JP2021012239A - Image forming apparatus - Google Patents

Abstract

To provide an image forming apparatus that has improved accuracy in detecting an image for measurement for image density detection.SOLUTION: An image forming apparatus 100 transfers an image formed on a photoconductor drum 1 to an intermediate transfer belt 7 to form the image on the intermediate transfer belt 7. The intermediate transfer belt 7 is stretched over a counter roller 14. The image forming apparatus 100 separates, from a first output value output from a density detection sensor 20 detecting a surface of the intermediate transfer belt 7 on which an image is not formed, a first frequency component of the intermediate transfer belt 7 and a second frequency component of the counter roller 14. The image forming apparatus 100 registers any one of the first frequency component and the first output value as grounding data. The image forming apparatus 100 creates a density correction table from the grounding data and a second output value output from the density detection sensor 20 detecting the intermediate transfer belt 7 on which an image for measurement for image density detection is formed.SELECTED DRAWING: Figure 1

Description

The present invention relates to an electrophotographic image forming apparatus having an image density control function.

The electrophotographic full-color image forming apparatus develops an electrostatic latent image formed on a plurality of photoconductors with a developer (toner) of a corresponding color, thereby developing a toner image of a color corresponding to each photoconductor. To form. The electrostatic latent image is formed by exposing a charged photoconductor with a laser beam. The toner image of each color is superimposed on the transfer body and transferred. As a result, a full-color toner image is formed. The full-color toner image on the transfer body is collectively transferred to the recording material. The toner image transferred to the recording material is heated and pressurized and fixed on the recording material. As a result, a printed matter in which an image is formed on the recording material is generated. The printed matter is ejected from the image forming apparatus.

In order to control the density of the image to be formed, the image forming apparatus has image forming conditions such as the exposure amount and exposure time when forming an electrostatic latent image on the photoconductor, the development bias voltage during development, and the charging voltage of the photoconductor. To control. However, even when the image formation conditions are the same, changes over time in various state quantities of the image forming apparatus such as toner charge amount, photoconductor temperature, and transfer efficiency, and changes in environmental conditions such as temperature and humidity. As a result, the image density changes. For this purpose, conventionally, a measurement image for image density detection is formed on a photoconductor or a transfer body, and the image density is controlled by feedback-controlling the image formation conditions based on the detection result of the measurement image by an optical sensor. The technology is adopted.

In an image carrier such as a photoconductor or a transfer body on which a toner image is formed, the glossiness of the surface on which the toner image is formed changes due to a large amount of image formation. For example, the surface of the image carrier has a decrease in glossiness and changes in glossiness at each position. Such a change in the glossiness of the image carrier affects the detection result of the measurement image. In Patent Document 1, the correction amount is calculated from the amount of reflected light from the surface of the image carrier whose glossiness has changed, and the amount of reflected light from the measurement image is corrected by this correction amount to control the image density. The device is disclosed.

Further, when the optical sensor is arranged to face the roller supporting the image carrier, the detection result of the amount of reflected light by the optical sensor may fluctuate depending on the rotation cycle of the roller due to the influence of the eccentricity of the roller or the like. Patent Document 2 discloses an image forming apparatus that controls the image density by removing the periodic fluctuation of the roller from the detection result of the optical sensor and detecting the amount of reflected light of the image carrier. When removing the periodic variation of the rollers, the amount of reflected light on the surface on which the toner image of the image carrier is formed is registered as the amount of reflected light on the base.

JP-A-2002-236402 Japanese Unexamined Patent Publication No. 2003-186350

When removing the periodic fluctuation of the roller from the detection result of the optical sensor, smoothing correction is uniformly performed to smooth the shading variation regardless of the degree of the periodic fluctuation of the roller that changes due to the manufacturing variation of the roller and the change with time. The unevenness of the reflected light amount of the image carrier is detected. Therefore, when the periodic fluctuation amount of the roller is small, the unevenness of the reflected light amount to be registered as the base may be blunted by the smoothing correction. In this case, the unevenness of the amount of reflected light on the base of the image carrier is not correctly acquired, and an error occurs when correcting the amount of reflected light from the detection result of the measurement image, so that the image formation conditions for density correction are accurately corrected. Becomes difficult.

In view of the above problems, the main object of the present invention is to provide an image forming apparatus having improved detection accuracy of a measurement image for image density detection.

In the image forming apparatus of the present invention, an image forming means for forming an image based on image forming conditions, an endless intermediate transfer belt in which the image is transferred to a surface and driven to rotate, and the intermediate transfer belt are stretched. The roller, the transfer means for transferring the image from the intermediate transfer belt to the recording material, and the surface of the intermediate transfer belt are irradiated with light, the reflected light is received, and the amount of the reflected light is received. An optical sensor that outputs an output value according to the above and the image forming means form a measurement image, and the optical sensor irradiates the measurement image formed on the intermediate transfer belt with light to obtain the measurement image. An generating means that acquires an image output value of the optical sensor corresponding to the reflected light from the optics and generates the image forming condition based on the image output value and the profile of the reflected light from the intermediate transfer belt, and the optics. The intermediate transfer belt in a state where the measurement image is not formed on the sensor is irradiated with light, the first output value of the optical sensor corresponding to the reflected light from the intermediate transfer belt is acquired, and the first output is obtained. The first output value as the profile when the separation means for separating the second output value of the predetermined frequency component corresponding to the rotation cycle of the roller from the value and the predetermined condition regarding the amplitude of the second output value are satisfied. Is set, and when the predetermined condition is not satisfied, the profile includes a control means for setting a third output value in which the predetermined frequency component is reduced from the first output value.

According to the present invention, the detection accuracy of the measurement image for image density detection is improved.

The block diagram of the image forming apparatus. Diagram of configuration of the concentration detection sensor. Configuration diagram of the controller. A flowchart showing the gradation correction process. The figure of the measurement image for image density detection. A flowchart showing an image density measurement process. Explanatory drawing of concentration value calculation processing. An example diagram of a sensor output / concentration conversion table. (A) and (b) are explanatory views of making an inverse conversion table. Explanatory drawing of the process of creating a new density correction table. A flowchart showing a method of registering background data. Explanatory drawing of background data.

An embodiment of the present invention will be described with reference to the drawings.

(Configuration of image forming apparatus)
FIG. 1 is a configuration diagram of an image forming apparatus of the present embodiment. The image forming apparatus 100 forms an image on the sheet-shaped recording material P. The image forming apparatus 100 includes a photosensitive drum 1 which is a drum-shaped photosensitive member. A charger 2, an exposure device 3, a developer 4, a primary transfer roller 5, a photoconductor cleaner 6, and a potential sensor 9 are arranged around the photosensitive drum 1. An intermediate transfer belt 7, which is an endless belt-shaped transfer body, is stretched below the photosensitive drum 1. A fixing device 13 is provided on the downstream side of the intermediate transfer belt 7 in the transport direction of the recording material P.

The photosensitive drum 1 is an amorphous silicon drum having a negative charging polarity, and is rotationally driven in the direction of arrow R1 by an electric motor (not shown). A bias voltage is applied to the charger 2 by a power source (not shown) to uniformly charge the surface of the rotating photosensitive drum 1. In the present embodiment, the potential sensor 9 measures the potential on the photosensitive drum 1 after charging so that the charging potential Vd on the surface of the photosensitive drum 1 becomes a predetermined constant value. By controlling the bias voltage applied to the charger 2 according to the measurement result by the potential sensor 9, the charging potential Vd is controlled within a predetermined range including a constant value.

The exposure device 3 is provided on the downstream side of the charger 2 in the rotation direction of the photosensitive drum 1. The exposure device 3 exposes the surface of the charged photosensitive drum 1 while controlling the emission of laser light based on, for example, image information representing an image to be formed. As a result, an electrostatic latent image corresponding to the image information is formed on the surface of the photosensitive drum 1.

The developer 4 is provided on the downstream side of the exposure device 3 and on the upstream side of the primary transfer roller 5 in the rotation direction of the photosensitive drum 1. The developer 4 includes a container 4a, a developing sleeve 4b, and a magnet roller 4c. The container 4a contains the one-component developer (black). The developing sleeve 4b is rotatably installed in the opening of the container 4a facing the photosensitive drum 1. The magnet roller 4c is provided in the developing sleeve 4b, and the developer is supported on the developing sleeve 4b. The magnet roller 4c is fixedly arranged non-rotating with respect to the rotation of the developing sleeve 4b. The developer has a negative electrode property, and is spiked by the magnetic force of the developing main electrode located in the developing region of the magnet roller 4c. The developing agent adheres to the dark part of the electrostatic latent image by rubbing the developing sleeve 4b on the surface of the photosensitive drum 1 and applying a developing bias voltage to the developing sleeve 4b by a power source (not shown). As a result, the electrostatic latent image is developed, and the toner image t is formed on the surface of the photosensitive drum 1.

The intermediate transfer belt 7 is stretched by an opposing roller 14, a driving roller 8, a backup roller 10, and a suspension roller 15. The concentration detection sensor 20 is arranged at a position facing the opposing roller 14 via the intermediate transfer belt 7. The backup roller 10 constitutes the secondary transfer unit P2 described later. The suspension roller 15 determines the amount of winding of the intermediate transfer belt 7 around the photosensitive drum 1. The intermediate transfer belt 7 travels in the R3 direction at the same speed as the photosensitive drum 1 by the driving roller 8, and the meandering correction control is performed by the opposing roller 14. A belt cleaner 12 is provided at a position facing the drive roller 8 via the intermediate transfer belt 7. A primary transfer unit P1 and a secondary transfer unit P2 are provided on the traveling path of the intermediate transfer belt 7. In the primary transfer unit P1, the intermediate transfer belt 7 is passed between the photosensitive drum 1 and the primary transfer roller 5. The secondary transfer unit P2 is composed of a backup roller 10 and a secondary transfer outer roller 11.

Both ends of the primary transfer roller 5 are pressed against the surface of the photosensitive drum 1 by pressing members such as springs (not shown). The primary transfer roller 5 is driven to rotate in the direction of arrow R2 as the photosensitive drum 1 rotates. A predetermined voltage is applied to the primary transfer roller 5 from the positive terminal of a power supply (not shown). By applying a voltage, the transfer charge determined by the surface potential of the primary transfer roller 5 is charged, and the toner image t on the photosensitive drum 1 is transferred to the surface of the intermediate transfer belt 7 in the primary transfer unit P1. The toner image t transferred to the intermediate transfer belt 7 is conveyed to the secondary transfer unit P2 by the rotation of the intermediate transfer belt 7.

Adhesions such as toner remaining on the surface of the photosensitive drum 1 after transfer are removed by the photoconductor cleaner 6. The photoconductor cleaner 6 includes a cleaner blade 6a. The cleaner blade 6a is in contact with the photosensitive drum 1 at a predetermined angle and pressure, and collects toner and the like remaining on the surface of the photosensitive drum 1.

In the secondary transfer unit P2, the intermediate transfer belt 7 is passed between the secondary transfer outer roller 11 and the backup roller 10. A predetermined voltage is applied to the secondary transfer outer roller 11 from a positive terminal of a power supply (not shown). By applying a voltage, the transfer charge determined by the surface potential of the secondary transfer outer roller 11 is charged, and the toner image t on the intermediate transfer belt 7 is transferred to the surface of the recording material P in the secondary transfer unit P2. ..

Adhesions such as toner remaining on the surface of the intermediate transfer belt 7 after transfer are removed by the belt cleaner 12. The belt cleaner 12 includes a cleaner blade 12a. The cleaner blade 12a is in contact with the intermediate transfer belt 7 laid around the drive roller 8 at a predetermined angle and pressure, and collects toner and the like remaining on the surface of the intermediate transfer belt 7.

The recording material P to which the toner image t is transferred is conveyed to the fixing device 13. The fuser 13 heats and pressurizes the recording material P to which the toner image t is transferred. As a result, a printed matter in which the toner image t is fixed on the surface of the recording material P is generated.

The density detection sensor 20 arranged on the opposite side of the facing roller 14 with the intermediate transfer belt 7 interposed therebetween is used for gradation correction control for keeping the image density and the gradation property constant. Details of the concentration detection sensor 20 will be described later.
Further, the intermediate transfer belt 7 of the present embodiment is provided with a reference position mark indicating the reference position of the intermediate transfer belt 7 on the inside (the back surface with respect to the front surface on which the toner image is transferred). Therefore, a reference position sensor 21 for reading the reference position mark is provided inside the intermediate transfer belt 7. In the present embodiment, the reference position sensor 21 and the concentration detection sensor 20 have the same configuration.

(Concentration detection sensor)
FIG. 2 is a configuration explanatory view of the concentration detection sensor 20. The density detection sensor 20 detects a measurement image for image density detection using both a specular reflection component and a diffuse reflection component. Therefore, the density detection sensor 20 is an optical sensor composed of a light emitting unit 201, a specularly reflected light receiving unit 202, and one light emitting and two light receiving units of the diffusely reflected light receiving unit 203. The light emitting unit 201 is, for example, an LED (Light Emitting Diode), and irradiates light having a wavelength corresponding to the reflectance characteristic of the toner toward the surface on which the toner image of the intermediate transfer belt 7 is formed. For example, the light emitting unit 201 irradiates light having a wavelength of 800 [nm] to 850 [nm].

The light emitted from the light emitting unit 201 is reflected by the object. The specularly reflected light from the object is received by the specularly reflected light receiving unit 202. The specularly reflected light receiving unit 202 outputs an output value (image output value) which is a voltage value corresponding to the intensity (light amount) of the specularly reflected light received. The diffusely reflected light from the object is received by the diffusely reflected light receiving unit 203. The diffusely reflected light receiving unit 203 outputs an output value (image output value) which is a voltage value corresponding to the intensity (light amount) of the received diffusely reflected light. The object is a surface on which the toner image of the intermediate transfer belt 7 is formed or a toner image (measurement image) formed on the surface.

(controller)
FIG. 3 is a configuration diagram of a controller that controls the operation of the image forming apparatus 100 having such a configuration. The controller of the image forming apparatus 100 includes a printer control unit 30 and an engine unit 31. The printer control unit 30 includes an image processing unit 301 and an engine control unit 303. The engine unit 31 includes the above-mentioned charger 2, exposure device 3, developer 4, transfer unit 50 (primary transfer unit P1, secondary transfer unit P2), fuser 13, reference position sensor 21, and concentration detection sensor 20. It is a configuration that includes. The image forming apparatus 100 is communicably connected to the host computer 32 that transmits image information. The image information is input to the image processing unit 301.

The image processing unit 301 holds a density correction table 302 (γLUT) for correcting the gradation characteristics of the engine unit 31 to ideal gradation characteristics in a predetermined memory. The image processing unit 301 performs image processing such as density correction on the image information by using the density correction table 302. The image processing unit 301 transmits the image information after image processing to the engine control unit 303. The engine control unit 303 includes a CPU (Central Processing Unit) 305 and a measurement image generation unit 304. The CPU 305 controls the operation of the image forming apparatus 100 by executing a predetermined computer program. The CPU 305 controls the operation of the engine unit 31 according to the image information acquired from the image processing unit 301 to form an image corresponding to the image information on the recording material P. Various parameters related to the operation of the image forming apparatus 100 are determined by the engine control unit 303 and transmitted to each part of the engine unit 31 during the image forming process.

The measurement image generation unit 304 generates image information of the measurement image for image density detection during the image density control process. When the image density is measured and the density is controlled, the CPU 305 acquires the image information of the measurement image from the measurement image generation unit 304, and controls the operation of the engine unit 31 according to the image information. An image for measurement is formed on the intermediate transfer belt 7. The measurement image formed on the intermediate transfer belt 7 is detected by the density detection sensor 20. The detection result of the measurement image by the density detection sensor 20 is transmitted to the CPU 305. The CPU 305 creates a density correction table 302 based on the detection result of the measurement image by the density detection sensor 20, and stores it in a predetermined memory of the image processing unit 301. The density correction table 302 is an example of a correction table for image formation conditions created according to the characteristics of the engine unit 31. The measurement image for image density detection is an example of the measurement image for detecting the correction value of the image formation condition.

During the image density control process, the reference position sensor 21 detects the reference position mark from the rotation-driven intermediate transfer belt 7. The detection result of the reference position mark is transmitted from the reference position sensor 21 to the CPU 305. The CPU 305 detects the reference position of the intermediate transfer belt 7 based on the detection result of the reference position mark. The CPU 305 associates the reference position of the intermediate transfer belt 7 with the detection result of the image density and stores it in the internal memory.

(Gradation correction processing)
The gradation correction process is one of the image density control processes in the image forming apparatus 100. Specifically, the image density control process is roughly divided into maximum density control and gradation correction control. The maximum density control is a process of adjusting the density of the solid part of the image to the target density. The gradation correction control is a process of adjusting the density of the halftone portion of the image to the target density. The image density control process includes a method of detecting the density of the measurement image formed on the recording material P using a reader or a measuring instrument, and a density detection sensor 20 for detecting the density of the measurement image formed on the intermediate transfer belt 7. There is a method of detecting and performing using. In the present embodiment, the density of the measurement image formed on the intermediate transfer belt 7 is detected by using the density detection sensor 20. In the present embodiment, the image density control process for one black color will be described, but the image density control process for other colors such as yellow, magenta, and cyan is also executed in the same manner.

FIG. 4 is a flowchart showing the gradation correction process. The gradation correction process is performed at a timing such as after passing a predetermined number of sheets (for example, 100 sheets), immediately after the power is turned on, or after returning from the sleep state.

The CPU 305 creates a measurement image for image density detection on the surface of the intermediate transfer belt 7 (S101). Therefore, the CPU 305 acquires the image information of the measurement image from the measurement image generation unit 304. Based on this image information, the CPU 305 controls the operation of each part of the engine part 31 under normal image forming conditions. The engine unit 31 forms a toner image of the measurement image on the photosensitive drum 1 under the control of the CPU 305, and transfers this to the intermediate transfer belt 7. As a result, a measurement image is created on the surface of the intermediate transfer belt 7. FIG. 5 is an example diagram of a measurement image for detecting image density. This measurement image has a configuration in which a plurality of rectangular patch images of 25 [mm] in the R3 direction, which is the rotation direction of the intermediate transfer belt 7, and 15 [mm] in the direction orthogonal to the R3 direction, are arranged in the R3 direction. .. In the present embodiment, the measurement image is composed of 10 patch images. Each patch image has a different image density. In the example of FIG. 5, the image density of each patch image is set in 10% increments from 100% to 10%. Each patch image is formed by using the density correction table 302 (γLUT before control) created in the previous gradation correction process.

The CPU 305 acquires the detection result of the measurement image formed on the intermediate transfer belt 7 from the density detection sensor 20 and measures the image density of each patch image (S102). The CPU 305 converts the detection result (output value) of the measurement image by the density detection sensor 20 into the image density (density value). FIG. 6 is a flowchart showing the image density measurement process.

The CPU 305 acquires an output value which is a detection result of each patch image of the measurement image from the density detection sensor 20 (S1201). The output value output by the density detection sensor 20 includes a specular reflection component and a diffuse reflection component. In the present embodiment, since the measurement image is black, the output value of the diffuse reflection component is extremely smaller than the output value of the specular reflection component. Therefore, the output value of the black measurement image can be represented only by the specular reflection component. The diffused reflection component is dominant in the output value of the measurement image of chromatic colors such as yellow, magenta, and cyan. In this case, the density can be detected without performing the background correction processing for the specular reflection component described later. The description of the image density detection of the chromatic color measurement image will be omitted.

The CPU 305 performs a background correction process on the output value using the background data registered in advance (S1022). The background data is the result (profile) of the density detection sensor 20 detecting the surface (base) on which the toner image of the intermediate transfer belt 7 is transferred in a state where the toner image is not transferred. The background data is Sigma. Base [i], the output value of the concentration detection sensor 20 is shown in Sig. When R [i] is set, the output value after the background correction processing (data after background correction) Sig. R'[i] is represented by the following equation (1).
Sig.R'[i] = Sig.R [i] /Sig.Base [i]… (1)

[I] is the address of the surface (base) of the intermediate transfer belt 7 represented by the reference position of the intermediate transfer belt 7 as “0”. For example, when one round of the intermediate transfer belt 7 is read by the concentration detection sensor 20 in 1024 divisions, the position of the intermediate transfer belt 7 on the base is 1023 at the maximum in the reading order of the output value from 0 in which the reference position is indicated. Represented by an address. It should be noted that the data after background correction Sigma. When R'[i] is converted into 10-bit data, the equation (1) is multiplied by 1023. The details of the method of registering the background data will be described later.

The CPU 305 uses the data Sig. The image density (density value) of each patch image of the measurement image is calculated based on R'[i] (S1023). FIG. 7 is an explanatory diagram of the density value calculation process of each patch image. The CPU 305 selects the patch formation addresses p1 to p1 + x representing the formation position of the patch image as the data of the first patch image. The patch formation address p1 is stored as an intermediate transfer belt address of the formation position of the first patch image when the measurement image is formed. The CPU 305 uses the data Sigma of the first patch image after background correction. R'[p1 to (p1 + x)] is converted into a concentration value using the sensor output / concentration conversion table. FIG. 8 is an example diagram of a sensor output / concentration conversion table. The larger the specular reflection component of the detection result (output value) of the density detection sensor 20, the lower the density value. The CPU 305 averages the density values of the first patch image and calculates the density D1 of the first patch image. The CPU 305 repeats such a process for the number of patch images constituting the measurement image (10 times in this embodiment), and calculates the density value of each patch image.

The CPU 305, which measures the image density (density value) of each patch image by the above processing, performs an inverse conversion operation on the density target so that the measured density value becomes an appropriate density, and then performs an inverse conversion table. Is created (S103). FIG. 9 is an explanatory diagram for creating an inverse conversion table. FIG. 9A illustrates the relationship between the concentration value and the concentration target. FIG. 9B illustrates an inverse conversion table (inverse conversion LUT) created from the concentration value and the concentration target of FIG. 9A.

The CPU 305 creates a new density correction table (γLUT) by the density correction table 302 (γLUT) created in the previous gradation correction process and the inverse conversion table created in the process of S103 (S104). FIG. 10 is an explanatory diagram of a process for creating a new density correction table. The new density correction table (γLUT) is created by multiplying the density correction table 302 (γLUT) created in the previous gradation correction process and the inverse conversion table.

The CPU 305 corrects the created new density correction table (γLUT) (S105). The CPU 305 prevents, for example, interpolation processing of numerical values in the density correction table when the number of patch images of the measurement image is insufficient with respect to the new density correction table (γLUT), and the occurrence of gradation steps. The smoothing process of the density correction table for this purpose is performed. The CPU 305 sets the corrected density correction table (γLUT) in the image processing unit 301.

The gradation correction process is performed as described above. The image forming apparatus 100 can perform gradation correction in consideration of the state of the engine unit 31 by performing gradation correction on the image information by the density correction table 302 (γLUT) at the time of image formation. Therefore, the image density is corrected according to the state of the engine unit 31, and an image with good image quality can be obtained.

(How to register background data)
The background data used in the background correction processing of S1022 of FIG. 6 will be described. FIG. 11 is a flowchart showing a method of registering background data. FIG. 12 is an explanatory diagram of the background data. As described above, the registered background data is used to correct the detection result (output value) of the measurement image by the density detection sensor 20. The base data represents unevenness in the detection result (output value) by the density detection sensor 20 for one round of the surface of the intermediate transfer belt 7. This unevenness also includes a component caused by the opposing roller 14 arranged to face the concentration detection sensor 20. The registration of the background data needs to be performed before the background of the intermediate transfer belt 7 may change. For example, the base data is registered at a timing such as after passing a predetermined number of sheets (for example, 1000 sheets), after starting, and before taking the density target on the intermediate transfer belt 7.

The CPU 305 detects the reference position of the intermediate transfer belt 7 based on the detection result of the reference position sensor 21 (S201). The reference position sensor 21 reads the reference position mark indicating the reference position of the intermediate transfer belt 7. The reading position of the reference position mark is the address 0, which is the reference position of the base of the intermediate transfer belt 7. In the present embodiment, as described above, one circumference of the intermediate transfer belt 7 is divided into 1024 readings. The position address i representing the position of the base of the intermediate transfer belt 7 is represented by 0 to 1023. The detection of the reference position is always performed when the density detection sensor 20 detects the measurement table image. The detection result (output value) of the density detection sensor 20 and the position address i on the intermediate transfer belt 7 are always associated with each other.

The CPU 305 acquires the detection result (output value) of the surface (base) of the intermediate transfer belt 7 by the density detection sensor 20 (S202). At this time, the intermediate transfer belt 7 is in a state in which the toner image is not transferred. From the detection result of the concentration detection sensor 20, the CPU 305 detects the driving distance for half a circumference of the opposing roller 14 + one circumference of the intermediate transfer belt 7 + half a circumference of the opposing roller 14. In the present embodiment, one round of the opposing roller 14 is divided into 61 divisions at the position address on the intermediate transfer belt 7, and the half circumference is divided into 30 divisions. That is, the detected drive distance is 30 divisions for half a circumference of the opposing roller 14 + 1024 divisions for one rotation of the intermediate transfer belt 7 + 30 divisions for half circumferences of the opposing roller 14 = 1084 divisions. The position address j of the output value is set to the reading start position 0 to 1083, and the output value Data [j] is associated with each position address j.

The detection start timing by the concentration detection sensor 20 of the present embodiment is defined as the timing at which the position address i of the intermediate transfer belt 7 is half a circumference before the opposing roller 14 (i = 994) from the reference position 0. This is because the output values before and after the address of interest are used in the subsequent processing. It should be noted that the detection may be performed for a long time in advance, and then the output value may be cut out from the timing of the position address i of the predetermined intermediate transfer belt 7. Data [j] in FIG. 12 is an output value detected by this process. The output value Data [j] is an analog value including a frequency component due to the rotation of the intermediate transfer belt 7 and a frequency component due to the rotation of the opposing roller 14.

The CPU 305 separates the output value Data [j] into a frequency component Data_ITB corresponding to the rotation cycle of the intermediate transfer belt 7 and a frequency component Data_Roller corresponding to the rotation cycle of the opposing roller 14 (S203). As shown in FIG. 12, the CPU 305 performs moving average processing on the output value Data [j] (j = 0 to 1083), and performs the moving average processing, and the average value Data_ITB [j] (j = 0) of the frequency components of the intermediate transfer belt 7. 1083) is calculated. The range of the output value Data [j] referred to in the moving average processing is one round of the opposing roller 14. The CPU 305 performs moving average processing using the output values (30 addresses) for half a circumference of the front and rear opposing rollers 14 with the attention address as the center. The average value Data_ITB [j] is calculated by the following formula.
Data_ITB [j] = AVERAGE (Data [j-30]: Data [j + 30])… (2)
AVERAGE (A: B) is a variable for calculating the average value from A to B. At this time, if there is no output value to be referenced, the calculation may be performed within the referenceable data range.

Next, the CPU 305 converts the output value Data [j] and the average value Data_ITB [j] into Data [i] and Data_ITB [i] based on the address of the intermediate transfer belt 7. The formula is shown below.
Data [i] (i = 0 ~ 1023) ← Data [j] (j = 30 ~ 1053)… (3)
Data_ITB [i] (i = 0 ~ 1023) ← Data_ITB [j] (j = 30 ~ 1053)… (4)

"←" represents an assignment. Here, since the value of each element constituting the data is not changed and only the address is changed, it is described using substitution. In the present embodiment, when the measurement image is detected, the detection start timing is set from the timing when the position address i of the intermediate transfer belt is half a circumference of the roller (i = 994) before the reference position 0. Therefore, it is possible to convert to Data [i] by cutting out the front and rear roller half circumferences of the output value Data [j]. The same applies to Data_ITB [j].

Next, the CPU 305 receives an output value Data_Roller [ i] is calculated. The formula is shown below.
Data_Roller [i] = Data [i]-Data_ITB [i]… (5)
At this time, Data_Roller [i] may take a negative value due to the difference.

After the component separation, the CPU 305 calculates the periodic fluctuation amount Amp by the opposing roller 14 from the Data_Roller [i] (S204). The formula is shown below.
Amp = MAX (Data_Roller)-MIN (Data_Roller)… (6)
MAX (A) is a variable for calculating the maximum value from the data constituting A. MIN (A) is a variable for calculating the minimum value from the data constituting A. As a measure against noise included in the output value, the calculation may be performed by excluding the upper few points and the lower several points from the data constituting A. The periodic fluctuation amount Amp represents the amplitude of the frequency component by the opposing roller 14.

The CPU 305 selects the background data to be registered depending on whether or not the periodic fluctuation amount Amp satisfies a predetermined condition. In the present embodiment, the CPU 305 compares the periodic fluctuation amount Amp with the switching threshold value of the background correction, and selects the background data to be registered according to the comparison result (S205). The switching threshold is a predetermined value set in advance. When the periodic fluctuation amount Amp is smaller than the switching threshold value (S205: Y), the CPU 305 registers the output value Data as the background data (S206). When the periodic fluctuation amount Amp is larger than the switching threshold value (S205: N), the CPU 305 registers the frequency component Data_ITB of the intermediate transfer belt 7 as the base data (S207). That is, when the amplitude of the frequency component by the opposing roller 14 is smaller than the threshold value, the base data becomes the output value which is the detection result of the surface (base) of the intermediate transfer belt 7. That is, when the amplitude of the frequency component by the opposing roller 14 is larger than the threshold value, the background data becomes the frequency component Data_ITB of the intermediate transfer belt 7 separated from the detection result of the surface (base) of the intermediate transfer belt 7.

By comparing the periodic fluctuation amount Amp and the switching threshold value of the background correction, when the periodic fluctuation amount Amp is small, the output value can be registered as the background data without blunting the unevenness of the intermediate transfer belt 7. When the periodic fluctuation amount Amp is large, the unevenness of the intermediate transfer belt 7 from which the influence of the opposing roller 14 is removed can be registered as the base data.

The CPU 305 corrects the output value of the density detection sensor 20 when the measurement image is detected based on such background data. Since the concentration correction table 302 (γLUT) is generated based on the corrected output value, the influence of the periodic fluctuation amount (amplitude) by the opposing roller 14 is suppressed, and the concentration detection accuracy using the concentration detection sensor 20 is improved. Can be improved. As described above, in the present embodiment, the concentration detection accuracy can be improved even when the output value of the concentration detection sensor 20 includes two or more frequency components having components having different cycles.

(Modification example)
The background data to be registered may be determined as follows, in addition to the comparison result between the periodic fluctuation amount Amp and the background correction switching threshold value. For example, the amount of periodic fluctuation by the opposing roller 14 is measured at the time of manufacturing the image forming apparatus 100, and the base data to be registered is determined based on the measured amount of periodic fluctuation. That is, at the manufacturing stage, the image forming apparatus 100 determines and fixes the base data to be registered in advance to either the output value Data and the intermediate transfer belt component Data_ITB. In addition to the method described in the process of S204, the measurement of the periodic fluctuation amount by the opposing roller 14 may be performed using, for example, a value measured by a dedicated jig for the eccentricity of the opposing roller 14.
In this case, when the periodic fluctuation amount and the switching threshold value are substantially the same value, it is possible to prevent the processing of S206 and the processing of S207 from being frequently switched and the registered background data to be frequently switched.

Further, the process of S203 for separating the output value Data into the frequency component Data_ITB of the intermediate transfer belt 7 and the frequency component Data_Roller of the opposing roller 14 may be performed by using FFT (Fast Fourier Transform). Specifically, an FFT is performed on the output value of the concentration detection sensor 20, and a filter calculation is executed using a frequency filter that reduces the frequency component of the period of the opposing roller 14. Alternatively, the frequency component of the opposing roller 14 is extracted by executing a filter calculation using a frequency filter that reduces the frequency component of the intermediate transfer belt 7 with respect to the FFT output value. As a result, the frequency component of the intermediate transfer belt 7 is extracted from the output value. Further, the frequency component of the opposing roller 14 is extracted by executing the filter calculation using the frequency filter that extracts only the frequency component of the period of the opposing roller 14 with respect to the FFT output value. In such a method, when the output value contains an unexpected third frequency component, the influence of the frequency component can be removed.

Claims (6)

An image forming means for forming an image based on an image forming condition,
An endless intermediate transfer belt in which the image is transferred to the surface and driven to rotate,
The roller on which the intermediate transfer belt is stretched and
A transfer means for transferring the image from the intermediate transfer belt to the recording material,
An optical sensor that irradiates light toward the surface of the intermediate transfer belt, receives the reflected light, and outputs an output value according to the amount of the reflected light.
The image forming means forms a measurement image, the optical sensor irradiates the measurement image formed on the intermediate transfer belt with light, and the image of the optical sensor corresponding to the reflected light from the measurement image. A generation means that acquires an output value and generates the image formation condition based on the image output value and the profile of the reflected light from the intermediate transfer belt.
The intermediate transfer belt in a state where the measurement image is not formed on the optical sensor is irradiated with light, and the first output value of the optical sensor corresponding to the reflected light from the intermediate transfer belt is acquired, and the first output value is obtained. A separation means for separating the second output value of a predetermined frequency component corresponding to the rotation cycle of the roller from one output value, and
When the predetermined condition regarding the amplitude of the second output value is satisfied, the first output value is set as the profile, and when the predetermined condition is not satisfied, the predetermined frequency component from the first output value is set as the profile. An image forming apparatus comprising: a control means for setting a third output value in which the frequency is reduced. The control means sets the first output value as the profile when the amplitude of the second output value is smaller than the predetermined threshold value, and when the amplitude of the second output value is larger than the predetermined threshold value. The third output value is set as the profile.
The image forming apparatus according to claim 1. The separation means calculates the output value of the frequency component corresponding to the rotation cycle of the intermediate transfer belt by performing a moving average process on the first output value, and from the first output value, the intermediate transfer belt The second output value is calculated by differentiating the output values of the frequency components corresponding to the rotation period.
The image forming apparatus according to claim 1 or 2. The separation means separates the second output value by performing a filter calculation on the first output value using a filter that reduces the frequency component corresponding to the rotation cycle of the intermediate transfer belt.
The image forming apparatus according to claim 1 or 2. The roller and the optical sensor are arranged at positions facing each other via the intermediate transfer belt.
The image forming apparatus according to any one of claims 1 to 4. The image forming condition is a density correction table for correcting the gradation characteristics of an image formed by the image forming means.
The image forming apparatus according to any one of claims 1 to 5.