Image forming apparatus

Abstract

To suitably reduce density unevenness in an image.SOLUTION: In an image forming apparatus 100, a reference selection unit 79 selects, from among a plurality of photoconductor drums 40, a drum in which a phase correction value becomes minimum as a result of calculation carried out by a relative phase difference calculation unit 78 as a reference drum 40S (reference image carrier). With a plurality of motors 71 being driven, and with reference to the reference drum 40S determined by the reference selection unit 79, a correction control unit 80 controls respective rotational positions of other drums than the reference drum 40S in the plurality of photoreceptor drums 40 to be the phase correction value.SELECTED DRAWING: Figure 5

Description

Technical Field

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

Background Art

[0002] In a tandem color type image forming apparatus having a plurality of photoreceptor drums as image carriers, periodic image density unevenness generated in the photoreceptor circumference may occur due to eccentricity of the photoreceptor drum or the like.

[0003] Patent Documents 1 and 2 disclose a configuration in which the target speed is varied so as to detect the periodic unevenness of the photoreceptor drum and correct the periodic unevenness.

Summary of the Invention

Problems to be Solved by the Invention

[0004] In the conventional methods described in Patent Documents 1, 2, etc., during the startup of the motor for driving the photoreceptor drum, by matching the periodic unevenness of each color such as Y, M, C, etc., the density unevenness of secondary colors or more is reduced and improved, but this is not sufficient.

[0005] An object of the present invention is to suitably reduce the density unevenness of an image.

Means for Solving the Problems

[0006] To solve the above-mentioned problems, an image forming apparatus according to one aspect of the present invention comprises: a plurality of image carriers; a plurality of motors for rotating each of the plurality of image carriers; a rotation position detection unit for detecting the rotational position of the plurality of image carriers; a density detection unit for detecting the density of toner images formed on the plurality of image carriers; a density unevenness detection unit for forming a predetermined image pattern and detecting the density unevenness of the rotational period of each of the plurality of image carriers and the density unevenness phase from a rotational position reference based on the detection results by the rotation position detection unit and the density detection unit; and from the density unevenness phase obtained by the density unevenness detection unit, each of the plurality of image carriers The system includes: a target phase calculation unit that calculates a target phase; a relative phase difference calculation unit that calculates the relative phase difference between each of the plurality of image carriers based on the rotation position obtained by the rotation position detection unit and the target phase calculated by the target phase calculation unit; a reference selection unit that selects the image carrier from the plurality of image carriers whose phase correction value is smallest according to the calculation result of the relative phase difference calculation unit as the reference image carrier; and a correction control unit that, while the plurality of motors are driven, controls the rotation position of each of the other image carriers of the plurality of image carriers to the phase correction value based on the reference image carrier obtained by the reference selection unit. [Effects of the Invention]

[0007] Image density unevenness can be effectively reduced. [Brief explanation of the drawing]

[0008] [Figure 1] A diagram showing an example of the configuration of an image forming apparatus according to the embodiment. [Figure 2] A diagram showing an example of the configuration of the image-making unit. [Figure 3] Block diagram showing an example of the hardware configuration of an image forming apparatus. [Figure 4] Diagram illustrating the overview of phase alignment control. [Figure 5] Conceptual diagram of phase alignment control [Figure 6] Functional block diagram of an image forming apparatus related to phase alignment control. [Figure 7]Flowchart for phase alignment control [Figure 8] Timing chart of various signals in phase alignment control shown in Figure 7 [Figure 9] Flowchart of the phase acquisition process for each drum during the phase detection period [Figure 10] This diagram illustrates the parameters used for phase correction in this embodiment. [Figure 11] Flowchart for calculating phase correction value [Modes for carrying out the invention]

[0009] The embodiments will be described below with reference to the attached drawings. To facilitate understanding of the explanation, the same reference numerals are used for identical components in each drawing whenever possible, and redundant explanations are omitted.

[0010] The following describes an embodiment using an electrophotographic image forming apparatus equipped with a secondary transfer mechanism known as a tandem system as an example.

[0011] This image forming apparatus is an MFP (Multifunction Peripheral / Printer / Product) that incorporates copying, printing, and facsimile functions into a single housing. While recording media can include plain paper commonly used for copying, overhead projector sheets (OHP sheets), cards, postcards, or envelopes, paper P will be used as an example recording medium in this explanation.

[0012] <Example of the configuration of the image forming apparatus 100> Figure 1 is a diagram showing an example of the configuration of an image forming apparatus 100 according to an embodiment, and is a cross-sectional view showing the main part of the image forming apparatus 100 according to an embodiment. As shown in Figure 1, the image forming apparatus 100 has an intermediate transfer unit in the center, and the intermediate transfer unit has an intermediate transfer belt 10 which is an endless belt. The intermediate transfer belt 10 is wrapped around three support rollers 14 to 16 and is driven to rotate clockwise.

[0013] Further, the image forming apparatus 100 includes an intermediate transfer body cleaning unit 17 that removes residual toner remaining on the intermediate transfer belt 10 after image transfer, to the left of the second support roller 15 among the three support rollers 14 to 16.

[0014] An image forming unit 20 including a yellow (Y) image forming unit, a magenta (M) image forming unit, a cyan (C) image forming unit, and a black (K) image forming unit is provided so as to face the intermediate transfer belt 10 disposed between the first support roller 14 and the second support roller 15, and the image forming units of each color are arranged along the moving direction of the intermediate transfer belt 10.

[0015] In addition, the image forming units of each color have the same configuration except that the color of the toner used is different. Therefore, in the description and drawings, the suffixes "Y", "M", "C", and "K" indicating the color of the toner used may be appropriately omitted in some cases.

[0016] The image forming unit 20 includes a photosensitive drum (image carrier) 40 (40Y, 40M, 40C, 40K) of each color, a charging roller 18 (18Y, 18M, 18C, 18K), a developing unit, and a cleaning unit, and is detachably attached to the image forming apparatus 100.

[0017] The image forming apparatus 100 is provided with a cover portion that can be opened and closed by tilting forward (the front side of the paper surface) in order to protect the inside of the image forming apparatus 100. A user or a service technician who performs maintenance of the image forming apparatus 100 can open the cover portion to access the inside of the image forming apparatus 100 and detach and attach the image forming unit 20 to a predetermined position inside the image forming apparatus 100.

[0018] This image forming unit 20 is, for example, an exchangeable process cartridge drum unit (hereinafter referred to as PCDU) according to the life of the photosensitive drum 40.

[0019] Further, above the image forming unit 20, the image forming apparatus 100 includes a light beam scanning unit 21. The light beam scanning unit 21 can form an electrostatic latent image corresponding to image data on each photosensitive drum 40 of each color by irradiating the photosensitive drum 40 of each color with a light beam (laser light) for image formation.

[0020] The electrostatic latent image on each photosensitive drum 40 of each color is developed by a developing unit, and the developed toner images of each color are superimposed and primarily transferred onto the intermediate transfer belt 10. Thereby, a color toner image is formed on the intermediate transfer belt 10. The toner image is carried on the intermediate transfer belt 10 as an example of an image carrier and is moved along the moving direction of the intermediate transfer belt 10. The configuration of the image forming unit 20 will be described in detail separately with reference to FIG. 2.

[0021] Also, the image forming apparatus 100 includes a secondary transfer unit 22 below the intermediate transfer belt 10. The secondary transfer unit 22 is arranged such that an endless secondary transfer belt 24 is spanned between two rollers 23 and presses up the intermediate transfer belt 10 against the third support roller 16. The secondary transfer belt 24 can secondarily transfer the toner image formed on the intermediate transfer belt 10 onto the paper P.

[0022] Furthermore, the image forming apparatus 100 includes a fixing unit 25 beside the secondary transfer unit 22. The fixing unit 25 fixes the toner image on the paper P conveyed in a state where the toner image has been secondarily transferred, onto the paper P. The fixing unit 25 includes an endless fixing belt 26, a heating roller, and a pressing roller 27, and can fix the toner image transferred onto the surface of the paper P onto the paper P by the heat and pressure of the fixing belt 26 and the pressing roller 27.

[0023] Also, the image forming apparatus 100 includes a sheet reversing unit 28 below the secondary transfer unit 22 and the fixing unit 25 for reversing the front and back of the paper P and feeding it out in order to form an image on the back surface of the paper P immediately after an image is formed on the front surface.

[0024] Next, we will explain the sequence of steps involved in the formation of an image on paper P in the image forming apparatus 100.

[0025] When the "Copy" start button on the operation unit (not shown) is pressed, the image forming apparatus 100, if a document is placed on the document feed tray 30 of the ADF (Auto Document Feeder) 400, which is the automatic document transport unit, will cause the ADF 400 to transport the document toward the contact glass 32. On the other hand, if no document is placed on the document feed tray 30, the image reading unit 300, which is equipped with a first carriage 33 and a second carriage 34, will be driven to read the document that has been manually placed on the contact glass 32.

[0026] In the image reading unit 300, the light source included in the first carriage 33 irradiates the contact glass 32 with light. The reflected light from the document surface is reflected towards the second carriage 34 by the first mirror included in the first carriage 33, and then reflected again by the mirror included in the second carriage 34. The reflected light from the document surface is then imaged on the imaging surface of the CCD (Charge Coupled Device) 36, which is the reading sensor, by the imaging lens 35. The CCD 36 captures an image of the document surface, and based on the image signal captured by the CCD 36, image data of each color, Y, M, C, and BK, is generated.

[0027] Furthermore, when the "print" start button is pressed, when an image forming instruction is received from an external device such as a PC (Personal Computer), or when a FAX (Facsimile) output instruction is received, the image forming apparatus 100 starts rotating the intermediate transfer belt 10 and prepares each unit of the image forming section 20 for image formation.

[0028] Subsequently, the image forming apparatus 100 starts the image formation process for each color. A laser modulated based on the image data of each color is shone onto the photosensitive drum 40 for each color, and an electrostatic latent image is formed. Then, the toner images of each color, which have been developed from the electrostatic latent images, are superimposed on the intermediate transfer belt 10 to form a single image.

[0029] Subsequently, the paper P is fed into the secondary transfer unit 22 at a time that coincides with the leading edge of the toner image on the intermediate transfer belt 10 entering the secondary transfer unit 22. The secondary transfer unit 22 then transfers the toner image from the intermediate transfer belt 10 to the paper P. The paper P, with the toner image transferred to it, is then fed into the fuser unit 25, where the toner image is fixed to the paper P.

[0030] Here, we will explain the feeding of the paper P up to the secondary transfer position. The paper P is fed from one of the multi-stage paper trays 44 provided in the paper feeding unit 43 by the rotational drive of one of the paper feed rollers 42 of the paper feed table 200. Then, one sheet is separated by the separation roller 45 and enters the transport roller unit 46, where it is transported by the transport roller 47. After that, it is guided to the transport roller unit 48 in the image forming apparatus 100, and after being stopped by the registration roller 49 of the transport roller unit 48, it is sent towards the secondary transfer unit 22 in accordance with the timing of the secondary transfer, as described above.

[0031] In addition, the user can feed paper P by inserting it into the manual feed tray 51. When the user inserts paper P into the manual feed tray 51, the image forming apparatus 100 rotates the feed roller 50 to separate one sheet of paper P from the manual feed tray 51 and pulls it into the manual feed path 53. Then, as described above, it abuts against the registration roller 49 to stop it briefly, and then sends it to the secondary transfer unit 22 in accordance with the timing of the secondary transfer described above.

[0032] The paper P, fixed and ejected by the fixing unit 25, is guided by the switching claw 55 to the ejection roller 56, ejected by the ejection roller 56, and stacked on the output tray 57. Alternatively, it is guided by the switching claw 55 to the sheet reversal unit 28, which reverses it and guides it back to the secondary transfer position. After an image is formed on the back side of the paper P, it is ejected onto the output tray 57 by the ejection roller 56.

[0033] Meanwhile, any residual toner remaining on the intermediate transfer belt 10 after image transfer is removed by the intermediate transfer cleaning unit 17, preparing the belt for further image formation.

[0034] The image forming apparatus 100 can thus form a color image on the paper P.

[0035] <Example of the configuration of the image-making unit 20> Next, the configuration of the image-forming unit 20 in the image forming apparatus 100 will be explained using Figure 2.

[0036] Figure 2 is a diagram showing an example of the configuration of the image-forming unit 20, and shows one example of the configuration of the image-forming unit for each color. As mentioned above, the image-forming units for the other three colors have similar configurations except for the different toner colors used, so their illustrations and explanations are omitted, and only one image-forming unit is explained. In other words, in the case of Figure 1, each symbol in Figure 2 is represented by the subscripts "Y", "M", "C", and "K" which indicate the toner color used.

[0037] The image-forming unit 20 comprises a photoreceptor drum 40, a charging roller 18, a developer 29, a cleaning blade 13, a static eliminator 19, and a primary transfer roller 62. A high-voltage power supply 181 for charging is electrically connected to the charging roller 18, and a high-voltage power supply 621 for transfer is electrically connected to the primary transfer roller 62.

[0038] As an example of an image carrier, the photoreceptor drum 40 is a negatively charged organic photoreceptor, with a photosensitive layer and the like provided on a drum-shaped conductive support. The photoreceptor drum 40 has a conductive support as the base layer, on which an insulating undercoat layer, a charge generation layer and a charge transport layer as the photosensitive layer, and a protective layer, i.e., a surface layer, are sequentially laminated. A conductive material with a volume resistivity of 10¹⁰ Ωcm or less can be used for the conductive support of the photoreceptor drum 40.

[0039] The charging roller 18 is a roller member formed by coating the outer circumference of a conductive core with a medium-resistance elastic layer. A predetermined voltage is applied from a high-voltage power supply 181 for charging, and the surface of the photoreceptor drum 40 facing the charging roller 18 is uniformly charged. A cleaning roller for removing dirt from the charging roller 18 may also be provided in contact with the charging roller 18.

[0040] A small gap is provided between the charging roller 18 and the photoreceptor drum 40, and the charging roller 18 is positioned in a non-contact state with respect to the photoreceptor drum 40. This charging method for charging the photoreceptor drum 40 is called a non-contact charging method.

[0041] Compared to the contact charging method, which charges the photoreceptor drum 40 by bringing the charging roller 18 into contact, the non-contact charging method makes it less likely for foreign substances such as toner and lubricant remaining on the photoreceptor drum 40 to adhere to the charging roller 18, thus suppressing uneven charging due to the adhesion of foreign substances. However, the embodiment is not limited to the non-contact charging method and may also be applied to the contact charging method. The high-voltage power supply 181 for charging applies a charging bias to the charging roller 18.

[0042] The developing unit 29 has a developing roller 29a facing the photosensitive drum 40. The developing roller 29a comprises a magnet fixed inside that forms magnetic poles on the roller's circumferential surface, and a sleeve that rotates around the magnet. Multiple magnetic poles are formed on the developing roller 29a by the magnet, and the developer is carried on the developing roller 29a.

[0043] The cleaning blade 13 mechanically scrapes off any untransferred toner or other deposits adhering to the surface of the photoreceptor drum 40. The cleaning blade 13 is a blade-shaped member made of a rubber material such as urethane rubber and formed in a roughly plate-like shape, and it contacts the surface of the photoreceptor drum 40 at a predetermined angle and with predetermined pressure.

[0044] The static eliminator 19 removes the charge from the surface of the photoreceptor drum 40 after the toner image has been transferred. The photoreceptor drum 40, uniformly charged by the charging roller 18, is exposed to a light beam from the light beam scanning unit 21 according to the image data. An electrostatic latent image is formed on the surface of the photoreceptor drum 40. The developer unit 29 deposits toner onto the electrostatic latent image formed on the surface of the photoreceptor drum 40. This develops a toner image on the surface of the photoreceptor drum 40.

[0045] The voltage generated by the high-voltage power supply 621 for transfer is applied to the primary transfer roller 62, transferring the toner image on the surface of the photoreceptor drum 40 to the intermediate transfer belt 10. The toner image on the intermediate transfer belt 10 is transferred to the paper P by the secondary transfer unit 22 and fixed to the paper P by the fuser unit 25. Any remaining toner on the surface of the photoreceptor drum is removed by the cleaning blade 13. In addition, any electric charge on the surface of the photoreceptor drum 40 is removed by the static eliminator 19.

[0046] <Example of hardware configuration of image forming apparatus 100> Next, the hardware configuration of the image forming apparatus 100 will be described. Figure 3 is a block diagram showing an example of the hardware configuration of the image forming apparatus 100.

[0047] As shown in Figure 3, the image forming apparatus 100 includes a controller 910, a short-range communication circuit 920, an engine control unit 930, an operation panel 940, a network interface 950, and a control board 960.

[0048] Of these components, the controller 910 includes the CPU 901, which is the main part of the computer; system memory (MEM-P) 902; northbridge (NB) 903; southbridge (SB) 904; ASIC (Application Specific Integrated Circuit) 906; local memory (MEM-C) 907, which is the storage unit; HDD controller 908; and HD 909, which is the storage unit. Furthermore, the NB 903 and the ASIC 906 are connected by an AGP (Accelerated Graphics Port) bus 921.

[0049] Of these, the CPU (Central Processing Unit) 901 is a control unit that performs overall control of the image forming apparatus 100. The NB903 is a bridge for connecting the CPU 901 with the MEM-P902, SB904, and AGP bus 921, and includes a memory controller that controls reading and writing to the MEM-P902, as well as a PCI (Peripheral Component Interconnect) master and an AGP target.

[0050] MEM-P902 consists of a ROM (Read Only Memory) 902a, which is a memory for storing programs and data that realize the various functions of the controller 910, and a RAM (Random Access Memory) 902b, which is used for program and data deployment and drawing during memory printing.

[0051] Furthermore, the program stored in RAM902b may be configured to be provided as an installable or executable file recorded on a computer-readable recording medium such as a CD-ROM, CD-R, or DVD.

[0052] SB904 is a bridge for connecting NB903 to PCI devices and peripheral devices. ASIC906 is an integrated circuit (IC) for image processing applications that has hardware elements for image processing and acts as a bridge connecting AGP bus 921, PCI bus 922, HDD908, and MEM-C907, respectively.

[0053] This ASIC906 consists of a PCI target and an AGP master, an arbiter (ARB) that forms the core of the ASIC906, a memory controller that controls the MEM-C907, multiple DMACs (Direct Memory Access Controllers) that perform image data rotation and other operations using hardware logic, and a PCI unit that performs data transfer between the scanner unit 931 and the printer unit 932 via the PCI bus 922.

[0054] Additionally, the ASIC906 can be connected to a USB interface or an IEEE 1394 (Institute of Electrical and Electronics Engineers 1394) interface.

[0055] MEM-C907 is local memory used as a copy image buffer and code buffer. HD909 is storage for storing image data, font data used during printing, and forms. HD909 controls data reading or writing to it according to the control of CPU901.

[0056] The AGP bus 921 is a bus interface for graphics accelerator cards proposed to accelerate graphics processing, and by providing high-throughput direct access to the MEM-P902, it can speed up graphics accelerator cards.

[0057] Furthermore, the short-range communication circuit 920 is equipped with a short-range communication circuit 920a. The short-range communication circuit 920 is a communication circuit such as NFC or Bluetooth (registered trademark).

[0058] Furthermore, the engine control unit 930 is composed of a scanner unit 931 and a printer unit 932. The image-making unit 20 described in Figure 2 is included in this printer unit 932.

[0059] The control panel 940 includes a panel display unit 940a, such as a touch panel, which displays current settings and selection screens and accepts input from the operator, and a control panel 940b, which consists of a numeric keypad that accepts setting values ​​for image formation conditions such as density settings, and a start key that accepts a copy start command.

[0060] The controller 910 controls the entire image forming apparatus 100, for example, controlling drawing, communication, and input from the operation panel 940. The scanner unit 931 or the printer unit 932 includes image processing parts such as error diffusion and gamma conversion.

[0061] Furthermore, the image forming apparatus 100 allows the user to sequentially switch between the document box function, copy function, printer function, and facsimile function using the application switching key on the operation panel 940.

[0062] When the document box function is selected, it will be in document box mode; when the copy function is selected, it will be in copy mode; when the printer function is selected, it will be in printer mode; and when the facsimile mode is selected, it will be in facsimile mode.

[0063] Furthermore, the network interface 950 is an interface for data communication using a network. The short-range communication circuit 920 and the network interface 950 are electrically connected to the ASIC 906 via the PCI bus 922.

[0064] <Overview of Phase Alignment Control> As shown in Figure 1, in a tandem color image forming apparatus 1 having multiple photoreceptor drums 40 (40Y, 40M, 40C, 40K) as an image carrier, periodic image density unevenness may occur along the circumference of the photoreceptor due to eccentricity of the photoreceptor drums 40. Therefore, in the image forming apparatus 1 of this embodiment, "phase alignment control" is implemented to synchronize the density phase unevenness of each color in order to suitably reduce image density unevenness.

[0065] Figure 4 is a diagram illustrating the overview of phase alignment control. Figures 4(A) and (B) show the overview of the conventional technology and the phase alignment control of this embodiment. Figures 4(C) and (D) show examples of printed images generated by the conventional technology and the phase alignment control of this embodiment. With regard to this technology, synchronizing the phase of density unevenness in Y, M, C, K, and S colors is also effective, but in Figure 4 only Y, M, and C are shown as an example.

[0066] In Figures 4(A) and (B), the horizontal axis represents time, and the vertical axis represents the density unevenness of the 40Y, 40M, and 40C photoreceptor drums for each color. In Figures 4(C) and (D), the horizontal axis represents the sub-scanning direction, and the vertical axis represents the brightness L* and chromaticity a* and b* of the printed image.

[0067] As shown in Figure 4(A), if each color photoreceptor is driven independently in response to toner deposition amount fluctuations (density unevenness) that occur in the photoreceptor cycle, the phases of each color remain out of sync because the phases are not synchronized. Therefore, as shown in Figure 4(C), when a superimposed image of two or more colors is formed, density unevenness is exaggerated, and large density fluctuations may occur. In the example in Figure 4(C), the hue components a* and b* fluctuate, making periodic color fluctuations noticeable.

[0068] On the other hand, in this embodiment, as shown in Figure 4(B), phase alignment control is performed to drive the photoreceptor drums 40 of each color so that the density uneven phases of each color photoreceptor match on the image. As shown in Figure 4(D), the brightness and chromaticity of the generated image are synchronized by the phase alignment control, and the hue components a* and b* can be suppressed, thereby suppressing density unevenness (ΔE) in the sub-scanning direction caused by the rotating body.

[0069] Figure 5 is an illustrative diagram of phase alignment control. Figure 5(A) shows the phases of each color photoreceptor drum 40C, 40M, and 40Y at the start of phase alignment control, and Figure 5(B) shows the phases of each color photoreceptor drum 40C, 40M, and 40Y at the completion of phase alignment control. Figure 5(C) is a diagram showing an example of the relationship between drum phase and density unevenness, with the horizontal axis representing drum phase and the vertical axis representing density unevenness. In Figure 5(C), the phase at which density unevenness is maximum is indicated by the symbol a. In Figure 5(C), a predetermined reference position for the drum phase is also shown. In Figures 5(A) and (B), the peak of density unevenness in each photoreceptor drum C, M, and Y, i.e., the position of phase a at which the density unevenness shown in Figure 5(C) is maximum, and the reference position are illustrated along the circular circumferential direction of each drum.

[0070] As shown in Figure 5(A), at the start of phase alignment control, the positions of the density unevenness peaks relative to the reference position are different for each photoreceptor drum 40C, 40M, and 40Y, and the phases of the density unevenness peaks for each photoreceptor drum 40C, 40M, and 40Y are also different.

[0071] In phase alignment control, the rotation speed of each photoreceptor drum 40C, 40M, and 40Y is adjusted so that the phases of the peaks of density unevenness on each photoreceptor drum 40C, 40M, and 40Y coincide on the printed image before each photoreceptor drum 40C, 40M, and 40Y contact the center belt.

[0072] The relationship between density unevenness (density unevenness phase) and the drum phase of each drum 40C, 40M, and 40Y can be obtained in advance through a separate adjustment operation. The example in Figure 5 illustrates a case where a reference position detection unit such as a home position sensor is used as the phase detection means for the photoreceptor drum, and a rotary encoder is used as the speed detection unit.

[0073] As shown in Figure 5(B), when the phase alignment control is completed, the phases of the peaks of density unevenness in each photoreceptor drum 40C, 40M, and 40Y are aligned to be the same.

[0074] <Specific configuration of phase alignment control> Figure 6 is a functional block diagram of an image forming apparatus related to phase alignment control.

[0075] In addition to the photoreceptor drum 40 described above, the image forming apparatus 100 includes a motor 71, a rotation speed detection unit 72, and a rotation position detection unit 73. Furthermore, the controller 910 has functions related to phase alignment control, including a determination unit 74, a density detection unit 75, a density unevenness detection unit 76, a target phase calculation unit 77, a relative phase difference calculation unit 78, a reference selection unit 79, and a correction control unit 80.

[0076] The motor 71 rotates each of the multiple photoreceptor drums 40. Specifically, the motor 71 consists of multiple units, each driving one of the multiple photoreceptor drums 40Y, 40M, 40C, and 40K, and is also referred to as multiple motors 71Y, 71M, 71C, and 71K. Figures 5(A) and (B) above illustrate the motors 71C, 71M, and 71Y corresponding to the photoreceptor drums 40C, 40M, and 40Y.

[0077] The rotational speed detection unit 72 detects the rotational speeds of multiple motors 71. Specifically, the rotational speed detection unit 72 is composed of multiple units for detecting the rotational speed of each of the multiple motors 71Y, 71M, 71C, and 71K, and is also referred to as multiple rotational speed detection units 72Y, 72M, 72C, and 72K. As the rotational speed detection unit 72, elements such as rotary encoders can be applied, for example, as shown in the speed detection unit illustrated in Figure 5. The rotational speed detection unit 72 outputs the information of the detected rotational speed of each motor 71 to the determination unit 74.

[0078] The rotation position detection unit 73 detects the rotational positions (θy, θm, θc shown in Figure 10, etc.) of the multiple photoreceptor drums 40. The rotation position detection unit 73 consists of multiple units for detecting the rotational position of each of the multiple photoreceptor drums 40Y, 40M, 40C, and 40K, and is also referred to as multiple rotation position detection units 73Y, 73M, 73C, and 73K. As the rotation position detection unit 73, elements such as a home position sensor can be applied, for example, as in the reference position detection unit shown in Figure 5. The rotation position detection unit 73 outputs the detected rotational position information of the photoreceptor drums 40 to the density unevenness detection unit 76 and the relative phase difference calculation unit 78.

[0079] The determination unit 74 determines whether the rotational speeds of the multiple photoreceptor drums 40Y, 40M, 40C, and 40K are within the target speed range. The determination unit 74 can, for example, use the rotational speed information of the motor 71 detected by the rotational speed detection unit 72 to calculate the rotational speed of the photoreceptor drum 40 and make the determination. Alternatively, the rotational speed detection unit 72 may directly measure the rotational speed of the photoreceptor drum 40, and the determination unit 74 may use the rotational speed of the photoreceptor drum 40 directly measured by the rotational speed detection unit 72 to make the determination. The determination unit 74 outputs the determination result to the correction control unit 80.

[0080] The density detection unit 75 detects the density of toner images formed on multiple photoreceptor drums 40Y, 40M, 40C, and 40K. The density detection unit 75 outputs information on the density of the detected toner images to the density unevenness detection unit 76.

[0081] The density unevenness detection unit 76 forms a predetermined image pattern and, based on the detection results from the rotation position detection unit 73 and the density detection unit 75, detects the density unevenness of each rotation period of the multiple photoreceptor drums 40Y, 40M, 40C, and 40K, as well as the density unevenness phase from the rotation position reference. The density unevenness detection unit 76 calculates the periodic density unevenness of each photoreceptor drum 40, as illustrated in Figures 4(A) and (B), based on the density information of the toner image detected by the density detection unit 75, for example. The density unevenness detection unit 76 also derives a reference position for each photoreceptor drum 40, as illustrated in Figures 5(A) and (B), based on the rotation position information of each photoreceptor drum 40 detected by the rotation position detection unit 73, for example. Then, it derives the density unevenness phase from the relationship between the periodic density unevenness and the rotation position reference, as illustrated in Figure 5(C). The concentration unevenness detection unit 76 outputs the detected concentration unevenness phase information to the target phase calculation unit 77.

[0082] The target phase calculation unit 77 calculates the target phases (θy_offset, θm_offset, θc_offset shown in Figure 10, etc.) for each of the multiple photoreceptor drums 40Y, 40M, 40C, and 40K from the density uneven phase acquired by the density uneven phase detection unit 76. The target phase calculation unit 77 outputs the calculated target phase information for each photoreceptor drum 40 to the relative phase difference calculation unit 78.

[0083] The relative phase difference calculation unit 78 calculates the relative phase differences (eYM, eYC, eMY, eMC, eCY, eCM shown in Figure 11) of the multiple photoreceptor drums 40Y, 40M, 40C, and 40K from the rotational positions of the multiple photoreceptor drums 40Y, 40M, 40C, and 40K acquired by the rotational position detection unit 73 and the target phase calculated by the target phase calculation unit 77. The relative phase difference calculation unit 78 outputs the calculated relative phase difference information of each photoreceptor drum 40 to the reference selection unit 79.

[0084] The reference selection unit 79 selects, from among the multiple photoreceptor drums 40Y, 40M, 40C, and 40K, the photoreceptor drum that has the smallest phase correction value (Max(eY#), Max(eM#), Max(eC#) shown in Figure 11) based on the calculation results of the relative phase difference calculation unit 78, as the reference image carrier, or reference drum 40S. The reference selection unit 79 outputs information about the selected reference drum 40S to the correction control unit 80.

[0085] The correction control unit 80, while driving the multiple motors 71Y, 71M, 71C, and 71K, uses the reference drum 40S determined by the reference selection unit 79 as a reference and controls the rotational position of each of the multiple photoreceptor drums 40Y, 40M, 40C, and 40K, other than the reference drum 40S, to the phase correction value determined by the reference selection unit 79. The correction control unit 80 can correct the rotational position of the photoreceptor drums 40Y, 40M, 40C, and 40K driven by each motor, for example, by outputting control commands to each of the motors 71Y, 71M, 71C, and 71.

[0086] In Figures 5(A) and (B), the photoreceptor drum 40M is selected as the reference drum 40S, and the rotational positions of the other photoreceptor drums 40C and 40Y are corrected so that the density unevenness phase aligns with that of the reference drum 40S.

[0087] Furthermore, the correction control unit 80 can determine, for example, based on the determination result of the determination unit 74 described above, that the motors 71Y, 71M, 71C, and 71K are driven when the rotational speeds of the multiple photoreceptor drums 40Y, 40M, 40C, and 40K are within the target speed range. When the correction control unit 80 determines the motor drive state in this way, it performs the correction of the rotational position of each drum as described above.

[0088] Figure 7 is a flowchart of the phase alignment control. In Figure 7, the example is used to explain the process when printing is performed with four colors: Y, M, C, and K, and phase alignment is performed using the Y, M, and C drums (photoreceptor drums 40Y, 40M, and 40C) (the same applies hereafter unless otherwise specified). In this embodiment, the explanation uses Y, M, and C, which have the greatest impact on image quality, as an example, but the phase alignment control method of this embodiment is also applicable to the K color and to cases with five or more colors.

[0089] First, in steps S101 to S104, the controller 910 activates each photoreceptor drum 40Y, 40M, 40C, and 40K. In step S105, the determination unit 74 determines whether the photoreceptor drums 40Y, 40M, and 40C, which are to be phase-aligned, are converging to the target speed. If they are not converging to the target speed (No. in step S105), they enter a waiting state.

[0090] If the determination unit 74 confirms convergence (Yes in step S105), it is determined that the motors 71 that rotate the photoreceptor drums 40Y, 40M, and 40C have been driven, and the phase alignment control process from step S106 onwards begins.

[0091] In step S106, the rotation position detection unit 73 starts phase detection of each photoreceptor drum 40Y, 40M, and 40C. If phase detection is not complete, the unit waits (as indicated in step S107).

[0092] Once phase detection is complete for each photoreceptor drum 40Y, 40M, and 40C (Yes in step S107), the phase correction value is calculated in step S108. Details of the process in step S108 will be described later with reference to Figure 11. In step S109, the correction control unit 80 starts the phase correction. If the phase correction is not complete (No in step S109), the system waits.

[0093] In step S108, the correction control unit 80 may be configured to decelerate and align each of the other photoreceptor drums 40, other than the reference drum 40S, when controlling the rotational position of each photoreceptor drum 40 to the phase correction value. Alternatively, the correction control unit 80 may be configured to accelerate and align each of the other photoreceptor drums 40, other than the reference drum 40S, when controlling the rotational position of each photoreceptor drum 40 to the phase correction value. The choice between deceleration or acceleration can be appropriately selected depending on the operating conditions of the motor 71 and the photoreceptor drums 40 during the execution of the phase alignment control. This enables more appropriate phase alignment control according to the operating environment.

[0094] Once the phase correction of each photoreceptor drum 40Y, 40M, and 40C is completed (Yes in step S109), the phase alignment is complete, and in step S111, the controller 910 brings each drum 40 into contact with the intermediate transfer belt (intermediate transfer belt 10), and in step S112, the printing operation begins.

[0095] Note that the number of colors used for printing does not have to be the colors listed in this flowchart, as long as there are two or more colors, and the startup order of the photoconductor drum 40 does not have to be the order listed in this flowchart.

[0096] Furthermore, the drive control of components other than the photoreceptor drum 40 has been omitted from the description.

[0097] In the flowchart of Figure 7, an example is shown in which, in step S105, the determination unit 74 confirms that the rotational speeds of the photoreceptor drums 40Y, 40M, and 40C, which are to be phase-aligned, have reached the target speed. This determines that the motors 71 that rotate the photoreceptor drums 40Y, 40M, and 40C have been driven, and the phase-alignment control process from step S106 onwards begins. However, in this embodiment, it is sufficient that the position-alignment control can be performed after the motors 71 that rotate the photoreceptor drums 40Y, 40M, and 40C have been driven, and the determination of the motor 71's driving state may be performed by a method other than the determination unit 74.

[0098] Figure 8 is a timing chart of various signals in the phase alignment control shown in Figure 7. Figure 8 shows an example of the time progression of the motor rotation speeds (Y), (M), and (C) of each photoreceptor drum 40Y, 40M, and 40C, the reference position signals (Y), (M), and (C), the rotation phase acquisition signal, the phase correction value calculation processing signal, and the phase correction value input signal. At the bottom of Figure 8, the states [1] to [5] of each photoreceptor drum 40Y, 40M, and 40C corresponding to the time axis of each of the above signals are shown.

[0099] Each photoreceptor drum 40Y, 40M, and 40C, after startup, sequentially transitions through [1] a startup period, [2] a phase detection period, [3] a calculation period, [4] a phase correction period, and [5] a speed control period. As shown in Figure 7, [1] the startup period corresponds to steps S101 to S105 in Figure 7, [2] the phase detection period corresponds to steps S106 to S107, [3] the calculation period corresponds to step S108, [4] the phase correction period corresponds to steps S109 to S110, and [5] the speed control period corresponds to steps S111 to S112.

[0100] As shown in Figure 8, [1] During the startup period, the drum motors 71Y, 71M, and 71C that perform phase matching control are started up (= converged to the same target speed). For example, if the speed stays within ±3% of the target speed for 50 milliseconds, it can be determined that the speed has converged to the target speed. In the example in Figure 8, the motor rotation speeds (Y), (M), and (C) of the drum motors 71Y, 71M, and 71C start up in this order, and when each reaches its target speed, it transitions to [2] the phase detection period.

[0101] [2] During the phase detection period, rotational phase is acquired at the rising edges of the reference position signals (Y), (M), and (C) of each photoreceptor drum 40Y, 40M, and 40C. In the example in Figure 8, the reference position signals (Y), (C), and (M) are detected in the order of photoreceptor drum 40Y → photoreceptor drum 40C → photoreceptor drum 40M, and the rotational phase signals are acquired at the timing of the rising edges of these reference position signals. The system transitions to the [3] calculation period when all rotational phase signals have been acquired.

[0102] [3] During the calculation period, the phase correction value calculation processing signal rises, and the phase correction value is calculated from the relationship between the rotational phase difference of each photoreceptor drum (40Y, 40M, 40C) and the respective density uneven phases. When the calculation of the phase correction value is completed, the system transitions to [4] the phase correction period.

[0103] [4] During the phase correction period, the phase correction values ​​calculated in the [3] calculation period are input to the respective drum motors 71Y, 71M, and 71C to adjust the rotational phase of each photoreceptor drum 40Y, 40M, and 40C. In the example in Figure 8, the phase correction value input signal is switched to the ON state, and the motor rotation speeds (Y), (M), and (C) of each motor 71Y, 71M, and 71C are adjusted accordingly, thereby correcting the rotational phase of each photoreceptor drum 40Y, 40M, and 40C. Once the correction of the rotational phase of each drum is complete, the motor rotation speeds (Y), (M), and (C) are returned to the target speed, and the program transitions to the [5] speed control period.

[0104] [5] During the speed control period, the speeds of the drum motors 71Y, 71M, and 71C are controlled to maintain a steady speed. Note that during the [4] phase correction period, the rotational speed of each motor 71 is changed, so contact between the photoreceptor drum 40 and the intermediate transfer belt 10 is made from the [5] speed control period after the rotational speed has stabilized. For example, if the rotational speed remains within ±3% of the target speed for 50 milliseconds continuously, it can be determined that the rotational speed has stabilized.

[0105] Figure 9 is a flowchart of the phase acquisition process for each drum during the phase detection period.

[0106] In the phase detection period [2] in Figure 8, the number of encoder pulses (angle) of one drum is recorded at the first HP interrupt of each photoreceptor drum 40Y, 40M, and 40C to determine the rotational phase difference of each drum. In Figure 9, the number of encoder pulses (angle) of drum Y (photoreceptor drum 40Y) is recorded as an example, but other colored drums can also be used.

[0107] Figure 10 illustrates the parameters used for phase correction in this embodiment. Figures 10(A), (B), and (C) are graphs of density unevenness for drums Y, M, and C (photoreceptor drums 40Y, 40M, and 40C), respectively. The horizontal axis of each graph represents the drum phase, and the vertical axis represents the density unevenness.

[0108] In this embodiment, as shown in Figure 10, the phases of the phase detection period for each photoreceptor drum 40Y, 40M, and 40C are stored as θy, θm, and θc, and the target phases of the density unevenness phase calculation results (acquired in advance) are set as θy_offset, θm_offset, and θc_offset. From Figure 10, the angles θy, θm, and θc can also be expressed as the rotation angles from the position at the start of the phase detection period to the reference positions (Y), (M), and (C) of each photoreceptor drum 40Y, 40M, and 40C. Furthermore, the target phases θy_offset, θm_offset, and θc_offset can also be expressed as the difference between the position where the density unevenness of each photoreceptor drum 40Y, 40M, and 40C is maximum and the reference positions (Y), (M), and (C).

[0109] Figure 11 is a flowchart of the phase correction value calculation process. The flowchart shown in Figure 11 corresponds to step S108 of the phase alignment control shown in Figure 7, and is the subroutine processing of step S108. Furthermore, the processing of the flowchart shown in Figure 11 is performed during the calculation period [3] shown in Figure 8.

[0110] During the calculation period shown in Figure 8 [3], the process of the flow in Figure 11 is performed to select the reference color for phase alignment and calculate the target phase correction amount to minimize it. If the target phase correction amount can be minimized, the time required for phase alignment will be shortened and the problem can be solved.

[0111] The flow in Figure 11 will be explained using the example where the parameters shown in Figure 10 are θy=200, θm=200, θc=90, θy_offset=200, θm_offset=180, and θc_offset=180.

[0112] In the preceding steps of the flowchart in Figure 11, the density detection unit 75 detects the density of the toner image formed on each photoreceptor drum 40. Next, the density unevenness detection unit 76 detects the density unevenness for each rotation period of each photoreceptor drum 40 and the density unevenness phase from the rotation position reference, based on the detection results from the rotation position detection unit 73 and the density detection unit 75, for example, as shown in the graph illustrated in Figure 10. Then, the target phase calculation unit 77 calculates the target phases θy_offset, θm_offset, and θc_offset for each photoreceptor drum 40 from the density unevenness phase acquired by the density unevenness detection unit 76. In addition, the phases θy, θm, and θc of each photoreceptor drum 40 are detected in advance by the rotation position detection unit 73 in steps S106 and S107 of Figure 7.

[0113] In step S301, the relative phase difference calculation unit 78 calculates the relative phase angles based on Y, M, and C. The relative phase difference calculation unit 78 determines the relative phase angles starting from each photoreceptor drum 40Y, 40M, and 40C.

[0114] When using the Y reference point starting from the photoconductor drum 40Y, the relative phase difference eYM of the photoconductor drum 40M and the relative phase difference eYC of the photoconductor drum 40C can be calculated using the following equations (1) and (2). eYM=(θy+θy_offset)-(θm+θm_offset) ···(1) eYC=(θy+θy_offset)-(θc+θc_offset) ···(2)

[0115] When using M as the reference point, starting from the photoconductor drum 40M, the relative phase difference eMY of the photoconductor drum 40Y and the relative phase difference eMC of the photoconductor drum 40C can be calculated using the following equations (3) and (4). eMY=(θm+θm_offset)-(θy+θy_offset) ···(3) eMC=(θm+θm_offset)-(θc+θc_offset) ···(4)

[0116] When using C as the reference point, starting from the photoconductor drum 40C, the relative phase difference eCY of the photoconductor drum 40Y and the relative phase difference eCM of the photoconductor drum 40M can be calculated using the following equations (5) and (6). eCY=(θm+θm_offset)-(θy+θy_offset) ···(5) eCM=(θm+θm_offset)-(θc+θc_offset) ···(6)

[0117] In the numerical examples of each parameter described above, after processing in step S301, the values ​​will be eYM=-20, eYC=-130, eMY=-20, eMC=110, eCY=-130, and eCM=-110.

[0118] In steps S302 to S305, the relative phase difference calculation unit 78 converts the relative phase angle calculated in step S301 to a value less than one full rotation (0 or more and less than 360).

[0119] The parameter "e##" described in steps S302 and S304 is a collective representation of the six parameters eYM, eYC, eMY, eMC, eCY, and eCM. If any parameter has a negative value (Yes in S302), 360 is added (S303). Also, if any parameter has a value of 360 or more (Yes in S304), 360 is subtracted (S305).

[0120] In the numerical examples of each parameter described above, after processing steps S302 to S305, the values ​​will be eYM=350, eYC=230, eMY=340, eMC=110, eCY=230, and eCM=250.

[0121] In step S306, the reference drum 40S is selected by the reference selection unit 79. For each starting drum, the larger of two relative phase angles is selected (Max(e##)), and further, the drum with the smallest color among the relative phase angles selected for each starting drum (Min(Max(eY#), Max(eM#), Max(eC#)) is designated as the reference drum 40S.

[0122] In the numerical examples of each parameter mentioned above, the largest values ​​for each criterion are Y criterion: eYM=350, M criterion: eMY=340, and C criterion: eCM=250. Since eCM is the smallest value among these, the photoreceptor drum 40C, which is the starting point for criterion C, is selected as criterion drum 40S.

[0123] In steps S307 to S309, the correction control unit 80 sets the target phase (phase correction value) of the other drums based on the reference drum 40S selected in step S307.

[0124] In step S307, the photoreceptor drum 40Y, which is the starting point of the reference Y, is selected as the reference drum 40S. Therefore, the target phase (phase correction value) is set to 0 for photoreceptor drum 40Y, -eYM for photoreceptor drum 40M, and -eYC for photoreceptor drum 40C.

[0125] In step S308, the photoreceptor drum 40M, which is the starting point of the reference M, is selected as the reference drum 40S. Therefore, the target phase (phase correction value) is set to -eMY for photoreceptor drum 40Y, 0 for photoreceptor drum 40M, and -eMC for photoreceptor drum 40C.

[0126] In step S309, the photoreceptor drum 40C, which is the starting point of reference C, is selected as the reference drum 40S. Therefore, the target phase (phase correction value) is set to -eCY for photoreceptor drum 40Y, -eCM for photoreceptor drum 40M, and 0 for photoreceptor drum 40C.

[0127] In the numerical examples of each parameter described above, since the reference C is selected in step S307, after processing in step S309, the target phases will be Y:-230, M:-250, and C:0.

[0128] In the image forming apparatus 100 of this embodiment, the reference selection unit 79 selects the drum from among the multiple photoreceptor drums 40 that has the smallest phase correction value based on the calculation result of the relative phase difference calculation unit 78 as the reference drum 40S (reference image carrier). With the multiple motors 71 driven, the correction control unit 80 controls the rotational position of each of the multiple photoreceptor drums 40 other than the reference drum 40S to the phase correction value, using the reference drum 40S determined by the reference selection unit 79 as a reference.

[0129] This configuration makes it possible to efficiently align the phases of multiple photosensitive drums 40, thereby effectively reducing unevenness in image density.

[0130] Furthermore, in this embodiment, the angles at the time when the position detection of the photoreceptor drum 40 is completed are stored as θy, θm, and θc, and the target phases of the density unevenness phase calculation results are set as θy_offset, θm_offset, and θc_offset. During calculation, the process shown in Figure 11 is performed to calculate the target position correction amount in order to minimize it. If the target position correction amount can be minimized, the time required for phase alignment can be shortened.

[0131] The embodiments have been described above with reference to specific examples. However, this disclosure is not limited to these specific examples. Modifications made to these specific examples by those skilled in the art are also included within the scope of this disclosure, as long as they retain the features of this disclosure. The elements, their arrangement, conditions, shapes, etc., of each of the aforementioned specific examples are not limited to those illustrated and can be modified as appropriate. The elements of each of the aforementioned specific examples can be combined in different ways as appropriate, as long as no technical inconsistencies arise. [Explanation of Symbols]

[0132] 100 Image forming apparatus 40 (40Y, 40M, 40C, 40K) Photoreceptor Drum (Image Carrier) 40S Reference Drum (Reference Image Carrier) 71 (71Y, 71M, 71C, 71K) Motor 72 (72Y, 72M, 72C, 72K) Rotational speed detection unit 73 (73Y, 73M, 73C, 73K) Rotation position detection unit 910 Controller 74 Judgment section 75 Concentration detection unit 76 Concentration unevenness detection unit 77 Target phase calculation section 78 Relative Phase Difference Calculation Unit 79. Criteria Selection Section 80 Correction Control Unit [Prior art documents] [Patent Documents]

[0133] [Patent Document 1] Japanese Patent Publication No. 2020-177098 [Patent Document 2] Japanese Patent Publication No. 2011-081171

Claims

1. Multiple image carriers, Multiple motors that rotate each of the multiple image carriers, A rotation position detection unit for detecting the rotational position of the plurality of image carriers, A density detection unit for detecting the density of toner images formed on the plurality of image carriers, A density unevenness detection unit forms a predetermined image pattern and detects the density unevenness of the rotational period of each of the multiple image carriers and the density unevenness phase from the rotational position reference based on the detection results from the rotational position detection unit and the density detection unit, A target phase calculation unit calculates the target phase for each of the multiple image carriers from the density unevenness phase acquired by the density unevenness detection unit, A relative phase difference calculation unit calculates the relative phase difference of each of the multiple image carriers based on the rotation position obtained by the rotation position detection unit and the target phase calculated by the target phase calculation unit. A reference selection unit selects, from among the plurality of image carriers, the image carrier whose phase correction value is minimized according to the calculation results of the relative phase difference calculation unit, as the reference image carrier. With the plurality of motors driven, a correction control unit controls the rotational position of each of the other image carriers of the plurality of image carriers to the phase correction value, using the reference image carrier determined by the reference selection unit as a reference. Equipped with, The relative phase difference calculation unit calculates multiple relative phase differences between each of the multiple image carriers, starting from each of the multiple image carriers, The reference selection unit selects the maximum relative phase difference from the plurality of relative phase differences for each of the image carriers designated as the starting point, and further selects the image carrier of the color with the smallest relative phase difference among the maximum relative phase differences selected for each of the image carriers designated as the starting point as the reference image carrier. The relative phase difference calculation unit calculates a value obtained by adding 360 degrees if the multiple relative phase differences take a negative value. Image forming apparatus.

2. A rotation speed detection unit for detecting the rotation speed of the multiple motors, A determination unit that determines whether the rotation speed of the multiple image carriers falls within the target speed, Equipped with, The correction control unit determines the state in which the plurality of motors are driven based on the determination result of the determination unit. The image forming apparatus according to claim 1.

3. The correction control unit, When controlling the rotational position to the phase correction value, each of the other image carriers is decelerated and aligned. The image forming apparatus according to claim 1 or 2.

4. The correction control unit, When controlling the rotational position to the phase correction value, each of the other image carriers is accelerated and aligned. The image forming apparatus according to claim 1 or 2.

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