PROCESS CONTROL24 February, 1999
2.1.2 POTENTIAL CONTROL
Overview
Potential control is the process of controlling the development potential to maintain
the density of the toner image on the drum. It does this by compensating for
variations in drum chargeability and toner chargeability.
The machine uses the image density (ID) sensor to measure drum reflectivity and
the density of a standard sensor pattern. It uses the potential sensor to detect the
potential on the standard sensor pattern (before the pattern is developed). These
tests are done during the process control self check, which is done at specific times
(such as after replacing the developer).
The ID and potential sensor outputs are used to calculate the development
potential. This is the difference between the development bias voltage and the
voltage of areas of the drum that have been discharged by laser exposure at full
power. If changes in this potential are not accounted for, the color balance will be
poor.
Depending on the development potential that is calculated, the machine uses a
look-up table in memory (called a pointer table) to adjust the following:
? VD: Drum potential without exposure - to adjust this, the machine adjusts the
charge corona grid volG)tage (V
? VL: Drum potential with the strongest exposure - to adjust this, the machine
adjusts the laser diode inpuLDt cur)rent (I
? VB: Development bias
Potential control controls the development potential so that the maximum amount
of toner applied to the drum is kept constant. However, the medium (greyscale)
range is ignored. To improve this situation, a new process called ?process control
gamma correction? is done after potential control. This process defines LD output
for all 256 grades of the greyscale (development bias and charge corona grid
potential are not affected).
Potential Control Timing
The machine carries out potential control and process control gamma correction
during the ?process control self check?. There are five types of process control self
check, categorized according to their execution times. Process control takes
approximately 3 minutes.
(1) Forced process control
After replacing the drum, the technician must do the forced process control
procedure (SP3-126). There is no need to use SP3-126 at installation, because
forced process control is included in the developer initialization process (SP2-225).
(See 6.6.3 Developer Collection Procedure for details.)
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