JPH03223713A - Image forming device - Google Patents

Abstract

PURPOSE:To use one laser beam generation part in common by providing a converting circuit which converts the output data of a buffer which holds the ON/OFF signal of a laser oscillator into an ON/OFF signal corresponding to requested main scanning density. CONSTITUTION:An image signal which is read out of a line buffer 1 is encoded by a pulse-width encoding circuit 2 while controlled with the fundamental clock outputted by a fundamental clock generating circuit 3. The pulse-width encoding circuit 2 consists of a shift register which outputs the data from a converting circuit converting data equivalent to three dots into data equivalent to four dots to the laser oscillator 4 in synchronism with the fundamental clock and a PWM (pulse-width modulating) circuit which turns on and off the laser oscillator 4 according to the output signal of the shift register. The image data is converted into the data corresponding tot he requested recording density and PWM processing is carried on according to the data. Consequently, this device is applicable to both a printer and a copying machine and one laser beam generation part can be used in common.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an image forming apparatus that exposes a photoreceptor to light by deflecting a light-modulated laser beam.

[Conventional technology]

There are various types of image forming devices, but among them, laser beam printers and laser beam copiers, which scan a light-modulated laser beam with a polygon mirror and expose it onto a photoreceptor, are capable of high speed and high recording density. It is attracting attention because it is capable of printing.

In a laser beam type image forming apparatus, to convert the recording density, the speed of the photoreceptor, the speed of the polygon mirror,
This can be done by varying the frequency of the pixel clock.

Incidentally, there are, for example, Japanese Patent Laid-Open Nos. 163482-1982 and 124667-1987, which perform main scanning density conversion, and control the light output and lighting time.

By the way, in a laser beam printer, for example, the scanning direction of the light beam is called main scanning, and the paper feeding direction (the moving direction of the photoreceptor) is called sub-scanning. In the main scanning direction, the recording density is determined by
The sub-scanning direction is determined by the basic clock that turns off and the characteristics of the fθ lens (located between the polygon mirror and the photoconductor, which corrects changes in the focal position due to changes in the distance from the polygon mirror to the exposure position). It is determined by the rotation speed of the polygon mirror and the moving speed of the photoreceptor.

Generally, in laser beam printers, the density is set to be the same for both main scanning and sub-scanning, and the number of dots per inch is
The unit is DPI (dots/inch), 180DP11
240DP11300DPI, 480DPI, etc. are used.

On the other hand, in digital copying devices that use a laser beam to scan image signals read by an optical document scanning device (hereinafter referred to as a scanner), both the scanner and the laser beam generating section are highly Resolution is required, and 400 DPI or higher is usually adopted.

Furthermore, in order to more faithfully reproduce the shading of the original,
There is a need for a laser beam generator that controls the power of a dot, and a PWM system that changes the pulse width of one dot has been proposed as one of them.

[Problem to be solved by the invention]

However, in the above-mentioned conventional technology, while laser beam printers use a fixed recording density (because there are many determining factors), copying machines use variable pulse width for one dot. The recording density is not compatible,
It is difficult to share one laser beam generating section for a printer and a copying machine.

SUMMARY OF THE INVENTION An object of the present invention is to provide an image forming apparatus in which a laser beam generating section can be used commonly for a printer and a copying machine.

[Means to solve the problem]

In order to achieve the above object, the present invention drives a laser oscillator in PWM for a predetermined period of a basic clock using a pulse width encoded signal generated based on image data, scans the laser beam with a polygon mirror, and scans the laser beam with a polygon mirror. f the light
In an image forming apparatus that forms an image on a photoreceptor through a θ lens, there is a buffer that holds the on/off signal of the laser oscillator, and the output data of this buffer is converted into an on/off signal corresponding to the required main scanning density. A conversion circuit for conversion is provided.

[Effect]

According to the above-mentioned means, the image data read from the buffer is converted into data corresponding to the required recording density, and PWM processing is performed based on this data. therefore,
It becomes possible to correspond to the recording density of both printers and copiers, and one laser beam generating section can be shared.

〔Example〕

FIG. 1 is a schematic configuration diagram showing an embodiment of the present invention, and FIG. 2 is a timing chart showing the modulation method of the present invention.

A pulse width encoding circuit 2 is connected to a line buffer l that stores on/off signals for two lines, and a basic clock generation circuit 3 is connected to this pulse width encoding circuit 2. The pulse width encoding circuit 2 is further connected to a laser oscillator 4 configured using a semiconductor laser or the like and generating a laser beam.

In front of the laser oscillator 4 in the beam irradiation direction, a polygon mirror 5, which has a polygonal outer peripheral surface and which rotates at a constant speed, is provided with a reflecting mirror. An fθ lens 6 is disposed in the beam output optical path of the polygon mirror 5, and a drum-shaped photoreceptor 7 is disposed further rearward along the irradiation direction.

In the above configuration, the image signal read from the line buffer 1 is encoded by the pulse width encoding circuit 2 while being controlled by the basic tarock output from the basic clock generation circuit 3. The laser oscillator 4 is driven in accordance with this encoding, and a modulated laser beam is irradiated toward the polygon mirror 5.

The polygon mirror 5 continuously reflects the incident beam at different output angles while rotating, and sequentially exposes the photoreceptor 7 from one end to the other end via the fθ lens 6, thereby completing the l-scan.

Upon completion of this 1 scan, the photoreceptor 7 rotates by one line and prepares for the next scan. Here, the mirror next to the polygon mirror 5 scans the beam from the laser oscillator 4 to perform the second exposure. Thereafter, exposure for the designated line is performed in the same manner.

Incidentally, the electrophotographic method utilizes the fact that the charged potential of a photoreceptor changes due to light reception, and toner adheres in accordance with the amount of this change. Therefore, in order to express shading by controlling the amount of light received by the photoreceptor, that is, the amount of light emitted from the laser oscillator,
A PWM (pulse width modulation) method is used.

Next, the operation of PWM will be explained with reference to FIG.

The basic clock means a time interval corresponding to one dot (recording density in the main scanning direction), and Figure (C) shows the PWM operation. In FA (c), the numbers 0°1.2.3 indicate IJ while the laser oscillator 4 is on, and are on for 1 dot (7), 2/3, and 1 (7), respectively. It shows that. The amount of light received per area equivalent to one dot on the photoreceptor is proportional to this pulse width, and the amount of light received per dot-equivalent area of the photoreceptor is
When the amount of light received for WM=3) is set to 1, PWM=0.1,2
．． 3, the amount of light received is also o, 1/3.2/3,1.

Since the amount of received light and the amount of toner deposited are not in a perfect proportional relationship, the density is not proportional to the amount of received light, but the density increases as the PWM number increases. This concentration is PWM=0
The relationship is <1<2<3.

In the present invention, the above-mentioned functions of the PWM system are utilized for density conversion in the main scanning direction, and the printer and copying machine can be used in common.

For example, the second method is to realize a 300DPI printer using a 400DPI PWM laser beam generator.
This will be explained using figures.

Since the printer controls the laser oscillator only with an on/off (Ilo) signal, if it is turned on/off every basic clock according to this signal, the result will be as shown in Figure (a). At this time, since the basic clock is equivalent to 40oDPI, as a result, the image is output at 400DPI and reduced by a factor equivalent to 374 times. Therefore, in the present invention, by temporarily storing the sent on/off signal in the line buffer 1 and generating a PWM signal as shown in FIG.
It is converted into a signal equivalent to PI. In figure (b), P
WM=1 indicates that the signal is normally turned on from the leading edge of the basic clock, whereas a negative sign indicates that the signal is turned on after the interval consisting of the leading edge of the basic clock. That is, when PWM=-1, 2/3 of the tip turns on after being turned off.

FIG. 3 is a block diagram showing details of the pulse width encoding circuit 2. As shown in FIG. Here, an example of converting 300DPI to 400DPI is shown.

The pulse width encoding circuit 2 includes a conversion circuit 8 that converts data equivalent to 3 dots from the line buffer 1 into data equivalent to 4 dots, and outputs the data from the conversion circuit 8 to the laser oscillator 4 in synchronization with the basic clock. It is composed of a shift register 9 and a PWM circuit IO that turns on/off the laser oscillator 4 according to the output signal of the shift register 9.

In the line buffer 1, while an on/off signal is being written to the first line, data is read from another line and converted. Next, writing/reading is performed independently by alternating the writing and reading lines. The first three dots of the data read from the line buffer l are converted into 300DP by the conversion circuit 8.
1 equivalent to I. 0. 1 is converted into a PWM code 3°i, -i, 3 corresponding to 400DPI, and the next 0.0.1 is converted into 0°0, -1.3.

The code converted by the conversion circuit 8 is transferred to the shift register 9, and is sequentially outputted every time the basic clock is input. According to the output signal of the shift register 9, the PWM circuit IO turns on/off the laser oscillator 4, and the waveform shown in FIG. 2(b) is obtained.

〔Effect of the invention〕

As explained above, according to the present invention, a pulse width encoded signal generated based on image data is PWM-driven for a laser oscillator for a predetermined period of a basic clock, and the laser beam is scanned by a polygon mirror, and the scanning light is fθ
In an image forming apparatus that forms an image on a photoreceptor through a lens, there is a buffer that holds the on/off signal of the laser oscillator, and the output data of this buffer is converted into an on/off signal corresponding to the required main scanning density. Since a conversion circuit is provided, it is possible to correspond to the recording density of both printers and copying machines, and one laser beam generating section can be shared.

[Brief explanation of drawings]

FIG. 1 is a schematic configuration diagram showing an embodiment of the present invention, FIG. 2 is a timing chart showing the modulation method of the present invention, and FIG. 3 is a block diagram showing details of the pulse width encoding circuit according to the present invention. be. ■... Line buffer, 2... Pulse width encoding circuit, 3... Basic clock generation circuit, 4... Laser oscillator, 5... Polygon mirror 6... fθ lens, 7
...Photoreceptor, 8...Conversion circuit, 9...Shift register, 10...PWM circuit. Figure 2 @X; yo=77!

Claims (1)

[Claims] A pulse width encoded signal generated based on image data is used to PWM drive a laser oscillator for a predetermined period of a basic clock, and the laser beam is scanned by a polygon mirror.
In an image forming apparatus that images the scanning light on a photoreceptor through an fθ lens, there is a buffer that holds the on/off signal of the laser oscillator, and an on/off signal that controls the output data of this buffer according to the required main scanning density. 1. An image forming apparatus comprising: a conversion circuit for converting the signal into an off signal.