Patent Publication Number: US-7583281-B2

Title: Image forming apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is a continuation of U.S. application Ser. No. 10/870,958, filed Jun. 21, 2004, which is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2003-196266, filed on Jul. 14, 2003, the entire contents of each of which are incorporated herein by reference. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an image forming apparatus, which uses a plurality of beams, such as a copier, a facsimile machine, and a printer, and in particular, to a method of improving the image quality of an image formed by the image forming apparatus. 
   2. Description of the Background Art 
   Conventionally, many image forming apparatuses of this type which scan with a plurality of beams concurrently and select a leading beam from among the plurality of beams such that color shifts may be prevented are disclosed in, for example, Japanese Patent Laid-Open No. 10-104537 and Japanese Patent Laid-Open No. 2002-072607. In this prior art, when image formation is performed by using a plurality of beams, writing is started from a headline of the image in each color with a different beam so as to decrease shifts between superimposed images. Nevertheless, when the writing of the beginning line of an image, i.e., a top line is started with different beams, there is a possibility that image unevenness arises in a superimposed image. 
   SUMMARY OF THE INVENTION 
   It is an object of the present invention to provide an image forming apparatus capable of decreasing the image unevenness of superimposed images when forming the beginning (head) lines of images in respective colors with beams which are different in order to decrease a shift of a superimposed image. 
   In accordance with the present invention, there is provided an image forming apparatus which generates a toner image from a latent image, formed on an image carrier by scan type writing means, by developing means, repeats multiple times steps of transferring the toner image on an intermediate transfer member for respective colors, and forms a color image with superimposing the toner images by turns for respective colors. The writing means to form the latent image is constituted such that a plurality of beams which is adjacent in a subscanning direction may perform concurrent scans, and can select from the plurality of beams a beam which becomes a head at the time of formation of an image in each color to perform tonality correction of each beam according to the selected beam. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description taken with the accompanying drawings in which: 
       FIG. 1  is a diagram showing an example of the basic construction of an image forming apparatus according to the present invention; 
       FIGS. 2A to 2C  are views showing schematically lines (solid) formed by dots of beams A and B; 
       FIG. 3  is a graph showing an example of a γ curve; 
       FIGS. 4A to 4D  are views showing an example of the timing (dot positions) of scanning beams to a belt mark signal; 
       FIG. 5  is a block diagram showing an example of the construction of beam selection control means; 
       FIG. 6  is a view for describing the operation of the beam selection control means; 
       FIG. 7  is a view for explaining the operation of beam selection; 
       FIG. 8  is a block diagram showing another example of the construction of the beam selection control means; 
       FIG. 9  is a timing chart for describing the operation of the beam selection control means shown in  FIG. 8 ; and 
       FIG. 10  is a diagram showing the construction of an image forming apparatus having two stations. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Hereafter, the present invention will be described specifically with reference to accompanying drawings. 
     FIG. 1  shows an example of the basic construction of an image forming apparatus according to the present invention. The image forming apparatus forms a color image by generating a toner image by developing means from a latent image formed on an image carrier by scan type writing means, repeating multiple times a step of transferring the toner image on an intermediate transfer member for every color, and superimposing the toner image by turns for every color. 
   As shown in  FIG. 1 , charging means  2 , writing means  3 , developing means  4 , transfer means  5 , and cleaning means  6  are located around an image carrier  1  such as a photoconductive drum. In addition, an intermediate transfer member  7  such as an endless belt is located above the image carrier  1 . Furthermore, the intermediate transfer member  7  is provided with a mark indicating a reference position. 
   In operation, a surface of the image carrier  1  which rotates in the direction shown by an arrow is charged by the charging means  2 . When the mark on the intermediate transfer member  7  is detected, the writing means  3  starts forming a latent image on the image carrier  1 . The latent image on the image carrier is transformed into a visualized image by the developing means  4  as a toner image, and the toner image is transferred to the intermediate transfer member  7  by the transfer means  5 . The residual toner on the image carrier  1  after transfer is removed by the cleaning means  6 . 
   When a color image is formed, the above-mentioned developing steps are repeated by the number of times of necessary colors by switching the developing means  4  with switching means not shown and the image in each color is superimposed on the intermediate transfer member  7 . 
   The image superimposed on the intermediate transfer member  7  is transferred to a recording medium such as transfer sheet by another transfer means not shown, and is fixed thereto by fixing means not shown. 
   The image formation on each color is started on the basis of the mark on the intermediate transfer member  7 . When the writing means  3  is a laser scanning optical system, the mark detection in the intermediate transfer member  7  and a synchronizing signal as a writing reference of the writing means  3  are asynchronous. Hence, even if the image formation is started on the basis of the mark of the intermediate transfer member  7 , a shift occurs between the superimposed images in respective colors. 
   An image forming apparatus provided with writing means, which uses a plurality of light sources and performs simultaneous scans with a plurality of beams adjoining in a subscanning direction, decreases the shift of the superimposed images by selecting abeam which forms an image head line. 
   Hereinafter, an example of an apparatus will be described with reference to the drawings. The apparatus performs simultaneous scans of two lines of beams A and B in one scan with the writing means to form an image and superimposes images in respective colors on the basis of a reference signal such as a belt mark. 
     FIGS. 2A to 2C  schematically show lines (solid) formed by dots of respective beams. Arrangement is made such that the beam A may precede the beam B in a subscanning direction when an image carrier is scanned. In this diagram, a dot line pair shown as the beam A and beam B is formed by a first scan, and dot line pairs of the beams A and B following the above-mentioned dot line pair therebelow are formed in order by second and later scans. 
   It is ideal to start to write all the images in respective colors from the same beam, for example, the beam A. However, there is a case that headlines of images may be written with different beams for reduction of the shift of a superimposed image. 
   It is assumed that a position where an image in a first color (first rotation) is formed with the beam A (black dot) as a leading beam is, for example, a position shown in  FIG. 2A  when performing the image formation with a first synchronizing signal to a mark signal. At this time, if a position where the formation of an image in a second color (second rotation) starts is one shown in  FIG. 2B , a shift by one dot (line) occurs in a superimposed image. As shown in  FIG. 2C , when image formation is performed by selecting the beam B as the beam as the head of an image, the shift of a superimposed image may be decreased. The beam selection is performed by sending empty data, i.e., dummy data to the beam A, and supplying to the beam B the data for the headline of the image. 
   That is, an even line and an odd line in the images in the first and second colors, are formed with different beams. Although there is no problem if dot shapes and exposure power of the beams A and B are completely the same, they are different actually. 
   It is noted that unevenness appears in a superimposed image of the images in the first and second colors. Then, when image formation is performed by changing a leading beam for each color image for color shift reduction, tonality correction is performed to each beam for subsequent images so as to decrease image unevenness due to the difference between shapes and power of respective beams. 
   The tonality correction is performed by modifying a γ curve or the emission power of a light source (LD) which generates a beam. For example, a modification may be made so that a portion of γ curve (represented as solid line in  FIG. 3 ) at low tonality region can be compensated, in the case where beams are superimposed which have thinner shapes at shorter lighting times in pulse width modulation, where there is no change in the dot shape in full lighting (pulse width: 1.0) for one dot of beam. Alternatively, the difference in dot shapes may be reduced by presetting the emission power of beams higher to some extent. 
   The modification table of the γ curve and the corrective values for the emission power change, described above, are obtained beforehand by outputting superimposed images for beams. In addition, a change of image processing is also made depending on the case. When the leading beam of an image to be formed is the same as that of a preceding color image, the tonality correction mentioned above is not performed. Third and later colors are performed similarly. 
   Also when the number of beams scanning concurrently is three or more, correction is made for each of beams that form dots overlapping in the subsequent image formation, thereby reducing image unevenness in the case where the order of formation beams depends on the color. 
   A specific method of selecting a leading beam will be described. 
   Timing of scanning beams with respect to a belt mark signal, which is the starting reference of image formation in each color, represented as dot positions, is illustrated in  FIGS. 4A to 4D . Hereinafter, the description is made while the generating timing of a synchronizing signal of the writing means is represented by the centers of dots. 
   Here,  FIG. 4A  shows the belt mark signal. In addition, time T in  FIG. 4B  is a line period. In addition, reference character A 1  denotes a dot position formed by the beam A in a first scan, B 1  denotes a dot position formed by the beam B in a fist scan, A 2  denotes a dot position formed by the beam A in a second scan, and B 2  denotes a dot position formed by the beam B in a second scan. 
   Suppose that the formation of an image in a first color is started in the timing t 1  in  FIG. 4B  with respect to the belt mark. In the image formation in a second or later color, when t 1 −t 2 &gt;3T/4 as shown in  FIG. 4C , the image formation is started with one-scan delayed scanning. That is, a first line of image data is written from a position A 2  shown in  FIG. 4C . This allows the reduction of color shift between superimposed images. In this case, tonality correction is not performed since the beam at the image head is the same beam A as that in the first color. 
   In addition, when T/4&lt;t 1 −t 3 &lt;3T/4 as shown in  FIG. 4D , image data is written from a position B 1 . Here, a headline of the image is written with the beam B different from the beam A with which the headline of the image in the first color was formed. Therefore, the tonality correction is performed. Note that, in the case of t 1 −t 3 &lt;T/4, image formation is started from the position Al which is the same as that in the first color. 
   Here, if the image in the first color is formed in the timing in  FIG. 4C , start timing in the second and later color as shown in  FIG. 4B  make it impossible to superimpose head lines. Then, image formation starts with a synchronized detection signal after a preset reference value, or T/2 in this case, has elapsed from the belt mark detection. 
     FIG. 5  shows the construction of beam selection control means which performs the control mentioned above. It comprises: mark detection means  21  for detecting a mark of the intermediate transfer member  7 ; first measuring means  22  for measuring elapsed time after mark detection whenever detecting the mark used as the reference of image formation start; first determining means  24  comparing a first reference value, set beforehand, with a measured value of the first measuring means  22  and judging their magnitude; first memory means  25  storing time (t 1 ) from detecting the mark to a synchronizing signal when or after a measured value of the first measuring means  22  reached the first reference value; calculation means  26  obtaining a difference between measured time (t 2 , t 3 ) in the first measuring means  22  from detecting the mark of the image starting reference in the second and later color to a synchronizing signal generated by the writing means, and the measurement result of the first memory means  25 ; and calculation means  26  outputting its absolute value and a positive or negative; second judging means  27  comparing the result of the calculation means  26  with a second reference value set beforehand and judging their magnitude; third judging means  28  comparing the result of the calculation means  26  with a third reference value set beforehand and judging their magnitude; fourth judging means  29  comparing the result of the calculation means  26  with a fourth reference value set beforehand and judging their magnitude; and beam selection unit  30  commanding beam selection to the writing controller from the result of the positive or negative of the difference, obtained by the calculation means  26 , and the result of the second judging means  27 , third judgment means  28 , and fourth judging means  29 . 
   Here, the first reference values is T/2 that is a half of a period T of the synchronizing signal of the writing means  3 , the second reference value of the second judging means  27  is 5T/4, the third reference value of the third judging means  28  is 3T/4, and the fourth reference value of the fourth judging means  29  is T/4. 
   The control means in  FIG. 5  will be described by using  FIG. 6 . 
   By the first determining means  24 , the writing of a first color image is started with a synchronizing signal at the time when T/2 elapses after the detection of the mark signal shown by (a) in  FIG. 6 . Hence, the start timing of the image in the first color may occur between (b) and (c). Here, (b) is the case that a synchronizing signal occurs immediately after T/2 elapses, and (c) is the case that a synchronizing signal occurs just before T/2 elapses. 
   Since the writing means forms two lines in one scan, dot positions generated in respective timing are made as shown by (f) and (i) in  FIG. 6 . An arrow denotes a subscanning direction. 
   The first memory means  25  measures and stores a time period from the belt mark to the writing start with the synchronizing signal after the first reference value (T/2) elapses (for example, t10min or t10max). The start timing of images in second and later colors is changed in a period between (d) and (e) in  FIG. 6  at the maximum similarly to the above. 
   The first measuring means  22  measures a time period from the detection of the mark to the generation of the synchronizing signal of the writing means  3  (for example, t20min or t20max). In addition, the calculation means  26  subtracts a time period until the generation of the synchronizing signal of the writing means  3  for second and later colors (for example, t20min or t20max) from a time period until the writing start of the image in the first color which the first memory means  25  measures and stores (for example, t10min or t10max), and obtains an absolute value and determines the sign such as positive or negative. 
   The second judging means  27  judges the magnitude between the result of the calculation means  26  and 5T/4 which is the second reference value, and outputs the result to the beam selection unit  30 . The third judging means  28  judges the magnitude between the result of the calculation means  26  and 3T/4 which is the third reference value, and outputs the result to the beam selection unit  30 . The fourth judging means  29  judges the magnitude between the result of the calculation means  26  and T/4 which is the fourth reference value, and outputs the result to the beam selection unit  30 . 
   In addition, the calculation means  26  outputs the positive or negative sign of the difference to the beam selection unit  30 . According to the result of each judging means and calculation means, the beam selection unit  30  selects a leading beam of images in the second and later colors as shown in  FIG. 7 , and controls the outputting order of image data in connection with it. Here, suppose that the beam A is used for the formation of a headline in the first color. 
   The operation of beam selection will be described with reference to  FIG. 7 . That is, if the result of the second judging means  27  is “larger”, the beam selection unit  30  outputs head line data to a subsequent beam (beam B) that is in a scan operation delayed by one scan from the synchronizing signal immediately after the belt mark detection to start image formation. If the result of the second judging means  27  is “smaller” and the result of the third judging means  28  is “larger”, the beam selection unit  30  starts image formation in a scan operation delayed by one scan from the synchronizing signal immediately after the belt mark detection. A leading beam at this time is the same as that in the first color. 
   If the result of the second judging means  27  and the result of the third judging means  28  are “smaller” and the result of the fourth judging means  29  is “larger”, the beam selection unit  30  selects a beam according to the positive or negative of the difference obtained by the calculation means  26 . If the difference is “positive”, the beam selection unit  30  outputs head line data to a subsequent beam (beam B), and starts image formation. If the difference is “negative”, the beam selection unit  30  outputs second line data to a leading beam (beam A), and starts the image formation. 
   When all the results of the second judging means  27 , third judging means  28 , and fourth judging means  29  are “smaller”, the beam selection unit  30  starts the image formation from the same leading beam (beam A) as that in the first color. 
   A specific example is shown in  FIG. 6 . Suppose that the start of images in second and later colors is (d) or (e) when the formation of an image in the first color is started in the timing of (b) in  FIG. 6 , In the case of (d), since time difference Δt holds T/4&lt;Δt&lt;3T/4, the results of the second judging means and third judging means are “smaller”, the result of the fourth judging means is “large”, and Δt is “positive”. Hence, the beam selection unit  30  writes headline data from a position of a consecutive beam B (dot G 2 ). 
   In the case of (e), the result of each judging means is the same as the case of (d), but since Δt is “negative”, image formation is performed from the second line data of an image in a position of the leading beam A (dot H 1 ). 
   Owing to this, the head line data of the image data in the second and later colors can be written in a position G 2  to the head line position (dot F 1 ) in the first color, or the second line data in the second color can be written in a position H 1  to a dot position F 2  of the second line in the first color. Hence, it is possible to decrease a positional error. 
   In addition, when the first color starts in the timing (dot I 1 ) of (c), the result of the second judging means  27  is “larger” when the start of images in second and later colors is in (d). Hence, the beam selection unit  30  starts writing with the consecutive beam B from a second synchronizing signal (delayed by one scan) after the mark detection. Also even when the number of scanning beams is three or more, similar control may be performed for reducing the position shift, by providing a plurality of reference values and judging means as mentioned above and setting conditions adequately. 
   In the previous description, although the formation of an image in the first color is started with a first synchronizing signal after the first reference value (here, T/2) elapses, there may be another examples. A setting value n is provided such that writing can be started with delaying a synchronizing signal of the writing means by n periods.  FIG. 8  shows other construction of beam selection control means of the present invention. In this beam selection control means, counting means  18  is further added to the construction in  FIG. 5 . 
   The counting means  18  is provided which counts the number of synchronizing signals of the writing means after the first reference value (period) elapses after the detection of the mark used as a writing starting reference during the formation of an image in the first color, and counts the number of synchronizing signals after the detection of the mark during the formation of images in second and later colors. The counting means is set at n, and the counting means instructs the writing controller to start image formation when the counted value reaches n. For example, when n=3, the formation of an image in the first color is started from pe when a synchronizing signal for the first color is the timing shown by (b) in  FIG. 9 . Moreover, in regard to the formation of images after second and later colors, calculation is performed with making pg 3  a reference synchronizing signal when it is the timing of (c), or making ph 3  a reference synchronizing signal when it is the timing of (d). Furthermore, image formation is started from the result of the calculation on the basis of  FIG. 7 . 
   Thus, the writing start position can be changed thereby avoiding the image formation on a connecting portion of an intermediate transfer member and use in the same portion. 
     FIG. 10  shows the construction of the image forming apparatus with two stations according to the present invention. This image forming apparatus has two image forming means (first station for (C, M) and second station for (Y, K)) under the intermediate transfer member  7 . Each image forming means comprises: one image carrier  1 ; writing beams  3 ; developing means  4  comprising at least two developing units each for developing an electrostatic latent image formed on this image carrier by each beam  3 ; and switching means which alternatively selects and drives a developing unit of the developing means  4 . This image forming apparatus generates a multiple-color image by superimposing images formed by a plurality of above-mentioned image forming means, on an intermediate transfer member. Therefore, according to the present invention, it becomes possible to easily superimpose toner images formed on the intermediate transfer member in respective colors in adequate accuracy, and hence, it is possible to achieve a high-definition full color image forming apparatus that is free of image unevenness. 
   As described above, the present invention has the following advantageous features: 
   (1) Different beams that form superimposed lines can also provide a superimposed image with reduced image unevenness. 
   (2) It is possible to prevent adverse effects caused by needless tonality correction processing. 
   (3) A simple construction can achieve the controlled reduction of position (color) shift of a superimposed image, and suppress image unevenness. 
   (4) Even when the writing means with a two-line simultaneous scan is used, it is possible to decrease a position (color) shift of a superimposed image easily. In addition, it is possible to reduce the variation in position shifts of superimposed images. 
   (5) It is possible to reduce the image unevenness of a superimposed image easily. 
   (6) It is possible to reduce a position (color) shift of a superimposed image easily, and to suppress image unevenness. 
   (7) Since change an image formation position on an intermediate transfer member can be changed, it is possible to prevent the degradation of the intermediate transfer member. 
   (8) Since the image formation position on the intermediate transfer member can be changed depending on the operating conditions, it is possible to prevent the degradation of the intermediate transfer member. 
   (9) Also in an image forming apparatus which is small, high-speed, and low cost, it is possible to form a high-definition image without image unevenness where a position shift (color shift) of superimposed images is reduced. 
   Various modifications will become possible for those skilled in the art after receiving the teachings of the present disclosure without departing from the scope thereof.