Patent Publication Number: US-7900913-B2

Title: Image forming apparatus and method for controlling the same

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority from U.S. provisional application 61/037,558, filed on Mar. 18, 2008, the entire contents of each of which are incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present invention relates to an image forming apparatus and a method for controlling the same, and, more particularly to an image forming apparatus and a method for controlling the same that can prevent, when skew (oblique shift) occurs in a sheet conveyed from a paper feeding unit, occurrence of paper jam by detecting the skew and estimating a cause of the skew. 
     BACKGROUND 
     In an image forming apparatus employing an electrophotographic system such as a copying machine, a printer, or a multi-function peripheral (MFP), a toner image formed on a photoconductive drum is transferred onto a sheet to perform printing. The transfer is performed in a transfer region near the outer circumference of the photoconductive drum. The sheet before the printing is stored in a paper feeding tray or the like. During the printing, the paper is picked up from the paper feeding tray and led to the transfer region through a conveying path configured by several rollers. 
     While the sheet is conveyed from the paper feeding tray to the transfer region, a paper jam may occur. 
     In the past, when a paper jam occurs, the paper jam is detected by using plural paper detection sensors arranged on a sheet conveying path. The respective paper detection sensors detect passing timing of the leading end or the trailing end of a sheet conveyed along the sheet conveying path. When the passing timing exceeds an allowable limit value set in advance, the paper detection sensors determine that a paper jam occurs. 
     However, a cause of a paper jam that occurs during sheet conveyance may be present in a place other than a place where the occurrence of the paper jam is actually detected. For example, because a sheet fed by a paper feeding unit is fed in a skewed state, the sheet may be wrinkled during conveyance or, in the worst case, a paper jam may occur during the conveyance. In such a case, a real cause of the occurrence of the paper jam is not present in a place where the paper jam occurs but is present in a place where the skew of the sheet occurs. However, the image forming apparatus in the past does not have means for notifying a user of the occurrence of the paper jam and the occurrence of the skew in association with each other. 
     JP-A-2006-16089 discloses a technique for determining detection timing obtained from plural sheet detection sensors set in different positions in a conveying direction, storing the detection timing as a conveyance history when the detection timing exceeds a predetermined value set in advance, and narrowing down, using the conveyance history, places as causes of occurrence of a paper jam including positions other than a position where the paper jam actually occurs. However, JP-A-2006-16089 does not disclose a technique for detecting occurrence of sheet skew. 
     On the other hand, for example, JP-A-07-276725 discloses a technique for detecting occurrence itself of sheet skew. However, JP-A-07-276725 does not disclose a technique for associating the occurrence of the sheet skew with occurrence of a paper jam. Further, in a skew detecting method disclosed by JP-A-07-276725, it is necessary to add a separate sensor for skew detection. This leads to an increase in cost of an apparatus. 
     SUMMARY 
     The present invention is devised in view of the circumstances and it is an object of the present invention to provide an image forming apparatus and a method for controlling the same that can detect occurrence of sheet skew without, for example, addition of a special sensor, estimate a cause of the occurrence of the paper jam in association with the sheet skew, and prevent occurrence of a paper jam in advance. 
     In order to attain the object, according to an aspect of the present invention, there is provided an image forming apparatus including: a first sensor that is provided in a conveying path for a sheet and detects a first passage time of the sheet; a second sensor that is provided in a position different from a position of the first sensor in both a conveying direction of the sheet and a direction orthogonal to the conveying direction and detects second passage time of the sheet; a skew determining unit that calculates a skew amount of the sheet 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 skew of the sheet on the basis of the skew amount; and a cause estimating unit that estimates, when the skew determining unit determines that the skew occurs, a cause of the occurrence of the skew. 
     According to another aspect of the present invention, there is provided a method for controlling an image forming apparatus including: detecting a first passage time of a sheet with a first sensor provided in a conveying path for the sheet; detecting a second passage time of the sheet with a second sensor provided in a position different from a position of the first sensor in both a conveying direction of the sheet and a direction orthogonal to the conveying direction; determining occurrence of skew of the sheet from a difference between the first passage time detected by the first sensor and the second passage time detected by the second sensor; and estimating, when it is determined that the skew occurs, a cause of the occurrence of the skew. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       In the accompanying drawings: 
         FIG. 1  is a perspective view of an external appearance example of an image forming apparatus according to an embodiment of the present invention; 
         FIG. 2  is a sectional view of a configuration example of the image forming apparatus according to the embodiment; 
         FIG. 3  is a side view of a configuration example and an arrangement example of rollers and sensors provided in a conveying path for a sheet; 
         FIG. 4  is a plan view of a configuration example and an arrangement example of the rollers and the sensors provided in the conveying path for a sheet; 
         FIG. 5  is a first diagram (counterclockwise skew) for explaining an operation principle of skew amount detection according to the embodiment; 
         FIG. 6  is a second diagram (clockwise skew) for explaining the operation principle of the skew amount detection according to the embodiment; 
         FIG. 7  is a flowchart of an example of processing necessary for determining occurrence of skew; 
         FIG. 8  is a diagram of sensor arrangement examples that can be applied to the image forming apparatus according to the embodiment and a sensor arrangement example that cannot be applied to the image forming apparatus; 
         FIG. 9  is a functional block diagram of, in particular, functions related to skew detection, paper jam detection, and estimation of causes of the skew and the paper jam among functions of the image forming apparatus according to the embodiment; 
         FIG. 10  is a flowchart of a processing example related to tray opening and closing detection; 
         FIG. 11  is a flowchart of a processing example mainly related to skew determination, conveying speed determination, cause determination, display, and conveyance control; and 
         FIGS. 12A to 12F  are diagrams of message display examples on a display unit. 
     
    
    
     DETAILED DESCRIPTION 
     An image forming apparatus and a method for controlling the same according to an embodiment of the present invention are explained below with reference to the accompanying drawings. 
     (1) Configuration of the Image Forming Apparatus 
       FIG. 1  is a diagram of an external appearance example of a copying machine (or a multi-function peripheral (MFP)) as a typical example of an image forming apparatus  1  according to this embodiment. 
     The image forming apparatus  1  includes a scanning unit  2 , an image forming unit  3 , a paper feeding unit  4 , and an operation unit  5 . 
     The scanning unit  2  optically scans an original document placed on a document table or an original document input to an ADF (Auto Document Feeder)  6  and generates image data. 
     The image forming unit  3  prints the image data on a sheet fed from the paper feeding unit  4  using an electrophotographic system. 
     A display unit  56  and various operation buttons are provided as a user interface in the operation unit  5 . 
     The paper feeding unit  4  includes plural (in the example in  FIG. 1 , two) paper feeding trays  7  that store sheets. In the following explanation, when the two paper feeding trays  7  are distinguished, the paper feeding trays  7  are referred to as paper feeding tray ( 1 )  7  and paper feeding tray ( 2 )  7 , respectively. Any one of the paper feeding trays  7  is selected according to a designated sheet size or the like. When the sheets are exhausted, a user opens the paper feeding tray(s)  7  to set sheets and closes the paper feeding tray(s)  7 . 
     The paper feeding unit  4  includes a manual feed tray  8  that opens to the outside of the apparatus. In performing manual feed printing, the user opens the manual feed tray  8  and places sheets on the opened manual feed tray  8 . 
       FIG. 2  is a schematic sectional view of an internal configuration example of the image forming unit  3 . 
     The image forming unit  3  includes, near the center thereof, a photoconductive drum  10  that rotates in an arrow direction shown in the figure. A charging device  11 , an exposing device  12 , a developing device  13 , a transfer device  14 , a charge removing device  15 , and a cleaner  16  are disposed around the photoconductive drum  10  in order from upstream to downstream of the rotation. 
     The charging device  11  uniformly charges the surface of the photoconductive drum  10  to a predetermined potential. The exposing device  12  irradiates a laser beam modulated according to the intensity of image data on the surface of the photoconductive drum  10 . When the laser beam is irradiated, the potential in a part on which the laser beam is irradiated falls. An electrostatic latent image is formed on the surface of the photoconductive drum  10 . 
     The developing device  13  deposits a developer on the surface of the photoconductive drum  10  to develop the electrostatic latent image. The developing device  13  develops the electrostatic latent image with a toner in the developer and forms a toner image on the surface of the photoconductive drum  10 . 
     Meanwhile, a sheet is conveyed from the paper feeding tray  7  of the paper feeding unit  4  to a transfer region  21  (a position where the photoconductive drum  10  and the transfer device  14  are opposed to each other). The transfer device  14  transfers the toner image on the photoconductive drum  10  onto the sheet. 
     Registration rollers  34  are provided immediately before the transfer region  21 . Skew of the sheet that occurs in a conveyance path (a conveying path  20 ) leading from the paper feeding unit  4  to the registration rollers  34  is corrected by the registration rollers  34 . 
     However, when a skew angle is too large, the registration rollers  34  cannot completely correct the skew angle. A paper jam may occur because of occurrence of skew. Therefore, the image forming apparatus  1  according to this embodiment performs processing for, for example, detecting occurrence of skew and, when a skew angle is equal to or larger than a predetermined reference value, displaying warning to a user or stopping conveyance of a sheet. Occurrence of a paper jam can be prevented by this processing. Even when a paper jam occurs, the image forming apparatus  1  performs processing for displaying, as an estimated cause of the occurrence of the paper jam, for example, presence or absence of occurrence of skew and a place estimated as a cause of the occurrence of the skew. Details of these kinds of processing are explained later. 
     The sheet having the toner image transferred thereon is conveyed to a fixing device  17  provided on a downstream side of the transfer device  14 . The sheet is heated and pressed and the toner image is fixed on the sheet. The sheet subjected to fixing processing is discharged to the outside by a paper discharge device  18 . 
     In the photoconductive drum  10  from which the transfer of the toner image onto the sheet is finished, surface charges are removed by the charge removing device  15  and the toner remaining on the surface is removed by the cleaner  16 . 
     It is possible to perform continuous printing by repeating the processing explained above. 
       FIG. 3  is a schematic diagram of a configuration example of the conveying path  20  of the image forming apparatus  1  according to this embodiment. A sheet before printing is stored in the paper feeding tray  7  provided in the paper feeding unit  4 . In the conveying path  20  leading from the paper feeding tray  7  to the transfer region  21 , a pickup roller  30 , paper feeding rollers (first rollers)  31 , conveying rollers ( 1 ) (second rollers)  32 , conveying rollers ( 2 ) (third rollers)  33 , the registration rollers  34 , and the like are provided in order from the paper feeding tray side. 
     When printing is instructed from the operation unit  5 , the sheet is picked up from the paper feeding tray  7  by the pickup roller  30  and thereafter conveyed by the paper feeding rollers  31 , the conveying rollers ( 1 )  32 , and the conveying rollers ( 2 )  33  to reach the position of the registration rollers  34 . 
     The rotation of the registration rollers  34  is stopped at a point when the printing is instructed. A leading end of the sheet conveyed from the conveying path  20  bumps into a nip portion of the stopped registration rollers  34 . When a skew occurs in the sheet, one end of a leading end line of the sheet bumps against the nip portion earlier than the other end of the leading end line. Thereafter, sheets are continuously fed from the conveying path  20  by the rotation of the conveying rollers ( 1 )  32  and the conveying rollers ( 2 )  33 . Therefore, deflection occurs near the end of the sheet that bumps into the nip portion earlier. The other end of the leading end line soon reaches the nip portion and the leading end line of the sheet is aligned with and becomes parallel to a line of the nip portion. Misalignment of the leading end line of the sheet is absorbed by the deflection that occurs near the end of the sheet and the skew is corrected. When it is determined that the skew is corrected and the leading end line of the sheet becomes parallel to the line of the nip portion (i.e., the correction of the skew is completed), the registration rollers  34  start rotation and convey the leading end line of the sheet to the transfer region. 
     The completion of the skew correction is determined according to, for example, whether a predetermined period (a skew correction period) elapses after the leading end of the sheet comes into contact with the nip portion of the registration rollers  34 . More specifically, a registration sensor  44  arranged in a position immediately before the registration rollers  34  detects passage of the leading end of the sheet and thereafter, when the set skew correction period elapses, the completion of the skew correction is determined. Then, the registration rollers  34  start rotation and draws the leading end of the sheet into the transfer region. 
     In the conveying path  20  for the sheet, besides the registration sensor  44 , a paper feed sensor (a first sensor)  41  is provided near the paper feeding rollers  31 , a conveyance sensor ( 1 ) (a second sensor)  42  is provided near the conveying rollers ( 1 )  32 , and a conveyance sensor ( 2 ) (a third sensor)  43  is provided near the conveying rollers ( 2 )  33 . 
     The passage of the sheet through the conveying path  20  is detected by using these four sensors. Each of the sensors includes, for example, a photosensor. When the leading end of the sheet is detected, each of the sensors is turned on and detects time of the turn-on as ON time. On the other hand, when the trailing end of the sheet passes the sensor, the sensor is turned off and detects time of the turn-off as OFF time. It is possible to determine a passing state of the sheet on the basis of ON times and OFF times detected by the sensors. When a paper jam occurs in the conveying path  20 , it is possible to specify not only presence or absence of the occurrence of the paper jam but also a position where the paper jam occurs. 
     Other than the conveying path  20  leading from the paper feeding tray  7  to the registration rollers  34 , a paper jam could also occur. Therefore, sensors similar to the above are also arranged in a conveying path further downstream than the registration rollers  34 . These sensors detect presence or absence of the occurrence of the paper jam and an occurrence position of the paper jam. 
     One of main causes of occurrence of a paper jam is skew of the sheet. In the image forming apparatus  1  according to this embodiment, the arrangement of the sensors on the conveying path  20  is contrived. The sensors are used not only for detection of a paper jam but also for detection of skew. 
       FIG. 4  is a diagram of a state of the arrangement of the paper feed sensor  41 , the conveyance sensor ( 1 )  42 , and the conveyance sensor ( 2 )  43  used for skew detection. In the figure, the state of the arrangement is two-dimensionally expanded in a conveying direction of the sheet and a direction orthogonal to the conveying direction. 
     As it is seen from  FIG. 4 , the paper feed sensor (the first sensor)  41  is set near the paper feeding rollers  31  and in a position away from the center of the two paper feeding rollers  31 . The conveyance sensor ( 1 ) (the second sensor)  42  is arranged in a position different from the position of the paper feed sensor  41  in the conveying direction of the sheet and in the direction orthogonal to the conveying direction. In other words, the paper feed sensor  41  and the conveyance sensor ( 1 )  42  are arranged obliquely to the conveying direction of the sheet. 
     On the other hand, the conveyance sensor ( 2 ) (the third sensor)  43  is arranged in a position different from the position of the conveyance sensor ( 1 )  42  in the conveying direction of the sheet but same as the position of the conveyance sensor ( 1 )  42  in the direction orthogonal to the conveying direction. In other words, the conveyance sensor ( 2 )  43  and the conveyance sensor ( 1 )  42  are arranged parallel to the conveying direction of the sheet. 
     (2) Skew Detecting Method 
     A method for controlling the image forming apparatus  1  configured as explained above, in particular, a skew detecting method for a sheet is explained below. 
       FIGS. 5 and 6  are diagrams for explaining a principle of the skew detecting method according to this embodiment.  FIG. 5  is a diagram of a state in which counterclockwise skew occurs.  FIG. 6  is a diagram of a state in which clockwise skew occurs. 
     A figure on the left side in  FIG. 5  is a diagram of a state in which the leading end of the sheet passes the position of the paper feed sensor  41  in a state skewed counterclockwise. A figure in the center in  FIG. 5  is a diagram of a state in which the sheet is further conveyed and the leading end of the sheet passes the position of the conveyance sensor ( 1 )  42 . A figure on the right side in  FIG. 5  is a diagram of a state in which the sheet is thereafter further conveyed and the leading end of the sheet passes the position of the conveyance sensor ( 2 )  43 . 
     As explained above, the conveyance sensor ( 1 )  42  and the conveyance sensor ( 2 )  43  are arranged parallel to the conveying direction of the sheet. Therefore, a period in which the sheet passes the two sensors indicates the same value in a normal state in which the sheet is not skewed and a state in which the sheet is skewed. 
     On the other hand, the paper feed sensor  41  and the conveyance sensor ( 1 )  42  are arranged obliquely to the conveying direction of the sheet. Therefore, a period in which the sheet passes the two sensors is different in the normal state in which the sheet is not skewed and the state in which the sheet is skewed. As shown in  FIG. 5 , a period T in which the leading end of the sheet passes between the paper feed sensor  41  and the conveyance sensor ( 1 )  42  when the sheet is skewed counterclockwise is longer than a period T 0  in which the leading end of the sheet passes between the paper feed sensor  41  and the conveyance sensor ( 1 )  42  when the sheet is not skewed (T&gt;T 0 ). 
     Similarly, as shown in  FIG. 6 , when the sheet is skewed clockwise, the period T in which the leading end of the sheet passes between the paper feed sensor  41  and the conveyance sensor ( 1 )  42  is also different from the passage period T 0  in the normal state in which the sheet is not skewed. However, in this case, the passage period T when the sheet is skewed is shorter than the passage period T 0  (T&lt;T 0 ) when the sheet is not skewed. 
     A skew amount s of the sheet is represented by a skew angle θ (see drawings in the centers of  FIGS. 5 and 6 ). A distance in the direction orthogonal to the conveying direction between the paper feed sensor  41  and the conveyance sensor ( 1 )  42  is represented as L and conveying speed for the sheet is represented as V 0 . Then, the skew angle θ is represented by the following formula.
 
tan(θ)= ABS ( T−T 0)·( V 0/ L )  Formula 1
 
where ABS( ) is an operator for calculating an absolute value. The normal passage period T 0  in which the sheet is not skewed, the conveying speed V 0  for the sheet, and the distance L are values given in advance.
 
     The passage period T in Formula 1 is a period calculated from a difference between time when the leading end of the sheet passes the paper feed sensor  41  (first passage time t 1 ) and time when the leading end of the sheet passes the conveyance sensor ( 1 )  42  (second passage time t 2 ) (T=t 2 −t 1 ). The skew angle θ can be calculated from Formula 1 on the basis of the period T and the passage period T 0 , the conveying speed V 0 , and the distance L. 
     The conveying speed V 0  for the sheet used in Formula 1 is normal conveying speed in which abrasion or the like does not occur in the rollers. Therefore, if abrasion or the like occurs in the rollers and actual conveying speed V substantially deviates from the normal conveying speed V 0 , it is likely that an error occurs when a skew angle is calculated by using Formula 1. 
     Therefore, in this embodiment, whether the actual conveying speed V substantially deviates from the normal conveying speed V 0  is determined from the time when the leading end of the sheet passes the conveyance sensor ( 1 )  42  (the second passage time t 2 ), and passage time of the conveying sensor ( 2 )  43  arranged in parallel in the conveying direction with respect to the conveyance sensor ( 1 )  42  (a third passage time t 3 ). 
       FIG. 7  is a flowchart of an example of the determination processing. In ACT  1 , conveying period of the sheet (or conveying speed of the sheet) between the conveyance sensor ( 1 )  42  and the conveyance sensor ( 2 )  43  is calculated from the passage time t 2  when the leading end of the sheet passes the conveyance sensor ( 1 )  42  and the passage time t 3  when the leading end of the sheet passes the conveyance sensor ( 2 )  43 . The conveying period is compared with a normal conveying period. If the difference in the conveying period (or the conveying speed) is not within a predetermined reference range, it is determined that the conveying period (or the conveying speed) is abnormal. In this case, it is assumed that some failure occurs in a sheet conveying function of the rollers because of abrasion or the like of the rollers. 
     If there is no abnormality in the conveying period of the sheet (or the conveying speed of the sheet) between the conveyance sensor ( 2 )  43  and the conveyance sensor ( 1 )  42 , the processing proceeds to skew determination processing in ACT  2 . In ACT  2 , conveying period between the paper feed sensor  41  and the conveyance sensor ( 1 )  42  is checked. If the conveying period is within a predetermined range, it is determined that skew does not occur (normal). If the conveying period is outside the predetermined range, it is determined that skew occurs (abnormal). In ACT  2 , the skew angle θ may be calculated on the basis of Formula 1 and normality or abnormality may be determined with respect to the skew angle θ. 
     In the above explanation, the sensors detect passage times of the leading end of the sheet. However, instead, the trailing end of the sheet may be detected. 
     The arrangement of the paper feed sensor  41 , the conveyance sensor ( 1 )  42 , and the conveyance sensor ( 2 )  43  is not limited to the arrangement shown in  FIG. 4  (or  FIGS. 5 and 6 ). 
       FIG. 8  is a diagram of arrangement examples that can be applied to the image forming apparatus  1  according to this embodiment (a pattern  1 , a pattern  2 , and a pattern  3 ) and an arrangement example that cannot be applied to the image forming apparatus  1  (a pattern  4 ). Two sensors are necessary to detect a skew angle. The two sensors are a first sensor S 1  and a second sensor S 2 . To separately detect a change in conveying speed of the sheet and skew, one more sensor (a third sensor S 3 ) is necessary. 
     The first sensor S 1  and the second sensor S 2  for detecting a skew angle need to be arranged in different positions in both the conveying direction of the sheet and the direction orthogonal to the conveying direction. In the arrangement example shown in  FIG. 4  (or  FIGS. 5 and 6 ), the first sensor S 1  and the second sensor S 2  corresponds to the paper feed sensor  41  and the conveyance sensor ( 1 )  42 , respectively. 
     On the other hand, the third sensor S 3  can be freely arranged as illustrated by the pattern  1 , the pattern  2 , and the pattern  3  unless the third sensor S 3  is arranged on a straight line connecting the first sensor S 1  and the second sensor S 2  as illustrated by the pattern  4 . In the arrangement example shown in  FIG. 4  (or  FIGS. 5 and 6 ), the third sensor S 3  corresponds to the conveyance sensor ( 2 )  43 . 
     (3) Configuration of the Image Forming Apparatus and a Method of Controlling the Same 
       FIG. 9  is a functional block diagram of, in particular, functions related to skew detection, paper jam detection, and estimation of causes of the skew and the paper jam among functions of the image forming apparatus  1  according to this embodiment. In order to realize these functions, the image forming apparatus  1  includes, besides the paper feed sensor  41 , the conveyance sensor ( 1 )  42 , and the conveyance sensor ( 2 )  43 , a skew determining unit  52 , a conveying-speed determining unit  53 , a paper-jam detecting unit  54 , a cause determining unit  55 , a display unit  56 , and a conveyance control unit  57 . 
     The image forming apparatus  1  further includes a tray-opening-closing detecting unit  50  and a manual-feed-tray detecting unit  51 . 
     Among the functional blocks, the skew determining unit  52  calculates the skew amount s of the sheet from a difference between the first passage time t 1  detected by the paper feed sensor  41  (the first sensor) and the second passage time t 2  detected by the conveyance sensor ( 1 )  42  (the second sensor) and determines occurrence of skew of the sheet on the basis of the skew amount s. Specifically, for example, the skew determining unit  52  calculates the skew amounts (the skew angle θ) according to Formula 1, compares the calculated skew amount s with predetermined thresholds Smax 1  and Smax 2  to determine occurrence of skew. 
     The conveying-speed determining unit  53  calculates the conveying speed V of the sheet according to the third passage time t 3  detected by the conveyance sensor ( 2 )  43  (the third sensor) and the second passage time t 2  detected by the conveyance sensor ( 1 )  42 . When a distance between the conveyance sensor ( 2 )  43  and the conveyance sensor ( 1 )  42  is represented as D 2-3 , the conveying speed of the sheet can be calculated from, for example, V=D 2-3 /(t 3 −t 2 ). 
     If the calculated conveying speed V of the sheet is within a predetermined range, the skew determining unit  52  performs skew determination. If the conveying speed V is outside the predetermined range, the skew determining unit  52  determines that some failure occurs in the sheet conveying function itself because of abrasion or the like of the rollers. In this case, the skew determining unit  52  causes the display unit  56  to display an alarm or instructs the conveyance control unit  57  to stop the conveyance of the sheet. 
     The paper-jam detecting unit  54  detects occurrence of a paper jam from passage time information of conveyance sensors  60  of the respective units in addition to the three sensors. When a paper jam occurs, the paper-jam detecting unit  54  instructs the conveyance control unit  57  to stop the conveyance and sends information for displaying the occurrence of the paper jam and an occurrence position of the paper jam to the display unit  56 . 
     The occurrence of the paper jam is often caused by skew of the sheet. Empirically, the skew of the sheet often occurs when sheets are supplied to the paper feeding tray  7  anew. Skew tends to occur when the sheets are not properly set in the paper feeding tray  7  or a sheet guide in the paper feeding tray  7  is not properly set. 
     Therefore, the tray-opening-closing detecting unit  50  detects that the paper feeding tray  7  is once opened and then closed to thereby determine whether sheets are supplied to the paper feeding tray  7  anew. When there are plural paper feeding trays  7  as in this embodiment, the tray-opening-closing detecting unit  50  can specify which of the paper feeding trays  7  is opened and then closed. 
     When manual feed printing is performed, skew may occur unless a setting condition of sheets placed on the manual feed tray  8 , such as a sheet direction or a setting condition of a sheet guide, is not proper. Therefore, the manual-feed-tray detecting unit  51  detects whether printing is performed by using the manual feed tray  8 . 
     If the skew determining unit  52  determines that skew occurs, the cause determining unit  55  estimates a cause of the occurrence of the skew. Specifically, when skew occurs after the paper feeding tray  7  is opened and then closed, the cause determining unit  55  estimates that the skew occurs because of the opened and then closed paper feeding tray and causes the display unit  56  to perform display to urge the user to check whether the sheets or the sheet guide in the paper feeding tray  7  is properly set. When skew occurs during the use of the manual feed tray  8 , the cause determining unit  55  estimates that the skew occurs because of the manual feed tray  8  and causes the display unit  56  to perform display to call attention of the user to a setting condition of the manually fed sheets, such as a sheet direction or a setting condition of a sheet guide. 
     The display unit  56  is a display device provided in the operation unit  5  and includes, for example, a liquid crystal panel. 
     The conveyance control unit  57  controls ON, OFF, and the like of a conveyance motor (not shown) that drives various conveying rollers. 
       FIGS. 10 and 11  are flowcharts of more specific processing examples of the functional blocks explained above.  FIG. 10  is a diagram of a processing example related to the tray-opening-closing detecting unit  50  (and the manual-feed-tray detecting unit  51 ). 
     In ACT  10  in  FIG. 10 , the tray-opening-closing detecting unit  50  sets a tray flag to OFF for initialization. In ACT  20 , the tray-opening-closing detecting unit  50  determines whether the paper feeding tray (n)  7  is opened and then closed. “n” is a reference number for identifying the respective paper feeding trays  7  when there are the plural paper feeding trays  7 . In the image forming apparatus  1  according to this embodiment shown in  FIG. 1 , the two paper feeding trays ( 1 ) and ( 2 ) are provided. Therefore, in ACT  11 , the tray-opening-closing detecting unit  50  specifies which of the paper feeding trays is opened and then closed. 
     In ACT  12 , the tray-opening-closing detecting unit  50  sets an opening and closing flag (n) corresponding to the opened and then closed paper feeding tray (n) to ON and waits for the start of printing (ACT  13 ). The opening and closing flag (n) is provided to correspond to each of the plural paper feeding trays (n). 
     If the paper feeding tray  7  is not opened and then closed, the tray-opening-closing detecting unit  50  proceeds to ACT  13  without setting the opening and closing flag (n) to ON and waits for the start of printing. 
     When printing is started, in ACT  14 , the tray-opening-closing detecting unit  50  determines whether the manual feed tray  8  is in use. If the manual feed tray  8  is used to perform printing, the tray-opening-closing detecting unit  50  sets the tray flag to ON (ACT  15 ) and proceeds to processing shown in  FIG. 11 . 
     If the manual feed tray  8  is not used to perform printing, the printing is performed by using any one of the paper feed trays  7 . In this case, the tray-opening-closing detecting unit  50  refers to the opening and closing flag (n) corresponding to the paper feeding tray (n) in use. If the opening and closing flag (n) is ON (YES in ACT  16 ), the tray-opening-closing detecting unit  50  sets the tray flag to ON and proceeds to the processing shown in  FIG. 11 . 
     On the other hand, if the opening and closing flag (n) corresponding to the paper feeding tray (n) in use is not ON, the tray-opening-closing detecting unit  50  proceeds to the processing shown in  FIG. 11  without setting the tray flag to ON. 
     As explained above, the tray flag is set to ON when the paper feed tray  7  presently used for printing is opened and then closed before the start of printing (i.e., when sheets are supplied to the paper feeding tray anew) or when the manual feed tray  8  is presently used to perform printing. 
     On the other hand, even when one paper feeding tray  7  is opened and then closed to feed sheets before the start of printing, if another paper feeding tray  7  is selected to perform printing, the tray flag is not set to ON. 
       FIG. 11  is a flowchart of a processing example of mainly the skew determining unit  52 , the conveying-speed determining unit  53  (including the paper-jam detecting unit  54 ), the cause determining unit  55 , the display unit  56 , and the conveyance control unit  57  among the functional blocks shown in  FIG. 9 . 
     In ACT  100 , the conveying-speed determining unit  53  waits for the paper feed sensor  41  (the first sensor) to be turned on and, when the paper feed sensor  41  (the first sensor) is turned on, stores ON time t 1  thereof (first passage time) in an appropriate memory. 
     In ACT  101 , the conveying-speed determining unit  53  waits for the conveyance sensor ( 1 )  42  (the second sensor) to be turned on and, when the conveyance sensor ( 1 ) is turned on, stores ON time t 2  thereof (second passage time) in an appropriate memory. 
     In ACT  102 , the conveying-speed determining unit  53  waits for the conveyance sensor ( 2 )  43  (the third sensor) to be turned on and, when the conveyance sensor ( 2 ) is turned on, stores ON time t 3  thereof (third passage time) in an appropriate memory. 
     In ACT  103 , the conveying-speed determining unit  53  calculates, from a difference between the ON time t 3  detected by the conveyance sensor ( 2 )  43  and the ON time t 2  detected by the conveyance sensor ( 1 )  42 , a period t R  in which the sheet is conveyed from the conveyance sensor ( 1 )  42  to the conveyance sensor ( 2 )  43  (t R =t 3 −t 2 ). 
     In ACT  104 , the conveying-speed determining unit  53  determines whether the conveyance period t R  calculated in ACT  103  is within a predetermined range (a range from a minimum value T R     —     min  to a maximum value T R     —     max ) (T R     —     min &lt;t R &lt;T R     —     max ). 
     If it is determined in ACT  104  that the conveyance period t R  is not within the predetermined range, it is estimated that the conveying speed V 0  of the sheet is not normal because of a cause such as abrasion that occurs in the rollers. 
     Therefore, in this case, a correct skew amount is not obtained even if skew amount determination based on the conveying speed V 0  is performed. Therefore, processing for the skew amount determination (processing in ACT  106 , ACT  107 , ACT  111 , and the like) is not performed. 
     In this case, a paper jam is considered to occur in the conveying path  20 . Therefore, the conveying-speed determining unit  53  may instruct the display unit  56  to display a message (A) that “paper jam occurs” (ACT  105 ). 
       FIGS. 12A to 12F  are diagrams of display examples of the display unit  56 . In ACT  105 , for example, a message shown in  FIG. 12A  is displayed. 
     Since it is determined that the conveying speed V 0  of the sheet is not normal, the conveying-speed determining unit  53  may instruct the conveyance control unit  57 , which controls the conveyance of the sheet, to stop the conveyance of the sheet (ACT  105 ). 
     On the other hand, if it is determined that the conveying speed V 0  of the sheet is normal (YES in ACT  104 ), the skew determining unit  52  determines the skew amount s (more specifically, the skew angle θ). 
     In ACT  106 , the skew determining unit  52  calculates the skew amount s (the skew angle θ) from the ON time t 2  of the conveyance sensor ( 1 )  42  and the ON time t 1  of the paper feed sensor  41  detected earlier. Specifically, the skew determining unit  52  calculates a difference between the ON time t 2  of the conveyance sensor ( 1 )  42  and the ON time t 1  of the paper feed sensor  41  (T=t 2 −t 1 ) (a passage period) and substitutes the calculated difference T in Formula 1 to calculate the skew amount s (the skew angle θ). 
     Standard conveying speed at normal time may be used as the conveying speed V 0  of the sheet in Formula 1. Alternatively, actual conveying speed at the present point (only if it is determined in ACT  104  that the conveying speed is normal) may be used from the conveyance period t R  calculated in ACT  103 . 
     In ACT  107 , the skew determining unit  52  determines whether the skew amount s (the skew angle θ) calculated in ACT  106  exceeds a predetermined limit value Smax 1 . The skew determination in ACT  107  is processing for determining occurrence of a relatively large amount of skew. If the skew amount s (the skew angle θ) exceeds the predetermined limit value Smax 1 , this means that the skew is so large that the skew correction is not properly performed by the registration rollers  34  and the skew is so large that a paper jam is caused by the skew. 
     If the skew amount s exceeds the predetermined limit value Smax 1 , in ACT  108 , the tray-opening-closing detecting unit  50  determines a setting state of the tray flag. 
     If the tray flag is set to ON, it is highly likely that a cause of occurrence of skew is present in the paper feeding tray  7  in use that has been opened and then closed before the start of printing. Therefore, while the skew determining unit  52  instructs the conveyance control unit  57  to stop the conveyance of the sheet, the cause determining unit  55  instructs the display unit  56  to display a message for urging the user to check the inside of the paper feeding tray  7  in use (ACT  109 ). 
     For example, when the paper feeding tray  7  in use is the paper feeding tray ( 2 ), as shown in  FIG. 12B , the cause determining unit  55  instructs the display unit  56  to display a message such as “The sheet is conveyed in a skewed state. Please check whether sheets in the tray  2  and the guide are properly set.” 
     With such message display, it is possible to prevent occurrence of a paper jam in advance by causing the user to check the paper feeding tray  7  estimated as a cause of occurrence of skew and removing the cause of the occurrence of the skew. 
     When skew occurs during printing performed by using the manual feed tray  8 , in ACT  109 , similar message display is performed. In this case, the cause determining unit  55  instructs the display unit  56  to display a message such as “The sheet is conveyed in a skewed state. Please check whether sheets are properly set in the manual feed tray.” 
     When the tray flag is off, skew occurs because of a cause other than the paper feeding tray  7 . Therefore, in this case, as in the above case, the skew determining unit  52  instructs the conveyance control unit  57  to stop the conveyance of the sheet. Meanwhile, as shown in  FIG. 12C , the cause determining unit  55  instructs the display unit  56  to display a message such as “The sheet is conveyed in a skewed state.” With this message display, the user can correct skew as a cause of a paper jam by finding a cause of occurrence of skew other than the paper feeding tray  7  or calling a serviceperson. 
     On the other hand, if the skew amount s (the skew angle θ) is equal to or smaller than the predetermined limit value Smax 1  (NO in ACT  107 ), this means that skew does not occur or, even if skew occurs, the skew is relatively small. 
     In ACT  111 , the skew determining unit  52  determines presence or absence of smaller skew using a reference value Smax 2  smaller than the limit value Smax 1 . 
     If small skew occurs (YES in ACT  111 ), presence or absence of a paper jam (ACT  112 ) is determined. Even if the skew is small in the conveying path  20  leading from the paper feeding tray  7  to the registration rollers  34 , the skew may increase on the downstream side of the conveying path (e.g., near the transfer device  21  or the fixing device  17 ) to cause a paper jam. In such a case, even if the skew is small on the upstream side (the conveying path  20 ), the skew causes a paper jam. 
     Therefore, in ACT  113 , the tray-opening-closing detecting unit  50  determines a state of the tray flag. If the tray flag is set to ON, the tray-opening-closing detecting unit  50  inform the user that an estimated cause of the occurrence of the skew is present in the paper feeding tray  7  opened and then closed previously (ACT  114 ). This is effective in terms of specifying a cause of occurrence of a paper jam. 
     In ACT  114 , the skew determining unit  52  instructs the conveyance control unit  57  to stop the conveyance of the sheet, and the cause determining unit  55  instructs the display unit  56  to display a message such as “Paper jam has occurred. Please check whether sheets and the guide in the tray ( 2 ) are properly set.”, as shown in  FIG. 12D . 
     If the tray flag is OFF (NO in ACT  113 ) the cause determining unit  55  instructs the display unit  56  to simply display a message such as “Paper jam has occurred.”, as shown in  FIG. 12E . 
     If small skew occurs but a paper jam does not occur (NO in ACT  112 ), the cause determining unit  55  instructs the display unit  56  to display a message such as “Pay attention to sheet conveyance because it is likely that the sheet is conveyed in a skewed state.”, as shown in  FIG. 12F  (ACT  116 ), then the end of the printing is determined (ACT  117 ). 
     If even small skew does not occur (NO in ACT  111 ), the end of the printing is determined without performing anything (ACT  117 ). 
     When the printing ends (YES in ACT  117 ), if the opening and closing flag (n) is set to ON, the tray-opening-closing detecting unit  50  resets the opening and closing flag (n) to OFF and finishes the processing. 
     As explained above, with the image forming apparatus  1  and the method for controlling the same according to this embodiment, it is possible to detect occurrence of sheet skew without, for example, addition of a special sensor and even if conveying period changes because of abrasion or the like of the rollers, estimate a cause of the occurrence of the skew and inform the user of the cause, and prevent occurrence of a paper jam. 
     The present invention is not limited to the embodiments per se. It is possible to modify and embody the elements without departing from the spirit of the present invention at an implementation stage. It is possible to form various embodiments of the invention according to appropriate combinations of the plural elements disclosed in the embodiments. For example, several elements may be deleted from all the elements described in the embodiments. The elements described in different embodiments may be appropriately combined.