Internal Process Controls - Control of Color Imaging Systems: Analysis and Design - page 470

9 Internal Process

Controls

9.1 INTRODUCTION
The output of digital printers drifts over time or deviates from the predetermined
optimum standards due to a variety of factors. These factors include environmental
conditions (temperature, relative humidity [RH], etc.), use patterns, the type of
media, variations in media, aging of the components, variations from original models
used in initialization, general wear, etc. To achieve predictable print quality time
after time, important internal parameters (states of the machine) are controlled by
applying feedback [1]. These loops maintain background, solid area development,
and tone reproduction curves (TRCs) of the individual primaries by adjusting various
internal process and image actuators that operate at varying frequency while making
prints.
Many earlier color products only used solid area control for low- and high-
density regions of the tone curve (e.g., Xerox product, 5775). This corresponds to
one-point TRC control without any hierarchy. The control algorithms for set-point
tracking were done through

rules; for example, when certain conditions are
met, dispense the toner, otherwise not. Control loops were single-input single-output
(SISO) although they were implemented on a coupled multiple-input multiple-output
(MIMO) system. On some occasions, proportional, integral control was used in the
SISO control loop. Today

on - off

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s digital production or entry production systems have
become more complex and require a multivariable, modular design approach to
implement stable feedback systems that can deal with a wide range of process
parameters and ensure the accuracy, consistency, and stability of the internal states
of the system.

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time hierarchy is preferred for simplifying the implementation
complexity of such a system.
In this chapter, theoretical methods are developed for analyzing the control
performance of processes within the xerographic printing system. First, we describe
how the system is represented in a system theoretic form, often called a state-space
model. These models are used to design a time hierarchical control system for
controlling all the processes that affect the images printed on paper. After that,
design of the key process control loops and their effects on the

a

final print quality
are described in detail. Unique strategies and methods to overcome the adverse
effects of loop interactions and avoid undesirable glitches in the controller perform-
ance are also presented.

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