Image forming apparatus

Abstract

When adjusting the light emission timing of each light-emitting element based on the density of the correction image, it is determined whether or not the image density is lower than expected. [Solution] The control unit determines the deviation of the writing position based on the density of the correction image, and performs a correction process to correct the deviation of the writing position by adjusting the light emission timing of each light-emitting element based on the determined deviation of the writing position. When performing the correction process, the control unit performs at least one of the first process and the second process. The control unit performs a first process to determine whether or not an image density unevenness has occurred on the scanned surface based on the density of the first image. The control unit performs a second process to determine whether or not an abnormality in light intensity has occurred in any of the multiple light-emitting elements based on the density of the second image.

Description

Technical Field

[0006] ,

[0007] ,

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

Background Art

[0002] An electrophotographic image forming apparatus includes an optical scanning device. The optical scanning device forms an electrostatic latent image on a scanned surface by scanning the scanned surface with a light beam. The electrostatic latent image on the scanned surface is developed into a toner image, and the toner image is transferred onto a sheet.

[0003] For example, a multi-beam type optical scanning device is installed in an image forming apparatus. The multi-beam type image forming apparatus scans a scanned surface with a plurality of light beams. Thereby, since exposure for a plurality of lines can be performed in one scan, high-speed printing can be achieved. Such an optical scanning device is disclosed in Patent Document 1.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] In Patent Document 1, a plurality of light emitting elements are arranged in a row at a constant interval with a predetermined angle with respect to the main scanning direction. Then, by adjusting the light emission timing of each light emitting element, the writing positions of each light emitting element in the main scanning direction are aligned.

[0006] For example, the interval in the main scanning direction (beam spot interval) of the beam irradiation position of each light emitting element with respect to the scanned surface may be deviated. In this case, if the light emission timing of each light emitting element is left as the initial setting, a deviation occurs in the writing position of each light emitting element in the main scanning direction, and the image quality deteriorates.

[0007] Therefore, in Patent Document 1, a correction image (evaluation chart) is formed, and the beam spot spacing is determined based on the density of the correction image. Then, the light emission timing of each light-emitting element is adjusted based on the determined beam spot spacing. This corrects the deviation in the output position of each light-emitting element in the main scanning direction.

[0008] For example, if an abnormality occurs during the development process, the image density may become lower than expected. In this state, the beam spot spacing cannot be determined accurately. In other words, there will be a difference between the beam spot spacing determined based on the density of the correction image and the actual beam spot spacing. As a result, the timing of each light-emitting element may be adjusted unnecessarily, which can lead to a decrease in image quality. Therefore, when adjusting the timing of each light-emitting element (i.e., correction processing), it is necessary to determine whether the image density is lower than expected.

[0009] The present invention was made to solve the above problems, and aims to provide an image forming apparatus that can determine whether or not the image density is lower than expected during the adjustment, in a configuration in which the light emission timing of each light-emitting element is adjusted based on the density of a correction image. [Means for solving the problem]

[0010] To achieve the above objective, one aspect of the present invention provides an optical scanning apparatus comprising: an image carrier having a surface to be scanned; a plurality of light-emitting elements arranged in a line at a predetermined angle with respect to the main scanning direction and at regular intervals, wherein the optical scanning apparatus forms an electrostatic latent image on the surface to be scanned by scanning the surface with light beams emitted from the plurality of light-emitting elements; a developing apparatus that forms a toner image by developing the electrostatic latent image with toner; and a control unit that performs a correction process to correct the deviation of the writing position of the plurality of light-emitting elements in the main scanning direction relative to the surface to be scanned. The control unit causes the optical scanning apparatus and the developing apparatus to form a correction image, which is a toner image used for the correction process, determines the deviation of the writing position based on the density of the correction image, and performs a correction process to correct the deviation of the writing position by adjusting the light emission timing of the plurality of light-emitting elements based on the determined deviation of the writing position. When performing the correction process, the control unit performs at least one of the first process and the second process. The control unit causes the optical scanning device and the developing device to form a first image, which is a toner image used in the first process, and performs a first process in which it determines whether or not there is unevenness in image density on the scanned surface based on the density of the first image. The control unit causes the optical scanning device and the developing device to form a second image, which is a toner image used in the second process, and performs a second process in which it determines whether or not there is an abnormality in the amount of light in any of the multiple light-emitting elements based on the density of the second image. [Effects of the Invention]

[0011] In this invention, in a configuration in which the light emission timing of each light-emitting element is adjusted based on the density of a correction image, it is possible to determine whether or not the image density is lower than expected during the adjustment. [Brief explanation of the drawing]

[0012] [Figure 1] This is a schematic diagram of an image forming apparatus according to an embodiment. [Figure 2] This is a schematic diagram of an image forming unit including a photoreceptor drum and developing apparatus according to an embodiment. [Figure 3] This is a schematic diagram of an optical scanning device according to an embodiment. [Figure 4]It is a perspective view of a light source unit according to an embodiment. [Figure 5] It is a diagram showing the arrangement position of a light emitting element according to an embodiment. [Figure 6] It is a diagram showing the arrangement position of a correction pattern image formed by an image forming apparatus according to an embodiment. [Figure 7] It is a detailed view of a correction pattern image formed by an image forming apparatus according to an embodiment. [Figure 8] It is a diagram showing the relationship between the deviation amount and the differential density obtained by an image forming apparatus according to an embodiment. [Figure 9] It is a detailed view of a correction pattern image formed by an image forming apparatus according to an embodiment (a diagram when deviation occurs). [Figure 10] It is a diagram showing the relationship between the deviation amount and the differential density obtained by an image forming apparatus according to an embodiment (including the density straight line when image density unevenness occurs). [Figure 11] It is a diagram showing the arrangement positions of a first image and a second image formed by an image forming apparatus according to an embodiment. [Figure 12] It is a detailed view of a first image formed by an image forming apparatus according to an embodiment. [Figure 13] It is a diagram showing the relationship between the deviation amount and the differential density obtained by an image forming apparatus according to an embodiment (including the density straight line when there is an abnormal light amount in the light emitting element). [Figure 14] It is a detailed view of a second image formed by an image forming apparatus according to an embodiment.

Embodiments for Carrying Out the Invention

[0013] Hereinafter, an image forming apparatus according to an embodiment of the present invention will be described by taking a tandem type color laser printer as an example. Note that the present invention is not limited to printers and is also applicable to multifunction devices having a copying function and the like. Further, the present invention is not limited to color devices and is also applicable to monochrome devices.

[0014] <Configuration of Image Forming Apparatus> As shown in FIG. 1, the image forming apparatus 100 of the present embodiment includes a sheet conveyance path. In FIG. 1, the sheet conveyance path is schematically shown by a dashed arrow. The image forming apparatus 100 supplies the sheet S housed in the cassette CS to the sheet conveyance path and conveys it along the sheet conveyance path. Then, the image forming apparatus 100 prints an image on the conveyed sheet S.

[0015] The image forming apparatus 100 includes four image forming units 1 corresponding to cyan, magenta, yellow, and black colors respectively. The image forming apparatus 100 also includes an optical scanning device 2.

[0016] Each image forming unit 1 has a configuration as shown in FIG. 2. Each image forming unit 1 includes a photosensitive drum 11, a charging device 12, a developing device 13, and a cleaning device 14. The photosensitive drum 11 corresponds to an "image carrier". Note that the basic configurations of the respective image forming units 1 are the same as each other. For this reason, in FIG. 2, only one image forming unit 1 is illustrated for convenience.

[0017] The photosensitive drum 11 is rotatably supported. The photosensitive drum 11 has its outer peripheral surface 10 as a surface to be scanned. In the following description, the outer peripheral surface 10 of the photosensitive drum 11 is simply referred to as the surface to be scanned 10. The charging device 12 charges the surface to be scanned 10.

[0018] The optical scanning device 2 forms an electrostatic latent image on the surface to be scanned 10 by scanning and exposing the surface to be scanned 10. The configuration of the optical scanning device 2 will be described in detail later!

[0019] The developing device 13 has a developing roller 130. The developing roller 130 rotates while carrying toner on its outer peripheral surface. The developing device 13 supplies the toner on the developing roller 130 to the surface to be scanned 10. In other words, the developing device 13 supplies toner to the electrostatic latent image. As a result, the electrostatic latent image is developed, and a toner image is formed on the surface to be scanned 10. The photosensitive drum 11 rotates while carrying the toner image on the surface to be scanned 10. The cleaning device 14 cleans the surface to be scanned 10.

[0020] Furthermore, as shown in Figure 1, the image forming apparatus 100 includes a transfer unit 30 that transfers the toner image to the sheet S. The transfer unit 30 transfers the toner image formed in each image forming unit 1 to the sheet S. As will be described in detail later, toner images such as the correction image P, the first image A, and the second image B can also be transferred to the sheet S.

[0021] The transfer unit 30 includes an intermediate transfer belt 3. The intermediate transfer belt 3 is an endless belt. The intermediate transfer belt 3 is rotatably supported. The intermediate transfer belt 3 is tensioned by a plurality of tension rollers.

[0022] The transfer unit 30 is equipped with four primary transfer rollers 31, each corresponding to cyan, magenta, yellow, and black. Each primary transfer roller 31 is positioned on the inner circumference of the intermediate transfer belt 3. Each primary transfer roller 31 presses against the photoreceptor drum 11 (scanned surface 10) which carries the toner image of the corresponding color, via the intermediate transfer belt 3.

[0023] The transfer unit 30 includes a secondary transfer roller 32. The secondary transfer roller 32 presses against the outer surface of the intermediate transfer belt 3, forming a transfer nip between itself and the intermediate transfer belt 3. During printing by the image forming apparatus 100, the sheet S is transported toward the transfer nip and passes through it.

[0024] When a print request is input to the image forming apparatus 100, each image forming unit 1 forms a toner image with the corresponding color toner. That is, a toner image is formed on each scanned surface 10. Each primary transfer roller 31 primary transfers the toner image on the corresponding scanned surface 10 onto the outer surface of the intermediate transfer belt 3. The intermediate transfer belt 3 rotates carrying the toner image on its outer surface. The secondary transfer roller 32 secondary transfers the toner image from the outer surface of the intermediate transfer belt 3 to the sheet S while the sheet S is passing through the transfer nip.

[0025] The image forming apparatus 100 includes a fixing roller pair 300. The fixing roller pair 300 includes a heating roller and a pressure roller. The heating roller has a built-in heater. The pressure roller presses against the heating roller, forming a fixing nip between them.

[0026] After the toner image is transferred to the sheet S, the sheet S passes through the fixing nip. At this time, the sheet S is heated and pressurized. This fixes the toner image to the sheet S. After that, the sheet S is discharged into the discharge tray ET.

[0027] The image forming apparatus 100 includes a control unit 4. The control unit 4 includes a CPU, ASIC, and memory. The control unit 4 controls printing by the image forming apparatus 100. The control unit 4 controls the feeding of the sheet S, the driving of various rotating bodies, exposure processing, development processing, primary transfer processing, secondary transfer processing, and fixing processing.

[0028] The image forming apparatus 100 also includes a density sensor 5. The density sensor 5 detects the density of the toner image on the outer surface of the intermediate transfer belt 3. The density sensor 5 is, for example, a reflective light sensor. The density sensor 5 emits light toward the outer surface of the intermediate transfer belt 3 and receives the light reflected from the outer surface of the intermediate transfer belt 3. If a toner image is present in the detection area of ​​the density sensor 5, the density sensor 5 receives the light reflected from that toner image. The type of density sensor 5 is not particularly limited. For example, the toner image on the outer surface of the intermediate transfer belt 3 may be imaged, and the density of the toner image may be detected based on the imaged image.

[0029] The density sensor 5 is connected to the control unit 4. The density sensor 5 outputs a value to the control unit 4 corresponding to the amount of light received. The output value of the density sensor 5 (in other words, the amount of light received) changes according to the density of the toner image present in the detection area of ​​the density sensor 5. Based on the output of the density sensor 5, the control unit 4 detects the density of the toner image on the outer surface of the intermediate transfer belt 3.

[0030] <Configuration of the optical scanning device> The optical scanning device 2 has the configuration shown in Figure 3. The optical scanning device 2 includes a laser scanning optical system LS. The laser scanning optical system LS forms an electrostatic latent image on the scanned surface 10 by deflecting and scanning the light beam. The laser scanning optical system LS includes a polygon mirror PM, an optical reflection mirror RM, and lenses SL (e.g., an fθ lens, a collimator lens, and a cylindrical lens). In Figure 3, the path of the light beam is schematically shown by a dashed line.

[0031] The optical scanning device 2 includes a light-emitting unit 200. The light-emitting unit 200 includes four light source units 20 (see Figure 4). The four light source units 20 correspond to cyan, magenta, yellow, and black, respectively.

[0032] Each light source unit 20 has the configuration shown in Figure 4. The basic configuration of each light source unit 20 is the same as that of the others. Therefore, here we will focus on the configuration of one light source unit 20 and explain its configuration, and the configurations of the other light source units 20 will be omitted, with the explanation below being used as a reference.

[0033] The light source unit 20 is equipped with multiple light-emitting elements (LDs). Each of the multiple light-emitting elements (LDs) emits a light beam. That is, the light source unit 20 emits multiple light beams. For example, the number of light-emitting elements (LDs) is four. However, it is not limited to this. The number of light-emitting elements (LDs) may be eight. When there are four light-emitting elements (LDs), four light beams are emitted from the light source unit 20.

[0034] The light source unit 20 is cylindrical, with axis CA extending in the direction of light beam emission as its central axis (cylindrical axis). In the following description, the direction perpendicular to the central axis CA will be referred to as the radial direction.

[0035] The light source unit 20 includes a cylindrical holder (reference numeral omitted) with a central axis CA as its axis, and emits a light beam from its tip surface 20a. In other words, multiple light-emitting elements LDs are arranged in the holder. The multiple light-emitting elements LDs are arranged in a line at regular intervals in the radial direction.

[0036] In this configuration, with the light source unit 20 mounted on the housing of the optical scanning device 2, as shown in Figure 5, multiple light-emitting elements (LDs) are arranged in a line at a predetermined angle to the main scanning direction D1 and at regular intervals. In other words, the scanning position of each light-emitting element (LD) on the surface to be scanned 10 is arranged in the sub-scanning direction D2, which is perpendicular to the main scanning direction D1. This allows for simultaneous scanning of multiple lines on the surface to be scanned 10.

[0037] In a configuration where multiple light-emitting elements (LDs) are arranged in a line at a predetermined angle to the main scanning direction D1 and at regular intervals, the beam pitch Ls of the light beam emitted from each light-emitting element (LD) in the sub-scanning direction D2 can be adjusted by rotating the light source unit 20 around the central axis CA. The beam pitch Ls is determined based on the resolution of the image to be printed by the image forming apparatus 100. In Figure 5, rotating the light source unit 20 clockwise reduces the beam pitch Ls. Conversely, rotating the light source unit 20 counterclockwise increases the beam pitch Ls.

[0038] Furthermore, in this configuration, the position of each light-emitting element LD is shifted in the main scanning direction D1. Therefore, when scanning by each light-emitting element LD is started simultaneously, the starting position of the light beam emitted from each light-emitting element LD in the main scanning direction D1 relative to the scanned surface 10 is shifted by the beam pitch Lm.

[0039] Therefore, the control unit 4 appropriately controls the light emission timing of each light-emitting element LD. In other words, the control unit 4 appropriately controls the writing timing of the light beam of each light-emitting element LD to the scanned surface 10. The control unit 4 staggers the writing timing of each light-emitting element LD.

[0040] The light-emitting unit 200 includes multiple driver circuits 21 (see Figure 3) corresponding to multiple light-emitting elements LDs. Each driver circuit 21 is connected to the corresponding light-emitting element LD. Each driver circuit 21 supplies current to the corresponding light-emitting element LD. As a result, each driver circuit 21 causes the corresponding light-emitting element LD to emit light.

[0041] Each driver circuit 21 is connected to the control unit 4. The control unit 4 controls each driver circuit 21 so that the writing position of each light-emitting element LD in the main scanning direction D1 relative to the scanned surface 10 is the same for all of them.

[0042] Each driver circuit 21 can increase or decrease the current flowing to the corresponding light-emitting element LD. In other words, each driver circuit 21 can increase or decrease the light intensity of the corresponding light-emitting element LD. The control unit 4 controls each driver circuit 21 to adjust the light intensity of each light-emitting element LD.

[0043] In the following explanation, when it is necessary to distinguish between multiple light-emitting diodes (LDs), sequential numbers 1 to 4 will be added to the end of the code of each light-emitting diode, starting from the upstream side to the downstream side in the sub-scanning direction D2.

[0044] <Correction process> In the manufacturing line of the optical scanning device 2, an optical sensor, acting as a manufacturing jig, is used to measure the distance between the beam irradiation positions of each light-emitting element LD on the scanned surface 10 in the main scanning direction D1 (hereinafter simply referred to as the beam spot interval). In this measurement, the jig (optical sensor) is placed at a position corresponding to the arrangement position of the scanned surface 10, and an optical beam is irradiated from each light-emitting element LD toward the jig. Measurement information (beam spot interval) showing the measurement result at this time is stored in the memory of the control unit 4. Based on the measurement information, the control unit 4 controls the light emission timing of each light-emitting element LD.

[0045] For example, due to factors such as assembly tolerances, the installation position of the photoreceptor drum 11 (scanned surface 10) may deviate from its ideal position. Similarly, the installation position of the optical scanning device 2 may also deviate from its ideal position. These factors can cause a difference between the beam spot spacing indicated by the measurement information and the actual beam spot spacing. In other words, in control based on measurement information, the output position of each light-emitting element LD in the main scanning direction D1 relative to the scanned surface 10 may be shifted. This can lead to a decrease in image quality.

[0046] To suppress the occurrence of such problems, after the image forming apparatus 100 is assembled, the control unit 4 performs a predetermined correction process. By performing the correction process, the control unit 4 corrects the deviation of the writing position of each light-emitting element LD in the main scanning direction D1 relative to the scanned surface 10.

[0047] Specifically, as shown in Figures 6 and 7, first, the control unit 4 causes the image forming unit 1 (including the developing unit 13) and the optical scanning unit 2 to form the correction image P. The correction image P is a toner image used in the correction process. For example, the image forming unit 1 corresponding to black is used to form the correction image P. The optical scanning unit 2 forms an electrostatic latent image of the correction image P on the scanned surface 10. The developing unit 13 develops the electrostatic latent image on the scanned surface 10 into a toner image to form the correction image P.

[0048] During the correction process, the density of the correction image P is detected. Then, the correction process is performed based on the density of the correction image P. For this reason, the correction image P is transferred onto the outer surface of the intermediate transfer belt 3. After that, for example, the density of the correction image P is detected by the density sensor 5. Although not shown in the figure, a density sensor capable of detecting the density of the toner image on the scanned surface 10 may be installed, and the density of the correction image P on the scanned surface 10 may be detected by that density sensor.

[0049] Alternatively, after the correction image P is secondarily transferred to the sheet S, the density of the correction image P transferred to the sheet S may be detected by a density sensor, and the detected density may be used for the correction process. In this case, although not shown in the figure, a density sensor (for example, a sensor similar to density sensor 5) for detecting the correction image P secondarily transferred to the sheet S is separately installed in the image forming apparatus 100. The detection position of the density sensor is set downstream of the fixing nip in the sheet transport direction of the sheet transport path.

[0050] Furthermore, after the correction image P is secondarily transferred to the sheet S, the sheet S with the transferred correction image P may be output from the image forming apparatus 100. Then, the correction image P transferred to the sheet S may be read, and the density of the correction image P may be detected based on the image data of the correction image P obtained from the reading. The density of the correction image P can be detected based on the brightness value of the correction image P in the image data.

[0051] The image forming apparatus 100 may also be a multifunction device equipped with an image reading device that reads an image and generates image data. In this case, the image reading device of the image forming apparatus 100 may be used to read the correction image P transferred to the sheet S, and the density of the correction image P may be detected based on the image data of the correction image P obtained from the reading.

[0052] Alternatively, the correction image P transferred to the sheet S may be read using an image reading device separate from the image forming apparatus 100, and the density of the correction image P may be detected based on the image data of the correction image P obtained from the reading. When using an image reading device separate from the image forming apparatus 100, after detecting the density of the correction image P, the density data of the correction image P can be input to the image forming apparatus 100, thereby causing the control unit 4 to detect the density of the correction image P.

[0053] The correction image P is an image used to detect the beam spot distance between two adjacent light-emitting elements LDs on the scanned surface 10 in the sub-scanning direction D2. Specifically, the correction image P includes image P1 for detecting the beam spot distance between light-emitting elements LD4 and LD1, image P2 for detecting the beam spot distance between light-emitting elements LD1 and LD2, image P3 for detecting the beam spot distance between light-emitting elements LD2 and LD3, and image P4 for detecting the beam spot distance between light-emitting elements LD3 and LD4. In the correction process, images P1, P2, P3, and P4 are formed in that order.

[0054] The correction image P includes multiple correction pattern images PT. These multiple correction pattern images PT are arranged in the main scanning direction D1 and the sub-scanning direction D2. Six correction pattern images PT constitute one set of images. For example, multiple images composed of six correction pattern images PT are arranged in the main scanning direction D1. Correction processing is performed using at least one of the multiple images arranged in the main scanning direction D1.

[0055] Here, we will explain in detail using image P2, which is used to detect the beam spot distance between light-emitting elements LD1 and LD2, as an example. In the following, when it is necessary to distinguish between the six correction pattern images PT that make up a set of images, we will add sequential numbers from 1 to 6 to the end of the code of each of the six correction pattern images PT in the explanation.

[0056] Correction pattern images PT1 and PT2 are adjacent to each other in the main scanning direction D1. Correction pattern images PT3 and PT4 are adjacent to each other in the main scanning direction D1. Correction pattern images PT5 and PT6 are adjacent to each other in the main scanning direction D1. In addition, correction pattern images PT1, PT3, and PT5 are arranged in this order in the sub-scanning direction D2. Correction pattern images PT2, PT4, and PT6 are arranged in this order in the sub-scanning direction D2.

[0057] Each of the six correction pattern images PT contains multiple (six) patch images PC. Each patch image PC consists of a first sub-image G1 and a second sub-image G2, each having a length of 3 pixels in the main scanning direction D1. The first sub-image G1 is obtained by developing the electrostatic latent image formed by scanning exposure with the light-emitting element LD1. The second sub-image G2 is obtained by developing the electrostatic latent image formed by scanning exposure with the light-emitting element LD2.

[0058] Correction pattern images PT3 and PT4 are symmetrical to each other. In correction pattern image PT1, the second sub-image G2 is shifted by one pixel to the negative (left) side of the main scanning direction D1 compared to correction pattern image PT3. In correction pattern image PT2, the second sub-image G2 is shifted by one pixel to the negative (left) side of the main scanning direction D1 compared to correction pattern image PT4. In correction pattern image PT5, the second sub-image G2 is shifted by one pixel to the positive (right) side of the main scanning direction D1 compared to correction pattern image PT3. In correction pattern image PT6, the second sub-image G2 is shifted by one pixel to the positive (right) side of the main scanning direction D1 compared to correction pattern image PT4.

[0059] After the formation of the correction image P, the control unit 4 detects the density of the correction image P based on the output of the density sensor 5. Hereinafter, the densities of the correction pattern images PT1, PT2, PT3, PT4, PT5, and PT6 will be denoted as Dn1, Dn2, Dn3, Dn4, Dn5, and Dn6, respectively. The difference density obtained by subtracting density Dn1 from density Dn2 will be referred to as the "upper difference density (Dupper)". The difference density obtained by subtracting density Dn3 from density Dn4 will be referred to as the "middle difference density (Dmid)". The difference density obtained by subtracting density Dn5 from density Dn6 will be referred to as the "lower difference density (Dlower)".

[0060] If there is no misalignment in the writing position of the main scanning direction D1 with respect to the scanned surface 10 between the light-emitting element LD1 and the light-emitting element LD2 (ideally), a correction image P as shown in Figure 7 is formed.

[0061] In this case, the patch image PC of the correction pattern image PT1 is smaller in the main scanning direction D1 compared to the patch image PC of the correction pattern image PT2, and the first small image G1 and the second small image G2 are more densely packed, resulting in a higher density of the patch image PC. In other words, the density of the correction pattern image PT1 is higher than that of the correction pattern image PT2.

[0062] The density of the patch image PC in the correction pattern image PT3 will be approximately the same as the density of the patch image PC in the correction pattern image PT4. In other words, the density of the correction pattern image PT3 will be approximately the same as the density of the correction pattern image PT4.

[0063] The patch image PC of correction pattern image PT5 has a lower density compared to the patch image PC of correction pattern image PT6, because the first small image G1 and the second small image G2 are scattered more widely in the main scanning direction D1. In other words, the density of correction pattern image PT5 is lower than that of correction pattern image PT6.

[0064] As a result, the relationship between the amount of deviation (X axis) and the difference density (Y axis) is as shown in Figure 8 (solid line). When the three points of the difference density (Dupper) in the upper row, the difference density (Dmid) in the middle row, and the difference density (Dlower) in the lower row are plotted and connected by a straight line (hereinafter referred to as the density line), the intersection of the density line and the X axis is approximately 0. In other words, when the intersection of the density line and the X axis is approximately 0, it can be said that there is no deviation in the writing position of the main scanning direction D1 with respect to the scanned surface 10 between the light-emitting element LD1 and the light-emitting element LD2.

[0065] On the other hand, if there is a discrepancy in the output position of the main scanning direction D1 relative to the scanned surface 10 between the light-emitting element LD1 and the light-emitting element LD2, a correction image P, such as the one shown in Figure 9, is formed. Here, we assume that the output position of the light-emitting element LD2 relative to the scanned surface 10 in the main scanning direction D1 is shifted by 1 pixel to the negative side (left side) of the main scanning direction D1.

[0066] If there is a shift in the output position (see Figure 9), the density Dn1 of correction pattern image PT1 will be higher and the density Dn2 of correction pattern image PT2 will be lower compared to when there is no shift in the output position (see Figure 5). The density Dn3 of correction pattern image PT3 will be higher and the density Dn4 of correction pattern image PT4 will be lower. The density Dn5 of correction pattern image PT5 will be higher and the density Dn6 of correction pattern image PT6 will be lower. As a result, the difference density in the upper row (=Dn2-Dn1) will be smaller, the difference density in the middle row (=Dn4-Dn3) will be smaller, and the difference density in the lower row (=Dn6-Dn5) will be smaller.

[0067] As a result, the relationship between the amount of shift (X-axis) and the difference in density (Y-axis) is shown in Figure 8 (dashed line). When there is no shift in the output position, the intersection of the density line (solid line) and the X-axis is approximately 0, whereas when there is a shift in the output position, the intersection of the density line (dashed line) and the X-axis shifts by approximately 0 to +1 pixels. From this, it can be concluded that the value obtained by inverting the sign of the X-coordinate of the intersection of the density line and the X-axis corresponds to the amount of shift in the output position.

[0068] Note that the method for calculating the deviation in the writing position (the method for detecting the beam spot interval) described here is just one example. Other methods may be used to determine the deviation in the writing position.

[0069] The control unit 4 determines the beam spot spacing between light-emitting element LD4 and light-emitting element LD1, between light-emitting element LD1 and light-emitting element LD2, between light-emitting element LD2 and light-emitting element LD3, and between light-emitting element LD3 and light-emitting element LD4. Based on each beam spot spacing, the control unit 4 adjusts the light emission timing of each light-emitting element LD so that the writing position of each light-emitting element LD in the main scanning direction D1 relative to the scanned surface 10 is aligned.

[0070] <First Processing> In the development process, which supplies toner to the electrostatic latent image on the scanned surface 10, the thickness of the toner layer on the developing roller 130 is restricted by a blade. In this configuration, foreign matter may get caught in the blade. If foreign matter gets caught in the blade, an abnormality occurs in the toner supply, resulting in insufficient toner supply in a part of the scanned surface 10. In other words, uneven image density occurs, and the image quality deteriorates.

[0071] For example, suppose an abnormality occurs in the toner supply at the location indicated by the white arrow in Figure 7 and its surrounding area. In this state, if we attempt to form the correction image P shown in Figure 7, the density of the correction pattern images PT1, PT3, and PT5 will be lower than expected, regardless of the beam spot spacing.

[0072] In this example, assume that there is no misalignment in the writing position of each light-emitting element LD in the main scanning direction D1 relative to the scanned surface 10. In this case, the upper differential density (Dupper), middle differential density (Dmid), and lower differential density (Dlower) will each be higher than the differential density when the toner supply is normal. As a result, as shown in Figure 10, when the toner supply is normal, the intersection point of the density line (solid line) and the X-axis is approximately 0, whereas when there is an abnormality in the toner supply, the intersection point of the density line (dashed line) and the X-axis shifts towards the negative direction.

[0073] If correction processing is performed based on this amount of deviation, the light emission timing of each light-emitting diode (LD) will be unnecessarily adjusted. As a result, the image quality will be worse than before the correction processing.

[0074] Therefore, when performing correction processing, the control unit 4 performs a first process. The first process is to determine whether or not image density unevenness occurs on the scanned surface 10 in the main scanning direction D1. This makes it possible to determine whether or not the image density is lower than expected when adjusting the light emission timing of each light-emitting element LD based on the density of the correction image P, that is, when performing correction processing.

[0075] When performing a correction process involving the first process, as shown in Figures 11 and 12, the control unit 4 causes the image forming unit 1 (including the developing unit 13) and the optical scanning device 2 to form the first image A in addition to the correction image P. The first image A is a toner image used in the first process. For example, the image forming unit 1 corresponding to black is used to form the first image A. The optical scanning device 2 forms an electrostatic latent image of the first image A on the scanned surface 10. The developing device 13 develops the electrostatic latent image on the scanned surface 10 into a toner image to form the first image A.

[0076] In the first process, the density of the first image A is detected. Then, the first process is performed based on the density of the first image A. As a result, the first image A is transferred onto the outer surface of the intermediate transfer belt 3. After that, for example, the density of the first image A is detected by the density sensor 5. However, although not shown in the figure, a density sensor capable of detecting the density of the toner image on the scanned surface 10 may be installed, and the density of the first image A on the scanned surface 10 may be detected by that density sensor.

[0077] Alternatively, the density of the first image A may be detected in the same manner as the density detection method for the correction image P. For example, the first image A may be transferred to a sheet S and output to the outside of the machine, the first image A transferred to the sheet S may be read by an image reader, and the density of the first image A may be detected based on the image data of the first image A obtained by the reading. The density detection method for the first image A is the same as the density detection method for the correction image P. Therefore, a detailed explanation of the density detection method for the first image A will be omitted, as it will refer to the explanation of the density detection method for the correction image P.

[0078] Here, multiple first images A are formed. These multiple first images A are arranged in the main scanning direction D1. The number of first images A formed is the same as the number of correction pattern images PT arranged in the main scanning direction D1. In the following, when it is necessary to distinguish between multiple first images A, each of the multiple first images A will be given a sequential number from 1 to 8 at the end of its code.

[0079] Each of the multiple correction pattern images PT is positioned in the same location relative to one of the first images A in the main scanning direction D1. In the example shown in Figure 11, the multiple correction pattern images PT in column a are positioned in the same location relative to the first image A1 in the main scanning direction D1. The multiple correction pattern images PT in column b are positioned in the same location relative to the first image A2 in the main scanning direction D1. The multiple correction pattern images PT in column c are positioned in the same location relative to the first image A3 in the main scanning direction D1. The multiple correction pattern images PT in column d are positioned in the same location relative to the first image A4 in the main scanning direction D1. The multiple correction pattern images PT in column e are positioned in the same location relative to the first image A5 in the main scanning direction D1. The multiple correction pattern images PT in column f are positioned in the same location relative to the first image A6 in the main scanning direction D1. The multiple correction pattern images PT in column g are positioned in the same location relative to the first image A7 in the main scanning direction D1. The multiple correction pattern images PT in column h are positioned in the same location as the first image A8 and in the main scanning direction D1.

[0080] Note that multiple instances of the first image A are identical to one another. That is, ideally, the density of each instance of the first image A will be the same. The content of the first image A is not particularly limited; it just needs to be that the area ratio of the printed dots is not 100% (i.e., it just needs to be that it is not a solid image).

[0081] The control unit 4 detects the density of each of the multiple first images A based on the output of the density sensor 5. The control unit 4 also determines the density difference (its absolute value) between two first images A adjacent to each other in the main scanning direction D1, and determines whether the determined density difference exceeds a predetermined first threshold. The control unit 4 then determines that image density unevenness has occurred in the main scanning direction D1 if the density difference between two first images A adjacent to each other in the main scanning direction D1 exceeds the first threshold. In other words, if the density of one of the two first images A adjacent to each other in the main scanning direction D1 has decreased due to an abnormality in toner supply, it is determined that image density unevenness has occurred.

[0082] In the example shown in Figure 11, the density differences between the first image A1 and the first image A2, between the first image A3 and the first image A4, between the first image A5 and the first image A6, and between the first image A7 and the first image A8 are calculated, and each of these density differences is compared with the first threshold. If any of the density differences exceed the first threshold, it is determined that there is an unevenness in image density.

[0083] When image density unevenness occurs in the main scanning direction D1, it is not possible to accurately determine the amount of deviation in the output position in the main scanning direction D1 (i.e., the beam spot spacing). Therefore, if correction processing is performed when image density unevenness occurs in the main scanning direction D1, the image quality may be lower than before the correction processing.

[0084] Therefore, if the first processing determines that image density unevenness has occurred in the main scanning direction D1, the control unit 4 stops the correction processing. This suppresses the deterioration of image quality caused by unnecessary adjustment of the light emission timing of each light-emitting element LD.

[0085] However, this is not limited to this. Depending on the extent of the image density unevenness, the system may be configured to continue the correction process.

[0086] Specifically, when the first processing determines that image density unevenness occurs in the main scanning direction D1, the control unit 4 recognizes the positions in the main scanning direction D1 of the two first images A whose density difference exceeds the first threshold as density unevenness locations. Then, the control unit 4 does not use the correction pattern image PT in which the position in the main scanning direction D1 is a density unevenness location in the correction processing, and performs the correction processing based on the density of the correction pattern image PT in which the position in the main scanning direction D1 is not a density unevenness location.

[0087] For example, in the example shown in Figure 11, suppose the density difference between the first image A1 and the first image A2 exceeds the first threshold. In this case, the position of each of the first images A1 and A2 in the main scanning direction D1 is recognized as the density unevenness location. Therefore, the correction pattern images PT in columns a and b are not used in the correction process, and the correction process is performed based on the respective densities of the correction pattern images PT in columns c to h.

[0088] In this configuration, even if correction processing is performed when image density unevenness occurs, it is possible to suppress a decrease in image quality compared to before the correction processing. Furthermore, in this configuration, since correction processing is performed, it is possible to suppress the decrease in image quality caused by the deviation of the write position in the main scanning direction D1.

[0089] <Second Processing> The light intensity of a malfunctioning light-emitting diode (LD) decreases from its initial value. In other words, the light intensity of a malfunctioning LD decreases more than that of other LDs. Therefore, if a light intensity anomaly (i.e., a decrease in light intensity) occurs in any of the multiple LDs, the image density will be lower than expected.

[0090] For example, suppose an abnormality in light intensity occurs in the light-emitting element LD1. If we try to form the correction image P shown in Figure 7 under these conditions, the density of the first sub-image G1 will decrease. In other words, the density of all correction pattern images PT1 to PT6 will decrease.

[0091] In this example, let's assume that toner was not applied to the formation position of the first small image G1. In this case, the densities of the correction pattern images PT1 to PT6 will be approximately the same. As a result, as shown in Figure 13, when all light-emitting elements LDs are functioning normally, a density straight line like the solid line is obtained, whereas when there is an abnormality in light-emitting element LD1, the upper differential density (Dupper), middle differential density (Dmid), and lower differential density (Dlower) all become approximately 0.

[0092] This result does not allow us to determine the amount of deviation in the writing position in the main scanning direction D1. In other words, the correction process cannot be performed accurately.

[0093] Therefore, when performing correction processing, the control unit 4 performs a second process. The second process is to determine whether or not an abnormality in light intensity has occurred in any of the multiple light-emitting elements LD. This makes it possible to determine whether or not the image density is lower than expected when adjusting the light emission timing of each light-emitting element LD based on the density of the correction image P, that is, when performing correction processing.

[0094] When performing a correction process involving a second process, as shown in Figures 11 and 14, the control unit 4 causes the image forming unit 1 (including the developing unit 13) and the optical scanning device 2 to form a second image B in addition to the correction image P. The second image B is a toner image used in the second process. For example, the image forming unit 1 corresponding to black is used to form the second image B. The optical scanning device 2 forms an electrostatic latent image of the second image B on the scanned surface 10. The developing device 13 develops the electrostatic latent image on the scanned surface 10 into a toner image to form the second image B.

[0095] In the second process, the density of the second image B is detected. Then, the second process is performed based on the density of the second image B. As a result, the second image B is transferred onto the outer surface of the intermediate transfer belt 3. After that, for example, the density of the second image B is detected by the density sensor 5. However, although not shown in the figure, a density sensor capable of detecting the density of the toner image on the scanned surface 10 may be installed, and the density of the second image B on the scanned surface 10 may be detected by that density sensor.

[0096] Alternatively, the density of the second image B may be detected in the same manner as the density detection method for the correction image P. For example, the second image B may be transferred to a sheet S and output to the outside of the machine, the second image B transferred to the sheet S may be read by an image reader, and the density of the second image B may be detected based on the image data of the second image B obtained from the reading. The density detection method for the second image B is the same as the density detection method for the correction image P. Therefore, a detailed explanation of the density detection method for the second image B will be omitted, as it will refer to the explanation of the density detection method for the correction image P.

[0097] Here, multiple images B are formed. Each of the multiple images B corresponds to a different light-emitting element LD. In other words, there are four types of images B. In the following explanation, when it is necessary to distinguish between the multiple images B, the number 1 will be added to the end of the code of the image B corresponding to light-emitting element LD1, the number 2 to the end of the code of the image B corresponding to light-emitting element LD2, the number 3 to the end of the code of the image B corresponding to light-emitting element LD3, and the number 4 to the end of the code of the image B corresponding to light-emitting element LD4.

[0098] Multiple second images B are obtained by developing the electrostatic latent image formed solely by scanning the corresponding light-emitting element LD with its light beam. For example, by illuminating and extinguishing the light-emitting elements LD1, LD2, LD3, and LD4 in this order, the electrostatic latent images of second images B1, B2, B3, and B4 are formed. Note that the number of second images B formed corresponding to each light-emitting element LD may be one or more. Figure 11 illustrates an example where two second images B are formed corresponding to each light-emitting element LD.

[0099] The control unit 4 detects the density of each of the multiple second image B based on the output of the density sensor 5. The control unit 4 also calculates the average value of the densities of the multiple second image B (hereinafter referred to as the "average density value"). Furthermore, for each of the multiple second image B, the control unit 4 determines whether the difference (absolute value) between the density of the second image B and the average density value exceeds a predetermined second threshold. The control unit 4 determines that a light intensity abnormality has occurred in the light-emitting element LD corresponding to the second image B whose difference from the average density value exceeds the second threshold.

[0100] For example, if a light intensity anomaly occurs in the light-emitting element LD1, the density of the second image B1 will decrease. Therefore, the difference between the density of the second image B1 and the average density value will exceed the second threshold. As a result, it is determined that a light intensity anomaly has occurred in the light-emitting element LD1.

[0101] When any of the light-emitting elements (LDs) experience an abnormal light intensity, it becomes impossible to accurately determine the amount of deviation in the output position in the main scanning direction D1 (i.e., the beam spot spacing). Therefore, if correction processing is performed when any of the light-emitting elements (LD1) experience an abnormal light intensity, the image quality may be reduced compared to before the correction processing.

[0102] Therefore, if the second processing determines that an abnormality in light intensity has occurred in any of the light-emitting LDs (i.e., there exists a second image B in which the difference from the average density value exceeds the second threshold), the control unit 4 stops the correction processing. This suppresses the deterioration of image quality caused by unnecessary adjustment of the light emission timing of each light-emitting LD.

[0103] However, this is not limited to this. The system may be configured to continue the correction process even if an abnormality in light intensity occurs in any of the light-emitting LDs.

[0104] Specifically, if the second processing determines that a light intensity abnormality has occurred in any of the light-emitting element LDs, the control unit 4 performs a light intensity adjustment process to adjust the light intensity of the light-emitting element LD experiencing the light intensity abnormality. Hereinafter, the light-emitting element LD subject to the light intensity adjustment process (i.e., the light-emitting element LD experiencing the light intensity abnormality) will be referred to as the target light-emitting element LD.

[0105] The control unit 4 increases the density of the second image B corresponding to the target light-emitting element LD by performing a light intensity adjustment process. In other words, the control unit 4 increases the light intensity of the target light-emitting element LD by performing a light intensity adjustment process. As a result, the control unit 4 makes the density of the second image B corresponding to the target light-emitting element LD the same as the average density value. After performing the light intensity adjustment process, the control unit 4 again causes the image forming unit 1 (including the developing device 13) and the optical scanning device 2 to form a correction image P and performs a correction process.

[0106] In this configuration, even if an abnormality in light intensity occurs in any of the light-emitting diodes (LDs), the light intensity adjustment process is performed on that LD, making it possible to determine the amount of deviation in the write position in the main scanning direction D1. In other words, even if an abnormality in light intensity occurs in any of the light-emitting diodes (LDs), correction processing can be performed if necessary.

[0107] However, in some cases, the light intensity of the target light-emitting element LD may decrease to a degree that cannot be addressed by the light intensity adjustment process. In such cases, it is preferable to discontinue the correction process. Therefore, the following process may be additionally performed as a second process. Alternatively, only the following process may be performed as a second process.

[0108] Specifically, the control unit 4 determines whether the density of each of the multiple second images B falls below a predetermined lower density value. The control unit 4 determines that a light intensity abnormality has occurred in the light-emitting element LD corresponding to the second image B whose density falls below the lower density value.

[0109] When the control unit 4 determines that an abnormality in light intensity has occurred in any of the light-emitting LDs (i.e., a second image B exists with a light intensity below the lower limit), it stops the correction process. This suppresses the degradation of image quality caused by unnecessary adjustment of the light emission timing of each light-emitting LD.

[0110] The embodiments disclosed herein should be considered in all respects to be illustrative and not restrictive. The scope of the present invention is indicated by the claims rather than by the description of the embodiments above, and furthermore, all modifications within the meaning and scope equivalent to the claims are included. [Explanation of Symbols]

[0111] 2. Optical scanning device 4. Control Unit 10 Scanned surface 11. Photosensitive drum (image carrier) 13. Developing device 30 Transfer section 100 Image forming apparatus A First image B. Second image D1 Main scanning direction D2 Sub-scanning direction LD light-emitting element Image for P correction PT correction pattern image S Seat

Claims

1. An image carrier having a scanning surface, An optical scanning device having a plurality of light-emitting elements arranged in a line at a predetermined angle with respect to the main scanning direction and at regular intervals, and which scans the surface to be scanned with light beams emitted from the plurality of light-emitting elements to form an electrostatic latent image on the surface to be scanned, A developing apparatus that develops the electrostatic latent image with toner to form a toner image, The system includes a control unit that performs a correction process to correct the deviation of the writing position of a plurality of light-emitting elements in the main scanning direction relative to the scanned surface, The control unit causes the optical scanning device and the developing device to form a correction image, which is the toner image used in the correction process, to determine the deviation of the writing position based on the density of the correction image, and performs a correction process to correct the deviation of the writing position by adjusting the light emission timing of the plurality of light-emitting elements based on the determined deviation of the writing position. When performing the correction process, the control unit performs at least one of the first process and the second process. The control unit causes the optical scanning apparatus and the developing apparatus to form a first image, which is the toner image used in the first process, and performs a process as the first process to determine whether or not image density unevenness occurs on the scanned surface based on the density of the first image. Image forming apparatus, wherein the control unit causes the optical scanning device and the developing device to form a second image, which is the toner image used in the second process, and performs a process as the second process to determine whether or not an abnormality in light intensity has occurred in any of the plurality of light-emitting elements based on the density of the second image.

2. Multiple images of the first image are formed, The multiple first images are arranged in the main scanning direction, The aforementioned correction image includes multiple correction pattern images, Each of the multiple correction pattern images has the same arrangement position in the main scanning direction as any of the first images. The control unit performs a first process which determines that image density unevenness has occurred when the density difference between two adjacent first images in the main scanning direction exceeds a predetermined first threshold. The image forming apparatus according to claim 1, wherein the control unit discontinues the correction process when it determines that the aforementioned image density unevenness has occurred.

3. Multiple images of the first image are formed, The multiple first images are arranged in the main scanning direction, The aforementioned correction image includes multiple correction pattern images, Each of the multiple correction pattern images has the same arrangement position in the main scanning direction as any of the first images. The control unit performs a first process which determines that image density unevenness has occurred when the density difference between two adjacent first images in the main scanning direction exceeds a predetermined first threshold. When the control unit determines that the aforementioned image density unevenness has occurred, it recognizes the arrangement positions in the main scanning direction of the two first images whose density difference exceeds the first threshold as density unevenness occurrence positions, and does not use the correction pattern image whose arrangement position in the main scanning direction is the density unevenness occurrence position in the correction process, and performs the correction process based on the density of the correction pattern image whose arrangement position in the main scanning direction is not the density unevenness occurrence position, as described in claim 1.

4. Multiple images of the second image are formed, Each of the multiple second images corresponds to a different light-emitting element, and is obtained by developing the electrostatic latent image formed solely by scanning the corresponding light-emitting element with a light beam. The control unit calculates the average density value of a plurality of second images, determines whether the difference between the density of each of the plurality of second images and the average density value exceeds a predetermined second threshold, and performs the second process which determines that the light intensity abnormality has occurred in the light-emitting element corresponding to the second image in which the difference with the average density value exceeds the second threshold. The image forming apparatus according to claim 1, wherein the control unit determines that there is a light-emitting element in which the aforementioned light intensity abnormality is occurring, and discontinues the correction process.

5. Multiple images of the second image are formed, Each of the multiple second images corresponds to a different light-emitting element, and is obtained by developing the electrostatic latent image formed solely by scanning the corresponding light-emitting element with a light beam. The control unit performs a second process which determines whether the density of each of the plurality of second images falls below a predetermined lower density value, and determines that the light intensity abnormality has occurred in the light-emitting element corresponding to the second image whose density falls below the lower density value. The image forming apparatus according to claim 1, wherein the control unit determines that there is a light-emitting element in which the aforementioned light intensity abnormality is occurring, and discontinues the correction process.

6. Multiple images of the second image are formed, Each of the multiple second images corresponds to a different light-emitting element, and is obtained by developing the electrostatic latent image formed solely by scanning the corresponding light-emitting element with a light beam. The control unit calculates the average density value of a plurality of second images, determines whether the difference between the density of each of the plurality of second images and the average density value exceeds a predetermined second threshold, and performs the second process which determines that the light intensity abnormality has occurred in the light-emitting element corresponding to the second image in which the difference with the average density value exceeds the second threshold. The image forming apparatus according to claim 1, which, when it determines that there is a light-emitting element in which the light intensity abnormality is occurring, performs a process to adjust the light intensity of the light-emitting element in which the light intensity abnormality is occurring, then causes the optical scanning device and the developing device to form the correction image, and performs the correction process.

7. The unit includes a transfer section for transferring the toner image onto a sheet, The density of the correction image is detected based on image data obtained by reading the correction image transferred to the sheet. The density of the first image is detected based on image data obtained by reading the first image transferred to the sheet. The image forming apparatus according to claim 1, wherein the density of the second image is detected based on image data obtained by reading the second image transferred to the sheet.

Images