Timing control of light beam scan

Details Timing control of light beam scan Timing control of light beam scan

2000-07-12

2002-10-15

Pham, Hai (Department: 2861)

Incremental printing of symbolic information

Light or beam marking apparatus or processes

Scan of light

C347S235000

Reexamination Certificate

active

06466247

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to a circuit for controlling scanning of light beams, an optical unit, and an image forming device. The present invention particularly relates to a circuit for controlling scanning of light beams so as to achieve desired scanning of a plurality of light beams, and further relates to an optical unit and an image forming device based on such a control circuit.
2. Description of the Related Art
Laser beam printers are required to be faster and to be better in terms of image resolution. To meet such requirements, scanning of a laser beam needs to be faster, which is achieved by rotating a polygon mirror of an optical unit at a faster rate. A rotation rate of a polygon mirror is around 30,000 rpm in a currently available optical unit, and this rate is close to an upper limit when physical and mechanical constraints are taken into account. Against this background, a multi-beam optical unit has been developed in order to scan a plurality of laser beams.
The multi-beam optical unit scans a plurality of laser beams on a photosensitive drum while slightly displacing each laser beam from the others, thereby create a latent image on the photosensitive drum. If relative positional relations between the laser beams are not correct, it results in a poor image quality. Relation of scan positions between laser beams thus needs to be controlled with accuracy.
FIG. 1
is a block diagram of a related-art laser printer.
The laser printer
1
includes a printer-sheet conveyor mechanism
2
, a photosensitive drum
3
, a charging device
4
, an optical unit
5
, a developing unit
6
, a transfer unit
7
, a fixing unit
8
, and a video controller
9
.
A printer sheet
10
is carried by the printer-sheet conveyor mechanism
2
in a direction shown by an arrow A. The printer sheet
10
first comes in contact with the photosensitive drum
3
, which rotates in a direction shown by an arrow B. The photosensitive drum
3
is electrically charged by the charging device
4
, and, rotates in the direction B. The optical unit
5
directs laser beams L
1
and L
2
onto the photosensitive drum
3
so as to create a latent image on a surface thereof.
The photosensitive drum
3
having the latent image created thereon is further rotated in the direction B, and has a toner image developed thereon by the developing unit
6
. The transfer unit
7
then transfers the toner image from the photosensitive drum
3
to the printer sheet
10
.
The printer-sheet conveyor mechanism
2
further carries the printer sheet
10
in the direction A, so that the printer sheet
10
is supplied to the fixing unit
8
. The fixing unit
8
fixes the transferred toner image permanently on the printer sheet
10
by applying heat, for example. The printer sheet
10
having the toner image fixed thereon is further carried in the direction A by the printer-sheet conveyor mechanism
2
until it is ejected.
In the following, a description of the optical unit
5
will be given.
The optical unit
5
includes laser diodes D
1
and D
2
, a polygon mirror
11
, a motor
12
, a motor driver
13
, mirrors
14
and
15
, a laser-beam-reference-position detecting unit
16
, laser-power-adjustment beam-detection units
17
and
18
, an optical control unit
19
, a mechanism control unit
20
, a power unit
21
, and switches SW
1
through SW
4
.
The laser diodes D
1
and D
2
are connected to the optical control unit
19
, and generate laser beams L
1
and L
2
, respectively. The laser beams L
1
and L
2
hit the polygon mirror
11
.
The polygon mirror
11
is rotated by the motor
12
in a direction shown by an arrow C at a constant rotation rate. The motor driver
13
controls the motor
12
to rotate at a constant rate such as 3,000 rpm. The laser beams L
1
and L
2
are reflected by the rotating polygon mirror
11
so that they are scanned in the direction C.
The polygon mirror
11
directs the laser beams L
1
and L
2
to the mirror
15
first. The mirror
15
reflects the laser beams L
1
and L
2
coming from the polygon mirror
11
so as to direct them to the laser-beam-reference-position detecting unit
16
. After the laser beams L
1
and L
2
illuminate the laser-beam-reference-position detecting unit
16
, the polygon mirror
11
further rotates in the direction C (i.e., the direction shown by the arrow C). The laser beams L
1
and L
2
are scanned in a direction indicated by an arrow D, and hit the mirror
14
. The mirror
14
reflects the laser beams L
1
and L
2
, and directs them to the photosensitive drum
3
.
As the polygon mirror
11
rotates in the direction C, the laser beams L
1
and L
2
are scanned in the direction D, so that they move on the mirror
14
in a direction indicated by an arrow E. As the laser beams L
1
and L
2
sweep in the direction E on the mirror
14
, their reflections are scanned in a direction shown by an arrow F on the photosensitive drum
3
. As the scanning of the laser beams L
1
and L
2
on the photosensitive drum
3
progresses, the laser beams L
1
and L
2
are controlled to be turned on or off appropriately.
The laser-beam-reference-position detecting unit
16
, which detects the laser beams L
1
and L
2
as previously described, supplies a detection signal to the optical control unit
19
.
The optical control unit
19
detects positions of the laser beams L
1
and L
2
on the photosensitive drum
3
based on the detection signal supplied from the laser-beam-reference-position detecting unit
16
.
The laser beams L
1
and L
2
are also supplied to the laser-power-adjustment beam-detection units
17
and
18
, respectively. The laser-power-adjustment beam-detection units
17
and
18
supply detection signals indicative of intensities of the laser beams L
1
and L
2
, respectively, to the optical control unit
19
. The optical control unit
19
monitors the intensities of the laser beams L
1
and L
2
, and attends to power control so as to achieve constant beam intensity.
The optical control unit
19
includes laser control circuits
22
and
23
and an optical control circuit
24
. The laser control circuits
22
and
23
control the laser diodes D
1
and D
2
, respectively, based on the detection signals supplied from the laser-power-adjustment beam-detection units
17
and
18
, respectively.
The optical control circuit
24
receives a video signal from the video controller
9
where the video signal represents an image to be reproduced on a sheet. Further, the optical control circuit
24
receives the detection signal from the laser-beam-reference-position detecting unit
16
. In response to the detection signal from the laser-beam-reference-position detecting unit
16
, the optical control circuit
24
controls an on/off state of the laser beams L
1
and L
2
in accordance with the video signal supplied from the video controller
9
.
The mechanism control unit
20
attends to control of various mechanisms such as drive control of the printer-sheet conveyor mechanism
2
and rotation control of the photosensitive drum
3
. Further, the mechanism control unit
20
includes relays R
1
through
5
, which are switched according to the switches SW
1
through SW
4
that detect an open/close status of a stack cover, a front cover, and an eject cover as well as a presence/absence of a transport unit. In the mechanism control unit
20
, the relays R
1
through R
5
are turned on if the stack cover, the front cover, and the eject cover are all closed, and if the transport unit is present. As the relays R
1
through R
5
are turned on, power is supplied from the power unit
21
to the optical control unit
19
.
In the following, the optical control circuit
24
will be described in connection with timing control of the laser beams L
1
and L
2
.
FIG. 2
is an illustrative drawing for explaining a displacement between the laser beams L
1
and L
2
in their scan direction shown by an arrow F.
FIG. 3
is an illustrative drawing for explaining a displacement between the laser beams L
1
and L
2
in their sub-scan directi

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