Abstract:
Provided is a method for forming an image containing the steps of: supplying a clear toner on a surface of an image forming support; heating the supplied clear toner on the surface of the image forming support to melt; close contacting the surface of the image forming support supplied with the clear toner with a belt; and cooling the clear toner in a state of close contacting with the belt so as to form a clear toner layer on the image forming support, wherein the aforesaid clear toner contains at least a resin formed with a polymerizable monomer represented by Formula (1) and a vinyl monomer, Formula (1): H 2 C═CR 1 —COOR 2 , wherein R 1  represents a hydrogen atom or a methyl group; and R 2  represents a chain alkyl group having 12 to 22 carbon atoms, or a cyclic alkyl group.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    The present invention is based on Japanese Patent Application No. 2011-235964 filed with Japanese Patent Office on Oct. 27, 2011, the entire contents of which being hereby incorporated by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to an image forming apparatus for shifting a sheet along a sheet width direction that is perpendicular to a sheet transporting direction before the sheet is transported at an image-forming position with the sheet being nipped. 
         [0004]    2. Description of Related Art 
         [0005]    An image forming apparatus having multiple functions which have functions of a printer, a scanner, a copier, a facsimile and the like combined together has been widely used in recent years. In the image forming apparatus, there may be a deflection in which when transporting the sheet from a paper feeder or a reversing path to a secondary transfer unit during a period of image-forming time, the sheet is deflected to a direction (hereinafter, also referred to as “sheet width direction”) perpendicular to a sheet-transporting direction of the sheet because of any mechanical factors in the apparatus. As the mechanical factors in the apparatus, for example, any errors in manufacturing the rollers and/or any wear on long-term deterioration are illustrated. When performing a printing operation while there is such a deflection of the sheet, an image is printed on the sheet while it is deflected from an image forming position thereof. 
         [0006]    In order to align the image and the image forming position of the sheet accurately by taking the deflection of the sheet into consideration, a registration roller shift correction is carried out by which the deflection of the sheet can be corrected by shifting the sheet to the sheet width direction with the registration roller nipping the sheet. For example, Japanese Patent Application Publication No. 2007-22680 has disclosed an image forming apparatus in which a registration roller is arranged at an upstream side from an image forming position and a line sensor is arranged at a downstream side of the registration roller as well as the deflection of the sheet can be corrected by shifting the sheet to the sheet width direction based on the deflection amount of the sheet detected by the line sensor. 
         [0007]    Here, the following will describe a configuration of a general shift mechanism for shifting the sheet using the registration roller.  FIG. 1  shows an outline configuration example of an important portion of such a general shift mechanism  300 , which is seen from a side thereof. As shown in  FIG. 1 , a driving roller  32  constituting the registration roller  30  is provided with a shift mechanism  300  for shifting the registration roller  30  to a sheet width direction D 2 . The shift mechanism  300  contains a rack  302  attached to an end of the driving roller  32 , a round pinion  304  that is arranged to engage with the teeth  302   a  formed on a side surface portion of the rack  302  and a motor, not shown, that drives the pinion  304  to rotate. 
         [0008]    When driving the motor, the pinion  304  rotates. The driving roller  32  fixed on the rack  302  is then shifted by rotation of this pinion  304  to the sheet width direction D 2 . The rack  302  is biased under any predetermined urging force F by a spring  306  to a front side (frontward direction) of the image forming apparatus to maintain the shift of the registration roller suitably. The registration roller  30  stays at a home position HP thereof under normal conditions in which the registration roller  30  is not shifted. 
       SUMMARY OF THE INVENTION 
       [0009]    In such a registration roller shift correction, however, when performing the registration roller shift correction, shift amounts of the sheet are differently measured based on shift directions of the sheet if the registration roller  30  is shifted backward beyond the home position HP of the registration roller  30  or if the registration roller  30  is shifted frontward from the home position HP of the registration roller  30 . This is because the registration roller  30  is biased by the spring  306  against a side thereof (for example, a frontward side thereof) to shift the registration roller  30  stably. 
         [0010]      FIG. 2  shows an example of a past relationship between a specified value of the shift amount of the sheet and an actually measured value thereof at a period of time of the registration roller shift correction. In  FIG. 2 , a horizontal axis indicates to the specified value of the shift amount of the sheet and a vertical axis indicates to the actually measured value of the shift amount of the sheet. Right upper region indicates to a case where the sheet is shifted backward beyond the home position HP and left lower region indicates to a case where the sheet is shifted frontward before the home position HP. 
         [0011]    As shown in  FIG. 2 , for example, if the sheet P is shifted backward when setting the specified value of the shift amount of the sheet as 2 mm, the actually measured value becomes 2.3 mm as shown by dotted line of  FIG. 2 . Further, if the sheet P is shifted frontward when setting the specified value of the shift amount of the sheet as 2 mm, the actually measured value becomes 3 mm as shown by dotted line of  FIG. 2 . Thus, even if the same shift amount of the sheet is set as the specified values of the shift amount of the sheet, the shift amounts of the sheet are differently measured under any influence by the spring  306  when the registration roller  30  is shifted backward or the registration roller  30  is shifted frontward. In order to make the actually measured shift amounts of the sheet P identical to each other, for example, 2 mm when the registration roller  30  is shifted backward and the registration roller  30  is shifted frontward, it is required to set the specified value as 1.7 mm when the registration roller  30  is shifted backward and to set the specified value as 1.3 mm when the registration roller  30  is shifted frontward, as shown by alternate short and long dash line of  FIG. 2 . A registration roller shirt correction taken the shift direction into consideration, however, has not been carried out in the past. As a result thereof, the sheet is not shitted according to the value specified by a user, particularly, frontward before or backward beyond the home position HP of the registration roller so that the shifted sheet P may be deflected from its image-forming position to fail in forming an image on the shifted sheet with high accuracy. 
         [0012]    This invention addresses the above-mentioned issue and has an object to provide an improved image forming apparatus which may carry out a registration roller shift correction with high accuracy with taking a shift direction into consideration. 
         [0013]    To achieve the above-mentioned object, an image forming apparatus reflecting one aspect of the present invention contains an image-forming portion that forms an image on a sheet, a detecting unit that detects an amount of deflection of the sheet from a previously set reference position along a sheet width direction which is perpendicular to a transporting direction of the sheet, the detecting unit being positioned at an upstream side of the image-forming portion, a registration roller that is biased to one side of the sheet width direction by a predetermined urging force to shift toward the sheet width direction, the registration roller being positioned at an upstream side of the detection unit, and a control unit that is configured to determine a required shift amount of the sheet on which the sheet is shifted to the reference position based on the amount of deflection of the sheet detected by the detecting unit and to control the registration roller to perform first and second registration roller shift corrections, to shift the sheet toward she sheet width direction based on the required shift amount of the sheet and to send the shifted sheet to the image-forming portion, wherein the control unit is also configured to perform the first registration roller shift correction on a first required shift amount of she sheet when shifting the sheer to a first direction on the sheet width direction to acquire a first specified shift value that takes into consideration the urging force by which the registration roller is biased or to perform the second registration roller shift correction, which is different from the first registration roller shift correction, on a second required shift amount of the sheet when shifting one sheet to a second direction on the sheet width direction, the second direction being opposed to the first direction, to acquire a second specified shift value that takes into consideration the urging force by which the registration roller is biased. 
         [0014]    It is desirable to provide the image forming apparatus wherein the control unit is configured to acquire the first specified shift value from the first required shift amount of the sheet using a first correction equation in which the first required shift amount of the sheet based on the amount of deflection corresponds to the first specified shift value that takes into consideration the urging force when shifting the sheet to the first direction on the sheet width direction, or the control unit is configured to acquire the second specified shift value from the second required shift amount of the sheer using a second correction equation in which the second required shift amount of the sheet based on the amount of deflection corresponds to the second specified shift value that takes into consideration the urging force when shifting the sheet to the second direction on one sheet width direction. 
         [0015]    It is also desirable to provide the image forming apparatus further containing a driving unit that shifts the registration roller to the sheet width direction, wherein the driving unit includes a stepping motor, and wherein the first and second specified values are respectively converted to a pulse signal supplied to the stepping motor. 
         [0016]    It is further desirable to provide the image forming apparatus further containing a first shift table in which the first required shift amount of the sheet corresponds to a correction value for correcting the first required shift amount that is set by taking into consideration the urging force when shifting the sheet to the first direction, and a second shift table in which the second required shift amount of the sheet corresponds to a correction value for correcting the second required shift amount that is set by taking into consideration the urging force when shifting the sheet to the second direction, wherein the control unit is configured to acquire the first specified shift value from the first required shift amount of the sheet using the first shift table when shifting the sheet to the first direction on the sheet width direction, or the control unit is configured to acquire the second specified shift value from the second required shift amount of the sheet using the second shift table when shifting the sheet to the second direction on the sheet width direction. 
         [0017]    It is additionally desirable to provide the image forming apparatus further containing a manipulation unit for changing any of the first and second specified shift values. 
         [0018]    It is still further desirable to provide the image forming apparatus wherein the detecting unit detects an amount of deflection of the sheet which has been shifted by any of the first and second specified shift values by the registration roller, and the control unit is configured to correct any of the first and second specified shift values of a next transported sheet based on the amount of deflection of the shifted sheet, the amount of deflection being detected by the detecting unit. 
         [0019]    The concluding portion of this specification particularly points out and directly claims the subject matter of the present invention. However, those skilled in the art will best understand both the organization and method of operation of the invention, together with further advantages and objects thereof, by reading the remaining portions of the specification in view of the accompanying drawing(s) wherein like reference characters refer to like elements. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0020]      FIG. 1  is a diagram showing an outline configuration example of an important portion of a general shift mechanism of registration roller, which is seen from a side thereof; 
           [0021]      FIG. 2  is a graph showing an example of a past relationship between a specified value of the shift amount of the sheet and an actually measured value of the shift amount of the sheet at a period of time of the registration roller shift correction; 
           [0022]      FIG. 3  is a diagram showing a configuration example of an image forming apparatus according to a first embodiment of the invention; 
           [0023]      FIG. 4  is a diagram showing a relationship between a sheet and a sheet-detecting portion when calculating an amount of deflection of the sheet from a reference position thereof; 
           [0024]      FIG. 5  is a block diagram of the image forming apparatus for showing a configuration example thereof; 
           [0025]      FIG. 6  is a graph showing correction equations set based on a required shift amount and a specified shift value; 
           [0026]      FIG. 7  is a diagram showing a configuration example of a management screen for changing a numerical value in the correction equations; 
           [0027]      FIG. 8  is a flowchart showing an operation example of a control unit during an operation time of registration roller shift correction; 
           [0028]      FIGS. 9A and 9B  are tables showing configuration examples of the shift tables corresponding to the shift directions, the tables being stored in a storage portion of the image forming apparatus according to a second embodiment of this invention; and 
           [0029]      FIG. 10  is a flowchart showing an operation example of a control unit during an operation time of registration roller shift correction in the image forming apparatus according to the second embodiment of this invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0030]    The following will describe embodiments of this invention with reference to the drawings. 
       1. First Embodiment 
     &lt;Configuration Example of Image Forming Apparatus&gt; 
       [0031]    The image forming apparatus  100  according to the first embodiment of this invention acquires an amount of deflection Δx from a reference position La of a sheet P, which a sheet-detecting portion (detecting unit)  70  has been detected. The image forming apparatus  100  calculates a specified shift value for shifting the sheet P to the reference position La based on the amount of deflection Δx and corrects the calculated specified shift value using a previously set correction equation according to any shift direction of the sheet P to perform a registration roller shift correction with any urging force F which urges a registration roller  30  being taken into consideration. 
         [0032]      FIG. 3  shows a configuration example of the image forming apparatus  100  according to the first embodiment of the invention. It is to be noted that dimensions and/or ratios in the drawings are exaggerated for convenience of explanation and they may be different from actual ones. As shown in  FIG. 3 , the image forming apparatus  100  is an image forming apparatus of tandem type and is provided with an automatic document feeder  101  and an image forming apparatus body  102 . The automatic document feeder  101  is arranged on the image forming apparatus body  102  and feeds a manuscript M set on a manuscript holder to an image-scanning portion  82  in the image forming apparatus body  102  by a transporting roller or the like. 
         [0033]    The image forming apparatus body  102  contains the image-scanning portion  82 , an image-forming portion  60 , an intermediate transfer belt  8  and a fixing portion  72 . The image-scanning portion  82  scans and exposes the manuscript mounted on the manuscript holder using an optical system of a scanning and exposure apparatus and reads an image on the scanned manuscript using a charge coupled device (CCD) image sensor to perform a photoelectric conversion so that an image information signal is generated. An image processing section, not shown, performs analog processing, analog/digital conversion processing (hereinafter, referred to as “A/D conversion processing”), shading processing, image compression processing and the like on the image information signal and then outputs it to the image-forming portion  60 . 
         [0034]    The image-forming portion  60  forms the image by an electrophotography method and contains an image-forming unit  10 Y which forms a yellow image (Y), an image-forming unit  10 M which forms a magenta image (M), an image-forming unit  10 C which forms a cyan image (C) and an image-forming unit  10 K which forms a black image (K). In this embodiment, the common functions concerning colors are indicated by Y, M, C and K, which respectively show colors to be formed, following a number, for example,  10 . 
         [0035]    The image-forming unit  10 Y contains a photosensitive drum  1 Y, a charging portion  2 Y which is arranged around the photosensitive drum  1 Y, an exposing (optically writing) portion  3 Y, a developing portion  4 Y and a cleaning portion  6 Y. 
         [0036]    The image-forming unit  10 M contains a photosensitive drum  1 M, a charging portion  2 M which is arranged around the photosensitive drum  1 M, an exposing portion  3 M, a developing portion  4 M and a cleaning portion  6 M. 
         [0037]    The image-forming unit  10 C contains a photosensitive drum  1 C, a charging portion  2 C which is arranged around the photosensitive drum  1 C, an exposing portion  3 C, a developing portion  4 C and a cleaning portion  6 C. 
         [0038]    The image-forming unit  10 K contains a photosensitive drum  1 K, a charging portion  2 K which is arranged around the photosensitive drum  1 K, an exposing portion  3 K, a developing portion  4 K and a cleaning portion  6 K. 
         [0039]    The respective photosensitive drums  1 Y,  1 M,  1 C and  1 K, the charging portions  2 Y,  2 M,  2 C and  2 K, the exposing portions  3 Y,  3 M,  3 C and  3 K, the developing portions  4 Y,  4 M,  4 C and  4 K and the cleaning portions  6 Y,  6 M,  6 C and  6 K in the image forming units  10 Y,  10 M,  10 C and  10 K have the respectively common configurations. They will be described with Y, M, C and K being omitted except for any cases in which they are required to be distinguished. 
         [0040]    Each of the charging portions  2  charges a static charge uniformly around a surface of each of the photosensitive drums  1 . Each of the exposing units  3  is composed of, for example, a laser scanning exposure device of polygon mirror type. The exposing units  3  scan the surfaces of the photosensitive drums  1  using laser beam based on the image information signal to form latent images. The developing portions  4  develop the latent images formed on the surfaces of the photosensitive drums  1  using toners. This enables toner images, which are visual images, to be formed on the photosensitive drums  1 . 
         [0041]    The intermediate transfer belt  8  is stretched across plural rollers so as to be able to run around them. By moving the intermediate transfer belt  8  when operating primary transfer rollers, the toner images formed on the photosensitive drums  1  are transferred to image transfer positions of the intermediate transfer belt  8  (Primary Transfer). 
         [0042]    A paper feeder  20  is provided with plural feeding trays  20 A,  20 B and  20 C, which respectively contain sheets P each having a predetermined size, for example, A 3  or A 4  . The paper feeder  20  feeds the sheet P from any of the feeding trays  20 A,  20 B and  20 C using transporting rollers  21 ,  22  and the like to the registration roller  30 , which is arranged at a downstream side along a transporting direction of the sheet, through the loop forming roller  40 . It is to be noted that numbers of the feeding trays are not limited to three. If necessary, a single or plural large capacity feeder(s) that can contain a large number of sheets P may be arranged. 
         [0043]    The registration roller  30  has a pair of driving roller  32  and driven roller  34  and forms a loop when a leading edge of the sheet P is hit against the registration roller  30  by the loop forming roller  40  so that the sheet P can be deskewed. Further, the registration roller  30  nips the sheet P and shifts it to a sheet width direction D 2  based on a detection result of the sheet-detecting portion  70  to correct the deflection of the sheet P (registration roller shift correction). In this moment, a control unit corrects specified values of the shift amount of the sheet P according to the shift direction of the sheet P based on the amount of the deflection Δx detected by the sheet-detecting portion  70  using any correction equations, which are set according to the shift direction of the sheet P. The correction equations will be described later. 
         [0044]    When the registration roller shift correction is complete, the sheet P is transported to a secondary transfer unit  36  at a predetermined timing. The secondary transfer unit  36  transfers all of the toner images which have been transferred to the image transfer positions of the intermediate transfer belt  8 , on a surface of the sheet P transported from the paper feeder  20  to form a color image (Secondary Transfer). The sheet P on which the secondary transfer is performed is transported to the fixing portion  72 . 
         [0045]    The fixing portion  72  fixes the color image on the sheet P by heating and pressing the sheet P to which the color image is transferred. The sheet P on which the fixing portion  72  has fixed is ejected by a paper election tray  25  through paper ejection roller  24 . 
         [0046]    Further, the image forming apparatus  100  is provided with a sheet-reversing unit  27  for performing a duplex printing or the like. When a duplex printing mode is set, the image forming apparatus  100  leads the sheet P which the fixing portion  72  has fixed to the sheet-reversing unit  27  and then reverses a surface thereof to be again transported to the secondary transfer unit  36  where a color image or the like is formed on a back surface of the sheet P. 
       [Configuration Example of Registration Roller and Sheet-Detecting Portion] 
       [0047]      FIG. 4  shows a configuration example of the registration roller  30  and the sheet-detecting portion  70  and a relationship between the sheet P and the sheet-detecting portion  70  when a controller  50  calculates the amount of deflection Δx of the sheet P from a reference position La thereof. In this image forming apparatus  100 , as shown in  FIG. 4 , the reference position La of the sheet P is set as a reference position when forming an image on the sheet P. In this embodiment, the sheet p is transported so that a left side end Pa of the sheet P on the sheet width direction D 2  of the sheet P is passed through the reference position La of the sheet P. It is to be noted that the reference position La of the sheet P is set so that it varies for every sheet having different sizes. 
         [0048]    The sheet-detecting portion  70  is arranged at a downstream side of the registration roller  30  along the transporting direction D 1  of the sheet P. The sheet-detecting portion  70  is positioned so that a longitudinal direction thereof is parallel with the sheet width direction D 2 . The sheet-detecting portion  70  is composed of a line sensor in which photoelectric transducers are arranged in a row or an image sensor in which photoelectric transducers are arranged in a matrix state. 
         [0049]    The sheet-detecting portion  70  detects the left side end of the passing sheet P and acquires as the amount of deflection Δx a distance on the sheet width direction D 2  between the detected left side end Pa of the sheet P and the reference position La of the sheet P. In this embodiment, a sign of the amount of deflection Δx of the sheet P when the left side end Pa of the sheet P is deflected to a right side of the transporting direction D 1  of the sheet P from the reference position La thereof is defined as plus, “+”. Conversely, a sign of the amount of deflection Δx of the sheet P when the left side end Pa of the sheet P is deflected to a left side of the transporting direction D 1  of the sheet P from the reference position La thereof is defined as minus, “−”. 
         [0050]    The registration roller  30  is positioned at an upstream side of the sheet-detecting portion  70  on the transporting direction of the sheet D 1 . A rotation axis of the registration roller  30  is arranged so as to be parallel with the sheet-width direction D 2 . This registration roller  30  stays at a home position HP shown in  FIG. 1  unless the registration roller shift correction is performed. In this embodiment, the home position HP is set as the reference position La of the sheet P, for example, at a part of an outer side of the registration roller  30 . The registration roller  30  shifts along the sheet-width direction D 2  from the home position HP. In this embodiment, the right side from the home position HP on the sheet-width direction D 2  is defined as backward or plus, “+”. Conversely, the left side from the home position HP on the sheet-width direction D 2  is defined as frontward (a front side of the image-forming apparatus  100 ) or minus, “−”. It is to be noted that the backward direction corresponds to a first direction on the sheet-width direction D 2  while the frontward direction corresponds to a second direction on the sheet-width direction D 2 . 
         [0051]    The registration roller  30  is always biased to the frontward direction on the sheet-width direction D 2  by a predetermined urging force F of a spring  306  to shift the sheet P toward the sheet width direction D 2  stably. Thus, even if the same shift value is specified in the backward direction and the frontward direction, when the registration roller  30  is shifted to the frontward direction, the spring  306  biases the registration roller  30  by the urging force F thereof stronger than a case where the registration roller  30  is shifted to the backward direction so that the registration roller  30  is slightly more shifted to the frontward direction. Accordingly, in this embodiment, by correcting the specified shift value using correction equations set according to the shift directions, a shift amount of the sheet P is adjusted by taking into consideration the urging force F by the spring  306 . 
       [Block Configuration Example of Image-Forming Apparatus] 
       [0052]      FIG. 5  illustrates a block configuration example of the image forming apparatus  100 . As shown in  FIG. 5 , the image forming apparatus  100  contains a control unit  50  controlling an operation of whole of the image forming apparatus  100 . The control unit  50  includes, for example, a central processing unit (CPU)  52 , a read only memory (ROM)  54  and a random access memory (RAM)  56 . The CPU  52  performs an image forming process and/or a registration roller shift correction process by reading any desired programs stored in the ROM  54  and extracting the programs in the RAM  54  to execute them. 
         [0053]    The control unit  50  connects a manipulation and display portion  80 , the automatic document feeder  101 , the image-scanning portion  82 , the image-forming portion  60 , the sheet-detecting portion  70 , a registration roller shifting portion  90 , the fixing portion  72 , the paper feeder  20 , a storage portion  84  and a communication portion  86 , respectively through a bus. 
         [0054]    The manipulation and display portion  80  is configured to have a touch panel in which a location input device of electrostatic system or resistive film system and a display device such as a liquid crystal display panel or an organic electroluminescence (EL) display panel are combined. The manipulation and display portion  80  detects any input information, based on the user&#39;s input operation and supplies a manipulation signal to the control unit  50 . For example, the manipulation and display section  80  inputs various kinds of conditions input by the user when performing the image forming process such as paper weight of the sheet P, size of the sheet P. The manipulation and display section  80  inputs any information on an inclination, an intercept and the like to set or change a correction equation in the registration roller shift correction. The manipulation and display portion  80  supplies a manipulation signal based on the input information to the control unit  50 . 
         [0055]    The storage portion  84  is configured to include a semiconductor memory, a hard disk drive (HDD) and the like. The storage portion  84  stores any information on a correction equation 1 or a correction equation 2, which are used when performing the registration roller shift correction, a program for carrying out these correction equations and the like. The above-mentioned information on a correction equation 1 or a correction equation 2 may be stored in the ROM  54 . It is to be noted that the correction equation 1 corresponds to a first correction equation and the correction equation 2 corresponds to a second correction equation. 
         [0056]    The registration roller shifting portion  90  is configured to include a stepping motor. The registration roller shifting portion  90  shifts the registration roller  30  to the sheet-width direction D 2  by rotating the motor based on a driving signal (specified shift value) corresponding to the amount of deflection Δx, which is supplied from the control unit  50  when performing the registration roller shift correction. The specified shift value is converted to a pulse signal formed on the basis of the amount of deflection Δx. This allows the sheet P to be shifted to the sheet-width, direction D 2 , thereby correcting the deflection of the sheet P. 
         [0057]    The sheet-detecting portion  70  detects the amount of deflection Δx from the reference position La of a transported sheet P and supplies a manipulation signal based on the amount of deflection Δx to the control unit  50 . 
         [0058]    The image-forming portion  60  includes image forming units  10 Y,  10 M,  10 C and  10 K and performs any image forming processes based on any control information supplied from the control unit  50 . 
         [0059]    The paper feeder  20  feeds the sheet P corresponding to the information on the sheet size input by the manipulation and display portion  80  or the like from a feeding tray to the image-forming portion  60  based on the control signal supplied from the control unit  50 . 
         [0060]    The fixing portion  72  fixes a toner image to the sheet P by performing pressure and heat processing on the sheet P in which the toner images have been fixed in the image-forming portion  60 . 
         [0061]    The communication portion  86  is configured to include various kinds of interfaces such as network interface card (NIC), modulator-demodulator (MODEM) and universal serial bus (USB). The communication portion  86  communicates external equipment such as a personal computer which is connected through the communication portion  86 . 
       [Correction Equations] 
       [0062]    The following will describe the correction equations for correcting a required shift amount of the sheet P according to the shift directions of the sheet P.  FIG. 6  is a graph for explaining the correction equations for correcting a required shift amount of the sheet P. A vertical axis thereof indicates a specified shift value, which is set by taking into consideration the urging force F by the spring  306 , after the correction has been performed. A horizontal axis thereof indicates a required shift amount based on the amount of the deflection Δx detected by the sheet-detecting portion  70 . 
         [0063]    As the correction equations, two different equations are defined according to the shift directions of the sheet P. These equations are a correction equation 1 and a correction equation 2, The correction equation 1 is used when shifting the sheet P to the backward side. The correction equation 2 is used when shifting the sheet P to the frontward side. For example, when the image forming apparatus  100  is manufactured, the sheet-detecting portion  70  detects the amounts of the deflection of the sheets actually. Actual measured values are acquired by performing the registration roller shift correction based on the specified shift values corresponding to the acquired amounts of the deflection of the sheets. The above equations are formed on any differences between each of the actual measured values thus acquired and each of the specified shift values. 
         [0064]    First, the correction equation 1 used for the backward side will be described. The correction equation 1 is defined as the following equation (1). 
         [0000]        Y   1   =a   1 (Δ X /(a shift amount of motor/one pulse)+ b   1 )  (1)
 
         [0065]    Here, Y 1  is a specified shift value, which is set by taking into consideration an urging force F by the spring  306 , after the correction has been performed and indicates a pulse signal to be supplied to the registration roller shifting portion  90 ; a 1  is a correction coefficient, i.e., an inclination and b 1  is an intercept. The intercept b 1  is determined by a backlash in the registration roller shifting portion  90  that drives the registration roller  30 , ΔX is a required shift amount (mm) corresponding to the amount of the deflection Δx detected by the sheet-detecting portion  70 . The shift amount of motor per one pulse is previously set on the basis of a specification of the registration roller shifting portion  90  to be used. 
         [0066]    For example, the control unit  50  calculates the required shift amount ΔX based on the amount of the deflection Δx detected by the sheet-detecting portion  70  when the sheet-detecting portion  70  acquires the amount of the deflection Δx from the reference position La of the sheet P. In this embodiment, the required shift amount ΔX is set as the amount of the deflection Δx. For example, if a calculated required shift amount is ΔXa, this required shift amount ΔXa is substituted for the above-mentioned equation (1) so that the specified shift value Y a  can be acquired as pulse signal from the required shift amount ΔXa. 
         [0067]    Next, the correction equation 2 used for the frontward side will be described. The correction equation 2 is defined as the following equation (2). 
         [0000]        Y   2   =a   2 (ΔX/(a shift amount of motor/one pulse)+ b   2 )  (2)
 
         [0068]    Here, Y 2  is a specified shift value, which is set by taking into consideration an urging force F by the spring  306 , after the correction has been performed and indicates a pulse signal to be supplied to the registration roller shifting portion  90 ; a 2  is a correction coefficient, i.e., an inclination and b 2  is an intercept. The intercept b 2  is determined by a backlash in the registration roller shifting portion  90  that drives the registration roller  30 . ΔX is a required shift amount (mm) corresponding to the amount of the deflection Δx detected by the sheet-detecting portion  70 . The shift amount of motor per one pulse is previously set on the basis of a specification of the registration roller shifting portion  90  to be used. For example, if a calculated required shift amount is ΔXb, this required shift amount ΔXb is substituted for the above-mentioned equation (2) so that the specified shift value Y b  can be acquired as pulse signal from the required shift amount ΔXb. 
         [0069]    In the correction equation 2, its inclination a 2  is set so as to be smaller than the inclination a 1  of the correction equation 1. This is because the registration roller  30  is biased to the frontward direction by the spring  306  and when shifting the sheet P to the frontward side, the urging force F by the spring  306  is more added to the sheet so that an amount of shift of the sheet P is increased as compared with a case where the sheet P is shifted to the backward side. Thus, in the correction equation 2, by making the inclination smaller, the specified shift value Y 2  is set so as to be limited by the urging force F by the spring  306 . As a result thereof, the specified shift values Y 1  and Y 2  after the correction at the frontward and backward sides are identical to each other. 
         [0070]    Here, the specified shift values Y 1  and Y 2  obtained by the correction equations 1 and 2 may include an amount of correction which is set by taking into consideration a difference between the specified shift value and the actually measured value thereof occurred by an influence of friction between the sheet P and a transporting path on which the sheet P is transported, in addition to the amount of correction for correcting an influence by the urging force F of the spring  306 . In other words, as the paper weight and size of the sheet P to be used are made larger or humidity in a space of the image forming apparatus (or humidity in a room in which the image forming apparatus is settled) is made higher, the friction between the sheet P and a transporting path on which the sheet P is transported is made larger so that the difference between the specified shift value and the actually measured value thereof is relatively increased. Thus, it is possible to correct the required shift amount suitably based on the paper weight and size of the sheet P. Further, when performing duplex printing, the sheet P is flexed at a period of registration roller shift correction time so that a resistance of the sheet P is made stronger as compared with a case where a simplex printing is performed. In this case, it is also preferable to correct the required shift amount suitably. For example, frictional force is increased and the shift amount is decreased for that so that the specified shift value Y may be set by adding the shift amount by an amount of frictional force to the required shift amount. 
       [Configuration Example of Management Screen] 
       [0071]      FIG. 7  shows a configuration example of a management screen  800  to be displayed on the manipulation and display portion  80  for inputting and/or changing a numerical value in the correction equations. In this embodiment, as the management screen  800 , a touch panel is adapted. 
         [0072]    As shown in  FIG. 7 , the management screen  800  includes a various setting button  802  for performing any basic setting on the image forming, a copy button  804  for fixing an execution of the image forming, a scan button  806  for executing a scan or facsimile, a reset button  808  for suspending a copy or a scanning or resetting all of the settings, a comment column  810  for displaying any comments for user operation aid, and a changing screen  812  for changing a situation of the sheet P and various numerical values in the correction equations 1 and 2. 
         [0073]    The following will describe the changing screen  812 . The changing screen  812  is displayed on the management screen  800  when, for example, a user selects a button for changing the numerical value in the equations after the user has selected the various setting button  802 . The changing screen  812  contains a feeding tray specifying button  814 , a sheet size specifying button  816 , a paper weight specifying button  818 , a shift amount adjustment button (specified shift value adjustment button)  820 , an equation displaying screen  822 , a numerical value changing screen  824 , a cancel button  826 , and an OK button  828 . The feeding tray specifying button  814  is a button for specifying a feeding tray to be set. The sheet size specifying button  816  and the paper weight specifying button  818  are respectively buttons for setting a sheet size and a paper weight for the feeding tray specified by the feeding tray specifying button  814 . 
         [0074]    The shift amount adjustment button  820  is a button for inputting or changing the inclination a 1  and intercept b 1  of the correction equation 1 and the inclination a 2  and intercept b 2  of the correction equation 2. When the user selects the shift amount adjustment button  820 , the equation displaying screen  822  and the numerical value changing screen  824  are displayed on. the changing screen  812 . On the numerical value changing screen  824 , items of the inclination a 1  and intercept b 1  of the correction equation 1, the specified shift value Y 1 , the inclination a 2  and intercept b 2  of the correction equation 2 and the specified shift value Y 2 , number buttons for inputting or changing the value, up/down buttons for allowing the user to be shifted between the items and the like are displayed. 
         [0075]    The user can set any of the correction equations by inputting the inclination and the intercept of each equation operating the number buttons and the like while seeing the equation displaying screen  822 . The correction equations may be previously set when the image forming apparatus is manufactured. The user can change the numerical values of the inclination and intercept of the set correction equation based on a result of the registration roller shift correction operation. This allows an accuracy of alignment of an image and the sheet P to be improved. 
       [Operation Example of Image Forming Apparatus] 
       [0076]    The following will describe an operation example of the control unit  50  when performing the registration roller shift correction.  FIG. 8  shows the operation example of the control unit  50  of the image forming apparatus  100  when performing the registration roller shift correction. 
         [0077]    As shown in  FIG. 8 , at a ST 100 , the control unit  50  acquires the amount of the deflection Δx from the reference position La of the sheet P, which is transported from the paper feeder  20  or a reverse transporting route, by the sheet-detecting portion  70 . The control unit  50  calculates the required shift amount ΔX which is required for shifting the sheet P to the reference position La of the sheet P based on the amount of the deflection Δx acquired from the sheet-detecting portion  70 . A sign “+” or “−” is added to this required shift amount ΔX according to the shift direction of registration roller. The control unit  50  goes to a step S 102  after the control unit  50  has calculated the required shift amount ΔX. 
         [0078]    At the step S 102 , the control unit  50  determines whether or not the shift direction of the registration roller is the frontward direction in the required shift amount ΔX thus calculated. The control unit  50  determines whether or not the shift direction of the registration roller is the frontward direction by checking that the sign added to the required shift amount ΔX is “+” or “−”. The control unit  50  determines that the shift direction of the registration roller is the backward direction if the sign added to the required shift amount ΔX is “+” and then, goes to a step S 106 . Contrary, the control unit  50  determines that the shift direction of the registration roller is the frontward direction if the sign added to the required shift amount ΔX is “−” and then, goes to a step S 104 . 
         [0079]    At the step S 104 , the control unit  50  reads the correction equation 2 (the above-mentioned equation (2)) corresponding to the frontward direction out of a memory in the storage portion  84  or the like. The control unit  50  then substitutes the required shift amount ΔX to the read correction equation 2 to acquire the specified shift value Y 2  which is set by taking into consideration the urging force F of the spring  306 . This specified shift value Y 2  is converted to a pulse signal to be given to the registration roller shifting portion  90 . When acquiring the specified shift value Y 2 , the control unit  50  goes to a step S 108 . 
         [0080]    On the other hand, at the step S 106 , the control unit  50  reads the correction equation 1 (the above-mentioned equation (1)) corresponding to the backward direction out of the memory in the storage portion  84  or the like. The control unit  50  then substitutes the required shift amount ΔX to the read correction equation 1 to acquire the specified shift value Y 1  which is set by taking into consideration the urging force F of the spring  306 . This specified shift value Y 1  is converted to a pulse signal to be given to the registration roller shifting portion  90 . When acquiring the specified shift value Y 1 , the control unit  50  goes to the step S 108 . 
         [0081]    At the step S 108 , the control unit  50  performs the registration roller shift correction based on the acquired specified shift value Y 1  or Y 2 . For example, the registration roller  30  nips the sheet P and the specified shift value Y 1  or Y 2  (pulse signal) thus acquired is supplied to the registration roller shifting portion  90 . The control unit  50  then shifts the registration roller  30  frontward or backward from its home position HP to shift the sheet P to the reference position La of the sheet P. When the registration roller shift correction in complete, the control unit  50  controls the registration roller  30  or the like to transport the sheet P to the secondary transfer unit  36 . At the same time, the control unit  50  controls the registration roller  30  to release the nip of the sheet P. The control unit  50  then controls the registration roller  30  to shift to its home position HP and to bring its condition to the close condition. The control unit  50  then goes to a step S 110 . 
         [0082]    At the step S 110 , the control unit  50  starts image forming process. The sheet P is transported to the secondary transfer unit  36  at a predetermined timing and an image which has been formed on the intermediate transfer belt  8  is transferred to the sheet P. In this moment, the deflection of the sheet P has been corrected by the registration roller shift correction so that the image can be transferred to the sheet P without any shear. 
         [0083]    At a step S 112 , the control unit  50  again acquires the amount of the deflection Δx of the sheet P by the sheet-detecting portion  70 . In other words, the control unit  50  acquires the amount of the deflection Δx from the reference position La of the sheet P on which the registration roller shift correction has been performed. This is because there may be a case where the deflection of the sheet P is not completely corrected even if the registration roller shift correction is performed on the sheet P. The control unit  50  goes to a step S 114  when acquiring the amount of deflection Δx of the sheet P. 
         [0084]    At the step S 114 , the control unit  50  calculates a correction value for feeding back the amount of deflection Δx to the next transported sheet P based on the amount of deflection Δx of the sheet P acquired by the sheet-detecting portion  70 . For example, the control unit  50  calculates the amount of deflection Δx from the difference between the shift amount (specified shift amount) corresponding to the specified shift amount and the actually measured value and feeds the amount of deflection Δx thus calculated as the correction value back to the specified shift value Y of the next transported sheet P. When performing the registration roller shift correction on the next transported sheet P, the control unit  50  corrects the calculated specified shift value Y based on the correction equation using the correction value to form a pulse signal which is supplied to the registration roller shifting portion  90  to perform a registration roller shift correction on the next transported sheet P. This enables the registration roller shift correction to be carried out with high accuracy. It is to be noted that the correction equation itself may be amended on the basis of the fed-back correction value. 
         [0085]    As described above, in the first embodiment, when performing the registration roller shift correction, the specified shift value (required shift amount) is corrected using the correction equation 1 if the registration roller  30  or the sheet P is shifted backward. The specified shift value (required shift amount) is also corrected using the correction equation 2 if the registration roller  30  or the sheet P is shifted frontward. For example, when shifting the sheet P by 2 mm, the control unit  50  forms the specified shift value Y 1  based on the correction equation 1 so that the shift amount on the backward direction is 1.7 mm and forms the specified shift value Y 2  based on the correction equation 2 so that the shift amount on the frontward direction is 1.3 mm. This enables the specified shift value Y to be adjusted with taking into consideration the urging force F (burden) on which the registration roller  30  is biased so that the shift amount can be adjusted without any deflection on the shift direction. Thus, an accuracy of the registration roller shift correction can be improved so that an image can be formed on the sheet P with high accuracy. 
       2.  Second Embodiment 
       [0086]    The second embodiment is different from the first embodiment in that the specified shift value is corrected using a shift table to be used for each shift direction. It is to be noted that other components and operations of the image forming apparatus according to this embodiment are identical to those of the first embodiment so that the identical components are indicated by the same reference numbers, a detailed explanation of which will be omitted. 
       [Configuration Example of Shift Tables] 
       [0087]      FIGS. 9A and 9B  show configuration examples of the shift tables for correcting the required shift amount (specified shift value) corresponding to the shift direction of the sheet P.  FIG. 9A  shows a table TB 1  used when shifting the registration roller  30  (the sheet P) to the backward direction and  FIG. 9B  shows a table TB 2  used when shifting the registration roller  30  (the sheet P) to the frontward direction. It is to be noted that the shift table TB 1  corresponds to a first shift table and the shift table TB 2  corresponds to a second shift table. 
         [0088]    The storage portion  84  stores the shift table TB 1  to be used when the sheet P is shifted to the backward direction and the shift table TB 2  to be used when the sheet P is shifted to the frontward direction, respectively. For example, during the manufacture of the image forming apparatus  100 , the sheet-detecting portion  70  actually detects the amounts of deflection Δx of plural sheets P. The control unit  50  then acquires an actually measured value of each sheet P by performing the registration roller shift correction on each sheet P based on the required shift amount ΔX corresponding to the acquired amount of deflection Δx. The shift tables TB 1  and TB 2  may be created on the basis of a difference between the actually measured value thus acquired and the required shift amount ΔX. It is to be noted that although, in this embodiment, the shift tables TB 1  and TB 2  are separately configured, they may be configured as one shift table. 
         [0089]    The following describe the shift table TB 1  for the backward direction. As shown in  FIG. 9A , a row indicates the required shift amount ΔX and a column indicates paper weight of the sheet P. In this embodiment, the required shift amount ΔX is partitioned into three categories: less than 1 mm; 1 mm or more through less than 3 mm; and 3 mm or more through less than 5 mm. The paper weight is also partitioned into three categories: less than 100 g/m 2 ; 100 g/m 2  or more through less than 200 g/m 2 ; and 200 g/m 2  or more. As the correction value (shift amount (mm) required for the correction) for correcting the required shift amount ΔX, numerical values corresponding to the required shift amount ΔX and the paper weight are stored. 
         [0090]    Here, the paper weight of the sheet P is taken into consideration because as the paper weight of the sheet P is heavier, a frictional force (guiding resistance) occurred between the sheet P and the transporting route has an effect on the difference between the specified value and the actually measured value. Particularly, as the paper weight of the sheet P is heavier, the guiding resistance by the sheet P is made larger so that the shift amount based on the specified value is indicated so as to be made smaller. Therefore, in this embodiment, as the paper weight of the sheet P is heavier, the subtracting correction value is set to be small so that the shift amount is made larger. 
         [0091]    The following describe the shift table TB 2  for the frontward direction. As shown in  FIG. 9B , a row indicates the required shift amount ΔX and a column indicates paper weight of the sheet P. In this embodiment, the required shift amount ΔX is partitioned into three categories; less than 1 mm; 1 mm or more through less than 3 mm; and 3 mm or more through less than 5 mm. The paper weight is also partitioned into three categories: less than 100 g/m 2 ; 100 g/m 2  or more through less than 200 g/m 2 ; and 200 g/m 2  or more. As the correction value (shift amount (mm) required for the correction) for correcting the required shift amount ΔX, numerical values corresponding to the required shift amount ΔX and the paper weight are stored. Also, in this shift table TB 2 , because of the same reason as that of the shift table TB 1 , the subtracting correction value is set to be small as the paper weight of the sheet P is heavier, so that the shift amount is made larger. 
         [0092]    The correction values for the shift table TB 2  for the frontward direction are set so as to be larger than those for the shift table TB 1  for the backward direction as a whole. This is because the registration roller  30  is biased to the frontward side by the spring  306  and when the sheet P is shifted to the frontward direction, the urging force F by the spring  306  is added to the registration roller  30  so that the shift amount of the sheet P is made larger than that shifted when the sheet P is shifted to the backward direction. Thus, for example, when the required shift amount ΔX is 2 mm and the paper weight is 150 g/m 2 , the correction value is −0.4 mm in the shift table TB 1  for the backward direction while the correction value is −0.8 mm in the shift table TB 2  for the frontward direction. 
       [Operation Example of Image Forming Apparatus] 
       [0093]    The following will describe an operation example of the control unit  50  when performing the registration roller shift correction in this embodiment.  FIG. 10  shows the operation example of the control unit  50  of the image forming apparatus  100  when performing the registration roller shift correction according to the second embodiment. It is to be noted that common operations to the operations of the control unit  50  when performing the registration roller shift correction according to the first embodiment shown in  FIG. 8  will be briefly described. 
         [0094]    As shown in  FIG. 10 , at a ST 200 , the control unit  50  acquires the amount of the deflection Δx from the reference position La of the sheet P, which is transported from the paper feeder  20  or a reverse transporting route, by the sheet-detecting portion  70 . The control unit  50  calculates the required shift amount ΔX which is required for shifting the sheet P to the reference position La of the sheet P based on the amount of the deflection Δx acquired from the sheet-detecting portion  70 . The control unit  50  goes to a step S 202  after the control unit  50  has calculated the required shift amount ΔX. 
         [0095]    At the step S 202 , the control unit  50  determines whether or not the shift direction of the registration roller is the frontward direction in the required shift amount ΔX thus calculated. The control unit  50  determines that the shift direction of the registration roller is the backward direction if the sign added to the required shift amount ΔX is “+” and then, goes to a step S 206 . Contrary, the control unit  50  determines that the shift direction of the registration roller is the frontward direction if the sign added to the required shift amount ΔX is “−” and then, goes to a step S 204 . 
         [0096]    At the step S 204 , using the shift table TB 2  for the frontward direction, the control unit  50  acquires the corrected specified shift value Y 2  which is set by taking into consideration the urging force F of the spring  306 . Specifically, the control unit  50  reads the shift table TB 2  out of a memory in the storage portion  84  or the like and acquires the paper weight set by the manipulation and display portion  80  out of the memory in the storage portion  84  or the like. The control unit  50  also reads the corresponding correction value from the paper weight and the required shift amount Δx by referring to the shift table TB 2 . The control unit  50  then subtracts the read correction value from the required shift amount ΔX to obtain the corrected shift amount and converts the corrected shift amount to a pulse signal to acquire the specified shift value Y 2 . When acquiring the specified shift value Y 2 , the control unit  50  goes to a step S 208 . 
         [0097]    On the other hand, at the step S 206 , using the shift table TB 1  for the backward direction, the control unit  50  acquires the corrected specified shift value Y 1  which is set by taking into consideration the urging force F of the spring  306 . Specifically, the control unit  50  reads the shift table TB 1  out of a memory in the storage portion  84  or one like and acquires the paper weight set by the manipulation and display portion  80  out of the memory in the storage portion  84  or the like. The control unit  80  also reads the corresponding correction value from the paper weight and the required shift amount ΔX by referring to the shift table TB 1 . The control unit  50  then subtracts the read correction value from the required shift amount ΔX to obtain the corrected shift amount and converts the corrected shift amount to a pulse signal to acquire the specified shift value Y 1 . When acquiring the specified shift value Y 1 , the control unit  50  goes to a step S 208 . 
         [0098]    At the step S 208 , the control unit  50  performs the registration roller shift correction based on the calculated specified shift value Y 1  or Y 2 . For example, the registration roller  30  nips the sheet P and the specified shift value Y 1  or Y 2  (pulse signal) thus calculated is supplied to the registration roller shifting portion  90 . The control unit  50  then shifts the registration roller  30  frontward or backward from its home position HP to shift the sheet P to the reference position La of the sheet P. When the registration roller shift correction is complete, the control unit  50  goes to a step S 210 . 
         [0099]    At the step S 210 , the control unit  50  starts image forming process. At a step S 212 , the control unit  50  again acquires the amount of the deflection Δx of the sheet P from the sheet-detecting portion  70 . At the step S 214 , the control unit  50  calculates a correction value for feeding back the amount of deflection Δx to the next transported sheet P based on the amount of deflection Δx of the sheet P acquired from the sheet-detecting portion  70 . Namely, the control unit  50  calculates the correction value for correcting the correction value in the shift tables TB 1  and TB 2 . In this embodiment, the control unit  50  repeats such a series of operation. 
         [0100]    As described above, in the second embodiment, when performing the registration roller shift correction, the specified shift value (required shift amount) is corrected using the shift table TB 1  if the registration roller  30  or the sheet P is shifted backward. The specified shift value (required shift amount) is also corrected using the shift table TB 2  if the registration roller  30  or the sheet P is shifted frontward. This enables the specified shift value (required shift amount) to be adjusted with taking into consideration the urging force F by which the registration roller  30  is biased so that the shift amount can be adjusted without any deflection on the shift direction. Thus, an accuracy of the registration roller shift correction can be improved so that an image can be formed on the sheet P with high accuracy. 
         [0101]    Although the present invention has been fully described by way of examples with reference to the accompanying drawings, it is to be noted that various changes and modifications will be apparent to those skilled in the art. Therefore, unless otherwise such changes and modifications depart from the scope of the present invention, they should be construed as being included therein. 
         [0102]    Although in the above-mentioned first embodiment, the user has changed the specified shift value obtained using the correction equations on the management screen  800  in the manipulation and display portion  80 , this invention is not limited thereto. For example, the user may change the numerical value of the correction value in the shift table TB 1  or TB 2  on the management screen  800  in the manipulation and display portion  80  as described in the second embodiment. 
         [0103]    Although, in the above-mentioned first embodiment, one correction equation has been used for a predetermined condition of the paper weight of the sheet P, this invention is not limited thereto. Plural correction equations may be set for every paper weight of the sheet P. Further, in addition to the paper weight of the sheet P, a size of the sheet and/or humidity in the apparatus may be applied thereto. 
         [0104]    It should be understood by those skilled in the art that various modifications, combinations, sub-combinations and alterations may occur depending on design requirements and other factors insofar as they are within the scope of the appended claims or the equivalents thereof.