Abstract:
The object of the present invention is to provide a sheet conveying apparatus capable of correcting the skew conveying of a sheet without causing a tugging. In order to accomplish the object, the sheet conveying apparatus includes a detection unit for detecting an inclination to a sheet conveying direction of a sheet to be conveyed; an inclination correction unit for correcting the inclination of the sheet by rotating in a state of nipping the sheet of an inclined state based on a detection signal from the detection unit; a first conveying unit, which is provided in the upstream side of the inclination correction unit and nips the sheet toward the inclination correction unit and conveys the same; a first nipping release unit for releasing a nipping state of the second sheet conveying unit, which is provided in the downstream side of the inclination correction unit and nips the sheet from the inclination correction unit and convey the same to the downstream side; a second nipping release unit for releasing the nipping state of the second sheet conveying unit; and a control unit for controlling the first nipping release unit and the second nipping release unit to selectively release the sheet nipping state of the first sheet conveying unit and the second sheet conveying unit according to an inclination correction operation by the inclination correction unit.

Description:
[0001]     This application claims priority from Japanese Patent Application No. 2003-198786 filed Jul. 17, 2003, which is hereby incorporated by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to a sheet conveying apparatus provided in an image forming apparatus, and more in particular, it relates to a constitution for correcting inclination (or skew) of the sheet to be conveyed.  
         [0004]     2. Related Background Art  
         [0005]     Conventionally, in general, an image forming apparatus and an image reading apparatus such as a copying machine, a printer, a facsimile, a scanner and the like comprise a sheet conveying apparatus for conveying a sheet such as a recording paper and an original to an image forming portion and an image reading portion. This sheet conveying apparatus is sometimes provided with correction means for performing a skew conveying correction of the sheet and a positional deviation correction of the sheet to align the attitude and position of the sheet before being conveyed to the image forming portion and the image reading portion.  
         [0006]     Here, as for a correction system of such correction means, there are those using a registration roller pair, and for example, in the case of the image forming apparatus, a so-called loop registration system has become a mainstreamer, wherein a leading edge of the sheet is abutted against a nip of the registration roller pair at rest so as to cause a deflection on the sheet, and the leading edge of the sheet is allowed to run parallel with the roller nip by elasticity of the sheet so as to perform a correction of the skew conveying, and after that, the registration roller pair is rotated at a predetermined timing so as to establish a synchronization between the sheet and the image.  
         [0007]     However, in such a loop registration system, a loop space for forming a loop has been certainly required, thereby making the apparatus large-sized. Further, when the loop space is not sufficiently secured, there is a problem in that a jam (sheet jam) due to a buckling particularly in the sheet such as a thin paper being weak in rigidity has occurred, and abnormal sound (so-called loop sound) has been emitted when the sheet was abutted against the registration roller pair.  
         [0008]     Furthermore, there is a problem in that a skew conveying correction capability ends up being changed depending on strength of the sheet rigidity. To be more specific, in the case of the thin paper, which is weak in rigidity, the leading edge of the sheet lacks an abutting force when abutting against the registration roller nip, and there are some cases where the leading edge of the sheet does not sufficiently abut against the registration roller pair, and in this case, it is totally impossible to perform the skew conveying correction.  
         [0009]     Further, in the case of a thick paper and the like which is strong in rigidity, there is a problem in that the sheet ends up going through the nip of the registration roller pair due to an impact caused by abutting against the nip of the registration roller. To prevent this from occurring, for example, when an attempt is made to give a load and the like to the registration roller pair by a brake member, it invites a cost-up of the product.  
         [0010]     Still further, when there is a curl or a crease on the leading edge of the sheet, the leading edge of the sheet is unable to precisely run along the nip portion of the registration roller, and as a result, there arises a problem in that a precise correction of the skew conveying cannot be performed, thereby lowering a print accuracy.  
         [0011]     On the other hand, in recent years, the image forming apparatus and the image reading apparatus have been digitalized to such an extent that, after an original is read once, its image information can be electrically encoded and stored in a memory. At an image forming time, the information inside the memory is read, and an image corresponding to the image information of the original is allowed to be formed on a photosensitive body by an exposing apparatus such as a laser beam, a LED array and the like. Hence, even in a plurality of copies, no mechanical movement such as an optical system is needed.  
         [0012]     This has made it possible that a sheet interval, which is an interval between the sheets, is closed up, and a number of sheets are processed within a shortest possible time. As a result, for example, in the case of the image forming apparatus, at an image forming time, the substantial improvement of an image forming speed has come to be attempted without increasing a processing speed.  
         [0013]     Nevertheless, in case of using an apparatus adopting the loop registration system described as above as the sheet conveying apparatus, a sheet is stopped once so as to form a loop, and therefore, a sheet interval is inevitably decided, thereby greatly affecting the improvement of the image forming speed (productivity).  
         [0014]     Consequently, to overcome such a defect, a sheet conveying apparatus adopting the registration system so as to automatically reform the skew conveying of the sheet has been disclosed in Japanese Patent Application Laid-Open No. 10-067448.  
         [0015]     Here, this sheet conveying apparatus comprises a conveying roller pair (registration rollers) for nipping and conveying the sheet, a sensor for detecting an amount of skew of the sheet provided at the downstream side in a conveying direction of the conveying roller, and correction means for correcting the inclination of the conveying roller such that the conveying roller is displaced so as to incline in a direction orthogonal to the conveying direction of the sheet, and in case of correcting the skew conveying of the sheet, based on the information of the skew amount detection sensor, the skew conveying of the sheet is corrected by displacing the conveying roller according to the inclination of the sheet.  
         [0016]     However, in such a conventional sheet conveying apparatus for correcting the skew conveying of the sheet by displacing the conveying roller, at the time when the conveying roller is displaced so as to correct the skew conveying of the sheet, in the case where the leading edge of the sheet is nipped by the roller positioned at the downstream portionside in a conveying direction of the conveying roller or the tailing edge portion of the sheet is nipped by the roller positioned at the upstream side in the conveying direction of the conveying roller, for example, if the conventional sheet conveying apparatus is applied to the image forming apparatus, there occurs the tugging of the sheet between the rollers, and this causes problems such as the occurrence of a crease on the sheet, the correction of the skew conveying not adequately performed, the appearance of the distortion in the image, and the print accuracy remarkably poor.  
       SUMMARY OF THE INVENTION  
       [0017]     Hence, the present invention has been made in view of the above-described situation, and the object of the invention is to provide a sheet conveying apparatus and an image forming apparatus, which can correct the skew conveying of a sheet without allowing the tugging of the sheet to occur.  
         [0018]     The present invention has the following constitution in the sheet conveying apparatus for conveying the sheet by a plurality of sheet conveying means arranged along a sheet conveying path; comprising: 
        detection means for detecting an inclination in a sheet conveying direction of the sheet to be conveyed;     inclination correction means for correcting the inclination of the sheet by rotating the sheet in an inclined state in a state of being nipped based on a detection signal from the detection means;     first conveying means which is provided in the upstream of the inclination correction means and conveys the sheet toward the inclination correction means;     first nipping release means for releasing a nipping state of the first sheet conveying means;     second sheet convey means which is provided in the downstream side of the inclination correction means and conveys the sheet from the inclination correction means to the downstream side;     second nipping release means for releasing a nipping state of the second sheet conveying means; and     control means for controlling the first nipping release means and the second nipping release means to selectively release the sheet nipping state of the first sheet conveying means and the second sheet conveying means according to the sheet inclination correction operation by the inclination correction means.        
 
         [0026]     The present invention has the following constitution in the sheet conveying apparatus for conveying the sheet by a plurality of sheet conveying means arranged along the sheet conveying path, comprising: 
        a sheet position detection sensor which detects the inclination of the sheet to be conveyed in the sheet conveying direction;     a correction roller pair which is provided rotatably, and corrects the inclination of the sheet by rotating in a state of nipping the sheet in an inclined state based on the detection signal from the sheet position detection sensor;     a upstream roller pair provided in the upstream side of the correction roller pair, the upstream roller pair being separable;     a downstream roller pair provided in the downstream side of the correction roller pair, the downstream roller pair being separable; and     wherein the upstream roller pair and the downstream roller pair are selectively separated according to the inclination correction operation of the sheet by the correction roller pair.       
 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0032]      FIG. 1  is a sectional view of a printer which is one example of an image forming apparatus comprising a sheet conveying apparatus according to a first embodiment of the present invention;  
         [0033]      FIG. 2  is a perspective view of a inclination correction roller portion of the sheet conveying apparatus;  
         [0034]      FIG. 3  is a side view of the inclination correction roller portion of the sheet conveying apparatus;  
         [0035]      FIG. 4  is a plan view showing a positional relationship between each roller of the inclination correction roller portion of the sheet conveying apparatus and a position detection sensor;  
         [0036]      FIG. 5  is a control block diagram of the printer;  
         [0037]      FIG. 6  is a flowchart to explain about a skew conveying correction operation of the sheet conveying apparatus;  
         [0038]      FIGS. 7A and 7B  are a first drawing to explain about the skew conveying correction operation of the sheet conveying apparatus;  
         [0039]      FIGS. 8A and 8B  are a second drawing to explain about the skew conveying correction operation of the sheet conveying apparatus;  
         [0040]      FIG. 9  is a flowchart to explain about the skew conveying correction operation of the sheet conveying apparatus according to a second embodiment of the present invention;  
         [0041]      FIGS. 10A and 10B  are a first drawing to explain about the skew conveying correction operation of the sheet conveying apparatus;  
         [0042]      FIG. 11  is a second drawing to explain about the skew conveying correction operation of the sheet conveying apparatus;  
         [0043]      FIG. 12  is a flowchart to explain about the skew conveying correction operation of the sheet conveying apparatus according to a third embodiment of the present invention;  
         [0044]      FIGS. 13A and 13B  are a first drawing to explain about the skew conveying correction operation of the sheet conveying apparatus;  
         [0045]      FIGS. 14A and 14B  are a second drawing to explain about the skew conveying correction operation of the sheet conveying apparatus; and  
         [0046]      FIG. 15  is a perspective view showing another constitution of the inclination correction roller portion of the sheet conveying apparatus. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0047]     Embodiments of the present invention will be described below in detail with reference to the drawings.  
         [heading-0048]     (First Embodiment)  
         [0049]      FIG. 1  is a sectional view of a printer, which is one example of an image forming apparatus comprising a sheet conveying apparatus according to a first embodiment of the present invention.  
         [0050]     In the drawing, reference numeral  1000  denotes a printer, and the printer  1000  comprises a printer body  1001  and a scanner  2000  arranged on the top surface of the printer body  1001 .  
         [0051]     Here, the scanner  2000  for reading an original comprises a scanning optical light source  201 , a platen glass  202 , an open-close original thick plate  203 , a lens  204 , a photodiode (photoelectric conversion)  205 , an image processing portion  206 , a memory portion  208  for storing an image processing signal processed at the image processing portion  206 , and the like.  
         [0052]     When reading the original, an illustrated original placed on the platen glass  202  is irradiated with light by the scanning optical light source  201  so as to be read. The read original image is processed by the image processing portion  206 , and after that, it is converted into an electrical signal  207  electrically encoded, and is transmitted to a laser scanner  111 , which is an image forming means. Note that the image information processed and encoded at the image processing portion  206  is stored once in the memory portion  208 , and can be also transmitted to the laser scanner  111  by the signal from a controller  120  as occasion demands.  
         [0053]     The printer  1001  comprises a sheet feeding apparatus  1002  for feeding a sheet S, a sheet conveying apparatus  1004  for conveying the sheet S fed by the sheet feeding apparatus  1002  to an image forming portion  1003 , the controller  120  as control means for controlling the printer  1000 , and the like.  
         [0054]     Here, the sheet feeding apparatus  1002  comprises a separation portion consisting of a cassette  100 , a pick up roller  101 , a feed roller  102 , and a retard roller  103 . The sheets S inside the cassette  100  are allowed to be separated and fed one sheet by one sheet by the operation of the pick up roller  101  moving up and down and rotating at a predetermined timing and the separation portion.  
         [0055]     The sheet conveying apparatus  1004  comprises a conveying roller pair  105  and a inclination correction roller portion  1  having a upstream roller pair  130  and a downstream roller pair  2 . The sheet S fed from the sheet feeding apparatus  1002  passes through a sheet conveying path  108  constituted by guide plates  106  and  107  by the conveying roller pair  105 , and after that, it is transferred to a sheet conveying path  110  constituted by guide plates  109  and  111 , and then, is led to the inclination correction roller portion  1 . In this inclination correction roller portion  1 , the sheet S has a skew conveying and a positional deviation corrected as to be described later, and after that, it is conveyed to the image forming portion  1003 .  
         [0056]     The image forming portion  1003  comprises a photosensitive drum  112 , the laser scanner  111 , a developing device  114 , a transfer charger  115 , a separation charger  116 , and the like. At the time of an image formation, a laser beam from the laser scanner  111  is turned back by a mirror  113 , and is irradiated to an exposing position  112   a  on the photosensitive drum rotating clockwise, thereby forming a latent image on the photosensitive drum, and moreover, the latent image formed on the photosensitive drum in this manner is subsequently allowed to be visualized as a toner image by the developing device  114 .  
         [0057]     Note that this toner image on the photosensitive drum is subsequently transferred on the sheet S by a transferring charger  115  in a transfer portion  112   b . Further, the sheet S transferred with the toner image in this manner is electrostatically separated from the photosensitive drum  112  by the separation charger  116 , and after that, it is conveyed to a fixing apparatus  118  by a conveying belt  117  so that a fixing of the toner image is performed, and then it is discharged by a discharge roller  119 .  
         [0058]     Note that, in  FIG. 1 , reference numeral  131  denotes an exposing start sensor for detecting the sheet S having passed through the restoration roller pair  2 , and when this exposing start sensor  131  detects the sheet S having passed through the downstream roller pair  2 , the irradiation of the laser beam by the laser scanner  111  is started.  
         [0059]     Here, a distance  11  from the exposing start sensor  131  to a transfer portion  112   b  is arranged at a position equal to a distance  12  from the laser beam irradiation position  112   a  of the photosensitive drum  112  to the transfer portion  112   b , and this makes it possible to establish a synchronization between the sheet S and the leading edge position of the image on the photosensitive drum  112 .  
         [0060]     Note that, in the present embodiment, though the printer body  1001  and the scanner  2000  are a separate body, there are sometimes the cases where the printer body  1001  and the scanner  2000  are in one united body. Further, regardless of whether the printer body  1001  is a body separate from the scanner  2000  or in one united body with the scanner  2000 , if the processing signal of the scanner  2000  is inputted to the laser scanner  111 , the scanner functions as a copying machine, and if the transmission signal of a FAX is inputted, it functions as the FAX. In addition, if the output signal of a personal computer is inputted, it functions as a printer.  
         [0061]     On the other hand, if the processing signal of an image processing portion  206  of the scanner  2000  is transmitted to another FAX, it functions as the FAX. Further, in the scanner  2000 , if an original automatic feeding apparatus  250  as shown by a two-dot chain line is mounted, the original can be automatically read.  
         [0062]      FIG. 2  is a perspective view of a inclination correction roller portion  1 ,  FIG. 3  is a side view thereof, and  FIG. 4  is a plan view showing a positional relationship between each roller of the inclination correction roller portion  1  and a position detection sensor to be describer later.  
         [0063]     In FIGS.  2  to  4 , reference numeral  301  denotes a correction roller pair which is inclination correction means constituted by correction rollers  301   a  and  301   b  which are two rotational members separably press-contacted. This correction roller pair  301  is arranged between the upstream roller pair  130  which is first sheet conveying means arranged at the upstream side and the downstream roller pair  2  which is second sheet conveying means arranged at the downstream side.  
         [0064]     Here, as shown in  FIG. 2 , correction rollers  301   a  and  301   b  are mounted with gears  15  and  16 , respectively at its one side, and the correction gears  301   a  and  301   b  are constituted by these gears  15  and  16  such that they are synchronized so as to be rotated. Further, a driving force from an unillustrated conveying motor is transmitted to a shaft  28  of the lower correction roller  301   b , and this allows the correction roller pair  301  to rotate in a sheet conveying direction.  
         [0065]     Reference numeral  10  denotes a frame which rotatably holds the upper correction roller  301   a  from among the correction roller pair  301  by both bent end portions  10   a , and is fixed with an arm  22 , on top of which is formed a rack gear portion  22   a  in the upper surface center portion. This frame  10  allows an engaging portion  22   b  formed on the arm  22  to engage with a rotational shaft  14  provided on a stay  13 , thereby being able to rotate with the rotational shaft  14  as a center.  
         [0066]     Here, the rack gear portion  22   a  of the arm  22  engages with a gear  23  fixed on the output shaft of a swing motor  24 , and when the swing motor  24  is driven in this manner, the frame  10  integral with the arm  22  swings (rotates, or pivotally moves) in a direction of an arrow mark A, and accompanied with this, the correction roller pair  301  is allowed also to swing (rotate, or pivotally moves) in a direction of the arrow mark A, that is, in a direction at angle with the direction orthogonal to the sheet conveying direction P.  
         [0067]     On the other hand, the stay  13  is fixed with a rack gear portion  18 , and this rack gear portion  18  is engaged with a lateral moving drive gear  19  which is fixed to the output shaft of a lateral moving motor  20  fixed to the printer body  1001 .  
         [0068]     When the lateral moving motor  20  rotates, the rotation of this lateral moving motor  20  is transmitted to the stay  13  through a lateral register drive gear  19  and the rack gear portion  18 , and this allows the stay  13  to move to a thrust direction which is the direction orthogonal to the sheet conveying direction P indicated by an arrow mark B, and the movement of this stay  13  allows the correction roller pair  301  also to move in the thrust direction.  
         [0069]     Further, this stay  13  is made movable up and down in an arrow mark direction C by a lift motor  404   b  and an unillustrated cam mechanism, and by moving the stay  13  up and down in this manner, the upper correction roller  301   a  can be separated and press-contacted with respect to the lower correction roller  301   b . This constitution is corresponding to correction nipping release means of the present invention.  
         [0070]     Note that the correction roller pair  301  has the nip line held by the controller  120  at a home position, which is parallel with a rotational center shaft of the photosensitive drum  112  or the nip line of the downstream roller pair  2  based on the signal from an unillustrated home position sensor at least before the conveyance of the sheet S starts.  
         [0071]     On the other hand, as shown in  FIG. 3 , the downstream roller pair  2  consists of two downstream rollers  2   a  and  2   b , and the lower downstream roller  2   b  from among two downstream rollers  2   a  and  2   b  constituting the downstream roller pair  2  is rotatably held on a movement end portion of a first pressure arm  324  swinging up and down with a shaft  324   a  provided on the upper portion of a fixed base  321  as a point of support. By swinging up and down this first pressure arm  324 , the lower downstream roller  2   b  is allowed to press-contact or separate with respect to the upper downstream roller  2   a.    
         [0072]     Further, the upstream roller pair  130  is constituted by two upstream rollers  130   a  and  130   b , and the lower upstream roller  130   b  from among two upstream rollers  130   a  and  130   b  constituting the upstream roller pair  130  is rotatably held on the movement end portion of a second pressure arm  325  swinging up and down with a shaft  325   a  provided on the upper portion of a fixed base  321  as a point of support. By swinging up and down this second pressure arm  325 , the lower upstream roller  130   b  is allowed to press-contact or separate with respect to the upper upstream roller  130   a.    
         [0073]     In  FIG. 3 , reference numeral  405   b  denotes a press-contacting/separating motor fixed to the printer body  1001 , and a belt pulley  361  is fixed on the output shaft of this press-contacting/separating motor  405   b . A belt  326   b  is suspended between this belt pulley  361  and a center pulley  331  provided on the upper portion of the fixed base  321 .  
         [0074]     Further, a belt  327  is suspended between the center pulley  331  and a pulley  322  mounted on a rotational shaft  341  provided at the side of the downstream roller pair of the fixed base  321 , and a belt  328  is suspended between the center pulley  331  and a pulley  330  mounted on a rotational shaft  342  provided at the side of the upstream roller pair of the fixed base  321 . Further, the rotational shaft  341  at the side of the downstream roller pair is mounted with the first pressure arm  324  and a cam  323  abutting from below, and the rotational shaft  342  at the side of the upstream roller pair is mounted with the second pressure arm  325  and a cam  329  abutting from below.  
         [0075]     When the belt pulley  361  is rotated clockwise by the press-contacting/separating motor  405   b  and the belt  326  is driven in the arrow mark direction in the drawing, the drive of this belt  326  allows the cam  323  to rotate integrally with the pulley  322  through the center pulley  331  and the belt  327  in the arrow mark direction, and by the rotation of the cam  323 , the first pressure arm  324  is pushed upward as shown in the drawing, and the lower downstream roller  2   b  is press-contacted to the upper downstream roller  2   a.    
         [0076]     Further, at the same time, the drive of the belt  326  allows the cam  329  to rotate integrally with the pulley  330  through the center pulley  331  and the belt  328 , and by the rotation of this cam  329 , as shown in the drawing, the second pressure arm  325  is lowered downward by a dead weight or an unillustrated biasing means, and the lower upstream roller  130   b  is separated from the upper upstream roller  130   a.    
         [0077]     Note that, after that, when the press-contacting/separating motor  405   b  is further rotated (or rotated in reverse), by the operation of the cams  323  and  329 , the first pressure arm  324  is lowered downward by the dead weight or the unillustrated biasing means, and is separated from the upper downstream roller  2   a , and the lower upstream roller  130   b  is press-contacted to the upper upstream roller  130   a.    
         [0078]     In this manner, in the present embodiment, by the press-contacting/separating motor  405   b , which is the same drive means, the press-contact and separation of the upstream roller pair  130  and the downstream roller pair  2  can be controlled. Note that, by changing the respective phases of the cams  323  and  329 , it is possible to push upward the lower downstream roller  2   b  and the lower upstream roller  130   b  in a pressuring direction at the same time or move them in a separating direction at the same time. Further, the press-contact and separation are possible at a different timing. Note that the constitution allowing these upstream roller pairs  130  to be separated is corresponding to the first nipping release means of the present invention, and the constitution separating the downstream roller pair  2  is corresponding to the second nipping release means of the preset invention.  
         [0079]     Further, in  FIG. 4 , reference numeral  3  denotes a sheet position detection sensor for detecting an inclination to the sheet conveying direction of the leading edge of the sheet S conveyed by the correction roller pair  301  and a positional deviation of the side end in a direction orthogonal to the sheet conveying direction of the sheet S. A sheet position detection sensor  3  constituting this detection means is arranged in a direction orthogonal to the sheet conveying direction at the downstream side of the conveying direction of the correction roller pair  301 .  
         [0080]      FIG. 5  shows a control block diagram of the printer  1000  comprising such a sheet conveying apparatus  1004  and the like, and as shown in the drawing, the photosensitive drum  112 , the conveying belt  117 , the fixing apparatus  118 , and the discharge roller  119 , all of which are previously described, are directly connected to a main motor M, and each element is possible to rotate in synchronization with the main motor M.  
         [0081]     Further, the controller  120 , which is control means, is connected with the sheet position detection sensor  3  and other various sensors, and is inputted with the detection signals, respectively, which are obtained by the sheet position detection sensor  3  and the like. In the controller  120 , for example, the amount of inclination of the sheet S is calculated by a calculation circuit  401  based on the detection signal from the sheet position detection sensor  3 .  
         [0082]     Further, the controller  120  is connected to the lateral moving motor  20  which is a correction motor, the swing motor  24 , a motor  404   b  for use of correction roller pressure, the press-contacting/separating motor  405   b  for use of the downstream roller pair  2  and the upstream roller pressure, and a conveying drive motor  406   b  for driving the correction roller pair  301 , respectively through the drive circuits  403   a ,  404   a  and  406   a , and outputs a necessary control signal based on the detection result of the sheet position detection sensor  3 , thereby allowing respective motors  20 ,  24 ,  404   b ,  405   b  and  406   b  to drive for a predetermined amount.  
         [0083]     Next, the skew conveying correction operation of the printer  1000  (sheet conveying apparatus  1004 ) having such a constitution will be described with reference to the flowchart shown in  FIG. 6 , and  FIGS. 7A and 7B , and  FIGS. 8A and 8B .  
         [0084]     First, when an unillustrated start button of the printer  1000  is pushed, the lateral moving motor  20  and the swing motor  24  are driven, and by an unillustrated home position sensor, the initializing operation of the swing direction of the correction roller pair  301  and the position in a thrust direction is performed (Step  1 ).  
         [0085]     Accompanied with this initializing operation, the motor  404   b  (refer to  FIG. 2 ) is driven, and the upper correction roller  301   a  is lifted so as to separate from the lower correction roller  301   b . Further, the press-contacting/separating motor  405   b  is driven so that a downstream roller pair  2  and the upstream roller pair  130  are press-contacted to one another (Step  2 ).  
         [0086]     In this state, as shown in  FIG. 7A , when the sheet S skewed by an angle θ (degrees) from the sheet conveying direction P is conveyed, this sheet S advances into the nip portion of the upstream roller pair  130  and is nipped there. After that, the sheet S nipped by the upstream roller pair  130  is conveyed along the sheet conveying direction P and advanced while remaining in an inclined state so as to be detected by the sheet position detection sensor  3  arranged in the downstream side of the upstream roller pair  130  (Step  3 ).  
         [0087]     Here, the detection signal from this sheet position detection sensor  3  is inputted to the controller  120 , and by the calculation circuit  401  of the controller  120 , a passing time point of the sheet leading edge, the inclination of the sheet S nipped by the correction roller pair  301 , and the amount of lateral reference deviation are calculated (Step  4 ).  
         [0088]     Next, the controller  120  determines the existence or non-existence of the skew conveying and the lateral reference deviation of the sheet S from this calculation result (Step  5 ), and if there exists no skew conveying nor any lateral reference deviation of the sheet S (N of Step  5 ), no correction operation is performed, but if there exist the skew conveying and the lateral reference deviation of the sheet S (Y of Step  5 ), the amount of correction related to this, that is, the amount of driving of the lateral moving motor  20  and the swing motor  24  is calculated (Step  6 ).  
         [0089]     Next, the time until the leading edge of the sheet S reaches the downstream roller pair  2  is calculated (Step  7 ), and after that, as shown in  FIG. 7B , the press-contacting/separating motor  405   b  is driven so as to separate the downstream roller pair  2  and the upstream roller pair  130 , respectively, and release the sheet nipping state, and the motor  404   b  is driven so that the correction roller pair  301  is press-contacted and put into a state of nipping the sheet (Step  8 ).  
         [0090]     The downstream roller pair  2  and the upstream roller pair  130  are separated, respectively, in this manner, and the correction roller pair  301  is put into a state of nipping the sheet, and after that, as shown in  FIG. 8A , the correction roller pair  301  is swung by an inclination θ (degrees) in an arrow mark A 2  direction, and is moved in an arrow mark A 1  by the amount &lt; of the lateral reference deviation (Step  9 ). The correction of the sheet finishes before the leading edge of the sheet reaches the downstream roller pair  2 .  
         [0091]     Next, after such a correction is performed, before it is the time when the sheet leading edge searched previously in Step  7  reaches the downstream roller pair  2 , the press-contacting/separating motor  405   b  is driven so that the upstream roller pair  130  and the downstream roller pair  2  are press-contacted and restored to a sheet nipping state, and the motor  404   b  is driven so as to separate the correction roller pair  301  (Step  10 ).  
         [0092]     In this manner, the conveyance of the sheet S is continued by the upstream roller pair  130 , and after that, the sheet S is further conveyed to the image forming portion  1003  at the downstream side by the downstream roller pair  2  nipping the sheet S as shown in  FIG. 8B .  
         [0093]     In this manner, the correction roller pair  301  is moved in a direction to correct the inclination of the sheet S in a state of nipping the sheet S in an inclined state based on the detection signal from the sheet position detection sensor  3 , and at the time when the correction roller pair  301  moves, for example, similarly to the present embodiment, when the inclination of the sheet S is corrected in a state of the sheet S being nipped by the upstream roller pair  130 , at least the upstream roller pair  130  is separated so as to release the sheet nipping state, so that the nipping state of the correction roller pair  301  is released after correcting the inclination of the sheet, and moreover, the upstream roller pair  130  released from the sheet nipping state is restored to the sheet nipping state, thereby making it possible to eliminate the tugging of the sheet S between the correction roller pair  301  and the upstream roller pair  130 .  
         [0094]     Consequently, when the correction roller pair  301  for correcting the skew conveying of the sheet S is moved, it is possible to prevent the crease on the sheet and the distortion in the image from occurring and perform a highly accurate skew conveying correction and a lateral reference correction. As a result, the print accuracy is improved, and the productivity can be enhanced. In addition, no loop sound is emitted, and a problem of the buckling at the time of the loop formation of the thin paper can be eliminated. Moreover, no loop space is needed, thereby making it possible to downsize the apparatus.  
         [heading-0095]     (Second Embodiment)  
         [0096]     Next, a second embodiment of the present invention will be described.  
         [0097]      FIG. 9  is a flowchart showing a skew conveying correction operation of a sheet conveying apparatus according to the present embodiment, and a sheet skew conveying correction according to the present embodiment will be described with reference to the same drawing.  
         [0098]     First, when an unillustrated start button of a printer  1000  is pushed, a lateral moving motor  20  and a swing motor  24  are driven, and by an unillustrated home position sensor, the initializing operation of the swing direction of a correction-roller pair  301  and the position in a thrust direction is performed (Step  1 ).  
         [0099]     Accompanied with this initializing operation, a motor  404   b  is driven, and an upper correction roller  301   a  is lifted so as to separate from a lower correction roller  301   b . Further, a press-contacting/separating motor  405   b  is driven so that a downstream roller pair  2  and a upstream roller pair  130  are press-contacted to one another (Step  2 ).  
         [0100]     In this state, as shown in  FIG. 10A , when a sheet S skewed by an angle θ (degrees) from a sheet conveying direction P is conveyed, this sheet S advances into the nip portion of a upstream roller pair  130  and is nipped there. After that, the sheet S nipped by the upstream roller pair  130  is conveyed along a sheet conveying direction P and advanced so as to be detected by a sheet position detection sensor  3  arranged in the downstream side of a upstream roller pair  130  (Step  3 ).  
         [0101]     Here, the detection signal from this sheet position detection sensor  3  is inputted to a controller  120 , and by a calculation circuit  401  of the controller  120 , a passing time point of the sheet leading edge, the inclination of the sheet S nipped by the correction roller pair  301 , and the amount of lateral reference deviation are calculated (Step  4 ).  
         [0102]     Next, the controller  120  determines the existence or non-existence of the skew conveying and the lateral reference deviation of the sheet S from this calculation result (Step  5 ), and if there exists no skew conveying nor any lateral reference deviation of the sheet S (N of Step  5 ), no correction operation is performed, but if there exist the skew conveying and the lateral reference deviation of the sheet S (Y of Step  5 ), the amount of correction related to this, that is, the amount of driving of the lateral moving motor  20  and the swing motor  24  is calculated (Step  6 ).  
         [0103]     Next, the time until the leading edge of the sheet S reaches a downstream roller pair  2  is calculated (Step  7 ), and when this calculation time has elapsed (Y of Step  8 ) and the leading edge of the sheet S reaches the downstream roller pair  2 , as shown in  FIG. 10B , the press-contacting/separating motor  405   b  is driven so as to separate the downstream roller pair  2  and the upstream roller pair  130 , respectively, as shown in  FIG. 11A , and a motor  404   b  is driven so that the correction roller pair  301  is put into a press-contacted state (Step  9 ).  
         [0104]     The downstream roller pair  2  and the upstream roller pair  130  are separated, respectively, in this manner, and the correction roller pair  301  is put into a press-contacted, and after that, the correction roller pair  301  is swung by an inclination θ (degrees) in an arrow mark A 2  direction, and is moved in an arrow mark A 1  by the amount &lt; of the lateral reference deviation (Step  10 ).  
         [0105]     Next, after such a correction is performed, as shown in  FIG. 11B , the press-contacting/separating motor  405   b  is driven so that the downstream roller pair  2  and the upstream roller pair  130  are press-contacted so as to nip the sheet S, and the motor  404   b  is driven so as to separate the correction roller pair  301  (Step  11 ).  
         [0106]     Here, similarly to the present embodiment, if the constitution is such that the sheet S is nipped by the downstream roller pair  2  immediately after the sheet position is corrected, in addition to the advantages of the first embodiment previously described, there occurs almost no positional deviation of the sheet after the correction, and it is possible to perform a highly accurate skew conveying correction and a lateral reference correction.  
         [heading-0107]     (Third Embodiment)  
         [0108]     Next, a third embodiment of the present invention will be described.  
         [0109]      FIG. 12  is a flowchart showing a skew conveying correction operation of a sheet conveying apparatus according to the present embodiment, and the skew conveying correction of a sheet according to the present embodiment will be described with reference to the drawing.  
         [0110]     First, when an unillustrated start button of a printer  1000  is pushed, a lateral moving motor  20  and a swing motor  24  are driven, and by an unillustrated home position sensor, the initializing operation of the swing direction of a correction roller pair  301  and the position in a thrust direction is performed (Step  1 ).  
         [0111]     Accompanied with this initializing operation, a motor  404   b  is driven, and an upper correction roller  301   a  is lifted so as to separate from a lower correction roller  301   b . Further, a press-contacting/separating motor  405   b  is driven so that a downstream roller pair  2  and a upstream roller pair  130  are press-contacted to one another (Step  2 ).  
         [0112]     In this state, as shown in  FIG. 13A , when a sheet S skewed by an angle θ (degrees) from a sheet conveying direction P is conveyed, this sheet S advances into the nip portion of a upstream roller pair  130  and is nipped there. After that, the sheet S nipped by the upstream roller pair  130  is, as shown in  FIG. 13B , conveyed along the sheet conveying direction P and advanced so as to be detected by a sheet position detection sensor  3  arranged in the downstream side of a upstream roller pair  130  (Step  3 ).  
         [0113]     Next, the detection signal from this sheet position detection sensor  3  is inputted to a controller  120 , and by a calculation circuit  401  of the controller  120 , a passing time point of a sheet leading edge, the inclination of the sheet S nipped by a correction roller pair  301 , and the amount of lateral reference deviation are calculated (Step  4 ).  
         [0114]     Next, the controller  120  determines the existence or non-existence of the skew conveying and the lateral reference deviation of the sheet S from this calculation result (Step  5 ), and if there exists no skew conveying nor any lateral reference deviation of the sheet S (N of Step  5 ), no correction operation is performed, but if there exist the skew conveying and the lateral reference deviation of the sheet S (Y of Step  5 ), the amount of correction related to this, that is, the amount of driving of the lateral moving motor  20  and the swing motor  24  is calculated (Step  6 ).  
         [0115]     Next, the time until the tailing edge of the sheet S passes through the upstream roller pair  130  is calculated (Step  7 ), and when this calculation time has elapsed (Y of Step  8 ) and, as previously described in  FIG. 13B , the tailing edge of the sheet S passes through the upstream roller pair  130 , as shown in  FIG. 14B , the press-contacting/separating motor  405   b  is driven so as to separate the downstream roller pair  2  and the upstream roller pair  130 , respectively, and a motor  404   b  is driven so that the correction roller pair  301  is put into a press-contacted state (Step  9 ).  
         [0116]     The downstream roller pair  2  and the upstream roller pair  130  are separated, respectively in this manner, and the correction roller pair  301  is put into a press-contacted state, and after that, the correction roller pair  301  is swung by an inclination θ (degrees) in an arrow mark A 2  direction, and is moved in an arrow mark A 1  by the amount &lt; of the lateral reference deviation (Step  10 ).  
         [0117]     Next, after such a correction is performed, as shown in  FIG. 14B , the press-contacting/separating motor  405   b  is driven so that the downstream roller pair  2  and the upstream roller pair  130  are press-contacted so as to nip the sheet S, and the motor  404   b  is driven so as to separate the correction roller pair  301  (Step  11 ).  
         [0118]     Here, similarly to the present embodiment, if the constitution is such that the correction of a sheet position is performed immediately after the tailing edge of the sheet S is pulled out from the nip portion of the upstream roller pair  130 , in addition to the advantages of the first and second embodiments previously described, there occurs almost no catch due to the friction of the sheet S with the upstream roller  130   a  or the upstream roller  130   b  at the moving time of the sheet S for the position correction even when the conveying path between the correction roller pair  301  and the upstream roller pair  130  is formed in a bending shape. This makes it possible to perform a highly accurate skew conveying correction and a lateral reference correction even in a bending path portion.  
         [0119]     Note that, in the present embodiment, the upstream roller pair  130  may be put into a press-contacted state after it is put into a state as shown in  FIG. 14A . In this case, since the next sheet is made conveyable, the sheet can be conveyed with the sheet interval made short, thereby increasing a throughput.  
         [0120]     Note that, in the first to the third embodiments described so far, while the correction roller pair  301  comprising two contactably separable correction rollers  301   a  and  301   b  is used as inclination correction means, the present invention is not limited to this, but a roller pair comprising two contactably separable friction members  311  and  312  as shown in  FIG. 15  may be used as the inclination correction means. This performs the correction of the positional deviation of the sheet by nipping the sheet by the two friction members  311  and  312  and allowing it to rotate and make a slide movement.  
         [0121]     When such inclination correction means is used, though it is not possible to perform the correction while conveying the sheet, since there is no need for a motor for rotationally driving the inclination correction means, gear rows, and bearings, even if it is an inexpensive and simple constitution, it is possible to perform a highly accurate skew conveying correction and a lateral reference correction.  
         [0122]     Further, in the descriptions made so far, while the application of the sheet conveying means to the image forming apparatus has been described in order that the sheet can be accurately conveyed to the image forming portion  1003  without an inclination and a positional deviation, the present invention is not limited to this, but for example, the sheet conveying means can be also adapted to the image reading apparatus in order that the sheet S can be accurately conveyed to the image reading portion for reading the sheet (original), which is a subsequent step with no inclination nor positional deviation.