JPH0746827B2 - Laser beam printer - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial field of application] The present invention includes a rotary polygon mirror for scanning a laser beam on a photoconductor, and modulation control for detecting the laser beam on the scanning upper side than the photoconductor. A laser beam printer provided with a light receiving section for light modulation, and a modulation control device for starting modulation based on an image signal for the laser beam based on a signal output by comparing the output from the light receiving section for modulation control with a set value. Regarding

[Conventional technology]

The laser beam printer described above ensures that the electrostatic latent image formed by scanning the photosensitive member moving in the sub-scanning direction with the laser beam a plurality of times in the main scanning direction becomes uniform in the sub-scanning direction. In addition, the timing of starting the modulation corresponding to the image information is synchronized for each scan.

In this type of laser beam printer, it is general to use an optical sensor such as a photodiode as a light receiving section for modulation control. Further, conventionally, the set value has been fixedly set. On the other hand, it is known that the output from the optical sensor to be compared with this set value is used in the linear region and used in the saturation region (for example, Japanese Patent Laid-Open No. 59-157614).

[Problems to be solved by the invention]

However, the conventional configuration described above has the following problems.

In other words, the rotating polygon mirror, which is often used as a device capable of high-speed scanning of the laser beam, may have some unevenness in reflectance or may fall over. Therefore, the intensity of the laser beam incident on the modulation control light-receiving unit may be reduced. Variation will occur.

In the case where the output from the optical sensor is used in the linear region, the output from the optical sensor has a slight rise delay. Therefore, if the intensity of the incident laser beam varies, the output is fixed. There is a possibility that the timing at which the set value exceeds the set value also varies, and the output image obtained as a result may be wavy in the sub-scanning direction.

This is shown in the graphs of FIGS. 4A and 4B. In FIG. 4 (a), two curves <I>,
<II> is one in which the output from the modulation control light receiving section by the laser beams reflected by the reflecting surfaces of the rotating polygon mirror having different reflectances is converted into a voltage signal. The set value in the modulation control device is fixedly set to a fixed value [v _s ]. Therefore, as shown in FIG. 4 (b), a deviation occurs between the rising timings [t ₁ ] and [t ₂ ] of the synchronization signal for starting the modulation.

On the other hand, the configuration that uses the output from the optical sensor in the saturation region is
For example, it amplifies the output from the optical sensor. In this case, the rise time of the output is shortened, so that even if the intensity of the incident laser beam varies, the variation in the timing of starting the modulation can be reduced. However, the detected laser beam has a Gaussian distribution in the energy distribution in the transverse direction. Therefore, there is a delay in the rise of the output due to the energy distribution characteristic of the laser beam itself. Therefore, if the intensity of the laser beam varies due to this,
It was difficult to sufficiently reduce the variation in the timing of starting the modulation.

In the case of such a configuration, in order to reduce the variation in the timing of starting the modulation as much as possible, it is necessary to make the amplification factor by the amplification means such as an amplifier necessary for using the output from the optical sensor in the saturation region extremely large. . In that case,
The output after amplification may become unstable, or noise from the outside may be easily picked up.

Moreover, since the output from the optical sensor is used in the saturation region, it takes some time to return to the steady state. Therefore, when the recording operation is performed at high speed, the presence of this time may deteriorate the followability of the output for controlling the modulation, making it difficult to perform the desired modulation control.

In view of the above situation, an object of the present invention is to synchronize the timing of starting modulation in each scan with higher accuracy.

[Means for solving problems]

In order to achieve the above-mentioned object, the present invention is a laser beam printer for scanning a laser beam on a photoconductor, and a modulation control light-receiving unit which is provided on the scanning side of the photoconductor and detects the laser beam, and the modulation control. A modulation control device for starting the modulation of an image signal for the laser beam based on a signal output by comparing the output from the light receiving section for use with a set value, and the modulation control apparatus is provided further on the scanning side than the light receiving section for modulation control. The set value correction light receiving unit and the maximum value of the signal output when the set value correction light receiving unit detects the laser beam, and the value corresponding to the held maximum value is set as the set value. And a set value correction means.

[Work]

The magnitude of the output signal from the set value correction light receiving portion varies depending on the intensity of the laser beam. Therefore, the set value correction means first holds the maximum value of the output signal from the set value correction light receiving section. Then, the set value is corrected according to the held maximum value. For example, the set value is set low when the intensity of the laser beam incident on the set value correction light receiving unit is small, and the set value is set high when the intensity of the laser beam is large. As a result, when the laser beam enters the modulation control light-receiving part, the time from the rise of the output signal of the modulation control light-receiving part to the time when it exceeds the set value becomes constant regardless of the intensity of the laser beam. Since the maximum value of the output signal of the correction light receiving unit is held and the set value is corrected according to the held maximum value, the modulation start timing is not affected by the transient specification of the output from the modulation control light receiving unit. Further, it is not necessary to adjust the intensity of the laser beam.

〔Example〕

 Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 shows a schematic configuration of a scanning device of a laser beam printer. The laser beam (B) oscillated from the semiconductor laser (1) that is modulated based on the input image information is
It is collimated by a collimator lens (2) and reflected by a reflecting surface having a polygon mirror (3) rotating at a high speed. This polygon mirror (3), which is an example of a rotating polygon mirror
The rotation of the laser beam changes the inclination of its reflecting surface with respect to the laser beam (B), and accordingly, the reflected laser beam (B) is directed in the longitudinal direction of the photoconductor drum (4) which is an example of the photoconductor. Scanning (this direction is the main scanning direction).

This laser beam (B) is converged by the fθ lens (5) and forms an image on the uniformly charged photosensitive drum (4) to reduce the charging potential at that position. Then, the electrostatic latent image is formed on the photosensitive drum (4) by repeating this scanning accompanying the rotation of the photosensitive drum (4) at a constant speed.

Further, in order to align the latent image formation start position for each scan with respect to the rotation direction of the photoconductor drum (4) (this direction is the sub-scanning direction), on the scanning side of the photoconductor drum (4). A modulation control light receiving portion (6) and a set value correction light receiving portion (7) are provided.

The output from the modulation control light receiving unit (6) is input to the modulation control device (8). Then, the modulation control device (8)
When the output from the modulation control light receiving unit (6) exceeds a set value, the driving unit (9) for the semiconductor laser (1)
In addition, a synchronizing signal (▲ ▼) is output. The drive section (9) receives this signal (▲ ▼), fetches data based on the input image signal from the memory (10), and directly drives the semiconductor laser (1) for modulation.

On the other hand, the set value correction light receiving section (7) is provided adjacent to the scanning control side of the modulation control light receiving section (6), and the output thereof is also input to the modulation control device (8). . Then, the modulation control device (8) is configured to correct the above-mentioned set value according to the output.

Next, the configuration and operation of this modulation control device (8) will be described using the block diagram shown in FIG.

In the figure, (6) and (7) are photodiodes, respectively, which constitute the above-mentioned light receiving section for modulation control and the light receiving section for setting value correction. Hereinafter, the photodiode forming the light receiving section for modulation control will be referred to as a first photodiode (6), and the photodiode forming the light receiving section for setting value correction will be referred to as a second photodiode (7).

When the laser beam (B) is incident on both photodiodes (6) and (7), an output current corresponding to the intensity of the laser beam (B) is generated. The respective output currents are input to the separate amplifier circuits (11) and (12). Both amplifier circuits (1
1) and (12) are each configured by arranging two stages of inverting amplifiers using operational amplifiers, and have equal amplification factors. Output currents from the photodiodes (6) and (7) are converted into voltages by the amplifier circuits (11) and (12) and amplified.

Amplifier circuit (11) corresponding to the first photodiode (6)
The output from is directly input to the comparison circuit (13).
On the other hand, the output from the amplifier circuit (12) corresponding to the second photodiode (7) passes through the peak hold circuit (14) and then is input to the comparison circuit (13). The peak hold circuit (14), which is an example of setting value correction means, includes a diode (D), a pair of resistors (R1) and (R2), and a capacitor (C). The constant of the hold time is set to be sufficiently longer than the light receiving time of the laser beam (B) by the photodiodes (6) and (7).

FIG. 3 (a) shows the above-mentioned two amplifier circuits (11),
The output voltage from (12) is shown. <a ₁ in the figure>,
<a ₂ curve the output voltage from the amplifier circuit (12) of>
The curves of <b ₁ > and <b ₂ > are the output voltage from the amplifier circuit (11). Also, <a _1>, <b ₁ curve of>, by laser beam reflected from the surface highly reflective of the reflective surface of the polygon mirror (3) (B), <a 2>, <b 2 > Curve is the laser beam (B) reflected from the surface with low reflectance.
It is due to.

When the intensity of the laser beam (B) entering the second photodiode (7) is large, i.e., when the output voltage from the amplifier circuit (12) is shown by the curve of FIG. 3 (b) Medium <a _1> , the peak-hold circuit (14), the maximum value of the output voltage [v _1] voltage drop due to the diode (D) from [v _0] value [v ₁ -v _0] minus is held.

Therefore, the output voltage (V _ref1 ) of the peak hold circuit (14) when the laser beam (B) is incident on the first photodiode (6) is V _ref1 = γ · (v ₁ −v ₀ ) where γ = Γ ₂ / (γ ₁ + γ ₂ ) where γ _{1 is} the resistance value of the resistance (R ₁ ) and γ _{2 is} the resistance value of the resistance (R 2).

Similarly, when the laser beam (B) intensity incident on the second photodiode (6) is small, i.e., the output voltage from the amplifying circuit (12) is curve of FIG. 3 (b) Medium <a _2> , The output voltage (V _ref2 ) of the peak hold circuit (14) is V _ref2 = γ · (v ₂ −v ₀ ) where v _{2 is} the maximum output voltage of the amplifier circuit (12). Be done.

Then, the resistance value [γ ₁ ] of the two resistors (R1) and (R2),
By appropriately setting [γ ₂ ] (where γ ₁ << γ ₂ ),
Output voltage of peak hold circuit (14) (V _ref1 ), (V
_ref2 ) is as shown in Fig. 3 (b). That is, the output timing of the synchronization signal (▲ ▼) as the output of the comparison circuit (13) is irrespective of the intensity of the laser beam (B),
It is almost always constant.

Therefore, even if each reflecting surface of the polygon mirror (3) has an error such as non-uniformity of reflectance or tilting, modulation based on the image signal for the laser beam (B) is always started at a constant timing. As a result, the output image can be arranged in the sub-scanning direction. Moreover, since the outputs from any of the photodiodes (6) and (7) are used in the linear region, the amplifier circuits (11) and (12) do not need to have a structure with a large amplification factor, and recording can be performed at high speed. Even when it is performed, there is no fear that a timing shift due to the delay in the return to the steady state occurs.

In the above embodiment, the configuration in which the output from the set value correction light receiving unit (7) is integrated and the set value is corrected according to the integration result has been described. For the correction, the peak value of the output from the set value correction light receiving section (7) or the rate of change of the rising edge may be used.

〔The invention's effect〕

As is clear from the above description, in the laser beam printer according to the present invention, the light receiving section for setting value correction is provided on the scanning side of the light receiving section for modulation control, and the maximum value held from the output of the light receiving section for setting value correction is held. The set value is corrected according to the value. Then, by comparing the corrected set value with the output of the modulation control light receiving unit, the modulation of the image signal with respect to the laser beam is started. Therefore, when the laser beam enters the modulation control light receiving unit, the time from the rise of the output signal of the modulation control light receiving unit to the time when the output signal exceeds the set value is constant regardless of the intensity of the laser beam. As a result, the variation in the modulation start timing can be made extremely small without being affected by the transient characteristics of the output from the modulation control light receiving section and the energy distribution characteristics of the laser beam. As a result, the output image can be excellent in quality with no deviation in the sub-scanning direction.

Further, since the output from the modulation control light receiving section can be used in the linear region, it is possible to stably perform the modulation control operation without causing a decrease in reliability due to the provision of the amplifier having a large amplification factor. it can.

In particular, since the set value is corrected in the apparatus of the present invention, it is not necessary to adjust the intensity of the laser beam. Therefore, even when the intensity of the laser beam is adjusted, the modulation start timing can be corrected well. Further, since the time for adjusting the intensity of the laser beam is not necessary, the scanning speed of the laser beam can be increased, and the followability when performing the recording operation at high speed is good. Also,
Since the distance between the modulation control light receiving portion and the setting value correction light receiving portion can be shortened, the entire laser beam can be downsized.

[Brief description of drawings]

1 to 3 (a) and (b) show an embodiment of a laser beam printer according to the present invention. FIG. 1 is a schematic diagram of a scanning device, FIG. 2 is a block diagram of a modulation control device, and FIG. FIG. 3A is a time chart of the output signal from the amplifier circuit of the modulation control device, and FIG. 3B is a time chart of the synchronization signal. FIGS. 4 (a) and 4 (b) are time charts for explaining the conventional technique, and FIG. 4 (a) is a time chart of the output signal from the light receiving unit for modulation control corresponding to FIG. 3 (a). FIG. 4 (b) is a time chart of the synchronization signal corresponding to FIG. 3 (b). (3) …… Rotating polygon mirror, (4) …… Photoconductor, (6) ……
Modulation control light receiver, (7) ...
(8) ... Modulation control device, (14) ... Set value correction means,
(B) ... Laser beam.

Claims (1)

[Claims] 1. A laser beam printer for scanning a laser beam on a photoconductor, the modulation control light receiving section being provided on the scanning side of the photoconductor to detect the laser beam, and the modulation control light receiving section. A modulation control device that starts modulation of an image signal for the laser beam based on a signal output by comparing the output with a set value, and a set value correction provided further on the scanning side than the light receiving section for modulation control. And a set value correction means for holding a maximum value of a signal output when the set value correction light receiving section detects a laser beam, and setting the value corresponding to the held maximum value as the set value. A laser beam printer comprising: