# Patent

## Title
Printing apparatus and conveying control method

## Abstract

Provided are a printing apparatus and a conveying-error controlling method capable of printing a high-quality image by correction reflecting the conveying amount of a printing medium. A provided inkjet printing apparatus prints images by printing scans for actual printing and by conveying the printing medium with a roller orthogonally to the printing-scan direction. In each printing scan, the printing medium is scanned with a print head having an array of nozzles from which the ink is ejected. The print head moving direction differs from the arranging direction of the nozzles in the array. The apparatus includes a conveying controller to control the conveying of the printing medium on the basis of a correction value used to correct a conveying error of the roller. The conveying controller changes the correction value to be applied in accordance with the conveying amount of the printing medium between two scans with the print head.

## Background
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a printing apparatus and a conveying control method. Specifically, the invention relates to a technique to apply a correction value to correct an error in conveying a printing medium used in an inkjet printing apparatus.
2. Description of the Related Art
An inkjet printing apparatus has a print head that has a fine-nozzle array, and ink is ejected from each nozzle in accordance with printing data. The ejected ink forms dots on the printing medium to form an image. Accordingly, to form a high-quality image, it is important that the dots should be formed on the printing medium at intended positions. The displacement of the dot-formation position has to be avoided as much as possible. Some of the various causes of such displacement deviation are: difference in shape amongst the nozzles of the print head; noise factors, such as the vibrations of the apparatus that occur while the printing is being carried out; and the distance between the printing medium and the print head. The inventors of the present invention have discovered that one of the significant causes for such displacement deviation of the dot-formation position is the lack of accuracy in conveying the printing medium. One of the commonly used conveying units for the printing medium is a roller (a conveying roller). Conveying the printing medium by a desired distance can be achieved by rotation of the conveying roller by a designated angle with the conveying roller being pressed onto the printing medium. Here, the accuracy in the conveying of the printing medium depends, to a significant extent, on the eccentricity of the conveying roller.
FIGS. 33, 34A and 34B, and 35 illustrate cross-sectional shapes of various conveying rollers. The conveying roller of FIG. 33 has a perfectly-circular cross-sectional shape, and has its central axis aligned exactly with its rotational axis. The conveying roller of FIGS. 34A and 34B has a cross-sectional shape that is not a perfect circle. The conveying roller of FIG. 35 has its rotational axis offset from its central axis.
Assume such a case as shown in FIG. 33, or, to be more specific, a case where the cross-sectional shape of the conveying roller is a perfect circle and where the central axis of the conveying roller is aligned exactly with its rotational axis. In addition, further assume that the rotational angle to convey the printing medium is uniform. Then, every rotation of the conveying roller by an angle R constantly gives a particular length (LO) in the circumferential directions (length of arc). Accordingly, every position within the conveying roller always gives a uniform amount of conveying the printing medium that is conveyed while being in contact with the conveying roller.
Contrasting outcomes are obtained by conveying rollers with an ellipsoidal cross-sectional shape such as those shown in FIGS. 34A and 34B. Such a conveying roller gives different amount of conveying even when the conveying roller rotates by the same angle R. This difference in the amount of conveying depends on the rotational position of the conveying roller. To be more specific, for the rotational position shown in FIG. 34A, the printing medium is conveyed by an amount L1 while for another rotational position shown in FIG. 34B, the printing medium is conveyed by an amount L2. Here, the lengths L0, L1, and L2 have such a relationship as L1>L0>L2. That is to say, a periodical variation in amount of conveying the printing medium occurs, and the variation depends on the period of the conveying roller.
Alternatively, as in the case of FIG. 35, the offsetting of the rotational axis of the conveying roller from the central axis O that is intended to be the rotational axis may sometimes cause the amount of conveying the printing medium to vary periodically in response to the period of the conveying roller. To be more specific, assume cases where the rotational axis is offset from the central axis O and is positioned at either the point A or the point B shown in FIG. 35. In these cases, the same rotational angle α produces different amounts of conveying. Such difference in conveying amount results in a periodical variation in the conveying of the printing medium. Here, the variation depends on the period of the conveying roller.
The eccentricity of the roller, which has been mentioned above, includes these above-described states. Specifically, included are a state where the roller has a cross-sectional shape that is not a perfect circle, and a state where the conveying roller has its rotational axis offset from its central axis. In the case of an ideal accuracy being achieved in conveying, the image should be printed in such a way as shown in the schematic diagram of FIG. 36A. With the above-mentioned eccentricity, however, the printed image will be an uneven image with stripes that appear periodically in the conveying direction as shown in FIG. 36B while the period is the same as the amount of conveying corresponding to a full rotation of the conveying roller.
The amount of eccentricity for the conveying roller is usually controlled so as to stay within a certain range. The stricter the standard for the amount of eccentricity is, the lower the yielding of the conveying roller becomes. Accordingly, the printing apparatus thus produced becomes more expensive. For this reason, an excessively strict standard for the amount of eccentricity is not preferable.
To address the above-mentioned problem, various measures have been proposed. Different correction values for the conveying errors are set for different phases of the conveying roller so that even an eccentric conveying roller can achieve a steady amount of conveying as similar to the case of a conveying roller with a perfectly-circular cross-sectional shape and with its rotation axis being aligned exactly with its central axis (Japanese Patent Laid-Open No. 2006-240055 and Japanese Patent Laid-Open No. 2006-272957). To be more specific, correction to reduce the amplitude of the fluctuation in amount of conveying with a period equivalent to the circumferential length of the conveying roller can be done by applying a periodic function with the same period and reversed polarity.
When printing is carried out on a front-end portion and on a rear-end portion of the printing medium, some inkjet printing apparatuses can reduce their respective numbers of nozzles to be used. When printing is carried out on these portions of the printing medium, the printing medium may be supported and conveyed by either the conveying roller or the discharge roller alone. In this state, the flatness of the printing medium may not be secured. As a consequence, fluctuations of not a small amount occur in the distance between the print head and the end portion that is not supported, and create a quite unstable state. This is one of the reasons for the reduction in the number of nozzles to be used. Another case of such reduction in the number of nozzles to be used is to achieve an improvement in printing quality for the printing on a particular portion of or the entire part of the printing medium.
Assume a case where the correction is carried out for an area to be printed with a reduced number of nozzles, while using the correction value for eccentricity and the correction value for outer-diameter which are equal to the respective values used for the printing with all the nozzles. In this case, despite the intention of the correction, streaks are caused in some areas. One of the reasons for the streaks is an occurrence of density unevenness resulting from inconsistency of the period, phase and amplitude of the eccentricity correction, with the amplitude at the time when the number of nozzles is changed by reducing the number of nozzles to be used.
SUMMARY OF THE INVENTION
The present invention was made in view of the above-described problems. The present invention, therefore, aims to carry out correction that reflects the amount of conveying the printing medium, and, eventually, to contribute to the achievement of the printing of a high-quality image.
To this end, an aspect of the present invention provides an inkjet printing apparatus that has the following features. The inkjet printing apparatus prints images by carrying out the printing scans and the conveying of the printing medium in a direction that is orthogonal to the direction of the printing scans. In the printing scans, printing is actually carried out while the printing medium is scanned with the print head. The print head has an array of nozzles from which the ink is ejected. The direction in which the print head used in the scans moves differs from the direction in which the nozzles in the array are aligned. In addition, the printing apparatus includes a conveying controller to control the conveying of the printing medium on the basis of a correction value used to correct the conveying error of the roller. The conveying controller changes the correction value to be actually applied in accordance with the amount by which the printing medium is actually conveyed between the two corresponding scans with the print head.
In the printing apparatus and the conveying control method that are provided with the above-described configuration, the correction is carried out in accordance with the actual amount of conveying the printing medium. As a result, the apparatus and the method can contribute to the achievement of the printing of high-quality images.
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 schematic perspective view illustrating the entire configuration of an inkjet printing apparatus according to an embodiment of the present invention;
FIG. 2 is an explanatory diagram schematically illustrating a print head which is employed in the embodiment shown in FIG. 1 and which is viewed from the side of a nozzle-formed face;
FIG. 3 is a block diagram illustrating an example of the configuration for a principal portion of a control system for the inkjet printing apparatus of FIG. 1;
FIG. 4 is a flowchart illustrating an outline of processing procedure to acquire a correction value for eccentricity and a correction value for outer-diameter according to the embodiment of the present invention;
FIG. 5 is an explanatory diagram illustrating an example of the test patterns used in this embodiment;
FIGS. 6A and 6B are explanatory diagrams for describing different states in which the printing medium is conveyed;
FIG. 6C is an explanatory diagram for describing the state in which the printing medium is released from an upstream-side conveying unit and comes to be conveyed by a downstream-side conveying unit alone;
FIG. 7 is an explanatory diagram for describing an aspect where the entire printing area of the printing medium is divided into two areas: an area on which the printing is done with the upstream-side conveying unit being involved in the action of conveying the printing medium; and another area on which the printing is done with the printing medium being conveyed by the downstream-side conveying unit alone;
FIG. 8 is an explanatory diagram illustrating another example of test patterns applicable to the embodiment of the present invention;
FIG. 9 is an explanatory diagram for describing the way nozzles are used when the test patterns are formed;
FIGS. 10A to 10E are explanatory diagrams for describing the way the test patterns, or the patches constituting the test patterns, are formed by using the upstream-side nozzle group NU and the downstream-side nozzle group ND;
FIGS. 11A and 11B are explanatory diagrams of, respectively, a patch element group for reference and a patch element group for adjustment each of which group is printed by a single main scan;
FIG. 12 is an explanatory diagram illustrating a test pattern including a group of patches each of which is composed of a patch element for reference and a patch element for adjustment. FIG. 12 illustrates, in an enlarged manner, one of the four test patterns shown in FIG. 5;
FIG. 13 is an explanatory diagram illustrating an enlarged patch element for reference or for adjustment;
FIG. 14 is an explanatory diagram illustrating the patch element of FIG. 13 in a further enlarged manner;
FIGS. 15A and 15B are explanatory diagrams for describing the change in density caused by the interference between the patch element for reference and the patch element for adjustment;
FIGS. 16A and 16B are explanatory diagrams for describing a problem caused by ejection failure that occurs in the nozzles used to form the test pattern;
FIGS. 17A and 17B are explanatory diagrams for describing that even when ejection failure in the nozzles used to form the test pattern causes a problem, the test pattern used in the embodiment can alleviate the problem;
FIG. 18 is a flowchart illustrating an example of arithmetic processing procedure to find the correction value for eccentricity according to the embodiment;
FIG. 19 is an explanatory diagram for illustrating, in a form of a graph, the conveying errors measured in numerical terms based on the information on density obtained from a certain test pattern;
FIG. 20 is an explanatory diagram for showing the difference that the conveying error for each value of n has with their average value;
FIG. 21 is an explanatory diagram for showing the absolute values of addition values Xn′′ for each value of n;
FIGS. 22A and 22B are explanatory diagrams for showing two examples of processing carried out to obtain a final correction value for eccentricity when plural test patterns are formed in the main-scanning direction;
FIG. 23 is a flowchart illustrating an example of arithmetic processing procedure to acquire a correction value for outer-diameter according to the embodiment;
FIG. 24 is an explanatory diagram for describing the occurrence of an error in the correction value for outer-diameter;
FIG. 25 is an explanatory diagram for describing the fact that the correction value for outer-diameter varies in response to the order of the acquiring of the correction value for eccentricity and the acquiring of the correction value for outer-diameter;
FIG. 26 is an explanatory diagram for describing a way to store a correction value for eccentricity according to the embodiment;
FIG. 27 is a flowchart showing an example of the conveying control procedure according to the embodiment;
FIG. 28 is an explanatory diagram for describing the way of applying the correction value for eccentricity to the conveying control;
FIG. 29 is a flowchart showing an embodiment of the processing procedure from the formation of a test pattern to the storing of a conveying-error correction value;
FIG. 30 is a flowchart showing another embodiment of the processing procedure from the formation of a test pattern to the storing of a conveying-error correction value;
FIG. 31 is a flowchart showing still another embodiment of the processing procedure from the formation of a test pattern to the storing of a conveying-error correction value;
FIG. 32 is an explanatory diagram for describing an alternative way of forming patches constituting the test pattern;
FIG. 33 is an explanatory diagram of a state of a conveying roller that has a perfectly-circular cross-sectional shape, and has its central axis aligned exactly with its rotational axis;
FIGS. 34A and 34B are explanatory diagrams of a state of conveying roller which has a cross-sectional shape that is not a perfect circle;
FIG. 35 is an explanatory diagram of a state of a conveying roller that has its rotational axis offset from its central axis;
FIGS. 36A and 36B are explanatory diagrams of images with and without unevenness caused by the eccentricity of the conveying roller, respectively;
FIGS. 37A to 37C are explanatory diagrams for describing printing areas;
FIG. 38 is a schematic top plan view of a platen;
FIGS. 39A to 39D are explanatory diagrams for illustrating printing areas according to a second embodiment of the present invention;
FIG. 40 is an explanatory diagram for showing a relationship between the range of nozzles to be used and the printing scans in the printing of a first embodiment of the present invention;
FIG. 41 is an explanatory diagram for showing a relationship between the range of nozzles to be used and the printing scans in the printing of the first embodiment of the present invention;
FIG. 42 is an explanatory diagram for showing a relationship between the range of nozzles to be used and the printing scans in the printing of the first embodiment of the present invention;
FIG. 43 is an explanatory diagram for showing a relationship between the range of nozzles to be used and the printing scans in the printing of the first embodiment of the present invention;
FIG. 44 is an explanatory diagram for showing a relationship between the range of nozzles to be used and the printing scans in the printing of the first embodiment of the present invention;
FIG. 45 is an explanatory diagram for showing a relationship between printing areas and the correction values in the first embodiment of the present invention;
FIG. 46 is an explanatory diagram for showing a relationship between the range of nozzles to be used and the printing scans in the printing of the first embodiment of the present invention;
FIG. 47 is an explanatory diagram for showing a relationship between the range of nozzles to be used and the printing scans in the printing of the first embodiment of the present invention;
FIGS. 48A and 48B are charts describing conveying errors;
FIGS. 49A and 49B are explanatory diagrams for describing a relationship between the magnitude of the rotation angle and the conveying amount;
FIGS. 50A and 50B are explanatory diagrams for describing a relationship between the magnitude of the rotation angle and the conveying amount;
FIGS. 51A to 51C are explanatory diagrams for describing a printing method according to the second embodiment of the present invention; and
FIG. 52 is an explanatory diagram for showing the relationship between the printing areas and the correction values according to other embodiments of the present invention.



DESCRIPTION OF THE EMBODIMENTS
Hereafter, the present invention will be described in detail with reference to accompanying drawings.
(1) Configuration of Apparatus
FIG. 1 is a schematic perspective view illustrating the entire configuration of an inkjet printing apparatus according to an embodiment of the present invention. When the printing is carried out, a printing medium P is held by and between a conveying roller 1—one of the plural rollers provided in the conveying path—and pinch rollers 2 that follow and are driven by the conveying roller 1. The printing medium P is guided onto a platen 3 by rotations of the conveying roller 1. The printing medium P is conveyed in a direction indicated by the arrow A in FIG. 1 while being supported on the platen 3. Though not illustrated in FIG. 1, a pressing member, such as a spring, is provided to elastically bias the pinch rollers 2 against the conveying roller 1. The conveying roller 1 and the pinch rollers 2 are components of a conveying unit on the upstream side.
The platen 3 is disposed at the printing position opposite to the face on which ejection openings are formed in a print head 4 provided in the form of an inkjet print head (hereafter the face is referred to as “ejection face”). The platen 3 thus disposed supports the back side of the printing medium P to keep a constant, or a predetermined, distance between the top surface of the printing medium P and the ejection face.
Once the printing is carried out on the printing medium P that has been conveyed onto the platen 3, the printing medium P is conveyed in the direction A, being held by and between a discharging roller 12 that rotates and spur rollers 13 that follow and are driven by the discharging roller 12. The printing medium P is thus discharged out onto an output tray 15. The discharging roller 12 and the spurring rollers 13 are components of conveying unit on the downstream side. It should be noted that only a single pair of the discharging roller 12 and the line of spurring rollers 13 is shown in FIG. 1, but that two pairs of them may be provided as will be described later.
A member 14 is disposed by one of the side ends of the printing medium P, and is used to set the reference line when the printing medium P is conveyed (the member will, therefore, be referred to as “conveying reference member 14”). Any printing medium P, irrespective of the width thereof, is conveyed with the above-mentioned side of the printing medium along the reference line set by the conveying reference member 14. Besides the role of setting the reference line, the conveying reference member 14 may also serve the purpose of restricting the rising-up of the printing medium P towards the ejection face of the print head 4.
The print head 4 is detachably mounted on a carriage 7 with its ejection face opposing to the platen 3, or the printing medium P. The carriage 7 is driven by a driving source—a motor—to reciprocate along two guide rails 5 and 6.The print head 4 may perform ink-ejection action during the reciprocating movement. The direction in which the carriage 7 moves is orthogonal to the direction in which the printing medium P is conveyed (in the direction indicated by the arrow A). Such a direction is usually referred to as “main-scanning direction” while the direction in which the printing medium P is conveyed is usually referred to as “sub-scanning direction.” The printing of images on the printing medium is carried out by repeating the alternation of main scan (printing scan) of the carriage 7, or the print head 4, and the conveying of the printing medium P (sub scan).
As the print head 4, for example, a print head that includes an element for generating thermal energy to be used for ejecting ink (an example of such element is a heat-generating resistor element) may be employed. The thermal energy causes a change in the state of the ink (that is, film boiling of the ink occurs). As another example, a print head that includes, as an element for generating energy, an element to generate mechanical energy may be employed. An example of such an element is a piezo element. The mechanical energy thus generated is used for the ejection of the ink.
The printing apparatus of this embodiment forms an image with pigment inks of ten colors. The ten colors are: cyan (C), light cyan (Lc), magenta (M), light magenta (Lm), yellow (Y), first black (K1), second black (K2), red (R), green (G), and gray (Gray). When a term “K-ink” is used, either the first black (K1) ink or the second black (K2) ink is mentioned. Here, the first and the second black inks (K1 and K2) may, respectively, be a photo black ink that is used to print a glossy image on glossy paper and a matt black ink suitable for matt coated paper without gloss.
FIG. 2 schematically illustrates the print head 4 used in this embodiment, and the print head 4 is viewed from the side of the nozzle-formed face. The print head 4 of this embodiment has two printing-element substrates H3700 and H3701, in each of which nozzle array for five colors of the above-mentioned ten colors formed. Each of the nozzle arrays H2700 to H3600 corresponds to each one of the ten different colors.
Nozzle arrays H3200, H3300, H3400, H3500, and H3600 are formed in one of the two substrates—specifically in the printing-element substrate H3700—to perform ink ejection with respective inks of gray, light cyan, the first black, the second black and light magenta being supplied to. Meanwhile, nozzle arrays H2700, H2800, H2900, H3000 and H3100 are formed in the other one of the two substrates—specifically, in the printing-element substrate H3701—to perform ink ejection with respective inks of cyan, red, green, magenta and yellow being supplied to. Each of the nozzle arrays is formed by 768 nozzles arranged in the direction of conveying the printing medium P at intervals of 1200 dpi (dot/inch) and ejects ink droplets each of which is approximately 3 picoliters. Each nozzle has an ejection opening with an opening area of approximately 100 μm2.
The above-described head configuration enables what is termed as “one-pass printing” to be carried out. In this way of printing, the printing on a single area of the printing medium P is completed in a single main scanning. However, what is termed as “multi-pass printing” is also possible for the purpose of improving the printing quality by reducing the negative influence of the nozzles that are formed with lack of uniformity. In this mode of printing, the printing on a single scanning area of the printing medium P is completed by carrying out main scanning plural times. When the multi-pass printing is selected, the number of passes is determined appropriately by taking account of conditions, such as the mode of printing.
Plural ink tanks corresponding to colors of inks to be used are detachably installed in the print head 4, independently. Alternatively, the inks may be supplied to the print head 4 via respective liquid-supply tubes from the corresponding ink tanks fixed somewhere in the apparatus.
A recovery unit 11 is disposed so as to be able to face the ejection face of the print head 4. The recovery unit 11 is disposed at a position within the area that the print head 4 can reach when the print head 4 moves in the main scanning direction. The position is located outside of side-edge portion of the printing medium P, or of the platen 3. That is, the position is in an area where no image is to be printed. The recovery unit 11 has a known configuration. Specifically, the recovery unit 11 includes a cap portion for capping the ejection face of the print head 4, a suction mechanism for sucking the inks with the ejection face being capped to force the inks out of the print head 4. A cleaning blade to wipe off the tainted ink-ejection face, among other members, is also included in the recovery unit 11.
FIG. 3 illustrates an example of the configuration for the principal portion of the control system for the inkjet printing apparatus according to this embodiment. A controller 100 controls each portions of the inkjet printing apparatus according to this embodiment. The controller 100 includes a CPU 101, a ROM 102, an EEPROM 103, and a RAM 104. The CPU 101 performs various arithmetic processing and determination for processing related to the printing action and the like including processing procedures that are to be described later. In addition, the CPU 101 performs the processing related to the print data and the like. The ROM 102 stores the programs corresponding to the processing procedures that are executed by the CPU 101, and also stores other fixed data. The EEPROM 103 is a non-volatile memory and is used to keep predetermined data even when the printing apparatus is switched off. The RAM 104 temporarily stores the print data supplied from the outside, and the print data developed in conformity with the configuration of the apparatus. The RAM 104 functions as a work area for the arithmetic processing performed by the CPU 101.
An interface (I/F) 105 is provided to connect the printing apparatus to an outside host apparatus 1000. Communications in both directions based on a predetermined protocol is carried out between the interface 105 and the host apparatus 1000. It should be noted that the host apparatus 1000 is provided by a known form, such as a computer. The host apparatus 1000 serves as a supply source of the print data on which the printing action of the printing apparatus of this embodiment is based. In addition, a printer driver—the program to cause the printing apparatus to execute the printing action—is installed in the host apparatus 1000. To be more specific, from the printer driver, the print data and the print set-up information, such as the information on the kind of printing medium P on which the print based on the print data is performed are sent. Also sent therefrom is the control command that causes the printing apparatus to control its action.
A linear encoder 106 is provided to detect the position of the print head 4 in the main-scanning direction. A sheet sensor 107 is provided in an appropriate position in the path of conveying the printing medium P. By detecting the front end and the rear end of the printing medium P with this sheet sensor 107, the conveying position (sub-scanning position) of the printing medium P can be determined. Motor drivers 108 and 112 and a head-driving circuit 109 are connected to the controller 100. The motor driver 108, under the control of the controller 100, drives a conveying motor 110, which serves as the driving source for conveying the printing medium P. The drive power is transmitted from the conveying motor 110 via a transmission mechanism, such as gears, to the conveying roller 1 and the discharge roller 12. The motor driver 112 drives a carriage motor 114, which serves as the driving source for the movement of the carriage 7. The drive power is transmitted from the carriage motor 114 via a transmission mechanism, such as a timing belt, to the carriage 7. The head-driving circuit 109, under the control of the controller 100, drives the print head 4 to execute the ink-ejection.
A rotary encoder 116 is mounted on each of the shafts of the conveying roller 1 and the discharge roller 12. Each of the rotary encoders 116 detects the rotational position and the speed of the corresponding roller so as to control the conveying motor 110.
A reading sensor 120 is provided to serve as detector for detecting the density of the images printed on the printing medium P. The reading sensor 120 may be provided in the form of a reading head mounted on the carriage 7 either along with or in place of the print head 4. Alternatively, the reading sensor 120 may be provided as an image-reading apparatus constructed as a body that is independent of the printing apparatus shown in FIG. 1.
(2) Outline of the Processing
In the printing apparatus with the above-described configuration, one of the biggest causes for the lowering of the accuracy in conveying is the eccentricity of a roller. The eccentricity of a roller is defined as a state where the rotational axis of a roller is offset from the central axis of the roller, that is, a state in which the axis of the rotational center of a roller deviates from the geometrical central axis of the roller. In addition, the eccentricity is defined as a state where the roller has a cross-sectional shape that is not a perfect circle. The eccentricity of a roller causes a periodical conveying error, and the period depends on the rotational angle from the reference position of the roller. Assume that such eccentricity exists. In this case, even when the roller is rotated by an equal angle, the length in the circumferential direction (lengths of arc) corresponding to the equal-angle rotation varies from one time to another. As a result, an error occurs in the amount of conveying the printing medium P. An error that occurs in this way prevents the formation, in the direction of conveying the printing medium P, of the dots in positions in which the dots are originally supposed to be formed. Dots are formed densely in some areas, and sparsely in others, in the direction of conveying the printing medium P. In summary, unevenness of printing occurs with a period equivalent to the amount of conveying corresponding to a full rotation of the roller.
Another example of the big causes for the lowering of the accuracy in conveying is a cause that derives from the error in the outer diameter of a roller. Assume that such an error in the outer diameter of a roller exists. In this case, even when the roller is rotated by a rotational angle that has been determined for a roller with a certain reference outer diameter, a predetermined amount of conveying which is supposed to be obtained cannot always be obtained. To be more specific, when a roller with an outer diameter that is larger than the reference outer diameter is used, the amount of conveying becomes larger than what is supposed to be. In this case, white stripes are likely to occur in the printed image. In contrast, when a roller with an outer diameter that is smaller than the reference outer diameter is used, the amount of conveying becomes smaller than what is supposed to be. In this case, black stripes are likely to occur in the printed image.
In view of what has just been described above, this embodiment of the present invention aims to provide a configuration that is capable of reducing variations in positions of dot formation, which derives from the lack of accuracy in conveying due to such causes as the eccentricity of the conveying roller 1 and of the discharge roller 12 as well as the errors in outer diameter of these rollers. For this purpose, in this embodiment, a first correction value is acquired to reduce the negative influence of the eccentricity of the rollers (hereafter, the first correction value is referred to as “correction value for eccentricity”). In addition, a second correction value is acquired to reduce the negative influence of the outer-diameter error (hereafter, the second correction value is referred to as “correction value for outer-diameter”). Then, these correction values are used to control the rotation of the rollers, or to be more precise, to control the driving of the conveying motor 110 when the printing is actually carried out.
FIG. 4 is a flowchart illustrating the outline of processing procedures to acquire the correction value for eccentricity and the correction value for outer-diameter. In this procedure, firstly, preparation for the start of printing action including the setting and the feed of the printing medium P is done (step S9). When the printing medium P is conveyed to a predetermined position for the printing, test patterns are printed (step S11). With these test pattern, simultaneous detection of the errors in the amount of conveying caused by both the eccentricity and the outer-diameter error (hereafter, also referred to as “conveying error”) is possible, and detail descriptions of the test patterns will be given later.
Subsequently, the test pattern is read using the reading sensor 120, and the information on the density of the test pattern is acquired (step S13). Then, on the basis of this density information, the acquiring of the correction value for eccentricity (step S15) and the acquiring of the correction value for outer-diameter (step S17) are carried out in this order.
(3) Test Pattern
FIG. 5 illustrates an example of the test patterns used in this embodiment. In this embodiment, test patterns used to detect the conveying error caused by the conveying roller 1 and test patterns used to detect the conveying error caused by the discharge roller 12 are formed side by side with each other in a direction, which is corresponding to the direction of conveying the printing medium P, that is, in the sub-scanning direction. Two test patterns are formed side by side with each other in a direction corresponding to the direction of the rotational axis of each roller, that is, in the main-scanning direction. One of the two test patterns is formed in a position near the conveying reference member 14, and the other is formed in a position far from the conveying reference member 14, so as to detect the conveying errors of the corresponding roller in the respective positions. To be more specific, in FIG. 5, a test pattern FR1 is provided to detect the conveying error of the conveying roller 1 in a position near the conveying reference member 14, and a test pattern ER1 is provided to detect the conveying error of the discharge roller 12 in a position near the conveying reference member 14. In addition, a test pattern FR2 is provided to detect the conveying error of the conveying roller 1 in a position far from the conveying reference member 14, and a test pattern ER2 is provided to detect the conveying error of the discharge roller 12 in a position far from the conveying reference member 14.
Now, some of the reasons why the test patterns for both the conveying roller 1 and the discharge roller 12 are printed will be given in the paragraphs that follow.
In the printing apparatus according to this embodiment, conveying units are respectively provided at the upstream and the downstream sides, in the direction of conveying the printing medium P, of the position where the printing is executed by the print head 4 (printing position). Accordingly, the printing medium P can be in any one of the following three states: first, the printing medium P is supported and conveyed by the upstream-side conveying unit alone: second, the printing medium P is supported and conveyed by the conveying units on both sides (FIG. 6A); and third, the printing medium P is supported and conveyed by the downstream-side conveying unit alone (FIG. 6B).
The conveying roller 1 and the discharge roller 12 have their respective main functions that are different from each other. So, the conveying accuracy of the conveying roller 1 frequently differs from that of the discharge roller 12. The main function of the conveying roller 1 is to set the printing medium P, for each stage of the printing scan action, in an appropriate position for the print head 4. Accordingly, the conveying roller 1 is formed with a roller diameter that is large enough to carry out the conveying action with relatively high accuracy. In contrast, the main function of the discharge roller 12 is to discharge the printing medium P with certainty when the printing on the printing medium P is finished. So, most frequently, the discharge roller 12 cannot rival the conveying roller 1 in the accuracy of conveying the printing medium P.
As evident from what has been described above, when the conveying roller 1 is actually involved in the action of conveying the printing medium P, the conveying accuracy for the conveying roller 1 affects the error of conveying the printing medium P. When, in contrast, only the discharge roller 12 is involved in the action of conveying the printing medium P, the conveying accuracy for the discharge roller 12 affects therefrom of conveying the printing medium P.
That is why, in this embodiment, the printing medium P is divided into two areas—an area I and an area II—as shown in FIG. 7. For the printing on the area I, the conveying roller 1 is involved in the conveying action. Meanwhile, the printing medium P is conveyed by the discharge roller 12 alone when the printing is done on the area II. The test patterns are printed while the printing medium P is conveyed by the rollers that are mainly involved in the conveying action for the printing on the respective areas I and II. From each of the test patterns, information on the density is acquired, and thus the correction values that are used in the actual printing of the respective areas are acquired. Incidentally, the printing apparatus according to this embodiment is designed to be capable of printing an image with no margins, that is, “margin less printing” in the front-end portion or in the rear-end portion of the printing medium P. The correction value is usable when the margin less printing is performed in the rear-end portion of the printing medium P. For this reason, acquiring the correction value for the occasion where the printing medium P is conveyed by the discharge roller 12 alone is useful.
FIG. 6B illustrates a state where the printing apparatus performs an actual printing action with the printing medium P being conveyed by the downstream-side conveying unit alone. In this case, the area where the test patterns used for detecting the conveying error of the discharge roller 12—specifically, the test patterns ER1 and ER2—are printed is limited to the area II. So, to secure an enough area to be used for this purpose, a state shown in FIG. 6C—the state where the printing medium P is conveyed by the downstream-side conveying unit alone—can be artificially created by releasing the pinch rollers 2 when the printing of the test patterns FR1 and FR2 is finished. This releasing may be done manually. Alternatively, the releasing action may be automatically executed by the printing apparatus configured as such.
When the printing medium P is conveyed by both the conveying roller 1 and the discharge roller 12, the conveying accuracy for the conveying roller 1 has a predominant influence on the conveying error. For this reason, the entire printing area is divided into such two areas as described above. However, the conveying error in a case where the conveying roller 1 alone is involved in the conveying of the printing medium P (printing is performed on the front-end portion of the printing medium P) may differ from the conveying error in a case where both the conveying roller 1 and the discharge roller 12 are involved in the conveying. Then, the area corresponding to both of the above-mentioned cases may be divided further into smaller portions to be processed independently.
To be more specific, as shown in FIG. 8, the area I can be, firstly, divided into two portions—a portion corresponding to the conveying done by the conveying roller 1 alone and another portion corresponding to the conveying done by both the conveying roller land the discharge roller 12. Then, test patterns are printed individually in both portions, and the density information and the correction values are acquired for each of the portions. In this case, to secure enough space to print test patterns corresponding to the state where the printing medium P is conveyed by the conveying roller 1 alone, the spur rollers 13 may be designed to be released from the discharge roller 12.
Now, some of the reasons why the test patterns for each of the conveying roller 1 and the discharge roller 12 are formed both in a position near the conveying reference member 14 and in a position far from the conveying reference member 14 will be given in the following paragraph.
Assume that each roller is manufactured within a predetermined design tolerance. Even in this case, the conveying error that derives from such factors as the amount of eccentricity and the state of eccentricity may sometimes differ between a position on the side of the printing apparatus near the conveying reference member (a position on the conveying-reference side) and a position on the side thereof far from the conveying reference member (a position on the non-conveying-reference side). Rollers, which are used in a large-scale inkjet printing apparatus that can print on a A3-sized (297 mm×420 mm) or larger printing medium P, tend to have such a difference that is more prominent than those used in other types of apparatus. A possible way to minimize the difference in the conveying error between a position on the conveying-reference side and a position on the non-conveying-reference side is that a single test pattern is printed in the central position in the main-scanning direction, that is, in the longitudinal direction of the roller, and then a correction value is acquired from the information on the density of the test pattern. In this embodiment, however, plural test patterns are printed in the main-scanning direction (for example, two test patterns are printed in this embodiment, but three, or more, are also allowable). Then, having compared those printed test patterns, a correction value is selected so as to reduce most the negative influence of the conveying error on the test pattern that is affected most prominently by the corresponding conveying error (this will be described later)
(4) Details of Test Pattern
Each of the test patterns shown in FIG. 5 is formed in the following way.
FIG. 9 is an explanatory diagram for describing the way how the nozzles are used when the test patterns are formed. When the test patterns are formed, by using, amongst the 768 nozzles included in the nozzle array H3500 for the second black ink, for example, a nozzle group NU that consists of a part of the 768 nozzles consecutively formed on the upstream side in the conveying direction and another nozzle group ND that consists of a part of the 768 nozzles consecutively formed on the downstream side in the conveying direction. The nozzle groups NU and ND are located with an in-between distance that is equal to each amount of conveying between every two printing scans multiplied by the number of printing scans done until patch elements, which are to be described later, are laid over each other. In this embodiment, the nozzle group located on the downstream side (the nozzle group ND) is made to be the nozzle group for reference, and 128 nozzles located in a range from the 65th to 193rd nozzle counted from the nozzle located in the most downstream position are used, in a fixed manner, to print plural patch elements for reference RPEs (first patch elements) The nozzle group located on the upstream side (the nozzle group NU) is made to be the nozzle group for adjustment. The number of nozzles, amongst the nozzle group NU, to be used is 128, which is the same number of nozzles to be used amongst those in the nozzle group ND. However, the range of nozzles of the nozzle group NU is shifted by one nozzle during the main scan. In this way, plural patch elements for adjustment APEs (second patch elements) are printed.
FIGS. 10A to 10E are explanatory diagrams for describing the way how the test patterns, or the patches constituting the test patterns, are formed by using the upstream-side nozzle group NU and the downstream-side nozzle group ND. Firstly, patch elements for adjustment is formed in a main scan at a certain conveying position (that is, by the first main scan), then printing medium P is conveyed by an amount corresponding to 128 nozzles, and thereafter patch elements for adjustment are further formed. When the above-described series of actions are repeated, the first ones of the patch elements for adjustment thus formed reach the position where the downstream-side nozzle group ND is located at the time of the fifth main scan. By forming patch elements for reference at this position, patches that are used to acquire the density information (the kind of patches of the first line) are completed.
Likewise, at the sixth main scan, the patch elements for adjustment formed at the second main scan reach the position where the downstream-side nozzle group ND is located. By forming patch elements for reference at this position, patches of the second line are completed. Patches of the third line onwards are formed in a similar way, and thus plural lines of patches are completed in the sub-scanning direction.
The above descriptions show that, to complete the patches, four times of conveying the printing medium P are necessary to be carried out between the scan to form the patch elements for adjustment and the scan to form the patch elements for reference. Accordingly, each of the patches reflects the conveying error caused by the sector of the roller used in the four times of conveying the printing medium P, which are carried out between the scan having formed the patch elements for adjustment and the scan having formed the patch elements for reference.
FIGS. 11A and 11B illustrate, respectively, a group of patch elements for reference printed by a single main scan and a group of patch elements for adjustment printed likewise. As FIG. 11A shows, the patch elements for reference RPEs are printed neatly in a line in the main-scanning direction. In contrast, FIG. 11B shows that when the patch elements for adjustment APEs are printed, each of the patch elements for adjustment APEs is shifted by a pitch corresponding to one nozzle. The group of patch elements for adjustment APEs includes a reference patch element for adjustment APEr that is printed by using 128 nozzles located in a range from the 65th nozzle to the 193rd nozzles that are counted from the nozzle located in the most upstream position.
Those patch elements for adjustment APEs located closer to the conveying reference member 14 than the reference patch element for adjustment APEr, those are located at the left side of the reference patch element for adjustment APEr in FIG. 11B, printed in the following way. Each patch element for adjustment APE is printed by using the nozzle group for adjustment NU, but the range of nozzles used to print a patch element for adjustment is shifted, by one nozzle towards the downstream side of the conveying, from the range of nozzles used to print the adjacent patch element for adjustment APE that is located at the right side thereof. Meanwhile, those patch elements for adjustment APEs located farther from the conveying reference member 14 than the reference patch element for adjustment APEr, those are located at the right side of the reference patch element for adjustment APEr in FIG. 11B, are printed in the following way. Each patch element for adjustment APE is printed by using the nozzle group for adjustment NU, but the range of nozzles used to print a patch element for adjustment is shifted, by one nozzle towards the upstream side of the conveying, from the range of nozzles used to print the adjacent patch element for adjustment APE that is located at the left side thereof. The range of nozzles is shifted by 3 nozzles for the conveying-reference side and by 4 nozzles for the non-conveying-reference side. When the shifting towards the upstream side is denoted as positive, the range of shifting, as a whole, is from −3 to +4.
Now, assume that the printing medium P is conveyed between two main scans, without any error, by a distance corresponding to a range of 128 nozzles arranged at a pitch of 1200 dpi (128/1200×25.4=2.709 [mm]). Then, the patch elements for reference RPEs that are printed at the fifth main scan is laid exactly over the reference patch element for adjustment APEr (shifting amount=0) printed at a main scan after the printing medium P is conveyed four times. Note that a positive amount of shifting corresponds to a case where the amount of conveying is larger than the above-mentioned distance while a negative amount of shifting corresponds to a case where the amount of conveying is smaller than the above-mentioned distance.
FIG. 12 illustrates a test pattern including plural patch elements, or including a group of patches each of which is composed of a patch element for reference and a patch element for adjustment. FIG. 12 illustrates, in an enlarged manner, one of the four test patterns shown in FIG. 5.
With the reference patch element for adjustment APEr, patch elements for adjustment APEs are printed by with the nozzles actually used for printing being shifted, by one nozzle, from the respective adjacent ones within a range from −3 to +4 nozzles. Accordingly, in each test pattern, 8 patches are formed in the main-scanning direction. In addition, the amount of conveying the printing medium P, in this embodiment, between each two main scans is set at 2.709 mm (as an ideal value) Main scans are repeatedly carried out 30 times in total to form 30 patches across the range in the sub-scanning direction (in the direction of conveying the printing medium P). Accordingly, each test pattern has a length in the sub-scanning direction of 2.709×30=81.27 mm (as an ideal amount). When a roller has, nominally, an outer diameter of 37.19 mm, the above-mentioned length of the test pattern corresponds to more than twice the circumference of the roller.
A patch column A shown in FIG. 12 includes the reference patch elements for adjustment APErs. Each of patch columns marked with A+1 to A+4 includes patch elements for adjustment APEs printed with the used range of the nozzle group for adjustment NU being shifted towards the upstream side in the direction of conveying the printing medium P from the reference patch elements for adjustment APErs by an amount corresponding to 1 nozzle to 4 nozzles. Each of patch columns marked with A−1 to A−3 includes patch elements for adjustment APEs printed with the used range of the nozzle group for adjustment NU being shifted towards the downstream side in the direction of conveying the printing medium P from the reference patch elements for adjustment APErs by an amount corresponding to 1 nozzle to 3 nozzles.
Patch rows B1 to B30 are formed with different sectors of the roller used to convey the printing medium P between the scan to form each patch element for adjustment APE and the scan to form the corresponding patch element for reference RPE. Assume that the conveying of the printing medium P after the printing of the patch element for adjustment APE of the patch row B1 is carried out from a reference position of the roller. In this case, for the patch row B1, the sector of the roller used between the scan to print the patch element for adjustment (APE) and the scan to print the patch element for reference (RPE) corresponds to a sector of the roller used to convey the printing medium P four times (0 mm to 10.836 mm) starting from the reference position of the roller. For the patch row B2, the sector of the roller used between the scan to print the patch element for adjustment (APE) and the scan to print the patch element for reference (RPE) corresponds to a sector of the roller used to conveying action of the printing medium P four times (2.709 mm to 13.545 mm) starting from a position away from the reference position by 2.709 mm. Likewise, for the patch row B3, a sector of the roller (5.418 mm to 18.963 mm) is used while for the patch row B4, another sector of roller (8.127 mm to 21.672 mm). In this way, for the different patch rows, different sectors of the roller are used between the scan to print the patch element for adjustment (APE) and the scan to print the patch element for reference (RPE).
In addition, patch rows that are adjacent to each other share, partially, a sector of the roller to be used between the scan to print the patch element for adjustment (APE) and the scan to print the patch element for reference (RPE). For example, both of the patch rows B1 and B2 use a common sector of the roller (2.709 mm to 10.836 mm).
The position of conveying after the printing of the patch element for reference (RPE) of the patch row B1 may be aligned with the reference position of the roller. In the formation of the test pattern, however, no such control as to make the above state accomplished is necessary. Alternatively, the conveying position after the printing of the patch element for reference of the patch row B1 may be printed and may be used as the reference to acquire the relations between the patch rows (positions to be used within a roller) and the conveying error, which relations are to be described later.
(5) Details of Patch
FIG. 13 illustrates the patch element for reference or the patch element for adjustment in enlarged manner. In FIG. 14, the patch element is illustrated in a further enlarged manner. The patch element is formed in a stair-shaped pattern with print blocks, as base units, each of which has dimensions of 2 dots in the sub-scanning direction and 10 dots in the main-scanning direction. In addition, a certain distance in the sub-scanning direction between each two stair-shaped patterns is secured by taking account of the range for shifting the group of nozzles to be used. In the example shown in FIG. 14, the group of nozzles to be used is shifted by 1 to 4 nozzles towards the upstream side of the conveying direction (+1 to +4) and by 1 to 3 nozzles towards the downstream side in the conveying direction. In response to this, a space of 6 nozzles is secured in the sub-scanning direction.
In this embodiment, such a patch element as shown in this drawing is printed in the upstream-side nozzle group NU and in the downstream-side nozzle group ND as well. Accordingly, the state of overlaying of the patch element for reference (RPE) and the patch element for adjustment (APE) is changed in response to the degree of conveying errors. As a result, in the test pattern, patches of various densities are formed as shown in FIG. 12.
Specifically, when the patch element for adjustment (APE) printed by the upstream-side nozzle group NU and the patch element for reference (RPE) printed by the downstream-side nozzle group ND are aligned exactly with each other as shown in FIG. 15A, the density (OD value) becomes low. In contrast, when these patches are misaligned as shown in FIG. 15B, the space that is supposed to be blank is filled, so that the density becomes high.
The reliability of the test pattern has to be enhanced so that the conveying error can be detected from the information on the density of the test pattern. To this end, it is preferable that the state of the nozzles of the print head 4 be less likely to affect the patches. In nozzles that are used continuously or used under certain conditions, such ejection failure as deflection in the ejection direction (dot deflection) and no ejection of ink may sometimes occur. When such ejection failure brings about a change in the information on the density of the patches, the correction value for conveying error can be calculated only incorrectly. It is, therefore, strongly desirable that patches to be formed are capable of reducing the change in information on the density even with the existence of such ejection failure as mentioned above. The patch element employed in this embodiment can respond such a demand. The reason for this will be described in the following paragraphs by using a simple model.
The patch element is formed in a pattern with spaces in the sub-scanning direction as shown in FIG. 16A so that the amount of offset in positions can be measured as the information on the density. However, when a particular nozzle does not eject any ink at all, all the area that is supposed to be printed with the particular nozzle becomes blank as shown in FIG. 16B.
To address the problem, the patch element is formed, as shown in FIG. 17A, of plural print blocks also with spaces placed between two adjacent blocks arranged in the main-scanning direction. In addition, the range of used nozzles is dispersed so that the patterns may not be adjacent to each other amongst print blocks. Thus, the negative influence of a particular nozzle on the pattern can be reduced. Specifically, even when there is ejection failure of a particular nozzle, a blank area, the blank area being produced because the patch elements for reference (RPEs) and the patch elements for adjustment (APEs) are not aligned with one another, is reduced (the example in FIG. 17B has half a blank area of that in FIG. 16B). Accordingly, the density of the patch elements, and eventually, that of the patch itself, can be prevented from being lowered. The pattern in FIG. 17B has an area factor (proportion of the area of the patch pattern to the patch area) that is equal to the area factor of the pattern in FIG. 16B. Here, the sum of the density for each unit area within the pattern or the average value thereof is made to be the density value for the entire area of the pattern. Then, the density value becomes the same even when the patterns are different.
Note that in this embodiment, the more the patch element for reference (RPE) and the patch element for adjustment (APE) are laid over each other, the smaller the area factor becomes and the lower the density of the patch thus formed becomes. In another allowable configuration, however, the more the patch element for reference (RPE) and the patch element for adjustment (APE) are laid over each other, the larger the area factor becomes and the higher the density of the patch thus formed becomes. In essence, any configuration is allowable as long as the information on the density can change sensitively in response to the degree of overlaying of, or the degree of offsetting (that is, the conveying error) of, the patch element for reference (RPE) and the patch element for adjustment (APE).
In addition, in this embodiment, each patch element is formed with print blocks arranged in a stair shape. Another arrangement, however, is allowable as long as the print blocks are not continuous in the direction of the scan for printing and as long as the arrangement can effectively reduce the negative influence of the ejection failure. For example, the print blocks may be arranged in a mottled fashion, or at random.
Moreover, in this embodiment, the matt black ink is used to form the test patterns. Any ink of a different color may be used for this purpose as long as the information on density can be acquired with a reading sensor in a favorable manner. In addition, inks of different colors may be used to print the patch elements for reference (RPEs) and to print the patch elements for adjustment (APEs), respectively.
Furthermore, regarding the numbers of the nozzle groups to be used and the positions of the nozzles to be used, the respective examples given in the above embodiment are not the only ones. Any number of nozzle groups and any positions of the nozzles are allowable as long as the change in information on density in response to the conveying error can be acquired in a favorable manner and as long as little negative influence is exerted by an ejection fault of the nozzle. To enhance the accuracy in detection of the conveying error caused by the eccentricity of the roller and by the outer-diameter error, the distance between the nozzle group used to print the patch elements for reference (RPEs) and the nozzle group used to print the patch elements for adjustment (APEs) is preferably made larger, and the two kinds of patch elements preferably have the same pattern.
(6) Correction Value for Conveying Error
In this embodiment, the density of each of the patches constituting the test pattern is measured with the reading sensor 120. In the measurement with the reading sensor 120, the test pattern is scanned with an optical sensor that includes a light emitter and a light detector thereon, and thus the density of each of the patches where the pattern for reference and the pattern for adjustment interfere with each other (FIGS. 15A and 15B) is determined. The density of the patch is detected as the amount of light reflected (intensity of reflected light) when light is emitted onto the patch. The detection operation may be executed only once for each area to be detected, or may be executed plural times to reduce the negative influence of the detection error.
Following the detection of the density of the patches, the densities of the respective plural patches printed in the main-scanning direction are compared with one another. Then, the error in conveying amount is calculated from the positions of, and from the difference in density between, the least dense patch and the second least dense patch. Here, the density values obtained from the least dense patch is denoted with N1, and the density value obtained from the second least dense patch is denoted with N2. Then, the difference in density (N=N2−N1) is compared with three threshold values T1, T2, and T3 (T1<T2<T3). When N<T1, little difference exists between N1 and N2. In this case, the conveying error is determined as the intermediate value of the offset amount for the least dense patch and the offset amount for the second least dense patch (the offset amount for the least dense patch+the length of 1⁄2 nozzles). When T1<N<T2, the difference between N1 and N2 is slightly larger than the difference in the previous case. In the case of T1<N<T2, the conveying error is determined as the value that is shifted further from the above-mentioned intermediate value to the side of the least dense patch by an amount of 1⁄4 nozzles (the offset amount for the least dense patch+the length of 1⁄4 nozzles). When T2<N<T3, the difference between N1 and N2 is even larger than the difference in the previous case. In the case of T2<N<T3, the conveying error is determined as the value of the offset amount for the least dense patch+the length of 1⁄8 nozzles. When T3<N, the difference in density N is significantly large. In this case, the conveying error is defined as the offset amount for the least dense patch.
As has been described above, three threshold values are set in this embodiment, and thus the detection of the conveying error is made possible with a unit of 2.64 μm, which is equivalent to the one eighth of the nozzle pitch, 9600 dpi (=1200×8). The processing is executed for each of the plural—30, to be more specific—patch rows that are formed in the sub-scanning direction. Thus, the conveying error is detected for each circumferential length (2.709 mm×4=10.836 mm) that is used in the four-time actions of conveying the printing medium P for each patch rows.
FIG. 19 is a chart illustrating the relationship between the patch rows Bn (n=1 to 30) and the conveying errors Xn detected from the respective patch rows Bn. In the chart, the horizontal axis shows the value of n and the vertical axis shows the value of conveying error Xn. The plotted values of conveying error Xn correspond to the respective values of n, which in turn correspond to the respective 1 to 30 patch rows Bn.
In FIG. 19, the value of the conveying error Xn fluctuates depending upon the values of n. This is because different amounts of conveying are produced by different rotational angles from the reference position of the roller, and this difference in the conveying amount derives from the eccentricity of the roller. Note that the fluctuation of the values of conveying error Xn derives from the eccentricity of the roller so that the fluctuation is a periodic one with a period corresponding exactly to a full rotation of the roller.
In addition, the values of the conveying error Xn, as a whole, are shifted either upwards or downwards in response to whether the outer diameter of the roller is larger or smaller than that for reference. When the outer diameter of the roller is larger than that for reference, the printing medium P is conveyed by an amount that is larger than a predetermined amount of conveying. Accordingly, the conveying errors Xn, as a whole, are shifted upwards in the chart. In contrast, when the outer diameter of the roller is smaller than that for reference, the conveying errors Xn, as a whole, are shifted downwards in the chart.
For the purpose of reducing the values of the conveying error Xn, it is necessary to reduce the amplitude, which is the fluctuation component of the conveying errors Xn, and to approximate the center value of the fluctuation to zero, that is, to the nominal value of the outer diameter of the roller. To this end, in this embodiment, an appropriate first correction value (correction value for eccentricity) to reduce the amplitude of the conveying errors Xn is acquired, and then a second correction value (correction value for outer-diameter) to approximate the central value of the fluctuation to zero is acquired.
In the following paragraphs, detailed descriptions of the processing to acquire these correction values will be given. The following descriptions will be given by taking the processing for the conveying roller 1 as an example, but similar processing can be carried out for the discharge roller 12. In addition, though the conveying roller 1 conveys the printing medium P in cooperation with the pinch rollers 2 and the conveying error is determined as an outcome of the combination of these rollers, the descriptions that follow are based, for convenience sake, on the assumption that the conveying error is of the conveying roller 1.
(7) Acquiring Correction Value for Eccentricity
To begin with, descriptions will be given as to the outline of the conveying control carried out in this embodiment by using the correction value for eccentricity and the correction value for outer-diameter that have been acquired previously. Though the details of this conveying control is to be given later, only the outline thereof will be given beforehand to describe the steps of acquiring the correction value for eccentricity and the correction value for outer-diameter.
In this embodiment, as shown in FIG. 28, the roller is divided into 110 sectors starting from a position for reference (thus formed are blocks BLK1 to BLK110). Then, a table is prepared to associate the blocks to their respective correction values for eccentricity. FIG. 26 shows an example of such a table. Correction values for eccentricity e1 to e110 are respectively assigned to the block BLK1 to BLK110.
In the conveying control of this embodiment, the base conveying amount is added with a correction value other than the correction value for eccentricity, that is, the correction value for outer-diameter, and then the rotation of the conveying roller 1 is calculated. In other words, from which of the blocks to which of the blocks the conveying roller 1 rotates is calculated. Then, correction value for eccentricity that corresponds to the blocks passing with this rotation is added. The value thus produced is made to be the final conveying amount, and the conveying motor 110 is driven to obtain this conveying amount.
As has just been described, to carry out the conveying control of this embodiment, correction values for eccentricity have to be acquired for each of the blocks created by dividing the circumferential length of the roller in 110 sectors, or, to put it other way, for the blocks each of which has a 0.338-mm (=37.19 mm/110) circumferential length of roller.
In this embodiment, however, the conveying error is detected, from the test pattern, for each circumferential length of roller used to convey the printing medium P four times for each of the patch rows (the length is 10.836 mm). In addition, two adjacent patch rows in the test pattern share part of their respective roller sectors used to carry out their respective four-time actions of conveying the printing medium P. So, following the procedures to be described below, correction values for eccentricity are acquired from the test pattern for the respective blocks of the roller, each of which blocks has a circumferential length (0.338 mm) formed by dividing the circumferential length of the roller into 110 sectors.
Incidentally, the period of the eccentricity appears in the form of a periodic function with period equivalent to the circumferential length of the roller. So, a periodic function having a periodic component that is equivalent to the circumferential length of the roller and having a polarity that is opposite to that of the function of the conveying error is to be obtained firstly in this embodiment (hereafter, such a function will be referred to as “correction function”). Then, the distance from the reference position of the roller is assigned to the correction function. Accordingly, the correction value for eccentricity is acquired for each of the blocks formed by the division into 110 sectors.
The correction function in this embodiment is obtained by selecting a combination of an amplitude A and an initial phase θ that are capable of reducing most the conveying error caused by the eccentricity of the roller—that is, the amplitude component of the conveying error Xn shown in FIG. 19—for a sine function, y=A sin (2π/L×T+θ). Here, L is the circumferential length of the roller (specifically, 37.19 mm for the conveying roller 1), and T is the distance from the reference position of the roller. Four different values—specifically, 0, 0.0001, 0.0002, and 0.0003—can be set for the amplitude A, while 22 different values—specifically, −5 m×2π/110 (m=0, 1, 2, 3, . . . , 21)—can be set for the initial phase θ. In summary, 66 different combinations of the amplitude and the initial phase without including the case of the amplitude A=0 are selectable in this embodiment, and 67 different combinations are selectable when the case of the amplitude A=0 is included. Amongst these different combinations, an optimum combination of the amplitude A and the initial phase θ for correcting the eccentricity of the roller is selected.
FIG. 18 illustrates an example of arithmetic processing procedure for finding the correction value for eccentricity.
Firstly, in step S21, a determination is made to judge whether an arithmetic processing is necessary to acquire the correction value for eccentricity, and this determination has to precede the acquirement of the correction value for eccentricity from the correction function. For example, when the conveying error caused by the eccentricity is smaller than a certain threshold value, such arithmetic processing to acquire the correction value for eccentricity is judged to be unnecessary. If this is the case, the amplitude of the correction function is set at zero, and the procedure is finished. In the embodiment, the procedure for determining the necessity of the arithmetic processing to acquire the correction value for eccentricity will be given in the following paragraphs.
Firstly, the average value Xn(ave) of the conveying errors Xn (n=1 to 30) shown in FIG. 19 is obtained, and the differences Xn′ between this average value Xn(ave) and the conveying errors Xn are calculated. FIG. 20 is a chart illustrating the relationship between the value of n and the difference Xn′ with the values of n on the horizontal axis and with the differences Xn′ on the vertical axis. Then, the absolute value |Xn′| of each of the differences Xn′ is squared, and the sum of this squared values Σ|Xn′|2 is calculated. When the sum Σ|Xn′|2 thus calculated is smaller than the certain threshold value mentioned above, a determination that the correction value for eccentricity is unnecessary is made.
In contrast, when the sum Σ|Xn′|2 thus calculated is larger than the certain threshold value mentioned above, the operational flow advances to the processing to acquire the correction function to correct the eccentricity of the roller. In a step S24, a correction function having an amplitude A and an initial phase θ that are optimum to correct the eccentricity of the roller is calculated. An example of the way to calculate this correction value will be given in the following paragraphs.
Firstly, for each of all the combinations (66 combinations without the case of the amplitude A=0) of the amplitude A and the initial phase θ in the above-described sine function, the values are obtained by assigning, to the variable T of the sine function, the 34 different values starting from 2.709 to 92.117 at the intervals of 2.709.
For example, values y1, y2, and y3 are obtained respectively by assigning 2.709, 5.418, and 8.128 to the variable T of the above-mentioned sine function with a certain amplitude A and a certain initial phase θ. The calculation continues until a value y34 is obtained by assigning 92.117 to the variable T. The processing has to be done for all the 66 different combinations of the amplitude A and the initial phase θ without the case of the amplitude A=0.
Then, four successive values of y in a certain combination of the amplitude A and the initial phase θ are added together to produce 30 integrated values Yn′. For example, y1′=y1+Y2+y3+y4, and y2′=y2+y3+y4+y5. In this way, values from y1′ to y30′ are calculated. The processing has to be done for all the 66 different combinations of the amplitude A and the initial phase θ.
Note that the values y1, y2, y3, and y4 are obtained by assigning, respectively, 2.709, 5.418, 8.128, and 10.836 to the variable T, where T is the distance from the reference position of the roller. Accordingly, in the sine function having a certain combination of the amplitude A and the initial phase θ, the value y1′ obtained by adding the values y1 to y4, together is a value that corresponds to a sector of the roller starting from the reference position and ending with the 10.836-mm position. Likewise, in the sine function having a certain combination of the amplitude A and the initial phase θ, the value y2′ obtained by adding the values y2 to y5 together is a value that corresponds to a sector starting from the 2.709-mm position and ending with the 13.545-mm position.
Subsequently, for each of the combinations of the amplitude A and the initial phase θ, the integrated values yn′ are added to the respective differences Xn′ between the conveying errors Xn and the average value. For example, y1′ is added to x1′, and y2′ is added to X2′. The following additions are carried out similarly until y30′ is added to X30′. Thus obtained are addition values Xn′′. Then, the absolute value of each of the addition values Xn′′ is squared, and the sum of this squared values Σ|Xn′′|2 is calculated. FIG. 21 shows a graph illustrating the relationship between the value of n and the squared absolute value |Xn′′|2 of the addition values while the values of n are on the horizontal axis and the values of |Xn′′|2 are on the vertical axis. By summing up the squared absolute values |Xn′′|2 corresponding to the respective values of n in this graph, the sum of the Σ|Xn′′|2 of the addition values Xn squared can be calculated.
In accordance with a procedure that is similar to the one described above, the sum Σ|Xn′′|2 of the squared absolute value of the addition values Xn is obtained for each of the all the 66 different combinations of the amplitude A and the initial phase θ. Then, one of the 66 combinations is selected so as to minimize the value of the square sum Σ|Xn′′|2. What can be obtained in this way is a correction function that can reduce most the conveying error caused by the eccentricity of the roller, that is, the amplitude component of the conveying error Xn. After that, the correction value for eccentricity for each block formed by dividing the roller into 110 sectors can be acquired by assigning the distance from the reference position for each of the blocks to the variable T of the correction function.
According to the above-described method of acquiring the correction value for eccentricity, the correction value for eccentricity for an area of the roller that is associated with the distance from the reference position of the roller can be obtained even with a test pattern, such as the one of this embodiment, in which:
the conveying error Xn detected from each of the patch rows corresponds to a circumferential length of the roller corresponding to plural times of the conveying action for the printing medium P; and
two adjacent patch rows share part of the sectors of the roller that are used to print the respective patch elements for reference and to print the respective patch elements for adjustment.
Subsequently, in step S25 in FIG. 18, a determination is made to judge whether there are plural test patterns in the main-scanning direction.
When only a single test pattern is printed in the main-scanning direction, a correction function is determined on the basis of the information on the density obtained from the test pattern so as to have an optimum combination of the amplitude A and the initial phase θ to correct the eccentricity. Then the correction value is arithmetically operated using the correction function (step S27).
Even for a roller manufactured within a predetermined design tolerance, the conveying error that derives from the amount and the state of eccentricity of the roller may sometimes vary between on the conveying-reference side and on the non-conveying-reference side of the printing apparatus. To address this phenomenon, two test patterns can be printed in the main-scanning direction in this embodiment. Accordingly, for each of the patterns, an optimum combination of the amplitude A and the initial phase θ to correct the eccentricity is obtained. Then, in step S29, the two combinations thus obtained are compared to determine whether the two combinations are the same or different. When the two combinations are the same, the correction value is arithmetically operated on the basis of the correction function with the common amplitude A and the common initial phase θ (step S31).
In contrast, there may be cases where the combination of the amplitude A and the initial phase θ on the conveying-reference side is different from the combination thereof on the non-conveying-reference side. In this case, selected is the combination of the amplitude A and the initial phase θ that minimizes the larger one of the values of square sum Σ|Xn′′|2 for the conveying-reference side and the non-conveying-reference side. The reason why such a way of selection is employed is avoiding the following inconvenience. It is possible to select the combination of the amplitude A and the initial phase θ that minimizes the smaller one of the values of square sum Σ|Xn′′|2 for the conveying-reference side and the non-conveying-reference side. Such selection may cause an unfavorable situation in which the conveying error caused by the eccentricity of the roller cannot be limited within the range of the design tolerance. When the combination of the amplitude A and the initial phase θ on the conveying-reference side is different from the combination thereof on the non-conveying-reference side, the processing described in the following paragraphs is carried out.
Firstly, for each of the three amplitude conditions (specifically, A=0.0001, A=0.0002, and A=0.0003), the square sum Σ|Xn′′|2 are plotted while the initial phase θ is changed. The plotting is done both for the conveying-reference side and for the non-conveying-reference side. The two curves thus obtained and representing the respective sides are compared with each other. From the two curves, sections of one of the two curves that have larger values than the values of the corresponding section of the counterpart curve are selected. The operation is schematically illustrated in FIGS. 22A and 22B.
FIGS. 22A and 22B illustrate the curves each of which obtained by plotting the square sum ΣXn′′|2 with the initial phase θ varying for each of the side near the conveying-reference and the side far from the conveying-reference. FIG. 22A is of a case where the curve for the conveying-reference side crosses the curve for the non-conveying-reference side. In this case, the sections represented by a thick solid line are the sections where the values of the square sum Σ|Xn′′|2 on the curve are larger than the corresponding values on the counterpart curve. FIG. 22B, on the other hand, illustrates a case where the curve for the conveying-reference side does not cross the curve for the non-conveying-reference side. In this case, the whole sector of one of the two curves constantly has the larger values of the square sum Σ|Xn′′|2, and is accordingly shown by a thick solid line in FIG. 22B.
Subsequently, within the selected sector, or sectors, having larger values of the square sum Σ|Xn′′|2 (shown by the thick solid line in FIGS. 22A and 22B), the value of the initial phase θ that makes the value of the square sum Σ|Xn′′|2 the lowest is selected as the optimum value under the amplitude condition of the case. When the two curves cross each other as shown in FIG. 22, one of the intersecting points that has the lowest value of the square sum Σ|Xn′′|2 is selected as the optimum value under the amplitude condition of the case. In the case shown in FIG. 22B, the value of the initial phase θ at the lowest-value point on the thick solid line is selected as the optimum value under the amplitude condition of the case.
The operation described above is carried out for each of the amplitude conditions. Then, the values of the square sum Σ|Xn′′|2 corresponding to the respective initial values determined individually for the amplitude conditions are compared with one another. Thereafter, the amplitude A and the initial phase θ of a case where the value of the square sum Σ|Xn′′|2 is the lowest are selected as the optimum values. After that, the correction value is arithmetically operated on the basis of the correction function having the optimum amplitude A and the optimum initial phase θ (step S33).
As has been described thus far, in this embodiment, the optimum values of the amplitude A and of the initial phase θ are obtained from a single test pattern or plural test patterns and then a correction function having such optimum values is determined. Then, on the basis of this correction function, the correction value for eccentricity is acquired.
In the above description, the correction value for eccentricity for each of the sectors formed by dividing the roller into 110 parts (blocks BLK1 to BLK110) is acquired while the correction values for eccentricity are associated with the respective distances from the reference position of the roller to the respective sectors. Note that this is not the only way to acquire the correction values for eccentricity. For example, the correction values for eccentricity may be acquired while the correction values for eccentricity are associated with the respective rotational angles from the reference position of the roller to the respective sectors.
In this embodiment, the rotary encoder 116 attached to the conveying roller 1 outputs 14080 pulses per rotation, for example. Then, the 14080 pulses are divided into groups each of which has 128 pulses so as to suit for the 110 sectors. Thus, the current position of the roller can be detected in accordance with the pulses outputted from the rotary encoder 116. Then, for each of the 110 sectors (blocks), the correction value for eccentricity is associated with the rotational angle eccentricity-correction-value table is formed by setting these correction values for eccentricity (step S35) in the table. Storing these set values in, for example, the EEPROM 103 (see FIG. 3), makes it possible to keep these values even when the apparatus itself is switched off. Updating the set values is also made possible according to this configuration.
(8) Acquiring Correction Value for Outer-Diameter
Besides the reduction of the conveying error caused by the eccentricity of the roller, the reduction of the conveying error caused by the outer-diameter error of the roller is effective for reducing the conveying error in total. The latter processing is the outer-diameter correction. Hereafter, descriptions will be given as to the way of acquiring the correction value for outer-diameter to use that processing and as to the reason why the acquiring of the correction value for eccentricity has to precede the processing for acquiring the correction value for outer-diameter.
FIG. 23 illustrates an example of arithmetic processing procedure to acquire the correction value for outer-diameter.
Firstly, contents of the eccentricity-correction-value table are applied to the conveying errors Xn detected from the respective patch rows of the test patterns, and the values thus obtained are denoted as Yn (step S41). Then, the average value of Yn are calculated and denoted as Yn(ave) (step S43). Note that, as has been described above, each of the conveying errors Xn is the conveying error for the circumferential length of the roller corresponding to the four-time conveying of the printing medium P. Accordingly, before being applied to the conveying errors, the correction values for eccentricity in the eccentricity-correction-value table have to be integrated so as to be suitable for the conveying errors Xn thus obtained.
Subsequently, a determination is made to judge whether there are plural test patterns in the main-scanning direction (step S45). When there is only a single test pattern printed in the main-scanning direction, the difference between a target value (the value of the roller with dimensions that are exactly equal to the nominal ones and, accordingly, without any conveying error) and the average value Yn(ave) are calculated. Then, on the basis of the calculated differences, the correction value for outer-diameter is determined (step S47).
Here, when the difference obtained by subtracting the average value Yn(ave) from the target value is positive, the roller has a circumferential length that is longer than the roller with dimensions equal to exactly nominal ones. To put it other way, even a single conveying action using the roller conveys the printing medium P more than the amount that is supposed to be conveyed. Accordingly, in this case, a correction value (correction values for outer-diameter) is determined in step S47 so as to make the average value Yn(ave) equal to the target value.
On the other hand, when plural test patterns (two test patterns in this embodiment) are printed in the main-scanning direction, the average values Yn(ave) obtained from the respective test patterns are added up to find the average value thereof (step S49). The difference between this average value thus obtained and the target value is used to produce determine the correction values for outer-diameter (step S51). This correction value for outer-diameter can also be stored in the EEPROM 103 (see FIG. 3).
Now, description will be given in the following paragraphs as to the reason why the acquiring of the correction values for eccentricity has to precede the acquiring of the correction values for outer-diameter.
In this embodiment, emphasis is put on the achievement of a high-accuracy conveying-error correction without sacrificing the versatility of the test pattern and of the printing method. Assume that a test pattern used here has a length in the sub-scanning direction that is equal to an integral multiplication of the circumferential length of the roller. With such a test pattern, acquiring high-accuracy conveying-error correction values is possible even when the order of the acquiring of the correction values for eccentricity and the acquiring of the correction values for outer-diameter is reversed.
The test pattern used in this embodiment, however, has an 80-mm length in the sub-scanning direction. When a roller with a nominal outer circumference of 37.19 mm is used, the 80-mm length exceeds an integral multiplication of the roller with the nominal outer circumference (exceeds the amount of two full rotations of the roller). Hence, in this embodiment, the conveying error is detected from the area, within the test pattern, corresponding to the two full rotations of the conveying roller and detected from the excess area corresponding to a small, beginning part of the third rotation.
Note that it is, in fact, difficult to form a test pattern with its length in the sub-scanning direction that is precisely equal to an integral multiplication of the circumferential length of the roller. In addition, the tolerance of the outer diameter of the conveying roller 1 may sometimes cause fluctuations in the period of the eccentricity of the conveying roller 1. It is, therefore, rather preferable that the test pattern have a larger length in the sub-scanning direction than an integral multiplication of the nominal circumferential length of the conveying roller 1. Nevertheless, when the test pattern has a length in the sub-scanning direction that is not equal to an integral multiplication of the circumferential length of the roller, or to put it other way, when the conveying error is detected from the test pattern including an excess area, such inconveniences as described in the following paragraph may possibly occur.
In FIG. 24, conveying errors (Xn) acquired from the test pattern in this embodiment are plotted. The area marked by a circle in FIG. 24 corresponds to the excess area. As has been described before, the correction value for outer-diameter is used to correct the amount of the conveying error for each rotation of the conveying roller 1, and is calculated by the average of the values of the conveying error. Acquiring a precise correction value for outer-diameter, however, is problematic when the eccentricity of the roller causes significantly large deviation, from the average value, of the conveying error for the excess area.
In this embodiment, to reduce the negative influence caused by the part of the excess area, the correction value for eccentricity is acquired. Then, after the correction value for eccentricity is applied, the arithmetic processing of the correction value for outer-diameter is carried out. Accordingly, a variation in conveying error in the excess area is suppressed. As a result, it is possible to reduce a difference between the conveying error and the average of the values of the conveying error, so that the influence of the eccentricity can be reduced.
FIG. 25 shows examples of correction values acquired through the processing, firstly, of the correction value for eccentricity and then through the processing of the correction value for outer-diameter as well as examples of correction values acquired through the two processing carried out in the reverse order. Here, for the sake of simplicity, outcomes of calculation on the test pattern FR1 on the conveying-reference side are compared.
Firstly, assume that the correction values are calculated in an order in which the processing for the correction value for outer-diameter precedes the processing for the correction value for eccentricity. In this case, when the average value Yn(ave) is calculated in a state shown in FIG. 24, the value becomes 9.31 μm. After the correction value for outer-diameter acquired on the basis of this value of 9.31 μm is reflected, an operation of eccentricity correction is carried out. In this case, a value of 0.0003 is selected for the amplitude A. Meanwhile, a value of n=13 is selected for the initial phase θ. In contrast, assume that the calculation of the correction value for eccentricity precedes the calculation of the correction values for outer-diameter, as in the case of this embodiment. In this case, a value of 0.0003 is selected for the amplitude A. Meanwhile, a value of n=13 is selected for the initial phase θ. Then, while the correction value for eccentricity is applied, the value of Yn(ave) is calculated. The resultant value becomes 8.74 μm (on the basis of this value Yn(ave) of 8.74 μm, the correction value for outer-diameter is acquired). The comparison of the procedures in different orders makes it clear that the correction values for eccentricity are the same but that the correction values for outer-diameter are different from each other.
Note that, here, the theoretical figure of the correction value for outer-diameter is 8.54 μm when the correction value for outer-diameter is calculated by extracting the value of Xn corresponding to two full rotations of the roller from the state in FIG. 24. Accordingly, as in the case of this embodiment, when the acquiring of the correction value for eccentricity precedes the acquiring of the correction values for outer-diameter, the correction value for outer-diameter can be acquired with the deviations from the theoretical figure being made smaller.
(9) Control of Conveying
As has been described above, in this embodiment, the rotary encoder 116 attached to the conveying roller 1 outputs 14080 pulses for each rotation. Then, in this embodiment, the 14080 pulses are divided into 110 circumferential sectors each of which has 128 pulses starting from the reference position of the rotary encoder 116. Subsequently, a table for storing the correction values for eccentricity acquired through the arithmetic processing for correction values for eccentricity is formed so as to make the correction values for eccentricity correspond to the respective above-mentioned circumferential sectors.
FIG. 26 shows an example of the table thus formed. Correction values for eccentricity e1 to e110 are allocated so as to correspond to the respective blocks BLK1 to BLK110 each of which has a rotational angle corresponding to 128 pulses of the rotary encoder 116. These correction values for eccentricity are reflected in the control of the conveying in a way described in the following paragraphs.
FIG. 27 shows an example of the procedure for the control of the conveying. FIG. 28 is an explanatory diagram for describing the operation corresponding to this procedure. Note that the procedure shown in FIG. 27 is executed for the purpose of determining the amount of conveying the printing medium P (sub scan) between every two printing scans, and can, accordingly, be done either during a printing scan or after the completion of a printing scan.
Firstly, in a step S61, the base amount of conveying is loaded. The base amount of conveying is a theoretical value of the sub-scanning amount between every two consecutive printing scans. Then, in a step S63, the base amount of conveying is added with a correction value other than the correction value for eccentricity, that is, the correction value for outer-diameter. Moreover, in a step S65, a calculation is executed so as to find to what position the conveying roller 1 rotates from the current rotational position in response to the resultant value of the above-mentioned addition. In the example shown in FIG. 28, the conveying roller 1 rotates from a position within the block BLK1 to a position within the block BLK4.
After that, in a step S67, the correction values for eccentricity corresponding to the blocks that are passed by during the rotation of this time are added. To be more specific, in the example shown in FIG. 28, the blocks BLK2 and BLK3 are passed by during the rotation, so that the correction values for eccentricity e2 and e3 are added. The resultant value from the addition is made to be the final amount of conveying, and then the conveying motor 110 is driven to obtain this amount of conveying (step S69).
Note that only the correction values for eccentricity for the blocks that are passed by are configured to be added in this embodiment, but another configuration is possible. In accordance with the position within the current block before the rotation (i.e. block BLK1) and the position within the block after the rotation (i.e. block BLK4), the correction values for eccentricity for these blocks are converted appropriately, and the values thus converted can be used for the addition. Nevertheless, the simple use of the correction values of the respective blocks that are passed by can be processed with more ease and in shorter time than such a fine-tune recalculation of the correction value can.
The correction values thus far described are those for the conveying roller 1, but the correction values for the discharge roller 12 can be obtained in a similar way and can be stored in the EEPROM 103. The stored correction value for the discharge roller 12 can be used when the roller, or rollers, used for the conveying is switched to the discharge roller 12 alone.
(10) Ways of Acquiring Correction Values
The correction value for eccentricity and the correction value for outer-diameter may be acquired on the basis of the information on density obtained by scanning the test pattern with a reading sensor 120 mounted, along with the print head 4, on the carriage 7. Alternatively, the correction value for eccentricity and the correction value for outer-diameter may be acquired on the basis of the information on density obtained by scanning the test pattern with a reading sensor 120 provided in the form of a reading head and mounted, in place of the print head 4, on the carriage 7.
FIG. 29 shows an example of the processing procedure corresponding to the configurations described above. At the start of this procedure, the printing medium P is set (step S101), and test patterns such as ones shown in FIG. 5 are printed (step S103). Then, the printing medium P with the test patterns formed thereon is set in the apparatus again, and the operation of reading the test patterns is executed to acquire the information on density (step S105). After that, on the basis of the information on density, the correction value for eccentricity and the correction value for outer-diameter are acquired in this order (steps S107 and S109), and then these correction values are stored (or updated) in the EEPROM 103 (step S111).
In a case where the printing apparatus has no built-in reading sensor (including a case where the printing apparatus are configured as a multi-function apparatus having a scanner apparatus unit integrated therewith), the printing medium P with the test patterns printed thereon is set in an outside scanner apparatus to carry out the reading.
FIG. 30 shows another example of the processing procedure corresponding to the configurations described above. The difference that this procedure has from the one described above is the provision of a process (step S125) in which the printing medium P with the test patterns formed thereon is set in an outside scanner apparatus followed by the inputting of the information on density thus read.
In addition, the arithmetic operation for the correction values may be executed not as a process done on the printing-apparatus side but as a process done by a printer driver operating within the host apparatus 1000 provided in the form of a computer connected to the printing apparatus.
FIG. 31 shows an example of the processing procedure in this case. In this procedure, the printing medium P with the test patterns formed thereon is read using an outside scanner apparatus, and the information on density thus read is then supplied to the host apparatus 1000 to operate arithmetically the correction values. The printing apparatus awaits the imputing of the correction values (step S135). In a case where such an input is actually done, the correction values are stored (updated) in the EEPROM 103 (step S111).
The above-described processes may be executed either in response to the instruction given by the user. Alternatively, the user may delegate a serviceman to do the processes on behalf of the user, or the user may carry the apparatus in the service center to do the job. In any case, storing the correction values in the EEPROM 103 enables the correction values to be updated when it is necessary. As a result, the deterioration with age of the roller can be addressed properly.
However, assume a case where the deterioration with time is not a real problem, and where no update is necessary. In this case, a default value for the correction value may be determined in an inspection process done before the printing apparatus is shipped from the factory. Then, the default value thus determined is stored in the ROM 102, which is installed in the printing apparatus. In this sense, “the method of acquiring the correction value for the conveying-amount error” characterized: by an arithmetic operation for the correction value for eccentricity; and by a determination of the correction value for outer-diameter that follows the above-mentioned arithmetic operation, is not necessarily carried out within the printing apparatus, but can also be carried out using an apparatus, or an inspection system, that is provided independently of the printing apparatus.
(11) Other Modifications
The above-described embodiment and the modified examples thereof described in various places in the course of the descriptions are not the only ways of carrying out the present invention.
For example, in the configuration described above, the conveying roller 1 and the discharge roller 12 are respectively provided on the upstream side and on the downstream side in the direction of conveying the printing medium P. The printing medium P is conveyed by various conveying units since the printing medium P is loaded till the printing is finished. Assume that units other than the two rollers mentioned above are involved in the conveying, and that the conveying errors caused by the eccentricity or the variation in the outer diameter of each unit may possibly affect the printing quality. If this is the case, a conveying-error correction value can be acquired for each of the rollers in consideration independently or in combination with others. Also in this case, in a similar way to the one employed in the case described above, test patterns are printed firstly, and then an correction value for eccentricity and an correction value for outer-diameter are acquired on the basis of the information on density of the test patterns. In summary, the printing of the test patterns and the acquiring of the correction values can be carried out in accordance with the number of and the combination of the conveying units involved in the conveying at the time when the printing is actually done. In this way, an even and high-quality printing is possible on all over the printing medium P.
For example, in a case where only a single roller is used to convey the printing medium P, the conveying is always carried out by the single roller alone. As a result, there are only one kind of the printing of the test patterns and one kind of the conveying-error correction value. When two rollers are used in the conveying, the processes to be done can be divided, as in the above-described case, into a case where the conveying roller 1 is involved in the conveying and a case where the discharge roller 12 alone is involved in the conveying. In addition, the processes to be done can also be carried out by further dividing the former of the two resultant cases above into a case where the conveying roller 1 alone is involved in the conveying and a case where the conveying roller 1 is involved in the conveying in cooperation with the discharge roller 12. In a case of three rollers, the processes to be done can be divided into five, at the maximum, cases (areas) in a similar manner. In general terms, when the conveying is carried out by n rollers (n>2), the processes to be done can be divided into 3+1⁄2[n(n−1)]areas at the maximum.
In addition, in the example described above, the correction value for eccentricity and the correction value for outer-diameter are acquired for the discharge roller 12 as well. Suppose, however, a case where the discharge roller 12 is made of rubber. Rubber is a material, which is susceptible to the changes in environment and to the deterioration with age, and where reflecting the correction value for eccentricity for the discharge roller 12 may have few, if any, effects. If this is the case, the arithmetic operation for or the application of the correction value for eccentricity for the discharge roller 12 can be omitted.
Moreover, in the example described above, the patch elements for adjustment (the second patch elements) are printed using a part of the nozzle arrays that is located on the upstream side in the conveying direction. Alternatively, for example, as shown in FIG. 32, a printing medium P with patch elements for adjustment RPEs' printed thereon in advance may be used. Then, patch elements for reference APEs are printed using, fixedly, a particular nozzle group of all the nozzle arrays, and thus the formation of the test patterns is completed. After that, on the basis of the test pattern thus formed, processes to acquire the correction values are carried out. Note that the patch elements printed in advance may be the patch elements for reference RPEs', and that the patch elements for adjustment APEs may be printed in the later process.
Furthermore, given in the descriptions provided above are only examples of: the number of color-tones (color, density and the like) of the inks; the kind of the inks; the number of nozzles; ways of setting the range of nozzles actually used and ways of setting the amount of conveying the printing medium P. Likewise, various numerical values given in the descriptions above are also just examples of those that can be used.
2. Characteristic Configuration
The correction values obtained in the way described above are applicable to the control of conveying the printing medium at the time of actual printing.
2.1 First Embodiment of Printing-Medium Conveying Control
(1) Printing Method in First Embodiment
In a first embodiment of the control of conveying a printing medium, the printing on the front-end portion and on the rear-end portion of the printing medium P is carried out by reducing appropriately the range of nozzles to be used.
In some cases of the printing on the front-end portion or the rear end portion of the printing medium P, either one of the conveying roller 1 and the discharge roller 12 is not actually involved in the conveying of the printing medium P. FIGS. 37A and 37C illustrate examples of these cases. When the printing medium P is supported and conveyed by only one of the conveying roller 1 and the discharge roller 12 as in the above-mentioned cases, the flatness of the printing medium P is not secured at a sufficient level. As a result, the distance between the print head and the end portion that is not supported (hereafter, also referred to as “head-to-paper distance”) varies in not a small amount, and the head-to-paper distance is in a quite unstable state. Assume that the printing is carried out on the central portion of the second area of the printing medium P as shown in FIG. 37B. In this case, while the printing medium P is supported and conveyed by both the conveying roller 1 and the discharge roller 12, the printing scan is executed with the ink being ejected. Here, the ink is ejected at timing corresponding to a predetermined head-to-paper distance that is supposed to be kept when the printing medium P is on the platen 3. The ink ejected at appropriate timing form dots on the printing medium P. When the dots thus formed are aligned with an appropriate pitch, an image is formed properly. However, in the second and the third areas, that is, in the front-end portion and in the rear-end portion, the unstable head-to-paper distance—the head-to-paper distance varies to a greater extent within the printing width—destabilizes the positions of the dots formed on the printing medium P. Consequently, such harmful effects as white or black streaks and roughness sometimes appear in the image thus formed. To prevent such degradation in image quality, in the printing apparatus of this embodiment, the range of nozzles to be used is reduced and the printing width of the print head is restricted during the printing on the front-end portion and the rear end portion of the printing medium P. To put it other way, the range of nozzles to be used is reduced and, at the same time, the amount of conveying the printing medium P is reduced. Thus, the variation in the head-to-paper distance is reduced so that the harmful effects to the image can be lowered down to the minimum.
FIG. 38 is a schematic top plan view of the platen 3. The printing medium P is conveyed from the lower side to the upper side in the drawing, that is, in the conveying direction indicated by the arrow. Accordingly, the conveying roller 1 and the discharge roller 12 are respectively disposed on the lower side and on the upper side in FIG. 38.
Nozzle arrays HN are formed in the print head 4. In FIG. 38, only the nozzle array corresponding to the ink of a particular, single color is illustrated for the sake of simplicity. An opening is formed in the platen 3 that supports the printing medium P when the printing medium P passes by the area, which is scanned with the nozzle array HN. Inside the opening, plural ribs P001 are formed as being raised so as to support the printing medium P. Ink absorber P002 is provided to receive the ink that goes off the edge, such as front, rear, and side edges, of the printing medium P when the margin less printing is carried out.
The ribs P001 are formed inside the opening of the platen 3. Specifically, a plural number of the ribs P001 are formed in each of the end portions on both upstream and downstream sides in the conveying direction. The distance between the line of the ribs P001 formed in the end portion on the upstream side and the line of the ribs P001 formed in the end portion on the downstream side is wider than the length corresponding to the maximum number of nozzles (768 nozzles in this embodiment) used for the printing of the central portion of the printing medium P. Accordingly, the ribs P001 are not stained by the ink that goes off from the right and left side edges of the printing medium P.
Also inside the openings, a plural number of the ribs P001 are disposed in the substantially central portion in the direction of conveying the printing medium P so as to support the printing medium P. These ribs P001 in the central portion are disposed so as not to be stained by the ink that goes off from the front and the rear edges as well as from the right-side and the left-side edges when the margin less printing is carried out. The disposition of the ribs P001 and the maximum number of the nozzles that can be involved in the printing on the front-end portion and the rear-end portion of the printing medium P are determined appropriately by taking account of the relationship between the disposition of the ribs P001 and the number of nozzles.
FIGS. 39A to 39D illustrate the printing areas at the time when the printing is carried out by the printing apparatus of this embodiment. With the printing apparatus of this embodiment, printing with no margin (margin less printing) is performed on an A4 printing medium P (294 mm×210 mm).
FIG. 39A illustrates the area in the front-end portion of the printing medium P. As shown in FIG. 37A, printing is performed on the area shown in FIG. 39A before the front end of the printing medium P starts to be supported by the discharge roller 12. FIG. 39B illustrates the area in the central portion of the printing medium P. As shown in FIG. 37B, printing is performed on the area shown in FIG. 39B with the printing medium P being supported by both the conveying roller 1 and the discharge roller 12. FIG. 39C illustrates the area where the printing is performed around the time when the printing medium P is released from the conveying roller 1. FIG. 39D illustrates the area in the rear-end portion of the printing medium P. As shown in FIG. 37C, printing is performed on the area shown in FIG. 39D after the rear end of the printing medium P is released from the conveying roller 1.
The printing on the front-end portion of the printing medium P is performed using 192 nozzles located in a range from the 64th nozzle to the 255th nozzle counted from the nozzle located at the most downstream position of the nozzle array HN shown in FIG. 39A. Such restriction on the range of nozzles to be used when printing is performed on the front-end portion of the printing medium P prevents the ink from being ejected onto the ribs P001.
FIG. 40 illustrates the relationship between the printing scans and the range of nozzles to be used in the printing on the front-end portion shown in FIG. 39A. As shown in FIG. 40, at the beginning of the printing in this case (corresponding to the left-hand side portion of the drawing), 192 nozzles located in a range from the 64th nozzle to the 255th nozzle counted from the nozzle located at the most downstream position are used for the scan. When the scan is done once, the printing medium P is conveyed in an amount corresponding to 48 nozzles (=192/4), and then another scan is done using the 192 nozzles on the downstream side. Every two scans are followed by the conveying of the printing medium P in an amount corresponding to 48 nozzles. Printing is performed by repeating the scan and the conveying.
Firstly, in the front-end portion of the printing medium P, the printing on the front-end portion of a 37.2-mm length is performed using the 192 nozzles positioned on the most downstream side of the nozzle array HN. Then, once the front end of the printing medium P gets supported by the discharge roller 12 as shown in FIG. 34B, the range of nozzles to be used is broadened gradually.
FIG. 41 illustrates the relationship between the printing scans and the range of nozzles used in the printing performed on a shifting area. On the shifting area, printing is performed once the front-end portion of the printing medium P starts to be supported by the discharge roller 12. While the printing on the shifting area is progressing, the range of the nozzles to be used is gradually widened. Specifically, use of the 192 nozzles is use of a part of all the 768 nozzles. This partial use of the nozzles has to be changed into the total use of the 768 nozzles. To this end, the range of nozzles to be used is widened while being shifted. As shown in FIG. 41, the scan at the time when the printing medium P starts to be supported by the discharge roller 12 (corresponding to the left-hand side portion of the drawing) is executed using the 192 nozzles located in a range from the 64th nozzle to the 255th nozzle counted from the nozzle located at the most downstream position. Then, gradually, the range of the nozzles to be used is widened to the upstream side by 32 nozzles for each time. Scans and the conveying in an amount corresponding to 48 nozzles are repeatedly carried out while the range of the nozzles to be used is widened. The scans are performed with the range of the nozzles to be used being widened until the range covers all the 768 nozzles.
Once the widening range of nozzles to be used comes to cover all the 768 nozzles, printing in the central portion of the printing medium P is performed using the entire nozzle array HN as shown in FIG. 39B. Note that the ribs P001 are disposed appropriately also for this case. The point is that no ribs are disposed corresponding to edges of any standard-sized printing medium, for example. For this reason, no ink is ejected onto any ribs P001.
Subsequently, when printing is performed on a portion of the printing medium P near the rear end portion thereof, the range of nozzles to be used is gradually reduced. The timing for starting the restriction on the range of the nozzles used for the printing in the rear-end portion of the printing medium P can be determined on the basis of the timing when a PE sensor detects the rear edge of the printing medium P. To be more specific, firstly, the point of time when the rear edge of the printing medium P is released from the position where the rear edge has been pinched by the conveying roller 1 and the pinch rollers 2 (rear-edge released time) can be identified on the basis of the above mentioned detected timing. Then, the point of time thus identified can be used to find the timing for starting the above-mentioned restriction.
FIG. 42 illustrates the relationship between the printing scan and the range of the nozzles to be used in the printing in another shifting area located near the rear-end portion of the printing medium P. The printing in this shifting area is performed while the range of the nozzles to be used is gradually reduced. Specifically, the range of the nozzles to be used is shifted and gradually reduced from the use of all the 768 nozzles to the use of 384 nozzles—just a part of all the 768 nozzles—located in the range from the 320th nozzle to the 703rd nozzle counted from the nozzle located at the most downstream position. As shown in FIG. 42, when the PE sensor detects the rear edge of the printing medium P (corresponding to the left-hand side portion of the drawing), all the 768 nozzles are used for the scan. Then, the range of the nozzles to be used is gradually reduced to the downstream side by 32 nozzles. Scans and the conveying in an amount corresponding to 48 nozzles are repeatedly carried out while the range of the nozzles to be used is reduced. The printing is performed with the range of the nozzles to be used being reduced until the range includes only 384 nozzles.
After the range of the nozzles to be used is reduced down to the 384 nozzles, the rear-end portion of the printing medium P is released from the conveying roller 1.
FIG. 43 illustrates the relationship between the printing scans and the range of the nozzles to be used in the printing at the time when the rear-end portion of the printing medium P is released from the conveying roller 1 as shown in FIG. 39C. The relieving of the printing medium P from the binding by the conveying roller 1 and the pinch rollers 2 produces an impact, which may in turn cause unevenness in the product print. The occurrence of such unevenness has to be prevented. When the rear edge of the printing medium P is released from the conveying roller 1, the printing medium P sometimes advances excessively in the discharging direction, so that the conveying amount becomes larger than what is supposed to be, that is, a predetermined amount. Accordingly, in the printing on such areas, the number of nozzles to be used is kept at 384, but the positions of the nozzles to be used are shifted. In this embodiment, when the rear edge is released, the amount of the conveying is made to correspond to 160 nozzles, and the printing is carried out using 384 nozzles located in the range from the 192nd nozzle to the 575th nozzle counted from the nozzle that is located at the most downstream position after the positions of the nozzles to be used have been shifted to the upstream side by 144 nozzles.
Printing of several scans is carried out after the rear edge of the printing medium P is released from the conveying roller 1. Then, as shown in FIG. 44, the range of the nozzles to be used is gradually reduced from the 384 nozzles to 192 nozzles located in the range from the 512th nozzle to the 703rd nozzle counted from the nozzle located at the most downstream position. Scans and the conveying in an amount corresponding to 16 nozzles are repeatedly carried out while the range of the nozzles to be used is reduced. The printing is performed with the range of the nozzles to be used being reduced until the range includes only 192 nozzles.
Then, once the printing medium P is supported by the discharge roller 12 alone as shown in FIG. 37C, 192 nozzles on the upstream side of the nozzle array HN are used for the printing as shown in FIG. 39D.
FIG. 45 illustrates the relationship between the printing scans and the range of the nozzles to be used in the printing on the front-end portion shown in FIG. 39C. As shown in FIG. 45, at the beginning of the printing in this case (corresponding to the left-hand side portion of the drawing), 192 nozzles located on the upstream side are used for the scan. When the scan is done once, the printing medium P is conveyed in an amount corresponding to 48 nozzles (=192/4), and then another scan is done using 192 nozzles on the upstream side. Every two scans are followed by the conveying of the printing medium P in an amount corresponding to 48 nozzles. Printing is performed by repeating the scan and the conveying.
As has been described above, the printing in this embodiment is carried out by changing the range of the nozzles to be used in response to the printing areas on which the printing is actually carried out.
(2) Details of Application of Correction Values
Subsequently, the application of the eccentricity correction and of the outer-diameter correction in this embodiment will be described in detail.
As has been described above, a smaller number of nozzles are used in the printing on the front-end portion and on the rear-end portion of the printing medium P in this embodiment. Here, amongst the areas on the printing medium P, the area on which the printing is done with the conveying roller 1 alone being used for the conveying of the printing medium P is defined as a first area (FIG. 37A). The area on which the printing is done with both of the conveying roller 1 and the discharge roller 12 being used for the conveying is defined as a second area (FIG. 37B). The area on which the printing is done with the discharge roller 12 alone being used for the conveying is defined as a third area (FIG. 37C). In this embodiment, different values of the correction values for eccentricity and of the correction values for outer-diameter are applied to the printing of the first to the third areas.
FIG. 46 shows the correction values for eccentricity and the correction values for outer-diameter, and each of these correction values is associated with one of the printing areas defined above. Following the above-described procedure, a first correction value for eccentricity and a first correction value for outer-diameter are obtained for the first area. Likewise, a second correction value for eccentricity and a second correction value for outer-diameter are obtained for the second area. Meanwhile, a third correction value for eccentricity and a third correction value for outer-diameter are obtained for the third area. In this embodiment, however, the application status of the correction values is switched from one to another in accordance with the range of nozzles to be used and the amount of conveying for the areas.
As shown in FIG. 47, all of the 768 nozzles are used for the printing on the second area. This results in a larger amount of conveying for each occasion, and the amount corresponds to 192 nozzles (first conveying amount). Accordingly, the conveying control is carried out by following the procedure described with reference to FIG. 31 and by applying a second correction value for eccentricity and a second correction value for outer-diameter, that is, the correction values for eccentricity and the correction values for outer-diameter of both the conveying roller 1 and the discharge roller 12, which are obtained by following the procedure described with reference to FIGS. 20 and 25. In contrast, the nozzles to be used in the printing of the first area are 192 nozzles in the range from the 64th nozzle to 255th nozzle counted from the nozzle located in the most downstream position. This results in a smaller amount of conveying for each occasion, and the amount corresponds to 48 nozzles (second conveying amount). Accordingly, the conveying control is carried out by applying only the correction value for outer-diameter (first correction value for outer-diameter) of the conveying roller 1 without applying the correction value for eccentricity (first correction value for eccentricity) of the conveying roller 1. Likewise, the nozzles to be used in the printing of the third area are 192 nozzles in the range from the 64th nozzle to 255th nozzle, but in this case, counted from the nozzle located in the most upstream position. The amount of conveying for each occasion for the third area is, as in the case of the first area, relatively small, and corresponds to 48 nozzles (second conveying amount). Accordingly, the conveying control is carried out by applying only the correction value for outer-diameter (third correction value for outer-diameter) of the discharge roller 12 without applying the correction value for eccentricity (third correction value for eccentricity) of the discharge roller 12.
As has just been described, no correction value for eccentricity is applied to the printing of the first area or of the third area, either. This may suggest that there is no necessity of acquiring the correction values for eccentricity for these areas. Nevertheless, when the correction value for outer-diameter is acquired by using the correction value for eccentricity as described above, acquirement of the correction values for eccentricity for the first and the third areas are preferable.
In the printing of the second area, the outer-diameter component of the conveying roller 1 has more dominant influence on the conveying of the printing medium P than the outer-diameter component of the discharge roller 12 does. Accordingly, the outer-diameter correction for both of the first and the second areas may be carried out with a single, common correction value for outer-diameter. To put it other way, the outer-diameter correction for the first area may be carried out, as in the case of the outer diameter correction for the second area, with the correction values for outer-diameter for both of the conveying roller 1 and the discharge roller 12 to carry out the conveying control.
As described above, the conveying control in each of the printing of the first and the third areas is carried out without applying the correction value for eccentricity. The conveying control for these areas may be carried out by applying no correction value for outer-diameter as well as by applying no correction value for eccentricity. If it is the case, no processing is needed for acquiring correction values for these areas.
Incidentally, the areas that correspond to the shifting process are narrow and the printing for these areas is carried out by actually using a reduced range of nozzles. As a consequence, the unevenness caused by eccentricity is hardly noticeable. Accordingly, the conveying control for these areas may be carried out without applying the correction values for eccentricity.
As described above, the conveying control, which is carried out with different application status of correction value for eccentricity and the correction value for outer-diameter for each of the areas, can bring about higher precision in the conveying for the front-end portion and the rear-end portion of the printing medium P. As a result, the printing of higher-quality images can be achieved.
Now, a description is given of some of the reasons why different correction values for eccentricity and different correction values for outer-diameter are applied in accordance with the width of the range of nozzles to be used and with the amount of conveying the printing medium P.
As has been described above, 768 nozzles that can be involved in the printing are arranged for each color in the print head f4 so that the printing of a 1200-dpi density can be achieved. Now, assume a case of repeating 12 times a process consisting of the conveying of the printing medium P in an amount corresponding to, for example, 64 nozzles (=768÷12) for each printing scan using this print head 4. In other words, assume a case where completion of the printing for a single image area on the printing medium by using the print head 4 requires printing of 12 passes.
FIGS. 48A and 48B are graphs describing conveying errors. FIG. 48A shows conveying precision errors. FIG. 48B shows integrated error in conveying in the case of using 768 nozzles and case of using 192 nozzles. The conveying errors shown in FIG. 48A affect the printing on each of the printing areas as being integrated for 768 nozzles to be used in the printing. A curve (1) in the FIG. 48B is obtained by shifting, by 64 nozzles for each time, the integrated values for the 768 nozzles. The period that the curve shows is regarded as the period of the unevenness caused by the eccentricity while the magnitude of the amplitude of this curve corresponds to the extent of the unevenness caused by the eccentricity. A curve (2) in the FIG. 48B, on the other, is obtained by shifting, by 16 nozzles for each time, the integrated values for the 192 nozzles. As FIGS. 48A and 48B demonstrate, an improvement in the unevenness caused by eccentricity can be achieved by reducing the range of nozzles to be involved in the printing and by, accordingly, narrowing the printing width for each scan. To put it other way, reducing the range of nozzles to be involved in the printing, and thus printing by a smaller width in each scan bring about a smaller amount of conveying the printing medium P between every two scans. In the case of the conveying roller 1, such reduction and such narrowing results in a smaller rotational angle for every two scans and a shorter circumferential length of the conveying roller 1 to be used for the completion of the printing for a strip corresponding to a single pass.
FIGS. 49A and 49B as well as FIGS. 50A and 50B are explanatory diagrams each of which describes a relationship between the magnitude of the rotation angle and the conveying amount.
In each of the cases shown in FIGS. 50A and 50B, the conveying roller 1 conveys the printing medium P by a rotational angle that is smaller than that in each of the cases shown in FIGS. 49A and 49B. As clearly shown in FIGS. 49A and 49B, different circumferential lengths PL (lengths of arc) are obtained corresponding to the same angle θ due to the eccentricity of the rotational axis Ec of the conveying roller 1. The difference in the cases of FIGS. 50A and 50B, however, is smaller than that in the cases of FIGS. 49A and 49B. In a case where the printing for an area on the printing medium P by multi-pass printing, such a reduction in the conveying amount in turn reduces the total amount of conveying required to complete the multi-pass printing. As a result, a reduction is also brought about in the integrated amount of the errors in the conveying amount.
As has been described above, the print head 4 has 768 nozzles arranged in a certain area, and has the maximum width by which the printing is possible corresponding to the above-mentioned area of the 768 nozzles. Now assume, as an example, a case where the range of nozzles to be actually involved in the printing is reduced to a range corresponding to 192 nozzles (=768−4) so that the zone to be actually printed may correspond to a quarter of the maximum possible width. Also in this case, a characteristic curve that is similar to the curve (2) shown in FIG. 48B can be obtained by an operation that is similar to the one described above, that is, by integrating the conveying errors shown in FIG. 48A for 192 nozzles and by thus obtaining the moving integrated value. The results clearly demonstrate that when the range of nozzles to be used is reduced to 192 nozzles, the amplitude becomes a quarter of that in the case of using all the 768 nozzles. In addition, the restricted use of 192 nozzles makes the unevenness caused by eccentricity become smaller. This result is reflected in the difference between the appearance of unevenness caused by eccentricity in the image printed by actually using 192 nozzles and the appearance thereof in the image printed by actually using 768 nozzles.
(3) Modified Example 1
As described above, in the front-end portion and the rear-end portion of the printing medium P, flatness of the printing medium P cannot always be secured while the printing medium P is being supported and conveyed by either the conveying roller 1 or the discharge roller 12 alone. That is why the printing on the front-end portion and on the rear-end portion in the first embodiment is carried out by reducing the range of nozzles to be used in printing. Note that the conveying of the printing medium P at that time is carried out without applying the correction value for eccentricity.
Incidentally, the printing on the front-end portion and on the rear-end portion is not the only occasion where the printing is carried out with a reduced range of nozzles to be used. For example, even the printing on the second area is sometimes carried out with a reduced range of nozzles to be used to achieve a high-quality printing. An example of such printing with a reduced range of nozzles is the printing on areas where the printing is carried out while the printing medium P is being conveyed in a smaller amount. The printing on the above-described shifting areas or the printing by using 384 nozzles on the areas immediately before the shifting area are some of the cases of a smaller conveying amount. In addition, in a case where dots cover a relatively small proportion of the surface of the printing medium P, such as in a case where a small number of ink-colors are used for the printing, the printing on all the areas are sometimes carried out by using only a restricted range of nozzles. One of the reasons for this is that in such a case of printing with a smaller number of colors—typically, in the case of monochrome printing—the unevenness becomes more visually noticeable as shown in FIG. 36B.
In these cases, the reduction in the range of nozzles to be used and the reduction in the conveying amount result in reduced extent of unevenness caused by eccentricity. That is why conveying control can be executed with only the correction value for outer-diameter being applied for the purpose.
(4) Modified Example 2
In the embodiment described above, the determination made on whether the eccentricity correction is carried out depends on an area where printing is going to be carried out. Here, the area is either an area that should be printed by using all the nozzles or an area that should be printed by using only a limited number of nozzles. The present invention, however, is not necessarily carried out in this way. In a possible alternative, the determination made on whether the eccentricity correction is carried out depends on whether the printing apparatus is set in a printing mode of using all the nozzles or in a printing mode of using a limited number of nozzles.
2.2 Second Embodiment of Printing-Medium Conveying Control
In general terms, the first embodiment and the modified examples for the first embodiment can be summarized as a case where the application status of the correction value for eccentricity and/or the correction value for outer-diameter (also including whether the application of a correction value is or is not carried out) is changed in accordance with the magnitude of the range of nozzles to be used or with the magnitude of the conveying amount related to the magnitude of the range. In the second embodiment of the printing-medium conveying control, however, the application status of the correction value for eccentricity and/or the correction value for outer-diameter is changed in accordance with the magnitude of conveying amount related to the difference in the number of passes of printing scan.
An ordinary printing apparatus has plural, selectable printing modes, such as “fine (high-quality printing),” “standard,” and “fast (high-speed printing).” In accordance with the selected mode, the printing apparatus can perform what is termed as one-pass printing (printing on a single scanning area on the printing medium P is completed by a single scan) or what is termed as multi-pass printing (printing on a single scanning area on the printing medium is completed by multiple scans). For example, as shown in FIG. 51A, when the fast mode is selected, the printing apparatus is made to perform the printing for a single scanning area by a single printing scan (one-pass printing). In the standard-quality printing mode, four printing scans are carried out to complete the printing (4-pass printing) as shown in FIG. 51B while in the high-quality printing mode, eight printing scans are carried out to complete the printing (8-pass printing) as shown in FIG. 51C.
The higher quality the selected mode is oriented to, that is, the larger the number of passes the printing is carried out by, the smaller the amount of conveying the printing medium P for each occasion becomes. This is because the conveying amount has to be smaller than the width of the array of nozzles to be used. As the conveying amount becomes smaller, the amount of conveying error becomes smaller and the unevenness caused by eccentricity becomes less noticeable. This may present conditions favorable to the conveying control that is carried out with only the correction value for outer-diameter being applied. In short, what is possible in a case where the printing is carried out by an increased number of passes for such purposes as the improvement in the printing quality is the conveying control that is similar to the control carried out for the first and the third areas, which has been described in the first embodiment.
2.3 Other Embodiments
As described above, in the conveying control of the present invention, the conveying corresponding to the printing on each of the shifting areas may accompany a correction either with the correction value for eccentricity having been acquired or without acquiring the correction value for eccentricity.
Logically, when the eccentricity correction is carried out in the case of the conveying corresponding the printing on each of the shifting areas, a higher quality image can be obtained. If it is the case, what is needed for the eccentricity correction for each of the shifting areas is a correction value added with the circumferential-length component of each of the rotational bodies for conveying that are used in the conveying corresponding to the printing areas respectively preceding and following each of the shifting areas in consideration. For example, a polynomial correction function, such as

y=a sin(2π/L1+θ)+b sin(2π/L2+φ),

is useful for the purpose. In addition, the outer-diameter correction for each of the shifting areas is preferably executed using a correction value added with the correction value for outer-diameter of each of the areas that are adjacent to the shifting area.
In the case of two rotational bodies for conveying (in the case of a conveying roller and a discharge roller, for example), the achievement of the eccentricity correction for each of the shifting areas can complete the eccentricity correction for all the areas on the printing medium as shown in FIG. 52. As a result, the eccentricity of each of the rotational bodies for conveying can be prevented from causing harmful effects on the image. Eventually, enhanced uniformity between printing areas can be brought about by the above-mentioned achievement.
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. 2007-103309, filed Apr. 10, 2007, which is hereby incorporated by reference herein in its entirety.

## Claims

1. An inkjet printing apparatus that prints an image on a printing medium by a plurality of scans of a print head relative to the printing medium, the print head having nozzles for ejecting ink, the apparatus comprising:
a roller that conveys the printing medium between the scans of the print head; and
a conveying controller that controls a conveying of the printing medium by the roller on the basis of a correction value used to correct a conveying error of the roller,
wherein the conveying controller determines a value to be used among a plurality of candidates for the correction value in accordance with a number of nozzles to be used for printing.

2. The printing apparatus according to claim 1, wherein a printing of a single image area on the printing medium is performed by the plurality of scans, between each of which the conveying of the printing medium is effected, the printing medium being conveyed after each scan by an amount that is smaller than the width of a nozzle array of the print head.

3. The printing apparatus according to claim 1, further comprising:
a unit configured to cause the print head to form, on the printing medium, a test pattern used to detect the conveying error of the roller; and
a unit configured to acquire the correction value by using the test pattern.

4. The printing apparatus according to claim 1, wherein the correction value includes:
a first correction value to correct a conveying error that depends on the eccentricity of the roller; and
a second correction value to correct a conveying error that depends on the outer diameter of the roller.

5. The printing apparatus according to claim 4, wherein the second correction value is acquired by using the first correction value.

6. The printing apparatus according to claim 4, wherein:
the conveying controller controls the conveying of the printing medium on the basis of both the first and the second correction values when a conveying amount is equal to a first conveying amount, and
the conveying controller controls the conveying of the printing medium on the basis of the second correction value alone when the conveying amount is equal to a second conveying amount that is smaller than the first conveying amount.

7. The printing apparatus according to claim 1, wherein
a plurality of rollers are provided, and
the conveying controller is capable of using the correction value corresponding to one of the plurality of rollers that is involved in the conveying of the printing medium.