Patent Publication Number: US-2022234847-A1

Title: Medium conveying apparatus for detecting a folding of a medium

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is continuation application of U.S. patent application Ser. No. 16/667,569, filed Oct. 29, 2019, which claims the benefit of priority of prior Japanese Patent Application No. 2018-238434, filed on Dec. 20, 2018, the entire contents of which are incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     Embodiments discussed in the present specification relate to medium conveyance. 
     BACKGROUND 
     A medium conveying apparatus, such as a scanner, for conveying a medium, such as a document, and reading an image of the conveyed medium has a function of separating and feeding a plurality of media. However, when a medium folded in two, a bound medium, such as a transportation slip or a passport, etc., is conveyed in a state in which the function of separating and feeding a plurality of media is enabled, the medium may not be separated, and a medium jam may occur. 
     A sheet feeding device including height detection sensors being located at positions different from one another with respect to a width direction perpendicular to a sheet feeding direction, each height detection sensor detecting a position of a highest sheet loaded on a loading surface in a height direction at each position in the width direction, is disclosed (see Japanese Unexamined Patent Publication (Kokai) No. 2018-122950). The sheet feeding device stops feeding of a document based on a detection result of the height detection sensors during feeding of the document. 
     SUMMARY 
     According to some embodiments, a medium conveying apparatus includes a housing, a medium tray, a feed roller to feed a medium on the medium tray, a first optical sensor located on an upper part of the housing and in a central part of the housing in a direction perpendicular to a medium conveying direction, and including a first light emitter for emitting first light and a first light receiver for generating a first signal based on receiving the first light, a second optical sensor located on an upper part of the housing and on a side of the first optical sensor in the direction perpendicular to the medium conveying direction, and including a second light emitter for emitting second light and a second light receiver for generating a second signal based on receiving the second light, a processor for detecting a folding of the medium based on at least the first signal or the second signal, and stopping feeding of the medium by the feed roller in accordance with a detection result of the folding of the medium. The first light emitter emits the first light toward a downstream side of the medium in the medium conveying direction, the medium being placed on the medium tray, and the second light emitter emits the second light toward an upstream side of the medium in the medium conveying direction, the medium being placed on the medium tray. 
     According to some embodiments, a method for detecting a folding of a medium includes feeding the medium on a medium tray by a feed roller, emitting first light by a first light emitter of a first optical sensor located on an upper part of a housing and in a central part of the housing in a direction perpendicular to a medium conveying direction, generating a first signal based on receiving the first light by a first light receiver of the first optical sensor, emitting second light by a second light emitter of a second optical sensor located on an upper part of the housing and on a side of the first optical sensor on the housing in the direction perpendicular to the medium conveying direction, generating a second signal based on receiving the second light by a second light receiver of the second optical sensor, detecting the folding of the medium based on at least the first signal or the second signal, and stopping feeding of the medium by the feed roller in accordance with a detection result of the folding of the medium. The first light emitter emits the first light toward a downstream side of the medium in the medium conveying direction, and the second light emitter emits the second light toward an upstream side of the medium in the medium conveying direction. 
     According to some embodiments, a computer program causes a medium conveying apparatus including a housing, a medium tray, a feed roller to feed a medium on the medium tray, a first optical sensor located on an upper part of the housing and in a central part of the housing in a direction perpendicular to a medium conveying direction, and including a first light emitter for emitting first light and a first light receiver for generating a first signal based on receiving the first light, a second optical sensor located on an upper part of the housing and on a side of the first optical sensor in the direction perpendicular to the medium conveying direction, and including a second light emitter for emitting second light and a second light receiver for generating a second signal based on receiving the second light, to execute a process including detecting a folding of the medium based on at least the first signal or the second signal, and stopping feeding of the medium by the feed roller in accordance with a detection result of the folding of the medium. The first light emitter emits the first light toward a downstream side of the medium in the medium conveying direction, and the second light emitter emits the second light toward an upstream side of the medium in the medium conveying direction. 
    
    
     
       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 each optical sensor. 
         FIG. 4  is a schematic diagram for illustrating each optical sensor. 
         FIG. 5  is a block diagram illustrating a schematic configuration of the medium conveying apparatus  100 . 
         FIG. 6  is a diagram illustrating a schematic configuration of a storage device  140  and a processing circuit  150 . 
         FIG. 7  is a flowchart illustrating an operation example of medium reading processing. 
         FIG. 8  is a flowchart illustrating an operation example of folding detection processing. 
         FIG. 9  is a flowchart illustrating an operation example of the folding detection processing. 
         FIG. 10  is a schematic diagram for illustrating a technical meaning of detecting a folding of a medium. 
         FIG. 11  is a schematic diagram for illustrating the technical meaning of detecting a folding of a medium. 
         FIG. 12A  is a schematic diagram for illustrating changes in an amount of folding of a medium over time. 
         FIG. 12B  is a schematic diagram for illustrating changes in an amount of folding of a medium over time. 
         FIG. 13A  is a schematic diagram for illustrating an amount of folding when a medium is loaded. 
         FIG. 13B  is a schematic diagram for illustrating an amount of folding when a medium is loaded. 
         FIG. 14A  is a schematic diagram for illustrating a signal value of each signal. 
         FIG. 14B  is a schematic diagram for illustrating a signal value of each signal. 
         FIG. 15A  is a schematic diagram for illustrating a signal value of each signal. 
         FIG. 15B  is a schematic diagram for illustrating a signal value of each signal. 
         FIG. 16  is a flowchart illustrating an operation example of first sensitivity change processing. 
         FIG. 17  is a flowchart illustrating an operation example of second sensitivity change processing. 
         FIG. 18  is a schematic diagram for illustrating a characteristic of an ultrasonic signal. 
         FIG. 19  is a flowchart illustrating an operation example of third sensitivity change processing. 
         FIG. 20  is a diagram illustrating a schematic configuration of a processing circuit  260  in another medium conveying apparatus. 
     
    
    
     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. Conveyed media include a brochure or a passport bound by a seam part, paper bound by a staple, paper in two by a crease part, and any other document. 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 an example of an upper part of a housing, is located in a position covering a top surface of the medium conveying apparatus  100 , and is engaged with the lower housing  101  by a hinge in such a way as to be able to open and close in a case of a medium being stuck, cleaning inside the medium conveying apparatus  100 , etc. Specifically, the upper housing  102  is located above the lower housing  101  and functions as a housing cover for covering the lower housing  101 . 
     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 medium tray  103  is provided in such a way that a placement surface  103   a  of a medium is tilted against an installation surface of the medium conveying apparatus  100 . The medium tray  103  includes side guides  107   a  and  b . Each of the side guides  107   a  and  b  is provided on the medium tray  103  in such a way as to be movable in a direction A 2  perpendicular to a medium conveying direction A 1  and also regulates a width direction of a medium placed on the medium tray  103 . Each of the side guides  107   a  and  b  is provided in such a way that a maximum height of each of the side guides  107   a  and  b  in a direction A 3  perpendicular to the placement surface  103   a  is greater than a maximum media loading capacity on the medium tray  103  supported by the medium conveying apparatus  100 . The side guides  107   a  and  107   b  may be hereinafter collectively referred to as side guides  107   a.    
     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 second optical sensors  111  and  112 , a first optical sensor  113 , a first medium detection sensor  114 , a pick arm  115 , feed rollers  116   a  and  b , brake rollers  117   a  and  b , a second medium detection sensor  118 , an ultrasonic transmitter  119   a , an ultrasonic receiver  119   b , a first center sensor  115 , a third medium detection sensor  120 , a fourth medium detection sensor  121 , a fifth medium detection sensor  122 , first conveyance rollers  123   a  and  b , second conveyance rollers  124   a  and  b , a second center sensor  120 , a first imaging device  125   a , a second imaging device  125   b , third conveyance rollers  126   a  and  b , and fourth conveyance rollers  127   a  and  b , etc. 
     The feed rollers  116   a  and  116   b  may be hereinafter collectively referred to as feed rollers  116 . Further, the brake rollers  117   a  and  117   b  may be collectively referred to as brake rollers  117 . Further, the first conveyance rollers  123   a  and  123   b  may be collectively referred to as first conveyance rollers  123 . Further, the second conveyance rollers  124   a  and  124   b  may be collectively referred to as second conveyance rollers  124 . Further, the first imaging device  125   a  and the second imaging device  125   b  may be collectively referred to as imaging devices  125 . Further, the third conveyance rollers  126   a  and  126   b  may be collectively referred to as third conveyance rollers  126 . Further, the fourth conveyance rollers  127   a  and  127   b  may be collectively referred to as fourth conveyance rollers  127 . 
     A top surface of the lower housing  101  forms a lower guide  108   a  of a medium conveyance path and functions as a medium conveyance surface  101   a . On the other hand, a bottom surface of the upper housing  102  forms an upper guide  108   b  of the medium conveyance path. 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 medium detection sensor  114  is located on the downstream side of the second optical sensors  111  and  112 , and the first optical sensor  113  in the medium conveying direction A 1 . The first medium detection sensor  114  includes a contact detection sensor and detects whether or not a medium exists at a position of the contact detection sensor. The first medium detection sensor  114  generates and outputs a first medium detection signal changing the signal value between a state in which a medium exists at the position and a state in which a medium does not exist at the position. 
     The pick arm  115  is provided on the upper housing  102  and is located on the downstream side of the first medium detection sensor  114  in the medium conveying direction A 1 . The pick arm  115  is an example of a pressing member and presses a medium placed on the medium tray  103 . The pick arm  115  is provided at a position facing the feed rollers  116  with the medium conveyance path in between and when a medium is not fed, separates from the feed rollers  116 . On the other hand, when a medium is fed, the pick arm  115  comes into contact with a medium placed on the medium tray  103  and presses the medium from above. Consequently, a moderate frictional force is generated between the feed rollers  116  and the medium, and the feed rollers  116  can satisfactorily feed the medium. 
     The feed rollers  116  are provided on the lower housing  101 , and the brake rollers  117  are provided on the upper housing  102  to face the feed rollers  116 . The feed rollers  116  and the brake rollers  117  are provided in such a way that a nip position is located on the downstream side of the pick arm  115  in the medium conveying direction A 1 . The feed rollers  116  and the brake rollers  117  are examples of a feeding part for separating and feeding media placed on the medium tray  103  and sequentially feed media placed on the medium tray  103  from the lowermost side. The feed rollers  116  are provided to be rotatable in a direction of an arrow A 4  in  FIG. 2  according to a driving force transmitted from a driving device, to be described later, and feed a medium placed on the medium tray  103  toward the medium conveying direction A 1 . On the other hand, the brake rollers  117  are provided to be rotatable in a direction of an arrow A 5  in  FIG. 2  according to a driving force transmitted from the driving device, and by rotating in the direction of the arrow A 5 , prevents feeding of a medium not in contact with the feed rollers  116  out of the media placed on the medium tray. 
     The feed rollers  116  are provided to be rotatable in a direction opposite to the direction of the arrow A 4  in  FIG. 2  in order to be able to reset a fed medium to the medium tray  103 . 
     Further, in order to be able to turn the separation function OFF, the brake rollers  117  are provided in such a way as to be able to interrupt a driving force from the driving device. For example, a driving force transmission mechanism, such as a gear group, for transmitting a driving force from the driving device to the brake rollers  117  is provided between the driving device and the brake rollers  117 . At least one gear in the gear group in the driving force transmission mechanism is provided to be movable, and the medium conveying apparatus  100  interrupts a driving force from the driving device to the brake rollers  117  by separating the gear from a gear engaged with the gear. 
     Alternatively, in order to be able to turn the separation function OFF, the brake rollers  117  may be provided in such a way as to be able to reduce a separating force by the brake rollers  117 . For example, two driving force transmission mechanisms, such as gear groups, each of which transmitting a driving force from the driving device to the brake rollers  117  are provided between the driving device and the brake rollers  117 . The driving force transmission mechanisms are provided with torque limiters having different torque limit values, respectively. Further, at least one gear in the gear group in the driving force transmission mechanism is provided to be movable. The medium conveying apparatus  100  reduces a separating force by the brake rollers  117  by moving the gear in each driving force transmission mechanism and, switching the driving force transmission mechanism transmitting a driving force from the driving device to the brake rollers  117 . 
     The second medium detection sensor  118  is located on the downstream side of the feed rollers  116  and the brake rollers  117  in the medium conveying direction A 1 . The second medium detection sensor  118  includes a contact detection sensor and detects whether or not a medium exists at a position of contact detection sensor. The second medium detection sensor  118  generates and outputs a second medium detection signal changing the signal value between a state in which a medium exists at the position and a state in which a medium does not exist at the position. The second medium detection sensor  118  is an example of a medium detection sensor located on the downstream side of a feeding part in the medium conveying direction. 
     The ultrasonic transmitter  119   a  and the ultrasonic receiver  119   b  are located on the downstream side of the second medium detection sensor  118 . The ultrasonic transmitter  119   a  and the ultrasonic receiver  119   b  are located close to the conveyance path of a medium in such a way as to face one another with the conveyance path in between. The ultrasonic transmitter  119   a  outputs an ultrasonic wave. On the other hand, the ultrasonic receiver  119   b  receives an ultrasonic wave being transmitted by the ultrasonic transmitter  119   a  and passing through a medium, and generates and outputs an ultrasonic signal being an electric signal corresponding to the received ultrasonic wave. The ultrasonic transmitter  119   a  and the ultrasonic receiver  119   b  may be hereinafter collectively referred to as an ultrasonic sensor  119 . 
     The third medium detection sensor  120  is located on the downstream side of the ultrasonic sensor  119  in the medium conveying direction A 1  and detects whether or not a medium exists at the position. The third medium detection sensor  120  includes a light emitter and a light receiver that are provided on one side of the medium conveyance path and a reflection member, such as a mirror, provided at a position facing the light emitter and the light receiver with the conveyance path in between. The light emitter emits light toward the conveyance path. On the other hand, the light receiver receives light emitted by the light emitter and reflected by the reflection member, and generates and outputs a third medium detection signal being an electric signal based on intensity of the received light. When a medium exists at a position of the third medium detection sensor  120 , light emitted by the light emitter is shaded by the medium, and therefore a signal value of the third medium detection signal varies between a state in which a medium exists at the position of the third medium detection sensor  120  and a state in which a medium does not exist. The light emitter and the light receiver may be provided at positions facing one another with the conveyance path in between, and the reflection member may be omitted. 
     The fourth medium detection sensor  121  is located at the same position as the third medium detection sensor  120  in the medium conveying direction A 1 . The fourth medium detection sensor  121  has a configuration similar to that of the third medium detection sensor  120 , and generates and outputs a fourth medium detection signal being an electric signal based on intensity of light received by a light receiver. 
     The fifth medium detection sensor  122  is located on the downstream side of the third medium detection sensor  120  and the fourth medium detection sensor  121  in the medium conveying direction A 1 . The fifth medium detection sensor  122  includes a contact detection sensor and detects whether or not a medium exists at a position of contact detection sensor. The fifth medium detection sensor  122  generates and outputs a fifth medium detection signal changing the signal value between a state in which a medium exists at the position and a state in which a medium does not exist at the position. The fifth medium detection sensor  122  is an example of a medium detection sensor located on the downstream side of a feeding part in the medium conveying direction. 
     The first imaging device  125   a  includes a reduction optical system type line sensor including an imaging element based on charge coupled devices (CCDs) linearly located in a main scanning direction. Further, the first imaging device  125   a  includes a lens for forming an image on the imaging element, and an ND converter for amplifying and analog-digital (ND) converting an electric signal output from the imaging element. The first imaging device  125   a  generates and outputs an input image imaging a back side of a conveyed medium. 
     Similarly, the second imaging device  125   b  includes a reduction optical system type line sensor including an imaging element based on CCDs linearly located in the main scanning direction. Further, the second imaging device  125   b  includes a lens for forming an image on the imaging element, and an ND converter for amplifying and ND converting an electric signal output from the imaging element. The second imaging device  125   b  generates and outputs an input image imaging a front side of a conveyed medium. 
     Only either of the first imaging device  125   a  and the second imaging device  125   b  may be located in the medium conveying apparatus  100  and only one side of a medium may be read. Further, a unity-magnification optical system type contact image sensor (CIS) including an imaging element based on a complementary metal oxide semiconductor (CMOS) may be used in place of the imaging element based on CCDs. 
     A medium placed on the medium tray  103  is conveyed between the lower guide  108   a  and the upper guide  108   b  in the medium conveying direction A 1  by the feed rollers  116  rotating in the direction of the arrow A 4  in  FIG. 2 . When a medium is conveyed, the brake rollers  117  rotate in the direction of the arrow A 3 , that is, a direction opposite to the medium feeding direction. By the workings of the feed rollers  116  and the brake rollers  117 , when a plurality of media are placed on the medium tray  103 , only a medium in contact with the feed rollers  116 , 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). 
     A medium is fed between the first conveyance rollers  123  and the second conveyance rollers  124  while being guided by the lower guide  108   a  and the upper guide  108   b . The medium is fed between the first imaging device  125   a  and the second imaging device  125   b  by the first conveyance rollers  123  and the second conveyance rollers  124  rotating in directions of an arrow A 6  and an arrow A 7 , respectively. The medium read by the imaging devices  125  is ejected on the ejection tray  104  by the third conveyance rollers  126  and the fourth conveyance rollers  127  rotating in directions of an arrow A 8  and an arrow A 9 , respectively. 
       FIG. 3  and  FIG. 4  are schematic diagrams for illustrating the second optical sensors  111  and  112 , and the first optical sensor  113 .  FIG. 3  is a schematic diagram of the upstream side of the medium conveying apparatus  100  viewed from side in a state in which the side guides  107  are removed.  FIG. 4  is a schematic diagram of the upstream side of the medium conveying apparatus  100  viewed from above. 
     As illustrated in  FIG. 3 , the second optical sensors  111  and  112  are provided on the upper housing  102 , that is, above the medium conveyance path and are located on the upstream side of the first optical sensor  113  in the medium conveying direction A 1 . Each of the second optical sensors  111  and  112  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 second optical sensors  111  and  112  include second light emitters  111   a  and  112   a , and second light receivers  111   b  and  112   b , respectively. Each of the second light emitters  111   a  and  112   a  emits second light (infrared light) toward the medium tray  103  or the lower housing  101 . On the other hand, each of the second light receivers  111   b  and  112   b  receives the second light emitted by each of the second light emitters  111   a  and  112   a  and reflected by the medium tray  103 , the lower housing  101 , or a medium placed on the medium tray  103 , and generates and outputs a second signal being an electric signal based on the received light, that is, based on receiving the second light. For example, the second signal indicates a time period from a time when each of the second light emitters  111   a  and  112   a  emits second light to a time when each of the second light receivers  111   b  and  112   b  receives the second light. 
     For example, a known infrared access distance sensor capable of measuring a distance in a range of 0 to 100 mm with a resolution of 1 mm may be used as each of the second optical sensors  111  and  112 . Only one of the second optical sensors  111  and  112  may be located and the other may not be located in the medium conveying apparatus  100 . 
     The first optical sensor  113  is provided on the upper housing  102 , that is, above the medium conveyance path and is located on the downstream side of the second optical sensors  111  and  112  in the medium conveying direction A 1  and on the upstream side of the pick arm  115  in the medium conveying direction A 1 . The first optical sensor  113  is an infrared access distance sensor similar to the second optical sensors  111  and  112 . The first optical sensor  113  includes a first light emitter  113   a  and a first light receiver  113   b . The first light emitter  113   a  emits first light (infrared light) toward the medium tray  103  or the lower housing  101 . On the other hand, the first light receiver  113   b  receives the first light emitted by the first light emitter  113   a  and reflected by the medium tray  103 , the lower housing  101 , or a medium placed on the medium tray  103 , and generates and outputs a first signal being an electric signal based on the received light, that is, based on receiving the first light. For example, the first signal indicates a time period from a time when the first light emitter  113   a  emits first light to a time when the first light receiver  113   b  receives the first light. 
     The first optical sensor  113  may be located on the upstream side of the second optical sensors  111  and  112 . Further, the first optical sensor  113  and the second optical sensors  111  and  112  may be located on the downstream side of the pick arm  115 . 
     An arrangement position of the second optical sensors  111  and  112 , and the first optical sensor  113  will be described in detail below. 
     The second optical sensors  111  and  112 , and the first optical sensor  113  are located on the upper housing  102  at positions facing the conveyance surface  101   a . For example, the first optical sensor  113  is located at a position P 2  on the upstream side of a contact position P 1  of the pick arm  115  and the feed rollers  116  by a predetermined distance D 1  in the medium conveying direction A 1 . For example, the predetermined distance D 1  is greater than or equal to 5 mm and less than or equal to 30 mm. When a distance D 2  between the contact position P 1  and the arrangement position P 2  of the first optical sensor  113  in the direction A 3  perpendicular to the conveyance surface  101   a  is 15 mm, an angle θ 1  formed by the direction A 3  and a straight line from the contact position P 1  toward the arrangement position P 2  is greater than or equal to 18° and less than or equal to 70°. Consequently, the first light emitter  113   a  can efficiently emit the first light toward the downstream side of a medium placed on the medium tray  103 , and the first light receiver  113   b  can efficiently receive the first light reflected by the medium. 
     On the other hand, the second optical sensors  111  and  112  are located at a position P 3  on the upstream side of the contact position P 1  of the pick arm  115  and the feed rollers  116  by a predetermined distance D 3  in the medium conveying direction A 1 . The predetermined distance D 3  is greater than the predetermined distance D 1  and is, for example, greater than or equal to 10 mm and less than or equal to 50 mm. Consequently, the second light emitters  111   a  and  112   a  can efficiently emit the second light toward the upstream side of a position to which the first light emitter  113   a  emits the first light, and the second light receivers  111   b  and  112   b  can efficiently receive the second light reflected by a medium. 
     Further, the first light emitter  113   a  is provided to emit the first light toward the downstream side of a medium placed on the medium tray  103  in the medium conveying direction A 1 . For example, the first light emitter  113   a  emits the first light toward a predetermined position P 4  in the conveyance surface  101   a , that is, toward the downstream side of a boundary position B between the conveyance surface  101   a  and the placement surface  103   a . In other words, the first light emitter  113   a  emits the first light toward the downstream side of a position corresponding to the boundary position B (a position where a straight line connecting the first light emitter  113   a  to the boundary position B passes) on a medium placed on the medium tray  103 . 
     A medium placed on the medium tray  103  is pressed by the pick arm  115  and therefore hardly bends on the downstream side of the pick arm  115 . Accordingly, it is preferable that the first light emitter  113   a  be provided to emit the first light toward a position on the upstream side of the pick arm  115 . 
     On the other hand, the second light emitters  111   a  and  112   a  are provided to emit the second light toward the upstream side of a medium placed on the medium tray  103  in the medium conveying direction A 1 . The second light emitters  111   a  and  112   a  emit the second light toward a position on the upstream side of the position P 4  to which the first light emitter  113   a  emits the first light on a medium placed on the medium tray  103 . Particularly, the second light emitters  111   a  and  112   a  emit the second light toward a predetermined position P 5  in the placement surface  103   a , that is, toward the upstream side of the boundary position B between the conveyance surface  101   a  and the placement surface  103   a . Specifically, the second light emitters  111   a  and  112   a  emit the second light toward the upstream side of a position corresponding to the boundary position B on a medium placed on the medium tray  103  (a position where a straight line connecting each of the second light emitters  111   a  and  112   a  to the boundary position B passes). 
     Further, the second light emitters  111   a  and  112   a  are provided in such a way that an angle θ 2  formed by the placement surface  103   a  and a light emission direction of each of the second light emitters  111   a  and  112   a  is greater than a predetermined angle (for example, 10°). Consequently, each of the second light receivers  111   b  and  112   b  can reliably receive the second light emitted by each of the second light emitters  111   a  and  112   a  and reflected by a medium placed on the medium tray  103 . Similarly, the first light emitter  113   a  is provided in such a way that an angle formed by the conveyance surface  101   a  and a light emission direction of the first light emitter  113   a  is greater than a predetermined angle. Consequently, the first light receiver  113   b  can reliably receive the first light emitted by the first light emitter  113   a  and reflected by a medium placed on the medium tray  103 . 
     The first optical sensor  113  is used for detecting a folding occurring when a medium the center of which is bound by a seam part, such as a passport in an opened state, is fed in such a way that the seam part is perpendicular to the medium conveying direction A 1 . The folding includes a bending. When such a medium is fed, a folding occurs on the downstream side of the seam part in the medium conveying direction A 1 . Accordingly, the first light emitter  113   a  may be provided to emit the first light toward the downstream side of the seam part of the passport placed on the medium tray  103  in such a way that the seam part is perpendicular to the medium conveying direction A 1 . In that case, a distance D 4  in the medium conveying direction A 1  between a nip position P 6  of the feed rollers  116  and the brake rollers  117  and the position P 4  to which the first light emitter  113   a  emits the first light is determined within a range of a length of a page of a passport in a widthwise direction (88 mm). 
     On the other hand, the second optical sensors  111  and  112  are used for detecting a folding occurring when a medium one end of which is bound by a seam part, such as a brochure in a closed state, is fed in such a way that the seam part is parallel with the medium conveying direction A 1 . When such a medium is fed, a folding occurs at an end facing the seam part (an end on the open side) near the central position in the medium conveying direction A 1 . Accordingly, the second light emitters  111   a  and  112   a  may be provided to emit the second light toward a range within a predetermined distance in the medium conveying direction A 1  from the center position of a medium with a predetermined size placed on the medium tray  103 , on the upstream side of the position to which the first light emitter  113   a  emits the first light. For example, the predetermined size includes A 3  to A 6  sizes. For example, the predetermined distance is 50 mm. 
     Considering a case of an A 4 -size brochure bound along a longitudinal direction being determined as a folding detection target, a length of an A 4 -size medium in a lengthwise direction is 297 mm, and a length from an end to the center position in the longitudinal direction is 148.5 mm. In that case, a distance D 5  from the nip position P 6  to the position P 5  to which the second light emitters  111   a  and  112   a  emit the second light in the medium conveying direction A 1  is determined to be within a range from 98.5 mm to 198.5 mm. 
     Further, as illustrated in  FIG. 4 , the first optical sensor  113  is located in the central part of the upper housing  102  in the direction A 2  perpendicular to the medium conveying direction. On the other hand, the second optical sensors  111  and  112  are located outside the first optical sensor  113 , that is, on a side of the first optical sensor  113  on the upper housing  102  in the direction A 2  perpendicular to the medium conveying direction. 
     As described above, the first optical sensor  113  is used for detecting a folding occurring when a medium the center of which is bound by a seam part is fed in such a way that the seam part is perpendicular to the medium conveying direction A 1 . Since the folding occurs along the direction A 2  perpendicular to the medium conveying direction, the first optical sensor  113  can satisfactorily detect the folding as long as the first optical sensor  113  is located at any position where the medium passes. In general, a user is highly likely to place a medium in the central part in the direction A 2  perpendicular to the medium conveying direction, and therefore it is preferable that the first optical sensor  113  be located at a position close to the center in the direction A 2  perpendicular to the medium conveying direction. Consequently, even when a small medium is fed, the first optical sensor  113  can satisfactorily detect a folding occurring on the medium. 
     Further, as described above, the second optical sensors  111  and  112  are used for detecting a folding occurring when a medium bound by a seam part is fed in such a way that the seam part is parallel with the medium conveying direction A 1 . When such a medium is fed, a folding occurs at an end facing the seam part (an end on the open side). Accordingly, it is preferable that the second optical sensors  111  and  112  be located close to an end of a medium with a predetermined size placed on the medium tray  103  in the direction A 2  perpendicular to the medium conveying direction. For example, the predetermined size includes A 3  to A 6  sizes. For example, a position close to an end ranges from a position apart from the end by a first distance to a position apart from the end by a second distance. For example, the first distance is 5 mm, and the second distance is 50 mm. 
     Considering a case of an A 4 -size brochure bound along a longitudinal direction being determined as a folding detection target, a length of an A 4 -size medium in a widthwise direction is 210 mm, and a length from the center position to an end in a widthwise direction is 105 mm. In that case, the second optical sensors  111  and  112  are v within a range in which a distance D 6  from the center position P 7  in the direction A 2  perpendicular to the medium conveying direction is greater than or equal to 55 mm and less than or equal to 100 mm. 
     On the other hand, the first optical sensor  113  is used for detecting a folding occurring inside an area on a fed medium where the second optical sensors  111  and  112  detect a folding. Accordingly, the first optical sensor  113  is located inside the second optical sensors  111  and  112  in the direction A 2  perpendicular to the medium conveying direction. Considering a case of an A 4 -size brochure bound along a longitudinal direction being determined as a target, the first optical sensor  113  is located in a range in which a distance from the center position P 7  in the direction A 2  perpendicular to the medium conveying direction is less than 55 mm. 
       FIG. 5  is a block diagram illustrating a schematic configuration of the medium conveying apparatus  100 . 
     The medium conveying apparatus  100  further includes a driving device  131 , an interface device  132 , a storage device  140 , and a processing circuit  150 , etc., in addition to the configuration described above. 
     The driving device  131  includes one or a plurality of motors, and conveys a medium by rotating the feed rollers  116 , the brake rollers  117 , and the first to fourth conveyance rollers  123 ,  124 ,  126 , and  127 , by a control signal from the processing circuit  150 . Further, the driving device  131  raises or lowers the pick arm  115  in accordance with a control signal from the processing circuit  150 . 
     For example, the interface device  132  includes an interface circuit conforming to a serial bus such as universal serial bus (USB), is electrically connected to an unillustrated information processing device (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  132 . For example, the predetermined communication protocol is a wireless local area network (LAN). 
     The storage device  140  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  150  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  140  from a computer-readable, non-transitory medium such as a compact disk read only memory (CD-ROM), a digital versatile disk read only memory (DVD-ROM), etc., by using a well-known setup program, etc. 
     For example, the processing circuit  150  is a processor, such as a central processing unit (CPU). The processing circuit  150  operates in accordance with a program previously stored in the storage device  140 . The processing circuit  150  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  150  is connected to the operation device  105 , the display device  106 , the second optical sensors  111  and  112 , the first optical sensor  113 , the first medium detection sensor  114 , the second medium detection sensor  118 , the ultrasonic sensor  119 , the third medium detection sensor  120 , the fourth medium detection sensor  121 , the fifth medium detection sensor  122 , the imaging devices  125 , the driving device  131 , the interface device  132 , the storage device  140 , the processing circuit  160 , etc., and controls each of these units. The processing circuit  150  performs drive control of the driving device  131 , imaging control of the imaging devices  125 , etc., acquires an image, and transmits the image to the information processing device through the interface device  142 . Further, the processing circuit  150  detects a folding of a fed medium based on a signal generated by the first optical sensor  113  or the second optical sensors  111  and  112 , and stops feed of the medium depending on the detection result. 
     The processing circuit  160  executes predetermined image processing on an image imaged by the imaging device  125  and stores the image on which the image processing is executed into the storage device  140 . A DSP, an LSI, an ASIC, an FPGA, etc., may be used in place of the processing circuit  160 . 
       FIG. 6  is a diagram illustrating schematic configurations of the storage device  140  and the processing circuit  150 . 
     As illustrated in  FIG. 6 , the storage device  140  stores a control program  141 , a folding detection program  142 , an image acquisition program  143 , a loaded amount detection program  144 , a multi-feed detection program  145 , a skew detection program  146 , etc. Each of these programs is a functional module implemented by software operating on a processor. The processing circuit  150  reads each program stored in the storage device  140  and operates in accordance with each read program. Consequently, the processing circuit  150  functions as a control module  151 , a folding detection module  152 , an image acquisition module  153 , a loaded amount detection module  154 , a multi-feed detection module  155 , and the skew detection module  156 . 
       FIG. 7  is a flowchart illustrating an operation example of medium reading processing in the medium conveying apparatus  100 . 
     Referring to the flowchart illustrated in  FIG. 7 , 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  150  in cooperation with each element in the medium conveying apparatus  100 , in accordance with a program previously stored in the storage device  140 . The operation flow illustrated in  FIG. 7  is periodically executed. 
     First, the control module  151  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  151  acquires a first medium detection signal from the medium detection sensor  111  and determines whether or not a medium is placed on the medium tray  103 , based on the acquired first medium detection signal (step S 102 ). 
     When a medium is not placed on the medium tray  103 , the control module  151  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  151  drives the driving device  131  and causes the pick arm  115  to descend and press the medium placed on the medium tray  103  (step S 103 ). 
     Next, the control module  151  acquires a first signal from the first optical sensor  113  (step S 104 ). The control module  151  acquires a first signal at a timing immediately before feeding of the medium. The timing immediately before feeding of the medium is an example of a predetermined timing. 
     Next, the control module  151  sets a first reference value and a second reference value based on a signal value of the acquired first signal (step S 105 ). For example, the control module  151  sets the signal value of the acquired first signal to the first reference value and sets a value acquired by multiplying the signal value of the first signal by a predetermined coefficient to the second reference value. The predetermined coefficient is set to a ratio of a distance between each of the second optical sensors and a position on the medium conveying apparatus  100  to which the second light is emitted by each of the second optical sensors to a distance between the first optical sensor  113  and a position on the medium conveying apparatus  100  to which the first light is emitted by the first optical sensor  113 . 
     The front edge of a medium is pressed by the pick arm  115  at a timing immediately before feeding of the medium, and therefore even when the front edge of a fed medium is curved (curled), the curved part becomes straight. Further, the first optical sensor  113  emits the first light toward the downstream side of a medium, and the emitted light is less likely to be blocked by a hand of a user setting the medium. Accordingly, by setting each reference value based on a signal value of a first signal acquired immediately before feeding of the medium, the control module  151  can suppress the reference values being set to improper values due to an effect of a state of the medium or a user operation. 
     Next, the control module  151  drives the driving device  131 , rotates the feed rollers  116 , the brake rollers  117 , and the first to fourth conveyance rollers  123 ,  124 ,  126 , and  127 , and feeds and conveys the medium (step S 106 ). When successively feeding and conveying a plurality of media and already rotating each roller, the control module  151  does not particularly execute processing. 
     Next, the control module  151  determines whether or not a folding flag is ON (step S 107 ). The folding flag is set to OFF before the medium reading processing is executed and is set to ON when occurrence of a folding is determined in folding detection processing, to be described later, executed by the folding detection module  152 . 
     When the folding flag is ON, the control module  151  stops the driving device  131  and stops feeding of a medium, and also sets the folding flag to OFF, as abnormality processing (step S 108 ). Thus, depending on a detection result of the folding detection module  152 , the control module  151  stops feeding of a medium by the feed rollers  116  and the brake rollers  117 . 
     Next, the control module  151  drives the driving device  131  and causes the feed rollers  116 , the brake rollers  117 , the first to fourth conveyance rollers  123 ,  124 ,  126 , and  127  to rotate in a direction opposite to the medium conveying direction and reversely feed the medium (step S 109 ). Thus, when stopping feeding of a medium by the feed rollers  116  and the brake rollers  117 , the control module  151  temporarily resets the medium to the medium tray  103 . 
     Next, the control module  151  switches the driving force transmission mechanism provided between the driving device  131  and the brake rollers  117 , interrupts a driving force from the driving device  131  to the brake rollers  117 , and turns the separation function OFF (step S 110 ). The control module  151  may switch the driving force transmission mechanism provided between the driving device  131  and the brake rollers  117 , reduce a separating force by the brake rollers  117 , and turn the separation function OFF. 
     Next, the control module  151  drives the driving device  131  and causes the feed rollers  116  and the first to fourth conveyance rollers  123 ,  124 ,  126 , and  127  to re-rotate in the medium conveying direction, and refeed and re-convey the medium (step S 111 ). At this time, the brake rollers  117  are driven by the feed rollers  116  and do not separate media. Thus, the control module  151  controls the feed rollers  116  and the brake rollers  117  in such a way that the rollers refeed the media without separation. In other words, when a medium folded in two, a bound medium, etc., is fed, and a folding of the medium is detected, the control module  151  automatically turns the separation function OFF and refeeds the medium. Consequently, a user does not need to turn the separation function OFF and refeed the medium, and therefore the control module  151  can improve user convenience. 
     When the folding flag is OFF in step S 107  or when a medium is refed in step S 111 , the image acquisition module  153  causes the imaging device  125  to image the conveyed medium and acquires an input image (step S 112 ). 
     Next, the image acquisition module  153  transmits the input image to the unillustrated information processing device through the interface device  132  (step S 113 ). When not being connected to the information processing device, the image acquisition module  153  stores the input image in the storage device  140 . 
     Next, the control module  151  determines whether or not a medium remains on the medium tray  103  based on a first medium detection signal acquired from the first medium detection sensor  114  (step S 114 ). When a medium remains on the medium tray  103 , the control module  151  returns the processing to step S 104  and repeats the processing in steps S 104  to S 114 . 
     On the other hand, when a medium does not remain on the medium tray  103 , the control module  151  stops the driving device  131  (step S 115 ) and ends the series of steps. 
     In steps S 104  and S 105 , the control module  151  may set the first reference value and the second reference value to predetermined fixed values. 
     Further, the control module  151  may omit the processing in steps S 109  to S 111  and after stopping feeding of the medium, may notify occurrence of an abnormality to a user by an unillustrated speaker, LED, etc., and end the series of steps. 
       FIG. 8  and  FIG. 9  are flowcharts illustrating an operation example of the folding detection processing. 
     The operation flow described below is executed mainly by the processing circuit  150  in cooperation with each element in the medium conveying apparatus  100 , in accordance with a program previously stored in the storage device  140 . The flowcharts illustrated in  FIG. 8  and  FIG. 9  are executed at predetermined time intervals after feeding of a medium is started in step S 106  in  FIG. 7 . By executing the folding detection processing only after starting feeding of a medium, the folding detection module  152  can prevent a detection error of a folding of a medium caused by light reflected by a hand of a user setting the medium on the medium tray  103 . Further, the front edge of a medium placed on the medium tray  103  is pressed by the pick arm  115  before the folding detection processing is executed, and therefore even when the front edge of a medium is curved (curled), the curved part becomes straight. Accordingly, the folding detection module  152  can prevent a detection error of a folding of a medium caused by a curved front edge of the medium. 
     First, the folding detection module  152  acquires a second medium detection signal from the second medium detection sensor  118  (step S 201 ). 
     Next, the folding detection module  152  determines whether or not a medium exists at a position of the second medium detection sensor  118  based on the acquired second medium detection signal (step S 202 ). 
     When a medium exists at the position of the second medium detection sensor  118 , the folding detection module  152  does not particularly execute processing and ends the series of steps. Thus, when a medium exists at the position of the second medium detection sensor  118 , that is, in a period from a time when the front edge of a medium passes the position of the second medium detection sensor  118  to a time when the rear edge of the medium passes the position of the second medium detection sensor  118 , the folding detection module  152  does not detect a folding of the medium. 
     In a case that the rear edge of a conveyed medium is curved (curled), when a folding of the medium is detected by use of light reflected by the rear edge, a folding may be mistakenly determined to have occurred at the time of feeding. Accordingly, it is preferable that the second light emitters  111   a  and  112   a  be provided to emit the second light toward the central part of a medium before feeding in the medium conveying direction A 1 , the medium being placed on the medium tray  103 . As described above, the folding detection module  152  does not detect a folding of a medium when the front edge of the medium passes the nip position of the feed rollers  116  and the brake rollers  117 , and reaches the position of the second medium detection sensor  118  located on the downstream side of the nip position. Consequently, the folding detection module  152  does not detect a folding of a medium based on light reflected at the rear edge of the medium, and therefore a detection error of a folding of the medium caused by light reflected at the rear edge of a medium in a curved state can be prevented. 
     On the other hand, when a medium does not exist at the position of the second medium detection sensor  118 , the folding detection module  152  acquires a first signal from the first optical sensor  113  (step S 203 ). Specifically, the folding detection module  152  acquires a first signal from the first optical sensor  113  at predetermined time intervals after feeding of a medium, in the folding detection processing. Each timing after feeding of a medium is an example of another timing different from the predetermined timing. 
     Next, the folding detection module  152  calculates a first variation based on a signal value of the acquired first signal and stores the calculated first variation into the storage device  140  (step S 204 ). The folding detection module  152  calculates a subtracted value acquired by subtracting the signal value of the acquired first signal from the first reference value as the first variation. In other words, a first variation indicates a magnitude of a change from the first reference value to a signal value of a latest first signal. 
     Next, the folding detection module  152  determines whether or not the calculated first variation is greater than a first upper limit threshold value (step S 205 ). For example, the first upper limit threshold value is previously set to a value greater than a first variation calculated when a folding of a medium occurs in an experiment of feeding various types of media. 
     When the first variation is greater than the first upper limit threshold value, the folding detection module  152  does not particularly execute processing and ends the series of steps. When a variation of a signal value of the first signal is greater than the first upper limit threshold value, the folding detection module  152  estimates that the first signal is generated based on light reflected by a hand of a user, etc., rather than a medium placed on the medium tray  103  and does not detect a folding of the medium. Consequently, the folding detection module  152  can prevent a detection error of a folding of a medium caused by light reflected by a hand of a user, etc. 
     On the other hand, when the first variation is less than or equal to the first upper limit threshold value, the folding detection module  152  calculates a first period based on each first variation stored in the storage device  140  (step S 206 ). When a latest first variation is greater than or equal to a first lower limit threshold value and less than or equal to the first upper limit threshold value, the folding detection module  152  calculates, as a first period, a period in which the first variation is successively greater than or equal to the first lower limit threshold value and less than or equal to the first upper limit threshold value up to the latest first variation. For example, the first lower limit threshold value is previously set to a value between a first variation calculated when a folding of a medium occurs and a first variation calculated when a folding of a medium does not occur in an experiment of feeding various types of media. On the other hand, when the latest first variation is less than the first lower limit threshold value, the folding detection module  152  sets the first period to 0. 
     Next, the folding detection module  152  determines whether or not the calculated first period is greater than or equal to a first period threshold value (step S 207 ). For example, the first period threshold value is previously set to a value between a first period calculated when a folding of a medium occurs and a first period calculated when a folding of a medium does not occur in an experiment of feeding various types of media. 
     When the first period is greater than or equal to the first period threshold value, the folding detection module  152  determines that a folding of a fed medium is occurring (step S 208 ). Next, the folding detection module  152  sets the folding flag to ON (step S 209 ) and ends the series of steps. 
     On the other hand, when the first period is less than the first period threshold value, the folding detection module  152  acquires a second signal from each of the second optical sensors  111  and  112  (step S 301 ). Specifically, the folding detection module  152  acquires a second signal from each of the second optical sensors  111  and  112  at predetermined time intervals after feeding of a medium, in the folding detection processing. Each timing after feeding of a medium is an example of another timing different from the predetermined timing. 
     Next, the folding detection module  152  calculates a height of the medium placed on the medium tray  103  from the placement surface  103   a  (a height in the direction A 3  perpendicular to the placement surface  103   a ) based on a signal value of each second signal acquired for each of the second optical sensors  111  and  112 . The folding detection module  152  determines whether or not either of the calculated heights is greater than a maximum loading capacity of media on the medium tray  103  supported by the medium conveying apparatus  100  (step S 302 ). For example, the medium conveying apparatus  100  stores, in the storage device  140 , a table associating each signal value of the second signal with a height of media based on a relation between a signal value of the second signal and a height of a medium, the relation being acquired in a previously performed experiment. The folding detection module  152  refers to the stored table and specifies a height of the medium related to the signal value of each of the acquired second signals. 
     When either of the calculated heights is greater than the maximum loading capacity, the folding detection module  152  does not particularly execute processing and ends the series of steps. As described above, the height of the side guide  107  is greater than the maximum loading capacity. Accordingly, when the height of the medium detected based on the second signal is greater than the maximum loading capacity, the folding detection module  152  estimates that the second signal is generated based on light reflected by the side guide  107  rather than the medium and does not detect a folding of the medium based on the second signal. Consequently, the folding detection module  152  can prevent a detection error of a folding of a medium caused by light reflected by the side guide  107 . 
     On the other hand, when both of the calculated heights are less than or equal to the maximum loading capacity, the folding detection module  152  calculates a second variation based on a signal value of each second signal acquired for each of the second optical sensors  111  and  112 , and stores the calculated second variation into the storage device  140  (step S 303 ). The folding detection module  152  calculates a subtracted value acquired by subtracting the signal value of the acquired second signal from the second reference value as the second variation. In other words, a second variation indicates a magnitude of a change from the second reference value to a signal value of a latest second signal. 
     Next, the folding detection module  152  determines whether or not either of the calculated second variations is greater than a second upper limit threshold value (step S 304 ). For example, the second upper limit threshold value is previously set to a value greater than a second variation calculated when a folding of a medium occurs in an experiment of feeding various types of medium. 
     When either of the second variations is greater than the second upper limit threshold value, the folding detection module  152  does not particularly execute processing and ends the series of steps. When a variation of a signal value of the second signal is greater than the second upper limit threshold value, the folding detection module  152  estimates that the second signal is generated based on light reflected by a hand of a user, etc., rather than a medium placed on the medium tray  103  and does not detect a folding of the medium. Consequently, the folding detection module  152  can prevent a detection error of a folding of a medium caused by light reflected by a hand of a user, etc. 
     On the other hand, when both of the second variations are less than or equal to the second upper limit threshold value, the folding detection module  152  calculates a second period based on each second variation stored in the storage device  140  for each of the second optical sensors  111  and  112  (step S 305 ). When a latest second variation is greater than or equal to a second lower limit threshold value and less than or equal to the second upper limit threshold value, the folding detection module  152  calculates, as a second period, a period in which the second variation is successively greater than or equal to the second lower limit threshold value and less than or equal to the second upper limit threshold value up to the latest second variation. For example, the second lower limit threshold value is previously set to a value between a second variation calculated when a folding of a medium occurs and a second variation calculated when a folding of a medium does not occur in an experiment of feeding various types of media. On the other hand, when the latest second variation is less than the second lower limit threshold value, the folding detection module  152  sets the second period to 0. 
     Next, the folding detection module  152  determines whether or not either of the calculated second periods is greater than or equal to a second period threshold value (step S 306 ). For example, the second period threshold value is previously set to a value between a second period calculated when a folding of a medium occurs and a second period calculated when a folding of a medium does not occur in an experiment of feeding various types of media. 
     When either of the second periods is greater than or equal to the second period threshold value, the folding detection module  152  determines that a folding of a fed medium is occurring (step S 307 ). Next, the folding detection module  152  sets the folding flag to ON (step S 308 ) and ends the series of steps. 
     On the other hand, when both of the second periods are less than the second period threshold value, the folding detection module  152  determines that a folding of a fed medium is not occurring (step S 309 ) and ends the series of steps. 
     Thus, the folding detection module  152  detects a folding of a medium based on at least either of a first signal generated by the first optical sensor  113  or second signals generated by the second optical sensors  111  and  112 . Particularly, the folding detection module  152  detects a folding of a medium by comparing a signal value of a first signal at a timing immediately before feeding of the medium with a signal value of the first signal or a signal value of the second signal at each timing after feeding of the medium. 
     The folding detection module  152  may omit the processing in steps S 201  and S 202 , and detect a folding of a medium regardless of whether or not a medium exists at the position of the second medium detection sensor  118 . Alternatively, the folding detection module  152  may acquire a fifth medium detection signal from the fifth medium detection sensor  122  in step S 201  and determine whether or not a medium exists at a position of the fifth medium detection sensor  122  based on the fifth medium detection signal in step S 202 . In that case, when a medium exists at the position of the fifth medium detection sensor  122 , the folding detection module  152  does not detect a folding of the medium. 
     Further, the folding detection module  152  may omit the processing in step S 205  and detect a folding of a medium regardless of whether or not a first variation is greater than the first upper limit threshold value. Similarly, the folding detection module  152  may omit the processing in step S 304  and detect a folding of a medium regardless of whether or not a second variation is greater than the second upper limit threshold value. Further, the folding detection module  152  may omit the processing in step S 302  and detect a folding of a medium regardless of whether or not a calculated height is greater than the maximum loading capacity. 
       FIG. 10  and  FIG. 11  are schematic diagrams for illustrating a technical meaning of detecting a folding of a medium. 
       FIG. 10  is a schematic diagram for illustrating a case of a passport  1000  in an opened state being fed in such a way that a seam part  1001  of the passport  1000  is perpendicular to the medium conveying direction A 1 . The upper-left diagram in  FIG. 10  is a schematic diagram of the fed passport  1000  viewed from above the medium conveying apparatus  100 . The upper-right diagram in  FIG. 10  is a schematic diagram of the fed passport  1000  viewed from the side of the medium conveying apparatus  100 . The lower-left diagram in  FIG. 10  is a schematic diagram of the fed passport  1000  viewed from the downstream side in the medium conveying direction A 1 . 
     As illustrated in  FIG. 10 , when the passport  1000  in an opened state is fed in such a way that the seam part  1001  of the passport  1000  is perpendicular to the medium conveying direction A 1 , pages on the front edge side of the passport  1000  are separated by the feed rollers  116  and the brake rollers  117 . Specifically, out of pages of the passport  1000  on the front edge side, a page  1002  in contact with the feed rollers  116  and another page  1003  are separated. Accordingly, when the passport  1000  is fed, the front edge  1004  of the page  1003  not in contact with the feed rollers  116  remain at the position of the brake rollers  117  and the seam part  1001  moves with the page  1002  in contact with the feed rollers  116 . Consequently, the central part  1005  of the page  1003  in the medium conveying direction A 1 , the page  1003  not being in contact with the feed rollers  116 , bends upward. 
     The first light emitter  113   a  in the first optical sensor  113  emits the first light toward the downstream side of a medium placed on the medium tray  103  in the medium conveying direction A 1 , and therefore the first light emitted by the first light emitter  113   a  irradiates the central part  1005  and is reflected by the central part  1005 . By the central part  1005  folding upward, a distance between the central part  1005  and the first optical sensor  113  is shortened. Consequently, as the central part  1005  bends more upward, a time taken by the first light emitted by the first light emitter  113   a  to be reflected by the central part  1005  and be received by the first light receiver  113   b  becomes shorter. Accordingly, the folding detection module  152  can detect a folding of a medium fed in such a way that a seam part  1001  is perpendicular to the medium conveying direction A 1 , in a short period and with high precision, based on a first signal generated by the first optical sensor  113 . 
       FIG. 11  is a schematic diagram for illustrating a case of a brochure  1100  in a closed state being fed in such a way that a seam part  1101  of the brochure  1100  is parallel with the medium conveying direction A 1 . The upper-left diagram in  FIG. 11  is a schematic diagram of the fed brochure  1100  viewed from above the medium conveying apparatus  100 . The upper-right diagram in  FIG. 11  is a schematic diagram of the fed brochure  1100  viewed from the side of the medium conveying apparatus  100 . The lower-left diagram in  FIG. 11  is a schematic diagram of the fed brochure  1100  viewed from the downstream side in the medium conveying direction A 1 . 
     As illustrated in  FIG. 11 , when the brochure  1100  in a closed state is fed in such a way that the seam part  1101  of the brochure  1100  is parallel with the medium conveying direction A 1 , pages of the brochure  1100  are separated by the feed rollers  116  and the brake rollers  117 . Specifically, out of pages of the brochure  1100 , a page  1102  in contact with the feed rollers  116  and another page  1103  are separated. Accordingly, when the brochure  1100  is fed, the front edge  1104  of the page  1103  not in contact with the feed rollers  116  remains at the position of the brake rollers  117 , and the seam part  1101  moves with the page  1102  in contact with the feed rollers  116 . Consequently, an end  1105  facing the seam part  1101  on the page  1103  not in contact with the feed rollers  116  bends upward with a central part  1106  in the medium conveying direction A 1  at the center. When paper folded in two is fed in such a way that a crease part of the paper is parallel with the medium conveying direction A 1 , the paper bends similarly to the brochure  1100 . 
     The second light emitters  111   a  and  112   a  are located outside in the direction A 2  perpendicular to the medium conveying direction and emit the second light toward the upstream side of a medium placed on the medium tray  103  in the medium conveying direction A 1 . Accordingly, the second light emitted by each of the second light emitters  111   a  and  112   a  irradiates the central part  1106  of the end  1105  and is reflected by the central part  1106 . By the central part  1106  folding upward, a distance between the central part  1106  and each of the second optical sensors  111  and  112  is shortened. Consequently, as the central part  1106  bends more upward, a time taken by the second light emitted by each of the second light emitters  111   a  and  112   a  to be reflected by the central part  1106  and be received by each of the second light receivers  111   b  and  112   b  becomes shorter. Accordingly, the folding detection module  152  can detect a folding of a medium fed in such a way that a seam part or a crease part is parallel with the medium conveying direction A 1 , in a short period and with high precision, based on second signals generated by the second optical sensors  111  and  112 . 
       FIG. 12A  and  FIG. 12B  are schematic diagrams for illustrating changes in an amount of folding of a medium over time, the medium being fed in such a way that a seam part or a crease part is parallel with the medium conveying direction A 1 . 
     A graph  1200  in  FIG. 12A  illustrates an amount of folding of a medium viewed from the downstream side in the medium conveying direction A 1  in the medium conveying apparatus  100 . In  FIG. 12A , the horizontal axis represents each position in the direction A 2  perpendicular to the medium conveying direction, and the vertical axis represents an amount of folding of the medium at each position. The left end in  FIG. 12A  represents a position of the seam part or the crease part of the medium. A graph  1201  illustrates an amount of folding at a time T 1 , a graph  1202  illustrates an amount of folding at a time T 2  after an elapse of a predetermined time from the time T 1 , and a graph  1203  illustrates an amount of folding at a time T 3  after a further elapse of a predetermined time from the time T 2 . 
     A graph  1210  in  FIG. 12B  illustrates an amount of folding of the medium viewed from side in the medium conveying apparatus  100 . In  FIG. 12B , the horizontal axis represents each position in the medium conveying direction A 1 , and the vertical axis represents an amount of folding of the medium at each position. In  FIG. 12B , the left side represents the downstream side, the right side represents the upstream side, and the left end represents the nip position of the feed rollers  116  and the brake rollers  117 . A graph  1211  illustrates an amount of folding at the time T 1 , a graph  1212  illustrates an amount of folding at the time T 2 , and a graph  1213  illustrates an amount of folding at the time T 3 . 
     When a medium is fed in such a way that the seam part or the crease part is parallel with the medium conveying direction A 1 , an amount of folding at a position closer to an end facing the seam part or the crease part is larger, as illustrated in the graph  1201  in  FIG. 12A . Further, in that case, an amount of folding at the end on the opposite side of the seam part or the crease part increases as time elapses, as illustrated in the graphs  1201  to  1203 . Further, in that case, an amount of folding at a position closer to a central part in the medium conveying direction A 1  is larger, as illustrated in the graph  1211  in  FIG. 12B , and an amount of folding on the upstream side in the medium conveying direction A 1  increases as time elapses, as illustrated in the graphs  1211  to  1213 . 
     As described above, the second light emitters  111   a  and  112   a  are located outside in the direction A 2  perpendicular to the medium conveying direction and emit the second light toward the upstream side of a medium placed on the medium tray  103  in the medium conveying direction A 1 . Accordingly, the folding detection module  152  can detect a folding of a medium fed in such a way that a seam part or a crease part is parallel with the medium conveying direction A 1 , in a short period and with high precision, based on second signals generated by the second optical sensors  111  and  112 . Furthermore, by the second light emitters  111   a  and  112   a  being provided to emit the second light toward a central part of a medium placed on the medium tray  103  in the medium conveying direction A 1 , the folding detection module  152  can detect a folding of the medium in a shorter period. 
       FIG. 13A  and  FIG. 13B  are schematic diagrams for illustrating an amount of folding when another medium is loaded on a medium on which a folding is occurring.  FIG. 13A  is a schematic diagram for illustrating an amount of folding of a medium fed in such a way that a seam part or a crease part is parallel with the medium conveying direction A 1 , and  FIG. 13B  is a schematic diagram for illustrating an amount of folding of a medium fed in such a way that a seam part is perpendicular to the medium conveying direction A 1 . 
     Each of a graph  1300  in  FIG. 13A  and a graph  1310  in  FIG. 13B  illustrates an amount of folding of a medium viewed from the side of the medium conveying apparatus  100 . In each of  FIG. 13A  and  FIG. 13B , the horizontal axis represents each position in the medium conveying direction A 1 , and the vertical axis represents an amount of folding of a medium at each position. In each of  FIG. 13A  and  FIG. 13B , the left side represents the downstream side, the right side represents the upstream side, and the left end represents the nip position of the feed rollers  116  and the brake rollers  117 . Each of the graph  1301  and graph  1311  illustrates an amount of folding when another medium is not loaded on a bound medium. On the other hand, each of a graph  1302  and a graph  1312  illustrates an amount of folding when 10 sheets of PPC paper are loaded on the bound medium, and each of a graph  1303  and a graph  1313  illustrates an amount of folding when 50 sheets of PPC paper are loaded on the bound medium. 
     As illustrated in the graphs  1302  and  1303  in  FIG. 13A , when another medium is placed on a medium fed in such a way that a seam part or a crease part is parallel with the medium conveying direction A 1 , the folding medium is pushed down by the other medium being placed, and an amount of folding decreases. However, even in that case, an amount of folding on the upstream side in the medium conveying direction A 1  is sufficiently large. Accordingly, even when another medium is placed on a folding medium, the folding detection module  152  can detect the folding of the medium with high precision based on second signals generated by the second optical sensors  111  and  112 . 
     Similarly, as illustrated in the graphs  1312  and  1313  in  FIG. 13B , when another medium is placed on a medium fed in such a way that a seam part is perpendicular to the medium conveying direction A 1 , the folding medium is pushed down by the other medium being placed, and an amount of folding decreases. However, even in that case, an amount of folding on the downstream side in the medium conveying direction A 1  is sufficiently large. Accordingly, even when another medium is placed on a folding medium, the folding detection module  152  can detect the folding of the medium with high precision based on a first signal generated by the first optical sensor  113 . 
     Accordingly, in the so-called bottom-first type medium conveying apparatus  100  sequentially feeding media placed on the medium tray  103  from the lowermost side, the folding detection module  152  can detect a folding occurring on the lowest placed medium out of the loaded media, with high precision. 
       FIG. 14A  is a schematic diagram for illustrating a signal value of each signal when a medium is fed in such a way that a seam part or a crease part is parallel with the medium conveying direction A 1 . 
     In  FIG. 14A , the horizontal axis represents time, and the vertical axis represents a signal value. Graph  1401  illustrates a signal value of a first signal generated by the first optical sensor  113 . A graph  1402  illustrates a signal value of a second signal generated by the second optical sensor  111  located on a seam part or crease part side, and a graph  1403  represents a signal value of a second signal generated by the second optical sensor  112  located on the opposite side of the seam part or the crease part. 
     As illustrated in  FIG. 14A , when a medium is fed in such a way that a seam part or a crease part is parallel with the medium conveying direction A 1 , a variation in a signal value of the first signal with respect to a first reference value R 1  is small and is less than B 1 . Accordingly, the folding detection module  152  cannot detect a folding of the medium based on the first signal. On the other hand, a variation of a signal value of each second signal with respect to a second reference value R 2  is large and is greater than or equal to a second lower limit threshold value B 2 . Particularly, a signal value of the second signal generated by the second optical sensor  112  located on the opposite side of the seam part or the crease part changes in a short period. A variation of a signal value of the second signal becomes greater than or equal to the second lower limit threshold value B 2  at a time T 4  and is always greater than or equal to the second lower limit threshold value B 2  until a time T 5  at which a second period threshold value C 2  elapses after the time T 4 . Accordingly, the folding detection module  152  can detect a folding of the medium based on the second signal generated by the second optical sensor  112  located on the opposite side of the seam part or the crease part. 
       FIG. 14B  is a schematic diagram for illustrating a signal value of each signal when a large number of unbound media are successively fed. 
     In  FIG. 14B , the horizontal axis represents time, and the vertical axis represents a signal value. A graph  1411  illustrates a signal value of a first signal generated by the first optical sensor  113 , and graphs  1412  and  1413  illustrate signal values of second signals generated by the second optical sensors  111  and  112 . As illustrated in  FIG. 14B , when a large number of unbound media are successively fed, every time a medium is fed one by one, an entire height of the media decreases. Accordingly, a distance between the highest position of the media and each of the first optical sensor  113  and the second optical sensors  111  and  112  gradually increases, and a signal value of each signal gradually increases. However, a variation of each signal for each fed medium is sufficiently small, and a folding of a medium is not mistakenly detected. 
       FIG. 15A  is a schematic diagram for illustrating a signal value of each signal in a case of, while a large number of unbound media are successively fed, another medium is loaded (replenished) on the media by a user. 
     In  FIG. 15A , the horizontal axis represents time, and the vertical axis represents a signal value. A graph  1501  illustrates a signal value of a first signal generated by the first optical sensor  113 . On the other hand, graphs  1502  and  1503  illustrate signal values of second signals generated by the second optical sensors  111  and  112 . In the example illustrated in  FIG. 15A , a medium is replenished by the user at a time T 6 . At the time T 6 , a hand of the user enters between the medium, and the first optical sensor  113  and second optical sensors  111  and  112 ; and a signal value of each signal momentarily and considerably changes. However, the considerable change in a signal value is for a short period, and the hand of the user entering between the medium, and the first optical sensor  113  and the second optical sensors  111  and  112  is not mistakenly detected as a folding of a medium. 
       FIG. 15B  is a schematic diagram for illustrating a signal value of each signal when a wrinkled medium is fed. 
     In  FIG. 15B , the horizontal axis represents time, and the vertical axis represents a signal value. A graph  1511  illustrates a signal value of a first signal generated by the first optical sensor  113 , and graphs  1512  and  1513  illustrate signal values of second signals generated by the second optical sensors  111  and  112 , respectively. In the example illustrated in  FIG. 15B , the second light is emitted to a wrinkle part of the medium by the second optical sensor  112  from a time T 7  to a time T 8 . Consequently, a signal value of the second signal generated by the second optical sensor  112  changes from the time T 7  to the time T 8 . However, a variation of a signal value is sufficiently less than a second lower limit threshold value and also a period in which the signal value changes is sufficiently less than a second period threshold value; and therefore the wrinkle formed on the medium is not mistakenly detected as a folding of the medium. 
     As described in detail above, while separating and feeding media, the medium conveying apparatus  100  detects a folding of a medium almost directly below by the first optical sensor  113  located in a central part and detects a folding of a medium at the rear end side by the second optical sensors  111  and  112  located outside. Then, the medium conveying apparatus  100  stops feeding of a medium depending on a detection result of a folding of the medium. Consequently, while separating and feeding media, the medium conveying apparatus  100  can detect a folding of a fed medium with high precision and when a medium not to be separated is fed, stop feeding of the medium with higher precision. 
       FIG. 16  is a flowchart illustrating an operation example of first sensitivity change processing according to another embodiment. 
     An operation flow described below is executed mainly by the processing circuit  150  in cooperation with each element in the medium conveying apparatus  100 , in accordance with a program previously stored in the storage device  140 . The flowchart illustrated in  FIG. 16  is periodically executed. Alternatively, the flowchart illustrated in  FIG. 16  may be executed between the processing in step S 202  and the processing in step S 203  in  FIG. 8 . 
     First, the loaded amount detection module  154  acquires a first signal from the first optical sensor  113  (step S 401 ). 
     Next, the loaded amount detection module  154  detects a loaded amount of media on the medium tray  103 , that is, a height of the media placed on the medium tray  103  from the placement surface  103   a  (a height in the direction A 3  perpendicular to the placement surface  103   a ) based on the acquired first signal (step S 402 ). For example, the medium conveying apparatus  100  stores, in the storage device  140 , a table associating each signal value of the first signal with a loaded amount of media based on a relation between a signal value of the first signal and a loaded amount of media, the relation being acquired in a previously performed experiment. The folding detection module  152  refers to the stored table and specifies a loaded amount of media related to a signal value of the acquired first signal. 
     Next, the folding detection module  152  changes a detection sensitivity for detecting a folding of a medium based on the calculated loaded amount (step S 403 ) and ends the series of steps. As a loaded amount increases, a fed medium is more firmly pressed by media loaded on the medium, and therefore an amount of folding of the medium decreases. Accordingly, as a loaded amount increases, the folding detection module  152  increases the detection sensitivity in order to facilitate detection of a folding of a medium. For example, the folding detection module  152  increases the detection sensitivity by changing the first lower limit threshold value and the second lower limit threshold value in such a way as to decrease each threshold value as a loaded amount increases. Further, the folding detection module  152  may increase the detection sensitivity by changing the first period threshold value and the second period threshold value in such a way as to decrease each threshold value as a loaded amount increases. Consequently, even when a large number of media are loaded on the medium tray  103 , the folding detection module  152  can detect a folding of a medium with high precision. 
       FIG. 17  is a flowchart illustrating an operation example of second sensitivity change processing according to another embodiment. 
     The operation flow described below is executed mainly by the processing circuit  150  in cooperation with each element in the medium conveying apparatus  100 , in accordance with a program previously stored in the storage device  140 . The operation flow illustrated in  FIG. 16  is periodically executed. When the flowchart illustrated in  FIG. 17  is executed, the processing in steps S 201  and S 202  in  FIG. 8  is omitted. Alternatively, in this case, whether or not a medium exists at the position of the fifth medium detection sensor  122  is determined based on a fifth medium detection signal from the fifth medium detection sensor  122  in step S 202 . The second sensitivity change processing is executed in place of the first sensitivity change processing or in addition to the first sensitivity change processing. 
     First, the multi-feed detection module  155  acquires an ultrasonic signal from the ultrasonic sensor  119  (step S 501 ). 
     Next, the multi-feed detection module  155  determines whether or not a signal value of the acquired ultrasonic signal is less than a multi-feed determination threshold value (step S 502 ). 
       FIG. 18  is a schematic diagram for illustrating a characteristic of an ultrasonic signal. 
     In a graph  1800  in  FIG. 18 , a solid line  1801  represents a characteristic of an ultrasonic signal when one sheet of paper is conveyed as a medium, and a dotted line  1802  represents a characteristic of an ultrasonic signal when multi-feed of paper is occurring. The horizontal axis of the graph  1800  indicates time, and the vertical axis indicates a signal value of an ultrasonic signal. Due to occurrence of multi-feed, a signal value of the ultrasonic signal in the dotted line  1802  declines in a section  1803 . The multi-feed determination threshold value is set to a value between a signal value S 1  of an ultrasonic signal when one sheet of paper is conveyed and a signal value S 2  of an ultrasonic signal when multi-feed of paper is occurring. By determining whether or not a signal value of an ultrasonic signal is less than the multi-feed determination threshold value, the multi-feed detection module  155  can determine whether or not multi-feed of a medium is occurring. 
     When a signal value of the ultrasonic signal is greater than or equal to the multi-feed determination threshold value, the multi-feed detection module  155  determines that multi-feed is not occurring (step S 503 ) and ends the series of steps. 
     On the other hand, when a signal value of the ultrasonic signal is less than the multi-feed determination threshold value, the multi-feed detection module  155  determines that multi-feed is occurring (step S 504 ). Thus, the multi-feed detection module  155  detects multi-feed of a medium based on an ultrasonic signal generated by the ultrasonic sensor  119 . 
     Next, the folding detection module  152  changes a detection sensitivity to detect a folding of a medium based on the detection result of multi-feed of the medium (step S 505 ) and ends the series of steps. 
     When multi-feed of media is occurring, it is highly likely that a medium folded in two or a bound medium is fed. Accordingly, the folding detection module  152  facilitates detection of a folding of a medium by setting a detection sensitivity in a case of multi-feed of media being detected higher than a detection sensitivity in a case of multi-feed of media not being detected. For example, the folding detection module  152  increases the detection sensitivity by changing the first lower limit threshold value and the second lower limit threshold value in such a way that each threshold value in a case of multi-feed of media being detected is less than each threshold value in a case of multi-feed of media not being detected, respectively. Further, the folding detection module  152  may increase the detection sensitivity by changing the first period threshold value and the second period threshold value in such a way that each threshold value in a case of multi-feed of media being detected is less than each threshold value in a case of multi-feed of media not being detected, respectively. Consequently, the folding detection module  152  can detect a folding of a medium with higher precision. 
       FIG. 19  is a flowchart illustrating an operation example of third sensitivity change processing according to another embodiment. 
     The operation flow described below is executed mainly by the processing circuit  150  in cooperation with each element in the medium conveying apparatus  100 , in accordance with a program previously stored in the storage device  140 . The operation flow illustrated in  FIG. 19  is periodically executed. When the flowchart illustrated in  FIG. 19  is executed, the processing in steps S 201  and S 202  in  FIG. 8  is omitted. Alternatively, in this case, whether or not a medium exists at the position of the fifth medium detection sensor  122  is determined based on a fifth medium detection signal from the fifth medium detection sensor  122  in step S 202 . The third sensitivity change processing is executed in place of the first sensitivity change processing or the second sensitivity change processing, or in addition to the first sensitivity change processing and the second sensitivity change processing. 
     First, the skew detection module  156  acquires a third medium detection signal from the third medium detection sensor  120  and acquires a fourth medium detection signal from the fourth medium detection sensor  121  (step S 601 ). 
     Next, the skew detection module  156  calculates a passage time difference between a time when the front edge of a medium passes the third medium detection sensor  120  and a time when the front edge passes the fourth medium detection sensor  121 , based on the acquired third medium detection signal and fourth medium detection signal (step S 602 ). 
     The skew detection module  156  detects a time when a signal value of the third medium detection signal out of signal values of the third medium detection signal acquired up to the present time changes from a value indicating a state in which a medium does not exist to a value indicating a state in which a medium exists, as a time when the front edge of the medium passes the third medium detection sensor  120 . Similarly, the skew detection module  156  detects a time when a signal value of the fourth medium detection signal out of signal values of the fourth medium detection signal acquired up to the present time changes from a value indicating a state in which a medium does not exist to a value indicating a state in which a medium exists, as a time when the front edge of the medium passes the fourth medium detection sensor  121 . The skew detection module  156  calculates, as a passage time difference, a time period from a time when the front edge of the medium passes either of the third medium detection sensor  120  or the fourth medium detection sensor  121  to a time when the front edge passes the other sensor. When the front edge of the medium has not yet passed one sensor, the skew detection module  156  calculates a time period from a time when the front edge of the medium passes the other sensor to the present time, as a passage time difference. 
     Next, the skew detection module  156  determines whether or not the calculated passage time difference is less than a skew determination threshold value (step S 603 ). 
     When the calculated passage time difference is less than the skew determination threshold value, the skew detection module  156  determines that a skew being an oblique movement of a medium is not occurring (step S 604 ) and ends the series of steps. 
     On the other hand, when the calculated passage time difference is greater than or equal to the skew determination threshold value, the skew detection module  156  determines that a skew is occurring (step S 605 ). Thus, the skew detection module  156  detects a skew of a medium based on a third medium detection signal generated by the third medium detection sensor  120  and a fourth medium detection signal generated by the fourth medium detection sensor  121 . 
     Next, the folding detection module  152  changes the detection sensitivity to detect a folding of a medium based on the detection result of a skew of the medium (step S 606 ) and ends the series of steps. 
     In a case of a medium folded in two or a bound medium being fed, it is highly likely that a skew of the medium occurs when the front edge of the medium passes the nip position of the feed rollers  116  and the brake rollers  117 . Accordingly, the folding detection module  152  facilitates detection of a folding of a medium by setting a detection sensitivity in a case of a skew of a medium being detected higher than a detection sensitivity in a case of a skew of a medium not being detected. For example, the folding detection module  152  increases the detection sensitivity by changing the first lower limit threshold value and the second lower limit threshold value in such a way that each threshold value in a case of a skew of a medium being detected is less than each threshold value in a case of a skew of a medium not being detected, respectively. Further, the folding detection module  152  may increase the detection sensitivity by changing the first period threshold value and the second period threshold value in such a way that each threshold value in a case of a skew of a medium being detected is less than each threshold value in a case of a skew of a medium not being detected, respectively. Consequently, the folding detection module  152  can detect a folding of a medium with higher precision. 
     The detection sensitivity may be changed by a user. In that case, the folding detection module  152  accepts setting of the detection sensitivity (the first lower limit threshold value, the second lower limit threshold value, the first period threshold value and/or the second period threshold value) using the operation device  105  by the user. The user may increase the detection sensitivity when a soft medium being less likely to bend, such as thin paper, is conveyed and decrease the detection sensitivity when a medium being more likely to be mistakenly determined to be folding, such as wrinkled paper, is conveyed. Consequently, the folding detection module  152  can detect a folding of a medium with higher precision. 
     As described in detail above, by operating in accordance with the flowcharts illustrated in  FIG. 16 ,  FIG. 17  and/or  FIG. 19 , the medium conveying apparatus  100  can detect a folding of a medium with higher precision. 
       FIG. 20  is a diagram illustrating a schematic configuration of a processing circuit  260  in a medium conveying apparatus according to yet another embodiment. The processing circuit  260  is used in place of the processing circuit  150  in the medium conveying apparatus  100  and executes the medium reading processing, the folding detection processing, the first sensitivity change processing, the second sensitivity change processing and the third sensitivity change processing in place of the processing circuit  150 . The processing circuit  260  includes a control circuit  261 , a folding detection circuit  262 , an image acquisition circuit  263 , a loaded amount detection circuit  264 , a multi-feed detection circuit  265 , and the skew detection circuit  266 , etc. 
     The control circuit  261  is an example of a control module and has a function similar to the control module  151 . The control circuit  261  receives an operation signal from an operation device  105 , a first medium detection signal from a first medium detection sensor  114 , and a detection result of a folding of a medium from the folding detection circuit  262 . The control circuit  261  drives a driving device  131  based on each of the received signals and also when a folding of a medium is detected, stops feeding of the medium. 
     The folding detection circuit  262  is an example of a folding detection module and has a function similar to the folding detection module  152 . The folding detection circuit  262  receives a first signal from a first optical sensor  113 , second signals from second optical sensors  111  and  112 , and a second medium detection signal from a second medium detection sensor  118 . Further, the folding detection circuit  262  receives a detection result of a loaded amount of media from the loaded amount detection circuit  264 , a detection result of multi-feed of media from the multi-feed detection circuit  265 , and a detection result of a skew of a medium from the skew detection circuit  266 . The folding detection circuit  262  detects a folding of a medium based on each piece of received information and outputs the detection result to the control circuit  261 . 
     The image acquisition circuit  263  is an example of an image acquisition module and has a function similar to the image acquisition module  153 . The image acquisition circuit  263  receives an input image from an imaging device  125  and stores the input image into a storage device  140 , and also transmits the input image to an unillustrated information processing device through an interface device  132 . 
     The loaded amount detection circuit  264  is an example of a loaded amount detection module and has a function similar to the loaded amount detection module  154 . The loaded amount detection circuit  264  receives a first signal from the first optical sensor  113 , detects a loaded amount of media based on the first signal, and outputs the detection result to the folding detection circuit  262 . 
     The multi-feed detection circuit  265  is an example of a multi-feed detection module and has a function similar to the multi-feed detection module  155 . The multi-feed detection circuit  265  receives an ultrasonic signal from an ultrasonic sensor  119 , detects multi-feed of media based on the ultrasonic signal, and outputs the detection result to the folding detection circuit  262 . 
     The skew detection circuit  266  is an example of a skew detection module and has a function similar to the skew detection module  153 . The skew detection circuit  266  receives a third medium detection signal from a third medium detection sensor  115 , a fourth medium detection signal I from a fourth medium detection sensor  121 . The skew detection circuit  274  detects a skew of a medium based on each received signal and outputs the detection result to the folding detection circuit  262 . 
     As described in detail above, even when using the processing circuit  260 , the medium conveying apparatus, can stop feeding of the medium with higher precision when a medium not to be separated is fed. 
     Each part included in the processing circuit may be independently configured with an integrated circuit, a microprocessor, firmware, etc. Further, some parts included in the processing circuit may be configured with a circuit, and other parts may be configured with a functional module implemented by software operating on a processor. 
     According to this embodiment, the medium conveying apparatus, the method, and the computer-readable, non-transitory medium storing the control program, according to the present invention, can, while separating and feeding media, stop feeding of media with higher precision when a medium not to be separated is fed. 
     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.