Information
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Patent Grant
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6418287
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Patent Number
6,418,287
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Date Filed
Tuesday, March 7, 200024 years ago
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Date Issued
Tuesday, July 9, 200222 years ago
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Inventors
-
Original Assignees
-
Examiners
-
CPC
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US Classifications
Field of Search
US
- 399 167
- 399 303
- 399 312
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International Classifications
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Abstract
A belt drive operative to rotate one or more photoconductor drums while carrying media sheets past the rotating drum or drums. For single pass color printers in which the different color planes are developed using a series of photoconductor drums, a substantially flat movable belt is positioned adjacent to and extends across each of the drums. The belt simultaneously engages each drum so that movement of the belt past the drums causes the drums to rotate together as the belt carries media sheets past the rotating drums. In one preferred version of the invention, grit applied to the ends of the drum(s) and/or to the edge of the belt is used to transfer driving force from the belt to the drums.
Description
FIELD OF THE INVENTION
The present invention is directed to a system for reducing banding and color plane registration errors in an image forming device and, more particularly, to a belt drive operative to rotate one or more photoconductor drums while carrying media sheets past the rotating drum or drums.
BACKGROUND
To print in color, electronic data representing a desired print image is initially separated into four distinct color planes; one cyan, one magenta, one yellow, and one black. Single pass full color laser printers generally include a sequence of developer stations each responsible for producing one color plane image. As a sheet of paper or other suitable media passes through the first developer station, the first color plane of the separated print image is applied. The full color print image is formed as the paper passes through the other three developer stations with each of the remaining color plane images being superimposed over the first.
Each developer station includes an insulating photoconducting material usually placed on a drum and a light source such as a scanning laser. Repeatedly scanning a beam across the drum in a series of precise lines, the scanning laser creates a latent image corresponding to one color plane on the drum's surface by selectively exposing areas of the photoconductor drum to light. A difference in electrostatic charge density is created between the areas on the drum exposed and not exposed to light. The visible image is developed by electrostatic toners. The photoconductor drum may be either positively or negatively charged, and the toner system similarly may contain negatively or positively charged particles. The toners are selectively attracted to the portions of the photoconducting surface either exposed or unexposed to light, depending on the relative electrostatic charges of the photoconductor drum and the toner. A transfer roller is given an electrostatic charge opposite that of the toner and is rotated close to the photoconductor drum. The transfer roller pulls the toner from the surface of the photoconductor drum onto a sheet of paper or other print media in the pattern of the color plane image developed from the photoconducting surface. The full color image is produced as each color plane image is transferred and fused to the media sheet.
Since laser printers are designed to run very quickly, problems can arise with even slight variations in photoconductor drum speeds. Variations in photoconductor drum speed appear on the printed page as increased or decreased spacing between lines and visually appear as “bands.” Banding can be a particularly severe problem for laser printers when printing full color images such as photographs. In addition to banding, varying photoconductor drum speeds cause color plane registration errors. To produce accurate color prints, each successive color plane image must be precisely aligned and superimposed over the prior color plane. Color plane registration errors exceeding as little as about fifty microns, for example, produces a detectable degradation in print quality.
The principle cause of varying drum speeds is gear noise. Photoconductor drums are generally driven by a stepper motor or a brushless DC motor in connection with a gear array. Gear noise results from imperfect spacing of gear teeth, variances in flexing of gear teeth as forces are transferred from one gear to the next, and other intrinsic variations in gear force transfer. The stepper motor can also contribute to the problem because, as it drives the gear array in a laser printer, it may have slight variations in angular velocity due to the multiple magnet positions for each step.
Past solutions to banding and color plane registration errors include providing helical gears or gears made of better materials or with greater precision. These gears add significantly more expense to the final product. Solutions specific to correct banding include providing sensors to detect undesired variations in the photoconductor drum speed and additional circuitry directed to compensate the modulation of the laser accordingly. Solutions specific to registration errors include sensing variations in the speeds of the photoconductor drums and correspondingly adjusting the timing of the scanning lasers to correct the placement of each color plane image. Unfortunately, each of these solutions, requiring additional circuitry, sensing capabilities, and precision components and machining, substantially increase the manufacturing costs of an image forming device.
SUMMARY
The present invention is directed to a belt drive operative to rotate one or more photoconductor drums while carrying media sheets past the rotating drum or drums. For single pass color printers in which the different color planes are developed using a series of photoconductor drums, a substantially flat movable belt is positioned adjacent to and extends across each of the drums. The belt simultaneously engages each drum so that movement of the belt past the drums causes the drums to rotate together as the belt carries media sheets past the rotating drums. In one preferred version of the invention, grit applied to the ends of the drum(s) and/or to the edge of the belt is used to transfer driving force from the belt to the drums. Conventional single pass color printers that use a paper transport belt to carry paper through the developer stations can be readily adapted to use the invention. It is expected that the invention will provide a cost effective alternative to conventional methods for reducing banding and registration errors.
DESCRIPTION OF THE DRAWINGS
FIG. 1
is a representational side sectional view of a single pass color printer incorporating the present invention.
FIG. 2
is a schematic side sectional view of media sheet transport belt driven photoconductor drums according to one embodiment of the present invention.
FIG. 3
is schematic view showing grit applied to the ends of the photoconductor drum to transfer driving force from the belt to the drum.
FIG. 4
is a schematic view showing grit applied to the sides of the transport belt to transfer driving force from the belt to the drum.
FIG. 5
is a schematic view showing grit applied to ends of the photoconductor drum and to the sides of the transport belt.
DETAILED DESCRIPTION
Although it is expected that the invention will be most useful in electrophotographic printing devices such as the single pass color laser printer illustrated in
FIG. 1
, the invention may be suitable for use in any single or multiple drum printer, copier, or other image forming device in which it is necessary of desirable to closely coordinate drum rotation.
FIG. 1
illustrates a single pass color laser printer, designated by reference number
10
. Color laser printer
10
includes a media sheet transport system that includes media tray
12
, pick roller
14
, follower rollers
16
,
18
, transport belt
20
, tensioning rollers
22
,
24
, and drive rollers
26
. Pick roller
14
picks a media sheet
30
, typically a sheet of paper, from tray
12
and advances sheet
30
toward follower rollers
16
and
18
. Sheet
30
is grabbed between follower rollers
16
,
18
and transport belt
20
and carried past developer stations
32
,
34
,
36
and
38
on upper run
20
a
of belt
20
. Drive rollers
26
or another suitable mechanism engage the lower run
20
b
of belt
20
urging belt
20
to circulate around tensioning rollers
22
,
24
.
The desired print image is applied to each sheet
30
at developer stations
32
,
34
,
36
, and
38
. Each developer station is the same except that each contains a different color toner and is responsible for transferring a different color plane image to media sheet
30
. For instance, developer station
32
contains black toner (K), developer station
34
contains yellow toner (Y), developer station
36
contains magenta toner (M), and developer station
38
contains cyan toner (C). Each developer station includes a photoconductor drum
40
, a charge roller
42
, a scanning laser
44
, a developer roller
46
and a transfer roller
47
. Each drum
40
is placed adjacent to one transfer roller
47
with transport belt
20
passing between the two. The toner supply for each developer station is maintained within a reservoir
48
.
In operation, as belt
20
carries media sheet
30
toward the black developer station
32
, charge roller
42
places a relative and uniform electrical charge on photoconductor drum
40
. Repeatedly scanning a light beam horizontally across photoconductor drum
40
in a series of precise lines, scanning laser
44
creates a latent image of the corresponding color plane, in this case black, on the surface of photoconductor drum
40
by selectively discharging portions of photoconductor drum
40
according to the black color plane image. A difference in electrostatic charge density is created between the areas on drum
40
exposed and not exposed to the beam. Each color plane image is developed by electrostatic toners. As photoconductor drum
40
rotates the charged image, it passes by developer roller
46
enabling toner to be taken up from roller
46
onto the exposed or not exposed portions of photoconductor drum
40
depending upon the relative electrostatic charges of drum
40
and the toner. Thereafter, the toner image is rotated into contact with media sheet
30
which is pressed between photoconductor drum
40
and adjacent transfer roller
47
. Transfer roller
47
is given an electrostatic charge opposite that of the toner. As media sheet
30
passes between photoconductor drum
40
and transfer roller
47
, transfer roller
47
pulls the toner onto media sheet
30
. The desired full color image is created as media sheet
30
passes through the remaining developer stations
34
,
36
,
38
, each functioning in a substantially identical manner. Once each color plane image is transferred to media sheet
30
, the toner is fused to media sheet
30
as the sheet passes between heated fusing rollers
50
, and media sheet
30
is released to output bin
52
.
Referring now to
FIGS. 2 and 3
, instead of being individually driven by a separate motor and gear array as in a conventional printer, photoconductor drums
40
are commonly rotated by transport belt
20
. Belt
20
engages each of the drums
40
simultaneously so that movement of belt
20
past drums
40
cause the drums to rotate while belt
20
carries media sheets
30
past the rotating drums. The transfer driving force from belt
20
to drums
40
is improved through grit
54
applied to one or both ends of drums
40
. Grit
54
increases friction between photoconductor drums
40
and transport belt
20
. It may be desirable in some operating environments that grit
54
extend in far enough to overlap media sheet
30
. The increased friction on sheet
30
allows photoconductor drum
40
to secure the margins of media sheet
30
against belt
20
to help prevent media sheet
30
from slipping as belt
20
carries it past drum
40
. The increased friction between photoconductor drum
40
and transport belt
20
also helps prevent photoconductor drum
40
from slipping as transport belt
20
circulates. As shown in
FIG. 3
, grit
54
is applied to contact and overlap transport belt
20
and, preferably, the margins of media sheet
30
as belt
20
carries media sheet
30
past photoconductor drum
40
.
In an alternative embodiment of the invention shown in
FIG. 4
, grit
56
is applied along one or both edges on the outer surface
58
of belt
20
. As above, grit
56
provides increased friction between photoconductor drum
40
, media sheet
30
, and transport belt
20
. Again, the size and placement of grit
56
can be selected to allow grit
56
to contact and overlap both media sheet
30
and photoconductor drum
40
as belt
20
transfers media sheet
30
past drum
40
. Grit
54
,
56
can be formed by depositing and affixing a plurality of grit particles such as sand, metal flake, rubber bits, or other suitable material to selected surface areas of photoconductor drum
40
and transport belt
20
. Alternatively, grit
54
,
56
can be formed by roughening, machine etching, grinding, or cutting those surface areas of transport belt
20
and photoconductor drums
40
. It is expected that a random distribution of grit particles applied to one or both ends of drum
40
in which the spacing between grit particles is less than ⅓ mm will be sufficient to transfer adequate driving force from belt
20
to drum
40
. A random distribution of grit particles is preferred to help prevent the engagement of the belt to the drums from creating continuous frequencies that might contribute to banding. Also, it has been observed that bandwidths ⅓ mm or less are not visible when the printed sheets are viewed from 20 inches or more. Hence, it is expected that grit particles spaced ⅓ mm or less should be sufficient to prevent banding that is visible from 20 inches or more.
Although the present invention has been shown and described with reference to the foregoing exemplary embodiments, other embodiments are possible. For example, the invention could be used in a monochrome printer having only one photoconductor drum. One of the tension rollers
22
or
24
could be used to drive the belt in lieu of separate drive rollers
26
. As shown in
FIG. 5
, grit may be applied to both drum
40
and belt
20
as may be necessary or desirable to transfer adequate driving force to drum
40
. The type, size and density of the grit particles will likely vary depending on the particular printing device and environmental conditions. Other friction enhancement/driving force transfer mechanisms might also be used. The extent of the engagement between belt
20
and drums
40
, the normal force exerted at the point of engagement between belt
20
and drums
40
and other factors may influence the specific characteristics of the engagement between belt
20
and drums
40
. It is to be understood, therefore, that other forms, details, and embodiments may be made without departing from the spirit and scope of the invention which is defined in the following claims.
Claims
- 1. A belt drive system for multiple photoconductor drums, comprising:a series of photoconductor drums; a series of transfer rollers each adjacent to one of the photoconductor drums; a first roller disposed upstream from the first in the series of drums and a second roller disposed downstream from the last in the series of drums; an endless loop belt wrapping the first and second rollers, the belt having a generally horizontal upper run that carries media sheets and passes between and extends across the transfer rollers and the photoconductor drums, the upper run of belt simultaneously engaging each drum so that movement of the belt past the drums causes the drums to rotate as the belt carries media sheets past the rotating drums; grit particles randomly distributed along a portion of the belt that engages each drum such that the spacing between the grit particles is less than or equal to ⅓ mm.
- 2. A single pass color printer, comprising:a series of developer stations each configured to transfer a different color plane toner image to a media sheet and each developer station having a photoconductor drum and a transfer roller adjacent to the photoconductor drums; a first roller disposed upstream from the first in the series of developer stations and a second roller disposed downstream from the last in the series of developer stations; an endless loop belt wrapping the first and second rollers, the belt having a generally horizontal upper run that carries media sheets and passes between and extends across the transfer rollers and the photoconductor drums, the upper run of belt simultaneously engaging each drum so that movement of the belt past the drums causes the drums to rotate as the belt carries media sheets past the rotating drums; grit particles randomly distributed along a portion of the belt that engages each drum such that the spacing between the grit particles is less than or equal to ⅓ mm; a media tray disposed upstream from the developer stations for holding media sheets; at least one feed roller operative to move media sheets from the media tray to the belt; and a fuser disposed downstream from the developer stations for fusing the toner images to the media sheets.
US Referenced Citations (7)