Patent Publication Number: US-8991813-B2

Title: Sheet transport device

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority from Japanese Patent Application No. 2012-273467, filed on Dec. 14, 2012, the entire subject matter of which is incorporated herein by reference. 
     TECHNICAL FIELD 
     Aspects of the present invention relates to a sheet transport device which reads an image on a sheet while transporting the sheet along a predetermined transport path. 
     BACKGROUND 
     There has been known a sheet transport device configured to read an image on a sheet using a reading unit while transporting the sheet along a predetermined transport path. 
     The sheet transport device separates a plurality of sheets set on a sheet supporting surface sheet by sheet, transports a separated sheet, and reads an image on the sheet during the transporting process. 
     In this type of sheet transport device, a size of a sheet which can be read by the sheet transport device is set as specification in advance. In other words, a maximum sheet size and a minimum sheet size of a sheet which can be transported in the transport path is set as specifications. In many cases, the maximum sheet size and the minimum sheet size, in particular, the minimum sheet size is determined by a distance of arrangement of transport roller pairs in a transport direction which are arranged at a plurality of positions on the transport path to transport the sheet. 
     In other words, in many cases, there is employed a method in which a driving roller and a driven roller of the transport roller pair nip a sheet to transport the sheet. In this configuration, in order to appropriately transport a sheet, it is necessary to pass the sheet from a nip portion of the transport roller pair arranged at an upstream side of the transport path to a nip portion of a subsequent transport roller pair arranged at a downstream side. Therefore, it is necessary that the transport roller pairs are arranged within a distance allowing the passing of the sheet and the sheet is laid across at least two of the nip portions at the upstream side and the downstream side. 
     When a user accidentally or forcibly sets a sheet smaller than the minimum sheet size set as specifications of the device on a sheet supporting surface, performs a reading start operation, and starts the transporting of the sheet, there could be a concern, for example, that the passing of the sheet cannot be performed and the transporting of the sheet is stopped at a middle of the transport path. When the transporting of the sheet is stopped in the middle of the transport path, it is difficult to take out the sheet. 
     SUMMARY 
     Accordingly, an aspect of the present invention provides a sheet transport device which may suppress forcible or accidental setting and transporting of a sheet smaller than a set minimum sheet size. 
     According to an illustrative embodiment of the present invention, there is provided a sheet transport device including a sheet supporting surface configured to support one or more sheets thereon, a separation roller configured to separate, one by one, the one or more sheets supported by the sheet supporting surface to transport a separated sheet in a predetermined transport direction, and a first detection unit disposed to the sheet supporting surface and configured to detect whether a sheet is set on the sheet supporting surface. The separation roller and the first detection unit are disposed to be separated at least in a sheet width direction orthogonal to the predetermined transport direction. A distance between the first detection unit and an end edge of the separation roller at a side of the first detection unit is longer than a short side of a non-allowable sheet which is smaller than a minimum sheet size within an allowable transport range set in the sheet transport device. 
     According to the above configuration, the sheet transport device may suppress forcible or accidental setting and transporting of a sheet smaller than a set minimum sheet size. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other aspects of the present invention will become more apparent and more readily appreciated from the following description of illustrative embodiments of the present invention taken in conjunction with the attached drawings, in which: 
         FIG. 1  is a perspective view of a multifunction printer mounted with a sheet transport device according to an illustrative embodiment of the present invention; 
         FIG. 2  is a perspective view of the multifunction printer in a state where the sheet transport device mounted with a sheet transport unit is opened; 
         FIG. 3  is a cross-sectional view showing a state where a sheet supporting cover of the sheet transport device is closed; 
         FIG. 4  is a cross-sectional view showing a state where the sheet supporting cover of the sheet transport device is opened; 
         FIG. 5  is a plan view showing a state where the sheet supporting cover and an extension tray of the sheet transport device are opened; 
         FIG. 6  is a perspective view showing a state where the sheet supporting cover, an ADF cover, and the extension tray of the sheet transport device are opened: 
         FIG. 7  is a cross-sectional view of a main part of a sheet transport portion of the sheet transport device; 
         FIG. 8  is an explanatory view showing a relationship of arrangement of a first detection unit and a second detection unit of the sheet transport device; 
         FIG. 9  is a block diagram of a control system of the multifunction printer mounted with the sheet transport device; and 
         FIG. 10  is a flowchart of a control routine of a control unit according to the illustrative embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, a sheet transport device according to an illustrative embodiment of the present invention, which is applied to a multifunction printer, will be described while referring to the accompanying drawings. The sheet transport device exemplified in the illustrative embodiment is configured as a part of the multifunction printer which has not only an image reading function (scanning function) but also other functions (for example, printing function, copying function, and facsimile transmission/reception function). Also, the illustrative embodiment shown hereinbelow is a preferred specific example of the sheet transport device of the present invention and there are cases in which various technically preferable limitations are given thereto, but the scope of the present invention is not limited to these aspects unless stated to limit the present invention. Further, components of the illustrative embodiment shown hereinbelow may be appropriately replaced by existing components, and various variations, including a combination with other existing components, may be made. 
     Specifically, there is no limitation in combining the various functions of the multifunction printer with each other and in providing additional functions by using options or the like (for example, a sheet feed cassette). Also, the maximum size (for example, A4 or A3) and the minimum size (for example, postcard size or business card size) of sheets (documents) which is allowed for the sheet transport function may be arbitrarily set. Therefore, the illustrative embodiment shown hereinbelow does not limit the contents of the invention described in the claims. The description hereinbelow will be based on the upper-lower, left-right, and front-rear directions shown in the drawings so as to facilitate the understanding of the relative positional relationship between parts of the multifunction printer. Also, each of the directions is defined on the assumption that the standing position of a user is the front side of the device. 
     [External Configuration of Multifunction Printer] 
     Referring to  FIG. 1  and  FIG. 2 , a multifunction printer  1  includes a main body unit  10 , a reading unit  20  which is arranged on an upper part of the main body unit  10 , and a sheet transport unit  30  (example of a sheet transport device) which is arranged on an upper part of the reading unit  20 . An image forming unit (not shown) is accommodated by the main body unit  10 . 
     [Configuration of Main Body Unit  10 ] 
     The main body unit  10  includes drawer-type recording sheet accommodating cassettes  11  and  12  which are arranged in two in the upper-lower direction, and an outlet  13  which is opened at an upper part of the recording sheet accommodating cassettes  11  and  12  and used to discharge recording sheet after an image forming process by the image forming unit. Also, over the outlet  13  and at a front surface side of the main body unit  10 , there is disposed an operation unit  16  including a switch  14  which is used for a user to input various instructions to the multifunction printer  1 , and a touch panel-type liquid crystal display panel  15 . 
     The recording sheet accommodating cassettes  11  and  12  may accommodate recording sheet of different sizes (for example, A4 size and A3 size, and the like). Also, the lower recording sheet accommodating cassette  11  is expandable by an option or the like, and also there is a case in which the lower recording sheet accommodating cassette is not disposed. The outlet  13  is an opening which is used to discharge the recording sheet after the image forming process, and, for example, may be a known sheet feed and discharge port which allows the manual feed of postcards and the like. The operation unit  16  may be arranged with, for example, known numeric keys and button-type switches such as mode selection switches for various functional modes (scanning function, printing function, copying function, facsimile transmission and reception function, and the like) of the multifunction printer  1  in combination with the liquid crystal display panel  15 . Further, in the illustrative embodiment, the operation unit  16  is arranged on a front cover  17  which is the upper part of the main body unit  10 . The front cover  17  is also used as a front cover of the reading unit  20 . Therefore, the front cover  17  may be integrally configured with a circumferential wall  21  of the reading unit  20  and the operation unit  16  may be arranged in the reading unit  20 . 
     The main body unit  10  includes driving mechanisms which are necessary to perform the various functional modes of the multifunction printer  1 , and a control unit (not shown) which controls the driving mechanism. The control unit controls not only the driving mechanisms arranged in the operation unit  16  and the main body unit  10  but also the various driving mechanisms disposed in the reading unit  20  and the sheet transport unit  30 . The driving mechanisms of the reading unit  20  and the sheet transport unit  30  may be subject to another control unit which may be interface-connected with the control unit of the main body unit  10  to ensure the convenience of option installation, unit exchange, and the like. 
     Within the main body unit  10 , a drive motor which drives a recording sheet transport mechanism portion which is used to discharge and transport the recording sheet accommodated in the recording sheet accommodating cassettes  11  and  12  through a sheet feed and discharge path reaching a sheet discharge port  13 , the image forming unit which is arranged in the transport path, various sensors which detect the transport timing and jamming of the recording sheet in the sheet feed and discharge path, and the like are arranged to be controlled by the control unit. Known techniques may be employed in the specific configurations and control examples thereof, and thus the description will be omitted herein. 
     [Configuration of Reading unit  20 ] 
     The reading unit  20  has a structure openable and closable with respect to the main body unit  10 , and the pivot center thereof is the axis (not shown) extending in the left-right direction at a rear surface side (rear side shown in  FIG. 1 ) of the main body unit  10  and the reading unit  20 . 
     When the reading unit  20  is displaced to an open position, an upper surface side of the main body unit  10  is opened, and maintenance works may be performed on each part, such as the image forming unit, built into the main body unit  10  although not shown in detail. 
     The reading unit  20  is surrounded at four sides by the circumferential wall  21 , and. As shown in  FIG. 2 , the reading unit  20  includes a first platen  22  formed of a transparent glass or the like, an abutting member  23  along one edge of the first platen  22 , and a second platen  24  formed of transparent glass or the like and arranged at the opposite side from the first platen  22  with respect to the abutting member  23  on an upper surface thereof. 
     The first platen  22  is used as a supporting surface which supports a still document, and the abutting member  23  is used in the positioning of the document when the user supports the fixed document with the first platen  22 . Also, the second platen  24  is used to read the sheets by using the sheet transport unit  30 . The first platen  22  has a rectangular shape extending in the front-rear and left-right directions, and the size thereof is larger than the size at which a document having the maximum document size (for example, A3 size) which is readable by the device can be supported. The first platen  22  and the second platen  24  may be integrally formed, and the abutting member  23  may be arranged on upper surfaces thereof by adhesion or the like. 
     As shown in  FIG. 3  and  FIG. 4 , a movable reading sensor  25  is arranged in a movable manner below the first platen  22  and the second platen  24 . The movable reading sensor  25  reciprocates by the driving of a moving motor, which is not shown, along a guide shaft  26  spanning in the left-right direction in an inner space surrounded by the circumferential wall  21  of the reading unit  20 . When the image on the sill document is read, the movable reading sensor  25  moves in the left-right direction (sub-scanning direction) within the reading unit  20  along a lower surface side of the first platen  22  as shown in  FIG. 3  to read the image on the still document. Also, when the image on the sheet transported by the sheet transport unit  30  is read, the movable reading sensor  25  moves to a position facing the second platen  24  as shown in  FIG. 4 , and reads the image on the sheet transported by the transport unit  30  at that position. The movable reading sensor  25  reads the image upward while the first platen  22  and the second platen  24  are arranged over the movable reading sensor  25 . 
     [Configuration of Sheet Transport Unit  30 ] 
     As shown in  FIG. 2 , the sheet transport unit  30  has a structure openable and closable with respect to the reading unit  20 , and the pivot center thereof is an axis P extending in the left-right direction at a back surface side of the reading unit  20  and the sheet transport unit  30 . Therefore, the sheet transport unit  30  may be displaced to a closed position as shown in  FIG. 1 , that is, a state where the upper surface of the reading unit  20  having the first platen  22  is covered (closed state) and to the open position as shown in  FIG. 2 , that is, a state where the upper surface of the reading unit  20  having the first platen  22  is exposed (open state). In a case where the sheet transport unit is at the closed position, the sheet transport unit  30  functions as a document cover which covers the first platen  22  and a sheet which is a reading object supported by the first platen  22 . 
     As shown in  FIG. 1 , the sheet transport unit  30  is arranged with a sheet supporting cover  31  to the left from around the center of the upper surface, a sheet discharge tray  32  to the right from around the center of the upper surface, and an extension tray  33  near a right end of the upper surface. The sheet transport unit  30  is surrounded at four sides by the frame wall  34 , and the frame wall  34  is formed by assembling a plurality of divided panels. In the illustrative embodiment, a left side surface portion is further divided vertically into a lower panel portion  34 A and an upper panel portion  35 A. Also, a driving unit cover portion  34 B is mounted on a rear surface of the sheet transport unit  30 . 
     The sheet supporting cover  31  may be displaced to the closed state shown in  FIG. 3  in which a left side from the center in the left-right direction of the sheet transport unit  30  is covered and the open state shown in  FIG. 4  and  FIG. 5  in which the transport sheet supporting surface  31 A is exposed. The sheet supporting cover  31  is pivotably attached to the sheet transport unit  30  using a pivot shaft  31 B which is engaged in the front-rear direction with a cover member (not shown) placed inside the frame wall  34  near the center in the left-right direction of the sheet transport unit  30  as a fulcrum. When the sheet supporting cover  31  is in the opened state, the sheet transport function may be used as an automatic document feeder (ADF). Therefore, when the sheets are held by the transport sheet supporting surface  31 A, the sheet supporting cover  31  is in the opened state with an inclination in which the pivot shaft  31 B side is down forward and the free end side is up rearward. Also, when the sheet supporting cover  31  is in the opened state, an ADF cover  35  integrally having the upper panel portion  35 A and a sheet guide surface  36  are exposed. On the sheet guide surface  36 , a pair of sheet guide members  37  which support both side edges along the transport direction of the sheets held by the transport sheet supporting surface  31 A are arranged. The pair of sheet guide members  37  suppress skew transport by guiding both side edges of the sheets held by the transport sheet supporting surface  31 A. For example, relative approach and separation may be made between a position corresponding to the width of the maximum allowable size (for example, A3 vertical) set in advance using the main body unit  10  or the sheet transport unit  30  as shown with the solid line in  FIG. 5  and a position corresponding to the width of the minimum allowable size (for example, postcard horizontal) shown with the chain line in  FIG. 5 . 
     Between the transport sheet supporting surface  31 A and the ADF cover  35 , a sheet feed inlet  38  for the sheet is formed. Between the transport sheet supporting surface  31 A and the sheet discharge tray  32 , a sheet discharge port  39  for the sheet is formed. 
     In a normal state, the ADF cover  35  covers the sheet transport portion  40  from the sheet feed inlet  38  to the sheet discharge port  39  from above. When maintenance is performed or sheet jamming or the like occurs, the sheet transport portion  40  is opened upward as shown in  FIG. 6 . The ADF cover  35  may be an attachable and detachable type instead of a pivoting type as long as the sheet transport portion  40  placed therebelow can be opened. 
     [Overall Configuration of Sheet Transport Portion  40 ] 
     Next, the specific configuration of the sheet transport portion  40  will be described while referring to  FIG. 7 . In the description below, the sheet transport direction is referred to as a downstream side based on the sheet transport direction (refer to the thick two-dot chain line) and the opposite side is referred to as an upstream side. 
     The sheet transport portion  40  includes a sheet feed unit  41  which is arranged at the downstream side of the sheet feed inlet  38 , a fixed reading unit  42  which is arranged at the downstream side of the sheet feed unit  41 , a U-turn transport unit  43  which is arranged at the downstream side of the fixed reading unit  42 , and a discharge unit  44  which is arranged at the downstream side of the U-turn transport unit  43  to reach the sheet discharge port  39 . 
     [Configuration of Sheet Feed Unit  41 ] 
     The sheet feed unit  41  performs sheet feed toward the downstream side while separating, one by one, the sheets set in a face-up state on the transport sheet supporting surface  31 A (sheet guide surface  36 ) sheet by sheet from the uppermost one. The sheet feed unit  41  includes a sheet feed roller  45 , a separation roller  46 , a separation pad  47 , a swinging holder  48 , and a relay roller pair  49 , a front sensor SW1 (example of a first detection unit), and a maximum sheet sensor SW2. 
     The sheet feed roller  45  is rotation-driven at a position along the sheet guide surface  36  at the transport direction upstream side of the sheet feed unit  41 . The sheet feed roller  45  is rotation-driven in a predetermined direction (clockwise direction in  FIG. 7 ), that is, a direction in which the sheets are drawn to the separation roller  46  by transmitting the driving force of a transport motor (not shown) of a known driving transmission mechanism. In this manner, the sheet feed roller  45  feeds the sheets set on the transport sheet supporting surface  31 A toward the separation roller  46 . 
     The separation roller  46  is rotatably supported to a frame, which is not shown, at the transport direction downstream side from the sheet feed roller  45 . The separation roller  46  is rotated in the same direction as the sheet feed roller  45  by the power transmission caused by the driving of the transport motor. 
     The separation pad  47  is arranged at a position facing the separation roller  46 . The separation pad  47  is biased by a biasing spring  50  toward an outer circumferential surface of the separation roller  46 . In this manner, the separation roller  46  operates together with the separation pad  47  and separates only the uppermost sheet in contact with the separation roller  46  from the other sheets to transport the uppermost sheet toward the downstream side. 
     The swinging holder  48  is supported by a rotating shaft  46 A of the separation roller  46  via a bearing, which is not shown. The swinging holder  48  extends from the rotating shaft  46 A of the separation roller  46  toward the transport direction upstream side, and rotatably supports the sheet feed roller  45 . The swinging holder  48  is swung about the rotating shaft of the separation roller  46  by the power transmission from the transport motor. 
     At the transport direction downstream side of the sheet feed unit  41 , the relay roller pair  49  transports the sheet after the separation toward the fixed reading unit  42 . The number of the relay roller pair  49  may be increased or decreased depending on the length of the transport path corresponding to the allowable size set in advance, for example, in the main body unit  10  and the sheet transport unit  30 . 
     The front sensor SW1 has a tip end projecting from an upper surface of the sheet guide surface  36  at a position close to the swinging holder  48 , and detects the setting of the sheet on the transport sheet supporting surface  31 A by detecting a tip end (lower end) of the sheet set on the transport sheet supporting surface  31 A. The maximum sheet sensor SW2 has a tip end projecting from the upper surface of the sheet guide surface  36  near an end portion (or near both ends) of the sheet guide surface  36  at the opposite side of the front sensor SW1 with respect to the swinging holder  48 , and detects that the sheet set on the sheet supporting surface  31 A is a document having a maximum sheet size. The maximum sheet sensor SW2 is used when automatically selecting which size of the recording sheet accommodated in the recording sheet accommodating cassettes  11  and  12  for the copying function. 
     [Configuration of Fixed Reading Unit  42 ] 
     If necessary, the fixed reading unit  42  reads the image on the sheet that is transported from the sheet feed unit  41 . The fixed reading unit  42  is arranged in a portion of the linear-shaped path at the transport direction downstream side than the relay roller pair  49  and at the upstream side than the U-turn transport unit  43 . The fixed reading unit  42  includes a fixed reading sensor  51 , a third platen  52 , a sheet pressing member  53  which faces the fixed reading sensor  51  across the third platen  52 , and a biasing spring  54  which biases the sheet pressing member  53  toward the third platen  52 , and a holder  55  which holds the fixed reading sensor  51  and the third platen  52 . 
     A contact image sensor (CIS) is used in the fixed reading sensor  51 . The fixed reading sensor  51  is arranged with a reading surface of the image directed upward. The reading range of the fixed reading sensor  51  in a main scanning direction (front-rear direction) is the length corresponding to a short side of the A3 size. Herein, the sheet which is transported by the relay roller pair  49  passes between the third platen  52  which faces the reading surface of the fixed reading sensor  51  and the sheet pressing member  53 , and the sheet is transported while being pressed to the third platen  52  side by the sheet pressing member  53 . The fixed reading sensor  51  reads the image on a back surface of the passing sheet in a case where the sheet has images on both sides. 
     The holder  55  holds the third platen  52  in a state where the fixed reading sensor  51  is received, and holds a pinch roller  49 A of the relay roller pair  49 , the separation pad  47 , and the biasing spring  50  to form these into an unit. The separation pad  47  is held displaceably by the holder  55  via a spring cover  56 A. 
     [Configuration of U-Turn Transport Unit  43 ] 
     The U-turn transport unit  43  includes a main transport roller  56 , a pinch roller  57 , and a pinch roller  58  so as to form a U-turn transport path  59 . 
     The main transport roller  56  is rotatably supported by a frame, which is not shown, placed inside the lower panel portion  34 A, and is rotation-driven by the sheet feed motor or an additional drive motor (not shown). In the illustrative embodiment, the range of approximately one-half to the left of an outer circumferential surface of the main transport roller  56  is used as the U-turn transport path  59 . The pinch roller  57  is arranged near an upper end of the main transport roller  56  to be in contact with the main transport roller  56 . The pinch roller  58  is arranged to be in contact with the main transport roller  56  from the lower left of the main transport roller  56 . Also, at a position which faces the main transport roller  56 , a plurality of guide ribs  35 C which project from an inner surface  35 B of the ADF cover  35  toward the main transport roller  56  side and extend in a direction along the transport direction and a guide surface  34 D which bends and extends inside from the lower panel portion  34 A and is shaped along the outer circumferential surface of the main transport roller  56  are arranged. Therefore, the guide ribs  35 C and the guide surface  34 D form the U-turn transport path  59  so as to face the main transport roller  56  with a gap. Also, the guide ribs  35 C fix a shaft  60  of the pinch roller  57 . 
     [Configuration of Discharge Unit  44 ] 
     The discharge unit  44  includes a guide panel  61  which spans in a range reaching the vicinity of the sheet discharge port  39  from a position which faces near a lower end of the main transport roller  56  at the transport direction downstream side than the pinch roller  58 , a lower guide panel portion  32 A which extends so as to face the guide panel  61  with a gap to form a sheet discharge path  62 , a sheet discharge roller  63 , and a pinch roller  64  which faces the sheet discharge roller  63 . The lower guide panel portion  32 A integrally has the sheet discharge tray  32  which is placed at the downstream side. 
     An upstream end side of the guide panel  61  is arranged with a sheet pressing member  65  which faces an upper surface of the second platen  24 , and a biasing spring  66  which biases the sheet pressing member  65  toward the second platen  24 . Also, the abutting member  23  (refer to  FIG. 2 ) is placed at a position of the guide panel  61  and specifically, at an upstream end of the downstream side lower guide panel portion  32 A. Further, a sheet pad  68  which has a white sheet  67  is attached to a surface of the lower guide panel portion  32 A of the sheet discharge tray  32  at an area facing the first platen  22 . The sheet pad  68  presses the sheet which is supported by the first platen  22  from above. 
     [Relationship of Arrangement of Front Sensor SW1 and Rear Sensor SW3] 
     Next, the relationship of arrangement of the front sensor SW1 and a rear sensor SW3 according to the illustrative embodiment will be described while referring to  FIG. 8 . In the description below, the minimum size of the sheet which can be transported by the sheet transport unit  30  is assumed to be the postcard size (100 mm×148 mm). Also, the shortest inner surface facing distance W1 between the sheet guides  37  is assumed to be the short side (100 mm) of the postcard. Further, there will be exemplified a case where a standard name card (55 mm×91 mm) which is a sheet smaller than the minimum sheet size (postcard size) is used to perform image reading. In  FIG. 8 , the front sensor SW1 and the rear sensor SW3 are arranged in a plan view for the convenience of illustration, and the dimensions (distance) are based on the center of each sensor. 
     In the sheet transport unit  30  according to the illustrative embodiment, the separation roller  46  and the front sensor SW1 are disposed to be separated from each other at least in the sheet width direction (front-rear direction) which is orthogonal to a predetermined transport direction of the sheet set on the sheet guide surface  36 , and a distance W2 between the front sensor SW1 and an end edge of the separation roller  46  at a side of the front sensor SW1, that is, a front side end edge of the separation roller  46  is longer than a short side width W3 of a name card NC. The distance W2 is the straight-line distance in the front-rear direction orthogonal to the transport direction. 
     Herein,  FIG. 8  assumes a case where the name card NC (1) is set toward the separation roller  46  such that a long side abuts onto the sheet guide  37  at the side where the front sensor SW1 is arranged, and the short side is arranged along the sheet width direction. In this case, even if the front sensor SW1 detects the setting of the sheet on the sheet guide surface  36  and the separation roller  46  is rotated by a user operation for the start of image reading, the name card NC (1) is not transported since the name card NC (1) is not in contact with the separation roller  46 . In this manner, the transporting of the name card NC (1) may be prevented even in a case where the name card NC (1) which is smaller than the minimum sheet size is set on the sheet guide surface  36 , and the separation roller  46  is driven. 
     Also, as shown in  FIG. 7  and  FIG. 8 , the rear sensor SW3 is disposed at the transport direction downstream side than the separation roller  46  and at the opposite side of the front sensor SW1 in the sheet width direction with respect to the separation roller  46 , and detects the sheet which is transported further toward the transport direction downstream side than the separation roller  46 . In a plan view of the rear sensor SW3, a sheet width direction distance W4 from the front sensor SW1 is longer than the long side of the name card NC (2), and more specifically, longer than a diagonal line which is longer than the long side as shown in the name card NC (3) considering a case where so-called skew transport is performed. 
     Even if the front sensor SW1 detects the horizontal name card NC (2) which is set on the sheet guide surface  36  and the separation roller  46  transports the name card NC (2), the long side direction does not reach the rear sensor SW3 and the rear sensor SW3 does not detect the name card NC (2). At this time, in the illustrative embodiment, the driving of each of the rollers including the main transport roller  56  is stopped when the rear sensor SW3 does not detect the name card NC (2). Therefore, the name card NC (2) is not continuously transported toward the transport direction downstream side and the transporting of the name card NC (2) is stopped by the nip caused by the main transport roller  56  and the pinch roller  57  operating together. Accordingly, even if the transporting of the name card NC (2) is started by the separation roller  46 , the transporting is stopped near the main transport roller  56 . By opening the ADF cover  35  and opening the transport path near the main transport roller  56 , that is, the upstream side path of the main transport roller  56  and the curved path  59 , the name card NC (2) which is stopped at the upstream side of the transport path than the rear sensor SW3 may be easily taken out. 
     In this manner, even in a case where the sheet smaller than the minimum sheet size is set on the sheet guide surface  36  and the transporting of the sheet is started, the inconvenience that the sheet is transported toward the transport path further to the downstream side of the main transport roller  56  may be suppressed. 
     The main transport roller  56  integrally configures the U-turn transport path  59  which U-turns the transport direction of the sheet. Also, since the rear sensor SW3 is used also for determining the start timing of the reading by the movable reading sensor  25 , the rear sensor is arranged at the downstream side of the U-turn transport path  59  near the movable reading sensor  25  which is at a stop position. Therefore, the ADF cover  35  has the upper panel portion  35 A which is a portion on the left side surface of the sheet transport unit  30  in an integral manner such that the range including the U-turn transport path  59  at the upstream side than the rear sensor SW3 is opened when the ADF cover  35  is opened. 
     In the case of a relatively thick and firm sheet such as the name card NC (2), the sheet is unlikely to fall by its weight to the U-turn transport path  59 . Therefore, if the distance from a nip portion between the main transport roller  56  and the pinch roller  57  to a nip portion between the main transport roller  56  and the pinch roller  58  is more separated than the short side W3 of the name card NC (2), the name card NC (2) which passes through the nip portion between the main transport roller  56  and the pinch roller  57  cannot reach the nip portion between the main transport roller  56  and the pinch roller  58  when the horizontal name card such as the name card NC (2) is set and transported. Accordingly, the transporting of the name card NC (2) to the downstream side of the main transport roller  56  may be suppressed. 
     An end portion of the ADF cover  35  at a side of the sheet feed inlet  38  has an end edge extending along the sheet width direction and formed with a projecting portion  35 D at a center part thereof, which projects in a direction along the sheet guide surface  36  with respect to both end portions thereof. 
     It may be preferable that the projecting portion  35 D is formed to be the range facing the separation roller  46 , that is, the width of the separation roller  46  or wider. It may be more preferable that the projecting portion is formed to be wider than the range having the length (distance W4) corresponding to the long side of the small sheet which is a non-allowable sheet or longer from a position facing the rear sensor SW3. In the illustrative embodiment, the projecting portion  35 D has approximately the same width as the inner surface facing distance W1 which is the shortest position at which the sheet guides  37  correspond to the sheet having the minimum sheet size. Also, the projecting length L1 of the projecting portion  35 D is set such that the distance L2 between the projecting portion  35 D and the nip portion of the separation roller  46  and the separation pad  47  is longer than the short side W3 of the name card NC (4). 
     Therefore, when the sheet is set from the sheet guide surface  36  to the separation roller  46 , the setting of the small non-allowable sheet on the sheet guide surface  36  may be suppressed by the projecting portion  35 D. Also, even if the sheet is forcibly set as in the case of the name card NC (4), it is difficult to set the name card NC (4) to reach the separation roller  46 , and the transporting of the small non-allowable sheet may be suppressed. In the illustrative embodiment, the small non-allowable sheet is the name card NC, and the name card NC is set on the sheet guide surface  36 . However, when a sheet larger in size than the name card NC is the small non-allowable sheet, there may be a case where the sheet is set from the sheet guide surface  36  across the sheet supporting surface  31 A. 
     [Configuration of Control System] 
     The various driving systems installed in the main body unit  10 , the reading unit  20 , and the sheet transport unit  30  and the reading data are controlled by the control unit installed in the main body unit  10 . 
     As shown in  FIG. 9 , the main body unit  10  includes a CPU  71 , ROM  72 , RAM  73 , and NVRAM  74 , and includes a timer  76 , an image forming unit  77 , a LAN communication unit  78 , a PSTN communication unit  79  and the like in addition to the above-described operation unit  16  (including the liquid crystal display panel  15 ) via a bus  75 . 
     The CPU  71  executes predetermined processing following a control program stored in the ROM  72 , and executes control on each portion of the multifunction printer  1  based thereon. By arranging the CPU or the like in the reading unit  20  and the sheet transport unit  30  and performing comprehensive control on the main body unit  10  with the CPU, the unit exchange of the reading unit  20 , versatility for the installation of an option on the sheet transport unit  30  or the like may be expanded. 
     The image forming unit  77  is received by the main body unit  10 , and includes an electro-photographic or inkjet printing mechanism. The LAN communication unit  78  is configured to have a communication interface device corresponding to a wireless LAN and a communication interface device corresponding to a wired LAN. The PSTN communication unit  79  is configured to have various equipments necessary for public switched telephone network (PSTN) connection such as a fax modem and a voice codec. 
     The CPU  71  controls the driving of the movable reading sensor  25  of the reading unit  20  and a reading motor  80  which moves the movable reading sensor  25 . The image data read by the movable reading sensor  25  is processed by an image processing circuit such as an analog front end (AFE) which is not shown and stored by, for example, the RAM  73  or the NVRAM  74 . In the case of the copying function with which the read image data is copied, the stored data is processed by the image forming unit  77  to perform image forming processing on the recording sheet. The specific configuration and the description of the control related to the movement of the movable reading sensor  25  are omitted since known techniques may be employed therein. Also, the CPU  71  causes the movable reading sensor  25  to execute white reference correction or the like. 
     The CPU  71  executes the same processing as the above-described movable reading sensor  25  on the image data read by the fixed reading sensor  51  of the sheet transport unit  30 , and controls the driving of a transport motor  81  which drives each of the rollers of the sheet transport portion  40 . Also, the CPU  71  controls the driving of the transport motor  81  and the reading timing of each of the reading sensors  25  and  51  according to the detection results of the front sensor SW1, the maximum sheet sensor SW2, and the rear sensor (second detection unit) SW3. 
     [Sheet Transport Operation] 
     In the above-described configuration, the front sensor SW detects the setting of the sheets when the sheets having a size allowed to be transported by the sheet transport unit  30  according to the illustrative embodiment is appropriately set on the sheet supporting surface  31 A across the sheet guide surface  36 . When a reading start operation (scanning function or copying function) is made in this state, the sheets set on the sheet supporting surface  31 A are drawn by the sheet feed roller  45 , and the uppermost sheet of the sheets is separated by the separation roller  46  and the separation pad  47  operating together and is transported toward the relay roller pair  49 . 
     The relay roller pair  49  transports the sheet which is transported toward downstream side. In a case of a double-sided reading, the image on the back surface side is read by the fixed reading sensor  51  when the sheet passes between the third platen  52  and the sheet pressing member  53 . Also, in a case of a single-sided reading, the fixed reading sensor  51  does not perform the reading of the image and transports the sheet as it is toward the U-turn transport path  59 . 
     In the U-turn transport path  59 , the main transport roller  56  is driven and rotated, and transports the sheet to U-turn the sheet toward the downstream side by operating together with the pinch roller  57 , the transport rib  35 C, the guide surface  34 D, and the pinch roller  58 . 
     When the rear sensor SW3 detects the tip end of the sheet passing through the U-turn transport path  59 , the image on the front surface side of the sheet is read by the movable reading sensor  25  when the sheet passes between the sheet pressing member  65  and the second platen  24 . Further, the sheet after the reading of the image is transported toward the discharge unit  44 , passes through the sheet discharge path  62  between the guide panel  61  and the lower guide panel portion  32 A, and the sheet is discharged from the sheet discharge port  39  toward the sheet discharge tray  32  as the sheet discharge roller  63  and the pinch roller  64  operate together with each other. 
     Next, the processing executed by the CPU  71  when the sheet smaller than the minimum sheet size is set on the sheet guide surface  36  will be described while referring to the flowchart of  FIG. 10 . 
     When the main power (not shown) of the operation unit  16  is turned on, the CPU  71  performs initial operations such as the confirmation of the LAN connection by the LAN communication unit  78 , the shading correction using white reference data and the like (step S 1 ), and proceeds to step S 2  in a standby state. 
     In step S 2 , the CPU  71  determines whether the sheet is set on the sheet guide surface  36  based on the detection by the front sensor SW1. When the front sensor SW1 detects the sheet, the process proceeds to step S 3 . When the front sensor SW1 does not detect the sheet, the routine continues to monitor whether the sheet is set. 
     In step S 3 , the CPU  71  displays that the sheet is set, on the liquid crystal display panel  15  or the like and proceeds to step S 4 . At this time, display is made to ask the user to select whether the reading of the sheet is to be performed on one side surface or both side surfaces and which mode (scanning mode, copying mode, and facsimile mode) is to be used. After the selection is made, the user is asked to enter the start operation to start the reading of the sheet. 
     In step S 4 , the CPU  71  monitors whether the mode necessary for the reading of the image on the sheet (scanning mode, copying mode, and facsimile mode) is selected and whether the reading start operation is made. When the reading start operation is made, the process proceeds to step S 5 . When the reading start operation is not made, the routine continues monitoring. Optionally, when a predetermined time between the detection of the sheet in step S 2  and the operation in step S 4  elapses, error notification, entering a sleep mode or the like may be made. 
     In step S 5 , the CPU  71  drives the sheet feed roller  45 , the separation roller  46 , the relay roller pair  49 , and the main transport roller  56  by controlling the transport motor  81 , and proceeds to step S 6  by starting the transporting of the sheet. 
     In step S 6 , the CPU  71  monitors whether the rear sensor SW3 detects the sheet. When the rear sensor SW3 detects the sheet, the process proceeds to step S 7 . When the rear sensor SW3 does not detect the sheet, the process proceeds to step S 13 . 
     In step S 7 , the CPU  71  starts the reading of the image on the sheet by the movable reading sensor  25  and proceeds to step S 8 . 
     In step S 8 , the CPU  71  monitors whether the front sensor SW1 detects the trailing edge of the sheet. When the front sensor SW1 becomes off by transporting of the sheet, the transporting of all of the set sheets is finished and the process proceeds to step S 9 . When the front sensor SW is not off, the process returns to step S 5  so as to continue the processing on the second and subsequent sheets. 
     In step S 9 , the CPU  71  monitors whether the rear sensor SW3 detects the trailing edge of the sheet. When the rear sensor SW3 is off by transporting of the sheet, the trailing edge of the sheet passes from the rear sensor SW3 and the process proceeds to step S 10 . When the sheet continues to be detected with the rear sensor SW3 remaining on, the process proceeds to step S 14 . 
     In step S 10 , the CPU  71  finishes the reading of the image on the sheet by the movable reading sensor  25  and proceeds to step S 11 . 
     In step S 11 , the CPU  71  monitors whether the sheet passes from the sheet discharge port  39  by a sheet discharge sensor which is not shown. When the sheet discharge is completed, the process proceeds to step S 12 . When the sheet discharge is not completed, the routine is continued to be monitored. When the sheet discharge sensor is turned from on to off in a state where the front sensor SW1 detects the sheet, the transporting of the subsequent sheets may be started if the second and subsequent sheets are present. 
     In step S 12 , the CPU  71  stops the driving of the transport motor  81  so as to stop the driving of the sheet feed roller  45 , the separation roller  46 , the relay roller pair  49 , and the main transport roller  56 , and finishes the processing. 
     When the rear sensor SW3 does not detect the sheet in step S 6 , the CPU  71  monitors in step S 12  whether the count by the timer  76  exceeds four seconds from the start of the transporting of the sheet in step S 4 . When the count by the timer  76  exceeds four seconds, the CPU  71  proceeds to step S 15 . When the count by the timer  76  does not exceeds four seconds, the process proceeds to step S 5  to continue the transporting of the sheet as assuming the sheet is being transported. 
     When the rear sensor SW3 detects the sheet in step S 9 , the CPU  71  monitors in step S 14  whether the total sheet transport distance (including the second and subsequent sheets) is within 900 mm from the start of the transporting of the sheet in step S 4 . In other words, since the maximum sheet size by the sheet transport unit  30  according to the illustrative embodiment is A3 (long side is 420 mm), the fact that the rear sensor SW3 does not become off means not the normal transporting of the sheet even if two sheets of A3 sheets are transported without being separated. Therefore, the CPU  71  determines whether to transporting has been made perform 900 mm, and continues the transporting of the sheet by proceeding to step S 9  if the distance is within 900 mm and proceeds to step S 15  when the distance exceeds 900 mm. 
     In step S 15 , the CPU  71  performs jamming notification by the liquid crystal display panel  15  or the like if the determination result in step S 13  or step S 14  is abnormal as jamming of the sheet occurs, proceeds to step S 12 , and stops the driving of the transport motor  81 . 
     According to this processing, when the rear sensor SW3 does not detect the sheet even if the front sensor SW1 detects the sheet and four seconds elapse after the start of the transporting of the sheet, the driving of the rollers is stopped and the transporting is stopped. At this time, the sheet is stopped at the upstream side of the transport path than the rear sensor SW3, and therefore, the sheet may be easily taken out by opening the ADF cover  35 . 
     According to the above configuration, in the sheet transport unit  30  according to the illustrative embodiment, the front sensor SW1 is arranged to be more separated than the short side width W3 of the name card NC from the end edge of the separation roller  46 , and the rear sensor SW3 is arranged to be more separated than the long side of the name card NC from the front sensor SW1. Therefore, the transporting of the name card NC which is smaller than the minimum sheet size may be suppressed, and, even if the sheet is transported, the transporting of the sheet is stopped at the upstream side than the rear sensor SW3 since the sheet is not detected by the rear sensor SW3. Accordingly, even in a case where the transporting of the sheet is started, the transporting of the sheet to a position where it is difficult for the sheet to be taken out may be suppressed or prevented. 
     While the present invention has been shown and described with reference to certain illustrative embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims. 
     For example, in the above-described illustrative embodiment, the minimum sheet size of the sheet transport unit  30  is the postcard size (100 mm×148 mm), the shortest inner surface facing gap W1 of the sheet guide  37  is the short side (100 mm) of the postcard, and the distance W2 between the front sensor W1 and the end edge of the separation roller  46  at a side of the front sensor SW1 is longer than the short side width W3 of the name card NC. However, these settings are optional.