Patent Publication Number: US-8540240-B2

Title: Sheet conveying apparatus and image forming apparatus

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a sheet conveying apparatus and an image forming apparatus such as a printer or photocopier including the same. 
     2. Description of the Related Art 
     In general, it is known that an image forming apparatus such as a laser beam printer or color photocopier includes a sheet conveying apparatus which conveys a sheet fed from a sheet feeding cassette or a manual feeding tray into the image forming apparatus. 
     In a sheet conveying apparatus, it is also known that the sheet is conveyed along a conveying path with the leading edge of the sheet in a skew feeding state slightly rotated with respect to the conveying direction due to such as a difference of the conveying speed by conveying rollers or a misalignment of the image transferring portion of the conveying rollers. If an image is formed with the sheet conveyed in a skew feeding state like this, it becomes a cause for forming a bad image such as an image formed on the sheet bent with respect to the sheet. 
     Therefore, conventionally, a sheet conveying apparatus has a skew correction apparatus which corrects for the skew feeding of a sheet to prevent the sheet from being conveyed in a skew feeding state. The skew correction apparatus includes a registration roller (downstream roller) for conveying the sheet to the transferring portion and an upstream roller for conveying the sheet to the registration roller. In addition, the sheet conveyed by the upstream roller abuts the registration roller and bends the sheet itself to form a loop therein, thereby correcting for the skew feeding of the sheet. 
     In the conventional skew correction apparatus, a loop is formed in the sheet by the leading edge of the sheet abutting a nip portion of the registration roller in a stop state (non-rotating state). Conveying the sheet to the registration roller by the upstream roller continues until the whole of the sheet leading edge abuts the nip portion of the registration roller, and the sheet itself is rotated to correct for the skew feeding of the sheet (See Japanese Patent Laid-Open Nos. 6-336353 and 6-345294). 
     However, in the conventional skew correction apparatus, if the rigidity of the sheet conveyed from the upstream roller is high and the leading edge of the sheet abuts the nip portion of the registration roller in a skew feeding state, the leading edge of the sheet encroaches on the nip portion of the registration roller. If the leading edge of the sheet encroaches on the nip portion of the registration roller like this (the state in which the sheet is squeezed into the registration roller), the sheet is made immovable by the registration roller. 
     Even if a loop is formed by bending the sheet in such a state, the sheet itself cannot rotate because it is made immovable by the registration roller, and the skew feeding of the sheet cannot be corrected for. 
     Therefore, conventionally, a sheet conveying apparatus, which is provided with a skew correction apparatus for preventing the sheet from encroaching on the nip portion of the registration roller by rotating the registration roller backward in a direction reverse to the rotation for conveying the sheet, has been devised (See Japanese Patent No. 4016621). 
     By rotating the registration roller backward, it is clearly possible to resolve the sheet-encroaching state even if the sheet has encroached on the nip portion of the registration roller, and it is also possible to prevent the sheet from encroaching on the registration roller. 
     However, in the sheet conveying apparatus described in the Japanese Patent No. 4016621, the registration roller is rotated backward a given amount regardless of the extent of the amount of encroachment of the sheet on the registration roller. Thus, if the registration roller is rotated backward a given amount even when the amount of encroachment of the sheet on the registration roller is small, there is a concern that damage such as “leading edge curling” or “leading edge folding” may occur in the sheet due to the backward rotation of the registration roller as illustrated in  FIGS. 13A and 13B . 
     Especially when the surface of the registration roller is made of rubber material, in case of the leading edge of the sheet being curled or the like, the leading edge of the sheet is caught by the surface of the roller because the friction coefficient of the rubber surface is high, whereby the above-mentioned “leading edge curling” or “leading edge folding” is apt to occur. 
     Thus, it is desirable to provide a sheet conveying apparatus in which the above-mentioned problem is solved by changing the backward rotation time for rotating the downstream roller backward based on the skew amount of the conveyed sheet. 
     SUMMARY OF THE INVENTION 
     The sheet conveying apparatus of the present invention, in which a loop is formed in a sheet between an upstream roller and a registration roller by the registration roller abutting the sheet conveyed by the upstream roller to correct for the skew feeding of the sheet, includes a driving apparatus which rotates the registration roller forward or backward with respect to the sheet conveying direction, a detection portion which detects a skew amount of the sheet before the sheet abuts the registration roller, and a controller which determines the backward rotation amount for the downstream roller to rotate backward based on the detection results of the detection portion and controls the driving apparatus to rotate the downstream roller backward based on the backward rotation amount determined by the controller with the sheet conveyed by the upstream roller abutting the downstream roller. 
     According to the present invention, the backward rotation time of the registration roller which the leading edge of the sheet abuts is changed to correct for the skew feeding of the sheet, based on the detected skew amount. By this, the registration roller makes a minimum required backward rotation at the time of skew correction of the sheet, so that it is possible to resolve the state in which the leading edge of the sheet encroaches on the nip portion of the registration roller and reduce the occurrence of “leading edge curling” or “leading edge folding” of the leading edge of the sheet. 
     Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view showing a schematic configuration of an image forming apparatus having a sheet conveying apparatus; 
         FIG. 2  is an exploded perspective view of a skew correction apparatus provided in the sheet conveying apparatus; 
         FIG. 3  is a block diagram of a controller provided in the sheet conveying apparatus; 
         FIG. 4  is a flowchart showing the operation of sheet skew correction of the sheet conveying apparatus; 
         FIG. 5A  is a side view showing the state in which the sheet has encroached on a nip portion of a registration roller, and  FIG. 5B  is a plan view showing the state in which the sheet has encroached on the nip portion of the registration roller; 
         FIG. 6  is a graph showing the relationship between the skew amount of the sheet and the encroachment amount of the sheet; 
         FIG. 7  is a view showing the state in which the sheet is conveyed askew on the registration roller; 
         FIG. 8  is a view showing the state in which the sheet is conveyed askew on the registration roller; 
         FIG. 9  is a time chart showing the rotating operation of the registration roller; 
         FIGS. 10A ,  10 B and  10 C are perspective views showing the operation of skew correction of the sheet of the skew correction apparatus provided in the sheet conveying apparatus, each of which shows a different state; 
         FIGS. 11A ,  11 B and  11 C are side views showing the operation of skew feeding correction of the skew correction apparatus included in the sheet conveying apparatus, each of which shows a different state; 
         FIG. 12A  is a side view of an apparatus which includes a contact image sensor as a sensor provided in the sheet conveying apparatus, and  FIG. 12B  is a plan view of the apparatus of  FIG. 12A ; and 
         FIG. 13A  is a plan view showing the state of “leading edge curling” or “leading edge folding” of a sheet generated by the operation of sheet skew correction in a conventional sheet conveying apparatus, and  FIG. 13B  is a side view of  FIG. 13A . 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings. 
     A digital color printer  1  (hereinafter referred to in brief as a “printer”), which is an example of an image forming apparatus according to an embodiment of the present invention, is formed in rectangular shape when seen from a side, as illustrated in  FIG. 1 . In the upper side of the printer, sheet discharging trays  140  and  141  are installed in two upper and lower stages respectively for discharging a sheet P with an image formed thereon, and in the lower side thereof, an image forming portion  2  for forming an image on the sheet P is installed. In the lower side of the image forming portion  2 , sheet feeding cassettes  111  and  112  are installed in two upper and lower stages for stacking the sheet P on which the image is to be formed. The sheets P of these sheet feeding cassettes  111  and  112  are conveyed to the image forming portion  2  and sheet discharging trays  140  and  141  by a sheet conveying apparatus  3 . 
     First, the image forming portion  2  will be described in detail. The image forming portion  2  installed in the printer  1  has laser beam scanners  103   a  to  103   d  of a four-drum full color type, as illustrated in  FIG. 1 . The image forming portion  2  also includes four image forming units including photosensitive drums  101   a  to  101   d , charging rollers  102   a  to  102   d , development devices  104   a  to  104   d  and cleaners  107   a  to  107   d . These four image forming units form toner images of four colors of yellow (Y), magenta (M), cyan (C), and black (Bk). The photosensitive drums  101   a  to  101   d  are configured to rotate in an arrow direction illustrated in  FIG. 1  by the driving force of a driving apparatus not illustrated. 
     As illustrated in  FIG. 1 , primary transferring rollers  105   a  to  105   d  are arranged facing the respective photosensitive drums  101   a  to  101   d . An intermediate transfer member belt  106 , which is supported by the respective photosensitive drums  101   a  to  101   d  and the primary transferring rollers  105   a  to  105   d , passes between the respective photosensitive drums  101   a  to  101   d  and the primary transferring rollers  105   a  to  105   d.    
     The intermediate transfer member belt  106  is wound on a driving roller  5 , a tension roller  6  and a secondary transferring opposite roller  109   b , the driving roller  5  rotates in the arrow direction as illustrated in the drawing, and the intermediate transfer member belt  106  also rotates in the same direction as the driving roller  5 . A fixing portion  110  is installed near a secondary transferring portion  118 . 
     Next, the image forming operation of the image forming portion  2  of the printer  1  configured as described above will be described. When the image forming operation starts and image signals of four colors of yellow (Y), magenta (M), cyan (C), and black (Bk) are input into the laser beam scanners  103   a  to  103   d , the laser beam scanners  103   a  to  103   d  emit laser beams. 
     The surfaces of the photosensitive drums  101   a  to  101   d  are charged with uniform electric charge beforehand by charging rollers  102   a  to  102   d  and the charged surfaces are irradiated with laser beams emitted by the laser beam scanners  103   a  to  103   d . By the irradiation with laser beams by the laser beam scanners  103   a  to  103   d , electrostatic latent images of yellow, magenta, cyan, and black are formed on the photosensitive drums  101   a  to  101   d.    
     The electrostatic latent images formed on the photosensitive drums  101   a  to  101   d  are developed by yellow, magenta, cyan, and black toners to be visualized by the development devices  104   a  to  104   d . The toners developed on the respective photosensitive drums  101   a  to  101   d  are sequentially transferred to the intermediate transfer member belt  106  as transfer bias is applied to the intermediate transfer member belt  106  from the primary transferring rollers  105   a  to  105   d , so as to form a full color image on the intermediate transfer member belt  106 . After being transferred, the toners remaining on the photosensitive drums  101   a  to  101   d  are removed by the cleaners  107   a  to  107   d  to be prepared for the next image forming. 
     Meanwhile, the sheet P to which the full color image formed on the intermediate transfer member belt  106  is to be transferred is fed from the sheet feeding cassettes  111  and  112  or a manual feeding portion  113 . The uppermost sheet P is separated from the sheet bundle stacked in the sheet feeding cassettes  111  and  112  by a pickup roller  150  and is fed to the conveying apparatus  3  by conveying rollers  114 . Further, by feeding from the manual feeding portion  113 , the same uppermost sheet P is separated to be fed to the sheet conveying apparatus  3 . 
     The skew of the sheet P fed to the sheet conveying apparatus  3  from the sheet feeding cassettes  111  and  112  or the manual feeding portion  113  is corrected for by the sheet conveying apparatus  3  before it is synchronized with the leading edge of the image on the intermediate transfer member belt  106  and is conveyed to the secondary transferring portion  118 . The image formed on the intermediate transfer member belt  106  is transferred to the sheet P conveyed to the secondary transferring portion  118  by the secondary transfer bias applied to the secondary transferring roller  109   a , and the sheet P is conveyed to the fixing portion  110 . The sheet P is heated and pressed in the fixing portion  110 , so that the toner image on the sheet P is melted and mixed for the image to be fixed on the sheet. 
     The sheet P with the image fixed in the fixing portion  110  passes the conveying path  25  to be discharged from the discharging portion  119   a  or  119   b  to the sheet discharging trays  140  and  141 . 
     Next, the sheet conveying apparatus  3  will be described in detail. The sheet conveying apparatus  3  includes a skew correction apparatus  30  which corrects for the skew of the conveyed sheet P. The skew correction apparatus  30  includes a registration roller  120  as a downstream roller which conveys the sheet P to the secondary transferring portion  118  and an upstream roller  115  which conveys the sheet P to the registration roller  120 , as illustrated in  FIG. 2 . The skew correction apparatus  30  includes a resist motor  61  as a driving apparatus which rotates the registration roller  120  forward or backward with respect to the sheet conveying direction. Further, the sheet conveying apparatus  3  includes resist sensors  117   a  to  117   d  which are a detection portion arranged across the sheet conveying direction in a position more upstream than the registration roller  120  to detect the leading edge of the sheet P and the skew amount S of the sheet. The resist sensors  117   a  to  117   d  (hereinafter referred to in brief as numeral  117  to indicate all of the resist sensors) include optical sensors, for instance CCD (Charge Coupled Device) image sensors. 
     Here, the registration roller  120  will be described in detail. The registration roller  120  includes, as illustrated in  FIG. 2 , a lower resist roller  10 , both ends of which are journaled, and which can rotate forward or backward with respect to the sheet conveying direction, and an upper resist roller  20 , both ends of which are also journaled, and which can rotate forward or backward with respect to the sheet conveying direction. 
     The lower resist roller  10  is formed as one body with a plurality of rubber rollers  10   b  attached to a metal shaft  10   a . A gear  71  is mounted at one end of the metal shaft  10   a , and is connected to an output shaft  61   a  of the resist motor through a gear  72 . The upper resist roller  20  is also formed as one body with a plurality of rubber rollers  20   b  attached to a metal shaft  20   a . The outer shape of the rubber roller  10   b  itself is formed to 20Ø, and the outer shape of the roller  20   b  itself is also formed to 20Ø like the rubber roller  10   b . Further, the roller  20   b  is made of polyacetal (POM). 
     The upper resist roller  20  and the lower resist roller  10  are arranged facing each other so that the roller  20   b  attached to the upper resist roller  20  and the rubber roller  10   b  attached to the lower resist roller  10  contact each other. The upper resist roller  20  and the lower resist roller  10  are pressed by springs  13  mounted respectively on a plurality of unillustrated bearing portions that bear the upper resist roller  20 . Therefore, as illustrated in  FIG. 2 , the nip portion  15  is formed at the position at which the rubber roller  10   b  and the roller  20   b  come into contact. 
     Next, a controller of the printer  1  will be described.  FIG. 3  is a block diagram of the controller  50  which is a control portion of the sheet conveying apparatus  3 . An operation portion  200  of the printer  1 , the resist motor  61 , a preresist motor  60 , a resist sensor  117  and a sheet feeding motor  54  are each connected to the controller  50  connected to an external computer  201  through a network. 
     The controller  50  outputs a signal to the sheet feeding motor  54  when a signal is output from the operation portion  200  or the connected external computer  201 . The controller  50  sets a backward rotation start timing for the registration roller  120  to start backward rotation, and the backward rotation time for the registration roller  120  to rotate backward, based on the detection results by the resist sensor  117 . Based on these, the controller  50  outputs signals to the resist motor  61  or the like to rotate forward or backward and stop the resist motor  61  with respect to the sheet conveying direction, and further controls backward start timing and backward rotation time (backward rotation amount). 
     Next, the operation of the sheet conveying apparatus  3  configured as described above will be described in detail. When the start of a print job is executed by the external computer  201  connected by the network to the operation portion  200  of the printer  1  or the printer  1 , supplying (feeding) of the sheet P starts (step  101  of  FIG. 4 , hereinafter referred to in brief as “SXXX”). When the feeding is started, the uppermost sheet is separated by the pickup roller  150  from the sheet bundle stacked in the sheet feeding cassettes  111  and  112  or the manual feeding portion  113  to be fed to the sheet conveying apparatus  3  (S 102 ). 
     The sheet P fed to the sheet conveying apparatus  3  is conveyed at a given sheet conveying speed (so-called process speed), and after being conveyed to the upstream roller  115 , is conveyed to the registration roller  120  by the upstream roller  115  (S 103 ). 
     The leading edge downstream in the sheet conveying direction of the sheet P (hereinafter referred to in brief as a “downstream leading edge”) conveyed by the upstream roller  115  is detected by the resist sensor  117 . As the leading edge of the sheet P is detected by each of the resist sensors  117   a  to  117   d , the skew amount S of the conveyed sheet is detected (S 104 ). 
     Here, the skew amount S refers, as illustrated in  FIG. 7 , to a tilt angle (amount) of a corner portion Pb of the sheet leading edge on the side being retarded (hereinafter, referred to in brief as a “retarded corner portion”) with respect to a corner portion Pa of the sheet leading edge on the side being advanced (hereinafter, referred to in brief as an “advanced corner portion”), in the conveying direction in the downstream leading edge of the sheet P. 
     Next, the phenomenon of encroachment of the sheet P on the registration roller  120  will be briefly described. The encroachment phenomenon refers, as illustrated in  FIG. 5A , to the sheet P being conveyed toward the registration roller  120  by the upstream roller  115  entering the nip portion  15  of the registration roller  120 . X 2  is a position of a center line of the registration roller  120 , X 3  is a position at which the sheet has not encroached on the registration roller  120  but has stopped when the leading edge of the sheet P abuts, and X 1  is a position at which the leading edge of the sheet P has encroached on the registration roller  120  and stopped. The distance from the X 1  to X 3  becomes the encroachment amount Lb. 
       FIG. 5B  is a view showing the state in which the leading edge of the sheet P has encroached on the nip portion  15  of the registration roller  120 . The greater the skew amount of the sheet P being conveyed is, the more easily the encroachment phenomenon occurs; the encroachment amount Lb depends greatly on the skew amount S. Like the graph illustrated in  FIG. 6 , the encroachment amount Lb of the sheet P into the nip portion  15  of the registration roller is increased almost in proportion to the skew amount S. 
     Next, detection of the skew amount S of the conveyed sheet P will be described in detail. Each of the resist sensors  117   a  to  117   d  detects the leading edge of the sheet P conveyed in the direction of arrow A from the upstream roller  115  in a skew feeding state. In the downstream leading edge of the sheet P, a time difference of passing time between both leading edges is obtained from a passing time of the leading edge P 1  on the side advanced in the conveying direction detected by the resist sensor  117   d  and a passing time of the leading edge P 2  on the side retarded in the conveying direction detected by the resist sensor  117   a . A delay amount s 1  is calculated by multiplying the time difference by the conveying speed of the sheet P conveyed. 
     As illustrated in  FIG. 7 , provided that the distance (interval) from the position at which the resist sensor  117   d  detects the advanced leading edge P 1  of the sheet P to the position at which the resist sensor  117   a  detects the retarded leading edge P 2  of the sheet P is L 1 , the skew amount S can be calculated (defined) from the skew amount s 1  by the equation below.
 
 S=a  tan( s 1 /L 1)  (Equation 1)
 
     When a sheet with the width narrower than the distance (interval) L 1  between the detection position of the resist sensor  117   a  and the detection position of the resist sensor  117   d  is conveyed, the resist sensor  117   c  and the resist sensor  117   b  detect the leading edge of the sheet. 
     In the downstream leading edge of the sheet P, a time difference of passing time between both leading edges is obtained from a passing time of the advanced leading edge P 3  in the conveying direction detected by the resist sensor  117   c  and a passing time of the retarded leading edge P 4  in the conveying direction detected by the resist sensor  117   b . A delay amount s 2  is calculated by multiplying the time difference by the conveying speed of the conveyed sheet P. 
     As illustrated in  FIG. 7 , if the distance (interval) from the position at which the resist sensor  117   c  detects the advanced leading edge P 3  of the sheet P to the position at which the resist sensor  117   b  detects the retarded leading edge P 4  of the sheet P is L 2 , the skew amount S can be calculated (defined) from the skew amount s 2  by the following equation.
 
 S=a  tan( s 2 /L 2)  (Equation 2)
 
     The controller  50  rotates the registration roller  120  backward to prevent the sheet P from encroaching on the nip portion  15  of the registration roller. The controller  50  sets the backward rotation time for the registration roller  120  to rotate backward from the skew amount S of the sheet P detected by the resist sensor  117  (S 105 ). 
     Next, the changing of the backward rotation time of the registration roller  120  will be described in detail. As described above, the encroachment amount Lb tends to be proportional to the skew amount S of the sheet P; the greater the skew amount S of the sheet P is, the more the encroachment amount of the sheet P on the nip portion  15  of the registration roller becomes. Therefore, the greater the skew amount S of the sheet P is, the longer the backward rotation time of the registration roller  120  (the greater the backward rotation amount) is set, with the leading edge of the sheet contacting the registration roller  120 . On the other hand, the smaller the skew amount S of the sheet P is, the shorter the backward rotation time of the registration roller  120  (the smaller the backward rotation amount) is set. The encroachment amount G of the sheet for calculating the backward rotation time (backward rotation amount) of the registration roller  120  can be calculated (defined) by the following equation using the skew amount S and a proportional constant a (S 105 ).
 
 G=a·S   (Equation 3)
 
     The backward rotation time of the registration roller  120  is set by the controller  50  to correspond to the encroachment amount G. In addition, the backward rotation timing for starting the backward rotation of the registration roller  120  is set to correspond to the backward rotation time of the registration roller  120  (S 105 ). 
     Next, the backward rotation operation of the registration roller  120  will be described in detail. After the leading edge of the conveyed sheet P is detected by the resist sensor  117  and the leading edge of the sheet has passed the resist sensor  117 , the registration roller  120  starts rotation in the direction opposite to the conveying direction of the sheet P by the backward rotation of the resist motor  61 . 
     At this time, if the registration roller  120  stops backward rotation before skew correction of the sheet P is completed, there is a concern that the sheet P will encroach on the nip portion  15  of the registration roller again, and the loop R will need to be formed to correct for the skew of the sheet P. In such a case, a sufficient effect of skew correction cannot be obtained. Therefore, setting is performed such that the backward rotation of the registration roller  120  continues until the retarded corner portion Pb of the downstream leading edge of the skewed sheet P abuts the nip portion  15  of the registration roller (S 105 ). That is, with the whole region of the downstream leading edge of the sheet abutting the nip portion of the registration roller, the controller  50  stops the backward rotation of the registration roller  120  once the skew of the sheet is corrected for. 
     The start time of the backward rotation, which is the start timing of the backward rotation of the registration roller  120 , is set based on the time at which the retarded corner portion Pb of the downstream leading edge of the sheet P abuts the nip portion  15  of the registration roller, when the skew correction of the sheet P is completed. For this, the time t_G 2  when the retarded corner portion Pb of the downstream leading edge of the sheet P abuts the nip portion  15  of the registration roller is calculated. 
     Next, calculation of the time t_G 2  will be described in detail. Time t_a is the time at which the leading edge  123  of the sheet P reaches the sheet leading edge detection position of the resist sensor  117   a , as illustrated in  FIG. 8 . As for the conveying direction of the sheet, the distance from the sheet leading edge detection position of the resist sensor  117  to the nip portion  15  of the registration roller is referred to as L 3 . As for the width direction of the sheet conveying direction, the distance from the leading edge  123  of the sheet P to the leading edge  122  of the retarded corner portion Pb of the sheet P that abuts the position  121  of the widthwise outer end portion of the roller  20   b  arranged outermost is referred to as L 4 . Further, the conveying speed of the sheet v and t_G 2  from the skew amount S is calculated by the following equation.
 
 t   —   G 2 =t   —   a +{( L 3 +L 4)tan( S )}/ v   (Equation 4)
 
     The time t_G 2  at which the leading edge  122  of the retarded corner portion Pb of the sheet P abuts the nip portion  15  of the registration roller, and the start time of the backward rotation of the registration roller  120  from the backward rotation time that is changed based on the skew amount S are determined. Based on these, the backward rotation start time is set such that the smaller the detected skew amount S is, the shorter the backward rotation time becomes (the smaller the backward rotation amount becomes), and the backward rotation stop of the registration roller  120  becomes later than t_G 2  (S 105 ). 
       FIG. 9  is an example of the timing chart for the operation of the registration roller that is set such that the rotation drive of the registration roller stops later than t_G 2 , the time at which the leading edge  122  of the retarded corner portion Pb of the sheet P abuts the nip portion  15  of the registration roller. In  FIG. 9 , t_G 1  is the time at which the advanced corner portion Pa of the sheet P abuts the nip portion  15  of the registration roller  120 . As illustrated in  FIG. 9 , the backward rotation of the registration roller  120  is started after time t_G 1  has passed. That is, the registration roller  120  rotates backward in a state in which the leading edge of the sheet abuts the registration roller  120 . In addition, the backward rotation of the registration roller  120  is stopped after time t_G 2  has passed. Unlike the example in  FIG. 9 , the stop of the backward rotation of the registration roller  120  becomes later than the time t_G 2 , even when the backward rotation of the registration roller  120  starts earlier than the time t_G 1  from the time t_G 2  and the backward rotation time of the registration roller  120 . 
     When the backward rotation time and the start time of the backward rotation for the registration roller  120  are set by the controller  50 , the skew correction of the sheet P starts (S 106 ), and the registration roller  120  starts backward rotation at the set start time of the backward rotation (S 107 ). 
     As illustrated in  FIGS. 10A and 11A , the sheet P conveyed to the registration roller  120  continues to be conveyed in the arrow A direction by the upstream roller  115  even after the leading edge of the sheet P abuts the nip portion  15  of the registration roller. By the force pushing out the sheet P ahead by the conveying of the upstream roller  115 , the sheet P is bent so as to form a loop, as illustrated in  FIGS. 10B and 11B . 
     By the force pushing out the leading edge of the sheet P by the loop R formed like this, the retarded corner portion Pb of the sheet P is pushed out toward the registration roller  120 . As the retarded corner portion Pb of the sheet P is pushed out toward the registration roller  120 , the whole of the leading edge downstream of the sheet conveying direction rotates to correct for the skew of the sheet. 
     After the skew of the sheet P is corrected, the registration roller  120  is rotated forward again by the resist motor  61  to convey the sheet P to the secondary transferring portion  118 , as illustrated in  FIGS. 10C and 11C . The sheet P that has the image transferred on the sheet P in the secondary transferring portion  118  is discharged out of the printer  1  after the image is fixed in the fixing portion  110  on the sheet (S 109 ), and printing by the printer  1  is completed (S 110 ). 
     As described above, the skew amount S of the sheet is detected, and based on the detected skew amount S, the backward rotation time of the registration roller  120  is changed to rotate backward until the retarded corner portion Pb of the downstream leading edge of the sheet P abuts the nip portion  15  of the registration roller. As the registration roller  120  is thus rotated backward at the backward rotation time reduced depending on the skew amount S of the sheet to suppress the damage such as “leading edge curling” or “leading edge folding” being generated on the sheet P, it is possible to resolve the state in which the leading edge of the sheet P encroaches on the nip portion  15  of the registration roller. By rotating the registration roller backward, it is also possible to prevent the leading edge of the sheet P from encroaching on the nip portion  15  of the registration roller. 
     Further, it is also possible to shorten the total skew correction time of the sheet P since the interval between the forward rotation and the backward rotation is shortened by setting the backward rotation time of the registration roller  120  to the minimum. It is also possible to increase the printing speed of the printer  1 , because the conveying efficiency of the sheet conveying apparatus is improved. 
     In the present embodiment, four resist sensors  117  which detect the sheet leading edge downstream of the sheet conveying direction were arranged across the sheet conveying direction, but only two resist sensors  117   b  and  117   c  that can detect the skew amount S of the sheet P of minimum width may be included. 
     In addition, the controller  50  included in the sheet conveying apparatus may be configured so as to have the backward rotation time changed such that the higher the rigidity Z of the sheet P conveyed to the registration roller  120  is, the longer the backward rotation time of the registration roller  120  becomes. 
     Here, the proportional constant a of Equation 3 is calculated from the following equation by using a given proportional constant b in a function of the rigidity Z of the sheet P.
 
 a=b·Z   (Equation 5)
 
     By using the calculated a, the backward rotation time is changed depending on the rigidity Z of the sheet P from the skew amount S. 
     As described above, change is performed such that the higher the rigidity Z of the conveyed sheet P is, the longer the backward rotation time of the registration roller  120  becomes. Thereby, if a sheet P of high rigidity is conveyed to the registration roller  120  as the rotary driving force of the resist motor  61  is transmitted to the gears  71  and  72 , as illustrated in  FIG. 2 , the encroachment amount becomes greater as the registration roller  120  is rotated by the backlash of the gears  71  and  72 . Like this, even if the sheet P of high rigidity, which makes the encroachment amount to the registration roller  120  greater, is conveyed, it is possible to resolve the encroachment of the sheet P on the registration roller with a high degree of precision. 
     The resist sensor that detects the leading edge of the conveyed sheet P may also be configured to be installed across the CIS (Contact Image Sensor), an optical line sensor, at right angles with the sheet conveying direction, as illustrated in  FIGS. 12A and 12B . 
     Here, the CIS will be described briefly. The CIS is one of optical sensors of light and compact construction, in which modules including a light source, a light system, and light amount detection system are made as one body. 
     The resist sensor  116  including the CIS detects the leading edge plane downstream in the conveying direction of the sheet P being conveyed to determine the backward rotation start timing of the registration roller  120  and also continually detects the change of the lateral cross section along the conveying direction to detect the skew amount S. 
     As for the sheet skew amount S, as illustrated in  FIG. 12 , the CIS detects the retarded corner portion Pb of the downstream leading edge of the sheet P and also detects the side edge along the conveying direction of the sheet P. When the advanced corner portion Pa of the sheet P abuts the nip portion  15  of the registration roller, the side edge P 5  along the conveying direction of the sheet P is detected. From the side edge P 5  along the conveying direction of the sheet P, a travel amount d that the side edge has moved along the conveying direction of the sheet P in the sheet width direction crossing the sheet conveying direction at right angles from the position of the detected retarded corner portion Pb is detected. 
     As illustrated in  FIG. 12 , if the travel amount is d and the distance (interval) from the detection position of the sheet P of the resist sensor  116  to the nip portion  15  of the registration roller is Lc, the skew amount S is calculated by the following equation.
 
 S=a  tan( d/Lc )  (Equation 6)
 
     As described above, the skew amount S of the sheet P can be detected properly by detecting the lengthwise side edge of the sheet P by the CIS. By using a relatively low-priced CIS for the resist sensor  116  that detects the leading edge of the sheet P, the manufacturing cost can be reduced. 
     Note that, the embodiment of the sheet conveying apparatus according to the present invention is described as a color digital printer, which is one of image forming apparatuses, as an example, but the present invention is not limited thereto, and can obviously be applied to a printer of an ink jet type as well. 
     While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all modifications, equivalent structures and functions. 
     This application claims the benefit of Japanese Patent Application No. 2011-195286, filed Sep. 7, 2011, which is hereby incorporated by reference herein in its entirety.