Patent Publication Number: US-8118297-B2

Title: Sheet feeder and image forming apparatus with side surface air mechanism

Description:
The present invention is a continuation application of U.S. patent application Ser. No. 12/552,123 filed in the U.S. Patent Office on Sep. 1, 2009, which itself claims priority to Japanese Patent Application JP 2008-287634 filed in the Japanese Patent Office on Nov. 10, 2008, the entire contents of both of which are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a sheet feeder used for an image forming apparatus such as a printer, a copier, a fax machine, or a multi-functional peripheral incorporating these functions. The present invention also relates to an image forming apparatus including the sheet feeder. 
     2. Description of the Related Art 
     To date, only sheets of high-quality paper, plain paper specified by copier manufacturers, or the like have been used as sheet recording medium that can be continuously fed in image forming apparatuses such as printers, copiers, and fax machines. Such sheets of high-quality paper, plain paper, or the like have low surface smoothness, whereby their inter-sheet adhesion is comparatively low. Thus, it has been comparatively easy to prevent double feeding that may occur when the cut sheets are fed out one at a time from a sheet loading section such as a sheet feed tray. The term “double feeding” refers to a phenomenon in which a plurality of cut sheets adhering to each other are simultaneously fed out. Moreover, even if double feeding occurs when such cut sheets are used, it is possible to separate doubly fed cut sheets by providing a separation roller, a separation pad, a separation claw, or the like to the sheet feeder so that the cut sheets can be smoothly fed one at a time. 
     However, the sheet recording medium has become diversified in recent years. Sheets having a low surface smoothness such as high-quality paper, plain paper, or the like are not the only sheets used as sheet recording medium. In particular, as the colorization technology for image forming apparatuses has improved, a paper having a high surface smoothness such as a coating paper can now be used. A coating paper is composite paper of which one or both sides are coated with a coating color, which is a coating material, so as to improve printability. A coating paper has a high whiteness and gloss. Thus, in recent years, demand has been increasing for feeding not only high-quality paper and plain paper, but also the above-described coating paper, film sheets, tracing paper, and the like in an image forming apparatus. Because coating paper, film sheet, tracing paper, and the like have a high adhesion between papers, it is difficult to prevent double feeding of such sheets. Therefore, it is necessary to introduce special measures in order to feed (in particular, to feed out) such sheets. 
     Moreover, a stack of sheets loaded on a sheet loading section is prone to absorb moisture because the upper surface and the outer periphery of the stack of sheets are exposed to the air outside. The upper surface and the side surfaces of the stack of sheets absorb moisture and swell, while the inside of the sheet stack swells to a lesser extent because the inside absorbs less moisture than the upper surface and the side surfaces. As a result, inner spaces of the sheet stack (spaces between sheets) enter a negative pressure state, which causes the sheets to adhere to each other. 
     In order to reduce adhesion between sheets and separate the sheets in a sheet stack before feeding the sheets, some large copiers and the like adopt sheet feeders including mechanisms (hereinafter referred to as “side warm-air assists”) for blowing warm air toward side surfaces of sheet stack. 
     For example, there is a known technique that increases the efficiency of sheet separation while fulfilling the requirement for reduction in size and power consumption. With this technique, movement speed of an air shielding member, which serves to partially close an opening through which blowing means blows air from an outlet thereof toward a side surface of a sheet stack, is changed so that air is effectively blown toward an upper part of the sheet stack. 
     However, with this sheet separation technique, for example, while a large number of sheets are being continuously fed, sheets in a lower part of a sheet stack may be fed without being separated and may cause jamming. This problem is particularly serious when art paper or coated paper, which has high inter-sheet adhesion, is used in a high-humidity environment. 
     SUMMARY OF THE INVENTION 
     The present invention, which has been achieved against the above-described background, provides a sheet feeder including a sheet separation mechanism that securely prevents jamming even when continuous feeding of sheets with high inter-sheet adhesion is performed, and an image forming apparatus including the sheet feeder. 
     A sheet feeder according to an aspect of the present invention includes a sheet loading plate for loading a stack of sheets thereon, a sheet feed mechanism capable of performing a continuous sheet feeding operation from an uppermost sheet in the stack on the sheet loading plate, a warm-air mechanism blowing air toward a side surface of the stack from an outlet, where the side surface is parallel to a sheet feeding direction, a lift mechanism displacing the sheet loading plate, and a controller controlling a sheet separating operation to perform every time a predetermined number of the sheets are fed during the continuous sheet feeding operation. 
     In the sheet separating operation, the lift mechanism displaces the sheet loading plate by while the warm-air mechanism blows warm air is blown to the side surface of the sheet stack. 
     Therefore, the sheet feeder is provided with a sheet separation mechanism that can securely prevent double feeding by separating sheets every time a predetermined number of sheets are fed during a continuous sheet feeding operation. This occurs even when sheets that are made of, for example, a paper having a high inter-sheet adhesion and a susceptibility to double-feeding, such as an art paper or a coated paper, are continuously fed. 
     It is preferable that the sheet feeder may further include a sheet identifying unit that identifies a type of sheet to be fed. The controller then determines whether or not to perform the sheet separating operation corresponding to the type of the sheet identified by the sheet identifying unit. 
     It is preferable that the sheet feeder may further include a sheet identifying unit that identifies a type of the sheet to be fed, with the controller changing the predetermined number corresponding to the type of sheet identified by the sheet identifying unit and carrying out control so as to perform the sheet separating operation. 
     It is preferable that the sheet feeder may further include a float amount detector that detects a float amount by which the sheet is floated when warm air is blown from the outlet. The controller may determine whether or not to perform the sheet separating operation corresponding to the float amount of the sheet detected by the float amount detector. In addition, the controller may change the predetermined number corresponding to the float amount of the sheet detected by the float amount detector and perform the sheet separating operation. 
     It is preferable that the lift mechanism of the sheet feeder may include a lifting member that lifts up the sheet loading plate. The sheet loading plate may be rotatably supported at an end thereof, the end being in an upstream side of the sheet loading plate with respect to the sheet feeding direction. The lifting member may be configured such that an end thereof is rotatably supported by a drive shaft and the other end thereof contacts the bottom surface of the sheet loading plate so as to lift up the sheet loading plate. 
     An image forming apparatus according to another aspect of the present invention includes a sheet feeder having any of the above-described configurations, and an image forming apparatus body that forms images on the sheets fed by the sheet feeder. 
     Since the image forming apparatus includes a sheet feeder having any of the above-described configurations, jamming can be effectively prevented even when a continuous feeding operation using sheets having a high inter-sheet adhesion is performed under a high humidity environment. 
     The present invention provides a sheet feeder that effectively prevents jamming even when a continuous feeding operation using sheets with high inter-sheet adhesion is performed, and an image forming apparatus including the sheet feeder. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an external perspective view of a printer including a sheet feeder according to an embodiment of the present invention; 
         FIG. 2  is a sectional view showing an internal structure of the printer shown in  FIG. 1 ; 
         FIG. 3  is a sectional view showing a structure of a sheet feeder according to an embodiment of the present invention; 
         FIG. 4  is a perspective view of a sheet feed cassette of the sheet feeder shown in  FIG. 3  in a state in which the sheet feed cassette has been pulled out from the body of the sheet feeder; 
         FIGS. 5A and 5B  are explanatory views showing position detection sensors incorporated in the sheet feeder shown in  FIG. 3 ; 
         FIG. 6  is an explanatory view showing a structure of a sheet feeder according to an embodiment of the present invention; 
         FIG. 7  is a horizontal sectional view of a main part of a side warm-air mechanism incorporated in the sheet feeder shown in  FIG. 6 ; 
         FIG. 8  is a vertical sectional view of a main structure of an upper warm-air mechanism incorporated in the sheet feeder shown in  FIG. 3 ; 
         FIG. 9  is a functional block diagram of a controller, which controls a warm-air blowing operation including a separating operation, according to an embodiment of the present invention; 
         FIG. 10  is a flowchart showing a control process exercised by the controller shown in  FIG. 9 ; 
         FIG. 11  is a flowchart showing another control process exercised by the controller shown in  FIG. 9 ; 
         FIG. 12  is a longitudinal sectional view of a main part of a sheet feeding unit according to an embodiment of the present invention; 
         FIG. 13  is a longitudinal sectional view of a main part of a sheet feeding unit according to an embodiment of the present invention; and 
         FIG. 14  is a longitudinal sectional view of a main part of a sheet feeding unit according to an embodiment of the present invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     First Embodiment 
     Referring to  FIGS. 1 and 2 , an image forming apparatus including a sheet feeder according to an embodiment of the present invention is described. 
       FIG. 1  is an external perspective view of an image forming apparatus including a sheet feeder according to an embodiment of the present invention.  FIG. 2  is a sectional view showing an internal structure of the image forming apparatus. 
     As shown in  FIG. 1 , a color printer  1 , which is an image forming apparatus according to an embodiment of the present invention, includes a printer body  200  and a sheet supply section  100 . The printer body  200  is connected to a personal computer (PC) (not shown), or the like, directly or through a Local Area Network (LAN). The sheet supply section  100 , which is disposed below the printer body  200 , can separately hold sheets P in different trays corresponding to the size. The color printer  1  includes other components used in a general color printer, such as a control circuit for controlling operation of the color printer  1 . 
     As shown in  FIG. 2 , the printer body  200  includes toner containers  900 Y,  900 M,  900 C,  900 K, an intermediate transfer unit  92 , an image forming unit  93 , an exposure unit  94 , a fusing unit  97 , a sheet ejection unit  96 , a housing  90  of the printer body, a top cover  911 , and a front cover  912 . 
     The image forming unit  93  includes and developing units  10 Y,  10 M,  10 C, and  10 K respectively for yellow, magenta, cyan, and black, disposed below the toner containers. 
     The image forming unit  93  further includes photosensitive drums  17  (photosensitive members on which latent images are formed by electrophotography) that bear toner images of respective colors. Photosensitive material of the photosensitive drums made of an amorphous silicon (a-Si) material can be used as the photosensitive drums  17 . Toners of yellow, magenta, cyan, and black colors are supplied to the photosensitive drums  17  from the corresponding developing units  10 Y,  10 M,  10 C, and  10 K. 
     As described above, the image forming unit  93  in this embodiment is capable of forming full-color images. However, an embodiment is not limited thereto, and an image forming unit that forms black-and-white images or non-full-color color images may be used. 
     Around the photosensitive drums  17 , chargers  16 , developing units  10  ( 10 Y,  10 M,  10 C, and  10 K), transfer units (transfer rollers)  19 , cleaning units  18 , and the like are disposed. The chargers  16  uniformly charge surfaces of the photosensitive drums  17 . The charged surfaces of the photosensitive drums  17  are exposed by the exposure unit  94  so that electrostatic latent images are formed thereon. The developing units  10 Y,  10 M,  10 C, and  10 K respectively develop (make visible) the electrostatic latent images formed on the photosensitive drums  17  using toner of the corresponding colors supplied from the toner containers  900 Y,  900 M,  900 C, and  900 K. The transfer rollers  19  and the photosensitive drums  17  nip intermediate transfer belts  921  so as to primarily transfer the toner images formed on the photosensitive drums  17  onto the intermediate transfer belt  921 . After the toner images have been transferred, the cleaning units  18  clean the peripheral surfaces of the photosensitive drums  17 . 
     Each of the developing units  10 Y,  10 M,  10 C, and  10 K includes a case  20  that contains a two-component developer including magnetic carrier and toner. Near the bottom of the case  20 , two stirring rollers  11  and  12  (developer stirring members) are disposed in parallel in such a manner that each of the stirring rollers  11  and  12  are rotatable around the longitudinal axis thereof. 
     A developer circulation path is made along the inner bottom of the case  20 , and the stirring rollers  11  and  12  are disposed in the circulation path. A partition wall  201  stands between the stirring rollers  11  and  12  so as to extend in the axis direction of the stirring rollers  11  and  12 . The partition wall  201  divides the circulation path so that the circulation path surrounds the partition wall  201 . The two-component developer is charged while the two-component developer is stirred and transported by the stirring rollers  11  and  12  along the circulation path. 
     The two-component developer circulates in the case  20  while the two-component developer is being stirred by the stirring rollers  11  and  12  so that the toner is charged, and the two-component developer on the stirring roller  11  is attracted to a magnetic roller  14  disposed above the stirring roller  11  and transported onto the magnetic roller  14 . The two-component developer attracted to the magnetic roller  14  forms a magnetic brush (not shown) on the magnetic roller  14 . The thickness of the magnetic brush is regulated by a doctor blade  13 , and a toner layer is formed on a developing roller  15  due to an electrical potential difference between the magnetic roller  14  and the developing roller  15 . Using the toner layer on the developing roller  15 , the electrostatic latent image on the photosensitive drum  17  is developed. 
     The exposure unit  94  includes various optical devices, such as a light source, polygon mirrors, reflection mirrors, and deflection mirrors. The exposure unit  94  irradiates peripheral surfaces of the photosensitive drums  17  disposed in the image forming unit  93  with light corresponding to the image data, so that the electrostatic latent images are formed on the photosensitive drums  17 . 
     The intermediate transfer unit  92  includes the intermediate transfer belt  921 , a drive roller  922 , and a driven roller  923 . Toner images are primarily transferred from the photosensitive drums  17  onto the intermediate transfer belt  921  in an overlapping manner. A secondary transfer unit  98  secondarily transfers the toner images onto the sheet P that is supplied by a sheet feeding unit  130 . The drive roller  922  and the driven roller  923  rotate the intermediate transfer belt  921 . The drive roller  922  and the driven roller  923  are rotatably supported by a case (not shown). 
     The sheet feeding unit  130  stores a sheet stack including the sheets P on which images are to be formed. The sheet feeding unit is detachably loaded into the housing  90 . 
     The fusing unit  97  fuses the toner images that have been secondarily transferred onto the sheet P conveyed from the intermediate transfer unit  92 . After a color image has been fixed on the sheet P, the sheet P is conveyed to the sheet ejection unit  96  disposed in an upper part of the printer body  200 . 
     The sheet ejection unit  96  ejects the sheet P that has been conveyed from the fusing unit  97  onto the top cover  911  serving as a sheet ejection tray. 
     The sheet supply section  100  includes a sheet feeder fixed to the printer body  200  and a plurality (in this embodiment, two) of the sheet feeding units (sheet feeders)  130  that are stacked on top of each other and removably loaded on the printer body  200 . Several sizes of the sheet stacks S are respectively stored in the sheet feeding units  130 . When one of the sheet feeding units  130  is selected, a pickup roller  40  disposed in the sheet feeding unit  130  is rotated so that the uppermost sheet P in the sheet stack S is picked up, fed out to a sheet conveying path  133 , and transported into the image forming unit  93 . 
     Each of the sheet feeding units  130  includes a conveying mechanism that can be mounted as an option to the bottom portion of the printer body  200  in a stacking manner, so that a desired number of the sheet feeding units  130  can be optioned to the printer body  200 . By thus stacking the sheet feeding units  130  under the printer body  200 , the transport mechanisms of the sheet feeding units  130  are connected to each other, so that the sheet conveying path  133  extending to the printer body  200  is formed. In this manner, the sheet feeding units  130  can be optioned to the printer body  200  in a stacking manner. 
     In the embodiment, the sheet supply section  100  includes three sheet feeding units  130 . However, the present invention is not limited thereto, and also applicable to an image forming apparatus, such as a printer, having the sheet supply section  100  including one, two, four, or more sheet feeding units  130 . 
     Referring to  FIGS. 1 ,  3 , and  5 , the structure of the sheet feeding unit (sheet feeder)  130  according to the embodiment, which is disposed into the sheet supply section  100  of the color printer  1 , is described in detail. 
       FIG. 3  is a sectional view showing a structure of the sheet feeder according to the embodiment.  FIG. 4  is a perspective view of a sheet feed cassette of the sheet feeder shown in  FIG. 3  in a state in which the sheet feed cassette has been pulled out from the body of the sheet feeder.  FIG. 5  is an explanatory view showing position detection sensors incorporated in the sheet feeder shown in  FIG. 3 . 
     As shown in  FIGS. 3 and 4 , the sheet feeding unit  130  includes a lift plate (sheet loading plate)  31  disposed on an inner bottom surface of a sheet container  35 . A sheet stack S including a plurality of sheets (sheet recording medium) P is placed on the lift plate  31 . The lift plate  31  is rotatably supported by supporting sections  38  at an upstream end thereof (left end in  FIG. 3 ) with respect to the sheet feeding direction. That is, the lift plate  31  is supported by the supporting sections  38  so that the lift plate is vertically rotatable in the sheet container  35  with a downstream end thereof acting as a free end. The supporting sections  38  are disposed on both side walls of the sheet container  35  disposed opposite each other in the width direction of the sheet P (the direction perpendicular to the sheet feeding direction). 
     A sheet feeding cassette  130 A of the sheet feeding unit  130  includes a pair of width-adjusting cursors  34   a  and  34   b  for positioning the sheets P in the sheet container  35  in the width direction, and a back-end cursor  33  for aligning back ends of the sheets P. The pair of width-adjusting cursors  34   a  and  34   b  are disposed so as to be reciprocally movable in the sheet width directions (shown by arrow AA′ in  FIG. 4 ) along a guide rail (not shown). The back-end cursor  33  is disposed so as to be reciprocally movable in directions parallel to the sheet feeding direction (shown by arrow BB′ in  FIG. 4 ) along guide rails  33   a  and  33   b  so that the sheets P can be fed in the direction of arrow B. By moving the pair of width-adjusting cursors  34   a  and  34   b  and the back-end cursor  33  according to the size of the sheet, the sheet stack S can be stored in the sheet feeding unit  130  at a predetermined position. The sheet feeding unit  130  includes a cassette cover  43 . A front surface (the front side when viewed in the direction of arrow C in  FIG. 4 ) of the cassette cover  43  is exposed to the outside and forms a part of the exterior surface of the color printer  1 . 
     A lift mechanism  30  ( FIG. 9 ), which lifts up the lift plate  31 , is disposed below a downstream portion of the lift plate  31  with respect to the sheet feeding direction. The lift mechanism  30  includes a drive shaft  36 , a lifting member  32 , and a drive connection member (not shown). A receiving member (not shown) corresponding to the drive connection member and a lift motor M ( FIG. 9 ) that is connected to the receiving member and rotatable in both directions is located on a sheet feeding unit body  130 B. When the sheet feeding cassette  130 A is inserted into the sheet feeding unit body  130 B, the drive connection member of the sheet container  35  of the sheet feeding cassette  130 A engages with the receiving member of the sheet feeding unit body  130 B. Thus, the power of the lift motor M can be transmitted to the drive shaft  36 . The drive shaft  36 , the lifting member  32 , the drive connection member, the receiving member, and the lift motor M constitute a lift mechanism that displaces the lift plate  31  between a sheet feed position and a retracted position. The term “sheet feed position” refers to a position at which the lift plate  31  is lifted up and the upper surface of the sheet stack S placed on the lift plate  31  contacts the pickup roller  40  so that a sheet can be fed out. The term “retracted position” refers to a position at which the lift plate  31  is lowered to the limit. 
     The type of the sheets P to be fed can be selected by using a sheet selecting unit (sheet identifying unit)  39 . The sheet selecting unit  39  includes a plurality of operation keys and a display unit (both of which are not shown). The sheet selecting unit  39  can be disposed, for example, on an operation panel (not shown) of the sheet feeding unit  130  or of the printer body  200 . 
     The lift motor M included in the lift mechanism  30  for lifting the lift plate  31  may be implemented as a stepping motor, a DC motor, or the like. 
     As shown in  FIG. 3 , the sheet feeding unit  130  includes a feed roller  41  disposed downstream of the pickup roller  40  with respect to the sheet feeding direction, and a separation roller  42  disposed below the feed roller  41 . Moreover, a pair of conveying rollers  44  and  45  is disposed downstream of the pickup roller  40  and the feed roller  41  with respect to the sheet feeding direction. The feed roller  41 , the pickup roller  40 , and the conveying roller  44  are disposed on the sheet feeding unit body  130 B, while the separation roller  42  and the conveying roller  45  are disposed on the sheet feeding cassette  130 A. When the sheet feeding cassette  130 A is loaded into the sheet feeding unit body  130 B, the feed roller  41  contacts the separation roller  42 . 
     The feed roller  41  serves to feed the sheet P that has been picked up with the pickup roller  40  to the pair of conveying rollers  44  and  45 . The feed roller  41  rotates in a direction that allows the sheet P to be fed downstream. In contrast, the separation roller  42  rotates in a direction that allows the sheet P to be fed upstream. Even if a plurality of the sheets P have been picked up by the pickup roller  40  in an overlapping manner, the separation roller  42  prevents the sheet P that is not at the uppermost position from being fed toward the pair of conveying rollers  44  and  45  so that only the uppermost sheet P can be fed toward the pair of conveying rollers  44  and  45  by the feed roller  41 . The pair of conveying rollers  44  and  45  conveys the sheet P to the sheet conveying path  133  (see  FIG. 2 ). 
     As shown in  FIGS. 5A and 5B , the sheet feeding unit  130  includes a first detecting sensor PS 1  for detecting whether the uppermost sheet P of the sheet stack S placed on the lift plate  31  is at the sheet feed position. The first detecting sensor PS 1  includes a light-shielding member PSA and an optical sensor PSB. The optical sensor PSB includes a light-emitting device fixed to a position near the pickup roller  40  and a light-receiving device for receiving light that is emitted from the light-emitting device. The light-shielding member PSA is disposed on a supporting member  50  that supports the pickup roller  40 . The supporting member  50  is rotatable around the rotation axis of the feed roller  41 . With this structure, when the lift plate  31  is lifted up, the upper surface of the sheet stack S placed on the lift plate  31  is moved to the sheet feed position shown in  FIG. 5B . The pickup roller  40  is pushed up by the uppermost sheet P and rotated around the rotation axis of the feed roller  41 , displacing slightly upward. At this time, the light-shielding member PSA is lifted up together with the pickup roller  40  so as to block the light path of the optical sensor PSB, thereby allowing the optical sensor to detect that the upper surface of the sheet stack S is at the sheet feed position. 
     In the sheet feeding unit  130 , when the lift motor M is driven, the lifting member  32  engages with the bottom surface of the lift plate  31  and lifts up a downstream end of the lift plate  31 . Thus, the upper surface of the sheet stack S placed on the lift plate  31  is displaced to the sheet feed position at which the upper surface of the sheet stack S contacts the pickup roller  40  disposed in an upper part of the sheet feeding cassette  130 A. 
     When the first detecting sensor PS 1  detects that the pickup roller  40  has displaced to the sheet feed position as shown in  FIG. 5B , the lift motor M is stopped. When the number of the sheets P decreases while the sheets P are being fed and the first detecting sensor PS 1  enters a non-detection state, the lift motor M is driven so as to lift up the sheet stack S to the sheet feed position. 
     The sheet feeding unit  130  according to the embodiment further includes a second detection sensor PS 2 . The second detection sensor PS 2  serves as a float amount detector for detecting a float amount by which the sheet P floats when warm air is blown toward the sheet P from a first warm-air outlet  155  of a side warm-air mechanism  150 . A sensor such as a reflective photosensor or an ultrasonic sensor can be used as the second detection sensor PS 2 . A reflective photosensor can detect the float amount by irradiating a surface of the sheet P serving as a reflection surface with light from a light source such as an LED and by detecting reflected light from the surface of the sheet P with a light-receiving device such as a photodiode. An ultrasonic sensor can detect the float amount by measuring an interval between the time when sound is emitted and the time when the sound that has been reflected by a surface of the sheet P serving as a reflection surface returns to the sensor. 
     As described below, detection results obtained by the first detecting sensor PS 1  and the second detection sensor PS 2  are output to a controller  300 . The sheet feeding unit  130  according to the embodiment appropriately controls the sheet separating operation corresponding to the float amount of the sheet P detected by the second detection sensor PS 2  while continuous feeding is being performed. 
     For example, in a case in which the sheets P to be continuously fed are a paper type such an art paper or a coated paper having a high inter-sheet adhesion or one having a weight equal to or greater than 100 g, the float amount of the sheet P when the side warm-air mechanism  150  blows warm air toward the sheet P is smaller than the case in which the sheets P are a plain paper having a low inter-sheet adhesion. Under a high-humidity environment (of a humidity equal to or greater than 50%), inter-sheet adhesion is high for the same type of sheets P. Thus, even if a warm air blowing operation is performed in the same manner, the float amount of the sheets P varies corresponding to the type of the sheets P to be fed and the difference in the environment. 
     Therefore, by changing the frequency with which the sheet separating operation is performed corresponding to the float amount of the sheets P being fed, it is possible to efficiently prevent jamming of the sheets P without significantly decreasing a continuous sheet feeding speed. 
     As shown in  FIGS. 2 ,  3 ,  6 , and  7 , the sheet feeding unit  130  according to the embodiment includes the side warm-air mechanism (warm-air mechanism)  150  serving as a sheet separation mechanism that utilizes blowing of warm air. 
       FIG. 6  is an explanatory view showing a structure of a sheet feeder according to the embodiment of the present invention.  FIG. 7  is a horizontal sectional view of a main part of the side warm-air mechanism incorporated in the sheet feeder according to the embodiment. 
     The side warm-air mechanism  150  is disposed on the sheet feeding unit body  130 B. As shown in  FIG. 6 , a top panel  56  is formed on top of the sheet feeding unit body  130 B in an area in which the side warm-air mechanism  150  and an upper warm-air mechanism (second warm-air mechanism)  140  described below are not disposed. The top panel  56  covers a sheet containing space. 
     As shown in  FIG. 6 , the side warm-air mechanism  150  is disposed on a side of the sheet feeding cassette  130 A parallel to the sheet feeding direction. As shown in  FIG. 7 , the side warm-air mechanism  150  includes a first fan (an air blowing section)  151  and a first heater (a heating section)  152 , both of which are disposed in a side warm-air chamber  153 . 
     As shown in  FIG. 7 , the side warm-air mechanism  150  draws air from the sheet feeding unit  130  through a first inlet (an air blowing section)  154  disposed in the sheet feeding unit  130 . When the first fan  151  rotates and air in the side warm-air chamber (air blowing section)  153  is moved toward the first heater  152 , air is drawn in from the sheet feeding unit  130  through the first inlet  154  to the side warm-air chamber  153 . Air that has been moved to the first heater  152  is heated with the first heater  152 , and blown toward a side surface of the sheet stack S through a first warm-air outlet (outlet, air blowing section)  155 . 
     The first warm-air outlet  155  of the side warm-air mechanism  150  from which warm air is blown toward a side surface of the sheet stack S at the sheet feed position is oriented toward a point N, which is shown in  FIG. 5B  on a sectional plane parallel to the sheet feeding direction, at which the pickup roller  40  contacts the upper surface of the sheet stack S. With this structure, the warm air can be intensively blown toward the side surface of the sheet stack S at a position at which a sheet is picked up by the pickup roller  40 , so that air can be efficiently blown into spaces between the sheets. Thus, even if the side warm-air mechanism  150  is not large, the sheet stack S can be efficiently separated before feeding. 
     As shown in  FIGS. 2 ,  3 ,  6 , and  8 , the sheet feeding unit  130  according to the embodiment includes the upper warm-air mechanism  140  serving as a sheet separation mechanism that utilizes blowing of warm air, in addition to the side warm-air mechanism  150 . 
       FIG. 8  is a vertical sectional view of a main part of the upper warm-air mechanism incorporated in the sheet feeder according to the embodiment. 
     As with the above-described side warm-air mechanism  150 , the upper warm-air mechanism  140  is disposed on the sheet feeding unit body  130 B. As shown in  FIG. 8 , the upper warm-air mechanism  140  draws air in through a second inlet (air blowing section)  144  and blows the warm air toward the upper surface of the sheet stack S contained in the sheet container  35  through a second warm-air outlet (air blowing section)  145  disposed above the upper surface of the sheet stack S. 
     The upper warm-air mechanism  140  includes a second fan (air blowing section)  141  and a second heater (a heating section)  142  in an upper warm air chamber (an air blowing section)  143 . The second inlet  144  is formed in an upper surface of the upper warm air chamber  143  above the second fan  141 . When the second fan  141  rotates and air in the upper warm air chamber  143  is moved toward the second heater  142 , outside air is drawn into the upper warm air chamber  143  through the second inlet  144 . Air that has been moved to the second heater  142  is heated with the second heater  142 , and blown toward the upper surface of the sheet stack S through the second warm-air outlet  145  disposed in a lower surface of the upper warm air chamber  143 . The upper warm-air mechanism  140  is attached to the sheet feeding unit  130  such that the second warm-air outlet  145  is positioned in a downstream portion of the upper warm-air mechanism  140  with respect to the sheet feeding direction. 
     With the above described structure, when a specific sheet feeding unit  130  is selected for image formation, the lift plate  31  is moved upward so that the sheet stack S is lifted toward the pickup roller  40 . Then, the upper warm-air mechanism  140  is driven so that the warm air is blown toward the upper surface of the sheet stack S through the second warm-air outlet  145 . 
     The upper surface and the outer periphery of the sheet stack S is prone to absorbing moisture because the upper surface and the outer periphery are in contact with outside air. Thus, the upper surface and the side surfaces of the sheet stack S absorb moisture and swell, while the inside of the sheet stack swells to a lesser extent because the inside absorbs less moisture than the upper surface and the side surfaces. As a result, inner spaces (spaces between sheets) of the sheet stack S enter a negative pressure state, which causes the sheets to adhere to each other. 
     However, since the sheet feeding unit  130  according to the embodiment includes the upper warm-air mechanism  140 , the relative humidity (the relative humidity on the upper surface and the outer periphery of the sheet stack) of the sheet stack S in the sheet feeding unit  130  can be instantaneously decreased. 
     That is, the upper warm-air mechanism  140  can intensively and uniformly blow air toward the upper surface and the outer periphery of the sheet stack S, where adhesion is particularly high. Thus, the moisture of the upper side and the outer periphery of the sheet stack S can be rapidly reduced so as to reduce swelling of these parts, whereby the relative humidity (humidity of the upper surface and the outer periphery of the sheet stack S) can be instantaneously decreased and the negative pressure state in the inner spaces (spaces between sheets) of the sheet stack S can be released. Therefore, inter-sheet adhesion can be reduced, so that the sheet stack S can be efficiently separated before feeding. 
     As shown in  FIG. 3 , the upper warm-air mechanism  140  according to the embodiment is disposed upstream of the pickup roller  40  with respect to the sheet feeding direction and in a rear portion of the sheet feeding unit  130  with respect to the sheet feeding direction. Since the second warm-air outlet  145  is disposed in a downstream portion of the upper warm-air mechanism  140  with respect to the sheet feeding direction as described above, the warm air can be efficiently blown toward the upper surface of the sheet stack S contained in the sheet container  35  through the second warm-air outlet  145 . By thus disposing the upper warm-air mechanism  140  having a high sheet-separation efficiency by utilizing an unused space in the sheet feeding unit  130 , a sheet separation mechanism that uses a warm-air assist and is applicable to a small sheet feeder can be realized. 
     Referring to  FIGS. 9 to 14 , a control process of a sheet separating operation according to an embodiment for warm air blowing is described. 
       FIG. 9  is a functional block diagram of a controller according to an embodiment of the present invention. The controller controls a warm-air blowing operation including a separating operation.  FIGS. 10 and 11  are flowcharts showing a control process exercised by the controller shown in  FIG. 9 .  FIGS. 12 to 14  are longitudinal sectional views of a main part of a sheet feeding unit according to an embodiment, where  FIG. 12  shows a state in which the sheet stack is being separated with blowing of warm air at the sheet feed position,  FIG. 13  shows a state in which the sheet stack is being separated with blowing of warm air at a separation position, and  FIG. 14  shows a state in which some sheets (those having high inter-sheet adhesion in an upper part of the sheet stack) float while adhering to each other. 
     As described below, the sheet feeding unit  130  according to the embodiment can perform an intermittent sheet separating operation in which a sheet separating operation is performed every time a predetermined number (for example, ten) of the sheets P are fed during a continuous feeding operation. 
     Since the sheets P are separated every time a predetermined number of the sheets P are fed, the sheet separation mechanism effectively prevents the sheets P from jamming even when a large number of the sheets P like art paper or coated paper having a high inter-sheet adhesion, for which prevention of double feeding is particularly difficult, are continuously fed. 
     First, referring to the functional block diagram of  FIG. 9  and the flow chart of  FIG. 10 , a sheet separating operation according to the embodiment is described. 
     The sheet feeding unit  130  includes the controller  300  that controls the lift mechanism  30  so as to perform a sheet separating operation in which the lift plate  31  is displaced so that a position on a side surface of the sheet stack S, the side surface being parallel to the sheet feeding direction, toward which warm air is blown from the first warm-air outlet (outlet)  155  of the side warm-air mechanism  150  is changed. The controller  300  controls the lift mechanism  30  so that the sheet separating operation is performed every time a predetermined number (for example, ten) of the sheets P are continuously fed during a continuous feeding operation. 
     As shown in the functional block diagram of  FIG. 9 , the controller  300  includes an I/O unit  85 , a warm-air controller  90 , a lift mechanism controller  80 , and a memory unit  84 . 
     Signals input to the I/O unit  85  includes a sheet type signal from the sheet selecting unit  39 , a position detection signal from the first detecting sensor PS 1 , a light detection signal from the second detection sensor PS 2 , a first timeout signal from a first timer  86 , a second timeout signal from a second timer  87 , an output signal from a first counter  88 , an output signal from a second counter  89 , a humidity signal from a humidity sensor HS, a warm-air request signal and a sheet feed command signal from a CPU  210  of the printer body  200 . 
     The warm-air controller  90  controls driving of the side warm-air mechanism  150  and the upper warm-air mechanism  140  corresponding to the sheet feed command signal and the warm-air request signal. In response to these input signals, the warm-air controller  90  outputs a control signal for driving the side warm-air mechanism  150  and the upper warm-air mechanism  140  to driving motors and heaters (not shown) of the warm-air mechanisms  140  and  150  through the I/O unit  85 . 
     The lift mechanism controller  80  includes a downward-drive determining section  82  and an upward-drive determining section  83 . The lift mechanism controller  80  controls the lifting movement of the lift mechanism  30  corresponding to the first timeout signal from the first timer  86 , the second timeout signal from the second timer  87 , the output signal from the first counter  88 , and the output signal from the second counter  89 , so that the lift mechanism  30  repeats a separating operation in which the lift plate  31  is moved between the sheet feed position and the separation position. 
     The downward-drive determining section  82  outputs a control signal for downwardly driving the lifting member  32  through the I/O unit  85  to the lift motor M corresponding to the sheet type signal and the first timeout signal. 
     The upward-drive determining section  83  outputs a control signal for driving the lift plate  31  upward using the lifting member  32  through the I/O unit  85  to the lift motor M corresponding to the sheet feed command signal and the second timeout signal. 
     The memory unit  84  stores, for example, a first timeout value for the first timer  86  and a second timeout value for the second timer  87  corresponding to the type of the sheets P selected with the sheet selecting unit  39 , an output signal from the first counter  88 , an output signal from the second counter  89 , and operation programs for the controllers. Moreover, the memory unit  84  includes a storage area for temporarily storing a determination result—and other data. 
     The controller  300  can be constituted by, for example, a CPU, a memory (ROM, RAM, etc.), an input interface, and an output interface. 
     In the embodiment, the type of the sheets P can be selected with the sheet selecting unit  39 . However, the present invention is not limited thereto. For example, the type of the sheets P to be fed may be determined by using a reflective photosensor, which irradiates a surface of the sheets P serving as a reflection surface with light from a light source such as an LED and detects reflected light from the surface of the sheets P with a light-receiving device such as a photodiode. 
     Referring to the flowchart of  FIG. 10 , a control process of the sheet separating operation of the controller  300  according to the embodiment is described. 
     First, when the sheet feeding cassette  130 A is loaded into the color printer  1  (S 1 ), the upward-drive determining section  83  of the lift mechanism controller  80  outputs a control signal for upwardly driving the lift plate  31  with the lifting member  32  through the I/O unit  85  to the lift motor M and the upward drive of the lift plate  31  (S 2 ) starts. 
     When it is determined that the lift plate  31  has lifted up to the sheet feed position on the basis of a position detection signal from the first detecting sensor PS 1  ( FIG. 5 ) (S 3 ), the upward-drive determining section  83  stops the lift motor M, whereby the upward drive of the lift plate  31  is stopped (S 4 ). The control process is held in a standby state in this feed position until a sheet feed command is issued. 
     When a control signal corresponding to the continuous feeding number (for example, 100 sheets) that a user has set with the operation panel and the type of the sheets to be fed that has been selected with the sheet selecting unit  39  is input through the I/O unit  85 , a sheet feed preparation period is started (S 5 ). At the same time, on the basis of a sheet feed command signal and a warm-air request signal, the warm-air controller  90  outputs control signals through the I/O unit  85  to the first fan  151  and the first heater  152  of the side warm-air mechanism  150  and to the second fan  141  and the second heater  142  of the upper warm-air mechanism  140  so as to drive the heaters and the fans (S 6 ). 
     Next, the downward-drive determining section  82  starts a downward drive of the lift plate  31  and reads from the memory unit  84  the data for a downward drive period as a first predetermined period, which corresponds to the selected type of the sheets P, on the basis of the sheet type signal from the sheet selecting unit  39 , and starts the first timer  86  (S 7 ). Then, the downward-drive determining section  82  continues the downward drive of the lift plate  31  for the first predetermined period. 
     That is, the downward-drive determining section  82  determines whether the first predetermined period has elapsed on the basis of the first timeout signal from the first timer  86  (S 8 ). If it is determined that the first predetermined period has elapsed on the basis of the first timeout signal (when the determination in S 8  is “YES”), the downward-drive determining section  82  stops the lift motor M so as to stop the downward drive of the lift plate  31  (S 9 ). 
     Next, the upward-drive determining section  83  determines whether the second predetermined period has elapsed on the basis of the second timeout signal from the second timer  87  (S 10 ). The second timer  87  continues to keep time until the second predetermined period elapses, while the lift plate  31  is held in the separation position. On the other hand, if it is determined that the second predetermined period has elapsed on the basis of the second timeout signal (when the determination in S 10  is “YES”), the upward-drive determining section  83  outputs a control signal for upwardly driving the lift plate  31  with the lifting member  32  through the I/O unit  85  to the lift motor M. Thus, the lift motor M is driven and an upward drive of the lifting member  32  is started (S 11 ). 
     Next, when it is detected that the upward drive of the lift plate  31  with the lifting member  32  to the sheet feed position has finished on the basis of the position detection signal from the first detecting sensor PS 1 , the upward-drive determining section  83  stops the lift motor M (stops the upward drive) (S 12 ). 
     If a predetermined number of separating operations have not finished (when the determination in S 13  is “NO”), the separating operation (S 7  to S 12 ), with which the lift plate  31  is moved up and down between the sheet feed position ( FIG. 12 ) and the separation position ( FIGS. 13 and 14 ), is repeated. 
     If a predetermined number of separating operations have finished (if the determination in S 13  is “YES”), a continuous feeding operation including an intermittent sheet separating operation is started (S 14 ). 
     The embodiment includes the side warm-air mechanism  150  and the upper warm-air mechanism  140 . However, needless to say, the present invention is applicable to a structure including only the side warm-air mechanism  150 . Moreover, for example, the upper warm-air mechanism  140  may be used only when the sheets P to be continuously fed are made of paper such as art paper or coated paper having a high inter-sheet adhesion. 
     In the embodiment, even after the continuous feeding operation is started, the steps S 6  to S 13  are performed as an intermittent sheet separating operation every time a predetermined number of sheets P are continuously fed. 
     Referring to the flowcharts of  FIGS. 10 and 11 , a control process of the continuous feeding operation including the intermittent sheet separating operation is described below. 
     When a predetermined number of separating operations have finished and the sheet feed preparation period has ended as shown in  FIG. 10 , continuous sheet feeding is started as shown in  FIG. 11  (S 20 ). Then, until a predetermined number (for example, a hundred) of sheets have been continuously fed, the first counter  88  counts up the number of sheets every time a sheet is fed, and the continuous feeding is performed until the number of sheets reaches a hundred, which is the final count at which the continuous feeding finishes (when the determination in S 21  is “YES”). 
     In the embodiment, until the continuous feeding of a hundred sheets P finishes, the lift mechanism  30  is controlled such that the sheet separating operation is performed every time a predetermined number (for example, ten) of the sheets P are continuously fed. 
     That is, from the time when the continuous feeding is started in S 20  to the time when a predetermined number (ten) of the sheets P have been continuously fed, the continuous feeding is continued (steps S 20  to S 22  are repeated until the determination in S 22  becomes “YES”). When the continuous feeding of the predetermined number (ten) of sheets finishes (when the determination in S 22  becomes “YES”), a sheet separating operation is performed (S 23 ). 
     Thus, in step S 23 , a sheet separating operation including the steps S 6  to S 13  shown in the flowchart of  FIG. 10  is performed. When the sheet separating operation in step S 23  finishes, a continuous feeding is started again (S 20 ). 
     In such a manner, until a continuous feeding of a predetermined number (a hundred) of sheets finishes, the sheet separating operation of step S 23  is intermittently inserted into the continuous feeding operation when the number of sheets that have been continuously fed becomes ten, twenty, thirty, . . . , ninety. 
     As described above, the sheet feeding unit  130  according to the embodiment includes the lift plate  31  on which the sheet stack S of a plurality of the sheets P are placed, a sheet feed mechanism being capable of performing a continuous sheet feeding operation starting from an uppermost sheet P in the sheet stack S placed on the lift plate  31 , the side warm-air mechanism  150  that blows air toward a side surface of the sheet stack S from the first warm-air outlet  155 , the side surface being parallel to a sheet feeding direction, a lift mechanism  30  that displaces the lift plate  31 , and the controller  300  that controls the lift mechanism  30  so as to perform a sheet separating operation in which the lift plate  31  is displaced so that a position on the side surface of the sheet stack S toward which warm air is blown from the first warm-air outlet  155  is changed, the side surface being parallel to the sheet feeding direction. The controller  300  controls the lift mechanism  30  so as to perform the sheet separating operation every time a predetermined number of the sheets P are fed during the continuous sheet feeding operation. 
     With this structure, an intermittent sheet separating operation, in which the sheet separating operation is performed every time a predetermined number of the sheets P are continuously fed, is performed during the continuous feeding operation. 
     Thus, for example, even when the sheets P made of paper, such as art paper or coated paper, having a high inter-sheet adhesion and for which prevention of double feeding is particularly difficult, are continuously fed, the sheets P can be separated every time a predetermined number of the sheets P are fed. Therefore, the sheet feeding unit  130  including the sheet separation mechanism can securely prevent sheet jamming. 
     Moreover, it is preferable that the sheet feeding unit  130  further includes the sheet identifying unit  39  that identifies a type of the sheets P to be fed, with the controller  300  determining whether or not to perform the sheet separating operation corresponding to the type of the sheets P identified by the sheet identifying unit  39 . 
     With this structure, control can be performed so that the sheet separating operation takes place, for example, when the sheets P to be continuously fed are made of paper such as art paper or coated paper having a high inter-sheet adhesion or made of paper having a weight equal to or greater than 100 g, while the sheet separating operation does not take place when the sheets P are made of paper having a low inter-sheet adhesion such as plain paper. In this case, the sheet separating operation is performed with a minimal frequency. Therefore, jamming of the sheets P can be efficiently prevented without excessively reducing the speed of continuous feeding. 
     Moreover, it is preferable that the controller  300  changes the predetermined number corresponding to the type of the sheets P identified by the sheet identifying unit  39  and performs the sheet separating operation. 
     With this structure, even if the same number (for example, a hundred) of the sheets P are to be continuously fed, control can be performed in such a manner that, when the sheets P are made of paper such as plain paper having a low inter-sheet adhesion, the sheet separating operation is performed every time twenty sheets are fed. Conversely, when the sheets P are made of paper such as art paper or coated paper having a high inter-sheet adhesion or made of paper having a weight equal to or greater than 100 g, the sheet separating operation is performed every time ten sheets are fed. 
     By changing the frequency of performing the sheet separating operation corresponding to the type of the sheets P to be fed, jamming of the sheets P can be efficiently prevented without excessively reducing the speed of continuous feeding. 
     It is preferable that the sheet feeding unit  130  according to the embodiment further include the second detection sensor SP 2  that detects a float amount by which the sheets P float when warm air is blown from the first warm-air outlet  155  of the side warm-air mechanism  150 , with the controller  300  determining whether or not to perform the sheet separating operation corresponding to the float amount of the sheets P detected by the second detection sensor SP 2 . 
     With this structure, for example, when the sheets P to be fed are made of paper such as plain paper having a low inter-sheet adhesion and the second detection sensor PS 2  detects that the sheets P have sufficiently floated due to blowing of warm air, the intermittent sheet separating operation can be omitted. 
     By thus performing the sheet separating operation only in case of necessity, jamming of the sheets P can be efficiently prevented without excessively reducing the speed of continuous feeding. 
     For example, when the sheets P to be continuously fed are made of paper such as art paper or coated paper having a high inter-sheet adhesion or made of paper having a weight equal to or greater than 100 g, the floating amount of the sheets P due to blowing of warm air by the side warm-air mechanism  150  is smaller than the floating amount in the case when the sheets P are made of paper such as plain paper having a low inter-sheet adhesion. For the same type of sheets P, inter-sheet adhesion is high under a high-humidity environment (for example, in an environment of a humidity equal to or greater than 50% RH). Therefore, even if the same warm air blowing operation is performed, a floating amount of the sheets P may differ corresponding to the type of the sheets P to be fed and the difference in environment. 
     With the above-described structure, even if the same number (for example, a hundred) of the sheets P are to be continuously fed, the controller can control in such a manner that, when it is detected that the sheets P have floated by a sufficient floating amount due to blowing of warm air, the sheet separating operation is not performed during the continuous feeding operation, and when the floating amount is insufficient, the sheet separating operation is performed during the continuous feeding operation. 
     By thus changing the frequency for performing the sheet separating operation corresponding to the floating amount of the sheets P to be fed, jamming of the sheets P can be efficiently prevented without excessively reducing the speed of continuous feeding. 
     The control may be performed in such a manner that the sheet separating operation takes place during the continuous feeding operation only when, for example, humidity equal to or greater than 50% RH is observed on the basis of the humidity signal from the humidity sensor HS. 
     In the above-described structure, the controller may change the predetermined number corresponding to the float amount of the sheets P detected by the second detection sensor PS 2  and carry out the control so as to perform the sheet separating operation. 
     For example, a float amount of the sheets P due to blowing of warm air during the sheet feed preparation period may be detected by the second detection sensor PS 2 , and the controller  300  may change the predetermined number on the basis of the detection by the second detection sensor PS 2  corresponding to the float amount of the sheets P and may perform the sheet separating operation. 
     For example, when the sheets P to be continuously fed are made of paper such as art paper or coated paper having a high inter-sheet adhesion or made of paper having a weight equal to or greater than 100 g, the floating amount of the sheets P due to blowing of warm air by the side warm-air mechanism  150  is smaller than the floating amount in the case when the sheet P is made of paper such as plain paper having a low inter-sheet adhesion. For the same type of sheets P, the inter-sheet adhesion is high under a high-humidity environment (for example, in an environment of a humidity equal to or greater than 50% RH). Therefore, even if the same warm air blowing operation is performed, floating amount of the sheets P may differ corresponding to the type of the sheets P to be fed and the difference in environment. 
     With this structure, for example, even if the same number (for example, a hundred) of the sheets P are to be continuously fed, control can be performed in such a manner that, when the floating amount of the sheets P is large, the sheet separating operation is performed, for example, every time twenty sheets are fed, and, when the floating amount of the sheets P is small, the sheet separating operation is performed every time ten sheets are fed. 
     By thus changing the frequency for performing the sheet separating operation corresponding to the floating amount of the sheets P to be fed, jamming of the sheets P can be efficiently prevented without excessively reducing the speed of continuous feeding. 
     The sheet feeder according to the embodiments of the present invention can be applied to various image forming apparatuses including printers, copiers, fax machines, and multi-functional peripherals having these functions. In particular, the sheet feeder is suitable for small image forming apparatuses.