Patent Publication Number: US-7591460-B2

Title: Sheet feeding apparatus, image forming apparatus and sheet feeding method

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   This application is based on and claims priority under 35 USC §119 from Japanese Patent Application No. 2007-152940 filed Jun. 8, 2007. 
   BACKGROUND 
   (i) Technical Field 
   The present invention relates to a sheet feeding apparatus for separating and supplying recording paper sheets one by one, an image forming apparatus having such a sheet feeding apparatus, and a sheet feeding method. 
   (ii) Related Art 
   An image forming apparatus in the background art, such as a copying machine and a printer, is provided with a sheet feeding apparatus which separates recording sheets stacked and accommodated in a sheet feeing portion one by one, so as to stably supply recording sheets, on which an image is formed, to an image forming portion. 
   SUMMARY 
   According to an aspect of the invention, there is provided a sheet feeding apparatus comprising: 
   a multiple-feed detecting section that detects recording sheets superposed with each other to be transported by plural rolls in a direction; 
   a separating section that, in a case where recording sheets are transported with being superposed with each other, separates an uppermost sheet of the recording sheets from the other; 
   a slippage detecting section that detects a slippage of the uppermost recording sheet with respect to a roll; and 
   a transporting-speed reduction suppressing section that suppresses a transporting-speed of the uppermost recording sheet from reducing from a value, 
   the separation section including: a transporting roll that transports recording sheets in a transporting direction; and a separation roll that is placed to be opposed to and to be press-contacted with the transporting roll through a recording sheet, 
   the transporting-speed reduction suppressing section changing a separation torque of the separation roll according based on a result of detection by the multiple-feed detecting section and a result of detection by the slippage detecting section. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Embodiments of the present invention will be described in detail based on the following figures, wherein: 
       FIG. 1  is a view illustrating a configuration of an image forming apparatus according to an exemplary embodiment of the invention; 
       FIG. 2  is a schematic view illustrating a configuration of a sheet feeding apparatus according to an exemplary embodiment of the invention and illustrating also an operation of controlling this sheet feeding apparatus; 
       FIG. 3  is a schematic view illustrating a relation between the friction coefficient and the transporting performance of a roll; 
       FIG. 4A  is a flowchart illustrating a control procedure according to Embodiment 1; 
       FIG. 4B  is a table summarizingly describing a relation between the number of recording sheets and the value of separation torque according to Embodiment 1; 
       FIG. 5A  is a flowchart illustrating a control procedure according to Embodiment 2; 
       FIG. 5B  is a table summarizingly describing the relation between the number of recording sheets and the value of separation torque according to Embodiment 2; 
       FIG. 6A  is a flowchart illustrating a control procedure according to Embodiment 3; 
       FIG. 6B  is a table summarizingly describing a relation between the number of recording sheets and the value of separation torque according to Embodiment 3; and 
       FIG. 7  is a flowchart illustrating a control procedure according to Embodiment 4, 
   

   wherein reference numerals and signs are set forth below.
       3 : intermediate transfer belt     4 : fixing device     5 : sheet feed tray     20 : photosensitive drum     21 : charging corotron     22 : laser beam scanner     25 : drum cleaner     30 : primary transfer roll     31 : secondary transfer roll     32 : backup roll     33 : belt cleaner     50 : sheet feeding apparatus     50 B: transporting belt     50 N: press-contact portion     50 P: sheet feeding roll     50 R: registration roll     50 T: drawing roll     51 : detection roll     52 : sheet number detection sensor     53 : transporting roll     54 : torque sensor     59 : arrival detecting section   CR: control unit   L: operating line   Lo: operating point   P: recording paper   R 1 : misfeed area   R 2 : multiple-feeding area   TL: load torque   TR: discharge tray   Vf: transporting roll rotation speed   Vt: drawing roll rotation speed   

   DETAILED DESCRIPTION 
   Hereinafter, embodiments according to the invention are described below with reference to the accompanying drawings. 
   First, a configuration of an image forming apparatus according to an exemplary embodiment of the invention is described below with reference to  FIG. 1 .  FIG. 1  illustrates the configuration of the entire image forming apparatus according to an exemplary embodiment of the invention. 
   As illustrated in  FIG. 1 , the image forming apparatus according to the invention includes an image input portion (IIT)  1 , which optically reads image information of an original  11  placed on a platen  10  and causes a charge-coupled device (CCD) sensor  12  to convert the image information into electrical image data, and includes also an image output portion (IOT)  2  that forms an image on a recording sheet P according to image data transferred from the image input portion  1 . Additionally, an automatic document feeder (ADF), which automatically feed the original  11  to the platen  10 , can be attached to the image input portion  1 . 
   The image output portion  2  forms a toner image on a photosensitive drum  20  according to image data transferred from the image input portion  1 . Subsequently, a primary transfer of such a toner image onto an endless intermediate transfer belt  3  is performed. Then, a secondary transfer of the toner image, which is formed on the intermediate transfer belt  3 , onto a recording sheet P is performed. Thus, a recording image is formed on the recording sheet P. Subsequently, the recording sheet P, onto which a toner image is transferred, is discharged onto a discharge tray TR through a fixing device  4 . 
   The photosensitive drum  20  is turned in a direction of an arrow at a predetermined process speed. Around the photosensitive drum  20 , the following devices are placed. That is, a charging corotron  21  which uniformly charges a surface of such a photosensitive drum  20  to a predetermined background portion potential level, a laser beam scanner  22  which exposes the photosensitive drum  20  by laser beams modulated according to image data so as to form an electrostatic latent image on the photosensitive drum  20 , a development device  23  which develops the electrostatic latent image formed on the photosensitive drum  20 , a transfer preprocessing corotron  24  which eliminates the potential from the surface of the photosensitive drum  20  before the primary transfer of a toner image onto the intermediate transfer belt  3 , and a drum cleaner  25 , which eliminates residual toner on the photosensitive drum  20  upon completion of performing the primary transfer of an toner image, are provided. 
   On the other hand, the intermediate transfer belt  3  is laid around plural rolls and is turned in the direction of an arrow. A toner image formed on the photosensitive drum  20  is transferred onto the intermediate transfer belt  3 . Subsequently, a secondary transfer of the toner image onto a recording sheet P from such an intermediate transfer belt  3  is performed. A primary transfer roll  30 , which forms a transfer electric-field extending therefrom to the photosensitive drum  20 , is provided to face the photosensitive drum  20  across the intermediate transfer belt  3 . Meanwhile, a secondary transfer roll  31  and a backup roll  32  are provided across the intermediate transfer belt  3  at a secondary transfer position at which a secondary transfer of a toner image is performed. The recording sheet P is inserted into between the secondary transfer roll  31  and the intermediate transfer belt  3 . A toner image primary-transferred onto the intermediate transfer belt  3  is secondary-transferred onto the recording sheet P. A belt cleaner  33  for cleaning paper powder and residual toner from a surface of the intermediate transfer belt  3  is provided at a part of a turning path of the intermediate transfer belt  3 , which is located between a primary transfer position and the secondary transfer position. 
   A sheet feeding portion for supplying recording sheets P to the image output portion  2  is provided under the image output portion  2 . The sheet feeding portion is equipped with four sheet feeding trays  5   a  to  5   d  respectively accommodating different-size recording sheets P. Recording sheets P of a size selected in a copying operation are sent from one of the sheet feeding trays to the image output portion  2  by turning a pickup roll  50 P. Plural sheet transporting rolls  50   t  are provided on a transporting path extending from each of the sheet feeding trays  5   a  to  5   d  to the secondary transfer position at which a toner image is transferred onto a recording sheet. Registration rolls  50 R are placed just anterior to the secondary transfer position. Such registration rolls  50 R feed recording sheets P supplied from the sheet feeding trays  5   a  to  5   d  to the secondary transfer position at predetermined timing synchronized with timing at which an electrostatic latent image is written to the photosensitive drum  20 . 
   Incidentally, in  FIG. 1 , reference numeral  26  designates an image processing portion for supplying image data to the laser beam scanner  22  after processing image data, which has been transferred from the image input portion  1  to the image output portion  2 , according to information representing a copying operation. Reference character  50 B denotes a sheet transporting belt for feeding a recording sheet P, onto which a toner image is secondary-transferred, to the fixing device  4 . Reference character  50 V represents an inverter path for feeding, when double-sided copying of a recording sheet P is performed, the recording sheet P from the fixing device  4  to the secondary transfer position by reversing the recording sheet P. Reference character TR 0  designates a manual feed tray used for manual feed of recording sheets P. Reference character CR denotes a device controller for controlling each component device. 
   In the image forming apparatus constituted as described above, the laser beam scanner  22  exposes the photosensitive drum  20  according to image information of an original, which is input by the image input portion  1 . An electrostatic latent image corresponding to the image information is written onto the photosensitive drum  20 . This electrostatic latent image is developed by the development device  23  so that developing timing lags a little behind the writing timing at which the electrostatic latent image is written to the drum. Then, a voltage of a polarity opposite to that of charged toner is applied to the base material of the intermediate transfer belt  3  by the primary transfer roll  30  in a primary transfer part in which the photosensitive drum  20  and the intermediate transfer belt  3  are press-contacted with each other. Thus, a toner image formed in this way is primary-transferred onto a surface of the intermediate transfer belt  3  by a press-contact force and an electrostatic attracting force. An unfixed-toner image primary-transferred onto the intermediate transfer belt  3  is transported by the rotation of the intermediate transfer belt  3  to a secondary transfer part that faces a transporting path on which recording sheets P are transported. Residual toner on the photosensitive drum  20 , onto which the toner image has been primary-transferred, is scraped off therefrom by an elastic cleaning blade of the drum cleaner  25 . Thus, the sheet feeding apparatus is prepared for the next image forming cycle. 
   In the secondary transfer part, the secondary transfer roll  31  is pressed through the intermediate transfer belt  3  against the backup roll  32  provided in a space surrounded by the intermediate transfer belt  3 . A recording sheet P carried out with predetermined timing as a recording medium is inserted into between the secondary transfer roll  31  and the intermediate transfer belt  3  by a registration roll  50 R. 
   Then, unfixed toner images held on the intermediate transfer belt  3  are electrostatically transferred onto a recording sheet P in the secondary transfer part by a transfer electric-field formed between the backup roll  32  and the secondary transfer roll  31 . 
   The recording sheet P, onto which the unfixed toner image is transferred, is fed into the fixing device  4  through the transporting belt  50 B. This toner image is fixed onto the recording sheet P by heat and pressure by the fixing device  4 . Subsequently, the recording sheet P, to which the toner image is fixed, is discharged to the discharge tray TR. Incidentally, residual toner on which the intermediate transfer belt  3  on which the transfer of the unfixed toner image onto the recording sheet P has been completed, is removed by the belt cleaner  33 . 
   Next, the configuration of a sheet feeding apparatus according to the present embodiment is described below with reference to  FIG. 2 . Incidentally,  FIG. 2  is a schematic view illustrating the general configuration of the sheet feeding apparatus according to the present embodiment and illustrating also an operation of controlling this sheet feeding apparatus. 
   As illustrated in  FIG. 2 , the sheet feeding apparatus  50  according to the present embodiment includes the sheet feeding trays  5   a  to  5   d  (hereunder referred to generically as the sheet feeding tray  5 , because the sheet feeding trays  5   a  to  5   d  basically have the same structure) for accommodating plural recording sheets P by stacking the recording sheets P, the sheet feeding rolls  50 P for drawing the top (uppermost) recording sheet P 1  from the recording sheets P accommodated in the sheet feeding tray  5  and for feeding the top recording sheet P 1  to the predetermined transporting path, a pair of the transporting roll  53  and the separation roll  55 , which are placed downstream in the transporting direction of the sheet feeding roll  50 P to face each other and constitute the separation section, and a pair of drawing-rolls  50 T,  50 T, which are provided downstream from the separation section and which draw a recording sheet P out of the press-contact portion  50 N between the transporting roll  53  and the separation roll  55  and transport the recording sheet P to a transporting roll in a subsequent stage. A detection roll  51 , which rotates with transporting the recording sheet P 1  and detects a movement speed or an amount of displacement of the top recording sheet P 1 , is provided in the vicinity of the sheet feeding roll  50 P. A sheet number detecting sensor  52  serving as the multiple-feed detecting section for detecting the number of multiply-fed recording sheets P being present in the press-contact portion  50 N is provided in the vicinity of the separation roll  55 . For example, conventionally known optical type, capacitance type, and mechanical type multiple-feed detecting sensors for determining the number of multiply-fed recording sheets by detecting a total thickness of multiply-fed recording sheets P 1 , P 2 , . . . can appropriately be used as the sheet number detecting sensor  52 . 
   The slippage detecting section according to the present embodiment detects the presence/absence of occurrence of slippage between a recording sheet P and a predetermined roll by comparing the rotation speed (or the amount of rotation) of the detecting roll  51  with the associated predetermined rotation speed (or the associated predetermined amount of rotation) of the transporting roll  53  or the drawing roll  50 T. 
   The sheet feeding tray  5  is constituted detachably from a casing of an image forming apparatus. Recording sheets P are accommodated in the sheet feeding tray  5 . The sheet feeding tray  5  is provided with a bottom plate (not shown) for lifting up the entire recording sheets P so that the uppermost recording sheet P 1  in the sheet feeding tray  5  is placed at a predetermined position. 
   On the other hand, the aforementioned sheet feeding roll  50 P is attached to the casing of the image forming apparatus, into which the sheet feeding tray  5  is inserted. The sheet feeding roll  50 P is contactable with the uppermost recording sheet P 1  lifted up to the predetermined position and can change the press-contact force that acts upon the recording sheet P 1 . The sheet feeding roll  50 P moves from a separation position to a contact position according to, for example, a transport start instruction issued from a control unit CR to rotate while being press-contacted with the uppermost recording sheet P 1 . The recording sheet P 1  is drawn out of the sheet feeding tray  5  in the predetermined transporting direction (hereunder referred to also as a forward direction) by this press-contact rotation force of the sheet feeding roll  50 P. In the case of occurrence of a state (hereunder referred to also as a multiple feed state) in which plural recording sheets P are present in the press-contact portion  50 N, the recording sheets P are separated into the uppermost recording sheet P 1  and the other recording sheets P 2 , P 3 , . . . when the recording sheets P pass through the press-contact portion  50 N. Thus, the recording sheets P are transported on the predetermined transporting path one by one. 
   The transporting roll  53  is constituted to be rotated at a predetermined rotation speed (or amount of rotation) by a drive source (not shown) so as to transport the uppermost recording sheet P 1  in the forward direction. A known torque sensor  54  is attached to the rotating shaft of the transporting roll  53  so as to be able to detect load torque when a recording sheet P is transported. 
   On the other hand, the separation roll  55  is constituted to be contacted with the bottom surfaces of the multiply-fed recording sheets P 2 , P 3  . . . to provide separation torque to the recording sheets P 2 , P 3  . . . so as to backwardly feed the recording sheets in a direction opposite to the predetermined transporting direction. The separation roll  55  is constituted to be drive-controlled by a drive portion  57  and a controller  58 , which constitute a feedback control system including a speed sensor, a torque limiter, and a direct-current (DC) motor so as to change the separation torque. More specifically, in the sheet feeding apparatus  50  according to the present embodiment, separation torque to be generated in the separation roll  55  is changed between the separation torque T 2  to be generated therein in the case of occurrence of multiple feed (i.e., in a case where two or more recording sheets are present in the press-contact portion  50 N), and the separation torque T 1  to be generated therein in the case of an ordinary transporting state in which no multiple feed occurs. Also, the separation torque T 2  to be generated in the case of occurrence of multiple feed is set to be larger than the separation torque T 1  to be generated in the case of an ordinary transporting state. Consequently, the separation performance at occurrence of a multiple-feed can be enhanced. Additionally, the device controller CR can be used also as the controller  58 . Apparently, another controller can alternatively be provided as the controller  58 . 
   Further, the drawing roll  50 T provided downstream from the press-contact portion  50 N is constituted as the pair of opposed rolls  50 T,  50 T, and is caused by a drive source (not shown) to rotate at a predetermined rotation speed (or a predetermined amount of rotation). According to the present embodiment, the drawing roll  50 T is constituted as the pair of opposed rolls  50 T,  50 T. However, as long as the drawing roll is constituted to be able to draw a recording sheet out of the press-contact portion  50 N of the separation section, any other drawing section can be employed. For example, the drawing roll  50 T can be press-contacted with the predetermined transporting path. 
   A known optical type sensor can additionally be provided as an arrival detecting section  59 . However, according to the present embodiment, the arrival detecting section is constituted by calculating a movement distance of a recording sheet P according to an amount of rotation of the detection roll  51 . That is, the detection roll  51  is used as both of the arrival detecting section  59  and a part of the aforementioned slippage detecting section. Consequently, the miniaturization and the cost reduction of a sheet feeding apparatus can be achieved by reducing the number of sensors. 
   Also, the transporting speed reduction suppressing section according to the present embodiment is constituted by causing the controller  58  to control predetermined component devices on the basis of appropriate information of the sensors according to information representing control operations that will be described later. Consequently, reduction in transporting-speed of an uppermost recording sheet P 1  from a predetermined transporting speed can effectively be suppressed. Hereinafter, practical control operations are described as those of examples. 
   EMBODIMENT 1 
   According to the present inventor&#39;s study, it has been found that generally, a primary cause of occurrence of a slippage is reduction in friction coefficient of the transporting roll due to abrasion thereof.  FIG. 3  illustrates the relation between reduction in the friction coefficient of the roll and the transporting performance thereof. 
   In  FIG. 3 , a straight line L represents an operating line of the separation section including the transporting roll  53  and the separation roll  55 . The separation section generates a predetermined forward-direction transporting force and an opposite-direction separation torque at an operating point L 0  on the straight line L. This operating point L 0  moves on the predetermined operating line L according to the separation torque of the separation roll  55 . 
   A region R 1  extending above the operating line L is an area (i.e., a misfeed area) in which a transport failure occurs due to insufficient transporting force. On the other hand, another region R 2  extending below the operating line L is an area (i.e., a multiple-feed area) in which a multiple-feed occurs due to reduction in separation performance. 
   According to the present inventor&#39;s study, it is found that the reduction in friction coefficient of the transporting roll  53  expands the range of the misfeed area R 1  and has substantially no effects on the operating line L and the multiple-feed area R 2 . More specifically, it is found that the gradient of a straight line L 1  representing a border of the misfeed area R 1  decreases with reduction of the friction coefficient, and is changed to a straight line indicated by a dashed line L 1  shown in  FIG. 3 , that consequently, the misfeed area R 1  is expanded, and that however, the straight line L and the region R 2  do not vary. 
   Accordingly, it is found out that it is effective to downwardly move the operating point L 0  of the separation section along the predetermined operating line L so as to avoid the misfeed area R 1 , which expands with reduction in the friction coefficient, to thereby prevent occurrence of a transport failure and a multiple-feed. More specifically, the separation torque of the separation roll  55  is set so that the operating point L 0  is on a part of the straight line L, which is above a line segment L v . Consequently, reduction in the transporting performance can be suppressed without degrading the separation performance. 
   Thus, according to the present embodiment, in a case where the slippage detecting section detects a slippage, the separation torque to be generated in the separation roll  55  is changed according to the number of recording sheets P that are present in the press-contact portion  50 N. A practical control operation according to the present embodiment is described below with reference to  FIGS. 4A and 4B .  FIG. 4A  is a flowchart illustrating a control procedure according to the present embodiment.  FIG. 4B  is a table summarizingly describing the relation between the number of recording sheets and the value of separation torque in a separation portion according to the present embodiment. 
   As illustrated in  FIG. 4A , first, in step ST 1 , the slippage detecting section detects the presence/absence of occurrence of a slippage according to a transport start instruction. More specifically, for example, a rotation speed V 0  of the detection roll  51  corresponding to the transporting speed of a recording sheet P is compared with a predetermined rotation speed Vf of the transporting roll  53  or with a predetermined rotation speed Vt of the drawing roll  50 T. In a case where Vf (or Vt)≈V 0 , the slippage detecting section determines that no slippage occurs. In a case where Vf (or Vt)&gt;V 0 , the slippage detecting section determines that a slippage occurs between the recording sheet P and the roll corresponding to a slippage to be detected. A reference value V 0  can be set to be a predetermined constant value, or to have a predetermined range including a fluctuation band. When the slippage detecting section determines whether a slippage occurs, the slippage detecting section can determine the presence/absence of occurrence of a slippage by comparing, for example, an amount of rotation of the detection roll  51  corresponding to an amount of movement of a recording sheet P with an amount of rotation of the transporting roll  53  or with an amount of rotation of the drawing roll  50 T, instead of comparing the rotation speed there between. 
   Next, in a case where it is determined that no slippage occurs, the sheet number detecting sensor  52  determines in step ST 2  the presence/absence of a multiple-feed (i.e., whether two or more recording sheets P are present in the press-contact portion  50 N formed by the transporting roll  53  and the separation roll  55 ). 
   Then, in a case where a multiple-feed occurs (i.e., two or more recording sheets P are present in the press-contact portion  50 N), the separation torque of the separation roll  55  is set at a predetermined value T 2  in step ST 3 . In a case where no multiple-feed occurs, the separation torque of the separation roll  55  is set at a predetermined value T 1  in step ST 4 . Incidentally, T 1 &lt;T 2 . A reason for generating the separation torque T 1  even in the case of occurrence of no multiple-feed is that the sheet feeding apparatus is enabled to immediately deal with a subsequent possible multiple-feed. However, the apparatus can be controlled so that the separation torque T 1  is not generated (i.e., the separation torque is set to be 0). 
   On the other hand, in a case where it is determined that a slippage occurs, continuously and similarly, the slippage detecting section determines the presence/absence of occurrence of a multiple-feed in step ST 5 . 
   Then, in a case where a multiple-feed occurs, the separation torque of the separation roll  55  is set at a predetermined value T 2   a  (T 2   a &lt;T 2 ) in step ST 6 . In a case where no multiple-feed occurs, the separation torque of the separation roll  55  is set at a predetermined value T 1   a  (T 1   a &lt;T 1 ) in step ST 7 . Incidentally, T 1   a &lt;T 2   a .  FIG. 4B  illustrates a table that summarizingly describes the relation between the number of recording sheets P, which are present in the press-contact portion  50 N, and the value of the separation torque generated at the separation roll  55 . 
   Thus, according to the multiple-feed state and to the presence/absence of occurrence of a slippage, the values T 1 , T 1   a , T 2 , and T 2   a  of the separation torque generated at the separation roll  55  are set on the line segments L v  shown in  FIG. 3  so as to meet the predetermined relations: T 1   a &lt;T 1 , T 2   a &lt;T 2 , and T 1 &lt;T 2 . Consequently, reduction in transporting performance due to a slippage can be suppressed without degrading the separation performance. 
   The multiple-feed detecting sensor according to the present embodiment does not always need to determine the number of recording sheets that are present in the press-contact portion  50 N. It is sufficient to determine whether the number of recording sheets being present in the press-contact portion  50 N is 1 or more. Consequently, simpler sensors can be used. 
   EMBODIMENT 2 
   The present embodiment is configured to change the separation torque of the separation roll  55  according to the load torque of the transporting roll  53 , in addition to the control operations performed in Embodiment 1, so as to achieve appropriate separation/transport even in a case where a forward direction transport failure occurs on a transporting path between the press-contact portion  50 N of the separation section and the drawing roll  50 T. Practical control operations are described below with reference to  FIGS. 5A and 5B .  FIG. 5A  is a flowchart illustrating a control procedure.  FIG. 5B  is a table summarizingly describing the relation between the number of recording sheets and the value of separation torque. Incidentally, examples of the forward direction transport failure are an increase in electrostatic transporting resistance due to an electrostatic-adsorption force acting between recording sheets, and an increase in physical transporting resistance, such as connection, on the transporting path. 
   As illustrated in  FIG. 5A , first, in step ST 11 , according to a transport start instruction, the arrival detecting section detects whether a recording sheet P has arrived the drawing roll  50 T. 
   In a case where a leading end of the recording sheet P has arrived at the drawing roll  50 T, the arrival detecting section determines that there is no increase in the transporting resistance which becomes an impediment. Then, the apparatus performs an operation of controlling separation torque, similarly to the aforementioned Embodiment 1. More specifically, as illustrated in  FIG. 5   b , the separation torque is set at one of the values T 1 , T 2 , T 1   a , and T 2   a  at step ST 12  according to the presence/absence of occurrence of a slippage and to that of occurrence of a multiple-feed. 
   In a case where a leading end of a recording sheet P has not arrived at the drawing roll  50 T, next, the torque sensor provided at the transporting roll  53  detects load torque TL and determines in step ST 13  whether the value of the load torque TL is equal to or more than a predetermined value TL 0 . 
   In a case where the value of the detected load torque TL is less than the predetermined value TL 0 , a separation torque control operation similar to that performed in step ST 12  is performed in step ST 14 . 
   Conversely, in a case where the value of the detected load torque TL is equal to or more than the predetermined value TL 0 , it is determined that there is an increase in the transporting resistance, which is an impediment, on the transporting path between the separation portion and the drawing portion. Then, the separation torque of the separation roll  55  is increased. Consequently, a retarding pressure (i.e., a press-contact pressure exerted on the transporting roll  53  from the separation roll  55 ) is increased. Thus, a frictional force, which acts between the transporting roll  53  and the recording sheet P, is increased. Consequently, a forward-direction transporting force exerted on the recording sheet P is increased. That is, the transporting force is increased, as competition with the increase in the transporting resistance. 
   More specifically, in the aforementioned state, the presence/absence of a slippage is detected in step ST 15 . In a case where a slippage occurs, further, the presence/absence of a multiple-feed (i.e., whether plural recording sheets are present in the press-contact portion  50 N) is detected in step ST 16 . In a case where a multiple-feed occurs, the separation torque to be generated at the separation roll  55  is set at the value T 2  in step ST 17 . In a case where no multiple-feed occurs, the separation torque is set at the value T 1  in step ST 18 . 
   On the other hand, in a case where the occurrence of a slippage is detected in step ST 15 , further, the presence/absence of occurrence of a multiple-feed is detected in step ST 19 . In a case where a multiple-feed occurs, the separation torque to be generated at the separation roll  55  is increased and is set at a value T 2   b  (incidentally, T 2   b &gt;T 2 ) in step ST 20 . Similarly, even in a case where no multiple-feed occurs, the separation torque is increased and is set at a value T 1   b  (T 1   b &gt;T 1 ) in step ST 21 . Thus, the forward-direction transporting force exerted on the recording sheet P by the transporting roll  53  can be increased, as competition with the increase in the transporting resistance, by increasing the values of the separation torque to those T 1   b  and T 2   b  in the case where the load torque, whose value is equal to or more than the predetermined value TL 0  of the load torque is detected. Consequently, the possibility of eliminating the impediment to the forward-direction transport can be enhanced. 
   Subsequently, it is determined again in step ST 22  whether the value of the load torque TL is equal to or more than a predetermined value TL 0 . In a case where the value of the load torque TL is less than the predetermined value TL 0 , it is determined that the impediment to the forward-direction transport is eliminated, a separation torque control operation similar to that performed in Embodiment 1 is performed in step ST 23 . In a case where the value of the load torque TL is equal to or more than the predetermined value TL 0 , it is determined that the impediment, which cannot be eliminated by such a separation torque control operation, to the forward-direction transport is caused. Then, for example, an emergency stop of the apparatus is performed in step ST 24 . Incidentally, in a case where it is determined in step ST 22  that the value of the load torque TL is equal to or more than the predetermined value TL 0 , a cycle of steps ST 15  to ST 22  can be repeated a predetermined number of times so as to enhance the possibility of eliminating the impediment to the forward-direction transport still more. 
   Even in the present embodiment, it is sufficient for the multiple-feed detecting sensor to determine whether the number of recording sheets being present in the press-contact portion  50 N is one or more. Thus, a simpler sensor can be used. Further, when the slippage detecting section detects the presence/absence of occurrence of a slippage, it is preferable from the viewpoint of more effectively suppressing reduction in the transporting speed due to the slippage to detect the presence/absence of occurrence of a slippage with respect to the drawing rolls  50 T in a case where a recording sheet P has arrived at the drawing rolls  50 T. In a case where a recording sheet P has not arrived at the drawing rolls  50 T, it is preferable to detect the presence/absence of occurrence of a slippage with respect to the transporting rolls  53 . 
   Thus, according to the separation control operation in the present embodiment, the determination of the status of the load torque at the transporting roll  53  is added to the control operation performed in the aforementioned Embodiment 1. Consequently, even transport abnormality occurring between the separation roll  55  and the drawing roll  50 T can appropriately be controlled according to the cause thereof. Accordingly, reduction in the transporting speed of a recording sheet can be suppressed more effectively and stably. 
   EMBODIMENT 3 
   Generally, in a case where the number of recording sheets being present in the press-contact portion  50 N is large (i.e., equal to or more than 3), an apparatus failure, such as a jam, is liable to occur. Thus, it is preferable to increase the separation torque so that the multiply-fed sheets P 2 , P 3  . . . other than the uppermost recording sheet P 1  is immediately and reversely fed. 
   On the other hand, in a case where the multiply-fed recording sheets is 2, the separation torque provided to the separation roll  55  is liable to cause reciprocating motions of a second sheet P 2 , which is separated by the separation section from the uppermost recording sheet, to repeat motions of going into and out of the press-contact portion  50 N. According to the present inventor&#39;s study, it has been found that the reciprocating motions around the press-contact portion  50 N are propagated to the separation roll  55  and the transporting roll  53  as transient oscillations and result in reduction in separation performance of the separation section. 
   Thus, according to the present embodiment, the control operations according to the aforementioned Embodiment 2 are improved so that the number of recording sheets P, which are present in the press-contact portion  50 N, and that when the number of multiply-fed recording sheets is 2, a second sheet position control operation of stopping a second recording sheet at a predetermined position is performed. Practical control operations according to the present embodiment are described below with reference to  FIGS. 6A and 6B .  FIG. 6A  is a flowchart illustrating a control procedure according to the present embodiment.  FIG. 6B  is a table summarizingly describing the relation between the number of recording sheets and the value of separation torque according to the present embodiment. 
   As illustrated in  FIG. 6A , first, in step ST 31 , according to a transport start instruction, the arrival detecting section detects whether a recording sheet P has arrived the drawing roll  50 T. 
   In a case where a leading end of the recording sheet P has arrived at the drawing roll  50 T, the sheet number detecting sensor  52  determines the number of recording sheets P, which are present in the press-contact portion  50 N, in step ST 32 . In a case where the detected number of recording sheets is other than 2 (i.e., the detected number of sheets 0, 1, or 3 or more), the apparatus performs an operation of controlling separation torque, similarly to the aforementioned Embodiment 1 in step ST 33 . In a case where a multiple-feed occurs at that time (i.e., in a case where the detected number of recording sheets is 3 or more), the value of the separation torque generated at the separation roll  55  may be T 2  or T 2   a , similarly to the aforementioned Embodiment 1. However, preferably, the value of the separation torque generated at the separation roll  55  is a larger value T 3  or T 3   a  (incidentally, T 3 &gt;T 2 , T 3   a &gt;T 2   a ). 
   On the other hand, in a case where the detected number of recording sheets is 2, the following second sheet position control operation is performed. That is, in a case where it is detected that the number of multiply-fed recording sheets is 2, an amount of displacement of a second recording sheet, which is performed since a detection point of time, is calculated according to the rotation speed of the separation roll  55 . A sequentially variable control operation of the separation torque to be generated at the separation roll  55  is performed in step ST  34  so that the leading end of the second recording sheet P 2  is stopped at a predetermined position between the press-contact portion  50 N and the drawing roll  50 T. 
   Next, in a case where the leading end of the recording sheet P has not arrived at the drawing roll  50 T, the torque sensor  54  provided at the transporting roll  53  detects the load torque TL and determines in step ST 35  whether the value of the load torque TL is equal to or more than a predetermined TL 0 . 
   In a case where the value of the detected load torque TL is less than the predetermined value TL 0 , operations of setting the separation torque (at T 1 , T 3 , T 1   a , or T 3   a ) or second sheet position control operations are performed in steps ST 36  to ST 38  according to the number of recording sheets, which are present in the press-contact portion  50 N, similarly to processing performed in steps ST 32  to ST 34 . 
   Conversely, in a case where the value of the detected load torque is equal to or more than the predetermined value TL 0 , it is determined in step ST 39  whether the number of recording sheets P being present in the press-contact portion is 2. In a case where the number of recording sheets P is other than 2 (i.e., 0, 1, or 3 or more), the separation torque is set (at T 1 , T 1   a , T 1   b , T 3 , T 3   a , or T 3   b ) according to the presence/absence of occurrence of a slippage, and to the presence/absence of occurrence of a multiple-feed, similarly to Embodiment 2. At that time, in a case where a multiple-feed occurs (i.e., in a case where the number of recording sheets being present in the press-contact portion is 3 in the case of the present embodiment), the value of the separation torque to be generated at the separation roll  55  may be T 2 , T 2   a , or T 2   b , similarly to the aforementioned Embodiment 2. However, preferably, the value of the separation torque to be generated at the separation roll  55  is a larger value T 3 , T 3   a , or T 3   b  (incidentally, T 3 &gt;T 2 , T 3   a &gt;T 2   a , and T 3   b &gt;T 2   b ). On the other hand, in a case where the number of recording sheets P being present in the press-contact portion is 2, the aforementioned second sheet position control operation is performed in step ST 41 . 
   Thus, according to the present embodiment, the second sheet position control operation of performing the sequentially variable control of the separation torque, which is to be generated at the separation roll  55 , so that the leading end of the second recording sheet P 2  is stopped at the predetermined position between the press-contact portion  50 N and the drawing roll  50 T. Consequently, oscillations due to reciprocating motions, which are likely to occur in a case where the number of multiply-fed recording sheets is 2, can be prevented from occurring. Accordingly, reduction in the separation performance and the transporting performance can be prevented. 
   EMBODIMENT 4 
   Generally, a drawing failure occurs in a case where the friction coefficient of the drawing rolls  50 T is reduced, so that the forward-direction transporting force is less than the separation torque generated at the separation roll  55 . 
   Thus, according to the present embodiment, in a case where the uppermost recording sheet P 1  arrives at the drawing roll  50 T, and where a slippage occurs between this recording sheet P 1  and the drawing roll  50 T, the transporting force is assisted by the sheet feeding roll  50 P. A practical control operation according to the present embodiment is described below with reference to  FIG. 7 . Incidentally, it is assumed that in the present embodiment, the sheet feeding roll  50 P is in contact with the uppermost recording sheet P 1  until the leading end portion of a recording sheet P arrives at the drawing roll  50 T and that subsequently, the sheet feeding roll  50 P is separated from the recording sheet P. 
   First, in step ST 51 , it is detected whether the uppermost sheet P 1  of recording paper arrives at the drawing roll  50 T. 
   In a case where the recording sheet P 1  does not arrive at the drawing roll  50 T, a separation torque control operation similar to that performed in the aforementioned Embodiment 1 is performed in step ST 52 . 
   On the other hand, in a case where the sheet P 1  of recording paper has arrived at the drawing roll  50 T, a predetermined amount of rotation of the drawing roll  50 T is compared with a detected amount of rotation detected using the detection roll  51 . Thus, it is determined in step ST 53  whether a slippage occurs between the recording sheet P 1  and the drawing roll  50 T. 
   In a case where no slippage occurs, a separation torque control operation similar to that performed in Embodiment 1 is conducted in step ST 54 , similar to step ST 52 . 
   On the other hand, in a case where a slippage occurs, a press-contact force exerted by the sheet feeding roll SOP on the recording sheet P 1  is increased. Consequently, in step ST 55 , the transporting force exerted on the uppermost recording sheet P 1  is assisted by being increased. Accordingly, reduction in the transporting speed is suppressed. 
   A first modification of the present embodiment is configured so that at a moment, at which the recording sheet P 1  arrives at the press-contact portion  50 N, the press-contact force to be exerted on the recording sheet P 1  by the sheet feeding roll  50 P is reduced to a minimum level sufficient to the extent that the sheet feeding roll  50 P is separated from the recording sheet P 1 . In a case where the slippage is detected at the moment at which the recording sheet P 1  arrives at the drawing roll  50 T, the transporting force is assisted by changing and increasing the press-contact force exerted by the sheet feeding roll  50 P to a predetermined value (incidentally, this predetermined value is substantially equal to a value of the strength of the press-contact force at the start of the sheet-feeding or is a certain predetermined value). Consequently, according to the first modification, reduction in the transporting speed can be suppressed. 
   Although the present embodiment is configured so that the sheet feeding roll SOP is in contact with the recording sheet P 1  until the recording sheet P 1  arrives at the drawing roll  50 T, this example can be configured so that the sheet feeding roll  50 P is separated from the recording sheet P 1 , for example, at a time, at which the recording sheet P 1  arrives at the press-contact portion  50 N, so as to reduce a transporting load. 
   A modification in this case (i.e., a second modification) can be configured to assist the forward-direction transporting force by press-contacting the sheet feeding roll  50 P with the uppermost recording sheet P 1  in a case where a slippage occurs at the time, at which the recording sheet P 1  arrives at the drawing roll  50 T. 
   Each of the aforementioned examples can singly be implemented. Apparently, appropriate combinations of the aforementioned examples can be implemented. For example, in a case where a slippage is detected when the recording sheet P 1  arrives at the drawing roll  50 T, the separation torque of the separation roll  55  can be reduced, in addition to the assisting the transporting force with the sheet feeding roll  50 P.