Image forming device

Abstract

To provide an image forming apparatus that can reduce periodic density unevenness despite its low-cost configuration.SOLUTION: An image forming apparatus has: test pattern forming means that forms a test pattern for detecting sub-scanning unevenness; density detection means that reads the density of the test pattern; calculation means that calculates the period and amplitude of unevenness in the density of a sub-scanning direction of the test pattern read by the density detection means; and a control unit that performs process control with the period and amplitude calculated by the calculation means. The image forming apparatus has: time measuring means that measures time; and phase prediction means that predicts the phase of the sub-scanning unevenness during image formation from the time measuring means and a result of reading of the test pattern, and the image forming apparatus determines the timing of the process control according to a result from the phase prediction means.SELECTED DRAWING: Figure 8

Description

[Technical field]

[0001] The present invention relates to an image forming apparatus for forming a toner image on a transfer medium. [Background technology]

[0002] In recent years, electrophotographic image forming devices have begun to spread in the printing industry, and the demand for high-speed output and high image quality is rapidly increasing. Among the requirements for high image quality, there is a strong demand for uniformity of density within a page, that is, uniformity of density of an image formed on a single sheet of recording medium such as paper, and it is important to minimize uneven density within a page.

[0003] It is known that this uneven density occurs due to various factors. For example, uneven charging due to uneven charging, uneven exposure of an exposure device, rotational vibration and uneven sensitivity of an image carrier such as a photoconductor, uneven resistance of a developer carrier such as a developing roller, uneven charging of toner, uneven transfer of a transfer roller, etc. Among these, uneven density caused by rotational vibration and uneven sensitivity of an image carrier has a short period, occurs periodically within a page, and is easy to see, so it is often the subject of a complaint. Therefore, it is particularly important to suppress uneven density caused by rotational vibration and uneven sensitivity of an image carrier.

[0004] Patent Document 1 shows a method of modulating the developing bias in accordance with the rotation period of the image carrier. Specifically, the method uses a rotation position detection sensor that detects the rotation position of the image carrier and a density detection sensor that detects the density of the image. In this method, the density unevenness detected by the density detection sensor is separated by the image carrier period, and the developing bias is periodically changed using a signal from the rotation position detection sensor as a trigger to cancel out electric field fluctuations caused by rotational vibration and the like to make the electric field constant in order to suppress the detected density unevenness.

[0005] As another example of this method, a method of modulating not only the developing bias but also the charging bias may be considered. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] JP 2000-98675 A Summary of the Invention [Problem to be solved by the invention]

[0007] However, while there are various factors that cause periodic unevenness, not just the image carrier, the above methods cannot deal with periodic unevenness that does not have a phase detection means. If a phase detection means were to be attached to the component that causes periodic unevenness, the device would become larger and more complicated, and the cost would be significantly increased.

[0008] An object of the present invention is to provide an image forming apparatus that can apply a control waveform that cancels density unevenness corresponding to the rotation period at accurate timing, and can correct density unevenness with high precision. [Means for solving the problem]

[0009] In order to achieve the above object, an image forming apparatus of the present invention includes a test pattern forming means for forming a test pattern on a rotating photoconductor, a density detection means for reading a density of the test pattern, a calculation means for calculating a period and an amplitude of density unevenness in the rotation direction of the photoconductor from the density read by the density detection means, a determination means for determining image formation conditions based on the period and amplitude calculated by the calculation means, a phase determination means for determining a phase for controlling the image formation conditions based on a reading result of the test pattern read by the density detection means, and a density unevenness detection mode for forming a first test pattern by the test pattern forming means and calculating the period and amplitude of the density unevenness using the calculation means from a first reading result of the first test pattern read by the density detection means, forming a second test pattern by the test pattern forming means, and a second reading result of the second test pattern read by the density detection means. , by the calculation means The first reading result The calculation result is calculated from From 、 and a control means for executing a phase determination sequence for determining a phase for controlling the image forming conditions using the phase determination means. Effect of the Invention

[0010] According to the present invention, it is possible to provide an image forming apparatus that makes it possible to apply a control waveform that cancels out density unevenness corresponding to the rotation period at a precise timing, thereby enabling correction of density unevenness with high precision. [Brief description of the drawings]

[0011] [Figure 1] 1 is a schematic diagram of an image forming apparatus according to an embodiment of the present invention. [Diagram 2] 1 is a schematic diagram of a concentration detection sensor according to a first embodiment. [Diagram 3] FIG. 1 is a functional block diagram according to a first embodiment. [Figure 4] 4 is a schematic diagram of a test pattern used in the density unevenness correction according to the first embodiment. FIG. [Diagram 5] 5 is a diagram showing the relationship between the output value of the density sensor according to the first embodiment and the image forming conditions created based on the output value. FIG. [Figure 6] 4 is a schematic diagram of a test pattern used in the phase detection sequence according to the first embodiment. FIG. [Figure 7] FIG. 4 is a diagram illustrating a flow of density unevenness correction according to the first embodiment. [Figure 8] 1 is a diagram illustrating a phase detection method according to a first embodiment. FIG. [Figure 9] FIG. 4 is a diagram illustrating a phase detection flow according to the first embodiment. [Figure 10] FIG. 4 is a diagram illustrating an image formation flow according to the first embodiment. [Figure 11] FIG. 4 is a diagram showing density fluctuation in the sub-scanning direction according to the first embodiment. [Figure 12] FIG. 11 is a schematic diagram of a potential sensor according to a second embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] The present invention will be described with reference to the drawings.

[0013] The present invention will be described in more detail below with reference to examples. Note that although these examples are examples of the best mode for carrying out the present invention, the present invention is not limited to the configurations of these examples. EXAMPLES

[0014] [Image forming device] 1 is a configuration diagram of an image forming apparatus. Image forming apparatus 200 can be realized by a printer, copier, multifunction machine, facsimile, etc. that forms color images by electrophotography. Image forming apparatus 200 is a so-called intermediate transfer tandem type image forming apparatus in which four image forming units Pa to Pd are arranged side by side on intermediate transfer belt 7.

[0015] Recording material S, such as a sheet on which an image is to be formed, is stacked in a recording material storage 60 and is fed by a pair of paper feed rollers 61 employing a friction separation method in accordance with the timing of image formation by image forming units Pa to Pd. The pair of paper feed rollers 61 conveys the recording material S to registration rollers 62 via a conveying path. The registration rollers 62 correct skew of the recording material S and adjust the timing to convey the recording material S to the secondary transfer unit T2.

[0016] The image forming apparatus 200 forms images by image forming units Pa to Pd. The image forming units Pa to Pd include photoconductors 1a to 1d, chargers 2a to 2d, exposure units 3a to 3d, developing units 100a to 100d, primary transfer units T1a to T1d, and photoconductor cleaners 6a to 6d. The chargers 2a to 2d uniformly charge the surfaces of the photoconductors 1a to 1d. The photoconductors 1a to 1d are driven to rotate, and are irradiated with light by the exposure units 3a to 3d. The exposure units 3a to 3d irradiate the photoconductors 1a to 1d with light modulated according to image information of the image to be formed. As a result, an electrostatic latent image according to the image is formed on the photoconductors 1a to 1d.

[0017] The developing units 100a-100d develop the electrostatic latent images formed on the photoconductors 1a-1d with a developer. In this embodiment, toner is used as the developer. The developing units 100a-100d develop the electrostatic latent images by attaching toner to the photoconductors 1a-1d on which the electrostatic latent images are formed, thereby forming toner images. The primary transfer units T1a-T1d are given a predetermined pressure amount and an electrostatic load bias, and transfer the toner images from the photoconductors 1a-1d to the intermediate transfer belt 7. At this time, the toner images formed on the photoconductors 1a-1d are transferred to the intermediate transfer belt 7 so as to be superimposed on each other.

[0018] Image forming unit Pa generates a yellow toner image. Image forming unit Pb generates a magenta toner image. Image forming unit Pc generates a cyan toner image. Image forming unit Pd generates a black toner image. However, the number of colors of the toner images formed is not limited to four. The developing units 100a to 100d of this embodiment contain a two-component developer that is a mixture of non-magnetic toner and magnetic carrier, but may be a one-component developer that contains only magnetic toner or non-magnetic toner.

[0019] A full-color toner image is formed by transferring superimposed toner images of yellow, magenta, cyan, and black onto the intermediate transfer belt 7. Toner remaining on the photoconductors 1a-1d after transfer is collected by photoconductor cleaners 6a-6d. When the amount of toner contained inside the developing units 100a-100d falls below a predetermined amount, toner is replenished from toner bottles Ta-Td, which are developer replenishment containers, through a replenishment path (not shown).

[0020] The intermediate transfer belt 7 is an endless belt provided in an intermediate transfer belt frame (not shown) and stretched by a secondary transfer inner roller 8, a tension roller 17, and a secondary transfer upstream roller 18. The intermediate transfer belt 7 is rotated in the direction of arrow R7 by the secondary transfer inner roller 8, the tension roller 17, and the secondary transfer upstream roller 18. The intermediate transfer belt 7 on which a full-color toner image has been formed conveys the toner image to secondary transfer unit T2 by rotating.

[0021] The toner image formed on the intermediate transfer belt 7 is transported at a timing that coincides with the recording material S at the secondary transfer portion T2. ​​The secondary transfer portion T2 is a transfer nip portion formed by a secondary transfer inner roller 8 and a secondary transfer outer roller 9 arranged opposite to each other, and causes the toner image to be attracted onto the recording material S by applying a predetermined pressure force and an electrostatic load bias. In this way, the secondary transfer portion T2 transfers the toner image on the intermediate transfer belt 7 to the recording material S. Toner remaining on the intermediate transfer belt 7 after transfer is collected by a transfer cleaner 11.

[0022] The recording material S onto which the toner image has been transferred is transported from the secondary transfer portion T2 to the fixing device 13 by the secondary transfer outer roller 9. The fixing device 13 applies a predetermined pressure and heat to the recording material S within a fixing nip formed by opposing rollers, melting and fixing the toner image on the recording material S. The fixing device 13 is equipped with a heater as a heat source, and is controlled so as to always maintain an optimum temperature. The recording material S onto which the toner image has been fixed is discharged onto a paper discharge tray 63. In the case of double-sided image formation, the recording material S is reversed by a reversing conveying mechanism and conveyed to registration rollers 62.

[0023] A concentration detection sensor 70 for detecting the toner concentration is provided near the intermediate transfer belt 7. The concentration detection sensor 70 is disposed between the photoconductor 1d and the outer secondary transfer roller 9 in order to detect the toner patterns of each color formed on the intermediate transfer belt 7.

[0024] The operation unit U allows the user to use the display unit and operation keys to not only issue a command to start image formation, but also to set the image quality and input information about the recording material S to be set in the paper feed cassette 60. The CPU 301 determines the image formation conditions based on the input information, and performs image formation under the predetermined conditions.

[0025] [Patch detection] FIG. 2 is a schematic diagram of the density detection sensor 70. The density detection sensor 70 is disposed opposite the intermediate transfer belt 7 and detects the toner pattern formed on the intermediate transfer belt 7. The density detection sensor 70 is composed of a light emitting diode LED71 that emits infrared rays, photodiodes PD72 and PD73 that receive infrared rays, and an electric board (not shown) on which they are mounted. The LED71 is disposed so as to irradiate the intermediate transfer belt 7 with infrared rays at an incident angle of 20°, and the PD72 is disposed so as to receive the specular reflected light of the light irradiated on the intermediate transfer belt 7 and the toner pattern at a position of a reflection angle of -20°. The PD73 is disposed so as to receive the diffuse reflected light of the light irradiated by the LED71 on the intermediate transfer belt 7 and the toner pattern at a position of a reflection angle of 50°. These optical elements are mounted on an electric board (not shown) composed of a drive circuit that supplies a current to the LED71, and a light receiving circuit having an IV conversion function that converts the current flowing according to the amount of light received by the PD72 and PD73 into a voltage.

[0026] [Block diagram] 3 is a schematic functional block diagram of the image forming apparatus 200. The CPU 301 has a function of generating various command signals and executing arithmetic processing to operate various sensors and motors of the image forming apparatus 200 in accordance with an electrophotographic process. The CPU 301 also has a built-in memory for storing data. The image data generating unit 302 has a function of converting various image data into signals for laser control and sending control signals to the laser driving units 303a to 303d. The image data generating unit 302 also has a function of generating a toner pattern for detecting toner concentration.

[0027] The laser driving units 303a to 303d have a function of driving the laser elements of the laser scanners 3a to 3d and controlling the light intensity and light emission of the lasers based on a signal sent from the image data generating unit 302. The sensor driving circuit 305 has a function of controlling the ON / OFF and driving current of the LED 71 in the density detection sensor 70 according to a command signal from the CPU 301. The density detection sensor detection circuit 305 amplifies the light receiving voltage signal from the density detection sensor 70 and sends it to the CPU 301.

[0028] The motor control unit 91 is electrically connected to each drive motor (not shown) and has a function of controlling the drive timing and drive speed. The high voltage control unit 92 has a function of controlling the output of biases required for the image formation process, such as a charging bias, a developing bias, and a transfer bias.

[0029] Furthermore, CPU 301 is electrically connected to sheet feed cassette 60, I / F unit 85, and timer 90, and is further connected to operation unit U through I / F unit 85. CPU 301 can perform image formation using recording material S stored in sheet feed cassette 60. Furthermore, operation unit U accepts operations by a user, and is configured, for example, with a liquid crystal touch panel. Note that operation unit U may be an external terminal such as a personal computer connected to the image forming apparatus.

[0030] The CPU 301 is also electrically connected to the controller 87 and the image processing unit 84. Image information 88 is sent to the CPU 301 through the controller 87. The CPU 301 can form an image by processing the received image information 88 in the image processing unit 84.

[0031] [Sub-scanning irregularity correction method] Next, a method of correcting periodic density unevenness in the sub-scanning direction according to this embodiment will be described.

[0032] In this embodiment, there are two types of modes: a density unevenness detection mode that detects sub-scanning unevenness, and a phase detection sequence that detects a control phase that cancels the density unevenness and determines the control timing. Sub-scanning unevenness is caused by rotational runout and sensitivity unevenness of key parts related to the image formation process, such as the image carrier, and the strength of the unevenness varies from one main unit to another. The density unevenness detection mode is executed for the purpose of detecting in advance the strength of the sub-scanning unevenness inherent to the main unit and its cause. Details will be described later, but this mode determines the process conditions in advance.

[0033] The process conditions determined in the density unevenness detection mode are controlled based on the phase detected by a phase detection sequence performed for each job. The phase detection sequence predicts the phase of sub-scanning unevenness that occurs in a job image at the start of the job and determines the control timing.

[0034] [Uneven density detection mode] Next, we will explain the density unevenness detection mode. The density unevenness detection mode detects the density that changes periodically in the sub-scanning direction in advance, and determines the control waveform E(t) (Equation 1) that periodically changes each process condition so as to reduce the detected sub-scanning unevenness. E(t)=Acos(Wt+θ) (Formula 1) where A is the amplitude of the waveform, W is the period, and θ is the phase.

[0035] In order to detect sub-scanning unevenness, a test pattern F as shown in FIG. 4 is used. Since the density of the test pattern F is measured by the density detection sensor 70 described above, the position of the test pattern F in the direction perpendicular to the rotation direction of the photoconductor (main scanning direction) is placed on the measurement position of the density sensor 70. In addition, each color is arranged in parallel so that four colors can be controlled simultaneously. The length of the test pattern F is set to twice the maximum circumference of the cause of density unevenness. This is to reduce the effects of sudden density streaks and other noise, unevenness of the intermediate transfer belt 7 that is the background of the pattern, and the like, in addition to periodic density unevenness. Note that the image density of the test pattern in this embodiment is set to 50% for each color.

[0036] The density of the test pattern F is measured by the density detection sensor 70. In this mode, for the cyan, magenta, and yellow patterns, the density of the pattern on the intermediate transfer belt 7 is measured by the PD73 that receives diffuse reflected light. On the other hand, for the black pattern, the density of the pattern on the intermediate transfer belt 7 is measured by the PD72 that receives specular reflected light. Since the PD72 detects both the specular reflected light component and the diffuse reflected light component, the specular reflected light component is calculated by removing the diffuse reflected light component detected by the PD73 from the reflected light component detected by the PD72 and performing a correction calculation. Since the reflected light from the intermediate transfer belt is large and there is almost no reflected light from the toner, the specular reflected light component detected by the PD72 decreases as the toner image density increases. Based on the results of detection in this way, the relationship between the toner image density and the diffuse reflected light and specular reflected light of each color is stored in advance, and the toner image density is calculated from the detected diffuse reflected light and specular reflected light.

[0037] 5 is an example of the result of detection of patch pattern F(C) by density detection sensor 70 in density unevenness correction mode of this embodiment. As shown in FIG. 5, sub-scanning unevenness is a waveform with a constant period and amplitude. In order to determine process conditions from the detection result, CPU 301 performs a Fourier transform on the waveform and extracts the period and amplitude of the sub-scanning unevenness. The Fourier transform result in this embodiment was a period of 63 mm and an amplitude of 0.2 V. At this time, if multiple unevennesses occur, multiple waveform results are extracted respectively.

[0038] Next, CPU 301 determines the process conditions to be changed and the values ​​of A and W in the control waveform E(t) from the results of these Fourier transforms. Since the period of the previous Fourier transform results matches that of photoconductor 1c, the process condition is set to the surface potential of photoconductor 1c. The surface potential of photoconductor 1c is changed by controlling the bias of charger 2c. Similarly, the period W in the control waveform E(t) is set to W=2π / 63.

[0039] In this embodiment, the amplitude A is determined using an amplitude conversion table that is stored in advance. In this embodiment, A=20 (V) when the amplitude is 0.2 V. These determined process conditions are stored.

[0040] The process conditions determined in this mode can be used to determine a phase θ that is shifted by half a wave with respect to the sub-scanning unevenness by a phase determining means (to be described later) and execute control to cancel the sub-scanning unevenness.

[0041] Next, the operation flow of the density unevenness detection mode will be described with reference to the flowchart in Fig. 7. The processing of this flowchart is realized by the CPU 301 executing a control program stored inside the CPU 301.

[0042] First, in step S1, the CPU 301 issues an instruction to the image forming apparatus to output the test pattern F, and causes the image forming apparatus to form the test pattern F.

[0043] Next, the CPU 301 causes a density detection sensor arranged on the intermediate transfer belt to detect the formed test pattern F (S2). When the CPU receives a detection signal, it uses a stored density conversion table to convert it into a density waveform (S3).

[0044] The converted concentration waveform is subjected to a Fourier transform to separate it into period and amplitude, and only periods with amplitudes equal to or greater than a predetermined value are extracted (S4), and the process control to be fed back is determined (S5). At the same time, the period W in the control waveform E(t) is also determined.

[0045] Finally, in order to determine the amplitude A in the waveform E(t) for the determined process control, a table that converts a pre-stored density amplitude into a control value is used to determine the control waveform E(t) (S6), and the density unevenness detection mode is terminated.

[0046] [Phase detection sequence] Next, the phase detection sequence will be described. The phase detection sequence is a mode that detects the density that changes periodically in the sub-scanning direction at the start of a job and determines the control phase θ of the control waveform E(t) that periodically changes each process condition so as to reduce the detected density unevenness.

[0047] In order to detect sub-scanning unevenness, a test pattern G as shown in Fig. 6 is used. In this embodiment, the sub-scanning length of test pattern G is set to be the same as the photoconductor perimeter, which is the maximum perimeter of the factor that causes density unevenness, but it may be the same as the above-mentioned test pattern F. Also, like the above-mentioned test pattern F, the arrangement in the main scanning direction is set on the density detection sensor arrangement.

[0048] Moreover, the density detection method is the same as that in the density unevenness correction mode described above, and therefore will not be described.

[0049] Next, a method for determining the phase θ will be described. Fig. 8 is a schematic diagram showing the relationship between the patch pattern in the phase detection mode of this embodiment and the JOB image input by the user. Fig. 8 also illustrates an example of correcting sub-scanning unevenness on the photoconductor 1c.

[0050] The phase detection sequence is performed for the job entered by the user, so it is performed during the first rotation of the job as shown in Figure 8. Therefore, the test pattern G(C) is output before the head of the job. At the same time, the timer 90 is used to measure the time t from the drive start timing.

[0051] When the density detection sensor 70 measures the test pattern G, fitting is performed using the stored waveform E(t) for the photoconductor 1c, and the peak time tp of the sub-scanning unevenness is extracted. Since θ is determined so that the control waveform E(t) cancels out the sub-scanning unevenness when time t=tp, θ=π-W×tp-Δts can be set taking into account the difference (Δts) between the detection time and the control time of the density detection sensor. Here, Δts is calculated as Δts=ds / Sp, based on the process speed Sp and the distance ds from the charger 2 to the density detection sensor 70.

[0052] Next, the flow of the phase detection sequence will be described with reference to the flowchart in Fig. 9. The processing of this flowchart is realized by the CPU 301 executing a control program stored inside the CPU 301.

[0053] When the phase detection sequence is started, the CPU 301 drives each motor (S11) and at the same time starts a timer to start measuring time (S12).

[0054] Next, the image forming apparatus is operated to form a phase detection test pattern G (S13), and the time when the image forming apparatus writes out an image is stored (S14).

[0055] When the density detection sensor 70 detects a density signal of the test pattern G (S15), the CPU 301 converts the signal into a density waveform and performs fitting with a pre-stored curve E(t) (S16). Furthermore, the CPU 301 extracts the density peak time tp of the curve E(t) from the fitting result (S17).

[0056] Thereafter, the CPU 301 determines the phase θ of the control waveform E(t) from the peak time (S18), and ends the phase detection sequence (S19).

[0057] [Image formation operation] Next, the image forming operation in this embodiment will be described with reference to the flow chart shown in FIG.

[0058] In this embodiment, the phase detection sequence is performed during the pre-rotation period from the start of a job to the start of image formation of the job image, as shown in the flowchart. This is because when the drive stops, a phase shift occurs due to overrun of each member or attachment / detachment of parts, so the detected phase is valid until the drive stops. However, in a configuration in which the phase after the drive stops can be controlled to some extent, for example, by reducing overrun using a brake mechanism or by having a mechanism for not attaching or detaching, it is not necessary to perform the phase detection every time. For example, the number of drive stops may be counted, and phase detection may be started when a predetermined number is reached.

[0059] First, when a job is started, the CPU 301 starts the above-mentioned phase detection sequence (S21).

[0060] When the phase detection is completed, the control for correcting density unevenness is started at a predetermined timing (S22, S23), and the control is continued until the job is completed (S24).

[0061] When the job is completed, the previously stored phase is cleared (S25), and the image forming operation is completed.

[0062] [Effects of the invention] Next, the effect of using the sub-scanning unevenness correction method according to this embodiment will be described, and a comparative example will be shown in which sub-scanning unevenness correction is not performed.

[0063] FIG. 11 is a graph showing a density profile in the sub-scanning direction at a given position when a full-surface solid image is output on GFC081, A4 size, using the image forming apparatuses of this embodiment and the comparative example.

[0064] As shown in FIG. 11, it can be seen that while large density unevenness occurs in the comparative example, the occurrence of density unevenness is greatly reduced in the example.

[0065] Moreover, the maximum in-plane unevenness of the chart at this time is ΔE00=2.0 in the comparative example, whereas the maximum is ΔE00=0.5 in the example.

[0066] As described above, by controlling the process control waveform determined from the density unevenness correction mode according to the phase timing determined using the phase detection sequence during rotation before the JOB, it is possible to reduce periodic density fluctuations that occur in the sub-scanning direction.

[0067] With regard to reading the pattern in the density unevenness correction mode, as shown in the first embodiment, a configuration using a density detection sensor on the intermediate transfer belt has been shown, but in order to measure density unevenness more precisely when correcting it, a configuration may also be used in which a test pattern is output on paper, and the test pattern on the paper is read by a sensor or scanner after fixing, and the amplitude and period are determined. EXAMPLES

[0068] In the first embodiment, the phase detection was performed by forming a test pattern and determining the control phase using the signal from the density detection sensor, but in this embodiment, a potential sensor is placed on the photoconductor, and the phase is detected from the result of directly measuring the potential fluctuation on the photoconductor without outputting a test pattern. Note that the same reference numerals are used for the same components as in the first embodiment, and the description will be omitted.

[0069] 12 is a schematic diagram of a potential sensor 110 for measuring the surface potential of the photosensitive drum 1. This measures the surface potential of the photosensitive drum 1 after the surface of the photosensitive drum 1, which has been uniformly charged by the charging roller 2, is exposed to laser light 3a by a laser scanner 3. This makes it possible to confirm whether the charging potential and exposure potential on the surface of the photosensitive drum 1 are actually at predetermined potentials.

[0070] As shown in FIG. 12, the potential sensor 110 has a tuning fork vibrator 802, which is a conductive vibrating object, disposed between a photoconductor 1, which is an object to be measured, and a measurement electrode 801.

[0071] The tuning fork vibrator 802 increases or decreases the amount of electric field lines incident on the measurement electrode 801 from the surface of the photoconductor 1, thereby measuring the electric field intensity on the surface of the photoconductor 1, which is the measurement target.

[0072] It is also possible to measure the surface potential of the photoconductor 1 using a potential measuring means other than the chopper type potential sensor shown in Fig. 12. For example, a capacitance variation type potential sensor is used to physically vibrate the measuring electrode 801 itself, changing the electric field distribution between the measuring electrode 801 and the photoconductor 1 to be measured, thereby measuring the electric field intensity on the surface of the photoconductor 1.

[0073] Next, a phase detection mode according to this embodiment will be described. In the first embodiment, the density that changes periodically in the sub-scanning direction is detected, but in this embodiment, a potential sensor is used to measure the potential on the photoconductor and determine the waveform.

[0074] Therefore, the charging roller 2 and the laser scanner 3 are only controlled so that the test pattern has a certain predetermined potential, and the developer is bias controlled so as not to develop toner. The sub-scanning length of the test pattern in this case should be equal to or longer than the circumferential length of the photoconductor 1 and the charging roller 2, which are factors that cause potential unevenness that can cause density unevenness. In this embodiment, it is set to the circumferential length of the photoconductor 1. Also, the thrust position is set to match the potential sensor.

[0075] The phase detection flow is the same except that the test pattern is not developed, so it will be omitted. [Explanation of symbols]

[0076] 7 Intermediate transfer belt 13 Fixing device 70 Concentration detection sensor (optical sensor) 71 Light emitting element (LED) 72, 73 Light receiving element (photodiode) 84 Printer image processing unit 110 Potential sensor 200 Image forming device 301 CPU

Claims

1. a test pattern forming means for forming a test pattern on a rotating photoreceptor; a density detection means for reading the density of the test pattern; a calculation means for calculating a period and an amplitude of density unevenness in a rotation direction of the photoconductor from the density read by the density detection means; a determination means for determining an image forming condition based on the period and the amplitude calculated by the calculation means; a phase determining means for determining a phase for controlling the image forming conditions based on a reading result of the test pattern read by the density detecting means; a density unevenness detection mode is executed in which a first test pattern is formed by the test pattern forming means, and a period and an amplitude of the density unevenness are calculated by the calculating means from a first reading result of the first test pattern read by the density detecting means; and a control means for executing a phase determination sequence in which a second test pattern is formed by the test pattern forming means, and a phase for controlling the image formation conditions is determined using the phase determination means based on a second reading result of the second test pattern read by the density detection means and a calculation result calculated from the first reading result by the calculation means.

2. 2 . The image forming apparatus according to claim 1 , wherein a length of the first test pattern in the rotation direction is longer than a length of the second test pattern in the rotation direction.

3. An image forming apparatus as described in claim 1, characterized in that the length of the second test pattern in the rotational direction is greater than or equal to the circumference of the photosensitive body.

4. An image forming apparatus as described in claim 1, characterized in that the length of the first test pattern in the rotational direction is twice the circumference of the photosensitive body.

5. 2. The image forming apparatus according to claim 1, wherein the control means executes the phase determination sequence during a period from the start of a job to the start of image formation of an image related to the job.

6. An image forming means for forming an image on a rotating photoreceptor; a density detection means for reading the density of a test pattern formed by the image forming means; a calculation means for calculating a period and an amplitude of density unevenness in a rotation direction of the photoconductor from the density read by the density detection means; a determination means for determining an image forming condition based on the period and the amplitude calculated by the calculation means; a potential applying means for applying a potential to the photoconductor to form a potential pattern on the photoconductor; a potential detection means for detecting a potential of the potential pattern in the rotation direction; a phase determining means for determining a phase for controlling the image forming conditions determined by the determining means based on a potential detection result of the potential pattern detected by the potential detecting means; and a control unit that controls the image forming unit based on the image forming condition determined by the determination unit and the phase determined by the phase determination unit.

7. An image forming apparatus as described in Claim 6, characterized in that the length of the test pattern in the rotation direction is twice the circumference of the photosensitive body.

8. The image forming unit further includes a charging roller for charging the photoconductor.

7. The image forming apparatus according to claim 6, wherein a length of the potential pattern in the rotation direction is equal to or greater than a circumferential length of the photoconductor and the charging roller.

Images