Patent Publication Number: US-9428357-B2

Title: Conveying device and conveying control method

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a conveying device and a conveying control method and particularly to a conveying device and a conveying control method in which a phase of a conveying roller and a position of a print medium are controlled. 
     2. Description of the Related Art 
     Conventionally, in an inkjet printer, a high-accuracy roller in which a metal shaft is coated with a grinding stone has been used as a main conveying roller. As a sub conveying roller also functioning as a discharge roller located on the downstream of the main conveying roller, a rubber roller having an accuracy lower than the main conveying roller and formed by rubber being attached to a metal shaft has been used. 
     According to this configuration, a conveying error is large in delivery of a print medium from the main conveying roller to the sub conveying roller and in conveyance only with the sub conveying roller, and it has been difficult to realize higher image quality and higher throughput. 
     As a measure against that, printing of a test pattern on a print medium and reading and analysis of the printed data by a scanner have been performed in recent years. A technology is proposed that characteristics (outer diameter, deflection, and the like) of the main and sub conveying rollers are obtained by performing the above-described analysis using a test pattern, and the obtained result is fed back as a correction value in printing and conveyance is performed (Japanese Patent Laid-Open No. 2005-007817). When this type of method is used, a technology of controlling a phase of the main conveying roller or the sub conveying roller and a position of the print medium becomes important for the following two reasons. 
     When the test pattern is printed, the sub conveying roller cannot print one cycle since a length of a single print medium is not sufficient. Thus, it is necessary to separate a print for one cycle into two sheets for printing. Thus, the first reason is that if the print for one cycle is separated into two sheets for printing, the phase of the sub conveying roller and the position of the print medium should be controlled so that there is no conveying error. 
     Moreover, a second reason is that the phases of the main conveying roller and the sub conveying roller should be fixed to optimal phases when the print medium is delivered from the main conveying roller to the sub conveying roller so that the conveying error is stabilized and can be easily corrected. This technology is proposed in Japanese Patent Laid-Open No. 2010-046994, for example. 
     According to the configuration described in Japanese Patent Laid-Open No. 2010-046994, the conveying roller and a feeding roller need to be driven and controlled, respectively in order to match the phase of the conveying roller with the position of the print medium. This can be realized by driving the conveying roller and the feeding roller by separate motors and by controlling the rollers individually, for example. Alternatively, this can be realized by coupling the conveying roller and the feeding roller with a motor through drive switching device, respectively, by switching driving of the motor by the drive switching device and by controlling rotation of the conveying roller or the feeding roller. 
     However, such realizing device has many demerits such as cost increase caused by provision of a plurality of motors, reduction in a throughput due to operation of the drive switching device, complication of drive transmitting device and control and the like. 
     SUMMARY OF THE INVENTION 
     Thus, in order to solve the above-described problems, the present invention has an object to provide a conveying device and a conveying control method that can control a phase of a main conveying roller and a position of a print medium with an inexpensive configuration and a fast throughput. 
     A conveying device of the invention of this application is a conveying device including first conveying device for conveying a print medium, second conveying device provided on the downstream side of the first conveying device in a conveying direction of the print medium and for conveying the print medium by means of rotation, detecting device provided between the first conveying device and the second conveying device in a conveying path of the print medium and for detecting a position of the print medium conveyed by the first conveying device, correcting device for correcting skewing of the print medium by bending the print medium between the first conveying device and the second conveying device in the conveying path of the print medium, and phase detecting device for detecting a phase of rotation of the second conveying device, in which on the basis of a result of detection by the phase detecting device, phase matching control is made so that the print medium comes to a desired position when the second conveying device is at a desired phase, and skewing of the print medium is corrected by the correcting device by the same control as the phase matching control. 
     Moreover, a conveying control method of the invention of this application is a conveying control method including a first conveying step for conveying a print medium, a second conveying step for conveying the print medium by means of rotation after the first conveying step in a conveying direction of the print medium, a detection step for detecting a position of the print medium conveyed in the first conveying step between the first conveying step and the second conveying step in a conveying path of the print medium, a correction step for correcting skewing of the print medium by bending the print medium between the first conveying step and the second conveying step in the conveying path of the print medium, and a phase detection step for detecting a phase in rotation in the second conveying step, in which on the basis of a result of detection in the phase detection step, a phase matching control step in which the print medium comes to a desired position when a phase of the rotation in the second conveying step is a desired phase and a step for correcting skewing of the print medium in the correction step by the same control as control in the phase matching control step are provided. 
     According to the present invention, the conveying device performs the phase matching control in which the print medium comes to a desired position when the second conveying device has a desired phase on the basis of the result of the detection by the phase detecting device and corrects skewing of the print medium by the correcting device by the same control as the phase matching control. 
     As a result, a conveying device and a conveying control method that can control the phase of the main conveying roller and the position of the print medium with an inexpensive configuration and a fast throughput can be realized. 
     Further features of the present invention will become apparent from the following description of exemplary embodiments (with reference to the attached drawings). 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of an embodiment of a sheet-feeding and conveying device of a printing apparatus in the present invention; 
         FIG. 2  is a longitudinal sectional view of the sheet-feeding and conveying device in  FIG. 1 ; 
         FIG. 3  is a perspective view of a loading portion in a state where a print medium has not been set yet when seen diagonally; 
         FIG. 4  is a perspective view of the loading portion in a state where the print medium is set when seen from diagonally above; 
         FIG. 5  is a back face view of sheet feeding device constituting a sheet feeding portion when seen from below; 
         FIG. 6  is a longitudinal sectional view of the sheet feeding portion, a separation portion, a reversing conveying portion, and a horizontal conveying portion; 
         FIG. 7  is a perspective view of an inner guide unit when seen from above; 
         FIG. 8  is a perspective view of a PF roller unit when seen from above; 
         FIG. 9  is an exploded view of the inner guide unit; 
         FIG. 10  is a sectional view of an outer guide unit constituting a part of the reversing conveying portion; 
         FIG. 11  is a perspective view of the outer guide unit when seen from diagonally front; 
         FIG. 12  is a graph illustrating a relationship between conveying resistance and a contact force between a roller and a pinch roller; 
         FIG. 13  is as perspective view of an entire driving row driving a PF roller, an LF roller, and a discharge roller when seen from a rear part above; 
         FIG. 14  is a perspective view of the driving row for transmitting driving from the LF roller to the PF roller unit when seen from above; 
         FIG. 15  is a perspective view of the driving row for transmitting driving from the LF roller to the PF roller unit when seen from above; 
         FIG. 16  is a facilitated sectional view describing a curved path of a print medium in a manner facilitated to a straight path; 
         FIG. 17  is a facilitated sectional view describing the curved path of the print medium in a manner facilitated to the straight path; 
         FIG. 18  is a facilitated sectional view describing the curved path of the print medium in a manner facilitated to the straight path; 
         FIG. 19  is a facilitated sectional view describing the curved path of the print medium in a manner facilitated to the straight path; 
         FIG. 20  is a facilitated sectional view describing the curved path of the print medium in a manner facilitated to the straight path; 
         FIG. 21  is a facilitated sectional view describing the curved path of the print medium in a manner facilitated to the straight path; 
         FIG. 22  is a facilitated sectional view describing the curved path of the print medium in a manner facilitated to the straight path; 
         FIG. 23  is a flowchart illustrating a sheet feeding operation in phase matching; 
         FIG. 24  is a block diagram illustrating an outline of printing operation control of an inkjet printing apparatus. 
         FIG. 25A  is a facilitated sectional view describing the curved path of the print medium in a manner facilitated to the straight path; and 
         FIG. 25B  is a facilitated sectional view describing the curved path of the print medium in a manner facilitated to the straight path. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     An embodiment of the present invention will be specifically described below by referring to drawings. The same reference numerals refer to the same or corresponding portions throughout the each drawing.  FIG. 1  is a perspective view of one embodiment of a sheet-feeding and conveying device of a printing apparatus in the present invention.  FIG. 2  is a longitudinal sectional view of the sheet-feeding and conveying device in  FIG. 1 . In  FIGS. 1 and 2 , a sheet-feeding and conveying device  10  is mainly composed of a print-medium loading portion  11 , a sheet feeding portion  12 , a separation portion  13 , a reversing conveying portion  14 , a double-sided conveying path  15 , and a horizontal conveying portion  16 . 
     (Explanation of Flow of Print Medium in Each Unit) 
     A bundle of print mediums set on the print-medium loading portion  11  is separated into a single print medium by the sheet feeding portion  12  and the separation portion  13  and fed to the reversing conveying portion  14 . Then, the front and back of the print medium are reversed in the reversing conveying portion  14 , fed to the horizontal conveying portion  16 , goes through an image forming portion  17  and is discharged. The image forming portion (hereinafter also referred to as an image processing portion)  17  is composed of a printable unit such as inkjet printing device. 
     (Explanation of Configuration of Loading Portion) 
     Subsequently, a configuration of the loading portion  11  will be described.  FIG. 3  is a perspective view of the loading portion  11  in a state where a print medium has not been set yet when seen diagonally. Moreover,  FIG. 4  is a perspective view of the loading portion  11  in a state where the print medium is set when seen from diagonally above. The print-medium loading portion  11  includes a loading surface  31  for holding a plurality of print mediums S substantially horizontally, side guides  32   a  and  32   b  for guiding both side faces of the print medium S, and a tip-end reference surface  33  for guiding the tip end of the print medium S. 
     The tip-end reference surface  33  is attached capable of swing, configured substantially perpendicularly to a sheet feeding direction except during a sheet-feeding operation and serves as an abutment reference when a user sets a print medium. On the other hand, the tip-end reference surface  33  is configured to retreat to the outside of a conveying path (conveying route) of the print medium S during sheet feeding. 
     (Explanation of Configuration of Sheet Feeding Portion) 
     Subsequently, a configuration of the sheet feeding portion  12  will be described.  FIG. 5  is a back face view of sheet feeding device constituting the sheet feeding portion  12  when seen from below. A swing arm  52  pivotally supports sheet feeding rollers  51   a  and  51   b  capable of swing. A biasing spring  53  is spring device stretched between a hook portion  52   a  of the swing arm  52  and a hook portion, not shown, of a sheet feeding base  58  (See  FIG. 2 ). A drive shaft  54  transmits driving to the sheet feeding rollers  51   a  and  51   b.    
     A one-way clutch  55  which transmits a torque only in one direction is attached between an input gear  54   b  for transmitting driving from a driving source, not shown, to the drive shaft  54  and an output gear  54   a . The one-way clutch  55  is configured such that the torque is transmitted when a torque from the input gear  54   b  to the output gear  54   a  rotates the sheet feeding rollers  51   a  and  51   b  in a sheet feeding direction. 
     On the swing arm  52 , idler gears  57   a  and  57   b  for transmitting the driving from an output gear portion  54   a  of the drive shaft  54  to a sheet-feeding roller gear  56  is pivotally supported. The swing arm  52  is attached to the lower surface side of the sheet feeding base  58  capable of rotary motion (swing). Moreover, the drive shaft  54  is also rotatably fitted and pivotally supported on the sheet feeding base  58  coaxially with a rotating fulcrum of the swing arm  52 . 
     (Explanation of Configuration of Separation Portion) 
       FIG. 6  is a longitudinal sectional view of the sheet feeding portion  12 , the separation portion  13 , the reversing conveying portion  14 , and the horizontal conveying portion  16 . By using  FIGS. 3 and 6 , a configuration of the separation portion  13  will be described. A separation method of the separation portion  13  in the embodiment is a cost-advantageous separation bank method. The separation portion  13  is composed of a separation bank surface  61  having a surface inclined to the sheet feeding direction and functioning as a separation bank, a separation assisting member  62  provided at the center of the separation bank surface  61 , and a separation print medium  63  attached to the loading surface  31  of the print medium S. 
     The separation assisting member  62  is attached slightly protruding from the inclined surface of the separation bank surface  61  and is configured such that, when the print medium S is fed, first, the separation assisting member  62  is first brought into contact with the print medium tip end and gives resistance. Moreover, the separation assisting member  62  is attached movably in the horizontal direction and is configured to retreat when being pressed with a load stronger than predetermined. 
     In the configuration, when the sheet feeding roller  51  is rotated and driven, the loaded print medium S is fed out while being pressed by the sheet feeding roller  51 . When the tip end of the print medium S is pressed onto the separation assisting member  62  and the separation bank surface  61  and receives resistance, elasticity of the print medium S and a friction force received at the print medium tip end separate the uppermost print medium S from the print mediums below (the subsequent print medium and after) and feeds only one sheet to the conveying path. 
     (Explanation of Configuration of Reversing Conveying Portion) 
     Subsequently, a configuration of the reversing conveying portion  14  will be described.  FIG. 7  is a perspective view of an inner guide unit  71  constituting a part of the reversing conveying portion  14  when seen from above.  FIG. 8  is a perspective view of a PF roller unit  74  when seen from the above.  FIG. 9  is an exploded view of the inner guide unit  71 .  FIG. 10  is a sectional view of an outer guide unit  72  constituting a part of the reversing conveying portion  14 .  FIG. 11  is a perspective view of the outer guide unit  72  when seen from diagonally front. 
     The reversing conveying portion  14  is composed of the inner guide unit  71  and the outer guide unit  72 . The inner guide unit  71  forms an inner guide of a reversing conveying path for reversing the print medium S and is composed of an inner guide  73  supporting each component which will be described later and the PF roller unit  74  (See  FIG. 6 ) for conveying the print medium in the reversing conveying path. The outer guide unit  72  forms an outer guide of the reversing conveying path and conveys the print medium S in collaboration with a PF pinch roller unit  76  which will be described later. 
     In  FIGS. 7, 8, and 9 , a PF roller  77  (first conveying device) has a high friction material such as rubber on an outer periphery and is pivotally supported at the tip end of a PF arm  78 . The PF arm  78  is supported capable of swing by means of shafts  78   a  and  78   b  formed integrally at the PF arm  78  pivotally supported by holes  73   a  and  73   b  of the inner guide  73 . One end of a PF shaft  79  is pivotally supported by a hole formed coaxially with the shaft  78   a  of the PF arm  78 , while the other end is pivotally supported by the inner guide  73  through a clutch portion  80 . 
     In this state, the PF arm  78  is rotatable within a predetermined range by a rotation regulation portion  91  (See  FIG. 6 ) formed on the inner guide  73  and an engagement portion of the PF arm  78 . Fulcrums  78   a  and  78   b  of the PF arm  78  is set on the upstream side using a contact point between the print medium S and the PF roller  77  as a reference with respect to the conveying direction of the print medium S conveyed on the reversing conveying path. Moreover, a PF roller output gear  81  is fixed to the other end of the PF shaft  79  and is meshed with a PF roller gear  82  rotating integrally with the PF roller  77 . 
     A flat engagement portion  80   a  is formed at one end of the clutch portion  80 , while a groove  84   a  engaged with the engagement portion  80   a  is formed in a PF gear shaft  84  rotating integrally with a PF input gear  83  connected to a driving source, not shown. A clutch spring  85  is attached to the PF gear shaft  84 , and rotation only in one direction is made possible by fixing one end of the clutch spring  85  to a driving frame, not shown. 
     With the above configuration, if the PF input gear  83  is rotated in a clockwise direction (CW direction in the figure), the clutch spring  85  is loosened, and the PF gear shaft  84  is made rotatable. Then, the rotation is transmitted to the PF shaft  79  through the engagement portion  80   a , the groove  84   a , and the clutch portion  80 , the PF roller output gear  81  is rotated in the clockwise direction, and the PF roller gear  82  and the PF roller  77  are rotated in a counterclockwise direction (CCW direction in the figure), that is, in the conveying direction. 
     On the other hand, if the PF roller  77  is rotated in the conveying direction (CCW direction) in a state where driving of the PF input gear  83  is stopped, driving of the PF shaft  79  and the PF gear shaft  84  is shut off by an action of the clutch  80 . Thus, a loosening torque of the clutch spring  85  does not work, and the PF roller  77  can rotate with a low driving torque. Further, if the PF roller  77  is rotated in the clockwise direction (CW direction) in a state where driving of the PF input gear  33  is stopped, the driving is transmitted to the PF gear shaft  84  by the action of the clutch portion  80 , but since the clutch spring  85  is closed, rotation is made impossible. 
     Moreover, a precompression spring  90  (See  FIG. 8 ) generating a biasing force in the clockwise direction (CW direction) in  FIG. 6  is attached to the PF arm  78 , and the PF arm  78  is stopped in a state in contact with a PF pinch roller which will be described later by an action of the precompression spring  90 . In the embodiment, a biasing force of 30 gf is generated by the precompression spring  90 . In  FIGS. 10 and 11 , the PF pinch roller  86  is pivotally supported at one end of a PF pinch roller holder  87 . 
     The PF pinch roller holder  87  is pivotally supported capable of swing by means of a shaft  87   a  formed integrally on the PF pinch roller holder  87  pivotally supported by a hole formed in the outer guide unit  72 . A PF pinch roller spring  88  is provided between a back surface  87   b  of the PF pinch roller holder  87  and an outer guide opposing portion  75   b . Then, the back surface  87   b  is biased in an arrow X direction, and the position of the PF pinch roller holder  87  is regulated by a stopper  89  provided on the outer guide  75 . 
     Subsequently, a relationship between conveying resistance and a contact force between the PF roller  77  and the PF pinch roller  86  will be described.  FIG. 12  is a graph illustrating the relationship between the conveying resistance and the contact force between the PF roller  77  and the PF pinch roller  86  generated in accordance with the conveying resistance. In  FIG. 6 , a fulcrum  78   c  of the PF arm  78  is set in a direction so that a couple generated by the conveying resistance F 2  of the print medium S increases a biasing force to the PF pinch roller  86  (the PF arm  78  rotates in the CW direction) and is configured such that a friction force according to the conveying resistance F 2  is generated. 
     As illustrated in  FIG. 12 , the contact force between the PF roller  77  and the PF pinch roller  86  is only a force generated by the precompression spring  90  in a standby state where there is no conveying resistance. If the conveying resistance F 2  increases in a print medium conveying state, the contact force is generated in accordance with the conveying resistance by the couple of the PF arm  78 . 
     Moreover, if the conveying resistance further increases and the contact force exceeds the biasing force generated in the PF pinch roller spring  88 , the PF pinch roller holder  87  rotates in the counterclockwise direction (CCW direction in the figure) around the fulcrum  87   a  and retreats. The PF arm  78  follows that and rotates in the clockwise direction (CW direction in the figure). If the resistance further increases, rotation of the PF arm  78  is regulated by the rotation regulation portion  91 , and the contact force no longer increases. 
     (Explanation of Configuration of Horizontal Conveying Portion) 
     Subsequently, the horizontal conveying portion  16  (See  FIG. 1 ) will be described. In  FIGS. 1 and 2 , an LF roller (second conveying device)  101  is provided on the downstream side in the conveying direction with respect to the PF roller  77  and is configured such that the surface of a metal shaft is coated with ceramic micro particles, and a metal portion of both shaft ends is supported by a bearing portion attached to a chassis. 
     On a pinch roller holder  103 , a plurality of pinch rollers  105  biased to the surface of the LF roller  101  by a pinch roller spring  104  are held, and the pinch roller  105  is brought into contact with the surface of the LF roller  101  and follows it. A discharge roller  106  is configured such that a plurality of rubber rollers are inserted into a metal shaft and fixed. 
     A plurality of spurs are attached to a spur holder  107 , and these spurs are pressed toward the discharge roller  106  by a spur spring in which a coil spring is provided in a rod state. A platen  108  is configured to support the lower surface of the print medium S between the LF roller  101  and the discharge roller  106 . 
     (Explanation of Configuration of Origin Seeking Device and Explanation of Operation of Origin Seeking Device) 
     Subsequently, a configuration of origin seeking device of the LF roller  101  will be described. An LF slider guide is rotatably attached with friction to an outer periphery of the LF roller  101 . An engagement portion is integrally formed on the LF slider guide, and a rotating angle is regulated by engagement between a first stopper and a second stopper formed on a left side chassis. If the LF roller rotates normally (counterclockwise direction), the engagement portion and the first stopper are brought into contact with each other, the LF slider guide is stopped, and only the LF roller rotates. 
     On the other hand, if the LF roller  101  rotates reversely (clockwise direction), the rotation of the LF slider guide is stopped at a position where the engagement portion and the second stopper are brought into contact with each other, and only the LF roller  101  rotates. The LF slider is attached capable of rotation and sliding in the rotating direction and the axial direction with respect to the outer periphery of the LF roller  101 . The LF slider is molded from a material with low friction with respect to metal (POM in the embodiment) and is capable of rotation and sliding with a low load. By engaging a rib formed on the LF slider guide with a groove in the LF slider, the LF slider rotates in synchronization with the LF slider guide. 
     In the LF roller gear, a projection engaged with a tip end portion of the LF slider is formed. In the above configuration, if the LF roller gear is rotated in the clockwise direction in a state where the LF slider is made to slide to an origin seeking position where the tip end portion is engaged with the projection, the tip end portion is brought into contact with the projection, and the LF roller  101  can no longer rotate. And this position is stored as an origin in a main body. When origin seeking is finished, the LF slider slides to a position where the LF roller gear can rotate by its own weight. 
     (Explanation of Configuration of Driving Row) 
     Subsequently, a configuration of a driving row will be described.  FIG. 13  is a perspective view of an entire driving row for driving the PF roller  77 , the LF roller  101 , and the discharge roller  106  when seen from the rear part above.  FIGS. 14 and 15  are perspective views of the driving row for transmitting driving from the LF roller  101  to the PF roller unit  74  when seen from above. Driving of a motor  201  which is a driving source is transmitted to a discharge roller gear  204  attached to one end of the discharge roller  106  through a pinion gear  202  and an idler gear  203 . Moreover, the idler gear  203  is also connected to an LF roller gear  205  attached to one end of the LF roller  101 , and the driving from the motor  201  is also transmitted to the LF roller  101  at the same time. 
     A rotation ratio between the LF roller  101  and the discharge roller  106  is configured to be 1:1. In addition, the rotation ratio between the LF roller gear  205  and the discharge roller gear  204  is also configured to be 1:1. As configured as above, a rotation cycle of the LF roller  101  becomes equal to the rotation cycle of the discharge roller  106  and the rotation cycle of a transmission gear, and a conveying amount error caused by eccentricity of the roller also occurs with the same cycle as roller rotation. A cord wheel having slits formed at a pitch of 150 to 360 lpi is directly connected coaxially with the LF roller  101 . And the number of times and timing of passage of the slit on the cord wheel are read by an LF roller encoder sensor, and a rotation amount and a rotation speed of the driving motor are controlled. 
     On the side opposite to the driving source sandwiching the LF roller  101  between them, a PF roller driving row  210  for transmitting driving to the PF roller  77  is arranged. The PF roller driving row  210  is composed of an LF output gear  211  attached to the other end of the LF roller  101 , an idler gear  212 , a pendulum gear unit  213 , and the PF roller gear  82 . The pendulum gear unit  213  is composed of a pendulum arm  214 , a planetary gear  216 , and a transmission gear  217  attached coaxially with a sun gear  215  through a one-way clutch. The one-way clutch can transmit the driving to the transmission gear  217  when the sun gear  215  rotates in the clockwise direction in the figure. 
     If the LF roller  101  rotates normally (rotation in the CW direction in the figure), the driving is transmitted to the sun gear  215  through the LF output gear  211  and the idler gear  212 , the sun gear  215  and the pendulum arm  214  rotate in the CW direction, and the planetary gear  216  rotates in the CCW direction. The pendulum arm  214  rotates in the CW direction and is brought into contact with a stopper, not shown, and stopped. Then, the driving is transmitted to the transmission gear  217  by the one-way clutch, and the PF input gear  83  rotates in the CCW direction. 
     If the LF roller  101  rotates reversely (rotation in the CCW direction in the figure) from this state, the sun gear  215  and the pendulum arm  214  are rotated in the CCW direction, and the planetary gear  216  is stopped by a stopper, not shown, at a position meshed with the PF input gear  83 . Then, the PF input gear  83  is rotated in the CW direction by the planetary gear  216 . At this time, the transmission gear  217  rotates in the CW direction by means of the PF input gear  83 , which is made possible since the driving from the sun gear  215  is not transmitted by the action of the one-way clutch. 
     That is, a delay mechanism is provided in which, if the LF roller  101  switches from normal rotation to reverse rotation, the PF input gear  83  is stopped once, the LF roller  101  is rotated by a predetermined amount and then, starts rotation. 
     On the other hand, when the LF roller  101  switches from the reverse rotation to the normal rotation, the driving is immediately transmitted to the PF input gear  83  without a time difference of the delay time. A predetermined amount of rotation of the LF roller  101  at that time is determined by a rotation angle of the pendulum arm  214  determined by the stopper of the pendulum arm  214 . The predetermined amount in the embodiment is set so that, when the LF roller  101  is rotated by 130 degrees (11 mm in the conveying length), the driving is transmitted to the PF input gear  83 . 
     (Explanation of Operation from Sheet Feeding to Discharge) 
     Subsequently, an operation from sheet feeding to discharge will be described. Speed ratios of the print medium conveyed by the sheet feeding roller  51 , the PF roller  77 , the LF roller  101 , and the discharge roller  106  are set as follows:
         When the LF roller  101  is rotating normally:   Sheet feeding roller:PF roller:LF roller:discharge roller=0.6:0.6:1:1,   when the LF roller  101  is rotating reversely:   PF roller:LF roller:discharge roller=1:1:1.       

     First, before feeding the print medium, an origin position of the LF roller  101  is detected by the origin seeking device, and a phase of the LF roller  101  is made obtainable all the time. Subsequently, sheet feeding is started by means of normal rotation of the sheet-feeding roller  51 , the PF roller  77 , and the LF roller  101 . Then, from the bundle of the print mediums S set with a surface (that is, an image printing surface or an image reading surface) faced downward on the loading portion  11 , one uppermost print medium is separated by means of an action of the sheet feeding roller  51  and the separation bank surface  61 . The separated print medium S passes through a guide surface of a double-sided discharge flapper  114  by the sheet feeding roller  51  and enters the reversing conveying path. 
     The print medium S then reaches a nip portion between the PF roller  77  and the PF pinch roller  86  and is conveyed further to the downstream by the PF roller  77 . If the print medium S is conveyed by the predetermined amount from the PF roller nip portion, driving of the sheet feeding roller  51  is shut off and stopped. Subsequently, when the print medium S is conveyed to the downstream of the reversing conveying path by the PF roller  77 , the print medium detecting device (detecting device)  221  detects the tip end of the print medium S, detects the phase of the LF roller at that time by the phase detecting device, and stores the detected result. 
     Then, after performing the skew correcting operation of the print medium and a phase matching operation for matching the phase of the LF roller and the position of the print medium S, the print medium S is conveyed to an image forming position. The print medium S having been conveyed to the image forming position is conveyed to the downstream by the LF roller  101  and the discharge roller  106 , and an image is formed by the image forming portion  17 . When image formation is then finished, the print medium S is discharged by the discharge roller  106 . 
     Subsequently, the skew correction and the LF roller phase matching will be described by using the figures. The LF roller phase matching is performed so that a phase of the LF roller  101  when the rear end of the print medium exits the LF roller  101  becomes optimal with few conveying errors. The phase when the print medium S exits the LF roller  101  is calculated from the phase when the print medium S is bitten by the LF roller  101  and the print medium length. 
     Since he length of the print medium S is determined in advance by the print medium S to be set, the phase matching is realized by controlling the phase when the print medium S is bitten by the LF roller  101  so that the phase when the print medium S exits the LF roller  101  becomes optimal. 
       FIGS. 16 to 22  are facilitated sectional views describing a curved path of the print medium S in a manner facilitated to a straight path in order to facilitate understanding of the LF phase matching operation. A curved path as in  FIG. 2  also functions similarly. 
     First, the tip end of the print medium S having been conveyed by the PF roller  77  is detected by the print medium end portion detecting device  221  and the phase of the LF roller  101  at that time is stored at the same time. As a result, the position of the print medium S and the phase of the LF roller  101  in the conveying path can be recognized. Then, after the print medium S has been conveyed by 10 mm from the tip end detected position, the PF roller  77  and the LF roller  101  are stopped ( FIG. 16 ). 
     The tip end position of the print medium S at this time is referred to as Pos 1  and the phase of the LF roller  101  to θ 1 . Further, the LF roller  101  is rotated α 1  normally (CCW direction in the figure) and the print medium S is conveyed, and the print medium position when the tip end of the print medium S is brought into contact with the LF roller nip is referred to as Pos 2  and the phase of the LF roller to θ 2  ( FIG. 17 ). Here, a distance from the print medium end portion detecting device  221  to the nip of the LF roller  101  is set to 17 mm. 
     Further, the LF roller  101  is normally rotated α 2  (CCW direction in the figure) and when the tip end of the print medium passes 2 mm from the LF roller nip, the LF roller  101  is stopped ( FIG. 18 ). The print medium position at this time is referred to as Pos 3  and the phase of the LF roller  101  to θ 3 . Then, the LF roller  101  is reversely rotated α 3  (CW direction in the figure) only by 6 mm in the conveying length, the print medium tip end is returned to the nip of the LF roller  101 , and skew is corrected by the correcting device ( FIG. 19 ). 
     The print medium position at this time is referred to as Pos 4  and the phase of the LF roller  101  to θ 4 . Here, assuming that the diameter of the LF roller  101  is 9.6 mm and normal rotation (CCW direction in the figure) of the LF roller is +, the following is obtained from the above-described peripheral speed difference between the LF roller and the PF roller:
 
θ2=θ1+α1=θ1+197°
 
θ3=θ2+α2=θ2+24°
 
θ4=θ2−α3=θ3−71°
 
     If θ 4  is expressed as θ 1 , assuming that
 
θ4=θ1+α1+α2−α3=θ1+197+24−71=θ1+150°
 
α1+α2−α3=α,
 
θ4=θ1+α.
 
     However, since the phase is 0 to 359°, if 359° (one rotation) is exceeded, 360° is subtracted. 
     That is, the phase of Pos 4  becomes a phase advanced by 150° with respect to the phase of the LF roller  101  at Pos 1 . Here, the phase of the LF roller  101  to be aligned with the print medium tip end is set to θ 5 , and a reverse rotation amount R required for matching the LF roller  101  to θ 5  by reverse rotation (rotation in the CCW direction) from the phase of θ 4  can be expressed as follows:
 
 R=θ 4−θ5=θ1+150°−θ5
 
θ5=θ4 −R=θ 1 +α−R  
 
     The skew can be corrected by rotation by adding this reverse rotation amount R to a reverse rotation amount of skew correction, and the LF roller  101  can be matched to a predetermined phase ( FIG. 20 ). 
     However, as described above, if the LF roller  101  is reversely rotated by 130° or more, the PF roller  77  starts normal rotation and thus, by considering the reverse rotation amount at θ 4  above, the print medium S is conveyed by the PF roller  77  if R exceeds 59°, and a loop becomes larger. This is because 130° (the rotation angle of the delay)−71° (the reverse rotation angle of the LF roller at the time of the formation of the skew correcting loop)=59°. 
     A maximum allowable loop amount at this time is decided by the skew correcting capability and a sheet path. For example, if the loop becomes excessively large as in  FIG. 21 , a direction of a force acting by the loop changes, and the tip end cannot be pressed to the LF roller  101  or cannot be bitten any longer when the LF roller  101  normally rotates, and the skew correcting capability deteriorates. 
     Moreover, a problem that the loop is brought into contact with the sheet path and is buckled also occurs. Furthermore, if the reverse rotation amount of the LF roller  101  after the skew correction increases, a problem of damage on the print medium tip end also occurs. Since the allowable maximum loop amount excluding 2 mm of the skew correction in the embodiment is 7 mm, reverse rotation by 143° in addition to the reverse rotation amount of the skew correction in the LF roller  101  is possible. This is based on the equation: 360°×7 mm/(9.6 mm×π)+59°=143°. 
     Here, the maximum value of the reverse rotation amount decided by the allowable maximum loop amount is assumed to be θmax. 
     In the case where the LF roller is matched to the desired phase θ 5  at Pos 4 , the LF roller need be further reversely rotated by an angle R. Here, the LF roller cannot be reversely rotated at a greater angle than θmax. When R is equal to or smaller than θmax, the LF roller is reversely rotated at the angle R at Pos 4 , so that the phase of the LF roller can be matched to θ 5 . 
     However, in order to match the LF roller  101  to an arbitrary phase, reverse rotation of 359° at the maximum is required. Thus, if the reverse rotation amount R exceeds 143° which is θmax, the LF roller  101  is reversely rotated by R−θmax (143°) before transition from Pos 1  to Pos 2 , and the phase of θ 1  is moved to θ 1 ′ ( FIG. 22 ). Here, when R−θmax exceeds the rotation angle (130°) of the delay, the print medium is adversely conveyed. Therefore, it is the rotation angle (130°) of the delay at most that the LF roller  101  can be reversely rotated in the state in which the tip end of the print medium is positioned at Pos 1 . When R−θmax is greater than the rotation angle (130°) of the delay, the LF roller  101  is rotated by 130° at Pos 1 . In contrast, when R−θmax is equal to or smaller than the rotation angle (130°) of the delay, the LF roller  101  is reversely rotated by R−θmax at Pos 1 , so that the phase is changed to θ 1 ′. Thus, the reverse rotation amount R in the LF phase matching becomes 143°. 
     When R−θmax is equal to or smaller than 130°, since the PF roller  77  does not rotate due to the delay mechanism at this time, only the phase θ 1  of the LF roller  101  can be changed to θ 1 ′ while the print medium position Pos 1  remains as it is. When R−θmax is greater than 130°, the LF roller  101  is rotated by the rotation angle (130°) of the delay at Pos 1 . Next, the LF roller is rotated forward at the rotation angle (130°) of the delay or greater, and thus, the delay is accumulated. For example, the sheet is conveyed up to Pos 2 , that is, the LF roller is rotated by 197°, so that the delay of 130° is accumulated. Therefore, the LF roller is reversely rotated at Pos 2  by R−the rotation angle (130°) of the delay−θmax (143°)=R−273. Also here, it is the rotation angle (130°) of the delay at most that the LF roller can be reversely rotated. When the rotation angle of the delay is 130°, the LF roller  101  is reversely rotated by R−273. Also here, the LF roller can be rotated within 130°. Thus, R can be covered within 359°. 
     Namely, the rotation angle of the delay is assumed to be θdel, the LF roller is reversely rotated by θdel at Pos 1 ; by R−θdel−θmax at Pos 2 ; and by θmax at Pos 4 .
 
θ1+α−θdel−( R −θdel−θmax)−θmax=θ1 +α−R=θ 5
 
     When the rotation angle of the delay is small, and further,
 
 R −θdel−θmax&gt;θdel,
 
     the LF roller is reversely rotated at θdel a plurality of times (n times) without any reverse conveyance of the print medium until the print medium reaches Pos 2 . When
 
 R−n ×θdel−θmax&gt;θdel,
 
     the LF roller is reversely rotated by R−n×θdel−θmax, and then, it is reversely rotated by θmax at Pos 4 .
 
θ1 +α−n ×θdel−( R−n ×θdel−θmax)−θmax=θ1+α− R=θ 5
 
     The phase matching operation as above is performed once or several times. 
     Subsequently, a flow of skew correction and phase matching will be described by using a flowchart.  FIG. 23  is a flowchart illustrating a sheet feeding operation when the phase matching is performed. When the sheet feeding is started, first, in step S 1 , the origin seeking of the LF roller  101  is performed by the LF roller origin seeking device, and a current phase of the LF roller is made obtainable by slit information of the cord wheel. 
     Subsequently, in step S 2 , a target phase θ 5  of the LF roller phase matching is obtained. Then, each roller is driven in step S 3 , and sheet feeding is started. When the print medium S exceeds the PF roller  77  and the tip end thereof reaches the print medium end detecting device  221 , the print medium end portion is detected in step S 4 . At the same time, the LF roller phase at that time is detected, and the print medium end portion and the LF roller phase are made controllable. Then, at a position where the print medium tip end exceeds the print medium detection portion and is conveyed by a predetermined amount (10 mm in the embodiment), the conveying is stopped. 
     Then, in step S 5 , the LF roller phase θ 1  at this time is obtained. Subsequently, in step S 6 , the above-described LF roller phase matching rotation amount R (θ 1 +150°−θ 5 ) is calculated from the obtained θ 1  and θ 5 . After that, in step S 7 , it is determined in comparison whether or not the rotation amount R is larger than θmax determined by the allowable maximum loop amount. If R is smaller than θmax, the LF phase matching and skew correction are performed in step S 8 . 
     After that, in step S 9 , indexing is performed, and sheet feeding is finished. On the other hand, if R is larger than θmax in step S 7 , the program proceeds to step S 11 , moves the phase of the LF roller only by R−θmax by reversing the LF roller (first phase matching) and changes the phase of θ 1  so that the phase matching rotation amount R becomes smaller than θmax. After that, in step S 12 , second LF roller phase matching and skew correction are performed. Then, in step S 9 , indexing is performed, and sheet feeding is finished. 
       FIG. 24  is a block diagram illustrating an outline of printing operation control of an inkjet printer product to which the embodiment is applied when an inkjet printing apparatus is used for the image forming portion. When a control portion B 1  receives a printing instruction from a PC (B 2 ) or an operation panel B 3  or performs an operation by a timer or the like in the control portion B 1 , the control portion B 1  issues an instruction to supply power to a conveying motor B 5  connected to a conveying driving transmission system B 6  through a driver B 4 . 
     In parallel, an instruction is given to supply power also to a printing portion motor B 12  connected to a printing portion B 13  through a printing portion motor driver B 11 . Moreover, the printing portion B 13  is connected so that driving switching of a conveying driving switching/transmission system B 8  is performed by the operation thereof. Furthermore, a printing feeding roller unit and a discharge roller unit B 7  to which driving is transmitted from the conveying motor B 5  through the conveying driving transmission system B 6  are configured to be able to convey the print medium S in printing and to transmit a rotation driving force to the conveying driving switching/transmission system B 8 . 
     The conveying driving switching/transmission system B 8  transmits the driving force transmitted from the printing feeding roller unit and the discharge roller unit B 7  to a sheet feeding roller unit B 9  and an intermediate roller unit B 10  by switching presence or absence of driving transmission and a rotating direction by an operation of the printing portion B 13 . A rotation state and a load state of each motor and a conveying state of a print medium are detected by various sensors B 14  provided at each spot in a printer, and information is sent to the control portion B 1  in a form of a signal. The control portion B 1  performs printing by controlling each motor on the basis of the instruction and the sensor information. 
     (Another Embodiment) 
     The description has been given of the adjustment of the phase of the LF roller that is reversely rotated for the purpose of the skew correction in the above-described embodiment. Another description will be given below of the adjustment of the phase of the roller that is not reversely rotated. 
     In  FIGS. 25A and 25B , reference numeral  121  designates a first roller, and  122  denotes a pinch roller that holds a sheet in cooperation with the first roller  121 . Moreover, reference numeral  123  designates a second roller located at a position downstream of the first roller in the conveying direction, and  124  denotes a pinch roller that holds a sheet in cooperation with the second roller  123 . 
     Like the above-described embodiment, the first roller and the second roller are driven by a motor serving as a common driving source. In the same manner as the above-described embodiment, the rotation of the driving source is transmitted to the first roller by a transmitting device including a delay mechanism. The delay mechanism can accumulate a delay angle in such a manner that the first roller cannot be rotated even if the second roller is reversely rotated by θdmax at the maximum. 
     In  FIG. 25A , a sheet P is held between the first roller  121  and the pinch roller  122 . The tip end of the sheet P is located at a position Pos 10  upstream of a nip defined between the second roller  123  and the pinch roller  124 . The driving source for the first roller  121  and the pinch roller  122  is forward rotated at this position, so that the sheet P is conveyed until the tip end of the sheet P reaches a position Pos 11  downstream of the nip defined between the second roller  123  and the pinch roller  124 . In the case where the phase of the second roller  123  is not adjusted, the phase of the second roller is assumed to be θ 11 , and therefore, the second roller  123  is assumed to be rotated by α 10 .
 
θ11=θ10+α10
 
     Assuming that the target phase of the second roller  123  when the tip end of the sheet P reaches Pos 11  is assumed to be θT,
 
θ T=θ 11 +R 2.
 
     In the case where 360°−R 2 ≦θdmax, the second roller  123  is reversely rotated by (360°−R 2 ) when the tip end of the sheet P is located at Pos 10 , so that the phase of the second roller  123  is changed from θ10 to θ 10 ′.
 
θ10′=θ10−(360 °−R 2)
 
     Since 360°−R 2 ≦θdmax, the tip end of the sheet P remains located at Pos 10 . Next, the second roller  123  is rotated by α 10 , the tip end of the sheet P reaches Pos 11 . At this time, the phase of the second roller  123  is:
 
θ10′+α10=θ10−(360 °−R 2)+α10=θ10+α10 +R 2−360°=θ11 +R 2−360 °=θT− 360°
 
     Here, θT−360° is equal to θT from the viewpoint of the phase, and therefore, the phase of the second roller  123  has been adjusted to θT. 
     Even if the second roller  123  is reversely rotated by (360°−R 2 ) at any timings after the tip end of the sheet P is conveyed from Pos 10  and before it is held at the nip defined between the second roller  123  and the pinch roller  124 , the phase of the second roller  123  at Pos 11  can be adjusted to θT. 
     In the case where 360°−R 2 &gt;θdmax, the second roller  123  is reversely rotated by (360°−R 2 ) per delay angle θd a plurality of times (n times) until the tip end of the sheet is held at the nip defined between the second roller  123  and the pinch roller  124 . 
     For example, in the case where θd≦θdmax and 360°−R 2 =n×θd, the second roller  123  is reversely rotated by the delay angle θd n times. When the second roller  123  is further reversely rotated after the reverse rotation, the sheet is conveyed to accumulate the delay angle, followed by next reverse rotation such that the second roller  123  can be further reversely rotated within the delay angle.
 
θ10 −n×θd+α 10=θ10−(360 °−R 2)+α1 °=θT− 360 °
 
     Hence, the phase of the second roller  123  has been adjusted to θT. Incidentally, in the case where 360°−R 2 ≦θdmax, n is equal to 1. 
     The above-described embodiment can be applied to any sheet feeding device as long as a print-medium shaped printing medium or a printing medium such as a manuscript is fed one by one from the loading portion in an image forming apparatus such as a printer, a facsimile machine, a copying machine and the like regardless of its form or operating method. Moreover, when the image forming portion  17  is composed of a printing portion, various printing methods can be employed for the printing portion as long as an image is printed on a print medium by printing device on the basis of image information. 
     For example, other than an inkjet printing apparatus which performs printing by ejecting ink to a print medium from a ejection port of a printing head, any method of printing devices such as a laser beam type, a thermal transfer type, a thermal type, a wire-dot type and the like can be employed. For the printing portion, either of a serial type in which printing is performed by using a printing head mounted on a reciprocally moving carriage or a line type in which printing is performed only by vertical scanning (conveying) of a print medium by using a printing head extending in the width direction of the print medium may be used. Further, the present invention can be applied not only to an image forming apparatus having a printing portion but also similarly to other image forming apparatuses having a configuration in which a single or a plurality of devices are integrated. 
     As described above, the LF roller phase is obtained, phase matching is controlled so that the print medium comes to a desired position when the LF roller is at a desired phase, and skewing of the print medium is corrected by the same control method as the control of phase matching. As a result, a conveying device and a conveying control method which can control the phase of a main conveying roller and a position of a print medium with an inexpensive configuration and a fast throughput can be realized. 
     While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions. 
     This application claims the benefit of Japanese Patent Application No. 2011-179779, filed Aug. 19, 2011, which is hereby incorporated by reference herein in its entirety.