JPS63293567A - Laser beam printer - Google Patents

Abstract

PURPOSE:To attain a stable gradation expression by detecting the density of a visual image developed on a sensitive body, which is a reference, and controlling a laser output to compensate the variation of the density. CONSTITUTION:The revealed picture, which is a reference, is formed on the sensitive drum 33 and the density is detected by a density sensor 27 in a proper before executing image formation. The detected value is composed with a prescribed value in a control part 14 and the contents of a buffer 18 which sets the laser output is corrected. Then, the laser modulation current value of a laser modulation circuit 30 is decided to obtain the laser output according to the prescribed value of the density. Therefore, the variation of the density of electrophotography sensitive body can be compensated and the stable gradation expression can be obtained.

Description

[Detailed Description of the Invention] [Industrial Application Field 1] The present invention relates to a laser beam printer that scans a semiconductor laser beam to form an image, and in particular aims at improving a control system for semiconductor laser output. be.

[Prior art] As a printer that is capable of high-speed recording and has low noise, it uses a laser beam to scan an electrophotographic photoreceptor.
2. Description of the Related Art Laser beam printers that form four latent images, develop this electrostatic latent image, and transfer it to a recording medium to obtain a visible image have been put into practical use. Particularly in recent years, semiconductor lasers have been widely used as laser light sources for such laser beam printers due to their miniaturization and ease of modulation.

Furthermore, since semiconductor lasers can be easily pulse width modulated, they have the advantage of being able to output images with high resolution and gradation.

[Problems to be Solved by the Invention] Semiconductor lasers generally have a forward current of -
Has light output characteristics. That is, as shown in the figure, the semiconductor laser has a large threshold current value Lh, and furthermore, this threshold current value Lh has a large temperature dependence. For this reason, semiconductor lasers require a large drive current, and when performing high-speed pulse width modulation, the pulse width of the laser output may be smaller than the pulse width of the human input signal depending on the temperature conditions, resulting in sufficient gradation. may not be able to be secured.

In order to cope with this problem, a device is known in which a constant bias current III lower than the light emission of the laser is previously applied to the semiconductor laser.

In addition, in order to stabilize the output of the laser, a means is provided to keep the temperature of the semiconductor laser element constant,
Conventionally, the laser output is monitored by a photodetector integrated with the semiconductor laser element, and the output is kept constant by controlling the laser current accordingly.

However, the former requires precise temperature detection and temperature control to keep the laser output constant. On the other hand, the latter is advantageous in terms of cost and size because the configuration can be simplified as long as the sensitivity of the photoreceptor is constant within the range where the laser wavelength changes due to temperature changes. It is also effective against but,
When the latter type of laser output control is performed, as mentioned above, the threshold current value 1gh for laser emission changes depending on the temperature, so even if the bias current ha is applied, the I
As Ia-Lhl changes, the pulse width of the laser output changes. Therefore, this also has an influence when expressing the gradation of an image, causing a problem in achieving stable gradation expression.

On the other hand, the sensitivity of electrophotographic photoreceptors generally changes depending on the environment and over time, and the development characteristics of the developer used for development also change, so in order to obtain stable gradation images, it is necessary to appropriately control the image density. It is desirable to do so.

The present invention solves these problems and appropriately controls laser output in order to compensate for fluctuations in the laser output of a laser generator due to changes in environmental conditions such as temperature and changes over time, and fluctuations in image density. The purpose of this invention is to provide a laser beam printer that is capable of stable gradation expression.

[Means for Solving the Problems] To this end, the present invention includes a photoconductor on which an electrostatic latent image is formed by charging treatment and scanning with a semiconductor laser beam, and In a laser beam printer that performs image formation according to the image signal, the laser beam printer includes a semiconductor laser beam generator, a laser output monitor circuit that detects the output of the generator, and a laser modulation current that is applied to the generator in accordance with the supply of image signals. A laser modulation circuit that drives the laser, a bias current circuit that is arranged in parallel with the laser modulation circuit to the generator and that supplies the generator with a bias current of a value less than the laser oscillation current, and a A first control means that determines the value of the bias current so that the output of the laser output monitor circuit becomes a preset value outside the drive period of the device, and detects the density of the formed image. The present invention is characterized by comprising a detection means and a second control means for determining a laser modulation current according to the detected concentration.

[Function] According to the present invention, the bias current circuit changes the bias current under the control of the first control means so that a predetermined laser output can be obtained regardless of the temperature or the like.

As a result, the difference between the threshold current value and the bias current value for laser oscillation becomes approximately constant regardless of temperature or the like.

Further, the laser output is controlled under the control of the second control means so that the density of the image is compensated, but at this time, this control does not change the above-mentioned difference, that is, the bias current value is changed to the current value. This is done while maintaining an appropriate value commensurate with the laser modulation current value.

These make it possible to stabilize the laser output according to the human power signal during image recording.

[Example] Hereinafter, an example of the present invention will be described in detail with reference to the drawings.

FIG. 1 is a block diagram showing an embodiment of the laser beam printer of the present invention, and FIG. 2 is a side view schematically showing the vicinity of the photosensitive drum. In the figure, 11 is a semiconductor laser device as a laser light source, and 12 and 13 are a photodiode and an amplifier, respectively, which constitute a laser output monitor that monitors the laser light output from the semiconductor laser device. 14 is a control unit;
A CPU 14a that executes the procedures described later in the figure, a ROM 14b that stores programs corresponding to the processing procedures, a bias current value (described later), control data, etc., and an A/D conversion for taking in the output of the amplifier 13. It has a RAM 14d and the like used for the work of the device 14C1.

17 is a bias current circuit for supplying the semiconductor laser device 11 with a bias current 6 lower than the laser oscillation; The current value IB is changed according to the analog control voltage.

Reference numeral 21 denotes a polygon mirror for flattening the laser beam I emitted from the semiconductor laser device 11. The polygon mirror 21 is fixed to a motor shaft (not shown) and rotates in the direction of the arrow in the figure, so that the beam is directed onto the photosensitive drum 23. ■ Scan. 22 is an f-θ lens that focuses the polarized laser beam I onto the photosensitive drum 23.

24 is a beam detector consisting of a light receiving diode for detecting the starting position of information writing on the photosensitive drum 23 by the laser beam ■; 25 is the beam detector 24;
This is an amplifier that amplifies the output of. 26 is a blanking circuit, which generates a blanking signal BL that turns off the irradiation of the laser beam (■) for a predetermined period when the beam detector 24 detects the laser beam (■), and sends this to the control unit 14 and the superimposing circuit 35. supply

A density sensor 27 detects the density of the visualized image formed on the photosensitive drum 23.

30 is a bias current circuit 17 for the semiconductor laser device.
A power supply circuit that changes the current value according to the control voltage of the analog amount by the D/A converter 19 that performs D/A conversion of the value set in the buffer 18 by the control unit 14. 31. It has a differential switch 32 and the like.

The signal superimposition circuit 35 generates a modulated signal S by superimposing a blanking signal BL on a pulse width modulated image signal (2) supplied from the control unit 14 or an image signal supply source (not shown). The differential switch 32 turns on/off the semiconductor laser device 11 using a modulation signal S obtained by superimposing a blanking signal BL on a pulse width modulated image signal V. The power supply circuit 31 keeps the current constant during the on-time period during the image period according to the setting value for the buffer 18, and this current flows through the resistor R when the power supply circuit 31 is off-state.

In FIG. 2, numeral 40 includes the parts 14 to 19 and 30 shown in FIG. 1, and constitutes an output controller for the semiconductor laser device 11. In FIG. 41 is a charger for charging the photosensitive drum 23; 43 is a developer; 45 is a transfer charger for transferring developer (toner) to the transfer paper 44; 46
A cleaning blade removes toner remaining on the photosensitive drum 23 after transfer.

FIG. 3 is a waveform diagram showing the on/off state of the laser beam (2), and the operation of the apparatus shown in FIGS. 1 and 2 will be explained using this diagram.

The laser beam I emitted from the semiconductor laser device 11 is polarized by the polygon mirror 21 and
The light is focused and scanned onto the photosensitive drum 23 by the lens 22 . That is, a latent image is formed on the photosensitive drum 23 by uniform charging by the charger 41 and scanning by the laser beam I. As shown in FIG.
During the non-image period, the light can be turned on by the blanking signal BL, and during the blanking period during which the light is turned on, the output of the semiconductor laser device 11 is monitored and the laser beam scan is detected. The blanking circuit 26 is the beam detector 2
4 deactivates the blanking signal BL for a predetermined period td when the laser beam I is detected, so that the on/off of the laser beam I during this period depends only on the input of the signal ■ during the image period.

The control unit 14 sets the buffer 18 so that the laser modulation circuit 30 supplies a predetermined value of current to the laser device 11 during the blanking period, and also outputs a current from the photodiode 12 via the amplifier 13 and the A/D converter 14c. The output value of the buffer 15 is read in the supplied laser output monitor value, and the output value of the buffer 15 is adjusted so that the value approaches a preset value.
That is, the bias value 1B is changed and set.

FIGS. 4(A) and CB) are examples of various value determination processing procedures for controlling the laser output by the control unit 14, and the steps are shown in FIGS. The procedure and the procedure for determining a buffer value according to temperature and the like are shown.

The process shown in FIG. 5A can be started at an appropriate time before image formation, for example, at predetermined intervals, or by forming a standard visual image during image formation. . When the procedure is started, first, in step SAI, the density of the visualized image formed by the potential sensor 27 is detected, and in step SA3, the detected value is compared with a predetermined value.

Here, when it is determined that the detected value is not an appropriate predetermined value, that is, when an appropriate density is not obtained for a certain laser output, the process proceeds to step SA5, and the contents of the buffer 18 for setting the laser output are is corrected and the process ends. That is, at the end of this procedure, the laser output is set to a value Ps that allows a preset density to be obtained.
The laser modulation current value Is of the laser modulation circuit 30 is determined.

Next, the process shown in FIG. 4(B) is performed during the blanking period, and is started in response to the detection of the rise of the blanking signal BL in step SBI.

At this time, in step SB2, the contents set in the buffer 18 are saved, for example, in a predetermined area of the RAM 14d, and a value corresponding to a constant current value rat is set in the buffer 18 instead. Next, in step SB3, the output of the photodiode 12 is amplified and A/D converted to a value (monitor value).
Load. Next, in step SB5, the monitor value and R
Predetermined value Pl! stored in OM14b! 1L, and if they match or are almost equal, the process proceeds to step SB9, where the contents of the buffer 18 are saved and set to a certain value, and the procedure ends.

On the other hand, if a negative determination is made in step SB5, the process moves to step SB7, where a control amount corresponding to the amount of deviation between the two is set in the buffer 15, and the bias current 1. The value of is changed, the process returns to step SB3, and the procedure is repeated until an affirmative determination is made in step SB5.

Here, since the curve shown in FIG. 7 has a substantially similar shape even when the temperature changes, the predetermined value can be a constant value regardless of the temperature. That is, in this example, the bias current value I6 is controlled in accordance with the monitor value so that the difference between the threshold current value 1sh and the corresponding bias current value 6 becomes approximately constant.

As mentioned above, the semiconductor laser output is very sensitive to temperature changes and changes over time, but the main change is the change in the laser oscillation threshold current value 1h.

According to this example, the bias current is controlled by the bias current circuit 17 which is arranged in parallel with the laser modulation circuit 30.
For example, when the laser output characteristic changes from (1) to (2) and the threshold current value changes from Ish to Itho, as shown in FIG.
The bias current value also changes from ■ to ■8°. That is, 1. Since h-1° and I*h'-111" can be made almost equal, and Lh-Ill can be made small, the laser output is faithful to the pulse width of the input signal and is always stable. This makes it possible to modulate the width.

Furthermore, as is clear from the above explanation, by being able to modulate input signals with narrow pulse widths at high speed and stably,
It becomes possible to stably obtain a high-resolution gradation image.

Then, during image formation, as shown in FIG.
s is controlled according to the settings of the buffer 18. Since changes in the sensitivity of the photosensitive drum 23 and changes in the development characteristics of the developer in the developing device 43 are sufficiently slow compared to the image forming time, it is possible to control the image density to a constant level by controlling the density as described above. becomes.

As explained above, assuming that the current value of the laser modulation circuit 30 is constant (IBL) during the planning and kinging periods,
By setting the bias current value ■8 so that the laser output is constant, the difference between the laser threshold current value Lh and the bias current value IB is always kept at a predetermined amount, so that f I The value of I-h-IBI can be made sufficiently small, and the output pulse can be stably reproduced with respect to the input pulse. Further, the image density is controlled by the laser output without changing l5h-18, that is, while keeping I8 at an appropriate value commensurate with Is.

With these features, it is possible to realize a laser beam printer that can output stable gradation images at high speed and high resolution.

In the above example, the value is controlled digitally by the control unit 14 including the CPU and the buffer 15, but the above configuration can also be performed by an appropriate logic circuit, or by using a comparator. It is also possible to change and set the value of I during the blanking period using, for example, the control in an analog manner.

Further, in the above example, the standard microscopic image formed on the photosensitive drum 23 is directly detected by the density sensor 27, but it is also possible to perform the same detection for an image transferred thereto.

FIG. 6 shows an embodiment of the present invention based on this.

In the figure, 47 is a belt that conveys the recording medium 44;
A is an image obtained by transferring the standard microscopic image on the photosensitive drum 23 onto the belt 47 and fixing it. The density of this image 47a is
If the density sensor 127 installed close to and facing the conveyance path is used for detection by reflection or by transmission when the belt 47 is transparent, a similar result can be obtained through the same process as in the first embodiment. It can be effective. After such detection is completed, the image 47a may be cleaned by a cleaner 48 provided to the belt 47.

[Effects of the Invention] As explained above, according to the present invention, the laser output can be adjusted appropriately to compensate for fluctuations in the laser output of the laser generator due to changes in environmental conditions such as temperature or changes over time, and fluctuations in image density. By performing control, a laser beam printer capable of stable gradation expression can be realized.

[Brief explanation of drawings]

1 and 2 are a block diagram and a side view, respectively, showing an embodiment of the laser beam printer of the present invention. FIG. 3 is a waveform diagram showing the on/off state of the laser beam according to the embodiment shown in FIG. , FIGS. 4(A) and (B) are flowcharts showing an example of the laser output control procedure according to the embodiment shown in the first drawing, and FIGS. 5(A) and (B) explain the effects of the embodiment shown in the first drawing. FIG. 6 is a side view showing another embodiment of the present invention, and FIG. 7 is a characteristic curve diagram showing the relationship between current and laser output for controlling the laser output according to the conventional example. FIG. 3 is a characteristic curve diagram showing the relationship with output. DESCRIPTION OF SYMBOLS 11... Semiconductor laser, 12... Photodiode, 13... Amplifier, 14... Control part, 14a... CPU. 14c...A/D converter, 15.18...Buffer, 16.19...-D/A converter, 17...Bias current circuit, 21...Polygon mirror, 22...f -θ lens, 23... Photosensitive drum, 24... Beam detector, 25... Amplifier, 26... Blanking circuit, 27... Density sensor, 30... Laser modulation circuit, 31... - Current source, 32... Differential switch, 40... Controller, 41... Charger. Fig. 2 Fig. 3 (A) () ゛Rankin 7゛ Desert Drug Field) CB) (Image aIv1) Fig. 5 Fig. 4 (f3) Fig. 7

Claims (1)

[Scope of Claims] 1) A laser beam printer that has a photoreceptor on which an electrostatic latent image is formed by charging processing and scanning with a semiconductor laser beam, and forms an image according to the formed electrostatic latent image. , the semiconductor laser beam generator; a laser output monitor circuit that detects the output of the generator; and a laser modulator that drives the generator by supplying a laser modulation current to the generator in accordance with the supply of an image signal. a bias current circuit that is arranged in parallel with the laser modulation circuit for the generator and supplies the generator with a bias current having a value less than or equal to the laser oscillation current; and the generator by the laser modulation circuit. a first control means for determining the value of the bias current so that the output becomes a preset value in accordance with the output of the laser output monitor circuit outside the driving period; and detecting the density of the formed image. A laser beam printer comprising: a detection means for detecting the concentration; and a second control means for determining the laser modulation current according to the detected concentration. 2) A laser beam printer according to claim 1, wherein the detection means includes means for directly detecting the density of the visualized image developed on the photoreceptor. 3) The laser beam printer according to claim 1, further comprising a conveying member for conveying a recording medium related to image formation, and transferring a visible image developed on the photoreceptor onto the conveying member; A laser beam printer characterized in that the density of the transferred image is detected by the detection means.