Patent Publication Number: US-7222849-B2

Title: Feeding method and apparatus for sheet-shaped recording material

Description:
This Nonprovisional application claims priority under 35 U.S.C. § 119(a) on patent application No(s). 2003-323066 filed in Japan on Sep. 16, 2003, the entire contents of which are hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method and an apparatus for feeding a sheet-shaped recording material, and in particular relates to operation control of roller pairs for nipping and feeding the recording material. 
     2. Description of the Related Art 
     A photo printer for recording an image on a sheet-shaped photosensitive material is widely known. In this kind of the photo printer, recording light is applied in a scanning direction while the photosensitive material is fed in a sub-scanning direction perpendicular to the scanning direction. The photo printer includes a plurality of feeding roller pairs comprising a capstan roller and a nip roller. The recording material is nipped and fed by the feeding roller pair. 
     In order to prevent density unevenness of a recording image to be caused by fluctuation of a feeding speed, it is necessary to accurately feed the photosensitive material at the time of image recording. In view of this, Japanese Patent Laid-Open Publication No. 2001-33883 teaches nip rollers respectively disposed at an upstream side and a downstream side of a record head for irradiating the recording light. The nip roller is movable between a nip position where the photosensitive material is nipped, and a release position where the nip is released. The respective nip rollers start to move when an anterior end or a posterior end of the photosensitive material has reached a predetermined position. In virtue of this, nipping and releasing the photosensitive material are performed at prescribed timing even if a length and a feeding speed of the photosensitive material are different. Thus, quality of the recording image may be kept in a good condition. 
     Meanwhile, Japanese Patent Laid-Open Publication No. 2002-3002 teaches nip rollers of an upstream side and a down stream side, which are moved by a single pulse motor. A movement speed of the nip roller is changed in accordance with a length of the photosensitive material. By doing so, shock to be caused to the photosensitive material is reduced when a feeding roller pair performs nipping and releasing. 
     In order to improve processing ability of the photo printer, it is preferable to increase the feeding speed of the photosensitive material. In addition, it is preferable to reduce an interval between the photosensitive materials successively fed. In this case, there is a possibility that drive timing of the respective nip rollers overlap. That is, timing for moving the upstream nip roller to the release position occurs while the downstream nip roller moves toward the nip position. 
     The drive timing of the nip rollers are usually designed so as not to overlap with each other. In case the drive timing of the nip rollers overlap, it is necessary to temporarily stop feeding the photosensitive material, since an operation is judged as being abnormal. Alternatively, it is necessary to stop the operation itself of the photo printer. In this case, the feeding speed changes while the image is recorded. Thus, the density unevenness is most likely to be caused. Further, there arises a problem in that the processing ability of the photo printer remarkably deteriorates, since feeding the photosensitive material is stopped. 
     SUMMARY OF THE INVENTION 
     In view of the foregoing, it is a primary object of the present invention to provide a feeding method and a feeding apparatus in which a recording material is stably fed without lowering a feeding speed even if drive timing of feeding roller pairs overlap. 
     In order to achieve the above and other objects, the feeding method according to the present invention comprises the steps of performing a movement operation of one of movable rollers, judging whether or not interrupt timing for moving the other movable roller occurs during the movement operation of the one of the movable rollers, and prioritizing a movement operation of the other movable roller when the interrupt timing has occurred. The movable rollers are an upstream movable roller and a downstream movable roller respectively constituting an upstream roller pair and a downstream roller pair, which are disposed at an upstream side and a downstream side of a recording position in a feeding direction of a sheet-shaped recording material. Each of the movable rollers is movable between a nip position for nipping and feeding the recording material, and a release position for releasing the nip of the recording material. 
     In a preferred embodiment, the movable rollers are moved by a single pulse motor. When the interrupt timing occurs during the movement operation of the one of the movable rollers, drive pulses, whose speed is determined based on the other movable roller, are supplied to the pulse motor. It is preferable that a number of the drive pulses to be successively supplied to the pulse motor is a total of drive-pulse numbers required for moving the one of the movable rollers and the other movable roller. 
     As to a movement speed of the downstream movable roller, it is preferable that the movement speed toward the nip position is slower than that toward the release position. In the meantime, as to a movement speed of the upstream movable roller, it is preferable that the movement speed toward the release position is slower than that toward the nip position. Further, it is preferable that an occurrence number of the interrupt timing is counted. 
     In the feeding apparatus according to the present invention, an upstream roller pair and a downstream roller pair are disposed at an upstream side and a downstream side of a recording position in a feeding direction of a sheet-shaped recording material. An upstream movable roller and a downstream movable roller constituting the upstream roller pair and the downstream roller pair are respectively movable between a nip position for nipping and feeding the recording material, and a release position for releasing the nip of the recording material. The feeding apparatus comprises a first moving mechanism, a second moving mechanism and a movement controller. The first moving mechanism moves one of the movable rollers. The second moving mechanism moves the other movable roller. The movement controller controls the first and second moving mechanisms in accordance with a position of the recording material. The controller judges whether or not interrupt timing for moving the other movable roller occurs during the movement operation of the one of the movable rollers. When the interrupt timing has occurred, the controller prioritizes the movement operation of the other movable roller. 
     According to the present invention, when the interrupt timing occurs during the movement operation of the one of the movable rollers, the movement operation of the other movable roller is prioritized. Thus, it is possible to make the feeding operation stable. Further, the drive pulse, whose speed is determined based on the movement operation of the other movable roller, is supplied to the pulse motor. Thus, a delay of timing for nipping/releasing the recording material is kept to the minimum. 
     The number of the drive pulses to be successively supplied to the pulse motor is a total of drive-pulse numbers required for moving the movable rollers. Thus, a stop position of the movable roller may be surely controlled even if the movement timing of the movable rollers overlap. 
     The movement speed of the downstream movable roller toward the nip position is slower than that thereof toward the release position, and the movement speed of the upstream movable roller toward the release position is slower than that thereof toward the nip position. Thus, it is possible to reduce a shock of the recording material to be caused in association with the nipping/releasing operation. Further, the occurrence number of the interrupt timing is counted. Thus, it is possible to specify a place where the movement timing of the movable rollers are likely to overlap. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above objects and advantages of the present invention will become apparent from the following detailed description of the preferred embodiments of the invention when read in conjunction with the accompanying drawings, in which: 
         FIG. 1  is an explanatory illustration schematically showing a structure of a printer; 
         FIG. 2  is a plan view schematically showing a structure of a feeding mechanism, wherein nip rollers of an upstream side and a downstream side are kept in a release position; 
         FIG. 3  is a plan view schematically showing the structure of the feeding mechanism, wherein the upstream nip roller is kept in a nip position and the downstream nip roller is kept in the release position; 
         FIG. 4  is a plan view schematically showing the structure of the feeding mechanism, wherein both of the nip rollers are kept in the nip position; 
         FIG. 5  is a plan view schematically showing the structure of the feeding mechanism, wherein the upstream nip roller is kept in the release position and the downstream nip roller is kept in the nip position; 
         FIG. 6  is a timing diagram showing speed variation of drive pulses to be supplied to a pulse motor; 
         FIGS. 7A ,  7 B and  7 C are timing diagrams showing speed variation of the drive pulses in a condition that movement timing of the downstream nip roller occurs while the upstream nip roller is moved; 
         FIGS. 8A and 8B  are timing diagrams showing speed variation of the drive pulses in a condition that movement timing of the upstream nip roller occurs while the downstream nip roller is moved; 
         FIGS. 9A and 9B  are timing diagrams showing speed variation of the drive pulses in a condition that movement timing of the downstream nip roller occurs while the upstream nip roller is moved; 
         FIGS. 10A and 10B  are timing diagrams showing speed variation of the drive pulses in a condition that movement timing of the upstream nip roller occurs while the downstream nip roller is moved; 
         FIG. 11  is a perspective view schematically showing another embodiment of the feeding mechanism; and 
         FIG. 12  is a plan view schematically showing a structure of the feeding mechanism shown in  FIG. 11 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S) 
     An embodiment of the present invention is described below, referring to the drawings.  FIG. 1  schematically shows a printer  11  of a photo printer. The printer  11  includes a paper-roll chamber  13 , a cutter  14 , a back-printing unit  15 , an image recorder  16  and a sorter  17 . 
     A magazine  20  disposed in the paper-roll chamber  13  contains a recording-paper roll  22  formed by rolling a sheet-shaped photosensitive recording paper  21 , which is used as a recording material. The recording paper  21  is formed such that a surface of at least an emulsion coating side (image recording side) of a substrate is covered with a composition, in which white pigment is mixed and dispersed in a resin including polyester or the like. The substrate is made of a base paper and so forth. Meanwhile, a paper-feed roller pair  23  is disposed near a paper mouth of the magazine  20 . When the paper-feed roller pair  23  is rotated by a drive motor, which is not shown, the recording paper  21  is drawn out of the recording-paper roll  22  and is advanced toward the cutter  14 . 
     The cutter  14  is constituted of a fixed blade  14   a  and a movable blade  14   b , which are disposed across a passage of the recording paper  21 . When an anterior end of the recording paper  21  is advanced from the cutter  14  by a predetermined length, a cutter driving mechanism not shown is actuated to move the movable blade  14   b  toward the fixed blade  14   a . Thereupon, the recording paper  21  is cut to produce a recording-paper sheet  24  having the predetermined length. The recording-paper sheet  24  is transported toward the back-printing unit  15  by advancing roller pairs  25  and along a guide rail, which is not shown. In the back-printing unit  15 , necessary information including film ID, a frame number and so forth is printed on a rear surface (opposite surface to an emulsion layer) of the recording-paper sheet  24 . 
     The recording-paper sheet  24  for which back printing has been performed is transported to the image recorder  16  by advancing roller pairs  26  and  27 . The image recorder  16  is constituted of an exposure device  28  for radiating recording light toward the recording-paper sheet  24 , and a feeding mechanism for moving the recording-paper sheet  24  in the image recorder  16 . The feeding mechanism comprises a first feeding roller pair  30  for feeding the recording-paper sheet  24  to an exposure position, and a second feeding roller pair  31  for feeding the exposed sheet  24  to a belt conveyor  32 . 
     The exposure device  28  comprises a well-known laser printer and an image memory for storing image data read by a film scanner, which is not shown. Alternatively, the image memory stores image data outputted from a recording medium of a memory card or the like, which is not shown. The laser printer irradiates the recording-paper sheet  24  with a laser, whose intensity is modulated on the basis of the image data stored in the image memory, to perform exposure recording of an image. The exposed sheet  24  is advanced to the belt conveyor  32 . The recording-paper sheets  24  are sorted into plural rows by the sorter  17  while transported by the belt conveyor  32 . And then, the recording-paper sheet  24  is advanced to a processing unit (not shown) wherein various processes of coloring, fixing and washing are performed. After these processes, a drying process is performed in the processing unit. Ultimately, the recording-paper sheet is discharged to the outside of the printer as a photo print. 
       FIG. 2  schematically shows the feeding mechanism of the image recorder  16 . The first feeding roller pair  30  is constituted of a capstan roller  33  and a nip roller  34  being as a driven roller, which are disposed so as to nip an upper side of a guide member  49 . Similarly, the second feeding roller pair  31  is constituted of a capstan roller  35  and a nip roller  36  being as a driven roller, which are disposed so as to nip the upper side of the guide member  49 . The capstan roller  33  of the first feeding roller pair  30  is connected to a motor  37  via a gear train, which is not shown. As to the motor  37 , is used a pulse motor having one hundred rotor teeth and five phases, for example. Drive pulses are supplied to the motor  37  so as to always rotate it at uniform velocity. Incidentally, an inelastic steal belt  38  is laid between the capstan rollers  33  and  35 , which are rotated at the uniform velocity by the sole motor  37 . 
     Upon activating the motor  37  to rotate the capstan rollers  33  and  35 , the recording-paper sheet  24  nipped by the nip rollers  34  and  36  is transported at a constant speed in a direction shown by an arrow (in a sub-scanning direction). When the recording-paper sheet  24  passes the exposure position  39 , the exposure device  28  applies the laser beam in a scanning direction (in a perpendicular direction relative to the drawing), which intersects with the sub-scanning direction at right angles, to perform the exposure one line by one line. Incidentally, the advancing roller pair  27  (see  FIG. 1 ) comprises a one-way clutch. A conveyance speed of the sheet  24  conveyed by the roller pair  27  is determined so as to be smaller than a feed speed of the sheet  24  fed by the first feeding roller pair  30 . Consequently, the advancing roller pair  27  becomes free at the moment that the recording-paper sheet is nipped and fed by the first feeding roller pair  30 . Thus, it is possible to hold down speed fluctuation to be caused by movement transition of the recording-paper sheet  24 . 
     The nip rollers  34  and  36  are rotatably supported by brackets  40  and  41  respectively disposed at both sides thereof in the scanning direction. The brackets  40  and  41  are guided by guide plates, which are not shown, so as to be vertically movable. The brackets  40  and  41  are urged toward the capstan rollers  33  and  35  with compression springs  42  and  43  by which the nip rollers  34  and  36  are pressed against the capstan rollers  33  and  35  to nip the recording-paper sheet  24 . 
     Elongate holes  40   a  and  41   a  formed in the brackets  40  and  41  respectively engage with guide pins  45   a  and  46   a  formed at ends of drive levers  45  and  46 . The drive levers  45  and  46  are rotatably attached to each other by an attachment shaft  48  so as to intersect. Cam followers  50  and  51  attached to the other ends of the drive levers  45  and  46  abut on a peripheral surface of an eccentric cam  52 . Upon rotating the eccentric cam  52 , the other ends of the drive levers  45  and  46  are pushed so that the drive levers  45  and  46  are rotated around the attachment shaft  48 . Owing to this, the nip rollers  34  and  36  are vertically moved between the nip position for nipping the recording-paper sheet  24 , and the release position for releasing the nip of the recording-paper sheet  24 . 
     A rotary shaft  53  of the eccentric cam  52  is connected to a pulse motor  55  via a gear train, which is not shown. The pulse motor  55  is connected to a controller  56  and is activated by receiving drive pulses from the controller  56 . When the pulse motor  55  is driven to rotate an output shaft thereof, the eccentric cam  52  is rotated around the rotary shaft  53  in a clockwise direction. The output shaft of the pulse motor  55  is rotated by a predetermined angle per one drive pulse. Thus, by counting a number of the drive pulses with a counter  57 , it is possible to detect a rotational position of the eccentric cam  52 , namely movement positions of the nip rollers  34  and  36 . Meanwhile, a reference-position sensor  60  is disposed near the pulse motor  55 . The reference-position sensor  60  comprises alight emitting portion and a light receiving portion to detect a passage of a projection  55   a  formed on the output shaft of the pulse motor  55 . 
     First and second position sensors  63  and  64  for detecting the passage of the recording-paper sheet  24  are respectively disposed at the upstream sides of the nip rollers  34  and  36 . Each of the first and second position sensors  63  and  64  is constituted of a light emitting portion and a light receiving portion. The light emitting portion radiates the light downward in the drawing, and the light receiving portion detects the reflected light. When intensity of the reflected light has changed, it is detected that the anterior end or the posterior end of the recording-paper sheet  24  has passed. 
     When the recording-paper sheet  24  is not fed, the eccentric cam  52  is stopped at a position where the reference-position sensor  60  detects the projection  55   a . At this time, the eccentric cam  52  depresses the other ends of the drive levers  45  and  46  so as to keep each of the nip rollers  34  and  36  in the releasing position. Thus, the nip rollers  34  and  36  are prevented, at the time of nonuse, from being pressed against the capstan rollers  33 ,  35  and from being deformed. 
     When the recording-paper sheet  24  is transported and the anterior end thereof is detected by the first position sensor  63 , the controller  56  supplies the drive pulse to the pulse motor  55  to rotate the eccentric cam  52  from the position shown in  FIG. 2  to the position shown in  FIG. 3 . By rotating the eccentric cam  52 , the drive lever  45  is rotated in a counterclockwise direction. The nip roller  34  of the upstream side is gradually moved downward and is stopped at the nip position shown in  FIG. 3 . 
     The recording-paper sheet  24  is nipped and fed by the first feeding roller pair  30  and the anterior end thereof passes the second position sensor  64 . Then, the eccentric cam  52  is rotated in the clockwise direction from the position shown in  FIG. 3  to the position shown in  FIG. 4  to merely rotate the drive lever  46 . Only the nip roller  36  of the downstream side is gradually moved toward the nip position in the state that the nip roller  34  of the upstream side is kept in the nip position. After the anterior end of the recording-paper sheet  24  has passed the second feeding roller pair  31 , the nip roller  36  reaches the nip position shown in  FIG. 4 . Thus, it is possible to eliminate a shock when the recording-paper sheet  24  enters the second feeding roller pair  31 . 
     The recording-paper sheet  24  is nipped by the first and second roller pairs  30  and  31 . In this state, the recording-paper sheet  24  is fed at a constant speed in the sub-scanning direction shown by an arrow in the drawing. And then, the posterior end of the recording-paper sheet  24  passes the first position sensor  63 . Thereupon, the pulse motor  55  is driven and the eccentric cam  52  is rotated from the position shown in  FIG. 4  to the position shown in  FIG. 5  to merely rotate the drive lever  45 . Owing to this, the nip roller  34  of the upstream side commences to move to the releasing position shown in  FIG. 5  in the state that the nip roller  36  of the downstream side is kept in the nip position. Before the posterior end of the recording-paper sheet  24  passes the first feeding roller pair  30 , the upstream nip roller  34  is separated from the recoding-paper sheet  24 . Thus, it is possible to eliminate a shock when the recording-paper sheet  24  passes through the first feeding roller pair  30 . 
     After that, the recording-paper sheet  24  is advanced only by the second feeding roller pair  31  of the downstream side. When it is detected that the posterior end of the recording-paper sheet  24  has passed the second position sensor  64 , the controller  56  supplies the drive pulse to the pulse motor  55  to rotate the eccentric cam  52  from the position shown in  FIG. 5  to the position shown in  FIG. 2 . The nip roller  36  of the downstream side commences to move to the release position in the state that the nip roller  34  of the upstream side is kept in the release position. Consequently, the nip of the second feeding roller pair  31  is released. 
     As shown in  FIG. 2 , a ROM  70  is connected to the controller  56  via a data bus  68 . The ROM  70  stores a control program for driving the nip rollers  34  and  36 . The control program is read at the time of recording an image. Meanwhile, a RAM  71  connected to the controller  56  via the data bus  68  stores data concerning speeds (S 1  and S 2  described later) of the drive pulses to be supplied to the pulse motor  55  for moving the nip rollers  34  and  36 . In addition, the RAM  71  stores the other data concerning numbers (P 1  through P 4  described later) of the drive pulses to be supplied to the pulse motor  55  in the respective movement stages of the nip rollers  34  and  36 . The RAM  71  further stores the other data concerning time lags (T 1  through T 5  described later) to be taken for driving/stopping the pulse motor  55  after the position sensors  63  and  64  have detected the recording-paper sheet  24 . 
     An operation of the feeding mechanism having the above structure is described below, referring to a timing diagram shown in  FIG. 6 . When the recording-paper sheet  24  is not fed or when the first position sensor  63  does not detect the anterior end of the sheet  24  notwithstanding the transport thereof, the reference-position sensor  60  makes the pulse motor  55  stop in a state that the output shaft thereof is set to an origin position. At this time, both the nip rollers  34  and  36  are kept in the release position to prevent the peripheral surface of the rollers from being deformed. 
     When the recording-paper sheet  24  is fed and the anterior end thereof is detected by the first position sensor  63 , driving the pulse motor  55  is commenced after a time T 1  to move the upstream nip roller  34  toward the nip position. The speed of the drive pulse to be supplied to the pulse motor  55  increases at a fixed rate and becomes a constant value S 1 . Incidentally, this drive-pulse speed corresponds to a movement speed of the nip roller  34 . After that, the speed of the drive pulse decreases at a fixed rate and becomes zero when the nip roller  34  reaches the nip position. The drive pulses of a predetermined number P 1  are supplied to the pulse motor  55  until the nip roller  34  reaches the nip position. It is possible to surely stop the nip roller  34  at the nip position by counting the number of the drive pulses, which are supplied to the pulse motor  55 , with the counter  57 . 
     The first feeding roller pair  30  of the nipping state feeds the recording-paper sheet  24  in the sub-scanning direction. When a leading edge of a recording area of the recording-paper sheet  24  has reached the exposure position  39 , the exposure device  28  is driven to record an image on the sheet  24  one line by one line. When the second position sensor  64  detects the anterior end of the recording-paper sheet  24 , the pulse motor  55  commences to rotate after a time T 2  so that the downstream nip roller  36  is moved toward the nip position. The speed of the drive pulse to be supplied to the pulse motor  55  increases at a fixed rate and becomes a constant value S 2 . Incidentally, this drive-pulse speed corresponds to a movement speed of the nip roller  36 . After that, the speed of the drive pulse decreases at a fixed rate and becomes zero when the nip roller  36  has reached the nip position (when the pulses of a number P 2  have been supplied). 
     As will be apparent from  FIG. 6 , in the drive sequence for moving the downstream nip roller  36  to the nip position, the drive-pulse speed S 2  is smaller than the drive-pulse speed S 1  for moving the upstream nip roller  34  to the nip position. Thus, it is possible to hold down a shock to be caused when the downstream nip roller  36  abuts on the recording-paper sheet  24  during the image recording. Consequently, exposure unevenness may be reduced. 
     The image recording is performed in the state that the recoding-paper sheet  24  is nipped and fed by the first and second roller pairs  30  and  31 . When the first position sensor  63  detects the posterior end of the recording-paper sheet  24 , the pulse motor  55  commences to rotate after a time T 3  to move the upstream nip roller  34  to the release position. The speed of the drive pulse to be supplied to the pulse motor  55  increases at a fixed rate and becomes the constant value S 2 . Incidentally, this drive-pulse speed corresponds to the movement speed of the nip roller  34 . After that, the speed of the drive pulse decreases at a fixed rate and becomes zero when the nip roller  34  has reached the release position (namely, when the pulses of a number P 3  have been supplied). Consequently, the nip roller  34  is stopped. The drive-pulse speed of this movement stage is also determined so as to be smaller, similarly to the stage for moving the downstream nip roller  36  to the nip position. Owing to this, it is possible to reduce a shock to be caused at the moment that the nip roller  34  is separated from the recording-paper sheet  24 . Thus, the exposure unevenness to be caused in association with the fluctuation of the feeding speed may be effectively held down. 
     After that, the recording-paper sheet  24  is transported in the sub-scanning direction by the second feeding roller pair  31  kept in the nip state. When the second position sensor  64  detects the posterior end of the recording-paper sheet  24 , the pulse motor  55  commences to rotate after a time T 4  to move the downstream nip roller  36  to the release position. The speed of the drive pulse to be supplied to the pulse motor  55  increases at a fixed rate and becomes the constant value S 1 . Incidentally, this drive-pulse speed corresponds to the movement speed of the nip roller  36 . And then, the speed of the drive pulse decreases at a fixed rate and becomes zero when the nip roller  36  has reached the release position (namely, when the pulses of a number P 4  have been supplied). Consequently, the nip roller  36  is stopped. In the stage for moving the downstream nip roller  36  to the release position, the pulse motor  55  is controlled so as to be stopped when the drive pulses of a number P 5  has been counted after detecting the projection  55   a  with the reference-position sensor  60 . 
     By the above-described operation, one sheet  24  passes the exposure device  28 . Successively, the similar operation is performed when the next sheet  24  reaches the exposure device  28 . In doing so, the recording-paper sheets  24  are fed in the condition that the shock to be caused by the nipping/releasing operation of the nip rollers  34  and  36  is held down. 
     When the length and the feeding speed of the recording-paper sheet  24  are within the limits of design, driving the pulse motor  55  pauses in accordance with a timing diagram shown in  FIG. 6  whenever the drive pulses of the predetermined numbers (P 1  to P 4 ) are supplied in the respective movement sequences. However, in case the length and the feeding speed of the recording-paper sheet  24  are out of the limits of design, movement timing of the nip rollers  34  and  36  sometimes overlap. In such a case, the nipping/releasing operations of the nip rollers  34  and  36  lag, so that it is judged that movement errors occur. Due to this, it has been necessary to temporarily stop feeding the recording-paper sheet  24  and to stop the whole operation of the photo printer. 
     In view of this, the printing process is adapted to be performed by continuously driving the pulse motor  55  without stopping the movement of the recording-paper sheet  24  when the movement timing of the nip rollers  34  and  36  overlap. For example, the system controller  56  detects the speed of the drive pulses supplied to the pulse motor  55  at the moment that the downstream nip roller  36  is moved to the nip position (at the moment that the time T 2  has passed after detecting the recording-paper sheet  24  with the second position sensor  64 ). And then, the system controller  56  changes the drive-pulse speed in accordance with the detected drive-pulse speed to drive the pulse motor at the speed S 2 . 
     Such as shown in  FIG. 7A , when the drive-pulse speed is S 1  at the moment that the downstream nip roller  36  is moved, the controller  56  decreases the drive-pulse speed to drive the pulse motor  55  at the pulse speed S 2 . And then, driving the pulse motor  55  is stopped when the number of the successively-supplied drive pulses becomes (P 1 +P 2 ). At this time, the downstream nip roller  36  reaches the nip position and stops in this state. 
     In the meantime, such as shown in  FIG. 7B , when the drive-pulse speed is between S 1  and S 2  at the moment that the downstream nip roller  36  is moved, the pulse speed is decreased at a fixed rate until the drive-pulse speed becomes S 2 . After that, the pulse motor  55  is driven at the speed S 2 . Meanwhile, when the drive-pulse speed is smaller than S 2  at the moment that the downstream nip roller  36  is moved (see  FIG. 7C ), the pulse speed is increased at a fixed rate until the drive-pulse speed becomes S 2 . After that, the pulse motor  55  is driven at the speed S 2 . In both of these cases, driving the pulse motor  55  is stopped when the number of the successively-supplied drive pulses becomes (P 1 +P 2 ). 
     When the timing for driving the downstream nip roller  36  occurs during the drive of the pulse motor  55 , a delay of timing for nipping the recording-paper sheet  24  with the nip roller  36  may be reduced by successively driving the pulse motor  55  at the speed S 2  in a prompt manner. In virtue of this, it is unnecessary to stop the movement of the recording-paper sheet  24  so that processing ability of the photo printer may be constantly maintained. Since the drive speed of the pulse motor  55  is adapted to be S 2 , a shock may be held down when the nip roller  36  abuts on the recording-paper sheet  24 . 
     When the timing for moving the upstream nip roller  34  to the release position occurs during the drive of the pulse motor  55 , the pulse motor  55  is successively driven in the similar manner. In other words, such as shown in  FIG. 8A , when the drive-pulse speed is S 2  at the moment that the upstream nip roller  34  is moved (at the moment that the time T 3  has passed after detecting the posterior end of the recording-paper sheet  24  with the first position sensor  63 ), the controller  56  keeps the drive-pulse speed at S 2 . When the number of the supplied pulses becomes (P 2 +P 3 ), the controller  56  stops driving the pulse motor  55 . At this time, the upstream nip roller  34  is stopped at the release position. Meanwhile, such as shown in  FIG. 8B , when the drive-pulse speed is smaller than S 2  at the moment that the upstream nip roller  34  is moved, the pulse speed is increased at a fixed rate until the drive-pulse speed becomes S 2 . After that, the pulse motor  55  is driven at the speed S 2 . When the number of the supplied pulses becomes (P 2 +P 3 ), driving the pulse motor  55  is stopped. Owing to this, a delay of timing for moving the nip roller  34  to the release position may be reduced. 
     When the timing for moving the downstream nip roller  36  to the release position occurs during the drive of the pulse motor  55 , the pulse motor  55  is successively driven without making the pulse speed zero. Such as shown in  FIG. 9A , when the drive-pulse speed is S 2  at the moment that the downstream nip roller  36  is moved (at the moment that the time T 4  has passed after detecting the posterior end of the recording-paper sheet  24  with the second position sensor  64 ), the controller  56  increases the drive-pulse speed up to S 1  to move the nip roller  36 . When the pulses of the number P 5  are supplied after detecting the projection  55   a  with the reference-position sensor  60 , driving the pulse motor  55  is stopped. At this time, both of the nip rollers  34  and  36  are stopped at the release position. Meanwhile, when the drive-pulse speed is smaller than S 2  at the moment that the downstream nip roller  36  is moved, the pulse speed is increased at a fixed rate until the drive-pulse speed becomes S 1 , such as shown in  FIG. 9B . After that, the pulse motor  55  is driven at the speed S 1 . And then, when the pulses of the number P 5  are supplied after detecting the projection  55   a  with the reference-position sensor  60 , driving the pulse motor  55  is stopped. 
     When the timing for moving the upstream nip roller  34  to the nip position occurs during the drive of the pulse motor  55 , the pulse motor  55  is successively driven without making the pulse speed zero. Such as shown in  FIG. 10A , when the drive-pulse speed is S 1  at the moment that the upstream nip roller  34  is moved (at the moment that the time T 1  has passed after detecting the recoding-paper sheet  24  with the first position sensor  63 ), the controller  56  keeps the drive-pulse speed at S 1  to move the nip roller  34 . And then, when the pulses of the number (P 5 +P 1 ) are supplied after detecting the projection  55   a  with the reference-position sensor  60 , namely when the number of the successively-supplied pulses becomes (P 4 +P 1 ), driving the pulse motor  55  is stopped. At this time, the upstream nip roller  34  is stopped at the nip position. Meanwhile, such as shown in  FIG. 10B , when the drive-pulse speed is smaller than S 1  at the moment that the upstream nip roller  34  is moved, the pulse speed is increased at a fixed rate until the drive-pulse speed becomes S 1 . After that, the pulse motor  55  is driven at the speed S 1 . And then, when the pulses of the number (P 5 +P 1 ) are supplied after detecting the projection  55   a  with the reference-position sensor  60 , driving the pulse motor  55  is stopped. 
     In the above embodiment, the pulse speed is increased and decreased on the basis of the dive-pulse speed detected at the moment that the nip rollers  34  and  36  are moved. However, the drive-pulse speed at the moment of moving the nip rollers  34  and  36  may be calculated by reading information, which concerns the supplied-pulse number, from the counter  57  when the position sensors  63  and  64  have detected the recording-paper sheet  24 . Since a maximum value and acceleration of the drive-pulse speed are predetermined in the respective movement stages, the drive-pulse speed to be set after the times T 1  to T 4  can be calculated by easy operation. 
     Incidentally, in the case the timing for moving the nip rollers have overlapped, it is preferable to record information concerning the overlap. Concretely, in the case of the movement timing shown in  FIGS. 7 through 10 , an occurrence number thereof is recorded in the RAM  71 . The information concerning the occurrence number is read from the RAM  71  at the time of checking the photo printer to detect the stage during which the movement timing of the nip rollers are likely to overlap. It is possible to prevent the movement timing from overlapping by changing the times T 1  to T 4  and the pulse speeds S 1  and S 2 . 
     The above embodiment relates to only the case in that two kinds of the movement timing overlap. The present invention, however, may be adopted to another case in that three or more kinds of the movement timing overlap. 
     The mechanism for moving the nip roller is not limited to the above embodiment. For example, it is possible to employ the mechanism shown in  FIGS. 11 and 12 . In this embodiment, an upstream nip roller  101  and a downstream nip roller  102  are vertically moved by rotating a cam unit  103  to nip the recording-paper sheet with a capstan roller, which is not shown, and to release the nip of the recording-paper sheet. 
     The cam unit  103  comprises a drive cam  105 , a first cam  106  and a second cam  107 , which are disposed in an axial direction of the nip rollers  101  and  102 . A timing belt  110  is laid between the drive cam  105  and two pulleys  108  and  109 . Upon driving a pulse motor  111  connected to the pulley  108 , the timing belt  110  is moved to rotate the drive cam  105  around a rotary shaft in a counterclockwise direction. At this time, the first cam  106  and the second cam  107  fixed to the drive cam  105  are rotated in the counterclockwise direction. 
     Peripheral surfaces of the first cam  106  and the second cam  107  respectively abut on first and second cam followers  113  and  114  urged toward the cam unit  103 . The first cam follower  113  and the upstream nip roller  101  are supported by a base member  115  and are capable of revolving around a rotary shaft  115   a . Similarly, the second cam follower  114  and the downstream nip roller  102  supported by a base member  116  are capable of revolving around a rotary shaft  116   a.    
     When the pulse motor  111  is driven to rotate the cam unit  103  in the counterclockwise direction, the first cam follower  113  moves in a radial direction of the cam unit  103 , abutting on the first cam  106 . In association with this movement, the upstream nip roller  101  moves between a nip position for nipping the recording-paper sheet with the capstan roller, which is not shown, and a release position for separating from the capstan roller. Similarly, the second cam follower  114  moves, abutting on the second cam  107 , so that the downstream nip roller  102  moves between the nip position and the release position. 
     In the moving mechanism having the above structure, when the timing for moving the nip roller occurs during the drive of the pulse motor  111 , it is possible to reduce a delay of timing for nipping the recording-paper sheet with the nip roller by successively driving the pulse motor  111  along the sequences shown in  FIGS. 7 to 10 . 
     The above embodiments are described with the feeding mechanism in which two nip rollers are moved by using the sole pulse motor. The present invention, however, may be applied to another feeding mechanism in which pulse motors for driving the respective nip rollers are provided. In this case, when the timing for driving one nip roller occurs during the movement of the other nip roller, the pulse motors are simultaneously driven to simultaneously move the nip rollers without stopping the recording-paper sheet as a result of judging an abnormal condition. 
     Although the present invention has been fully described by way of the preferred embodiments thereof with reference to the accompanying drawings, various changes and modifications will be apparent to those having skill in this field. Therefore, unless otherwise these changes and modifications depart from the scope of the present invention, they should be construed as included therein.