JP2013029729A - Image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To make a correction margin larger in the front and rear surfaces of a sheet of paper, when correcting the meandering of the sheet.SOLUTION: An image forming apparatus includes: an image forming unit forming an image and transferring the image to a sheet of paper; a paper feed conveyance part feeding and conveying the sheet; a reverse conveyance part reversing and conveying the sheet after being conveyed by the paper feed conveyance part and passing through the image forming unit, to convey the sheet back to the paper feed conveyance part; a meandering correction unit moving the sheet under conveyance in a main scan direction, to correct the meandering, for the formation of the image; a paper position measurement unit measuring a position of the sheet under conveyance, whose front surface is printed or rear surface is printed in the main scan direction; and a control unit controlling the formation of the image. The control unit receives the measurement result of the paper position measurement unit, to determine the positional tendency of the sheet whose front surface is printed or rear surface is printed and decide an image reference position for printing the front surface or the rear surface in the formation of the image, according to the positional tendency.

Description

  BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming apparatus that forms an image and transfers the image onto a sheet, and more particularly to an image forming apparatus capable of correcting a shift of a sheet position in the main scanning direction of the sheet in preparation for image formation.

In an electrophotographic image forming apparatus such as a copying machine, a printer, a facsimile machine, and a multifunction machine having these functions, a latent image corresponding to a document is formed on a photoconductor in an image forming unit. The toner image is visualized by applying toner to the toner image, and the visualized toner image is transferred to a sheet. Thereafter, the toner image on the paper is fixed by the fixing unit and discharged.
The paper is normally stored in a paper feed tray, and when an image is formed, the paper is fed from the paper feed tray by the paper feed transport unit and transported to the image forming unit. In addition, the sheet on which the image is formed on the front side can be reversed and conveyed by a reversing conveyance unit as desired, and can be recirculated to the paper feeding conveyance unit, whereby image formation can be performed on the back side.

When replenishing paper to the paper feed tray, the paper is not firmly pressed because the guide plate in the main scanning direction is poorly pressed or the guide plate itself is loose due to durability problems. Then, the paper is shifted in the main scanning direction in the paper feed tray, and appears as a deviation at the time of paper feeding. Further, during conveyance after paper feeding, it shifts in the main scanning direction due to vibration, deterioration of parts over time, etc., and appears as a deviation during conveyance.
Therefore, there are two main causes of the deviation corrected by the front surface registration portion, that is, the deviation at the time of feeding and the deviation at the time of conveyance. However, since paper is frequently supplied, Misalignment is often a factor.

For the above-mentioned deviation, a deviation correction mechanism has been proposed that corrects deviation by moving a sheet in the middle of conveyance in a direction orthogonal to the conveyance direction (main scanning direction) (see, for example, Patent Documents 1 and 2). ).
However, since there is a limit on the amount of offset that can be corrected mechanically in the offset correction mechanism, for example, if the limit is 5 mm on both sides from the center of the correction mechanism, the offset will be 2 mm away from the offset during feeding. The deviation during conveyance is only allowed up to 3 mm. That is, if a deviation exceeding 3 mm occurs during conveyance, there arises a problem that correction cannot be performed.
On the other hand, in Patent Document 3, the position of the sheet in the main scanning direction is measured to determine the position tendency of the sheet, the end reference position is determined, and a piece of the sheet is determined based on the measurement position and the end reference position. There has been proposed an image forming apparatus that corrects a shift. This makes it possible to form an appropriate image even on a sheet having a deviation larger than the limit in a deviation correction mechanism having a limit in the amount of deviation that can be mechanically corrected.

Japanese Patent Laid-Open No. 03-94275 JP 2003-81489 A JP 2008-32913 A

  However, in the image forming apparatus, when the paper is conveyed by the above-described reverse conveyance unit and printing is performed on the back side of the paper, when the deviation of the paper in the reverse conveyance path is likely to occur due to deterioration of the parts, etc. There is a case where it is necessary to correct a large amount of paper, and a correction margin on the back side cannot be secured. In the image forming apparatus disclosed in Japanese Patent Laid-Open No. 2004-228561, only correction of deviation based on the paper position tendency when printing on one side of the paper is considered, and no consideration is given to the paper position tendency during double-sided printing.

  The present invention has been made against the background of the above circumstances, and provides an image forming apparatus capable of appropriately transferring an image onto a sheet by properly correcting a deviation of the sheet on which the back side printing is performed together with the front side printing. The purpose is to do.

That is, among the image forming apparatuses of the present invention, the first aspect of the present invention includes an image forming unit that forms an image and transfers the image onto a sheet, a sheet feeding and conveying unit that feeds and conveys the sheet,
A reversing conveyance unit that reverses and conveys the sheet that has been conveyed by the sheet feeding conveyance unit and passed through the image forming unit, and circulates to the sheet feeding conveyance unit;
In preparation for the image formation, a misalignment correction unit that corrects misalignment by moving the paper in the middle of conveyance in the main scanning direction;
A paper position measuring unit for measuring the position in the main scanning direction of the front surface printing and back surface printing paper in the middle of conveyance;
A control unit for controlling the image formation,
The control unit receives the measurement result of the paper position measurement unit, determines the positional tendency of the paper for front side printing and back side printing, and for front side printing and back side printing in the image formation according to the positional tendency The image reference position is determined.

  The image forming apparatus according to a second aspect of the present invention is the image forming apparatus according to the first aspect of the present invention, wherein the paper feeding / conveying unit includes a registration unit that aligns the leading end position of the paper during the conveyance, It is intended to correct a deviation of the sheet passing through the registration portion.

  An image forming apparatus according to a third aspect of the present invention is characterized in that, in the second aspect of the present invention, the deviation correction unit is configured by a mechanism that swings a registration roller provided in the registration unit.

  The image forming apparatus according to a fourth aspect of the present invention is the image forming apparatus according to any one of the first to third aspects of the invention, wherein the control unit is configured to measure the measurement result of the paper position measuring unit on the surface printing paper and the surface printing paper. Based on the image reference position, the deviation correction unit corrects the deviation of the front surface printing paper, and the measurement result of the paper position measurement unit on the back side printing paper and the back side printing image reference position, Based on the above, the misalignment correction unit performs control to correct the misalignment of the back surface printing paper.

  An image forming apparatus according to a fifth aspect of the present invention is characterized in that, in the fourth aspect of the present invention, the control unit performs control to correct a deviation of the sheet so that the position of the sheet is aligned with the image reference position. The image forming apparatus according to claim 4.

  The image forming apparatus according to a sixth aspect of the present invention is the image forming apparatus according to any one of the first to fifth aspects, wherein the control unit responds to a positional tendency of the front-side printing paper and a rear-side printing paper. The image reference position for front side printing and the image reference position for back side printing are respectively determined.

  The image forming apparatus according to a seventh aspect of the present invention is the image forming apparatus according to any one of the first to fifth aspects, wherein the control unit responds to a positional tendency of the front-side printing paper and a rear-side printing paper. Thus, an image reference position common to front side printing and back side printing is determined.

  The image forming apparatus according to an eighth aspect of the present invention is the image forming apparatus according to the seventh aspect, wherein the control unit operates at a common image reference position for front side printing and back side printing in a job in which single-sided printing and double-sided printing are mixed. It is characterized by doing.

  In an image forming apparatus according to a ninth aspect of the present invention, in any one of the first to eighth aspects of the present invention, the control unit determines a positional tendency of the paper between a predetermined number of sheets after the start of the job, The image reference position on the subsequent paper is determined according to the tendency.

  The image forming apparatus according to a tenth aspect of the present invention is the image forming apparatus according to the ninth aspect, wherein the control unit is configured to perform single-sided printing or double-sided printing for a predetermined number of sheets after the job starts in a job in which single-sided printing and double-sided printing are mixed. Regardless of this, all the sheets are operated in a duplex mode that passes through the paper feed conveyance unit and the reverse conveyance unit, and the positional tendency of the paper is determined for each of the front surface printing and the back surface printing.

  An image forming apparatus according to an eleventh aspect of the invention is characterized in that, in the ninth or tenth aspect of the invention, the predetermined number is the number in the job after the job is started.

  In the image forming apparatus according to a twelfth aspect of the present invention, in the first aspect of the present invention, the control unit stores the determined image reference position in association with a paper feed tray that feeds paper. In the case of a job for feeding paper from a paper tray, image formation and misalignment correction are performed using a stored image reference position.

  An image forming apparatus according to a thirteenth aspect of the present invention is the image forming apparatus according to any one of the first to twelfth aspects of the present invention, wherein the control unit has a measurement mode for conveying a sheet without performing image formation. The image reference position is determined in a measurement mode.

  As described above, according to the present invention, the positional trend of the front surface printing paper and the back surface printing paper is determined based on the measurement result of the paper position measuring unit, and the front surface printing in image formation is performed according to the positional trend. The image reference position for printing and backside printing can be determined, and in both front side printing and backside printing, a sufficient margin amount for misalignment correction can be secured and the image can be transferred onto the paper satisfactorily. There is.

That is, in front side printing, the image reference position is determined according to the paper position tendency, so that the deviation at the time of paper feeding can be corrected by image shift. As a result, a sufficient correction margin can be secured by the deviation correction unit, and even if a large deviation occurs suddenly during conveyance, the deviation correction unit can correct it.
Even when printing on the back side, by determining the image reference position according to the paper position trend, it is possible to secure a large correction margin of the deviation correction unit, and a large deviation suddenly occurs during conveyance in the reverse conveyance path. Even if it occurs, the deviation correction unit can correct it.
In the case of a single-sided and double-sided job, if the image reference position is common to both sides, the problem of stacking a single-sided sheet and a double-sided sheet at the time of paper discharge can be solved.

1 is a schematic diagram illustrating an image forming apparatus according to an embodiment of the present invention. Similarly, it is a figure which shows the control block of an image forming apparatus. FIG. 6 is a diagram illustrating a state of deviation of a sheet with respect to an image center, and a diagram illustrating a state in which the center of the sheet is aligned with the center of an image by conventional deviation correction. It is a figure which shows the misalignment correction margin at the time of aligning the paper center with the image center by the conventional misalignment correction. It is a figure which shows the shift | offset | difference correction margin at the time of aligning the paper center with the corrected image center by this invention. Similarly, it is a diagram illustrating an example in which the image center is determined on each of the front and back surfaces in a job in which one side and both sides are mixed. Similarly, it is a figure explaining the example which determined the image center common to front and back in the job where one side and both sides were mixed. Similarly, it is a flowchart showing a control procedure for performing deviation correction.

Hereinafter, an embodiment of the present invention will be described with reference to the accompanying drawings.
FIG. 1 is a diagram showing a mechanical configuration of an image forming apparatus 1 according to the present invention, which will be described below.
In FIG. 1, 10 is an image forming unit, and 30 is a document reading unit.
The document reading unit 30 reads an image of a document transported by an automatic document transport device (not shown in FIG. 1) or a document placed on the platen 31, and temporarily records it in an image memory (not shown).
The image forming unit 10 includes photoconductors 11C, 11M, 11Y, and 11K prepared for respective colors (cyan, magenta, yellow, black, and the like), and the peripheral portions of the photoconductors 11C, 11M, 11Y, and 11K. Are provided with charging units 12C, 12M, 12Y, and 12K, writing units 13C, 13M, 13Y, and 13K, and developing units 14C, 14M, 14Y, and 14K, which are charged by the charging units 12C, 12M, 12Y, and 12K. The surfaces of the photoreceptors 11C, 11M, 11Y, and 11K are subjected to image exposure by the writing units 13C, 13M, 13Y, and 13K based on the image information of the document recorded in the image memory or the like, and the photoreceptors 11C, 11M, 11Y, and 11K are exposed. A latent image is formed on the 11K surface. The latent image is developed by developing units 14C, 14M, 14Y, and 14K to become a toner image. The toner image is transferred to the intermediate transfer belt 16, and transferred from the intermediate transfer belt 16 to a sheet conveyed by the secondary transfer roller 18 through the paper feed conveyance path 22. The transferred paper is heated and fixed by the fixing device 19 and an image forming output (printing) is performed on the paper discharge unit. The paper feed conveyance path 22 constitutes a part of the paper feed conveyance unit of the present invention.

In the image forming unit 10, corresponding to each photoconductor, each photoconductor on the rotation direction side with respect to the contact position with the intermediate transfer belt 16 and on the rotation direction opposite side with respect to the chargers 12C, 12M, 12Y, and 12K. Cleaning units 15C, 15M, 15Y, and 15K that contact the 11C, 11M, 11Y, and 11K to remove residual toner are disposed. Further, a cleaning unit 17 for removing residual toner on the intermediate transfer belt 16 is disposed on the rotation direction side of the intermediate transfer belt 16 with respect to the sheet transfer position and on the rotation direction opposite side of the transfer position with each photoconductor. .
Each of the photoreceptors 11C, 11M, 11Y, and 11K is rotated by a drive motor (not shown), and the intermediate transfer belt 16 is also driven by a drive motor (not shown).

In the image forming apparatus 1, a plurality of paper feed trays 21... 21 storing sheets are disposed below the image forming unit 10. Further, a paper feed path 22 from each of the paper feed trays 21 to 21 to the image forming unit 10 and from the image forming unit 10 to the paper discharge unit is provided.
The paper feeding / conveying path 22 feeds and conveys the paper. The paper stored in each paper feeding tray 21 is fed, conveyed through the registration rollers 23 to the secondary transfer roller 18, and the paper A color image on the intermediate transfer belt 16 is transferred thereon. The sheet on which the color image is transferred is fixed with the toner image on the sheet by applying heat and pressure by the fixing device 19 and discharged outside the apparatus in the single-sided mode face up.

In addition, the image forming apparatus 1 includes a paper reversing conveyance path 24. The fixed sheet is guided from the fixing device 19 to the paper reversing conveyance path 24 so that the front and back sides are reversed and ejected face-down. It is possible to perform image formation on both sides. The paper reversal conveyance path 24 joins the paper feed conveyance path 22 on the upstream side of the registration roller 23, and can recirculate the paper. The sheet reversal conveyance path 24 constitutes a part of the reversal conveyance unit of the present invention. The duplex mode is a mode in which a sheet on which an image is printed on one side is conveyed again to the image forming unit 10 and an image is printed on the back side.
In addition, an operation unit 140 is installed on the upper part of the main body of the image forming apparatus 1, and various setting inputs and displays can be performed.

Next, FIG. 2 is a block diagram showing an electrical configuration of the image forming apparatus of the present invention. This will be described below.
The image forming apparatus 1 is input / output between a copier main body having a control block 110, a scanner unit 130, an operation unit 140, and a printer unit 150, and an external device 2 (for example, a terminal) through the LAN 50 as main components. And image processing means (print & scanner controller) 160 for processing image data.

The control block 110 has a PCI bus 112, and the PCI bus 112 is connected to the DRAM control IC 111 in the control block 110. Further, the control block 110 includes an image control CPU 113, and the DRAM control IC 111 is connected to the image control CPU 113. Further, a nonvolatile memory 115 is connected to the image control CPU 113. The nonvolatile memory 115 stores a program for operating the image control CPU 113, setting data for the image forming apparatus 1, data such as setting conditions such as process control parameters, and the like.
The image control CPU 113 controls the entire image forming apparatus 1 and grasps the state of the entire image forming apparatus 1, and performs image formation control, job print management, and the like. That is, the image control CPU 113 functions as a control unit of the present invention together with the nonvolatile memory 115.

  The scanner unit 130 includes a CCD 131 that performs optical reading, and a scanner control unit 132 that controls the entire scanner unit 130. The scanner control unit 132 is connected to the image control CPU 113 so as to be capable of serial communication, and is controlled by the image control CPU 113. The scanner control unit 132 can be configured by a CPU or a program for operating the CPU. The image data read by the CCD 131 is subjected to data processing by the reading processing unit 116. A compression IC 118 that compresses image data is connected to the reading processing unit 116, and the compression IC 118 is connected to the DRAM control IC 111 described above.

  The operation unit 140 includes a touch panel type LCD 141 and an operation unit control unit 142. The LCD 141 and the operation unit control unit 142 are connected, and the operation unit control unit 142 and the image control CPU 113 are serially connected. It is connected so that it can communicate. With this configuration, the operation control unit 140 is controlled by the image control CPU 113. The operation unit control unit 142 can be configured by a CPU, a program for operating the CPU, and the like. The operation unit 140 can input operation control conditions such as settings and operation commands in the image forming apparatus, and can display setting contents, machine states, information, and the like. Be controlled. With this operation unit 140, a predetermined operation or the like can be performed.

  The DRAM control IC 111 is connected to an image memory including a compression memory 120 and a page memory 121. In the image memory, image data acquired by the scanner unit 130 and image data acquired through the LAN 50 are stored. As described above, the image memory is a storage area for image data, and stores image data of a job to be printed. The DRAM control IC 111 can store image data relating to a plurality of jobs in an image memory. That is, the image memory can store image data of a reserved job.

  Further, a decompression IC 125 that decompresses compressed image data is connected to the DRAM control IC 111, and a write processing unit 126 is connected to the decompression IC 125. The writing processing unit 126 is connected to the LD 152 (corresponding to the writing units 13C, 13M, 13Y, and 13K in FIG. 1) of the printer unit 150, and processes data used for the operation of the LD 152. The printer unit 150 includes a printer control unit 151 that controls the entire printer unit 150. The printer control unit 151 is connected to the image control CPU 113 and is controlled. That is, the printing operation is started / stopped according to the parameters given from the image control IC 113. The printer unit 150 includes the image forming unit 10 and the paper feeding unit 20 described above. The operation of the printer unit 150 is controlled by the image control CPU 113.

  Further, the printer control unit 151 is connected to a post-processing control unit (FNS control unit) 171 included in the post-processing device (FNS unit) 170. As a result, the post-processing device 170 can be controlled by the image control CPU 113 through the printer control unit 151 and the post-processing control unit 171. Note that the printer control unit 151 and the post-processing control unit 171 can be configured by a CPU or a program for operating the CPU.

  Further, a DRAM control IC 161 of the image processing means (print & scanner controller) 160 is connected to the PCI bus 112 connected to the DRAM control IC 111. In the image processing means (print & scanner controller) 160, an image memory 162 is connected to the DRAM control IC 161. In the image processing means (print & scanner controller) 160, a controller control CPU 163 is connected to the DRAM control IC 161, and a LAN interface 165 is connected to the DRAM control IC 161. The LAN interface 165 is connected to the LAN 50.

Next, the basic operation of the image forming apparatus 1 will be described.
First, a procedure for storing image data in the image forming apparatus 1 will be described.
When the scanner unit 130 reads an image of a document and generates image data, the scanner unit 130 optically reads the image of the document from the document by the CCD 131. At this time, the operation of the CCD 131 is controlled by the scanner control unit 132 that receives a command from the image control CPU 113. The image read by the CCD 131 is subjected to data processing by the reading processing unit 116, and the image data subjected to the data processing is compressed by a predetermined method in the compression IC 118 and stored in the compression memory 120 via the DRAM control IC 111. The image data stored in the compression memory 120 can be managed as a job by the image control CPU 113.

  When acquiring image data from the outside, for example, image data transmitted from the external device 2 through the LAN 50 is stored in the image memory 162 by the DRAM control IC 161 via the LAN interface 165. Data in the image memory 162 is temporarily stored in the page memory 121 via the DRAM control IC 161, the PCI bus 112, and the DRAM control IC 111. The data stored in the page memory 121 is sequentially sent to the compression IC 118 via the DRAM control IC 111 and compressed, stored in the compression memory 120 via the DRAM control IC 111, and managed by the image control CPU 113 in the same manner as described above. Made.

When the image forming apparatus 1 outputs an image, that is, when used as a copying machine or a printer, the image data stored in the compression memory 120 is sent to the decompression IC 125 via the DRAM control IC 111 to decompress and decompress the data. The data is sent to the writing processing unit 126, and the LD 152 performs writing to each of the photoreceptors 11C, 11M, 11Y, and 11K.
In the printer unit 150, each unit is controlled by the printer control unit 151 that receives a command from the image control CPU 113. In the image forming unit 10, the toner image written on each photoconductor is transferred to the intermediate transfer belt 16, and then transferred to the paper supplied by the paper feed conveyance path 22, and fixed by the fixing device 19. The paper on which the image has been formed is transported to the post-processing device 170 via the paper feed transport path 22, and post-processing is performed when post-processing is designated, and if post-processing is not required, it is discharged to the paper output tray as it is. Paper. When there are a plurality of reserved jobs, the image output is sequentially performed according to the setting order.
In each of the photoconductors 11C, 11M, 11Y, and 11K, after the toner image is transferred to the intermediate transfer belt 16, the residual toner is removed by the cleaning units 15C, 15M, 15Y, and 15K. Similarly, in the intermediate transfer belt 16, after the toner image is transferred onto the paper, the residual toner is removed by the cleaning unit 17.

Next, the configuration in the vicinity of the registration roller 23 in the paper feed conveyance path 22 will be described with reference to FIG.
In the paper feed conveyance path 22, a registration portion is provided on the front side in the conveyance direction of the secondary transfer roller 18, and the registration portion is constituted by a pair of registration rollers 23. The registration roller 23 corrects the skew of the sheet by forming a loop by contacting the leading end of the sheet conveyed through the sheet feeding conveyance path 22. After completion of the loop creation by the registration roller 23, the registration roller 23 is rotationally driven in the conveyance direction according to the image on the intermediate transfer belt 16, and the paper P is conveyed to the secondary transfer roller 18. Further, the registration roller 23 can move (swing) the paper P in the main scanning direction with the paper P interposed therebetween. Accordingly, the registration roller 23 constitutes a deviation correction unit of the present invention. The swing of the registration roller 23 is controlled by the image control CPU 113. However, in the present invention, the configuration of the deviation correction unit is not limited to the registration roller.
A line sensor 230 composed of a CCD or the like is disposed along the main scanning direction immediately after the registration roller 23 is conveyed. The line sensor 230 reads the edge position of the paper P, and the result is transmitted to the image control CPU 113. The line sensor 230 corresponds to the paper position measuring unit of the present invention.

FIG. 3A shows a state in which the sheet P that has reached the registration roller 23 is shifted by a shift amount x.
The line sensor 230 detects the end of the sheet along the transport direction of the sheet P transported as described above, and transmits the detection result to the image control CPU 113. The image control CPU 113 selects a predetermined size of paper at the time of paper feeding and knows the size of the paper being conveyed. Then, the image control CPU 113 determines the position of the sheet center P0 from the sheet size and the detection result by the line sensor 230. Further, the image control CPU 113 sets an image center G0 that is imaged on the intermediate transfer belt 16, and in this example, the image center G0 coincides with the center line of the intermediate transfer belt 16. The image control CPU 113 calculates the difference between the paper center P0 and the image center G0. This difference is the amount of deviation x of the paper.

FIG. 3B shows a state where the offset amount x is corrected.
In the conventional image forming apparatus, the image control CPU 113 determines the rocking amount −x of the registration roller 23 so that the paper center P0 is aligned with the image center G0 based on the deviation amount x, and then the determined rocking amount. The registration roller 23 is swung by −x to move the paper P by the correction amount −x. As a result, the image center G0 coincides with the paper center P0, and the paper fed by the registration roller 23 is fed to the secondary transfer roller 18. In this way, the deviation of the paper P in the main scanning direction can be corrected and adjusted to the image position.

  In the present invention, the tendency of the paper position is determined for each of the paper for front side printing and the paper for back side printing, and the image reference position for front side printing and the image reference position for back side printing are separately determined based on the tendency. Alternatively, it is determined in common, and by correcting the position of the sheet with respect to this image reference position, the margin of the correction amount can be increased when performing front and back printing. An example of determining the image reference position according to the present invention will be described below.

(Example 1)
In this example, a paper position tendency for front side printing and a paper position tendency for back side printing are measured, and each image reference position is determined according to the tendency.
The paper position tendency is determined by measuring a predetermined number (n in this example) of front and back paper positions, determining an image reference position, and shifting the image in the main scanning direction.
Table 1 shows the paper position history on the front and back sides during continuous printing. The paper position is shown as the paper center, and the paper position tendency is shown by the average value. Note that the present invention is not limited to the one in which the paper position tendency is expressed as an average value.

  Specifically, in continuous printing, the front and back paper positions are stored and left as a history. Table 1 shows the front and back sheet position history. The image center in this table is a number representing the center (image center G0) of the image formed by image processing with reference to the center line of the intermediate transfer belt 16, and initially 0 mm on both sides (= the middle of the intermediate transfer belt 16). The image will be described as an image). The sheet center P0 is a value calculated from the edge of the sheet P read by the line sensor 230 and the sheet main scanning length after rotation of the registration roller 23 is started, and is a number expressed with the center line of the intermediate transfer belt 16 as a reference. is there. Therefore, the difference between the center of the image and the center of the paper is the main scanning deviation amount of the paper (= the amount of rocking by the rocking mechanism).

  In this example, since the image center G0 and the paper center G0 are shifted by 1.6 to 1.7 mm up to the nth sheet on the front side, the paper P is swung by 1.6 to 1.7 mm, and the paper center P0 is imaged. The deviation is corrected by matching with the center G0 (state of FIG. 4). As for the paper position tendency at the time of surface printing, the paper center P0 is shifted by 1.6 mm on the average from the center line of the intermediate transfer belt 16 (corresponding to the center position 18a of the secondary transfer roller 18) to the left in the transport direction. Indicated. In this example, the image reference position is described as being the image center, but the image reference position is not limited to the image center.

  This time, the deviation of 1.6 to 1.7 mm is assumed to be a deviation at the time of paper feeding that occurs because the paper setting state of the paper feeding tray is bad. As described above, it is possible to correct the deviation by swinging the paper P. However, since the deviation at the time of surface printing includes not only the deviation at the time of paper feeding but also the deviation that occurs at the time of conveyance, If the deviation amount 1.6 to 1.7 mm generated at the time of paper feeding is corrected by swinging, the amount that can be corrected for the deviation generated at the time of conveyance is reduced. Specifically, in the case of a mechanism capable of correcting by 5 mm on both sides around the center 23a (HP position) of the registration roller 23 which is a swing mechanism, the offset of 1.6 to 1.7 mm at the time of paper feeding is swung. Within the correction by the mechanism, the deviation generated at the time of conveyance is only allowed to be 3.3 to 3.4 mm, and if the deviation at the time of conveyance that suddenly exceeds 3.5 mm occurs, The image cannot be corrected and the image is shifted.

  Further, since the image center G0 and the paper center P0 are shifted by 0.6 to 0.7 mm up to the nth sheet on the back surface, the paper is swung by 0.6 to 0.7 mm and the paper center P0 is aligned with the image center G0. The offset is corrected. The deviation of 0.6 to 0.7 mm is a deviation at the time of conveyance that occurred during conveyance of the sheet in the sheet reversal conveyance path 24.

When the nth front and back paper centers P0 are determined, the average value of the front and back paper center positions is calculated. In the example of Table 1, the front surface is 1.6 mm and the back surface is 0.7 mm. The amount of rocking by the rocking mechanism can be reduced by shifting the image in the main scanning direction so that the image center G0 matches the average value of the paper center position. Specifically, the image center G0 of the surface of the next image (n + 1 sheet in the example of Table 1) is +1.6 mm from 1.6 mm (center line of the intermediate transfer belt 16 (center 18a of the secondary transfer roller 18)). ), The paper center P0 and the image center G0 substantially coincide with each other, so that the swinging amount by the swinging mechanism is greatly reduced (state shown in FIG. 5).
In other words, the misalignment of 1.6 to 1.7 mm that occurred at the time of paper feeding was absorbed by changing the image center G0, and the correction margins close to 5 mm were secured on both sides of the swing mechanism for subsequent sheets. The state can be maintained. Thus, even if a large deviation occurs suddenly during conveyance, it can be corrected by the swing mechanism.

  In addition, as with the front surface, the back surface average value is calculated and the image center G0 is changed as in the front surface. Specifically, up to the nth sheet, the surface swings the sheet P, and the sheet center P0 is set to the image center G0 (center line of the intermediate transfer belt 16 (center 18a of the secondary transfer roller 18)). On the other hand, since the image center G0 is aligned with the paper center P0 after the (n + 1) th sheet, the paper center P0 is shifted by 1.6 mm from the center line of the intermediate transfer belt 16 to the paper reversal conveyance path 24. Will be sent. Therefore, on the back side, 2.3 mm, which is the average of the paper center position average of 0.7 mm and the average paper center position of the front side of 1.6 mm, is used as the center of the image on the back side. The correction margin can be secured.

FIGS. 4 and 5 show the margin for misalignment correction in the above example.
FIG. 4 shows a state of deviation correction before the nth sheet. In the figure, LL indicates the left swing limit of the registration roller 23, and RL indicates the right swing limit of the registration roller 23. As shown in Table 1, before the nth sheet, the registration roller 23 swings on the left side. In the state shown in FIG. 4, the center 23a of the registration roller 23 approaches the left swing limit LL, The amount of correction margin to the left side is small. For this reason, there is a possibility that the correction may be difficult when there is a symptom in which the paper is suddenly shifted to the right side.

  FIG. 5 shows a state of deviation correction for the (n + 1) th and subsequent sheets. As shown in Table 1, after the (n + 1) th sheet, the image center G0 is shifted to the right according to the paper center P0 indicating the paper position tendency based on the paper position tendency, and the swinging amount by the registration roller 23 is reduced. For this reason, the correction margin amount to the left and right can be assured to be close to 5 mm, and correction can be easily performed when there is a symptom in which the paper P suddenly shifts to one side.

(Example 2)
In the above example, the image reference position is determined for each of the front and back sides based on the front and back paper position trends. However, a common image reference position can be determined from the front and back paper position trends.
In the following example, a description will be given of a case where an image reference position for each of the front and back sides and a common image reference position for the front and back sides are determined for a job in which single-sided printing and double-sided printing are mixed.
The paper position tendency is determined by measuring a predetermined number (n in this example) of front and back paper positions, determining an image reference position, and shifting the image in the main scanning direction.

The table of FIG. 6 shows the paper position history of the front and back during continuous printing when determining the image reference position for each of the front and back. The paper position is shown as the paper center, and the paper position tendency is shown by the average value.
The table of FIG. 7 shows the paper position history of the front and back during continuous printing when determining the image reference position common to the front and back. The paper position is shown as the paper center, and the paper position tendency is shown by the average value.
Further, FIG. 6 shows a conveyance image when the image reference position is determined for each of the front and back sides according to the paper position tendency and a single-sided and double-sided job is executed.
FIG. 7 shows a conveyance image when a common image reference position is determined according to the paper position tendency and a single-sided and double-sided job is executed.

  As described in Example 1, by determining the image centers of the front and back sides from the front and back sheet position trends, it is possible to secure a swing margin of the mechanism on both the front and back sides. However, if a single-sided / double-sided job is executed in this state, the paper is ejected at the center of the front image in single-sided mode, whereas it is ejected at the back image reference position in double-sided mode. become. Therefore, as shown in FIG. 6, the single-sided mode paper and the double-sided mode paper are stacked in a state of being shifted at the paper discharge port, and there is a possibility that the subsequent workability may be deteriorated.

  Therefore, in a single-sided and double-sided job, it can be determined that the front and back image centers are aligned. Specifically, in the normal job, the center of the front image is determined as 1.6 mm and the center of the back image is determined as 2.3 mm as shown in FIG. 6, whereas in the mixed job of one side and both sides, the front image is displayed as shown in FIG. An intermediate position of 1.95 mm between the center G01 (1.6 mm position) and the back image center G02 (2.3 mm position) is determined as the common image center G00. By doing so, it is possible to eliminate the paper shift at the paper discharge port while minimizing the amount of paper swinging by the mechanism on both the front and back sides. Note that the image reference position common to the front and back is not limited to the average of the image reference positions of the front and back.

(Example 3)
In each of the above examples, the paper position tendency is determined by measuring a predetermined number of paper positions on each side, and the image reference position is determined. However, in a single-sided and double-sided mixed job, the predetermined number of sheets is measured. You may make it operate | move in double-sided mode all during.
In the second example, the operation of measuring a predetermined number of front and back paper position trends in a single-sided and double-sided mixed job and determining an image reference position common to the front and back sides is described. However, if an attempt is made to measure n sheets in a single-sided / double-sided mixed job in actual control, the number of measured sheets on the front and back sides inevitably deviates because of the mixed job, and the control becomes complicated. For example, if an attempt is made to determine when either of the front and back reaches n sheets, there is a possibility that the number of samples on one side is small and the tendency is not accurately taken. In addition, if both reach n and try to determine them, the determination timing of the image reference position will be delayed, so that the state in which the correction margin of the swing mechanism is small will continue for a long time.

  Therefore, as shown in Table 2, in a job in which single-sided and double-sided coexist, it is unconditionally operated in the double-sided mode regardless of single-sided or double-sided for the predetermined number of sheets after the start of the job (trend measurement period). End measurement of front and back trends as soon as possible. After that, one side or both sides are operated according to each instruction. By doing so, it is possible to determine the optimal image reference position common to the front and back at an early stage.

(Modification)
In the above embodiment, the tendency is to measure the offset every time a job is started. However, the measurement result once measured is stored in association with the used paper feed tray, and the same paper feed tray is used thereafter. In the job to be performed, the image reference position may be set using the stored measurement result.
In each of the above examples, the tendency to shift is measured during job execution. However, a measurement mode for measuring the trend may be provided separately. In this case, control is performed so that the image is not formed and the measurement is performed only by passing the paper. The measurement result is stored in association with the paper feed tray in the same manner as described above and used in subsequent jobs.

With these modified examples, the swing amount can be reduced from the job start time.
However, when the previous measurement results are used as in these modified examples, the sheet setting position is changed when the sheet replacement in the sheet feeding tray is detected after the measurement results are stored (the tendency to shift). Therefore, the stored measurement result is not used, and it is preferable to perform the measurement again at the beginning of the job or in the above measurement mode. In addition, even if the paper is replaced, the tendency to shift due to the reverse conveyance path does not change, so only the surface may be measured.

Next, an example of the offset correction procedure according to the present invention will be described with reference to the flowchart of FIG. Note that the control in the following procedure is executed by the image control CPU 113.
The image forming apparatus starts operation and waits for the start of printing (step s1). When printing is started (step s1, YES), the line sensor 230 measures the edge along the transport direction of the paper P transported through the paper feed transport path 22, and the image control CPU 113 sets the paper center as the paper reference position. Is calculated (step s2). Next, it is determined whether or not it is a measurement period (step s3)). For example, a predetermined number or less can be set as the measurement period. The predetermined number of pieces of data can be stored in a storage unit such as the non-volatile memory 115, and the data may be set and changed by the user through the operation unit 140.

  If it is not within the measurement period (NO in step s3), the swing control is performed in step s8. If within the measurement period (step s3, YES), the measured paper center is stored in a storage unit such as a RAM (step s4). Next, it is determined whether or not the print is the nth sheet (step s5). If the print is not the nth sheet (step s5, NO), the process proceeds to step s8 to perform the swing control. (Step s5, YES), an image reference position determination process is performed (step s6).

  In the image reference position determination process (step s6), the image control CPU 113 that manages the job determines whether the job is a single-sided / double-sided mixed job (step s60). If the job is a mixed job (step s60, YES), RAM or the like. The paper center tendency of the front and back sides is determined based on the paper center stored in step S61, and the image center common to the front and back sides is determined based on the paper position tendency (step s61). If the job is not a mixed job (NO in step s60), the front and back paper position trends are determined based on the paper center stored in the RAM and the like, and the optimum image center for each front and back is determined based on the front and back paper position trends (step s62). To do. In this example, the image center is set as the image reference position. However, the present invention is not limited to the image center as long as it is a reference position for image formation.

After the determination of the image reference position, an image shift is instructed from the image next to the image currently being created in the image processing according to the image reference position (step s7).
Next, control is performed to swing the paper so that the paper center matches the image center in accordance with the image reference position (step s8), and then the operation mode is determined (step s9).

  In determining the operation mode, the image control CPU 113 that manages the job determines whether the job is a single-sided / double-sided mixed job (step s90). If the job is a mixed job (step s90, YES), it is determined whether the job is within the measurement period. Is performed (step s91). If it is within the measurement period (step s91, YES), the double-sided operation is fixed (step s92). That is, a double-sided operation using a double-sided conveyance path is performed regardless of single-sided or double-sided printing. However, if the printing is not duplex printing, the image is not transferred to the back surface. If it is not a single-sided / double-sided mixed job (step s90, NO), if it is a mixed job and not in the measurement period (step s91, NO), the specified mode (single-sided mode if single-sided is specified, double-sided mode if double-sided is specified) ) (Step s93).

  After the operation mode is determined, it is determined whether or not the printing is finished (step s10). If the printing is not finished (NO in step s10), the process returns to step s2, the edge of the sheet is measured, and the subsequent processing is repeated in the same manner as described above. If printing is finished (step s10, YES), the process is finished.

  Although the present invention has been described based on the above embodiment, the present invention is not limited to the description of the above embodiment, and appropriate modifications are possible without departing from the scope of the present invention. .

DESCRIPTION OF SYMBOLS 1 Image forming apparatus 10 Image forming part 16 Intermediate transfer belt 18 Secondary transfer roller 19 Fixing device 20 Paper feed conveyance path 22 Paper feed conveyance path 23 Registration roller 230 Line sensor 24 Paper reverse conveyance path 113 Image control CPU
115 Nonvolatile memory

Claims (13)

An image forming unit that forms an image and transfers the image to a sheet;
A paper feeding and conveying unit that feeds and conveys the paper;
A reversing conveyance unit that reverses and conveys the sheet that has been conveyed by the sheet feeding conveyance unit and passed through the image forming unit, and circulates to the sheet feeding conveyance unit;
In preparation for the image formation, a misalignment correction unit that corrects misalignment by moving the paper in the middle of conveyance in the main scanning direction;
A paper position measuring unit for measuring the position in the main scanning direction of the front surface printing and back surface printing paper in the middle of conveyance;
A control unit for controlling the image formation,
The control unit receives the measurement result of the paper position measurement unit, determines the positional tendency of the paper for front side printing and back side printing, and for front side printing and back side printing in the image formation according to the positional tendency An image forming apparatus characterized by determining an image reference position.   The paper feeding / conveying unit includes a registration unit that aligns the leading end position of the sheet in the middle of conveyance, and the deviation correction unit corrects deviation of the sheet passing through the registration unit. The image forming apparatus according to claim 1.   The image forming apparatus according to claim 2, wherein the deviation correction unit includes a mechanism that swings a registration roller provided in the registration unit.   The control unit corrects a deviation of the front surface printing paper by the deviation correction unit based on the measurement result of the paper position measurement unit and the front surface image reference position on the front surface printing paper. In addition, based on the measurement result of the paper position measuring unit and the image reference position for the back side printing on the paper for back side printing, control is performed to correct the side deviation of the paper for back side printing by the deviation correction unit. The image forming apparatus according to any one of claims 1 to 3.   The image forming apparatus according to claim 4, wherein the control unit performs control to correct a deviation of the sheet so that the position of the sheet is aligned with the image reference position.   The control unit determines the image reference position for front side printing and the image reference position for back side printing in accordance with the positional tendency of the front side printing paper and the rear side printing paper. The image forming apparatus according to claim 1, wherein:   The control unit determines an image reference position common to front side printing and back side printing according to a positional tendency of front side printing paper and a rear side printing paper. The image forming apparatus according to any one of 1 to 5.   The image forming apparatus according to claim 7, wherein the control unit operates at a common image reference position for front side printing and back side printing in a job in which single-sided printing and double-sided printing are mixed.   The control unit determines a position tendency of the sheet between a predetermined number of sheets after the start of a job, and determines an image reference position on a subsequent sheet according to the tendency. The image forming apparatus according to claim 1.   In the case of a job in which single-sided printing and double-sided printing are mixed, the control unit performs both sides of the paper feeding conveyance unit and the reverse conveyance unit for a predetermined number of sheets after the job starts regardless of single-sided printing or double-sided printing. 9. The image forming apparatus according to claim 8, wherein the positional tendency is determined by operating in a mode for each of the front-side printing paper and the back-side printing paper.   The image forming apparatus according to claim 9, wherein the predetermined number is a number in the job after the job starts.   The control unit stores the determined image reference position in association with a paper feed tray that feeds paper, and uses the stored image reference position for a job that feeds paper from the paper feed tray. The image forming apparatus according to claim 1, wherein image formation and misalignment correction are performed.   13. The control unit according to claim 1, wherein the control unit has a measurement mode in which a sheet is conveyed without performing image formation, and the image reference position is determined in the measurement mode. The image forming apparatus described in 1.

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