JP4200030B2 - Image forming apparatus - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a beam scanning image forming apparatus such as a laser printer, a digital copying machine, and a facsimile machine, and more particularly to a shading correction technique.
[0002]
[Prior art]
As an image forming apparatus such as a laser printer, a digital copying machine, or a facsimile machine, a laser beam is scanned in a main scanning direction on a scanned surface such as a photosensitive member by a scanning means such as a polygon mirror, and the scanned surface is scanned in a sub scanning direction. And a beam scanning type image forming apparatus equipped with a function of writing an image for each line on a photosensitive member.
In this beam scanning type image forming apparatus, the laser beam is deflected at a constant angular velocity by a polygon mirror, and an fθ lens and an fθ mirror are used in order to keep the scanning velocity on the surface to be scanned constant. The laser light passing through the fθ lens and the fθ mirror has a constant scanning speed on the surface to be scanned, but the intensity of the laser light on the surface of the photoreceptor varies depending on the image height. This is because the light utilization efficiency such as reflectance and transmittance of optical elements such as glass, lenses, and mirrors that pass from the time when the laser light is emitted from the laser diode to the surface of the photoreceptor varies depending on the incident angle of the laser light, fθ This is because the lens thickness varies depending on the image height. The intensity of the beam light intensity depending on the image height is called a shading characteristic. However, since it affects the density of the formed image, the beam scanning image forming apparatus is required to correct the shading characteristic.
As for the shading correction method, for example, as the prior application of the same applicant as the present application, the effective writing area is divided, and the light amount correction data corresponding to each divided area is written in advance in the RAM address, and the laser beam In the effective writing area scanning, the light amount correction data corresponding to each divided region is read from the designated address, and the analog voltage converted by the digital-analog converter (DAC) is input to the LD modulation unit as a light amount correction signal. Techniques for adjusting the amount of laser light have been proposed.
[0003]
[Problems to be solved by the invention]
As described above, conventionally, in order to correct shading, the effective writing area is divided, the light amount correction data corresponding to each divided area is converted by a digital-to-analog converter (DAC), and the converted analog voltage is converted. A technique has been proposed in which a laser light quantity is adjusted by inputting a light quantity correction signal to an LD modulator.
However, in the prior art, the conversion of the light amount correction data by the DAC is performed inside the LD write control IC, which takes a lot of time to create and verify the LD write control IC, and further increases the chip area and cost. It was a factor.
An object of the present invention is to provide a beam scanning image forming apparatus that can easily realize a shading correction function with a low-cost configuration.
[0004]
[Means for Solving the Problems]
In order to achieve the above object, the invention described in claim 1 is directed to a light source means for generating laser light, a deflection means for deflecting and scanning laser light from the light source means, and a laser beam from the deflection means to be scanned. A beam scanning type image forming apparatus having scanning imaging optical means for condensing scanning on a surface, wherein the means for correcting shading characteristics, which is a laser intensity displacement within a laser beam scanning width in the scanning imaging optical means, is a laser. An image forming apparatus including a pulse width modulation unit that generates a pulse corresponding to the amount of light and a low-pass filter that smoothes the output from the pulse width modulation unit is the main feature.
The invention according to claim 2 is characterized in that the image forming apparatus according to claim 1 is characterized in that the time constant of the low-pass filter is variable.
[0005]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the drawings.
FIG. 1 is a front view showing a writing optical system of an image forming apparatus according to an embodiment of the present invention, and FIG. 2 is a top view showing the control system. The configuration will be described together with the operation.
Laser light as a light source is shaped and emitted from the LD 2 (light source means) of the laser diode (LD) unit 1. The laser light passes through the cylinder lens 7 and passes through the polygon mirror shield glass 9 for use in soundproofing the polygon mirror 8 (deflecting means) as scanning means. The polygon mirror 8 is rotationally driven by a polygon motor 10 as a rotational drive unit and rotates at a predetermined rotational speed, and deflects and scans the laser light incident from the polygon mirror shield glass 9.
The laser light from the polygon mirror 8 passes through the polygon mirror shield glass 9 again, passes through the fθ lens 11, passes through the barrel toroidal lens 12 for correcting the surface tilt of the polygon mirror 8, and is reflected by the folding mirror 13. The light is reflected, further passes through the dust-proof glass 14, and is condensed on the photosensitive member 15 (scanned surface) as a recording medium. The laser beam from the polygon mirror 8 is incident on the beam detector 16 as a beam detection signal.
For example, a photosensitive drum is used as the photosensitive member 15, and the image is written by being repeatedly driven in the main scanning direction by a laser beam after being rotated and rotated uniformly by a rotation driving unit and uniformly charged by a charger. As a result, an electrostatic latent image is formed. The electrostatic latent image on the photosensitive drum 15 is developed by a developing device to become a toner image, and the toner image is transferred onto a recording material such as transfer paper by a transfer unit and fixed on the recording material by a fixing device.
[0006]
In this embodiment, the laser light is deflected at a constant angular velocity by the polygon mirror 8, and the polygon mirror 8, the polygon mirror shield glass 9, and fθ through which the laser light passes to make the scanning speed on the surface to be scanned constant. Optical elements such as a lens 11, a barrel toroidal lens 12, a folding mirror 13, and a dustproof glass 14 are used. In the claims, the scanning imaging optical means means the fθ lens 11, the barrel toroidal lens 12, and the folding mirror 13.
Since the light utilization efficiency such as reflectance and transmittance of these optical elements varies depending on the incident angle of light, the intensity of the laser light is varied depending on the image height. The intensity of the beam light intensity depending on the image height is called a shading characteristic and affects the density of the formed image.
[0007]
FIG. 3 shows the laser light intensity according to the image height on the surface to be scanned (the surface of the photosensitive drum 15) when the LD 2 is illuminated with the same amount of light at the entire image height. There are effective writing areas on the left and right with the image height 0 being the center of the photosensitive drum 15 in the main scanning direction. By correcting the amount of laser light so that the shading characteristics shown in FIG. 3 are opposite to those shown in FIG. 4, the beam intensity on the surface to be scanned becomes constant.
In this embodiment, light amount correction data corresponding to each scanning position of the laser beam is read during scanning of the effective writing area of the laser beam. In the pulse width modulation (PWM) unit 17 (pulse width modulation means) of the LD writing control IC 6 shown in FIG. 2, pulse width modulation corresponding to the light amount correction data is performed. This PWM output is smoothed by a low-pass filter (LPF) 18 outside the LD write control IC and becomes an analog voltage light amount correction signal to correct the laser light amount.
At this time, if the time constant of the LPF 18 is too small, the PWM output becomes difficult to be smoothed and ripples increase, and if the time constant of the LPF 18 is too large, a light amount correction signal is generated for a target LD light amount change. However, the time constant of the LPF 18 is derived and set to an optimum value, whereby a light amount correction signal that smoothly changes as shown in FIG. 4 can be obtained.
[0008]
Thus, in this embodiment, the LD unit 1 as a light source means for generating a light beam composed of laser light, the polygon mirror 8 as a deflection means for deflecting and scanning the light beam from the light source means, and the deflection means Scanning imaging optical means comprising an fθ lens 11, a barrel toroidal lens 12, and a folding mirror 13 for condensing a light beam as a light spot on the surface of the photoconductor 15 as a scanning surface and scanning the scanning surface with the light spot. And a pulse that performs pulse width modulation corresponding to the scanning position of the laser beam based on the light amount correction data of the light source means given in advance corresponding to the scanning position of the laser beam. Since the width modulation means 17 and the LPF means 18 for smoothing the PWM output are provided, the shading characteristics by the optical system can be simplified. It can be corrected, the image quality can be improved to be able to beam intensity for each image height to reduce the non-uniformity of the image density due to different.
In this embodiment, since only the pulse width modulation (PWM) signal needs to be output from the LD write control IC 6, the creation and verification of the LD write control IC 6 is facilitated. Cost can be reduced.
In this embodiment, by adjusting the value of the LPF 18, the PWM output from the pulse width modulation unit 17 is smoothed, and the laser light amount unevenness at the division boundary of the effective writing area is reduced. Shading correction can be realized. The pulse width modulator 17 and the LPF 18 can be used not only for shading correction but also for general LD light amount adjustment.
According to the present embodiment, the LD write control IC 6 performs only pulse width modulation (PWM), and the low-pass filter (LPF) 18 outside the LD write control IC 6 performs a pulse width modulation unit inside the LD write control IC. Since the PWM output from 17 is smoothed, it is possible to provide a beam scanning image forming apparatus that can easily realize the shading correction function with a low-cost configuration.
[0009]
【The invention's effect】
According to the first aspect of the present invention, the light source means for generating laser light, the deflecting means for deflecting and scanning the laser light from the light source means, and condensing and scanning the laser light from the deflecting means on the surface to be scanned. A scanning image forming optical means, wherein the shading characteristic correcting means, which is a laser intensity displacement within a laser beam scanning width in the scanning image forming optical means, outputs a pulse corresponding to the laser light quantity. By configuring the generated pulse width modulation means and the low-pass filter for smoothing the output from the pulse width modulation means, the intended purpose can be achieved.
According to the second aspect of the present invention, by adjusting the time constant of the low-pass filter, the PWM output from the pulse width modulation means can be smoothed, and the unevenness of the laser light quantity that occurs when the pulse width is switched can be eliminated. it can.
[Brief description of the drawings]
FIG. 1 is a front view showing a writing optical system of an image forming apparatus according to an embodiment of the present invention.
FIG. 2 is a top view showing the same control system.
FIG. 3 is a diagram illustrating the laser light intensity according to the image height on the surface to be scanned (the surface of the photosensitive drum) when the LD is illuminated with the same amount of light at the entire image height.
FIG. 4 is a diagram illustrating a light amount correction signal.
[Explanation of symbols]
1 LD unit (light source means)
8 Polygon mirror (deflection means)
11 fθ lens (scanning imaging optical means)
12 Barrel toroidal lens (scanning imaging optical means)
13 Folding mirror (scanning imaging optical means)
15 Photoconductor (scanned surface)
17 Pulse width modulation section (pulse width modulation means)
18 Low-pass filter

Claims (2)

Beam scanning having light source means for generating laser light, deflecting means for deflecting and scanning laser light from the light source means, and scanning imaging optical means for condensing and scanning the laser light from the deflecting means on the surface to be scanned In this type of image forming apparatus, the correction means for shading characteristics, which is the laser intensity displacement within the scanning width of the laser beam in the scanning imaging optical means, and a pulse width modulation means for generating a pulse corresponding to the amount of laser light, and this pulse An image forming apparatus comprising: a low-pass filter that smoothes an output from a width modulation unit. 2. The image forming apparatus according to claim 1, wherein the time constant of the low-pass filter is variable.

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