Image forming apparatus

Abstract

To accurately achieve correction processing that is performed by forming an image for measurement.SOLUTION: An image forming apparatus 100 has a function that can change the size of an image to be formed in a sub-scanning direction. The image forming apparatus 100 comprises an image forming unit PY that forms an image on a photoconductor drum 1Y under a predetermined image forming condition, and a control unit 110. When forming an image instructed in a print job, the control unit 110 changes the size in the sub-scanning direction of the image and causes the image forming unit PY to form the image, and when forming an image for measurement for correcting the image forming condition, causes the image forming unit PY to form the image for measurement without changing the size in the sub-scanning direction of the image for measurement.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to an image forming apparatus such as a copying machine, a multifunction machine, a printer, etc.

Background Art

[0002] An image forming apparatus employing an electrophotographic method forms an image by scanning a photosensitive drum, which is a drum-shaped photoreceptor having a photosensitive layer on its surface, with a laser beam. The image forming apparatus uniformly charges the photosensitive layer of the photosensitive drum rotating about a drum axis and then irradiates (scans) it with a laser beam to form an electrostatic latent image on the photosensitive layer of the photosensitive drum. The electrostatic latent image is developed with toner to form a toner image, which is transferred onto a sheet of paper. The sheet of paper onto which the toner image is transferred is, for example, heated and pressurized so that the toner image melts and adheres. In this way, an image is formed (printed) on the sheet of paper.

[0003] In such an image forming apparatus, uneven charging during charging of the photosensitive drum, uneven exposure during laser beam scanning, uneven development during development, etc. may occur. These unevennesses cause uneven image density in a predetermined direction of the image formed on the sheet of paper. For example, uneven image density occurs in the main scanning direction and the sub-scanning direction. The main scanning direction is the direction in which the laser beam scans the photosensitive drum and is the drum axis direction. The sub-scanning direction is the direction intersecting the main scanning direction and is the rotation direction of the photosensitive drum.

[0004] In order to correct uneven image density, a sheet of paper on which a measurement image including a pattern image for measuring uneven image density is formed by dividing the image forming range into a plurality of regions is used. Based on the measurement results of the pattern images for each region, the laser beam amount is adjusted so that there is no image density difference for each region, thereby correcting the uneven image density. For example, Patent Document 1 proposes a technique for correcting uneven image density in the main scanning direction. Patent Documents 2 and 3 propose techniques for correcting uneven image density in the sub-scanning direction.

[0005] In Patent Document 1, the unevenness of image density in the main scanning direction is corrected based on the measurement results of a plurality of pattern images arranged in the main scanning direction. In Patent Document 2, the unevenness of image density in the sub-scanning direction generated in the rotation cycle of the developing sleeve is corrected. The developing sleeve is a member that rotates in a driven manner following the rotation of the photosensitive drum and attaches toner to the electrostatic latent image. In Patent Document 3, the unevenness of image density in the sub-scanning direction is corrected based on the measurement results of the period and amplitude of the unevenness of image density in the sub-scanning direction.

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

Patent Document 2

Patent Document 3

Summary of the Invention

Problems to be Solved by the Invention

[0007] For the correction of the unevenness of image density in the sub-scanning direction, it is necessary to accurately align the rotation phase of the rotating members such as the photosensitive drum and the developing sleeve that cause the unevenness of image density with the detection position of the unevenness of image density in the sub-scanning direction.

[0008] On the other hand, in order to correct the deviation of the paper margin of the image of the final product caused by the unevenness of the paper conveyance speed and the unevenness of the peripheral speed between the rotating members in the image forming apparatus, the image forming apparatus has a function called sub-scanning magnification variation that varies the laser irradiation interval in the sub-scanning direction. In such a case, depending on the set amount of the sub-scanning magnification variation, the measurement image for detecting the unevenness of image density in the sub-scanning direction extends in the sub-scanning direction. This causes a deviation between the rotation phase of the photosensitive drum and the measurement image. Due to such a deviation, the accurate correction amount of the unevenness of image density cannot be determined.

[0009] Such problems caused by sub-scanning magnification change can occur not only in correction of image density unevenness in the sub-scanning direction, but also in all image correction control performed by forming a measurement image on an image carrier such as a photosensitive drum. One example is correction processing performed by forming a measurement image to determine the image shape conditions for forming an electrostatic latent image on an image carrier in order to correct image density.

[0010] SUMMARY OF THE INVENTION In view of the above-mentioned problems, it is a primary object of the present invention to provide an image forming apparatus that can form a measurement image and perform correction processing with high accuracy. [Means for solving the problem]

[0011] The image forming apparatus of the present invention is an image forming apparatus having a function capable of changing the size of an image to be formed in a predetermined direction, and is characterized by comprising: an image forming means for forming an image based on predetermined image forming conditions; and a control means for, when forming an image specified in a print job, changing the size of the image in the predetermined direction using the function and causing the image forming means to form the image of the image; and, when forming a measurement image for correcting the image forming conditions, causing the image forming means to form the measurement image without applying the function. [Effects of the Invention]

[0012] According to the present invention, it is possible to form a measurement image and perform correction processing with high accuracy. [Brief explanation of the drawings]

[0013]

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[0014] An embodiment of the present invention will be described with reference to the drawings. In this embodiment, an electrophotographic laser beam printer will be described as an example of an image forming apparatus. However, the image forming apparatus is not limited to a laser beam printer, and may be any other electrophotographic printer such as an LED (Light Emitting Diode) printer.

[0015] (First embodiment) FIG. 1 is a diagram showing the configuration of an image forming apparatus according to this embodiment. The image forming apparatus 100 includes a reader A, a printer B, and an operation unit 20. The printer B prints an image on paper S. The reader A reads the image from paper (document G) on which the image is printed. The operation unit 20 is a user interface. The operation unit 20 includes various key buttons and a touch panel as an input interface. The operation unit 20 includes a display unit 218 as an output interface. A user uses the operation unit 20 to issue instructions to start copying and to make various settings.

[0016] (Leader) The scanner A includes a document table glass 102 on which a document G is placed, a light source 103 that irradiates light onto the document G placed on the document table glass 102, an optical system 104, a light receiving unit 105, and an image processing unit 108. The scanner A also includes a CPU (Central Processing Unit) 214, a RAM (Random Access Memory) 215, and a ROM (Read Only Memory) 216. The light source 103, the optical system 104, and the light receiving unit 105 constitute an image reading unit that reads an image of the document G. At the edge of the document table glass 102, a positioning member 107 that prevents the document G from being obliquely placed by abutting one side of the document G and a reference white plate 106 used for shading correction of the image reading unit are arranged.

[0017] The optical system 104 forms an image of the reflected light of the light irradiated from the light source 103 by the document G on the reading surface of the light receiving unit 105. The light receiving unit 105 has a photoelectric conversion element such as a CCD (Charge Coupled Device) sensor, etc., and outputs an image signal obtained by converting the received reflected light into an electrical signal. For example, the photoelectric conversion elements of the light receiving unit 105 are arranged in three rows corresponding to red (R), green (G), and blue (B). The light receiving unit 105 generates color component signals of each color of R, G, and B as image signals. The image reading unit reads the image of the document G placed on the document table glass 102 line by line while moving in the arrow direction R103.

[0018] The image signal generated by the light receiving unit 105 is input to the image processing unit 108. The image processing unit 108 performs image processing such as A / D conversion, shading correction, and color conversion on the image signal obtained from the light receiving unit 105. The image processing unit 108 transmits the image signal after image processing to the printer B.

[0019] The CPU 214 controls the operation of the scanner A by executing a computer program stored in the ROM 216. The RAM 215 is a work memory when the CPU 214 executes processing. The scanner A has various operations for reading an image of the document G controlled by the CPU 214.

[0020] (Printer) Printer B includes an image forming units PY, PM, PC, PK, an intermediate transfer belt 6, a secondary transfer roller 64, a fixing unit 11, a paper feed cassette 65, and a printer control unit 109. Printer B is a tandem type intermediate transfer full-color printer in which the image forming units PY, PM, PC, PK are arranged along the intermediate transfer belt 6. The image forming unit PY forms a yellow image (toner image). The image forming unit PM forms a magenta image (toner image). The image forming unit PC forms a cyan image (toner image). The image forming unit PK forms a black image (toner image).

[0021] The intermediate transfer belt 6 is an image carrier supported by being wound around a tension roller 61, a driving roller 62, and a counter roller 63. A belt cleaner 68 is provided facing the tension roller 61. The intermediate transfer belt 6 is driven by the driving roller 62 and rotates in the direction of arrow R2 at a predetermined process speed. The images (toner images) formed by each of the image forming units PY, PM, PC, PK are sequentially transferred and overlaid on the intermediate transfer belt 6 at a timing corresponding to the rotation speed of the intermediate transfer belt 6. Thereby, a full-color image (toner image) is formed on the intermediate transfer belt 6.

[0022] The counter roller 63 forms a secondary transfer portion T2 with the secondary transfer roller 64. The images of each color transferred to the intermediate transfer belt 6 are conveyed to the secondary transfer portion T2 and collectively transferred to the paper S. By applying a positive DC voltage to the secondary transfer roller 64, the images (toner images) of each color charged negatively and carried on the intermediate transfer belt 6 are collectively transferred to the paper S. The developer remaining on the intermediate transfer belt 6 after transfer is removed by the belt cleaner 68. The belt cleaner 68 recovers the transfer residual toner remaining on the intermediate transfer belt 6 after passing through the secondary transfer portion T2 by rubbing a cleaning blade against the intermediate transfer belt 6.

[0023] The sheet S is stored in the paper feed cassette 65 and fed one by one. A separation roller 66 and a registration roller 67 are provided on the conveyance path along which the sheet S is conveyed. The sheet S is fed from the paper feed cassette 65, separated one by one by the separation roller 66, and conveyed to the registration roller 67. The registration roller 67 receives and waits for the sheet S in a stopped state, and conveys the sheet S to the secondary transfer unit T2 according to the timing when the image carried on the intermediate transfer belt 6 is conveyed to the secondary transfer unit T2.

[0024] The sheet S on which the image has been transferred is conveyed to the fixing unit 11 via the conveyance belt 10 by the secondary transfer roller 64. The fixing unit 11 melts and fixes the image on the sheet S by heating and pressing the sheet S. The sheet S on which the image has been fixed is discharged to the outside of the printer B body.

[0025] On the downstream side of the image forming unit PK in the rotation direction of the intermediate transfer belt 6, an image density sensor 69 as an image sensor is disposed at a position facing the driving roller 62 with the intermediate transfer belt 6 interposed therebetween. The image density sensor 69 is used to measure the image density of the unfixed toner image transferred to the intermediate transfer belt 6.

[0026] The image formation by the image forming units PY, PM, PC, and PK will be described. The image forming units PY, PM, PC, and PK only differ in the color of the developer (toner in this case) used for development, and have the same configuration and perform the same operations. In the following description, when distinguishing colors, subscripts Y, M, C, and K are added to the end of the reference numerals, and when not distinguishing colors, Y, M, C, and K at the end of the reference numerals are omitted.

[0027] FIG. 2 is a configuration explanatory diagram of the image forming unit P. The image forming unit P includes a photosensitive drum 1, a charger 2, an exposure device 3, a developing device 4, a reflected light amount sensor 12, a primary transfer roller 7, and a drum cleaner 8. The intermediate transfer belt 6 is sandwiched between the photosensitive drum 1 and the primary transfer roller 7. The charger 2, the exposure device 3, the developing device 4, the reflected light amount sensor 12, the primary transfer roller 7, and the drum cleaner 8 are disposed around the photosensitive drum 1.

[0028] The photosensitive drum 1 of this embodiment is an image carrier in which a photosensitive layer having a negative charge polarity is formed on the outer peripheral surface (surface) of an aluminum cylinder. The photosensitive drum 1 rotates in the direction of arrow R1 around the drum axis at a predetermined process speed. The photosensitive drum 1 is, for example, an OPC photosensitive member having a reflectance of approximately 40% for near-infrared light (960 [nm]). Note that the photosensitive drum 1 may also be an amorphous silicon-based photosensitive member or the like having a similar reflectance.

[0029] The charger 2 in this embodiment is a scorotron charger, which irradiates the photosensitive drum 1 with charged particles generated by corona discharge, thereby charging the photosensitive layer on the surface of the photosensitive drum 1 to a uniform negative potential. The scorotron charger has a wire to which a high voltage is applied, a grounded shield portion, and a grid portion to which a desired voltage is applied. A predetermined charging bias voltage is applied to the wire of the charger 2 from a charging bias power supply (not shown). A predetermined grid bias voltage is applied to the grid portion of the charger 2 from a grid bias power supply (not shown). Although it depends on the voltage applied to the wire, the photosensitive drum 1 is charged to approximately the voltage applied to the grid portion.

[0030] The exposure device 3 reflects laser light with a rotating mirror to scan the surface of the charged photosensitive drum 1 in the drum axial direction, forming an electrostatic latent image on the surface of the photosensitive drum 1. For this reason, the drum axial direction of the photosensitive drum 1 is the main scanning direction. The sub-scanning direction, which intersects with the main scanning direction, is the direction of rotation of the photosensitive drum 1. The sub-scanning direction is also parallel to the transport direction in which the paper S is transported. A potential sensor 5, which is a potential detector, is provided near the photosensitive drum 1. The potential sensor 5 can detect the potential of the electrostatic latent image formed on the photosensitive drum 1.

[0031] The developing device 4 forms an image (toner image) on the photosensitive drum 1 by attaching toner to the electrostatic latent image of the photosensitive drum 1 when a developing bias voltage is applied. The developing device 4 includes a developing sleeve 41, a first conveying screw 42, and a second conveying screw 43 in a developer container 45 for accommodating toner. The developer container 45 of the present embodiment accommodates a two-component developer in which non-magnetic toner and a magnetic carrier are mixed. The developer container 45 is divided into two chambers by a partition wall 46, with the first conveying screw 42 provided on one side and the second conveying screw 43 provided on the other side. The partition wall 46 is provided with two openings, and toner can flow into each other between the two chambers through the openings. The first conveying screw 42 and the second conveying screw 43 rotate to stir and mix the developer while circulating it inside the developer container 45.

[0032] The developing sleeve 41 is disposed close to the photosensitive drum 1 and rotates so as to be driven by the photosensitive drum 1. The developing sleeve 41 carries a developer in which toner and a carrier are mixed. The developer carried on the developing sleeve 41 develops the electrostatic latent image on the photosensitive drum 1 when a developing bias voltage is applied to the developing sleeve 41. The developing bias voltage is applied by a power supply unit 44. The power supply unit 44 controls the application of the developing bias voltage under the control of a control unit 110 (CPU 111) described later.

[0033] The developing device 4 includes a toner amount sensor 14 for measuring the amount of toner in the developer container 45. For example, a magnetic permeability sensor that detects the magnetic permeability of the developer is used as the toner amount sensor 14. The developing device 4 is connected to a toner supply container 33 via a supply path 32. When the measurement result of the toner amount by the toner amount sensor 14 is less than a predetermined amount, toner is supplied from the toner supply container 33 to the developer container 45 via the supply path 32.

[0034] The reflected light quantity sensor 12 is an optical sensor having a light emitting part 12a and a light receiving part 12b, and is used for measuring the image density of the toner image formed on the photosensitive drum 1. The reflected light quantity sensor 12 irradiates light from the light emitting part 12a onto the toner image on the photosensitive drum 1. The light receiving part 12b receives the reflected light by the toner image and outputs an output signal corresponding to the received reflected light quantity.

[0035] The primary transfer roller 7 presses against the inner surface of the intermediate transfer belt 6 to form a primary transfer part T1 between the photosensitive drum 1 and the intermediate transfer belt 6. When a positive-polarity DC voltage is applied to the primary transfer roller 7, the negative-polarity toner image carried on the photosensitive drum 1 is transferred to the intermediate transfer belt 6 passing through the primary transfer part T1. In this way, the image forming part P forms a toner image of the color corresponding to the photosensitive drum 1. The toner image is transferred from the photosensitive drum 1 to the intermediate transfer belt 6. The drum cleaner 8 rubs a cleaning blade against the photosensitive drum 1 to collect the residual transferred toner remaining on the photosensitive drum 1 after the transfer to the intermediate transfer belt 6.

[0036] The operation of such an image forming part P is controlled by a printer control part 109 and a control part 110 provided in the printer A. The printer control part 109 controls the operation of the printer B. The control part 110 controls the operation of the entire image forming apparatus 100. The control part 110 is connected to the printer control part 109 and the image processing part 108 of the reader A. Also, an operation part 20 is connected to the control part 110. The operation part 20 is also connected to the CPU 214 of the reader A. Although not shown in the figure, the CPU 214 of the reader A is also connected to the control part 110.

[0037] The control unit 110 includes a CPU 111, a RAM 112, and a ROM 113. The CPU 111 controls the operation of the image forming apparatus 100 by executing a computer program stored in the ROM 113. The RAM 112 is a work memory when the CPU 111 executes processing. The reader A and the printer B of the image forming apparatus 100 are controlled for various operations by the CPU 214. The printer control unit 109 includes a light amount control unit 190, a pattern generator 192, and a pulse width modulator 191. The image processing unit 108 includes a video counter 220 and a gamma correction unit 209.

[0038] The exposure device 3 of the present embodiment is a laser scanner having a rotary mirror. The exposure amount of the exposure device 3 is determined by the light amount control unit 190 so that a predetermined image density level is obtained for the laser output signal. In the present embodiment, in order to suppress image density unevenness in the main scanning direction and the sub-scanning direction, the light amount setting (LPW) is managed by enabling the light amount to be set in units of approximately 30 [mm] in each direction. Further, the exposure device 3 outputs more laser light according to the pulse width determined by the pulse width modulator 191 based on the drive signal generated using the tone correction table (LUT) of the gamma correction unit 209.

[0039] The laser output signal is determined based on the tone correction table held in the gamma correction unit 209. The tone correction table shows the relationship between the laser output signal and the image density level of the image to be formed, and the laser output signal is determined according to the image density of the image to be formed.

[0040] The printer control unit 109 acquires the image signal generated by the image processing unit 108. The printer control unit 109 performs pulse width modulation (PWM) on the laser light output from the exposure device 3 based on the image signal, and forms an image with image density gradation by area gradation. Therefore, the printer control unit 109 generates and outputs a laser output signal with a width (time width) corresponding to the level of the image signal for each pixel by the pulse width modulator 191. The laser output signal is a laser drive pulse signal. For an image signal indicating high image density, the laser output signal is a pulse signal with a wide width. For an image signal indicating low image density, the laser output signal is a pulse signal with a narrow width. For an image signal indicating intermediate image density, the laser output signal is a pulse signal with an intermediate width.

[0041] The laser output signal (laser drive pulse signal) output from the pulse width modulator 191 is supplied to the light source of the laser light of the exposure device 3 (for example, a semiconductor data). The semiconductor laser outputs laser light for a time corresponding to the pulse width of the laser output signal. Therefore, the semiconductor laser is driven for a long time for pixels with high image density and for a short time for pixels with low image density. As a result, the dot size (area) of the electrostatic latent image formed on the photosensitive drum 1 varies according to the image density of the pixels. The exposure device 3 exposes a longer range in the main scanning direction for pixels with high image density and a shorter range in the main scanning direction for pixels with low image density.

[0042] The pattern generator 192 generates an image signal of a measurement image formed to correct the image formation conditions. When forming the measurement image, the pulse width modulator 191 generates a laser output signal based on the image signal of the measurement image acquired from the pattern generator 192. The measurement image in this embodiment is, for example, an image for correcting image density unevenness in the sub-scanning direction or an image for correcting image density.

[0043] (Magnification setting for sub-scanning variable magnification) As described above, in the image forming apparatus 100, unevenness in the conveyance speed of the sheet S or unevenness in the peripheral speed between the rotating members such as the photosensitive drum 1 may cause deviation of the image on the sheet S in the sheet margin. In order to correct the deviation of the sheet margin, there is a function called sub-scanning magnification change that changes the laser irradiation interval in the sub-scanning direction.

[0044] The magnification setting value of the sub-scanning magnification change is a unique value adjusted for each image forming apparatus 100 by a service technician when installing the image forming apparatus 100. At the time of image formation, the exposure pitch interval in the sub-scanning direction is adjusted according to the magnification setting value of the sub-scanning magnification change. In addition, the sub-scanning magnification change can also be performed by correcting the image signal in addition to adjusting the exposure pitch interval.

[0045] (Shading function) In the present embodiment, correction is performed using the shading function of the exposure device 3 for uneven image density in the sub-scanning direction. The light amount control unit 190 acquires a correction value of the light amount corresponding to each exposure position and the phase in the sub-scanning direction from the ROM 113 of the control unit 110, and controls the exposure with a light amount setting (LPW) based on the correction value. The correction value of the light amount corresponding to each exposure position is obtained by uneven image density correction described later. In the present embodiment, correction values for light amount setting are stored in the ROM 113 at intervals of about 30 [mm] in each of the main scanning direction and the sub-scanning direction. For uneven image density in the main scanning direction, it is handled by shading correction in the main scanning direction. In the shading correction in the main scanning direction, the light amount control unit 190 acquires a correction value of the light amount corresponding to each exposure position in the main scanning direction from the ROM 113 of the control unit 110, and controls the exposure with a light amount setting based on the correction value.

[0046] (Uneven image density correction) In the present embodiment, in order to suppress uneven image density occurring in a predetermined direction (here, the sub-scanning direction), the control unit 110 performs uneven image density correction processing using the shading function. The control unit 110 performs, for example, exposure amount correction processing of the exposure device 3 at the time of image formation, measurement image formation processing for detecting uneven image density, uneven image density detection processing, and calculation processing of the uneven image density correction amount.

[0047] Multiple types of sensors can be used as the detection sensor for the image density unevenness detection process. In this embodiment, for example, a potential sensor 5 that detects the potential unevenness of the photosensitive drum 1, which is one of the causes of image density unevenness, and an image density sensor 69 that detects the image density unevenness of the image carried on the intermediate transfer belt 6 can be used as the detection sensor for the image density unevenness detection process. Here, the case of using the potential sensor 5 will be described.

[0048] FIG. 3 is a flowchart showing the image density unevenness correction process in the sub-scanning direction. Note that the image density unevenness correction in the sub-scanning direction is performed for each color.

[0049] When the control unit 110 starts the image density unevenness correction process in the sub-scanning direction, it starts forming a measurement image for detecting the image density unevenness in the sub-scanning direction (S201). FIG. 4 is an exemplary diagram of the measurement image. The measurement image is a strip image having a predetermined width in the main scanning direction and extending a predetermined length in the sub-scanning direction, and is formed based on an image signal indicating a uniform image density. This image signal is generated by the pattern generator 192. The strip images (pattern images) of each color are arranged at predetermined intervals in the main scanning direction. In this embodiment, the pattern images of each color are formed by an image signal with an image density of 40%. The example of FIG. 4 shows a state where the pattern images of each color are transferred to the intermediate transfer belt 6. On the photosensitive drum 1 corresponding to each color, a monochromatic pattern image of the corresponding color is formed.

[0050] The image formation conditions in the sub-scanning direction need to associate the position of the measurement image for detecting image density unevenness with the rotational phase of the rotating body, which is the cause of image density unevenness. In this embodiment, phase control of the image carrier (here, the photosensitive drum 1) is performed to align the writing position of the pattern image with the home position of the rotational phase. Further, as one of the features of this embodiment, when forming the pattern image (measurement image), image formation is performed without correction based on the set value of the sub-scanning magnification. That is, the image signal of the measurement image generated by the pattern generator 192 is not processed based on the set value of the sub-scanning magnification.

[0051] As a result, it is possible to obtain image density unevenness information representing image density unevenness that accurately corresponds to the phase of one rotation of each color image carrier (here, the photosensitive drum 1). This is because it is necessary to accurately associate the pattern image formed on the photosensitive drum 1, which is the most upstream of the image formation process, with the rotational phase of the photosensitive drum 1, and it is not necessary to apply the setting of the sub-scanning magnification set to correct the peripheral speed difference of the intermediate transfer belt 6 that occurs downstream.

[0052] The control unit 110 that has started forming the measurement image measures the potential of the electrostatic latent image of the pattern image with the potential sensor 5 (S202). FIG. 5 is an explanatory diagram of the potential measurement in the sub-scanning direction of the pattern image. The measurement in the sub-scanning direction is performed by dividing a predetermined length (here, about 300 [mm]) corresponding to one rotation of the image carrier (photosensitive drum 1) into 10 equal parts and dividing them into 1 to 10 every about 30 [mm] from the upstream side in the sub-scanning direction. When measuring the pattern image (measurement image), the potential sensor 5 measures the potential of the electrostatic latent image of the pattern image in unit sections.

[0053] The pattern image is formed with a length in the sub-scanning direction (sub-scanning length) of 300 [mm] in order to detect the image density of about 300 [mm], which is the circumferential length of the photosensitive drum 1. When the sub-scanning magnification is set to extend the image by 10% in the sub-scanning direction, the pattern image is extended by 10% in the sub-scanning direction and formed at 330 [mm]. The measurement position of the potential sensor 5 can be shifted according to the pattern image, but it will deviate from the phase of the photosensitive drum 1 by up to about 30 [mm]. Therefore, when performing sub-scanning magnification, it becomes difficult to detect the image density with high accuracy, and it becomes difficult to correct the image density unevenness in the sub-scanning direction. For this reason, in this embodiment, sub-scanning magnification is not performed when forming the measurement image.

[0054] The control unit 110 acquires the measurement result of the electrostatic latent image of the pattern image from the potential sensor 5, and calculates the average value of each potential for each delimiter unit (S203). The control unit 110 calculates the potential difference Δ between the average value and the potential of each delimiter region (regions 1 to 10) (S204). The control unit 110 calculates the correction amount (ΔLPW) corresponding to the calculated potential difference Δ (S205). Through the above processing, the control unit 110 determines the correction amount (ΔLPW) of the light amount for correcting the image density unevenness in the sub-scanning direction for each of the regions 1 to 10 (S206).

[0055] As described above, the image forming apparatus 100 according to the first embodiment can detect image density unevenness in the sub-scanning direction with high accuracy according to the potential of the electrostatic latent image on the photosensitive drum 1. In this case, the measurement image is not subjected to sub-scanning magnification. Therefore, the image forming apparatus 100 can determine an optimal correction value for image density unevenness in the sub-scanning direction, and can form a high-quality image with suppressed image density unevenness in the sub-scanning direction.

[0056] (Second Embodiment) In the second embodiment, image density unevenness is suppressed by detecting the amount of change in image density and correcting the gradation correction table. Since the configuration of the image forming apparatus 100 is the same as that of the first embodiment, the description thereof is omitted.

[0057] FIG. 6 is an explanatory diagram of a measurement image for measuring image density. Here, for the purpose of explaining the yellow image, FIG. 6 illustrates a yellow measurement image 101 formed on the photosensitive drum 1Y on which the yellow image (toner image) is formed. In this embodiment, the measurement image 101 is formed at a timing other than during the image forming operation according to the print job. The measurement image 101 is composed of a plurality of pattern images having different image densities each including a halftone pattern. The plurality of pattern images are arranged at a predetermined interval in the rotation direction of the photosensitive drum 1y.

[0058] The toner amount (image density) adhering to the measurement image 101 is measured by detecting the amount of reflected light from the measurement image 101 with the reflected light amount sensor 12y. The reflected light amount sensor 12y is an optical sensor including the light emitting part 12a and the light receiving part 12b as described above. The light emitting part 12a irradiates the photosensitive drum 1Y with light. The light receiving part 12b detects only the specularly reflected light of the light irradiated from the light emitting part 12a by the photosensitive drum 1Y. The reflected light amount sensor 12y measures the amount of reflected light of the measurement image 101 in this way.

[0059] The reflected light amount sensor 12y is arranged at a position where the image density can be measured between the development position by the developing device 4Y and the primary transfer roller 7Y. The amount of reflected light is measured at the timing when the measurement image 101 formed outside the image formation area on the photosensitive drum 1Y passes through the measurement range of the reflected light amount sensor 12y. Based on the amount of reflected light, yellow gradation correction (γ correction) is executed, which is estimated to obtain a predetermined constant image density (amount of reflected light).

[0060] Similarly, for the photosensitive drums 1M, 1C, 1K corresponding to other colors, the reflected light amount sensors 12m, 12c, 12k are arranged at positions where the image density can be measured between the development positions by the developing devices 4M, 4C, 4K and the primary transfer rollers 7M, 7C, 7K. The image density of the images (toner images) formed on the photosensitive drums 1M, 1C, 1K is detected by the reflected light amount sensors 12m, 12c, 12k. Based on the detected image density (amount of reflected light), magenta, cyan, and black gradation correction (γ correction) is executed, which is estimated to obtain a predetermined constant image density (amount of reflected light).

[0061] FIG. 7 is an explanatory diagram of a signal processing unit that processes the detection result (output signal) by the reflected light amount sensor 12. The signal processing unit 70 is provided between the reflected light amount sensor 12 or inside the control unit 110. The signal processing unit 70 includes an A / D converter 15 and an image density conversion unit 16. The signal processing unit 70 is connected to the CPU 111.

[0062] The signal processing unit 70 acquires an output signal, which is an electrical signal representing the detection result of the image density, from the reflected light quantity sensor 12. The output signal is an analog signal having a value corresponding to the amount of reflected light received by the reflected light quantity sensor 12, and takes a voltage value of, for example, 0 to 5 [V]. The output signal is input to the A / D converter 15.

[0063] The A / D converter 15 converts the output signal acquired from the reflected light quantity sensor 12 into, for example, an 8-bit digital signal. The 8-bit digital signal is transmitted to the image density conversion unit 16. The image density conversion unit 16 converts the digital signal acquired from the A / D converter 15 into an image density signal. The image density conversion unit 16 has a table 16a showing the relationship between the digital signal (output signal) corresponding to the amount of reflected light and the image density signal (image density). The image density conversion unit 16 converts the digital signal into an image density signal according to the table 16a.

[0064] In addition to the operation unit 20, the ROM 113, and the RAM 112, the CPU 111 is connected to an I / O interface 21 and a LUT (Look Up Table) 25. The LUT 25 is a table serving as correction conditions for setting the writing image dot density of yellow, cyan, magenta, and black at the time of image formation in order to obtain an image with an appropriate image density for the image signal representing the image to be formed.

[0065] The tone control with such a configuration will be described. FIG. 8 is a graph showing the relationship between the image density of the image (toner image) formed on the photosensitive drum 1 and the output signal from the reflected light quantity sensor 12. This graph shows the relationship between the image density and the output signal when the image density of the measurement image 101 formed on the photosensitive drum 1 is changed stepwise by the area tone of each color. The voltage value of the output signal output from the reflected light quantity sensor 12 when no toner is attached to the photosensitive drum 1 is set to 5 [V] (255 levels in an 8-bit digital signal). As the area coverage ratio by the toner increases and the image density increases, the voltage value of the output signal output from the reflected light quantity sensor 12 decreases.

[0066] The image density conversion unit 16 has registered therein a table 16a dedicated to each color, which has the characteristics of the graph shown in Fig. 8. The image density conversion unit 16 converts the output signal output from the reflected light amount sensor 12 into an image density signal for each color using the table 16a dedicated to each color. This allows the image density of the measurement image 101 for each color to be detected with high accuracy.

[0067] In this embodiment, the CPU 111 forms the measurement image 101 outside the timing of image formation according to the print job, detects the image density, and corrects the table data of the LUT 25. A feature of this embodiment is that the measurement image 101 is formed without applying the sub-scanning magnification setting described in the first embodiment.

[0068] 9 is a diagram illustrating the relationship between the input image signal and the image density (output image density) detected from the measurement image. The difference in output image density between when the sub-scanning magnification setting is applied and when it is not applied will be described.

[0069] When the measurement image 101 is formed with the sub-scanning magnification setting applied, the reading position of the reflected light intensity sensor 12y is shifted in the sub-scanning direction from the formation position of the measurement image 101. The measurement image 101 is formed on the photosensitive drum 1Y with five pattern images, each with a different image density, lined up in the sub-scanning direction. If the size of the pattern images is 30 mm in the sub-scanning direction and the spacing between the pattern images is 20 mm in the sub-scanning direction, the total length of the measurement image 101 in the sub-scanning direction is 230 mm. When the measurement image 101 is formed with the sub-scanning magnification setting set to 10%, the formation position of the measurement image 101 is shifted in the sub-scanning direction by up to 23 mm from the reading position of the reflected light intensity sensor 12y.

[0070] As shown in the figure, the image density of each pattern image is measured from the results of multiple image density detections by the reflected light intensity sensor 12. For example, the image density of each pattern image is calculated as the average value of the multiple detection results. If the formation position of the measurement image 101 is shifted in the sub-scanning direction, the number of measurements for each pattern image will change, and the accuracy of the image density detection results will decrease. This will result in detection errors in the image density detected from the measurement image 101, making it impossible to accurately correct the table data in the LUT 25. For this reason, in this embodiment, the measurement image 101 is formed without applying sub-scanning magnification settings, enabling highly accurate image density detection.

[0071] When forming the measurement image 101, the CPU 111 sets appropriate image signal amounts for the image signals of each color (cyan, magenta, yellow, and black) using the LUT 25, just as when forming a normal image. The table data of the LUT 25 used when forming the measurement image 101 is the same as the table data used during normal image formation at that time. In other words, the correction results from the previous image density correction control are used as the table data of the LUT 25.

[0072] The image signal is corrected by the LUT 25 to reduce the difference between the output image density detected from the measurement image 101 and the target value of the output image density. For example, if the input image density of the measurement image 101 (image density specified by the image signal) is 128 levels and the target value is 128, the LUT 25 corrects the image signal level (image signal amount) so that the output image density becomes 128. However, the image characteristics of the image forming apparatus 100 are unstable and may constantly change. For this reason, the table data of the LUT 25 is corrected based on the difference ΔD between the input image density and the measurement result (output image density) of the measurement image 101. The difference ΔD is the difference between the target value obtained from the measurement image 101 formed using the previous LUT 25 and the image density detected from the measurement image 101 formed using the new LUT 25.

[0073] In this embodiment, a LUT correction table for correcting the deviation of LUT25 is stored in advance in RAM112. When performing control, CPU111 calculates a correction amount corresponding to the deviation amount ΔD for all image density signals of the LUT correction table stored in RAM112 up to the previous time. Based on the calculated correction amount, CPU111 corrects the previous LUT correction table by the amount of deviation ΔD to create the current LUT correction table. Such rewriting (correction) of LUT25 is performed for each color at the timing when the creation of the LUT correction table corresponding to the deviation amount ΔD is completed.

[0074] FIG. 10 is a flowchart showing an image forming process including a process for creating a LUT correction table. The process for creating a LUT correction table is executed while forming an image corresponding to a print job and before forming the next image in accordance with a normal image forming process corresponding to the print job. In FIG. 10, the process in the case of forming an image by one type of image processing is illustrated.

[0075] When starting the process according to the print job, CPU111 corrects the table data of LUT25 based on the following formula (1) using the LUT correction table obtained by the previous process (S21). CPU111 sets the corrected table data in LUT25 (S22). CPU111 forms an image of the print job using this LUT25 (S23). (Table data) = LUT + (LUT correction table up to the previous time) …(1)

[0076] After image formation, CPU111 forms a measurement image 101 outside the image formation area (between images) on the photosensitive drum 1, which is the area between the rear end of the image and the front end of the next image (S24). CPU111 controls the reflected light amount sensor 12 to read the measurement image 101 (S25). Based on the reading result of the measurement image 101, CPU111 calculates the deviation amount ΔD between the image density of the measurement image 101 and the target value of the image density (S26). Based on the calculated deviation amount ΔD and the LUT correction table up to the previous time, CPU111 creates a new LUT correction table (S27).

[0077] In this way, the image forming apparatus 100 of the second embodiment can perform optimal image density correction table (LUT) correction by detecting the measurement image 101 with high accuracy. As a result, it is possible to consistently form highly stable images with reduced changes in image density. Because the image density at each position in the sub-scanning direction can be detected with high accuracy, it is possible to accurately correct image density deviations in the sub-scanning direction.

[0078] In the first embodiment, image density unevenness correction suppresses density unevenness based on the potential of the electrostatic latent image on the photosensitive drum 1 measured by the potential sensor 5. In the second embodiment, image density unevenness correction suppresses density unevenness based on the density of the measurement image 101 on the photosensitive drum 1 measured by the reflected light amount sensor 12. However, image density unevenness correction may be configured to suppress density unevenness in the sub-scanning direction using a sensor other than these sensors. For example, the image forming apparatus 100 may have an in-line sensor downstream of the fixing unit 11 in the direction in which the paper S is transported, and density unevenness in the sub-scanning direction may be adjusted based on the reading result of the test image on the paper S read by the in-line sensor.

[0079] As described in the first and second embodiments, when forming an image according to a print job, the image forming apparatus 100 performs sub-scanning magnification to suppress fluctuations in the image size in the sub-scanning direction of the image formed on the paper S. However, sub-scanning magnification becomes a factor that prevents highly accurate detection of the image density at each position in the sub-scanning direction when detecting the image density in the sub-scanning direction of the image carrier corresponding to each position in the sub-scanning direction. This prevents suppression of uneven image density in the sub-scanning direction.

[0080] Therefore, when forming a measurement image for detecting the image density in the sub-scanning direction, the image forming apparatus 100 forms the measurement image without changing the image size in the sub-scanning direction. As a result, the rotational phase of the rotating body and the image density at each position in the sub-scanning direction of the measurement image are associated with each other, and the image density at each position in the sub-scanning direction can be detected with high precision. Therefore, it is possible to highly accurately suppress unevenness in image density in the sub-scanning direction. As a result, it is possible to suppress unevenness in image density and stably obtain an image with an appropriate image density.

Claims

1. An image forming apparatus having a function of changing the size of a formed image in a predetermined direction, image forming means for forming an image based on predetermined image forming conditions, when forming an image instructed by a print job, the function is used to change the size of the image in the predetermined direction and cause the image forming means to form the image, and when forming a measurement image for correcting the image forming conditions, the image forming means is caused to form the measurement image without applying the function, and control means, characterized in that it comprises, image forming apparatus.

2. The image forming means, a photoreceptor, exposure means for exposing the rotating photoreceptor to form an electrostatic latent image on the photoreceptor, development means for developing the electrostatic latent image to form a toner image on the photoreceptor, and comprises, the function is a function capable of changing the size of an image in the rotation direction of the photoreceptor, the control means, when forming the image, changes the size of the image in the rotation direction by the function and causes the image forming means to form the image, and when forming the measurement image, causes the image forming means to form the measurement image without applying the function, characterized in that, The image forming apparatus according to claim 1.

3. further comprising potential detection means for detecting the potential of the electrostatic latent image formed on the photoreceptor, the control means corrects the image forming conditions based on the potential detected from the electrostatic latent image of the measurement image by the potential detection means, characterized in that, The image forming apparatus according to claim 2.

4. the length of the measurement image in the rotation direction is equal to or more than one circumference in the rotation direction, the control means divides the length of the measurement image in the rotation direction into predetermined lengths, and corrects the image forming conditions based on the potential detected by the potential detection means in units of division, characterized in that, The image forming apparatus according to claim 3.

5. the control means determines a correction amount for correcting the image forming conditions based on the potential difference between the average value of each potential of the division unit of the measurement image and each potential of the division unit, characterized in that, The image forming apparatus according to claim 4.

6. the control means determines the correction amount for correcting the exposure amount by the exposure means, characterized in that, The image forming apparatus according to claim 5.

7. The apparatus further includes measuring means for measuring the image density from the toner image formed on the photoreceptor. The control means corrects the image forming conditions based on the image density detected from the measurement result of the toner image of the measurement image by the measurement means. The image forming apparatus according to claim 2.

8. The toner image of the measurement image is constituted by arranging a plurality of pattern images having different image densities side by side at a predetermined interval in the rotational direction. The measurement means measures each of the plurality of pattern images. The control means corrects the image forming conditions based on the respective image densities detected from the measurement results of the plurality of pattern images by the measurement means. The image forming apparatus according to claim 7.

9. The measurement means measures each of the plurality of pattern images a plurality of times. The control means detects the image density of each of the plurality of pattern images by the average value of the image densities detected from the plurality of measurement results of each of the plurality of pattern images by the measurement means, and corrects the image forming conditions based on the detected image density of each of the plurality of pattern images. The image forming apparatus according to claim 8.

10. The control means determines a correction amount for correcting the exposure amount by the exposure means based on the detected image density of each of the plurality of pattern images. The image forming apparatus according to claim 9.

11. The control means causes the image forming means to form an image corresponding to the print job, and after the image is formed, causes the image forming means to form the measurement image in an area between the rear end of the image and the front end of the next image. The image forming apparatus according to claim 10.

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