JP2006305811A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To enable exposure with stable PWM characteristics even in the wide range of the used amount of light from an initial sensitivity of a photoconductor to sensitivity after a deterioration in durability. <P>SOLUTION: A laser scanning optical system is provided with a mode by which a PWM characteristic correction table is rewritten or switched by acquiring the PWM characteristics depending on the amount of light for use. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention guides laser light, which is pixel-modulated from a laser light source based on an input image signal, onto an image bearing surface such as a photosensitive member or an electrostatic recording medium, and on the surface, for example, The present invention relates to an image forming apparatus that forms image information including an electrostatic latent image.

  Conventionally, the laser light amount range used in an electrophotographic image forming apparatus is not so wide, and is at most from half the maximum rated light amount to the maximum light amount. As long as the amount of laser light was used in the vicinity of half of the rating or more, the linearity characteristic during PWM light emission of the laser did not change that much, and one type of table for correcting the PWM characteristic was sufficient.

  This will be described with reference to FIG. In the figure, the horizontal axis represents data, 1 represents 100% duty (1 pixel ON), 0 represents 0% duty (1 pixel OFF), and the vertical axis is a graph obtained by normalizing the light amount. Conventionally, for example, a laser is used only in the range of 20 mW and 10 mW in the figure, and it can be seen that the PWM characteristics are almost equivalent. Therefore, in order to make this PWM characteristic ideal, conventionally, the input image data is PWMed using the output image data through one type of conversion table (described as a correction curve) as shown in FIG. It was. As shown in the figure, for example, when data A1 is input, the light amount is Pa when not passing through the conversion table, but when passing through the conversion table, B ′ is output by the table, and B ′ can be understood from the graph. Since B ′ = B1, the amount of light is Pb.

  This will be explained with reference to the block diagram of FIG. 6. When the multi-value image data A1 (S601) is input to the conversion table 601, the conversion table 601 outputs the multi-value PWM data B1 (S602), which generates PWM. The laser drive unit 603 drives the laser 43 in accordance with the PWM signal S603 input to the unit 602 and output from the PWM generation unit 602, and has an optical waveform as in S604. The light quantity of this optical waveform is the light quantity Pb after PWM correction, and the PWM characteristic is converted to the ideal characteristic by the conversion table.

Similarly, whenever the multi-value image data S601 is input (A2, A3,...), The conversion table 601 outputs the corrected multi-value PWM data S602 (B2, B3,...) The PWM generator 602 generates a PWM signal S603 according to the data, and the laser driver 603 drives the laser 43. While the laser is being driven, the sequence controller 600 inputs a signal from the PD sensor 604 and controls the laser light emission amount to be constant (auto power control) (see, for example, Patent Document 1).
Japanese Patent Laid-Open No. 11-216912

  However, in recent years, there has been a gradual need to further extend the life of image forming devices, and the light intensity range is wide enough to cover the initial sensitivity of the photoreceptor to the sensitivity after endurance degradation (from the lowest light quantity that can emit laser light to the maximum rated light quantity). Up to the full range) is being demanded of lasers. However, the PWM linearity characteristics of lasers tend to be extremely downward at low light amounts, as shown by the 5 mW and 1 mW characteristics in FIG. 4, and highlight skipping tends to occur. The characteristics must be corrected.

  Therefore, it has become necessary to rewrite or switch the PWM characteristic correction table according to the amount of light used.

  The present invention has been made paying attention to the above points, and provides an image forming apparatus capable of performing exposure with stable PWM characteristics even in a wide light usage range from the initial sensitivity of the photoreceptor to the sensitivity after durability deterioration. The purpose is to do.

Therefore, in the present invention,
In an electrophotographic image forming apparatus that forms an image on a recording medium by scanning an image carrier with a laser beam output according to a PWM signal based on input multivalued image data.
A lookup table for converting and outputting the input multi-value image data to multi-value PWM data in accordance with the laser emission characteristics;
Means for generating a PWM signal in accordance with the multi-value PWM data output from the lookup table;
Means for driving a laser in response to a PWM signal from the PWM generation means;
Means for detecting the amount of light emitted when the laser scans the image carrier;
Means for A / D converting the output of the light emission amount detection means;
The laser emits light with at least one point of PWM data between the maximum PWM data and the minimum PWM data, the light emission amount detecting means detects the laser light emission amount, and the A / D conversion means converts it to digital data Mode to obtain PWM characteristics by
Provided,
Multi-level PWM data output using the look-up table corresponding to the PWM characteristic data acquired in the mode is converted into a PWM signal by the PWM signal generating means, and the laser driving means is converted according to the PWM signal. And forming an image by driving the laser.
Further, the light emission amount detecting means is a surface potential detector of a photoreceptor,
According to the PWM characteristic data acquired in the mode, the look-up table is rewritten,
Furthermore, it is characterized by selecting from a plurality of lookup tables prepared in advance according to the PWM characteristic data acquired in the mode.

  According to the first aspect of the invention, the laser is caused to emit light with at least one point of PWM data between the maximum PWM data and the minimum PWM data, the light emission amount detecting means detects the light emission amount of the laser, and By providing a mode for acquiring PWM characteristics by converting to digital data by A / D conversion means, it is possible to grasp the PWM characteristics according to the light quantity of the laser that is actually used. In order to use the look-up table according to the PWM characteristic data, the PWM characteristic can be corrected according to the light quantity of the laser actually used, and a stable PWM characteristic can be obtained even when using a low light quantity. Is possible.

  According to the second aspect of the present invention, since the light emission amount detecting means is a surface potential detector of the photoconductor, the actual exposure amount on the photoconductor can be measured, and the laser is accurately measured. It is possible to acquire the PWM characteristics of the.

  According to the third aspect of the present invention, the look-up table is rewritten in accordance with the PWM characteristic data acquired in the mode, so that the PWM characteristic can be corrected according to the actually used exposure amount. become.

  According to the fourth aspect of the present invention, the maximum PWM data and the minimum PWM are selected from the plurality of lookup tables prepared in advance according to the PWM characteristic data acquired in the mode. Only by turning on PWM data with a small number of points between data, the PWM characteristics can be predicted according to the exposure amount actually used, and correction can be made.

  Hereinafter, the best mode for carrying out the present invention will be described in detail with reference to examples.

[First embodiment]
The best mode of the image forming apparatus and the control method thereof according to the present invention will be described below.

  FIG. 1 is a cross-sectional view showing the overall configuration of the image forming apparatus according to the present invention. The basic operation of the digital copying machine will be described with reference to FIG.

  In the figure, reference numeral 1 denotes a document feeder, and the documents stacked on the document feeder 1 are sequentially conveyed onto the surface of the document table glass 2 one by one. When the document is conveyed, the lamp 3 in the scanner portion is turned on, and the document is irradiated even if the scanner unit 4 moves.

  The reflected light of the original passes through the lens 8 via the mirrors 5, 6, and 7, and then the image signal input to the image sensor unit 9 is stored directly in the image memory (not shown) or read out again. Is input to the exposure control unit 10.

  The latent image created on the photoconductor 11 by the irradiation light generated by the exposure control unit 10 is monitored by a potential sensor (potential detection sensor) 100 to determine whether the potential on the photoconductor 11 has a desired value, and then The developing unit 13 develops the latent image on the photoconductor 11.

  A transfer member (transfer material) such as a recording sheet or a recording film is conveyed from the transfer member stacking unit 14 or the transfer member stacking unit 15 in synchronization with the latent image, and the developed toner is transferred in the transfer unit 16. The machine is transferred onto the transfer member. The transferred toner image is fixed to the transfer member by the fixing unit 17, and then discharged to the outside of the apparatus from a paper discharge unit (also referred to as a paper discharge roller) 18 through a paper discharge conveyance path 21.

  The surface of the photoconductor 11 after the transfer is cleaned by a cleaner 25, and the surface of the photoconductor 11 cleaned by the cleaner 25 is neutralized by an auxiliary charger 26 so that a good charge can be obtained in a primary charger 28. The residual charge on the photoconductor 11 is erased by the pre-exposure lamp 27, the surface of the photoconductor 11 is charged by the primary charger 28, and a plurality of images are formed by repeating this process.

  Reference numeral 19 is a sensor for detecting the trailing edge of the transfer material, and 20 is a flapper for switching between discharging the single-side printed transfer member as it is or guiding it to the transport paths 22, 23, 24. Occasionally, when the trailing edge of the transfer material is detected by the sensor 19, the paper discharge roller 18 is rotated in the reverse direction, and the flapper 20 is switched to the direction leading to the delivery paths 22, 23, and 24.

  FIG. 2 is a diagram showing a configuration of the exposure control unit 10 of FIG.

  In FIG. 2, 31 is a semiconductor laser light emitting part. The chip 43 of the semiconductor laser generator 31 is provided with a PD sensor (light amount detecting means) for detecting a part of the laser light, and APC control (abbreviation for auto power control) of the laser light emitting diode using the PD detection signal. Meaning automatic light control).

  The laser beam emitted from the semiconductor laser generator 31 becomes substantially parallel light by the collimator lens 35 and the diaphragm 32, and enters the rotary polygon mirror 33 with a predetermined beam diameter. The rotary polygon mirror 33 rotates at a constant angular velocity in the direction indicated by the arrow, and with this rotation, the incident light beam is reflected as a deflected beam that continuously changes its angle.

  The light that has become the deflected beam is focused by the f-θ lens 34. On the other hand, since the f-θ lens simultaneously corrects distortion aberration so as to guarantee the temporal linearity of scanning, the light beam is directed on the photosensitive member 11 as an image carrier in the direction of the arrow in the figure. Combined scanning is performed at high speed.

  Reference numeral 36 denotes a beam detect (hereinafter referred to as BD) sensor for detecting reflected light from the rotary polygon mirror 33. The detection signal of the BD sensor 36 synchronizes the rotation of the rotary polygon mirror 33 and the writing of image data. Is used as a synchronization signal.

  Next, the sequence in the present embodiment will be described with reference to FIG. 3, FIG. 7, and FIG.

  3, when the PWM characteristic acquisition mode is entered (300), the sequence controller 600 in FIG. 7 sets the PWM data S704 to be acquired in the register 701, and the register 701 outputs the set PWM data S704 as S701. The sequence controller 600 sets the selector 702 with the select signal S703, and S701, which is PWM data, is output as S702. The output PWM data S702 is input to the PWM generation unit 602 as described above, and is output as the PWM signal S603. The PWM signal S603 is input to the laser driving unit 603, and the laser 43 is driven by the laser driving signal S706 from the sequence controller 600 (302 in FIG. 3).

  As described above, the driven laser beam scans the photosensitive member via the polygon mirror, the lens, and the mirror. The scanned laser beam forms a latent image on the photosensitive member with an exposure amount corresponding to the set PWM signal. The formed latent image is detected as a potential corresponding to the exposure amount by the potential sensor 100 (303 in FIG. 3). The detected potential is input to a CPU (not shown), A / D converted (304 in FIG. 3), and stored in a RAM (not shown) (305 in FIG. 3). If the data acquisition is not completed, the process returns to 301 in FIG. 3, data different from the previously set PWM data is set in the register 701, and the above sequence is repeated. When data acquisition is completed (306 in FIG. 3), PWM characteristics are obtained from the acquired data. Correction data is created according to the PWM data (307 in FIG. 3).

  For example, FIG. 8 shows a potential graph when the PWM data S702 as input data acquires minimum PWM data, maximum PWM data, and a total of 9 points of PWM data from C1 to C7 in between. In the figure, the horizontal axis and the vertical axis are normalized so that the maximum value is 1. In the case of FIG. 9, as shown in FIG. 9, correction data that is line-symmetric with respect to a straight line connecting (0, 0) to (1, 1) may be generated for the obtained PWM characteristic data (white circles). Show).

  8 and 9, there are 7 points other than the maximum and minimum PWM data, but it is easy to understand that if the number of points is increased, more accurate PWM characteristic data can be obtained and the correction data can be more accurate. I think that the. Then, the sequence controller 600 writes the generated correction data in the conversion table 601 (308 in FIG. 3), and ends the PWM characteristic acquisition mode (309 in FIG. 3).

  If the mode is not the PWM characteristic acquisition mode, the sequence controller 600 outputs a select signal S703 for selecting and outputting the signal S602 from the conversion table 601 to the selector 702. When the image data S601 is input to the conversion table 601, PWM data S702 corresponding to the correction data written in the conversion table 601 by the above-described method is output, and the laser 43 is output according to the corrected PWM data S702. When driven, an image is formed with linear PWM characteristics.

  As described above, by providing the PWM characteristic acquisition mode, it is possible to monitor the PWM light emission amount at that time as the potential on the photosensitive member even when the light quantity is low, and to acquire accurate PWM characteristics. Since correction data corresponding to the acquired PWM characteristic is written in the conversion table and used, correction of the PWM characteristic corresponding to the light emission amount of the laser becomes possible.

[Second Embodiment]
In the present invention, a plurality of correction tables are prepared in advance in the conversion table, and in the PWM characteristic acquisition mode, it is relatively easy to emit light with minimum PWM data, maximum PWM data, and one point of PWM data therebetween. Since it can be determined whether the correction table should be used, the conversion table can be set in a short time. This will be described with reference to FIGS.

  FIG. 10 shows three types of correction tables of 1 mW, 5 mW, and 10 to 20 mW. FIG. 11 is almost the same as the sequence diagram of FIG. 3, but there is no correction data generation of 307 and table rewriting of 308, 1101 correction data search and 1102 table selection.

  In FIG. 10, in the PWM characteristic acquisition mode, PWM light emission is first performed with the minimum PWM data, the maximum PWM data, and the duty data X between them in the order of the sequence of FIG. After the measurement, the data obtained by A / D conversion is normalized. The normalized data will be described along the numbers in the graph.

  First, A / D conversion data Y after measurement of the potential sensor is obtained for the PWM data X ([1]) emitted ([2]). Next, the CPU (not shown) reads the data stored in the conversion table 601 when the data Y is input data ([3]) ([4]). In this case, since there are three correction tables, the values A, B, and C are read in order. Of these read values, the table that is close to X is the optimum correction table (the correction table search 1101 in FIG. 11 indicates this). In FIG. 10, since C is equal to X, the 1 mW correction table can be determined as the optimal correction table, and the CPU (not shown) selects this correction table (1102 in FIG. 11), and ends the PWM characteristic acquisition mode. (309 in FIG. 11).

  By doing so, it took time until the PWM correction table was determined because PWM light emission was made at a large number of points in the first embodiment, but the correction table was determined with a small number of points in the second embodiment. Therefore, the time required for the PWM characteristic acquisition mode can be shortened.

Sectional drawing which shows the structure of the image forming apparatus which concerns on this invention The figure which shows the structure of the exposure control part of FIG. The figure which shows the sequence of a 1st Example The figure which shows the PWM emission characteristic of the laser with respect to input data Diagram showing emission characteristic correction curve of conventional example The figure which shows the laser control block diagram of the conventional example The figure which shows the laser control block diagram of 1st Example The figure which shows the PWM data in 1st Example, and a photoreceptor surface potential The figure which shows the correction | amendment curve in 1st Example The figure which shows the correction curve in 2nd Example The figure which shows the sequence of a 2nd Example.

Explanation of symbols

43 Laser diode 600 Sequence controller 601 Conversion table 602 PWM generation unit 603 Laser drive unit 604 PD sensor (light quantity detection means)
701 register 702 selector

Claims (4)

In an image forming apparatus that scans an image carrier with a laser beam output according to a PWM signal based on input multi-value image data and forms an image on a recording medium.
A lookup table that converts and outputs input multi-value image data to multi-value PWM data that matches the laser emission characteristics;
Generating means for generating a PWM signal according to the multi-level PWM data output from the lookup table;
Driving means for driving the laser in response to a PWM signal from the generating means;
Detecting means for detecting a light emission amount when the laser scans the image carrier;
Conversion means for A / D converting the output of the detection means,
The driving means drives the laser with at least one point of PWM data between the maximum PWM data and the minimum PWM data, detects the light emission amount by the detecting means, and acquires PWM characteristic data converted by the converting means. Set acquisition mode,
An image forming apparatus, wherein image formation is performed using the look-up table corresponding to PWM characteristic data acquired in the acquisition mode.   The image forming apparatus according to claim 1, wherein the detection unit is a detector that detects a surface potential of the image carrier.   The image forming apparatus according to claim 1, wherein the look-up table is rewritten based on PWM characteristic data acquired in the acquisition mode.   The image forming apparatus according to claim 1, wherein the image forming apparatus is selected from a plurality of the lookup tables prepared in advance based on the PWM characteristic data acquired in the acquisition mode.

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