Abstract:
A medium conveying apparatus for conveying a recording medium includes an apparatus body and a conveying control unit. A paper feeding unit is detachable mounted to the apparatus body and includes a record medium enclosing section and a paper feeding section. First and second conveying sections are downstream from the paper feeding section. A third conveying section is between the paper feeding section and the first conveying section. The conveying control unit controls the third conveying section to convey the record medium before the front edge of the record medium arrives at the first conveying section, controls the first and the third conveying sections to convey the record medium before the front edge of the record medium arrives at the second conveying section and stops the third conveying section from operating after the front edge of the record medium arrives at the second conveying section.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a medium conveying apparatus. 
     2. Related Background Art 
     Hitherto, in an image forming apparatus such as printer, copying apparatus, facsimile apparatus, or the like, as a medium conveying apparatus for conveying a recording medium to an image forming unit, one recording medium is separated from a plurality of recording media stacked and enclosed on a medium tray, fed out, and thereafter, conveyed by conveying rollers as a plurality of conveying units. 
     To separate one recording medium from the plurality of stacked recording media and feed it out, when a paper feed roller is pressed toward a front edge portion of the top recording medium and the paper feed roller is rotated by a driving force transfer unit having a one-way clutch, the top recording medium is fed out, the front edge of the recording medium passes through the conveying roller arranged on the downstream side from the paper feed roller, and thereafter, the driving of the paper feed roller is stopped. 
     After the driving of the paper feed roller is stopped, the recording medium which is conveyed by the conveying rollers drives the paper feed roller. When a rear edge of the recording medium passes through the paper feed roller, the paper feed roller is stopped, and the recording medium is separated (for example, refer to JP-A-9-263336). 
       FIG. 2  is a perspective view showing a main section of the conventional medium conveying apparatus.  FIG. 3  is a perspective view showing a main section of another conventional medium conveying apparatus. 
     In  FIG. 2 , F denotes a supporting frame and reference numeral  15  denotes a first conveying roller unit rotatably supported to the supporting frame F. The first conveying roller unit  15  comprises a conveying roller  101  and a pinch roller  102 . The conveying roller  101  comprises rubber rollers  103  arranged in predetermined positions of a shaft sh 1 . Reference numeral  16  denotes a second conveying roller unit rotatably supported to the supporting frame F. The second conveying roller unit  16  comprises a conveying roller  104  and a pinch roller  105 . A recording medium (not shown) is supplied to the first conveying roller unit  15  in the direction shown by an arrow A, ejected from the first conveying roller unit  15 , thereafter, guided by a curved guide  108 , supplied to the second conveying roller unit  16 , and ejected from the second conveying roller unit  16  in the direction shown by an arrow B. 
     Although a paper feed roller (not shown) is arranged on the upstream side from the first conveying roller unit  15  in the conveying direction of the recording medium, as mentioned above, when the rotation of the paper feed roller is stopped, the first conveying roller unit  15  needs to solely convey the recording medium in the state where loads of the paper feed roller and other devices have been applied. 
     As shown in  FIG. 2 , however, since the rubber roller  103  is small, a conveying force of the recording medium cannot be increased in the conveying roller  101 . Therefore, as shown in  FIG. 3 , a rubber roller  113  is enlarged so as to sufficiently assure an area touch the recording medium, thereby increasing the conveying force of the recording medium. 
     However, in the conventional medium conveying apparatus, since an amount of rubber material which is used is increased by an enlargement amount of the rubber roller  113 , costs rise. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide a medium conveying apparatus which can solve the problem of the conventional medium conveying apparatus as mentioned above, smoothly convey a recording medium in a conveying unit, and reduce costs. 
     According to the present invention, there is provided a medium conveying apparatus comprising:
         a recording medium enclosing unit which encloses a plurality of recording media;   a paper feeding unit which feeds out the recording media in the recording medium enclosing unit;   a first conveying unit which is arranged on the downstream side from the paper feeding unit and conveys the fed-out recording medium;   a second conveying unit which is arranged on the downstream side from the first conveying unit;   a medium front edge detecting unit which detects a front edge of the recording medium which is being conveyed; and   a operation stopping unit which stops the operation of the paper feeding unit at predetermined timing.       

     In the medium conveying apparatus, the operation stopping unit stops the operation of the paper feeding unit after the front edge of the recording medium passed through the second conveying unit. 
     Moreover, The medium conveying apparatus may further comprise a third conveying unit which is arranged between the paper feeding unit and the first conveying unit, and wherein the operation stopping unit stops the operation of the third conveying unit at predetermined timing. 
     Moreover, the operation stopping unit may stop the operation of the paper feeding unit after the front edge of the recording medium passed through the second conveying unit and stops the operation of the third conveying unit. 
     Moreover, the operation stopping unit may stop the operation of the paper feeding unit after the front edge of the recording medium passed through the third conveying unit and stops the operation of the third conveying unit after the front edge of the recording medium passed through the second conveying unit. 
     Moreover, the medium conveying apparatus may further comprise a medium length detecting unit which detects a length of recording medium in the recording medium enclosing unit, and wherein the operation stopping unit stops the operation of the paper feeding unit after the front edge of the recording medium passed through the third conveying unit and stops the operation of the third conveying unit after the recording medium was conveyed by a distance corresponding to the length of recording medium detected by the medium length detecting unit after the front edge of the recording medium had passed through the third conveying unit. 
     Moreover, the medium conveying apparatus may further comprise a medium rear edge detecting unit which detects a rear edge of the recording medium which is being conveyed, and wherein the operation stopping unit stops the operation of the paper feeding unit after the front edge of the recording medium passed through the third conveying unit and stops the operation of the third conveying unit after the rear edge of the recording medium passed through the third conveying unit. 
     Moreover, in the medium conveying apparatus, the third conveying unit has two rollers which face each other, a pressing member for pressing one of the rollers to the other roller, and a driving unit for rotating at least one of the rollers, and the operation stopping unit stops the operation of the paper feeding unit after the front edge of the recording medium passed through the third conveying unit, cancels a pressing force to the pressing member after the front edge of the recording medium passed through the first conveying unit, and applies the pressing force to the pressing member after a rear edge of the recording medium passed through the third conveying unit. 
     That is, according to the invention, the medium conveying apparatus comprises: the recording medium enclosing unit which encloses a plurality of recording media; the paper feeding unit which feeds out the recording media in the recording medium enclosing unit; the first conveying unit which is arranged on the downstream side from the paper feeding unit and conveys the fed-out recording medium; the second conveying unit which is arranged on the downstream side from the first conveying unit; the medium front edge detecting unit which detects the front edge of the recording medium which is being conveyed; and the operation stopping unit which stops the operation of the paper feeding unit at the predetermined timing. 
     In this case, since the front edge of the recording medium which is being conveyed is detected and the operation of the paper feeding unit is stopped at the predetermined timing, a load which is applied to the first conveying unit can be reduced and the recording medium can be smoothly conveyed by the first conveying unit. 
     Since the load which is applied to the first conveying unit can be reduced, there is no need to enlarge a rubber roller of the first conveying unit. Therefore, the costs of the medium conveying apparatus can be reduced. 
     The above and other objects and features of the present invention will become apparent from the following detailed description and the appended claims with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a time chart showing the operation of a medium conveying apparatus in the first embodiment of the invention; 
         FIG. 2  is a perspective view showing a main section of a conventional medium conveying apparatus; 
         FIG. 3  is a perspective view showing a main section of another conventional medium conveying apparatus; 
         FIG. 4  is a schematic diagram of a printer in the first embodiment of the invention; 
         FIG. 5  is a block diagram of a control apparatus of the printer in the first embodiment of the invention; 
         FIG. 6  is a perspective view of the medium conveying apparatus for conveying a recording medium in an optional paper feed tray unit in the first embodiment of the invention; 
         FIG. 7  is a perspective view of a medium conveying apparatus for conveying a recording medium in an optional paper feed tray unit in the second embodiment of the invention; 
         FIG. 8  is a time chart showing the operation of the medium conveying apparatus in the second embodiment of the invention; 
         FIG. 9  is a time chart showing the operation of a medium conveying apparatus in the third embodiment of the invention; 
         FIG. 10  is a time chart showing the operation of a medium conveying apparatus in the fourth embodiment of the invention; 
         FIG. 11  is a perspective view of a medium conveying apparatus for conveying a recording medium in an optional paper feed tray unit in the fifth embodiment of the invention; 
         FIG. 12  is a side elevational view of the medium conveying apparatus for conveying the recording medium in the optional paper feed tray unit in the fifth embodiment of the invention; 
         FIG. 13  is a perspective view showing a main section of the medium conveying apparatus for conveying the recording medium in the optional paper feed tray unit in the fifth embodiment of the invention; and 
         FIG. 14  is a time chart showing the operation of the medium conveying apparatus in the fifth embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiments of the invention will be described in detail hereinbelow with reference to the drawings. In this case, a printer as an image forming apparatus will be described. 
       FIG. 4  is a schematic diagram of the printer in the first embodiment of the invention. 
     In the diagram, reference numeral  11  denotes a printer main body showing an apparatus main body;  12  a sheet cassette as a recording medium enclosing unit for enclosing recording media (not shown) such as paper, OHP sheets, or the like; and  13  a paper feed roller as a first paper feeding unit for feeding out the recording media in the sheet cassette  12  one by one. The paper feed roller  13  has a one-way clutch function and propagates a rotating force only to one direction. The paper feed roller  13  is raced in the opposite direction and does not propagate the rotating force. 
     Reference numerals  14  and  17  denote medium sensors as first and second detecting units serving as first and second medium front edge detecting units for detecting a front edge of the recording medium in order to recognize passage of the recording medium;  45  a first conveying roller unit having functions of conveying the recording medium fed out of the sheet cassette  12  and correcting a skew of the recording medium;  108  the guide for deflecting the recording medium ejected from the first conveying roller unit  45  and guiding it in the horizontal direction; and  16  the second conveying roller unit for supplying the recording medium guided by the guide  108  to each image forming unit  18 . 
     Reference numeral  30  denotes an optional paper feed tray unit which is detachably arranged to/from the printer main body  11 ;  31  a sheet cassette as a recording medium enclosing unit for optional paper feeding; and  32  a paper feed roller as a second paper feeding unit for feeding out the recording media in the sheet cassette  31  one by one. The paper feed roller  32  has a one way clutch function in a manner similar to that of the paper feed roller  13 . 
     The first conveying roller unit  45  is arranged on the downstream side from the paper feed rollers  13  and  32  and constructs the first conveying unit. The second conveying roller unit  16  is arranged on the downstream side from the first conveying roller unit  45  and constructs the second conveying unit. The first conveying roller unit  45  has a conveying roller  121  and a pinch roller  122  pressed to the conveying roller  121  by a spring (not shown). The second conveying roller unit  16  has the conveying roller  104  and the pinch roller  105  pressed to the conveying roller  104  by a spring (not shown). An electromagnetic clutch (not shown) is arranged coaxially with the first conveying roller unit  45 . By turning on/off (engaging/disengaging) the electromagnetic clutch, a rotating force to the first conveying roller unit  45  from the paper feed motor is propagated or disconnected. Since the conveying roller  121  has a small structure in a manner similar to the rubber roller  103  shown in  FIG. 2 , an amount of rubber which is used can be reduced. 
     Each image forming unit  18  has a photosensitive drum  112  as an image holding member, a transfer roller  115  as a transfer apparatus arranged so as to face the photosensitive drum  112 , and the like in order to form a visible toner image of each color by thermally meltable toner of each color and transfer it onto the recording medium. A transfer belt  114  is arranged between the photosensitive drum  112  and the transfer roller  115  so that it can run freely. When the recording medium is conveyed on the transfer belt  114 , the toner images are transferred onto the recording medium. 
     Reference numeral  19  denotes a fixing unit for fixing the color toner images formed by the image forming units  18  onto the recording medium, thereby forming a color image. The fixing unit  19  has a heat roller  116  and a pressing roller  118 . Reference numeral  20  denotes an ejecting roller unit for ejecting out the recording medium to the outside from the printer main body  11  and  21  indicates a running route of the recording medium in the case where the paper is fed out of the sheet cassette  12  mounted in the printer main body  11 . 
     Further, reference numeral  33  denotes a medium sensor as a third detecting unit serving as an optional medium front edge detecting unit for detecting a front edge of the recording medium in order to recognize the passage of the recording medium and  34  indicates an optional conveying roller unit having functions of conveying the recording medium fed out of the sheet cassette  31  and correcting the skew of the recording medium. The conveying roller unit  34  constructs the third conveying unit. An electromagnetic clutch (not shown) is arranged coaxially with the third conveying roller unit  34 . By turning on/off (engaging/disengaging) the electromagnetic clutch, the rotating force to the conveying roller unit  34  from the paper feed motor is propagated or disconnected. The conveying roller unit  34  comprises a conveying roller  125  and a pinch roller  35  pressed to the conveying roller unit  34  by a spring (not shown). Reference numeral  36  denotes a running route of the recording medium in the case where the paper is fed out of the optional paper feed tray unit  30 . In the printer main body  11 , the medium conveying apparatus for conveying the recording medium is constructed by the paper feed roller  13 , the first conveying roller unit  45 , the second conveying roller unit  16 , and the like. In the optional paper feed tray unit  30 , the medium conveying apparatus for conveying the recording medium is constructed by the paper feed roller  32 , the conveying roller unit  34 , and the like. 
     A control apparatus of the printer will now be described. 
       FIG. 5  is a block diagram of the control apparatus of the printer in the first embodiment of the invention. 
     In the diagram, reference numeral  50  denotes a print control unit comprising a microprocessor, a ROM, a RAM, input/output ports, a timer, and the like (not shown). The print control unit  50  receives print data and a control command from an upper apparatus, controls the whole printer, and executes the printing operation. 
     Reference numeral  51  denotes an interface (I/F) control unit for transmitting printer information to the upper apparatus, analyzing control commands transmitted from an upper apparatus, and processing the print data transmitted from the upper apparatus;  52  a data processing unit for converting the print data into an image signal necessary in an image forming unit control unit  55 ;  53  an operation unit comprising a display device such as LED or LCD (not shown) for displaying a state of the printer and switches for giving instructions to the printer from the operator; and  54  various sensors to detect a status of the medium conveying apparatus for conveying the recording medium and states such as temperature, humidity, and the like in the printer main body  11  ( FIG. 4 ). Outputs of the sensors  54  are sent to the print control unit  50 . 
     Reference numeral  55  denotes the image forming unit control unit for controlling each image forming unit  18  and  56  indicates a fixing unit control unit for controlling the fixing unit  19 . The fixing unit control unit  56  detects a temperature of the heat roller  116  by a thermistor on the basis of an instruction of the print control unit  50 , controls the on/off operations of a heater built in the heat roller  116 , keeps the heat roller  116  at a predetermined temperature, and controls a motor to rotate the heat roller  116  and the pressing roller  118 . 
     Reference numeral  57  denotes a conveyance control unit for driving a paper feed motor of the printer main body  11 , turning on/off the electromagnetic clutch of the first conveying roller unit  45 , driving a paper feed motor of the optional paper feed tray unit  30 , and turning on/off the electromagnetic clutch of the conveying roller unit  34 . 
     The operation of the printer with the above construction will now be described. 
     First, when the print control unit  50  receives the control command and the print data transmitted from the upper apparatus through the interface control unit  51 , it issues an instruction to the fixing unit control unit  56  and allows the heater to start heating. When the temperature of the heat roller  116  rises to a predetermined temperature, the fixing unit control unit  56  issues an instruction to the motor of the fixing unit  19  and drives the motor so as to rotate the heat roller  116  and the pressing roller  118 . 
     Subsequently, the print control unit  50  discriminates a type of recording media enclosed on the sheet cassette  12  on the basis of sensor outputs of a sheet residual amount sensor as a medium residual amount detecting unit and a sheet size sensor as a medium size detecting unit and issues an instruction to the conveyance control unit  57  in order to start the conveyance of the recording medium according to the type of recording medium. 
     Subsequently, the recording medium fed out of the sheet cassette  12  is conveyed to the image forming unit  18  through the first conveying roller unit  45 , the guide  108 , and the second conveying roller unit  16 . In the image forming unit  18 , electrophotographic processes such as charging, exposure, development, transfer, and the like are executed on the basis of the image signal converted by the data processing unit  52  and the toner image is transferred onto the recording medium. 
     In the fixing unit  19 , the recording medium onto which the toner image has been transferred passes between the heat roller  116  and the pressing roller  118 . The toner image is fixed onto the recording medium by the heat of the heat roller  116 . Subsequently, the recording medium onto which the toner image has been fixed is further conveyed and ejected to the outside of the printer through the ejecting roller unit  20 . 
     The medium conveying apparatus for conveying the recording medium in the printer main body  11  and the optional paper feed tray unit  30  will now be described. In the embodiment, since the structure of the medium conveying apparatus for conveying the recording medium in the printer main body  11  and that of the medium conveying apparatus for conveying the recording medium in the optional paper feed tray unit  30  are the same, only the medium conveying apparatus for conveying the recording medium in the optional paper feed tray unit  30  will now be described. 
       FIG. 6  is a perspective view of the medium conveying apparatus for conveying the recording medium in the optional paper feed tray unit in the first embodiment of the invention. 
     As shown in the diagram, an output shaft (not shown) of a paper feed motor  37  as a driving unit for paper feeding is coupled with the paper feed roller  32  through gears  39  and  40  and, further, coupled with the conveying roller  125  through gears  41  and  42 . The paper feed roller  32  has a shaft sh 2  and a rubber roller  131 . An electromagnetic clutch  38  is arranged coaxially with the conveying roller  125 . By turning on/off the electromagnetic clutch  38 , the rotating force to the conveying roller  125  from the paper feed motor  37  is propagated or disconnected. The conveying roller  125  comprises a shaft sh 3  and a rubber roller  132 . 
     When the recording medium is fed into the printer main body  11  from the optional paper feed tray unit  30  ( FIG. 4 ), if the driving of the paper feed motor  37  is immediately stopped after the front edge of the recording medium reaches the first conveying roller unit  45 , the first conveying roller unit  45  fetches the recording medium into the printer main body  11  while receiving loads which are applied by a separating member, the paper feed roller  32 , the conveying roller unit  34 , and the like of the optional paper feed tray unit  30  side. Therefore, a sufficient conveying force cannot be assured in the first conveying roller unit  45 . 
     In the embodiment, therefore, even after the front edge of the recording medium reached the first conveying roller unit  45 , the driving of the paper feed motor  37  is also continued, thereby enabling the recording medium to be smoothly conveyed in the first conveying roller unit  45 . 
       FIG. 1  is a time chart showing the operation of the medium conveying apparatus in the first embodiment of the invention. 
     First, when the recording medium is fed into the printer main body  11  from the optional paper feed tray unit  30  ( FIG. 4 ), the conveyance control unit  57  ( FIG. 5 ) drives the paper feed motor  37  ( FIG. 6 ) so as to rotate the paper feed roller  32 . The recording media enclosed on the sheet cassette  31  of the optional paper feed tray unit  30  are fed out one by one by the paper feed roller  32 . At this time, since the electromagnetic clutch  38  is OFF (has been disconnected), the front edge of the fed-out recording medium is abutted on the conveying roller unit  34  and temporarily stopped, so that the skew is corrected. Subsequently, the conveyance control unit  57  turns on the electromagnetic clutch  38  so that the rotating force generated by the paper feed motor  37  is propagated to the conveying roller unit  34 , thereby rotating the paper feed roller  32  and the conveying roller unit  34  and feeding the recording medium into the printer main body  11 . In association with it, the conveyance control unit  57  drives the paper feed motor of the printer main body  11  at predetermined timing, thereby rotating the first and second conveying roller units  45  and  16 . Thus, the recording medium is conveyed by the paper feed roller  32 , the conveying roller unit  34 , and the first conveying roller unit  45 . 
     When the recording medium subsequently reaches the second conveying roller unit  16  and the medium sensor  17  detects the front edge of the recording medium and is turned on, the conveying force by the second conveying roller unit  16  can be used. Therefore, paper feeding unit operation stop processing means which serves as a operation stopping unit of the conveyance control unit  57  executes a paper feeding unit operation stopping process, stops the operations of the paper feed roller  32  and the conveying roller unit  34 , stops the driving of the paper feed motor  37 , and turns off the electromagnetic clutch  38  at predetermined timing, in the embodiment, after the front edge of the recording medium passed through the second conveying roller unit  16 . Therefore, the paper feed roller  32  is raced by the built-in one-way clutch and the conveying roller unit  34  is raced due to the turn-off of the electromagnetic clutch  38 , respectively. The recording medium is conveyed by the first and second conveying roller units  45  and  16  and fed to the image forming unit  18 . 
     As mentioned above, according to the embodiment, for a period of time until the recording medium reaches the second conveying roller unit  16  after it reached the first conveying roller unit  45 , the paper feed roller  32  and the conveying roller unit  34  are rotated and the recording medium is pushed from behind by the paper feed roller  32  and the conveying roller unit  34 , so that the recording medium can be conveyed by the paper feed roller  32 , the conveying roller unit  34 , and the first conveying roller unit  45 . Therefore, for the period of time until the recording medium reaches the second conveying roller unit  16 , the load of the optional paper feed tray unit  30  side which is applied to the first conveying roller unit  45  can be reduced. After the recording medium reached the second conveying roller unit  16 , the load of the optional paper feed tray unit  30  side can be distributed by the first and second conveying roller units  45  and  16 . Thus, the recording medium can be smoothly conveyed by the first conveying roller unit  45 . 
     Since the load which is applied to the first conveying roller unit  45  can be reduced, there is no need to enlarge the rubber roller (not shown) of the conveying roller  121 . Thus, the costs of the medium conveying apparatus can be reduced. 
     In the first embodiment, on the other hand, for the period of time until the front edge of the recording medium reaches the second conveying roller unit  16 , the paper feed roller  32  and the conveying roller unit  34  need to be rotated on the optional paper feed tray unit  30  side. Therefore, when the recording medium is short, even if the feed-out of the recording medium is finished, the paper feed roller  32  continues to rotate and the next recording medium is fed out. Consequently, restriction occurs in the sheet size as a medium size. 
     The second embodiment of the invention will now be described. Component elements having the same structures as those of the first embodiment are designated by the same reference numerals and their detailed description is omitted. Since the second embodiment has substantially the same structure, effects similar to those in the first embodiment are also obtained. 
       FIG. 7  is a perspective view of the medium conveying apparatus for conveying the recording medium in an optional paper feed tray unit in the second embodiment of the invention. 
     As shown in the diagram, the electromagnetic clutch  38  is arranged coaxially with the conveying roller  125  in the conveying roller unit  34  constructing the third conveying unit. By turning on/off the electromagnetic clutch  38 , the rotating force to the conveying roller  125  from the paper feed motor  37  as a driving unit for the paper feeding can be propagated or disconnected. A planet gear  61  is arranged between the paper feed roller  32  as a second paper feeding unit and the conveying roller  125  so as to be movable in the directions shown by arrows c and d. When the paper feed motor  37  is forwardly rotated (driven in the direction shown by an arrow a), the planet gear  61  is moved in the direction shown by the arrow c and come into engagement with a gear  62 . The rotating force generated by the paper feed motor  37  is propagated to the gear  62  through the gears  39  to  41  and the planet gear  61 . On the other hand, when the paper feed motor  37  is reversely rotated (driven in the direction shown by an arrow b), the planet gear  61  is driven in the direction shown by the arrow d and come into engagement with a gear of the electromagnetic clutch  38 . The rotating force generated by the paper feed motor  37  is propagated to the electromagnetic clutch  38  through the gears  39  to  41  and the planet gear  61 . By turning on/off the electromagnetic clutch  38 , the rotating force is selectively propagated from the paper feed motor  37  only to the paper feed roller  32 , only to the conveying roller unit  34 , or to both of the paper feed roller  32  and the conveying roller unit  34 . Reference numeral  63  denotes a subroller which is arranged in contact with the paper feed roller  32  and assists the conveyance of the recording medium. 
       FIG. 8  is a time chart showing the operation of the medium conveying apparatus in the second embodiment of the invention. 
     First, when the recording medium is fed into the printer main body  11  from the optional paper feed tray unit  30  ( FIG. 4 ), the conveyance control unit  57  ( FIG. 5 ) drives the paper feed motor  37  ( FIG. 7 ) so as to rotate the paper feed roller  32 , so that the recording media enclosed on the sheet cassette  31  as a recording medium enclosing unit are fed out one by one. At this time, since the electromagnetic clutch  38  is OFF, the front edge of the fed-out recording medium is abutted on the conveying roller unit  34  and temporarily stopped, so that the skew is corrected. Subsequently, the conveyance control unit  57  turns on the electromagnetic clutch  38  so as to forwardly rotate the paper feed motor  37 . The rotating force generated by the paper feed motor  37  is propagated to the conveying roller unit  34 , thereby rotating the paper feed roller  32  and the conveying roller unit  34  and conveying the recording medium by an arbitrary short distance. 
     As mentioned above, when the recording medium passes through the conveying roller unit  34 , the paper feeding unit operation stop processing means of the conveyance control unit  57  reversely rotates the paper feed motor  37 . At this time, the operation of the paper feed roller  32  is stopped and raced by the one-way clutch. Only the conveying roller unit  34  is rotated, thereby feeding the recording medium into the printer main body  11 . 
     In association with it, the conveyance control unit  57  drives the paper feed motor of the printer main body  11 , thereby rotating the first and second conveying roller units  45  and  16  constructing the first and second conveying units. Thus, the recording medium is conveyed by the conveying roller unit  34  and the first conveying roller unit  45 . 
     When the recording medium subsequently reaches the second conveying roller unit  16  and the medium sensor  17  as a second medium front edge detecting unit detects the front edge of the recording medium and is turned on, the conveying force by the second conveying roller unit  16  can be used. Therefore, the paper feeding unit operation stop processing means stops the driving of the paper feed motor  37  and turns off the electromagnetic clutch  38 . Therefore, the operation of the conveying roller unit  34  is stopped and the conveying roller unit  34  is raced due to the turn-off of the electromagnetic clutch  38 . The recording medium is conveyed by the first and second conveying roller units  45  and  16  and fed to the image forming unit  18 . 
     As mentioned above, according to the embodiment, the planet gear  61  is arranged near the conveying roller unit  34  and by forwardly or reversely rotating the paper feed motor  37 , the recording medium can be pushed from behind by the conveying roller unit  34  until the front edge of the recording medium reaches the second conveying roller unit  16 . When the recording medium passes through the conveying roller unit  34 , the paper feed motor  37  is stopped. Therefore, if the recording medium is short, after the feed-out of the recording medium was finished, the next recording medium is not fed out. Consequently, a range of the sheet size which can be pushed from behind can be widened. 
     Further, although the load that is applied to the first conveying roller unit  45  increases by arranging the subroller  63 , in the embodiment, the load that is applied to the first conveying roller unit  45  by the subroller  63  can be absorbed by the conveying roller unit  34 . 
     Although the recording medium is pushed from behind until the front edge of the recording medium reaches the second conveying roller unit  16  in the first and second embodiments mentioned above, in the case of a recording medium of a relatively large sheet size (for example; Legal 14 [inches] size or the like), even if the recording medium is pushed from behind until the front edge of the recording medium reaches the second conveying roller unit  16 , the load also remains after that. 
     The third embodiment of the invention will now be described. Since a structure of a medium conveying apparatus in the third embodiment is substantially the same as that of the medium conveying apparatus in the second embodiment, it will be explained also with reference to  FIG. 7 . 
       FIG. 9  is a time chart showing the operation of the medium conveying apparatus in the third embodiment of the invention. 
     In this case, a length of recording medium in the conveying direction is detected as a sheet size by the sheet size sensor as a medium length detecting unit and sent to the conveyance control unit  57  through the print control unit  50  ( FIG. 5 ). The number of pulses corresponding to the length of the detected sheet size is set into a counter of the conveyance control unit  57 . The operator can also manually input the pulse number without detecting the sheet size. 
     First, when the recording medium is fed into the printer main body  11  from the optional paper feed tray unit  30  ( FIG. 4 ), the conveyance control unit  57  drives the paper feed motor  37  as a driving unit for the paper feeding and rotates the paper feed roller  32  as a second paper feeding unit, thereby feeding out the recording media enclosed on the sheet cassette  31  as a recording medium enclosing unit. At this time, since the electromagnetic clutch  38  is OFF, the front edge of the fed-out recording medium is abutted on the conveying roller unit  34  constructing the third conveying unit and temporarily stopped, so that the skew is corrected. Subsequently, the conveyance control unit  57  turns on the electromagnetic clutch  38  so as to forwardly rotate the paper feed motor  37 . The rotating force generated by the paper feed motor  37  is propagated to the conveying roller unit  34 , thereby rotating the paper feed roller  32  and the conveying roller unit  34  and conveying the recording medium by an arbitrary short distance. In association with the start of the rotation of the conveying roller unit  34 , the conveyance control unit  57  starts the counting operation by the counter. 
     Subsequently, when the paper feeding unit operation stop processing means of the conveyance control unit  57  reversely rotates the paper feed motor  37 , the operation of the paper feed roller  32  is stopped by the one-way clutch, the paper feed roller  32  is raced, and only the conveying roller unit  34  is rotated, thereby feeding the recording medium into the printer main body  11 . 
     In association with it, the conveyance control unit  57  drives the paper feed motor of the printer main body  11  and rotates the first and second conveying roller units  45  and  16  constructing the first and second conveying units, so that the recording medium is conveyed by the conveying roller unit  34  and the first conveying roller unit  45 . 
     Subsequently, when the recording medium reaches the second conveying roller unit  16  and the medium sensor  17  as a second medium front edge detecting unit detects the front edge of the recording medium and is turned on, the conveying force by the second conveying roller unit  16  can be used. However, the apparatus enters the standby mode until a count value of the counter is equal to the number of pulses set every recording medium. When the count value is equal to the number of pulses set every recording medium and the recording medium is conveyed by the distance corresponding to the length of sheet size, the paper feeding unit operation stop processing means stops the driving of the paper feed motor  37  and turns off the electromagnetic clutch  38 . Therefore, the operation of the conveying roller unit  34  is stopped due to the turn-off of the electromagnetic clutch  38  and the conveying roller unit  34  is raced. The recording medium is conveyed by the first and second conveying roller units  45  and  16  and fed to the image forming unit  18 . 
     As mentioned above, according to the embodiment, since the driving of the paper feed motor  37  can be continued for a period of time corresponding to the sheet size, even in the case of a recording medium of a relatively large sheet size, the recording medium can be sufficiently pushed from behind. 
     In the embodiment, since the number of pulses is set on the basis of the sheet size, there is a case where a slip occurs during the conveyance of the recording medium due to not only the type, thickness, stiffness, surface state, and the like of the recording medium but also ambient environments (temperature, humidity, etc.) and a conveyance error occurs. In such a case, a conveyance amount of the recording medium is smaller than a presumed amount. 
     Therefore, the fourth embodiment of the invention in which the recording medium can be accurately conveyed by the distance corresponding to the presumed amount will now be described. Since a structure of a medium conveying apparatus in the fourth embodiment is substantially the same as that of the medium conveying apparatus in the second embodiment, it will be explained also with reference to  FIG. 7 . 
       FIG. 10  is a time chart showing the operation of the medium conveying apparatus in the fourth embodiment of the invention. 
     First, when the recording medium is fed into the printer main body  11  from the optional paper feed tray unit  30  ( FIG. 4 ), the conveyance control unit  57  ( FIG. 5 ) drives the paper feed motor  37  as a driving unit for the paper feeding and rotates the paper feed roller  32  as a second paper feeding unit, thereby feeding out the recording media enclosed on the sheet cassette  31  as a recording medium enclosing unit. At this time, since the electromagnetic clutch  38  is OFF, the front edge of the fed-out recording medium is abutted on the conveying roller unit  34  constructing the third conveying unit and temporarily stopped, so that the skew is corrected. Subsequently, the conveyance control unit  57  turns on the electromagnetic clutch  38  so as to forwardly rotate the paper feed motor  37 . The rotating force generated by the paper feed motor  37  is propagated to the conveying roller unit  34 , thereby rotating the paper feed roller  32  and the conveying roller unit  34  and conveying the recording medium by an arbitrary short distance. 
     Subsequently, when the paper feeding unit operation stop processing means of the conveyance control unit  57  reversely rotates the paper feed motor  37 , the operation of the paper feed roller  32  is stopped by the one-way clutch, the paper feed roller  32  is raced, and only the conveying roller unit  34  is rotated, thereby feeding the recording medium into the printer main body  11 . 
     In association with it, the conveyance control unit  57  drives the paper feed motor of the printer main body  11  and rotates the first and second conveying roller units  45  and  16  constructing the first and second conveying units, so that the recording medium is conveyed by the conveying roller unit  34  and the first conveying roller unit  45 . 
     Subsequently, until the recording medium passes through the medium sensor  33  serving as an optional medium front edge detecting unit and as a third detecting unit and the medium sensor  33  functions as a medium rear edge detecting unit, detects a rear edge of the recording medium, and is turned off, the paper feeding unit operation stop processing means continues to turn on the electromagnetic clutch  38 , continues to reversely rotate the paper feed motor  37 , conveys the recording medium by a predetermined distance, in the embodiment, by a distance of ten and a few [mm], turns off the electromagnetic clutch  38  at that timing, and stops the driving of the paper feed motor  37 . After that, the recording medium is conveyed by the first and second conveying roller units  45  and  16  and sent to the image forming unit  18 . 
     As mentioned above, in the embodiment, after the rear edge of the recording medium is detected by the medium sensor  33 , the recording medium is conveyed only by the first and second conveying roller units  45  and  16 . Therefore, the recording medium can be pushed from behind in correspondence to the length of each recording medium irrespective of the slip. Thus, the recording medium can be accurately conveyed by the distance corresponding to the presumed amount. 
     The fifth embodiment of the invention will now be described. Component elements having the same structures as those of the second embodiment are designated by the same reference numerals and their detailed description is omitted. Since the fifth embodiment has substantially the same structure, effects similar to those in the second embodiment are also obtained. 
       FIG. 11  is a perspective view of a medium conveying apparatus for conveying the recording medium in an optional paper feed tray unit in the fifth embodiment of the invention.  FIG. 12  is a side elevational view of the medium conveying apparatus for conveying the recording medium in the optional paper feed tray unit in the fifth embodiment of the invention.  FIG. 13  is a perspective view showing a main section of the medium conveying apparatus for conveying the recording medium in the optional paper feed tray unit in the fifth embodiment of the invention. 
     In this case, in a manner similar to the second embodiment of the invention, the electromagnetic clutch  38  is arranged coaxially with the conveying roller  125  as a first roller in the conveying roller unit  34  constructing the third conveying unit and by turning on/off the electromagnetic clutch  38 , the rotating force to the conveying roller  125  from the paper feed motor  37  as a driving motor for the paper feeding can be propagated or disconnected. The planet gear  61  is arranged between the paper feed roller  32  as a second paper feeding unit and the conveying roller  125  so as to be movable in the directions shown by the arrows c and d. When the paper feed motor  37  is forwardly rotated (driven in the direction shown by the arrow a), the planet gear  61  is moved in the direction shown by the arrow c and come into engagement with the gear  62 . The rotating force generated by the paper feed motor  37  is propagated to the gear  62  through the gears  39  to  41  and the planet gear  61 . On the other hand, when the paper feed motor  37  is reversely rotated (driven in the direction shown by the arrow b), the planet gear  61  is driven in the direction shown by the arrow d and come into engagement with a gear of the electromagnetic clutch  38 . The rotating force generated by the paper feed motor  37  is propagated to the electromagnetic clutch  38  through the gears  39  to  41  and the planet gear  61 . By turning on/off the electromagnetic clutch  38 , the rotating force is selectively propagated from the paper feed motor  37  only to the paper feed roller  32 , only to the conveying roller unit  34 , or to both of the paper feed roller  32  and the conveying roller unit  34 . The rotating force is propagated to a clutch  66  through a reduction gear  65  by using the rotating force which is propagated from the paper feed motor  37  to the gear  62 . 
     The pinch roller  35  has a shaft sh 4  and a rubber roller  135  and is supported by a frame guide  73  (which is guided by long holes formed in a plurality of positions). Therefore, both sides of the pinch roller  35  is supported by bearings  71  and pressed toward the conveying roller  125  by springs  72  as pressing members. 
     A link  70  is arranged on the further outside from the bearing  71  of the shaft sh 4  and pressed ordinarily in the direction shown by an arrow g by the bearing  71  and the spring  72  while setting a portion CN of the frame guide  73  to a rotational center. 
     A solenoid lever  67  is pressed ordinarily in the direction shown by an arrow e by a spring  74  as a pressing member. When a solenoid  68  is turned on, the solenoid lever  67  is rotated in the direction shown by an arrow f, propagates the rotating force from the paper feed motor  37  to a shaft  75 , and is fixed to the shaft  75 . A cam  76  is arranged so as to be rotatable in the directions shown by arrows  1  and j together with the shaft  75 . The solenoid lever  67  rotates the cam  76  in the direction shown by the arrow i by about a half turn, thereby rotating the link  70  in the direction shown by an arrow h. 
       FIG. 14  is a time chart showing the operation of the medium conveying apparatus in the fifth embodiment of the invention. 
     First, when the recording medium is fed into the printer main body  11  from the optional paper feed tray unit  30  ( FIG. 4 ), the conveyance control unit  57  ( FIG. 5 ) drives the paper feed motor  37  and rotates the paper feed roller  32 , thereby feeding out the recording media enclosed on the sheet cassette  31  as a recording medium enclosing unit. At this time, since the electromagnetic clutch  38  is OFF, the front edge of the fed-out recording medium is abutted on the conveying roller unit  34  and the medium is temporarily stopped, so that the skew is corrected. Subsequently, the conveyance control unit  57  turns on the electromagnetic clutch  38  so as to forwardly rotate the paper feed motor  37 . The rotating force generated by the paper feed motor  37  is propagated to the conveying roller unit  34 , thereby rotating the paper feed roller  32  and the conveying roller unit  34  and conveying the recording medium by an arbitrary short distance. 
     Subsequently, when the paper feeding unit operation stop processing means of the conveyance control unit  57  reversely rotates the paper feed motor  37 , the paper feed roller  32  is stopped by the one-way clutch and raced, and only the conveying roller unit  34  is rotated, thereby feeding the recording medium into the printer main body  11 . 
     Subsequently, when the front edge of the recording medium reaches the medium sensor  14  as a first medium front edge detecting unit and the medium sensor  14  is turned on and the front edge of the recording medium reaches the first conveying roller unit  45 , the paper feeding unit operation stop processing means turns on the solenoid  68  ( FIG. 12 ), thereby rotating the solenoid lever  67  in the direction shown by the arrow f. Thus, the clutch  66  is made operative, the rotating force is propagated to the shaft  75  ( FIG. 13 ), the cam  76  is rotated in the direction shown by the arrow i, and the link  70  is rotated in the direction shown by the arrow h around the portion CN as a rotational center, thereby setting the cam  76  into a state shown by a solid line. At this time, the pressing force by the spring  72  is cancelled, the pinch roller  35  is removed away from the conveying roller  125 , and the solenoid  68  is turned off, thereby rotating the solenoid lever  67  in the direction shown by the arrow e. In this manner, the operation of the conveying roller unit  34  is stopped. 
     The rotation of the cam  76  is controlled on the basis of the number of pulses of the paper feed motor  37  or by a sensor arranged on the shaft  75 . 
     In the state where the pinch roller  35  is away from the conveying roller  125  as mentioned above, the recording medium is conveyed only by the first conveying roller unit  45  and sent to the second conveying roller unit  16 . 
     When the rear edge of the recording medium passes through the medium sensor  33  and the medium sensor  33  is turned off, the conveyance control unit  57  drives the paper feed motor  37  again, turns on the solenoid  68 , rotates the cam  76  in the direction shown by the arrow j from the state shown by the solid line, returns the cam  76  to the original position, and stops the paper feed motor  37 . Also in this case, the rotation of the cam  76  is controlled on the basis of the number of pulses of the paper feed motor  37  or by the sensor arranged on the shaft  75 . 
     Subsequently, the conveyance control unit  57  drives the paper feed motor  37 , rotates the paper feed roller  32 , and conveys the next recording medium. 
     As mentioned above, in the embodiment, since the pinch roller  35  pressed to the conveying roller  125  is constructed as a movable roller, after the recording medium reached the first conveying roller unit  45  in the printer main body  11 , the load of the whole optional paper feed tray unit  30  is reduced and the recording medium can be fetched into the printer main body  11  only by the conveying force applied by the first conveying roller unit  45 . 
     Although one optional paper feed tray unit  30  is attached to the printer main body  11  in each of the foregoing embodiments, a plurality of optional paper feed tray units can be also attached. 
     The present invention is not limited to the foregoing embodiments but many modifications and variations are possible within the spirit and scope of the appended claims of the invention.