Image forming apparatus

Abstract

To provide an image forming apparatus capable of suppressing the occurrence of positional deviation between an additional printing image formed on paper and a base image in additional printing.SOLUTION: An image forming apparatus includes: a conveyance section; an eye mark detection section 39 that is disposed in a conveyance path and detects an eye mark formed on paper P in advance; an image forming section that forms an additional printing image at a prescribed position on the paper P with reference to the position of the eye mark on the paper P; and a speed detection section 38 that has a speed detection roller 381 that is made to abut on the paper P and detects conveyance speed of the paper P. The eye mark detection section 39 is disposed from a position where the image forming section forms the additional printing image on the paper P to a position away by an integral multiple of a roller circumferential length of a driver roller 371 and away by non-integral multiple of a roller circumferential length of the roller 381.SELECTED DRAWING: Figure 7

Description

[Technical Field]

[0001] The present disclosure relates to an image forming apparatus. [Background technology]

[0002] 2. Description of the Related Art Conventionally, there has been known an image forming apparatus that performs additional printing on a sheet on which a base image has been printed in advance, which is called overprinting printing.

[0003] 1 and 2 are diagrams showing an example of overprinting. Fig. 1 shows an example of an eye mark Ra, a base image Rs, and an overprinted image Rt formed on a continuous paper P. Fig. 2 shows an example of overprinting on a continuous paper P by electrophotographic printing.

[0004] Overprinting is applied to, for example, continuous paper. In overprinting, a base image is formed on the surface of the continuous paper by analog printing such as screen printing, and then an overprint image is formed on the surface of the continuous paper by digital printing such as electrophotographic printing.

[0005] The base image and the overprinted image are aligned with each other so that a balanced overall image is formed. Specifically, when the base image is formed on the continuous paper, a reference mark called an eye mark is printed. Then, in the overprinting printing, the overprinted image is formed at a position on the continuous paper a predetermined distance away from the eye mark. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Publication No. 2020-99996 Summary of the Invention [Problem to be solved by the invention]

[0007] Conventionally, image forming devices used in this type of overprinting are provided with an eye mark detection unit for detecting eye marks formed on paper, as shown in Figure 2. The image forming device then determines the position at which the overprint image is to be formed on the paper based on the paper transport time after the eye mark is detected by the eye mark detection unit. Note that, hereinafter, the position at which the overprint image is to be formed on the paper by the image forming unit is also referred to as the "image transfer point."

[0008] However, in this type of image forming apparatus, variations in the paper transport time from the detection of the eye mark to the image transfer point can occur. In such cases, the transfer timing does not match the timing when the paper is transported to the target position, resulting in a discrepancy between the formation position of the base image and the formation position of the overprinted image. In particular, in image forming apparatuses that use an intermediate transfer belt for electrophotographic printing, after detecting the eye mark, an image is written onto the photoreceptor, the image is transferred from the photoreceptor to the intermediate transfer belt, and the image is transferred from the intermediate transfer belt onto the paper. As a result, the paper transport time from the detection of the eye mark to the image transfer point is long, making the paper transport time prone to variations.

[0009] In light of this, in image forming apparatuses according to conventional techniques, a configuration has been proposed in which the paper transport speed is detected and the rotation speed of a drive roller in a transport unit that transports the paper is feedback-controlled in order to suppress speed fluctuations during paper transport time (see, for example, Patent Document 1).

[0010] However, conventional image forming devices do not adequately analyze the causes of speed fluctuations in paper transport time, and countermeasures for each cause are insufficient. As a result, depending on the situation, significant misalignment occurs between the overprinted image and the base image formed on the paper. In such cases, the printed material itself printed on the continuous paper is treated as a defective product.

[0011] The present invention has been made in consideration of the above-mentioned problems, and aims to provide an image forming apparatus that can suppress misalignment between an overprinted image formed on a sheet and a base image during overprinting. [Means for solving the problem]

[0012] The main invention that solves the above-mentioned problems is: An image forming apparatus applied to form an additional image on a recording medium, a conveying section that conveys the recording medium along a conveying path by a drive roller; an eyemark detection unit disposed in the transport path and detecting an eyemark formed on the recording medium in advance; an image forming unit disposed downstream of the eye mark detecting unit in the conveying path, the image forming unit forming the overprinted image at a predetermined position on the recording medium based on the position of the eye mark on the recording medium; a speed detection unit having a roller for detecting speed that is brought into contact with the recording medium in the conveyance path, and detecting the conveyance speed of the recording medium based on the rotation speed of the roller; Equipped with The eye mark detection unit is disposed at a position that is an integral multiple of the roller circumference of the drive roller and a non-integral multiple of the roller circumference of the roller, from a position where the image forming unit forms the overprint image on the recording medium. An image forming apparatus. [Effects of the Invention]

[0013] According to the image forming apparatus of the present invention, it is possible to suppress the occurrence of misalignment between an overprinted image formed on a sheet and a base image during overprinting. [Brief explanation of the drawings]

[0014] [Figure 1] FIG. 1 shows an example of overprinting. [Figure 2] FIG. 1 shows an example of overprinting. [Figure 3]FIG. 1 is a diagram showing the overall configuration of an image forming system according to an embodiment of the present invention. [Figure 4] FIG. 1 is a diagram showing the configuration of a control system of an image forming system according to an embodiment of the present invention. [Figure 5] FIG. 1 is a diagram showing the configuration of a control unit of an image forming system according to an embodiment of the present invention. [Figure 6] FIG. 1 is a diagram showing the configuration of a speed detection unit of an image forming system according to an embodiment of the present invention. [Figure 7] FIG. 1 is a diagram for explaining the position of an eye mark detection unit according to an embodiment of the present invention; [Figure 8A] FIG. 10 is a diagram illustrating the effect of the eccentricity of the drive roller on the position where an additionally printed image is formed on a sheet of paper. [Figure 8B] FIG. 10 is a diagram illustrating the effect of the eccentricity of the drive roller on the position where an additionally printed image is formed on a sheet of paper. [Figure 9] FIG. 1 is a diagram illustrating the design concept of the roller diameter in one embodiment of the present invention. [Figure 10] A diagram showing an example of the t-ΔV waveform of the paper transport speed observed when the diameter of the drive roller is enlarged. [Figure 11] FIG. 10 is a diagram showing an example of a control flow by an image formation timing adjustment function of a control unit according to an embodiment of the present invention. [Figure 12] FIG. 10 is a diagram showing an example of a control flow by a conveying speed correction function of a control unit according to an embodiment of the present invention. [Figure 13] FIG. 10 is a diagram showing an example of a speed fluctuation component included in measurement data of a paper conveyance speed; [Figure 14] FIG. 10 is a diagram showing an example of the frequency analysis results of measurement data of paper conveyance speed. [Figure 15] FIG. 10 is a diagram showing an example of a frequency setting table that defines frequencies corresponding to one rotation period of a drive roller and a roller; [Figure 16] FIG. 10 is a diagram showing an example of a speed fluctuation canceling waveform according to an embodiment of the present invention; [Figure 17] FIG. 10 is a diagram showing a modified example of the control flow by the conveying speed correction function of the control unit according to the embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0015] Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. In this specification and drawings, components having substantially the same functions are designated by the same reference numerals, and redundant description will be omitted.

[0016] An example of the configuration of an image forming system according to one embodiment of the present invention (hereinafter referred to as image forming system 1) will be described below with reference to FIGS.

[0017] In this embodiment, continuous paper (hereinafter referred to as paper P) is used as the recording medium on which an overprinted image is formed in the image forming system 1. However, the recording medium is not limited to paper media, and may be, for example, cloth, film, or the like.

[0018] It should be noted that when the base image is formed on the paper P, an eye mark is simultaneously formed. As described above, the eye mark is an image that indicates a reference position on the paper P in the transport direction. The eye mark may take any form, and may be, for example, an I mark extending in the width direction of the paper P as shown in FIG. 1, or a T mark or any other mark. In this embodiment, the eye mark is formed as a black I mark at the separation position between pages in the non-image forming area on the paper P (for example, at the edge in the width direction of the paper P).

[0019] Fig. 3 is a diagram showing the overall configuration of the image forming system 1. Fig. 4 is a diagram showing the configuration of a control system of the image forming system 1. Fig. 5 is a diagram showing the configuration of a control unit 31 of the image forming system 1. Fig. 6 is a diagram showing the configuration of a speed detection unit 38 of the image forming system 1.

[0020] The image forming system 1 includes a paper feed device 10, a paper feed adjustment device 20, an image forming device 30, a paper discharge adjustment device 40, and a winding device 50. These devices are connected in order from the upstream side to the downstream side in the transport direction of the paper P.

[0021] The paper feeder 10 stores and holds the paper P wound in a roll (that is, the roll paper R0), and supplies the paper P.

[0022] The paper feed adjustment device 20 holds the paper P with slack and adjusts the supply of the paper P to the image forming device 30. That is, the paper feed adjustment device 20 has a buffer function that absorbs minute differences in the conveying speed of the paper P between the paper feed device 10 and the image forming device 30, as well as deviations of the paper P.

[0023] The image forming apparatus 30 forms an image based on image data on a sheet P using a known imaging process such as an electrophotographic process.

[0024] The paper discharge adjustment device 40 holds the paper P in a slack state and adjusts the supply of the paper P to the rewinding device 50. That is, like the paper feed adjustment device 20, the paper discharge adjustment device 40 has a buffer function that absorbs minute differences in the conveying speed of the paper P between the image forming device 30 and the rewinding device 50, as well as bias of the paper P.

[0025] The winding device 50 winds the discharged paper P into a roll to form the roll paper R1.

[0026] Next, the image forming apparatus 30 will be described in detail.

[0027] The image forming apparatus 30 includes a control unit 31, a storage unit 32, a communication unit 33, an operation display unit 34, a conveying unit 35, an image forming unit 36, a fixing unit 37, a speed detection unit 38, and an eye mark detection unit 39. The components are interconnected via a bus for exchanging signals.

[0028] The control unit 31 includes a CPU, and controls the above-mentioned components and executes various arithmetic processing in accordance with various programs stored in the storage unit 32, and also controls the image forming system 1 in an integrated manner.

[0029] For example, the control unit 31 controls the sheet feeding device 10, the sheet feeding adjustment device 20, the sheet discharge adjustment device 40, and the winding device 50, thereby controlling the transport of the sheet P.

[0030] Here, the control unit 31 has an image formation timing adjustment function 311 and a conveyance speed adjustment function 312 (see FIG. 5). The image formation timing adjustment function 311 is a function that controls the image forming unit 36 ​​so that an additional image is transferred onto the paper P after a predetermined timing, triggered by the timing when an eye mark on the paper P is detected by the eye mark detection unit 39. The conveyance speed adjustment function 312 is a function that feedback controls the rotation speed of the drive roller (here, the fixing roller 371) based on the conveyance speed of the paper P successively detected by the speed detection unit 38. The image formation timing adjustment function 311 will be described later with reference to FIG. 11. The conveyance speed adjustment function 312 will be described later with reference to FIGS. 12 to 16.

[0031] The storage unit 32 includes a ROM for storing various programs and data in advance, a RAM for temporarily storing programs and data as a working area, and a hard disk for storing various programs and data.

[0032] The communication unit 33 includes an interface for communicating with other devices such as a user's PC, etc. The communication unit 33 receives, for example, a print job from a user's PC.

[0033] The operation display unit 34 is configured, for example, with a liquid crystal display with a touch panel, and functions as a display unit 34a and an operation unit 34b. The display unit 34a displays various operation screens, image status, the operating status of each function, information related to printing, etc., in accordance with a display control signal input from the control unit 31. The operation unit 34b has various operation keys such as a numeric keypad and a start key, and accepts various input operations by the user and outputs operation signals to the control unit 31.

[0034] The transport section 35 includes a plurality of transport rollers arranged along the transport path, and transports the paper P along the transport path.

[0035] The image forming unit 36 ​​forms an image based on the input image data on the paper P conveyed by the conveying unit 35. The image forming unit 36 ​​is an electrophotographic image forming unit including, for example, photosensitive members 361 for each color, an intermediate transfer belt 362, a transfer roller 363, and an opposing roller 364.

[0036] Specifically, the image forming unit 36 ​​records a color image on the paper P by overlaying the four colors of yellow (Y), magenta (M), cyan (C), and black (K) on the paper P at a predetermined number of gradations based on the input image data.

[0037] The image forming unit 36 ​​charges the photoconductor 361 for each color, and then scans the photoconductor 361 with a light beam emitted by an optical scanning unit based on an original image, thereby forming an electrostatic latent image. The image forming unit 36 ​​then forms an image on the photoconductor 361 by supplying a color material such as toner and developing it. The image forming unit 36 ​​transfers the images formed on the four photoconductors 361 onto the intermediate transfer belt 362 in succession, superimposing them on top of each other. As a result, an image made up of each color is formed on the intermediate transfer belt 362.

[0038] Intermediate transfer belt 362 is an endless belt supported by multiple rollers so that it can run, and transports an image transferred by photoconductor 361 in the primary transfer area to a secondary transfer area. Transfer roller 363 is disposed in the secondary transfer area, and forms a nip between itself and opposing roller 364, which faces intermediate transfer belt 362. Transfer roller 363 then transfers the image transported by intermediate transfer belt 362 onto paper P passing through the nip.

[0039] The image forming unit 36 ​​is disposed on the transport path downstream of the eye mark detection unit 39. Under the control of the control unit 31, the image forming unit 36 ​​forms an additional image at a predetermined position on the paper P, using the position of the eye mark on the paper P as a reference.

[0040] The fixing unit 37 includes a fixing roller 371 and a pressure roller 372, and applies heat and pressure to the paper P on which an image has been formed by the image forming unit 36, thereby fixing the image to the paper P. The fixing roller 371 is heated by an internally disposed heater or an externally disposed heating roller (not shown), etc. The pressure roller 372 forms a nip between itself and the opposing fixing roller 371, and the paper P passing through the nip is heated and pressurized.

[0041] The fixing roller 371 is driven by a drive motor and also functions as a drive roller for conveying the paper P, constituting part of the conveying unit 35. Because the fixing roller 371 needs to heat and pressurize the paper P, it is usually set to have the strongest conveying force compared to other motor-driven conveying rollers. For example, the transfer roller 363 also has a paper conveying force, but the conveying force of the transfer roller 363 is smaller than the conveying force of the fixing roller 371. That is, in the image forming apparatus 30 according to this embodiment, the rotation speed of the fixing roller 371 essentially determines the conveying speed of the paper P. For example, a stepping motor capable of controlling the rotation speed for each rotation position is used as the drive motor for operating the fixing roller 371.

[0042] In this embodiment, the conveying speed of the paper P is controlled by controlling the rotation speed of the fixing roller 371. Hereinafter, the fixing roller 371 is also referred to as the "drive roller 371."

[0043] The speed detection unit 38 detects the transport speed of the paper P. The speed detection unit 38 includes, for example, a roller 381 and a rotary encoder 382 connected to the roller 381. The roller 381 is rotatably supported at a fixed point on the transport path where it abuts against the paper P. The roller 381 rotates due to frictional force received from the paper P as the paper P is transported. The rotary encoder 382 is positioned by a fulcrum 383, and outputs a pulse signal each time it detects rotation of the roller 381 by a predetermined angle. In other words, the output signal of the rotary encoder 382 changes based on the rotation speed of the roller 381, and the transport speed of the paper P is successively detected.

[0044] The speed detection unit 38 transmits measurement data of the paper transport speed that is successively detected to the control unit 31. The control unit 31 refers to the measurement data of the paper transport speed that is successively detected, and performs feedback control of the rotation speed of the drive roller 371 to suppress speed fluctuations in the paper transport speed.

[0045] It is preferable that rollers 381 be disposed upstream of fixing unit 37 in the conveyance direction of paper P and spaced apart from fixing unit 37. The atmosphere near fixing unit 37 is likely to become hot due to the influence of the heater within fixing unit 37. If the temperature of rollers 381 rises, the diameter of rollers 381 will change due to thermal expansion, adversely affecting the detection accuracy of the conveyance speed. From this perspective, in this embodiment, rollers 381 are disposed outside the main body of image forming apparatus 30, upstream of transfer roller 363 in the conveyance direction. However, it is sufficient that rollers 381 are disposed upstream of transfer roller 363 in the conveyance direction.

[0046] The eye mark detection unit 39 is provided upstream of the image forming unit 36 ​​on the transport path, and detects eye marks formed on the paper P. As the eye mark detection unit 39, for example, a reflective optical sensor is used, which irradiates the upper surface of the paper P with light and detects eye marks formed on the paper P from the reflected light observed at that time. However, as the eye mark detection unit 39, a transmissive optical sensor or other sensors may also be used.

[0047] It should be noted that the eye mark detection unit 39 transmits the detection result to the control unit 31 every time an eye mark on the paper P is detected.

[0048] <Location of the eye mark detection unit 39> Here, the location of the eye mark detector 39 in the image forming apparatus 30 will be described.

[0049] As described above, variations in the paper transport time from when the eye mark is detected at the eye mark detection position to when the image is transferred are caused by fluctuations in the paper transport speed. Normally, in the image forming device 30, the rotational speed of the drive roller 371 of the transport unit 35 is controlled so that the transport speed of the paper P is constant at a reference speed. However, in reality, variations in the rotational speed due to eccentricity of the drive roller 371, variations in the rotational speed due to changes in the diameter of the drive roller 371, and variations in the rotational speed due to eccentricity of the roller 381 occur within the image forming device 30, and these variations cause variations in the paper transport speed.

[0050] Here, the eccentricity of the drive roller 371 is caused by, for example, a processing error during the manufacture of the drive roller or an installation error during assembly. The eccentricity of the drive roller 371 can also occur when foreign matter adheres to the drive roller 371 while the image forming apparatus 30 is in use. In this embodiment, the fixing roller 371 functions as the drive roller.

[0051] Furthermore, the change in diameter of the drive roller 371 is caused, for example, by a change in temperature of the drive roller 371 while the image forming apparatus 30 is in operation, which causes the drive roller 371 to thermally expand. Generally, in the image forming apparatus 30, the fuser roller 371 often serves as the drive roller, and typically, the drive roller 371 is placed in a high temperature state while the image forming apparatus 30 is in operation. Therefore, while the image forming apparatus 30 is in operation, the drive roller 371 tends to thermally expand and expand in diameter as the temperature rises. When the diameter of the drive roller 371 expands, the paper transport amount per rotation of the drive roller 371 changes, causing fluctuations in the paper transport speed.

[0052] Furthermore, the eccentricity of the speed detection roller 381 is caused by, for example, processing errors during manufacturing or installation errors during assembly, just like the drive roller 371. Eccentricity of the roller 381 can also occur when foreign matter adheres to the roller 381 while the image forming apparatus 30 is in use. The paper transport speed detected by the roller 381 is generally fed back to the control unit 31 and used to control the rotational speed of the drive roller 371 that transports the paper P. Therefore, if the roller 381 is eccentric, it will induce a measurement error in the paper transport speed, which will in turn cause an error in the manipulated variable when the control unit 31 controls the rotational speed of the drive roller 371.

[0053] In the image forming apparatus 30 according to this embodiment, the position of the eye mark detector 39 is set with the aim of improving robustness against such fluctuations in the sheet transport speed.

[0054] FIG. 7 is a diagram for explaining the position where the eye mark detector 39 is disposed.

[0055] The eye mark detection unit 39 is disposed at a position that is an integer multiple of the roller circumference of the drive roller 371 and a non-integer multiple of the roller circumference of the roller 381, from the position where the image forming unit 36 ​​forms the overprinted image on the paper P. In other words, the position where the eye mark detection unit 39 is disposed is set so as to satisfy the following formulas (1) and (2).

[0056] L=π×D1×n1…Equation (1) (L represents the distance from the mark detection position to the image transfer point. D1 represents the diameter of the drive roller 371. n1 represents any integer.)

[0057] L≠π×D2×n2…Equation (2) (L represents the distance from the eyemark detection position to the image transfer point. D2 represents the diameter of the roller 381. n2 represents any integer.)

[0058] First, the relationship in equation (1) will be explained.

[0059] 8A and 8B are diagrams illustrating the influence of the eccentricity of the drive roller 371 on the position where the additionally printed image is formed on the paper P. FIG.

[0060] FIG. 8A shows the t-ΔV waveform when the positional relationship between the eyemark detection position and the image transfer point satisfies the following formula (1). FIG. 8B shows the t-ΔV waveform when the positional relationship between the eyemark detection position and the image transfer point does not satisfy the following formula (1). The upper and lower figures in FIG. 8A show the t-ΔV waveform observed at different timings. The upper and lower figures in FIG. 8B show the t-ΔV waveform observed at different timings.

[0061] In the t-ΔV waveform, the horizontal axis t represents the time axis, and the vertical axis ΔV represents the speed fluctuation component of the paper transport speed relative to the reference speed V0 (same below). From the t-ΔV waveforms in Figures 8A and 8B, the behavior of speed fluctuations in the paper transport speed due to the eccentricity of the drive roller 371 can be read from the time when the eye mark is detected until the image is transferred. Note that Tb in Figures 8A and 8B represents the paper transport time from the eye mark detection position (point A) to the image transfer point (point B). Hereinafter, this paper transport time Tb will also be referred to as the "point A-point B transport time Tb."

[0062] The control unit 31 is set with a waiting time Ta from when the eye mark is detected until image transfer is performed. This waiting time Ta is set assuming that the paper P is transported at a reference speed V0. Ideally, the waiting time Ta and the transport time Tb between point A and point B are equal. Hereinafter, the waiting time Ta set in the control unit 31 from when the eye mark is detected until image transfer is performed will also be referred to as the "waiting time Ta."

[0063] If the drive roller 371 is eccentric, the paper transport speed will not be constant at the reference speed V0, but will vary sinusoidally with the period of one rotation of the drive roller 371. Therefore, due to this speed fluctuation ΔV, the actual paper transport time Tb from the eye mark detection position to the image transfer point may deviate from the waiting time Ta calculated when the paper P is transported at the constant reference speed V0. In other words, in this case, there is a risk of misalignment occurring in the image printed on the paper P.

[0064] Therefore, in the image forming apparatus 30 according to this embodiment, the distance from the eye mark detection position to the image transfer point is set to an integer multiple of the roller circumference of the drive roller 371, thereby canceling the speed fluctuation ΔV in the paper transport speed due to the eccentricity of the drive roller 371.

[0065] That is, in this case, the number of times that the speed fluctuation ΔV included in the transport time Tb between points A and B fluctuates up and down relative to the reference speed V0 is determined by one rotation period of the drive roller 371. Therefore, by setting the positional relationship of the above formula (1), the number of times that the speed fluctuation ΔV included in the transport time Tb between points A and B fluctuates up and down relative to the reference speed V0 is the same even at different timings, as shown in the upper and lower diagrams of Figure 8A. Note that Figure 8A shows that the transport time Tb between points A and B always includes a time period in which the speed increases relative to the reference speed V0 for five peaks, and also includes a time period in which the speed decreases relative to the reference speed V0 for five peaks.

[0066] Therefore, in a state where the above formula (1) is satisfied, the paper transport time Tb from the eye mark detection position to the image transfer point is approximately the same as when the drive roller 371 is not eccentric, even if the drive roller 371 is eccentric. In other words, even when the drive roller 371 is eccentric, the actual paper transport time Tb from the eye mark detection position to the image transfer point is approximately the same as the waiting time Ta preset in the control unit 31. Therefore, no misalignment of the image formation position on the paper P occurs.

[0067] On the other hand, in a state where the above formula (1) is not satisfied, if the eye mark detection timing is different, the number of times that the speed fluctuation ΔV included in the transfer time Tb between point A and point B fluctuates up and down relative to the reference speed V0 will differ. In Figure 8B, the upper diagram includes a time period during which the transfer time Tb between point A and point B increases from the reference speed V0 for 5.5 peaks, and also includes a time period during which the transfer time Tb decreases from the reference speed V0 for 5.5 peaks. On the other hand, the lower diagram includes a time period during which the transfer time Tb between point A and point B increases from the reference speed V0 for 5 peaks, and also includes a time period during which the transfer time Tb decreases from the reference speed V0 for 6 peaks.

[0068] Therefore, in a state where the above formula (1) is not satisfied, if the drive roller 371 is eccentric, the transport time Tb between points A and B changes depending on the timing of the eye mark detection. In other words, if the drive roller 371 is eccentric, the actual paper transport time from the eye mark detection position to the image transfer point will differ from time to time from the waiting time Ta preset in the control unit 31. This will result in a positional deviation of the image formation position on the paper P.

[0069] In this way, in the image forming apparatus 30 according to this embodiment, the position of the eye mark detection unit 39 is set so as to satisfy the above formula (1), thereby increasing the robustness of image formation against misalignment due to the eccentricity of the drive roller 371.

[0070] Next, the relationship in equation (2) will be explained.

[0071] In the image forming apparatus 30 according to this embodiment, the diameter of the roller 381 is designed based on the relationship with the diameter of the drive roller 371.

[0072] Figure 9 is a diagram explaining the design concept of the diameter of roller 381. The top part of Figure 9 shows the fluctuation in paper conveyance speed caused by the eccentricity of drive roller 371 in the form of a t-ΔV waveform when the diameter of drive roller 371 is set to 100. The bottom part of Figure 9 shows the fluctuation in paper conveyance speed caused by the eccentricity of roller 381 in the form of a t-ΔV waveform when the diameter of roller 381 is set to 100 / 50 / 30.

[0073] In the above, we have explained countermeasures for the eccentricity of the drive roller 371 as the only factor that causes fluctuations in the paper transport speed. However, according to the knowledge of the inventors of the present application, in reality, the paper transport speed is also significantly affected by changes in the diameter of the drive roller 371 and the eccentricity of the speed detection roller 381.

[0074] To address these issues, it is first necessary to separate the speed fluctuation component caused by the drive roller 371 and the speed fluctuation component caused by the roller 381 contained in the measurement data of the paper conveyance speed obtained by the speed detection unit 38.

[0075] If the roller 381 is eccentric, this will induce a measurement error in the paper transport speed, which will in turn cause an error in the manipulated variable when feedback controlling the rotational speed of the drive roller 371. Therefore, for example, as shown in Figure 9, the actual paper transport speed will not be constant at the reference speed V0, but will vary sinusoidally with the period of one rotation of the roller 381.

[0076] From this perspective, the control unit 31 according to this embodiment performs frequency analysis on the measurement data of the paper conveyance speed obtained by the speed detection unit 38 to separate the speed fluctuation component caused by the drive roller 371 and the speed fluctuation component caused by the roller 381 contained in the measurement data. Then, when controlling the paper conveyance speed, a method is adopted to selectively reduce the speed fluctuation component caused by the drive roller 371. Note that if the diameter of the drive roller 371 changes, the relationship of equation (1) does not hold, and therefore it becomes necessary to reduce the speed fluctuation component caused by the drive roller 371.

[0077] Therefore, in the image forming apparatus 30 according to this embodiment, the diameter of the drive roller 371 is set to be a non-integer multiple of the diameter of the rollers 381, as shown in the following formula (3).

[0078] D1≠D2×n3…Equation (3) (D1 represents the diameter of the drive roller 371, D2 represents the diameter of the roller 381, and n3 represents any integer.)

[0079] FIG. 9 is a diagram for explaining this point in a schematic manner.

[0080] In this embodiment, the diameter of the roller 381 of the speed detection unit 38 is designed to be smaller than the diameter of the drive roller 371. This is because, in general, the eccentricity of a small-diameter roller 381 has a smaller effect on the measurement accuracy of the paper transport speed than a large-diameter roller 381, and the accuracy error as a component is also smaller. Therefore, it is preferable to use a small-diameter roller 381 in order to measure the transport speed of the paper P with high accuracy.

[0081] When the diameter of drive roller 371 is the same as the diameter of roller 381 or is an integer multiple of the diameter of roller 381 (in FIG. 9, the diameter of roller 381 is 100 or 50), the speed fluctuation of the paper conveyance speed behaves in sync with the period of drive roller 371 and the period of roller 381. In this case, during frequency analysis, the speed fluctuation component caused by roller 381 is superimposed on the speed fluctuation component caused by drive roller 371, making it difficult to separate the two.

[0082] In this regard, by designing the diameter of drive roller 371 and / or the diameter of roller 381 as in the above formula (3), it becomes possible to perform frequency analysis of the measurement data of the paper conveyance speed with high accuracy in a short time. Note that, in Figure 9, for the above reasons, in relation to the diameter of drive roller 371, the diameter of roller 381 is incompatible when it is 100 or 50, and only the diameter of roller 381 is compatible when it is 30.

[0083] By substituting the relationship in formula (3) into formula (1), the relationship in formula (2) is derived. That is, it is clear that the eye mark detection unit 39 must be disposed at a position that is a non-integer multiple of the circumferential length of the roller 381 away from the position where the additional image is formed on the paper P.

[0084] L=π×D1×n1…Equation (1) D1≠D2×n3…Equation (3) ⇒L≠π×D2×n2…Equation (2)

[0085] 10 is a diagram showing an example of the t-ΔV waveform of the paper transport speed observed when the diameter of the drive roller 371 is enlarged. Note that the lines in FIG. Dotted line: Paper transport speed under normal conditions Solid line: Paper conveying speed when the diameter of the drive roller 371 is enlarged Dot-dash line: Paper conveying speed when the drive roller 371 is eccentric

[0086] When the diameter of drive roller 371 changes, the actual paper conveyance speed behaves differently from when eccentricity occurs in drive roller 371. When eccentricity occurs in drive roller 371, as described above, the paper conveyance speed does not remain constant at reference speed V0, but changes sinusoidally with the period of one rotation of drive roller 371 (dotted line).

[0087] On the other hand, when the diameter of the drive roller 371 is expanded, the average speed of the paper transport speed becomes higher than the reference speed V0, and the amplitude of the speed fluctuation relative to the reference speed V0 becomes larger (solid line). Therefore, even if the distance from the eye mark detection position to the image transfer point is set to the relationship of equation (1) based on the non-expansion standard of the drive roller 371, the speed fluctuation ΔV of the paper transport speed cannot be canceled.

[0088] From this perspective, the control unit 31 according to this embodiment performs frequency analysis of the measurement data of the paper conveying speed, detects the speed fluctuation ΔV of the paper conveying speed that accompanies the change in the diameter of the drive roller 371, and adopts a method of selectively reducing this fluctuation, as will be described later with reference to FIG.

[0089] <Image formation timing adjustment function of the control unit 31> Next, the image formation timing adjustment function 311 of the control unit 31 will be described.

[0090] The image formation timing adjustment function 311 is a function that controls the image forming unit 36 ​​so that an additional printing image is transferred onto the paper P after a predetermined waiting time Ta, triggered by the timing when the eye mark on the paper P is detected by the eye mark detection unit 39.

[0091] FIG. 11 is a diagram showing an example of a control flow by the image formation timing adjustment function 311 of the control unit 31.

[0092] In this embodiment, the eye marks are formed as black I marks at the delimiting positions of each page in the non-image forming area on the paper P, as shown in Fig. 1. The control unit 31 forms an image of each page (an additional printing image) at a predetermined position on the paper P, based on the position of the eye mark added to each page on the paper P. The processing of steps S1 to S3 represents a loop processing for the ith page (where i is a variable).

[0093] In step S1, the control unit 31 determines whether or not an eye mark has been detected on the paper sheet P by the eye mark detection unit 39. If an eye mark has been detected on the paper sheet P (S1: YES), the control unit 31 proceeds to step S2. On the other hand, if an eye mark has not been detected on the paper sheet P (S1: NO), the control unit 31 waits until an eye mark on the paper sheet P is detected.

[0094] In step S2, the control unit 31 determines whether the waiting time Ta has elapsed since the eye mark on the paper P was detected until the paper P reached the image transfer point. If the waiting time Ta has elapsed (S2: YES), the control unit 31 proceeds to step S3. On the other hand, if the waiting time Ta has not elapsed (S2: NO), the control unit 31 waits for the waiting time Ta until the paper P reaches the image transfer point to elapse. The waiting time Ta is a paper transport time set on the assumption that the paper P is transported from the eye mark detection position to the image transfer point at a reference speed V0.

[0095] In step S3, the control unit 31 controls the image forming unit 36 ​​to form an image at a predetermined position on the paper P. Then, the control unit 31 increments the variable i for the ith page by one page, returns to the process in step S1, and executes the image forming process for the next page.

[0096] The control unit 31 performs such image formation processing in order from the first page of the paper P to the last page (n-th page), thereby forming images on all pages of the paper P.

[0097] <Conveyance speed correction function of the control unit 31> Next, the conveying speed adjusting function 312 of the control unit 31 will be described.

[0098] The transport speed adjustment function 312 is a function that performs feedback control of the rotation speed of the drive roller 371 based on the paper transport speed successively detected by the speed detection unit .

[0099] As described above, when the diameter of the drive roller 371 is enlarged, the behavior is different from when eccentricity occurs in the drive roller 371. Furthermore, the measurement data of the paper conveyance speed includes a speed fluctuation component due to the eccentricity of the roller 381.

[0100] From this perspective, the control unit 31 performs frequency analysis of the measurement data of the paper conveyance speed. Then, the control unit 31 detects the speed fluctuation component of the paper conveyance speed that accompanies the change in diameter of the drive roller 371, and feedback controls the rotation speed of the drive roller 371 so as to selectively cancel out this component.

[0101] More specifically, the control unit 31 sets a speed fluctuation cancellation waveform (FIG. 16) to selectively cancel out the speed fluctuation component of the conveyance speed that changes sinusoidally in one rotation period of the drive roller 371. The control unit 31 then adjusts the reference phase of the speed fluctuation cancellation waveform to the paper conveyance speed that is successively detected, and performs feedback control of the rotation speed of the drive roller 371. This cancels out the speed fluctuation component of the paper conveyance speed that occurs as the diameter of the drive roller 371 increases.

[0102] At this time, the control unit 31 calculates, for example, the moving average speed of the sequentially detected paper transport speed per unit time (for example, 1 second) that is equal to or greater than the rotation period of the drive roller 371. Then, the control unit 31 feedback-controls the rotation speed of the drive roller 371 so that the moving average speed approaches the reference speed. This cancels out the increase in the average paper transport speed that occurs as the diameter of the drive roller 371 increases.

[0103] FIG. 12 is a diagram showing an example of a control flow by the conveying speed adjustment function 312 of the control unit 31.

[0104] Fig. 13 is a diagram showing an example of a speed fluctuation component included in the measurement data of the paper transport speed. Fig. 13A is a diagram showing the original data of the measurement data of the paper transport speed. Fig. 13B is a diagram showing an example of a speed fluctuation component included in the measurement data of the paper transport speed and observed at a frequency corresponding to one rotation period of the drive roller 371. Fig. 13C is a diagram showing an example of a speed fluctuation component included in the measurement data of the paper transport speed and observed at a frequency corresponding to one rotation period of the roller 381.

[0105] In the following, the frequency corresponding to one rotation period of the drive roller 371 (i.e., the reciprocal of the rotation period) is referred to as the "rotation frequency of the drive roller 371." The frequency corresponding to one rotation period of the roller 381 (i.e., the reciprocal of the rotation period) is referred to as the "rotation frequency of the roller 381."

[0106] FIG. 14 is a diagram showing an example of the frequency analysis result of the measurement data of the paper conveyance speed.

[0107] FIG. 15 is a diagram showing an example of a frequency setting table that defines the rotational frequencies of the drive roller 371 and the rollers 381. As shown in FIG.

[0108] The data of this frequency setting table is calculated in advance based on the diameter of the drive roller 371 and the diameter of the rollers 381, and is stored in the storage unit 32. The rotation frequency of the drive roller 371 is calculated, for example, by the following formula (4). The rotation frequency of the rollers 381 is calculated, for example, by the following formula (5).

[0109] V0 / (π×D1)=f1…Equation (4) (V0 represents the reference speed for the paper transport speed. D1 represents the diameter of the drive roller 371. f1 represents the rotation frequency of the drive roller 371.)

[0110] V0 / (π×D2)=f2…Equation (5) (V0 represents the reference speed of the paper transport speed. D2 represents the diameter of the roller 381. f2 represents the rotation frequency of the roller 381.)

[0111] FIG. 16 is a diagram showing an example of a speed fluctuation canceling waveform.

[0112] The speed fluctuation cancellation waveform is, for example, a sinusoidal waveform that defines a speed change value that changes periodically with the rotation frequency of the drive roller 371. In this speed fluctuation cancellation waveform, an amplitude r1 of the speed fluctuation component of the rotation frequency f1 of the drive roller 371 calculated by frequency analysis is set.

[0113] It should be noted that reference waveform data for setting the speed fluctuation canceling waveform is stored in advance in the storage unit 32. However, the waveform shape of the speed fluctuation canceling waveform is not limited to a sine wave, and a triangular wave or the like may also be used.

[0114] A description will be given of each process in the control flow of Fig. 12. The flowchart shown in Fig. 12 is a process that is repeatedly executed by the control unit 31 at predetermined intervals (for example, every 100 msec) according to a computer program, for example.

[0115] In step S11, the control unit 31 acquires measurement data of the paper transport speed obtained by the speed detection unit 38. Note that the speed detection unit 38 outputs time-series data of the paper transport speed that is successively detected as measurement data.

[0116] In step S11, the control unit 31 acquires measurement data of the paper transport speed from the present time up to a predetermined time in the past (for example, 1 second). The measurement data of the paper transport speed includes a rotational speed fluctuation component of the drive roller 371 and a rotational speed fluctuation component of the roller 381, as shown in FIG.

[0117] In step S12, the control unit 31 performs frequency analysis (for example, FFT analysis) on the measurement data.

[0118] In step S12, as shown in Fig. 14, the signal strength value (i.e., amplitude) of each frequency component is calculated as each speed fluctuation component included in the measurement data of the paper conveyance speed. That is, this separates the speed fluctuation component at the rotation frequency of drive roller 371 from the speed fluctuation component at the rotation frequency of roller 381.

[0119] In step S13, the control unit 31 sets a speed fluctuation canceling waveform based on the frequency analysis in step S12.

[0120] In step S13, the control unit 31 first identifies the speed fluctuation component at the rotational frequency of the drive roller 371 from the results of frequency analysis of the measurement data of the paper conveyance speed, based on the frequency setting table shown in Fig. 15. The control unit 31 sets, for example, as the speed fluctuation cancellation waveform, a sinusoidal waveform of frequency f1 corresponding to one predetermined rotation period of the drive roller 371, as shown in Fig. 16. At this time, the control unit 31 sets, as the amplitude of the speed fluctuation cancellation waveform, amplitude r1 at the rotational frequency of the drive roller 371 calculated by frequency analysis. Note that here, the moving average speed of the paper conveyance speed calculated in step S14 is set as the center value of the speed fluctuation cancellation waveform.

[0121] In step S14, the control unit 31 calculates, from the measurement data of the paper conveying speed, a moving average speed V1 of the paper conveying speed, which is set to a unit time equal to or greater than the rotation period of the drive roller 371. Then, the control unit 31 calculates the deviation between the moving average speed V1 and the reference speed V0.

[0122] The purpose of step S14 is to calculate the increase in moving average speed V1 of the paper transport speed relative to the reference speed V0 that occurs as the diameter of drive roller 371 increases. Therefore, moving average speed V1 of the paper transport speed is preferably the moving average speed over a relatively long period of time that is at least one rotation period of drive roller 371. Furthermore, it is more preferable that this length of time be an integer multiple of one rotation period of drive roller 371. In step S14, control unit 31 calculates moving average speed V1 from the measurement data of the paper transport speed for, for example, the most recent one second.

[0123] Furthermore, the deviation between the moving average speed V1 of the paper transport speed and the reference speed V0 is used to calculate the average target value of the rotational speed of the drive roller 371 to bring the actual value of the moving average speed V1 of the paper transport speed closer to the reference speed V0.

[0124] In step S15, the control unit 31 executes feedback control of the rotational speed of the drive roller 371 based on the speed fluctuation canceling waveform and the deviation between the moving average speed V1 of the paper conveying speed and the reference speed V0. The control method of the feedback control itself is the same as that of conventionally known feedback control.

[0125] In step S15, the control unit 31 first calculates the average target value of the rotation speed of the drive roller 371 from the deviation between the moving average speed V1 of the paper transport speed and the reference speed V0, for example.

[0126] The control unit 31 then calculates the deviation between the speed change value of the speed fluctuation cancellation waveform and the currently detected value of the paper conveyance speed, which is acquired sequentially, and calculates the integral value of the deviation over one rotation period of the drive roller 371. The control unit 31 then sets the reference phase of the speed fluctuation cancellation waveform based on the calculated integral value. The timing at which the integral value of the deviation over one rotation period of the drive roller 371 is minimized is the timing at which the speed fluctuation cancellation waveform and the waveform of the speed fluctuation component associated with diameter changes of the drive roller 371 overlap. Therefore, by setting the reference phase of the speed fluctuation cancellation waveform to a value that is opposite in phase to the timing at which the integral value of the deviation over one rotation period of the drive roller 371 is minimized, the speed fluctuation cancellation waveform acts to cancel out the speed fluctuation component associated with diameter changes of the drive roller 371.

[0127] That is, the control unit 31 superimposes the fluctuation component corresponding to the rotational position of the drive roller 371, calculated from the speed fluctuation cancellation waveform, on the average target value of the rotational speed of the drive roller 371, and sets this as the target value of the rotational speed corresponding to the rotational position of the drive roller 371. Then, the control unit 31 operates the drive motor that drives the drive roller 371 in accordance with this target value. This makes it possible to selectively cancel out the speed fluctuation component of the paper transport speed that accompanies changes in the diameter of the drive roller 371. That is, this makes it possible to operate the drive roller 371 so that the paper transport speed becomes the reference speed V0.

[0128] The control unit 31 constantly performs this feedback control during printing based on the current detection value of the paper transport speed successively acquired by the speed detection unit 38. The control unit 31 cancels the speed fluctuation component of the paper transport speed that occurs as the diameter of the drive roller 371 increases.

[0129] 12 periodically, and calculates the deviation between the moving average speed V1 of the paper transport speed and the reference speed V0 calculated in step S13, or the speed fluctuation canceling waveform set in step S14. Then, when a change occurs in the deviation between the moving average speed V1 of the paper transport speed and the reference speed V0 or the speed fluctuation canceling waveform, the control unit 31 updates these values ​​and continues the same feedback control.

[0130] <Effects> As described above, the image forming apparatus 30 according to this embodiment can reduce the deviation in the transport time from the eye mark detection position to the image transfer point of the paper P. This can prevent the formation position of the overprinted image from being misaligned with the base image.

[0131] In particular, in the image forming apparatus 30 according to this embodiment, a frequency analysis is performed on the sequentially detected conveyance speed of the paper P. Then, based on the results of this frequency analysis, a speed fluctuation cancellation waveform is set so as to selectively cancel out the speed fluctuation component of the conveyance speed that changes sinusoidally in one rotation period of the drive roller 371. Then, the rotation speed of the drive roller 371 is feedback-controlled in accordance with the speed fluctuation cancellation waveform.

[0132] This makes it possible to separate the speed fluctuation component of the paper transport speed caused by the eccentricity of roller 381 from the speed fluctuation component of the paper transport speed caused by the change in diameter of drive roller 371. In other words, this makes it possible to perform highly accurate feedback control of the rotation speed of drive roller 371 so as to maintain a constant transport time from the eye mark detection position to the image transfer point.

[0133] <Modification> In the above embodiment, the control unit 31 (conveyance speed correction function) focuses on only the rotation frequency component of the drive roller 371 among the speed fluctuation components of the paper conveyance speed, and performs feedback control of the rotation speed of the drive roller 371.

[0134] In this regard, the main cause of the speed fluctuation component of the paper conveyance speed may change over time. For example, foreign matter may adhere to the speed detection roller 381, causing the roller 381 to become eccentric, which may increase the rotational speed fluctuation, resulting in increased speed fluctuation of the paper conveyance speed. In addition, other factors (for example, foreign matter adhering to the transfer roller) may also increase the speed fluctuation of the paper conveyance speed.

[0135] From this perspective, the control unit 31 may switch the control mode for controlling the rotation speed of the drive roller 371 based on the results of frequency analysis of the measurement data of the paper conveyance speed.

[0136] FIG. 17 is a diagram showing a modified example of the control flow by the transport speed correction function of the control unit 31.

[0137] The flowchart in FIG. 17 differs from the flowchart in FIG. 12 in that the control mode for feedback controlling the rotational speed of the drive roller 371 is switched based on the analysis result of the frequency analysis obtained in the process of S12.

[0138] Specifically, if the result of the frequency analysis shows that the amplitude r1 of the rotational frequency component (f1) of the drive roller 371 in the measurement data of the paper conveying speed is greater than the threshold value RR1, the control unit 31 performs the processes of S13 to S15, as in the above embodiment.

[0139] On the other hand, if the result of the frequency analysis indicates that the amplitude r2 of the rotation frequency component (f2) of the roller 381 in the measurement data of the paper conveyance speed is greater than the threshold value RR2, the control unit 31 performs the process of step S23.

[0140] Here, step S23 means stopping the feedback control of the rotation speed of the drive roller 371. That is, the control unit 31 controls the rotation speed of the drive roller 371 so that it becomes the reference speed, without relying on the detection value of the paper conveyance speed obtained from the roller 381.

[0141] In addition, the reason why the feedback control of the rotational speed of the drive roller 371 is stopped in this step S23 is because it is highly likely that feedback control of the rotational speed of the drive roller 371 will become difficult when the eccentricity of the roller 381 is large.

[0142] On the other hand, if the results of the frequency analysis show that the amplitude r1 of the rotational frequency component (f1) of the drive roller 371 is less than or equal to the threshold value RR1 and the amplitude r2 of the rotational frequency component (f2) of the roller 381 is less than or equal to the threshold value RR2, the control unit 31 performs processing of step S33.

[0143] Here, step S33 means switching to a mode in which normal feedback control is used to control the rotational speed of drive roller 371. Normal feedback control means a mode in which the rotational speed of drive roller 371 is controlled so that the detected value of the paper transport speed, which is detected sequentially, approaches the reference speed and the deviation between the detected value of the paper transport speed and the reference speed becomes zero.

[0144] In step S33, when the eccentricity of the drive roller 371 and the roller 381 is not large, it is often possible to smooth out fluctuations in the paper conveying speed by performing normal feedback control.

[0145] Then, the control unit 31 periodically performs the operation of the flowchart in FIG. 17, and switches the control mode for feedback-controlling the rotation speed of the drive roller 371 based on the analysis result of the frequency analysis obtained in the process of S12.

[0146] As described above, according to the image forming apparatus 30 of this modification, it is possible to smooth out fluctuations in the paper conveyance speed by switching the control mode for controlling the rotational speed of the drive roller 371 depending on the situation within the apparatus. In other words, this makes it possible to suppress deviations in the conveyance time from the eye mark detection position to the image transfer point.

[0147] (Other embodiments) Although the image forming apparatus according to the present invention has been described using the illustrated embodiment, the present invention is not limited to this. For example, the present invention can also be applied to overprinting in an inkjet image forming apparatus.

[0148] Although specific examples of the present invention have been described above in detail, these are merely examples and do not limit the scope of the claims. The technology described in the claims includes various modifications and alterations of the specific examples exemplified above. [Industrial Applicability]

[0149] According to the image forming apparatus of the present invention, it is possible to suppress the occurrence of misalignment between an overprinted image formed on a sheet and a base image during overprinting. [Explanation of symbols]

[0150] 1. Image forming system 10 Paper feeder 20 Paper feed adjustment device 30 Image forming device 31 Control Unit 32 Storage section 33 Communications Department 34 Operation display section 35 Conveying section 36 Image forming unit 37 Fixing section 38 Speed ​​detection unit 39 Eye mark detector 40 Paper discharge adjustment device 50 Paper ejection device 311 Image formation timing adjustment function 312 Conveying speed adjustment function 361 Photoreceptor 362 Intermediate transfer belt 363 Transfer roller 364 Opposing Roller 371 Fuser roller (drive roller) 372 Pressure roller 381 Coro 382 rotary encoder P paper

Claims

1. An image forming apparatus applied to form an additional image on a recording medium, a conveying section that conveys the recording medium along a conveying path by a drive roller; an eyemark detection unit disposed in the transport path and detecting an eyemark formed on the recording medium in advance; an image forming unit disposed downstream of the eye mark detecting unit in the conveying path, the image forming unit forming the overprinted image at a predetermined position on the recording medium based on the position of the eye mark on the recording medium; a speed detection unit having a roller for detecting speed that is brought into contact with the recording medium in the conveyance path, and detecting the conveyance speed of the recording medium based on the rotation speed of the roller; Equipped with The eye mark detection unit is disposed at a position that is an integral multiple of the roller circumference of the drive roller and a non-integral multiple of the roller circumference of the roller, from a position where the image forming unit forms the overprint image on the recording medium. Image forming device.

2. The control unit further includes a control unit that performs frequency analysis on the conveyance speed of the recording medium that is detected sequentially, and controls the rotation speed of the drive roller based on the analysis result of the frequency analysis. The image forming apparatus according to claim 1 .

3. The control unit sets a speed fluctuation cancellation waveform based on the analysis result of the frequency analysis so as to selectively cancel a speed fluctuation component of the conveying speed that changes with the rotation period of the drive roller, and feedback controls the rotation speed of the drive roller in accordance with the speed fluctuation cancellation waveform. The image forming apparatus according to claim 2 .

4. the speed fluctuation canceling waveform is a sinusoidal waveform having a frequency corresponding to a rotation period of the drive roller and an amplitude of the speed fluctuation component of the conveying speed that changes with the rotation period of the drive roller calculated by the frequency analysis. The image forming apparatus according to claim 4 .

5. The control unit further calculates a moving average speed of the conveying speed of the recording medium detected successively per unit time equal to or greater than the rotation period of the drive roller, and feedback controls the rotation speed of the drive roller so that the moving average speed approaches a reference speed. The image forming apparatus according to claim 3 .

6. The control unit switches a control mode for feedback controlling the rotation speed of the drive roller based on the analysis result of the frequency analysis. The image forming apparatus according to claim 2 .

7. The diameter of the roller is smaller than the diameter of the drive roller. The image forming apparatus according to claim 1 .

8. The drive roller is a fixing roller that fixes the overprinted image onto the recording medium. The image forming apparatus according to claim 1 .

9. The image forming unit is an electrophotographic image forming unit having an image carrier that carries a toner image, an endless intermediate transfer belt that receives the toner image from the image carrier, and a transfer roller around which the intermediate transfer belt is stretched and which is disposed at a position where the overprinted image is formed on the recording medium. The image forming apparatus according to claim 8 .

10. The conveying force of the transfer roller is smaller than the conveying force of the fixing roller. The image forming apparatus according to claim 9 .

11. The fixing roller is disposed downstream of the transfer roller in the conveying direction. The image forming apparatus according to claim 9 .

12. The roller is disposed upstream of the transfer roller in the conveying direction and outside the housing of the image forming apparatus. The image forming apparatus according to claim 9 .

13. The recording medium is continuous paper. The image forming apparatus according to claim 1 .

Images