Image forming apparatus and method for controlling image forming apparatus

Abstract

To provide an image forming apparatus that can accurately detect an image for detection even in a case of double-sided printing.SOLUTION: An image forming apparatus comprises: a printer 150 that forms images on both sides of recording paper based on an image forming condition; a reader 160 that reads an image for detection formed on the recording paper for adjusting the image forming condition; and a controller 110 that adjusts the image forming condition based on a result of reading of the image for detection performed by the reader 160 and causes the printer 150 to form images based on the adjusted image forming condition. When the controller 110 causes the printer 150 to form the image for detection on the recording paper, the controller causes the printer 150 to form an image in an area on a surface opposite to a surface on which the image for detection is formed, the area corresponding to an area in which the image for detection is formed so as to make the image density of the image constant.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a technique for stabilizing image quality in an image forming apparatus. [Background technology]

[0002] An image forming apparatus using an electrophotographic process forms an image on a recording paper through the processes of charging, exposing, developing, transferring, and fixing to generate a product. The toner image formed through the processes of charging, exposing, and developing is transferred to the recording paper through the transfer process, and fixed to the recording paper through the fixing process. In an image forming apparatus, the characteristics of each process change due to the aging of parts and environmental changes, and the image quality, such as the image density of the image on the recording paper, may change. Therefore, an image forming apparatus generally performs a process called image stabilization control. The image stabilization control is a process in which a detection image for detecting the image density is formed on a photosensitive drum or an intermediate transfer belt, and the image forming conditions are adjusted so that the image density is appropriate based on the result of reading the detection image by an optical sensor. The image forming conditions are, for example, the charge amount during the charging process and the amount of emission energy of the laser light during the exposure process.

[0003] Image stabilization control is performed using image density detected from a detection image, which is a toner image before being transferred to a recording sheet. Therefore, the influence of processes after the transfer process on the image density is outside the scope of image stabilization control. For example, the influence of environmental fluctuations on the transfer efficiency during the transfer process cannot be adjusted by image stabilization control. Therefore, conventional image stabilization control cannot suppress variations in the image density of the image finally formed on the recording sheet. In response to this, Patent Document 1 discloses an image forming apparatus that forms a detection image on a recording sheet, and adjusts image formation conditions so that the image density of the image on the recording sheet is appropriate based on the results of reading the detection image formed on the recording sheet by an optical sensor.

[0004] Some image forming devices are capable of double-sided printing, forming images on both sides of a recording sheet. When detecting image density by forming detection images on both sides of a recording sheet, the detection image on the first side may affect the image density, chromaticity, and spectral value detection accuracy of the detection image on a second side different from the first side. In response to this, Patent Document 2 discloses an image forming device that forms a detection image on the first side of a recording sheet and forms a detection image on the second side that has a higher image density than the detection image formed on the first side, thereby reading the detection image formed on the second side while suppressing the effects of show-through. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] JP 2012-53089 A [Patent Document 2] JP 2013-15578 A Summary of the Invention [Problem to be solved by the invention]

[0006] A recording paper may have a detection image printed on the first side and an image according to a print job (hereinafter referred to as a "user image") printed on the second side. In this case, the user image may show through in the area where the detection image on the first side is printed. As a result, the image density detected from the detection image on the first side may be affected by the user image, and the detection accuracy may decrease. The decrease in detection accuracy of the detection image hinders the adjustment of appropriate image formation conditions.

[0007] The present invention has been made in consideration of the above-mentioned problems, and has an object to provide an image forming apparatus that can detect a detection image with high accuracy even in the case of double-sided printing. [Means for solving the problem]

[0008] The image forming apparatus of the present invention includes a receiving means for receiving image data, an image forming means for forming an image on a sheet based on image forming conditions, a reading means for reading a test image formed on the sheet by the image forming means, and a generating means for generating the image forming conditions based on a reading result of the test image read by the reading means, wherein the image forming means forms a first image corresponding to the image data and a first test image for generating the image forming conditions on a first side of the sheet, and forms a second image corresponding to the image data on a second side opposite to the first side of the sheet, without forming a second test image for generating the image forming conditions, When the first test image is formed on the first surface of the sheet and the second image is formed on the second surface of the sheet, the image forming means The second image is formed in a second area of ​​the second surface corresponding to a first area of ​​the first test image formed on the first surface. and an image is formed by removing the image in the second region. It is characterized by: Another aspect of the image forming apparatus of the present invention comprises a receiving means for receiving image data, an image forming means for forming an image on a sheet based on image formation conditions, a reading means for reading a test image formed on the sheet by the image forming means, and a generating means for generating the image formation conditions based on the reading result of the test image read by the reading means, wherein the image forming means forms a first image corresponding to the image data and a first test image for generating the image formation conditions on a first side of the sheet, and forms a second image corresponding to the image data on a second side of the sheet opposite the first side to form a second test image for generating the image formation conditions, and when the first test image is formed on the first side of the sheet and the second image is formed on the second side of the sheet, the image forming means does not form the second image in a second area of ​​the second side corresponding to a first area of ​​the first test image formed on the first side, and forms an image in which the second area is replaced with a blank area in which no image is formed. Effect of the Invention

[0009] According to the present invention, it is possible to detect the detection image with high accuracy even in the case of double-sided printing. [Brief description of the drawings]

[0010] [Figure 1] FIG. 1 is a diagram illustrating the configuration of a printing system. [Diagram 2] FIG. 1 is a diagram illustrating the configuration of an image forming apparatus. [Diagram 3] FIG. [Figure 4] FIG. 2 is a diagram illustrating the configuration of a line sensor. [Diagram 5] 11 is a flowchart showing a process of calculating a correction value for image density. [Figure 6] 5A and 5B are diagrams illustrating an example of a user image and a detection image formed on a recording paper. [Figure 7] FIG. 4 is a configuration diagram of a concentration detection processing unit. [Figure 8] FIG. 4 is an explanatory diagram of a storage range of image data. [Figure 9] 6 is a diagram illustrating the amount of skew of a detection image with respect to a line sensor unit. [Figure 10] 4 is a flowchart showing an image density detection process. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] Hereinafter, the embodiments will be described in detail with reference to the drawings. Note that the following embodiments do not limit the invention according to the claims, and all of the combinations of features described in the embodiments are not necessarily essential to the solution of the invention.

[0012] (Printing System) 1 is a diagram illustrating the configuration of a printing system including an image forming apparatus according to this embodiment. The printing system includes an image forming apparatus 100 and a host computer 101. The image forming apparatus 100 and the host computer 101 are communicatively connected via a network 105. The network 105 is, for example, a communication line such as a LAN (Local Area Network), a WAN (Wide Area Network), or a public communication line. Note that a plurality of image forming apparatuses 100 and a plurality of host computers 101 may be connected to the network 105.

[0013] The host computer 101 is, for example, a server device, and transmits a print job to the image forming apparatus 100 via a network 105. The print job includes various information necessary for printing, such as image data, the type of recording paper used for printing, the number of copies to be printed, and an instruction for double-sided or single-sided printing.

[0014] Image forming apparatus 100 includes controller 110, operation panel 120, paper feed unit 140, printer 150, and reader 160. Image forming apparatus 100 controls the operation of printer 150 based on a print job acquired from host computer 101, and forms an image on recording paper according to image data. Controller 110, operation panel 120, paper feed unit 140, printer 150, and reader 160 are connected to each other via system bus 116 so as to be able to communicate with each other.

[0015] The controller 110 controls the operation of each unit of the image forming apparatus 100. The controller 110 is an information processing device including a read only memory (ROM) 112, a random access memory (RAM) 113, and a central processing unit (CPU) 114. The controller 110 also includes a communication control unit 111 and a storage 115. Each module is connected to each other via a system bus 116 so as to be able to communicate with each other.

[0016] The communication control unit 111 is a communication interface that communicates with the host computer 101 and other devices via the network 105. The storage 115 is a large-capacity storage device such as a hard disk drive (HDD) or a solid state drive (SSD). The storage 115 stores computer programs and various data used in image forming processing (printing processing). The CPU 114 executes computer programs stored in the ROM 112 and the storage 115 to control the operation of the image forming apparatus 100. The RAM 113 provides a work area when the CPU 114 executes the computer programs.

[0017] Operation panel 120 is a user interface and includes an input interface and an output interface. The input interface is, for example, an operation button, a numeric keypad, a touch panel, etc. The output interface is, for example, a display such as an LCD (Liquid Crystal Display), a speaker, etc. A user can input print jobs, commands, print settings, etc. to image forming apparatus 100 through operation panel 120. Operation panel 120 displays a setting screen and the status of image forming apparatus 100 on the display.

[0018] Paper feed unit 140 includes a plurality of paper feed stages (described below) that store recording paper. Paper feed unit 140 feeds paper from a paper feed stage that stores the type of recording paper specified by a print job. A paper feed stage stores a plurality of sheets of recording paper (a stack of recording papers), and the sheets are fed in order starting from the topmost sheet. Paper feed unit 140 transports the recording paper fed from the paper feed stage to printer 150. Each paper feed stage may store the same type of recording paper, or may store different types of recording paper.

[0019] The printer 150 prints an image on recording paper supplied from the paper feed unit 140 based on image data included in the print job to generate a product. The reader 160 is an image reading device that reads an image from the product generated by the printer 150 and transmits the reading result to the controller 110. The image read by the reader 160 is an image (detection image) for adjusting the image formation conditions when the printer 150 performs image formation. The controller 110 detects the image condition such as image quality from the result of reading the detection image by the reader 160, and adjusts the image formation conditions based on the detected image condition. In this embodiment, the image density is detected from the detection image, and the image formation conditions are adjusted based on the detected image density.

[0020] (Image forming device) 2 is a configuration diagram of image forming apparatus 100. Image forming apparatus 100 includes, in order from the upstream side in the conveying direction of recording paper, paper feed stages 140a to 140e, printer 150, reader 160, and finisher 190. Paper feed stages 140a to 140e configure paper feed section 140. Here, finisher 190 is a post-processing device that performs post-processing on an output from printer 150. Finisher 190 performs, for example, stapling and sorting of multiple sheets of output, and cutting of an area where an image for detection, described later, is formed.

[0021] Printer 150 includes multiple image forming units that form images of different colors. Printer 150 of this embodiment includes four image forming units to form images of four colors: yellow, magenta, cyan, and black. Each image forming unit has a similar configuration and performs a similar operation, except that the colors of the images it forms are different.

[0022] One image forming unit includes a photosensitive drum 153, a charger 220, an exposure device 223, and a developing device 152. The photosensitive drum 153 is a drum-shaped photosensitive body having a photosensitive layer on its surface, and is rotated in the direction of an arrow R1 by a motor (not shown). The charger 220 charges the surface (photosensitive layer) of the rotating photosensitive drum 153. The exposure device 223 exposes the charged surface of the photosensitive drum 153 to a laser beam. The laser beam scans the surface of the photosensitive drum 153 in the axial direction of the photosensitive drum 153. The direction in which the laser beam scans the surface of the photosensitive drum 153 is the main scanning direction (depth direction in FIG. 2) of the printer 150. As a result, an electrostatic latent image is formed on the surface of the photosensitive drum 153. The developing device 152 develops the electrostatic latent image using a developer (toner). As a result, an image (toner image) in which the electrostatic latent image is visualized is formed on the surface of the photosensitive drum 153.

[0023] Printer 150 includes intermediate transfer belt 154 onto which toner images generated by each image forming unit are transferred. Intermediate transfer belt 154 is driven to rotate in the direction of arrow R2. The toner images of each color are transferred at a timing corresponding to the rotation of intermediate transfer belt 154. As a result, a full-color toner image in which the toner images of each color are superimposed is formed on intermediate transfer belt 154. The full-color toner image is transported to a nip portion formed by intermediate transfer belt 154 and transfer roller 221 by the rotation of intermediate transfer belt 154. The full-color toner image is transferred to recording paper by the nip portion.

[0024] The recording paper is stored in paper feed stages 140a, 140b, 140c, 140d, and 140e of paper feed section 140, and is fed in accordance with the timing of image formation by each image forming unit. The paper feed stage that feeds the recording paper is specified by the print job. The recording paper is transported to the nip portion at the same time that the full-color toner image is transported to the nip portion. This causes the toner image to be transferred to a predetermined position on the recording paper. The transport direction of the recording paper is the sub-scanning direction perpendicular to the main scanning direction.

[0025] The printer 150 includes a first fixing device 155 and a second fixing device that fix a toner image to a recording paper by applying heat and pressure. The first fixing device 155 includes a fixing roller with a built-in heater and a pressure belt for pressing the recording paper against the fixing roller. The fixing roller and the pressure belt are driven by a motor (not shown) to sandwich and transport the recording paper. The second fixing device 156 is disposed downstream of the first fixing device in the transport direction of the recording paper. The second fixing device 156 is used to increase the gloss and ensure fixability of the image on the recording paper that has passed through the first fixing device 155. The second fixing device 156 includes a fixing roller with a built-in heater and a pressure roller with a built-in heater. Depending on the type of recording paper, the second fixing device 156 is not used. In this case, the recording paper is not transported to the second fixing device 156 but is transported to the transport path 130. For this reason, a flapper 131 that guides the recording paper to either the transport path 130 or the second fixing device 156 is provided downstream of the first fixing device 155.

[0026] A conveying path 135 and a discharge path 139 are provided downstream of the position where the conveying path 130 joins downstream of the second fixing device 156. For this purpose, a flapper 132 for guiding the recording paper to either the conveying path 135 or the discharge path 139 is provided at the position where the conveying path 130 joins downstream of the second fixing device 156. For example, in a double-sided printing mode, the flapper 132 guides the recording paper with an image formed on its first side to the conveying path 135. For example, in a face-up discharge mode, the flapper 132 guides the recording paper with an image formed on its first side to the discharge path 139. For example, in a face-down discharge mode, the flapper 132 guides the recording paper with an image formed on its first side to the conveying path 135.

[0027] The recording paper conveyed to the conveying path 135 is conveyed to the inverting section 136. After the conveying operation of the recording paper conveyed to the inverting section 136 is temporarily stopped, the recording paper is switched back to reverse the conveying direction. The recording paper is guided from the inverting section 136 to either the conveying path 135 or the conveying path 138 by the flapper 133. For example, in the double-sided printing mode, the flapper 133 guides the switched back recording paper to the conveying path 138 to print an image on the second side. The recording paper conveyed to the conveying path 138 is conveyed toward the nip portion between the intermediate transfer belt 154 and the transfer roller 221. As a result, the front and back of the recording paper are inverted when passing through the nip portion, and an image is formed on the second side. For example, in the face-down paper discharge mode, the flapper 133 guides the switched back recording paper to the conveying path 135. The recording paper conveyed to the conveying path 135 by the flapper 133 is guided to the discharge path 139 by the flapper 134 .

[0028] The recording paper on which an image is formed by the printer 150 is transported from the discharge path 139 to the reader 160. The reader 160 reads the detection image of the image density printed together with the user image printed on the recording paper in accordance with the print job. The recording paper transported from the printer 150 to the reader 160 is transported to a transport path 313 inside the reader 160. The reader 160 includes an original detection sensor 311 and line sensor units 312a and 312b on the transport path 313. The reader 160 reads the detection image by the line sensor units 312a and 312b while transporting the recording paper along the transport path 313. The recording paper on which the detection image is printed will be described in detail later.

[0029] The document detection sensor 311 is, for example, an optical sensor having a light emitting element and a light receiving element. The document detection sensor 311 detects the leading edge of the recording paper in the transport direction transported on the transport path 313. The detection result of the leading edge of the recording paper by the document detection sensor 311 is transmitted to the controller 110. The controller 110 starts a reading operation by the reader 160 (line sensor units 312a, 312b) based on the timing when the document detection sensor 311 detects the leading edge of the recording paper.

[0030] The detection image can be printed on both sides of the recording paper, the first side and the second side. Line sensor unit 312a and line sensor unit 312b are provided at positions sandwiching conveyance path 313 in order to read the detection images on both sides of the recording paper in one conveyance. When performing image density adjustment, image forming apparatus 100 reads the detection images with line sensor units 312a and 312b, and detects the image densities of the detection images on both sides of the recording paper from the reading results. Controller 110 adjusts image formation conditions based on the detection results of image density to control image formation processing so that the image printed on the recording paper has an appropriate density.

[0031] (Leader) 3 is a diagram illustrating the configuration of the reader 160. The reader 160 includes line sensor units 312a and 312b and an original detection sensor 311, as well as an image memory 303 and a density detection processing unit 305. The operations of the line sensor units 312a and 312b, the image memory 303, the density detection processing unit 305, and the original detection sensor 311 are controlled by the CPU 114 of the controller 110.

[0032] The line sensor unit 312a includes a line sensor 301a, a memory 300a, and an AD converter 302a. The line sensor unit 312b includes a line sensor 301b, a memory 300b, and an AD converter 302b. The line sensors 301a and 301b are, for example, contact image sensors (CIS). The memories 300a and 300b store correction information such as variations in the amount of light between pixels of the corresponding line sensors 301a and 301b, steps between pixels, and distances between pixels.

[0033] The AD converters 302a and 302b acquire analog signals that are the results of reading by the corresponding line sensors 301a and 301b. The AD converters 302a and 302b convert the acquired analog signals into digital signals and send them to a density detection processing unit 305. The digital signals are image data of R (red), G (green), and B (blue). The density detection processing unit 305 calculates an average luminance value of RGB of the detection image portion from the RGB image data and sends it to the CPU 114. The density detection processing unit 305 is composed of an FPGA (Field-Programmable Gate Array), an ASIC (Application Specific Integrated Circuit), etc. The image memory 303 stores image data necessary for image processing in the CPU 114.

[0034] 4 is a diagram illustrating the configuration of line sensor 301a. Line sensor 301a includes light emitting units 400a and 400b, light guides 402a and 402b, a lens array 403a, and a sensor chip group 401a. Line sensor 301a is a rectangular parallelepiped and reads an image with its longitudinal direction as the main scanning direction. Line sensor 301b has a similar configuration.

[0035] The light emitting units 400a and 400b are light sources constituted by, for example, LEDs (Light Emitting Diodes) that emit white light. The light emitting unit 400a is disposed at an end of the light guide 402a, and the light emitted from the light emitting unit 400a is irradiated toward the recording paper. The light emitting unit 400b is disposed at an end of the light guide 402b, and the light emitted from the light emitting unit 400b is irradiated toward the recording paper. The light guides 402a and 402b are formed linearly in the main scanning direction. Therefore, the line sensor 301 irradiates the recording paper in a straight line in the main scanning direction. The main scanning direction of the line sensor unit 312a and the main scanning direction of the printer 150 are the same direction.

[0036] Lens array 403a guides the reflected light of light emitted from light emitting units 400a and 400b by the recording paper to sensor chip group 401a. Sensor chip group 401a is configured with multiple photoelectric conversion elements (sensor chips) lined up in a straight line in the main scanning direction. One sensor chip reads an image of one pixel. The multiple sensor chips are configured in three lines. One line is coated with an R (red) color filter, the other line is coated with a G (green) color filter, and the other line is coated with a B (blue) color filter. The light guided by lens array 403a is imaged on the light receiving surface of each sensor chip in sensor chip group 401a.

[0037] The light emitted from the light emitting units 400a and 400b diffuses inside the light guides 402a and 402b, and is emitted from the portions having a curvature, illuminating the entire area of ​​the recording paper in the main scanning direction. The light guides 402a and 402b are arranged on either side of the lens array 403a in the sub-scanning direction perpendicular to the main scanning direction. For this reason, the line sensor 301a has a double-sided illumination configuration in which light is irradiated onto the lens array 403a (image reading line) from two directions in the sub-scanning direction. The sub-scanning direction of the line sensor unit 312a and the sub-scanning direction of the printer 150 are the same direction.

[0038] (Calculation of image density correction value) 5 is a flowchart showing the image density correction value calculation process. This process is started when CPU 114 acquires a print job including the recording paper size and print mode set by the user operating operation panel 120. Here, the process when performing copy processing will be explained. To perform copy processing, printer 150 is provided with a scanner that reads an image from a document to be copied. Note that when performing print processing, CPU 114 acquires a print job from host computer 101 and performs this process.

[0039] Based on the acquired print job, CPU 114 sets each device to an operation mode required for executing the print job (S500). When the user operates operation panel 120 to instruct the start of copying (S501: Y), CPU 114 acquires this instruction, reads an image from an original with a scanner (not shown), and starts image formation processing based on image data representing the read image. CPU 114 initializes page count value P (P=0) indicating the number of sheets of recording paper on which the image density has been detected (S502). CPU 114 forms a user image and an image for detection on recording paper (S503). Details will be described later.

[0040] When the leading edge of the recording paper on which an image has been formed is detected by the document detection sensor 311 of the reader 160, the CPU 114 increments the page count value P by 1 (S504). The document detection sensor 311 changes its output value (e.g., from 0 to 1) upon detecting the recording paper. The CPU 114 can know that the leading edge of the recording paper has been detected by the document detection sensor 311 from the change in the output value of the document detection sensor 311.

[0041] When the document detection sensor 311 detects the recording paper, the CPU 114 detects the image density of the detection image from the recording paper using the line sensor units 312a and 312b (S505). The details of the image density detection process will be described later. The CPU 114 checks the page count value P, and repeats the processes of S504 to S506 until the page count value P reaches or exceeds the predetermined number (S506: N). When the page count value P reaches or exceeds the predetermined number (S506: Y), the CPU 114 detects the image density based on the reading result of the detection image, and calculates a correction value for correcting the image density (S507). The correction value is calculated from the difference in image density based on the reading result of the detection image relative to the reference image density, for example. The predetermined number is set in advance. That is, the image density correction value is calculated every time the image formation process is performed for the predetermined number of sheets. The image density correction value is calculated as described above.

[0042] FIG. 6 is an example diagram of a user image and a detection image formed on a recording paper in the process of S503. The shaded portion is an area where the user image is printed. The detection image is printed in the shaded portion near the edge of the shaded portion. The detection image is composed of test images of yellow, magenta, cyan, and black. The test image of each color is composed of a plurality of patch images whose image density changes stepwise. The patch image at one end of the test image is the patch image with the highest density. The patch image at the other end is the patch image with the second highest density. When the detection image overlaps with the user image, the detection image is prioritized over the user image. That is, the detection image is formed on the user image. The area where the detection image is formed is an area that is finally cut and discarded by the finisher 190. Therefore, the detection image does not remain in the final deliverable.

[0043] The detection images of recording sheets S1 to S3 are formed by forming test images of each color on the edge of the recording sheet. The detection image of recording sheet S1 is formed by forming test images of each color of yellow, magenta, cyan, and black lined up in the sub-scanning direction along one side of the recording sheet in the main scanning direction. The detection image of recording sheet S2 is formed by forming test images of two colors each lined up in the sub-scanning direction on both sides of the recording sheet in the main scanning direction. The detection image of recording sheet S3 is formed by forming test images of two colors each lined up in the main scanning direction on both sides of the recording sheet in the sub-scanning direction.

[0044] Recording paper S4 indicates the back side of recording paper S1. Recording paper S5 indicates the back side of recording paper S2. Recording paper S6 indicates the back side of recording paper S3. When a user image is formed in an area on the back side of the area where the detection image is formed, the show-through of the user image affects the detection result of the detection image. For this reason, in this embodiment, a user image is not formed in an area corresponding to the area where the detection image is formed on the opposite side to the side where the detection image is formed. In recording papers S4 to S6, the area is blanked out. In order to suppress the effect of show-through on the detection result of the detection image, the area is not limited to being blanked out, and a solid black color (solid black) can be expected to be effective. In addition, even if the image has a uniform image density, the effect of show-through on the detection result of the detection image can be suppressed.

[0045] (Image density detection process) Fig. 7 is a block diagram of density detection processing unit 305. The image density detection processing of S505 will be described with reference to Fig. 7. Since the image density detection processing using the detection images on the first side (front side) and second side (back side) of the recording paper is the same, the image density detection processing on the front side will be described here, and the description of the back side will be omitted. Density detection processing unit 305 includes a brightness value storage unit 710, a skew amount detection unit 720, a readout unit 730, and an average brightness calculation unit 740.

[0046] The luminance value storage unit 710 stores image data output from the line sensor unit 312a. The luminance value storage unit 710 includes a color selection unit 711, a left edge coordinate detection unit 712, a luminance value storage area determination unit 713, a luminance value writing unit 714, and a memory 715.

[0047] The color selection unit 711 selects one color of image data from the three RGB color image data output from the line sensor unit 312a. Any color may be selected, but it is preferable to select according to the color of the recording paper in order to improve the accuracy of the left edge coordinate detection.

[0048] The left edge coordinate detection unit 712 detects the left edge coordinate of the detection image from the image data of the color selected by the color selection unit 711. The left edge coordinate detection unit 712 detects the left edge by performing threshold value judgment on the image data in order from the first pixel in the main scanning direction. Since the brightness is high on the recording paper and the brightness is low in the detection image, the left edge coordinate is detected by detecting pixels where the brightness value falls. If the detection accuracy of the left edge coordinate is low, the left edge coordinate detection unit 712 may detect the falling edges of the brightness values ​​of multiple lines and detect the coordinate from multiple data.

[0049] A luminance value storage area determination unit 713 determines the ranges in the main scanning direction and sub-scanning direction of image data to be stored in memory 715 based on the first left edge coordinate (the coordinate of the upper left corner of the image for detection) detected by left edge coordinate detection unit 712 and the size of the image for detection. Fig. 8 is an explanatory diagram of the storage range of image data.

[0050] The shaded area in the test image T1 indicates the area where the average luminance value for each patch image is calculated. In order to eliminate the effect of flare due to the periphery of the image, the average value is calculated only for the luminance values ​​in the center part of the patch image as shown in the test image T1. The diagonal line portion of the test image T2 illustrates the storage range determined by the brightness value storage area determination unit 713. The storage range is obtained by expanding the area in which the average brightness value is calculated in the main scanning direction. The reason why the storage range is expanded from the area in which the average brightness value is calculated is to adjust the area used in calculating the average brightness value based on the amount of skew of the detection image with respect to the line sensor units 312a and 312b. The reason why the area is not expanded in the sub-scanning direction is because the effect of the amount of skew in the sub-scanning direction is small and can be ignored. However, the storage range is not limited to being expanded in the main scanning direction, and may be expanded in both the main scanning direction and the sub-scanning direction.

[0051] A luminance value writing unit 714 writes RGB image data within the storage range in the main scanning direction and sub-scanning direction determined by the luminance value storage area determination unit 713 into a memory 715. The luminance value storage area determination unit 713 determines the storage range by taking into consideration the amount of skew, rather than the entire image area of ​​the detection image, so that the capacity of the memory 715 used to store image data can be reduced.

[0052] The skew amount detection unit 720 includes a left edge coordinate storage area determination unit 721 , a left edge coordinate writing unit 722 , a memory 723 , and a skew amount calculation unit 724 .

[0053] A left edge coordinate storage area determination unit 721 determines a range in the sub-scanning direction for storing left edge coordinates in memory 723, based on the first left edge coordinate (the coordinate of the upper left corner of the image for detection) of the patch image detected by the left edge coordinate detection unit 712 and the size of the image for detection. The left edge coordinate written to memory 723 by the left edge coordinate writing unit 722 is used to detect the amount of skew of the image for detection with respect to the line sensor unit 312a.

[0054] 9 is an explanatory diagram of the amount of skew of the detection image with respect to the line sensor unit 312a. At least two left edge coordinates are required to detect the amount of skew of the detection image. The two left edge coordinates are detected, for example, using the first and last high density patch images P1 and P2 that allow the left edge coordinates to be detected with high accuracy. The area for storing the left edge coordinates is two lines, the first line of the first patch image P1 and the first line of the last patch image P2.

[0055] In Fig. 9, two lines passing through coordinates Y1 and Y2 in the sub-scanning direction are used. The storage area is not limited to the above area, and may be a plurality of consecutive lines. By using the average coordinate of the left end coordinates of a plurality of consecutive lines, the detection accuracy of the left end coordinate is improved, and as a result, the detection accuracy of the amount of skew is also improved. The left end coordinate writing unit 722 writes the left end coordinate value of the patch image detected by the left end coordinate detection unit 712 within the area in the sub-scanning direction determined by the left end coordinate storage area determination unit 721 to the memory 723.

[0056] The skew amount calculation unit 724 obtains two left edge coordinate values ​​from the memory 723, and calculates the amount of skew of the detection image on the recording paper with respect to the line sensor unit 312a. The amount of skew is calculated from two coordinates, the left edge coordinate (X1, Y1) of one line of the first patch image P1, and the left edge coordinate (X2, Y2) of one line of the final patch image P2, as shown in Fig. 9. The amount of skew θskew is calculated, for example, by the following formula. θskew=(Y1-Y2) / (X1-X2) …(Formula 1)

[0057] The reading unit 730 determines the range from which image data is read based on the amount of skew calculated by the skew amount detection unit 720, and reads image data from the memory 715 based on the determined range. The range from which image data is read is determined by adding the amount of deviation due to the amount of skew to the range in the main scanning direction set in advance. For example, if the range in the main scanning direction set in advance is A to B, and the amount of deviation due to the amount of skew is a, the range to be read is A+a to B+a. The amount of deviation a due to the amount of skew is expressed as a=b×(DC), where C is the sub-scanning coordinate of the left edge coordinate, D is the sub-scanning coordinate of the density patch, and b is the amount of skew.

[0058] The average luminance calculation unit 740 calculates an average luminance value for each patch image from each of the RGB image data read by the reading unit 730. When the test image is composed of seven patch images as in Fig. 8, the average luminance calculation unit 740 calculates seven average luminance values ​​for each of RGB, thereby calculating a total of 21 average luminance values.

[0059] 10 is a flowchart showing the image density detection process of S505 by the density detection processing unit 305. This process is performed every line period.

[0060] The density detection processing unit 305 initializes a count value M of the patch images to 0 (S101). The count value M of the patch images is used to grasp the number of patch images for which image density detection has been performed. When the count value M reaches the number of patch images formed on one sheet of recording paper, the image density detection process ends.

[0061] The density detection processing unit 305 initializes a line count value H to 0 (S102). The line count value H is used to grasp the detection position of the test images of each color of yellow, magenta, cyan, and black. When the line count value H reaches the number of lines of the patch image, reading of one patch image is completed.

[0062] When the initialization of the count value M and the line count value H of the patch image is completed, the line sensor units 312a and 312b read the detection image under the control of the controller 110. The density detection processing unit 305 acquires image data from the line sensor units 312a and 312b (S103).

[0063] The density detection processing unit 305 extracts and stores the luminance value and the skew amount from the acquired image data by the luminance value storage unit 710 and the skew amount detection unit 720 (S104). As described above, the luminance value in the area determined by the luminance value storage area determination unit 713 is stored in the memory 715, and the left edge coordinate (skew amount) in the area determined by the left edge coordinate storage area determination unit 721 is stored in the memory 723.

[0064] The density detection processing unit 305 increments the line count value H by 1 (S105). The density detection processing unit 305 determines which line has been detected since the start of image data acquisition from the line count value H. The density detection processing unit 305 determines whether the line count value H is equal to or greater than a predetermined set value (S106). The set value is set in advance and indicates the number of lines from the start of image data acquisition until the reading of one patch image is reliably completed. In other words, when the line count value H is equal to or greater than the set value, it means that the reading of one patch image of each color on the recording paper has been completed. If the line count value H is less than the set value (S106:N), the processes of S103 to S106 are repeated until the line count value H is equal to or greater than the set value.

[0065] When the line count value H becomes equal to or greater than the set value (S106: Y), the density detection processing unit 305 calculates the amount of skew of the detection image using the skew amount calculation unit 724 (S107). The density detection processing unit 305 determines the range of image data to be read based on the amount of skew calculated by the density detection processing unit 305 using the readout unit 730, and reads image data (brightness value) within the determined range from the memory 715 (S108). The density detection processing unit 305 calculates the average value (average brightness value) of the brightness values ​​of each patch image from the image data read out by the readout unit 730 using the average brightness calculation unit 740 (S109). The density detection processing unit 305 increments the count value M of the patch image by 1 (S110).

[0066] This completes the calculation process of the average luminance value of one patch image. The density detection processing unit 305 determines whether the count value M of the patch images is equal to a set value that is the number of patch images in the detection image (S111). If they are not equal (S111: N), the processes of S102 to S111 are repeated until the count value M of the patch images becomes equal to the number of patch images in the detection image.

[0067] When the count value M of the patch images becomes equal to the number of patch images in the detection image (S111: Y), the calculation process of the average luminance value for all patch images on the recording paper is completed, and the density detection process ends. The average luminance value for each patch image calculated by the density detection processing unit 305 is transmitted to the controller 110.

[0068] As described above, when the image forming apparatus 100 of the present embodiment prints a detection image and a user image on both sides of a recording sheet, the area on the back side of the detection image is blanked out or the image density is made constant, thereby stabilizing the accuracy of the detection result of the detection image. By stabilizing the detection result, the image forming conditions by the image forming apparatus 100 can be appropriately adjusted. As a result, the image forming apparatus 100 can provide a product with stable image quality.

[0069] In the present embodiment, the printer 150 and the reader 160 are configured separately, but they may be configured as one unit. For example, the conveying path 135 and the discharging path 139 may be provided with the line sensor units 312a and 312b.

Claims

1. A receiving means for receiving image data; an image forming means for forming an image on a sheet based on image forming conditions; a reading means for reading a test image formed on a sheet by the image forming means; a generating unit that generates the image forming conditions based on a reading result of the test image read by the reading unit, the image forming means forms a first image corresponding to the image data and a first test image for generating the image forming conditions on a first surface of the sheet, and forms a second image corresponding to the image data on a second surface opposite to the first surface of the sheet, without forming a second test image for generating the image forming conditions; An image forming apparatus characterized in that, when the first test image is formed on the first surface of the sheet and the second image is formed on the second surface of the sheet, the image forming means does not form the second image in a second area of ​​the second surface corresponding to a first area of ​​the first test image formed on the first surface, and forms an image in which the image in the second area has been removed.

2. A receiving means for receiving image data; an image forming means for forming an image on a sheet based on image forming conditions; a reading means for reading a test image formed on a sheet by the image forming means; a generating unit that generates the image forming conditions based on a reading result of the test image read by the reading unit, the image forming means forms a first image corresponding to the image data and a first test image for generating the image forming conditions on a first surface of the sheet, and forms a second image corresponding to the image data on a second surface opposite to the first surface of the sheet, without forming a second test image for generating the image forming conditions; An image forming apparatus characterized in that, when the first test image is formed on the first surface of the sheet and the second image is formed on the second surface of the sheet, the image forming means does not form the second image in a second area of ​​the second surface corresponding to a first area of ​​the first test image formed on the first surface, but forms an image in which the second area is replaced with a blank area in which no image is formed.

3. the image forming apparatus has a transport path for transporting the sheet on which the test image has been formed from an image forming position where the image forming means forms an image on the sheet to a reading position where the reading means reads the image on the sheet, 3. The image forming apparatus according to claim 1, wherein the reading means reads the test image on the sheet conveyed to the conveying path.

4. 3. The image forming apparatus according to claim 1, wherein the reading means reads the test image from the sheet being conveyed.

5. the image forming unit has a fixing unit for fixing the image on the sheet by heating the image, 3. The image forming apparatus according to claim 1, wherein the reading unit reads the test image downstream of the fixing unit in the conveying direction in which the sheet is conveyed.

6. 3. The image forming apparatus according to claim 1, wherein the test image is formed in an edge area on the sheet in a conveying direction in which the sheet is conveyed.

7. 3. The image forming apparatus according to claim 1, wherein the test image is formed in an edge area on the sheet in a direction perpendicular to a conveying direction in which the sheet is conveyed.

8. 3. The image forming apparatus according to claim 1, wherein the image forming unit forms the first image on the first side so as not to overlap the first test image in a double-sided printing job in which images are formed on both sides of a sheet.

9. A method for controlling an image forming apparatus that forms an image on a sheet based on image forming conditions, comprising: A receiving step of receiving image data; an image forming step of forming an image and a test image on a sheet; a reading step of reading the test image on the sheet; a generating step of generating the image forming conditions based on a reading result of the test image read in the reading step, In the image forming step, a first image corresponding to the image data and a first test image for generating the image forming conditions are formed on a first surface of the sheet, and a second image corresponding to the image data is formed on a second surface of the sheet opposite to the first surface, but a second test image for generating the image forming conditions is not formed; A control method characterized in that, when the first test image is formed on the first side of the sheet and the second image is formed on the second side of the sheet, the second image is not formed in a second area of ​​the second side corresponding to a first area of ​​the first test image formed on the first side, and an image with the image of the second area removed is formed.

10. A method for controlling an image forming apparatus that forms an image on a sheet based on image forming conditions, comprising: A receiving step of receiving image data; an image forming step of forming an image and a test image on a sheet; a reading step of reading the test image on the sheet; a generating step of generating the image forming conditions based on a reading result of the test image read in the reading step, In the image forming step, a first image corresponding to the image data and a first test image for generating the image forming conditions are formed on a first surface of the sheet, and a second image corresponding to the image data is formed on a second surface of the sheet opposite to the first surface, but a second test image for generating the image forming conditions is not formed; A control method characterized in that, when the first test image is formed on the first side of the sheet and the second image is formed on the second side of the sheet, the second image is not formed in a second area of ​​the second side corresponding to a first area of ​​the first test image formed on the first side, and an image is formed in which the second area is replaced with a blank area in which no image is formed.

11. 11. The control method according to claim 9, wherein the reading step reads the test image on the sheet while conveying the sheet.

12. 11. The control method according to claim 9, wherein the image forming step forms the test image in an edge region on the sheet in a conveying direction in which the sheet is conveyed.

13. 11. The control method according to claim 9, wherein the image forming step forms the test image in an edge area on the sheet in a direction perpendicular to a conveying direction in which the sheet is conveyed.

14. The control method according to claim 9 or 10, wherein the image forming step forms the first image on the first side so as not to overlap the first test image in a double-sided printing job in which images are formed on both sides of a sheet.

Images