Abstract:
A sheet conveying apparatus including: a skew feed amount detecting portion for detecting a skew feed amount of a sheet; a pair of skew feed correcting portions for correcting the skew feed by conveying the sheet by their independent rotations; a conveying amount detecting portion for detecting the sheet conveying amounts by the correcting portions; and a controller for controlling rotations of the correcting portions based on detection results obtained by the skew feed amount detecting portion and the conveying amount detecting portion, wherein, after a skew feed correcting operation-is started, the controller detects a sheet correction amount by the correcting portions based on a detection result obtained by the conveying amount detecting portion, and controls the rotations of the correcting portions so that the sheet correction amount equals to the skew feed amount of the sheet detected by the skew feed amount detecting portion.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a sheet conveying apparatus, and more particularly, to a sheet conveying apparatus for correcting skew feed of a sheet such as a recording sheet or an original conveyed to an image forming portion or an image reading portion.  
         [0003]     2. Related Background Art  
         [0004]     Conventionally, an image forming apparatus or an image reading apparatus, such as a copying machine, a printer, or a facsimile apparatus, is equipped with a sheet conveying apparatus for conveying a sheet such as a recording sheet or an original to an image forming portion or an image reading portion. Some of such sheet conveying apparatuses are equipped with a skew feed correcting unit for effecting sheet skew feed correction to correctly adjust the attitude and position of the sheet until the sheet is conveyed to the image forming portion or the image reading portion.  
         [0005]     In a system of such the skew feed correcting unit, a leading edge of a sheet is caused to abut a nip of a roller pair at rest to generate bending in the sheet, and the leading edge of the sheet is caused to extend along the roller nip by utilizing an elasticity of the sheet, thereby correcting skew feed. In another system, a shutter member is retractably provided in a sheet conveying path, and the leading edge of a sheet conveyed is caused to abut the shutter member -to thereby effect skew feed correction. In the system using the shutter member, there is no need to stop the sheet being conveyed, so it is possible to reduce an interval between the sheets, making it possible to enhance throughput in image formation, etc.  
         [0006]     Further, as disclosed in Japanese Patent Application Laid-open No. H04-277151, there has been proposed an active skew feed correction system, in which skew feed is corrected while conveying a sheet without stopping it, making it possible to effect skew feed correction more reliably and with high accuracy.  
         [0007]      FIG. 8  shows an example of a conventional skew feed correcting unit for correcting skew feed of a sheet by such the active skew feed correction system.  
         [0008]     Two sheet leading edge detecting sensors  201   a  and  201   b  are provided in a sheet conveying path. The two sheet leading edge detecting sensors  201   a  and  201   b  are arranged in a direction (i.e., width direction of the sheet being conveyed) which is perpendicular to the sheet conveying direction indicated by an arrow A.  
         [0009]     Further, there are provided skew feed correcting rollers  222   a  and  222   b . The skew feed correcting rollers  222   a  and  222   b  are arranged in the direction perpendicular to the sheet conveying direction and coaxially at a predetermined interval. The skew feed correcting rollers  222   a  and  222   b  are driven independently by separate drive sources  221   a  and  221   b,  respectively.  
         [0010]     In this skew feed correcting unit, constructed as described above, when a leading edge of a sheet S conveyed from a downstream side traverses the sheet leading edge sensors  201   a  and  201   b , signals are output from the sheet leading edge detecting sensors  201   a  and  201   b , respectively. Based on those leading edge detecting signals, the skew feed amount (i.e., skew angle) of the sheet leading edge is calculated, and, based on the calculated value, rotating speeds of the skew feed correcting rollers  222   a  and  222   b  are respectively controlled to thereby correct the skew feed of the sheet S. That is, by producing a difference in the speed at which the sheet is conveyed by the skew feed correcting rollers  222   a  and  222   b , the sheet is turned by an amount corresponding to the skew feed amount to thereby correct the skew feed.  
         [0011]      FIG. 9  is a control block diagram showing the skew feed correcting unit. A skew feed amount detecting portion  215  is formed by the two sheet leading edge detecting sensors  201   a  and  201   b  and a skew feed detecting portion  200 . The sheet leading edge detection signals from the sheet leading edge detecting sensors  201   a  and  201   b  are input to the skew feed detecting portion  200 , which outputs a skew feed detection signal to a control portion  210 .  
         [0012]     After this, the control portion  210  calculates the skew feed amount by a skew feed amount calculating portion  210   a  based on the skew feed detection signal. Further, after that, based on the calculated skew feed amount, the respective rotating speeds of two skew feed correcting portions  220  formed by the skew feed correcting rollers  222   a  and  222   b  and the drive sources  221   a  and  221   b  are determined by a control amount calculating portion  210   b . By driving the skew feed correcting rollers  222   a  and  222   b  at the respective determined rotating speeds, skew feed correction is effected on the sheet.  
         [0013]     However, this control involves the following problem: the respective rotating speeds of the skew feed correcting rollers  222   a  and  222   b  are determined based on the detection signals from the skew feed amount detecting portion  215  formed by the sheet leading edge detecting sensors  201   a  and  201   b  and the skew feed detecting portion  200 , and the skew feed is corrected according to a difference in the speed at which the sheet is conveyed by the skew feed correcting rollers  222   a  and  222   b . Thus, when, for example, there has been generated a change in conveyance efficiency due to wear of the rollers, etc., even if the rotating speeds of the skew feed correcting rollers  222   a  and  222   b  are set, the sheet conveying speeds may differ from those when the rollers were in initial states, resulting in a deterioration in accuracy in skew feed correction.  
         [0014]     In view of this, as disclosed, for example, in Japanese Patent Application Laid-open NO. H06-191684, there has been proposed a skew feed correcting unit, which uses, instead of the sheet leading edge detecting sensors  201   a  and  201   b  shown in  FIG. 8 , line sensors  301   a  and  301   b  as shown in  FIG. 10 .  FIG. 12  is a side view of a skew feed correcting portion.  
         [0015]     By using such the line sensors  301   a  and  301   b , it is possible to continuously detect the leading edge of the sheet S, whereby it is possible to perform control so as to reduce the sheet skew feed amount (i.e., skew angle) to 0 (zero).  
         [0016]      FIG. 11  is a block diagram showing such the skew feed correcting unit. A skew feed amount detecting portion  315  is formed by two line sensors  301   a  and  301   b  and a skew feed detecting portion  300 . Sheet leading edge detection signals from the line sensors  301   a  and  301   b  are input to the skew feed detecting portion  300 , and the skew feed detecting portion  300  outputs a skew feed detection signal to a control portion  310 .  
         [0017]     After this, the control portion  310  calculates the skew feed amount based on the skew feed detection signal by a skew feed amount calculating portion  310   a . Further, after that, based on the skew feed amount calculated, the respective rotating speeds of two skew feed correcting portions  320 , formed by skew feed correcting rollers  322   a  and  322   b  and drive sources  321   a  and  321   b , are determined by a control amount calculating portion  310   b . Then, the skew feed correcting rollers  322   a  and  322   b  are driven at the determined rotating speeds, and the sheet is turned according to a difference in sheet conveying speed between the skew feed correcting rollers  322   a  and  322   b  to thereby effect skew feed correction.  
         [0018]     Here, due to the use of the line sensors  301   a  and  301   b , after the start of the skew feed correcting operation, signals are continuously input to the skew feed detecting portion  300  from the line sensors  301   a  and  301   b . Then, a result of the skew feed correction by a skew feed correcting portion  320  is input to the control portion  310  from the skew feed detecting portion  300 . As a result, it is possible to make a judgment as to whether the skew feed amount of the sheet leading edge has been reduced to 0 or not, thus making it possible to effect the skew feed correction with high accuracy.  
         [0019]     However, the conventional sheet conveying apparatus equipped with a skew feed correcting unit using the line sensors  301   a  and  301   b  has the following problems.  
         [0020]     In order to arrange the line sensors  301   a  and  301   b  within a requisite correcting section for correcting the skew feed of the sheet, it is necessary to provide a linear sheet conveying path. That is, to always detect the leading edge of the sheet, it is necessary to provide, as shown in  FIG. 12 , the line sensors  301   a  and  301   b  that are of the same length as the correcting section X, which is of the requisite length for skew feed correction. To arrange the line sensors  301   a  and  301   b , it is necessary to provide a sheet conveying path whose correcting section X is linear. This correcting section X must be made longer in proportion to the level of accuracy for sheet skew feed correction to be attained.  
         [0021]     Further, when the line sensors  301   a  and  301   b  are used, deterioration in detection accuracy is involved due to curling of the leading edge of the sheet, etc. Thus, to detect the Leading edge of the sheet with high accuracy, it is necessary to press the sheet S against the line sensors  301   a  and  301   b . This requires a device for pressing the sheet. Further, all the while the sheet is conveyed through the correcting section X, the sheet S is rubbed against the line sensors  301   a  and  301   b , with the result that marks, etc. are undesirably left on the sheet surface.  
         [0022]     Further, it is impossible for the line sensors  301   a  and  301   b  -to extend to an image transfer position, which locates between a photosensitive drum  21  and a transfer charger  22  shown in  FIG. 12 , with the result that there is generated a conveying section Y where the skew feed correction is impossible. In this case, the skew feed correcting rollers  321   a  and  321   b  may involve a difference in conveying amount as a result of long-term use. When such the difference in conveying amount is generated, there is a possibility of skew feed being newly generated in the conveying section Y.  
         [0023]     Thus, while allowing feedback of the skew feed correction results, the method using the line sensors  301   a  and  301   b  involves limitations in terms of arrangement, and cannot be easily applied to an image forming apparatus, such as a copying machine or a printer, or an image reading apparatus, such as a scanner, which is nowadays being increasingly reduced in size.  
       SUMMARY OF THE INVENTION  
       [0024]     The present invention has been made in view of the above-mentioned problems in the conventional art. It is therefore an object of the present invention to provide a sheet conveying apparatus, an image forming apparatus, and an image reading apparatus capable of realizing a reduction in size and performing sheet skew feed correction with high accuracy.  
         [0025]     According to the present invention, there is provided a sheet conveying apparatus including:  
         [0026]     a skew feed amount detecting portion configured to detect a skew feed amount of a sheet;  
         [0027]     a pair of skew feed correcting portions arranged in a direction perpendicular to a sheet conveying direction, configured to correct skew feed of the sheet by conveying the sheet through independent rotations;  
         [0028]     a conveying amount detecting portion configured to detect the respective sheet conveying amounts of the skew feed correcting portions; and  
         [0029]     a control portion configured to control rotations of the skew feed correcting portions according to detection results obtained by the skew feed amount detecting portion and the conveying amount detecting portion, and  
         [0030]     in the sheet conveying apparatus, after a skew feed correcting operation by the skew feed correcting portions is started, the control portion calculates a sheet correction amount by which the skew feed of the sheet is to be corrected by the pair of skew feed correcting portions based on a detection result obtained by the conveying amount detecting portion, and controls the rotations of the skew feed correcting portions such that the sheet correction amount becomes equal to the skew feed amount of the sheet detected by the skew feed amount detecting portion. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0031]      FIG. 1  is a sectional view of a printer constituting an example of an image forming apparatus equipped with a sheet conveying apparatus according to a first embodiment of the present invention;  
         [0032]      FIG. 2  is a perspective view showing the construction of a skew feed correcting unit provided in the sheet conveying apparatus;  
         [0033]      FIG. 3  is a block diagram showing the skew feed correcting unit;  
         [0034]      FIG. 4  is a side view of the skew feed correcting unit;  
         [0035]      FIG. 5  is a perspective view of a skew feed correcting unit provided in a sheet conveying apparatus according to a second embodiment of the present invention;  
         [0036]      FIG. 6  is a block diagram of the skew feed correcting unit;  
         [0037]      FIG. 7  is a side view of the skew feed correcting unit;  
         [0038]      FIG. 8  is a perspective view showing the construction of a conventional skew feed correcting unit;  
         [0039]      FIG. 9  is a block diagram showing the conventional skew feed correcting unit;  
         [0040]      FIG. 10  is a perspective view showing the construction of another conventional skew feed correcting unit;  
         [0041]      FIG. 11  is a block diagram of still another conventional skew feed correcting -unit; and  
         [0042]      FIG. 12  is a side view of still another conventional skew feed correcting unit. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0043]     In the following, most preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings.  
         [0044]      FIG. 1  is a sectional view of a printer, which constitutes an example of an image forming apparatus equipped with a sheet conveying apparatus according to the first embodiment of the present invention.  
         [0045]     In  FIG. 1 , a printer  1 , which is an image forming apparatus, is equipped with a printer main body  2  provided with an image forming portion  3 , a scanner  11  arranged on the upper surface of the printer main body  2 , and a feeding deck  12  which is arranged on one side of the printer main body  2  and which accommodates a large amount of sheets S in a stacked state.  
         [0046]     Here, the printer main body  2  is equipped with the image forming portion  3  provided with a photosensitive drum  21  serving as the image bearing member, and retard separation type sheet feeding apparatuses  1 . 6  and  17  for feeding the sheets S. Further, the printer main body  2  is equipped with a sheet conveying apparatus  4  for conveying the sheets S.,which are fed by the sheet feeding apparatuses  16  and  17 , to the image forming portion  3 .  
         [0047]     Here, the sheet feeding apparatuses  16  and  17  are equipped with cassettes  13  and  14  containing a plurality of sheets S, feeding rollers  16   a  and  17   a  for sending out the sheets S from the cassettes  13  and  14 , and the like. Further, the sheet conveying apparatus  4  is equipped with a conveying roller  41  and a skew feed correcting unit  18 , and the sheets S sent out from the sheet feeding apparatuses  16  and  17  are sent to the skew feed correcting unit  18  by the conveying roller  41 . The sheets contained in the feeding deck  12  are sent out by a feeding roller  10 , and sent to the skew feed correcting unit  18  by a retard separation type sheet feeding apparatus  15  and a conveying roller  12   a.    
         [0048]     Then, after undergoing skew feed correction by the skew feed correcting unit  18 , the sheet S is transmitted to a transfer portion of the image forming portion  3 , which is formed by the photosensitive drum  21  and the transfer charger  22 . Then, at the transfer portion, a toner image previously formed on the photosensitive drum  21  is transferred. After that, the sheet S to which the toner image has been transferred is sent to a fixing device  24  by a conveyor belt  23 , and the transferred toner image is fixed to the sheet S by the fixing device  24 .  
         [0049]     Here, the printer  1  is provided with a two-sided copying mode in which copying is performed on both sides of the sheet S. In the case of the normal copying mode (i.e., one-side copying mode), the sheet S that has undergone the fixing process is delivered onto a delivery tray  27  outside the printer  1  by a delivery roller pair  26 .  
         [0050]     In the case of the two-sided copying mode, the sheet S is temporarily stacked on an intermediate tray  31  by an in-delivery roller pair  25  or a switchback roller pair  29  through a re-feed path  28  and a duplex transport path  30 . After that, the sheet S placed on the intermediate tray  31  is conveyed to the skew feed correcting unit  18  again by the re-feeding apparatus  32  for image formation, and from this onward, the sheet S undergoes the same process as in the one-side copying mode before being delivered to the outside.  
         [0051]     Next, the skew feed correcting unit  18  will be described.  
         [0052]      FIG. 2  is a perspective view of the skew feed correcting unit  18 . In  FIG. 2 , two optical sheet leading edge detecting sensors  101   a  and  101   b  are arranged at a fixed interval in a direction (i.e., width direction of the sheet being conveyed) which is perpendicular to the sheet conveying direction indicated by an arrow A. Driving rollers  122   a  and  122   b  are arranged coaxially in a direction perpendicular to the sheet conveying direction indicated by the arrow A and independently driven by pulse motors  121   a  and  121   b  serving as drive sources.  
         [0053]     Conveyor belts  123   a  and  123   b  are respectively stretched between the driving rollers  122   a  and  122   b , and driven rollers  124   a  and  124   b  arranged on the upstream side of the driving rollers  122   a  and  122   b . Driven rotary members  131   a  and  13 !b are held in press contact with the conveyor belts  123   a  and  123   b  by a pressurizing device (not shown). The driven rotary members  131   a  and  131   b  are associated with the rotation of the conveyor belts  123   a  and  123   b  to rotate respectively. The driving rollers  122   a  and  122   b , the driven rollers  124   a  and  124   b , the conveyor belts  123   a  and  123   b , and the driven rotary members  131   a  and  131   b  constitute a pair of right and left skew feed correcting portions  18   a  and  18   b.    
         [0054]     A curved path is formed by the conveyor belts  123   a  and  123   b  and the driven rotary members  131   a  and  131   b  arranged on the inner side, making it possible to correct skew feed while conveying the sheet in a curved state.  
         [0055]     Then, the sheet S sent out and conveyed by the sheet feeding apparatuses  15 ,  16 , and  17  is conveyed while being nipped by the conveyor belts  123   a  and  123   b , and the driven rotary members  131   a  and  131   b.    
         [0056]     Rotary encoders  132   a  and  132   b  are rotation amount detecting portions connected to the driven rotary members  131   a  and  131   b . The respective rotation amounts of the driven rotary members  131   a  and  131   b  are detected by the rotary encoders  132   a  and  132   b.    
         [0057]      FIG. 3  is a control block diagram showing the skew feed correcting unit  18 , constructed as described above. A skew feed amount detecting portion  105  for detecting the skew feed amount (i.e., skew angle) of the sheet is formed by the two sheet leading edge detecting sensors  101   a  and  101   b  serving as sheet detecting devices and a skew feed detecting portion  100  to which signals from the sheet leading edge detecting sensors  101   a  and  101   b  are input. A control portion  110  calculates the skew feed amount of the sheet based on a signal from the skew feed detecting portion  100 , and which determines the respective control amounts of the driving rollers  122   a  and  122   b  for the skew feed correcting portions  18   a  and  18   b  to correct the skew feed of the sheet based on the calculated skew feed amount.  
         [0058]     Here, the control amounts of the driving rollers  122   a  and  122   b  are the respective rotating speeds of the driving rollers  122   a  and  122   b  for correcting the skew feed of the sheet S in a predetermined period of time. That is, in the control portion, there is calculated the amount by which the sheet is to be conveyed by the conveyor belts  123   a  and  123   b  to correct the skew feed of the sheet in a predetermined period of time. Then, the respective rotating speeds of the driving rollers  122   a  and  122   b  are set such that the sheet conveying amount of the conveyor belts  123   a  and  123   b  is the calculated sheet conveying amount. By driving the driving rollers  122   a  and  122   b  at the set rotating speeds, there is generated a difference in sheet conveying speed (i.e., peripheral speed), and the sheet S is turned for a predetermined period of time, whereby the skew feed of the sheet is corrected. Here, the predetermined period of time is set as appropriate to a period of time that allows sheet skew feed correction within the correction section X for correcting the skew feed of the sheet until the sheet from the detecting position P 1  reaches the transfer portion formed by the photosensitive drum  21  and the transfer charger  22 . The control portion  110  is equipped with a comparing portion  110   c  and a storage portion  110   d  described below.  
         [0059]     Here, the operation of the skew feed correcting unit  18  will be described with reference to  FIG. 4 , which is a side view of the skew feed correcting unit  18 .  
         [0060]     The sheet S, which is conveyed from the downstream side by the sheet feeding apparatuses  15 ,  16 , and  17 , is conveyed along a curved path which is formed by arranging the conveyor belts  123   a  and  123   b  on the outer side and arranging the driven rotary members  131   a  and  131   b  on the inner side. Then, the leading edge of the sheet S traverses the sheet leading edge detecting sensors  101   a  and  101   b . As a result, the sheet leading edge detecting sensors  101   a  and  101   b  output signals indicating the traversing of the sheet to the skew feed detecting portion  100 .  
         [0061]     Here, for example, when the sheet leading edge traverses the detecting position P 1  of the two sheet leading edge detecting sensors  101   a  and  101   b  with different timings, the detection signals are output from the sheet leading edge detecting sensors  101   a  and  101   b  to the skew feed detecting portion  100  with different timings.  
         [0062]     When the detection signals are thus output with different timings, the skew feed detecting portion  100  outputs an instantaneous skew feed detection signal to the control portion  110 , and the control portion  110  calculates the skew feed amount (i.e., skew angle) by a skew feed amount calculating portion  110   a  based on the skew feed detection signal. Based on the skew feed amount thus calculated, the respective control amounts for the skew feed correcting portions  18   a  and  18   b  of the skew feed correcting unit  18  are determined by a control amount calculating portion  110   b . That is, the amount by which the sheet is to be conveyed by the conveyor belts  123   a  and  123   b  in the predetermined period of time to correct the skew feed amount of the sheet S is calculated, and the respective rotating speeds of the driving rollers  122   a  and  122   b  for achieving this are determined.  
         [0063]     Based on this control amount, the respective rotating speeds of the pulse motors  121   a  and  121   b  are controlled, and the conveyor belt  123   a  of the front-side skew feed correcting portion  18   a  and the conveyor belt  123   b  of the depth-side skew feed correcting portion  18   b , shown in  FIG. 2 , are driven for a predetermined period of time with a difference in sheet conveying speed (i.e., peripheral speed). By thus producing a difference in the conveying amount of the sheet S (obtained by sheet conveying speed×time) between the front-side skew feed correcting portion  18   a  and the depth-side skew feed correcting portion  18   b , the sheet is turned, and the skew feed is corrected.  
         [0064]     This difference in sheet conveying speed (i.e., peripheral speed) may be controlled so as to be always at a fixed value, or may be controlled such that the difference in sheet conveying speed is increased according to the skew feed amount. Further, skew feed correction can also be effected by performing control to delay the advancing end portion side of the sheet S, control to promote the delayed end portion side thereof, or both controls. Further, control is possible if the pulse motors  121   a  and  121   b  serving as the drive sources are DC motors or AC motors.  
         [0065]     When skew feed correction is thus effected on the sheet S by the skew feed correcting unit  18 , there occurs a difference between the rotation amount of the front-side driven rotary member  131   a  and the rotation amount of the depth-side driven rotary member  131   b , both of which rotate with the sheet conveyed by the conveyor belts  123   a  and  123   b . Accordingly, the respective detection values of the rotary encoders  132   a  and  132   b  vary accordingly.  
         [0066]     Here, the difference in detection value between the rotary encoders  132   a  and  132   b  corresponds to a difference in the amount by which the sheet is conveyed by the conveyor belts  123   a  and  123   b , so this difference constitutes the correction amount to be obtained through actual correction on the sheet S. Then, the detection value detected by the conveying amount detecting portion  130  including the driven rotary members  131   a  and  131   b  and the rotary encoders  132   a  and  132   b  is output to the control portion  110 , where the actual difference in sheet conveying amount is calculated as the correction amount result.  
         [0067]     Further, according to the detection results obtained by the skew feed amount detecting portion  105  and the conveying amount detecting portion  130 , the comparing portion  110   c  of the control portion  110  compares the actual correction amount result with the skew feed amount which is detected by the skew feed amount detecting portion  105  and then calculated by the skew feed amount calculating portion  110   a . Then, a judgment is made as to whether the skew feed correction has been effected to a sufficient degree or as to whether overshoot (i.e., getting beyond the proper position) in skew feed correction has been generated or not. That is, when determining the control amount by the control amount calculating portion  110   b , the difference in the amount by which the sheet is conveyed by the conveyor belts  123   a  and  123   b  is calculated from the skew feed amount of the sheet S. Thus, the difference between this calculated sheet conveying amount and the actual sheet conveying amount input from the conveying amount detecting portion  130  are compared with each other to make a judgment as to whether the skew feed correction has been performed properly or not.  
         [0068]     When the comparison result shows that the skew feed amount is insufficient, the difference in the sheet conveying speed (i.e., peripheral speed) at which the sheet is conveyed by the conveyor belts  123   a  and  123   b  is maintained. When overshoot has been generated, the pulse motors  121   a  and  121   b  are controlled such that the relationship of the difference in the sheet conveying speed is reversed. This operation is continued until the calculated sheet conveying amount and the actual sheet conveying amount become equal to each other; even after they are judged to be equal, correction control is conducted again when any skew feed of the sheet is detected.  
         [0069]     It is also possible for the control portion  110  to calculate the actual sheet skew feed correction amount (i.e., correction angle) from the detection value detected by the conveying amount detecting portion  130 , and to compare the correction amount with the skew feed amount (i.e., skew angle) calculated by the skew feed amount calculating portion  110   a  based on the skew feed detection signal from the skew feed detecting portion  100 . Based on this comparison, a judgment is made as to whether the sheet skew feed correction is proper or not. In this case also, when the comparison result shows that the skew feed amount is insufficient, the difference in conveying speed (i.e., peripheral speed) at which the sheet is conveyed by the conveyor belts  123   a  and  123   b  is maintained. When overshoot has been generated, the pulse motors  121   a  and  121   b  are controlled such that the relationship of the difference in sheet conveying speed is reversed. Then, this operation is continued until the skew angle based on the detection by the skew feed detecting portion  100  and the actually corrected correction angle become equal to each other. Even after those angles are judged to be equal, correction control is conducted again if any skew feed of the sheet is detected.  
         [0070]     This operation can be continued through the correction section X from the moment that skew feed of the leading edge of the sheet is detected at the detecting position P 1  shown in  FIG. 4  to the moment that the leading edge of the sheet reaches the transfer portion formed by the photosensitive drum  21  and the transfer charger  22 .  
         [0071]     When, in the skew feed correcting portions  18   a  and  18   b , there is no sheet S between the conveyor belts  123   a ,  123   b  and the driven rotary member  131   a ,  131   b , the driven rotary members  131   a  and  131   b  follow the movement of the conveyor belts  123   a  and  123   b . Thus, by detecting the signals from the rotary encoders  132   a  and  132   b  in this sate, it is possible to control the sheet conveying speed of the conveyor belts  123   a  and  123   b . For example, when the surfaces of the conveyor belts  123   a  and  123   b  have been worn as a result of long-term use, and the sheet conveying speed is changed, it is possible to maintain the sheet conveying speed of the conveyor belts  123   a  and  123   b  at a desired value by the conveying amount detecting portion  130  and the control portion  110 .  
         [0072]     Further, the rotational accuracy of the driven rotary members  131   a  and  131   b  is strictly controlled, and variation in the rotation of the conveyor belts  123   a  and  123   b  is stored in the storage port-on  110   d  provided in the control portion  110 , controlling the pulse motors  121   a  and  121   b  so as to cancel the variation in rotation. As a result, it is possible to equalize the rotating speeds of the two driving rollers  122   a  and  122   b , and to obtain a desired stable sheet conveying speed free from variation in rotation. Further, in the case of this construction, by making the cycle of all the rotary members related to the skew feed correcting unit  18  and of all the rotary members related to the conveying amount detecting portion  130  an integral multiple, it is possible to shorten the cycle of the variation in rotation.  
         [0073]     In this way, after the start of the skew feed correcting operation by the skew feed correcting unit  18 , the difference in the sheet conveying amount between the skew feed correcting portions  18   a  and  18   b  of the skew feed correcting unit  18  is detected based on a signal from the conveying amount detecting portion  130 . Then, the skew feed correcting portions  18   a  and  18   b  of the skew feed correcting unit  18  are controlled such that the difference in the detected sheet conveying amount becomes equal to the skew feed amount detected by the skew feed detecting portion  105 . This makes it possible to achieve a reduction in size and to perform skew feed correction with which accuracy. Further, it is possible to perform skew feed correction until the sheet leading edge reaches the transfer position.  
         [0074]     In this embodiment, as shown in  FIG. 4 , the rotation center of the driven rotary members  131   a  and  131   b  and of the rotary encoders  132   a  and  132   b  is provided at a center of a circle along which the curved sheet conveying path (i.e., curved path) L extends. By arranging the conveyor belts  123   a  and  123   b  and the driven rotary members  131   a  and  131   b  in the curved portion of the sheet conveying path L, the contact area between the conveyor belts  123   a  and  123   b  and the sheet S and the driven rotary members  131   a  and  131   b  is increased.  
         [0075]     Due to this arrangement, it is possible to mitigate slippage between the sheet S and the driven rotary members  131   a ,  131   b  and between the conveyor belts  123   a ,  123   b  and the driven rotary members  131   a ,  131   b . The same effect can be obtained even if the positional relationship of the skew feed correcting unit  18  and the conveying amount detecting portion  130  with respect to the sheet conveying path L is reversed.  
         [0076]     In this case, the conveying amount detecting portion  130  is formed not by the rotary encoders  132   a  and  132   b  and the driven rotary members  131   a  and  131   b  but by the rotary encoders  132   a  and  132   b  and the conveyor belts  123   a  and  123   b . In the case in which the skew feed correcting unit  18  and the conveying amount detecting portion  130  are provided in the substantially linear portion of the sheet conveying path, both may be provided as a belt structure.  
         [0077]     Further, instead of using the conveyor belts  123   a  and  123   b , it is also possible to convey the sheet, with a single driving roller  122   a ,  122   b  and a plurality of driven rotary members arranged along the peripheral surfaces of the driven rotary members  131   a  and  131   b.    
         [0078]     Next, the second embodiment of the present invention will be described.  
         [0079]      FIG. 5  is a perspective view of a skew feed correcting unit provided in a sheet conveying apparatus according to an embodiment of the present invention, and  FIG. 6  is a block diagram thereof. In  FIGS. 5 and 6 , the reference symbols that are the same as those of  FIGS. 2 and 3  indicate the same or equivalent components.  
         [0080]     In  FIG. 5 , skew feed correcting rollers  125   a  and  125   b  constitute a pair of skew feed correcting portions  20   a  and  20   b  of a skew feed correcting unit  20 . The skew feed correcting rollers  125   a  and  125   b  are arranged coaxially in a direction perpendicular to the sheet conveying direction indicated by the arrow A, and are driven independently by pulse motors  121   a  and  121   b  serving as the drive sources.  
         [0081]     Conveying rotary members  126   a  and  126   b  are held in press contact with the skew feed correcting rollers  125   a  and  125   b  by a pressurizing device (not shown). The sheet S conveyed by the sheet feeding apparatuses  15 ,  16 , and  17  is conveyed while being sandwiched between the conveying rotary members  126   a  and  126   b  and the skew feed correcting rollers  125   a  and  125   b.    
         [0082]     In this embodiment, the axial width of the skew feed correcting rollers  125   a  and  125   b  is larger than the axial width of the conveying rotary members  126   a  and  126   b . Due to this difference in width, there are generated, in the skew feed correcting rollers  125   a  and  125   b , portions that are not in contact with the conveying rotary members  126   a  and  126   b , and laser Doppler type sensors  133   a  and  133   b  are arranged so as to face these portions.  
         [0083]     Here, the laser Doppler type sensors  133   a  and  133   b  are sensors capable of detecting fluctuation in the speed of an object in a non-contact fashion. By using the sensors  133   a  and  133   b , it is possible to detect the conveying speed of the sheet S. Further, the sensors  133   a  and  133   b  are adapted to undergo a change in output when the sheet S is conveyed thereto. Due to this change in output, it is possible to detect any sheet S conveyed thereto. That is, in this embodiment, the skew feed amount detecting portion also serves as the conveying amount detecting portion, and the skew feed amount detecting portion  115  of the present invention is formed by the sensors  133   a  and  133   b  and the skew feed detecting portion  100 .  
         [0084]     In the skew feed correcting unit  20 , constructed as described above, when the sheet S conveyed by the sheet feeding apparatuses  15 ,  16 , and  17  is conveyed to the detecting point P 2  of the sensors  133   a  and  133   b  as shown in  FIG. 7 , the values detected by the sensors  133   a  and  133   b  change.  
         [0085]     Here, when the sheet S is conveyed in a skew feed state, the leading edge of the sheet S traverses the two sensors  133   a  and  133   b  with different timings, whereby detection signals are output from the sensors  133   a  and  133   b  with different timings.  
         [0086]     As in the first embodiment described above, when the detection signals are thus output with different timings, the skew feed detection signals are input to the skew feed detecting portion  100  shown in  FIG. 6  from the sensors  133   a  and  133   b , and the skew feed detecting portion  100  outputs an instantaneous skew feed detection signal to the control portion  110 . After this, the control portion  110  calculates the skew feed amount based on this skew feed detection signal. Then, based on the skew feed amounts thus calculated, there is determined the control amount with respect to the skew feed correcting portions  20   a  and  20   b  of the skew feed correcting unit  20 , formed by the pulse motors  121   a  and  121   b , the skew feed correcting rollers  125   a  and  125   b , etc. Here, as in the first embodiment, the term “control amount” means a rotating speed set for each of the skew feed correcting rollers  125   a  and  125   b  in order to correct the skew feed of the sheet S in a predetermined period of time.  
         [0087]     Based on this control amount, the rotating speeds of the pulse motors  121   a  and  121   b  are controlled to produce a difference in a rotating speed between the front-side skew feed correcting roller  125   a  and the depth-side skew feed correcting roller  125   b , thereby producing a difference in the sheet conveying amount between the front side and the depth side. In this way, the sheet S is turned, and the skew feed thereof is corrected.  
         [0088]     Here, the sheet conveying speeds (i.e., peripheral speeds) of the front-side skew feed correcting roller  125   a  and the depth-side skew feed correcting roller  125   b  are respectively detected by the sensors  133   a  and  133   b . By calculating the sheet conveying amount based on the detection result, it is possible to detect the actual correction amount.  
         [0089]     The comparing portion  110   c  of the control portion  110  compares the actual correction result input from the sensors  133   a  and  133   b , also serving as the conveying amount detecting portion, with the skew feed amount detected by the skew feed amount detecting portion  115  and calculated by the skew feed amount calculating portion  110   a . In this way, it is checked whether the skew feed correction has been effected to a sufficient degree, or whether any overshoot in a skew feed correction has been generated or not.  
         [0090]     When the skew feed amount is insufficient, the difference in the sheet conveying speed (i.e., peripheral speed) is maintained; when an overshoot has been generated, the pulse motors  121   a  and  121   b  are controlled so as to reverse the relationship of the difference in the sheet conveying speed. This operation is continued until the skew feed amount becomes equal to the correction amount. Even after the skew feed amount and the correction amount are judged to be equal, correction control is conducted again if any skew feed of the sheet is detected.  
         [0091]     Due to this construction, it is possible, as in the first embodiment described above, to provide a sheet conveying apparatus and a printer (i.e., image forming apparatus) allowing a reduction in size and capable of conducting sheet skew feed correction with high accuracy.  
         [0092]     In the above-mentioned embodiments, after the start of the skew feed correcting operation, a difference in the sheet conveying amount of the skew feed correcting unit is detected based on the detection by the conveying amount detecting portion, and the skew feed correcting unit is controlled such that the difference in the sheet conveying amount detected becomes equal to the skew feed amount detected. As a result, it is possible to achieve a reduction in the size of a sheet conveying apparatus, and to,perform sheet skew feed correction with high accuracy.  
         [0093]     When it is rather difficult to detect the sheet conveying speeds by the laser Doppler type sensors  133   a  and  133   b  due to the material of the skew feed correcting rollers  125   a  and  125   b , the width of the conveying rotary members  126   a  and  126   b  is made larger than that of the skew feed correcting rollers  125   a  and  125   b . In this case, it is also possible to arrange the laser Doppler type sensors  133   a  and  133   b  on the skew feed correcting rollers  125   a  and  125   b  side, and to detect the sheet conveying speeds of the conveying rotary members  126   a  and  126   b  to thereby control the pulse motors  121   a  and  121   b.    
         [0094]     While in the first and second embodiments described above, the sheet conveying apparatus of the present invention is provided in a printer (i.e., image forming apparatus). However, the apparatus according to the present invention should not be construed restrictively. The present invention is also applicable, for example, to an image reading apparatus, such as a scanner, which is equipped with an image reading portion for reading the image of an original.  
         [0095]     This application claims priority from Japanese Patent Application No. 2005-218847 filed Jul. 28, 2005, which is hereby incorporated by reference herein.