Incremental printing of symbolic information – Electric marking apparatus or processes – Electrostatic
Reexamination Certificate
2001-07-11
2003-08-05
Lee, Susan S. Y. (Department: 2852)
Incremental printing of symbolic information
Electric marking apparatus or processes
Electrostatic
C399S049000, C399S072000, C399S301000
Reexamination Certificate
active
06603495
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an image forming apparatus, and in particular, to correction of aberration of an image position.
2. Related Background Art
For color image forming apparatuses, various systems have been proposed which have a plurality of image forming parts in order to increase processing speed and which sequentially transfer images of different colors onto a recording material held on a conveying belt.
Problems with such apparatuses having the plurality of image forming parts include color aberration (one of position aberration) in which when the color images are placed on one another, the color images are not overlapped at a common position due to an irregular movement of a plurality of photosensitive drums or the conveying belt associated with mechanical accuracy or the like or due to the variation, for each color, of a relationship between an outer peripheral surface of the photosensitive drum and the movement of the conveying belt in the transfer positions of the image forming parts.
In particular, in apparatuses including a plurality of image forming parts each having a laser scanner and a photosensitive drum, there are errors in the distance between the laser scanner and the photosensitive drum between the image forming parts. If these errors are different among the image forming parts, the laser scanning width on the photosensitive drum varies, resulting in color aberration.
FIGS. 2A
,
2
B,
2
C and
2
D show an example of a color aberration. Reference numeral
7
denotes an original image position, and reference numeral
8
(
8
a
-
8
d
) denotes an image position observed when the color aberration occurs. Additionally,
FIGS. 2A
,
2
B and
2
C show cases in which the color aberration occurs in a scanning direction. In those figures the two lines are separately drawn in a conveying direction for easy understanding.
FIG. 2A
indicates an inclination of a scanning line which occurs when an optical part and the photosensitive drum are inclined relative to each other. This inclination can be corrected to the arrowed direction by, for example, adjusting the position of the optical part, the photosensitive drum, or a lens.
FIG. 2B
indicates a scale aberration caused by variations in the distance between the optical part and the photosensitive drum. The case is likely to occur when the optical part comprises laser scanner. This aberration can be corrected to the arrowed direction by, for example, fine-tuning image frequency (increasing the frequency if the scanning width is large) to change the length of the scanning line.
FIG. 2C
indicates a write start position error in the main-scanning direction. This error can be corrected to the arrowed direction by, for example, adjusting a write start timing with respect to a beam detected location if the optical part comprises a laser scanner.
FIG. 2D
indicates a sub-scanning direction (sheet conveying direction) aberration. This can be corrected to the arrowed direction by the arrow by, for example, adjusting the write start timing with respect to detection of a leading end of a sheet for each color.
To correct these color aberrations, position aberration detecting patterns for corresponding colors are conventionally formed on the conveying belt and are detected by a pair of optical sensors provided on the respective sides of a downstream part of the conveying belt so that various adjustments as described above are carried out depending on the amount of aberration detected.
FIG. 3
shows an example of such position aberration detecting patterns. Reference numerals
9
and
10
denote patterns for detecting position aberration in the sub-scanning direction. Reference numerals
11
and
12
denote patterns for detecting position aberration in the scanning direction, which are inclined from a conveying direction of belt
3
through 45° in FIG.
3
. The patterns
9
to
11
have been transferred onto the conveying belt.
Reference numerals
9
a,
10
a,
11
a,
and
12
a
denote black patterns (hereafter referred to as “Bk”), reference numerals
9
b,
10
b,
11
b,
and
12
b
denote yellow patterns (hereafter referred to as “Y”), reference numerals
9
c,
10
c,
11
c,
and
12
c
denote magenta patterns (hereafter referred to as “M”), and reference numerals
9
d,
10
d,
11
d,
and
12
d
denote cyan patterns (hereafter referred to as “C”).
Reference numerals tsf
1
to tsf
4
, tmf
1
to tmf
4
, tsr
1
to tsr
4
, and tmr
1
to tmr
4
denote timings with which sensors
6
a
and
6
b
detect the patterns. The arrow A denotes the direction in which a conveying belt
3
moves.
Here, the movement speed of the conveying belt
3
is defined as v mm/s, Bk is assumed to denote a reference color, and the theoretical distances between each of the color pattern for the sheet conveying direction and the Bk pattern is defined as dsY mm, dsM mm, and dsC mm. The measured distances between each of the color pattern for the sheet conveying direction and a corresponding one of the patterns for the scanning direction are defined as dmfBk mm, dmfY mm, dmfM mm, and dmfC mm on the left side and as dmrBk mm, dmrY mm, dmrM mm, and dmrC mm on the right side.
When the Bk is assumed to denote the reference color, aberration &dgr;es in the sub-scanning direction for each color is expressed by:
&dgr;
esY=v
*{(
tsf
2
−
tsf
1
)+(
tsr
2
−
tsr
1
)}/2−
dsY
(1)
&dgr;
esM=v
*{(
tsf
3
−
tsf
1
)+(
tsr
3
−
tsr
1
)}/2−
dsM
(2)
&dgr;
esC=v
*{(
tsf
4
−
tsf
1
)+(
tsr
4
−
tsr
1
)}/2−
dsC
(3)
Further, left and right aberrations &dgr;emf and &dgr;emr in the main-scanning direction for each color are expressed by the following (12) to (17) on the basis of the following (4) to (11).
dmfBk=v
*(
tmf
1
−
tsf
1
) (4)
dmfY=v
*(
tmf
2
−
tsf
2
) (5)
dmfM=v
*(
tmf
3
−
tsf
3
) (6)
dmfC=v
*(
tmf
4
−
tsf
4
) (7)
dmrBK=v
*(
tmf
1
−
tsf
1
) (8)
dmrY=v*
(
tmf
2
−
tsf
2
) (9)
dmrM=v
*(
tmf
3
−
tsf
3
) (10)
dmrC=v
*(
tmf
4
−
tsf
4
) (11)
&dgr;
emfY=dmfY−dmfBK
(12)
&dgr;
emfM=dmfM−dmfBK
(13)
&dgr;
emfC=dmfC−dmfBK
(14)
&dgr;
emrY=dmrY−dmrBK
(15)
&dgr;
emrM=dmrM−dmrBK
(16)
&dgr;
emrC=dmrC−dmrBK
(17)
Thus, the aberration directions can be determined depending on whether the results of the calculations are positive or negative. The &dgr;emf is used to correct the write start position, whereas the &dgr;emr−&dgr;emf is used to correct the scanning width.
In the case when the scanning width has an error, the write start position is calculated by using not only the &dgr;emf but also the variation of the image frequency associated with the correction of the scanning width.
In the following description, if the color aberration detecting patterns in
FIG. 3
are used, the line width thereof is assumed to be 35 dots, the length thereof in the main-scanning direction is assumed to be 100 dots, and the space between the patterns is assumed to correspond to 50 dots.
The conventional examples, however, have the following disadvantages.
Due to the eccentricity of a belt driving roller, the irregular rotation of a driving part, or the like, the movement speed v mm/s of the conveying belt
3
is not always fixed, resulting in a detection error proportional to the temporal difference between the patterns.
If this detection error is caused by periodically irregular driving, it can be eliminated by arranging the plural sets of position aberration detecting patterns at appropriate locations, calculating the misalignments thereof, and averaging them. For non-periodic irregularity, however, the detection error cannot be eliminated even with the averaging
Canon Kabushiki Kaisha
Fitzpatrick ,Cella, Harper & Scinto
Lee Susan S. Y.
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