Image reading device and image forming apparatus

Abstract

To provide an image reading device that can appropriately determine the ground color of a medium to be conveyed without being affected by the attitude of the medium.SOLUTION: An image reading device comprises: conveying units (112, 113) that convey a medium (P) on which detection marks are formed along a conveyance path; a background member (133) that is arranged at a position facing the conveyance path; a luminance value sensor (134) that detects a luminance value of the background member (133) or the medium (P); and a controller (100) that determines a luminance value of the background color of the medium (P). The controller (100) causes the luminance value sensor (134) to detect the luminance value at a plurality of timings during the conveyance of the medium (P) performed by the conveying units (112, 113), and determines the luminance value of the background color based on a determination area including the first peak of the plurality of luminance values detected by the luminance value sensor (134).SELECTED DRAWING: Figure 2

Description

Technical Field

[0001] The present invention relates to an image reading apparatus and an image forming apparatus.

Background Art

[0002] Conventionally, an image reading apparatus that reads an image formed on a medium, and an image forming apparatus that analyzes the image read by the image reading apparatus and then controls subsequent image forming processing are known.

[0003] For example, a detection mark and a surface image are formed on the surface of a medium, the position of the detection mark formed on the surface of the medium is read by a scanner, and based on the position of the read detection mark, a back surface image formed on the back surface of the medium or the posture of the medium is corrected. An image forming apparatus is known (see, for example, Patent Document 1).

[0004] In addition, the following method is known as a method for determining a detection threshold for detecting an edge or a detection mark of a medium. Specifically, the conveyance amount after detecting the leading edge position of the medium with a leading edge detection sensor is counted by a rotary encoder, and assuming that the background color of the medium faces the scanner at the timing when the medium has been conveyed by a predetermined conveyance amount, a method is used in which the midpoint between the luminance value of the background color read by the scanner and the luminance value of the background member is set as the detection threshold.

Summary of the Invention

Problems to be Solved by the Invention

[0005] However, when the conveyed medium is skewed or the edge of the leading end of the medium is not perpendicular to the conveyance direction due to a cutting error or the like, the background color of the medium may not face the scanner at the above-described timing. As a result, at the above-described assumed timing, the background member or the detection mark may face the scanner. Therefore, there is a problem that the luminance value of the background color of the medium is misrecognized and an appropriate detection threshold cannot be determined.

[0006] The present invention has been made to solve such problems, and an object thereof is to provide an image reading apparatus capable of appropriately determining the background color of a medium without being affected by the posture of the medium being conveyed.

Means for Solving the Problems

[0007] In order to solve the above problems, one aspect of the present invention is has a portion where no image exists at the tip in the conveying direction, and among said portions, a plurality of a conveyance unit that conveys a medium having a detection mark formed at a position separated from the leading end in the conveyance direction along a conveyance path, a background member disposed at a position facing the conveyance path, and a luminance value sensor disposed at a position facing the background member with the conveyance path therebetween, the luminance value sensor detecting the luminance value of the background member or the medium, and a controller that determines the luminance value of the background color of the medium. The controller causes the luminance value sensor to detect a luminance value at a plurality of timings during the conveyance of the medium by the conveyance unit, and among the luminance values included in the determination region that includes the first peak of the plurality of luminance values detected by the luminance value sensor and is before the second peak , having different luminance values on the surface and is characterized by the above. , when the detection mark having a luminance value lower than the ground color is formed on the medium and the background member having a luminance value lower than the ground color faces the conveyance path, determine the maximum luminance value as the luminance value of the ground color, and when the detection mark having a luminance value higher than the ground color is formed on the medium and the background member having a luminance value higher than the ground color faces the conveyance path, determine the minimum luminance value as the luminance value of the ground color

Advantages of the Invention

[0008]

Brief Description of the Drawings

Brief Description of the Drawings

[0009]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Embodiments for Carrying Out the Invention

[0010] FIG. 1 is a schematic diagram showing the overall configuration of the image forming apparatus 1 according to the present embodiment. As shown in FIG. 1, the image forming apparatus 1 mainly includes a paper feed tray 101, a paper discharge tray 102, a conveyance unit 110, an image forming unit 120, and a reading unit (image reading device) 130. The paper feed tray 101 stores a plurality of sheets of paper P before an image is formed thereon in a stacked state. The paper discharge tray 102 stores the sheet of paper P on which an image has been formed.

[0011] The sheet of paper P is an example of a medium that is conveyed by the conveyance unit 110, on which an image is formed by the image forming unit 120, and on which luminance values are read by the reading unit 130. The sheet of paper P is, for example, cut paper that has been cut in advance to a predetermined size (e.g., A4, B5, etc.). Further, the sheet of paper P is paper or cloth woven with fibers that expand and contract when an image is formed thereon.

[0012] Also, inside the image forming apparatus 1, there is a main conveyance path R which is a space through which the sheet of paper P is conveyed. 1 and a reverse conveyance path R 2 are formed. The main conveyance path R 1 is a path that extends from the paper feed tray 101 through the image forming unit 120 to the paper discharge tray 102. The reverse conveyance path R 2 branches off from the main conveyance path R at a branch point BP on the downstream side in the conveyance direction from the reading unit 130, and rejoins the main conveyance path R on the upstream side in the conveyance direction from the image forming unit 120. 1 from the main conveyance path R 1 and rejoins the main conveyance path R on the upstream side in the conveyance direction from the image forming unit 120.

[0013] More specifically, the reverse conveyance path R 2 is a so-called switchback path that reverses the front and back of the sheet of paper P on which an image has been formed on the surface and guides it back to the image forming unit 120 again. Note that the sheet of paper P is reversed in the reverse conveyance path R 2 so that the front end and the rear end in the conveyance direction are swapped, and is guided to a position facing the image forming unit 120 again.

[0014] The conveying unit 110 conveys the sheet P along the main conveying path R 1 and the reverse conveying path R 2 Specifically, the conveying unit 110 conveys the sheet P accommodated in the paper feed tray 101 to a position facing the image forming unit 120 along the main conveying path R 1 Further, the conveying unit 110 conveys the sheet P with an image formed on its front surface through the reverse conveying path R 2 to a position facing the image forming unit 120 again after inverting the front and back. Furthermore, the conveying unit 110 discharges the sheet P with an image formed on its back surface to the paper discharge tray 102 along the main conveying path R 1 .

[0015] The conveying unit 110 includes a plurality of conveying rollers 111, 112, and 113. The conveying rollers 111, 112, and 113 are composed of, for example, a driving roller that rotates when the driving force of a motor is transmitted and a driven roller that abuts against the driving roller and rotates passively. Then, by sandwiching and rotating the sheet P between the driving roller and the driven roller, the sheet P is conveyed along the conveying paths R 1 , R 2 .

[0016] The conveying roller 111 is arranged on the upstream side in the conveying direction from the image forming unit 120. The conveying rollers 112 and 113 are arranged on the downstream side in the conveying direction from the image forming unit 120 and on the upstream side in the conveying direction from the branch point BP. However, the conveying unit 110 further includes other conveying rollers such as a conveying roller that conveys the sheet P along the reverse conveying path R 2 .

[0017] The image forming unit 120 is arranged to face the main conveying path R 1 between the conveying rollers 111 and 112. The image forming unit 120 forms an image on each of the front surface and the back surface of the sheet P conveyed by the conveying unit 110. The image forming unit 120 according to the present embodiment forms an image on the sheet P conveyed along the main conveying path R 1 by an electrophotographic method.

[0018] The image forming unit 120 according to the present embodiment has a configuration in which photosensitive drums 121Y, 121M, 121C, and 121K of respective colors (hereinafter, these are collectively referred to as "photosensitive drum 121") are arranged along a conveyance belt 122 which is an endless moving means, and is a so-called tandem type. That is, a plurality of photosensitive drums 121Y, 121M, 121C, and 121K are arranged in order from the upstream side in the conveyance direction of the conveyance belt 122 along the conveyance belt 122 on which an intermediate transfer image for transferring to the paper P fed from the paper feed tray 101 is formed.

[0019] Images of respective colors developed by toner which is a colorant on the surfaces of the photosensitive drums 121 of respective colors are superposed and transferred onto the conveyance belt 122, whereby a full-color image is formed. The full-color image formed on the conveyance belt 122 in this way is transferred to the paper P by the function of the transfer roller 123 at the position closest to the main conveyance path R 1 and is transferred to the paper P.

[0020] Furthermore, the image forming unit 120 includes a fixing roller 124 disposed on the downstream side in the conveyance direction from the transfer roller 123. The fixing roller 124 includes a driving roller driven by a motor and a driven roller that abuts against the driving roller and is driven. Then, in the process of sandwiching and rotating the paper P by the driving roller and the driven roller, the image transferred by the transfer roller 123 is fixed to the paper P by heating or pressing the paper P.

[0021] The reading unit 130 is an image reading device that reads the image formed on the paper P by the image forming unit 120. The reading unit 130 is disposed facing the main conveyance path R on the downstream side in the conveyance direction from the fixing roller 124 and on the upstream side in the conveyance direction from the branch point BP. 1 In other words, the reading unit 130 is disposed at a position where the paper P with an image formed only on the surface and the paper P with images formed on both the surface and the back surface both pass through in common.

[0022] Referring to FIGS. 2 and 3, the details of the reading unit 130 will be described. FIG. 2 is a side view of the reading unit 130. FIG. 3 is a plan view of the reading unit 130. The reading unit 130 according to the present embodiment mainly includes conveying rollers 112 and 113 which are part of the conveying unit, a leading edge detection sensor 131, a rotary encoder 132, a background member 133, and scanners (luminance detection sensors) 134a and 134b. Further, the leading edge detection sensor 131, the rotary encoder 132, and the scanners 134a and 134b are connected to a controller 100 which will be described later.

[0023] The leading edge detection sensor 131 is disposed facing the main conveyance path R at the center in the width direction orthogonal to the conveyance direction of the sheet P. 1 The leading edge detection sensor 131 detects the leading edge in the conveyance direction of the sheet P conveyed by the conveying unit 110. The leading edge detection sensor 131 is, for example, a reflection type photo interrupter that irradiates light toward the main conveyance path R and receives the reflected light. More specifically, the leading edge detection sensor 131 does not receive the reflected light when not facing the sheet P, and receives the reflected light when facing the sheet P. Then, while receiving the reflected light, the leading edge detection sensor 131 outputs a detection signal to the controller 100. 1

[0024] The rotary encoder 132 detects the amount of rotation of the driven roller constituting the conveying roller 112. In other words, the rotary encoder 132 outputs a pulse signal interlocked with the rotation of the driven roller to the controller 100. The controller 100 can specify the conveyance amount of the sheet P by the conveying roller 112 based on the value obtained by integrating the number of these pulse signals (hereinafter referred to as "encoded value").

[0025] The background member 133 is located on the downstream side in the conveyance direction from the leading edge detection sensor 131 with respect to the main conveyance path R. 1The background member 133 is composed of a rotatable holder 133a and a plurality of background rollers 133b, 133c, 133d, and 133e held by the holder 133a. The background rollers 133b to 133e are arranged to extend in the width direction. The background rollers 133b to 133e have different surface colors (i.e., brightness values). By rotating the holder 133a, one of the background rollers 133b to 133e is aligned along the main transport path R. 1 (more specifically, scanners 134a, 134b).

[0026] For example, when the background color of the paper P is white, the black background roller 133b (i.e., the luminance value is lower than that of the paper P) is connected to the main transport path R. 1 As another example, when the background color of the paper P is black, the white background roller 133c (that is, the luminance value of the paper P is higher) faces the main transport path R. 1 Face them to each other.

[0027] The scanners 134a and 134b are located downstream of the leading edge detection sensor 131 in the conveying direction and are aligned along the main conveying path R. 1 The scanners 134a and 134b are disposed at a position facing the background member 133 with the paper P therebetween. Moreover, the scanners 134a and 134b are disposed at positions symmetrical with respect to the center in the width direction of the paper P. More specifically, the scanner 134a is disposed at a position where it can face the detection marks 2a and 2c and the left edge of the paper P. Moreover, the scanner 134b is disposed at a position where it can face the detection marks 2b and 2d and the right edge of the paper P.

[0028] The scanners 134a and 134b are an example of a luminance value sensor that detects the luminance value of the background member 133 or the paper P. The scanners 134a and 134b detect the luminance value of the member they face (the background member 133 or the paper P) and output a luminance value signal indicating the detection result to the controller 100. The scanners 134a and 134b are, for example, CIS (Contact Image Sensors) configured by arranging a plurality of imaging elements in the width direction. However, specific examples of the luminance value sensor are not limited to the scanners 134a and 134b, and may be a reflective photointerrupter, a camera, or the like.

[0029] FIG. 4 is a diagram showing the hardware configuration of the image forming apparatus 1. As shown in FIG. 4, in the image forming apparatus 1, a CPU (Central Processing Unit) 10, a RAM (Random Access Memory) 20, a ROM (Read Only Memory) 30, an HDD (Hard Disk Drive) 40, and an I / F 50 are connected via a common bus 90. The CPU 10, the RAM 20, the ROM 30, and the HDD 40 constitute a controller 100.

[0030] The CPU 10 is an arithmetic means and controls the operation of the entire image forming apparatus 1. The RAM 20 is a volatile storage medium capable of high-speed reading and writing of information, and is used as a work area when the CPU 10 processes information. The ROM 30 is a non-volatile storage medium for read-only, and stores programs such as firmware. The HDD 40 is a non-volatile storage medium capable of reading and writing information and having a large storage capacity, and stores an OS (Operating System), various control programs, application programs, and the like.

[0031] The image forming apparatus 1 processes a control program stored in the ROM 30, an information processing program (application program) loaded from a storage medium such as the HDD 40 into the RAM 20, and the like by an arithmetic function provided in the CPU 10. By this processing, a software control unit including various functional modules of the image forming apparatus 1 is configured. A functional block that realizes the functions of the image forming apparatus 1 is configured by a combination of the software control unit configured in this way and the hardware resources mounted on the image forming apparatus 1.

[0032] I / F 50 is an interface that connects the LCD 60, the operation unit 70, the conveyance unit 110, the image forming unit 120, and the reading unit 130 to the common bus 90. The LCD 60 is a display that displays various screens for notifying the user of information. The operation unit 70 is an input interface that receives input of various information from the user, and includes a touch panel superimposed on the LCD 60, push buttons, and the like.

[0033] FIG. 5 is a functional block diagram of the controller 100. As shown in FIG. 5, the controller 100 includes, as functional blocks (functional modules), an arithmetic means 200, a luminance value storage means 210, a conveyance means 220, an image forming means 230, a leading edge position detection means 240, and a reading means 250.

[0034] The arithmetic means 200 performs various calculations and instructs other functional blocks to operate. The arithmetic means 200 includes a ground color determination means 201 for determining the luminance value of the ground color of the sheet P, a threshold determination means 202 for determining a detection threshold value, and an image correction means 203 for correcting an image (mainly, the back side image) formed on the sheet P.

[0035] The luminance value storage means 210 performs storage, reading, and updating of the registered luminance values described later with respect to the HDD 40 based on an instruction from the arithmetic means 200.

[0036] The conveyance means 220 operates the conveyance unit 110 based on a conveyance instruction from the arithmetic means 200. More specifically, the conveyance means 220 drives a motor for rotating a drive roller. Further, the conveyance means 220 includes a conveyance amount detection means 221 that detects the conveyance amount of the sheet P based on a pulse signal output from the rotary encoder 132 and notifies the detected conveyance amount to the arithmetic means 200.

[0037] The image forming means 230 operates the image forming unit 120 based on the image forming instruction from the arithmetic means 200. More specifically, the image forming means 230 develops an image on the surface of the photosensitive drum 121, transfers the image developed on the photosensitive drum 121 to the conveyance belt 122, transfers the image transferred to the conveyance belt 122 to the paper P using the transfer roller 123, and fixes the image transferred to the paper P using the fixing roller 124.

[0038] The leading edge position detection means 240 detects the leading edge position of the paper P using the leading edge detection sensor 131 based on the detection instruction from the arithmetic means 200, and notifies the arithmetic means 200 of the detected leading edge position.

[0039] The reading means 250 causes the scanners 134a and 134b to read an image based on the reading instruction from the arithmetic means 200, and notifies the arithmetic means 200 of the image data indicating the read image. Further, the reading means 250 includes luminance value detection means 251 that detects the luminance value of the background member 133 or the paper P using the scanners 134a and 134b, and notifies the arithmetic means 200 of the detected luminance value.

[0040] Next, with reference to FIGS. 6 to 8, the continuous printing process according to the present embodiment will be described. FIG. 5 is a flowchart of the continuous printing process. FIG. 7 is a flowchart of the detection threshold determination process. FIG. 8 is a diagram showing the temporal change of the luminance value detected by the scanners 134a and 134b in the detection threshold determination process.

[0041] The continuous printing process is a process of forming images on a plurality of sheets of paper P in order. For example, when the controller 100 is instructed by the operator through the operation unit 70 to make continuous copies of a plurality of sheets of paper P, or when the controller 100 is instructed by an external device (e.g., a PC) to make continuous prints of a plurality of sheets of paper P, the continuous printing process is started. In continuous copying or continuous printing, the number of sheets of paper P on which an image is to be formed (hereinafter, referred to as "designated number N") and the images to be formed on the front and back surfaces of each sheet of paper P are specified.

[0042] First, the controller 100 initializes the variable M stored in the RAM 20 (=1) (S601). The variable M is a variable indicating the number of sheets of paper P on which an image is to be formed next in the continuous printing process. Next, the controller 100 forms four detection marks 2a, 2b, 2c, 2d and a designated surface image on the surface of the M(=1)th sheet of paper P (S602).

[0043] More specifically, the conveying means 220 of the controller 100 conveys the paper P accommodated in the paper feed tray 101 to the conveying unit 110 to a position facing the image forming unit 120 through the main conveying path R. 1 Then, the image forming means 230 of the controller 100 causes the detection marks 2a to 2d and the designated surface image to be formed on the surface of the conveyed paper P by the image forming unit 120. Further, the conveying means 220 of the controller 100 further conveys the paper P with an image formed on its surface by the conveying unit 110, so that the detection marks 2a to 2d and the surface image are fixed as the paper P passes through the fixing roller 124.

[0044] As shown in FIG. 3, the detection marks 2a to 2d are substantially L-shaped images formed at the four corners of the paper P. Further, the detection marks 2a to 2d are arranged at positions spaced apart from the ends in the conveying direction of the paper P. That is, the detection marks 2a and 2b are located behind the leading edge of the paper P. Also, the detection marks 2c and 2d are located in front of the trailing edge of the paper P.

[0045] The surface image is formed in the region surrounded by the detection marks 2a to 2d. On the other hand, no image is formed outside the detection marks 2a to 2d. That is, among the surfaces of the paper P, the regions in front of the detection marks 2a and 2b and the regions behind the detection marks 2c and 2d remain the ground color even after the execution of step S602.

[0046] Next, the controller 100 executes the detection threshold determination process shown in FIG. 7 (S603). The detection threshold determination process is a process for determining a detection threshold for detecting the edge of the paper P or the detection marks 2a to 2d in the back surface image correction process (S605) described later.

[0047] Hereinafter, assuming that the background color of the sheet P is white and the black background roller 133b faces the main conveyance path R 1 and the black detection marks 2a to 2d are formed on the surface of the sheet P, the detection threshold determination process will be described. That is, in the present embodiment, it is assumed that the luminance values of the detection marks 2a to 2d and the background member 133 are set lower than the luminance value of the background color of the sheet P.

[0048] The conveyance means 220 of the controller 100 conveys the sheet P on which the detection marks 2a to 2d and the surface image are formed to the conveyance unit 110. Then, the leading edge position detection means 240 of the controller 100 causes the leading edge detection sensor 131 to detect the leading edge position of the sheet P in the process of conveying the sheet P to the conveyance unit 110 (S701). More specifically, the leading edge position detection means 240 determines that the leading edge of the sheet P has reached the position facing the leading edge detection sensor 131 at the timing when a detection signal is output from the leading edge detection sensor 131 (S701: Yes).

[0049] Next, in response to the leading edge position of the sheet P being detected by the leading edge detection sensor 131 (S701: Yes), the luminance value detection means 251 of the controller 100 causes the scanners 134a and 134b to start detecting the luminance value (S702). Then, the controller 100 causes the scanners 134a and 134b to detect the luminance value at a plurality of timings until the sheet P is conveyed by a predetermined threshold conveyance amount (S703: No). Note that the controller 100 may detect the luminance value while conveying the sheet P, or may interrupt the conveyance of the sheet P when detecting the luminance value.

[0050] That is, as shown in FIG. 8, the controller 100 stores a plurality of luminance value signals continuously output from the scanners 134a and 134b in the RAM 20 in time series order. Further, the threshold conveyance amount is set to a conveyance amount sufficient for the detection marks 2a and 2b to face the scanners 134a and 134b. Furthermore, that the sheet P has been conveyed by the threshold conveyance amount may be detected by counting the pulse signal output from the rotary encoder 132, or may be detected by a software timer implemented in the controller 100.

[0051] Note that the conveyance direction distance between the leading edge detection sensor 131 and the scanners 134a and 134b is set to a length such that the sheet P does not reach the position facing the scanners 134a and 134b at the start of step S702 even if there is skew or cutting error in the sheet P. That is, at the start of step S702, the luminance value signal output from the scanners 134a and 134b indicates the luminance value of the background roller 133b.

[0052] That is, in steps S702 to S703, the luminance values detected by the scanners 134a and 134b (hereinafter referred to as "detection values") change as shown by the solid line in FIG. 8. More specifically, the detection value is low while facing the background member 133, rises by facing the ground color area of the sheet P, drops by facing the detection marks 2a and 2b, rises again by facing the ground color area of the sheet P, and drops by facing the surface image.

[0053] Next, the ground color determination means 201 of the controller 100 determines the luminance value of the ground color of the sheet P based on a determination area set within the change locus of the luminance values shown in FIG. 8 (S704). The determination area is an area including the first peak of a plurality of detection values. More specifically, the determination area is an area including a predetermined number of detection values with the first peak in between.

[0054] A peak refers to a detected value between the rising and falling trajectories of the luminance value. That is, when the luminance values of the detection marks 2a to 2d and the background member 133 are lower than the luminance value of the ground color of the sheet P, the peak refers to the maximum luminance value detected by the scanners 134a and 134b. On the other hand, when the luminance values of the detection marks 2a to 2d and the background member 133 are higher than the luminance value of the ground color of the sheet P, the peak refers to the minimum luminance value detected by the scanners 134a and 134b.

[0055] As an example, when the luminance values of the detection marks 2a to 2d and the background member 133 are lower than the luminance value of the ground color of the sheet P, the ground color determination means 201 determines the maximum luminance value included in the determination area as the luminance value of the ground color. Further, when the luminance values of the detection marks 2a to 2d and the background member 133 are higher than the luminance value of the ground color of the sheet P, the ground color determination means 201 determines the minimum luminance value included in the determination area as the luminance value of the ground color.

[0056] As another example, in the HDD40 which is an example of the memory, a registered luminance value which is a previously detected luminance value of the ground color may be stored. Then, the ground color determination means 201 may determine, as the luminance value of the ground color, the luminance value having the smallest difference from the registered luminance value registered in the HDD40 among the plurality of luminance values included in the determination area.

[0057] Further, the HDD40 may store a plurality of registered luminance values associated with the type of the sheet P. Then, the ground color determination means 201 may receive an input of the type of the sheet P from the operator through the operation unit 70, and compare the registered luminance value associated with the input type of the sheet P with the plurality of luminance values included in the determination area.

[0058] Furthermore, the luminance value storage means 210 of the controller 100 updates the registered luminance value stored in the HDD40 with the luminance value of the ground color determined in step S704 (S705). Note that the luminance value storage means 210 may use a representative value (for example, an average value, a moving average value, a median value, etc.) of a plurality of past registered luminance values and the luminance value of the ground color determined this time as the new registered luminance value. However, when no registered luminance value is stored in the HDD, the process of step S705 is omitted.

[0059] Next, the threshold determination means 202 of the controller 100 determines a detection threshold based on the luminance value of the background color determined in step S704 (S706). Then, the controller 100 causes the HDD 40 to store the determined detection threshold. The threshold determination means 202 according to the present embodiment determines, for example, the midpoint (average value) between the luminance value V of the background color and the luminance values L of the detection marks 2a to 2d as the detection threshold (= (V + L) / 2). Thereby, as shown in FIG. 8, a detection threshold smaller than the luminance value of the background color and larger than the luminance values of the detection marks 2a to 2d can be obtained. However, the method for determining the detection threshold is not limited to the above example as long as it is based on the luminance value of the background color.

[0060] Next, returning to FIG. 6, the controller 100 specifies the positions of the detection marks 2a to 2d formed at the four corners of the front surface of the sheet P using the detection threshold determined in step S706 (S604). Next, the controller 100 corrects the shape of the back surface image according to the relative positions of the detection marks 2a to 2d specified in step S605 (S605). Since steps S604 to S605 are already well-known processes, detailed description thereof is omitted.

[0061] Next, the controller 100 forms the back surface image corrected in step S605 on the back surface of the sheet P (S606). More specifically, the conveyance means 220 of the controller 100 guides the sheet P to a position facing the image forming unit 120 by inverting the front and back of the sheet P by conveying the sheet P along the reverse conveyance path R 2 to the conveyance unit 110. Then, the image forming means 230 of the controller 100 causes the corrected back surface image to be formed on the back surface of the conveyed sheet P by the image forming unit 120. Further, the conveyance means 220 of the controller 100 discharges the sheet P on which the back surface image has been formed to the discharge tray 102 by the conveyance unit 110.

[0062] Next, the controller 100 determines whether the variable M has reached the specified number of sheets N (S607). If the variable M has not yet reached the specified number of sheets N (S607: No), the controller 100 increments the variable M by 1 (S608) and executes the processes after step S602. On the other hand, when the variable M reaches the specified number of sheets N (S607: Yes), the controller 100 ends the continuous printing process.

[0063] According to the above embodiment, for example, the following operational effects can be achieved.

[0064] According to the above embodiment, a determination area is set within the change locus of the luminance values detected at a plurality of timings during the conveyance of the sheet P, and the luminance value of the background color of the sheet P is determined based on the set determination area. As a result, even if the conveyed sheet P is skewed or the edge of the leading end of the sheet P is not perpendicular to the conveyance direction due to a cutting error or the like, the background color of the sheet P can be appropriately determined.

[0065] In addition, when determining the maximum luminance value or the minimum luminance value as the luminance value of the background color based on the magnitude relationship between the detection marks 2a to 2d and the background member 133 and the background color of the sheet P, the process of determining the luminance value of the background color is simplified.

[0066] On the other hand, when determining the luminance value of the background color by comparing the luminance values detected by the scanners 134a and 134b with the registered luminance value, the luminance value of the background color can be determined more accurately. Furthermore, by updating the registered luminance value with the determined luminance value of the background color, changes in the luminance value due to aging deterioration of the scanners 134a and 134b can be absorbed.

[0067] In addition, according to the above embodiment, since the reverse side image correction process is executed using the detection threshold value determined based on the determined luminance value of the background color, the positions of the edges of the sheet P and the detection marks 2a to 2d can be appropriately detected. As a result, the reverse side image can be appropriately corrected.

[0068] Further, in step S702, the controller 100 may detect the luminance value using one of the plurality of image sensors included in the scanners 134a and 134b, or may detect the luminance value using a plurality of image sensors spaced apart in the width direction. By detecting the luminance value at a plurality of positions in the width direction, the influence of skew and cutting errors can be reduced.

[0069] Furthermore, the image forming method of the image forming unit 120 is not limited to the electrophotographic method, and an inkjet method may also be used. In this case, the paper P may expand or contract as the landed ink dries. That is, the present invention can be applied regardless of whether the image forming unit 120 uses the electrophotographic method or the inkjet method.

Explanation of Signs

[0070] 1: Image forming apparatus 2a, 2b, 2c, 2d: Detection marks 10: CPU 20: RAM 30: ROM 40: HDD 50: I / F 60: LCD 70: Operation unit 90: Common bus 100: Controller 101: Paper feed tray 102: Paper discharge tray 110: Conveying unit 111, 112, 113: Conveying rollers 120: Image forming unit 121: Photoconductor drum 121C, 121K, 121M, 121Y: Photoconductor drums 122: Conveying belt 123: Transfer roller 124: Fixing roller 130: Reading unit 131: Leading edge detection sensor 132: Rotary encoder 133: Background member 133a: Holder 133b, 133c, 133d, 133e: Background roller 134a, 134b: Scanner 200: Arithmetic means 201: Ground color determination means 202: Threshold determination means 203: Image correction means 210: Luminance value memory means 220: Conveying means 221: Conveying amount detection means 230: Image forming means 240: Tip position detection means 250: Reading means 251: Luminance value detection means

Prior art documents

Patent documents

[0071]

Patent Document 1

Claims

A transport unit that transports a medium having a portion without an image at the tip in the transport direction and having a detection mark formed at a position spaced apart from the tip in the transport direction of the portion along a transport path; A plurality of background members arranged at a position facing the transport path and having different luminance values on the surface; A luminance value sensor arranged at a position facing the background member with the transport path therebetween, for detecting the luminance value of the background member or the medium; A controller for determining the luminance value of the background color of the medium, comprising: The controller: Causes the luminance value sensor to detect a luminance value at a plurality of timings during the transport of the medium by the transport unit; Among the luminance values included in the determination region including the first peak of the plurality of luminance values detected by the luminance value sensor and before the second peak, When the detection mark having a luminance value lower than the background color is formed on the medium and the background member having a luminance value lower than the background color faces the transport path, determines the maximum luminance value as the luminance value of the background color; An image reading apparatus characterized in that when the detection mark having a luminance value higher than the background color is formed on the medium and the background member having a luminance value higher than the background color faces the transport path, determines the minimum luminance value as the luminance value of the background color.

2. An operation unit that receives an operation for inputting the type of the medium transported by the transport unit; A memory that stores a plurality of registered luminance values associated with the type of the medium; The controller determines, as the luminance value of the background color, the luminance value having the smallest difference from the registered luminance value stored in the memory in association with the type of the medium input through the operation unit among the plurality of luminance values included in the determination region. The image reading apparatus according to claim 1.

3. The controller updates the registered luminance value stored in the memory in association with the type of the medium input through the operation unit with the determined luminance value of the background color. The image reading apparatus according to claim 2.

4. An image reading apparatus according to any one of claims 1 to 3; An image forming apparatus characterized by comprising an image forming unit that forms an image on each of the front and back surfaces of the medium transported by the transport unit.

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