Abstract:
An image forming apparatus includes: a recording section which records therein image data, and processing position information of a sheet; an image forming section which forms an image on the sheet on the basis of the image data recorded in the recording section; a position detector which detects a pre-processed position on a sheet to be fed; and a controller which compares the processing position information with a value detected by the position detector, judges whether or not an abnormality is present, and corrects an image forming position of the image forming section for each block of a plurality of blocks surrounded by either processed portions of a sheet or a processed portion and an edge of the sheet.

Description:
[0001]    This application is based on Japanese Patent Application No. 2006-155795 filed on Jun. 5, 2006, which is incorporated hereinto by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    The present invention relates to an image forming apparatus, such as a copying machine, a facsimile, and a printer, and in particular, to an image forming apparatus which forms an image on a perforated sheet. 
         [0003]    In recent years, image forming apparatuses incorporating an electro-photographic method are used in the field of short-run printing, such as a POD (Print On Demand), whereby the image forming apparatus is required to meet the various needs relating to image quality and recording members. Specifically concerning the recording members, a previously perforated or a previously folded sheet is used, hereinafter, such recording sheet is referred to as a “pre-processed sheet”. 
         [0004]    When such pre-processed sheet is used, and when image formation is conducted based on a leading edge of the pre-processed sheet, and when a processed position varies against the leading edge, image formation may be adversely conducted on the processed position. If image formation is conducted on such processed position, in the case of perforated sheets, the image cannot be recognized after separation, and in the case of folded sheet, toner is not completely fixed on the folded section in a post-finishing process after image formation. 
         [0005]    In order to overcome these problems, Unexamined Japanese Patent Application Publication No. 02-175,171 discloses a method in which when image formation is conducted on continuous perforated sheets, the perforation is detected by a detecting circuit using ultrasonic waves, whereby image formation is so controlled as not to be conducted on the perforation. Further, Unexamined Japanese Patent Application Publication No. 05-229,193 discloses a method in which the sheet is registered after the perforation is detected by laser beams. 
         [0006]    Further, another technology is proposed to register the sheet and the image, in Unexamined Japanese Patent Application Publication No. 07-131,599, a format sheet is used which carries a previously printed filling frame, a reading section reads the position of the filling frame, and signals detected by the reading section are compared with previously set frame information, whereby the image position is correctly positioned. 
         [0007]    However, Unexamined Japanese Patent Application Publication Nos. 02-175,171, and 05-229,193 disclose the perforation detection and registration method on the series of perforated sheets whose perforation is perpendicular to the conveyance direction of the perforated sheets, however, both of which do not disclose the application of the series of perforated sheets whose perforation is parallel to the conveyance direction of the perforated sheets. Further, in Unexamined Japanese Patent Application Publication No. 07-131,599, the formatted sheet is limited in use of the previously printed filling frame. 
         [0008]    Additionally, in Unexamined Japanese Patent Application Publication No. 07-131,599, the intended objects to be corrected are a starting position and timing, for main scanning and sub-scanning, that is, the position of the total image is corrected on the sheet, but no printing position is corrected by using plural frame references. 
       SUMMARY OF THE INVENTION 
       [0009]    One aspect of the present invention is an image forming apparatus, including: a recording section which records therein image data, and processing position information of a sheet; an image forming section which forms an image on the sheet on the basis of the image data recorded in the recording section; a position detector which detects a pre-processed position on the sheet; and a controller which compares the processing position information with a value detected by the position detector, judges whether or not an abnormality is present, and corrects an image forming position of the image forming section for each block of a plurality of blocks surrounded by either processed portions of a sheet or a processed portion and an edge of the sheet. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  shows relevant parts of an image forming apparatus relating to an embodiment of the present invention. 
           [0011]      FIG. 2  is a block diagram which shows a control sequence of an image forming apparatus relating to an embodiment of the present invention. 
           [0012]      FIGS. 3(   a ) and  3 ( b ) show the relationship between processing position detector  20  and sheet S. 
           [0013]      FIG. 4  is a flow chart which shows the sequence of the image forming apparatus relating to an embodiment of the present invention. 
           [0014]      FIG. 5  shows an inputted example of processing position information. 
           [0015]      FIGS. 6(   a ) and  6 ( b ) show an example of image allotment and layout on the sheet in  FIG. 5 . 
           [0016]      FIG. 7  shows an example of a display screen of an inputting section relating to an embodiment of the present invention. 
           [0017]      FIG. 8  shows an example of correction reference positions and image position corrections. 
           [0018]      FIG. 9  is a flow chart which shows the sequential operation of a processing position information detecting mode relating to an embodiment of the present invention. 
           [0019]      FIG. 10  is a flow chart which shows the sequential operation when abnormalities occur on an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0020]    The embodiment of the present invention will now be detailed while referring to the drawings.  FIG. 1  shows the relevant parts of the image forming apparatus relating to an embodiment of the present invention. 
         [0021]    Numeral  1  represents a drum shaped photoconductor, numeral  2  represents an electrical charger which evenly charges photoconductor  1 , numeral  3  represents an imagewise exposure unit which exposes an image onto charged photoconductor  1 , numeral  4  represents a developing unit which develops the electrostatic latent image on photoconductor  1 , and forms a toner image, numeral  5  represents a transfer and separation unit which transfers the toner image formed on photoconductor  1  onto sheet S and separates sheet S from photoconductor  1 , numeral  6  represents a cleaning unit which removes remaining toner from photoconductor  1 , and numeral  7  represents a fixing unit which permanently fixes the toner image onto sheet S. Numeral  18  represents a finisher which has plural ejection trays  18   a  and  18   b.    
         [0022]    While photoconductor  1  rotates clockwise, charging, exposure, and development are conducted on photoconductor  1  whereby a toner image is formed on photoconductor  1 , after which the toner image is transferred to sheet S conveyed to synchronize for proper image formation, and the transferred toner image is fixed as a permanent image onto sheet S. After the image transfer, photoconductor  1  is cleaned by cleaning unit  6 . 
         [0023]    Sheet S is conveyed from sheet storing section  8   a , represented by a cassette or a tray, and is supplied to paired registration rollers  13  via paired conveyance rollers  10 . Sheet S is temporarily stopped by paired registration rollers  13 , and is synchronously conveyed with proper image formation, after which sheet S is fed to transfer and separation unit  5 . Numeral  14  represents paired conveyance rollers which further convey sheet S from paired registration rollers  13  to a transfer section. Sheet S, carrying the transferred toner image, is fixed by fixing unit  7 , and then ejected onto ejection tray  18   a  outside of the apparatus via conveyance rollers  15  and  16 . 
         [0024]    In a double-sided image forming mode, after an image is formed on a first surface, sheet S is oriented downward by switching gate G 1 , and is conveyed to sheet flipping section  8   c . Flipped sheet S is conveyed by paired conveyance rollers  11 , and interflows to conveyance path  9   d , via conveyance path  9   c . Sheet S is conveyed again to the transfer section from paired registration rollers  13 , whereby image formation is conducted on a second surface, and the toner image on the second surface is also fixed by fixing unit  7 . Sheet S, carrying the fixed images on both surfaces, is ejected onto ejection tray  18   a  via conveyance rollers  15  and  16 . In this case, switching gates G 1  and G 2  are driven by solenoids which are not illustrated, and send sheet S to the desired path. 
         [0025]    Numeral  20  represents a position detector which detects processing position information on sheet S. The position detector is mounted upstream of paired registration rollers  13  with respect to the sheet conveyance direction, and its length is longer than the width of sheet S. 
         [0026]      FIG. 2  is a block diagram which shows the control of the image forming apparatus relating to the embodiment of the present invention. In  FIG. 2 , various sections and their relationships which are necessary to explain the operation of the present embodiment are illustrated, and well known sections as the image forming apparatus are not illustrated. 
         [0027]    Numeral  100  represents an image forming apparatus which forms an image on a sheet based on image data. Numeral  120  represents a finisher (which corresponds to finisher  18  in  FIG. 1 ) which selects the sheet ejection tray, or folds the sheet, as the post-finishing operation for the sheet carrying the formed image. 
         [0028]    In image forming apparatus  100 , numeral  101  represents a controller to conduct various controls such as (i) allotment and layout of image data in a recording section, (ii) judgment of presence of abnormality on processing position, (iii) correction of image forming positions, (iv) enabling an operator to select sheet type using an inputting section and (v) selection of ejection trays, numeral  106  represents a scanner which reads original image to form the image data, numeral  107  represents an inputting section having LEDs to display various information of the apparatus, through which an operator inputs various operational information, numeral  108  represents an interface section of a communication section which communicates to outer sections via a network, and numeral  110  represents an image forming section (which corresponds to the drawing from which finisher  18  is omitted in  FIG. 1 ) which forms the image on the sheet. 
         [0029]    In  FIG. 2 , controller  101  includes control section  102  being a CPU to control various sections, image memory  103  serving as various recording sections, control memory  104 , and nonvolatile memory  105 . 
         [0030]    Various controls are conducted by programs previously memorized in control memory  104 . Image memory  103  memorizes image data inputted through scanner  106  or interface section  108 , and further memorizes composite image data as a single page carrying said image data to which after-mentioned “allotting processing” has been conducted. Nonvolatile memory  105  memorizes plural information of the sheet, such as sheet size, processing position information, and processing type information. 
         [0031]    The operator inputs various operational information through inputting section  107 , after the specific image data in image memory  103  is selected based on previously inputted information, image forming unit  110  forms an image based on the selected image data. 
         [0032]      FIGS. 3(   a ) and  3 ( b ) show the relationship between position detector  20  and sheet S.  FIG. 3(   a ) is a cross sectional view, while  FIG. 3(   b ) is a plane view. Numeral  20   a  represents a light emitting section including plural LEDs, and numeral  20   b  represents a light receiving section in which plural photodiodes are aligned on a line. Since light amount entering light receiving section  20   b , through the perforated section of sheet S differs to that through non-perforated section, the position of the perforation can be detected. 
         [0033]    LEDs  20   a  and photodiodes  20   b , both of which form position detector  20  are arranged to sandwich sheet S, and are longer than the widest expected sheet to be conveyed by the image forming apparatus, due to this, width, the position and the length of the processing position in the total area of the widest sheet can be detected. 
         [0034]      FIG. 4  is a flow chart which shows the operation of the image forming apparatus relating to the embodiment of the present invention. In step S 301 , sheet information, such as the type of sheet and the size of sheet, are inputted via inputting section  107 . Sheet information is inputted to correspond to each sheet storing section  8   a  (hereinafter referred to as a sheet feeding tray). For example, when image formation is to be conducted on a perforated sheet, the perforated sheet and the sheet feeding tray are selected on a sheet type selecting screen. In addition, when the sheets are set in the sheet feeding tray, the size of sheet is automatically detected by sheet size detecting sensor (which is not illustrated) mounted on each sheet feeding tray. The type of sheet means not only a type of sheet, such as a perforated sheet, a folded sheet, a normal sheet, a high quality sheet and a coated sheet, but also means concepts including weight of the sheet. 
         [0035]    When “perforated sheet” is selected by the operator on the sheet type selection screen, an image screen for inputting processing position information of the perforation is displayed on the operation screen, the operator inputs necessary processing position information (step S 302 ), and the inputted information is recorded in nonvolatile memory  105  which serves as a recording section. Processing position information includes the number of blocks, the processing position of perforation (by X-Y coordinates), and processing types (which are the diameter and the interval between individual perforations). 
         [0036]    “Block” means the area surrounded by the perforations on the sheet, or the area surrounded by the perforations and the edges of sheet. The number of blocks means the number of such areas. Next, an inputting method of the processing position relative to the perforations will now be detailed while referring to  FIG. 5 . 
         [0037]      FIG. 5  shows an inputted example of processing position information. Firstly, overall sheet size information is obtained. When the sheets are set into the sheet feeding tray, the sheet size is automatically noted by a size detecting sensor mounted on the sheet feeding tray. Otherwise, when a sheet type and a sheet size, being of finite form, are selected via the inputting section, the sheet size information can be designated. Alternatively, the sheet type and the sheet size can be numerically inputted via the inputting section image screen. 
         [0038]    Next, the processing positions of the perforations are inputted. The processing positions are sequentially inputted via X-Y coordinates of intersecting points. The intersecting point is the point where an edge of the sheet and a perforation intersect, or the point where a perforation intersects another perforation. 
         [0039]    Further, processing type of perforation is inputted. The processing type means the diameter of perforation, or the interval between perforations, which normally are not necessary to be inputted. However, when the diameter of perforation is extraordinarily large, a printable block size on the sheet is reduced based on the diameter of perforation. Further, when image formation is conducted on the perforation and if no problem occurs due to the wider interval of the perforation, the block size can be increased to the perforation. 
         [0040]    The example in  FIG. 5  shows sheet  40  as A3 size, representing a finite form, and there are four blocks. P 1  and P 2  represent the perforations, and numerals  41 - 49  represent the intersecting points. Since intersecting point  41  represents original point (0,0), the sheet size is automatically determined in the case of the finite form sheet, because the coordinate of intersecting point  49  corresponds to information stored in nonvolatile memory  105 . 
         [0041]    In  FIG. 5 , in finite formed sheet A3 (297×420 mm), intersecting point  49  is represented by coordinate (297, 420). Since the perforations are made parallel to the edges of the sheet, if the operator inputs coordinate (148.5, 210) as intersecting point  45 , the positions of perforations are established, and the coordinates of other intersecting points are not inputted. Further, as another inputting method, if the number of the blocks is 4, 6 or 9, and each block is the same shape, when the operator inputs the size of sheet and the number of blocks, the positions of the perforations are automatically determined. However, if the perforations are not parallel to the edges of the sheet, above described method is not usable, that is, the operator must input the coordinates of all intersecting points, to specify the processing positions of the perforations. 
         [0042]    Further, in the sheet type selection which was described above, if the operator selects “perforated sheet”, the operator can easily retrieve processing position information. That is, previously inputted processing position information, such as the diameter of perforation holes, the interval, and the position, is associated with the name (brand and production number) of the specific perforated sheet, and said associated information is stored in nonvolatile memory  105 , serving as a recording section. Therefore, when the operator selects a name of the perforated sheet, processing position information is easily retrieved. 
         [0043]    Accordingly, since the present invention has a sheet type selector, described above, by which the operator can select the sheet type via the inputting section, and easily retrieves processing position information stored in the recording section related to the sheet type, whereby the operator can easily input processing position information. Further, when different images are to be formed on a sheet of the same processing position, the operator needs not to input the processing position, which allows the image forming apparatus to be more operable. 
         [0044]    Returning to  FIG. 4 , the operational flow will be detailed. In step S 303 , the images are laid out into each block as follows. Firstly, plural image data are read out from image memory  103 , being the recording section. The image data is sequentially laid out by the inputting section into the number of blocks which was set in step S 301 . While the image data are laid out, the image data is allotted to a specific block, and the reference position is set in the block for image formation (which will be detailed in  FIG. 8 ). “Allot” means “each image is laid out on a specific block”, while “lay out” means “the allotted image data is to be arranged at a specific position of a specific block of the sheet”. 
         [0045]    When an image size is greater than its laid out block size, the image size can be reduced, or a portion of the image data is trimmed, which options are selectable. Therefore, the allotted image can be fitted onto the block. 
         [0046]    In this case, allotted image data of each block on a single sheet is combined so that combined image data (hereinafter, referred to as “a page image data”) is formed in a single sheet. 
         [0047]    Next, the operator sets the number of sheets to be outputted (step S 304 ), and presses a start button on the operation panel. Then image formation starts in accordance with the inputted setting conditions, and sheets are conveyed from sheet storing section  8   a  (step S 305 ). 
         [0048]    Next, image forming position correcting functions will be detailed (in steps S 306 -S 310 ). Position detector  20  detects a processing position from the edge of sheet S (step S 306 ), which is compared with processing position information of the perforations of sheet S (step S 307 ). If the detected processing position is within a predetermined range of error, (“Yes” in step S 308 ), the image forming position correction is conducted as below (S 309 ). 
         [0049]    In addition, in step S 307 , when the difference between the processing information and the detected value by the detector  20  is out of the predetermined range of error (“No” in step S 308 ), abnormality countermeasures are conducted (step S 330 ), which will be detailed later. 
         [0050]    In image forming position correcting step S 309 , the position of the image data in each block is adjusted based on changes of the processing position, and the image data of each block on a single sheet is combined to become a new page image data. Based on the new page image data, image formation is conducted on the sheet based on a leading edge reference (step S 310 ), and the sheet carrying the formed image is ejected onto tray  1  ( 18   a  in  FIG. 1 ) (being step S 311 ). 
         [0051]    The above described operations in steps S 305 -S 311  are repeated (when step S 312  is “No”), until the number of set sheets is completed. When the number of sheets set is counted up (“Yes” in step S 312 ), image formation is completed. 
         [0052]    Instead of the above leading edge reference, operational timing of paired registration rollers  13  will be used for the reference, or an output from position detector  20  will be used for the reference. 
         [0053]    In the above described explanation, the processing position is detected for each sheet. However, for one job, in which image formation is repeated under predetermined conditions until the number of sheets set is completed, the image forming position correction can be conducted for only the top sheet, and the correction conducted for the top sheet is repeated on the sheets from second sheet, that is, page image data after the re-combining operation can be repeatedly outputted on the following sheets to form the images. 
         [0054]    When the number of sheets set reaches the number inputted in step S 304 , image formation is completed (“Yes” in step S 312 ). As another example which is not the case of the one job operation, image forming position can be corrected for each stack of sheets, that is, every each sheet feeding tray. 
         [0055]    Further, it is possible for the present invention to mount position detector  20  between paired registration rollers  13  and paired conveyance rollers  14 . However, since detecting timing of position detector  20  is more delayed than exposure timing of imagewise exposure unit  3 , the image forming position is not corrected on real time. To overcome this problem, with respect to one job or the top sheet of stacked sheets, only the processing position is detected by position detector  20 , while no image formation is conducted, and the image forming positions from the second sheet are corrected by processing position information of the top sheet. 
         [0056]    Still further, the above operation can be conducted in the double-sided image forming mode. In the double-sided image forming mode, after the above operation is conducted on the front surface (the first surface), operations from step S 305  to step S 310  are conducted on the rear surface (the second surface). 
         [0057]    That is, in the double-sided image forming mode, the image forming position is corrected by the image forming position correcting section, whereby image formation can be conducted with high accuracy even on the rear surface by adjusting the image to the processing position. 
         [0058]    Still further, since the perforations penetrate the sheet, the processed positions on the front surface and the rear surface are equal. Accordingly, the detected value of the processed position of the front surface can also be used as processing position information of the rear surface, by flipping the surface and changing back to front, that is, the processing position detection by position detector  20  of the front rear surface can be neglected. 
         [0059]    As described above, the image forming position can be corrected based on the detected value of position detector  20  of the front surface, whereby the image forming position on both surfaces can be effectively corrected. 
         [0060]      FIGS. 6(   a ) and  6 ( b ) show an example of image layout conducted on the sheet in  FIG. 5 . Numerals  401 - 404  of  FIG. 6(   a ) represent blocks on the sheet which are surrounded by perforations P 1  and P 2 , and the edges of sheet  40 . Images  201 - 204  stored in image memory  103  are laid out onto the blocks as shown in  FIG. 6(   b ). 
         [0061]    When images  201 - 204  are allotted, plural sections on sheet  40  can be used as the correction reference positions. For example, perforations P 1  and P 2  serve the correction reference positions of images  201  and  203 , while perforation P 2  and a right edge (which is shown by a straight line on intersecting points  43 ,  46  and  49 ) of sheet  40  serve as the correction reference positions of images  202  and  204 . Accordingly, since plural members are used as the correction reference positions, highly accurate image formation can be conducted, while the images are adjusted within the processing positions. 
         [0062]      FIG. 7  shows an example of a display screen of an inputting section relating to the embodiment of the present invention. The inputting section in  FIG. 7  corresponds to inputting section  107  in  FIG. 2 . To input an operational direction, the operator uses a mouse or presses buttons on a key-board, or touches specific positions on a touch panel, superposed on an LCD of the inputting section. 
         [0063]    Display screen  70  is displayed by the LCD, character display sections  71 - 78  correspond to the touch panels, which, when the operator touches one of them, another display screen (which is not illustrated) appears, and changes to a screen to enable input of a file name or reference position. Further, when the operator touches “preview” button  71  on the touch panel, the preview screen is displayed in which the image data arranged for each block is displayed as a thumb-nail data. 
         [0064]    When the operator inputs the file name, selected image  72 , representing the image data, is retrieved from image memory  103 . Block numbers represent the number which is automatically assigned to each block on the sheet from the upper left. Image arrangement represents designated references to arrange the image data within each block, and to designate the relative position against the references. 
         [0065]    In the example shown in  FIG. 7 , image data  201  (file name xyz 0001 ) in  FIGS. 6(   a ) and  6 ( b ) is instructed to be arranged within block  401  (block number  001 ) in  FIG. 6(   a ), under the reference of right perforation P 1  and bottom perforation P 2  in block  401 , and with the interval of 3 mm from the reference. 
         [0066]    By touching “repeat” button  76  on the touch panel to change the screen display (which is not illustrated), the operator can input all data about a full sheet to be repeatedly arranged of the same selected image. 
         [0067]      FIG. 8  shows an example of the correction reference position and the image position correction.  FIG. 8  shows an enlarged portion of  FIG. 6(   b ). The image data is arranged with interval of 3 mm from each reference, based on the instruction which was set in  FIG. 7 , under the correction reference position in direction X is perforation P 1 , while that of direction Y is perforation P 2 . In this case, image  201  is created from write-start point  51  (coordinate x1, y1). 
         [0068]    If the detected positions of perforations P 1  and P 2 , which were detected by position detector  20 , are the same as the processing position information, the correction becomes zero, and write-start point  51  of image  201  has still coordinate (x1, y1). 
         [0069]    If the detected positions of perforation P 1  and P 2  are shifted from processing position information for “a” in direction X, and “b” in direction Y, corrected write-start point  51  is shown by coordinate (x1+a, y1+b). Due to this, the position of image  201  is kept in the desired positional relationship with the perforations. 
         [0070]    As described above, based on the sheet processing position and allotted image information, both of which are set by the inputting section, and also on the detected results of the edge of sheet or the processing position, which is detected by the position detector, the image forming position correcting function of the controller controls a timing and a position for starting image formation, and it forms an image on the sheet based on the allotted image data stored in the recording section. 
         [0071]      FIG. 9  is a flow chart which shows the operations of the processing position information detecting mode relating to the embodiment of the present invention. The flow chart shown in  FIG. 9  corresponds to steps S 301 -S 303  in the flow chart of  FIG. 4 . In order to not need to repeat explanation, the steps of  FIG. 9  which are common to those in  FIG. 4  are designated by the same number. 
         [0072]    When the operator touches a mode switching button (which is not illustrated) on inputting section  107 , the processing position information detecting mode is switched from the normal condition. In the processing position information detecting mode, after sheet information, such as the sheet size, is inputted (step S 301 ), the sheet is supplied (step S 321 ), and the processing position is detected (step S 322 ). Using the detected value, processing position information of the sheet is recorded in nonvolatile memory  105  which serves as the recording section. Since the positions and the sizes of the blocks are understood based on said recorded processing position information, the image data are allotted onto each block via inputting section  107  (step S 303 ). 
         [0073]    Accordingly, it is possible to structure a system in such a way that, instead of inputting processing position information by the operator, after the sheet is supplied, processing position information is obtained by the value detected by position detector  20 . By this structure, processing position information is inputted easily. 
         [0074]      FIG. 10  is a flow chart which shows the operation when abnormal situations occur on the embodiment of the present invention. “Abnormal situation” means that the difference between processing position information of the sheet and detected processed position value by position detector  20 , is greater than a predetermined value, which corresponds to the operation of step S 330  in the flowchart of  FIG. 4 . When the difference between processing position information of the sheet and detected processing position value by position detector  20  is greater than a predetermined value (“No” in step  308 ), the controller determines that an abnormal situation exists, and an abnormal sign is displayed on the LCD of the inputting section (step S 331 ). The predetermined value for determining abnormality may, for example, be 2 mm, or can be set by the operator to be any reasonable value. 
         [0075]    As the procedure of the above case, when the processing position is beyond the normal range, no image formation is conducted on the sheet, and a sheet carrying no image is ejected onto ejection tray  2  (which is  18   b  in  FIG. 1 ), different from ejection tray  1  (which is  18   a  in  FIG. 1 ) used for normal situation (step S 332 ). 
         [0076]    After this, the procedure returns to step S 305 , and new sheets are supplied for subsequent image formation. 
         [0077]    In addition, as one embodiment, the procedure is explained in which the display of abnormality (step S 331 ) and ejection onto ejection tray  2  (step S 332 ) are conducted, but as another embodiment, the procedure can be conducted in which either one of them is carried out. Further, when an abnormality is judged to exist, the current operation can be stopped, without returning to step S 305 . 
         [0078]    As explained above, when the processing position varies on the sheet, the controller compares processing position information with the value detected by the position detector. When the difference between them is greater than the predetermined value, the display shows the abnormality, as well as the sheet carrying the varied processing position is ejected onto the tray, differing for the tray for the normal operation, that is, sheets carrying the abnormal processing position can be separated from sheets carrying the acceptable processing position. Further when an abnormality is detected, no image formation is conducted on the sheet, which reduces waste. 
         [0079]    In  FIGS. 3(   a ) and  3 ( b ), as an example of processing position detector  20 , a transparent detector is explained in which light source  20   a  and light detector  20   b  sandwich the conveyed sheet. However, not limiting to the above example, a reflection detector can be employed in which light source  20   a  and light detector  20   b  are mounted on one side of the sheet. Further, a CCD sensor can be employed to read the surface condition of the sheet, whereby the CCD sensor automatically catches patterns such as perforations. 
         [0080]    Still further, a perforated sheet is used for the pre-processed sheet in the above explanation. However, not limiting to the perforated sheet, a folded sheet or a folded sheet carrying perforations can also be employed, while the CCD sensor detects the surface condition of the sheet. 
         [0081]    Still further, not to limit the above-described image forming apparatus using the electro-photographic method, the present invention can be applied onto any image forming apparatus which forms images and conveys the sheet by the paired rollers, such as a thermal-transfer printer, and an inkjet printer. 
         [0082]    As described above, based on the present invention, the image forming apparatus can conduct image formation while accurately adjusting the image to each of several processing position, on a pre-processed sheet on which perforations or folds have been created, and in particular, on a single sheet divided into plural processing positions.