Patent Publication Number: US-2010111546-A1

Title: Image forming apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of U.S. Provisional Application No. 61/112,109, filed Nov. 6, 2008. 
    
    
     TECHNICAL FIELD 
     The present invention relates to an image forming apparatus, such as a printer or a digital multi-function peripheral, in which a sheet is fed from a containing section selected according to a set value of sheet size and an image is formed on the fed sheet. 
     BACKGROUND 
     Hitherto, an image forming apparatus, such as a printer or a digital multi-function peripheral, often has plural containing sections for containing sheets on which images are formed. The respective containing sections can contain sheets with sizes different from each other. In the image forming apparatus, in order to select a suitable size sheet, the sizes of sheets to be contained in the respective containing sections are set. However, there is a possibility that a user erroneously sets a sheet with a size different from the size set in the containing section. In the related art image forming apparatus, when a sheet with a size different from the set sheet size is set in the containing section, since there is a possibility that conveyance control is performed based on the set sheet size, a possibility of occurrence of a disadvantage such as a jam becomes high. For example, when the set sheet size is different from the size of the sheet actually set in the containing section, in the related art image forming apparatus, the rotation of a conveyance roller is stopped before the sheet is completely carried out, or a preceding sheet and a subsequent sheet collide with each other, and a jam can occur. As techniques to resolve the disadvantage due to the mismatch between the set sheet size and the actually set sheet size, the following techniques are disclosed. 
     JP-A-2001-348129 or JP-A-2003-246475 discloses a technique in which the length of a fed sheet in a conveyance direction is measured and the presence or absence of a size error is determined based on the measured result. 
     JP-A-2008-65309 discloses a technique in which a sheet is pulled into a reverse path without performing image formation, and the length of the sheet in the conveyance direction is measured, and further, the sheet pulled into the reverse path is again fed, and printing on the sheet is made possible. 
     However, in the related art, each time the printing process (print job) is performed, at least one sheet must be conveyed for the measurement of the length. 
     For example, in the image forming apparatus disclosed in JP-A-2001-348129 or JP-A-2003-246475, the length of the sheet under conveyance is measured while an image is being printed on the first sheet. In JP-A-2001-348129 or JP-A-2003-246475, when the set size and the actual sheet size are different from each other, it is conceivable that the first image is formed on the sheet (first sheet) with a size not intended by the user. 
     Besides, in the image forming apparatus disclosed in JP-A-2008-65309, each time the printing process (print job) occurs, the first sheet is made to pass through the reverse path, and therefore, it is conceivable that the performance is reduced. 
     SUMMARY 
     According to an aspect of the invention, an image forming apparatus includes a sheet feed section which feeds a sheet on which an image is formed, an image formation section which forms the image on the sheet fed from the sheet feed section, a first conveyance section which conveys the sheet from the sheet feed section to a discharge port through the image formation section, a second conveyance section which branches from a sheet conveyance path of the first conveyance section at a downstream side of the image formation section and again conveys the sheet to the image formation section, a storage section which stores a sheet size to be set for the sheet feed section, a measurement section which measures, if there is a possibility that a new sheet is set in the sheet feed section, a length of a sheet in a conveyance direction, which is conveyed by the first conveyance section, without performing image formation by the image formation section to the first sheet fed from the sheet feed section, and a control section which again conveys the sheet by the second conveyance section to the image formation section to form an image if the length of the sheet measured by the measurement section coincides with the sheet size stored in the storage section, and discharges the sheet if the length of the sheet measured by the measurement section does not coincide with the sheet size stored in the storage section. 
     Besides, according to another aspect of the invention, a control method used for an image forming apparatus includes storing a sheet size to be set for a sheet feed section to feed a sheet on which an image is formed in a memory, detecting a possibility that a new sheet is set in the sheet feed section, feeding a first sheet from the sheet feed section if it is detected that there is a possibility that the new sheet is set in the sheet feed section, conveying the first sheet fed from the sheet feed section through a first path including an image formation position, measuring a length of the first sheet in a conveyance direction, which is conveyed through the first path, without performing image formation on the sheet, again conveying the sheet through a second path branching from the first path to the image formation position to form an image if the measured length of the sheet coincides with the sheet size stored in the memory, and discharging the sheet if the measured length of the sheet does not coincide with the sheet size stored in the memory. 
     Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate embodiments of the invention, and together with the general description given above and the detailed description of the embodiments given below, serve to explain the principles of the invention. 
         FIG. 1  is a sectional view showing a structural example of a printer  1  as an image forming apparatus. 
         FIG. 2  is a block diagram showing a structure of a control system of a digital multi-function peripheral including a printer. 
         FIG. 3  is a flowchart for explaining a confirmation process of sheet width. 
         FIG. 4  is a flowchart for explaining a confirmation process of sheet length. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, embodiments will be described in detail with reference to the drawings. 
     First, a structure of a printer  1  as an image forming apparatus will be described. 
       FIG. 1  is a sectional view showing a structure example of the printer  1  as the image forming apparatus. It is supposed that the printer  1  shown in  FIG. 1  is applied to a digital multi-function peripheral including a scanner, an external interface and the like. 
     The printer  1  includes an upper cassette  11 , a lower cassette  12  and a manual feed tray  13 , which are a sheet feed section to feed a sheet on which an image is printed. The upper cassette  11  and the lower cassette  12  include cassettes as containing sections to contain sheets. The user sets sheets in the upper cassette  11  and the lower cassette  12 . The user is required to set the sheets with sizes specified for the upper cassette  11  and lower cassette  12  into the containing section. However, as a physical structure, a sheet with a size other than the specified sheet size can also be set in the upper cassette  11  and the lower cassette  12 . 
     The upper cassette  11  includes an open and close detector  11   a,  a tray  11   b,  a conveyance guide  11   c  and a sheet detection sensor  11   d.  The lower cassette  12  includes an open and close detector  12   a,  a tray  12   b,  a conveyance guide  12   c  and a sheet detection sensor  12   d.    
     Each of the upper cassette  11  and the lower cassette  12  includes a mechanism (open and close mechanism) by which the sheet containing section can be pulled out so that the user sets a sheet. For example, when the sheet is set in the upper cassette  11 , the user pulls out the upper cassette  11 . After setting the sheet in the pulled-out upper cassette  11 , the user returns the upper cassette  11  to the original position. In a similar procedure, the user can also set sheets in the lower cassette  12 . 
     The open and close detector  11   a  detects that the upper cassette  11  is pulled out. The open and close detector  12   a  detects that the lower cassette  12  is pulled out. In an after-mentioned control system, it is determined by the detection result of the detector  11   a  or  12   a  that the once pulled-out upper cassette  11  or lower cassette  12  is again set. 
     The user sets sheets in the state where the upper cassette  11  or the lower cassette  12  is pulled out. Thus, when the open and close detector  11   a  or  12   a  detects that the upper cassette  11  or the lower cassette  12  is pulled out (opened), it is possible to determine that there is a possibility that a sheet is newly set in the pulled-out upper cassette  11  or lower cassette  12 . 
     Besides, in a state where the power source of the printer  1  is off, the open and close detector  11   a  or  12   a  does not function. Thus, in the state where the power source of the printer  1  is turned off, the opening is not detected by the open and close detector  11   a  or  12   a,  and there is a possibility that a new sheet is set in the upper cassette  11  or the lower cassette  12 . That is, it is possible to determine that immediately after the power source of the printer  1  is turned on, there is a possibility that a sheet is newly set in the upper cassette  11  or the lower cassette  12 . 
     The trays  11   b  and  12   b  respectively have functions to push up sheets contained in the upper cassette  11  and the lower cassette  12 . The sheets pushed up by the trays  11   b  and  12   b  are pressed against an upper sheet feed roller  14  and a lower sheet feed roller  15 . 
     The conveyance guides  11   c  and  12   c  guide a sheet in a sheet width direction (direction orthogonal to a sheet conveyance direction). Each of the conveyance guides  11   c  and  12   c  includes a movement mechanism to be moved according to the set sheet size. The user adjusts the conveyance guides  11   c  and  12   c  according to the width of the set sheet. 
     Each of the conveyance guides  11   c  and  12   c  has a function to output information indicating the present position (corresponding to the set sheet width). Since the conveyance guides  11   c  and  12   c  are adjusted according to the sheet width, it may be determined that the present positions of the conveyance guides  11   c  and  12   c  indicate the sheet widths set for the conveyance guides  11   c  and  12   c.  That is, the conveyance guides  11   c  and  12   c  function also as width detectors to detect the set sheet width. 
     The sheet detection sensors  11   d  and  12   d  are sensors to detect the presence or absence of a sheet. The sheet detection sensors  11   d  and  12   d  outputs signals indicating whether or not a sheet exists in the upper cassette  11  or the lower cassette  12 . 
     The manual feed tray  13  is a tray on which the user can place a sheet in an untouched state (that is, without pull out). The user is required to place a sheet with a set size on the manual feed tray  13 . However, as a physical structure, a sheet with a size other that the set sheet size can also be set on the manual feed tray  13 . 
     The manual feed tray  13  includes a conveyance guide  13   c  and a sheet detection sensor  13   d.  The manual feed tray  13  includes a mechanism to push up the placed sheet. The sheet set on the manual feed tray  13  is pushed up and contacts with a manual sheet feed roller  16 . 
     The conveyance guide  13   c  guides the sheet, which is set on the manual feed tray  13 , in the width direction (direction orthogonal to the sheet conveyance direction). The conveyance guide  13   c  includes a movement mechanism moved according to the set sheet size. The user adjusts the conveyance guide  13   c  according to the width of the set sheet. The conveyance guide  13   c  has a function to output information indicating the present position (corresponding to the set sheet width). Since the conveyance guide  13   c  is adjusted according to the sheet width, it may be determined that the present position of the conveyance guide  13   c  indicates the width of the sheet set on the manual feed tray  13 . That is, the conveyance guide  13   c  functions also as a width detector to detect the width of the sheet set on the manual feed tray  13 . 
     The sheet detection sensor  13   d  detects the presence or absence of a sheet on the manual feed tray  13 . The sheet detection sensor  13   d  outputs a signal indicating whether or not a sheet exists on the manual feed tray  13 . 
     The upper sheet feed roller  14  takes out sheets contained in the upper cassette  11  one by one. The lower sheet feed roller  15  takes out sheets contained in the lower cassette  12  one by one. The manual feed roller  16  takes out sheets placed on the manual feed tray  13  one by one. A lower conveyance roller  17  conveys the sheet taken out by the lower sheet feed roller  15  to an upper conveyance roller  18 . A manual conveyance roller  19  conveys the sheet taken out by the manual feed roller  16  to the upper conveyance roller  18 . The upper conveyance roller  18  conveys the sheet taken out by the upper sheet feed roller  14 , the sheet conveyed by the lower conveyance roller  17 , or the sheet conveyed by the manual conveyance roller  19  to a register roller  20 . 
     The register roller  20  conveys the sheet at a desired timing (timing when an image is transferred to the sheet). The sheet conveyed by the upper conveyance roller  18  is once stopped at the time point when the leading edge of the sheet reaches the register roller  20 . The register roller  20  starts, at a desired timing, to convey the sheet whose conveyance is once stopped. The register roller  20  rotates to convey the sheet at a constant conveyance speed. That is, the conveyance speed of the sheet by the register roller  20  is the constant conveyance speed. 
     A register sensor  20   a  is installed just before the register roller  20 . The register sensor  20   a  detects the sheet reaching the register roller  20  and the sheet conveyed by the register roller  20 . For example, the register sensor  20   a  outputs an on signal in a state where the sheet is detected, and outputs an off signal in a state where the sheet is not detected. When the output signal of the register sensor  20   a  is changed from off to on, it is determined that the sheet conveyed by the upper conveyance roller  18  reaches the register roller  20 . When the output signal of the register sensor  20  is changed from on to off, it is determined that the conveyance of the sheet by the register roller  20  is ended. The after-mentioned control system has a function to determine the length of the sheet by the conveyance speed of the sheet and the time from turning-on of the register sensor  20   a  to turning-off thereof (that is, the time in which the register roller  20  conveys the sheet). 
     Plural image formation sections ( 21 K,  21 C,  21 M,  21 Y) are disposed to face an exposure unit  22  and an intermediate transfer belt  23 . The exposure unit  22  forms electrostatic latent images as respective color images on image carriers in the respective image formation units ( 21 K,  21 C,  21 M,  21 Y). The intermediate transfer belt  23  is an intermediate transfer body as a transfer target body. A drive roller  24   a  and a support roller  24   b  hold the intermediate transfer belt  23  as the transfer target body at a suitable tension. The intermediate transfer belt  23  is driven in accordance with the rotation of the drive roller  24   a.  The respective image formation sections ( 21 K,  21 C,  21 M,  21 Y) form toner images on the intermediate transfer belt  23  by developing the electrostatic latent images with respective color toners. 
     The respective image formation sections  21 Y ( 21 M,  21 C,  21 K) respectively include photoconductive drums Y 1  (M 1 , C 1 , K 1 ), charging rollers Y 2  (M 2 , C 2 , K 2 ), exposure sections Y 3  (M 3 , C 3 , K 3 ), developing units Y 4  (M 4 , C 4 , K 4 ), transfer rollers Y 5  (M 5 , C 5 , K 5 ), cleaners Y 6  (M 6 , C 6 , K 6 ) and toner cartridges Y 7  (M 7 , C 7 , K 7 ). The respective image formation sections  21 Y,  21 M,  21 C and  21 K have the same structure. Thus, the image formation section  21 Y will be described, and the detailed description of the respective image formation sections  21 M,  21 C and  21 K other than the image formation section  21 Y will be omitted. 
     The photoconductive drum Y 1  is the image carrier. The photoconductive drum Y 1  includes an organic or amorphous silicon photoconductive layer on a conductive substrate. For example, the photoconductive drum Y 1  is an organic photoconductive body charged to a minus polarity. The charging roller Y 2  uniformly charges the surface of the photoconductive drum Y 1  rotated by a not-shown motor. The exposure unit  22  emits image modulated laser light. The exposure unit  22  irradiates the laser light to the exposure section Y 3  of the photoconductive drum Y 1  through a polygon mirror and a not-shown optical member. The exposure section Y 3  is a surface area of the photoconductive drum Y 1  which was charged by the charging roller Y 2 . An electrostatic latent image is formed on the surface of the photoconductive drum Y 1  exposed by the laser light in the exposure section Y 3 . 
     The developing unit Y 4  develops (visualizes) the electrostatic latent image formed on the surface of the photoconductive drum Y 1 . The developing unit Y 4  develops the electrostatic latent image by a two-component developing system in which a nonmagnetic toner charged to a negative polarity and a magnetic toner are mixed. The toner does not adhere to a part (unexposed part) of the surface of the photoconductive drum Y 1  where the laser light is not irradiated, and the toner adheres to a part (exposed part) irradiated with the laser light. The photoconductive drum Y 1  holds the toner image appearing by the adhesion of the toner. Incidentally, the developing system is not limited to the two-component developing system, and a system such as contact one-component development, non-contact one-component development, or conductive one-component development can also be applied. 
     The toner image formed on the surface of the photoconductive drum Y 1  is transferred to the intermediate transfer belt  23  as the transfer target body. The transfer roller Y 5  as the transfer member contacts with the back of the intermediate transfer belt  23  when viewed from the photoconductive drum Y 1  side. The transfer roller Y 5  supplies an electric field for transferring the toner image from the back of the intermediate transfer belt  23 . The cleaner Y 6  removes the remaining toner remaining on the surface of the photoconductive drum Y 1  after transfer. The cleaner Y 6  is provided on this side of the charging roller Y 2  in the rotation direction of the photoconductive drum Y 1 . Besides, the toner cartridge Y 7  contains the toner supplied to the developing unit Y 4 . 
     The respective image formation sections  21 Y,  21 M,  21 C and  21 K superimpose the toner images developed with the respective color toners on the intermediate transfer belt  23  and transfer them (primary transfer). A color image in which the respective color toner images are superimposed on each other is formed on the intermediate transfer belt  23 . The color image of the plural color toners transferred on the intermediate transfer belt  23  is transferred to the sheet at a secondary transfer position. The secondary transfer position is the position where the toner images on the intermediate transfer belt  23  are transferred to the sheet. The secondary transfer position is the position where the support roller  24   b  and a secondary transfer roller  25  face each other. 
     The register roller  20  conveys the sheet to a secondary transfer position  25   a  as the image transfer position in timing with the toner image on the intermediate transfer belt  23 . The toner image on the intermediate transfer belt  23  is transferred to the sheet at the secondary transfer position  25   a.  When the support roller  24   b  is grounded, a positive (+) polarity bias is applied to the secondary transfer roller  25  in order to transfer the toner to the sheet P as the second transfer target body. Incidentally, a structure may be such that the secondary transfer roller  25  is grounded, and a negative polarity bias is applied to the support roller  24   b.  The sheet on which the toner image is transferred at the secondary transfer position passes through a fixing unit  26 . The fixing unit  26  fixes the toner image transferred on the sheet to the sheet. 
     The fixing unit  26  conveys the sheet to a paper discharge roller  27 . The paper discharge roller  27  discharge the sheet to a discharge section  28 . A reverse gate  29  is provided between the fixing unit  26  and the paper discharge roller  27 . The reverse gate  29  guides the sheet conveyed from the fixing unit  26  to the paper discharge roller  27 . The reverse gate  29  guides the sheet switched back from the paper discharge roller  27  to an auto duplex unit (ADU)  30 . The ADU  30  includes plural conveyance rollers  31 ,  32  and  33 . The plural conveyance rollers  31 ,  32  and  33  convey the sheet switched back from the paper discharge roller  27  to the register roller  20 . 
     Incidentally, the structure example shown in  FIG. 1  is the printer to form an image on a sheet by an electrophotographic system. However, the embodiment is not limited to the printer of the electrophotographic system. For example, the embodiment can be applied also to an ink jet system printer or a thermal transfer system printer. 
     Next, the structure of the control system in the printer  1  will be described. 
       FIG. 2  is a block diagram showing the structure of the control system of the digital multi-function peripheral including the printer  1 . 
     As shown in  FIG. 2 , the control system of the digital multi-function peripheral includes a main control section  41 , an operation section  42 , a scanner  43  and the printer  1 . 
     The operation section  42  is a user interface to which an operation instruction from a user is inputted. The operation section  42  includes, for example, a hard key such as a numeric keypad, and a display section  42   a  having a built-in touch panel. The display section  42   a  of the operation section  42  displays an operation guide, a touch key (icon) capable of being selected by the touch panel or the like. For example, the operation section  42  detects through the touch panel that the user touches the touch key (icon) displayed on the display section  42   a.  Besides, the display section  42   a  of the operation section  42  displays a guide indicating that printing is possible, together with the guide of the operation, and displays a guide of error contents when a print error occurs. 
     The scanner  43  functions as an image read section to convert an image on a document surface into image data. The scanner  43  converts, for example, the image of the document into color or monochrome digital image data. The scanner  43  includes a scanning section (not shown) to optically scan the document surface, a photoelectric conversion section (not shown), such as a CCD line sensor, to convert the reflected light from the document surface optically scanned by the scanning section into an electric signal, and the like. 
     The main control section  41  controls the whole digital multi-function peripheral including the printer  1 . The main control section  41  includes a main CPU  51 , a RAM  52 , a ROM  53 , a non-volatile memory  54 , an image processing section  55 , a page memory  56 , a HDD  57  and an external I/F  58 . 
     The main CPU  51  performs control of the respective sections in the main control section  41 , data processing and the like. The main CPU  51  executes a control program stored in the ROM  53 , the non-volatile memory  54  or the HDD  57 , and realizes various processes and various functions. For example, the main CPU  51  controls the scanner  43  and the printer  1  by a control program for copying, and realizes copying as one of print jobs. Besides, the main CPU  51  controls the external I/F  58  and the printer  1  by a control program for printing, and realizes network printing as one of the print jobs. 
     The RAM  52  is a memory for temporarily storing working data or for storing reference data. The ROM  53  is an unrewritable non-volatile memory. The ROM  53  stores, for example, the control program for controlling the digital multi-function peripheral, control data and the like. 
     The non-volatile memory  54  is a rewritable non-volatile memory. The non-volatile memory  54  includes an EEPROM or a flash ROM. The non-volatile memory  54  stores system setting information. In this embodiment, the non-volatile memory  54  stores set values of sizes of sheets to be set in the upper cassette  11  and the lower cassette  12  and on the manual feed tray  13  which are plural sheet feed sections. Incidentally, the set values of the sizes of the sheets in the upper cassette  11 , the lower cassette  12  and the manual tray  13  are values indicating at least the sheet width and the sheet length in the sheet conveyance direction. 
     The image processing section  55  performs various image processes on image data. The image processing section  55  performs image processes such as correction of image data, compression thereof or expansion thereof. For example, when a copying process is performed, the image processing section  55  performs image processes, such as shading correction, gradation correction, and in-line correction, on the image data read by the scanner  43 . 
     The page memory  56  is a memory having a storage area in which image data of at least one page is expanded. The HDD  57  is a large capacity storage device. The HDD  57  is used also as a backup memory for various data. The HDD  57  may store various set data or management data. 
     The external I/F  58  is an interface to perform network communication. The external I/F  58  is, for example, an interface for data communication with an external device connected to a local area network (LAN) through network cable or wireless communication. For example, when the printer  1  functions as a network printer, the external I/F  58  receives image data for printing. 
     The control system of the printer  1  includes a printer CPU  61 , a RAM  62 , a ROM  63 , the ADU  30 , a conveyance control section  64 , an exposure control section  65 , a development control section  66 , a transfer control section  67 , a fixing control section  68  and the like. 
     The printer CPU  61  controls the respective sections in the printer  1 . The printer CPU  61  realizes various functions by executing a control program stored in the ROM  63  or the like. The printer CPU  61  performs a process of forming an image on a sheet according to an instruction from the main control section  41 . Besides, the conveyance control section  64 , the exposure control section  65 , the development control section  66 , the transfer control section  67  and the fixing control section  68  may be made functions realized by execution of the control program by the printer CPU  61 . 
     The printer CPU  61  inputs detection signals of the respective detectors (the open and close detectors  11   a  and  12   a,  the sheet detection sensors  11   d,    12   d  and  13   d,  and the conveyance guides  11   c,    12   c  and  13   c  which function as the width detectors). The printer CPU  61  notifies the detection signals of the respective detectors to the main control section  41 . However, the detection signals of the respective detectors may be directly inputted to the main CPU  51  of the main control section  41 . 
     The RAM  62  is a memory for temporarily storing working data or for storing reference data. The ROM  63  is an unrewritable non-volatile memory. The ROM  63  stores, for example, a control program for controlling the printer  1 , control data and the like. 
     The conveyance control section  64  controls the conveyance of sheets taken out from the respective sheet feed cassettes  11  and  12  and the manual feed tray  13 . The conveyance control section  64  controls the operations of the sheet feed rollers  14 ,  15  and  16 , the conveyance rollers  17 ,  18  and  19 , the register roller  20  and the like based on the operation instructions of the printer CPU  61 . For example, the conveyance control section  64  drive controls the respective conveyance rollers in the ADU  30 , and performs conveyance control to again supply the sheet, which was once conveyed to the paper discharge roller  27 , to the register roller  20 . 
     The exposure control section  65  controls the operation of the exposure unit  22 . The exposure control section  65  controls the exposure unit  22 , so that laser light is irradiated to the surfaces of the respective photoconductive drums Y 1 , M 1 , C 1  and K 1  as the image carriers and electrostatic latent images are formed. The development control section  66  controls so that the respective developing units Y 4 , M 4 , C 4  and K 4  supply toners to the respective photoconductive drums Y 1 , M 1 , C 1  and K 1 . The transfer control section  67  performs control to transfer the toner images formed on the respective photoconductive drums Y 1 , M 1 , C 1  and K 1  onto the transfer belt  23 , and control to transfer the toner images formed on the transfer belt  23  to the sheet. The fixing control section  68  controls the fixing process of the toner images to the sheet. 
     Next, the operation of the printer  1  will be described. 
       FIG. 3  and  FIG. 4  are flowcharts for explaining a confirmation process of a size of a sheet set in the upper cassette  11  or the lower cassette  12  or on the manual feed tray  13 . 
     First, the process of confirming the sheet width (length of the sheet in a direction orthogonal to the conveyance direction) will be described. 
       FIG. 3  is a flowchart for explaining the process to confirm the sheet width (length of the sheet in the direction orthogonal to the conveyance direction). 
     Immediately after the power source of the printer  1  is turned on (ACT  10 , YES), the main CPU  51  detects the widths of the sheets set in the upper cassette  11 , the lower cassette  12  and the manual feed tray  13  (ACT  11 ). Besides, also immediately after a sheet is put in and out (ACT  10 , YES), the main CPU  51  detects the width of the sheet set in the cassette  11 ,  12  or the tray  13  in which the sheet is put in and out (ACT  11 ). The main CPU  51  stores information indicating the detected sheet width in the memory such as the RAM  52  or the non-volatile memory  54  (ACT  12 ). 
     Here, as shown in  FIG. 1 , the upper cassette  11 , the lower cassette  12 , and the manual feed tray  13  include the conveyance guides  11   c,    12   c  and  13   c  adjusted according to the sheet width. The respective conveyance guides  11   c,    12   c  and  13   c  output information indicating the present position (corresponding to the sheet width). The main CPU  51  detects the width of the sheet set in the upper cassette  11  or the lower cassette  12  or on the manual feed tray  13  based on the information indicating the position (corresponding to the sheet width) from the respective conveyance guides  11   c,    12   c  and  13   c.    
     When the width of the sheet set in the upper cassette  11  or the lower cassette  12  or on the manual feed tray  13  is detected, the main CPU  51  determines whether or not the detected sheet width coincides with the set value for each of the upper cassette  11 , the lower cassette  12  and the manual feed tray  13  (ACT  13 ). 
     When it is determined in the above determination that the detected sheet width and the set value coincide with each other (ACT  13 , YES), the main CPU  51  causes the display section  42   a  of the operation section  42  to display that printing is possible (ACT  14 ). In the state where the display section  42   a  displays that printing is possible, the main CPU  51  becomes in the state where the printing process can be executed. That is, when a request for print starting is received (ACT  16 , YES), and when the state is such that the display section  42   a  displays that printing is possible (ACT  17 , NO), the main CPU  51  starts to print. When printing is started, the main CPU  51  gives a print job indicating print contents to the printer CPU  61 . 
     When it is determined that the detected sheet width does not coincide with the set value (ACT  13 , NO), the main CPU  51  causes the display section  42   a  of the operation section  42  to display an error guide indicating that the sheet size is different from the set value (ACT  15 ). In the state where the display section  42   a  displays the error guide, the main CPU  51  makes print starting impossible. That is, when a request for print starting is received (ACT  16 , YES), and when the state is such that the display section  42   a  displays the error guide (ACT  17 , YES), the main CPU  51  makes print execution impossible. 
     When the display section  42   a  displays the error guide (that is, when printing becomes impossible due to the inconsistency in size), the user again sets a sheet with a width matching with the set value or changes the set value of the sheet size. 
     For example, when the user again sets a sheet, the main CPU  51  detects the putting in and out of the sheet by the cassette detector  11   a,    12   a  or the sheet detector  11   d,    12   d,    13   d,  and again executes the process from ACT  10 . 
     When the user changes (sets again) the set value of the sheet size (ACT  18 , YES), the main CPU  51  determines whether or not the detected sheet width coincides with the sheet size set again (again set value) (ACT  19 ). 
     When it is determined in the above determination that the detected sheet width coincides with the again set value (ACT  19 , YES), the main CPU  51  causes the display section  42   a  of the operation section  42  to display that printing is possible, instead of the error guide (ACT  20 ). When the display section  42   a  displays that printing is possible, the main CPU  51  becomes in the state where the print process can be executed. When it is determined that the detected sheet width does not coincide with the again set value (ACT  19 , NO), the main CPU  51  causes the display section  42   a  to display the error guide, which is being displayed, as it is (ACT  21 ). When the display section  42   a  displays the error guide, the main CPU  51  makes print starting impossible. 
     According to the process, in the printer, immediately after the power source is turned on or immediately after the putting in and out of the sheet is performed, the width of the actually set sheet is detected, and it is checked whether the detected sheet width coincides with the set value. As a result of this, when it is determined that the sheet with the width different from the set value is set at power-on or immediately after the sheet is set, printing is made impossible, and inconsistency in sheet width can be notified to the user. 
     Besides, in the printer, the result of detection of the width of the actually set sheet is stored in the memory, and in neither the case of immediately after power-on nor the case of immediately after putting in and out of the sheet, but immediately after the sheet size is again set, it is checked whether the detected sheet width stored in the memory coincides with the again set value. As a result of this, when the setting of the sheet size is changed, even if the sheet width is not detected, it can be determined whether the width of the actually set sheet coincides with the set value. 
     Next, the process of confirming the length of a sheet in the conveyance direction will be described. 
       FIG. 4  is a flowchart for explaining the process of confirming the length of the sheet (length of the sheet in the conveyance direction). 
     When print starting is possible, the main CPU  51  gives a print job indicating print contents to the printer CPU  61 . In the print job, for example, a cassette or a tray as a sheet feed source is specified. When the print job is given, the printer CPU  61  determines whether or not the state is immediately after the power source of the printer  1  is turned on or immediately after putting in and out of the sheet is performed in the cassette or tray specified by the print job (ACT  31 ). When it is determined that the state is immediately after the power source is turned on or immediately after the putting in and out of the sheet is performed (ACT  31 , YES), the printer CPU  61  starts an operation of feeding the first sheet from the specified cassette  11 ,  12  or the manual feed tray  13  (ACT  32 ). 
     When the first sheet is fed (ACT  33 , YES), the printer CPU  61  guides the sheet to the ADU  30  (ACT  34 ), and measures the length of the sheet in the conveyance direction (ACT  35 ). The length of the sheet in the conveyance direction is calculated based on a period of time from a time when the register roller  20  starts to convey the sheet to a time when the register sensor  20   a  stops detecting the sheet, and the conveyance speed. The printer CPU  61  stores information indicating the measured length of the sheet into the memory such as the RAM  62 , the RAM  52  or the non-volatile memory  54  (ACT  36 ). 
     When the length of the sheet is measured, the printer CPU  61  determines whether or not the sheet length obtained as the measurement result coincides with the set value of the sheet size for the cassette  11 ,  12  or the manual feed tray  13  from which the sheet is fed (ACT  37 ). 
     When it is determined in the above determination that the measured sheet length coincides with the set value (ACT  37 , YES), the printer CPU  61  prints an image of a first page to the first sheet conveyed to the register roller  20  again by the ADU  30  (ACT  38 ). When printing of the image of the first page is successful, the printer CPU  61  continuously performs, as the print process of the second and subsequent pages, the process of sequentially printing images of the second and subsequent pages without measuring the sheet length (ACT  38 ). 
     When it is determined that the measured sheet length does not coincide with the set value (ACT  37 , NO), the printer CPU  61  does not print on the sheet conveyed to the register roller  20  again by the ADU  30 , and discharges the sheet to the outside (ACT  39 ). Incidentally, in the printer in which the sheet length can be measured without guiding the sheet to the ADU  30 , when it is determined that the measured sheet length does not coincide with the set value, the printer CPU  61  may discharge the sheet without causing it to pass through the ADU  30 . 
     When the sheet is discharged without printing of an image, the printer CPU  61  notifies the main CPU  51  that a print error occurs because the measured sheet length does not coincide with the set value. The main CPU  51  receiving the notification of the print error cause the display section  42   a  of the operation section  42  to display the error guide indicating that the measured sheet length is different from the set value (ACT  40 ). When the display section  42   a  displays the error guide, the main CPU  51  makes printing impossible. 
     When the error guide indicating that the measured sheet length is different from the set value is displayed (that is, when printing becomes impossible due to inconsistency in size), the user again sets a sheet with a size coincident with the set value for the cassette  11 ,  12  or the manual feed tray  13  in which the error occurs, or changes the set value of the sheet size for the cassette  11 ,  12  or the manual feed tray  13  in which the error occurs. 
     For example, when the user again sets the sheet, the main CPU  51  detects the putting in and out of the sheet by the cassette detector  11   a,    12   a  or the sheet detector  11   d,    12   d,    13   d.  When the putting in and out of the sheet is detected, the printer CPU  61  again executes the process from ACT  32  in the immediate print job. 
     Besides, when the user changes (again sets) the set value of the sheet size (ACT  41 , YES), in the print job immediately after the setting is again performed, the printer CPU  61  determines whether or not the measured value of the sheet length stored in the memory such as the RAM  62  coincides with the again set sheet size (again set value) (ACT  42 ). Incidentally, immediately after only the setting of the sheet size is again performed, the printer  1  is in neither the state of immediately after power-on nor the state of immediately after putting in and out of the sheet. Thus, it is conceivable that even if the operation of actually measuring the sheet length is not performed, the printer CPU  61  has only to determine whether the measurement value of the sheet length stored in the memory coincides with the again set value. 
     When it is determined in the above determination that the measurement value of the sheet length stored in the memory coincides with the again set value (ACT  42 , YES), the printer CPU  61  causes the display section  42   a  of the operation section  42  to display that printing is possible, instead of the error guide, and executes the print process corresponding to the print job (ACT  43 ). 
     When it is determined that the measured sheet length does not coincide with the again set value (ACT  42 , NO), the printer CPU  61  notifies the main CPU  51  of the print error due to inconsistency between the sheet length and the set value. The main CPU  51  receiving the notification of the print error causes the display section  42   a  of the operation section  42  to display the error guide indicating that the measured sheet length is different from the again set value (ACT  44 ). When the display section  42   a  displays the error guide, the main CPU  51  makes printing impossible. 
     According to the above process, in the printer  1 , immediately after the power source is turned on, or immediately after the putting in and out of the sheet is performed, the fed sheet is once conveyed to the ADU to actually measure the length of the sheet, and it is checked whether the measured sheet length coincides with the set value. As a result of this, when the sheet with the length different from the set value is set at power-on or immediately after the sheet is set, printing is stopped, and the inconsistency in sheet size can be informed to the user. 
     Besides, in the printer, the result of measurement of the length of the actually set sheet is stored in the memory, and in neither the state of immediately after power-on nor the state of immediately after putting in and out of the sheet, immediately after the sheet size is again set, it is checked whether the measured sheet length stored in the memory coincides with the again set value. As a result of this, when the setting of the sheet size is changed, even if the measurement operation of the sheet length is not performed, it can be determined whether the length of the actually set sheet coincides with the set value. 
     Incidentally, the process of ACT  10  to ACT  17  shown in  FIG. 3  is premised on the structure in which the sensor to detect the width of the sheet is provided in the sheet feed section (the upper cassette  11 , the lower cassette  12  or the manual feed tray  13 ) of the printer  1 . However, the printer  1  can have a structure in which the sensor to detect the width of the sheet is not provided in the upper cassette  11 , the lower cassette  12  or the manual tray  13 . In the printer having the structure as stated above, the process shown in  FIG. 3  is omitted, and the process shown in  FIG. 4  described later may be performed. Even when the process of  FIG. 3  is omitted, since the length in the conveyance direction can be confirmed, the effect of preventing the occurrence of a jam or the like in conveyance can be obtained. 
     Besides, although the manual feed tray provided in the printer often has the conveyance guide capable of detecting the sheet width, it seldom has a sensor to detect the sheet length. That is, in many manual feed trays used for printers, only the sheet width can be detected. In view of such circumstances, it is conceivable that the process as shown in  FIG. 4  is suitable for the process of checking the size of the sheet set on the manual feed tray. 
     Besides, the manual feed tray has the structure in which the user can easily set sheets. Thus, a sheet with a size different from the set sheet size is liable to be erroneously set on the manual feed tray. Further, in the manual feed tray having the conveyance guide, it is conceivable that although an error in the sheet width hardly occurs by virtue of the conveyance guide, an error in the sheet length is liable to occur. Accordingly, it is conceivable that when the process shown in  FIG. 4  is applied to the sheet fed from the manual feed tray, many disadvantages due to inconsistency between the set sheet size and the actual sheet size can be prevented. 
     Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiment shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.