Misregistration amount detector, misregistration amount detecting method, and storage medium

A misregistration amount detector in a tandem image forming apparatus having a conveying body configured to convey paper on which a color image is to be printed is disclosed. The misregistration amount detector includes an image creating part configured to create, on the conveying body, a first pattern for detecting the amount of misregistration of an image position related to a first color relative to an image position related to a second color other than the first color and a second pattern for measuring the conveyance speed of the conveying body, the first pattern and the second pattern being side by side in the conveyance direction of the conveying body, and a reading part configured to read the first pattern and the second pattern created on the conveying body by the image creating part.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a unit configured to detect the amount of misregistration between multiple color images in the case of obtaining an image visualized by superposing multiple colors in a tandem-type image forming apparatus.

2. Description of the Related Art

According to a so-called “tandem” image forming apparatus, a different image forming part is used for each of four colors, and a (composite) color image is formed by superposing toner images directly on paper or on an intermediate transfer belt.

Such a tandem image forming apparatus cannot produce stable color images if the color toner images are superposed at slightly different positions. Therefore, such misregistration correction is commonly performed as detecting misregistration correction patterns for the respective colors formed on the conveyor body (conveyor belt) of the image forming apparatus and superposing the four colors on the same position. In general, the results of detection of color patterns (cyan, magenta, and yellow) and the result of detection of a reference color pattern (black) are compared, and the amounts of misregistration of the color patterns relative to the reference color pattern are calculated. (See, for example, Japanese Laid-Open Patent Application No. 2003-266798 and Japanese Patent No. 3186587.)

On the other hand, since the conveyor belt, on which the misregistration correction patterns are formed, is not uniform in thickness, the surface velocity of the conveyor belt varies to cause detection error.

Therefore, in order to cancel the effect of the above-described detection error due to the non-uniform thickness of the conveyor belt, misregistration correction patterns are formed on the conveyor belt over its overall length or more (for one turn or more), and then the amount of misregistration is detected.

However, forming misregistration correction patterns on the conveyor belt over its overall length or more and detecting the amount of registration as described above cause the problem of a longer time for forming and detecting the misregistration correction patterns.

SUMMARY OF THE INVENTION

According to one embodiment of the present invention, a misregistration amount detector, a misregistration amount detecting method, and a storage medium storing a program for causing a computer to execute the misregistration amount detecting method are provided in which the above-described problem may be solved or reduced.

According to one embodiment of the present invention, a misregistration amount detector, a misregistration amount detecting method, and a storage medium storing a program for causing a computer to execute the misregistration amount detecting method are provided that make it possible to detect misregistration in a shorter period of time without detection error due to the unevenness (non-uniformity) of the thickness of a conveyor belt.

According to one embodiment of the present invention, a misregistration amount detector in a tandem image forming apparatus having a conveying body configured to convey paper on which a color image is to be printed is provided that includes an image creating part configured to create, on the conveying body, a first pattern for detecting an amount of misregistration of an image position related to a first color relative to an image position related to a second color other than the first color and a second pattern for measuring a conveyance speed of the conveying body, the first pattern and the second pattern being side by side in a conveyance direction of the conveying body, and a reading part configured to read the first pattern and the second pattern created on the conveying body by the image creating part.

According to one embodiment of the present invention, a misregistration amount detecting method in a tandem image forming apparatus having a conveying body configured to convey paper on which a color image is to be printed is provided that includes the steps of (a) creating, on the conveying body, a first pattern for detecting an amount of misregistration of an image position related to a first color relative to an image position related to a second color other than the first color and a second pattern for measuring a conveyance speed of the conveying body, the first pattern and the second pattern being side by side in a conveyance direction of the conveying body, and (b) reading the first pattern and the second pattern created on the conveying body by the image creating part.

According to one embodiment of the present invention, a computer-readable storage medium storing a program for causing a computer to execute a misregistration amount detecting method in a tandem image forming apparatus having a conveying body configured to convey paper on which a color image is to be printed is provided, wherein the misregistration amount detecting method includes the steps of (a) creating, on the conveying body, a first pattern for detecting an amount of misregistration of an image position related to a first color relative to an image position related to a second color other than the first color and a second pattern for measuring a conveyance speed of the conveying body, the first pattern and the second pattern being side by side in a conveyance direction of the conveying body, and (b) reading the first pattern and the second pattern created on the conveying body by the image creating part.

Thus, according to one aspect of the present invention, there are provided a misregistration amount detector and misregistration amount detecting method that make it possible to perform misregistration detection without detection error due to the unevenness (non-uniformity) of the thickness of a conveyor belt in a shorter period of time, and a computer-readable storage medium storing a program for causing a computer to execute the misregistration amount detecting method.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A description is given, with reference to the accompanying drawings, of an embodiment of the present invention.

(Operating Principle of Misregistration Amount Detector According to this Embodiment)

A description is given, with reference toFIG. 1, of the operating principle of a misregistration amount detector100according to this embodiment.FIG. 1is a diagram illustrating the operating principle of the misregistration amount detector100according to this embodiment.

The misregistration amount detector100includes an image creating part200, a reading part210, a first calculation part220, a second calculation part230, a third calculation part240, a storage part250, and a calculation part260.

The image creating part200creates (forms) a misregistration detection pattern and a conveyance speed detection pattern parallel to each other in a conveyance direction on the conveyor (conveying body) of a tandem color image forming apparatus. The misregistration detection pattern is for detecting the amount of misregistration between the image position of a specific color and the image position of a color other than the specific color in the image forming apparatus. The conveyance speed detection pattern is for detecting the conveyance speed of the conveyor.

The reading part210is a sensor that reads the misregistration detection pattern and conveyance speed detection pattern created by the image creating part200.

The first calculation part220calculates the amount of misregistration in the conveyance direction and the amount of misregistration in a direction perpendicular to the conveyance direction, using the position information of the misregistration detection pattern read by the reading part210.

The second calculation part230calculates the actual conveyance speed of the conveyor, using the position information of the conveyance speed detection pattern read by the reading part210.

The third calculation part240corrects the amounts of misregistration calculated by the first calculation part220, using the conveyance speed calculated by the second calculation part230.

The calculation part260calculates the actual conveyance speed of the conveyor, using the position information of the conveyance speed detection pattern read by the reading part210, and calculates the amount of misregistration in the conveyance direction and the amount of misregistration in a direction perpendicular to the conveyance direction, using the calculated conveyance speed and the position information of the misregistration detection pattern read by the reading part210.

The storage part250stores the amounts of misregistration corrected by the third calculation part240in a storage unit (storage medium) such as a random access memory (RAM) (not graphically illustrated inFIG. 1). The storage part250stores the amounts of misregistration corrected by the calculation part260in the storage unit.

According to one operation of the misregistration amount detector100of this embodiment, the image creating part200creates a misregistration detection pattern and a conveyance speed detection pattern on the conveyor. Then, the reading part210reads the position information of these patterns. Next, the first calculation part220calculates the above-described two kinds of amounts of misregistration from the position information of the misregistration detection pattern read by the reading part210, and the second calculation part230calculates the actual conveyance speed of the conveyor from the position information of the conveyance speed detection pattern read by the reading part210. Then, the third calculation part240corrects the two kinds of amounts of misregistration calculated by the first calculation part220, using the actual conveyance speed calculated by the second calculation part230. Finally, the storage part250stores the corrected two kinds of amounts of misregistration in the storage unit. Thereby, the misregistration amount detector100ends processing.

According to another operation of the misregistration amount detector100of this embodiment, the image creating part200creates a misregistration detection pattern and a conveyance speed detection pattern on the conveyor. Then, the reading part210reads the position information of these patterns. Next, the calculation part260calculates the actual conveyance speed of the conveyor from the position information of the conveyance speed detection pattern read by the reading part210, and calculates the two kinds of amounts of misregistration based on this calculated conveyance speed and the position information of the misregistration detection pattern read by the reading part210. Finally, the storage part250stores these two kinds of amounts of misregistration in the storage unit. Thereby, the misregistration amount detector100ends processing.

A detailed description is given below of processing operations of these parts200through260.

(a) Processing Operation of Image Creating Part200of Misregistration Amount Detector100

A description is given, with reference toFIG. 2, of the image creating part200of the misregistration amount detector100according to this embodiment.FIG. 2is a schematic diagram illustrating a configuration of the misregistration amount detector100(and a color image forming apparatus) according to this embodiment.

According to this embodiment, the misregistration amount detector100may be provided in a tandem color image forming apparatus to correct a misregistration between the positions of color images created therein. Accordingly, the image creating part200of the misregistration amount detector100and the image creation part of the tandem color image forming apparatus perform the same processing operation using the same mechanism. Therefore, a description is given below of the configuration and processing operation of the image creation part of the color image forming apparatus.

The color image forming apparatus according to this embodiment includes a paper feed tray1, a paper feed roller2, separation rollers3, recording paper4, a conveyor belt5, image forming parts6BK,6M,6C, and6Y, a drive roller7, a driven roller8, photosensitive body drums9BK,9M,9C, and9Y, chargers10BK,10M,10C, and10Y, an exposure unit11, developing units12BK,12M,12C, and12Y, dischargers13BK,13M,13C, and13Y, transfer units15BK,15M,15C, and15Y, a fuser16, and sensors17,18, and19. Laser (light) beams14BK,14M,14C, and14Y, which are exposure beams for corresponding image colors, are emitted from the exposure unit11. Here, those referred to by suffixes BK, M, C, and Y correspond to color images of black, magenta, cyan, and yellow, respectively.

According to the color image forming apparatus of this embodiment, the image forming parts6BK,6M,6C, and6Y, which form respective color images of black, which is a reference color, magenta, cyan, and yellow, which are other colors, are arranged along the conveyor belt5, which is an endless carrier, in its conveyance direction as illustrated inFIG. 2. That is, the image forming parts6BK,6M,6C, and6Y are disposed along the conveyor belt5, which conveys the paper (recording paper)4separated and fed from the paper feed tray1by the paper feed roller2and the separation rollers3, in this order from the upstream side of the conveyance direction of the conveyor belt5.

These image forming parts6BK,6M,6C, and6Y are only different in the color of toner for forming an image, and have the same internal configuration. Accordingly, in the following, a specific description is given of each component (element) of the image forming part6BK, and a description of the other image forming parts6M,6C, and6Y is omitted because the image forming parts6M,6C, and6Y perform the same processing operation as the image forming part6BK.

The conveyor belt5is an endless belt wound (engaged) around the drive roller7and the driven roller8. The drive roller7is rotated by a drive motor (not graphically illustrated), so that the drive motor, the drive roller7, and the driven roller8operate as a drive unit that causes the conveyor belt5, which is an endless carrier, to move.

At the time of forming images, the sheets of paper4contained in the paper feed tray1are fed one after another beginning with one at the top. Each sheet of paper4is attracted and adhered to the conveyor belt5by electrostatic attraction and adhesion, and is conveyed by the rotating conveyor belt5to the first image forming part6BK, where a black toner image is transferred onto the sheet of paper4.

The image forming part6BK includes the photosensitive body drum9BK as a photosensitive body and the charger10BK, the exposure unit11, the developing unit12BK, a photosensitive body cleaner (not graphically illustrated), and the discharger13BK, which are disposed around the photosensitive body drum9BK. The exposure unit11is configured to emit the laser beams14BK,14M,14C, and14Y, which are exposure beams corresponding to the colors of the images formed by the image forming parts6BK,6N,6C, and6Y, respectively.

Here, a description is given, with reference toFIG. 3, of the exposure unit11.FIG. 3is a diagram illustrating the inside of the exposure unit11. The laser beams14BK,14M,14C, and14Y are emitted from laser diodes21BK,21M,21C, and21Y, respectively, which are light sources. The emitted laser beams14BK,14M,14C, and14Y are reflected by a reflecting mirror20to go through corresponding optical systems22BK,22M,22C, and22Y, thereby having their respective optical paths adjusted so as to scan the surfaces of the photosensitive body drums9BK,9M,9C, and9Y. The reflecting mirror20, which is a polygon mirror of a hexahedron, can rotate to cause an exposure beam to scan for one line in the main scanning direction per polygon mirror surface. Further, scanning is performed with the single polygon mirror20with respect to the four laser diodes21BK,21M,21C, and21Y serving as light sources.

The laser beams14BK and14M perform scanning using one of opposed reflecting surfaces of the polygon mirror20and the laser beams14C and14Y perform scanning using the other one of the opposed reflecting surfaces of the polygon mirror20. As a result, it is possible to expose the four different photosensitive body drums9BK,9M,9C, and9Y to light simultaneously. Each of the optical systems22BK,22M,22C, and22Y includes an fθ lens that makes constant the scanning speed of the corresponding laser beam (light)22BK,22M,22C, or22Y on the corresponding photosensitive body drum BK,9M,9C, or9Y and a deflecting mirror that deflects the corresponding laser beam (light)22BK,22M,22C, or22Y.

At the time of forming an image, the exterior cylindrical surface of the photosensitive body drum9BK is evenly charged by the charger10BK in a dark place, and is thereafter exposed to the laser beam14BK corresponding to a black image from the exposure unit11, so that an electrostatic latent image is formed on the exterior cylindrical surface of the photosensitive body drum9BK. The developing unit12BK makes this electrostatic latent image visible with black toner, so that a black toner image is formed on the photosensitive body drum9BK.

This toner image is transferred onto the paper4at a transfer position, or where the photosensitive body drum9BK and the paper4on the conveyor belt5come into contact, through the action of the transfer unit15BK. As a result of this transfer, the black image is formed on the paper4with the black toner.

The paper4, onto which the black toner image has been transferred in the image forming part6BK, is conveyed to the next image forming part6M by the conveyor belt5. In the image forming part6M, a magenta toner image is formed on the photosensitive body drum9M in the same image forming process as in the image forming part6BK, and the magenta toner image is transferred onto the paper4so as to be superposed on the black image formed thereon.

The paper4is further conveyed to the next image forming parts6C and6Y, and a cyan toner image formed on the photosensitive body drum9C and a yellow toner image formed on the photosensitive body drum9Y are transferred onto the paper4so as to be superposed on the black and magenta images in the same manner. Thus, a full color image is formed on the paper4. The paper4on which this full color superposed (composite) image is formed is removed from the conveyor belt5to have the image fused (fixed) in the fuser16, and is thereafter discharged outside the image forming apparatus.

On the other hand, color toner images may not be superposed where they should be superposed, so that there may be misregistration between colors. If there is color misregistration, it is necessary to correct the misregistration between color toner images. This correction is performed by aligning the position of each of magenta, cyan, and yellow images with the position of a black image.

(b) Configuration of Misregistration Correction Pattern

Next, a description is given, with reference toFIG. 4, of a misregistration correction pattern.FIG. 4is a diagram illustrating a misregistration correction pattern25according to this embodiment. (See alsoFIG. 1.) The misregistration correction pattern25includes misregistration correction pattern columns (lines)26FC_R and26FC_L for detecting the amount of misregistration between the position of a black image and each of the positions of magenta, cyan, and yellow images, and a conveyance speed detection pattern column (line)26BK_C for detecting the conveyance speed of the conveyor belt5.

According to this embodiment, the pattern column26BK_C is formed of black rectilinear patterns25BK_E perpendicular to the conveyance direction as illustrated inFIG. 4. Alternatively, however, the rectilinear patterns25BK_E may be created in any of magenta, cyan, yellow, and may be oblique patterns having an inclination relative to the conveyance direction. Further, the pattern column26BK_C may include multiple colors if the individual patterns of the pattern column26BK_C are each single-colored, are equally shaped, and are created at regular intervals.

Further, according to this embodiment, as illustrated inFIG. 4, each of the pattern columns26FC_R and26FC_L includes first misregistration detection patterns25BK_Y1,25M_Y1,25C_Y1,25Y_Y1,25BK_Y2,25M_Y2,25C_Y2, and25Y_Y2that are rectilinear patterns perpendicular to the conveyance direction, second misregistration detection patterns25BK_S1,25M_S1,25C_S1, and25Y_S1that are oblique patterns having an inclination angle of π/4 with respect to a direction perpendicular to the conveyance direction, and third misregistration detection patterns25BK_S2,25M_S2,25C_S2, and25Y_S2that are oblique patterns having an inclination angle of 3π/4 with respect to the direction perpendicular to the conveyance direction. Hereinafter, the first misregistration detection patterns25BK_Y1and25BK_Y2may be collectively referred to by reference numeral25BK_Y. The first misregistration detection patterns25M_Y1and25M_Y2,25C_Y1and25C_Y2, and25Y_Y1and25Y_Y2may also be collectively referred to by reference numerals25M_Y,25C_Y, and25Y_Y, respectively.

Further, inFIG. 4, the pattern column26FC_L, the pattern column26BK_C, and the pattern column26FC_R, each extending in the conveyance direction, are graphically illustrated in this order relative to the direction perpendicular to the conveyance direction (from left to right). However, this order may be changed.

Here, in the misregistration amount detector100according to this embodiment, the misregistration correction pattern25, which is made up of black, magenta, cyan, and yellow toner images, is formed on the conveyor belt5in the same process as a color image is formed on the paper4. Further, each of the image forming parts6BK,6M,6C, and6Y corresponds to the image creating part200in this embodiment.

Further, according to this embodiments the conveyor belt5may be an intermediate transfer belt. In this case, the image creating part200creates the misregistration correction pattern25on the intermediate transfer belt.

Conventionally, a misregistration correction pattern is formed over the overall length or more (one turn or more) of a conveyor belt in order to cancel the effect of detection error due to the non-uniformity of the thickness of the conveyor belt. According to this embodiment, the overall length of the misregistration correction pattern25in the conveyance direction (overall length38BK_E) may be an integral multiple of the circumference of the photosensitive body drum9BK. In the case illustrated inFIG. 4, the overall length38BK_E is twice the circumference of the photosensitive body drum9BK, and is less than or equal to a third of the overall length (one turn) of the conveyor belt5.

Therefore, according to this embodiment, the misregistration correction pattern25is created and detected in a shorter period of time than in misregistration correction using a conventional misregistration correction pattern.

(a) Overview of Reading Part210

A description is given, with reference toFIG. 5andFIG. 6, of operations and configurations of sensors that are the reading part210of the misregistration amount detector100according to this embodiment.FIG. 5is an enlarged view of one of the sensors17,18, and19.FIG. 6is a schematic diagram illustrating the sensors17,18, and19and their peripheral part.

Referring toFIG. 5, each of the sensors17,18, and19includes a light-emitting part23and a light-receiving part24. The light-emitting part23emits exposure light onto the misregistration correction pattern formed on the conveyor belt5, and the reflected light is received by the light-receiving part24, so that each of the sensors17,18, and19detects the misregistration correction pattern25.

Further, as illustrated inFIG. 6, the sensors17,18, and19are provided on the downstream side of the image forming part6Y so as to be opposed to the conveyor belt5. The sensors17,18, and19are supported on the same substrate (not graphically illustrated) so as to be along a direction perpendicular to the conveyance direction of the paper4.

A description is given, with reference toFIG. 7, of the principle of detection of a misregistration correction pattern according to this embodiment.FIG. 7is a diagram for illustrating the principle of detection of the misregistration correction pattern25by the sensor17(18,19).

Referring toFIG. 7, (c) shows a result of detection of reflected light received by the light-receiving part24(detection result31), and (b) shows detected intensity of light of diffuse reflection received by the light-receiving part24(detected intensity32) and detected intensity of light of specular reflection received by the light-receiving part24(detected intensity33). Here, the detection result31is the sum of the detected intensity32and the detected intensity33.

Further, in the graph of (c) ofFIG. 7, the vertical axis represents the intensity of light received by the light-receiving part24and the horizontal axis represents time.

Here, the light of specular reflection refers to light reflected in a direction opposite to the direction of incidence of exposure light at the same angle as the angle of incidence of the exposure light (that is, reflected light whose angle of reflection is π−θ, where θ is an angle of incidence). The light of diffuse reflection refers to reflected light other than the light of specular reflection.

Each of the sensors17,18, and19determines that an edge of the misregistration correction pattern25is detected at each of positions37BK_1,37BK_2,37M_1(37C_1and37Y_1), and37M_2(37C_2and37Y_2) where the detection result31crosses a predetermined threshold36. According to this embodiment, the midpoint of two edges (for example, the midpoint of the positions37BK_1and37BK_2) detected from the misregistration correction pattern25is determined as the position of an image. Alternatively, each of the positions (edges)37BK_1,37BK_2,37M_1(37C_1and37Y_1), and37M_2(37C_2and37Y_2) detected from the misregistration correction pattern25may be determined as the position of an image. The positions37BK_1and37BK_2may correspond to the edges of the first misregistration detection pattern25BK_Y of (a) ofFIG. 7. The positions37M_1(37C_1and37Y_1) and37M_2(37C_2and37Y_2) may correspond to the edges of the first misregistration detection pattern25M_Y (25C_Y and25Y_Y) of (a) ofFIG. 7.

Further, referring to (a) ofFIG. 7, a line width29of each of the color misregistration detection patterns25BK_Y,25M_Y,25C_Y, and25Y_Y in the conveyance direction is substantially equal to a spot size 27 of exposure light in order to improve the signal-to-noise ratio at the time of detection of the misregistration detection patterns25BK_Y,25M_Y,25C_Y, and25Y_Y. Further, if two of the misregistration detection patterns25BK_Y,25M_Y,25C_Y, and25Y_Y are simultaneously exposed to light of diffuse reflection, pattern detection cannot be performed normally. Accordingly, an interval (distance)30between misregistration detection patterns is greater than a spot size 28 of light of diffuse reflection.

(a) Calculation of Misregistration Amount

A description is given, with reference toFIGS. 8A and 8B, of calculation of the amount of misregistration using misregistration detection patterns.FIGS. 8A and 8Bare diagrams illustrating the principle of calculating the amount of misregistration using misregistration detection patterns.

InFIG. 8A, by way of example, the amount of misregistration is calculated from black and magenta misregistration detection patterns. By replacing the magenta misregistration detection patterns with cyan misregistration detection patterns or yellow misregistration detection patterns, the amount of misregistration relative to a black image can also be detected for a cyan image or a yellow image in the same manner as in the case of a magenta image.

FIG. 8Aillustrates the first black misregistration detection patterns25BK_Y1and25BK_Y2, the first magenta misregistration detection patterns25M_Y1and25M_Y2, the second black misregistration detection pattern25BK_S1, the second magenta misregistration detection pattern25M_S1, the third black misregistration detection pattern25BK_S2, and the third magenta misregistration detection pattern25M_S2. As described above, the magenta misregistration detection patterns25M_Y1,25M_Y2,25M_S1, and25M_S2may be replaced with the cyan misregistration detection patterns25C_Y1,25C_Y2,25C_S1, and25C_S2, respectively, or the yellow misregistration detection patterns25Y_Y1,25Y_Y2,25Y_S1, and25Y_S2, respectively.

Referring toFIG. 8A, there is an interval (distance)42BK_1between the first black misregistration detection pattern25BK_Y1and the second black misregistration detection pattern25BK_S1, and there is an interval (distance)42BK_2between the first black misregistration detection pattern25BK_Y2and the third black misregistration detection pattern25BK_S2. Further, there is an interval (distance)42M_1between the first magenta misregistration detection pattern25M_Y1and the second magenta misregistration detection pattern25M_S1, and there is an interval (distance)42M_2between the first magenta misregistration detection pattern25M_Y2and the third magenta misregistration detection pattern25M_S2. Here, the position of each misregistration detection pattern used to calculate the above-described misregistration detection pattern intervals42BK_1,42BK_2,42M_1, and42M_2is the midpoint of the leading (front) edge and the trailing (rear) edge of the misregistration detection pattern detected by the sensor17or19.

In the case of using the cyan misregistration detection patterns25C_Y1,25C_Y2,25C_S1, and25C_S2, there is an interval42C_1between the first cyan misregistration detection pattern25C_Y1and the second cyan misregistration detection pattern25C_S1, and there is an interval42C_2between the first cyan misregistration detection pattern25C_Y2and the third cyan misregistration detection pattern25C_S2as parenthesized inFIG. 8A. In the case of using the yellow misregistration detection patterns25Y_Y1,25Y_Y2,25Y_S1, and25Y_S2, there is an interval42Y_1between the first yellow misregistration detection pattern25Y_Y1and the second yellow misregistration detection pattern25Y_S1, and there is an interval42Y_2between the first yellow misregistration detection pattern25Y_Y2and the third cyan misregistration detection pattern25Y_S2as parenthesized inFIG. 8A.

Then, considering that the second magenta misregistration detection pattern25M_S1and the third magenta misregistration detection pattern25M_S2are at angles of π/4 and 3π/4, respectively, with a direction perpendicular to the conveyance direction, an amount of misregistration43D_1and an amount of misregistration43D_2in the direction perpendicular to the conveyance direction, calculated from the respective misregistration detection patterns sets, are expressed by:
43D—1=42BK—1−42M—1, and
43D—2=42M—2−42BK2, respectively.

InFIG. 8A, the detection point of the sensor17or19is indicated by a one-dot chain line500. If there is no misregistration, the second magenta misregistration detection pattern25M_S1is created as indicated by a dotted line. However, since there is the amount of misregistration43D_1, a midpoint O1of the second magenta misregistration is detection pattern25M_S1deviates in the conveyance direction by a distance OM1. As illustrated inFIG. 8B, which is an enlarged view of the broken-line circled portion B ofFIG. 8A, this distance OM1is equal to the amount of misregistration43D_1because the second magenta misregistration detection pattern25M_S1is at an angle of π/4 with the direction perpendicular to the conveyance direction. Since the actual value of the interval42M_1is less than what the interval42M_1is supposed to be, which is equal to the interval42BK_1, by the distance OM1, the amount of misregistration43D_1is obtained by subtracting the interval42M_1from the interval42BK_1. Likewise, the third magenta misregistration detection pattern25M_S2without misregistration is indicated by a dotted line. For the same reason as in the case of the second magenta misregistration detection pattern25M_S1, a midpoint deviation (distance) OM2is equal to the amount of misregistration43D_2. Since the actual value of the interval42M_2is more than what the interval42M_2is supposed to be, which is equal to the interval42BK_2, by the distance OM2, the amount of misregistration43D_2is obtained by subtracting the interval42BK_2from the interval42M_2.

The amount of misregistration DA of the magenta image relative to the black image in the direction perpendicular to the conveyance direction is expressed by the average of the amount of misregistration43D_1and the amount of misregistration43_D2as follows:
DA=(43—D1+43—D2)/2.

Further, the amount of misregistration DB of the magenta image relative to the black image in the conveyance direction is determined by calculating the difference between a detected value44D_1of a distance between the first black misregistration detection pattern25BK_Y1and the first magenta misregistration detection pattern25M_Y1and a desired distance between the first black misregistration detection pattern25BK_Y1and the first magenta misregistration detection pattern25M_Y1(supposed to be created by the misregistration amount detector100), or by calculating the difference between a detected value44D_2of a distance between the first black misregistration detection pattern25BK_Y2and the first magenta misregistration detection pattern25M_Y2and a desired distance between the first black misregistration detection pattern25BK_Y2and the first magenta misregistration detection pattern25M_Y2(supposed to be created by the misregistration amount detector100). The amount of misregistration DB of the magenta image relative to the black image in the conveyance direction may also be determined by calculating and averaging the detected values44D_1and44D_2.

(b) Correction of Calculated Amount of Misregistration

First, a description is given, with reference toFIG. 9, of calculation of the surface conveyance speed (velocity) of the conveyor belt5using conveyance speed detection patterns according to this embodiment.FIG. 9is a diagram illustrating the principle of calculating the conveyance speed of the conveyor belt5from the conveyance speed detection patterns25BK_E.

Referring to (a) ofFIG. 9as well asFIG. 4, each conveyance speed detection pattern25BK_E has a width29BK_E, the conveyance speed detection patterns25BK_E are arranged at regular intervals30BK_E, and there is the interval (distance)38BK_E between the start point and end point of the conveyance speed detection pattern column26BK_C. Further, referring to the graph of (b) ofFIG. 9, reference numeral50indicates the conveyance speed of the conveyance belt5at each point of time calculated from the detection result of the pattern column26BK_C, reference numeral53indicates the average conveyance speed in the interval38BK_E between the start point and end point of the pattern column26BK_C, and reference numeral54indicates a desired conveyance speed of the conveyance belt5. Further, the ratio of the average conveyance speed53to the desired conveyance speed54(average conveyance speed53/desired conveyance speed54) is expressed by a correction coefficient55for correcting detection error. Further, the vertical axis represents speed and the horizontal axis represents the time of detection of the conveyance speed detection pattern26BK_E.

The conveyance speed50, or the conveyance speed of the conveyor belt5at each point of time, is calculated by dividing the total value (sum) of the width29BK_E of the conveyance speed detection pattern25BK_E and the interval30BK_E between each adjacent two of the conveyance speed detection patterns25BK_E by the difference between a time39BK_E(t) at which one conveyance speed detection pattern25BK_E is detected and a time39BK_E(t−1) at which a conveyance speed detection pattern25BK_E immediately preceding the one conveyance speed detection pattern25BK_E is detected. The average conveyance speed53is calculated by averaging the thus calculated conveyance speed50of the conveyor belt5over the entire interval.

Here, the conveyor belt5is not uniform in thickness over its length. Therefore, there is an error between the average conveyance speed53, which is the surface velocity of the conveyor belt5, and the desired conveyance speed54of the conveyor belt5.

The amount of misregistration DA of the magenta image relative to the black image in the direction perpendicular to the conveyance direction and the amount of misregistration DB of the magenta image relative to the black image in the conveyance direction, which are calculated using the desired conveyance speed54of the conveyor belt5, therefore contain a detection error corresponding to the difference between the average conveyance speed53and the desired conveyance speed54. For example, the interval42M_1(FIG. 8A) used to determine the amount of misregistration DA may be calculated by multiplying the desired conveyance speed54by the difference in time between the first magenta misregistration detection pattern25M_Y1and the second magenta misregistration detection pattern25M_S1detected by the sensor17or19. For example, the distance (detected value)44D_1may be calculated by multiplying the desired conveyance speed54by the difference in time between the first black misregistration detection pattern25BK_Y1and the first magenta misregistration detection pattern25M_Y1detected by the sensor17or19.

Therefore, an amount of misregistration DA′ of the magenta image relative to the black image in the direction perpendicular to the conveyance direction, which excludes the above-described detection error, and an amount of misregistration DB′ of the magenta image relative to the black image in the conveyance direction, which excludes the above-described detection error, can be expressed by:
DA′=Detection Error Correction Coefficient 55×DA, and
DB′=Detection Error Correction Coefficient 55×DB, respectively.

Thus, by correcting the amount of misregistration calculated from the detection result of misregistration detection patterns using the detection result of conveyance speed detection patterns, it is possible to improve the accuracy of detection of the amount of misregistration.

On the other hand, the average conveyance speed53calculated using the detection result of conveyance speed detection patterns may be used in place of the desired conveyance speed54in calculating the amount of misregistration DA of the magenta image relative to the black image in the direction perpendicular to the conveyance direction and the amount of misregistration DB of the magenta image relative to the black image in the conveyance direction. As a result, it is possible to calculate the amount of misregistration that does not contain detection error caused by the difference between the average conveyance speed53and the desired conveyance speed54.

(c) Calculation Unit of Misregistration Amount Detector100

Next, a description is given, with reference toFIG. 10, of a configuration and operations of a calculation unit2000of the misregistration amount detector100according to this embodiment.FIG. 10is a block diagram illustrating a configuration of the calculation unit2000, which includes the first calculation part220, the second calculation part230, the third calculation part240, and the calculation part260.

The calculation unit2000according to this embodiment includes an amplifier60, a filter61, an analog-to-digital (A/D) conversion part62, a sampling control part63, a first-in first-out (FIFO) memory64, an input/output (I/O) port65, a data bus66, a central processing unit (CPU)67, a RAM68, a read-only memory (ROM)69, and a light emission amount control part70.

A reflected light signal received by the reception part24(alsoFIG. 5) is amplified by the amplifier60. Then, only a signal component detecting the misregistration correction pattern25is extracted from the amplified signal using the filter61. Next, the reflected light signal is converted from analog data into digital data by the A/D conversion part62. Data sampling accompanying this A/D conversion is controlled by the sampling control part63. The sampled signal is stored in the FIFO memory64.

After all the misregistration correction pattern25formed of the four colors of black, magenta, cyan, and yellow has been detected, the data contained in the FIFO memory64are loaded into the RAM68through the I/O port65and the data bus66. The CPU67performs a computation that calculates the above-described amounts of misregistration on the data loaded into the RAM68.

The ROM69contains various programs for controlling the misregistration amount detector100according to this embodiment, such as a program for calculating the above-described amounts of misregistration. Further, the CPU67monitors a detection signal from the light-receiving part24with appropriate timing, and controls the amount of light emission through the light emission amount control part70so as to ensure detection even if the conveyor belt5and the light-emission part23degrade, thereby making the level of the received light signal from the light-reception part24always constant. Thus, the CPU67and the ROM69operate as a control part that controls the operation of the entire misregistration amount detector100.

(Operating Procedure of Misregistration Amount Detector100According to this Embodiment)

A description is given, with reference toFIG. 11, of calculation of the amount of misregistration (first processing) in the misregistration amount detector100according to this embodiment.FIG. 11is a flowchart illustrating a procedure for calculating the amount of misregistration (first processing) in the misregistration amount detector100according to this embodiment.

Here, calculation of the amount of misregistration (first processing) refers to the processing of calculating the amount of misregistration using the detection results of the misregistration detection pattern columns26FC_R and26FC_L and the desired conveyance speed54, thereafter correcting this amount of misregistration using the average conveyance speed53calculated using the detection result of the conveyance speed detection pattern column26BK_C, and calculating the amount of misregistration from which detection error is removed.

Referring toFIG. 11, in step S1, the misregistration amount detector100starts processing. In step S2, the image creating part200(the image forming parts6BK,6M,6C, and6Y) creates the images of the misregistration detection pattern columns26FC_R and26FC_L and the conveyance speed detection pattern column26BK_C according to this embodiment illustrated inFIG. 4on the conveyor belt5. First, the image forming part6BK forms black toner images, then the image forming part6M forms magenta toner images, then the image forming part6C forms cyan toner images, and finally the image forming part6Y forms yellow toner images on the conveyor belt5moving in the conveyance direction.

Here, the conventional misregistration correction pattern is created on the conveyor belt for the overall length or more (one turn or more) of a conveyor belt in order to cancel the effect of detection error due to the non-uniformity of the thickness of the conveyor belt. On the other hand, according to this embodiment, the detection error is eliminated by calculating the actual conveyance speed of the conveyor belt5by detecting the conveyance speed detection patterns25BK_E. Accordingly, as illustrated inFIG. 4, the overall length38BK_E of the misregistration correction pattern25in the conveyance direction may be twice the circumference of the photosensitive body drum9BK and less than or equal to a third of the overall length (one turn) of the conveyor belt5.

In step S3, the reading part210reads the misregistration correction pattern25. The sensor17reads the misregistration detection pattern column26FC_L created on the conveyor belt5, the sensor18reads the conveyance speed detection pattern column26BK_C created on the conveyor belt5, and the sensor19reads the misregistration detection pattern column26FC_R created on the conveyor belt5. Specifically, the light-emitting part23of each of the sensors17,18, and19emits exposure light onto the misregistration correction pattern25formed on the conveyor belt5, and the light-receiving part24receives its reflected light, so that the misregistration correction pattern25is detected. Since the sensor19operates in the same manner and performs the same processing on the received light signal as the sensor17, a description of the sensor19is omitted in the following description.

The signal of the light received by the sensor17in step S3is provisionally stored in the FIFO memory64through the amplifier60, the filter61, and the A/D conversion part62. The signal of the light received by the sensor18is also provisionally stored in the FIFO memory64through the amplifier60, the filter61, and the A/D conversion part62.

Then, in step S4, it is determined whether the sensor17has detected a single set of misregistration detection patterns (eight misregistration detection patterns, for example, the first misregistration detection patterns25BK_Y1,25M_Y1,25C_Y1, and25Y_Y1and the second misregistration detection patterns25BK_S1,25M_S1,25C_S1, and25Y_S1, or the first misregistration detection patterns25BK_Y2,25M_Y2,25C_Y2, and25Y_Y2and the third misregistration detection patterns25BK_S2,25M_S2,25C_S2, and25Y_S2). If it is determined in step34that the sensor17has detected a single set of misregistration detection patterns (YES in step S4), in step S5, the received light signal of the sensor17stored in the FIFO memory64is stored in the RAM68through the I/O port65and the data bus66. Simultaneously, the received light signal of the sensor18stored in the FIFO memory64is also stored in the RAM68through the I/O port65and the data bus66.

If it is determined in step S4that the sensor17has not detected a single set of misregistration detection patterns (NO in step S4), in step S3, the sensor17continues to read misregistration detection patterns and the sensor18also continues to read conveyance speed detection patterns.

Next, in step S6, it is determined whether the sensor17has read all the misregistration detection patterns25BK_Y1,25M_Y1,25C_Y1,25Y_Y1,25BK_Y2,25M_Y2,25C_Y2,25Y_Y2,25BK_S1,25M_S1,25C_S1,25Y_S1,25BK_S2,25M_S2,25C_S2, and25Y_S2illustrated inFIG. 4and the entire signal of the light received by the sensor17has been stored in the RAM68. If it is determined in step S6that the sensor17has read all the misregistration detection patterns and the entire signal of the light received by the sensor17has been stored in the RAM68(YES in step S6), in step S7, the sensor17ends the reading of misregistration detection patterns and the sensor18also ends the reading of conveyance speed detection patterns.

If it is determined in step S6that the sensor17has not read all the misregistration detection patterns (NO in step S6), in step S3, the sensor17continues to read misregistration detection patterns and the sensor18also continues to read conveyance speed detection patterns.

In step S8, the first calculation part220(FIG. 1) calculates the amount of misregistration43D_1(FIG. 8A) with respect to each of magenta, cyan, and yellow colors using the position information of the first misregistration detection patterns25BK_Y1,25M_Y1,25C_Y1, and25Y_Y1and the second misregistration detection patterns25BK_S1,25M_S1,25C_S1, and25Y_S1(the former two sets inFIG. 4) retained in the RAM68. In the case of magenta, the amount of misregistration43D_1is calculated with respect to the black misregistration detection patterns25BK_Y1and25BK_S1and the magenta misregistration detection patterns25M_Y1and25M_S1included in the former two sets ofFIG. 4on a set-by-set basis, and the average of the amounts of misregistration43D_1is calculated. The same processing is performed with respect to cyan and yellow.

In step S8, the first calculation part220further calculates the amount of misregistration43D_2(FIG. 8A) with respect to each of magenta, cyan, and yellow colors using the position information of the first misregistration detection patterns25BK_Y2,25M_Y2,25C_Y2, and25Y_Y2and the third misregistration detection patterns25BK_S2,25M_S2,25C_S2, and25Y_S2(the latter two sets inFIG. 4) retained in the RAM68. In the case of magenta, the amount of misregistration43D_2is calculated with respect to the black misregistration detection patterns25BK_Y2and25BK_S2and the magenta misregistration detection patterns25M_Y2and25M_S2included in the latter two sets ofFIG. 4on a set-by-set basis, and the average of the amounts of misregistration43D_2is calculated. The same processing is performed with respect to cyan and yellow.

Then, the first calculation part220calculates the average DA of the amount of misregistration43D_1and the amount of misregistration43D_2with respect to each of magenta, cyan, and yellow.

At the same time, the first calculation part220calculates the amount of misregistration DB of each of magenta, cyan, and yellow images relative to a black image in the conveyance direction. In calculating the amount of misregistration DB, the distance (detected value)44D_1inFIG. 8Ais calculated with respect to each detection result (each set) of misregistration detection patterns and the average of the distances44D_1is calculated for each of magenta, cyan, and yellow.

In step S9, the second calculation part230calculates the average conveyance speed53using the position information of the conveyance speed detection patterns25BK_E retained in the RAM68. The second calculation part230calculates the conveyance speed50of the conveyor belt5at each point of time inFIG. 9by dividing the total value (sum) of the width29BK_E of the conveyance speed detection pattern25BK_E and the interval30BK_E between each adjacent two of the conveyance speed detection patterns25BK_E by the difference between the time39BK_E(t) at which one conveyance speed detection pattern25BK_E is detected and the time39BK_E(t−1) at which a conveyance speed detection pattern25BK_E immediately preceding the one conveyance speed detection pattern25BK_E is detected, the difference being retained in the RAM68. Then, the second calculation part230calculates the average of the thus calculated conveyance speed50over the entire interval, and determines the calculated average as the average conveyance speed53.

In step S10, the third calculation part240multiplies the above-described amounts of misregistration DA and DB by the detection error correction coefficient55, which is the ratio of the average conveyance speed53to the desired conveyance speed54calculated in step S9, with respect to each of magenta, cyan, and yellow, thereby calculating the amounts of misregistration DA′ and DB′, from which the detection error caused by the difference between the actual conveyance speed53and the desired conveyance speed54of the conveyor belt5is eliminated, with respect to each of magenta, cyan, and yellow.

In step S11, the storage part250stores the amounts of misregistration DA′ and DB′ calculated in step S10in the RAM68, and in step S12, the processing of the misregistration amount detector100according to this embodiment ends.

Thereby, the misregistration amount detector100can be provided, which makes it possible to detect misregistration in a shorter period of time than the misregistration correction based on the conventional misregistration correction pattern without detection error due to the unevenness (non-uniformity) of the thickness of a conveyor belt.

A description is given, with reference toFIG. 12, of calculation of the amount of misregistration (second processing) in the misregistration amount detector100according to this embodiment.FIG. 12is a flowchart illustrating a procedure for calculating the amount of misregistration (second processing) in the misregistration amount detector100according to this embodiment.

Here, according to the calculation of the amount of misregistration (first processing), the amount of misregistration is provisionally calculated using the desired conveyance speed54, and thereafter, the amount of misregistration is corrected by multiplying the amount of misregistration by the detection error correction coefficient55, thereby eliminating detection error due to the unevenness (non-uniformity) of the thickness of the conveyor belt5. On the other hand, according to the calculation of the amount of misregistration (second processing), the amount of misregistration is calculated using the actual conveyance speed53of the conveyor belt5calculated from the detection results of the conveyance speed detection pattern column26BK_C in place of the desired conveyance speed54.

Referring toFIG. 12, in step S21, the misregistration amount detector100starts processing. In step S22, the image creating part200(the image forming parts6BK,6M,6C, and6Y) creates the images of the misregistration detection pattern columns26FC_R and26FC_L and the conveyance speed detection pattern column26BK_C according to this embodiment illustrated inFIG. 4on the conveyor belt5. First, the image forming part6BK forms black toner images, then the image forming part6M forms magenta toner images, then the image forming part6C forms cyan toner images, and finally the image forming part6Y forms yellow toner images on the conveyor belt5moving in the conveyance direction.

Here, the conventional misregistration correction pattern is created on the conveyor belt for the overall length or more (one turn or more) of a conveyor belt in order to cancel the effect of detection error due to the non-uniformity of the thickness of the conveyor belt. On the other hand, according to this embodiment, the detection error is eliminated by calculating the actual conveyance speed of the conveyor belt5by detecting the conveyance speed detection patterns25BK_E. Accordingly, as illustrated inFIG. 4, the overall length38BK_E of the misregistration correction pattern25in the conveyance direction may be twice the circumference of the photosensitive body drum9BK and less than or equal to a third of the overall length (one turn) of the conveyor belt5.

In step S23, the reading part210reads the misregistration correction pattern25. The sensor17reads the misregistration detection pattern column26FC_L created on the conveyor belt5, the sensor18reads the conveyance speed detection pattern column26BK_C created on the conveyor belt5, and the sensor19reads the misregistration detection pattern column26FC_R created on the conveyor belt5. Specifically, the light-emitting part23of each of the sensors17,18, and19emits exposure light onto corresponding misregistration detection patterns or conveyance speed detection patterns formed on the conveyor belt5, and the light-receiving part24receives its reflected light, so that the misregistration detection patterns and the conveyance speed detection patterns are detected. Since the sensor19operates in the same manner and performs the same processing on the received light signal as the sensor17, a description of the sensor19is omitted in the following description.

The signal of the light received by the sensor17in step S23is provisionally stored in the FIFO memory64through the amplifier60, the filter61, and the A/D conversion part62. The signal of the light received by the sensor18is also provisionally stored in the FIFO memory64through the amplifier60, the filter61, and the A/D conversion part62.

Then, in step S24, it is determined whether the sensor17has detected a single set of misregistration detection patterns (eight misregistration detection patterns, for example, the first misregistration detection patterns25BK_Y1,25M_Y1,25C_Y1, and25Y_Y1and the second misregistration detection patterns25BK_S1,25M_S1,25C_S1, and25Y_S1, or the first misregistration detection patterns25BK_Y2,25M_Y2,25C_Y2, and25Y_Y2and the third misregistration detection patterns25BK_S2,25M_S2,25C_S2, and25Y_S2). If it is determined in step S24that the sensor17has detected a single set of misregistration detection patterns (YES in step S24), in step S25, the received light signal of the sensor17stored in the FIFO memory64is stored in the RAM68through the I/O port65and the data bus66. Simultaneously, the received light signal of the sensor18stored in the FIFO memory64is also stored in the RAM68through the I/O port65and the data bus66.

If it is determined in step S24that the sensor17has not detected a single set of misregistration detection patterns (NO in step S24), in step S23, the sensor17continues to read misregistration detection patterns and the sensor18also continues to read conveyance speed detection patterns.

Next, in step S26, it is determined whether the sensor17has read all the misregistration detection patterns25BK_Y1,25M_Y1,25C_Y1,25Y_Y1,25BK_Y2,25M_Y2,25C_Y2,25Y_Y2,25BK_S1,25M_S1,25C_S1,25Y_S1,25BK_S2,25M_S2,25C_S2, and25Y_S2illustrated inFIG. 4and the entire signal of the light received by the sensor17has been stored in the RAM68. If it is determined in step S26that the sensor17has read all the misregistration detection patterns and the entire signal of the light received by the sensor17has been stored in the RAM68(YES in step S26), in step S27, the sensor17ends the reading of misregistration detection patterns and the sensor18also ends the reading of conveyance speed detection patterns.

If it is determined in step S26that the sensor17has not read all the misregistration detection patterns (NO in step S26), in step S23, the sensor17continues to read misregistration detection patterns and the sensor18also continues to read conveyance speed detection patterns.

In step S28, the calculation part260(FIG. 1) calculates the average conveyance speed53using the position information of the conveyance speed detection patterns25BK_E retained in the RAM68. The calculation part260calculates the conveyance speed50of the conveyor belt5at each point of time inFIG. 9by dividing the total value (sum) of the width29BK_E of the conveyance speed detection pattern25BK_E and the interval30BK_E between each adjacent two of the conveyance speed detection patterns25BK_E by the difference between the time39BK_E(t) at which one conveyance speed detection pattern25BK_E is detected and the time39BK_E(t−1) at which a conveyance speed detection pattern25BK_E immediately preceding the one conveyance speed detection pattern25BK_E is detected, the difference being retained in the RAM68. Then, the calculation part260calculates the average of the thus calculated conveyance speed50over the entire interval, and determines the calculated average as the average conveyance speed53.

In step S29, the calculation part260calculates the amount of misregistration43D_1(FIG. 8A) with respect to each of magenta, cyan, and yellow colors using the position information of the first misregistration detection patterns25BK_Y1,25M_Y1,25C_Y1, and25Y_Y1and the second misregistration detection patterns25BK_S1,25M_S1,25C_S1, and25Y_S1(the former two sets inFIG. 4) and the average conveyance speed53retained in the RAM68. In the case of magenta, the amount of misregistration43D_1is calculated with respect to the black misregistration detection patterns25BK_Y1and25BK_S1and the magenta misregistration detection patterns25M_Y1and25M_S1included in the former two sets ofFIG. 4on a set-by-set basis, and the average of the amounts of misregistration43D_1is calculated. The same processing is performed with respect to cyan and yellow.

In step S29, the calculation part260further calculates the amount of misregistration43D_2(FIG. 8A) with respect to each of magenta, cyan, and yellow colors using the position information of the first misregistration detection patterns25BK_Y2,25M_Y2,25C_Y2, and25Y_Y2and the third misregistration detection patterns25BK_S2,25M_S2,25C_S2, and25Y_S2(the latter two sets inFIG. 4) and the average conveyance speed53retained in the RAM68. In the case of magenta, the amount of misregistration43D_2is calculated with respect to the black misregistration detection patterns25BK_Y2and25BK_S2and the magenta misregistration detection patterns25M_Y2and25M_S2included in the latter two sets ofFIG. 4on a set-by-set basis, and the average of the amounts of misregistration43D_2is calculated. The same processing is performed with respect to cyan and yellow.

Then, the calculation part260calculates the average DA of the amount of misregistration43D_1and the amount of misregistration43D_2with respect to each of magenta, cyan, and yellow.

At the same time, the calculation part260calculates the amount of misregistration DB of each of magenta, cyan, and yellow images relative to a black image in the conveyance direction, using the detection results of the first misregistration detection patterns25BK_Y1,25M_Y1,25C_Y1and25Y_Y1and the average conveyance speed53. In calculating the amount of misregistration DB, the distance (detected value)44D_1inFIG. 8Ais calculated with respect to each detection result (each set) of misregistration detection patterns and the average of the distances44D_1is calculated for each of magenta, cyan, and yellow.

In step S30, the storage part250stores the amounts of misregistration DA and DB in the RAM68, and in step S31, the processing of the misregistration amount detector100according to this embodiment ends.

Thereby, the misregistration amount detector100can be provided, which makes it possible to detect misregistration in a shorter period of time than the misregistration correction based on the conventional misregistration correction pattern without detection error due to the unevenness (non-uniformity) of the thickness of a conveyor belt.

The present application is based on Japanese Priority Patent Application No. 2007-334400, filed on Dec. 26, 2007, the entire contents of which are hereby incorporated by reference.