Abstract:
An image forming apparatus includes: a common conveyance path through which a sheet fed from a sheet feed tray is conveyed from a registration roller to a transfer section; a direct conveyance path through which the sheet is directly fed from the sheet feed tray to the common conveyance path; a delay conveyance path in which the sheet after being branched from the direct conveyance path is joined to the common conveyance path, and is caused to delay in being fed from the sheet feed tray to the common sheet conveyance path; a conveyance section which conveys the sheet fed from the sheet feed tray to the transfer section; and a controller which controls the conveyance section to convey once the sheet fed from the sheet feed tray from the direct conveyance path to the delay conveyance path, and thereafter to convey to the common conveyance path.

Description:
This application is based on Japanese Patent Application Nos. 2007-250980 filed on Sep. 27, 2007, and 2008-111045 filed on Apr. 22, 2008, which are incorporated hereinto by reference. 
     BACKGROUND OF THE INVENTION 
     The present invention relates to an image forming apparatus wherein a sheet stacked on a sheet feed tray is fed and an image is formed on the sheet. 
     Image forming apparatuses such as a copying machine, a printer and facsimile machine are introduced in many offices, and a higher speed of the image forming apparatus and higher image quality are demanded by users. In recent years, electrophotographic image forming apparatuses are introduced also in the print industry, and a higher speed and higher image quality are demanded by users. 
     As an art to realize higher image quality, there is a technology in which physical properties of each sheet fed out of a sheet feed tray including, for example, a width and a whiteness degree, or, positions such as deviations, for example, are detected to correct image data, and an image is formed on the sheet based on the corrected image data. 
     In the technology described in Unexamined Japanese Patent Publication Application No. 2005-41582, for example, a sheet deviation sensor is provided on a conveyance path for sheets, then, image data are shifted in the main scanning direction on the memory to be corrected based on the results of the detection by the sensor, and an image is formed on a sheet based on the corrected image data. This technology makes it possible to take a deviation of each sheet into consideration and thereby to form a high image quality that is constantly at a fixed position from an edge of the sheet. 
     For forming an image by considering physical properties and positions of each sheet fed out of a sheet feed tray, it is necessary to detect a sheet that is fed from sheet feed tray  1000  by detection sensor  3000  installed on conveyance path  2000  as shown in  FIG. 10 , for example, and to correct image data based on results of the detection. Further, when realizing a higher speed while conducting correction of image data, it is necessary to feed sheets continuously from sheet feed tray  1000  by shortening an conveyance interval of sheets, and to detect physical properties and positions of each sheet with detection sensor  3000 . 
     Since a certain period of time is needed from the moment when physical properties and others of each sheet are detected by the detection sensor  3000  to the moment when an image formed on photoreceptor  4000 A arrives at transfer section  5000  after image data are corrected, it is necessary to detect physical properties and others of a sheet early, and to form images on photoconductor  4000 A based on results of the detection in succession, for forming images on sheets fed continuously at short intervals. 
     When taking this point into consideration, it is necessary to arrange the detection sensor  3000  at the upstream side of the conveyance path  2000 , and to make a length of a conveyance path between the detection sensor  3000  and the transfer section  5000  to be as long as possible so that a plurality of sheets may stay between the detection sensor  3000  and the transfer section  5000 . In this way, physical properties of sheets can be detected early, and a higher speed can be realized. 
     However, if the conveyance path between the detection sensor  3000  and the transfer section  5000  is made to be longer linearly, an apparatus needs to be large in size. In addition, if the conveyance path between the detection sensor  3000  and the transfer section  5000  is deflected to be made longer for preventing a large-sized apparatus, removal of jammed sheets is difficult when a sheet jam takes place at the deflected portion. 
     SUMMARY OF THE INVENTION 
     Therefore, an objective of the invention is to provide a compact image forming apparatus wherein physical properties or positions of each sheet which has been fed are detected, and an image can be formed at high speed based on results of the detection. 
     For attaining the aforesaid objective, an image forming apparatus relating to the invention forming an image on a sheet has a following structure wherein a sheet feed section having a tray on which sheets are loaded, a sheet detector that detects physical properties or positions of each sheet fed from the aforesaid sheet feed section, an image forming section that forms an image based on results of the detection conducted by the sheet detector, a transfer section that transfers an image formed by the aforesaid image forming section onto a sheet, a common conveyance path that conveys a sheet fed from the sheet feed section to the transfer section from a registration roller, a direct conveyance path that feeds a sheet directly from the common conveyance path from the sheet feed section, a delay conveyance path that joins the common conveyance path after branching to the direct conveyance path, and feeds a sheet by delaying a sheet from the sheet feed section to the common conveyance path, a conveyance section that conveys a sheet fed from the sheet feed section to the transfer section and a controller that controls the aforesaid conveyance section are provided and the controller controls the conveyance section so that a sheet fed from the sheet feed section may be conveyed temporarily from the direct conveyance path to the delay conveyance path, and then, may be conveyed to the common conveyance path. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a central sectional view showing the internal constitution of an image forming apparatus. 
         FIG. 2  is a block diagram of a control system of the image forming apparatus. 
         FIG. 3  is an enlarged diagram of a conveyance path in an apparatus main body and an optional sheet feed device. 
         FIG. 4  is a flow chart showing operations for conveying a sheet that is fed out of a sheet feed tray by switching a conveyance path. 
         FIG. 5  is an illustration showing configuration for conveying a sheet to an optional conveyance path temporarily from a conveyance path of a main body. 
         FIG. 6  is an illustration showing configuration for conveying a sheet to a reversal conveyance path temporarily from a conveyance path of a main body. 
         FIG. 7  is a flow chart showing operations to switch a conveyance path and to convey for a sheet that is fed from an optional sheet feed tray. 
         FIG. 8  is an illustration showing configuration for conveying a sheet temporarily to a conveyance path of a main body from an optional conveyance path. 
         FIG. 9  is an illustration showing configuration for conveying a sheet temporarily to a reversal conveyance path from an optional conveyance path. 
         FIG. 10  is a schematic structural diagram of an ordinary image forming apparatus. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       FIG. 1  is a central sectional view showing the internal constitution of an image forming apparatus  1 . 
     The image forming apparatus  1  is composed of apparatus main body  2  and optional sheet feed device  3 . 
     The apparatus main body  2  is an apparatus main body of a tandem type having intermediate transfer belt  50 . 
     A document set on document feed table of automatic document feeder  10  is conveyed to image reading section  30  by each conveyance roller. 
     The apparatus main body  2  has, at its lower portion, plural main body sheet feed trays  20 . Image forming section  40  and intermediate transfer belt  50  are provided over the main body sheet feed tray  20 , and image reading section  30  is provided on the upper portion of the apparatus main body. 
     The main body sheet feed tray  20  is of the structure to be capable of being drawn toward the front side of the apparatus (toward this side on the page in  FIG. 1 ). In a plurality of main body sheet feed trays  20 , there are stacked sheets S such as white sheets which are classified according to the size. Sheets S loaded in main body sheet feed trays  20  are fed by sheet feed roller  21 . 
     Optional sheet feed device  3  is connected to the outside of the apparatus main body  2 . In the optional sheet feed device  3 , there are provided optional sheet feed trays  210  each having therein a large number of sheets S stacked. When printing a large number of sheets continuously, sheets S are fed out of the optional sheet feed trays  210  and images are formed on the sheets S. 
     Image forming section  40  has four sets of image forming units  400 Y,  400 M,  400 C and  400 K which form respectively toner images in respective colors of Y, M, C and K. The image forming units  400 Y,  400 M,  400 C and  400 K are arranged downward linearly in this order from the top, and each of them is of the same structure. If image forming unit  400 Y for yellow color is taken as an example for the structure, the image forming unit  400 Y has therein photoreceptor  410  that rotates counterclockwise, scorotron charging section  420 , exposure section  430  and developing section  440 . 
     Cleaning section  450  is arranged to include an area that faces the lowermost portion of photoreceptor  410 . 
     Intermediate transfer belt  50  positioned at a central portion of the apparatus main body is in an endless shape and has a prescribed volume resistivity. Primary transfer electrode  510  is installed at a position that faces photoreceptor  410  through the intermediate transfer belt  50 . 
     Next, an image forming method for forming a color image will be explained. 
     The photoreceptor  410  is driven to rotate by drum drive motor (not shown), and is charged by electric discharge of scorotron charging section  420  at negative polarity of, for example, −800 V. Next, optical writing corresponding to image information is conducted by exposure section  430  on photoreceptor  410 , and an electrostatic latent image is formed on the photoreceptor  410 . When the electrostatic latent image thus formed passes through developing section  440 , the latent image is developed reversely by toner charged to be at negative polarity in the developing section through impression of negative polarity developing bias, whereby, a toner image is formed on the photoreceptor  410 . The toner image thus formed is transferred onto intermediate transfer belt  50  that comes in pressure contact with the photoreceptor  410 . Toner staying on the photoreceptor  410  after the transfer is removed by cleaning section  450 . 
     When toner images which are respectively formed by image forming units  400 Y,  400 M,  400 C and  400 K are transferred onto intermediate transfer belt  50  to be superimposed, a color image is formed on the intermediate transfer belt  50 . Sheet S is fed one sheet by one sheet from main body sheet feed tray  20  or from optional sheet feed tray  210 , and is conveyed to loop roller  90  and to registration roller  60 . The sheet S hits the registration roller  60  to be stopped temporarily, thus, a loop is formed by loop roller  90 , and a skew of the sheet S is corrected. The sheet S is fed from the registration roller  60  at timing when the toner image on the intermediate transfer belt  50  synchronizes with the image position. 
     The sheet S fed by the registration roller  60  is guided by a guide plate to be fed in a transfer nip position that is formed by intermediate transfer belt  50  and transfer section  70 . The transfer section  70  constituted by the roller presses the sheet S toward the intermediate transfer belt  50 . When bias (for example, +500 V) whose polarity is opposite to that of toner is impressed on transfer section  70 , a toner image on intermediate belt  50  is transferred onto sheet S by actions of electro static force. The sheet S is neutralized by a separating device (not shown) composed of a neutralizing needle to be separated from intermediate transfer belt  50 , and is sent to fixing section  80  having a heat roller and a pressure roller. As a result, a toner image is fixed on sheet S, and the sheet S on which an image is formed is ejected to the outside of the apparatus. 
     Incidentally, image forming apparatus  1  in the present embodiment is one that forms a color image on a sheet through an electrophotographic system, but, an image forming apparatus relating to the invention is not limited to the present embodiment, and it may also be an image forming apparatus that forms a monochrome image. 
       FIG. 2  is a block diagram of a control system of image forming apparatus  1 , and in this case, typical ones only are shown. 
     CPU (Central Processing Unit)  101  is connected to ROM (Read Only Memory)  102  and to RAM (Random Access Memory)  103  through system bus  109 . This CPU  101  reads out various programs stored in ROM  102  to develop them on RAM  103 , and it controls operations of each section. Further, CPU  101  carries out various types of processes in accordance with programs developed in RAM  103 , and it stores results of the processes in RAM  103 , and displays them on operation and display section  105 . Then, CPU  101  preserves the results of the processes stored in RAM  103  in the prescribed destination for preservation. In the meantime, in the present embodiment, CPU  101  constitutes a control section together with ROM  102  and RAM  103 . 
     ROM  102  stores programs and data in advance, and it is composed typically of a semiconductor memory. 
     RAM  103  forms a work area that stores temporarily data processed by various types of programs carried out by CPU  101 . 
     HDD  104  has a function to store image data of document images which are read by image reading section  30 , and to store outputted image data. HDD  104  is of the structure wherein a plurality of metal discs each being coated or evaporated with magnetic materials are superimposed at regular intervals, and when a magnetic head approaches HDD  104  that is rotated by a motor at high speed, data are read or written. 
     Operation and display section  105  is one that allows various types of configurations. The operation and display section  105  is of a touch panel type, for example, and when a user inputs through the operation and display section  105 , conditions concerning a color print or a monochrome print are established. The operation and display section  105  displays various types of information including network setting information. 
     The image reading section  30  reads out document images optically, and converts them into electric signals. When reading out a color document, the image reading section  30  generates image data having luminance information of 10 bits per one pixel for each of R, G and B. 
     Image data generated by the image reading section  30  and image data transmitted from a personal computer connected to image forming apparatus  1  are subjected to image processing conducted by image processing section  106 . When color printing is carried out on image forming apparatus  1 , image data respectively for R (Red), G (Green) and B (Blue) generated by the image reading section  30  are inputted in color conversion LUT in image processing section  106 , and R, G and B data are color-converted into image data respectively for Y (Yellow), M (Magenta), C (Cyan) and Bk (Black). Then, for the color-converted image data, a tone reproduction characteristic is corrected, or, screen processing such as halftone is carried out referring to density correction LUT, and edge processing for fine-line enhancement is conducted. 
     Main body sheet sensor SS 1  that functions as a sheet detector is arranged on main body conveyance path R 1  for sheet S that is fed from main body sheet feed tray  20 , and it detects a shift, a width and a degree of whiteness of each sheet conveyed. When the shift of sheet S is detected by main body sheet sensor SS 1 , image processing section  106  shifts image data in the main scanning direction on image memory, based on results of the detection, to correct it. Due to this, an image is formed at an appropriate position for a sheet. 
     Further, after a width of sheet S is detected by main body sheet sensor SS 1 , image processing section  106  corrects a magnification of image data, and after a degree of whiteness of sheet S is detected by main body sheet sensor SS 1 , image processing section  106  corrects a color tone of image data. After the magnification and the color tone of image data are corrected, images with high image quality can be formed on the sheet. 
     Main body conveyance section  107  conveys sheet S that is fed from main body sheet feed tray  20  arranged. A controller controls operations of the main body conveyance section  107 . 
     Communication section  108  of the apparatus main body is connected with communication section  201  of an optional sheet feed device, and operations of optional sheet feed device  3  are controlled by a controller of apparatus main body  2 . 
     Optional conveyance section  202  conveys sheet S that is fed from optional sheet feed tray  210 . Operations of the optional conveyance section  202  are controlled by a controller. 
     Optional sheet sensor SS 2  that functions as a sheet detector detects a shift, a width and a degree of whiteness of each sheet conveyed, in the same way as in the main body sheet sensor SS 1 , and results of the detections are reflected on correction of image data conducted by image processing section  106 . 
     Incidentally, in the present embodiment, main body conveyance section  107 , optional conveyance section  202  and switching sections G 1 , G 2 , G 3  and G 4  shown in  FIG. 3  constitute a conveyance section. 
       FIG. 3  is an enlarged diagram of a conveyance path in apparatus main body  2  and optional sheet feed device  3 . 
     R 1  represents a main body conveyance path through which sheet S fed from main body sheet feed tray  20  is conveyed, and it is a conveyance path covering from each main body sheet feed tray  20  to loop roller  90 . R 2  represents an optional conveyance path through which sheet S fed from optional sheet feed tray  210  is conveyed, and it is a conveyance path covering from each optional sheet feed tray  20  to loop roller  90 . Further, R 3  represents a reversing conveyance path that reverses sheet S on which an image has been formed on its front side, and it is a conveyance path covering from a diverging point at the downstream side of fixing section  80  to loop roller  90 . R 4  represents a common conveyance path through which sheet S conveyed from each conveyance path is conveyed to transfer section  70 , and it is a conveyance path covering from loop roller  90  to transfer section  70 . R 5  represents the first joint conveyance path that conveys a sheet to optional conveyance path R 2 , diverging from main body conveyance path R 1 . Further, the first joint conveyance path R 5  can convey a sheet to main body conveyance path R 1  by diverging from optional conveyance path R 2 . R 6  represents the second joint conveyance path that conveys a sheet to the reversing conveyance path R 3  by diverging from main body conveyance path R 1  or from optional conveyance path R 2 . 
     SS 1  represents a main body sheet sensor that detects a shift of sheet S conveyed by main body conveyance path R 1 . SS 2  represents an optional detection sensor that detects a shift of sheet S conveyed by optional conveyance path R 2 . The main body sheet sensor SS 1  is arranged at the upstream side of the common conveyance path R 4  in the sheet conveyance direction. In the same way, the optional sheet sensor SS 2  is arranged at the upstream side of the common conveyance path R 4  in the sheet conveyance direction. 
     On the conveyance path, there are installed a plurality of switching sections G 1 , G 2 , G 3  and G 4  by which a conveyance path for sheet S is switched. The switching section G 1  switches so that the sheet S conveyed from main body conveyance path R 1  may be conveyed to optional conveyance path R 2 , or to common conveyance path R 4 . The switching section G 2  switches so that the sheet S conveyed from the optional conveyance path R 2  may be conveyed to the main body conveyance path R 1 , or to the common conveyance path R 4 . The switching section G 3  switches so that the sheet S conveyed from the main body conveyance path R 1  or from the optional conveyance path R 2  may be conveyed to reversal conveyance path R 3 , or to the common conveyance path R 4 . 
     When realizing speeding up by detecting a shift and others of each sheet that is fed from a sheet feed tray and by correcting image data, it is necessary to feed sheets from the sheet feed tray continuously by shortening an interval for conveyance, and to detect a shift and others of each sheet one after another with a sheet detector. 
     Since it takes a fixed amount of time for an image formed on photoreceptor  410  to arrive at transfer section  70  after shift and others of sheet S are detected by a sheet detector and image data are corrected, it is necessary to detect a shift and others of the sheet S in their early stages, and to form images on the photoreceptor  410  in order based on results of the detection. 
     When the foregoing is taken into consideration, a conveyance distance between the sheet detector and transfer section  70  needs to be long so that plural sheets S may stay at a space between the sheet detector and the transfer section to be delayed. For the reason mentioned above, conveyance paths are switched by switching sections G 1 , G 2 , G 3  and G 4  so that a conveyance distance between the sheet detector and transfer section  70  may be controlled to be long. 
     The aforesaid conveyance control for delaying the sheets may be conducted for a specific print job. The specific print job is a print job wherein a high quality mode is established and a low speed mode is not established in establishment information for print job. The high quality mode is a mode wherein a shift, a width and a degree of whiteness of sheet S fed out of a sheet feed tray are read, then, image data are corrected and an image is formed on the sheet S, thus, printing is carried out on a high image quality basis and at a prescribed position on the sheet. In the low speed mode, the sheet is conveyed at a low speed for avoiding troubles, because sheet jamming tends to be caused by conditions of the conveyance path when sheet S is a thick sheet. Further, some users allow a sheet to be outputted at a low speed provided that a quality of the sheet is high. 
     These points are explained as follows, referring to  FIG. 4 . 
       FIG. 4  is a flow chart showing operations for conveying by switching a conveyance path for sheet S that is fed out of main body sheet feed tray  20 . 
     First, actions for the print job established in operation and display section  105  of image forming apparatus  1  and for the print job received from PC representing an outer terminal are started (step S 1 ). 
     Next, establishment information of the print job is judged whether it is of a high quality mode or not, based on establishment information of the print job (step S 2 ). 
     When the print job is judged not to be of a high quality mode in step S 2  (Step S 2 ; No), image forming is conducted without detecting a sheet with main body sheet sensor SS 1 , because the print job is one wherein no importance is attached to quality. Specifically, for the print job which is not of a high quality mode, namely, the print job other than a specific print job, sheet S is fed from main body sheet feed tray  20  (step S 3 ), then, sheet S is conveyed directly to common conveyance path R 4  without conveying the sheet S to a delay conveyance path (step S 4 ), and a toner image is transferred onto sheet S by transfer section  70  (step S 5 ). In the explanation by using  FIG. 4 , sheet S is fed from main body sheet feed tray  20 , and sheet S is conveyed directly to common conveyance path R 4  to be conveyed to transfer section  70 , without being conveyed to optional conveyance path R 2  representing a delay conveyance path and reversal conveyance path R 3 . Operations covering from step S 3  to step S 5  are repeated until the print job is completed. 
     On the other hand, when the establishment information of the print job is judged to be of a high quality mode in step S 2  (Step S 2 ; Yes), the print job is judged next whether it is of a low speed mode or not based on the establishment information (step S 7 ). The print job is judged in step S 7  whether it is of a low speed mode or not in step S 7 , and switching of conveyance path is controlled. 
     When the print job is judged not to be of a low speed mode in step S 7  (Step S 7 ; No), sheet S is conveyed by considering that a plurality of sheets S are detected by main body sheet sensor SS 1  to acquire information such as a shift in its early stage and by considering a period of time during which image data are corrected and an image formed on photoreceptor  410  arrives at transfer section  70 . Specifically, sheet S is fed from main body sheet feed tray  20  (step S 8 ), and the sheet S thus fed is detected by main body sheet sensor SS 1  (step S 9 ). Then, for securing the conveyance distance to be long, sheet S is conveyed temporarily to a delay conveyance path, and then, sheet S is conveyed to common conveyance path R 4  (step S 10 ), and a toner image is transferred onto sheet S by transfer section  70  (step S 11 ). Operations covering from step S 8  to S 11  will be explained in detail as follows, referring to  FIG. 5  and  FIG. 6 . 
       FIG. 5  is an illustration showing how sheet S is conveyed temporarily from main body conveyance path R 1  to optional conveyance path R 2 .  FIG. 6  is an illustration showing how sheet S is conveyed temporarily from main body conveyance path R 1  to optional conveyance path R 2 .  FIG. 6  is an illustration showing how sheet S is conveyed temporarily from main body conveyance path R 1  to reversal conveyance path R 3 . Incidentally, in  FIG. 5 , main body conveyance path R 1  corresponds directly to the conveyance path, and optional conveyance path R 2  corresponds to a delay conveyance path. In  FIG. 6 , main body conveyance path R 1  corresponds directly to the conveyance path, and reversal conveyance path R 3  corresponds to a delay conveyance path. 
     First, in the explanation of the embodiment shown in  FIG. 5 , sheet S fed from main body sheet feed tray  20  advances through main body conveyance path R 1  in the direction a, after being detected by main body sheet sensor SS 1  for a shift and others. Then, sheet S is caused to advance through first joint conveyance path R 5  in the direction b by operations of switching section G 1 , and is conveyed temporarily to optional conveyance path R 2  representing a delay conveyance path. Next, sheet S follows a route of a switchback to advance in the direction c, to be conveyed to transfer section  70  through common conveyance path R 4 . If the conveyance path between main body sheet sensor SS 1  and transfer section  70  is secured to be long temporarily, by switching the conveyance path so that sheet S may be conveyed through optional conveyance path R 2 , it is possible to feed sheets S continuously from main body sheet feed tray  20  by shortening a conveyance interval, and thereby to acquire information such as a shift of each of plural sheets in its early stage. It is further possible to cause sheet S to stay on the conveyance path and thereby to delay the conveyance thereof for the period of time during which the image data are corrected and an image formed on photoreceptor  410  arrives at transfer section  70 . 
     Next, in the explanation about an embodiment shown in  FIG. 6 , sheet S fed from main body sheet feed tray  20  is detected in terms of a shift by main body sheet sensor SS 1 , and advances through main body conveyance path R 1  in the direction d. Then, owing to operations of switching sections G 1  and G 3 , sheet S advances from second joint conveyance path R 6  in direction e, and is conveyed temporarily to reversal conveyance path R 3  representing a delay conveyance path. Next, sheet S follows a route of a switchback to advance in the direction f, and is conveyed to transfer section  70  through common conveyance path R 4 . If the conveyance path between main body sheet sensor SS 1  and transfer section  70  is secured to be long temporarily, by conveying sheet S to reversal conveyance path R 3 , it is possible to feed sheets S continuously from main body sheet feed tray  20  by shortening a conveyance interval, and thereby to acquire information such as a shift of each of plural sheets in its early stage. It is further possible to cause sheet S to stay on the conveyance path and thereby to delay the conveyance thereof for the period of time during which the image data are corrected and an image formed on photoreceptor  410  arrives at transfer section  70 . 
     By practicing actions of steps S 8  to S 11  shown in  FIG. 4  as explained above, it is possible to detect a shift and others for each sheet fed, and thereby, to reflect results of the detection on image forming and to form images at high speed. 
     The explanation will be continued by returning to  FIG. 4 . When a mode is judged to be a low speed mode in step S 7  (step S 7 ; Yes), sheets S are fed from main body sheet feed tray  20  without shortening a conveyance interval (step S 13 ), because a print job is one wherein no importance is attached to a speed, or one wherein a sheet is thick, and detection is practiced by main body sheet sensor SS 1  (step S 14 ). Then, sheet S is conveyed directly to common conveyance path R 4  without conveying to a delay conveyance path (step S 15 ), and a toner image is transferred onto sheet S by transfer section  70  (step  16 ). In the explanation referring to  FIG. 4 , for a print job other than a specific print job, sheet S is fed from main body sheet feed tray  20 , and is conveyed directly to common conveyance path R 4  from main body conveyance path R 1 , to be conveyed to transfer section  70 , without being conveyed to optional conveyance path R 2  representing a delay conveyance path and to reversal conveyance path R 3 . Operations for steps S 13  to S 16  are repeated until the print job is completed. 
     Operations for feeding sheet S from main body sheet feed tray  20  in apparatus main body  2  have been explained as mentioned above. Next, operations for feeding sheet S from optional sheet feed tray  210  in optional sheet feed device  3  will be explained. 
       FIG. 7  is a flow chart showing operations to switch a conveyance path and to convey for a sheet that is fed from an optional sheet feed tray  210 . 
     First, a print job established on operation and display section  105  of image forming apparatus  1  and a print job received from PC that is an outer terminal are started (step S 21 ). 
     Next, judgments are made, based on establishment information for the print job, whether the print job is of a high quality mode or not (step S 22 ). 
     When the print job is judged not to be of the high quality mode in step S 22  (step S 22 ; No), image forming is conducted without detecting a sheet with optional sheet sensor SS 2 , because the print job is one wherein no importance is attached to image quality. Specifically, for the print job which is not of a high quality mode, namely, the print job other than a specific print job, sheet S is fed from optional sheet feed tray  210  (step S 23 ), then, sheet S is conveyed directly to common conveyance path R 4  without conveying the sheet S to a delay conveyance path (step S 24 ), and a toner image is transferred onto sheet S by transfer section  70  (step S 25 ). In the explanation by using  FIG. 8 , sheet S is fed from optional sheet feed tray  210 , and sheet S is conveyed directly to common conveyance path R 4  to be conveyed to transfer section  70 , without being conveyed to main body conveyance path R 1  representing a delay conveyance path and reversal conveyance path R 3 . Operations covering from step S 23  to step  25  are repeated until the print job is completed. 
     On the other hand, when the establishment information of the print job is judged to be of a high quality mode in step S 22  (Step S 22 ; Yes), the print job is judged next whether it is of a low speed mode or not based on the establishment information (step S 27 ). 
     When the print job is judged not to be of a low speed mode in step S 27  (Step S 27 ; No), sheet S is conveyed, after switching a conveyance path, by considering that a plurality of sheets S are detected by optional sheet sensor SS 2  to acquire information such as a shift in its early stage and by considering a period of time during which image data are corrected and an image formed on photoreceptor  410  arrives at transfer section  70 . Specifically, sheet S is fed from main body sheet feed tray  210  (step S 28 ), and the sheet S thus fed is detected optional sheet sensor SS 2  (step S 29 ). Then, for securing the conveyance distance to be long, sheet S is conveyed temporarily to a delay conveyance path, and then, sheet S is conveyed to common conveyance path R 4  (step S 30 ), and a toner image is transferred onto sheet S by transfer section  70  (step S 31 ). Operations covering from step S 28  to step S 31  will be explained in detail as follows, referring to  FIG. 8  and  FIG. 9 . 
       FIG. 8  is an illustration showing how sheet S is conveyed temporarily from optional conveyance path R 2  to main body conveyance path R 1 , and  FIG. 9  is an illustration showing how sheet S is conveyed temporarily from optional conveyance path R 2  to reversal conveyance path R 3 .  FIG. 6  is an illustration showing how sheet S is conveyed temporarily from main body conveyance path R 1  to reversal conveyance path R 3 . Incidentally, in  FIG. 8 , optional conveyance path R 2  corresponds directly to the conveyance path, and main body conveyance path R 1  corresponds to a delay conveyance path. In  FIG. 9 , optional conveyance path R 2  corresponds directly to the conveyance path, and reversal conveyance path R 3  corresponds to a delay conveyance path. 
     First, in the explanation of the embodiment shown in  FIG. 8 , sheet S fed from optional sheet feed tray  210  advances through optional conveyance path R 2  in the direction g, after being detected by optional sheet sensor SS 2  for a shift and others. Then, sheet S is caused to advance through first joint conveyance path R 5  in the direction h by operations of switching sections G 2  and G 1 , and is conveyed to main body conveyance path R 1  representing a delay conveyance path. Next, sheet S follows a route of a switchback to advance in the direction i, to be conveyed to common conveyance path R 4 . If the conveyance path between optional sheet sensor SS 2  and transfer section  70  is secured to be long temporarily, by switching the conveyance path so that sheet S may be conveyed through main body conveyance path R 1 , it is possible to feed sheets S continuously from optional sheet feed tray  20  by shortening a conveyance interval, and thereby to acquire information such as a shift of each of plural sheets in its early stage. It is further possible to cause sheet S to stay on the conveyance path and thereby to delay the conveyance thereof for the period of time during which the image data are corrected and an image formed on photoreceptor  410  arrives at transfer section  70 . 
     Next, in the explanation about an embodiment shown in  FIG. 9 , sheet S fed from optional sheet feed tray  210  is detected in terms of a shift by optional sheet sensor SS 2 , and advances through optional conveyance path R 2  in the direction j. Then, owing to operations of switching sections G 2  and G 3 , sheet S advances through second joint conveyance path R 6  in direction k, and is conveyed to reversal conveyance path R 3  representing a delay conveyance path. Next, sheet S follows a route of a switchback to advance in the direction  1 , and is conveyed to transfer section  70  through common conveyance path R 4 . If the conveyance path between optional sheet sensor SS 2  and transfer section  70  is secured to be long temporarily, by switching a conveyance path so that sheet S may be conveyed to reversal conveyance path R 3 , it is possible to feed sheets S continuously from optional sheet feed tray  210  by shortening a conveyance interval, and thereby to acquire information such as a shift of each of plural sheets in its early stage. It is further possible to cause sheet S to stay on the conveyance path and thereby to delay the conveyance thereof for the period of time during which the image data are corrected and an image formed on photoreceptor  410  arrives at transfer section  70 . 
     By practicing actions of steps S 28  to S 31  shown in  FIG. 7  as explained above, it is possible to detect a shift and others for each sheet fed, and thereby, to reflect results of the detection on image forming and to form images at high speed. 
     The explanation will be continued by returning to  FIG. 7 . When a mode is judged to be a low speed mode in step S 27  (step S 27 ; Yes), sheets S are fed from optional sheet feed tray  210  without shortening a conveyance interval (step S 33 ), (step S 34 ). Then, sheet S is conveyed to common conveyance path R 4  without conveying to a delay conveyance path (step S 35 ), and a toner image is transferred onto sheet S by transfer section  70  (step  36 ). In the explanation referring to  FIG. 7 , for a print job other than a specific print job, sheet S is fed from optional sheet feed tray  210 , and is conveyed directly to common conveyance path R 4  from optional conveyance path R 2 , to be conveyed to transfer section  70 , without being conveyed to main body conveyance path R 1  representing a delay conveyance path and to reversal conveyance path R 3 . Operations for steps S 33  to S 36  are repeated until the print job is completed. 
     In the meantime, the invention is not limited to the present embodiment, and any modification and any addition which do not depart from the spirit and scope of the invention can be include in the invention. 
     The invention makes it possible to provide a compact image forming apparatus wherein physical properties or positions of each sheet fed therein can be detected, and image forming at high speed can be realized based on the results of the aforesaid detections.