Patent Publication Number: US-9846380-B2

Title: Image forming apparatus that adjusts relative positions of images by detecting test pattern

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to control of correction of positions of images formed on an image carrier. 
     Description of the Related Art 
     In image forming apparatuses using an electrophotographic method, a temperature increase caused by continuous image formation and the expansion/contraction of components caused by, for example, fluctuations in the environmental temperature lead to the occurrence of color misregistration, which is misalignment of relative positions of images of different colors used in image formation. In view of this, the image forming apparatuses form a test pattern that is used to detect a misregistration amount on an image carrier, and perform misregistration correction to reduce misregistration based on the result of the detection of the test pattern. 
     Misregistration correction is started based on a period over which an image forming apparatus operated continuously, and/or the number of recording mediums on which images have been formed. Japanese Patent Laid-Open No. 2013-25128 discloses a configuration in which, to perform misregistration correction during image formation, the image formation is temporarily suspended, an interval to the next recording medium (hereinafter referred to as a paper interval) is increased, and misregistration correction is performed by forming a test pattern in the increased interval. Japanese Patent Laid-Open No. 2006-171352 discloses a configuration in which, to perform misregistration correction during continuous image formation, a test pattern that fits within a paper interval is formed. 
     The configuration described in Japanese Patent Laid-Open No. 2013-25128 lowers the productivity of image formation because image formation is suspended each time misregistration correction is performed. With the configuration described in Japanese Patent Laid-Open No. 2006-171352, misregistration correction is performed while image formation is in operation. With this configuration, if a recording medium enters a nip region between a secondary transfer unit, such as a secondary transfer roller, and an intermediate transfer belt, the speed of the intermediate transfer belt could possibly deviate from a target speed. If misregistration correction is performed in a state where the speed of the intermediate transfer belt differs from the target speed, misregistration of relative positions of images cannot be corrected with high precision. 
     SUMMARY OF THE INVENTION 
     According to an aspect of the present invention, an image forming apparatus includes an image forming unit configured to form an image by overlapping a first image and a second image, the image forming unit including a first image forming unit configured to form the first image using toner of a first color, and a second image forming unit configured to form the second image using toner of a second color that is different from the first color; an image carrier configured to carry the image formed by the image forming unit; a transfer unit configured to transfer the image carried by the image carrier to a sheet; a detection unit configured to detect a test pattern including a first detection image and a second detection image that have been formed on the image carrier by the first image forming unit and the second image forming unit, respectively; and an adjustment unit configured to control the first image forming unit and the second image forming unit to form the test pattern, and adjust relative positions of the first image and the second image based on a detection result of the test pattern by the detection unit. The adjustment unit is further configured to control the image forming unit to form the test pattern after the image forming unit has formed on the image carrier an image to be transferred to a sheet with a basis weight less than a threshold, and before the image forming unit forms on the image carrier an image to be transferred to a sheet with a basis weight greater than the threshold. 
     Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a schematic configuration of an image forming apparatus according to an embodiment. 
         FIG. 2  shows a configuration related to control of an image forming apparatus according to an embodiment. 
         FIG. 3  shows fluctuations in torques of driving rollers associated with a basis weight. 
         FIG. 4  shows fluctuations in torques of driving rollers associated with a basis weight. 
         FIG. 5  shows misregistration amounts associated with a basis weight. 
         FIG. 6  shows misregistration amounts associated with a basis weight. 
         FIG. 7  shows misregistration amounts associated with a basis weight. 
         FIGS. 8A and 8B  are timing charts showing exposure processing according to an embodiment. 
         FIG. 9  is a flowchart of processing executed by a control unit according to an embodiment. 
         FIG. 10  is a timing chart showing exposure processing according to an embodiment. 
         FIG. 11  is a flowchart of processing executed by a control unit according to an embodiment. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     The following describes exemplary embodiments of the present invention with reference to the drawings. Note that the following embodiments serve as examples, and are not intended to limit the present invention to the contents thereof. Furthermore, constituents that are not necessary for the description of the embodiments are omitted in the drawings described below. 
     First Embodiment 
       FIG. 1  shows a schematic configuration of an image forming apparatus  101  according to the present embodiment. Image forming units Pa, Pb, Pc, and Pd form yellow, cyan, magenta, and black toner images, respectively, on an intermediate transfer belt  7 . Note that the image forming units Pa, Pb, Pc, and Pd are configured in the same manner, and differ from one another only in the colors of toner they use; therefore, they will hereinafter be described collectively. First of all, a photosensitive member  1  is an image carrier, and is driven and rotated during image formation. The photosensitive member  1  is charged by a charger  2  to have a uniform electric potential. An exposure unit  3  forms an electrostatic latent image on the photosensitive member  1  by scanning the charged photosensitive member  1  with light, that is to say, exposing it to light. A developer  4  forms a toner image by developing the electrostatic latent image formed on the photosensitive member  1  using toner. A primary transfer unit  5  transfers the toner image formed on the photosensitive member  1  to the intermediate transfer belt  7 . A cleaner  6  removes toner that has not been transferred to the intermediate transfer belt  7  and thus remained on the photosensitive member  1 . 
     The intermediate transfer belt  7  is an image carrier that is hung in a stretched state by a tension roller  30 , a secondary transfer opposite roller  31 , and a driving roller  8 , and is rotated in conformity to the rotation of the driving roller  8  during image formation. The intermediate transfer belt  7  has a nip region  35  in which the intermediate transfer belt  7  is in contact with a secondary transfer belt  9 . Along with the rotation of the intermediate transfer belt  7 , toner images transferred to the intermediate transfer belt  7  are conveyed to the nip region  35 . Meanwhile, a sheet, or a recording medium, is fed from cassettes  11  and conveyed to the nip region  35  along a conveyance path. When a sensor  19  detects a tip of the sheet, the conveyance of the sheet is temporarily suspended. Thereafter, rollers  12  restart the conveyance of the sheet so that the sheet arrives at the nip region  35  at the time of arrival of the toner images on the intermediate transfer belt  7  to the nip region  35 . Note that the nip region  35  is equivalent to a transfer position at which the images are transferred to the sheet. 
     The secondary transfer belt  9  is hung in a stretched state by a plurality of rollers, and is driven and rotated in conformity to the rotation of a driving roller  32 . Note that the driving roller  32  and the driving roller  8  are driven by different driving sources. Here, a transfer member that is driven and rotated is not limited to the secondary transfer belt  9 , and may be a roller. A moving speed Vb of a surface of the intermediate transfer belt  7  is set to be higher than a moving speed Vtr of a surface of the secondary transfer belt  9  in order to improve the transfer characteristic of transfer to the sheet. That is to say, the relationship Vb&gt;Vtr holds. 
     When supplied with a transfer voltage from a non-illustrated power unit, the secondary transfer belt  9  transfers the toner images on the intermediate transfer belt  7  to the sheet. A cleaner  10  removes toner that has not been transferred to the sheet and thus remained on the intermediate transfer belt  7 . A conveyance belt  13  conveys the sheet to which the toner images have been transferred to a fixing unit  14 . The fixing unit  14  fixes the toner images onto the sheet by heating and pressurizing the sheet. Thereafter, the sheet is discharged to a discharge tray  15  outside the apparatus. The image forming apparatus  101  also includes a detection unit  100  that detects a test pattern formed on the intermediate transfer belt  7  during misregistration correction. The detection unit  100  includes an LED that illuminates the intermediate transfer belt  7  with light, and a light receiving unit that receives light reflected from the test pattern on the intermediate transfer belt  7 , and the detection unit  100  outputs, to a control unit  111  ( FIG. 2 ), a sensor output value based on the intensity of light received by the light receiving unit. Note that the test pattern includes yellow, magenta, cyan, and black detection images. 
       FIG. 2  shows a configuration related to control of the image forming apparatus. The control unit  111  controls the overall image formation by the image forming apparatus. The control unit  111  also determines whether it is necessary to perform misregistration correction. To cause the image forming apparatus to perform misregistration correction, the control unit  111  forms a test pattern on the intermediate transfer belt  7 , obtains the detection result from the detection unit  100 , and obtains the relative misregistration amounts of images of different colors based on the detection result. Then, based on the obtained misregistration amounts, the control unit  111 , for example, adjusts the timings to form electrostatic latent images on the photosensitive members  1  so as to reduce misregistration. Note that in  FIG. 2 , a forming unit  110  represents the image forming units Pa, Pb, Pc, and Pd shown in  FIG. 1 . A storage unit  112  stores attribute information of sheets. For example, the storage unit  112  stores basis weights of the sheets. That is to say, the storage unit  112  stores information indicating the relationship between the sheets and the basis weights of the sheets. 
       FIG. 3  shows the measurement result of fluctuations in torques of the driving rollers  8  and  32  when toner images have been transferred to a sheet with a basis weight of 128 g/m 3 .  FIG. 4  shows the measurement result of fluctuations in torques of the driving rollers  8  and  32  when images have been transferred to a sheet with a basis weight of 350 g/m 3 . Note that the size of the recording medium is A3 in both cases. Furthermore, at the time of the measurement, the moving speed Vb of the surface of the intermediate transfer belt  7  is set to be higher than the moving speed Vtr of the surface of the secondary transfer belt  9  as mentioned earlier. In  FIGS. 3 and 4 , time T 1  denotes time when a tip of the sheet starts to enter the nip region  35 . As shown in  FIG. 3 , in the case of the sheet with a basis weight of 128 g/m 3 , the entry of the sheet into the nip region  35  does not cause any torque fluctuation. On the other hand, as shown in  FIG. 4 , in the case of the sheet with a basis weight of 350 g/m 3 , the driving roller  8  that drives the intermediate transfer belt  7  is subjected to large torque fluctuations after the sheet enters the nip region  35 . Such torque fluctuations could possibly cause the intermediate transfer belt  7  to slip with respect to the driving roller  8 . If the intermediate transfer belt  7  slips, the speed of the surface of the intermediate transfer belt  7  deviates from the target speed. 
       FIG. 5  shows the measurement result of the misregistration amounts of black (K) with respect to yellow (Y) when images have been formed continuously on five A3 sheets with a basis weight of 128 g/m 3 .  FIG. 6  shows the measurement result of the misregistration amounts of black (K) with respect to yellow (Y) when images have been formed continuously on five A3 sheets with a basis weight of 350 g/m 3 .  FIG. 7  shows the measurement result of the misregistration amounts of black (K) with respect to yellow (Y) when images have been formed continuously on ten A3 sheets with a basis weight of 350 g/m 3 . At the time of the measurement shown in  FIG. 7 , the paper interval between the sixth and seventh sheets is increased compared with other paper intervals. Note that in the image forming apparatus, when the first sheet enters the nip region  35 , the intermediate transfer belt  7  carries toner images to be transferred to the third sheet. 
     As is apparent from  FIG. 5 , in the case of sheets with a relatively small basis weight of 128 g/m 3 , there is no large difference among the misregistration amounts of images formed on five sheets. On the other hand, as shown in  FIG. 6 , in the case of sheets with a large basis weight of 350 g/m 3 , the misregistration amounts on the fourth and fifth sheets are larger than the misregistration amount on the first to third sheets. This is attributed to torque fluctuations caused by the entry of the sheets into the nip region  35 . In  FIG. 7 , due to the increased paper interval between the sixth and seventh sheets, the misregistration amount on the seventh sheet is not much different from the misregistration amount on the first sheet. This is because, due to the increased paper interval, fluctuations in the moving speed Vb of the surface of the intermediate transfer belt  7  caused by the torque fluctuations have been subdued when images to be transferred to the seventh sheet are formed on the intermediate transfer belt  7 . Furthermore, as images to be transferred to the ninth sheet are already transferred to the intermediate transfer belt  7  before the seventh sheet enters the nip region  35 , the misregistration amounts on the eighth and ninth sheets are not much different from the amount of color misalignment on the first sheet. However, the misregistration amount of images on the tenth sheet is large due to the influence of the entry of the seventh sheet into the nip region  35 . Note that the intermediate transfer belt  7  is controlled so that the moving speed of its surface matches a predetermined speed. That is to say, the driving roller  8  is controlled to achieve a predetermined moving speed, regardless of the basis weights of sheets to which images carried by the intermediate transfer belt  7  are to be transferred. 
     In view of the above, depending on the basis weights of sheets P, the image forming apparatus controls misregistration correction that is performed during continuous formation of a plurality of images. For example, when all of the basis weights of sheets on which images designated by a print job are to be formed are equal to or smaller than a threshold, the control unit  111  performs misregistration correction by forming a test pattern in a paper interval without suspending the print job. That is to say, while forming images to be transferred to sheets on the intermediate transfer belt  7 , the control unit  111  performs misregistration correction by forming a test pattern on the intermediate transfer belt  7  in a region between the images to be transferred to the sheets. Note that misregistration correction may be performed without suspending a print job if all of the basis weights of sheets to which the images satisfying the following condition are to be transferred are equal to or smaller than the threshold: image formation is performed on the photosensitive members  1  after the control unit  111  has determined that misregistration correction is necessary. On the other hand, if any of the basis weights of sheets on which images designated by a print job are to be formed is larger than the threshold, the control unit  111  performs misregistration correction while the print job is suspended. Specifically, after images to be transferred to a sheet with a basis weight larger than the threshold are formed on the intermediate transfer belt  7 , the print job is suspended and a test pattern is formed. Then, misregistration correction is performed based on the detection result of the test pattern. After completion of misregistration correction, the control unit  111  restarts the print job. The foregoing configuration maintains the precision of misregistration correction when a sheet enters the nip region  35 . Although the threshold is hereinafter set to 128 g/m 3 , the threshold is not limited to such a particular value. More specifically, the threshold can be determined based on a basis weight that starts to influence misregistration correction due to fluctuations in the moving speed of the surface of the intermediate transfer belt  7  caused by the entry of a sheet into the nip region  35 . 
       FIGS. 8A and 8B  are timing charts showing the operations of the exposure units  3  for each color in misregistration correction according to the present embodiment. Note that  FIG. 8A  depicts a case in which the basis weights of sheets designated by a print job issued by a user&#39;s instruction are equal to or smaller than 128 g/m 3 , whereas  FIG. 8B  depicts a case in which at least one of the basis weights of sheets designated by a print job is larger than 128 g/m 3 . Here, the relationship between the types and basis weights of the sheets is stored in advance as the attributed information of the sheets in the storage unit  112 . In  FIGS. 8A and 8B , high-level periods are periods in which the photosensitive members  1  are exposed to light to form electrostatic latent images thereon. Furthermore, among the high-level periods, periods indicated by solid lines are periods in which exposure is performed to form images to be transferred to the sheets, whereas periods indicated by dot lines are periods in which exposure is performed to form test patterns. Upon the start of image formation, each exposure unit  3  starts to expose the corresponding photosensitive member  1  to light. Note that in the present embodiment, as shown in  FIG. 1 , the photosensitive members  1  for yellow, cyan, magenta, and black are arranged in this order, with the photosensitive member  1  for yellow being most upstream in the direction of movement of the intermediate transfer belt  7 . Therefore, the timings at which the exposure units  3  for yellow and cyan expose the corresponding photosensitive members  1  to light differ from each other by a pitch period. The pitch period is obtained by dividing a distance L between two neighboring photosensitive members  1  in  FIG. 1  by a process speed, that is to say, the moving speed Vb of the surface of the intermediate transfer belt  7 . 
     If a controller determines that misregistration correction is necessary on satisfaction of predetermined conditions, in the case of  FIG. 8A , a test pattern for misregistration correction is formed in a paper interval without suspending the print job. On the other hand, in the case of  FIG. 8B  in which at least one of the basis weights of the sheets to be printed is larger than 128 g/m 3 , a test pattern is formed while the print job is temporarily suspended. This test pattern may be the same as or different from the test pattern that is formed in a paper interval without suspending the print job. If the former is different from the latter, the former may be longer than the latter in the direction of movement of the surface of the intermediate transfer belt  7 . Such a longer test pattern can improve the precision of misregistration correction. In the case of  FIG. 8B , upon completion of misregistration correction, the print job, that is to say, formation of images to be transferred to the sheets, is restarted. 
       FIG. 9  is a flowchart of processing executed by the control unit  111  in the present embodiment. The control unit  111  starts the processing of  FIG. 9  if it determines that misregistration correction is necessary during image formation based on a print job. Note that the control unit  111  determines whether misregistration correction is necessary based on whether the states of the image forming apparatus satisfy predetermined conditions. More specifically, the image forming apparatus starts misregistration correction when, for example, the following elements have reached predetermined values: an amount of change in the internal temperature since the last misregistration correction; the number of printed sheets or an elapsed period since the start of image formation based on the print job; and the cumulative number of printed sheets or a cumulative elapsed period since power ON. 
     First, in step S 10 , the control unit  111  determines whether at least one of the basis weights of sheets to be printed based on the print job is larger than the threshold. If none of the basis weights is larger than the threshold, misregistration correction that utilizes paper intervals is performed in step S 14 . That is to say, misregistration correction is performed without suspending the print job. On the other hand, if the print job includes printing on at least one sheet with a basis weight larger than the threshold, the control unit  111  suspends the print job, that is to say, temporarily suspends image formation processing, after forming images to be transferred to the sheet with the basis weight larger than the threshold in step S 11 . Then, the control unit  111  performs misregistration correction by forming a test pattern on the intermediate transfer belt  7  in step S 12 . After completion of misregistration correction, the control unit  111  restarts the print job, that is to say, formation of images to be formed on the sheets, in step S 13 . 
     Second Embodiment 
     A description is now given of a second embodiment, with a focus on differences from the first embodiment. In the present embodiment, the sensor  19  is configured to detect the basis weights of sheets. Based on the result of the detection by the sensor  19 , the control unit  111  determines whether to perform misregistration correction without suspending a print job, or to perform misregistration correction while the print job is suspended. 
       FIG. 10  is a timing chart showing the operations of the exposure units  3  for each color in misregistration correction according to the present embodiment. Note that this figure is illustrated in a manner similar to  FIG. 8A . Upon the start of image formation, each exposure unit  3  starts to expose the corresponding photosensitive member  1  to light. If the controller determines that misregistration correction is necessary on satisfaction of predetermined conditions, the control unit  111  determines whether it is necessary to suspend the print job based on the basis weight detected by the sensor  19 , specifically, the basis weight of a sheet that is to enter the nip region next. In  FIG. 10 , image formation is performed for two sheets after the start of misregistration correction. However, as the sensor  19  detects the basis weight larger than the threshold, image formation is suspended for the third sheet after the start of misregistration correction, and then misregistration correction is performed. Similarly to the first embodiment, the control unit  111  restarts the print job upon completion of misregistration correction. 
       FIG. 11  is a flowchart of processing executed by the control unit  111  in the present embodiment. The control unit  111  starts the processing of  FIG. 11  if it determines that misregistration correction is necessary during image formation based on a print job. Once the control unit  111  has started misregistration correction during the print job, it determines whether the sensor  19  has detected a sheet with a basis weight larger than the threshold in step S 20 . If such a sheet has not been detected, the control unit  111  performs misregistration correction by forming test patterns in paper intervals without suspending the print job in step S 24 . In this case, the control unit  111  determines whether misregistration correction has been completed in step S 25 , and repeats the processing from step S 20  if misregistration correction has not been completed. In misregistration correction that forms test patterns in paper intervals, image forming positions are corrected by averaging the detection results of test patterns formed in at least two paper intervals. Therefore, the control unit  111  does not determine that misregistration correction has been completed until the detection unit  100  detects test patterns formed in at least two paper intervals. If misregistration correction is completed before the sensor  19  detects a sheet with a basis weight larger than the threshold, the control unit  111  ends the processing. On the other hand, the control unit  111  suspends the print job in step S 21  if a sheet with a basis weight larger than the threshold is detected in step S 20  after the start of misregistration correction, or if a basis weight larger than the threshold is detected in step S 20  before completion of misregistration correction that utilizes test patterns formed in paper intervals. Then, the control unit  111  performs misregistration correction by forming a test pattern on the intermediate transfer belt  7  in step S 22 , and restarts the print job in step S 23  upon completion of misregistration correction. 
     The above-described embodiments use a basis weight as a criterion to determine whether to perform misregistration correction while a print job is suspended, or to perform misregistration correction without suspending the print job. It is also possible to use attribute information that is related to an arbitrary sheet and enables determination of whether the rotation of the intermediate transfer belt  7  is influenced by the entry of a sheet into the nip region  35 . Specifically, information indicating the basis weights, thicknesses, and stiffnesses of sheets can be used as attribute information. 
     Other Embodiments 
     Embodiments of the present invention can also be realized by a computer of a system or apparatus that reads out and executes computer executable instructions (e.g., one or more programs) recorded on a storage medium (which may also be referred to more fully as a ‘non-transitory computer-readable storage medium’) to perform the functions of one or more of the above-described embodiments and/or that includes one or more circuits (e.g., application specific integrated circuit (ASIC)) for performing the functions of one or more of the above-described embodiments, and by a method performed by the computer of the system or apparatus by, for example, reading out and executing the computer executable instructions from the storage medium to perform the functions of one or more of the above-described embodiments and/or controlling the one or more circuits to perform the functions of one or more of the above-described embodiments. The computer may comprise one or more processors (e.g., central processing unit (CPU), micro processing unit (MPU)) and may include a network of separate computers or separate processors to read out and execute the computer executable instructions. The computer executable instructions may be provided to the computer, for example, from a network or the storage medium. The storage medium may include, for example, one or more of a hard disk, a random-access memory (RAM), a read only memory (ROM), a storage of distributed computing systems, an optical disk (such as a compact disc (CD), digital versatile disc (DVD), or Blu-ray Disc (BD)™), a flash memory device, a memory card, and the like. 
     While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions. 
     This application claims the benefit of Japanese Patent Application No. 2015-153517, filed on Aug. 3, 2015, which is hereby incorporated by reference herein in its entirety.