Image forming apparatus

Abstract

Suppresses concentration unevenness with higher precision than conventional methods. [Solution] The CPU 201 determines the timing when the test image 301f reaches the detection position of the density sensor 9f based on the digital signal D2 output by detecting the trigger image 1001r formed on the intermediate transfer belt 5 with the density sensor 9r, acquires profile data output from the density sensor 9f based on this timing, and generates correction data to suppress density unevenness based on the profile data.

Description

Technical Field

[0001] The present invention relates to a technique for correcting density unevenness in the sub-scanning direction of an image to be formed by an image forming apparatus.

Background Art

[0002] In an image forming apparatus that transfers an image formed on a rotating body to a sheet, density unevenness corresponding to the rotation period of the rotating body may occur in the image. According to Patent Document 1, a technique has been proposed for detecting periodic density unevenness from a toner image formed on an intermediate transfer belt and correcting control parameters so that the density unevenness is reduced.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] According to Patent Document 1, the output signal of an optical sensor is acquired in synchronization with the output signal of a home position sensor that detects the home position of a photosensitive drum as a rotating body. That is, sampling of the output signal of the optical sensor is started after a certain time from the output signal of the home position sensor. This is because it is assumed that the image always arrives at the detection position of the optical sensor at a constant conveyance time.

[0005] However, if slippage occurs between the photosensitive drum and the image carrier, the time at which the image reaches the detection position will deviate from the ideal time. As a result, the relationship between the measurement phase of density unevenness and the rotation phase of the photosensitive drum becomes misaligned, reducing the accuracy of density unevenness measurement. If exposure correction data is created from such density unevenness data, the exposure will be corrected using correction data created from density unevenness with a rotation phase different from the correct rotation phase. As a result, density unevenness will not be corrected accurately.

[0006] Therefore, the present invention aims to suppress concentration unevenness with higher precision than conventional methods. [Means for solving the problem]

[0007] To solve the above problems, the present invention provides an image forming apparatus comprising: an image forming means for forming an image on a rotating image carrier; a transfer means for transferring the image on the image carrier to a sheet; a first output means for receiving reflected light from a test image on the image carrier that passes through a first detection position and outputting an output signal based on the result of receiving the reflected light; a second output means for receiving reflected light from a test image on the image carrier that passes through a second detection position different from the first detection position in the rotation direction of the image carrier and outputting an output signal based on the result of receiving the reflected light; and a test image pattern that passes through both the first detection position and the second detection position as the image carrier rotates, wherein the test image pattern passes through the first detection position The image forming means is characterized by comprising: a determination means for determining the timing of reaching a position based on an output signal output by the first output means; an acquisition means for acquiring an output signal from the second output means, which is to be used to generate correction data, based on the timing determined by the determination means; a generation means for generating correction data to correct density unevenness in the rotation direction of the image to be formed by the image forming means, based on the output signal acquired by the acquisition means; and a control means for suppressing density unevenness in the rotation direction of the image to be formed by the image forming means, based on the correction data generated by the generation means. [Effects of the Invention]

[0008] According to the present invention, concentration unevenness can be suppressed with higher precision than in the conventional method. [Brief explanation of the drawing]

[0009] [Figure 1] Schematic cross-sectional view of an image forming apparatus [Figure 2] Control block diagram of an image forming apparatus [Figure 3] Reference diagram to explain the phase shift in concentration variations. [Figure 4] Schematic diagram illustrating the main components of the density sensor in an image forming apparatus. [Figure 5] Schematic diagram of the test image [Figure 6] Timing chart showing the start of profile data acquisition. [Figure 7] Flowchart showing the concentration unevenness correction sequence [Modes for carrying out the invention]

[0010] The embodiments will be described in detail below with reference to the attached drawings. Note that the following embodiments do not limit the invention as defined in the claims. While the embodiments describe multiple features, not all of these features are essential to the invention, and the features may be combined in any way. Furthermore, in the attached drawings, identical or similar configurations are given the same reference numerals, and redundant descriptions are omitted.

[0011] (Description of the image forming apparatus 100) Figure 1 is a schematic cross-sectional view of the image forming apparatus 100. In Figure 1, the letters abcd appended to the end of the reference numerals correspond to the toner colors: yellow "Y", cyan "C", magenta "M", and black "K". Therefore, when explaining matters common to all four colors, the letters abcd may be omitted from the reference numerals.

[0012] The photoreceptor drum 1 is an image carrier that rotates while carrying an electrostatic latent image or toner image. The diameter of the photoreceptor drum 1d for the black image is larger than the diameters of the photoreceptor drums 1a, 1b, and 1c for the other colors. Generally, black images are formed more frequently. Therefore, increasing the diameter of the photoreceptor drum 1d for the black image extends the lifespan of the photoreceptor drum 1d for the black image.

[0013] The charging roller 4 functions as a charging member that uniformly charges the surface of the photoreceptor drum 1. The exposure unit 2 has a light source that irradiates the surface of the photoreceptor drum 1 with laser light corresponding to the image signal to form an electrostatic latent image. The light source of the exposure unit 2 functions as an exposure unit that exposes the photoreceptor to form an electrostatic latent image. The developer unit 3 has a developing sleeve that develops the electrostatic latent image using toner to form a toner image. The developing sleeve of the developer unit 3 functions as a developing unit that develops the electrostatic latent image on the photoreceptor. The primary transfer roller 12 transfers the toner image from the photoreceptor drum 1 to the intermediate transfer belt 5. The drum cleaner 10 is a cleaning member that cleans the toner remaining on the photoreceptor drum 1. A full-color image is formed by superimposing the yellow toner image, magenta toner image, cyan toner image, and black toner image.

[0014] As the intermediate transfer belt 5 rotates, the toner image 6 is transported to the secondary transfer section. The secondary transfer section is a nip formed by the contact between the secondary transfer roller 11 and the intermediate transfer belt 5. The secondary transfer roller 11 transfers the toner image 6 to the sheet P. The fuser 15 heats and pressurizes the toner image and the sheet P to fix the toner image onto the sheet P. The fuser 15 has a heater for heating and two opposing rotating bodies (e.g., rollers, cylindrical film) for pressurizing. Toner remaining on the intermediate transfer belt 5 is cleaned by the belt cleaner 13.

[0015] As described above, density unevenness may occur in the toner image 6 according to the period of the rotating body. Hereinafter, as an example of the rotating body, the photosensitive drum 1 is adopted. The image forming apparatus 100 forms a toner image (test image) for detecting density unevenness on the intermediate transfer belt 5. The density sensor 9 acquires data (profile data) of the density of the test image corresponding to one rotation of the photosensitive drum 1 by detecting reflected light from the test image. The density sensor 9 is an optical sensor that receives reflected light from the test image formed on the image carrier and outputs an output signal based on the light reception result of the reflected light.

[0016] Based on the profile data, the image forming apparatus 100 corrects control parameters for adjusting the density of the toner image so as to suppress density unevenness. Examples of the control parameters include exposure amount, charging voltage, and developing voltage. In the present embodiment, as an example, the exposure amount is corrected.

[0017] The HP sensor 7 is a phase sensor (detector) for detecting the rotation phase of the photosensitive drum 1. HP is an abbreviation of home position. The home position corresponds to a reference phase in the rotation phase. A mark indicating the home position is formed at a certain position on the surface of the photosensitive drum 1. The mark may be an optical mark or a magnetic mark. The image forming apparatus 100 creates profile data by associating the rotation phase of the photosensitive drum 1 acquired by the HP sensor 7 with output values corresponding to different positions on the test image 301 in the sub-scanning direction. The HP sensor 7 functions as rotation phase detection means used for detecting the rotation phase of the photosensitive drum 1.

[0018] When the image forming apparatus 100 forms an image (user image) arbitrarily prepared by the user, the image forming apparatus 100 determines a correction value of the exposure amount corresponding to the rotation phase of the photosensitive drum 1 acquired by the HP sensor 7 based on the profile data and corrects the exposure amount. Alternatively, a correction value corresponding to the rotation phase may be read from correction data which is a set of correction values created in advance from the profile data.

[0019] FIG. 2 is a control block diagram of the image forming apparatus 100. The CPU 201 detects the home position (reference phase) of the photoreceptor drums 1a to 1d by detecting the output signals of the HP sensors 7a to 7d. Thereby, the CPU 201 can identify which position on the surface of the photoreceptor drum 1 is being exposed. The CPU is an abbreviation for a central processing unit. The CPU 201 may be called a controller or a control board. The CPU 201 may be realized by a hardware circuit such as a specific application integrated circuit (ASIC) or a field programmable gate array.

[0020] The density sensor 9 includes a density sensor 9f and a density sensor 9r at different positions in the rotation direction of the intermediate transfer belt 5. The density sensor 9f has a light emitting element 91 that irradiates light toward the test image, and a light receiving element 92 that receives reflected light from the test image and the surface (base) of the intermediate transfer belt 5. The position on the intermediate transfer belt 5 irradiated with the light from the light emitting element 91 is the first detection position. The light receiving element 92 functions as a first output means that receives (detects) the reflected light from the test image passing through the first detection position and outputs an output value (output signal) based on the light receiving result (detection result) of the reflected light from the test image. The density sensor 9r is also an optical sensor having a light emitting element 93 and a light receiving element 94, similar to the density sensor 9f. The position on the intermediate transfer belt 5 irradiated with the light from the light emitting element 93 is the second detection position. The second detection position is a position different from the first detection position in the rotation direction of the intermediate transfer belt 5. The light receiving element 94 functions as a second output means that receives (detects) the reflected light from the test image passing through the second detection position and outputs an output value (output signal) based on the light receiving result (detection result) of the reflected light from the test image.

[0021] The CPU 201 controls the lighting and extinguishing of the light-emitting elements 91 and 93. The CPU 201 may also have an analog-to-digital converter (ADC) 208. The ADC 208 is a conversion circuit that converts the current values ​​(analog signals) output from the light-receiving elements 92 and 94 into voltage values ​​(digital signals). The ADC 208 may also convert analog current values ​​to digital current values. The light-receiving elements 92 and 94 are not limited to those that output current values, but may also be elements that output voltage values. In this case, the ADC 208 may convert analog voltage values ​​to digital voltage values.

[0022] The image forming control unit 209 includes an exposure control unit 212, a development control unit 213, a drum control unit 214, and a belt control unit 215. The exposure control unit 212 controls the on / off state and exposure amount of the exposure units 2a-2d according to commands output from the CPU 201. The exposure amount is a control parameter that affects the density of the toner image. Typically, the exposure amount is the amount of exposure that can achieve the maximum density in the toner image. By modulating the laser light of this exposure amount according to the user image, toner images with various gradations are formed. The development control unit 213 controls the development voltage of the development units 3a-3d according to commands output from the CPU 201. The development voltage is a control parameter that controls how easily toner adheres to the electrostatic latent image. The drum control unit 214 controls the motor that rotates the photoreceptor drums 1a-1d and controls the charging voltage of the charging rollers 4a-4d. The belt control unit 215 controls the motor that drives the intermediate transfer belt 5 and controls the primary transfer voltage of the primary transfer rollers 12a-12d.

[0023] The CPU 201 controls the image forming apparatus 100 by executing a control program stored in the ROM 210. ROM stands for read-only memory and is an example of non-volatile memory. The RAM 211 stores temporary data when the control program is executed. RAM stands for random access memory and is an example of volatile memory.

[0024] ROM 210 may store image data of a test image used to detect density unevenness. CPU 201 reads the image data from ROM 210, generates an image signal, and outputs it to exposure control unit 212. Exposure control unit 212 controls the exposure unit 2 to modulate the laser light according to the image signal and scan the laser light on the photoreceptor drum 1. This forms an electrostatic latent image of the test image. Subsequently, the electrostatic latent image is developed to form the toner image of the test image. The test image is transferred to the intermediate transfer belt 5 and detected by the density sensor 9. The test image may be a halftone image of a predetermined gradation (e.g., intermediate gradation).

[0025] The ADC208 converts the analog signal output from the photodetector 92 of the density sensor 9 into a digital signal (digital value). The CPU201 associates this digital value with the rotational phase of the photoreceptor drum 1 to create profile data and stores it in the RAM211. The CPU201 calculates the difference between the profile data and the target image density characteristics (gradation characteristics), and corrects the control parameters that affect the image density based on the calculation result. Here, correction values ​​for each rotational phase of the photoreceptor drums 1a-1d are determined, and correction data, which is a set of correction values ​​for one rotation of the photoreceptor drums 1a-1d, may be created in advance. Alternatively, correction values ​​may be determined in real time based on the profile data.

[0026] Note that profile data is created individually for each photoreceptor drum 1a-1d. This is because the density unevenness characteristics may differ for each of the photoreceptor drums 1a-1d.

[0027] (Explanation of phase shift in density unevenness correction) Figure 3(A) is a timing chart of the signals used when creating profile data. Here, the output signal of the HP sensor 7, the exposure / non-exposure status of the exposure unit 2, and the density detection result (analog signal) of the density sensor 9f are shown as examples.

[0028] Time t0 is the falling edge timing of the output signal of the HP sensor 7. Time t0 corresponds to the reference phase. The CPU 201 may also determine the rotation phase of the photoreceptor drum 1 using a counter that is reset each time the output signal of the HP sensor 7 falls. In other words, the counter's count value indicates the rotation phase of the photoreceptor drum 1. The HP sensor 7 may also be an encoder attached to the rotation axis of the photoreceptor drum 1.

[0029] Time t1 is the timing when a predetermined time Tstart has elapsed from time t0. CPU201 starts exposure with exposure unit 2 at time t1. Here, the exposure amount is controlled to be constant so that the test image becomes a halftone image with a predetermined number of tones. CPU201 continues exposure from time t1 to time t3. Here, the time Trot from time t1 to time t3 corresponds to the time required for the photoreceptor drum 1 to complete one rotation (one cycle). As a result, a test image (electrostatic latent image) with a length at least equivalent to the circumference of the photoreceptor drum 1 is formed on the photoreceptor drum 1. A halftone image is used as the test image because the density of halftone images tends to vary more easily.

[0030] Time t2 is the point in time Tcount that has elapsed since time t1. At time t2, the leading edge of the test image arrives at the detection position of the density sensor 9f. At time t2, the CPU 201 starts detecting the test image with the density sensor 9. Tcount is the time obtained by dividing the sum of the distance on the surface of the photoreceptor drum 1 from the exposure position to the primary transfer position and the distance on the surface of the intermediate transfer belt 5 from the primary transfer position to the detection position by the process speed. Here, the process speed is the same as the surface speed of the photoreceptor drum 1 or the surface speed of the intermediate transfer belt 5. The exposure position is the position on the photoreceptor drum 1 to which exposure light (laser light) from the exposure unit 2 is irradiated. The primary transfer position is the contact position between the photoreceptor drum 1 and the intermediate transfer belt 5.

[0031] The CPU 201 creates profile data for one rotation of the photoreceptor drum 1 by sampling the detection results of the density sensor 9f from time t2 to time t4. The density detected at time t2 is the density of the rotation phase of the photoreceptor drum 1 that corresponds to time t1.

[0032] When forming a user image, the CPU 201 reads profile data associated with the rotation phase corresponding to the exposure start timing (time t1) from the RAM 211 and corrects the exposure amount according to the profile data. Thereafter, each time the rotation phase advances, the CPU 201 reads profile data from the RAM 211 and corrects the exposure amount according to the profile data. This corrects density unevenness according to the rotation phase of the photoreceptor drum 1.

[0033] Here, instead of exposure amount, development voltage or charging voltage may be used as the control parameter for correcting density unevenness. If the control parameter is development voltage, the correction value of the development voltage over one rotation of the photoreceptor drum 1a-1d corresponds to the correction data. If the control parameter is charging voltage, the correction value of the charging voltage over one rotation of the photoreceptor drum 1a-1d corresponds to the correction data.

[0034] Figure 3(B) shows the positional relationship between the test image 301, the intermediate transfer belt 5, and the density sensor 9f. The test image 301 is formed on the surface of the intermediate transfer belt 5 at a position readable by the density sensor 9f. Here, the direction in which the photoreceptor drum 1 rotates (rotation direction) is called the sub-scanning direction. The direction of travel of the intermediate transfer belt 5 (rotation direction) is also parallel to the sub-scanning direction, and the direction of travel of the intermediate transfer belt 5 (rotation direction) can also be called the sub-scanning direction. The direction perpendicular to the sub-scanning direction is called the main scanning direction. The length of the test image 301 in the main scanning direction is greater than the diameter of the detection spot of the density sensor 9. The length Lti of the test image 301 in the sub-scanning direction is at least greater than or equal to the circumference of the photoreceptor drum 1.

[0035] However, when correcting exposure based on profile data acquired before forming a user image, the following problem arises: In order to correct density unevenness over the entire rotation of the photoreceptor drum 1, the phase of the profile data must match the rotation phase of the photoreceptor drum 1. The method for matching the phase of the profile data with the rotation phase of the photoreceptor drum 1 is as described in relation to Figure 3(A).

[0036] However, as the distance between the detection position of the density sensor 9f and the exposure position on the surface of the photoreceptor drum 1 increases, phase shifts, as shown in Figure 3(A), are more likely to occur. A phase shift is a phenomenon in which the phase difference between the exposure start phase and the density detection start phase deviates from the ideal phase difference. In Figure 3(A), the exposure start phase corresponds to the phase at time t1. The ideal detection start phase corresponds to the phase at time t2. When a phase shift occurs, the leading edge of the toner image arrives at the detection position at time t2', which is different from time t2. If profile data is acquired from time t2 despite the occurrence of a phase shift, the relationship between the rotation phase and density unevenness in the profile data will be misaligned. Using such profile data may reduce the accuracy of density unevenness correction.

[0037] Factors contributing to phase shift include tolerances in the detection position of the density sensor 9f (and similarly, the density sensor 9r) relative to the intermediate transfer belt 5, and tolerances in the contact position (primary transfer position) between the photoreceptor drum 1 and the intermediate transfer belt 5. These tolerances may also be called relative position tolerances. Furthermore, the slippage of the intermediate transfer belt 5 relative to the photoreceptor drum 1 can cause a shift in the transfer position of the toner image transferred from the photoreceptor drum 1 on the intermediate transfer belt 5, which also contributes to phase shift.

[0038] The relative position tolerance depends on the number of components involved in toner image transport. The rotation axis of the photoreceptor drum 1, the rotation axis of the rotating roller of the intermediate transfer belt 5, and the density sensors 9f (and 9r) are supported by the main frame of the image forming apparatus 100. Reducing the number of support components supporting these components reduces the relative position tolerance. As a result, the phase shift, converted to length, becomes approximately 0.2 mm. If further slippage occurs, the phase shift, converted to length, can exceed 1 mm.

[0039] Furthermore, the components supporting the intermediate transfer belt 5 and the drum unit including the photoreceptor drum 1 are replaced when their lifespans expire. As a result, the relative position tolerances also change. Therefore, the profile data acquired when the image forming apparatus 100 is shipped from the factory cannot correctly correct for density unevenness. For this reason, if the intermediate transfer belt 5 or the components supporting the intermediate transfer belt 5 are replaced, or if the drum unit including the photoreceptor drum 1 is replaced, it is necessary to acquire new profile data.

[0040] (Description of the embodiment) The following description explains a method for creating profile data with higher accuracy than conventional methods using density sensors 9f and 9r on the intermediate transfer belt 5. In order to match the phase in the profile data with the rotational phase in the photoreceptor drum 1, the CPU 201 identifies the rotational phase at which exposure began in the analog signal output from the density sensor 9.

[0041] Figure 4 is an illustrative diagram showing an excerpt of the part involved in acquiring profile data for CPU 201 and concentration sensor 9f. The part involved in acquiring profile data for CPU 201 and concentration sensor 9r is similar, so its explanation is omitted here. The output terminal of the light-receiving element 92 is connected to the input terminal of ADC 208 and the negative terminal of comparator 400. The light-receiving element 92 converts a detection current corresponding to the amount of light received (light intensity) into a detection voltage V1 and outputs it.

[0042] The positive terminal of comparator 400 has a threshold voltage Vth applied to it. The threshold voltage Vth is generated by dividing the power supply voltage V_logic using voltage divider resistors R1 and R2. Vth=V_logic×R2 / (R1+R2) (Formula 1)

[0043] One end of the voltage divider resistor R1 is connected to the power supply voltage V_logic. The other end of the voltage divider resistor R1 is connected to the positive terminal of comparator 400 and one end of the voltage divider resistor R2. The other end of the voltage divider resistor R2 is connected to the frame ground (GND). As a result, comparator 400 compares the analog signal (output signal) output in time series from the photodetector 92 of the concentration sensor 9f with the threshold voltage Vth (threshold). Comparator 400 functions as a comparison unit that compares the detection voltage V1 output by the concentration sensor 9f with the threshold voltage Vth.

[0044] The output terminal of comparator 400 has an open collector or a similar output circuit. The output terminal of comparator 400 is connected to the power supply voltage V_logic via a pull-up resistor R3. Furthermore, the output terminal of comparator 400 is connected to timer 401 of CPU 201. The output terminal of comparator 400 outputs a binarized signal (digital signal D1) obtained by binarizing the analog signal, the detection voltage V1, by the threshold voltage Vth.

[0045] When the detected voltage V1 falls below the threshold voltage Vth, the output terminal of comparator 400 does not draw current. Therefore, the digital signal D1 is at a high level. On the other hand, when the detected voltage V1 exceeds the threshold voltage Vth, the output terminal of comparator 400 draws current. Therefore, the digital signal D1 is at a low level.

[0046] The CPU 201 identifies or determines the timing when the level of the digital signal D1 changes from a high level to a low level as the exposure start timing of the test image 301. Here, the timing when the level of the digital signal D1 changes from a high level to a low level is the timing when the test image 301 on the intermediate transfer belt 5 reaches the detection position of the density sensor 9f. The comparator 400 is a detection unit that detects the timing when the test image 301 on the intermediate transfer belt 5 reaches the detection position of the density sensor 9f. From the exposure start timing, the CPU 201 starts the ADC 208 and begins sampling of the analog signal V1. When the timer 401 has measured a predetermined time Trot from the exposure start timing, the CPU 201 stops the ADC 208.

[0047] Figure 5 is a schematic diagram of the test images 301f and 301r transferred to the intermediate transfer belt 5, and the trigger images 1001f and 1001r transferred to the intermediate transfer belt 5. Below, the yellow test images 301f and 301r and the yellow trigger images 1001f and 1001r, which are used to correct density unevenness in the yellow image, will be explained. Similarly, the process of forming test images 301f and 301r and trigger images 1001f and 1001r for magenta, cyan, and black is the same, and their explanation will be omitted below.

[0048] The trigger image 1001r is used to determine the start of acquiring profile data obtained from the test image 301f, and the trigger image 1001f is used to determine the start of acquiring profile data obtained from the test image 301r. The trigger image 1001f is transferred upstream of the test image 301f in the direction of travel of the intermediate transfer belt 5. The trigger image 1001r is transferred downstream of the test image 301r in the direction of travel of the intermediate transfer belt 5. The trigger image 1001f has a higher density than the test image 301f, and the trigger image 1001r has a higher density than the test image 301r. In addition, the length of the test images 301f and 301r in the sub-scanning direction is longer than the circumference of the photoreceptor drum 1.

[0049] Figure 6 is a timing chart showing the start of profile data acquisition using test images 301f and 301r and trigger images 1001f and 1001r shown in Figure 5. Here, the rotation phase of the photoreceptor drum 1, exposure timing, analog signal V1 of the density detection result, analog signal V2 of the density detection result, digital signal D1 of the density detection result, and digital signal D2 of the density detection result are shown.

[0050] Here, analog signal V1 is a detection voltage (V1) corresponding to the amount of light received (light intensity) output from the photodetector 92. Analog signal V2 is a detection voltage (V2) corresponding to the amount of light received (light intensity) output from the photodetector 94. Digital signal D1 is a binarized signal showing the comparison result of analog signal V1 and threshold voltage Vth. Digital signal D2 is a binarized signal showing the comparison result of analog signal V2 and threshold voltage Vth.

[0051] At time t10, the signal output from HP sensor 7 falls to a falling edge. At the timing when a predetermined time Tstart has elapsed from time t10 (time t11), CPU 201 starts exposure for test image 301f on exposure unit 2. Here, the exposure amount for test image 301f is exposure amount E1. CPU 201 performs exposure from time t11 to time t13. The time from time t11 to time t13 is longer than the time required for the photoreceptor drum 1 to complete one rotation (one cycle). Here, the time from time t11 to time t13 is defined as the time required for the photoreceptor drum 1 to complete one rotation (Trot) plus three times the time obtained by dividing the diameter of the detection spot of density sensor 9f by the process speed (peripheral speed of the intermediate transfer belt 5).

[0052] Furthermore, by exposing the image for a longer time than the time Trot, the exposure unit 2 forms an electrostatic latent image of the test image 301f on the photoreceptor drum 1, with a length at least equivalent to the circumference of the photoreceptor drum 1. A halftone image is used as the test image 301f because midtone images tend to have more variation in density.

[0053] Here, time t12 is the point in time Tcount that has elapsed since time t11. If there is no phase shift, the leading edge of the test image 301f will arrive at the detection position of the density sensor 9f at time t12.

[0054] Furthermore, at a predetermined time elapsed from time t11 (time t14), the CPU 201 causes the exposure unit 2 to start exposure for the trigger image 1001r. Here, the exposure amount E2 for the trigger image 1001r is greater than the exposure amount E1. Here, the trigger image 1001r and the test image 301f are formed at different positions in the main scanning direction. In other words, the exposure unit 2 switches between exposure amounts E1 and E2 according to the position of the photoreceptor drum 1 in the main scanning direction. As a result, the trigger image 1001r and the test image 301f do not overlap on the photoreceptor drum 1.

[0055] When forming the trigger image 1001r, the exposure unit 2 exposes the photoreceptor drum 1 only from time t14 to time Tsp. Time Tsp is the time obtained by dividing the diameter of the detection spot of the density sensor 9r by the process speed (peripheral speed of the intermediate transfer belt 5). In other words, the diameter of the detection spot of the density sensor 9r and the length of the trigger image 1001r in the sub-scanning direction are equal. Time t15 is the timing when time Tcount has elapsed from time t14. If no phase shift has occurred, the leading edge of the trigger image 1001r will arrive at the detection position of the density sensor 9r at time t14.

[0056] At the moment (time t16) when the analog signal V2 of the density sensor 9r, based on the detection result of the trigger image 1001r, exceeds the threshold voltage, the level of the digital signal D2 changes from High to Low. Time 16 is the actual timing when the trigger image 1001r reaches the detection position of the density sensor 9r, with a phase shift occurring. In other words, the time difference between time t15 and time t16 corresponds to the delay time of the timing when the trigger image 1001r on the intermediate transfer belt 5 reaches the detection position of the density sensor 9r, caused by the phase shift. The time difference between time t15 and time t16 is the delay time of the intermediate transfer belt 5, and is therefore equal to the delay time when the test image 301f reaches the detection position of the density sensor 9f.

[0057] Here, time Tsp is the time from time t16 to time t16 during which the trigger image 1001r is detected by the density sensor 9r. In order to acquire profile data for one cycle of the photoreceptor drum 1, the CPU 201 starts acquiring profile data A of the test image 301f using the density sensor 9f from time t17, after a margin time Tmag has elapsed from time t16. The CPU 201 acquires profile data A over time Trot. At time t18, the CPU 201 completes the acquisition of profile data using the density sensor 9f.

[0058] Here, the CPU 201 functions as a determination means that determines the timing when the test image 301f reaches the detection position of the density sensor 9f, based on the digital signal D2 output from the density sensor 9r when the density sensor 9r detects the trigger image 1001r.

[0059] Next, at time t20, the signal output from HP sensor 7 falls. At the timing when a predetermined time Tstart has elapsed from time t20 (time t21), CPU 201 causes exposure unit 2 to start exposure for test image 301r. Here, the exposure amount for test image 301r is exposure amount E1. CPU 201 performs exposure from time t21 to time t23. The time from time t21 to time t23 is longer than the time required for the photoreceptor drum 1 to complete one rotation (one cycle). Here, the time from time t21 to time t23 is defined as the time required for the photoreceptor drum 1 to complete one rotation (Trot) plus three times the time obtained by dividing the diameter of the detection spot of density sensor 9r by the process speed (peripheral speed of the intermediate transfer belt 5).

[0060] Furthermore, by exposing the image for a longer time than the time Trot, the exposure unit 2 forms an electrostatic latent image of the test image 301r on the photoreceptor drum 1, with a length at least equivalent to the circumference of the photoreceptor drum 1. A halftone image is used as the test image 301r because midtone images tend to have more variation in density.

[0061] Here, time t22 is the point in time Tcount that has elapsed since time t21. If there is no phase shift, the leading edge of the test image 301r will arrive at the detection position of the density sensor 9r at time t22.

[0062] Furthermore, at a predetermined time elapsed from time t21 (time t24), the CPU 201 instructs the exposure unit 2 to begin exposure for the trigger image 1001f. Here, the exposure amount E2 for the trigger image 1001f is greater than the exposure amount E1. The trigger image 1001f and the test image 301r are formed at different positions in the main scanning direction. In other words, the exposure unit 2 switches between exposure amounts E1 and E2 depending on the position of the photoreceptor drum 1 in the main scanning direction. As a result, the trigger image 1001f and the test image 301r do not overlap on the photoreceptor drum 1.

[0063] When forming the trigger image 1001f, the exposure unit 2 exposes the photoreceptor drum 1 only from time t24 to time Tsp. Time Tsp is the time obtained by dividing the diameter of the detection spot of the density sensor 9f by the process speed (peripheral speed of the intermediate transfer belt 5). In other words, the diameter of the detection spot of the density sensor 9f is equal to the length of the trigger image 1001f in the sub-scanning direction. Time t25 is the timing when time Tcount has elapsed from time t24. If no phase shift occurs, the leading edge of the trigger image 1001f arrives at the detection position of the density sensor 9f at time t24.

[0064] At the moment (time t26) when the analog signal V1 of the density sensor 9f, based on the detection result of the trigger image 1001f, exceeds the threshold voltage, the level of the digital signal D1 changes from High to Low. Time 26 is the actual timing when the trigger image 1001f reaches the detection position of the density sensor 9f, with a phase shift occurring. In other words, the time difference between time t25 and time t26 corresponds to the delay time of the timing when the trigger image 1001f on the intermediate transfer belt 5 reaches the detection position of the density sensor 9f, caused by the phase shift. The time difference between time t25 and time t26 is the delay time of the intermediate transfer belt 5, and is therefore equal to the delay time when the test image 301f reaches the detection position of the density sensor 9f.

[0065] Here, time Tsp from time t26 is the time during which the trigger image 1001f is detected by the density sensor 9f. In order to acquire profile data for one cycle of the photoreceptor drum 1, the CPU 201 starts acquiring profile data of the test image 301f using the density sensor 9f from time t27, after a margin time Tmag has elapsed from time t26. The CPU 201 acquires profile data over time Trot. At time t28, the CPU 201 completes the acquisition of profile data using the density sensor 9f.

[0066] Here, the CPU 201 functions as a determination means that determines the timing when the test image 301r reaches the detection position of the density sensor 9r, based on the digital signal D1 output from the density sensor 9f when the density sensor 9f detects the trigger image 1001f.

[0067] Finally, CPU201 generates profile data by taking the detection results (profile data) of test image 301f and the detection results (profile data) of test image 301r and averaging the detection results for the same phase.

[0068] Figure 7 is a flowchart showing the density uniformity correction sequence executed by the CPU 201 according to the control program. The control program is stored in the ROM 210.

[0069] In step S801, CPU201 determines whether HP (reference phase) has been detected based on the output signal of HP sensor 7. For example, if the output signal of HP sensor 7 changes from a high level to a low level, CPU201 proceeds to process S801 to S802. Note that the relationship between high and low levels may be reversed.

[0070] In S802, CPU201 obtains the time t10 from a timer (not shown) and stores it in RAM211, then starts timing Tstart. The timing of Tstart may be performed based on a counter circuit or the like.

[0071] At S803, the CPU 201 begins forming the test image 301f. Specifically, the CPU 201 converts the image data of the test image 301f into an image signal and outputs it to the exposure control unit 212. This initiates exposure of the photoreceptor drum 1 for the test image 301f.

[0072] In S804, CPU201 obtains the time t11 at which the formation of test image 301f began from timer 401, and stores the time difference between time t11 and time t10 as Tstart in RAM211.

[0073] At step S805, the CPU 201 begins forming the trigger image 1001r. Specifically, the CPU 201 converts the image data of the trigger image 1001r into an image signal and outputs it to the exposure control unit 212. This initiates exposure of the photoreceptor drum 1 for the trigger image 1001r.

[0074] In S806, CPU201 waits until the level of digital signal D2 changes from high to low. Once the level of digital signal D2 changes from high to low, CPU201 proceeds to process S806 to S807 after a predetermined margin time Tmar has elapsed.

[0075] In S807, CPU201 acquires profile data A by starting sampling of the analog signal V1 output from density sensor 9f. CPU201 stores the digital signal D1, which is the density detection result, as profile data A in RAM211, associated with the time (rotation phase of photoreceptor drum 1) obtained from a timer (not shown). In profile data A, density data I, which is the detection result of density sensor 9f, is stored linked to the rotation phase θ of photoreceptor drum 1. In this case, CPU201 converts the time to the rotation phase θ.

[0076] In S808, the CPU201 determines whether the acquisition of profile data A is complete. For example, if a predetermined time (Trot) has elapsed since the start of sampling of the analog signal V1, the CPU201 determines that the acquisition of profile data A is complete. Alternatively, the CPU201 may determine that the acquisition of profile data A is complete when a predetermined number of detection results (digital signal D1) have been acquired. If the acquisition of profile data A is not complete, the CPU201 proceeds to process S809 from S808.

[0077] At S809, CPU201 waits for approximately Tdistace before proceeding with processing from S809 to S807. Meanwhile, if the acquisition of profile data A is complete, CPU201 proceeds with processing from S808 to S810.

[0078] In the S810, CPU201 modifies profile data A stored in RAM211 and stores it back in RAM211. As a result, profile data A becomes profile data A with the reference phase as the initial phase (starting point).

[0079] After the formation of the test image 301f is completed in step S803, the process of forming the trigger image 1001f and the test image 301r begins. In S811, the CPU 201 determines whether HP (reference phase) has been detected based on the output signal of the HP sensor 7. For example, when the number of times the output signal of the HP sensor 7 has changed from a High level to a Low level reaches 2 from time t10, the CPU 201 proceeds from S811 to S812. Note that the relationship between High level and Low level may be reversed.

[0080] In S812, CPU201 obtains the time t20 from a timer (not shown) and stores it in RAM211, then starts timing Tstart. The timing of Tstart may be performed based on a counter circuit or the like.

[0081] In step S813, the CPU 201 begins forming the test image 301r. Specifically, the CPU 201 converts the image data of the test image 301r into an image signal and outputs it to the exposure control unit 212. This initiates exposure of the photoreceptor drum 1 for the test image 301r.

[0082] In S814, CPU201 obtains the time t21 at which the formation of test image 301r began from timer 401, and stores the time difference between time t21 and time t20 as Tstart in RAM211.

[0083] At S815, the CPU 201 begins forming the trigger image 1001f. Specifically, the CPU 201 converts the image data of the trigger image 1001f into an image signal and outputs it to the exposure control unit 212. This initiates exposure of the photoreceptor drum 1 for the trigger image 1001f.

[0084] In S816, CPU201 waits until the level of digital signal D1 changes from high to low. Once the level of digital signal D1 changes from high to low, CPU201 proceeds to process S815 to S816 after a predetermined margin time Tmar has elapsed.

[0085] In S817, the CPU201 acquires profile data B by starting sampling of the analog signal V2 output from the density sensor 9r. The CPU201 stores the digital signal D2, which is the density detection result, as profile data B in the RAM211, associated with the time (rotation phase of the photoreceptor drum 1) obtained from a timer (not shown). In profile data B, the density data I, which is the detection result of the density sensor 9r, is stored linked to the rotation phase θ of the photoreceptor drum 1. In this case, the CPU201 converts the time to the rotation phase θ.

[0086] In S818, the CPU201 determines whether the acquisition of profile data B is complete. For example, if a predetermined time (Trot) has elapsed since the start of sampling of the analog signal V2, the CPU201 determines that the acquisition of profile data B is complete. Alternatively, the CPU201 may determine that the acquisition of profile data B is complete when a predetermined number of detection results (digital signal D2) have been acquired. If the acquisition of profile data B is not complete, the CPU201 proceeds to S819 from S818.

[0087] At S819, CPU201 waits for approximately Tdistace before proceeding to process S819 through S817. Meanwhile, if the acquisition of profile data is complete, CPU201 proceeds to process S818 through S820.

[0088] In the S820, CPU201 corrects the phase of profile data B stored in RAM211 and stores it back in RAM211. As a result, profile data B becomes profile data B with the reference phase as the initial phase (starting point).

[0089] In S821, CPU201 creates correction data for control parameters (e.g., exposure) based on profile data A and profile B. It averages the density data of the same phase in profile A and profile B and identifies it as the profile data for that phase. Then, CPU201 creates exposure correction data starting from HP (reference phase) from the average density of each phase so that the density data (average) of each phase becomes the target density data, and stores it in RAM211.

[0090] The CPU 201 completes the density unevenness correction sequence through the above processing. Next, when a job to form a user image on the sheet is executed, the CPU 201 corrects the exposure amount of the exposure unit 2 based on the correction data stored in the RAM 211, thereby forming an image (user image) with suppressed density unevenness.

[0091] Similarly, for magenta, cyan, and black as described above, test images and trigger images are formed, and profile data is generated based on the detection results of density sensors 9f and 9r.

[0092] Although CPU201 generated correction data using test images 301f and 301r and trigger images 1001f and 1001r, correction data may also be generated from profile data A acquired using test image 301f and trigger image 1001r. Test image 301f and trigger image 1001r correspond to test image patterns that pass through both the detection position of density sensor 9f and the detection position of density sensor 9r due to the rotation of the intermediate transfer belt 5.

[0093] This configuration allows for the creation of toner consumption data and also reduces the time required for sequencing.

[0094] Furthermore, in the above explanation, the position detection of trigger image 1001f is performed using the timing of the digital signal D1, whose logic changes when the analog signal V1 of the intermediate transfer belt density sensor 9f exceeds the threshold voltage Vth. However, position detection may also be performed using the time between the timing when the level of the digital signal D1 changes from High to Low and the timing when it changes from Low to High. The same applies to the position detection of trigger image 1001r.

[0095] Furthermore, if there is a tilt between the detection position of density sensor 9f and the detection position of density sensor 9r, the above profile creation may be generated taking this tilt amount into consideration. The tilt amount is calculated using a reference image that passes through both the detection position of density sensor 9f and the detection position of density sensor 9r. The reference image is detected by density sensors 9f and 9r, and the CPU 201 calculates the tilt amount from the difference between the timing when the level of digital signal D1 logically changes from High to Low and the timing when the level of digital signal D2 logically changes from High to Low.

[0096] According to the configuration described above, profile data can be acquired with higher accuracy than in conventional configurations, and density unevenness in the sub-scanning direction can be appropriately suppressed. [Explanation of symbols]

[0097] 5. Intermediate Transfer Form 11. Secondary transfer roller 9f concentration sensor 9r concentration sensor 201 CPU

Claims

1. Image forming means for forming an image on a rotating image carrier, A transfer means for transferring the image on the image carrier to a sheet, A first output means that receives reflected light from a test image on the image carrier that passes through a first detection position and outputs an output signal based on the result of receiving the reflected light, A second output means receives reflected light from a test image on the image carrier that passes through a second detection position different from the first detection position in the rotational direction of the image carrier, and outputs an output signal based on the result of receiving the reflected light. The image forming means causes the image carrier to form a test image pattern that passes through both the first detection position and the second detection position by rotation, and the determination means determines the timing when the test image pattern reaches the first detection position based on the output signal output by the first output means. Based on the timing determined by the determination means, an acquisition means acquires an output signal from the second output means that is to be used to generate correction data, which is output in time series from the second output means, based on the timing. A generation means for generating correction data for correcting density unevenness in the rotational direction of an image to be formed by the image forming means, based on the output signal acquired by the acquisition means, An image forming apparatus characterized by having a control means for suppressing density unevenness in the rotational direction of an image to be formed by the image forming means based on the correction data generated by the generation means.

2. The test image pattern consists of a test image formed on the image carrier so as to pass through the first detection position and a trigger image formed on the image carrier so as to pass through the second detection position. The image forming apparatus according to claim 1, characterized in that the density of the test image and the trigger image are different.

3. The test image pattern consists of a test image formed on the image carrier so as to pass through the first detection position and a trigger image formed on the image carrier so as to pass through the second detection position. The image forming apparatus according to claim 1, characterized in that the length of the trigger image in the rotational direction is shorter than the length of the test image in the rotational direction.

4. The image forming means forms an image on a rotating photoreceptor and transfers the image from the photoreceptor to the image carrier. The image forming apparatus according to claim 3, characterized in that the length of the test image in the rotational direction is longer than the circumference of the photoreceptor.

5. The image forming means forms an image on a rotating photoreceptor and transfers the image from the photoreceptor to the image carrier. The system further comprises rotational phase detection means used to detect the rotational phase of the photoreceptor, The image forming apparatus according to claim 1, characterized in that the control means suppresses density unevenness in the rotational direction of the image to be formed by the image forming means based on the correction data and the detection result of the rotational phase detection means.

6. The image forming apparatus according to claim 1, characterized in that the determination means has a comparison unit that compares the output signal output from the first output means with a threshold value, and the timing is determined based on the comparison result by the comparison unit.

7. The image forming means includes a rotating photoreceptor, a charging unit for charging the photoreceptor, an exposure unit for exposing the photoreceptor charged by the charging unit to form an electrostatic latent image on the photoreceptor, and a developing unit for developing the electrostatic latent image on the photoreceptor. The image forming apparatus according to claim 1, characterized in that the control means controls the amount of light that the exposure unit exposes to the photoreceptor based on the correction data.

Images