Abstract:
An image reading apparatus includes a reading unit, a conveyance unit, a carriage, a motor, a motor gear, a conveyance gear, a carriage gear, a switching gear, and a control device. The conveyance unit is configured to convey a document from a document tray. The switching gear is configured to be placed in either one of a first position where the motor gear is communicated with the conveyance gear and a second position where the motor gear is communicated with the carriage gear. The control device is configured to: control the motor to rotate the motor gear in a first direction to have the switching gear free from the conveyance gear in response to a determination that the document jam has occurred; and control the motor to rotate the motor gear in a second direction to have the switching gear communicate with the conveyance gear.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority from Japanese Patent Application No. 2013-016406 filed on Jan. 31, 2013. The entire content of the priority application is incorporated herein by reference. 
     TECHNICAL FIELD 
     The present invention relates to an image reading apparatus. 
     BACKGROUND 
     An image reading apparatus described in Japanese Patent Application Publication No. 2006-86817 is well-known in the art. The conventional image reading apparatus includes a reading unit such as a CIS for reading a document, a conveyance unit for conveying a document to the reading unit, a carriage for moving the reading unit, a single motor for driving the moving unit and the conveyance unit, and a switching gear serving as a transmission unit for transmitting drive power of the motor. The switching gear switches its position between a conveyance position at which the transmission unit transmits the power to the conveyance unit and a carriage position at which the transmission unit transmits the power to the moving unit, whereby the conveyance unit and the moving unit can be driven by a single motor. 
     SUMMARY 
     Upon a jam occurrence in the conveyance unit, a user can easily clear the jam in the conveyance unit by moving the switching gear away from the conveyance position in a configuration in which both the transfer unit and the conveyance unit are driven by one motor, that is, in a configuration in which a position of the switching gear is switched between the carriage position and the conveyance position. However, an appropriate position for retaining the switching gear after clearing the jam has not yet been discussed in the configuration. 
     In view of the foregoing, it is an object of the present invention to provide an image reading apparatus capable of retaining the switching gear at an appropriate position after clearing the jam in a configuration in which the position of the switching gear is switched between the carriage position and the conveyance position. 
     In order to attain the above and other objects, the invention provides an image reading apparatus. The image reading apparatus includes a reading unit, a conveyance unit, a carriage, a motor, a motor gear, a conveyance gear, a carriage gear, a switching gear, and a control device. The conveyance unit is configured to convey a document from a document tray. The carriage is configured to support the reading unit. The motor gear is coupled to the motor. The conveyance gear is configured to communicate with the conveyance unit. The carriage gear is configured to communicate with the carriage. The switching gear is configured to be placed in either one of a first position where the motor gear is communicated with the conveyance gear via the switching gear and a second position where the motor gear is communicated with the carriage gear via the switching gear. The control device configured to: determine whether to occur a document jam; control the motor to rotate the motor gear in a first direction to have the switching gear free from the conveyance gear in response to a determination that the document jam has occurred; determine whether the document jam is cleared; determine whether the document is present on the document tray; and control the motor to rotate the motor gear in a second direction different from the first direction to have the switching gear communicate with the conveyance gear in response to a determination that the document is present on the document tray when determining that the document jam is cleared. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The particular features and advantages of the invention as well as other objects will become apparent from the following description taken in connection with the accompanying drawings, in which: 
         FIG. 1  is an enlarged schematic cross-sectional view of an image reading apparatus according to a preferred embodiment of the present invention; 
         FIG. 2  is a schematic perspective view of the image reading apparatus in a state where a document cover is closed according to the preferred embodiment of the present invention 
         FIG. 3  is a schematic perspective view of the image reading apparatus in a state where the document cover is open according to the preferred embodiment of the present invention; 
         FIG. 4  is a schematic view of a drive transmission mechanism of the image reading apparatus in a state where a planetary gear is positioned at a FB position according to the preferred embodiment of the present invention; 
         FIG. 5  is a schematic view of the drive transmission mechanism in a state where the planetary gear is positioned at an ADF position to the preferred embodiment of the present invention; 
         FIG. 6  is a block diagram illustrating an electric configuration of the image reading apparatus according to the preferred embodiment of the present invention; 
         FIG. 7  is a flowchart illustrating an ADF reading process of the image reading apparatus according to a first embodiment of the present invention; 
         FIG. 8  is a flowchart illustrating an ADF reading process of an image reading apparatus according to a second embodiment of the present invention; 
         FIG. 9  is a flowchart illustrating a document determination process  1  according to the first embodiment of the present invention; 
         FIG. 10  is a flowchart illustrating a document determination process  2  according to the second embodiment of the present invention; 
         FIG. 11  is a flowchart illustrating a restart prompting process  1  according to the second embodiment of the present invention; and 
         FIG. 12  is a flowchart illustrating a restart prompting process  2  according to the second embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     An embodiment of the present invention will be described with reference to the drawings. 
     [External Configuration of Image Reading Apparatus] 
     The term “top” of an image reading apparatus  1  is defined as top side of the  FIG. 1 , the term “left” as left side of  FIG. 1 , and the term “front” as front side of  FIG. 1 . As shown in  FIGS. 1 to 3 , the image reading apparatus  1  includes a reading unit  7 , a carriage  8 , a moving mechanism  9  for conveying the carriage  8 , an Auto Document Feeder  40  (hereinafter abbreviated to “ADF”), and a document platen  3 . The image reading apparatus  1  has a front side portion provided with an operation unit  11  and a display unit  12  such as a liquid crystal display. The operation unit  11  is provided with a read start key  11 A, a power key  11 B, and a clear reception key  11 C. 
     The image reading apparatus  1  has, as an image reading mode, a carriage-moving mode (hereinafter, referred to as “FB (Flat-Bed) reading mode, example of a second mode”) in which the reading unit  7  reads an image of a document conveyed by the carriage  8  and a sheet-conveying mode (hereinafter, referred to as “ADF reading mode”, example of a first mode) in which the reading unit  7  reads an image of a document conveyed by a conveying unit  44  (see  FIG. 1 ) of the ADF  40 . The image reading apparatus  1  may be a stand-alone scanner or copier, or a part of so-called a multifunctional device having printer and facsimile functions. 
     As shown in  FIG. 3 , the document platen  3  has a platen  3 B such as a glass or acrylic transparent as a first reading window. The platen  3 B defines a placement surface  3 A on which a document is placed. A document cover  5  is assembled immediately above the document platen  3  via a hinge mechanism  5 A. 
     The document cover  5  is pivotally movable between a closed position ( FIG. 2 ) covering the document platen  3  and a remote position ( FIG. 3 ) away from the document platen  3 . In the FB reading mode, a user needs to manually move the document cover  5  upward and place a document on the document platen  3 . 
     As shown in  FIG. 3 , the image reading apparatus  1  further includes a drive transmission mechanism  13  and a load generation unit  25 . The load generation unit  25  includes a pair of first contacted portions  25 A provided in the reading unit  7  and a pair of first stoppers  25 B provided in the document platen  3  each corresponding to the pair of first contacted portions  25 A. 
     The reading unit  7  moving along the placement surface  3 A is provided immediately below the placement surface  3 A (see  FIG. 1 ). The reading unit  7  emits the light to a document and receives emitted light reflected from the document and generates an electric signal based on the received light. The image reading apparatus  1  converts characters and the like written on the document into the electric signal generated in the reading unit  7 . 
     The reading unit  7  is adapted to read the document with a CIS (Contact Image Sensor) system. The reading unit  7  includes a linear image sensor  7 C having a plurality of light receiving elements, a light source  7 A constituted by RGB three-color light emitting diodes (LEDs), and a rod lens array  7 B that focuses light reflected from the document onto the light receiving elements of the linear image sensor  7 C, and those are linearly arranged in a direction perpendicular to a paper surface, i.e. front-to-rear direction. 
     The carriage  8  for supporting the reading unit  7  is coupled to a toothed belt  9 A described later and moves in a conveyance direction, i.e., an arrow A and arrow B as depicted in  FIG. 3 , with movement of the toothed belt  9 A. The toothed belt  9 A is driven by a drive power transmission mechanism to be described later through a toothed pulley  9 B (see  FIG. 4 ). The image reading system of the reading unit  7  is not limited to the CIS system, and the reading unit  7  may adopt so-called a CCD system using an optical reduction system and a CCD (Charge-Coupled Device) image sensor. 
     When reading the document placed on the platen  3 B, that is, in the FB reading mode, the reading unit  7  reads the document while being conveyed, at a constant speed, by the carriage  8  coupled to the toothed belt  9 A in the conveyance direction (direction of the arrow A of  FIG. 1 ) parallel to a plate surface of the platen  3 B from a waiting position WP. The reading unit  7  can read the document within a read range in the conveyance direction defined between a read start position PS and a read end position PE (see  FIG. 1 ). In the present embodiment, the read start position PS is fixed irrespective of a read range of the document, and the read end position PE is changed according to the read range of the document. 
     On the other hand, when reading the document conveyed by the ADF  40 , that is, in the ADF reading mode, the image reading unit  7  reads the document while being retained, by the carriage  8 , at a conveyance reading position (hereinafter, referred to as “ADF reading position”) RP immediately below a second reading window  3 C. 
     As shown in  FIG. 3 , a document pressing member  46  is provided at a position immediately above the second reading window  3 C, i.e., at a position opposite to the reading unit  7  positioned at the ADF reading position RP with respect to the second reading window  3 C. The document pressing member  46  is adapted to push the document during the ADF reading mode. The reading unit  7  is positioned at the ADF reading position RP and reads the document pressing member  46  in the absence of the document. In the present embodiment, the document pressing member  46  has a confronting surface in confrontation with reading unit  7  and provided with a white reference board  46 A. 
     The moving mechanism  9  illustrated in  FIG. 3  includes a first and second toothed pulleys  9 B (see  FIG. 4) and 9C  which are fixed to the document platen  3  and the toothed belt  9 A looped around the first and second toothed pulleys  9 B and  9 C. The toothed belt  9 A moves upon the rotation of the first toothed pulley  9 B. The carriage  8  is connected to the toothed belt  9 A, thereby moving depending on a moving direction of the toothed belt  9 A. 
     The second reading window  3 C is also closed by a transparent platen such as a glass similarly to the first reading window, i.e., placement surface  3 A. As shown in  FIG. 3 , the placement surface  3 A and the second reading window  3 C are separated by a beam-like partitioning member  3 D, and the second reading window  3 C is provided between the partitioning member  3 D and a left end portion  3 E of the document platen  3 . 
     As shown in  FIG. 1 , the partitioning member  3 D has an adjustment reference board  55 . The adjustment reference board  55  is provided for readjusting reference of color and shading at a reading process for reading the document by the reading unit  7  and a reference position of the reading unit  7 . 
     The adjustment reference board  55  includes a white tape  55 A and a black tape  55 B arranged in a sub-scan direction (left-right direction of  FIG. 1 ). In the present embodiment, the waiting position WP is a position corresponding to the white tape  55 A in the left-right direction of  FIG. 1 . The waiting position WP is a position where the carriage  8  stays during a non-execution of the FB reading mode and the ADF reading mode. 
     As shown in  FIG. 1 , the document cover  5  is provided with the ADF  40 . The ADF  40  includes a conveyance path  4 , an ADF cover  41  (example of a cover), a supply tray  42  (example of a document tray), a supply roller  44 A, conveyor rollers  44 B and  44 C, a discharge roller  44 D, and a discharge tray  43  utilizing a top surface of the document cover  5 . Further, the document cover  5  is further provided with a plurality of driven rollers  45  opposite to the conveyor roller  44 C and the discharge roller  44 D. 
     The ADF cover  41  is provided with a front sensor  47  (example of a document sensor), such as a photosensor, adapted to detect whether the document is placed on the supply tray  42  and then output a signal. The front sensor  47  is turned ON upon the detection of the document. 
     The conveyance unit  44  is provided with a rear sensor  48  (example of a document sensor), such as a photosensor, adapted to detect whether the document conveyed by the conveyer rollers  44 B and  44 C is presence in the conveyance path  4 . The rear sensor  47  is turned ON upon the detection of the document. 
     In the conveyance path  4 , the rear sensor  48  is provided on a downstream of the supply tray  42  and an upstream of the reading unit  7  in a document conveyance direction of the conveyance path  4 . The rear sensor  48  mainly detects the jammed document in the conveyance path  4 . 
     [Configuration of Drive Power Transmission Mechanism] 
     In the present embodiment, the moving mechanism  9  and the conveyance unit  44  are driven by a single motor  31 . That is, the drive transmission mechanism  13  selectively transmits drive power, i.e., a motor torque, generated in the motor  31  to the moving mechanism  9  and the conveyance unit  44 . 
     As shown in  FIG. 4 , the drive transmission mechanism  13  according to the present embodiment includes: a planetary gear mechanism having a sun gear  15 , a planetary gear  17 , and an engagement portion  19 ; a FB side transmission gear  21 ; and an ADF side transmission gear  23 . The sun gear  15 , the planetary gear  17 , the FB side transmission gear  21 , and an ADF side transmission gear  23  respectively corresponds to a motor gear, a switching gear, a carriage gear, and a conveyance gear of the present invention. 
     The sun gear  15  transmits the power supplied from the motor  31  (see  FIG. 6 ) to the planetary gear  17  and rotates without a displacement relative to the platen  3 . The sun gear  15  rotates by obtaining drive power from the motor  31 . In the present embodiment, a rotation direction of the motor  31  and that of the sun gear  15  coincide with each other, and thus the sun gear  15  rotates in a normal rotation direction and a reverse rotation direction in accordance with the rotation direction of the motor  31 . 
     In the FB reading mode, the FB side transmission gear  21  transmits the power from the motor  31  to the carriage  8 . On the other hand, in the ADF reading mode, the ADF side transmission gear  23  transmits the power from the motor  31  to the conveyance unit  44 . 
     The planetary gear  17  switches its position as follows. That is, in the ADF reading mode, the planetary gear  17  is engaged with the ADF side transmission gear  23  at a ADF position (example of a first position) so as to connect the sun gear  15  with the ADF side transmission gear  23  in  FIG. 5 ; while, in the FB reading mode, the planetary gear  17  is engaged with the FB side transmission gear  21  at a FB position (example of a second position) so as to connect the sun gear  15  with the FB side transmission gear  21  in  FIG. 4 . Further, the planetary gear  17  can rotate about a center thereof while being engaged with the sun gear  15  and revolve between the FB position illustrated in  FIG. 4  and the ADF position illustrated in  FIG. 5  about the sun gear  15  as a revolution center. 
     When the sun gear  15  rotates, the planetary gear  17  is applied with a rotation force for rotating the planetary gear  17  and a revolution force for revolving the planetary gear  17 . Thus, when the sun gear  15  rotates in the normal direction (clockwise direction in  FIG. 4 ), the planetary gear  17  is applied with a revolution force directed from the ADF position to FB the position (i.e., clockwise direction in  FIG. 4 ). 
     On the other hand, when the sun gear rotates in the reverse direction (counterclockwise direction in  FIG. 4 ), the planetary gear  17  is applied with a revolution force directed from the FB position to ADF position (i.e., counterclockwise direction in  FIG. 4 ). When the revolution force becomes larger, the planetary gear  17  revolves in the direction of the revolution force. While the revolution force is small, the planetary gear  17  rotates without the revolution. 
     When the sun gear  15  rotates in the normal direction, the planetary gear  17  rotates in a normal rotation direction. Similarly, when the sun gear  15  rotates in the reverse direction, the planetary gear  17  rotates in a reverse rotation direction. The normal rotation direction of the planetary gear  17  and the normal direction of the sun gear  15  are opposed to each other, and the reverse rotation direction of the planetary gear  17  and the reverse direction of the sun gear  15  are opposed to each other, respectively. 
     The planetary gear  17  is supported on an arm  18  so as to be rotatable and revolvable. The arm  18  has one end in an extending direction thereof rotatably supported coaxially on the sun gear  15  and the other end where the planetary gear  17  is rotatably assembled. 
     The document platen  3  has a second stopper  3 H and a third stopper  3 J each restricting excessive rotation of the arm  18 . On the other hand, the arm  18  has a second contacted portion  18 A in contact with the second stopper  3 H and a third contacted portion  18 B in contact with the third stopper  3 J. 
     As shown in  FIG. 4 , when the planetary gear  17  is positioned at the FB position, the second stopper  3 H is in contact with the second contacted portion  18 A to restrict further clockwise rotation of the arm  18 . Further, as shown in  FIG. 5 , when the planetary gear  17  is positioned at the ADF position. the third stopper  3 J is in contact with the third contacted portion  18 B to restrict further counterclockwise rotation of the arm  18 . 
     A first spring  16  is provided for urging the arm  18  so as to prevent the revolving of the planetary gear  17  when the planetary gear  17  is positioned at the FB position or the ADF position. More specifically, in the FB reading mode, the first spring  16  prevents the planetary gear  17  from being separated from the FB side transmission gear  21  while the sun gear  15  rotates in the reverse direction. That is, the first spring  16  applies to the planetary gear  17  a first inhibiting force preventing the planetary gear  17  from revolving toward the ADF position at least when the planetary gear  17  is positioned at the FB position. 
     The first spring  16  used in the present embodiment is an extension coil spring. The first spring  16  has a one end in an extension direction thereof connected to one side of the arm  18  opposite to the planetary gear  17  with respect to a swing center of the arm  18  and the other end connected to the document platen  3 . Thus, the first spring  16  applies to the arm  18  a second inhibiting force preventing the planetary gear  17  from revolving toward the FB position when the planetary gear  17  is positioned at the ADF position. 
     As described later, in a state where the planetary gear  17  is positioned at the ADF position, a drive force is transmitted to the conveyance unit  44 , and the sun gear  15  is rotating in the reverse direction. While the sun gear  15  rotates in the reverse direction, the planetary gear  17  is applied with a force for revolving from the FB position to the ADF position. Thus, in the present embodiment, even if the second inhibiting force is absent, the planetary gear  17  stays at the ADF position at least while the drive force is transmitted to the conveyance unit  44 , that is, while the sun gear  15  rotates in the reverse direction. 
     In view of the above, in the present embodiment, the first inhibiting force for preventing the revolution of the planetary gear  17  when the planetary gear is positioned at the FB position due is made larger than the second inhibiting force for preventing the revolution of the planetary gear  17  when the planetary gear  17  is positioned at the ADF position. 
     Specifically, the one end and the other end each of the first spring  16  are located such that a deformation amount of the first spring  16  when the planetary gear  17  is positioned at the FB position ( FIG. 4 ) is larger than that of the first spring  16  when the planetary gear  17  is positioned at the ADF position ( FIG. 5 ). 
     The engagement portion  19  is engaged with the planetary gear  17  while the planetary gear  17  revolves between the FB position and the ADF position. In the present embodiment, the engagement portion  19  is constituted by an internally-toothed gear. As shown in  FIG. 4 , the engagement portion  19  is provided with a plurality of projections  19 A projecting toward the sun gear  15 , and the projections  19 A are arranged along a revolution path of the planetary gear  17 . 
     The engagement portion  19  is assembled to the document platen  3  so as to be movable relative to the sun gear  15 . Specifically, in the present embodiment, the engagement portion  19  is movable along the revolution path of the planetary gear  17  about the sun gear  15 . A second spring  19 B for urging the engagement portion  19  to an original position thereof is provided. 
     The ADF side transmission gear  23  is provided on the hinge mechanism  5 A relative to the FB side transmission gear  21  in a direction parallel to the placement surface  3 A and perpendicular to the moving direction of the reading unit  7  (i.e., front-to-rear direction in the present embodiment). Further, the ADF side transmission gear  23  is a gear rotating only in one direction. Thus, the ADF side transmission gear  23  has a mechanism that allows rotation in an X-direction (counterclockwise direction in  FIG. 5 ) by which the conveyance unit  44  conveys the document and prevents rotation in a direction reverse to the X-direction. For example, as the mechanism for preventing the reverse rotation, a known reverse rotation preventing claw (not shown) can be employed. 
     That is, as shown in  FIG. 4 , the FB side transmission gear  21  is provided at a position opposed to the ADF side transmission gear  23  with respect to the sun gear  15 . The sun gear  15 , the planetary gear  17 , the FB side transmission gear  21 , and the ADF side transmission gear  23  respectively have a rotational axis orthogonal to the placement surface  3 A. 
     Further, as shown in  FIG. 4 , the FB side transmission gear  21  is engaged with the planetary gear  17  when the planetary gear  17  is positioned at the FB position. Thus, when the planetary gear  17  is positioned at the FB position, the drive power is transmitted from the sun gear  15  to the FB side transmission gear  21  through the planetary gear  17 . Then, the FB side transmission gear  21  drives the first toothed pulley  9 B to thereby activate the moving mechanism  9 . 
     When the sun gear  15  rotates in the normal direction, the moving mechanism  9  moves the carriage  8 , i.e., the reading unit  7  in the direction of the arrow A of  FIG. 3 , while when the sun gear  15  rotates in the reverse direction, the moving mechanism  9  moves the carriage  8 , i.e., the reading unit  7  in the direction of the arrow B of  FIG. 3 . That is, the moving direction of the reading unit  7  is determined depending on the rotational direction of the sun gear  15 . 
     As shown in  FIG. 5 , the ADF side transmission gear  23  is engaged with the planetary gear  17  when the planetary gear  17  is positioned at the ADF position. Thus, when the planetary gear  17  is positioned at the ADF position, the drive power is transmitted from the sun gear  15 , through the planetary gear  17 , to the ADF side transmission gear  23  to thereby activate the conveyance unit  44 . 
     As shown in  FIG. 2  and the like, the load generation unit  25  is provided for increasing a rotational resistance of the FB side transmission gear  21 . The load generation unit  25  increases the rotational resistance of the FB side transmission gear  21  when the carriage  8 , i.e., the reading unit  7  is positioned at the ADF reading position RP as compared to that when the reading unit  7  is at a position other than the ADF reading position RP. 
     That is, in the present embodiment, the load generation unit  25  includes the first contacted portions  25 A provided in the reading unit  7  and the first stoppers  25 B provided in the document platen  3 . The first contacted portions  25 A and the first stoppers  25 B contact each other, as shown in  FIG. 4 . 
     Thus, when the reading unit  7  is positioned at the ADF reading position RP while the sun gear  15  rotates in the reverse direction to bring the first contacted portions  25 A and the first stoppers  25 B into contact with each other, the movement of the reading unit  7  is restricted, which increases the rotational resistance of the FB side transmission gear  21 . 
     Further, as shown in  FIG. 2  and the like, each the first stoppers  25 B of the load generation unit  25  is positioned at an end position in a range within which the carriage  8  is movable in a predetermined direction (right-to-left direction) and restricts the movement of the carriage  8  when the position of the planetary gear  17  is switched from the FB position to the ADF position. 
     [Operation of Drive Power Transmission Mechanism] 
     The FB reading mode will be described in detail. When the image reading apparatus  1  is in an inactive state, the reading unit  7  is positioned at the waiting position WP, and the planetary gear  17  is positioned at the FB position. When a user pushes the read start key  11 A to start the FB reading, a CPU  20  described later rotates the motor  31  in the normal direction to rotate the sun gear  15  in the normal direction. 
     As a result, the reading unit  7  is moved from the waiting position WP toward the read end position PE. At this time, the planetary gear  17  is applied with the revolution force directed from the ADF position to the FB position. However, the second stopper  3 H and the second contacted portion  18 A contact with each other, and thus the planetary gear  17  rotates in the normal direction while staying at the FB position without revolution. 
     Then, the CPU  20  (see  FIG. 6 ) rotates the motor  31  in the reverse direction when, for example, the number of drive steps of the motor  31  reaches a predetermined value to rotate the sun gear  15  in the reverse direction. Further, the CPU  20  stops the motor  31  when the reading unit  7  reaches the waiting position WP. As a result, the reading unit  7  is moved from the read end position RE to the waiting position WP. 
     While the sun gear  15  rotates in the reverse direction, the planetary gear  17  is applied with the revolution force directed from the FB position to the ADF position, that is, the revolution force in a direction away from the FB side transmission gear  21 . However, the above revolution force is canceled by the first spring  16 , so that the planetary gear  17  rotates in the reverse direction while staying at the FB position without revolution. 
     Next, the ADF reading mode will be described in detail. When the image reading apparatus  1  is in an inactive state, the reading unit  7  is positioned at the waiting position WP, and the planetary gear  17  is positioned at the FB position. When a user pushes the read start key  11 A to start the ADF reading, the CPU  20  rotates the motor  31  in the reverse direction to rotate the sun gear  15  in the reverse direction. 
     As a result, the reading unit  7 , i.e., the carriage  8 , is moved from the waiting position WP to the ADF reading position RP. Then, when the reading unit  7  reaches the ADF reading position RP to bring the first stoppers  25 B and the first contacted portions  25 A into contact with each other, the movement of the reading unit  7  is restricted to increase the rotational resistance of the FB side transmission gear  21 . 
     As the rotational resistance of the FB side transmission gear  21  increases, the rotation force of the planetary gear  17  is reduced and the revolution force of the planetary gear  17  to revolve from the FB position to the ADF position is increased. Then, when the revolution force exceeds the first inhibiting force of the first spring  16 , the planetary gear  17  is brought into engagement with the engagement portion  19 , whereby the planetary gear  17  starts to revolve toward the ADF position. 
     When the planetary gear  17  revolves to bring the third stopper  3 J and the third contacted portion  18 B into contact with each other, the revolution of the planetary gear  17  is stopped, and then the planetary gear  17  is brought into engagement with the ADF side transmission gear  23 . Thus, drive power is transmitted to the conveyance unit  44 , whereby conveyance of the document is started. 
     When the ADF reading is ended, the CPU  20  rotates the motor  31  in the normal direction. As a result, the planetary gear  17  is applied with the revolution force directed from the ADF position to the FB position. 
     Then, the revolution force exceeds the second inhibiting force of the first spring  16 , the planetary gear  17  revolves toward the FB position. When the planetary gear  17  is positioned at the FB position, the planetary gear  17  is brought into engagement with the FB side transmission gear  21 , so that the reading unit  7  is moved in the direction of the arrow A of  FIG. 2  from the ADF reading position RP. 
     [Electrical Configuration of Image Reading Apparatus] 
     As shown in  FIG. 6 , the image reading apparatus  1  includes the CPU (example of a control device)  20 , a ROM  26 , a RAM  27 , an NVRAM (Non-Volatile RAM)  28 , and a network interface (hereinafter, referred to as “network I/F”)  24 . The above components are connected with the reading unit  7 , the operation unit  11 , the display unit  12 , a counter  35 , the front sensor  47 , the rear sensor  48 , an ADF cover sensor  49  (example of a cover sensor), and a motor drive IC  30 . The motor drive IC  30  is connected with the motor  31 . 
     The operation unit  11  receives an instruction inputted by a user such as power ON/OFF, setting of reading resolution, start of reading operation, and clear of the jam. 
     The network I/F  24  is connected to an external user computer through a communication line (not illustrated), allowing data communication to be performed between the image reading apparatus  1  and the external user computer through the network I/F  24 . The reading start instruction can be received from the external user computer through the network I/F  24 . 
     The ROM  26  stores therein various programs for executing an ADF reading process described later and controlling operation of the image reading apparatus  1 . The CPU  20  performs control for each unit of the image reading apparatus  1  according to a program read from the ROM  26  while storing a process result of the program in the RAM  27  or NVRAM  28 . For example, the ROM  26  stores therein the number of steps for step-driving the motor  31 . The storage device for storing above program may be a CD-ROM, a hard disk device, and a flash memory instead of the ROM  26 . 
     The CPU  20  stores in the RAM  27 , as setting data, conveyance document data related to the presence or absence of the document in the conveyance unit  44 , tray document data related to a status of the supply tray  42 , and cover status data related to a status of the ADF cover  41  during a jam occasion period from when the document jam occurs to when a user pushes the clear reception key  11 C. The tray document data for the status of the supply tray  42  indicates whether or not the document is placed on the supply tray  42 , and cover status data indicates whether the ADF cover  41  is open or closed. 
     The motor  31  is a stepping motor. The motor drive IC  30  controls drive of the motor  31  under control of the CPU  20 . The counter  35  counts the number of steps to control the motor  31 . The ADF cover sensor  49  (example of a cover sensor) detects an open state of the ADF cover  41 . The ADF cover sensor  49  is turned ON upon the detection of the open state of the ADF cover  41 . 
     The CPU  20  controls the motor drive IC  30  to control a torque and a rotation direction of the motor  31 . A motor drive current and a motor torque have a proportional relationship, and thus increasing the drive current causes the motor torque to increase. Alternatively, the motor drive speed and the motor torque have an inverse relationship, and thus reducing the drive speed causes the motor torque to increase. 
     [ADF Reading Process] 
     The following describes embodiments of ADF reading process in the image reading apparatus  1  with reference to flowcharts of  FIGS. 7 to 12 . The ADF reading process in the following embodiments is a process that the CPU  20  executes according to a program stored in the ROM  26  when a user places the document on the supply tray  42 , pushes the read start key  11 A and then the CPU  20  receives an ADF reading instruction. 
     First Embodiment 
     The ADF reading process according to a first embodiment will be described. As illustrated in  FIG. 7 , upon reception of the ADF reading instruction, the CPU  20  executes a pre-reading process (S 2 ). In the pre-reading process, the CPU  20  executes acquisition process for acquiring initialization data initializing the reading unit  7  or correction data correcting reading data to be used in the ADF reading process. 
     The CPU  20  executes a switching process in which the motor  31  drives such that the planetary gear  17  is engaged with the ADF side transmission gear  23 , i.e., the motor  31  is rotated in the reverse direction to switch the position of the planetary gear  17  from the FB position to ADF position (S 4 ). In the switching process for switching the position of the planetary gear  17  from the FB position to the ADF position, reaction force from the FB side transmission gear  21  generated by the rotation of the planetary gear  17  according to the rotation of the sun gear  15  in the reverse direction needs to be larger than the inhibiting force of the first spring  16  inhibiting the revolution of the planetary gear  17 . Thus, the motor torque of the motor  31  in the switching process is previously determined by, e.g., experiments such that the reaction force from the FB side transmission gear  21  is larger than the inhibiting force of the first spring  16  inhibiting the revolution of the planetary gear  17 . 
     The reaction force from the FB side transmission gear  21  is caused by the rotational resistance of the FB side transmission gear  21  due to the load generation unit  25 . That is, in this state, the FB side transmission gear  21  hardly rotates due to the load generation unit  25 , so that the planetary gear  17  receives, upon start of the rotation thereof, the reaction force acting in a direction substantially opposite to a direction of the rotation thereof from the FB side transmission gear  21 . 
     As described above, utilizing the reaction force from the FB side transmission gear  21  whose rotation is suppressed to the planetary gear  17  in the switching process, the position of the planetary gear  17  can appropriately be switched from the FB position to the ADF position without loss of synchronization of the planetary gear  17  with the motor  31 . 
     Then, the CPU  20  rotates the rollers  44 A to  44 D of the conveyance unit  44  to start conveying the document placed on the supply tray  42  along the conveyance path  4  (S 6 ). When the conveyance of the document is started, the document is drawn into the conveyance path  4 , and reading of the document is executed in the conveyance path  4  (S 8 ). 
     The document conveyed along the conveyance path  4  may be jammed in the conveyance path  4 , that is, a document jam may occur. The CPU  20  determines whether or not the document jam occurs in the conveyance path  4  during reading of the document (S 10 ). Specifically, the CPU  20  determines that the document jam occurs when the rear sensor  48  is turned ON for a predetermined time or more or when the rear sensor  48  is turned OFF for a prescribed time or more after the document is drawn into the conveyance path  4 . 
     If in S 10  the document jam occurs (S 10 :YES), the CPU  20  immediately stops rotation of the rollers  44 A to  44 D of the conveyance unit  44  to stop the conveyance of the document in the conveyance path  4  (S 12 ). Then, the CPU  20  executes a drive process for driving the motor  31  in a direction in which the planetary gear  17  is engaged with the FB side transmission gear  21 , that is, drives the motor in the normal direction to switch the position of the planetary gear  17  from the ADF position to the FB position (S 14 ). This releases the engagement between the ADF side transmission gear  23  and the planetary gear  17 , thereby preventing the rotation of the ADF side transmission gear  23  by the motor  31  while the user removes the document jammed in the conveyance path  4 . The process executed in S 14  is an example of the drive process. 
     In the present embodiment, while executing the drive process of S 14 , the CPU  20  controls the display unit  12  to display message such as “jam is detected, and removal of the jammed document is facilitated”. This allows the user to understand the drive process executed in the image reading apparatus  1  when the drive process for switching the position of the planetary gear  17  from the ADF position to the FB position after the stop of the document conveyance. 
     The CPU  20  then determines whether or not the operation unit  11  receives a signal indicating the document jam is cleared (S 16 ). That is, the CPU  20  determines whether or not the user pushes the clear reception key  11 C. If the operation unit  11  does not receive the signal indicating the clear of the document jam (S 16 :NO), the CPU  20  repeatedly executes the process of S 16  until the clear reception key  11 C is pushed. If the operation unit  11  receives the signal indicating the clear of the document jam (S 16 :YES), the CPU  20  executes a document determination process  1  to be described later (S 18 ). Hereinafter, the process executed in S 16  is referred to as “jam clear determination process”. 
     After completion of the document determination process  1 , the CPU  20  determines whether the document is present in the conveyance unit  44  (S 20 ). Specifically, the CPU  20  reads from the RAM  27  the conveyance document data. The conveyance document data is stored in the document determination process  1  to be described later and then the CPU  20  determines whether or not the conveyance document data indicates “presence”. If the document is present in the conveyance unit  44 , that is, if the conveyance document data read from the RAM  27  indicates “presence” (S 20 :YES), the CPU  20  returns to S 4  and then executes once again the process of switching the position of the planetary gear  17  from the FB position to the ADF position (S 4 ). The process of S 4  that the CPU  20  executes once again is an example of re-drive process. 
     As described above, the CPU  20  executes the re-drive process of switching the position of the planetary gear  17  from the FB position to the ADF position when the conveyance document data indicates “presence” after the clear of the document jam. This allows the CPU  20  to advance to operation of discharging the document left in the conveyance unit  44  more immediately after the clear of the document jam. 
     After that, conveyance of the document is started (S 6 ), and the document is conveyed to the conveyance path  4  (S 8 ). By executing the process of conveying the document after the re-drive process, the document left in the conveyance path  4  is discharged to the discharge tray  43 . The process of S 8  that the CPU  20  executes once again is an example of discharging process. 
     If in S 20  the document is absent in the conveyance unit  44 , that is, if the conveyance document data read from the RAM  27  does not indicate “presence” (S 20 :NO), the CPU  20  executes post-reading process (S 26 ). For example, in the post-reading process, the CPU  20  detects the waiting position WP of the carriage  8  and transfers the carriage  8  to the detected waiting position WP. After completion of the post-reading process, the CPU  20  ends the ADF reading process. The post-reading process that the CPU  20  executes in S 26  is an example of transfer process. 
     On the other hand, if in S 10  the document jam does not occur (S 10 :NO), the CPU  20  determines whether or not the ADF reading or the discharging process is completed for all the documents to be subjected to the ADF reading, that is, all the documents placed on the supply tray  42  (S 22 ). Specifically, when, for example, the front sensor  47  or the rear sensor  48  is continuously turned ON, the CPU  20  determines that the ADF reading or discharging process for all the documents placed on the supply tray  4  is not completed. 
     If in S 22  the reading or conveyance of the document in the conveyance path  4  is not completed (S 22 :NO), the CPU  20  returns to S 8  and then executes reading or conveyance of the document left in the supply tray  42  or conveyance path  4  (S 8 ). If in S 22  the reading or conveyance of the document in the conveyance path  4  is completed (S 22 :YES), the CPU  20  executes the same process as that executed in S 14 , that is, rotates the motor  31  in the normal direction to switch the position of the planetary gear  17  from the ADF position to the FB position (S 24 ). After completion of the process of S 24 , the CPU  20  executes the above-described post-reading process (S 26 ). After completion of the post-reading process, the CPU  20  ends the ADF reading process. 
     [Document Determination Process  1 ] 
     The following describes the document determination process  1  that the CPU  20  executes in S 18  with reference to  FIG. 9 . The CPU  20  determines whether or not the document is left in the conveyance path  4  (S 32 ). Specifically, the CPU  20  determines whether or not the rear sensor  48  detects the document to be turned ON. 
     If in S 32  the document is left in the conveyance path  4 , that is, if the rear sensor  48  detects the document to be turned ON (S 32 :YES), the CPU  20  sets the conveyance document data to “presence”, stores the conveyance document data in the RAM  27  (S 38 ), and then ends the document determination process  1 . 
     On the other hand, if in S 32  the document is not left in the conveyance path  4 , that is, if the rear sensor  48  is in OFF (S 32 :NO), the CPU  20  determines whether or not the document is placed on the supply tray  42  (S 34 ). Specifically, the CPU  20  determines whether a detection signal indicating the presence or absence of the document is output from the front sensor  47 . 
     If in S 34  the document is placed on the supply tray  42 , that is, if the front sensor  47  detects the document to be turned ON (S 34 :YES), the CPU  20  sets the conveyance document data to “presence”, stores the conveyance document data in the RAM  27  (S 38 ), and then ends the document determination process  1 . 
     On the other hand, if in S 34  the document is not placed on the supply tray  42 , that is, if the front sensor  47  is in OFF (S 34 :NO), the CPU  20  sets the conveyance document data to “absence”, stores the conveyance document data in the RAM  27  (S 36 ), and then ends the document determination process  1 . 
     Effects of First Embodiment 
     The following describes effects of the ADF reading process according to the first embodiment. The image reading apparatus  1  of the present embodiment has a configuration in which the carriage  8  and the conveyance unit  44  are driven by single motor  31 . When the document jam occurs in the conveyance path  4 , engagement between the ADF side transmission gear  23  and the planetary gear  17  is released, facilitating for the user the clear of the document jam. After that, when receiving the signal indicating the clear of the document jam by the user, the CPU  20  executes the document determination process  1  to determine whether or not the document is left in the conveyance unit  44  or the supply tray  42 . 
     If the document is present in the conveyance unit  44  in the document determination process  1 , that is, if the conveyance document data indicates “presence”, the CPU  20  executes the re-drive process for driving the motor  31  in a direction in which the planetary gear  17  is engaged with the ADF side transmission gear  23 . This allows the CPU  20  to advance the discharge process for discharging the document left in the conveyance unit  44  immediately after the clear of the document jam. As described above, the image reading apparatus  1  of the present embodiment has a configuration in which the carriage  8  and the conveyance unit  44  are driven by the single motor  31 , as well as, the planetary gear  17  can be retained at an appropriate position after the clear of the jam. 
     Further, the image reading apparatus  1  of the present embodiment is provided with the rear sensor  48  adapted to detect the presence or absence of the document to be conveyed by the conveyance unit  44  and outputs the detection signal thereof. The CPU  20  determines, based on the output from the rear sensor  48 , the presence or absence of the document in the conveyance unit  44  in the re-drive process. As described above, the presence or absence of the document can be detected correctly by the rear sensor  48 , thereby increasing a possibility that the CPU  20  executes the re-drive process when necessary. 
     Further, the image reading apparatus  1  of the present embodiment is provided with the display unit  12 . When executing the drive process corresponding to the process of S 14 , the CPU  20  controls the display unit  12  to display message indicating that the jam clear operation can be easily performed. The execution of the drive process causes the motor  31  to be rotated in a direction so that the planetary gear  17  disengages the ADF side transmission gear  23 , facilitating the clear of jam. According to the image reading apparatus  1  of the present embodiment, the user can get the message indicating that the jam clear operation can be facilitated at a timing when the jam clear operation becomes easy to perform. 
     Further, in the image reading apparatus  1  of the present embodiment, the carriage  8  is positioned at the waiting position WP when the ADF reading or the FB reading is not executed. If the front sensor  47  and the rear sensor  48  are each in OFF, the CPU  20  sets the conveyance document data to “absence”. Then, the CPU  20  controls the gear switching to retain the planetary gear  17  at the FB position and then transfers the carriage  8  to the waiting position WP. As described above, in the present embodiment, the carriage  8  is transferred to the waiting position WP when no document is left in the conveyance unit  44 , thereby advancing to the FB reading process for other documents immediately. 
     Second Embodiment 
     The ADF reading process according to a second embodiment will be described. The ADF reading process of the second embodiment is the same as that of the first embodiment except for the process of S 18  (see  FIG. 7 ) in the flowchart of the ADF reading process of the first embodiment. Thus, descriptions of the same process procedures as those of the first embodiment are omitted. 
     In the ADF reading process of the second embodiment, if, in the jam clear determination process to be executed in S 16 , the signal for the clear of the document jam is received (S 16 :YES), the CPU  20  executes at least one of restart prompting process  1  and restart prompting process  2  each described later (S 19 ). After completion of the process of S 19 , the routine advances to S 20 . The process of S 20  and subsequent process procedures are the same as those in the first embodiment. Which one of the restart prompting process  1  and the restart prompting process  2  is executed or whether or not the restart prompting process  1  and restart prompting process  2  are executed in combination may be previously set in a program stored in the ROM  26 . 
     [Restart Prompting Process  1 ] 
     The following describes the restart prompting process  1  that the CPU  20  executes in S 19  with reference to  FIG. 11 . In the restart prompting process  1 , the CPU  20  sets the tray document data stored in the RAM  27  to “absence” and stores the tray document data in the RAM  27  (S 52 ). 
     Subsequently, the CPU  20  determines whether or not the document is placed on the supply tray  42  (S 54 ). Specifically, the CPU  20  determines whether or not the front sensor  47  detects the document to be turned ON (S 54 ). Hereinafter, the same process as that executed in S 54  is referred to as “tray document detection process”. 
     If in S 54  the document is placed on the supply tray  42 , that is, if the front sensor  47  detects the document to be turned ON (S 54 :YES), the CPU  20  sets the tray document data to “presence”, stores the tray document data in the RAM  27  (S 56 ), and advances to S 58 . On the other hand, if in S 54  the document is not placed on the supply tray  42 , that is, if the front sensor  47  is in OFF (S 54 :NO), the CPU  20  advances to S 64 . 
     In S 58 , the CPU  20  executes the jam clear determination process for determining whether or not the operation unit  11  receives the signal indicating the clear of the document jam (S 58 ). If in S 58  the signal indicating the clear of the document jam is received (S 58 :YES), the CPU  20  advances to S 82 . On the other hand, if in S 58  the signal indicating the clear of the document jam is not received (S 58 :NO), the CPU  20  advances to S 60  and then executes the tray document detection process (S 60 ). 
     If in S 60  the document is placed on the supply tray  42  (S 60 :YES), the CPU  20  returns to S 58  and then executes the jam clear determination process. On the other hand, if in S 60  the document is not placed on the supply tray  42  (S 60 :NO), the CPU  20  sets the tray document data to “presence→absence”, stores the tray document data in the RAM  27  (S 62 ), and advances to S 70 . 
     In S 64 , the CPU  20  executes the jam clear determination process (S 64 ). If in S 64  the signal indicating the clear of the document jam is received (S 64 :YES), the CPU  20  advances to S 82 . On the other hand, if in S 60  the signal indicating the clear of the document jam is not received (S 64 :NO), the CPU  20  advances to S 66  and then executes the tray document detection process (S 66 ). 
     If in S 66  the document is placed on the supply tray  42  (S 66 :YES), the CPU  20  sets the tray document data to “presence”, and stores the tray document data in the RAM  27  (S 68 ). Then, the CPU  20  advances to S 58  and then executes the jam clear determination process. On the other hand, if in S 66  the document is not placed on the supply tray  42  (S 66 :NO), the CPU  20  returns to S 64  and then executes the jam clear determination process. 
     In S 70 , the CPU  20  executes the jam clear determination process (S 70 ). If in S 70  the signal indicating the clear of the document jam is received (S 70 :YES), the CPU  20  advances to S 82 . On the other hand, if in S 70  the signal indicating the clear of the document jam is not received (S 70 :NO), the CPU  20  advances to S 72  and then executes the tray document detection process (S 72 ). 
     If in S 72  the document is not placed on the supply tray  42  (S 72 :NO), the CPU  20  returns to S 70  and then executes the jam clear determination process. On the other hand, if in S 72  the document is placed on the supply tray  42  (S 72 :YES), the CPU  20  sets the tray document data to “presence→absence→presence”, stores the tray document data in the RAM  27  (S 74 ), and advances to S 76 . 
     In S 76 , the CPU  20  executes the jam clear determination process (S 76 ). If in S 76  the signal indicating the clear of the document jam is received (S 76 :YES), the CPU  20  advances to S 82 . On the other hand, if in S 76  the signal indicating the clear of the document jam is not received (S 76 :NO), the CPU  20  advances to S 78  and then executes the tray document detection process (S 78 ). 
     If in S 78  the document is placed on the supply tray  42  (S 78 :YES), the CPU  20  returns to S 76  and then executes the jam clear determination process. On the other hand, if in S 78  the document is not placed on the supply tray  42  (S 78 :NO), the CPU  20  sets the tray document data to “absence” and stores the tray document data in the RAM  27  (S 80 ). Then, the CPU  20  advances to S 64  and then executes the jam clear determination process. 
     As described above, in the restart prompting process  1 , when the CPU  20  determines that the signal indicating the clear of the document jam is received in the jam clear determination process in S 58 , S 64 , S 70 , and S 76 , the CPU  20  advances to S 82 . In S 82 , the CPU  20  reads, from the RAM  27 , the tray document data and determines whether or not the read tray document data indicates “presence” (S 82 ). 
     If the tray document data indicates “presence” (S 82 :YES), the CPU  20  sets the conveyance document data stored in the RAM  27  to “presence” (S 84 ) and then ends the restart prompting process  1 . 
     On the other hand, if the tray document data does not indicate “presence” (S 82 :NO), that is, the tray document data indicates “presence→absence”, “presence→absence→presence”, or “absence”, the CPU  20  sets the conveyance document data stored in the RAM  27  to “absence” (S 86 ) and then ends the restart prompting process  1 . 
     [Restart Prompting Process  2 ] 
     The following describes the restart prompting process  2  which the CPU  20  executes in S 19  with reference to  FIG. 12 . In the restart prompting process  2 , the CPU  20  sets the cover status data stored in the RAM  27  to “close” and stores the cover status data in the RAM  27  (S 152 ). 
     Then, the CPU  20  determines whether or not the ADF cover  41  is open (S 154 ). Specifically, the CPU  20  determines whether or not the ADF cover sensor  49  detects the open state of the ADF cover  41  to be turned ON. Hereinafter, the same process as that executed in S 154  is referred to as “cover state detection process”. 
     If in S 154  the ADF cover  41  is open, that is, if the ADF cover sensor  49  detects the open state of the ADF cover  41  to be tuned ON (S 154 :YES), the CPU  20  sets the cover status data to “open”, stores the cover status data in the RAM  27  (S 156 ), and advances to S 158 . On the other hand, if in S 154  the ADF cover  41  is closed, that is, if the ADF cover sensor  49  is in OFF (S 154 :NO), the CPU  20  advances to S 164 . 
     In S 158 , the CPU executes the jam clear determination process (S 158 ). If in S 158  the signal indicating the clear of the document jam is received (S 158 :YES), the CPU  20  advances to S 182 . On the other hand, if in S 158  the signal indicating the clear of the document jam is not received (S 158 :NO), the CPU  20  advances to S 160  and then executes the cover state detection process (S 160 ). 
     If in S 160  the ADF cover  41  is open (S 160 :YES), the CPU  20  returns to S 158  and then executes the jam clear determination process. On the other hand, if in S 160  the ADF cover  41  is not open (S 160 :NO), the CPU  20  sets the cover status data to “open→close” and stores the cover status data in the RAM  27  (S 162 ). Then, the CPU  20  advances to S 164  and then executes the jam clear determination process. 
     In S 164 , the CPU  20  executes the jam clear determination process (S 164 ). If in S 164  the signal indicating the clear of the document jam is received (S 164 :YES), the CPU  20  advances to S 182 . On the other hand, if in S 164  the signal indicating the clear of the document jam is not received (S 164 :NO), the CPU advances to S 166  and then executes the cover state detection process (S 166 ). 
     If in S 166  the ADF cover  41  is open (S 166 :YES), the CPU  20  sets the cover status data to “close→open”, stores the cover status data in the RAM  27  (S 168 ), and then advances to S 170 . On the other hand, if in S 166  the ADF cover  41  is closed (S 166 :NO), the CPU  20  returns to S 164  and then executes the jam clear determination process. 
     In S 170 , the CPU  20  executes the jam clear determination process (S 170 ). If in S 170  the signal indicating the clear of the document jam is received (S 170 :YES), the CPU  20  advances to S 182 . On the other hand, if in S 170  the signal indicating the clear of the document jam is not received (S 170 :NO), the CPU  20  advances to S 172  and then executes the cover state detection process (S 172 ). 
     If in S 172  the ADF cover  41  is open (S 172 :YES), the CPU  20  returns to S 170  and then executes the jam clear determination process. On the other hand, if in S 172  the ADF cover  41  is closed (S 172 :NO), the CPU  20  sets the cover status data to “close→open→close”, stores the cover status data in the RAM  27  (S  174 ), and advances to S 176 . 
     In S 176 , the CPU  20  executes the jam clear determination process (S 176 ). If in S 176  the signal indicating the clear of the document jam is received (S 176 :YES), the CPU  20  advances to S 182 . On the other hand, if in S 176  the signal indicating clear of the document jam is not received (S 176 :NO), the CPU  20  advances to S 178  and then executes the cover state detection process (S 178 ). 
     If in S 178  the ADF cover  41  is closed (S 178 :NO), the CPU  20  returns to S 176  and then executes the jam clear determination process. On the other hand, if in S 178  the ADF cover  41  is open (S 178 :YES), the CPU  20  sets the cover status data to “open” and stores the cover status data in the RAM  27  (S 180 ). Then, the CPU  20  returns to S 158  and then executes the jam clear determination process. 
     As described above, in the restart prompting process  2 , when the CPU  20  determines that the signal indicating the clear of the document jam is received in the jam clear determination process in S 158 , S 164 , S 170 , and S 176 , the CPU  20  advances to S 182 . In S 182 , the CPU  20  reads, from the RAM  27 , the cover status data and determines whether or not the read cover status data indicates “open” or “close→open” (S 182 ). 
     If the cover status data indicates “open” or “close→open” (S 182 :YES), the CPU  20  controls the display unit  12  to display an error message (S  184 ). Then, the CPU  20  advances to S 158  and then executes the jam clear determination process. In a case where the read cover status data indicates “open” or “close→open” upon the reception of the signal indicating the clear of the document jam, the ADF cover  41  is open, so that the document cannot be conveyed by the conveyance unit  44 . In such a case, the display unit  12  displays the error message, thereby informing the user that the document cannot be conveyed by the conveyance unit  44 . 
     On the other hand, if in S 182  the read cover status data does not indicate “open” or “close→open” (S 182 :NO), the CPU  20  determines whether or not the cover status data read in S 182  indicates “close” (S  186 ). 
     In S 186 , if the cover status data read in S 182  indicates “close” (S 186 :YES), the CPU  20  executes the document determination process  1  described above (S 190 ) and then ends the restart prompting process  2 . On the other hand, if the cover status data read in S 182  does not indicate “close” (S 186 :NO), that is, the cover status data read in S 182  indicates “open→close” or “close→open→close”, the CPU  20  executes the document determination process  2  (S 188 ) and then ends the restart prompting process  2 . 
     [Document Determination Process  2 ] 
     The following describes the document determination process  2  that the CPU  20  executes in S 188  with reference to  FIG. 10 . In the document determination process  2 , the CPU  20  determines whether or not the document is left in the conveyance path  4  (S 42 ). Specifically, the CPU  20  determines whether or not the rear sensor  48  detects the document to be turned ON. 
     If in S 42  the document is left in the conveyance path  4 , that is, if the rear sensor  48  detects the document to be turned ON (S 42 :YES), the CPU  20  sets the conveyance document data to “presence”, stores the conveyance document data in the RAM  27  (S 48 ), and then ends the document determination process  2 . 
     On the other hand, if in S 42  the document is not left in the conveyance path  4 , that is, if the rear sensor  48  in OFF (S 42 :NO), the CPU  20  sets the conveyance document data to “absence”, stores the conveyance document data in the RAM  27  (S 46 ), and then ends the document determination process  2 . 
     As described above, in the document determination process  1 , the CPU  20  determines the presence or absence of the document in the conveyance unit  44  using both the rear sensor  48  and the front sensor  47 ; while, in the document determination process  2 , the CPU  20  determines the presence or absence of the document in the conveyance unit  44  using only the rear sensor  48 . Thus, the document determination process  2  is reduced in load on the CPU  20  as compared to the document determination process  1 . 
     Effects of Second Embodiment 
     Effects obtained when the restart prompting process  1  is executed in the ADF reading process of the present embodiment will be described. The image reading apparatus  1  includes the supply tray  42  on which the document to be conveyed by the conveyance unit  44  is placed, the discharge tray  43  to which the document that has been conveyed by the conveyance unit  44  and read by the reading unit  7  is discharged, and the front sensor  47  adapted to detect the presence or absence of the document placed on the supply tray  42  and outputs a signal indicating a detection result. 
     In the image reading apparatus  1  of the present embodiment, if the CPU  20  determines that the front sensor  47  detects the document to be turned ON without determining that the front sensor  47  is in OFF during a jam occasion period from when the document jam occurs to when the signal indicating the clear of the document jam is received, the CPU  20  executes the discharging process for discharging the document left in the conveyance unit  44  and in the supply tray  42  to the discharge tray  43  after the re-drive process. In other words, if the tray document data read from the RAM  27  indicates “presence” upon the reception of the signal indicating the clear of the document jam, the CPU  20  executes the discharging process of discharging the document left in the conveyance unit  44  and in the supply tray  42  to the discharge tray  43  after the re-drive process. 
     When the document is left in the supply tray  42  before and after occurrence of the document jam, the user may accidentally leave the document on the supply tray  42 . Thus, in the image reading apparatus  1  of the present embodiment, the discharging process for discharging the document left by the user on the supply tray  42  is executed after the clear of the jam. As a result, the document left on the supply tray  42  is discharged to the discharge tray  43 , so that no document is placed on the supply tray  42 , thereby allowing subsequent ADF reading process to be restarted quickly. 
     Further, in the image reading apparatus  1  of the present embodiment, when the CPU  20  determines, at least once, that the front sensor  47  is in OFF during the jam occasion period from when occurrence of the document jam is determined in the jam determination process executed in S 10  to when the signal indicating the clear of the document jam is received, the CPU  20  does not execute the discharging process. In other words, in a case where the tray document data read from the RAM  27  does not indicate “presence” upon the reception of the signal indicating the clear of the document jam, the CPU  20  does not execute the discharging document. 
     In such a case, there is a high possibility that a new document to be read is placed on the supply tray  42 . If the discharging process is executed, the new document to be read and placed on the supply tray  42  is erroneously discharged. In the image reading apparatus  1  of the present embodiment, execution of such unnecessary discharging process can be avoided. 
     Further, in the image reading apparatus  1  of the present embodiment, when the CPU  20  determines, at least once, that the front sensor  47  is in OFF during the jam occasion period from when occurrence of the document jam is determined in the jam determination process executed in S 10  to when the signal indicating the clear of the document jam is received, the CPU  20  does not execute the re-drive process, and then the planetary gear  17  has been retained at the FB position. 
     When the planetary gear  17  is retained at the FB position, the discharging process cannot be executed. That is, in the image reading apparatus  1  of the present embodiment, since the planetary gear  17  is retained at the FB position, thereby avoiding the unnecessary discharging process and the unnecessary switching of the position of the planetary gear  17  to the ADF position. 
     Further, in the image reading apparatus  1  of the present embodiment, in a case where the front sensor  47  detects the document to be turned ON and is then turned OFF during the jam occasion period and the front sensor  47  detects the document once again to be turned ON when the signal indicating the clear of the document jam is received, the CPU  20  does not execute the discharging process. That is, in a case where the tray document data read from the RAM  27  indicates “presence→absence→presence” upon the reception of the signal indicating the clear of the document jam, the CPU  20  does not execute the discharging process. 
     In such a case, there is a high possibility that the user places once again a document that has been taken out of the conveyance unit  44  at the time of the jam clear on the supply tray  42 , together with other documents that have been placed on the supply tray  42  in order to read the documents once again. As described above, in the image reading apparatus  1  of the present embodiment, execution of such unnecessary discharging process for the documents placed again on the supply tray  42  can be avoided. 
     Next, effects obtained when the restart prompting process  2  is executed in the ADF reading process of the present embodiment will be described. The image reading apparatus  1  includes the openable ADF cover  41  provided in the conveyance unit  44  and the ADF cover sensor  49  adapted to detect the state of the ADF cover  41  and outputs a signal indicating a detection result. 
     In the image recording apparatus of the present embodiment, in a case where the ADF cover sensor  49  detects the close state of the ADF cover  41  to be turned ON and is then turned OFF during the jam occasion period and the ADF cover sensor  49  detects the close state of the ADF cover  41  once again to be turned ON upon the reception of the signal indicating the clear of the document jam, the CPU  20  retains the planetary gear  17  at the FB position. That is, in a case where the cover status data read from the RAM  27  indicates “open→close” or “close→open→close” upon the reception of the signal indicating the clear of the document jam, the CPU  20  retains the planetary gear  17  at the FB position. 
     When the signal indicating the clear of the document jam is received after the ADF cover  41  is once open, there is a high possibility that the user clears the jam after opening the ADF cover  41  and removing the jammed document from the conveyance unit  44 . In such a case, there is a low possibility that the document is left in the conveyance unit  44 , and there is less need to detect the presence or absence of the document with a plurality of sensors. Thus, according to the image reading apparatus  1  of the present embodiment, unnecessary use of a plurality of sensors for detection can be prevented, thereby preventing extra load from being applied to the CPU  20 . 
     Further, in the image reading apparatus  1  of the present embodiment, in a case where the ADF cover sensor  49  detects a close state of the ADF cover  41  to be turned ON and is then turned OFF during the jam occasion period and the ADF cover sensor  49  detects the close state of the ADF cover  41  once again to be turned ON upon the reception of the signal indicating the clear of the document jam, the CPU  20  does not execute the discharging process. 
     As described above, when the signal indicating the clear of the document jam is received after the ADF cover  41  is once open, there is a high possibility that the user clears the jam after opening the ADF cover  41  and removing the jammed document from the conveyance unit  44 . In such a case, there is a low possibility that the document is left in the conveyance unit  44 , and thus there is less need to execute the discharging process. Thus, according to the image reading apparatus  1  of the present embodiment, unnecessary execution of the discharge process can be avoided. 
     Other Embodiments 
     The technology disclosed in the present specification is not limited to the embodiment described above with reference to the drawings but includes the following various embodiments. 
     (1) In the above embodiments, the position of the planetary gear is switched from the ADF side to FB side in the drive process. However, the drive process is not limited to the embodiment in which the planetary gear is switched to the FB position to be engaged with the FB side transmission gear. The drive process only needs to be processing that drives the motor in a direction in which the FB side transmission gear and planetary gear are engaged with each other to release engagement between the ADF side transmission gear and the planetary gear. 
     (2) In the second embodiment, one of the restart prompting process  1  and restart prompting process  2  is executed in S 18 . However, the restart prompting process  1  and the restart prompting process  2  may be executed in combination in S 18 . 
     (3) In the above embodiments, the CPU uses the front and rear sensors provided in the conveyance unit to detect the presence or absence of the document in the conveyance unit. However, another sensor may be provided in the conveyance unit. In this case, the CPU may detect the presence or absence of the document in the conveyance unit by using only another sensor or by using the other sensor in combination of the front and rear sensors. 
     In the above embodiments, the CPU determines in S 10  whether or not the jam occurs by using the rear sensor. However, the present invention is not limited to this configuration. The CPU  10  may use another method to determine the occurrence of the jam in S 10 . 
     (5) In the above embodiments, the CPU receives a signal indicating the jam is cleared by depression of the clear reception key by the user. However, the present invention is not limited to this configuration. The CPU may receive the signal indicating the jam is cleared by another method. 
     (6) In the above embodiment, the CPU  20  is used as an example of a control device. However, the present invention is not limited to this configuration. The controller may be constituted by a plurality of circuits including an ASIC or may be constituted by the CPU and other circuits.