JP4470406B2 - Image forming apparatus and image forming method - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an image forming apparatus and an image forming method for controlling tone correction characteristics of an apparatus based on a density detection result of a patch image.
[0002]
[Prior art]
In an electrophotographic image forming apparatus such as a printer, a copying machine, and a facsimile machine, a small test image (patch image) having a predetermined image pattern is formed as needed, and the image density is adjusted by a density sensor. A predetermined image quality can be stably obtained by detecting and adjusting the operating condition of each part of the apparatus based on the detection result.
[0003]
For example, in the image processing apparatus described in Patent Document 1 according to the application of the present applicant, gradation correction characteristics of the apparatus are controlled based on the density detection result of the patch image. That is, the density of the test image of a predetermined pattern is detected by the patch sensor, and a correction conversion table for tone correction is generated based on the detection result. The image signal is converted into laser drive pulse width data while referring to the correction conversion table newly generated in this way. As a result, it is possible to obtain an optimum printing result corresponding to the variation of the gamma characteristic of the apparatus.
[0004]
[Patent Document 1]
JP 2000-333012 A (Page 6, FIG. 7)
[0005]
[Problems to be solved by the invention]
In recent years, the demand for image quality has increased, and accordingly, establishment of a more accurate gradation correction technique is required. For this purpose, it is necessary to include the print results at as many gradation levels as possible in the patch image and to obtain the image density corresponding to each gradation level with high accuracy.
[0006]
In patch images formed with multiple gradation levels in this way, the density difference between each gradation level is small, so the detection results are affected by density fluctuations and noise caused by variations in device characteristics and fluctuations in the surrounding environment. There is a problem that it is easy to receive. However, in order to perform such gradation correction characteristic control with high accuracy, what kind of patch image is preferably used has not been sufficiently studied so far.
[0007]
The present invention has been made in view of the above problems. In an image forming apparatus and an image forming method for controlling gradation correction characteristics based on a density detection result of a patch image, a patch image suitable for control is formed. It is an object of the present invention to provide a technique for obtaining good gradation correction characteristics based on the density detection result.
[0008]
[Means for Solving the Problems]
According to the present invention, an image carrier that carries an electrostatic latent image on its surface and a toner carrier that carries toner on its surface are arranged to face each other at a predetermined facing position, and the toner carrier is circulated in a predetermined direction. In an image forming apparatus and an image forming method for forming a toner image by developing the electrostatic latent image with the toner by moving the toner carried on the toner carrier to the image carrier while moving the toner. In order to achieve the above object, the tone correction characteristics of the apparatus are controlled based on the density detection result of the toner image formed as a patch image, and the patch image is the surface of the toner carrier surface at the facing position. A gradation image portion whose gradation level gradually changes along the moving direction, and a gradation image portion formed prior to the formation of the gradation image portion, and having a length along the moving direction. Above the length corresponding to the circumference of the carrier Has a single gray level A toner image having a header portion Forming and detecting the density at each position in the header part and the gradation image part, and correcting the density detection result at each position in the gradation image part by using the density detection result at each position in the header part. The gradation correction characteristic is controlled based on the density detection result at each position in the gradation image portion. It is characterized by that.
[0009]
In the invention configured as described above, the header portion is formed prior to the formation of the gradation image portion, and the length of the header portion is set to be equal to or longer than the length corresponding to the peripheral length of the toner carrier. ing. That is, after the header portion is formed using the toner corresponding to one round of the toner carrier, the gradation image portion with the gradation level changed is formed. The reason for this is as follows. That is, in this image forming apparatus and image forming method, density unevenness may occur in the formed image due to non-uniformity of the toner layer on the surface of the toner carrier. In particular, such density unevenness is likely to occur after leaving the apparatus for a long time without image formation.
[0010]
If such density unevenness occurs in a patch image formed by changing the gradation level for the purpose of controlling the gradation correction characteristics, the gradation characteristics of the device cannot be correctly grasped from the density detection result, As a result, the gradation reproducibility of the apparatus is degraded. Therefore, according to the present invention, the toner layer non-uniformity is eliminated by consuming the toner for one round of the toner carrying member for the formation of the header portion. Re Using the formed uniform toner layer, a gradation image portion in which the gradation level is changed is formed. Therefore, density unevenness hardly appears in the gradation image portion, and this gradation image portion represents the gradation characteristics of the apparatus with high fidelity. Further, since the periodic density unevenness that can occur at each position in the gradation image portion also appears in the header portion, it can be corrected using the density detection result at each position in the header portion. This was corrected By controlling the tone correction characteristics based on the density detection result, this image forming apparatus and image forming method can obtain good tone correction characteristics. As described above, the patch image formed in the image forming apparatus and the image forming method according to the present invention has an aspect suitable as a patch image used for controlling the gradation correction characteristics of the apparatus.
[0011]
In the above description, the “length corresponding to the circumferential length of the toner carrier” means the following. That is, the length is the length along the moving direction that the toner image formed while rotating the toner carrier just once has on the image carrier. In a non-contact development type image forming apparatus in which a toner carrier and an image carrier are spaced apart from each other, the length of a toner image formed during one rotation of the toner carrier is not necessarily the same as that of the toner carrier. This is because it does not always coincide with the circumference.
[0012]
In order to improve the homogeneity of the re-formed toner layer, the header portion is preferably an image having a uniform image pattern. By doing so, the amount of toner remaining on the toner carrier after formation of the header portion becomes substantially uniform, and as a result, it becomes easy to obtain uniformity of the toner layer formed thereafter. As such an image pattern of the header part, for example, an image having a single gradation level can be used. In particular, if the header portion is a solid image with the maximum gradation level, more toner on the toner carrier is consumed for forming the header portion, and the effect of eliminating density unevenness is enhanced.
[0013]
Further, when the header portion having such a uniform image pattern is formed, density unevenness caused by the above-described inhomogeneity of the toner layer and other causes appears in the header portion. Therefore, the density detection result in the header part is obtained from the density detection result, that is, at least one of the density fluctuation period and the density fluctuation amount in the header part, and the density detection result in the gradation image part is corrected based on the result. It is also possible to obtain the gradation characteristics of the apparatus with higher accuracy.
[0014]
In particular, when the header portion is a solid image of the maximum gradation level and an image in which the gradation level changes from the maximum level to the minimum level along the moving direction is formed as the gradation image portion, An effect is obtained. That is, in such a patch image, the gradation level changes continuously and smoothly from the header portion to the gradation image portion, and accordingly, the density change of the formed image also becomes smooth. If a patch image having a smooth density change image pattern is formed in this way, it becomes easier to determine the influence of density unevenness appearing in the density detection result.
[0015]
From this, for example, the image density is sampled at each of a plurality of sampling positions that are different from each other in the moving direction in the patch image, and the obtained sampling data string is smoothed, and the level is calculated based on the result. It is possible to control the tone correction characteristics. In this way, it is possible to accurately control the tone correction characteristics while suppressing the influence of density unevenness, noise, and the like.
[0016]
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 is a view showing an embodiment of an image forming apparatus according to the present invention. FIG. 2 is a block diagram showing an electrical configuration of the image forming apparatus of FIG. This apparatus forms a full color image by superposing four color toners (developers) of yellow (Y), cyan (C), magenta (M), and black (K), or only black (K) toner. The image forming apparatus forms a monochrome image using In this image forming apparatus, when an image signal is given to the main controller 11 from an external device such as a host computer, the CPU 101 provided in the engine controller 10 controls each part of the engine unit EG in response to a command from the main controller 11. Then, a predetermined image forming operation is executed, and an image corresponding to the image signal is formed on the sheet S. In this embodiment, the engine controller 10 and the main controller 11 integrally function as the “control unit” of the present invention.
[0017]
In the engine unit EG, the photosensitive member 22 is provided to be rotatable in the arrow direction D1 in FIG. A charging unit 23, a rotary developing unit 4 and a cleaning unit 25 are arranged around the photosensitive member 22 along the rotation direction D1. The charging unit 23 is applied with a predetermined charging bias, and uniformly charges the outer peripheral surface of the photoconductor 22 to a predetermined surface potential. The cleaning unit 25 removes the toner remaining on the surface of the photosensitive member 22 after the primary transfer, and collects it in a waste toner tank provided inside. The photosensitive member 22, the charging unit 23, and the cleaning unit 25 integrally constitute the photosensitive member cartridge 2, and the photosensitive member cartridge 2 is detachably attached to the apparatus main body as a whole.
[0018]
Then, the light beam L is irradiated from the exposure unit 6 toward the outer peripheral surface of the photosensitive member 22 charged by the charging unit 23. The exposure unit 6 exposes the light beam L onto the photosensitive member 22 in accordance with an image signal given from an external device, and forms an electrostatic latent image corresponding to the image signal. Thus, in this embodiment, the photoconductor 22 functions as the “image carrier” of the present invention.
[0019]
The electrostatic latent image thus formed is developed with toner by the developing unit 4. That is, in this embodiment, the developing unit 4 is configured as a support frame 40 that is rotatably provided about a rotation axis center orthogonal to the paper surface of FIG. A yellow developing device 4Y, a cyan developing device 4C, a magenta developing device 4M, and a black developing device 4K are provided. The developing unit 4 is controlled by the engine controller 10. Based on the control command from the engine controller 10, the developing unit 4 is driven to rotate, and the developing units 4Y, 4C, 4M, and 4K are selectively brought into contact with the photoreceptor 22 or have a predetermined gap. When positioned at a predetermined development position that is opposed to each other, a developing roller 44 that is a “toner carrier” that is provided in the developing unit and carries toner of a selected color is disposed to face the photoreceptor 22. At the facing position, toner is applied from the developing roller 44 to the surface of the photoreceptor 22. As a result, the electrostatic latent image on the photosensitive member 22 is visualized with the selected toner color.
[0020]
The toner image developed by the developing unit 4 as described above is primarily transferred onto the intermediate transfer belt 71 of the transfer unit 7 in the primary transfer region TR1. The transfer unit 7 includes an intermediate transfer belt 71 stretched between a plurality of rollers 72 to 75, and a drive unit (not shown) that rotates the intermediate transfer belt 71 in a predetermined rotation direction D2 by rotationally driving the roller 73. It has. When a color image is transferred to the sheet S, each color toner image formed on the photosensitive member 22 is superimposed on the intermediate transfer belt 71 to form a color image and taken out from the cassette 8 one by one. The color image is secondarily transferred onto the sheet S conveyed to the secondary transfer region TR2 along the conveyance path F.
[0021]
At this time, in order to correctly transfer the image on the intermediate transfer belt 71 to a predetermined position on the sheet S, the timing of feeding the sheet S to the secondary transfer region TR2 is managed. Specifically, a gate roller 81 is provided on the transport path F on the front side of the secondary transfer region TR2, and the gate roller 81 rotates in accordance with the timing of the circumferential movement of the intermediate transfer belt 71. S is sent to the secondary transfer region TR2 at a predetermined timing.
[0022]
Further, the sheet S on which the color image is thus formed is conveyed to the discharge tray portion 89 provided on the upper surface portion of the apparatus main body via the fixing unit 9, the pre-discharge roller 82 and the discharge roller 83. Further, when images are formed on both sides of the sheet S, when the rear end portion of the sheet S on which the image is formed on one side as described above is conveyed to the reversal position PR behind the pre-discharge roller 82. The rotation direction of the discharge roller 83 is reversed, whereby the sheet S is conveyed in the direction of the arrow D3 along the reverse conveyance path FR. Then, the sheet is again placed on the transport path F before the gate roller 81. At this time, the surface of the sheet S to which the image is transferred by contacting the intermediate transfer belt 71 in the secondary transfer region TR2 is first transferred. It is the opposite surface. In this way, images can be formed on both sides of the sheet S.
[0023]
In addition, the apparatus 1 includes a display unit 12 controlled by the CPU 111 of the main controller 11 as shown in FIG. The display unit 12 is constituted by, for example, a liquid crystal display, and in accordance with a control command from the CPU 111, the operation guidance to the user, the progress of the image forming operation, the occurrence of an abnormality in the apparatus, the replacement timing of any unit, etc. A predetermined message for notification is displayed.
[0024]
In FIG. 2, reference numeral 113 denotes an image memory provided in the main controller 11 for storing an image given from an external device such as a host computer via the interface 112. Reference numeral 106 is a ROM for storing a calculation program executed by the CPU 101, control data for controlling the engine unit EG, and the like. Reference numeral 107 is a RAM for temporarily storing calculation results in the CPU 101 and other data. is there.
[0025]
A cleaner 76 is disposed in the vicinity of the roller 75. The cleaner 76 can be moved toward and away from the roller 75 by an electromagnetic clutch (not shown). Then, the blade of the cleaner 76 abuts on the surface of the intermediate transfer belt 71 that is stretched over the roller 75 while moving to the roller 75 side, and the toner that remains on the outer peripheral surface of the intermediate transfer belt 71 after the secondary transfer. Remove.
[0026]
Further, a density sensor 60 is disposed in the vicinity of the roller 75. The density sensor 60 is provided to face the surface of the intermediate transfer belt 71 and measures the image density of the toner image formed on the outer peripheral surface of the intermediate transfer belt 71 as necessary. Based on the measurement results, this apparatus uses the operating conditions of each part of the apparatus that affect the image quality, such as the developing bias applied to each developing device, the intensity of the exposure beam L, and the tone correction characteristics of the apparatus. Adjustments are being made.
[0027]
The density sensor 60 is configured to output a signal corresponding to the image density of a region of a predetermined area on the intermediate transfer belt 71 using, for example, a reflection type photosensor. The CPU 101 can detect the image density of each part of the toner image on the intermediate transfer belt 71 by periodically sampling the output signal from the density sensor 60 while rotating the intermediate transfer belt 71.
[0028]
In order to improve the accuracy of density detection by suppressing the influence of light incident from the outside, the density sensor 60 uses an optical path of irradiation light from the density sensor 60 toward the intermediate transfer belt 71 and the intermediate transfer belt 71. Polarized beam splitters (not shown) are respectively provided on the optical paths of the reflected light that is reflected by and incident on the density sensor 60. Thus, while limiting the polarization component of the irradiation light and receiving only a specific polarization component of the reflected light, the S / N ratio of the output signal can be increased, and the density detection can be performed with high accuracy. In the density sensor 60 of this embodiment, irradiation light having a single polarization component (hereinafter referred to as “p-polarization component”) is irradiated toward the intermediate transfer belt 71, and the same p as the irradiation light among the reflected light. A polarization component and a polarization component perpendicular to the polarization component (hereinafter referred to as “s-polarization component”) are individually received, and a voltage corresponding to each received light amount is output as an output signal. In the following, the output voltage of the density sensor 60 corresponding to the p and s polarization components will be expressed as Vp and Vs, respectively.
[0029]
FIG. 3 is a diagram showing a gradation processing block of the image forming apparatus. The main controller 11 includes functional blocks such as a color conversion unit 114, a gradation correction unit 115, a halftoning unit 116, a pulse modulation unit 117, a gradation correction table 118, and a correction table calculation unit 119.
[0030]
In addition to the CPU 101, ROM 106, and RAM 107 shown in FIG. 2, the engine controller 10 includes a laser driver 121 for driving a laser light source provided in the exposure unit 6 and a detection result of the engine unit EG based on the detection result of the density sensor 60. A gradation characteristic detecting unit 123 that detects a gradation characteristic indicating a gamma characteristic is provided.
[0031]
In the main controller 11 to which the image signal is given from the host computer 100, the color conversion unit 114 converts the RGB gradation data indicating the gradation level of the RGB component of each pixel in the image corresponding to the image signal into the corresponding CMYK. Conversion into CMYK gradation data indicating the gradation level of the component. In this color conversion unit 114, the input RGB gradation data is, for example, 8 bits per pixel per color component (that is, representing 256 gradations), and the output CMYK gradation data is similarly 8 bits per pixel per color component ( That is, it represents 256 gradations). The CMYK gradation data output from the color conversion unit 114 is input to the gradation correction unit 115.
[0032]
The gradation correction unit 115 performs gradation correction on the CMYK gradation data of each pixel input from the color conversion unit 114. That is, the gradation correction unit 115 refers to the gradation correction table 118 registered in advance in the nonvolatile memory, and in accordance with the gradation correction table 118, the input CMYK gradation data of each pixel from the color conversion unit 114. Is converted into corrected CMYK gradation data indicating the corrected gradation level. The purpose of the gradation correction is to compensate for the change in the gamma characteristic of the engine unit EG configured as described above, and to keep the overall gamma characteristic of the image forming apparatus always ideal. The corrected CMYK gradation data corrected in this way is input to the halftoning unit 116. The halftoning unit 116 performs halftoning processing such as an error diffusion method, a dither method, and a screen method, and inputs halftone CMYK gradation data of 8 bits per pixel to the pulse modulation unit 117.
[0033]
The CMYK gradation data after halftoning input to the pulse modulation unit 117 indicates the size and arrangement of toner dots of CMYK colors to be attached to each pixel, and the pulse modulation unit 117 that has received such data receives the data. The halftone CMYK gradation data is used to create a video signal for pulse width modulation of the exposure laser pulses of the CMYK color images of the engine unit EG and output to the engine controller 10 via a video interface (not shown). To do. Upon receiving this video signal, the laser driver 121 controls ON / OFF of the semiconductor laser of the exposure unit 6 to form an electrostatic latent image of each color component on the photosensitive member 22. In this way, normal printing is performed.
[0034]
In addition, this image forming apparatus has a gradation correction mode that is executed at an appropriate timing, for example, immediately after the power is turned on, and forms a patch image for gradation correction to change and set the gradation correction table. In this gradation correction mode, a gradation patch image for gradation correction prepared in advance for measuring the gamma characteristic is formed on the intermediate transfer belt 71 by the engine unit EG for each toner color. The density sensor 60 reads the image density of the image, and based on the signal from the density sensor 60, the gradation characteristic detection unit 123 associates the gradation level of each gradation patch image with the detected image density ( The gamma characteristics of the engine unit EG are created and output to the correction table calculation unit 119 of the main controller 11.
[0035]
In this embodiment, the data of the gradation patch image is programmed in the ROM of the main controller 11, for example, and the surface of the intermediate transfer belt 71 is executed by executing the above-described image forming operation based on this image data. A gradation patch image having a predetermined pattern is formed on the substrate.
[0036]
FIG. 4 is a diagram showing a gradation patch image. As shown in FIG. 4A, the gradation patch image Ip in this embodiment has a strip shape extending along the moving direction D2 of the intermediate transfer belt 71, and its gradation level is not uniform and is moving. It is formed so as to continuously change from the maximum level (level 255) to the minimum level (level 0) along the direction D2. However, a “header portion” Ih, which is a solid image formed with the gradation level maintained at the maximum value 255, is provided in contact with the portion where the gradation level changes at the head portion (x = 0 to x0) of the image. Yes. The length of the header portion Ih is not less than the circumferential length Ldr of the developing roller 44, and the reason will be described later. Of the gradation patch image Ip, the portion Ig excluding the header portion Ih described above corresponds to the “gradation image portion” of the present invention.
[0037]
That is, in the process of forming the gradation patch Ip, first, a header portion Ih, which is a solid image having a constant width and a length Ldr or more along the direction D2, is formed on the photoconductor 22, and subsequently, the gradation is patched. A gradation image portion Ig having the same width (or narrower than the header portion) whose level gradually changes from the maximum level to the minimum level is formed. A tone patch image Ip is a transfer of the image formed in this way onto the intermediate transfer belt 71. As the gradation patch image Ip, in addition to the image shown in FIG. 4A whose gradation level continuously changes, an image whose gradation level changes stepwise can be used.
[0038]
As will be described later, in this image forming apparatus, the density sensor 60 detects the image density of the gradation patch image Ip. That is, the CPU 101 samples the output signal from the density sensor 60 at a constant period while rotating the intermediate transfer belt 71 carrying the patch image Ip. Thereby, as shown in FIG. 4B, a sampling data string corresponding to the image density of each of a plurality of regions (hereinafter referred to as “detection regions”) P in the patch image Ip is obtained. In addition, as shown in FIG. 4B, the detection areas P that are adjacent to each other may partially overlap each other or may be separated from each other.
[0039]
FIG. 5 is a flowchart showing the gradation correction mode. This gradation correction mode has the following characteristics: (1) The patch image density is normalized by the evaluation value calculated from the density sensor output signal using a predetermined calculation formula, and based on this (2) In consideration of the influence of the surface state of the intermediate transfer belt 71, which is the base on which the patch image is formed, on the density detection result, the patch image is formed before and after the patch image formation at the same position. The evaluation value is calculated based on the sampling result; (3) A predetermined correction process is performed on the sampling result in consideration of the possibility that the sampling result includes the influence of density fluctuation and noise. Hereinafter, each processing step in the gradation correction mode will be described in detail.
[0040]
In this embodiment, in view of the great difference in optical characteristics between the color toner and the achromatic black toner, the patch image formed with the color toner differs from the patch image formed with the black toner. Processing is performed using a calculation formula. However, since the basic concept is common, the processing for patch images with color toner will be mainly described below, and different calculation formulas for black toner will be collectively shown later.
[0041]
In step S1, the background density is detected. That is, the CPU 101 samples the output signal from the density sensor 60 at regular intervals (8 msec intervals in this embodiment) while rotating the intermediate transfer belt 71 that does not carry a toner image. Sampling is performed for approximately one turn of the intermediate transfer belt 71. The density sensor outputs Vp and Vs at the sampling position x (x = 0, 1, 2,...) At this time are expressed as Tp1 (x) and Ts1 (x), respectively. Among these, for the sample value at the same position as the detection area where the gradation patch image is formed and the density is detected later, the sample value is stored separately for each position.
[0042]
For the sample values Tp1 (x) and Ts1 (x) for one turn of the intermediate transfer belt 71, the average values Tave and Tpave are obtained and stored according to the following expressions (Expression 1) and (Expression 2). :
Tave = Sg.AVG (Tp1 (x) -Vp0) -AVG (Ts1 (x) -Vs0) (Formula 1)
Tpave = AVG (Tp1 (x) −Vp0) (Formula 2)
Here, AVG (f (x)) is defined as an operator for obtaining an average value of the function f (x) having x as a variable in the entire range of x. The values Vp0 and Vs0 are the “dark output” of the density sensor 60, that is, the output voltages Vp and Vs output from the density sensor 60 when the irradiation light quantity is zero, and the output offset of the output circuit of the density sensor 60. Is a value corresponding to.
[0043]
The coefficient Sg is a gain correction coefficient of the density sensor 60. In this embodiment, the S / N ratio of the signal is improved in consideration of the fact that the amount of the s-polarized light component is smaller than the p-polarized light component that is the same polarization component as the irradiated light among the polarized light components included in the reflected light. Therefore, the s-polarized component light receiving unit of the density sensor 60 is given a larger gain than the p-polarized component light receiving unit. The above-mentioned gain correction coefficient Sg is a coefficient for compensating for this gain difference and making the weights of both polarization components the same.
[0044]
Next, the engine unit EG is operated to form the patch image Ip shown in FIG. 4A on the intermediate transfer belt 71 (step S2). Then, for a plurality of detection areas P (FIG. 4B) in the patch image Ip formed in this way, the output signal of the density sensor 60 corresponding to the image density of each detection area is sampled (step S3). The density sensor outputs Vp and Vs at the sampling position x at this time are represented as Dp1 (x) and Ds1 (x), respectively.
[0045]
In subsequent step S4, background correction is performed by subtracting the influence of the background (intermediate transfer belt 71) from the sampling result of the patch image. The basic concept of this background correction is as follows. That is, the sampling result of the patch image Ip includes the influence of light that is transmitted through the patch image Ip and reflected from the surface of the intermediate transfer belt 71. In particular, the influence is large in a portion formed at a relatively low gradation level (in the patch image Ip shown in FIG. 4A, it is formed at a gradation level lower to the right). Therefore, the net image density of the patch image is obtained by correcting the sampling results Dp1 (x) and Ds1 (x) for the patch image Ip using the sampling results Tp1 (x) and Ts1 (x) for the background, respectively. Can be requested.
[0046]
Specifically, density data Fc (x) after correction for a color toner patch image is obtained by the following calculation formula:
Fc (x) = Sg · {Dp1 (x) −Vp0} − {Ds1 (x) −Vs0} × Tave / [Sg · {Tp1 (x) −Vp0} − {Ts1 (x) −Vs0}] ( Formula 3)
Here, the subscript c on the left side indicates a value corresponding to the color toner, and the calculation formula itself is common to cyan, magenta, and yellow toner colors, but the value of the density data Fc (x) is Needless to say, each toner color has a different value. The same applies to the following calculations.
[0047]
Next, the density data Fc (x) obtained in this way is converted into an evaluation value G1c (x) (step S5). This “evaluation value” is expressed by normalizing the patch image density with a value from 0 to 1. In the density sensor 60 using the reflection type photosensor, the output voltage decreases as the amount of received light increases, so that the value of the density data Fc (x) described above becomes smaller as the image density is higher and is difficult to handle. Therefore, an evaluation value G1c (x) indicating a larger value as the image density is higher is introduced. The evaluation value G1c (x) is calculated by the following formula:
G1c (x) = 1-Fc (x) / Tave (Formula 4)
Here, due to an error occurring in the process of sampling and calculation processing, the result of (Expression 4) may be a negative value or a value exceeding 1. In such a case, the calculation result is rounded to 0 or 1, respectively.
[0048]
On the other hand, the same can be considered for black toner. In this case, the following formulas (Formula 3A) and (Formula 4A) can be used instead of the above formulas (Formula 3) and (Formula 4), respectively:
Fk (x) = {Dp1 (x) −Vp0} × Tpave / {Tp1 (x) −Vp0} (Formula 3A)
G1k (x) = 1-Fk (x) / Tpave (Formula 4A)
Here, the subscript k on the left side indicates a value corresponding to black toner.
[0049]
The subsequent treatment for the evaluation values G1c (x) and G1k (x) thus obtained is common to the color toner and the black toner. Therefore, in the following description, the evaluation value G1c (x) corresponding to the color toner and the evaluation value G1k (x) corresponding to the black toner are not particularly distinguished, and in any case, G1 ( x).
[0050]
Subsequently, correction processing is performed on the evaluation value G1 (x) thus obtained (step S6). This correction process is a correction process for canceling the influence of periodic density fluctuations caused by the developing roller 44. The density fluctuation caused by the developing roller 44 occurs, for example, as follows.
[0051]
If the developing roller 44 is eccentric, periodic density fluctuations occur as the developing roller 44 rotates. In a contact development type image forming apparatus in which development is performed in a state where the developing roller 44 and the photosensitive member 22 are in contact with each other, the contact pressure between the developing roller 44 and the photosensitive member varies due to the eccentricity of the developing roller 44. The probability of toner movement to 22 fluctuates. On the other hand, in a non-contact development type image forming apparatus in which development is performed in a state where the developing roller 44 and the photosensitive member 22 are spaced apart from each other, the strength of the electric field formed in the gap portion varies and the flying property of the toner changes. To do. Therefore, in any type of apparatus, the eccentricity of the developing roller 44 causes fluctuations in image density.
[0052]
Further, non-uniformity of the toner layer on the surface of the developing roller 44 such as the amount of toner carried on the developing roller 44 and the charge amount thereof also causes density fluctuation. This is because if the amount of toner carried on the surface of the developing roller 44 or the amount of charge varies, the amount of toner moving to the photosensitive member 22 will also differ partially. Such variations are caused by unevenness of the surface condition of the developing roller 44, such as manufacturing variations and the presence of toner stuck to the developing roller surface, and the apparatus stops the rotation of the developing roller 44. It is also caused by being left in the state for a long time.
[0053]
In this embodiment, when the patch image Ip is formed, the header portion Ih, which is a solid image, is provided prior to the formation of the gradation image portion Ig for data collection used for controlling the gradation correction characteristics. Yes. The length along the direction D2 of the header portion Ih is set to be not less than the developing roller circumferential length Ldr. Therefore, the header portion Ih of the patch image Ip is formed using the toner carried on the surface of the developing roller 44 before the patch image is formed. Then, new toner is supplied to the surface of the developing roller 44 that has consumed the toner in the developing device. At this time, since the header portion Ih is a uniform solid image, the toner on the surface is consumed almost uniformly over one or more rotations of the developing roller 44. Therefore, the toner newly carried on the surface of the developing roller 44 has high uniformity in the circumferential direction of the developing roller 44.
[0054]
On the other hand, the gradation image portion Ig of the patch image Ip is formed by using the highly uniform toner thus newly supplied to the developing roller 44. Therefore, in this image forming apparatus, density unevenness due to non-uniformity of the toner layer on the surface of the developing roller 44 is difficult to appear, and the gradation characteristics of the apparatus can be obtained with high accuracy from the density detection result in the gradation image portion Ig. Is possible.
[0055]
As described above, in this embodiment, the header portion Ih is provided in the patch image Ip, and the length thereof is not less than the developing roller circumferential length Ldr, whereby density unevenness due to the non-uniformity of the toner on the surface of the developing roller 44 is achieved. Can be prevented from reaching the gradation image portion Ig. However, density fluctuations due to other causes such as the eccentricity of the developing roller 44 described above may not be completely removed.
[0056]
Such fluctuations in image density caused by the developing roller 44 appear repeatedly in synchronization with the rotation cycle of the developing roller 44. Therefore, it is possible to reduce the influence of density fluctuations by performing correction using the periodicity on the sampling data. Here, it is necessary to pay attention to the following. When the moving speeds of the developing roller 44 and the surface of the photosensitive member 22 that rotate with respect to each other are the same, the fluctuation cycle of the image density on the photosensitive member 22 is the same as the circumferential length of the developing roller 44. However, when the moving speeds of the two are different, the fluctuation cycle of the image density on the photosensitive member 22 is longer or shorter than the circumference of the developing roller 44 depending on the speed ratio.
[0057]
In this embodiment, the moving speeds of the developing roller 44 and the photosensitive member 22 are the same. Therefore, fluctuations in image density appear as repetitions at the circumference of the developing roller 44. In the following, two examples, ie, correction process 1 and correction process 2, will be described as correction processes for reducing the influence of such periodic fluctuations.
[0058]
FIG. 6 is a principle diagram for explaining the correction process 1. In this correction process 1, the following smoothing process is performed on the evaluation values for the respective positions of the previously obtained patch image Ip, thereby canceling fluctuations appearing at a repetition period corresponding to the developing roller circumferential length Ldr.
[0059]
As shown by the solid line in FIG. 6, the evaluation value for the patch image Ip is substantially constant at the sampling position (0 ≦ x ≦ x0) corresponding to the header portion Ih (FIG. 4), and corresponds to the gradation image portion Ig. At the sampling position (x> x0), it should gradually decrease as the gradation level decreases. However, because of the density fluctuation of the patch image Ip caused by the developing roller 44, as shown by the broken line in FIG. 6, the calculation result of the evaluation value G1 (x) obtained previously (shown by a white circle) is also the developing roller circumferential length. Periodic fluctuations are shown corresponding to Ldr. Therefore, in this correction process 1, the evaluation value G1 (x) obtained at one sampling position is used as an average value G2 () of the evaluation values at each sampling position included in the range of the developing roller circumferential length Ldr with the position as the center. Such a periodic variation is canceled by performing a correction to be replaced with x). FIG. 6 shows a case where eleven sampling positions are included in the range of the developing roller circumferential length Ldr. For example, the evaluation value G2 (x1) after correction at the sampling position (x = x1) (indicated by a black circle) ) Is expressed as:
G2 (x1) = {G1 (x1-5) + G1 (x1-4) + ... + G1 (x1) + ... + G1 (x1 + 4) + G1 (x1 + 5)} / 11 (Formula 5)
Here, G1 (x1-5), G1 (x1-4),... Are evaluations at the sampling positions (5 samples before, 4 samples before,. Value.
[0060]
This smoothing processing is a processing technique generally known as “moving average processing”. In the smoothing process by the moving average process, it is possible to smooth the variation included in the continuous sampling data string. Here, since the range of the sampling data used for processing corresponds to the developing roller circumferential length Ldr, it is possible to cancel the influence of the periodic density fluctuation caused by the rotation of the developing roller 44.
[0061]
Note that the smoothing process based on the moving average process has a problem that the steep change in data becomes dull. In the gradation patch image Ip of this embodiment, since the gradation level of the gradation image portion Ig is continuously changed, only the both ends of the gradation image portion Ig are problematic. Among these, the tip portion with the maximum gradation level is further provided with a header portion Ih made of a solid image in contact with the front thereof, so that the evaluation value at the end portion changes slowly. In addition, as for the rear end portion, the evaluation value gradually approaches zero as the gradation level is lowered, so that a rapid change in the value does not occur. Thus, as the patch image Ip, the header part Ih at the maximum gradation level and the gradation image part Ig whose gradation level sequentially changes from the maximum level to the minimum level in contact with the rear end of the header part Ih. By configuring as an image having, it is possible to avoid the problem of dull data change associated with the smoothing process.
[0062]
In addition, when the required number of sampling data after the rear end portion of the gradation image portion Ig cannot be obtained, the number of data used for the smoothing process in the vicinity of the rear end portion may be appropriately changed. For example, when there are only n data (n is a natural number smaller than 10) behind the sampling position, the data corresponding to the sampling position and each of n data before and after the data are (2n + 1) data. Smoothing processing can be performed.
[0063]
FIG. 7 is a flowchart showing the correction process 1. In this embodiment, since the sampling is actually performed 21 times within the length corresponding to the developing roller circumferential length Ldr, the moving average process using 21 sampling data is performed. That is, assuming that the sampling position at which the process is started is (x = x0) (step S101), the evaluation values at the sampling position at a total of 21 sampling positions are extracted (step S102), and the average value is calculated. (Step S103). The average value thus obtained is set as the corrected evaluation value G2 (x) at the position (step S104). By repeating this process until all sampling positions in the gradation image portion Ig are completed while moving the position x (steps S105 and S106), a new variation in which the periodic fluctuation due to the developing roller 44 has been removed is removed. An evaluation value G2 (x) is obtained.
[0064]
As a result, the sampling result at the header portion Ih is used in the data processing near the leading end of the gradation image portion Ig, while the rear position of the gradation patch image Ip is further behind in the data processing near the trailing end portion. The sampling result at is used. As described above, there is little data dullness due to this. Therefore, by this correction process 1, an evaluation value that indicates the image density for each gradation level from the maximum gradation level to the minimum gradation level is calculated with high accuracy by canceling the periodic fluctuation caused by the developing roller 44. be able to. When the corrected evaluation value G2 (x) (where 0 ≦ x ≦ x0) in the header part Ih is required, the average value of the evaluation values G1 (x) at each position in the header part Ih is used for each position. The corrected evaluation value G2 (x) may be used. This is because the image density should be constant in the header part Ih.
[0065]
Next, a second embodiment of the correction process for removing the influence of density fluctuation caused by the developing roller 44 will be described with reference to FIGS.
[0066]
FIG. 8 is a principle diagram for explaining the correction process 2. FIG. 9 is a flowchart showing the correction process 2. In this correction processing 2, the degree of density fluctuation appearing in the patch image Ip is determined from the sampling result in the header part Ih, and the density detection result in the gradation image part Ig is corrected using the result, thereby developing roller circumferential length Ldr. The periodic fluctuation of the evaluation value corresponding to is removed. Specifically, correction is performed as follows.
[0067]
First, an average value Gave of evaluation values G1 (x) at each sampling position in the header part Ih is obtained. This average value Gave corresponds to an evaluation value in the header part Ih when there is no period fluctuation. Then, the ratio of the evaluation value G1 (x) at each sampling position in the header part Ih to the average value Gave is obtained for each sampling position. For example, as shown in FIG. 8, it is assumed that the evaluation value G1 (x2) at the sampling position (x = x2) is A times the average value Gave. That is, G1 (x2) = A · Gave. This indicates that the evaluation value G1 (x2) at this position appears as A times the original value Gave due to the influence of density fluctuation.
[0068]
If the fluctuation of the evaluation value occurs at a cycle corresponding to the developing roller circumferential length Ldr, the same fluctuation as described above appears also at a position (x = x 2 + Ldr) corresponding to the rear of the developing roller 44 by exactly one turn. It should be. That is, the evaluation value G1 (x2 + Ldr) at this position is considered to be A times the original value. Therefore, by correcting the evaluation value G1 (x2 + Ldr) at this position by the following equation (Equation 6), a corrected evaluation value G3 (x2 + Ldr) that is not affected by the density fluctuation can be obtained:
G3 (x2 + Ldr) = G1 (x2 + Ldr) / A (Formula 6)
Further, the evaluation value G1 (x2 + N · Ldr) at the position (x = x2 + N · Ldr) corresponding to the N circumferences (where N is a natural number) of the developing roller 44 with respect to the sampling position (x = x2) Similarly, by dividing this by the ratio A, the corrected evaluation value G3 (x2 + N · Ldr) can be obtained.
[0069]
The evaluation values at other sampling positions in the gradation image portion Ig are also corrected by performing the correction according to the ratio between the evaluation value G1 (x) and the average value Gave at the position in the header portion Ih corresponding to the position. The periodic fluctuation of the evaluation value is eliminated.
[0070]
A specific processing method will be described with reference to FIG. First, an average value Gave of the evaluation values G1 (x) obtained at each position of the header part Ih is calculated (step S201). Then, for each position in the header part Ih, the ratio of the evaluation value G1 (x) at that position to the average value Gave is determined, and the correction coefficient at that position is determined from the ratio (step S202). In the above example, it is assumed that the influence of density fluctuation appears at the same ratio on the developing roller 44 at the same position, and the ratio itself obtained above is used as the correction coefficient. You may ask for.
[0071]
Then, the position where correction processing is started is set as (x = x0) (step S203), and the evaluation value G1 (x) at the position is corrected by the correction coefficient obtained for the position in the header portion Ih corresponding to the position. A new evaluation value G3 (x) is obtained (step S204). This process is repeated until the sampling is completed for all sampling positions in the gradation image portion Ig while moving the position x (steps S205 and S206), whereby a new variation in which the periodic fluctuation due to the developing roller 44 has been removed is removed. An evaluation value G3 (x) is obtained.
[0072]
By either of these two correction processes, the periodic variation component can be removed from the evaluation value data. In order to acquire data to be used for these correction processes and to suppress density fluctuations that occur in the gradation image portion Ig, the length is equal to or longer than the developing roller circumferential length Ldr prior to the formation of the gradation image portion Ig. It is desirable to form a header portion Ih having
[0073]
Returning to FIG. 5, the description of the gradation correction mode will be continued. Based on the corrected evaluation value G2 (x) or G3 (x) obtained in this way, in this embodiment, the gradation correction table 118 is updated as necessary (step S7). That is, the necessary correction is performed by comparing the actual image density for each gradation level with its ideal value from the actually measured evaluation value for each sampling position obtained above.
[0074]
FIG. 10 is a diagram illustrating the gradation characteristic of the engine unit and its correction characteristic. When the evaluation value G2 (x) or G3 (x) calculated corresponding to each point in the gradation patch image Ip as described above is plotted corresponding to the gradation level, for example, the curve a in FIG. As shown in the graph, a curve indicating the gradation characteristics in this apparatus is obtained. The actually measured gradation characteristics may not match the originally desired ideal gradation characteristics (for example, the curve b shown in FIG. 10) due to individual differences among devices, changes with time, changes in the surrounding environment, and the like. is there. Therefore, for example, as shown by a curve c in FIG. 10, the gradation of the input image signal is faithfully reproduced by applying gradation correction to the image signal in advance based on the inverse characteristic of the actually measured gradation characteristic. It is possible to form an image.
[0075]
Specifically, the correction table calculation unit 119 compensates the actually measured tone characteristic of the engine unit EG based on the tone characteristic given from the tone characteristic detection unit 123 to obtain an ideal tone characteristic. Tone correction table data is calculated, and the content of the tone correction table 118 is updated to the calculation result. Thus, the gradation correction table 118 is changed and set.
[0076]
At this time, if the measured gradation characteristic data of the apparatus includes a variation caused by the developing roller 44, the gradation correction table data calculated based on the data is also different from the original value. As a result, an image having an unnatural gradation is formed. On the other hand, in this embodiment, since such fluctuation components are removed in advance by correction processing and used for calculation, such a problem does not occur.
[0077]
In the subsequent image forming operation, the input CMYK gradation data of each pixel from the color conversion unit 114 is corrected with reference to the updated gradation correction table 118, and an image is generated based on the corrected CMYK gradation data. By performing the formation, a high-quality image with excellent gradation can be formed. In addition, by updating the gradation correction table 118 as needed, ideal gradation correction can always be performed in accordance with the gamma characteristic of the engine unit EG that changes over time, and an image with stable image quality can be obtained. Formation can be performed.
[0078]
As described above, in the image forming apparatus according to this embodiment, the gradation patch image Ip for the gradation correction mode includes the gradation image portion Ig whose gradation level gradually changes from the maximum level to the minimum level, and this level. Prior to the formation of the toned image portion Ig, an image having a header portion Ih having a length equal to or longer than the developing roller circumferential length Ldr is formed. By forming the header portion Ih in this way, the toner for one round of the developing roller is consumed, and the gradation image portion Ig is formed using the newly supplied toner. Is difficult to appear.
[0079]
The gradation correction characteristic of the apparatus is controlled based on the image density detection result of the gradation image portion Ig. At this time, a smoothing process is performed to correct density unevenness that occurs in the rotation cycle of the developing roller. Therefore, the gradation characteristics of the apparatus can be obtained with high accuracy, and ideal gradation correction characteristics can always be obtained. As a result, this image forming apparatus can stably form a toner image with excellent gradation and good image quality.
[0080]
Further, since the header portion Ih is a solid image having a constant gradation level and the gradation level of the gradation image portion Ig is gradually changed from the maximum to the minimum, the end portion of the gradation image portion Ig is configured. Therefore, the smoothing process can be performed with higher accuracy while using the density detection result in the header portion Ih.
[0081]
The present invention is not limited to the above-described embodiment, and various modifications other than those described above can be made without departing from the spirit of the present invention. For example, the gradation image portion Ig of the above-described embodiment is configured such that the gradation level gradually decreases, but conversely, the gradation level may gradually increase. This is because the effect of making the toner on the developing roller uniform by forming the header portion is obtained regardless of the image pattern of the gradation image portion. However, from the viewpoint of performing the smoothing process on the density detection result of the gradation image portion, it is desirable to obtain a patch image in which the image density continuously changes from the header portion to the gradation image portion. As described above, it is preferable to use a gradation image portion in which the gradation level gradually decreases.
[0082]
Moreover, in the said embodiment, although the header part Ih is made into the solid image, the image pattern of a header part is not limited to this, It is good also as another image pattern. For example, when the gradation image portion is configured to start from a gradation level other than the maximum gradation level, an image having the same gradation level as the head portion of the gradation image portion may be used as the header portion. However, from the viewpoint of increasing the uniformity of the toner supplied to the developing roller after forming the header portion, it is preferable to form an image having a uniform pattern with a density as high as possible as the header portion. Further, in order to use the density detection result of the header part for correction processing of the detection result of the gradation image part, the header part preferably has a uniform image pattern.
[0083]
In the above-described embodiment, the gradation image portion Ig whose gradation level continuously changes from the maximum level to the minimum level is formed. However, the gradation image portion Ig is typically formed at several gradation levels. A gradation image portion may be formed. In this case, the density detection result of the header portion having a length equal to or greater than the circumferential length of the developing roller at one gradation level may be used as data representing the image density at that gradation level.
[0084]
In the above embodiment, the density detection of the patch image carried on the intermediate transfer belt 71 is performed. However, the present invention is not limited to this. For example, the patch carried on the photosensitive member 22 is not limited thereto. Image density detection may be performed. Further, instead of the intermediate transfer belt 71, in an apparatus including another transfer medium such as a transfer drum, the density detection of the patch image may be performed on the transfer medium.
[0085]
In each of the above embodiments, the present invention is applied to an apparatus that forms an image using toners of four colors, yellow, magenta, cyan, and black. However, the types and number of toner colors are limited to those described above. It is optional. In addition to the rotary development type apparatus as in the present invention, the so-called tandem type image forming apparatus in which developing units corresponding to the respective toner colors are arranged in a line along the sheet conveying direction. The present invention is also applicable. Furthermore, the present invention is not limited to the electrophotographic apparatus as in the above embodiment, but can be applied to all image forming apparatuses.
[Brief description of the drawings]
FIG. 1 is a diagram showing an embodiment of an image forming apparatus according to the present invention.
FIG. 2 is a block diagram showing an electrical configuration of the image forming apparatus of FIG.
FIG. 3 is a diagram illustrating a gradation processing block of the image forming apparatus.
FIG. 4 is a diagram illustrating a gradation patch image.
FIG. 5 is a flowchart showing a gradation correction mode.
FIG. 6 is a principle diagram for explaining correction processing 1;
FIG. 7 is a flowchart showing a correction process 1;
FIG. 8 is a principle diagram for explaining correction processing 2;
FIG. 9 is a flowchart showing correction processing 2;
FIG. 10 is a diagram illustrating tone characteristics and correction characteristics of an engine unit.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 10 ... Engine controller (control means) 11 ... Main controller (control means) 22 ... Photoconductor (image carrier), 44 ... Developing roller (toner carrier), Ig ... Gradation image part, Ih ... Header part, Ip ... patch image, Ldr ... circumference of developing roller 44

Claims (6)

An image carrier capable of carrying an electrostatic latent image on its surface;
A toner carrier that conveys the toner to a position facing the image carrier by moving around in a predetermined direction while carrying toner on the surface;
The toner carried on the toner carrier is moved to the image carrier to visualize the electrostatic latent image with the toner to form a toner image, and the density of the toner image formed as a patch image Control means for controlling the tone correction characteristics of the apparatus based on the detection result,
The patch image is
Formed along the direction of movement of the surface of the toner carrier at the facing position;
A gradation image portion whose gradation level gradually changes along the moving direction;
A header portion that is formed prior to the formation of the gradation image portion, and that has a length along the moving direction that is equal to or greater than the peripheral length of the toner carrier and has a single gradation level. And
The control means detects the density at each position in the header part and the gradation image part, and the density detection result at each position in the gradation image part as viewed from the position of the toner carrier on the image carrier. Correction is performed based on the density detection result at the position in the header portion separated by N times (N is a natural number) the length corresponding to the circumference, and based on the density detection result at each position in the gradation image portion after the correction. And controlling the gradation correction characteristics.   The control means obtains a ratio of the density at the position to the average density of the header part from the density detection result at each position in the header part, and corrects the density detection result at each position in the gradation image part based on the ratio. The image forming apparatus according to claim 1. An image carrier capable of carrying an electrostatic latent image on its surface;
A toner carrier that conveys the toner to a position facing the image carrier by moving around in a predetermined direction while carrying toner on the surface;
The toner carried on the toner carrier is moved to the image carrier to visualize the electrostatic latent image with the toner to form a toner image, and the density of the toner image formed as a patch image Control means for controlling the tone correction characteristics of the apparatus based on the detection result,
The patch image is
Formed along the direction of movement of the surface of the toner carrier at the facing position;
A gradation image portion whose gradation level gradually changes along the moving direction;
A header portion formed prior to the formation of the gradation image portion and having a single gradation level with a length along the moving direction equal to or longer than a peripheral length of the toner carrier;
Have
The patch image is formed such that gradation levels are continuous between the header portion and the gradation image portion,
The image forming apparatus according to claim 1, wherein the control unit corrects the density detection result at each position in the gradation image portion by a moving average process in which a length corresponding to the circumference of the toner carrier is a processing range.   The image forming apparatus according to claim 1, wherein the header portion is a solid image.   The image forming apparatus according to claim 4, wherein in the gradation image portion, a gradation level changes from a maximum level to a minimum level along the moving direction. An image carrier that carries an electrostatic latent image on its surface and a toner carrier that carries toner on its surface are arranged facing each other at a predetermined facing position, and the toner carrier is moved around in a predetermined direction, In the image forming method of forming the toner image by moving the toner carried on the toner carrier to the image carrier to visualize the electrostatic latent image with the toner,
Based on the density detection result of the toner image formed as a patch image, the gradation correction characteristic of the apparatus is controlled,
A gradation image portion whose gradation level gradually changes along the moving direction of the surface of the toner carrier at the facing position;
A header portion formed prior to the formation of the gradation image portion and having a single gradation level with a length along the moving direction equal to or longer than a peripheral length of the toner carrier. Forming a toner image,
The length corresponding to the circumference of the toner carrier on the image carrier when the density is detected at each position in the header part and the gradation image part and the density detection result at each position in the gradation image part is viewed from the position. Is corrected based on the density detection result at a position in the header portion separated by N times (N is a natural number), and the gradation correction characteristic is based on the density detection result at each position in the gradation image portion after the correction. An image forming method characterized by controlling the above.

Images