Patent Publication Number: US-7896566-B2

Title: Printing apparatus and printing method

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a printing apparatus and a printing method capable of correcting a transport amount of a print medium when an image is printed on the print medium with transporting the print medium. 
     2. Description of the Related Art 
     In recent years, printing apparatus such as printers are finding a growing range of applications in printing picture images. Particularly, ink jet printers have achieved a significant progress in reducing the size of ink droplets ejected from a print head and thus are able to print images equal to or higher in image quality than silver salt pictures. In such ink jet printers, an image is formed on a sheet (print medium) by transporting the sheet along a transport path passing through a print position that faces the print head and ejecting ink onto the sheet being transported from the print head kept out of contact with the sheet. 
     To print an image up to a rear end area of the sheet with respect to the transport direction, another transport means (downstream transport means) needs to be provided on a downstream side of the sheet transport direction in addition to a separate transport means (upstream transport means) installed on an upstream side of the sheet transport direction. The upstream and downstream transport means may, for example, be a construction including rollers that hold the sheet in between as they rotate. If the transport means are installed on the upstream and downstream sides of the transport direction, the sheet is first transported only by the upstream transport means. Then, after the front end of the sheet reaches the transport position of the downstream transport means, it is transported by both of the upstream and downstream transport means. After the rear end of the sheet gets out of the transport position of the upstream transport means, it is transported only by the downstream transport means. 
     In the construction in which the sheet is transported in the sheet transport direction by the upstream and downstream transport means, the print position of an image on the rear end area of the sheet may deviate, degrading an image quality. That is, as the sheet gets out of the upstream transport means, the transport amount of the sheet may greatly vary from a predetermined value, as by backlash of gears in a drive system of the rollers included in the upstream transport means, shifting the print position of the image and degrading the image quality. When the sheet is being transported only by the downstream transport means after the rear end of the sheet has come out of the upstream transport means, a sheet transport accuracy becomes lower than when the sheet is transported by both of the upstream and downstream transport means, making the image print position more likely to deviate, giving rise to a problem of image degradation. 
     Japanese Patent Laid-Open No. 2002-254736 proposes a method for preventing image degradations in a rear end area of the sheet caused by such a deteriorated transport accuracy in a serial scan type ink jet printing apparatus. This method involves reducing the transport amount before the rear end of the sheet comes out of the upstream transport means situated on the upstream side of the transport direction and, when the rear end of the sheet leaves the upstream transport means and the transport amount of the sheet changes, changing a amount the downstream transport means transports the sheet. Further, a range of nozzles in the print head that are allowed to be used is limited and the positions of the nozzles in operation are shifted at the same time that the transport amount of the sheet is changed. 
     Further, the transport amount when the sheet is transported only by the downstream transport means is set equal to the transport amount when the rear end of the sheet leaves the upstream transport means so that it is smaller than when the sheet is transported by both the upstream and downstream transport means. 
     With the construction described in Japanese Patent Laid-Open No. 2002-254736, the image degradations in the rear end area of the sheet can therefore be minimized by reducing the sheet transport errors. That is, when the rear end of the sheet leaves the upstream transport means or during the transporting of the sheet after it has left the upstream transport means, possible reductions in the transport accuracy can be minimized. 
     SUMMARY OF THE INVENTION 
     For a further quality enhancement of printed image, however, the printed image degradations in the rear end region of the sheet caused by the deteriorated transport accuracy of the sheet need to be reduced even if condition (for example, roller diameter or friction coefficient of roller) of the printing apparatus is changed from initial condition. 
     An object of this invention is to provide a printing apparatus and a printing method capable of printing a quality image in the rear end region of the print medium by minimizing a degradation in transport accuracy of the sheet as the rear end of the sheet leaves the upstream transport means situated on the upstream side of the transport direction or during the sheet transporting after it has left the upstream transport means. 
     In a first aspect of the present invention, there is provided a printing apparatus for printing an image on a print medium by repetitively performing a scan operation that scans a print head in a scan direction and a transport operation that transports the print medium in a transport direction crossing the scan direction, the printing apparatus comprising: 
     first transport means for transporting the print medium, the first transport means being installed upstream of the print head with respect to the transport direction; 
     second transport means for transporting the print medium, the second transport means being installed downstream of the print head with respect to the transport direction; 
     printing means for printing on the print medium a test pattern used to detect a difference between a transport amount in a first transporting operation and a transport amount in a second transporting operation, the first transporting operation transporting the print medium by the first transport means and the second transport means, the second transporting operation transporting the print medium in which a transporting state is switched from a transporting state of the first transporting operation to a transporting state transporting the print medium not by the first transport means but by the second transport means; and 
     correction means for correcting the transport amount in the second transporting operation based on a printed result of the test pattern. 
     In a second aspect of the present invention, there is provided a printing apparatus for printing an image on a print medium by repetitively performing a scan operation that scans a print head in a scan direction and a transport operation that transports the print medium in a transport direction crossing the scan direction, the printing apparatus comprising: 
     first transport means for transporting the print medium, the first transport means being installed upstream of the print head with respect to the transport direction; 
     second transport means for transporting the print medium, the second transport means being installed downstream of the print head with respect to the transport direction; 
     printing means for printing on the print medium a test pattern used to detect a difference between a transport amount in a first transporting operation and a transport amount in a second transporting operation, the first transporting operation transporting the print medium by the first transport means and the second transport means, the second transporting operation transporting the print medium not by the first transport means but by the second transport means; and 
     correction means for correcting the transport amount in the second transporting operation based on a printed result of the test pattern. 
     In a third aspect of the present invention, there is provided a printing apparatus for printing an image on a print medium by repetitively performing a scan operation that scans a print head in a scan direction and a transport operation that transports the print medium in a transport direction crossing the scan direction, the printing apparatus comprising: 
     first transport means for transporting the print medium, the first transport means being installed upstream of the print head with respect to the transport direction; 
     second transport means for transporting the print medium, the second transport means being installed downstream of the print head with respect to the transport direction; 
     detecting means for detecting a transport amount in a first transporting operation and a transport amount in a second transporting operation, the first transporting operation transporting the print medium by the first transport means and the second transport means, the second transporting operation transporting the print medium in which a transporting state is switched from a transporting state of the first transporting operation to a transporting state transporting the print medium not by the first transport means but by the second transport means; and 
     correction means for correcting the transport amount in the second transporting operation based on a result of detection by the detection means. 
     In a fourth aspect of the present invention, there is provided a printing apparatus for printing an image on a print medium by repetitively performing a scan operation that scans a print head in a scan direction and a transport operation that transports the print medium in a transport direction crossing the scan direction, the printing apparatus comprising: 
     first transport means for transporting the print medium, the first transport means being installed upstream of the print head with respect to the transport direction; 
     second transport means for transporting the print medium, the second transport means being installed downstream of the print head with respect to the transport direction; 
     detection means for detecting a transport amount in a first transporting operation and a transport amount in a second transporting operation, the first transporting operation transporting the print medium by the first transport means and the second transport means, the second transporting operation transporting the print medium not by the first transport means but by the second transport means; and 
     correction means for correcting the transport amount in the second transporting operation based on a result of detection by the detection means. 
     In a fifth aspect of the present invention, there is provided a printing apparatus for printing an image on a print medium by repetitively performing a scan operation that scans a print head in a scan direction and a transport operation that transports the print medium in a transport direction crossing the scan direction, the printing apparatus comprising: 
     first transport means for transporting the print medium, the first transport means being installed upstream of the print head with respect to the transport direction; 
     second transport means for transporting the print medium, the second transport means being installed downstream of the print head with respect to the transport direction; 
     printing means for printing on the print medium a test pattern used to detect a difference between transport amounts of the print medium in a first transporting operation and a second transporting operation or a difference between transport amounts of the print medium in the first transporting operation and a third transporting operation; and 
     correction means for correcting the transport amount in the second transporting operation or the transport amount in the third transporting operation based on a printed result of the test pattern; 
     wherein the first transporting operation is a transporting operation for transporting the print medium by the first transport means and the second transport means; 
     wherein the third transporting operation is a transporting operation for transporting the print medium not by the first transport means but by the second transport means; 
     wherein the second transporting operation is a transporting operation interposed between the first transporting operation and the third transporting operation. 
     In a sixth aspect of the present invention, there is provided a printing apparatus for printing an image on a print medium by repetitively performing a scan operation that scans a print head in a scan direction and a transport operation that transports the print medium in a transport direction crossing the scan direction, the printing apparatus comprising: 
     first transport means for transporting the print medium, the first transport means being installed upstream of the print head with respect to the transport direction; 
     second transport means for transporting the print medium, the second transport means being installed downstream of the print head with respect to the transport direction; 
     detection means for detecting any one of transport amounts of the print medium in a first transporting operation and a second transporting operation, transport amounts of the print medium in the first transporting operation and a third transporting operation, or transport amounts of the print medium in the first transporting operation, the second transporting operation and the third transporting operation; and 
     correction means for correcting the transport amount in the second transporting operation or the third transporting operation based on a result of detection by the detection means; 
     wherein the first transporting operation is a transporting operation for transporting the print medium by the first transport means and the second transport means; 
     wherein the third transporting operation is a transporting operation for transporting the print medium not by the first transport means but by the second transport means; 
     wherein the second transporting operation is a transporting operation interposed between the first transporting operation and the third transporting operation. 
     In a seventh aspect of the present invention, there is provided a printing apparatus for printing an image on a print medium by repetitively performing a scan operation that scans a print head in a scan direction and a transport operation that transports the print medium in a transport direction crossing the scan direction, the printing apparatus comprising: 
     first transport means for transporting the print medium, the first transport means being installed upstream of the print head with respect to the transport direction; 
     second transport means for transporting the print medium, the second transport means being installed downstream of the print head with respect to the transport direction; 
     correction means for correcting a transport amount in a second transporting operation based on a correction value, the correction value being set according to a difference between a transport amount in a first transporting operation and the transport amount in the second transporting operation, the first transporting operation transporting the print medium by the first transport means and the second transport means, the second transporting operation including a transporting state transporting the print medium not by the first transport means but by the second transport means. 
     In a eighth aspect of the present invention, there is provided a printing method for printing an image on a print medium by repetitively performing a scan operation that scans a print head in a scan direction and a transport operation that transports the print medium in a transport direction crossing the scan direction, the printing method comprising the steps of: 
     transporting the print medium by a first transport means installed upstream of the print head with respect to the transport direction or by a second transport means installed downstream of the print head with respect to the transport direction; and 
     correcting a transport amount in a second transporting operation based on a correction value, the correction value being set according to a difference between a transport amount in a first transporting operation and the transport amount in the second transporting operation, the first transporting operation transporting the print medium by the first transport means and the second transport means, the second transporting operation including a transporting state transporting the print medium not by the first transport means but by the second transport means. 
     According to this invention, the transport amount of the sheet is corrected when the rear end of the sheet leaves the upstream transport means situated on the upstream side of the transport direction or during the sheet transporting after it has left the upstream transport means. This minimizes degradations in the sheet transport accuracy and thereby enables a quality image to be formed in a rear end area of the print medium. 
     Further features of the present invention will become apparent from the following description of exemplary embodiments (with reference to the attached drawings). 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a printing apparatus in a first embodiment of this invention; 
         FIG. 2  is a perspective view showing a mechanism in the printing apparatus of  FIG. 1 ; 
         FIG. 3  is a cross-sectional view of the printing apparatus of  FIG. 1 ; 
         FIG. 4  is a block configuration diagram of a control system in the printing apparatus of  FIG. 1 ; 
         FIG. 5  is an explanatory diagram showing an example of a printed test pattern in the first embodiment of this invention; 
         FIG. 6A  is an explanatory diagram showing first dots in the test pattern of  FIG. 5  and  FIG. 6B  is an explanatory diagram showing second dots in the test pattern of  FIG. 5 ; 
         FIGS. 7A and 7B  are explanatory diagrams showing other printed test patterns in  FIG. 5 ; 
         FIG. 8  is an explanatory diagram showing an example of a printed test pattern in a second embodiment of this invention; 
         FIG. 9  is an explanatory diagram showing an example of a printed test pattern in a third embodiment of this invention; 
         FIG. 10  is an explanatory diagram showing a construction of a transport amount sensor in a fourth embodiment of this invention; 
         FIG. 11  is an explanatory diagram showing an example of a printed test pattern in a fifth embodiment of this invention, and 
         FIG. 12  is a flow chart showing a method of correcting a transport amount in the fifth embodiment of this invention. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Now, embodiments of this invention will be described by referring to the accompanying drawings. 
     First Embodiment 
       FIG. 1  through  FIG. 7B  represent the first embodiment of this invention.  FIG. 1  is a perspective view of a printing apparatus to which this embodiment can be applied;  FIG. 2  is a perspective view showing a mechanism of the printing apparatus;  FIG. 3  is a cross-sectional view of the printing apparatus;  FIG. 4  is a block configuration diagram of a control system of the printing apparatus; and  FIGS. 5 to 7B  are explanatory diagrams showing a transport amount correction control. 
     The printing apparatus  1  of this embodiment comprises a paper feed unit  2 , a paper transport unit  3 , a carriage unit  5 , a paper discharge unit  4 , an automatic U-turn transport unit  8 , a cleaning unit  6  for a print head  7 , and an enclosure  9 . Each of these components will be explained as follows (see  FIG. 1  to  FIG. 3 ). 
     (A) Paper Feed Unit 
     A base  20  of the paper feed unit  2  is provided with a pressure plate  21  for stacking sheets (of print medium) P, a paper feed roller  28  for feeding sheets P, a separation roller  241  for separating sheets P, and a return lever (not shown) to return sheets P to the stack position. A feed tray  26  for holding the stacked sheets P is mounted on the base  20  or the enclosure. The feed tray  26  is of multistage type and is drawn out for use. 
     The paper feed roller  28  is a rod arc-shaped in cross section which has a separation roller rubber near a reference position of sheet to feed the sheet. A motor installed in the paper feed unit  2  and commonly used also by the cleaning unit  6  (hereinafter referred to also as a “transport motor”) transmits a drive force to the paper feed roller  28  through a drive force transmission gear and a planetary gear not shown. 
     The pressure plate  21  has a movable side guide  23  which limits the stack position of the sheets P. The pressure plate  21  is rotatable about a rotating shaft connected to the base  20  and is urged by a pressure plate spring not shown toward the paper feed roller  28 . A part of the pressure plate  21  facing the paper feed roller  28  is provided with a separation sheet of a material having a large friction coefficient, such as artificial leather, to prevent two or more sheets P from being fed at one time. The pressure plate  21  is driven by a pressure plate cam not shown to engage and disengage from the paper feed roller  28 . 
     Further, a separation roller holder mounted with the separation roller  241  to separate sheets P one at a time is rotatable about a rotating shaft mounted to the base  20  and is urged toward the paper feed roller  28  by a separation roller spring not shown. The separation roller  241  has a clutch spring that allows a portion of the holder attached with the separation roller  241  to rotate when applied with more than a predetermined load. The separation roller  241  is driven by a separation roller release shaft and a control cam not shown to engage and disengage from the paper feed roller  28 . The positions of the pressure plate  21 , the return lever not shown to return the sheets P to the stack position and the separation roller  241  are detected by an ASF (auto sheet feeder) sensor not shown. 
     The return lever to return the sheets P to the stack position is rotatably mounted on the base  20  and is urged in a release direction by a return lever spring not shown. Further, the return lever is pivoted by the control cam when returning the sheets P to the stack position. 
     In a normal standby state, the pressure plate  21  is released by the pressure plate cam, the separation roller  241  is released by the control cam and the return lever blocks a stack opening so as to prevent the sheets P from moving further inward when they are stacked. When, in this state, the paper feed operation starts, the motor drives the separation roller  241  to engage the paper feed roller  28 . Then, the return lever is released and the pressure plate  21  engages the paper feed roller  28 . In this state the sheets P begin to be fed. The sheets P are restricted by a front stage separation unit provided on the base  20  and only a predetermined number of sheets P are fed to a nip portion formed by the paper feed roller  28  and the separation roller  241 . The sheets P are separated by the nip portion and only the top sheet P is fed. 
     When the sheet P reaches a position of a transport roller  36  and pinch rollers  37 , the pressure plate  21  is released by the pressure plate cam and the separation roller  241  is released by the control cam. The return lever is returned to the stack position by the control cam. At this time the sheets P that have reached the nip portion between the paper feed roller  28  and the separation roller  241  can be returned to the stack position. 
     (B) Paper Transport Unit 
     The paper transport unit  3  is mounted on a chassis  11  formed of a bent metal plate. The paper transport unit  3  has the transport roller  36  for transporting sheets P and a PE (paper end) sensor not shown. The transport roller  36  is a metal shaft coated with fine ceramic particles on its surface and the metal portions of the shaft ends are mounted on the chassis  11  through bearings. To assure a stable transport by applying a load to the transport roller  36  during its rotation, a transport roller tension spring is provided between the transport roller  36  and its bearing to give a bias to the transport roller  36  to apply a predetermined load. 
     The transport roller  36  is engaged with a plurality of pinch rollers  37  that are driven by the transport roller. The pinch rollers  37  are pressed against the transport roller  36  by a pinch roller spring biasing the pinch roller holder to generate a sheet P transporting force. The pinch roller holder holds the pinch rollers  37  and rotates about bearings of the chassis. At an inlet of the paper transport unit  3  to which the sheets P are transported, a paper guide flapper  33  to guide the sheets P and a platen  34  are installed. The pinch roller holder is provided with a PE sensor lever that is operated by the front and rear ends of the sheet P as they pass, activating the PE sensor  32 . The platen  34  is mounted and positioned on the chassis  11 . The paper guide flapper  33  is rotatable about predetermined bearing portions in the transport roller  36  and is positioned when it engages the chassis  11 . 
     In this construction, the sheet P fed to the paper transport unit  3  is guided by the pinch roller holder and the paper guide flapper  33  and fed between the transport roller  36  and the pinch rollers  37 . By detecting the front end of the sheet P based on the operation of the PE sensor lever, the print position of the sheet P is set. As the rollers  36 ,  37  are driven by the transport motor, the sheet P is transported over the platen  34 , which is formed with ribs that form a reference surface for transporting the sheet P. 
     A drive force of the transport motor (DC motor) is transmitted to the transport roller  36  through a timing belt and a pulley. On the rotating shaft of the transport roller  36  is provided a code wheel  362  that is formed with marks at 150-300 lpi (line per inch). The chassis  11  is mounted with an encoder sensor  363  that detects the marks on the code wheel  362  to determine the amount of transport by the transport roller  36 . The transport motor is provided with an encoder and, according to a pulse signal representing a slit position or slit number of the encoder, is rotated a predetermined angle stepwise. In the sheet transport amount correction control described later, the pulse signal representing the slit position or slit number is changed according to a correction value to correct the transport amount of the sheet P. 
     On a downstream side of the transport roller  36  in the transport direction of the sheet P, the print head  7  to form an image according to image information is installed. As the print head  7  an ink jet print head is used which allows a plurality of ink tanks  71  of different color inks to be replaced individually. This print head  7  can apply heat to ink by an electrothermal transducer (heater). The heat causes a film boiling in ink and a pressure change generated by an expansion or contraction of a bubble in ink is used to eject ink from each nozzle in the print head  7  to form an image on the sheet P. The construction of the print head  7  is not particularly limited. It may be an ink jet print head using a piezoelectric element to eject ink or a thermosensitive print head. 
     (C) Carriage Unit 
     The carriage unit  5  has a carriage  50  in which the print head  7  can be mounted. The carriage  50  is guided by a guide shaft  52  and a guide rail  11  so that it can be moved in a main scan direction X. The main scan direction crosses the transport direction Y of the sheet P (in this case, at right angles). 
     The carriage  50  is driven by a carriage motor  54  mounted on the chassis  11  through a timing belt. To detect the position of the carriage  50 , a code strip  561  formed with marks at a pitch of 150-300 lpi is provided parallel to the timing belt, and a carriage substrate attached with an encoder sensor is mounted on the carriage  50 . The encoder sensor reads the marks on the code strip  561 . The print head  7  is fixed to the carriage  50  by pivoting a head set lever  51 . The carriage  50  is mounted with an automatic registration sensor that optically reads a pattern printed on the sheet P to adjust landing positions of ink droplets ejected from the print head  7 . 
     In this construction, an image is formed on the sheet P by transporting the sheet P to the image forming position by the rollers  36 ,  37  and ejecting ink from the print head  7  while the carriage unit  5  is moved in the main scan direction by the carriage motor. 
     (D) Paper Discharge Unit 
     The paper discharge unit  4  comprises two discharge rollers  40 ,  41 , spurs  42  engaged with and driven by these rollers, and a gear train to transmit the rotating force of the transport roller  36  to the discharge rollers  40 ,  41 . 
     The discharge rollers  40 ,  41  are mounted to the platen  34 . The discharge roller  40  situated on the upstream side of the transport direction of the sheet P is driven by the rotating force of the transport roller  36  transmitted through an idler gear. To the discharge roller  41  situated on the downstream side of the discharge roller  40  a drive force is transmitted from the discharge roller  40  through an idler gear. 
     The spurs  42  are thin SUS plates having protruding portions at their periphery and formed integral with a resin portion. They are attached to a spur holder  43 . The spurs  42  are attached to the spur holder  43  by spur springs and pressed against the discharge rollers  40 ,  41  by a biasing force of the spur springs. The spurs  42  include ones having first and second functions. The first function is mainly to generate a sheet P transporting force by arranging the spurs at positions corresponding to the elastic portions such as rubber portions of the discharge rollers  40 ,  41 . The second function is to keep the paper P from floating as the sheet P is printed, by arranging the spurs at positions facing other than the elastic portions  411  of the discharge rollers  40 ,  41 . A paper end support for holding both ends of the sheet P at elevated positions is arranged between the discharge rollers  40  and  41 . 
     In this construction, the sheet P formed with an image by the carriage unit  5  is gripped by the nip portion between the discharge roller  40  and the spurs  42  and the nip portion between the discharge roller  41  and the spurs  42  and transported and discharged onto a discharge tray  46 . The discharge tray  46  is accommodated in a front cover  95  and drawn out for use, as shown in  FIG. 1 . The discharge tray  46  is formed such that it is higher toward the front end, with both sides raised, to improve a stacking performance of the discharged sheets P and prevent the printed surface from being rubbed and smeared. 
     (E) Automatic U-Turn Transport Unit 
     The cassette  81  provided on the front side of the apparatus accommodates the sheets P. To separate the sheets P for feeding, the cassette  81  has a pressure plate that presses the sheet P against the feed roller  821 . A UT base of the apparatus body is provided with the feed roller  821  for feeding the sheet P, a separation roller for separating the individual sheets P, a return lever for returning the sheets P to the stack position, and a means for controlling the pressing action on the pressure plate. 
     The cassette  81  has a two-stage contractible construction that can be used in different ways according to the size of the sheets P. When the sheets P is small in size or when the cassette is not in use, the cassette  81  is contracted and accommodated in the enclosure  9  of the apparatus body. 
     The feed roller  821  is a bar arc-shaped in cross section and has a separation roller rubber near a reference position of the sheet for sheet feeding. An automatic U-turn transport motor installed in the automatic U-turn transport unit  8  transmits a drive force to the feed roller  821  through a transmission gear and planetary gear. 
     The pressure plate is provided with a movable side guide that restricts the stack position of the sheets P. The pressure plate is rotatable about a rotating shaft connected to the cassette  81  and is urged toward the feed roller  821  by a pressing and control means that is installed on the UT base and made up of a pressure plate spring. A part of the pressure plate facing the feed roller  821  is provided with a separation sheet of a material having a large friction coefficient, such as artificial leather, to prevent two or more sheets near the bottom from being fed at one time. The pressure plate is driven by a pressure plate cam to engage and disengage from the feed roller  821 . 
     A separation roller holder having a separation roller to separate sheets P one at a time is rotatable about a rotating shaft mounted to the separation base and is urged toward the feed roller  821  by a separation roller spring. The separation roller has a clutch spring that allows a portion of the holder attached with the separation roller to rotate when applied with more than a predetermined load. The separation roller is driven by a separation roller release shaft and a control cam to engage and disengage from the feed roller  821 . The positions of the pressure plate, the return lever and the separation roller are detected by a UT sensor. 
     The return lever to return the sheets P to the stack position is rotatably mounted on the UT base and is urged in a release direction by a return lever spring. Further, the return lever is pivoted by the control cam when returning the sheets P to the stack position. 
     In a normal standby state, the pressure plate is released by the pressure plate cam and the separation roller is released by the control cam. The return lever returns the sheets P and blocks a stack opening so as to prevent the sheets P from moving further inward when they are stacked. When, in this state, the paper feed operation starts, the motor drives the separation roller to engage the feed roller  821 . Then, the return lever is released and the pressure plate  21  engages the feed roller  821 . In this state the sheets P begin to be fed. The sheets P are restricted by a front stage restriction means provided on the base and only a predetermined number of sheets P are fed to a nip portion formed by the feed roller  821  and the separation roller  831 . The sheets P thus fed are separated by the nip portion and only the top sheet P is fed. 
     When the sheet P that was separated and fed reaches a position of a U-turn intermediate roller ( 1 )  86  and a U-turn pinch roller  861 , the pressure plate is released by the pressure plate cam and the separation roller is released by the control cam. The return lever is returned to the stack position by the control cam. At this time, the sheets P that have reached the nip portion between the feed roller  821  and the separation roller can be returned to the stack position. 
     On the downstream side of the paper feed unit, two transport rollers, i.e., the U-turn intermediate roller ( 1 )  86  and a U-turn intermediate roller ( 2 )  87  are arranged to further transport the sheet P that has been fed. These rollers include metal shafts having EPDM of rubber hardness of 40-80° attached to four to six locations on their cores. At positions corresponding to the rubber portions U-turn pinch rollers  861 ,  871  are mounted by spring shafts. These U-turn pinch rollers  861 ,  871  are urged toward the corresponding U-turn intermediate roller ( 1 )  86  and U-turn intermediate roller ( 2 )  87 . An inner guide is arranged on the inner side of the transport path of the sheet P, and an outer guide is arranged on the outer side of the transport path. 
     At a merge point between the transport path and the paper feed unit  2  there is a flapper that is constructed to ensure that their paths merge smoothly. When the front end of the sheet P is fed between the transport roller  36  and the pinch rollers  37 , it engages the nip portion of the roller pairs at rest so as to perform the sheet registration operation (positioning operation). 
     The sheet P that was printed as it was transported by the transport roller  36  and the pinch rollers  37  now passes between the rollers. In an automatic two-sided printing that prints both the front and back sides of the sheet P, the rear end of the sheet P is gripped between the transport roller  36  and the pinch rollers  37  and transported again. At this time, the pinch rollers  37  are moved up by a raise/lower mechanism  884  to allow the sheet P to be transported smoothly. 
     The sheet P that was fed again is gripped between a two-sided printing roller  891  and pinch rollers and transported, guided by a guide member. The transport path for the two-sided printing merges with the U-turn transport path, and the operation of the transport path after the merge is the same as described above. 
     (F) Cleaning Unit 
     The cleaning unit  6  comprises a pump  60  to clean the print head  7 , a cap  61  to keep the print head  7  from drying, and a blade to clean a face (nozzle forming surface) of the print head  7  in which nozzle openings are formed. 
     The cleaning unit  6  is driven mainly by the transport motor described above. The cleaning unit  6  has a one-way clutch that basically allows the pump  60  to be operated when the transport motor rotates in one direction and also allows the blade to be operated and the cap  61  to be raised or lowered when it rotates in the opposite direction. 
     The pump  60  generates a negative pressure by squeezing two tubes with a pump roller. The pump  60  is connected to the cap  61  through a valve or the like. With the cap  61  in hermetic contact with the print head  7 , the pump  60  is activated to suck out unnecessary ink from the print head  7 . The cap  61  is provided with an ink absorbent to minimize the amount of ink remaining on the face of the print head  7  after the suction operation. To prevent the ink remaining in the absorbent from solidifying and causing troubles, the ink remaining in the cap  61  is drawn out with the cap open. The waste ink sucked out by the pump  60  is absorbed and held in a waste ink absorbent provided in a lower case. 
     A series of operations, including the operation of the blade and the raise/lower operation of the cap  61 , is controlled by a main cam having a plurality of cams on the shaft. With the main cam acting on the cams and arms, the predetermined operations are performed. The position of the main cam can be detected by a position sensor such as photo interrupter. When the cap  61  is lowered, the blade is moved perpendicular to the scan direction of the carriage unit  5  to clean the face of the print head  7 . Two or more of the blades are provided, of which one is assigned to clean the nozzle openings and another cleans the entire face of the print head. When the blades have moved to the farthest end of their stroke, they engage a blade cleaner that removes ink adhering to the blades. 
     A drive force to open and close the valve between the pump  60  and the cap  61  is transmitted and controlled in connection with the rotation of the discharge roller  40 . Two or more of the caps  61  are provided to match a plurality of faces formed with nozzles that eject different colors of inks. These caps  61  are connected to the pump  60  through the associated valves. Therefore, by selectively controlling these valves, it is possible to perform an overall ink suction for all colors and individual suctions of desired ink colors as the situation demands. The positions of these valves are detected by valve position sensors. 
     (G) Enclosure 
     These units of the above constructions are built into the chassis  11  to form the mechanism portion of the printer. An enclosure is arranged to enclose the chassis  11 . The enclosure comprises mainly a lower case  99 , an upper case  98 , an access cover  97 , a connector cover, a front cover  95  and side covers  93 . 
     The front cover  95  accommodates the discharge tray  46  so that the discharge tray can be retracted and drawn out. When the printer is not in use, the paper discharge opening is closed. The open or closed state of the front cover  95  is detected by a sensor not shown. 
     The access cover  97  is rotatably attached to the upper case  98 . The upper case  98  has an opening in a part of its top surface, through which the ink tanks  71  and the print head  7  can be replaced. Further, to detect the opening and closing action of the access cover  97 , the upper case  98  has a door switch lever, an LED guide  982  to transmit and display LED light and a key switch  983  to act on a switch of the substrate. The upper case  98  also has the feed tray  26  rotatably mounted thereon. When the paper supply unit is not in use, the feed tray  26  is retracted to be used as a cover of the paper supply unit. 
     The upper case  98  and the lower case  99  are mounted by elastic engagement claws. Portions of these cases  98 ,  99  where connectors are provided are enclosed by a connector cover. 
     The side covers  93  are mounted to left and right sides of the apparatus body to cover the upper case  98  and the lower case  99 . 
     (Configuration of Control System) 
       FIG. 4  is an outline block configuration of a control system in the printing apparatus that can apply the present invention. 
     In  FIG. 4 , a CPU  100  executes control processing on the operations of the printing apparatus, including the correction control of the transport amount described later, and data processing. A ROM  101  stores programs dictating the procedures of these processing. A RAM  102  is used as a work area by the CPU when executing the processing. The print head  7  ejects ink according to drive data (image data) and a drive control signal (heat pulse signal) for heating elements. These data and signal are supplied to a head driver  7 A by the CPU  100 . The CPU  100  controls the carriage motor  54  for driving the carriage  50  in the main scan direction through a motor driver  54 A and also controls the transport motor  104  for transporting the sheet P in the transport direction through a motor driver  104 A. 
     (Transport Amount Correction Control) 
     Next, the control to correct the transport amount of the sheet P will be explained by referring to  FIG. 5  to  FIG. 7B . 
     As described earlier, the sheet P is fed from the paper feed unit  2  or the cassette  81  on the front side. The sheet P fed is transported a predetermined amount by the transport roller  36  and then printed by the print head  7  mounted on the carriage  50 . In a full width printing (marginless printing), the ink ejected outside the edges of the sheet P is absorbed in the platen absorbent on the platen  34 . That is, all the ink that was ejected outside the four side edges of the sheet P is absorbed in the platen absorbent. As the printing operation proceeds, the rear end Pr of the sheet P approaches and comes out of the nip portion between the transport roller  36  and the pinch rollers  37 . At this time, backlashes of gears in the roller drive system and an outer diameter difference between the transport roller  36  and the discharge roller  40  may cause the transport amount of the sheet P to deviate greatly from a predetermined amount. After the rear end Pr of the sheet P has come out of the nip portion between the transport roller  36  and the pinch rollers  37 , the transport accuracy of the sheet P is determined by the outer diameter accuracy of the discharge roller  40 . 
     Depending on the code wheel  362  directly connected to the transport roller  36  and the encoder sensor  363 , the minimum unit of the transport amount of the sheet P is, for example, 600 dpi (4.2 μm). 
     The carriage  50  is mounted with a reflection type transport amount correction sensor  53  composed of a light emitting element and a light receiving element. With this sensor  53 , the pattern printed on the sheet P can be read. The sensor  53  also doubles as a paper width sensor or a position sensor during a CD-R direct printing operation. 
     Next, a series of operations performed to correct the transport amount of the sheet P will be explained. 
     The sheet P set in the paper feed unit  2  or cassette  81  is fed to the position of the transport roller  36  which in turn transports the sheet. In this state, a test pattern is printed. This test pattern is used for correcting the transport amount of the sheet when the rear end Pr of the sheet P comes out of the nip portion between the transport roller  36  and the pinch rollers  37 . This test pattern, as shown in  FIG. 5 , is printed on the sheet P at a position near its rear end Pr. That is, the test pattern PA is printed in five blocks (PA- 1  to PA- 5 ) arrayed in the scan direction (X direction) of the carriage  50 . 
     The test pattern PA is printed in two scans of the print head  7  as follows. 
     First, the rear end Pr of the sheet P is detected by the PE sensor and, based on the detection timing, the time when the rear end Pr comes out of the nip portion between the transport roller  36  and the pinch rollers  37  is determined. Then, in the first scan immediately before the rear end Pr of the sheet P comes out of the nip portion between the transport roller  36  and the pinch rollers  37 , first-applied dots D 1  are formed as shown in  FIG. 6A . These first-applied dots D 1  are formed simultaneously in five blocks PA- 1  to PA- 5 . 
     Next, the sheet P is transported a predetermined amount by the transport roller  36  to cause the rear end Pr to come out of the nip portion between the transport roller  36  and the pinch rollers  37 . Then, in a second scan of the print head  7  as shown in  FIG. 6B , subsequent dots D 2  are formed to overlap the first-applied dots D 1  of  FIG. 6A . In this case, the subsequent dots D 2  can be formed by transporting the sheet P half the length of the nozzle column of the print head  7  after the first-applied dots D 1  have been formed in the first scan and then performing the second scan of the print head  7 . 
     In forming the subsequent dots D 2  in the five blocks PA- 1  to PA- 5  following the printing of the first-applied dots D 1  in the first scan, the transport amount of the sheet P is changed. When the five blocks PA- 1  to PA- 5  are printed in the order from left to right, for example, the subsequent dots D 2  are formed as follows. 
     First, the first-applied dots D 1  are formed in the five blocks PA- 1  to PA- 5  in the first scan and then the sheet P is transported an amount which is (2×Aμm) shorter than a predetermined transport amount, after which the subsequent dots D 2  are formed in the first block PA- 1 . The predetermined transport amount of the sheet P is determined in advance for intermittent transporting of the sheet P in a serial scan type printing apparatus. 
     Next, the carriage  50  is stopped temporarily and the sheet P is transported an amount of Aμm, after which the subsequent dots D 2  are formed in the second block PA- 2 . Next, the carriage  50  is stopped temporarily and the sheet P is transported Aμm, after which the subsequent dots D 2  are formed in the third block PA- 3 . Next, the carriage  50  is stopped temporarily and the sheet P is transported Aμm, after which the subsequent dots D 2  are formed in the fourth block PA- 4 . Then, the carriage  50  is stopped temporarily and the sheet P is transported Aμm, after which the subsequent dots D 2  are formed in the fifth block PA- 5 . 
     Therefore, the subsequent dots D 2  in the blocks PA- 1  and PA- 2  are formed after the sheet P has been transported amounts, which are (2×A)μm and Aμm shorter than the predetermined transport amount respectively. The subsequent dots D 2  in the block PA- 3  are formed after the sheet P has been transported the predetermined transport amount. The subsequent dots D 2  in the blocks PA- 4  and PA- 5  are formed after the sheet P is transported amounts, which are Aμm and (2×A)μm longer than the predetermined transport amount respectively. The scans to form the first-applied dots D 1  and the subsequent dots D 2  are not limited to ones immediately before and after the sheet P comes out the nip portion. The first-applied dots D 1  may be formed in any scan before the sheet P is transported to come out of the nip portion and the subsequent dots D 2  may be formed in any scan after the sheet P has been transported and come out of the nip portion. 
     After the test pattern PA has been printed as described above, densities of the pattern blocks are measured by the transport amount correction sensor  53 . That is, as the carriage  50  is moved in the main scan direction, the sensor  53  detects the density of the test pattern PA. If the sheet P is correctly fed the predetermined transport amount when the rear end Pr of the sheet P comes out of the nip portion, the dots D 1 , D 2  are formed in the block PA- 3  as shown in  FIG. 6B . This represents a block with the highest density. In this case, in the block PA- 1  and block PA- 2  for which the sheet P is transported smaller amounts, the dots D 1 , D 2  are formed as shown in  FIG. 7A . In the block PA- 4  and block PA- 5  for which the sheet P is transported large amounts, the dots D 1 , D 2  are formed as shown in  FIG. 7B . In these blocks PA- 1 , PA- 2 , PA- 4 , PA- 5  the dots D 1 , D 2  overlap incompletely, leaving unprinted portions S to appear, lowering the density. 
     Therefore, if the block with the highest density is not PA- 3 , the correction value for the transport amount when the sheet P comes out of the nip portion can be determined by detecting a block with the highest density. Suppose the block PA- 2  has the highest density, for example. Based on the correction value determined by this block, the transport amount of the sheet P is set Aμm smaller than the predetermined transport amount, so that possible degradations in the transporting accuracy when the rear end Pr of the sheet P leaves the transport roller  36  can be minimized. The correction may be done in a manner that corrects the transport amount when the rear end Pr of the sheet P leaves the transport roller  36  so as to reduce a difference between that transport amount and the predetermined transport amount. This method can also minimize the degradations in the transporting accuracy when the rear end Pr of the sheet P leaves the transport roller  36 . Such a correction value is stored in the memory in the printing apparatus body. 
     When in subsequent printing operations a rear end area of the sheet P is printed, the transport amount used when the rear end Pr of the sheet P leaves the nip portion is corrected based on this correction value. As explained above, the transport amount of the sheet P can be corrected by changing, based on such a correction value, the slit position of the encoder attached to the transport motor or the pulse signal representing the number of slits. By minimizing the degradations in the transporting accuracy when the rear end Pr of the sheet P leaves the nip portion between the transport roller  36  and the pinch rollers  37 , as described above, a quality image can be printed in the rear end area of the sheet P. 
     The method of printing the test pattern PA (PA- 1  to PA- 5 ) is not limited to the above configuration. For example, the sheet P may be transported Aμm at a time to form the first-applied dots D 1  in the blocks PA- 1  to PA- 5  so that the dots in these blocks are shifted Aμm between the blocks. After this, the sheet P is transported a amount 2 Aμm smaller than the predetermined transport amount. Then, the subsequent dots D 2  may be formed in the blocks PA- 1  to PA- 5  in one scan. In this case, there is no need to transport the sheet Aμm at a time by only the discharge rollers  40 ,  41  to form the subsequent dots D 2 . This reduces transporting errors resulting from the transporting of the sheet P by only the discharge rollers  40 ,  41 . 
     The test pattern PA may also be printed by successively completing the individual blocks PA- 1  to PA- 5 . That is, the PA- 1  block is first printed with the first-applied dots D 1  and then the sheet P is transported a amount 2 Aμm smaller than the predetermined transport amount, after which the same block is printed with the subsequent dots D 2 . The sheet P with the PA- 1  block printed is again set in the printing apparatus and the next PA- 2  block is printed by forming the first-applied dots D 1  in the PA- 2  block, transporting the sheet P a amount Aμm smaller than the predetermined transport amount and forming the subsequent dots D 2  in the PA- 2  block. The sheet P with the PA- 1  and PA- 2  blocks printed is again set in the printing apparatus before proceeding to print the next PA- 3  block. That is, after forming the first-applied dots D 1  in the PA- 3  block, the sheet P is transported the predetermined transport amount and the subsequent dots D 2  are formed in the PA- 3  block. For the blocks PA- 4  and PA- 5  the similar printing procedure is performed. By repeating the process of completing the individual blocks PA- 1  to PA- 5  successively, the test pattern PA is formed. With this test pattern printing method, each time the individual block PA- 1  to PA- 5  is completed, the sheet transporting is done to cause the rear end Pr of the sheet P to leave the nip portion, as in the normal printing operation. Therefore, based on the same sheet transporting operation as performed during the actual printing operation, the correction value for the transport amount can be determined. 
     The control to correct the transport amount of the rear end area of the sheet P after having printed the test pattern PA on the sheet may, for example, be performed automatically when the power of the printing apparatus is first turned on. It is also possible to provide a command unit in the printing apparatus  1  (see  FIG. 1 ) and/or in a host device connected to the printing apparatus  1  and to freely perform the correction control on the transport amount of the sheet P according to commands of an operator from the command unit. That is, with the mechanism for printing a test pattern on the print medium (sheet P), the printing apparatus  1  of this embodiment can perform the correction control on the print medium transport amount at a desired timing according to the command from the operator. 
     Second Embodiment 
     In the first embodiment, the method of correcting the transport amount when the rear end Pr of the sheet P leaves the transport roller  36  has been explained. In the second embodiment the process of correcting the transport amount when the sheet P is fed only by the discharge rollers  40 ,  41  after the rear end Pr of the sheet P have left the nip portion between the transport roller  36  and the pinch rollers  37 . The construction similar to the one already explained above will be omitted in the following description. 
     In  FIG. 8 , PB (PB- 1  to PB- 5 ) represents a test pattern used to correct a transport amount when the sheet P is fed only by the discharge rollers  40 ,  41 . As with the test pattern PA described above, the test pattern PB is also printed in two scans. This test pattern PB, as shown in  FIG. 8 , comprises five blocks (PB- 1  to PB- 5 ) arrayed near the rear end Pr of the sheet P in the scan direction (X direction) of the carriage  50 . This test pattern PB is located closer to the rear end Pr than the position where the test pattern PA is printed. 
     The printing of the blocks PB- 1  to PB- 5  in both the first scan and the second scan is performed after the rear end Pr of the sheet P has left the nip portion between the transport roller  36  and the pinch rollers  37 . That is, between the two scans the sheet P is transported by only the discharge rollers  40 ,  41 . 
     First, the rear end Pr of the sheet P is detected by the PE sensor and, based on the detection timing, the time when the rear end Pr comes out of the nip portion between the transport roller  36  and the pinch rollers  37  is determined. Then, after the rear end Pr of the sheet P has come out of the nip portion between the transport roller  36  and the pinch rollers  37 , the first-applied dots D 1  are formed in the blocks PB- 1  to PB- 5  of the test pattern PB in the first scan of the print head  7 . Then, in the second scan after the sheet P has been transported differing amounts, the subsequent dots D 2  are formed in the blocks PB- 1  to PB- 5  respectively. The amount by which the transport amounts of the sheet P are differentiated may be Aμm as described above, or other value. 
     After the blocks PB- 1  to PB- 5  have been completely printed in two scans, their densities are measured by the transport amount correction sensor  53 . By detecting a block with the highest density, the correction value for the amount of transporting by the discharge rollers  40 ,  41  after the rear end Pr of the sheet P has left the nip portion can be determined. This correction value is stored in the memory in the printing apparatus body. In subsequent printing operations, when a rear end area of the sheet P is printed, the transport amount after the rear end Pr of the sheet P has left the nip portion is corrected based on this correction value in such a way that the transport amount is equal to the predetermined transport amount or their difference is reduced. By correcting the transport amount of the sheet P in this manner, it is possible to minimize degradations in the transporting accuracy when the sheet P is transported only by the discharge rollers  40 ,  41  and thereby print a quality image in the rear end area of the sheet P. 
     Third Embodiment 
     Next, a control to correct the transport amount used when the rear end Pr of the sheet P leaves the nip portion between the transport roller  36  and the pinch rollers  37  and a control to correct the transport amount used after the sheet P has left the nip portion will be explained. Such a correction of the transport amount can be executed by combining the constructions of the first embodiment and the second embodiment. That is, from the test pattern PA a correction value for the transport amount of the sheet P used when the rear end Pr of the sheet P leaves the nip portion between the transport roller  36  and the pinch rollers  37  is determined. Then, based on the correction value, the transport amount when the rear end Pr of the sheet P leaves the nip portion of the rollers  36 ,  37  is corrected. Further, from the test pattern PB, a correction value for the transport amount used when the sheet P is transported only by the discharge rollers  40 ,  41  after it has left the nip portion is determined. Then, based on the correction value, the transport amount when the sheet P is transported only by the discharge rollers  40 ,  41  is corrected. Constructions similar to those described above are omitted in the following description. 
     These test patterns PA, PB for the transport amount correction are printed near the rear end Pr of the sheet P. The test patterns PA (PA- 1  to PA- 5 ) and PB (PB- 1  to PB- 5 ) are each comprised of five blocks arrayed in the scan direction (X direction) of the carriage  50 . PA (PA- 1  to PA- 5 ) and PB (PB- 1  to PB- 5 ) are shifted in position from each other in the transport direction (Y direction). 
     First, in the same procedure as in the first embodiment, the test pattern PA is printed on the sheet P. Then, densities of the pattern blocks are measured by the transport amount correction sensor  53 . By detecting a block with the highest density, a correction value for the transport amount used when the sheet P leaves the nip portion can be determined as described above. Then, based on the correction value, the transport amount when the rear end Pr of the sheet P leaves the nip portion is corrected in such a way that the transport amount is equal to the predetermined transport amount or their difference is reduced. 
     Next, the test pattern PB is printed on the sheet P. The procedure for printing the test pattern PB is the same as in the second embodiment. Using the transport amount correction sensor  53 , a block with the highest print density is detected from among the blocks PB- 1  to PB- 5  of the test pattern PB. As a result, a correction value for the transport amount used when the sheet P is transported only by the discharge rollers  40 ,  41  after the rear end Pr of the sheet P has left the nip portion can be determined. Based on this correction value, the transport amount of the sheet P after the rear end Pr of the sheet P has left the nip portion is corrected in such a way that the transport amount is equal to the predetermined transport amount or their difference is reduced. 
     From the test pattern PA, a transport amount correction value for the sheet P used when the rear end Pr of the sheet P leaves the nip portion between the transport roller  36  and the pinch rollers  37  (transport means on the upstream side with respect to the transport direction) is determined. Then, based on the correction value, the transport amount used when the rear end Pr of the sheet P leaves the nip portion between the transport roller  36  and the pinch rollers  37  can be corrected. Further, from the test pattern PB, a transport amount correction value used when the sheet P is transported only by the discharge rollers  40 ,  41  after it has left the nip portion is determined. Then, based on the correction value, the transport amount used when the sheet P is transported only by the discharge rollers  40 ,  41  can be corrected. As a result, degradations in the transporting accuracy after the rear end of the sheet P has left the transport means on the upstream side with respect to the transport direction can be minimized. This in turn minimizes deteriorations of image quality in the rear end area of the sheet P. 
     Fourth Embodiment 
     The printing apparatus of this embodiment has a transport amount sensor  300  to directly measure the distance that the sheet P has moved. With this sensor, it is possible to correct the transport amount used when the rear end Pr of the sheet P leaves the transport means (upstream transport means) on the upstream side with respect to the transport direction and after it has left the upstream transport means. Constructions similar to those described above are omitted in the following description. 
     The construction having the transport amount sensor  300  is not limited to the correction of the transport amount used when the rear end Pr of the sheet P leaves the upstream transport means and after it has left the upstream transport means. That is, this embodiment can also be applied to correcting the transport amount used when the rear end Pr of the sheet P leaves the nip portion and to correcting the transport amount used after the rear end Pr of the sheet P has left the nip potion. 
     Referring to  FIG. 10 , an example construction of the transport amount sensor  300  is explained. In the transport amount sensor  300  of this example, an LED  301  as a light source and a light receiver  302  are arranged as shown in  FIG. 10 . At the position between the transport roller  36  and the discharge rollers  40 ,  41 , the transport amount sensor  300  can directly measure the distance (transport amount) the sheet P has traveled. The transport amount sensor  300  is mounted, for example, on a side surface of the carriage  50  in the printing apparatus  1 . Alternatively, the transport amount sensor  300  may be arranged at a position facing the ejection face of the print head  7  to measure the travel amount (transport amount) from the back of the sheet P. As the light receiver  302  may be used a line sensor having light receiving elements arrayed one dimensionally or an area sensor having the light receiving elements arranged two dimensionally. As the light receiving elements, CCD or CMOS may be used. 
     The transport amount sensor  300  emits light from the LED  301  to the sheet P transporting to the right or left in  FIG. 10  during the printing operation and receives reflected light from the sheet P by the light receiver  302  at a predetermined interval. The sheet P is printed with an image by repetitively performing the transporting of the sheet P and the printing scan of the print head  7 . Thus, by detecting the printed image on the sheet P by the transport amount sensor  300 , the transport amount or travel distance of the sheet P before and after a printing scan can be detected. That is, data of the reflected light taken in from the light receiver  302  at each timing is subjected to image processing to extract features of individual images, determining a distance by which each of the images is deviated from the previous event (previous data intake timing). 
     Extraction of image features may apply a variety of methods. They may include, for example, a method involving Fourier-transforming the data taken in from the light receiver  302  and checking a match at each frequency, or a method involving extracting only a peak of the data and determining a positional deviation of that peak. Another popular method involves binarizing the data of the printed image taken in and checking a match with binarized pattern. In any of these methods it is also possible to determine an instantaneous value of a transporting speed of the sheet P from the sheet travel distance per unit time obtained or calculate acceleration from a change in the transporting speed. 
     The use of the reflection type optical sensor enables the measurement of the transporting speed and travel amount (transport amount) of the sheet P at each unit time. At this point this embodiment differs greatly from the embodiment that uses a rotary angle sensor. The rotary angle sensor measures the time it takes for the sheet to move a unit distance (unit rotary angle), so it is difficult to perform a highly precise control during a low-speed operation. With this embodiment, however, the transport amount of the sheet P can be determined with a stable accuracy whatever the transporting speed of the sheet P. 
     In performing the printing operation, the rear end Pr of the sheet P is detected by the PE sensor to determine the timing at which the rear end Pr of the sheet P leaves the nip portion between the transport roller  36  and the pinch rollers  37 . Then, using the transport amount sensor  300 , the transport amount appearing before the rear end Pr of the sheet P leaves the nip portion, the transport amount appearing when it leaves the nip portion and the transport amount appearing after it has left the nip portion are measured. Based on differences among these measured transport amount, the transport amount used when the rear end Pr of the sheet P leaves the nip portion and the transport amount after it has left the nip portion are corrected. 
     By correcting the transport amount as the rear end area of the sheet is printed, as described above, degradations in transporting accuracy that may occur when the rear end Pr of the sheet P leaves the upstream transport means and after it has left the upstream transport means can be minimized. As a result, image deteriorations in the rear end area of the sheet P can be suppressed. 
     The control to correct the transport amount of the rear end area of the sheet P by directly measuring the transport amount of the sheet P, as described above, may be performed automatically, for example, when the power of the printing apparatus is first turned on. It is also possible to provide a command unit in the printing apparatus  1  or in a host device connected to the printing apparatus  1  and to freely perform the correction control on the transport amount of the sheet P according to commands of an operator from the command unit. That is, using the detection mechanism such as the transport amount sensor  300 , the printing apparatus  1  of this embodiment can perform the correction control on the transport amount at a desired timing according to the command from the operator. 
     Fifth Embodiment 
     In this embodiment, an operator recognizes a printed result of a test pattern for correcting a transport amount of the rear end of the sheet P and enters a result of his or her check. 
     In this embodiment test patterns PA, PB are printed as shown in  FIG. 11 . That is, to ensure that the blocks in the test patterns can clearly be checked, the sheet P is printed with reference symbols A and B at the upper and lower tiers of the test pattern and also with reference numbers  1 ,  2 ,  3 ,  4  and  5  for columns of blocks arranged from left to right, in addition to the test patterns PA, PB. 
       FIG. 12  is a flow chart showing the process of correcting the transport amount by using such test patterns. First, a test sample is printed which includes the test patterns PA, PB, reference symbols A, B and reference numbers  1 - 5  (step  101 ). The operator checks the printed result of the test sample, finds a block with the highest density from among the A blocks of the test pattern PA, and enters the column number of that block (step  102 ). According to the input value entered, the transport amount is corrected (step  103 ) to secure the transporting accuracy when the rear end of the sheet P leaves the transport roller  36 . Next, the operator determines a block with the highest density from among the B blocks of the test pattern PB and enters a column number of that block (step  104 ). As a result, the transport amount is corrected according to the value entered (step  105 ), thus securing the transporting accuracy after the rear end of the sheet P has left the transport roller  36 . 
     The information on the test patterns that the operator enters may be entered from an operation unit provided in the printing apparatus  1 . The printing apparatus  1  can correct the transport amount according to the value entered and thus prevent possible degradations of image in the rear end area of the sheet P. Alternatively, the operator may enter information on the test pattern into a host device connected to the printing apparatus  1  and transfer this information from the host device to the printing apparatus  1  for the similar processing described above. 
     With this embodiment, there is no need to provide the transport amount correction sensor  53  in the printing apparatus, so the cost of the printing apparatus can be lowered. Other constructions and actions are similar to those of the preceding embodiments. 
     It is noted that this embodiment is not limited to a case where both of the test pattern PA for correcting the transport amount used when the rear end Pr of the sheet P leaves the transport roller  36  and the test pattern PB used to correct the transport amount after the rear end Pr of the sheet P has left the transport roller  36  are printed. That is, this embodiment can also be applied to cases where only the test pattern PA or only the test pattern PB is printed. 
     Other Embodiments 
     The correction of the transport amount performed on the rear end area of the sheet P, described in the first to fifth embodiment, may be combined with the method disclosed in Japanese Patent Laid-Open No. 2002-254736. That is, based on a change in the transport amount appearing when the rear end Pr of the sheet P leaves the nip portion, the transport amount associated with the rear end area of the sheet P is corrected and at the same time the range of nozzles in the print head  7  that can be used is limited to reduce the transport amount associated with the rear end area of the sheet P. Further, with this construction the transporting accuracy of the rear end area of the sheet P can be enhanced compared with that of Japanese Patent Laid-Open No. 2002-254736, making it possible to increase the number of nozzles to be operated and the transport amount to give a priority to the printing speed. 
     The printing apparatus of this invention may have a construction for printing test patterns on a print medium and a mechanism such as a transport amount sensor for detecting a difference between transport amounts. The latter mechanism is one that detects a difference between a transport amount appearing before the sheet P leaves the nip portion of the transport means disposed on the upstream side with respect to the transport direction and a transport amount appearing when the sheet P leaves the nip portion or appearing after it has left the nip portion. With this construction, it is possible to correct the transport amount according to the operating condition of the printing apparatus even if the transport amount changes due to changing conditions of the printing apparatus when the sheet P leaves the nip portion or after it has left the nip portion. The operating condition of the printing apparatus varies when a roller diameter changes with temperature and humidity variations or when a roller friction coefficient changes as the number of sheets printed increases. 
     While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions. 
     This application claims the benefit of Japanese Patent Application No. 2005-362426, filed Dec. 15, 2006, and Japanese Patent Application No. 2006-271189, filed Oct. 2, 2006, which are hereby incorporated by reference herein in their entirety.