Printing apparatus and printing method

A printing apparatus and method are capable of printing an image in a rear end area of a print medium. A sheet is transported by a transport roller and pinch rollers situated upstream of a print head with respect to the transport direction, and by a discharge roller and spurs situated downstream of the print head. After the sheet has left the transport roller and the pinch rollers, it is transported by the discharge roller and spurs. The sheet is printed with a test pattern for detecting a difference between a transport amount appearing before the sheet leaves the transport roller and pinch rollers and a transport amount appearing when the sheet leaves them. Based on a correction value set according to a printed result of the test pattern, the transport amount used when the sheet leaves the transport roller and pinch rollers is corrected.

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.

DESCRIPTION OF THE EMBODIMENTS

Now, embodiments of this invention will be described by referring to the accompanying drawings.

First Embodiment

FIG. 1throughFIG. 7Brepresent the first embodiment of this invention.FIG. 1is a perspective view of a printing apparatus to which this embodiment can be applied;FIG. 2is a perspective view showing a mechanism of the printing apparatus;FIG. 3is a cross-sectional view of the printing apparatus;FIG. 4is a block configuration diagram of a control system of the printing apparatus; andFIGS. 5 to 7Bare explanatory diagrams showing a transport amount correction control.

The printing apparatus1of this embodiment comprises a paper feed unit2, a paper transport unit3, a carriage unit5, a paper discharge unit4, an automatic U-turn transport unit8, a cleaning unit6for a print head7, and an enclosure9. Each of these components will be explained as follows (seeFIG. 1toFIG. 3).

(A) Paper Feed Unit

A base20of the paper feed unit2is provided with a pressure plate21for stacking sheets (of print medium) P, a paper feed roller28for feeding sheets P, a separation roller241for separating sheets P, and a return lever (not shown) to return sheets P to the stack position. A feed tray26for holding the stacked sheets P is mounted on the base20or the enclosure. The feed tray26is of multistage type and is drawn out for use.

The paper feed roller28is 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 unit2and commonly used also by the cleaning unit6(hereinafter referred to also as a “transport motor”) transmits a drive force to the paper feed roller28through a drive force transmission gear and a planetary gear not shown.

The pressure plate21has a movable side guide23which limits the stack position of the sheets P. The pressure plate21is rotatable about a rotating shaft connected to the base20and is urged by a pressure plate spring not shown toward the paper feed roller28. A part of the pressure plate21facing the paper feed roller28is 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 plate21is driven by a pressure plate cam not shown to engage and disengage from the paper feed roller28.

Further, a separation roller holder mounted with the separation roller241to separate sheets P one at a time is rotatable about a rotating shaft mounted to the base20and is urged toward the paper feed roller28by a separation roller spring not shown. The separation roller241has a clutch spring that allows a portion of the holder attached with the separation roller241to rotate when applied with more than a predetermined load. The separation roller241is driven by a separation roller release shaft and a control cam not shown to engage and disengage from the paper feed roller28. The positions of the pressure plate21, the return lever not shown to return the sheets P to the stack position and the separation roller241are 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 base20and 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 plate21is released by the pressure plate cam, the separation roller241is 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 roller241to engage the paper feed roller28. Then, the return lever is released and the pressure plate21engages the paper feed roller28. In this state the sheets P begin to be fed. The sheets P are restricted by a front stage separation unit provided on the base20and only a predetermined number of sheets P are fed to a nip portion formed by the paper feed roller28and the separation roller241. 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 roller36and pinch rollers37, the pressure plate21is released by the pressure plate cam and the separation roller241is 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 roller28and the separation roller241can be returned to the stack position.

(B) Paper Transport Unit

The paper transport unit3is mounted on a chassis11formed of a bent metal plate. The paper transport unit3has the transport roller36for transporting sheets P and a PE (paper end) sensor not shown. The transport roller36is a metal shaft coated with fine ceramic particles on its surface and the metal portions of the shaft ends are mounted on the chassis11through bearings. To assure a stable transport by applying a load to the transport roller36during its rotation, a transport roller tension spring is provided between the transport roller36and its bearing to give a bias to the transport roller36to apply a predetermined load.

The transport roller36is engaged with a plurality of pinch rollers37that are driven by the transport roller. The pinch rollers37are pressed against the transport roller36by a pinch roller spring biasing the pinch roller holder to generate a sheet P transporting force. The pinch roller holder holds the pinch rollers37and rotates about bearings of the chassis. At an inlet of the paper transport unit3to which the sheets P are transported, a paper guide flapper33to guide the sheets P and a platen34are 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 sensor32. The platen34is mounted and positioned on the chassis11. The paper guide flapper33is rotatable about predetermined bearing portions in the transport roller36and is positioned when it engages the chassis11.

In this construction, the sheet P fed to the paper transport unit3is guided by the pinch roller holder and the paper guide flapper33and fed between the transport roller36and the pinch rollers37. 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 rollers36,37are driven by the transport motor, the sheet P is transported over the platen34, 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 roller36through a timing belt and a pulley. On the rotating shaft of the transport roller36is provided a code wheel362that is formed with marks at 150-300 lpi (line per inch). The chassis11is mounted with an encoder sensor363that detects the marks on the code wheel362to determine the amount of transport by the transport roller36. 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 roller36in the transport direction of the sheet P, the print head7to form an image according to image information is installed. As the print head7an ink jet print head is used which allows a plurality of ink tanks71of different color inks to be replaced individually. This print head7can 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 head7to form an image on the sheet P. The construction of the print head7is 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 unit5has a carriage50in which the print head7can be mounted. The carriage50is guided by a guide shaft52and a guide rail11so 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 carriage50is driven by a carriage motor54mounted on the chassis11through a timing belt. To detect the position of the carriage50, a code strip561formed 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 carriage50. The encoder sensor reads the marks on the code strip561. The print head7is fixed to the carriage50by pivoting a head set lever51. The carriage50is 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 head7.

In this construction, an image is formed on the sheet P by transporting the sheet P to the image forming position by the rollers36,37and ejecting ink from the print head7while the carriage unit5is moved in the main scan direction by the carriage motor.

(D) Paper Discharge Unit

The paper discharge unit4comprises two discharge rollers40,41, spurs42engaged with and driven by these rollers, and a gear train to transmit the rotating force of the transport roller36to the discharge rollers40,41.

The discharge rollers40,41are mounted to the platen34. The discharge roller40situated on the upstream side of the transport direction of the sheet P is driven by the rotating force of the transport roller36transmitted through an idler gear. To the discharge roller41situated on the downstream side of the discharge roller40a drive force is transmitted from the discharge roller40through an idler gear.

The spurs42are thin SUS plates having protruding portions at their periphery and formed integral with a resin portion. They are attached to a spur holder43. The spurs42are attached to the spur holder43by spur springs and pressed against the discharge rollers40,41by a biasing force of the spur springs. The spurs42include 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 rollers40,41. 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 portions411of the discharge rollers40,41. A paper end support for holding both ends of the sheet P at elevated positions is arranged between the discharge rollers40and41.

In this construction, the sheet P formed with an image by the carriage unit5is gripped by the nip portion between the discharge roller40and the spurs42and the nip portion between the discharge roller41and the spurs42and transported and discharged onto a discharge tray46. The discharge tray46is accommodated in a front cover95and drawn out for use, as shown inFIG. 1. The discharge tray46is 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.

The cassette81provided on the front side of the apparatus accommodates the sheets P. To separate the sheets P for feeding, the cassette81has a pressure plate that presses the sheet P against the feed roller821. A UT base of the apparatus body is provided with the feed roller821for 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 cassette81has 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 cassette81is contracted and accommodated in the enclosure9of the apparatus body.

The feed roller821is 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 unit8transmits a drive force to the feed roller821through 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 cassette81and is urged toward the feed roller821by 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 roller821is 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 roller821.

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 roller821by 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 roller821. 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 roller821. Then, the return lever is released and the pressure plate21engages the feed roller821. 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 roller821and the separation roller831. 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)86and a U-turn pinch roller861, 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 roller821and 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)86and a U-turn intermediate roller (2)87are 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 rollers861,871are mounted by spring shafts. These U-turn pinch rollers861,871are urged toward the corresponding U-turn intermediate roller (1)86and 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 unit2there 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 roller36and the pinch rollers37, 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 roller36and the pinch rollers37now 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 roller36and the pinch rollers37and transported again. At this time, the pinch rollers37are moved up by a raise/lower mechanism884to allow the sheet P to be transported smoothly.

The sheet P that was fed again is gripped between a two-sided printing roller891and 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 unit6comprises a pump60to clean the print head7, a cap61to keep the print head7from drying, and a blade to clean a face (nozzle forming surface) of the print head7in which nozzle openings are formed.

The cleaning unit6is driven mainly by the transport motor described above. The cleaning unit6has a one-way clutch that basically allows the pump60to be operated when the transport motor rotates in one direction and also allows the blade to be operated and the cap61to be raised or lowered when it rotates in the opposite direction.

The pump60generates a negative pressure by squeezing two tubes with a pump roller. The pump60is connected to the cap61through a valve or the like. With the cap61in hermetic contact with the print head7, the pump60is activated to suck out unnecessary ink from the print head7. The cap61is provided with an ink absorbent to minimize the amount of ink remaining on the face of the print head7after the suction operation. To prevent the ink remaining in the absorbent from solidifying and causing troubles, the ink remaining in the cap61is drawn out with the cap open. The waste ink sucked out by the pump60is 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 cap61, 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 cap61is lowered, the blade is moved perpendicular to the scan direction of the carriage unit5to clean the face of the print head7. 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 pump60and the cap61is transmitted and controlled in connection with the rotation of the discharge roller40. Two or more of the caps61are provided to match a plurality of faces formed with nozzles that eject different colors of inks. These caps61are connected to the pump60through 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.

These units of the above constructions are built into the chassis11to form the mechanism portion of the printer. An enclosure is arranged to enclose the chassis11. The enclosure comprises mainly a lower case99, an upper case98, an access cover97, a connector cover, a front cover95and side covers93.

The front cover95accommodates the discharge tray46so 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 cover95is detected by a sensor not shown.

The access cover97is rotatably attached to the upper case98. The upper case98has an opening in a part of its top surface, through which the ink tanks71and the print head7can be replaced. Further, to detect the opening and closing action of the access cover97, the upper case98has a door switch lever, an LED guide982to transmit and display LED light and a key switch983to act on a switch of the substrate. The upper case98also has the feed tray26rotatably mounted thereon. When the paper supply unit is not in use, the feed tray26is retracted to be used as a cover of the paper supply unit.

The upper case98and the lower case99are mounted by elastic engagement claws. Portions of these cases98,99where connectors are provided are enclosed by a connector cover.

The side covers93are mounted to left and right sides of the apparatus body to cover the upper case98and the lower case99.

(Configuration of Control System)

FIG. 4is an outline block configuration of a control system in the printing apparatus that can apply the present invention.

InFIG. 4, a CPU100executes control processing on the operations of the printing apparatus, including the correction control of the transport amount described later, and data processing. A ROM101stores programs dictating the procedures of these processing. A RAM102is used as a work area by the CPU when executing the processing. The print head7ejects 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 driver7A by the CPU100. The CPU100controls the carriage motor54for driving the carriage50in the main scan direction through a motor driver54A and also controls the transport motor104for transporting the sheet P in the transport direction through a motor driver104A.

Next, the control to correct the transport amount of the sheet P will be explained by referring toFIG. 5toFIG. 7B.

As described earlier, the sheet P is fed from the paper feed unit2or the cassette81on the front side. The sheet P fed is transported a predetermined amount by the transport roller36and then printed by the print head7mounted on the carriage50. 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 platen34. 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 roller36and the pinch rollers37. At this time, backlashes of gears in the roller drive system and an outer diameter difference between the transport roller36and the discharge roller40may 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 roller36and the pinch rollers37, the transport accuracy of the sheet P is determined by the outer diameter accuracy of the discharge roller40.

Depending on the code wheel362directly connected to the transport roller36and the encoder sensor363, the minimum unit of the transport amount of the sheet P is, for example, 600 dpi (4.2 μm).

The carriage50is mounted with a reflection type transport amount correction sensor53composed of a light emitting element and a light receiving element. With this sensor53, the pattern printed on the sheet P can be read. The sensor53also 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 unit2or cassette81is fed to the position of the transport roller36which 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 roller36and the pinch rollers37. This test pattern, as shown inFIG. 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-1to PA-5) arrayed in the scan direction (X direction) of the carriage50.

The test pattern PA is printed in two scans of the print head7as 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 roller36and the pinch rollers37is 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 roller36and the pinch rollers37, first-applied dots D1are formed as shown inFIG. 6A. These first-applied dots D1are formed simultaneously in five blocks PA-1to PA-5.

Next, the sheet P is transported a predetermined amount by the transport roller36to cause the rear end Pr to come out of the nip portion between the transport roller36and the pinch rollers37. Then, in a second scan of the print head7as shown inFIG. 6B, subsequent dots D2are formed to overlap the first-applied dots D1ofFIG. 6A. In this case, the subsequent dots D2can be formed by transporting the sheet P half the length of the nozzle column of the print head7after the first-applied dots D1have been formed in the first scan and then performing the second scan of the print head7.

In forming the subsequent dots D2in the five blocks PA-1to PA-5following the printing of the first-applied dots D1in the first scan, the transport amount of the sheet P is changed. When the five blocks PA-1to PA-5are printed in the order from left to right, for example, the subsequent dots D2are formed as follows.

First, the first-applied dots D1are formed in the five blocks PA-1to PA-5in 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 D2are 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 carriage50is stopped temporarily and the sheet P is transported an amount of Aμm, after which the subsequent dots D2are formed in the second block PA-2. Next, the carriage50is stopped temporarily and the sheet P is transported Aμm, after which the subsequent dots D2are formed in the third block PA-3. Next, the carriage50is stopped temporarily and the sheet P is transported Aμm, after which the subsequent dots D2are formed in the fourth block PA-4. Then, the carriage50is stopped temporarily and the sheet P is transported Aμm, after which the subsequent dots D2are formed in the fifth block PA-5.

Therefore, the subsequent dots D2in the blocks PA-1and PA-2are 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 D2in the block PA-3are formed after the sheet P has been transported the predetermined transport amount. The subsequent dots D2in the blocks PA-4and PA-5are 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 D1and the subsequent dots D2are not limited to ones immediately before and after the sheet P comes out the nip portion. The first-applied dots D1may be formed in any scan before the sheet P is transported to come out of the nip portion and the subsequent dots D2may 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 sensor53. That is, as the carriage50is moved in the main scan direction, the sensor53detects 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 D1, D2are formed in the block PA-3as shown inFIG. 6B. This represents a block with the highest density. In this case, in the block PA-1and block PA-2for which the sheet P is transported smaller amounts, the dots D1, D2are formed as shown inFIG. 7A. In the block PA-4and block PA-5for which the sheet P is transported large amounts, the dots D1, D2are formed as shown inFIG. 7B. In these blocks PA-1, PA-2, PA-4, PA-5the dots D1, D2overlap 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-2has 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 roller36can 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 roller36so 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 roller36. 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 roller36and the pinch rollers37, 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-1to 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 D1in the blocks PA-1to PA-5so 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 D2may be formed in the blocks PA-1to PA-5in one scan. In this case, there is no need to transport the sheet Aμm at a time by only the discharge rollers40,41to form the subsequent dots D2. This reduces transporting errors resulting from the transporting of the sheet P by only the discharge rollers40,41.

The test pattern PA may also be printed by successively completing the individual blocks PA-1to PA-5. That is, the PA-1block is first printed with the first-applied dots D1and 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 D2. The sheet P with the PA-1block printed is again set in the printing apparatus and the next PA-2block is printed by forming the first-applied dots D1in the PA-2block, transporting the sheet P a amount Aμm smaller than the predetermined transport amount and forming the subsequent dots D2in the PA-2block. The sheet P with the PA-1and PA-2blocks printed is again set in the printing apparatus before proceeding to print the next PA-3block. That is, after forming the first-applied dots D1in the PA-3block, the sheet P is transported the predetermined transport amount and the subsequent dots D2are formed in the PA-3block. For the blocks PA-4and PA-5the similar printing procedure is performed. By repeating the process of completing the individual blocks PA-1to PA-5successively, the test pattern PA is formed. With this test pattern printing method, each time the individual block PA-1to PA-5is 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 apparatus1(seeFIG. 1) and/or in a host device connected to the printing apparatus1and 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 apparatus1of 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 roller36has been explained. In the second embodiment the process of correcting the transport amount when the sheet P is fed only by the discharge rollers40,41after the rear end Pr of the sheet P have left the nip portion between the transport roller36and the pinch rollers37. The construction similar to the one already explained above will be omitted in the following description.

InFIG. 8, PB (PB-1to PB-5) represents a test pattern used to correct a transport amount when the sheet P is fed only by the discharge rollers40,41. As with the test pattern PA described above, the test pattern PB is also printed in two scans. This test pattern PB, as shown inFIG. 8, comprises five blocks (PB-1to PB-5) arrayed near the rear end Pr of the sheet P in the scan direction (X direction) of the carriage50. 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-1to PB-5in 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 roller36and the pinch rollers37. That is, between the two scans the sheet P is transported by only the discharge rollers40,41.

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 roller36and the pinch rollers37is determined. Then, after the rear end Pr of the sheet P has come out of the nip portion between the transport roller36and the pinch rollers37, the first-applied dots D1are formed in the blocks PB-1to PB-5of the test pattern PB in the first scan of the print head7. Then, in the second scan after the sheet P has been transported differing amounts, the subsequent dots D2are formed in the blocks PB-1to PB-5respectively. 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-1to PB-5have been completely printed in two scans, their densities are measured by the transport amount correction sensor53. By detecting a block with the highest density, the correction value for the amount of transporting by the discharge rollers40,41after 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 rollers40,41and 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 roller36and the pinch rollers37and 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 roller36and the pinch rollers37is 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 rollers36,37is 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 rollers40,41after 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 rollers40,41is 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-1to PA-5) and PB (PB-1to PB-5) are each comprised of five blocks arrayed in the scan direction (X direction) of the carriage50. PA (PA-1to PA-5) and PB (PB-1to 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 sensor53. 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 sensor53, a block with the highest print density is detected from among the blocks PB-1to PB-5of 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 rollers40,41after 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 roller36and the pinch rollers37(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 roller36and the pinch rollers37can be corrected. Further, from the test pattern PB, a transport amount correction value used when the sheet P is transported only by the discharge rollers40,41after 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 rollers40,41can 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 sensor300to 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 sensor300is 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 toFIG. 10, an example construction of the transport amount sensor300is explained. In the transport amount sensor300of this example, an LED301as a light source and a light receiver302are arranged as shown inFIG. 10. At the position between the transport roller36and the discharge rollers40,41, the transport amount sensor300can directly measure the distance (transport amount) the sheet P has traveled. The transport amount sensor300is mounted, for example, on a side surface of the carriage50in the printing apparatus1. Alternatively, the transport amount sensor300may be arranged at a position facing the ejection face of the print head7to measure the travel amount (transport amount) from the back of the sheet P. As the light receiver302may 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 sensor300emits light from the LED301to the sheet P transporting to the right or left inFIG. 10during the printing operation and receives reflected light from the sheet P by the light receiver302at 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 head7. Thus, by detecting the printed image on the sheet P by the transport amount sensor300, 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 receiver302at 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 receiver302and 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 roller36and the pinch rollers37. Then, using the transport amount sensor300, 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 apparatus1or in a host device connected to the printing apparatus1and 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 sensor300, the printing apparatus1of 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 inFIG. 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 numbers1,2,3,4and5for columns of blocks arranged from left to right, in addition to the test patterns PA, PB.

FIG. 12is 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 numbers1-5(step101). 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 (step102). According to the input value entered, the transport amount is corrected (step103) to secure the transporting accuracy when the rear end of the sheet P leaves the transport roller36. 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 (step104). As a result, the transport amount is corrected according to the value entered (step105), thus securing the transporting accuracy after the rear end of the sheet P has left the transport roller36.

The information on the test patterns that the operator enters may be entered from an operation unit provided in the printing apparatus1. The printing apparatus1can 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 apparatus1and transfer this information from the host device to the printing apparatus1for the similar processing described above.

With this embodiment, there is no need to provide the transport amount correction sensor53in 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 roller36and the test pattern PB used to correct the transport amount after the rear end Pr of the sheet P has left the transport roller36are 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 head7that 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.

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.