Patent Publication Number: US-8540239-B2

Title: Sheet conveying apparatus and image forming apparatus

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a sheet conveying apparatus which has a position detecting apparatus which detects an end part position of a sheet, and an image forming apparatus having this sheet conveying apparatus. 
     2. Description of the Related Art 
     A conventional image forming apparatus pulls out sheets on which images are formed, one by one from a sheet cassette by means of feeding rollers to feed to the image forming portion. Therefore, there are cases where sheets to be fed (skewed and fed) are conveyed in an inclined state with respect to a sheet conveying direction due to the difference between outer diameters of feeding rollers or the difference between feeding speeds produced by friction of feeding rollers, or the influence of the sliding resistance between sheets and a conveying guide which conveys sheets. 
     When sheets are skewed and fed, if, for example, a toner image on a photosensitive drum is transferred onto a sheet, the toner image is printed in a state where the image is inclined with respect to the sheet. Hence, the image forming apparatus has a shutter member in a pair of registration rollers to correct skew feeding of sheets (hereinafter, also “skew feeding correction”), and aligns the front end of a sheet to be adjusted in a direction orthogonal to the conveying direction to convey to the image forming portion. 
     However, with the above skew feeding which is performed by having the front end of a sheet hit the shutter member, although it is possible to perform skew feeding correction of conveying sheets virtually parallel to the sheet conveying direction, it is not possible to correct the position of the sheet misaligned in a direction orthogonal to the sheet conveying direction. 
     Further, when, for example, duplex printing of sheets is performed, an image is formed on the first face, then the sheet is reversed and an image is formed on the second face, and therefore a conveying route for an image to be formed on the second face is longer than the first face. Therefore, the second face is susceptible to the influence of various rollers and conveying guide. By this means, sheets are likely to be skewed and fed, or misaligned. Further, sheets on which toner images are fixed in a fixing portion are contracted due to heat and pressure of the fixing portion, and is smaller upon printing of the second face than upon printing of the first face. By this means, there are cases where the end part position of a sheet is changed in a direction orthogonal to the conveying direction. 
     By contrast with this, a position detecting apparatus is discussed which corrects position misalignment of sheets by detecting an end part position of a sheet in a direction orthogonal to the conveying direction (Japanese Patent Laid-Open No. 2000-335010 and FIG. 1 of Japanese Patent Laid-Open No. 62-40475). With the position detecting apparatus discussed in Japanese Patent Laid-Open No. 2000-335010, line sensors are arranged in a line to detect end part positions of the minimum size to maximum size of sheets to be conveyed, and detect a side edge portion of a sheet based on a light blocked state by irradiating one face of the sheet with light. According to Japanese Patent Laid-Open No. 62-40475, a plurality of sensors is closely arranged in a width direction to detect an end part position of a sheet. 
     However, the position detecting apparatus discussed in Japanese Patent Laid-Open No. 2000-335010 detects side edge portions of conveyable sheets of all sizes, and therefore requires line sensors having a length which enables detection of end portions of all sizes. Therefore, there is a problem that cost of sensors becomes high, and, as a result, cost of the entire apparatus becomes high. According to Japanese Patent Laid-Open No. 62-40475, while a detecting portion which is provided with a plurality of sensors aligned in a width direction is moved according to a sheet size and therefore the number of sensors is comparatively a little, more sensors are necessary to precisely detect positions of sheets, and therefore cost increases. 
     By contrast with this, a position detecting apparatus is discussed which detects an end part position in a sheet width direction orthogonal to a sheet conveying direction using a photointerrupter in which light emitting elements and light receiving elements are arranged to oppose to each other (Japanese Patent Laid-Open No. 5-132193). The position detecting apparatus discussed in Japanese Patent Laid-Open No. 5-132193, moves a photointerrupter such that a sheet end part crosses between pairs of light emitting elements and light receiving elements arranged to oppose to each other, and detects an end part position of a sheet based on the distance from a reference position to an optical path blocking position. 
     However, the position detecting apparatus discussed in Japanese Patent Laid-Open No. 5-132193 detects a position by means of pairs of light emitting elements and light receiving elements, and needs to increase the moving distance of the photointerrupter to measure sheet end parts of a plurality of sheet sizes. Further, to secure the moving distance of the photointerrupter, for example, a moving mechanism needs to be made larger. Therefore, the apparatus becomes larger. 
     SUMMARY OF THE INVENTION 
     The present invention provides a sheet conveying apparatus which has a position detecting apparatus which can precisely detect a position with a cheap and simple configuration which is easy to assemble, and an image forming apparatus. 
     The present invention provides a sheet conveying apparatus which detects an end part position of a sheet in a sheet width direction orthogonal to a conveying direction of the sheet conveyed on a sheet conveying path, comprising: a plurality of sensor portions which includes light emitting portions and light receiving portions, and which is arranged in the sheet width direction; a holding portion which holds the plurality of sensor portions such that the sheet conveyed on the sheet conveying path can pass between the light emitting portions and the light receiving portions; a plurality of aperture stops which is disposed between the light emitting portions and the light receiving portions and which is provided on the holding portion; and a driving portion which causes a movement of the holding portion in the sheet width direction. 
     According to the present invention, when an aperture stop is provided between light emitting portion and a light receiving portion, so that it is possible to precisely detect a position of the end part with a cheap and simple configuration which is easy to assemble. 
     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 sectional view schematically illustrating the entire structure of a laser beam printer according to a first embodiment of the present invention. 
         FIG. 2  is a perspective view illustrating a sheet end part detecting portion of a laser beam printer according to the first embodiment. 
         FIG. 3  is a partial enlarged view illustrating a sensor unit of a sheet end part detecting portion according to the first embodiment. 
         FIG. 4  is a view for describing an operation range of a sheet end part detecting portion according to the first embodiment. 
         FIG. 5  is an assembly diagram for describing the state before a sensor portion is assembled according to the first embodiment. 
         FIG. 6A  is a perspective view illustrating the state where cover members and a supporting plate of a sensor portion are assembled according to the first embodiment, and  FIG. 6B  is a perspective view illustrating the state where a supporting plate and a holder are assembled according to the first embodiment. 
         FIG. 7  is a perspective view illustrating a sheet end part detecting portion of a laser beam printer according to a second embodiment. 
         FIG. 8  is a partial enlarged view illustrating a sensor unit of a sheet end part detecting portion according to the second embodiment. 
         FIG. 9  is a view for describing an operation range of a sheet end part detecting portion according to the second embodiment. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Hereinafter, an image forming apparatus according to embodiments of the present invention will be described with reference to the drawings. The image forming apparatus according to embodiments of the present invention is a copying machine, printer, facsimile or all-in-one machine of these having a sheet conveying apparatus including a sheet end part detecting portion which can detect an end part position of a sheet. The following embodiments will be described using a laser beam printer  100  as an image forming apparatus. 
     First Embodiment 
     The laser beam printer  100  according to the first embodiment of the present invention will be described with reference to  FIGS. 1 to 4 . First, the entire structure of the laser beam printer  100  according to the first embodiment will be described with reference to  FIG. 1 .  FIG. 1  is a sectional view schematically illustrating the entire structure of the laser beam printer  100  according to the first embodiment of the present invention. 
     As illustrated in  FIG. 1 , the laser beam printer  100  has a sheet feeding portion  10  which feeds sheets S, and an image forming portion  11  which forms images on the sheets S fed from the sheet feeding portion  10 . Further, the laser beam printer  100  has a fixing portion  210  which fixes images, a discharge portion  14  and a conveying portion  15  which is a sheet conveying apparatus. 
     The sheet feeding portion  10  has a sheet cassette  204  which accommodates the sheets S, a pair of feed rollers  206  which feed the sheets S accommodated in the sheet cassette  204  to the image forming portion  11 , and a separating portion (not illustrated) which separates the sheet S one by one. The sheet feeding portion  10  feeds the sheets S accommodated in the sheet cassette  204 , to the image forming portion  11  by means of a pair of feed rollers  206  while separating the sheets S one by one in the separating portion. 
     The image forming portion  11  has a pair of conveying rollers  209 , an exposure portion  201 , a process cartridge  203  and a transfer roller  205 . A pair of conveying rollers  209  convey the sheets S fed from the sheet feeding portion  10 . The process cartridge  203  has a photosensitive drum  202 , a charging portion (not illustrated), a developing portion (not illustrated) and a cleaning portion (not illustrated). The photosensitive drum  202  is formed with a metal cylinder on the surface of which a photosensitive layer of a negative charging polarity is formed. The charging portion evenly charges the drum surface of the photosensitive drum  202  which is an image bearing member. The exposure portion  201  irradiates the photosensitive drum  202  with laser beam based on image information and forms an electrostatic latent image thereon. The developing portion attaches toner to the electrostatic latent image to visualize as a toner image. The transfer roller  205  transfers the toner image on the photosensitive drum  202  to the sheet S. The cleaning portion removes toner left on the surface of the photosensitive drum  202  after toner is transferred. 
     The fixing portion  210  has a driving roller  211  and a fixing roller  212  in which a heater is built in. The fixing portion  210  fixes to the sheet S the toner image transferred by being heated and pressured against the passing sheets S. 
     The discharge portion  14  has a pair of inner discharge rollers  213 , an outer discharge roller  214  and a discharge tray  215 . The discharge portion  14  discharges the sheets S after one face or duplex fixing processing, onto the discharge tray  215  through a pair of inner discharge rollers  213  and the outer discharge roller  214 . 
     The conveying portion  15  has a reversing unit  13  which reverses the two sides of a sheet P 1 , and a sheet end part detecting portion  1  which is a position detecting apparatus. The reversing unit  13  has a pair of switch-back rollers  216 , a re-feeding path  217 , a duplex conveying path  218 , an intermediate tray  219  and a re-feeding apparatus  220 . The reversing unit  13  reverses the two sides of the sheet S after one face fixing processing in duplex printing processing. The sheet S after one face fixing processing is temporarily accommodated on the intermediate tray  219  by being fed through the re-feeding path  217  and duplex conveying path  218  by a pair of inner discharge rollers  213  and a pair of switch-back rollers  216 . The sheet S accommodated on the intermediate tray  219  is conveyed to form an image again by the re-feeding apparatus  220 , and an image is formed on the second face by the image forming portion. The sheet S on which an image is formed is discharged onto the discharge tray  215  by the discharge portion  14 . 
     The sheet end part detecting portion  1  is provided on the downstream side of the re-feeding apparatus  220 . The sheet end part detecting portion detects the end part position on the Y 2  side in the sheet width direction Y in which the sheet is misaligned in a direction (the Y direction illustrated in  FIG. 2  and hereinafter “sheet width direction Y”) orthogonal to the sheet conveying direction (the X direction illustrated in  FIG. 2  and hereinafter “conveying direction X”). 
     Next, the sheet end part detecting portion  1  will be specifically described with reference to  FIGS. 2 to 4 .  FIG. 2  is a perspective view illustrating the sheet end part detecting portion  1  of the laser beam printer  100  according to the first embodiment.  FIG. 3  is a partial enlarged view illustrating the sensor unit  8  of the sheet end part detecting portion  1  according to the first embodiment.  FIG. 4  is a view for describing an operation range of the sheet end part detecting portion  1  according to the first embodiment. 
     As illustrated in  FIGS. 2 and 3 , the sheet end part detecting portion  1  has a plurality of sensor portions  2 , cover members  3  of aperture members, a holder  4  which is a holding portion, a driving unit  5  which is a driving portion, and a pair of conveying guides  6  (see  FIG. 1 ). 
     The holder  4  is formed such that the cross-section in the sheet width direction Y is formed in a nearly U shape, and an upper surface  40  and lower surface  41  are formed a predetermined interval apart from each other to form the sheet conveying path  7  for the sheets S between the upper surface  40  and lower surface  41 . In addition, the upper surface  40  of the holder  4  is a base plate to which a plurality of light emitting elements is attached. 
     A pair of conveying guides  6  have a first rectangular conveying guide  60  and a second rectangular conveying guide  61 . With a pair of conveying guides  6 , the first conveying guide  60  is arranged on the upper surface  40  side between the upper surface  40  and lower surface  41  of the holder  4 , and the second conveying guide  61  is arranged on the lower surface  41  side in a state where the second conveying guide  61  is spaced a predetermined interval apart from the first conveying guide  60 . Thus, a pair of conveying guides  60  form the sheet conveying path  7  on which the sheets S can pass. Further, the first conveying guide  60  and second conveying guide  61  are made of a transparent material. 
     A plurality of sensor portions  2  is arranged in positions associated with sheet sizes at intervals (predetermined intervals) associated with the sheet sizes in the sheet width direction Y. For example, to support four types of sheet sizes, four sensors are arranged at predetermined intervals associated with each size. Further, as illustrated in  FIG. 3 , the sensor portion  2  has a light receiving element  20  which is a light receiving portion, and a light emitting element  21  which is a light emitting portion. The light receiving elements  20  are arranged at predetermined intervals in the sheet width direction Y between the upper surface  40  of the holder  4  and the first conveying guide  60 . The light emitting elements  21  are arranged to oppose to the light receiving elements  20 , between the lower surface  41  of the holder  4  and the second conveying guide  61 . That is, the light receiving elements  20  and light emitting elements  21  are each arranged to form pairs. In addition, a plurality of sensor portions  2  is aligned and arranged in the width direction in this way. This arrangement of the sensor portions aligned in the width direction means that the positions of a plurality of sensor portions  2  are different in the width direction, and a plurality of sensor portions  2  is shifted and arranged in the conveying direction. 
     The cover member  3  is made of a non-permeable material which does not allow transmission of light, and covers the light receiving element  20 . The cover member  3  is attached to cover the light receiving element  20 , and positioned with respect to the light emitting element  21 . A plurality of cover members  3  is individually attached to cover the light receiving elements and positioned, so that the positions of hole portions (aperture stop)  30  formed in the cover members  3  are precise. Further, in the cover member  3 , a hole portion  30  is formed in the position meeting the front end of the light receiving element  20  in a state where the cover member  3  covers the light receiving element  20 . The hole portion  30  restricts the orientation of light emitted from the light emitting element  21  arranged to oppose to the hole portion  30 , and narrows down light such that only the light receiving element  20  arranged to oppose to the hole portion  30  receives light. In other words, the hole portion  30  blocks part of light emitted by the light emitting element  21  arranged to oppose to the hole portion  30 , so that the light receiving element  20  arranged to oppose to the hole portion  30  linearly receives light through the hole portion  30  of the cover member  3 . 
     The driving unit  5  causes round trip movement of the holder  4 . The driving unit  5  has a stepping motor  50 , a pinion  51  attached to the stepping motor  50  and a driving train  52  meshing with the pinion  51 . The driving train  52  is coupled to the holder  4 , and is formed to move the holder  4  in the sheet width direction Y. The driving unit  5  rotates the stepping motor  50 , so that the driving train  52  meshing with the pinion moves the holder  4  in the sheet width direction Y. The end of the driving train  52  is provided with a cam which contacts part of the holder  4 , and the holder  4  is moved following rotation of the cam rotated by the stepping motor  50 . At this time, as illustrated in  FIG. 4 , the driving unit  5  causes round trip movement of the holder  4  with a predetermined stroke amount M required to detect an end part position of one sheet size. The cover member  3  positioned with respect to the light receiving element  20  moves together with the holder  4 . 
     Next, the operation of the sheet end part detecting portion  1  will be described with reference to  FIG. 4 . As illustrated in  FIG. 4 , the holder (hereinafter, also “sensor unit  8 ”)  4  in which a plurality of sensor portions  2  is arranged detects the light blocking state of the sheets S in a smaller operating area, and therefore the sheet end part detecting portion  1  needs to cause high speed round trip movement of the holder  4 . Hence, the driving unit  5  causes round trip movement of the sensor unit  8  of the holder  4  with a predetermined stroke amount M required to detect an end part position of one sheet size. 
     The round trip movement of this sensor unit  8  causes round trip movement of a plurality of sensor portions  2  fixed and arranged in the sensor unit  8  at the same time. As illustrated in  FIG. 4 , the distance the sensor unit  8  moves covers a distance N from the maximum size to the minimum size of each sheet conveyed from the image forming portion  11 , and one of the light emitting elements  20  is configured to cross the end part of the conveyed sheet S. 
     Further, the position where the sensor unit  8  makes round trip movement is controlled from the default position where the round trip movement starts such that this position is calculated based on the number of pulses of the stepping motor  50  which drives the holder  4 . Consequently, brightness and darkness of light are produced when the sheet S blocks the optical path formed between the light emitting elements  21  and light receiving elements  20 , so that the light receiving elements  20  can detect the end part position of the sheet S. At this time, the light receiving elements  20  are covered by the cover members  3 , and receive only light incident through the hole portions  30  formed in the cover members  3 . The end part position of a sheet is detected based on the distance between the default position of the reference position and the position where the end part of the conveyed sheet is detected by the light emitting elements  21  and light receiving elements  20  (for example, the position where the optical path is blocked by the sheet) during movement of the sensor unit  8 . 
     In a controlling operation, the reference position of the sensor portion  2  outside a sheet conveying area and a default value of a driving pulse of the stepping motor  50  are set. Further, assuming that the average position (average distance) of the end part position (distance) of the sheet S detected a plurality of times while one sheet is conveyed is the end part position of the sheet S (distance to the end part position), a correcting apparatus corrects the exposure position of the image forming portion  11  exposed by the photosensitive drum  202 . By this means, it is possible to obtain adequate images without being misaligned in the sheet width direction Y. 
     When conveyance of the sheet S is finished, the stepping motor  50  is stopped and then the driving train  52  is stopped, and, accompanying this, the sensor unit  8  also stops. At this time, the position of the sensor unit  8  (holder  4  and light receiving elements  20 ) which is stopped is recognized based on the number of pulses, and is set as the default position to start the following round trip movement of the holder  4 . 
     Next, the method of fitting the light receiving elements  20  and cover members  3  will be described. A plurality of cover members  3  described above is coupled by a coupling member  301 . The coupling member  301  is provided with elastic portions  312  which are elastically deformable. The plurality of cover members  3  which is coupled by the coupling member  301  is attached to a transparent supporting plate  302  which is a supporting member. Then, the base plate provided with a plurality of light receiving elements  20  and the supporting plate  302  which supports a plurality of cover members  3  coupled by the elastic portions  312  are assembled to join each other. Further, the elastic portions  312  of a plurality of cover members  3  positioned with respect to the supporting plate  302  deform, and, consequently, the cover members  3  are respectively positioned with respect to the base plate (light receiving elements). 
     Hereinafter, the method of fitting the light receiving elements  20  and cover members  3  will be described in detail with reference to  FIGS. 5 to 6B .  FIG. 5  is an assembly diagram for describing the state before the sensor portion  2  is assembled according to the first embodiment.  FIG. 5  illustrates the state before the cover members  3  are attached to the light receiving elements  20 , and illustrates that a plurality of holders  4  and cover members  3  provided with a plurality of light receiving elements  20  are coupled to a coupling member  301  and the supporting plate  302  by the elastic portions  312 . In addition, part of a side where the light emitting elements  21  of the holders  4  are provided is not displayed. 
     Although the elastic portion  312  is made of the same material as the cover member  3 , the elastic portion  312  has elasticity by molding the elastic portion  312  thin. In addition, with the present embodiment, as described above, although elasticity is provided by molding the same material thinner, it is also possible to provide the effect of the present invention by molding members other than the cover members  3  using another material and coupling these members. 
     Positioning portions  303  and  304  which are used to determine the positions with respect to the supporting plate  302  are provided in the coupling member  301 . Further, positioning holes  305  and  306  which determine the position of the coupling member  301  are also provided in the supporting plate  302 . Further, positioning portions  309  and  310  are provided in the supporting plate  302  such that positioning holes  307  and  308  provided in the holder  4  are positioned. 
       FIG. 6A  is a view illustrating that the coupling member  301  is attached to the supporting plate  302 . As described above, the positioning portions  303  and  304  of a plurality of aperture members and the positioning holes  305  and  306  provided in the supporting plate mate. Further, a lead-in shape  311  of a chamfered shape is provided such that the light receiving element  20  easily fits in each cover member  3 . In addition, the lead-in shape  311  leads the light receiving element  20  in the cover member  3  even if the position variation with respect to the holder  4  of the light receiving element and positioning variation of the transparent member are taken into account. 
       FIG. 6B  is a view illustrating that the supporting plate  302  is assembled to the holder  4  provided with a plurality of light receiving elements. In addition,  FIG. 6B  displays the cover members  3  and light receiving elements  20  by making a notch in the supporting plate  302 . The positioning holes  307  and  308  of the holder  4  and supporting plate  302  described in  FIG. 5  mate, so that arrangement of the supporting plate  302  and holders  4  is determined. At this time, as described in  FIG. 6A , the cover member  3  has the lead-in shape  311  of the chamfered shape, and the light receiving element  20  is led in the cover member  3  and is directly positioned. 
     At this time, the cover members  3  are elastically coupled, and therefore the cover members  3  are not positioned by the supporting plate  302  and are directly positioned with respect to each light receiving element. By so doing, necessary position precision of the cover members  3  and light receiving elements  20  is required and the cover members  3  do not need to be assembled to the light receiving elements  20  one by one. That is, it is possible to substantially improve easiness of assembly. In addition, with the present embodiment, although the number of light emitting elements is four, it is possible to provide the effect of the present invention irrespectively of the number of light emitting elements as long as the number of light emitting elements is plural. A configuration is possible where the whole of coupling member is elastically deformable. A configuration is possible where each of the cover members  3  is positioned by the light receiving element  20  when the supporting plate  302  to which the cover members  3  is attached is combined with the holder  4 . 
     The laser beam printer  100  according to the first embodiment employing the above configuration provides the following effect. The sheet end part detecting portion  1  of the laser beam printer  100  according to the first embodiment detects an end part position of the sheet S by causing round trip movement of a plurality of sensor portions  2 . Consequently, it is possible to reduce the number of sensors compared to cases where line sensors associated with sheet sizes are used. By this means, it is possible to reduce cost of sensors, and reduce manufacturing cost of the entire apparatus. 
     Further, in the sheet end part detecting portion  1  according to the first embodiment, the light receiving elements  20  and light emitting elements  21  are arranged at predetermined intervals in the sheet width direction Y. Further, the light emitting members and light receiving members are configured separately, and respective members are arranged on the upper side and lower side of the sheet conveying path. Consequently, it is not necessary to have the conveyed sheet S stand by in a position where the sheet S does not overlap in the width direction unlike a conventional photointerrupter, or perform control of stopping the sheet S before the end part of the sheet S hits after the end part position of the sheet S is detected. By this means, it is possible to provide a sheet end part detecting portion with a simple configuration. 
     Further, in the sheet end part detecting portion  1  according to the first embodiment, the cover members  3  are positioned at the positions (outer periphery) of the light receiving elements  20 . Consequently, even when, for example, the sheet S floats on the sheet conveying path  7 , it is possible to prevent unnecessary light from entering. By this means, it is possible to precisely detect the end part position of a sheet without decreasing reading detection precision. Further, even when the lengths of corresponding sizes in the width direction need to be adjacent and the light emitting elements  21  and light receiving elements  20  need to be arranged adjacently, the direction of emitted light is restricted, so that it is possible to precisely detect the end part position of the sheet S. 
     For example, with a plurality of light receiving elements and light emitting elements arranged to oppose to each other, there are cases where light emitted by the light emitting elements is diffused and a light emitting element receives light from the adjacent light emitting element to which this light emitting element does not oppose. Particularly when the light receiving elements and light emitting elements make round trip movement and the end part position of a sheet is detected based on the distance between the reference position and the position where the optical path is blocked, the light receiving element is more likely to receive light of the adjacent light emitting element. Therefore, there is a concern that precision to measure the end part position of a sheet decreases. By contrast with this concern, with the present embodiment, the respective light receiving elements  20  are covered by the cover members  3  in which the hole portions  30  are formed. Further, the light receiving element  20  receives only light which has passed the hole portion  30  formed in the cover member  3 . By this means, detection precision of the sheet end part detecting portion  1  according to the present embodiment improves. 
     Further, the sheet end part detecting portion  1  according to the first embodiment detects the end part position of the sheet S by causing round trip movement of a plurality of sensor portions  2  with a simple small stroke. Consequently, the configuration of the driving train  52  becomes simple, and the configuration of the complicated and large moving mechanism is not required. Further, it is possible to detect the end part position of the sheet S a plurality of times while one sheet S passes, so that it is possible to precisely detect the end part position of the sheet S by skewed feeding of the sheet S. Consequently, it is possible to form images of precise positions. As a result, with a cheap and simple configuration, it is possible to provide a sheet end part detecting portion  1  which can precisely detect the position and the laser beam printer  100  having the sheet end part detecting portion  1 . 
     Second Embodiment 
     Next, a laser beam printer  100 A according to a second embodiment of the present invention will be described with reference to  FIGS. 7 to 9  in addition to  FIG. 1 .  FIG. 7  is a perspective view illustrating a sheet end part detecting portion  1 A of the laser beam printer  100 A according to the second embodiment.  FIG. 8  is a partial enlarged view illustrating a sensor unit  8 A of the sheet end part detecting portion  1 A according to the second embodiment.  FIG. 9  is a view for describing an operation range of the sheet end part detecting portion  1 A according to the second embodiment. 
     The laser beam printer  100 A according to the second embodiment differs from the laser beam printer  100  according to the first embodiment in covering light emitting elements  21  by means of cover members  3 A. Hence, with the second embodiment, the difference from the first embodiment, that is, light emitting elements  21  covered by the cover members  3 A, will be mainly described. 
     In addition, with the second embodiment, the same configuration as the laser beam printer  100  according to the first embodiment will employ the drawings used in the first embodiment and will be assigned the same reference numerals, and description thereof will not be repeated. By this means, with the second embodiment, the same configuration as the first embodiment provides the same effect as the first embodiment. 
     As illustrated in  FIG. 1 , the laser beam printer  100 A according to the second embodiment has the sheet feeding portion  10  which feeds the sheets S, and the image forming portion  11  which forms images on the sheets S fed from the sheet feeding portion  10 . Further, the laser beam printer  100 A has the fixing portion  210  which fixes images, the discharge portion  14  and a conveying portion  15 A which is a sheet conveying apparatus. 
     As illustrated in  FIG. 7 , the conveying portion  15 A has a plurality of sensor portions  2 , cover members  3 A of aperture members, a holder  4  which is the holding portion, the driving unit  5  which is the driving portion and a pair of conveying guides  6  (see  FIG. 1 ). 
     As illustrated in  FIG. 8 , the cover member  3 A is made of a non-permeable material which does not allow transmission of light, and covers the light emitting element  21 . The cover member  3 A covers the light emitting element  21  and thereby is positioned with respect to the light receiving element  20 . Further, in the cover member  3 A, the hole portion  30 A is formed in the position meeting the front end of the light emitting element  21  in the state where the cover member  3 A covers the light emitting element  21 . The hole portion  30 A restricts the orientation of light emitted from the light emitting element  21 , such that the receiving element  20  arranged to oppose to the hole portion  30 A receives light. In other words, the hole portion  30 A blocks part of light emitted by the light emitting element  21 , so that the light receiving element  20  arranged to oppose to the hole portion  30 A linearly receives light through the hole portion  30 A of the cover member  3 A. 
     Next, the operation of the sheet end part detecting portion  1 A will be described with reference to  FIG. 9 . As illustrated in  FIG. 9 , the holder (hereinafter, also “sensor unit  8 A”)  4  in which a plurality of sensor portions  2  is arranged detects the light blocking state of the sheet S in a smaller operating area, and therefore the sheet end part detecting portion  1 A needs to cause high speed round trip movement of the holder  4 . Hence, the driving unit  5  causes round trip movement of the sensor unit  8 A of the holder  4  with a predetermined stroke amount M required to detect an end part of one sheet size. 
     The round trip movement of this sensor unit  8 A causes round trip movement of a plurality of sensor portions  2  fixed and arranged in the sensor unit  8 A at the same time. Here, as illustrated in  FIG. 9 , the operating area of the sensor unit  8 A covers a distance N from the maximum size to the minimum size of each sheet conveyed from the image forming portion  11 , and one of the light emitting elements  20  is configured to cross the end part of the conveyed sheet S. 
     Further, the position where the sensor unit  8 A makes round trip movement is controlled from the default position where the round trip movement starts such that this position is calculated based on the number of pulses of the stepping motor  50  which drives the holder  4 . Consequently, brightness and darkness of light are produced when the sheet S blocks the optical path formed between the light emitting elements  21  and light receiving elements  20 , so that the light receiving elements  20  can detect the end part position of the sheet S. At this time, the light emitting element  21  is covered by the cover member  3 A, so that the light receiving element  20  arranged to oppose to the hole portion  30 A linearly receives only light incident through the hole portion  30 A formed in the cover member  3 A. The end part position of a sheet is detected based on the distance between the default position of the reference position and the position where the end part of the conveyed sheet is detected by the light emitting elements  21  and light receiving elements  20  (for example, the position where the optical path is blocked by the sheet) during movement of the sensor unit  8 A. 
     In a controlling operation, the reference position of the sensor portion  2  outside a sheet conveying area and a default value of a driving pulse of the stepping motor  50  are set. Further, assuming that the average position (average distance) of the end part position (distance) of the sheet S detected a plurality of times while one sheet is conveyed is the end part position of the sheet S (distance to the end part position), a correcting apparatus corrects the exposure position of the image forming portion  11  exposed by the photosensitive drum  202 . By this means, it is possible to obtain adequate images without being misaligned in the sheet width direction Y. 
     When conveyance of the sheets S is finished, the stepping motor  50  is stopped and then the driving train  52  is stopped, and, accompanying this, the sensor unit  8 A also stops. At this time, the position of the sensor unit  8 A (holder  4  and light receiving elements  20 ) which is stopped is recognized based on the number of pulses, and is set as the default position to start the following round trip movement of the holder  4 . 
     In addition, the light receiving elements  20  are only changed to the light emitting elements  21  in the fitting method described in the first embodiment, and therefore the method of fitting the light emitting elements  21  and cover members  3  according to the second embodiment will not be described. 
     The laser beam printer  100 A according to the second embodiment employing the above configuration provides the following effect in addition to the effect provided from the same configuration as the laser beam printer  100  according to the first embodiment. In the sheet end part detecting portion  1 A of the laser beam printer  100 A according to the second embodiment, the cover members  3 A are positioned in the positions (outer periphery) of the light emitting elements  21 . Consequently, even when, for example, the sheet S floats on the sheet conveying path  7 , light is linearly radiated from the light emitting elements  21  to the light receiving elements  20 , so that it is possible to prevent unnecessary light from entering. By this means, it is possible to precisely detect the end part position of a sheet without decreasing reading detection precision. Further, even when the lengths of corresponding sizes in the width direction are adjacent and the light emitting elements  21  and light receiving elements  20  need to be arranged adjacently, it is possible to precisely detect the end part position of the sheet S. 
     Although the embodiments of the present invention have been described above, the present invention is by no means limited to the above-described embodiments. Further, the most exemplary effects produced from the present invention have only been described as the effects discussed in the embodiments of the present invention, and the effect of the present invention is by no means limited to the effect discussed in the embodiments of the present invention. 
     Although a configuration is employed with the present embodiment where the cover members  3  are attached to one of the light receiving elements  20  and light emitting elements  21 , the present invention is by no means limited to this. For example, a configuration is possible where the cover members  3  are attached to both of the light receiving elements  20  and light emitting elements  21 . 
     Further, although a configuration has been employed with the present embodiment where the light emitting elements  20  are arranged on the upper surface  40  of the holder  4  and the light emitting elements  21  are arranged on the lower surface  41  of the holder  4 , the present invention is by no means limited to this. For example, a configuration is possible where the light receiving elements  20  are arranged on the lower surface  41  of the holder  4  and the light emitting elements  21  are arranged on the upper surface  40  of the holder  4 . 
     Further, although, for example, LEDs can be illustrated as the light emitting elements  21  according to the present embodiment, any light emitting elements are possible as long as the light receiving elements  20  can receive light. Further, although the holder  4  is driven using the stepping motor  50  with the present embodiment, the present invention is by no means limited to this. A configuration is possible where the holder  4  is moved by a driving source other than the stepping motor  50 . 
     Further although, with the present embodiment, an exposure position is corrected as the average position of the end part position of the sheet S obtained by detecting the end part position of the sheet S a plurality of times, the present invention is by no means limited to this. For example, a configuration is possible where control to correct the exposure position of the end part position of the sheet S is performed based on the relationship between the position of the sheet in the conveying direction X upon detection and the end part position of the sheet. 
     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. 2010-150246, filed Jun. 30, 2010, No. 2011-071782, filed Mar. 29, 2011 which are hereby incorporated by reference herein in their entirety.