these 10
readings are averaged and this value becomes TD sensor output voltage Vt(10).
The machine compares Vt(10) with Vref. If Vt(10) is greater than Vref, the toner
concentration in the development unit judged to be low. When Vt(10) is detected to
be greater than Vref 20 times, then this indicates that the toner concentration is
consistently low, Vref is incremented by 0.1V, and the conditions are checked
again. The result of this check determines the value of K, the toner supply rate
coefficient, which is one of the factors that is used in the toner supply motor on-
time calculation.
ID Sensor. In addition to comparing Vt(10) from the TD sensor and Vref, after
every 10 copies the ID sensor, located at the lower right area of the drum, checks
both the reflectivity (Vsg) and the pattern on the drum (Vsp), created by the laser
diodes and charge roller. If the reflected light is too strong, this indicates that toner
is low and toner is added to the development unit. (The frequency of these checks
can be adjusted with SP2-210 (ID Sensor Pattern Interval).
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