PATENT DOCUMENT

Abstract:
A recording apparatus including feeding unit that feeds a medium, conveying unit that conveys the fed medium, recording unit that performs recording on the medium, and controlling unit that controls the feeding unit and the conveying unit, a measuring unit that measures a distance between the previous medium and the next medium after completing the preparatory feeding; and determining unit that determines whether or not the measured distance is a predetermined distance or greater, wherein, if the distance is the predetermined distance or greater, the controlling unit completely feeds the next medium.

Full Description:
BACKGROUND 
       [0001]    1. Technical Field 
         [0002]    The present invention relates to a method of feeding a medium in a recording apparatus, which starts to feed a subsequent medium while recording is being performed on a previous medium being fed, and to a recording apparatus. 
         [0003]    2. Related Art 
         [0004]    A printer, which is a known example of recording apparatuses includes an auto sheet feeder (hereinafter, referred to as ASF) (for example, JP-A-2003-72964 or the like). When printing starts, the ASF is driven to feed an uppermost sheet from among sheets stacked in a cassette, and a leading end of the sheet is positioned at a printing start position. 
         [0005]    The ASF starts to feed a subsequent sheet after a previous sheet has been printed and discharged. In the feeding method which starts to feed the subsequent sheet after the previous sheet has been printed, however, a relatively long standby time is present between the start of discharge of the previous sheet and the start of printing of the subsequent sheet. Accordingly, printing throughput is deteriorated. 
         [0006]    In order to solve this problem, JP-A-2003-72964 discloses a recording apparatus that simultaneously performs a discharge operation of a previous sheet and a feeding operation of a subsequent sheet while maintaining a predetermined gap between the previous sheet and the subsequent sheet. That is, in the recording apparatus of JP-A-2003-72964, the position of a trailing end of the previous sheet is calculated on the basis of a transport distance of the previous sheet and sheet length data. Then, if two conditions that the trailing end of the previous sheet has passed through a specified position and a discharge command has been received are satisfied, the feeding operation of the subsequent sheet starts. According to this recording apparatus, the discharge operation of the previous sheet and the feeding operation of the subsequent sheet are simultaneously performed, while an inter-paper distance between the previous sheet and the subsequent sheet is ensured. Therefore, a standby time from the start of discharge of the previous sheet and the start of printing of the subsequent sheet can be shortened, and as a result printing throughput can be improved. 
         [0007]    JP-A-2005-22792 (paragraphs [0029] to [0054]) discloses a sheet feeding device in which a leading end of a subsequent sheet is positioned in front of a feed/separation roller beforehand. In this case, before an instruction to control a feeding operation of the subsequent sheet is input, a pickup roller is driven to start a preliminary feeding operation. Then, if a pre-separation sensor detects a leading end of the subsequent sheet fed by the preliminary feeding operation, the pickup roller is stopped. In this sheet feeding device, if a post-separation sensor detects that the previous sheet has passed through the feed/separation roller, a control device starts to drive the pickup roller and the feed/separation roller. 
         [0008]    JP-A-2001-278472 and JP-A-2002-145469 disclose a page printer in which, in order to improve throughput, a feeding operation of a next page starts before recording on a previous page is completed (so-called preceding feeding). 
         [0009]    According to the recording apparatus of JP-A-2003-72964, if recording is performed to the end of the previous sheet (recordable last row), the discharge command may be received a long time after transporting of the previous sheet was started. For this reason, a gap between the previous sheet and the subsequent sheet exists, and printing throughput is deteriorated. 
         [0010]    In the recording apparatus of JP-A-2005-22792, after the subsequent sheet is preliminary fed, the feeding operation of the subsequent sheet starts when the post-separation sensor detects the passage of the previous sheet. The gap between the previous sheet and the subsequent sheet is defined by a gap between the pre-separation sensor and the post-separation sensor. The inter-sensor gap is not necessarily identical to a gap which should be ensured between the previous sheet and the subsequent sheet. For this reason, at some positions of the sensors in the recording apparatus, when the feeding operation of the subsequent sheet starts on the basis of the instruction to control the feeding operation, a necessary gap between the previous sheet and the subsequent sheet may not be ensured. As described in the JP-A-2003-72964, an insufficient inter-paper gap results in a paper detection sensor not being able to detect the leading end of the subsequent sheet, and accordingly, it is difficult to manage the transport position of the subsequent sheet. 
       SUMMARY 
       [0011]    An advantage of some aspects of the invention is that it provides a method of feeding a medium in a recording apparatus, which is capable of preventing a delay of start of a transport operation while maintaining a gap between a previous medium and a subsequent medium, thereby preventing throughput from being deteriorated, and a recording apparatus. 
         [0012]    According to an aspect of the invention, a recording apparatus including feeding unit that feeds a medium, conveying unit that conveys the fed medium, recording unit that performs recording on the medium, and controlling unit that controls the feeding unit and the conveying unit, wherein, when a rear edge of a previous medium which is previously fed reaches a preparatory feed start position, the controlling unit drives the feeding unit to preparatorily feed a next medium until when a front edge of the next medium reaches a target position, the recording apparatus characterized by comprising: measuring unit that measures a distance between the previous medium and the next medium after completing the preparatory feeding; and determining unit that determines whether or not the measured distance is a predetermined distance or greater, wherein a conveying distance between the preparatory feed start position and the target position and the predetermined distance satisfy a relationship of the conveying distance&lt;the predetermined distance, and wherein, if the distance is the predetermined distance or greater, the controlling unit completely feeds the next medium when the controlling unit performs a conveying operation of the previous medium, and if the distance is smaller than the predetermined distance, the controlling unit does not completely feed the next medium when the controlling unit performs the conveying operation of the previous medium. Herein, the recording operation includes an operation of the recording unit to perform recording onto the medium and an operation to transport the medium. Moreover, “alternation of recording and transport” is a concept including a case in which the recording operation of the recording unit and the transport operation of the medium are alternately performed, and a case in which the recording operation and the transport operation are substantially alternately performed but partially temporally overlap each other. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    The invention will be described with reference to the accompanying drawings, wherein like numbers reference like elements. 
           [0014]      FIG. 1  is a perspective view of a printer according to an embodiment of the invention. 
           [0015]      FIG. 2  is a schematic side sectional view showing an auto sheet feeder and a paper transport mechanism. 
           [0016]      FIG. 3  is a schematic side view of a feeder for explaining constants to be used to calculate an inter-paper distance. 
           [0017]      FIG. 4  is a block diagram showing the electrical configuration of the printer. 
           [0018]      FIG. 5  is a timing chart showing a feed control processing for ensuring an inter-paper distance. 
           [0019]      FIG. 6  is a timing chart showing a feed control processing for ensuring an inter-paper distance. 
           [0020]      FIG. 7  is a timing chart showing a feed control processing for ensuring an inter-paper distance. 
           [0021]      FIG. 8  is a timing chart showing a feed control processing for ensuring an inter-paper distance. 
           [0022]      FIG. 9  is a flowchart showing a printing processing. 
           [0023]      FIG. 10  is a flowchart showing a feed control processing (paper transport processing). 
           [0024]      FIG. 11  is a flowchart showing a feed control processing (paper transport processing). 
       
    
    
     DESCRIPTION OF EXEMPLARY EMBODIMENTS 
       [0025]    Hereinafter, an embodiment in which the invention is embodied will be described with reference to  FIGS. 1 to 11 . 
         [0026]      FIG. 1  is a perspective view of a printer according to this embodiment. As shown in  FIG. 1 , a printer  11  which is an example of recording apparatuses has a rectangular boxlike main body  12 . A carriage  13  is provided in a central portion of the main body  12  so as to freely reciprocate in a main scanning direction (left-right direction in  FIG. 1 ) along a guide shaft  14 . 
         [0027]    As shown in  FIG. 1 , a long plate-shaped platen  15  is disposed at a lower position opposing the carriage  13  in the main body  12 . In a lower portion on a front surface of the printer  11  (a surface on a near side in  FIG. 1 ), a sheet feeding cassette  16  is detachably mounted in a concave mounting portion  12 A. A sheet feeding tray  17  is provided in an upper portion on a rear surface of the main body  12 . In this embodiment, the printer selectively performs a feeding operation from the sheet feeding cassette  16  in the front portion thereof and a feeding operation from the sheet feeding tray  17  in the rear portion thereof. 
         [0028]    A plurality of ink cartridges  18  are loaded in a cover  12 B which covers a front right surface of the main body  12 . Ink in the ink cartridges  18  is supplied to the carriage  13  through a plurality of ink supply tubes (not shown) which are provided in a flexible wiring board  19 , and ink droplets are ejected (discharged) from a recording head  20  (shown in  FIG. 2 ) which is provided below the carriage  13 . In the recording head  20 , a pressurization element (piezoelectric element, electrostatic element, or heater element) for applying an ejection pressure to ink is incorporated in each nozzle. If a predetermined voltage is applied to the pressurization element, ink droplets are ejected (discharged) from the corresponding nozzle. 
         [0029]    During printing, ink droplets are ejected from the recording head  20  onto a sheet fed from the sheet feeding cassette  16  and positioned on the platen  15  while the carriage  13  is reciprocating, and thus printing for one line is performed. After printing for one line is completed, the sheet is transported to a printing position of a next row. In this way, a printing operation achieved by one scanning operation of the carriage  13  and a paper transport operation to transport the sheet to the printing position of the next row are alternately performed, thereby performing printing on the sheet. Various operating switches  21  including a power switch are provided in a lower portion on a front left surface of the main body  12 . The printing operation and the paper transport operation may be temporally independently performed. In this embodiment, the printing operation and the paper transport operation are performed such that the other operation starts before one operation is completed, and the operations partially overlap each other at the start and end of the operations. 
         [0030]      FIG. 2  is a side view showing the overall configuration of the printer. Hereinafter, the overall configuration of the printer  11  will be described in detail with reference to  FIG. 2 . The printer  11  includes a rear feeder  22  in the rear portion thereof and a front feeder  23  in the bottom portion thereof. A sheet P (mainly, single sheet) serving as a recording medium is fed from one of the two feeders  22  and  23  to a pair of transport rollers  25 . The sheet P is transported to a recording section  24  by the pair of transport rollers  25 , and after recording is performed, is discharged to a stacker (not shown) by a pair of discharge rollers  26 . 
         [0031]    Hereinafter, the components on a paper transport path will be further described in detail. 
         [0032]    The rear feeder  22  includes a hopper  31 , a feed roller  32 , a retard roller  33 , and a sheet returning lever  34 . The hopper  31  pivots around a pivot fulcrum  31   a  in an upper portion thereof, and is switched between a posture in which the sheet P obliquely supported by the hopper  31  is pressed against the feed roller  32 , and a posture in which the sheet P is positioned away from the feed roller  32 . 
         [0033]    The retard roller  33  is provided to have predetermined rotation resistance, and forms a nip point with the feed roller  32  to separate an uppermost sheet P to be fed from a next sheet P. The sheet returning lever  34  is rotatably provided, when a sheet feeding path is viewed in side view. The next sheet P separated by the retard roller  33  is returned to an upstream side by the rotation of the sheet returning lever  34 . 
         [0034]    The front feeder  23 , which is provided in the bottom of the printer  11  and in which the sheet is set from the front side of the printer  11 , includes the sheet feeding cassette  16 , a pickup roller  35 , an intermediate roller  36 , a retard roller  37  serving as a separation unit, a sheet returning lever  38 , and an assist roller  39 . 
         [0035]    A plurality of sheets P (a maximum number of sheets ranging from 300 to 800) are stacked in the sheet feeding cassette  16  which is mounted on and removed from the front side, and the sheets P are delivered from the sheet feeding cassette  16  by the pickup roller  35 , which is driven by an ASF motor  54  (see  FIG. 4 ), one by one starting from the uppermost one. The pickup roller  35  is provided in a pivot member  40  which pivots around a pivot shaft  40   a . When the pivot member  40  pivots while being urged toward the sheet by an urging unit (not shown), the pickup roller  35  is in constant contact with the uppermost sheet. The height of the pickup roller  35  in contact with the uppermost sheet from among the sheets stacked in the sheet feeding cassette  16  changes depending on a residual sheet amount, and accordingly the pivot member  40  pivots around the pivot shaft  40   a  between a highest position when a maximum number of sheets are loaded and a lowest position when a minimum number of sheets are loaded, as indicated by two-dot-chain lines in  FIG. 2 . As described above, in this embodiment, if a relatively large number of sheets are loaded in the sheet feeding cassette  16 , a paper feeding distance is different by a distance corresponding the thickness of a maximum number of sheets between when a sheet is fed at a position where the pickup roller  35  is in contact with the top surface of the uppermost sheet from among the maximum number of sheets and when a sheet is fed at a position where the pickup roller  35  is in contact with the top surface of a last sheet in the sheet feeding cassette  16 . 
         [0036]    The sheet P which is delivered by the pickup roller  35  constituting a feed unit is preliminarily separated by a separation inclined surface  16   a,  and travels toward the retard roller  37 . The retard roller  37  is provided at a position opposing a peripheral surface of the intermediate roller  36  so as to advance and retreat with respect to the intermediate roller  36 . When the sheet is delivered from the sheet feeding cassette  16 , the retard roller  37  is pressed against the intermediate roller  36  so as to form the nip point, such that the uppermost sheet P (previous page) to be fed and a next sheet P are separated from each other. 
         [0037]    The sheet returning lever  38  is rotatably provided, when the paper feeding path is viewed in side view, such that when the sheet returning lever  38  rotates, the nip point of the intermediate roller  36  and the retard roller  37  falls within the trace of a leading end of the lever. At a feeding standby position, the sheet returning lever  38  takes a posture in which the leading end thereof protrudes toward the feeding path, as indicated by a solid line in  FIG. 2 . When the sheet P is fed, the sheet returning lever  38  rotates to a position indicated by a two-dot-chain line in a clockwise direction in  FIG. 2 , and retreats from the paper feeding path to open the paper feeding path. When a predetermined time (or predetermined distance) elapses after the paper feeding operation starts, the sheet returning lever  38  rotates to a position indicated by the solid line in a counterclockwise direction of  FIG. 2 , that is, rotates in a direction to close the paper feeding path. Accordingly, the leading end of the next sheet at the nip point between the retard roller  37  and the intermediate roller  36  is returned to the upstream side (the sheet feeding cassette  16 ). 
         [0038]    The intermediate roller  36  which constitutes a transport unit for further delivering the sheet P fed by the pickup roller  35  to the downstream side, together with the pair of transport rollers  25 , is driven by a PF motor  53  (shown in  FIG. 4 ), flexes and inverts the sheet to be fed, and delivers the sheet P to the pair of transport rollers  25  on the downstream side. The assist roller  39  is in contact with the intermediate roller  36  to assist the transport of the sheet P to the downstream side by the intermediate roller  36 . 
         [0039]    The pair of transport rollers  25  includes a transport driving roller  41  that is rotated by the PF motor  53  ( FIG. 4 ), and a transport driven roller  42  that is rotated while being pressed against the transport driving roller  41  when the transport driven roller  42  rotates. The sheet P whose leading end has reached the pair of transport rollers  25  is transported to the recording section  24  on the downstream side by the rotation of the transport driving roller  41  while being nipped by the transport driving roller  41  and the transport driven roller  42 . 
         [0040]    The recording section  24  includes a recording head  20  that ejects ink onto the sheet P, and a platen  15  that supports the sheet P to restrict a distance between the sheet P and the recording head  20 . The recording head  20  is provided in a bottom portion of the carriage  13 . The carriage  13  is driven to reciprocate in a main scanning direction by a carriage motor  52  (see  FIG. 4 ) while being guided by a guide shaft  14  extending in the main scanning direction (a direction perpendicular to the paper plane of  FIG. 2 ). In this example, a so-called off-carriage type in which the ink cartridges  18  are provided in the main body  12  is used, but a so-called on-carriage type in which the ink cartridges are mounted on the carriage may be used. 
         [0041]    A pair of discharge rollers  26  provided on the downstream side of the recording section  24  includes a discharge driving roller  43  that is rotated by the PF motor  53  ( FIG. 4 ), and a discharge driven roller  44  that is in contact with the discharge driving roller  43  and is rotated when the discharge driving roller  43  rotates. The sheet P on which recording was performed by the recording section  24  is discharged to a stacker (not shown) provided on the front side of the printer  11  by the rotation of the discharge driving roller  43  while being nipped by the discharge driving roller  43  and the discharge driven roller  44 . 
         [0042]      FIG. 3  is a schematic view of an auto paper feeder (front feeder) and a transport device as viewed from a side surface. In the printer  11  of this embodiment, inter-page control processing is performed in which, while a gap between a previous sheet P 1  serving as a previous medium and a subsequent sheet P 2  serving as a subsequent medium is maintained small, a feeding operation of the subsequent sheet P 2  is performed during performance of a recording operation on the previous sheet P 1 . Hereinafter, various positions and distances to be defined in the inter-page control processing will be described with reference to  FIG. 3 . The previous sheet P 1  indicates a first sheet from among two sheets P to be successively fed during multi-sheet printing, and the subsequent sheet P 2  indicates a second sheet to be fed subsequent to the previous sheet P 1 . 
         [0043]    In a paper transport path with the nip point interposed between the intermediate roller  36  and the retard roller  37 , a trailing end sensor  45  is provided at a position on a downstream side to detect a trailing end of the previous sheet P 1 , and a leading end sensor  46  is provided at a position on an upstream side to detect a leading end of the subsequent sheet P 2 . The distance between the trailing end sensor  45  and the leading end sensor  46  in the transport path is set to A (mm) (for example, a value ranging from 10 to 30 mm). 
         [0044]    A paper detection sensor  47  is provided at a predetermined position between the assist roller  39  and the pair of transport rollers  25  in the paper transport path. The paper detection sensor  47  is positioned opposing the transport path of the sheet P to be fed from the rear feeder  22  (see  FIG. 2 ) or the front feeder  23 , and detects the leading end and the trailing end of the sheet P. In this embodiment, the trailing end sensor  45 , the leading end sensor  46 , and the paper detection sensor  47  are formed of non-contact sensors, such as optical sensors. An optical sensor includes a pair of a photoreceiver and a phototransmitter. When light emitted from the phototransmitter is shielded by the sheet P and not received by the photoreceiver, a state “paper present” is detected, and when light is not shielded by the sheet P and is received by the photoreceiver, a state “paper absent” is detected. The sensors  45  to  47  are not limited to non-contact sensors, but at least one of them may be changed to a contact sensor. 
         [0045]    The trailing end sensor  45  detects the trailing end of the sheet P (the previous sheet P 1 ) when a detection state is switched from “paper present” to “paper absent”. The leading end sensor  46  detects the leading end of the sheet P (the subsequent sheet P 2 ) when a detection state is switched from “paper absent” to “paper present”. The paper detection sensor  47  detects the leading end of the sheet P (the previous sheet P 1 ) when a detection state is switched from “paper absent” to “paper present”, and detects the trailing end of the sheet P (the previous sheet P 1 ) when the detection state is switched from “paper present” to “paper absent”. 
         [0046]    In the printer  11  of this embodiment, a plurality of printing modes are set. Of these, in a fast printing mode (a draft printing mode), paper feed control is used in which, if the previous sheet P 1  has been transported to a prescribed position, a feeding operation of the subsequent sheet P 2  starts even though printing is being performed on the previous sheet P 1 . That is, if the trailing end of the previous sheet P 1  is detected by the trailing end sensor  45 , the pickup roller  35  is driven to start the feeding operation of the subsequent sheet P 2 . Then, the subsequent sheet P 2  is stopped at a position a prescribed distance B (mm) (for example, a value ranging 0 to 10 mm) more advanced from a position at which the leading end is detected by the leading end sensor  46 . The prescribed distance B (mm) is set such that the leading end of the subsequent sheet P 2  is not nipped between the intermediate roller  36  and the retard roller  37 . When the trailing end of the previous sheet P 1  is detected by the trailing end sensor  45 , and a preliminary feeding operation of the subsequent sheet P 2  starts, the retard roller  37  is in contact with the intermediate roller  36 , and the sheet returning lever  38  rotates from a closed position indicated by the solid line in  FIG. 2  to an open position indicated by the two-dot-chain line. 
         [0047]    In this embodiment, an inter-paper distance Lg between the previous sheet P 1  and the subsequent sheet P 2  is ensured by a prescribed amount K longer than a distance (A-B) mm. For this reason, there is a case in which the inter-paper distance Lg between the subsequent sheet P 2  preliminarily fed and the previous sheet P 1  does not meet the prescribed amount K. For this reason, in this embodiment, after the preliminary feeding operation, the inter-paper distance Lg is calculated before the previous sheet P 1  is next transported (paper transport), and it is determined whether or not the condition Lg≧K is satisfied. If the condition Lg≧K is not satisfied, during the next transport operation, only the previous sheet P 1  is transported while the subsequent sheet P 2  is stopped. If the condition Lg≧K is satisfied, when the previous sheet P 1  is transported, the subsequent sheet P 2  is fed by the same distance. After the condition Lg≧K is satisfied, each time the previous sheet P 1  is transported, the subsequent sheet P 2  is fed by the same distance while maintaining the inter-paper distance Lg. In this way, the inter-paper distance Lg between the sheets P 1  and P 2  is ensured by the prescribed amount K or more, and thus the leading end of the subsequent sheet P 2  can be reliably detected by the paper detection sensor  47 . Therefore, if a subsequent transport distance is counted on the basis of the detection position of the leading end of the subsequent sheet P 2 , a transport position of the subsequent sheet P 2  can be grasped. 
         [0048]    The feeding operation of the subsequent sheet P 2  does not start immediately when the condition Lg≧K is established during the paper transport operation, but the feeding operation of the subsequent sheet P 2  starts after the next transport operation of the previous sheet P 1  starts. The reason is as follows. If the pickup roller  35  is driven during the paper transport operation in which the intermediate roller  36  is rotating at a predetermined speed, a difference in speed occurs between a portion of the subsequent sheet P 2  nipped between the intermediate roller  36  and the retard roller  37  and a portion of the subsequent sheet P 2  in contact with the pickup roller  35  being accelerated on the upstream side in the feeding direction. This difference in speed may cause the subsequent sheet P 2  being fed to be pulled between the portions and the subsequent sheet P 2  may be damaged. In order to solve this problem, the feeding operation of the subsequent sheet P 2  starts at the same timing as the timing at which the paper transport operation of the previous sheet P 1  starts. 
         [0049]    Next, the electrical configuration of a printer having an auto paper feeder will be described with reference to  FIG. 4 . 
         [0050]    As shown in  FIG. 4 , the printer  11  includes a control section  50  that performs various kinds of control. The control section  50  is communicably connected to a host computer  48  (PC) through an interface  51 , and controls the printer  11  on the basis of print data received from the host computer  48 . 
         [0051]    The control section  50  is connected to the carriage motor  52 , the PF motor  53  (paper transport motor), the ASF motor  54  (automatic feeding motor), and a sub motor  55  (ASF-SUB motor) as an output system. The control section  50  is also connected to a linear encoder  56 , encoder  57  and  58 , the trailing end sensor  45 , the leading end sensor  46 , and the paper detection sensor  47  as an input system. 
         [0052]    The control section  50  includes a controller  60 , a head driver  61 , and motor drivers  62 ,  63 ,  64 , and  65 . The controller  60  drives the recording head  20  on the basis of print data through the head driver  61 , and draws an image or a document based on print data by dots of ink droplets. The controller  60  drives the carriage motor  52  through the motor driver  62 , and controls the movement of the carriage  13  in the main scanning direction. At this time, input pulses from the linear encoder  56  are counted by a counter (not shown), and accordingly the controller  60  grasps a movement position of the carriage  13  with respect to an origin position (home position). The input pulses from the linear encoder  56  are also used to generate an ejection timing signal of the recording head  20 . 
         [0053]    The controller  60  also drives the PF motor  53  through the motor driver  63 . An output shaft of the PF motor  53  is connected to the transport driving roller  41 , the discharge driving roller  43 , and the intermediate roller  36  through a series of wheels (not shown) so as to transmit power to them. If the PF motor  53  is forward driven, the transport driving roller  41 , the discharge driving roller  43 , and the intermediate roller  36  are rotated in the paper transport direction. If the PF motor  53  is reversely driven, the transport driving roller  41  and the discharge driving roller  43  are reversely driven due to the action of a clutch  66 , but the intermediate roller  36  is not reversely driven. 
         [0054]    The controller  60  also drives the ASF motor  54  through the motor driver  64 . An output shaft of the ASF motor  54  is connected to the feed roller  32  and the pickup roller  35  through a series of wheels (not shown) so as to transmit power to them. A clutch  67  is interposed in a power transmission path between the ASF motor  54  and each of the rollers  32  and  35 . When the ASF motor  54  is driven, a selected one of the rollers  32  and  35  is rotated in the paper feeding direction due to the movement of the clutch  67 . Therefore, if the ASF motor  54  is forward driven, one of the feed roller  32  and the pickup roller  35  selected by the clutch  67  is rotated in the paper feeding direction. 
         [0055]    The controller  60  also drives the sub motor  55  through the motor driver  65 . An output shaft of the sub motor  55  is connected to the hopper  31  and the retard rollers  33  and  37  through a series of wheels (not shown) so as to transmit power to them. When the sub motor  55  is driven, one of a power transmission path of the rear feeder  22  and a power transmission path of the front feeder  23  is selected on the basis of the movement of a clutch  68 . If the power transmission path of the rear feeder  22  is selected, the sub motor  55  is forward/reversely driven by a predetermined amount. Then, the hopper  31 , the retard roller  33 , and the sheet returning lever  34  are driven between a retreat position and a feeding position. If the power transmission path of the front feeder  23  is selected, the retard roller  37  and the sheet returning lever  38  are driven from the retreat position to the feeding position when the sub motor  55  is forward driven by a predetermined amount. Meanwhile, when the sub motor  55  is reversely driven by a predetermined amount, the retard roller  37  and the sheet returning lever  38  are driven from the feeding position to the retreat position. 
         [0056]    During printing, a user can activate a printer driver (not shown) in the host computer  48  to select the rear (sheet feeding tray) and the front (sheet feeding cassette) as a sheet feeding source by an operation of an input device. The controller  60  receives, from the host computer  48 , print data which includes information regarding the selected sheet feeding source as one of printing conditions. The controller  60  controls a driving system to select the designated sheet feeding source on the basis of print data. That is, the controller  60  selects the connection states of the clutches  66  to  68  to select a sheet feeding source to be driven from among the rear feeder  22  and the front feeder  23 . 
         [0057]    The printer driver of the host computer  48  acquires various printing parameters, such as sheet size, sheet type, and layout, which are set by an operation of the user with the input device, and if an instruction to perform printing is received, generates printing image data by predetermined processing, such as resolution conversion, color conversion, halftone, and rasterization. Then, a command is attached to a header with printing image data as a body, thereby generating print data. The header includes various printing parameters starting with sheet type and sheet feeding source designation information, as well as the command. 
         [0058]    The controller  60  includes a head controller  71 , a carriage controller  72 , a transport controller  73 , a paper feed controller  74 , a first controller  75 , a second controller  76 , a third controller  77 , a PF counter  78 , an ASF counter  79 , a trailing end detection state monitoring section  80 , a leading end detection state monitoring section  81 , a paper feed start condition determining section  82 , an inter-paper distance calculator  83 , a motor driving state determining section  84 , a paper feed driving condition determining section  85 , and a memory  86 . The controller  60  includes, for example, a CPU, an ASIC (Application Specific IC (specific-use IC)), a ROM, a RAM, a nonvolatile memory, and the like. The controller  60  is configured such that the CPU executes a program which is stored in the ROM, and shown in flowcharts of  FIGS. 9 to 11 . The controller  60  is not limited to software. For example, the controller  60  may be formed of hardware, such as an electronic circuit (for example, a custom IC), or a combination of software and hardware. 
         [0059]    The head controller  71  drives the recording head  20  through the head driver  61 . The carriage controller  72  drives the carriage motor  52  through the motor driver  62 . 
         [0060]    The first to third controllers  75  to  77  are a control section for a paper transport system. The first controller  75  drives the PF motor  53  through the motor driver  63 . The second controller  76  drives the ASF motor  54  through the motor driver  64 . The third controller  77  drives the sub motor  55  through the motor driver  65 . 
         [0061]    The rotation of the PF motor  53  is detected by the encoder  57  (rotary encoder), and a detection signal (encoder signal) is input to the PF counter  78 . The PF counter  78  counts pulse edges of the encoder signal, and obtains a value corresponding to a paper transport amount with a sheet position during reset as an origin. 
         [0062]    The rotation of the ASF motor  54  is detected by the encoder  58  (rotary encoder), and a detection signal (encoder signal) is input to the ASF counter  79 . The ASF counter  79  counts pulse edges of the encoder signal, and obtains a value corresponding to a paper transport amount with a sheet position during reset as an origin. 
         [0063]    The trailing end detection state monitoring section  80  monitors on the basis of a detection signal input from the trailing end sensor  45  whether or not the trailing end sensor  45  detects the trailing end of the previous sheet P 1 . Specifically, the trailing end detection state monitoring section  80  monitors whether or not the detection state of the trailing end sensor  45  is switched from “paper present” to “paper absent”, and if the detection state is switched to “paper absent”, changes a monitoring flag from “0” to “1”. The leading end detection state monitoring section  81  monitors on the basis of a detection signal input from the leading end sensor  46  whether or not the leading end sensor  46  detects the leading end of the subsequent sheet P 2 . Specifically, the leading end detection state monitoring section  81  monitors whether or not the detection state of the leading end sensor  46  is switched from “paper absent” to “paper present”, and if the detection state is switched to “paper present”, changes a monitoring flag from “0” to “1”. 
         [0064]    The paper feed start condition determining section  82  inputs the monitoring results (monitoring flags) of the trailing end detection state monitoring section  80  and the leading end detection state monitoring section  81 . In this embodiment, when the detection state of the trailing end by the trailing end sensor  45  is switched from “paper present” to “paper absent” during the paper transport operation of the previous sheet P 1 , the feeding operation of the subsequent sheet P 2  starts. On the other hand, there may be a case in which, during the feeding operation of the previous sheet P 1 , the subsequent sheet P 2  is double fed. In this embodiment, when double feeding occurs, the leading end of the subsequent sheet P 2  is in contact with the sheet returning lever  38  in the closed position, and thus the position of the subsequent sheet P 2  is restricted. In this case, however, the subsequent sheet P 2  already passes by the preliminary feeding position (target position) (in  FIG. 3 , a position by a distance B away from the leading end sensor  46 ). For this reason, even though double feeding occurs and the trailing end of the previous sheet P 1  is detected, the preliminary feeding operation of the subsequent sheet P 2  is not performed. 
         [0065]    The paper feed start condition determining section  82  determines whether or not to permit or inhibit the preliminary feeding operation of the subsequent sheet P 2 . That is, if the monitoring flag from the trailing end detection state monitoring section  80  is changed from “0” to “1”, the paper feed start condition determining section  82  starts the determination processing. If the monitoring flag from the leading end detection state monitoring section  81  is “0” (leading end non-detection state), it is determined that a preliminary feeding start condition is established. If the monitoring flag is “1” (leading end detection state) it is determined that the preliminary feeding start condition is not established. 
         [0066]    The paper feed start condition determining section  82  sends the determination result to the paper feed controller  74 . The paper feed controller  74  selects one of the second and third controllers  76  and  77  as a destination of a motor driving instruction in accordance with the determination result. That is, if the preliminary feeding start condition is not established, the motor driving instruction is not output to the second controller  76 , and the preliminary feeding operation of the subsequent sheet P 2  is inhibited. If the preliminary feeding start condition is established, the motor driving instruction is output to both the second and third controllers  76  and  77  to start the preliminary feeding operation of the subsequent sheet P 2 . For this reason, if the preliminary feeding start condition is established, the second controller  76  drives the ASF motor  54 , and the pickup roller  35  is forward driven in the feeding direction. In addition, the third controller  77  drives the sub motor  55 . Accordingly, the sheet returning lever  38  is driven from the closed position (feeding restriction position) to the open position (feeding permission position), and the retard roller  37  is driven from the retreat position to the feeding position. 
         [0067]    The paper feed controller  74  performs control the start and stop of the preliminary feeding operation. That is, after the preliminary feeding operation starts, the paper feed controller  74  monitors the flag of the leading end detection state monitoring section  81 . Then, if the leading end sensor  46  detects the leading end of the subsequent sheet P 2  and the detection state of the leading end sensor  46  is switched from “paper absent” to “paper present” (that is, if the flag is changed from “0” to “1”), the paper feed controller  74  resets the ASF counter  79 . In addition, if the count value of the ASF counter  79  has reached a value corresponding to the prescribed distance B, in order to stop the feeding operation of the subsequent sheet P 2 , the paper feed controller  74  transmits an instruction to stop motor driving to the second controller  76 . For this reason, the subsequent sheet P 2  is stopped when the leading end thereof passes through the detection position of the leading end sensor  46  by the prescribed distance B (mm). The third controller  77  is stopped when the sheet returning lever  38  is driven to the feeding permission position and the retard roller  37  is driven to the feeding position. 
         [0068]    When the subsequent sheet P 2  is positioned at the feeding standby position (target position), it is determined in advance whether or not a main feeding start condition is established on which the transport operation of the previous sheet P 1  and the feeding operation of the subsequent sheet P 2  can be simultaneously performed during the next transport operation. If the main feeding start condition is established, the feeding operation is performed simultaneously with the next transport operation. The determination regarding whether or not the main feeding start condition is established is performed on the basis of the calculation value of the inter-paper distance Lg. For this calculation, the inter-paper distance calculator  83  is provided. The inter-paper distance calculator  83  calculates the inter-paper distance Lg on the basis of the count value of the PF counter  78 , the count value of the ASF counter  79 , and the set value stored in the memory  86 . The memory  86  stores various kinds of set data, such as the transport distance between the trailing end sensor  45  and the leading end sensor  46  and the like, which are used to calculate the inter-paper distance. 
         [0069]    When the transport controller  73  is requested to perform the next transport operation of the previous sheet, if the ASF motor  54  is not being driven, inter-paper distance calculation is performed immediately before the next transport operation starts. If the inter-paper distance Lg of a prescribed amount C or more is ensured, the main feeding operation is performed during the next transport operation. When the transport controller  73  is requested to perform the next transport operation of the previous sheet, if the ASF motor  54  is being driven, the inter-paper distance Lg is not calculated, and the transport operation of the previous sheet P 1  is immediately performed, without waiting for until the preliminary feeding operation of the subsequent sheet P 2  is stopped. In any cases, the time when the paper transport processing of the previous sheet is defined based on when the transport controller  73  is requested to perform the next transport operation of the previous sheet. A feeding distance until the subsequent sheet P 2  reaches the feeding standby position varies depending on the number of sheets in the sheet feeding cassette  16  at the time of the start of the feeding operation. That is, when a small number of sheets remain in the sheet feeding cassette  16 , as shown in  FIGS. 2 and 3 , an uppermost sheet is supplied from a low position close to the bottom of the sheet feeding cassette  16 . Accordingly, as shown in  FIG. 2 , the feeding distance extends extra (for example, 40 to 80 mm), as compared with an uppermost sheet from among a substantially maximum number of sheets stacked in the sheet feeding cassette  16  near the maximum number of sheets. In such a case, the trailing end of the previous sheet P 1  is detected by the trailing end sensor  45  during the transport operation of the previous sheet P 1 , and the transport operation ends when the preliminary feeding operation of the subsequent sheet P 2  starts. Accordingly, even though it comes a time to start the next transport operation, that is, even though it comes a time to calculate the inter-paper distance, the preliminary feeding operation of the subsequent sheet P 2  may be still continuing. In this case, the position (stop position) of the subsequent sheet P 2  during the preliminary feeding operation is not fixed, and as a result, the inter-paper distance Lg cannot be calculated. 
         [0070]    For this reason, in this embodiment, the driving state of the ASF motor  54  is monitored, and if the ASF motor  54  is being driven even though it comes a time to calculate the inter-paper distance Lg, the transport operation of the previous sheet P 1  immediately start without waiting for until the preliminary feeding operation of the subsequent sheet P 2  is stopped. When it comes a time to calculate the inter-paper distance Lg, the motor driving state determining section  84  determines the driving state of the ASF motor  54 . Before the next transport operation, if it is determined that the ASF motor  54  is stopped, the motor driving state determining section  84  transmits a calculation start instruction to the inter-paper distance calculator  83 . Meanwhile, if the ASF motor  54  is being driven and is not stopped until a predetermined time limit in the next transport operation reaches, the calculation start instruction is not transmitted. For this reason, if the instruction to start calculation is not received until the predetermined time limit elapses, the inter-paper distance calculator  83  does not calculate the inter-paper distance Lg. 
         [0071]    If the preliminary feeding start condition is established and the preliminary feeding operation is performed, or the preliminary feeding start condition is not established and the preliminary feeding operation is not performed is indicated by the determination signal from the paper feed start condition determining section  82 , the inter-paper distance calculator  83  changes a computational expression to be used to calculate the inter-paper distance Lg according to the details. That is, when the preliminary feeding operation is performed, a first computational expression is used on an assumption that the leading end of the subsequent sheet P 2  is at the feeding standby position. Meanwhile, when the preliminary feeding operation is not performed, a second computational expression is used on an assumption that the leading end of the subsequent sheet P 2  is at the feeding restriction position at which it is in contact with the sheet returning lever  38  in the closed state. The first computational expression and the second computational expression are described below. 
       First Computational Expression 
       [0072]        Lg=n+A−B    (1) 
       Second Computational Expression 
       [0073]        Lg=n+A−C    (2) 
         [0074]    Here, n is a PF driving distance from a detection position, at which the detection state of the trailing end sensor  45  is switched from “paper present” to “paper absent”, to the position of the trailing end of the previous sheet Pl. “A” is a transport distance between a trailing end sensor  45  and a leading end sensor  46 , and “B” is a prescribed distance. “C” is a transport distance (mm) from the leading end sensor  46  to the feeding restriction position (medium restriction position), at which the leading end of the subsequent sheet P 2  is positioned when the leading end is in contact with and is restricted by the sheet returning lever  38 . The distances A, B, and C are constants which are uniquely defined in design in accordance with the positions of the sensors  45  and  46  or the operation position of the sheet returning lever  38 . In this example, the condition B&lt;C&lt;A is satisfied. When the inter-paper distance is calculated, the inter-paper distance calculator  83  sends the calculated inter-paper distance Lg to the paper feed driving condition determining section  85 . 
         [0075]    The paper feed driving condition determining section  85  determines on the basis of the inter-paper distance Lg whether to perform the feeding operation or not. In this embodiment, it is necessary to ensure the inter-paper distance Lg of the prescribed amount K (mm) or more. After the preliminary feeding operation is completed, the start of the main feeding operation is determined on the basis of whether or not the main feeding start condition Lg≧K is satisfied. Here, a minimum gap exists so as to ensure the paper detection sensor  47  to reliably detect the leading end of the subsequent sheet P 2 . The prescribed amount K (mm) is obtained by adding a predetermined margin to the minimum gap. The prescribed amount K is also set such that a skew removal operation is performed during the feeding operation of the subsequent sheet P 2  without damaging the sheet. The skew removal operation indicates a series of operations, including nip and release operations, in which a part of the leading end of the subsequent sheet P 2  is temporarily nipped between the pair of transport rollers  25 , and the pair of transport rollers  25  are reversely driven to release the leading end of the subsequent sheet P 2 . In this example, when the previous sheet P 1  is at a last row printing position according to the paper size, the prescribed amount K is set under a condition that the inter-paper distance Lg exists and the leading end of the subsequent sheet P 2  on the upstream side in the transport direction is not nipped between the pair of transport rollers  25 . For example, if the prescribed amount is set to such a value that the leading end of the subsequent sheet P 2  is nipped between the pair of transport rollers  25 , a relatively large amount of the leading end protrudes toward the downstream side in the transport direction from the nip point of the subsequent sheet P 2  due to the release operation in the skew removal operation after last row printing. Accordingly, it is necessary to increase the amount of reverse rotation of the pair of transport rollers  25  for the release operation. In this embodiment, the intermediate roller  36  is only rotatable forward (paper transport direction) but is not rotatable reversely. If the amount of reverse rotation of the pair of transport rollers  25  is excessive, the subsequent sheet P 2  may be excessively flexed between the pair of transport rollers  25  and the intermediate roller  36  during the release operation and may be damaged. In contrast, the prescribed amount K is set such that the amount of reverse rotation of the pair of transport rollers  25  during the release operation is not excessive. Therefore, the subsequent sheet P 2  can be prevented from being excessively flexed and damaged during the release operation. The paper feed driving condition determining section  85  sends a main feeding instruction signal to the paper feed controller  74  only if it is determined the main feeding start condition Lg≧K is satisfied. 
         [0076]    If the main feeding instruction signal is received from the paper feed driving condition determining section  85 , the paper feed controller  74  drives the ASF motor in synchronization with driving of the PF motor during the next transport operation, and transmits the motor driving instruction to the second controller  76  such that the feeding operation is performed simultaneously with the transport operation. If the main feeding instruction signal is not received, no motor driving instruction is transmitted to the first to third controllers  75  to  77 . For this reason, the inter-paper distance Lg of the prescribed amount K or more is ensured, and thus the main feeding operation is performed. 
         [0077]    The next transport operation is as follows. If the instruction to start the transport operation is received from the carriage controller  72 , the transport controller  73  transmits the motor driving instruction to the first controller  75  to drive the PF motor  53 , and accordingly the pair of transport rollers  25 , the pair of discharge rollers  26 , and the intermediate roller  36  are forward driven at a predetermined speed profile in the transport direction. In this way, the next transport operation is performed. At this time, the second controller  76  acquires information regarding the amount of the next transport operation from the transport controller  73 , and controls the speed of the ASF motor  54  at a feeding speed profile conforming to a transport speed profile defined by the information regarding the transport amount so as to be synchronous with the PF motor  53 , such that the subsequent sheet P 2  is fed at the same speed, in the same amount, and at the same transport timing as the previous sheet P 1 . At this time, in view of a difference in reduction ratio due to a difference in roller diameter between the PF system and the ASF system, the PF motor  53  and the ASF motor  54  are controlled such that the transport speed, the transport distance, and the transport timing are identical. 
         [0078]    For example, if the main feeding operation is performed in a state where the inter-paper distance Lg is insufficient (Lg&lt;K) and the subsequent sheet P 2  is temporarily nipped between the intermediate roller  36  and the retard roller  37 , the intermediate roller  36  is forward driven each time the previous sheet P 1  is transported. For this reason, the inter-paper distance Lg is fixed to the insufficient initial value (Lg&lt;K). The insufficient inter-paper distance Lg causes various problems. In this embodiment, therefore, the subsequent sheet P 2  preliminarily feeds to the feeding standby position (target) near to the nip point between the intermediate roller  36  and the retard roller  37  and stands by at the feeding standby position. Then, after it is determined that the inter-paper distance Lg satisfies the condition Lg≧K, the main feeding operation is performed. 
         [0079]    Next, the operation of the printer  11  will be described. First, a printing processing of the printer  11  will be described with reference to a flowchart of  FIG. 9 . If print data is received, the controller  60  executes a program shown in  FIG. 9  and drives a printer engine on the basis of print data to perform the printing processing. 
         [0080]    First, a paper feed processing is performed (Step S 10 ). That is, in a state where the sub motor  55  is driven, and the retard roller  37  and the sheet returning lever  38  are at the feeding position indicated by the two-dot-chain line of  FIG. 2 , the ASF motor  54  and the PF motor  53  are driven. Then, the pickup roller  35  rotates, and accordingly the uppermost sheet P in the sheet feeding cassette  16  is fed. The leading end of the sheet P 1  is detected by the paper detection sensor  47 , and then the sheet P 1  is transported by a predetermined distance. Thus, the paper feed processing ends. For example, if the sheet P 1  is transported to a position to be nipped between the pair of transport rollers  25  and the paper feed processing ends, the sub motor  55  is driven. Then, the retard roller  37  is separated from the intermediate roller  36 , and the sheet returning lever  38  is at the closed position to close a feeding port. 
         [0081]    Next, a leading end setting processing is performed (Step S 20 ). With a position of the sheet P 1  at the time of end of the feeding operation as an origin, if a count value corresponding to a distance from the origin to a leading end setting position is counted by the PF counter  78 , the PF motor  53  is stopped, and the sheet P 1  is set to the leading end setting position. The sheet P 1  is positioned at the printing start position by the leading end setting processing, and thus a paper transport processing in Step S 30  is not performed in the leading end setting processing. 
         [0082]    Next, a printing processing is performed (Step S 40 ). That is, the carriage motor  52  is driven to move the carriage  13  in the main scanning direction, and ink droplets are ejected are ejected from the nozzles of the recording head  20  while the carriage  13  is moving. In this way, printing for one pass is performed. 
         [0083]    It is determined whether or not printing for one page is completed (Step S 50 ), and if printing is not completed, the paper transport processing (Step S 30 ) and the printing processing (Step S 40 ) are alternately performed until a discharge command is received and it is determined that printing for one page is completed. During the paper transport processing, the ASF motor  54  and the PF motor  53  are driven in accordance with a paper transport command, and the sheet is transported by the instructed transport amount. 
         [0084]    If the discharge command is received and printing for one page is completed, it is determined whether or not the paper feeding operation is performed during the paper transport operation (Step S 60 ). That is, when the trailing end of the previous sheet P 1  is detected by the trailing end sensor  45  during the transport operation, it is determined whether or not the paper feeding operation of the subsequent sheet P 2  is performed. If the paper feeding operation is not performed during the paper transport operation, the previous sheet P 1  is not transported to a position at which the trailing end of the previous sheet P 1  is detected by the trailing end sensor  45 . In this case, therefore, a paper discharge processing is performed (Step S 70 ). If the paper feeding operation is performed during the paper transport operation, the previous sheet P 1  already passes by a position at which the trailing end of the previous sheet P 1  is detected by the trailing end sensor  45 . In this case, the paper discharge processing is not performed, and the process progresses to the paper feed processing (Step S 10 ). Then, the subsequent sheet P 2  is fed by the paper feed processing, and the previous sheet P 1  is discharged. 
         [0085]    After the paper discharge processing, it is determined whether or not all pages are printed (Step S 80 ). If all the pages are not printed, the paper feed processing of a next page is performed (Step S 10 ). If all the pages are printed, the routine ends. 
         [0086]    Next, the feed control processing in the printer  11  will be described.  FIGS. 5 to 8  are timing charts when the feed control processing is performed. In the feed control of this embodiment, four kinds of control are branched off depending on the situations (a difference in transport amount, a difference in residual length of the sheet, presence/absence of double feeding, and the like).  FIGS. 5 and 6  show a processing in a cast in which the preliminary feeding operation and the main feeding operation of the subsequent sheet P 2  start during the paper transport operation with detection of the trailing end of the previous sheet P 1  as a trigger.  FIG. 7  shows a processing in a case in which the preliminary feeding operation is not performed in a state where the leading end sensor  46  is already in a detection state at the time of detection of the trailing end of the previous sheet P 1 .  FIG. 8  shows a processing in a case in which the ASF motor  54  is continuously driven when it comes a time to calculate the inter-paper distance after the preliminary feeding operation of the subsequent sheet P 2  starts with detection of the trailing end of the previous sheet P 1  as a trigger, and before the next transport operation. 
         [0087]    Hereinafter, the feed control processing of the printer in the above-described cases will be sequentially described with reference to  FIGS. 5 to 8 . 
         [0088]      FIG. 5  is a timing chart showing the operation timing of the recording head  20 , the carriage motor  52  (in the drawing, CR motor), the PF motor  53 , the ASF motor  54 , and the sub motor  55  during the feed control processing, together with the detection states of the trailing end sensor  45  and the leading end sensor  46 . The operation timing of the carriage motor  52  and the recording head  20  is shown only in  FIG. 5 . 
         [0089]    During printing, the printing operation and the paper transport operation are alternatively performed, and then printing is performed on the previous sheet P 1 . For this reason, the carriage motor  52  and the PF motor  53  are alternately driven. In  FIG. 5 , during a constant-speed period in which the carriage motor  52  is driven at a constant speed, ink droplets are ejected from the recording head  20  (in  FIG. 5 , a hatched region). The PF motor  53  for the transport operation of the previous sheet P 1  starts to be driven after the ink droplets are ejected from the recording head  20 . At this time, the transport amount is defined by the command in print data, and the previous sheet P 1  is transported to a printing position of a next row (next line). 
         [0090]    During the transport operation of the previous sheet P 1 , it is monitored whether or not the detection state of the trailing end sensor  45  is switched from “paper present” to “paper absent”. As shown in  FIG. 5 , if the previous sheet P 1  is transported during printing, the trailing end of the previous sheet P 1  reaches a preliminary feeding start position Q, and the trailing end sensor  45  detects the trailing end of the previous sheet P 1 . In this state, if it is detected that the detection state of the trailing end sensor  45  is switched from “paper present” to “paper absent”, the ASF motor  54  and the sub motor  55  are driven. As the ASF motor  54  is driven, the preliminary feeding operation of the subsequent sheet P 2  starts from the set position in the sheet feeding cassette  16 . During the preliminary feeding operation, it is monitored whether or not the detection state of the leading end sensor  46  is switched from “paper absent” to “paper present”. If it is detected that the detection state of the leading end sensor  46  is switched from “paper absent” to “paper present”, the ASF counter  79  starts to measure the ASF transport distance. If the measured distance has reached the prescribed distance B (mm), the ASF motor  54  is stopped. With this preliminary feeding operation, the subsequent sheet P 2  is delivered to the feeding standby position W (target position). 
         [0091]    As the sub motor  55  is driven, the retard roller  37  is raised and positioned at the feeding position (a position indicated by a two-dot-chain line in  FIG. 2 ) to be in contact with the intermediate roller  36 . Simultaneously, the sheet returning lever  38  is positioned at the open position (a position indicated by a two-dot-chain line in  FIG. 2 ) and the feeding port is opened. As the sheet returning lever  38  is opened, the subsequent sheet P 2  can enter a gap (nip point) between the intermediate roller  36  and the retard roller  37 , and the main feeding operation to further transport the subsequent sheet P 2  from the feeding standby position is prepared. 
         [0092]    During the preliminary feeding operation, the subsequent sheet P 2  is delivered to the feeding standby position W in front of the nip point between the intermediate roller  36  and the retard roller  37 . For this reason, even though the intermediate roller  36  which has the same power source (PF motor  53 ) as the transport driving roller  41  rotates during the transport operation of the previous sheet P 1 , the subsequent sheet P 2  is not fed. In this state, the subsequent sheet P 2  is fed when the ASF motor  54  is driven. 
         [0093]    Subsequently, the printing operation is performed and it comes a calculation time before a predetermined time (for example, 5 to 20 milliseconds) from the next transport operation, the inter-paper distance Lg is calculated. That is, the inter-paper distance Lg=n+A−B is calculated by the first computational expression (Expression (1)). In this case, the PF counter  78  is reset when the leading end sensor  46  detects the trailing end of the previous sheet P 1 , and subsequently, counts the pulse edges of the signal input from the encoder  57 . In this way, the PF driving distance “n” corresponding to the amount of rotation of the PF motor  53  from the detection position of the trailing end of the previous sheet P 1  (the preliminary feeding start position Q) is obtained as the count value. The inter-paper distance Lg is calculated on the basis of the PF driving distance n and the constants A and B by the first computational expression. 
         [0094]      FIG. 5  shows an example where, during an initial transport operation after the preliminary feeding operation, the main feeding start condition is satisfied, that is, the inter-paper distance Lg is equal to or more than the prescribed amount K (Lg≧K). 
         [0095]    If the condition Lg≧K is satisfied, and the necessary inter-paper distance Lg is ensured, as shown in  FIG. 5 , the ASF motor  54  is driven in synchronization with the PF motor  53  for the next transport operation of the previous sheet P 1  is driven. Then, the main feeding operation in which the transport operation of the previous sheet P 1  and the feeding operation of the subsequent sheet P 2  are simultaneously performed is performed. In this case, the PF motor  53  and the ASF motor  54  are controlled such that the transport speed of the previous sheet P 1  is substantially identical to the feeding speed of the subsequent sheet P 2 . For this reason, during the main feeding operation, the inter-paper distance Lg between the previous sheet P 1  and the subsequent sheet P 2  is maintained. Subsequently, each time the PF motor  53  is driven for the transport operation, the ASF motor  54  is simultaneously driven. Therefore, the transport operation of the previous sheet P 1  and the feeding operation of the subsequent sheet P 2  are simultaneously performed while the inter-paper distance Lg is maintained. 
         [0096]      FIG. 6  shows an example in which, during an initial transport operation after the preliminary feeding operation, the inter-paper distance Lg does not satisfy the main feeding start condition Lg≧K. Up to the preliminary feeding operation of the subsequent sheet P 2  is the same as the example of  FIG. 5 . However, if the inter-paper distance Lg calculated by the first computational expression before the next transport operation is less than the prescribed amount K (Lg&lt;K), and an insufficient inter-paper distance is ensured, as shown in  FIG. 6 , when the PF motor  53  for the next transport operation is driven, the ASF motor  54  is not driven, and only the transport operation of the previous sheet P 1  is performed. As a result, the inter-paper distance Lg between the previous sheet P 1  and the subsequent sheet P 2  increases by the transport amount of the previous sheet P 1 . 
         [0097]    Before the next transport operation, the inter-paper distance Lg is recalculated by the first computational expression. In this case, the PF driving distance n represented by the count value of the PF counter  78  increases by the previous transport amount. If the calculated inter-paper distance Lg is equal to or more than the prescribed amount K (Lg≧K), as shown in  FIG. 6 , the ASF motor  54  is driven in synchronization with the PF motor  53  for the next transport operation, and the transport operation of the previous sheet P 1  and the feeding operation of the subsequent sheet P 2  are simultaneously performed. As a result, the previous sheet P 1  and the subsequent sheet P 2  are transported together while the inter-paper distance Lg is maintained. Subsequently, each time the PF motor  53  for the transport operation is driven, the ASF motor  54  is simultaneously driven, and thus the transport operation of the previous sheet P 1  and the feeding operation of the subsequent sheet P 2  are simultaneously performed while the inter-paper distance Lg is maintained. Meanwhile, if Lg&lt;K, only the transport operation of the previous sheet P 1  is performed again. That is, only the transport operation of the previous sheet P 1  is performed until the inter-paper distance Lg calculated before the next transport operation satisfies the main feeding start condition Lg≧K. Then, if the condition Lg≧K is satisfied, during a subsequent transport operation, the feeding operation of the subsequent sheet P 2  is performed together while the inter-paper distance Lg is maintained. 
         [0098]      FIG. 7  shows a processing in a case in which, even though it comes to a time to calculate the inter-paper distance Lg, the ASF motor  54  for the preliminary feeding operation is continuously driven. In this case, when the ASF motor  54  is stopped and the position of the subsequent sheet P 2  is not decided, the inter-paper distance Lg may not be decided, and the inter-paper distance Lg may not be calculated. When this happens, if it waits for until the ASF motor  54  is stopped, a time to start the next transport operation is delayed and throughput is deteriorated. In this embodiment, if the ASF motor  54  is continuously driven when it comes a time to calculate, the PF motor  53  is driven immediately without waiting for until the ASF motor  54  is stopped. With this transport operation, the trailing end of the previous sheet P 1  is moved by the transport amount toward the downstream side in the transport direction. 
         [0099]    When it comes a time to calculate before the next transport operation, if the ASF motor  54  is stopped, the inter-paper distance Lg is calculated by the first computational expression. If the calculated inter-paper distance Lg satisfies the main feeding start condition Lg≧K, the ASF motor  54  is driven in synchronization with the PF motor  53  for the next transport operation. Therefore, the transport operation of the previous sheet P 1  and the feeding operation of the subsequent sheet P 2  are simultaneously performed while the inter-paper distance Lg is maintained. If the main feeding start condition Lg≧K is not satisfied, the ASF motor  54  is not driven, and only the PF motor  53  is driven to perform the transport operation of the previous sheet P 1 . With this transport operation, the inter-paper distance Lg increases by the transport amount. Subsequently, the same processing as that in  FIG. 6  is performed. 
         [0100]      FIG. 8  shows a processing in a case in which the preliminary feeding operation is not performed in a state where the detection state of the leading end sensor  46  is already “paper present” at the time of detection of the trailing end of the previous sheet P 1 . For example, when the subsequent sheet P 2  is double fed while the previous sheet P 1  is fed, the subsequent sheet P 2  is separated from the previous sheet P 1  by the retard roller  37 . Therefore, there is no case in which subsequent sheet P 2  exceeds the retard roller  37  toward the downstream side in the transport direction. If the previous sheet P 1  is fed, the sub motor  55  is driven, and the retard roller  37  is lowered and separated from the intermediate roller  36 . Simultaneously, the sheet returning lever  38  is rotated to the closed position. As a result, the leading end of the double fed subsequent sheet P 2  is in contact with the sheet returning lever  38 . In addition, when the subsequent sheet P 2  is double fed at the time of the transport operation of the previous sheet P 1  after the sheet returning lever  38  is closed, the leading end of the subsequent sheet P 2  is in contact with the sheet returning lever  38 . Therefore, the subsequent sheet P 2  is restricted so as to be no longer transported toward the downstream side in the transport direction. 
         [0101]    As shown in  FIG. 8 , if the detection state of the trailing end sensor  45  is switched from “paper present” to “paper absent” during the transport operation of the previous sheet P 1 , when the detection state of the leading end sensor  46  is already “paper present”, it may be considered that the subsequent sheet P 2  has reached the feeding restriction position R and is in contact with the sheet returning lever  38  due to double feeding. In this case, at the feeding restriction position R, the leading end of the subsequent sheet P 2  exceeds the feeding standby position W by a predetermined distance toward the downstream side in the transport direction, and thus the ASF motor  54  for the preliminary feeding operation is not driven. 
         [0102]    If it comes a time to calculate before the next transport operation, the inter-paper distance Lg is calculated. In this case, the leading end of the subsequent sheet P 2  is regarded as being at the feeding restriction position R at which the subsequent sheet P 2  is in contact with the sheet returning lever  38 , and accordingly the second computational expression Lg=n+A−C is used. In the second computational expression, the constant C is identical to the ASF driving distance between the detection position of leading end of the subsequent sheet P 2  and the feeding restriction position R. With the second computational expression, the inter-paper distance Lg which is identical to a transport distance between the feeding restriction position R and the position of the trailing end of the previous sheet P 1  is calculated. 
         [0103]    It is determined whether or not the calculated inter-paper distance Lg is equal to or more than the prescribed amount K. If the condition Lg≧K is established, the PF motor  53  and the ASF motor  54  are simultaneously driven. If the condition Lg≧K is not established, the ASF motor  54  is not driven, and only the PF motor  53  is driven. When the subsequent sheet P 2  is double fed, the subsequent sheet P 2  is already transported to the feeding restriction position R beyond the feeding standby position W. For this reason, the inter-paper distance Lg is relatively short, and the main feeding start condition Lg≧K is likely to be established, as compared with the subsequent sheet P 2  is at the feeding standby position W. If the condition Lg≧K is not established, while the position of the subsequent sheet P 2  is maintained, only the transport operation of the previous sheet P 1  is performed. Thus, the inter-paper distance Lg increases. If the inter-paper distance Lg calculated before a subsequent transport operation satisfies the condition Lg≧K, the ASF motor  54  is driven in synchronization with the PF motor  53 . Therefore, the transport operation of the previous sheet P 1  and the feeding operation of the subsequent sheet P 2  are simultaneously performed, while the inter-paper distance Lg is maintained. 
         [0104]      FIGS. 10 and 11  are flowcharts showing the feed control processing. Hereinafter, the feed control processing of the printer will be described with reference to  FIGS. 10 and 11 , in addition to  FIGS. 5 to 8  with respect to the above-described cases. In the following description, the driving of the PF motor  53  may be referred to as “PF driving”, and the driving of the ASF motor  54  may be referred to as “ASF driving”. 
         [0105]    In Step S 110  of  FIG. 10 , it is determined whether or not the detection state of the trailing end sensor  45  is switched from “paper present” to “paper absent” during the PF driving of the previous transport operation. This determination is performed on the basis of the value of the flag for storing the monitoring result of the trailing end detection state monitoring section  80 , which monitors the detection state of the trailing end sensor  45 . The trailing end detection state monitoring section  80  monitors the detection state of the trailing end sensor  45  during the PF driving. If the detection state is “paper present”, a trailing end flag is set to “1”, and if the detection state is “paper absent”, the trailing end flag is set to “0”. If the value of the flag is changed from “1” to “0” , a previous transport flag is changed from “0” to “1”. The determination in Step S 110  is performed by the paper feed start condition determining section  82  on the basis of the value of the previous transport flag. If the detection state is changed from “paper present” to “paper absent” during previous PF driving (that is, the previous transport flag=1), the process progresses to Step S 160  of  FIG. 11 . If the switching of the detection state from “paper present” to “paper absent” is not detected (that is, the previous transport flag=0), the process progresses to Step S 120 . The previous transport flag is changed from “1” to “0” when the detection state of the trailing end sensor  45  is switched from “paper absent” to “paper present” during the PF driving. 
         [0106]    In Step S 120 , the PF motor  53  is driven to transport the previous sheet P 1  by a designated transport distance. In this case, the ASF motor  54  is not driven, and only the transport operation of the previous sheet P 1  is performed. Step  120  corresponds to performing of a transport operation. 
         [0107]    In Step S 130 , it is determined whether or not the detection state of the trailing end sensor  45  is switched from “paper present” to “paper absent” during the PF driving. This determination is performed by the trailing end detection state monitoring section  80 . If the detection state of the trailing end sensor  45  is switched from “paper present” to “paper absent” during the PF driving, the process progresses to Step S 140 . If the switching of the detection state is not detected, the paper transport processing ends. In Step S 130 , if the determination is false, the trailing end detection state monitoring section  80  changes the previous transport flag from “0” to “1”. 
         [0108]    In Step S 140 , it is determined whether or not the detection state of the leading end sensor  46  is “paper absent”. This determination is performed by the paper feed start condition determining section  82  on the basis of the monitoring result of the leading end detection state monitoring section  81 . The leading end detection state monitoring section  81  monitors the detection state of the leading end sensor  46 . If the detection state is “paper present”, a leading end flag is set to “1”, and if the detection state is “paper absent”, the leading end flag is set to “0”. The paper feed start condition determining section  82  performs the determination in Step S 140  on the basis of the value of the leading end flag. If the determination result is “paper absent”, the process progresses to Step S 150 , and if the determination result is “paper absent” (that is, “paper present”), the paper transport processing ends. As in the example of  FIG. 8 , when the subsequent sheet P 2  is double fed while the previous sheet P 1  is fed and transported, if the detection state of the trailing end sensor  45  is switched from “paper present” to “paper absent”, the detection state of the leading end sensor  46  is already “paper present”. In such a case, during the PF driving, the ASF driving is not performed. Steps S 130  and S 140  correspond to a processing in which the paper feed start condition determining section  82  determines on the monitoring results of the trailing end detection state monitoring section  80  and the leading end detection state monitoring section  81  whether or not the paper feed start condition for starting the preliminary feeding operation of a sheet in the sheet feeding cassette  16  is established. 
         [0109]    If the detection state of the leading end sensor  46  is “paper absent” (that is, the paper feed start condition is established), in Step S 150 , the ASF motor  54  is driven. Specifically, Step S 150  is performed by the paper feed controller  74 . In Step S 150 , when receiving a paper feed start instruction from the paper feed start condition determining section  82 , the paper feed controller  74  executes a predetermined paper feed sequence and outputs an instruction to the second controller  76  and the third controller  77 . The paper feed controller  74  executes the predetermined paper feed sequence to first drive the ASF motor  54 . During the ASF driving, if the fact that the detection state of the leading end sensor  46  is switched from “paper absent” to “paper present” is acquired from the leading end detection state monitoring section  81 , the ASF counter  79  is reset. If the ASF counter  79  has reached a count value corresponding to the prescribed distance B (mm) the ASF motor  54  is stopped. In this way, the subsequent sheet P 2  is preliminary fed to the feeding standby position shown in  FIG. 3 , at which the leading end of the subsequent sheet P 2  is positioned on the downstream side in the transport direction by the prescribed distance B (mm) from the leading end sensor  46  (the detection position of the leading end). This corresponds to the “preliminary feeding operation” in which the ASF motor  54  is initially driven, in the examples of  FIGS. 5 to 7 . The uppermost sheet (subsequent sheet P 2 ) in the sheet feeding cassette  16  is fed from the set position. Then, the leading end of the sheet reaches the detection position of the leading end sensor  46  and is further fed by the prescribed distance B (mm) after the detection state of the leading end sensor  46  is switched from “paper absent” to “paper present”. The determination in Step S 130  and the ASF driving in Step S 150  correspond to preliminary feeding of a subsequent medium. 
         [0110]    In Step S 110 , if the detection state of the trailing end sensor  45  is switched from “paper present” to “paper absent” during the PF driving of the previous transport operation (in Step S 110 , if the determination is false), the process progresses to Step S 160  of  FIG. 11 . That is, when the ASF motor  54  is driven during the previous PF driving to start the preliminary feeding operation, the process progresses to Step S 160 . 
         [0111]    In Step S 160 , it is determined whether or not the ASF motor  54  is stopped. This determination is performed by the motor driving state determining section  84 . If the ASF motor  54  is stopped, the process progresses to Step S 170 , and if the ASF motor  54  is being driven, the process progresses to Step S 220 . 
         [0112]    In Step S 220 , the PF motor  53  is driven to transport the previous sheet P 1  by the designated transport distance. In this case, the ASF motor  54  is not driven for the main feeding operation, and only the transport operation of the previous sheet P 1  is performed. That is, as shown in  FIG. 7 , even though it comes a time to calculate before the next transport operation starts, when the ASF motor  54  is still driving (that is, the preliminary feeding operation) if it comes a time to start the transport operation, the PF motor  53  is driven to start the transport operation of the previous sheet P 1 , and places priority on printing throughput of the previous sheet P 1 , without waiting for until the preliminary feeding operation is completed. Steps S 160  and S 220  correspond to placing priority on a transport operation. 
         [0113]    In Step S 170 , it is determined whether or not the ASF motor  54  is driven when the detection state of the trailing end sensor  45  is switched from “paper present” to “paper absent”. That is, it is determined whether or not the preliminary feeding operation is performed when the trailing end of the previous sheet P 1  is detected. When the detection state of the leading end sensor  46  is “paper absent”, the preliminary feeding operation is not performed. Meanwhile, when the detection state of the leading end sensor  46  is “paper present”, the subsequent sheet P 2  is regarded as being already fed to the feeding restriction position R due to double feeding, and thus the preliminary feeding operation is not performed. When the ASF driving (the preliminary feeding operation) is performed, the paper feed start condition determining section  82  set an ASF driving flag to “1”, and the paper feed driving condition determining section  85  performs determination on the basis of the value of the ASF driving flag. When the ASF driving is performed (the determination is false), the process progresses to Step S 180 . When the ASF driving is not performed (the determination is true), the process progresses to Step S 190 . 
         [0114]    In Step S 180 , the inter-paper distance Lg is calculated by the first computational expression. That is, the inter-paper distance Lg is calculated by the expression Lg=n+A−B. In the examples of  FIGS. 5 to 7 , in which the preliminary feeding operation is performed, and the leading end of the subsequent sheet P 2  is positioned at the feeding standby position on the downstream side in the transport direction by the prescribed distance B from the leading end detection position, in Step S 180 , the inter-paper distance Lg is calculated by the first computational expression. 
         [0115]    In Step S 190 , the inter-paper distance Lg is calculated by the second computational expression. That is, the inter-paper distance Lg is calculated by the expression Lg=n+A−C. In the example of  FIG. 8 , in which the sheets P 1  and P 2  are double fed, the preliminary feeding operation is not performed, and the leading end of the subsequent sheet P 2  is positioned at the feeding restriction position R at which the leading end is in contact with the sheet returning lever  38 , in Step S 190 , the inter-paper distance Lg is calculated by the second computational expression. Steps S 180  and S 190  correspond to acquiring of a determination value (measuring). 
         [0116]    In Step S 200 , it is determined whether or not the inter-paper distance Lg is equal to or more than the prescribed amount K. If the condition Lg≧K is satisfied, the process progresses to Step S 210 . If the condition Lg≧K is not satisfied (that is, Lg&lt;K), the process progresses to Step S 220 . 
         [0117]    In Step S 210 , the PF motor  53  and the ASF motor  54  are driven together. In this case, the transport controller  73  drives the PF motor  53  by the designated transport distance, and the paper feed controller  74  drives the ASF motor  54  in synchronization with the PF motor  53  such that the transport speed and amount of the previous sheet P 1  are the same as the transport speed and amount of the subsequent sheet P 2 . With this driving, the previous sheet P 1  and the subsequent sheet P 2  are transported by the designated transport distance while the inter-paper distance Lg is maintained. 
         [0118]    If the inter-paper distance Lg is less than the prescribed amount K (Lg&lt;K), in Step S 220 , the PF motor  53  is driven to transport the previous sheet P 1  by the designated transport distance. In this case, the ASF motor  54  is not driven, and only the transport operation of the previous sheet P 1  is performed. For example, as shown in  FIGS. 6 and 8 , with respect to the inter-paper distance Lg calculated before the initial transport operation after the trailing end of the previous sheet P 1  is detected (in  FIG. 6 , after the preliminary feeding operation starts), if Lg&lt;K, during the driving of the PF motor  53  for initial transport operation after the trialing end is detected, the ASF motor  54  is not driven. As a result, while the position of the subsequent sheet P 2  (for example, the feeding standby position or the feeding restriction position) is maintained, only the transport operation of the previous sheet P 1  is performed. Thus, the inter-paper distance Lg increases by the transport distance. After the paper transport processing (Step S 220 ), when a subsequent transport operation is performed, similarly, the inter-paper distance Lg is calculated (Step S 180  or S 190 ), and the inter-paper distance Lg is determined (Step S 200 ). If the condition Lg≧K is satisfied, during the corresponding transport operation, the PF motor  53  and the ASF motor  54  are synchronously driven. Therefore, the previous sheet P 1  and the subsequent sheet P 2  are transported together while the inter-paper distance Lg is maintained. Steps S 200  and S 220  correspond to performing of main feeding control. 
         [0119]    In this way, during the printing of the previous sheet P 1 , the feeding operation of the subsequent sheet P 2  (at least the preliminary feeding operation from among the preliminary feeding operation and the main feeding operation) is performed. If one page of the previous sheet P 1  is printed (YES in Steps S 50  and S 60  of  FIG. 9 ), the process progresses to the paper feed processing (Step S 10 ) of the subsequent sheet P 2 , not the paper discharge processing (Step S 70 ). During the paper feed processing (Step S 10 ) of the subsequent sheet P 2  and the leading end setting processing (Step S 20 ), the previous sheet P 1  is discharged. While the last page is being printed, the feeding operation of the subsequent sheet P 2  is not performed during the transport operation. Therefore, after printing is completed, the paper discharge processing (Step S 70 ) is performed. When the page is printed before the trailing end of the previous sheet P 1  is detected by the trailing end sensor  45 , the paper discharge processing (Step S 70 ) is performed. The paper discharge processing is performed to a position at which the trailing end of the subsequent sheet P 2  is detected by the trailing end sensor  45  or the leading end sensor  46 . Subsequently, the process progresses to the paper feed processing (Step S 10 ). 
         [0120]    For example, the feeding distance of the subsequent sheet P 2  varies depending on whether a maximum number of sheets or a minimum number of sheets are stacked in the sheet feeding cassette  16 . That is, as shown in  FIG. 2 , when a maximum number of sheets are stacked, the pickup roller  35  is positioned at a position indicated by the upper two-dot-chain line near to the intermediate roller  36 . Meanwhile, when a minimum number of sheets are stacked, the pickup roller  35  is positioned at a position indicated by the lower two-dot-chain line (the same as the position of the pickup roller in  FIG. 3 ) away from the intermediate roller  36 . When a minimum number of sheets are stacked, the transport distance of the subsequent sheet P 2  extends. In this case, when the trailing end of the previous sheet P 1  passes through the preliminary feeding start position Q, the preliminary feeding operation is performed to deliver the subsequent sheet P 2  to the feeding standby position in advance. Subsequently, the previous sheet P 1  and the subsequent sheet P 2  are simultaneously transported while the necessary inter-paper distance Lg of the prescribed amount K or more is ensured. Therefore, only if the trailing end of the previous sheet P 1  passes by the preliminary feeding start position Q, even though printing of the previous sheet P 1  ends at some point, the subsequent sheet P 2  is fed to a position on the upstream side in the transport direction by the inter-paper distance Lg from the trailing end of the previous sheet P 1 . As a result, the feeding distance after the paper feed processing of the subsequent sheet P 2  is performed can be shortened, without depending on the number of sheets in the sheet feeding cassette, and thus printing throughput can be improved. 
         [0121]    As described above in detail, according to this embodiment, the following effects are obtained. 
         [0122]    (1) The trailing end sensor  45  and the leading end sensor  46  are individually provided on the downstream side and the upstream side in the transport direction with the position opposing the retard roller  37  serving as a separation unit in the feeding path interposed therebetween. If the trailing end of the previous sheet P 1  is detected by the trailing end sensor  45 , the feeding operation of the subsequent sheet P 2  starts from the set position in the sheet feeding cassette  16 , and the subsequent sheet P 2  is further fed by the prescribed distance B (mm) after the leading end of the subsequent sheet P 2  is detected by the leading end sensor  46 . Next, the inter-paper distance Lg is calculated before the next transport operation, and it is confirmed that the calculated inter-paper distance Lg is equal to or more than the prescribed amount K. Subsequently, the PF driving and the ASF driving are simultaneously performed, and the transport operation of the previous sheet P 1  and the feeding operation of the subsequent sheet P 2  are performed while the inter-paper distance Lg is maintained. As a result, when the previous sheet P 1  (one page) is printed, the subsequent sheet P 2  is immediately fed at the inter-paper distance Lg. For this reason, if the paper feed processing is performed, the subsequent sheet P 2  is set to a printing start position in a relatively small transport amount, and thus printing on the subsequent sheet P 2  can early start. Therefore, printing throughput can be improved. 
         [0123]    (2) After the feeding operation to the feeding standby position, the inter-paper distance Lg is calculated before the next transport operation starts, and it is determined whether or not the calculated inter-paper distance Lg is equal to or more than the prescribed amount K. If the condition Lg≧K is satisfied, during the transport operation, the ASF motor  54  is driven together with the PF motor  53 , and the feeding operation of the subsequent sheet P 2  is performed. If the condition Lg≧K is not satisfied (that is, Lg&lt;K), as the PF motor  53  is driven, the ASF motor  54  is not driven, and only the transport operation of the previous sheet P 1  is performed. Thus, the inter-paper distance Lg increases. In this way, the feeding operation of the subsequent sheet P 2  is performed while it is confirmed that a necessary inter-paper distance Lg is ensured. Therefore, even though the transport distance between the detection position of the trailing end of the previous sheet P 1  and the feeding standby position, which is the target position of the subsequent sheet P 2  to be preliminary fed, is less than the prescribed amount K, a necessary inter-paper distance Lg can be reliably ensured. In addition, during the main feeding operation, the ASF driving and the PF driving are performed at the substantially same driving distance, driving start timing, and driving speed. As a result, the feeding operation of the subsequent sheet P 2  can be performed while the necessary inter-paper distance Lg can be ensured. 
         [0124]    (3) Even though it comes a time to calculate the inter-paper distance Lg set after the preliminary feeding operation starts and immediately before the next transport operation start, when the ASF motor  54  is continuously driven (the preliminary feeding operation is still continuing), the PF motor  53  for the next transport operation starts, without waiting for until the ASF motor  54  is stopped. For this reason, printing of a next row onto the previous sheet P 1  can early start, as compared with a case in which the transport operation starts after the ASF motor  54  is stopped, and thus printing throughput can be improved. 
         [0125]    (4) When the trailing end sensor  45  detects the trailing end of the previous sheet P 1 , it is determined whether or not the leading end sensor  46  detects the leading end of the subsequent sheet P 2 . If the leading end of the subsequent sheet P 2  is detected, the ASF motor  54  is not driven, and the preliminary feeding operation is not performed. Accordingly, even though the sub motor  55  for preparation of the main feeding operation is driven with detection of the trailing end of the previous sheet P 1  as a trigger, and the retard roller  37  and the sheet returning lever  38  are positioned at the time of the feeding operation, it is possible to prevent the subsequent sheet P 2  from being nipped between the intermediate roller  36  and the retard roller  37  before the inter-paper distance Lg is confirmed. For example, if the transport operation is necessarily performed in a predetermined amount enough to reach the target position when the trailing end sensor  45  detects the previous sheet P 1 , the subsequent sheet P 2  is nipped between the intermediate roller  36  and the retard roller  37 . Accordingly, even though the inter-paper distance Lg does not meet the prescribed amount K, when the PF motor  53  for the transport operation of the previous sheet P 1  is driven, the intermediate roller  36  is rotated with the PF motor  53  as a driving source. In this case, even though the ASF motor  54  is not driven, the subsequent sheet P 2  is forcibly fed. According to this embodiment, however, if the leading end of the subsequent sheet P 2  is already detected, the ASF motor  54  is not driven. As a result, it is possible to prevent the subsequent sheet P 2  from being fed when the inter-paper distance Lg does not meet the prescribed amount K. 
         [0126]    (5) When the trailing end of the previous sheet P 1  is detected, if the leading end of the subsequent sheet P 2  is not detected, and the preliminary feeding operation is performed, the inter-paper distance Lg is calculated by the first computational expression Lg=n+A−B with the prescribed distance B corresponding to the target position as a constant. Meanwhile, when the trailing end of the previous sheet P 1  is detected, if the leading end of the subsequent sheet P 2  is already detected, and the preliminary feeding operation is not performed, the leading end of the subsequent sheet P 2  is regarded as being at the feeding restriction position R at which the leading end of the subsequent sheet P 2  is in contact with the sheet returning lever  38 . In this case, the inter-paper distance Lg is calculated by the second computational expression Lg=n+A−C with the distance C from the trailing end detection position to the feeding restriction position R as a constant. Therefore, even though the subsequent sheet P 2  exceeds the target position due to double feeding before the preliminary feeding operation is performed, the inter-paper distance Lg between the previous sheet P 1  and the subsequent sheet P 2  can be relatively accurately calculated. 
         [0127]    (6) Even though the inter-paper distance Lg is equal to or more than the prescribed amount K, during the transport operation, the main feeding operation does not start. Specifically, after the condition Lg≧K is satisfied, when it comes a time to start the next transport operation of the previous sheet P 1 , the main feeding operation starts such that the feeding start timing of the subsequent sheet P 2  is synchronized with the start timing of the next transport operation. For example, if the feeding operation starts during the transport operation, the driving of the ASF motor  54  starts in a state where the PF motor  53  is already rotated at high speed. In this case, the subsequent sheet P 2  may be pulled between the intermediated roller  36 , which rotates at high speed with the PF motor  53  as a driving source, and the pickup roller  35 , which rotates at constant speed in the course of acceleration with the ASF motor  54  as a driving source, and be damaged due to a difference in speed between the intermediate roller  36  and the pickup roller  35 . In this embodiment, however, it is possible to prevent the subsequent sheet P 2  from being damaged due to excessive tension caused by the difference in speed between the rollers. In particular, in this embodiment, at the time of main feeding, the PF motor  53  and the ASF motor  54  are controlled such that the PF motor  53  and the ASF motor  54  have the substantially same driving start timing, transport speed, and driving stop timing. Therefore, it is possible to reliably prevent the subsequent sheet P 2  from being damaged due to excessive tension caused by the difference in speed between the rollers. 
         [0128]    The invention is not limited to the embodiment, but the following modifications may be applicable. 
         [0129]    (Modification 1) The target position is not limited to a fixed position, but it may be variable. For example, the target position may vary depending on a target transport position of the previous sheet P 1 . That is, when the trailing end sensor  45  detects the trailing end of the previous sheet P 1  during the transport operation of the previous sheet P 1 , a position on the downstream side in the feeding direction at a necessary inter-paper distance (for example, the prescribed amount K) from the trailing end position of the previous sheet P 1  after completion of the transport operation defined by the target transport position of the previous sheet P 1  at that time may be calculated as the target position, and the preliminary feeding operation may be performed in accordance with the calculated target position. In this case, even though the transport distance from the detection position of the trailing end of the previous sheet P 1  when the preliminary feeding operation is performed and the target transport position varies depending on the transport amount at that time, the subsequent sheet P 2  can be preliminarily fed to the target position separated by an appropriate inter-paper distance substantially identical the prescribed amount from the trailing end of the previous sheet P 1 . Therefore, when an initial (next) transport operation after the preliminary feeding operation starts, the inter-paper distance Lg can be appropriately ensured, and the inter-paper distance can be prevented from excessively increasing. In this case, even though it comes a time to calculate the inter-paper distance Lg, if the preliminary feeding operation of the subsequent sheet is still continuing, the transport operation of the previous sheet starts immediately after it comes a time to start the transport operation, without waiting for until the preliminary feeding operation is stopped. 
         [0130]    (Modification 2) In the foregoing embodiment, a necessary inter-paper distance (that is, the prescribed amount K) is fixed, but it may be variable. For example, the prescribed amount K may vary depending on a printing mode, a transport speed, or a paper size. Like the sheet feeding device described in JP-A-2005-22792 (paragraphs [0029] to [0054]), when the inter-paper distance is determined in accordance with the detection position of the sensor, it is difficult to set the inter-paper distance variable. In this embodiment, however, if the trailing end position of the previous sheet P 1  is measured, an inter-paper distance is calculated on the basis of the measurement value (that is, the PF driving distance n), and the start timing of the main feeding operation is determined on the basis of the calculated inter-paper distance, a prescribed amount can be selected from a plurality of prescribed amount Kn(n=1, 2,. . . ) stored in a memory in accordance with the printing condition. Therefore, a necessary inter-paper distance can be relatively simply variable. 
         [0131]    (Modification 3) The determination value is not limited to the gap. For example, a distance between the leading ends of the sheets, an inter-center distance of the sheets, or a distance between the leading end of the previous sheet P 1  and the trailing end of the subsequent sheet P 2  may be used as the determination value. 
         [0132]    (Modification 4) The calculation time (measurement time) when the inter-paper distance is calculated is not limited to immediately before start of the next transport operation. For example, any time from when the current transport operation is completed with the trailing end of previous sheet detected until the final calculation start time, which is permitted so as to complete calculation and determination of the inter-paper distance before the next transport operation starts may be set. In addition, even though the ASF motor is driven at the calculation time, when a standby time (for example, several  100  milliseconds) exists until the transport operation starts, a second calculation time is set immediately before the transport operation starts. Even though the second calculation time comes, if the ASF motor is continuously driven, the transport operation starts immediately when it comes a time to start the transport operation. Meanwhile, when the second calculation time comes, if the ASF motor is stopped, the main feeding operation may be performed. In this case, the second calculation time may be set several times. 
         [0133]    (Modification 5) The trailing end sensor and the leading end sensor may not be separately provided, but may be formed of a single common sensor. If the common sensor detects the trailing end of the previous sheet, and it is detected that the trailing end has reached the preliminary feeding start position, the preliminary feeding operation of the subsequent sheet starts. If the leading end of the subsequent sheet is detected by the common sensor, or if the leading end of the subsequent sheet is detected by the common sensor, and then the subsequent sheet is further fed by the prescribed distance B and reaches the target position, a structure for stopping the preliminary feeding operation may be used. In addition, after the trailing end of the previous sheet P 1  is detected, the transport amount of the previous sheet P 1  may be measured to confirm that the trailing end moves to a predetermined position separated by a predetermined transport distance from the sensor detection position toward the downstream side in the transport direction, and then the preliminary feeding operation of the subsequent sheet P 2  may start. The paper detection sensor  47  may be used for the single common sensor. 
         [0134]    (Modification 6) The trailing end sensor and the leading end sensor are provided on both sides of the paper transport path with the intermediate roller, but at least the leading end sensor may be positioned on the upstream side in the transport direction by the intermediate roller. That is, with respect to a transport unit (roller) (in the foregoing embodiment, the intermediate roller  36 ), which is positioned on an uppermost stream side in the transport direction, from among a transport unit (in the foregoing embodiment, the transport driving roller  41 , the discharge driving roller  43 , and the intermediate roller  36 ), which is driven to transport the previous sheet during the recording operation, the leading end sensor may be positioned on the upstream side in the transport direction. The target position at the time of the preliminary feeding operation may be positioned on the upstream side with respect to the transport unit (roller) on the uppermost stream side in the transport direction. 
         [0135]    (Modification 7) Even if the inter-paper distance is equal to or more than the prescribed amount, each time the transport operation is performed, the inter-paper distance may be measured in advance, and it may be determined on the basis of measured inter-paper distance whether to perform the transport operation of the previous sheet and the feeding operation of the subsequent sheet together or not. 
         [0136]    (Modification 8) In the foregoing embodiment, the prescribed distance B is set, and the subsequent sheet P 2  is further fed from the leading end detection position by the prescribed distance B and then stopped, but the prescribed distance may not be provided. That is, when the leading end sensor  46  detects the leading end of the subsequent sheet P 2 , the ASF motor  54  may be stopped. In this case, in a printer in which a distance in the transport path between the sensors  45  and  46  (that is, a distance between the feeding start position and the target position at the time of the preliminary feeding operation) is less then the prescribed amount K required as the inter-paper distance, a sufficient inter-paper distance can also be ensured. 
         [0137]    (Modification 9) The invention may be applied to a printer in which the distance between the feeding start position and the target position at the time of the preliminary feeding operation is equal to or more than the prescribed amount K required as the inter-paper distance. For example, even though the previous sheet and the subsequent sheet are double fed, a sufficient inter-paper distance can also be ensured. 
         [0138]    (Modification 10) The computational expression for calculating the inter-paper distance Lg varies depending on whether or not the detection state of the leading end sensor  46  is “paper present” when the trailing end of the previous sheet P 1  is detected. Alternatively, the same computational expression may be used insofar as a necessary inter-paper distance Lg is ensured. For example, if the second computational expression is constantly used, when double feeding occurs, the time to start the main feeding operation is delayed once, but a necessary inter-paper distance can be reliably ensured. 
         [0139]    (Modification 11) The PF motor  53  and the ASF motor  54  are provided, and the PF driving system and the ASF driving system use separate driving sources. Alternatively, the same driving source (same motor) may be used, and a clutch may be used to switch power transmission to separately drive the PF driving system and the ASF driving system. 
         [0140]    (Modification 12) In case of a serial printer, a dot impact recording type or a thermal transfer recording type may be applicable, in addition to an ink jet recording type. 
         [0141]    (Modification 13) A recording apparatus is not limited to the printer. Alternatively, the invention may be applied to another liquid ejection type recording apparatus for ejecting a liquid other than ink. Herein, “recording” is not limited to recording based on printing. For example, “recording” includes an operation to form a wiring pattern or an image on a circuit board serving as a medium by ejecting a liquid-state material including a material having a predetermined characteristic. For example, the invention may be applied a liquid ejection apparatus (recording apparatus) for ejecting a liquid-state material, in which a material, such as an electrode material or a color material is dispersed or dissolved, used to manufacture a liquid crystal display, an EL (Electro Luminescence) display, and a field emission display. When a feeding unit sequentially feeds sheet-like substrates one by one, and a recording unit forms a predetermined pattern on a substrate to be fed, throughput can be improved while a gap between the substrates serving as a medium can be ensured. As a result, productivity can be improved. 
         [0142]    Hereinafter, technical ideas capable of being understood from the embodiment and the modifications will be described. 
         [0143]    (1) In the method according to any one of claims  1  to  5 , the transport unit and the feeding unit include separate driving sources. 
         [0144]    (2) In the method according to any one of claims  2  to  5 , when the main feeding operation is performed in the controlling of the main feeding operation, before every transport operation of the previous medium, the calculating and the controlling of the main feeding operation are repeatedly performed until the main feeding operation is performed. 
         [0145]    (3) In the method according to any one of claims  1  to  5 , in the measuring, the trailing end position of the previous medium is measured by a trailing end measuring unit ( 78 ), and a distance in a transport path between the trailing end position and the target position is measured as the gap by using the measurement value of the trailing end position.