Patent Publication Number: US-8528899-B2

Title: Sheet detecting apparatus and image forming apparatus

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a sheet detecting apparatus for detecting the sheet conveyed and an image forming apparatus having the same. 
     2. Description of the Related Art 
     In general, a sheet conveying portion of an image forming apparatus includes a sheet detecting apparatus capable of detecting a front end position of the sheet in order to match the timing for sending the sheet to a transfer position and the timing for sending the image formed in the image forming portion to the transfer position (see U.S. Pat. No. 6,011,948). 
     Here,  FIGS. 26 and 27A  to  27 C illustrate the sheet detecting apparatus of the related art. As illustrated in  FIG. 26 , the sheet detecting apparatus of the related art is provided on the downstream side in the sheet conveying direction of a pair of conveying rollers  618  and  619  closest to the transfer position at which the image formed in the image forming portion is transferred. The sheet detecting apparatus includes a lever member  623  that abuts on the sheet, an optical sensor  624 , a light-shielding flag  625  for shielding an optical path starting from the light-emitting portion of the optical sensor  624  and ending at the light-receiving portion, and a stopper portion  626  for positioning the lever member  623  at the home position. The lever member  623  is rotatably formed by a rotational axis  627  and is adapted to return to the home position by a pressing force of a return spring  628  even when it rotates. The light-shielding flag  625  is formed integrally with the lever member  623  and rotates along with the lever member  623 . 
     As illustrated in  FIG. 27A , if a front end of the sheet S is brought into contact with the lever member  623 , the lever member  623  rotates in the direction of the arrow of  FIG. 27A  with respect to the rotational axis  627  from the home position, and the light-shielding flag  625  blocks off the optical path of the optical sensor  624 . If the optical sensor  624  detects that the optical path is blocked, the sheet detecting apparatus recognizes that the front end of the sheet S has arrived at the lever member  623 . Then, the sheet S moves while being in contact with the front end of the lever member  623 . If the rear end of the sheet S is separated from the lever member  623 , the lever member  623  rotates in the direction of the arrow of  FIG. 27C  by the return spring  628  and returns to the home position. At this moment, the light-shielding flag  625  retracts from the optical path, and the light-receiving portion of the optical sensor  624  receives the light emitted from the light-emitting portion again so that the sheet detecting apparatus recognizes that the rear end of the sheet S has passed through the lever member  623 . 
     However, in recent years, users demand still higher throughput for the image forming apparatus. In order to improve the throughput in the image forming apparatus, it is necessary to improve the conveying speed of the sheet or to shorten the distance (hereinafter, referred to as a “sheet interval”) between the rear end of the preceding sheet and the front end of the subsequent sheet. Therefore, it is necessary for the sheet detecting apparatus to return the lever member  623  to the home position within a period corresponding to a short sheet interval after the preceding sheet S has passed. 
     Meanwhile, as the front end of the sheet S having passed a pair of conveying rollers  618  and  619  abuts on an abutting portion, the lever member  623  of the related art rotates by being pressed by the sheet S. As the rear end of sheet S is separated from the abutting portion, the lever member  623  is reversely rotated and returns to the home position. For this reason, a distance necessary as the sheet interval becomes the sum of the distances D 1  and D 2 , in which D 1  denotes a distance between the position where the rear end of the preceding sheet passes through the abutting portion of the lever member  623  and the home position where the front end of the subsequent sheet abuts on the abutting portion, and D 2  denotes a distance at which the subsequent sheet is conveyed therebetween (refer to  FIG. 27B ). 
     Here, the distance D 2  is set to Δt×v, in which Δt denotes the time taken for the lever member  623  to move by the distance D 1 , and v denotes the sheet conveying speed. When the lever member  623  performs a reciprocating movement, the distance D 1  for returning the lever member  623  to the home position is generated, and distance D 2  at which the subsequent sheet S is conveyed in the return operation is lengthened as the sheet conveying speed increases. For this reason, the sheet detecting apparatus of the related art has a problem in that the distance of sheet interval becomes longer as the conveying speed of the sheet S becomes faster. This suppresses further improvement in throughput. 
     In this regard, the invention provides a sheet detecting apparatus capable of improving the throughput by suppressing the sheet interval from being lengthened even when the sheet conveying speed increases and an image forming apparatus having the same. 
     SUMMARY OF THE INVENTION 
     The present invention provides a sheet detecting apparatus that detects a sheet conveyed by a conveying portion that conveys the sheet, the sheet detecting apparatus comprising: a lever member having an abutting surface that abuts against a leading end of a sheet conveyed by the conveying portion; a biasing portion that applies a biasing force to the lever member to position the lever member at a first position where the abutting surface abuts against the leading end of the sheet; a supporting mechanism that movably supports the lever member so that the lever member moves in an order of the first position, a second position to which the lever member moves by the sheet being conveyed against the biasing force of the biasing member, and a third position where the lever member abuts on a surface of the sheet being conveyed and waits in order to move to the first position when a trailing end of the sheet passes the lever member, while keeping the abutting surface facing upstream in a sheet conveying direction; an interlocking portion that interlock with the lever member; and a detector that detects a position of the interlocking portion. 
     According to the invention, it is possible to shorten the time between a point in time when the sheet has passed and a point in time when the lever member is positioned in the first position which is a standby position. Therefore, it is not necessary to obtain a long distance as the sheet interval is reduced, and thus possible to improve the throughput. 
     Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view schematically illustrating the entire structure of the image forming apparatus according to a first embodiment of the invention; 
         FIG. 2A  is a perspective view illustrating the sheet conveying portion of the image forming apparatus according to the first embodiment of the invention;  FIG. 2B  is a perspective view illustrating the sheet conveying portion of  FIG. 2A  as seen from the opposite side; 
         FIG. 3  is an exploded perspective view illustrating a part of the sheet detector according to the first embodiment of the invention; 
         FIG. 4A  is a diagram illustrating a state that the sheet is conveyed to the sheet conveying portion according to the first embodiment of the invention;  FIG. 4B  is a diagram illustrating a rotating lever in a state that the sheet is conveyed to the sheet conveying portion;  FIG. 4C  is a diagram illustrating a light-shielding flag in a state that the sheet is conveyed to the sheet conveying portion; 
         FIG. 5A  is a diagram illustrating a sheet conveying portion in a state that the front end of the sheet abuts on the abutting surface of the rotating lever;  FIG. 5B  is a diagram illustrating a rotating lever in a state that the front end of the sheet abuts on the abutting surface;  FIG. 5C  is a diagram illustrating a light-shielding flag in a state that the front end of the sheet abuts on the abutting surface of the rotating lever; 
         FIG. 6A  is a diagram illustrating the sheet conveying portion in a state that the rotating lever is rotated while it is pressed by the front end of the sheet;  FIG. 6B  is a diagram illustrating the rotating lever rotated by the pressing of the front end of the sheet;  FIG. 6C  is a diagram illustrating the light-shielding flag in a state that the rotating lever is rotated while it is pressed by the front end of the sheet; 
         FIG. 7A  is a diagram illustrating the sheet conveying portion in a state that the rotating lever is located in the second position while it is pressed by the front end of the sheet;  FIG. 7B  is a diagram illustrating the rotating lever in a state that the rotating lever is located at the second position by the pressing of the front end of the sheet;  FIG. 7C  is a diagram illustrating the light-shielding flag in a state that the rotating lever is located at the second position by the pressing of the front end of the sheet; 
         FIG. 8A  is a diagram illustrating the sheet conveying portion in a state that the rotating lever moves from the second position to the third position;  FIG. 8B  is a diagram illustrating the rotating lever which moves from the second position to the third position;  FIG. 8C  is a diagram illustrating the light-shielding flag in a state that the rotating lever moves from the second position to the third position; 
         FIG. 9A  is a diagram illustrating the sheet conveying portion in a state that the rotating lever moves from the third position to the first position;  FIG. 9B  is a diagram illustrating the rotating lever in a state that it moves from the third position to the first position;  FIG. 9C  is a diagram illustrating the light-shielding flag in a state the rotating lever moves from the third position to the first position; 
         FIG. 10  is a diagram illustrating a rotating trajectory of the abutting surface of the rotating lever which cyclically moves from the first position to the third position while the abutting surface is directed to an upstream side; 
         FIG. 11A  is a perspective view illustrating the sheet conveying portion of the image forming apparatus according to a second embodiment of the invention;  FIG. 11B  is a perspective view illustrating the sheet conveying portion of  FIG. 11A  as seen from the opposite side; 
         FIG. 12A  is a diagram illustrating the sheet conveying portion of the image forming apparatus according to the second embodiment of the invention;  FIG. 12B  is a diagram illustrating a state that the rotating lever of the sheet conveying portion of  FIG. 12A  waits at the third position; 
         FIG. 13A  is a perspective view illustrating the sheet conveying portion of the image forming apparatus according to a third embodiment of the invention;  FIG. 13B  is a perspective view illustrating the sheet conveying portion of  FIG. 13A  as seen from the opposite side; 
         FIG. 14  is an exploded perspective view illustrating a part of the sheet detector according to the third embodiment of the invention; 
         FIG. 15A  is a diagram illustrating a state that the sheet is conveyed in the sheet conveying portion according to the third embodiment of the invention;  FIG. 15B  is a diagram illustrating the rotating lever in a state that the sheet is conveyed to the sheet conveying portion;  FIG. 15C  is a diagram illustrating the light-shielding flag in a state that the sheet is conveyed to the sheet conveying portion; 
         FIG. 16A  is a diagram illustrating the sheet conveying portion in a state that the front end of the sheet abuts on the abutting surface of the rotating lever;  FIG. 16B  is a diagram illustrating the rotating lever in a state that the front end of the sheet abuts on the abutting surface;  FIG. 16C  is a diagram illustrating the light-shielding flag in a state that the front end of the sheet abuts on the abutting surface; 
         FIG. 17  is a diagram illustrating a rotating trajectory of the abutting surface of the rotating lever which cyclically moves from the first position to the third position while the abutting surface is directed to an upstream side; 
         FIG. 18A  is a perspective view illustrating the sheet conveying portion of the image forming apparatus according to a fourth embodiment of the invention;  FIG. 18B  is a perspective view illustrating the sheet conveying portion of  FIG. 18A  as seen from the opposite side; 
         FIG. 19  is an exploded perspective view illustrating a portion of the sheet detector according to the fourth embodiment of the invention; 
         FIG. 20A  is a diagram illustrating the second rotating lever in a state that the sheet is conveyed to the sheet conveying portion;  FIG. 20B  is a diagram illustrating the first rotating lever in a state that the sheet is conveyed to the sheet conveying portion;  FIG. 20C  is a diagram illustrating the light-shielding flag in a state that the sheet is conveyed to the sheet conveying portion;  FIG. 20D  is a diagram illustrating the biasing portion in a state that the sheet is conveyed to the sheet conveying portion; 
         FIG. 21A  is a diagram illustrating the second rotating lever in a state that the front end of the sheet abuts on the abutting surface of the first rotating lever;  FIG. 21B  is a diagram illustrating the first rotating lever in a state that the front end of the sheet abuts on the abutting surface;  FIG. 21C  is a diagram illustrating the light-shielding flag in a state that the front end of the sheet abuts on the abutting surface of the first rotating lever;  FIG. 21D  is a diagram illustrating the biasing portion in a state that the front end of the sheet abuts on the abutting surface of the first rotating lever; 
         FIG. 22A  is a diagram illustrating a state that the abutting surface of the first rotating lever is pressed by the front end of the sheet so that the second rotating lever is rotated along with the first rotating lever;  FIG. 22B  is a diagram illustrating a state that the first rotating lever is rotated by the pressing of the front end of the sheet;  FIG. 22C  is a diagram illustrating the light-shielding flag in a state that the first rotating lever is rotated by the pressing of the front end of the sheet;  FIG. 22D  is a diagram illustrating the biasing portion in a state that the first rotating lever abuts on the front end of the sheet and is rotated; 
         FIG. 23A  is a diagram illustrating the second rotating lever in a state that the first rotating lever is rotated to the second position;  FIG. 23B  is a diagram illustrating the first rotating lever rotated to the second position;  FIG. 23C  is a diagram illustrating the light-shielding flag in a state that the first rotating lever is rotated to the second position;  FIG. 23D  is a diagram illustrating the biasing portion in a state that the first rotating lever is rotated to the second position; 
         FIG. 24A  is a diagram illustrating a state that the front end of the second rotating lever rotated along with the first rotating lever abuts on the front end of the conveyed sheet and waits at the third position;  FIG. 24B  is a diagram illustrating the first rotating lever in a state that the second rotating lever waits;  FIG. 24C  is a diagram illustrating the light-shielding flag in a state that the second rotating lever waits at the third position;  FIG. 24D  is a diagram illustrating the biasing portion in a state that the second rotating lever waits at the third position. 
         FIG. 25A  is a diagram illustrating a state that the second rotating lever is rotated to the first position;  FIG. 25B  is a diagram illustrating the first rotating lever in a state that the second rotating lever is rotated to the first position;  FIG. 25C  is a diagram illustrating the light-shielding flag in a state that the second rotating lever is moved to the first position;  FIG. 25D  is a diagram illustrating the biasing portion in a state that the second rotating lever is moved to the first position; 
         FIG. 26  is a perspective diagram illustrating the sheet conveying portion of the image forming apparatus in the related art; 
         FIG. 27A  is a diagram illustrating a state that the front end of the sheet abuts on the light-shielding flag of the sheet detector of the sheet conveying portion in the related art;  FIG. 27B  is a diagram illustrating the light-shielding flag in a state that it waits until the sheet passes; and  FIG. 27C  is a diagram illustrating a state that the sheet passes, and the light-shielding flag is returned to the home position. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Hereinafter, the image forming apparatus including the sheet conveying portion according to the embodiment of the invention will be described with reference to the accompanying drawings. The image forming apparatus according to the embodiment of the invention is an image forming apparatus including a sheet conveying portion having a sheet detection function capable of detecting a position of the conveyed sheet, such as a copier, a printer, a facsimile, and a multi-function peripheral having a combination of the functions thereof. In the following embodiments, it will be described using the electrophotographic image forming apparatus that forms a toner image of four colors. 
     First Embodiment 
     The image forming apparatus  100  according to a first embodiment of the invention will be described with reference to  FIGS. 1 to 10 . First, the entire structure of the image forming apparatus  100  according to the first embodiment of the invention will be described with reference to the  FIG. 1 .  FIG. 1  is a cross-sectional view schematically illustrating the entire structure of the image forming apparatus  100  according to the first embodiment of the invention. 
     As illustrated in  FIG. 1 , the image forming apparatus  100  according to the first embodiment of the invention includes a sheet feeding portion  8  that feeds the sheet S and a sheet conveying portion  9  which conveys the sheet S fed from the sheet feeding portion  8  and detects the position of the sheet S. The image forming apparatus  100  includes an image forming portion  14  that forms a toner image on the sheet S conveyed from the sheet conveying portion  9 , a fixing portion  10  that fixes an unfixed toner image formed in the image forming portion  14  onto the sheet, and a sheet discharge portion  13  that discharges the sheet where the toner image is fixed. 
     The sheet feeding portion  8  includes a sheet cassette  80  where the sheets S are housed, a feeding roller  81  that feeds the sheets S housed in the sheet cassette  80  to the sheet conveying portion  9 , and a separating portion (not illustrated) that separates the sheets S one by one. The sheet feeding portion  8  separates the sheets S housed in the sheet cassette  80  one by one and feeds the sheets S to the sheet conveying portion  9  using the feeding roller  81 . 
     The sheet conveying portion  9  is provided on the downstream side of the sheet feeding portion  8 . The sheet conveying portion  9  conveys the sheet S fed from the sheet feeding portion  8  or the sheet S conveyed from the duplex conveying path  15   b  described below. In addition, the sheet conveying portion  9  includes a sheet detector  200  as a sheet detecting apparatus for detecting that the sheet passes through a predetermined position. Moreover, the sheet detector  200  will be described in detail in conjunction with the sheet conveying portion  9  described below. 
     The image forming portion  14  initiates the image forming operation at a predetermined timing when the sheet detector  200  detects that the sheet S arrives at a predetermined position. Specifically, the image forming portion  14  forms the toner image based on predetermined image information at a predetermined timing and transfers the toner image onto the sheet S conveyed by the sheet conveying portion  9 . The image forming portion  14  includes photosensitive drums  1   a ,  1   b ,  1   c , and  1   d , charging portions  2   a ,  2   b ,  2   c , and  2   d , exposing portions  3   a ,  3   b ,  3   c , and  3   d , development portions  4   a ,  4   b ,  4   c , and  4   d , transfer rollers  5   a ,  5   b ,  5   c , and  5   d , and cleaning portions  6   a ,  6   b ,  6   c , and  6   d . In addition, the image forming portion  14  includes a transfer belt  14   a.    
     The photosensitive drums  1   a  to  1   d  serving as an image bearing member are formed by coating an organic photo conductor (OPC) layer on the outer circumferential surface of an aluminum cylinder. Both ends of photosensitive drums  1   a  to  1   d  are rotatably supported by the flanges and are rotationally driven in a counterclockwise direction in  FIG. 1  by transmitting a driving force from a driving motor (not illustrated) to the one end. Charging portions  2   a  to  2   d  make the conductive roller formed in a roller shape abut on the surface of photosensitive drums  1   a  to  1   d  and uniformly charge the surfaces of photosensitive drums  1   a  to  1   d  by applying a charging bias voltage using a power supply (not illustrated). The exposing portions  3   a  to  3   d  form an electrostatic latent image on the photosensitive drums  1   a  to  1   d  by irradiating a laser beam based on image information. 
     The development portions  4   a  to  4   d  include toner storage portions  4   a   1 ,  4   b   1 ,  4   c   1 , and  4   d   1 , and developing roller portions  4   a   2 ,  4   b   2 ,  4   c   2 , and  4   d   2 . The toner storage portions  4   a   1  to  4   d   1  house black, cyan, magenta, and yellow toners in each color. The developing roller portions  4   a   2  to  4   d   2  are arranged adjacently to the photosensitive member surface, and the toner in each color is adhered to the electrostatic latent image on the photosensitive drums  1   a  to  1   d  by applying a developing bias voltage to develop the latent image as a toner image. 
     The transfer rollers  5   a  to  5   d  are arranged in the inner side of the transfer belt  14   a  so as to abut on the transfer belt  14   a  oppositely to the photosensitive drums  1   a  to  1   d . The transfer rollers  5   a  to  5   d  are connected to the power supply for the transfer bias (not illustrated), and the positive charge from the transfer rollers  5   a  to  5   d  is applied to the sheet S through the transfer belt  14   a . By this electric field, each negative color toner image on the photosensitive drums  1   a  to  1   d  is sequentially transferred onto the sheet S making contact with the photosensitive drums  1   a  to  1   d  so as to form the color image. The cleaning portions  6   a  to  6   d  remove the toner remained on the surface of the photosensitive drums  1   a  to  1   d  after transferring. 
     In addition, according to the present embodiment, the photosensitive drums  1   a  to  1   d , the charging portions  2   a  to  2   d , the development portions  4   a  to  4   d , and the cleaning portions  6   a  to  6   d  integrally constitute the process cartridge portions  7   a  to  7   d.    
     The fixing portion  10  fixes the unfixed toner image by heating the sheet S where the unfixed toner image has been transferred. The sheet discharge portion  13  includes a pair of discharge rollers  11  and  12  by which the sheet S having an image is conveyed by positive rotation or is inverted by reverse rotation and a discharging portion  13   a  from which the sheet S having an image is discharged. 
     In addition, the image forming apparatus  100  includes a sheet conveying path  15   a  which conveys the sheet S and the like where the toner image has been formed in the image forming portion  14 , a duplex conveying path  15   b , a pair of oblique feeding rollers  16 , and a pair of U-turn rollers  17 . The sheet conveying path  15   a  is a conveying path for conveying the sheet S fed from the sheet feeding portion  8  or the sheet S conveyed from the duplex conveying path  15   b  and the like and is provided with the sheet conveying portion  9  and the image forming portion  14 . The duplex conveying path  15   b  is a conveying path for conveying the sheet S reversed in a pair of discharge rollers  11  and  12  in order to perform duplex printing to the sheet conveying path  15   a . A pair of oblique feeding rollers  16  is arranged in the duplex conveying path  15   b  and conveys the reversed sheet S. A pair of U-turn rollers  17  is arranged in the duplex conveying path  15   b  and re-conveys the sheet S conveyed through the duplex conveying path  15   b  to the sheet conveying path  15   a.    
     The sheet S fed to the sheet conveying path  15   a  from the sheet feeding portion  8  is conveyed to the image forming portion  14  through the sheet detector  200  of the sheet conveying portion  9 . In the sheet detector  200 , the front end position of the sheet S is detected. As the front end position of the sheet S is detected in the sheet detector  200 , the forming of the toner image (image forming operation) by the image forming portion  14  is initiated at the timing when the sheet S arrives at the transfer rollers  5   a  to  5   d . After the toner image is formed, as the sheet S reaches the transfer roller  5   a  to  5   d , each color of toner image on the photosensitive drums  1   a  to  1   d  is sequentially transferred onto the sheet S. Then, the unfixed toner image is fixed to the sheet S in the fixing portion  10 , and the sheet S is discharged to the discharging portion  13   a  by a pair of discharge rollers  11  and  12 . 
     In addition, at the time of duplex printing, the unfixed toner image is fixed to the sheet S in the fixing portion  10 , and then, a pair of discharge rollers  11  and  12  are reversely rotated before the sheet S is discharged to the discharging portion  13   a  by the a pair of discharge rollers  11  and  12 . As a result, the sheet S is conveyed to the duplex conveying path  15   b . The sheet S conveyed to the duplex conveying path  15   b  is re-conveyed to the image forming portion  14  through the sheet detector  200  by a pair of oblique rollers  16  and a pair of U-turn rollers  17  so as to perform duplex printing. 
     Next, the sheet conveying portion  9  will be described in detail with reference to  FIGS. 2A to 10 . First, the entire structure of the sheet conveying portion  9  will be described with reference to  FIGS. 2A to 3 .  FIG. 2A  is a perspective view illustrating the sheet conveying portion  9  of the image forming apparatus  100  according to the first embodiment.  FIG. 2B  is a perspective view of the sheet conveying portion  9  of  FIG. 2A  as seen from the opposite side.  FIG. 3  is an exploded perspective view illustrating a part of the sheet detector  200  according to the first embodiment of the invention. In addition, arrows illustrated in the  FIGS. 2A and 2B  represent the conveying direction of the sheet S. 
     As illustrated in  FIGS. 2A and 2B , the sheet conveying portion  9  includes a paper feeding frame  20 , a guide frame  28 , a pair of conveying rollers  18  and  19  as a conveying portion for conveying the sheet S conveyed along the sheet conveying path  15   a  to the image forming portion  14 , and a sheet detector  200 . The paper feeding frame  20  and the guide frame  28  are arranged in the vicinity of the upstream side of the image forming portion  14  with respect to the sheet conveying path  15   a  and support a pair of conveying rollers  18  and  19  and the sheet detector  200 . A pair of conveying rollers  18  and  19  includes a plurality of conveying rollers  19  and a plurality of conveying rollers  18  arranged oppositely to each of the conveying rollers  19 . The conveying roller  19  is fixed to the rotational axis  19   a  axially supported in parallel with the rotational axial direction of the photosensitive drums  1   a  to  1   d  and rotates integrally with the rotational axis  19   a . The conveying roller  18  is rotatably and axially supported by the paper feeding frame  20 . In addition, the conveying roller  18  is biased to the conveying roller  19  by the conveying roller spring  21  mounted on the paper feeding frame  20  and serves as a follower rotating member of the conveying roller  19  for conveying the sheet S using this biasing force. 
     The sheet detector  200  is supported by the paper feeding frame  20  in the downstream side of the sheet conveying direction and the guide frame  28  from a pair of conveying rollers  18  and  19  along the sheet conveying path. The sheet detector  200  detects the front end position of the sheet S conveyed to the image forming portion  14  by a pair of conveying rollers  18  and  19 . The sheet detector  200  includes a rotating lever  23  as a lever member, a support axis  31  serving as a support mechanism, a pair of rotating members  24  and  25  serving as a rotating member of a support mechanism, and rotating axes  24   a  and  25   a  which rotate a pair of rotating members  24  and  25 . The support mechanism movably supports the rotating lever  23  so that rotating lever  23  can perform a crank-movement. In addition, the sheet detector  200  includes a lever driving member  26  as a connection rotating member, a bias spring  27  as a biasing member, a light-shielding flag  29  as an interlocking portion, and an optical sensor  30  as a sensor. 
     The rotating lever  23  includes a main portion  23   e  formed in a long plate shape (straight line shape), an abutting portion  23   a  formed integrally with the main portion  23   e  at one end of the main portion  23   e  in the longitudinal direction, and a long hole portion  23   b  formed in the other end side. In addition, the rotating lever  23  includes a connected portion  23   c  formed between the abutting portion  23   a  and the long hole portion  23   b.    
     The abutting portion  23   a  includes an abutting surface  23   d  capable of abutting on the front end of the sheet S conveyed along the sheet conveying path  15   a  by a pair of conveying rollers  18  and  19 . The abutting surface  23   d  is arranged to protrude to the sheet conveying path  15   a  so as to abut against the front end (leading end) of the sheet S conveyed by a pair of conveying rollers  18  and  19 . Hereinafter, a position where the front end of the sheet S in the downstream side of the sheet conveying direction of a pair of conveying rollers  18  and  19  abuts on the abutting surface  23   d  is called a “first position.” 
     The long hole portion  23   b  is formed along the longitudinal direction of the main portion  23   e  in the other end portion of the main portion  23   e , and the fixed support axis  31  positioned in the paper feeding frame  20  is slidably engaged thereto (refer to  FIG. 2B ). The connected portion  23   c  is rotatably connected to a pair of rotating members  24  and  25  such that the rotating lever  23  can rotate along with a pair of rotating members  24  and  25 . The support axis  31  serves as a slide support portion for supporting the main portion  23   e  of the rotating lever  23  slidably. 
     A pair of rotating members  24  and  25  is formed in a disk shape and rotating axes  24   a  and  25   a  is connected to each center of rotation. The rotating member  24  is provided with a connecting axis  24   b  formed to protrude as a connecting portion that can penetrate the connected portion  23   c  of the rotating lever  23  at a position (eccentric position) offset from the rotational center of the rotating member  24  to a radial direction. The rotating member  25  is provided with a connection hole  25   b  where the connecting axis  24   b  that penetrates the connected portion  23   c  can be connected. The connection hole  25   b  is formed at a position (eccentric position) offset from the rotational center of the rotating member  25  to a radial direction. A pair of rotating members  24  and  25  are connected to the rotating lever  23  by making the connection axis  24   b  be formed in an eccentric position from the rotational center penetrate the connected portion  23   c  and fit to the connection hole  25   b  formed in an eccentric position from the rotational center by insertion. 
     The lever driving member  26  is formed in a disk shape and is fixed to the end of the rotational axis  25   a  so as to match the rotational axis  25   a  with the rotational center of the lever driving member  26 . According to the present embodiment, the lever driving member  26  is fixed to the rotational axis  25   a  by pressedly inserting the D-shaped portion formed in the front end of the rotational axis  25   a  into the D-cut hole formed in the rotational center of the lever driving member  26 . The lever driving member  26  includes a connection support portion  26   a  formed to protrude in a position (eccentric position) offset from the rotational center to a radial direction. The connection supporting portion  26   a  is connected to one end of the biasing spring  27  and is provided to make the abutting surface  23   d  locate at the first position while the biasing spring  27  is at the minimum biasing state (no extending state). 
     One end of the biasing spring  27  is connected to the connection supporting portion  26   a  of the lever driving member  26 , and the other end is fixedly positioned in the paper feeding frame  20 . The biasing spring  27  applies a force to the rotating lever  23  through the lever driving member  26 , the rotating axes  24   a  and  25   a , and a pair of rotating members  24  and  25  in the upstream side of the sheet conveying direction so that the abutting surface  23   d  is located at the first position. For example, the biasing spring  27  makes the abutting portion  23   a  locate at the first position by applying a force to the rotating lever  23 , rotated by a collision to the front end of the sheet, in the Z 3  direction (refer to  FIG. 7B  described below) through the lever driving member  26 . 
     The light-shielding flag  29  blocks the light path L of the optical sensor  30 . The light-shielding flag  29  is fixed to the rotational axis  24   a  and rotates integrally with a pair of rotating members  24  and  25  with respect to the rotating axes  24   a  and  25   a . That is, the light-shielding flag  29  interlocks with rotation of the rotating lever  23 . In addition, the light-shielding flag  29  includes a slit portion  29   a  for transmitting the light of the optical sensor  30 . The slit portion  29   a  is formed to transmit the light of the optical sensor  30  when the abutting surface  23   d  of the abutting portion  23   a  provided in the rotating lever  23  is located at the first position (refer to  FIG. 4C  described below). The light-shielding flag  29  is configured to be rotated along with the rotating lever  23  when the rotating lever  23  is pressed and rotated by the front end of the sheet S so as to block the light path L of the optical sensor  30 . 
     The optical sensor  30  is provided in the middle of the rotating path of the light-shielding flag  29  and includes a light-emitting portion (not illustrated) that emits the light and a light-receiving portion (not illustrated) that receives the light emitted from the light-emitting portion. The light emitted from the light-emitting portion is received at the light-receiving portion so as to form the light path L. As the light-shielding flag  29  blocks the light emitted from the light-emitting portion, the signal (optical signal) output from the light-emitting portion is blocked, and the received signal is changed. The optical sensor  30  detects the movement position of the light-shielding flag  29  based on the change of the received signal. 
     Next, the operation of the sheet conveying portion  9  will be described with reference to  FIGS. 4A to 10  in addition to  FIG. 1 .  FIG. 4A  is a diagram illustrating a state that the sheet S is conveyed to the sheet conveying portion  9  according to the first embodiment of the invention.  FIG. 4B  is a diagram illustrating a rotating lever  23  in a state that the sheet S is conveyed to the sheet conveying portion  9 .  FIG. 4C  is a diagram illustrating the light-shielding flag  29  in a state that the sheet S is conveyed to the sheet conveying portion  9 .  FIG. 5A  is a diagram illustrating a sheet conveying portion  9  in a state that the front end of the sheet S abuts on the abutting surface  23   d  of the rotating lever  23 .  FIG. 5B  is a diagram illustrating a rotating lever  23  in a state that the front end of the sheet S abuts on the abutting surface  23   d .  FIG. 5C  is a diagram illustrating the light-shielding flag  29  in a state that the front end of the sheet S abuts on the abutting surface  23   d  of the rotating lever  23 .  FIG. 6A  is a diagram illustrating the sheet conveying portion  9  in a state that the rotating lever  23  is rotated by the pressing of the front end of the sheet S.  FIG. 6B  is a diagram illustrating the rotating lever  23  rotated by the pressing of the front end of the sheet S.  FIG. 6C  is a diagram illustrating the light-shielding flag  29  in a state that the rotating lever  23  is rotated by the pressing of the front end of the sheet S. 
       FIG. 7A  is a diagram illustrating the sheet conveying portion  9  in a state that the rotating lever  23  is located at the second position by the pressing of the front end of the sheet S.  FIG. 7B  is a diagram illustrating the rotating lever  23  in a state that the rotating lever  23  is located at the second position by the pressing of the front end of the sheet S.  FIG. 7C  is a diagram illustrating the light-shielding flag  29  in a state that the rotating lever  23  is located at the second position by the pressing of the front end of the sheet S.  FIG. 8A  is a diagram illustrating the sheet conveying portion  9  in a state that the rotating lever  23  moves from the second position to the third position.  FIG. 8B  is a diagram illustrating the rotating lever  23  which moves from the second position to the third position.  FIG. 8C  is a diagram illustrating the light-shielding flag  29  in a state that the rotating lever  23  moves from the second position to the third position.  FIG. 9A  is a diagram illustrating the sheet conveying portion  9  in a state that the rotating lever  23  moves from the third position to the first position.  FIG. 9B  is a diagram illustrating the rotating lever  23  in a state that it moves from the third position to the first position.  FIG. 9C  is a diagram illustrating the light-shielding flag  29  in a state that the rotating lever  23  moves from the third position to the first position.  FIG. 10  is a diagram illustrating a rotating trajectory T of the abutting surface  23   d  of the rotating lever  23  which cyclically moves from the first position to the third position in a state that it is directed to the upstream side. 
     As illustrated in  FIGS. 4A and 4B , the abutting portion  23   a  of the rotating lever  23  is held at the first position in a standby state by virtue of a biasing force of the biasing spring  27  while the front end of the sheet S does not abut on the abutting surface  23   d  of the rotating lever  23 . In addition, at the first position, as illustrated in  FIG. 4C , the light path L of the optical sensor  30  is held in a state that the light does not blocked by the slit portion  429   b  of the light-shielding flag  29 . 
     Next, as illustrated in  FIG. 5A , as the front end of the sheet S conveyed by a pair of conveying rollers  18  and  19  abuts on the abutting surface  23   d  of the rotating lever  23 , the sheet S presses the abutting surface  23   d  against the holding force of the lever driving member  26  biased by the biasing spring  27 . As the abutting surface  23   d  is pressed by the sheet S, the lever driving member  26  is rotated to the arrow direction r of  FIG. 5A  against the biasing force of the biasing spring  27 . At this time, the light-shielding flag  29  of  FIG. 5C  is also rotated to the arrow direction r of  FIG. 5C . As the lever driving member  26  and the light-shielding flag  29  are rotated to the arrow direction r, the long hole portion  23   b  is guided to the support axis  31   b  and slides, and the rotating lever  23  is rotated. As the rotating lever  23  is rotated, the abutting surface  23   d  moves to the arrow direction z 1  of  FIG. 5B . 
     At this time, the front end of the sheet S is guided by the sheet-passing guide arranged in the downstream side of the sheet conveying direction of a pair of conveying rollers  18  and  19 . As illustrated in  FIG. 5C  the sheet-passing guide includes a paper feeding frame  20  and a guide frame  28 . For this reason, the front end of the sheet S from being separated may be prevented from the abutting surface  23   d  and rotation while the abutting surface  23   d  of the rotating lever  23  is reliably pressed may be provided by the front end of the sheet S. 
     As illustrated in  FIGS. 6A and 6B , as the abutting surface  23   d  is pressed by the front end of the sheet S, the long hole portion  23   b  is guided to the support axis  31   b  and slides, and the rotating lever  23  is rotated against the biasing force of the biasing spring  27 . As the rotating lever  23  is rotated, the abutting surface  23   d  moves to the arrow direction z 2  of  FIG. 6B . Similarly, the lever driving member  26  is rotated to the arrow direction r of  FIG. 6A , and the light-shielding flag  29  is also rotated to the arrow direction r of  FIG. 6C . 
     As the rotating lever  23  is further rotated, as illustrated in  FIGS. 7A and 7B , the connected portion  23   c  of the rotating lever  23  arrives at the top dead point (hereinafter, refer to as a “second position”) of the lever driving member  26  where the biasing spring  27  is biased at maximum. As the rotating lever  23  arrives at the second position, the force for rotating the lever driving member  26  is switched from the force of the sheet S for pressing the rotating lever  23  to the biasing force of the biasing spring  27  for returning the abutting portion  23   a  to the first position. Moreover, the abutting surface  23   d  of the rotating lever  23  moves to the arrow direction z 3  of  FIG. 7B  by virtue of the biasing force of the biasing spring  27 , the abutting portion  23   a  is retracted from the sheet conveying path  15   a , and the abutting surface  23   d  is retracted from the front end of the sheet S. Similarly, as illustrated in  FIG. 7C , the light path L of the optical sensor  30  is blocked by the light-shielding flag  29 . If the light path L of the optical sensor  30  is blocked, the sheet detector  200  detects that the rotating lever  23  is rotated to a predetermined rotating position, and the front end of the sheet S is conveyed to a desired position. In addition, a predetermined signal is transmitted to the image forming portion  14 , and the image forming portion  14  initiates formation of the toner image when the image forming portion  14  receives this signal. 
     Here, although the rotating lever  23  moves to the arrow direction z 3  of  FIG. 7B  by the biasing force of the biasing spring  27 , the sheet S is conveyed by a pair of conveying rollers  18  and  19  (while it passes through the first position in sheet conveying path  15   a ). Therefore, as illustrated in  FIGS. 8A and 8B , the rotating lever  23  waits in a state that the front end of the abutting portion  23   a  abuts on the surface of the sheet S while the rotating lever  23  is biased by the biasing spring  27  (hereinafter, referred to as a “third position”). In addition, as illustrated in  FIG. 8C , the light path L of the optical sensor  30  is blocked by the light-shielding flag  29  even in this state. 
     Moreover, as the rear end (trailing end) of the sheet S passes through the front end of the abutting portion  23   a , the rotating lever  23  initiates rotation to make the abutting portion  23   a  locate at the first position by the biasing force of the biasing spring  27 . Furthermore, as the rear end of the sheet S recedes from the abutting portion  23   a , the rotating lever  23  has a state that the abutting portion  23   a  protrudes to the sheet conveying path  15   a , and the abutting surface  23   d  waits at the first position for aligning the front end of the next sheet S as illustrated in  FIGS. 9A and 9B . At this time, in the light path L of the optical sensor  30 , the light-blocking by the light-shielding flag  29  is removed, and the optical sensor  30  generates a transmission signal as illustrated in  FIG. 9C . As a result, the rear end of the sheet S may be detected. 
     In this manner, by repeating the state illustrated in  FIGS. 4A to 9C , the rotating lever  23  cyclically moves to the first position, to the second position, and to the third position along a rotating trajectory T of  FIG. 10  while the abutting surface  23   d  is directed to the upstream side of the sheet conveying direction. In other words, the abutting surface  23   d  performs an approximate elliptical movement by the unidirectional rotation of a pair of rotating members  24  and  25  and the lever driving member  26 . 
     The image forming apparatus  100  according to the first embodiment having the configuration described above, it is possible to obtain the following effects. The sheet detector  200 , of the image forming apparatus  100  according to the first embodiment of the invention cyclically moves to the first position, to the second position, and to the third position while the abutting surface  23   d  of the rotating lever  23  is directed to the upstream side, and waits in the third position of the upstream side until the sheet S passes. Then, the abutting portion  23   a  is located at the first position as the sheet S passes through the front end of the rotating lever  23 . Therefore, it is possible to shorten the time that the rotating lever  23  returns to the first position from the standby position until the sheet S passes through the abutting portion  23   a  in comparison with a reciprocating movement of the related art. As a result, it is possible to suppress the sheet interval from being lengthened even when the conveying speed of the sheet S increases. Therefore, it is possible to return the abutting portion  23   a  to the first position within a short sheet interval under a fast conveying speed condition unlike the related art. As a result, it is possible to improve the throughput. 
     For example, according to the first embodiment of the invention, it is possible to reduce the sheet interval to approximately a half compared to the rotating lever of the related art which performs the reciprocating movement. Therefore, it is possible to satisfy user&#39;s demands for further improving the throughput of the image forming apparatus. In addition, as illustrated in  FIG. 10 , it is possible to reduce the rotating trajectory T of the abutting surface  23   d  of the rotating lever  23  in a small elliptical shape. Therefore, it is possible to arrange the image forming apparatus, for example, even in areas where space or arrangement is limited. 
     According to the first embodiment of the invention, the rotating lever  23  is supported by the supporting mechanism having the support axis  31  and a pair of rotating members  24  and  25 . Therefore, it is possible to transmit the rotational driving force to the rotating lever  23  with a simple configuration. As a result, it is possible to, for example, manufacture the image forming apparatus at low cost or suppress a manufacturing cost. In addition, according to the embodiment described above, the main portion  23   e  of the rotating lever  23  is slidably supported by forming the long hole portion  23   b  in the rotating lever  23  and fitting the support axis  31  of the paper feeding frame  20  to the long hole portion  23   b  by insertion. However, for example, the main portion  23   e  of the rotating lever  23  may be slidably configured by forming, in the paper feeding frame  20 , the long hole where a pin protruding from the rotating lever  23  is fitted by insertion. 
     Second Embodiment 
     Next, the image forming apparatus  100 A according to a second embodiment of the invention will be described with reference to  FIGS. 11A to 12B  along with  FIG. 1 . The image forming apparatus  100 A according to the second embodiment of the invention is different from that of the first embodiment in that the follower roller  22  is provided in the front end of the rotating lever  223  as a follower roller. For this reason, in the second embodiment, a description will be made by focusing on a difference from the first embodiment, that is, the follower roller  22  provided in the front end of the rotating lever  223 . In the second embodiment, like reference numerals denote like elements as in the image forming apparatus  100  according to the first embodiment, and the description thereof will not be repeated. According to the second embodiment of the invention, it is possible to obtain the effects similar to those of the first embodiment in the configuration similar to that of the first embodiment. 
     First, the entire structure of the image forming apparatus  100 A according to the second embodiment will be described with reference to  FIGS. 11A to 12B .  FIG. 11A  is a perspective view illustrating the sheet conveying portion  9 A of the image forming apparatus  100 A according to the second embodiment of the invention.  FIG. 11B  is a perspective view illustrating the sheet conveying portion  9 A illustrated in  FIG. 11A  as seen from the opposite side.  FIG. 12A  is a diagram illustrating the sheet conveying portion  9 A of the image forming apparatus  100 A according to the second embodiment of the invention.  FIG. 12B  is a diagram illustrating a state in which the rotating lever  223  of the sheet conveying portion  9 A illustrated in  FIG. 12A  waits in the third position. Arrows illustrated in  FIGS. 11A and 11B  represent the conveying direction of the sheet S. 
     As illustrated in  FIG. 1 , the image forming apparatus  100 A includes the sheet feeding portion  8 , the sheet conveying portion  9 A which conveys the sheet S while detecting the front or rear end of the sheet S fed from the sheet feeding portion  8 , the image forming portion  14 , the fixing portion  10 , and the sheet discharge portion  13 . As illustrated in  FIGS. 11A and 11B , the sheet conveying portion  9 A includes the paper feeding frame  20 , the guide frame  28 , a pair of conveying rollers  18  and  19 , and the sheet detector  200 A. The sheet detector  200 A includes the rotating lever  223 , the support axis  31 , a pair of rotating members  24  and  25 , the rotating axes  25   a  and  24   a , the lever driving member  26 , the biasing spring  27 , the light-shielding flag  29 , and the optical sensor  30 . 
     As illustrated in  FIG. 12A , the rotating lever  223  includes the main portion  23   e , the abutting portion  23   a , the follower roller  22  provided in the front end of the abutting portion  23   a , the long hole portion  23   b , and the connected portion  23   c . The follower roller  22  is formed such that the roller surface of the follower roller  22  is contacted by rolling with the surface (rear surface) of sheet S conveyed along the sheet conveying path  15   a  when the rotating lever  23  waits in the third position as illustrated in  FIG. 12B . 
     Next, the operation of the sheet conveying portion  9 A of the image forming apparatus  100 A according to the second embodiment of the invention will be described. Since the basic operation of the sheet conveying portion  9 A is similar to that of the first embodiment, the description thereof will not be repeated, and the operation of the rotating lever  223  of the sheet detector  200 A at the third position will be only described. As illustrated in  FIG. 12B , in the third position, while the rotational force is generated in the rotating lever  223  by the biasing spring  27  and the lever driving member  26 , the rotating lever  223  is held in balance between the rotational force and the stiffness of the sheet S. In this state, the follower roller  22  provided in the front end of the rotating lever is contacted by rolling with the sheet S in the middle of conveying, and the rotating lever  223  waits in the third position while the front end is contacted by rolling. Then, as the rear end of the sheet S passes, the rotating lever  223  is rotated to the first position from the third position. 
     In the image forming apparatus  100 A according to the second embodiment having the configuration described above, it is possible to obtain the following effects. In the sheet detector  200 A of the image forming apparatus  100 A according to the second embodiment, the follower roller  22  is provided in the front end of the rotating lever  3 . For this reason, the front end of the rotating lever  23  can wait in the third position in contact with the sheet S while the front end of the rotating lever  223  is contacted by rolling with the surface (rear surface) of the sheet S conveyed along the sheet conveying path  15   a . As a result, it is possible to suppress formation of the contact traces or the like on the surface (rear surface) of the sheet S generated when the surface (rear surface) of the sheet S and the front end of the rotating lever  23  rub with each other. 
     Third Embodiment 
     Next, the image forming apparatus  100 B according to a third embodiment of the invention will be described with reference to  FIGS. 13A to 17  along with  FIG. 1 . The image forming apparatus  100 B according to the third embodiment of the invention is different from the first embodiment in terms of the rotating lever  323  and the supporting mechanism of the rotating lever  323 . For this reason, in the third embodiment, description will be made by focusing on a difference from the first embodiment, that is, the rotating lever  323  and the supporting mechanism of the rotating lever  323 . In the third embodiment, like reference numerals denote like elements as in the image forming apparatus  100  according to the first embodiment, and the description thereof will not be repeated. In the third embodiment, it is possible to obtain the effects similar to those of the first embodiment in the configuration similar to that of the first embodiment. 
     Next, the entire structure of the image forming apparatus  100 B according to the third embodiment of the invention will be described with reference to  FIGS. 13A to 15C  along with  FIG. 1 .  FIG. 13A  is a perspective view illustrating the sheet conveying portion  9 B of the image forming apparatus  100 B according to the third embodiment of the invention.  FIG. 13B  is a perspective view illustrating the sheet conveying portion  9 B of  FIG. 13A  as seen from the opposite side.  FIG. 14  is an exploded perspective view illustrating a portion of the sheet detector  200 B according to the third embodiment of the invention.  FIG. 15A  is a diagram illustrating a state in which the sheet S is conveyed in the sheet conveying portion  9 B according to the third embodiment of the invention.  FIG. 15B  is a diagram illustrating the rotating lever  323  in a state in which the sheet S is conveyed in the sheet conveying portion  9 B.  FIG. 15C  is a diagram illustrating the light-shielding flag  29  in a state in which the sheet S is conveyed in the sheet conveying portion  9 B. Arrows illustrated in  FIGS. 13A and 13B  represent the conveying direction of the sheet S. 
     As illustrated in  FIG. 1 , the image forming apparatus  100 B includes the sheet feeding portion  8 , the sheet conveying portion  9 B which conveys the sheet S while detecting the front or rear end of the sheet S fed from the sheet feeding portion  8 , the image forming portion  14 , the fixing portion  10 , and the sheet discharge portion  13 . As illustrated in  FIGS. 13A and 13B , the sheet conveying portion  9 B includes the paper feeding frame  20 , the guide frame  28 , a pair of conveying rollers  18  and  19 , and the sheet detector  200 B. 
     The sheet detector  200 B includes the rotating lever  323 , the rotating member  25 , the rotational axes  25   a  and  324   a , the lever driving member  26 , the biasing spring  27 , the light-shielding flag  29 , and the optical sensor  30 . In addition, the sheet detector  200 B includes the first and second gears  324  and  332  as the first and second rotating members which configure a support mechanism, and the linking gear  333  as the linking member which links the first and second gears  324  and  332  to rotate in the same direction at the same phase. 
     As illustrated in  FIG. 14 , the rotating lever  323  includes the main portion  323   e  formed in a strip shape, the abutting portion  323   a  formed in one end of the main portion  323   e  in the longitudinal direction, and the first connected portion  323   b  formed in the other end side of the main portion  323   e . In addition, the rotating lever  323  includes the second connected portion  323   c  formed in a position parallel to the first connected portion  323   b . As illustrated in  FIGS. 15A and 15B , the abutting portion  323   a  is provided to protrude to the sheet conveying path  15   a  at the first position and includes the abutting surface  323   d  where the front end of the sheet S which moves along the sheet conveying path  15   a  can abut at the first position. The second connected portion  323   c  is formed in the vicinity of a base end of the abutting portion  323   a  in the rotating lever  323 . The first connected portion  323   b  is formed in the opposite end of the second connected portion  323   c.    
     The first gear  324  is connected to the rotational axis  324   a  and includes the first connecting axis  324   b  as the first connecting portion which extends in parallel with the rotational axis  324   a  in a position offset radially from the center of rotation (eccentric position). The first connecting axis  324   b  is formed to penetrate the second connected portion  323   c  and is fitted by insertion to the connection hole  25   b  of the rotating member  25  after penetrating the second connected portion  323   c . As a result, the rotating lever  323  can rotate together with the first gear  324 . 
     The second gear  332  is arranged around the axis parallel to the first gear  324 . In addition, the second gear  332  includes the second connecting axis  332   b  as the second connecting portion which extends in parallel with the rotational axis  324   a  in a position radially offset from the center of rotation (eccentric position). The second connecting axis  332   b  is formed to be connectable to the first connected portion  323   b , and rotates the rotating lever  323  along with the second gear  332 . Moreover, the first and second gears  324  and  332  are formed to have a gear ratio of 1:1. 
     The linking gear  333  is arranged around the axis parallel to the first and second gears  324  and  332  and meshes with the first and second gears  324  and  332  to rotate the first and second gears  324  and  332  in the same direction at the same phase. The first and second gears  324  and  332  are rotated in the same direction at the same cycle by the linking gear  333 . 
     Next, the operation of the sheet detector  200 B will be described with reference to  FIGS. 16A to 17  along with  FIGS. 15A to 15C .  FIG. 16A  is a diagram illustrating the sheet conveying portion  9 B in a state in which the front end of the sheet S abuts on the abutting surface  323   d  of the rotating lever  323 .  FIG. 16B  is a diagram illustrating the rotating lever  323  in a state in which the front end of the sheet S abuts on the abutting surface  323   d .  FIG. 16C  is a diagram illustrating the light-shielding flag  29  in a state in which the front end of the sheet S abuts on the abutting surface  323   d .  FIG. 17  is a diagram illustrating a rotating trajectory T 2  of the abutting surface  323   d  of the rotating lever  323  cyclically moving from the first position to the third position while the abutting surface is directed to an upstream side. 
     As illustrated in  FIGS. 15A and 15B , the abutting portion  323   a  of the rotating lever  323  is held at the first position in a standby state by virtue of a biasing force of the biasing spring  27  while the front end of the sheet S does not abut on the abutting surface  323   d  of the rotating lever  323 . In addition, at the first position, as illustrated in  FIG. 15C , the light path L of the optical sensor  30  is not blocked by the slit portion  29   a  of the light-shielding flag  29 . 
     Next, as illustrated in  FIG. 16A , as the front end of the sheet S conveyed by a pair of conveying rollers  18  and  19  abuts on the abutting surface  323   d  of the rotating lever  323 , the sheet S presses the abutting surface  323   d  against the holding force of the lever driving member  26  biased by the biasing spring  27 . As the abutting surface  323   d  of the sheet S is pressed, the lever driving member  26  is rotated to the arrow direction r of  FIG. 16A  against the biasing force of the biasing spring  27 . At this time, the light-shielding flag  29  of  FIG. 16C  is also rotated to the arrow direction r of  FIG. 16C . The first gear  324  is rotated as the lever driving member  26  and the light-shielding flag  29  are rotated to the direction r, and the second gear  332  is rotated by the linking gear  333  as the first gear  324  is rotated. The rotating lever  323  is rotated when the first gear  324  and the second gear  332  rotate. The abutting surface  323   d  moves to the arrow direction z 1  of  FIG. 16B  as the rotating lever  323  is rotated. 
     At this time, the front end of the sheet S, as illustrated in  FIG. 16C , includes the paper feeding frame  20  and the guide frame  28  and is guided by the sheet-passing guide arranged on the downstream side in the sheet conveying direction of a pair of conveying rollers  18  and  19 . For this reason, it is possible to prevent the front end of the sheet S from being separated from the abutting surface  323   d  and provide rotation while the abutting surface  323   d  of the rotating lever  323  is reliably pressed by the front end of the sheet S. 
     As the rotating lever  323  is further rotated, the connected portion  323   c  of the rotating lever  323  arrives at the top dead point (second position) of the lever driving member  26  where the biasing spring  27  is biased at maximum. As the rotating lever  323  arrives at the second position, the force for rotating the lever driving member  26  is switched from the force that the sheet S presses the rotating lever  323  to the biasing force that the biasing spring  27  returns the abutting portion  323   a  to the first position. Moreover, the abutting surface  323   d  of the rotating lever  323  moves by the biasing force of the biasing spring  27 , the abutting portion  323   a  is retracted from the sheet conveying path  15   a , and the abutting surface  323   d  is retracted from the front end of the sheet S. Similarly, the light path L of the optical sensor  30  is blocked by the light-shielding flag  29 . As the light path L of the optical sensor  30  is blocked, the sheet detector  200 B detects that the rotating lever  323  is rotated to a predetermined rotating position and the front end of the sheet S is conveyed to a desired position. In addition, a predetermined signal is transmitted to the image forming portion  14 , and the image forming portion  14  initiates formation of the toner image as the image forming portion  14  receives the signal. According to the present embodiment, the light path L of the optical sensor  30  is blocked by the light-shielding flag  29  at the second position. 
     Here, although the rotating lever  323  rotates around the rotating axes  25   a  and  324   a  by the biasing force of the biasing spring  27 , the sheet S is conveyed by a pair of conveying rollers  18  and  19  (while passing through the first position in the sheet conveying path  15   a ). For this reason, the rotating lever  323  waits at the third position while it is biased by the biasing spring  27 , and the front end of the abutting portion  323   a  abuts on the surface of the sheet S. In addition, the light path L of the optical sensor  30  is blocked by the light-shielding flag  29  even in this state. 
     Moreover, as the rear end of the sheet S passes through the front end of the abutting portion  323   a , the rotating lever  323  initiates rotation such that the abutting portion  323   a  is located at the first position by the biasing force of the biasing spring  27 . Furthermore, as the rear end of the sheet S recedes from the abutting portion  323   a , the rotating lever  323  enters a state in which the abutting portion  323   a  protrudes to the sheet conveying path  15   a , and the abutting surface  323   d  waits at the first position where the abutting surface  323   d  can abut on the front end of the subsequent sheet S. At this time, in the light path L of the optical sensor  30 , the light blocking by the light-shielding flag  29  is released, and the optical sensor  30  generates a transmission signal. As a result, it is possible to detect that the sheet S passes. 
     In this manner, by repeating the process described above, the rotating lever  323  cyclically moves to the first position, to the second position, and to the third position along a rotating trajectory T 2  of  FIG. 17  while the abutting surface  323   d  is directed to the upstream side in the sheet conveying direction. In other words, the abutting surface  323   d  performs a circular movement by the unidirectional rotation of the first gear  324 , the second gear  332 , the linking gear  333 , the rotating member  25 , and the lever driving member  26 . 
     In the image forming apparatus  100 B according to the third embodiment having the configuration described above, it is possible to obtain the following effects. The sheet detector  200 B of the image forming apparatus  100 B according to the third embodiment includes the first gear  324 , the second gear  332 , and the linking gear  333  in order to rotate the rotating lever  323 . Therefore, it is possible to rotate the rotating lever  323  smoothly. Moreover, as illustrated in  FIG. 17 , it is possible to reduce the rotating trajectory T 2  of the front end of the rotating lever  323  in the sheet conveying direction (vertical direction of  FIG. 17 ), compared to the first embodiment, and alleviate restriction in space or arrangement of the image forming apparatus  100 B. 
     Fourth Embodiment 
     Next, the image forming apparatus  100 C according to a fourth embodiment of the invention will be described with reference to  FIGS. 18A to 25D  along with  FIG. 1 . In the image forming apparatus  100 C according to the fourth embodiment, the rotating member and the biasing portion which applies a force to the rotating lever are different from those of the first embodiment. For this reason, in the fourth embodiment, a description will be made by focusing on a difference from the first embodiment, that is, the rotating member and the biasing portion which applies a force to the rotating lever. In addition, in the fourth embodiment, like reference numerals denote like elements as in the image forming apparatus  100  according to the first embodiment, and the description thereof will not be repeated. In the fourth embodiment, it is possible to obtain the effects similar to those of the first embodiment using the configuration similar to that of the first embodiment. 
     First, the entire structure of the image forming apparatus  100 C according to the fourth embodiment of the invention will be described with reference to  FIGS. 18A to 20D  along with  FIG. 1 .  FIG. 18A  is a perspective view illustrating the sheet conveying portion  9 C of the image forming apparatus  100 C according to the fourth embodiment of the invention.  FIG. 18B  is a perspective view illustrating the sheet conveying portion  9 C of  FIG. 18A  as seen from the opposite side.  FIG. 19  is an exploded perspective view illustrating a portion of the sheet detector  200 C according to the fourth embodiment of the invention.  FIG. 20A  is a diagram illustrating the second rotating lever  523  in a state where the sheet S is conveyed to the sheet conveying portion  9 C.  FIG. 20B  is a diagram illustrating the first rotating lever  423  in a state where the sheet S is conveyed to the sheet conveying portion  9 C.  FIG. 20C  is a diagram illustrating the light-shielding flag  429  in a state where the sheet S is conveyed to the sheet conveying portion  9 C.  FIG. 20D  is a diagram illustrating a biasing portion in a state where the sheet S is conveyed to the sheet conveying portion  9 C. 
     As illustrated in  FIG. 1 , the image forming apparatus  100 C includes the sheet feeding portion  8 , the sheet conveying portion  9 C conveying the sheet S while it detects the front or rear end of the sheet S fed from the sheet feeding portion  8 , the image forming portion  14 , the fixing portion  10 , and the sheet discharge portion  13 . As illustrated in  FIGS. 18A and 18B , the sheet conveying portion  9 C includes the paper feeding frame  20 , the guide frame  28 , a pair of conveying rollers  18  and  19 , and the sheet detector  200 C. 
     The sheet detector  200 C includes the first rotating lever  423  as the first lever member, the second rotating lever  523  as the second lever member, the support axis  31 , the rotating axes  424   a  and  426   c , the lever driving member  426 , and a pair of rotating members  425  and  424 . In addition, the sheet detector  200 C includes the light-shielding flag  429 , the optical sensor  30 , the plate cam  430  as a rotating member, the cam follower  436 , the pressing member  435 , and the biasing spring  427 . Moreover, the biasing spring  427 , the pressing member  435 , and the cam follower  436  configure the biasing portion. 
     The first and second rotating levers  423  and  523  are similar to the rotating lever  23  according to the first embodiment so that the description thereof will not be repeated. The first and second rotating levers  423  and  523  are arranged in a symmetrical position with respect to the rotational center of the plate cam  430  so as to alternately and cyclically move to the first position. The lever driving member  426  is formed in a disk shape and fixed to the end of the rotational axis  426   c  so as to match the rotational axis  426   c  and the rotational center of the lever driving member  426 . The lever driving member  426  includes the first connecting axis  426   b  of a pair of connecting portions formed to protrude to a position (eccentric position) offset radially from the rotational center. 
     The rotating member  424  is formed in a disk shape and is fixed to the end of the rotational axis  424   a  so as to match the rotational axis  424   a  and the rotational center of the rotating member  424 . The rotating member  424  includes the second connecting axis  424   b  of a pair of connecting portions formed to protrude to a position (eccentric position) offset radially from the rotational center. The second connecting axis  424   b  is formed to penetrate the connected portion  323   c  of the first rotating lever  423 . 
     The rotating member  425  includes the first rotating member  425   a , the second rotating member  425   b , and the connecting portion  425   c  which connects the first rotating member  425   a  and the second rotating member  425   b . The first rotating member  425   a  is formed in a disk shape and includes an insertion hole through which the second connecting axis  424   b  is fitted at a position (eccentric position) offset radially from the rotational center. The second rotating member  425   b  is formed in a disk shape and includes an insertion hole through which the first connecting axis  426   b  is fitted at a position (eccentric position) offset radially from the rotational center. 
     The light-shielding flag  429  blocks the light path L of the optical sensor  30 . The light-shielding flag  429  is fixed to the rotational axis  424   a  and is rotated integrally with a pair of rotating members  424  and  425  around the rotational axis  424   a . That is, the light-shielding flag  429  is rotated in synchronization with the first rotating lever  423  and the second rotating lever  523 . In addition, the light-shielding flag  429  includes the first and second slit portions  429   a  and  429   b  that transmit the light of the optical sensor  30 . The first slit portion  429   a  is formed to transmit the light of the optical sensor  30  when the abutting surface  423   d  of the abutting portion  423   a  provided in the first rotating lever  423  is located at the first position (refer to  FIGS. 20A and 20C ). The second slit portion  429   b  is formed to transmit the light of the optical sensor  30  when the abutting surface  523   d  of the abutting portion  523   a  provided in the second rotating lever  523  is located at the first position (refer to  FIGS. 25A and 25C  described below). The light-shielding flag  429  is configured to rotate along with the first and second rotating levers  423  and  523  when the first and second rotating levers  423  and  523  are rotated by the pressing of the front end of the sheet S to alternatively block the light path L of the optical sensor  30 . 
     The plate cam  430  is formed in an elliptical shape having two top dead points and two bottom dead points. The rotational axis  424   a  is fixed to the rotational center of the plate cam  430 . Specifically, the plate cam  430  is rotated so as to be alternately located between the top dead point and the bottom dead point with respect to the rotational axis  424   a . The cam follower  436  is mounted on the pressing member  435  and is engaged with the outer circumferential surface of the plate cam  430 . The pressing member  435  is rotatably mounted on the paper feeding frame  20  in the base end, and the front end thereof is engaged with the biasing spring  427 . The pressing member  435  swingably supports the cam follower  436 . In other words, the pressing member  435  swings by the cam follower  436 . One end of the biasing spring  427  is fixed to the paper feeding frame  20 , and the other end thereof is connected to the pressing member  435 . The biasing spring  427  makes the first and second rotating levers  423  and  523  locate at the first position. 
     Next, the operation of the sheet detector  200 C will be described with reference to  FIGS. 21A to 25D  along with  FIGS. 20A to 20D .  FIG. 21A  is a diagram illustrating the second rotating lever  523  in a state where the front end of the sheet S abuts on the abutting surface  423   d  of the first rotating lever  423 .  FIG. 21B  is a diagram illustrating the first rotating lever  423  in a state where the front end of the sheet S abuts on the abutting surface  423   d .  FIG. 21C  is a diagram illustrating the light-shielding flag  429  in a state where the front end of the sheet S abuts on the abutting surface  423   d  of the first rotating lever  423 .  FIG. 21D  is a diagram illustrating the biasing portion in a state where the front end of the sheet S abuts on the abutting surface  423   d  of the first rotating lever  423 .  FIG. 22A  is a diagram illustrating a state in which the abutting surface  423   d  of the first rotating lever  423  is pressed by the front end of the sheet S so that the second rotating lever  523  is rotated along with the first rotating lever  423 .  FIG. 22B  is a diagram illustrating a state in which the first rotating lever  423  is rotated by the pressing of the front end of the sheet S.  FIG. 22C  is a diagram illustrating the light-shielding flag  429  in a state where the first rotating lever  423  is rotated by the pressing of the front end of the sheet S.  FIG. 22D  is a diagram illustrating the biasing portion in a state where the front end of the sheet S abuts on the biasing portion, and the first rotating lever  423  is rotated. 
       FIG. 23A  is a diagram illustrating the second rotating lever  523  in a state where the first rotating lever  423  is rotated to the second position.  FIG. 23B  is a diagram illustrating a state in which the first rotating lever  423  is rotated to the second position.  FIG. 23C  is a diagram illustrating the light-shielding flag  429  in a state where the first rotating lever  423  is rotated to the second position.  FIG. 23D  is a diagram illustrating the biasing portion while the first rotating lever  423  is rotated to the second position.  FIG. 24A  is a diagram illustrating a state in which the front end of the second rotating lever  523  rotated along with the first rotating lever  423  abuts on the front end of the conveyed sheet S and waits at the third position.  FIG. 24B  is a diagram illustrating the first rotating lever  423  in a state where the second rotating lever  523  waits.  FIG. 24C  is a diagram illustrating the light-shielding flag  429  in a state where the second rotating lever  523  waits at the third position.  FIG. 24D  is a diagram illustrating the biasing portion in a state where the second rotating lever  523  waits at the third position. 
       FIG. 25A  is a diagram illustrating a state in which the second rotating lever  523  is rotated to the first position.  FIG. 25B  is a diagram illustrating the first rotating lever  423  in a state where the second rotating lever  523  is rotated to the first position.  FIG. 25C  is a diagram illustrating the light-shielding flag  429  in a state where the second rotating lever  523  is moved to the first position.  FIG. 25D  is a diagram illustrating the biasing portion in a state where the second rotating lever  523  is moved to the first position. 
     As illustrated in  FIGS. 20A and 20B , the abutting portion  423   a  of the first rotating lever  423  is held at first position in a standby state by virtue of a holding force of the biasing spring  427  while the front end of the sheet S does not abut on the abutting surface  423   d  of the first rotating lever  423 . In addition, when the first rotating lever  423  waits at the first position, the second rotating lever  523  waits at the position that the abutting surface  523   d  is retracted from the sheet conveying path  15   a . At this time, as illustrated in  FIG. 20C , the light path L of the optical sensor  30  is not blocked by the first slit portion  429   a  of the light-shielding flag  429 . Moreover, as illustrated in  FIG. 20D , the plate cam  430  is positioned such that the bottom dead point of the one end is engaged with the cam follower  436 , and the biasing spring  427  holds the plate cam  430  at the first position through the cam follower  436  and the pressing member  435 . 
     Next, as illustrated in  FIGS. 21A and 21B , as the front end of the sheet S conveyed by a pair of conveying rollers  18  and  19  abuts on the abutting surface  423   d  of the first rotating lever  423 , the sheet S presses the abutting surface  423   d  against the holding force of the biasing spring  427  in order to hold the plate cam  430  through the pressing member  435  and the cam follower  436 . If the abutting surface  423   d  is pressed by the sheet S, the plate cam  430  is rotated in the z 3  direction of  FIG. 21D , and the outer circumferential surface of the plate cam  430  presses the biasing spring  427  through the cam follower  436  and the pressing member  435 . At this time, the light-shielding flag  429  is also rotated in the arrow direction z 3  of  FIG. 21C . In addition, as the abutting surface  423   d  is pressed by the sheet S, the first rotating lever  423  is rotated. As the first rotating lever  423  is rotated, the abutting surface  423   d  moves in the z 2  direction of  FIG. 21B , the second rotating lever  523  is rotated, and the abutting surface  523   d  moves in the arrow direction z 1  of  FIG. 21A . 
     At this time, the front end of the sheet S includes the paper feeding frame  20  and the guide frame  28  and is guided by the sheet-passing guide arranged on the downstream side in the sheet conveying direction of a pair of conveying rollers  18  and  19  as illustrated in  FIG. 21C . For this reason, it is possible to prevent the front end of the sheet S from being separated from the abutting surface  423   d . Further, it is possible to rotate the abutting surface  423   d  of the first rotating lever  423  while being reliably pressed by the front end of the sheet S. 
     Subsequently, as illustrated in  FIGS. 22A and 22B , the first rotating lever  423  is further rotated in the z 2  direction. Then, the light-shielding flag  429  illustrated in  FIG. 22C  is rotated in the z 3  direction to block the light path L of the optical sensor  30  so as to synchronize with the arrival of the plate cam  430  illustrated in  FIG. 22D  at the second position. As the first rotating lever  423  is further rotated and arrives at the top dead point (the second position) of the plate cam  430  where the biasing spring  427  is biased at maximum as illustrated in  FIG. 23D , the first rotating lever  423  arrives at the second position as illustrated in  FIG. 23B . At the same time, as illustrated in  FIG. 23A , movement to the arrow z 1  direction is initiated such that the abutting surface  523   d  of the second rotating lever  523  is located in the sheet conveying path  15   a . As a result, as illustrated in  FIG. 23C , the light path L of the optical sensor  30  is blocked by the light-shielding flag  429 . If the light path L of the optical sensor  30  is blocked, the sheet detector  200 C detects that the first rotating lever  423  is rotated to a predetermined rotating position, and the front end of the sheet S is conveyed to a desired position. In addition, a predetermined signal is transmitted to the image forming portion  14 , and the image forming portion  14  initiates formation of the toner image as the image forming portion  14  receives the signal. 
     As illustrated in  FIGS. 24A ,  24 C, and  24 D, as the first rotating lever  423  arrives at the second position, the force for rotating the plate cam  430  is switched to the biasing force for retracting the abutting surface  423   d  of the first rotating lever  423  from the sheet conveying path  15   a . Similarly, the force for rotating the plate cam  430  is switched to the biasing force for positioning the abutting surface  523   d  of the second rotating lever  523  to the first position. 
     Here, although the second rotating lever  523  receives the biasing force of the biasing spring  427  and is rotated to the first position, the sheet S is conveyed by a pair of conveying rollers  18  and  19  at this timing (while the sheet S passes through the first position in the sheet conveying path  15   a ). Therefore, the second rotating lever  523  waits at the third position where the front end of the abutting portion  523   a  abuts on the surface (or rear surface) of the sheet S while it is biased by the biasing spring  427  as illustrated in  FIG. 24A . In addition, as illustrated in  FIG. 24C , the light path L of the optical sensor  30  is blocked by the light-shielding flag  429  even in this state. 
     Moreover, as the rear end of the sheet S passes through the front end of the abutting portion  523   a , the second rotating lever  523  initiates rotation so that the abutting portion  523   a  is located at the first position by the biasing force of the biasing spring  427  as illustrated in  FIG. 25A . Furthermore, as the rear end of the sheet S recedes from the abutting portion  523   a , in the second rotating lever  523 , the abutting portion  523   a  protrudes to the sheet conveying path  15   a , and the abutting surface  523   d  waits at the first position where the abutting surface  523   d  can abut on the front end of the subsequent sheet S. At this time, in the light path L of the optical sensor  30 , the light blocking by the light-shielding flag  429  is released, and the optical sensor  30  generates a transmission signal as illustrated in  FIG. 25C . As a result, it is possible to detect that the sheet S passes. In addition, as illustrated in  FIG. 25D , since the bottom dead point of the other end of the plate cam  430  is engaged with the cam follower  436 , the biasing spring  427  holds the plate cam  430  at the first position through the cam follower  436  and the pressing member  435 . For this reason, the second rotating lever  523  is held at the first position. Similarly, as illustrated in  FIG. 25B , the first rotating lever  423  is held in a retracted state from the sheet conveying path  15   a.    
     In the image forming apparatus  100 C according to the fourth embodiment having the configuration described above, it is possible to obtain the following effects in addition to the effects of the first embodiment. The sheet detector  200 C according to the fourth embodiment uses the plate cam as the rotating member and includes the pressing member  435 , the cam follower  436 , and the biasing spring  427  as the biasing member. For this reason, for example, it is possible to improve the position accuracy in a state where the rotating lever stops at the first position. 
     In addition, the sheet detector  200 C according to the fourth embodiment includes the first rotating lever  423  and the second rotating lever  523 . Using a plurality of rotating levers in this manner, for example, it is possible to suppress the scraping generated when sheet S passes through the rotating lever. 
     While the invention has been particularly shown and described with reference to exemplary embodiments thereof, it is noted that the invention is not limited to the above-described embodiments. The effects described in the embodiment of the invention are merely listed the most suitable effects resulting from the invention, and the effect of the invention is not limited to those described in the embodiment of the invention. 
     For example, in the first embodiment, although formation of the toner image (image forming process) is initiated in the image forming portion  14  when the sheet detector  200  detects that the front end of the sheet S is conveyed to a desired position, the invention is not limited thereto. The image forming apparatus  100  may form the toner image (image forming process) using the image forming portion  14  in advance, and the image may be transferred to the transfer rollers  5   a  to  5   d  at the timing when the sheet S arrives at the transfer rollers  5   a  to  5   d  if sheet S is detected by the sheet detector  200 . 
     Moreover, for example, in the present embodiment, although the biasing spring  27  makes the rotating lever wait at the first position, the invention is not limited thereto. For example, by controlling the weight balance of the rotating lever, the abutting surface of the rotating lever may wait at the first position using the weight. In addition, an elastic force of a plate spring, rubber, and the like may be used. 
     In the third embodiment, although the linking gear  333  is used as the linking member, the invention is not limited thereto. For example, the linking member may be a timing belt, a timing chain, and the like. The linking member may rotate the first and second rotating members in the same direction at the same phase. 
     Furthermore, for example, in the present embodiment, although the sheet conveying portion  9  (sheet detector  200 ) is provided on the upstream side of the image forming portion  14 , the invention is not limited thereto. For example, the sheet conveying portion  9  (sheet detector  200 ) may be provided on the downstream side of the fixing portion  10 . For example, if the sheet detector  200 A according to the second embodiment is provided in the downstream side of the fixing portion  10 , the sheet detector  200 A waits in rolling contact with the surface of the sheet S where the toner image has been formed after the fixing. Therefore, it is possible to suppress a damage of the fixed toner image. 
     While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions. 
     This application claims the benefit of Japanese Patent Application No. 2011-004918, filed Jan. 13, 2011, which is hereby incorporated by reference herein in its entirety.