JP2009234024A - Test print for evaluating eccentricity degree of roller and method of evaluating eccentricity degree of roller - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a test print for evaluating eccentricity degree of a roller, the test print facilitating the observation and evaluation of the eccentricity degree of a print conveying roller. <P>SOLUTION: The test print for evaluating eccentricity degree of the roller has many comparison dot lines 91 extending in a main scanning direction and formed with one-dot width by a first nozzle unit, and many evaluation dot lines 93 extending in the main scanning direction and formed with one-dot width by a second nozzle unit. The comparison dot lines 91 are formed in such a way that the dot lines 91 are repeated for each rotation angle of the print conveying roller that provides a sub-scanning feed pitch having a space equal to the width of one or more dots. The evaluation dot lines 93 are formed in such a way that the dot lines 93 are repeated for each rotation angle of the print conveying roller, which are superposed exactly on the comparison dot lines 93. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  According to the present invention, a first nozzle unit having a large number of ink nozzles that form dot lines in the main scanning direction with respect to the recording medium and a second nozzle that is arranged at a predetermined distance in the sub-scanning direction from the first nozzle unit. Eccentricity of a print transport roller in a printing apparatus having a print head having a nozzle unit, a print transport roller for transporting the recording medium in a sub-scanning direction, and a drive unit for rotating the print transport roller at a predetermined pitch The present invention relates to a roller eccentricity evaluation test print and a roller eccentricity evaluation method.

  As a technique for preventing the deterioration of image quality due to the eccentricity of the recording paper transport roller, the recording paper transport roller for reciprocating the recording paper and the same area on the recording paper when the recording paper moves forward by the recording paper transport roller A recording unit that performs line-by-line print recording in different colors, a detection unit that detects a rotational angle position of the recording conveyance roller, a registration mark recording unit that records registration marks on a recording sheet at regular time intervals, and Based on the reading means for reading the registration mark, calculation means for calculating the moving speed displacement of the recording paper at each rotation angle position of the recording paper conveyance roller from the reading information of the registration mark, and based on the calculated moving speed displacement of the recording paper Data for creating correction data for line-by-line print recording timing by the recording means at each rotation angle position of the recording paper transport roller And forming means, the image recording apparatus is known which is provided and control means for controlling the printing recording by said recording means on the basis of the correction data created by said data creating means (for example, see Patent Document 1). In this image recording apparatus, first, a registration mark is created on the recording paper at the same timing as the time of image recording, and the registration mark is read, and the movement speed displacement of the recording paper at each rotational angle position of the recording paper transport roller is obtained. To create recording timing correction data for recording dots. Based on the correction data, image recording is performed by correcting the recording timing from the time of image recording of the first color. In this image recording device, the eccentricity of the recording paper conveyance roller is obtained by measuring the individual intervals of the registration marks formed at regular time intervals irrespective of the conveyance speed. In order to obtain the above, it is necessary to shorten the interval between the registration marks and increase the measurement accuracy of the interval. However, in order to increase the measurement accuracy of the interval between the registration marks formed at a short interval, it is necessary to make the registration mark position detector have high performance, which is costly.

  In addition, a recording medium transport rotating body that transports the recording medium, an encoder that detects position or speed information of the recording medium transport rotating body, and a driving force that engages with the recording medium transport rotating body and transmits a driving force. A recording medium conveyance system for moving the recording medium by a predetermined amount by a transmission means and a motor engaged with the driving force transmission means is provided. A drive unit that rotates; a transport unit that transports a recording medium by adding an eccentricity detection amount to a predetermined amount of the drive unit; and a process that performs printing for each transport unit of the recording medium, Ink having means for associating the eccentricity correction value of the recording medium conveyance rotating body with the position information of the encoder by comparing the formed image on the recording medium by the printing process for each rotation of the conveyance rotating body. Jet recording apparatus is also known (e.g., see Patent Document 2). In this ink jet recording apparatus, wide print patterns obtained by performing wide printing for each divided drive amount of the recording medium transport rotator are sequentially superposed while shifting in the main scanning direction, and by eccentricity of the recording medium transport rotator. The degree of overlap of each print pattern that changes is evaluated. Since the overlapping portion of the print pattern that changes due to the eccentricity of the recording medium conveyance rotating body is a small area of the entire print pattern, it is difficult to accurately evaluate the eccentricity from the density change due to the overlap.

JP-A-8-101618 (paragraph number 0075-0104, FIG. 6) Japanese Patent Laying-Open No. 2006-192807 (paragraph numbers 0036-0042, FIG. 7)

  In view of the above-described circumstances, an object of the present invention is to provide a test print for evaluating roller eccentricity that makes it easy to observe and evaluate the eccentricity of a print conveying roller that conveys a recording medium in the sub-scanning direction. The present invention provides a technique for outputting a roller eccentricity test print and evaluating the roller eccentricity.

A first nozzle unit having a large number of ink nozzles that form dot lines in the main scanning direction with respect to the recording medium, and a second nozzle unit that is arranged at a predetermined distance in the sub-scanning direction from the first nozzle unit. An eccentricity of the print transport roller is evaluated in a printing apparatus having a print head having the print head, a print transport roller that transports the recording medium in the sub-scanning direction, and a drive unit that rotates the print transport roller at a predetermined pitch. In order to solve the above problems in a test print for evaluating roller eccentricity, a test print for evaluating roller eccentricity according to the present invention is:
A large number of comparison dot lines extending in the main scanning direction formed with one dot width by the first nozzle unit, and a large number of evaluation dot lines extending in the main scanning direction formed with one dot width by the second nozzle unit. The comparative dot line is repeatedly formed for each rotation angle of the print transport roller to create a sub-scan feed pitch in which the gap between each other is 1 dot width or more, and the evaluation dot line is It is repeatedly formed for each rotation angle of the print transport roller that just overlaps the comparative dot line.
A dot here is a minimum print pixel formed by a print head. For example, in the case of an ink jet printing apparatus, one dot is a pixel formed on a recording medium by one unit of ink droplets ejected from an ink nozzle. It is. Therefore, the dot line here means a line segment created by continuously forming such dots in the main scanning direction.

  In this test print for evaluating the degree of eccentricity of a roller, a dot line for comparison, which is a dot line of 1 dot width extending in the main scanning direction, is sequentially formed by the first nozzle unit with a gap of 1 dot width or more in the sub scanning direction. Furthermore, an evaluation dot line having a width of 1 dot is formed by the second nozzle unit for each rotation angle of the print transport roller that just overlaps the comparison dot line. The comparison dot line is formed by the ink nozzles of the first nozzle unit, whereas the evaluation dot line is a second nozzle unit provided at a predetermined distance in the sub-scanning direction from the first nozzle unit. Since the print transport roller is not decentered and the recording medium can be transported with a constant sub-scan feed length in any rotation angle range, the evaluation dot line is always compared. Will overlap just above the dot line. As a result, the density distribution in the sub-scanning direction of the chart portion composed of a large number of comparative dot lines and evaluation dot lines is uniform. However, when the print transport roller is decentered and the recording medium is transported with a different sub-scan feed length depending on the rotation angle range, the sub-scan position of the evaluation dot line is shifted from the target position. The evaluation dot line does not overlap directly above the use dot line, and a shift occurs. When the difference between the comparison dot line and the evaluation dot line becomes large, the area of the dot line in the predetermined unit region becomes relatively large, and the density of the predetermined unit region becomes high. Since the behavior of eccentricity is repeated with one rotation of the print transport roller as a cycle, the cycle in which a unit region that appears to be higher in density than other unit regions also corresponds to that cycle. In other words, in the test print for evaluating the degree of eccentricity of the roller according to the present invention, a high density region is periodically generated in the sub-scanning direction according to the degree of eccentricity of the print conveying roller. The degree is easy to observe and evaluate.

  In a high-resolution printing apparatus, if the width of one dot line (dot width) is generally 30 microns or less and the error of sub-scan feed by the print transport roller due to eccentricity is considered to be about several tens of microns, If the sub-scan feed pitch for forming the comparison dot line in the sub-scanning direction is about 3 dots wide, even if the evaluation dot line is largely displaced due to the eccentricity of the print transport roller, the evaluation dot line is adjacent to the comparison dot. It does not overlap the line, and the density change based on the eccentricity can be expressed well.

  The present invention is not limited to the above-described test print for evaluating the degree of eccentricity of a roller, but also a technique for evaluating the degree of eccentricity of a roller by outputting such a test print. For example, a first nozzle unit having a large number of ink nozzles that form dot lines in the main scanning direction with respect to the recording medium and a second nozzle unit that is shifted from the first nozzle unit by a predetermined distance in the sub-scanning direction. In the printing apparatus, the print transport roller includes: a print head including: a print head including: a print transport roller that transports the recording medium in a sub-scanning direction; and a drive unit that rotates the print transport roller at a predetermined pitch. In the evaluation method, in order to solve the above problems, in the present invention, a comparative dot line forming step in which a number of comparative dot lines extending in the main scanning direction are formed on the recording medium by the first nozzle unit; A number of dot lines for evaluation extending in the scanning direction are recorded by the second nozzle unit. A dot line for evaluation formed on the body, and the dot line for comparison is repeatedly formed for each rotation angle of the print transport roller that creates a sub-scan feed pitch with a gap of 1 dot or more between each other. The evaluation dot line is repeatedly formed for each rotation angle of the print transport roller that just overlaps the comparison dot line, and the density of the recording medium on which the comparison dot line and the evaluation dot line are formed. And the degree of roller eccentricity is evaluated from the density distribution in the sub-scanning direction on the recording medium. This roller eccentricity evaluation method also has the advantage that the eccentricity of the print transport roller can be easily observed and evaluated for the reasons described above.

Hereinafter, an embodiment of a printing apparatus employing an eccentricity evaluation technique according to the present invention will be described with reference to the drawings.
As shown in FIGS. 1 and 2, this printing apparatus feeds print paper P as a recording medium stored in a roll shape to a printing mechanism 2 by a feeding and conveying mechanism 1 to print an image. A discharge conveyance mechanism 4 is provided that sequentially conveys the paper P to the cutting unit C, the back printing unit B, and the decurling unit D and discharges the paper P to a discharge tray 31 attached to the outside of the housing 30. The discharge / conveyance mechanism 4 includes a plurality of nipping / conveying roller pairs 40.

  Since the printing apparatus is configured as an ink jet system, the print mechanism 2 includes an ink jet print head 20 as shown in FIGS. 3 and 4 that reciprocates in the main scanning direction. Inside the housing 30, there are provided a suction unit 33, an ink storage unit IB, and a control unit 5 that are arranged at positions facing the print head 20 across the transport path of the print paper P. The suction unit 33 incorporates an electric fan (not shown) that sucks air from a large number of openings formed on the upper surface of the case-like unit and sends it out downward.

  The feed conveyance mechanism 1 includes a print conveyance roller 10 on the upstream side of the printing mechanism 2 in the conveyance direction. Two press rollers 10a and 10b are attached to the print transport roller 10, and the print paper P is sandwiched and transported by pressing the print paper P against the print transport roller 10 by the press rollers 10a and 10b. The print transport roller 10 creates the transport of the print paper P in the sub-scanning direction during printing, and the rotational displacement amount of the roller peripheral surface becomes the displacement amount of the print paper P in the sub-scanning direction. The print transport roller 10 uses a transport motor M1 formed of a DC servo motor as a drive source. A rotary encoder E is attached to the rotation shaft of the print transport roller 10 and has a resolution capable of generating 32768 pulses by one rotation of the print transport roller 10. The feed conveyance mechanism 1 is further provided with a pull-in conveyance roller pair 13, whereby the cut paper set on the manual feed tray 32 is drawn in and delivered to the print conveyance roller 10, whereby the cut paper is transferred to the print mechanism 2. Can be supplied.

  The cutting unit C includes a disk-shaped cutter 34 that cuts the print paper P, and an operating mechanism 35 that reciprocates the disk-shaped cutter in the main scanning direction. The back print unit B includes a print head 3 that prints information on the back side of the print paper P. In order to correct the curl of the print paper P, the decurling unit D is configured to convey the print paper P while curving it in a direction opposite to the winding direction of the roll-shaped print paper P, and a guide plate 42. And. A plurality of ink cartridges (not shown) are set in the ink reservoir IB, and ink sent from the ink reservoir IB is supplied from an intermediate tank (not shown) to the print head 20 via a flexible tube. .

  The print head 20 includes a carriage 21 and an ink discharge unit 23 that is connected to the carriage 21 and discharges ink onto the print paper P to print information, and has a carriage cover 22 that covers the carriage 21. Yes. The first nozzle unit 23a and the second ink nozzle unit 23b are arranged on the lower surface of the ink discharge unit 23 at two positions spaced apart in the sub-scanning direction. As shown in FIG. 5, a large number of ink nozzles 24 for ejecting ink are formed in each of the ink nozzle units 23a and 23b.

  In the print mechanism 2, a pair of guide rods 26 having a circular cross-sectional shape are arranged in a parallel posture in the main scanning direction. The carriage 21 is supported so as to be slidable with respect to the pair of guide rods 26. One side surface of the carriage 21 in the sub-scanning direction is provided with a pair of slide guides 25 in which circular holes are formed so as to be fitted on one guide rod 26, and the other guide rod 26 is provided on the other side surface. The slide block 25 is provided with a recess that is engaged with the slide block 25. A belt driving mechanism 27 including a driving pulley 27a, a driven pulley 27b, and a timing belt type driving belt 27c wound around the driving pulley 27a is arranged along the main scanning direction on the side of the carriage. The drive belt 27c and the carriage 21 are connected, and a rotational driving force is transmitted to the drive pulley 27a from the drive motor M2.

  In order to construct the linear encoder system 28 that acquires the position of the print head 20 in the main scanning direction, a belt-like linear strip 28a is fixed to the housing 30 at a position near the print head 20 in a posture parallel to the main scanning direction. A sensor unit 28b for optically acquiring scale information of the linear strip 28a is supported by the carriage 21 of the print head 20. The linear encoder system 28 is configured to specify the position of the print head 20 in the main scanning direction by counting the scale information of the linear strip 28a when the print head 20 moves.

  The control unit 5 acquires information such as images and characters to be printed and converts them into print data. The control unit 5 controls the transport motor M2 while checking the pulses from the encoder E based on the print data, thereby realizing the sub-scan transport of the print paper P by the feed transport mechanism 1, and further from the linear encoder system 28. By controlling the drive motor M2 based on the above signal, the reciprocation of the print head 20 in the main scanning direction is realized. In synchronism with these movements, the control unit 5 controls the ink ejection mechanism built in the ink ejection unit 23 based on the print data, thereby controlling the ink nozzles 24 of the first nozzle unit 23a and the second nozzle unit 23b. Ink is ejected to form an image of dot groups on the print paper P. More specifically, the print head 20 is moved in one of the main scanning directions, and printing is performed while ejecting ink through the ink nozzles 24. As a result, when the print head 20 reaches the moving end, the print paper P is transported by a predetermined pitch in the sub-scanning direction by the feed transport mechanism 1. After this conveyance, the print head 20 is moved to the other side of the main scanning direction (in the reverse direction), and the image is again printed on the print paper P while ejecting ink through the ink nozzles 24 of the first nozzle unit 23a and the second nozzle unit 23b. Form.

  The control unit 5 of the printing apparatus also has a function of evaluating the eccentricity of the print transport roller 10. FIG. 6 is a functional block diagram illustrating functions related to the roller eccentricity evaluation in the control unit 5. This functional block diagram includes an image data input unit 51, a memory 52 for developing input image data, an image processing unit 53 for processing input image data, and print data for generating print data from the processed image data The generation unit 54, the print head control unit 55 that controls the ink ejection mechanism of the print head 20 and the drive motor M2 that moves the print head 20 in the main scanning direction based on the print data, and the print conveyance roller that performs the sub-scan conveyance of the print paper P Some of them are also used for normal image printing, such as a conveyance motor control unit 56 that controls ten conveyance motors M1 based on a pulse signal from an encoder E. A test print output management unit 57 that manages processing for outputting a roller eccentricity evaluation test print TP with this printing apparatus as shown in FIG. 7 is used for evaluating the roller eccentricity. It is included. While the output test print TP for evaluating roller eccentricity is visually observed, adjustment data for compensating for roller eccentricity may be manually input. However, test print TP for evaluating roller eccentricity may be used. In the case of adopting a configuration in which adjustment data for automatically compensating for roller eccentricity is generated and the rotation of the print conveying roller 10 is adjusted, an eccentricity evaluation means 58 is incorporated in the control unit 5. .

  In this embodiment, the drive unit that rotates the print transport roller 10 at a predetermined pitch includes a transport motor M 1, an encoder E, and a transport motor control unit 56.

  The test print management unit 57 stores test print data for a roller eccentricity evaluation test print TP (hereinafter simply referred to as a test print TP) shown in FIG. 7, and this test print data is stored as necessary. The print data generation unit 54 outputs the test print TP. The test print TP has a strip-shaped chart portion 80 that is long in the sub-scanning direction. As is apparent from FIG. 8 in which the test print TP is enlarged, the chart portion 80 has a large number of comparative dot lines 91 extending in the main scanning direction and formed by the ink nozzles 24 of the first nozzle unit 23a with a width of one dot. And a plurality of evaluation dot lines 93 extending in the main scanning direction and having a width of 1 dot formed by the second nozzle unit 23b. Both the comparative dot line 91 and the evaluation dot line 93 are formed with a width of one dot. The length of the chart unit 80 in the sub-scanning direction is set to correspond to the sub-scan feed length of one or more rotations of the print transport roller 10.

  FIG. 9 schematically shows a procedure for forming the comparative dot line 91 and the evaluation dot line 93. The comparative dot line 91 is repeatedly formed by the first nozzle unit 23a for each rotation angle of the print transport roller 10 that creates a sub-scan feed pitch in which the gap between each other is 1 dot width or more, preferably approximately 3 dot width. . The comparative dot line 91 formed by the first nozzle unit 23a approaches the specific ink nozzle 24 of the comparative dot line 91 through the sub-scan feed of the test print TP, and the comparative dot line reaches the specific ink nozzle 24. An evaluation dot line 93 is formed at the timing of reaching the facing position. This timing starts from the pulse position that defines the rotation angle of the print transport roller 10 when the corresponding comparison dot line 91 is formed, and the ink of the ink nozzle 24 of the first nozzle unit 23a and the ink of the second nozzle unit 23b. It is determined when the necessary number of pulses are counted up to the rotation angle of the print conveying roller 10 that creates a circumferential length corresponding to the distance in the sub-scanning direction from the nozzle 24. That is, the evaluation dot line 93 is repeatedly formed for each rotation angle of the print transport roller 10 that just overlaps the comparison dot line 91. In this embodiment, the distance in the sub-scanning direction between the ink nozzle 24 of the first nozzle unit 23a and the ink nozzle 24 of the second nozzle unit 23b is approximately half the length of the print transport roller 10. .

Observation of the test print TP having the comparative dot line 91 and the evaluation dot line 93 formed as described above reveals the following.
First, when the print transport roller 10 has no eccentricity and the test print TP is transported at a constant sub-scan feed length in any rotation angle range, the evaluation dot line 93 is always the comparison dot line 91. Overlapping just above. As a result, the density distribution of the chart unit 80 including the comparative dot line 91 and the evaluation dot line 93 is uniform.
However, when the print transport roller 10 is decentered and the test print TP is transported with a different sub-scan feed length depending on the rotation angle range, the sub-scan position of the evaluation dot line 93 is shifted from the target position. Thus, the evaluation dot line 93 does not overlap directly above the comparison dot line 91, and a deviation occurs according to the degree of eccentricity. When the comparison dot line 91 and the evaluation dot line 93 are shifted, the area of the dot line in the predetermined unit region is relatively increased, and as a result, the density of the predetermined unit region is increased.
Further, it is possible to grasp whether the feeding speed of the print conveying roller 10 is faster or slower depending on the deviation direction of the evaluation dot line 93 with respect to the comparative dot line 91 in the sub-scanning direction. That is, when the feeding speed of the print transport roller 10 is increased due to eccentricity, the evaluation dot line 93 is shifted downward with respect to the comparison dot line 91 with respect to FIG. On the contrary, when the feeding speed of the print transport roller 10 becomes slower, the evaluation dot line 93 is shifted upward with respect to the comparative dot line 91 with respect to FIG.

  Since the eccentric behavior of the print transport roller 10 is repeated with one rotation of the print transport roller as a cycle, the cycle in which a region that appears to be higher in density than other regions also corresponds to that cycle. Accordingly, when the density distribution of the output test print TP is observed, the degree of eccentricity of the print transport roller 10 can be grasped. The presence of eccentricity can be understood by observing the chart portion 80 in the sub-scanning direction, where a slightly darker band appears periodically. At that time, in order to specify the eccentricity portion of the print transport roller 10 from the test print TP, a marker that defines the reference rotation angle position is set on the print transport roller 10 and a marker detector MD that detects this marker is set. It is preferable that the first dot line is formed in response to the detection signal from.

  The degree of eccentricity of the print transport roller 10 can be automatically evaluated by the density distribution in the sub-scanning direction of the chart portion 80 of the test print TP. That is, the output test print TP is converted into a digital image by using an image reading device such as a flatbed scanner 60, and is developed as test print image data in the memory 52 through the image data input unit 51. The density distribution is obtained, and the eccentricity of the print transport roller 10 from the reference rotation angle position can be converted into data based on the density distribution. The eccentricity converted into data is sent to an adjustment unit 56a built in the conveyance motor control unit 56, so that the adjustment unit 56a controls the conveyance motor M1 so as to compensate the roller eccentricity of the print conveyance roller 10. can do. As a result, accurate sub-scanning conveyance is realized regardless of the roller eccentricity of the print conveyance roller 10.

A perspective view schematically showing the configuration of an ink jet printing apparatus Side view schematically showing the configuration of an ink jet printing apparatus The perspective view explaining the operation mechanism of the print head Side view explaining the configuration of the print head Bottom view showing nozzle unit and nozzle Functional block diagram showing the functions of the control unit Plan view showing test print for evaluating roller eccentricity Enlarged view of the chart part of the test print for evaluating roller eccentricity Explanatory drawing explaining formation of a dot line for comparison and formation of a dot line for evaluation

Explanation of symbols

E Encoder P Print paper (recording medium)
TP roller eccentricity evaluation test print M1 conveyance motor MD marker detector 10 print conveyance roller 20 print head 51 image data input unit 56 conveyance motor control unit 56a adjustment unit 57 print output management unit 58 eccentricity evaluation means 80 chart unit 91 dot line for comparison 93 dot line for evaluation

Claims (3)

A first nozzle unit having a large number of ink nozzles that form dot lines in the main scanning direction with respect to the recording medium, and a second nozzle unit that is arranged at a predetermined distance in the sub-scanning direction from the first nozzle unit. An eccentricity of the print transport roller is evaluated in a printing apparatus having a print head having the print head, a print transport roller that transports the recording medium in the sub-scanning direction, and a drive unit that rotates the print transport roller at a predetermined pitch. In the test print for roller eccentricity evaluation for
A large number of comparison dot lines extending in the main scanning direction formed with one dot width by the first nozzle unit, and a large number of evaluation dot lines extending in the main scanning direction formed with one dot width by the second nozzle unit. And
The comparative dot line is repeatedly formed for each rotation angle of the print conveying roller that creates a sub-scan feed pitch in which the gap between each other is 1 dot width or more,
The evaluation dot line is a test print for evaluating roller eccentricity, which is repeatedly formed for each rotation angle of the print transport roller that just overlaps the comparison dot line.   The roller eccentricity evaluation test print according to claim 1, wherein the sub-scan feed pitch for forming the comparative dot line in the sub-scan direction is 3 dots wide. A first nozzle unit having a large number of ink nozzles that form dot lines in the main scanning direction with respect to the recording medium, and a second nozzle unit that is arranged at a predetermined distance in the sub-scanning direction from the first nozzle unit. A method for evaluating the degree of eccentricity of the print transport roller in a printing apparatus, comprising: a print head having a print transport roller that transports the recording medium in a sub-scanning direction; and a drive unit that rotates the print transport roller at a predetermined pitch. In
A comparison dot line forming step in which a number of comparison dot lines extending in the main scanning direction are formed on the recording medium by the first nozzle unit, and a number of evaluation dot lines extending in the main scanning direction are the second nozzles. A dot line forming step for evaluation formed on the recording medium by a unit,
The comparative dot line is repeatedly formed for each rotation angle of the print transport roller to create a sub-scan feed pitch where the gap between each other is 1 dot width or more,
The dot line for evaluation is repeatedly formed for each rotation angle of the print transport roller that just overlaps the dot line for comparison,
A roller eccentricity evaluation method in which the density of the recording medium on which the comparative dot line and the evaluation dot line are formed is measured, and the roller eccentricity is evaluated from a density distribution in the sub-scanning direction on the recording medium.