Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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TUNABLE SCOROTRON FOR DEPOSITING UNIFORM CHARGE POTENTIAL
This invention relates generally to a scorotron charging device,
and more particularly to an adjustable grid scorotron that applies a uniform
charge to a charge retentive surface.
BACKGROUND AND SUMMARY OF THE INVENTION
The present invention controls the uniformity and magnitude of
corona charging of a charge retentive, photo responsive surface. The
tunable scorotron makes use of an open screen grid as a control electrode,
to establish a reference potential, so that when the receiver surface reaches
the grid's reference potential, the corona generated electric fields no
longer drive ions to the receiver, but rather to the grid. Many factors can
contribute to charge nonuniformity across the surface of a photoresponsive
member. For example, nonuniformity in the thickness of the
photo responsive layers and edge effects both impact the charging
characteristics of a photo responsive member. Furthermore, the
nonuniformity can be exacerbated upon aging of the photo responsive
member due to the higher charge levels needed to produce a desired
potential on the photoresponsive surface.
Heretofore, numerous variations of scorotron charging systems
have been developed, of which the following disclosures may be relevant:
US-A-2,777,957
Patentee: Walkup
Issued: Jan. 15, 1957
A
212543
US-A-2,965,754
Patentee: Bickmore et al.
Issued: Dec. 20, 1960
US-A-3,937,960
Patentee: Matsumoto et al.
Issued: Feb. 10, 1976
US-A-4,112,299
Patentee: Davis
Issued: Sep. 5, 1978
US-A-4,456,365
Patentee: Yuasa
Issued: Jun. 26, 1984
US-A-4,638,397
Patentee: Foley
Issued: Jan. 20, 1987
US-A-5,025,155
Patentee: Hattori
Issued: Jun. 18, 1991
Xerox Disclosure Journal
Vol. 10, No. 3
May/June 1985
Xerox Disclosure Journal
Vol. 17, No. 3
May/June 1992
_2_
Xerox Disclosure Journal
Vol. 17, No. 4
Jynaugust 1992
IBM Technical Disclosure Bulletin
Vol. 19, No. 8
January 1977
The relevant portions of the foregoing patents may be briefly
summarized as follows:
US-A-2,777,957 discloses a corona discharge device for
electrically charging an insulating layer. A conductive grille is interposed
between the ion source, for example, the corona discharge electrode, and
the insulating layer, preferably a photoconductive insulating layer. The
grille is maintained at a potential below the voltage of the corona
discharge electrode and produces a uniform charge potential across the
insulating layer.
US-A-2,965,754 describes a double screen corona device having a
pair of corona screens to substantially eliminate charge nonuniformity,
referred to as charge streaking. The screens, inserted between the corona
element and an insulating layer, are arranged in a parallel fashion
overlapping one another so as to diffuse the ions emitted by the corona
element before they are deposited on an insulating layer. Both screens may
be maintained at slightly different potentials, however, the screen closest
to the insulating layer is maintained at a potential between four and ten
times the maximum potential to which the insulating layer is to be raised.
US-A-3,937,960 discloses a charging device for an
electrophotographic apparatus having a movable control plate. The
control plate, commonly referred to as a shield, is formed of a flexible
conductive material. The control plate may be moved relative to a corona
producing wire, such that the movement of the plate produces a
corresponding variation in the ion flow from the wire.
3-
215432
US-A-4,112,299 teaches a corona charging device having an
elongated wire and a surrounding conductive shield which is segmented in
a direction parallel to the wire. Each of the conductive shield segments may
be biased at different potentials in order to produce a universal corona
generating device which is adaptable to a variety of situations.
US-A-4,456,365 discloses a corona charging device for uniformly
charging an image forming member which includes a corona wire and a
conductive shield which partially surrounds the wire. The image forming
member is uniformly charged by applying an AC voltage to the corona wire,
along with an additional DC bias voltage.
US-A-4,638,397 describes a scorotron where the wire grid is
connected to ground via a plurality of Zener diodes and a variable resistor.
The control circuit employed effectively limits the charge potential which is
deposited on a photoconductive layer by varying the voltage applied to a
control grid as a fraction of the nominal voltage applied to the grid.
US-A-5,025,155 teaches a corona charging device for charging
the surface of a moving member which includes a plurality of corona
generating electrodes and a grid electrode located between the moving
member and the wire electrodes. Increased surface potential is achieved on
the moving member utilizing a plurality of wire electrodes, where the
distance between the grid electrode and the moving member is shortest
beneath the downstream electrode.
Xerox Disclosure Journal (Vol. 10, No. 3; May/June 1985) teaches,
at pp. 139-140, a charging scorotron employing a scorotron grid which is
segmented on one end thereof in order to selectively avoid the creation of
unused charged areas on an adjacent photoreceptor. The two disclosed
segments at the end of the scorotron are switchably connected to a
potential source so that in all cases the photoreceptor width corresponding
to the image size of the smallest copy sheet is always charged.
Xerox Disclosure Journal (Vol. 17, No. 3; May/June 1992)
illustrates, at pp 139-140, a micrometer adjustment suitable for leveling
the scorotron in an imaging device. The micrometer head may be used to
~, .
F 21 254 32
accurately adjust the scorotron wire with respect to the surface of a
reprographic element.
Xerox Disclosure Journal (Vol. 17, No. 4; July/August 1992)
describes, at pp. 239-240 a corrugated scorotron screen having corrugations
which run orthogonal to the process direction of a charge receptor. As
noted, the added strength and rigidity provided by the corrugations within
the screen help to maintain flatness and rigidity of the screen.
IBM Technical Disclosure Bulletin (Vol. 19, No. 8; Jan. 1977)
discloses, at pp. 2907-2908, a scorotron used in a xerographic process to
charge a photoconductor. Accurate positioning of the scorotron grid wires
is achieved by using a plastic block along with separate mechanical locating
means to position the wires.
In accordance with an aspect of the present invention, there is provided a
scorotron charging apparatus adapted to apply a uniform charge to a
charge retentive surface. The apparatus comprises corona producing
means, spaced from the charge retentive surface, for emitting corona ions,
and a flexible grid, interposed between said corona producing means and
the charge retentive surface in a nonplanar fashion, with the spacing
between said grid and the charge retentive surface being variable along a
region of said grid.
In accordance with another aspect of the present invention,
there is provided arr electrophotographic imaging apparatus for producing
a toned image, including a photoconductive member, means for charging a
surface of said photoconductive member, means for exposing the charged
surface of said photoconductive member to record an electrostatic latent
image thereon, and means for developing the electrostatic latent image
recorded on said photoconductive member with toner to form a toned
image thereon. The charging means includes corona producing means,
spaced from the surface of said photoconductive member, for emitting
corona ions, and a flexible grid, interposed between said corona producing
means and the surface of said photoconduct~ve member in a nonplanar
fashion, with the spacing between said grid and the surface of said
21 25~+ 3~
photoconductive member being variable along at least a region of said grid.
Other aspects of this invention are as follows:
a scorotron charging apparatus for producing a uniform charge on a charge
retentive
surface; comprising:
corona producing means, spaced from the charge retentive surface, for emitting
corona ions; and
a flexible grid, interposed between said corona producing means and the charge
retentive surface in a nonplanar fashion, said flexible grid being movable
with respect to the
charge retentive surface and the corona producing means so as to vary the
spacing between a
longitudinal portion of said flexible grid and the charge retentive surface in
order to apply a
uniform charge to the charge retentive surface.
Therefore, various aspects of the invention are provided as follows:
a scorotron charging apparatus for producing a uniform charge on a charge
retentive
surface, further comprising:
corona producing means, spaced from the charge retentive surface, for emitting
corona ions;
a flexible grid, interposed between said corona producing means and the charge
retentive surface in a nonplanar fashion, said flexible grid being movable
with respect to the
charge retentive surface and the corona producing means so as to vary the
spacing between a
portion of said flexible grid and the charge retentive surface in order to
apply a uniform
charge to the charge retentive surface;
grid adjusting means for altering the spacing between selected regions of said
flexible
grid and the charge retentive surface; and
means for generating an adjustment signal, wherein said grid adjusting means
automatically responds to the adjustment signal to alter the spacing between
said flexible grid
and the charge retentive surface.
An electrophotographic imaging apparatus for producing a toned image,
including:
a photoconductive member;
means for charging a surface of said photoconductive member, said charging
means
including:
corona producing means, spaced from the surface of said photodonductive
member,
for emitting corona ions;
a flexible grid interposed between said corona producing means and the surface
of
-6-
21 254 32
said photoconductive member in a nonplanar fashion said flexible grid being
movable with
respect to the surface of said photoconductive member and the corona producing
means so as
to vary the spacing between a longitudinal portion of said grid and the
surface of said
photoconductive member;
means for exposing the charged surface of said photoconductive member to
record an
electrostatic latent image thereon; and
means for developing the electrostatic latent image recorded on said
photoconductive
member with toner to form a toned image thereon.
Therefore, various aspects of the invention are provided as follows:
an electrophotographic imaging apparatus for producing a toned image,
including:
a photoconductive member;
means for charging a surface of said photoconductive member, said charging
means
including
corona producing means, spaced from the surface of said photoconductive
member,
for emitting corona ions;
a flexible grid, interposed between said corona producing means and the
surface of
said photoconductive member in a nonplanar fashion with the spacing between
said grid and
the surface of said photoconductive member to record an electrostatic latent
image on said
surface;
means for developing the electrostatic latent image recorded on said
photoconductive
member with toner to form a toned image thereon;
means for detecting a charge nonuniformity across the surface of said
photoconductive member and generating a signal indicative thereof; and
means for automatically adjusting the spacing between a longitudinal portion
of said
flexible grid and the surface of said photoconductive member as a function of
the signal from
said detecting means.
BRIEF DESCRIPION OF THE DRAWINGS
Figures 1, 2, 3 and 4 illustrate various perspective and orthographic views of
an
illustrative embodiment of the present invention;
Figure 5 is an illustration of a portion of a photoreceptor illustrating
various regions
on the surface thereof;
Figure 6 is a graph illustrating the thickness profile of the photoreceptor
depicted in
Figure 5;
-6a-
2125432
Figure 7 is a,graph illustrating expected voltage and charge profiles across
the surface
of the photoreceptor depicted in Figure 5 using an ideal scorotron device,
while Figure 8 is a
graph illustrating similar voltage and charge profiles for a scorotron device
employing the
present invention; and
Figure 9 is a schematic elevational view showing an electrophotographic
printing
machine incorporating the features of the present invention therein.
The present invention will be described in connection with a preferred
embodiment,
however, it will be understood that there is no intent to limit the invention
to the embodiment
described. On the contrary, the intent is to cover all alternatives,
modifications, and
equivalents as may be included within the spirit and scope of the invention as
defined by the
appended claims.
-6b-
21 254 32
DESCRIPTION OF THE PREFERRED EMBODIMENT
For a general understanding of the present invention, reference
is made to the drawings. In the drawings, like reference numerals have
been used throughout to designate identical elements. Figure 9 shows a
schematic elevational view of an electrophotographic printing machine
incorporating the features of the present invention therein. It will become
evident from the following discussion that the present invention is equally
well suited for use m a wide variety of printing systems, and is not
necessarily limited in its application to the particular system shown herein.
-6c-
iJ.
3. j
z1z5~3z
Turning first to Figure 9, during operation of the printing
system, a multicolor original document 38 is positioned on a raster input
scanner (RIS), indicated generally by the reference numeral 10. The RIS
contains document illumination lamps, optics, a mechanical scanning drive,
and a charge coupled device (CCD array). The RIS captures the entire image
from original document 38 and converts it into a series of raster scan lines
and, moreover, measures a set of primary color densities (i.e. red, green and
blue densities) at each point of the original document. This information is
transmitted as electrical signals to an image processing system (IPS),
indicated generally by the reference numeral 12. IPS 12 converts the set of
red, green and blue density signals to a set of colorimetric coordinates. The
IPS contains control electronics which prepare and manage the image data
flow to a raster output scanner (ROS), indicated generally by the reference
numeral 16. A user interface (UI), indicated generally by the reference
numeral 14, is in communication with IPS 12. UI 14 enables an operator to
control the various operator adjustable functions. The operator actuates
the appropriate keys of UI 14 to adjust the parameters of the copy. UI 14
may be a touch screen, or any other suitable control panel, providing an
operator interface with the system. The output signal from UI 14 is
transmitted to IPS 12. The IPS then transmits signals corresponding to the
desired image to ROS 16, which creates the output copy image.
ROS 16 includes a laser with rotating polygon mirror blocks. The
ROS illuminates, via mirror 37, the charged portion of a photo responsive
belt 20 of a printer or marking engine, indicated generally by the reference
numeral 18, at a resolution of about 400 pixels per inch, to achieve a set of
subtractive primary latent images. The ROS will expose the
photoconductive belt to record three latent images which correspond to
the signals transmitted from IPS 12. One latent image is developed with
cyan developer material. Another latent image is developed with magenta
developer material and the third latent image is developed with yellow
developer material. These developed images are transferred to a copy
sheet in superimposed registration with one another to form a
7_
~12543Z
multicolored image on the copy sheet. This multicolored image is then
fused to the copy sheet forming a color copy.
With continued reference to Figure 9, printer or marking engine
18 is an electrophotographic printing machine. Photoresponsive belt 20 of
marking engine 18 is preferably made from a polychromatic photo-
conductive material. The photoconductive belt moves in the direction of
arrow 22 to advance successive portions of the photoconductive surface
sequentially through the various processing stations disposed about the
path of movement thereof. Photoconductive belt 20 is entrained about
transfer rollers 24 and 26, tensioning roller 28, and drive roller 30. Drive
roller 30 is rotated by a motor 32 coupled thereto by suitable means such as
a belt drive. As roller 30 rotates, it advances belt 20 in the direction of
arrow 22. The speed of the belt is monitored in conventional fashion, and
directly controlled by motor 32.
Describing now the operation of the printing engine, initially, a
portion of photoconductive belt 20 passes through a charging station,
indicated generally by reference numeral 33. At charging station 33, a
scorotron 34 charges photoconductive belt 20 to a relatively high,
substantially uniform potential. Specific details of scorotron 34 will be
further described with respect to the remaining drawing figures.
Next, the charged photoconductive surface is rotated to an
exposure station, indicated generally by the reference numeral 35.
Exposure station 35 receives a modulated light beam corresponding to
information derived by RIS 10 having a multicolored original document 38
positioned thereat. The modulated light beam impinges on the surface of
photoconductive belt 20. The beam illuminates the charged portion of
photoconductive belt to form an electrostatic latent image. The
photoconductive belt is exposed at least three times to record latent images
thereon.
After the electrostatic latent images have been recorded on
photoconductive belt 20, the belt advances such latent images to a
development station, indicated generally by the reference numeral 39. The
development station includes four individual developer units indicated by
a
X125432
reference numerals 40, 42, 44 and 46. The developer units are of a type
commonly known as "magnetic brush development units." Typically, a
magnetic brush development system employs a magnetizable developer
material including magnetic carrier granules having toner particles
adhering triboelectrically thereto. The developer material is continually
advanced through a directional flux field to form a brush of developer
material. The developer material is constantly moving so as to continually
provide the brush with fresh developer material.
Development is achieved by bringing the brush of developer
material into contact with the photoconductive surface. Developer units
40, 42, and 44, respectively, apply toner particles of a specific color which
correspond to the compliment of the specific color separated electrostatic
latent image recorded on the photoconductive surface. The color of each
of the toner particles is adapted to absorb light within a preselected
spectral region of the electromagnetic wave spectrum. For example, an
electrostatic latent image formed by discharging the portions of charge on
the photoconductive belt corresponding to the green regions of the
original document will record the red and blue portions as areas of
relatively high charge density on photoconductive belt 20, while the green
areas will be reduced, or discharged, to a voltage level ineffective for
development. The remaining charged areas are then made visible by
having developer unit 40 apply green absorbing (magenta) toner particles
onto the electrostatic latent image recorded on photoconductive belt 20, as
is commonly referred to as charged area development. Similarly, during a
subsequent development cycle, a blue separation is developed by
developer unit 42 with blue absorbing (yellow) toner particles, while during
yet another development cycle the red separation is developed by
developer unit 44 with red absorbing (cyan) toner particles. Developer unit
46 contains black toner particles and may be used to develop the
electrostatic latent image formed from a black and white original
document, or that portion of the color image determined to be
representative of black regions ach of the developer units is moved into
and out of an operative position. In the operative position, the magnetic
g_
z~ z~43z
brush is positioned substantially adjacent the photoconductive belt, while
in the nonoperative position, the magnetic brush is spaced apart
therefrom. More specifically, in Figure 9, developer unit 40 is shown in the
operative position with developer units 42, 44 and 46 being in
nonoperative positions. During development of the color separations
associated with each of the electrostatic latent images, only one developer
unit is in the operative position, the remaining developer units are in the
nonoperative position. This insures that each electrostatic latent image is
developed with toner particles of the appropriate color without
commingling.
After development, the toner image is moved to a transfer
station, indicated generally by the reference numeral 65. Transfer station
65 includes a transfer zone 64, where the toner image is transferred to a
sheet of support material, such as plain paper. At transfer station 65, a
sheet transport apparatus, indicated generally by the reference numeral 48,
moves the sheet into contact with photoconductive belt 20. Sheet
transport 48 has a pair of spaced belts 54 entrained about a pair of
substantially cylindrical rollers 50 and 52. A sheet gripper (not shown)
extends between belts 54 and moves in unison therewith. A sheet is
advanced from a stack of sheets 56 disposed on a tray. A friction retard
feeder 58 advances the uppermost sheet from stack 56 onto a pre-transfer
transport 60. Transport 60 advances the sheet to sheet transport 48 in
synchronism with the movement of the sheet gripper. In this way, the
leading edge of a sheet arrives at a preselected position, i.e. a loading
zone,
to be received by the open sheet gripper. The leading edge of the sheet is
secured releasably by the sheet gripper. As belts 54 move in the direction of
arrow 62, the sheet moves into contact with the photoconductive belt, in
synchronism with the toner image developed thereon. In transfer zone 64,
a corona generating device 66 sprays ions onto the backside of the sheet so
as to charge the sheet to the proper magnitude and polarity for attracting
the toner image from photoconductive belt 20 thereto. The sheet remains
secured to the sheet gripper so as to move in a recirculating path for three
cycles. In this way, three different color toner images are transferred to the
10-
z~z~~~z
sheet in superimposed registration with one another. One skilled in the art
will appreciate that the sheet may move in a recirculating path for four
cycles when under-color or black removal is used. Each of the electrostatic
latent images recorded on the photoconductive surface is developed with
the appropriately colored toner and transferred, in superimposed
registration with one another, to the sheet to form the multicolor copy of
the colored original document.
After the last transfer operation, the sheet transport system
directs the sheet to vacuum conveyor 68 which transports the sheet, in the
direction of arrow 70, to fusing station 71, where the transferred toner
image is permanently fused to the sheet. The fusing station includes a
heated fuser roll 74 and a pressure roll 72. The sheet passes through the nip
defined by fuser roll 74 and pressure roll 72. The toner image contacts fuser
roll 74 so as to be affixed to the sheet. Thereafter, the sheet is advanced by
a pair of rolls 76 to a catch tray 78 for subsequent removal therefrom by the
machine operator.
The last processing station in the direction of movement of belt
20, as indicated by arrow 22, is a cleaning station, indicated generally by
the
reference numeral 79. A rotatably mounted fibrous brush 80 is positioned
in the cleaning station and maintained in contact with photoconductive
belt 20 to remove residual toner particles remaining after the transfer
operation. Cleaning station 79 may also employ pre-clean corotron 81, in
association with brush 80, to further neutralize the electrostatic forces
which attract the residual toner particles to belt 20, thereby improving the
efficiency of the fibrous brush. Thereafter, lamp 82 illuminates
photoconductive belt 20 to remove any residual charge remaining thereon
prior to the start of the next successive cycle.
Referring now to Figure 1, in conjunction with Figures 2 through
4, which depict various portions of the tunable scorotron of Figure 1,
scorotron 34 is comprised of a flexible grid 102, and a corona generating
element 104 enclosed within a U-shaped shield 106. Flexible grid 102 may
be made from any flexible, conductive, perforated material, and is
preferably formed from a thin metal film having a pattern of regularly
21 x.5432
spaced perforations opened therein, as illustrated in Figure 4. As
illustrated, corona generating element 104 is a commonly known wire or
thin rod-like member, however, a variety of comb-shaped pin
arrangements may also be employed as the corona generating element.
The three primary elements of the tunable scorotron, 34; the flexible grid,
the shield, and the corona generating element, are maintained in electrical
isolation from one another so as to prevent electrical current from flowing
directly from one to another. More specifically, corona element mounts
108 are used to electrically insulate the corona generating element from
shield 106, as well as, to rigidly position corona element 104 with respect to
the shield. Similarly, the flexible grid, while being generally supported by
or suspended from shield 106, is insulated therefrom by insulators 110
which form natural extensions of the legs of shield 106. Furthermore, the
entire scorotron assembly, 34, is positioned in a direction parallel to the
surface of photoreceptor belt 20, yet perpendicular to the direction of
travel of the belt.
As indicated by the simplified electrical schematic depicted in
Figure 2, both the shield 106 and the corona element 104 are maintained at
a high voltage potential by power supply 114, the difference in potential
between the two is controlled by resistor R, which may be any fixed or
variable resistor suitable for use in the high voltage circuit. Typically, the
potential of high voltage power supply 114 is in the range of 1 to 10
kilovolts (kV), preferably at about 6 kV, thereby maintaining the corona
element at a potential of about 6 kV and the shield in the range of about 0
to 1 kV. Likewise, grid 102 is also maintained at a predetermined voltage
potential by high voltage supply 116, typically in the range of 0.5 kV to 1.5
kV, and preferably at about 1.0 kV. More importantly, as described by R.M.
Schaffert in Electro~hotograahy, Focal Press, London ( 1971 ),
the ion current (IP) passing from the corona element to the
surface of photoconductive belt 20 is represented as follows:
IP = IS - Ig, Eq.1
~12~432
where Is is the corona current generated by the corona effusing element
104, and Ig is the ion current flowing to the grid. More specifically,
Is = As (V - Vs) (V - Vs - Vo), and Eq. 2
Ig = Ag (V - Vg) (V - Vg - Vo), Eq. 3
where Vo is the critical corona onset voltage, V is the voltage potential on
corona element 104, Vs the potential of the photoreceptor surface, and Vg
the grid potential. Furthermore, constants As and Ag are dependent upon
the geometry and spacing of the Wire and grid, respectively, and their
relationship with other elements in close proximity. Specifically, Ag
pertains to the grid geometry, for example, the pattern of the grid (Figure
4), the area of the open space in the grid, as well as the spatial
relationships
between the grid and the corona element and the grid and the
photoreceptor surface.
As further illustrated in Figures 1 and 2, for example, thumb
screws 118, positioned on each end of scorotron 34, may be used to adjust
the position of the end sections of the grid. Effectively, this allows the
central region of grid 104, as indicated by reference numeral 120 in Figure
1, to be held in a generally planar position, While the opposite ends of the
grid may be independently adjusted up or down in order to vary the spatial
relationship between the grid and the photoreceptor belt surface. In
addition, alternative methods of adjusting the location of the
unconstrained grid ends are understood to exist, and would include a
plurality of spaced-apart ratcheting teeth (not shown) disposed in a linear
direction for releasably constraining an interior portion of aperture 122
through which they would extend.
Referring now to Figures 5, 6, 7, and 8, photoreceptor belt 20 is
generally coated within and extending slightly beyond a center imaging
region 140, which forms the usable imaging area thereon. Along one side,
belt 20 further includes a ground strip region 142 which is uncoated by the
photo responsive layers present in the imaging region, in order to allow the
belt to be grounded by contacting brush 126, or a similar grounding device,
as illustrated in Figure 2. Along both edges of imaging region 140, for
example the region identified by reference numeral 144, there may be a
21_25432
characteristic "fall-off" in the thickness profile of the photoconductive
layer present on the surface of the belt, as illustrated in Figure 6. coupled
with the proximity of the ground strip, the thickness profile nonuniformity
would result in the charge density and voltage profiles represented in
Figure 7 as curves A and B, respectively, when subjected to an "ideal"
scorotron charging device. Such a device would be capable of supplying
copious amounts of charged ions to bring the voltage potential to a
uniform level across the coated surface of photoresponsive belt 20. For
example, at a point where the thickness of the photoconductive coating is
thinner than a nominal thickness of about 24 microns, portions of region
144, a higher charge density will be deposited, whereas the opposite will be
true for thicker photoconductive regions. Thus, for an "ideal" scorotron
charging system, the charge density profile will be inversely proportional to
the thickness of the photoconductive layer being charged.
However, for typical scorotron charging devices, there is a
practical limit to the ion current which can be generated. Hence, the
charge density nonuniformity during a charging operation with a common
planar scorotron device would be less pronounced than the edge
nonuniformity illustrated in curve A of Figure 7. Similarly, there would be
an impact to the charge potential distribution, resulting in a characteristic
decrease in the potential near the edges of the photoconductive coating,
generally proportional to the change in thickness of the photoconductive
coati ng.
On the other hand, it is possible, using the tunable features of
the present invention, to adjust the grid-to-photoreceptor spacing to
achieve a more uniform charge density or voltage profile across the entire
width of imaging region 140, as needed for the particular development
system used. As an example, with the fall-off in charge density exhibited in
curve A of Figure 7, the left end of flexible grid 102 would be adjusted so as
to allow more space between the grid and the surface of photoreceptor
belt 20. The voltage deposited by a scorotron at a point on the surface of
the photoreceptor depends upon the separation of grid and the
photoreceptor surface, following the general rule that the greater the
14-
2125432
distance between the two, the lower the voltage potential that will be
reached on the surface. Therefore by locally increasing the distance
between the grid and the photoreceptor the charge will also be reduced
locally. If it is desirable to reduce the charge density peaks near the edges
of the photoreceptor, as shown in curve A of Fig 7, the separation distance
is increased resulting in a slight depression of the voltage near the edges
and lowering the charge density. In other case, where it is desired to have
a uniform voltage profile, which is not obtainable by a less than "ideal"
scorotron, it is possible to adjust the distance suitably to give a more
uniform profile of voltage. Similarly, the grid spacing may be slightly
increased or decreased on the right side of the scorotron as well, to
compensate for any charge density nonuniformity occurring within the
right side of imaging area 140. The resulting charge density and charge
potential profiles are represented by curves A' and B', respectively, in
Figure
8. There, the impact of the thickness variation in the photoconductive
coating is controlled at least within the imaging region so as to
significantly
reduce or eliminate the deleterious effects on copy quality caused by the
nonuniformity.
In another embodiment, the thumbscrews, 118, used to adjust
the position of the grid ends to alter the grid-to-photoreceptor spacing
may be replaced with servomotor mechanisms, so that the adjustment of
the spacing may be made automatically. More specifically, the
servomotors, or any similar electro-mechanical adjusting means, may be
responsive to a control signal which controls the direction in which the grid
ends are adjusted, over a predetermined range of motion. The control
signal may be generated in response to a manual operator input,
performed at user interface 14, or as an automated response to the
detection of unacceptable charge nonuniformity at the edges of the
imaging region. While it is known that the charging nonuniformity is
measurable using an electrostatic voltmeter, it is also possible to sense the
result of the charging nonuniformity, namely developed toner in the
background regions along the edge of the photoreceptor, in the case of a
discharged area development system. Using commonly known reflectance-
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type toner density measurements, for example, those described
in US-A-4,318,610 to Grace (Issued Mar. 9, 1982), the presence
of developed toner could be detected along
the edges of the imaging area. In response to the detection of toner at the
edges, the control signal would be generated to alter the grid-to-
photoreceptor spacing until the reflectance had increased to a desirable
level, due to the lack of unnecessarily developed toner in the background
regions of the image area. Similarly, using an electrostatic voltmeter to
monitor the potential levels at the edges of the imaging region, the control
signal could be generated to alter the spacing as necessary to achieve more
desirable charge density and charge potential profiles needed for uniform
copy quality, such as those indicated by curves A' and B' in Figure 8.
In recapitulation, the present invention is an apparatus for
altering the relative spacing between a flexible scorotron grid and a charge
retentive surface, such as a photoreceptor, in order to achieve a desired
charge density and charge potential profile across the usable portion of the
surface. More specifically, the relative spacing may be manually or
automatically adjusted by altering the position of the ends of the flexible
grid so as to deform the grid from a nominally planar configuration.
It is, therefore, apparent that there has been provided, in
accordance with the present invention, an apparatus for tuning or altering
the charge potential limiting effect that a scorotron grid has upon an
adjacent charge receiving surface. While this invention has been described
in conjunction with preferred embodiments thereof, it is evident that many
alternatives, modifications, and variations will be apparent to those skilled
in the art. Accordingly, it is intended to embrace all such alternatives,
modifications and variations that fall within the spirit and broad scope of
the appended claims
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