Patent Publication Number: US-8523171-B2

Title: Paper feeder and image forming apparatus including the same

Description:
BACKGROUND OF THE INVENTION 
     This application claims priority under 35 U.S.C. §119(a) on Patent Application No. 2011-115890 filed in Japan on May 24, 2011, the entire contents of which are herein incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a paper feeder for pulling out and sending papers from a bundle of papers loaded on a paper tray, and an image forming apparatus including this paper feeder. 
     2. Description of the Related Art 
     An example of this kind of paper feeders is disclosed in JP H6-1467 (hereinafter referred to as Patent Literature 1). Here, a paper transport belt is arranged above the paper tray, air is breathed from an air hole on the paper transport belt through an air intake duct towards a fan, and papers in a bundle of papers on the paper tray is attached to the paper transport belt and thus transported. The air exhausted by the same fan is blown through an exhaust duct against the sides of the bundle of papers on the paper tray, thus the air is caused to flow into the respective papers in the bundle of papers to reduce the adhesion between the papers, thereby facilitating pulling out of each paper. Both the air breathed and the air exhausted by a single fan are used to save the number of parts. 
     Further, a valve is provided to the air intake duct, and an air path in the air intake duct is narrowed by this valve, thereby adjusting the amount of air that flows in the air path in the air intake duct. 
     Incidentally, with the configuration where a single fan breaths air from the air hole on the paper transport belt through the air intake duct and causes air to blow through the exhaust duct against the sides of a bundle of papers on the paper tray, the amounts of air breathed by the fan and that exhausted by the fan are approximately the same, and one of the amounts of air decreases as the other decreases. Therefore, when a sheet of paper is attached to the paper transport belt and blocks the air hole on the paper transport belt and the amount of the breathed air decreases, the amount of the air blown against the sides of the bundle of papers on the paper tray accordingly decreases, the air is not caused to flow into the papers, the adhesion between the papers is not reduced, and a transportation error where a plurality of papers are simultaneously pulled out occurs in some cases. 
     Also in Patent Literature 1, the amount of air that flows in the air path in the air intake duct is adjusted by narrowing the air path in the air intake duct with the valve. However, if the air hole on the paper transport belt is blocked and the amount of the air breathed towards the air intake duct reduces, the amount of air is not increased even if the valve is opened. 
     The present invention was made in light of the foregoing conventional problems, and it is an object thereof to provide a paper feeder in which it is assumed a configuration where papers are attached to the paper transport belt and air is blown against the sides of a bundle of papers using the air breathed and exhausted with a single fan, and even if a sheet of paper is attached to the paper transport belt, the amount of air blown against the sides of the bundle of papers does not reduce, and an image forming apparatus including this paper feeder. 
     SUMMARY OF THE INVENTION 
     To solve the foregoing problems, in a paper feeder of the present invention, an air hole that allows air to pass through is formed on a paper transport belt for transporting a sheet of paper, a fan for breathing and exhausting air and an exhaust duct forming an opening for exhausting air are connected to each other, air is exhausted from the fan through the exhaust duct, the fan and an air intake duct for breathing air are connected to each other, air is breathed from the air hole through the air intake duct towards the fan, and a sheet of paper is transported by the paper transport belt by causing the paper to be attached to the paper transport belt. This paper feeder includes: an air state detection unit for detecting at least one of an air pressure and an air flow rate in the air intake duct; an opening and closing unit provided to the air intake duct for opening and closing the opening; and a control unit for opening and closing the opening based on a result of detection by the air state detection unit. 
     Incidentally, in a paper feeder, in some cases the air hole on the paper transport belt is blocked when a sheet of paper is attached to the paper transport belt, and the amount of breathed air reduces. At this time, the amount of air exhausted through the exhaust duct also reduces, and the exhausted air cannot be utilized. For example, in the case where exhausted air is blown through the exhaust duct against the respective papers to reduce the adhesion between the papers, the adhesion between the papers is not reduced, and a transportation error of simultaneously drawing a plurality of papers occurs. 
     Therefore, in the present invention, considering the fact that if the amount of breathed air reduces, the air pressure and the air flow rate in the air intake duct varies, at least one of the air pressure and the air flow rate is detected, and the opening and closing unit is opened or closed based on the detection result. In other words, the opening and closing unit is controlled to open or close the opening in the air intake duct based on the detection result. Thus, if the amount of breathed air reduces, air can be caused to flow from the opening into the air intake duct, the amount of air breathed and exhausted from the fan is increased and restored, the air is blown between the papers, and the transportation error of simultaneously drawing a plurality of papers is prevented. 
     In the paper feeder of the present invention, if either one of the air pressure and the air flow rate in the air intake duct detected by the air state detection unit becomes lower than a predetermined threshold, the control unit controls the opening and closing unit to open the opening, and if either one of the air pressure and the air flow rate in the air intake duct becomes equal to or higher than the predetermined threshold, the control unit controls the opening and closing unit to close the opening. 
     The threshold compared with the air pressure or the air flow rate in the air intake duct may be predetermined, and the opening may be opened if the air pressure or the air flow rate in the air intake duct becomes less than the threshold. Thus, if the amount of breathed air reduces, air can be caused to flow from the opening into the air intake duct. Further, the opening is closed after the air pressure or the air flow rate in the air intake duct becomes the threshold or higher, that is, after the air pressure or the air flow rate in the air intake duct is increased and restored. Therefore, the air suction force from the air hole on the paper transport belt is hardly reduced, and it is possible to suppress reduction of the attachment force of papers to the paper transport belt and keep the state where each paper is attached to the paper transport belt. 
     Further, in the paper feeder of the present invention, if either one of the air pressure and the air flow rate in the air intake duct detected by the air state detection unit becomes lower than a predetermined threshold, the control unit controls the opening and closing unit to open the opening, and when a predetermined time elapses from at time the opening is opened, the control unit controls the opening and closing unit to close the opening. 
     The opening may be opened if the air pressure or the air flow rate in the air intake duct becomes lower than a predetermined threshold. Thus, if the amount of breathed air reduces, air can be caused to flow from the opening into the air intake duct. Further, by closing the opening after a lapse of a predetermined time after the opening is opened, the air suction force from air hole on the paper transport belt is hardly reduced, and it is possible to suppress reduction of the attaching force of papers to the paper transport belt and keep the state where each paper is attached to the paper transport belt. 
     Further, in the paper feeder of the present invention, when the control unit controls the opening and closing unit to open the opening, the control unit controls the fan to accelerate a rotational speed of the fan. 
     In this case, when the opening is opened, air is caused to flow from the opening into the air intake duct, and the amount of air exhausted from the fan is increased and restored. By accelerating the rotational speed of the fan at this time, the amount of air exhausted by the fan can be more promptly restored. 
     Further, in the paper feeder of the present invention, the air state detection unit may be provided near the opening in the air intake duct. 
     Thus, variation in the air pressure or the air flow rate caused by opening the opening can be detected with accuracy. 
     In the paper feeder of the present invention, the air state detection unit may be provided downstream of the opening in the air intake duct in a direction of air flow in the air intake duct. 
     In this case, if the opening is opened, air is caused to flow from the opening through the air intake duct towards the fan. Therefore, by providing the air state detection unit downstream of the opening in a direction of air flow, the variation in the air pressure or the air flow rate can be detected most promptly. 
     The image forming apparatus of the present invention includes the above-described paper feeder of the present invention. Such an image forming apparatus of the present invention also has the same effect as that of the above-described paper feeder of the present invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view of an image forming apparatus to which one embodiment of a paper feeder of the present invention is applied. 
         FIG. 2  is a schematic side view of the paper feeder in the present embodiment. 
         FIG. 3  is a plan view of the paper feeder shown in  FIG. 2 . 
         FIG. 4  is a front view of the paper feeder shown in  FIG. 2 . 
         FIG. 5  is an oblique view of the paper feeder in a state where a paper pull-out unit is detached, viewed diagonally from the back. 
         FIG. 6  is a simplified side view of a paper tray, a wire, an idler pulley, and a take-up pulley in the paper feeder. 
         FIG. 7  includes  FIGS. 7A ,  7 B, and  7 C.  FIGS. 7A and 7B  are simplified side view and plan view of a height position sensor for detecting the height of a bundle of papers loaded on the paper tray, respectively.  FIG. 7C  is a side view showing an operating state of the height position sensor. 
         FIG. 8  is an oblique view of the paper pull-out unit viewed diagonally from the upper front. 
         FIG. 9  is an oblique view of the paper pull-out unit viewed diagonally from the upper back. 
         FIG. 10  is an oblique view of the paper pull-out unit viewed diagonally from the lower back. 
         FIG. 11  is an enlarged plan view of a paper transport belt and the like in the paper pull-out unit. 
         FIG. 12  is a schematic front view of the paper pull-out unit. 
         FIG. 13  includes  FIGS. 13A and 13B .  FIGS. 13A and 13B  are schematic front views of an opening and an opening and closing body in an air intake duct. 
         FIG. 14  is a block diagram showing a configuration of a control system of the paper feeder. 
     
    
    
     DESCRIPTION OF REFERENCE NUMERALS 
       1  Image forming apparatus 
       2  Original reader 
       11  Printing unit 
       12  Paper transport unit 
       13  Paper supply unit 
       14  Large capacity cassette (LCC) 
       71  Paper feeder 
       72  Outer frame body 
       73  Bottom plate 
       74  Paper tray 
       75  Paper pull-out unit 
       76  Paper trailing edge guide 
       77 ,  78  Assist duct 
       79 ,  80  Assist fan 
       81  Paper transport belt 
       82 ,  83  Roller 
       84  Air intake and exhaust fan (fan) 
       85  Air intake duct 
       85   k  Opening 
       86  Exhaust duct 
       92  Pulse motor 
       93  Height position sensor 
       101  Opening and closing body (opening and closing unit) 
       102  Air pressure sensor (air state detection unit) 
       106  Solenoid 
       107  Transport motor 
       111  Control unit 
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     An embodiment of the present invention will be hereinafter described in detail with reference to the accompanying drawings. 
       FIG. 1  is a cross-sectional view of an image forming apparatus to which one embodiment of a paper feeder of the present invention is applied. This image forming apparatus  1  is designed to print a monochrome image represented by image data on a recording paper, and its configuration is roughly divided into an original reader  2 , a printing unit  11 , a paper transport unit  12 , a paper supply unit  13 , and a large capacity cassette (LCC)  14 . 
     This image forming apparatus  1  generates image data by reading an original image with the original reader  2 , or acquires image data by receiving image data input from an external terminal apparatus or the like, performs various types of image processing on the image data, and then prints the image represented by the image data on a recording paper with the printing unit  11 . 
     A photosensitive drum  21  is arranged at the approximate center of the printing unit  11 , and a charging device  22 , a laser exposure device  23 , a development device  24 , a transfer roller  25 , and a cleaning device  26  are arranged around the photosensitive drum  21 . 
     A fixing device  27  is provided in the upper part of the printing unit  11 . The fixing device  27  has a heating roller  28  and a pressing roller  29  that are pressed against each other, heats and presses a recording paper held at a nip area between the heating roller  28  and the pressing roller  29 , and fixes a toner image transferred onto the recording paper. This recording paper is transported from the fixing device  27  to a discharge roller pair  36 , and discharged to a discharge tray  37  by the discharge roller pair  36 . 
     The paper supply unit  13  has a plurality of paper feed cassettes  38 . The paper feed cassettes  38  are trays for containing recording papers and provided on a plurality of stages in the lower part of the image forming apparatus  1 . Each paper feed cassette  38  is provided with a pickup roller  39  and the like for pulling out and sending recording papers one by one, and a pulled-out recording paper is sent out to a transport path  33  in the paper transport unit  12 . 
     The large capacity cassette (LCC)  14  is capable of containing a large amount of recording papers, and pulls out and sends recording papers one by one to the transport path  33  in the paper transport unit  12  in a similar manner to the paper feed cassettes  38 . 
     The paper transport unit  12  includes a plurality of transport roller pairs  31  for transporting recording papers, a registration roller pair  32 , a transport path  33 , an alternative path  34 , a branch claw  35 , a discharge roller pair  36 , a discharge tray  37 , and the like. In this transport path  33 , a recording paper pulled out from any of the paper feed cassettes is transported in a paper transport direction C and delivered to the registration roller pair  32 . The leading end of the recording paper is caused to abut against the registration roller pair  32  that is temporarily stopped and thus bent such that the leading end of the recording paper is aligned parallel to the registration roller pair  32  due to the elastic force of the recording paper. Then, the registration roller pair  32  starts to rotate and transports the recording paper to the nip area between the photosensitive drum  21  and the transfer roller  25 . This recording paper passes through the nip area between the photosensitive drum  21  and the transfer roller  25 , and the toner image is transferred onto the recording paper. It then passes through the nip area between the heating roller  28  and the pressing roller  29 , and the toner image is fixed on the recording paper. It is then transported by the discharge roller pair  36  in the forward direction A and discharged to the discharge tray  37 . 
     Further, if an image is also printed on the back face of the recording paper, the discharge roller pair  36  is stopped while the recording paper is being transported in the forward direction A and discharged to the discharge tray  37 . In other words, the discharge roller pair  36  is stopped in a state where the recording paper is held between the rollers. The branch claw  35  is shifted obliquely downward, and the discharge roller pair  36  is then rotated in a reverse direction to transport the recording paper in a reverse direction B and leads it to the alternative path  34 , and then the recording paper is again led to the transport path  33  through the alternative path  34  and returned to the registration roller pair  32 . 
     Such shifting of the transport direction of recording papers is referred to as “switch-back transport,” with which a recording paper is turned over and simultaneously its leading end and back end are switched. Accordingly, when the turned recording paper is returned, the back end of the recording paper is caused to abut against the registration roller pair  32  and aligned parallel to the registration roller pair  32 . The recording paper is transported from its back end by the registration roller pair  32  to the nip area between the photosensitive drum  21  and the transfer roller  25 , the image is printed on the back face of the recording paper, the toner image is fixed on the back face of the recording paper by the heating roller  28  and the pressing roller  29 , and the recording paper is discharged to the discharge tray  37  through the discharge roller pair  36 . 
     Next, a configuration of the paper feeder built in the large capacity cassette  14  in the present embodiment is described in detail. This paper feeder is designed to contain a large amount of recording papers and pull out and send the recording papers one by one to the transport path  33  (shown in  FIG. 1 ). 
       FIG. 2  is a schematic side view of the paper feeder in the present embodiment. As shown in  FIG. 2 , the paper feeder  71  includes an outer frame body  72 , a bottom plate  73 , a paper tray  74  arranged inside the outer frame body  72 , a paper pull-out unit  75  arranged at one end on the upper side of the outer frame body  72 , and the like. 
     The paper tray  74  is designed to be loaded with a large amount of recording papers (a bundle of papers), and is provided to be capable of moving up and down inside the outer frame body  72 . 
     The paper pull-out unit  75  has a paper transport belt  81  for transporting papers, a pair of rollers  82  and  83  for stretching the paper transport belt  81 , an air intake and exhaust fan (not shown) for taking in and exhausting air, an air intake duct  85  for taking in air, an exhaust duct  86  for taking out air, and the like. The paper transport belt  81  is provided with a number of air holes  81   a  for letting air pass through. Air is breathed from the air holes  81   a  on the paper transport belt  81  through the air intake duct  85  towards the air intake and exhaust fan, and a recording paper in a bundle of papers on the paper tray  74  is attached to the lower surface of the paper transport belt  81 . The paper transport belt  81  is intermittently revolving, and pulls out, in a pull-out direction E, the recording paper attached to the lower surface thereof and transports the recording paper to the transport roller pair  31 . This recording paper is transported to the registration roller pair  32  through the transport path  33  shown in  FIG. 1 . 
     The air exhausted from the air intake and exhaust fan is led through the exhaust duct  86 , and blown from an exhaust port  86   b  in the exhaust duct  86  against the upper layer of the leading end side of the bundle of papers on the paper tray  74  and flow into the respective recording papers to separate them. Thus, the adhesion between the respective recording papers in the upper layer of the bundle of papers is lowered, and it becomes easier to pull out recording papers from the bundle of papers and to pull out those papers one by one. 
     Accordingly, with a single air intake and exhaust fan, air is breathed from the air holes  81   a  on the paper transport belt  81  through the air intake duct  85  and blown through the exhaust duct  86  against the side of the bundle of papers on the paper tray  74 . 
       FIGS. 3 and 4  are a plan view and a front view of the paper feeder in the present embodiment. As shown in  FIGS. 3 and 4 , the paper tray  74  is provided with a long hole  74   a  that extends in a recording paper pull-out direction (paper feeding and transporting direction) E. A paper trailing edge guide  76  is supported on the bottom plate  73  reciprocatably along the recording paper pull-out direction E, and protrudes upward through the long hole  74   a  in the paper tray  74 . Note that the “front” indicates the recording paper pull-out direction E, and the “back” indicates the opposite direction of the pull-out direction E. 
     Both sides of the paper tray  74  are respectively provided with recesses  74   b , where assist ducts  77  and  78  are arranged respectively. Each of the assist ducts  77  and  78  is supported reciprocatably in a direction orthogonal to the pull-out direction E on both sides of the outer frame body  72 , and is moved in conjunction with each other to come close to or away from each other. 
     The paper pull-out unit  75  includes four paper transport belts  81 , a pair of rollers  82  and  83  for stretching the paper transport belts  81 , an air intake and exhaust fan  84 , an air intake duct  85 , an exhaust duct  86 , and the like. Each paper transport belt  81  is provided with a number of air holes  81   a , and air is breathed from the air holes  81   a  on the paper transport belts  81  through air intake duct  85  towards the air intake and exhaust fan  84 . The air exhausted from the air intake and exhaust fan  84  is led through the exhaust duct  86  and blown in the direction opposite the pull-out direction E (backward) from the exhaust duct  86  towards the inside of the outer frame body  72 . 
       FIG. 5  is an oblique view of the outer frame body  72 , the bottom plate  73 , the paper tray  74 , and the like viewed from the diagonal back in a state where the paper pull-out unit  75  is detached. As shown in  FIG. 5 , assist fans  79  and  80  are provided outside the assist ducts  77  and  78 , respectively. Each of the assist ducts  77  and  78  is a hollow body and has an air path inside, and the air breathed by the assist fans  79  and  80  is sent to the air paths in the assist ducts  77  and  78  and blown towards the inside of the outer frame body  72  from exhaust ports  77   a  and  78   a  respectively in the assist ducts  77  and  78 . 
     Further, as shown in  FIGS. 3 and 5 , the paper trailing edge guide  76  can reciprocate along the recording paper pull-out direction E, and is positioned at an arbitrary location along the pull-out direction E. Furthermore, as shown in  FIGS. 3 and 5 , the assist ducts  77  and  78  can reciprocate in the direction orthogonal to the pull-out direction E and are positioned at an arbitrary location along the direction orthogonal to the pull-out direction E. 
     Here, when a bundle of papers is loaded on the paper tray  74 , the paper trailing edge guide  76  is moved backward such that the space between a column  76   a  in the paper trailing edge guide  76  and an attachment plate  72   b  in the outer frame body  72  is opened wide, and the assist ducts  77  and  78  are moved in the directions opposite to each other such that the space between the assist ducts  77  and  78  is opened wide. In this state, a bundle of papers is loaded on the paper tray  74 , then the paper trailing edge guide  76  is moved in the pull-out direction E, the back end of the bundle of papers is pressed by the column  76   a  in the paper trailing edge guide  76  in the pull-out direction E, thereby causing the bundle of papers to slip and move on the paper tray  74 , the leading end of the bundle of papers is caused to abut against the attachment plate  72   b  in the outer frame body  72 , and the bundle of papers is positioned by holding the leading end and the back end thereof between the column  76   a  in the paper trailing edge guide  76  and the attachment plate  72   b  in the outer frame body  72 . Further, the assist ducts  77  and  78  are moved to close to each other, and the bundle of papers is positioned by holding the both ends thereof between the assist ducts  77  and  78 . 
     Further, as shown in  FIG. 5 , both sides of the paper tray  74  are provided with two protruding pieces  74   c , and these protruding pieces  74   c  protrude from rectangular holes  72   a  on both sides of the outer frame body  72 . On one side of the outer frame body  72 , two wires  87  are connected to the respective protruding pieces  74   c  on one side of the paper tray  74 , and each wire  87  is caught and extended around a plurality of idler pulleys  88  and connected to the take-up pulley  89 . Also on the other side of the outer frame body  72 , other two wires  87  are connected to the respective protruding pieces  74   c  on the other side of the paper tray  74 , and each wire  87  is caught and extended around a plurality of other idler pulleys  88  and connected to another take-up pulley  89 . Each take-up pulley  89  is secured to both ends of a rotatably supported common shaft  91 . Rotation of the shaft  91  is driven by a pulse motor  92 , thereby rotating the take-up pulleys  89 , and the wires  87  are reeled by the take-up pulleys  89  or unreeled from the take-up pulleys  89 . 
       FIG. 6  is a simplified side view of the paper tray  74 , the wires  87 , the idler pulleys  88  and the take-up pulley  89 . As is obvious from  FIG. 6 , when rotation of the shaft  91  is driven by the pulse motor  92  and the take-up pulleys  89  are rotated clockwise, the wires  87  are reeled by the take-up pulleys  89  and the paper tray  74  moves upward. Meanwhile, when the take-up pulleys  89  are rotated anticlockwise, the wires  87  are unreeled from the take-up pulleys  89  and the paper tray  74  moves downward. Further, the rotation angle of the take-up pulleys  89 , rotation of which is driven by the pulse motor  92 , is associated with the height of the paper tray  74 . Accordingly, the height of the paper tray  74  can be adjusted by controlling the direction and angle of rotation of the pulse motor  92 . 
     Further, as shown in  FIGS. 3 and 5 , the head portion  76   b  of the paper trailing edge guide  76  is provided with a height position sensor  93  for detecting the height of the uppermost paper of the bundle of papers on the paper tray  74 .  FIGS. 7A and 7B  are simplified side view and plan view of the height position sensor  93  provided to the head portion  76   b  of the paper trailing edge guide  76 . As shown in  FIGS. 7A and 7B , the height position sensor  93  has an L-shaped lever  95  rotatably supported by a horizontal shaft  94 , and a light-emitting element  96  and a light-receiving element  97  arranged opposite to each other with a rotational movement area of an end portion  95   a  of the L-shaped lever  95  therebetween. A leading end  95   c  of the L-shaped lever  95  faces towards the attachment plate  72   b  in the outer frame body  72 , and the bending portion  95   b  in the L-shaped lever  95  protrudes downward from the head portion  76   b  of the paper trailing edge guide  76 . Therefore, in the state where the bundle of papers on the paper tray  74  is held between the column  76   a  in the paper trailing edge guide  76  and the attachment plate  72   b  of the outer frame body  72 , the L-shaped lever  95  is located above the back end of the bundle of papers. 
     Here, in the state where, as shown in  FIG. 6 , the take-up pulleys  89  are rotated anticlockwise by the pulse motor  92 , the paper tray  74  moves downward, and the bundle of papers on the paper tray  74  is separated from the L-shaped lever  95  in the height position sensor  93 , as shown in  FIG. 7C , the L-shaped lever  95  rotates around the shaft  94  anticlockwise due to its own weight, the L-shaped lever  95  abuts against a stopper  98 , the end  95   a  of the L-shaped lever  95  shifts out of an optical path between the light-emitting element  96  and the light-receiving element  97 , and the light from the light-emitting element  96  is received by the light-receiving element  97 . Further, if the take-up pulleys  89  are rotated clockwise by the pulse motor  92 , the paper tray  74  moves upward, and the uppermost paper of the bundle of papers on the paper tray  74  reaches a detection reference height, as shown in  FIG. 7A  the uppermost paper comes into contact with the bending portion  95   b  in the L-shaped lever  95  and lifts up the bending portion  95   b , the L-shaped lever  95  rotates clockwise around the shaft  94 , the end portion  95   a  of the L-shaped lever  95  interrupts the optical path between the light-emitting element  96  and the light-receiving element  97 , and the light from the light-emitting element  96  is not received by the light-receiving element  97 . Moreover, if the paper tray  74  further moves upward and the uppermost paper of the bundle of papers on the paper tray  74  exceeds the detection reference height, the L-shaped lever  95  further rotates clockwise, the end portion  95   a  of the L-shaped lever  95  shifts out of the optical path between the light-emitting element  96  and the light-receiving element  97 , and the light from the light-emitting element  96  is received by the light-receiving element  97 . 
     Accordingly, whether or not the uppermost paper of the bundle of papers on the paper tray  74  is at the detection reference height can be detected based on a variation in output of the light received by the light-emitting element  97 . 
     Next, the configuration of the paper pull-out unit  75  is described in detail.  FIG. 8  is an oblique view of the paper pull-out unit  75  viewed obliquely from the upper front.  FIG. 9  is an oblique view of the paper pull-out unit  75  viewed obliquely from the upper back, and  FIG. 10  is an oblique view of the paper pull-out unit  75  viewed obliquely from the lower back. 
     Referring to  FIGS. 8 ,  9 , and  10 , the air intake duct  85  is a hollow body, which is internally provided with a long air path extending in the direction orthogonal to the pull-out direction E. One side end  85   a  of the air path is connected to the air intake and exhaust fan  84 , and as indicated by arrow F, air is breathed from the air path in the air intake duct  85  through the side end  85   a  towards an air intake port (not shown) in the air intake and exhaust fan  84 . 
     An upper surface  85   b  of the air intake duct  85  is made flat, and a lower surface  85   g  of the air intake duct  85  is also made flat. Further, a front end  85   c  and a back end  85   d  of the air intake duct  85  are provided with recesses  85   h  respectively, where the rollers  82  and  83  are respectively arranged and supported rotatably around shafts. 
     The lower surface  85   g  of the air intake duct  85  is provided with a plurality of ribs  85   j  that extend in the pull-out direction E and are arranged along the direction orthogonal to the pull-out direction E. Further, as shown in the enlarged plan view of  FIG. 11 , a plurality of air intake holes  85   e  that communicate with the air path in the air intake duct  85  are provided between the respective ribs  85   j  on the lower surface  85   g  of the air intake duct  85 . 
     The transport motor  107  drives rotation of the roller  82  on the front side in an arrow direction D, the roller  83  on the back side rotates following the driven rotation of the roller  82 , and the paper transport belts  81  rotate in the arrow direction D. At this time, the paper transport belts  81  are in slidable contact with the ribs  85   j  provided on the lower surface  85   g  of the air intake duct  85 . 
     Here, as shown in the enlarged plan view of  FIG. 11 , the air holes  81   a  on the paper transport belts  81  are arranged in multiple lines in the pull-out direction E, those lines are formed at the same interval as that of the ribs  85   j , and the paper transport belts  81  stretch such that the lines are located between the respective ribs  85   j . Accordingly, a space surrounded by the ribs  85   j , the paper transport belts  81 , and the lower surface  85   g  of the air intake duct  85  is formed between the respective ribs  85   j , and in this space, the air holes  81   a  on the paper transport belts  81  and the air intake holes  85   e  on the lower surface  85   g  of the air intake duct  85  are the entrance and exit of air. Therefore, if air inside the air intake duct  85  is breathed by the air intake and exhaust fan  84 , the air flows from the air holes  81   a  on the paper transport belts  81  through the space into the air intake holes  85   e  on the lower surface  85   g  of the air intake duct  85 , and further flows through the air intake duct  85  towards the air intake and exhaust fan  84 . Thus, recording papers can be attached to the lower surface of the paper transport belts  81 . 
     Meanwhile, the exhaust duct  86  is also a hollow body, which is internally provided with a long air path that extends in the direction orthogonal to the pull-out direction E. One side end  86   a  of the air path is connected to the air intake and exhaust fan  84 , and as indicated by arrow K, air is sent in from an exhaust port (not shown) in the air intake and exhaust fan  84  through the side end  86   a  of the exhaust duct  86  towards the air path in the exhaust duct  86 . 
     An internal wall  86   d  of the exhaust duct  86  is provided with exhaust ports  86   b  that communicate with the air path in the exhaust duct  86 . The exhaust ports  86   b  are arranged side by side in the direction orthogonal to the pull-out direction E. The internal wall  86   d  of the exhaust duct  86  is arranged to cover the outer surface of the attachment plate  72   b  (shown in  FIG. 5 ) in the outer frame body  72 , and the exhaust ports  86   b  in the exhaust duct  86  face towards the inside of the outer frame body  72  through a cutout portion  72   c  in the attachment plate  72   b  in the outer frame body  72 . If air is sent in from the air intake and exhaust fan  84  towards the exhaust duct  86 , this air is blown backward from the exhaust ports  86   b  towards the inside of the outer frame body  72 . 
     Further, both the side end  85   a  of the air intake duct  85  and the side end  86   a  of the exhaust duct  86  are connected to the air intake and exhaust fan  84 , and the other side end  85   f  of the air intake duct  85  and the other side end  86   c  of the exhaust duct  86  are connected to each other, and thus the air intake and exhaust fan  84 , the air intake duct  85 , and the exhaust duct  86  are integrated. 
     In the above-described paper feeder  71 , it is assumed that, as shown in the side view of  FIG. 2 , a bundle of papers is loaded on the paper tray  74 , the leading end and the back end of the bundle of papers are held between the column  76   a  in the paper trailing edge guide  76  and the attachment plate  72   b  in the outer frame body  72  and thus positioned, and the both sides of the bundle of papers are held between the assist ducts  77  and  78  and thus positioned. Then, the take-up pulleys  89  are rotated clockwise by the pulse motor  92  to move the paper tray  74  upward, and if the uppermost paper of the bundle of papers on the paper tray  74  is detected by the height position sensor  93 , that is, if the uppermost paper reaches the detection reference height, the pulse motor  92  is stopped and the uppermost paper is positioned at the detection reference height. Further, air is sent in from the assist fans  79  and  80  towards the assist ducts  77  and  78  and blown from the exhaust ports  77   a  and  78   a  in the assist ducts  77  and  78  against the both sides of the upper layer near the leading end of the bundle of papers loaded on the paper tray  74 , such that the air gets in between the respective recording papers and those papers separate from one another. Further, air is sent from the air intake and exhaust fan  84  towards the exhaust duct  86  and blown from the exhaust ports  86   b  in the exhaust duct  86  against the leading end of the upper layer of the bundle of papers such that air gets in (i.e., air is caused to flow) between the respective recording papers and those papers separate from one another. Thus, the adhesion (force generated due to stacking, or the like) between the recording papers in the upper layer of the bundle of papers is lowered, and it becomes easier to pull out recording papers from the bundle of papers and to pull out recording papers one by one. 
     In this state, when air is breathed from the air intake duct  85  towards the air intake and exhaust fan  84 , the rollers  82  and  83  are intermittently rotated and the paper transport belts  81  are intermittently rotated while taking the air in through the air holes  81   a  on the paper transport belts  81  and the air intake holes  85   e  on the lower surface  85   g  of the air intake duct  85 , a recording paper is attached to the lower surface of the paper transport belts  81 , pulled out by the paper transport belts  81  in the pull-out direction E, transported to the transport roller pair  31 , and then transported through the transport path  33 . Subsequently, the next recording paper is attached to the lower surface of the paper transport belts  81 , pulled out by the paper transport belts  81  in the pull-out direction E, and transported to the transport roller pair  31 . After that, similarly, recording papers are attached to the lower surface of the paper transport belts  81 , and pulled out and transported by the paper transport belts  81  in the pull-out direction E. 
     Incidentally, the paper feeder  71  has a configuration where, with the single air intake and exhaust fan  84 , air is breathed from the air holes  81   a  on the paper transport belt  81  through the air intake duct  85 , as well as blown through the exhaust duct  86  against the sides of the bundle of papers on the paper tray  74 . Accordingly, the amount of air breathed by the air intake and exhaust fan  84  and the amount of air exhausted thereby are approximately the same, and if one of those air amounts reduces, the other also reduces. Therefore, when a sheet of paper is attached to the paper transport belts  81 , if the air holes  81   a  on the paper transport belts  81  are blocked and the amount of the air breathed towards the air intake duct  85  reduces, the amount of the air blown from the exhaust ports  86   b  in the exhaust duct  86  against the sides of the bundle of papers on the paper tray  74  accordingly reduces. If such a phenomenon where the amount of the air breathed and exhausted reduces is left as it is, air is not caused to flow into the recording papers in the upper layer of the bundle of papers on the paper tray  74  and the adhesion between the recording papers is not lowered, and a transportation error occurs where a plurality of recording papers are simultaneously pulled out. 
     Therefore, in the present embodiment, an air pressure sensor for detecting the air pressure in the air intake duct  85  in the paper pull-out unit  75  is provided. Further, as shown in  FIGS. 3 ,  8 , and so on, an opening  85   k  is formed on the wall on the upper surface  85   b  side of the air intake duct  85 , and an opening and closing body  101  for opening and closing the opening  85   k  is provided. 
     Here, if the air holes  81   a  on the paper transport belts  81  are blocked and the amount of the air breathed towards the air intake duct  85  reduces, the air pressure in the air intake duct  85  is lowered, and so whether or not the amount of the air breathed towards the air intake duct  85  has reduced can be determined based on the air pressure detected by the air pressure sensor. Then, if it is determined that the amount of the air breathed towards the air intake duct  85  has reduced, the opening and closing body  101  is moved to open the opening  85   k  on the upper surface  85   b  side of the air intake duct  85 , thereby letting air flow through the opening  85   k  into the air intake duct  85 , and with the air thus caused to flow, the amount of the air breathed and exhausted by the air intake and exhaust fan  84  can be increased. As a result, the reduced air amount is restored, the amount of the air blown from the exhaust ports  86   b  in the exhaust duct  86  against the sides of the bundle of papers on the paper tray  74  is also restored, air is blown between the respective recording papers, thereby reducing the adhesion between the recording papers, and thus the transportation error where a plurality of recording papers are simultaneously pulled out is prevented. 
     Next, the control system of the air pressure sensor, the opening and closing body  101 , and the paper feeder  71  is described in detail.  FIG. 12  is a schematic front view of the paper pull-out unit  75  in the paper feeder  71 . As shown in  FIG. 12 , both the side end  85   a  of the air intake duct  85  and the side end  86   a  of the exhaust duct  86  are connected to the air intake and exhaust fan  84 , thereby letting the air intake duct  85  communicate with the inside of the paper transport belts  81 . 
     In an air path  85   p  in the air intake duct  85 , air that flows in through the air holes  81   a  on the paper transport belts  81  and the air intake holes  85   e  on the air intake duct  85  flows in the direction indicated by arrow F and is breathed towards the air intake and exhaust fan  84 . The opening  85   k  is formed downstream of the paper transport belts  81  in the direction of arrow F, and the opening and closing body  101  for opening and closing the opening  85   k  is provided. Further, the air pressure sensor  102  is arranged downstream of the opening  85   k  in the direction of arrow F in which the air flows and near the opening  85   k , and this air pressure sensor  102  is secured on the inner wall of the air intake duct  85 . With the position of the air pressure sensor  102 , variations in the air pressure in the air intake duct  85  when the air holes  81   a  on the paper transport belts  81  are blocked and when the opening  85   k  is opened and air flows in through the opening  85   k  can be detected in the fastest manner and with accuracy. 
     Further, in the air path  86   p  in the exhaust duct  86 , air exhausted from the air intake and exhaust fan  84  flows in the direction of arrow K and is brown out from the exhaust ports  86   b.    
       FIGS. 13A and 13B  are schematic plan views of the opening  85   k  and the opening and closing body  101  in the air intake duct  85 . The opening  85   k  in the air intake duct  85  is formed in an approximately triangle shape, and the opening and closing body  101  is also formed in an approximately triangle shape in accordance with the shape of the opening  85   k.    
     The opening and closing body  101  is rotatably supported by a shaft  103  caused to pass through a hole in a corner  101   a  of the opening and closing body  101 . Further, the shaft  103  is caused to pass through a ring  104   a  formed at the center of an L-shaped kick spring  104 , an end  104   b  of the kick spring  104  is secured on the external wall of the air intake duct  85 , the other end  104   c  of the kick spring  104  is secured to the opening and closing body  101 , and the opening and closing body  101  is biased anticlockwise by the kick spring  104 . Further, a solenoid  106  is secured on the external wall of the air intake duct  85 , and a plunger  107  in the solenoid  106  communicates with the opening and closing body  101  via a dumper  108 . 
     If the solenoid  106  is de-energized, as shown in  FIG. 13A  the plunger  107  protrudes in the direction of arrow Q, the opening and closing body  101  is moved by the kick spring  104  to rotate anticlockwise until the opening and closing body  101  abuts against a stopper  105 , and the opening  85   k  in the air intake duct  85  is closed by the opening and closing body  101 . 
     Meanwhile, if the solenoid  106  is energized, as shown in  FIG. 13B  the plunger  107  draws back in a direction opposite arrow Q, the opening and closing body  101  is moved to rotate clockwise against the elastic force of the kick spring  104 , and the opening  85   k  in the air intake duct  85  is opened. 
     Accordingly, the opening  85   k  can be closed or opened by de-energizing or energizing the solenoid  106 . 
       FIG. 14  is a block diagram showing a configuration of the control system of the paper feeder  71 . Referring to  FIG. 14 , the control unit  111  is designed to control the paper feeder  71  and the like in an integrated manner, and includes a CPU, a RAM, a ROM, various interfaces, and the like. For example, the control unit  111  controls the pulse motor  92  based on the height of the uppermost paper of a bundle of papers on the paper tray  74  detected by the height position sensor  93  to adjust the height of the uppermost paper of the bundle of papers, and de-energizes or energizes the solenoid  106  based on the air pressure in the air intake duct  85  detected by the air pressure sensor  102  to rotate the opening and closing body  101  and open or close the opening  85   k.    
     Next, the control of the paper feeder  71  by the control unit  111  is described in detail. First, the control unit  111  controls driving of the pulse motor  92 , rotates the take-up pulleys  89 , and moves up the paper tray  74 . Then, if the height position sensor  93  detects that the uppermost paper of a bundle of papers on the paper tray  74  reaches the detection reference height, the control unit  111  stops the pulse motor  92  to stop moving up the paper tray  74 , and positions the uppermost paper of the bundle of papers on the paper tray  74  at the detection reference height. At this time, as shown in  FIG. 2 , a clearance h 0  between the uppermost paper of the bundle of papers on the paper tray  74  and the lower surface of the paper transport belts  81  is a predetermined distance ha. 
     Further, the control unit  111  drives the assist fans  79  and  80  to cause air to blow from the exhaust ports  77   a  and  78   a  in the assist ducts  77  and  78  against the both sides in the upper layer near the leading end of the bundle of papers on the paper tray  74 . Further, the control unit  111  drives the air intake and exhaust fan  84  to cause air to blow through the exhaust duct  86  from the exhaust ports  86   b  in the exhaust duct  86  against the both sides in the upper layer of the bundle of papers on the paper tray  74 , and to breath air through the air holes  81   a  on the paper transport belts  81  and the air intake holes  85   e  on the lower surface  85   g  of the air intake duct  85  towards the air intake duct  85 . Thus, the adhesion between the papers in the upper layer of the bundle of papers is lowered, and the recording papers are attached to the lower surface of the paper transport belts  81 . 
     Then, the control unit  111  controls driving of the transport motor  107  to intermittently rotate the rollers  82  and  83  and intermittently rotate the paper transport belts  81 . Thus, the recording papers are attached to the lower surface of the paper transport belts  81 , pulled out by the paper transport belts  81 , and transported to the transport path  33  in the image forming apparatus  1 . Such pulling out and transportation of recording papers are repeated. 
     After the recording papers are thus sequentially pulled out from the bundle of papers on the paper tray  74  and transported, the height of the uppermost paper of the bundle of papers on the paper tray  74  is lowered, and the height position sensor  93  detects that the uppermost paper of the bundle of papers on the paper tray  74  has become lower than the detection reference height (h 0 &gt;ha). In response thereto, the control unit  111  controls driving of the pulse motor  92  to move up the paper tray  74  again until the height position sensor  93  detects that the uppermost paper of the bundle of papers on the paper tray  74  is at the detection reference height, and resets the clearance h 0  between the uppermost paper of the bundle of papers on the paper tray  74  and the lower surface of the paper transport belts  81  to the predetermined distance ha. 
     After that, similarly, the recording papers are sequentially pulled out from the bundle of papers on the paper tray  74  and transported, and if the uppermost paper of the bundle of papers on the paper tray  74  becomes lower than the detection reference height (h 0 &gt;ha), the paper tray  74  is moved up, the uppermost paper of the bundle of papers on the paper tray  74  is positioned at the predetermined height, and the clearance h 0  is set to the predetermined distance ha. 
     Accordingly, the clearance h 0  is substantially kept at the predetermined distance ha. This predetermined distance ha is a distance suitable for causing the recording papers to be attached to the lower surface of the paper transport belts  81 , and the recording papers on the paper tray  74  can be quickly breathed and pulled out one by one by the paper transport belts  81 . 
     Furthermore, while the paper feeder  71  is operating, the control unit  111  monitors the air pressure in the air intake duct  85  detected by the air pressure sensor  102  and compares the detected air pressure in the air intake duct  85  with a predetermined threshold. If the detected air pressure in the air intake duct  85  is kept at or higher than the threshold, the control unit  111  keeps de-energizing the solenoid  106  and closing the opening  85   k  in the air intake duct  85 . Meanwhile, if the detected air pressure in the air intake duct  85  becomes lower than the threshold, the control unit  111  energizes the solenoid  106  to open the opening  85   k  in the air intake duct  85  and cause air to flow through the opening  85   k  into the air intake duct  85 . Simultaneously, the control unit  111  controls driving of the air intake and exhaust fan  84  to accelerate its rotational speed and increase the amount of air breathed and exhausted by the air intake and exhaust fan  84 . Thus, the amount of air caused to flow through the opening  85   k  into the air intake duct  85  is promptly increased, and the air pressure in the air intake duct  85  is promptly increased. 
     Then, after the air pressure in the air intake duct  85  detected by the air pressure sensor  102  is restored to the threshold or higher, the control unit  111  de-energizes the solenoid  106  to close the opening  85   k  in the air intake duct  85 , and also decelerates the rotational speed of the air intake and exhaust fan  84  to the normal speed. 
     Here, if the paper transport belts  81  are intermittently rotated and the recording papers are attached one by one to the lower surface of the paper transport belts  81  and pulled out, the air holes  81   a  on the paper transport belts  81  are blocked due to attachment of the recording papers, and in some cases the amount of air breathed towards the air intake duct  85  is reduced, and the amount of air blown from the exhaust ports  86   b  in the exhaust duct  86  is accordingly reduced. 
     In this case, the air pressure in the air intake duct  85  detected by the air pressure sensor  102  becomes lower than the threshold. Therefore the solenoid  106  is energized to open the opening  85   k  in the air intake duct  85 , the rotational speed of the air intake and exhaust fan  84  is accelerated, and air is promptly caused to flow through the opening  85   k  into the air intake duct  85 . Thus, the amount of air breathed and exhausted by the air intake and exhaust fan  84  is increased and restored, the amount of air blown from the exhaust ports  86   b  in the exhaust duct  86  against the sides of the bundle of papers on the paper tray  74  is also restored, the air is caused to flow into the respective recording papers, the adhesion between the recording papers is reduced, and the transportation error where a plurality of recording papers are simultaneously pulled out is thus prevented. 
     Then, after the air pressure in the air intake duct  85  is restored to the threshold or higher due to the air flow from the opening  85   k , the solenoid  106  is de-energized to close the opening  85   k  in the air intake duct  85  again, and the rotational speed of the air intake and exhaust fan  84  is returned to the normal speed. Thus, the amount of air breathed from the air holes  81   a  on the paper transport belts  81  through the air intake duct  85  towards the air intake and exhaust fan  84  is restored, and the attaching force of the recording papers to the lower surface of the paper transport belts  81  is restored. 
     Subsequently, the same operation is repeated, thereby keeping the air pressure in the air intake duct  85  approximately at or higher than the threshold and preventing the air pressure in the air intake duct  85  from becoming much lower than the threshold. 
     Therefore, the amount of air sent from the air intake and exhaust fan  84  to the exhaust duct  86  is always sufficient, the air can be blown from the exhaust ports  86   b  in the exhaust duct  86  against the upper layer of the bundle of papers on the paper tray  74 , and thus the transportation error where a plurality of recording papers are simultaneously pulled out does not occur. Further, the recording papers can be continuously attached to the lower surface of the paper transport belts  81 . 
     As described above, the paper feeder  71  in the present embodiment is provided with the air pressure sensor  102  for detecting the air pressure in the air intake duct  85 , as well as provided with the opening  85   k  on the wall of the air intake duct  85  and the opening and closing body  101  for opening and closing the opening  85   k . If the air pressure in the air intake duct  85  detected by the air pressure sensor  102  becomes lower than the threshold, the solenoid  106  is energized to open the opening  85   k  in the air intake duct  85  and accelerate the rotational speed of the air intake and exhaust fan  84 , and air is promptly caused to flow through the opening  85   k  into the air intake duct  85 . Meanwhile, after the air pressure in the air intake duct  85  detected by the air pressure sensor  102  is restored to the threshold or higher, the solenoid  106  is de-energized to close the opening  85   k  in the air intake duct  85  and restore the rotational speed of the air intake and exhaust fan  84  to the normal speed. Therefore, a transportation error can be prevented by causing air to blow from the air holes  81   a  on the exhaust duct  86  against the upper layer of the bundle of papers on the paper tray  74 , and the recording papers can be continuously attached to the lower surface of the paper transport belts  81 . 
     In the above-described embodiment, the opening  85   k  in the air intake duct  85  is closed when the air pressure in the air intake duct  85  detected by the air pressure sensor  102  is restored to the threshold or higher. However, the elapsed time from when the opening  85   k  in the air intake duct  85  is opened may be measured, and the opening  85   k  in the air intake duct  85  may be closed when this elapsed time reaches a predetermined time. Alternatively, the opening  85   k  in the air intake duct  85  may be closed when the air pressure in the air intake duct  85  detected by the air pressure sensor  102  is restored to the threshold or higher or when the elapsed time reaches the predetermined time. This is for the purpose of preventing the attaching force of the recording papers to the paper transport belts  81  from being significantly lowered by limiting the period of time to open the opening  85   k  to a short period of time because, in the state where the opening  85   k  in the air intake duct  85  is open, air flows from the opening  85   k  towards the air intake and exhaust fan  84 , the amount of air breathed from the air holes  81   a  on the paper transport belts  81  decreases, and the attaching force of the recording papers to the paper transport belts  81  is lowered. 
     Furthermore, the air pressure in the air intake duct  85  is detected by the air pressure sensor  102 , but alternatively, the air flow rate in the air intake duct  85  may be detected by a flow sensor. If the air holes  81   a  on the paper transport belts  81  are blocked due to the attachment of the recording papers, the amount of air breathed towards the air intake duct  85  is reduced and the air flow rate detected by the flow sensor is also reduced. Therefore, the opening  85   k  is opened when the air flow rate detected by the flow sensor becomes lower than a predetermined threshold, thus air is caused to flow through the opening  85   k  into the air intake duct  85 , and the opening  85   k  is closed after the air flow rate detected by the flow sensor is increased and restored to the threshold or higher. 
     The preferable embodiment of the present invention has been described with reference to the accompanying drawings, but needless to say the present invention is not limited the above-described examples. It is obvious that a person skilled in the art would arrive at various modified or revised examples within the scope stated in the claims, and it is understood that those modified or revised examples also naturally belong to the technical scope of the present invention.