Developing unit and developing method

A developing unit includes a housing enclosing a mesh belt stretched across first and second rollers. The belt passes between a pair of friction plates that can be used to tribocharge toner particles carries on the mesh belt. Charging occurs over the large width of the entire mesh belt and the friction plates. Toner is transported on the surface of the mesh belt to a developing roller which, in turn, transports the toner to the surface of a photoconductor. The configuration of this developing unit reduces the loading associated with the tribocharging operation and the belt transport system reduces the volume of the developing unit.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to electrophotographic processes and, more 
particularly, to an apparatus and a method for developing images in an 
electrophotographic process. 
2. Description of the Related Art 
Most computer systems, and most particularly small or "personal" computer 
systems, have been making increasing use of colors in the display of 
information. The increasing use of color in personal computers reflects 
both the more ready availability of low price color displays and the more 
powerful and faster microprocessors that are used in personal computers. 
The technology for providing printed color output, i.e., "hard copy" 
output such as color printouts on paper, plastic or other materials, has 
not kept pace with the technology for the color display of information. 
Presently, the options available for color hard copy output either do not 
present sufficiently high quality output or are undesirably expensive for 
home or small office use. Examples of this conventional color output 
technology include ink jet printing, whether using liquid or solid inks, 
as well as a few different implementations of color electrophotographic 
printing. Ink jet printing is comparatively inexpensive, at least when 
using liquid inks, but tends to be slow and it is difficult to obtain 
acceptably high quality output using liquid ink jet printing technology. 
Color electrophotographic printing is commercially available, but tends to 
be expensive and slow. For example, multipass color electrophotographic 
printing is a process by which multiple photoconductor exposures and 
multiple developing processes are used to create a multicolor image on the 
surface of a paper sheet. In essence, conventional multipass color 
electrophotographic printing consists of repeated application of single 
color or monochrome electrophotographic printing processes using different 
colors for each successive application or pass through the multipass 
printer. Traditional monochrome (black and white) electrophotographic 
printing forms an image on the optically active surface of a 
photoconductor by exposing the photoconductor using a laser or an 
equivalent high intensity light source. Before exposure, a uniform charge 
distribution is provided over the surface of the photoconductor and, after 
exposure, a charge pattern corresponding to the exposure image to be 
printed is present on the surface of the photoconductor. The latent image 
corresponding to the charge pattern on the surface of the photoconductor 
is converted to a physical image by a developer which adheres charged 
toner particles to the charge pattern on the photoconductor in a pattern 
corresponding to the latent image. The toner image is transferred onto a 
paper sheet using an electrostatic transfer process and then fusing is 
performed to fix the toner image on the paper sheet. In a multipass color 
laser printer or other similar electrophotographic apparatus, this 
printing process would be repeated several times. 
FIG. 1 illustrates a conventional, multipass color electrophotographic 
apparatus which is assumed to be a laser printer for the purposes of this 
discussion. The multipass laser printer of FIG. 1 includes a 
photoconductive drum 10, a charger 14 for creating a uniform charge 
distribution on the surface of the photoconductive drum 10, a laser beam 
scanning unit 16 for exposing the surface of the photoconductive drum with 
an optical image, a developing device 20 including a plurality of single 
color developing units 22, 24, 26, 28 for developing a latent image, a 
transfer charger 30 for applying a transfer electrical field, and a fuser 
40 for fixing an image onto a recording medium 32 such as a paper sheet. 
The photoconductive drum 10 is generally a metal cylinder covered by an 
optically active material 12 known as the photoconductor. The 
photoconductor generally is highly insulative in the dark while developing 
a substantial level of conductivity under illumination. Thus, the 
photoconductor 12 can hold a charge on its surface in the dark, but charge 
on the surface of the photoconductor is discharged under illumination. 
In operation, a uniform charge is applied to the surface of the 
photoconductor 12 at the beginning of each pass of the multipass color 
printing process. Charging of the photoconductor surface is accomplished 
with charger 14, which typically uses corona charging or a similar 
technique to provide charge to the surface of the photoconductor 12. After 
the charging operation, the photoconductor 12 has on its surface a uniform 
charge distribution corresponding to a voltage of .+-.600.about..+-.800 V. 
When the photoconductor 12 is exposed by the laser beam scanning unit 16, 
a laser beam 18 directed by the scanning unit 16 illuminates a specified 
area of the photoconductor 12 in accordance with an image modulation 
pattern generated by a controller (not shown). The voltage on the portions 
of the photoconductor 12 illuminated by the laser beam 18 is discharged to 
approximately 0.about..+-.150 V. 
Multipass color laser printing is accomplished by successively forming on 
the surface of the photoconductor 12 successive monochrome images so that, 
when all of the monochrome images are combined together on the 
photoconductor 12, the combined image provides an acceptable color image. 
Typical multipass color electrophotographic strategies use four printing 
passes, with each successive pass applying a different optical image to 
the photoconductor corresponding to a different monochrome image 
component. Each successive image is developed after the exposure portion 
of the pass with a developer having the appropriate color of toner 
corresponding to that monochrome portion of the image. To effect this 
strategy, it is necessary to provide four different developing units 22-28 
as shown in FIG. 1 having four distinct colors of toner to be applied in 
successive ones of four different passes. Thus, four developing units 
22-28 corresponding to yellow (Y) toner, magenta (M) toner, cyan (C) toner 
and black (K) toner respectively are provided for the FIG. 1 printers. The 
reproduced image is therefore made up of a plurality of colors applied in 
varying concentrations to achieve various gray levels. 
In a first pass of the multicolor printing process of FIG. 1, the laser 
beam scanning unit 16 exposes the surface of the photoconductor with 
modulated laser light 18 to create a first latent image component 
corresponding to the first monochrome component of the image to be 
printed. After the photoconductor 12 is exposed with the first latent 
image component, the first component of the image pattern is developed 
using a first developer 22, described in greater detail below, to provide 
a first color of toner to the surface of the photoconductor. After the 
first monochrome component of the color image to be printed has been 
provided on the surface of the photoconductor, a second pass is performed 
to provide a second monochrome component of the color image to be printed. 
The photoconductor 12 on the drum is charged to provide a new uniform 
charge distribution on the photoconductor. The laser beam scanning unit 16 
then scans the laser beam 18 over the surface of the photoconductor to 
expose the photoconductor 12 with a second latent image component. A 
second color of toner is applied by the second developing unit 24 so that 
it adheres to the photoconductor 12 in a pattern corresponding to the 
second latent image component and overlies the first toner image. This 
process is repeated for the third and fourth components of the image, 
using the third and fourth developing units 26 and 28, respectively, to 
provide four different overlaid monochrome toner images on the 
photoconductor. The four color toner image is then transferred onto the 
surface of a recording medium 32 such as a paper sheet at the transfer 
charger 30 and the toner image is fused to the recording medium 32 at 
fuser 40. To accomplish fusing, the recording medium 32 is passed between 
the heating roller 42 and the pressing roller 44 that make up the fuser 
40. A heater, such as a halogen lamp, is provided in the heating roller 42 
to heat a surface of the roller to a predetermined high temperature 
sufficient to at least soften the developer, when combined with the 
pressure applied by the pressure roller 44. The high temperature and 
pressure between these two rollers cause the toner to melt and to be fixed 
onto the recording medium 32, thereby forming a color image on the 
recording medium. 
For the four pass color laser printer illustrated in FIG. 1, four 
developing processes are needed to create an image. Because the FIG. 1 
printer essentially requires four complete and independent printing 
processes to create an image, the FIG. 1 printer is about four times as 
slow as a conventional monochrome laser printer. As such, the color image 
reproduction rate for the FIG. 1 printer is generally unacceptably slow. 
FIG. 2 is a schematic view showing a different conventional implementation 
of a color electrophotographic apparatus. The "tandem" color 
electrophotographic apparatus of FIG. 2 is similar to the apparatus of 
FIG. 1 in that the printed color image is built up over the course of 
repeated distinct monochrome printing processes. For the FIG. 2 apparatus, 
however, the four distinct printing processes are performed in series on 
four distinct drums 10 having associated chargers 14, laser beam scanning 
units 16, four distinct developers 22, 24, 26, 28 carrying four different 
colors of toner, and four transfer chargers 30. Operation of the FIG. 2 
apparatus is generally similar to that of the FIG. 1 apparatus, with the 
primary exception that each component of the latent image is formed on 
different photoconductive drums by dedicated laser beam scanning units in 
the FIG. 2 apparatus. Like elements in FIG. 2 are represented by like 
reference numerals in FIG. 1. A first color image is formed and then 
transferred to a recording medium 32 from the first photoconductive drum 
10, and then second, third and fourth color images are successively 
transferred from the second, third and fourth photoconductive drums in 
sequence. As illustrated, the FIG. 2 structure can accommodate a linear 
transport path for the recording medium 32 so that the four color 
components of the image can be transferred to the recording medium in a 
single transport operation. Thereafter, a fuser 40 fixes the resultant 
four component image on the recording medium 32. The apparatus of FIG. 2 
is advantageous in that the reproduction rate thereof is much higher than 
that of the apparatus shown in FIG. 1 since the four developing processes 
can proceed simultaneously. The tandem color electrophotographic apparatus 
of FIG. 2 has undesirable characteristics, however. Although the FIG. 2 
apparatus is improved over the FIG. 1 apparatus, there remains in the FIG. 
2 apparatus a difficulty in obtaining a desirable level of registration 
between the successive images transferred onto the paper sheet or other 
recording medium due to the need to align four different drums with the 
recording medium for the four successive toner transfer operations. More 
importantly, the FIG. 2 apparatus can be undesirably large and expensive 
due to the need to provide multiple complete print stations. 
A different implementation of a color electrophotographic apparatus is 
illustrated in FIG. 3. The illustrated color laser printer provides four 
color components for a printed color image while using a single pass 
printing operation. The FIG. 3 laser printer includes a photoconductive 
drum 10 with four printing stations, each including a charger 50-56, a 
laser beam scanning unit 58-64 and a developer 60-72, all arranged around 
the circumference of the drum. At the first printing station, the drum 10 
is charged to an initial uniform voltage by the charger 50, the drum is 
exposed according to a first component of the image to provide a first 
latent image component, and the first latent image component is developed 
by the developer 66 to produce a first toner pattern on the surface of the 
drum. The second printing station repeats this process using a second 
charger 52 to reproduce a uniform charge distribution over the surface of 
the drum 10, including over the portions of the drum covered by the first 
toner image. As second toner image is created on the surface of the drum 
and is overlaid with the first toner image. Third and fourth color image 
components are created on the surface of the drum 10 as the drum rotates 
through the third and fourth developing stations to provide a four 
component, four color toner image on the surface of the drum. The four 
color toner image is then transferred onto the recording medium 32 by 
transfer charger 30, and four color image is fused onto the recording 
medium 32 at fuser 40. 
The FIG. 3 single pass color printer provides color output at higher speeds 
than the multipass design illustrated in FIG. 1. As a practical matter, 
however, it is difficult to achieve the necessary registration of images 
for the FIG. 3 apparatus because of the exacting alignment accuracy 
required to obtain registration of the images created at successive ones 
of the printing stations. To obtain acceptable levels of registration for 
the different image components of the FIG. 3 design, it is necessary to 
arrange the four different laser beam scanning units 58-64 so that the 
laser beams trace lines on the surface of the drum that are parallel to 
the cylindrical axis of the drum to a very high degree. Because this 
alignment requires precise positioning of five objects in relation to each 
other in three dimensional space, it is very difficult to achieve an 
acceptable level of alignment, so that the FIG. 3 apparatus is not 
amenable to high volume manufacturing techniques. The difficulty of 
aligning the four laser beam paths is heightened because the lasers are 
typically scanned using high speed rotating polygon mirrors, with cach 
mirror rotating independently of all others. Small variations in mirror 
position will thus be magnified by the long path traced by the laser beam 
reflected from the mirror to the surface of the drum. 
A further difficulty with the apparatus of FIG. 3 is that it tends to be 
large. The drum 10 must be made sufficiently large so that the four 
different print stations and the transfer charger 30 can be arranged about 
its periphery. Thus, it is difficult to make the apparatus of FIG. 3 in a 
small enough form factor to comfortably fit into the home and small office 
operating environments. One attempt to address this issue is illustrated 
in U.S. Pat. Nos. 5,541,722 and 5,557,394, which modify the design of the 
FIG. 3 apparatus by providing light emitting diode arrays as optical 
exposure units within a transparent drum that has a photoconductor formed 
on its surface. While this modification achieves reduced size, it requires 
the use of a transparent drum, which is generally undesirable for reasons 
of both cost and performance. Additionally, the design strategy of these 
two patents is limited in its potential for success because the 
modifications do nothing to reduce the space required by the developing 
units, which typically occupy a far larger volume than the optical 
exposure units. 
It is thus desirable to provide an improved configuration for an 
electrophotographic apparatus. An associated, but distinct consideration 
in the design of high performance, low cost electrophotographic systems is 
the desirability of providing a more compact and higher performance 
developer. To appreciate this design consideration, it is useful to 
consider the design of a conventional developing unit used in some 
electrophotographic processes, illustrated in FIG. 4. The developing unit 
of FIG. 4 generally comprises a conductive roller that transports a 
developer consisting of a mixture of magnetic carrier particles and 
plastic or other toner ink particles to the surface of a photoconductive 
drum which carries a latent image. The developing unit includes roller, 
stirrers or other mechanisms to agitate the developer. The agitated 
developer adheres to the surface of the roller and a predetermined 
thickness of developer is maintained on the roller by use of a doctoring 
blade. The toner is triboelectrically charged and the toner and carrier 
are carried from a reservoir within the developing unit to a position 
adjacent the photoconductive drum by the roller as the roller rotates in 
an opposite sense to the photoconductive drum. A developing bias from a 
d.c. power supply and/or an a.c. power supply is applied between the 
photoconductive drum and the developing unit to form an electrical field 
that transports the developer from the roller to the photoconductive drum. 
FIG. 4 shows an exemplary structure of a conventional developing unit in 
which a tribocharging blade 80 and a feed roller 82 are provided adjacent 
to a developing roller 84 within a housing 86. The tribocharging blade 80 
is held against a surface of the developing roller 84, for example, by a 
spring mechanism. A stirrer 88 is provided within the reservoir of the 
developer mixture of carrier and toner 90. During developing, the toner 90 
is agitated by the stirrer 88 and then distributed on a surface of the 
feed roller 82. Next, the toner 90 on the surface of the feed roller 82 is 
conveyed to the surface of the developing roller 84 by the feed roller. At 
the same time, a strong shearing force between the tribocharging blade 80 
and the developing roller 84 causes the toner 90 to be tribocharged. 
Subsequently, the toner 40 is selectively transferred to a surface of a 
photoconductor 10 by the electrostatic mechanism discussed above, thereby 
achieving the developing operation. 
In the conventional developing unit discussed above, there are a range of 
problems observed: 
(1) The load caused by the frictional force between the tribocharging blade 
80 and the developing roller 84 is large and the fluctuation in that load 
can be dramatic, resulting in variations in the charging of the toner 90. 
(2) A nip between the tribocharging blade 80 and the developing roller 84 
is small, introducing further instability to the tribocharging of the 
toner. 
(3) Toner particles arc crushed by the strong shearing force between the 
tribocharging blade 80 and the developing roller 84, resulting in a 
fogging effect and reducing the image quality. 
(4) The reservoir for storing the toner is separated from the other 
developing mechanisms. The configuration of the developing unit tends to 
be complicated, resulting in wasted space and in residual toner being left 
in the developing unit that cannot be accessed in developing operations. 
It is therefore desirable for a developing unit to have a smaller volume, 
improved integration and improved performance as a part of an overall 
strategy to facilitate the production of a high performance, readily 
manufactured color electrophotographic printing apparatus. 
SUMMARY OF THE PREFERRED EMBODIMENTS 
It is an object of the present invention to provide a simple and compact 
developing unit which has improved performance. 
It is another object of the present invention to provide a developing unit 
which has comparatively low internal loading and which can smoothly and 
stably charge the contents of the developing unit. 
It is another object of the invention to provide a developing method which 
does not apply an unacceptable level of shearing force to toner and which 
provides sufficiently undamaged toner as to produce high quality images. 
Particularly preferred embodiments of the present invention provide a 
transportation device within the developing unit, such as a mesh belt, for 
moving toner within the developing unit. For such embodiments, it is 
possible to achieve charging of the toner by causing the belt or other 
transportation belt to carry the toner between opposed friction plates. 
One aspect of the present invention provides a developing unit of the type 
that might be used for electrophotography having a housing for enclosing 
components of the developing unit, the housing having an opening through 
which toner can pass. First and second rollers are provided in the housing 
with a belt extending over the first and second rollers so that the belt 
is translated around the first and second rollers in response to rotation 
of the first and second rollers. The belt is capable of transporting toner 
on a surface of the belt. A charging device is provided adjacent at least 
a portion of the belt, the charging device imparting a charge to toner on 
the belt passing adjacent the charging device. An output device transports 
toner from the surface of the belt to the opening in the housing.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The present invention provides an electrophotographic apparatus that 
produces color hard copy output, preferable in a single pass process and 
most preferably using a highly manufacturable and compact configuration. 
In one aspect, the present invention provides an electrophotographic 
system having a photoconductor with a planar surface along at least part 
of its path. The associated laser beam scanning units or other optical 
exposure units for the electrophotographic system are positioned so that 
the exposure light from each of the optical exposure units extends 
perpendicularly to the planar surface of the photoconductor. By 
positioning all of the optical exposure units so that their light outputs 
are incident on the planar portion of the photoconductor, alignment of the 
optical exposure units requires only that the units be adjusted so that 
their output beams are parallel to one another. This is a much simpler 
alignment problem than is presented by the single pass electrophotography 
system illustrated in FIG. 3. In aligning the system of FIG. 3, it is 
necessary to precisely select the four distinct planes traced by the four 
beam scanning systems in a three dimensional space and to position those 
four planes with respect to a cylindrical surface. Aligning 
electrophotographic systems formed in accordance with the present 
invention requires only that four optical exposure mechanisms be 
positioned so that their optical output beams trace parallel lines on the 
surface of the planar portion of the photoconductor. From one point of 
view, alignment of systems in accordance with this aspect of the invention 
is like solving a two-dimensional problem, while the alignment of the FIG. 
3 apparatus requires solution of a harder three-dimensional problem. Those 
skilled in the art of optics and alignment of optical systems will 
recognize that making the four or more exposure beams of preferred 
embodiments of this invention trace parallel optical paths is a much 
simpler task than is involved in aligning the FIG. 3 system. In addition, 
such preferred embodiments will have far greater stability than 
conventional systems like the FIG. 3 apparatus and so will tend to retain 
their alignment better. Thus, preferred embodiments of the present 
invention are expected to not only be less expensive to manufacture but 
also to have better durability. 
Several other aspects of the present invention facilitate the 
implementation of a photoconductor having a planar surface in conjunction 
with four exposure units disposed to output parallel beams. One of these 
aspects is the use of a smaller developing unit. In the system illustrated 
in FIG. 3, the developing units occupy large volumes as a natural 
consequence of the design of these developing units. Thus, it is likely 
difficult to modify the drum of the FIG. 3 apparatus to provide a planar 
surface on which images can be formed without making the apparatus 
undesirably large. By using a more efficient and compact design for the 
developing unit, preferred embodiments of the present invention 
accommodate the use of a planar configuration for the photoconductor and 
other parts of a print engine. Briefly, particularly preferred embodiments 
of certain aspects of the present invention provide a developing unit 
having a mesh or web belt for the internal transport of toner within the 
developing unit, allowing for different configurations of developing units 
to be used which more efficiently use space. The use of a more active 
toner transport mechanism also allows the shape of the developing unit to 
be selected to optimize the integration of the overall system. The 
developing unit can be made still more compact in some embodiments by 
using a single component developer, in contrast to the more conventional 
multicomponent developer. If the developer has only toner particles and no 
carrier particles, the reservoir for developer will be used more 
efficiently, allowing the reservoir to me made smaller and thus allowing 
the developing unit to be made smaller. 
Another aspect of the present invention which facilitates the use of a 
planar printing surface for the photoconductor is the use of highly 
integrated optical exposure units. In one configuration, the optical 
exposure units might include four polygon mirrors that, along with the 
shaft on which the mirrors are mounted and which is used to rotate the 
mirrors, are formed from a single piece of plastic in a process that 
readily produces mirror faces that are aligned and parallel to a high 
degree. Thus, when four laser beams are scanned by this configuration of 
optical exposure unit, the four scanned beams trace through four parallel 
planes with comparatively little alignment required. In another 
configuration, the optical exposure unit might include a rigid frame on 
which four LED arrays ("bars") are mounted. It is well within the 
characteristics of the LED manufacturing process and within typical 
mechanical tolerances to align the LED arrays so that the LED arrays 
optical output extends along parallel planes. These and other 
configurations of optical exposure units for the present invention 
facilitate the planar configuration of the print engine of some preferred 
embodiments of the present invention. In addition, these optical exposure 
units are readily manufactured and integrated with the system to make the 
manufacture of the entire electrophotography system less expensive. 
These and other aspects of the present invention are now discussed with 
particular reference to certain preferred embodiments and with reference 
to the figures. Implementations of the present invention use many 
components which are commercially available or are otherwise well known. 
As such, detailed descriptions of these components are not provided here 
so that the description is more concise and better emphasizes the 
distinctive aspects of the invention's electrophotographic apparatus. It 
should be appreciated that the following description emphasizes a four 
color laser printer, but other electrophotographic systems might also 
benefit from the principles of the present invention. For example, a color 
copier based on scanned images which are separated into different color 
components stored in different memories would readily implement aspects of 
the present invention. In addition, the present invention is not limited 
to four colors. Embodiments of the present invention readily incorporate 
additional (or fewer) printing stations with only simple modifications. 
FIG. 5 shows a color electrophotographic apparatus according to the present 
invention. As shown, the color electrophotographic apparatus comprises 
flexible photoconductor 100 extending as an endless belt around a roller 
set including a first roller 102 and a second roller 104. The rollers 102, 
104 are spaced apart by a sufficient distance that the planar surface 
defined on one side of the photoconductor 100 is large enough to allow the 
various print stations to be positioned along that planar printing 
surface. Stability of the planar printing surface can be maintained by 
positioning a tensioning plate on the opposite surface of the 
photoconductor between the rollers. Further improvement in the stability 
of the planar printing surface can be provided by positioning a plate 
adjacent to and behind the planar printing surface. The lateral position 
of the photoconductor 100 is maintained by one or more limiting pins 
placed on one or more of the rollers 102, 104 to prevent lateral movement 
of the photoconductor 100 during rotation. In the illustrated embodiment, 
the rollers are driven, or one of the rollers is driven, to translate the 
photoconductor through the printing region continuously in a downward 
direction. The translation direction defines an ordering for the elements 
within the four illustrated print stations. The print stations include 
chargers 106-112 positioned as the first element of each of the print 
stations. Four developing units 114-120 are interposed between the 
chargers 106-112. Four laser beam scanners are provided for the four print 
stations which are formed of semiconductor lasers 122-128 for emitting 
laser beams 130-136 that pass through gaps between the chargers 106-112 
and the corresponding developing units 114-120 and reach the planar 
printing surface of the flexible photoconductor 100. The illustrated 
electrophotographic system also includes a conventional transfer charger 
138, and, for example, a conventional fuser 140 including a heating roller 
142 and a pressing roller 144. The chargers, laser beam scanners, and 
developing units together make up four distinct print stations for 
providing (in order along the direction of rotation) yellow (Y), magenta 
(M), cyan (C) and black (K) toner image components to the photoconductor. 
According to a particularly preferred embodiment of the present invention, 
the flexible photoconductor 100 comprises a belt-shaped flexible substrate 
coated with an organic photoconductor. The substrate of the belt is 
typically chosen to be highly conductive and might consist of a metal mesh 
or web or a plastic coated with a film of metal so that the substrate can 
act as a ground plane for the photoconductor. When the substrate of the 
belt is formed from a plastic, it may also be desirable to choose a 
conductive plastic, so long as the plastic provides the desired 
flexibility characteristics. The conductive substrate of the belt is 
coated with a photoconductive material such as an amorphous semiconductor 
or an organic photoconductor. Often, organic photoconductors are preferred 
because they tend to be more flexible and have a longer history of use. 
The photoconductor preferably is substantially insulative so that charge 
provided onto the surface of the photoconductor remains on the surface of 
the photoconductor for sufficient time to pass through a print station, so 
long as the photoconductor in maintained in the dark. It is also desirable 
for the photoconductor to be rapidly discharged upon application of light 
of the wavelength and intensity of the laser beams 122-128. Organic 
photoconductors are commercially available from, for example, Eastman 
Kodak Company and Mitsubishi Chemical Corporation. Of course, other 
photoconductors having similar flexibility and photoconductive properties 
can be used. 
Each of the laser beam scanners is associated with one of the polygon 
mirrors and with the well known focusing and collimating optics used with 
semiconductor lasers within laser printers. A shaft 158 passes through the 
aligned central axes of the polygon mirrors 150-156 and is driven by a 
motor 160 to rotate the polygon mirrors. Alternately, if higher rotational 
speeds are desired, the shaft may be rotated on an air bearing as is known 
in the electrophotographic arts. The illustrated mirrors 150-156 might 
each have a square cross section and the entire assembly might be formed 
from a single piece of plastic. In one simplistic approach, the mirror 
assembly might be formed from a starting rectangular prism block of 
plastic having a uniform, square cross section by turning the block using 
a lathe to define the shaft portions between the mirrors. The mirror faces 
would then be rendered reflective, for example, by sputter coating the 
faces with aluminum and then the mirror assembly is ready for use. The 
resulting mirror assembly will have four polygon mirrors held in fixed 
alignment with parallel and aligned faces. More practically, the mirror 
assembly can be formed by injection molding of plastic followed by coating 
the mirror surfaces with a metal film such as aluminum. Such a molding 
process can be accomplished with sufficient accuracy to ensure that the 
mirror faces will be aligned and parallel within the required tolerances. 
The use of a fixed mirror assembly or optical exposure unit facilitates the 
alignment of the FIG. 5 electrophotographic system, since alignment can be 
accomplished by properly positioning the mirror assembly with respect to 
the planar printing surface of the photoconductor and then performing 
relatively simple focusing and alignment operations for each of the 
semiconductor lasers 122-128. Therefore, the array of four parallel laser 
beams 130-136 emitted from the laser beam scanners can scan the surface of 
the photoconductor 100 in a direction perpendicular to the moving 
direction of the photoconductor 100 as they are reflected by the rotating 
polygon mirrors 150-156 with a high degree of accuracy. 
As is known in the art, including the art discussed in the background 
above, each of the lasers 122-128 is connected to a modulating circuit 
that modulates the lasers with the information necessary to create yellow, 
magenta, cyan and black image components, respectively. The commercially 
implemented modulation schemes of available color laser printers are in 
most regards adequate for use with the present system and so will not be 
described further herein. On the other hand, certain modifications to the 
known modulation schemes facilitate the assembly and alignment of the FIG. 
5 system. Vertical registration between the successive yellow, magenta, 
cyan and black image components is obtained by varying the timing between 
the respective modulation patterns on the basis of the transport speed of 
the photoconductor 100. In particularly preferred embodiments of the 
present invention, the delays between the successive color image 
components are programmed into nonvolatile memories (EPROM, EEPROM or 
flash) at the time of manufacture so that the registration can be 
empirically determined for each printer. Thus, an initial alignment of the 
system is accomplished and then testing is performed to determine 
alignment between successive color image components. This testing might be 
accomplished by printing test patterns, as is known in the art. 
Adjustments in the relative delays optimize the resignation between 
successive color image components and then these optimized delays are 
stored in the nonvolatile memories. By this strategy, registration with 
one half pixel or dot accuracy is accomplished between all four color 
image components during the initial set up of the FIG. 5 system. 
Horizontal registration can be optimized in a similar manner. The data used 
to generate the modulation signals used to drive cach of the lasers 
122-128 are stored in line memories corresponding to each of the lasers. 
Four different line memories are provided for the respective lasers and 
color image components to be generated and each of the line memories is 
configured as illustrated in FIG. 6 with a preshift register 170, a line 
memory register 172 and a postshift register 174. The general 
representation of the line memory shown in FIG. 6 is appropriate whether 
the modulation data are single bit data stored in a shift register or are 
data words stored in an array of shift registers. By use of the 
illustrated register array, the desired modulation data an be shifted to 
the right or to the left in the memory to advance or delay the modulation 
signal with respect to the other line memories. The advance or delay of 
each horizontal line of each color image component can thus be adjusted to 
obtain better horizontal registration between all of the color image 
components. In particularly preferred embodiments of the present 
invention, the relative advance and delay of each of the line memories is 
adjusted during manufacture and stored, for example in a nonvolatile 
memory, to obtain horizontal registration between the different color 
image components with one half pixel or dot accuracy. 
The image reproduction operation of the FIG. 5 color electrophotographic 
apparatus according to the present invention proceeds as follows. At 
first, the rollers 102 and 104 are rotated to move the flexible 
photoconductor 100 and then the first charger 106 applies a voltage of 
-600.about.-800 V (in this embodiment, for example, -700 V) to a leading 
portion of the planar printing surface of the photoconductor 100. When the 
uniformly charged surface of the photoconductor 100 passes through a gap 
between the first charger 106 and the first developing unit 114, the first 
laser beam 130 modulated with information for the first color image 
component impinges upon the charged area of the photoconductor 100 and 
discharges the photoconductor) in accordance with the information 
corresponding to the image to be printed. At this time, the voltage on the 
photoconductor corresponding to a blank (toner will not adhere) portion of 
the latent image is about -700 V while the voltage on the photoconductor 
corresponding to a non-blank (toner will adhere) portion is between 
.+-.150 and 0 V. Subsequently, when the charged area carrying the first 
latent image passes by the first developing unit 114, developer containing 
yellow toner is conveyed by the first developing unit 114 to the charged 
surface of the photoconductor 100 carrying the latent image, thereby 
developing the yellow image component. Next, magenta developer, cyan 
developer and black developer images are applied to the surface of the 
photoconductor, each color in sequence using a series of charging, 
discharging and developing operations like that described for yellow, so 
that a four color toner image is formed on the planar printing surface of 
the photoconductor 100. A recording medium 162, which is generally a sheet 
of pater, is transported to a space between the roller 104 and the 
transfer charger 138. The Y, M, C and K toner images on the surface of the 
photoconductor 100 are transferred to the recording medium 162 by means of 
an electric field produced between the photoconductor and the recording 
medium by the transfer charger 138. Then, the toner images are fused by 
the high temperature and high pressure provided by the fuser 140, fixing 
the four color toner image on the recording medium 162. Typically, the 
photoconductor belt is blanket discharged and cleaned of remnant toner or 
other debris at a cleaning station 164 positioned on a portion of the 
photoconductor 100 away from the planar printing surface. This cleaning 
station might also include a tensioning unit for maintaining an 
appropriate tension on the photoconductor during its transport. 
In the embodiments of the present invention illustrated by FIG. 5, laser 
beam scanners having semiconductor lasers scanned by polygon mirrors are 
used. However, other light sources such as light emitting diode LEDs) or a 
light source modulated by liquid crystal devices (LCDs) can be utilized 
instead. FIG. 7 shows a color electrophotographic apparatus according to a 
second embodiment of the present invention wherein the laser beam scanners 
in the first embodiment are replaced with independently controllable 
arrays of LEDs or light sources modulated by LCDs 180-186. In this case, 
the semiconductor lasers 122-128, the polygon mirrors 150-156 and the 
associated optics in the FIG. 5 embodiments can be omitted. Therefore, in 
comparison with the electrophotographic apparatus according to the first 
embodiment, the apparatus of the second embodiments is advantageous in 
that the spatial volume thereof is smaller and the manufacturing cost is 
lower. Alternately, the developing units might be made larger to provide 
additional toner capacity. The LED arrays LCD modulated light sources 
180-186 are mounted to a single rigid frame so that the optical exposure 
units 180-186 are held in fixed relationship to one another. Most 
preferably, the LED arrays are imaged onto the planar printing surface of 
the photoconductor 100 using cylindrical optics (such as fiber lenses) 
which extend across the entire LED array. Other optics might be necessary 
when using LCD modulation, depending on the nature of the light source. 
The optical exposure units 180-186 can be mounted to the rigid frame so 
that the optical output of the array on average propagates along parallel 
planes in a direction perpendicular to the planar printing surface of the 
photoconductor. As such, the optical exposure units 180-186 present 
similar levels of alignment simplification over the conventional system of 
FIG. 3 as are provided by use of the mirror assembly used in the FIG. 5 
system. 
As discussed above, the configuration of the electrophotographic systems of 
FIG. 5 and FIG. 7 is facilitated by use of a reduced volume developing 
unit. Such a reduced volume developing unit is provided in accordance with 
a different aspect of the present invention. While preferred embodiments 
of the present invention's developing unit find particular application in 
the systems illustrated in FIGS. 5 and 7, it should be appreciated that 
embodiments of the developing unit of the present invention can be used in 
other configurations of these and other electrophotographic systems. FIG. 
8 shows a developing unit according to an embodiment of the present 
invention including a first roller 200, a second roller 202, a 
transportation device 204 which is typically a mesh or similar material, a 
friction device 206 positioned around a friction region 208 of the 
transportation device, toner 210 within a reservoir, a developing roller 
212, and a thickness controlling blade 214. The FIG. 8 developing unit is 
provided within a housing 216. The illustrated transportation device 204 
is a mesh or web belt stretched between the first roller 200 and the 
second roller 202, where the opening in the mesh or web of the belt are 
large enough that toner can readily pass though the openings on the 
surface of the belt. The first roller 200 is connected to a motor during 
operation to serve as a driving roller for the transportation device 204 
and as a feed roller for conveying charged toner to a surface of the 
developing roller 212. The second roller 202 is used to restore the 
transportation device 204 in the illustrated embodiment. 
The toner 210 carried by the transportation device 204 enters and passes 
through the friction device 206 before the toner reaches the developing 
roller. The friction device 206 includes two plates that sandwich the 
transportation device 204, with the plates being formed from a material 
selected so that the toner 210 is charged as it passes between the plates 
of the friction device 206. In some embodiments, the plates are metal and 
are connected to a bias V.sub.B which typically has a value of, for 
example, between about -300 to -700 V. Alternately, the charging plates 
might be formed from a dielectric (nonconductive) material such as 
synthetic fiber, paper, acrylic resin and the like. In this case, no bias 
is applied to the friction device 206 and charge is supplied to the toner 
solely by tribocharging. 
An important aspect of the FIG. 8 developing unit design is that the 
charging plates of the friction device 206 face on the transportation 
device 204 over a large width and over a considerable length. This allows 
for high levels of tribocharging while imposing a relatively light and 
constant load on the transportation device. The charged toner 210 is 
conveyed to the first roller 200 and then from the transportation device 
204 to a surface of the developing roller 212. The thickness controlling 
blade 214 exerts pressure on the developing roller 212 to control the 
thickness of the toner layer carried on the developing roller 212. The 
thickness of the toner layer controlled by the thickness controlling blade 
213 is between 10 microns to 100 microns on the surface of developing 
roller 212. The developing roller 212 faces on the photoconductor 220 
though a window 222 and the window may be controlled to selectively eject 
toner 210 onto photoconductor 220. 
Provision of the transportation device 204 for moving toner through the 
developing unit allows the developing unit of FIG. 8 to be made smaller 
and/or shaped differently than the conventional developing unit. The 
transportation device 204 also allows the FIG. 8 developing unit to be 
made without a stirring mechanism. In addition, the transportation device 
allows a single component developer, that is, a toner ink without carrier, 
to be used in the developing unit, allowing the developing unit to be made 
smaller. In presently preferred embodiments of the invention, the 
transportation device is a mesh belt formed from a conductive material. 
Each of the toner particles has a diameter ranging from several microns to 
tens of microns. The mesh of the transportation devices 204 allows the 
toner particles to pass though, and the openings in the mesh preferably 
range from tens of microns to hundreds of microns. 
FIG. 8 shows a developing unit according to another embodiment of the 
present invention generally similar to the developing unit shown in FIG. 8 
except that a different configuration of a friction device 230 is used 
instead of the friction device 206 and the second roller 202 of the FIG. 8 
embodiment. In the FIG. 9 embodiment, a transportation device 204 is 
driven by a first roller 200, and the friction device includes a cylinder 
230 and a hemispherical shell 232, with the transportation device 204 
running between the cylinder 230 and the hemispherical shell 232. The 
friction device stretches the transportation device 204 to maintain an 
appropriate tension on the transportation device and also tribocharges the 
toner. The cylinder 230 rotates as the transportation device 204 is move 
so that the toner carried by the transportation device 204 is tribocharged 
by the hemispherical shell 232. Other aspects of the developing operation 
using the FIG. 9 apparatus are the same as were describes with respect to 
the FIG. 8 first embodiment, and so the description of those aspects is 
not repeated. 
FIG. 10 shows another variation on a developing unit according to the 
present invention and having a structure substantially the same as that of 
the FIG. 8 embodiment except that a "fur" or soft roller 230 is added 
between the first roller 200 and the developing roller 212. The surface of 
the fur or soft roller 240 might, for example, consist of a polyester or 
other fiber having a nap on the order of about 0.5-2.0 millimeters. Toner 
is stored temporarily within the nap on the soft roller as an intermediate 
reservoir which ensures that there will be a substantially constant supply 
of toner available to the developing roller 212. Toner 210 is carried by 
the transportation device 204 and tribocharged by a friction device 206. 
Next, the charged toner 210 is conveyed to a first roller 200 and then to 
the soft roller 240. The charged toner is fed from the soft roller 240 to 
a surface of a developing roller 212, and then provided to the 
photoconductor 220 via a window 222. 
In each of the above embodiments of developing units illustrated in FIGS. 
8-10, the contact area of the friction device is large and an endless mesh 
belt is used to carry toner as it is charged. Consequently, the toner 
carrying operation is smooth, the load of the developing unit is stable 
and the power consumption of the developing unit is reduced. Further, 
reduction in the driving load of the developing unit decreases the size 
and weight of the developing unit, and the manufacturing cost is also 
decreased. In addition, since the forces applied to toner particles are 
reduced, less damage to the toner particles occurs so that the image 
quality is improved. 
The various aspects of the present invention have bee described in terms of 
certain presently preferred embodiments. Those of ordinary skill will 
appreciate that modifications to and variations from the preferred 
embodiments might be made while remaining consistent with the basic 
teachings of the present invention. As such, the scope of the present 
invention is not to be limited to the particular described embodiments. 
Rather, the scope of the present invention is to be determined from the 
claims, which follow.