1. Field of the Invention
The present invention relates to a printing apparatus and a method of acquiring correction value. Specifically, the invention relates to a technique to acquire 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 unit 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 (L0) 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 a conveying roller with an ellipsoidal cross-sectional shape as ones 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 has 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.
Besides the eccentricity that is mentioned above, variations in outer circumference, or outer diameter, of the roller is another important cause for lowering the accuracy in conveying. With such variations in outer diameter of the roller, rotation of a roller by a rotation angle determined for a roller with a reference outer diameter will not produce a predetermined amount of conveying. Specifically, use of a roller with an outer diameter that is larger than the standard outer diameter produces a larger amount of conveying while use of a roller with an outer diameter that is smaller than the standard outer diameter produces a smaller amount of conveying. Accordingly, even when the amplitude of the variation is reduced by the above-described correction, the range of variations which is maximum and which exceeds a certain amount of conveying error causes unevenness that appears in the image. This means that to achieve the printing of a high-quality image without unevenness requires not only the lowering of the influence of the eccentricity but also the lowering of the influence of the variations in the outer diameter of the conveying roller.
An example of the techniques to achieve the printing of a high-quality image reducing unevenness is disclosed in Japanese Patent Laid-Open No. 2002-273956. In the disclosed technique, the correction value to correct the conveying error caused by the variations in the outer diameter of the conveying roller (correction value for outer-diameter) is acquired. Also acquired is the correction value to correct the conveying error caused by the eccentricity (correction value for eccentricity).
The inventors of the present invention, however, have found out that a simple application of the technique disclosed in Japanese Patent Laid-Open No. 2002-273956 has difficulty acquiring a more precise correction value for correcting the conveying error caused by the outer diameter of the conveying roller. If a test pattern with a length equal to an integer multiple of the circumferential length of the roller in the conveying direction is used, a precise correction value of the conveying error can be acquired with the acquiring of correction values for eccentricity and outer diameter being in reverse sequence. In practice, however, it is difficult to form a test pattern with a length that is precisely equal to an integer multiple of the circumferential length of the roller. There has to be, in the test pattern, an excess area that exceeds the area corresponding to an integer multiple of the circumferential length of the roller. Although the correction value for outer diameter can be calculated from the average value of the conveying errors, the part of the above-mentioned excess area must have an influence on the calculation.