Patent Publication Number: US-11643290-B2

Title: Sheet feeder and image forming apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority from Japanese Patent Application No. 2019-158615 filed on Aug. 30, 2019, the content of which is incorporated herein by reference in its entirety. 
     TECHNICAL FIELD 
     Aspects disclosed herein relate to a sheet feeder and an image forming apparatus including the sheet feeder. 
     BACKGROUND 
     Some sheet feeders are known that feed a sheet from outside an apparatus to inside the apparatus. Such a sheet feeder includes a sheet tray for supporting sheets to be fed, a photosensor including a light emitter and a light receiver facing each other, and an actuator including a contact element and a shielding plate. The contact element is pivotable by contact with a sheet supported on the sheet tray. The shielding plate is configured to pass through between the light emitter and the light receiver in association with the rotation of the contact element. The sheet feeder is configured to detect the presence or absence of a sheet on the sheet tray depending on whether the shielding plate is between the light emitter and the light receiver. 
     The photosensor of the sheet feeder is configured to detect the presence or absence of a sheet on the sheet tray. The light receiver may, however, receive light from outside, and thereby false detection may occur. 
     Thus, it is known that, to reduce such false light detection, the actuator further includes a light shield to cover the photosensor. 
     SUMMARY 
     Aspects of the disclosure provide a sheet feeder to further help reducing effects of outside light on a photosensor. 
     According to one or more aspects of the disclosure, a sheet feeder includes a sheet tray, a feed roller, a sheet feed frame, a contact element, a guide frame, a photosensor, a shield, and a linkage. The feed roller is configured to feed a sheet on the sheet tray. The sheet feed frame holds the feed roller. The contact element is pivotable relative to the sheet feed frame. The guide frame is disposed below the sheet feed frame. The photosensor includes a light emitter for emitting a light and a light receiver for receiving the emitted light. The shield is movable between a light transmission position and a light shield position. The linkage connects the contact element and the shield such that the shield is movable in association with pivoting of the contact element. The light receiver receives the light emitted from the light emitter when the shield is at the light transmission position. The shield at the light shield position shields the light receiver from the light emitted from the light emitter. 
     This configuration may reduce effects of light from outside on the photosensor, and thus may prevent false light detection. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a cross-sectional view of an image forming apparatus including a sheet feeder according to an illustrative embodiment of the disclosure. 
         FIG.  2    is a perspective view of the sheet feeder. 
         FIG.  3    is a perspective view of the sheet feeder viewed from a guide frame. 
         FIG.  4    is a perspective view of a sheet detector of the sheet feeder. 
         FIG.  5 A  is a cross-sectional view of the sheet feeder. 
         FIG.  5 B  is a cross-sectional view of the sheet detector when no sheets are supported on a MP tray. 
         FIG.  6 A  is a cross-sectional view of the sheet detector when one sheet is supported on the MP tray. 
         FIG.  6 B  is a cross-sectional view of the sheet detector when the maximum number of sheets are supported on the MP tray. 
         FIG.  7    is a graph illustrating a relationship between the angle of a contact element and the angle of a shield, wherein the contact element and the shield are included in the sheet detector. 
         FIG.  8 A  is a cross-sectional view of the sheet detector when the angle of the contact element is θ 1 A. 
         FIG.  8 B  is a cross-sectional view of the sheet detector when the angle of the contact element is θ 1 B. 
         FIG.  9 A  is a cross-sectional view of the sheet detector when the angle of the contact element is θ 1 C. 
         FIG.  9 B  is a cross-sectional view of the sheet detector when the angle of the contact element is θ 1 D. 
         FIG.  10 A  is a cross-sectional view of the sheet detector when the angle of the contact element is θ 1 E. 
         FIG.  10 B  is a cross-sectional view of the sheet detector when the angle of the contact element is θ 1 F. 
         FIG.  11 A  is a perspective view of a sheet detector according to an alternative embodiment of the disclosure, wherein the sheet detector includes a second extension having a first shaft and a second shaft.  FIG.  11 B  is a cross-sectional view of the sheet detector when a contact element is at its initial state. 
         FIG.  12 A  is a perspective view of the sheet detector.  FIG.  12 B  is a cross-sectional view of the sheet detector, wherein a shield moves integrally with the contact element pivoting. 
         FIG.  13 A  is a perspective view of the sheet detector.  FIG.  13 B  is a cross-sectional view of the sheet detector, wherein the contact element pivots after the shield stops pivoting. 
     
    
    
     DETAILED DESCRIPTION 
     Illustrative embodiments of the disclosure will be described with reference to the accompanying drawings. 
     Image Forming Apparatus 
     An image forming apparatus  1  illustrated in  FIG.  1    is an example of an image forming apparatus including a sheet feeder according to an aspect of the disclosure. The image forming apparatus  1  includes a casing  2 , a sheet feed unit  3 , an image forming unit  5 , a discharge unit  7 , and a sheet feeder  4 . 
     In the following description, right and left sides of the page of  FIG.  1   , a side facing out of the page of  FIG.  1   , and a side facing into the page of  FIG.  1    are defined respectively as front, rear, left, and right sides of the image forming apparatus  1 . Upper and lower sides of the page of  FIG.  1    are defined respectively as upper and lower sides of the image forming apparatus  1 . 
     The casing  2  is box-shaped, and accommodates the sheet feed unit  3 , the image forming unit  5 , the discharge unit  7 , and the sheet feeder  4 . The casing  2  has an opening  2 A at its front and a multi-purpose tray (hereinafter referred to as a MP tray)  21  configured to open and close the opening  2 A. The MP tray  21  is an example of a sheet tray configured to support a sheet. The casing  2  has an upper surface covered by an upper cover  23 . 
     The MP tray  21  is rotatable about a rotation axis  21 A located at its lower end and extending horizontally in a left-right direction. The MP tray  21  is movable between a closed position to close the opening  2 A and an open position to open the opening  2 A. The MP tray  21  at the open position is configured to support one or more sheets S. FIG.  1  illustrates the MP tray  21  at the open position. The upper cover  23  includes a sheet discharge tray  23 A inclined downward to the rear. 
     The sheet feed unit  3  includes a sheet cassette  31 , a feed roller  32 , a separation roller  33 , a separation pad  33 A, and a registration roller pair  35 , and is configured to feed a sheet S from the sheet cassette  31  to the image forming unit  5 . The casing  2  defines therein a conveyance path P 1  extending from the sheet cassette  31  via the image forming unit  5  to the sheet discharge tray  23 A. 
     The sheet cassette  31  is configured to accommodate a stack of sheets S. The feed roller  32  is configured to feed a sheet S from the sheet cassette  31 , and the separation roller  33  and the separation pad  33 A separate the sheet S from subsequent sheets S, so that the sheet S is singly conveyed toward the conveyance path P 1 . 
     The sheet S is then conveyed along the conveyance path P 1  by the registration roller pair  35 , which is located downstream from the separation roller  33 , toward the image forming unit  5 . The registration roller pair  35  temporarily stops feeding the sheet, aligns the leading end of the sheet S, and then starts rotating at a predetermined timing to convey the sheet S toward a transfer position in the image forming unit  5 . 
     The image forming unit  5  is disposed above the sheet cassette  31  and configured to form an image on the sheet S. The image forming unit  5  includes a process cartridge  50  configured to transfer an image on a sheet S conveyed from the sheet feed unit  3 , an exposure unit  56  configured to expose a surface of a photosensitive drum  54  in the process cartridge  50 , and a fixing unit  60  configured to fix the image transferred on the sheet S by the process cartridge  50 . 
     The process cartridge  50  includes a developing roller  53 , the photosensitive drum  54 , and a transfer roller  55 . The exposure unit  56  includes a laser diode, a polygon mirror, a lens, and a reflecting mirror, and is configured to expose a surface of the photosensitive drum  54  by irradiating the surface with a laser beam based on image data inputted in the image forming apparatus  1  to expose the surface. 
     The photosensitive drum  54  is disposed adjacent to the developing roller  53 . The surface of the photosensitive drum  54  is positively and uniformly charged by a charger, and then exposed by the exposure unit  56 . Exposed areas on the surface of the photosensitive drum  54  are lower in electric potential than the other areas thereon, so that an electrostatic latent image is formed on the surface of the photosensitive drum  54  based on the image data. The electrostatic latent image on the surface of the photosensitive drum  54  is developed into a visible developer image with positively charged toner supplied from the developing roller  53 . 
     The transfer roller  55  is disposed facing the photosensitive drum  54 , and receives a negative transfer bias from a power source. While a sheet S is nipped at a transfer position between the transfer roller  55  and the photosensitive drum  54 , the developer image on the photosensitive drum  54  is transferred to the sheet S due to the transfer bias. 
     The fixing unit  60  includes a heat roller  61  and a pressure roller  62 . The heat roller  61  is configured to rotate by a drive force from the image forming apparatus  1 , and is configured to be heated by a heater. The pressure roller  62  is disposed facing the heat roller  61  and is rotatable. The sheet S having the transferred developer image is conveyed to the fixing unit  60 , in which the sheet S is nipped and conveyed by the heat roller  61  and the pressure roller  62 , and thus the developer image is fixed onto the sheet S due to the heat. 
     The discharge unit  7  includes a discharge roller pair  71  configured to discharge the sheet S conveyed from the fixing unit  60  to the outside of the casing  2 , specifically, to the sheet discharge tray  23 A. 
     The sheet feeder  4  is disposed adjacent to the opening  2 A of the casing  2  and configured to feed one or more sheets S on the MP tray  21  at the open position through the opening  2 A of the casing  2  toward the image forming unit  5 . The sheet feeder  4  includes a feed roller  41  to feed a sheet S, a separation roller  42  disposed downstream from the feed roller  41  in a conveyance direction in which a sheet S is conveyed, and a separation pad  43  disposed facing the separation roller  42 . The feed roller  41 , the separation roller  42 , and the separation pad  43  are disposed substantially in the center of the casing  2  in the left-right direction corresponding to a width direction of a sheet S orthogonal to a sheet feed direction. 
     The sheet feeder  4  includes a sheet detector  9  to detect presence or absence of a sheet S on the MP tray  21 . The sheet detector  9  includes a contact element  91  contactable with the sheet S on the MP tray  21 . The sheet detector  9  is configured to detect the presence of the sheet S on the MP tray  21  when the contact element  91  contacts the sheet S. The casing  2  defines inside a sheet feed path P 2 , which is a part of the conveyance path P 1 , extending from the separation roller  42  to the registration roller pair  35 . 
     The feed roller  41  is configured to feed a sheet S from the MP tray  21 . The separation roller  42  and the separation pad  43  separate the sheet S from subsequent sheets S, so that the sheet S is singly conveyed toward the sheet feed path P 2 . The registration roller  35  is configured to further convey the sheet S conveyed along the sheet feed path P 2  toward the image forming unit  5 . 
     The sheet feeder  4  includes a sheet feed frame  25  located above the sheet feed path P 2  and a guide frame  27  located below the sheet feed path P 2 . That is, the guide frame  27  is disposed below the sheet feed frame  25 . The guide frame  27  includes an internal space. The feed roller  41 , the separation roller  42 , and the contact element  91  are held by the sheet feed frame  25 . Leading ends of sheets S on the MP tray  21  contact the guide frame  27 . A sheet S fed by the feed roller  41  is guided by the guide frame  27  and conveyed into the sheet feed path P 2 . 
     As illustrated in  FIGS.  2  and  3   , the sheet feeder  4  has a sheet feed range W extending in the left-right direction. The sheet feed range W corresponds to a maximum size, in the left-right direction, of a sheet supportable on the MP tray  21 . The guide frame  27  is disposed within the sheet feed range W in the left-right direction. 
     Sheet Detector 
     As illustrated in  FIGS.  2  to  5 B , the sheet detector  9  includes the contact element  91 , a photosensor  92 , a shield  93 , and a linkage  94 . 
     The contact element  91  is a tongue-shaped part extending downward from the sheet feed frame  25 , and pivotably supported by the sheet feed frame  25 . The contact element  91  is located upstream from the feed roller  41  in a sheet feed direction in which a sheet S is fed. As illustrated in  FIG.  5 B  where no sheets S are on the MP tray  21 , an end  91   a  of the contact element  91  is located below a sheet support surface  21 B of the MP tray  21 . The contact element  91  is located on one side of the separation roller  42  in the left-right direction. In this embodiment, the contact element  91  is disposed to the right of the separation roller  42 . 
     The photosensor  92  is located in the internal space of the guide frame  27  and includes a light emitter  921  for emitting light and a light receiver  922  for receiving light emitted from the light emitter  921 . The light emitter  921  and the light receiver  922  are spaced from each other in the left-right direction. 
     The shield  93  is a plate-like member located in the internal space of the guide frame  27  and is movable, e.g., pivotable, between a light transmission position to allow transmission of light emitted from the light emitter  921  to the light receiver  922  and a light shield position to shield the light receiver  922  from light emitted from the light emitter  921 . 
     The shield  93  at the light shield position is located between the light emitter  921  and the light receiver  922  to shield the light receiver  922  from light emitted from the light emitter  921 . The photosensor  92  is configured to, when the light receiver  922  is shielded from the light emitted from the light emitter  921 , detect that no sheets S are on the MP tray  21 . 
     The shield  93  at the light transmission position is located away from a position between the light emitter  921  and the light receiver  922 , thereby allowing the light receiver  922  to receive light emitted from the light emitter  921 . The photosensor  92  is configured to, in response to that the light receiver  922  receives the light emitted from the light emitter  921 , detect that a sheet S is on the MP tray  21 . 
     The linkage  94  connects the contact element  91  and the shield  93  such that the shield  93  is movable, e.g., pivotable, in association with pivoting of the contact element  91 . The linkage  94  includes a first extension  95 , a second extension  96 , and a connection  97 . 
     The first extension  95  is a shaft extending from the contact element  91  toward one side in the left-right direction beyond the sheet feed range W. In this embodiment, the first extension  95  extends from the contact element  91  to the right. The first extension  95  is rotatably supported by the sheet feed frame  25 . The contact element  91  is pivotable about the first extension  95 . 
     The second extension  96  is a shaft extending from the shield  93  toward one side in the left-right direction beyond the sheet feed range W. In this embodiment, the second extension  96  extends from the shield  93  to the right. The second extension  96  is rotatably supported by the guide frame  27 . The shield  93  is pivotable about the second extension  96 . The first extension  95  is located upstream from the second extension  96  in the sheet feed direction and above the second extension  96  in an up-down direction. 
     The connection  97  connects the first extension  95  and the second extension  96  outside the sheet feed range W in the left-right direction. The connection  97  is located outside the sheet feed range W in the left-right direction, and thus may be prevented from interfering with a sheet S to be fed in the sheet feed range W. The connection  97  includes a first connection  971 , a second connection  972 , a boss  973 , and a slide groove  974 , which are all located outside of the sheet feed range W in the left-right direction. 
     The first connection  971  extends from the first extension  95  toward the second extension  96 . The first connection  971  is pivotable about the first extension  95  as the contact element  91  pivots. The second connection  972  extends from the second extension  96  toward the first extension  95 . The second connection  972  and the shield  93  are integrally pivotable about the second extension  96  integrally with the shield  93 . 
     The boss  973  is located at the first connection  971  and protrudes from the first connection  971  to the right. The slide groove  974  is defined in the second connection  972  and receives the boss  973  slidably. The boss  973  located at the first connection  971  is engaged in the slide groove  974  defined in the second connection  972 , thereby the first extension  95  communicating with the second extension  96 . 
     The shield  93  is located downstream from the contact element  91  in the sheet feed direction. The boss  973  is located downstream from the contact element  91  in the sheet feed direction, more specifically, closer to the shield  93  than the contact element  91 . The slide groove  974  is located upstream from the shield  93  in the sheet feed direction, more specifically, closer to the contact element  91  than the shield  93 . 
     The contact element  91  pivots integrally with the first connection  971  as contacting with a sheet S on the MP tray  21 , and the boss  973  pivots as the first connection  971  pivots. The boss  973  slides in the slide groove  974  while pivoting, thereby pivoting the second connection  972  which in turn pivots the shield  93 . 
     In this embodiment, the boss  973  is located at the first connection  971  and the slide groove  974  is defined in the second connection  972 . In some embodiments, the boss  973  may be located at the second connection  972  and the slide groove  974  may be defined in the first connection  971 . 
     In the sheet detector  9  structured as above, when no sheets S are on the MP tray  21 , that is, when the contact element  91  is in an initial state out of contact with any sheet S, as illustrated in  FIG.  5 B , the contact element  91  protrudes downward from the first extension  95  and the end  91   a  of the contact element  91  is located below the sheet support surface  21 B of the MP tray  21 . When no sheets S are on the MP tray  21 , the shield  93  is at the light shield position so the light receiver  922  is shielded from the light emitted from the light emitter  921 . In this state, the light receiver  922  does not receive the light emitted from the light emitter  921  so the photosensor  92  detects that no sheets S are on the MP tray  21 . 
     As illustrated in  FIG.  6 A , in a state where one sheet S is on the MP tray  21 , the contact element  91  pivots, as contacting with the sheet S, further downstream in the sheet feed direction than when no sheets S are on the MP tray  21 . The shield  93  pivots further frontward than when no sheets S are on the MP tray  21 , that is, moves from the light shield position to the light transmission position. In this state, the light receiver  922  receives the light emitted from the light emitter  921  so the photosensor  92  detects that a sheet S is on the MP tray  21 . 
     As illustrated in  FIG.  6 B , in a state where the maximum number of sheets S are on the MP tray  21  (i.e., the MP tray  21  is in its full load state), the contact element  91  pivots, as contacting with a topmost sheet S, further downstream in the sheet feed direction than when one sheet S is on the MP tray  21 . The shield  93  pivots further frontward than when one sheet S is on the MP tray  21 , and maintains its light transmission position. In this state, the light receiver  922  receives the light emitted from the light emitter  921  so the photosensor  92  detects that a sheet S is on the MP tray  21 . 
     In the sheet detector  9 , the photosensor  92  is located inside the guide frame  27 , i.e., located in the internal space of the guide frame  27 , which is disposed below the sheet feed frame  25 . The guide frame  27  shields and prevents stray light from outside from entering the light receiver  922 . This structure sufficiently reduces effects of light from outside the photosensor  92 , and thus reduces false detection of a sheet S on the MP tray  21 . 
     As the number of sheets S on the MP tray  21  changes from zero to one or more, the contact element  91  protruding downward pivots downstream in the sheet feed direction. The greater the number of sheets S on the MP tray  21 , the greater the angle of the contact element  91  pivoting downward in the sheet feed direction. The shield  93  is configured to pivot to move from the light shield position to the light transmission position, as the linkage  94  communicates the pivoting movement of the contact element  91  to the shield  93 . 
     In this embodiment, an angle of the contact element  91  relative to the vertical direction is represented by θ 1 , and an angle of the shield  93  relative to the vertical direction is represented by θ 2  as shown in  FIG.  6 A . As illustrated in  FIG.  7   , the amount of change in the angle θ 2  relative to the amount of change in the angle θ 1  is less when the number of sheets S on the MP tray  21  changes from one to the maximum than when the number of sheets S on the MP tray  21  changes from zero to one. In  FIG.  7   , the angle θ 1 C of the contact element  91  indicates the angle θ 1  of the contact element  91  when one sheet S is on the MP tray  21 . 
     The linkage  94  is configured to communicate movement of the contact element  91  to the shield  93  such that a pivoting amount of the shield  93  according to a pivoting amount of the contact element  91  is less when the number of sheets S on the MP tray  21  changes from one to two or more than when the number of sheets S on the MP tray  21  changes from zero to one. 
     As the number of sheets S on the MP tray  21  changes from zero to one, to enable the photosensor  92  to detect that a sheet S is on the MP tray  21 , the shield  93  is preferably structured to have a relatively great amount of movement such that the shield  93  moves from the light shield position to the light transmission position. 
     In contrast, while the number of sheets on the MP tray  21  changes from one and reaches the maximum, the shield  93  moves slightly and is maintained at the light transmission position such that the movement of the shields  93  produces no significant effect on sheet detection. Because the moving amount of the shield  93  according to the pivoting amount of the contact element  91  is small, a small space is enough to allow the shield  93  moving inside the guide frame  27 , thereby facilitating accommodation of the shield  93  inside the guide frame  27 . 
     Thus, the moving amount of the shield  93  according to the pivoting amount of the contact element  91  is set to be great enough when the number of sheets S on the MP tray  21  changes from zero to one, and is set to be less while the number of sheets Son the MP tray  21  reaches the maximum from one than when the number of sheets S on the MP tray  21  changes from zero to one. 
     The connection  97  of the linkage  94  is structured so that the moving amount of the shield  93  according to the pivoting amount of the contact element  91  when the number of sheets S changes from one to two or more is less than when the number of sheets S on the MP tray  21  changes from zero to one. 
     Movement of Shield Corresponding to Pivoting of Contact Element 
       FIG.  8 A  illustrates the sheet detector  9  when no sheets S are on the MP tray  21 , that is, when the contact element  91  is in the initial state out of contact with any sheet S. As illustrated in  FIG.  8 A , the slide groove  974  defined in the second connection  972  of the connection  97  has a first groove portion  974 A and a second groove portion  974 B. 
     The first groove portion  974 A extends in the sheet feed direction from a position of the boss  973  located when the contact element  91  is in the initial state out of contact with any sheet S, toward the shield  93 . In other words, when the contact element  91  is in the initial state, the first groove portion  974 A is elongated in the front-rear direction along the sheet feed direction. 
     The second groove portion  974 B extends continuously from the first groove portion  974 A diagonally downward relative to the sheet feed direction. When the contact element  91  is in the initial state, the second groove portion  974 B is inclined relative to the horizontal direction and elongated diagonally downward to the rear. 
     The boss  973  is slidably engageable in the slide groove  974  having the first groove portion  974 A and the second groove portion  974 B, thereby moving the shield  93  in association with the pivoting of the contact element  91 . 
     As illustrated in  FIG.  8 A , the contact element  91  in the initial state extends downward and the angle θ 1  of the contact element  91  relative to the vertical direction is θ 1 A)(=0°). In the initial state, the boss  973  is in a front portion of the first groove portion  974 A, and the shield  93  is at the light shield position. When the contact element  91  pivots from the initial state in the sheet feed direction, the first connection  971  having the boss  973  pivots to the rear about the first extension  95 . When the first connection  971  pivots to the rear, the boss  973  moves diagonally upward to the rear. 
     When the contact element  91  is in the initial state, an inner peripheral surface of the first groove portion  974 A and a moving direction of the boss  973  form an angle represented by θ 3   a . When the contact element  91  in the initial state starts pivoting, the second connection  972  having the slide groove  974  pivots upward about the second connection  96  and the shield  93  moves to the front. 
     As illustrated in  FIG.  8 B , when the contact element  91  in the initial state pivots until the angle θ 1  of the contact element  91  relative to the vertical direction becomes θ 1 B, where θ 1 B is greater than θ 1 A, the boss  973  is in a rear end portion of the first groove portion  974 A and the angle formed between the inner peripheral surface of the first groove portion  974 A and the moving direction of the boss  973  is θ 3 B. In this case, the second connection  972  pivots upward about the second extension  96  by the pivoting of the contact element  91 , and thus the shield  93  moves to the front. 
     As the contact element  91  further pivots from the state illustrated in  FIG.  8 B  so that the angle θ 1  becomes θ 1 C, where θ 1 C is greater than θ 1 B, the boss  973  moves from the first groove portion  974 A to the second groove portion  974 B and reaches a middle portion of the second groove portion  974 B, as illustrated in  FIG.  9 A . When the angle θ 1  of the contact element  91  relative to the vertical direction is θ 1 C, one sheet S is on the MP tray  21 . 
     In this state, the angle formed between the inner peripheral surface of the first groove portion  974   a  and the moving direction of the boss  973  is θ 3 C. In this case, the second connection  972  pivots upward about the second extension  96  by the pivoting of the contact element  91 , and thus the shield  93  moves to the front. This allows the shield  93  to move, i.e., pivot, from the light shield position to the light transmission position. 
     The angles θ 3 A, θ 3 B, and θ 3 C  3  correspond to the angles θ 1 A, θ 1 B, and θ 1 C, respectively, and each of the angles θ 3 A, θ 3 B, and θ 3 C has a relatively great value. 
     As the contact element  91  further pivots from the state illustrated in  FIG.  9 A  so that the angle θ 1  becomes θ 1 D, where θ 1 D is greater than θ 1 C, the boss  973  moves along the second groove portion  974 B from a position illustrated in  FIG.  9 A  and reaches a position opposite to the first groove portion  974 A in the up-down direction, as illustrated in  FIG.  9 B . In this state, the angle formed between the inner peripheral surface of the first groove portion  974 A and the moving direction of the boss  973  is θ 3 D. 
     In this case, the second connection  972  pivots upward about the second extension  96  as the contact element  91  pivots, and thus the shield  93  moves to the front. The angle θ 3 D is less than any of the angles θ 3 A, θ 3 B, and θ 3 C, and the moving amount of the shield  93  according to the pivoting amount of the contact element  91  is less than when the angle θ 1  is in the range of θ 1 A to θ 1 C. 
     As the contact element  91  further pivots from the state illustrated in  FIG.  9 B  so that the angle θ 1  becomes θ 1 E, where θ 1 E is greater than θ 1 D, the boss  973  moves along the second groove portion  974 B toward the first groove portion  974 A and reaches an end portion of the second groove portion  974 B adjacent to the first groove portion  974 A, as illustrated in  FIG.  10 A . In this state, the angle formed between the inner peripheral surface of the first groove portion  974 A and the moving direction of the boss  973  is θ 3 E. 
     In this case, the second connection  972  pivots upward about the second extension  96  as the contact element  91  pivots, and thus the shield  93  moves to the front. When the angle θ 1  is θ 1 E, the angle θ 3 E is less than any of the angles θ 3 A, θ 3 B, and θ 3 C, and the moving amount of the shield  93  according to the pivoting amount of the contact element  91  is less than when the angle θ 1  is in the range of θ 1 A to θ 1 C. 
     As the contact element  91  further pivots from the state illustrated in  FIG.  9 B  so that the angle θ 1  becomes θ 1 F, where θ 1 F is greater than θ 1 E, the boss  973  moves along the second groove portion  974 B toward the first groove portion  974 A and reaches an end portion of the second groove portion  974 B adjacent to the first groove portion  974 A, as illustrated in  FIG.  10 A . When the angle θ 1  of the contact element  91  relative to the vertical direction is θ 1 F, the MP tray  21  is in the full load state so that the maximum number of sheets S are on the MP tray  21 . 
     In this state, the angle formed between the inner peripheral surface of the first groove portion  974 A and the moving direction of the boss  973  is θ 3 F. In this case, the second connection  972  pivots upward about the second extension  96  as the contact element  91  pivots, and thus the shield  93  moves to the front. When the angle θ 1  is θ 1 F, the angle θ 3 F is less than any of the angles θ 3 A, θ 3 B, and θ 3 C, and the moving amount of the shield  93  according to the pivoting amount of the contact element  91  is less than when the angle θ 1  is in the range of θ 1 A to θ 1 C. 
     Thus, in the sheet detector  9 , each of the angles θ 3 D, θ 3 E, and θ 3 F between the inner peripheral surface of the first groove portion  974 A and the moving direction of the boss  973  when the angle θ 1  of the contact element  91  relative to the vertical direction is in a range of θ 1 D to θ 1 F, is less than any of the angles θ 3 A, θ 3 B, and θ 3 C when the angle θ 1  is in a range of θ 1 A to θ 1 C. 
     Thus, the moving amount of the shield  93  according to the pivoting amount of the contact element  91  can be set smaller when the angle θ 1  is in a range of θ 1 D to θ 1 F than when the angle θ 1  is in a range of θ 1 A to θ 1 C. 
     In other words, the moving amount of the shield  93  corresponding to the pivoting amount of the contact element  91  can be determined by altering the shapes of the first groove portion  974 A and the second groove portion  974 B of the slide groove  974 . This embodiment illustrates that the first groove portion  974 A extends in the sheet feed direction from the position of the boss  973  located when the contact element  91  is in the initial state out of contact with any sheet S, toward the shield  93 , and the second groove portion  974 B extends continuously from the first groove portion  974 A diagonally downward relative to the sheet feed direction. 
     This configuration enables adjustment of the moving amount of the shield  93  according to the pivoting amount of the contact element  91  such that the moving amount of the shield  93  of when the number of sheets S on the MP tray  21  changes from one to two or more is less than the moving amount of the shield  93  of when the number of sheets S on the MP tray  21  changes from zero to one. This facilitates accommodation of the shield  93  inside the guide frame  27 . 
     The boss  973  is pivotable together with the contact element  91 . The slide groove  974  is defined in the connection  97  integrally movable with the shield  93 . Slidable engagement of the boss  973  in the slide groove  974  enables easy adjustment of the moving amount of the shield  93  according to the pivoting amount of the contact element  91 . 
     Alternative Embodiments 
     The above embodiment illustrates that the shield  93  and the second connection  972  of the connection  97  are connected by the second extension  96  and are thus integrally movable, e.g., pivotable, about the second extension  96 . In some embodiments, however, the shield  93  and the second connection  972  may be connected as follows. 
     Referring to  FIGS.  11 A to  13 B , a sheet detector  9  according to an alternative embodiment will be described. In the following description, elements identical to those described in the above embodiment are designated by identical reference numerals and thus the detail description thereof may be eliminated for the sake of brevity. As illustrated in  FIG.  11 A , the sheet detector  9  includes a second extension  98  that connects the shield  93  and the second connection  972 . The second extension  98  is a shaft extending in the left-right direction and includes a first shaft  981  and a second shaft  982 . The first shaft  981  has an insertion shaft portion  981 A inserted into the second shaft  982  rotatably relative thereto. The first shaft  981  and the second shaft  982  are rotatably connected by inserting the insertion shaft portion  981 A into the second shaft  982 . 
     The insertion shaft portion  981   a  is located at a left end portion of the first shaft  981 . The second connection  972  is fixed at a right end portion of the first shaft  981 . The shield  93  is fixed at a left end portion of the second shaft  982 . The first shaft  981  has a first engagement piece  981 B protruding from its outer surface, and the second shaft  982  has a second engagement piece  982 A protruding from its outer surface. 
     A coil spring  99  is wound around the first shaft  981  and located between the first engagement piece  981 B and the second engagement piece  982 A. The coil spring  99  has a first engagement portion  991  engaging with the first engagement piece  981   b  and a second engagement portion  992  engaging with the second engagement piece  982 A. 
     The coil spring  99  urges the first shaft  981  and the second shaft  982  in their circumferential directions to increase an acute angle D formed between the first engagement piece  981   b  and the second engagement piece  982 A as shown in  FIG.  11 B . The insertion shaft portion  981 A of the first shaft  981  has a restriction piece  981 C to restrict the angle D from increasing any further by contact with the second engagement piece  982 A of the second shaft  982 . 
     The coil spring  99  urges the second engagement piece  982 A and the restriction piece  981 C so as to contact with each other. When no external force acts on the shield  93 , the first shaft  981  and the second shaft  982  are rotatable integrally. 
     As illustrated in  FIG.  11 B , when the contact element  91  is in the initial state where the contact element  91  protrudes downward from the first extension  95  because no sheet S is on the MP tray  21 , the restriction piece  981   c  of the first shaft  981  and the second engagement piece  982 A of the second shaft  982  are in contact with each other and the shield  93  is located at the light shield position. 
     As the contact element  91  pivots from the initial state because some sheets are placed on the MP tray  21 , the second connection  972  pivots about the first shaft  981  integrally with the first shaft  981 , as illustrated in  FIGS.  12 A and  12 B . As the first shaft  981  rotates, the second shaft  982  also rotates due to the urging force of the coil spring  99 . The pivoting of the contact element  91  allows the first shaft  981  and the second shaft  982  to rotate integrally, thereby moving the shield  93  from the light shield position to the light transmission position. 
     As the contact element  91  further pivots while the shield  93  is at the light transmission position, the shield  93  also pivots and contacts an inner surface  27 A of the guide frame  27 , as illustrated in  FIG.  13 B . When the shield  93  contacts the inner surface  27 A of the guide frame  27 , the shield  93  and the second shaft  982  are restricted from rotating any further and stop rotating. 
     In contrast, the contact element  91  and the first shaft  981  rotate against the urging force of the coil spring  99 . The rotation of the contact element  91  and the first shaft  981  after the shield  93  and the second shaft  982  stop rotating causes the first engagement piece  981 B to reach near the second engagement piece  982 A in the circumferential direction, and reduces the angle D formed between the first engagement piece  981   b  and the second engagement piece  982 A to be less than that illustrated in  FIGS.  11 B and  12 B . Thus, the restriction piece  981   c  of the first shaft  981  moves away from the second engagement piece  982 A of the second shaft  982 . 
     The shield  93  stops moving during the pivoting of the contact element  91 , thereby reducing the moving amount of the shield  93  according to the pivoting amount of the contact element  91 . This also reduces the moving amount of the shield  93  in the guide frame  27 , facilitating accommodation of the shield  93  inside the guide frame  27 .