Patent Publication Number: US-9836005-B2

Title: Sheet feeding apparatus, sheet feeding method and image formation system

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is entitled to and claims the benefit of Japanese Patent Application No. 2015-172803, filed on Sep. 2, 2015, the disclosure of which including the specification, drawings and abstract is incorporated herein by reference in its entirety. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a sheet feeding apparatus, a sheet feeding method and an image formation system. 
     2. Description of Related Art 
     Conventionally, as a sheet feeding apparatus for supplying sheets to an image forming apparatus such as a copier and a printer, sheet feeding apparatuses of an air system have been known (see, for example, Japanese Patent Publication No. 4952524). As disclosed in Japanese Patent Publication No. 4952524, a sheet feeding apparatus of an air system (hereinafter referred to as “air sheet feeding apparatus”) includes a sheet storage section, a suction conveyance section, a floating air blowing section, a separation air blowing section, an outlet roller section and the like. 
     The suction conveyance section includes an endless conveyance belt having a large number of absorption ports, and an air absorption section installed inside the conveyance belt. The suction conveyance section is disposed above the sheet storage section. The suction conveyance section sucks, on the conveyance belt, sheets one by one from the sheet bundle loaded in the sheet storage section, and conveys the sheets. The floating air blowing section applies air to the sides of the sheet bundle from a direction orthogonal to the sheet conveyance direction (from the both ends of the sheet width direction) so as to float several upper sheets of the sheet bundle. The separation air blowing section applies air to a plurality of sheets sucked on the conveyance belt from the downstream side in the sheet conveyance direction and separates only the uppermost sheet. The outlet roller section is disposed on the downstream side of the suction conveyance section in the sheet conveyance direction, and configured to further output the sheet conveyed by the suction conveyance section toward the downstream side. 
     In the air sheet feeding apparatus, only the uppermost sheet which has been separated by the air of the separation air blowing section is sucked on the conveyance surface (suction surface) of the conveyance belt by the air absorption section. When the conveyance belt travels in this state, the uppermost first sheet is conveyed. 
     However, when the length of the sheet (the length in the sheet conveyance direction: sheet length) is longer than the length of the conveyance surface of the conveyance belt in the sheet conveyance direction, the sheet exceeding the conveyance surface of the conveyance belt hangs, and the hanging first sheet and the second sheet make contact with each other. As the sheet length increases, the range of the hanging part increases, and the contacting range with the second sheet increases. As a result, the frictional resistance between sheets increases, thus causing overlapped-sheets feeding in which the second sheet is sent together with the first sheet. In the case where air is applied to the second sheet to prevent the overlapped-sheets feeding, when the intensity of the air jetting is too high, the rear end of the first sheet which is not sucked on the conveyance surface bounces (sheet bounce), and deviates from the guide. Consequently, the sheet makes contact with the upper side structures and the like, and sliding resistance is generated. 
     In view of this, a technique has been provided in which a suction conveyance section is provided over the entire sheet surface, and the suction is performed in accordance with the sheet length so as to prevent the sheet bounce. For example, Japanese Patent Application Laid-Open No. 6-9083 discloses a technique in which an air absorption section installed inside a conveyance belt has a shutter for opening and closing an absorption port provided on the upstream side and downstream side in the sheet conveyance direction, and the absorption port is appropriately opened and closed with the shutter to selectively suck the sheet located at a position on the upstream side or downstream side in the sheet conveyance direction on the conveyance surface of the conveyance belt, and convey the sheet. In addition, Japanese Patent Application Laid-Open No. 9-216750 discloses a technique in which a slide valve is opened or closed in accordance with the length of the conveyed sheet, and an absorption force is generated only at a downward position of the sheet so that the sheet does not bounce. 
     However, in the techniques disclosed in Japanese Patent Application Laid-Open No. 6-9083 and Japanese Patent Application Laid-Open No. 9-216750 in which the entirety of the sheet surface is sucked in accordance with the sheet length to prevent sheet bounce, when sending of the uppermost first sheet with the conveyance belt is started, the first sheet is sequentially ejected out from the rear end (upstream) side of the conveyance surface of the conveyance belt in the sheet conveyance direction, and the second sheet is sucked with the absorption force on the conveyance belt at the conveyance surface from which the first sheet is ejected. Consequently, when the conveyance belt travels, overlapped feeding of the second sheet and the first sheet is caused. As such, it is necessary to partially stop the suction with a shutter and the like, or, stop the absorption fan itself for the purpose of preventing the suction of the second sheet. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a sheet feeding apparatus, a sheet feeding method and an image formation system which can prevent overlapped-sheets feeding. 
     To achieve the abovementioned object, a sheet feeding apparatus reflecting one aspect of the present invention includes: a sheet storage section configured to store a plurality of sheets; a first conveyance belt on which to suck the sheet stored in the sheet storage section, the first conveyance belt being disposed above the sheet storage section and configured to convey the sucked sheet in a sheet conveyance direction; a first absorption section configured to absorb the sheet stored in the sheet storage section to suck the sheet on the first conveyance belt; a second conveyance belt on which to suck the sheet stored in the sheet storage section, the second conveyance belt being disposed above the sheet storage section on an upstream side of the first conveyance belt in the sheet conveyance direction, and configured to convey the sucked sheet in the sheet conveyance direction; a second absorption section configured to absorb the sheet stored in the sheet storage section to suck the sheet on the second conveyance belt; and a control section configured to perform a stopping operation for stopping at least one of a conveyance operation of the second conveyance belt and an absorption operation of the second absorption section during a conveyance operation of the first conveyance belt. 
     Desirably, in the sheet feeding apparatus, the control section performs the stopping operation when a rear end of the sheet passes through a position of a front end of the second conveyance belt in the sheet conveyance direction. 
     Desirably, in the sheet feeding apparatus includes: a sheet rear end detection section configured to detect whether the rear end of the sheet passes through the position of the front end of the second conveyance belt. The control section performs the stopping operation when the sheet rear end detection section detects that the rear end of the sheet passes through the position of the front end of the second conveyance belt. 
     Desirably, in the sheet feeding apparatus, the control section performs the stopping operation when the control section determines that the rear end of the sheet passes through the position of the front end of the second conveyance belt based on a length of the sheet and a sending time of the sheet by the second conveyance belt. 
     Desirably, in the sheet feeding apparatus, a length of a conveyance surface of the second conveyance belt in the sheet conveyance direction is equal to or smaller than a length of a conveyance surface of the first conveyance belt in the sheet conveyance direction. 
     Desirably, in the sheet feeding apparatus, the control section moves the second conveyance belt and the second absorption section in accordance with a length of the sheet stored in the sheet storage section. 
     Desirably, in the sheet feeding apparatus, a space is provided between the first conveyance belt and the second conveyance belt in the sheet conveyance direction; and, after the sheet is sucked on the first conveyance belt by the first absorption section and the sheet is sucked on the second belt by the second absorption section, the control section performs an operation for setting a conveyance speed of the second conveyance belt to a speed higher than a conveyance speed of the first conveyance belt such that the sheet is deflected in the space. 
     Desirably, in the sheet feeding apparatus, the first conveyance belt includes a first curved part configured to protrude to the space side at a predetermined curvature, and the second conveyance belt includes a second curved part configured to protrude to the space side at a predetermined curvature, the sheet feeding apparatus further comprising: a third absorption section configured to absorb the sheet conveyed in the space to suck the sheet on the first curved part; and a fourth absorption section configured to absorb the sheet conveyed in the space to suck the sheet on the second curved part. 
     Desirably, in the sheet feeding apparatus further includes a blowing section configured to apply air to the sheet to deflect the sheet in the space. 
     Desirably, in the sheet feeding apparatus, the blowing section applies the air in a horizontal direction orthogonal to the sheet conveyance direction. 
     Desirably, in the sheet feeding apparatus, the control section performs the operation for setting the conveyance speed of the second conveyance belt to a speed higher than the conveyance speed of the first conveyance belt such that a deflection amount of the sheet in the space is changed in accordance with a length of the sheet stored in the sheet storage section. 
     Desirably, in the sheet feeding apparatus, the control section performs the operation for setting the conveyance speed of the second conveyance belt to a speed higher than the conveyance speed of the first conveyance belt such that a deflection amount of the sheet in the space is changed in accordance with a type of the sheet stored in the sheet storage section. 
     To achieve the abovementioned object, a sheet feeding method reflecting one aspect of the present invention includes: storing a plurality of sheets in a sheet storage section; absorbing by a first absorption section the sheet stored in the sheet storage section to suck the sheet on a first conveyance belt; absorbing by a second absorption section the sheet stored in the sheet storage section to suck the sheet on the second conveyance belt disposed on an upstream side of the first conveyance belt in the sheet conveyance direction; conveying by the first conveyance belt and the second conveyance belt the sheet sucked on the first conveyance belt and the second conveyance belt in the sheet conveyance direction; and stopping at least one of a conveyance operation of the second conveyance belt and an absorption operation of the second absorption section during a conveyance operation of the first conveyance belt. 
     To achieve the abovementioned object, an image formation system reflecting one aspect of the present invention includes: the above-mentioned sheet feeding apparatus; and an image forming apparatus connected with the sheet feeding apparatus and configured to form an image on a sheet fed from the sheet feeding apparatus. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  illustrates a general configuration of an image formation system of an embodiment of the present invention; 
         FIG. 2  illustrates a principal part of a control system of a sheet feeding apparatus of the embodiment of the present invention; 
         FIG. 3  is a perspective view illustrating a sheet feed unit which is a main part of the sheet feeding apparatus of the embodiment of the present invention; 
         FIG. 4  is a flowchart of an exemplary sheet feeding process of the embodiment of the present invention; 
         FIG. 5  schematically illustrates a sheet feed unit at the start of sheet conveyance; 
         FIG. 6  schematically illustrates the sheet feed unit at a timing before a rear end of a sheet passes through a position of the conveyance surface of the conveyance belt; 
         FIG. 7  illustrates a sheet which is sucked only on a downstream conveyance belt; 
         FIG. 8  schematically illustrates the sheet feed unit at a timing when a rear end of a sheet passes through a position of the conveyance surface of the conveyance belt; 
         FIG. 9  is a flowchart of an exemplary sheet feeding process of modification 1; 
         FIG. 10  schematically illustrates the sheet feed unit according to modification 1; 
         FIG. 11  schematically illustrates the sheet feed unit according to modification 1; 
         FIG. 12  schematically illustrates the sheet feed unit according to modification 2; 
         FIG. 13  schematically illustrates the sheet feed unit according to modification 3; 
         FIG. 14  schematically illustrates the sheet feed unit according to modification 4; 
         FIG. 15  schematically illustrates the sheet feed unit according to modification 4; 
         FIG. 16  schematically illustrates the sheet feed unit according to modification 5; and 
         FIG. 17  illustrates a sheet sucked on both of an upstream side conveyance belt and a downstream side conveyance belt. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the following, an embodiment of the present invention is described in detail with reference to the accompanying drawings.  FIG. 1  schematically illustrates a general configuration of image forming system  1  according to the embodiment of the present invention. As illustrated in  FIG. 1 , image formation system  1  has a configuration in which external large capacity sheet feeding apparatus  10  (hereinafter referred to as “sheet feeding apparatus  10 ”) is connected to a lateral side (in  FIG. 1 , right side) of image forming apparatus  20 . 
     Sheet feeding apparatus  10  includes therein triple-stage sheet feed units  10 A to  10 C, and feeds sheets to image forming apparatus  20  one by one. As illustrated in  FIG. 2 , sheet feeding apparatus  10  includes control section  100  having central processing unit (CPU)  101 , read only memory (ROM)  102 , random access memory (RAM)  103  and the like. Control section  100  integrally controls an operation of each block of sheet feeding apparatus  10  in conjunction with control section  28  of image forming apparatus  20 . To be more specific, control section  100  controls operations of sheet storage section  11 , suction conveyance section  12 , floating air blowing section  13 , separation air blowing section  14 , and outlet roller section  15  on the basis of a control signal from image forming apparatus  20 , an input signal from sheet height detection section  181 , conveyance state detection section  182 , and sheet detection section  183  described later or the like. Details of sheet feed units  10 A to  10 C are described later. 
     Image forming apparatus  20  is a color-image forming apparatus of an intermediate transfer system using electrophotographic process technology. A longitudinal tandem system is adopted for image forming apparatus  20 . In the longitudinal tandem system, respective photoconductor drums corresponding to the four colors of YMCK are placed in series in the travelling direction (vertical direction) of an intermediate transfer belt, and the toner images of the four colors are sequentially transferred to the intermediate transfer belt in one cycle. That is, image forming apparatus  20  transfers (primary-transfers) toner images of yellow (Y), magenta (M), cyan (C), and black (K) formed on the photoconductor drums to the intermediate transfer belt, and superimposes the toner images of the four colors on one another on the intermediate transfer belt. Then, image forming apparatus  20  transfers (secondary-transfers) the resultant image to a sheet, to thereby form an image. 
     Image forming apparatus  20  includes image reading section  21 , operation display section  22 , image processing section  23 , image forming section  24 , fixing section  25 , sheet feeding section  26 , sheet conveyance section  27 , and control section  28 . 
     Control section  28  includes a central processing unit (CPU), a read only memory (ROM), a random access memory (RAM) and the like. The CPU reads a program suited to processing contents out of the ROM, develops the program in the RAM, and integrally controls an operation of each block of image forming apparatus  20  in cooperation with the developed program. In addition, control section  28  controls the operation of sheet feeding apparatus  10  in conjunction with control section  100  of sheet feeding apparatus  10 . 
     Image reading section  21  includes an auto document feeder (ADF), a document image scanning device (scanner) and the like. In image reading section  21 , a document conveyed from the auto document feeder onto the contact glass or a document placed on the contact glass is read by the document image scanning device, and input image data is generated. 
     Operation display section  22  is composed of a liquid crystal display (LCD) with a touch panel, and functions as a display section and an operation section, for example. 
     Image processing section  23  performs on the input image data a digital image process such as a compression process, and various kinds of correction processes such as tone correction, color correction and shading correction in accordance with the initial setting or the user setting. Image forming section  24  is controlled on the basis of the image data that has been subjected to the above-mentioned processes. 
     Image forming section  24  forms images of colored toners of Y component, M component, C component, and K component on the basis of the image data. Image forming section  24  includes a photoconductor drum, a charging device, an exposing device, a developing device, and an intermediate transfer device. In image forming section  24 , the surface of the photoconductor drum is uniformly charged by the charging device. The charged photoconductor drum is irradiated with laser light based on the image data by the exposing device, and thus an electrostatic latent image is formed on the surface of the photoconductor drum. Then, the photoconductor drum on which an electrostatic latent image is formed is supplied with toner by the developing device, and thus the electrostatic latent image is visualized to form a toner image. This toner image is transferred to a sheet by the intermediate transfer device having the intermediate transfer belt and the like. 
     Fixing section  25  includes an upper fixing section having a fixing side member disposed on a fixing surface (the surface on which a toner image is formed) side of a sheet, a lower fixing section having a back side supporting member disposed on the rear surface (the surface opposite to the fixing surface) side of a sheet, a heating source, and the like. The back side supporting member is brought into pressure contact with the fixing side member, whereby a fixing nip for conveying a sheet in a tightly sandwiching manner is formed. At the fixing nip, fixing section  25  applies heat and pressure to a sheet on which a toner image has been secondary-transferred to fix the toner image on the sheet. 
     Sheet feeding section  26  includes a plurality of (in  FIG. 1 , three) sheet feed trays. Sheets (standard type sheets and special type sheets) discriminated on the basis of their basis weight, size (such as the length and the width) and the like are stored in the sheet feed trays on a predetermined type basis. 
     Sheet conveyance section  27  conveys the sheet fed from sheet feeding section  26  or sheet feeding apparatus  10  to image forming section  24 . At the time when the sheet passes through the secondary transfer section of image forming section  24 , the toner image on the intermediate transfer belt is secondary transferred to one surface (front surface) of the sheet at a time, and a fixation process is performed at fixing section  25 . The sheet on which an image has been formed is ejected out of the apparatus by a sheet ejection roller. In the case where images are formed on the both sides of a sheet, a sheet on which an image has been formed on one surface thereof is conveyed to a rear surface conveyance path, and then conveyed to image forming section  24  in an inverted state. 
     Now sheet feed units  10 A to  10 C are described.  FIG. 3  is a perspective view illustrating sheet feed units  10 A to  10 C which are a main part of sheet feeding apparatus  10 . In  FIG. 3 , suction conveyance section  12  is shifted to the downstream side in the sheet conveyance direction. In the following, the “downstream side in the sheet conveyance direction” is also referred to as “front end side” or referred simply to as “downstream side,” and the “upstream side in the sheet conveyance direction” is also referred to as “rear end side” or referred simply to as “upstream side.” 
     As illustrated in  FIG. 3 , each of sheet feed units  10 A to  10 C includes sheet storage section  11 , suction conveyance section  12 , floating air blowing section  13 , separation air blowing section  14 , outlet roller section  15  (see  FIG. 1 ), guide section  16  (see  FIG. 1 ) and the like. 
     Sheet storage section  11  includes sheet stand  111 , front end restriction member  112 , rear end restriction member  113 , and side end restriction members  114  and  115 , and stores a large number of sheets. For example, 1,300 sheets are stored in sheet storage section  11  of sheet feed unit  10 A, and 1,850 sheets are stored in each sheet storage section  11  of sheet feed units  10 B and  10 C, and in total, about 5,000 sheets are stored. Sheet storage section  11  can be drawn out from sheet feeding apparatus  10  with a guide rail (not illustrated). While sheet feed units  10 A to  10 C are different from each other in the number of stored sheets as described above, sheet feed units  10 A to  10 C have configurations substantially identical to each other. In the following description, sheet feed unit  10 A is described as an example, and the description of sheet feed units  10 B and  10 C is omitted. 
     Sheet stand  111  is vertically movable such that the upper end surface (uppermost sheet) of sheet bundle SS placed thereon is always located at a predetermined position. At the time of replenishment of sheets, sheet stand  11  goes down to the lowermost position. The vertical movement operation of sheet stand  111  is controlled by control section  100 . Front end restriction member  112  is fixed at the front end of sheet stand  111 , and sets the position of the front end of sheet bundle SS. 
     Rear end restriction member  113  is configured to be movable in the sheet conveyance direction, and is disposed to conform to the sheet length of sheet bundle SS. Rear end restriction member  113  softly presses sheet bundle SS from the rear end side of sheet bundle SS to set the position of the rear end of sheet bundle SS. Rear end restriction member  113  has a height and a shape which always allow for the setting of the position of the rear end of the sheet even when the sheet is floated by air blow of floating air blowing section  13  or separation air blowing section  14 . 
     Height detection section  181  is disposed at rear end restriction member  113 . Height detection section  181  detects the height of the uppermost part of sheet bundle SS loaded on sheet stand  111 . Control section  100  controls the vertical movement operation of sheet stand  111  on the basis of the detection result of sheet height detection section  181 . Side end restriction members  114  and  115  are configured to be movable in the sheet width direction, and are disposed to conform to the sheet width of sheet bundle SS. Side end restriction members  114  and  115  softly press sheet bundle SS from the both sides in the sheet width direction to set the side end position of sheet bundle SS. 
     Side end restriction members  114  and  115  have a hollow structure, and floating air blowing section  13  is disposed inside side end restriction members  114  and  115 . Air blow ports  114   a  and  115   a  for the air blowing from floating air blowing section  13  are provided at an upper part of the internal surface (which makes contact with sheet bundle SS) of side end restriction members  114  and  115 . Air blow ports  114   a  and  115   a  are disposed such that air blow ports  114   a  and  115   a  at least partly overlap suction conveyance section  12  in the sheet conveyance direction. 
     Suction conveyance section  12  includes a plurality of suction conveyance sections  12 A disposed side by side in the sheet width direction on the downstream side in the sheet conveyance direction, and a plurality of suction conveyance sections  12 B disposed side by side in the sheet width direction on the upstream side in the sheet conveyance direction. Suction conveyance section  12  is disposed above sheet stand  111 . Control section  100  individually controls suction conveyance section  12 A and suction conveyance section  12 B. To be more specific, depending on the position of the sheet being conveyed, suction conveyance section  12 A and suction conveyance section  12 B may simultaneously convey the same sheet, or suction conveyance section  12 B may stop the conveyance of the next sheet while suction conveyance section  12 A is conveying a sheet. These states are determined by conveyance state detection section  182  described later. 
     The relationships between the sheet length of sheet bundle SS (sheet length) loaded on sheet stand  111  and installation of suction conveyance sections  12 A and  12 B are as follows. The first case is a case where the leading end of the sheet exceeds the conveyance surface of conveyance belt  121 B. The second case is a case where the leading end of the sheet falls within the range of the conveyance surface of conveyance belt  121 B. The third case is a case where the leading end of the sheet does not reach the conveyance surface of conveyance belt  121 B of suction conveyance section  12 B (described later). The first case will be mainly described below, and, regarding the second and third cases, the configurations different from the first case will be described. 
     Suction conveyance section  12 A disposed on the downstream side and suction conveyance section  12 B disposed on the upstream side have configurations substantially identical to each other. Suction conveyance section  12 A will be described below as an example, and, regarding suction conveyance section  12 B, the configurations different from those of suction conveyance section  12 A will be mainly described. 
     Suction conveyance section  12 A includes endless conveyance belt  121 A (which corresponds to “first conveyance belt” of the embodiment of the present invention) and air absorption section  123 A (which corresponds to “first absorption section” of the embodiment of the present invention) disposed inside conveyance belt  121 A. 
     Conveyance belt  121 A is provided with a large number of absorption ports  122  over the entire surface thereof. Conveyance belt  121 A is wound around roller  125  provided on the upstream side of air absorption section  123 A in sheet conveyance direction, and roller  126  provided on the downstream side of air absorption section  123 A in the sheet conveyance direction. Rollers  125  and  126  are formed in a drum shape (crown shape) for the purpose of preventing the lateral shifting of conveyance belt  121 A wound on the rollers  125  and  126 . Furthermore, rollers  125  and  126  may have flanges at the peripheries of the both end portions in the axis direction. Roller  126  rotates to follow the travelling of conveyance belt  121 A. The sheet conveyance surface of conveyance belt  121 A is a horizontal surface. 
     A plurality of rollers  125  corresponding to a plurality of conveyance belts  121 A are provided, and attached on common roller shaft  125   a . Roller shaft  125   a  is connected with a drive motor through a power transmission mechanism (not illustrated). When the drive motor is driven into rotation by control section  100 , rollers  125  rotate and conveyance belts  121 A travel in a certain direction. 
     A plurality of rollers  126  corresponding to a plurality of conveyance belts  121 A are provided, and are attached on common roller shaft  126   a.    
     Air absorption section  123 A includes absorption duct  124  and an absorption fan (not illustrated). Absorption duct  124  extends through conveyance belts  121 A toward the depth side of the device, and has on the bottom surface thereof absorption ports disposed at positions corresponding to conveyance belts  121 A. The absorption fan is disposed at the deepest position of absorption duct  124 . When the absorption fan operates, the internal pressure of absorption duct  124  becomes negative, and a sheet is absorbed toward conveyance belt  121 A through absorption port  122  and sucked on conveyance belt  121 A. The operation of the absorption fan is controlled by control section  100 . 
     Subsequently, the configurations of suction conveyance section  12 B different from those of suction conveyance section  12 A will be mainly described. Sheet feeding apparatus  10  includes a sheet rear end detection section configured to detect a passing of the rear end of a sheet through conveyance belt  121 B of suction conveyance section  12 B on the upstream side in the sheet conveyance direction (which corresponds to “second conveyance belt” of the embodiment of the present invention”). The sheet rear end detection section is disposed at a position between the front end (downstream end) and the rear end (upstream end) of the conveyance surface (suction surface) of conveyance belt  121 B, and detects suction of a second sheet (which is a sheet conveyed next to the uppermost sheet of sheet bundle SS placed on sheet stand  111 ) by the air absorption section  123 B (which corresponds to “second absorption section” of the embodiment of the present invention) immediately after the rear end of a first sheet (which is the uppermost sheet) passes through that position. Accordingly, the position where the sheet rear end detection section is disposed is a position where suction of the second sheet is started, on the most upstream side in the region from the front end to the rear end of the conveyance surface of conveyance belt  121 B. The position where the sheet rear end detection section is disposed is changed by the suction force of air absorption section  123 B for the second sheet and the like. The position of the sheet rear end detection section is as follows, for example. In the case where suction of the second sheet is started immediately after the rear end of the first sheet passes through a center portion of the conveyance surface, the position of the sheet rear end detection section is located at a center portion of the conveyance surface. In the case where suction of the second sheet is started immediately after the first sheet has completely passed over the conveyance surface of conveyance belt  121 B, the position of the sheet rear end detection section is located at the front end of the conveyance surface. In the case where suction of the second sheet is started immediately after the rear end of the first sheet has passed through the rear end of the conveyance surface of conveyance belt  121 B, the position of the sheet rear end detection section is located at the rear end of conveyance surface. 
     In the present embodiment, the sheet rear end detection section is composed of conveyance state detection section  182  (see  FIG. 1 ) configured to detect the conveyance state of a sheet on conveyance belt  121 B, and includes a sensor. In conveyance state detection section  182 , the sensor is disposed in the close proximity of a sheet sucked on second conveyance belt  121 B at a position between conveyance belts  121 B and  121 B on the inner side of four conveyance belts  121 B disposed side by side, for example. It is to be noted that a micro switch which turns on when a sensor (contact member) receives a contact pressure from a sheet and turns off when the contact pressure is not received is used for conveyance state detection section  182 , for example. 
     Techniques of an optical mouse are applicable to conveyance state detection section  182 , for example. That is, conveyance state detection section  182  includes a light emission section and a light reception section (image sensor) as a sensor, and detects the conveyance state (whether the sheet is being conveyed or stopped) of the sheet sucked on conveyance belt  121 B on the basis of the variation between new and old image information obtained from reflection light from a sheet on the conveyance belt. 
     In addition, techniques of a mechanical mouse are applicable for conveyance state detection section  182 , for example. In this case, conveyance state detection section  182  includes a rotational body (such as a roller and a ball, for example) as a sensor, and detects the conveyance state of the sheet sucked on conveyance belt  121 B on the basis of the rotation of the rotational body. 
     Now, the detection timing of conveyance state detection section  182  will be described. When the rear end of the first sheet (the uppermost sheet of sheet bundle SS placed on sheet stand  111 ) has not passed through a position immediately below conveyance state detection section  182 , the detection object is the first sheet, and therefore conveyance state detection section  182  detects that the sheet is being conveyed. On the other hand, after the rear end of the first sheet has passed through a position immediately below conveyance state detection section  182 , the detection object is the second sheet, and therefore conveyance state detection section  182  detects that the sheet is stopped. That is, the timing when the detection result of conveyance state detection section  182  is changed from “conveyance” to “stop” is the timing when the rear end of the first sheet passes through a center portion of the conveyance surface of conveyance belt  121 B and immediately before the second sheet is sucked on the second conveyance belt by air absorption section  123 B, that is, an optimum timing for stopping the travelling of conveyance belt  121 B. When the travelling of conveyance belt  121 B is stopped at the above-mentioned timing, the second sheet sucked on conveyance belt  121 B is not conveyed, and thus overlapped feeding of the second sheet and the first sheet can be prevented from occurring. 
     Floating air blowing section  13  includes air sending fan  131  and air introduction duct  132 . Floating air blowing section  13  is disposed inside both of side end restriction members  114  and  115 .  FIG. 3  illustrates only floating air blowing section  13  disposed in side end restriction member  115 . Air sending fan  131  can control the air volume on the basis of the sheet length, the sheet type, the basis weight and the like for example, and can send air with an optimum air volume. The operation of air sending fan  131  is controlled by control section  100 . 
     Air introduction duct  132  of floating air blowing section  13  disposed inside side end restriction member  115  is communicated with air blow port  115   a . An air introduction duct (not illustrated) of the floating air blowing section (not illustrated) disposed inside side end restriction member  114  is communicated with air blow port  114   a . Floating air blowing section  13  is disposed inside side end restriction members  114  and  115 , and therefore moves along with the movement of side end restriction members  114  and  115  when the sheet length is changed. 
     When air sending fan  131  operates in floating air blowing section  13 , floating air is sent upward, and the direction of the floating air is changed by 90° at air introduction duct  132 . Then, the floating air is sent from the both sides in the sheet width direction to the upper part of sheet bundle SS through blow ports  114   a  and  115   a . Thus, several upper sheets of sheet bundle SS are floated. 
     Separation air blowing section  14  includes air sending fan  141  and air introduction duct  142 . Separation air blowing section  14  is disposed on the downstream side of front end restriction member  112  in the sheet conveyance direction. Air sending fan  141  can control the air volume on the basis of the size (such as the length and the width), the sheet type, the basis weight and the like of the sheet, and can send air with an optimum air volume. The operation of air sending fan  141  is controlled by control section  100 . 
     Separation air blowing section  14  may include a wind direction switching plate (not illustrated) such that air the air sending direction can be switched between the region around the front end of suction conveyance section  12 A on the downstream side, and a region around the front end of sheet bundle SS. In this case, separation air blowing section  14  sends floating air to a region around the front end of sheet bundle SS when floating air is sent by floating air blowing section  13 , and sends separation air to a region around the front end of suction conveyance section  12 A when the floating air sent from floating air blowing section  13  is stopped. That is, separation air blowing section  14  also functions as a floating air blowing section. 
     When air sending fan  141  operates in separation air blowing section  14 , air is sent to a region around the front end of sheet bundle SS or a region around the front end of suction conveyance section  12  through air blow port  142   a  of air introduction duct  142 . By sending floating air to a region around the front end of sheet bundle SS, several upper sheets of sheet bundle SS can be efficiently floated. In addition, by sending separation air to a region around the front end of suction conveyance section  12 , the second and subsequent sheets can be separated from the multiple sheets sucked on conveyance belt  121 A, and thus only the first sheet can be conveyed by sucking the sheet on conveyance belt  121 A. 
     Outlet roller section  15  includes upper conveyance roller  151  (see  FIG. 1 ) and lower conveyance roller  152  (see  FIG. 1 ) which makes contact with upper conveyance roller  151 . Upper conveyance roller  151  is a driving roller, and lower conveyance roller  152  is a driven roller. Outlet roller section  15  sandwiches a sheet conveyed by suction conveyance sections  12 A and  12 B between upper conveyance roller  151  and lower conveyance roller  152 , and sends the sheet toward the downstream side in the sheet conveyance direction. 
     Guide section  16  is disposed on the upstream side of output roller section  15  in the sheet conveyance direction. Guide section  16  includes upper side guide plate  161  (see  FIG. 1 ) and lower side guide plate  162  (see  FIG. 1 ), and guides a sheet conveyed by suction conveyance sections  12 A and  12 B to outlet roller section  15 . A sheet is conveyed through a gap formed by upper side guide plate  161  and lower side guide plate  162 . 
     Sheet detection section  183  (see  FIG. 1 ) is disposed on the upstream side of output roller section  15  in the sheet conveyance direction. Sheet detection section  183  (so-called feed sensor) is composed of a reflection-type light sensor for example, and detects the presence and absence of a sheet on the basis of the intensity of received reflection light. 
     Sheet conveyance states, and control of suction conveyance section  12 A and suction conveyance section  12 B will be described with reference to  FIG. 5  to  FIG. 8 .  FIG. 5  illustrates a state where the uppermost sheet (the first sheet) of sheet bundle SS placed on sheet stand  111  is sucked by suction conveyance section  12 A and suction conveyance section  12 B.  FIG. 6  illustrates a state where suction conveyance section  12 A and suction conveyance section  12 B convey the first sheet.  FIG. 7  illustrates a problem in a conventional conveyance device.  FIG. 8  illustrates a state where suction conveyance section  12 A conveys a first sheet, and suction conveyance section  12 B sucks the next sheet (the second sheet). 
     In  FIG. 5 , length L 1  indicates the length of the rear end of a hanging first sheet which is not sucked on downstream conveyance belt  121 A and upstream conveyance belt  121 B, and makes contact with the second sheet. Meanwhile, in  FIG. 7 , length L 2  indicates a total length of an intermediate portion and a rear end portion of a hanging first sheet which is not sucked on downstream conveyance belt  121 A, and makes contact with the second sheet. When a first sheet is conveyed, a frictional resistance is caused by the contact between the first and the second sheets. Length L 1  of the contact between the first and the second sheets illustrated in  FIG. 5  is considerably smaller than length L 2  illustrated in  FIG. 7 , and therefore the frictional resistance at this time is small. Thus, overlapped feeding of the second sheet and the first sheet can be prevented from occurring. In addition, damaging of sheets due to friction can be prevented. 
     In the state illustrated in  FIG. 5 , the first sheet is sucked on suction conveyance section  12 A and suction conveyance section  12 B (first state). After the first state, suction conveyance section  12 A and suction conveyance section  12 B convey the sucked sheet as illustrated in  FIG. 6  (second state). At this time, when the rear end of the sheet passes through conveyance state detection section  182 , the travelling of conveyance belt  121 B is stopped while the absorption of air absorption section  123 B is continued (third state). In this state, when the sheet is completely removed from the rear end of suction conveyance section  12 B (removed from air absorption section  123 B), absorption of air absorption section  123 B is still continued, and accordingly suction conveyance section  12 A conveys the first sheet whereas suction conveyance section  12 B sucks the second sheet on suction conveyance section  12 B as illustrated in  FIG. 8  (fourth state). In the fourth state, suction conveyance section  12 A conveys the first sheet whereas suction conveyance section  12 B sucks the second sheet on suction conveyance section  12 B, and thus the first sheet and the second sheet can be surely separated from each other. In addition, the second sheet separated from the third sheet is sucked on suction conveyance section  12 B, and therefore, when suction conveyance section  12 A absorbs the second sheet after conveyance of the first sheet, the sheet is easily absorbed in comparison with absorption of a second sheet using the conventional suction conveyance section. As a result, the productivity is improved in terms of sheet conveyance. 
       FIG. 4  is a flowchart of an exemplary sheet feeding process in the present embodiment. The sheet feeding process illustrated in  FIG. 4  is achieved when CPU  101  executes a predetermined program stored in ROM  102  in response to input of sheet feeding start information from image forming apparatus  20 , for example. While sending and stopping of the floating air performed in the sheet feeding process are not mentioned here, several upper sheets of sheet bundle SS stored in sheet storage section  11  are floated against the own weight of the sheets by sending the floating air at an optimum timing. Likewise, while sending and stopping of the separation air performed in the sheet feeding process are not mentioned here, the second and subsequent sheets are separated from a plurality of sheets sucked on conveyance belt  121 A by sending the separation air at an optimum timing. Further, the sheet length of the sheet to be fed is greater than the length of the conveyance surface of conveyance belt  121 B. 
     At step S 110 , control section  100  controls downstream air absorption section  123 A and upstream air absorption section  123 B to start air absorption. Air absorption sections  123 A and  123 B perform air absorption at all times until the sheet feeding process is completed. As illustrated with outlined arrow in  FIG. 5 , with the air absorption, the uppermost first sheet is sucked on downstream conveyance belt  121 A and upstream conveyance belt  121 B (first state). 
     At step S 120 , control section  100  controls suction conveyance sections  12 A and  12 B to move conveyance belts  121 A and  121 B. Thus, the first sheet is conveyed in a state where it is sucked on conveyance belts  121 A and  121 B (second state). In  FIG. 6 , the travelling of conveyance belts  121 A and  121 B is illustrated with the arrow of the rotational direction of roller  126 , and in addition, the conveyance of the first sheet is illustrated with the arrow of the sheet conveyance direction. It is to be noted that control section  100  performs an operation for starting the rotation of outlet roller section  15 . 
     At step S 130 , control section  100  determines whether the first (uppermost) sheet has passed over the center portion of the conveyance surface of conveyance belt  121 B on the basis of the detection result of conveyance state detection section  182 . When conveyance state detection section  182  detects “conveyance” of a sheet (S 130 : NO), it can be determined that the rear end of the first sheet has not yet passed through the center portion of the conveyance surface. Accordingly, the process is returned to step S 120 , and control section  100  controls suction conveyance sections  12 A and  12 B to continue the travelling of conveyance belts  121 A and  121 B. When the detection result of conveyance state detection section  182  is changed from “conveyance” to “stop,” (S 130 : YES), it can be determined that the rear end of the first sheet has passed over the center portion of the conveyance surface, and the process is advanced to step  140 . 
     At step S 140 , control section  100  controls suction conveyance section  12 B to stop the travelling of conveyance belt  121 B (third state). The second sheet receives a force in the sheet conveyance direction due to the frictional resistance with the first sheet; however, since control section  100  controls suction conveyance section  12 B to stop the travelling of conveyance belt  121 B, the second sheet sucked on conveyance belt  121 B does not move (fourth state). Thus, overlapped-sheets feeding can be prevented from occurring (see  FIG. 8 ). In  FIG. 8 , the travelling of conveyance belt  121 A is illustrated with the arrow of the rotational direction of roller  126 , and stop of the travelling of conveyance belt  121 B is represented by not showing the arrow of the rotational direction of roller  126 . 
     Control section  100  controls suction conveyance section  12 A to continue travelling of control conveyance belt  121 A, and in addition, operates to continue the sheet ejection of outlet roller section  15 . Thus, the first sheet is ejected to the downstream side in the sheet conveyance direction. 
     At step S 150 , control section  100  determines the presence and absence of the first sheet on the basis of the detection result of sheet detection section  183 . When sheet detection section  183  detects the “presence” of first (uppermost) sheet (S 150 : NO), control section  100  again determines the presence and absence of the first sheet. When sheet detection section  183  detects the “absence” of the first sheet (S 150 : YES), the process is advanced to step S 160 . 
     At step S 160 , control section  100  controls suction conveyance section  12 A to stop the travelling of conveyance belt  121 A. 
     At step S 170 , control section  100  determines whether the sheet feeding request has been made on the basis of the sheet feeding information sent from image forming apparatus  20 . When control section  100  determines that no sheet feeding request has been made (S 170 : NO), the sheet feeding process is ended. When control section  100  determines that a sheet feeding request has been made (S 170 : YES), the process is advanced to step S 120 . 
     In this manner, sheet feeding apparatus  10  of the present embodiment includes control section  100  that operates to stop the travelling of conveyance belt  121 B when it is determined that the rear end of the sheet has passed over the center portion of the conveyance surface of conveyance belt  121 B on the basis of the detection result of conveyance state detection section  182  during conveyance of the first (uppermost) sheet. When conveyance belt  121 B is in a stopped state, the second sheet is sucked on conveyance belt  121 B, and therefore, the movement of the second sheet is limited when the first sheet is conveyed. In this manner, overlapped-sheets feeding can be prevented from occurring. 
     In addition, during the conveyance of the first sheet, a part of the second sheet is sucked by upstream air absorption section  123 B to perform the floating. Thus, the suction force between the second sheet and the third sheet is reduced, and by air absorption of downstream air absorption section  123 A, floating of the second sheet is facilitated. As a result, the time until suction of the sheet on downstream conveyance belt  121 A is reduced, and feeding of the second sheet can be immediately started, and consequently, the conveyance time of the second sheet can be reduced. In contrast, in the case of a sheet feeding apparatus having a conveyance belt of an entire surface suction type in which the conveyance surface (suction surface) is provided over the entire surface of the sheet, the feeding operation of the second sheet is started after the conveyance of the first sheet is completed. That is, in sheet feeding apparatus  10  of the present embodiment, the time for the conveyance of the second sheet can be reduced by starting the feeding operation of the second sheet during the conveyance of the first sheet, and therefore the productivity can be improved in comparison with the sheet feeding apparatus of the entire surface suction type. 
     Furthermore, in a conventional technique disclosed in Japanese Patent Application Laid-Open No. 6-9083, it is necessary to upsize the apparatus and additionally provide the air absorption section so that the air absorption section conforms to the maximum length of the sheet to be used. In addition, in view of using sheets ranging from a sheet such as a post card having a short length to a long sheet which is long in the sheet conveyance direction, the structure is complicated, and the number of components and the cost increase. In addition, in the conventional technique disclosed in Japanese Patent Application Laid-Open No. 9-216750, a slide valve corresponding to the length of the sheet to be conveyed is required, and consequently the number of components and the cost increase. In contrast, in the above-mentioned image formation system according to the embodiment, it is only necessary to dispose suction conveyance section  12 A on the downstream side in the sheet conveyance direction and suction conveyance section  12 B on the upstream side in the sheet conveyance direction, and it is not necessary to provide the suction conveyance section over the entirety of the surface of the sheet. Consequently, in comparison with the case of the conventional technique where the suction conveyance section and the slide valve are provided over the entirety of the surface of the sheet, the increase of the number of components and the cost can be suppressed. 
     The case (first case) where the leading end of the sheet (sheet length) exceeds the conveyance surface of conveyance belt  121 B has been described above. In the case where the leading end of the sheet falls within the range of the conveyance surface of conveyance belt  121 B (second case), the sheet can be conveyed as with the first case. It should be noted that, in the case where control section  100  controls air absorption sections  123 A and  123 B to suck the sheet on conveyance belts  121 A and  121 B, when the rear end of the sheet is located at a position downstream side of the center portion of the conveyance surface of conveyance belt  121 B, control section  100  naturally controls suction conveyance section  12 B to stop the travelling of conveyance belt  121 B. In addition, in the case where the leading end of the sheet does not reach the conveyance surface of conveyance belt  121 B of suction conveyance section  12 B (third case), naturally, control section  100  controls air absorption section  123 B to stop the air absorption, and control section  100  controls suction conveyance section  12 B to stop the travelling of conveyance belt  121 B. 
     It is desirable to set the positional relationship between suction conveyance section  12 A and suction conveyance section  12 B in consideration of the length of hanging without being sucked on upstream conveyance belt  121 B (L 1  of  FIG. 5 ), and the length of hanging without being sucked on downstream conveyance belt  121 A. To be more specific, the positions of suction conveyance section  12 A and suction conveyance section  12 B can be set in accordance with the length of the sheet storage section, and when the positions are set such that the length of the hanging without being sucked on upstream conveyance belt  121 B and the length of the hanging without being sucked on downstream conveyance belt  121 A are minimized, generation of the frictional resistance due to the contact between the first and the second sheets can be suppressed. In addition, since a sheet, which has been floated once by suction conveyance section  12 , generates less frictional resistance due to the contact between the first and the second sheets in comparison with a sheet which has not been floated, it is possible to set the positions such that the length of the hanging without being sucked on upstream conveyance belt  121 B is smaller than the length of the hanging without being sucked on downstream conveyance belt  121 A. 
     Modification 1 
       FIG. 9  is a flowchart of an exemplary sheet feeding process of modification 1. In the sheet feeding process described in  FIG. 9 , processes different from the sheet feeding process of the above-mentioned embodiment described in  FIG. 4  will be mainly described, and the steps in which the same processes are performed are denoted with the same reference numerals and the description thereof will be omitted. 
     In the above-mentioned embodiment, control section  100  determines whether the rear end of the sheet has passed through the center portion of the conveyance surface of conveyance belt  121 B on the basis of the detection result of conveyance state detection section  182 . In contrast, in modification 1, the determination is made on the basis of the sheet length and the sending time of the sheet by conveyance belt  121 B. To be more specific, a timer (not illustrated) measures the time elapsed after the start of sheet conveyance (which corresponds to “sending time of the sheet by the second conveyance belt” of the embodiment of the present invention). A preliminarily set time based on the sheet length (sheet length) and the travelling speed of conveyance belt  121 B (the sending speed of a sheet by conveyance belt  121 B) is stored in an internal memory (not illustrated) of control section  100 . Control section  100  reads out the preliminarily set time from the internal memory, and compares the preliminarily set time with the elapsed time measured with the timer to determine whether the rear end of the sheet has passed through the center portion of the conveyance surface. 
     At step S 131 , control section  100  determines whether the preliminarily set time has elapsed from the start of sheet conveyance. When control section  100  does not determine that the preliminarily set time has elapsed from the start of sheet conveyance (S 131 : NO), the process is advanced to step S 120 . When control section  100  determines that the preliminarily set time has elapsed from the start of sheet conveyance (S 131 : YES), the process is advanced to step S 141 . 
     At step S 141 , control section  100  controls upstream air absorption section  123 B to stop the air absorption (see  FIG. 10 ). In  FIG. 10 , the stop of air absorption is indicated by not showing the outlined arrow at air absorption section  123 B. Since the absorption force by air is reduced, the second sheet is not sufficiently sucked on conveyance belt  121 B, or is separated from conveyance belt  121 B by the own weight. As a result, the second sheet is not conveyed by the travelling of conveyance belt  121 B, and thus overlapped-sheets feeding can be prevented from occurring. 
     At step S 142 , control section  100  controls suction conveyance section  12 B to stop the travelling of conveyance belt  121 B, and, controls upstream air absorption section  123 B to restart the air absorption (see  FIG. 11 ). In  FIG. 11 , air absorption is indicated by showing the outlined arrow at air absorption section  123 B, and a state where the travelling of conveyance belt  121 B of stopped is indicated by not showing the arrow in the rotational direction of roller  126 . It is to be noted that the timing at which the travelling of conveyance belt  121 B is stopped, and, the air absorption is restarted may be a timing immediately after the air absorption is stopped at step S 141 , or may be a timing after a predetermined time has elapsed from the stop of the absorption, such as a timing at which the first sheet has completely passed over the conveyance surface of conveyance belt  121 B, for example. 
     Since the air absorption is restarted during the conveyance of the first sheet to suck a part of the second sheet on conveyance belt  121 B, productivity can be improved as with the above-mentioned embodiment. 
     Modification 2 
       FIG. 12  schematically illustrates sheet feed units  10 A to  10 C according to modification 2. In modification 2, on the basis of information (included in sheet feeding information sent from image forming apparatus  20 ) on the length of the sheet stored in sheet storage section  11 , control section  100  moves upstream conveyance belt  121 B to a position in the sheet conveyance direction in accordance with the sheet length. It is to be noted that conveyance belt  121 B is moved by moving the entirety of suction conveyance section  12 B. Distance L 5  in  FIG. 12  indicates the movement length from the position of conveyance belt  121 B before the movement illustrated by broken line to the position of conveyance belt  121 B after the movement illustrated by solid line. Here, the “position in the sheet conveyance direction in accordance with the sheet length” is a position where the area (hanging total area) of the range in which a hanging part of the first sheet which is out of the conveyance surfaces of conveyance belts  121 A and  121 B (for example, a rear end portion or an intermediate portion of the sheet portion) makes contact with the second sheet is minimized. The position where the total area is minimized can be empirically determined on the basis of the sheet length, the basis weight, the type of the sheet, the length of the conveyance surface of conveyance belt  121 B in the sheet conveyance direction and the like. The position where the total area is minimized is stored in coordination with the sheet length and the like in an internal memory (not illustrated) of control section  100 . By minimizing the total area of the hanging, the frictional resistance between the first sheet and the second sheet is reduced, and a state where overlapped-sheets feeding is not easily generated can be established. 
     Modification 3 
       FIG. 13  schematically illustrates sheet feed units  10 A to  10 C according to modification 3. In modification 3, length L 3  of the conveyance surface of conveyance belt  121 B (suction surface) in sheet conveyance direction is smaller than length L 4  of the conveyance surface of conveyance belt  121 A (L 3 ≦L 4 ). By setting length L 3  of the conveyance surface of conveyance belt  121 B to a length smaller length L 4  of the conveyance surface of conveyance belt  121 A, downstream conveyance belt  121 A can have a wide conveyance surface (suction surface) as the main force of the conveyance force in sheet conveyance. On the other hand, upstream conveyance belt  121 B can have a narrow conveyance surface as an auxiliary force in the conveyance force. In addition, by reducing the length of the conveyance surface, the weight of suction conveyance section  12 B is reduced, and, advantageously, suction conveyance section  12 B can be easily moved. Further, by setting length L 3  of the conveyance surface of conveyance belt  121 B and length L 4  of the conveyance surface of conveyance belt  121 A to the same length, the conveyance belt and the like can be shared. 
     Modification 4 
       FIG. 14  and  FIG. 15  schematically illustrate sheet feed units  10 A to  10 C according to modification 4. 
     In modification 4, control section  100  controls air absorption section  123 A such that the front end side of a sheet is sucked on conveyance belt  121 A, and in addition, controls air absorption section  123 B such that the rear end side of a sheet is sucked on conveyance belt  121 B. Further, control section  100  controls suction conveyance sections  12 A and  12 B such that the first sheet (for example, an intermediate portion thereof) is deflected in a direction opposite to the second sheet (that is, the upward direction) in a space between conveyance belts  121 A and  121 B in the sheet conveyance direction. Thus, the uppermost first sheet can be sufficiently separated from the second sheet, and separativeness can be improved, and, a state where overlapped-sheets feeding is not easily generated can be established. By deflecting an intermediate portion of a sheet, a rear end portion of the sheet is moved to the downstream side, and along with this movement, the hanging amount of the sheet hanging from the rear end of conveyance belt  121 B is reduced. Also in this manner, a state where overlapped-sheets feeding is not easily generated can be established. 
     It is to be noted that the intermediate portion of the sheet is not required to be deflected in the case where the reading end of the sheet does not reach the conveyance surface of conveyance belt  121 B (second case). However, depending on the basis weight and the type of the sheet, when an intermediate portion of the sheet makes contact with the second sheet when deflected in a direction toward the second sheet (that is, downward direction), the sheet may be deflected in a direction opposite to the second sheet (that is, upward direction). 
     The method for deflecting an intermediate portion of the sheet into the space includes a method in which conveyance belt  121 B on the downstream side is moved with downstream conveyance belt  121 A stopped as illustrated in  FIG. 14 . Alternatively, another method illustrated in  FIG. 15  may also be adopted in which downstream conveyance belt  121 A is moved at a predetermined speed and upstream side travelling of conveyance belt  121 B is moved at a speed higher than the speed of downstream conveyance belt  121 A. In the methods in  FIG. 14  and  FIG. 15 , after predetermined deflection of an intermediate portion of a sheet is formed, downstream conveyance belt  121 A is moved at a speed identical to that of upstream conveyance belt  121 B. In this manner, the sheet is conveyed with the deflected shape of the intermediate portion of the sheet maintained, and thus overlapped-sheets feeding can be prevented from occurring. 
     Modification 5 
       FIG. 16  schematically illustrates sheet feed units  10 A to  10 C according to modification 5. In the sheet feeding process, when the first (uppermost) sheet is sucked on conveyance belts  121 A and  121 B with air absorption sections  123 A and  123 B, the first sheet may possibly hang in a direction toward the second sheet (that is, downward direction) (see  FIG. 17 ). When, in such a hanging state, control section  100  controls suction conveyance sections  12 A and  12 B to move conveyance belt  121 B at a speed higher than that of conveyance belt  121 A as described in modification 4, the intermediate portion of the first sheet is deflected in a direction toward the second sheet (downward direction), and the separation of the sheet is reduced. The deflection of the intermediate portion of the first sheet in a direction toward the second sheet (downward direction) can be prevented by controlling, by control section  100 , suction conveyance sections  12 A and  12 B such that the intermediate portion of the first sheet is deflected in advance in the space in a direction opposite to the second sheet (that is, upward direction) before conveyance belt  121 B is moved at a speed higher than that of conveyance belt  121 A. 
     In modification 5, to preliminarily deflect the intermediate portion of the first sheet upward in the space, any of the following devices is provided. The first device is an air absorption section which sucks a sheet to a curved part wound around roller  125  in conveyance belt  121 A (only the air absorption direction is illustrated in  FIG. 16 ). The second device is an air absorption section which sucks a sheet to a curved part wound around roller  126  in conveyance belt  121 B (only the air absorption direction is illustrated in  FIG. 16 ). The third device is blowing section  127  which applies air jet in a substantially horizontal direction substantially orthogonal to the sheet conveyance direction. It is to be noted that the above-mentioned first to third devices may be used in combination. In addition, air blow ports  114   a  and  115   a  (see  FIG. 3 ) may be used as blowing section  127 . 
     While the embodiment of the present invention has been described in detail, the embodiment of the present invention is not limited to the above-mentioned configurations. For example, control section  100  may determine the hardness of the sheet on the basis of the basis weight and the type of the sheet (for example, plain paper, coated paper) included in sheet feeding information sent from image forming apparatus  20  to, on the basis of the hardness of the sheet, adjust the deflecting amount of an intermediate portion of a sheet by suction conveyance sections  12 A and  12 B. For example, in the case where the sheet is hard, the amount of hanging is slight, and therefore it is not necessary to deflect the sheet. Further, when an end portion of a sheet protrudes from the conveyance surface of conveyance belt  121 B to the upstream side in the sheet conveyance direction, control section  100  may control suction conveyance sections  12 A and  12 B to adjust the deflecting amount of an intermediate portion of a sheet in accordance with the protruding amount. By increasing the deflecting amount, the protruding amount is reduced, and the total area of sheet hanging is reduced. Consequently, overlapped-sheets feeding can be prevented from occurring. 
     While suction conveyance section  12 A is disposed on the downstream side in the sheet conveyance direction, and suction conveyance section  12 B is disposed on the upstream side in the sheet conveyance direction in the above-described embodiment, the present invention is not limited to this configuration. For example, three or more suction conveyance sections may be disposed in a range from the upstream side to the downstream side. By increasing the number of the suction conveyance sections in accordance with the sheet length, the total area of the sheet hanging can be reduced. 
     The embodiments disclosed herein are merely exemplifications and should not be considered as limitative. While the invention made by the present inventor has been specifically described based on the preferred embodiments, it is not intended to limit the present invention to the above-mentioned preferred embodiments but the present invention may be further modified within the scope and spirit of the invention defined by the appended claims. 
     REFERENCE SIGNS LIST 
     
         
           1  Image formation system 
           10  Sheet feeding apparatus 
           11  Sheet storage section 
           12 ,  12 A,  12 B Suction conveyance section 
           100  Control section 
           121 A,  121 B Conveyance belt 
           123 A,  123 B Air absorption section 
           181  Sheet height detection section 
           182  Conveyance state detection section 
           183  Sheet detection section