Patent Publication Number: US-10759619-B2

Title: Image forming apparatus

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present disclosure relates to an image forming apparatus. 
     Description of the Related Art 
     In general, a copying machine, a printing apparatus, or other such image forming apparatus includes a sheet feeding cassette configured to store sheets. The sheet feeding cassette is configured so as to be able to store sheets of A-series sizes (A3, A4, and A5), B-series sizes (B4 and B5), sizes defined in inches (LGL and LTR), and other various sizes from large sizes to small sizes. Moreover, size information on currently stored sheets is set for the sheet feeding cassette in advance, which prevents a sheet of a wrong size from being fed to be subjected to image formation. 
     In Japanese Patent Application Laid-open No. Hei 7-112844, the following method is proposed as a method of setting a sheet size. That is, in an image forming apparatus of Japanese Patent Application Laid-open No. Hei 7-112844, a trailing edge guide and a side guide, which are configured to regulate the movement of a trailing edge and a side edge of sheets, respectively, are provided to a cassette, and the size of the sheets stored in the cassette is automatically detected based on the positions of the trailing edge guide and the side guide. There is also generally known a method of setting sheet size information (user set size), which is input by a user through use of an operating device, for the sheet feeding cassette. 
     When the trailing edge guide or the side guide is not correctly set at a position at which the trailing edge guide or the side guide abuts against the trailing edge or the side edge of sheets, respectively, there are a fear that one sheet may move in a direction reverse to a feeding direction in reaction to the feeding of another sheet and a fear that skew feeding of a sheet may occur while the sheet is being fed. In U.S. Pat. No. 9,475,663 B2, there is proposed a method of displaying a warning screen when it is determined that the automatically detected sheet size and the sheet size set by the user do not match each other. 
     SUMMARY OF THE INVENTION 
     An image forming apparatus according to the present disclosure includes: a container configured to store a sheet; an image forming unit configured to form an image on the sheet fed from the container; a trailing edge regulating plate, which is movable in a feeding direction of the sheet inside the container, and is configured to regulate a position of a trailing edge of the sheet in the feeding direction; a detector configured to detect a size of the sheet based on the position of the trailing edge regulating plate; a receiver configured to receive a size of the sheet to be set for the container based on a user instruction; an attachment, which is to be mounted to the trailing edge regulating plate, and is configured to regulate the position of the trailing edge of the sheet having a sheet size smaller than an applicable sheet size of the trailing edge regulating plate; and a determination unit, wherein the determination unit is configured to: determine, in a case where a length in the feeding direction of the size received by the receiver is longer than a predetermined length, whether to notify a user of a warning based on the size of the sheet received by the receiver and the size of the sheet detected by the detector; and determine, in a case where the length in the feeding direction of the size of the sheet received by the receiver is shorter than the predetermined length, whether to notify the user of the warning based on the size of the sheet received by the receiver, the size of the sheet detected by the detector, and a length of the attachment in the feeding direction. 
     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. 1A  is a sectional view of an image forming apparatus, and  FIG. 1B  is a top view of an operating device illustrated in  FIG. 1A . 
         FIG. 2A  is a control block diagram of the image forming apparatus, and  FIG. 2B  is a block diagram for illustrating details of an operation controller illustrated in  FIG. 2A . 
         FIG. 3A ,  FIG. 3B , and  FIG. 3C  are each a configuration view of a sheet feeding cassette. 
         FIG. 4A ,  FIG. 4B , and  FIG. 4C  are each an illustration of a setting screen for a sheet size. 
         FIG. 5A  and  FIG. 5B  are each a table for showing a relationship between a detection result of the sheet size and a user set size. 
         FIG. 6  is an illustration of a sheet size error screen. 
         FIG. 7  is an explanatory flow chart for illustrating processing for comparing the user set size and an automatically detected size. 
         FIG. 8A  and  FIG. 8B  are schematic views for illustrating how an auxiliary regulating member is mounted to the sheet feeding cassette. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Image Forming Apparatus 
     Now, a preferred embodiment of the present invention is described with reference to the accompanying drawings.  FIG. 1A  is a sectional view for illustrating a schematic configuration of a full-color copying machine being an example of an image forming apparatus. The image forming apparatus of  FIG. 1A  includes a reader  1001 , a printer  1002 , and an operating device  1003 . The reader  1001  includes an image reading unit  150 , and is configured to optically read an image on an original, and generate image data by converting the read image into an electric signal. The reader  1001  further includes an automatic document feeder (hereinafter referred to as “ADF”)  160 , and is configured to convey originals S stacked on an original tray  161  to an original flow reading glass  154  one by one by a conveyor  164  indicated by the broken line in  FIG. 1A . 
       FIG. 1A  is an example of an image forming apparatus according to a first embodiment of the present invention, and is a vertical sectional view for illustrating the schematic configuration of the full-color copying machine. The image forming apparatus includes the reader  1001 , the printer  1002 , and the operating device  1003 . The reader  1001  includes the image reading unit  150  and the ADF  160 . The image reading unit  150  is configured to optically read an original image by illuminating an original, and create image data by converting the read image into an electric signal. The reader  1001  includes the ADF  160 , and is configured to convey the originals S stacked on the original tray  161  to the original flow reading glass  154  one by one by the conveyor  164  indicated by the broken line in  FIG. 1A . Then, the image on the original conveyed onto the original flow reading glass  154  is read by an optical scanner unit  151  located below the original flow reading glass  154 . The original is read by the optical scanner unit  151 , and then is delivered onto an original delivery tray  162  by the conveyor  164 . A delivered-original detection sensor  163  is configured to detect the presence or absence of the original delivered onto the original delivery tray  162 . Moreover, an original presence detection sensor  165  is configured to detect whether or not the originals S are stacked on the original tray  161 . 
     The printer  1002  includes a laser exposure device  101 , an imager  102 , and a fixer  103 , each of which is indicated by the broken line frame part, sheet feeding cassettes  120   a  and  120   b , and a feeder/conveyor  104  (solid line frame part) including a manual feed tray  120   c . The sheet feeding cassettes  120   a  and  120   b  and the manual feed tray  120   c  are sheet containers configured to store sheets of a plurality of sizes. The imager  102  is an image forming unit including photosensitive drums  110   a ,  110   b ,  110   c , and  110   d  (hereinafter denoted by “ 110   a  to  110   d ”). 
     The laser exposure device  101  causes laser light modulated in accordance with the image data to enter a rotary polygon mirror  111  rotating at a constant angular velocity, and to thereby apply the laser light to the photosensitive drums  110   a  to  110   d  as reflected scanning light indicated by the one-dot chain line in  FIG. 1A  to form latent images thereon. The imager  102  drives the photosensitive drums  110   a  to  110   d  to rotate, causes a charger to charge the photosensitive drums  110   a  to  110   d , causes the laser exposure device  101  to develop the latent images formed on the photosensitive drums  110   a  to  110   d  through use of toners, and transfers toner images onto a sheet fed from the sheet container. The four photosensitive drums  110   a  to  110   d  are arrayed for forming images of yellow (Y), magenta (M), cyan (C), and black (K) in the stated order, and are configured to sequentially perform an image forming operation after elapse of a predetermined time period from the start of image formation. Under this timing control, the toner images of the respective colors are transferred onto the sheet while being superimposed on one another to form a full-color toner image. The fixer  103  is formed of a combination of a roller and a belt, and includes a halogen heater or other such heat source. The fixer  103  is configured to melt and fix an unfixed full-color toner image, which is formed on the sheet by the imager  102 , by heat and pressure. 
     The feeder/conveyor  104  is configured to separate the sheets stored in any one of the containers, namely, the sheet feeding cassettes  120   a  and  120   b  and the manual feed tray  120   c , one by one, and convey the separated sheet to the imager  102 . As described above, the toner images of the respective colors are transferred onto the sheet conveyed to the imager  102 , fixed thereto by the fixer  103 , and delivered onto a delivery tray  130 . 
     The operating device  1003  is a receiver configured to receive a user instruction, and includes a key input device  4000  and a touch panel  4001 . The key input device  4000  is operated by a user touching a key. A power switch  401  is a switch for switching between power-on and power-off of the entire image forming apparatus. A start key  402  is a key for instructing to start execution of a copy job for performing an original reading operation and a printing operation, and a stop key  403  is a key for instructing to interrupt the copy job. Numeric keys  404  indicated by the broken line frame part are keys for setting the number of copies. A clear key  405  is a key for returning a set value of a copy mode to an initial value. A user mode key  406  is a key for transitioning to a screen for performing system setting, various kinds of adjustment, and other such operations. The touch panel  4001  allows the user to input information by touching a touch panel  301 , and also functions as a display for displaying information. On the touch panel  4001 , the number of copies, a magnification, a selected sheet, a copy density, the copy mode, system conditions, and other such information are displayed. 
     Control Blocks 
       FIG. 2A  is a control block diagram of the image forming apparatus according to this embodiment. The image forming apparatus includes a CPU  201 , a ROM  202 , a RAM  203 , an operation controller  204 , a reader controller  205 , an image processor  206 , an external I/F controller  208 , and a printer controller  207 , which are connected to one another via a system bus  200 . The CPU  201  is a controller configured to control the image forming apparatus. A program for executing processing of a flow chart (described later) is written to the ROM  202 . The CPU  201  is configured to execute the program to provide various functions. The RAM  203  is used as a work memory to be used when the CPU  201  performs various kinds of arithmetic operation processing. The operation controller  204  is an electric circuit configured to control the components of the operating device  1003 . The reader controller  205  and the printer controller  207  are each an electric circuit including an input/output port and being configured to control the components of the reader  1001  and the printer  1002 , respectively. The printer controller  207  is configured to control detectors such as a width volume sensor  514 , sheet presence sensors  515 ,  516 , and  517 , a sheet width detection sensor  818 , a sheet length detection sensor  817 , and a sheet presence sensor  820 . The printer controller  207  includes respective drive circuits for pickup rollers  811  illustrated in  FIG. 3 . The CPU  201  is configured to control the reader controller  205  and the printer controller  207  based on the program stored in the ROM  202  to perform the image forming operation. 
     The image processor  206  is configured to perform various kinds of image processing on digital data on the original image converted by the reader controller  205 , and output the data subjected to the image processing to the printer controller  207 . The external I/F controller  208  is an electric circuit configured to connect the image forming apparatus to a server, a personal computer, or other such external device via a LAN cable or a USB cable, and control communication to/from the external device. 
     Block Diagram of Operation Controller 
       FIG. 2B  is a block diagram for illustrating details of the operation controller  204 . The touch panel  301  and a hardware key group  303  (corresponding to  402  to  406  in  FIG. 1B ) are connected to the system bus  200  via an operation input device  306 . Then, coordinate information indicating a pressing position of the touch panel  301  and key information indicating a pressed key of the hardware key group  303  are output to the system bus  200  via the operation input device  306 . An LCD  302  is connected to the system bus  200  via a display controller  305 . The display controller  305  and the operation input device  306  are connected to the CPU  201 , the ROM  202 , and the RAM  203  via the system bus  200 . The ROM  202  is configured to store not only the control program but also data on a setting screen, data on a display key, and other such data for various kinds of operation modes. The RAM  203  is configured to store a current setting status of the image forming apparatus. When the coordinate information and the key information are input from the operation input device  306 , the CPU  201  operates in the following manner. The CPU  201  selects the setting screen and the display key corresponding to the current setting status of the image forming apparatus and an operation performed on the touch panel  301  or the hardware key group  303  by the user, and transfers the setting screen and the display key to the display controller  305  as the display data. The display controller  305  transmits the display data transferred by the CPU  201  to the LCD  302 , and the LCD  302  visibly displays the transferred display data. 
     Configuration of Sheet Feeding Cassette 
     With reference to  FIG. 3 , a description is given of a configuration of the sheet feeding cassette  120   a  being an example of a container in this embodiment.  FIG. 3A  is a top view of the sheet feeding cassette  120   a  with the front of the image forming apparatus at the bottom, and  FIG. 3B  and  FIG. 3C  are each a back side view of the sheet feeding cassette  120   a  obtained when the sheet feeding cassette  120   a  is virtually viewed through from the front of the image forming apparatus.  FIG. 3C  is also a front view of the sheet feeding cassette  120   a  obtained when the sheet feeding cassette  120   a  is inserted into the image forming apparatus. 
     The sheet feeding cassette  120   a  enables sheets to be supplied thereto under a state in which the sheet feeding cassette  120   a  is pulled out from a main body  819  on the image forming apparatus side, and enables the printing operation to be performed under a state in which the sheet feeding cassette  120   a  is accommodated in the main body  819  on the image forming apparatus side. 
     A plurality of sheets can be stacked on the sheet feeding cassette  120   a , and are fed to the imager  102  in order from the top sheet by the pickup rollers  811  provided in the vicinity of the main body  819 . Side edge regulating plates  812  and  813  are configured so as to be slidable in directions indicated by the arrows  8 Y 1  and  8 Y 2  in conjunction with each other, and are configured to sandwich the stacked sheets at side edges thereof in a width direction thereof, to thereby prevent a sheet from being conveyed while being skew-fed. 
     The side edge regulating plates  812  and  813  are provided so as to be movable in the directions indicated by the arrows  8 Y 1  and  8 Y 2 , which correspond to the width direction perpendicular to a feeding direction of the sheet, and are configured to regulate the stacked sheets at the side edges in the width direction, to thereby prevent a sheet from being fed while being skew-fed. Moreover, the side edge regulating plates  812  and  813  are coupled to a sheet width detection plate  815  illustrated in  FIG. 3B  via a link mechanism (not shown), and the sheet width detection plate  815  is configured to slide in the directions indicated by the arrows  8 Y 1  and  8 Y 2 , which correspond to a direction perpendicular to the feeding direction of the sheet, in accordance with the movement of the side edge regulating plates  812  and  813 . In this embodiment, the side edge regulating plates  812  and  813  are capable of regulating sheets having a sheet width ranging from 98.5 mm to 215.0 mm. 
     A trailing edge regulating plate  814  is configured so as to be movable in directions indicated by the arrows  8 X 1  and  8 X 2 , which correspond to a feeding direction of a sheet, and is configured to regulate the position of a trailing edge of the sheet in the feeding direction of the sheet. With this configuration, the trailing edge regulating plate  814  prevents one sheet from moving in a direction reverse to the feeding direction in reaction to the feeding of another sheet from the stacked sheets. Moreover, the trailing edge regulating plate  814  is coupled to a sheet length detection plate  816  via a link mechanism (not shown), and the sheet length detection plate  816  is configured to slide in the directions indicated by the arrows  8 X 1  and  8 X 2  in conjunction with the position of the trailing edge regulating plate  814 . In this embodiment, the trailing edge regulating plate  814  is capable of regulating the trailing edge of sheets having a sheet length ranging from 190.5 mm to 355.6 mm. 
     The sheet width detection sensor  818  provided to the main body  819  is formed of a triple switch, namely, switches SW 0 , SW 1 , and SW 2 , and is configured to detect the sheet width under the state in which the sheet feeding cassette  120   a  is inserted in the main body  819 . Specifically, a combination of on/off states of the switches SW 0 , SW 1 , and SW 2  is detected as information on a shape formed by positions on the broken lines illustrated in  FIG. 3B  of the sheet width detection plate  815 , which is configured to slide in the directions indicated by the arrows  8 Y 1  and  8 Y 2  in conjunction with the side edge regulating plates  812  and  813 . 
     Similarly, the sheet length detection sensor  817  provided to the main body  819  is formed of a triple switch, namely, switches SW 3 , SW 4 , and SW 5 , and is configured to detect the sheet length under the state in which the sheet feeding cassette  120   a  is inserted in the main body  819 . Specifically, a combination of on/off states of the switches SW 3 , SW 4 , and SW 5  is detected as information on a shape formed by positions on the broken lines illustrated in  FIG. 3C  of the sheet length detection plate  816 , which is configured to slide in the directions indicated by the arrows  8 X 1  and  8 X 2  in conjunction with the trailing edge regulating plate  814 . 
     The sheet presence sensor  820  provided to the main body  819  is configured to detect a sheet surface of a sheet placed at the top surface among a plurality of sheets stored in the sheet feeding cassette  120   a . The CPU  201  is configured to drive or stop a lift-up motor (not shown) based on detection results obtained by the sheet width detection sensor  818 , the sheet length detection sensor  817 , and the sheet presence sensor  820 . With this configuration, the CPU  201  performs a lift-up operation for lifting up the sheets stacked on the sheet feeding cassette  120   a  to such a height as to enable sheet feeding by the pickup rollers  811 . Then, the CPU  201  performs an operation for determining the presence or absence of a sheet on the sheet feeding cassette  120   a  with the sheets being lifted up to such a height as to enable sheet feeding. 
     User Set Size 
     In this embodiment, a mechanism for automatically detecting the size of sheets stored in a sheet feeding cassette is provided, but it is also possible to set a sheet size designated by the user for the sheet feeding cassette. 
     With reference to  FIG. 4 , a description is given of an example of setting the sheet size based on a user instruction.  FIG. 4A  to  FIG. 4C  are each a sheet size setting screen to be displayed on the operating device  1003 . 
     In  FIG. 4A , a sheet feeding cassette key  402  is a key for selecting a sheet feeding cassette for which the sheet size is to be set. On the sheet feeding cassette key  402 , information on the sheet size registered for the sheet feeding cassette is displayed. When the sheet feeding cassette key  402  is pressed by the user, the sheet feeding cassette corresponding to the pressed key is selected as the sheet feeding cassette for which the sheet size is to be set.  FIG. 4A  is an illustration of a state in which the sheet feeding cassette  120   a  has been selected as the sheet feeding cassette for which the sheet size is to be set. A sheet information display area  404  is an area in which detailed information on sheets set in the selected sheet feeding cassette is displayed. Not only the information on the sheet size but also information on a sheet type and a basis weight are displayed in the sheet information display area  404 . A user set size key  403  is a key for allowing the user to input the sheet size to be set for the selected sheet feeding cassette. When the user set size key  403  is pressed by the user, a transition is made to a user set size screen  701  illustrated in  FIG. 4B . 
       FIG. 4B  is an illustration of a screen to be displayed when the user set size key  403  is pressed. In  FIG. 4B , a sheet size input area  406  is an area for inputting information on a length (sheet length) in a direction for feeding a sheet (feeding direction X), and a sheet size input area  407  is an area for inputting information on a length (sheet width) of a sheet in a width direction Y perpendicular to the feeding direction. Through input of the information on the sheet length and the sheet width to the sheet size input areas  406  and  407 , it is possible to set an irregular size as well as a regular size. The user uses the numeric keys  4040  or a software keyboard displayed in  FIG. 4B  to designate the sheet sizes in the feeding direction X and the width direction Y. An OK key  405  is a key for setting the designated sheet size. When the OK key  405  is pressed, the CPU  201  sets the sheet size input on a user set size screen for the sheet feeding cassette for which the sheet size is to be set, and as illustrated in  FIG. 4C , displays the information on the sheet size in the sheet information display area  404 . 
     Sheet Size Detection for Sheet Feeding Cassette 
     Next, a description is given of processing for determining a match or a mismatch between the sheet size designated by the user (hereinafter referred to as “user set size”) and the automatically detected sheet size (hereinafter referred to as “automatically detected size”).  FIG. 5A  is a correspondence table between detection results obtained by the switches SW 3  to SW 5  of the sheet length detection sensor  817  and the user set sizes. The correspondence table of  FIG. 5A  and a correspondence table of  FIG. 5B  are each stored in the ROM  202  or the RAM  203 , and the CPU  201  determines a sheet size error based on the information of those correspondence tables. 
       FIG. 5A  is a table for showing a relationship between the length in the feeding direction of the automatically detected size and the length in the feeding direction of the user set size.  FIG. 5B  is a table for showing a relationship between the length in the feeding direction of the automatically detected size and the length in the feeding direction of the user set size. 
     The first column of the table of  FIG. 5A  indicates the sheet length identified from the output states of the switches SW 3  to SW 5  of the sheet length detection sensor  817 . The second to fourth columns of the table indicate the output states of the switches SW 3  to SW 5 , respectively. The fifth column and the subsequent columns each indicate the user set size. In  FIG. 5A , “N/A” indicates that the sheet length set as the user set size and an automatic detection result exhibit a mismatch (not applicable), namely, a setting error is determined. When a setting error is determined, the CPU  201  causes the operating device  1003  to display a sheet size error screen illustrated in  FIG. 6  to notify the user of the setting error. “A” indicates that the sheet length set as the user set size and the automatic detection result exhibit a match, namely, no setting error is determined. In this case, the sheet size error screen is not displayed, and the user set size is determined as the set size of the sheet feeding cassette. 
       FIG. 5B  is the correspondence table between the sheet size automatically detected based on the position of the trailing edge regulating plate  814  and the user set size. The first column of the table indicates a setting range of the sheet width. The second to fourth columns of the table indicate the output states of the switches SW 0  to SW 2  of the sheet length detection sensor  817 , respectively. The fifth column and the subsequent columns each indicate the user set size. In  FIG. 5B , “N/A” indicates that the sheet width set as the user set size and a detection result of the sheet size exhibit a mismatch, namely, a setting error is determined. When a setting error is determined, the CPU  201  causes the operating device  1003  to display the sheet size error screen illustrated in  FIG. 6  to notify the user of the setting error. “A” indicates that the sheet width set as the user set size and the automatic detection result exhibit a match, namely, no setting error is determined. 
     For example, in a case where the detection results indicate that the switch SW 3  is OFF, the switch SW 4  is ON, and the switch SW 5  is ON, it is understood from the table of  FIG. 5A  that the trailing edge regulating plate  814  is located at a position for regulating sheets having a length of 190.5 mm to 221.5 mm in the feeding direction. When the length in the feeding direction set as the user set size input on the sheet size setting screen is 190.5 mm to 221.5 mm, the automatically detected sheet size and the user set size match each other. Therefore, it is determined that the trailing edge regulating plate  814  is correctly set, and the sheet size error screen is not displayed. However, in a case where the length in the feeding direction set as the user set size input on the sheet size setting screen is longer than 221.5 mm, the user set size does not match the automatically detected sheet size, and hence the CPU  201  causes the operating device  1003  to display the sheet size error screen of  FIG. 6 . Through display of the sheet size error screen of  FIG. 6 , it is possible to prevent an occurrence of a feeding failure or a jam ascribable to the trailing edge regulating plate  814  (or side edge regulating plates  812  and  813 ) that is not set at a correct position. 
     Case of Placing Auxiliary Regulating Member 
     The trailing edge regulating plate  814  is formed so as to be movable by a distance between the arrows  8 X 1  and  8 X 2  illustrated in  FIG. 3A , and is capable of regulating a trailing edge position of the sheets having a length of 190.5 mm to 355.6 mm. However, a moving range of the trailing edge regulating plate  814  has a limit defined by an applicable maximum size and an applicable minimum size, and is not capable of regulating the sheets having a small size of less than 190.5 mm. For example, in a case where A5 sheets having a sheet length of 148.0 mm or C 6  envelopes having a sheet length of 162.0 mm or other such format are stored in the sheet feeding cassette, the trailing edge position of the sheets cannot be regulated by the trailing edge regulating plate  814 . In this manner, in a case where sheets having a size smaller than the sheet size that can be regulated by the trailing edge regulating plate  814  are used, in this embodiment, an auxiliary regulating member  901  is placed between the trailing edge regulating plate  814  and the sheets. The auxiliary regulating member  901  is an attachment mountable to the sheet feeding cassette  120   a , which is provided as an accessory to the sheet feeding cassette  120   a , and serves to regulate the trailing edge position of the sheets in place of the trailing edge regulating plate  814 . 
       FIG. 8A  and  FIG. 8B  are views for illustrating an example of how the auxiliary regulating member  901  is mounted to the trailing edge regulating plate  814  inside the sheet feeding cassette  120   a . The broken line of  FIG. 8B  indicates a position for the minimum size to which the trailing edge regulating plate  814  is allowed to slide. It is assumed that the auxiliary regulating member  901  may be placed between the sheets and the trailing edge regulating plate  814  in order to stack sheets having a size (for example, A5 size being 148.0 mm) less than the minimum size to the position of which the trailing edge regulating plate  814  is allowed to slide. 
     Flow Chart for Illustrating Processing for Displaying Sheet Size Error Screen 
       FIG. 7  is an explanatory flow chart for illustrating processing for displaying the sheet size error screen based on the user set size and a result of determination of the automatically detected size. The processing of  FIG. 7  is executed when the image forming apparatus is powered on, when the sheet feeding cassette  120   a  is closed, or when the user sets the sheet size. The steps illustrated in the flow chart of  FIG. 7  are executed by the CPU  201  in accordance with the program stored in the ROM  202 . 
     In Step S 1401 , the CPU  201  initializes information on the sheet feeding cassette  120   a  stored in the RAM  203  to “no sheet being present” and “sheet size being undefined”. Subsequently, in Step S 1402 , the CPU  201  determines whether or not the sheet feeding cassette  120   a  is mounted to the main body  819  based on detection results obtained by the switches SW 0 , SW 1 , and SW 2  of the sheet width detection sensor  818  and the switches SW 3 , SW 4 , and SW 5  of the sheet length detection sensor  817 . When all the switches SW 0  to SW 5  are in an off state, the CPU  201  determines that the sheet feeding cassette  120   a  is not mounted to the main body  819 . Meanwhile, when any one of the switches SW 0  to SW 5  is in an on state, the CPU  201  determines that the sheet feeding cassette  120   a  is mounted. When it is determined in Step S 1402  that the sheet feeding cassette  120   a  is mounted, the CPU  201  advances to the processing of Step S 1403 . In Step S 1403 , the CPU  201  detects the size of sheets based on the positions of the trailing edge regulating plate  814  and the side edge regulating plates  812  and  813 . This detection method is as described above with reference to  FIG. 5 . The sheet size detected in Step S 1403  is referred to as “automatically detected size”. 
     In Step S 1404 , the CPU  201  examines a sheet setting for each sheet feeding cassette, and determines whether or not the sheet setting is equal to a sheet setting made by the user via a sheet size setting screen of  FIG. 4B . When a result of the determination is “Yes” in Step S 1404 , the CPU  201  advances to Step S 1405 . When a result of the determination is “No” in Step S 1404 , the CPU  201  advances to Step S 1410 . When a result of the determination is “No” in Step S 1404 , the CPU  201  sets the information on the automatically detected sheet size (automatically detected size) for the sheet feeding cassette  120   a . That is, the sheet size identified in accordance with the detected positions of the trailing edge regulating plate  814  and the side edge regulating plates  812  and  813  is determined as the sheet size for the sheet feeding cassette  120   a.    
     In Step S 1405 , the CPU  201  determines whether or not the sheet length of the user set size in the feeding direction is less than a minimum length (predetermined length) that enables the movement of the trailing edge regulating plate  814 . Specifically, the CPU  201  determines whether or not the length (sheet length) in the feeding direction X of the user set size is in a range of equal to or greater than 148.0 mm and less than 190.5 mm. A sheet size within this range is a size less than the size of the sheets that can be regulated by the trailing edge regulating plate  814 . Therefore, the auxiliary regulating member  901  is assumed to be placed between the trailing edge regulating plate  814  and the sheets. In this embodiment, a comparison is performed with the automatically detected size in consideration of the length of the auxiliary regulating member  901  in the feeding direction X. In this embodiment, the length of an auxiliary regulating member in the feeding direction is 42.5 mm. In view of this, a length obtained by adding 42.5 mm to the user set size is regarded as the user set size. That is, in Step S 1405 , the CPU  201  sets a length having a value obtained by adding 42.5 mm to the user set size as the sheet length. 
     After that, in Step S 1408 , the CPU  201  compares the user set size and the automatically detected sheet size to determine whether or not both match each other. This determination is performed with reference to the tables of  FIG. 5A  and  FIG. 5B . When it is determined in Step S 1408  that the user set size and the automatically detected size do not match each other, the CPU  201  advances to the processing of Step S 1409 . In Step S 1409 , the CPU  201  causes the operating device  1003  to display the sheet size error screen illustrated in  FIG. 6 . The sheet size error screen illustrated in  FIG. 6  is a screen for notifying that the user set size received by the operation input device  306  and the automatically detected size exhibit a mismatch. The sheet size error screen illustrated in  FIG. 6  is also a screen for prompting the user to adjust the trailing edge regulating plate  814  and the side edge regulating plates  812  and  813  at positions suitable for the user set size. In this case, the CPU  201  maintains the state of the sheet feeding cassette  120   a  at the state of “no sheet being present” and “sheet size being undefined” set in the initialization of Step S 1401 . This imposes a limitation on the image formation on a sheet fed from the sheet feeding cassette  120   a.    
     Meanwhile, when it is determined in Step S 1408  that the user set size and the automatically detected size match each other, the CPU  201  advances to the processing of Step S 1410 . In Step S 1410 , the CPU  201  determines that the user set size is the size of the sheets stored in the sheet feeding cassette  120   a.    
     In Step S 1411 , as described above, the CPU  201  starts the lift-up operation for lifting up the sheets stacked on the sheet feeding cassette  120   a  to such a height as to enable sheet feeding. In Step S 1412 , the CPU  201  determines the presence or absence of a sheet based on the detection result obtained by the sheet presence sensor  820 . When it is determined in Step S 1412  that no sheet is present based on the detection result obtained by the sheet presence sensor  820 , the CPU  201  determines in Step S 1413  whether or not a predetermined time period has elapsed. When determining that the predetermined time has not elapsed, the CPU  201  returns to the processing of Step S 1412 . When it is determined in Step S 1413  that the predetermined time has elapsed, that is, when a sheet surface cannot be detected from the sheets stacked on the sheet feeding cassette  120   a  by the sheet presence sensor  820  even after a lapse of the predetermined time period, the CPU  201  stops the lift-up operation in Step S 1414 . In Step S 1415 , the CPU  201  determines that no sheet is stacked on the sheet feeding cassette  120   a . The CPU  201  replaces the information of “sheet size being undefined” set in Step S 1401  by the user set size or the automatically detected sheet size. That is, the CPU  201  sets the state of the sheet feeding cassette  120   a  to a state of having a sheet present, and stores the user set size or the automatically detected sheet size in the RAM  203 . 
     Meanwhile, when determining in Step S 1412  that a sheet is present based on the detection result obtained by the sheet presence sensor  820 , the CPU  201  advances to the processing of Step S 1416 . The CPU  201  determines that a sheet surface has been detected from the sheets stacked on the sheet feeding cassette  120   a , and the CPU  201  stops the lift-up operation in Step S 1416 . In Step S 1417 , the CPU  201  sets the user set size or the automatically detected sheet size as the sheet size for the sheet feeding cassette  120   a . That is, the CPU  201  sets the state of the sheet feeding cassette  120   a  to the state of having a sheet present, and stores the information on the user set size or the automatically detected size in the RAM  203 . Then, the CPU  201  controls the printer  1002  to execute image formation based on the information on the sheet size stored in the RAM  203 . 
     In Step S 1418 , the CPU  201  determines whether or not the sheet feeding cassette  120   a  is mounted to the main body  819  of the container. Specifically, the CPU  201  determines that the sheet feeding cassette  120   a  is mounted when any one of the switches SW 0  to SW 5  is in an on state based on the detection results obtained by the sheet width detection sensor  818  and the sheet length detection sensor  817 . Meanwhile, when all the switches SW 0  to SW 5  are in an off state, the CPU  201  determines that the sheet feeding cassette  120   a  is not mounted. When it is determined in Step S 1418  that the sheet feeding cassette  120   a  is absent, the CPU  201  returns to the processing of Step S 1401 , and the CPU  201  sets the state of the sheet feeding cassette  120   a  to the state of having no sheet present and the sheet size undefined. 
     According to this embodiment, when the length (sheet length) in the feeding direction of the user set size is longer than the predetermined length, a comparison is performed between the user set size and the automatically detected size. Meanwhile, when the length (sheet length) in the feeding direction of the user set size is shorter than the predetermined length, a comparison is performed between a value obtained by adding the length (42.5 mm) of the auxiliary regulating member to the user set size and the automatically detected size. That is, when sheets having a size smaller than a movable range of a trailing edge regulating plate is set, a comparison is performed between the value obtained by adding the length of the auxiliary regulating member to the user set size and the automatically detected size. With this configuration, the image forming apparatus including the container in which the auxiliary regulating member can be placed between the trailing edge regulating plate and the sheets can be caused to appropriately display a warning. 
     This embodiment has been described by taking the length (190.0 mm) of a minimum sheet that can be regulated by the trailing edge regulating plate  814  as an example of the predetermined length, but another reference value may be set as the predetermined length. For example, a length determined based on the size of the auxiliary regulating member  901  provided as an accessory may be set as the predetermined length. 
     Further, in this embodiment, the value obtained by adding the length of the auxiliary regulating member to the user set size is compared with the automatically detected size, but a value obtained by subtracting the length of the auxiliary regulating member from the automatically detected size may be compared with the user set size. 
     Further, as an example of displaying a warning, the example of displaying information for prompting the user to examine whether or not the trailing edge regulating plate and the side edge regulating plates are set at the positions suitable for the sheet size and the example of displaying the information for instructing the user to examine whether or not the user set size matches the size of the sheets supplied to the sheet feeding cassette have been described. However, contents to be displayed as the warning are not limited thereto. For example, only a warning for notifying the user of a sheet setting error may be displayed. 
     As described above, according to the present invention, the image forming apparatus including the sheet container in which an attachment can be mounted to the trailing edge regulating plate can be caused to appropriately display a warning. 
     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. 2017-199602, filed Oct. 13, 2017, which is hereby incorporated by reference herein in its entirety.