Abstract:
An image forming apparatus and image forming method including a low-noise mode at paper sheet reverse section to decrease a reverse convey speed of reverse conveyance by a reverse roller pair, where necessary, a reverse convey speed of reverse conveyance.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   The present application is a Divisional of U.S. application Ser. No. 10/462,679, filed Jun. 17, 2003 now U.S. Pat. No. 6,931,230, the entire contents of which is incorporated herein by reference. 
   This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2002-176624, filed Jun. 18, 2002, the entire contents of which are incorporated herein by reference. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an image forming apparatus such as a digital copying machine or a printer, and to a control method for the image forming apparatus. 
   2. Description of the Related Art 
   In a conventional digital copying machine, when a copying operation is performed, an original is fed to a scan mechanism and a paper sheet is fed to a print/output mechanism. Thus, the copying operation is executed. In this operation, there is a case where a sheet-reversing section for reversing a paper sheet is provided in front of the output mechanism. 
   Specifically, in order to successively output copied paper sheets in the order of page numbers, the obverse and reverse sides of a conveyed paper sheet are turned upside down by the sheet-reversing section. In addition, in order to perform double-side printing, a paper sheet having an image on its one side is reversed by the sheet-reversing section and brought to an automatic double-side unit. 
   However, when a paper sheet is set in a sheet feed cassette of a digital copying machine, for example, when a thick paper sheet is set, the thick sheet (e.g. 209 g sheet), which has a greater thickness (and a greater resiliency) than an ordinary paper sheet, may cause friction with the guide member of the convey path. As a result, large friction noise is produced when the thick paper sheet passes through a guide-shaped R-portion (reversing section). 
   Besides, in a modern high-speed machine, a sheet feed interval of paper sheets is decreased to increase a copy productivity (CPM: copy per minute). In this case, in order to reverse and output (or discharge) the sheet, the sheet convey speed at the reversing section needs to be increased. To achieve this, a speed acceleration control is executed to accelerate the sheet convey speed at the time of reversing the sheet. Specifically, a paper sheet on which an image is formed is fed at a constant speed until it passes through a fixing device. After a rear end of the sheet comes out of the fixing device, the convey speed is accelerated at a predetermined timing. 
   In the apparatus where the sheet-reverse section is used to effect both operations for the reversed-sheet output and the sheet reverse conveyance to the automatic double-side unit, the sheet-reversing positions for the respective operations are determined. The sheet-reversing positions are determined by the timing provided by sensors disposed in the convey path. In short, as the convey speed increases, a variation increases in the sheet-reversing position due to an error in timing or a slip of rollers. 
   In addition, in general, a sheet-reverse position for re-feeding the sheet to the automatic double-side unit is set on the downstream side of a sheet-reverse position for reversing the sheet and outputting the reversed sheet. In a case where an LD sheet with a large length is used, if the sheet-reverse position shifts to the downstream side, a front edge of the sheet abuts upon an end wall of the convey path, resulting in folding of the sheet or noise due to abutment. 
   On the other hand, if the sheet-reverse position shifts to the upstream side, the reverse conveyance to the automatic double-side unit would begin before the sheet does not completely come out of the convey path. As a result, a jam may occur in the vicinity of the entrance of the automatic double-side unit or noise of abutment may occur in the convey path. 
   Furthermore, the sheet-reverse position alters due to the convey speed. If there is a variance among machines due to precision of parts, such as a roller diameter, or assembling, the sheet-reverse position would vary. As a result, like the above-mentioned case, such problems as folding of paper, jamming or noise due to abutment may arise. 
   BRIEF SUMMARY OF THE INVENTION 
   The object of the present invention is to provide an image forming apparatus and an image forming method thereof, which can reduce noise at a sheet-reversing section and can prevent problems such as jamming. 
   In order to achieve the object, the present invention may provide an image forming apparatus and an image forming method which forms an image on a paper sheet that is fed, comprising: 
   an image forming section that forms on a paper sheet an image; and 
   a reversing section that reverses the paper sheet in convey direction, on one side of which an image is formed by the image forming section, wherein reverses the paper sheet in a first reverse convey speed (V 1 ) when a first mode is performed, and reverses the paper sheet in a second reverse convey speed (V 2 ) that is slower than the first reverse convey speed (V 1 ) when a second image formation mode is performed. Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter. 

   
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
     The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate presently preferred embodiments of the invention, and together with the general description given above and the detailed description of the preferred embodiments given below, serve to explain the principles of the invention. 
       FIG. 1  is a cross-sectional view schematically showing the structure of a digital copying machine according to an embodiment of the present invention; 
       FIG. 2  is a block diagram schematically showing electrical connection of the digital copying machine and flow of signals for control; 
       FIG. 3  is a flow chart illustrating a control operation according to a first embodiment of the invention; 
       FIG. 4  illustrates convey speed controls for a reverse roller pair in different modes; 
       FIG. 5  is a flow chart illustrating a control operation according to a second embodiment of the invention; 
       FIG. 6  illustrates a speed control at a time of sheet reverse conveyance in the sheet output direction and a speed control at a time of sheet reverse conveyance to an automatic double-side unit; 
       FIG. 7  is an enlarged view of a reverse convey path in the digital copying machine; and 
       FIG. 8  is a flow chart illustrating a control operation for an optimal reverse position. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Embodiments of the present invention will now be described with reference to the accompanying drawings. 
     FIG. 1  shows a schematic structure of a digital copying machine  10  including an automatic double-side unit  1  according to an embodiment of the invention. The automatic double-side unit (ADU)  1  receives a paper sheet, on one side of which an image is formed, from a printer section  4  (that contains an image forming section) (to be described later) in the digital copying machine  10 . The automatic double-side unit  1  automatically reverses the sheet and feeds it to the printer section  4  once again. 
   As is shown in  FIG. 1 , the digital copying machine  10  includes a scanner section  2  that reads an image on an original and acquires image data; the aforementioned printer section  4  that forms on a paper sheet an image; the automatic double-side unit  1  that successively reverses paper sheets, on one side of each of which an image is formed by the printer section  4 , and feeds them to the printer section  4  once again; and a sheet feed section  6  that feeds paper sheets of desired sizes to the printer section  4 . In addition, an automatic document feeder (ADF)  8  is openably disposed on top of the digital copying machine  10 . The ADF  8  serves as a cover for holding an original placed on an original table  3 , and automatically feeds a plurality of originals one by one onto the original table  3 . 
   The scanner section  2  includes a first carriage  11  formed to be movable in parallel with the original table  3  under the original table  3 ; a second carriage  12  that is movable following movement of the first carriage  11 ; a lens  13  that provides predetermined focusing characteristics to reflective light (image light) from the original, which is sent from the first and second carriages  11  and  12 ; and a photoelectric conversion element (CCD sensor)  14  that photoelectrically converts the image light which is provided with the predetermined focusing characteristics by the lens  13 , thus acquiring image data. 
   The original placed on the original table  3  is illuminated by a light source  15  provided on the first carriage  11  as one piece. Image light reflected from the original is successively reflected by a first mirror  11   a  mounted on the first carriage  11 , and second and third mirrors  12   a  and  12   b  mounted on the second carriage  12 . The reflective light is then focused on the CCD sensor  14  via the lens  13 . At this time, the first carriage  11  and second carriage  12  are moved along the original table  3  at predetermined speed. Thereby, image light associated with the entire surface of the original is received by means of the CCD sensor  14 , and image data relating to the image on the entire surface of the original is acquired. 
   The printer section  4  includes an image forming section comprising: an exposing device  21  that emits a laser beam; a photosensitive drum  20  that is scanned and exposed with the laser beam emitted from the exposing device  21  so that an electrostatic latent image is formed on an outer peripheral surface  20   a  of the photosensitive drum  20 , which is precharged with a predetermined potential; a developing device  22  that applies toner to, and thus develops, the electrostatic latent image formed on the outer peripheral surface  20   a  of the photosensitive drum  20 ; a transfer belt  23  that transfers the developed toner image onto a paper sheet fed from the sheet feed section  6  (to be described later) at a predetermined timing; and a fixing device  24  that fixes the toner image, which has been transferred on the paper sheet, on the paper sheet. 
   The electrostatic latent image formed on the outer peripheral surface  20   a  of the photosensitive drum  20  by the exposure/scan by the exposing device  21  is developed into a visible toner image by the toner supplied from the developing device  22 . The visible toner image on the outer peripheral surface  20   a  is moved by the rotation of the photosensitive drum  20 , and transferred onto the paper sheet fed from the sheet feed section  6  (to be described later). The toner image transferred on the sheet is heated and fused by the fixing device  24 , and thus the toner image is fixed on the sheet. 
   The sheet, on one side of which an image is formed by the fixation of the toner image, is delivered to a direction-switching gate  26  via an image-fixed sheet output roller pair  25 . The direction-switching gate  26  is switched to output the sheet to the outside of the machine via an output roller pair  27 , or feeds the sheet to the automatic double-side unit  1  via a reverse convey path  28 , a reverse roller pair  29  and an ADU reverse roller pair  30 , which are described later. 
   An actuator-type sensor  41  is provided near the image-fixed sheet output roller pair  25 . An actuator-type sensor  42  is provided immediately after the reverse roller pair  29  in a forward convey direction of the sheet. An actuator-type sensor  43  is provided immediately after the ADU reverse roller pair  30  in the forward convey direction of the sheet. 
   A reversing section that reverses the paper sheet, on one side of which an image is formed by the image forming section, when the setting section sets one of relating to a reverse sheet output single-side image formation mode (normal mode) that is a first mode, when the setting section sets double-side image formation mode that is a second mode reverses a conveying direction of the paper sheet and conveys the paper toward the automatic double-side unit  1 . 
   Further, there are provided a motor  51  (to be described later) for driving the fixing device  24  and image-fixed sheet output roller pair  25 , and a motor  52  (to be described later) for driving the reverse roller pair  29  and ADU reverse roller pair  30 . With this structure, an optimal control for a fixation speed and a reverse convey speed can be performed in the present invention. 
   The automatic double-side unit  1  has a plurality of convey roller pairs  5 . The sheet feed section  6  includes a plurality of sheet feed cassettes  31 ,  32 ,  33  and  34  containing a plurality of paper sheets of different sizes. 
   Pick-up rollers  31   b ,  32   b ,  33   b  and  34   b  for picking up sheets one by one from the uppermost ones, which are contained in the associated sheet feed cassettes  31 ,  32 ,  33  and  34 , are provided near feed-side end portions (right-hand end portions in  FIG. 1 ) of the sheet feed cassettes  31 ,  32 ,  33  and  34 . Sheet feed rollers  31   a ,  32   a ,  33   a  and  34   a  are provided adjacent to the pick-up rollers  31   b ,  32   b ,  33   b  and  34   b  on the downstream side of the pick-up rollers  31   b ,  32   b ,  33   b  and  34   b  in the direction in which the sheets are taken out. A paper sheet selectively taken out of the sheet feed cassettes  31 ,  32 ,  33  and  34  by the pick-up rollers  31   b ,  32   b ,  33   b  and  34   b  and sheet feed rollers  31   a ,  32   a ,  33   a  and  34   a  is conveyed upward (in  FIG. 1 ) via a plurality of convey roller pairs  36  provided along a sheet convey path  35 . The conveyed sheet is fed to an aligning roller pair  37  provided in front of the photosensitive drum  20  of the printer section  4 . 
   A manual feed device  39  for manually feeding a paper sheet is provided upward of the sheet feed cassette  31 . The paper sheet fed via the manual feed device  39  is delivered to the aligning roller pair  37 . 
   A front edge of the paper sheet fed to the aligning roller pair  37  from the sheet feed cassette,  31 ,  32 ,  33 ,  34 , of the sheet feed section  6  or from the manual feed device  39  is once aligned by the aligning roller pair  37 . Then, the aligning roller pair  37  is rotated in synchronism with the timing of the image forming operation in the printer section  4 . Thus, the sheet is fed to a transfer region between the transfer belt  23  and photosensitive drum  20 . In this manner, the above-mentioned toner image is transferred onto the sheet fed to the transfer region. 
     FIG. 2  is a block diagram schematically showing electrical connection of the digital copying machine  10  shown in  FIG. 1  and flow of signals for control. In  FIG. 2 , a control system of the digital copying machine  10  comprises three CPUs: a main CPU  91  provided in a main control section  90 ; a scanner CPU  100  in the scanner section  2 ; and a printer CPU  110  in the printer section  4 . The main CPU  91  performs bi-directional communication with the printer CPU  110  via a shared RAM  95 . The main CPU  91  issues an operational instruction, and the printer CPU  110  returns status data. Serial communication is performed between the printer CPU  110  and scanner CPU  100 . The printer CPU  110  issues an operational instruction, and the scanner CPU  100  returns status data. 
   An operation panel  80  is connected to the main CPU  91 . The operation panel  80  comprises a print key  82  that instructs the start of a copying operation, a panel CPU  83  that controls the entirety of the operation panel  80 , and a liquid crystal display (LCD) section  84  having a touch panel for operational inputs. 
   The main control section  90  comprises the main CPU  91 , a ROM  92 , a RAM  93 , an NVRAM  94 , a shared RAM  95 , an image processing section  96 , a page memory control unit  97 , a page memory  98 , a printer controller  99 , and a printer font ROM  121 . 
   The main CPU  91  controls the entirety of the main control section  90 . The ROM  92  stores control programs. The RAM  93  temporarily stores various data. 
   The NVM (Non-Volatile RAM)  94  is a non-volatile memory backed up by a battery (not shown). Even when power is not supplied to the NVM  94 , stored data is maintained. 
   The shared RAM  95  is used to perform bidirectional communication between the main CPU  91  and printer CPU  110 . 
   The page memory controller  97  stores and reads out image data in and from the page memory  98 . The page memory  98  has areas capable of storing image data of a plurality of pages. The page memory  98  can store compressed data in units of a page, which is obtained by compressing image data from the scanner section  2 . 
   The printer font ROM  121  stores font data corresponding to print data. 
   The printer controller  99  develops print data, which is sent from an external device  122  such as a personal computer, into image data using the font data stored in the printer font ROM  121  with a resolution corresponding to resolution data added to the print data. 
   The scanner section  2  comprises the scanner CPU  100  for controlling the entirety of the scanner section  2 ; a ROM  101  storing control programs, etc.; a data storage RAM  102 ; a CCD driver  103  for driving the CCD sensor  14 ; a scan motor driver  104  for controlling the rotation of a scan motor for moving the light source  15 , first mirror  11   a , second mirror  12   a , third mirror  12   b , etc.; and an image correction unit  105 . 
   The image correction section  105  comprises an A/D converter for converting analog signals output from the CCD sensor  14  to digital signals; a shading correction circuit for correcting a variance in the CCD sensor  14 , or a variation in threshold level due to ambient temperature variation relative to the output signal from the CCD sensor  14 ; and a line memory for temporarily storing shading-corrected digital signals from the shading correction circuit. 
   The printer section  4  comprises the printer CPU  110  for controlling the entirety of the printer section  4 ; a ROM  111  storing control programs, etc.; a data storage RAM  112 ; a laser driver  113  for turning on/off the exposing device  21  that emits a laser beam; a polygon motor driver  114  for controlling the rotation of the polygon motor of the exposing device  21 ; a sheet convey unit  115  for controlling conveyance of the sheet; a development process section  116  for controlling charging, developing and transferring processes using the developing device  22  and transfer belt  23 ; a fixation control unit  117  for controlling the fixing device  24 ; and an option unit  118 . 
   The aforementioned sensors  41 ,  42  and  43  are included in the sheet convey unit  115 . The aforementioned motors  51  and  52  are included in the printer section  4 . 
   The image processing section  96 , page memory  98 , printer controller  99 , image correction section  105 , and laser driver  113  are connected over an image data bus  120 . 
   A first embodiment of the present invention with the above-described structure will now be described. 
   Sheet reverse conveyance is described referring to  FIG. 1 . 
   In an upstream side of the reverse section, a sheet on which an image is formed by the printer section  4  is conveyed by the image-fixed sheet output roller pair  25 , and then brought to the reverse convey path  28  via the direction-switching gate  26 . In a sheet feed interval T 1  (sheet feed timing) in a normal mode that is the first mode, a sheet convey speed is 400 mm/sec. At the time of reverse conveyance, the sheet convey speed is controlled and accelerated up to 800 mm/sec. In the reverse convey path  28 , the sheet is fed forward at high speed by the reverse roller pair  29  for a prescribed time after passing through the nip between the image-fixed sheet output rollers  25 . After the sensor  42  detects the rear edge of the sheet being fed by the reverse roller pair  29 , the reverse rollers are rotated in the reverse direction at a preset timing. The reverse roller pair  29  convey the sheet backwards. The sheet, thus fed by the reverse roller pair  29 , is output from the machine as the direction-switching gate  26  is switched. 
   The sheet conveyed is guided by a guide (not shown) for ensuring exact conveyance. 
   In the first embodiment, in the sheet reverse conveyance, noise occurring at the time of forming a thick-sheet copy (roller noise at the time of reversing, noise of friction between the guide and thick sheet, etc.) is reduced. Compared to the aforementioned normal mode that is the first, a thick paper sheet mode (low-speed mode or low-noise mode) that is the second mode is set in the reverse conveyance. 
   For example, when a 209 g/m 2  sheet is fed as a thick sheet, compared to an ordinary sheet, within the digital copying machine  10 , noise of friction between the guide and the thick sheet occurs due to the thickness of the sheet (high resiliency of the thick sheet). In particular, large noise occurs when the thick sheet passes along a guide-shaped R section (reversing section). 
   To cope with this problem, in the thick paper sheet mode (low-speed mode or low-noise mode) of the first embodiment, when the reverse conveyance is performed, the sheet feed interval T  2  (feed timing) in the thick paper sheet mode (that is the second mode) is controlled to become longer than the sheet feed interval T  1  in the normal mode that is the first mode. In the normal mode, a first reverse convey speed (V 1 ) is 800 mm/sec, whereas in the thick paper mode (low-speed mode or low-noise mode) is set at 600 mm/sec that is a second reverse convey speed (V 2 ) which slower than the first revere convey speed (V 1 ). 
   The control operation in the first embodiment with the above structure will now be described with reference to a flow chart of  FIG. 3 . 
   To start with, the thick paper sheet mode (low-speed mode or low-noise mode) that is a second mode is set in any one of the sheet feed cassettes  34 . Assume that the thick sheet is set in the sheet feed cassette  34 . 
   The sheet feed cassette  34  is selected through the LCD section  84  of the operation panel  80  and the thick paper sheet mode is selected, and the print key  82  is depressed (ST 1 ). Then, the main CPU  91  determines whether the thick paper sheet mode is set through the LCD section  84  (ST 2 ). 
   If the thick paper sheet mode is set, the printer CPU  110  delays the timing of sheet feed from the sheet feed cassette  34  by a predetermined time, compared to the normal mode (ST 3 ). 
   Further, the printer CPU  110  controls the motor  52  to set the reverse convey speed of the reverse roller pair  29  at 600 mm/sec (V 2 ), which is slower than in the normal mode (the reverse sheet output single-side image formation mode) that is the first mode (ST 4 ). 
   In the case of the normal mode in step ST 2 , the printer CPU  110  controls the sheet feed timing and the reverse convey speed at values for the normal mode (ST 5 ). 
     FIG. 4  illustrates convey speed controls for the reverse roller pair  29  in different modes. In  FIG. 4 , the solid line indicates how the sheet-conveying speed is controlled in the normal mode. In the normal mode, the sheet is conveyed, first at 400 mm/sec and then faster at 800 mm/sec, and is conveyed at 800 mm/sec (V 1 ) when the conveying direction is reversed. 
   In  FIG. 4 , too, the broken line indicates how the sheet-conveying speed is controlled in the low-speed mode. In the low-speed mode, the sheet is conveyed, first at 400 mm/sec and then faster at 600 mm/sec, and is conveyed at 600 mm/sec (V 2 ) when the conveying direction is reversed. The sheet may be conveyed at 400 mm/sec when the conveying direction is reversed, depending on the type of the sheet. 
   As has been described above, according to the first embodiment, if the thick paper mode (low-speed mode or low-noise mode) is selected, noise of the reverse roller pair at the time of reversing, noise of friction between the guide and thick sheet, etc., can be reduced. 
   A second embodiment of the present invention will now be described. 
   In the second embodiment, a paper sheet is conveyed in the reverse convey path  28  that is reversing section, is reversed with a second reverse convey speed (V 2 ) that is slower than the first revere convey speed (V 1 ) to the automatic double-side unit  1   
   The reverse conveyance to the sheet output side due to the first mode has already been described above. 
   The reverse conveyance to the automatic double-side unit  1  will be described referring to  FIG. 1 . 
   A paper sheet, on one side of which an image is formed by the printer section  4 , is conveyed by the image-fixed sheet output roller pair  25 , and then brought to the reverse convey path  28  via the direction-switching gate  26 . 
   The sheet conveyed along the reverse convey path  28  passes through the reverse roller pair  29  and the ADU reverse roller pair  30 , and is sensed by the sensor  43 . At a predetermined timing from the sensing by the sensor  43 , the sheet is reversely conveyed by the ADU reverse roller pair  30 . The sheet reversely conveyed by the ADU reverse roller pair  30  is conveyed by the convey roller pairs  5  of the automatic double-side unit  1 . 
   The sheet convey path for the double-side image formation will be described below. 
   (1) A paper sheet, on one side of which an image is formed by the printer section  4 , is conveyed by the image-fixed sheet output roller pair  25 , and then guided to the reverse convey path  28  via the direction-switching gate  26 . The sheet passes through the reverse roller pair  29  and is then reversely conveyed by the ADU reverse roller pair  30  into the automatic double-side unit  1 . Up to three paper sheets are brought into the automatic double-side unit  1 . 
   (2) If three paper sheets are brought into the automatic double-side unit  1 , the sheet that was first brought in the automatic double-side unit  1  is fed once again to the printer section  4 . An image is thus formed on the other side (reverse side) of the sheet in the printer section  4 . 
   (3) The paper sheet, on both sides of which images have been formed, is conveyed by the image-fixed sheet output roller pair  25  and output to the outside of the machine via the direction-switching gate  26  and output roller pair  27 . 
   (4) Subsequently, another paper sheet, on one side of which an image is formed by the printer section  4 , is conveyed by the image-fixed sheet output roller pair  25 , and then guided to the reverse convey path  28  via the direction-switching gate  26 . The sheet passes through the reverse roller pair  29  and is then reversely conveyed by the ADU reverse roller pair  30  into the automatic double-side unit  1 . As a result, the number of paper sheets brought in the automatic double-side unit  1 , on one side of each of which an image is formed, becomes three once again. 
   Then, the next sheet in the automatic double-side unit  1  is fed to the printer section  4  once again. An image is thus formed on the other side (reverse side) of the sheet in the printer section  4 . 
   The double-side image formation is performed in the order of the above steps (2), (3) and (4). 
   The control operation in the second embodiment with the above structure will now be described with reference to a flow chart of  FIG. 5 . 
   To start with, a plurality of originals are set in the ADF  8 . Setting for an, whether double-side image formation mode or reverse sheet output with single-side image formation mode is to be performed, is instructed through the LCD section  84  of the operation panel  80  (ST 11 ). 
   When the print key  82  is depressed, the printer CPU  110  controls the motor  51  to convey the sheet at the set convey speed (400 mm/sec) (ST 12 ). 
   If the setting in step S 11  is the double-side image formation mode (ST 13 ), the printer CPU  110  causes the image-fixed sheet output roller pair  25  to convey the sheet, on one side of which an image is formed, at the same convey speed of 400 mm/sec, thus bringing the sheet to the reverse convey path  28  via the direction-switching gate  26  (ST 14 ). 
   The sheet with the image on one side passes through the nip of the reverse roller pair  29  and the nip between the ADU reverse roller pair  30  at a higher speed of 600 mm/sec. The sheet is then detected by the sensor  43 . Upon lapse of a prescribed time from the detection of the sheet, the printer CPU  110  causes the ADU reverse roller pair  30  to rotate in reverse direction to convey the sheet at 600 mm/sec (V 2 ) that is slower than the first revere convey speed (V 1 ) (ST 15 ). 
   Thus, the printer CPU  110  causes the reversely conveyed sheet to be taken into the automatic double-side unit  1  (ST 16 ). 
   If the setting in step ST 11  is the reverse sheet output with single-side image formation mode (ST 13 ), the printer CPU  110  causes the image-fixed sheet output roller pair  25  to convey the sheet with the image (on one side alone) at the same convey speed of 400 mm/sec, thus bringing the sheet to the reverse convey path  28  via the direction-switching gate  26  (ST 17 ). 
   The sheet with the image on one side only is conveyed by the reverse roller pair  29 , faster at 800 mm/sec and is detected by the sensor  42 . Upon lapse of a prescribed time from the detection of the sheet, the printer CPU  110  causes the reverse roller pair  29  to rotate in reverse direction to convey the sheet faster at 800 mm/sec (V 1 ) (ST 18 ). 
   Thus, the printer CPU  110  outputs the reverse-conveyed sheet to the outside of the machine via the direction-switching gate  26  and the output roller pair  27  (ST 19 ). 
     FIG. 6  illustrates a speed control at a time of sheet reverse conveyance to the sheet output side and a speed control at a time of sheet reverse conveyance to the automatic double-side unit  1 . The left-hand portion in  FIG. 6  illustrates the convey speed control for the sheet reverse conveyance to the sheet output side. In this case, the sheet is conveyed at 400 mm/sec, and 800 mm/sec at the time of reverse conveyance. 
   The right-hand portion in  FIG. 6  illustrates the convey speed control for the sheet reverse conveyance to the automatic double-side unit  1 . In this case, the sheet is conveyed at 400 mm/sec, and 600 mm/sec at the time of reverse conveyance. 
   As has been described above, according to the second embodiment, slower reverse convey speeds are set than the reverse convey speed to the sheet output side, and the reverse conveyance performs to the automatic double-side unit  1 . Thereby, paper jam at the time of reverse conveyance to the automatic double-side unit is prevented, and noise due to, e.g. abutment, at the time of reverse feed operation, can be reduced. 
     FIG. 7  is an enlarged view of the reverse convey path in the digital copying machine  10 . 
   In  FIG. 7 , a sheet reverse position R at the time of reverse conveyance to the sheet output side is variable due to a convey speed of the image-fixed sheet output roller pair  25  of the fixing device  24  and a convey speed of the reverse roller pair  29 . Similarly, a sheet reverse position A at the time of reverse conveyance to the automatic double-side unit  1  is variable due to a convey speed of the image-fixed sheet output roller pair  25  of the fixing device  24  and a convey speed of the ADU reverse roller pair  30 . 
   If the convey speed of the image-fixed sheet output roller pair  25  of the fixing device  24 , the reverse roller pair  29  or ADU reverse roller pair  30  varies, the sheet reverse position (R, A) alters, leading to jamming or other problems. In addition, there is a variance among copying machines with respect to the diameter of each roller, fixation convey speed and reverse convey speed. 
   An optimal sheet reverse position is automatically adjusted at the time of reverse conveyance to the sheet output side or to the automatic double-side unit  1 . In addition, such an optimal sheet reverse position can be adjusted by a serviceman. 
   The optimal reverse position control operation with the above-described structure will now be described with reference to a flow chart of  FIG. 8 . 
   To start with, a plurality of originals are set in the ADF  8 . Setting for an image forming operation is instructed through the LCD section  84  of the operation panel  80  (ST 31 ). 
   The printer CPU  110  controls the fixing device  24  and image-fixed sheet output roller pair  25  at a convey speed set in the printer section  4 . Specifically, the printer CPU  110  controls the motor  51  to convey the sheet at a set convey speed (ST 32 ). The sheet is sensed by the sensor  41  when it passes through the image-fixed sheet output roller pair  25  (ST 33 ). 
   In addition, the printer CPU  110  controls the motor  52  on the basis of the set convey speed of the fixing device  24  and image-fixed sheet output roller pair  25  driven by the motor  51 , thereby controlling the convey speed of the reverse roller pair  29  and ADU reverse roller pair  30  (ST 34 ). 
   The printer CPU  110  determines whether the sheet conveyed by the image-fixed sheet output roller pair  25  is to be reversely conveyed to the sheet output side or to the automatic double-side unit  1  (ST 35 ). 
   If it is determined in step ST 35  that the sheet is to be reversely conveyed to the sheet output side, the printer CPU  110  guides the sheet to the reverse convey path  28  via the direction-switching gate  26  (ST 36 ). 
   When the sheet passes through the reverse roller pair  29  and is sensed by the sensor  42 , the printer CPU  110  controls the motor  52  to drive the reverse roller pair  29  so that the sheet may come to the optimal reverse position R in accordance with the sensing by the sensors  41  and  42  (ST 37 ). In this case, the printer CPU  110  controls the driving of the motor  52  by computing a time period from the sensing of the rear end of the sheet by the sensor  41  to the reaching of the rear end to the optimal reverse position R on the basis of a time period between the sensing by the sensor  41  and the sensing by the sensor  42 . 
   On the other hand, if it is determined in step ST 35  that the sheet is to be reversely conveyed to the automatic double-side unit  1 , the printer CPU  110  guides the sheet to the reverse convey path  28  via the direction-switching gate  26  (ST 38 ). 
   When the sheet passes through the reverse roller pair  29  and ADU reverse roller pair  30  and is sensed by the sensor  43 , the printer CPU  110  controls the motor  52  to drive the reverse roller pair  29  so that the sheet may come to the optimal reverse position A in accordance with the sensing by the sensors  41  and  42  (ST 39 ). In this case, the printer CPU  110  controls the driving of the motor  52  by computing a time period from the sensing of the rear end of the sheet by the sensor  42  to the reaching of the rear end to the optimal reverse position A on the basis of a time period between the sensing by the sensor  41  and the sensing by the sensor  43 . 
   A serviceman may adjust the control by the CPU  110  to set the optimal reverse position R, A in accordance with the individual copying machine. Specifically, the serviceman causes the LCD section  84  of operation panel  80  to display adjustment codes, and adjusts the speeds of the motors  51  and  52  in accordance with roller diameters, fixation speeds and reverse convey speeds of individual copying machines. 
   As has been described above, optimal reverse positions can be controlled in accordance with the convey speed at the fixing device and the convey speed at the time of sheet reversing. 
   According to the above-described embodiments of the invention, when a thick sheet is selected at the time of sheet setting relating to sheet feed cassettes, the reverse convey speed is controlled and decreased. Thereby, noise of the roller pair at the time of reversing, noise of friction between the guide and thick sheet, etc., can be reduced. 
   In addition, different reverse-convey speeds are set between the reverse conveyance to the sheet output side and the reverse conveyance to the automatic double-side unit. Thereby, paper jam at the time of reverse conveyance to the automatic double-side unit is prevented, and noise due to, e.g. abutment, at the time of reverse feed operation, can be reduced. 
   Furthermore, the optimal reverse position for stably conveying the sheet at the reverse position is always controlled by computation, whereby paper jamming or other problems can be prevented. 
   Besides, the adjustment mode may be set through the operation panel, thereby to adjust the speeds of the motors that are reversely driven. Thereby, paper jamming or other problems can be prevented. 
   Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.