Incremental printing of symbolic information – Light or beam marking apparatus or processes – Scan of light
Reexamination Certificate
2000-01-24
2002-12-10
Lee, Susan S. Y. (Department: 2852)
Incremental printing of symbolic information
Light or beam marking apparatus or processes
Scan of light
C347S232000, C347S249000
Reexamination Certificate
active
06493019
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an image forming apparatus, and more particularly to an apparatus for forming an image by using a plurality of light beams.
2. Related Background Art
Of such apparatus, a color image forming apparatus of a recent electronic photograph type has a plurality of image forming units in order to speed up the image forming operation. Various methods have been proposed for sequentially transferring different color images to a recording medium held on a transport belt.
An apparatus having a plurality of image forming units has the following problems. Because of mechanical precision and the like, a change in motion of a plurality of photosensitive drums and a transport belt, a change in motion relation between a circumferential surface of each photosensitive drum and the transport belt at a transfer position of each image forming unit, and the like occur differently at each color. Therefore, when images of different colors are superposed, color aberration (position misalignment) occurs.
There is an error of an optical distance between a laser scanner and a photosensitive member of each image forming unit. If this error of each image forming unit is different, there is a difference of a main scan magnification of a beam on the photosensitive drum so that the color aberration (position misalignment) occurs. If there is a position displacement of a laser scanner and a photosensitive member of each image forming unit relative to the beam scan direction (hereinafter called a main scan direction) and this position displacement is different at each image forming unit, color aberration (position misalignment) appears on the final images.
In order to reduce the color aberration (position misalignment) to be caused by a difference of the main scan magnification, a method of correcting the main scan magnification has been proposed (e.g., JP-B-06-57040) in which as shown in
FIG. 1
, a video clock generator for video signals is provided for each color, and the frequency of the video clock for each color is independently changed and adjusted to correct the main scan magnification.
With reference to
FIG. 1
, a conventional method of changing a video clock frequency of an image forming apparatus will be described.
FIG. 1
is a block diagram showing the structure of a PLL (phase locked loop) for changing the frequency of a video clock signal of a conventional image forming apparatus described in JP-B-06-57040.
In
FIG. 1
, a crystal oscillator
1007
generates a clock signal
14
at a frequency fin. A 1/M frequency divider
1002
divides the frequency of a clock signal
14
output from the crystal oscillator
1007
by M. A 1/N frequency divider
1006
divides the frequency of a clock signal (video clock)
15
output from a voltage controlled oscillator
1005
by N. A phase comparator
1003
compares the phase of the clock signal
14
output from the crystal oscillator
1007
and divided by M with the phase of the video clock
15
output from the voltage controlled oscillator
1005
and divided by N.
A low-pass filter
1004
receives a comparison result from the phase comparator
1003
and changes an input voltage of the voltage controlled oscillator
1005
. For example, if the phase of the clock signal
14
output from the crystal oscillator
1007
and divided by M advances from the phase of the video clock
15
output from the voltage controlled oscillator
1005
and divided by N, the input voltage to the voltage controlled oscillator
1005
is raised to advance the phase of the video clock
15
.
The PLL
1008
has the 1/M frequency divider
1002
, phase comparator
1003
, low-pass filter
1004
, voltage controlled oscillator
1005
and 1/N frequency divider
1006
.
The operation of PLL will be described more specifically.
The clock signal
14
output from the crystal oscillator
1007
and divided by M and the video clock signal
15
divided by N are input to the phase comparator
1003
. An output of the phase comparator
1003
is passed through the low-pass filter
1004
which supplies a voltage to the voltage controlled oscillator
1005
. For example, if the phase of the clock signal
14
output from the crystal oscillator
1007
and divided by M advances from the phase of he video clock
15
divided by N, the input voltage to the voltage controlled oscillator
1005
is raised to advance the phase of the video clock
15
.
If the frequency of the clock signal
14
output from the crystal oscillator
1007
is represented by fin, and that of the video signal
15
is represented by fout, then the following equation is satisfied:
f
out=
f
in×
N/M
The values of N and M are adjusted in accordance with a detected main scan width to thereby adjust the video clock frequency and correct the main scan width.
In order to speed up an image forming operation, a plurality of lines are scanned at the same time by using a plurality of beams.
A scanner optical system capable of scanning a photosensitive drum with a plurality of beams, particularly with two beams, will be described briefly.
FIG. 2
is a perspective view showing the outline structure of a scanner optical system capable of scanning a photosensitive drum with a plurality of beams, particularly with two beams.
In
FIG. 2
, laser sources (semiconductor laser)
81
(
81
a
,
81
b
) emit a plurality of laser beams (hereinafter simply called beams)
87
a
and
87
b
. A collimator lens
82
collimates the plurality of beams
87
a
and
87
b
output from the laser source
81
. A polygon mirror
83
scans a plurality of beams
87
a
and
87
b
collimated by the collimator lens
82
. An f&thgr; lens
84
adjusts the scan speeds of the plurality of beams
87
a
and
87
b
scanned by the polygon mirror
83
. The plurality of beams
87
a
and
87
b
scanned via the f&thgr; lens form latent images corresponding to the video signals on the surface of a photosensitive drum
1
. A position detection sensor (hereinafter called a BD sensor)
86
detects the plurality of scanned beams
87
a
and
87
b
and outputs horizontal synchronization signals (BD signals (BD(A), BD(B)).
The operation of the scanner optical system will be described more specifically.
The plurality of beams
87
a
and
87
b
emitted from the laser source
81
(
81
a
,
81
b
) are collimated by the collimator lens
82
and thereafter scanned by the polygon mirror
83
. The scan speeds of the plurality of scanned beams
87
a
and
87
b
are adjusted by the f&thgr; lens
84
. Latent images corresponding to video signals are eventually formed on the photosensitive drum
1
.
Scanning the photosensitive drum
1
with the plurality of beams as shown in
FIG. 2
is associated with some problem. Because of a difference of an emission light wavelength between semiconductor lasers (in
FIG. 2
, semiconductor lasers
81
a
and
81
b
) of the laser sources, the main scan magnification for each beam becomes different (JP-A-06-227037). To solve this problem, a method of adjusting the main scan magnification for each beam by changing the video clock frequency for each beam has been proposed (JP-A-06-227037).
FIG. 3
is a block diagram showing the structure of video clock frequency variable unit of an image forming apparatus described in JP-A-06-227037. In
FIG. 3
, like elements to those shown in
FIG. 2
are represented by using identical reference symbols.
In
FIG. 3
, crystal oscillators
91
a
and
91
b
output clock signals whose frequencies were adjusted so that each beam
87
a
,
87
b
has the same main scan magnification. Horizontal synchronization units (BD synchronization units)
92
a
and
92
b
synchronize the clock signals output from the first and second crystal oscillators
91
a
and
91
b
with BD signals
95
output from the BD sensors
86
.
Laser drive units (LDA driver unit, LDB driver unit)
93
a
and
93
b
drive the semiconductor lasers (LDA
81
a
, LDB
81
b
) in accordance with video signals. As shown in
FIG. 2
, a single laser scanner optical system has two semiconductor lasers (LDA
81
a
, LD
Canon Kabushiki Kaisha
Lee Susan S. Y.
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