An Overview of Digital Printing Systems - Control of Color Imaging Systems: Analysis and Design - page 11

In the EP process, as the electrostatic image is developed with a charged
pigmented thermoplastic powder that is transferred and fused to paper under heat
and pressure, mass of the toner particles on the paper can vary. The control functions
should maintain the mass by adjusting the electrostatic charge, development eld,

and transfer currents. Although dependency of various parameters to output quality
was reasonably well understood by developers, no closed-loop control was applied
to the early copiers and printers (the Xerox

914, 813, and 2400 copiers) to adjust
these control actuators since the sensors were not reliable. In 1970, automatic density
control (ADC) sensors were introduced with Xerox 4000 duplicators and subse-
quently used in various forms in the 5600 and 9200 families [8]. Using the ADC
sensors, somewhat frequent manual adjustments were made to the toner control
system. Since selenium alloy PRs were used, the system was fairly stable. No
separate charge control was required in those printers.
The use of different PRs and the demand for improved copy quality led to the
need for better controls in the 1980s. At that time, copy quality tune up was required
every 50k prints and process quality drifted due to environmental conditions. A
two-patch control system was developed in the 1980s that controlled the toner
concentration (TC), electrostatic charge, and hence the developability on the PR.
Low- and high-density patches were created as surrogates to customer images on the
PR. Low-density patches were measured by the sensor, and the data were used in a
single-input single-output (SISO) closed-loop con

'

guration to control the electro-
static charge. Similarly, a high-density patch was used to adjust the TC in the
developer housing at a lower rate, which subsequently improved developability.
Although high-density patch measurements are sensitive to electrostatic charge and
TC, the coupling was removed by running the electrostatic charge control loop at a
much faster rate.
The presence of a two-patch control scheme in the Xerox 1075, 1090, and 4050
copiers helped in reducing service calls for background and density variation by
more than a factor of 10 and resulted in lower subsystem costs. Due to architectural
constraints and unstable charge on the PR, Xerox developed a low cost charge-
measuring sensor called an electrostatic voltmeter (ESV). A reduced-cost version of
the infra red density (IRD) sensor was also developed. The charge control was done
using a separate ESV sensor followed by the developability control with the low cost
IRD sensor, which appeared in the 1065 marathon copiers in 1987. This control
strategy was subsequently adopted in various forms in the 5090, the DocuTech 135
(1990), 4135 (1991), 5390 (1993), and the DocuPrint 4635 (1994). The same sensors
were also used with more advanced software in the 5100 (1991). In addition to
runtime controls, these sensors helped to accurately set up the process and perform
good diagnostics to reduce cost. With runtime controls, the perceptible page-to-page
differences were minimized. In many accounts, automated setups changed PR replace-
ments from a 45 min service call to a 10 min customer operation, which contributed to
increased customer satisfaction rating.
For color printers, toner mass has to be tightly regulated so that the printer maps
the desired tone to the actual output. This is achieved by creating and implementing
inverse maps in a one-dimensional (1-D) coordinate space, called a tone reproduction
curve (TRC), at various stages in the printer path. At the least, a midpoint tone