Image forming apparatus

Abstract

To accurately correct the partial magnification of an image formed on a sheet.SOLUTION: An image forming apparatus has: image forming means that forms an image on an image carrier; transfer means that has a roller to form a nip part between the image carrier and itself and transfers the image on the image carrier to the recording material; a driving source for rotating the roller; control means that controls the rotation speed of the roller rotated by the driving source; acquisition means that causes the image forming means to form a test chart, causes the control means to control the rotation speed of the roller to a different rotation speed, causes the transfer means to transfer the test chart to the recording material, and acquires read data related to the test chart on the recording material; and determination means that, based on the read data acquired by the acquisition means, determines a target speed of the rotation speed controlled by the control means.SELECTED DRAWING: Figure 8

Description

[Technical Field]

[0001] The present invention relates to a technique for correcting magnification in the sub-scanning direction. [Background technology]

[0002] In an electrophotographic image forming apparatus, a toner image is generally fixed onto a sheet of paper through the steps of charging, exposure, development, transfer, and fixing.

[0003] Image forming devices using the intermediate transfer method employ an image formation process in which a toner image formed in the image forming unit is primarily transferred onto an intermediate transfer belt, and then the toner image formed on the intermediate transfer belt is secondarily transferred onto paper such as paper media that has been fed or transported.

[0004] Secondary transfer is performed in a secondary transfer nip formed by pressing the surface of the intermediate transfer belt against a secondary transfer member (inner secondary transfer roller) that stretches the backside of the intermediate transfer belt with an opposing roller (outer secondary transfer roller).

[0005] Here, since the shape of the secondary transfer nip (secondary transfer nip) differs depending on the paper being passed through, it is known that various issues arise when performing secondary transfer depending on the paper conditions (stiffness, basis weight, surface properties, etc.).

[0006] For example, when paper of different thicknesses, such as thin paper or thick paper, is passed through, or when paper of different surface properties, such as high-quality paper or glossy paper, is passed through, the conveying force applied to the paper from the intermediate transfer belt changes, and the paper conveying speed also changes.

[0007] Furthermore, when toner is present between the intermediate transfer belt and the paper at the secondary transfer nip, the intermediate transfer belt applies a transport force to the paper via the toner, and the transport speed also changes.

[0008] Furthermore, the paper transport speed also changes depending on the surface properties of the paper on the side of the outer secondary roller (i.e., the second side of double-sided printing), and whether or not there is a toner layer that has undergone a fixing process on the surface of the paper. This difference becomes a problem, especially when drive is applied to the outer secondary roller.

[0009] When the fluctuations in the paper transport speed described above occur within the plane of the paper, they appear as partial expansion and contraction of the image in the paper transport direction (the sub-scanning direction of the image) (partial magnification fluctuations), and when the speed fluctuates across the entire paper plane, they appear as overall expansion and contraction of the image in the sub-scanning direction (overall magnification fluctuations).

[0010] As a means of solving the above problems and meeting high image quality specifications for the increasingly diverse paper types in the commercial printing market in recent years, Patent Document 1 describes a method of detecting the linear speed of the intermediate transfer belt and the paper transport speed downstream of the transfer section, and controlling the speed of the secondary transfer roller based on the detection results to suppress fluctuations in magnification. [Prior art documents] [Patent documents]

[0011] [Patent Document 1] Japanese Patent Application Laid-Open No. 2018-200396 Summary of the Invention [Problem to be solved by the invention]

[0012] In the above-mentioned invention, a sensor is provided to measure the speed of the paper after the transfer nip, the average speed within the paper surface is calculated, and the conveying speed of the secondary outer drive roller is determined, thereby making it possible to suppress fluctuations in the overall magnification even if the thickness of the paper changes.

[0013] However, this configuration calculates the average speed of one sheet of paper, so it cannot follow partial magnification fluctuations within the surface of the paper. Therefore, when printing an image with a significantly different amount of toner applied within the surface of the paper, partial magnification fluctuations within the surface may occur.

[0014] Therefore, an object of the present invention is to correct the partial magnification of an image formed on a sheet with high precision. [Means for solving the problem]

[0015] In order to achieve the above object, the image forming apparatus of the present invention is characterized by comprising: an image forming means for forming an image on an image carrier; a transfer means having a roller that forms a nip portion between the image carrier and the transfer means and that transfers the image on the image carrier to a recording material; a drive source for rotating the roller; a control means for controlling the rotational speed of the roller rotated by the drive source; an acquisition means for causing the image forming means to form a test chart, causing the control means to control the drive source to change the rotational speed of the roller to different rotational speeds, causing the transfer means to transfer the test chart to the recording material, and acquiring read data related to the test chart on the recording material; and a determination means for determining a target speed for the rotational speed controlled by the control means based on the read data acquired by the acquisition means. [Effects of the Invention]

[0016] According to the present invention, the partial magnification of an image formed on a sheet can be corrected with high precision. [Brief explanation of the drawings]

[0017] [Figure 1] Schematic cross-sectional view of an image forming apparatus [Figure 2] Test chart diagram [Figure 3] Test chart diagram [Figure 4] A graph showing the sub-scanning magnification deviation obtained from a test chart. [Figure 5] FIG. 10 is a graph showing the speed sensitivity of the outer roller [Figure 6] Schematic diagram of velocity profile [Figure 7] Schematic diagram of the front and back sides of the test chart [Figure 8] Control block diagram of an image forming apparatus [Figure 9]Figure showing an example of a screen displayed on the operation unit [Figure 10] Schematic diagram of the transfer unit drive configuration [Figure 11] Figure showing an example of a screen displayed on the operation unit DETAILED DESCRIPTION OF THE INVENTION

[0018] Hereinafter, an embodiment of an image forming apparatus according to the present invention will be described with reference to the drawings. Note that, although the following describes an example in which the present invention is applied to an electrophotographic full-color image forming apparatus having multiple photosensitive drums, the present invention is not limited to this, and can also be applied to various types of image forming apparatuses, monochrome image forming apparatuses, etc.

[0019] First, the schematic configuration of the image forming apparatus of this embodiment will be described with reference to FIG.

[0020] Figure 1 is a diagram showing a full-color image forming apparatus according to this embodiment. In the following description, the longitudinal direction of the image forming apparatus or the components that make it up is the width direction perpendicular to the recording material conveyance direction within the plane of the recording material conveyance path. The short side direction is the direction parallel to the recording material conveyance direction. The front is the surface of the apparatus as seen from the recording material inlet side, the back is the opposite side (the recording material outlet side), and the left and right are the left and right as seen from the front of the apparatus. The upstream side and downstream side are the upstream side and downstream side with respect to the recording material conveyance direction.

[0021] Image forming apparatus 1000 comprises image reading device 300 and image forming apparatus main body 400. Image reading device 300 reads an original placed on platen glass 302, and light emitted from light source 303 is reflected by the original and forms an image on CCD sensor 305 via optical system components 304 such as lenses. This optical system unit scans in the direction of the arrow, converting the original into an electric signal data string for each line.

[0022] The image signal obtained by the CCD sensor 305 is sent to the image forming apparatus main body 400, where it is processed by the image processing unit 500 in accordance with the image processing required for each image forming unit (described below). The image processing unit 500 can also receive external image signals from a print server or the like. The image forming apparatus main body 400 includes multiple image forming units Pa, Pb, Pc, and Pd, each of which forms an image based on the image signal. Specifically, the image processing unit 500 transmits the image signal to the control unit 309, which converts it into a PWM (pulse-width modulation) laser beam. In FIG. 1, a polygon scanner 310 serves as an exposure device, scanning the laser beam in accordance with the image signal. The laser beam is then irradiated onto the photosensitive drums 200a-200d serving as image carriers for the image forming units Pa-Pd.

[0023] Note that Pa is a yellow (Y) image forming unit, Pb is a magenta (M) image forming unit, Pc is a cyan (C) image forming unit, and Pd is a black (Bk) image forming unit, each of which forms an image of the corresponding color. Since the image forming units Pa to Pd are substantially identical, the Y image forming unit Pa will be described in detail below, and descriptions of the other image forming units will be omitted. In the Y image forming unit Pa, 200a is a photosensitive drum, and as will be described below, a toner image is formed on the surface based on an image signal.

[0024] Reference numeral 201a denotes a primary charger, which charges the surface of the photosensitive drum 200a to a predetermined potential to prepare for the formation of an electrostatic latent image. A laser beam from a polygon scanner 310 forms an electrostatic latent image on the surface of the photosensitive drum 200a, which has been charged to a predetermined potential. Reference numeral 202a denotes a developer, which develops the electrostatic latent image on the photosensitive drum 200a to form a toner image. Reference numeral 203a denotes a transfer roller, which discharges from the back of the intermediate transfer belt 204, which serves as an image carrier, and applies a primary transfer bias of opposite polarity to the toner, thereby transferring the toner image on the photosensitive drum 200a onto the intermediate transfer belt 204. After transfer, the surface of the photosensitive drum 200a is cleaned by a cleaner 207a.

[0025] The toner image on intermediate transfer belt 204 is then transported to the next image forming unit, where the toner images of each color formed in each image forming unit are transferred in the order of Y, M, C, and Bk, forming a four-color image on its surface. The toner image that has passed through the Bk image forming unit is secondarily transferred to paper P in a secondary transfer unit formed by a pair of secondary transfer rollers 205 and 206 by applying a secondary transfer electric field of the same polarity as the toner image on intermediate transfer belt 204 from the intermediate transfer belt side. The fed paper waits in registration unit 208, and then the CPU controls the timing to align the position of the toner image on the intermediate transfer belt with the paper, and the paper is transported from the registration unit. The toner image on the paper is then fixed to the paper in fixing device 700, which serves as an image heating device.

[0026] In the case of a job that prints on both sides of a sheet of paper, once the toner transfer and fixing process for the first side (first side) of image formation is completed, the sheet is tucked into a reversing unit 209 provided inside the image forming device after fixing, stopped once, and the sheet transport direction is reversed and the sheet is transported to a reversing path, where the leading and trailing ends of the sheet are reversed.The sheet then passes through a double-sided transport unit 210, where it is turned over, the toner transfer and fixing process for the second side (second side) of image formation is completed, and the sheet is then ejected from the machine.

[0027] (Magnification fluctuation in the sub-scanning direction) As mentioned above, variations in partial magnification in the sub-scanning direction and variations in overall magnification due to variations in paper transport speed at the secondary transfer nip are caused by differences in paper thickness and surface properties, and toner density on the intermediate transfer belt. Changes in the transport force the paper receives from the intermediate transfer belt cause changes in paper speed, which in turn causes variations in the difference between the intermediate transfer belt (toner image) speed and the paper speed. Note that this invention controls the speed of the paper, and the expansion / contraction components of the image itself are not subject to control, so a description of this will be omitted.

[0028] To correct this fluctuation in the speed of the paper relative to the intermediate transfer belt, the drive source M rotates the secondary outer roller. The secondary outer roller can be changed independently of the speed of the intermediate transfer belt. Therefore, if the paper speed is too fast relative to the intermediate transfer belt, the rotation speed of the secondary outer roller is slowed down, and if the paper speed is too slow relative to the intermediate transfer belt, the rotation speed of the secondary outer roller is increased, thereby correcting the sub-scanning magnification. Note that, as long as the image transferred onto the paper is at the correct magnification (1.00x), the peripheral speed of the intermediate transfer belt and the rotation speed of the secondary outer roller are not necessarily set to the same value.

[0029] (Sub-scanning magnification correction) Next, correction of the partial magnification in the sub-scanning direction in the present invention will be described.

[0030] First, the user stores the paper they want to use in the paper feed unit, sets the paper information on the operation unit, and presses the speed profile acquisition button displayed on the operation unit. This causes the control unit 309 of the image forming device to start printing a test chart for acquiring the speed profile.

[0031] (Test chart) As shown in Figure 2, the test chart has scale images printed at regular intervals in the paper feed direction. To confirm the effect of toner coverage on the magnification in the sub-scanning direction, the chart is printed with varying toner coverage in the paper feed direction. For example, the test chart is printed with scale images printed at 2 dots per 3 spaces on an image forming device with a writing resolution of 2400 dpi. Therefore, the spacing (Lnominal) between scale images is 0.053 mm. The area of ​​the paper other than the areas printed with scale images on both ends is divided into multiple areas in the paper feed direction, with the toner density uniform within each area. From the leading edge of the paper in the feed direction, the toner coverage on the paper is 0%, 50%, 100%, and 200%. Hereinafter, the greater the toner coverage, the more the paper is considered to be covered with toner, and the toner coverage rate on the paper is referred to as coverage.

[0032] In the test chart, the position and feature amount of the scale image, the image width, and the distribution of coverage within the paper can all be selected as appropriate.

[0033] The image of the first side of the test chart is transferred to the paper by the transfer unit, fixed in the fixing unit, then reversed by the reversing unit 209, passes through the double-sided conveying unit 210 again and the transfer and fixing units, the second side is formed, and the chart is discharged outside the machine. As with the first side, the second side also has a scale image formed across the conveying direction, making it a chart with coverage distributed in the conveying direction.

[0034] (Partial magnification correction and velocity profile acquisition) FIG. 3 is a diagram illustrating the calculation of the deviation between the partial magnification and the overall magnification from the scale image. Now, when a user prints a test chart on paper of their choice and causes the image reading device 300 to read the printed paper, the control unit 309 acquires read data related to the test chart. The control unit 309 determines the interval Lr of the scale images actually printed on the paper from the read data. The control unit 309 then calculates the partial magnification at each point based on the difference between the interval Lr and the interval Lnominal between the scale images in the image data. In addition, the total magnification is ΣΔLi (i = 1 to n), which is the sum of the differences ΔLi between the interval Lri and Lnominal of the scale images at each point (i = 1 to n). Partial magnification deviation ΔLi=Lri-Lnominal (Equation 1) Overall magnification deviation ΣΔLi=Σ(Lri-Lnominal) =ΣLri-n×Lnominal (Formula 2) (i=1~n)

[0035] This test chart was acquired at two double outer roller speeds, V1 and V2, and the first side was scanned and numerically processed. The results are shown in Figure 4. The vertical axis of the graph in this figure represents the deviation ΔLi in the sub-scanning magnification at that point, and the horizontal axis represents the position on the test chart. Note that ΔLi varies even within the same coverage band due to measurement error and the vibration components of each roller. For convenience, however, a value obtained by averaging within the same toner concentration is used. In other words, the deviation ΔLi of the scale image at each position is uniquely determined by numerically processing within the same coverage area. This value is referred to as the sub-scanning magnification deviation S. This figure shows that the sub-scanning magnification varies depending on the coverage. The amount of deviation in the sub-scanning magnification also changes depending on the speed of the double outer roller. The difference in the deviation in the sub-scanning magnification due to the two double outer roller speeds is represented as ΔS.

[0036] The reason why the secondary outer roller speed is obtained at two different speeds, V1 and V2, is that the slip curve between the secondary outer roller and the paper varies depending on the speed of the secondary outer roller, and when adjusting the speed of the secondary outer roller to keep the paper speed constant, the effect on the actual paper speed varies depending on the degree to which the speed of the secondary outer roller is changed. Therefore, here, the speed sensitivity is calculated from the amount of deviation in the sub-scanning magnification at the two outer transfer roller speeds V1 and V2 within the same coverage area. Figure 5 shows the speed sensitivity of the secondary outer roller drive, and it turns out that this speed sensitivity also changes depending on the toner coverage.

[0037] Here, if the difference between V1 and V2 is expressed as ΔV, Secondary outer roller drive speed sensitivity = ΔS / ΔV (Equation 3) Since the coverage is distributed on the test chart, ΔS according to the coverage is obtained.

[0038] Next, as shown in the speed profile (Fig. 6), the target speed of the secondary outer roller corresponding to the toner coverage is calculated from the deviation amount of the sub-scanning magnification corresponding to the toner coverage obtained above and the sensitivity of the secondary outer roller drive speed corresponding to the toner concentration. For example, in an area where the toner concentration is 50%, in order to make the deviation of the sub-scanning magnification zero, the sub-scanning magnification must deviate by S(50) when the secondary outer roller speed is V1. Furthermore, there is a deviation in the sub-scanning magnification of ΔS(50) due to the speed difference ΔV between the secondary outer roller speeds V1 and V2. From this, the target speed V(50) of the secondary outer roller when the toner concentration is 50% is V(50) = V1 - (S(50) / ΔS(50) × ΔV) (Equation 4) In other words, the outer secondary roller speed V(α) at ​​any toner concentration α is V(α) = V1 - (S(α) / ΔS(α) × ΔV) (Equation 5) Here, the sub-scanning magnification deviation S takes a positive value because the intervals between the scale images printed on the paper are extended, that is, if the paper speed is faster than the intermediate transfer belt speed, the image is stretched when transferred, and if the paper speed is slower than the intermediate transfer belt speed, the image is shrunk when transferred, so it takes a negative value.

[0039] In this way, the target secondary outer roller speed is determined according to each toner coverage. The above operation is performed for the first side, and the target speed of the secondary outer roller when transferring the first side of the image is determined. The distribution of the target speeds of the secondary outer roller for this toner coverage on the first side is called the first side speed profile fs(c). Here, c indicates that it is a function of coverage.

[0040] As with the first side, the scale image on the test chart for the second side is read by an internal or external reading device. For the second side, the contact ratio between the outer secondary roller and the paper / toner changes depending on the toner coverage of the image formed on the first side, so the drive transmission efficiency to the paper changes. Therefore, the speed profile fr(c) for the second side is the speed profile fs(c) for the first side minus the effect of the toner coverage of the first side during image formation on the second side.

[0041] Let f'r(c) be the velocity profile obtained from the image formed on the second side, and f's(c) be the coverage impact profile on the first side. f's(c) = f'r(c)-fs(c) (Equation 6) However, this can be considered as a coverage impact on the first page.

[0042] Therefore, the velocity profile fr(c) on two surfaces is fr(cs)=fs(cs)-f's(cr) (Formula 7) Here, cs is the coverage amount on the intermediate transfer belt side (second side), and cr is the coverage amount on the outer secondary roller side (first side when forming an image on the second side).

[0043] As shown in Figure 7, in this example, the same image is printed on both the first and second sides. The speed profiles for the first and second sides would be the same if there was no image on the outer secondary roller side, and the difference between the two is the effect of the coverage on the outer secondary roller side. Therefore, it is easier and more convenient to print the same image on both sides. Furthermore, because the first-side paper is inverted, when the image on the second side is formed, the side with the larger toner coverage on the outer secondary roller side is the leading edge of the paper. Therefore, when calculating the speed profile, the coverage changes from 200% to 100% to 50% to 0% in the conveyance direction, while the coverage on the image side of the second side changes from 0% to 50% to 100% to 200%.

[0044] Taking this test chart as an example, the target speed of the outer roller for 50% coverage on the second side is 100% coverage on the first side of the chart, since the image on the 50% coverage side of the chart is 100% coverage. fr(50)=fs(50)-f's(100)...(Formula 8) This becomes:

[0045] Although the method described above uses a reading device of the image forming apparatus, it is also possible to calculate the scale image using an external reading device and input the deviation of the scale image from an operation unit of the image forming apparatus, etc. Also, it is possible to automatically read it using a reading device provided in the transport path of the image forming apparatus main body.

[0046] Next, the control block diagram of the image forming apparatus will be explained using Figure 8. The control unit 309 receives the toner coverage distribution in the transport direction from the image processing unit. It also references the first-side speed profile and the second-side speed profile from the media library information stored in the storage unit 501, and controls the rotation speed of the drive motor for the outer secondary roller by matching the paper transport timing, paper coverage information, and speed profile.

[0047] The operation unit 601 is a user interface (UI, input means, display means) that exchanges electrical information with the CPU 600. The operation unit 601 allows a user (operator, user) to set an image formation mode and input instructions to the CPU 600. The CPU 600 also notifies the user of the device status via the operation unit 601. The CPU 600 controls all of the mechanical units of the image forming apparatus 1 in an integrated manner.

[0048] The user selects the paper and paper feed tray they want to use from the operation unit, and based on the selected media information, the control unit 309 uses the speed profile associated with the selected paper in the media library in the storage unit. For paper used for the first time, a default speed profile is set in advance, which is a general profile predicted from information such as the paper's basis weight and surface properties. Creating a profile for the paper that will actually be used is necessary to reduce variations in sub-scanning magnification.

[0049] Therefore, the image forming apparatus can overwrite the speed profile information in the media library with the acquired speed profile for the paper selected by the user. By doing so, the speed profile linked to the paper brand is stored and correction is made including machine differences such as the secondary transfer roller diameter, so that fluctuations in the sub-scanning magnification can be suppressed with higher precision.

[0050] 9 shows an example of a screen displayed on the operation unit 601 for confirming that speed profile information is stored in the media library. The system has an input unit (operation panel) 1004 and a display unit (display: UI screen) 1005. The input unit 1004 is provided with various operation keys (not shown), such as a numeric keypad for entering values, a print start button, a stop key, and a power saving button. The display unit 1005 is a touch panel type liquid crystal screen, which displays various information such as a paper display that allows selection of the paper to be used, as well as various operation buttons. Various settings for the operations performed by the image forming apparatus are also input to the control unit 309 using the displayed operation buttons.

[0051] As shown in Fig. 9, it is displayed that the speed profile linked to the paper brand is stored together with the acquisition date and time and temperature and humidity information from the on-board sensor. The paper sub-scanning magnification is affected to a certain extent by the temperature and humidity of the paper, and is also affected by fluctuations in parts such as the outer diameter of the outer secondary roller, so the profile is stored to detect such changes.

[0052] When the image forming apparatus receives a print from an external processing device or performs a copy operation in a reading device, image information is digitally processed, and the image processing unit notifies the control unit 309 of the toner coverage distribution information of the paper. The control unit 309 controls the speed of the outer secondary roller based on the toner coverage distribution information and the speed profile information stored in the memory unit. The timing of the speed control of the outer secondary roller and the paper position can be determined by using the timing when the paper and image are aligned in the registration unit, or by providing a transport sensor upstream of the secondary roller.

[0053] For example, when a 100% coverage image is transferred on the first side, the secondary outer roller operates at the target speed calculated from fs(100). When a 50% coverage image is transferred on the second side of this paper, the target speed of the secondary outer roller is fr(50) = fs(50) - {f'r(100) - fs(100)}. Naturally, the more coverage information there is in the paper transport direction, that is, the more divisions there are in the transport direction, the more precise the control of the secondary outer roller drive speed becomes, enabling more detailed partial magnification correction.

[0054] In the image forming apparatus described above, the drive speed of the secondary outer roller was changed and the drive was input when acquiring the speed profiles for the first and second sides, but it is also possible to have a means for switching the drive connection of the secondary outer roller and use driven rotation, as shown in Fig. 10. By monitoring the rotation speed of the secondary outer roller with a rotary encoder while keeping the secondary outer roller driven, the runout component of the secondary outer roller can be calculated and superimposed on the speed profile.

[0055] Furthermore, if fluctuations in sub-scanning magnification are still a concern even when using the speed profile stored in the media library, the user can manually adjust the speed profile from the operation unit, as shown in FIG. [Explanation of symbols]

[0056] 101 Power supply 102 control circuit section 204 Intermediate transfer belt 205 Transfer roller 206 outer transfer roller 209 Inversion section 210 Double-sided conveying section 309 Control unit (control means, reflection means) 500 Image Processing Unit 501 Storage section 601 Operation section 700 Fixing unit 800 Encoder 801 Electromagnetic clutch (drive connection switching means) 1000 Image forming device M1 and M2 driving methods

Claims

1. an image forming means for forming an image on an image carrier; a transfer means having a roller that forms a nip portion between itself and the image carrier, and that transfers the image on the image carrier to a recording material; a drive source for rotating the roller; a control means for controlling the rotation speed of the roller rotated by the driving source; an acquiring unit that causes the image forming unit to form a test chart, causes the control unit to control the driving source to rotate the roller at different rotation speeds, causes the transfer unit to transfer the test chart onto the recording material, and acquires read data related to the test chart on the recording material; and a determining unit that determines a target speed of the rotation speed controlled by the control unit based on the read data acquired by the acquiring unit.

2. 2. The image forming apparatus according to claim 1, wherein the test chart includes a plurality of scale images formed along the conveying direction of the recording material as the recording material passes through the nip portion.

3. 2. The image forming apparatus according to claim 1, wherein the test chart includes a plurality of images of different densities along the conveying direction of the recording material as the recording material passes through the nip portion, and a plurality of scale images formed along the conveying direction.

4. the acquiring means acquires read data relating to an image of a first density and read data relating to an image of a second density different from the first density, 2. The image forming apparatus according to claim 1, wherein the determining means determines the relationship between the density of the image to be formed on the sheet and the target speed based on the read data relating to the image of the first density and the read data relating to the image of the second density.

Images