JP2004341171A - Image forming device - Google Patents

Abstract

In a multi-beam scanning type image forming apparatus, even when a plurality of light sources and their driving circuits have individually different characteristics, it is possible to prevent the occurrence of density unevenness which may occur due to the difference in characteristics and to improve the quality of an output image. Aim.
An image is formed by dividing a region on an image forming surface and forming images by a plurality of LDs individually by one LD (four LDs of ch1 to ch4 in FIG. 7), and comparing the images by the LDs with each LD. The density variation is determined in advance, and the light emission amount is individually adjusted according to the determination result to eliminate the density variation. In addition, images formed by a plurality of LDs are individually formed on the photoconductor one by one LD, the image density is detected by an optical density sensor, and the light emission amount is controlled for each LD so as to reach a target density based on the detection result. Density correction to eliminate density variations.
[Selection diagram] FIG.

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an image forming apparatus (for example, a copying machine, a printer, a facsimile, or a multi-function machine based on these, etc.) using light beam writing, and more specifically, it is possible to control each lighting based on image data. In multi-beam scanning type image formation, in which an image is formed on the photoreceptor surface by light emitted from a plurality of light sources such as laser diodes, to eliminate density unevenness due to variations in the light emission characteristics of the plurality of light sources and improve the image quality of the image The present invention relates to the image forming apparatus provided with the means.
[0002]
[Prior art]
Conventionally, image writing on the surface of a photosensitive image carrier (hereinafter sometimes simply referred to as a "photoconductor") is performed by a beam scanning method of light emitted from a laser light source (laser diode). Widely used in image forming apparatuses such as printers.
In the laser beam scanning method, a laser beam output from a laser light source driven by image data is made to act on a rotating polygon mirror to be periodically deflected, thereby scanning the photoconductor surface in a main scanning direction, while scanning the photoconductor in a sub-scanning direction. The image is written by raster scanning over the surface by moving in the direction. In this method, as a method for responding to the demand for higher performance and higher speed, basically, it is necessary to increase the rotation speed of the polygon motor or the frequency of the image signal (image data).
However, there are limitations on the number of rotations of the polygon motor and the frequency of the image signal, and when writing is performed using a single beam, there is a restriction directly due to the limitation.
In recent years, demands for higher writing speeds and higher writing densities have been at a high level beyond the limit of single-beam writing, and in order to meet such demands, the development of a multi-beam writing method has been developed. Is being promoted.
The multi-beam writing method uses a plurality of laser light sources that are located at substantially the same position in the main scanning direction and are separated by a predetermined distance in the sub-scanning direction, and simultaneously writes images on a plurality of lines to thereby write information amount per rotation of the polygon mirror. And speed up.
In addition, in the multi-beam writing method, since the amount of information that can be written simultaneously can be increased, the number of rotations of the polygon motor and the frequency of the image signal can be reduced in comparison to the case where the same amount of information is written in the single beam method. And stable images can be processed at high speed.
[0003]
[Problems to be solved by the invention]
However, in the case of multi-beam writing, since the characteristics of a plurality of laser diodes used and the characteristics of the circuit for driving the laser diodes also vary, the amount of light output from each laser diode and illuminating the photosensitive surface during writing is constant. Does not occur, and in such a case, it appears as uneven image density.
Also, in the initial stage, even if the variation in the amount of light is small and the density unevenness is not a problem, non-negligible density unevenness may occur due to a change with time thereafter. That is, when the sensitivity is reduced due to deterioration of the photoreceptor with the lapse of use, a method of adjusting the output of the laser diode may be used to correct this, and this occurs when the output is adjusted by this method. The output adjustment of the laser diode performed here detects the sensitivity decrease of the photoreceptor by measuring the density of the reference image formed on the photoreceptor by the density sensor, and responds to the density measurement value indicating the decrease fluctuation. The output is increased to a determined light amount. In this case, the measured density value detected by the density sensor corresponds to the density of a reference image formed by a plurality of beams and indicates an average value. Accordingly, the detected density value is uniformly applied to a plurality of laser diodes, and the output light amount of each laser diode is adjusted to an amount necessary for correcting the sensitivity of the photoconductor. However, even if the same adjustment amount is used uniformly for a plurality of laser diodes, the light amount does not change uniformly due to the difference in the characteristics of the respective laser diode drive circuits, causing variations in the light amount and causing density unevenness. .
As described above, conventionally, when assembling a device of the multi-beam writing system, the influence of the difference between the individual characteristics is suppressed by adopting a plurality of laser diodes and ones having no variation in the characteristics of the driving circuit thereof. In the case where the influence of a plurality of laser diodes having different characteristics and the drive circuit thereof actually occurs, the influence cannot be controlled individually and the occurrence of density unevenness cannot be effectively suppressed.
The present invention has been made in view of the above-mentioned problems of the related art in an image forming apparatus that forms an image by irradiating modulated (image) light emitted from a plurality of light sources onto a photosensitive image carrier surface by a multi-beam scanning method. The purpose is to prevent the occurrence of density unevenness that can occur due to the difference in characteristics even when using a plurality of light sources having different characteristics and their driving circuits, and to improve the quality of the output image. It is to plan.
[0004]
[Means for Solving the Problems]
According to a first aspect of the present invention, an image is formed by irradiating light emitted from a plurality of light sources, each of which can control lighting of each image data as a modulation signal, to a photosensitive image carrier surface by a multi-beam main / sub scanning method. Means for varying the light emission amount of each of the plurality of light sources, and sequentially turning on the light sources one by one in the main scanning direction in a state where the light emission amounts of each of the plurality of light sources are set to a predetermined setting. An image forming apparatus comprising: means for outputting a test pattern by an image forming operation in which control is repeatedly performed in a sub-scanning direction.
[0005]
According to a second aspect of the present invention, there is provided an image forming an image by irradiating a light emitted from a plurality of light sources, each of which can be controlled to be turned on with image data as a modulation signal, on a photosensitive image carrier surface by a multi-beam main / sub scanning method. Means for varying the amount of light emission of each of the plurality of light sources, a density sensor for detecting the density of an image carried on the photosensitive image carrier surface, and a predetermined setting of the amount of light emission of the plurality of light sources. When only one of the light sources is turned on, the operation of detecting the density of the image carried on the photosensitive image carrier surface by the density sensor is performed for each of the plurality of light sources, and based on the detected density data, An image forming apparatus comprising: a density correction unit that determines an amount to be set in a unit that changes a light emission amount in order to control a density of an image carried on a photosensitive image carrier surface when a light source is turned on. A.
According to a third aspect of the present invention, in the image forming apparatus according to the second aspect, when the density correction unit turns on only one of the plurality of light sources to carry an image on the photosensitive image carrier surface, This is characterized in that the speed of the image carrier in the sub-scanning direction is made lower than that in a normal image forming operation.
[0006]
BEST MODE FOR CARRYING OUT THE INVENTION
An image forming apparatus according to the present invention will be described based on the following embodiments shown in the accompanying drawings.
Here, as an embodiment of the image forming apparatus of the present invention, a laser emitted by a plurality of laser diodes which can control the lighting of each of the optical writing apparatus constituting a main part of the image forming apparatus, that is, image data as a modulation signal. 1 shows an apparatus for generating an image by irradiating light to a photosensitive image carrier surface by a multi-beam main / sub scanning method. Note that an image processing unit (for example, in the case of a copying machine, a scanner for inputting a document image, a processing unit for generating image data for writing, and in the case of a printer, a print command from a host is provided in front of the optical writing device. An image forming process unit (usually a toner controller), which is generally used as an electrophotographic process for receiving and creating image data for writing, and creating an image on recording paper at a later stage and outputting the image on a paper medium Development → transfer → fixation).
First, the configuration of the optical writing apparatus according to the present embodiment, and the outline of the operation and functions thereof will be described.
[0007]
FIG. 1 is a schematic diagram showing a configuration of a laser scanning optical system unit in the optical writing device of the present embodiment.
The configuration will be described with reference to FIG. 1. The laser scanning optical system unit shown in FIG. 1 includes a laser diode array element having a plurality of laser light sources (as shown in FIG. 2 to FIG. And a multi-beam laser scanning optical system.
In this optical scanning optical system, reference numeral 1 denotes a laser diode array element, in which a plurality of laser diodes (hereinafter abbreviated as "LD") as laser light sources are housed in one package, and each lighting can be controlled by using image data as a modulation signal. Then, a laser beam bearing an image is emitted (output) from each LD.
A plurality of laser beams emitted from the LD array 1 are converted into a parallel light beam by a collimating lens 2 constituting a scanner as an optical scanning means, and then an aperture having a slit corresponding to the size of a writing dot (scanning density). 8, the extra laser beam is cut. Each beam for writing an image in the main scanning direction by the cylindrical lens 3 is converted into a narrow beam having a predetermined cross-section on the surface (photoconductor surface) of the photoconductor drum 7 of each parallel light beam shaped by the aperture 8. Then, the light is irradiated to the polygon mirror 4.
[0008]
Since the polygon mirror 4 is continuously rotated at a constant speed by a polygon motor (not shown), the laser beam incident on the polygon mirror 4 is periodically deflected, and passes through the reflection mirror 9 in the main scanning direction (photosensitive drum 7). (Movement scanning) in X direction. At this time, the laser beam deflected by the polygon mirror 4 is converted by a pair of Fθ lenses 5 from equiangular motion to constant-velocity motion, and is subjected to surface tilt correction by a surface tilt correction lens 6, and then to a reflection mirror 9. The image is formed in a spot on the surface of the photosensitive drum 7 with a predetermined beam diameter.
Since the LD array 1 has a plurality of LDs, a plurality of laser beams having a pitch (position difference) in the sub-scanning direction (rotation direction of the photosensitive drum 7) Y on the surface of the photosensitive drum 7 An irradiation locus is drawn.
In FIG. 1, the laser beam immediately before scanning on the photosensitive drum 7 is synchronized with the laser beam provided on the main scanning start point side laser optical path outside the main scanning writing area (outside the predetermined main scanning width) with respect to the surface of the photosensitive drum 7. Since the laser beam passes through the synchronization detection sensor 11 via the mirror 10, the synchronization detection sensor 11 detects the laser beam and generates and outputs a synchronization detection signal for defining a writing start position in the main scanning direction. Then, a control unit (not shown) uses the synchronization detection signal from the synchronization detection sensor 11 to control the emission start timing of the laser beam for image writing for each scan (periodically).
[0009]
FIG. 2 is an enlarged perspective view of the LD array 1 of FIG. LDs having a plurality of (four in this example) light emitting points are formed on the heterodyne junction surface 1A of the LD array 1, and the laser beam emitted from each light emitting point of the LD is as shown in the figure. B1, B2, B3, and B4. In this embodiment, the LD array 1 having four light-emitting points is used. Instead, an LD array having two or three LDs or a single LD unit is used in the same number as in the array. You may.
FIG. 3 is a diagram showing a circuit configuration of the LD array. Four LDs (LD1 to LD4) linearly arranged on one chip have their negative electrodes commonly grounded. In this circuit, there is a light receiving element PD for detecting the amount of light emitted from the LD. One light receiving element PD is used to monitor the light emission amount of the LD in common to four LDs (LD1 to LD4). If possible, if the LDs are driven independently, the light amount of each LD can be detected individually.
[0010]
FIG. 4 is an exploded view showing a configuration of a laser diode (LD) drive unit unit assembled in the apparatus. As described above, the LD array 1 has four light emitting points 1. _a1 ~ 1 _a4 The LD drive unit is composed of the LD array 1, the holder 12, the drive circuit board 13, the holding member 14, the collimating lens 2, the aperture 8, the bracket 17, and the like.
The LD array 1 is pressed down and fixed at approximately the center of the holder 12 by a pressing member 14 with two screws. When the LD array 1 is attached to the holder 2, four light emitting points 1 provided on the LD array 1 are provided. _a1 ~ 1 _a4 Are fixed by a positioning jig (not shown) so that they are aligned in the sub-scanning direction of arrow B as shown.
The collimating lens 2 is fixed to the flange of the holder 12 using an ultraviolet curing adhesive 25. At that time, with the LD array 1 emitting light, the collimating lens 2 is moved in the directions of three vertical axes A, B, and C before the adhesive is cured, and the optical axis position and the collimating position are determined. After that, the adhesive 25 is irradiated with ultraviolet rays to be hardened, thereby fixing the collimating lens.
[0011]
FIG. 5 is a block diagram illustrating an example of a write control unit in the optical writing device according to the present embodiment.
The write control unit shown in FIG. 5 includes an LD array 1 having the above configuration, and an LD drive unit 24 that drives each LD (LD1 to LD4) of the LD array 1. ₁ ~ 24 ₄ The components necessary for controlling the LD drive unit 21, which is a basic element, include a CPU 50, a video signal processing unit 20, a PWM modulation unit 22 ₁ ~ 22 ₄ Having.
The video signal processing unit 20 includes an LD driving unit 24 for driving the LD array 1 based on image data transmitted from the preceding image processing unit. ₁ ~ 24 ₄ Modulation section 22 that generates a PWM (pulse width modulation) signal for controlling ₁ ~ 22 ₄ Performs control processing and operation of video signals input to.
The CPU 50 is a microcomputer including a processor, a ROM, a RAM, and the like, and generally controls the entire image forming apparatus including the above-described multi-beam writing device. That is, according to the operation mode instructed by the input operation unit (not shown), designation and setting of operation conditions are performed for each unit in the video signal processing unit 20 and the LD drive unit 21. Therefore, in addition to the normal image forming function by multi-beam writing, it functions as a means for realizing a density correction function corresponding to a density variation between multi-beams described in detail in “Embodiments 1 to 3” below.
[0012]
The basic operation of the writing control unit shown in FIG. 5 will be described. Based on a synchronization detection signal for determining a recording (writing) start position in the main scanning direction, the video signal processing unit 20 is provided with a preceding image processing unit (not shown). ), Image data for four lines is sequentially input. The synchronization detection signal uses a signal obtained by detecting the scanning beam output from the LD by the synchronization detection sensor 11 (see FIG. 1).
In the video signal processing unit 20, after the image data for four channels is temporarily stored in the internal line memory, the image data for four lines whose timing has been converted from the line memory in accordance with the rotation timing of the polygon mirror 4 is stored. Each of the four PWM modulators 22 simultaneously ₁ ~ 22 ₄ Is input to At this time, the video signal processing unit 20 converts the image data based on the synchronization detection signal obtained from the synchronization detection sensor 11 into the PWM modulation unit 22. ₁ ~ 22 ₄ At the same time, a reference timing signal (image data transfer reference signal) for transferring image data to the video signal processing unit 20 is generated for an image processing unit (not shown) at the preceding stage.
PWM modulator 22 ₁ ~ 22 ₄ The modulated signal subjected to pulse width modulation based on the image data is supplied to an LD driving unit (hereinafter referred to as “LDD”) 24 in the LD driving unit 21. ₁ ~ 24 ₄ Is input to LD drive unit 24 ₁ ~ 24 ₄ Is the PWM modulator 22 ₁ ~ 22 ₄ The LDs (LD1 to LD4) that emit four beams in the LD array 1 are respectively driven based on the PWM modulation signal from.
In this way, by simultaneously writing four lines by the multi-beam scanning method, it is possible to increase the amount of information to be written per rotation of the polygon mirror, thereby increasing the speed or stabilizing the writing process.
[0013]
The optical writing device described so far with reference to FIGS. 1 to 5 exemplifies a multi-beam scanning type writing device which is a premise of the present invention. According to the present invention, on the premise of writing by the multi-beam scanning method as in the above-described apparatus (FIGS. 1 to 5), a plurality of light sources (LDs) used therein and their driving circuits have different characteristics. It is an object of the present invention to prevent the occurrence of density unevenness that may occur due to the difference in characteristics when the image has such characteristics and to improve the quality of an output image.
Hereinafter, embodiments of an image forming apparatus having a density correction function that is a means for solving the problem of preventing the occurrence of density unevenness that may occur due to density variations between multiple beams will be referred to as “Embodiments 1 to 3”. Show.
“Embodiments 1 to 3” are the same in that a light emission amount adjusting unit is used for each light source having a variable light emission amount in order to perform correction for eliminating the density unevenness. Is determined using a different method.
That is, in the “first embodiment”, by dividing an area on an image forming surface and forming an image by a plurality of light sources individually for each light source, the images from the respective light sources are compared to determine a variation in density for each light source. Is what makes it possible.
In the second embodiment, images of a plurality of light sources are individually formed on the surface of the photosensitive image carrier one by one light source, and the density of the image carried thereon is detected by a density sensor to quantify the variation in density. The third embodiment is different from the second embodiment in that the accuracy of the detection by the density sensor is further improved in the third embodiment. Means for improving the above.
[0014]
"Embodiment 1"
The present embodiment makes it possible to divide an area on an image formation surface and individually form images by a plurality of LDs one by one LD, thereby comparing the images by the LDs and determining a variation in density for each LD. The light emission amount is individually adjusted according to the result of the determination to realize a correction function for eliminating the density variation.
Before describing the image formation for determining the density variation, first, a description will be given of a light emission amount adjustment unit for each LD (LD1 to LD4) of the LD array 1.
Referring to FIG. 5 showing the present embodiment, the LD drive unit 21 includes a means for adjusting the light emission amount of LD1 to LD4 in the unit. This adjusting means comprises a D / A converter (hereinafter referred to as DAC) 28 and a variable resistor 29, and enables to set the resistance value of the variable resistor 29 and the voltage output from the DAC 28 individually for LD1 to LD4. , The drive voltages of LD1 to LD4 are individually varied to adjust the light emission amount. In this example, the output voltage of the DAC 28 is determined by a digital input to the DAC 28 set by the CPU 50, and a variable resistor 29 that can be set by a manual operation is employed.
[0015]
The adjustment operation for equalizing the light emission amount of each of LD1 to LD4 to a predetermined value is performed as follows.
First, in the initial adjustment, before the LD drive unit is incorporated into the device, the light emission amount of each LD is equalized to a predetermined value with respect to the unit alone. The adjustment method at this time is as follows: the setting value (digital input) of the DAC 28 for each LD is the same, each LD is sequentially turned on, and the light emission amount is measured (measurement can be performed based on the detection value of the PD of the LD array 1). The light emission amount of each LD is made uniform by changing the variable resistance so that the measured value becomes a predetermined value.
FIG. 6 is a graph showing the relationship between the set value of the DAC 28 and the light emission amount in the light amount adjusting means of the present embodiment. As shown in FIG. 6, the relationship between the DAC setting value of each LD and the light emission amount is linear, but the slope of each characteristic line is A, A due to the variation of the LDD circuit characteristics and external device characteristics. It is known that they differ from B and C, and have an offset (light quantity at DAC set value = 0).
[0016]
FIG. 6 shows a characteristic line of the LD after the above-described initial adjustment. That is, the CPU 50 adjusts each variable resistor 29 with the set value DAC0 set in the DAC 28 by the variable resistor 29 so that each LD has a constant light emission amount P0. As a result, the characteristic line of each LD as shown in the drawing is obtained. Have the same value in DAC0. Here, after the resistance is set in the initial adjustment, the adjustment by the variable resistor 29 is not performed in principle, so that the characteristic of the LD is defined by the characteristic line shown in FIG. The data is stored in the ROM in the CPU 50 that performs the operation, and is used in the subsequent adjustment.
In other words, the CPU 50 subsequently changes the light emitting amount of the LD in order to improve the image quality in response to the deterioration of the photoconductor or the like that changes with use. Will be determined.
For example, when the light amount of each LD is changed, if the DAC setting value of each LD is changed with the same value, the light amount will be different for each LD, resulting in a variation in density. This is because, as shown in the example of FIG. 6, when the light amount is changed from P0 to P1, the setting for the DAC 28 is changed for each LD to a setting corresponding to the difference in the characteristics such as DAC1, DAC2, and DAC3. This can be avoided.
In this case, it is assumed that there is no variation in the characteristic line of FIG. 6. If this assumption is satisfied, the appropriate density is maintained by adjusting the amount of light emission by the above-described method, and there is no density variation. Although it is possible to obtain an image, if the characteristic line of the LD fluctuates due to the aging of each LD and its driving circuit, and shows individual fluctuations, even if the above-described method is applied, the density variation may occur. Can not be avoided.
[0017]
Therefore, this problem is solved by a method in which a variation in density can be determined by actually forming an image by each LD, and the light emission amount of each LD is individually adjusted based on the determination result. .
In the present embodiment, the user or the administrator of the apparatus determines which LD among the images formed by a plurality of LDs from the output image formed on the recording paper causes the density variation, An image is formed in a form that can be determined by looking at the image.
Specifically, on the image forming surface of the recording paper, the recording area is divided and images by a plurality of LDs are individually formed one by one to compare the images by the LDs and determine a variation in density for each LD. And
FIG. 7 illustrates a test pattern formed on a recording sheet. As shown in FIG. 7, an image is formed by dividing the recording area for each channel for driving each LD (in this example, four channels of ch1 to ch4).
The operation at the time of forming the test pattern (see FIG. 7) will be described. This operation mode is performed as a test operation mode different from the normal image forming operation mode. A screen is provided, and the setting can be instructed by a key operation or the like performed according to a display for setting a test operation mode prepared in the menu.
When a test operation mode is instructed by an input operation of a user or the like on an operation unit (not shown), the CPU 50 starts a program for performing an operation of forming a test pattern in response to the instruction.
[0018]
In the test operation mode, the CPU 50 causes the image processing unit (not shown) to perform an image forming operation using image data generated by a test pattern generation function provided therein (in a normal image forming operation mode, The image processing unit performs processing and operation using image data input from outside via a scanner, a printer controller, or the like).
This test pattern generation function is a function of generating image data for dividing a recording area and individually forming images by a plurality of LDs one by one. In the case of the test pattern illustrated in FIG. Based on the detection signal, the respective LDs (LD1 to LD4) of the LD array 1 are sequentially turned on and scanned one by one in the main scanning direction, and the lighting of each LD having the same pattern is repeated in the sub-scanning direction to form an image on the photoreceptor. This function is for generating image data for four lines to be formed. That is, the recording area of one cycle of main scanning of four multi-beams is divided into four parts, the first area is only LD1 (ch1), the second area is only LD2 (ch2), the third area is only LD3 (ch3), Only the LD4 (ch4) is turned on in the four areas, and image data that repeats this in the sub-scanning direction is generated.
The video signal processing unit 20 controls the LD drive unit based on the image data generated by the test pattern generation function and performs the same processing as in the normal image forming operation in the multi-beam scanning method to write an image. (Refer to the description of the image writing operation based on FIG. 5), whereby a test image can be formed on recording paper.
[0019]
When the test operation mode was performed as described above and a test image was formed on the recording paper, the difference in density caused by the light amount difference between the LDs (LD1 to LD4) of the LD array 1 was actually formed on the recording paper. The image can be viewed in the form of an image, and it is possible to appropriately and easily determine a density variation by a user or the like. FIG. 7 illustrates a case where the light amount of ch3 is smaller than that of the other LD beams and the density is reduced.
When the density variation is determined by the determination method based on the test pattern formed on the recording paper, correction for eliminating the variation is performed. At this time, the light amount of the LD corresponding to the pattern in which the variation is recognized is adjusted. In this example, the adjustment of the light amount of the LD can be instructed from an operation unit (not shown). For example, a density adjustment screen is prepared on the user operation screen of the operation unit described above, and a key operation on this screen changes the set value to the DAC 28 which determines the light amount of the LD at the time of test pattern formation (constant). The value may be increased / decreased) by instructing each LD.
After instructing the density adjustment by such a method, a test pattern is formed again according to the changed setting conditions, the result is determined, and when it is determined that the density difference has disappeared, the test operation is terminated.
[0020]
"Embodiment 2"
In the above-described “Embodiment 1”, the method of determining the density variation between the multi-beams (LDs) based on the test pattern formed on the recording paper is used. Individually forming a test image on the photosensitive image carrier surface and detecting the density of the test image carried by the density sensor to quantify the variation in image density by each LD with respect to the target density, based on this detection result This makes it possible to change the setting of the LD light emission amount and perform correction to the target density.
FIG. 8 is a block diagram illustrating an example of a write control unit in the optical writing device according to the present embodiment.
The writing control unit shown in FIG. 8 includes, as constituent elements unique to this embodiment, a density sensor 40 for an image carried on the photoconductor surface, a unit for individually forming a test image on the photoconductor surface by one LD, and a unit formed on the photoconductor surface. Means for detecting the density deviation (relative to the target density) based on the density data detected by the density sensor 40 from the obtained test image, and stabilizing the density for the LD in which the density deviation is detected (correction to the target density) To adjust the amount of light emission.
Except for the components specific to the present embodiment described above, components necessary for a normal image forming operation in the multi-beam scanning method have the same components as those of the writing control unit shown in FIG. That is, the video signal processing unit 20 performs processing of the video signal based on the image data transmitted from the preceding image processing unit, and the video signal processing unit 20 sends the PWM signal to the PWM modulation unit 22. ₁ ~ 22 ₄ There is no change in the LD light emission operation of the LD drive unit 21 by the drive control signal input through the. Therefore, the description of the image writing apparatus based on FIG. 5 for controlling the LD drive unit and writing an image by the same processing as in the normal image forming operation in the multi-beam scanning method will be referred to. Omitted.
[0021]
Also in the present embodiment, in order to make the density of the image formed by each LD (LD1 to LD4) of the LD array 1 constant (correct the target density), the light emission amount adjustment operation for each LD is performed. The means for adjusting the light emission amount for each of the LD4 can be implemented in the same manner as in the above-described "Embodiment 1".
That is, the light amount adjustment can be performed by a digital input to the DAC 28 variably set by the CPU 50 and the resistance value of the variable resistor 29 by using a unit including the DAC 28 and the variable resistor 29. In the initial adjustment, the initial value DAC0 is stored in the DAC 28. When set, the resistance value is adjusted so that the light emission amount of each LD becomes constant, and there is no functional change. Therefore, the description of the means for adjusting the amount of light emission is the same as that of "Embodiment 1" relating to this means, and therefore, the above description is referred to for details. In the case of the present embodiment, in order to equalize the density of the image formed on the photoconductor by the LD1 to LD4, an image density of a test image described later is detected by the density sensor 40, and based on the detection result. When a feedback control operation is performed as an operation mode for performing a control operation for adjusting the light emission amount of each LD, the control operation can be performed without considering the LD drive characteristics (see FIG. 6). It is.
[0022]
A description will be given of an embodiment of a series of operation procedures relating to a density correction process for correcting the image density of each LD by adjusting the light emission amount of each LD and performing correction to a target density. In the above-described writing apparatus (FIG. 8), a series of operation procedures relating to the density correction processing are performed as an operation mode different from a normal image forming operation mode. It is possible to automatically or instruct the execution of the operation mode by a key operation or the like from an operation unit (not shown) automatically at the time of starting the operation or as needed.
When an operation mode of the density correction processing is instructed by a key operation of the user or the like, the CPU 50 receives the instruction and starts a program for executing an operation procedure of the density correction processing.
In this operation procedure, it is required to perform an operation suitable for a process for eliminating a density variation of a plurality of LDs of the LD array 1 and guaranteeing a target density value. Here, each LD (LD1 to LD1) of the LD array 1 is required. The LD 4) is individually corrected to the target density value. Therefore, the operation procedure of the density correction processing described below is sequentially executed one LD at a time.
[0023]
In the operation procedure of the density correction processing, for one LD, first, a procedure of detecting the density of an image formed by the current setting (the density depends on the light emission amount determined by the setting value of the DAC 28) is performed. In the present embodiment, a test image is formed to accurately detect the image density.
This test image needs to be formed on the entire surface of a region whose size is such that a correct detection operation is guaranteed by the density sensor 40 (details will be described later) at the subsequent stage. A test image generation function for generating image data that causes the light to scan is prepared in an image processing unit (not shown).
The video signal processing unit 20 controls the LD drive unit by the same processing as in a normal image forming operation in the multi-beam scanning method based on the image data generated by the test image generating function, and writes an image. (Refer to the description of the image writing operation based on FIG. 5), a test image can be formed on the photoconductor.
[0024]
As the next operation procedure, the density of the test image formed by the above procedure is detected. The density sensor 40 for detecting the density of the test image is formed by attaching toner to an electrostatic latent image (the potential level of the latent image depends on the light amount of the LD) generated on the photoconductor by optical writing. This is a means for detecting the density of the toner image, and an optical sensor described below can be used as an effective means.
The optical sensor for detecting the toner concentration is composed of a semiconductor laser, an optical system for selecting specularly reflected light from the photoreceptor surface irradiated with the laser, a photosensor for photoelectrically converting reflected light input via the optical system, and the like. Become. The optical density sensor converts the amount of reflected light, which changes according to the density of the toner image formed on the photoconductor, into an electrical quantity and detects the quantity. When the density of the toner image is low, the sensor of the specular reflection detection method increases the photoelectric detection output by the specular reflection light from the glossy photoconductor surface, and when the density of the toner image is high, Part of the light applied to the toner image is absorbed by the toner, and the rest is diffusely reflected (scattered) on the toner surface, so that almost no regular reflection component is generated. A) A certain quantitative relationship holds between the density and the photoelectric detection output. The detection accuracy of the optical density sensor should be at least such that it can detect a density change corresponding to a change in the light emission amount of the LD (determined by the resolution of the set value of the DAC 28) which is adjusted for the density correction described later. desirable.
[0025]
As a next operation procedure, the light amount of each LD is adjusted based on the density of the test image detected in the above-described procedure in order to equalize the density of each LD.
In the present embodiment, the light amount of each LD is controlled so that the image density becomes the target density for each LD that is sequentially subjected to the density correction processing.
The light amount control of the LD is performed by changing the set value to the DAC 28 and adjusting the drive voltage to the LD as described in the light amount adjustment described above.
As a method of controlling the light amount of the LD, a set value to the DAC 28 for obtaining the target density is determined by referring to the LD drive characteristics obtained in advance from the density detection result of the test image by the density sensor 40 and the target density. An open-loop control method for determining or a closed-loop control method for feeding back the test image density to the target density by feeding back the density detection result of the test image can be appropriately adopted.
The operation procedure for adjusting the test image density to the target density by controlling the light quantity of the LDs in this manner is sequentially performed for each LD (LD1 to LD4) of the LD array 1, and the operation of all LDs is completed. In some cases, the image density of each LD is adjusted to the target density, and the density is made uniform to the target density. At this stage, the operation procedure of the density correction processing ends.
[0026]
"Embodiment 3"
In the above “Embodiment 2”, images by a plurality of LDs are individually formed one by one on the photoconductor surface, the density of the image carried thereon is detected by a density sensor, and the light emission amount of the LD is determined based on the detection result. By performing the control, it is possible to perform the density correction to the target value. However, in the present embodiment, a means for further improving the detection accuracy by the density sensor is added to the “second embodiment”. is there.
As described in the above “Embodiment 2”, it is desirable that the test image formed for detecting the density be formed over the entire area of a size in which the correct detection operation is guaranteed by the density sensor 40. A test image generation function for generating image data for lighting and scanning all lines in the area in the sub-scanning direction is provided in an image processing unit (not shown).
Further, in the “second embodiment”, when a test image by a plurality of LDs (image data is generated by the above-described test image generation function) is formed on the photoconductor individually by one LD, a normal multi-beam scanning method The LD drive unit is controlled by the same processing as in the image forming operation to write an image. Therefore, since the test image written by this method is individual for each LD, compared to an image (solid image) in which all of the multi-beams are turned on over the entire detection region, one test beam (in the example shown in FIG. (4 beams), that is, an image on the horizontal line may be detected, and the density of the background portion of the photoconductor may be detected. Therefore, accurate density may not be detected by the optical density sensor 40 in some cases.
Thus, in the present embodiment, it is possible to form an image closer to a solid image, and to improve the detection accuracy of the optical density sensor 40.
As a means for achieving this, the linear speed of the photoconductor at the time of writing the test image, that is, the sub-scanning speed, is reduced so that the pitch of the main scanning line is written small and dense. For example, if the linear velocity is reduced to 1/4 in the illustrated embodiment of the 4-beam system, a test image equivalent to a solid image can be formed.
[0027]
【The invention's effect】
(1) Effects corresponding to the first aspect of the invention
The light sources are sequentially turned on one by one in the main scanning direction in a state where the light emission amounts of the plurality of light sources are set to a predetermined setting, and the images by the respective light sources are compared by a test pattern obtained by an image forming operation in which this lighting control is repeatedly performed in the sub-scanning direction. Since it is possible to determine the density variation of the image and adjust the light emission amount for each light source according to the determination result, even if a plurality of light sources and their driving circuits have individually different characteristics, the light source may have different characteristics. It is possible to prevent the occurrence of density unevenness that can occur and to improve the quality of an output image.
(2) Effects corresponding to the second aspect of the invention
Only one of the plurality of light sources is turned on, and at that time, the operation of detecting the density of the image carried on the photoreceptor surface by the density sensor is performed for each of the plurality of light sources, and each light source is determined based on the detected density data. Control the amount of light emitted from the light source and automatically correct the density of the image carried on the surface of the photosensitive image carrier when it is turned on, so that multiple light sources and their driving circuits have different characteristics. , It is possible to prevent the occurrence of density unevenness that may occur due to the difference in characteristics, and to improve the quality of an output image.
(3) Effects corresponding to the invention of claim 3
When only one light source of the plurality of light sources is turned on to carry an image on the photosensitive image carrier surface, the speed of the photosensitive image carrier in the sub-scanning direction is reduced from that in a normal image forming operation. With one light source, it becomes possible to record an image close to a solid image on the photoreceptor, it is possible to detect the density more accurately, and it is possible to output an image without density unevenness.
[Brief description of the drawings]
FIG. 1 is a schematic diagram showing a configuration of a laser scanning optical system unit in an optical writing device as an embodiment of the present invention.
FIG. 2 is an enlarged perspective view of an LD array used for multi-beam writing.
FIG. 3 shows a circuit configuration of an LD array used for multi-beam writing.
FIG. 4 is an exploded view showing a configuration of a laser diode (LD) drive unit unit assembled in the apparatus.
FIG. 5 is a block diagram illustrating an example of a write control unit in the optical writing device according to the embodiment of the present invention.
FIG. 6 is a graph showing a relationship between a DAC setting value in a light amount adjusting unit and a light emission amount.
FIG. 7 illustrates a test pattern for density variation determination formed on recording paper.
FIG. 8 is a block diagram showing another example of the write control unit in the optical writing device as the embodiment of the present invention.
[Explanation of symbols]
1. Laser diode (LD) array element
4 ... polygon mirror 7 ... photoconductor drum
11: synchronization detection sensor, 20: video signal processing unit,
21: LD drive unit, 22 ₁ ~ 22 ₄ ... PWM modulator
24 ₁ ~ 24 ₄ ... LD drive unit, 28 ... D / A converter (DAC), 29 ... variable resistance, 40 ... concentration sensor,
50 ... CPU.

Claims (3)

An image forming apparatus that forms an image by irradiating light emitted from a plurality of light sources, each of which can be controlled to be turned on as image data as a modulation signal, to a photosensitive image carrier surface by a multi-beam main / sub scanning method, Means for varying the light emission amount of each light source, and sequentially turning on the light sources one by one in the main scanning direction with the light emission amount of each of the plurality of light sources set to a predetermined setting, and repeating the lighting control in the main scanning direction in the sub-scanning direction An image forming apparatus comprising: means for outputting a test pattern by an image forming operation to be performed. An image forming apparatus that forms an image by irradiating light emitted from a plurality of light sources, each of which can be controlled to be turned on as image data as a modulation signal, to a photosensitive image carrier surface by a multi-beam main / sub scanning method, Means for varying the amount of light emitted by each light source, a density sensor for detecting the density of the image carried on the photosensitive image carrier surface, and turning on only one of the plurality of light sources with a predetermined amount of light emitted When the light source is turned on, the operation of detecting the density of the image carried on the photosensitive image carrier surface by the density sensor is performed for each of the plurality of light sources, and based on the detected density data, the photosensitive image is turned on when each light source is turned on. An image forming apparatus comprising: a density correction unit that determines an amount to be set in a unit that varies a light emission amount in order to control a density of an image carried on a carrier surface to be constant. 3. The image forming apparatus according to claim 2, wherein the density correction unit performs sub-scanning of the photosensitive image carrier when only one of the plurality of light sources is turned on to carry an image on the surface of the photosensitive image carrier. An image forming apparatus, characterized in that the speed in the direction is made lower than that in a normal image forming operation.

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