Patent Publication Number: US-8523172-B2

Title: Image forming apparatus

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an image forming apparatus, and more particularly, to a structure for reducing noise when a sheet cassette detachably mounted on the apparatus main body is extracted. 
     2. Description of the Related Art 
     Nowadays, among image forming apparatuses, such as a copying machine, a printer, and a facsimile, widely used image forming apparatuses are configured in such a manner that a sheet feeding device feeds a sheet to an image forming portion to form an image. Generally in such an image forming apparatus, a sheet cassette is detachably mounted on the apparatus main body, and sheets stored in the sheet cassette are fed to the image forming portion by a feeding roller. 
     For example, there is a known sheet cassette in which a sheet stacking portion stacks sheets inside the cassette main body and the sheets are pressed to the feeding roller can be lifted and lowered. In addition, when a sheet is fed, the sheet stacking portion is lifted so that the sheet is pressed to the sheet feeding roller and thus the sheet is fed by virtue of a pressing force (hereinafter, referred to as a feeding pressure) between the feeding roller and the top surface of the sheet. In an exemplary image forming apparatus, a drive source for lifting/lowering the sheet stacking portion is provided in the apparatus main body side, and, when the sheet cassette is mounted on the apparatus main body, the drive source lifts the sheet stacking portion so that the sheet is pressed to the feeding roller, and a feeding pressure is generated. 
     However, some of the image forming apparatuses of the related art have a sheet feeding device which transfers the sheet from the drive source to a rotation shaft at a predetermined timing. Such sheet feeding devices include a drive gear and a tooth-chipped gear. The drive gear is connected to the drive source such as a motor, and the tooth-chipped gear meshes with the drive gear and is rotated by the changes of the clutch mechanism. In addition, when the sheet is fed, the tooth-chipped gear is rotated by the clutch mechanism so as to mesh with the drive gear. In this way, the feeding roller is rotated (refer to U.S. Pat. No. 6,070,867). 
       FIG. 18  illustrates such a sheet feeding device of the related art. The sheet feeding device includes a feeding roller  920  and an intermediate lifting/lowering cam  908  which are attached to the rotation shaft  905 . The sheet feeding device further includes a liftable intermediate plate  900  which is upwardly biased by a biasing member (not illustrated) and is provided with protrusions  909  at both ends. In addition, as the rotation shaft  905  is rotated, the intermediate plate  900  is lifted by the protrusion  909  and the intermediate lifting/lowering cam  908  which rotates in synchronization with the rotation shaft  905 , and the sheets S stacked on the intermediate plate  900  are pressed to the feeding roller  920 . Then, as the feeding roller  920  is rotated, and in consequence the sheets S are fed. 
     However, the sheet feeding device includes a drive gear  901  and a tooth-chipped gear  902 . The drive gear  901  is connected to the drive source such as a motor (not illustrated). The tooth-chipped gear  902  is fixed to the rotation shaft  905 , has a clutching mechanism, and meshes with the drive gear  901  by the changes of the clutch mechanism. The solenoid  906  illustrated in  FIG. 18  includes an armature  907 , a coil  913  that generates magnetism when an electric current flows across it, a frame  910  that efficiently transmits the generated magnetism, and a stator  914  that generates a magnetic force as illustrated in  FIGS. 19A and 19B . The armature  907  is attached to the support point  911  of the frame  910  and is biased in the direction of an arrow F by the biasing force of the spring  912 . 
     The armature  907  has a locking portion  904  at the leading edge thereof, and the tooth-chipped gear  902  has a locking claw  915  which engages with the locking portion  904  of the armature  907 . Here, the tooth-chipped gear  902  is biased to rotate in the direction of an arrow W by the biasing member (not illustrated). However, the tooth-chipped gear  902  is held at a position where the tooth-chipped gear  902  does not mesh with the drive gear  901  because the locking portion  904  of the armature  907  engages with the locking claw  915  as illustrated in  FIG. 19A  until sheet feeding is initiated. 
     Then, as an electric current flows to the solenoid  906  for the sheet feeding, a magnetic force is generated in the stator  914  due to the magnetism generated from the coil  913 , and the armature  907  is attracted toward the stator  914  as illustrated in  FIG. 19B . In this manner, as the armature  907  is attracted to the stator  914 , the locking between the locking portion  904  of the armature  907  and the locking claw  915  of the tooth-chipped gear  902  is released, and the tooth-chipped gear  902  is rotated in the direction of the arrow W and meshes with the drive gear  901 . 
     As a result, the tooth-chipped gear  902  is rotated by the drive gear  901 , which brings about the rotation of the rotation shaft  905  engaged with the tooth-chipped gear  902 , and in consequence the intermediate lifting/lowering cam  908  integrated with the rotation shaft  905  becomes rotated. In this manner, as the intermediate lifting/lowering cam  908  is rotated, the intermediate plate  900  is lifted by the protrusions  909  provided at both ends of the intermediate plate  900 , and the sheets S stacked on the intermediate plate  900  are is pressed to the feeding roller  920 . In addition, since the tooth-chipped gear  902  and the feeding roller  920  are fixed to the rotation shaft together, the feeding roller  920  is rotated along with the tooth-chipped gear  902 , thereby feeding the sheets S that are pressed to the feeding roller  920 . 
     However, in such an image forming apparatus of the related art, the armature  907  of the solenoid  906  is locked when the apparatus is not used, and the position of the tooth-chipped gear  902  is held. Therefore, the tooth-chipped gear  902  does not mesh with the drive gear  901 . However, if a strong vibration or impact is applied, for example, when the apparatus is transported after the apparatus is manufactured, or when the image forming apparatus is moved, the armature  907  of the solenoid  906  may be released from the locking claw  915  of the tooth-chipped gear  902 . 
     If the armature  907  of the solenoid  906  is released as described above, the tooth-chipped gear  902  meshes with the drive gear  901 . In this case, the sheet may be fed just by rotating the motor even when the solenoid is not driven during use of the apparatus, and therefore, it may generate a sheet jam and consume sheets uselessly. 
     SUMMARY OF THE INVENTION 
     In this regard, the invention provides an image forming apparatus capable of preventing unnecessary sheet feeding when the power is turned on. 
     An image forming apparatus that forms an image on a sheet fed by a feeding roller from a sheet cassette mounted on an apparatus main body, the image forming apparatus includes a drive portion that is provided in the apparatus main body to drive the feeding roller, a drive transmission portion that is capable of switching, by virtue of driving of the drive portion, from a first state in which the driving of the drive portion is transmitted to the feeding roller to a second state in which the driving of the drive portion is not transmitted to the feeding roller, a switching portion that switches the drive transmission portion from the second state to the first state when the sheet is fed, and makes the drive transmission portion, returned from the first state to the second state by virtue of driving of the drive portion, stay in the second state if the sheet is fed, and a controller that drives the drive portion for a time necessary to return the drive transmission portion from the first state to the second state so as to make the drive transmission portion be in the second state when power is turned on. 
     According to the invention, it is possible to prevent unnecessary sheet feeding when power is turned on by driving the drive portion for the time necessary to return the drive transmission portion from the first state to the second state. This ensures that the drive transmission portion will be in the second state as the power is turned on. 
     Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an entire configuration diagram of the full-color laser beam printer as an exemplary image forming apparatus according to a first embodiment of the invention; 
         FIG. 2  is a diagram illustrating a configuration of the sheet cassette of the above-described full-color laser beam printer; 
         FIG. 3  is a diagram illustrating a configuration of the sheet feeding device of the above-described full-color laser beam printer; 
         FIG. 4  is a diagram illustrating a configuration of a lifter drive mechanism of the above-described sheet feeding device; 
         FIG. 5  is a diagram illustrating a configuration of the pressing drive unit of the above-described lifter drive mechanism; 
         FIGS. 6A to 6E  are diagrams illustrating a relationship between the operational position of the rack provided in the above-described lifter drive mechanism and the response of the photointerrupter; 
         FIG. 7  is a diagram illustrating output results of the photointerrupter depending on the operational position of the above-described rack; 
         FIG. 8  is a diagram illustrating a configuration of the drive portion of the above-described sheet feeding device; 
         FIG. 9  is a diagram illustrating a configuration of the tooth-chipped gear unit provided in the drive portion of the above-described sheet feeding device; 
         FIG. 10  is a diagram illustrating a state that the tooth-chipped gear unit and a feeding solenoid of the above-described drive portion are at a standby position; 
         FIG. 11  is a diagram illustrating a state that the feeding solenoid of the above-described drive portion is operated; 
         FIG. 12  is a control block diagram illustrating the above-described full-color laser beam printer; 
         FIG. 13  is a flowchart illustrating pre-rotation control when a typical printer main body of the above-described full-color laser beam printer is powered on; 
         FIG. 14  is a flowchart illustrating control when the print job of the above-described full-color laser beam printer is received; 
         FIG. 15  is a flowchart illustrating control performed when the above-described full-color laser beam printer is initially powered on; 
         FIG. 16  is a flowchart illustrating control of the sheet feeding device provided in the image forming apparatus according to the second embodiment of the invention; 
         FIG. 17  is a flowchart illustrating pre-rotation control of the above-described sheet feeding device; 
         FIG. 18  is a schematic diagram illustrating a sheet feeding device of the related art; and 
         FIGS. 19A and 19B  are diagrams illustrating operations of the drive portion of the above-described sheet feeding device of the related art. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Hereinafter, embodiments of the invention will be described in detail with reference to the accompanying drawings.  FIG. 1  is an entire configuration diagram illustrating the full-color laser beam printer as an exemplary image forming apparatus according to a first embodiment of the invention. In  FIG. 1 , the full-color laser beam printer main body  1 A (hereinafter, referred to as a printer main body) as a main body of the a full-color laser beam printer  1  includes an image forming portion  1 B for forming an image on a sheet, a sheet feeding device  20  for feeding sheets, and the like. 
     The image forming portion  1 B includes process cartridges P (PY, PM, PC, and PBk) for forming toner images for four colors including yellow, magenta, cyan, and black, respectively. The process cartridge P includes photosensitive drums  26  ( 26 Y,  26 M,  26 C, and  26  Bk) as an image bearing member and is detachably mounted on the printer main body  1 A. In addition, the image forming portion  1 B has a scanner unit  28  arranged immediately below the process cartridge P to form an electrostatic latent image on the photosensitive drum  26  by irradiating a laser beam based on image information. 
     In  FIG. 1 , an intermediate transfer belt unit  31  includes an intermediate transfer belt  30  and primary transfer rollers  52  ( 52 Y,  52 M,  52 C, and  52 Bk) arranged at the inner side of the intermediate transfer belt  30 . In addition, the intermediate transfer belt  30  is stretched between a drive roller  100  and a tension roller  105 , and the tension roller  105  is configured to move horizontally depending on the length of the intermediate transfer belt  30 . In addition, the primary transfer roller  52  is provided oppositely to each photosensitive drum  26  so that a transfer bias is applied by a bias application unit (not illustrated). In addition, since the primary transfer bias is applied to the intermediate transfer belt  30  using the primary transfer roller  52 , each color toner image on the photosensitive drum is sequentially transferred to the intermediate transfer belt  30  so that a full-color image is formed on the intermediate transfer belt. 
     The secondary transfer portion  1 C includes a drive roller  100  and a secondary transfer roller  27  to transfer the full-color image sequentially formed on the intermediate transfer belt  30  to the sheet. A fixing portion  25  fixes the toner image formed on the sheet by adding heat and pressure. 
     The sheet feeding device  20  includes a sheet cassette  200  detachably mounted on the installation space provided under the printer main body  1 A, a feeding roller  21  included in a sheet feeding portion for feeding the sheets S stored in the sheet cassette  200 . When the sheets S stored in the sheet cassette  200  are fed, the sheets S are fed by rotating the feeding roller  21  pressed on the sheets S. In addition, the sheets S fed in this manner are separated one by one using a separation portion including the feeding roller  21  and the separation roller  22  and then conveyed to a pair of registration rollers  23 . 
     Next, an image forming operation of the full-color laser beam printer  1  configured in this manner will be described. As an image signal is input to the scanner unit  28  from a PC (not illustrated), laser light corresponding to the image signal is irradiated onto the photosensitive drum from the scanner unit  28 . At this moment, the surface of the photosensitive drum  26  is uniformly charged with a predetermined polarity and electric potential in advance, and an electrostatic latent image is formed on the surface by irradiating the surface with laser light from the scanner unit  28 . Then, the electrostatic latent image is developed by the developing unit provided in the process cartridge P and is visualized. 
     For example, first, the photosensitive drum  26 Y is irradiated from the scanner unit  28  with laser light based on the yellow component color image signal of an original and a yellow electrostatic latent image is formed on the photosensitive drum. Then, the yellow electrostatic latent image is developed using a yellow toner from the developing unit to visualize the yellow toner image. Then, if the toner image reaches the primary transfer portion where the photosensitive drum  26 Y and the intermediate transfer belt  30  abut as the photosensitive drum  26 Y is rotated, the yellow toner image is transferred to the intermediate transfer belt with the primary transfer bias applied to the first transfer roller  52 Y. 
     Then, as the portion bearing the yellow toner image of the intermediate transfer belt  30  moves, similarly, a magenta toner image formed on the photosensitive drum  26 M is transferred to the intermediate transfer belt  30  from the yellow toner image as described above. Similarly, as the intermediate transfer belt  30  moves, the cyan toner image and the black toner image are transferred overlappingly on the yellow toner image and the magenta toner image in each primary transfer portion. As a result, a full-color toner image is formed on the intermediate transfer belt. In addition, in the intermediate transfer belt  30  to which the toner image has been secondarily transferred, transfer residual toner remaining on the surface is removed by a belt cleaner (not illustrated) provided in the vicinity of the tension roller  105 . 
     Along with the toner image forming operation, the sheet S stored in the sheet cassette  200  is fed by the feeding roller  21 , and then, conveyed to a pair of registration rollers  23 . Then, the sheet conveyed to a pair of registration rollers  23  is timed by a pair of registration rollers  23  and conveyed to the secondary transfer portion  1 C. In addition, in the secondary transfer portion  1 C, toner images of four colors on the intermediate transfer belt  30  are secondarily transferred to the conveyed sheet S by applying a positive bias to the secondary transfer roller  27 . 
     The sheet S to which the toner image is transferred is conveyed to the fixing portion  25 . In the fixing portion  25 , a full-color toner image is fixed on the surface thereof as a permanent image by applying heat and pressure on the surface. Then, after the full-color toner image is fixed as a permanent image, the sheet S is discharged to a discharge tray  41  through a pair of discharge rollers  40 . 
     Here, the sheet cassette  200  is detachably attachable in the near front direction of the printer main body  1 A, that is, in the direction perpendicular to the sheet feeding direction. As the sheet cassette  200  is mounted on the printer main body  1 A, an image forming controller which is a controller of the printer main body  1 A as illustrated in  FIG. 12  described below determines that the sheet cassette  200  is mounted on the printer main body  1 A using a presence/absence detection unit described below. 
       FIG. 2  is a diagram illustrating a configuration of the sheet cassette  200 . The sheet cassette  200  can store the sheets having an A 5  longitudinal size to a letter longitudinal size. Here, the sheet cassette  200  has a cassette main body  200   a  for storing a plurality of sheets. In the cassette main body  200   a , an intermediate plate  201  which is a sheet stacking portion for stacking the sheets is provided pivotably (liftably) with respect to a pivot point  201   a.    
     In the cassette main body  200   a , a rear edge control member  251  for controlling the rear edge, which is an upstream side end in the sheet feeding direction of the sheet on the intermediate plate  201 , is provided slidably in the sheet feeding direction. The rear edge control member  251  is configured to slidably move to the upstream and downstream sides in the sheet feeding direction when a user manipulates the lever  252  of the rear edge control member  251  so as to arrange the sheets at the position depending on the sheet size. 
     In the sheet cassette  200 , a front side control plate  261  and a back side control plate  263  constituting a pair of side end control portions for controlling positions in the width direction perpendicular to the sheet feeding direction of the sheet on the intermediate plate  201  are provided slidably in the width direction. Here, the front side control plate  261  and the back side control plate  263  are connected to each other using a rack portion and a pinion gear (not illustrated). Therefore, as a user manipulates a lever  262  provided in the front side control plate  261 , the front side control plate  261  and the back side control plate  263  move in the width direction in synchronization. 
     In addition, in the downstream side of the intermediate plate  201 , as illustrated in  FIG. 3 , a pressing lever  202  for pushing up the intermediate plate  201  toward the feeding roller  21  is provided. In addition, the pressing lever  202  has the leading edge thereof contacting the bottom surface of the intermediate plate  201  at the center of the intermediate plate  201 . In addition, the pressing lever  202  is connected to a pressing arm  203  for pivoting the intermediate plate  201  in the vertical direction by pivoting the pressing lever  202  in the vertical direction, as illustrated in  FIG. 2 , in the downstream (hereinafter, referred to as the back side) of the cassette installation direction of the cassette main body  200   a.    
     Here, the pressing arm  203  is driven (operated) by the lifter drive mechanism  200 A, which is an operation mechanism illustrated in  FIG. 2 . According to the present embodiment, the pressing lever  202 , the pressing arm  203 , and the lifter drive mechanism  200 A constitute a lift mechanism for pushing up the intermediate plate  201  to make the sheets which are stacked on the intermediate plate  201  pressed to the feeding roller  21 . As the sheet cassette  200  is mounted, the lift mechanism is connected to a drive transmission gear  301  described below so as to lift the intermediate plate  201 . As the connection with the drive transmission gear  301  is released when the sheet cassette  200  is extracted, the intermediate plate  201  is lowered. 
     As illustrated in  FIG. 4 , the lifter drive mechanism  200 A has a pressing spring  205  which is a tensile coil spring of which one end is locked in the pressing arm  203  so as to set, depending on the sheet size, a pressing force for making the sheet on the intermediate plate pressed to the feeding roller  21 . In addition, a rack  204  is provided on a wall surface  200   b  of the back side of the sheet cassette  200 , such that the other end of the pressing spring  205  is locked, and the rack  204  is movable along the sheet feeding direction as illustrated in the arrows P 1  and P 2 . In addition, using the pressing spring  205  and the rack  204 , the sheets on the intermediate plate (on the sheet stacking portion) are pressed to the feeding roller  21  with a pressing (feeding pressure) in a magnitude depending on the sheet size by the pressing arm  203  and the pressing lever  202 . 
     In  FIG. 4 , a cassette gear  206  is provided on the back side wall surface  200   b  of the sheet cassette  200  and meshes with the gear  204   b  of the rack  204  to serve as a pinion gear. As the sheet cassette  200  is mounted, the cassette gear  206  meshes with the drive transmission gear  301  provided in the printer main body  1 A of  FIG. 2 . In addition, as the drive transmission gear  301  is rotated by a pressing drive unit  300  provided in the printer main body  1 A, the drive is transmitted to the cassette gear  206  so that the cassette gear  206  is rotated. As a result, the rack  204  moves, and accordingly, the intermediate plate  201  is lifted or lowered by the pressing arm  203  and the pressing lever  202 . 
     Here, as illustrated in  FIG. 5 , the pressing drive unit  300  includes a pressing drive motor  75  having a worm gear  302 , a deceleration gear  303  meshing with the worm gear  302 , a deceleration gear  304 , and a drive transmission gear  301  meshing with the cassette gear  206 . In addition, the driving of the pressing drive motor  75  which is a lifting/lowering drive portion for lifting/lowering the intermediate plate  201  is transmitted to the cassette gear  206  through the worm gear  302 , the deceleration gear  303 , the deceleration gear  304 , and the drive transmission gear  301  so as to move the rack  204 . 
     A detection protrusion  220  of  FIG. 4  is provided on the wall surface  200   b  of the back side of the sheet cassette  200 , and the photointerrupter  221  is fixed to the printer main body  1 A. As the sheet cassette  200  is mounted on the printer main body  1 A, the detection protrusion  220  at the sheet cassette side is detected by the photointerrupter  221 . In this manner, according to the present embodiment, a presence/absence detection unit for detecting presence/absence of the sheet cassette  200  is configured using the detection protrusion  220  and the photointerrupter  221 . The image forming controller  72  (CPU  721  thereof) of  FIG. 12  described below determines whether the sheet cassette  200  is mounted on the printer main body  1 A based on the signal from the photointerrupter  221 . 
     As illustrated in  FIG. 5 , the rack  204  includes detection protrusions  204   c  and  204   d . In addition, in the printer main body  1 A, the two photointerrupters  210   a  and  210   b  of  FIG. 4  are provided on a movement locus of the detection protrusions  204   c  and  204   d . Here, the detection protrusions  204   c  and  204   d  and the photointerrupters  210   a  and  210   b  as a sensor constitute an operational position detection unit so as to detect the operational position of the rack  204  by combining responses from the photointerrupters  210   a  and  210   b.    
     Next, a relationship between the operational position of the rack  204  and the responses of the photointerrupters  210   a  and  210   b  will be described with reference to  FIGS. 6A to 6E .  FIG. 6A  illustrates a position of the rack  204  at a home position. The signal of the photointerrupters  210   a  and  210   b  at this time corresponds to the response A in  FIG. 7 . That is, when the rack  204  is at the home position, the photointerrupter  210   a  is marked as o (detection signal is present), and the photointerrupter  210   b  is marked as x (detection signal is absent). 
       FIG. 6B  illustrates an exemplary position of the rack  204  between the home position and a first pressing position and corresponds to the response B in  FIG. 7 . That is, when the rack  204  is positioned between the home position and the first pressing position, the photointerrupters  210   a  and  210   b  are marked as x.  FIG. 6C  illustrates a position of the rack  204  at the first pressing position and corresponds to the response C in  FIG. 7 . That is, when the rack  204  is positioned at the first pressing position, the photointerrupter  210   a  is marked as x, and the photointerrupter  210   b  is marked as o. 
       FIG. 6D  illustrates a position of the rack  204  at the second pressing position and corresponds to the response D in  FIG. 7 . That is, when the rack  204  is positioned at the second pressing position, the photointerrupters  210   a  and  210   b  are marked as o. In addition,  FIG. 6E  illustrates a position of the rack  204  when the sheet cassette  200  is mounted on the printer main body  1 A and corresponds to the response A in  FIG. 6A . That is, according to the present embodiment, the response A in  FIG. 7  corresponds to two kinds of rack positions. 
     According to the present embodiment, when the sheet cassette  200  is inserted into the printer main body  1 A, the detection protrusion  204   d  is detected by the photointerrupter  210   a . In this case, a pressing force for applying a necessary feeding pressure is read from data such as a sheet size and a basis weight, input from a user, based on a data table recorded in the ROM  722 . In addition, control is performed such that the rack is moved to a predetermined pressing position during the sheet feeding so as to obtain a pressing force necessary to feed the sheet. 
     Next, a configuration of the drive portion of the sheet feeding device  20  will be described. In  FIG. 8 , the tooth-chipped gear unit  14  is attached to rotate in synchronization with a sheet-feeding shaft  15  and serves as a tooth-chipped gear unit for transmitting the driving of the drive gear  12  to the feeding roller  21 . The drive gear  12  for transmitting the driving force to the tooth-chipped gear unit  14  is rotated at all times in the counterclockwise direction by the gear array and the sheet-feeding drive motor, which is a drive portion for driving the feeding roller of  FIG. 12  described below, and has teeth across the entire circumference. 
     The tooth-chipped gear unit  14  includes a drive tooth-chipped gear  141 , a control tooth-chipped gear  142  pivotably attached at a certain angle with respect to the drive tooth-chipped gear  141 , and a tooth-chipping control cam  143  integrated with the control tooth-chipped gear  142 . In addition, the sheet-feeding shaft  15  is connected to the feeding roller  21  as illustrated in  FIG. 2  and is rotated in synchronization with the feeding roller  21 . 
     In  FIG. 8 , the solenoid  19  locks the tooth-chipping control cam  143  and stops the tooth-chipped portions  141   a  and  142   a  of the drive tooth-chipped gear  141  and the control tooth-chipped gear  142  at the position of the drive gear  12 . The solenoid  19  includes an armature  192 , an armature holding portion  19   c  for pivotably holding the armature  192 , and a solenoid spring  193  for applying a recovery force to the armature  192 . 
     Here, the leading edge of the armature  192  is locked with the locking portion  143   a  of the tooth-chipping control cam  143  until the sheet feeding operation is initiated and as the sheet feeding operation is terminated. The locking with the locking portion  143   a  is released as the sheet feeding operation is initiated. As the locking with the armature  192  is released, as described below, the tooth-chipping control cam  143  is rotated. Accordingly, the sheet-feeding shaft  15  is rotated in synchronization with the feeding roller  21 . In this manner, the initiation and stopping of the operation of the feeding roller  21  is controlled by the armature  192 . 
     The drive tooth-chipped gear  141  and the control tooth-chipped gear  142  are assembled such that the hole of the control tooth-chipped gear  142  is rotatably fit into a shaft  141   b  of the drive tooth-chipped gear  141  as illustrated in  FIG. 9 . When the control tooth-chipped gear  142  is assembled in this manner, the control tooth-chipped gear  142  is biased by the spring force of the tooth-chipped spring  144  provided in the drive tooth-chipped gear  141  and bumps into the stopper  141   s  provided in the drive tooth-chipped gear  141 . As the control tooth-chipped gear  142  bumps into the stopper  141   s  in this manner, the control tooth-chipped gear  142  and the drive tooth-chipped gear  141  are rotated in synchronization. 
     Here, the control tooth-chipped gear  142  is resistant to the spring force of the tooth-chipping spring  144  and held at the position separated from the stopper  141   s  while the armature  192  is locked with the tooth-chipping control cam  143  as described below. In addition, as the locking of the armature  192  is released, the control tooth-chipped gear  142  is rotated in the direction meshing with the drive gear  12  by virtue of the spring force of the tooth-chipping spring  144  as an initial rotation applying portion (biasing portion) and bumps into the stopper  141   s  of the drive tooth-chipped gear  141 . 
     The rotation angle of the control tooth-chipped gear  142  against the drive tooth-chipped gear  141  is an angle from the standby position where the control tooth-chipped gear  142  is locked with the armature  192  through the tooth-chipping control cam  143  to the position where the control tooth-chipped gear  142  is released from being locked with the armature  192 , and the control tooth-chipped gear  142  bumps into the stopper  141   s  of the drive tooth-chipped gear  141 . That is, the control tooth-chipped gear  142  is rotated with a certain angle until it bumps into the stopper  141   s  as the locking of the armature  192  is released. 
     Here, the rotation angle of the control tooth-chipped gear  142  is set to an angle at which it meshes with the drive gear  12  when the control tooth-chipped gear  142  bumps into the stopper  141   s . As the control tooth-chipped gear  142  meshes with the drive gear  12  in this manner, then, the drive tooth-chipped gear  141  as well as the control tooth-chipped gear  142  is rotated in synchronization. 
     In this manner, according to the present embodiment, the drive transmission portion  14 A for transmitting the driving of the sheet-feeding drive motor to drive the feeding roller  21  during the sheet feeding includes the drive gear  12 , the drive tooth-chipped gear  141 , and the control tooth-chipped gear  142  meshing with the drive gear  12  to rotate the feeding roller  21 . The drive transmission portion includes the tooth-chipping spring  144  and the solenoid  19  releasably locked with the control tooth-chipped gear  142 . In addition, by virtue of the drive transmission portion having such a configuration, the control tooth-chipped gear  142  and the drive tooth-chipped gear  141  after the sheet feeding are rotated to the standby position where the tooth-chipped portions  141   a  and  142   a  face the drive gear  12  as described below and are locked by the armature  192  of the solenoid  19  to stop. 
     Next, a rotational drive operation of the feeding roller  21  using the drive portion configured in this manner will be described.  FIG. 10  is a diagram illustrating a state at which the solenoid  19  and the tooth-chipped gear unit  14  are at the standby position. At this state, the control tooth-chipped gear  142  stops at the standby position when the leading edge  192   t  of the armature  192  is caught by the locking portion  143   a  of the tooth-chipping control cam  143 . In addition, the armature  192  at that time is at the open position where the absorption is opened when the solenoid  19  is powered off. 
     When the feeding roller  21  is rotated, the solenoid  19  is powered on. As a result, the armature  192  is absorbed and separated from the locking portion  143   a  of the tooth-chipping control cam  143  of the control tooth-chipped gear  142  as illustrated in  FIG. 11  so that the locking with the locking portion  143   a  is released (absorption position). The released control tooth-chipped gear  142  is rotated by the spring force of the tooth-chipping spring  144  so as to mesh with the drive gear  12  and rotate in synchronization with the drive gear  12 . 
     Then, when the control tooth-chipped gear  142  is rotated by a predetermined angle (here, about 35°) from the standby position, it bumps into the stopper  141   s  of the drive tooth-chipped gear  141 . As a result, the control tooth-chipped gear  142  is connected to the drive tooth-chipped gear  141  and is rotated in synchronization. That is, the control tooth-chipped gear  142  becomes free by virtue of the operation of the solenoid  19 , first, the control tooth-chipped gear  142  is rotated by the force of the tooth-chipping spring  144 , and at last, meshes with the drive gear  12  so that the rotation is initiated. Then, when the control tooth-chipped gear  142  is rotated by about 35°, the control tooth-chipped gear  142  bumps into the stopper  141   s  and is connected to the drive tooth-chipped gear  141 . Then, accordingly, the drive tooth-chipped gear  141  is rotated from the standby position to the position where it meshes with the drive gear  12 . As a result, a driving force is applied to the feeding roller  21 . 
     Meanwhile, when the feeding roller  21  is rotated by one turn after that, the control tooth-chipped gear  142  makes the locking portion  143   a  of the tooth-chipping control cam  143  be caught by the armature  192  of the solenoid  19 . When the armature  192  of the solenoid  19  is caught in this manner, the tooth-chipping control cam  143  stops. Then, the drive tooth-chipped gear  141  is rotated by about 35° and stops at the standby position where the tooth-chipped portion faces the drive gear  12 . 
     In  FIG. 11  which illustrates a state in which the control tooth-chipped gear  142  is rotated by about 35°, the radius Ron ranges from the rotation center of the tooth-chipping control cam  143  to the leading edge position of the armature  192  in the solenoid absorption state. In addition, the radius Ra of a first cam surface  143   b  ranges from the rotation center of the tooth-chipping control cam  143  to a meshing start portion where the control tooth-chipped gear  142  meshes with the drive gear  12 . 
     Here, according to the present embodiment, the radius Ron to the leading edge position of the armature  192  in the solenoid absorption state is set to be larger than the radius Ra to the meshing start position of the tooth-chipping control cam  143 . As a result of such setting, it is possible to prevent the leading edge of the armature  192  and the tooth-chipping control cam  143  from making contact with each other and generating operational errors in the operational area where the control tooth-chipped gear  142  is rotated by the force of the initial tooth-chipping spring  144 . 
       FIG. 12  is a control block diagram illustrating the full-color laser beam printer  1 . The image forming controller  72  controls the image forming portion  1 B based on the image data from the image processing controller  71  which processes the image information input from an external host device  70 . The image forming controller  72  includes a CPU  721 , a ROM  722  which internally stores a control program corresponding to the flowchart of  FIG. 13  described below, and a RAM  723  used as a work area for the operation caused by the control or as an area for temporarily holding the control data. The image forming controller  72  as a controller is connected to the main drive motor  74 , the solenoid  19 , the pressing drive motor  75 , and the sheet-feeding drive motor  76 , in addition to the image forming portion  1 B. According to the present embodiment, the sheet-feeding drive motor  76  is a stepping motor. 
     Next, control operations of the present embodiment will be described. First, pre-rotation control when a typical printer main body  1 A is powered on will be described with reference to the flowchart of  FIG. 13 . The pre-rotation control is necessary to advance various actuators to the home position or check that the sheet S does not remain in a conveying path to make the printer main body  1 A be in a standby (ready) state where the printer main body  1 A is enabled to perform the image forming operation. 
     In the pre-rotation control, first, after the power is turned on, the above-described presence/absence detection unit determines whether the cassette is present (S 50 ). Here, if it is determined that the cassette  200  is absent (N in S 50 ), an error indication is carried out (S 53 ). Otherwise, it is determined that the cassette is present (Y in S 50 ), the pressing drive motor  75  is rotated forward (S 51 ), and the rack  204  moves to the home position. Then, if it is detected that the response of the photointerrupters  210   a  and  210   b  in  FIG. 7  becomes A and the rack  204  moves to the home position as illustrated in  FIG. 6A  (Y in S 52 ), the pressing drive motor  75  stops. As a result, the sheet feeding device  2  is in the standby state. 
     Then, the control in the case where the print job JOB is received after the standby state will be described with reference to the flowchart of  FIG. 14 . When the print job JOB is received, first, the sheets S stacked on the intermediate plate  201  are pressed to the feeding roller  21  with a predetermined pressing force. Therefore, the pressing drive motor  75  is driven forward (S 61 ). In addition, the rack  204  stops at the first or second pressing position in  FIG. 6C  or  6 D, that is, the position where the response of the photointerrupters  210   a  and  210   b  becomes C or D in  FIG. 7 . 
     Then, when the rack  204  stops at the first or second pressing position, that is, when the position of the rack  204  is set to the first or second pressing position (Y in S 62 ), the pressing drive motor  75  stops (S 63 ), and the sheet-feeding drive motor  76  is driven so as to rotate the feeding roller  21  (S 64 ). In addition, in synchronization with the image formation, the solenoid  19  is turned on (S 65 ), and the locking of the solenoid  19  with the armature  192  described above is released. As a result, the drive transmission portion  14 A switches from the first state in which the driving of the sheet-feeding drive motor  76  is transmitted to the feeding roller  21  to the second state in which the driving of the sheet-feeding drive motor  76  is not transmitted to the feeding roller  21 . Then, until a predetermined number of sheets S are fed, that is, until the sheet-feeding operation is terminated (N in S 66 ), the solenoid  19  is turned on. When the sheet-feeding operation is terminated (Y in S 66 ), the sheet-feeding drive motor  76  stops (S 67 ). 
     Then, the pressing drive motor  75  is driven backward (S 68 ). In addition, when the rack  204  is at the home position illustrated in  FIG. 6A , that is, the position where the response of the photointerrupters  210   a  and  210   b  becomes A in  FIG. 7  (Y in S 69 ), the pressing drive motor  75  stops (S 70 ). In this case, the sheet S and the feeding roller  21  are separated (at the standby state) so that the print operation is terminated. 
     Here, the time of rotation of the sheet-feeding drive motor  76  can be shortest considering operational sound, power consumption, reduction of lifetimes caused by partial cutting of components such as gears or rollers moving as the sheet-feeding drive motor  76  is rotated. For this reason, according to the present embodiment, when the drive control of the feeding roller  21  is performed, the sheet-feeding drive motor  76  is rotated as much as necessary to feed the sheet S. 
     For example, if a strong vibration or impact is applied when the full-color laser beam printer  1  is transported after the manufacture, or the full-color laser beam printer  1  is moved, the armature  192  of the solenoid  19  may be deviated from the locking portion  143   a  of the tooth-chipping control cam  143 . That is, if a strong vibration or impact is applied, and when the drive transmission portion  14 A is switched from the second state to the first state and the sheet is fed, the solenoid  19  which is a switching portion for holding, in the second state, the drive transmission portion  14 A returning from the first state to the second state by virtue of the driving of the sheet-feeding drive motor  76 , is unlocked. 
     Here, in a case where the armature  192  of the solenoid  19  is unlocked in this manner, the control tooth-chipped gear  142  is rotated to mesh with the drive gear  12 . In addition, in the control described above, if the sheet-feeding drive motor  76  is rotated in this state, the feeding roller  21  is also rotated at the same time as the start of rotation of the sheet-feeding drive motor  76 . In this case, an unintended feeding of the sheet S occurs. 
     In this regard, according to the present embodiment, when the printer main body  1 A is initially powered on after the manufacture, an unusual control is performed. Next, the control performed when the printer main body  1 A is initially powered on will be described with reference to the flowchart illustrated in  FIG. 15 . 
     In the pre-rotation control performed when the printer main body  1 A is initially powered on after the manufacture, delivery, and installation, it is first determined whether the cassette  200  is present after the power is supplied (S 71 ). If it is determined that the cassette  200  is absent (N in S 71 ), an error indication is displayed (S 72 ). If it is determined that the cassette  200  is present (Y in S 71 ), the sheet-feeding drive motor  76  is rotated (S 73 ). According to the present embodiment, whether the printer main body  1 A is initially powered on after the manufacture is determined based on information stored in the RAM  723  of the image forming controller  72  of the printer main body  1 A. 
     Then, the sheet-feeding drive motor  76  is rotated for a predetermined time (S 74 ). Here, the predetermined time for rotating the sheet-feeding drive motor  76  may be set to be equal to or longer than the time (which is taken for one turn) necessary to cut off a rotational force from the drive gear  12  to the drive tooth-chipped gear  141  and the control tooth-chipped gear  142 . That is, the predetermined time may be the time taken for returning the drive transmission portion  14 A from the first state to the second state. By rotating the sheet-feeding drive motor  76  for a predetermined time in this manner, the drive tooth-chipped gear  141  and the control tooth-chipped gear  142  reach the standby position where the tooth-chipped portion faces the drive gear  12 . That is, by rotating the sheet-feeding drive motor  76  for a predetermined time taken to return the drive transmission portion from the first state to the second state, it is possible to return the drive tooth-chipped gear  141  and the control tooth-chipped gear  142  to the standby position even when the locking of the armature  192  is released. 
     Then, when the sheet-feeding drive motor  76  is rotated for a predetermined time in this manner (Y in S 74 ), the sheet-feeding drive motor  76  stops (S 75 ). Then, the pressing drive motor  75  is driven forward (S 76 ). Then, as illustrated in  FIG. 6A , if it is detected that the rack  204  is positioned at the home position (Y in S 77 ), the pressing drive motor  305  stops. As a result, the pre-rotation of the sheet feeding device  2  is terminated to enter the standby state. 
     In this manner, according to the present embodiment, when the printer main body  1 A is initially powered on after the manufacture, the sheet-feeding drive motor  76  is driven before the pressing drive motor  75  is rotated, so that the rotational force from the drive gear  12  to the tooth-chipped gears  141  and  142  is cut off. That is, when power is initially supplied after the manufacture, the intermediate plate  201  is lifted, and the sheet-feeding drive motor  76  is driven before the sheet abuts the feeding roller  21  so as to return the drive tooth-chipped gear  141  and the control tooth-chipped gear  142  to the standby position. That is, when power is initially supplied after the manufacture, the drive transmission portion  14 A is returned from the first state to the second state by driving the sheet-feeding drive motor  76 . 
     As a result, even when the sheet-feeding drive motor  76  is driven while the locking by the armature  192  of the solenoid  19  is released due to vibration during the transportation or movement work, the drive tooth-chipped gear  141  and the control tooth-chipped gear  142  are returned to the standby position. Therefore, the feeding roller  21  is not rotated. In addition, when the drive tooth-chipped gear  141  and the control tooth-chipped gear  142  are returned to the standby position, the feeding roller  21  is also rotated. However, according to the present embodiment, the operation of the intermediate plate  201  and the rotational operation of the feeding roller  21  are controlled independently from each other. For this reason, the feeding roller  21  and the sheet S are not allowed to abut each other. As a result, since the drive tooth-chipped gear  141  and the control tooth-chipped gear  142  are returned to the standby position, the sheet S is not fed even when the feeding roller  21  is rotated. As a result, it is possible to prevent a jam and useless consumption of the sheet S. 
     As described above, according to the present embodiment, when the printer main body  1 A is initially powered on after the manufacture, the tooth-chipped gears  141  and  142  are rotated and stop at the standby position where the tooth-chipped portion faces the drive gear  12 . That is, according to the present embodiment, as the printer main body  1 A is initially powered on after the manufacture, the sheet-feeding drive motor  76  is driven for the time necessary to return the drive transmission portion  14 A from the first state to the second state so as to make the drive transmission portion  14 A be in the second state. If the power is turned on in this manner, by making the drive transmission portion  14 A be in the second state, it is possible to prevent unnecessary sheet feeding when the power is turned on without causing additional costs such as modification or addition of devices or components. 
     Although description has been made by assuming that the above-described control is performed when the full-color laser beam printer  1  is initially powered on after the installation, the invention is not limited thereto. For example, the above-described control may be performed when the power is turned on in a case where the full-color laser beam printer  1  is provided in a significantly vibrating place and is not used for a long time. 
     However, the image forming controller  72  of the full-color laser beam printer  1  according to the present embodiment stops the sheet feeding immediately when an accident such as a jam occurs or when a user opens the access door of the printer main body  1 A during the print operation (during the sheet feeding). 
     As a result, it is possible to prevent unrelated sheets from being fed during the jam, prevent a user from feeling difficulty in removing the jammed sheet, prevent the components of the printer main body  1 A from being damaged, and prevent the sheets from being uselessly consumed. In addition, the printer main body  1 A is controlled by the image forming controller  72  to immediately stop the operation even during the print operation when the door is opened for replacing consumables or removing the jam during the print operation. 
     Here, in a case where the paper feeding operation is initiated, and the print operation stops soon as described above, the drive tooth-chipped gear  141  and the control tooth-chipped gear  142  are not returned to the standby position. That is, the sheet-feeding drive motor  76  is not rotated after the armature  192  of the solenoid  19  is released and until it returns to the standby position. In this case, the drive tooth-chipped gear  141  and the control tooth-chipped gear  142  stop in a state where they mesh with the drive gear  12 . 
     Even in this state, if a user completely closes the access door after jam recovery, the sheet feeding is resumed. However, at this moment, the printer main body  1 A performs the pre-rotation operation. In this case, even when the pre-rotation control described in conjunction with  FIG. 13  is performed, the drive tooth-chipped gear  141  and the control tooth-chipped gear  142  are kept in meshing engagement with the drive gear  12 . For this reason, after that, if the print job JOB is initiated, the feeding roller  21  is also rotated at the same time with the rotation of the sheet-feeding drive motor  76 . As a result, an unintended feeding of the sheet S and a jam occurs again. 
     In this regard, when the drive tooth-chipped gear  141  and the control tooth-chipped gear  142  are not returned to the standby position as described above, it is necessary to return the drive tooth-chipped gear  141  and the control tooth-chipped gear  142  to the standby position without feeding the sheet. 
     Next, a second embodiment according to the invention will be described, in which the drive tooth-chipped gear  141  and the control tooth-chipped gear  142  are returned to the standby position without feeding the sheet when the drive tooth-chipped gear  141  and the control tooth-chipped gear  142  are not returned to the standby position. 
       FIG. 16  is a flowchart illustrating the control of the sheet feeding device provided in the image forming apparatus according to the present embodiment. When a print job JOB is received, first, the sheets S stacked on the intermediate plate  201  are pressed to the feeding roller  21  with a predetermined pressing force, and thus, the pressing drive motor  75  is driven forward (S 81 ). In addition, the rack  204  stops at the first or second pressing position of  FIG. 6C  or  6 D, that is, the position where the response of the photointerrupters  210   a  and  210   b  becomes C or D in  FIG. 7 . 
     Then, as the rack  204  stops at the first or second pressing position, that is, the position of the rack  204  is set to the first or second pressing position (Y in S 82 ), the pressing drive motor  75  stops (S 83 ), and the sheet-feeding drive motor  76  is driven (S 84 ). In addition, the solenoid  19  is turned on (S 85 ) at the image formation timing, and the locking by the armature  192  of the solenoid  19  is released. Then, according to the present embodiment, the number of steps in the sheet-feeding drive motor  76  after the solenoid  19  is turned on is stored in the RAM  723  (S 86 ). 
     Here, the number of steps in the sheet-feeding drive motor  76  are stored in this manner, and it is possible to compute the rotation amount of the drive tooth-chipped gear  141  and the control tooth-chipped gear  142  after the armature  192  of the solenoid  19  is released. In addition, based on the result of this computation, that is, based on the rotation amount of the drive tooth-chipped gear  141  and the control tooth-chipped gear  142 , it is possible to determine whether the drive tooth-chipped gear  141  and the control tooth-chipped gear  142  are returned to the standby position. 
     According to the present embodiment, although the stored information is the number of steps for driving the sheet-feeding drive motor  76 , the time (driving time) taken after the solenoid  19  is turned on may be stored. In a case where the time information is stored (measured) in this manner, the stored time is compared with a predetermined time necessary to return the drive tooth-chipped gear  141  and the control tooth-chipped gear  142  to the standby position. As a result, it is possible to determine whether the drive tooth-chipped gear  141  and the control tooth-chipped gear  142  are returned to the standby position. That is, according to the present embodiment, the RAM  723  forms a detection portion for detecting the positions of the drive tooth-chipped gear  141  and the control tooth-chipped gear  142  when the sheet feeding stops and is resumed. 
     Then, until a predetermined number of sheets are fed, that is, until the sheet feeding operation is terminated (N in S 87 ), the solenoid  19  is turned on. When the sheet feeding operation is terminated (Y in S 87 ), the sheet-feeding drive motor  76  stops (S 88 ). Then, the pressing drive motor  75  is driven backward (S 89 ). When the rack  204  is at the home position as illustrated in  FIG. 6A  (Y in S 90 ), the pressing drive motor  75  stops (S 91 ). At this moment, the sheet S and the feeding roller  21  are separated from each other (standby), and the print operation is terminated. 
     However, according to the present embodiment, in a case where the drive tooth-chipped gear  141  and the control tooth-chipped gear  142  are likely to stop while they mesh with the drive gear  12  after the jam is removed or an emergency stop occurs, the control illustrated in the flowchart of  FIG. 17  is performed during the pre-rotation operation. That is, for example, if a user closes the access door after the jam recovery, it is determined whether the cassette  200  is present (S 100 ). If it is determined that the cassette  200  is absent (N in S 100 ), the error indication is displayed (S 101 ). Otherwise, if it is determined that the cassette  200  is present (Y in S 100 ), it is determined whether the next sheet S is separated from the feeding roller  21 , that is, whether the rack  204  is at the home position (S 102 ). Here, if it is determined that the rack  204  is not at the home position (N in S 102 ) and the sheet S and the feeding roller  21  abut each other, first, the pressing drive motor  75  is rotated (S 103 ), and the rack  204  is returned to the home position. 
     Then, if it is determined that the rack  204  is at the home position (Y in S 102 ), it is determined whether the drive tooth-chipped gear  141  and the control tooth-chipped gear  142  mesh with the drive gear  12  based on the information on the number of steps of the sheet-feeding drive motor  76  illustrated in  FIG. 16 . That is, it is determined whether a state that the drive tooth-chipped gear  141  and the control tooth-chipped gear  142  do not mesh with the drive gear  12 , that is, whether the drive tooth-chipped gear  141  and the control tooth-chipped gear  142  are at the standby position where they do not mesh with the drive gear  12  (S 104 ). 
     If it is determined that the drive tooth-chipped gear  141  and the control tooth-chipped gear  142  are not at the standby position (N in S 104 ), the sheet-feeding drive motor  76  is rotated for a predetermined time (S 106 ). The predetermined time may be set to the time necessary to cut off the rotational force from the drive gear  12  to the drive tooth-chipped gear  141  and the control tooth-chipped gear  142 , that is, the time (which is taken for one turn) for preventing the drive tooth-chipped gear  141  and the control tooth-chipped gear  142  from meshing with the drive gear  12 . Then, if the sheet-feeding drive motor  76  is rotated for a predetermined time (Y in S 106 ), the sheet-feeding drive motor  76  stops (S 107 ). As a result, the pre-rotation of the sheet feeding device  2  is terminated, and the sheet feeding device  2  is in the standby state. 
     In this manner, according to the present embodiment, in a case where the sheet feeding operation stops while the sheet is fed, the rack  204  is returned to the home position, and the drive tooth-chipped gear  141  and the control tooth-chipped gear  142  are returned to the standby position. Through the above-described control, even when the drive tooth-chipped gear  141  and the control tooth-chipped gear  142  are pre-rotated while they mesh with the drive gear  12 , it is possible to return the tooth-chipped gears  141  and  142  to the standby position without feeding the sheet S. At this moment, although the feeding roller  21  is rotated, the feeding roller  21  and the sheet S do not abut each other. Therefore, it is possible to prevent a jam and useless consumption of the sheet S without feeding the sheet S. 
     As described above, according to the present embodiment, even when an accident such as a jam occurs, the drive tooth-chipped gear  141  and the control tooth-chipped gear  142  are returned to the standby position. That is, according to the present embodiment, even when an accident occurs, the sheet-feeding drive motor  76  is driven for the time necessary to return the drive transmission portion  14 A from the first state to the second state so as to make the drive transmission portion  14 A have the second state. As a result, it is possible to further prevent a jam or useless consumption of the sheet and make the printer main body have a standby state. 
     While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all modifications, equivalent structures and functions. 
     This application claims the benefit of Japanese Patent Application No. 2010-288431, filed Dec. 24, 2010, which is hereby incorporated by reference herein in its entirety.