JP3673665B2 - Image forming apparatus - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
  The present invention relates to an image forming apparatus that forms an image by exposing a photosensitive member to a photoconductor in accordance with an image signal synchronized with a video clock adjusted for each of a plurality of scanning units.
[0002]
[Prior art]
Conventional image forming apparatuses such as electrophotographic color printers and color copiers have a plurality of image forming units for outputting color images at high speed, and are formed on the photosensitive drums of the image forming units. Various methods for sequentially transferring toner images of different colors onto a recording material held on a conveying belt have been proposed.
[0003]
In such a conventional image forming apparatus having a plurality of image forming units, due to machine accuracy or the like, the photosensitive drums or the conveyance belts of the plurality of image forming units have uneven rotation or movement, and the transfer positions of the image forming units. The amount of movement between the outer peripheral surface of the photosensitive drum and the transport belt in the image forming apparatus is different for each image forming unit, and the toner images of each color formed on the photosensitive drum of each image forming unit are transferred to a recording material. Sometimes, the toner images of the respective colors do not match and color misregistration (positional misregistration) occurs.
[0004]
In particular, in an image forming apparatus having a plurality of image forming units having a laser scanner and a photosensitive drum, there is an error in the distance between the laser scanner and the photosensitive drum in the image forming unit, and if this error differs between the image forming units, A difference occurs in the scanning width of the laser on the photosensitive drum of each image forming unit, and a color shift (position shift) occurs.
[0005]
In order to reduce this color misregistration (positional misregistration), as shown in FIG. 9 to be described later, a video clock generator for image signals is provided for each color, and the frequency of the video clock can be varied independently for each color. An image forming apparatus that corrects the laser scanning width by adjusting the frequency has been proposed (for example, disclosed in Japanese Patent Publication No. 6-57040).
[0006]
FIG. 9 is a block diagram illustrating a configuration of a video clock generator in a conventional image forming apparatus. The video clock generator is composed of a PLL (Phase Locked Loop) circuit.
[0007]
In the figure, 101 is a crystal oscillator, 102 is a 1 / M frequency divider, 103 is a phase comparator, 104 is a low pass filter, 105 is a voltage controlled oscillator (VCO), 106 is a 1 / N frequency divider, and 14 is an output signal. And output from the crystal oscillator 101. Reference numeral 15 denotes a video clock, which is an output of the video clock generator and is a video clock whose frequency is variable.
[0008]
Hereinafter, a method for changing the video clock frequency will be described. A signal obtained by dividing the output signal 14 of the crystal oscillator 101 by M by the 1 / M divider 102 and a signal obtained by dividing the video clock 15 by N by the 1 / N divider 106 are input to the phase comparator 103. The output of the phase comparator 103 is input to the voltage controlled oscillator 105 through the low pass filter 104.
[0009]
For example, when the phase of the signal obtained by dividing the output signal 14 of the crystal oscillator 101 by M is ahead of the phase of the signal obtained by dividing the video clock 15 by N, the input voltage of the voltage controlled oscillator 105 rises and the phase of the video clock is increased. To proceed. When the frequency of the crystal oscillator 101 is fin and the frequency of the video clock 15 is fout,
[0010]
[Expression 2]
fout = fin × N / M (1)
It becomes. By adjusting the value of N / M according to the detected main scanning width, the frequency of the video clock 15 output from the video clock generator is variable.
[0011]
For example, according to the above Japanese Patent Publication No. 6-57040, it is written that the counter of the frequency divider needs 10 bits when it is desired to change the video clock by 0.1%. It is also written that the variable minimum frequency of the video clock is determined by the division ratio. That is, for example, by setting N / M = 1000/1000 → N / M = 999/1000, a variable step of −0.1% is obtained.
[0012]
According to this idea, when the video clock is to be changed by 0.001%, the frequency division ratio may be set to about 100,000. For example, when “main scanning width of reference color = 300,000 μm (300 mm)”, “main scanning width of adjustment color = 30,0003 μm”, and “N / M of adjustment color = 1”, N / M of the adjustment color is set to “N / M M = 0.99999 (corresponding to −0.001%), for example, N = 99999, M = 100000 ”.
[0013]
Further, when “main scanning width of reference color = 300000 μm (300 mm)”, “main scanning width of adjustment color = 299997 μm”, and “N / M of adjustment color = 1”, N / M of the adjustment color is set to N /M=1.0001 (corresponding to + 0.001%), for example, N = 100001 and M = 100000.
[0014]
[Problems to be solved by the invention]
However, as N increases, the feedback amount of the PLL loop decreases. As in the above-described example, when N is N≈100,000 and fout is fout≈20 MHz, feedback to the phase comparator 103 is performed only once every 5 msec.
[0015]
At this time, if the jitter of the VCO 105 itself is large, jitter other than when feedback is applied deteriorates.
[0016]
FIG. 10 is a diagram for explaining the deterioration of the jitter of the video clock 15 when the feedback amount is small.
[0017]
In the figure, the vertical axis represents the jitter of the video clock 15, and the horizontal axis represents time (msec).
[0018]
Since feedback to the phase comparator 103 is performed every 5 msec, the jitter of the video clock 15 is improved only every 5 msec. When the jitter is not improved, the jitter of the VCO 105 itself appears. As described above, in the PLL circuit, the jitter can be reduced to the same level as that of the reference clock. However, when N≈100,000 as described above, the feedback amount becomes small, so that the jitter is smaller than that of the reference clock. growing.
[0019]
As described above, the conventional image forming apparatus has the following drawbacks.
[0020]
To reduce the main scanning magnification variable step amount in order to increase the correction accuracy of the main scanning magnification, it is necessary to increase M and N of the frequency dividers 102 and 106 shown in FIG. However, when N is set to a large value, the feedback amount of the loop of the PLL is reduced, and as a result, there is a problem that the jitter of the VCO 105 itself appears as jitter in the video clock 15. Furthermore, there is a problem that the jitter of the video clock 15 appears in the form of positional deviation or color deviation in the formed image.
[0021]
In order to reduce the jitter of the video clock 15, a VCO having a small jitter may be used for the PLL circuit. However, in this case, there is a problem that the cost becomes high.
[0022]
  The present invention has been made to solve the above problems, and the object of the present invention is to reduce the number of pitches smaller than the number of pitches required when the frequency of the video clock is varied in units of a predetermined frequency variable pitch. By setting the frequency dividing ratio of the first frequency dividing means and the second frequency dividing means, the video clock jitter is deteriorated even when the frequency of the video clock is varied in smaller pitch units. The present invention provides an image forming apparatus capable of adjusting a video clock with high accuracy and obtaining a high-quality image with accurate color correction with an inexpensive configuration.
[0023]
[Means for Solving the Problems]
  According to a first aspect of the present invention, an image forming unit including a plurality of scanning units, one or more first clock generating units that generate a reference clock, and a frequency of the reference clock are adjusted for each of the scanning units. A plurality of second clock generating means for generating a video clock, and each scanning section exposes the photosensitive member in accordance with an image signal synchronized with the video clock generated by the second generating means. An image forming apparatus for forming an image, wherein each of the second clock generators includes a first divider for dividing the reference clock, a second divider for dividing the video clock, and the first Phase comparison means for comparing the phases of the clock obtained by dividing the reference clock by the frequency dividing means and the clock obtained by dividing the video clock by the second frequency dividing means, and outputting a signal corresponding to the comparison result; Filter means for outputting a low frequency component of the output of the phase comparison means, voltage controlled oscillation means for outputting a clock having a frequency corresponding to the voltage output from the filter means as the video clock, and each second clock generating means Control means for changing the frequency ratio of the video clock by changing the frequency dividing ratios of the first frequency dividing means and the second frequency dividing means, and the control means has a frequency of the video clock. Is made variable in steps of a predetermined frequency variable pitch unit, a number smaller than the maximum step number determined in the frequency variable pitch unit is used as the frequency dividing ratio of the first frequency dividing unit and the second frequency dividing unit. It is characterized by setting.
[0024]
  According to a second aspect of the present invention, the control means increases the frequency of the video clock in the predetermined frequency variable pitch unit, and decreases the video clock frequency in the predetermined frequency variable pitch unit. In this case, the frequency dividing ratio is set in the first frequency dividing means and the second frequency dividing means.
[0025]
  A third invention according to the present invention is characterized in that the control means obtains a frequency division ratio set in the first frequency dividing means and the second frequency dividing means based on a predetermined arithmetic expression.
[0026]
  According to a fourth aspect of the present invention, the control means stores the first frequency division means and the second frequency division ratio that are preliminarily stored in the frequency division ratios set in the first frequency division means and the second frequency division means. It is characterized by selecting from combinations of frequency division ratios with frequency dividing means.
[0027]
  According to a fifth aspect of the present invention, the arithmetic expression is M_kIs the frequency dividing ratio of the first frequency dividing means, N_kIs the frequency dividing ratio of the second frequency dividing means, b, c, k are integers, N_o , M_o Is a natural number
[Expression 1]
N_k = N_o + Kb
        M_k = M_o + Kc
It is characterized by being shown by.
[0035]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
[0036]
[First Embodiment]
Hereinafter, with reference to FIG. 1, the entire image forming apparatus showing an embodiment of the present invention will be described.
[0037]
FIG. 1 is a block diagram illustrating a schematic configuration of an image forming apparatus according to a first embodiment of the present invention. In the present embodiment, a color image forming apparatus such as a color printer having an image forming unit of four colors, that is, yellow Y, magenta M, cyan C, and black K will be described as an example.
[0038]
In the figure, reference numerals 1a to 1d denote photosensitive drums, in which electrostatic latent images for black K are formed for the photosensitive drum 1a, cyan C for the photosensitive drum 1b, magenta M for the photosensitive drum 1c, and yellow Y for the photosensitive drum 1d. Reference numerals 2a to 2d denote first to fourth laser scanners that perform exposure according to image signals to form electrostatic latent images on the photosensitive drums 1a to 1d.
[0039]
The black image forming unit includes a photosensitive drum 1a, a first laser scanner 2a, and the like. The cyan image forming unit includes a photosensitive drum 1b, a second laser scanner 2b, and the like. The magenta image forming unit includes a photosensitive drum 1c, a third laser scanner 2c, and the like. The yellow image forming unit includes a photosensitive drum 1d, a fourth laser scanner 2d, and the like.
[0040]
Reference numeral 3 denotes an endless conveyance belt, which also serves as a transfer belt that sequentially conveys a sheet (not shown) to the image forming units for each color. Reference numeral 4 denotes a driving roller, which is connected to driving means (not shown) including a motor and gears, and drives the conveying belt 3. Reference numeral 5 denotes a driven roller that rotates according to the movement of the conveyor belt 3 and applies a constant tension to the conveyor belt 3. Reference numerals 6a and 6b denote a pair of optical sensors, which are provided on both sides of the conveyance belt 3 and are formed on the conveyance belt 3 with a displacement detection pattern (position displacement detection patterns 7a to 10a shown in FIG. 3 to be described later). 7b to 10b).
[0041]
A controller 1000 includes a CPU 1001, a RAM 1002, a ROM 1003, and the like. The CPU 1001 performs overall control of the color printer based on a control program stored in the ROM 1003. The RAM 1002 is used as a work area for the CPU 1001.
[0042]
Hereinafter, the operation of the image forming apparatus of this embodiment will be described.
[0043]
When data to be printed is sent from a computer (PC) or the like (not shown) to the color printer and image processing according to the system of the printer engine is completed and the printer is ready, paper is supplied from a paper cassette (not shown) to the transport belt 3. The paper reaches the image forming portions of the respective colors sequentially by the transport belt 3.
[0044]
The image signals of the respective colors are sent to the respective laser scanners 2a to 2d in synchronism with the conveyance of the paper by the conveying belt 3, and the first to fourth laser scanners 2a to 2d irradiate the laser beams to respectively output the photosensitive drums 1a to 1d. An electrostatic latent image is formed thereon, a developing device (not shown) develops the electrostatic latent image formed on each of the photosensitive drums 1a to 1d with toner, and a toner image on the photosensitive drums 1a to 1d at a transfer unit (not shown). Is transferred onto the paper transported by the transport belt 3.
[0045]
In the color printer shown in this embodiment, images are sequentially formed in the order of Y, M, C, and K. Thereafter, the sheet on which the toner image has been transferred is separated from the conveyance belt 3, and the toner image is fixed on the sheet by heat by a fixing device (not shown) and discharged to the outside.
[0046]
Hereinafter, with reference to FIG. 2 and FIG. 3, a description will be given of an error in scanning width that occurs when the distance between the first to fourth laser scanners 2 a to 2 d and the photosensitive drums 1 a to 1 d is different in each image forming unit.
[0047]
FIG. 2 is a diagram for explaining an example of an error in scanning width on the photosensitive drums 1a to 1d when the distances between the first to fourth scanner units 2a to 2d and the photosensitive drums 1a to 1d shown in FIG. It is.
[0048]
In the figure, α is an incident angle, which is an incident angle of the scanning laser irradiated from the scanner unit 2 (2a to 2d) shown in FIG. 1 onto the photosensitive drum 1 (1a to 1d). ΔL is a distance error between the laser scanner 2 and the photosensitive drum 1. δs is an error in scanning width caused by α and ΔL,
[0049]
[Expression 4]
δs = ΔL × tan α (1)
It becomes. For example, when ΔL = 0.3 mm and α = 30 degrees, δs = 173 μm.
[0050]
FIG. 3 is a diagram for explaining a difference in scanning width occurring in an image transferred to a sheet conveyed by the conveying belt 3 shown in FIG.
[0051]
In the figure, reference numeral 11 denotes paper, which is transported by the transport belt 3 shown in FIG. 1, and each color toner image is transferred by each image forming unit. Reference numeral 12 denotes an image, which is a black image transferred to a sheet 11 in a certain color, for example, a black image forming unit. Reference numeral 13 denotes an image, which is a color different from a certain color, for example, a magenta image transferred to the paper 11 by a magenta image forming unit.
[0052]
For example, in the same condition as described above, that is, when the difference between the distance between the photosensitive drum 1a of the black image forming unit and the laser scanner 2a and the distance between the photosensitive drum 1c of the magenta image forming unit and the laser scanner 2c is ΔL. In addition, the error in the scanning width between the image 12 and the image 13 is 2 × δs = 346 μm.
[0053]
As described above, when the distance between the laser scanner of each image forming unit and the photosensitive drum is different, even if an image having the same width is formed, an image having a different width is formed on the sheet.
[0054]
In order to reduce the color misregistration (positional misalignment), a misregistration detection pattern as shown in FIG. 4 to be described later is formed on the conveying belt 3 shown in FIG. 1 and 1 provided on both sides of the conveying belt 3. Reading is performed by the pair of optical sensors 6a and 6b, and the amount of positional deviation of each color is detected.
[0055]
FIG. 4 is a diagram for explaining a misregistration detection pattern transferred to the conveyance belt 3 shown in FIG.
[0056]
In the figure, reference numerals 7a and 7b denote misregistration detection patterns, which are linear patterns extending in the laser scanning direction and the conveying direction transferred to the conveying belt 3 by the black image forming unit. Reference numerals 8a and 8b denote misregistration detection patterns, which are linear patterns extending in the laser scanning direction and the conveyance direction transferred to the conveyance belt 3 by the cyan image forming unit.
[0057]
Reference numerals 9a and 9b denote misregistration detection patterns, which are linear patterns that are transferred to the conveyance belt 3 by the magenta image forming unit and extend in the conveyance direction. Reference numerals 10a and 10b denote misregistration detection patterns, which are linear patterns extending in the laser scanning direction and the conveyance direction transferred to the conveyance belt 3 by the yellow image forming unit.
[0058]
The misregistration detection patterns 7a to 10a are patterns on the near side of the conveyor belt 3 shown in FIG. 1, and are read by the optical sensor 6a shown in FIG. The misregistration detection patterns 7b to 10b are patterns on the back side of the conveyor belt 3 shown in FIG. 1, and are read by the optical sensor 6b shown in FIG.
[0059]
By the misalignment detection patterns read by the optical sensors 6a and 6b shown in FIG. 1, the controller 1000 shown in FIG. 1 extends the misalignment amount in the carrying direction from the linear pattern extending in the laser scanning direction. The amount of positional deviation in the laser scanning direction is detected from the straight line pattern. The scanning width of each color is calculated from the distance between the linear patterns 7a and 7b, 8a and 8b, 9a and 9b, 10a and 10b, and the distance between the optical sensors 6a and 6b extending in the transport direction.
[0060]
As shown in FIGS. 5 and 6, which will be described later, the laser scanning width is corrected by having a video clock generator for an image signal for each color, making the frequency of the video clock variable for each color, and adjusting the frequency. To do.
[0061]
FIG. 5 is a diagram for explaining the flow of image signals sent to the first to fourth scanner units 2a to 2d shown in FIG.
[0062]
In the figure, reference numeral 601 denotes a crystal oscillator, which is a high-precision source clock supply source, and outputs an output signal to the first to fourth video clock generators 602a to 602d. The first to fourth video clock generators 602a to 602d have a configuration as shown in FIG. 6, which will be described later, and generate a video clock and output it to the first to fourth image data control units 603a to 603d.
[0063]
The first to fourth image data control units 603a to 603d output image signals to the first to fourth laser scanners 2a to 2d according to the frequency of the video clock from the first to fourth video clock generators 602a to 602d. . The first to fourth laser scanners 2a to 2d control the lighting of the laser according to the image signal.
[0064]
As described above, the image signal is sent to the first to fourth laser scanners 2a to 2d in accordance with the frequency of the video clock output from the first to fourth video clock generators 602a to 602d for each color, and the laser is turned on. Be controlled.
[0065]
FIG. 6 is a block diagram illustrating a PLL circuit provided in the first to fourth video clock generators 602a to 602d shown in FIG. In addition, the same code | symbol is attached | subjected to the same thing as FIG.
[0066]
In the figure, 702 is a 1 / M frequency divider, 703 is a phase comparator, 704 is a low pass filter, 705 is a voltage controlled oscillator, 706 is a 1 / N frequency divider, and 14 is an output signal from the crystal oscillator 601. , 15 are outputs of the first to fourth video clock generators 602a to 602d, which are variable video clocks.
[0067]
Reference numeral 700 denotes a video clock frequency control unit, reference numeral 710 denotes a RAM, and reference numeral 720 denotes a ROM.
[0068]
Hereinafter, a method of changing the video clock frequency in the first to fourth video clock generators 602a to 602d will be described.
[0069]
A signal obtained by dividing the output signal 14 of the crystal oscillator 601 by M by the 1 / M divider 702 and a signal obtained by dividing the video clock 15 by N by the 1 / N divider 706 are input to the phase comparator 703, and the phase The output of the comparator 703 is input to the voltage controlled oscillator 705 through the low pass filter 704.
[0070]
For example, when the phase of the signal obtained by dividing the output signal 14 of the crystal oscillator 601 by M is ahead of the phase of the signal obtained by dividing the video clock 15 by N, the input voltage of the voltage controlled oscillator 705 increases, Advance the phase.
[0071]
Here, if the frequency of the output signal 14 from the crystal oscillator 601 is fin and the frequency of the video clock 15 is fout,
[0072]
[Equation 5]
fout = fin × N / M (2)
It becomes.
[0073]
The video clock frequency can be varied by adjusting the value of N / M in accordance with the detected main scanning width.
[0074]
Here, as described above, increasing the value of N causes problems such as an increase in jitter. Therefore, in this embodiment, the feedback amount is increased and the jitter is reduced by the following.
[0075]
For example, when “main scanning width of reference color = 300,000 μm (300 mm)”, “main scanning width of adjustment color = 30,0003 μm”, “N / M of adjustment color = 952/949”, N / M of adjustment color N / M = 955/952.
[0076]
In the case of “main scanning width of reference color = 300,000 μm (300 mm)”, “main scanning width of adjustment color = 299997 μm”, and N / M of adjustment color = 952/949, N / M of adjustment color is N / M = 949/946 may be set. By doing so, the feedback amount becomes about 100 times larger than the case where N≈100,000 as shown in the prior art section. As a result, jitter can be reduced.
[0077]
As described above, by appropriately changing both the frequency division ratios M and N of the two frequency dividers 702 and 706, the video clock frequency can be finely adjusted without increasing the jitter.
[0078]
An example of a method for determining M and N will be described in detail based on the above embodiment.
[0079]
b, c, k are integers, N_o , M_o Is a natural number, the frequency division ratio K of the 1 / M frequency divider 702_k , 1 / N divider 706 divider ratio N_k Is
[0080]
[Formula 6]
N_k = N_o + Kb
K_k = M_o + Kc (3)
Is obtained by calculating. For example, b, c, M in the above formula (3)_o "B = -3", "c = -3", "M_o = N_o +3 ”
[0081]
[Expression 7]
N_k = N_o -3k
K_k = N_o + 3-3k
It is. Here, considering the case of k = 0 and k = 1, the video clock frequency variable pitch is expressed by the following equation.
[0082]
[Equation 8]
{(N₁ / M₁ )-(N_o / M_o )} / (N_o / M_o ) (4)
However, M₁ = N_o , N₁ = N_o -3.
[0083]
  In the above embodiment, the desired video clock frequency variable pitch is 0.001%. Therefore, when No and Mo are calculated so that the absolute value of Equation (4) is 0.001%, No is approximately 949 and Mo is approximately 952. At this time, when k = 0 to k = 1, N1 = 946 and M1 = 949, and the video clock frequency variable pitch is 9.99 ppm. Deviation from the desired video clock frequency variable pitch is 0.01 ppm. The value of each variable of the above equation (3) is determined so that a desired video clock frequency variable pitch can be obtained and the deviation of the video clock frequency variable pitch falls within a desired range.
[0084]
[Second Embodiment]
In the first embodiment, the case has been described in which the frequency division ratios M and N of the frequency dividers are obtained by calculating several sequences, but the combination of the frequency division ratios M and N of the frequency dividers is calculated in advance. It may be configured to store in a memory, read the combination of the division ratios stored as necessary, and change the video clock frequency. The embodiment will be described below.
[0085]
Note that the image forming apparatus of the present embodiment also has a configuration similar to that of the first embodiment.
[0086]
Hereinafter, an example of a combination of M and N when the video clock frequency variable step (pitch) is set to 35 ppm will be described with reference to FIG.
[0087]
FIG. 7 is a diagram for explaining an example of a combination table of frequency division ratios stored in the ROM 720 shown in FIG.
[0088]
The left column shows the combination of the division ratios M and N, and the right column shows the video clock frequency variable step when the division ratio is changed.
[0089]
Such a combination of the frequency division ratios M and N is calculated in advance and recorded in a memory such as the RAM 710, the ROM 720, and a hard disk (not shown) shown in FIG. The values of M and N are read from this memory as necessary, and the video clock frequency is changed.
[0090]
For example, consider the case where M = 1134 and N = 1168. Here, if it is necessary to reduce the main scanning width by 35 ppm, the video clock frequency control means 700 shown in FIG. 6 reads M = 1066, N = 1098 from the ROM 720 and converts the value into a 1 / M frequency divider 702. The 1 / N frequency divider 706 may be set. On the contrary, if it is necessary to increase the main scanning width by 35 ppm, the video clock frequency control means 700 reads M = 935, N = 963 from the ROM 720, and calculates the value as 1 / M divider 702, 1 / N divider. What is necessary is just to set to 706.
[0091]
If the combination of the frequency division ratios M and N recorded in the memory is increased, the variable range of the video clock frequency can be widened.
[0092]
Hereinafter, the procedure for adjusting the laser scanning width of the image forming apparatus will be described with reference to the flowchart of FIG.
[0093]
FIG. 8 is a flowchart for explaining an example of the first data processing procedure in the image forming apparatus according to the present invention. In addition, (1)-(2) shows each step.
[0094]
First, the video clock frequency control means 700 shown in FIG. 6 uses a combination of the frequency division ratios M and N of the 1 / M frequency divider 702 and 1 / N frequency divider 706 suitable for a desired video clock frequency variable pitch. Find (1). In step (1), the combination of the division ratios M and N may be obtained by calculating a number sequence as described above, or an appropriate division ratio from among the combinations of division ratios stored in advance. A combination of M and N may be selected.
[0095]
Next, the frequency division ratio of the 1 / M frequency divider 702 and the 1 / N frequency divider 706 is changed to the frequency division ratio selected in step (1) (2), and the process ends.
[0096]
By performing the processing in this manner, the frequency of the video clock can be finely adjusted without deteriorating the jitter, the positional deviation of the image formed in each image forming unit is reduced, and a high quality image is obtained. be able to.
[0097]
[Other Embodiments]
In each of the above embodiments, the main scanning width is corrected when appropriate. For example, the effect can be obtained when the environment such as the interval between papers, temperature and humidity changes, when a certain period of time has passed, when starting up the image forming apparatus, or when making adjustments before shipment at the factory. In factory pre-shipment adjustment, adjustment at a lower cost than mechanical adjustment of optical components and scanners can be expected.
[0098]
In each of the above-described embodiments, the configuration including the four-color image forming units has been described. However, the present invention is effective even in a configuration including two-color or three-color image forming units. The present invention is also effective in a configuration including image forming units of five or more colors.
[0099]
In each of the above embodiments, the case where N / M is close to 1 has been described. However, this configuration is effective even when N / M≈1 is not satisfied. For example, by setting N / M≈4, a desired video clock can be obtained with a reference clock having a frequency that is approximately ¼ of the video clock.
[0100]
In each of the above embodiments, the configuration including two frequency dividers has been described. However, the number of frequency dividers does not have to be two, and only two functions of a reference clock frequency dividing unit and a voltage controlled oscillator clock frequency dividing unit may be used as functions. For example, when a prescaler is used as the voltage controlled oscillator clock dividing means, the voltage controlled oscillator clock dividing means is composed of two or more frequency dividers. Furthermore, the effectiveness of this configuration does not change even when a device such as a pulse swallow counter that functions as one frequency divider with two or more frequency dividers is used.
[0101]
Further, in each of the above-described embodiments, the case has been described in which the positional deviation of the image formed by each image forming unit is detected by the positional deviation detection pattern including the straight lines in the main scanning and sub-scanning directions as shown in FIG. However, the present invention is effective even if other patterns are used as long as they are patterns that can detect positional deviation in the main scanning direction and the sub-scanning direction.
[0102]
In each of the above embodiments, an example in which the registration detection pattern is formed on the conveyance belt has been described. However, the present invention is not limited to this example. For example, a registration detection pattern is formed on a belt or drum of an apparatus that forms an image on a belt and then transfers the image onto an image recording medium such as paper, that is, an intermediate transfer belt or an intermediate transfer drum. Even in this case, the present invention is effective.
[0103]
In each of the above embodiments, the cause of the change in the main scanning magnification is that the distance between the scanner and the photosensitive member is shifted. However, the present invention is not limited to this cause, and the present invention is also effective for correction when the main scanning magnification changes due to, for example, a positional shift of an optical component or a change or deformation of a refractive index of a lens or the like.
[0104]
Furthermore, in each of the above embodiments, this configuration is used for correcting the main scanning magnification. However, it is effective to adjust the main scanning magnification positively when it is desired to adjust the image magnification. In this case, the image magnification can be adjusted in both the main scanning direction and the sub-scanning direction by adjusting the conveyance speed of the recording medium and changing the magnification in the sub-scanning direction.
[0105]
In each of the above-described embodiments, the configuration in which the laser scanner optical system and the photoconductor are in one-to-one correspondence is shown. However, the present invention can also be applied when the laser scanner optical system and the photosensitive member do not correspond one-to-one. For example, by using an endless photoreceptor or a drum-shaped photoreceptor, an image forming unit can be configured by a plurality of laser scanner optical systems and one photoreceptor. Even in this case, the present invention is effective.
[0106]
As described above, the image forming apparatus described in each of the above embodiments includes the first to fourth video clock generators 602a to 602a provided individually for the laser scanner optical systems (first to fourth laser scanners 2a to 2d). The video clock frequency can be changed by varying the frequency dividing ratio of the frequency dividers 702 and 706 included in 602d by the video clock frequency control means 700.
[0107]
In this way, by exposing the photosensitive drums 1a to 1d with the beam modulated with the image signal synchronized with the video clock whose frequency is changed, the scanning width on the photosensitive member can be changed, and each laser scanner optical system It is possible to correct an error in the scanning width due to variations in the number. Further, by correcting the scanning width, it is possible to correct the color shift and the position shift of the image formed by each image forming unit.
[0108]
At this time, as described above, the first and second frequency division ratios M and N of the 1 / N frequency divider 706 and the 1 / M frequency divider 702 that divide the output of the voltage controlled oscillator 705 are changed to change the first. Since the video clock frequency can be finely adjusted while increasing the feedback amount of the feedback circuit included in the fourth video clock generators 602a to 602d, color shift and position shift can be corrected with high accuracy at low cost. High image quality can be achieved.
[0109]
Moreover, you may comprise so that it may implement combining said each embodiment.
[0110]
Further, in each of the above embodiments, the image forming apparatus is described as an example, but various image forming apparatuses such as an electrophotographic apparatus, a digital copying machine, a monochrome copying machine, a color laser copying machine, a laser beam printer, This book is for color laser printers, facsimile machines, composite copiers with copy and / or print functions and / or facsimile functions, etc., and control devices, information processing devices, data processing devices, etc. for controlling various image forming devices. You may comprise so that the technique shown by embodiment may be applied.
[0111]
As described above, a storage medium storing software program codes for realizing the functions of the above-described embodiments is supplied to the system or apparatus, and the computer (or CPU or MPU) of the system or apparatus stores the storage medium in the storage medium. It goes without saying that the object of the present invention can also be achieved by reading and executing the program code.
[0112]
In this case, the program code itself read from the storage medium realizes the novel function of the present invention, and the storage medium storing the program code constitutes the present invention.
[0113]
As a storage medium for supplying the program code, for example, a floppy disk, a hard disk, an optical disk, a magneto-optical disk, a CD-ROM, a CD-R, a magnetic tape, a nonvolatile memory card, a ROM, an EEPROM, or the like is used. it can.
[0114]
Further, by executing the program code read by the computer, not only the functions of the above-described embodiments are realized, but also an OS (operating system) or the like running on the computer based on the instruction of the program code. It goes without saying that a case where the function of the above-described embodiment is realized by performing part or all of the actual processing and the processing is included.
[0115]
Further, after the program code read from the storage medium is written to a memory provided in a function expansion board inserted into the computer or a function expansion unit connected to the computer, the function expansion is performed based on the instruction of the program code. It goes without saying that the case where the CPU or the like provided in the board or the function expansion unit performs part or all of the actual processing and the functions of the above-described embodiments are realized by the processing.
[0116]
The present invention may also be applied to a system composed of a plurality of devices, such as a printing system, an image processing system, an image forming system, a control system, a printing control system, an image processing control system, and an image forming control system. You may apply to the apparatus which consists of one apparatus, for example, a printing apparatus, an image processing apparatus, an image forming apparatus, a control apparatus, a printing control apparatus, an image processing control apparatus, an image formation control apparatus etc. Needless to say, the present invention can be applied to a case where the present invention is achieved by supplying a program to a system or apparatus. In this case, by reading the storage medium storing the program represented by the software for achieving the present invention into the system or apparatus, the system or apparatus can enjoy the effects of the present invention. .
[0117]
Furthermore, by downloading and reading a program represented by software for achieving the present invention from a database on a network by a communication program, the system or apparatus can enjoy the effects of the present invention. .
[0118]
【The invention's effect】
  As described above, according to the present invention, a number smaller than the number of pitches required for changing the frequency of the video clock in a predetermined frequency variable pitch unit is set to the first frequency dividing means and the second frequency divided. By setting the frequency division ratio, the video clock can be adjusted with high accuracy without deteriorating the jitter of the video clock, even when the video clock frequency is varied in smaller pitch units. Thus, it is possible to obtain a high-quality image in which color misregistration is accurately corrected with an inexpensive configuration.
[Brief description of the drawings]
FIG. 1 is a block diagram illustrating a schematic configuration of an image forming apparatus according to a first embodiment of the present invention.
2 is a diagram illustrating an example of an error in scanning width on a photosensitive drum when the distance between the first to fourth scanner units shown in FIG. 1 and the photosensitive drum is different. FIG.
FIG. 3 is a diagram illustrating a difference in scanning width that occurs in an image transferred to a sheet conveyed by the conveyance belt illustrated in FIG. 1;
FIG. 4 is a diagram illustrating a misregistration detection pattern transferred to a conveyance belt or the like shown in FIG.
FIG. 5 is a diagram for explaining the flow of image signals sent to the first to fourth scanner units shown in FIG. 1;
6 is a block diagram illustrating a PLL circuit provided in the first to fourth video clock generators shown in FIG. 5. FIG.
7 is a diagram for explaining an example of a combination table of frequency division ratios stored in a ROM or the like shown in FIG. 6; FIG.
FIG. 8 is a flowchart illustrating an example of a first data processing procedure in the image forming apparatus according to the present invention.
FIG. 9 is a block diagram illustrating a configuration of a video clock generator in a conventional image forming apparatus.
FIG. 10 is a diagram for explaining deterioration of video clock jitter when the feedback amount is small;
[Explanation of symbols]
1a to 1d photosensitive drum
2a to 2d 1st to 4th laser scanner
3 Conveyor belt
6a, 6b Optical sensor
7a, 7b-10a, 10b Misalignment detection pattern
601 crystal oscillator
700 Video clock frequency control means
702 1 / M frequency divider
703 Phase comparator
704 Low-pass filter
705 Voltage controlled oscillator
706 1 / N divider
1000 controller

Claims (5)

An image forming unit having a plurality of scanning units, one or more first clock generating units for generating a reference clock, and a plurality of second clocks for generating a video clock by adjusting the frequency of the reference clock for each of the scanning units. An image forming apparatus configured to form an image by exposing each photoconductor to a photoconductor in accordance with an image signal synchronized with the video clock generated by the second generating unit.
Each of the second clock generation means includes
First dividing means for dividing the reference clock;
Second dividing means for dividing the video clock;
A phase comparison that compares the phase of the clock obtained by dividing the reference clock by the first divider and the clock obtained by dividing the video clock by the second divider and outputs a signal corresponding to the comparison result Means,
Filter means for outputting a low frequency component of the output of the phase comparison means;
Voltage-controlled oscillation means for outputting, as the video clock, a clock having a frequency corresponding to a voltage output from the filter means;
Control means for changing the frequency division ratio of the first frequency dividing means and the second frequency dividing means of the second clock generating means to vary the frequency of the video clock,
When the frequency of the video clock is varied in steps of a predetermined frequency variable pitch unit, the control unit sets a number smaller than the maximum step number determined in the frequency variable pitch unit to the first frequency dividing unit and the An image forming apparatus characterized in that it is set as a frequency dividing ratio of the second frequency dividing means. The control means sets the frequency division ratio when the frequency of the video clock is increased by the predetermined frequency variable pitch unit and when the frequency of the video clock is decreased by the predetermined frequency variable pitch unit. 2. The image forming apparatus according to claim 1, wherein the image forming apparatus is set to a first frequency dividing unit and a second frequency dividing unit. Said control means, the image forming apparatus according to claim 1 or 2, wherein the mel determined based on the frequency division ratio to be set in the first division unit and the second dividing means to a predetermined arithmetic expression . The control means is a combination of the frequency division ratios of the first frequency dividing means and the second frequency dividing means, in which the frequency dividing ratios set in the first frequency dividing means and the second frequency dividing means are stored in advance. The image forming apparatus according to claim 1, wherein the image forming apparatus is selected from the following. The arithmetic expression is as follows: M _k is a frequency dividing ratio of the first frequency dividing means, N _k is a frequency dividing ratio of the second frequency dividing means, b, c, k are integers, N _o , M _o Is a natural number
[Expression 1]
N _k = N _o + Kb
M _k = M _o + Kc
The image forming apparatus according to claim 3, wherein

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