Image forming apparatus and image forming method

The image forming apparatus includes: an image holder that is movably provided; a first image forming unit that forms a first image on the image holder; a second image forming unit that forms a second image on the image holder including the first image formed thereon; a transfer portion that transfers, onto a recording medium, the first image and the second image formed on the image holder; a supply unit that transports the recording medium and that supplies the recording medium to the transfer portion; an acquisition unit that acquires information on a position of displacement on the recording medium in a transport direction of the recording medium, the displacement occurring between the first image and the second image transferred onto the recording medium; and an adjustment unit that changes a supply speed of the recording medium at the supply unit on the basis of the information on the position.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is based on and claims priority under 35 USC §119 from Japanese Patent Application No. 2009-16967 filed Jan. 28, 2009.

BACKGROUND

1. Technical Field

The present invention relates to an image forming apparatus such as a printer or a copy machine, and an image forming method.

2. Related Art

In an image forming apparatus having multiple image forming units, color misregistration occurs sometimes because of displacement of transfer positions which occurs due to a non-uniform rotational speed or the like of photoconductive drums or the like. The color misregistration refers to a case where images do not coincide with one another when the images are superimposed on one another. In this respect, various techniques have been proposed to address this problem.

SUMMARY

According to an aspect of the present invention, there is provided an image forming apparatus including: an image holder that is movably provided; a first image forming unit that forms a first image on the image holder; a second image forming unit that forms a second image on the image holder including the first image formed thereon; a transfer portion that transfers, onto a recording medium, the first image and the second image formed on the image holder; a supply unit that transports the recording medium and that supplies the recording medium to the transfer portion; an acquisition unit that acquires information on a position of displacement on the recording medium in a transport direction of the recording medium, the displacement occurring between the first image and the second image transferred onto the recording medium; and an adjustment unit that changes a supply speed of the recording medium at the supply unit on the basis of the information on the position, the information acquired by the acquisition unit.

DETAILED DESCRIPTION

FIG. 1is a diagram showing an example of an overall configuration of an image forming apparatus1to which the exemplary embodiment is applied. The image forming apparatus1shown inFIG. 1is a so-called tandem-type color printer, and includes: an image-formation process unit10that performs image formation in accordance with color image data; a controller30(as an example of an acquisition unit and an adjustment unit) that controls operations of the entire image forming apparatus1; an image processor35that is connected to an external device such as a personal computer (PC)3or an image capturing apparatus4, and that performs image processing on image data received from the external device; and a main power supply36that supplies power to each component. The image forming apparatus1also includes a UI50that is configured of a display panel or the like and that receives information from a user and also displays information for the user.

The image-formation process unit10includes four image forming units11Y,11M,11C and11K (also collectively referred to as an “image forming unit11”) that are arranged side by side at certain intervals along the moving direction of an intermediate transfer belt20. Each of the image forming units11as an example of toner image forming units includes: a photoconductive drum12that is rotatably arranged, forms an electrostatic latent image and holds a toner image; a charging device13that uniformly charges the surface of the photoconductive drum12; an LED print head (LPH)14that is arranged along an axis direction (first scan direction) of the photoconductive drum12, and that exposes, on the basis of image data, the surface of the photoconductive drum12charged by the charging device13; a developing device15that develops the electrostatic latent image formed on the photoconductive drum12; and a drum cleaner16that cleans the surface of the photoconductive drum12after transfer.

In addition, the image forming units11respectively form toner images of yellow (Y), magenta (M), cyan (C) and black (K). Note that, these four image forming units11Y,11M,11C and11K are arranged from the upstream side in the moving direction of a later-described intermediate transfer belt20toward the downstream side therein in this order. Moreover, the image forming unit11Y may be taken as a first image forming unit, for example, and the image forming unit11K may be taken as a second image forming unit.

Furthermore, the image-formation process unit10includes: the intermediate transfer belt20as an example of an image holder onto which multiple layers of color toner images formed on the photoconductive drums12of the image forming units11are transferred; a drive roll21that drives the intermediate transfer belt20; a tension roll22that provides tension to the intermediate transfer belt20; a backup roll23that is provided for secondarily transferring superimposed color toner images onto a sheet P; and a belt cleaner25that removes residual toner or the like existing on the intermediate transfer belt20.

The intermediate transfer belt20is wound around the drive roll21, the tension roll22, and the backup roll23under a tension set in advance. The intermediate transfer belt20is also circularly driven at a speed set in advance in a direction of an arrow B by the drive roll21rotationally driven by a dedicated drive motor excellent in constant speed property (not shown in the figure). Note that, as a material of the intermediate transfer belt20, a belt material (rubber or resin) that does not charge up and whose resistance is adjusted is used.

Moreover, the image-formation process unit10includes: primary transfer rolls24that respectively form primary transfer portions T1at which the color toner images formed in the respective image forming units11are sequentially transferred (primarily transferred) onto the intermediate transfer belt20; and a secondary transfer roll26that collectively transfers (secondarily transferred) the superimposed toner images transferred onto the intermediate transfer belt20onto the sheet P which is a recording medium (recording sheet). In addition, the image-formation process unit10includes a fixing device45(as an example of a fixing unit) that fixes the superimposed toner images having been secondarily transferred, onto the sheet P while transporting the sheet P. Here, the fixing device45includes: a fixing roll451that is rotationally driven by a first motor M1; and a pressure roll452that is arranged to press this fixing roll451, and that forms a nip portion between itself and the fixing roll451. The sheet P passes through this nip portion. The fixing device45also includes a heater (not shown in the figure) that heats the fixing roll451.

In the image forming apparatus1of the present exemplary embodiment, image data inputted from the PC3or the image capturing device4is subjected to image processing performed by the image processor35, and then is supplied to the image forming units11via an interface not shown in the figure. Then, in the black (K) image forming unit11K, for example, the photoconductive drum12is uniformly charged by the charging device13while rotating in a direction of an arrow A, and then is exposed to light emitted from the LPH14on the basis of the image data transmitted from the image processor35. Thereby, an electrostatic latent image for the black (K) color image is formed on the photoconductive drum12. Then, the developing device15develops the electrostatic latent image formed on the photoconductive drum12, hence, forming the black (K) color toner image on the photoconductive drum12. In the same manner, yellow (Y), magenta (M) and cyan (C) color toner images are formed in the image forming units11Y,11M and11C, respectively.

The color toner images formed respectively in the image forming units11are sequentially and electrostatically adsorbed onto the intermediate transfer belt20moving in the direction of the arrow B with use of the respective primary transfer rolls24, and then a composite toner image obtained by superimposing the color toner images on one another is formed on the intermediate transfer belt20. Then, the composite toner image on the intermediate transfer belt20is transported along with the moving of the intermediate transfer belt20to an area (secondary transfer portion T2) at which the secondary transfer roll26is arranged.

Meanwhile, the sheet P is taken out from a sheet supplying unit40by a pickup roll41for feeding sheets. Then, this sheet P is transported to a position of the registration rolls42that regulate the position of the sheet P along a transport path R0. Then, the sheet P is transported toward the secondary transfer portion T2by the registration rolls42in synchronization with a timing at which the composite toner image is transported to the secondary transfer portion T2. At the secondary transfer portion T2, the composite toner image is electrostatically transferred (secondarily transferred) onto the sheet P at once by action of a transfer electric field formed between the backup roll23and the secondary transfer roll26having the secondary transfer bias voltage applied thereto. Note that, the registration rolls42as examples of a supply unit are configured of a drive registration roll421that is rotationally driven by a second motor M2, and a driven registration roll422that is arranged to press the drive registration roll421and that follows and rotates with the drive registration roll421. Then, the registration rolls42transport the sheet P while holding the sheet P between the drive registration roll421and the driven registration roll422, and supplies the sheet P to the secondary transfer portion T2.

Thereafter, the sheet P having the composite toner image electrostatically transferred thereonto is peeled from the intermediate transfer belt20and then transported to the fixing device45. The composite toner image on the sheet P transported to the fixing device45is subjected to fixing processing with heat and pressure by the fixing device45, and is thereby fixed onto the sheet P. Then, the sheet P having the fixed image formed thereon is transported to an output sheet stack unit43provided at an output unit of the image forming apparatus1. Meanwhile, the toner (transfer residual toner) attached to the intermediate transfer belt20after the secondary transfer is removed from the surface of the intermediate transfer belt20by the belt cleaner25after the secondary transfer finishes, and the intermediate transfer belt20is thus made ready for the next image forming cycle. In this way, the image formation in the image forming apparatus1is repeatedly performed for a designated number of sheets.

Incidentally, in the above-described image forming apparatus1, that is, in the so-called tandem type image forming apparatus1, non-uniform rotation of the photoconductive drums12, non-uniform movement of the intermediate transfer belt20or the like caused by machine accuracy or the like occurs, hence causing color misregistration to appear on the sheet P when the color toner images are superimposed and then transferred onto the sheet P in some cases.

In order to address this problem and to make a multi color print with high image quality, a write-start position control technique for correcting color misregistration is necessary. As an example of such a write-start position control technique, there is known a technique in which toner images of specific patterns are formed on the intermediate transfer belt20or the like, which patterns are then detected by use of a CCD camera or the like, and the amount of color misregistration is calculated from the result of detection of the patterns, for example (refer to Japanese Patent Application Laid Open Publication No. 8-286523). In this technique, correction amount data are sent to each correction system, and adjustment is made.

Moreover, the types of color misregistration include one that occurs when the toner images superimposed on the intermediate transfer belt20and the like are recorded (transferred) onto the sheet P. Here, this type of color misregistration is not judged until the image data are actually outputted onto the sheet P.

In order to cope with this type of color misregistration, there have been proposed various methods in which patterns that make color misregistration easily visible are recorded on the sheet P, and then, the amount of color misregistration on the outputted sheet P is measured. For example, the following methods are proposed: a method in which groups of mutually different color lines are shifted in each area in a stepwise manner, and a correction amount is determined depending on a position where the different colors overlap with each other (Japanese Patent Application Laid Open Publication No. 2001-109218); a method in which an aspect of overlapped patterns changes depending on the amount of the color misregistration, thereby causing a correction value to emerge (Japanese Patent Application Laid Open Publication No. 2005-315972), or a method in which the amount of color misregistration is judged by use of cross shaped patterns having different colors.

Furthermore, color misregistration also occurs due to a change in the moving speed of the intermediate transfer belt20in some cases. For example, in the image forming apparatus1of the present exemplary embodiment, which employs an intermediate transfer belt system, the sheet P is sent to the secondary transfer portion T2by the registration rolls42. At the secondary transfer portion T2, the sheet P is then held between the intermediate transfer belt20and the secondary transfer roll26. At this time, when the transport speed of the sheet P transported by the registration rolls42is lower than the moving speed of the intermediate transfer belt20, the sheet P is pulled by the registration rolls42. As a result of this, the moving speed of the intermediate transfer belt20decreases to a speed less than a moving speed set in advance. Then, in a case where a toner image is formed on the intermediate transfer belt20at this time, the position at which this toner image is formed is displaced, hence causing color misregistration.

In addition, when the transport speed of the sheet P transported by the registration rolls42is higher than the moving speed of the intermediate transfer belt20, the registration rolls42pushes the sheet P toward the intermediate transfer belt20. As a result, the moving speed of the intermediate transfer belt20becomes higher. Moreover, when the bottom end of this sheet P passes through the registration rolls42, the force to push the sheet P no longer exists, so that the moving speed of the intermediate transfer belt20decreases. As a result, color misregistration occurs in this case as well.

Specifically, in a case where the transport speed of the sheet P is high or low as compared with the moving speed of the intermediate transfer belt20, the force to pull or to push the sheet P is brought into effect, thereby causing a change in the moving speed of the intermediate transfer belt20. As a result, color misregistration occurs as described above. In particular, image forming apparatuses in recent years have been made to be much smaller in size, so that the distance between the secondary transfer portion T2and the registration rolls42tends to be shorter in this case. The change in the moving speed of the intermediate transfer belt20more likely occurs in this case, hence more likely causing color misregistration. Note that, the larger the basis weight of the sheet P and the harder the stiffness of the sheet P, the more likely the change in the moving speed of the intermediate transfer belt20occurs.

Here, color misregistration is prevented by selecting an optimum value for the rotation speed of the registration rolls42. However, in a case where the dimensions of the registration rolls42vary for example, the rotation speed of the registration rolls42changes. In addition, the rotation speed of the registration rolls42sometimes changes due to a change with the passage of time or depending on use environment.

In this respect, the rotation speed of the registration rolls42is measured by an encoder or the like, and the speed of the registration rolls42may be controlled on the basis of the result of the measurement, for example. However, in this case, the costs and the size of the apparatus increase. Note that, in the techniques using the aforementioned test patterns, which have been proposed heretofore, this type of color misregistration partially occurring on the sheet P is not considered as a target color misregistration. Accordingly, whether or not the rotation speed of the registration rolls42is optimum may not be determined by the techniques. In addition, there is a concern that detection of the amount of correction, which is the original purpose, may not be performed.

Here, descriptions will be given of the color misregistration attributable to the rotation speed of the registration rolls42.

FIGS. 2A to 2D,3A to3C,4,5A to5D,6A and6B,7A and7B, and8are diagrams for explaining the color misregistration attributable to the rotation speed of the registration rolls42.

Firstly, with reference toFIGS. 2A to 2D,3A to3C, and4, descriptions will be given of color misregistration that occurs when the rotation speed of the registration rolls42is lower than a speed set in advance.

Note that, inFIGS. 2A to 2D, and3A to3C, and later-described5A to5D,6A and6B,7A and7B, and the like, the intermediate transfer belt20, the secondary transfer roll26and the registration rolls42are shown in a simplified manner. In addition, in these drawings, for the purpose of making the illustrations easily viewable, an illustration of the image forming unit11M is omitted. Moreover, only the photoconductive drums12are illustrated without illustrating the entire image forming units11Y,11C, and11K. In addition, the photoconductive drums12in the image forming units11Y,11C, and11K are denoted by reference numerals12Y,12C and12K, respectively. Furthermore, for the purpose of making the illustrations easily viewable, the amount of displacement of a position between the color toner images is shown larger than the amount of displacement of a position that actually occurs.

Here,FIG. 2Ashows a state before a first sheet P1enters the secondary transfer portion T2. In this state, the composite toner image is formed on the intermediate transfer belt20without having any color misregistration. Then, as shown inFIG. 2B, when the sheet P1enters the secondary transfer portion T2, a state where the sheet P1is pulled occurs because the transport speed of the sheet P1transported by the registration rolls42is lower than the moving speed of the intermediate transfer belt20. As a result, the moving speed of the intermediate transfer belt20decreases because of this sheet P1. Note that, the decrease in the moving speed of the intermediate transfer belt20is shown by a broken line inFIG. 2B.

Thereafter, as shown in a solid line inFIG. 2C, the moving speed of the intermediate transfer belt20returns to the speed set in advance. However, the toner images influenced by the decrease in the speed of the intermediate transfer belt20arrive at the respective primary transfer portions T1with delay. As a result, as shown by an arrow C1inFIG. 2C, the black toner image is positioned at the downstream side in the moving direction of the intermediate transfer belt20as compared to the cyan toner image and the like, hence causing color misregistration. In addition, as shown by an arrow C2inFIG. 2C, the cyan toner image is positioned at the downstream side in the moving direction of the intermediate transfer belt20as compared to the yellow toner image, hence causing color misregistration.

Thereafter, as shown inFIG. 2D, the toner images are further transferred from the photoconductive drums12K and12C. However, as shown by an arrow D1ofFIG. 2D, color misregistration occurs between the black toner image transferred from the photoconductive drum12K and the toner images (yellow and cyan toner images) influenced by the decrease in the speed of the intermediate transfer belt20. Moreover, as shown by an arrow D2, color misregistration occurs between the cyan toner image transferred from the photoconductive drum12C and the toner image (yellow toner image) influenced by the decrease in the speed of the intermediate transfer belt20.

Then, the intermediate transfer belt20further moves and becomes in a state shown inFIG. 3A. As shown by an arrow A1inFIG. 3Aas well, color misregistration occurs between the black toner image transferred from the photoconductive drum12K and the toner image (yellow toner image) influenced by the decrease in the speed of the intermediate transfer belt20. Moreover, the intermediate transfer belt20further moves and becomes in a state shown inFIG. 3B. As shown by an arrow B1inFIG. 3Bas well, color misregistration occurs between the black toner image transferred from the photoconductive drum12K and the toner image (yellow toner image) influenced by the decrease in the speed of the intermediate transfer belt20.

Here,FIG. 3Cshows toner images transferred onto the sheet P1. Because of the reasons described above, color misregistration (displacement of positions) occurs between toner images. As shown inFIG. 3C, the color misregistration occurs on the center portion, in the transport direction, of the sheet P1.

In addition, when the speed of the intermediate transfer belt20decreases, the amount of influence given to the yellow toner image by the decrease in the speed is larger than that given to the cyan toner image. With reference toFIG. 2B, when the decrease in the speed occurs, the yellow toner images positioned between the photoconductive drums12K and12Y are influenced by the decrease in the speed. Meanwhile, the cyan toner images positioned between the photoconductive drums12K and12C are influenced by the decrease in the speed. Here, since the photoconductive drum12Y is positioned more apart from the photoconductive drum12K than the photoconductive drum12C, a larger amount of influence is given to the yellow toner images by the decrease in the speed of the intermediate transfer belt20.

For this reason, in the present exemplary embodiment, the area in which color misregistration occurs between the black toner image and the yellow toner image (refer to C1inFIG. 3C) is larger than the area in which color misregistration occurs between the black toner image and the cyan toner image (refer to C2inFIG. 3C). Additionally, in the image forming apparatus1of the present exemplary embodiment, the area in which color misregistration occurs between the toner image (black toner image) transferred onto the intermediate transfer belt20from the image forming unit11K positioned at the most downstream side and the toner image (yellow toner image) transferred onto the intermediate transfer belt20from the image forming unit11Y positioned at the most upstream side is the largest. To be more specific, the area in which color misregistration occurs becomes the largest in a case where the image forming units11K and11Y which are positioned most apart from each other are used.

As detailed descriptions will be given later, in the present exemplary embodiment, an yellow pattern image PY and a black pattern image PK are formed on a sheet P, and color misregistration that occurs between the yellow pattern image PY and the black pattern image PK is used for judging whether the rotation speed of the registration rolls42is high or low. Here, in a case where the black toner image and the yellow toner image are used, the area in which color misregistration occur becomes larger, as mentioned above. As a result of this, visibility in judging whether the rotation speed of the registration rolls42is high or low is enhanced.

Here,FIG. 4is a diagram showing a summary of the states of color misregistration occurring when the rotation speed of the registration rolls42is lower than a speed set in advance.

As shown inFIG. 4, when the top end of the first sheet P1enters the secondary transfer portion T2, the moving speed of the intermediate transfer belt20temporarily deceases. Due to this decrease in the speed, color misregistration in which the black toner image is shifted to a position closer to the top end of the sheet than the other color toner images (cyan, magenta and yellow) occurs at the top end side of the sheet in the center portion of the sheet P1. Moreover, color misregistration in which the black and cyan toner images are shifted to positions closer to the top end side of the sheet than the other color toner images (magenta and yellow) occurs at the center of the center portion of the sheet P1.

Moreover, color misregistration in which the black, cyan and magenta toner images are shifted to positions closer to the top end of the sheet than the other color toner image (yellow) occurs at the bottom end side of the sheet in the center portion of the sheet P1. In addition, when the top end of a second sheet P2enters the secondary transfer portion T2, the moving speed of the intermediate transfer belt20temporarily decreases, and the same color misregistration as those described in the case of the first sheet P1occur. Note that, as shown inFIG. 4, the area in which the color misregistration occurs between the black and yellow toner images is the largest.

Next, with reference toFIGS. 5A to 5D,6A and6B,7A and7B, and8, descriptions will be given of color misregistration occurring when the rotation speed of the registration rolls42is higher than a speed set in advance.

As shown inFIG. 5A, when the top end of the first sheet P1enters the secondary transfer portion T2, the registration rolls42(not shown in the figure) start to push the sheet P1. Thereby, the moving speed of the intermediate transfer belt20increases as shown by a broken line inFIG. 5A. Because of this increase in the speed, the color toner images on the intermediate transfer belt20move to positions at the downstream side in the moving direction than originally planned positions. As a result, as shown by an arrow A1, displacement of a position occurs between the black toner image transferred from the photoconductive drum12K and the toner images (yellow and cyan toner images) influenced by the increase in the speed of the intermediate transfer belt20. In addition, as shown by an arrow A2, displacement of a position occurs between the cyan toner image transferred from the photoconductive drum12C and the toner image (yellow toner image) influenced by the increase in the speed of the intermediate transfer belt20.

Thereafter, as shown inFIG. 5B, the intermediate transfer belt20further moves at the increased speed, and as shown by an arrow B1inFIG. 5B, displacement of a position occurs between the black toner image transferred from the photoconductive drum12K and the toner images (yellow and cyan toner images) influenced by the increase in the speed of the intermediate transfer belt20. In addition, as shown by an arrow B2, displacement of a position occurs between the cyan toner image transferred from the photoconductive drum12C and the yellow toner image. Thereafter, the intermediate transfer belt20further moves at the increased speed, and as the toner images are sequentially transferred from the photoconductive drums12Y,12C and12K, displacement of positions occurs between the respective toner images (refer toFIGS. 5Cand D, andFIGS. 6Aand B).

Incidentally, as shown inFIG. 7A, after the bottom end of the first sheet P1passes through the registration rolls42, the force to push this sheet P1no longer exists, so that the speed of the intermediate transfer belt20temporarily decreases as shown by a dot-dash line inFIG. 7A. Then, the decrease in the speed gives an influence to the yellow toner image transferred onto the intermediate transfer belt20for the second sheet P2, and causes the timing at which this yellow toner image arrives at the photoconductive drum12C to delay.

As a result of this, as shown inFIG. 7B, displacement of a position occurs between the cyan toner image transferred from the photoconductive drum12C and the toner image (yellow toner image) influenced by the decrease in the speed of the intermediate transfer belt20. Moreover, although the illustration thereof is omitted herein, displacement of a position occurs between the yellow toner image influenced by the decrease in the speed of the intermediate transfer belt20and the black toner image to be transferred from the photoconductive drum12K later.

FIG. 8is a diagram showing a summary of the states of color misregistration occurring when the rotation speed of the registration rolls42is higher than a speed set in advance.

As shown inFIG. 8, when the top end of the first sheet P1enters the secondary transfer portion T2, the moving speed of the intermediate transfer belt20increases. Because of this increase in the speed, a visually recognizable color misregistration occurs at the bottom end portion (refer to an arrow8A) of the first sheet P1due to the displacement of the position between the black and yellow toner images. In addition, a visually recognizable color misregistration occurs at the bottom end portion (refer to an arrow8C) of the second sheet P2due to the displacement of the position between the black and yellow toner images.

Furthermore, a visually recognizable color misregistration caused by the displacement of the position between the black and yellow toner images occurs at the top end portion (refer to an arrow8B) of the second sheet P2because of the decrease in the speed of the intermediate transfer belt20, which occurs along with passing of the first sheet P1through the registration rolls42.

Note that, at the portion denoted by an arrow8D inFIG. 8, for example, displacement of a position between the black and yellow toner images occurs. However, the amount of the displacement of the position is small. Thus, a visually recognizable color misregistration does not occur at the portion denoted by the arrow8D.

Note that, in the displacement of the positions occurring at the bottom end side of the first sheet P1(refer to the arrow8A), for example, the amount of the displacement of the position occurring between the black and yellow toner images is larger than that occurring between the black and cyan toner images, as shown by an arrow B1inFIG. 7B. Additionally, in the displacement of the positions occurring at the bottom end side of the sheet P1, the amount of the displacement of the position occurring between the toner image (black toner image) transferred onto the intermediate transfer belt20from the image forming unit11K arranged at the most downstream side and the toner image (yellow toner image) transferred onto the intermediate transfer belt20from the image forming unit11Y arranged at the most upstream side is the largest.

As described above, when the rotation speed of the registration rolls42is high or low, color misregistration between toner images formed on the sheet P occurs. In the present exemplary embodiment, as described below, the rotation speed of the registration rolls42is adjusted while attention is paid to this color misregistration.

Here, in the present exemplary embodiment, for the adjustment of the rotation speed of the registration rolls42, pattern images set in advance are formed for the sheet P.

FIG. 9is a diagram for explaining positions on the sheet P where pattern images are formed. In the present exemplary embodiment, when the pattern images to be described later are formed, the pattern images are formed at a first region R1, a second region R2and a third region R3shown inFIG. 9. Here, each of the first to third regions R1to R3is formed into a rectangular shape. Furthermore, the first to third regions R1, R2and R3are positioned at the top end portion of the sheet P, the center portion (center portion in the transport direction of the sheet P) of the sheet P and the bottom end portion of the sheet P, respectively.

Subsequently, descriptions will be given of the pattern images formed on the sheet P.

FIGS. 10A to 10Care diagrams for explaining the pattern images.

As shown inFIG. 10A, in the present exemplary embodiment, by use of the yellow image forming unit11Y, yellow (first color) pattern images PY (first images) are sequentially formed on the moving intermediate transfer belt20. Moreover, by use of the black image forming unit11K, black (second color) pattern images PK (second images) are formed on the yellow pattern images PY moving along with the movement of the intermediate transfer belt20.

In a case where the rotation speed of the registration rolls42is different from the rotation speed set in advance, for example, displacement of positions in the transport direction of the sheet P occurs between the pattern images PY and PK as shown inFIG. 10C, and then, yellow becomes recognizable. On the other hand, in a case where the registration rolls42rotate at the rotation speed set in advance, no displacement of positions occurs between the pattern images PY and PK as shown inFIG. 10B. Accordingly, yellow does not become recognizable in this case.

Note that, each of the pattern images PY and PK in the present exemplary embodiment is formed into a line shape. In addition, the pattern images PY and PK are arranged in a direction orthogonal to (intersecting with) the transport direction of the sheet P (moving direction of the intermediate transfer belt20). Moreover, a space is provided between the pattern images PK adjacent to each other. Then, when displacement of a position occurs, the pattern image PY appears in this space. Note that, the shape of the pattern images PY and PK is not limited to a line shape. For example, the pattern image of a dot-shape is employable. However, when the pattern images are formed into a line shape and arranged in the direction orthogonal to the transport direction of the sheet P as in the present exemplary embodiment, the visibility increases as compared with a dot-shape.

Here,FIGS. 11A to 11Care diagrams showing pattern images formed on the sheets P.

When the rotation speed of the registration rolls42is high and the registration rolls42push the sheet P, the color misregistration in which the positions of the black toner image and the yellow toner image are shifted from each other occurs at the bottom end side of the first sheet P1as described above. In addition, the color misregistration in which the positions of the black toner image and the yellow toner image are shifted with each other occurs at the top end side and the bottom end side of the second sheet P2. Accordingly, in a case where the aforementioned pattern images PK and PY are formed on the sheet P, the pattern image PY becomes visible on the bottom end side (the third region R3) of the first sheet P1, as shown inFIG. 11A. In addition, the pattern image PY becomes visible on the top end side (the first region R1) and the bottom end side (the third region R3) on the second sheet P2.

Note that, in the color misregistration respectively occurring at the bottom end sides of the first sheet P1and the second sheet P2, the pattern images PY are positioned closer to the top end of the first sheet P1and the second sheet P2than the pattern images PK (also refer toFIG. 7B). Moreover, in the color misregistration occurring at the top end side of the second sheet P2, the pattern image PY is positioned closer to the bottom end of the sheet P2than the pattern image PK (also refer toFIG. 7B).

On the other hand, in a case where the rotation speed of the registration rolls42is low and the sheet P is pulled by the registration rolls42, the color misregistration in which the positions of the black toner image and the yellow toner image are shifted from each other occurs at the center portion of the sheet P as described above. Specifically, as shown inFIG. 11C, the pattern images PY become visible at the respective center portions (the second regions R2) of the first sheet P1and the second sheet P2. Here, in these color misregistration, the pattern images PY are positioned closer to the bottom ends of the sheets than the pattern images PK (also refer toFIG. 3C).

Note that, in a case where the transport speed of the sheet P transported by the registration rolls42coincides with the moving speed of the intermediate transfer belt20, the color misregistration between the pattern images PY and the pattern images PK do not occur. Specifically, as shown inFIG. 11B, the pattern images PY are not visible while only the pattern images PK are visible.

Here, the controller30in the image forming apparatus1of the present exemplary embodiment forms the aforementioned pattern images PY and PK by use of the image forming units11Y and11K, respectively, at the aforementioned first to third regions R1to R3of the sheet P on the basis of an instruction from a user via the UI50, for example.

FIG. 12is a diagram showing control blocks of the controller30. Note that, inFIG. 12, blocks related to color misregistration between toner images are illustrated.

The controller30includes: a central processing unit (CPU)301; a read only memory (ROM)302; and a random access memory (RAM)303. The CPU301of the controller30executes processing while performing data communication with the RAM303in accordance with a program stored in the ROM302.

The controller30receives information from the UI50through an input and output interface304. In addition, the controller30displays information on the UI50through the input and output interface304. Moreover, the controller30controls a first motor M1and a second motor M2(also refer toFIG. 1) provided in the image-formation process unit10through the input and output interface304. Further, the controller30controls image forming units11K and11Y provided in the image-formation process unit10. Furthermore, the controller30controls a sheet transporting mechanism (not shown in the figure) and the like provided in the image-formation process unit10.

Next, descriptions will be specifically given of processing performed by the controller30.

Here,FIG. 13shows an example of processing executed by the controller30for adjusting the rotation speed of the registration rolls42.

Firstly, the controller30that receives an instruction to create pattern images via the UI50outputs an instruction to create the aforementioned pattern images PK and PY to the image-formation processing unit10(step101) Thereby, the first sheet P1and the second sheet P2which apply to any one of the three aspects respectively shown inFIGS. 11A to 11Care outputted. Then, the controller30displays an input screen for understanding the aspect and degree of the color misregistration on the UI50(step102), and then waits for an input from the user. Then, the controller30understands the aspect of the color misregistration (position (area) where the color misregistration occurs) and the degree (amount) of the color misregistration on the basis of the information inputted by the user via the UI50(step103).

To be more specific, in step102, the controller30schematically displays the three aspects shown inFIGS. 11A to 11Con the UI50, for example, and also displays, on the UI50, an operation screen for the user to select one of the three aspects, the one applying to each of the outputted first sheet P1and the second sheet P2. Then, after one of the aspects is selected by the user, the controller30displays an operation screen for inputting, by the user, information about the degree of the color misregistration on the UI50. For example, numeric numbers 1 to 10 corresponding to the degrees of the color misregistration are displayed. Then, the controller30understands the degree of the color misregistration on the basis of the numeric number selected by the user.

Thereafter, the controller30changes the rotation speed of the registration rolls42(the speed at which the sheet P is supplied by the registration rolls42) on the basis of the aspect of the color misregistration and the degree of the color misregistration understood in step103(step104). For example, when the aspect of the color misregistration is one shown inFIG. 11A, for example, the controller30performs a setting to decrease the rotation speed of the registration rolls42. More specifically, the controller30performs a setting to reduce the number of rotations of the second motor M2. Note that, the greater the degree of the color misregistration understood in step103is, the greater the number of rotations of the second motor M2is reduced. Here, when the rotation speed of the registration rolls42is changed in the aforementioned manner, the amount of change in the moving speed of the intermediate transfer belt20decreases as compared to the case where the rotation speed is not changed. Additionally, when the rotation speed of the registration rolls42is changed in the aforementioned manner, the moving speed of the intermediate transfer belt20is adjusted.

In addition, when the aspect of the color misregistration is one shown inFIG. 11C, for example, the controller30performs a setting to increase the number of rotations of the registration rolls42. Note that, the greater the degree of the color misregistration understood in step103is, the more the number of rotations is increased.

Next, the controller30monitors an output from the UI50, and thereby, determines whether or not an instruction to create pattern images is issued again (step105). Then, when an instruction to create pattern images is issued again, the controller30executes the processing from steps101to104again. On the other hand, when an instruction to create pattern images is not issued in a certain period of time, the controller30stops processing related to the creation of pattern images and returns the contents of the display on the UI50to the previous contents.

Note that, in the descriptions given above, the descriptions of the cases where the three aspects shown inFIGS. 11A to 11Care displayed on the UI50are provided. In other words, an example of the case where display for the six sheets P is performed is described. However, whether the rotation speed of the registration rolls42is high or low may be determined by the second sheet P2. For example, when the state of the second sheet P2is the state shown inFIG. 11A, the rotation speed of the registration rolls42may be determined to be high. Moreover, when the state of the second sheet P2is the state shown inFIG. 11C, the rotation speed of the registration rolls42may be determined to be low. Accordingly, only the display for the second sheet P2may be performed without the display for the first sheet P1. In addition, the display for the first sheet P1and the second sheet P2inFIG. 11Bmay be omitted. In a case where display for the six sheets are performed as described above, there is a concern that the contents displayed on the UI50are hard to view. The visibility on the UI50increases by reducing the number of sheets P displayed on the UI50.

Incidentally, a change in the speed of the photoconductive drums12Y and12K occurs in some cases. When such a change in the speed occurs, color misregistration occurs even when the speed of the registration roll42is a speed set in advance. Specifically, in some cases, the positions of the pattern images PY and PK are displaced due to a factor other than the registration rolls42, resulting in a situation where the yellow toner image becomes visible. In this respect, pattern images as shown below may be formed.

Here,FIGS. 14A to 14Care diagrams showing other forms of the pattern images.

As shown inFIG. 14A, the height (the width in the transport direction of the sheet) of each of the yellow pattern images PY is set smaller than that of each of the black pattern images PK in the present exemplary embodiment. Specifically, the width of the yellow pattern image PY is set narrower than that of the black pattern image PK. Moreover, the yellow pattern image PY is arranged so as to be closer to one edge of the black pattern image PK in the transport direction of the sheet. Specifically, the yellow pattern image PY is arranged so as to be closer to one edge of the black pattern image PK in the width direction. To be more specific, these pattern images PY and PK are arranged in a way that the edge of the black pattern image PK in the transport direction of the sheet coincides with the edge of the yellow pattern image PY in the transport direction of the sheet.

To be more specific, with reference toFIG. 14B, when the pattern images PK and PY are formed on the second region R2of the first sheet P1, each pattern image PY is arranged so as to be closer to one edge of the corresponding black pattern image PK at the bottom end side of the sheet. In addition, on the third region R3of the first sheet P1, each pattern image PY is arranged so as to be closer to one edge of the corresponding black pattern image PK at the top end side of the sheet.

Furthermore, on the first region R1of the second sheet P2, each pattern image PY is arranged so as to be closer to one edge of the corresponding black pattern image PK at the bottom end side of the sheet. On the second region R2of the second sheet P2, each pattern image PY is arranged so as to be closer to one edge of the corresponding black pattern image PK at the bottom end side of the sheet. Moreover, on the third region R3of the second sheet P2, each pattern image PY is arranged so as to be closer to one edge of the corresponding black pattern image PK at the top end side of the sheet.

Here, in a case where the speed of one of the photoconductive drums12Y and12K changes when the pattern images PY and PK formed at the third region R3on the second sheet P2are transferred onto the intermediate transfer belt20, for example, the transfer position of the pattern image PY may be displaced to a lower position than the original transfer position in some cases, as shown by reference numeral C2inFIG. 14C. In this case, if the pattern images shown inFIG. 10Aare employed, the yellow pattern image PY appears. Specifically, the yellow pattern image PY appears regardless of the rotation speed of the registration rolls42.

However, with the pattern images of this aspect, since the pattern image PY is arranged so as to be closer to the one edge of the black pattern image PK, so that even if the original transfer position of the pattern image PY is shifted, the pattern image PY do not appear as shown by C2inFIG. 14C. In other words, the state in which the yellow pattern image PY and the black pattern image PK are overlapped with each other is kept and the color appearance does not change. More specifically, the yellow pattern image PY is prevented from appearing due to a change in the speed of one of the photoconductive drums12Y and12K.

On the other hand, in a case where the rotation speed of the registration rolls42is higher than a speed set in advance, the yellow pattern image PY is positioned closer to the top end side of the sheet than the black pattern image PK, the yellow color becomes visible in the white ground as shown by C1inFIG. 14C. In other words, the color appearance changes. Thus, the state in which the rotation speed of the registration rolls42is higher may be recognized.

Note that, it is also conceivable to arrange the yellow pattern image PY on the third region R3of the second sheet P2in a way that each yellow pattern image PY is arranged so as to be closer to the edge of the corresponding black pattern image PK at the bottom end side of the sheet, for example. In this case, however, there may occur a situation where the yellow pattern image PY does not appear even if the transfer position of the yellow pattern image PY moves to the top end side of the sheet because the rotation speed of the registration rolls42is high. That is, even if the position of the pattern image PY is displaced due to the rotation speed of the registration rolls42, the pattern images PY and PK are kept in a state where they are overlapped with each other, and thus, the pattern image PY does not appear. For this reason, in the present exemplary embodiment, the pattern image PY is arranged at a position closer to a side to which the position thereof is expected to be displaced. To be more specific, in a case where the pattern image PY is displaced in the third region R3of the second sheet P2due to the registration rolls42, the pattern image PY is displaced to the top end side of the sheet (refer toFIG. 11A). For this reason, the pattern image PY is arranged on the third region R3of the second sheet P2in a way that each pattern image PY is arranged so as to be closer to the edge of the corresponding pattern image PK at the top end side of the sheet.

Furthermore, when color misregistration occurs in the third region R3of the first sheet P1due to the registration rolls42, for example, the yellow pattern image PY is positioned closer to the top end side of the sheet than the black pattern image as shown inFIG. 11A. For this reason, on the third region R3of the first sheet P1, the pattern image PY is arranged so as to be closer to the edge of the pattern image PK at the top end side of the sheet as shown inFIG. 14B.

In addition, when color misregistration occurs in the second region R2of the second sheet P2due to the registration rolls42, for example, the yellow pattern image PY is positioned closer to the bottom end side of the sheet than the black pattern image as shown inFIG. 11C. For this reason, on the second region R2of the second sheet P2, the pattern image PY is arranged so as to be closer to the edge of the pattern image PK on the bottom end side of the sheet as shown inFIG. 14B.

Incidentally, although the descriptions have been given of the case where the positions of toner images are displaced due to the registration rolls42, color misregistration also occurs due to the fixing device45(refer toFIG. 1).

Here,FIGS. 15A and 15B,16,17A and17B,18A and18B and19are diagrams for explaining color misregistration that occurs due to the fixing device45.

Note that,FIGS. 15A and 15Band16are diagrams for explaining color misregistration occurring when the transport speed of the sheet P transported by the fixing device45is lower than the moving speed of the intermediate transfer belt20. In addition,FIGS. 17A and 17B,18A and18B and19are diagrams for explaining color misregistration occurring when the transport speed of the sheet P transported by the fixing device45is higher than the moving speed of the intermediate transfer belt20.

In a case where the transport speed of the sheet P transported by the fixing device45is lower than the moving speed of the intermediate transfer belt20(the rotation speed of the fixing roll451(refer toFIG. 1) of the fixing device45is lower than a rotation speed set in advance), as shown inFIG. 15A, the transport speed (moving speed) of the first sheet P1decreases when the first sheet P1enters the fixing device45. Then, because of the decrease in the transport speed, the moving speed of the intermediate transfer belt20temporarily decreases as shown by a broken line inFIG. 15A.

Thereafter, as shown by a solid line inFIG. 15B, the moving speed of the intermediate transfer belt20returns to the speed set in advance. However, the toner images influenced by the decrease in the speed of the intermediate transfer belt20arrive at the respective primary transfer portions T1with delay. As a result, as shown by an arrow B1inFIG. 15B, the black toner image is positioned at the downstream side in the moving direction of the intermediate transfer belt20as compared to the cyan toner image and the like, hence causing color misregistration. Moreover, as shown by an arrow B2, the cyan toner image is positioned at the downstream side in the moving direction of the intermediate transfer belt20as compared to the yellow toner image, hence causing color misregistration. In other words, in a case where the transport speed of the sheet P transported by the fixing device45is lower than the moving speed of the intermediate transfer belt20, the color misregistration occurs in the toner images transferred onto the sheet P as in the case where the rotation speed of the registration rolls42is low.

Here,FIG. 16is a diagram showing a summary of the states of color misregistration occurring when the transport speed of the sheet P transported by the fixing device45is lower than a speed set in advance.

As shown inFIG. 16, when the top end of the first sheet P1enters the fixing device45, the moving speed of the intermediate transfer belt20temporarily decreases. Then, because of this decrease in the speed, the black toner image is positioned closer to the top end of the sheet than the other color (cyan, magenta and yellow) toner images, so that color misregistration occurs at the center of the center portion of the sheet P1. In addition, the black and cyan toner images are positioned closer to the top end side of the sheet than the other color (magenta and yellow) toner images, so that color misregistration occurs at the bottom end side of the sheet in the center portion of the sheet P1. In addition, the black, cyan and magenta toner images are positioned closer to the top end side of the sheet than the other color (yellow) toner image, so that color misregistration occurs at the bottom end side of the sheet P1. Moreover, the same color misregistration as those occurring on the first sheet P1also occur on the second sheet P2.

Note that, when color misregistration occurs due to the fixing device45, the position at which the color misregistration starts to occur on the sheet P is displaced so as to be closer to the bottom end of the sheet than the position at which color misregistration due to the registration rolls42starts to occur. In other words, when the position at which the color misregistration starts to occur due to the registration rolls42is denoted as color misregistration occurrence start position SR (refer toFIG. 4), color misregistration occurrence start position ST (refer toFIG. 16), which is the position at which the color misregistration starts to occur due to the fixing device45, is displaced so as to be closer to the bottom end of the sheet than the color misregistration occurrence start position SR.

Next, descriptions will be given of color misregistration occurring when the transport speed of the sheet P transported by the fixing device45is higher than the moving speed of the intermediate transfer belt20.

In a case where the transport speed of the sheet P transported by the fixing device45is higher than the moving speed of the intermediate transfer belt20, as shown inFIG. 17A, the first sheet P1is pulled by the fixing device45when the top end of the first sheet P1enters the fixing device45. Thereby, the moving speed of the intermediate transfer belt20increases as shown by a broken line inFIG. 17A. Because of this increase in the speed, the color toner images on the intermediate transfer belt20proceed to positions closer to the downstream side in the moving direction than planned positions.

As a result of this, as shown inFIG. 17B, the yellow toner image advances by the largest distance, followed by the cyan toner image on the sheet P1. In addition, among the toner images to be transferred onto the top end portion of the second sheet P2as well, the yellow toner image advances by the largest distance, followed by the cyan toner image on the sheet P2.

Then, as shown inFIG. 18A, after the bottom end of the first sheet P1passes through the secondary transfer portion T2, the speed of the intermediate transfer belt20temporarily decreases as shown by a dot-dash line inFIG. 18A. Then, the cyan and yellow toner images transferred onto the intermediate transfer belt20for the second sheet P2are influenced by this decrease in the speed. In other words, timings at which the cyan and yellow toner images arrive respectively at the photoconductive drums12K and12C are delayed.

As a result of this, as shown by an arrow B1inFIG. 18B, the black toner image transferred from the photoconductive drum12K advances by the largest distance, and the cyan toner image arrives with the largest delay, so that color misregistration occurs between the black toner image and the cyan toner image. Then, the toner images between which the color misregistration occurs are transferred onto the top end portion of the second sheet P2.

Here,FIG. 19is a diagram showing a summary of the states of color misregistration occurring when the transport speed of the sheet P transported by the fixing device45is higher than a speed set in advance.

As shown inFIG. 19, when the top end of the first sheet P1enters the fixing device45, the moving speed of the intermediate transfer belt20increases. Moreover, after the bottom end of the sheet P1passes through the secondary transfer portion T2, the moving speed of the intermediate transfer belt20temporarily decreases. Due to the decrease in the speed, as shown by an arrow19A and as described above, a large amount of color misregistration that is visually recognizable occurs due to displacement of the positions of the black toner image and the cyan toner image at the top end portion of the second sheet P2.

Here, for determining whether the transport speed of the sheet P in the fixing device45is high or low, the same processing as the one shown inFIG. 13is executed. Then, the transport speed of the sheet P transported by the fixing device45is changed. To be more specific, the number of rotations of the first motor M1(refer toFIG. 1) is set to a new number of rotations. Note that, for determining the transport speed of the sheet P transported by the fixing device45, a cyan pattern image PC and a black pattern image PK are formed on the sheet P by use of the image forming unit11C and the image forming unit11K, respectively. To be more specific, the cyan pattern image PC and the black pattern image PK are formed on the sheet P in the aforementioned step101(refer toFIG. 13).

Here,FIGS. 20A to 20Care diagrams showing pattern images formed on the sheets P.

For example, when the transport speed of the sheet P at the fixing device45is high, color misregistration occurs between the black toner image and the cyan toner image at the top end portion of the second sheet P2as shown by an arrow19A inFIG. 19. For this reason, when the cyan pattern image PC and the black pattern image PK are formed on the sheet P, color misregistration occurs in the first region R1(top end portion of the sheet P2) of the second sheet P2, as shown inFIG. 20A. Thus, the cyan pattern image PC becomes visible.

Note that, when the rotation speed of the registration rolls42is different from a rotation speed set in advance, color misregistration occurs between the cyan pattern image PC and the black pattern image PK as well. However, the position at which the color misregistration occurs due to the rotation speed of the registration rolls42is different from the position at which the color misregistration occurs due to the fixing device45. For example, when the rotation speed of the registration rolls42is low, the color misregistration occurs at a position closer to the bottom end side of the sheet (refer to the arrow4C inFIG. 4). Accordingly, when the color misregistration occurs between the cyan pattern image PC and the black pattern image PK at the top end portion of the second sheet P2, it may be determined that the transport speed of the sheet P at the fixing device45is high.

Moreover, when the transport speed of the sheet P at the fixing device45is low, color misregistration occurs between the black toner image and the cyan toner image in the center portions of the first sheet P1and the second sheet P2as shown by arrows16A and16B inFIG. 16, respectively. For this reason, when the color misregistration between the black pattern image PK and the cyan pattern image PC occurs on the center portion (second region R2) of the first sheet P1and the center portion (second region R2) of the second sheet P2as shown inFIG. 20C, it may be determined that the transport speed of the sheet P at the fixing device45is low.

Note that, when the rotation speed of the registration rolls42is different from a rotation speed set in advance, color misregistration occurs between the cyan pattern image PC and the black pattern image PK as well. However, the position at which the color misregistration occurs due to the rotation speed of the registration rolls42is different from the position at which the color misregistration occurs due to the fixing device45. For example, when the rotation speed of the registration rolls42is low, the color misregistration occurs at a position closer to the top end side of the sheet (refer to the arrows4C and4D inFIG. 4). Accordingly, when the cyan pattern image PC is recognizable on the center portion of the sheet P, it may be determined that the transport speed of the sheet P at the fixing device45is low.

Furthermore, when the rotation speed of the registration rolls42is high, displacement of a position between the black pattern image PK and the cyan pattern image PC occurs at the center portion of the sheet P (refer to the arrow8D inFIG. 8). However, even if the displacement of a position of this kind occurs, the amount of the displacement is so small that the displacement does not become visually recognizable. For this reason, when the cyan pattern image PC is recognized on the center portion of the sheet P, it may be determined that the transport speed of the sheet P at the fixing device45is low.

Note that, when the transport speed of the sheet P at the fixing device45is high or low, color misregistration between the black toner image and the cyan toner image occurs at the center portion or the top end portion of the sheet P, and no color misregistration occurs at the bottom end portion of the sheet P, as shown inFIGS. 16 and 19. For this reason, formation of the pattern images on the bottom end portion of the sheet P may be omitted as shown inFIGS. 20A to 20C. In this case, the amount of toner consumption is reduced.

Note that, although the cyan and black pattern images are used for determining the transport speed of the sheet P at the fixing device45in the above descriptions, the yellow and black pattern images may be used, as in the case of the registration rolls42. However, an area in which color misregistration between the black and yellow toner images occurs is wider as described above. As a result, there occurs a situation where it is difficult to judge whether the color misregistration occurring between the black and yellow toner images is due to the fixing device45or the registration rolls42. For this reason, the black pattern image PK and the cyan pattern image PC are used for determining the transport speed of the sheet P at the fixing device45because the area in which the color misregistration occur therebetween is the smallest. More specifically, the black image forming unit11K and the cyan image forming unit11C, which are adjacent to each other and are most closely related to each other, are used to form the pattern images.

Note that, although the black pattern image is used as one of the two pattern images in the aforementioned exemplary embodiment, a pattern image of a color other than black may be employed. However, when a pattern image other than black is used, a situation where it is difficult to judge whether or not color misregistration exists may occur. For this reason, the black pattern image may be particularly used as one of the two pattern images.

In addition, for determining the transport speed of the sheet P at the fixing device45, the pattern images described inFIGS. 14A to 14Cmay be used.

Furthermore, whether or not the transport speed of the sheet P transported by the registration rolls42is a speed set in advance, and whether or not the transport speed of the sheet P at the fixing device45is a speed set in advance may be determined by outputting the sheet P once.

FIG. 21is a diagram for explaining arrangement regions of pattern images formed on the sheet P.

In a case where the determination is desired to be made by outputting the sheet Ponce, as shown inFIG. 21, the pattern images are formed on regions A and B, which are provided at the top end portion of the second sheet P2and are adjacent to each other in a direction orthogonal to (intersecting with) the transport direction of the sheet P2. To be more specific, the aforementioned pattern images PK and PC are formed on the region A. The pattern images PK and PY are formed on the region B. In addition, the pattern images PK and PC are formed on a region C, which is the top half portion (top end side of the sheet) of the center portion of the sheet P2. Furthermore, the pattern images PK and PC are formed on a region D, which is the center of the center portion of the sheet P2.

Here, when the transport speed of the sheet P at the fixing device45is low, for example, color misregistration between the black toner image and the cyan toner image occurs at the center of the center portion of the second sheet P2(refer toFIG. 16). Accordingly, if the cyan pattern image PC is recognized in the region D inFIG. 21, it may be determined that the transport speed of the sheet P at the fixing device45is low.

In addition, when the transport speed of the sheet P at the fixing device45is high, for example, color misregistration between the black toner image and the cyan toner image occurs at the top end portion of the second sheet P2(refer toFIG. 19). Accordingly, if the cyan pattern image PC is recognized in the region A inFIG. 21, it may be determined that the transport speed of the sheet P at the fixing device45is high.

In addition, when the transport speed of the sheet P transported by the registration rolls42is low, color misregistration between the black toner image and the cyan toner image occurs at the top half portion (top end side of the sheet) of the center portion of the second sheet P2(refer toFIG. 4). Accordingly, if the cyan pattern image PC is recognized in the region C inFIG. 21, it may be determined that the transport speed of the sheet P transported by the registration rolls42is low.

Moreover, when the transport speed of the sheet P transported by the registration rolls42is high, color misregistration between the black toner image and the yellow toner image occurs at the top end portion of the second sheet P2(refer toFIG. 8). Accordingly, if the yellow pattern image PY is recognized in the region B inFIG. 21, it may be determined that the transport speed of the sheet P transported by the registration rolls42is high.