Patent Publication Number: US-8971732-B2

Title: Image forming apparatus with a sheet detection unit that detects a sheet in the conveyance path and in the storage unit

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an image forming apparatus which forms an image on a sheet. 
     2. Description of the Related Art 
     A sheet cassette in an image forming apparatus stores sheets on which images are to be formed. The image forming apparatus feeds the sheets one by one from the sheet cassette to an image forming unit, and the image forming unit forms images on the sheets. 
     Japanese Patent Application Laid-Open No. 2006-182463 discusses a sensor disposed in the sheet cassette, which detects whether a sheet or sheets are present in the sheet cassette. The sensor detects whether the sheets in the sheet cassette have run out while the sheets are sequentially fed from the sheet cassette. The image forming apparatus temporarily stops an image forming operation based on detection by the sensor, and notifies a user that the sheets in the sheet cassette have run out. 
     Japanese Patent Application Laid-Open No. 2006-182463 discusses disposing a dedicated sensor for detecting whether there is a sheet in the sheet cassette. Further, a dedicated space is necessary for arranging the sensor in the apparatus. It is thus desirable to commonly use the sensor so that cost can be reduced and the apparatus can be downsized. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to providing a low-cost and compact image forming apparatus. 
     According to an aspect of the present invention, an image forming apparatus that forms an image on a sheet using an image forming unit includes a storage unit configured to store sheets to be fed to the image forming unit, a feeding unit configured to feed the sheets stored in the storage unit, a sheet guide disposed above the storage unit and configured to form a sheet conveyance path, on which a sheet is conveyed, a detection unit disposed above the sheet guide and including a light emitting portion configured to emit light towards the storage unit and a light receiving portion configured to receive light, a light passage portion disposed in the sheet guide, through which light emitted from the light emitting portion in the detection unit passes, a reflection portion disposed below the storage unit and configured to reflect, towards the light receiving portion, light emitted from the light emitting portion and having passed through the light passage portion, and a control unit configured to determine, based on an amount of light received by the light receiving portion, whether there is a sheet in the sheet conveyance path and whether there is a sheet stored in the storage unit. 
     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 illustrating an image forming apparatus according to a first exemplary embodiment of the present invention. 
         FIG. 2  is a cross-sectional view illustrating a state in which a manual feed tray is open in the image forming apparatus according to the first exemplary embodiment. 
         FIG. 3  illustrates a configuration of a sheet detection unit according to the first exemplary embodiment. 
         FIG. 4  illustrates detection of whether there is a sheet in a sheet cassette according to the first exemplary embodiment. 
         FIG. 5  illustrates detection of whether there is a sheet in the sheet cassette according to the first exemplary embodiment. 
         FIG. 6  illustrates an output from a sheet detection sensor according to the first exemplary embodiment. 
         FIG. 7  illustrates detection of whether there is a sheet in a manual feed conveyance path according to the first exemplary embodiment. 
         FIG. 8  illustrates detection of whether there is a sheet in the manual feed conveyance path according to the first exemplary embodiment. 
         FIG. 9  illustrates an output from the sheet detection sensor according to the first exemplary embodiment. 
         FIG. 10  illustrates detection of whether there is a sheet in the manual feed conveyance path according to the first exemplary embodiment. 
         FIG. 11  illustrates an output from the sheet detection sensor according to the first exemplary embodiment. 
         FIG. 12  illustrates a state in which the last sheet is conveyed from the sheet cassette in the image forming apparatus according to the first exemplary embodiment. 
         FIG. 13  illustrates a control block diagram according to the first exemplary embodiment. 
         FIG. 14  is a flowchart illustrating control performed for detecting whether there is a sheet according to the first exemplary embodiment. 
         FIG. 15  illustrates detection of whether there is a sheet according to a first modification example. 
         FIG. 16  illustrates detection of whether there is a sheet according to the first modification example. 
         FIG. 17  illustrates detection of whether there is a sheet according to a second modification example. 
         FIG. 18  is a cross-sectional view illustrating conveying of the last sheet from the sheet cassette in the image forming apparatus according to a comparative example. 
         FIG. 19  is a cross-sectional view illustrating an image forming apparatus according to a second exemplary embodiment of the present invention. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     According to the first exemplary embodiment of the present invention, a color image forming apparatus employing an intermediate transfer belt will be described as an example. 
     &lt;Configuration of the Image Forming Apparatus&gt; 
       FIG. 1  is a cross-sectional view illustrating an image forming apparatus  100 . Referring to  FIG. 1 , a sheet feeding unit is disposed in a lower portion of the image forming apparatus. The sheet feeding unit, i.e. a first feeding unit, includes a sheet cassette  52 , which stores the sheets, and a feeding roller  50  which feeds the sheets stored in the sheet cassette  52 . The sheet cassette  52  includes an elevating plate  54 , which supports a leading edge portion with respect to a feeding direction of the sheet stored in the sheet cassette  52 . The elevating plate  54  is attached to a cassette frame  52   a  to be swingable around a fulcrum  54   a  disposed upstream with respect to the sheet feeding direction. 
     A manual feed tray  73 , which is rotatable around a lower edge as the fulcrum, is disposed on a side of the image forming apparatus  100 . As illustrated  FIG. 2 , a manual feeding unit  70 , i.e., a second feeding unit, which feeds the sheets stacked on the opened manual feed tray  73 , is arranged opposite the lower edge of the manual feed tray  73 . Referring to  FIG. 2 , when the manual feed tray  73  is opened, a feed port  95  leading to inside the image forming apparatus main body becomes exposed on the side of the image forming apparatus  100 . The manual feeding unit  70  separates the sheets that are stacked on the manual feed tray  73  via the feed port  95 , and feeds each sheet to inside the image forming apparatus  100 . 
     Manual feed conveyance paths  76 ,  77 , and  78 , i.e., the sheet conveyance paths for guiding the sheet fed from the manual feeding unit  70 , are disposed above the sheet cassette  52  and extend in an approximately horizontal direction. Conveyance roller pairs  74  and  75  and a paper position detection flag  79  are arranged in the manual feed conveyance paths  76 ,  77 , and  78 . A sheet detection sensor  60 , i.e., a detection unit for detecting the sheet conveyed on the manual feed conveyance path  76  and detecting whether there is a sheet in the sheet cassette  52 , is disposed above the manual feed conveyance path  76 . The configuration and the operation of the sheet detection sensor  60  will be described in detail below. 
     Further, the image forming apparatus  100  includes an image forming unit G, which forms an image on the sheet fed from the sheet cassette  52 , i.e., the storage unit that stores the sheets, or from the manual feed tray  73 . The image forming unit G includes four sets of photosensitive drums  1  and developing devices  4 , an intermediate transfer belt  10  located opposite the photosensitive drums  1 , a laser scanner  3  arranged below the intermediate transfer belt  10 , and a fixing device  30  arranged above a nip portion formed between the intermediate transfer belt  10  and a secondary transfer roller  20 . 
     &lt;Feeding Sheets from the Sheet Cassette&gt; 
     The feeding roller  50  feeds to the image forming unit G the sheets stacked and stored in the sheet cassette  52 , one by one from the top sheet. The elevating plate  54  rotates around the fulcrum  54   a  in a clockwise direction illustrated in  FIG. 1  as the number of sheets stored in the sheet cassette  52  decreases. The elevating plate  54  thus maintains the top sheet at a predetermined position appropriate for being fed by the feeding roller  50 . 
     &lt;Feeding Sheets from the Manual Feed Tray&gt; 
     When manually feeding the sheets from the manual feed tray  73 , the user opens the manual feed tray  73  to a right side of the image forming apparatus  100 , and sets the sheets on the manual feed tray  73 . The manual feeding unit  70  feeds via the feed port  95  the sheets stacked on the manual feed tray  73  to inside the image forming apparatus  100 . The conveyance roller pairs  74  and  75  then convey the fed sheet through the manual feed conveyance paths  76 ,  77 , and  78 , and the sheet temporarily stops directly below the feeding roller  50 . The position where the sheet temporarily stops is determined by stopping the rotation of the conveyance roller pairs  74  and  75  according to detection by the paper position detection flag  79  arranged downstream with respect to the conveyance roller pair  75 . When a control unit  100 M (illustrated in  FIG. 13 ) in the image forming apparatus  100  outputs a signal to re-feed the sheet, the feeding roller  50  and the conveyance roller pairs  74  and  75  restart feeding the sheet. 
     The image forming unit G forms an image on the sheet fed from the manual feed tray  73  similarly as on the sheet fed from the sheet cassette  52 . 
     &lt;Image Forming Operation&gt; 
     The drum-type electrophotographic photosensitive member (hereinafter referred to as a photosensitive drum)  1  is rotationally driven in a direction indicated by an arrow illustrated in  FIG. 1  at a predetermined circumferential speed (process speed). 
     While the photosensitive drum  1  is rotated, a charging roller  2  uniformly charges the photosensitive drum  1  to predetermined polarity and potential, and the laser scanner  3 , i.e., an image exposure unit, performs image exposing on the photosensitive drum  1 . As a result, an electrostatic latent image corresponding to a first color (i.e., yellow) component image of a target color image is formed. The developing device  4  then develops the electrostatic latent image in a developing position, and the electrostatic latent image is made visible as a toner image. 
     The intermediate transfer belt  10  is an endless belt, which is stretched by stretching members (i.e., a drive roller  11 , a tension roller  12 , and auxiliary rollers  18  and  19 ). The intermediate transfer belt  10  is rotationally driven at an opposing portion in which the intermediate transfer belt  10  is in contact with the photosensitive drum  1 , in the same direction and at approximately the same circumferential speed as the photosensitive drum  1 . 
     When the toner image formed on the photosensitive drum  1  passes through a contacting portion between the photosensitive drum  1  and the intermediate transfer belt  10 , the toner image is transferred (i.e., primary-transferred) to the intermediate transfer belt  10  by a primary transfer voltage applied to a primary transfer roller  14 . A cleaning device  5  cleans and removes residual toner remaining on the surface of the photosensitive drum  1  after the primary transfer, and the removed toner is used in the image forming operation including the charging process and subsequent processes. 
     A second color (magenta) toner image, a third color (cyan) toner image, and a fourth color (black) toner image are similarly formed, and sequentially superimposed and transferred to the intermediate transfer belt  10 . A combined color image corresponding to the target color image is thus acquired. 
     The four-color toner image on the intermediate transfer belt  10  passes through a secondary transfer nip formed between the intermediate transfer belt  10  and a secondary transfer roller  20 . As a result, the four-color toner image is transferred to the surface of the sheet fed from the sheet cassette  52  via the feeding roller  50 , or the sheet fed from the manual feed tray  73 . More specifically, a secondary transfer power source applies a secondary transfer voltage on the secondary transfer roller  20 , so that the toner image on the intermediate transfer belt  10  is collectively transferred (secondary transferred) to the surface of a sheet P. In such a case, drive timing or a conveying speed of a registration roller pair  51  is adjusted to align the toner image formed on the intermediate transfer belt  10  with the position of the sheet fed from the feeding roller  50 . 
     The sheet P to which the four-color toner image has been transferred is introduced to the fixing device  30 , and heat-pressed therein, so that the four color toners are melted and mixed, and fixed to the sheet P. As a result of the above-described operation, a full-color print image is formed. A discharge roller  31  discharges to a top surface of the apparatus the sheet on which the image has been fixed by the fixing device  30 . 
     Further, a transfer belt cleaning device  16  cleans and removes secondary transfer residual toner remaining on the surface of the intermediate transfer belt  10 , and stores the removed toner in a toner collecting container  17 . 
     According to the present exemplary embodiment, the laser scanner  3  and the photosensitive drum  1  are arranged below the intermediate transfer belt  10  in the image forming apparatus. However, the laser scanner  3  and the photosensitive drum  1  may be arranged above the intermediate transfer belt  10  in the image forming apparatus. 
     &lt;Sheet Detection Sensor  60 &gt; 
     The sheet detection sensor  60 , which is a detection unit that detects the sheet conveyed on the manual feed conveyance path  76  and detects whether there is a sheet in the sheet cassette  52 , will be described below with reference to  FIGS. 3 ,  4 ,  5 ,  6 ,  7 ,  8 ,  9 ,  10 , and  11 .  FIGS. 3 ,  4 ,  5 ,  7 , and  8  are schematic cross-sectional views illustrating the configuration and the operation of the sheet detection sensor  60 . More specifically,  FIG. 3  illustrates a state in which there is no sheet in the sheet cassette  52  and in the manual feed conveyance path  76 .  FIG. 4  illustrates a state in which there is only one sheet in the sheet cassette, and  FIG. 5  illustrates a state in which the one sheet in the sheet cassette  52  is being conveyed.  FIG. 7  illustrates a state in which there are sheets in the sheet cassette  52  and there is no sheet in the manual feed conveyance path  76 .  FIG. 8  illustrates a state in which there are sheets in the sheet cassette  52  and a sheet in the manual feed conveyance path  76 . 
     Referring to  FIG. 3 , the sheet detection sensor  60  is arranged above the manual feed conveyance path  76 . The sheet detection sensor  60  includes a light emitting portion  60   a , which emits light downwards towards the sheet cassette  52 , and a light receiving portion  60   b , which receives the light from below. 
     The manual feed conveyance path  76  is formed of an upper guide  76   a  and a lower guide  76   b , i.e., sheet guides. Further, a hole  76   h , i.e., a light passage portion through which light  60   c  emitted from the sheet detection sensor  60  can pass upwards and downwards is formed in an area where the sheet is guided by the upper guide  76   a  and the lower guide  76   b.    
     A reflecting plate  61  is a reflecting unit that reflects the light. The reflecting plate  61  is disposed in the area in which a sheet of a minimum size storable in the sheet cassette  52  is stacked at the bottom surface of a cassette frame  52   a  of the sheet cassette  52 , on a light path from the light emitting portion  60   a  in the sheet detection sensor  60 . The reflecting plate  61  reflects the light emitted from the light emitting portion  60   a  towards the light receiving portion  60   b . As illustrated in  FIG. 3 , when there is no sheet in the sheet cassette  52  and the manual feed conveyance path  76 , the reflecting plate  61  reflects the light emitted from the light emitting portion  60   a , and the light receiving portion  60   b  receives the reflected light. The reflecting plated  61  is attached to a bottom plate so that the upper surface of the reflective plate  61  is approximately parallel to the bottom surface of the cassette frame  52   a  of the sheet cassette  52 . The upper surface of the reflecting plate  61  and the upper surface of the top sheet stacked on the sheet cassette  52  thus become approximately parallel. 
     The upper guide  76   a  and the lower guide  76   b  are disposed so that a conveyance direction  76 Y of the sheet guided by the upper guide  76   a  and the lower guide  76   b  becomes tilted with respect to the upper surface of the reflecting plate  61 . According to the present exemplary embodiment, the upper guide  76   a  and the lower guide  76   b  are disposed so that the conveyance direction  76 Y is tilted by 18° with respect to the upper surface of the reflecting plate  61 . As a result, the sheet fed from the manual feed tray  73  is guided between the upper guide  76   a  and the lower guide  76   b  in a tilted orientation with respect to the upper surface of the reflecting plate  61 . Since the upper surface of the reflecting plate  61  and the upper surface of the top sheet stacked on the sheet cassette  52  are approximately parallel, the sheet guided between the upper guide  76   a  and the lower guide  76   b  becomes tilted with respect to the upper surface of the top sheet stacked on the sheet cassette  52 . 
     The upper surface of the reflecting plate  61  is perpendicular to an optical axis  60 S, which is a line connecting the sheet detection sensor  60  and the reflecting plate  61 . The optical axis  60 S is a line which passes through the sheet detection sensor  60  and which is perpendicular to the reflecting plate  61 . If sensor light is emitted from the light emitting portion  60   a  and specularly-reflected by the reflecting plate  61  towards the light receiving portion  60   b  (i.e., travels on a specular reflection light path), the following occurs. The light emitted from the light emitting portion  60   a  to the reflecting plate  61  and the light reflected by the reflecting plate  61  to the light receiving portion  60   b  pass through symmetrical positions with respect to the optical axis  60 S. According to the present exemplary embodiment, an angle θ which is 15° is formed between the light emitted from the light emitting portion  60   a  to the reflecting plate  61  and the light reflected by the reflecting plate  61  to the light receiving portion  60   b  in the specular reflection light path. 
     As illustrated in  FIGS. 1 and 2 , the sheet detection sensor  60 , the sheet cassette  52 , and the reflecting plate  61  are arranged so that the sensor light emitted from the light emitting portion  60   a  irradiates, among the sheets stored in the sheet cassette  52 , a trailing edge portion of the sheet not supported by the elevating plate  54 . According to the present exemplary embodiment, the position which the sheet detection sensor  60  irradiates in a sheet conveyance direction of the sheet cassette is set as follows. When the sheet cassette  52  is loaded in the apparatus main body and B5-size sheets, which are the minimum size sheets storable in the sheet cassette  52 , are set in the sheet cassette  52 , the position which the sheet detection sensor  60  irradiates with the sensor light is arranged to be approximately 5 mm downstream from an upstream edge of the B5-size sheet. 
       FIG. 13  illustrates a control block diagram according to the present exemplary embodiment. Referring to  FIG. 13 , the sheet detection sensor  60  is connected to the control unit  100 M, which controls the image forming apparatus  100 . The control unit  100 M determines, based on an amount of light received by the light receiving portion  60   b  in the sheet detection sensor  60 , whether there is a sheet in the sheet cassette  52  and in the manual feed conveyance path  76 . The control unit  100 M controls the operations of each unit in the image forming unit G and an operation display unit  201 . The control unit  100   m  includes a central processing unit (CPU)  501 , a read-only memory (ROM)  502  storing programs, and a random access memory (RAM)  503  used as a work memory when executing the programs stored in the ROM  502 . 
     &lt;Detection of a Sheet by the Sheet Detection Sensor  60 &gt; 
     (A) Detection of Whether there is a Sheet in the Sheet Cassette  52   
     Detection of whether there is a sheet stored in the sheet cassette  52  will be described below with reference to  FIGS. 4 and 5 . 
     Referring to  FIG. 4 , only one A4-size sheet P is in the sheet cassette  52 , and there is no sheet in the manual feed conveyance path  76 . In such a case, the sensor light emitted from the light emitting portion  60   a  is reflected on the upper surface of the sheet P in the sheet cassette  52 , and the light receiving portion  60   b  receives the reflected light. 
     On the other hand,  FIG. 5  illustrates a state in which the trailing edge of sheet P has passed the reflecting plate  61  after the state illustrated in  FIG. 4 . In such a case, the sensor light emitted from the light emitting portion  60   a  is reflected by the reflecting plate  61 , and the light receiving portion  60   b  receives the reflected light (i.e., the same state as illustrated in  FIG. 3 ). 
       FIG. 6  illustrates a change in the amount of light received by the sheet detection sensor  60  when the state shifts from the state illustrated in  FIG. 4  to that in  FIG. 5 . Referring to  FIG. 6 , after the trailing edge of the sheet P passes through the reflected light path  60   c , the sheet detection sensor  60  switches from a state of receiving the reflected light from the sheet P to the state of receiving the reflected light from the reflecting plate  61 . The amount of light received by the sheet detection sensor  60  thus increases. If the amount of received light is greater than a preset threshold value A (i.e., a first predetermined value), the control unit  100 M in the image forming apparatus  100  determines that there is no sheet in the sheet cassette  52 . The control unit  100 M thus stops, based on the determination result, the image forming operation with respect to the subsequent sheet. The control unit  100 M then notifies, using the operation display unit  201 , the user that there is no sheet in the sheet cassette  52 . As described above, the control unit  100 M determines whether there is a sheet in the sheet cassette  52  based on the amount of light received by the light receiving portion  60   b  in the sheet detection sensor  60 . 
     (B) Detection of a Sheet in the Manual Feed Conveyance path  76   
     (B-1) The method for detecting a sheet in the manual feed conveyance path  76  in the case where there is a sheet P in the sheet cassette  52  will be described below with reference to  FIGS. 7 and 8 . 
       FIG. 7  illustrates a state in which there is the sheet P in the sheet cassette  52 , and there is no sheet in the manual feed conveyance path  76 . In such a case, the sensor light emitted from the light emitting portion  60   a  is reflected by the upper surface of the sheet P in the sheet cassette  52 , and the light receiving portion  60   b  receives the reflected light. 
     On the other hand,  FIG. 8  illustrates the state in which there is a sheet PT in the manual feed conveyance path  76 , blocking a reflecting light path  60   c . In such a case, the sensor light emitted from the light emitting portion  60   a  is reflected by the sheet PT guided between the upper guide  76   a  and the lower guide  76   b  in the manual feed conveyance path  76 , and the light receiving portion  60   b  receives the reflected light. 
       FIG. 9  illustrates a change in the amount of light received by the sheet detection sensor  60  when the state shifts from the state illustrated in  FIG. 7  to that in  FIG. 8 . Referring to  FIG. 9 , when the leading edge of the sheet PT passes through the reflected light path  60   c , the sheet detection sensor  60  switches from the state of receiving the reflected light from the sheet P in the sheet cassette  52  to the state of receiving the reflected light from the sheet PT in the manual feed conveyance path  76 . The amount of received light thus decreases. 
     In such a case, if the amount of received light becomes less than a preset threshold value B (i.e., a second predetermined value), the control unit  100 M in the image forming apparatus  100  determines that the sheet PT is in the manual feed conveyance path  76 . 
     The sheet in the manual feed conveyance path  76  is detected for detecting a conveyance delay jam in the manual feeding unit  70  or a jam caused by the sheet PT remaining in the manual feed conveyance path  76 . In other words, the control unit  100 M controls the image forming apparatus as follows based on the result of determining whether there is a sheet in the manual feed conveyance path  76 . For example, if the sheet PT conveyed by the manual feeding unit  70  does not reach the sheet detection sensor  60  in a predetermined time, the control unit  100 M notifies the user of the jam, and stops the image forming operation on the photosensitive drum  1 . Further, if the sheet PT to be conveyed by the feeding roller  50  continues to remain below the sheet detection sensor  60  after a predetermined time has elapsed in the case where the feeding roller  50  conveys the sheet PT in the manual feed conveyance path  76  downstream, the control unit  100 M notifies the user of the jam, and stops the image forming operation. 
     When the sheet detection sensor  60  switches from the state of receiving the light reflected by the sheet P on the sheet cassette  52  to the state of receiving the light reflected by the sheet PT on the manual feed conveyance path  76 , the amount of received light decreases for the following reason. According to the present exemplary embodiment, the manual feed conveyance path  76  is arranged to be tilted with respect to the optical axis  60  connecting the sheet detection sensor  60  and the reflecting plate  61 . As a result, the specularly-reflected light among the reflected light from the sheet PT is reflected in a direction  60   d  illustrated in  FIG. 8 , so that the light-receiving portion  60   b  cannot receive the specularly-reflected light and thus only receives diffusely-reflected light. Since the light-receiving portion  60   b  does not receive the specularly-reflected light, the amount of light received by the sheet detection sensor  60  greatly decreases. 
     According to the present exemplary embodiment, the manual feed conveyance path  76  is arranged to be tilted by 18° with respect to the reflecting plate  61 . However, it is not limited thereto, as long as the manual feed conveyance path  76  is arranged to be tilted with respect to the reflecting plate  61  by an angle greater than the angle θ formed by the specularly-reflected light path of the sheet detection sensor  60  (15° according to the present exemplary embodiment), so that the specularly-reflected light moves away from the light receiving portion  60   b.    
     (B-2) The method for detecting a sheet in the manual feed conveyance path  76  in the case where there is no sheet P in the sheet cassette  52  will be described below with reference to  FIGS. 10 and 11 . 
       FIG. 10  illustrates a state in which there is no sheet P in the sheet cassette  52 , and there is the sheet PT in the manual feed conveyance path  76 , blocking the reflecting light path  60   c .  FIG. 11  illustrates a change in the amount of light received by the sheet detection sensor  60  when the state shifts from the state illustrated in  FIG. 3  to that in  FIG. 10 . Referring to  FIG. 11 , whether there is a sheet is determined using a threshold value B similarly as illustrated in  FIG. 9 . Since the jam determination and processing performed based on determining whether there is a sheet are similar to those illustrated in  FIG. 9 , the description will be omitted. 
     As described above, the control unit  100 M determines whether there is a sheet in the manual feed conveyance path  76  based on the amount of light received by the light receiving portion  60   b  in the sheet detection sensor  60 . 
     &lt;Sheet Detection Process&gt; 
     The process for detecting whether there is a sheet in the sheet cassette  52  and in the manual feed conveyance path  76  based on the control block diagram illustrated in  FIG. 13  will be described below with reference to  FIG. 14 . The process to be described below is realized by the CPU  501  executing the program stored in the ROM  502 . 
     In step S 101 , the control unit  100 M determines whether a sheet is fed from the sheet cassette  52 . More specifically, the control unit  100 M determines whether the sheet is fed from the sheet cassette  52  according to a user input to the operation display unit  201 , or from the manual feed tray  73 . In addition, an open/close sensor which detects an open/close state of the manual feed tray  73  may be provided. In such a case, if the open/close sensor detects that the manual feed tray  73  is closed, the control unit  100 M determines that the sheet is fed from the sheet cassette  52 . Further, a set sensor which detects whether there is a sheet set on the manual feed tray  73  may be provided. In such a case, if the set sensor detects that the sheet is set on the manual feed tray  73 , the control unit  100 M determines that the sheet is not fed from the sheet cassette  52 . 
     If the control unit  100 M determines that the sheet is fed from the sheet cassette  52  (YES in step S 101 ), the process proceeds to step S 102 . In step S 102 , the control unit  100 M determines whether the amount of light received by the light receiving portion  60   b  in the sheet detection sensor  60  is greater than the threshold value A. If the amount of light received by the light receiving portion  60   b  is greater than the threshold value A (YES in step S 102 ), the control unit  100 M determines that there is no sheet in the sheet cassette  52 . In such a case, the process proceeds to step S 104 . In step S 104 , the control unit  100 M controls the image forming unit G to stop the image forming operation with respect to the photosensitive drum  1 , and displays on the operation display unit  201  that there is no sheet. If the amount of light received by the light receiving portion  60   b  is not greater than the threshold value A (NO in step S 102 ), the process proceeds to step S 103 . In step S 103 , the control unit  100 M determines whether the image forming job has ended. If the job has ended (YES in step S 103 ), the process ends. If the job has not ended (NO in step S 103 ), the process returns to step S 102 . 
     If the control unit  100 M determines that the sheet is not fed from the sheet cassette  52  (NO in step S 101 ), the process proceeds to step S 105 . In step S 105 , the control unit  100 M causes the manual feeding unit  70  to feed the sheet from the manual feed tray  73 . In step S 106 , the control unit  100 M determines whether a first predetermined time has elapsed from when the manual feeding unit  70  has started feeding the sheet. 
     If the first predetermined time has elapsed (YES in step S 106 ), the process proceeds to step S 107 . In step S 107 , the control unit  100 M determines whether the amount of light received by the light receiving portion  60   b  in the sheet detection sensor  60  is less than the threshold value B. According to the present exemplary embodiment, the first predetermined time is set as the time between when the manual feeding unit  70  has started feeding the sheet and when the sheet reaches a detecting position of the sheet detection sensor  60 . The first predetermined time is set based on the distance from the manual feeding unit  70  to the sheet detection sensor  60  and the sheet conveying speed. If the amount of light received by the light receiving portion  60   b  is not less than the threshold value B (NO in step S 107 ), the process proceeds to step S 110 . In step S 110 , the control unit  100 M stops the image forming operation of the image forming unit G and displays on the operation display unit  201  that a paper jam has occurred. If the amount of light received by the light receiving portion  60   b  is not less than the threshold value B when the first predetermined time has elapsed, it indicates that the sheet fed from the manual feeding unit  70  has not reached the sheet detection sensor  60 . It is thus determined that the paper jam has occurred while the sheet is being conveyed. 
     If the control unit  100 M determines that the amount of light received by the light receiving portion  60   b  in the sheet detection sensor  60  is less than the threshold value B when the first predetermined time has elapsed from the manual feeding unit  70  starting to feed the sheet (YES in step S 107 ), the process proceeds to step S 108 . In step S 108 , the control unit  100 M determines whether a second predetermined time has elapsed from when causing the manual feeding unit  70  to start feeding the sheet. 
     If the second predetermined time has elapsed (YES in step S 108 ), the process proceeds to step S 109 . In step S 109 , the control unit  100 M determines whether the amount of light received by the light receiving portion  60   b  in the sheet detection sensor  60  is less than the threshold value B. According to the present exemplary embodiment, the second predetermined time is set as the time between when the manual feeding unit  70  has started feeding the sheet and when the trailing edge of the sheet passes the detecting position of the sheet detection sensor  60 . The second predetermined time is set based on the distance from the manual feeding unit  70  to the sheet detection sensor  60 , the sheet conveying speed, and a sheet size (i.e., the length of the sheet in the sheet feed direction). If the control unit  100 M determines that the amount of light received by the light receiving portion  60   b  is less than the threshold value B (YES in step S 109 ), the process proceeds to step S 110 . In step S 110 , the control unit  100 M stops the image forming operation of the image forming unit G and displays on the operation display unit  201  that a paper jam has occurred. If the amount of light received by the light receiving portion  60   b  is less than the threshold value B when the second predetermined time has elapsed, it indicates that the sheet to be conveyed downstream and to pass the sheet detection sensor  60  continues to remain in the detecting position. It is thus determined that a paper jam has occurred while the sheet is being conveyed. 
     If the control unit  100 M determines that the amount of light received by the light receiving portion  60   b  is not less than the threshold value B (NO in step S 109 ), the process proceeds to step S 111 . In step S 111 , the control unit  100 M determines whether the subsequent sheet is present on the manual feed tray  73 . If the subsequent sheet is present on the manual feed tray  73  (YES in step S 111 ), the process returns to step S 105 . If there is no subsequent sheet (NO in step S 111 ), the process ends. 
     &lt;Advantageous Effects of the Present Exemplary Embodiment&gt; 
     (1) One common sheet detection sensor  60  detects whether there is a sheet in the sheet cassette  52  and whether there is a sheet in the manual feed conveyance path  76 . A low-cost and compact image forming apparatus can thus be provided as compared to a configuration in which the sheet detection sensors are separately included. 
     (2) A light-reflecting sensor is employed as the sheet detection sensor  60 . As a result, as compared to employing a flag-type sensor using a sensor flag that rotates by being pressed by the sheet, the space for turning the sensor flag becomes unnecessary, so that a compact apparatus can be provided. 
     (3) Whether a sheet is present in the sheet cassette can be more promptly detected as compared to a comparative example to be described below with reference to  FIG. 18 . Wasteful consumption of the toner can thus be reduced, and the toner collecting container can be downsized, resulting in a decrease in the size and cost of the apparatus main body. The reduction of wasteful toner consumption by the image forming apparatus according to the present exemplary embodiment will be described below. 
       FIG. 12  illustrates the last sheet being conveyed from the sheet cassette  52  in the image forming apparatus according to the present exemplary embodiment.  FIG. 18  illustrates the last sheet being conveyed from the sheet cassette  52  in the image forming apparatus according to a comparative example. 
     In both  FIGS. 12 and 18 , dotted lines illustrated near the intermediate transfer belt  10  and the photosensitive drum  1  indicate the toner before the secondary transfer. Referring to  FIG. 12 , the trailing edge of a sheet fed from the sheet cassette  52  has passed the reflecting plate  61 , and the sheet detection sensor  60  detects that there is no sheet in the sheet cassette  52 . Further, toner T 0  to be secondary-transferred to a preceding sheet P 0 , and toner T 1 , at an image interval from the toner T 0 , to be secondary-transferred to a sheet P 1 , are primary-transferred on the intermediate transfer belt  10 . When the sheet detection sensor  60  detects that there is no sheet, the image forming operation on the photosensitive drum  1  with respect to the sheet P 1  is not yet completed. In other words, when the sheet detection sensor  60  detects that there is no sheet, the image forming operation on the photosensitive drum  1  with respect to the sheet subsequent to the sheet P 1  has not yet been started. In such a state, the control unit  100 M can acquire the information that there is no sheet. The image forming operation on the photosensitive drum  1  with respect to the sheet subsequent to the sheet P 1  is thus not performed until the sheet is set (replenished) in the sheet cassette  52 . As a result, it can be detected that there is no sheet in the sheet cassette  52  before starting the image forming operation on the photosensitive drum  1  with respect to the sheet subsequent to the sheet P 1 , so that wasteful toner consumption can be reduced. 
     On the other hand, when the control unit  100 M determines that there is a paper jam due to delayed sheet feeding from the manual feed tray, or a sheet run-out jam on the manual feed tray, the image forming operation is not yet started. As a result, wasteful toner consumption is prevented similarly as in the case where the sheet cassette  52  feeds the sheet. 
     The comparative example will be described below with reference to  FIG. 18 . Referring to  FIG. 18 , the last sheet is being conveyed from the sheet cassette  52  similarly as in  FIG. 12 . According to the present comparative example, a sensor flag  80 , which is rotatable by being pressed by the fed sheet, and a photo sensor (not illustrated) are employed to detect whether there is a sheet. The sensor flag  80  is arranged below the elevating plate  54 , which vertically moves the sheet. Since it is difficult to arrange the movable sensor flag  80  at a position near a movable range of a trailing edge regulating member  53 , which regulates the trailing edge of the sheet, the sensor flag  80  is arranged as described above. Further, the sensor flag  80  is arranged below the elevating plate  54 , which vertically moves the sheet when feeding the sheet, so that the space below the sheet cassette  52  which is only used as the movable range of the sensor flag  80  can be minimized. 
     According to the present comparative example, whether there is a sheet is detected using the leading edge of the sheet, based on the movement of the sensor flag  80 . As a result, when the sensor flag  80  detects that there is no sheet, the image forming operation on the photosensitive drum  1  with respect to the sheet P 1  is completed, and the image forming operation on the photosensitive drum  1  with respect to the sheet subsequent to the sheet P 1  is being performed. In such a state, the control unit acquires the information that there is no sheet, and stops the image forming operation for the sheet subsequent to the sheet P 1 . According to the comparative example, the toner is thus wasted every time the last sheet in the sheet cassette is detected. Further, when the sheet is manually fed, a manual feed sensor  81  is separately arranged so that throughput is not greatly reduced, and the toner is not wasted due to the paper run-out jam. 
     Modification Examples 
     The image forming apparatus according to the modification examples of the present invention will be described below with reference to  FIGS. 15 ,  16 , and  17 . Only the portions different from the first exemplary embodiment will be described below, and the configurations similar to that of the first exemplary embodiment will be assigned the same reference numerals and the description thereof will be omitted. 
       FIGS. 15 and 16  illustrate the configuration of the manual feed conveyance path  76  and the sheet conveying state according to a first modification example. According to the above-described exemplary embodiment, the manual feed conveyance path  76  is arranged to be tilted with respect to the optical axis  60   c . The specularly-reflected light reflected by the sheet PT is thus reflected in the direction  60   d , and the specularly-reflected light amount is not input to the light receiving portion  60   b , so that the amount of light received by the sheet detection sensor  60  is reduced. According to the first modification example, the attitude of the sheet in the manual feed conveyance path  76  is stabilized, so that the specularly-reflected light from the sheet in the manual feed conveyance path  76  is more surely prevented from being input to the light receiving portion  60   b.    
     According to the present modification example, conveyance assist rollers  90  and  91  are arranged in the manual feed conveyance path  76  as pressing units for pressing the sheet onto the lower guide  76   b . The conveyance assist rollers  90  and  91  are arranged near the position where the light emitting portion  60   a  irradiates the sheet conveyed on the manual feed conveyance path  76  with light. The conveyance assist rollers  90  and  91  are biased toward the lower guide  76   b  by springs (not illustrated), and press the sheet onto the lower guide  76   b . The sheet conveying speed of the conveyance roller pair  74  is set to be higher than the sheet conveying speed of the manual feeding unit  70 . 
       FIG. 15  illustrates the leading edge of the sheet PT conveyed in the manual feed conveyance path  76  entering the conveyance assist roller  91 . As the sheet PT is conveyed by the conveyance roller pair  74 , the sheet PT is pulled by the conveyance roller pair  74 , so that the sheet PT is stretched between the conveyance assist rollers  90  and  91 . The attitude of the sheet PT thus becomes stable as illustrated in  FIG. 16 . 
     Further,  FIG. 17  illustrates a second modification example. The manual feed conveyance path  76  is formed by the upper guide  76   a  and the lower guide  76   b . Referring to  FIG. 17 , a projecting portion  76   c  is disposed instead of the conveyance assist rollers  90  and  91 . The projecting portion  76   c  is disposed so that a path interval of the manual feed conveyance path  76  (the gap between the upper guide  76   a  and the lower guide  76   b ) becomes partially narrow, i.e., becomes narrow as compared to other areas. Further, the projection portion  76   c  stabilizes the attitude of the sheet PT. 
     According to the above-described exemplary embodiment, the upper guide  76   a  and the lower guide  76   b  are arranged so that the surface of the sheet in the manual feed conveyance path  76  becomes tilted with respect to the upper surface of the sheet in the sheet cassette  52 . By such an arrangement, the angle of the light reflected by the sheet in the sheet cassette  52  and the angle of the light reflected from the sheet in the manual feed conveyance path become different, so that the sheet in the manual feed conveyance path  76  is surely detected. A sensor capable of detecting whether the light is reflected by the sheet in the manual feed conveyance path  76  or in the sheet cassette  52  based on a difference in a loss of light amount due to the optical path length difference in the sensor light may also be employed. 
     According to the above-described exemplary embodiment, the reflecting plate  61  is disposed on the bottom surface of the cassette frame  52   a  of the sheet cassette  52 . However, the reflecting plate may be disposed below the storage unit (i.e., a storing space) which stores the sheets. For example, an opening may be formed on the bottom surface of the cassette frame  52   a , and the reflecting plate may be disposed at a position facing the opening in a lower portion of a chassis of the image forming apparatus main body. Further, according to the above-described exemplary embodiment, the light passage portion which allows the light from the sheet detection sensor  60  to pass through is formed by the hole  76   h  disposed in the upper guide  76   a  and the lower guide  76   b . However, a transparent portion may be disposed in the upper guide and the lower guide as the light passage portion through which the light can pass. 
     A second exemplary embodiment of the present invention will be described below with reference to  FIG. 19 . According to the first exemplary embodiment, the sheet detection sensor  60  is disposed above the manual feed conveyance path  76 . According to the second exemplary embodiment, the sheet detection sensor  60  is disposed in a re-feed conveyance path  98 . Since the position of the sheet detection sensor  60  is only a difference between the second exemplary embodiment and the first exemplary embodiment, the difference will be described below, and the description on the other portions will be omitted. 
     If the image forming unit G is to form an image on a second side of the sheet of which an image has been formed on a first side (i.e., two-sided image forming is to be performed), the sheet is reversed, and the re-feed conveyance path  98  re-conveys the sheet to the image forming unit G. The re-feed conveyance path  98  is disposed above the sheet cassette  52 , which stores the sheet P. 
     The sheet detection sensor  60  is arranged above the re-feed conveyance path  98 , and is arranged to emit light towards a sheet stacking surface of the sheet cassette  52 . The hole  76   h  is disposed in the sheet guide of the re-feed conveyance path  98  for allowing the light from the sheet detection sensor  60  to pass through. The reflecting plate  61  for reflecting the light emitted from the sheet detection sensor  60  is disposed on the sheet stacking surface of the sheet cassette  52 . As a result, the light emitted from the sheet detection sensor  60  is reflected by the reflecting plate  61  and received by the sheet detection sensor  60 . 
     The re-feed conveyance path  98  may be arranged to be tilted with respect to the sheet stacking surface of the sheet cassette  52 . The functions and the effect acquired according to the above-described configuration are similar to those in the first exemplary embodiment. 
     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. 2012-128130 filed Jun. 5, 2012, which is hereby incorporated by reference herein in its entirety.