Abstract:
50 An image forming apparatus having a pair of sheet transporting members having rotary shafts on the same axis in a direction perpendicular to the transport direction of a sheet, and rotatively driven independently of each other to thereby transport the sheet, a detector provided along a cross direction perpendicular to the transport direction of the sheet for detecting the transported state of the sheet transported by the sheet transporting members, and a controller for drive-controlling the pair of sheet transporting members on the basis of the detection information of the detector, and effecting the correction of a sheet position in the cross direction and the correction of the skew feed posture of the sheet relative to the transport direction.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The invention relates to a sheet transporting apparatus, and particularly to the correction of the position of a sheet in the cross direction thereof perpendicular to a sheet transport direction and the correction of the skew feed posture of the sheet relative to the sheet transport direction.  
           [0003]    2. Description of the Related Art  
           [0004]    In an image forming apparatus such as a copying machine, or an image reading apparatus, a registration (skew feed correction) device for the posture correction and alignment of a sheet being transported is provided immediately before the image forming portion or the image reading portion thereof. As such a registration device, there is, for example, an active registration system for correcting the skew feed of a sheet while transporting the sheet. This system is such that two original detecting sensors are disposed in a sheet transport path in the cross direction (hereinafter referred to as the main scan direction) of the sheet perpendicular to a sheet transport direction, and detect the skew of the leading edge of the sheet on the basis of a signal produced by the leading edge of the sheet crossing the respective sensors, and also the sheet transporting speed of skew feed correction rollers (hereinafter referred to as the registration rollers) disposed in the main scan direction and drivable independently of each other is controlled to thereby correct the skew feed of the sheet.  
           [0005]    According to this system, skew feed correction can be effected while the sheet is transported without being once stopped and therefore, the throughput of the sheet is not reduced. FIG. 13 of the accompanying drawings is a typical view illustrating a method of correcting the skew feed of a sheet transported by the active registration system.  
           [0006]    As the above-described conventional skew feed correction control method, there is an acceleration and deceleration control method or the like as shown in FIG. 13 wherein a transport roller  14  on one side preceding by skew feed is deceleratedly driven (the arrow D in FIG. 13) and at the same time, the accelerated drive of a transporting roller  13  (the arrow A in FIG. 13) on the other side delayed is effected.  
           [0007]    On the other hand, the skew feed direction, the detected skew feed amount (Nb), etc. of an original transported by rollers  13  and  14  are measured by original detecting sensors  15  and  16 . As drive sources for rotatively driving the rollers  13  and  14 , use is usually made of pulse motors  11  and  12 , and the aforementioned detected skew feed amount Nb is measured by counting the time from after one of the original detecting sensors  15  and  16  has detected the original until the other sensor detects the original, by counting clocks driving the pulse motors  11  and  12 .  
           [0008]    Further, the original detecting sensors  15  and  16  are disposed at distances equal from the center of the main scan direction to the right and left and therefore, there is the characteristic that when skew feed control is effected in conformity with the detected skew feed amount Nb calculated from the detection information of these detecting sensors  15  and  16 , the transport of the original in the central portion of the main scan direction of the original does not differ from the ordinary transport thereof, in the case of the acceleration and deceleration control method.  
           [0009]    However, an improvement in the sheet transporting speed has sometimes caused, in addition to the skew feed of the sheet, the deviation of the sheet position in the main scan direction.  
           [0010]    By the conventional active registration system, it has been impossible to detect the deviation of the sheet position in the main scan direction and therefore, to correct the deviation of the sheet position.  
         SUMMARY OF THE INVENTION  
         [0011]    The present invention has been made in view of the above-noted problem and an object thereof is to provide a sheet transporting apparatus which can efficiently effect the correction of the positional deviation of a sheet in the main scan direction and the correction of the skew feed of the sheet.  
           [0012]    (1) In order to achieve the above object, a sheet transporting apparatus according to the present invention has a pair of sheet transporting members having rotary shafts on the same axis in a direction perpendicular to the transport direction of a sheet and rotatively driven independently of each other to thereby transport the sheet, detecting means provided along a cross direction perpendicular to the sheet transport direction for detecting the transported state of the sheet transported by the sheet transporting members, and control means for drive-controlling the pair of sheet transporting members on the basis of the detection information of the detecting means, and effecting the correction of a sheet position in the cross direction and the correction of the skew feed posture of the sheet relative to the transport direction.  
           [0013]    (2) In the above item (1), the control means may preferably give a transporting speed difference between the pair of transporting members.  
           [0014]    (3) In the above item (1), the detecting means may preferably be a line sensor disposed in parallelism to the cross direction and the size of a detectable area in the cross direction by the line sensor may preferably be larger than the size of at least an area through which the sheet passes when transported in the cross direction.  
           [0015]    (4) In the above item (1), the detecting means may preferably be disposed upstream of the pair of transporting members with respect to the sheet transport direction.  
           [0016]    (5) In the above item (1), the detecting means may preferably be disposed downstream of the pair of transporting members with respect to the sheet transport direction.  
           [0017]    (6) In the above item (1), the control means may preferably effect deviation correction control in the cross direction and skew feed direction of the sheet caused by the pair of transporting members in parallel with each other.  
           [0018]    (7) In the above item (1), the sheet transporting apparatus may preferably have calculating means for calculating the movement direction and the movement amount of the sheet in the cross direction, and the skew feed direction and the skew feed amount of the sheet, relative to a normal transport position, on the basis of the detection information of the detecting means.  
           [0019]    (8) In the above item (7), the calculating means may preferably calculate the skew feed direction and the skew feed amount after the deviation correction control in the cross direction of the sheet to thereby calculate a total skew feed direction and a total skew feed amount, and the control means may preferably effect the deviation correction control in the skew feed direction of the sheet on the basis of the total skew feed direction and the total skew feed amount.  
           [0020]    Other objects and features of the present invention will become apparent from the following description and the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]    [0021]FIG. 1 is a schematic view showing the construction of the essential portions of a first embodiment of the present invention.  
         [0022]    [0022]FIG. 2 is a block diagram showing the control construction of the sheet transporting apparatus of FIG. 1.  
         [0023]    [0023]FIGS. 3A and 3B are typical views illustrating a main scan deviation detecting method and a skew feed detecting method in the sheet transporting apparatus of FIG. 1.  
         [0024]    [0024]FIG. 4 is a flow chart illustrating the control procedure of effecting main scan deviation correction and skew feed correction in the first embodiment of the present invention.  
         [0025]    [0025]FIG. 5 is a schematic view illustrating the construction of the essential portions of a second embodiment of the present invention.  
         [0026]    [0026]FIG. 6 is a block diagram illustrating the control construction of the second embodiment of the present invention.  
         [0027]    [0027]FIGS. 7A and 7B are typical views illustrating a main scan deviation detecting method and a skew feed detecting method in the second embodiment of the present invention.  
         [0028]    [0028]FIG. 8 is a flow chart illustrating the control procedure of effecting main scan deviation correction and skew feed correction in the second embodiment of the present invention.  
         [0029]    [0029]FIG. 9 is a schematic view illustrating the construction of the essential portions of a third embodiment of the present invention.  
         [0030]    [0030]FIG. 10 is a block diagram illustrating the control construction of the third embodiment of the present invention.  
         [0031]    [0031]FIGS. 11A and 11B are typical views illustrating a main scan deviation detecting method and a skew feed detecting method in the third embodiment of the present invention.  
         [0032]    [0032]FIG. 12 is a flow chart illustrating the control procedure of effecting main scan deviation correction and skew feed correction in the third embodiment of the present invention.  
         [0033]    [0033]FIG. 13 is a typical view illustrating a method of correcting the skew feed of a sheet transported by a conventional sheet transporting apparatus. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     First Embodiment  
       [0034]    Description will first be made of a first embodiment of a sheet transporting apparatus which achieves the objects of the present invention.  
         [0035]    [0035]FIG. 1 is a schematic view illustrating the construction of the essential portions of the sheet transporting apparatus according to the present embodiment. In FIG. 1, the same members as the members shown in FIG. 13 are given the same reference numerals.  
         [0036]    In FIG. 1, the sheet transporting apparatus according to the present embodiment has transporting rollers  13  and  14  frictionally contacting with a sheet to thereby transport the sheet in a predetermined transport direction, and a sheet detecting line sensor  17 , which is perpendicular to the transport direction of the sheet and disposed at a distance equal from the center of a transport path along which the sheet is normally transported to the right and left in order to measure the deviation amount and deviation direction in a direction perpendicular to the sheet transport direction (hereinafter referred to as the main scan direction), and a skew feed direction and a skew feed amount N. Also, this sheet detecting line sensor  17  is disposed downstream of the transporting rollers  13  and  14  in the transport path of the sheet.  
         [0037]    Also, the transporting rollers  13  and  14  each have a center of rotation on a straight line extending in the main scan direction (cross direction) on the transport path of the sheet, and are disposed in opposed relationship with each other with an interval of a distance “a” therebetween, and are directly connected to pulse motors  11  and  12 , respectively, and are drive-controlled independently of each other.  
         [0038]    The detectable area “d” by the sheet detecting line sensor  17  is wider than at least the widthwise length of the transport path of the sheet.  
         [0039]    [0039]FIG. 2 is a block diagram illustrating the control construction of the sheet transporting apparatus according to the present embodiment. In FIG. 2, a skew feed amount calculating portion  201  calculates the skew feed direction and skew feed amount N of the sheet being transported relative to the normal transport posture thereof on the basis of detection information from the sheet detecting line sensor  17 . A main scan deviation amount calculating portion  203  calculates a main scan deviation direction which is a direction deviating from a normal transport position in the cross direction of the sheet, and a main scan deviation amount X which is a deviation amount in the main scan deviation direction, on the basis of the detection information from the sheet detecting line sensor  17 .  
         [0040]    A correction control portion  202  controls driving pulse numbers individually given to the pulse motors  11  and  12 , and corrects the main scan deviation amount and skew feed amount of the sheet during transport, on the basis of the signals of the skew feed direction and the skew feed amount N transmitted from the skew feed amount calculating portion  201 , and the signals of the main scan deviation direction and the main scan deviation amount X transmitted from the main scan deviation amount calculating portion  203 .  
         [0041]    Each of the skew feed amount calculating portion  201 , the correction control portion  202  and the main scan deviation amount calculating portion  203  may be comprised, for example, of a controller provided with a CPU, a ROM, a RAM etc., and storing therein a program for effecting various kinds of control in accordance with the procedure of a flow chart which will be described later.  
         [0042]    [0042]FIGS. 3A and 3B are typical views illustrating a method of detecting the main scan deviation direction, the main scan direction amount X, the skew feed direction and the skew feed amount N (hereinafter referred to as the transported state) of the sheet in the sheet transporting apparatus according to the present embodiment, and in these figures, the same portions are given the same reference numerals and need not be described.  
         [0043]    For the calculation of the main scan deviation amount X, when first, with one side (the pulse motor  11  side) of the sheet detecting line sensor  17  as the reference, as shown in FIG. 3A, the position at which the sheet detecting line sensor  17  has first detected the sheet is defined as α, and the normal positions in the main scan direction which the sheet should originally pass are defined as “b” and “c” (“b” is the position of the end portion of the sheet on the pulse motor  11  side, and “c” is the position of the end portion of the sheet on the pulse motor  12  side), the main scan deviation amount X can be obtained from  
           X=c−α (or  X=b−α ). 
         [0044]    Next, for the calculation of the skew feed amount N, when after the sheet portion has first been detected by the sheet detecting line sensor  17 , as shown in FIG. 3B, the position at which the detected position of the sheet by the sheet detecting line sensor  17  assumes a minimum value (or a maximum value) is defined as β, the time until the position β is assumed is counted at a constant frequency “f” to thereby measure a detected skew feed count number C.  
         [0045]    Next, a detected skew feed amount Nb detected by the sheet detecting line sensor  17  is calculated from Nb=C×V/f by the use of transporting speed V of the sheet. Then, the skew feed amount N in the transporting roller portion actually used for the correction control portion  202  to control the pulse motors  11  and  12  is calculated from N=Nb×(α−β)/a by the use of the detected skew feed amount Nb detected by the sheet detecting line sensor and the distance between the transporting rollers  13  and  14 .  
         [0046]    The correction control portion  202  then controls the driving pulses of the pulse motors  11  and  12  on the basis of the thus obtained transport state to thereby correct the main scan deviation amount and skew feed amount of the transported sheet.  
         [0047]    A correction control method for the main scan deviation amount and the skew feed amount in the sheet transporting apparatus according to the present embodiment will hereinafter be described with reference to a flow chart shown in FIG. 4.  
         [0048]    In the case of FIGS. 3A and 3B, the sheet is in a skew feed posture rotated leftwardly relative to its normal position and therefore, the corner portion of the sheet which is far from the reference position of the sheet detecting line sensor  17  is first detected and the value thereof becomes a maximum value, but in an opposite skew feed posture, the corner portion of the sheet which is near is first detected and therefore the value thereof becomes a minimum value.  
         [0049]    In FIG. 4, the main scan deviation correction amount obtained on the basis of the main scan deviation amount X is represented by Xo, the main scan deviation allowable value determining the allowable range of the main scan deviation amount X is represented by Xm, and the skew feed allowable value determining the allowable range of the skew feed amount N is represented by Nm.  
         [0050]    When at a step  1 , the sheet detecting line sensor  17  detects the sheet, the main scan deviation amount calculating portion  203  calculates the main scan deviation direction and the main scan deviation amount X.  
         [0051]    When at a step  2 , the detected position of the sheet by the sheet detecting line sensor  17  has assumed a minimum value (or a maximum value), the skew feed amount calculating portion  201  calculates the skew feed direction and the skew feed amount N.  
         [0052]    At a step  3 , the correction control portion  202  judges whether the main scan deviation amount X is within a range of −Xm≦X≦Xm, and when it is judged to be within this allowable range, at a step  8 , the correction control portion  202  judges whether the skew feed amount N is within an allowable range of −Nm≦N≦Nm, and when it is judged to be within the allowable range processing is ended.  
         [0053]    On the other hand, when at the step  3 , the main scan deviation amount X is judged to be not within the allowable range of −Xm≦X≦Xm, at a step  4 , the correction of the main scan deviation amount (hereinafter referred to as the main scan deviation correction) is effected for a predetermined period by the correction control portion  202 .  
         [0054]    Then, at a step  5 , a total skew feed amount is calculated from the skew feed amount caused by the main scan deviation correction and the skew feed amount calculated by the skew feed amount calculating portion  201 , and at a step  6 , the correction of the total skew feed amount (hereinafter referred to as the skew feed correction) is effected by the correction control portion  202 .  
         [0055]    Subsequently, at a step  7 , the correction control portion  202  judges whether the main scan deviation correction amount Xo is within an allowable range of X−Xm≦Xo≦X+Xm, and when it is judged to be within the allowable range, processing is ended.  
         [0056]    When at the step  7 , the main scan deviation correction amount correction Xo is judged to be not within the allowable range of X−Xm≦Xo≦X+Xm, return is made to the step  4 , where the main scan deviation correction is again effected for a predetermined period by the correction control portion  202 , whereafter similar steps are executed.  
         [0057]    When at a step  8 , the skew feed amount N is judged to be not within the allowable range of −Nm≦N≦Nm, at a step  9 , the skew feed correction is effected by the correction control portion  202  by an amount corresponding to the skew feed amount N calculated by the skew feed amount calculated portion  201 , and processing is ended.  
         [0058]    As described above, according to the present embodiment, the pulse motors  11  and  12  are drive-controlled in conformity with the transported state of the sheet calculated on the basis of the detection information of the transported sheet to thereby effect the widthwise deviation correction and the skew feed correction, whereby the sheet can be corrected into its normal transported state and be transported.  
       Second Embodiment  
       [0059]    A second embodiment will now be described. While in the above-described first embodiment, description has been made of the correction control in which the main scan deviation correction and the skew feed correction are repeated and effected little by little on the basis of the deviation direction in the main scan direction, the deviation amount in the main scan direction, the skew feed direction and the skew feed amount calculated by detecting the sheet transported sheet by the sheet detecting line sensor as the sheet detecting means downstream of the transporting rollers as the driving means for transporting the sheet, in the present embodiment, the sheet detecting line sensor  17  as the sheet detecting means is disposed upstream of the transporting rollers as the sheet driving means with respect to the transport direction, and design is made so as to effect the main scan deviation correction and the skew feed correction by transporting rollers downstream of the sheet detecting line sensor  17  with respect to the transport direction, i.e., transporting rollers discrete from the transporting rollers which have caused skew feed, on the basis of the deviation direction in the main scan direction, the deviation amount in the main scan direction, the skew feed direction and the skew feed amount calculated by detecting the sheet.  
         [0060]    [0060]FIG. 5 is a schematic view illustrating the construction of the essential portions of a sheet transporting apparatus according to the present embodiment, and in FIG. 5, the above-described portions are given the same reference numerals and need not be described. The sheet detecting line sensor  17  in the present embodiment is disposed upstream of the transporting rollers  13  and  14  for effecting the main scan deviation correction and the skew feed correction on the transport path of the sheet with respect to the transport direction, and effects the detection of the transported state of the sheet transported by transporting means, not shown.  
         [0061]    [0061]FIG. 6 is a block diagram illustrating the control construction of the sheet transporting apparatus according to the present embodiment.  
         [0062]    In FIG. 6, the reference numeral  601  designates a skew feed amount calculating portion which calculates the skew feed direction and skew feed amount N of the sheet on the basis of detection information from the sheet detecting line sensor  17 . The reference numeral  603  denotes a main scan deviation amount calculating portion which calculates the main scan deviation direction and main scan deviation amount X of the sheet on the basis of the detection information from the sheet detecting line sensor  17 . A correction control portion  602  controls driving pulse numbers given to the pulse motors  11  and  12  on the basis of the signals of the skew feed direction and the skew feed amount N transmitted from the skew feed amount calculating portion  601 , and the main scan deviation direction and the main scan deviation amount X transmitted from the main scan deviation amount calculating portion  603 , and corrects the main scan deviation amount and skew feed amount of the transported sheet.  
         [0063]    Each of the skew feed amount calculating portion  601 , the correction control portion  602  and the main scan deviation amount calculating portion  603  may be comprised, for example, of a controller provided with a CPU, a ROM and a RAM, and may be controlled in accordance with the procedure of a flow chart which will be described later.  
         [0064]    [0064]FIGS. 7A and 7B are typical views illustrating a method of detecting the transported state of the sheet in the sheet transporting apparatus according to the present embodiment, and in these figures, the same members as the above-described members are given the same reference numerals and need not be described.  
         [0065]    For the calculation of the main scan deviation amount X, first, when the position at which the sheet has first been detected by the sheet detecting line sensor  17  is defined as α, and the normal widthwise positions which the sheet should originally passes at the sheet detecting line sensor  17  are defined as “b” and “c” (“b” is the position of the end portion of the sheet on the pulse motor  11  side, and “c” is the position of the end portion of the sheet on the pulse motor  12  side), X is calculated from  
           X=c−α (or  X=b−α ). 
         [0066]    For the calculation of the skew feed amount N, a position β at which the detected position of the sheet by the sheet detecting line sensor  17  assumes a minimum value (or a maximum value) and the time until the sheet becomes undetected by the sheet detecting line sensor  17  are counted at a constant frequency “f” to thereby measure a detected skew feed count number C. Also, the position at which the sheet has been detected lastly by the sheet detecting line sensor  17  is defined as Υ.  
         [0067]    Next, the detected skew feed amount Nb in the sheet detecting line sensor  17  itself is calculated from Nb=C×V/f by the use of the transporting speed of the sheet. The skew feed amount N in the transporting roller portion actually used for the correction control portion  602  to control the pulse motors  11  and  12  is calculated from N=Nb×(Υ−β)/a by the use of the detected skew feed amount Nb detected by the sheet detecting line sensor and the distance “a” between the transporting rollers.  
         [0068]    The correction control portion  602  controls driving pulse numbers given to the pulse motors  11  and  12  on the basis of the signals of the thus obtained skew feed direction and skew feed amount N, and the main scan deviation direction and the main scan deviation amount X, and corrects the main scan deviation amount and skew feed amount of the transported sheet.  
         [0069]    A method of correcting the main scan deviation amount and skew feed amount in the sheet transporting apparatus according to the present embodiment will hereinafter be described with reference to a flow chart shown in FIG. 8.  
         [0070]    [0070]FIG. 8 is a flow chart illustrating the procedure of correcting the main scan deviation amount and skew feed amount in the sheet transporting apparatus according to the present embodiment.  
         [0071]    A main scan deviation allowable value determining the allowable range of the main scan deviation amount X is represented by Xm, and a skew feed allowable value determining the allowable range of the skew feed amount N is represented by Nm.  
         [0072]    At a step  11 , when the sheet detecting line sensor  17  detects the sheet, the main scan deviation amount calculating portion  603  calculates the main scan deviation direction and the main scan deviation amount X.  
         [0073]    At a step  12 , the correction control portion  602  judges whether the main scan deviation amount X is within an allowable range of −Xm≦X≦Xm, and when it is judged to be within the allowable range, the main scan deviation correction is not effected, but the sheet is intactly transported.  
         [0074]    Next, when at a step  14 , the detected position of the sheet by the sheet detecting line sensor  17  assumes a minimum value (or a maximum value) and thereafter the sheet becomes undetected by the sheet detecting line sensor  17 , the skew feed amount calculating portion  601  calculates the skew feed direction and the skew feed amount N. Subsequently, at a step  15 , the correction control portion  602  judges whether the skew feed amount N is within an allowable range of −Nm≦N≦Nm, and when it is judged to be within the allowable range, processing is ended.  
         [0075]    On the other hand, when at the step  12 , the main scan deviation amount X is judged to be not within the allowable range of −Xm≦X ≦Xm, at a step  13 , the main scan deviation correction is effected by the correction control portion  602  by an amount corresponding to the main scan deviation amount X calculated by the main scan deviation amount calculating portion  603 , whereafter the sheet is transported.  
         [0076]    When at the step  15 , the skew feed amount N is judged to be not within the range −Nm≦N≦Nm, at a step  16 , the skew feed correction is effected by the correction control portion  602  by an amount corresponding to the skew feed amount N calculated by the skew feed amount calculating portion  601 , and processing is ended.  
         [0077]    As described above, according to the present embodiment, the sheet detecting line sensor as the detecting means is disposed upstream of the transporting rollers  13  and  14  with respect to the transport direction, and the pulse motors  11  and  12  are drive-controlled in conformity with the transported plate obtained on the basis of the detection information of the transported sheet to thereby effect the widthwise deviation correction, whereafter the skew feed correction is effected, whereby the sheet can be transported with the main scan deviation and skew feed thereof corrected.  
       Third Embodiment  
       [0078]    A third embodiment will now be described. While in the above-described first embodiment, description has been made of the correction control in which the main scan deviation correction and the skew feed correction are repeated and effected little by little on the basis of the deviation direction of the main scan direction, the deviation amount of the main scan direction, the skew feed direction and the skew feed amount calculated by detecting the transported sheet by the sheet detecting line sensor as the sheet detecting means downstream of the transporting rollers as the driving means for transporting the sheet, the present embodiment is designed such that the main scan deviation correction is effected on the basis of the deviation direction of the main scan direction, the deviation amount of the main scan direction, the skew feed direction and the skew feed amount calculated by detecting the transported sheet, whereafter a total skew feed amount is calculated from the aforementioned calculated skew feed amount and the skew feed amount caused by the main scan deviation correction to thereby effect the skew feed correction.  
         [0079]    In the present embodiment, the main scan deviation correction is effected on the basis of the transported state obtained by detecting the transported sheet, whereafter the total skew feed amount is calculated from the aforementioned calculated skew feed amount and the skew feed amount caused by the main scan deviation correction to thereby effect the skew feed correction.  
         [0080]    [0080]FIG. 9 illustrates the construction of a sheet transporting apparatus according to the present embodiment, and in FIG. 9, the same members as the above-described members are give the same reference numerals, and need not be described. FIG. 10 is a block diagram illustrating the control construction of the sheet transporting apparatus according to the present embodiment.  
         [0081]    In FIG. 10, the reference numeral  17  designates a sheet detecting line sensor which effects the detection of the transported state of the sheet. The reference numeral  1001  denotes a skew feed amount calculating portion which calculates the skew feed direction and skew feed amount N of the sheet on the basis of detection information from the sheet detecting line sensor  17 . The reference numeral  1003  designates a main scan deviation amount calculating portion which calculates the main scan deviation direction and main scan deviation amount X of the sheet on the basis of the detection information from the sheet detecting line sensor  17 .  
         [0082]    The reference numeral  1004  denotes a total skew feed amount calculating portion which calculates the skew feed direction and skew feed amount M of the sheet after the main scan deviation correction based on the main scan deviation direction and the main scan deviation amount X transmitted from the main scan deviation amount calculating portion  1003  has been effected, and calculates a total skew feed direction as an overall skew feed direction and a total skew feed amount L as an overall skew feed amount from the skew feed direction and the skew feed amount N transmitted from the skew feed amount calculating portion  1001  and the skew feed direction and the skew feed amount M caused by the main scan deviation correction.  
         [0083]    A correction control portion  1002  increases or decreases driving pulse numbers given to the right and left pulse motors  11  and  12 , on the basis of the total skew feed direction and the total skew feed amount L transmitted from the total skew feed amount calculating portion  1004 , and the main scan deviation direction and the main scan deviation amount X transmitted from the main scan deviation amount calculating portion  1003 , and corrects the main scan deviation amount and skew feed amount of the transported sheet.  
         [0084]    While in the present embodiment, the total skew feed amount calculating portion  1004  is of a construction discrete from the correction control portion  1002 , this is not restrictive, but for example, the total skew feed amount calculating portion  1004  may be of a construction included in the correction control portion  1002 .  
         [0085]    Each of the skew feed amount calculating portion  1001 , the correction control portion  1002 , the main scan deviation amount calculating portion  1003  and the total skew feed amount calculating portion  1004  may be comprised, for example, of a controller provided with a CPU, a ROM, a RAM, etc., and may be of a construction which controls in accordance with the procedure of a flow chart which will be described later.  
         [0086]    [0086]FIGS. 11A and 11B are typical views illustrating a method of detecting a sheet position in the sheet transporting apparatus shown in FIG. 9, and in these figures, the same members as those in FIG. 9 are given the same reference numerals.  
         [0087]    For the calculation of the main scan deviation amount X, first, when the position at which the sheet has been first detected by the sheet detecting line sensor  17  is defined as α, and the widthwise positions which the sheet should originally pass are defined as “b” and “c” (“b” is the pulse motor  11  side, and “c” is the pulse motor  12  side), X is calculated from X=c−α(or X=b−α).  
         [0088]    Next, for the calculation of the skew feed amount N, the time from after the sheet position has been first detected by the sheet detecting line sensor  17  until the detected position of the sheet by the sheet detecting line sensor  17  assumes a minimum value (or a maximum value) is counted at a constant frequency f to thereby measure a skew feed count number C. Next the detected skew feed amount Nb in the sheet detecting line sensor  17  itself is calculated from Nb=C×V/f by the use of the transporting speed V of the sheet. The skew feed amount N in the transporting roller portion actually used for the correction control portion  1002  to control the pulse motors  11  and  12  is calculated from N=Nb×b/a by the use of the detected skew feed amount Nb detected by the sheet detecting line sensor  17 , the distance “a” between the transporting rollers and the distance “b” from the sheet detecting line sensor.  
         [0089]    A method of correcting the main scan deviation amount and the skew feed amount in the sheet transporting apparatus according to the present embodiment will hereinafter be described with reference to a flow chart shown in FIG. 12.  
         [0090]    [0090]FIG. 12 is a flow chart illustrating the procedure of the method of correcting the main scan deviation amount and the skew feed amount in the sheet transporting apparatus according to the present embodiment.  
         [0091]    In FIG. 12, a main scan deviation allowable value determining the allowable range of the main scan deviation amount X is represented by Xm, and a skew feed allowable value determining the allowable range of the skew feed amount N is represented by Nm.  
         [0092]    When at a step  21 , the sheet detecting line sensor  17  detects the sheet, the main deviation amount calculating portion  1003  calculates the main scan deviation direction and the main scan deviation amount X.  
         [0093]    When at a step  22 , the detected position of the sheet by the sheet detecting line sensor  17  assumes a minimum value (or a maximum value), the skew feed amount calculating portion  1001  calculates the skew feed direction and the skew feed amount N.  
         [0094]    At a step  23 , the correction control portion  1002  judges whether the main scan deviation amount X is within an allowable range of −Xm≦X≦Xm, and when it is judged to be within the allowable range, at a step  29 , the correction control portion  1002  judges whether the skew feed amount N is within an allowable range of −Nm≦N≦Nm, and when it is judged to be within the allowable range, processing is ended.  
         [0095]    When at the step  23 , the main scan deviation amount X is judged to be not within the allowable range of −Xm≦X≦Xm, at a step  24 , main scan deviation correction is effected by the correction control portion  1002 .  
         [0096]    Next, at a step  25 , the skew feed direction and the skew feed amount M caused by the main scan deviation correction are calculated by the total skew feed amount calculating portion  1004 , and subsequently at a step  26 , the total skew feed direction and the total skew feed amount L are calculated by the total skew feed amount calculating portion  1004  with the skew feed direction and the skew feed amount N obtained from the skew feed amount calculating portion  1001  being taken into account.  
         [0097]    Next, at a step  27 , the correction control portion  1002  judges whether the total skew feed amount L is within an allowable range of −Nm≦L≦Nm, and when it is judged to be within the allowable range, processing is ended.  
         [0098]    When at the step  27 , the total skew feed amount L is judged to be not within the allowable range of −Nm≦L≦Nm, at a step  28 , skew feed correction is effected by the correction control portion  1002  by an amount corresponding to the total skew feed amount L calculated by the total skew feed amount calculating portion  1004 .  
         [0099]    When at a step  29 , the skew feed amount N is judged to be not within an allowable range of −Nm≦N≦Nm, at a step  30 , skew feed correction is effected by the correction control portion  1002  by an amount corresponding to the skew feed amount N calculated by the skew feed amount calculating portion  1001 , and processing is ended.  
         [0100]    Thus, main scan deviation correction is effected on the basis of the widthwise deviation direction, the widthwise deviation amount X, the skew feed direction and the skew feed amount N calculated on the basis of the detection of the transported sheet, whereafter the pulse motors  11  and  12  are drive-controlled in conformity with the total skew feed amount L from the above-calculated skew feed amount N and the skew feed amount M caused by the main scan deviation correction to thereby effect the correction of the main scan deviation amount and the correction of the skew feed amount at a time, whereby the sheet can be transported in a normal transported state.  
         [0101]    It is also possible to supply a storage medium having recorded therein a program code of software for realizing the operation of the sheet transporting apparatus according to each of the above-described embodiment to a system or an apparatus having a computer or the like, and read out and execute the program code stored in this storage medium by the computer (or the CPU or MPU) of the system or the apparatus. In this case, the above-described system is not restricted to one having a single apparatus, but may be one having a plurality of apparatuses.  
         [0102]    The sheet transporting apparatus according to each of the above-described embodiments can also be used as sheet transporting means in an image reading apparatus such as a scanner, or an image forming apparatus such as a printer or a facsimile.