Patent Publication Number: US-7912384-B2

Title: Image forming device adjusting conveying gap between consecutively fed sheets

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority from Japanese Patent Application No. 2005-213220 filed Jul. 22, 2005. The entire content of each of these priority applications is incorporated herein by reference. 
     TECHNICAL FIELD 
     The disclosure relates to an image forming device, such as a laser printer and a copier machine. 
     BACKGROUND 
     In an image-forming process performed in a conventional image-forming device such as a laser printer, a paper-feeding means feeds a sheet of recording medium to a conveying means, and the conveying means conveys the sheet between a transfer roller and a photosensitive drum where a toner image is transferred onto the sheet of recording medium. Subsequently, the toner is heated and melted as the sheet passes between a heating roller and a pressure roller, thereby fixing the toner image to the sheet of recording medium. In order to allocate sufficient time for developing print data and to prevent paper jams when feeding sheets of the recording medium consecutively, the image-forming device opens a prescribed gap (30 mm, for example) between a preceding sheet and a succeeding sheet. Further, since the frictional resistance against the sheet of recording medium on a conveying path differs based on the sheet thickness, this difference will cause a slight variance in the conveying speed and the pressure applied by the pressure roller. Hence, in order to form images of uniform quality on the recording medium, conventional image-forming devices adjust various image-forming conditions, such as the developing bias, transfer bias, and fixing temperature, based on the thickness of the sheet, as disclosed in Japanese unexamined patent application publications Nos. 2003-223022 and HEI-11-49388. 
     As shown in  FIG. 12 , a conventional image-forming device typically has a sheet sensor  202  for detecting the trailing edge of a sheet  201  that is fed by a feeding means (not shown) and conveyed by a conveying means (not shown). When the sheets  201  of recording medium are fed consecutively, the sheet sensor  202  detects the trailing edges of the sheets  201 , and the image-forming device adjusts the gap between conveyed sheets  201  by controlling the timing at which each sheet  201  is fed from the feeding means to the conveying means based on the detection results. 
     The sheet sensor  202  is disposed downstream of the feeding means. In order to keep costs down, the sheet sensor  202  includes an actuator  203  and a detector  204 . The actuator  203  is capable of pivoting in a direction indicated by arrows N in  FIG. 12  and has a front end that protrudes into a conveying path of the sheet  201 . The detector  204  is for detecting the rear end of the actuator  203 . When the sheet  201  contacts the front end of the actuator  203 , the actuator  203  pivots clockwise in  FIG. 12 . At this time, the detector  204  switches from an OFF state to an ON state, effectively detecting the leading edge of the sheet  201 . After the sheet  201  passes over the actuator  203 , the actuator  203  returns to its original position by pivoting counterclockwise in  FIG. 12 . At this time, the detector  204  changes from the ON state to the OFF state, effectively detecting the trailing edge of the sheet  201 . 
     An image forming device further includes a fixing device having a heating roller and a pressure roller. When the heating roller and the pressure roller apply heat and pressure to the sheet  201  in order to fix an image on the sheet  201 , the sheet  201  curls. There is a possibility that the curled sheet  201  may cause a paper jam by catching on some component in the image-forming device while being conveyed from the image-forming position to a discharge position. To resolve this problem, the image-forming device is provided with a discharge sensor having the same structure as the sheet sensor  202  described above disposed between the image-forming position and the discharge position in order to monitor the conveyed state of the sheet  201  based on the ON/OFF state of the discharge sensor. 
     SUMMARY 
     In the conventional image forming device described above, despite opening a prescribed gap (30 mm, for example) between successively conveyed sheets of recording medium, paper jams still frequently occur in conventional image-forming devices. After studying this problem, the inventors of the invention discovered that thin sheets of recording medium lacking body are more likely to cause jams. As they investigated the cause, the inventors determined that the paper jams occurred due to the relationship between the sheet sensor and the sheet thickness of the recording medium. The cause of this problem is described in more detail with reference to  FIGS. 13 through 15 . 
     A thin sheet  201  (such as a thin sheet of paper) is more yielding than a thick sheet (such as a thick sheet of paper) and is more likely to deform. Hence, when the sheet  201  contacts the sheet sensor  202 , the leading edge of the sheet  201  may deform and ride up on the sheet sensor  202 , as shown in  FIG. 13 . This causes a delay in the timing at which the sheet  201  switches the sheet sensor  202  from an OFF state to an ON state, thereby delaying detection of the leading edge. Further, when the trailing edge of the sheet  201  approaches the sheet sensor  202 , the restorative force of the actuator  203  flips up the trailing edge as shown in  FIG. 14 , causing the trailing edge to deform before the sheet  201  has completely passed over the actuator  203  and to ride up on the sheet sensor  202 . This speeds up the timing at which the sheet sensor  202  changes from the ON state to the OFF state, causing the sheet sensor  202  to detect the trailing edge of the sheet  201  too early. 
     When the sheet sensor  202  detects the leading edge of the sheet  201  too late as shown in  FIG. 13 , the image-forming device perceives the length of the sheet  201  to be a distance L 1  shorter than the actual length. Further, when the sheet sensor  202  detects the trailing edge of the sheet  201  too early as shown in  FIG. 14 , the image-forming device perceives the length of the sheet  201  to be a distance L 2  shorter than the actual length. If the sheet sensor  202  errs on detecting both the leading edge and the trailing edge, then the length of the sheet  201  perceived by the image-forming device is doubly shortened by the distances L 1  and L 2 . In such a case, the image-forming device feeds a succeeding sheet  201  to the conveying means before the prescribed gap is formed between the preceding sheet  201  and the succeeding sheet  201 . As a result, as shown in  FIG. 15 , the prescribed conveying gap N 1  between consecutively fed sheets  201 A and  201 B is reduced by a length N 3 , resulting in a conveying gap N 2  between the sheets  201 A and  201 B. 
     The image-forming device can increase the number of printed sheets per unit time by further shrinking the gap between the conveyed sheets  201  from the gap formed at the time of feeding (when the sheets are initially fed to the conveying means) just prior to the image-forming position, thereby reducing the amount of time loss caused by the conveying gap. However, if the conveying gap between the sheets  201  is reduced at the time of feeding, the succeeding sheet  201 B may be too near or may overlap the trailing edge of the preceding sheet  201 A at or just prior to the image-forming position. Since the process of removing the sheets  201  is troublesome when an actual paper jam occurs, the image-forming device may determine that a paper jam has occurred when the sheet  201 B becomes too close to or overlaps the sheet  201 A and may forcibly halt the printing operation at that time. 
     In addition, an actuator of the discharge sensor is considerably long in order to prevent the curled sheet  201  from floating up off the discharge sensor and escaping detection. Hence, the actuator of the discharge sensor requires a longer time to displace from an ON state position to an OFF state position. 
     As shown in  FIG. 15 , when the conveying gap N 2  between consecutively fed sheets  201 A and  201 B is shorter than the prescribed conveying gap N 1  when the sheets are fed, the conveying gap between the sheets  201 A and  201 B conveyed from the image-forming position to the discharge position becomes even shorter, and the succeeding sheet  201 B may press against the actuator of the discharge sensor before the actuator can return to the OFF state after passage of the sheet  201 A. Therefore, the discharge sensor may not detect the trailing edge of the preceding sheet  201 A. In such a case, the image-forming device perceives the sheet  201 A to be longer than its actual length and incorrectly determines that a paper jam has occurred. While this problem can be resolved by widening the conveying gap between the sheets  201 , it may not be possible to achieve the required throughput. 
     In view of the foregoing, it is an object of the invention to provide an image forming device capable of reliably controlling conveyance of recording medium while maintaining the maximum throughput. 
     In order to attain the above and other objects, the invention provides an image-forming device including a feeding member that feeds a recording sheet, a conveying member that conveys the recording sheet fed by the feeding member, an image-forming member that forms an image on the recording sheet conveyed by the conveying member, and a controller that adjusts, when the conveying member conveys recording sheets consecutively, a conveying gap between the consecutively fed recording sheets based on a thickness of the recording sheet. 
     The invention also provides a control method for controlling a conveyance of a recording sheet in an image forming device. The control method including determining a thickness of a recording sheet, and adjusting a conveying gap between consecutively fed recording sheets based on the determined thickness. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Illustrative aspects in accordance with the invention will be described in detail with reference to the following figures wherein: 
         FIG. 1  is a perspective view of a personal computer connected to a laser printer according to some aspects of the invention; 
         FIG. 2  is a side cross-sectional view showing the laser printer in  FIG. 1 ; 
         FIG. 3  is a perspective view of a paper cassette employed in the laser printer of  FIG. 1 ; 
         FIG. 4  is an enlarged view of a region A indicated in  FIG. 2 ; 
         FIG. 5  is a cross-sectional view showing a first orientation of a pivot link employed in the laser printer in  FIG. 1 ; 
         FIG. 6  is a cross-sectional view showing a second orientation of the pivot link in  FIG. 5 ; 
         FIG. 7  is a cross-sectional view showing a third orientation of the pivot link in  FIG. 5 ; 
         FIG. 8  is a block diagram showing a control hardware configuration of the laser printer in  FIG. 1 ; 
         FIG. 9  is a table illustrating a data structure for a conveying gap memory area shown in  FIG. 8 ; 
         FIG. 10  is a flowchart illustrating steps in a process for modifying the conveying gap executed according to a conveying gap modification program shown in  FIG. 8 ; 
         FIG. 11  is a block diagram showing a control hardware configuration of a laser printer according to additional aspects of the invention; 
         FIG. 12  is an explanatory diagram showing a construction for detecting a recording sheet; 
         FIG. 13  is an explanatory diagram illustrating detection of the leading edge of the recording sheet; 
         FIG. 14  is an explanatory diagram illustrating detection of the trailing edge of the recording sheet; and 
         FIG. 15  is an explanatory diagram illustrating a reduction in a conveying gap between consecutively conveyed recording sheets. 
     
    
    
     DETAILED DESCRIPTION 
     A laser printer  1  as an image forming device according to some aspects of the invention will be described while referring to the accompanying drawings. 
     In the following description, a depth direction of the laser printer  1  will be referred to as the X direction (the front surface side being +X), a width direction will be referred to as the Z direction (the near right side in  FIG. 1  being +Z), and a height direction will be referred to as the Y direction (the upper side in  FIG. 1  being +Y). 
     As shown in  FIG. 1 , the laser printer  1  is connected to a personal computer  141  via a cable  140  so that the personal computer  141  and the laser printer  1  can communicate with each other. The personal computer  141  includes a keyboard  142 , a mouse  143 , a main system  144 , and a display  145 . A user can input data through the keyboard  142  and the mouse  143 . The main system  144  has a built-in central processing unit (CPU), a random access memory (RAM), a read only memory (ROM), and the like for performing processes and arithmetic computations on this data. The display  145  is for displaying data outputted from the main system  144 . The personal computer  141  can output printing instructions to the laser printer  1  via the cable  140  when the user has inputted such printing instructions using the keyboard  142  and the mouse  143 , and the laser printer  1  can print a sheet of paper  3  ( FIG. 2 ) based on the inputted data. 
     Here, “printing instructions” denotes data that the laser printer  1  requires for forming an image and includes a “print command” for the laser printer  1 , “recording sheet data,” and “print data.” 
     The “recording sheet data” denotes information on a “recording sheet,” which is a sheet like recording medium, and includes information on the type of recording sheet (thick paper, normal paper, thin paper, transparency, etc.), information on the sheet thickness, and information on the size and standard of the recording sheet. 
     “Print data” denotes data for images (hereinafter including text) to be formed on a recording sheet. 
     As shown in  FIG. 1 , the exterior of the laser printer  1  is configured of a main casing  2 , a front cover  7 , and a paper cassette  9 . 
     As shown in  FIG. 2 , the laser printer  1  further includes a feeder unit  4  and an image forming unit  5  within the main casing  2 . In the laser printer  1 , paper  3  is picked up one sheet at a time from the feeder unit  4 , printed with images, and then discharged onto a discharge tray  53  formed on top of the main casing  2 . 
     An access opening  6  is formed in a top wall (wall on the +Y side) of the main casing  2  for inserting and removing a process cartridge  20  described later. The main casing  2  rotatably supports the front cover  7  so that the front cover  7  can open and close over the access opening  6 . 
     A cassette-accommodating section  2 A is provided in the bottom section of the main casing  2  and is open on the front side. Hence, a user can insert the paper cassette  9  into the cassette-accommodating section  2 A or remove the paper cassette  9  therefrom through operations performed on the front side of the main casing  2 . 
     The feeder unit  4  includes the paper cassette  9 , a separating roller  10 , a feeding roller  12 , and a pinch roller  13 .  FIG. 3  is a perspective view of the paper cassette  9 . Because the separating roller  10 , the feeding roller  12 , and the pinch roller  13  are mounted in the main casing  2 , these components would not naturally appear in  FIG. 3 , but have been shown in  FIG. 3  to illustrate their relationships with the paper cassette  9  or a paper dust roller  8  provided in the paper cassette  9 . 
     As shown in  FIG. 3 , the paper cassette  9  includes a main cassette body  71  and a wall portion  75 . The main cassette body  71  is shaped like a shallow tray for accommodating stacked sheets of the paper  3 . The main cassette body  71  has a bottom wall  71 A. A sheet-pressing plate  15  is mounted on the bottom wall  71 A in the front region thereof. The rear end of the sheet-pressing plate  15  is fixed to the bottom wall  71 A, while the front end is not fixed and can move vertically. 
     As shown in  FIG. 2 , a lever  17  is provided between the free end (front end) of the sheet-pressing plate  15  and the bottom wall  71 A of the paper cassette  9 . The lever  17  can rotate about a lever shaft  18 . 
     The lever shaft  18  is engaged with a paper-feeding gear train  16  (see  FIG. 3 ) disposed on the outer side wall of the paper cassette  9 . The paper-feeding gear train  16  is engaged with a paper-feeding motor (not shown). 
     The paper-feeding gear train  16  is driven to rotate by the paper-feeding motor. When the rotational drive force of the paper-feeding gear train  16  (counterclockwise in  FIG. 1 ) is applied to the lever shaft  18 , the lever  17  pivots about the lever shaft  18 . As a result, the front end of the lever  17  lifts the front end of the sheet-pressing plate  15  and, consequently, lifts the paper  3  stacked in the paper cassette  9  so that the paper  3  contacts the feeding roller  12  with sufficient pressure for the feeding roller  12  to pick up a sheet of the paper  3 . 
     As shown in  FIG. 3 , the wall portion  75  is provided on the front of the main cassette body  71 . The wall portion  75  includes a flat front plate  76  and a rear member  77 . The rear member  77  has a sloped surface  77 A on the opposite side from the flat front plate  76  that slopes downward toward the main cassette body  71 . In the widthwise center region of the sloped surface  77 A are provided the paper dust roller  8  and a separating pad  11  disposed in a vertical arrangement, and guide pieces  115  that protrude from the sloped surface  77 A toward the center of the paper cassette  9  (leftward in  FIG. 3 ). Five of the guide pieces  115  are disposed in intervals along the width of the paper cassette  9 . As shown in  FIG. 2 , the guide pieces  115  are sloped to follow the path through which the front end of the sheet-pressing plate  15  passes when moving vertically and function to align the front edges of the paper  3  lifted by the sheet-pressing plate  15 . 
     As shown in  FIG. 2 , a coil spring  78  is accommodated in the wall portion  75  for urging the separating pad  11  toward the separating roller  10 . The feeding roller  12 , the separating roller  10 , and the pinch roller  13  are arranged in the order given on a front section of a ceiling wall constituting the cassette-accommodating section  2 A (+X side), that is, at positions opposing the wall portion  75  of the paper cassette  9 . 
     A brief description of the roller support structure will be described with reference to  FIG. 3 . The separating roller  10  and the pinch roller  13  are arranged so that central shafts C 1  and C 2  of the separating roller  10  and the pinch roller  13  respectively extend in a width direction W of the paper cassette  9 . Both ends of the central shafts C 1  and C 2  are supported on the main casing  2  in a direction orthogonal to the paper-conveying direction. Unlike the separating roller  10  and the pinch roller  13 , the feeding roller  12  ( FIG. 2 ) does not have a central shaft that is linked to the main casing  2 , but is rotatably held on the central shaft C 1  by a holder  12 A formed as three sides of a rectangle. 
     As will be described later, a pivoting link  80  is fitted over the central shaft C 1  together with the separating roller  10 . 
     When the paper cassette  9  is accommodated in the cassette-accommodating section  2 A as shown in  FIG. 2 , the flat front plate  76  of the paper cassette  9  is flush with the front surface of the main casing  2  and covers the opening to the cassette-accommodating section  2 A. At this time, the separating roller  10  is positioned in opposition to the separating pad  11  and the paper dust roller  8  in opposition to the pinch roller  13 . As a result, the guide pieces  115  of the wall portion  75  together with the paper dust roller  8 , the separating roller  10 , the feeding roller  12 , and the pinch roller  13  form a conveying path  56 . Additionally, due to the urging force of the coil sprint  78 , the separating pad  11  presses against the separating roller  10  in order to produce a suitable frictional force between the separating roller  10  and the paper  3  to prevent a plurality of overlapped sheets of paper  3  from being supplied onto the conveying path  56 . 
     The laser printer  1  is also provided with a motor M shown in  FIG. 2 . A drive torque generated by the motor M is transmitted to the central shafts C 1  and C 2  via a drive transmission gear (not shown) and, consequently, to the separating roller  10 , the feeding roller  12 , and the pinch roller  13  for driving the separating roller  10 , the feeding roller  12 , and the pinch roller  13  to rotate. As shown in  FIG. 2 , the conveying path  56  curves back toward the rear near the paper dust roller  8  to form a U-shape. The conveying path  56  passes through a pair of registration rollers  14  disposed upstream of the image-forming unit  5 . With this construction, the paper  3  is conveyed along the conveying path  56  by the driving of the motor M toward the image-forming unit  5  described later. A front registration sensor  61  and a rear registration sensor  66  described later are disposed on upstream and downstream sides respectively of the registration roller  14 . 
     Driving of the motor M is controlled by a control unit  120  ( FIG. 8 ) described later. According to the aspects, when the personal computer  141  inputs printing instructions into the laser printer  1  via the cable  140 , a motor drive circuit  136  (see  FIG. 8 ) of the control unit  120  drives the motor M. Driving of the motor M is halted when the printing has completed. The main casing  2  incorporates a mechanism for interrupting power supply to the motor drive circuit  136  when the paper cassette  9  is removed. Hence, driving of the motor M is halted when the paper cassette  9  is not mounted in the cassette-accommodating section  2 A. 
     As shown in  FIG. 2 , the image forming unit  5  includes a scanner unit  19 , the process cartridge  20 , and a fixing unit  21 . The scanner unit  19  includes a light source (not shown), a polygon mirror  22 , lenses, and reflection mirrors. 
     The process cartridge  20  includes a photosensitive drum  29 , a Scorotron charger  30 , a developing cartridge  31 , a transfer roller  32 , and a cleaning brush  33 . The photosensitive drum  29  has a photosensitive layer on its surface and is rotatably supported. The charger  30  is for charging the surface of the photosensitive drum  29 . The developing cartridge  31  has a developing roller  41  and an accommodating chamber  39  accommodating toner T as developer. The transfer roller  32  is disposed in confrontation with the photosensitive drum  29  so as to form a nip portion between the transfer roller  32  and the photosensitive drum  29  at a transfer position. 
     The fixing unit  21  includes a heat roller  49  including a halogen lamp or the like for generating heat and a pressure roller  50  disposed in press contact with the heat roller  49 . 
     In the image forming unit  5 , first the charger  30  uniformly charges the entire surface of the photosensitive drum  29 . Then, a laser beam is emitted from the light source (not shown) based on image data. The laser beam is redirected by the polygon mirror  22 , and passes through or reflected by the lenses and the reflection mirrors so as to irradiate, in a high speed scanning operation, the surface of the photosensitive drum  29 . As a result, an electrostatic latent image corresponding to the image data is formed on the surface of the photosensitive drum  29 . 
     Then, the developer roller  41  supplies the toner T accommodated in the accommodating chamber  39  onto the surface of the photosensitive drum  29 . As a result, a toner image (visible image) corresponding to the electrostatic latent image is formed on the photosensitive drum  29 . As the paper  3  transferred from the registration rollers  14  passes through the transfer position between the photosensitive drum  29  and the transfer roller  32 , the toner image (toner T) on the photosensitive drum  29  is transferred onto the paper  3 . At this time, the transfer roller  32  is applied with a transfer bias. Toner T remaining on the surface of the photosensitive drum  29  after the transfer operation is removed by the cleaning brush  33 . In this way, the photosensitive drum  29  is capable of forming the following image. 
     The paper  3  formed with the toner image on its surface is conveyed to the fixing unit  21 . In the fixing unit  21 , the toner image on the paper  3  is thermally fixed on the paper  3  as the paper  3  passes between the heat roller  49  and the pressure roller  50 . 
     After the toner T is fixed on the paper  3 , the paper  3  is conveyed along a discharge path  51  extending vertically (Y direction) toward the top surface of the main casing  2 . Since the paper  3  has a curl due to the heat and pressure applied in the fixing unit  21 , a pair of pinch rollers  156  is provided along the discharge path  51  to remove the curl. Subsequently, a pair of discharge rollers  52  disposed near the top of the discharge path  51  discharge the paper  3  onto the discharge tray  53 . A discharge sensor  151  described later is disposed along the discharge path  51  upstream of the discharge rollers  52  and the pinch rollers  156 . 
     The laser printer  1  is also provided with a sheet detecting mechanism for detecting the conveyed state of the paper  3 . As shown in  FIG. 2 , the sheet detecting mechanism includes the front registration sensor  61 , the rear registration sensor  66 , the pivoting link  80  (see  FIG. 3 ), and the discharge sensor  151 . 
       FIG. 4  is an enlarged view of a region A shown in  FIG. 2 . The front registration sensor  61  is disposed on the upstream side of the registration rollers  14  for detecting the leading edge of a sheet of paper  3  supplied toward the registration rollers  14 . The rear registration sensor  66  is disposed on the downstream side of the registration rollers  14  for detecting the paper  3  supplied from the registration rollers  14  toward the image-forming unit  5 . 
     The front registration sensor  61  and the rear registration sensor  66  include respective actuators  62  and  67 , support shafts  63  and  68 , light-shielding members  64  and  69 , and detectors  65  and  70  (photo interrupters in the aspects). The actuators  62  and  67  have prescribed lengths. The support shafts  63  and  68  are fixed inside the main casing  2  for pivotably supporting the respective actuators  62  and  67  so that front ends of the actuators  62  and  67  protrude into the conveying path  56 . The light-shielding members  64  and  69  are provided on the rear ends of the respective actuators  62  and  67  and are capable of rotating integrally with the respective actuators  62  and  67 . The detectors  65  and  70  are disposed beneath the respective actuators  62  and  67  along paths through which the rear ends of the actuators  62  and  67  move. When the front registration sensor  61  and the rear registration sensor  66  are in a mounted state (that is, when the actuators  62  and  67 , the light-shielding members  64  and  69 , and the detectors  65  and  70  are installed and when a sheet of paper  3  is not being conveyed), the actuators  62  and  67  hang in a substantially vertical orientation by their own weight so that the light-shielding members  64  and  69  block the optical path of a detection light emitted from the detectors  65  and  70 . 
     Hence, when a sheet of the paper  3  is not pressing against the actuators  62  and  67 , the actuators  62  and  67  hang in a substantially vertical orientation with the light-shielding members  64  and  69  interrupting light emitted by the detectors  65  and  70 . Accordingly, detection results by the detectors  65  and  70  indicate an OFF state. However, when a sheet of paper  3  presses against the actuators  62  and  67 , rotating the actuators  62  and  67  counterclockwise in  FIG. 4  (indicated by arrows D 1  and D 2 ), the actuators  62  and  67  rotate into a tilted orientation. Since the light-shielding members  64  and  69  move integrally with the actuators  62  and  67 , the optical paths of the detectors  65  and  70  open up, and the detection results by the detectors  65  and  70  indicate an ON state. Once the sheet of paper  3  no longer presses against the actuators  62  and  67 , the actuators  62  and  67  rotate clockwise in  FIG. 4  (the directions opposite those indicated by arrows D 1  and D 2 ) by their own weight and return to their original vertical orientation. 
     As shown in  FIG. 5 , the pivoting link  80  is fitted over the central shaft C 1  with a slight gap so as to be capable of rotating freely (so as not to rotate together with the central shaft C 1 ). The pivoting link  80  includes an arm  84 , shielding plate  85 , and a protruding plate part  95 . 
     The arm  84  extends upward from a base part of the pivoting link  80  supported around the central shaft C 1 . The shielding plate  85  is provided on the top end of the arm  84 . Here, a description will be given for a photoelectric sensor  100  positioned opposite the shielding plate  85  for detecting a light-blocking object. The photoelectric sensor  100  includes a light-emitting element and a light-receiving element disposed in positions facing each other. In the aspects, a transparent photointerrupter in which the photoelectric elements have been packaged is used. 
     The photoelectric sensor  100  is fixed to the upper wall of the cassette-accommodating section  2 A at a position above the pivoting link  80  and extends along the central shaft C 1 . When the components are assembled (that is, when the photoelectric sensor  100 , the pivoting link  80 , and the paper cassette  9  are installed and a sheet of paper  3  is not being conveyed), the shielding plate  85  of the pivoting link  80  is positioned between the light-emitting element and the light-receiving element of the photoelectric sensor  100 . 
     The protruding plate part  95  extends toward the wall portion  75  of the paper cassette  9 . An end  95 A of the protruding plate part  95  is bent slightly. A receiving part  79  is formed in the wall portion  75  by depressing the side wall downward at a position opposite the protruding plate part  95 . The bottom portion of the receiving part  79  functions as a seat surface  79 A. 
     Changes in the orientation of the pivoting link  80  will be described. 
     (First Orientation) 
     As shown in  FIG. 5 , when the paper cassette  9  is mounted in the cassette-accommodating section  2 A, the protruding plate part  95  intersects the conveying path  56 , and the bent end  95 A of the protruding plate part  95  rests on the seat surface  79 A of the receiving part  79 . At this time, the arm  84  is in a substantially vertical orientation, and the shielding plate  85  blocks the path of detection light emitted from the light-emitting element (hereinafter, this state will be referred to as an “OFF state” for the sensor output). The pivoting link  80  is in a first orientation when the protruding plate part  95  is supported on top of the seat surface  79 A of the receiving part  79  as described above. 
     The laser printer  1  also includes a coil spring  110  shown in  FIG. 5 , having one end attached to the upper wall of the cassette-accommodating section  2 A and the other end engaged in a spring fastener  88  of the pivoting link  80 . When the pivoting link  80  is in the first orientation, the coil spring  110  urges the pivoting link  80  in a direction indicated by an arrow S in  FIG. 5 . 
     (Second Orientation) 
     Hence, before a sheet of the paper  3  is conveyed, the protruding plate part  95  extends across the conveying path  56 . When a sheet of the paper  3  is conveyed, the protruding plate part  95  is flipped upward by the paper  3 , causing the pivoting link  80  to rotate in the direction indicated by an arrow R in  FIG. 6 . When the pivoting link  80  rotates in this direction, the shielding plate  85  is retracted from the optical path of the detection light, enabling the photoelectric sensor  100  to receive light (hereinafter referred to as an “ON state” of the sensor output). The second orientation of the pivoting link  80  is the orientation in which the protruding plate part  95  is flipped upward as shown in  FIG. 6 . 
     While the paper  3  is conveyed, the pivoting link  80  is maintained in the second orientation, as the lower surface of the protruding plate part  95  is supported on the paper  3 . After the trailing edge of the paper  3  passes the protruding plate part  95  and the support of the paper  3  is removed (the protruding plate part  95  is in a free state), the pivoting link  80  returns to its original first orientation shown in  FIG. 5  due to the urging force of the coil spring  110  described above. 
     In this construction, the pivoting link  80  is disposed with the protruding plate part  95  downstream of the separating roller  10 . Therefore, even when the feeding roller  12  picks up a plurality of sheets of paper  3  from the paper cassette  9 , the separating roller  10  can supply the paper  3  to the pinch roller  13  one sheet at a time, enabling the pivoting link  80  to detect the leading edge and the trailing edge of the paper  3  one sheet at a time. 
     (Third Orientation) 
     If the paper cassette  9  is removed by moving the entire paper cassette  9  from the state shown in  FIG. 5  toward the right in the drawing, the support of the receiving part  79  is removed from beneath the protruding plate part  95  as shown in  FIG. 7 . Immediately after the support is removed, the pivoting link  80  rotates in the direction indicated by an arrow S in  FIG. 7  due to the urging force of the coil spring  110 . A stopper (not shown) is also provided on the pivoting link  80  for contacting an end face of the separating roller  10 , halting the rotation of the pivoting link  80 . At this time, the shielding plate  85  is retracted from the optical path of the detection light, enabling the photoelectric sensor  100  to receive light (ON state). This position is the third orientation of the pivoting link  80 . 
     However, when the paper cassette  9  is inserted into the cassette-accommodating section  2 A from this orientation, the bent end  95 A of the protruding plate part  95  is contacted by a sloped guiding surface  79 B of the wall portion  75 . As the protruding plate part  95  is guided up the sloped guiding surface  79 B, the pivoting link  80  rotates in the direction R against the urging force of the coil spring  110 . By the time the paper cassette  9  is completely accommodated in the cassette-accommodating section  2 A, the protruding plate part  95  has slid over the sloped guiding surface  79 B and is supported from below by the seat surface  79 A of the receiving part  79 . In other words, the pivoting link  80  is in the first orientation shown in  FIG. 5 . 
     The orientation of the pivoting link  80  and changes therein are identified as follows. The pivoting link  80  is determined to be in the first orientation when the photoelectric sensor  100  is in an OFF state. However, when the photoelectric sensor  100  changes from an OFF to an ON state, then the pivoting link  80  has rotated either in the R direction shown in  FIG. 6  or the S direction shown in  FIG. 7  from the first orientation. Therefore, the orientation of the pivoting link  80  is determined to be either the second orientation shown in  FIG. 6  or the third orientation shown in  FIG. 7 . Since the driving of the motor M halts when the paper cassette  9  is removed, the pivoting link  80  is determined to be in the second orientation shown in  FIG. 6  when the photoelectric sensor  100  is in the ON state and the motor M is being driven and in the third orientation shown in  FIG. 7  when the photoelectric sensor  100  is in the ON state and the motor M is stopped. 
     The discharge sensor  151  will be described with reference to  FIG. 2 . The discharge sensor  151  is disposed on the upstream side of the pinch rollers  156  for detecting the paper  3  supplied from the fixing unit  21  toward the discharge rollers  52 . 
     The discharge sensor  151  includes an actuator  152 , a shaft  153 , a light-shielding member  154 , and a detector  155  (photointerrupter in the aspects). The actuator  152  has a prescribed length. The shaft  153  is fixed inside the main casing  2  for pivotably supporting the actuator  152  so that a front end of the actuator  152  protrudes into the conveying path  51 . The light-shielding member  154  is provided on the rear end of the actuator  152  and is capable of rotating integrally with the actuator  152 . The detector  155  is disposed along a path through which the rear end of the actuator  152  moves. When the discharge sensor  151  is in a mounted state (that is, when the actuator  152 , the light-shielding member  154 , and the detector  155  are installed and when a sheet of paper  3  is not being conveyed), the actuator  152  hangs in a substantially vertical orientation by its own weight so that the light-shielding member  154  blocks the optical path of a detection light emitted from the detector  155 . 
     In the discharge sensor  151  having this construction, the actuator  152  hangs in a substantially vertical orientation when not being pressed by a sheet of paper  3 , with a front end protruding into the conveying path  51 , as shown in  FIG. 2 . At this time, the light-shielding member  154  blocks the path of light from the detector  155  so that the detection results from the detector  155  indicate an OFF state. When a sheet of paper  3  presses against the actuator  152 , the actuator  152  rotates counterclockwise in  FIG. 2  so that the front end is retracted from the conveying path  51 . Since the light-shielding member  154  moves together with the actuator  152 , the optical path of the detector  155  becomes clear, and the detection results from the detector  155  change from an OFF state to an ON state. When the paper  3  no longer presses against the actuator  152 , the actuator  152  rotates clockwise in  FIG. 2  by its own weight, returning to the original orientation. At this time, the detection results from the detector  155  change from an ON state to an OFF state. 
     The actuator  152  is considerably long so as to protrude far into the conveying path  51  in order to prevent the curled paper  3  from floating up off the discharge sensor  151  and escaping detection. Hence, the actuator  152  requires a longer time to displace from an ON state position to an OFF state position. 
     Next, a hardware construction for electrically controlling the laser printer  1  will be described.  FIG. 8  is a block diagram showing the hardware structure. 
     As shown in  FIG. 8 , the laser printer  1  includes the control unit  120 , mentioned earlier, configured around a CPU  121  for performing data processing and arithmetic computations. The CPU  121  is connected to a ROM  122 , a RAM  123 , the personal computer  141 , an image data development memory device  127 , and a conveying gap memory device  128 . 
     The ROM  122  stores various programs, initial values, and the like. The programs include an image-forming program  124 , a drive control program  125 , and a conveying control program  126 . 
     The image-forming program  124  is for controlling the timing for forming electrostatic latent images on the photosensitive drum  29  and the timing for forming images on the paper  3  based on the timing at which the rear registration sensor  66  detects the leading edge of the paper  3 . 
     The drive control program  125  is for controlling the driving of the motor M and operations of first through fifth clutch devices  131 ,  132 ,  133 ,  134 , and  135  provided in the drive transmission gear (not shown) in order to control the rotation of the separating roller  10 , the feeding roller  12 , the pinch roller  13 , and the discharge roller  52 . More specifically, the drive control program  125  the timing for supplying the paper  3  to the pinch roller  13  by controlling the rotation of the feeding roller  12  and the separating roller  10 . 
     The conveying control program  126  is for adjusting a conveying gap between consecutively fed sheets of paper  3  based on a sheet thickness. The conveying control program  126  will be described later in greater detail. 
     The RAM  123  is for temporarily storing data. The personal computer  141  is the host device, and printing instructions received from the personal computer  141  is stored in the RAM  123 . The image data development memory device  127  stores print data that the CPU  121  extracts from the printing instructions. The CPU  121  sequentially deletes the print data stored in the image data development memory device  127  as the print data is printed. 
     The conveying gap memory device  128  stores a table such as that shown in  FIG. 9  defining conveying gaps based on the type of recording sheet. 
     There are various types of recording sheets, such as thick paper, normal paper, and thin paper, each with a differing sheet thickness. It is well known that the stiffness of a recording sheet decreases for thinner sheets. Further, while transparency sheets may be stiffer than paper sheets due to the type of material, because transparencies exist in different thicknesses, some of which are more yielding than thin paper. The table in  FIG. 9  lists thick paper, normal paper, thin paper, and transparencies (OHP) as examples of types of recording sheets. 
     Here, “conveying gap” denotes the distance between the trailing edge of a sheet of paper  3  conveyed first and the leading edge of a sheet of paper  3  conveyed after the first sheet. In the aspects, the conveying gap is managed by time, as shown in  FIG. 9 . The conveying gap for thick paper is set as a reference value in  FIG. 9 , while conveying gaps for the thinner, more yielding normal paper, thin paper, and transparencies are defined as the amount of expansion from the reference value. More specifically, the conveying gap for thick paper is used as the reference value and is set to 300 msec in the aspects. As shown in the table in  FIG. 9 , the conveying gap for normal paper, which is thinner than thick paper, is stipulated as the reference value (300 msec) +30 msec, and the conveying gap for thin paper, which is thinner than normal paper, is stipulated as the reference value (300 msec) +60 msec. The conveying gap for thin transparencies having less stiffness than thin paper is defined as the reference value (300 msec) +80 msec. 
     As shown in  FIG. 8 , the CPU  121  is also connected to and acquires detection results from various sensors, including the photoelectric sensor  100 , the detector  65 , the detector  70 , and the detector  155  described above. 
     The CPU  121  is also connected to the motor drive circuit  136  for controlling the driving of the motor M. The CPU  121  is connected to the first through fifth clutch devices  131 ,  132 ,  133 ,  134 , and  135  for individually controlling the drive force transmitted to each of the separating roller  10 , the feeding roller  12 , the pinch roller  13 , the registration rollers  14 , and the discharge roller  52 . 
     For example, by connecting the second through fifth clutch devices  132 - 135  and disconnecting the first clutch device  131 , the feeding roller  12 , the separating roller  10 , the pinch roller  13 , and the discharge roller  52  are driven to rotate while the registration rollers  14  are halted. As a result, the leading edge of the paper  3  supplied from the paper cassette  9  onto the conveying path  56  runs into the halted registration rollers  14 , producing a small amount of slack in the leading edge side of the paper  3  so that the leading edge of the paper  3  is orthogonal to the conveying path  56 . Subsequently, the first clutch device  131  is connected so that the registration rollers  14  can rotate and convey the paper  3  to the image-forming unit  5 . This operation removes any skew in the paper  3  to obtain proper registration. 
     The CPU  121  has a built-in paper gap timer  130 . The paper gap timer  130  has a timer value set to the conveying gap corresponding to the type of recording sheet in  FIG. 9  for each sheet of paper  3 . The paper gap timer  130  begins counting down the time value when the trailing edge of the paper  3  is detected, that is, when the photoelectric sensor  100  changes from an ON state to an OFF state for adjusting the timing at which the next sheet of paper  3  is supplied. In other words, the paper gap timer  130  adjusts the conveying gap between consecutively fed sheets of paper  3 . 
     Next, a process to adjust the conveying gap between consecutively fed sheets of paper  3  will be described with reference to the flowchart in  FIG. 10 . The CPU  121  performs the process in  FIG. 10  by reading and executing the conveying control program  126  shown in  FIG. 8 . The conveying control program  126  is repeatedly executed at prescribed intervals (5 msec in the aspects) while power is supplied to the laser printer  1 . 
     If the power to the laser printer  1  is turned ON while the paper cassette  9  is mounted in the cassette-accommodating section  2 A, then in S 1  the CPU  121  determines whether a sheet of paper  3  is present in the section of the pivoting link  80  by confirming the ON/OFF state of the photoelectric sensor  100 . If the laser printer  1  is started up while the paper cassette  9  is mounted in the cassette-accommodating section  2 A, then the protruding plate part  95  of the pivoting link  80  extends through the conveying path  56 , with the bent end  95 A resting on the seat surface  79 A of the receiving part  79 . Further, the arm  84  is substantially vertical in orientation, and the shielding plate  85  blocks the optical path of the detection light emitted from the light-emitting element so that output from the photoelectric sensor  100  indicates the OFF state. Hence, since a sheet of paper  3  does not exist in the pivoting link  80  at this time (S 1 : NO), then in S 2  the CPU  121  determines whether any print data exists. Since no printing instructions have been received from the personal computer  141  at this time (S 2 : NO), then in S 4  the CPU  121  starts the paper gap timer  130  and returns to S 1 . Hence, the CPU  121  is essentially in a wait state until printing instructions are received from the personal computer  141 . Although the paper gap timer  130  is started in S 4  during each loop of the wait state, the timer value has not been set in the paper gap timer  130  and remains at zero. 
     When the laser printer  1  receives printing instructions from the personal computer  141 , the CPU  121  stores the printing instructions in the RAM  123 , extracts the print data from the printing instructions, and stores the print data in the image data development memory device  127 . Since the feeding roller  12  and the separating roller  10  have yet to pick up a sheet of paper  3  from the paper cassette  9  at the moment printing instructions are received and, hence, the output from the photoelectric sensor  100  indicates the OFF state, the CPU  121  determines that a sheet of paper  3  does not exist in the section of the pivoting link  80  (S 1 : NO) and advances to S 2 . 
     In S 2  the CPU  121  again determines whether print data exists. This determination is made by confirming whether the image data development memory device  127  holds print data. Since the print data is stored in the image data development memory device  127  when the printing instructions are received, the CPU  121  determines that print data exists (S 2 : YES) and advances to S 3 . Note that if the process for determining the existence of print data in S 2  is performed after executing the process in S 5  and S 6  described later, this determination is performed based on print data included with the next printing instructions. 
     In S 3  the CPU  121  determines whether the time counted by the paper gap timer  130  has elapsed. Since the timer value for the paper gap timer  130  has yet to be set and remains at zero when the printing instructions are just received, the CPU  121  determines that the time has elapsed (S 3 : YES), and advances to S 5 . 
     In S 5  the CPU  121  sets the timer value in the paper gap timer  130  based on the type of paper  3 . Specifically, since the printing instructions include recording sheet data, the CPU  121  extracts the recording sheet data from the printing instructions stored in the RAM  123  and determines the type of paper  3  based on the extracted recoding sheet data. Then, the CPU  121  references the table shown in  FIG. 9  using the determined type of paper  3  as an index and sets the timer value in the paper gap timer  130  to a value for producing the conveying gap corresponding to the type of paper  3  in the table. For example, if the CPU  121  determines that the paper  3  is “thin paper,” then the CPU  121  sets the timer value in the paper gap timer  130  to the reference value (300 msec) +60 msec. 
     In S 6  of  FIG. 10 , the CPU  121  connects the second through fifth clutch devices  132 - 135  to rotate the feeding roller  12 , the separating roller  10 , the pinch roller  13 , and the discharge roller  52 , beginning a feeding operation for picking up and feeding the paper  3 . Since the topmost sheet of paper  3  in the paper cassette  9  is in contact with the feeding roller  12 , the feeding roller  12  feeds the topmost sheet to the separating roller  10 , and the sheet is subsequently conveyed to the pinch roller  13 . After completing this process, the CPU  121  returns to S 1 . 
     In the next process, the paper  3  has been supplied from the separating roller  10  to the pinch roller  13 , contacting the pivoting link  80  and changing the orientation of the pivoting link  80  to the second orientation. Since the output of the photoelectric sensor  100  has changed from an OFF state to an ON state due to the change in orientation of the pivoting link  80 , in S 1  the CPU  121  determines that the paper  3  exists in the region of the pivoting link  80  (S 1 : YES). At this time, the CPU  121  disconnects the third and fourth clutch devices  133  and  134  to temporarily halt rotation of the feeding roller  12  and the separating roller  10  so that a subsequent sheet of paper  3  is not supplied toward the pinch roller  13  after the current sheet. Then the CPU  121  loops back to S 1 . 
     In this way, the CPU  121  continues to loop back to S 1  and does not reach S 4  until the paper  3  has passed through the pivoting link  80 . Therefore, the paper gap timer  130  is not started until this time. 
     When the paper  3  passes through the pivoting link  80 , the pivoting link  80  is no longer pressed by the paper  3  and returns from the second orientation to the first orientation, and the photoelectric sensor  100  changes from the ON state to the OFF state. Since the paper  3  is no longer present in the region of the pivoting link  80  (S 1 : NO), the CPU  121  advances to S 2 . 
     In S 2  the CPU  121  determines whether print data accompanying the next printing instructions exists in the image data development memory device  127 . If the next printing instructions have been transmitted from the personal computer  141  at this time, then print data exists in the image data development memory device  127  (S 2 : YES). Accordingly, in S 3  the CPU  121  determines whether the time set in the paper gap timer  130  has elapsed, that is, whether the paper gap timer  130  has counted down to zero. Since the paper gap timer  130  has not been started since the timer value was previously set in this example, the paper gap timer  130  has not counted down to zero (S 3 : NO). Therefore, the CPU  121  starts the paper gap timer  130  in S 4  and returns to S 1 . On the other hand, if the personal computer  141  has not transmitted the next printing instructions, then the next print data does not exist in the image data development memory device  127  (S 2 : NO). Accordingly, the CPU  121  starts the paper gap timer  130  in S 4  and returns to S 1 . 
     Assuming that the personal computer  141  has transmitted the next printing instructions (S 2 : YES), the CPU  121  repeatedly performs the processes S 1 -S 4  described above until the time set in the paper gap timer  130  has elapsed (S 3 : YES). The paper gap timer  130  arriving at zero signifies that the conveying gap corresponding to the sheet thickness of the previously conveyed sheet of paper  3  has opened up behind the trailing edge of the same sheet. For example, when the paper  3  is thin paper, then the elapsed time in the paper gap timer  130  indicates that a conveying gap corresponding to the reference value (300 msec) +60 msec has opened up after the trailing edge of the previously conveyed sheet of paper  3 . 
     In S 5  the CPU  121  extracts the recording sheet data from the next printing instructions stored in the RAM  123 , determines the type of paper  3  based on the recording sheet data, and resets the timer value in the paper gap timer  130 . In S 6  the CPU  121  connects the third and fourth clutch devices  133  and  134  to rotate the feeding roller  12  and the separating roller  10  again. The feeding roller  12  picks up the topmost sheet of paper  3  in the paper cassette  9 , and the separating roller  10  supplies one sheet of the paper  3  to the pinch roller  13 . Subsequently, the CPU  121  returns to S 1 . 
     The process in S 1 -S 6  performed with the preceding sheet of paper  3  is similarly performed on the succeeding sheet of paper  3  so that a prescribed conveying gap corresponding to the sheet thickness of the succeeding sheet of paper  3  is opened up between this succeeding sheet and the sheet following this succeeding sheet. When subsequent printing instructions are not transmitted from the personal computer  141 , the CPU  121  determines that there is no next print data (S 2 : NO). Therefore, in S 4  the CPU  121  starts the paper gap timer  130 , returns to S 1 , and repeatedly performs this process. Hence, the printing operation on the sheet of paper  3  ends without feeding another sheet. 
     As described above, according to the invention, when the pinch roller  13  continuously conveys paper  3  supplied from the separating roller  10  and the feeding roller  12 , the laser printer  1  adjusts the conveying gap between the consecutively fed sheets of paper  3  based on the thickness of each sheet. In this way, the laser printer  1  can prevent paper jams due to different thicknesses in the sheets of paper  3 . 
     That is, when sheets of paper  3  are fed consecutively, the laser printer  1  detects the paper  3  in the section of the pivoting link  80  based on the ON/OFF state of the photoelectric sensor  100 . When the paper  3  is thin paper, for example, the leading edge or the trailing edge of the paper  3  can ride up on the pivoting link  80  as shown in  FIGS. 13 and 14 , throwing off the timing at which the pivoting link  80  detects the leading edge or the trailing edge of the paper  3 . Therefore, when the pivoting link  80  is late in detecting the leading edge of the paper  3  as shown in  FIG. 13 , the laser printer  1  perceives the length of the paper  3  to be shorter by a length L 1 . Further, when the pivoting link  80  is early in detecting the trailing edge of the paper  3 , the laser printer  1  perceives the length of the paper  3  to be shorter by a length L 2 . If the pivoting link  80  is off in detecting both the leading edge and the trailing edge, the laser printer  1  perceives the overall length of the paper  3  to be doubly shorter by the length L 1  and L 2 . 
     Therefore, in the laser printer  1  according to the aspects, when the paper  3  is thin paper, the conveying gap between consecutively fed sheets of paper  3  is set to a gap equivalent to the reference value of 300 msec (the conveying gap for thick paper) plus a value of 60 msec considered equivalent to the amount the paper  3  deforms based on the sheet thickness and material. Accordingly, even when the length of the paper  3  is perceived to be shorter than the actual length, the laser printer  1  can allocate a conveying gap between the preceding sheet and the succeeding sheet corresponding to at least the reference value (300 msec). Hence, even when the laser printer  1  reduces the conveying gap between papers  3  at the registration roller  14  after the feeding roller  12  and the separating roller  10  have supplied the paper  3  to the pinch roller  13  in order to reduce time loss and increase the number of printed sheets per unit time, the laser printer  1  can prevent a succeeding sheet of paper  3  from being too close to or overlapping the trailing edge of the preceding sheet at the image-forming unit  5  or just before the image-forming unit  5 , thereby preventing paper jams. In this way, the laser printer  1  according to the aspects feeds consecutive sheets of a thin paper  3  while allocating at least the conveying gap for thick paper. Accordingly, the laser printer  1  does not feed paper at a faster rate than its capacity; images formed on the paper  3  are not cut off on the trailing edge; and overruns are unlikely to occur in image processing, thereby achieving excellent printing quality. 
     Further, the laser printer  1  according to the aspects opens a conveying gap (the reference value of 300 msec +60 msec, for example) between consecutively fed sheets of paper  3  corresponding to the sheet thickness at the time of feeding. Accordingly, a prescribed gap can be maintained between the consecutively fed sheets from the image-forming position at the photosensitive drum  29  to the discharge tray  53 , as well. As a result, even when consecutive sheets of thin paper  3  are fed, a succeeding sheet of paper  3  will not press against the actuator  152  after the preceding sheet has passed and before the actuator  152  can fully return from its orientation in the ON state to its orientation in the OFF state, thereby enabling the discharge sensor  151  to reliably detect the trailing edge of the preceding paper  3 . Accordingly, the laser printer  1  can properly perceive the length of the paper  3  without mistakenly detecting a paper jam. 
     When expanding the conveying gap between consecutively fed sheets of paper  3  in this way, the conveying time increases by the amount of expansion, reducing printing efficiency. However, the laser printer  1  according to the aspects appropriately modifies the conveying gap between consecutively fed sheets of paper  3  based on the sheet thickness and can therefore obtain the maximum printing efficiency corresponding to the thickness of the paper  3  without needlessly increasing the conveying gap. 
     Hence, the laser printer  1  according to the aspects can reliably control conveyance of the paper  3  while maintaining the maximum throughput. Future trends in image-forming devices will likely call for further improvements in printing speed, leading to a shorter gap between conveyed sheets of paper  3 . However, by modifying the conveying gap based on the thickness of the paper  3 , it will be possible to minimize time loss to meet the demand for faster printing rates while preventing paper jams from occurring with thin sheets of paper  3 , thereby improving printing efficiency. 
     Further, in the laser printer  1  according to the aspects, the sheet-pressing plate  15  presses the paper  3  in contact with the feeding roller  12 ; the feeding roller  12  rotates to feed the sheets of paper  3  to the separating roller  10 ; and the separating roller  10  supplies the paper  3  to the pinch roller  13  one sheet at a time. Hence, by controlling the timing at which the feeding roller  12  and the separating roller  10  are rotated and halted, the laser printer  1  can adjust the timing at which the feeding roller  12  and the separating roller  10  supply the paper  3  to the pinch roller  13 . Speeding up the timing for feeding sheets of paper  3  decreases the conveying gap between consecutively fed sheets, while slowing down the timing expands the conveying gap. Hence, the laser printer  1  according to the aspects can easily adjust the conveying gap using the feeding roller  12  and the separating roller  10  in the existing technology. 
     Further, since the conveying gap between sheets of paper  3  is adjusted based on the recording sheet data transmitted from the personal computer  141 , the laser printer  1  according to the above aspects can easily determine the sheet thickness based on data inputted by a user and transmitted from the personal computer  141 . 
     Next, a laser printer according to additional aspects of the invention will be described. Note that except for the structure of the control hardware, the remaining construction of the laser printer according to the additional aspects is identical to the laser printer  1  according to the above aspects. Therefore, parts and components identical to those used in the above aspects are designated with the same reference numerals to avoid duplicating description. The following description will focus on the points of difference. 
       FIG. 11  is a block diagram showing the control hardware structure employed in the laser printer according to the additional aspects. The laser printer according to the additional aspects has a control unit  120 A. The control unit  120 A is provided with a sheet thickness detecting function  161  for detecting the sheet thickness of the paper  3 . Specifically, the sheet thickness detecting function  161  detects the conveying time required to convey the paper  3  from the paper cassette  9  to the registration rollers  14  by measuring the time beginning from when the photoelectric sensor  100  of the pivoting link  80  changes from an OFF state to an ON state until the time that the detector  65  of the front registration sensor  61  changes from an OFF state to an ON state, and detects the sheet thickness of the paper  3  based on this conveying time. Specifically, the sheet thickness detecting function  161  determines that the paper  3  is a thick paper if the conveying time exceeds an upper limit, determines the paper  3  is a normal paper if the conveying time is less than or equal to the upper limit and greater than or equal to a lower limit, and determines that the paper  3  is a thin paper or a transparency if the conveying time is less than the lower limit. It is also possible to differentiate the thin paper from the transparency based on the conveying time or by detecting the transmittance or reflectance of light incident on the paper  3 . 
     This type of laser printer can automatically determine the thickness of the paper  3  conveyed from the paper cassette  9  to the conveying path  56  using the sheet thickness detecting function  161 , even when paper of different thicknesses is combined in the paper cassette  9 , and can adjust the conveying gap between consecutively fed sheets of paper  3  based on this determination. Hence, compared to the laser printer  1  according to the above aspects, the laser printer of the additional aspects reduces the operating load on the user by eliminating the need for the user to check the type of paper  3  in the paper cassette  9  and input this type in the personal computer  141  each time the user outputs printing instructions. The laser printer of the additional aspects can also eliminate incorrect settings for the type of recording sheet caused by errors in input operations, thereby improving the printing efficiency. 
     While the invention has been described in detail with reference to the above aspects thereof, it would be apparent to those skilled in the art that various changes and modifications may be made therein without departing from the spirit of the invention. 
     For example, in the above aspects, the laser printer adjusts the feeding timing by controlling the rotation of the separating roller  10  and the feeding roller  12  in order to adjust the conveying gap between consecutively fed sheets of paper  3 . Alternatively, it is possible to adjust the conveying gap by controlling the rotation of the registration rollers  14 . More specifically, the laser printer determines that a sheet of paper  3  exists in the section of the front registration sensor  61  when the detector  65  of the front registration sensor  61  changes from an OFF state to an ON state. At this time, the laser printer connects the first clutch device  131  to rotate the registration rollers  14  and convey the paper  3  to the image-forming unit  5 . Subsequently, the laser printer determines that the paper  3  no longer exists in the section of the front registration sensor  61  when the detector  65  of the front registration sensor  61  changes from the ON state to the OFF state. At this time, the laser printer disconnects the first clutch device  131  to temporarily halt rotation of the registration rollers  14  and prevent the succeeding sheet of paper  3  from being conveyed to the image-forming unit  5 . In the meantime, the succeeding sheet is picked up from the paper cassette  9  and conveyed to the registration rollers  14 . When the time set in the paper gap timer  130  reaches zero, the laser printer again connects the first clutch device  131  to rotate the registration rollers  14  and convey the succeeding sheet of paper  3  to the image-forming unit  5 . In this way, the laser printer can easily adjust the conveying gap formed between the trailing edge of a preceding sheet of paper  3  and the leading edge of a succeeding sheet of paper  3  using the existing registration rollers  14  by adjusting the timing at which the registration rollers  14  convey the paper  3  to the image-forming unit  5 . It is also possible to adjust the conveying gap using the separating roller  10 , the feeding roller  12 , and the registration rollers  14 . 
     While a laser printer is used as an example of the image-forming device in the above aspects, the image-forming device may be a color laser printer, an inkjet printer, a facsimile machine, a copier, or a multifunction device having such functions as a facsimile function, a scanner function, a copier function, and a printer function. 
     Although a sheet of paper  3  is detected using contact sensors in the above aspects, wherein the paper  3  contacts the pivoting link  80 , the front registration sensor  61 , the rear registration sensor  66 , and the discharge sensor  151 , it is possible to detect the paper  3  using a non-contact method in which the paper  3  passes between optical sensors. 
     While the personal computer  141  serves as the host device in the above aspects, the host device may be an inputting device provided directly on the laser printer  1 , such as a control panel. 
     The timing values representing conveying gaps in  FIG. 9  are merely examples and can be adjusted as appropriate. Further, although the conveying gap is controlled according to time in the above aspects, the conveying gap may be controlled by actually measuring the gap between consecutively fed sheets of paper  3  or based on the conveying speed. 
     In the above aspects, changes in the orientation of the pivoting link  80  are detected based on the state of the photoelectric sensor  100  and the driving state of the motor M. However, a sensor may be disposed near the pivoting link  80  for detecting positional changes in the pivoting link  80  relative to the sensor (displacement of the free end when the pivoting link  80  rotates). With this construction, the laser printer can identify the orientation of the pivoting link  80  from the amount of displacement in the free end of the pivoting link  80 . This method may also be employed in the detector  65  of the front registration sensor  61  and the detector  70  of the rear registration sensor  66 . 
     In the above aspects, light in the photoelectric sensor  100  is blocked when the pivoting link  80  is in the first orientation and unimpeded when the pivoting link  80  is in other orientations. However, the photoelectric sensor  100  may be configured so that light is received when the pivoting link  80  is in the first orientation and blocked when the pivoting link  80  is in other orientations. In this case, the pivoting link  80  must be provided with a plurality of light-shielding parts. The same configuration may be employed with the detector  65  of the front registration sensor  61  and the detector  70  of the rear registration sensor  66 . 
     In the above aspects, the coil spring  110  is provided in the cassette-accommodating section  2 A to improve the response of the pivoting link  80  to conveyance of the paper  3  and removal of the paper cassette  9 . However, the coil spring  110  is not essential, and the orientation of the pivoting link  80  may be changed using its own weight. In contrast, the orientations of the front registration sensor  61 , the rear registration sensor  66 , and the discharge sensor  151  are controlled by the weight of these components, but an urging force may be applied to the front registration sensor  61 , the rear registration sensor  66 , and the discharge sensor  151  using a coil spring or other urging member to increase response. 
     In the above aspects, the laser printer detects the trailing edge of the paper  3  to control the conveying gap, but it is also possible to control the conveying gap by detecting the leading edge of the paper  3  or both the leading edge and the trailing edge. 
     In the additional aspects described above, the laser printer detects a conveying time for the paper  3  based on ON/OFF states for the photoelectric sensor  100  of the pivoting link  80  and the detector  65  of the front registration sensor  61  and detects the sheet thickness of the paper  3  based on the conveying time. However, the sheet thickness of the paper  3  can also be detected directly using an optical sensor or the like.