Printing System Models - Control of Color Imaging Systems: Analysis and Design - page 556

10 Printing System Models

10.1 INTRODUCTION
The xerographic printing process is a unique discipline that incorporates domain-
speci

c ideas from physical sciences to engineering. Developing a complete model
of the printer is very dif cult and can limit its use for analysis purposes. We therefore

present a nonlinear model of the printing system with reasonable abstractions of key
elements of the digital color printing processes and yet retain the simplicity neces-
sary to study the effects of control techniques on each of these processes. We

rst
develop the underlying physics of the processes used for developing a colored dot
that is fundamental to the creation of digital images for the color CMYK process. The
process models are then cascaded in sequence where the output of one process model
becomes the input to the next one. The dot spread is then modeled using a halftoning
strategy and modulation transfer functions of the key segments of the electrophoto-
graphic process (EP). These models are used for designing feedback controllers for
each of the major subsystems, such as controllers for generating multidimensional
pro les, to understand their interactions, to manage the complexity of the system

through careful design of control loops, and to achieve the overall system objective.
We present the transformations that an electronic image goes through, before being
reproduced on paper using analytical abstractions of the printing processes.

10.2 PROCESS MODELS
There are at least six fundamental steps to monochrome (B W) digital EP printing.

Color digital printing carried out in several different architectures [1,2] involves
different combinations of these steps. The photoconductor

=

is charged in the

charging station, and the electrostatic latent image is formed on the photoconductor

'' ''

in the station. Then, in the next step, the latent image is rendered into a real

visible image on the photoconductor in the

exposure

'' ''

station using electro-
statically charged toner cloud. The developed latent image is transferred to the
media in a

development

'' ''

station. The transferred image is fused to the media by heat
and mechanical pressure in the

transfer

'' ''

fusing station. The residual toner on the photo-

'' ''

conductor is removed in the station. While each of the six stations of EP

process is critical to the proper functioning of the monochrome printing, we ignore the
cleaning station for our modeling purposes. The remaining

cleaning

'' ''

five steps are modeled as
nonlinear lumped localized transfer functions (LTF models) with actuators as key
inputs, and sensed or measured parameters as key outputs. The LTF models capture
only the local aspect of the color

The spatial aspects (i.e., dot growth,
dot spread, edge enhancement) of these processes are captured using modulation
transfer function (MTF) models.

'' dot printer. ''

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