An Overview of Digital Printing Systems - Control of Color Imaging Systems: Analysis and Design - page 16

1.8 PRINT ENGINE-BASED PROCESSING
The print engine and the DFE are responsible for most of the constraints regarding
image quality, color balance, and color stability. These constraints limit both nom-
inal device performance and the ability to achieve that performance repeatedly.
Nominal performance is a function of engineering trade-offs in the design process.
Repeatability is a function of the process control system used in the print engine.
The control algorithms employed to control the process are often customized for
the underlying system architecture to achieve optimum stability results. A generic
implementation is described in Refs. [10

12], which contains time-based hierarchy
which is architecturally named as levels 1, 2, and 3 controls [11] and not associated
with any particular print job. In real-time, controls of this nature run at a much faster
rate and include charge, density (or dot gain), background, and developed tone
reproduction control functions for each of the separations and provide information
for online remote interactive diagnostics. Surrogate patches, placed in the interdocu-
ment zone (IDZ) in between images, are utilized to provide the appropriate feedback
for process control. For example, PR voltages are read using an ESV sensor.
Charge control loop adjusts the PR charge and the intensity of the laser in a level 1
subsystem loop as indicated in Figure 1.7 so that the voltages on the PRs maintain or
track the desired values within a small tolerance (generally less that 1%) to prevent the
appearance of unwanted variations in prints. The amount of toner mass deposited on
the PR is measured at different tone levels using calibrated optical sensors. This
information is then used to control the dot gain and development reproduction curves
by actuating the charging and development system actuators. These are called level 2
control loops. TC, which is the ratio of toner mass to carrier plus toner mass, is
maintained to some desired set point for each of the color station. This is accom-
plished with digital controllers in the developer housing using TC sensors and
actuating the dispense rate. This is perhaps one of the most complex digital SISO
control loops to analyze which comprises of unstable time varying plant with time
delays, actuator saturation, sensor noise, disturbance due to demand for toner usage
(coming from each page of the job), and a variable actuation cycle, as in a typical
inventory management system. In addition to the real-time process adjustments, to
control primary color mixtures for optimum color quality, the number of actuators
required is more than those currently available in levels 1 and 2. One obvious place
to look for more actuators is in the image, since electronically produced color
documents contain pixels that are described in a 3-D

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RGB space. These color pixels

are transformed to corresponding digital values to a printable form, before

being sent to the printer. So, by linearizing the tone levels to each of the input tone
values of the primaries, we can achieve improved controls for all separations
individually [21]. This type of tone adjustment is called level 3 controls and can
be performed on the PR belt or on the paper, depending on the sensing method. For
levels 1, 2, and 3, only individual separations are controlled. Chapter 9 describes
relevant theory and practical controller design based on state variable methods with
pole-placement and regular linear quadratic design. We also show how observers can
be used to compensate for time delays in the TC control system.

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