Patent Publication Number: US-7913993-B2

Title: Sheet feeding device, sheet feeding unit and image forming apparatus connected with the sheet feeding unit with a controlled floating air blowing mechanism

Description:
This application is based on Japanese Patent Application No. 2008-316053 filed on Dec. 11, 2008, which is incorporated hereinto by reference. 
     BACKGROUND OF THE INVENTION 
     The present invention relates to a sheet feeding device wherein sheets stacked on a sheet stacking table are sucked to a suction surface by air to be fed out, and in particular, to an object wherein an air volume of floating air is made to be reasonable based on the state of floating in the case of blowing floating air against sheets stacked on the sheet stacking table. 
     An air sheet feeder has been used as a sheet feeding device of an image forming apparatus such as a copying machine. In the sheet feeding device, there is employed a mechanism wherein air is blown against a side edge surface of stacked sheets, and a sheet thus floated is sucked on a suction surface such as a perforated belt on which a suction opening that draws air is formed, and the belt is driven to rotate to convey the sheet. 
     With respect to the air sheet feeder of this kind, there has been proposed a technology in which an air volume is made to be variable depending on a sheet type (for example, see Unexamined Japanese Patent Application Publication No. 2005-75540). Further, there has been proposed a technology in which sheet feeding timing is detected to judge presence or absence of sheet feeding delay, and an air blowing operation is carried out (for example, see Unexamined Japanese Patent Application Publication No. 2005-96993). 
     In addition, there has been proposed a technology in which a period of time up to suction for sheet suction detection is measured to compare the measured time with a reference value, thus, the air volume is made to be variable (for example, see Unexamined Japanese Patent Application Publication No. S60-56739). There has further been proposed a technology to control an air volume by detecting a sheet floating position (for example, see Unexamined Japanese Patent Application Publication No. H07-89625). 
     In the technology described in Unexamined Japanese Patent Application Publication No. 2005-75540, it is impossible to cope with dispersion of the states in the same sheet type and with an influence of the state of keeping for sheets, because conditions are fixed by the sheet type. Further, in the technology described in Unexamined Japanese Patent Application Publication No. 2005-96993, it is impossible to judge whether floating of the sheet is appropriate or not, because a judgment is one under the state of sheet feeding. In the technologies described in Unexamined Japanese Patent Application Publication Nos. S60-56739 and H07-89625, a state is detected for the uppermost sheet among stacked sheets, and states of the second sheet and thereafter which are fed continuously are not reflected on the control of air volume. 
     Therefore, there have been generated dispersions on states of floating by air, and sure separation and conveyance of sheets have been impossible, resulting in problems. 
     SUMMARY OF THE INVENTION 
     The first aspect for solving the aforesaid problems is a sheet feeding device having therein a sheet stacking table that has thereon stacked plural sheets and goes up and down in the direction of stacking sheets, a sucking and conveying mechanism that sucks a sheet stacked on the sheet stacking table from the upper surface, then, causes the uppermost sheet to be sucked on a suction surface and conveys the sheet sucked on the suction surface to a sheet conveyance path, a floating air blowing mechanism that blows floating air against the sheet from the side of sheets stacked on the sheet stacking table, a sheet position detecting sensor that detects an upper surface position of sheets stacked on the sheet stacking table and detects the state of floating of the sheet floated by the aforesaid floating air, and a controller that judges the state of floating of the sheet detected by the aforesaid sheet position detection sensor in floating state detecting period interlocked with blowing out of the aforesaid floating air, and controls an air volume of floating air blown out of the floating air blowing mechanism based on the floating state of the sheet. 
     The second aspect is a sheet feeding unit having therein a single or plural sheet trays which will store sheets, a sheet stacking table that has thereon stacked plural sheets which are stacked on the sheet tray and goes up and down in the direction of stacking sheets, a sucking and conveying mechanism that sucks sheets stacked on a sheet stacking table from the upper surface, then, causes the uppermost sheet to be sucked on a suction surface and conveys the sheet sucked on the suction surface to a sheet conveyance path, a floating air blowing mechanism that blows floating air against the sheet from the side of sheets stacked on the sheet stacking table, a sheet position detecting sensor that detects an upper surface position of sheets stacked on the sheet stacking table and detects the state of floating of the sheet floated by the aforesaid floating air, and a controller that judges the state of floating of the sheet based on the state of detection of the sheet position detecting sensor during the floating state detecting period that interlocks with blowing out of the aforesaid floating air, and controls an air volume of floating air blown out of the floating air blowing mechanism based on the floating state of the sheet. 
     The third aspect is an image forming apparatus equipped with an image forming section that forms an image on a sheet, a sheet feeding device that feeds a sheet to an image forming section and a controller that feeds a sheet to the image forming section from the sheet feeding device, and controls forming of the image in the image forming section, wherein the sheet feeding device has therein a sheet stacking table that has thereon plural stacked sheets and goes up and down in the direction of sheet stacking, a sucking and conveying mechanism that sucks sheets stacked on the sheet stacking table from the upper surface, then, sucks the uppermost sheet on a suction surface and conveys the sheet sucked on the suction surface to a sheet conveyance path, a floating air blowing mechanism that blows floating air from the side of sheets stacked on the sheet stacking table and a sheet position detecting sensor that detects a position of an upper surface of sheets stacked on the sheet stacking table and detects the state of floating of the sheet floated by the aforesaid floating air, and the controller judges the state of floating of the sheet by the state of detection of the sheet position detecting sensor during a period of detection of the state of floating that is interlocked with blowing out of the aforesaid floating air, and controls an air volume of floating air blown out from a floating air blowing mechanism based on the state of floating of the sheet. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a side view showing an example of a sheet feeding device of the present embodiment. 
         FIG. 2  is a front view showing an example of a sheet feeding device of the present embodiment. 
         FIG. 3  is a perspective view of a sheet storage section showing an example of a sheet feeding device of the present embodiment. 
         FIG. 4  is a functional block diagram showing an example of a control system of the sheet feeding device of the present embodiment. 
         FIG. 5  is an illustration showing an example of initial setting condition table Tb 1 . 
         FIG. 6  is an illustration showing an example of detection state table Tb 2 . 
         FIG. 7  is a flow chart showing an example of operations of a sheet feeding device of the present embodiment. 
         FIG. 8  is a time chart showing an example of operations of a sheet feeding device of the present embodiment. 
         FIG. 9  is a structural diagram showing an example of an image forming apparatus and a sheet feeding device equipped with a sheet feeding unit of the present embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Embodiments for the sheet feeding device of the invention, the sheet feeding unit equipped with the sheet feeding device and for the image forming apparatus to which the sheet feeding unit is connected, will be explained as follows, referring to the drawings. 
     &lt;Example of Construction for the Present Embodiment&gt; 
       FIG. 1  is a side view showing an example of a sheet feeding device of the present embodiment,  FIG. 2  is a front view showing an example of a sheet feeding device of the present embodiment and  FIG. 3  is a perspective view of a sheet storage section showing an example of a sheet feeding device of the present embodiment. 
     The sheet feeding device  1 A of the present embodiment is equipped with sucking and conveying mechanism  3  that sucks sheet P stacked on sheet stacking table  2  and feeds out and with floating air blowing mechanism  4  that blows floating air A 1  against sheet P stacked on the sheet stacking table  2 . Further, the sheet feeding device  1 A is equipped with separation air blowing mechanism  5  that blows separation air A 2  against sheet P sucked by sucking and conveying mechanism  3  from the front portion for the conveyance direction shown with arrow F for sheet P. 
     The sheet feeding device  1 A blows out floating air A 1  from floating air blowing mechanism  4 , so that the state of sheet P floated by floating air A 1  can be detected by the operation to float sheet P stacked on the sheet stacking table  2 , and thereby controls so that floating air A 1  may be blown out at appropriate air volume. 
     Details of sheet feeding device  1 A of the present embodiment will be explained as follows. In the sheet feeding device  1 A, sheet stacking table  2  is provided on sheet storage section  20  on which a space capable of storing sheets to be in a form where a prescribed number of sheets P are stacked is formed. 
     The sheet stacking table  2  is caused by an unillustrated rise and fall mechanism to go up and down in the direction of stacking of sheets P. Sheet leading edge stopper surface  21  that regulates a position of the leading edge of sheet P stacked on sheet stacking table  2  is formed on the sheet storage section  20  in the direction of going up and down of sheet stacking table  2 . 
     Sheet feeding device  1 A is equipped with sheet position detection sensor  22  that detects upper surface position Pu of sheet P stacked on sheet stacking table  2  and detects the state of sheet P floated by floating air A 1 . The sheet position detection sensor  22  is composed of a pair of optical sensors, for example, whose detection position is formed, in the stacking direction of sheet P, at a prescribed position of sheet leading edge stopper surface  21 , and is arranged at a position for detecting that upper surface position Pu of sheet P stacked on the sheet stacking table  2  is at suction possibility height H 1  by sucking and conveying mechanism  3 . With respect to sheet position detection sensor  22 , its output, for example, is OFF under the condition where sheet P is absent at suction possibility height H 1 , and its output is ON under the condition where sheet P is present at suction possibility height H 1 . 
     In the operations to float sheet P stacked on sheet stacking table  2  by blowing out floating air A 1  from floating air blowing mechanism  4 , an output (ON/OFF) of sheet position detection sensor  22  is changed during a prescribed period of time from the start of blowing out of floating air A 1 . For example, after blowing out of floating air A 1  is started, a plurality of sheets P are floated, and an output of the sheet position detection sensor  22  is changed from ON to OFF. Among plural sheets P floated by floating air A 1 , those other than the uppermost sheet P sucked by sucking and conveying mechanism  3  are stacked again on the sheet stacking table  2 , and an output of the sheet position detection sensor  22  is changed from OFF to ON. 
     Timing for output of the sheet position detection sensor  22  to change to ON and OFF during the prescribed period of time from the start of blowing out of floating air A 1  is varied depending on a sheet type and a sheet thickness of sheet P. Owing to this, the state of sheet P floated by floating air A 1  is detected from timing of ON/OFF of the sheet position detection sensor  22 . 
     When a height of stacked sheets P on the sheet stacking table  2  is reduced by feeding out of sheets P stacked on the sheet stacking table  2  conducted by sucking and conveying mechanism  3 , sheet feeding device  1 A causes the sheet stacking table  2  to rise to the position where upper surface position Pu of sheet P is detected by the sheet position detection sensor  22 , and upper surface position Pu of stacked sheets P on the sheet stacking table  2  is controlled to reach the suction possibility height H 1 . 
     The sheet feeding device  1 A is equipped with sheet sensor  23  that detects presence or absence of sheep P stacked on the sheet stacking table  2 . The sheet sensor  23  is composed, for example, of a pair of optical sensors to be mounted on the sheet stacking table  2 , and its output is changed depending on presence or absence of sheet P on the sheet stacking table  2 . When sheet P is absent on the sheet stacking table  2 , an output of the sheet sensor  23 , for example, is OFF, while, when sheet P is present on the sheet stacking table  2 , an output is ON. 
     Sucking and conveying mechanism  3  is equipped with conveyance belt  30  on the upper portion of sheet storing section  20 . Further, the sucking and conveying mechanism  3  is equipped with drive roller  31  on which the conveyance belt  30  is wound around, first driven roller  32  and second driven roller group  33  having two driven rollers. 
     The conveyance belt  30  is endless, and suction openings  30   a  each passing through the conveyance belt  30  are arranged in a row in the lateral direction of the conveyance belt  30 , and plural suction openings  30   a  arranged in a row in the lateral direction of the conveyance belt  30  are formed repeatedly on the total area in the longitudinal direction of the conveyance belt  30 . 
     The drive roller  31  has a shaft in the direction perpendicular to the conveyance direction shown with arrow F for sheet P, and is driven to be rotated by a motor which will be explained later. Each of the first driven roller  32  and second driven roller group  33  has a shaft that is in parallel with the shaft of the drive roller  31 , and it is of the structure of free rotation driven by rotation of the conveyance belt  30  that is caused when the drive roller  31  is driven to rotate. 
     In sucking and conveying mechanism  3 , second driven roller group  33  is arranged to be more front side than sheet leading edge hitting surface  21  is, for the conveyance direction for sheet P shown with arrow F. Further, the drive roller  31  is arranged on the upper portion of sheet stacking table  2 . Further, first driven roller  32  is arranged on the upper portion of the sheet stacking table  2  between second driven roller group  33  and drive roller  31 . 
     With respect to the sucking and conveying mechanism  3 , the conveyance belt  30  is spread to be in parallel with the conveyance direction for sheet P between drive roller  31  and second driven roller group  33 . Owing to this, a trailing edge side wound around drive roller  31  of the conveyance belt  30  is positioned at the upper part of sheet P stacked on sheet stacking table  2 , while, a leading edge side wound around second driven roller  33  of the conveyance belt  30  is positioned to be more front side than sheet leading edge stopper surface  21  is. Further, on the sucking and conveying mechanism  3 , two conveyance belts  30  are caused to stand in a row at left and right for the conveyance direction for sheet P. 
     Then, when the drive roller  31  is driven to rotate in the direction shown with an arrow, each conveyance belt  30  rotates, and the side of the conveyance belt  30  facing the sheet stacking table  2  moves in the conveyance direction for sheet P shown with arrow F. 
     The sucking and conveying mechanism  3  is composed so that a lower end position on a circumferential surface of the drive roller  31  and a lower end position on a circumferential surface of the first driven roller  32  may be almost the same in terms of a height. In contrast to this, a lower end position in a circumferential surface of the driven roller on the lower side among the second driven roller group  33  is composed to be higher than a lower end position of the first driven roller  32  by a prescribed amount. 
     Due to this, the sucking and conveying mechanism  3  is almost in parallel with a surface of sheet P stacked on sheet stacking table  2 , between drive roller  31  and first driven roller  32 , on the side facing the sheet stacking table  2  of the conveyance belt  30 . In contrast to this, the sucking and conveying mechanism  3  is inclined upward along the conveyance direction for sheet P, between the first driven roller  32  and the second driven roller group  33 , and the conveyance belt  30  takes a curved form on the portion where the conveyance belt is wound around the first driven roller  32 . 
     The sucking and conveying mechanism  3  is equipped with suction chamber  34  into which air that sucks sheet P on conveyance belt  30  is sucked. With respect to the suction chamber  34 , a space into which air is sucked by an unillustrated fan is formed inside the conveyance belt  30 , then, the lower side facing the conveyance belt  30  positioned at the side facing sheet stacking table  2  is opened, and air is sucked in through suction opening  30   a  of the conveyance belt  30  on the side facing the sheet stacking table  2 . 
     In the sucking and conveying mechanism  3 , when air in the suction chamber  34  is sucked in by an unillustrated fan, pressure in the suction chamber  34  turns out to be negative, thus, air is sucked in through suction opening  30   a  of the conveyance belt  30  positioned on the side facing sheet stacking table  2 , and a flow of air that sucks sheet P on the conveyance belt  30  located at the side facing sheet stacking table  2  is generated. 
     Due to this, with respect to the sucking and conveying mechanism  3 , suction surface  30   b  that sucks sheet P is formed by the conveyance belt  30  on the side facing the sheet stacking table  2  where air is drawn in suction chamber  34  from suction opening  30   a.    
     Sheet feeding device  1 A is equipped with sheet conveyance path  35  through which the sheet P fed out by the sucking and conveying mechanism  3  is conveyed. The sheet conveyance path  35  is equipped with a guide member that guides sheet P sucked and fed out by sucking and conveying mechanism  3  to be conveyed, and sheet entering inlet  36  through which the sheet P enters is formed between the conveyance belt  30  on the side facing the sheet stacking table  2  and an upper end of sheet leading edge stopper surface  21 . 
     In the sheet feeding device  1 A, sheet conveyance path  35  is equipped with conveyance roller  37  and with driven roller  38  that faces the conveyance roller  37 . The conveyance roller  37  is driven to rotate by the motor which will be explained later, and sheet P fed out by the sucking and conveying mechanism  3  is interposed by the conveyance roller  37  and the driven roller  38  to be conveyed. 
     In the sheet feeding device  1 A, sheet conveyance path  35  has therein sheet detection sensor  39 . The sheet detection sensor  39  is composed, for example, of a pair of optical sensors whose detecting place is formed at the upstream side of the conveyance roller  37  and the driven roller  38 , and sheet P that is fed out by the sucking and conveying mechanism  3  and is conveyed by the conveyance roller  37  is detected. When a leading edge of sheet P fed out by the sucking and conveying mechanism  3  arrives at the sheet detection sensor  39 , an output of the sheet detection sensor  39 , for example, turns out to be ON, while, when a trailing edge of sheet P conveyed by the sucking and conveying mechanism  3  passes over the sheet detection sensor  39 , an output turns out to be OFF. 
     In floating air blowing mechanism  4 , floating air nozzle  40  is formed on the side of sheet storage section  20 . The floating air blowing mechanism  4  blows out air sucked by air blowing fan  41  from floating air nozzle  40  as floating air A 1 , and blows floating air A 1  against sheet P stacked on sheet stacking table  2  from the side way, to cause sheet P to be floated. 
     In the floating air blowing mechanism  4 , shutter  42  that changes over presence or absence of blowing out of floating air A 1  and air volume is driven by a solenoid which will be explained later, and the floating air nozzle  40  is opened and closed. In the floating air blowing mechanism  4 , floating air A 1  is blown out when floating air nozzle  40  is opened, and blowing out of floating air A 1  is stopped when floating air nozzle  40  is closed. 
     Separation air blowing mechanism  5  is equipped with separation air nozzle  50  through which separation air A 2  is blown out from sheet entering gate  36 . In the separation air nozzle  50 , air sucked in by air sending fan  51  is blown out obliquely against conveyance belt  30  toward sheet storage section  20 . Separation air A 2  blown out from the separation air nozzle  50  hits conveyance belt  30  obliquely, and is blown against sheet P from the front side of the sheet P sucked on suction surface  30   b  composed of the conveyance belt  30  on the side facing sheet stacking table  2 . 
     Sheet feeding device  1 A is equipped with suction detection sensor  6  that detects presence or absence of sheet P sucked on conveyance belt  30  by sucking and conveying mechanism  3 . The suction detection sensor  6  detects presence or absence of adsorption of sheet P on conveyance belt  30 , when detector  60  that is composed of conveyance belt  30  and is protruded from suction surface  30   b  composed of conveyance belt  30  is pressed by sheet P sucked on conveyance belt  30  and is displaced. With respect to the suction detection sensor  6 , its output turns out to be OFF under the condition that sheet P is not sucked on conveyance belt  30 , and its output turns out to be ON under the condition that sheet P is sucked on conveyance belt  30 . 
       FIG. 4  is a functional block diagram showing an example of a control system of the sheet feeding device of the present embodiment. The sheet feeding device  1 A is equipped with controller S 1  that conducts sheet feeding control to feed out sheets P stacked on sheet stacking table  2  one by one explained in  FIG. 1 , based on output of each sensor and with operation section S 2  wherein sheet types such as a basis weight or the like are selected. 
     The controller S 1  detects that sheet P is sucked on conveyance belt  30  in sucking and conveying mechanism  3  shown in  FIG. 1 , from an output of suction detection sensor  6 . Further, the controller S 1  detects that a leading edge and a trailing edge of sheet P fed out by sucking and conveying mechanism  3  arrive respectively at their prescribed positions, from an output of sheet detection sensor  39 . 
     The controller S 1  changes over presence or absence of blowing out of floating air A 1  by floating air blowing mechanism  4  explained in  FIG. 1 , based on a position of conveyance of sheet P detected by sheet detection sensor  39  and on presence or absence of suction of sheet P on conveyance belt  30  detected by suction detection sensor  6 . In the floating air blowing mechanism  4 , shutter  42  that opens and closes floating air nozzle  40  is driven by solenoid S 10 , and presence or absence of blowing out of floating air A 1  is changed over. 
     The controller S 1  detects the state of floating of sheet P floated by floating air A 1  from an output of sheet position detecting sensor  22 , during the floating state detecting period that is interlocked with blowing out of floating air A 1 . The controller S 1  changes over air volume of floating air A 1 , based on types of sheet P selected in operation section  52  and on the state of floating of sheet P detected by sheet position detecting sensor  22 . In the floating air blowing mechanism  4 , air volume of floating air A 1  is changed over, when motor M 1  that drives air blowing fan  41  is controlled and the number of revolution per minute of the air blowing fan  41  is changed. Further, air volume of floating air A 1  can be changed over by divergencies and opening and closing of shutter  42  that opens and closes floating air nozzle  40 . 
     The controller S 1  detects a position of an upper surface of sheet P stacked on sheet stacking table  2  from an output of sheet position detecting sensor  22 , during the detecting period for sheet height that is interlocked with a stop of blowing out of floating air A 1 . The controller S 1  controls motor M 2  that causes sheet stacking table  2  to go up and down based on the position of an upper surface of sheet P detected by the sheet position detecting sensor  22 , to make the position Pu of an upper surface of sheet P stacked on the sheet stacking table  2  to be equal to sheet feeding possibility height H 1 . 
     The controller S 1  controls motor M 3  that drives conveyance belt  30  and motor M 4  that drives conveyance roller  37 , based on presence or absence of suction of sheet P on conveyance belt  30  detected by suction detecting sensor  6  and on a conveyance position of sheet P detected by sheet feeding detecting sensor  39 . 
     Incidentally, sheet feeding device  1 A may possess either one of temperature sensor S 4  and humidity sensor S 5 , or may possess both of them, as an environmental detecting section. The temperature sensor S 4  and the humidity sensor S 5  detect respectively temperature and humidity in surroundings of the sheet feeding device  1 A, or in sheet storage section  20 . Further, the controller S 1  controls a motor that drives an air sending fan of sucking and conveying mechanism  3  and a motor that drives an air sending fan  51  of separation air blowing mechanism  5 , which is not shown in  FIG. 4 . In this case, the controller S 1  may also be constituted of a controller of an image forming apparatus which will be explained later. 
     The controller S 1  judges whether the state of floating of sheet P detected by timing of ON/OFF of sheet position detecting sensor  22  is appropriate or not. Namely, with respect to the controller S 1 , reference air volume information for blowing out floating air A 1  at an air volume conforming to sheet types such as basis weight of sheet P is established in advance. Further, there is established floating state reference value information showing the state of floating that is to be detected by the sheet position detecting sensor  22  when floating air A 1  is blown out at air volume based on reference air volume information that is specified by sheet types of sheet P. 
     The controller S 1  compares floating state detecting information detected by an output of sheet position detecting sensor  22  with floating state reference value information established in advance based on sheet types of sheet P, to judge whether the state of floating of sheet P is proper or not. 
       FIG. 5  is an illustration showing an example of initial setting condition table Tb 1  on which reference air volume information is established. In the initial setting condition table Tb 1 , there is established basis weight of sheet P as sheet type information of sheet P, which is stored in memory section S 3 . Further, as reference air volume information, there is established the number of revolutions per minute of a fan of air sending fan  41  for blowing out floating air A 1  at air volume corresponding to basis weight of sheet P. 
     After the basis weight is selected as a sheet type of sheet P in operation section S 2 , the controller S 1  establishes the number of revolutions per minute of a fan corresponding to the selected basis weight of sheet P, referring to the initial setting condition table Tb 1  stored in the memory section S 3 . 
       FIG. 6  is an illustration showing an example of detection state table Tb 2  where floating state reference value information is established. On the detection state table Tb 2 , there is established basis weight of sheet P as type information of sheet P, and the basis weight is stored in storing section S 3 . Further, as floating state reference value information, there are established the number of times of OFF/ON for an output of sheet position detecting sensor  22  corresponding to basis weight of sheet P and a reference value of a period of time during which an output of sheet position detecting sensor  22  stays to be ON. 
     The controller S 1  refers to the detection state table Tb 2  stored in memory section S 3  to compare floating state detection information for sheet P detected by sheet position detecting sensor  22  during the floating state detecting period that is interlocked with blowing out of floating air A 1  with floating state reference value information corresponding to basis weight of sheet P established in advance. 
     As a result, the controller S 1  judges that the floating state in the case of floating sheet P with floating air A 1  at air volume established in accordance with basis weight of sheet P is appropriate, if the floating state detection information detected during the floating state detecting period is equal to the floating state reference value information. 
     In contrast to this, the controller S 1  judges that the floating state in the case of floating sheet P with floating air A 1  at air volume established in accordance with basis weight of sheet P is not appropriate, if the floating state detection information detected during the floating state detecting period is different from the floating state reference value information. 
     During the floating state detecting period that is interlocked with blowing out of floating air A 1 , sheet P on sheet stacking table  2  is floated by floating air A 1 . With respect to detection of floating state by sheet position detecting sensor  22 , OFF/ON takes place within a given period of time, if sheet P is floated appropriately, because a surface of a leading edge of sheet P is detected. 
     With respect to the number of times of OFF/ON for sheet position detecting sensor  22 , its reference value is in a range from one to several times for one sheet of sheet P to be fed, under the cycle to feed sheets one by one by floating sheet P. Under the conditions where the number of times of OFF/ON of sheet position detecting sensor  22  is large, an output is ON constantly and a period of time for OFF is long, an air volume of floating air A 1  is changed because the state of floating is not appropriate. 
     In general, when air volume of floating air A 1  is large, sheet P tends to behave violently, and the number of times of OFF/ON for an output of sheet position detecting sensor  22  grows greater, in the case of thin paper. In the case of thick paper, an output of sheet position detecting sensor  22  stays to be ON for a long time and is not changed to OFF, because sheets P are floated in a form of a bundle if air volume of floating air A 1  is large. If air volume of floating air A 1  is small in the case of thick paper, an output of sheet position detecting sensor  22  stays to be OFF for a long time. Based on these conditions mentioned above, floating state reference value information is established for detection state table Tb 2 . 
     &lt;An Example of Operations for Sheet Feeding Device in the Present Embodiment&gt; 
       FIG. 7  is a flow chart showing an example of operations of a sheet feeding device of the present embodiment, while,  FIG. 8  is a time chart showing an example of operations of a sheet feeding device of the present embodiment, and an example of operations of sheet feeding device  1 A of the present embodiment will be explained next, referring to respective drawings. 
     In sheet feeding device  1 A, a sheet type selection screen on which a sheet type of sheet P stacked on sheet stacking table  2  is selected is displayed, for example, on operation section S 2 . When basis weight is selected on the sheet type selection screen as a sheet type of sheet P stacked on sheet stacking table  2  in the present example, the controller S 1  establishes the number of revolutions per minute of a fan for blowing out floating air A 1  at air volume corresponding to basis weight of sheet P by referring to initial setting condition table Tb 1  stored in memory section S 3  in step SA 1 . 
     When a start of sheet feeding instruction is received in step SA 2 , the controller S 1  controls motor M 2  that causes sheet stacking table  2  to go up and down in step SA 3 , and judges whether upper surface position Pu of sheet P stacked on the sheet stacking table  2  has been made to be equal to suction possibility height H 1  by sucking and conveying mechanism  3  or not based on an output of sheet position detecting sensor  22 , in step SA 4 . Then, if the upper surface position Pu of sheet P has been made to be equal to the suction possibility height H 1 , motor M 2  is stopped in step SA 5  to stop ascent and descent of the sheet stacking table  2 . 
     When sheet feeding operations are started in step SA 6 , the controller S 1  controls motor M 1  that drives air blowing fan  41  of floating air blowing mechanism  4 , to blow out floating air A 1  in step SA 7 . Further, air sending fan  51  of separation air blowing mechanism  5  is driven, and separation air A 2  is blown out. In addition, an unillustrated fan in sucking and conveying mechanism  3  is driven to suck air from suction chamber  34 . Owing to this, floating and suction operations for sheet P are carried out. 
     In the sucking and conveying mechanism  3 , when air in the suction chamber  34  is sucked in by an unillustrated fan, pressure in the suction chamber  34  turns out to be negative, thus, air is sucked in through suction opening  30   a  of the conveyance belt  30  positioned on the side facing sheet stacking table  2 , and a flow of air that sucks sheet P on the conveyance belt  30  located at the side facing sheet stacking table  2  is generated. Further, in floating air blowing mechanism  4 , floating air A 1  is blown from the side of sheet P stacked on sheet stacking table  2 . 
     Due to this, in sheets P stacked on the sheet stacking table  2 , sheets P stacked on the upper portion are floated, and uppermost sheet P stacked on the sheet stacking table  2  is sucked on conveyance belt  30  constituting suction surface  30   b.    
     The controller S 1  detects the number of times of OFF/ON for an output of sheet position detecting sensor  22  and a period of time during which an output of sheet position detecting sensor  22  is ON, during floating state detecting period t 1  interlocked with blowing out of floating air A 1 , in step SA 8 . 
     The controller S 1  compares floating state detection information for sheet P detected by sheet position detecting sensor  22  during floating state detecting period t 1  with floating state reference value information corresponding to basis weight of sheet P established in advance, by referring to detection state table Tb 2  stored in memory section S 3 , in step SA 9 . 
     If the floating state detection information detected during floating state detecting period t 1  and the floating state reference value information are the same, the controller S 1  judges that the floating state in the case of floating sheet P with floating air A 1  at air volume established in accordance with basis weight of sheet P is appropriate. 
     If the floating state detection information detected during floating state detecting period t 1  and the floating state reference value information are different from each other, the controller S 1  judges that the floating state in the case of floating sheet P with floating air A 1  at air volume established in accordance with basis weight of sheet P is not appropriate. 
     If the controller S 1  judges that floating state detection information detected during floating state detecting period t 1  and floating state reference value information are different from each other in step SA 9 , the controller S 1  judges, in step SA 10 , whether the number of times of OFF/ON for an output of sheet position detecting sensor  22  and a period of time during which an output of the sheet position detecting sensor  22  is ON are greater or smaller than floating state reference value information, in step SA 10 . 
     If the controller S 1  judges that the number of times of OFF/ON for an output of sheet position detecting sensor  22  and a period of time during which an output of the sheet position detecting sensor  22  is ON are greater than the floating state reference value information, in step SA 10 , the controller S 1  establishes the number of revolutions per minute of a fan so that air volume for floating air A 1  may be reduced, in step SA 11 . 
     If the controller S 1  judges that the number of times of OFF/ON for an output of sheet position detecting sensor  22  and a period of time during which an output of the sheet position detecting sensor  22  is ON are smaller than the floating state reference value information, in step SA 10 , the controller S 1  establishes the number of revolutions per minute of a fan so that air volume for floating air A 1  may be increased, in step SA 12 . 
     For establishment of the number of revolutions per minute of a fan, it is also possible to control the number of revolutions per minute of motor M 1  that drives air blowing fan  41  of floating air blowing mechanism  4 . It is further possible to arrange so that air volume can be adjusted by divergencies and opening and closing of shutter  42  that opens and closes floating air nozzle  40 . Namely, responsiveness for opening and closing of shutter  42  by solenoid S 10  is high for control signals. Therefore, when air volume of floating air A 1  is controlled based on floating state detection information, it is possible to acquire the targeted air volume more rapidly, in comparison with an occasion of continuous sheet feeding. Due to this, it is possible to cause a change of air volume to follow continuous sheet feeding, even when changing air volume of floating air A 1  on the half way of continuous sheet feeding. 
     In this case, if a residue of sheets P stacked on sheet stacking table  2  is different, there is a difference in terms of the state of floating of sheet P when floating air A 1  is blown under the same air volume. Therefore, the controller S 1  detects a residue of sheets P stacked on sheet stacking table  2 , based on a height of the sheet stacking table  2  and on the number of fed sheets. Then, it is also possible to correct the floating state reference value information based on a residue of sheets P to judge whether the state of floating of a sheet is appropriate or not. 
     Further, if temperature and humidity in a place where sheet feeding device  1 A is installed are changed, the state of floating for sheet P is varied even when floating air A 1  is blown under the same air volume. Therefore, the controller S 1  detects temperature and humidity based on outputs of temperature sensor S 4  and humidity sensor S 5 . Then, it is also possible to judge whether the state of floating for a sheet is appropriate or not, by correcting the floating state reference value information based on the temperature and humidity. 
     After detecting that sheet P has been sucked on conveyance belt  30  based on output of suction detecting sensor  6  at timing Ta 1 , the controller S 1  drives solenoid S 10  to close floating air nozzle  40  and stops blowing out of floating air A 1  from the floating air nozzle  40 , in step SA 13 . Operations of sucking sheets P by sucking and conveying mechanism  3  are continued. 
     After the blowing out of floating air A 1  has been stopped in step SA 13 , the controller S 1  detects a position of an upper surface of sheet P stacked on sheet stacking table  2  based on an output of the sheet position detecting sensor  22  during sheet height detection period t 2  interlocked with discontinuance of blowing out of floating air A 1 , in step SA 14 . 
     The controller S 1  controls motor M 2  that causes sheet stacking table  2  to go up and down, based on positions of an upper surface of sheet P obtained through detections by sheet position detecting sensor  22  in steps SA 15  and SA 16 , and causes upper position Pu of sheet P stacked on sheet stacking table  2  to be equal to sheet feeding possibility height H 1 . 
     Conveyance belt  30  positioned on the side facing sheet stacking table  2  takes a curved shape on the portion where the conveyance belt is wound around the first driven roller  32 , and the conveyance belt  30  positioned between the first driven roller  32  and the second driven roller group  33  is inclined upward in the direction of conveyance of sheet P. Owing to this, sheet P sucked on the conveyance belt  30  takes a curved shape at the portion where the conveyance belt  30  is wound around the first driven roller  32 . 
     When two or more sheets P are sucked to the conveyance belt  30  by sucking and conveying mechanism  3 , plural sheets P are in the state wherein the plural sheets P stick to each other. Due to this, one uppermost sheet P sucked on the conveyance belt  30  takes a curved shape, following the portion where the conveyance belt  30  is wound around the first driven roller  32 , because suction force of suction air directly applies on the uppermost sheet. 
     In contrast to this, suction force by suction air does not apply directly on the second sheet and thereafter each sticks to the uppermost sheet P and is sucked by the sucking and conveying mechanism  3 . Owing to this, the second sheet P and thereafter do not follow the curved shape of the conveyance belt  30  in the portion where the conveyance belt is wound around the first driven roller  32 , thus, clearances are generated between the one uppermost sheet P and the second sheet P and thereafter, and the sheets are separated. 
     In separation air blowing mechanism  5 , when air sending fan  51  is driven, air sucked in by air sending fan  51  is blown out from separation air nozzle  50 . Separation air A 2  blown out of separation air nozzle is oriented to be in parallel with the direction of the conveyance belt  30  on the side facing sheet stacking table  2 , to be blown against sheet P in the direction that is almost horizontal, from the front of sheet P sucked on conveyance belt  30  on the side facing the sheet stacking table  2 , in the sucking and conveying mechanism  3 . 
     When two or more sheets P are sucked on conveyance belt  30  in sucking and conveying mechanism  3 , a clearance is generated between the leading edge surface of the first one uppermost sheet P 1  and the leading edge surface of the second sheet P 2  and thereafter as described above. Due to this, separation air A 2  blown out against sheet P in the mostly horizontal direction from separation air blowing mechanism  5  is blown against a space between the uppermost one sheet P sucked on conveyance belt  30  and the second sheet P and thereafter. 
     On the uppermost one sheet P sucked on conveyance belt  30 , suction force by suction air for conveyance belt  30  operates, and it remains in the state where the force is sucked in conveyance belt  30 . On the other hand, second sheet P and thereafter sucked on conveyance belt  30  while following the first sheet P are separated by separation air A 2 . 
     When a prescribed separation standing by period of time tb 1  has passed after sheet P is sucked on conveyance belt  30  in sucking and conveying mechanism  3 , the controller S 1  causes motor M 3  to drive roller  31  of sucking and conveying mechanism  3  to rotate in step SA 17 , and causes motor M 4  to drive conveyance roller  37  to rotate, to start sheet feeding operations. 
     When the drive roller  31  is driven to rotate in the direction shown with an arrow, the conveyance belt  30  is rotated, and the side of the conveyance belt  30  facing the sheet stacking table  2  moves in the direction shown with arrow F. Due to this, sheet P sucked on conveyance belt  30  by sucking and conveying mechanism  3  is fed out in the conveyance direction shown with arrow F, in sucking and conveying mechanism  3 . 
     When sucked sheet P is fed out by conveyance belt  30  in sucking and conveying mechanism  3 , a leading edge of sheet P thus fed out is detected by sheet feeding detection sensor  39 , and sheet P fed out is interposed by conveyance roller  37  and by driven roller  38 . 
     When the controller S 1  detects that a leading edge of the first sheet P fed out by conveyance belt  30  has arrived at conveyance roller  37 , floating air nozzle  40  is opened and flowing out of floating air A 1  is started again by the controller S 1 , in step SA 18 , when sheet feeding is continuous. When flowing out of floating air A 1  is started again, detection of the state of floating of sheet P and control of air volume of floating air A 1  corresponding to the state of floating are conducted during floating state detecting period t 1 . 
     When prescribed standby time established by considering a period of time up to the moment when sheet P is interposed by conveyance roller  37  and driven roller  38  has passed after arrival of a leading edge of sheet P fed out by conveyance belt  30  at sheet detection sensor  39  was detected, the controller S 1  stops drive for rotation by drive roller  31 . On the other hand, drive for rotation by conveyance roller  37  is continued. 
     Due to this, first sheet P interposed by conveyance roller  37  and driven roller  38  is conveyed. Meanwhile, in sheet feeding operations for sheet P, suction operation for sheet P by sucking and conveying mechanism  3  is continued, and thereby, force to attract sheet P on conveyance belt  30  is acting. However, conveying force by interposing between the conveyance roller  37  and the driven roller  38  is stronger than suction force, whereby, sheet P is drawn out under the condition that conveyance belt  30  is stopped. 
     When the first sheet P interposed between the conveyance roller  37  and the driven roller  38  is conveyed, second sheet P is sucked on conveyance belt  30  in the case of continuous sheet feeding, and the same control as that in the first sheet P is conducted thereafter. 
     &lt;Construction Examples of Image Forming Apparatus and Sheet Feeding Unit in the Present Embodiment&gt; 
       FIG. 9  is a structural diagram showing examples of a sheet feeding unit and an image forming apparatus each being equipped with a sheet feeding device of the present embodiment. Image forming apparatus  100  is equipped with image forming apparatus main body A, image reading unit SC, automatic document feeding unit DF and large capacity sheet feeding unit  90 . 
     Sheet feeding unit  90  is equipped with a plurality of sheet trays constituting sheet storage section  20  shown in  FIG. 1 , and in the present example, the sheet feeding unit  90  is equipped with three sheet trays  220  which are provided in the vertical direction. Each sheet tray  220  is equipped with sheet position detecting sensor  22  that detects a position of an upper surface and the state of floating of sheet P stacked on sheet stacking table  2  that constitutes sheet feeding device  1 A of the present embodiment. Further, each sheet tray  220  is equipped with sucking and conveying mechanism  3  having conveyance belt  30  shown in  FIG. 1  or the like. 
     Further, there are provided floating air blowing mechanism  4  that blows floating air from the side of sheet P stacked on sheet tray  220  and separation air blowing mechanism  5  that blows separation air against a space between the uppermost one sheet and the second sheet and thereafter when two or more sheets P are sucked by sucking and conveying mechanism  3 . 
     Image forming apparatus main body A is equipped with an image forming section having therein photoconductor  101  representing an image carrier, charging section  102 , exposure unit  103 , developing unit  104 , transfer section  105  and cleaning section  106  and with a fixing device  107  and a sheet conveyance section. The image forming apparatus main body A is further equipped with controller S 1  that controls image forming apparatus  100  and with operation section S 2  where various operations are carried out. 
     The sheet conveyance section is composed of sheet feeding tray  110 , first sheet feeding section  111 , second sheet feeding section  112 , sheet ejection section  114 , conveyance path switching section  115 , circulation sheet re-feeding section  116  and reversing ejection section  117 . In this case, the sheet feeding tray  110  may also be equipped with sucking and conveying mechanism  3 , floating air blowing mechanism  4 , separation air blowing mechanism  5  and sheet position detection sensor  22 , which constitute sheet feeding device  1 A of the present embodiment. 
     Document “d” placed on a document platen of automatic document feeder DF is conveyed by a sheet feeding section, and images on one side or on both sides of document “d” are given exposure by an optical system of image reading device SC, so that the images are read in by image sensor CCD. Analog signals obtained through photoelectric conversion by image sensor CCD are subjected to analog processing, A/D conversion, shading correction and image compression processing in image processing section  120 , and then, image signals are sent to exposure unit  103 . 
     In the image forming section, there are conducted processes for charging, exposure, development, separation and cleaning. In the image forming section, a surface of photoconductor  101  is electrified by charging section  102 , and an electrostatic latent image is formed on the surface of photoconductor  101  by illumination of a laser beam coming from exposure unit  103 , and the electrostatic latent image is visualized by developing unit  104  to become a toner image. Then, the sheet P stored in sheet feeding tray  110  is conveyed from the first sheet feeding section  111 . Or, sheet P stored in sheet feeding unit  90  is conveyed from sucking and conveying mechanism  3 . The sheet P is synchronized with the toner image in the second sheet feeding section  112  composed of a registration roller, to be conveyed. After that, the toner image is transferred onto the sheet P in transfer section  105 , and is fixed by fixing device  107 . 
     The sheet P after being subjected to fixing is ejected out of the apparatus by sheet ejection section  114 . On the other hand, toner staying on photoconductor  101  after transfer is removed by cleaning section  106 . Meanwhile, in the case of duplex copying, sheet P which has undergone image forming on its first surface is sent in circulation sheet re-feeding section  116  to be reversed to undergo image forming again on its second surface in the image forming section, and is ejected out of the apparatus by the sheet ejection section  114 . In the case of reverse sheet ejection, sheet P which has branched off the ordinary sheet ejection path is ejected out of the apparatus by the sheet ejection section  114  after being reversed inside out through switchback in reverse sheet ejection section  117 . 
     Incidentally, though an image forming apparatus has been one for forming a monochrome image in the foregoing, it may also be an image forming apparatus forming color images. 
     The present invention is applied on an image forming apparatus equipped with a sheet feeding device that feeds out stacked sheets by sucking a sheet with air. 
     In the sheet feeding device of the present embodiment, when floating a sheet by blowing floating air from the side of the sheet stacked on a sheet stacking table, it is possible to control air volume of floating air based on the state of floating of the sheet by detecting the state of floating of the sheet by a sheet position detecting sensor. Owing to this, it is possible to optimize air volume of floating air in accordance with the actual state of floating of the sheet, thus, the state of floating of a sheet by floating air can be stabilized, which makes sure separation and conveyance of a sheet to be possible. Further, it is possible to optimize air volume without being influenced by conditions peculiar to the sheet such as sheet types and by external conditions such as the state of storage of sheets and the state of stacking of sheets. 
     In the sheet feeding unit of the present embodiment, double feeding of sheets can be prevented surely by providing the aforesaid sheet feeding device. Further, in the image forming apparatus of the present embodiment, it is possible to prevent double feeding surely and to improve image quality by providing the aforesaid sheet feeding device.