Patent Publication Number: US-2023159289-A1

Title: Sheet feeding apparatus and image forming apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This patent application is based on and claims priority pursuant to 35 U.S.C. § 119(a) to Japanese Patent Application No. 2021-191038, filed on Nov. 25, 2021, in the Japan Patent Office, the entire disclosure of which is hereby incorporated by reference herein. 
     BACKGROUND 
     Technical Field 
     Embodiments of the present disclosure relate to a sheet feeding apparatus and an image forming apparatus. 
     Related Art 
     A sheet feeding apparatus is known that includes a sheet stacker, an air blower, and a suction feeder. Multiple sheets are stacked on the sheet stacker. The air blower blows air to the multiple sheets stacked on the sheet stacker from a lateral side of the sheets to float an uppermost sheet of the sheets. The suction feeder is disposed above the sheet stacker and attracts the sheet floated by the air blower to feed the sheet in a feed direction. 
     In such a sheet feeding apparatus, when the number of sheets stacked on the sheet stacker decreases, air blown from the air blower passes above the sheets, and the sheets may not be properly floated. For this reason, a technology has been disclosed in which a sheet stacker is lifted by a constant elevation amount each time a sheet is fed when the number of sheets stacked on the sheet stacker is small. 
     SUMMARY 
     According to an embodiment of the present disclosure, a sheet feeding apparatus includes a sheet stacker, an air blower, a suction feeder, a lifting mechanism, an elevation detection sensor, a feed detection sensor, and processing circuitry. A plurality of sheets are stacked on the sheet stacker. The air blower blows air from a lateral side of the plurality of sheets stacked on the sheet stacker to the plurality of sheets to float an uppermost sheet of the plurality of sheets. The suction feeder is disposed above the sheet stacker and sucks the uppermost sheet floated by the air blower and feeds the sheet in a feed direction. The lifting mechanism lifts the sheet stacker. The elevation detection sensor detects that the plurality of sheets stacked on the sheet stacker has reached a detection position located above the sheet stacker and below the suction feeder. The feed detection sensor detects the sheet fed by the suction feeder. The processing circuitry controls operations of the air blower, the suction feeder, and the lifting mechanism based on detection results of the elevation detection sensor and the feed detection sensor. The processing circuitry drives the lifting mechanism such that the sheet stacker is lifted by an elevation amount determined based on a sheet thickness of the sheets stacked on the sheet stacker, each time a sheet is detected by the feed detection sensor when a number of sheets detected by the feed detection sensor is smaller than a threshold number of sheets and stops lifting of the sheet stacker until a sheet is not detected by the elevation detection sensor when the number of sheets reaches the threshold number of sheets, while repeatedly performing processing to feed the uppermost sheet floated by the air blower to the suction feeder and count the number of sheets detected by the feed detection sensor. 
     According to another embodiment of the present disclosure, an image forming apparatus includes the sheet feeding apparatus and an image forming device configured to form an image on a sheet fed by the sheet feeding apparatus. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       A more complete appreciation of the disclosure and many of the attendant advantages and features thereof can be readily obtained and understood from the following detailed description with reference to the accompanying drawings, wherein: 
         FIG.  1    is a schematic diagram illustrating an internal configuration of an image forming apparatus according to an embodiment of the present disclosure; 
         FIG.  2    is a schematic diagram illustrating a configuration of a feeder according to an embodiment of the present disclosure; 
         FIGS.  3 A,  3 B,  3 C, and  3 D  are diagrams illustrating how the feeder of  FIG.  2    operates; 
         FIG.  4    is a block diagram illustrating a hardware configuration of the image forming apparatus of  FIG.  1   ; 
         FIG.  5    is a functional block diagram of a controller according to an embodiment of the present disclosure; 
         FIG.  6    is a flowchart of elevation amount calculation processing according to an embodiment of the present disclosure; 
         FIG.  7    is a graph illustrating a correspondence relation between basis weight and range of sheet thickness stored in a memory, according to an embodiment of the present disclosure; and 
         FIG.  8    is a flowchart illustrating feeding processing according to an embodiment of the present disclosure. 
     
    
    
     The accompanying drawings are intended to depict embodiments of the present invention and should not be interpreted to limit the scope thereof. The accompanying drawings are not to be considered as drawn to scale unless explicitly noted. Also, identical or similar reference numerals designate identical or similar components throughout the several views. 
     DETAILED DESCRIPTION 
     In describing embodiments illustrated in the drawings, specific terminology is employed for the sake of clarity. However, the disclosure of this specification is not intended to be limited to the specific terminology so selected and it is to be understood that each specific element includes all technical equivalents that have a similar function, operate in a similar manner, and achieve a similar result. 
     Referring now to the drawings, embodiments of the present disclosure are described below. As used herein, the singular forms “a,” “an,” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. 
     Embodiments of the present disclosure are described below with reference to the attached drawings.  FIG.  1    is a schematic diagram illustrating an internal configuration of an image forming apparatus  100  according to an embodiment of the present disclosure. As illustrated in  FIG.  1   , the image forming apparatus  100  typically includes a feeder  110  as a sheet feeder, a conveyor  120 , an image forming device  130 , and an output tray  140 . In the feeder  110 , multiple sheets M as sheets of paper on which no images have been yet formed are stacked and stored. The sheet M on which an image has been formed is stored in the output tray  140 . 
     The sheet M is an example of a sheet that is fed from the feeder  110 , conveyed by the conveyor  120 , and on which an image is formed by the image forming device  130 . However, the sheet M is not limited to a sheet of paper, and may be, for example, an overhead projector (OHP) sheet, or cloth. A conveyance path R 1  that is a space in which the sheet M is conveyed is formed inside the image forming apparatus  100 . The conveyance path R 1  is a path extending from the feeder  110  to the output tray  140  via a position facing the image forming device  130 . 
     The feeder  110  stacks and stores multiple sheets M and feeds and supplies the stacked sheets M one by one to the conveyor  120 . More specifically, the feeder  110  floats an uppermost sheet M of the stacked sheets M to feed the sheet M. A detailed configuration of the feeder  110  will be described below with reference to  FIGS.  2  and  3   . 
     The conveyor  120  conveys the sheet M fed from the feeder  110  in the conveyance path R 1 . Specifically, the conveyor  120  conveys the sheet M stored in the feeder  110  to the position facing the image forming device  130  in the conveyance path R 1 . The conveyor  120  ejects the sheet M on which an image has been formed by the image forming device  130  to the output tray  140  in the conveyance path R 1 . 
     The conveyor  120  includes multiple conveyance roller pairs  121  and  122 . Each of the conveyance roller pairs  121  and  122  includes, for example, a driving roller to which a driving force of a motor is transmitted to rotate, and a driven roller that contacts the driving roller to be driven to rotate. The driving rollers and the driven rollers rotate while nipping the sheet M to convey the sheet M in the conveyance path R 1 . 
     The conveyance roller pair  121  is disposed upstream from the image forming device  130  in the conveyance direction. The conveyance roller pair  122  is disposed downstream from the image forming device  130  in the conveyance direction. However, positions at which the conveyance roller pair  121  and the conveyance roller pair  122  are disposed are not limited to the two positions illustrated in  FIG.  1   . 
     The image forming device  130  is disposed between the conveyance roller pair  121  and the conveyance roller pair  122  at a position facing the conveyance path R 1 . The image forming device  130  forms an image on a surface of a sheet M conveyed by the conveyor  120 . The image forming device  130  according to the present embodiment forms an image on the sheet M conveyed in the conveyance path R 1  by an electrophotographic method. However, the image forming method of the image forming device  130  may be an inkjet recording method in which ink is discharged onto a sheet M to form an image. 
     More specifically, in the image forming device  130 , photoconductor drums  131 Y,  131 M,  131 C, and  131 K (referred to collectively as photoconductor drums  131  in the following description) for the respective colors are arranged along a transfer belt  132  that is an endless moving conveyor. In other words, the multiple photoconductor drums  131 Y,  131 M,  131 C, and  131 K are arranged along the transfer belt  132 , on which an intermediate transfer image to be transferred to the sheet M fed from the feeder  110  is formed, in this order from upstream in a conveyance direction of the transfer belt  132 . 
     Toner contained in toner bottles is supplied to the photoconductor drums  131 . Images of the colors developed by the toner on the surfaces of the photoconductor drums  131 Y,  131 M,  131 C, and  131 K of the respective colors are superimposed and transferred onto the transfer belt  132 . Thus, a full-color image is formed on the transfer belt  132 . The full-color image formed on the transfer belt  132  is transferred to the sheet M by a transfer roller  133  at a position closest to the conveyance path R 1 . 
     Further, the image forming device  130  includes a fixing roller pair  134  disposed downstream from the transfer roller  133  in the conveyance direction. The fixing roller pair  134  includes a driving roller that is driven by a motor to rotate, and a driven roller that contacts the driving roller to be driven to rotate by the driving roller. Then, the driving roller and the driven roller rotate while the sheet M is nipped by the driving roller and the driven roller. At this time, the sheet M is heated and pressed and the image transferred by the transfer roller  133  is fixed onto the sheet M. 
       FIG.  2    is a schematic diagram illustrating the feeder  110  according to the present embodiment.  FIGS.  3 A,  3 B,  3 C, and  3 D  are diagrams illustrating how the feeder  110  operates, according to the present embodiment. The feeder  110  feeds sheets M one by one to the conveyance path R 1  through a feed path R 0 . As illustrated in  FIG.  2   , the feeder  110  typically includes a sheet stacker  111  as a sheet stacker, an air blower  112 , a suction feeder  113 , a nip feeder  114 , a lifting mechanism  115 , an elevation detection sensor  116 , a feed detection sensor  117 , and a remaining amount detection sensor  118 . 
     The sheet stacker  111  is an output tray or a sheet feed cassette on which multiple sheets M can be stacked. Sheets M can be replenished in the sheet stacker  111  by a user. Further, the sheet stacker  111  is supported by a frame of the feeder  110  so as to be moved up and down within a predetermined elevation range by the lifting mechanism  115 . 
     The air blower  112  is disposed above the sheet stacker  111  and below the suction feeder  113 . In addition, the air blower  112  is disposed at a position at which the air blower  112  can face the sheets M stacked on the sheet stacker  111  in the horizontal direction. As illustrated in  FIG.  3 A , the air blower  112  blows air from a lateral side of the sheets M to the multiple sheets M stacked on the sheet stacker  111  to float an uppermost sheet M. 
     The air blower  112  includes, for example, a float blower  112   a  and a blower port  112   b . The float blower  112   a  generates air to float the uppermost sheet M. The blower port  112   b  blows the air generated by the float blower  112   a  obliquely upward toward the sheets M stacked on the sheet stacker  111 . Then, the sheet stacker  111  is lifted or lowered by the lifting mechanism  115  so that the uppermost sheet M is positioned in a path in which the air is blown from the blower port  112   b  Thus, the uppermost sheet M is floated. 
     The suction feeder  113  is disposed above the sheet stacker  111 , the air blower  112 , and the elevation detection sensor  116 . Further, the suction feeder  113  is disposed upstream from the nip feeder  114  and the feed detection sensor  117  in the feed direction. The suction feeder  113  attracts a sheet M floated by the air blower  112  and conveys the sheet M in the feed direction in the feed path R 0 . The feed path R 0  is connected to the conveyance path R 1 . 
     The suction feeder  113  includes, for example, a driving pulley  113   a , a driven pulley  113   b , an endless annular belt  113   c , a feeding motor  113   d , a suction port  113   e , and a suction fan  113   f  The driving pulley  113   a  and the driven pulley  113   b  are each rotatably supported at positions spaced apart in the feed direction. The endless annular belt  113   c  is wound around the driving pulley  113   a  and the driven pulley  113   b . Multiple through-holes are formed on the surface of the endless annular belt  113   c . The feeding motor  113   d  rotates the driving pulley  113   a . The suction port  113   e  is disposed inside the endless annular belt  113   c  and is opened downward. The suction fan  113   f  sucks air from below the suction feeder  113  through the suction port  113   e  and the through-holes of the endless annular belt  113   c.    
     The suction fan  113   f  is driven to generate an upward air flow, as illustrated in  FIG.  3 B . Accordingly, a sheet M floated by the air blower  112  is attracted to a lower surface of the endless annular belt  113   c . Further, the feeding motor  113   d  is driven to rotate the driving pulley  113   a , in other words, the feeding motor  113   d  is driven to rotate the endless annular belt  113   c  counterclockwise, as illustrated in  FIG.  3 C . Accordingly, the sheet M attracted to the lower surface of the endless annular belt  113   c  is conveyed in the feed path R 0  and supplied to the nip feeder  114 . 
     The nip feeder  114  is disposed downstream from the suction feeder  113  in the feed direction and upstream from the feed detection sensor  117  in the feed direction. The nip feeder  114  feeds the sheet M supplied from the suction feeder  113  in the feed direction in the feed path R 0 . The nip feeder  114  includes, for example, a driving roller  114   a , a driven roller  114   b , and a feeding motor  114   c.    
     The driving roller  114   a  and the driven roller  114   b  are rotatably supported by each other. The driving roller  114   a  and the driven roller  114   b  are in contact with each other with the feed path R 0  interposed between the driving roller  114   a  and the driven roller  114   b . The feeding motor  114   c  rotates the driving roller  114   a . The driving roller  114   a  and the driven roller  114   b  of the nip feeder  114  nip and feed a sheet M that enters between the driving roller  114   a  and the driven roller  114   b . Thus, the sheet M is fed to the conveyance path R 1 . 
     The lifting mechanism  115  lifts or lowers the sheet stacker  111 . The lifting mechanism  115  includes, for example, a lifting motor  115   a  and a driving force transmitter that transmits the driving force of the lifting motor  115   a  to the sheet stacker  111 . The driving force transmitter may include, for example, a pulley that is rotatably supported, and a belt that is wound around the pulley, with one end of the belt being connected to the sheet stacker  111  and the other end of the belt being connected to an output shaft of the lifting motor  115   a . Then, as illustrated in  FIG.  3 D , the lifting mechanism  115  causes the lifting motor  115   a  to rotate in a first direction to lift the sheet stacker  111 . Further, the lifting mechanism  115  rotates the lifting motor  115   a  in a second direction opposite to the first direction to lower the sheet stacker  111 . 
     The elevation detection sensor  116  is fixed at a detection position located above the sheet stacker  111  and below the suction feeder  113 . More specifically, the elevation detection sensor  116  is located at a position higher than the sheet stacker  111  in the horizontal direction when the sheet stacker  111  is located at an upper end of the elevation range of the sheet stacker  111 . Further, the elevation detection sensor  116  is disposed at a position lower than the lower surface of the endless annular belt  113   c  in the horizontal direction by a height h (see  FIG.  2   ). Further, the elevation detection sensor  116  is disposed at a position at which the elevation detection sensor  116  can face the sheets M stacked on the sheet stacker  111  in the horizontal direction. Accordingly, the elevation detection sensor  116  detects whether the uppermost sheet M of the sheets M stacked on the sheet stacker  111  reaches the detection position. 
     The elevation detection sensor  116  is, for example, a reflection-type optical sensor including a light emitter and a light receiver. The light emitter emits light in the horizontal direction from the detection position. The light receiver receives the light emitted from the light emitter and reflected by the sheets M stacked on the sheet stacker  111 . When the light receiver receives the light, the elevation detection sensor  116  outputs an arrival signal indicating that the uppermost sheet M has reached the detection position to a controller  150 , which will be described later. On the other hand, when the light receiver does not receive the light, the elevation detection sensor  116  stops outputting the arrival signal to the controller  150 . 
     The feed detection sensor  117  is disposed downstream from the suction feeder  113  and the nip feeder  114  in the feed direction. Further, the feed detection sensor  117  is disposed to face the feed path R 0 . The feed detection sensor  117  detects whether a sheet M has passed through the feed path R 0 , in other words, whether the sheet M has been fed. 
     The feed detection sensor  117  is, for example, a reflection type optical sensor including a light emitter and a light receiver. The light emitter emits light toward the feed path R 0 . The light receiver receives the light emitted from the light emitter and reflected by the sheet M that passes through the feed path R 0 . When the light receiver receives the light, the feed detection sensor  117  outputs a feed signal indicating that the sheet M has been fed to the controller  150 . On the other hand, when the light receiver does not receive the light, the feed detection sensor  117  stops outputting the feed signal to the controller  150 . 
     The remaining amount detection sensor  118  is disposed at a position at which the remaining amount detection sensor  118  can face the sheets M stacked on the sheet stacker  111  in the horizontal direction. The remaining amount detection sensor  118  is movable up and down together with the sheet stacker  111  at a position slightly above the upper surface of the sheet stacker  111  in the horizontal direction. The remaining amount detection sensor  118  detects the remaining amount of the sheets M stacked on the sheet stacker  111 . The remaining amount of the sheets M is indicated by, for example, a ratio when a maximum amount of the sheets M, e.g., a maximum number of the sheets M, that can be stacked on the sheet stacker  111  is set to 100%. 
     The remaining amount detection sensor  118  is, for example, a reflection-type optical sensor including a light emitter and a light receiver. The light emitter emits light in the horizontal direction. The light receiver receives the light emitted from the light emitter and reflected by the sheets M stacked on the sheet stacker  111 . When the light receiver receives the light, the remaining amount detection sensor  118  outputs a remaining amount signal to the controller  150 . The remaining amount signal indicates that the remaining amount of the sheets M stacked on the sheet stacker  111  is equal to or greater than a threshold remaining amount of X %. On the other hand, when the light receiver does not receive the light, the remaining amount detection sensor  118  stops outputting the remaining amount signal to the controller  150 . 
       FIG.  4    is a block diagram illustrating a hardware configuration of the image forming apparatus  100  according to the present embodiment. The image forming apparatus  100  includes a central processing unit (CPU)  101  as a controller, a random access memory (RAM)  102  as a memory, a read only memory (ROM)  103  as a memory, a hard disk drive (HDD)  104  as a memory, and an interface (I/F)  105 . The CPU  101 , the RAM  102 , the ROM  103 , the HDD  104 , and the I/F  105  are connected to each other via a common bus  109  as a communication member. The CPU  101 , the RAM  102 , the ROM  103 , and the HDD  104  collectively serve as the controller  150 . 
     The CPU  101  is an arithmetic unit and controls the entire operation of the image forming apparatus  100 . The RAM  102  is a volatile recording medium capable of reading and writing data at high speed and is used as a work area when the CPU  101  processes the data. The ROM  103  is a read-only non-volatile recording medium in which programs such as firmware are stored. The HDD  104  is a large-capacity non-volatile recording medium capable of reading and writing data and stores, for example, an operating system (OS), various control programs, and application programs. 
     The image forming apparatus  100  processes programs such as a control program stored in the ROM  103 , a data-processing program, which is an application program, loaded into the HDD  104  from a recording medium such as the RAM  102  by a calculation function included in the CPU  101 . Such processing as described above is performed by a software controller that includes various functional modules of the image forming apparatus  100 . A functional block that implements the functions of the image forming apparatus  100  includes a combination of the software controller as described above and the hardware resources installed in the image forming apparatus  100 . 
     The I/F  105  is an interface that connects the feeder  110 , the conveyor  120 , the image forming device  130 , and an operation panel  160  to the common bus  109 . In other words, the controller  150  controls operations of the feeder  110 , the conveyor  120 , the image forming device  130 , and the operation panel  160  via the I/F  105 . 
     The operation panel  160  serves as a user interface that includes a display that displays, for example, current setting values, a selection screen and an operation panel that includes, for example, a touch panel and push buttons, that receives an input operation from a user. 
       FIG.  5    is a functional block diagram of the controller  150 , according to the present embodiment. The controller  150  typically includes a feed processing unit  151 , a counter  152 , a correction value acquisition unit  153 , a thickness value acquisition unit  154 , an elevation amount determination unit  155 , a threshold value determination unit  156 , and an elevation processing unit  157 . Each of the controller  150 , the feed processing unit  151 , the counter  152 , the correction value acquisition unit  153 , the thickness value acquisition unit  154 , the elevation amount determination unit  155 , the threshold value determination unit  156 , and the elevation processing unit  157  as the functional blocks that constitutes the controller  150  is implemented by, for example, the CPU  101  that executes programs stored in the memory. The controller  150 , the feed processing unit  151 , the counter  152 , the correction value acquisition unit  153 , the thickness value acquisition unit  154 , the elevation amount determination unit  155 , the threshold value determination unit  156 , and the elevation processing unit  157  as the functional blocks operate in conjunction with each other to feed multiple sheets M stacked on the sheet stacker  111  to the conveyance path R 1  one by one, as illustrated in  FIG.  5   . 
     As illustrated in  FIG.  5   , the feed processing unit  151  drives the float blower  112   a , the suction fan  113   f , and the feeding motors  113   d  and  114   c  to feed the multiple sheets M stacked on the sheet stacker  111  to the conveyance path R 1  in order one by one. 
     The counter  152  counts the number of sheets M fed by the feed processing unit  151 . Specifically, the counter  152  increments the number of sheets fed N, stored in the HDD  104  each time when a feed signal is output from the feed detection sensor  117 . The number of sheets fed N is reset when sheets M are replenished to the sheet stacker  111  or in step S 809  of  FIG.  8    and an initial value zero is assigned as the number of sheets fed N. 
     The correction value acquisition unit  153  acquires correction values α1 and α2 through the operation panel  160  input by a user of the image forming apparatus  100 . The correction values α1 and α2 according to the present embodiment are numerical values larger than one (α1&gt;1, α2&gt;1). Further, the correction value α2 is larger than the correction value α1 (α2&gt;α1). 
     The thickness value acquisition unit  154  acquires a sheet thickness t of sheets M stacked on the sheet stacker  111  through the operation panel  160  input by a user. As an example, a user may directly input the sheet thickness t through the operation panel  160 . As another example, a user may input a basis weight of the sheet M through the operation panel  160 . Then, the thickness value acquisition unit  154  may read the sheet thickness t corresponding to the input basis weight (for example, a sheet thickness tmin, a sheet thickness tavg., and a sheet thickness tmax in  FIG.  7   ) from the memory. The sheet thickness tmax, the sheet thickness tmin, and the sheet thickness tavg. are a maximum value, a minimum value, and an average value, respectively, of the sheet thickness t corresponding to the input basis weight. The feeder  110  may include a thickness detection sensor that detects the sheet thickness t. The thickness value acquisition unit  154  may acquire the sheet thickness t detected by the thickness detection sensor. 
     The elevation amount determination unit  155  determines elevation amounts H 1  and H 2  of the sheet stacker  111  lifted by the elevation processing unit  157  based on the correction values α1 and α2 acquired by the correction value acquisition unit  153  and the sheet thickness t acquired by the thickness value acquisition unit  154 . The elevation amount H 1  as a first elevation amount is an elevation amount of the sheet stacker  111  when the remaining amount of sheets M detected by the remaining amount detection sensor  118  is equal to or greater than the threshold remaining amount of X %. The elevation amount H 2  as a second elevation amount is an elevation amount of the sheet stacker  111  when the remaining sheet amount of sheets M detected by the remaining amount detection sensor  118  is smaller than the threshold remaining amount of X %. The elevation amount H 2  is set to a value larger than the elevation amount H 1 . 
       FIG.  6    is a flowchart illustrating how an elevation amount calculation processing is performed according to the present embodiment. The elevation amount determination unit  155  acquires the correction values α1 and α2 through the correction value acquisition unit  153  (S 601 , S 602 ). In addition, the elevation amount determination unit  155  acquires the thickness t through the thickness value acquisition unit  154  (S 603 ). Then, the elevation amount determination unit  155  multiplies the sheet thickness t by the correction value α1 to determine the elevation amount H 1  (S 604 ). Further, the elevation amount determination unit  155  multiplies the sheet thickness t by the correction value α2 to determine the elevation amount H 2  (S 605 ). Each of the correction values α1 and α2 is larger than one. Accordingly, each of the elevation amounts H 1  and H 2  is larger than the sheet thickness t. 
     However, the method of determining the elevation amounts H 1  and H 2  is not limited to the example of  FIG.  6   . As another example, the elevation amount determination unit  155  may add the correction amount α1 to the sheet thickness t to determine the elevation amount H 1 , and may add the correction value α2 to the sheet thickness t to determine the elevation amount H 2 . The correction values α1 and α2 in this case are positive values. As still another example, the elevation amount determination unit  155  may acquire the elevation amounts H 1  and H 2  from a user through the operation panel  160 . 
     The threshold value determination unit  156  determines a threshold number of sheets Nth. The threshold number of sheets Nth is a value equivalent to the number of sheets fed N that indicates the number of sheets fed M when the processing of lifting the sheet stacker  111  is stopped. In other words, the threshold number of sheets Nth is a value to be compared with the number of sheets fed N. The threshold number of sheets Nth may be a fixed value. However, the threshold number of sheets Nth can be determined by, for example, the following method described below. 
       FIG.  7    is a graph illustrating a correspondence relation between basis weight and range of sheet thickness stored in the memory. As illustrated in  FIG.  7   , the correspondence relations between multiple basis weights 0, 1, 2, 3, 4, 5, 6, 7, 8, and 9 and the ranges of sheet thicknesses are stored in the HDD  104  serving as memory. The basis weight refers to a weight per 1 m 2  of the sheet M. The range of the sheet thickness ranges from a maximum value, i.e., the sheet thickness tmax to a minimum value, i.e., the sheet thickness tmin of a sheet M having a corresponding basis weight. An average value, i.e., the sheet thickness tavg. of sheets M corresponding to each corresponding one of the basis weights may be stored in the HDD  104 . Further, an actual sheet thickness of a sheet M fed from the feeder  110  is set as a set sheet thickness t0 as a set sheet thickness value. For example, the set sheet thickness t0 may be set by a user through the operation panel  160  or may be set to a sheet thickness value corresponding to the basis weight stored in the HDD  104 . 
     For example, the threshold value determination unit  156  reads the sheet thickness tmin, which corresponds to a basis weight input through the operation panel  160 , from the HDD  104 . Then, the threshold value determination unit  156  determines the threshold number of sheets Nth based on the following formula 1. Note that a in the formula 1 described below is one of the correction values α1 and α2 acquired by the correction value acquisition unit  153 . 
       Threshold number of sheets  Nth=h× 1000/(α× t 0− t min)  Formula 1
 
     As another example, the threshold value determination unit  156  may determine the threshold number of sheets Nth based on the following formula 2. Note that a in the formula 2 described below is one of the correction values α1 and α2 acquired by the correction value acquisition unit  153 . In this case, the correspondence relation illustrated in  FIG.  7    can be omitted. 
       Threshold number of sheets  Nth=h× 1000/(α× t 0− t )  Formula 2
 
     The elevation processing unit  157  causes the lifting mechanism  115  to lift the sheet stacker  111  based on signals output from the elevation detection sensor  116 , the feed detection sensor  117 , and the remaining amount detection sensor  118 , the number of sheets fed N counted by the counter  152 , the elevation amounts H 1  and H 2  determined by the elevation amount determination unit  155 , and the threshold number of sheets Nth determined by the threshold value determination unit  156 . In addition, the elevation processing unit  157  causes the lifting mechanism  115  to lower the sheet stacker  111  at a timing when sheets M are replenished to the sheet stacker  111 . 
       FIG.  8    is a flowchart of feeding processing according to the present embodiment. The controller  150  executes the feeding processing at a timing when an image forming instruction is input to the image forming apparatus  100 . The controller  150  repeatedly executes the feeding processing when images are formed on multiple sheets M. Note that the feeding processing is executed by the feeding processing unit  151 , the counter  152 , and the elevation processing unit  157 . On the other hand, the processing of the correction value acquisition unit  153 , the thickness value acquisition unit  154 , the elevation amount determination unit  155 , and the threshold value determination unit  156  is executed before the feeding processing starts. 
     First, the feeding processing unit  151  drives the float blower  112   a , the suction fan  113   f , and the feeding motors  113   d  and  114   c  (S 801 ). Thus, as illustrated in  FIGS.  3 A,  3 B, and  3 C , one sheet M is fed to the feed path R 0 . Then, the execution of the feeding processing after step S 803  is put on standby until a feeding signal is output from the feed detection sensor  117  (NO in S 802 ). 
     When the feed signal is output from the feed detection sensor  117  (YES in S 802 ), the counter  152  increments the number of sheets fed N stored in the HDD  104  (N=N+1) (S 803 ). Further, in response to the output of the feed signal from the feed detection sensor  117  (YES in S 802 ), the elevation processing unit  157  executes the processing of steps S 804 , S 805 , S 806 , S 807 , S 808 , and S 809 . Further, in parallel with the processing in steps S 803 , S 804 , S 805 , S 806 , S 807 , S 808 , and S 809 , the conveyor  120  and the image forming device  130  convey the sheet M fed from the feeder  110  in the conveyance path R 1  and form an image on the sheet M. 
     The elevation processing unit  157  compares the number of sheets fed N counted by the counter  152  with the threshold number of sheets Nth determined by the threshold value determination unit  156  (S 804 ). When the number of sheets fed N is smaller than the threshold number of sheets Nth (NO in S 804 ), the elevation processing unit  157  determines whether a remaining amount signal is output from the remaining amount detection sensor  118 , in other words, whether the sheet remaining amount is equal to or greater than the threshold remaining amount of X % (S 805 ). 
     Then, when the remaining amount signal is output from the remaining amount detection sensor  118 , in other words, the remaining amount of sheets M is equal to or greater than the threshold remaining amount of X % (YES in S 805 ), the elevation processing unit  157  drives the lifting mechanism  115  so that the sheet stacker  111  is lifted by the elevation amount H 1  determined by the elevation amount determination unit  155  (S 806 ). Further, when the output of the remaining amount signal from the remaining amount detection sensor  118  is stopped, in other words, the remaining amount of sheets M is smaller than the threshold remaining amount of X % (NO in S 805 ), the elevation processing unit  157  drives the lifting mechanism  115  so that the sheet stacker  111  is lifted by the elevation amount H 2  determined by the elevation amount determination unit  155  (S 807 ). 
     On the other hand, when the number of sheets fed N reaches the threshold number of sheets Nth (YES in S 804 ), the elevation processing unit  157  determines whether an arrival signal is output from the elevation detection sensor  116 , in other words, whether sheets M are present at the detection position (S 808 ). When the arrival signal is output from the elevation detection sensor  116 , in other words, the sheets M are present at the detection position (YES in S 808 ), the elevation processing unit  157  ends the feeding processing without executing the processing of steps S 805 , S 806 , S 807 , S 808 , and S 809 . In addition, when the output of the arrival signal from the elevation detection sensor  116  is stopped, in other words, the sheets M are not present at the detection position (NO in S 808 ), the elevation processing unit  157  resets the number of sheets fed N stored in the HDD  104  and the initial value zero is assigned as the number of sheets fed N without executing the processing of steps S 805 , S 806 , and S 807  (S 809 ). 
     In other words, when the number of sheets fed N counted by the counter  152  is smaller than the threshold number of sheets Nth (NO in S 804 ) while the feeding processing is repeatedly performed, the elevation processing unit  157  lifts the sheet stacker  111  each time the feed signal is output from the feed detection sensor  117  (S 805 , S 806 , S 807 ). When the number of sheets fed N counted by the counter  152  reaches the threshold number of sheets Nth while the feeding processing is repeatedly performed (YES in S 804 ), the elevation processing unit  157  stops the lifting of the sheet stacker  111 . Further, the elevation processing unit  157  restarts the lifting of the sheet stacker  111  from the next feeding processing in which the number of sheets fed N is reset (S 809 ). 
     According to the above-described embodiments, for example, the following operational effects can be achieved. 
     The above-described embodiments allow the sheet stacker  111  to be lifted each time one sheet M is fed. Accordingly, an uppermost sheet M stacked on the sheet stacker  111  can be positioned in a path in which air is blown from the air blower  112 . Accordingly, non-feeding of the sheet M in the feeding processing can be prevented. In addition, setting the elevation amounts H 1  and H 2  to values greater than the sheet thickness t can effectively prevent the sheet M from not being fed in the feeding processing. 
     However, when the lifting of the sheet stacker  111  is repeated, an error between the total of the sheet thicknesses t of the multiple fed sheets M and the total of the elevation amounts of the sheet stacker  111  is accumulated. Accordingly, as in one of the above-described embodiments, the lifting of the sheet stacker  111  is temporarily stopped when the threshold number of sheets Nth is fed. Thus, the accumulated error can be reset. Accordingly, double feeding caused by multiple sheets M floating together can be prevented. 
     Non-feeding or double feeding of sheets M is likely to occur when the stacking height of the sheets M on the sheet stacker  111  is low. For this reason, as described in the above embodiments, the elevation amount H 2  when the remaining amount of sheets M is small, is set to a value greater than the elevation amount H 1  when the remaining amount of sheets M is large. Thus, the sheets M can be reliably fed even when the stacking height of the sheets M on the sheet stacker  111  is low. However, the elevation amount of the sheet stacker  111  may be set to a constant value regardless of the remaining amount of sheets M on the sheet stacker  111 . In other words, steps S 602  and S 605  in  FIG.  6    and steps S 805  and S 807  in  FIG.  8    can be omitted. 
     Furthermore, as described in the above-described embodiments, the threshold number of sheets Nth using formula 1 or formula 2 is set and the lifting of the sheet stacker  111  is stopped when the number of sheets fed N reaches the threshold number of sheet Nth. Accordingly, double feeding of sheets M caused by the sheet stacker  111  and the endless annular belt  113   c  moving too close to each other can be prevented. 
     Each of the functions that have been described in the above-described embodiments can be implemented by one processing circuit or multiple processing circuits. In the embodiments of the present disclosure, the processing circuit includes a processor programmed to execute each of the functions by software such as a processor implemented by an electronic circuit, and a device such as an application specific integrated circuit (ASIC), a digital signal processor (DSP), a field programmable gate array (FPGA), or a conventional circuit module designed to execute each function described above. 
     Note that the present disclosure is not limited to the above-described embodiments, and various modifications can be made without departing from the technical gist of the present disclosure, and all technical matters included in the technical idea described in the claims are the object of the present disclosure. It is therefore to be understood that the disclosure of the present specification may be practiced otherwise by those skilled in the art than as specifically described herein. Such embodiments and modifications thereof are included in the scope and gist according to the embodiments of the present disclosure and are included in the embodiments described in claims and the equivalent scope thereof. 
     The above-described embodiments are illustrative and do not limit the present disclosure. Thus, numerous additional modifications and variations are possible in light of the above teachings. For example, elements and/or features of different illustrative embodiments may be combined with each other and/or substituted for each other within the scope of the present disclosure. Any one of the above-described operations may be performed in various other ways, for example, in an order different from the one described above. 
     The present disclosure can be implemented in any convenient form, for example using dedicated hardware, or a mixture of dedicated hardware and software. The present disclosure may be implemented as computer software implemented by one or more networked processing apparatuses. The processing apparatuses include any suitably programmed apparatuses such as a general purpose computer, a personal digital assistant, a Wireless Application Protocol (WAP) or third-generation (3G)-compliant mobile telephone, and so on. Since the present disclosure can be implemented as software, each and every aspect of the present disclosure thus encompasses computer software implementable on a programmable device. The computer software can be provided to the programmable device using any conventional carrier medium (carrier means). The carrier medium includes a transient carrier medium such as an electrical, optical, microwave, acoustic or radio frequency signal carrying the computer code. An example of such a transient medium is a Transmission Control Protocol/Internet Protocol (TCP/IP) signal carrying computer code over an IP network, such as the Internet. The carrier medium may also include a storage medium for storing processor readable code such as a floppy disk, a hard disk, a compact disc read-only memory (CD-ROM), a magnetic tape device, or a solid state storage medium.