Patent Publication Number: US-2021188576-A1

Title: Medium conveying apparatus for changing torque limit value of torque limiter

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority of prior Japanese Patent Application No. 2019-229556, filed on Dec. 19, 2019, the entire contents of which are incorporated herein by reference. 
     TECHNICAL FIELD 
     Embodiments discussed in the present specification relate to medium conveyance. 
     BACKGROUND 
     In a medium conveying apparatus that feeds a medium using a feed roller and a brake roller, when the number of media placed on a medium tray is large, the frictional force between the media increases, and multi-feed of the media may occur. For example, the occurrence of multi-feed is suppressed by increasing the maximum torque applied to the brake roller, but when the maximum torque applied to the brake roller is increased, a jam of the medium tends to occur when thin paper, etc., is conveyed as a medium. 
     A medium supply apparatus including a separation force generating device to generate a rotational load on a brake roller in a direction opposite to the conveying direction, and capable of changing the rotational load of the brake roller by switching an electromagnetic clutch is disclosed (Japanese Unexamined Patent Publication (Kokai) No. 2013-193837). 
     A medium feeding device including a torque limiter to idle a separation roller in a first rotation direction when a rotational torque applied to the separation roller in the first rotation direction exceeds the limit torque which is a predetermined torque upper limit value, and capable of changing a separation setting is disclosed (Japanese Unexamined Patent Publication (Kokai) No. 2019-116383). 
     SUMMARY 
     According to some embodiments, a medium conveying apparatus includes a medium tray, a feed roller to feed media placed on the medium tray in order from the lower side, a brake roller located to face the feed roller, a torque limiter to control a load applied to the brake roller, and a changing member to change a limit value of a torque of the torque limiter according to a height of the media placed on the medium tray. 
     According to some embodiments, a medium conveying apparatus includes a medium tray, a feed roller to feed media placed on the medium tray in order from the lower side, a brake roller located to face the feed roller, a torque limiter to control a load applied to the brake roller, and a processor to change a limit value of a torque of the torque limiter according to a height of the media placed on the medium tray. 
     According to some embodiments, a method for controlling conveying a medium includes feeding media placed on a medium tray in order from the lower side, by a feed roller, a brake roller located to face the feed roller, controlling a load applied to a brake roller located to face the feed roller by a torque limiter, and changing a limit value of a torque of the torque limiter according to a height of the media placed on the medium tray. 
     According to some embodiments, a computer-readable, non-transitory medium stores a computer program. The computer program causes a medium conveying apparatus including a medium tray, a feed roller to feed media placed on the medium tray in order from the lower side, a brake roller located to face the feed roller, a torque limiter to control a load applied to the brake roller, to execute a process including changing a limit value of a torque of the torque limiter according to a height of the media placed on the medium tray. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a perspective view illustrating a medium conveying apparatus  100  according to an embodiment. 
         FIG. 2  is a diagram for illustrating a conveyance path inside the medium conveying apparatus  100 . 
         FIG. 3  is a schematic diagram for illustrating a driving mechanism of each roller. 
         FIG. 4  is a schematic diagram for illustrating a driving mechanism of each roller. 
         FIG. 5  is a schematic diagram for illustrating a pick arm  112 , etc. 
         FIG. 6  is a schematic diagram for illustrating each torque limiter. 
         FIG. 7  is a schematic diagram for illustrating the operations of the pick an  112 , etc. 
         FIG. 8  is a schematic diagram for illustrating the operations of the pick arm  112 , etc. 
         FIG. 9  is a schematic diagram for illustrating the operations of the brake roller  115 , etc. 
         FIG. 10  is a block diagram illustrating a schematic configuration of the medium conveying apparatus  100 . 
         FIG. 11  is a diagram illustrating schematic configurations of the storage device  160  and the processing circuit  170 . 
         FIG. 12  is a flowchart illustrating an operation example of the medium reading processing. 
         FIG. 13  is a flowchart illustrating an operation example of the setting processing. 
         FIG. 14  is a schematic diagram for illustrating another stopper  232 . 
         FIG. 15  is a schematic diagram for illustrating the operations of the stopper  232 , etc. 
         FIG. 16  is a schematic diagram for illustrating the operations of the stopper  232 , etc. 
         FIG. 17  is a diagram illustrating a schematic configuration of yet another processing circuit  270 . 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory, and are not restrictive of the invention, as claimed. 
     Hereinafter, a medium conveying apparatus, a method and a computer-readable, non-transitory medium storing a computer program according to an embodiment, will be described with reference to the drawings. However, it should be noted that the technical scope of the invention is not limited to these embodiments, and extends to the inventions described in the claims and their equivalents. 
       FIG. 1  is a perspective view illustrating a medium conveying apparatus  100  configured as an image scanner. The medium conveying apparatus  100  conveys and images a medium being a document. A medium is paper, thin paper, thick paper, a card, a brochure, a brochure, a passport, etc. The medium conveying apparatus  100  may be a fax machine, a copying machine, a multifunctional peripheral (MFP), etc. A conveyed medium may not be a document but may be an object being printed on etc., and the medium conveying apparatus  100  may be a printer etc. 
     The medium conveying apparatus  100  includes a lower housing  101 , an upper housing  102 , a medium tray  103 , an ejection tray  104 , an operation device  105 , and a display device  106 . 
     The upper housing  102  is located at a position covering the upper surface of the medium conveying apparatus  100  and is engaged with the lower housing  101  by hinges so as to be opened and closed at a time of medium jam, during cleaning the inside of the medium conveying apparatus  100 , etc. The medium tray  103  is engaged with the lower housing  101  in such a way as to be able to place a medium to be conveyed. The ejection tray  104  is engaged with the lower housing  101  in such a way as to be able to hold an ejected medium. 
     The operation device  105  includes an input device such as a button, and an interface circuit acquiring a signal from the input device, receives an input operation by a user, and outputs an operation signal based on the input operation by the user. The display device  106  includes a display including a liquid crystal or organic electro-luminescence (EL), and an interface circuit for outputting image data to the display, and displays the image data on the display. 
       FIG. 2  is a diagram for illustrating a conveyance path inside the medium conveying apparatus  100 . 
     The conveyance path inside the medium conveying apparatus  100  includes a first sensor  111 , a pick arm  112 , a second sensor  113 , a plurality of feed rollers  114   a ,  114   b , a plurality of brake rollers  115   a ,  115   b , a plurality of first conveyance rollers  116   a ,  116   b , a plurality of second conveyance rollers  117   a ,  117   b , a first imaging device  118   a , a second imaging device  118   b , a plurality of third conveyance rollers  119   a ,  119   b  and a plurality of fourth conveyance rollers  120   a ,  120   b , etc. 
     The feed rollers  114   a  and  114   b  may be hereinafter collectively referred to as feed rollers  114 . Further, the brake rollers  115   a  and  115   b  may be collectively referred to as brake rollers  115 . The first conveyance rollers  116   a  and  116   b  may be collectively referred to as first conveyance rollers  116 . The second conveyance rollers  117   a  and  117   b  may be collectively referred to as second conveyance rollers  117 . Further, the first imaging device  118   a  and the second imaging device  118   b  may be collectively referred to as imaging devices  118 . The third conveyance rollers  119   a  and  119   b  may be collectively referred to as third conveyance rollers  119 . The fourth conveyance rollers  120   a  and  120   b  may be collectively referred to as fourth conveyance rollers  120 . 
     A top surface of the lower housing  101  forms a lower guide  107   a  of a conveyance path of a medium, and a bottom surface of the upper housing  102  forms an upper guide  107   b  of the conveyance path of a medium. An arrow A 1  in  FIG. 2  indicates a medium conveying direction. An upstream hereinafter refers to an upstream in the medium conveying direction A 1 , and a downstream refers to a downstream in the medium conveying direction A 1 . 
     The first sensor  111  is provided on the upper housing  102 , that is, above the conveyance path of the medium, and is located on the upstream side of the pick arm  112 . The first sensor  111  is a sensor to detect a height of the media placed on the medium tray  103 . The first sensor  111  is an infrared access distance sensor and measures a distance from an object existing at a facing position, based on a time difference between emission and reflection of infrared rays. The first sensor  111  includes a light emitter and a light receiver. The light emitter irradiates light (infrared light) toward the medium tray  103  or the lower housing  101 . On the other hand, the light receiver receives the light emitted by the light emitter and reflected by the medium tray  103 , the lower housing  101 , or the medium placed on the medium tray  103 , and generates and output an optical signal being an electric signal based on the received light. The optical signal indicates a time period from a time when the light emitter emits the light to a time when the light receiver receives the light. Since the time period from a time when the light emitter emits the light to a time when the light receiver receives the light changes according to the height of the media placed on the medium tray  103 , the optical signal varies according to the height of the media placed on the medium tray  103 . Therefore, the medium conveying apparatus  100  can detect the height of the media placed on the medium tray  103  based on the optical signal. The number of the first sensors  111  is not limited to one, and a plurality of the first sensors  111  may be located at intervals in the width direction perpendicular to the medium conveying direction A 1 . The first sensor  111  may be omitted. 
     The pick arm  112  is provided in the upper housing  102  and is located on the downstream side of the first sensor  111  and on the upstream side of the nip position of the feed roller  114  and the brake roller  115 , particularly, at a position facing the feed roller  114  across the medium conveyance path. The pick arm  112  moves in the height direction A 8  perpendicular to the lower guide  107   a  in accordance with control from a processing circuit described later, and urges (pushes) the medium placed on the medium tray  103  from above. The pick arm  112  separates from the feed roller  114  when the medium is not fed and abuts against the medium placed on the pedestal  103  to urge the medium from above when the medium is fed. Consequently, a moderate frictional force is generated between the feed rollers  114  and the medium, and the feed rollers  114  can satisfactorily feed the medium. 
     The second sensor  113  is located on the upstream side of the feed roller  114  and the brake roller  115 . The second sensor  113  includes a contact detection sensor and detects whether or not a medium is placed on the medium tray  103 . The second sensor  113  generates and outputs a medium signal changing the signal value between a state in which a medium is placed on the medium tray  103  and a state in which a medium is not placed. 
     The feed rollers  114  are provided on the lower housing  101  and feed media placed on the medium tray  103  in order from the lower side. The brake rollers  115  are provided on the upper housing  102  and each of the plurality of brake rollers  113  is located to face a corresponding one of the feed rollers  114 . 
     The first imaging device  118   a  includes a line sensor based on a unity-magnification optical system type contact image sensor (CIS) including an imaging element based on a complementary metal oxide semiconductor (CMOS) linearly located in a main scanning direction. Further, the first imaging device  118   a  includes a lens for forming an image on the imaging element, and an A/D converter for amplifying and analog-digital (A/D) converting an electric signal output from the imaging element. The first imaging device  118   a  generates and outputs an input image imaging a front side of a conveyed medium, in accordance with control from a processing circuit to be described later. 
     Similarly, the second imaging device  118   b  includes a line sensor based on a unity-magnification optical system type CIS including an imaging element based on a CMOS linearly located in a main scanning direction. Further, the second imaging device  118   b  includes a lens for forming an image on the imaging element, and an A/D converter for amplifying and A/D converting an electric signal output from the imaging element. The second imaging device  118   b  generates and outputs an input image imaging a back side of a conveyed medium, in accordance with control from a processing circuit to be described later. 
     Only either of the first imaging device  118   a  and the second imaging device  118   b  may be located in the medium conveying apparatus  100  and only one surface of a medium may be read. Further, a line sensor based on a unity-magnification optical system type CIS including an imaging element based on charge coupled devices (CCDs) may be used in place of the line sensor based on a unity-magnification optical system type CIS including an imaging element based on a CMOS. Further, a line sensor based on a reduction optical system type line sensor including an imaging element based on CMOS or CCDs. The first imaging device  118   a  and the second imaging device  118   b  may be collectively referred to as imaging devices  118 . 
     A medium placed on the medium tray  103  is conveyed between the lower guide  107   a  and the upper guide  107   b  in the medium conveying direction A 1  by the feed rollers  114  rotating in a direction of an arrow A 2  in  FIG. 2 , that is, a medium feeding direction. When a medium is conveyed, the brake rollers  115  rotate in a direction of an arrow A 3 , that is, a direction opposite to the medium feeding direction. By the workings of the feed rollers  114  and the brake rollers  115 , when a plurality of media are placed on the medium tray  103 , only a medium in contact with the feed rollers  114 , out of the media placed on the medium tray  103 , is separated. Consequently, the medium conveying apparatus  100  operates in such a way that conveyance of a medium other than the separated medium is restricted (prevention of multi-feed). 
     The medium is fed between the first conveyance rollers  113  and the second conveyance rollers  114  while being guided by the lower guide  107   a  and the upper guide  107   b . The medium is fed between the first imaging device  118   a  and the second imaging device  118   b  by the first conveyance rollers  116  and the second conveyance rollers  117  rotating in directions of an arrow A 4  and an arrow A 5 , respectively. The medium read by the imaging devices  118  is ejected on the ejection tray  104  by the third conveyance roller  119  and the fourth conveyance roller  120  rotating in directions of an arrow A 6  and an arrow A 7 , respectively. 
       FIGS. 3 and 4  are schematic views for illustrating a driving mechanism of the feed roller  114 , the brake roller  115 , the first conveyance roller  116 , and the third conveyance roller  119 .  FIG. 3  is a perspective view of the driving mechanism of each roller from the upstream side, and  FIG. 4  is a perspective view of the driving mechanism of each roller from the above and downstream side. 
     As shown in  FIGS. 3 and 4 , the driving mechanisms of the brake rollers  115 , the first conveyance rollers  116 , and the third conveyance rollers  119  include a first motor  151 , first to fourth pulleys  141   a  to  141   d , first to second belts  142   a  to  142   b , first to ninth gears  143   a  to  143   i , an electromagnetic clutch  144 , first to fifth shafts  145   a  to  145   e , a first torque limiters  146 , a ratchet gear  147 , and second torque limiters  148   a  to  148   b . On the other hand, the driving mechanism of the feed rollers  114  has a second motor  152 , fifth to sixth pulley  141   e  to  141   f , a third belt  142   c , tenth to sixteenth gears  143   j  to  143   p  and a sixth shaft  145   f , etc. 
     The first motor  151  generates a driving force for rotating the brake roller  115 , the first conveyance rollers  116 , and the third conveyance rollers  119  by a control signal from a processing circuit to be described later. The first motor  151  generates a first driving force for rotating the brake roller  115  in a direction A 3  opposite to the medium feeding direction and rotating the first conveyance rollers  116  and the third conveyance rollers  119  in the medium conveying directions A 4  and A 6 . The first motor  151  may further rotate the second conveyance rollers  117  and the fourth conveyance rollers  120  in the medium conveying directions A 5  and A 7  by the first driving force. Also, some or all of the first to fourth conveyance rollers  116 ,  117 ,  119 , and  120  may be rotated by the driving force generated by the second motor  152  or other motor. 
     The first pulley  141   a  is attached to a rotation shaft of the first motor  151 , and the first belt  142   a  is stretched between the first pulley  141   a  and a pulley portion having a larger outer diameter of the second pulley  141   b . The second belt  142   b  is stretched between the pulley portion having the smaller outer diameter of the second pulley  141   b , a pulley portion of the third pulley  141   c , and a pulley portion of the fourth pulley  141   d.    
     A gear portion of the third pulley  141   c  is engaged with the first gear  143   a . The first gear  143   a  is engaged with the second gear  143   b , the second gear  143   b  is engaged with the third gear  143   c , and the third gear  143   c  is engaged with the electromagnetic clutch  144 . The electromagnetic clutch  144  is attached to the first shaft  145   a , and the fourth gear  143   d  is further attached to the first shaft  145   a . The fourth gear  143   d  is engaged with the fifth gear  143   e , and the fifth gear  143   e  is engaged with the sixth gear  143   f  The sixth gear  143   f  is attached to the second shaft  145   b , and the seventh gear  143   g  is further attached to the second shaft  145   b . The seventh gear  143   g  is engaged with the eighth gear  143   h , and the eighth gear  143   h  is engaged with the ninth gear  143   i . The ninth gear  143   i  is attached to the third shaft  145   c , and the brake rollers  115   a  and  115   b  are further attached to the third shaft  145   c  via the first torque limiter  146 , the ratchet gear  147 , and the second torque limiters  148   a  and  148   b.    
     The electromagnetic clutch  144  is a clutch in which the limit value of the torque can be electromagnetically changed according to a control signal from a processing circuit to be described later, and transmits a driving force from the first motor  151  to the brake roller  115 . The electromagnetic clutch  144  is, for example, a micro powder clutch. The electromagnetic clutch  144  may be another type of clutch, such as a hysteresis clutch. In the present embodiment, the limit value of the torque of the electromagnetic clutch  144  is always set to a sufficiently high value (a value higher than the limit value of the first torque limiter  146  and the second torque limiters  148   a  to  148   b ) by the control signal from the processing circuit. 
     The 7th to 9th gears  143   g  to  143   i  is an example of a transmission member to transmit the drive force occurred by the first motor  151  to the first torque limiter  146 . The transmission member may include the first to fourth pulleys  141   a  to  141   d , the first to second belts  142   a  to  142   b , the first to sixth gears  143   a  to  143   f , and the first to fifth shafts  145   a  to  145   e . The transmission member may be composed of only gears or only pulleys and belts. 
     The third pulley  141   c  is attached to the fourth shaft  145   d , and the first conveyance rollers  116  are further attached to the fourth shaft  145   d . The fourth pulley  141   d  is attached to the fifth shaft  145   e , and the third conveyance rollers  119  are further attached to the fifth shaft  145   e.    
     The second motor  152  generates a driving force for rotating the feed roller  114  by a control signal from the processing circuit to be described later. The second motor  152  generates a second driving force for rotating the feed roller  114  in the medium feeding direction A 2 . 
     The fifth pulley  141   e  is attached to a rotation shaft of the second motor  152 , and a third belt  142   c  is stretched between the fifth pulley  141   e  and a pulley portion of the sixth pulley  141   f . A gear portion of the sixth pulley  141   f  is engaged with the tenth gear  143   j , the tenth gear  143   j  is engaged with the eleventh gear  143   k , the eleventh gear  143   k  is engaged with the twelfth gear  143   l , and the twelfth gear  143   l  is engaged with the thirteenth gear  143   m . The thirteenth gear  143   m  is engaged with the fourteenth gear  143   n , the fourteenth gear  143   n  is engaged with the fifteenth gear  143   o , and the fifteenth gear  143   o  is engaged with the sixteenth gear  143   p . The sixteenth gear  143   p  is attached to the sixth shaft  145   f , and the feed rollers  114  are further attached to the sixth shaft  145   f.    
     The medium conveying apparatus  100  also further includes a support member  131 . One end of the spring  131   a  is supported by the upper housing  102 , and the other end of the spring  131   a  is attached to an upper surface of the support member  131 . The support member  131  and the brake rollers  115  are urged by the spring  131   a  downward in the height direction A 8 , that is, toward the feed rollers  114 . The spring  131   a  is an example of a pressing member to press the brake rollers  115  toward the feed rollers  114 . Instead of the spring  131   a , rubber, etc., may be used as the pressing member. 
     Hereinafter, the operations of each roller and the driving mechanism of each roller will be described. 
     When the first motor  151  generates the first driving force, the first pulley  141   a  rotates in the direction of the arrow B 1 , and the second to fourth pulleys  141   b  to  141   d  accordingly rotate in the direction of the arrow B 1 , respectively. Also, the first to third gears  143   a  to  143   c  and electromagnetic clutch  144  rotate in the direction of the arrows B 2  to B 5 , the fourth to sixth gears  143   d  to  143   f  rotate in the direction of the arrows B 5  to B 7 , and the seventh to ninth gears  143   g  to  143   i  rotate in the direction of the arrows B 7  to B 9 , respectively. As a result, the brake rollers  115  are rotated in the direction A 3  opposite to the medium feeding direction by the first driving force from the first motor  151 . 
     Further, the first conveyance rollers  116  rotate in the medium conveying direction A 4 , by the third pulley  141   c  rotating in the direction of the arrow B 1 . The third conveyance rollers  119  rotate in the medium conveying direction A 6 , by the fourth pulley  141   d  rotating in the direction of the arrow B 1 . 
     On the other hand, when the second motor  152  generates the second driving force, the fifth pulley  141   e  rotates in the direction of the arrow B 11 , and the sixth pulley  141   f  and the tenth gear  143   j  accordingly rotate in the direction of the arrows B 1  Iand B 12 , respectively. In addition, the feed rollers  114  rotate in the medium feeding direction A 2 , by the eleventh to sixteenth gears  143   k  to  143   p  rotating in the directions of the arrows B 13  to B 18 , respectively. 
       FIG. 5  is a schematic diagram for illustrating the pick arm  112  and the support member  131 .  FIG. 5  is a perspective view of a drive mechanism of the pick arm  112 , the support member  131 , and the brake rollers  115  from the upstream side. In  FIG. 5 , pick arm  112  and support member  131  are shown in dotted lines. 
     As shown in  FIG. 5 , the medium conveying apparatus  100  further includes a stopper  132 . 
     The pick arm  112  has rollers  112   a  and an abutting portion  112   b.    
     Each of the rollers  112   a  is provided at a position facing each of the feed rollers  114  across the medium conveyance path. Each of the rollers  112   a  separates from the feed rollers  114  when the medium is not fed, and abuts on the medium placed on the medium tray  103  when the medium is fed. 
     The abutting portion  112   b  is provided at a position facing the stopper  132 . The abutting portion  112   b  moves in conjunction with the movement of the pick arm  112  (the rollers  112   a ) in the height direction A 8 . The abutting portion  112   b  separates from the stopper  132  when the roller  112   a  is located at a position exceeding a predetermined height (e.g., 4 mm from a mounting surface of the medium tray  103 ) in the height direction A 8 . When the rollers  112   a  are located at a position equal to or less than a predetermined height in the height direction A 8 , the abutting portion  112   b  abuts against the stopper  132  to rotate (swing) the stopper  132 . 
     The stopper  132  is rotatably (swingably) supported with the support member  131 . The stopper  132  has an abutted portion  132   a  and a locking portion  132   b.    
     The abutted portion  132   a  is provided at a position facing the abutting portion  112   b  of the pick arm  112 . A force in which the abutted portion  132   a  is directed upward in the height direction A 8  by a torsion coil spring (not shown), is applied to the stopper  132 . When the roller  112   a  is located at a position exceeding a predetermined height in the height direction A 8 , the abutted portion  132   a  separates from the abutting portion  112   b  and located upward by the force of the torsion coil spring. On the other hand, when the roller  112   a  is located at a position less than the predetermined height in the height direction A 8 , the abutting portion  132   a  is pressed downward by the abutting portion  112   b  with a force larger than the force of the torsion coil spring, and is located downward. 
     The locking portion  132   b  is provided at a position facing the ratchet gear  147 . When the abutted portion  132   a  is apart from the abutting portion  112   b , the locking portion  132   b  abuts against the ratchet gear  147  to lock the ratchet gear  147 . On the other hand, the locking portion  132   b  separates from the ratchet gear  147  when the abutted portion  132   a  abuts against the abutting portion  112   b  and is located downward by the abutting portion  112   b.    
     When the ratchet gear  147  is locked by the locking portion  132   b , the ratchet gear  147  is configured to be rotatable only in the direction A 3  opposite to the medium feeding direction, and not to rotate the medium feeding direction (a direction opposite to the arrow A 3 ). 
     The support member  131  is a member made of resin or metal, etc., and has a first to third side surfaces  131   b  to  131   d . The support member  131  supports the brake rollers  115 , the seventh to ninth gears  143   g  to  143   i , the first torque limiter  146 , the ratchet gear  147 , and the second torque limiters  148   a  to  148   b  by the first to third side surfaces  131   b  to  131   d . The second side surface  131   c  and the third side surface  131   d  are attached to the second shaft  145   b  rotatably (swingably) with the second shaft  145   b  as a rotation (swing) axis, respectively. The first side surface  13   ib  rotates (swings) in conjunction with the rotation (swing) of the second side surface  131   c  and the third side surface  131   d . Both ends of the third shaft  145   c  are attached to the first side surface  131   b  and the third side surface  131   d . Thus, the support member  131  is provided to be rotatable (swing) about the second shaft  145   b , and swingably supports the brake roller  115 . 
       FIG. 6  is a schematic diagram for illustrating the first torque limiter  146  and the second torque limiters  148   a  to  148   b .  FIG. 6  is a cross-sectional view of the third shaft  145   c  provided with the brake rollers  115 , the ninth gear  143   i , the first torque limiter  146 , the ratchet gear  147 , and the second torque limiters  148   a  to  148   b.    
     The first torque limiter  146  and the second torque limiters  148   a  to  148   b  are an example of a torque limiter, which controls the load applied to the brake rollers  115 . As shown in  FIG. 6 , the first torque limiter  146  and the second torque limiters  148   a  to  148   b  are located on a transmission path of the driving force from the first motor  151  to the brake rollers  115 . Specifically, the first torque limiter  146  and the second torque limiters  148   a  to  148   b  are located on the third shaft  145   c  which is a rotational axis of the brake rollers  115 . Since there is no gear row between each torque limiter and the brake rollers  115 , it is suppressed that the separation force applied to the brake rollers  115  fluctuates due to manufacturing error, etc., for each part. Consequently, the medium conveying apparatus  100  can separate a medium with high precision regardless of a manufacturing error for each part. The first torque limiter  146  and the second torque limiters  148   a  to  148   b  may not necessarily be located on the coaxial axis. The first torque limiter  146  may be, for example, located on the second shaft  145   b.    
     The first torque limiter  146 , the ratchet gear  147 , the second torque limiters  148   a  to  148   b , the brake rollers  115  and the third shart 145   c  may be hereinafter collectively referred to as a brake unit. 
     The first torque limiter  146  is located on the transmission path of the driving force from the seventh to ninth gears  143   g  to  143   i  to the second torque limiters  148   a  to  148   b . The limit value of the torque of the first torque limiter  146  is a first limit value. A ratchet gear  147  is located on the transmission path of the driving force from the first torque limiter  146  to the second torque limiters  148   a  to  148   b.    
     The second torque limiters  148   a  to  148   b  are located on the transmission path of the driving force from the first torque limiter  146   a  to the brake rollers  115 . The second torque limiters  148   a  and  148   b  are provided separately between the third shaft  145   c  and the respective brake rollers  115   a  and  115   b . In other words, the second torque limiters  148   a  and  148   b  are provided corresponding to the brake rollers  115   a  and  115   b , respectively. The limit value of the torque of each second torque limiter  148   a ,  148   b  is less than the first limit value, and the sum of the limit values of the torque of the second torque limiters  148   a ,  148   b  is equal to a second limit value greater than the first limit value. For example, the first limit value is set to 500 gf·cm, the second limit value is set to 700 gf·cm, and the limit value of the torque of each of the second torque limiters  148   a  and  148   b  is set to 350 gf·cm. A common second torque limiter may be provided for the brake rollers  115   a  and  115   b  instead of separate second torque limiters  148   a  and  148   b  being provided for the brake rollers  115   a  and  115   b , respectively. 
     The first limit value is set to a value by which a turning force through the first torque limiter  146  is cut off when there is one medium, and a turning force through the first torque limiter  146  is transmitted when there are a plurality of media. Similarly, the second limit value is set to a value by which a turning force through the second torque limiters  148   a ,  148   b  is cut off when there is one medium, and a turning force through the second torque limiters  148   a ,  148   b  is transmitted when there are a plurality of media. Consequently, when only one medium is conveyed, the brake rollers  115  do not rotate according to the first driving force and are driven by the feed rollers  114 . On the other hand, when a plurality of media are conveyed, the brake rollers  115  prevents occurrence of media multi-feed by rotating in the direction A 3  opposite to the medium feeding direction and separating a medium in contact with the feed rollers  114  from the other media. At this time, the outer peripheral surfaces of the brake rollers  115  may be apply a force in the direction A 3  opposite to the medium feeding direction to the media in a state in which the outer peripheral surfaces are not rotating in the direction A 3  opposite to the medium feeding direction and are stopped. 
       FIGS. 7 and 8  are schematic views for illustrating the operations of the pick arm  112 , the stopper  132  and the brake roller  115 .  FIGS. 7 and 8  show a state in which the pick arm  112  moves downward in the height direction A 8  in accordance with the control of the processing circuit and presses the medium placed on the medium tray  103  from above.  FIG. 7  shows a state in which the height of the media M 1  placed on the medium tray  103  is equal to or lower than a predetermined height, and  FIG. 8  shows a state in which the height of the media M 2  placed on the medium tray  103  exceeds a predetermined height. 
     As shown in  FIG. 7 , when the height of the media M 1  placed on the medium tray  103  is equal to or less than a predetermined height, the roller  112   a  of the pick arm  112  abutting the uppermost surface of the media M 1  is located downward in the height direction A 8 . As a result, the abutting portion  112   b  abuts the abutted portion  132   a  of the stopper  132  and pushes the abutted portion  132   a  downward. As a result, the locking portion  132   b  separates from the ratchet gear  147 , and the rotation of the ratchet gear  147  is not limited. In this state, when the first motor  151  and the second motor  152  generate the first driving force and the second driving force, the first driving force is transmitted to the brake rollers  115  via the first torque limiter  146  and the second torque limiters  148   a  and  148   b . However, the limit value (first limit value) of the torque of the first torque limiter  146  is smaller than the sum (second limit value) of the limit values of the torques of the second torque limiters  148   a  and  148   b . Therefore, the limit value of the torque of the entire torque limiters is the first limit value which is the limit value of the torque of the first torque limiter  146 . 
     On the other hand, as shown in  FIG. 8 , when the height of the media M 2  placed on the medium tray  103  exceeds a predetermined height, the roller  112   a  of the pick arm  112  abutting the uppermost surface of the media M 2  is located upward in the height direction A 8 . As a result, the abutting portion  112   b  separates from the abutted portion  132   a  of the stopper  132 , and the abutting portion  132   a  is located upward by the force applied by the torsion coil spring. As a result, the locking portion  132   b  abuts the ratchet gear  147 , and locks the ratchet gear  147 . The ratchet gear  147  locked by the locking portion  132   b  can rotate only in the direction A 3  opposite to the medium feeding direction, and cannot rotate in the medium feeding direction (the direction opposite to the arrow A 3 ). In this state, when the first motor  151  and the second motor  152  generate the first driving force and the second driving force, the first driving force is transmitted to the brake rollers  115  via the second torque limiters  148   a  and  148   b . However, the force for causing the brake roller  115  to be driven and rotated in the medium feeding direction (the direction opposite to the arrow A 3 ) by the feed roller  114  rotating in the medium feeding direction A 2  by the second driving force is interrupted by the ratchet gear  147  and is not transmitted to the first torque limiter  146 . Therefore, the limit value of the torque of the entire torque limiters becomes the second limit value which is the limit value of the torque of the second torque limiters  148   a  and  148   b.    
     Thus, the stopper  132  functions as a regulating member to prevent the force for rotating the brake rollers  115  by the feed rollers  114  in the medium feeding direction from being transmitted to the first torque limiter  146 . 
     The pick arm  112  functions as a setting member to set a torque limiter that defines the maximum torque applied to the brake roller  115  to the first torque limiter  146  or the second torque limiters  148   a ,  148   b . In particular, the pick arm  112  uses the stopper  132  to set whether or not a force for rotating the brake roller  115  by the feed roller  114  in the medium feeding direction is transmitted to the first torque limiter  146 . The pick arm  112  sets the torque limiter that defines the maximum torque applied to the brake rollers  115  to the first torque limiter  146 , by setting so that the force is transmitted to the first torque limiter  146 . On the other hand, the pick arm  112  sets the torque limiter that defines the maximum torque applied to the brake rollers  115  to the second torque limiters  148   a  and  148   b  by setting the force not to be transmitted to the first torque limiter  146 . 
     In other words, the pick arm  112  functions as a changing member to change the limit value of the torque of the torque limiter to the first limit value or the second limit value. In particular, the pick arm  112  uses the stopper  132  to change whether or not a force for rotating the brake rollers  115  by the feed rollers  114  in the medium feed direction is transmitted to the first torque limiter  146 . 
     By using the pick arm  112  and the stopper  132 , the medium conveying apparatus  100  can change the maximum torque applied to the brake rollers  115  with a simple structure, thereby reducing the device size and equipment cost. 
     Further, the pick arm  112  sets the torque limiter that defines the maximum torque applied to the brake roller  115  to the first torque limiter  146  or the second torque limiters  148   a ,  148   b , in accordance with the height of the media placed on the medium tray  103 . In other words, the pick arm  112  changes the limit value of the torque of the torque limiter according to the height of the media placed on the medium tray  103 . The higher the height of the media placed on the pedestal  103 , the greater the weight of the medium placed on the medium to be fed and the greater the frictional force between the medium to be fed and the medium placed thereon. In the so-called bottom-first type medium conveying apparatus  100  in which the media placed on the medium tray  103  are fed in order from the lower side, the greater the frictional force between the medium to be fed and the medium placed thereon, the more likely the media are multi-fed. The medium conveying apparatus  100  increases the maximum torque applied to the brake rollers  115  when the height of the media placed on the medium tray  103  exceeds a predetermined height than the maximum torque applied to the brake rollers  115  when the height of the media placed on the medium tray  103  is equal to or less than the predetermined height. Thus, the medium conveying apparatus  100  can suppress the occurrence of multi-feed of the medium. 
     Conversely, the medium conveying apparatus  100  decreases the maximum torque applied to the brake rollers  115  when the height of the media placed on the medium tray  103  is equal to or lower than the predetermined height than the maximum torque applied to the brake rollers  115  when the height of the media placed on the medium tray  103  exceeds the predetermined height. Thus, the medium conveying apparatus  100  can suppress the occurrence of a jam in the medium when thin paper, etc., is conveyed as a medium. Accordingly, when only one medium exists between the brake rollers  115  and the feed rollers  114 , the brake rollers  115  are easily driven to the feed rollers  114 , and it is suppressed that slip occurs between the brake rollers  115  and the medium. Therefore, the medium conveying apparatus  100  can suppress the shortening of the component life of the brake rollers  115 . 
     Further, the pick arm  112  urges the medium placed on the medium tray  103  from above, and changes the limit value of the torque of the torque limiter, in accordance with the height at which the pick arm  112  is located. Therefore, the medium conveying apparatus  100  can automatically change the limit value of the torque limiter in conjunction with the height of the media placed on the medium tray  103 . The medium conveying apparatus  100  can appropriately change the maximum torque applied to the brake rollers  115  without changing it by the user, thereby improving the convenience of the user. 
       FIG. 9  is a schematic diagram for illustrating the operations of the seventh to ninth gears  143   g  to  143   i , the support member  131  and the brake rollers  115 . 
     As described above, when the first motor  151  generates the first driving force, the seventh to ninth gears  143   g  to  143   i  rotate in the direction of the arrows B 7  to B 9 , respectively, and the brake rollers  115  rotates in the direction A 3  opposite to the medium feeding direction. The gear group including the seventh to ninth gears  143   g  to  143   i  and the brake rollers  115  is supported by a support member  131  which is rotatably (swingably) provided about the second shaft  145   b  to which the seventh gear  143   g  is attached. Therefore, a force directed in the direction of the arrow C 1  is applied to the eighth gear  143   h , by the seventh gear  143   g  rotating in the direction of the arrow B 7 . As a result, a force for rotating about the second shaft  145   b  in the direction of the arrow C 1  is applied to the first side surface  131   b  to which the eighth gear  143   h  is attached. As a result, a force for rotating in the direction of arrow C 1  about the second shaft  145   b  is applied to the support member  131 , and a force in a direction (direction of arrow C 1 ) away from the feed rollers  114  is applied to the brake rollers  115 . 
     The number of gears located between the second shaft  145   b  serving as the swinging shaft of the brake rollers  115  and the third shaft  145   c  serving as the rotating shaft of the brake rollers  115  is not limited to three, and may be any odd number of three or more. As a result, while the brake rollers  115  rotate in the same direction A 3  as the rotation direction B 7  of the seventh gear  143   g , a force directed in the same direction C 1  as the rotation direction B 7  of the seventh gear  143   g  is applied to the brake rollers  115 . Thus, the brake roller unit is swingably supported with respect to the second shaft  145   b  so that a predetermined force acts on the brake roller  115  in a direction away from the feed rollers  114  when the first driving force is transmitted from the seventh to ninth gears  143   g  to  143   i.    
     The support member  131  and the brake rollers  115  are pressed by a spring  13   a  toward the feed roller  114 . Thus, the brake rollers  115  can feed the medium without separating from the feed rollers  114 . 
     Hereinafter, the force acting on the brake rollers  115  will be described. 
     As shown in  FIG. 9 , the first to third forces F 1  to F 3  act on the brake rollers  115 . The first force F 1  is a pressing force by which the spring  131   a  presses the brake rollers  115  toward the feed rollers  114  side. The first force F 1  is a static force determined according to the spring constant, etc., of the spring  131   a , and does not change regardless of whether the torque limiter that defines the maximum torque applied to the brake rollers  115  is the first torque limiter  146  or the second torque limiters  148   a  to  148   b.    
     The second force F 2  is a force that causes the brake rollers  115  to bite into the feed rollers  114 , which is generated by a load (separation torque) applied to the brake rollers  115  that tries to rotate in the direction A 3  opposite to the medium feed direction in the medium feed direction A 1 . The second force F 2  is applied in a direction in which the brake rollers  115  press the feed rollers  114  and changes dynamically in accordance with the maximum torque applied to the brake roller  115 . That is, the second force F 2  when the second torque limiters  148   a  to  148   b  define the maximum torque applied to the brake rollers  115  is greater than the second force F 2  when the first torque limiter  146  defines the maximum torque applied to the brake rollers  115 . 
     The third force F 3  is a force that tries to float the brake roller  115  upward, which is generated by the gear transmission torque of the gear group including the seventh to ninth gears  143   g  to  143   i . The third force F 3  is added in a direction in which the brake rollers  115  are apart from the feed rollers  114 , and dynamically changes in accordance with the torques applied to the gear group including the seventh to ninth gears  143   g  to  143   i . The medium conveying apparatus  100  uses a ratchet gear  147  located between the first torque limiter  146  and the second torque limiters  148   a  to  148   b  to change the torque limiter that defines the maximum torque applied to the brake rollers  115 . The first limit value of the first torque limiter  146  is smaller than the second limit value of the second torque limiters  148   a  to  148   b . Therefore, the maximum torque applied to the gear group including the seventh to ninth gears  143   g  to  143   i  is the first limit value, regardless of whether or not the force for rotating the brake rollers  115  in the medium feeding direction by the feed rollers  114  is interrupted by the ratchet gear  147 . Therefore, the third force F 3  does not change regardless of whether the torque limiter that defines the maximum torque applied to the brake rollers  115  is the first torque limiter  146  or the second torque limiters  148   a  to  148   b.    
     A force of a magnitude acquired by subtracting the magnitude of the third force F 3  from the sum of the magnitude of the first force F 1  and the magnitude of the second force F 2 , is applied to the brake roller  115  in a direction in which the brake rollers  115  press the feed rollers  114 . For example, it is possible to change the maximum torque applied to the brake rollers by providing two driving force transmission paths in which torque limiters having torque limit values different from each other are located between the gear group and the brake rollers, and switching the driving force transmission path. However, in that case, if the maximum torque applied to the brake rollers increases, the maximum torque applied to the gear group also increases, and the third force F 3  generated in the direction in which the brake rollers are spaced apart from the feed rollers also increases. That is, in this case, the third force F 3  is also increased together with the second force F 2 , the force acting in the direction in which the brake rollers  115  press the feed rollers  114  is not sufficiently large as a whole. Therefore, the maximum torque applied to the brake rollers  115  is too large relative to the pressing force for pressing the brake rollers  115  to the feed rollers  114  side. Thus, when only one medium is present between the brake rollers  115  and the feed rollers  114 , the brake rollers  115  are less likely to be driven by the feed rollers  114 , and thus slipping of the medium is likely to occur. 
     On the other hand, in the medium conveying apparatus  100 , even when the maximum torques applied to the brake rollers  115  are changed, the third force F 3  does not change since the maximum torques applied to the gear group including the seventh to ninth gears  143   g  to  143   i  do not change. Therefore, when increasing the maximum torque applied to the brake rollers  115 , the pressing force for pressing the brake rollers  115  to the feed rollers  114  side also increases. Thus, since the brake rollers  115  and the feed rollers  114  interpose the medium with a sufficient force, it is suppressed that slip occurs between the brake rollers  115  and the medium. Accordingly, the medium conveying apparatus  100  limits the maximum torque applied to the brake rollers  115  to an appropriate size, relative to the pressing force for pressing the brake rollers  115  to the feed rollers  114  side, and suppresses that the slip of the medium is likely to occur. 
       FIG. 10  is a block diagram illustrating a schematic configuration of the medium conveying apparatus  100 . 
     The medium conveying apparatus  100  further includes an interface device  153 , a storage device  160 , and a processing circuit  170 , etc., in addition to the configuration described above. 
     For example, the interface device  153  includes an interface circuit conforming to a serial bus such as universal serial bus (USB), is electrically connected to an unillustrated information processing apparatus (for example, a personal computer or a mobile information terminal), and transmits and receives an input image and various types of information. Further, a communication module including an antenna transmitting and receiving wireless signals, and a wireless communication interface device for transmitting and receiving signals through a wireless communication line in conformance with a predetermined communication protocol may be used in place of the interface device  153 . For example, the predetermined communication protocol is a wireless local area network (LAN). 
     The storage device  160  includes a memory device such as a random access memory (RAM) or a read only memory (ROM), a fixed disk device such as a hard disk, or a portable storage device such as a flexible disk or an optical disk. Further, the storage device  160  stores a computer program, a database, a table, etc., used for various types of processing in the medium conveying apparatus  100 . The computer program may be installed on the storage device  160  from a computer-readable, non-transitory medium such as a compact disc read only memory (CD-ROM), a digital versatile disc read only memory (DVD-ROM), etc., by using a well-known setup program, etc. 
     The processing circuit  170  operates in accordance with a program previously stored in the storage device  160 . The processing circuit  170  is, for example, a CPU (Central Processing Unit). The processing circuit  170  may be a digital signal processor (DSP), a large scale integration (LSI), an application specific integrated circuit (ASIC), a field-programmable gate array (FPGA), etc. 
     The processing circuit  170  is connected to the operating device  105 , the display device  106 , the first sensor  111 , the second sensor  113 , the imaging device  118 , the electromagnetic clutch  144 , the first motor  151 , the second motor  152 , the interface device  153  and the storage device  160 , etc., and controls each of these units. The processing circuit  170  performs drive control of the first motor  151  and the second motor  152 , imaging control of the imaging device  118 , etc., controls the conveyance of the medium, generates an input image, and transmits the input image to the information processing apparatus via the interface device  153 . 
       FIG. 11  is a diagram illustrating schematic configurations of the storage device  160  and the processing circuit  170 . 
     As illustrated in  FIG. 11 , a control program  161 , an image acquisition program  162 , etc., are stored in the storage device  160 . Each of these programs is a functional module implemented by software operating on a processor. The processing circuit  170  reads each program stored in the storage device  160  and operates in accordance with each read program. Thus, the processing circuit  170  functions as a control module  171  and an image acquisition module  172 . 
       FIG. 12  is a flowchart illustrating an operation example of medium reading processing in the medium conveying apparatus  100 . 
     Referring to the flowchart illustrated in  FIG. 12 , an operation example of the medium reading processing in the medium conveying apparatus  100  will be described below. The operation flow described below is executed mainly by the processing circuit  170  in cooperation with each element in the medium conveying apparatus  100 , in accordance with a program previously stored in the storage device  160 . The operation flow illustrated in  FIG. 12  is periodically executed. 
     First, the control module  171  stands by until an instruction to read a medium is input by a user by use of the operation device  105 , and an operation signal instructing to read the medium is received from the operation device  105  (step S 101 ). 
     Next, the control module  171  acquires the medium signal from the second sensor  113  and determines whether or not the medium is placed on the medium tray  103  based on the acquired medium signal (step S 102 ). 
     When a medium is not placed on the medium tray  103 , the control module  171  returns the processing to step S 101  and stands by until newly receiving an operation signal from the operation device  105 . 
     On the other hand, when a medium is placed on the medium tray  103 , the control module  171  drives the first motor  151  and the second motor  152  (step S 103 ). The control module  171  causes the first motor  151  to generate the first driving force to rotate the brake rollers  115  in the direction A 3  opposite to the medium feeding direction, and causes the first to fourth conveyance rollers  116 ,  117 ,  119 , and  120  to rotate in the medium conveying directions A 4  to A 7 . Further, the control module  171  causes the second motor  152  to generate the second driving force to rotate the feed rollers  114  in the medium feeding direction A 2 . Thus, the control module  171  performs feeding and conveying of the medium. Further, the control module  171  drives a motor (not shown) to move the pick arm  112  downward, to urge the medium placed on the medium tray  103  from above. 
     Next, the image acquisition module  172  causes the imaging device  118  to start imaging of the medium, and acquires an input image from the imaging device  118  (step S 104 ). 
     Next, the image acquisition module  172  transmits the input image to the information processing apparatus through the interface device  153  (step S 105 ). When not being connected to the information processing apparatus, the image acquisition module  162  stores the input image in the storage device  160 . 
     Next, the control module  171  determines whether or not the medium remains on the medium tray  103  based on the medium signal acquired from the second sensor  113  (step S 106 ). When a medium remains on the medium tray  103 , the control module  171  returns the processing to step S 104  and repeats the processing in steps S 104  to S 106 . 
     On the other hand, when the medium does not remain on the medium tray  103 , the control module  171  stops the first motor  151  and the second motor  152  (step S 107 ), and ends the series of steps. 
     As described in detail above, the medium conveying apparatus  100  changes the limit value of the torque of the torque limiter according to the height of the media placed on the medium tray  103 . Thus, the medium conveying apparatus  100  can suppress the occurrence of a medium jam when a thin paper, etc., is conveyed as a medium while suppressing the occurrence of multi-feed of the medium when the weight of the medium placed on the medium to be fed is large. Thus, the medium conveying apparatus  100  can more appropriately feed the medium. 
     Further, in the medium conveying apparatus  100 , when the torque limiter is changed, the torque limiter is provided so that the force in the direction in which the brake rollers  115  are spaced apart from the feed rollers  114  does not change while the force in the direction in which the brake rollers  115  press the feed rollers  114  is changed. Thus, the medium conveying apparatus  100  can suppress that the slip of the medium by the large maximum torque applied to the brake rollers  115  relative to the pressing force for pressing the brake rollers  115  to the feed rollers  114  side, is likely to occur. Thus, the medium conveying apparatus  100  can more appropriately feed the medium. 
     The medium conveying apparatus  100  also uses the mechanical first torque limiter  146  and second torque limiters  148   a  to  148   b  to change the maximum torque applied to the brake rollers  115 . Thus, the medium conveying apparatus  100  can change the maximum torque applied to the brake rollers  115  without the use of expensive components such as an electromagnetic clutch or electromagnetic brake, thereby reduce the device cost. 
       FIG. 13  is a flowchart illustrating an example of an operation of a setting process of a medium conveying apparatus according to another embodiment. 
     Referring to the flowchart illustrated in  FIG. 13 , an operation example of the setting processing in the medium conveying apparatus  100  will be described below. The operation flow described below is executed mainly by the processing circuit  170  in cooperation with each element in the medium conveying apparatus, in accordance with a program previously stored in the storage device  160 . The operation flow illustrated in  FIG. 13  is periodically executed. 
     In the present embodiment, in the medium conveying apparatus, the abutting portion  112   b  of the pick arm  112 , the stopper  132 , the first torque limiter  146 , the ratchet gear  147  and the second torque limiter  148   a  to  148   b  are omitted. In the present embodiment, the electromagnetic clutch  144  functions as a torque limiter of the torque applied to the brake rollers  115  and defines a maximum torque applied to the brake rollers  115 . Before the flow of the operation shown in  FIG. 13  is executed, the limit value of the torque of the electromagnetic clutch  144  is set to an initial value within the first limit value or the second limit value. 
     First, the control module  171  acquires an optical signal from the first sensor  111 , and detects the height of the media placed on the medium tray  103 , based on the acquired optical signal (step S 201 ). A table showing the relationship between the signal value of the optical signal and the height of the media placed on the medium tray  103  is stored in advance in the storage device  160 , based on a prior experiment of acquiring an optical signal while changing the height of the media placed on the medium tray  103 . The control module  171  specifies the height of the media placed on the medium tray  103  based on the table stored in the storage device  160 . 
     Next, the control module  171  determines whether the height of the media exceeds a predetermined height (step S 202 ). The predetermined height is, for example, set to the minimum value of the height of the media at which multi-feed of the medium occurred in a prior experiment in which the medium is conveyed while changing the height of the media placed on the medium tray  103  with the maximum torque applied to the brake rollers  115  set to the first limit value. 
     When the height of the media is equal to or less than the predetermined height, the control module  171  determines whether or not the limit value of the torque of the electromagnetic clutch  144  is set to the first limit value (step S 203 ). When the limit value of the torque of the electromagnetic clutch  144  is set to the first limit value, the control module  171  terminates a series of steps without executing the processing in particular. On the other hand, when the limit value of the torque of the electromagnetic clutch  144  is not set to the first limit value, the control module  171  changes the limit value of the torque of the electromagnetic clutch  144  by setting the limit value to the first limit value (step S 204 ), and ends the series of steps. 
     On the other hand, when the height of the media exceeds the predetermined height, the control module  171  determines whether or not the limit value of the torque of the electromagnetic clutch  144  is set to the second limit value (step S 205 ). When the limit value of the torque of the electromagnetic clutch  144  is set to the second limit value, the control module  171  terminates a series of steps without executing the processing in particular. On the other hand, when the limit value of the torque of the electromagnetic clutch  144  is not set to the second limit value, the control module  171  changes the limit value of the torque of the electromagnetic clutch  144  by setting the limit value to the second limit value (step S 206 ), and ends the series of steps. 
     Thus, in the present embodiment, the control module  171  functions as a changing module to change the limit value of the torque limiter according to the height of the media placed on the medium tray  103 . Thus, the medium conveying apparatus  100  can change the limit value of the torque applied to the brake rollers  115  without using a plurality of torque limiters, thereby, the structure of the apparatus can be simplified, and the apparatus size and the apparatus weight can be reduced. 
     The control module  171  may change the limit value of the torque in any stage of three or more stages instead of changing the limit value in two stages of the first limit value and the second limit value. In that case, the control module  171  changes the limit value so that the higher the height of the media placed on the medium tray  103  is, the larger the limit value. Thus, the medium conveying apparatus  100  can more flexibly change the limit value of the torque applied to the brake rollers  115 . 
     An electromagnetic brake may also be used in place of the electromagnetic clutch  144 . The electromagnetic brake is a brake capable of electromagnetically changing the limit value of the torque in accordance with a control signal from the processing circuit  170 , and transmits a driving force from the first motor  151  to the brake roller  115 . The electromagnetic brake is, for example, a micro powder brake. The electromagnetic brake may be another type of brake, such as a hysteresis brake. 
     In addition, the control module  171  may detect the height of the media placed on the medium tray  103  by using a third sensor for detecting the height at which the rollers  112   a  (pick arm  112 ) is located, instead of the first sensor  111 . In this case, the pick arm  112  is provided with a shield portion that moves in conjunction with the movement of the rollers  112   a . The third sensor has a light emitter and the light receiver provided so as to face each other across the shielding portion located in a predetermined position. The light emitter emits light toward light receiver. The light receiver receives the light emitted by the light emitter, and outputs a second optical signal which is an electrical signal corresponding to the intensity of the received light. When there is a shielding portion between the light emitter and the light receiver, the light emitted by the light emitter is shielded by the shielding portion. Therefore, the signal value of the second optical signal changes according to the position of the shielding portion, that is, according to the height at which the rollers  112   a  of the pick arm  112  are located (the height of the media). The control module  171  acquires the second optical signal from the third sensor, and detects the height of the media placed on the medium tray  103 , based on the acquired second optical signal. 
     As described in detail above, the medium conveying apparatus can more appropriately feed the medium even when using the electromagnetic clutch  144  or the electromagnetic brake. 
       FIG. 14  is a schematic diagram for explaining a stopper  232  of a medium conveying apparatus according to still another embodiment.  FIG. 14  is a perspective view of the stopper  232  from the upstream side. The stopper  232  is used in place of the stopper  132  of the medium conveying apparatus  100 . In the present embodiment, the abutting portion  112   b  of the pick an  112  is omitted in the medium conveying apparatus. 
     The stopper  232  is rotatably (swingably) supported with the support member  131 . The stopper  132  has a grip portion  232   a  and a locking portion  232   b.    
     The grip portion  232   a  is provided so as to rotate and move about the rotation axis of the stopper  232  in response to an operation by the user. 
     The locking portion  232   b  is provided at a position facing the ratchet gear  147 , and is provided so as to rotate in conjunction with the rotational movement of the grip portion  232   a.    
       FIGS. 15 and 16  are schematic views for illustrating the operations of the stopper  232  and the brake rollers  115 . 
     As shown in  FIG. 15 , when the grip portion  232   a  is located at the first predetermined position in response to the operation by the user, the locking portion  232   b  is separated from the ratchet gear  147 , and the rotation of the ratchet gear  147  is not limited. In this case, the torque limiter that defines the maximum torque applied to the brake rollers  115  is set to the first torque limiter  146 . On the other hand, as shown in  FIG. 16 , when the grip portion  232   a  is located at a second predetermined position different from the first predetermined position in response to the operation by the user, the locking portion  132   b  abuts against the ratchet gear  147  to lock the ratchet gear  147 . The ratchet gear  147  locked by the locking portion  132   b  is rotatable only in the direction A 3  opposite to the medium feeding direction, and cannot rotate in the medium feeding direction (the direction opposite to the arrow A 3 ). In this case, the torque limiter that defines the maximum torque applied to the brake rollers  115  are set to the second torque limiters  148   a  and  148   b.    
     That is, in the present embodiment, the grip portion  232   a  functions as a setting member to set the torque limiter that defines the maximum torque applied to the brake roller  115  to either the first torque limiter  146  or the second torque limiter  148   a  to  148   b  in response to the operation by the user. The medium conveying apparatus can more flexibly set the limit value of the torque applied to the brake rollers  115  in response to the operation by the user. 
     As described in detail above, the medium conveying apparatus can more appropriately feed the medium even when the limit value of the torque applied to the brake rollers  115  is set in response to the operation by the user. 
       FIG. 17  is a diagram illustrating a schematic configuration of a processing circuit  270  in a medium conveying apparatus according to yet another embodiment. The processing circuit  270  is used in place of the processing circuit  170  in the medium conveying apparatus  100  and executes the medium reading processing and the setting processing in place of the processing circuit  170 . Processing circuit  270  includes a control circuit  271  and an image acquisition circuit  272 , etc. Note that each unit may be configured by an independent integrated circuit, a microprocessor, firmware, etc. 
     The control circuit  271  is an example of a control module and has a function similar to the control module  171 . The control circuit  271  receives the operation signal from the operating device  105 , the optical signal from the first sensor  111 , and the medium signal from the second sensor  113 . The control circuit  271  drives the first motor  151  and the second motor  152  in accordance with the received signals, and sets the limit value of the torque of the electromagnetic clutch  144 . 
     The image acquisition circuit  272  is an example of an image acquisition module and has a function similar to the image acquisition module  172 . The image acquisition circuit  272  receives the input image from the imaging device  118  and stores the input image into the storage device  160 , and also transmits the input image to the information processing apparatus through the interface device  153 . 
     As described in detail above, the medium conveying apparatus can more appropriately feed the medium even when the processing circuit  270  is used. 
     In accordance with embodiments, the media conveying apparatus, the method, and the computer-readable non-temporary medium storing the control program, can more appropriately feed the medium. 
     All examples and conditional language recited herein are intended for pedagogical purposes to aid the reader in understanding the invention and the concepts contributed by the inventor to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of the superiority and inferiority of the invention. Although the embodiment(s) of the present inventions have been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.