Patent Publication Number: US-9405258-B2

Title: Image forming apparatus

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an image forming apparatus including a storage unit that once stores a sheet having an image formed thereon in the apparatus. 
     2. Description of the Related Art 
     Conventionally, an image forming apparatus such as a copying machine or a printer includes a storage unit that once stores sheets each having an image formed thereon in the apparatus so that a user can receive only the his/her own sheets such that others do not see the sheets. 
     Japanese Patent Application Laid-Open No. 7-125909 discusses an image forming apparatus including a plurality of storage units that once stores sheets each having an image formed thereon in the apparatus in addition to a normal discharge tray provided on an upper surface of an apparatus body and commonly used by a plurality of users. The sheets stored in the storage units cannot be seen from outside the apparatus. In the image forming apparatus, the storage units are respectively assigned to users, and sheets are distributed into the different storage units for each of the users. When receiving the sheets, the user instructs the image forming apparatus to discharge the sheets so that the sheets stored in the storage unit corresponding to the user who has issued the discharge instruction are discharged out of the apparatus. Thus, the user can receive only the his/her own sheets each having the image formed thereon such that others do not see the sheets. 
     In Japanese Patent Application Laid-Open No. 7-125909, one storage unit is assigned to one user. Thus, if a larger number of users than the number of storage units instruct an image forming apparatus to perform printing, sheets cannot be distributed into the different storage units for each of the users. If four users instruct an image forming apparatus including three storage units to perform printing, for example, sheets of the remaining one user cannot be stored in the storage units. More specifically, only sheets of one user can be stored in one storage unit. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to an image forming apparatus capable of distinguishing and storing a plurality of sheets in one storage unit. 
     According to an aspect of the present invention, an image forming apparatus includes a storage unit configured to store, in an apparatus body of the image forming apparatus, a sheet having an image formed on the sheet, a conveyance unit configured to convey the sheet having the image to the storage unit, an opening configured to expose the sheet stored in the storage unit to an outside of the apparatus body, and a movement unit configured to be movable to a first position where the sheet conveyed to the storage unit by the conveyance unit can be stored in the storage unit and a second position which is closer to the opening than the first position, wherein the sheet stored in the storage unit is moved when the movement unit moves from the first position to the second position, and is stopped in an exposed state where a part of the sheet is exposed to the outside of the apparatus body from the opening, and wherein the movement unit moves from the second position to the first position when the sheet is in the exposed state. 
     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  illustrates a configuration of an image forming apparatus according to an exemplary embodiment of the present invention. 
         FIG. 2  illustrates a configuration of a storage device in the exemplary embodiment of the present invention. 
         FIG. 3  is a perspective view of a storage unit in the exemplary embodiment of the present invention. 
         FIG. 4  is a block diagram illustrating a control unit and a functional configuration in the image forming apparatus according to the exemplary embodiment of the present invention. 
         FIG. 5  is a detail view of a storage device control unit in the exemplary embodiment of the present invention. 
         FIG. 6  is a flowchart during sheet printing in the exemplary embodiment of the present invention. 
         FIGS. 7A and 7B  illustrate how the storage device operates during sheet exposure in the exemplary embodiment of the present invention. 
         FIG. 8  is a perspective view of the image forming apparatus during sheet exposure in the exemplary embodiment of the present invention. 
         FIG. 9  is a flowchart in a first exemplary embodiment of the present invention. 
         FIGS. 10A, 10B, and 10C  illustrate specific examples in the first exemplary embodiment of the present invention. 
         FIG. 11  is a flowchart in a second exemplary embodiment of the present invention. 
         FIGS. 12A, 12B, and 12C  illustrate specific examples in the second exemplary embodiment of the present invention. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Various exemplary embodiments, features, and aspects of the invention will be described in detail below with reference to the drawings. 
     A first exemplary embodiment of the present invention will be described in detail below with reference to the drawings. 
     (Configuration Diagram of Image Forming Apparatus) 
       FIG. 1  illustrates a configuration of an image forming apparatus including a storage unit according to a first exemplary embodiment of the present invention. In the present exemplary embodiment, a laser beam printer is illustrated as the image forming apparatus. 
     An image forming apparatus  100  includes an image forming unit  101 , a feeding unit  102  that feeds a sheet S to the image forming unit  101 , and a discharge unit  104  that discharges the sheet S having an image formed thereon by the image forming unit  101 . The sheet S includes paper, an overhead projector (OHP) sheet, and a cloth. A storage device  200  includes a plurality of storage units  201  to  203  that once stores the sheet S having the image formed thereon in the apparatus. The storage device  200  is provided above the image forming unit  101 . The image forming apparatus  100  further includes a conveyance unit  105  that conveys the sheet S having the image formed thereon to the storage device  200 . 
     The image forming unit  101  includes a photosensitive drum  111  that rotates in a clockwise direction (CW direction) in  FIG. 1 , a charging roller  112  that charges a surface of the photosensitive drum  111 , and an exposure device  113  that irradiates the photosensitive drum  111  with light to form an electrostatic latent image. Further, the image forming unit  101  includes a development device  114  that attaches toner to the electrostatic latent image to form a toner image on the photosensitive drum  111 , and a transfer roller  115  that transfers the toner image onto the conveyed sheet S. The image forming unit  101  further includes a fixing roller  116 , a pressure roller  117  that abuts on the fixing roller  116 , and a fixing/discharge roller  118 . The image forming unit  101  fixes the toner image, which has been transferred onto the sheet S, to the sheet S. The image forming unit  101  forms the toner image on the sheet S using such an image forming process. In the image forming apparatus  100  according to the present exemplary embodiment, the photosensitive drum  111 , the charging roller  112 , the development device  114 , and a toner storage unit (not illustrated) for storing toner are integrated as a cartridge C, and is detachably attached to the image forming apparatus  100 . A user can replace the cartridge C when the toner runs out. The present invention is not limited to the image forming apparatus  100  of such a cartridge type, and is also applicable to an image forming apparatus  100  having a configuration in which members such as the photosensitive drum  111 , the charging roller  112 , and the development device  114  are fixedly installed (of such a type that the members need not be replaced). 
     The feeding unit  102  includes a feeding cassette  106  that stores a plurality of sheets S used for image formation in a stacked state, a feeding roller  107 , a conveyance guide  109 , and a registration roller  110 . 
     The discharge unit  104  includes a first switching member  120 , a conveyance roller  121 , a discharge guide  122 , a discharge roller  123 , and a discharge tray  124 . The first switching member  120  is switchable, by an actuator (not illustrated), between a position indicated by a solid line in  FIG. 1  and a position indicated by a broken line. From the position indicated by a solid line, the sheet S having the image formed thereon is directed toward the storage device  200 . From the position indicated by a broken line, the sheet S is directed toward the discharge tray  124 . The discharge tray  124  is provided on an upper surface of the image forming apparatus  100 , and can be used jointly by a plurality of users. The sheet S is discharged onto the discharge tray  124  with a surface (front surface) having the image formed thereon directed downward (face down). 
     The conveyance unit  105  includes a second switching member  133  and a third switching member  134  for switching a conveyance destination of the sheet S, and conveyance guides  128  to  132  that guide the sheet S to each of the storage units  201  to  203 . Each of the second switching member  133  and the third switching member  134  is switchable by an actuator (not illustrated) to a position indicated by a solid line and a position indicated by a broken line in  FIG. 1 . When the sheet S is conveyed to the first storage unit  201 , for example, the second switching member  133  and the third switching member  134  are respectively located at the positions indicated by the solid lines in  FIG. 1 . The sheet S is conveyed to the first storage unit  201  after passing through the conveyance guides  129  and  130  in this order from the conveyance guide  128 . If the sheet S is conveyed to the second storage unit  202 , only the third switching member  134  is switched to the position indicated by the broken line. In this case, the sheet S is conveyed to the second storage unit  202  after passing through the conveyance guides  128 ,  129 , and  131  in this order. Each of the storage units  201  to  203  stores the sheet S face down, similarly to the discharge tray  124 . 
     (Configuration Diagram of Storage Device) 
       FIG. 2  illustrates a configuration of the storage device  200 . In the storage device  200  according to the present exemplary embodiment, the storage units  201  to  203  are stacked in a plurality of stages and arranged in a vertical direction. Respective configurations of the storage units are the same. The configuration of the first storage unit  201  will be described below. 
     The first storage unit  201  includes a conveyance roller  211  for conveying the sheet S, a stack tray  221  for stacking and once storing the sheet S, and a sheet presence/absence sensor  231  that detects whether the sheet S is stored on the stack tray  221 . Further, the first storage unit  201  includes a sheet movement unit  241  that presses a trailing edge of the stored sheet S (an edge on the upstream side in a conveyance direction of the sheet S) and exposes a leading edge of the stored sheet S (an edge on the downstream side in the conveyance direction of the sheet S) to the outside of the image forming apparatus  100 . The sheet movement unit  241  moves the sheet S up to a position where the user can receive the sheet S, i.e., until the leading edge of the sheet S passes through an opening  250 . Thus, the sheet S can be exposed by a predetermined length to the outside of the apparatus. The predetermined length by which the sheet S is exposed to the outside of the apparatus is set to 30 mm in the present exemplary embodiment. The predetermined length 30 mm is an example, and may be set to such a length that the user can grasp the exposed sheet S and the sheet S is not greatly bent. 
     The stack tray  221  is bent, and is in a shape having a horizontal plane and an inclined plane, as illustrated in  FIG. 2 . The stack tray  221  is set to have such a length that the leading edge of the sheet S is not exposed from the opening  250  even if the sheet S of the maximum size, which can be stored in the first storage unit  201 , is stacked. When the sheet S is stacked on the stack tray  221 , and the sheet presence/absence sensor  231  falls to a position indicated by a broken line, the sheet presence/absence sensor  231  is turned on. When the sheet movement unit  241  moves the sheet S, and the sheet presence/absence sensor  231  returns to a position indicated by a solid line, the sheet presence/absence sensor  231  is turned off. Relative to the leading edge of the sheet S, an opening sensor  236  installed in the vicinity of the opening  250  falls to a position indicated by a broken line, the opening sensor  236  is turned on. When the sheet S, which has been exposed to the outside of the apparatus, is removed, and the opening sensor  236  returns to a position indicated by a solid line, the opening sensor  236  is turned off. The sheet movement unit  241  is positioned at a stacking position indicated by a solid line if the sheet S is sequentially conveyed to the first storage unit  201 . On the other hand, the sheet movement unit  241  moves toward the opening  250  along the conveyance direction of the sheet S, and is movable to an exposure position indicated by a broken line when the stored sheet S is exposed. A location of the exposure position, i.e., a distance that the sheet movement unit  241  moves is determined depending on the length by which the sheet S is exposed and the size in the conveyance direction of the sheet S. 
       FIG. 3  is a perspective view of the storage unit  201 . In  FIG. 3 , the sheet movement unit  241  is located at a position between the stacking position and the exposure position. The sheet movement unit  241  includes two sheet trailing edge pressing units  241   a  and  241   b  in a width direction of the sheet S. The sheet movement unit  241  integrally includes a rack  246 . The rack  246  engages with a pinion  247 , and the pinion  247  is connected to an actuator serving as a driving unit (not illustrated) in  FIG. 3 . The sheet movement unit  241  can reciprocate between the stacking position and the exposure position by driving the actuator in a forward direction or a backward direction. 
     (Block Diagram of Control Unit and Functional Configuration) 
       FIG. 4  is a block diagram illustrating a control unit and a functional configuration in the present exemplary embodiment. The image forming apparatus  100  includes an image forming apparatus control unit  301  as its control unit. The image forming apparatus control unit  301  includes a controller  302 , an engine control unit  303 , and a storage device control unit  304 . 
     The controller  302  communicates with an external device  300  such as a host computer, to receive print data  352 , and stores the received print data  352  in a memory  305  (e.g., a random access memory (RAM)). The controller  302  analyzes the print data  352  stored in the memory  305 , to generate a printing condition. The printing condition includes information representing the number of sheets S to be fed, a discharge destination of a sheet S having an image formed thereon, and a density of an image to be printed. The controller  302  designates through a serial I/F the printing condition generated from the print data  352  for the engine control unit  303 . The engine control unit  303  controls each of mechanisms according to the printing condition received from the controller  302 . More specifically, the image forming unit  101  is controlled to form the image on the sheet S, and the feeding unit  102  and the discharge unit  104  are controlled to feed and discharge the sheet S. 
     The controller  302  analyzes the print data  352  stored in the memory  305 , and generates a storage condition and a discharge condition of each of the storage units  201  to  203 . The controller  302  designates the storage condition and the discharge condition generated from the print data  352  for the storage device control unit  304  through a serial I/F. The storage condition includes information representing a storage destination of a sheet S having an image formed therein and the number of sheets S to be stored. The discharge condition includes information representing a distance by which each of sheet movement units  241  to  243  is moved to expose the sheet S from the opening  250 . The storage device control unit  304  controls each of the mechanisms according to the storage condition and the discharge condition that have been received from the controller  302 . More specifically, the conveyance unit  105  is controlled to convey the sheet S having the image formed thereon to each of the storage units  201  to  203 , and the storage device  200  including the sheet movement unit  241  is controlled to move the sheet S stored in each of the storage units  201  to  203  to the opening  250 . The operation unit controller  306  performs control to notify the controller  302  of various types of setting and a discharge instruction given by the user at an operation unit  307 . 
     (Details of Storage Device Control Unit) 
       FIG. 5  is a detail view of the storage device control unit  304  in the present exemplary embodiment. The storage device control unit  304  includes a central processing unit (CPU)  350 , and communicates with the controller  302  via a serial communication unit  351 . The serial communication unit  351  connects the CPU  350  and the controller  302  to each other via a plurality of signal lines. In the present exemplary embodiment, the serial communication unit  351  has three signal lines for transmitting a carry-in preview signal  353 , a storage destination signal  354 , and a discharge instruction signal  357 , described below. 
     Control performed at a time when the storage device  200  stores the sheet S will be described. When the controller  302  is notified of the print data  352  via the external device  300 , the controller  302  once stores the print data  352  in the memory  305 . Then, the controller  302  analyzes the stored print data  352 , and notifies the CPU  350  of the carry-in advance signal  353  and the storage destination signal  354  via the serial communication unit  351 . The CPU  350  controls each of actuators, described below, based on the notified signal, and conveys the printed sheet S to each of the storage units  201  to  203 . 
     Control performed at a time when the sheet S is exposed from the storage device  200  will be described below. When the user issues an instruction to discharge the sheet S stored in the storage unit, at the external device  300  or the operation unit  307 , the controller  302  is notified of a discharge instruction signal  357 . The controller  302  notifies the CPU  350  of the discharge instruction signal  357  via the serial communication unit  351  after determining the storage unit from which the sheet S is to be discharged, and issues an instruction to discharge the sheet S from the determined storage unit. The CPU  350  controls each of the actuators, described below, to expose the sheet S stored in the storage unit to the outside of the apparatus from the opening  250 . 
     Each of the actuators connected to the CPU  350  will be described below. 
     A motor driver  358  is connected to an output terminal of the CPU  350 . The motor driver  358  drives a conveyance motor  359 . When the conveyance motor  359  rotates, the conveyance rollers  211 ,  212 , and  213  rotate to convey the sheet S to each of the storage units  201  to  203 . 
     A motor driver  360  is connected to the output terminal of the CPU  350 . The motor driver  360  drives a discharge motor  361 . When the discharge motor  361  is rotated in the clockwise direction (CW direction), the sheet movement unit  241  in the first storage unit  201  moves toward the opening  250 . When the discharge motor  361  is rotated in a counterclockwise direction (CCW direction), the sheet movement unit  241  in the first storage unit  201  moves in a direction opposite to a direction toward the opening  250 . Similarly, motor drivers  362  and  364  are connected to the output terminal of the CPU  350 , to respectively drive discharge motors  363  and  365 . The discharge motor  363  controls a sheet movement unit  242  in the second storage unit  202 , and the discharge motor  365  controls the sheet movement unit  243  in the third storage unit  203 . 
     The sheet presence/absence sensor  231  uses a pull-up resistor  366 , to input information indicating whether the sheet S is stored in the first storage unit  201 , to the CPU  350  via a buffer  367 . Similarly, a sheet presence/absence sensor  232  inputs information indicating whether the sheet S is stored in the second storage unit  202  to the CPU  350 , and a sheet presence/absence sensor  233  inputs information indicating whether the sheet S is stored in the third storage unit  203  to the CPU  350 . 
     The opening sensor  236  uses a pull-up resistor  375 , to input information indicating whether the sheet S is exposed to the outside of the apparatus from the opening  250 , to the CPU  350  via a buffer  376 . 
     The actuator for switching the second switching member  133  is connected to the output terminal of the CPU  350 . When the actuator is turned on, the second switching member  133  is switched so that the sheet S is conveyed toward the conveyance guide  129 . When the actuator is turned off, the second switching member  133  is switched so that the sheet S is conveyed toward the conveyance guide  132 . Similarly, the actuator for switching the third switching member  134  is connected to the output terminal of the CPU  350 . The third switching member  134  is switched so that the sheet S is conveyed toward the conveyance guide  130  when the actuator is turned on and is conveyed toward the conveyance guide  131  when the actuator is turned off. 
     (Description of Operation of Storage Device) 
     As described above, in the image forming apparatus, the user can select from the external device  300  or the operation unit  307  either one of a buffer mode in which the sheet S is once stored in the storage device  200  and a normal mode in which the sheet S is discharged onto the discharge tray  124 . The selected mode is stored in the memory  305 .  FIG. 6  illustrates a flowchart at a time when the user issues an instruction to print the sheet S. The controller  302  described in  FIG. 4  performs control based on the flowchart based on a program stored in the memory  305 . 
     If the user first issues an instruction to print the sheet S via the external device  300 , then in step S 401 , the controller  302  determines whether the print data  352  has been transmitted thereto. If the controller  302  receives the print data  352  (YES in step S 401 ), then in step S 402 , the controller  302  refers to information stored in the memory  305 , and confirms whether the buffer mode has been selected. If the buffer mode has been selected (YES in step S 402 ), then in step S 403 , the controller  302  once stores the sheet S in the storage device  200 . Control in step S 403  will be described in detail below in  FIG. 9 . If the normal mode has been selected (NO in step S 402 ), then in step S 404 , the controller  302  discharges the sheet S to the discharge tray  124 . With that, the control based on the flowchart ends. While the flowchart illustrated in  FIG. 6  presupposes a configuration in which the user previously selects the mode, the present invention is not limited to this configuration. For example, a configuration may be used in which the user determines which of the modes is used for discharging the sheet S every time the user issues an instruction to perform printing. 
     In the present exemplary embodiment, when the sheet S is stored in the storage device  200 , the sheet S is distributed into the storage unit that differs for each job number of the sheet S. When the sheet S is exposed from the storage device  200 , the sheet S of the user who has issued a discharge instruction is exposed to the outside of the apparatus from the opening  250 . The user can issue the discharge instruction by inputting a password previously set to the external device  300  or the operation unit  307 . Alternatively, the user can also issue the discharge instruction by causing an ID card reading unit (not illustrated) provided in the operation unit  307  to read his/her own ID card to perform user authentication. In the present exemplary embodiment, the storage units  201  to  203  respectively are provided with the separate actuators that drive the sheet movement units  241  to  243 , as described above. Therefore, even if the sheets S of the same user are stored in a plurality of storage units, actuators provided therein are driven such that the user can collectively receive the sheets S. A job number of the sheet S and information about the user who has issued an instruction to print the sheet S are stored in the memory  305  provided in the controller  302 . When the user issues the discharge instruction, the controller  302  specifies the sheet S to be discharged by referring to the memory  305  and instructs the storage device  200  to discharge the sheet S. 
       FIG. 7  illustrates an example of the operation of the storage device  200 . In  FIG. 7A , the storage unit  201  stores sheets S of a user A, and each of the storage units  202  and  203  stores sheets S of a user B. Among the sheets S which the user B instructs the image forming apparatus  100  to print, the storage unit  202  stores the sheets S with a job number 1, and the storage unit  203  stores the sheets S with a job number 2. In  FIG. 7B , when an instruction to discharge the sheets S of the user B is issued, the sheet movement units  242  and  243  in the storage units  202  and  203  move toward the opening  250 , and a bundle SB of the sheets S is exposed from the opening  250 . 
       FIG. 8  is a perspective view of the image forming apparatus  100  at this time. A bundle SJ of sheets S printed by a plurality of users is stacked on the display tray  124 . A leading edge SB 2  of the sheet bundle SB discharged from each of the storage units  202  and  203  is exposed from the opening  250 . The user can receive the sheet bundle SB by gasping and pulling out the leading edge SB 2  exposed to the outside of the apparatus. 
     If the user issues an instruction to store a larger number of sheets S than the number of sheets that can be stored in the one storage unit, the sheets S are distributed into the difference storage units even if they have the same job number. The sheets S of the user B with different job numbers respectively are stored in the storage units  202  and  203  in  FIG. 7A , for example. If the number of the sheets S with the job number 1 is more than the upper-limit number in the storage unit  202 , however, the sheets S with the job number 1 are also distributed into the storage unit  203 , on the premise that other sheets S are not stored in the storage unit  203  at this time. 
     The storage device  200 , excluding the opening  250  for exposing the stored sheets S, is encompassed. Therefore, the user cannot see information printed on the sheets S in each of the storage units  201  to  203  with the sheets S stored in the storage unit. Thus, other users do not see the information printed on the his/her own sheets S so that confidentiality of the information can be enhanced. 
     On the other hand, there is an image forming apparatus that starts image formation after performing user authentication using an ID card to enhance confidentiality of information. However, compared to such an image forming apparatus, the image forming apparatus  100  according to the present exemplary embodiment may only discharge the sheets S each having the image already formed thereon from each of the storage units  201  to  203 . Therefore, the user can quickly take out the sheets S without waiting until the image is formed after user authentication is performed. 
     Further, when the user issues a discharge instruction to the image forming apparatus  100 , the user can take out only the his/her own sheets S. Thus, time and labor required for the user to search for the his/her own sheets S from the discharge tray  124  on which the sheets S and sheets of the others are mixed are saved. 
     (Control in Buffer Mode) 
     Characteristic control according to the present exemplary embodiment performed at a time when the buffer mode has been selected will be described below.  FIG. 9  illustrates a flowchart in the present exemplary embodiment. The flowchart illustrated in  FIG. 9  is a sub process of the flowchart illustrated in  FIG. 6 , and corresponds to step S 403 . The controller  302  illustrated in  FIG. 4  performs control based on the flowcharts based on the program stored in the memory  305 . 
     When the controller  302  first shifts to the buffer mode, then in step S 501 , the controller  302  searches for a free storage unit. In step S 502 , the controller  302  determines whether there is a free storage unit. If there is a free storage unit (YES in step S 502 ), then in step S 508 , the controller  302  conveys sheets S each having an image formed thereon to the free storage unit. If there is no free storage unit (NO in step S 502 ), then in step S 503 , the controller  302  determines whether the user has issued an instruction to discharge the sheets S from the storage unit, at the external device  300  or the operation unit  307 . If the discharge instruction has been issued (YES in step S 503 ), the processing proceeds to step S 504 . In step S 504 , the controller  302  notifies the storage device control unit  304  of the discharge instruction signal  357  when sheets S of the user who have issued the discharge instruction are stored in the first storage unit  201 , for example. The CPU  350  included in the storage device control unit  304  rotates the discharge motor  361  in the clockwise direction (CW direction) via the motor driver  360  when it receives the discharge instruction signal  357 , and moves the sheet movement unit  241  to the exposure position from the stacking position. The controller  302  exposes some of the sheets S stored in the storage unit  201  to the outside of the apparatus from the opening  250 . In step S 505 , the controller  302  determines whether subsequent sheets can be stored in the storage unit  201 . The controller  302  calculates a size L4 of the sheet that can be stored in the stack tray  221  using a calculation equation of L4=(L3+α)−(L1−L2), where L1 is a size in a conveyance direction of the exposed sheet, L2 is a length exposed from the opening  250 , of the sheet, L3 is a size in a conveyance direction of the stack tray  221 , and α is a length from a tip end of the stack tray  221  to the opening  250 . If a size in a conveyance direction of the subsequent sheets is not more than the size L4 of the sheets that can be stored in the stack tray  221 , the controller  302  determines that the subsequent sheets can be stored. If the subsequent sheets can be stored (YES in step S 505 ), then in step S 506 , the CPU  350  rotates the discharge motor  361  in the counterclockwise direction (CCW direction) via the motor driver  360  even if the opening sensor  236  remains turned on, and returns the sheet movement unit  241  to the stacking position. If the sheet movement unit  241  returns to the stacking position, then in step S 507 , the controller  302  starts to form an image on the subsequent sheets, and conveys the sheets to the storage unit  201 . 
       FIG. 10  illustrates a specific example of the present exemplary embodiment. In  FIG. 10A , the storage unit  201 , the storage unit  202 , and the storage unit  203  respectively store sheets of a user A, sheets of a user B, and sheets of a user C. In this state, suppose a user D issues a printing instruction to the image forming apparatus  100  in the buffer mode. The storage device  200  has no more space that stores printed sheet separately from other sheets, and thus cannot store sheets of the user D. In  FIG. 10B , when an instruction to discharge the sheets of the user A is issued, the sheet movement unit  241  in the storage unit  201  moves toward the opening  250 , to expose the sheets of the user A from the opening  250 . If a size in a conveyance direction of the sheets of the user D is smaller than that of a free space (L4 in  FIG. 10B ) at this time, the sheets of the user D can be stored in the storage unit  201 .  FIG. 10C  illustrates a state where the sheet movement unit  241  has been returned to the stacking position, and the sheets of the user D have actually been conveyed to the storage unit  201 . 
     A plurality of sheets can be distinguished and stored in the one storage unit by returning the sheet movement unit  241  to the stacking position before the user issues the discharge instruction to take out the sheets exposed from the opening  250  or before the opening sensor  236  is turned on from off. 
     In the above-mentioned first exemplary embodiment, control has been described which is performed at a time when the sheets are exposed from the one storage unit while the storage device  200  is fully-stacked. In a second exemplary embodiment, control performed at a time when sheets are exposed from a plurality of storage units will be described. Description of a principal part of the second exemplary embodiment is similar to that in the first exemplary embodiment, and only a different part of the second exemplary embodiment from the first exemplary embodiment will be described below.  FIG. 11  illustrates a flowchart in the present exemplary embodiment. The flowchart illustrated in  FIG. 11  is a sub process of  FIG. 6 , and corresponds to step S 403 . The controller  302  illustrated in  FIG. 4  performs control based on the flowcharts based on a program stored in the memory  305 . 
     Steps S 601  to S 603  in the flowchart illustrated in  FIG. 11  are similar to steps S 501  to S 503  in the flowchart illustrated in  FIG. 9 , and hence description thereof is not repeated. In step S 604 , the controller  302  determines whether the number of storage units serving as a discharge target is plural. If the number of storage units serving as a discharge target is one (NO in step S 604 ), then in steps S 606  to S 609 , the controller  302  performs control similar to that in the first exemplary embodiment. If the number of storage units serving as a discharge target is plural (YES in step S 604 ), then in step S 605 , the controller  302  exposes sheets stored in each of the storage units to the outside of the apparatus from the opening  250  overlapping with one another. At this time, a length by which the discharge target sheets stored in the one storage unit are exposed from the opening  250  is set to a normal length (e.g., 30 mm). A length by which the sheets stored in the other discharge target storage unit are exposed from the opening  250  is set to a length larger than the normal length (e.g., 40 mm). When sheets of a user who has issued a discharge instruction are stored in the first storage unit  201  and the third storage unit  203 , for example, the controller  302  notifies the storage device control unit  304  of the discharge instruction signal  357 . The CPU  350  included in the storage device control unit  304  rotates the discharge motors  361  and  365  in a clockwise direction (CW direction) via the motor drivers  360  and  364  and moves the sheet movement unit  241  and  243  toward the opening  250  when it receives the discharge instruction signal  357 . At this time, a period of time during which the CPU  350  rotates the discharge motor  365  is longer by ΔT than a period of time during which the CPU  350  rotates the discharge motor  361  is rotated, so that the CPU  350  moves the sheet movement unit  243  to a position closer to the opening  250  than the sheet movement unit  241 . As a result, the sheets stored in the storage unit  203  are greatly exposed by ΔL from the opening  250  compared with the sheets stored in the storage unit  201 . In step S 607 , the controller  302  then determines whether the subsequent sheets can be stored in the sheet movement unit  241  in the one storage unit from which the sheets have been exposed by a normal length, i.e., the storage unit  201 . The determination in step S 607  is made by the controller  302  under the same condition as that in the first exemplary embodiment. If the subsequent sheets can be stored (YES in step S 607 ), then in step S 608 , the CPU  350  rotates the discharge motor  361  in a counterclockwise direction (CCW direction) via the motor driver  360  and returns the sheet movement unit  241  to a stacking position even if the opening sensor  236  remains turned on. At this time, the sheet movement unit  243  is not returned to a stacking position and is not moved from an exposure position. If the sheet movement unit  241  returns to the stacking position, then in step S 609 , the CPU  350  starts to form an image on the subsequent sheets and conveys the sheets to the storage unit  201 . 
     A specific example of the present exemplary embodiment is illustrated in  FIG. 12 . In  FIG. 12A , sheets of a user A and sheets of a user B respectively are stored in the storage units  201  and  203  and the storage unit  202 . Among the sheets which the user A instructs the image forming apparatus  100  to print, sheets S 1  with a job number 1 and sheets S 2  with a job number 2 respectively are stored in the storage units  201  and  203 . In this state, suppose a user C issues a printing instruction to the image forming apparatus  100  in a buffer mode. The storage device  200  has no more space that stores printed sheets distinguished from other sheets, and thus cannot store sheets of the user C. In  FIG. 12B , when an instruction to discharge the sheets S 1  and S 2  of user A is issued, the sheet movement units  241  and  243  move toward the opening  250 , to expose the sheets S 1  and S 2  of the user A from the opening  250  with the sheets overlapping one another. In  FIG. 12B , the sheets S 2  with the job number 2 in the storage unit  203  are greatly exposed from the opening  250  by ΔL compared with the sheets S 1  with the job number 1 in the storage unit  201 . If a size in a conveyance direction of the sheets of the user C is smaller than that of a free space (L4 in  FIG. 12B ) at this time, the sheets of the user C can be stored in the storage unit  201 .  FIG. 12C  illustrates a state where the sheet movement unit  241  has been returned to the stacking position, and the sheets of the user C have actually been conveyed to the storage unit  201 . The sheet movement unit  243  has not moved from the exposure position. 
     The sheet movement unit  241  is located at the stacking position. Thus, if the user erroneously pushes the exposed sheets S 1  into the apparatus when taking out the sheets S 1 , the sheet S 1  may move into the storage device  200 , which may prevent extraction from the opening  250 . On the other hand, the sheet movement unit  243  is located at the exposure position. Thus, even if the user erroneously pushes the exposed sheets S 2  into the apparatus, the sheets S 2  do not move. In the present exemplary embodiment, the sheets S 2  are greatly exposed from the opening  250  compared with the sheets S 1  so that the user touches the sheets S 2  before touching the sheets S 1 . Thus, the user does not easily touch the sheets S 1 , and can be prevented from pushing the sheets S 1 . 
     As described above, the sheets S 2  stored in the storage unit  203  are greatly exposed from the opening  250  compared with the sheets S 1  stored in the storage unit  201 . The storage unit  201  can distinguish and store the plurality of sheets by returning the sheet movement unit  241  to the stacking position and conveying the subsequent sheets to the storage unit  201 . Further, the user can be prevented from pushing the sheets, by retaining the sheet movement unit  243  at the exposure position. 
     In the above-mentioned second exemplary embodiment, the respective discharge target sheets stored in the plurality of storage units are all the same in size. If the respective sheets stored in the storage units are different in size, when control is performed to convey the subsequent sheets to the storage unit storing the sheets of the smallest size, the possibility that the plurality of sheets can be distinguished and stored in the one storage unit is increased. 
     In the above-mentioned second exemplary embodiment, the subsequent sheets are stored in the one storage unit. On the other hand, it is conceivable that the one storage unit cannot store the subsequent sheets. In the case, instead of returning only the sheet movement unit  241  to the stacking position, as in the second exemplary embodiment, the sheet movement unit  243  may also be returned to the stacking position to convey the subsequent sheets to the storage units  201  and  203 . 
     While the length exposed from the opening  250 , of the sheets is set to 30 mm in the above-mentioned exemplary embodiments, the length may be made variable depending on a size in a conveyance direction of the subsequent sheets. That is, if the size of the subsequent sheets is large, the length exposed from the opening  250 , of the sheets may be increased to enlarge a space so that the subsequent sheets can be conveyed. 
     In the above-mentioned exemplary embodiments, the stack tray is bent, and is of a shape having the horizontal plane and the inclined plane. When an angle of the inclined plane is increased, the sheets, which have been partially exposed to the outside of the apparatus from the opening  250 , may be returned into the apparatus as the sheet movement unit moves from the exposed position to the stacking position. Thus, the controller  302  may determine whether the sheets can return into the apparatus before the sheet movement unit moves from the expose position to the stacking position. For example, when one-half or more of each of the exposed sheets in the conveyance direction is put on the horizontal plane of the stack tray, the controller  302  may determine that the exposed sheets are not returned into the apparatus even if the sheet movement unit is returned to the stacking position. In the above-mentioned exemplary embodiments, the stack tray may be in a shape having only the horizontal plane. 
     In the above-mentioned exemplary embodiments, the sheet movement units in the storage units respectively have separate actuators. Thus, the sheets stored in the plurality of storage units can be exposed while overlapping one another by simultaneously driving the actuators. On the other hand, one actuator may selectively move the plurality of sheet movement units by providing a smaller number of actuators than the number of the storage units and providing a drive transmission switching unit such as a clutch (not illustrated). In the above-mentioned exemplary embodiments, the controller  302  is provided with the memory  305 . However, the memory  305  may be provided in the engine control unit  303  or the storage device control unit  304 . Alternatively, the memory  305  may be independently provided within the image forming apparatus control unit  301 . 
     While the engine control unit  303  and the storage device control unit  304  are separately configured in the above-mentioned exemplary embodiments, the engine control unit  303  and the storage device control unit  304  may collectively be configured. In the case, the engine control unit  303  may control the conveyance unit  105  and the storage device  200 . 
     While sheet conveyance paths are joined on the downstream side of each of the storage units and the number of openings is one in the above-mentioned exemplary embodiments, a plurality of openings may be separately provided. Sheets stored in the storage units respectively may be exposed from the separate openings. 
     While the three storage units are provided in the above-mentioned exemplary embodiments, the number of storage units is not limited to three. The number of storage units may be set to match an environment where the apparatus body is used, the number of users who commonly use the apparatus, or a spec for the apparatus body. 
     In the above-mentioned exemplary embodiments, the storage device  200  is configured integrally with the image forming apparatus  100 . On the other hand, the storage device  200  may be provided being detachably attached to the image forming apparatus  100 . In the case, the control unit provided in the image forming apparatus  100  may control an operation of the storage device  200 . The operation of the storage device  200  may be controlled by providing a control unit independent of the storage device  200  to communicate with the control unit  301  provided in the image forming apparatus  100 . 
     While the laser beam printer is illustrated as an example in the above-mentioned exemplary embodiments, the image forming apparatus for which the present invention is employed is not limited to this. The image forming apparatus may be a printer of another type such as an inkjet printer or a copying machine. 
     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. 2013-225590 filed Oct. 30, 2013, which is hereby incorporated by reference herein in its entirety.