Patent Publication Number: US-2010110506-A1

Title: Image reading apparatus, image forming apparatus and method for image reading

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority from U.S. provisional application 61/112019, filed on Nov. 6, 2008, the entire contents of each of which are incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present invention relates to an image reading apparatus, an image forming apparatus and a method for image reading, and particularly to an image reading apparatus including an ADF (Auto Document Feeder), an image forming apparatus and a method for image reading. 
     BACKGROUND 
     Some image reading apparatuses such as scanners or some image forming apparatuses such as copiers or MFPs (Multi Function Peripheral) include an ADF for continuously reading plural original documents. 
     In general, the ADF includes a guide for aligning the position and orientation of an original document and the original document is set along this guide, so that stable and reliable reading of the original document can be performed. However, when the original document is disorderly set on the ADF without using the guide, a document skew (phenomenon in which the original document is conveyed in an obliquely inclined state) can occur. In order to handle a large number of original documents in a short time, it is necessary to increase the conveyance speed of the original documents or to shorten the interval between the conveyed original documents. This results in that the document skew becomes large. 
     When the original document is skewed, an image of the read original document may be distorted, or paper clogging (jam) may occur. 
     JP-A 2001-358914 discloses a technique to correct the distortion of an image due to a skew of an original document. In this technique, the skew of the original document is determined based on the position and width of the original document detected from the output of a line sensor to read the image of the original document, and the image of the read original document is corrected based on the determined skew. 
     However, the technique disclosed in JP-A2001-358914 is only for correcting the distortion of the image due to the skew, and the paper jam due to the skew can not be prevented. Many original documents such as handwritten original documents are one and only, and if they are torn or wrinkled by the paper jam, a very serious damage may be caused. 
     SUMMARY 
     The present invention is made in view of the above, and it is an object of the invention to provide an image reading apparatus, an image forming apparatus and a method for image reading, in which when a document skew occurs in the inside of an ADF, the occurrence of the document skew is detected without adding a specific sensor, the distortion of an image is corrected, and the occurrence of a paper jam due to the skew can be prevented. 
     In order to achieve the object, according to an aspect of the invention, an image reading apparatus includes an auto document feeder to automatically feed an original document along a conveyance path, a first sensor that is provided on the conveyance path and detects a first passage time of the original document, a second sensor that is provided at a position different from a position of the first sensor in a conveyance direction of the original document and also different therefrom in a direction perpendicular to the conveyance direction and detects a second passage time of the original document, and a skew determination section that obtains a skew amount of the original document from a difference between the first passage time detected by the first sensor and the second passage time detected by the second sensor and determines occurrence of a skew of the original document based on the skew amount. 
     Besides, according to another aspect of the invention, an image forming apparatus includes an auto document feeder to automatically feed an original document along a conveyance path, a first sensor that is provided on the conveyance path and detects a first passage time of the original document, a second sensor that is provided at a position different from a position of the first sensor in a conveyance direction of the original document and also different therefrom in a direction perpendicular to the conveyance direction and detects a second passage time of the original document, a skew determination section that obtains a skew amount of the original document from a difference between the first passage time detected by the first sensor and the second passage time detected by the second sensor and determines occurrence of a skew of the original document based on the skew amount, a line sensor to read the original document fed by the auto document feeder, and an image forming section to print image data of the original document read by the line sensor onto a sheet. 
     Besides, according to another aspect of the invention, a method for image reading includes automatically feeding an original document along a conveyance path by an auto document feeder, detecting a first passage time of the original document by a first sensor provided on the conveyance path, detecting a second passage time of the original document by a second sensor provided at a position different from a position of the first sensor in a conveyance direction of the original document and also different therefrom in a direction perpendicular to the conveyance direction, obtaining a skew amount of the original document from a difference between the first passage time detected by the first sensor and the second passage time detected by the second sensor, and determining occurrence of a skew of the original document based on the skew amount. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       In the accompanying drawings: 
         FIG. 1  is a perspective view showing an outer appearance example of an image forming apparatus of an embodiment of the invention; 
         FIG. 2  is a sectional view showing a structural example of the image forming apparatus of the embodiment of the invention; 
         FIG. 3  is a sectional view showing a structural example of an image reading apparatus of the embodiment of the invention; 
         FIG. 4  is a plan view showing a structural example and an arrangement example of rollers and sensors provided on a conveyance path of an original document; 
         FIGS. 5A to 5D  are first views (clockwise skew) for explaining the operation principle of skew amount detection of the embodiment; 
         FIGS. 6A to 6D  are second views (counterclockwise skew) for explaining the operation principle of skew amount detection of the embodiment; 
         FIG. 7  is a flowchart showing an example of a process required to determine the occurrence of a skew; 
         FIG. 8  is a view showing an arrangement example of sensors which can be applied to the image forming apparatus of the embodiment and an arrangement example which can not be applied thereto; 
         FIG. 9  is a function block diagram showing functions mainly relating to skew detection and skew correction in the functions of the image forming apparatus of the embodiment; 
         FIGS. 10A and 10B  are views for explaining the concept of skew correction; and 
         FIG. 11  is a flowchart showing an example of a detailed process relating to skew determination, conveyance speed determination, display, conveyance control and skew correction. 
     
    
    
     DETAILED DESCRIPTION 
     An image reading apparatus, an image forming apparatus, and a method for image reading according to an embodiment of the invention will be described with reference to the accompanying drawings. 
     (1) Structure of the Image Forming Apparatus and the Image Reading Apparatus 
       FIG. 1  is a view showing an outer appearance example of a copier (or MFP: Multi-Function Peripheral) as a typical example of an image forming apparatus  1  of the embodiment. 
     The image forming apparatus  1  includes an image reading apparatus  2 , an image forming section  30 , a paper feed section  40 , an operation section  50  and the like. 
     The image reading apparatus  2  optically reads an original document placed on a document table or an original document inputted to an ADF (Auto Document Feeder)  5  and generates image data. 
     The image forming section  30  prints the image data by using an electrophotographic system onto a sheet fed from the paper feed section  40 . 
     The operation section  50  includes a display section as a user interface and various operation buttons.  FIG. 2  is a schematic sectional view showing an inner structural example of the image forming section  30 . 
     The image forming section  30  includes a photoconductive drum  31  rotating in an illustrated arrow direction in the vicinity of a center part. A charging device  32 , an exposure device  33 , a developing device  34 , a transfer device  35 , a charge removing device  36 , and a cleaner  37  are sequentially disposed around the photoconductive drum  31  from an upstream side to a downstream side in the rotation direction. 
     The charging device  32  uniformly charges the surface of the photoconductive drum  31  to a specified potential. The exposure device  33  irradiates the surface of the photoconductive drum  31  with a laser light modulated according to the magnitude of image data. When the laser light is irradiated, the potential at that portion is reduced, and an electrostatic latent image is formed on the surface of the photoconductive drum  31 . 
     The developing device  34  causes a developer to adhere to the surface of the photoconductive drum  31 , and develops the electrostatic latent image. The electrostatic latent image is developed with toner in the developer, and a toner image is formed on the surface of the photoconductive drum  31 . 
     Meanwhile, a sheet is conveyed to a transfer position (the position where the photoconductive drum  31  and the transfer device  35  face each other) from a paper feed tray  7  of the paper feed section  40 , and the toner image on the photoconductive drum  31  is transferred to the sheet by the transfer device  35 . 
     The sheet on which the toner image is transferred is conveyed to a fixing device  38  disposed downstream of the transfer device  35 , and is heated and pressed here, and the toner image is fixed to the sheet. The sheet subjected to the fixing process is discharged to the outside by a paper discharge device  39 . 
     After the transfer to the sheet is completed, the surface charge of the photoconductive drum  31  is removed by the charge removing device  36 , and the toner remaining on the surface is removed by the cleaner  37 . 
     By repeating the above process, continuous printing can be performed. 
       FIG. 3  is a sectional view showing a structural example of the image reading apparatus  2  of the embodiment. A document read table  3  is disposed at an upper part of a main body housing  25  of the image reading apparatus  2 , and a contact glass is disposed at an upper part of the document read table  3 . An openable and closable document press plate  4  is provided at an upper part of the document read table  3 , and the document press plate  4  presses an original document set on the document read table  3  so that the document is brought into close contact with the contact glass of the document read table  3 . 
     The auto document feeder (ADF)  5  is disposed at an upper part of the document press plate  4 . The auto document feeder  5  includes a document tray  6 . The original document  301  placed on the original document tray  6  is picked up by a pickup roller  7 , and is separated one by one by a paper feed roller  8 . The document is conveyed by a conveyance roller  9 , an intermediate conveyance roller  10  and a pre-read roller  11  to a position (read position) where it contacts with the document read table  3 . Thereafter, the document is conveyed by a post-read roller  12  and a paper discharge roller  13  onto a storage tray  15 . 
     A white reference roller  16  for shading correction is disposed at the read position between the auto document feeder  5  and the document read table  3 . The auto document feeder  5  has an integral structure with the document press plate  4 , and when the document press plate  4  is opened or closed, the auto document feeder  5  is also opened or closed. 
     An exposure scanning optical unit  22  including a first carriage  17 , a second carriage  18 , a lens  19 , a CCD (Charge Coupled Device) sensor  20 , and a read control section  21  is disposed in the inside of the main body housing  25 . The first carriage  17  includes a light source  17   a  and a mirror  17   b,  and the second carriage  18  includes a mirror  18   a  and a mirror  18   b.    
     When the original document  301  placed on the document read stand  3  is read, the first carriage  17  and the second carriage  18  are moved in the horizontal direction (sub-scanning direction). The light source  17   a  of the first carriage  17 , for example, a fluorescent lamp irradiates a light to the original document  301  while moving. The reflected light from the original document  301  is sequentially reflected by the mirror  17   b  of the first carriage  17 , and the mirror  18   a  and the mirror  18   b  of the second carriage  18 , and is incident on the lens  19 . The lens  19  condenses the incident light on the CCD sensor  20 . In the CCD sensor  20 , CCD elements as photoelectric conversion elements are arranged one-dimensionally, and the CCD sensor photoelectric-converts the incident light incident from the lens  19  and outputs analog image data (image signal) to the read control section  21 . The exposure scanning optical unit  22  irradiates a light to the white reference roller  16  before scanning in the sub-scanning direction is performed, the reflected light from the white reference roller  16  is made incident on the CCD sensor  20  similarly to the above, and white reference data is obtained. 
     When the original document is automatically read, that is, when the original document  301  is set on the document tray of the auto document feeder  5 , first, the light source  17   a  is turned on, and reading of the white reference roller  16  is performed to obtain the white reference data. Meanwhile, the set original document  301  is conveyed at a constant speed, and is conveyed to the read position (position of the white reference roller  16 ). At this time, the first carriage  17  and the second carriage  18  are in a stop state, and the light source  17   a  irradiates a light to the original document  301  passing through the read position. The reflected light from the original document  301  is incident on the CCD sensor  20  through the mirror  17   b , the mirror  18   a , the mirror  18   b  and the lens  19 , and the image data of the image of the original document  301  is generated. 
     (2) Detection of Skew of the Original Document 
     As stated above, the original document  301  set on the ADF  5  is conveyed to the read position by the five rollers, that is, the pickup roller  7 , the paper feed roller  8 , the conveyance roller  9 , the intermediate conveyance roller  10  and the pre-read roller  11 . Besides, four paper detection sensors, that is, a pre-feed sensor  101 , a paper feed sensor  102 , a conveyance sensor  103  and an intermediate conveyance sensor  104  are provided on the conveyance path of the original document. These paper detection sensors are, for example, photo sensors, and are put in an ON state when the leading edge of the original document  301  passes, and are put in an OFF state when the trailing edge of the original document  301  passes. By detecting a time when each of the paper detection sensors is changed from the OFF state to the ON state, a passage time of the original document  301  is obtained together with the paper detection sensor which detects the passage time. The respective paper detection sensors are originally provided to detect the occurrence of a paper jam of the original document  301 . 
     In the image forming apparatus  1  and the image reading apparatus  2  of the embodiment, the arrangement of the paper detection sensors is contrived such that the occurrence of skew of the original document  301  is determined based on the passage time information of the original document  301  detected by the respective paper detection sensors, in addition to the detection of the occurrence of a paper jam. Further, when the skew occurs, not only the skew occurrence but also a skew amount (skew angle) is detected. 
       FIG. 4  is a view illustrating an arrangement of the pre-feed sensor  101 , the paper feed sensor  102 , the conveyance sensor  103  and the intermediate conveyance sensor  104 , which are used for skew detection, is shown and which is two-dimensionally expanded in a sheet conveyance direction and a direction perpendicular thereto.  FIG. 4  shows also the arrangement of the pickup roller  7 , the paper feed roller  8 , the conveyance roller  9 , the intermediate conveyance roller  10 , and the pre-read roller  11 . 
     As is understood from  FIG. 4 , the paper feed sensor  102  (second sensor) is arranged at a position different from a position of the pre-feed sensor  101  (first sensor) in the sheet conveyance direction and also different therefrom in the direction perpendicular to the conveyance direction. That is, the paper feed sensor  102  and the pre-feed sensor  101  are arranged obliquely to the sheet conveyance direction. 
     In contrast, the conveyance sensor  103  (third sensor) is arranged at a position which is different from the position of the paper feed sensor  102  (second sensor) in the sheet conveyance direction but is equal thereto in the direction perpendicular to the conveyance direction. That is, the conveyance sensor  103  and the paper feed sensor  102  are arranged in parallel to the sheet conveyance direction. 
     Meanwhile, the relative positional relation between the intermediate conveyance sensor  104  and the conveyance sensor  103  is the same as the positional relation between the paper feed sensor  102  (second sensor) and the pre-feed sensor  101  (first sensor). The intermediate conveyance sensor  104  is arranged at a position different from the position of the conveyance sensor  103  in the sheet conveyance direction and also different therefrom in the direction perpendicular to the conveyance direction. A skew detection method of the original document  301  using the sensors arranged as stated above will be described below. 
       FIGS. 5A to 5D  and  FIGS. 6A to 6D  are views for explaining the principle of the skew detection method of the embodiment. Among these,  FIGS. 5A to 5D  show a state where a clockwise skew occurs, and  FIGS. 6A to 6D  show a state where a counterclockwise skew occurs. 
       FIG. 5A  shows a state where the leading edge of the original document  301  having a clockwise skew passes through the position of the pre-feed sensor  101 .  FIG. 5B  shows a state where the sheet is further conveyed and the leading edge of the original document  301  passes through the position of the paper feed sensor  102 .  FIG. 5C  shows a state where the leading edge of the original document  301  passes through the position of the conveyance sensor  103 .  FIG. 5D  shows a state where the original document  301  is further conveyed, and the leading edge thereof passes through the position of the intermediate conveyance sensor  104 . 
     As stated above, since the pre-feed sensor  101  and the paper feed sensor  102  are arranged obliquely to the conveyance direction of the original document  301 , a period required for the document to pass the two sensors is different between an original document  301 a (original document indicated by a broken line in  FIG. 5A ) in a normal state where a skew does not occur and the skewed original document  301 . As shown in  FIG. 5A , a period T required for the leading edge of the original document  301  to pass the pre-feed sensor  101  and the paper feed sensor  102  when the original document  301  is skewed clockwise is shorter than a passage period T 0  when the document is not skewed (T&lt;T 0 ). 
     On the other hand, since the paper feed sensor  102  and the conveyance sensor  103  are arranged in parallel to the conveyance direction of the original document  301 , a period required for the sheet to pass the two sensors is the same between a normal state where the sheet is not skewed and a state where the sheet is skewed. 
     The relative positional relation between the conveyance sensor  103  and the intermediate conveyance sensor  104  is the same as the positional relation between the pre-feed sensor  101  and the paper feed sensor  102  as stated above. Accordingly, a period T required for the original document  301  skewed in the clockwise direction to pass the two sensors is shorter than a passage period T 0  when the document is not skewed (T&lt;T 0 ). 
     Also when the original document  301  skews in the counterclockwise direction, as shown in  FIGS. 6A to 6D , a period T required for the leading edge of the original document  301  to pass the pre-feed sensor  101  and the paper feed sensor  102  is different from the passage period T 0  in the normal state where the sheet is not skewed. However, in this case, the passage period T when the document is skewed is longer than the passage period T 0  when the document is not skewed (T&gt;T 0 ). The same applies to a period for the original document  301  to pass the conveyance sensor  103  and the intermediate conveyance sensor  104 . 
     A sheet skew amount s can be expressed by a skew angle θ (see  FIG. 5A ,  FIG. 6A ). When a distance between the pre-feed sensor  101  and the paper feed sensor  102  (or between the conveyance sensor  103  and the intermediate conveyance sensor  104 ) in the direction perpendicular to the conveyance direction is denoted by L, and a sheet conveyance speed is denoted by V 0 , the skew angle θ is expressed by a following equation. 
       tan(θ)=( T 0− T )·( V 0 /L )   (equation 1) 
     Where, the normal passage period T 0  when there is no skew, the sheet conveyance speed V 0 , and the distance L are previously given values. 
     The passage period T in equation 1 is the period (T=t 2 −t 1 ) calculated from a difference between a time (first passage time t 1 ) when the leading edge of the original document  301  passes the pre-feed sensor  101  (first sensor) and a time (second passage time t 2 ) when the leading edge passes the paper feed sensor  102  (second sensor). The skew angle θ can be calculated from this period (and T 0 , V 0  and L described above) based on equation 1. When the skew angle θ is positive, the skew is clockwise, and when the angle is negative, the skew is counterclockwise. 
     The skew angle θ can also be calculated from a period calculated from a difference between a time when the leading edge of the original document  301  passes the conveyance sensor  103  and a time when the leading edge passes the intermediate conveyance sensor  104 , and T 0 , V 0  and L described above. 
     The conveyance speed V 0  of the sheet used in equation 1 is a normal conveyance speed when a conveyance function failure due to abrasion or the like does not occur in the respective rollers. Accordingly, when abrasion or the like occurs in a roller and the actual conveyance speed V is remarkably deviated from the normal conveyance speed V 0 , there is a fear that an error occurs when the skew angle is obtained by using equation 1. 
     Then, in this embodiment, whether the actual conveyance speed V is remarkably deviated from the normal conveyance speed V 0  is determined from the time (second passage time t 2 ) when the leading edge of the original document  301  passes the paper feed sensor  102  and a passage time (third passage time t 3 ) when the leading edge passes the conveyance sensor  103 , the sensors  102  and  103  being arranged to be parallel to the conveyance direction. 
       FIG. 7  is a flowchart showing an example of the determination process. At ACT  1 , a conveyance time of the original document  301  between the two sensors  102  and  103  is obtained from a difference between the time t 2  when the leading edge of the original document  301  passes the paper feed sensor  102  and a passage time t 3  when the leading edge passes the conveyance sensor  103 , and comparison check with a normal conveyance time is performed. Instead of the conveyance time, a conveyance speed may be used to perform the determination. It is determined to be abnormal when the difference of the conveyance time (or the conveyance speed) is not within a specified reference range. In this case, it is estimated that some trouble occurs in the paper conveyance function of the roller due to roller abrasion or the like. 
     When there is no abnormality in the conveyance time (or the conveyance speed) of the original document  301  between the paper feed sensor  102  and the conveyance sensor  103 , an advance is made to a skew determination process at ACT  2 . At ACT  2 , a conveyance time between the pre-feed sensor  101  and the paper feed sensor  102  is checked. When this conveyance time is within a specified range, it is determined that a skew does not occur (normal), and when the conveyance time is outside the specified range, it is determined that a skew occurs (abnormal). At ACT  2 , the skew angle θ is obtained based on equation  1 , and normality or abnormality may be determined based on the skew angle θ. 
     In the above description, although each of the sensors detects the passage time of the leading edge of the original document  301 , the trailing edge of the sheet may be detected instead of this. Besides, the skew determination at ACT  2  may be performed using a passage time between the conveyance sensor  103  and the intermediate conveyance sensor  104 . 
     The arrangement of the respective sensors such as the pre-feed sensor  101 , the paper feed sensor  102  and the conveyance sensor  103  is not limited to the arrangement of  FIG. 4  (or  FIG. 5A  to  FIG. 5D ,  FIG. 6A  to  FIG. 6D ). 
       FIG. 8  is a view showing arrangement examples (pattern  1 , pattern  2 , pattern  3 ) which can be applied to the image forming apparatus  1  of the embodiment, and an arrangement example (pattern  4 ) which can not be applied thereto. In order to detect the skew angle, two sensors are required, and these are designated as a first sensor S 1  and a second sensor S 2 . Besides, in order to separate the detection of the change of the sheet conveyance speed due to the roller abrasion or the like from the detection of the skew, one more sensor (this is designated as a third sensor S 3 ) is required. 
     The first sensor S 1  and the second sensor S 2  for detecting the skew angle are required to be arranged at positions different from each other in the conveyance direction of the original document  301  and the direction perpendicular to this. In the arrangement example of  FIG. 4  (or  FIG. 5A  to  FIG. 5D ,  FIG. 6A  to  FIG. 6D ), the first sensor S 1  and the second sensor S 2  correspond to the pre-feed sensor  101  and the paper feed sensor  102  (or the conveyance sensor  103  and the intermediate conveyance sensor  104 ). 
     On the other hand, the third sensor S 3  can be freely arranged as illustrated in pattern  1 , pattern  2  and pattern  3  unless the third sensor is arranged on a straight line connecting the first sensor S 1  and the second sensor S 2  as in pattern  4 . In the arrangement example of  FIG. 4  (or  FIGS. 5A to 5D ,  FIGS. 6A to 6D ), the third sensor S 3  corresponds to the conveyance sensor  103 . 
     (3) Functions Relating to Skew Detection and Detailed Operation 
       FIG. 9  is a block diagram particularly showing functions relating to skew detection and skew correction in the functions of the image reading apparatus  2  of the embodiment. 
     In order to realize these functions, in addition to the pre-feed sensor  101 , the paper feed sensor  102 , the conveyance sensor  103 , and the intermediate conveyance sensor  104  described before, the image reading apparatus  2  includes a skew determination section  52  (a skew determination section  52   a  to determine a skew at the time of paper feeding, and a skew determination section  52   b  to determine a skew immediately before reading), a conveyance speed determination section  53 , a skew correction section  54 , a display section  55  and a conveyance control section  56 . 
     In the function blocks, the skew determination section (skew at the time of paper feeding)  52   a  obtains a skew amount s 1  from the difference between the first passage time t 1  detected by the pre-feed sensor  101  (first sensor) and the second passage time t 2  detected by the paper feed sensor  102  (second sensor), and determines the occurrence of a skew at the time of paper feeding of the original document  301  based on the skew amount s 1 . Specifically, for example, the skew amount s 1  (skew angle θ) is obtained by the foregoing equation 1, the absolute value of the obtained skew amount s 1  is compared with a threshold Smax 1 , and the occurrence of the skew is determined. The skew determination section  52   a  detects the skew which occurs at a relatively upstream side in the conveyance path of the original document  301  extending from the original document tray  6  to the read position. When it is determined that the skew occurs, an instruction is given so that the display section  55  displays to that effect, and in order to previously prevent a paper jam due to the skew from occurring, an instruction is given so that the conveyance control section  56  stops the conveyance of the original document  301 . 
     The conveyance speed determination section  53  obtains the conveyance speed V of the original document  301  based on the third passage time t 3  detected by the conveyance sensor  103  (third sensor) and the passage time t 2  detected by the paper feed sensor  102 . When the distance between the conveyance sensor  103  and the paper feed sensor  102  is denoted by D 2-3 , the conveyance speed V can be obtained from, for example, V=D 2-3 /(t 3 −t 2 ). 
     When the obtained conveyance speed V is within a specified range, the skew determination section  52   a  (and the skew determination section  52   b ) performs the skew determination. On the other hand, when the conveyance speed V is outside the specified range, it is determined that some trouble occurs in the conveyance function itself of the original document  301  because of roller abrasion or the like. In this case, an instruction is given so that the display section  55  displays an alarm, or the conveyance control section  56  stops the conveyance of the original document  301 . 
     The skew determination section (skew immediately before reading)  52   b  determines the presence or absence of the occurrence of a skew at a position just before the read position of the original document  301 . For the skew determination section  52   b,  the conveyance sensor  103  functions as the first sensor, and the passage time of the original document  301  detected by the conveyance sensor  103  becomes the first passage time t 1 . Besides, the intermediate conveyance sensor  104  functions as the second sensor, and the passage time of the original document  301  detected by the intermediate conveyance sensor  104  becomes the second passage time t 2 . A skew amount s 2  (skew angle θ) of the original document  301  immediately before reading is obtained from the difference between the passage times detected by the two sensors, and the absolute value of the skew amount s 2  is compared with a threshold Smax 2  to determine the occurrence of the skew. When it is determined that the skew occurs, similarly to the skew determination section  52   a,  an instruction is given so that the display section  55  displays to that effect, and in order to previously prevent a paper jam due to the skew from occurring, an instruction is given so that the conveyance control section  56  stops the conveyance of the original document  301 . 
     When the skew determination sections  52   a  and  52   b  determine that the skew occurs, the skew correction section  54  performs a process to correct the inclination of image data read by the CCD sensor  20 . 
     As exemplified in  FIG. 10A , the image data read in the state where the skew occurs in the original document  301  is inclined according to the skew angle θ. Then, the skew correction section  54  corrects the inclination of the image data based on the skew angle θ obtained by the skew determination section (skew immediately before reading)  52   b  and can return it in an erected state. The skew determination section (skew at the time of paper feeding)  52   a  also obtains the skew angle θ, and the correction can be performed using this skew angle. However, when the correction is performed using the skew angle obtained just before the reading of the original document  301  by the CCD sensor  20 , the correction with higher accuracy can be performed. 
       FIG. 9  is a view showing a structural example of the image reading apparatus  2  such as a scanner, and the respective function structures of the skew determination section  52 , the conveyance speed determination section  53 , the skew correction section  54 , the display section  55 , and the conveyance control section  56  are provided in, for example, the read control section  21  of the image reading apparatus  2 . On the other hand, in the image forming apparatus  1  such as a copier or an MFP, the image forming section  30  to print read image data to a sheet is provided at a subsequent stage of the image reading apparatus  2 , and the image forming section  30  also includes a control section for performing various controls and a processing section. Then, the respective function structures of the skew determination section  52 , the conveyance speed determination section  53 , the skew correction section  54 , the display section  55 , and the conveyance control section  56  shown in  FIG. 9  may be provided in the control section and the like of the image forming section  30 . 
       FIG. 11  is a flowchart mainly showing a process example which is performed by the skew determination section  52 , the conveyance speed determination section  53 , the skew correction section  54 , the display section  55 , and the conveyance control section  56  in the function blocks of  FIG. 9 . 
     At ACT  100 , waiting is made for turning on of the pre-feed sensor  101 , and when it is turned on, the ON time t 1  is stored in an appropriate memory. 
     At ACT  101 , waiting is made for turning on of the paper feed sensor  102 , and when it is turned on, the ON time t 2  is stored in an appropriate memory. 
     At ACT  102 , waiting is made for turning on of the conveyance sensor  103 , and when it is turned on, the ON time t 3  is stored in an appropriate memory. 
     At ACT  103 , a period t R  required for the original document  301  to be conveyed from the paper feed sensor  102  to the conveyance sensor  103  is calculated from the difference between the ON time t 3  detected by the conveyance sensor  103  and the ON time t 2  detected by the paper feed sensor  102  (t R =t 3 −t 2 ). 
     At ACT  104 , it is determined whether the conveyance period t R  obtained at ACT  103  is within a specified range (range of from the minimum value T R     —     min  to the maximum value T R     —     max ) (T R     —     min &lt;t R &lt;T R   max ). 
     At ACT  104 , when it is determined that the conveyance period t R  is not within the specified range, since it is estimated that the conveyance speed V 0  of the original document  301  is not normal because of a factor such as abrasion occurring in the roller, an error message of a conveyance function failure or the like is displayed. Besides, an instruction may be given so that the conveyance control section  56  to control the conveyance of the original document  301  stops the conveyance (ACT  105 ). Besides, in this case, even if the skew amount determination based on the conveyance speed V 0  is performed, a correct skew amount can not be obtained. Thus, the skew amount determination and skew correction process (process after ACT  106 ) is not performed. 
     On the other hand, when it is determined that the conveyance speed of the original document  301  is normal (YES at ACT  104 ), the determination of the skew amount s 1  (more specifically, the skew angle θ) at the time of paper feeding is performed. 
     At ACT  106 , the skew amount s 1  is calculated from the previously detected ON time t 2  of the paper feed sensor  102  and the ON time t 1  of the pre-feed sensor  101 . Specifically, the difference (T=t 2 −t 1 ) (passage period) between the ON time t 2  of the paper feed sensor  102  and the ON time t 1  of the pre-feed sensor  101  is obtained, and the obtained T is substituted into equation 1 to obtain the skew amount s 1  at the time of paper feeding. 
     For the conveyance speed V 0  in equation 1, a standard conveyance speed at the normal time may be used, or an actual conveyance speed at the present time (only when it is determined at ACT  104  that the conveyance speed is normal) based on the conveyance period t R  obtained at ACT  103  may be used. 
     At ACT  107 , it is determined whether the absolute value of the skew amount s 1  at the time of paper feeding calculated at ACT  106  does not exceed the specified limit value Smax 1 . When the skew amount s 1  exceeds the specified limit value Smax 1 , a message indicating that a skew occurs is displayed, and in order to previously prevent a paper jam due to the skew from occurring, the conveyance of the original document  301  is stopped (ACT  108 ). 
     On the other hand, when the absolute value of the skew amount s 1  at the time of paper feeding does not exceed the limit value Smax 1  (NO at ACT  107 ), waiting is made for turning on of the intermediate conveyance sensor  104 , and when it is turned on, the ON time t 4  is stored in an appropriate memory (ACT  109 ). 
     At ACT  110 , a period T required for the original document  301  to be conveyed from the conveyance sensor  103  to the intermediate conveyance sensor  104  is obtained from a difference between the ON time t 4  detected by the intermediate conveyance sensor  104  and the ON time t 3  detected by the conveyance sensor  103 , and the obtained T is substituted into equation 1 to calculate the skew amount s 2  immediately before reading. 
     It is determined whether the absolute value of the calculated skew amount s 2  immediately before reading does not exceed the specified limit value Smax 2  (ACT  111 ). When the skew amount s 2  exceeds the specified limit value Smax 2 , a message indicating that a skew occurs is displayed, and in order to previously prevent a paper jam due to the skew from occurring, the conveyance of the original document  301  is stopped (ACT  112 ). 
     On the other hand, when the skew amount s 2  does not exceed the specified limit value Smax 2 , although the conveyance of the original document  301  is continued, there is a possibility that the image data is inclined by the skew amount s 2  not larger than the limit value Smax 2 . Then, the skew correction process to return the inclination of the image data to the erected state is performed by using the skew amount s 2  immediately before reading, which is calculated at ACT  111  (ACT  112 ). 
     At ACT  114 , the presence or absence of a next original document is determined, and when there is a next original document, a return is made to ACT  100 , and the foregoing process is repeated. 
     As stated above, according to the image reading apparatus  2  and the image forming apparatus  1  of the embodiment, by using the paper detection sensors originally arranged for the purpose of detecting a paper jam, the skew amount of the original document  301  can be detected without newly increasing a sensor. Besides, in the sensor arrangement including at least two sensor arrangement sections different from each other in an angle formed between a straight line connecting two sensors of the arranged paper detection sensors and a straight line parallel to the conveyance direction, the conveyance time of each sensor section and the conveyance time of the section in the normal state are compared with each other, and comparison results of two or more sensor sections are combined to make a determination. As a result, the skew amount can be accurately calculated in view of the change of the sheet conveyance time due to the roller abrasion or the like. The skew correction of the read image data can be accurately performed based on the correctly calculated skew amount. Besides, the allowable value of the skew amount is previously determined, and the calculated skew amount is compared with the allowable value. When the skew amount exceeds the allowable value, the conveyance of the original document  301  can be stopped. As a result, the occurrence of paper wrinkle due to the excessive skew, and the occurrence of a paper jam can be prevented in advance, and the damage of the original document  301  can be avoided. 
     Besides, when the skew at the time of paper feeding is detected, the display section displays to that effect and can urge the user to take suitable measures for addressing the skew. For example, the display section urges the user to check the placement state of the original document on the original document tray, or urges the user to contact a maintenance serviceman. 
     The invention is not limited to the above embodiments, but the components can be modified and embodied at the practical stage within the scope not departing from the gist. Besides, the invention of various embodiments can be formed by suitable combination of plural components disclosed in the embodiments. For example, some components may be deleted from all components disclosed in the embodiment. Further, components in different embodiments may be suitably combined.