Image forming apparatus which executes a color registration correction process and a deviation correction process

An image forming apparatus having: a plurality of image forming sections each of which forms images of respective colors; an intermediate transfer belt; and a control section which executes a color registration correction process and a deviation correction process, wherein the color correction process includes: forming test images on the intermediate transfer belt by the plurality of image forming sections; measuring the test images thereafter; and correcting image forming positions of the image forming sections in response to the measurement results; and wherein the deviation correction process includes: detecting a deviation of the intermediate transfer belt; and correcting the detected deviation, and wherein, when an amount of correction in the deviation correction process exceeds a predetermined level during the execution of the color registration correction process, the control section executes the color registration correction process again.

This application is based on Japanese Patent Applications No. 2007-315111 filed with Japanese Patent Office on Dec. 5, 2007, the content of which is incorporated herein by reference.

BACKGROUND

1. Field of the Invention

The present invention relates to an image forming apparatus that forms a color image by superimposing images of various colors on a transfer belt, particularly to an image forming apparatus capable of correcting the color misregistration and deviation of a transfer belt.

2. Description of the Related Art

In an image forming apparatus such as a tandem color photocopier using an electrophotographic process, an image forming section including a photosensitive drum, charging device, optical scanning device and development device is provided for each of yellow (Y), magenta (M), cyan (C) and black (K) colors. These devices are arranged along an endless intermediate transfer belt, and a color image formed by superimposing the images of Y, M, C and K colors on the rotating intermediate transfer belt is transferred from the intermediate transfer belt to transfer paper.

When images of various colors are superimposed to form a color image as described above, the image formation positions of different colors must be correctly aligned. Otherwise, color misregistration will occur and a high-quality image cannot be obtained. To solve this problem, a color registration correction method (Japanese Unexamined Patent Application Publication No. 1-142679) has been used as required, wherein a test image for color registration correction known under the name of a registration mark is formed on the intermediate transfer belt, and this image is read by an optical sensor to get the required correction value, whereby the image forming positions for image forming sections of different colors are aligned.

Another solution to the aforementioned problem is provided by the correction control method (Japanese Unexamined Patent Application Publication No. 9-48533) wherein stable traveling of the intermediate transfer belt is achieved by preventing meandering and deviation.

The correction control technique for the deviation of the intermediate transfer belt is normally carried out throughout the process of image formation in order to avoid color misregistration resulting from the meandering or deviation of the intermediate transfer belt. However, if the deviation of the intermediate transfer belt is corrected during the correction of the color registration, the amount of color misregistration measured according to the registration mark will be equal to the sum of the color misregistration caused by the misalignment of the image forming position due to the image forming section and the color misregistration caused by the traveling of the intermediate transfer belt due to deviation. Thus, if color registration is executed according to the measured color misregistration, an error results from the amount of corrections having been made.

In the meantime, if the correction control for the deviation of the intermediate transfer belt is suspended during the correction of color registration, and the intermediate transfer belt is misaligned during the correction of color registration, the amount of color registration correction will be inappropriate. Not only that, this will require much time and effort in returning the deviated transfer belt to the appropriate position upon completion of correcting the color registration.

The object of the present invention is to solve the aforementioned problems and to provide an image forming apparatus capable of providing adequate correction of color registration while preventing the deviation of an intermediate transfer belt.

SUMMARY

The object of the present invention can be achieved by the following invention.

An image forming apparatus that carries out color image formation, comprising: a plurality of image forming sections each of which forms images of respective colors; an intermediate transfer belt on which plural images of respective colors formed by the plurality of image forming sections are transferred and are superimposed to form a color image; and a control section which executes a color registration correction process and a deviation correction process, wherein the color correction process comprises: forming test images on the intermediate transfer belt to correct position errors of each color image of the images formed by the plurality of image forming sections; measuring the test images on the intermediate transfer belt thereafter; and correcting image forming positions of the image forming sections in response to the measurement results; and wherein the deviation correction process comprises: detecting a deviation of the intermediate transfer belt; and correcting the detected deviation, and wherein, when an amount of correction in the deviation correction process exceeds a predetermined level during the execution of the color registration correction process, the control section executes the color registration correction process again.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The following describes the embodiments of the present invention with reference to the drawings.

FIG. 1shows the cross section of an image forming apparatus10as an embodiment of the present invention.FIG. 2shows the electric schematic structure of the image forming apparatus10. The image forming apparatus10is an apparatus called the color digital photocopier, and includes an automatic document feed apparatus11, reading section12, display/operation section13, control section14and printing section20.

The automatic document feed apparatus11(FIG. 1) feeds the documents2stacked on a document placement11a, one by one to the reading position of the reading section12, and ejects the documents having been read, to a sheet ejection tray11b.

The reading section12reads the document in color. The reading section12includes an exposure scanning section15made up of a light source and mirror; a color line image sensor16for receiving the light reflected from the document and for outputting the electric signal for each color conforming to the light intensity; various forms of mirrors17for leading the light reflected from the document to the line image sensor16; and a condensing lens18.

The printing section20is a tandem image forming apparatus, and includes an endless intermediate transfer belt21; a plurality of image forming sections30Y,30M,30C and30K each forming a single colored toner image on the intermediate transfer belt21; a sheet feed section22for feeding transfer sheets; a conveyance section23for conveying the transfer sheets having been fed; and a fixing apparatus24.

The image forming section30Y forms a yellow (Y) image on the intermediate transfer belt21. The image forming section30M forms a magenta (M) image on the intermediate transfer belt21. The image forming section30C forms a cyan (C) image on the intermediate transfer belt21. The image forming section30K forms a black (K) image on the intermediate transfer belt21.

The image forming section30Y contains a photoreceptor31Y as a cylindrical electrostatic latent image carrier on the surface of which an electrostatic latent image is formed; a charging device32Y arranged on the periphery thereof; a development device33Y; and a cleaning device34Y. The image forming section30Y also contains a laser unit35Y mode up of a laser diode, polygon mirror, lenses, mirror and the like.

Driven by a drive section (not illustrated), the photoreceptor31Y rotates in a predetermined direction (arrow A in the drawing), and the charging device32Y charges the photoreceptor31Y uniformly. As shown inFIG. 3, the laser unit35Y ensures that the laser beam emitted from a laser diode36Y is reflected by a rotating polygon mirror37Y, whereby the surface of the cylindrical photoreceptor31Y is repeatedly scanned by the laser beam in the axial direction (main scanning direction).

The surface of the photoreceptor31Y is scanned by the laser beam which is turned on or off in response to the yellow image data, whereby an electrostatic latent image is formed on the photoreceptor31Y. The development device33Y develops the electrostatic latent image on the photoreceptor31Y using the yellow toner. The toner image formed on the surface of the photoreceptor31Y is transferred onto the intermediate transfer belt21at the position in contact with the intermediate transfer belt21. The cleaning device34Y uses a blade or the like to scrape off, remove and collect the toner remaining on the surface of the photoreceptor31Y after transfer.

The image forming section30M, image forming section30C and image forming section30K are structured in the same manner as the image forming section30Y, except that the toner colors are different, and the laser beam is turned on or off according to the image data corresponding to each of the colors. Accordingly, the image forming sections30M,30C and30K will be described to avoid duplicated description. In the drawing, the components having the same structure but with different colors have the same reference numerals with subscript M, C or K added thereto instead of Y.

The intermediate transfer belt21is applied to a plurality of rollers and is rotated thereby. In the process of image formation, the intermediate transfer belt21rotates in the direction of arrow B. In the process of rotation, the images of different colors (toner images) are formed on the intermediate transfer belt21in the order of (Y), (M), (C) and (K) so as to be superimposed on one other by the image forming sections30Y,30M,30C and30K. At the secondary transfer position D, this color image is transferred to the transfer paper from the intermediate transfer belt21.

A pair of registration sensors41aand41bmade up of a reflection type optical sensor for detecting the registration mark on the intermediate transfer belt21are installed on the upstream side of the secondary transfer position D. Two registration sensors41aand41bare installed separately from each other across the intermediate transfer belt21. A belt cleaning device42for removing the toner remaining on the intermediate transfer belt21subsequent to transfer is installed further down on the downstream side of the registration sensors41aand41b.

The sheet feed section22has a plurality of sheet feed cassettes22afor accommodating the transfer sheets used for printing, and feeds the transfer sheets one by one to the conveyance section23from the selected sheet feed cassette22a. The conveyance section23includes a normal path23athat allows the transfer sheets fed from the sheet feed cassette22ato pass through the secondary transfer position D and fixing apparatus24, and ejects them out of the machine; and a reversing path23bthat, after reversing the transfer sheets having passed through the fixing apparatus24, merges them into with the normal path23aagain on the upstream side of the secondary transfer position D. This is designed to meet the double-sided printing requirements.

As shown inFIG. 2, the control section14includes a main body control section14aand an image forming section14b. The main body control section14acontrols the entire operation of the image forming apparatus10, and is made up of such major components as a CPU (Central Processing Unit), ROM (Read Only Memory) and RAM (Random Access Memory). The image forming section14bapplies image processing to the image data of different colors inputted from the line image sensor16of the reading section12and stores the image data temporarily. After that, the image data for the (Y), (M), (C) and (K) colors obtained by extending this data is outputted to the image forming sections30Y,30M,30C and30K of the printing section20by this image forming section14b.

The control section14is connected with the reading section12, display/operation section13, printing section20and others. The display/operation section13receives various forms of operations from the user and displays various types of operation screens, setting screens and guide screens for the user. For example, the display/operation section13is composed of a liquid crystal display whose surface is provided with a touch panel for detecting the depressed position, and other switches.

The control section14controls the operation of the printing section20through the CPU51of the printing section20. The CPU51is connected with the ROM52storing the programs or various forms of fixed data, and the RAM53serving as a work area when the CPU51executes a program, as well as the electric components (not illustrated) of the sensors and driving parts related to the sheet feed section22, conveyance section23and fixing apparatus24.

The control section14is responsible for corrections to the deviation of the intermediate transfer belt21through the CPU51of the printing section20, and color registration corrections for detecting and correcting the misregistration of different colors superimposed on the intermediate transfer belt21. To put it in greater detail, the CPU51is connected with: a belt drive motor54for driving the intermediate transfer belt21; a stepping motor56for driving the mechanism that corrects the deviation of the intermediate transfer belt21; and a pressing/releasing section57for switching between the mode of pressing wherein a transfer roller (or secondary transfer belt) for pressing the transfer sheets against the intermediate transfer belt21at the secondary transfer position D is pressed against the intermediate transfer belt21, and the mode for releasing wherein the transfer roller is released and separated from the transfer sheets. The pressing/releasing section57uses a motor and solenoid to execute the pressing and releasing functions.

The CPU51is connected with the correction sections58Y,58M,58C and58K, and the aforementioned registration sensors41aand41b, wherein these correction sections are mounted in response to the image forming sections30Y,30M,30C and30K and are used to correct color misregistration through fine adjustments to the time interval for turning on or off the laser beam conforming to the image data.

In the first place, the following describes the method for controlling the deviation of the intermediate transfer belt21(hereinafter referred to as “steering control”).

FIG. 4schematically shows the drive mechanism of the intermediate transfer belt21. The intermediate transfer belt21is applied to a plurality of cylindrical rollers, and constitutes a circulating path (FIGS. 1 and 9). Of the rollers, the drive roller61is driven by the belt drive motor54, and other rollers are driven motors that have no driving force. The adjusting roller62is mounted so that the inclination of the axis can be changed about the one end62athereof. The other end62bof the adjusting roller62is shaft-supported by the movable bearing63made up of a gear, stepping motor54and others. By driving the stepping motor56in the forward or reverse direction, the angle of the shaft of the adjusting roller62can be adjusted within a predetermined angle (plus/minus) with reference to the position parallel to the shaft of the drive roller61.

An deviation sensor55is provided to detect the position of the intermediate transfer belt21(position relative to the reference position) in the direction wherein deviation is likely to occur (across the width of the intermediate transfer belt21). A photo sensor for detecting the end of the intermediate transfer belt21, for example, is used as the deviation sensor55.

FIG. 5shows a specific example of the steering control that detects and corrects the deviation of the position across the intermediate transfer belt21. Steering control is provided in such a way that, when the intermediate transfer belt21has been misaligned across the width, the misalignment G is detected by the deviation sensor55, and the inclination of the adjusting roller62is adjusted so that the intermediate transfer belt21moves in the direction wherein the misalignment is corrected. For example, in the example ofFIG. 5, control is provided in such a way that, when the intermediate transfer belt21has been misaligned in the direction J of deviation, the inclination of the adjusting roller62is changed (correction Q in the diagram) so that the belt will move in the belt traveling direction opposite thereto. As described above, force in the direction P of the diagram is applied to the intermediate transfer belt21by changing the inclination of the adjusting roller62, whereby the misalignment of the intermediate transfer belt21is changed. The steering control is implemented by the CPU51. Steering control is carried out throughout the image forming operation (during printing operation).

The following describes the method for correcting the color registration.

During the correction of color misregistration, the registration marks70Y,70M,70C and70K of respective colors are formed by the image forming sections30Y,30M,30C and30K and transferred onto the intermediate transfer belt21as test images for color registration as shown inFIGS. 6 and 7. These marks are read by the registration sensors41aand41bto detect the misregistration of the registration marks70Y,70M,70C and70K. The result of this detection provides a basis for correcting the positions wherein the images of respective colors are formed, to ensure that there will be no color misregistration.

To put it in greater detail, the registration marks70Y,70M,70C and70K are formed in a zigzag manner wherein the crosswise line image of the intermediate transfer belt21and oblique line image are alternately repeated (four times in the diagram). The registration marks70Y,70M,70C and70K are formed in the vicinity of both ends of the intermediate transfer belt21across the width. They are read and detected by the registration sensors41aand41b.

FIG. 8shows how the position errors of images of respective colors and the lateral magnification are determined from the registration marks. The positional relationship in the sub-scanning direction of the K- and C-colored images (in the direction wherein the intermediate transfer belt is conveyed) is identified, for example, based on the length of time T1from the detection of the first line image71K of the K-colored registration mark70K across the width to the detection of the first line image71C of the C-colored registration mark70C across the width.

The positional relationship in the main scanning directions of the K- and C-colored images (across the width of the intermediate transfer belt21) is identified from the relationship between the time T2from the detection of the oblique first line image72K of the K-colored registration mark70K to the detection of the oblique first line image72C of the C-colored registration mark70C, and the time T1having been detected previously. If T1=T2, there is no position error in the main scanning direction. If T2is shorter than T1as shown in Ta of the diagram, the C-colored image is shown to have been misaligned to the left of the K-colored image, as indicated by the broken line of the diagram. Further, the amount of position error can be identified from the time difference between T1and T2.

Further, the skew (inclination) of the K-colored image can be detected from the difference (T3) between the time when the left registration sensor41ahas detected the crosswise first line image71K on the left row of the K-colored registration mark70K, and the time when the right registration sensor41bhas detected the crosswise first line image71KR on the left row. The lateral magnification of the K-colored image can be identified from the difference between the difference (T4) between the time when the left registration sensor41ahas detected the oblique first line image72K on the left row of the K-colored registration mark70K, and the time when the right registration sensor41bhas detected the oblique first line image72KR on the right row, and the aforementioned T3. In the same manner, the position errors, skew and magnification of other colors can be detected.

FIG. 9schematically shows the components related to color registration correction. The CPU51calculates the position error of the image (color misregistration) in the aforementioned manner, and provides the correction sections58Y,58M,58C and58K with the instructions specifying the amount F of correcting in such a way that the image formation position error will be corrected (the position error will be reduced to zero). For example, when an image has a color misregistration in the main scanning direction, the correction sections58Y,58M,58C and58K adjust the time from inputting of the horizontal synchronization signal denoting the time interval when the laser beam has crossed the reference position in the main scanning direction, to the initiation of the on-off control of the laser beam in response to the image data of each line.

If the steering control of the intermediate transfer belt21is started during the correction of the aforementioned color registration, the amount of color misregistration measured based on the registration marks70Y,70M,70C and70K will be the sum of the amount of color misregistration resulting from the image formation position error caused by the image forming sections30Y,30M,30C and30K, and the amount of color misregistration resulting from the traveling of the intermediate transfer belt21caused by the steering control. If color registration is corrected based on the measured amount of the color misregistration, the amount of correction may be inadequate. In the meantime, if the deviation correction control of the intermediate transfer belt21is suspended during the process of correcting the color registration, the amount of correction to the color registration will be inadequate when the intermediate transfer belt is considerably misregistered during the process of correcting the color registration. Not only that, it will be very difficult to get the considerably lopsided intermediate transfer belt21back to the adequate position after correction of the color registration. To solve this problem, the control section14executes the process of correcting the deviation (steering control) during the process of correcting the color registration, but the color registration correction is started over again if the amount of correction to the deviation (steering amount) exceeds the tolerance.

FIG. 10shows the flow of control by the control section14in the process of correcting the color registration. In this case, the steering control is implemented during the process of correcting the color registration. In the first place, a step is taken to determine whether or not the time is right to correct the color registration (Step S101). Color registration correction is executed for a predetermined number of prints (e.g., 1,000 sheets). Alternatively, it is executed when correction is determined to be necessary, according to the temperature or humidity inside the apparatus, or an instruction to execute the correction has been received from the user.

When the time is not right to execute the process of correcting the color registration (N in Step S101), this processing is terminated (END). When the time has come to correct the color registration (Y in Step S101), a step is taken to initiate the measurement of the amount of correcting for the deviation by steering control (Step S102), whereby color registration is corrected (Step S103).

When the color registration correction has terminated, measurement of the amount of deviation correction by steering control is terminated (Step S104). Thus, the system acquires the amount of deviation correction executed during the time period from the start of measurement in Step S102to the termination of measurement in Step S104. More preferably, measurement starts with the start of forming the registration mark70on the intermediate transfer belt21, and terminates with the last registration mark having been read by the registration sensors41aand41b.

A step is taken to determine whether or not the amount of deviation correction having been acquired has exceeded a predetermined tolerance. If it has exceeded the tolerance (N in Step S105), the system goes back to Step S102to start the procedure over again. If this amount has not exceeded the tolerance (Y in Step S105), the system considers that this correction of color registration has been executed adequately while the intermediate transfer belt21is placed under stable conditions, and terminates this processing (END).

As described above, color registration correction process is executed while steering control is implemented. This eliminates the possibility of considerable misregistration of the intermediate transfer belt21during the color registration correction. If the amount of deviation correction has exceeded the tolerance, color registration is corrected while traveling of the intermediate transfer belt21is unstable. In this case, however, color registration correction process is started over again. This arrangement eliminates the possibility of color registration being caused by the printing operation executed based on the possible inadequate color registration correction.

The amount of deviation correction to be compared with the tolerance can be (1) the sum of the amount of the deviation corrections from the start of measurement to the termination of measurement (preferably the sum of the absolute value of the deviation corrections), or (2) an amount of the deviation correction per unit time from the start of measurement to the termination of measurement (preferably the maximum value thereof). Either of the two can be utilized. It is also possible to arrange such a configuration that both are used in parallel, and if one of them exceeds the tolerance or both of them exceed the tolerance, color registration correction process is started over again.

During the printing operation, the transfer roller for pressing the transfer sheet against the intermediate transfer belt21, and the secondary transfer belt are pressed against the intermediate transfer belt21in the secondary transfer position D. Color registration correction is executed when this pressurized condition is released. If the pressure is applied or released, the traveling condition of the intermediate transfer belt21is affected. Thus, it is preferred to adopt such a structure that the pressing/releasing section57is controlled to release the pressure; then, steering control is executed for a predetermined period of time until the traveling condition of the intermediate transfer belt21is stabilized. After that, the measurement of Step S102is started and the color registration correction is executed.

In the aforementioned embodiment of the present invention, the control section implements the color registration correction while the deviation correction control is executed. If the amount of deviation corrections executed during this period has exceeded the tolerance, the color registration correction is started over again. Since the deviation correction control is executed during the color registration correction, any serious position error of the transfer belt does not occur during this time.

If there is a large amount of deviation corrections, this means that color registration has been executed while the intermediate transfer belt is traveling under the unstable condition. The control section allows the color registration correction to be started over again. In the meantime, if the amount of deviation corrections is below the tolerance, this means that color registration has been executed while the intermediate transfer belt is traveling under the stable condition. The control section adopts the results of this color registration correction as an adequate operation.

The image forming apparatus of the present invention prevents deviation of the intermediate transfer belt, and ensures appropriate color registration corrections.

The embodiment of the present invention has been described with reference to diagrams. It is to be expressly understood, however, that the present invention is not restricted thereto. The present invention can be embodied in a great number of variations with appropriate modifications or additions, without departing from the technological spirit and scope of the invention claimed.

The method of deviation correction of the intermediate transfer belt21is not restricted to that using the mechanism indicated inFIG. 4. Any other method can be used if the deviation is corrected. Further, the shape of the registration mark during the color registration correction or the method of evaluating the amount of color registration are not restricted to those exemplified in the description of the embodiment.

In the aforementioned embodiment, after termination of the color registration correction, evaluation is made to see whether or not the amount of deviation correction is abnormal (exceeding the tolerance). It is also possible to adopt such a structure that evaluation is made constantly during the process of color registration correction, and if the tolerance has been exceeded, color registration correction is suspended, and the procedure is started again.

The image forming apparatus10is not restricted to the multi-functional peripheral. Any image forming apparatus having a color printing function can be utilized.