Patent Publication Number: US-8985577-B2

Title: Sheet conveyor device and image forming apparatus

Description:
INCORPORATION BY REFERENCE 
     The present application claims priority under 35 U.S.C. §119 to Japanese Patent Application No. 2013-091148, filed Apr. 24, 2013. The contents of this application are incorporated herein by reference in their entirety. 
     BACKGROUND 
     The present disclosure relates to a sheet conveyor device to convey a sheet to an image forming section for image formation and an image forming apparatus including it. 
     For image forming apparatuses, such as printers, a sheet conveyor device is provided which conveys a sheet to an image forming section for image formation. Specifically, the sheet conveyor device includes a registration roller, an intermediate roller, a sheet detection section, etc. The registration roller conveys a sheet toward a transfer section. The intermediate roller conveys a sheet toward the registration roller. The sheet detection section detects a sheet between the intermediate roller and the registration roller. 
     Further, inclination (skew) of a sheet may be corrected by drive control on the registration roller and the intermediate roller in some sheet conveyor device. For example, when a predetermined period elapses from sheet detection by the sheet detection section after the intermediate roller conveys a sheet, while rotation of the registration roller is suspended, the registration roller starts being driven. This causes the sheet to be pushed against the registration roller under suspension, thereby forming a deflection in the sheet. Accordingly, inclination of the sheet is corrected. An optical sensor of reflection type that optically detects the presence/absence of a sheet may be used as the sheet detection section. 
     SUMMARY 
     A sheet conveyor device according to the first aspect of the present disclosure includes a registration roller, an intermediate roller, a first sheet detection section, a first drive controller, a second drive controller, and a third drive controller. The registration roller conveys a sheet toward an image forming section for image formation. The intermediate roller conveys the sheet toward the registration roller. The first sheet detection section is used for optically detecting the sheet between the registration roller and the intermediate roller. The first drive controller drives the intermediate roller, while suspending driving of the registration roller. The second drive controller drives the registration roller when a predetermined conveyance wait period elapses from detection of the sheet by the first sheet detection section. The third drive controller drives the registration roller when a predetermined forced conveyance period elapses from a start of conveyance of the sheet loaded on a sheet loading section. 
     A sheet conveyor device according to the second aspect of the present disclosure further includes a second sheet detection section and an abnormality detection section in the above sheet conveyor device. The second sheet detection section is used for detecting the sheet between the registration roller and the image forming section. The abnormality detection section detects abnormality in sheet conveyance where the second sheet detection section detects no sheet until a predetermined abnormality conveyance period elapses from the start of conveyance of the sheet loaded on the sheet loading section. 
     An image forming apparatus according to the third aspect of the present disclosure includes the sheet conveyor device according to the first aspect and the image forming section. 
     An image forming apparatus according to the fourth aspect of the present disclosure includes the sheet conveyor device according to the second aspect and the image forming section. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram showing a configuration of a multifunction peripheral according to one embodiment of the present disclosure. 
         FIG. 2  is a block diagram showing a system configuration of the multifunction peripheral according to one embodiment of the present disclosure. 
         FIG. 3A  is a flowchart depicting a process of sheet conveyance processing performed in the multifunction peripheral according to one embodiment of the present disclosure. 
         FIG. 3B  is a flowchart depicting a process of the sheet conveyance processing performed in the multifunction peripheral according to one embodiment of the present disclosure. 
         FIG. 4  is a timing chart showing an operation of the multifunction peripheral according to one embodiment of the present disclosure. 
         FIG. 5  is a timing chart showing an operation of the multifunction peripheral according to one embodiment of the present disclosure. 
         FIG. 6  is an illustration for explaining an operation of the multifunction peripheral according to one embodiment of the present disclosure. 
         FIG. 7  is an illustration showing another configuration of the multifunction peripheral according to one embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     With reference to the accompanying drawings, embodiments of the present disclosure will be described below to aid in understanding the present disclosure. It should be noted that the same numerals may be used for the same or corresponding elements in the drawings to omit duplicate description. The following embodiments are mere specific examples of the present disclosure and are therefore not intended to limit the technical scope of the present disclosure. 
     [Schematic Configuration of Multifunction Peripheral  10 ] 
     With reference to  FIG. 1 , the configuration of a multifunction peripheral  10  according to one embodiment of the present disclosure will be described first. 
     As shown in  FIG. 1 , the multifunction peripheral  10  is an image forming apparatus including an auto document feeder (ADF)  1 , an image reading section  2 , an electrographic image forming section  3 , a sheet feed cassette  4 , a control section  5 , an operation display section  6 , a sheet conveyance section  7 , etc. It is noted that the present disclosure is applicable also to any other image forming apparatuses, such as printers, facsimile machines, and copiers. Further note that the sheet conveyance section  7  and the control section  5  in the present embodiment correspond to a sheet conveyor device according to the present disclosure. 
     The ADF  1  includes a document setting section  11 , a conveyance roller  12   a , a plurality of conveyance roller pairs  12   b , an original cover  13 , a document discharging section  14 , etc. In the ADF  1 , when a motor (not shown) drives the conveyance roller  12   a  and the conveyance roller pairs  12   b , an original document loaded on the document setting section  11  is conveyed to the document discharging section  14 . At that time, the original document passes through an image data reading point  20  in the image reading section  2 . Thus, the image reading section  2  can read image date from the original document conveyed by the ADF  1 . 
     The image reading section  2  includes a document table  21 , a light source unit  22 , mirrors  23  and  24 , an optical lens  25 , a charge coupled device (CCD)  26 , etc. The document table  21  is a document loading section provided on top of the device main body. The light source unit  22  is capable of being moved in a sub scanning direction R 1  by a motor (not shown). Further, the light source unit  22  includes an LED light source  221  and a minor  222 . The LED light source  221  includes multiple white LEDs arranged in the main scanning direction (depth direction of the sheet of  FIG. 1 ). The LED light source  221  irradiates light to the surface of an original document on the document table  21 . The mirror  222  reflects the light reflected by the surface of the original document toward the minor  23 . Then, the mirror  24  guides the light reflected by the mirror  23  to the optical lens  25 . The optical lens  25  collects the incident light and allows it to enter the CCD  26 . The CCD  26  includes a photoelectric conversion element, etc. The CCD  26  inputs electric signals to the control section  5  according to the amount of received light entering from the optical lens  25  as image data of the original document. 
     The image forming section  3  forms an image on a sheet P on the basis of image data read by the image reading section  2  or image data input from an external information processing device, such as a personal computer. Specifically, the image forming section  3  includes a photosensitive drum  31 , a charger  32 , a laser scanning unit (LSU)  33 , a developing device  34 , a transfer roller  35 , a static eliminator  36 , a fixing roller  37 , a pressure roller  38 , etc. The image forming section  3  forms an image on a sheet P supplied from the sheet feed cassette  4  or the manual feed tray  76  in the following manner. It is noted that the sheet feed cassette  4  and the manual feed tray  76  are examples of a sheet loading section on which a sheet P is to be loaded. 
     First, the charger  32  uniformly charges the photosensitive drum  31  at a predetermined potential. Next, the LSU  33  irradiates light based on image data to the surface of the photosensitive drum  31 . This forms an electrostatic latent image corresponding to the image data on the surface of the photosensitive drum  31 . Then, the developing device  34  develops (visualizes) the electrostatic latent image on the photosensitive drum  31  as a toner image. The developing device  34  is replenished with toner (developer) from a toner container  34 A detachable from the image forming section  3 . Subsequently, the transfer roller  35  transfers the toner image formed on the photosensitive drum  31  to a sheet P. Then, the sheet P passes between the fixing roller  37  and the pressure roller  38 . At that time, the fixing roller  37  and the pressure roller  38  applies heat and pressure, respectively, to the toner image transferred to the sheet P for melting and fixing. The static eliminator  36  removes the charge on the photosensitive drum  31 . It is noted that the image forming section  3  is not limited to the electrographic image forming section and may be an image forming section of inkjet type, for example. 
     The control section  5  is a computer including control devices, such as a CPU, a ROM, a RAM, an EEPROM, etc. The CPU is a processor that executes various types of arithmetic processing. The ROM is a volatile storage section. The ROM stores in advance information, such as control programs for allowing the CPU to execute the various types of arithmetic processing. The RAM is a volatile storage section. The EEPROM is a nonvolatile storage section. The RAM and the EEPROM are used as temporary storage memories (work areas) for the various types of arithmetic processing that the CPU executes. 
     The control section  5  totally controls the multifunction peripheral  10  by executing various types of control programs stored in advance in the ROM with the use of the CPU. The control section  5  may be an electronic circuit, such as an integrated circuit of ASIC, DSP, or the like. Alternatively, the control section  5  may have a configuration including a main controller to totally control the multifunction peripheral  10  and a separate engine controller to control the image forming section  3  and the like. 
     The ROM or the EEPROM stores in advance a control program to allow the CPU to execute sheet conveyance processing. The sheet conveyance processing will be described later with reference to  FIGS. 3A and 3B . The control program may be read from a computer readable recording medium, such as a CD, DVD, flash memory, etc. and be installed in the EEPROM or a storage, such a hard disk (not shown). It may be considered that the present disclosure is directed to a method for executing various processes for the sheet conveyance processing in the multifunction peripheral  10 , a program to allow the control section  5  to execute the various processes for the sheet conveyance processing, or a computer readable recording medium that records the program. 
     The operation display section  6  includes a display section, such as a liquid crystal display, and an operation section, such as a hard key or a touch panel. The display section displays various information according to control instructions from the control section  5 . The operation section inputs various information to the control section  5  according to user&#39;s operation. 
     The sheet conveyance section  7  includes a sheet feed roller  71 , a conveyance roller pair  72 , an intermediate roller pair  73 , a registration roller pair  74 , a sheet detection section  75 , a manual feed tray  76 , a manual feed roller  77 , and a drive section  78  (see  FIG. 2 ). 
     The sheet feed roller  71  takes out sheets P from the sheet feed cassette  4  on sheet-by-sheet basis. The conveyance roller pair  72  rotates while sandwiching a sheet P taken out by the sheet feed roller  71 . This conveys the sheet P toward the intermediate roller pair  73 . 
     On the manual feed tray  76 , a sheet P may be loaded as a target for image formation in the image forming section  3 . The manual feed tray  76  is used particularly for loading a sheet of specific type, such as a viewgraph. A sheet P loaded on the manual feed tray  76  is conveyed generally straight in the interior of the multifunction peripheral  10 . The manual feed roller  77  conveys the sheet P loaded on the manual feed tray  76  toward the intermediate roller pair  73 . 
     The intermediate roller pair  73  rotates while sandwiching a sheet P conveyed from the conveyance roller pair  72  or the manual feed roller  77 . Thus, the sheet P is conveyed toward the registration roller pair  74 . 
     The registration roller pair  74  conveys a sheet P conveyed from the intermediate roller pair  73  toward the photosensitive drum  31 . Further, the registration roller pair  74  is used for synchronization control between timing of image formation by the image forming section  3  and timing of sheet conveyance, for correction of sheet inclination (skew), etc. 
     The sheet detection section  75  is arranged between the registration roller pair  74  and the intermediate roller pair  73 . The sheet detection section  75  optically detects a sheet P conveyed from the intermediate roller pair  73  toward the registration roller pair  74 . The sheet detection section  75  corresponds to a first sheet detection section. 
     Specifically, the sheet detection section  75  is a reflective type optical sensor. The reflective type optical sensor includes a light emitting section and a light receiving section. The light emitting section irradiates light toward a sheet conveyance path between the registration roller pair  74  and the intermediate roller pair  73 . The light receiving section receives light reflected by a sheet P. Upon detection of light, of which light amount is equal to or larger than a predetermined amount, by the light receiving section, the sheet P is detected. It is noted that the sheet detection section  75  may be an optical sensor of light shielding type that includes a light emitting section and a light receiving section, which are arranged to face each other. In this case, when a sheet P shields the light irradiated from the light emitting section to the light receiving section, the sheet P is detected. 
     Signals from the sheet detection section  75  are input to the control section  5 . Thus, the control section  5  can control timing of driving the registration roller pair  74 , timing of image formation by the image forming section  3 , etc. on the basis of timing of sheet detection by the sheet detection section  75 . 
     The drive section  78  will be described next with reference to  FIG. 2 . The drive section  78  includes a motor  81 , a sheet feed clutch  82 , a conveyance clutch  83 , an intermediate clutch  84 , a registration clutch  85 , and a manual feed clutch  86 . The control section  5  controls the motor  81 , the sheet feed clutch  82 , the conveyance clutch  83 , the intermediate clutch  84 , the registration clutch  85 , and the manual feed clutch  86 . 
     The motor  81  drives the sheet feed roller  71 , the conveyance roller pair  72 , the intermediate roller pair  73 , the registration roller pair  74 , and the manual feed roller  77  through a gear (not shown) and the like. The motor  81  may be a DC motor, for example. 
     The sheet feed clutch  82  is an electromagnetic clutch to switch transmission of the drive force from the motor  81  to the sheet feed roller  71  between on and off states. Similarly, the conveyance clutch  83  and the intermediate clutch  84  are electromagnetic clutches to switch transmission of the drive force from the motor  81  to the conveyance roller pair  72  and the intermediate roller pair  73  between on and off states, respectively. The registration clutch  85  and the manual feed clutch  86  are electromagnetic clutches to switch transmission of the drive force from the motor  81  to the registration roller pair  74  and the manual feed roller  77  between on and off states, respectively. It is noted that although the present embodiment describes a configuration in which the electromagnetic clutches switch transmission of the drive force from the motor  81  to the respective rollers between on and off states as one example, dedicated stepping motors may drive the respective rollers independently. 
     Further, as shown in  FIG. 2 , the control section  5  includes a first drive controller  51 , a second drive controller  52 , a third drive controller  53 , and a sheet type setting section  54 . Specifically, the control section  5  executes a control program stored in the ROM with the use of the CPU to function as the first drive controller  51 , the second drive controller  52 , the third drive controller  53 , and the sheet type setting section  54 . The first drive controller  51  drives the intermediate roller pair  73 , while suspending the rotation of the registration roller pair  74 . The second drive controller  52  drives the registration roller pair  74  when a predetermined conveyance wait period elapses from sheet detection by the sheet detection section  75 . The third drive controller  53  drives the registration roller pair  74  when a predetermined forced conveyance period elapses from a start of conveyance of a sheet P loaded on the sheet feed cassette  4  or the manual feed tray  76 . The sheet type setting section  54  receives an operation of setting a sheet type of a to-be-conveyed sheet P. The operation of setting a sheet type is performed through user&#39;s input to the operation display section  6 . 
     [Sheet Conveyance Processing] 
     One example of a process of the sheet conveyance processing that the control section  5  executes will be described below with reference to the flowcharts of  FIGS. 3A and 3B . It is noted that steps S 1 , S 2 , . . . each denote the number of a process operation (step) that the control section  5  executes. The sheet conveyance processing is executed by the control section  5  when image formation processing is performed in the multifunction peripheral  10 , such as printing, coping, or faxing. One example will be described herein in which sheet supply from the manual feed tray  76  is set in advance in the control section  5  in accordance with user&#39;s manipulation performed on the operation display section  6 , and then, the image formation processing is performed. 
     &lt;Step S 1 &gt; 
     First, at a step S 1 , the control section  5  causes conveyance of a sheet P from the manual feed tray  76  to the registration roller pair  74 , while suspending rotation of the registration roller pair  74 . The first drive controller  51  executes the operation of the step S 1 . Specifically, the control section  5  turns off the registration clutch  85 , turns on the intermediate clutch  84  and the manual feed clutch  86 , and drives the motor  81 . Thus, the manual feed roller  77  and the intermediate roller pair  73  convey the sheet P loaded on the manual feed tray  76  toward the registration roller pair  74 . 
     It is noted that in the case where supply of a sheet P from the sheet feed cassette  4  is set in the control section  5  in advance, the control section  5  turns on the sheet feed clutch  82 , the conveyance clutch  83 , and the intermediate clutch  84  and drives the motor  81 , while turning off the registration clutch  85 . Thus, the sheet feed roller  71 , the conveyance roller pair  72 , and the intermediate roller pair  73  convey the sheet P accommodated in the sheet feed cassette  4  toward the registration roller pair  74 . 
     &lt;Step S 2 &gt; 
     At a step S 2 , the control section  5  starts measuring a predetermined forced conveyance period T 1  from the start of conveyance of the sheet P loaded on the manual feed tray  76 . Then, the routine proceeds to a step S 3 . The forced conveyance period T 1  is a predetermined logical value as a time period from a start of sheet conveyance in the step S 1  to the time when a deflection with a first predetermined volume is formed in the sheet P in a manner that the sheet P abuts on and is pushed further against the registration roller pair  74 . Specifically, the forced conveyance period T 1  is set in advance according to the sheet conveyance speed, the length of the conveyance path, and the like in the sheet conveyance section  7 . Further, the forced conveyance period T 1  is set in advance for each of the manual feed tray  76  and the sheet feed cassette  4 . The control section  5  changes the forced conveyance period T 1  according to which the sheet P is conveyed from, the manual feed tray  76  or the sheet feed cassette  4 . It is noted that when the control section  5  changes the sheet conveyance speed in the sheet conveyance section  7  according to the sheet type of a sheet P, the forced conveyance period T 1  is also changed according to the sheet type of the sheet P. 
     The forced conveyance period T 1  is a time period (T 1 =T 2 +T 3 +T 4 ) longer by a predetermined delay period T 4  than a sum of a conveyance period T 2  and a conveyance wait period T 3 . The conveyance period T 2  is a logical value predetermined as a period from a start of sheet conveyance to detection of the sheet P by the sheet detection section  75 . The conveyance wait period T 3  will be described later. It is desirable to set the delay period T 4  within a time range where a deflection formed in a sheet P does not collide with a conveyance guide  70  (see  FIG. 6 ) of the sheet conveyance section  7 . Thus, generation of collision noise, which may be caused at collision of a deflection in a sheet P with the conveyance guide  70 , can be prevented. 
     &lt;Step S 3 &gt; 
     At the step S 3 , the control section  5  determines whether the forced conveyance period T 1  elapses. When the control section  5  determines that the forced conveyance period T 1  elapses (Yes in S 3 ), the routine proceeds to a step S 7 . By contrast, when the control section  5  determines that the forced conveyance period T 1  does not elapse yet (No in S 3 ), the routine proceeds to a step S 4 . 
     &lt;Step S 4 &gt; 
     At the step S 4 , the control section  5  determines whether the sheet P is present at or absent from the position of the sheet detection section  75  with the use of the signals from the sheet detection section  75 . When the control section  5  determines that the sheet P is present at the position of the sheet detection section  75  (Yes in S 4 ), the routine proceeds to a step S 5 . By contrast, when the control section  5  determines that the sheet P is absent from the position of the sheet detection section  75  (No in S 4 ), the routine returns to the step S 3 . Accordingly, where the sheet P is undetected in the sheet detection section  75 , the routine proceeds to a step S 7  after the forced conveyance period T 1  elapses. 
     &lt;Step S 5 &gt; 
     At the step S 5 , the control section  5  starts measuring the predetermined conveyance wait period T 3  from determination that the sheet P is present at the position of the sheet detection section  75  with the use of the signals from the sheet detection section  75 . Then, the routine proceeds to a step S 6 . The conveyance wait period T 3  is a logical value predetermined as a period from determination that a sheet P is present at the position of the sheet detection section  75  to formation of a deflection with a second predetermined volume in a sheet P. Specifically, the conveyance wait period T 3  is a sum of a first period and a second period. The first period is a period from determination that a sheet P is present at the position of the sheet detection section  75  to time when the leading edge of the sheet P abuts on the registration roller pair  74 . The second period is a period from abutment of the leading edge of the sheet P on the registration roller pair  74  to time when the rear edge of the sheet P is conveyed, and a deflection with a second predetermined volume is formed in the sheet P. As described above, the conveyance wait period T 3  is a period obtained by subtracting the conveyance period T 2  and the delay period T 4  from the forced conveyance period T 1 . 
     &lt;Step S 6 &gt; 
     At the step S 6 , the control section  5  determines whether the conveyance wait period T 3  elapses. When the control section  5  determines that the conveyance wait period T 3  elapses (Yes in S 6 ), it is determined that a deflection with the second predetermined volume is formed. Then, the routine proceeds to the step S 7 . By contrast, until the conveyance wait period T 3  elapses (No in S 6 ), the control section  5  suspends the processing at the step S 6 . 
     &lt;Step S 7 &gt; 
     At the step S 7 , the control section  5  determines whether the sheet P being conveyed by the sheet conveyance section  7  corresponds to the first sheet P in a job that is currently being carried out. If the sheet P is the first one, the multifunction peripheral  10  may be in a state immediately after the power is turned on or in a state immediately after return from the power saving mode Immediately after turning on the power or returning from the power saving mode, preparation for image formation may not be completed yet in the image forming section  3  because the fixing temperature of the fixing roller  37  may not reach the predetermined temperature, the rotation of the polygon mirror in the LSU  33  may be unstable yet, or so. In view of this, at the step S 7 , whether or not the possibility that the image forming section  3  is still in preparation for image formation remains is determined according to whether the sheet P is the first one. This branches the routine. 
     When it is determined that the sheet P is the first one (Yes in S 7 ), the control section  5  determines that there is a possibility that the image forming section  3  is still in preparation for image formation, and then, the routine proceeds to a step S 8 . By contrast, when it is determined that the sheet P is not the first one (No in S 7 ), the control section  5  determines that preparation for image formation in the image forming section  3  is completed, and then, the routine proceeds to a step S 71 . 
     &lt;Step S 71 &gt; 
     At the step S 71 , the control section  5  turns on the registration clutch  85  to start driving the registration roller pair  74 . Thus, the sheet P stopped at the registration roller pair  74  is conveyed toward the image forming section  3 . In this case, the registration clutch  85  is turned on in the state in which the intermediate roller pair  73  is continued to be driven. Accordingly, an error in conveyance amount, which may be caused due to response delay in off time of the intermediate clutch  84 , can be prevented. 
     As described above, in the multifunction peripheral  10 , the registration roller pair  74  is driven in a first situation (Yes in S 6 ) in which the conveyance wait period T 3  elapses from detection of a sheet P by the sheet detection section  75  or in a second situation (Yes in S 3 ) in which the forced conveyance period T 1  elapses from a start of conveyance of a sheet P loaded on the manual feed tray  76  or the sheet feed cassette  4 . The second drive controller  52  drives the registration roller pair  74  in the first situation. By contrast, the third drive controller  53  drives the registration roller pair  74  in the second situation. 
     Further, in driving the registration roller pair  74 , the control section  5  executes positioning of a write starting point where an image is written to the photosensitive drum  31  and the leading edge of a sheet P. Specifically, the control section  5  controls timing of a writing start when the LSU  33  starts writing an electrostatic latent image to the photosensitive drum  31  by reference to timing of a driving start when the registration roller pair  74  starts being driven at the step S 71 . Thus, formation of an electrostatic latent image on the photosensitive drum  31  can be started so that an image is formed at a predetermined position of the sheet P. 
     &lt;Step S 8 &gt; 
     At the step S 8 , the control section  5  determines whether the image forming section  3  completes preparation for image formation. In one example, the control section  5  determines that preparation for image formation is completed if the temperature of the fixing roller  37  reaches a predetermined fixing temperature, and the polygon minor in the LSU  33  is stably rotated. 
     When the control section  5  determines that preparation for image formation is completed (Yes in S 8 ), the routine proceeds to the step S 71 . By contrast, when the control section  5  determines that preparation for image formation is not yet completed (No in S 8 ), the routine proceeds to a step S 9 . Accordingly, in the case where the multifunction peripheral  10  continuously performs a plurality of jobs that accompany the image formation processing, if the current job is the second or following job, the registration roller pair  74  can be driven without suspending the driving of the intermediate roller pair  73  even when the currently conveyed sheet P is the first one in the job. 
     &lt;Step S 9 &gt; 
     At the step S 9 , the control section  5  suspends rotation of the intermediate roller pair  73  to stop conveyance of the sheet P. Then, the routine proceeds to a step S 10 . Specifically, the control section  5  turns off the intermediate clutch  84  or suspends driving of the motor  81  to stop conveyance of the sheet P. In other words, the driving of the intermediate roller pair  73  is suspended to stop conveyance of the sheet P after the conveyance wait period T 3  elapses from the time when the control section  5  determines that the sheet P is present at the position of the sheet detection section  75 . Alternatively, the driving of the intermediate roller pair  73  is suspended to stop conveyance of the sheet P when the control section  5  determines that preparation for image formation is not yet completed after elapse of the forced conveyance period T 1  from a start of conveyance of the sheet P. 
     &lt;Step S 10 &gt; 
     At the step S 10 , the control section  5  starts driving the intermediate roller pair  73  and the registration roller pair  74  after the image forming section  3  completes preparation for image formation. This allows the sheet P to be conveyed toward the photosensitive drum  31 . Specifically, the control section  5  drives the motor  81  and turns on the intermediate clutch  84  and the registration clutch  85  to start driving the intermediate roller pair  73  and the registration roller pair  74 . 
     In this case, in driving the registration roller pair  74 , the control section  5  also executes positioning of a write starting point where an image is written to the photosensitive drum  31  and the leading edge of the sheet P. Specifically, the control section  5  controls timing of a writing start when the LSU  33  starts writing an electrostatic latent image to the photosensitive drum  31  by reference to timing of a driving start when the registration roller pair  74  starts being driven in the step S 10 . Thus, formation of an electrostatic latent image on the photosensitive drum  31  can be started so that an image is formed at a predetermined position of the sheet P. 
     &lt;Step S 11 &gt; 
     After the registration roller pair  74  starts to be driven, the control section  5  determines whether all jobs that accompany the image information processing are completed in the step  11 . When it is determined that all jobs that accompany the image formation processing are completed (Yes in S 11 ), the control section  5  terminates the series of sheet conveyance processing. By contrast, when the control section  5  determines that not all jobs that accompany the image formation processing are completed (No in S 11 ), the routine returns to the step S 1  so that the sheet conveyance processing is repeated for any remaining to-be-printed sheets P. 
     As has been described so far, when the control section  5  determines that a sheet P is present at the position of the sheet detection section  75 , the sheet P is conveyed to the image forming section  3  after a deflection with a predetermined first volume is formed in the sheet P in the conveyance wait period T 3 . Also, even when the control section  5  determines that a sheet P is absent from the position of the sheet detection section  75 , the sheet P is conveyed to the image forming section  3  after a deflection with the second predetermined volume is formed in the forced conveyance period T 1 . Accordingly, control on timing of a driving start of the registration roller pair  74  according to a detection result by the sheet detection section  75  can avoid disadvantages, such as a sheet jam, sheet bending, etc. in a structure to form a deflection in a sheet P. 
     Forced driving of the registration roller pair  74  after elapse of the forced conveyance period T 1  may be done regardless of the type of a sheet P. Alternatively, the forced driving of the registration roller pair  74  may be done only when the type of a sheet P is a specific sheet type set in advance as a sheet type that is difficult to be detected by the sheet detection section  75 . Specifically, the control section  5  receives setting of a sheet type of a to-be-conveyed sheet P through user&#39;s manipulation on the operation display section  6 . This processing is executed by the sheet type setting section  54 . The control section  5  changes, for example, the conveyance speed of the sheet conveyance section  7 , the rotation speed of the photosensitive drum  31 , the fixing temperature of the fixing roller  37 , or the like according to the type of a sheet P. Further, the control section  5  executes the steps S 2  and S 3  only when the type of a sheet P is the predetermined specific sheet type, such as a viewgraph. The steps S 2  and S 3  are omitted when the sheet P is any of the other sheet types. In other words, according to the sheet type of the sheet P, the control section  5  enables/disables forced driving of the registration roller pair  74  after the forced conveyance period T 1  elapses. Accordingly, unnecessary forced driving of the registration roller pair  74  can be prevented from being performed on a sheet P of a sheet type that the sheet detection section  72  can detect inherently reliably, such as paper. Thus, influence on other control can be prevented which may be caused by such unnecessary forced driving. 
     [Sheet Conveyance Operation in Multifunction Peripheral  10 ] 
     With reference to  FIGS. 4-6 , description will be made below about operation examples of the multifunction peripheral  10  when the sheet conveyance processing is performed.  FIG. 4  shows an operation example where the sheet detection section  75  detects a sheet P.  FIG. 5  shows an operation example where the sheet detection section  75  detects no sheet P.  FIG. 6  is an illustration conceptually showing the sheet conveyance path in the sheet conveyance section  7 . In  FIG. 6 , the intermediate roller pair  73 , the registration roller pair  74 , and the photosensitive drum  31  are shown at the same height level. It is noted that the direction from right to left in  FIG. 6  is the sheet conveyance direction. One example will be described herein in which supply of a sheet P from the manual feed tray  76  is set in advance in the control section  5  through user&#39;s manipulation on the operation display section  6 , and then, image formation is performed. 
     [Operation when a Sheet is Detected] 
     With reference to  FIGS. 4 and 6 , description will be made first about an operation example of the multifunction peripheral  10  when the sheet detection section  75  detects a sheet P in the state in which the image forming section  3  completes preparation for image formation. 
     When the motor  81  is driven first, and the manual feed clutch  86  and the intermediate clutch  84  are turned on (time t 1 ), as shown in  FIG. 4 , a sheet P is conveyed from the manual feed tray  76  toward the registration roller pair  74 . 
     Next, when the sheet P is conveyed up to a predetermined point on the upstream side of the registration roller pair  74  in the sheet conveyance direction, the sheet P is detected by the sheet detection section  75  (time t 2 ). At the time t 2 , the registration clutch  85  is turned off, so that the driving of the registration roller pair  74  is suspended. 
     Subsequently, the on state of the intermediate clutch  84  and the off state of the registration clutch  85  continue until the conveyance wait period T 3  elapses, so that a deflection with the second predetermined volume is formed in the sheet P, as indicated by the solid line in  FIG. 6 . Thus, the leading edge of the sheet P is aligned to the nip part of the registration roller pair  74 , thereby correcting inclination (skew) of the sheet P. 
     When the conveyance wait period T 3  lapses (time t 3 ), the registration clutch  85  is turned on, while the intermediate clutch  84  remains turned on. Thus, the sheet P is conveyed toward the photosensitive drum  31 . At this time, the control section  5  controls synchronization between timing of turning on the registration clutch  85  and timing of a start of image formation by the image forming section  3  for a start of formation of an electrostatic latent image on the photosensitive drum  31 . Thus, an image is formed at a predetermined position of the sheet P. 
     [Operation when No Sheet is Detected] 
     With reference to  FIGS. 5 and 6 , description will be made next about an operation example of the multifunction peripheral  10  when the sheet detection section  75  detects no sheet P in the state in which the image forming section  3  completes preparation for image formation. As described above, where a sheet P is a specific sheet, such as a viewgraph or the like, the sheet detection section  75  may not be able to detect the sheet P. 
     Although a sheet P loaded on the manual feed tray  76  is conveyed to the sheet detection section  75 , if the sheet detection section  75  cannot detect the sheet P, the registration roller pair  74  does not start being driven (time t 2 ), as shown in  FIG. 5 . Accordingly, the intermediate roller pair  73  remains driven, while the driving of the registration roller pair  74  remains suspended, thereby forming a deflection with a volume larger than the second predetermined volume. 
     The above sheet conveyance processing by the control section  5  in the multifunction peripheral  10  results in turning on the registration clutch  85  at the time t 4  when the forced conveyance period T 1  elapses from the start of conveyance of the sheet P loaded on the manual feed tray  76 . This causes the sheet P to be conveyed toward the photosensitive drum  31 . 
     Accordingly, a deflection with the first predetermined volume is formed in the sheet P as indicated by the broken line in  FIG. 6 . The deflection with the first predetermined volume is a deflection of which volume is limitedly increased from the second predetermined volume by the delay period T 4 . It is noted that until the image forming section  3  completes preparation for image formation, driving of the intermediate roller pair  73  is suspended to restrict formation of a deflection in the sheet P. 
     Thus, where the sheet detection section  75  in the multifunction peripheral  10  detects no sheet P, the conveyance of the sheet P is limited. This can avoid disadvantages, such as a sheet jam, sheet bending, etc. As described above, it is desirable to set the delay period T 4  within a range that causes no sheet bending and also causes no collision of a deflection in a sheet P with the conveyance guide  70 , as shown in  FIG. 6 . Thus, noise, which may be caused due to collision of a sheet P with the conveyance guide  70 , can be prevented from being generated. 
     [Other Embodiments] 
     Other embodiments of the present disclosure will be described next with reference to  FIG. 7 . As shown in  FIG. 7 , the sheet conveyance section  7  of the multifunction peripheral  10  according to another embodiment includes a sheet detection section  79  arranged between the registration roller pair  74  and the photosensitive drum  31 . The sheet detection section  79  corresponds to a second sheet detection section. 
     The sheet detection section  79  is a sensor to detect the presence/absence of a sheet P between the registration roller pair  74  and the photosensitive drum  31 . In one example, the sheet detection section  79  may be a displacement type sensor to mechanically detect the presence/absence of a sheet P. In the case where the sheet detection section  79  is a displacement type sensor, the sheet detection section  79  includes an actuator (contact portion) that displaces upon contact with a sheet P and an optical sensor to detect displacement of the actuator. Alternatively, the sheet detection section  79  may be an image sensor or the like to detect the presence/absence of a sheet P on the basis of a captured image. 
     The control section  5  determines the position of the leading edge of a sheet P according to a detection result by the sheet detection section  79 . The control section  5  controls timing of image formation by the image forming section  3  on the basis of the determined position of the leading edge of the sheet P. More specifically, the control section  5  includes, as shown in  FIG. 2 , a timing controller  55  to control timing of image formation by the image forming section  3  according to timing of sheet detection by the sheet detection section  79 . Thus, even when a response of, for example, the registration clutch  85  is delayed, an image can be formed at a predetermined position of a sheet P regardless of its influence. Accordingly, the multifunction peripheral  10  is especially suitable for high speed printing. 
     Further, the control section  5  may include an abnormality detection section  56 , as shown in  FIG. 2 . The abnormality detection section  56  detects abnormality in sheet conveyance where the sheet detection section  79  detects no sheet P until a predetermined abnormality conveyance period elapses from a start of conveyance of a sheet P loaded on the manual feed tray  76  or the sheet feed cassette  4 . This enables detection of the fact that a sheet P does not actually reach the registration roller pair  74  when the registration roller pair  74  is driven without sheet detection by the sheet detection section  75 . 
     Furthermore, the control section  5  may include a re-conveyance controller  57 , as shown in  FIG. 2 . The re-conveyance controller  57  allows a sheet P to be conveyed again from the manual feed tray  76  or the sheet feed cassette  4  upon detection of abnormality in sheet conveyance. This can allow continuation of image formation without need of user&#39;s labor in the case where the cause of non-detection of a sheet P is that the sheet P is not actually conveyed to the registration roller pair  74 . It is noted that the control section  5  executes a control program stored in the ROM with the use of the CPU to function as the timing controller  55 , the abnormality detection section  56 , and the re-conveyance controller  57 .