Patent Publication Number: US-10308458-B2

Title: Image forming apparatus

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to an image forming apparatus which performs image formation while continuously conveying sheets, such as a copying machine and a printer. 
     Description of the Related Art 
     Conventionally, in an image forming apparatus such as a copying machine and a laser beam printer, the recording position of an image with respect to a recording sheet sometimes deviates due to variations in sheet conveying accuracy, positional accuracy of a toner image to be recorded and so on. Therefore, various techniques have been proposed regarding the registration of the toner image with respect to the sheet. 
     For example, the technique is proposed by Japanese Patent Laid-Open No. S60-120369 in which the registration is carried out in the configuration in which a sheet detection sensor is disposed in the sheet conveying path, and once a sheet reaches the detection sensor, the pair of registration rollers is temporarily stopped, and the pair of registration rollers is driven for conveying again taking into consideration the arrival timing of the formed toner image. 
     In addition, the technique has been proposed by Japanese Patent Application Laid-open No. H03-36559 in which an image is formed with the timing based on a detection signal from a sheet detection sensor, and the formed toner image is transferred to a sheet. 
     In these techniques, it is common to design the apparatus such that the feeding control of sheets from a sheet stacking portion at the timing as early as possible and a large margin of delay for sheet conveying from the sheet stacking section to the pair of registration rollers is allowed. This is because the sheet jam is suppressed by presuming a reduction in conveying efficiency caused by roller slip due to abrasion or paper dust adhesion of rollers up to the registration roller pair and separation failure due to the sheets on the sheet stacking portion sticking to each other. 
     However, when the control for starting the sheet feeding at the timing as early as possible is performed, if the image formation interval becomes large by for example adjustment control for stabilizing the color tone, the sheet conveyed to the pair of registration roller has to wait there for a time longer than usual. In such a case, special paper such as coated paper has a possibility that the surface of the sheet is deformed due to long-time roller gripping and image defects occurs due to transfer failure of the deformed portion. Similarly, when a standby state continues for a long time with a sheet being in contact with the registration roller, there is a possibility that a loop shape is made and remains at a tip portion of the sheet thereby causing a transfer failure. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide an image forming apparatus capable of suppressing image defects even when an image formation interval becomes large in a case where special paper such as coated paper is used. 
     An image forming apparatus according to the present invention, comprising: 
     a feeding portion configured to feed a sheet; 
     an image forming portion configured to form an image on the sheet fed by the feeding portion; 
     a conveying portion configured to stop the sheet fed by the feeding portion at a predetermined position and to convey the sheet to the image forming portion in accordance with timing of an image formation by the image forming portion; 
     a type discriminating portion configured to discriminate a type of the sheet to be fed; and 
     a controlling portion configured to change a sheet feeding start timing of the feeding portion in accordance with the type of the sheet discriminated by the type discriminating portion such that a time period in which the sheet is being stopped by the conveying portion is changed. 
     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  is a diagram showing a cross-sectional view of an image forming apparatus body according to an embodiment of the present invention. 
         FIG. 2A  is a cross-sectional view of a vicinity of a feeding tray in the embodiment of the present invention, and  FIG. 2B  is a plan view of a vicinity of a feeding tray in the embodiment of the present invention. 
         FIG. 3  is a control block diagram according to the embodiment of the present invention. 
         FIGS. 4A to 4E  are diagrams of an operation portion of the embodiment of the present invention. 
         FIG. 5  is a table showing the relationship between sheet types and feeding types. 
         FIG. 6  is a timing chart for feeding by feeding type A. 
         FIG. 7  is a timing chart for feeding by feeding type A with image adjustment. 
         FIG. 8  is a diagram showing an image defect. 
         FIG. 9  is a timing chart for feeding by feeding type B. 
         FIG. 10  is a flowchart showing an operation for triggering feeding control. 
         FIG. 11  is a flowchart showing an operation for triggering conveying control. 
         FIG. 12  is a timing chart of an embodiment in which the feeding timing is changed when image adjustment is performed. 
         FIG. 13  is a flowchart for feeding in a case where image adjustment is performed. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Next, embodiments of an image forming apparatus according to the present invention will be explained with reference to the drawings. 
     {First Embodiment} &lt;Overall configuration of the image forming apparatus&gt; First of all, an overall configuration of an image forming apparatus as well as the operation thereof will be briefly explained.  FIG. 1  is a cross-sectional view of an image forming apparatus according to the present embodiment. The sheets S set in the sheet cassettes  1   a  and  1   b  mounted on the lower portion of the main body of the image forming apparatus shown in  FIG. 1  or on the feeding tray  2  are fed to the image forming portion  3  in synchronization with the image forming operation where the formed image is recorded on a conveyed sheet. 
     The image forming portion  3  of the present embodiment employs an electro-photographic system in which toner images of respective colors of yellow, magenta, cyan, and black are sequentially formed on four photosensitive drums  3   a  arranged side by side, and the toner images are transferred and superposed on the intermediate transfer belt  3   b  so as to form a color image, and the color image is transferred to the conveyed sheet. The sheet on which the toner image has been transferred is conveyed upward, heated and pressed by the fixing portion  4  thereby fixing the toner image thereon, and then discharged to the discharge portion  5 . 
     The image reading portion  6  is arranged on the upper part of the apparatus. The image reading portion  6  sequentially feeds an original document sheet set in the original document tray  6   a , optically reads the original document sheet with the scanner portion  6   b , converts the read information into a digital signal, and sends the digital signal to the image forming portion  3 . 
     (Sheet feeding apparatus) Next, a sheet feeding configuration for feeding a sheet to the image forming portion  3  will be explained.  FIGS. 2A and 2B  are explanatory diagrams of the sheet feeding apparatus.  FIG. 2A  is a cross-sectional view of the sheet feeding apparatus and  FIG. 2B  is a plan view of the sheet feeding apparatus. As shown in  FIG. 2 , when the sheet S is placed on the feeding tray  2  and the sheet position is regulated with the side regulating member  10 , the sheet presence/absence sensor  9  detects the sheet. When the sheet feeding signal is created, the feeding motor  11  is driven to rotate the pickup roller  12  and the feeding roller  13  thereby feeding the sheet. The fed sheet is brought in contact with the nip portion of the registration roller  14  (see  FIG. 1 ) whose rotation is stopped and a loop is formed thereby correcting skew feeding of the sheet. 
     The registration roller  14  is a registration member and constitutes a conveying portion. The fed sheet is temporarily stopped at a predetermined position (the nip portion of the roller in the present embodiment) to stand by and the registration roller  14  is driven and rotated in synchronization with the timing of imaging of the toner image by the image forming portion  3  thereby conveying the sheet S again. As a result, the sheet is fed to the secondary transfer portion  15  in synchronization with the toner image formed by the image forming portion  3  and the toner image is transferred to a proper position of the sheet. 
     In the above explanation, as the configuration of the sheet feeding apparatus, the sheet feeding configuration from the feeding tray  2  is explained, however, the sheet feeding configuration from the sheet cassettes  1   a  and  1   b  is the same as that explained above. 
     (Controlling portion)  FIG. 3  is a block diagram showing a configuration of a controlling portion which controls the driving of the image forming apparatus. The control portion  300  performs a system control of the image forming apparatus and has the CPU  301  which performs the system control of the image forming apparatus including a sheet feeding control, the ROM  302  on which a control program is written, the RAM  303  which stores data and the timer  291  which performs time count. 
     The CPU  301  performs the original document feeding control of the image reading portion  6  via the original document feeding control portion  480 . Further, the CPU  301  reads the image of the fed original document with the image sensor via the image reader control portion  280 . 
     In the copy operation, the image signal control portion  281  processes a signal from the image sensor or the external computer  283  and outputs the processed signal to the printer control portion  285  via the external interface  282 . The printer control portion  285  instructs the image forming portion  3  to form an image based on an instruction from the CPU  301 . The image forming portion  3  drives the photosensitive drums  3   a  and so on based on the input video signal. 
     Also, based on an instruction from the CPU  301 , the sheet feeding and the conveying control are performed for the sheet conveying portion  270  which controls driving of the pickup roller  12 , the registration roller  14  and so on, which constitute the sheet feeding apparatus. 
     The operation portion  330  is used for selecting a color mode of forming an image, for displaying the state of the image forming apparatus, and for instructing a start of a copy and so on. Further, when the CPU  301  detects the sheet set on the feeding tray  2 , a sheet size selection screen is displayed. The mode setting selected here is stored in the RAM  303 . 
     &lt;Operation portion&gt; Next, the operation portion  330  will be explained.  FIG. 4A  is a front view of the operation portion  330 . The start key  306  for starting a copy operation, the stop key  307  for stopping the copy operation and the numeric keys  313  for setting a number are arranged on the operation portion  330 . Further, the display portion  311  on which a touch panel is formed is arranged on the operation portion  330 . Software keys can be created on the screen of the display portion  311 . Further, the CPU  301  performs print preparatory operation control in response to an operation on the numeric keys  313  for setting a number or the like. 
       FIGS. 4B and 4C  are a sheet size selection screen displayed on the display unit  311  when the sheet S is set on the feeding tray  2 . As shown in these figures, the A 4  button  321 , the A 4 R button  322 , the A 3  button  323  and the size OK button  325  are arranged. By pressing the size OK buttons  325  with any other button being selected, the sheet size is determined and the selected sheet size is stored in the RAM  303 . 
       FIG. 4B  shows a screen displayed when the CPU  301  detects that the sheet size set on the feeding tray  2  is A 4  or A 3  based on the position of the side regulating plate  421  explained above. In this case, as shown in  FIG. 4B , the A 4  button  321  and the A 3  button  323  can be selected, and the A 4 R button  322  is grayed out to be in a state in which the A 4 R button  322  is not selectable. On the other hand,  FIG. 4C  is a screen displayed when the CPU  301  detects that the sheet size set on the feeding tray  2  is A 4 R based on the position of the side regulating plate  421  explained above. In this case, as shown in  FIG. 4C , the A 4 R button  322  can be selected, and the A 4  button  321  and the A 3  button  323  are grayed out to be in a state in which the A 4  button  321  and the A 3  button  323  are not selectable. 
     After the sheet size has been determined, when the CPU  301  detects no sheet based on the state of the sheet presence/absence sensor  9 , the sheet size information is made uncertain and this information is also stored in the RAM  303 . Thereafter, when the sheet S is set, the sheet size selection screen is displayed again. The printing operation cannot be started until the sheet size is determined. 
       FIGS. 4D and 4E  are a screen for selecting the type of sheets set in the image forming apparatus. When the sheet type selection screen  340  arranged on the display portion  311  is pressed, the feeding stage selection screen ( FIG. 4D ) is displayed. By pressing any of the feeding stage  1  button  341 , the feeding stage  2  button  342  and the manual feeding button  343  and by pressing the feeding stage OK button  345 , the sheet type selection screen ( FIG. 4E ) for selecting the sheet type is displayed. 
     On the sheet type selection screen, the thin paper button  331 , the plain paper button  332 , the thick paper button  333 , the coated paper button  334 , the recycled paper button  335 , the carbon paper button  336 , the carbonless paper button  337 , the OHP button  338  and the type OK button  339  are arranged. By selecting one of the buttons  331  to  338  and then pressing the type OK button  339 , the preselected feeding stage and sheet type are determined and this information is stored in the RAM  303 . In the present embodiment, the type of sheet of each feeding stage is previously selected. However, the apparatus may be configured such that after the size OK button  325  is pressed, for example, the sheet type selection screen is displayed and the sheet type may be chosen on the sheet type selection screen. Further, the default sheet type of the present embodiment is set as plain paper. 
     In the present embodiment, the type of the sheet to be fed is determined, and the feeding timing is changed according to the type of the sheet. Further, in the present embodiment, the type determining portion determines the type of sheet based on the designation from the operation portion  330 . 
     In the present embodiment, two sheet type groups for which the feeding start timing is changed are provided. The two sheet type groups consist of a first sheet type group including at least plain paper and a second sheet type group including at least coated paper. On the surface of a sheet whose type belongs to the first sheet type group including plain paper, deformation or the like hardly occurs on even when the sheet waits for a predetermined time at the position of the registration roller  14 . Further, on the surface of a sheet whose type belongs to the second sheet type group including coated paper, deformation or the like easily occurs when the sheet waits for a predetermined time at the position of the registration roller  14 . 
     In the present embodiment, as shown in  FIG. 5 , among the types of sheets set on the selection screen of  FIG. 4E , thin paper, plain paper, thick paper, recycled paper belong to the first sheet type group. Sheet feeding with the timing for feeding a sheet whose type belongs to the first sheet type group is referred to as feeding type A. On the other hand, coated paper, carbon paper, carbonless paper and OHP belong to the second sheet type group. Sheet feeding with the timing for feeding a sheet whose type belongs to the second sheet type group is referred to as feeding type B. 
     &lt;Feeding timing of a sheet whose type belongs to the first sheet type group&gt; As explained above, in the present embodiment, the feeding timing performed in feeding type A when a sheet whose type belongs to the first sheet type group is different from the feeding timing performed in feeding type B when a sheet whose type belongs to the second sheet type group. Next, the feeding type A and the feeding type B will be explained in detail. 
     First, the feeding type A for feeding a sheet which belongs to the first sheet type group will be explained.  FIG. 6  is a sheet feeding timing chart of the feeding type A. The vertical axis represents the timing of image formation in the image forming portion  3 , the timing of the image formation and the sheet in the secondary transfer portion  15  and the sheet conveying timings of the page (n−1), the page (n) and the page (n+1) (n is an integer). 
     First, the image forming timing in the image forming portion  3  will be explained. At timing T 11 , the image formation of the page (n−1) is started, and image formation of the page (n) is started at timing T 14  while maintaining a predetermined image formation interval. Similarly, the image formation of the page (n+1) is performed at the timing T 18 . 
     In the secondary transfer portion  15 , the toner images formed by the image forming portion  3  are matched with the conveyed sheets and secondarily transformed on the conveyed sheets at the timing T 15  (page (n−1)), the timing T 20  (page (n)) and the timing T 22  (page (n+1)), respectively. 
     The feeding control is performed for the page (n−1) from the timing T 00 . Specifically, the timing T 00  is determined by calculation from the timing T 11  and the timing T 13  so as to be the same as the timing of the following page. 
     In the sheet feeding control, a sheet from the feeding tray  2  is fed and the sheet is conveyed to the registration roller  14 . In the present embodiment, when the sheet is conveyed to the registration roller  14 , the sheet temporarily stops (waits) there with the leading edge of the sheet being in contact with the nip portion of the registration roller  14  thereby forming a loop. Then, the time t_r is determined based on the image formation start timing T 11  of the current page so as to match the timing T 15  at which the toner image of the page (n−1) reaches the secondary transfer portion  15 . At the timing T 13  at which the time t_r has elapsed from the image formation start timing T 11 , the sheet conveying control is restarted and at the timing T 15 , the sheet and the image are matched at the secondary transfer portion  15 . As a result, the page (n−1) waits for the time t_w 1  from the end of the feeding control to the restart of the conveying control. In the conveying control according to the present embodiment, a control is made for the conveying of the sheet from the registration roller  14  to the secondary transfer portion  15 . 
     The page (n) waits for the time t_s from the image formation start timing T 11  of the previous page (n−1) and the feeding control is started at the timing T 12 . This means that the feeding control of the target page starts from the image formation start timing of the previous page of the target page. Similarly to the previous page (n−1), the conveying control is restarted at the timing T 17  at which the time t_r has elapsed from the image formation start timing T 14  of the current sheet and at the timing T 20 , the toner image and the image are matched at the secondary transfer portion  15 . As a result, the page (n) also waits for the time t_w 1  from the end of the feeding control to the restart of the conveying control. 
     The feeding control for the page (n+1) is started at the timing T 16  at which the page (n+1) has waited for the time t_s from the image formation start timing T 14  of the previous page (n). Similarly to the other pages, the conveying control is restarted at the timing T 21  at which the time t_r has elapsed from the image formation start timing T 18  of the current sheet and at the timing T 22 , the toner image and the image are matched at the secondary transfer portion  15 . As a result, the page (n+1) also waits for the time t_w 1  from the end of the feeding control to the restart of the conveying control. 
     As explained above, in the case of the feeding type A which feeds a sheet whose type belongs to the first sheet type group, the sheet is fed from the feeding tray  2  prior to the image formation start timing. In this manner, by performing the sheet feeding control from the feeding tray  2  at an earlier timing, it is possible to perform retrying a plurality of times for a feeding error from the feeding tray  2 , so that the apparatus is stopped less frequently due to feeding errors. 
     Next, the sheet feeding timing in the case where the image adjustment is performed while the images of a plurality of pages are continuously formed will be explained. In the image adjustment, for example, adjustment control for stabilizing the color taste is performed and the image formation is stopped during that time. Therefore, when image adjustment is not performed, the image formation is continuously performed at a predetermined interval, whereas when image adjustment is performed, the image formation interval becomes longer than the predetermined interval. 
       FIG. 7  is a timing chart of the feeding type A in the case where the image adjustment is performed when an image is formed on a sheet whose type belongs to the first sheet type group. 
     Similarly to  FIG. 6 , the vertical axis represents the timing in the image forming portion  3 , the timing in the secondary transfer portion  15  and the sheet conveying timings for the page (n−1), the page (n) and the page (n+1). Further, the image adjustment is performed between the page (n) and the page (n+1). 
     At timing T 31 , the image forming portion  3  starts the image formation of the page (n−1), and starts the image formation of the page (n) at the timing T 34  while maintaining a predetermined image formation interval. Then, the image adjustment is performed from the timing T 38  at which image formation of the page (n) has ended. Thereafter, the image formation on the page (n+1) is performed from the timing T 41  at which the image adjustment is completed. 
     In the secondary transfer portion  15 , the toner images formed by the image forming portion  3  are matched with the conveyed sheets and secondarily transformed on the conveyed sheets at the timing T 35  (page (n−1)), the timing T 40  (page (n)) and the timing T 43  (page (n+1)), respectively. 
     The feeding for the sheet of page (n−1) is performed at the timing T 30 . Specifically, the timing T 30  is determined by calculation based at the timing T 31  or the timing T 33  so as to be the same as the timing of the following page. The time t_r is determined based the image formation start timing T 31  of the current page so as to match the timing T 35  at which the toner image of the page (n−1) reaches the secondary transfer portion  15 . At the timing T 33  at which the time t_r has elapsed from the image formation start timing T 31 , the sheet conveying control is restarted and at the timing T 35 , the sheet and the image are matched at the secondary transfer portion  15 . As a result, the page (n−1) waits for the time t_w 1  from the end of the feeding control to the restart of the conveying control. 
     The sheet of the page (n) waits for the time t_s from the image formation start timing T 31  of the previous page (n−1) and the feeding control is started at the timing T 32 . This means that the feeding control of the target page starts from the image formation start timing of the previous page of the target page. Similarly to the previous page (n−1), the conveying control is restarted at the timing T 37  at which the time t_r has elapsed from the image formation start timing T 31  of the current sheet and at the timing T 40  the toner image and the image are matched at the secondary transfer portion  15 . As a result, the page (n) also waits for the time t_w 1  from the end of the feeding control to the restart of the conveying control. 
     The feeding control for the sheet of the page (n+1) is started at the timing T 36  at which the page (n+1) has waited for the time t_s from the image formation start timing T 34  of the previous page (n). The feeding control is completed at the timing T 39 . However, because the image adjustment is performed before the image formation of the page (n+1), the image formation of the page (n+1) is not immediately started. When the image formation of the page (n+1) is started at the timing T 41  at which the image adjustment has ended, the conveying control is restarted at the timing T 42  at which the time t_r has elapsed and at the timing T 43 , the toner image and the image are matched at the secondary transfer portion  15 . 
     The feeding control for the sheet of the page (n+1) after the image adjustment is performed as explained above is started at the image formation start timing of the page (n). However, the restart of the conveying control is performed at the timing T 42  and, as a result, the sheet of the page (n+1) waits for the time t_w 2 . Since the inequality t_w 1 &lt;t_w 2  holds, the sheet waits for a toner image for a longer time period at the registrationroller  14 . However, in the case of a sheet whose type belongs to the first sheet type group such as plain paper, the surface shape is not deformed even if it is pressed against the registration roller  14  for a little longer period, so that a transfer failure or the like does not occur at the secondary transfer portion  15 . 
     &lt;Feeding timing of a sheet whose type belongs to the second sheet type group&gt; When the type of a sheet to be fed belongs to the second sheet type group including coated paper and carbonless paper, there is a possibility that deformation or bending of the surface and so on may occur due to the sheet being pressed against the registration roller  14  for a long time. As a result, as shown in  FIG. 8 , there is a possibility that an image defect (arrow A in  FIG. 8 ) occurs at the secondary transfer portion  15  due to transfer failures. 
     In the present embodiment, it is exemplified that the sheet is stopped with the leading edge of the sheet being pressed to the nip portion of the registration roller  14 . However, the leading edge of the sheet may not be pressed to the nip portion roller and the sheet may stand by while being nipped between a pair of rollers including the registration roller  14 . That is, if the sheet is stopped for a long time with the sheet being nipped by the registration roller, there is a possibility that an image defect may occur due to a change in the surface property of the nipped portion of the sheet by the pressure of the roller pair. 
     Therefore, in the present embodiment, the sheet feeding start timing of the feeding type B for feeding a sheet whose type belongs to the second sheet type group is different from that of the feeding type A.  FIG. 9  is a sheet feeding timing chart of the feeding type B, illustrating the case where image adjustment is performed between the page (n) and the page (n+1). 
     Similarly to  FIGS. 6 and 7 , the vertical axis represents the timing in the image forming portion  3 , the timing in the secondary transfer portion  15  and the sheet conveying timings for the page (n−1), the page (n) and the page (n+1). 
     First, at timing T 50 , the image forming portion  3  starts the image formation of the page (n−1), and starts the image formation of the page (n) at the timing T 53  while maintaining a predetermined image formation interval. Subsequently, the image adjustment is performed from the timing T 57  and the image formation of the page (n+1) is performed from the timing T 59 . 
     In the secondary transfer portion  15 , the toner images formed by the image forming portion  3  are matched with the conveyed sheets and secondarily transformed on the conveyed sheets at the timing T 54  (page (n−1)), the timing T 58  (page (n)) and the timing T 62  (page (n+1)), respectively. 
     In the feeding type B, the sheet feeding control for the page (n−1) is performed from the image formation start timing T 50  of the page (n−1). In the feeding type B, unlike the feeding type A, the feeding control is performed with the image formation start timing of the current sheet as a trigger. Specifically, the feeding operation is started immediately after the image formation start timing of the current sheet. 
     Then, the time t_r is determined based on the image formation start timing T 50  of the current page so as to match the timing T 54  at which the toner image of the page (n−1) reaches the secondary transfer portion  15 . At the timing T 62  at which the time t_r has elapsed from the image formation start timing T 50 , the conveying control is restarted and at the timing T 54 , the sheet and the image are matched at the secondary transfer portion  15 . As a result, the page (n−1) waits for the time t_w 3  from the end of the feeding control to the restart of the conveying control. 
     In this case, it is exemplified that the feeding operation is started immediately after the image formation start timing of the current sheet. However, it suffices if the feeding operation is performed accompanying the start of the image formation of the current sheet. For example, the feeding operation may be started after a predetermined time period elapsed from the image formation start timing of the current sheet. Also, the feeding operation may be started simultaneously with the image formation start timing of the current sheet, or the feeding operation may be started immediately before the image formation start timing of the current sheet. 
     Similarly, the feeding control for the page (n) is started from the image formation start timing T 53  of the page (n). Next, the conveying control is restarted at the timing T 56  at which the time t_r has elapsed from the image formation start timing T 53  of the current sheet and at the timing T 58  the toner image and the image are matched at the secondary transfer portion  15 . As a result, the page (n) also waits for the time t_w 3  from the end of the feeding control to the restart of the conveying control. 
     Similarly, the feeding control for the page (n+1) is started on the image formation start timing T 59  of the current page. In this case, unlike the feeding type A, the feeding control is performed with the image formation start timing of the current sheet after the adjustment as a trigger. When the image formation for the page (n+1) is started at the timing T 59 , the conveying control is restarted at the timing T 61  at which the time t_r has elapsed and the toner image and at the timing T 62 , the image are matched at the secondary transfer portion  15 . As a result, the page (n+1) also waits for the time t_w 3  from the end of the feeding control to the restart of the conveying control. 
     As explained above, in the feeding type A for feeding a sheet whose type belongs to the first sheet type group, the sheet feeding start timing of the sheet is set with the image formation start of the previous page as a trigger and the sheet is fed from the feeding tray  2  at a first timing prior to the image formation start timing of the current sheet. On the other hand, in the feeding type B for feeding a sheet whose type belongs to the second sheet type group, the image formation start timing of the current sheet is used as a trigger and feeding is started at a second timing which is later than the first timing. 
     As explained above, by changing the sheet feeding timing between the case where the type of a sheet belongs to the first sheet type group and the case where the type of a sheet belongs to the second sheet type group, the inequality t_w 3 &lt;t_w 1 &lt;t_w 2  holds for the waiting time periods at the registration roller  14 . As a result, in the feeding type B which feeds a sheet whose type belongs to the second sheet type group, it is possible to reduce the waiting time as compared with the control of the feeding type A, and it is possible to prevent the image defect as shown in  FIG. 8 . On the other hand, it is possible to realize a stable sheet conveying control without reducing paper jam margin due to the delay in feeding control when a sheet fed in the feeding type A in which no image defect occurs. 
     &lt;Flowchart showing sheet feeding&gt; Here, with reference to  FIGS. 10 and 11 , a flowchart of the start trigger of the sheet feeding control and a flowchart of the start trigger of the conveying control will be explained. The operations shown in the flowcharts of  FIG. 10  and  FIG. 11  are performed each time image formation of every page is performed. 
       FIG. 10  is a flowchart showing a start trigger of the sheet feeding control. At the step S 900 , the image formation of the page (n) is started and the feeding type is confirmed which is set depending on whether the type of a sheet selected at the step S 901  belongs to the first sheet type group or the second sheet type group. When the sheet is fed in the feeding type A, the process advances to the step S 903  where the process waits for the time t_s and then the process advances to the step S 904 . At the step S 904 , it is confirmed whether the next sheet exists or not. When the next sheet exists in the step S 904 , the feeding control of the next page (n+1) is started at the step S 905 . On the other hand, if it is determined at the step S 901  that the feeding type is B, since the feeding control of the current sheet is not started from the operation from the step S 903 , the feeding control of the current sheet (n) is started at the step S 902 . 
     As explained above, it is possible to decide whether the feeding control is started at the image formation start timing of the previous sheet or the feeding control is started at the image formation start timing of the current sheet depending on the feeding type. 
       FIG. 11  is a flowchart for creating a start trigger for conveying control for conveying a sheet from the registration roller  14  to the secondary transfer portion  15 . The image formation of the page (n) is started at the step S 1000 . The process waits for the time t_r at the step S 1001 . The conveying control of the page (n) is started at the step S 1002 . Then the process ends at the step S 1003 . 
     As explained above, according to the present embodiment, it is possible to perform a stable sheet conveying while preventing image defects by switching waiting time at the registration roller depending on whether the sheet is of a type in which it is highly possible that image defects such as transfer failure at the secondary transfer portion occur or not. 
     {Second Embodiment} Next, a second embodiment of the present invention will be explained with reference to  FIGS. 12 and 13 . In the second embodiment, the sheet feeding timing in the feeding type B is different from that in the first embodiment, and the sheet feeding timing of the feeding type A is the same as that in the first embodiment. Therefore, only the sheet feeding timing in the feeding type B will be explained next. 
     In the first embodiment, in feeding a sheet whose type belongs to the second sheet type group, the sheet feeding timing is set with the image formation timing of the current sheet as a trigger irrespective of whether the image adjustment has occurred or not. However, in the present embodiment, when the image formation interval is equal to or less than the predetermined interval, the feeding is performed with the image formation timing of the previous sheet as a trigger, and when the image formation interval becomes longer than the predetermined interval due to image adjustment or the like, the feeding is performed with the image formation timing of the current sheet as a trigger. Whether image adjustment is to be performed or not is determined at the stage of image formation at least several pages before the image adjustment is performed. 
     Similarly to the other timing charts which are explained above, the vertical axis in  FIG. 12  represents the timing in the image forming portion  3 , the timing in the secondary transfer portion  15  and the sheet conveying timings for the page (n−1), the page (n) and the page (n+1). Further, the image adjustment is performed between the page (n) and the page (n+1). 
     First, the timing of image formation in the image forming portion  3  will be explained. At timing T 71 , the image formation of the page (n−1) is started and the image formation of the page (n) is started at the timing T 74  while maintaining a predetermined image formation interval. Then, the image adjustment is performed from the timing T 77 . The image formation of the page (n+1) is performed from the timing T 79 . 
     (Feeding timing without image adjustment) In the secondary transfer portion  15 , the toner images formed by the image forming portion  3  are matched with the conveyed sheets and secondarily transformed on the conveyed sheets at the timing T 75  (page (n−1)), the timing T 78  (page (n)) and the timing T 81  (page (n+1)), respectively. 
     The feeding for the sheet of the page (n−1) is performed at the timing T 70 . Specifically, the timing T 70  is determined by calculation based at the timing T 71  or the timing T 73  so as to be the same as the timing of the following page (not shown). The time t_r is determined based the image formation start timing T 71  of the current page so as to match the timing T 75  at which the toner image of the page (n−1) reaches the secondary transfer portion  15 . At the timing T 73  at which the time t_r has elapsed from the image formation start timing T 71 , the sheet conveying control is restarted and at the timing T 75 , the sheet and the image are matched at the secondary transfer portion  15 . As a result, the page (n−1) waits for the time t_w 4  from the end of the feeding control to the restart of the conveying control. 
     The page (n) waits for the time t_s from the image formation start timing T 71  of the previous page (n−1) and the feeding control is started at the timing T 72 . This means that the feeding control of the target page is started from the image formation start timing of the previous page of the target page. Similarly to the previous page (n−1), the conveying control is restarted at the timing T 76  at which the time t_r has elapsed from the image formation start timing T 71  of the current sheet and at the timing T 78 , the toner image and the image are matched at the secondary transfer portion  15 . As a result, the page (n) also waits for the time t_w 4  from the end of the feeding control to the restart of the conveying control. 
     (Feeding Timing with Image Adjustment) The image adjustment is performed between the page (n) and the page (n+1). When it is determined that the image adjustment is to be performed, the control portion performs the feeding control not at the timing at which the time t_s has elapsed from the image formation start timing T 74  of the previous page (n) as explained above, but at the image formation timing T 79  of the current sheet. 
     When the image formation of the page (n+1) is started at the timing T 79 , the conveying control is restarted at the timing T 80  at which the time t_r has elapsed, and at the timing T 81 , the toner image and the image are matched at the secondary transfer portion  15 . As a result, the page (n+1) waits for the time t_w 5  from the end of the feeding control to the restart of the conveying control. 
     As explained above, the feeding control is performed for the page (n+1) with the image formation timing of the current sheet as a trigger, the inequality t_w 5 &lt;t_w 4  holds. Thus, even if the image adjustment is performed between the pages, it is possible to prevent image defects as shown in  FIG. 8  because the time in which the conveying is stopped is short. 
       FIG. 13  is a flowchart showing the start trigger of the feeding control. At the step S 1300 , the image formation of the page (n) is started. First, at the step S 1308 , the feeding type is judged. When the feeding type is A, the process shifts to the step S 1304 , and when the feeding type is B, the process shifts to the step S 1301 . At the step S 1301 , it is determined whether the procedure the image forming portion  3  has just performed is the image formation or the image adjustment. When the previous procedure is the image adjustment, the process proceeds to the step S 1302 , and when the previous procedure is the normal image formation, the process proceeds to the step S 1303 . At the step S 1302 , the sheet feeding control of the current page (n) is started simultaneously with the image formation of the current page because the image forming is performed after the image adjustment. 
     At the step S 1303 , it is determined whether the image formation or the image adjustment is to be performed next. When the image adjustment is to be performed next, the feeding control for the page (n) is ended, when the image formation is to be performed next, the process proceeds to the step S 1304  where the process waits for the time t_s and determines whether there is a next sheet at the step S 1305 . When a next sheet exists, the process advances to the step S 1306  where the feeding control of the next page (n+1) is started. 
     As explained above, when the image adjustment is to be performed next, the process proceeds to the next step without performing the feeding control. 
     As explained above, according to the present embodiment, when the image forming interval is equal to or less than a predetermined time even in the feeding type B, the image formation timing of the previous page is used as a trigger and a sheet is fed from the feeding tray  2  at an early timing. As a result, it is possible to perform retrying a plurality of times for a feeding error, so that the apparatus is stopped less frequently due to feeding errors. Further, when the image formation interval is longer than a predetermined interval, it is possible to prevent image defects by shortening the conveying stop time. 
     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 modifications, equivalent structures and functions. 
     This application claims the benefit of Japanese Patent Application No. 2016-116132, filed Jun. 10, 2016, which is hereby incorporated by reference herein in its entirety.