Electrophotography – Control of electrophotography process – Of plural processes
Reexamination Certificate
1999-08-12
2001-03-06
Brase, Sandra (Department: 2852)
Electrophotography
Control of electrophotography process
Of plural processes
C399S046000
Reexamination Certificate
active
06198886
ABSTRACT:
This invention relates to electrophotographic printing and more specifically to a process for hybrid scavengeless development wherein selectively adjustable process parameters are monitored and adjusted for closed loop feedback control of the development process to maintain a tone reproduction curve in the marking process that enhances the effectiveness and range of distinguishable densities in a printing device. More specifically, the invention relates to the control of process parameters affecting a toner powder cloud height disposed intermediate a donor roll and a photoreceptor for engendering a substantially linear response in the transfer function between the spectrum of input signals and printed output densities.
In the well-known process of electrophotographic printing, a charge retentive surface, typically known as a photoreceptor, is electrostatically charged, and then exposed to a light pattern of an original image to selectively discharge the surface in accordance therewith. The resulting pattern of charged and discharged areas on the photoreceptor form an electrostatic charge pattern, known as a latent image, conforming to the original image. The latent image is developed by contacting it with a finely divided electrostatically attractable powder known as “toner”. Toner is held on the image areas by the electrostatic charge on the photoreceptor surface. Thus, a toner image is produced in conformity with a light image of the original being reproduced. The toner image may then be transferred to a substrate or support member such as paper, and the image affixed thereto to form a permanent record of the image to be reproduced. Subsequent to development, excess toner left on the charge retentive surface is cleaned from the surface. The process is useful for light lens copying from an original document or for printing electronically generated or stored originals such as with a raster output scanner (ROS), where a charged surface may be imagewise discharged in a variety of ways.
In such electrophotographic printing, the step of conveying toner to the latent image on the photoreceptor is known as “development”. The object of effective development of a latent image on the photoreceptor is to convey toner particles to the latent image at a controlled rate so that the toner particles effectively adhere electrostatically to the charged areas on the latent image. A commonly used technique for development is the use of a two-component developer material, which comprises, in addition to the toner particles which are intended to adhere to the photoreceptor, a quantity of magnetic carrier beads. The toner particles adhere triboelectrically to the relatively large carrier beads, which are typically made of steel. When the developer material is placed in a magnetic field, the carrier beads with toner particles thereon form what is known as a magnetic brush, wherein the carrier beads form relatively long chains which resemble the fibers of a brush. This magnetic brush is typically created by means of a “developer roll”. The developer roll is typically in the form of a cylindrical sleeve rotating around a fixed assembly of permanent magnets. The carrier beads form chains extending from the surface of the developer roll, and the toner particles are electrostatically attracted to the chains of carrier beads. When the magnetic brush is introduced into a development zone adjacent the electrostatic latent image on the photoreceptor, the electrostatic charge on the photoreceptor will cause the toner particles to be pulled off the carrier beads and onto the photoreceptor.
An important variation to the general principle of development is the concept of “scavengeless” development. In a scavengeless development system, toner is detached from a donor roll by applying an AC electric field to self-spaced electrode structures, commonly in the form of wires positioned in the nip between a donor roll and photoreceptor. This forms a toner powder cloud adjacent thereto. Because there is no physical contact between the development apparatus and the photoreceptor, scavengeless development is useful for devices in which different types of toner are supplied onto the same photoreceptor such as in “tri-level”, “recharge, expose and develop”, “highlight”, or “image on image” color xerography.
In hybrid scavengeless development, “hybrid” refers to the combining of concepts from single component development (applying toner to the latent image using a donor roll loaded with toner) and from two component development (applying toner to a surface from a mixture of toner and carrier).
With all development systems it is desirable to identify a control parameter for closed loop feedback control of area development. The height of the toner powder cloud is one such characteristic parameter which can be affected by a variety of related process parameters such as electrode wire AC and its amplitude, electrode wire AC frequency, electrode wire DC offset relative to the donor roll, the donor roll AC bias relative to the photoreceptor, the toner Q/M and the donor roll photoreceptor gap. Other parameters could also affect the toner powder cloud. From an alternative perspective, these same parameters represent variables which affect the tone reproduction curve of the development process. For color applications, the form of the tone reproduction curve (TRC) is extremely important to maintain for successful printing. Although it is known that the TRC of the printing device can be corrected with image processing, the development response is ideally maintained in a reasonable range so that the printer can utilize all available input gray levels (usually 256) to output distinguishable densities for each of the half-tone dot levels. Where a TRC is configured so that all the gray levels do not generate distinguishable output densities, the printing device lacks the ability to maximize a printing spectrum of useful and available output half-tone dot levels.
With particular reference to
FIGS. 1A and 1B
exemplary TRCs are shown which can better illustrate these concerns. Each curve relates to a particular half-tone design. The “area coverage input (%)” relates to the input signal with respect to a number of pixels which are intended to be turned on from the cell so that at 100 percent area coverage, i.e,. all pixels are turned on, the maximum density output is shown representing a fully printed solid patch. In the exemplary Figures, the “density output” range from 0 to 1.5 is a function of the measurement instrumentation used in generating these curves. Assuming that 0 percent area coverage is defined as input signal number 255 and 100 percent area coverage is defined by a signal number
0
, it can be appreciated that 256 different input signals are possible. An optimal system would provide 256 visually equally spaced and distinguishable density outputs. In pictorial printing, and in particular, color pictorial printing, visually distinguishable density outputs for each possible input signal level are highly desired for higher quality printing.
FIG. 1B
shows the TRC which due to its substantial nonlinearity, lacks the desired distinguishment between density outputs for the range of possible inputs. More particularly, it can be seen that nearly all of the possible range of density output is associated with only one-half of the available area coverage input range. If area coverage input percentage between 50 percent and 100 percent were associated with one-half of the available 256 input levels, i.e., only 128 of the input signals, then almost all the distinguishable density outputs are generated with merely one-half the input signals. Between 0 and 50 percent area coverage input would be associated with density outputs of between 0 and approximately 0.2. More dramatically, for only one-half of the TRC, 50 percent of the input signal levels control over 80 percent of the range of density outputs, while the other half of the input signal levels control less than 20 percent of the range of density output. For monochrome text printing, w
Brase Sandra
Fay Sharpe Fagan Minnich & McKee LLP
Xerox Corporation
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