U.S. patent application number 09/750292 was filed with the patent office on 2002-07-04 for image forming apparatus for maintaining a constant beam scanning state.
Invention is credited to Kawai, Koji, Komiya, Kenichi, Sakakibara, Jun, Tanimoto, Koji.
Application Number | 20020085081 09/750292 |
Document ID | / |
Family ID | 25017256 |
Filed Date | 2002-07-04 |
United States Patent
Application |
20020085081 |
Kind Code |
A1 |
Tanimoto, Koji ; et
al. |
July 4, 2002 |
IMAGE FORMING APPARATUS FOR MAINTAINING A CONSTANT BEAM SCANNING
STATE
Abstract
An image forming apparatus maintains a constant beam scanning
state so that light beams scan each drum always in the same state
once a desired color overlapping accuracy is obtained by
controlling color registration in color image formation. This
mechanism achieves constantly fine color overlapping without
forming a test pattern several times or consuming toner
unnecessarily. A beam passage switch is provided in a laser
scanning system. Sensors provided near each photosensitive drum
determine whether each light beam has scanned a certain portion
with no inclination or a certain scanning area, or whether each
image magnification provides a desired size. Sensors for detecting
scanning light beam obtains the center of light beam in accordance
with information on a point at which the sensors start to respond
and another point at which they cease responding, for performing
several types of control based on the positional information, which
provides correct information even though the size or power is
different over sevral light beam.
Inventors: |
Tanimoto, Koji;
(Kawasaki-Shi, JP) ; Komiya, Kenichi;
(Kawasaki-Shi, JP) ; Sakakibara, Jun; (Tokyo-To,
JP) ; Kawai, Koji; (Fujisawa-shi, JP) |
Correspondence
Address: |
Johnny A. Kumar
FOLEY & LARDNER
Washington Harbour
3000 K Street, N.W., Suite 500
Washington
DC
20007-5109
US
|
Family ID: |
25017256 |
Appl. No.: |
09/750292 |
Filed: |
December 29, 2000 |
Current U.S.
Class: |
347/115 ;
347/116; 347/232 |
Current CPC
Class: |
H04N 1/506 20130101;
G03G 2215/0119 20130101; H04N 1/233 20130101; H04N 1/295 20130101;
H04N 2201/33378 20130101; H04N 2201/0471 20130101; H04N 2201/04744
20130101; H04N 1/1135 20130101; H04N 1/2369 20130101; H04N
2201/04729 20130101; H04N 1/12 20130101 |
Class at
Publication: |
347/115 ;
347/116; 347/232 |
International
Class: |
G03G 015/01; G03G
015/04 |
Claims
What is claimed is:
1. An image forming apparatus for selectively outputting color and
monochrome images comprising: a first light beam generator to
generate a first light beams; a second light beam generator to
generate a second light beams; a scanner to control the first and
the second light beams generated by the first and the second light
beam generators, respectively, for scanning; a first scanning state
changer to change a scanning state of the first light beam; a
second scanning state changer to change a scanning state of the
second light beam; a first member to be exposed, scanned with the
first light beam or both the first and the second light beams while
the first member transferring in a perpendicular direction with
respect to a beam scanning direction; a second member to be
exposed, scanned with the second light beam while the second member
transferring in the perpendicular direction with respect to the
beam scanning direction; a first mode image generator to scan the
first member to be exposed with the first and the second light
beams, thus generating a monochrome image in a first image
formation mode; a second mode image generator to scan the first
member to be exposed with the first light beam and scan the second
member to be exposed with the second light beam, thus generating a
color image in a second image formation mode; a first light beam
detector to detect the scanning state of the first light beam or
the first and the second light beams to the first member to be
exposed; a second light beam detector to detect the scanning state
of the second light beam to the second member to be exposed; a
selector to select either the first image formation mode or the
second image formation mode; and a controller, in the first image
formation mode, to detect the scanning state of the first and the
second light beams to the first member to be exposed through the
first detector, thus controlling the first and the second scanning
state changers so that each light beam is set in a certain scanning
state, in the second image formation mode, to detect the scanning
state of the first light beam to the first member to be exposed
through the first detector and detect the scanning state of the
second light beam to the second member to be exposed through the
second detector, thus controlling the first and the second scanning
state changers so that each light beam is set in a certain scanning
state.
2. The image forming apparatus according to claim 1, wherein the
first and the second scanning state changers change a scanning
position of each light beam.
3. The image forming apparatus according to claim 1, wherein the
first and the second scanning state changers change an image
forming area for each light beam.
4. The image forming apparatus according to claim 1, wherein the
first and the second scanning state changers change image
magnification of each light beam.
5. The image forming apparatus according to claim 1, wherein the
first and the second scanning state changers change a scanning
direction corresponding to inclination of each light beam.
6. The image forming apparatus according to claim 1, wherein a
transfer speed of the first member to be exposed is variable and a
transfer speed in the first image formation mode is higher than
another transfer speed in the second image formation mode.
7. An image forming apparatus capable of switching a color image
formation mode and a monochrome image formation mode and storing a
control state in each mode, the apparatus comprising: a first light
beam generator to generate a first light beams; a second light beam
generator to generate a second light beams; a scanner to control
the first and the second light beams generated by the first and the
second light beam generators, respectively, for scanning; a first
scanning state changer to change a scanning state of the first
light beam; a second scanning state changer to change a scanning
state of the second light beam; a first member to be exposed,
scanned with the first light beam or both the first and the second
light beams while the first member transferring in a perpendicular
direction with respect to a beam scanning direction; a second
member to be exposed, scanned with the second light beam while the
second member transferring in the perpendicular direction with
respect to the beam scanning direction; a first mode image
generator to scan the first member to be exposed with the first and
the second light beams, thus generating a monochrome image in a
first image formation mode; a second mode image generator to scan
the first member to be exposed with the first light beam and scan
the second member to be exposed with the second light beam, thus
generating a color image in a second image formation mode; a first
light beam detector to detect a scanning state of the first light
beam or the first and the second light beams to the first member to
be exposed; a second light beam detector to detect a scanning state
of the second light beam to the second member to be exposed; a
selector to select either the first image formation mode or the
second image formation mode; a first memory to store the scanning
state of the first light beam to the first member to be exposed in
the first image formation mode; a second memory to store the
scanning state of the first light beam to the first member to be
exposed and the scanning state of the second light beam to the
second member to be exposed in the second image formation mode; and
a controller to control the first and the second scanning state
changers in accordance with information stored in the first or the
second memory when the selector performs selection of image
formation mode.
8. An image forming apparatus capable of switching a color image
formation mode and a monochrome image formation mode and outputting
a test pattern, the apparatus comprising: a first light beam
generator to generate a first light beams; a second light beam
generator to generate a second light beams; a scanner to control
the first and the second light beams generated by the first and the
second light beam generators, respectively, for scanning; a first
scanning state changer to change a scanning state of the first
light beam; a second scanning state changer to change a scanning
state of the second light beam; a first member to be exposed,
scanned with the first light beam or both the first and the second
light beams while the first member transferring in a perpendicular
direction with respect to a beam scanning direction; a second
member to be exposed, scanned with the second light beam while the
second member transferring in the perpendicular direction with
respect to the beam scanning direction; a first mode image
generator to scan the first member to be exposed with the first and
the second light beams, thus generating a monochrome image in a
first image formation mode; a second mode image generator to scan
the first member to be exposed with the first light beam and scan
the second member to be exposed with the second light beam, thus
generating a color image in a second image formation mode; a first
light beam detector to detect a scanning state of the first light
beam or the first and the second light beams to the first member to
be exposed; a second light beam detector to detect a scanning state
of the second light beam to the second member to be exposed; a
selector to select either the first image formation mode or the
second image formation mode; a image positional relationship
detector to determine whether a position of an image formed by the
first light beam and another image formed by the second light beam
lie in a predetermined positional relationship in the second image
formation mode; a memory to store the scanning states of the first
and the second light beams when the first and the second scanning
state changers are controlled so that the image formed by the first
light beam and the other image formed by the second light beam lie
in the predetermined positional relationship in the second image
formation mode; and a controller, in the first image formation
mode, to detect the scanning states of the first and the second
light beams to the first member to be exposed through the first
detector, thus controlling the first and the second scanning state
changers so that each light beam is set in a certain scanning
state, in the second image formation mode, to control the first and
the second scanning state changers in accordance with information
stored in the memory.
9. An image forming apparatus capable of switching a color image
formation mode and a monochrome image formation mode, outputting a
test pattern and storing a control state in each mode, the
apparatus comprising: a first light beam generator to generate a
first light beams; a second light beam generator to generate a
second light beams; a scanner to control the first and the second
light beams generated by the first and the second light beam
generators, respectively, for scanning; a first scanning state
changer to change a scanning state of the first light beam; a
second scanning state changer to change a scanning state of the
second light beam; a first member to be exposed, scanned with the
first light beam or both the first and the second light beams while
the first member transferring in a perpendicular direction with
respect to a beam scanning direction; a second member to be
exposed, scanned with the second light beam while the second member
transferring in the perpendicular direction with respect to the
beam scanning direction; a first mode image generator to scan the
first member to be exposed with the first and the second light
beams, thus generating a monochrome image in a first image
formation mode; a second mode image generator to scan the first
member to be exposed with the first light beam and scan the second
member to be exposed with the second light beam, thus generating a
color image in a second image formation mode; a first light beam
detector to detect a scanning state of the first light beam or the
first and the second light beams to the first member to be exposed;
a second light beam detector to detect a scanning state of the
second light beam to the second member to be exposed; a selector to
select either the first image formation mode or the second image
formation mode; a first memory to store the scanning states of at
least the first and the second light beams to the first member to
be exposed or control states of the first and the second scanning
state changers in the first image formation mode; an image
positional relationship detector to detect a positional
relationship of an image formed by the first light beam and another
image formed by the second light beam in the second image formation
mode; a memory to store the scanning states of the first and the
second light beams when the first and the second scanning state
changers are controlled so that the image formed by the first light
beam and the other image formed by the second light beam lie in the
predetermined positional relationship in the second image formation
mode; and a controller, when the selector selects the first image
formation mode, to control the first and the second scanning state
changers in accordance with information stored in the first memory,
whereas when the selector selects the image formation mode, to
control the first and the second scanning state changers in
accordance with information stored in the second memory.
Description
BACKGROUND OF THE INVENTION
[0001] The present invention relates to an image forming apparatus
with a multi-beam scanning system. Particularly, this invention
relates to an image forming apparatus including a printer and a fax
machine, etc., that overcomes a problem in that a formed image is
blurred due to positional shifting of dots on each line which
occurs when image magnification is not constant over light beams in
simultaneous formation of dots on several lines in an image-forming
area with light beams. Moreover, the present invention relates to
an image forming apparatus that overcomes a problem in that an
image-forming area will be shifted due to difference in image
magnification on a plane to be exposed to light beams, by detecting
scanning light beams on the plane at least two points in the
vicinity of the entire plane.
[0002] Several techniques have been proposed for image forming
apparatus, for example, a plane paper copy machine (called PPC
hereinafter) or a laser printer that prints out data from an image
data supplying apparatus, such as a personal computer (called PC
hereinafter) and a digital camera, on regular sheets or OHP sheets.
Known single-beam exposing system has gradually been replaced with
multi-beam exposing system for enhancing image-forming speed in
this type of image forming apparatus.
[0003] The multi-beam exposing system enhances an image-forming
speed by simultaneous scanning several lines with laser beams. Such
multi-beam exposing system is proposed, for example, in Japanese
Unexamined Publication NO. 9-258125 disclosing digital
beam-positional control, Japanese Un-examined Publication NO.
9-314901 disclosing beam-positional control in main scanning
direction, Japanese Unexamined Publication NO. 10-76704 disclosing
ladder-pattern sensors, Japanese Un-examined Publication NO.
10-142535 disclosing techniques for eliminating operational
amplifier-offsetting, Japanese Un-examined Publication NO. 11-84280
disclosing power control, Japanese Un-examined Publication NO.
11-951432 disclosing several resolutions, Japanese Un-examined
Publication NO. 11-202229 (Japanese Application NO. 10-6776)
disclosing beam-positional control in main scanning direction, and
Japanese Un-examined Publication NO. 10-356022 disclosing relative
beam-positional control in sub scanning direction. These proposals
mainly relate to light exposed position control. Particularly,
Japanese Un-examined Publication NO. 11-202229 discloses precise
beam-positional control in main scanning direction used in light
beam scanning apparatus and image forming apparatus.
[0004] The known image forming apparatus, however, has the
following drawbacks:
[0005] Light beams used for control are usually detected at edges
on an area to be scanned due to optical system structure. The known
image forming apparatus cannot generate constant light-beam shape
or power on this area. Inconstant light-beam shape or power causes
inaccurate detection of each beam position, thus resulting in
inaccurate light beam control. Such problem mostly occur in using a
compact polygon mirror having a small reflecting surface for light
beam scanning for higher beam scanning speed. A compact and thus
light polygon mirror relieves its load for increasing rotation
number.
[0006] Moreover, difference in wavelength or power of several laser
sources causes a problem in that a scanning-finishing point
fluctuates even though each beam scanning-starting point is
precisely controlled. In other words, it causes fluctuation in
image-forming area, or image magnification for each beam due to
difference in reflectivity of lenses or glasses caused by
difference in wavelength over several light beams.
[0007] Furthermore, monochrome-color switching machines have a
problem of increase in toner consumption in color registration
control.
SUMMARY OF THE INVENTION
[0008] In order to overcome the foregoing problems for known image
forming apparatus, a first purpose of the present invention is to
decrease in toner consumption in color registration control for a
color image forming apparatus, in detail, to attain high color
overlapping accuracy by color registration control with minimum
test pattern generation, followed by beam-positional control.
[0009] A second purpose of the present invention is to provide an
image forming apparatus for efficient and rapid monochrome and
color image forming. In other words, it is the purpose to solve
problems on two types of color image forming apparatus most popular
at present, thus providing an image forming apparatus with high
color image-forming speed and much higher monochrome image-forming
speed.
[0010] A third purpose of the present invention is to attain higher
image quality in high-speed image-forming apparatus using
multi-beam scanning system. An image forming apparatus with high
beam positional-detection accuracy even at edges on an area to be
scanned on which light beam shape or power tend to fluctuate
enhances beam positional-control accuracy. Moreover, an
image-forming apparatus having a detector for detecting an
image-forming area (image magnification) for each beam to detect
image magnification for each light beam provides image-forming
areas (image magnification) matched for several light beams for
forming high image of quality irrespect of difference in wavelength
of light beams.
[0011] In order to attain the first purpose, the image forming
apparatus according to the present invention maintains a constant
beam scanning state so that light beams scan each drum always in
the same state when a desired color-overlapping accuracy is
obtained by one-time color registration control, thus achieving
constantly fine color overlapping without generating a test
patterns several times or consuming toner unnecessarily. The beam
scanning state is monitored by sensors provided in the vicinity of
a photosensitive drum.
[0012] An optimum scanning state may be different for each light
beam in a high-speed monochrome image formation mode and a color
image formation mode. A control state for each light beam in each
mode is stored in memory for reproduction of the same state without
performing again color registration control when the monochrome
image formation mode is switched to the color image formation mode,
thus consuming no toner unnecessarily and also shortening a waiting
time for image formation when the mode is switched.
[0013] In order to attain the second purpose, the image forming
apparatus according to the present invention achieves efficient and
high-speed color image formation based on 4-tandem system in
combination of multi-beam exposure technique. Color images are
formed by scanning 4-color drums with four light beams like a
conventional 4-tandem system at a speed the same as the
conventional system. On the other hand, monochrome images are
formed with light beams including beams which usually used for
colors other than monochrome image formation. For example, in
formation of K-image only, light beams used for Y-, M- and C-image
formation are all used for the K-image, thus achieving four-fold
image formation performance. For such image formation, a transfer
speed of each photosensitive drum and transfer section is adjusted
to match an image-recording speed for high-speed operation with
optimum control of other requirements.
[0014] Accordingly, the present invention is provided with a
beam-passage switch in the laser scanning system for operating the
apparatus efficiently. Moreover, sensors are provided in the
vicinity of the photosensitive drum to determine whether each light
beam has scanned a certain portion with no inclination or a certain
scanning area (range), or whether each image magnification has
provided a desired size, and also a control mechanism is provided
for precisely scanning each photosensitive drum with each light
beam based on information always correct when detected.
[0015] In order to attain the third purpose, the present invention
provides an image forming apparatus for accurately detecting beam
positions in a light beam scanning direction also accurately
detecting image magnification. In general, sensors for detecting
scanning light beams output different response waveforms depending
on beam shape or power at edges of a scanning area at which beam
shape and power relatively fluctuate, which provides incorrect
information. The present invention therefore obtains a beam center
position from information on a point at which sensors start to
respond and another point at which they cease responding and, based
on the center positional information, performs several types of
control. This achieves several types of accurate control based on
correct beam positional information even at the edges of scanning
area at which beam size and power fluctuate, thus forming images at
high accuracy. In other words, the image forming area is shifted to
detect a point at which the sensors start to respond and another
point at which the sensors cease responding for each light beam
and, based on this information, define an image forming area for
each light beam, thus forming images at high accuracy even though
beam size and power are different over the light beams.
[0016] Against a problem of difference in image forming area, the
present invention is provided with a detector to detect an image
forming area (image magnification) for each light beam for image
forming area setting for each light beam so that the centers of
image forming areas of light beams meet each other for minimizing
the effects of difference in detected image magnifications for the
light beams. Moreover, adjustments to the clock frequency which is
the basis for defining the image forming area for each light beam
allows the image forming areas for the light beams to meet each
other precisely. The present invention also provides selection of
an optimum clock frequency as the basis for defining the image
forming area for each light beam from among several clocks.
BRIEF DESCRIPTION OF DRAWINGS
[0017] FIG. 1 is a schematic view of an embodiment of image forming
apparatus in color mode according to the present invention;
[0018] FIG. 2 is a schematic view of an embodiment of image forming
apparatus in monochrome mode according to the present
invention;
[0019] FIG. 3A is a schematic view of an ideally exposed alphabet
"T", FIGS. 3B to 3D are schematic views of incorrectly formed
alphabet "T" in inaccurate control of scanning position, inaccurate
control of exposure timing, and inaccurate control of both scanning
position and exposure timing, respectively;
[0020] FIG. 4 is a block diagram of a light beam control system in
embodiments of image forming apparatus according to the present
invention;
[0021] FIG. 5 is a flow chart indicating a control flow of
four-light beam scanning in monochrome image formation mode;
[0022] FIG. 6 illustrates the relationship among motion of each
light beam, printing areas and respective signals in monochrome
image formation mode;
[0023] FIG. 7 is a block diagram of mechanism of adjusting image
magnification and printing area;
[0024] FIG. 8 is another block diagram of mechanism of adjusting
image magnification and printing area;
[0025] FIG. 9A is plan view illustrating the relationship between
the first and the second light beam position detection sections and
several light beams, and
[0026] FIG. 9B is a time chart of signals corresponding to the beam
positional relationship in FIG. 9A;
[0027] FIG. 10 is an explanatory view of printing and exposed areas
corresponding to the first and the second sensors with a time chart
of respective signals;
[0028] FIG. 11 is another explanatory view of printing and exposed
areas corresponding to the first and the second sensors with a time
chart of respective signals;
[0029] FIG. 12 is still another explanatory view of printing and
exposed areas corresponding to the first and the second sensors
with a time chart of respective signals;
[0030] FIG. 13 is further another explanatory view of printing and
exposed areas corresponding to the first and the second sensors
with a time chart of respective signals;
[0031] FIG. 14 is a flow chart indicating image magnification
matching process;
[0032] FIG. 15A is a schematic view indicating a process of
matching printing areas in early stage in the scanning direction,
FIG. 15B is a schematic view indicating a process of making small
the maximum value of shifting by making small the shifting of the
sheet center, and FIG. 15C is a schematic view indicating a process
of canceling the shifting in the sheet center with image
magnification matching;
[0033] FIG. 16A is a schematic view of a degraded sample image
formed when the sensors 48 and 49 are attached in the same
direction with respect to the light beam scanning direction, and
FIG. 16B is another schematic view of a degraded sample image
formed when the sensors 48 and 49 are attached in different
directions with respect to the light beam scanning direction;
[0034] FIG. 17A is a schematic view of sensor pattern for detecting
inclination of the sensor 48, and FIG. 17B is another schematic
view of sensor pattern for detecting inclination of the sensor 49;
and
[0035] FIG. 18 is a flow chart indicating a process flow of light
beam control in color image formation mode.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0036] Preferred embodiments of image forming apparatus according
to the present invention will be disclosed with reference to the
attached drawings.
[0037] FIG. 1 is a schematic view of the first embodiment of image
forming apparatus according to the present invention. This is an
example of application of the present invention to a 4-tandem color
fax machine. In FIG. 1, an image forming apparatus 1 includes an
optical unit 2, developers 3Y, 3M, 3C and 3K for forming an image
for each color, photosensitive drums 4Y, 4M, 4C and 4K, chargers
5Y, 5M, 5C and 5K, cleaners 6Y, 6M, 6C and 6K, a transfer belt 7, a
sucking roller 8, a transfer belt rollers 9 and 10, a transfer belt
cleaner 11, aligning rollers 12 and 13, a paper feed roller 14, a
paper feed cassette 15 for stocking and feeding sheets P, a fixing
device 16, a fiber sensor 17, and transfer rollers 18Y, 18M, 18C
and 18K.
[0038] The optical unit 2 consists of a polygon motor 20 and a
polygon mirror 21, both described later, as the basic components.
Light beams emitted by several light sources housed in the unit 2
scan the photosensitive drums 4Y, 4M, 4C and 4K corresponding to
respective colors.
[0039] The 4-tandem color copy machine having the foregoing
structure performs a color image-forming process which will be
disclosed briefly.
[0040] The photosensitive drums 4Y, 4M, 4C and 4K and the transfer
belt 7 rotate by a drive motor (not shown) at a certain speed on
their outer periphery. The chargers 5Y, 5M, 5C and 5K arranged as
facing the surfaces of the photosensitive drums 4Y, 4M, 4C and 4K,
respectively, charge the photosensitive drums 4Y, 4M, 4C and 4K at
certain potentials. Four light beams emitted by the optical unit 2
are focussed, as scanning beams having a spot of necessary
resolution, on positions to be exposed on the photosensitive drums
4Y, 4M, 4C and 4K which are materials to be exposed, thus the drums
being exposed by scanning. The exposure forms electrostatic latent
images corresponding to an image signal on the photosensitive drums
4Y, 4M, 4C and 4K.
[0041] The electrostatic latent images formed on the photosensitive
drums 4Y, 4M, 4C and 4K are developed using toner, a developing
agent, supplied from the developers 3Y, 3M, 3C and 3K, thus forming
toner images. The electrostatic latent image, for example, formed
on the photosensitive drum 4Y is developed by the developer 3Y as a
yellow toner image. Likewise, the electrostatic latent images
formed on the photosensitive drums 4M, 4C and 4K are developed by
the developers 3M, 3C and 3K, respectively, thus, being formed as a
magenta toner image, a cyan toner image and a black toner
image.
[0042] The sheets P stocked in the paper feed cassette 15 are
conveyed to the aligning rollers 12 and 13 by the rotating paper
feed roller 14, and after the edges are aligned (positional
adjustment), conveyed to the sucking roller 8 and the transfer belt
roller 9 by the rotating aligning rollers 12 and 13. The sheets P
are conveyed by the rotating sucking roller 8 and the transfer belt
roller 9 towards the transfer belt roller 10 and further conveyed
in the reverse direction by the roller 10 while being sucked on the
transfer belt 7 while a predetermined potential difference is being
applied between the sucking roller 8 and the transfer belt roller
9.
[0043] The toner images formed on the photosensitive drums 4Y, 4M,
4C and 4K, which have been developed by the developers 3Y, 3M, 3C
and 3K, respectively, are transferred on a sheet P at positions on
which the transfer belt 7 and the transfer rollers 18Y, 18M, 18C
and 18K touch each other. A predetermined potential applied to each
of the transfer rollers 18Y, 18M, 18C and 18K generates static
charges which serves to transfer the toner images on the
photosensitive drums 4Y, 4M, 4C and 4K on the sheet P. The toner
imaqes of the four colors Y, M, C and K are overlapped with each
other on the sheet P while being sucked on the transfer belt 7 by
the foregoing operations, thus a color image being formed
thereon.
[0044] The sheet P is then heated and pressed while passing through
the fixing device 16, thus the toner images on the sheet P being
melted and infallibly fixed thereon. The cleaners 6Y, 6M, 6C and 6K
eliminate the residual toner on the photosensitive drums 4Y, 4M, 4C
and 4K, which have completed image transfer on the sheet P,
respectively. The photosensitive drums 4Y, 4M, 4C and 4K are then
reset to the initial state, thus being ready for the next
image-forming process. The transfer belt cleaner 11 eliminates
extra toner attached on the transfer belt 7, which has completed
sheet transfer, while passing through the cleaner 11, thus the
transfer belt 7 being ready for the next sheet transfer. Moreover,
the transfer belt 7 is subjected to several potential applications
at the sheet sucking section and the toner transfer section, etc.,
however, since the belt 7 is made of a semiconductor material and
the transfer belt roller 10 is grounded, the belt 7 returns to the
initial state (ground potential) on completion of extra toner
elimination. The foregoing process is repeated for successive
color-image formation.
[0045] Disclosed next are the detailed structure of the optical
unit 2 and an optical passage in image formation. The optical unit
2 houses, for example, four semiconductor lasers (not shown). A
light beam emitted by each semiconductor laser is reflected at the
surface of the polygon mirror 21 while rotating by the polygon
motor 20 and scans the surfaces (to be exposed) of the
photosensitive drums 4Y, 4M, 4C and 4K.
[0046] Here, light beams that have a possibility of reaching the
photosensitive drums 4Y, 4M, 4C and 4K are called light beams BM-Y,
BM-M, BM-C and BM-K, respectively. Scan beams controlled by the
polygon mirror 21 pass through senses LN1, LN2 and LN3. Each lens
exhibits the characteristics to reform each light beam into a small
diameter-beam and the f .theta. -characteristics to control the
beam at a constant scanning speed on each photosensitive drum.
[0047] The light beam BM-Y that has a possibility of reaching the
photosensitive drum 4Y passes through the lenses LN1, LN2 and LN3
and is reflected by a half mirror HM-Y by about 50% of its light
components, thus the reflected light components becoming a light
beam BM-Y1. The light beam BM-Y1 is then reflected by mirrors MR-Y1
and MR-Y2 and reaches the photosensitive drum 4Y. On the other
hand, a light beam BM-Y2 that has passed through the half mirror
HM-Y is shut out by a shield member (shutter) SHT-Y1 so that it
will not reach any drum.
[0048] The light beam BM-M that has a possibility of reaching the
photosensitive drum 4M passes through the lenses LN1, LN2 and LN3
and is reflected by a half mirror HM-M by about 50% of its light
components, thus the reflected light components becoming a light
beam BM-M1. The light beam BM-M1 is then reflected by mirrors MR-M1
and MR-M2 and reaches the photosensitive drum 4M. On the other
hand, a light beam BM-M2 that has passed through the half mirror
HM-M is shut out by a shield member (shutter) SHT-M2 so that it
will not reach any drum.
[0049] The light beam BM-C that has a possibility of reaching the
photosensitive drum 4C passes through the lenses LN1, LN2 and LN3
and is reflected by a half mirror HM-C by about 50% of its light
components, thus the reflected light components becoming a light
beam BM-C1. The light beam BM-C1 is then reflected by mirrors MR-C1
and MR-C2 and reachs the photosensitive drum 4C. On the other hand,
a light beam BM-C2 that has passed through the half mirror HM-C is
shut out by a shield member (shutter) SHT-C1 so that it will not
reach any drum.
[0050] The light beam BM-K that has a possibility of reaching the
photosensitive drum 4K passes through the lenses LN1, LN2 and LN3
and is reflected by a half mirror HM-K, thus reaching the drum 4K.
As disclosed, the light beams emitted by four semiconductor lasers
(not shown) are reflected by the surface of the polygon mirror 21
rotated by the polygon mirror 20 and then pass through the
respective passages to become light beams to scan the respective
photosensitive drums, thus enabling color image formation.
[0051] The shield members (shutters) SHT-Y2, SHT-M2, SHT-C2 and
SHT-K shut out the light beams that go to the photosensitive drums
4Y, 4M, 4C and 4K but not shut them out in color image formation.
In detail, the shield members (shutters) SHT-Y2, SHT-M2 and SHT-C2
shut light beams in monochrome image formation which will be
described later. On the other hand, the shield member (shutter)
SHT-K shuts alight beam during several operations to the light beam
before monochrome image formation which will be described later, or
shuts a light beam so that the photosensitive drum will not suffer
from unnecessary exposure.
[0052] The copy machine having the structure described above would
differ in power of the light beams that scan the photosensitive
drums 4Y, 4M and 4C and the light beam that scans the
photosensitive drum 4K due to difference in efficiency on their
passages if the four semiconductor generate light of beams of equal
power. In this embodiment, however, a semiconductor laser power to
emit a light beam for scanning the photosensitive drum 4Y is set
about 50% of the other semiconductor laser power or the
reflectivity of the miller MK-K is set about 50% of the other
millers, thus enabling scanning the photosensitive drum 4K using
the beam power the same as the other light beams.
[0053] Disclosed so far are the structure and operations of the
image forming apparatus and the passages of light beams in the
optical unit 2 in color image formation. Disclosed next with
reference to FIG. 2 are operations and passages of light beams in
optical system in high-speed monochrome image formation for the
copy machine having the same structure. The monochrome image
formation in this embodiment uses black toner.
[0054] The photosensitive drum 4K, the transfer belt 7 and the
fixing device 16 are rotated by a drive motor (not shown) at a
speed on their periphery four times higher than the color image
formation. On the other hand, the photosensitive drums 4Y, 4M and
4C which will not be used do not rotate and also developing rollers
of the developers 3Y, 3M and 3C do not rotate.
[0055] The charger 5K provided as facing the surface of the
photosensitive drum 4K charges the surface thereof at a certain
potential. The chargers 5Y, 5M and 5C provided as facing the
surfaces of the photosensitive drums 4Y, 4M and 4C (not rotating)
will not work. Unused components that are not be operated as
described above protect the copy machine from degradation which may
otherwise occur.
[0056] Different from color image formation, all four light beams
emitted by the optical unit 2 are focussed, as scanning beams
having a spot of necessary resolution, a position to be exposed on
the photosensitive drum 4K, thus the drum being exposed by
scanning. In other words, the photosensitive drum 4K is
simultaneously scanned by and exposed to the four light beams, thus
forming an electrostatic latent image in accordance with an image
signal. An optical beam passage in the optical unit 2 in this case
will be explained later. The electrostatic latent image formed on
the photosensitive drum 4K is developed using toner (a developing
agent) supplied from the developer 3K, thus forming a K-toner
image.
[0057] The sheets P stocked in the paper feed cassette 15 are
conveyed to the aligning rollers 12 and 13 by the rotating paper
feed roller 14, and after the edges are aligned, conveyed to the
sucking roller 8 and the transfer belt roller 9 by the rotating
aligning rollers 12 and 13. The sheets P are conveyed by the
rotating sucking roller 8, the transfer belt roller 9 and the
aligning rollers 12 and 13 towards the transfer belt roller 10 and
further conveyed in the reverse direction by the roller 10 while
being sucked on the transfer belt 7 with a predetermined potential
difference applied between the sucking roller 8 and the transfer
belt roller 9.
[0058] The transfer operation through sheet conveyance passage in
monochrome image formation mode is different from that through
sheet conveyance passage in color image formation mode. In detail,
as illustrated in FIG. 2, the transfer belt 7, the sucking roller
8, the transfer belt roller 9, and the transfer rollers 18Y, 18M
and 18C are shifted downwardly by a drive motor (not shown) in the
monochrome mode so that they will not touch the photosensitive
drums 4Y, 4M and 4C, while the transfer belt is being controlled so
that it touches the photosensitive drum 4K only.
[0059] The photosensitive drums 4Y, 4M and 4C (not rotating) are
separated from the transfer belt 7 as above so that they will not
touch each other, protects the drums from degradation which may
otherwise be caused by friction. As disclosed, this embodiment
selects an appropriate sheet conveyance passage either for color or
monochrome in accordance with image formation mode.
[0060] The toner image on the photosensitive drum 4K, which has
been developed by the developer 3K is transferred onto a sheet P
where the transfer belt 7 and the transfer roller 18K touch each
other. A predetermined potential applied to the transfer rollers
18K generates static charges which serves to transfer the toner
image on the photosensitive drum 4K onto the sheet P. As disclosed,
a black K-toner is transferred on the sheet P while being sucked by
the transfer belt 7.
[0061] The sheet P is then heated and pressed while passing through
the fixing device 16, thus the toner image on the sheet P being
melted and infallibly fixed thereon. A fixing roller housed in the
fixing device 16 is rotated at a speed four times higher than the
color image formation. The cleaner 6K eliminates the residual toner
on the photosensitive drum 4K which has completed image transfer to
the sheet P. The photosensitive drum 4K is then reset to the
initial state, thus being ready for the next image-forming
process.
[0062] The transfer belt cleaner 11 eliminates extra toner attached
on the transfer belt 7, which has completed sheet transfer, while
passing through the cleaner 11, thus the transfer belt 7 being
ready for the next sheet transfer. Moreover, the transfer belt 7 is
subjected to several potential applications at the sheet sucking
section and the toner transfer section, etc., however, since the
bel 7 is made of a semiconductor material and the transfer belt
roller 10 is grounded, the belt 7 returns to the initial state
(ground potential) at these sections. The foregoing process is
repeated for successive monochrome image formation at a speed four
times higher than the color image formation.
[0063] Disclosed here is the features and structure of the
monochrome (black) image-forming section. Monochrome images are
formed more often compared to color images when thinking of
frequency of color and monochrome image formation. Moreover, a
image-forming speed for monochrome images is four times higher than
that for color images in this embodiment, thus requiring high-speed
components for the image forming apparatus. Particularly, the
monochrome image-forming section requires high-speed processing,
high durability and also a large stock of consumables, such as
toner, compared to the other image processing section. This
embodiment has the following structure to meet such
requirements.
[0064] The photosensitive drum 4K is larger than the other
photosensitive drums 4Y, 4M and 4C. In this embodiment, the
photosensitive drum 4K is about 1.6 times larger than the other
photosensitive drums 4Y, 4M and 4C. The present invention is,
however, not limited to such a ratio and it is preferable to set a
ratio of size in consideration of frequency of use, image-forming
speed, etc. The reason for assembling the photosensitive drum 4K
for black toner-image formation larger than the photosensitive
drums 4Y, 4M and 4C for the other color toner-image formation is
that a lager photosensitive drum under the same requirements offers
high durability and longer life.
[0065] The developer 3K also has a large and high-capacity
developing roller compared to the developers 3Y, 3M and 3C for the
other colors. This requirement for the developer 3K in image
formation at a speed four times higher than the other developers
attains high durability with high-speed supply of a lot of
toner.
[0066] A distance DK between a point to be exposed to another point
to be developed in black is set as longer than distances DY, DM and
DC each between a point to be exposed to another point to be
developed in another color. The distance settings prevent a period
from exposure to developing on the photosensitive drum 4K from
being short.
[0067] As well known, whenever the surface of a photosensitive
material is charged by a charger and exposed, the surface potential
on the exposed surface will vary. Such change in surface potential
requires a limited period of time. This causes a developing error
when a point to be developed is developed before the surface
potential varies enough. In other words, the photosensitive drum 4K
sometimes rotates at a speed higher (four times higher in this
embodiment) than the other photosensitive drums 4Y, 4M and 4C, thus
requiring the distance DK between a point to be exposed to another
point to be developed being set as longer enough than distances DY,
DM and DC each between a point to be exposed to another point to be
developed in the other colors. For example, when the distance each
between a point to be exposed to another point to be developed for
each of the photosensitive drums 4Y, 4M and 4C is set at a distance
almost corresponding to a response speed of each photosensitive
drum, the photosensitive drum 4K requires the distance between a
point to be exposed to another point to be developed for four times
longer than those distances.
[0068] In this embodiment, since there is enough distance between a
point to be exposed to another point to be developed for the colors
Y, M and C other than black, the distance DK between a point to be
exposed to another point to be developed for the color black K is
set as two times or a little bit more longer than the distances DY,
DM and DC for the photosensitive drums 4Y, 4M and 4C.
[0069] Disclosed next is the passage of light beams in the optical
unit for monochrome (black) image formation.
[0070] The deference in passage between color image formation and
monochrome image formation lies in the locations of the shield
members SHT-Y1, SHT-Y2, SHT-M1, SHT-M2, SHT-C1, and SHT-C2. The
light beam BM-YL which has reached the photosensitive drum 4Y in
color image formation will not reach the photosensitive drum 4Y
because it is shielded by the shielding members. On the other hand,
the light beam BM-M2 that has passed the half mirror HM-M is
reflected by the mirror MRM3 and reaches the photosensitive drum
4K. The light beam BM-K which has reached the photosensitive drum
4K in color image formation reaches the photosensitive drum 4Y with
no change.
[0071] As disclosed above, in monochrome image formation mode,
optical passages to the photosensitive drums 4Y, 4M and 4C are shut
by the shield members SHT-Y2, SHT-M2, and SHT-C2 whereas the shield
members SHT-Y1, SHT-M1, and SHT-C1 are shifted to provide an
optical passage to the photosensitive drum 4K. The shield members
SHT-Y1, SHT-M1, SHT-C1, SHT-Y2, SHT-M2, and SHT-C2 are opened or
closed by a driver (not shown) and controlled by a main controller
which will be disclosed later.
[0072] As disclosed, the light beams from the four semiconductor
lasers (not shown) are reflected at the surface of the polygon
mirror 21 while being rotated by the polygon motor 20 and then
these all four beams scan the photosensitive drum 4K after passing
through each optical passage, thus enabling high-speed monochrome
image formation.
[0073] The image forming apparatus as having the structure
described above would have a difference in power for scanning and
exposing the photosensitive drum 4K due to the existence or
non-existence of half mirror if the four semiconductor lasers
output light beam of the same power. In order to solve such a
problem, the reflectivity of the mirror for the light beam BM-K is
lowered to about 50% of that for the other mirrors, thus enabling
scanning the photosensitive drum 4K with the same power for all
light beams.
[0074] It is preferable to detect each beam power while scanning
the photosensitive drum 4K and control the power to attain the same
power for all the beams, to provide the same power precisely, which
will be disclosed later. Disclosed so far are the structure of the
image forming apparatus and light beam passage in the optical unit
2 in monochrome image formation.
[0075] Disclosed next is arrangement of light beams on the
photosensitive drum 4K in monochrome image formation. Images are
formed using four light beams in monochrome image formation as
described above. FIGS. 3A to 3D illustrate the relationship between
a scanning track of each of four light beams and dots (points to be
exposed) that form an image, for example, a character "T" one of
the alphabets using the four light beams. FIG. 3A illustrates ideal
exposure in which all the light beams BM-Y2, BM-M2, BM-C2 and MB-K
ideally scan the exact positions with ideal exposure timing for dot
formation, thus forming the character "T" correctly.
[0076] Contrary to this, FIGS. 3B to 3D illustrates errors in
formation of the character "T". In detail, FIGS. 3B to 3D teach an
error in control of positions to be scanned, in control of exposure
timing, and in control of both positions to be scanned and exposure
timing, respectively. It is apparent from these figures that no
correct character "T" has been formed. Accordingly, the important
factors for image formation using several light beams are positions
to be scanned and exposure timing (main scanning position) for dot
formation, and also uniform light beam power for several beams. The
present invention installs the system to detect positions to be
scanned by each light beam, exposure timing and exposure power for
precise control.
[0077] Light beam control is described briefly with reference to
FIG. 4. A semiconductor laser LD-Y for emitting a light beam BM-Y
is driven by a laser driver LDR-Y. The light emitted by the laser
driver LDR-Y passes through a finite lens (not shown) and then
incident to a galvanomirror GLV-Y (a scanning position changer). A
light beam reflected by the galvanomirror GLV-Y passes through the
half mirrors HM-1 and HM-2 and then incident to the polygon mirror
21 rotated by the polygon motor 20.
[0078] A semiconductor laser LD-M for emitting a light beam BM-M is
driven by a laser driver LDR-M. The light emitted by the laser
driver LDR-M passes through a finite lens (not shown) and then
incident to a galvanomirror GLV-M (a scanning position changer). A
light beam reflected by the galvanomirror GLV-M is further
reflected by the half mirror HM-1 but passes through the half
mirror HM-2, and then incident to the polygon mirror 21 rotated by
the polygon motor 20.
[0079] A semiconductor laser LD-C for emitting a light beam BM-C is
driven by a laser driver LDR-C. The light emitted by the laser
driver LDR-C passes through a finite lens (not shown) and then
incident to a galvanomirror GLV-C (a scanning position changer). A
light beam reflected by the galvanomirror GLV-C passes through the
half mirror HM-3 but reflected by the half mirror HM-2, and then
incident to the polygon mirror 21 rotated by the polygon motor
20.
[0080] A semiconductor laser LD-K for emitting a light beam BM-K is
driven by a laser driver LDR-K. The light emitted by the laser
driver LDR-K passes through a finite lens (not shown) and then
incident to a galvanomirror GLV-K (a scanning position changer). A
light beam reflected by the galvanomirror GLV-K is further
reflected by the half mirrors HM-3 and HM-2, and then incident to
the polygon mirror 21 rotated by the polygon motor 20.
[0081] As described, the structure shown in the middle section of
FIG. 4 can change/adjust the positions to be scanned on the
photosensitive drum 4K by varying angles of the galvanomirrors
GLV-Y, GLV-M, GLV-C and GLV-K for the corresponding light beams.
The passages of light beams after the polygon mirror 21 is the same
as described with reference to FIGS. 1 and 2, hence description is
omitted here. In color image formation, the photosensitive drums
4Y, 4M and 4C for respective color toners are scanned by the
corresponding light beams whereas, in monochrome image formation,
the photosensitive drum 4K is scanned by all light beams as
described above.
[0082] Therefore, change in position to be scanned on the
photosensitive drums 4Y, 4M, 4C and 4K can be performed by the
adjustment of angles of the galvanomirrors GLV-Y, GLV-M, GLV-C and
GLV-K. This suggests that color registration control (color
overlapping control) can be precisely performed at or narrower than
one line which is otherwise be performed at one line only if no
galvanomirrors are provided.
[0083] Disclosed with reference to again FIG. 4 is the passage of
the light beams after the polygon mirror 21, taking the light beam
BM-K as an example from among the four beam. A light beam reflected
by the polygon mirror 21 passes through several lenses, for
example, LN1, LN2 and LN3 (not shown) but reflected by a mirror
MR-K, and then scans the photosensitive drum 4K. The mirror MR-K is
adjusted its angel by an actuator 55, as shown in FIG. 4. The
change in angle of the mirror MR-K can vary an angle of a track of
the scanning light beam BM-K on the photosensitive drum 4K.
[0084] The light beam BM-K scans the photosensitive drum 4K almost
perpendicular to a rotation direction of the photosensitive drum
4K. Change in angle of the mirror MR-K can adjust at a minute
amount the slant angle of the track (scanning line) of the scanning
light beam. On the other hand, change in angle of the galvanomirror
GLV-K does not adjust the angle of the track (scanning line) of the
scanning light beam BM-K on the photosensitive drum 4K but shifts
the track in parallel. According to the mechanism described above,
the track (scanning line) of the scanning light beam BM-K on the
photosensitive drum 4K can be controlled on its scanning positions
and angle of incidence by adjusting the angles of the galvanomirror
GLV-K and the mirror MR-K.
[0085] The mechanism described above is not only for the light beam
BM-K but also for the other light beams BM-Y2, BM-M2 and BM-C2. In
detail, the track (scanning line) of the scanning light beam BM-Y2
on the photosensitive drum 4K can be controlled on its scanning
positions and angle of incidence by adjusting the angles of the
galvanomirror GLV-Y and the mirror MR-Y3. The track (scanning line)
of the scanning light beam BM-M2 on the photosensitive drum 4K can
be controlled on its scanning positions and angle of incidence by
adjusting the angles of the galvanomirror GLV-M and the mirror
MR-M3. Moreover, the track (scanning line) of the scanning light
beam BM-C2 on the photosensitive drum 4K can be controlled on its
scanning positions and angle of incidence by adjusting the angles
of the galvanomirror GLV-C and the mirror MR-C3.
[0086] Described so far with reference to FIG. 4 is that all light
beams scan the photosensitive drum 4K in monochrome image
formation. The structure shown in FIG. 4 is simplified for brevity.
Omitted here after the polygon mirror are the lenses LN1, LN2 and
LN3, the half mirrors HM-Y, HM-M and HM-C, the light beams BM-Y2,
BM-M2 and BM-C2, the mirrors MR-Y1, MR-M1 and MR-C1, the shied
members SHT-Y2, SHT-M2 and SHT-C2, and so on. Mirror actuators 52,
53 and 54 in the figure control the angles of the mirrors MR-Y3,
MR-M3 and MR-C3 (not shown). Like the mirror actuator 55 that
controls the angle the mirror MR-K, a main controller 61 shifts
each mirror up and down at its edge using mirror-actuator drivers
56, 57 and 58 to control the angle of a track (scanning line) of
each scanning light beam on the photosensitive drum 4K.
[0087] Described briefly with reference to the flow chart shown in
FIG. 5 is control in which four light beams ideally scan the
photosensitive drum 4K in monochrome image formation mode. As shown
in FIG. 5, the main controller 61 controls the power of the light
beams so that the beams have the equal power while scanning the
photosensitive drum 4K in step ST1. The control in step ST1 is
disclosed in detail for the sequence in which a light beam power is
detected and adjusted to a certain value for the light beam
MM-K.
[0088] The main controller 61 rotates the polygon motor and drives
the laser LD-K to emit the light beam BM-K so that it scans a
sensor 48. The polygon mirror is rotated by a polygon motor driver
60 in response to a rotation command from the main controller 61.
The light beam is emitted by the laser LD-K driven by the laser
driver DR-K in response to an emission command from the main
controller 61. It is preferable to control the laser LD-K so as not
to emit a light beam at the scanning timing of the light beam BM-K
on the photosensitive drum 4K so as not to expose the
photosensitive drum 4K or the shied member SHT-K to shut out the
light beam BM-K from going to the photosensitive drum 4K.
Accordingly, the sensor 48 detects the light beam power while the
light beam BM-K is being controlled so that it scans the sensor 48
but not the photosensitive drum 4K.
[0089] The sensor 48 outputs a signal proportional to the detected
light beam power to a sensor output processor 50. The processor 50
converts the signal from the sensor 48 into a digital signal
corresponding to the light beam power and sends it to the main
controller 61. The main controller 61 recognizes the power of the
light beam BM-K on the sensor 48 based the digital signal sent from
the sensor output processor 50 and outputs a command to the laser
driver LDR-K to adjust the power if deviated from a predetermined
power. Accordingly, the main controller 61 controls the power of
the light beam BM-K while scanning the photosensitive drum 4K.
[0090] Likewise, the main controller 61 controls the power of the
light beams BM-Y1, BM-M2 and BM-C2 at respective predetermined
power while scanning the photosensitive drum 4K. When the sensor 48
cannot detect the scanning light beams, which means the positions
to be scanned are out of place from the sensor, the corresponding
gavanomirror driver 44 or 47 is driven to adjust the sensor 48 so
that it can detect the light beam power. The power of four light
beams are thus made equal as disclosed above.
[0091] Again in the flow chart of FIG. 5, the main controller 61
controls the light beams to adjust inclination of their tracks
(scanning lines) of scanning. The control sequence of track of
scanning beams is disclosed in detail taking the light beam BM-K as
an example. The main controller 61 rotates the polygon mirror 21
and drive the laser LD-K to emit the light beam BM-K so that it
scans the sensor 48 and also a sensor 49. The polygon mirror 21 is
rotated by the polygon motor driver 60 in response to a rotation
command from the main controller 61. The light beam LD-K is emitted
by the laser LD-K driven by the laser driver DR-K in response to an
emission command from the main controller 61.
[0092] Like the sequence of detecting light beam power disclosed
above, it is preferable to control the laser LD-K so as not to emit
a light beam at the scanning timing of the light beam BM-K on the
photosensitive drum 4K so as not to expose the photosensitive drum
4K or the shied member SHT-K to shut out the light beam BM-K from
going to the photosensitive drum 4K. Accordingly, the inclination
of the track of the scanning light beam BM-K based on the output of
the sensors 48 and 48 while the light beam BM-K is being controlled
to scan the sensors 48 and 49.
[0093] Disclosed next is control of inclination of the track of
scanning light beam based on the principle of inclination detection
and the detected value. The sensors 48 and 49 have the function of
varying each output in accordance with a position through which a
light beam passes in addition to detection of light beam power.
Each of the sensors 48 and 49 outputs a signal in accordance with a
position through which a light beam passes to the sensor output
processor 50. The processor 50 converts the sensor output into a
digital signal in accordance with the position through which the
light beam passes to the main controller 61.
[0094] The main controller 61 recognizes the inclination of the
light beam based on the balance between the two digital signals
converted from the sensor output supplied from the sensor output
processor 50. For example, when the two sensor output are equal to
each other, this suggests that the track (scanning line) of
scanning of light beam is not inclined with respect to the
photosensitive drum 4K, or a desired angle relationship is
maintained. On the other hand, when the two digital signals based
on the output of the sensors 48 and 49 are out of balance, this
suggests that the track (scanning line) of scanning of the light
beam BM-K is inclined with respect to the photosensitive drum
4K.
[0095] When the track of scanning of light beam is inclined, the
main controller 61 adjusts the angle of the mirror MR-K to correct
the inclination of the optical axis. A correction drive signal in
accordance with the detected inclination is sent to a mirror
(MR-K)-actuator driver 59 to drive the driver 55 for adjustment of
the inclination of the mirror MR-K, thus controlling the two
digital signals, or the output of the sensors 48 and 49 becoming
equal to each other. Likewise, for the inclination of the other
light beams BM-Y2, BM-M2 and BM-C2, drive signals are sent to
mirror actuator drivers 56 to 58 to drive actuators 52 to 54,
respectively, for adjusting the angle of the mirror MR-K so that
two output from the sensors 48 and 49 become equal to each other.
When impossible to detect a light beam that is scanning the sensors
48 and 49, it means that the scanning position of the light beam is
out of place from the sensors, so that the corresponding
gavanomirror GLV-Y, GLV-M, GLV-C or GLV-K is driven by the
corresponding valvanomirror driver 44, 45, 46 or 47 for the sensors
48 and 49 to accurately detect the light beam power. As disclosed,
the inclination of the optical axes of the four light beams are
adjusted for their tracks (scanning lines) of scanning.
[0096] Again, in FIG. 5, the main controller 61 controls the
scanning position of each light beam at a certain position. The
main controller 61 rotates the polygon mirror 21 and drives the
laser LD-Y to emit the light beam BM-Y so that it scans the sensor
48. The polygon mirror 21 is rotated by the polygon motor 20 which
is rotated by the polygon motor driver 60 when a rotation command
is sent thereto from the main controller 61. The light beam is
emitted by the laser LD-Y driven by the laser driver DR-Y in
response to an emission command from the main controller 61. Like
detection of power and inclination of optical axis, it is
preferable to control the laser LD-K so as not to emit a light beam
at the scanning timing of the light beam on the photosensitive drum
4K so as not to expose the photosensitive drum 4K or the shied
member (shutter) SHT-K to shut out the light beam from going to the
photosensitive drum 4K. Accordingly, the sensor 48 detects the
light beam power while the light beam BM-Y2 is being controlled so
that it scans the sensor 48, thus detecting the optical axis
position of the light beam BM-Y2 for the track (scanning line) of
scanning based on the sensor 48.
[0097] When the detected position scanned by the light beam BM-Y2
is shifted from a desired position, the main controller 61 controls
the angle of the galvanomirror GLV-Y so that the light beam BM-Y2
passes the desired position. Next, the main controller turns off
the laser LD-Y and drives the laser LD-M instead so that the light
beam BM-M2 scans the sensor 48. The light beam is emitted by the
laser LD-M driven by the laser driver DR-M in response to an
emission command from the main controller 61. It is also preferable
to control the laser LD-M so as not to emit a light beam at the
scanning timing of the light beam BM-M2 on the photosensitive drum
4K so as not to expose the photosensitive drum 4K or the shied
member (shutter) SHT-K to shut out the light beam from going to the
photosensitive drum 4K. Accordingly, the sensor 48 detects the
position of track (scanning line) of the scanning light beam BM-M2
while being controlled to scan the sensor 48. When the detected
position scanned by the light beam BM-M2 is out of place from the
desired position, the angle of the galvanomirror GLV-M is
controlled so that the light beam BM-M2 passes the desired
position. Likewise, the light beams BM-C2 and BM-K are controlled
with detection of positions they pass, and thus the four light
beams are controlled so that they pass through (scan) the
predetermined positions on the sensor 48. The predetermined
positions on the sensor 48 are decided based on recording pitch
(resolution), for example, the positions are located at an interval
of 42.3 .mu.m scanned by the light beams in image formation at
resolution of 600 dpi.
[0098] As disclosed, the four light beams BM-Y2, BM-M2, BM-C2 and
BM-K are controlled for scanning in parallel at a predetermined
interval. It is, however, preferable that detection of inclination
of and interval between the light beams is repeatedly performed to
correct them with fine adjustment control to overcome a problem in
that the inclination may vary at a minute amount after the interval
control. In the decision step ST4, decision is made any time as to
whether or not inclination and position of the track of scanning
light beam or optical axis is correct. If determined that
inclination of the track of scanning light beam or optical axis is
not correct, the process returns to step ST2 for adjustment of
inclination of the track of scanning light beam. And, in step ST3,
detection and adjustment of scanned positions are performed. If
determined that inclination and position of the track of scanning
light beam are not correct in decision sequence of step ST4, the
process goes to step ST5.
[0099] In step ST5, the main controller 61 controls exposure timing
(printing area) for each light beam in image formation. This
control has the following two purposes:
[0100] 1) adjust shifting of the relative positions of light beams
in the scanning direction (main scanning direction offsetting
adjustment), and 2) adjust variation in magnification of images for
light beams (main scanning direction magnification adjustment).
[0101] The former is to adjust shifting which could happen due to
complex mechanism of optically combining light beams emitted by
different lasers for simultaneous scanning. The latter is to adjust
variation which could happen due to difference in refractive index
at lenses of light beams caused by different laser wavelength or
difference in optical length of light beams.
[0102] As described briefly, the process from steps ST1 to ST5
achieves light beam control for accurate monochrome image
formation. The control state up to printing area control in step
ST5 is stored in a memory in step ST6. Optimum values can be
retrieved from the memory for effective control.
[0103] Disclosed next with reference to FIGS. 6, 7 and 8 is the
printing area control, or adjustment to variation in image
magnification and shifting of printing area performed in step ST5
in the flow chart shown in FIG. 5. FIG. 6 teaches the relationship
among motion of each light beam, printing area and respective
signals. FIGS. 7 and 8 show circuit block diagrams for adjustment
to image magnification and printing area. FIG. 6 illustrates time
charts with the positional relationship between sensor patterns S1
and S2 of the sensor 48 and the photosensitive drum 4K and also the
positional relationship among exposure (printing) areas for the
light beams BM-Y2, BM-M2, BM-C2 and BM-K decided by a sampling
timer which will be disclosed later, light emitting areas based on
image data and on-drum emission prohibition timer output.
[0104] As shown in FIG. 6, the sampling timer is reset by the
output of the sensor pattern S1 of the sensor 48, to start counting
clocks (not shown) from "0". The sampling timer outputs "HIGH" when
the counted number reaches a predetermined number to drive the four
lasers LD-Y, LD-M, LD-C and LD-K for emission. The value set on the
sampling timer is usually is a value, as shown in FIG. 6, for
emission of each light beam before the sensor 48 is scanned with
and exposed to each light beam sent from the next surface of the
polygon mirror after each light beam has passed the photosensitive
drum 4K.
[0105] Scanning of each light beam starts from the next surface of
the polygon mirror and the head light beam reaches the sensor
pattern S1, thus the sampling timer being reset for repetition of
the operations disclosed above. Each of the lasers LD-Y, LD-M, LD-C
and LDF-K are forcibly emits a light beam for a predetermined
period for each line within an area which is not related to image
formation. Performed within this forcible emission period is
automatic power control (APC) for maintaining light beam emission
power of each laser.
[0106] An on-drum emission prohibition timer is disclosed. forcible
emission includes emission with sampling timer output and also, as
already disclosed, forcible emission operation of the laser drivers
LDR-Y, LDR-M, LDR-C and LDR-K directly controlled by the main
controller 61. The main controller 61 controls any laser for
emission in this forcible emission operation for checking each
laser condition and also for scanning the sensor 48 with light
beams in positional control of passing (scanning) light beams or
power control of each light beam.
[0107] Successive emission from the lasers LD-Y, LD-M, LD-C and
LD-K will, however, expose the photosensitive drum 4K, thus causing
the following problems:
[0108] The exposure of the photosensitive drum 4K while it is
stopped intensively expose a particular portion of the drum 4K,
thus causing local degradation thereof. On the other hand, the
exposure of the photosensitive drum 4K while it is rotating could
cause attachment (consumption) of a large amount of toner or
attachment of carriers (charges).
[0109] The on-drum emission prohibition timer is used to prevent
such problems. As shown in the time charts in FIG. 6, the on-drum
emission prohibition timer prohibits forcible emission by the main
controller 61 in an area covering the photosensitive drum area. In
detail, based on the output of the sensor pattern S1 of the sensor
48, forcible emission is prohibited (on-drum emission prohibition
timer output: HIGH) at timing before a light beam almost reaches
the photosensitive drum 4K after passing over the sensor 48 (after
T.sub.OFF1 elapses for S1 output) and the prohibition is released
(on-drum emission prohibition timer output: LOW) at timing in which
the light beam has passed over the photosensitive drum 4K (after
T.sub.OFF2 elapses from S1 output).
[0110] Emission with image data (including test image data) is,
usually performed in a printing area on the photosensitive drum 4K.
Although not disclosed in detail, the positional relationship among
the light beams in the main scanning direction is not constant in
the mechanism in which several light beams are combined by a half
mirror for scanning, as already disclosed.
[0111] FIG. 6 illustrates that the light beam BM-Y2 is the head
beam followed by the light beams BM-M2, BM-C2 and BM-K in this
order. In FIG. 6, the light beams BM-M2, BM-C2 and BM-K are delayed
by .DELTA.Tym, .DELTA.Tyc and .DELTA.Tyk, respectively, from the
light beam BM-Y2.
[0112] Exact matching of exposure areas by the light beams having
such positional (phase) relationship requires, as shown in the
figure, shifting of the light beams BM-M2, BM-C2 and BM-K by
.DELTA.Tym, .DELTA.Tyc and .DELTA.Tyk, respectively, with respect
to the light beam BM-Y2 in emission timing using image data.
[0113] This exposure area setting is usually performed for each
clock (one pixel) based on reference clocks. However, in the
optical mechanism in this example, the light beams may be or may
not be shiftied from each other by one clock, thus further finer
adjustment being required. Moreover, image magnification different
over light beams requires to be set as constant by deciding the
reference clock frequency for each light beam.
[0114] FIG. 7 shows a circuit diagram for printing (exposure)
setting per unit finer than one clock at the same image
magnification, which prevents exposure of drum by forcible exposure
disclosed above. In FIG. 7, the sensor output processor 50 includes
a first counter 111, a second counter 112, a latch 113, and so on,
for printing area control.
[0115] A synchronous circuit 550 includes four crystal oscillators
114a to 114d, selectors 115a to 115d for selecting the oscillators
114a to 114d, clock synchronous circuits 116a to 116d, delay lines
117a to 117d, four delayed clock selectors 118a to 118d, four image
transfer clock generators (printing area setting sections) 119a to
119d, a sampling timer 120, an OR gate 121, an on-drum emission
prohibition timer 122, and so on.
[0116] Disclosed below is the mechanism of printing (exposure) area
setting per unit finer than one clock. As already disclosed, the
sensor pattern S1 of the sensor 48 is exposed to any one (or more)
of the light beams BM-Y2, BM-M2, BM-C2 and BM-K which is being
forcible emitted by the sampling timer 120, thus the output signal
of the sensor pattern S1 being switched from "LOW" to "HIGH", as
shown in FIGS. 9A and 9B. This signal is supplied to the sampling
timer 120 as disclosed to release the lasers LD-Y, LD-M, LD-C and
LD-K from forcible emission.
[0117] The light beams BM-Y2, BM-M2, BM-C2 and BM-K are then turned
off, thus the output of the sensor pattern S1 becoming a pulse (a
pulse may be obtained by passing of the head light beam when the
sampling timer 120 exhibits a slow response). The sensor pattern S1
output is also supplied to the clock synchronous circuits 116a to
116d in the synchronous circuit 550. The clock synchronous circuits
116a to 116d output clocks, as shown in 9B, at a frequency the same
as the output clocks of the crystal oscillator in synchronism with
the output of the sensor pattern S1. As shown, the output
synchronous clocks start to rise but is delayed by
.DELTA.T.sub.SYCN from the falling edge of the output of the sensor
pattern S1. Therefore, the clock synchronous circuits 116a to 116d
generate the same synchronous clocks when the selectors 115a to
115d select the output of the same crystal oscillator.
[0118] Disclosed below is based on the assumption that the
selectors 115a to 115d select the output of the same crystal
oscillator.
[0119] The synchronous clocks are supplied to the delay lines 117a
to 117d for delaying the input signal by a predetermined period.
The shown delay lines 117a to 117d have ten output taps. In
response to the input synchronous clocks, a delayed clock D1 output
from the first tap is delayed by .DELTA. td and a delayed clock D2
output from the second tap is further delayed by another .DELTA.
td.
[0120] A delayed clock D10 output from the last (tenth) tap is
delayed by 10 .DELTA. td from the input synchronous clocks. In this
example, {fraction (1/10)} of one cycle of the synchronous clocks
is almost equal to .DELTA. td. In other words, the delayed clock
D10 is almost equal to the input synchronous clocks in phase and
shifted one clock from the input synchronous clocks. A delay amount
of the delay lines 117a to 117d are set at {fraction (1/10)} of one
clock in this embodiment, however, it can be smaller for each tap
while increasing the number of taps.
[0121] The output of the delay lines 117a to 117d, or the delayed
clocks D1 to D10 are supplied to the delayed clock selectors 118a
to 118d corresponding to the light beams BM-Y2, BM-M2, BM-C2 and
BM-K. The delayed clock selectors 118a to 118d select clocks which
are to be supplied to the next stage image transfer clock generator
(printing area setting section) 119a to 119d based on a delayed
signal selection signal output from the main controller 61. In
other words, the main controller 61 can freely select clocks for
printing area setting from among the delayed clocks D1 to D10 for
each of the light beams BM-Y2, BM-M2, BM-C2 and BM-K.
[0122] The delayed clocks output by the delay lines 117a to 117d
are actually independent from each other, or each delay line
outputs delayed clocks D1 to D10 in actual application. The delayed
clocks are, however, treated as the same for the delay lines have
based on the assumption that the same crystal oscillator has been
selected.
[0123] Disclosed next is the image transfer clock generators
(printing area setting sections) 119a to 119d. The main controller
61 sets a printing area for each of the light beams BM-Y2, BM-M2,
BM-C2 and BMK per one clock (one pixel), using a printing area
setting signal, thus enabling setting of output timing of image
transfer clocks and the number of outputs. They are set in usual
image formation so that an emission area for each of the light
beams BM-Y2, BM-M2, BM-C2 and BM-K is located on a certain position
on the photosensitive drum 4K. The certain position depends on
paper size, margin setting, etc.
[0124] The image transfer clocks (printing area signal) obtained as
above are sent to an image I/F which then outputs image data (a
laser-modulation signal) corresponding to each of the light beams
BM-Y2, BMM2, BM-C2 and BM-K in synchronism with the image transfer
clocks (laser-modulation signal). The laser drivers LDR-Y, LDR-M,
LDR-C and LDR-K then modulate the lasers LD-Y, LD-M, LD-C and LD-K,
respectively, by using the image data (laser-modulation
signal).
[0125] Accordingly, the main controller 61 sets a printing area per
one clock (one pixel) using respective printing area setting
signals to the image transfer clock generators (printing area
setting sections) 119a to 119d. Moreover, the main controller 61
sets a printing area per {fraction (1/10)} clocks ({fraction
(1/10)} pixels) separately to each of the light beams BM-Y2, BM-M2,
BM-C2 and BM-K by using the delayed clock selection signals to the
delayed clock selectors 118a to 118d.
[0126] Disclosed next with reference to FIG. 10 is a principle of
obtaining main scanning direction-beam position information for the
light beams BM-Y2, BMM2, BM-C2 and BM-K for obtaining a matched
printing area (including image magnification) per one clock (one
pixel) or {fraction (1/10)} clocks ({fraction (1/10)} pixels).
[0127] FIG. 10 illustrates the principle in which a value set at
the image transfer clock generator (printing area setting section)
119a already disclosed is extremely smaller than that for usual
image formation. As shown, the main controller 61 selects a delayed
clock D1, for example, for the light beam BM-Y2 and sets printing
areas at "1" to "5", and then instructs all black as a test
printing command to the image data I/F, thus the portions
corresponding to the printing areas "1" to "5" being exposed to the
light beam BM-Y2.
[0128] Accordingly, the light beam detection device 48 is exposed
to the light beam which will not reach the area of the
photosensitive drum 4K at a small printing area setting value such
as mentioned above. Monitoring the output of the sensor pattern S2
located next to the sensor pattern SI while the light beam
detection device 48 is being exposed to the light beam as above
teaches the main controller 61 that, to what size the printing area
is set, the sensor patten S2 responses. The embodiment shown in
FIG. 10 teaches that the sensor pattern S2 start to respond to the
printing areas "1" to "10". Accordingly, the main controller 61
detects the relative positional relationship between the light beam
BM-Y2 and the output of the sensor pattern S1 for each one clock
(one pixel).
[0129] Disclosed next with reference to FIG. 12 is detection of the
relative positional relationship between the light beam BM-Y2 and
the output of the sensor pattern SI for each one clock (one pixel)
or less. As described with reference to FIG. 10, the sensor pattern
S2 responds to the light beam BM-Y2 at the printing areas "6" to
"10" when the delayed clock D1 is selected. The main controller 61
resets the printing areas down to "5" to "9". Moreover, the
controller 61 selects other delayed clocks, such as, from D1 to D2
and to D3, as shown FIG. 12. The printing area is thus shifted to
the right for each {fraction (1/10)} clocks ({fraction (1/10)}
pixels). The sensor pattern S2 starts to respond when the delayed
clock D5 is selected.
[0130] Accordingly, the main controller 61 determines that the
sensor patten S2 is exposed to the light beam BM-Y2 at the
rightmost edge of a five pixel-printing area when the delayed clock
D5 is selected at the areas "5" to "9" based on the output of the
sensor pattern S1. The sensor pattern S1 could be exposed again in
a certain printing area, which depends on the positional
relationship between the head light beam and the sensor pattern S1,
thus causing the second pulse being output. This is, however,
avoided by a circuit for allowing the first pulse only being output
(not disclosed in detail).
[0131] The foregoing operation reveals the positional relationship
between each of the light beams BM-Y2, BM-M2, BM-C2 and BM-K and
the output of the sensor pattern S1 caused by the head light beam
when the operation is applied to each light beam. Disclosed so far
is a method of detecting main scanning-direction beam position
information for each light beam, or the position with respect to
the sensor pattern S2 at accuracy of {fraction (1/10)} clocks
({fraction (1/10)} pixels). Moreover, disclosed so far is the
constant light beam power over light beams as the requirements for
higher accuracy. A precise detection of the positional relationship
may, however, not be realized at the constant light beam power when
the light beams are different in diameter or shape. The diameter or
shape tend to differ for each light beam at both edges of the light
beam-scanning area at which the sensors 48 and 49 are located when
the lens characteristics is incomplete. Moreover, the light beam
power tends to be lowered due to a bad condition of light beam
incident to the polygon mirror at both edges of the light
beam-scanning area.
[0132] Disclosed further is a method of accurately detecting a
position of each light beam in the main scanning direction even
though light beams are different in diameter or shape as discussed
above.
[0133] Like shown in FIG. 10, in FIG. 11, a value extremely smaller
than that for usual image formation is set at the image transfer
clock generator (printing area setting section) 119a. In FIG. 10,
the main controller 61 detects the point at which the pattern S2 of
the sensor 48 starts to respond to the light beam BM-Y2 by
selecting the delayed clock D1, setting the printing areas at "1"
to "5" and shifting the setting step by step. The information on
the detected point serves to detect a period (positional
relationship) from when the head light beam has exposed (passed)
the sensor pattern S1 to when the light beam BM-Y reaches the
sensor pattern S2.
[0134] Contrary to this, in FIG. 11, the printing area set at "7"
to "11" is shifted to the right step by step to detect the point at
which the sensor pattern S2 ceases responding. The detection
operation in FIG. 10 is to detect the response of the sensor
pattern S2 with respect to the right side (former edge) of the
light beam BM-Y2 whereas that in FIG. 11 is to detect the response
of the sensor pattern S2 with respect to the left side (latter
edge) of the light beam BM-Y2. In FIG. 11, for example, the sensor
pattern S2 has responded at the printing area setting from "11" to
"15", however, it does not respond at the printing area setting
after "12" to "16" because the printing area does not overlap (is
not exposed) with the sensor pattern S2. Accordingly, the main
controller 61 detects the positional relationship between the left
side (latter edge) of the light beam BM-Y2 and the sensor pattern
S2 for each one clock (one pixel).
[0135] Another disclosure is made with reference to FIG. 13. Like
shown in FIG. 12, FIG. 13 illustrates a method of detection of the
relationship between the left side (latter edge) of the light beam
BM-Y2 and the sensor pattern S2 for each one clock (one pixel) or
less. As disclosed with respect to FIG. 12, the sensor pattern S2
ceases responding to the light beam BM-Y2 at the printing area from
"12" to "16" when the delayed clock D1 is selected. The main
controller 61 then returns the printing area setting to "11" to
"15" for switching the selection of delayed clock.
[0136] As illustrated in FIG. 13, like FIG. 12, the printing area
is shifted to the right per {fraction (1/10)} clocks ({fraction
(1/10)} pixels) as the selected delayed clock is shifted from D1 to
D2 and to D3. In this embodiment, the sensor pattern S2 ceases
responding when the delayed clock D3 is selected.
[0137] The main controller 61 thus detects the point at which the
sensor pattern S2 ceases responding at accuracy of {fraction
(1/10)} clocks ({fraction (1/10)} pixels).
[0138] Accordingly, the main controller 61 detects the positional
relationship between the light beam BM-Y2 and the sensor pattern S2
based on the output of the sensor pattern S1 at accuracy of
{fraction (1/10)} clocks ({fraction (1/10)} pixels) on both right
side (former edge) and left side (latter edge). Discussed below is
beam diameter or beam power and the sensor pattern (S2)
response.
[0139] The sensor pattern S2 starts to respond earlier at a large
beam diameter or power than at a small beam diameter or power when
the printing area is shifted to the right. However, when the
printing area is shifted further to the right, the sensor pattern
S2 ceases responding slower at a large beam diameter or power than
a small beam diameter or power.
[0140] This is easily conjectured based on the fact that the sensor
pattern (S2) response requires a certain amount of light incident
to the sensor pattern. Care must be taken, however, that the sensor
pattern S2 may not respond when a beam power is small but a beam
diameter is large.
[0141] It is important here that the light beam center position can
be accurately detected based on a point at which the sensor pattern
S2 starts to respond and another point at which it ceases
responding irrespective of the beam dimeter size and power
magnitude.
[0142] As disclosed with respect to FIGS. 10, 11, 12 and 13, the
light beam center position can be accurately detected based on the
center of the point at which the sensor pattern S2 starts to
respond and the other point at which it ceases responding
irrespective of the beam dimeter size and power magnitude.
[0143] Disclosed so far is a method of accurately detecting the
positional relationship between each light beam and the sensor
pattern S2 of the sensor 48 irrespective of the beam dimeter size
and power magnitude.
[0144] Disclosed next are a method of detecting the positional
relationship between each light beam and a sensor pattern S3 of the
sensor 49 and also a method of obtaining image magnification for
each light beam.
[0145] As schematically illustrated in FIGS. 6 and 7, provided on
the sensor 49 is the detection pattern S3 extending in the
perpendicular direction with respect to the light beam scanning
direction, like the sensor patterns SI and S2 provided on the
sensor 48.
[0146] The main controller 61 accurately detects the position of
each light beam with respect to the sensor pattern S3 on the sensor
49, like the methods disclosed with respect to FIGS. 10, 11, 12 and
13. In other words, the position can be detected for each one clock
(one pixel) and {fraction (1/10)} clocks ({fraction (1/10)} pixels)
based on pulse signals output by the sensor pattern SI on the
sensor 48.
[0147] Once the positions of each light beam with respect to the
sensor pattern S2 on the sensor 48 and the sensor pattern S3 on the
sensor 49 are detected as above, a period of time (the number of
clocks) required for each light beam to scan from the sensor
pattern S2 to the sensor pattern S3 is obtained. The obtained
period of time (the number of clocks) provides image magnification
for each light beam. A light beam that takes long (a large number
of clocks) to scan from the sensor pattern S2 to the sensor pattern
S3 compared to the other light beams exhibits a slow scanning
speed. If such a light beam performed an exposure operation based
on image data, like the other light beams, the image would shrink
with respect to the light beam scanning direction. This suggests
that the period of time required for a light beam to scan from the
sensor pattern S2 to the sensor pattern S3 is inversely
proportional to the image magnification. For example, a light beam
that takes 1% more of time for scanning from S2 to S3 will produce
a 1%-shrunk image whereas a light beam that takes 1% less of time
for scanning from S2 to S3 will produce a 1%-stretched image.
[0148] In order to solve such a problem, this embodiment selects a
clock frequency for each light beam in image formation. In detail,
in FIG. 6, the selector 115a selects one of the outputs of the
crystal oscillators 1 to 4 (114a to 114d) for the light beam BM-Y2,
the selected output then being supplied to the clock synchronous
circuit 116a. Likewise, for the light beams BM-M2, BM-C2 and BM-K,
the selectors 115b, 115c and 115d select one of the outputs of the
crystal oscillators 1 to 4 (114a to 114d) and supply the selected
output to the clock synchronous circuits 116b, 116c and 116d,
respectively.
[0149] Frequencies oscillated by the crystal oscillators 1 to 4
(114a to 114d) are different from each other by 1% for example.
Therefore, at the same scanning speed for the light beams, a
crystal oscillator oscillating a 1%-higher frequency provides a
1%narrower printing area whereas a crystal oscillator oscillating a
1%-lower frequency provides a 1%-wider printing area. As disclosed,
the main controller 61 is provided with the means (the selectors
115a to 115d) for selecting one of the outputs of a plurality of
crystal oscillators for each light beam as the means for varying
the printing area (image magnification) for each light beam.
Therefore, the image magnification for each light beam is measured
as already disclosed and then one of the crystal oscillators 115a
to 115d is selected for each light beam by the selectors 115a to
115d so that the same image magnification is provided for the light
beams, thus obtaining a uniform image magnification for all light
beams as much as possible.
[0150] This embodiment is provided only four types of crystal
oscillators for brevity, however, the types and the number of
crystal oscillators can be increased, which are selected by the
selectors 115a to 115d for higher accurate control in a wide
range.
[0151] Another embodiment is, as shown in FIG. 8, provided with
voltage control oscillators (VCO) for varying frequency to the
corresponding light beams, to obtain an optimum frequency to each
light beam.
[0152] Disclosed so far is measuring the image magnification for
each light beam using the output of the sensors 48 and 49 and
selecting a clock frequency used for each light beam in image
formation using the measuring result, thus obtaining a uniform
image magnification to all light beams.
[0153] Disclosed below with reference to FIG. 14 is a control flow
for accurate matching of image magnification over the light beams
BM-Y2, BM-M2, BM-C2 and BM-K with summary of the foregoing
disclosure. FIG. 14 shows a process of obtaining a uniform image
magnification. In step ST11, the main controller 61 applies the
same crystal oscillator output (clocks) through the selectors 115a
to 115d.
[0154] Next, measured in step ST12 is the number of clocks required
for the sensor pattern S2 on the sensor 48 to be exposed to each
light beam (over which each beam passes) based on time (timing) for
the sensor pattern S1 on the sensor 48 to be exposed to the head
light beam (over which the head beam passes). The unit of
measurement is the number of clocks of crystal oscillator output
applied to each light beam. The measurement is performed in unit of
{fraction (1/10)} clocks for finer than one clock. The measurement
of time is, like disclosed with reference to FIGS. 10, 11, 12 and
13, to detect (measure) both the point (timing) at which the sensor
pattern S2 starts to respond and the point (timing) at which it
ceases responding to each light beam, thus accurately measuring
time irrespective of each light beam condition.
[0155] Next, measured in step ST13 is, like step ST12, the number
of clocks required for the sensor pattern S3 on the sensor 49 to be
exposed to each light beam (over which each beam passes) based on
time (timing) for the sensor pattern S1 on the sensor 48 to be
exposed to the head light beam (over which the head beam passes).
The unit of measurement is, like step ST12, the number of clocks
and {fraction (1/10)} clocks of crystal oscillator output applied
to each light beam. Moreover, the measurement of the number of
clocks is, like step ST12, to detect (measure) both the point
(timing) at which the sensor pattern S3 starts to respond and the
point (timing) at which it ceases responding to each light beam,
thus accurately measuring time irrespective of each light beam
condition in the vicinity of the sensor pattern S3 on the sensor
49.
[0156] Next, in step ST14, the main controller 61 calculates the
number of clocks required for each light beam to reach from the
sensor pattern S2 to the sensor pattern S3 based on the measured
data from each light beam in steps ST12 and 13.
[0157] For example, for the light beam BM-Y2, in step ST12, the
sensor pattern S2 starts to respond at the point (timing) when the
delayed clock D5 is selected at the printing areas "5" to "9" as
disclosed with respect to FIGS. 10 and 12 whereas it ceases
responding at the point (timing) when the delayed clock D3 is
selected at the printing areas "11" to "15" as disclosed with
respect to FIGS. 11 and 13. These two information suggest that, for
example, the point at which the sensor pattern S2 starts to respond
based on the timing at which the sensor pattern S1 is exposed to
the head light beam is 9.5-th clock whereas the point at which it
ceases responding is 11.3-th clock. The average of these two values
provides "10.4" as the value for the sensor pattern S2 to respond
to the light beam BM-Y2. In step ST13, if, for example, the point
at which the sensor pattern S3 on the sensor 49 starts to respond
to the light beam BM-Y2 is "7509.7"-th clock and the point at which
it ceases responding is "7511.5"-th clock, the average of the two
values provides the value "7510.6" as the point (timing) at which
the sensor pattern S3 responds. The value thus obtained provides
the number of clocks for which the light beam BM-Y2 reaches the
sensor pattern S3 from the sensor pattern S2. In detail, the value
"10.4" is subtracted from the value "7510.6" to obtain the value
"7500.2". This is the value as the time (the number of clocks)
required for the light beam BM-Y2 to reach the sensor pattern S3
from the sensor pattern S2. Likewise, the time (the number of
clocks) required for the light beam BM-Y2 to reach the sensor
pattern S3 from the sensor pattern S2 is obtained for each of the
light beams BM-Y2, BM-C, BM-K.
[0158] Next, in decision step ST15, the main controller 61 compares
the number of clocks required for each light beam to reach the
sensor pattern S3 from the sensor pattern S2 thus obtained in step
ST14 with a predetermined value, to determine whether or not the
difference in the number of clocks lies in a predetermined range.
The predetermined value is, for example, "7500", and if the
compared result is the difference within .+-.0.3 clocks, the
process goes to step ST17, otherwise goes to step ST16.
[0159] The main controller performs the following process in step
ST16. A high frequency-crystal oscillator output (clocks) is
applied to a light beam for which the number of clocks has been
determined as being smaller than the value "7499.7" in step ST15
through the selectors 115a to 115d. On the other hand, a low
frequency-crystal oscillator output (clocks) is applied to a light
beam for which the number of clocks has been determined as being
larger than the value "7500. 3" in step ST15 through the selectors
115a to 115d. The process means as follows: The light beam for
which the number of clocks has been determined as being smaller in
step ST15 is a light beam that has reached the sensor pattern S3
from the sensor pattern S2 for a small number of clocks (or a short
time) at a high scanning speed (a moving speed of light beam) with
respect to a selected crystal oscillator output (clocks). Image
formation with this light beam will generate an image stretching in
the beam scanning direction (at a large image magnification). A
high frequency crystal oscillator output (clocks) applied to such a
light beam offers a high image data transfer rate, so that the
image will shrink in the beam scanning direction, thus adjusting
the extension of image.
[0160] Contrary to this, the light beam for which the number of
clocks has been determined as being larger in step ST15 is a light
beam that, in step ST16, has reached the sensor pattern S3 from the
sensor pattern S2 for a large number of clocks (or a long time) at
a low scanning speed (a moving speed of light beam) with respect to
a selected crystal oscillator output (clocks). Image formation with
this light beam will generate an image shrinking in the beam
scanning direction (at a small image magnification). A low
frequency crystal oscillator output (clocks) applied to such a
light beam as disclosed offers a low image data transfer rate, so
that the image will stretch in the beam scanning direction, thus
adjusting the shrinkage of image.
[0161] The crystal oscillator output (clocks) to be applied is
switched and the process goes back to step ST12 to again measure
the number of clocks for which a light beam reaches to the sensor
pattern S3 from the sensor pattern S2 with comparison to the
predetermined value. The process is repeated until the measuring
result lies in the predetermined range.
[0162] When the number of clocks for which a light beam reaches the
sensor pattern S3 from the sensor pattern S2 is determined as lying
in the predetermined range in step ST15, the light beam exhibits
image magnification within a certain accuracy, for example, .+-.0.3
pixels to 7500 pixels in this embodiment. This embodiment records
images at resolution of 600 dpi, thus providing images of the size
(image magnification) at accuracy of
.+-.(25.4/600).times.(0.3+0.0127 mm=.+-.12. 7 .mu.m to
(25.4/600).times.=7500=317.5 mm. Disclosed so far is with respect
to FIG. 7, however, not only that, crystal oscillator-frequency
switching for each light beam like disclosed with respect to FIG. 8
attains the same the advantages.
[0163] Accordingly, the main controller 61 obtains the matched
image magnification and further matches image recording (printing)
areas for the light beams.
[0164] A method (process) of matching printing areas in an early
stage in the scanning direction is disclosed in Japanese
Un-examined Patent Publication No. 11-202229 (Japanese Patent
Application No. 10-6776), thus being not disclosed here. Discussed
below is several points to be improved on the method of matching
printing areas disclosed in Japanese Un-examined Patent Publication
No. 11-202229.
[0165] Image-forming area matching in an early stage in the
scanning direction with incomplete image magnification matching for
light beams will cause shifting of image-forming area in a later
stage in the scanning direction (sheet right edge in this
embodiment), as illustrated in FIG. 15A. In order to mask this
phenomenon as much as possible, it is preferable to distribute
image-forming area shifting to right and left, as illustrated in
FIG. 15B. The shifting of image-forming area for each light beam is
the same as in FIG. 15A, however, a small shifting on the sheet
center makes small the maximum shifting. This is achieved by
checking the image magnification for each light beam in accordance
with the process as disclosed above and, for incompletely matched
image magnification, the image forming area is defined for each
light beam so that the shifting is distributed to right and
left.
[0166] This method is effective to minimize the shifting even if
the image forming areas for light beams do not match each other.
FIG. 15C illustrates an image for which shifting has been canceled
on the sheet center with image magnification matching.
[0167] Discussed next is inclination of attached sensors. Images
illustrated in FIGS. 16A and 16B are degraded sample images caused
by inclined sensors 48 and 49 with respect to the scanning
direction.
[0168] FIG. 16A illustrates an image of vertical lines when the
sensors 48 and 49 have been inclined in the same direction. The
image forming starting and finishing points are set in accordance
with the sensor patterns S2 and S3 on the sensors 48 and49,
respectively, thus forming vertical (zigzag) lines inclined like
the sensors.
[0169] FIG. 16B illustrates an image of vertical lines when the
sensors 48 and 49 have been inclined in different direction. Image
magnification and printing area are set in accordance with the
sensor patterns S2 and S3 on the sensors 48 and 49, respectively,
thus forming vertical (zigzag) lines inclined in the opposite
directions in right and left, as illustrated in FIG. 16B, even
though under exact control.
[0170] The sensor patterns S2 and S3 on the sensors 48a and 49,
respectively, require to be set so that light beams pass thereover
vertically to avoid such problems. This is realized by a mechanism
of checking whether or not the sensors 48 and 49 are inclined to
the beam scanning.
[0171] For example, as illustrated in FIG. 17A, the sensor 48 is
provided with sensor patterns SA, SB, SC and SD, and, as
illustrated in FIG. 17B, and the sensor 49 is provided with sensor
patterns SA', SB', SC' and SD' for checking whether or not the
sensors are inclined to the beam scanning.
[0172] When a light beam scans an straight-line arrow A, the
difference in sensor pattern is in balance, that is,
(SA-SB)=(SC-SD), thus the sensor having been not inclined. On the
other hand, when a light beam scans an straight-line arrow B, the
difference in sensor pattern is out of balance, that is,
(SA-SB).noteq.(SC-SD), thus the sensor having been inclined.
[0173] Also, as illustrated in FIG. 17B for the sensor 49, when a
light beam scans an straight-line arrow A, the difference in sensor
pattern is in balance, that is, (SA'-SB')=(SC'-SD'), thus the
sensor having not been inclined. On the other hand, when a light
beam scans an straight-line arrow B, the difference in sensor
pattern is out of balance, that is, (SA'-SB').noteq.(SC'--SD'),
thus the sensor having been inclined.
[0174] As disclosed, the sensor patterns for detecting the
inclination of the sensors 48 and 49 to the beam scanning offers
inclination adjustment to the sensors 48 and 49 while checking the
output of the sensors 48 and 49, which is combined with the
sequential control described above for forming further accurate
images.
[0175] Disclosed lastly with reference to FIG. 18 is a process flow
of a beam control operation in color image formation mode.
Determined in step ST21 is whether or not the previous process
result has been stored in memory. If so, in step ST22, each light
beam is controlled in scanning state stored in memory, and the
color image formation starts immediately.
[0176] If the previous process result is not stored in memory in
step ST21, the process goes to step ST23, to print a test pattern
for color registration control. Next, determined in step ST24 is
whether or not color overlapping is accurate. If so, the process
goes to step ST25 to store the scanning state for each light beam
and ends. This initiates color image formation.
[0177] On the other hand, if color overlapping is not accurate in
step ST24, the process goes to step ST26 to control scanning state
of each light beam, such as, inclination, scanning position,
printing area and magnification. The process then returns to step
ST23 to print the test pattern, and then make decision in step
ST24. When color overlapping is determined as accurate, the process
ends the control operation via step ST25.
[0178] As disclosed, when the previous control state has not been
stored in memory, color registration control is performed to store
in memory the scanning state for each light beam in which color
overlapping has been accurate.
[0179] Thereafter, each light beam is controlled in the scanning
state stored in memory in color image formation mode, thus
requiring no toner consumption which is required in general for
test pattern formation.
* * * * *