Patent Publication Number: US-7909320-B2

Title: Feeding apparatus, and image forming apparatus incorporating feeding apparatus

Description:
RELATED APPLICATION 
     This is a divisional of application Ser. No. 11/183,837, filed on Jul. 19, 2005 now U.S. Pat. No. 7,429,040, and claims priority under 35 U.S.C. §119 to Japanese patent Application No. JP2004-340195 filed on Nov. 25, 2004, the contents of both of which are hereby incorporated by reference. 
    
    
     BACKGROUND 
     The present invention relates to a feeding apparatus for feeding sheets, such as original documents and image recording sheets, which applies to copiers, printers, facsimile machines and multi-functional machines which include the above apparatuses. 
     DESCRIPTION OF THE RELATED ART 
     Feeding apparatuses used in the image forming apparatus of a copier, and other image forming apparatuses are generally designed to supply a sheet from a sheet storage device one by one. Accordingly, developed have been technologies for preventing double feed being plural sheets fed at one time when a single paper sheet is intended to be fed. 
     In typical paper feeding apparatuses incorporating a device to prevent double feeding, a feeding roller structured by a friction roller, such as a rubber surfaced roller, feeds a single sheet of paper. Further the sheet of paper is separated into a single sheet by a driving mechanism having a torque limiter including a separation roller for separating sheets by rotating the separation roller, which is pressed onto the feeding roller, in a reverse direction against the rotational direction of the feeding roller. 
     Japanese non-examined Patent Publication No. H04-313548 discloses a technology which controls pressure of the separation roller based on occurrence probability of conveyance failure to prevent the double feeding in a friction separation system. Further Japanese non-examined Patent Publication No. H09-67037 discloses a technology which controls the threshold value of the torque limiter for transmitting rotational force to the separation roller based on detected results of friction coefficients between sheets, detected results of separation failures and conveyance rate. 
     According to the prior art described in the above patent references, the device for preventing double feed device is operated based on the results of detected conveyance failure, etc., and since it is not a device to prevent the double feeding of sheets during conveyance, the double feeding cannot be effectively prevented. 
     Further, an image forming apparatus, such as a copier, etc., including a feeding apparatus and an image reading apparatus operate under various situations. Accordingly, there is a case that double feeding and non-feeding, which is when no sheet is conveyed, occur based on ambient temperature. 
     SUMMARY 
     An object of the present invention is to provide a sheet feeing apparatus, which stably operates without being affected by an ambient temperature change. 
     An object of the present invention can be attained by a feeding apparatus including a feeding roller for conveying a sheet in a sheet feeding direction, a separation roller for conveying the sheet in a reverse direction against the sheet feeding direction based on rotational force transmitted trough a torque limiter, the separation roller being opposed to the feeding roller, a transmission mechanism for transmitting the rotational force to the feeding roller, a limited force reverse transmission mechanism for transmitting the rotational force to the separation roller through the torque limiter, a forced reverse transmission mechanism for transmitting the rotational force to the separation roller without passing through the torque limiter, a temperature detector for detecting ambient temperature and a controller for selecting a first feeding operation or a second feeding operation based on detected information from the temperature detector, wherein the first feeding operation is to rotate the feeding roller by applying the transmission mechanism and to temporarily rotate the separation roller by applying the forced reverse transmission mechanism after the limited force reverse transmission mechanism transmits the rotational force to the separation roller, then the limited force reverse transmission mechanism transmits the rotational force to the separation roller, and the second feeding operation is to rotate the feeding roller by applying the transmission mechanism and the limited force transmission mechanism transmits the rotational force to the separation roller. 
     Another object of the present invention can be attained by a feeding apparatus including a feeding roller for conveying a sheet in a sheet feeding direction, a separation roller for conveying the sheet in a reverse direction against the sheet feeding direction based on rotational force transmitted trough a torque limiter, the separation roller being opposed to the feeding roller, a transmission mechanism for transmitting the rotational force to the feeding roller, a first sensor for detecting existence of the sheet, the first sensor being arranged downstream against the feeding roller in a sheet conveyance direction, a temperature detector for detecting ambient temperature, and a controller for controlling a feeding operation based on detected information from the temperature detector, 
     wherein the controller temporarily stops transmitting the rotational force to the separation roller in case the first sensor does not detect the sheet within a first predetermined time interval, which is a time interval from a moment when starting feeding operation or from a reference time set after starting the feeding operation to a moment when the first predetermined time interval has elapsed, and re-transmits the rotational force to the separation roller. 
     Another object of the present invention can be attained by a feeding apparatus including, a feeding roller for conveying a sheet in a sheet feeding direction, a separation roller for conveying the sheet in a reverse direction against the sheet feeding direction based on rotational force transmitted through a torque limiter, the separation roller being opposed to the feeding roller, a transmission mechanism for transmitting the rotational force to the feeding roller, a limited force reverse transmission mechanism for transmitting the rotational force to the separation roller through the torque limiter, a forced reverse transmission mechanism for transmitting the rotational force to the separation roller without passing through the torque limiter, a temperature detector for detecting ambient temperature, and a controller for selecting a third feeding operation or a fourth feeding operation based on detected information from the temperature detector, wherein the third feeding operation is to rotate the feeding roller by applying the transmission mechanism and to rotate the separation roller by applying the forced reverse transmission mechanism for a fourth predetermined time interval after the limited force reverse transmission mechanism transmits the rotational force to the separation roller, and the fourth feeding operation is to determine whether the leading edge of the sheet reaches to a predetermined place within a first predetermined time interval which is measured from a moment when starting feeding operation or from a reference time set after starting the feeding operation to a moment when the leading edge of the sheet reaches to the predetermined place, after the transmission mechanism rotates the feeding roller and the limited forced reverse transmission mechanism rotates the separation roller, and when the sheet has reached to the predetermined place, the feeding operation is continued, and when the sheet has not reached to the predetermined place, the rotational force is re-transmitted to the separation roller after the transmission of the rotational force to the feeding roller is temporarily stopped, and the separation roller rotates following to the feeding roller. 
     Another object of the present invention can be attained by a feeding apparatus including a feeding roller for conveying a sheet in a sheet feeding direction, a separation roller for conveying the sheet in a reverse direction against the sheet feeding direction based on rotational force transmitted trough a torque limiter, the separation roller being opposed to the feeding roller, a transmission mechanism for transmitting the rotational force to the feeding roller, a limited force reverse transmission mechanism for transmitting the rotational force to the separation roller through the torque limiter, a forced reverse transmission mechanism for transmitting the rotational force to the separation roller without passing through the torque limiter, a temperature detector for detecting ambient temperature, and 
     a controller for selecting a first feeding operation or a second feeding operation based on detected information from the temperature detector, wherein the first feeding operation is to rotate the feeding roller by applying the transmission mechanism and to temporarily rotate the separation roller by applying the forced reverse transmission mechanism after the limited force reverse transmission mechanism transmits the rotational force to the separation roller, and then the limited force reverse transmission mechanism transfers rotational force to the separation roller, and the second feeding operation is to rotate the feeding roller by applying the transmission mechanism and to transmit the rotational force to the separation roller by applying limited force reverse transmission mechanism. 
     Another object of the present invention can be attained by an image forming apparatus including a feeding roller for conveying a sheet in a sheet feeding direction, a separation roller for conveying the sheet in a reverse direction against the sheet feeding direction based on rotational force transmitted trough a torque limiter, the separation roller being opposed to the feeding roller, a transmission mechanism for transmitting the rotational force to the feeding roller, a first sensor for detecting existence of the sheet, the first sensor being arranged downstream against the feeding roller in a sheet conveyance direction, a temperature detector for detecting ambient temperature, and a controller for controlling a feeding operation based on detected information from the temperature detector, wherein the controller temporarily stops transmitting the rotational force to the separation roller in case the first sensor does not detect the sheet within a first predetermined time interval being a time interval from a moment when starting feeding operation or from a reference time set after starting the feeding operation to a moment when the first predetermined time interval has elapsed, and re-transmits the rotational force to the separation roller. 
     Another object of the present invention can be attained by an image forming apparatus including a feeding roller for conveying a sheet in a sheet feeding direction, a separation roller opposed to the feeding roller, the separation roller being arranged to receive rotational force for rotating the separation roller in a reverse direction against the sheet feeding direction through a torque limiter, a transmission mechanism for transmitting the rotational force to the feeding roller, a limited force reverse transmission mechanism for transmitting the rotational force to the separation roller through the torque limiter, a forced reverse transmission mechanism for transmitting the rotational force to the separation roller without passing through the torque limiter, a temperature detector for detecting ambient temperature, and a controller for selecting a third feeding operation or a fourth feeding operation based on detected information from the temperature detector, wherein the third feeding operation is to rotate the feeding roller by applying the transmission mechanism and to rotate the separation roller by applying the forced reverse transmission mechanism for a fourth predetermined time interval after the limited force reverse transmission mechanism transmits the rotational force to the separation roller, and the fourth feeding operation is to rotate the feeding roller by applying the transmission mechanism and to determine whether the leading edge of the sheet reaches to a predetermined place within a first predetermined time interval being a time interval from a moment when starting feeding operation or from a reference time set after starting the feeding operation to a moment when the first predetermined time interval has elapsed, and when the sheet has reached to the predetermined place, the feeding operation is continued, and when the sheet has not reached to the predetermined place, the rotational force is re-transmitted to the separation roller after the transmission of the rotational force to the separation roller is temporarily stopped, and the separation roller rotates following to the feeding roller. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates an overall view of an image forming apparatus employing the feeding apparatus of the invention. 
         FIG. 2  illustrates a side view of embodiments 1, 2 and 4. 
         FIG. 3  illustrates a front view of embodiment 1 of the invention. 
         FIG. 4  illustrates a block diagram of embodiment 1 showing the circuit configuration of the feeding apparatus of the invention. 
         FIG. 5  illustrates a flow chart of embodiment 1 of the feeding apparatus of the invention. 
         FIG. 6  illustrates a side view of embodiment 2 of the feeding apparatus of the invention. 
         FIG. 7  illustrates a circuit diagram of embodiment 2 of the present invention. 
         FIG. 8  illustrates a flow chart of embodiment 2 of the feeding apparatus of the invention. 
         FIG. 9  illustrates a side view of embodiment 3 of the present invention. 
         FIG. 10  illustrates a circuit diagram of embodiment 3 of the present invention. 
         FIG. 11  illustrates a flow chart of embodiment 3 of the feeding apparatus of the invention. 
         FIG. 12  illustrates a front view of embodiment 4 of the invention. 
         FIG. 13  illustrates a circuit diagram of embodiment 4 of the present invention. 
         FIG. 14  illustrates a flow chart of embodiment 4 of the feeding apparatus of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiments of the present invention will be described below. However, the present invention is not limited to the embodiments. 
     &lt;Image Forming Apparatus&gt; 
       FIG. 1  illustrates an overall view of an image forming apparatus employing the feeding apparatus of the invention. 
     Automatic original document feeder  1  comprises original document table  11  for storing original documents, original document separating device  12  for separating original documents placed on original document table  11 , original document conveyance device  13  including plural rollers for conveying original documents separated by original document separating device  12 , original document ejecting device  14  for ejecting original documents conveyed by original document conveyance device  13 , ejected original document table  15  for storing original documents ejected by original document ejecting device  14  and original document flipping device  16  structured by a pared rollers for flipping the front and the back of a double sided original document. 
     Original document separating device  12  separates plural original documents (not shown) placed on original document placing table  11  into individual original document sheets one by one. After which, original document conveyance device  13  conveys the original document to the image reading section. 
     The image reading section is provided under original document conveyance device  13 . Slit  21  of image reading apparatus  2  formed in the image reading section is designed to read images on the original document. 
     Automatic original document feeder  1  is structured to pivot upward. Automatic original document feeder  1  is designed to lift automatic original document feeder  1  to clear the area on platen glass  22  so that the original document directly placed can be copied. 
     Image reading apparatus  2  is an apparatus for obtaining image data by reading images on the original documents. Image reading apparatus  2  comprises first mirror unit  23  structured by lamp  231  for irradiating the original document through slit  21  and first mirror  232  to reflect reflected light from the original document, second mirror unit  24  structured by second mirror  241  for reflecting reflected lights from first mirror  232  and third mirror  242 , imaging lens  25  for forming images on a CCD being a solid state image sensor and CCD  26  structured in a line shape to convert an optical image into electric signals. 
     Analog signals are converted from optical signals to electric signals via CCD  26 , which then converted into digital signals (A/D conversion). Then the digital signals are stored in an image memory as image data after image processing is conducted on the digital signals. 
     In an embodiment of the invention where a original document sent by automatic original document feeder  1  is read by image reading apparatus  2 , first mirror unit  23  and second mirror unit  24  are located as shown in  FIG. 1 . 
     An original document placed on platen glass  22  is read by moving first mirror unit  23  and second mirror unit  24  while maintaining a constant distance between first mirror unit  23  and second mirror unit  24 . 
     Image forming section  3  comprises photosensitive drum  31  incorporating a photoconductive sensitization layer on its surface serving as an image carrier, charging device  32  including a charging electrode for uniformly charging the surface of photosensitive drum  31 , writing device  33 , being an exposing device for forming an electrically static latent image (it is also simply called a latent image) by exposing the surface of photosensitive drum  31  based on image data after completing the image processing, developing apparatus  34  forming a toner image by reversibly developing the latent image formed on photosensitive drum  31 , transferring device  35  for transferring the toner image onto a normal paper sheet being a normal paper sheet (which will simply be call a sheet hereinafter), separating device  36  for separating the sheet from photosensitive drum  31  by conducting corona discharge based on AC voltage (alternate current) or DC voltage onto which AC is superimposed, onto a back side surface of the sheet onto which toner images are transferred and cleaning device  37  which cleans photosensitive drum  31  after completing a transferring process. 
     Ejecting roller  38  is a roller for conveying the paper sheet after separation to a fixing apparatus, for example fixing apparatus  9  featuring a heating roller and ejecting roller  38  is a roller for ejecting a paper sheet. 
     Fixing apparatus  9  collectively comprises heating source H, upper fixing roller  900  and lower fixing roller  903  which rotates while contacting with upper roller  900  as main components, and switching device  62 , which changes the paper conveyance route. 
     The image forming process described above is performed via the following steps including, forming a latent image corresponding to the original document image via dot exposure by writing device  33  after charging photosensitive drum  331  via charging device  32 , then, developing the latent image into a toner image via developing apparatus  34 . After that, transferring device  35  transfers the toner image onto a paper sheet fed via registration roller  56  as a second feeding device when registration roller  56  starts rotating. 
     Practically, the process to start development of the toner image on photosensitive drum  31  is arranged to synchronize with the timing of paper feeding operation associated with the rotation start of registration roller  56  under the condition that the paper sheet has reached to registration roller  56 . 
     Accordingly, the distance from the writing position on photosensitive drum  31  via writing device  33  is designed to be equal to the distance from the registration roller  56  to transferring device  35  and the linear velocity of photosensitive drum  31 , registration roller  56  and pre-transferring roller  57  are set to be equal. 
     The paper sheet on to which the toner images have been transferred is separated from photosensitive drum  31  by the action of separating device  36  and ejected out of the apparatus after completion of heating-and-pressing action of fixing apparatus  9 . 
     Photosensitive drum  31  continues rotating after passing through a transfer area and preparing for the next image formation after cleaning device  37  removes any residual toner remaining on photosensitive drum  31 . 
     Continuing of the description of the structure of the image forming apparatus, three separate sheet-feeding trays  40  for storing sheets PA provided in a vertically stacked state are arranged in paper storing section  4 , and each sheet-feeding tray  40  is designed to be easily removed. 
     Each sheet-feeding tray  40  includes feeding apparatus  100 , which feeds sheet PA, and sheet conveyer  5  conveys sheet PA to image forming section  3 . Feeding apparatus  100  will be described later. 
     Paired conveyance rollers  55  are located at the junction of paper-feeding section  5  and reverse conveyance section  8 . 
     Reverse ejection/re-feeder  6  functions to turn over sheet PA after completing of the transferring and fixing processes and re-feeds sheet PA based on the double sided image forming mode. Reversing conveyance section  8  has a function to turn over the sheet PA fed into reversing ejection-and-re-feeding section  6  and to covey to paired registration rollers  56 . However, since these are not directly related to the invention, detailed description will be omitted. 
     Temperature sensor TS is a sensor serving as a temperature detecting device for detecting temperature of an inside of an image forming apparatus. In the example shown in  FIG. 1 , temperature sensor TS is provided to detect the surface temperature of photosensitive drum  31  in a position adjacent to an exposing section. 
     Embodiment 1 
     Embodiment 1 will now be described by referring to  FIGS. 2-5 .  FIG. 2  is a side view of feeding apparatus  100 . 
     As shown in  FIG. 2 , a feeding apparatus  100  is arranged adjacent to a sheet-feeding tray  40  capable of storing a large number of sheets of sheet PA stacked horizontally in the sheet-feeding tray  40 . A sheet feeding roller  104  for feeding sheet PA into the feeding apparatus  100  is arranged over the sheet-feeding tray  40 . The sheet feeding roller  104  is in contact with sheet PA on the top sheet-feeding tray  40  due to the own weight. The sheet feeding roller  104  is arranged to rotate in a counterclockwise and to basically feed a single sheet PA on the sheet-feeding tray  40  by rotating while being in contact with the sheet placed on the top of the sheet-feeding tray  40  into the feeding apparatus  100  one by one. 
     A sheet feeing roller  105  for conveying sheet PA sent by the sheet feeding roller  104  to the next process is provided downstream of the sheet feeding roller  104 , in the conveyance direction of sheet PA. A separation roller  106  for separating a sheet from sheets PA located in a lower portion of plural sheets PA when they are fed in an overlapped state. The sheet feeding roller  105  is in contact with the separation roller  106  on their outer periphery. The sheet feeding roller  105  is arranged to rotate in a counterclockwise, the same as the sheet feeding roller  104  as shown in  FIG. 2  and to convey sheets PA fed by the sheet feeding roller  104  from the left side to the right end in  FIG. 2 . 
     The separation roller  106  in  FIG. 2  is arranged to have rotational force to rotate counterclockwise to covey sheets PA by the sheet feeding roller  105  in the reverse direction of the feeding direction. In  FIG. 2 , a torque limiter  107  for limiting rotational force to be transmitted to the separation roller  106  is provided on the right end of the separation roller  106 . Accordingly, the separation roller  106  constantly rotates in a counterclockwise with limited rotational force. Further, with regard to the sheet feeding roller  105  and the separation roller  106 , the diameter of the sheet feeding roller  105  is arranged to be relatively larger than that of the separation roller  106 . The outer periphery of the sheet feeding roller  105  is structured by a material whereby the friction coefficient of the outer periphery of the feeding roller  105  against sheet PA is greater than the friction coefficient of outer periphery of the separation roller  106  against sheet PA. The tolerance for frictional wear can be broadened by setting the diameter of the sheet feeding roller  105  larger than that of the separation roller  106 . It therefore becomes possible to securely feed a sheet while preventing superposition of other sheets by setting the friction coefficient against sheets PA of the sheet feeding roller  105  to be greater than that of the separation roller  106 . Rubber structures the outer peripheries of both sheet feeding roller  105  and separation roller  106 . As an example, the outer periphery of the sheet feeding roller  105  can be structured by ethylene-propylene rubber (EPM, EPDM), while the outer periphery of the separation roller  106  can be structured by urethane rubber. However it is possible that the same kind of rubber or different types of rubber as described above may structure the sheet feeding roller  105  and the separation roller  106 . It is preferable that the rubber applied for the outer periphery of the separation roller  106  is harder than that of the sheet feeding roller  105 . The frictional wear can be suppressed by applying a harder rubber on the outer periphery of the separation roller  106  than that of the sheet feeding roller  105 . 
     A conveyance roller  108  for conveying sheets PA conveyed by the sheet feeding roller  105  to the next process, and a following roller  109  rotating following to the rotation of the conveyance roller  108  are provided downstream of the sheet feeding roller  105  and the separation roller  106  located in the conveyance direction of sheet PA. The conveyance roller  108  and the following roller  109  are in contacted at the outer peripheries of both rollers. The conveyance roller  108  is arranged to rotate in counterclockwise, being same rotational direction of the abovementioned sheet feeding roller  105  as shown in  FIG. 2 . The conveyance roller  108  and the following roller  109  are arranged to covey sheets PA from the conveyance roller  105  from left to right as shown in  FIG. 2 . The following roller  109  is arranged to freely rotate clockwise following to the rotation of the conveyance roller  108  as shown in  FIG. 2 . 
     Three guide plates  110 ,  111  and  112  for guiding sheets PA from left to right in  FIG. 2  are provided along the conveyance path of sheets PA. Each guide plate  110 ,  111  or  112  is extended from the front to the rear (or from the rear to the front) of  FIG. 2 . A guide plate  110  enables a conveyance of sheets PA from the sheet tray  40  to the separation roller  106 , the guide plate  111  enables the conveyance path from the separation roller  106  downstream of the following roller  109  and the guide plate  112  enables the conveyance path from the above the mid-point of the guide plate  110  to end of the guide plate  111 . According to the embodiment, sheets PA are arranged to be conveyed from the sheet-feeding tray  40  to the outside of the feeding apparatus  100  via the sheet feeding roller  105 , the separation roller  106 , the conveyance roller  108  and the following roller  109  while sheets PA are guided by the guide plates  110 ,  111  and  112 . 
     Further, three sensors  113 ,  114  and  115  are provided along the conveyance path of sheets PA. A first sensor  113  is provided adjacent to the conveyance roller  108  and in upstream of the conveyance roller  108  in the conveyance direction of sheets PA. A second sensor  114  is provided adjacent to the conveyance roller  105  and upstream of the first sensor  113  in a conveyance direction of sheets PA. A third sensor  115  is provided adjacent to the conveyance roller  108  and downstream of the conveyance roller  108  in the conveyance direction of sheets PA. Particularly, the second sensor  114  is arranged so that the minimum distance between the second sensor  114  to the conveyance path is equal to or less than the radius of the sheet feeding roller  105 , so as to instantly detect the presence of a sheet passing the contacting area of the sheet feeding roller  105  and the separation roller  106 . 
     The configuration of a driving system of feeding apparatus  100  will be described by referring to  FIG. 3 . 
       FIG. 3  illustrates a front view of the feeding apparatus  100 . 
     The feeding apparatus  100  includes a motor  120  as a driving source of the sheet feeding roller  104 , the sheet feeding roller  105  and the separation roller  106 . A gear  121  is fixed on the output shaft of the motor  120 . The gear  121  is geared to a transmission gear  122 , which transmits rotational force of the motor  120  to all rollers, such as, the sheet feeding roller  104 , the sheet feeding roller  105  and the separation roller  106 . The feeding apparatus  100  includes a transmission mechanism  116 A for feeding sheets PA from the sheet tray  40  by transmitting rotational force of the motor  120  from the gear  122  to the sheet feeding roller  104  and the sheet feeding roller  105 , and the feeding transmission mechanism  116 B for returning sheets PA to the sheet tray  40  by transmitting rotational force of the motor  120  from the transmission gear  122  to the separation roller  106 . 
     The transmission mechanism  116 A comprises a shaft  123  which can freely rotate. An electro-magnetic clutch  124  is provided at the left edge of the shaft  123  in  FIG. 3 , and the electro-magnetic clutch  124  switches whether transmitting the rotational force of the motor  120  to the sheet feeding roller  104  and the sheet feeding roller  105  or not. A clutch gear  125  of the electro-magnetic clutch  124  engages the transmission gear  122 . The shaft  123  and a rotational shaft  126  of the sheet feeding roller  104  respectively have pulleys  127  and  128 , each of which is fixed on to the respective shafts. Endless belt  129  is entertained around respective pulleys  127  and  128 . 
     In the transmission mechanism  116 A, when the electro-magnetic clutch  124  is engaged, while the motor  120  is in rotation operation, and since the shaft  123  and the electro-magnetic clutch  124  are geared to transmit rotational force of the motor  120  to the shaft  123  via the transmission gear  122  and the clutch gear  125 , the shaft  123  rotates. While the shaft  123  rotates, the sheet feeding roller  105  rotates and the rotational force is, transmitted to the rotational shaft  126  via the belt  129 . Accordingly, the sheet feeding roller  104  and the sheet feeding roller  105  simultaneously rotate in the same direction. 
     On the other hand, when the motor  120  is in operation and the electro-magnetic clutch  124  is turned off to stop operation, the connection between the shaft  123  and the electro-magnetic clutch  124  is released and the clutch gear  125  freewheels. As a result, the rotation of shaft  123  comes to stop. At this time, since rotational force of the transmission gear  122  to the shaft  123  is turned off, the sheet feeding roller  104  and the sheet feeding roller  105  also simultaneously stops. Namely, in the transmission mechanism  116 A, the sheet feeding roller  104  synchronizes with the sheet feeding roller  105  to conduct operation based on the engagement or disengagement of the electro-magnetic clutch  124 . 
     Shafts  130  and  131  capable of rotating around the center axis of each shaft are provided in a limited reverse transmission mechanism  116 B and a forced reverse transmission mechanism  116 C. Gears  132  and  133  having different diameters, at a predetermined interval between them, are provided at the left end of shaft  130  as shown in  FIG. 3 . The Gear  132  has a larger diameter than that of the gear  133  and engages the transmission gear  122 . A torque limiter  107  is provided on the right end of the shaft  130  in  FIG. 3 . A gear  134  is provided on the left end of the torque limiter  107  and engages a gear  135 . A gear  136  is provided on the rotational shaft of the separation roller  106  and the gear  136  engages the gear  135 . 
     In the limited reverse transmission mechanism  116 B including the shaft  130 , rotational force of the motor  120  is transmitted to the shaft  130  via the gears  122  and  132  to rotate the shaft  130 . Once the shaft  130  rotates, the rotational force is transmitted to the separation roller  106  under condition limited by the torque limiter  107  via the gears  134 ,  135  and  136 . The separation roller  106  is arranged to rotate in a reverse direction to the paper feeding direction set by the sheet feeding roller  104  and the sheet feeding roller  105 . Namely, since the torque limiter is provided between the shaft  130  and the separation roller  106  in the limited reverse transmission mechanism  116 B which uses the shaft  130 , the full rotational force of the motor  120  is not transmitted to the separation roller  106  and thereby limited but constant rotational force is transmitted from the motor  120  to the separation roller  106 . 
     In the forced reverse transmission mechanism  116   c , an electro-magnetic clutch  137  functions as being a forced reverse transmission electro-magnetic clutch to switch the selection whether forcefully transmit rotational force of the motor  120  to the separation roller. The clutch gear  138  of the electro-magnetic clutch  137  engages gear  133 . In  FIG. 3 , a gear  139  is provided on the right end of the shaft  131  and the gear  139  engages the gear  134  of the torque limiter  107 . 
     In the forced reverse transmission mechanism  116 C applying the shaft  131 , when the electro-magnetic clutch  137  is turned on, while the motor  120  is in rotation operation, since the shaft  131  and the electro-magnetic clutch  137  are connected, rotational force of the motor  120  is transmitted to the shaft  131  via the transmission gears  132 ,  133  and the clutch gear  138 , whereby the shaft  131  rotates. Once the shaft  131  rotates, rotational force is transmitted to the separation roller  106  via gears  134 ,  135  and  136 . The separation roller  106  rotates so that the separation roller  106  conveys sheets PA to a reverse direction against paper feed direction by the sheet feeding roller  104  and the sheet feeding roller  105 . 
     On the contrary, the electro-magnetic clutch  137  is turned off to stop operation of the electro-magnetic clutch  137  while the motor  120  is in non-operation, and the connection between the shaft  131  and the electro-magnetic clutch  137  comes to release. As a result, the clutch gear  138  freewheels and rotational force from the shaft  131  to the separation roller  106  is intercepted. 
     In the abovementioned limited reverse transmission mechanism  116 B, rotational force of the motor  120  is transmitted to the separation roller  106  via the torque limiter  107 . In the abovementioned forced reverse transmission mechanism  116 C, rotational force of the motor  120  is transmitted to the separation roller  106  without via the torque limiter  107 . 
     Further, even though it is not shown in  FIGS. 2 and 3 , the feeding apparatus  100 , separately includes members, such as the shaft, the gear and an electro-magnetic clutch  141  (please refer to  FIG. 4 ) for transmitting rotational force of the motor  120  to the sheet feeding roller  104 , the sheet feeding roller  105  and the separation roller  106 , the same as the abovementioned sheet feeding mechanism  116 A. The conveyance roller  108  is arranged to freewheel based on an operation of the electro-magnetic clutch  141 . 
     A circuit configuration of the feeding apparatus  100  will be described by referring to  FIG. 4 . 
       FIG. 4  is a block diagram showing circuit configuration of the feeding apparatus  100 . 
     A controlling device  150  includes a controller  151  configured by a general-purpose CPU (Central Processing Unit), a ROM (Read Only Memory), a RAM (Random Access Memory) and a circuit, such as a non-volatile memory (not shown). The controller  151  is designed to transfer a processing-program from the ROM to the RAM whereby the CPU executes the processing program. 
     Concretely, the controller  151  is arranged to control the motor  120  and respective components connected to the electro-magnetic clutches  124 ,  137  and  141  based on the processing program. Particularly, in the feeding apparatus  100 , the sensors  113 - 115  are connected to the controller  151  which is arranged to control above clutches  124 ,  137  and  141  based on the detected results of the sensors  113 - 115 . 
     A temperature sensor TS is a temperature detecting device for detecting ambient temperature associated with the image forming apparatus. In  FIG. 1 , the temperature sensor TS detects the surface temperature of the photosensitive drum  31 . However, it is possible to appropriately set the temperature sensor TS in other places in the image forming apparatus. The temperature sensor TS is connected to the controller  151  which is arranged to control the electro-magnetic clutch  137  based on the detected results of the temperature sensor TS. Concretely, the controller  151  is arranged to operate the electro-magnetic clutch  137  for a fourth predetermined time interval when the temperature is not less than the first predetermined temperature, and not to operate the electro-magnetic clutch  137  when the temperature is less than the first predetermined temperature. 
     Sheet feeding operation of the feeding apparatus  100  will be described by referring to  FIG. 5 . 
       FIG. 5  is a flow chart showing tasks, which have been divided into plural processes according to elapsed time. 
     The motor  120  starts rotating (step  1 ) when a command signal for feeding sheets PA is inputted to the controller  151  of the control device  150 . In the limited reverse transmission mechanism  116 B of the fist transmission mechanism shaft  130 , rotational force of the motor  120  is transmitted to the shaft  130  via the transmission gear  122  and the gear  132 , thereby the shaft  130  starts rotating based on this command. Then, the rotational force is transmitted to the separation roller  106  under the condition that the rotational force is limited by the torque limiter  107 , and transmitted to the shaft  130  via the gears  134 ,  135  and  136 . As a result the separation roller  106  rotates counterclockwise as shown in  FIG. 2 . At this moment, in the transmission mechanism  116 A, since the electro-magnetic clutch  124  does not operate, the shaft  123  is in a state in which the shaft  123  freely rotes, and the sheet feeding roller  105  rotates according to the rotation of the separation roller  106 . 
     After that, the electro-magnetic clutch  124  operates to connect the shaft  123  to the electro-magnetic clutch  124  and the rotational force of the motor  120  is transmitted to the shaft  123  via the transmission gear  122  and the clutch gear  125 . Then the shaft  123  starts rotating. As a result, the rotational force is transmitted to the sheet feeding roller  104  and the sheet feeding roller  105 , and the sheet feeding roller  104  and the sheet feeding roller  105  simultaneously start rotating in counterclockwise as shown in  FIG. 2  (step  2 ). At this moment, even though the rotational force of the motor  120  is transmitted to the separation roller  106  via the torque limiter  107 , since the rotational force of the motor  120  is transmitted to the paper feeding roller  106  as it has been by the operation of the electro-magnetic clutch  124 , rotational force of the sheet feeding roller  105  is greater than that of the separation roller  106 . Consequently, the separation roller  106  being in contacted with the sheet feeding roller  105  rotates according to the rotation of the sheet feeding roller  105 . 
     When the sheet feeding roller  104  starts rotating, a single of sheets PA placed on the top of sheet-feeding tray  40  is sent to contacting portion of the sheet feeding roller  105  and the separation roller  106  from the sheet-feeding tray  40 . The single of sheets PA is conveyed to the conveyance roller  108  side while being sandwiched by the sheet feeding roller  105  and the separation roller  106 . 
     After that, the controller  151  determines whether any sheets PA exist based on the output of the second sensor  114  (step S 3 ). When the second sensor  114  does not detect sheets PA (step S 3 : No), the operation of step S 3  continues until the second sensor  114  detects sheet PA. When the second sensor  114  detects the leading edge of sheets PA (step S 3 : Yes), the controller  151  determines whether the temperature measured by the temperature sensor TS is not less than first predetermined temperature T 1  (step S 4 ). 
     When temperature is not less than first predetermined temperature T 1  (step S 4 : YES), the rotational force of the motor  120  is transmitted to the shaft  131  via the transmission gear  122  and the gears  132 ,  133  and the clutch gear  138  by the operation of the electro-magnetic clutch  137  for connecting the shaft  131  to the electro-magnetic shaft  137 . Consequently, the shaft  131  starts rotating. Then the rotational force is transmitted to the separation roller  106  from the gear  139  via the gears  134 ,  135  and  136 . The rotational force is applied to the separation roller  106  so that the separation roller  106  conveys sheets PA in a direction which is reverse to the paper feeding direction of the sheet feeding roller  105 . 
     Before the electro-magnetic clutch  137  starts operation, the rotational force of the shaft  130  is transmitted to the separation roller  106  via the torque limiter  107 . Once the electro-magnetic clutch  137  operates in the operation of step S 5 , the separation roller  106  is forcefully driven by the forced reverse transmission mechanism  116 C. Accordingly, the sheet feeding roller  105  and the separation roller  106  acts against sheet PA with the conveyance forces being reverse directions each other in the operation of step S 5 . 
     In this situation described above, when a single sheet of sheets PA is fed from the sheet-feeding tray  40 , since the outer periphery of the sheet feeding roller  105  is structured by a material having a larger friction coefficient than that of the separation roller  106 , the separation roller  106  rotates as the separation roller  106  slips on the lower surface of the sheet while the sheet feeding roller  105  continue to convey the sheet to the side of conveyance roller  108 . When plural sheets PA being overlapped are sent out from the sheet feeding tray  40 , the sheet feeding roller  105  continues rotating counterclockwise to convey the sheet of sheets PA positioned on the top of sheets PA to the conveyance roller  108  side as shown in  FIG. 2 . The separation roller  106  continues rotating counterclockwise as shown in  FIG. 2 , and to convey the sheets positioned in the lower side of sheet PA to the sheet feeding tray  40  while being in touch with double fed sheets PA. Namely, when the electro-magnetic clutch  137  operates in the operation of step S 5  and overlapped plural sheets PA are fed out from the sheet feeding tray  40 , a sheet positioned on the top of the plural sheets PA is sent out to the conveyance roller  108  side. The other sheets PA are separated from the sheet positioned on the top and returned to the sheet feeding tray  40  side. 
     In the operation of step S 5 , the electro-magnetic clutch  137  continues operating for a fourth predetermined time interval and the operation of the electro-magnetic clutch  137  stops when the fourth predetermined time interval has passed (step S 6 ). Then, the connection between the shaft  131  and the electro-magnetic clutch  137  is released. As a result, the clutch gear  138  freewheels and the transmission of rotational force from the shaft  131  to the separation roller  106  is intercepted. Namely, it comes to a state that the rotational force being transmitted through the torque limiter  107  to the shaft  130  is transmitted to the separation roller  106 . 
     When the temperature is less than a predetermined temperature T 1  in step S 4  (step S 4 : NO), steps S 5  and S 6  described above do not operate and will be omitted. 
     After that, the controller  151  determines whether the first sensor  113  detects a sheet or not. When first sensor  113  does not detect sheets PA (step S 7 : No), the operation of step S 7  is repeated until the first sensor  113  detects the edge of sheets PA. When the first sensor  113  detects the edge of sheets PA (step S 7 : YES), the electro-magnetic clutch  141  (refer to  FIG. 7 ) operates and the conveyance roller  108  rotates in a counterclockwise as shown in  FIG. 2  (step S 8 ). Then, the edge of sheets PA being passed through the pressing section of the sheet feeding roller  105  and the separation roller  106  is sandwiched between the conveyance roller  108  and the following roller  109 . As a result, the sheet PA passes through the contacting portion of the conveyance roller  108  and the following roller  109  in accordance with the rotation of the conveyance roller  108 . 
     After that, the operation of the electro-magnetic clutch  124  stops (step S 9 ). Then the connection between the shaft  123  and the electro-magnetic clutch  124  is released and the clutch gear  125  freewheels. The transmission of rotational force from the transmission gear  122  to the sheet feeding roller  104  and the sheet feeding roller  105  is intercepted. 
     After that, the controller  151  determines whether the third sensor  115  detects any sheet PA (step S 10 ). When the third sensor  115  does not detect any sheets PA (step S 10 : NO), the operation of step S 10  repeats until the third sensor  115  detects sheets PA. When the third sensor  115  detects sheets PA (step S 10 : YES), controller  151  determines whether the third sensor  115  detects any sheet PA again (step S 11 ). While the third sensor detects sheet PA (step S 11 : NO), the operation of step S 11  continues. 
     When the third sensor  115  comes to the condition that the third sensor  115  does not detect any sheets PA (step S 11 : YES), namely, the rear edge of sheet PA passes through the portion where the third sensor  115  detects sheet PA, the operation of the electro-magnetic clutch  141  stops (step S 12 ). Then the rotational force from the motor  120  to the conveyance roller  108  is intercepted. 
     After that, the controller  151  determines whether there is a signal for directing the next sheet feeding operation (step S 13 ). When the signal for directing the next sheet feeding operation has been inputted (step S 13 : YES), the abovementioned operations of steps  2 - 12  repeat. When no signal for direction the next sheet feeding operation has been inputted (step S 13 : NO), the rotation of the motor  120  stops (step S 14 ) and the operations of the feeding apparatus  100  finishes. 
     In the feeding apparatus  100 , when temperature detected by the temperature sensor TS is not less than the first predetermined temperature T 1 , the separation roller  106  is forcefully rotated by the rotational force from the motor  120  via the forced reverse transmission mechanism  116 C based on the temporary operation of the electro-magnetic clutch  137  of the forced reverse transmission mechanism  116 C in the operation of step S 5  while rotational force of the motor  120  is transmitted to the separation roller  106  via the limited reverse transmission mechanism  116 B. Accordingly, more larger reverse transmission force is applied to sheet PA while a predetermined reverse transmission force is applied to the abovementioned sheet PA. Consequently, when the predetermined reverse transmission force cannot separate plural sheets PA being overlapped in the lower side between the sheet feeding roller  105  and the separation roller  106 , more larger reverse transmission force than the predetermined reverse transmission force can be applied to the sheet PA being in contact with the separation roller  106 . As a result, the sheets PA being overlapped in the plural sheets PA in the lower side can be securely separated. 
     In higher temperature, the separability of overlapped sheets is lowered and so called double-feed which is phenomenon that overlapped sheets having more than two sheets are fed from the feeding apparatus  100 , tends to occur. The double feed in a higher temperature can be prevented by forcefully driving the separation roller  106  in a reverse direction. With regard to the threshold of the determination, is, for example, set at T 1 =29° C. 
     Further, since in the feeding apparatus  100 , reverse transmission force applied to the separation roller  106  for reversely transmitting sheet PA can be transmitted through only two simple configurations which are the limited reverse transmission mechanism  116 B for transmitting rotational force to the separation roller  106  and the forced reverse transmission mechanism  116 C, it is not necessary to have a complicated configuration to adjust pressing force of the separation roller  106  being given to the sheet feeding roller  105  and forced power to be applied to the separation roller  106  as they have been conducted in a conventional configuration. Accordingly, it becomes possible to securely prevent that overlapped plural sheets PA are conveyed without adjusting pressing force of the separation roller  106  against the sheet feeding roller  105  and rotational force applied to the separation roller  106 . 
     The present invention is not limited to the above embodiment and various changes and modification may be made without departing from the scope of the invention. 
     In the embodiment described above, the operation time period of the clutch  137  in step S 5  is set at the fourth predetermined time interval. However it may be changed in accordance with the kind of sheet PA. In this case, the time interval may be changed by a keyboard or a touch panel in accordance with the kind of sheets. It is also possible to configure a system for memorizing time table having time corresponding to the kind of sheet PA in advance, and automatically selecting the operation time of the electro-magnetic clutch  137  after the kind of sheets PA is inputted by the operations of the keyboard or the touch panel. 
     Further it is also possible to configure a system having sensors for detecting the transmission factor, such as, reflectivity, thickness and the size of sheet PA. Based on the detected results of these sensors, the control device  150  and the controller  151  may determine the kind of sheet PA, the existence of the action and the operation time period of the electro-magnetic clutch  137 . In accordance with the embodiment described above, double feeding at higher temperature where the separability of overlapped sheets is lowered can be prevented and high rate paper feeding can be securely conducted. Since the driving time interval of separation by the forced reverse transmission mechanism is precisely controlled, conveyance of sheets becomes stable and double feeding can be securely prevented. Further, the operation time interval of the forced reverse transmission mechanism is appropriately arranged for various conditions, double feeding under various conditions can be steadily prevented. 
     Embodiment 2 
     Embodiment 2 will be described by referring  FIGS. 6-8 . 
     Since  FIG. 2  illustrates a side view of the feeding apparatus of the embodiment, the description used for embodiment 1 illustrated in  FIG. 2  will be used. However, a driving system is different from the one used in embodiment 1. 
       FIG. 6  is a front view of a feeding apparatus  100 . 
     The feeding apparatus  100  comprises a sheet feeding roller  104 , a sheet feeding roller  105  and a motor  120  as a driving power source for a separation roller  106 . An output shaft of the motor  120  includes the gear  121 . The gear  121  engages a transmission gear  122  for transmitting rotational force of the motor  120  to the sheet feeding roller  104  and the sheet feeding roller  105 . A transmission gear  119  for transmitting rotational force of the motor  120  to the separation roller  106  is fixed on a rotational shaft  118 . The feeding apparatus  100  includes a feeding force transmission mechanism  116 A for transmitting rotational force of the motor  120  from the transmission gear  122  to the sheet feeding roller  104  and the sheet feeding roller  105 , and a limited reverse transmission mechanism  116 B for transmitting rotational force of the motor  120  from the transmission gear  119  to the separation roller  106 . 
     In the feeding force transmission mechanism  116 A, a shaft  123  which can freely rotate around the shaft is arranged. In  FIG. 6 , there is provided an one-way clutch  142  for limiting the rotational direction of the shaft  123  in one way and the electro-magnetic clutch  124  for switching transmission of rational force of the motor  120  to the sheet feeding roller  104  and the sheet feeding roller  105 . A clutch gear  125  of the electro-magnetic clutch  124  engages the transmission gear  122 . In  FIG. 6 , the sheet feeding roller  105  is fixed on the right of the shaft  123 . Pulleys  127  and  128  are respectively fixed on the shaft  123  and a rotational shaft  126  of the sheet feeding roller  104 , and an endless belt  129  is entertained around pulleys  127  and  128 . 
     In the feeding force transmission mechanism  116 A, when the electro-magnetic clutch  124  is turned on for operation while the motor  120  is rotating, the shaft  123  comes to connect with the electro-magnetic clutch  124 , and rotational force of the motor  120  is transmitted to the shaft  123  via the transmission gear  122  and the clutch gear  125  to rotate the shaft  123 . Once the shaft  123  rotates, the sheet feeding roller  105  rotates. Further the rotational force is transmitted to the rotational shaft  126  via the belt  129 . As a result the sheet feeding roller  104  and the sheet feeding roller  105  simultaneously rotate in the same direction. 
     When the electro-magnetic clutch  124  turns off to stop operation while the motor  120  is in an operation mode, the connection between the shaft  123  and the electro-magnetic clutch  124  is released, the clutch gear  125  freewheels and the rotation of the shaft  123  stops. Since the transmission of rotational force from the transmission gear  122  to the shaft  123  is intercepted, the rotation of the sheet feeding roller  104  and the sheet feeding roller  105  simultaneously stops. Namely, in the feeding force transmission mechanism  116 A, the sheet feeding roller  104  and the sheet feeding roller  105  are arranged to synchronously operate based on the operation and halt of electro-magnetic clutch  124 . Further, since the one-way clutch  142  is provided on the shaft  123 , the sheet feeding roller  105  does not reversibly rotate to follow the rotation of the separation roller  106 . 
     On the other contrary, in the limited reverse force transmission mechanism  116 B, the shaft  130  capable of freely rotating around the axis of the shaft is provided. In  FIG. 6 , there is provided the electro-magnetic clutch  140 , which is an electro-magnetic clutch  140  used for torque limiter, the electro-magnetic clutch  140  being used to switch for transmitting the rotational force of the motor  120  to the separation roller  106  or not. The clutch gear  132  of the electro-magnetic clutch  140  engages transmission the gear  119 . In  FIG. 6 , a torque limiter  107  is provided on the right end of the shaft  130 . A gear  134  is provided in the left side of the torque limiter  107  and the gear  134  engages a gear  135 . A gear  136  is fixed on the shaft of the separation roller  106  and the gear  136  engages the gear  135 . 
     In the limited force transmission mechanism  116 B, when the electro-magnetic clutch  140  turns on to start operation, while the motor  120  is in an operation mode, the shaft  130  is arranged to rotates as the shaft  130  comes to contact with the electro-magnetic clutch  140  and rotational force of the motor  120  transfers to the shaft  130  via the transmission gear  119  and the clutch gear  132 . Once the shaft  130  comes to rotate, the rotational torque is transmitted to the separation roller  106  via the gears  134 ,  135  and  136  while the rotational torque is in a limited mode. The separation roller  106  is arranged to rotate to convey sheet PA to the reverse direction against the sheet conveyance direction of the sheet feeding roller  104  and the sheet feeding roller  105 . Namely, in the limited force transmission mechanism  116 B being different from above the feeding force transmission mechanism  116 A, since the torque limiter  107  is provided between the shaft  130  and the separation roller  106 , all of rotational force of the motor  120  does not transmit to the conveyance roller  106 . Accordingly, only limited and constant rotational force is arranged to transmit from the motor  120  to the separation roller  106 . 
     When the electro-magnetic clutch  140  turns off to stop operation while the motor  120  is in an operation mode, and the connection between the shaft  130  and the electro-magnetic clutch  140  is released, the clutch gear  132  freewheels and the rotation of the shaft  130  stops. Since the transmission of rotational force from the transmission gear  119  to the shaft  130  is intercepted, the separation roller  106  and the shaft  130  are arranged to rotate following to the rotation of the sheet feeding roller  105 . 
     Although they are not shown in  FIGS. 2 and 6 , in the feeding apparatus  100 , the same as the feeding force transmission mechanism  116 A described above, the shafts, the gears and an electro-magnetic clutch  141  (refer to  FIG. 7 ) etc., are provided. A conveyance roller  108  is arranged to rotate based on the operation of the electro-magnetic roller  141 . 
     Next, the circuit configuration of the feeding apparatus  100  will be described referring to  FIG. 7 . 
       FIG. 7  is a block diagram showing the configuration of a control device  150  of the feeding apparatus  100 . 
     The controlling device  150  includes a controller  151  configured by a general-purpose CPU (Central Processing Unit), a ROM (Read Only Memory), a RAM (Random Access Memory) and a circuit, such as, a non-volatile memory (not shown). The controller  151  is designed to transfer a processing-program from the ROM to the RAM whereby the CPU executes the processing program. 
     Concretely, the controller  151  is arranged to the operation of the above motor  120  and components of the each electro-magnetic clutches  124 ,  140  and  141 . Particularly, in the feeding apparatus  100 , sensors  113 - 115  are connected to the controller  151  and the controller  151  is arranged to control the each electro-magnetic clutch  125 ,  140  and  141  based on the detected result of the each first to third sensors  113 - 115 . 
     A temperature sensor TS is a sensor to detect ambient temperature and it detects the surface temperature of the photosensitive drum  1  as shown in  FIG. 1 . The temperature sensor TS can be appropriately placed on any place other than the place shown in  FIG. 1 . 
     Next, the feeding operation of the feeding apparatus  100  will be described by referring to  FIG. 8 . 
       FIG. 8  is the flowchart showing tasks, which have been divided into plural processes according to the elapsed time. 
     When a signal for directing sheet feeding of sheet PA is inputted to the controller  151  of the control device  150 , the motor  120  starts rotating (step S 21 ). After that, the electro-magnetic clutch  124  operates to connect the shaft  123  to the electro-magnetic clutch  124 , and rotational force of the motor  120  transmits to the shaft  123  via the transmission gear  122  and the clutch gear  125 . Then the shaft  123  starts rotating. Then the rotational force transmits from the shaft  123  to the sheet feeding roller  104  and the sheet feeding roller  105 . The sheet feeding roller  104  and the sheet feeding roller  105  simultaneously start rotating counterclockwise as shown in  FIG. 2  (step S 22 ). When the sheet feeding roller  104  starts rotating, a single sheet PA placed on the top of the sheet-feeding tray  40  is sent to contacting portion of the sheet feeding roller  105  and the separation roller  106  from the sheet-feeding tray  40 . The sheet PA is conveyed to the conveyance roller  108  side while being sandwiched by the sheet feeding roller  105  and the separation roller  106 . 
     After that, the controller  151  determines whether the sheet PA is detected by the second sensor  114  or not (step S 23 ). When the second sensor  114  does not detect the sheet PA (step S 23 : No), the operation of step S 23  continues until the second sensor  114  detects the sheet PA. When the second sensor  114  detects the leading edge of sheet PA (step S 23 : Yes), the electro-magnetic clutch  140  operates to connect the shaft  130  to the electro-magnetic clutch  140  and rotational force of the motor  120  transmits from the transmission gear  119  to the shaft  130  via the clutch gear  132 . Consequently the shaft  130  starts rotating. Then, the rotational force transmits from the shaft  130  to the separation roller  106  via the gears  134 ,  135  and  136 . As a result, the separation roller  106  receives rotational force so that the separation roller  106  conveys sheet PA in a reverse direction against the sheet feeding direction of the sheet feeding roller  105  (step  24 ). 
     When a single sheet of sheets PA is fed from the sheet-feeding tray  40  in this situation described above, the sheet feeding roller  105  continue conveying sheet PA to the conveyance roller  108  side and since the separation roller  106  receives rotational force of the motor  120 , which is limited by the torque limiter  107 , the separation roller  106  rotates following to the rotation of the sheet feeding roller  105  while the separation roller  105  is in touch with sheet PA. On the contrary, when plural sheets PA being overlapped are sent out from the sheet feeding tray  40 , the sheet feeding roller  105  continues rotating counterclockwise to convey the single sheet of sheets PA positioned on the top of sheets PA to the conveyance roller  108  side as shown in  FIG. 2 . The separation roller  106  continues rotating counterclockwise as shown in  FIG. 2  and to convey sheets positioned in the lower side of sheets PA to the sheet feeding tray  40  side while being in touch with overlapped sheets PA. Namely, when overlapped plural sheets PA are fed out from the sheet feeding tray  40 , a sheet positioned on the top of the plural sheets PA is sent out to the conveyance roller  108  side. The other sheets PA (sheet PA positioned lower side among plural overlapped sheets PA) are separated from the sheet positioned on the top. 
     After that, the controller  151  determines whether the time interval from the moment when the second sensor  114  detects the leading edge of sheet PA in the operation of step  23  to the moment when the first sensor  113  detects the leading edge of sheet PA falls within the first predetermined time interval (step  25 ). Here, the first predetermined time interval means a barometer used for determining whether conveyance of sheet PA is correctly performed and the value of the first predetermined time interval is stored in the non-volatile memory in the control device  150 . 
     When the time interval from the moment when the second sensor  114  detects the leading edge of the sheet PA in the operation of step  23  to the moment when the first sensor  113  detects the leading edge of the paper is within the first predetermined time interval (step S 25 : YES), namely, the first sensor  113  detects the leading edge of sheet PA within the first predetermined time interval, the controller  151  starts measuring time (step S 31 ). 
     The first predetermined time interval is set corresponding to the ambient temperature, and the table of the first predetermined time interval corresponding to the temperature is stored in the non-volatile memory of the control device  150 . The controller  151  determines the first predetermined time interval by referring to the table based on the detected temperature of the temperature sensor TS. 
     In low temperature, conveyance rollers, such as sheet feeding roller  104  and sheet feeding roller  105  and sheets tend to slip each other. Accordingly, conveyance power of the conveyance roller lowers and no-feed tends to occur. In the low temperature, by setting the first predetermined time interval short and speedily conducting sheet-feed under the condition that the resistance of the separation roller is released, no-feed can be well prevented. 
     Namely, it becomes possible to securely prevent no-feed by setting the first predetermined time interval based on the ambient temperature for the determination in step  25 . 
     The first predetermined time interval may be continuously changed or changed stepwise. 
     After that, the controller  151  determines whether the elapsed time from the moment when starting measurement of the elapsed time reaches to the second predetermine time interval (step  32 ). When determining that the elapsed time has not reached to the second predetermined time (step  32 : NO), the controller  151  continues the process of step  32  from the moment when starts measuring time interval at step  31  to the moment when time has reaches to the predetermined time interval. 
     When the controller  151  determines that the elapsed time has reached to the second predetermine time interval from the moment when starts measuring of time interval in step S 31  (step S 32 : YES), the operation of the electro-magnetic clutch  124  is stopped (step S 33 ). Then the connection between the shaft  123  and the electro-magnetic clutch  124  is released and the clutch gear  125  freewheels. Accordingly, rotational force from the transmission gear  122  to the sheet feeding roller  104  and the sheet feeding roller  105  is intercepted. 
     After that, the controller  151  determines whether a signal for directing the restart of feeding sheet PA is inputted (step S 34 ). When determined that the directing signal has not inputted (step S 34 : NO), the controller  151  continues the process of step  34  until the signal for directing the restart of feeding sheet PA is inputted. When the controller  151  determines that the signal for directing the restart of feeding sheet PA has inputted (step S 34 : YES), the electro-magnetic clutch  141  operates to rotate the conveyance roller  108  counterclockwise as shown in  FIG. 2  (step S 36 ). 
     On the contrary, when the elapsed time from the moment when the second sensor  114  detects the leading edge of sheet PA to the moment when the first sensor  113  detects the leading edge of sheet PA does not fall within the first predetermined time interval (step S 25 : NO), namely, the first sensor  113  does not detects the leading edge after the first predetermined time interval has passed, the controller  151  stops operation of the electro-magnetic clutch  140  (step S 27 ). Then the connection between the shaft  130  and the electro-magnetic clutch  140  is released and the clutch gear  132  freewheels. As a result, rotational force from the transmission gear  119  to the separation roller  106  is intercepted. 
     In this situation, since the sheet feeding roller  105  continues rotating even though the separation roller  106  stops rotating, sheet PA fed out from the sheet-feeding  40  is conveyed by the action of the sheet roller  105  to the conveyance roller  108  side, whichever the number of sheet PA is a peace of sheet or plural sheets. 
     After that, the controller  151  determines whether the time from the moment when the second sensor  114  detects the leading edge of sheet PA in step S 23  to the moment when the first sensor  113  detects the leading edge of sheet PA is equal to or less than a third predetermined time being an erroneous time (step S 28 ). Here, the erroneous time is a parameter for determining whether conveyance error of sheet PA occurs and the third erroneous time is set longer than the first predetermined time. The value of the third erroneous time is stored in the non-volatile memory in the control device  150  as the same as the above first predetermined time. 
     When the time from the moment when the second sensor  114  detects the leading edge of sheet PA in step S 23  to the moment when the first sensor  113  detects the leading edge of sheet PA is equal to or less than third predetermined time interval (step  28 : YES), namely, the first sensor  113  detects the leading edge of sheet PA within the third predetermined time interval, the electro-magnetic clutch  140  restarts operation so that the rotational force transmits to the separation roller  106  to convey sheet PA in the direction being reverse direction against the paper feeding direction of the feeding roller  105  (step S 29 ). 
     In this situation, when a single sheet of sheet PA is fed from the sheet-feeding tray  40 , the sheet feeding roller  105  continues rotating to convey sheet PA to the conveyance roller  108  side as the same situation after the operation of step  24  and the separation roller  106  rotates following to the rotation of the sheet feeding roller  105 . When plural sheets of sheet PA are fed out from the sheet-feeding tray  40 , the sheet feeding roller  105  continues rotating to convey the single sheet of sheet PA positioned on the top of the plural sheets to the conveyance roller  108  side, and the separation roller  106  returns overlapped sheets positioned lower side of the plural sheets of sheet PA to the sheet-feeding tray  40  side. 
     When the elapsed time from the moment when the second sensor  114  detects the leading edge of sheet PA to the moment when the first sensor  113  detects the leading edge of sheet PA does not fall within the first predetermined time interval (step S 28 : NO.), namely, the first sensor  113  does not detects the leading edge after the first predetermined time interval has passed, the controller  151  conducts the process for sending information of conveyance failure of sheet PA (step S 30 ). In this embodiment, mechanical operations of each member of the sheet feeding apparatus  100  stop. However, a display (not shown) provided in the sheet feeding apparatus  100  may display the message and a buzzer (not shown) provided in the sheet feeding apparatus  100  may automatically sound. In step  23 , the controller  151  determines whether the time interval from the moment when the second sensor  114  detects the leading edge of sheet PA to the moment when the first sensor  113  detects the leading edge of sheet PA is equal to or less than the third predetermined time. However, in step S 28 , in the case of the apparatus arranged to temporally stop driving the sheet feeding roller  104  or the sheet feeding roller  105 , etc., when the second sensor  114  detects the leading edge of sheet PA, the controller  151  determines whether the time interval from the moment when restarting the drive of the sheet feeding roller  104  or the sheet feeding roller  105  to the moment when the first sensor  113  detects the leading edger of sheet PA is equal to or less than the third predetermined time interval. 
     After the operation of step S 29 , the operations of steps S 31  through step S 36  are conducted. When the roller  108  rotates based on the operation of step S 36 , the leading edge of sheet PA passed through the pressing portion of the sheet roller  105  and the separation roller  106  is sandwiched between the conveyance roller  108  and the following roller  109 . The sheet PA is conveyed to the outside of the sheet feeding apparatus  100  through the pressing portion of in accordance with the rotation of the conveyance roller  108 . 
     After that, the controller  151  determines whether a sheet PA is detected by the third sensor  115  or not (step S 37 ). When the third sensor  115  does not detect sheet PA (step S 37 : NO), the operation of step S 37  continues until the third sensor  115  detect sheet PA. When the third sensor  115  detects the leading edge of sheet PA (step S 37 : YES), the controller  151  determines whether the sheet PA is detected by the sensor  115  or not, again (step S 38 ). The operation of step  38  continues while the third sensor  115  is detecting sheet PA (step  38 : NO). 
     When the third sensor  115  becomes not to detect sheet PA (step S 38 : YES), namely the rear edge of sheet PA has passed through the detecting portion of sheet PA by the third sensor  115 , the operations of two electro-magnetic clutches  140  and  141  stop (step S 39 ). Then the connection between the shaft  130  and the electro-magnetic clutch  140  is released and the clutch gear  132  freewheels, and the transmission of rotational force from the transmission gear  119  to the separation roller  106  is intercepted. At the same time, the rotational force from the motor  120  to the conveyance roller  108  is intercepted. 
     After that, the controller  151  determines whether there is a signal for directing the feeding of next sheet PA (step S 40 ). When the signal for feeding sheet PA is inputted (step S 40 : YES), the operations of each member of steps S 22 -S 39  repeatedly continue. When the signal for directing the feeding of next sheet PA has not been inputted (step S 40 : NO), rotation of the motor  120  stops (step S 41 ) and feeding operation of the sheet feeding apparatus  100  completes. 
     In the sheet feeding apparatus  100 , when first sensor  113  does not detect sheet PA while the rotational force is transmitted to the separation roller  106  based on the operation of the electro-magnetic clutch  140  in the operation of step S 24 , since the rotation of the electro-magnetic clutch  140  stops, no reverse force (no load) in the reverse direction against the conveyance direction is applied to sheet PA passing between the sheet feeding roller  105  and the separation roller  106 . Accordingly, at this moment, since only rotational force of the sheet feeding roller  105  conveys sheet PA, it is possible to securely transmit rotational force of the sheet feeding roller  105  to sheet PA. As a result, since the sheet feeding roller  105  does not slip on the contacting surface with sheet PA, conveyance failure of sheet PA can be prevented. 
     From the different point of view, when the first sensor  113  does not detect sheet PA within the first predetermined time interval under the condition that rotational force is transmitted to the separation roller  106  by the operation of the electro-magnetic clutch  140  in step  24 , the electro-magnetic clutch  140  temporarily stops operation and restarts after the first sensor  113  detects sheet PA. Accordingly, rotational forces before and after the temporary stop of the electro-magnetic clutch  140 , are transmitted to the separation roller  106 . As a result, the reverse force being in the direction opposed to the conveyance direction is applied twice to sheet PA passing through the contact portion of the feeding roller  105  and the separation roller  106 . 
     Accordingly, when plural sheets of sheet PA pass through the contacting portion of the sheet feeding roller  105  and the separation roller  106 , for example, in case that even though the reverse feeding force of the separation roller  106  firstly applied to the plural sheets, does not separate all plural sheets of sheet PA being overlapped under the top sheet of sheet PA, the latter reverse feeding force of the separation roller  106  can sufficiently separate overlapped sheets under sheet PA. As a result, it becomes possible to sufficiently prevent the conveyance of plural sheet of sheet PA. 
     Since first predetermined time interval for determining operation/non-operation of the electro-magnetic clutch  140  is set based on the detected information of the temperature sensor TS, no-feed caused by slipping between a sheet and a conveyance roller at low temperature can be well prevented. In accordance with current embodiment of the invention, no-feed caused by the characteristic that conveyance rollers and a sheet tend to slip each other in low temperature can be well prevented and high speed sheet feeding can be conducted. Since the management time for controlling the freewheel of the separation roller can be precisely set, conveyance becomes stable and no-feed can be securely prevented. 
     Embodiment 3 
     Embodiment 3 of the invention will be described by using.  FIGS. 6 ,  9 ,  10  and  11 . 
     In  FIGS. 6 ,  9  and  10 , with regard to commonly used components and the circuit, detailed explanations will be omitted by referring to the description described above.  FIG. 6  illustrating the side view of embodiment 2 shows the driving system of the embodiment. 
       FIG. 9  is a side view of the sheet feeding apparatus  100  and  FIG. 10  is a circuit diagram of embodiment 3 of the sheet feeding apparatus  100 . 
     A different point between the sheet feeding apparatus  100  shown in  FIG. 9  and the sheet feeding apparatus  100  shown in  FIGS. 2 and 4  is that the second sensor  114  is omitted from the sheet feeding apparatus  100  shown in  FIGS. 2 and 4 , and others are the same as the sheet feeding apparatus  100  shown in  FIGS. 2 and 4 . In  FIG. 10 , first and third sensors  113  and  115  are connected to the controller  151 , which is arranged to control electro-magnetic clutches  124 ,  140  and  141  based on the detected results of the first and third sensors  113  and  115 . 
     The operation of the sheet-feeding apparatus  100  will be described by referring to  FIG. 11 . 
       FIG. 11  is a flowchart sequentially expressing plural processes of feeding operations of the sheet feeding apparatus  100 . 
     When a signal for directing the feed of sheet PA is inputted to the controller  151  of the control device  150 , the motor  120  stats rotating (step S 51 ), then the electro-magnetic clutches  124  and  140  operate (step S 52 ). When the electro-magnetic clutch  124  operates, the shaft  123  comes to contact with the electro-magnetic clutch  124  to transmit rotational force of the motor  120  to the sheet feeding roller  104  and the sheet feeding roller  105 . The sheet feeding roller  104  and the sheet feeding roller  105  simultaneously stat rotating counterclockwise as shown in  FIG. 9 . When the electro-magnetic clutch  140  starts operating, the shaft  130  connects to the electro-magnetic clutch  140  to transmit rotational force of the motor  120  to the separation roller  106  via the torque limiter  107 . As a result, the separation roller  106  receives a rotating force from the motor  120  in the reverse direction against the sheet feeding direction of the sheet feeding roller  105 . 
     When the sheet feeding roller  104  starts rotating, a single sheet of sheet PA positioned on the top of the sheet feeding tray  40  is fed into the pressing portion of the sheet feeding roller  105  and the separation roller  106 . The sheet feeding roller  105  conveys the single sheet of sheet PA to the conveyance roller  108  side while the single sheet of sheet PA is sandwiched between the paper feeding roller  105  and the separation roller  106 . 
     When the single sheet of sheets PA is fed from the sheet-feeding tray  40  in this situation described above, the sheet feeding roller  105  continues conveying sheet PA to the conveyance roller  108  side and since the separation roller  106  receives rotational force of the motor  120 , which is limited by the torque limiter  107 , the separation roller  106  rotates following to the rotation of the sheet feeding roller  105  while the separation roller  105  is in touch with sheet PA. On the contrary, when plural sheets PA being overlapped are sent out from the sheet feeding tray  40 , the sheet feeding roller  105  continues rotating counterclockwise to convey sheets PA positioned on the top of sheets PA to the conveyance roller  108  side as shown in  FIG. 9 . The separation roller  106  continues rotating counterclockwise as shown in  FIG. 9  and to convey the sheets positioned in the lower side of sheet PA to the sheet feeding tray  40  side while being in touch with overlapped sheets PA. 
     After that, the controller  151  determines whether the time interval from the moment when the electro-magnetic clutch  124  starts operating in the operation of step  52  to the moment when the first sensor  113  detects the leading edge of sheet PA falls within the first predetermined time interval (step  53 ). Here, the first predetermined time interval is a barometer used for determining whether conveyance of sheet PA is correctly performed, the same as the first predetermined time interval described in the operation of step  25  of the first embodiment of the invention, and the value of the table corresponding to ambient temperature is stored in the non-volatile memory in the control device  150 . 
     When the time from the start of operation of the electro-magnetic clutch  124  to the moment when the first sensor  113  detects the leading edge of sheet PA is equal to or less than the first predetermined time interval (step  53 : YES), namely the first sensor  113  detects sheet PA within the first predetermined time interval, the controller  151  starts measuring time (step S 58 ). 
     After that, the controller  151  determines whether elapsed time from the moment when stats measuring time of the process of step  58  has reached the second predetermined time interval (step S 59 ). When determined that the elapsed time has not reached to the second predetermined time interval (step S 59 : NO), the process of step S 59  repeats until the time reaches to the second predetermined time interval. 
     When the controller  151  determines that the elapsed time has reached to the second predetermine time from the moment when starts measuring time in step S 58  (step S 59 : YES), the operation of the electro-magnetic clutch  124  is stopped (step S 60 ). Then the connection between the shaft  123  and the electro-magnetic clutch  124  is released and the clutch gear  125  freewheels. Accordingly, rotational force from the transmission gear  122  to the sheet feeding roller  104  and the sheet feeding roller  105  is intercepted. 
     After that, the controller  151  determines whether signal for directing the restart of feeding sheet PA has been inputted (step S 61 ). When determined that the directing signal has not inputted (step S 61 : NO), the controller  151  continues the process of step  34  until the signal for directing the restart of feeding sheet PA is inputted. When the controller  151  determines that the signal for directing the restart of feeding sheet PA has inputted (step S 61 : YES), the electro-magnetic clutch  141  operates to rotate the conveyance roller  108  in counterclockwise as shown in  FIG. 5  (step S 62 ). 
     On the contrary, when the elapsed time from the moment when the electro-magnetic clutch  124  starts operating to the moment when the first sensor  113  detects the leading edge of sheet PA does not fall within the first predetermined time interval (step S 53 : NO), namely, the first sensor  113  does not detects the leading edge after the first predetermined has passed, the controller  151  stops operation of the electro-magnetic clutch  140  (step S 54 ). Then the connection between the shaft  130  and the electro-magnetic clutch  140  is released and the clutch gear  132  freewheels. As a result, rotational force from the transmission gear  119  to the separation roller  106  is intercepted. 
     In this situation, since the sheet feeding roller  105  continues rotating even though the separation roller  106  stops rotating, sheet PA fed out from the sheet-feeding tray  40  is conveyed by the action of the sheet roller  105  to the conveyance roller  108  side, whichever the number of sheet PA is a peace of sheet or plural sheets. 
     After that, the controller  151  determines whether the time from the moment when the electro-magnetic clutch  124  starts operation in step S 52  to the moment when the first sensor  113  detects the leading edge of sheet PA is equal to or less than third predetermined time interval being an erroneous time (step S 55 ). Here, the erroneous time is a parameter for determining whether conveyance error of sheet PA occurs, the same as the third predetermined time interval described in step S 28  of the embodiment 2, and the third erroneous time interval is set longer than above first predetermined time interval. The value of the third erroneous time interval is stored in the non-volatile memory in control device  150  as the same as the above first predetermined time. 
     After that, the same operation of the sheet feeding apparatus  100  shown in the embodiment 2 is conducted. In respective operations of steps S 56 , S 57  and S 63 -S 68 , the operation of step S 56  corresponds to the operation of step S 29 ; the operation of step  57  corresponds to the operation of step  30 ; the operation of step S 63  corresponds to the operation of step S 37 ; the operation of step S 64  corresponds to the operation of step S 38 ; the operation of step S 65  corresponds to the operation of step S 39 ; the operation of step  66  corresponds to the operation of step S 40 ; and the operation of step  67  corresponds to the operation of step S 41 . 
     Namely, in the sheet feeding apparatus  100  of embodiment 2, the first predetermined time interval and the erroneous time interval at steps  25  and  28  are corresponding to the time from the moment when the second sensor  114  detects the sheet PA to the moment when the first sensor  113  detects the sheet PA. In stead, in the sheet feeding apparatus  100  of embodiment 3, in the sheet feeding apparatus  100  of embodiment 3, the first predetermined time interval and the erroneous time interval at steps  53  and  55  are corresponding to the time from the moment when the electro-magnetic clutch  124  starts operating to the moment when the first sensor  113  detects the sheet PA. This is a point in which the operation of the sheet feeding apparatus  100  and that of the sheet feeding apparatus  1  is different. Accordingly, in the above sheet feeding apparatus  100 , the same as the sheet feeding apparatus  1 , the conveyance failures of sheet PA can be prevented. (Conveyance of plural overlapped sheets of sheet PA can be prevented.) In accordance with an embodiment of the present invention, no-feed caused by the phenomenon that conveyance rollers and paper sheets tend to slip each other in low temperature can be well prevented and high speed sheet feeding-conveyance can be stably conduced. 
     Embodiment 4 
     An embodiment 4 of the invention will be described by applying FIGS.  2  and  12 - 14 . 
       FIG. 2  is the side view of a sheet feeding apparatus of this embodiment of the invention.  FIG. 12  illustrates a front view of a driving system. 
     This embodiment is arranged to selectively uses the limited reverse transmission mechanism  116 B or the forced reverse transmission mechanism  116 C based on ambient temperature and selectively use the separation roller  106  or makes the separation roller  106  free to follow the rotation of the sheet feeding roller  105  based on the ambient temperature. This embodiment is an image forming apparatus having an embodiment including the combination of embodiments 1, 2 and 3. 
     A sheet feeding apparatus  100  of the embodiment shown in  FIG. 12  has the same structure as the sheet feeding apparatus shown in  FIG. 3  except that the limited reverse transmission mechanism  116 B includes the electro-magnetic  140  for limiting torque. 
     In the sheet feeding apparatus shown in  FIG. 12 , the rotational force of the motor  120  transmits to the separation roller  106  via the torque limiter  107  when the electro-magnetic clutch  140  operates and rotational force being a reverse direction against the sheet feeding direction transmits to the separation roller  106 . When the electro-magnetic clutch  140  comes to stop, the separation roller  106  becomes free and follows the rotation of the sheet feeding roller  105 . 
       FIG. 13  illustrates a block diagram of control circuit in an embodiment. As shown in  FIG. 13 , the control circuit is the same as the control circuit shown in  FIG. 4  except it has an electro-magnetic clutch  140  being an electro-magnetic clutch for limiting torque. 
     The operation of the embodiment of the invention will be described by referring to  FIG. 14 , which is the flowchart showing the flowchart of the control flow of the embodiment. 
     When a signal directing the feed of sheet PA is inputted to the controller  151  of the control device  150 , the motor  120  starts rotating (step S 1 ). 
     After that, an electro-magnetic clutch  124  used for an electro-magnetic clutch of a paper feeding roller and, an electro-magnetic clutch  140  being an electro-magnetic clutch for limiting torque operate to connect a shaft  123  with the electro-magnetic clutch  124 . Then the rotational force of the motor  120  transmits via a transmission gear  122  and a clutch-gear  125  to the shaft  123 . Once the shaft  123  starts rotating, the rotational force transmits to the shaft  130  and the sheet feeding roller  104  and the sheet feeding roller  105  simultaneously start rotating counterclockwise as shown in  FIG. 2  (step S 2 ). 
     At this moment, even though the rotational force of the motor  120  transmits to a separation roller  106  via a torque limiter  107 , since due to the operation of the electro-magnetic clutch  124 , the rotational force of the motor  120  directly transmits to the sheet feeding roller  105  as it is, the rotational force of the sheet feeding roller  105  is greater than that of the separation roller  106 , and the separation roller  106  which is pressed to the sheet feeding roller  105  rotates following to the rotation of the sheet feeding roller  105 . 
     When the sheet feeding roller  104  starts rotating, sheet PA placed on the top of the sheet-feeding tray  40  is sent to contacting portion of the sheet feeding roller  105  and the separation roller  106  from the sheet-feeding tray  40 . Sheet PA is conveyed to a conveyance roller  108  side while being sandwiched by the sheet feeding roller  105  and the separation roller  106 . 
     After that, the controller  151  determines whether a second sensor  114  detects a sheet PA or not (step S 3 ). When the second sensor  114  does not detect a sheet PA (step S 3 : No), the operation of step S 3  repeats until the second sensor  114  detects a sheet PA. When the second sensor  114  detects the leading edge of sheet PA (step S 3 : Yes), the controller  151  determines whether the temperature measured by a temperature sensor TS is not less than first predetermined temperature T 1  (step. S 4 ). 
     When temperature is not less than first predetermined temperature T 1  (step S 4 : YES), the rotational force of the motor  120  is transmitted to the shaft  131  via a transmission gear  122 , gears  132 ,  133  and a clutch gear  138  by the operation of an electro-magnetic clutch  137  for connecting the shaft  131  to electro-magnetic clutch  137 . Consequently, the shaft  131  starts rotating. Then the rotational force is transmitted to the separation roller  106  from a gear  139  via gears  134 ,  135  and  136 . The rotational force is applied to the separation roller  106  so that the separation roller  106  conveys sheets PA in a reverse direction against the sheet feeding direction of the sheet feeding roller  105  (step S 5 ). 
     Before the electro-magnetic clutch  137  starts operation, the rotational force of the shaft  130  is transmitted to the separation roller  106  via the torque limiter  107 . Once the electro-magnetic clutch  137  starts operating in the operation of step S 5 , the separation roller  106  is forcefully driven by a forced reverse transmission mechanism  116 C. Accordingly, the sheet feeding roller  105  and the separation roller  106  act against sheet PA with conveyance forces being reverse directions each other in the operation of step S 5 . 
     In this situation described above, when a single sheet of sheets PA is fed from the sheet-feeding tray  40 , since the outer periphery of the sheet feeding roller  105  is structured by a material having a larger friction coefficient than that of the separation roller  106 , the separation roller  106  rotates as the separation roller  106  slips on the lower surface of the sheet while the sheet feeding roller  105  continue to convey the sheet to the side of the conveyance roller  108 . When plural sheets PA being overlapped are sent out from the sheet feeding tray  40 , the sheet feeding roller  105  continues rotating counterclockwise to convey the sheet of sheets PA positioned on the top of sheets PA to a conveyance roller  108  side as shown in  FIG. 2 . The separation roller  106  continues rotating counterclockwise as shown in  FIG. 2 , and to convey the sheets positioned in the lower side of sheet PA to the sheet feeding tray  40  while being in touch with overlapped sheets PA. Namely, when the electro-magnetic clutch  137  operates in the operation of step S 5  and overlapped plural sheets PA are fed out from the sheet feeding tray  40 , a sheet positioned on the top of the plural sheets PA is sent out to the conveyance roller  108  side. The other sheets PA are separated from the sheet positioned on the top and returned to the sheet feeding  40  side. 
     In the operation of step S 5 , the electro-magnetic clutch  137  continues operating for a fourth predetermined time interval and the operation of the electro-magnetic clutch  137  stops when the fourth predetermined time interval has passed (step. S 6 ). Then, the connection between the shaft  131  and the electro-magnetic clutch  137  is released. As a result, a clutch gear  138  freewheels and the transmission of rotational force from the shaft  131  to the separation roller  106  is intercepted. Namely, it comes to a state that the rotational force being transmitted through the torque limiter  107  to the shaft  130  is transmitted to the separation roller  106 . 
     As described above, paper feeding is conducted while forcefully driving the separation roller  106  in the reverse direction being against to the paper feeding direction when temperature is not less than the first predetermined temperature T 1 . 
     At high temperature, there is a tendency that the separation of overlapped sheets becomes difficult. However, the separation at high temperature, namely, double feed can be well prevented by forcefully driving the separation roller  106  as described above. For example, the first predetermined temperature T 1  is set at 29° C. and the fourth predetermined time interval is, for example, set at 100 msec. 
     After that, the controller  151  starts measuring time at step S 31 , and determines whether elapsed time reaches to the second predetermined time interval (step S 32 ). When determined that the elapsed time has not reached to the second predetermined time interval (step S 32 : NO), the process of step S 32  will be repeated until the elapsed time of step S 31  reaches to the predetermined time interval. 
     When the controller  151  determines that the elapsed time has reached to the second predetermine time interval from the moment when starts measuring time in step S 31  (step S 32 : YES), the operation of the electro-magnetic clutch  124  is stopped (step S 33 ). Then the connection between the shaft  123  and the electro-magnetic clutch  124  is released and the clutch gear  125  freewheels. Accordingly, rotational force from the transmission gear  122  to the sheet feeding roller  104  and the sheet feeding roller  105  is intercepted. 
     After that, the controller  151  determines whether a signal for directing the restart of feeding sheet PA is inputted (step S 34 ). When determined that the directing signal has not inputted (step S 34 : NO), the controller  151  repeats the process of step  34  until the signal for directing the restart of feeding sheet PA is inputted. When the controller  151  determines that the signal for directing the restart of feeding sheet PA has inputted (step S 34 : YES), the electro-magnetic clutch  141  operates to rotate the conveyance roller  108  counterclockwise as shown in  FIG. 2  (step S 36 ). 
     After that, the controller  151  determines whether the third sensor  115  detects the sheet PA or not. When the third sensor  115  does not detect the sheet PA (step S 37 : No), the operation of step S 37  is repeated until the third sensor  115  detects the leading edge of sheet PA. When the third sensor  115  detects the leading edge of sheet PA (step S 37 : YES), the controller  151  determines whether the third sensor  115  detects the sheet PA or not, again (step S 38 ). The operation of step  38  is repeated while the third sensor  115  is detecting sheet PA (step S 38 : NO). 
     When the third sensor  115  comes to a condition that the third sensor does not detect sheet. PA (step S 38 : YES), namely, the rear edge of sheet PA has passed through a portion where the third sensor  115  detects sheet PA, the operation of the electro-magnetic clutches  140  and  141  stop (step S 39 ). Then, the connection between the shaft  130  and the electro-magnetic clutch  140  is released and the clutch gear  132  freewheels. As a result, the transmission of rotational force from the transmission gear  119  to the separation roller  106  is shut off. Simultaneously, the rotational force from the motor  120  to the conveyance roller  108  is also shut off. 
     After that, the controller  151  determines whether there is a signal for directing the feed of next sheet PA (step S 40 ). When the signal for directing the next feed of sheet PA has been inputted (step S 40 : YES), abovementioned operations of steps  22 - 39  repeat. When no signal for direction the feed of next sheet PA has been inputted (step S 40 : NO), the rotation of the motor  120  stops (step S 41 ) and the operations of the feeding apparatus  100  complete. 
     In the determination of step S 4 , when temperature T detected by the temperature sensor TS is lower than the first predetermined temperature T 1 , the controller  151  determines whether the detected temperature is less than the second predetermined temperature T 2 . 
     When the detected temperature is less than the second predetermined temperature T 2  (step S 70 : YES), the controller  151  changes the first predetermined time interval to a shorter time interval (step S 71 ). When the detected temperature is not less than the second predetermined temperature (step S 70 : NO), the controller  151  does not change the first predetermined time interval while setting the first predetermined time interval as a standard setting time interval. 
     After that, the controller  151  determines whether the time interval from the moment when the second sensor  114  detects the leading edge of sheet PA in the operation of step  23  to the moment when the first sensor  113  detects the leading edge of sheet PA falls with the first predetermined time (step  25 ). However, in the case of an apparatus in which the sheet feeding roller  104  or the sheet feeding roller  105  etc., are arranged to be stopped when the second sensor  114  detects the leading edge of sheet PA, the controller  151  determines whether the time interval from the moment when restarting the rotation of the feeding roller  104  and the feeding roller  105 , to the moment when the first sensor  113  detects the leading edge of sheet PA, falls within the first predetermined time interval. 
     When the detected time interval by the first sensor  113  is equal to or less than the first predetermined time interval, the controller  151  starts measuring time interval (step S 31 ). The controller  151  determines whether the time interval from the moment when starts measuring the time interval reaches to the second predetermined time interval (step S 32 ). When the time interval has not reached to the second predetermined time interval (step S 32 : NO), the controller  151  repeats the process of step S 32  until the time interval reaches to the predetermined time. 
     When the controller  151  determines that the time interval has reached to the second predetermine time from the moment when starts measuring time interval in step S 31  (step S 32 : YES), the operation of the electro-magnetic clutch  124  is stopped (step S 33 ). Then the connection between the shaft  123  and the electro-magnetic clutch  124  is released and the clutch gear  125  freewheels. Accordingly, rotational force from the transmission gear  122  to the sheet feeding roller  104  and the sheet feeding roller  105  is intercepted. 
     After that, the controller  151  determines whether signal for directing the restart of feeding sheet PA has been inputted (step S 34 ). When determined that the directing signal has not inputted (step S 34 : NO), the controller  151  repeats the process of step  34  until the signal for directing the restart of feeding sheet PA is inputted. When the controller  151  determines that the signal for directing the restart of feeding sheet PA has inputted (step S 34 : YES), the electro-magnetic clutch  141  operates to rotate the conveyance roller  108  counterclockwise as shown in  FIG. 2  (step S 36 ). 
     On the contrary, when the time interval from the moment when the second sensor  114  detects the leading edge of sheet PA to the moment when the first sensor  113  detects the leading edge of sheet PA does not fall within the first predetermined time (step S 25 : NO), namely, the first sensor  113  does not detects the leading edge after the first predetermined has passed, the controller  151  stops operation of the electro-magnetic clutch  140  (step S 27 ). Then the connection between the shaft  130  and the electro-magnetic clutch  140  is released and the clutch gear  132  freewheels. As a result, rotational force from the transmission gear  119  to the separation roller  106  is intercepted. 
     In this situation, since the sheet feeding roller  105  continues rotating even though the separation roller  106  stops rotating, sheet PA fed out from the sheet-feeding tray  40  is conveyed by the action of the sheet feeding roller  105  to the conveyance roller  108  side, whichever the number of sheet PA is a single sheet or plural sheets. 
     After that, the controller  151  determines whether the time interval from the moment when the second sensor  114  detects the leading edge of sheet PA in step S 25  to the moment when the first sensor  113  detects the leading edge of sheet PA is equal to or less than third predetermined time interval being an erroneous time interval (step S 28 ). Here, the erroneous time interval is a parameter for determining whether conveyance error of sheet PA occurs and the third erroneous time is set longer than the first predetermined time. The value of the third erroneous time interval is stored in the non-volatile memory in the control device  150  the same as the first predetermined time. 
     When the time interval from the moment when the second sensor  114  detects the leading edge of sheet PA in step S 23  to the moment when the first sensor  113  detects the leading edge of sheet PA is equal to or less than the third predetermined time (step  28 : YES), namely, the first sensor detects the leading edge of sheet PA within the third predetermined time, the electro-magnetic clutch  140  restarts operation so that the rotational force transmits to the separation roller  106  to convey sheet PA in the direction being reverse direction against the paper feeding direction of the feeding roller  105  (step S 29 ). 
     In this situation, when a single sheet of sheet PA is fed from the sheet-feeding tray  40 , the sheet feeding roller  105  continues rotating to convey the sheet PA to the conveyance roller  108  side the same as the situation after the operation of step  24 , and the separation roller  106  rotates following to the rotation of the sheet feeding roller  105 . When plural sheets of sheet PA are fed out from the sheet-feeding tray  40 , the sheet feeding roller  105  continues rotating to convey a single sheet of sheet PA positioned on the top of the plural sheets to the conveyance roller  108  side, and the separation roller  106  returns overlapped sheets positioned lower side of the plural sheets PA to the sheet-feeding tray  40  side. 
     When the elapsed time from the moment when the second sensor  114  detects the leading edge of sheet PA to the moment when the first sensor  113  detects the leading edge of sheet PA does not fall within the first predetermined time interval (step S 28 : NO), namely, In the case of that the first sensor  113  does not detects the leading edge of the sheet PA when the first predetermined time interval has passed, the controller  151  conducts process of sending information of conveyance failure of sheet PA (step S 30 ). In this embodiment, mechanical operations of each component of the sheet feeding apparatus  100  stop. However, a display (not shown) provided in the sheet feeding apparatus  100  may display the message and a buzzer (not shown) provided in the sheet feeding apparatus  100  may automatically sound. Still, in step S 28 , the controller  151  determines whether the time interval from the moment when the second sensor  114  detects the leading edge of sheet PA to the moment when the first sensor detects the leading edge of sheet PA falls within the third predetermined time interval. However, in the case of an apparatus in which the sheet feeding roller  104  or the sheet feeding roller  105  etc., are arranged to be stopped when the second sensor  114  detects, the controller  151  determines whether the time interval from the moment when restarting the rotation of the feeding roller  104  and the feeding roller  105  to the moment when the first sensor  113  detects the leading edge of sheet PA, falls within the third predetermined time interval. 
     After the operation of step S 29 , operations of steps S 31  through step S 36  are conducted. When the conveyance roller  108  rotates based on the operation of step S 36 , the leading edge of sheet PA passed through the pressing portion of the sheet roller  105  and the separation roller  106  is sandwiched between the conveyance roller  108  and the following roller  109 . The sheet PA is conveyed to the outside of the sheet feeding apparatus  100  through the pressing portion between the conveyance roller  108  and the following roller  109  in accordance with the rotation of the conveyance roller  108 . 
     After that, the controller  151  determines whether the third sensor  115  detects the sheet PA (step S 37 ). When third sensor  115  does not detect sheet PA (step S 37 : NO), the operation of step S 37  repeats until the third sensor  115  detects sheet PA. When the third sensor  115  detects the leading edge of sheet PA (step S 37 : YES), the controller  151  determines whether the third sensor  115  detects the sheet PA or not, again (step S 38 ). The operation of step  38  repeats while the third sensor  115  is detecting the sheet PA (step  38 : NO). 
     When the third sensor  115  becomes not to detect the sheet PA (step S 38 : YES), namely the rear edge of sheet PA has passed through the detecting portion of the sheet PA by the third sensor  115 , the operations of two electro-magnetic clutches  140  and  141  stop (step S 39 ). Then the connection of the shaft  130  and the electro-magnetic clutch  140  is released and the clutch gear  132  freewheels. As a result the transmission of rotational force from the transmission gear  119  to the separation roller  106  is intercepted. 
     After that, the controller  151  determines whether there is a signal directing for feeding sheet PA (step S 40 ). When the signal for directing for feeding sheet PA is input (step S 40 : YES), the operations of each task of steps S 22 -S 39  repeats. When the signal for directing for feeding sheet PA has not been inputted (step S 40 : NO), rotation of the motor  120  stops (step S 41 ) and the feeding operation of sheet feeding apparatus  100  finishes. 
     In the processes from step S 27  to step S 29 , the sheets are conveyed while the reverse action of the separation roller  106  is released. Accordingly, no-feed can be well prevented. 
     The change of the first predetermined time interval at steps S 70  and S 71  is conducted to execute the processes of steps S 27 -S 29  at low temperature in an early stage based on the determination whether the controller  151  executes the processes of steps S 27 -S 29  and the timing of. Accordingly, no-feed can be prevented. 
     The second predetermined temperature for applying the determination of step  70  is set, for example, at 19° C. In step S 71 , the standard first predetermined time interval is set at, for example, 85 msec, and the first predetermined time interval will be change to, for example, 80 msec when temperature is low. In accordance with the embodiments of the invention, since the double feed in which the separability of overlapped sheets lowers at high temperature can be well prevented and no-feed caused by the characteristic that a conveyance roller and a sheet tend to slip each other at low temperature can be well prevented, high speed paper feed and conveyance become to be stably conducted.