Patent Publication Number: US-9896285-B2

Title: Medium conveyance device

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority based on 35 USC 119 from prior Japanese Patent Application No. 2015-109558 filed on May 29, 2015, entitled “MEDIUM CONVEYANCE DEVICE”, the entire contents of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This disclosure relates to a medium conveyance device that convey a medium one by one out of media stacked on a medium stacker. 
     2. Description of Related Art 
     When a conventional medium conveyance device performs a medium conveyance (sheet feeding) operation to send a medium (a sheet) one by one from a medium stacker (a sheet container) in which media stacked, the medium conveyance device sends out (conveys) the lowest medium (at the bottom position) among the media so that a user can add media on the top of the stacked media even during the operation. In the medium conveyance operation, a conveyance belt comes into contact with a bottom surface of the medium located at the bottom, and applies a conveyance force in a conveyance direction to the medium. Receiving the conveyance force, the medium moves in the conveyance direction and passes through a passage (a clearance) defined between the conveyance belt and a separator opposed thereto. Thus, the single medium is separated from the rest of the media. Then, the medium thus separated is discharged to the outside of the medium conveyance device (see Japanese Patent Application Publication No. 2001-97563, for example). 
     SUMMARY OF THE INVENTION 
     However, the conventional medium conveyance device requires cumbersome adjustments, such as changing a height of the separator (a thickness of the passage) and lifting up tail ends of the stacked media, depending on states of the media including the medium type (a type of the media categorized by the thickness and material thereof), the medium length (a length of each medium), the amount of the stacked media (the remaining amount of the media), and so forth. If the adjustments are not carried out, the medium conveyance device is more likely to discharge multiple sheets of media at a time (multi-feeding). 
     An object of an embodiment of the invention is to provide a medium conveyance device which can reliably convey the lowest medium one by one among stacked media without requiring cumbersome adjustments depending on states of the media. 
     An aspect of the invention is a medium conveyance device that includes: a first conveyance unit that applies a first conveyance force in a conveyance direction to a lowest medium among media on a medium stacker; a second conveyance unit disposed downstream in the conveyance direction of the first conveyance unit and that applies a second conveyance force in the conveyance direction to the medium being conveyed in the conveyance direction; a detector that detects that a rear end of the lowest medium receiving the first conveyance force passes through the first conveyance unit; and a controller that controls the first and second conveyance units. Upon determining that the rear end of the lowest medium receiving the first conveyance force passes through the first conveyance unit, the controller switches the first conveyance unit from a conveyance state of applying the first conveyance force to a non-conveyance state of not applying the first conveyance force. 
     According to this aspect of the invention, the medium conveyance device can reliably convey the lowest medium one by one among stacked media without requiring cumbersome adjustments depending on states of the media. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram schematically illustrating a configuration of a medium conveyance device according to a first embodiment of the invention. 
         FIG. 2  is a vertical cross-sectional view schematically illustrating the configuration of the medium conveyance device according to the first embodiment. 
         FIGS. 3A to 3D  are diagrams illustrating operations of the medium conveyance device according to the first embodiment. 
         FIG. 4  is a flowchart illustrating the operations of the medium conveyance device according to the first embodiment. 
         FIG. 5  is a flowchart illustrating the operations (the operations subsequent to  FIG. 4 ) of the medium conveyance device according to the first embodiment. 
         FIG. 6  is a flowchart illustrating the operations (the operations subsequent to  FIG. 5 ) of the medium conveyance device according to the first embodiment. 
         FIG. 7  is a vertical cross-sectional view schematically illustrating a configuration of a medium conveyance device according to a second embodiment of the invention. 
         FIGS. 8A to 8C  are vertical cross-sectional views schematically illustrating states of the medium conveyance device according to the second embodiment. 
         FIG. 9  is a flowchart illustrating operations of the medium conveyance device according to the second embodiment. 
         FIG. 10  is a vertical cross-sectional view schematically illustrating a configuration of a medium conveyance device according to a third embodiment of the invention. 
         FIGS. 11A to 11C  are vertical cross-sectional views schematically illustrating states of the medium conveyance device according to the third embodiment. 
         FIG. 12  is a flowchart illustrating operations of the medium conveyance device according to the third embodiment. 
         FIG. 13  is a table illustrating relations between situations of medium detection by sensors and control of conveyance units in the medium conveyance device according to the third embodiment. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Descriptions are provided hereinbelow for embodiments based on the drawings. In the respective drawings referenced herein, the same constituents are designated by the same reference numerals and duplicate explanation concerning the same constituents is omitted. All of the drawings are provided to illustrate the respective examples only. 
     &lt;&lt;1&gt;&gt; First Embodiment 
     &lt;&lt;1-1&gt;&gt; Configuration 
       FIG. 1  is a diagram schematically illustrating a configuration of medium conveyance device  1  according to a first embodiment of the invention. As illustrated in  FIG. 1 , medium conveyance device  1  includes medium conveyor  20  configured to feed a medium one by one out of media stacked on medium stacker  10 , and controller  80 . Each of the media is a sheet of paper, for example. Medium conveyor  20  includes: auxiliary conveyance unit (first conveyance unit)  40  configured to apply a first conveyance force in a predetermined conveyance direction E to medium  11  which is the lowest medium among the media stacked on medium stacker  10 ; main conveyance unit (second conveyance unit)  50  disposed downstream in the conveyance direction E of auxiliary conveyance unit  40  and configured to apply a second conveyance force in the conveyance direction E to medium  11  being conveyed in the conveyance direction; and medium detection unit (detector)  30  configured to detect that rear end  11   a  in the conveyance direction E of medium  11  passes through auxiliary conveyance unit  40 . Controller  80  switches auxiliary conveyance unit  40  from a conveyance state of applying the first conveyance force to a non-conveyance state (a stopped state) of not applying the first conveyance force when controller  80  determines from a result of detection by medium detection unit  30 , that rear end  11   a  in the conveyance direction E of medium  11  receiving the first conveyance force passes through auxiliary conveyance unit  40  (when controller  80  determines that rear end  11   a  is located downstream of a position of a medium sensor, for example). 
     In the meantime, medium conveyance device  1  preferably includes medium separator (movement restrictor)  60  and discharger (sheet delivery unit)  70 . Medium separator  60  defines passage (clearance)  63  between medium separator  60  and main conveyance unit  50  to allow medium  11  to pass through passage  63 . Medium separator  60  has a function to restrict movements in the conveyance direction E of the media, which are stacked on medium stacker  10  and placed on medium  11  being the medium at the bottom, and thereby to separate only medium  11  from the rest of the media thereon. 
     Controller  80  receives information concerning locations of the media (information on whether or not the rear end of media  11  passes through auxiliary conveyance unit  40  and a reference position, or information indicating whether or not any of the media is present) I 30  from medium detection unit  30 , and sends drive commands C 40 , C 50 , and C 70  to auxiliary conveyance unit  40 , main conveyance unit  50 , and discharger  70 , respectively, based on received information I 30 . Moreover, controller  80  includes control IF (interface) unit  81 . Medium conveyance device  1  communicates with host device  2  by using control IF unit  81 . For example, host device  2  is a device (such as a printer or a facsimile machine) configured to transmit a medium feed instruction signal to medium conveyance device  1  and to receive a sheet as a medium from medium conveyance device  1 . 
       FIG. 2  is a vertical cross-sectional view schematically illustrating a configuration of medium conveyance device  1  according to the first embodiment. In  FIG. 2 , constituents which are identical or correspond to the constituents illustrated in  FIG. 1  are designated by the same reference numerals as those in  FIG. 1 . 
     As illustrated in  FIG. 2 , auxiliary conveyance unit  40  as the first conveyance unit includes first auxiliary roller  41  and second auxiliary roller  43  which come into contact with a bottom surface of medium  11 , being the lowest medium among the media stacked on medium stacker  10 , and apply the conveyance force in the conveyance direction E (the first conveyance force) to medium  11 . Moreover, auxiliary conveyance unit  40  includes first auxiliary motor (first driver)  42  configured to rotate or stop rotation of first auxiliary roller  41 , and second auxiliary motor (second driver)  44  configured to rotate or stop rotation of second auxiliary roller  43 . An outer peripheral surface of each of first auxiliary roller  41  and second auxiliary roller  43  is made of a material containing a natural rubber raw material, a urethane raw material, and the like, for example. Although  FIG. 2  illustrates the example in which auxiliary conveyance unit  40  includes first auxiliary roller  41  and second auxiliary roller  43 , auxiliary conveyance unit  40  may include one auxiliary roller or three or more auxiliary rollers instead. 
     As illustrated in  FIG. 2 , medium detection unit  30  includes: first medium sensor  31  disposed at a first reference position located upstream in the conveyance direction E of first auxiliary roller  41  and configured to detect a state of medium  11  at the first reference position to which the first conveyance force is applied; and second medium sensor  32  disposed at a second reference position between first auxiliary roller  41  and second auxiliary roller  43  and configured to detect a state of medium  11  at the second reference position to which the first conveyance force is applied. Controller  80  can determine the position of medium  11  by using results of the detection by first medium sensor  31  and second medium sensor  32 . 
     Each of first medium sensor  31  and second medium sensor  32  is a displacement sensor which detects a change in position of a detection target at the first reference position or the second reference position, for example. The displacement sensor is, for example, an optical sensor which detects the displacement of the detection target by measuring a change in time between a point of emission of a laser beam onto the medium and a point of reception of the laser beam reflected from the medium. Alternatively, the displacement sensor may be a mechanical sensor which detects the displacement of the detection target by bringing a measurement probe into contact with the bottom surface of the medium and measuring a change in position of the medium when the medium is conveyed. First medium sensor  31  and second medium sensor  32  may be sensors of other types as long as such sensors can detect the passage of rear end  11   a  of medium  11 . 
     Medium conveyance device  1  includes medium stacker (sheet container)  10  which contains the media stacked thereon. Moreover, as main conveyance unit  50 , medium conveyance device  1  includes: conveyance belt  51  which applies a second conveyance force in the conveyance direction E to the medium out of the media stacked on the medium stacker  10 , the medium being conveyed in the conveyance direction; conveyance belt rollers  52  and  53  on which conveyance belt  51  is wound; hopping clutch  54  which switches between stop and drive of conveyance belt rollers  52  and  53 ; and hopping motor (third driver)  55  which rotates conveyance belt rollers  52  and  53 . Hopping clutch  54  transmits a driving force generated by hopping motor  55  to conveyance belt roller  52 , thereby rotating conveyance belt roller  52  and thus rotating conveyance belt  51  (establishing a conveyance state). Meanwhile, hopping clutch  54  refrains from transmitting the driving force generated by hopping motor  55  to conveyance belt roller  52 , thereby stopping conveyance belt  51  (establishing a non-conveyance state). 
     Moreover, medium conveyance device  1  includes discharger  70 . Discharger  70  includes medium sensor  71 , paired registration rollers  72  which send medium  11  out to host device  2 ; and registration clutch  73  which switches between stop and drive of registration rollers  72 . Here, the driving force of hopping motor  55  is also transmitted to paired registration rollers  72  via registration clutch  73 . Accordingly, hopping motor  55  also has a function as discharger  70 . Meanwhile, medium sensor  71  is configured to detect whether or not the medium being conveyed is located at a detecting position. Accordingly, medium sensor  71  also has a function as medium detection unit  30 . Medium sensor  71  is used for detecting that front end  11   b  of medium  11  passes through the detecting position for medium sensor  71 . 
     Medium conveyance device  1  includes medium separator (movement restrictor)  60 . Medium separator  60  includes separation plate  61  as a first separation unit, and a separation piece  62  as a second separation unit. Separation plate  61  is disposed in such away as to be opposed to conveyance belt  51  while defining a first distance, which is an distance of passage (clearance)  64 , between separation plate  61  and conveyance belt  51 . Passage  64  has the distance that enables some media to pass therethrough. Separation piece  62  is disposed downstream in the conveyance direction of separation plate  61  and in such a way as to be opposed to conveyance belt  51  while defining a second distance, which is the distance of passage  63 , between separation piece  62  and conveyance belt  51 . The distance of passage  63  is narrower than the distance of passage  64 , and enables only one medium to pass therethrough. For example, passage  64  defined by separation plate  61  is adjusted to a thickness in a range from twice to five times as large as a thickness of each medium so that some (two to five, for example) lowest media among the media stacked on medium stacker  10  can pass through passage  64 . Meanwhile, the distance of passage  63  defined by separation piece  62  is adjusted to a value which is greater than the thickness of each medium but smaller than twice the thickness of each medium, so that the medium being conveyed in the conveyance direction can pass therethrough. In other words, an distance between conveyance belt  51  of second conveyance unit  50  and a bottom surface of separation piece  62  being apart of movement restrictor  60  located closest to conveyance belt  51  is adjusted to the value greater than the thickness of each medium but smaller than twice the thickness of each medium. Nonetheless, medium separator  60  does not always have to be formed from the two components (separation plate  61  and separation piece  62 ), and may be formed from a single component or three or more components instead. 
     Meanwhile, controller  80  of medium conveyance device  1  is formed from a control circuit, for example. Controller  80  drives or stops first auxiliary motor  42 , second auxiliary motor  44 , and hopping motor  55  based on information received from first medium sensor  31 , second medium sensor  32 , and medium sensor  71 . Each of motors  42 ,  44 , and  55  is a stepping motor, for example. In the meantime, controller  80  turns hopping clutch  54  and registration clutch  73  on (for connection) and off (for disconnection). Here, each of hopping clutch  54  and registration clutch  73  may adopt any mechanism as long as such a mechanism can switch between a state of transmitting the driving force of hopping motor  55  and a state of not transmitting the driving force thereof. 
     An distance between conveyance belt roller  53  and second auxiliary roller  43  is in a range from 50 mm to 70 mm, for example. An distance between second auxiliary roller  43  and first auxiliary roller  41  is in a range from 40 mm to 60 mm, for example. An distance between conveyance belt roller  53  and second medium sensor  32  is in a range from 20 mm to 35 mm, for example. An distance between second auxiliary roller  43  and second medium sensor  32  is in a range from 20 mm to 35 mm, for example. An distance between first auxiliary roller  41  and first medium sensor  31  is in a range from 20 mm to 35 mm, for example. However, the invention is not limited to the above-mentioned configurations and can be modified as appropriate depending on the medium type. The medium type subjected to the conveyance by the medium conveyance device applying the invention includes, but is not limited to, sheets of paper, name cards, postcards, envelopes, and the like. Further, the number of the auxiliary rollers is not limited to two, and the medium sensors are not limited only to the two sensors of the first medium sensor and the second medium sensor. The numbers of the auxiliary rollers and the medium sensors may be set to three or more depending on the medium type, for example. 
     &lt;&lt;1-2&gt;&gt; Operations 
       FIGS. 3A to 3D  are diagrams illustrating operations of medium conveyance device  1  according to the first embodiment. In  FIGS. 3A to 3D , constituents which are identical or correspond to the constituents illustrated in  FIG. 2  are designated by the same reference numerals as those in  FIG. 2 . As illustrated in  FIGS. 3A to 3D , the position of separation plate  61  relative to conveyance belt  51  is adjusted in such a way as to define passage  64  that allows some (about two) stacked media to pass therethrough. Meanwhile, the position of separation piece  62  relative to conveyance belt  51  is adjusted in such a way as to define passage  63  that allows a single medium to pass therethrough. 
     Medium conveyance device  1  starts the conveyance of the medium by driving first auxiliary roller  41 , second auxiliary roller  43 , and conveyance belt  51 . By rotation of first auxiliary roller  41 , second auxiliary roller  43 , and conveyance belt  51  as illustrated in  FIG. 3A , the conveyance force is applied to medium  11  which is the lowest medium among the media stacked on medium stacker  10 . Thus, medium  11  is conveyed in the conveyance direction E. Some media (such as media  12  and  13 ) that are stacked on medium  11  move in the conveyance direction E together with medium  11 . 
     As illustrated in  FIG. 3B , medium  11  and medium  12  located thereon move in the conveyance direction E while passing through passage  64  defined by separation plate  61 . As illustrated in  FIG. 3B , medium  13 , which is the third medium from the bottom, is not conveyed in the conveyance direction E because separation plate  61  restricts its movement in the conveyance direction E. After first medium sensor  31  detects rear end  11   a  of medium  11  (after the conveyance of medium  11  over a distance L 1  following the detection of rear end  11   a , for example), controller  80  stops first auxiliary motor  42  so as to switch first auxiliary roller  41  from the conveyance state of being rotated and applying the conveyance force to the non-conveyance state of not being rotated and not applying the conveyance force. 
     As illustrated in  FIG. 3C , medium  11  passes through passage  63  defined by separation piece  62 . In the meantime, medium  12  moves in the conveyance direction E together with medium  11 . Here, first auxiliary roller  41  is in contact with contact point  41 P on a bottom surface of medium  12  after the passage of medium  11 . Since first auxiliary roller  41  stops the rotation at this point, medium  12  does not receive the conveyance force in the conveyance direction E from first auxiliary roller  41 . Meanwhile, first auxiliary roller  41  in the non-conveyance state is not rotated by a force received from the stacked media. As a consequence, if medium  12  attempts to move in the conveyance direction E together with medium  11 , medium  12  receives a force from contact point  41 P in an opposite direction to the conveyance direction E. As described above, first auxiliary roller  41  in the non-conveyance state functions as a brake to prevent the media other than medium  11  from moving in the conveyance direction E. Accordingly, even when medium  12  moves in the conveyance direction E together with medium  11 , a distance of movement of medium  12  becomes smaller than a distance of movement of medium  11 . Here, the conveyance force in the conveyance direction E from medium  12  to be received by the medium located on medium  12  also becomes small. Thus, the medium located on medium  12  is prevented from entering the gap between separation plate  61  and medium  12 . 
     After second medium sensor  32  detects rear end  11   a  of medium  11  (after the conveyance of medium  11  over a distance L 2  following the detection of rear end  11   a , for example), medium conveyance device  1  stops second auxiliary motor  44  so as to switch second auxiliary roller  43  from the conveyance state of being rotated and to the non-conveyance state of not being rotated by changing the conveyance force and not applying the conveyance force. 
     As illustrated in  FIG. 3D , medium  11  is conveyed toward registration rollers  72  of discharger  70 . Meanwhile, after the passage of medium  11 , contact point  43 P of second auxiliary roller  43  is in contact with the bottom surface of medium  12 . Here, since second auxiliary roller  43  stops the rotation at this point, medium  12  does not receive the conveyance force in the conveyance direction E from second auxiliary roller  43 . Meanwhile, second auxiliary roller  43  in the non-conveyance state is not rotated by the force received from the stacked media. As a consequence, if medium  12  attempts to move in the conveyance direction E together with medium  11 , medium  12  receives a force from contact point  43 P in the opposite direction to the conveyance direction E. As described above, as with first auxiliary roller  41  in the non-conveyance state, second auxiliary roller  43  in the non-conveyance state functions as a brake to prevent the media other than medium  11  from moving in the conveyance direction E. 
     As described above, when medium  12  stacked on medium  11  passes through passage  64  defined by separation plate  61 , the conveyance force in the conveyance direction E to be received by medium  12  is limited since first auxiliary roller  41  and second auxiliary roller  43  are in the non-conveyance state. The sufficient conveyance force for allowing medium  12  to pass through the gap between medium  11  and separation piece  62  does not act on medium  12 . For this reason, by using separation piece  62 , medium conveyance device  1  can restrict the movement in the conveyance direction E of medium  12  stacked on medium  11 , and reliably convey medium  11  only. 
       FIGS. 4 to 6  are flowcharts illustrating the operations of medium conveyance device  1  according to the first embodiment.  FIGS. 4 to 6  illustrate the processing from a start of the conveyance of medium  11  by medium conveyance device  1  to the completion of the conveyance. In the following description, references are also made to  FIGS. 1 to 3D . 
       FIG. 4  illustrates the processing from the start of the conveyance of medium  11  to a stop of the drive of first auxiliary roller  41 . Upon receipt of a sheet feeding command sent from host device  2 , controller  80  drives hopping motor  55  (step S 1 ). Controller  80  turns hopping clutch  54  on (step S 2 ), and rotates conveyance belt  51 . Meanwhile, controller  80  starts the drive of first auxiliary motor  42  and second auxiliary motor  44  (step S 3 ), thereby rotating first auxiliary roller  41  and second auxiliary roller  43 . Thus, controller  80  causes first auxiliary roller  41 , second auxiliary roller  43 , and conveyance belt  51  to convey medium  11  in the conveyance direction E. 
     When first medium sensor  31  detects rear end  11   a  of medium  11  (YES in step S 4 ), controller  80  determines whether or not first auxiliary motor  42  performed α1-step rotation (rotation of the number of steps equal to α1) (step S 5 ). The α1-step rotation corresponds to the distance L 1  illustrated in  FIG. 3B , for example. In this way, from a result of the detection by first medium sensor  31 , controller  80  can determine that rear end  11   a  of medium  11  passed through the first reference position where first medium sensor  31  is disposed, and is located downstream of the first reference position. After first auxiliary motor  42  performed the α1-step rotation, controller  80  stops the drive of first auxiliary motor  42  (step S 6 ), and stops the rotation of first auxiliary roller  41 . Controller  80  establishes the non-conveyance state of first auxiliary roller  41 , and causes second auxiliary roller  43  and conveyance belt  51  to convey medium  11  in the conveyance direction E. 
       FIG. 5  illustrates the processing to the point where controller  80  drives registration rollers  72 . When second medium sensor  32  detects rear end  11   a  of medium  11  (YES in step S 7 ), controller  80  determines whether or not second auxiliary motor  44  performed α2-step rotation (rotation of the number of steps equal to α2) (step S 8 ). The α2-step rotation corresponds to the distance L 2  illustrated in  FIG. 3C , for example. In this way, from a result of the detection by second medium sensor  32 , controller  80  can determine that rear end  11   a  of medium  11  passed through the second reference position where second medium sensor  32  is disposed, and is located downstream of the second reference position. After second auxiliary motor  44  performed the α2-step rotation, controller  80  stops the drive of second auxiliary motor  44  (step S 9 ), and stops the rotation of second auxiliary roller  43 . Controller  80  establishes the non-conveyance state of second auxiliary roller  43 , and causes conveyance belt  51  to convey medium  11  in the conveyance direction E. 
     As medium  11  is conveyed, medium sensor  71  detects that front end  11   b  of medium  11  passes above medium sensor  71  (YES in step S 10 ). When medium sensor  71  detects front end  11   b  of medium  11 , controller  80  determines whether or not hopping motor  55  performed α3-step rotation (rotation of the number of steps equal to α3) (step S 11 ). After hopping motor  55  performed the α3-step rotation, controller  80  turns registration clutch  73  on (step S 12 ), and rotates registration rollers  72 . Controller  80  causes conveyance belt  51  and registration rollers  72  to convey medium  11  in the conveyance direction E. 
       FIG. 6  illustrates the processing to the point where controller  80  conveys medium  11  to host device  2 . Controller  80  determines whether or not hopping motor  55  performed α4-step rotation (rotation of the number of steps equal to α4) (step S 13 ). After hopping motor  55  performed the α4-step rotation, controller  80  turns hopping clutch  54  off (step S 14 ), and stops the drive of conveyance belt  51 . Controller  80  causes registration rollers  72  to convey medium  11  in the conveyance direction E. 
     After turning hopping clutch  54  off, controller  80  determines whether or not hopping motor  55  performed α5-step rotation (rotation of the number of steps equal to α5) (step S 15 ). After hopping motor  55  performed the α5-step rotation, controller  80  turns registration clutch  73  off (step S 16 ), and stops the rotation of registration rollers  72 . Thus, medium.  11  is conveyed from medium conveyance device  1  to host device  2 . 
     The respective values of α1, α2, α3, α4, and α5 indicating the numbers of steps corresponding to rotational angles are determined based on the size of the medium, the distance between medium sensor  71  and conveyance belt  51 , a friction coefficient between conveyance belt  51  and the medium, and the like. 
     In the above description, the timings to stop first auxiliary roller  41  and second auxiliary roller  43  are determined while designating the timings of the detection of rear end  11   a  of medium  11  by first medium sensor  31  and second medium sensor  32  as starting points, respectively. However, the invention is not limited to this configuration. The invention may also be configured to receive information on the length of the medium from host device  2 , to calculate the timings at which rear end  11   a  of medium  11  will pass through the first reference position and the second reference position based on the information on the length of the medium, and further to stop first auxiliary motor  42  and second auxiliary motor  44  after each of first auxiliary motor  42  and second auxiliary motor  44  performed rotation in a certain number of steps. In this case, first medium sensor  31  and second medium sensor  32  may be omitted. 
     &lt;&lt;1-3&gt;&gt; Effects 
     As described above, medium conveyance device  1  according to the first embodiment stops the rotation of first auxiliary roller  41  after first medium sensor  31  detects rear end  11   a  of medium  11 , and stops the rotation of second auxiliary roller  43  after second medium sensor  32  detects rear end  11   a  of medium  11 . As a consequence, even when medium  12  on medium  11  comes into contact with first auxiliary roller  41  and second auxiliary roller  43 , medium  12  does not receive the conveyance force in the conveyance direction E from first auxiliary roller  41  or second auxiliary roller  43 . Meanwhile, first auxiliary roller  41  and second auxiliary roller  43  in the non-conveyance state are not rotated by the force received from the stacked media. Accordingly, when medium  12  attempts to move in the conveyance direction E together with medium  11 , medium  12  receives the force in the opposite direction to the conveyance direction E from contact points  43 P and  41 P as a consequence. As described above, the conveyance force in the conveyance direction E received by medium  12  is restricted. Accordingly, medium conveyance device  1  can convey and discharge only medium  11  being the lowest medium among the stacked media, without performing cumbersome adjustments of separation plate  61 , separation piece  62 , and the like. Thus, medium conveyance device  1  can reliably prevent the media stacked on medium  11  from being discharged. 
     &lt;&lt;2&gt;&gt; Second Embodiment 
     &lt;&lt;2-1&gt;&gt; Configuration 
       FIG. 7  is a vertical cross-sectional view schematically illustrating a configuration of medium conveyance device  1   a  according to a second embodiment of the invention. In  FIG. 7 , constituents which are identical or corresponding to the constituents illustrated in  FIG. 2  are designated by the same reference numerals as those in  FIG. 2 . As illustrated in  FIG. 7 , medium conveyance device  1   a  according to the second embodiment is different from medium conveyance device  1  according to the first embodiment. Medium conveyance device  1   a  includes medium conveyor  20   a , which is provided with a third medium sensor  33  disposed at a third reference position between conveyance belt  51  and second auxiliary roller  43 . Third medium sensor  33  is configured to detect whether or not a medium is present at the third reference position. Each of conveyance belt  51 , second auxiliary roller  43 , and first auxiliary roller  41  is determined to be driven depending on a length in the conveyance direction E of the media stacked on medium stacker  10 . Other features of medium conveyance device  1   a  according to the second embodiment are the same as those of medium conveyance device  1  according to the first embodiment. Therefore, reference is also made to  FIG. 1  in the description of the second embodiment. 
     &lt;&lt;2-2&gt;&gt; Operations 
       FIGS. 8A to 8C  are vertical cross-sectional views schematically illustrating states of medium conveyance device  1   a  according to the second embodiment.  FIG. 8A  illustrates a state in which media  11  to  13  having a large medium length are stacked (set) on medium stacker  10 .  FIG. 8B  illustrates a state in which media  11  to  13  having a medium length shorter than the length in  FIG. 8A  are stacked on medium stacker  10 .  FIG. 8C  illustrates a state in which media  11  to  13  having a medium length shorter than the length in  FIG. 8B  are stacked on medium stacker  10 . In  FIGS. 8A to 8C , constituents which are identical or corresponding to the constituents illustrated in  FIGS. 3A to 3C  are designated by the same reference numerals as those in  FIGS. 3A to 3C . 
     In the case of  FIG. 8A , third medium sensor  33  detects that any of the media is present at the third reference position, being its detecting position. Meanwhile, second medium sensor  32  detects that any of the media is present at the second reference position, being its detecting position, and first medium sensor  31  detects that any of the media is present at the first reference position, being its detecting position. In this case, operations of medium conveyance device  1   a  are the same as the operations of medium conveyance device  1  according to the first embodiment. 
     In the case of  FIG. 8B , third medium sensor  33  detects that any of the media is present at the third reference position, being its detecting position. Meanwhile, second medium sensor  32  detects that any of the media is present at the second reference position, being its detecting position, and first medium sensor  31  detects that the media are not present at the first reference position, being its detecting position. In this case, the weight of the media stacked on medium stacker  10  is smaller than the weight in the case of  FIG. 8A . For this reason, controller  80  can convey medium  11  in the conveyance direction E by driving conveyance belt  51  and second auxiliary roller  43  while keeping first auxiliary roller  41  stopped. Meanwhile, when medium  11  moves in the conveyance direction E and first auxiliary roller  41  comes into contact with the bottom surface of medium  12 , first auxiliary roller  41  in the non-conveyance state can also function as the brake to prevent the media, other than medium  11 , from moving in the conveyance direction E. Accordingly, medium conveyance device  1   a  can convey and discharge only medium  11  being the lowest medium among the stacked media, and reliably prevent the media stacked on medium  11  from being discharged. 
     In the case of  FIG. 8C , third medium sensor  33  detects that any of the media is present at the third reference position, being its detecting position. Meanwhile, second medium sensor  32  detects that the media are not present at the second reference position, being its detecting position, and first medium sensor  31  detects that the media are not present at the first reference position, being its detecting position. In this case, the weight of the media stacked on medium stacker  10  is smaller than the weight in the case of  FIG. 8B . For this reason, controller  80  can convey medium  11  in the conveyance direction E by driving conveyance belt  51  while keeping second auxiliary roller  43  and first auxiliary roller  41  stopped. Meanwhile, as with first auxiliary roller  41  in the non-conveyance state, second auxiliary roller  43  in the non-conveyance state can also function as the brake to prevent the media, other than medium  11 , from moving in the conveyance direction E. Accordingly, medium conveyance device  1   a  can convey and discharge only medium  11  being the lowest medium among the stacked media, and reliably prevent the media stacked on medium  11  from being discharged. 
       FIG. 9  is a flowchart illustrating operations of medium conveyance device  1   a  according to the second embodiment. Note that reference is also made to  FIG. 7  and  FIGS. 8A to 8C  in the following description. 
     First, after the media are set on medium stacker  10  (step S 21 ), controller  80  determines whether or not any of the media is present at the third reference position opposed to third medium sensor  33  based on a detection signal from third medium sensor  33  (step S 22 ). When controller  80  determines that any of the media is present at the third reference position from a result of detection by third medium sensor  33  (YES in step S 22 ), controller  80  determines conveyance belt  51  as an object to be driven (step S 23 ). 
     Next, controller  80  determines whether or not any of the media is present at the second reference position opposed to second medium sensor  32  based on a detection signal from second medium sensor  32  (step S 24 ). When controller  80  determines that any of the media is present at the second reference position from a result of the detection by second medium sensor  32  (YES in step S 24 ), controller  80  determines second auxiliary roller  43  as an object to be driven (step S 25 ) and the processing proceeds to step S 26 . On the other hand, when controller  80  determines that the media are not present at the second reference position from the result of the detection by second medium sensor  32  (NO in step S 24 ), controller  80  drives conveyance belt  51  and conveys medium  11  in the conveyance direction E by using conveyance belt  51  (step S 30 ) as illustrated in  FIG. 8C . 
     In step S 26 , controller  80  determines whether or not any of the media is present at the first reference position opposed to first medium sensor  31  based on a detection signal from first medium sensor  31 . When controller  80  determines that any of the media is present at the first reference position from a result of the detection by first medium sensor  31  (YES in step S 26 ), controller  80  determines first auxiliary roller  41  as an object to be driven (step S 27 ). Next, as illustrated in  FIG. 8A , controller  80  drives conveyance belt  51 , second auxiliary roller  43 , and first auxiliary roller  41 , and conveys medium  11  in the conveyance direction E by using conveyance belt  51 , second auxiliary roller  43 , and first auxiliary roller  41  (step S 28 ). In the case of  FIG. 8A , medium conveyance device  1   a  conveys medium  11  in accordance with steps S 1  to step S 16  as illustrated in  FIGS. 4 to 6 . 
     Meanwhile, in the case illustrated in  FIG. 8B , medium conveyance device  1   a  conveys the medium in accordance with steps S 7  to step S 16  as illustrated in  FIGS. 5 and 6 . 
     &lt;&lt;2-3&gt;&gt; Effects 
     As described above, according to medium conveyance device  1   a  of the second embodiment, controller  80  determines the objects to be driven out of conveyance belt  51 , second auxiliary roller  43 , and first auxiliary roller  41  depending on the medium length of the media set on medium stacker  10 . The conveyance force in the conveyance direction E to be received by the media is restricted depending on the medium length as described above. Accordingly, medium conveyance device  1   a  of the second embodiment can convey and discharge only medium  11  being the lowest medium among the stacked media depending on the state of the media, without performing cumbersome adjustments of separation plate  61 , separation piece  62 , and the like. Thus, medium conveyance device  1   a  can reliably prevent the media stacked on medium  11  from being discharged. 
     &lt;&lt;3&gt;&gt; Third Embodiment 
     &lt;&lt;3-1&gt;&gt; Configuration 
       FIG. 10  is a vertical cross-sectional view schematically illustrating a configuration of medium conveyance device  1   b  according to a third embodiment of the invention. In  FIG. 10 , constituents which are identical or correspond to the constituents illustrated in  FIG. 7  are designated by the same reference numerals as those in  FIG. 7 . As illustrated in  FIG. 10 , medium conveyance device  1   b  according to the third embodiment is different from medium conveyance device  1   a  according to the second embodiment in that medium conveyance device  1   b  includes first height sensor  91  and second height sensor  92  each configured to detect the height of the media stacked on medium stacker  10 . Each of conveyance belt  51 , second auxiliary roller  43 , and first auxiliary roller  41  is determined to be driven depending on the height of the media stacked on medium stacker  10 . For example, first height sensor  91  and second height sensor  92  are arranged in the height direction on separation plate  61 . Other features of medium conveyance device  1   b  according to the third embodiment are the same as those of medium conveyance device  1   a  according to the second embodiment. Therefore, reference is also made to  FIG. 7  in the description of the third embodiment. 
     &lt;&lt;3-2&gt;&gt; Operations 
       FIGS. 11A to 11C  are vertical cross-sectional views schematically illustrating states of medium conveyance device  1   b  according to the third embodiment.  FIG. 11A  illustrates a state in which the media are stacked (set) to a position higher than a height D 2  on medium stacker  10 .  FIG. 11B  illustrates a state in which the media are stacked to a position lower than the height D 2  and higher than a height D 1  on medium stacker  10  (where D 2 &gt;D 1 ).  FIG. 11C  illustrates a state in which the media are stacked to a position lower than the height D 1  on medium stacker  10 . In  FIGS. 11A to 11C , constituents which are identical or correspond to the constituents illustrated in  FIGS. 8A to 8C  are designated by the same reference numerals as those in  FIGS. 8A to 8C . Although the third embodiment describes the case of providing first height sensor  91  and second height sensor  92 , it is also possible to provide a single height sensor or three or more height sensors instead. 
     As illustrated in  FIG. 11A , first height sensor  91  is disposed at a first height reference position D 1  which is a position having the height D 1 . Second height sensor  92  is disposed at a second height reference position D 2  which is a position having the height D 2 . In the case of  FIG. 11A , first height sensor  91  detects that any of the media is present at the first height reference position, being its detecting position. Meanwhile, second height sensor  92  detects that any of the media is present at the second height reference position, being its detecting position. In this case, the weight of the media stacked on medium stacker  10  is larger than the weights in the cases of  FIGS. 11B and 11C  to be described below. Here, medium conveyance device  1   b  has to apply a large conveyance force to medium  11 . For this reason, controller  80  of medium conveyance device  1   b  drives conveyance belt  51 , second auxiliary roller  43 , and first auxiliary roller  41  as in the case of the first embodiment, and conveys medium  11  in the conveyance direction E by using conveyance belt  51 , second auxiliary roller  43 , and first auxiliary roller  41 . In this case, the operations of medium conveyance device  1   b  are the same as the operations of medium conveyance device  1  according to the first embodiment. 
     In the case of  FIG. 11B , first height sensor  91  detects that any of the media is present at the first height reference position, being its detecting position. Meanwhile, second height sensor  92  does not detect that the media are present at the second height reference position, being its detecting position. In this case, the weight of the media stacked on medium stacker  10  is lower than the weight in the case of  FIG. 11A . For this reason, controller  80  can convey medium  11  in the conveyance direction E by driving conveyance belt  51  and second auxiliary roller  43  while keeping first auxiliary roller  41  stopped. Accordingly, controller  80  conveys the medium by driving conveyance belt  51  and second auxiliary roller  43  while keeping first auxiliary roller  41  in the non-conveyance state without driving first auxiliary roller  41 . Thus, by restricting the conveyance force in the conveyance direction E to be applied to the medium in advance, medium conveyance device  1   b  can convey and discharge only medium  11  being the lowest medium among the stacked media, and reliably prevent the media stacked on medium  11  from being discharged. 
     In the case of  FIG. 11C , neither first height sensor  91  nor second height sensor  92  detects that any of the media is present at its detecting position. In this case, the weight of the media stacked on medium stacker  10  is lower than the weight in the case of  FIG. 11B . For this reason, controller  80  can convey medium  11  in the conveyance direction E by driving conveyance belt  51  while keeping second auxiliary roller  43  and first auxiliary roller  41  stopped. Accordingly, controller  80  conveys the medium by driving conveyance belt  51  while keeping second auxiliary roller  43  and first auxiliary roller  41  in the non-conveyance state without driving second auxiliary roller  43  or first auxiliary roller  41 . Thus, by restricting the conveyance force in the conveyance direction E to be applied to the medium in advance, medium conveyance device  1   b  can convey and discharge only medium  11  being the lowest medium among the stacked media, and reliably prevent the media stacked on medium  11  from being discharged. 
       FIG. 12  is a flowchart illustrating operations of medium conveyance device  1   b  according to the third embodiment. Note that reference is also made to  FIG. 10  and  FIGS. 11A to 11C  in the following description. 
     First, after the media are set on medium stacker  10  (step S 31 ), controller  80  determines whether or not any of the media is present at the third reference position opposed to third medium sensor  33  based on the detection signal from third medium sensor  33  (step S 32 ). When controller  80  determines that any of the media is present at the third reference position from the result of the detection by third medium sensor  33  (YES in step S 32 ), controller  80  determines conveyance belt  51  as the object to be driven (step S 33 ). 
     Next, controller  80  determines whether or not any of the media is present at the first height reference position opposed to first height sensor  91  based on a detection signal from first height sensor  91  (step S 34 ). When controller  80  determines that any of the media is present at the first height reference position from a result of the detection by first height sensor  91  (YES in step S 34 ), controller  80  determines second auxiliary roller  43  as the object to be driven (step S 35 ), and the processing proceeds to step S 36 . On the other hand, when controller  80  determines that the media are not present at the first height reference position from the result of the detection by first height sensor  91  (NO in step S 34 ), controller  80  drives conveyance belt  51  and conveys medium  11  in the conveyance direction E by using conveyance belt  51  (step S 40 ) as illustrated in  FIG. 11C . 
     In step S 36 , controller  80  determines whether or not any of the media is present at the second height reference position opposed to second height sensor  92  based on a detection signal from second height sensor  92 . When controller  80  determines that any of the media is present at the second height reference position from a result of the detection by second height sensor  92  (YES in step S 36 ), controller  80  determines first auxiliary roller  41  as the object to be driven (step S 37 ). As illustrated in  FIG. 11A , controller  80  drives conveyance belt  51 , second auxiliary roller  43 , and first auxiliary roller  41 , and conveys medium  11  in the conveyance direction E by using conveyance belt  51 , second auxiliary roller  43 , and first auxiliary roller  41  (step S 38 ). In the case of  FIG. 11A , medium conveyance device  1   b  conveys medium  11  in accordance with steps S 1  to step S 16  as illustrated in  FIGS. 4 to 6 . 
     Meanwhile, in the case illustrated in  FIG. 11B , medium conveyance device  1   b  conveys the medium in accordance with steps S 7  to step S 16  illustrated in  FIGS. 5 and 6 . 
     Note that the processing from steps S 33  to S 40  illustrated in  FIG. 12  is also applicable to the case where more media are additionally stacked on medium stacker  10  while medium conveyance device  1   b  is conveying the medium. 
       FIG. 13  is a table illustrating relations between situations of medium detection by the sensors and control of conveyance units in medium conveyance device  1   b  according to the third embodiment. A status A 1  illustrated in  FIG. 13  indicates the case where third medium sensor  33 , first height sensor  91 , and second height sensor  92  detect the presence of the media. In this case, controller  80  drives conveyance belt  51 , second auxiliary roller  43 , and first auxiliary roller  41  as illustrated in  FIG. 11A  and in step S 38  in  FIG. 12 . 
     As medium conveyance device  1   b  conveys the media, the number of the media stacked on medium stacker  10  is decreased and the height of the media stacked on medium stacker  10  is reduced. A status A 2  illustrated in  FIG. 13  indicates the case where third medium sensor  33  and first height sensor  91  detect the presence of the media, whereas second height sensor  92  does not detect the presence of the media. In this case, controller  80  stops first auxiliary roller  41 , and drives conveyance belt  51  and second auxiliary roller  43  as illustrated in  FIG. 11B  and in step S 39  in  FIG. 12 . 
     When medium conveyance device  1   b  continues the conveyance of the media, the number of the media stacked on medium stacker  10  is further decreased and the height of the stacked media is further reduced. A status A 3  illustrated in  FIG. 13  indicates the case where third medium sensor  33  detects the presence of the media whereas first height sensor  91  and second height sensor  92  do not detect the presence of the media. In this case, controller  80  stops first auxiliary roller  41  and second auxiliary roller  43 , and drives conveyance belt  51  as illustrated in  FIG. 11C  and in step S 40  in  FIG. 12 . 
     A status A 4  illustrated in  FIG. 13  indicates the case where third medium sensor  33 , first height sensor  91 , and second height sensor  92  do not detect the presence of the media. This case represents the situation where no media are stacked on medium stacker  10 . Controller  80  stops first auxiliary roller  41 , second auxiliary roller  43 , and conveyance belt  51 . 
     There may be a case where more media are additionally stacked on medium stacker  10  while medium conveyance device  1   b  is conveying the medium. For example, if the status A 3  transitions to the status A 1 , medium conveyance device  1   b  determines first auxiliary roller  41  and second auxiliary roller  43  as the objects to be driven, and causes first auxiliary roller  41  and second auxiliary roller  43  to convey the media. 
     &lt;&lt;3-3&gt;&gt; Effects 
     As described above, according to medium conveyance device  1   b  of the third embodiment, controller  80  determines the objects to be driven out of conveyance belt  51 , second auxiliary roller  43 , and first auxiliary roller  41  depending on the stacked amount (the height) of the media set on medium stacker  10 . The conveyance force in the conveyance direction E to be received by the media is restricted depending on the stacked amount of the media as described above. Accordingly, medium conveyance device  1   b  of the third embodiment can convey and discharge only medium  11 , being the lowest medium among the stacked media, even though cumbersome adjustments of separation plate  61 , separation piece  62 , and the like are not performed depending on the state of the media. Thus, medium conveyance device  1   b  can reliably prevent the media stacked on medium  11  from being discharged. 
     &lt;&lt;4&gt;&gt; Modified Examples 
     Although each of medium conveyance devices  1 ,  1   a , and  1   b  in the first to third embodiments is described as a device provided separately from host device  2 , each of medium conveyance devices  1 ,  1   a , and  1   b  may be a medium feeder constituting a part of a printer, a facsimile, or a multifunction peripheral, for example. 
     The invention includes other embodiments in addition to the above-described embodiments without departing from the spirit of the invention. The embodiments are to be considered in all respects as illustrative, and not restrictive. The scope of the invention is indicated by the appended claims rather than by the foregoing description. Hence, all configurations including the meaning and range within equivalent arrangements of the claims are intended to be embraced in the invention.