Multi-beam control method

Details Multi-beam control method Multi-beam control method

2000-09-18

2002-12-17

Pham, Hai (Department: 2861)

Incremental printing of symbolic information

Light or beam marking apparatus or processes

Scan of light

C347S248000

Reexamination Certificate

active

06496213

ABSTRACT:

CROSS-REFERENCE TO RELATED APPLICATIONS
This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 11-274880, filed Sep. 28, 1999, the entire contents of which are incorporated herein by reference.
BACKGROUND OF THE INVENTION
The present invention relates to an image formation apparatus which forms an image using multiple laser beams and more specifically to a multi-beam control method for controlling the positions in the sub-scanning direction of the laser beams on a photosensitive drum with galvanomirrors and an image formation apparatus which uses the control method.
A multi-beam optical system is equipped with a plurality of laser beam sources and a single polygon mirror and is required to perform beam position control in order to prevent image misalignment due to the relative displacement of beams in the image plane. In this specification, “beam position” refers to the position of each beam in the sub-scanning direction that is scanned across the image plane, such as the surface of the photosensitive drum, in the main scanning direction.
Methods of controlling the positions of beams in the sub-scanning direction include a method of setting the spacing between each beam through the use of a plurality of galvanomirrors each having its reflecting surface (mirror) arranged to be rotatable in any direction as disclosed in, for example, Japanese Unexamined Patent Publication No. 10-76704 and Japanese Unexamined Patent Application No. 2000-147398.
In the sub-scanning control in the multi-beam optical system, coarse adjustment is made first. The coarse adjustment is intended to, when a beam is greatly off a target course and hence does not pass over a photosensor, in which case beam fine adjustment can not be made, shift the beam over the photosensor.
In general, in the coarse adjustment, a directive value, for example, a lower limiting directive value, is set in the galvanomirror and then the setting value is changed in large steps toward an upper limiting directive value to shift the transit position of a beam in the sub-scanning direction. For example, a step size of 100 bits results in a change of 176 &mgr;m in the image plane position. As shown in
FIGS. 17A and 17B
, a beam reflected by the galvanomirror is directed onto the surface of the photosensitive drum to form a latent image on it. The latent image is developed and then transferred onto paper. The sensor surface and the paper surface are set equidistant from the galvanomirror. The amount by which the beam position is displaced on the sensor surface appears on paper as it is. Thus, the beam position on the sensor surface is also called the image plane position.
FIG. 18
is a flowchart for the conventional coarse adjustment. This coarse adjustment is made automatically by the apparatus. First, of four beams, a beam to be adjusted is turned on (step S
110
). Next, a determination is made as to whether horizontal synchronization (HSYNC) is established (step S
101
). If established, then the coarse adjustment of the next beam is made.
If synchronization is not detected in step S
101
, then a lower limiting directive value is first set in the galvanomirror so that it is turned below the image plane at the maximum swing angle and then the setting value is increased in steps of 100 bits (corresponding to about 176 &mgr;m) (step S
106
).
The above operation is repeated until HSYNC has been established for all the beams. The control is repeated in the order of, for example, beam
1
, beam
2
, beam
3
, beam
4
, beam
1
and so on. If there are beams for which adjustment has been made, they are skipped and the control is repeated in the order of, for example, beam
1
, beam
3
, beam
4
, beam
1
and so on. When, even if the setting value is changed until the upper limiting value is reached, no HSYNC is detected, repair is needed. In this case, the necessity for service call is displayed on the apparatus.
After the termination of the coarse adjustment for all the beams, fine adjustment is made.
FIG. 19
is a flowchart for the conventional fine adjustment. The fine adjustment is made to drive the beam which has been allowed to pass over the sensor surface by the coarse adjustment into a target range of ±10 &mgr;m of a target value. The fine adjustment is essential in obtaining correct output images.
As with the coarse adjustment, only a beam to be controlled is turned on (step S
110
). The beam position information in the sensor is read (step S
111
). A determination is made as to whether the position over which the beam passes is within the target range (step S
112
). If the beam position lies within the target range, then the adjustment is terminated; otherwise, the galvanomirror is instructed so as to change the beam position on the sensor surface in small steps, thereby driving the beam position into the target range (step S
113
).
FIG. 20
shows a plot of output versus input of the galvanomirror. The input to the galvanomirror is given in voltage. Voltages from −10.6 to +10.6V are made correspond to 0EB8H to 3333H in hexadecimal notation. These hexadecimal numbers are handled as input values to the galvanomirror. These input values are converted into voltages by a D/A converter and then applied to the galvanomirror. The input values below 0EB8H or above 3333H correspond to voltages in the vicinity of supply voltages (±12V). For these input values, the output of the A/D converter is not proportional to the input. Thus, these input values are not generally used.
That is, 1 bit corresponds to an input voltage of 16 mV. The output is given in swing angles of the galvanomirror from −39.46 mrad to +39.46 mrad. In correspondence to these swing angles, the image plane position changes from −8.68 mm to +8.68 mm. Namely, when the input value is changed by 1 bit, the beam position on the image plane is changed by 1.76 &mgr;m.
For countermeasures against the galvanomirror being heated, the use of 5% portions of the driving range at both ends thereof is prohibited by software. Thus, the actual input ranges from −10.1 to +10.1 V and the output ranges from −37.49 to +37.49 mrad (the amount by which the image plane position is changed from −8.25 to +8.25 mm).
FIG. 21
illustrates the direction of operation of the galvanomirror, variations in the beam direction, and input data (0EB8H to 3146H) and input voltages (−10.1 to +10.1V) to the galvanomirror. When the input value is changed from 0EB8H to 3146H, the input voltage changes from −10.1V to +10.1V and the direction of the beam changes upward by 74.98 mrad.
The galvanomirrors have characteristics that greatly vary from galvanomirror to galvanomirror. As shown in
FIG. 22
, the amount by which the image plane position is changed per unit change (1 bit) in the input varies from 1.23 to 2.22 &mgr;m, the maximum distance moved by the image plane varies from 12.11 to 21.86 mm, the swing angle per unit change in the input varies from 6.22 to 11.23 &mgr;rad, and the maximum swing angle varies from 55.04 to 99.38 mrad. Thus, the galvanomirrors contain an individual difference of about ±30% in their characteristics.
The time of response to input of a directive value greatly varies from galvanomirror to galvanomirror if only the mixing ratio of dumping materials varies slightly.
FIGS. 23
,
24
and
25
show response characteristics when the mixing ratio is 1:1.1, 1:1.2, and 1:1.3, respectively. The time (mS) is shown on the horizontal axis and the amount (&mgr;m) by which the image plane position is changed is shown on the vertical axis. A change in the image plane position about 10 mS, the data sampling interval in conventional control, after the galvanomirror has been given a directive value is about 130 &mgr;m in
FIG. 23
, about 100 &mgr;m in
FIG. 24
, and about 80 &mgr;m in FIG.
25
. It will therefore be seen that a little change in the mixing ratio results in variations in the response characteristic of the galvanomirror.
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