Abstract:
An image forming apparatus which can reduce the appearance of a vertical regist shift in a multiple beam writing system. A time interval detector is provided and is configured to detect a time interval between a start signal for scanning in a sub-scanning direction and a synchronous signal for scanning in a scanning direction. Further, this time interval is compared with at least first and second threshold values. Writing by the multiple beam sources is then controlled based on the results of the comparison. By utilizing such an operation the appearance of a vertical regist shift can be reduced.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention is related to an apparatus, method, and system for adjusting a vertical regist shift between lines, pages or colors in a multiple beam recording system, and more particularly is directed to controlling the writing of information from the multiple beams to limit the vertical regist shift. 
     2. Discussion of the Background Art 
     A multiple beam optical scanning system for writing information onto a photoconductive drum is shown in FIG.  1 . In this multiple beam optical system, two light sources  25  and  26  are provided for generating laser beams for effecting writing onto the photoconductive drum  14 . The laser beams output of the light sources  25  and  26  pass through a collimator lens  27 , reflect off a polygonal mirror  28 , pass through a further lens  29 , and then impinge on the photoconductive drum  14 . The polygonal mirror  28  is rotated by a motor  30  and the photoconductive drum  14  is rotated by a motor  32 . Further, a photodetector  31  is provided for detecting a synchronous signal to effect scanning across the photoconductive drum  14  in the scanning direction. With this structure, the light sources  25  and  26  generate laser beams which scan across the photoconductive drum  14  in a scanning direction, and which generate different lines of information in a sub-scanning direction (i.e. the direction of rotation of the photoconductive drum  14 ). 
     This system shown in FIG. 1, however, may suffer from a problem of a vertical regist shift during the following situation. The detected signal from the photodetector  31  is utilized as a synchronizing signal for the scanning direction. An image forming start signal for the sub-scanning direction, to start rotation of the photoconductive of drum  14 , is also generated. Further, when the image forming start signal is output image data is read out from a memory (a RAM, ROM, etc.) or from a memory via registers and the image forming operation starts. However, the situation may arise that the image forming start signal is not synchronized with the detected signal from photodetector  31 . As a result, situations as shown in FIGS. 2 a  and  2   b  arise in an image forming operation, and during a series of image forming operations. Certain kinds of vertical regist shifts may occur based on the above lack of synchronization between the image forming start signal and the detected signal from the photodetector  31 . 
     When the situations as shown in FIGS. 2 a  and  2   b  exist in an image forming operation, a vertical regist shift may occur on output sheets of paper on which a same image is printed, on lines on a sheet of paper, or on colors on a sheet of paper. When the image forming device prints the same image on plural sheets of paper by plural image forming operations, a vertical regist shift may occur between the same position of each line of each paper. Also, when a color image forming device outputs color images on a single sheet of paper, a vertical regist shift may occur between each color image on the sheet. For example, a vertical regist shift may occur between a cyan color image and a magenta color image. A vertical regist shift may also occur between lines in a sheet of paper on a monochrome image forming device. 
     A normal operation and an operation which results in the vertical resist shift are explained in further detail with reference to FIGS. 2 a  and  2   b  which show two cases in the operation of the system of FIG.  1 . FIGS. 2 a  and  2   b  disclose a range of a vertical regist shift based upon a relation between the image forming start signal and the synchronous signal for the scanning direction (i.e., the detected signal from the photodetector  31 ). 
     In the operation of the system of FIG. 1 shown in the second case of FIG. 2 b , the synchronous signal for the scanning direction (as detected by photoconductor  31 ) is generated just after the image forming start signal for the subscanning direction (which starts rotation of the photoconductive drum  14 ). As a result, in this operation of FIG. 2 b  the first and second lines of information are immediately written at their appropriate positions. 
     In this operation of the system of FIG. 1 the writing on the photoconductive drum  14  is determined to start after a predetermined number of synchronous signals are detected (i.e., when a designated synchronous signal is detected) after the image forming start signal for the sub-scanning direction is detected. That is, in the operation of the system shown in FIG. 1 the photodetector  31  detects a signal to start writing onto the photoconductive drum  14 , and thereby the photodetector  31  provides an image forming start signal for the scanning direction. Further, the system as shown in FIG. 1 is designed so that a predetermined number of synchronous signals for the scanning direction are generated prior to writing of the first and second lines of information onto the photoconductive drum  14 . In the operation shown in FIG. 2 b  such a predetermined number of synchronous signals prior to the designated synchronous signal for writing are not shown. 
     In this first case of FIG. 2 a  a synchronous signal (to initiate writing by the laser beams output of light sources  25  and  26  onto the photoconductive drum  14 ) for the scanning direction occurs just prior to the image forming start signal for the sub-scanning direction (which starts rotation of the photoconductive drum  14 ). As a result, the designated synchronous signal for the scanning direction which is detected after the image forming start signal is a significant time tl after the image forming start signal. In this situation, the first and second lines are not written onto the photoconductive drum  14  until approximately one time cycle later than desired. This problem is significant in a multiple beam scanning system because each time cycle for writing corresponds to a shift of two lines if two light sources  25 ,  26  are being used, or a shift of three lines if three light sources are being used, etc. 
     One system for addressing this vertical regist shift operation is disclosed in Japanese Laid-Open Patent Application 08-142412. A control operation executed in this Japanese Laid-Open Patent Application 08-142412 is described in FIGS. 3 a  and  3   b  of the present specification. 
     In the system of this further background art a time interval detector for detecting a time from an image forming start signal for a sub-scanning direction and a synchronous signal for the scanning direction is measured, and a light source selector is provided to select between activating the light sources  25  and  26  based on this detected time interval. 
     More particularly, in this background art in a second case of FIG. 3 b  if a certain time interval t2 occurs between the image forming start signal for the sub-scanning direction and the synchronous signal for the scanning direction (which time interval t2 is detected by a time interval detector), a first line is written by only one of the light sources, and then for subsequent lines both of the light sources operate to write lines simultaneously. In the example shown in FIG. 3 b , with the time interval t2 the first line is written onto the photoconductive drum  14  by the light source  26 , and then the second and third lines are simultaneously written by both the light sources  25  and  26  after a second synchronous signal is detected, and so on. 
     This operation disclosed in Japanese Laid-Open Patent Application No. 08-142412 provides a benefit that a vertical regist shift between lines, colors or pages can be decreased to one line. However, this reduction in the vertical regist shift may still be unacceptable in a multiple beam writing system, and particularly if the multiple beam writing system of FIG. 1 is utilized in a color copier where different colors must be precisely superimposed upon one another. The contents of this reference, Japanese Laid-Open Patent Application No. 08-142,412, are incorporated herein by reference. 
     SUMMARY OF THE INVENTION 
     Accordingly, one object of the present invention is to provide a novel image forming apparatus which can overcome the above discussed drawbacks in the background art. 
     A further object of the present invention is to provide a novel image forming apparatus which can further reduce any vertical regist shift in a multiple beam writing system,. 
     To achieve these and other objects, the present invention implements a system in which an image forming apparatus includes a time interval detector configured to detect a time interval between a start signal for scanning in a sub-scanning direction and a synchronous signal for scanning in a scanning direction. Further, this time interval is compared with at least first and second threshold values in the present invention. Writing by the multiple beam sources is then controlled in the present invention based on the results of the comparison. By utilizing such an operation in the present invention a vertical regist shift can be reduced. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A more complete appreciation of the present invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein: 
     FIG. 1 shows a general configuration of a multiple beam writing system; 
     FIGS. 2 a  and  2   b  show one background implementation of a control of the multiple beam writing system of FIG. 1; 
     FIGS. 3 a  and  3   b  show a further background implementation of a control of the multiple beam writing system of FIG. 1 which reduces vertical regist shift; 
     FIG. 4 shows a block diagram construction of an embodiment of the present invention; 
     FIGS. 5 a - 5   b  explains a control of a writing operation in an embodiment of the present invention; 
     FIGS. 6 a - 6   d  further show the writing operation in an embodiment of the present invention; 
     FIGS. 7 a - 7   d  shows timing charts of the operations in FIGS. 5 and 6; 
     FIG. 8 shows a block diagram construction of the present invention in a multicolor image forming apparatus; 
     FIG. 9 shows one example of an image forming apparatus in which the present invention can be implemented; 
     FIG. 10 shows a further example of an image forming apparatus in which the present invention can be implemented; 
     FIG. 11 shows a further example of an image forming apparatus in which the present invention can be implemented; 
     FIG. 12 shows a further example of an image forming apparatus in which the present invention can be implemented; 
     FIG. 13 shows a further example of an image forming apparatus in which the present invention can be implemented; 
     FIG. 14 shows a further example of an image forming apparatus in which the present invention can be implemented; 
     FIG. 15 shows specific elements in certain image forming apparatuses in which the present invention can be implemented; 
     FIG. 16 shows specific elements in certain image forming apparatuses in which the present invention can be implemented; and 
     FIG. 17 shows specific elements in certain image forming apparatuses in which the present invention can be implemented. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The preferred embodiments of the present invention will now be explained in further detail below, in which like reference numerals designate identical or corresponding parts throughout the several views. 
     FIG. 4 shows an overall block diagram construction of the multiple beam image forming apparatus of the present invention. The image forming apparatus of the present invention includes an overall image scanning structure as shown in FIG. 1 in which a photodetector  31  is provided to generate a synchronous signal for starting scanning in the scanning direction, i.e., to begin writing onto the photoconductive drum  14  of FIG.  1 . 
     Further, a multiple beam light source  67  is provided which includes the first and second light sources  25  and  26 . The present invention further includes a start signal generator  61  which generates a start signal for a sub-scanning direction (to start rotation of the photoconductive drum  14  of FIG. 1) and a time interval detector  62  for detecting a time interval between the start signal for the sub-scanning direction and the synchronous signal for the scanning direction. The output of this time interval detector  62  is provided to an overall controller  63  which includes both a light source select signal generator  64  and a delay signal generator for start signal  65 . The light source select signal generator  64  can output a signal to selectively select one of the light sources  25  and  26  to operate so that a first line can be written by only one of the light sources  25  and  26 . The delay signal generator for the start signal  65  can issue a signal to delay the writing by both of the light sources  25  and  26 . A processor  66  receives the output of the light source select signal generator  64  and the delay signal generator  65  and controls the multiple light sources  25  and  26  based on such received signals and based on the received image data. 
     The specific control operation executed by the overall controller  63  in the present invention is now described in detail with respect to FIGS. 5,  6 , and  7 . First, FIG. 5 a  and FIG. 6 a  set forth an ideal operation in which the designated synchronous signal to begin writing onto the photoconductive drum  14  occurs at the most proper point after the image forming start signal for the sub-scanning direction, and writing by both of the multiple light sources  25  and  26  is started at the most correct time. In FIG. 6 the circles which are cross-hatched represent data written by the light sources  25  and  26 , and the hollow circles represent blank (non-written) areas. In this operation in the present invention in FIGS. 5 a ,  6   a , during a first scanning operation both of the light sources  25  and  26  simultaneously record the lines  68  and  69  shown in FIG. 6 a , and then simultaneously record the lines  70  and  71  in a next scanning operation. The starting of the writing occurs at the proper start point, as shown in both FIGS. 5 a  and  6   a.    
     In the embodiments of the present invention as now discussed, and particularly with reference to FIGS. 5 and 7, the time interval T is defined as a time interval between two successive synchronizing signals for the scanning direction. That is, the time interval “T” is a time interval between two successive signals output from the photodetector  31 . Further, the time interval “t” is a time interval from the image forming start signal for the sub-scanning direction, as generated by the start signal for sub-scanning direction generator  61 , until a designated synchronizing signal for the scanning direction to begin writing onto the photoconductive drum  14 , as output by the photodetector  31 . As noted above, in such a device as in the present invention after an image forming start signal for the sub-scanning direction is generated, a certain number of synchronizing signals may be generated until a designated synchronizing signal, which indicates to start writing onto the photoconductive drum  14 , is generated. This time period “t” is the time period from the image forming start signal for the sub-scanning direction to the designated synchronizing signal for the scanning direction to begin writing relative to one cycle for generating the synchronizing signal for the scanning direction, as shown in FIGS. 5 and 7. 
     FIGS. 5 b ,  6   b , and  7   b  show a situation in which the time interval “t” from generating the image forming start signal for the sub-scanning direction (as generated by the start signal for the sub-scanning direction generator  61 ) to the designated synchronous signal for the scanning direction (as generated by the photodetector  31 ) is greater than three-fourths T (t&gt;¾ T). In this situation, control is effectuated in the present invention such that both the multiple light sources  25  and  26  write signals after the designated synchronous signal. In this situation, a negative vertical regist shift does slightly occur such that the writing is begun slightly early. That is, in this operation a top margin on a sheet is actually slightly reduced. A second case as shown in FIG. 5 c , FIG. 6 c , and FIG. 7 c  is that the time interval “t” from the image forming start signal for the sub-scanning direction to the designated synchronous signal for the scanning direction (which is to indicate the starting of writing) is less than three-fourths T but is greater than one-fourth T (T/4&lt;t&lt;¾ T). In this situation, the light source select generator  64  outputs a signal to the processor  66  such that only the second light source  26  generates a light beam based on a writing signal corresponding to the first line. In this situation there is also a slightly negative vertical regist shift, i.e., corresponding to the top margin on a sheet is again slightly reduced. 
     A third case as shown in FIG. 5 d , FIG. 6 d , and FIG. 7 d  is that the time interval “t” from the image forming start signal for the sub-scanning direction to the designated synchronous signal for the scanning direction indicating the starting of writing is less than one-fourth T (t&lt;¼ T). In this situation, the delay signal generator for the start signal  65  outputs a delay signal to the processor  66 , and based on this output delay signal the processor  66  delays the writing from both of the light sources  25  and  26 . In this situation, there is a slight positive vertical regist shift from the beginning of writing by the light sources  25  and  26 , however this vertical regist shift is small and is less than one-half of a line. 
     In the operations as discussed above, by considering the four cases as noted above, the amount of vertical regist shift can be varied from anywhere from minus one-half line (i.e. a reduced top margin of up to one-half line width) to plus one-half line (i.e. an increased top margin of up to one-half line width). This is a significant improvement over the background art in which a regist shift can be up to plus one line. The operation in the present invention as discussed above allows an operation in which writing by the light sources  25  and  26  can occur slightly early, and as a result an appearance of the total amount of the vertical regist shift in the present invention is reduced. 
     FIG. 8 shows the implementation in the present invention when utilized in a multicolor image forming apparatus. As shown in FIG. 8, the present invention can be easily implemented in a full color image forming apparatus by utilizing one of the overall control system as shown in FIG. 4 for each of the desired colors of, i.e., yellow (Y), cyan (C), magenta (M), and black (K). The structure of the multicolor system of FIG. 8 operates in a similar manner as the system of FIGS. 4-7 discussed above, and thus a redundant explanation of the operation of the structure of FIG. 8 is omitted here. 
     Moreover, the present invention can be implemented in several types of image forming apparatus which utilizes a multiple beam optical unit. Examples of image forming apparatuses in which the present invention can be implemented are now provided, although it is noted that the present can be implemented in other examples of image forming apparatuses than as expressly noted herein. 
     FIG. 9 shows a monocolor image forming apparatus which utilizes a multiple laser beam optical system  12  (which can be implemented as the multiple laser beam optical system of FIG.  1 ). This device includes an image forming station  11  which in turn includes an image forming unit  13 . The image forming unit  13  includes a photoconductive drum  14 , a charger  15 , a developing unit  16 , and a cleaning unit  17 . Further, paper sheets are supplied from a sheet supply cartridge  18  after being fed from a feed roller  19 . The paper sheets are fed through a pair of regist rollers  20  and are detected by a sheet detector  21 . In this image forming apparatus as shown in FIG. 9, the output of the sheet detector  21  corresponds to the start signal for the sub-scanning direction generator  61  of FIG.  4 . The image forming apparatus shown in FIG. 1 further includes a toner image transfer charger  22 , an image fixing roller  23 , and a sheet eject tray  24 . 
     FIG. 10 shows a full color image forming apparatus in which the present invention can also be implemented. The full color image forming apparatus shown in FIG. 10 includes a multiple beam optical unit  42  (which also can be implemented as the multiple laser beam optical system of FIG. 1) included in an image forming station  41 . Further, a developing unit  45  is also provided with individual developing units  45 Y,  45 M,  45 C,  45 K for each of the colors yellow (Y), magenta (M), cyan (C) and black (K). An image is formed on the photoconductive drum  43 , which is surrounded by an electrical charger  44  and a cleaning unit  46 . Formed opposite the image forming unit  41  is an intermediate transfer belt  47  on which images are intermediately formed. This intermediate transfer belt  47  is supported by rollers  48 ,  49 , and  50 . Further, an intermediate transfer charger  51  effectuates transfer of images from the photoconductive drum  43  to the intermediate transfer belt  47 . The full color image from the intermediate transfer belt  47  is then provided to a sheet of paper  53  under the control of a transfer charger  52 . 
     This image forming apparatus as shown in FIG. 10 can employ the belt of FIG. 17 which includes a register mark  78  which is detected by a register mark detector  77 . This register mark detector  77 C in this image forming apparatus corresponds to the start signal for sub-scanning direction generator  61  of FIG.  4 . 
     FIG. 11 shows a further full color image forming apparatus in which the present invention can be implemented. The image forming apparatus of FIG. 11 also includes the multiple beam optical unit  42 , developing unit  45 , and photoconductive drum  43 . In this full color image forming apparatus a sheet  53 , e.g., a paper sheet, is disposed between the photoconductive drum  43  and an intermediate transfer drum  54 , and an image is transferred to the sheet  53  by a transfer charger  51 . This embodiment shown in FIG. 11 can employ the specific circuitry of FIG. 15 in which a position of the photoconductive drum  43  is detected by a position detector  72 . The position detector  72  includes a rotating plate  74  which rotates with the photoconductive drum and which includes a slit  73 . A light source  75  is aligned with a light detector  76  and the light detector  76  reads a signal when the slit is disposed between the light source  75  and the light detector  76 . Thus, the output of the light detector  76  provides an indication of the position of the photoconductive drum  43 . The output of the light detector  76  in this embodiment corresponds to the start signal for the sub-scanning direction generator  61  of FIG.  4 . 
     FIG. 12 shows a further full color image forming apparatus in which the present invention can be implemented and which also utilizes the multiple beam optical unit  42 . This embodiment is similar to the embodiment of FIG. 10 except that no intermediate transfer belt is provided, and as a result transfer of an image from photoconductive drum  43  is directly effectuated to a sheet recording medium  53 . This system can also employ the system of FIG. 15 to determine the position of the photoconductive drum  43 . 
     FIG. 13 shows a further full color image forming apparatus in which the multicolor structure of the present invention as shown in FIG. 8 can be implemented. In the embodiment shown in FIG. 13 each color image forming unit  41 Y− 41 K includes its own respective multiple beam optical unit  42 Y− 42 K. Each beam optical unit also includes its own developing unit  55 Y− 55 K, and an intermediate transfer belt  47  is formed opposite intermediate transfer chargers  51 Y− 51 K. This full color image forming apparatus as shown in FIG. 13 can implement the intermediate transfer belt as shown in FIG. 16 which includes a register mark  78 , and which further includes four register mark detectors  77 Y− 77 K. The outputs of the four register mark detectors  77 Y— 77 K correspond to the start signal for sub-scanning generators  61 Y— 61 K shown in FIG. 8 of the present specification. 
     FIG. 14 shows a further full color image forming apparatus to which the present invention can be implemented similar to that shown in FIG. 13 except that no intermediate transfer belt is provided, i.e., a direct transfer to a paper sheet is implemented. 
     Each of the above-noted monocolor and full color image forming apparatuses can employ the control system of the present invention to minimize a vertical regist shift. 
     Obviously, numerous additional modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the present invention may be practiced otherwise than as specifically described herein. 
     The present document is based on Japanese Priority Document 10-11155 filed in the Japanese Patent Office on Jan. 23, 1998, the entire contents of which are incorporated herein by reference.