JP5274139B2 - Image forming apparatus - Google Patents

Description

  The present invention relates to an electrophotographic image forming apparatus, and more particularly to an image forming apparatus using an exposure technique for exposing a photosensitive member with a light beam.

[Laser drive by PWM]
Conventionally, an electrophotographic image forming apparatus is known. In this type of image forming apparatus, a laser is turned on / off by a PWM signal that is pulse-width modulated for each pixel based on image data, the photoconductor is exposed by the laser beam, and an electrostatic latent image is formed on the photoconductor. An image is formed. This electrostatic latent image is developed as a visible image by a developer such as toner and transferred to a recording sheet.

  The PWM signal is generated by a pulse width modulation circuit (PWM circuit) as shown in FIG. This PWM circuit outputs a pulse signal having a value corresponding to the value (density value) of the input image data, and outputs the pulse signal to a laser driver for laser driving.

  That is, the PWM circuit includes a D / A conversion circuit 401, a triangular wave generation circuit 402, a comparator 403, and an oscillator 404. The D / A conversion circuit 401 converts, for example, input 8-bit digital image data into analog image data and outputs the analog image data to the comparator 403. The triangular wave generation circuit 402 generates a triangular wave having the same period as the clock signal from the oscillator 404, and outputs this triangular wave as a reference wave to the comparator 403.

  The comparator 403 compares the D / A converted image data (D / A output signal) with a triangular wave as a reference wave, and outputs a PWM signal that becomes H level only when the image data is large to a laser driver (not shown). Output to. For example, when the image data is represented by 00h to FFh, if the magnitude relationship between the image data and the triangular wave is the relationship shown in the upper part of FIG. 11, the comparator 403 has the pulse width shown in the lower part of FIG. A modulation signal (PWM signal) is output.

  That is, in the period related to the pixel 51, since the image data> triangular wave over the entire period, the PWM signal becomes “H” over the entire period of this pixel. In the period related to the pixel denoted by reference numeral 52, since the image data is smaller than the triangular wave over the entire period, the PWM signal is “L” over the entire period of the pixel. Further, in the period related to the pixel of reference numeral 53, the image data <triangular wave in the first and last part of the period and the image data> triangular wave in the central part, the PWM signal is the first and last part of the period of this pixel. The last part is “L” and the middle part is “H”.

  The laser driver turns on (turns on) the laser when the input PWM signal is “H”, and turns off (turns off) the laser during the “L” period.

[Multiple exposure method]
2. Description of the Related Art In an image forming apparatus using a ROS (Raster Output Scanner) exposure apparatus, the use of a multi-beam exposure light source is progressing as a technique corresponding to the high speed and high resolution of the apparatus. With this multi-beam configuration, a plurality of scanning lines can be formed by one scan, so that high resolution and high speed can be achieved without increasing the number of rotations of the polygon mirror.

  However, with multi-beams, streaks occur in the scanning direction in the output image when there is variation in the amount of light of the individual light beams. Further, when there is a variation in angle on the mirror surface of the polygon mirror, a variation occurs in the scanning interval, and a stripe unevenness corresponding to the variation occurs.

  In addition, with a single beam, the scanning interval is narrow and inconspicuous so that it is difficult to cause an image defect, but with a multi-beam, the scanning interval is wide and conspicuous, so that an image defect is likely to occur. Furthermore, when a multi-beam surface emitting laser is used, it is difficult to increase the amount of light compared to a single beam laser.

In order to solve these problems, a multiple exposure method has been proposed in which a plurality of light beams from different light sources are used to expose the same pixel position on the photosensitive member a plurality of times (see Patent Documents 1 and 2).
JP 2002-264391 A JP 2004-109680 A

  By the way, the laser needs to apply a current (threshold current) of a predetermined value or more as a laser emission condition. For this reason, when the pulse width of the PWM signal is narrow, the laser emission may not be obtained depending on the emission characteristics of the laser due to the emission delay. Therefore, conventionally, a bias current that does not exceed the threshold current has been applied to the laser in advance. By applying this bias current, the amount of current required to reach the threshold current due to the switching current (drive current based on the PWM signal) can be reduced, and laser emission can be obtained even if the pulse width of the PWM signal is narrow.

  However, since this threshold current varies depending on the temperature, for example, if the bias current is set to the same level as the threshold current, the laser may emit light depending on the temperature. Therefore, the set value of the bias current has to be lowered to some extent, and even when the bias current is applied, the pulse width of the PWM signal, that is, the “H” period (ON period) in order to emit laser light. Needed to be above a certain level.

  On the other hand, if the pulse width of the PWM signal is too wide in the period related to one pixel, the laser is in the entire period of one pixel even though the laser off period exists in the pixel period due to the turn-off delay. It will be on for a long time. This is because the “L” period of the PWM signal within one pixel period is short and the laser driver cannot respond completely.

  That is, when the laser is driven by the PWM signal, a light emission delay and a light-off delay occur. This problem will be described with reference to FIG. FIG. 12 is a diagram illustrating the relationship between the value of conventional image data and the amount of laser light emission. Note that the value of the image data indicates a density value, and is proportional to the “H” duty of the PWM signal, that is, the value of the PWM signal during the current supply time.

  In FIG. 12, there is no laser emission during the current supply time when the value of the image data is in the range of “00h to 30h”, and the output light quantity of the laser is constant. In the current supply time when the value of the image data is in the range of “30h to B0h”, the relationship between the value of the image data and the output light amount of the laser is linear.

  In addition, in the current supply time in the case where the value of the image data is in the range of “B0h to D0h”, the linearity between the value of the image data and the output light amount of the laser is not impaired, but the slope increases rapidly, It is difficult to strictly control the amount of laser output. Further, in the current supply time when the value of the image data is in the range of “D0h to FFh”, the output light amount of the laser is saturated at the maximum light amount.

  For this reason, in the conventional image forming apparatus, the relationship between the value of the image data “00h to FFh” and the output light amount of the laser is linear as shown in the range “30h to B0h” in FIG. Thus, the triangular wave as the reference wave and the D / A output signal are corrected.

  That is, by giving an offset component to the triangular wave, the pulse width of the actual PWM signal when the value of the image data is “00h”, and the pulse when the value of the D / A output signal of FIG. 11 is “30h” Set to correspond to the width. On the other hand, by adjusting the value of the D / A output signal with respect to the value of the image data, the pulse width of the actual PWM signal when the value of the image data is “FFh” is set to the value of the D / A output signal of FIG. Is set to correspond to the pulse width when “B0h” is set. Thereby, the relationship between the value of the image data and the amount of laser output light is kept linear.

  For example, when outputting a line image or the like in which the value of image data continuous in the main scanning direction is “FFh”, the laser emits intermittent light emission corresponding to the pulse width “B0h” in each pixel period, and a non-lighting period occurs. To do. As shown in FIG. 13, this non-lighting period occurs in the first and last period of one pixel period, that is, the period following the adjacent pixel. For this reason, there is a problem that the line is thinned or cut off, and the image quality of the line image or the character image is deteriorated.

  The present invention has been made under such a background, and an object of the present invention is an image that can be subjected to multiple exposure with a plurality of laser beams driven by a PWM signal without degrading the image quality of a line image or a character image. It is to provide a forming apparatus.

To solve the above problems, the present invention emits an image forming apparatus that forms an electrostatic latent image by the multiple exposure of the same pixel position on sensitive optical body based on the image data, a plurality of light beam light source when the deflection unit in which the plurality of light beams emitted from the light source to deflect the plurality of light beams to scan over the photosensitive member, in the case of forming the character image, to each pixel on the photosensitive body The light beam from the light source so that the exposure area of the first exposure and the exposure area of the second exposure performed on the basis of the corresponding image data are relatively displaced in the scanning direction of the light beam that scans on the photoconductor. When a gradation image is formed by controlling the emission timing of the first exposure, the exposure area of the first exposure and the exposure area of the second exposure performed based on the image data corresponding to each pixel on the photoconductor Characterized by further comprising a control means for controlling the emission timing of the light beam from the light source to be the same position in the direction.

  According to the present invention, multiple exposure can be performed with the light of a plurality of lasers driven by a PWM signal without degrading the image quality of a line image or a character image.

  The best mode for carrying out the invention will be described below with reference to the drawings.

  FIG. 1 is a diagram showing a schematic configuration of an image forming apparatus according to an embodiment of the present invention.

  The image forming apparatus 100 of FIG. 1 includes an exposure device 20, a developing device 21, a charger 22, a transfer roller 23, a fixing device 24, and a photosensitive drum 25.

  The photosensitive drum 25 has a structure in which a photosensitive layer is laminated on a cylindrical metal conductive substrate, and rotates in the direction of an arrow. The charger 22 is a corona charger and uniformly charges the photosensitive drum 25 to a desired potential by a voltage applied by a bias power source. For example, the bias power supply uniformly charges the surface of the photosensitive drum 25 at about −800 V (dark potential: Vd) at a position facing the photosensitive drum 25.

  The exposure apparatus 20 applies pulse width modulation to the photosensitive drum 25 charged at the dark potential by the charger 22 by a plurality of pulse width modulation circuits (PWM circuits) 400 (FIG. 4), which will be described later, based on image data. The exposed laser beam (light beam) is exposed and scanned. By this exposure scanning, an electrostatic latent image reflecting the image data is formed on the surface of the photosensitive drum 25. The potential of the laser beam (bright potential: VL) is about −200V.

  Thus, by irradiating the laser beam (about -200V) whose absolute value is lower than the dark potential (about -800V), the potential of the electrostatic latent image formed on the irradiated portion (exposed portion). Is lower than the dark potential (about −800 V).

  The electrostatic latent image formed on the surface of the photosensitive drum 25 is developed by the developer 21 with a developer such as toner. The developing device 21 has a developing sleeve for charging the toner. A developing bias (for example, DC = −500 V and AC) is applied to the developing sleeve by a developing bias power source.

  The transfer roller 23 transfers the toner image formed on the photosensitive drum 25 to a recording medium P such as recording paper. The transfer roller 23 is composed of a core metal to which a bias power source is applied by a bias power source, and a medium resistance elastic layer formed on the surface layer thereof.

  After the transfer, the recording medium P is sent to the fixing device 24 via the transport belt. The fixing device 24 includes a fixing roller and a pressure roller, and fixes the toner image on the recording medium P by pressing and heating.

  FIG. 2 is a view showing a schematic configuration of the exposure apparatus 20 included in the image forming apparatus 100 of FIG.

  The exposure apparatus 20 shown in FIG. 2 schematically includes a surface emitting laser 31 as a multi-beam laser light source, a collimator lens 35, an aperture (optical aperture) 32, a polygon mirror 33, and an fθ lens group 34. To do. Thus, the image forming apparatus 100 including the exposure device 20 is configured to scan the photosensitive drum 25. The exposure apparatus 20 includes a photodetector (BD sensor) 36 and a laser control unit 41. The laser control unit 41 controls light emission (emission) of the surface emitting laser 31 based on a signal from the image processing unit 40 included in the image forming apparatus 100.

  The surface emitting laser (VCSEL) 31 has a plurality of lasers (L1 to L8) arranged two-dimensionally as will be described later with reference to FIG. 3, and the plurality of laser light sources are individually controlled to emit light based on image data. . The collimator lens 35 (beam shaping means) converts a plurality of laser beams emitted from the surface emitting laser 31 into parallel laser beams. The aperture 32 adjusts the cross-sectional shape (spot shape and spot diameter) of the laser beam from the collimator lens 35.

  The polygon mirror 33 is rotated at a constant speed (constant angular speed) in the direction of the arrow 33a by a drive motor (not shown). The fθ lens group 34 corrects distortion so as to guarantee temporal linearity of scanning on the photosensitive drum 25.

  Next, the optical path will be described in order. The laser beam emitted from the surface emitting laser 31 is converted into substantially parallel light by the collimator lens 35 and the aperture 32 and then enters the polygon mirror 33 with a predetermined beam diameter. The polygon mirror 33 is rotated at a constant angular velocity in the direction indicated by the arrow 33a in the figure. As the polygon mirror 33 rotates, the incident laser beam is continuously converted into a deflected beam whose angle is changed and reflected. Is done. The deflected beam is focused by the fθ lens group 34. Further, the fθ lens group 34 corrects the distortion aberration so as to guarantee the temporal linearity of scanning on the photosensitive drum 25 simultaneously with respect to the laser beam. The laser beam thus corrected is combined and scanned on the photosensitive drum 25 in the direction of the arrow 37 at a constant speed.

  The laser beam passing through the fθ lens group 34 from the polygon mirror 33 is received by a photodetector (BD sensor) 36.

  The photodetector 36 generates a horizontal synchronization signal (BD signal) that serves as a writing reference in the main scanning direction that is the longitudinal direction (axial direction) of the photosensitive drum 25 at the timing when the laser beam is incident. The horizontal synchronization signal is used as a horizontal synchronization signal for synchronizing the rotation of the polygon mirror 33 and the writing of image data to the photosensitive drum 25. The image signal is output from the image processing unit 40 to the laser control unit 41 with reference to this synchronization signal.

  The laser control unit 41 further drives the current value and drive of the drive (light emission) signal of the surface emitting laser 31 based on the image signal input from the image processing unit 40 in the image section where the latent image is formed on the photosensitive drum 25. Control the time. The laser beam emitted from the surface emitting laser 31 is converted into substantially parallel light by the collimator lens 35 and the aperture (optical aperture) 32 and then enters the polygon mirror 33 with a predetermined beam diameter.

  In addition, a PD (not shown) is disposed as a light receiving element in a peripheral end region of the aperture (optical diaphragm) 32. The PD detection signal is used for light emission amount control of the surface emitting laser 31, that is, auto power control (APC) control for determining the current value of the drive (light emission) signal.

  FIG. 3 is a view showing a light emitting surface of the surface emitting laser 31 in the exposure apparatus 20 shown in FIG. On the light emitting surface of the surface emitting laser 31, eight lasers L1 to L8 as light sources are two-dimensionally arranged in a lattice shape. The lasers L1 and L5 are arranged at the same position in the sub-scanning direction, that is, on the same main scanning line. Similarly, the lasers L2 and L6, the lasers L3 and L7, and the lasers L4 and L8 are also arranged at the same position (on the same main scanning line) in the sub-scanning direction.

  The lasers L1 to L4 are arranged so that the interval in the sub-scanning direction is one pixel. Similarly, the lasers L5 to L8 are arranged so that the interval in the sub-scanning direction is one pixel. Further, the lasers L1 to L4 are arranged so that the interval in the main scanning direction is about 2/3 pixels. Similarly, the lasers L5 to L8 are arranged so that the interval in the main scanning direction is about 2/3 pixels. Further, lasers L1 and L5, lasers L2 and L6, lasers L3 and L7, and lasers L4 and L8, which will be described later, are arranged apart from each other by about 3.3 pixels.

  Further, the specifications and performance of the eight lasers L1 to L8 are the same standard, and for example, the oscillation wavelength is unified to about 700 to 800 nm.

  The eight lasers L1 to L8 individually have laser drive circuits LD1 to LD8 included in the laser driver 16, respectively. Under the control of the laser control unit 41, these laser drive circuits LD1 to LD8 can turn on / off (select) the corresponding lasers L1 to L8 and switch their emission intensities. The laser control unit 41 controls the laser emission by the surface emitting laser 31 based on the image data (rasterization signal) from the image processing unit 40. The image processing unit 40 performs various correction processes on the image data, and rasterizes the image data as vector information to convert it into bitmap information.

  The laser control unit 41 drives and controls the eight lasers L1 to L8 of the surface emitting laser 31 based on the image data of one pixel rasterized by the image processing unit 40. In other words, the laser control unit 41 uses the light emission patterns of the lasers L1 to L8 based on rasterized image data of one pixel (starting point of scanning within one pixel) as image attribute data (color, character / non-character). Etc.), based on information such as density data.

  In the following description, when the lasers L1 to L8 are generally described without being distinguished, they are referred to as lasers L. The PWM signals for driving the lasers L1 to L8 are also referred to as PWM signals S1 to S8 when distinguished from the lasers L1 to L8, and are generally described without distinction. Called S.

  The exposure apparatus 20 drives and controls the surface emitting laser 31 so that the lasers L1 and L5 (first light beam and second light beam) perform multiple exposure of the same pixel. Similarly, the exposure apparatus 20 drives and controls the surface emitting laser 31 so that the lasers L2 and L6, the lasers L3 and L7, and the lasers L4 and L8 respectively perform multiple exposure on the same pixel. The “same pixel” is a term indicating the same data component in the image data and the same position component on the photoconductor (photosensitive drum 25). The image resolution in this embodiment is 600 dpi for both main scanning and sub-scanning.

  Lasers L1 to L8 of the surface emitting laser 31 are individually driven by the PWM circuit 400 shown in FIG. That is, in the present embodiment, eight PWM circuits 400 are provided corresponding to the eight lasers L1 to L8. In other words, a plurality of light beams related to multiple exposure are individually generated from a plurality of light sources.

  The image data 1 as density data and the image identification data 7 as image attribute data are input from the image processing unit 40 to the PWM circuit 400 in FIG. Image data 1 is 8-bit data and has 16 levels of density. The image data 1 has been subjected to predetermined image processing such as gamma correction.

  The image data 1 is converted into an image data conversion signal 4 by the conversion table 3 of the image data conversion unit 2 and input to the D / A converter 5. The image data conversion signal 4 functions as a signal for determining the pulse width of the PWM signal S (period in which the value of the PWM signal S is “H” described later). The D / A converter 5 converts the digital image data conversion signal 4 into an analog signal. Thereafter, a D / A output signal 6 a obtained by adjusting the value of the analog image data conversion signal 6 by the volume circuit 13 is output to the comparator 12.

  The image identification data 7 is 2-bit data, and the image data 1 currently input to the PWM circuit 400 is a binary image (line image, for example, a character image), a gradation image (for example, a non-character image). Which type of image data is indicated. The image identification data 7 is used as a signal for selecting one of three reference waves (middle) 9M, a reference wave (left) 9L, and a reference wave (right) 9R, which will be described later. That is, in the image identification data 7, the reference wave (middle) 9M is center growth, the reference wave (left) 9L is left growth, and the reference wave (right) 9R is right growth. Used as a signal for selection.

  That is, the image identification data 7 is input to the selector 10 of the reference wave supply unit 8. The reference wave supply unit 8 includes generation circuits 10M, 10L, and 10R that generate three reference waves (middle) 9M, a reference wave (left) 9L, and a reference wave (right) 9R. The reference wave (middle) 9M, the reference wave (left) 9L, and the reference wave (right) 9R are input to the selector 10. The selector 10 selects one of the three reference waves (middle) 9M, the reference wave (left) 9L, and the reference wave (right) 9R based on the image identification data 7, and the selected reference wave Is output to the comparator 12.

  The comparator 12 compares the D / A output signal 6 a output from the volume circuit 13 with the reference wave 11 related to the selection output from the selector 10. The comparator 12 outputs to the laser driver 16 a PWM signal S that is “H” while the D / A output signal 6 a is larger than the reference wave 11 and is “L” during other periods.

  The laser driver 16 drives the corresponding laser L based on the input PWM signal S. That is, the laser driver 16 drives and controls the corresponding laser L so that the PWM signal S is turned on (emits light) during the “H” period and is turned off (extinguishes) during the “L” period. 4 shows all the PWM signals S1,..., S8 and all the lasers L1,..., L8 for convenience, but as described above, eight PWMs corresponding to each of the lasers L1 to L8. Circuit 400 actually exists. Also in the following description, these eight PWM circuits 400 are referred to as PWM circuits 400 without being associated with the lasers L1 to L8.

  The offset generation circuit 14 is a circuit that gives an offset component to the selected reference wave 11, and the offset amount can be adjusted by the volume circuit 15. In the present embodiment, this offset adjustment enables adjustment of the pulse width of the PWM signal when the value of the image data is “00h”, that is, the minimum pulse width of the PWM signal S (00h adjustment).

  Further, by inputting the D / A output signal 6 a obtained by adjusting the output of the D / A conversion circuit 5 by the volume circuit 13 to the comparator 12, the width of the PWM signal when the value of the image data is “FFh”, that is, PWM The maximum pulse width of the signal S can be adjusted (FFh adjustment).

  By the 00h adjustment and the FFh adjustment described above, the width of the PWM signal changes in a region where the relationship between the image data and the laser light amount is linear, in the region of “30h” to “B0h” in the example of FIG.

  A timing signal 17 is generated based on a signal from the PD sensor 36 (FIG. 2), and the timing signal 17 is input to the PWM circuit 400. Based on the timing signal 17, the PWM circuit 400 drives and controls the corresponding laser L so that an electrostatic latent image is formed at a desired position corresponding to the image data on the photosensitive drum 25.

  That is, the lasers L1 and L5, the lasers L2 and L6, the lasers L3 and L7, and the lasers L4 and L8 that perform multiple exposure are arranged apart from each other by about 3.3 pixels as described above. On the other hand, the light beams from the lasers L1 and L5 are applied to the photosensitive drum 25 as a deflected beam by the rotation of the polygon mirror 33. Accordingly, the PWM circuit 400 related to the lasers L1 and L5 is supplied with the image data 1, the timing signal 17, a reference wave described later, and the like, with the timing shifted by about 3.3 pixels. As a result, the same point (same pixel position) on the photosensitive drum 25 can be subjected to multiple exposure with the light beam relating to the same pixel of the same image data from the lasers L1, L5.

  However, in the time charts of FIGS. 5, 6, 7, and 8 to be described later, the time axes of the lasers L1 and L5 are aligned with reference to the same pixel for convenience of explanation. The same applies when multiple exposure is performed by the lasers L2 and L6, the lasers L3 and L7, and the lasers L4 and L8.

  In the present embodiment, when a binary image is formed, as shown in FIG. 5, the two lasers L that perform multiple exposure generate the PWM signal S with reference to different reference waves. That is, the two lasers L are controlled so as to have different scanning start points. FIG. 5 shows waveforms of a reference wave and a pulse width modulation signal (PWM signal) when a 600 dpi line image (binary image) is formed in the main scanning direction by the lasers L1 and L5, and the obtained line image. The shape is shown. Here, a case where multiple exposure is performed using the lasers L1 and L5 will be described. However, multiple exposure may be performed using two lasers arranged on the same main scanning line on the light emitting surface of the surface emitting laser 31. For example, it is not limited to this. For example, multiple exposure may be performed by lasers L2 and L6, lasers L3 and L7, and lasers L4 and L8.

  The selector 10 of the PWM circuit 400 corresponding to the laser L1 selects the reference wave (right) 9R when data indicating a line image is input as the image identification data 7. When a line image is formed, “FFh” is input as image data 1 to the PWM circuit 400 corresponding to the laser L1 for all pixels. However, as described above, the D / A output signal 6 a is adjusted by the volume circuit 13. For this reason, as shown in FIG. 5, a value smaller than the maximum potential of the reference wave (right) 9R is set as the potential of the D / A output signal 6a corresponding to the value “FFh” of the image data 1. .

  For this reason, even if the value of the image data 1 is “FFh”, the laser L is not turned on for 100% of the period of one pixel. For example, the PWM signal S1 output from the comparator 12 of the PWM circuit 400 corresponding to the laser L1 indicates that the laser L1 is lit for 80% of the period of one pixel as shown in FIG. . That is, a PWM signal having a width of 80% of one pixel is output by right growth starting from the right end of the pixel.

  The laser L5 performs multiple exposure on the pixels exposed by the laser L1. When the data indicating the line image is input as the image identification data 7, the selector 10 of the PWM circuit 400 corresponding to the laser L 5 selects the reference wave (left) 9 L. As the value of the image data 1, “FFh” is input to the PWM circuit 400 corresponding to the laser L5 as in the PWM circuit 400 corresponding to the laser L1. The PWM signal S5 output from the PWM circuit 400 corresponding to the laser L5 is as shown in FIG. That is, as described above, the D / A output signal 6 a is adjusted by the volume circuit 13. For this reason, as shown in FIG. 5, a value smaller than the maximum potential of the reference wave (left) 9L is set as the potential of the D / A output signal 6a corresponding to the value “FFh” of the image data 1. . For this reason, even if the value of the image data 1 is “FFh”, the laser L is not turned on for 100% of the period of one pixel. That is, the PWM signal S1 output from the comparator 12 of the PWM circuit 400 corresponding to the laser L5 indicates that the laser L5 is lit for 80% of the period of one pixel as shown in FIG. . That is, a PWM signal having a width of 80% of one pixel is output with left growth starting from the left end of the pixel.

  Here, in the PWM circuit 400 related to the lasers L1 and L5, a sawtooth waveform having a symmetrical relationship with each other is used as the reference wave. In other words, the PWM circuit 400 related to the lasers L1 and L5 uses the reference waves in the state where the phases are mutually inverted. Accordingly, the PWM circuit 400 related to the lasers L1 and L5 outputs the PWM signals S1 and S5 in which the “L” portion (light-out period) appears in the form in which the phases are inverted in the period of each pixel. Specifically, in the PWM signal S1, the first part of the period (area) of each pixel is a light-off period, and the PWM signal S5 is the last part of the period (area) of each pixel. Become. That is, the lasers L1 and L5 are controlled so as to have different scanning start points.

  Therefore, when a line image is formed by multiple exposure of the same pixel with the lasers L1 and L5, focusing on one pixel that is subjected to multiple exposure, the exposure position of each of the lasers L1 and L5 is shifted. The laser light is never turned off (no exposure: extinguished). That is, at least one of the lasers L1 and L5 is turned on (exposure: lighting) over the entire period of one pixel. For this reason, in the formed image, although the line width is slightly narrowed between the pixels formed by the laser L1 and the pixels formed by the laser L5, the line is not interrupted in the multiple exposure pixels. There is no continuous line. In other words, it is possible to perform multiple exposure with a plurality of laser beams related to the PWM signal S without degrading the image quality of the line image or character image.

  FIG. 6 is a diagram showing various signal waveforms and the shape of an output image in the case of a binary image in the comparative example with respect to FIG. That is, it is assumed that both the PWM circuits 400 related to the lasers L1 and L5 that perform multiple exposure use the reference wave (right) 9R. In this case, in the PWM circuit 400 related to the lasers L1 and L5 that perform multiple exposure, the reference wave (right) 9R having the same waveform is used, and these reference waves (right) 9R are comparators as waveform data of synchronized pixel positions. 12 is supplied.

  Accordingly, the lasers L1 and L5 that perform multiple exposure are driven and controlled by the PWM signals S1 and S5 having the same waveform and the same phase, and focusing on the pixels that are subjected to multiple exposure, the exposure positions of the lasers L1 and L5 are the same. In the pixel to be exposed, an unexposed portion is generated in the period of one pixel. For this reason, even in the formed image, as shown in FIG. 6, a binary image (a line image or a character image) having a discontinuity and poor image quality is obtained.

  On the other hand, when forming a gradation image, multiple exposure is performed on the same position in the same pixel by lasers L1 and L5 as shown in FIG. FIG. 7 shows a case where the potential of the D / A output signal 6a corresponds to the value “30h” of the image data 1.

  In FIG. 7, the selector 10 of the PWM circuit 400 related to the lasers L <b> 1 and L <b> 5 performing multiple exposure selects the same reference wave based on the image identification data 7 indicating a gradation image (for example, a non-character image). Here, both reference waves (right) 9R are selected. These reference waves (right) 9R are synchronized for each pixel. Therefore, the PWM signals S1 and S5 output from the PWM circuit 400 related to the lasers L1 and L5 are exactly the same in pulse waveform and phase. As a result, the same position in the same pixel is subjected to multiple exposure by the lasers L1 and L5, and the formed image faithfully reflects the density indicated by the original image data. That is, the lasers L1 and L5 are controlled so as to have the same scanning start point.

  This is because in a gradation image, the area stability as a dot expressing density is more important than the shape of the formed image. Further, in order to stabilize the dots against potential unevenness and other disturbances in the electrophotographic system, it is preferable to take a deep potential of the electrostatic latent image. Therefore, in the case of a gradation image, it is better to perform multiple exposure at the same position as described above.

  Further, in order to maintain continuity of image density with respect to image data, it is preferable to perform multiple exposure at the same position. This is due to the characteristic of electrophotography that the density varies depending on the size of the area to be exposed even if the total amount of light to be exposed per pixel is the same. Further, if the irradiation position of multiple exposure is changed with a certain gradation value with respect to the gradation of density, the continuity of density change may be interrupted. From this point, it is better to perform multiple exposure at the same position in the gradation image.

  On the other hand, when a binary image such as a character image or a line image is formed, since the image is normally formed with the maximum light quantity, the density is stable, and shape accuracy and resolution are more important than density. Therefore, in this embodiment, when forming a binary image, the accuracy of the shape is emphasized, and lines are continuously formed. That is, in the present embodiment, a good image without interruption is formed in a character image or a line image while realizing the linearity of the laser light quantity with respect to the image data.

  When this embodiment is applied, it is desirable to set the maximum light amounts of the lasers L1 to L8 so that a line image and a character image can be formed not only by multiple exposure but also by single exposure. This is because, when a line image and a character image cannot be formed by a single exposure, the image quality may be lowered due to the density.

  It should be noted that the reference wave can be switched for each pixel of the image data. For example, as shown in FIG. 8, it is also possible to form an image by appropriately changing the combination of various reference waves for each pixel. That is, in the above description, the reference wave (middle) 9M, the reference wave (left) 9L, and the reference wave (right) 9R are selected in units of one line in the main scanning direction according to the image identification data 7. However, it is not limited to such image formation. For example, when a character part and a non-character part, that is, a binary image and a gradation image are provided side by side in the main scanning direction, a reference wave (medium) 9M according to the image identification data 7 in the middle of one line, reference The wave (left) 9L and the reference wave (right) 9R may be switched. Further, the reference wave (middle) 9M, the reference wave (left) 9L, and the reference wave (right) 9R may be selected in units of pixels. In FIG. 8, the reference wave (right) 9R is selected for the first and second pixels, the reference wave (middle) 9M is selected for the third pixel, the reference wave (left) 9L is selected for the fourth pixel, and the five pixels This is an example when the reference wave (right) 9R is selected by the eyes. In other words, according to the image identification data 7, it may be controlled so as to have different scanning start points for each adjacent pixel, or controlled so that each laser L at the time of multiple exposure has different scanning start points. Also good.

  The present invention is not limited to the above embodiment, and for example, a surface emitting laser 31 configured as shown in FIG. 9 can be used.

  In the surface emitting laser 31 shown in FIG. 3, two lasers L1 and L5, lasers L2 and L6, lasers L3 and L7, and lasers L4 and L4 that are formed on the same main scanning line and are not separated in the sub-scanning direction. Multiple exposures were performed by L8. On the other hand, in the surface emitting laser 31 shown in FIG. 9, two lasers L1 and L5, lasers L2 and L6, lasers L3 and L7, which are arranged on different main scanning lines and separated in the sub-scanning direction, and Multiple exposure is performed by lasers L4 and L8, respectively.

  That is, in the surface emitting laser 31 shown in FIG. 9, the lasers L1 to L8 are arranged at intervals of 1 pixel in the sub-scanning direction and at intervals of 2/3 pixels in the main scanning direction. When exposure scanning is performed on the image carrier, the exposure scanning by the lasers L1 to L4 is performed so as to overlap the main scanning line that has been exposed and scanned by the lasers L5 to L8 in the previous exposure scanning.

  In other words, the laser signals L1 and L5, the lasers L2 and L6, the lasers L3 and L7, and the lasers L4 and L8 are PWM signals based on image data related to the same main scanning line in the previous exposure scan and the current exposure scan, respectively. Is supplied, and the same pixel is subjected to multiple exposure. This exposure scanning method is disadvantageous in terms of image forming speed because it is half the speed in the case of this embodiment, but has the advantage that streak images due to variations in scanning intervals are not noticeable. The reference wave selection method and the like are the same as in the case of the surface emitting laser 31 in FIG.

  Further, the PWM circuit 400 may be a digital PWM circuit that is driven at a clock frequency that is sufficiently higher than one pixel scanning time, for example, instead of a circuit that uses a reference wave as shown in FIG. Thereby, it is also possible to form a PWM signal with an arbitrary phase so that the non-exposed portions of the light beam related to multiple exposure do not overlap.

  In the above embodiment, when a binary image is formed, the light emission timing in each pixel is made different between the light beams related to multiple exposure, and in the case of forming a gradation image, the light emission timing in each pixel. Were the same. In contrast, in an arbitrary image in which the value of the image data is in a certain range, such as a line image with a low density, and the shape is prioritized over the stability of the density, the light emission timing in each pixel is set between the light beams related to multiple exposure. You may make it differ.

  Furthermore, even when multiple exposures are performed on the same pixel position using three or more light beams, the above-described embodiment can be applied to eliminate a non-exposed pixel region when forming a binary image or the like. It is.

1 is a diagram illustrating a schematic configuration of an image forming apparatus according to an embodiment of the present invention. FIG. 2 is a view showing a schematic configuration of an exposure apparatus in the image forming apparatus of FIG. It is a figure which shows the light emission surface of the surface emitting laser in the exposure apparatus shown in FIG. It is a block diagram which shows the structure of the PWM circuit in an exposure apparatus. It is a figure which shows the shape of the various signal waveforms and output image in the case of the binary image in embodiment of this invention. It is a figure which shows the shape of the various signal waveforms in the case of the binary image in the comparative example with respect to FIG. 5, and the output image. It is a figure which shows the shape of the various signal waveforms and output image in the case of the gradation image in embodiment of this invention. It is a figure which shows the shape of the various signal waveform and output image at the time of switching the reference waveform in embodiment of this invention for every pixel of image data. It is a figure which shows the modification of the light emission surface of the surface emitting laser of FIG. It is a block diagram which shows the structure of the conventional PWM circuit. It is a figure showing the relationship between the reference waveform in a conventional PWM circuit, a D / A output signal, a PWM signal waveform, and the shape of an output image. It is a figure which shows the relationship between the value of the conventional image data, and a laser emission light quantity. It is a figure which shows the various signal waveforms and output image in the case of the binary image in the past.

Explanation of symbols

8 Reference Wave Supply Unit 10 Selector 12 Comparator 20 Exposure Device 25 Photosensitive Drum 31 Surface Emitting Lasers L1 to L8 Laser

Claims (2)

In an image forming apparatus for forming an electrostatic latent image by multiple exposure of the same pixel position on a photoconductor based on image data,
A light source that emits a plurality of light beams;
A deflection means for the plurality of light beams emitted from the light source to deflect the plurality of light beams to scan over the photosensitive member,
When a character image is formed, a light beam is scanned on the photoconductor by the exposure area of the first exposure and the exposure area of the second exposure performed on the photoconductor based on image data corresponding to each pixel. When the gradation timing image is formed by controlling the emission timing of the light beam from the light source so as to be relatively shifted in the direction, the first exposure is performed based on image data corresponding to each pixel on the photoreceptor. An image forming apparatus comprising: a control unit configured to control the emission timing of the light beam from the light source so that the exposure area of the second exposure area and the exposure area of the second exposure are in the same position in the scanning direction.   The image forming apparatus according to claim 1, wherein the plurality of light sources are arranged at different positions in a sub-scanning direction that is a rotation direction of the photoconductor.

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