JP2005091767A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming apparatus which ensures highly accurate calibration without keeping toner density unnecessarily high or decreasing operability. <P>SOLUTION: The image forming apparatus executes toner image density control before calibration, thereby allowing the calibration while maintaining satisfactory color reproducibility. In a case where a difference between the highest patch density and the highest target image density is large, the target toner image density that is a control target value in the toner image density control is corrected, thereby altering the image density of a test image used for the calibration. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to image density control of an image forming apparatus using an electrophotographic method.

  In copying machines and printers, recording methods such as a thermal method, an electrophotographic method, and an ink jet method are used. Recently, with the progress of colorization, various requests have been made by users, and in each of the above-mentioned methods, efforts are being made to improve image quality by taking advantage of the respective characteristics. In particular, the electrophotographic recording system uses the electrostatic phenomenon of toner, which is a fine particle, so that the image density fluctuates or is used by being strongly affected by environmental changes such as temperature and humidity and changes over time. There is a tendency that differences in image density are likely to occur due to minute variations in the performance of the component parts. Therefore, generally, a process called toner image density control and a process called calibration are performed in order to stably maintain the image density.

  Toner image density control compares the density of a toner image formed on an image carrier such as a photoreceptor or an intermediate transfer body with a target toner image density that is a control target value based on an image signal of a predetermined level. If there is a difference between them, the charging potential of the photosensitive member, the irradiation light amount of the exposure device, and the like are controlled to make them coincide. Normally, this toner image density control is automatically executed at the timing when the image forming apparatus is powered on, and thereafter periodically.

Next, calibration is a process of reading out the density of the test image formed on the recording paper and adjusting the level of the image signal so that the image density becomes a predetermined value to perform gradation correction. . This calibration is usually performed by an operator giving an instruction to perform calibration to the image forming apparatus.
FIG. 14 shows a general calibration flowchart. As shown in the figure, the image forming apparatus first forms a test image composed of a plurality of reference patches having different densities on a recording sheet (step S1). This test image is an image made up of a plurality of reference patches whose density changes stepwise as shown in FIG. When the operator places a recording paper on which such a test image is formed on the image reading unit of the image forming apparatus and instructs to read the image, the image forming apparatus reads the density of each reference patch (step S2). It compares with the target image density which is each target value (step S3). Next, the image forming apparatus creates a gradation correction curve for correcting the input image signal so that the density of the read reference patch matches the target image density (step S4). Thereafter, an image having a desired density can be obtained by outputting the input image signal after correcting the input image signal based on such a gradation correction curve.

Here, an example of the gradation correction curve used for the correction as described above is shown in FIGS. FIG. 16 shows a gradation correction curve when the reference patch density corresponding to the maximum density is higher than the target image density. FIG. 17 shows the reference patch density corresponding to the maximum density higher than the target image density. The gradation correction curve in the case of being thin is shown. In these drawings, the right scale on the vertical axis indicates the density, and the left scale indicates the output image signal level.
For example, in FIG. 16, when the input image signal of the maximum level (100%) is input, the target image density that is the target value is “1.7”, whereas the reference patch density formed on the recording paper is It means “1.8” (that is, the reference patch density is higher than the target image density). Therefore, in the gradation correction curve shown in FIG. 16, an output image signal of 80% level is associated with an input image signal of 100% level. That is, for an input image signal of 100% level, if an output image signal for an input image signal of 80% level is output, the density corresponding to the original input image signal of 100% level (1.7) ) Is obtained. As described above, when the reference patch density is higher than the target image density, correction is performed such that the output image signal level is replaced with a lower value by the gradation correction curve. It is possible to make the corrected density coincide with the target image density in all regions up to 100%.
On the other hand, if the reference patch density is lower than the target image density, proper correction may not be performed even if calibration is performed. For example, as shown in FIG. 17, when an input image signal of 80% level is already corrected to an output image signal of 100% level, the input image signal of 100% level or higher is originally exceeded. Therefore, all the input image signals of 80% or more (80% or more and 100% or less) can only be corrected to the output image signal of the same level. That is, in a high density region such as 80% or more and 100% or less, the density after correction cannot be matched with the target image density.

  In order to solve such a problem, for example, in the technique described in Patent Document 1, a margin is provided in advance for the maximum value of the target image density, and the output image signal level is higher than the target image density (reference patch density). I try not to be thin. Furthermore, in the technique described in Patent Document 1, calibration is executed twice, the maximum density is corrected by the first calibration, and the intermediate density is corrected by the second calibration, thereby correcting the corrected image density. Are made to coincide with the target image density in all level regions. The idea of performing calibration twice as described above is also described in Patent Document 2.

JP-A-7-264427 JP-A-5-40396

However, the following problems can be considered in the above-described technique.
First, in the method of providing a margin for the target image density, the image density before calibration is always set high. If the image density is increased more than necessary, the toner mixing ratio in the developing device increases, and toner contamination due to toner scattering occurs, or the photosensitive member is loaded due to the necessity of keeping the charged potential of the photosensitive member high. There is a problem that it takes. In addition, performing image formation processing at a reduced density by constant calibration means that image formation processing can always be performed with a smaller number of gradations than the number of gradations originally provided in the image forming apparatus. As a result, the number of gradations of the image finally formed on the recording paper is reduced. Such a problem also occurs in the conventional calibration. That is, when the reference patch density is always higher than the target image density as shown in FIG. 16, the number of gradations is smaller than the number of gradations originally provided in the image forming apparatus, as described above. The image formation continues to be executed, and the performance of the image forming apparatus is not fully exhibited.
Further, in the method of executing the calibration twice, it is necessary to perform the test image forming process and the reading process twice, so that the operation becomes complicated and takes time.

  The present invention has been made under such a background, and an object of the present invention is to perform highly accurate calibration (tone correction) without unnecessarily maintaining a high toner density or reducing operability. An object is to provide an image forming apparatus.

In order to solve the above-described problem, an image forming apparatus according to the present invention measures a toner density, and controls a plurality of density different from a toner density control unit that controls the density so that the measured density approaches a target value. A test image having areas is formed on the recording medium, the density of each density area of the formed test image is read, the read density is compared with the target value, and the image is recorded on the recording medium based on the comparison result. An image forming apparatus including a gradation correction unit that corrects a level of an output image signal output to form an image according to a comparison result between an image density in a maximum density region of the test image and a target value thereof. Correction means for correcting the toner density controlled by the toner density control means.
In a more preferred aspect, the correction means corrects the toner density controlled by the toner density control means to be high when the image density in the maximum density region of the test image is lower than the target value. To do.
At this time, the toner density control means measures the density of the toner using a sensor for detecting the density of the toner image formed on the image carrier, and the correction means measures the measured toner density. Alternatively, the toner density controlled by the toner density control unit is corrected by correcting the target value of the density.
Alternatively, the toner density control means may be configured to expose the charged potential of the image carrier on which a toner image is formed, the exposure potential after the charged image carrier is neutralized by exposure, and the image carrier. The density of the toner is controlled by controlling at least one of a developing bias potential applied to develop the electrostatic latent image formed thereby, and the correcting means is configured to control the charging potential or the exposure. By controlling the potential, the toner density controlled by the toner density control means is corrected.
Alternatively, the toner density control means uses the sensor for detecting the mixing ratio of the toner and the carrier in the developing device for developing the electrostatic latent image formed by exposing the image carrier. Measure the mixing ratio of toner and carrier in the developing device, and the correction means corrects the measured mixing ratio or the target value of the mixing ratio to thereby adjust the toner density controlled by the toner density control means. to correct.
As a result, the image forming apparatus can execute calibration without unnecessarily maintaining a high toner density or reducing operability.

In a more preferred aspect of the image forming apparatus according to the present invention, the correction unit stores an upper limit value of a correction amount with respect to toner density, and performs the correction with a correction amount equal to or less than the upper limit value.
According to this aspect, it is possible to correct the density of the output image so that it is difficult for humans to detect.

In a more preferred aspect of the image forming apparatus according to the present invention, the gradation correction unit weakens a correction amount for the high density region of the image when the correction unit performs the correction.
At this time, more preferably, the gradation correction unit weakens the correction amount for the high density region of the image according to the correction amount of the correction unit when the correction unit performs the correction.
At this time, more preferably, the amount of weakening the correction amount for the high density region is less than or equal to the correction amount by the correction means.
Alternatively, in a more preferred aspect of the image forming apparatus according to the present invention, the gradation correction unit is configured to perform the high density region of the image according to the comparison result between the image density in the maximum density region of the test image and its target value. Decrease the correction amount for.
At this time, more preferably, the amount of weakening the correction amount for the high density region is less than or equal to the difference between the image density in the maximum density region of the test image and its target value.
According to this aspect, it is possible to correct the density of the output image so that it is not easily perceived by humans.

In a more preferred aspect of the image forming apparatus according to the present invention, toner density control by the toner density control unit is executed before correction by the tone correction unit.
According to this aspect, it is possible to always perform a calibration by forming a test image with an appropriate density.
At this time, more preferably, the correction by the correction unit is executed after a toner density control by the toner density control unit is performed and then a predetermined steady state is obtained.
According to this aspect, even in an image forming apparatus that performs toner density control with poor responsiveness, calibration can be executed without unnecessarily maintaining a high toner density or reducing operability. .

(1) First Embodiment Hereinafter, a first embodiment of the present invention will be described in detail with reference to the drawings.

(1-1) Configuration FIG. 1 is a diagram illustrating a configuration of an image forming apparatus 1 according to the present embodiment. Hereinafter, the configuration of the image forming apparatus 1 will be described in detail with reference to FIG. In the figure, a dotted line P indicates a recording paper conveyance path.
The image forming apparatus 1 is roughly divided into an image forming unit 10 and an image reading unit 20. The image forming unit 10 has a so-called printer function, and the image reading unit 20 has a so-called scanner function.

  The image reading unit 20 includes a platen glass 210 on which an original to be read is placed, a platen cover 220 that blocks external light, a lamp 230 that irradiates light on the original on the platen glass 210, and reflected light from the original. A sensor 240 that receives light and converts it into an image signal, and optical systems 250 and 251 that collect reflected light from the document and guide it to the sensor 240 are provided. With the above configuration, the image reading unit 20 reads an image on a document, converts the image into an image signal, and outputs the image signal to the image forming unit 10.

  The image forming unit 10 includes image forming units 110Y, 110M, 110C, and 110K that form toner images of four colors of Y (yellow), M (magenta), C (cyan), and K (black), and an image forming unit 110Y. , 110M, 110C, 110K, an intermediate transfer belt 120 that is circulated and moved, a drive roll 121 that circulates and moves the intermediate transfer belt 120, and a plurality of driven rolls 122 that are driven and stretched by the intermediate transfer belt 120. The primary transfer rolls 130Y, 130M, 130C, and 130K that primarily transfer the toner images formed by the image forming units 110Y, 110M, 110C, and 110K to the intermediate transfer belt 120, and the toner that is primarily transferred onto the intermediate transfer belt 120. A secondary transfer roll 140 for secondary transfer of the image onto the recording paper, and a toner image on the recording paper A fixing device 150 for fixing by heating, a paper feed tray 170 for supplying recording paper, a plurality of transport rolls 171 for transporting recording paper, a paper discharge tray 172 for discharging recording paper on which an image is printed, and an operator An operation unit 160 that accepts an operation according to the above, and a gradation control unit 180 and a toner image density control unit 190 that are characteristic portions of the present embodiment. Note that the image forming units 110Y, 110M, 110C, and 110K are different only in the toner used, and the structure is common. Therefore, in the following, the image forming unit 110 is generically referred to unless otherwise distinguished. Similarly, the primary transfer rolls 130Y, 130M, 130C, and 130K are also collectively referred to as the primary transfer roll 130.

FIG. 2 is a diagram showing details of the image forming unit 110, the gradation control unit 180, the toner image density control unit 190, and the peripheral portions thereof. As shown in the figure, the image forming unit 110 includes a photosensitive drum 111, a charging device 112, an exposure device 113, a developing device 114, and a toner box 115.
The photosensitive drum 111 is a rotatable roll member having a photoconductive layer formed on the surface thereof. The charging device 112 charges the surface of the photosensitive drum 111 to a predetermined charging potential. The exposure device 113 emits light of a light amount corresponding to an image signal of a desired image to lose the charge of the irradiated portion, and the surface of the photosensitive drum 111 charged by the charging device 112 is set to a predetermined exposure potential. An electrostatic latent image is formed on the surface of the photosensitive drum 111. The developing device 114 charges and discharges the toner. Since the discharged toner adheres according to the charge on the surface of the photosensitive drum 111, the electrostatic latent image is visualized by the toner, and a toner image is formed here. The toner box 115 includes a motor (not shown), and supplies a predetermined amount of toner to the developing device 114 by driving the motor.
The above is the function of each part of the image forming unit 110. The image forming unit 110 forms a toner image of a desired image on the surface of the photoconductive drum 111 by causing each unit to function as described above while rotating the photoconductive drum 111 in the direction of arrow R in the drawing. The toner image is transferred to the intermediate transfer belt 120 in contact with the photosensitive drum 111 at a portion where the photosensitive drum 111 faces the primary transfer roll 130.

In this manner, the image forming unit 10 forms a desired image on the recording paper by forming a toner image of the desired image, and transferring and fixing the toner image on the recording paper.
Further, by using the image reading unit 20 and the image forming unit 10 in combination, the image forming apparatus 1 functions as a so-called copying machine that reads a document, copies it, and outputs it on a recording sheet.

The toner image density control unit 190 stores a toner image density sensor 192 that detects the density of the toner image on the intermediate transfer belt 120 and a target toner image density that is a control target value of the toner image on the intermediate transfer belt 120. A toner image density storage unit 191. The toner image density control unit 190 compares the toner image density on the intermediate transfer belt 120 with the target toner image density, and if there is a difference between them, the toner image density matches the target toner image density. At least one of the charging potential of the charging device 112, the exposure potential of the exposure device 113, the developing bias potential of the developing device 114, and the motor driving of the toner box 115 is controlled.
This control is referred to as “toner image density control” in the present embodiment.

FIG. 3 is a block diagram showing the configuration of the gradation control unit 180. As shown in the figure, the gradation control unit 180 includes a test image forming unit 181, a target image density storage unit 182, a gradation correction curve creation unit 183, a gradation correction curve storage unit 184, and a maximum density. A determination unit 185 and a target toner image density correction unit 186 are provided.
The target image density storage unit 182 stores the target image density, that is, the image density to be output at each signal level of the input image signal input to the image forming unit 10. The test image forming unit 181 outputs an image signal for forming a test image as shown in FIG. 15 to the exposure device 113 based on the target image density stored in the target image density storage unit 182. The gradation correction curve creating unit 183 compares the image density (hereinafter referred to as test image density) of the test image obtained by reading the test image formed on the recording paper by the image reading unit 20 with the target image density. Then, a gradation correction curve is created based on the comparison result. Since the test image is formed by a plurality of reference patches having different densities, the test image density includes a plurality of reference patch densities. The tone correction curve storage unit 184 stores the tone correction curve created by the tone correction curve creation unit 183 and performs tone correction on the image signal output to the exposure device 113 based on the tone correction curve. .
The maximum density determination unit 185 obtains the reference patch density having the maximum image density of the test images and the maximum density of the target image density, and calculates a difference D between these densities (hereinafter, the respective densities are calculated). Referred to as “maximum patch density” and “maximum target image density”). The target toner image density correction unit 186 corrects the target toner image density stored in the target toner image density storage unit 191 based on the calculation result of the maximum density determination unit 185.

Here, the above-described density value will be described. In this embodiment, the density refers to the reflection density. That is, when light enters the reflector, the density d when the incident light quantity is I ₀ and the reflected light emission quantity is I is expressed by the following formula 1.
[Equation 1]
d = −log (I / I ₀ )
In Equation 1, (I / I ₀ ) is the reflectance, that is, the density d when the reflectance is “0.1” is “1.0”.
The difference D is a dimensionless amount as with the above-described density d, but when the maximum patch density is thinner than the maximum target image density, the difference D is positive, and conversely, the maximum patch density is the maximum target image density. If thinner, the difference D is negative.

As described above, the gradation control unit 180 compares the test image density with the target image density, forms a gradation correction curve so that the test image density matches the target image density, and outputs the output ratio of the image signal. So-called calibration is performed.
Also, the gradation control unit 180 compares the maximum density of the test image density and the target image density, and corrects the target toner image density stored in the target toner image density storage unit 191 if there is a difference between them. .

(1-2) Operation Based on the configuration described above, the image forming apparatus 1 of the present embodiment operates as follows.
When the image forming apparatus 1 repeatedly performs image forming processing, color reproducibility changes due to environmental changes such as temperature and humidity, and changes with time. For this reason, the image forming apparatus 1 appropriately executes toner image density control and calibration, and maintains color reproducibility in a good state.

FIG. 4 is a flowchart of calibration executed in the present embodiment. In the figure, the same reference numerals are assigned to the same steps in order to clarify common points and differences with the general calibration shown in FIG.
When this process is compared with general calibration, there are two differences. That is, the process of step S0 is performed at the start of the process, and the determination process of step S31 is performed instead of step S3 of FIG. 14, and the process of step S32 is performed according to the determination result. Therefore, in the following, these two operations and their effects will be described first, and then the entire calibration operation will be described.

  First, in step S0, the toner image density control unit 190 of the image forming apparatus 1 performs toner image density control. The toner image density control is as already described. By performing toner image density control at the start of calibration execution, the following effects can be obtained.

FIG. 5 is a diagram for explaining the timing at which general toner image density control and calibration are executed. In the drawing, “x” marks indicate timings at which toner image density control is performed, and “◯” marks indicate timings at which calibration is performed. It is assumed that t ₁ << t ₂ .
As described above, generally, toner image density control is automatically executed at the timing when power is supplied to the image forming apparatus, and thereafter periodically executed at predetermined intervals. In general, the calibration is executed when an instruction to perform calibration is given to the image forming apparatus by the operator. Thus, if each process is independent, there exists a possibility that a calibration may not be performed at an appropriate timing.
For example, in FIG. 5, when the calibration is executed at the timing indicated by the point A, since the time t ₁ has elapsed since the toner image density control is performed, the test image to be printed is The toner image density is appropriately controlled. However, when the calibration is executed at the timing indicated by the point B, since a considerable time t ₂ has elapsed since the toner image density control is performed, the toner image density output during this time changes. There is a possibility. Even if calibration is performed in such a state, since the test image to be formed does not have an appropriate toner image density, an image having no good color reproducibility is eventually output.
The process of step S0 is a process performed to solve this, and by performing toner image density control immediately before the formation of the test image by calibration, calibration with better color reproducibility is performed. Making it possible.

  Next, in step S31, when comparing the test image density with the target image density, the maximum density determination unit 185 of the image forming apparatus 1 compares the maximum patch density with the maximum target image density, and the magnitude of these differences is large. Different processing is performed depending on the situation. Specifically, when the maximum density determination unit 185 determines that the absolute value of the difference D between the maximum patch density and the maximum target image density is greater than or equal to the first threshold value Δd1 (step S31; YES), the process proceeds to step S32. When the density correction process is performed and the maximum density determination unit 185 determines that the absolute value of the difference D between the maximum patch density and the maximum target image density is less than Δd1 (step S31; NO), the process of step S4 is performed. . That is, the first threshold Δd1 is a threshold provided for determining whether or not to perform density correction processing.

FIG. 6 is a flowchart for explaining the density correction processing in step S32. Hereinafter, a description will be given with reference to FIG.
This process is executed when the maximum density determination unit 185 determines that the absolute value of the difference D between the maximum patch density and the maximum target image density is equal to or greater than Δd1. First, the maximum density determination unit 185 determines whether or not the absolute value | D | of the above-described difference D is equal to or greater than the second threshold value Δd2 (step S321). Here, Δd2 is a value satisfying Δd1 ≦ Δd2. Then, the absolute value of the difference D | D | is [Delta] d1 ≦ | D | if the value that satisfies ≦ .DELTA.d2 (step S321; NO), the target toner image density correction unit 186 substitutes the difference D to the variable x _D (step S322), the value of the target toner image density stored in the target toner image density storage unit 191 of the toner image density control unit 190, is changed to a value such that the test image density changes by an amount corresponding to the variable x _D, A new target toner image density is stored (step S323). On the other hand, if the value of the absolute value of the difference D satisfies D> .DELTA.d2 (step S321; NO), the target toner image density correction unit 186 variable x _D, the .DELTA.d2 if positive difference D is also the difference D There substituting -Δd2 if it is negative (step S324), such as the value of the target toner image density stored in the target toner image density storage unit 191, the test image density changes by an amount corresponding to the variable x _D The value is changed to a value and stored as a new target toner image density (step S323). That is, the second threshold value Δd2 is a threshold value provided in order to give an upper limit to the correction amount and prevent a large density correction from being performed at once in this density correction process.
The density correction process is thus completed, and the process of step S4 is continued.

  By performing this density correction processing, when the difference between the maximum patch density and the maximum target image density is large, the target toner image density correction unit 186 corrects the target toner image density stored in the target toner image density storage unit 191. can do. The reason why this correction is performed and the effect thereof will be described below.

  In general, even if the toner image density on the intermediate transfer belt 120 is controlled to match the target toner image density, the maximum patch density of the test image output during calibration does not always match the maximum target image density. . For example, if the secondary transfer condition when the toner is transferred from the intermediate transfer belt 120 to the recording paper or the fixing condition in the fixing device 150 changes, the density of the test image on the paper changes.

As an example, let us consider a case where the maximum patch density is lower than the maximum target image density in such a state. If the maximum patch density becomes lighter than the maximum target image density, the target toner image density itself does not change. Therefore, even if the toner image density control unit 190 continues to control the toner image density appropriately, The maximum patch density continues to be lower than the maximum target image density. As a result, even if calibration is executed, the target density cannot be obtained in the high density region as shown in FIG.
However, if the above-described density correction processing is performed, correction is performed to increase the target toner image density itself when the difference between the maximum patch density and the maximum target image density is large, so that the next toner image density control is performed. Makes it possible to darken the test image itself output during calibration.

Furthermore, in addition to the above processing, if the maximum patch density becomes higher than the maximum target image density due to variations in secondary transfer conditions or fixing conditions, correction may be performed to reduce the target toner image density. good. As a result, the test image can be thinned.
As described above, by performing this density correction processing, the maximum patch density can be set to an appropriate density even if the secondary transfer condition or the fixing condition fluctuates.

Further, specific values of Δd1 and Δd2 are preferably about “0.05” to “0.10”. For example, Δd1 is “0.05” and Δd2 is about “0.10”. This is based on the sensitivity to human image density.
Human sensitivity to image density differs between low density areas and high density areas. Although there are individual differences, humans can sense a density difference of about 0.02 in the low density region, but can finally be sensed with a density difference of about 0.05 in the high density region.

Δd1 is “0”, that is, density correction processing may be executed in all cases, but in this embodiment, the target toner image density is changed excessively due to minute repeated variations in the maximum patch density on the recording paper. In order to prevent this from happening, in the case of a density difference that cannot be detected by humans, that is, a density difference that is less than 0.05, the process of step S32 is not executed. On the other hand, if Δd2 is set to a value exceeding “0.10”, a significant level of density change occurs in the output image before and after toner image density control. Therefore, in this embodiment, Δd2 is set to “0.10”. However, if it is desired to reduce the density change before and after toner image density control, Δd2 is set to a smaller value, and “0.05” which is equal to Δd1 at the minimum. It is possible to set a value up to "."
By setting Δd1 and Δd2 to appropriate values as described above, it is possible to control the density of the output image without causing a large density change. In such a method, if the density difference between the maximum patch density and the maximum target image density is large, a desired toner image density cannot be obtained by one toner image density control, but gradually by repeating the toner image density control. It is possible to approach the desired concentration.

Based on the above, the calibration of the present embodiment will be described with reference to FIG.
The image forming apparatus 1 receives a calibration execution instruction from the operator from the operation unit 160 and executes this processing. At this time, in the image forming apparatus 1, each unit operates as follows.
First, the toner image density control unit 190 performs toner image density control, and prepares for the generation of a test image by controlling the toner image density to match the target toner image density (step S0). When this preparation is completed, the test image forming unit 181 subsequently forms a test image having a plurality of density reference patches on the recording paper (step S1). When the test image is formed on the recording paper, the image reading unit 20 reads it (step S2), and outputs the obtained image signal of each density to the maximum density determination unit 185 and the gradation correction curve creation unit 183. Here, the maximum density determination unit 185 compares the maximum patch density with the maximum target image density, and determines whether or not the difference D is equal to or greater than Δd1 (step S31). If this determination is affirmative, the above-described density correction process (see FIG. 6) is performed to correct the target toner image density (step S32). If this determination is negative, this density correction process is omitted. Finally, a gradation correction curve for correcting the input image signal so that the reference patch density matches the target image density is created based on the input image signal by the gradation correction curve creating unit 183 (step S4). .
By operating as described above, the image forming apparatus 1 of the present embodiment can perform calibration without unnecessarily maintaining a high toner density or reducing operability.

(2) Second Embodiment Subsequently, a second embodiment of the present invention will be described.
The reason for performing toner image density control and calibration in the present invention is to correct changes in image density due to environmental changes and changes over time. In the first embodiment described above, the toner image density sensor 192 is used as means for detecting this change in image density. In this embodiment, instead of detecting a change in image density by the toner image density sensor 192, a change in environment and time, which is a direct factor of the change in image density, is detected. The specific configuration is shown below.

  The image forming apparatus 1a of the present embodiment has the same configuration as the image forming apparatus 1 of the first embodiment for the most part. Therefore, below, only a different part from the structure of 1st Embodiment is demonstrated. The same parts as those in the first embodiment are denoted by the same reference numerals.

FIG. 7 is a block diagram showing the configuration of the toner image density control unit 190a of this embodiment. As shown in the figure, the toner image density control unit 190a of this embodiment includes an environment sensor 193 and a cycle counter 194 instead of the toner image density sensor 192 of the first embodiment. The environment sensor 193 includes a temperature sensor and a humidity sensor, and measures the temperature and humidity inside the image forming apparatus 1. The cycle counter 194 measures the number of cycles of the photosensitive drum 111.
The toner image density control unit 190a stores the charging characteristics and photosensitivity characteristics of the photosensitive drum 111 with respect to variations in temperature, humidity, and cycle number. The photosensitive drum 111 is obtained from the environment sensor 193 and the cycle counter 194. An appropriate exposure potential is calculated in the exposure device 113 when the data is in the state of data, and the charging device 112 or the exposure device 113 is controlled. As a result, an appropriate target toner image density can be obtained regardless of variations in temperature, humidity, and cycle number.
Under the above configuration, the toner image density control unit 190a is equivalent in step S323 of FIG. 6, the value of the target toner image density stored in the target toner image density storage unit 191, the test image density in the variable x _D The charging potential of the charging device 112 and the exposure potential of the exposure device 113 are controlled so as to change by the amount to be changed. For example, when the test image density is changed by “0.1”, the exposure potential may be changed by 20V. Other operations are the same as those in the first embodiment.
According to the second embodiment, the same effect as the first embodiment can be obtained.

(3) Third Embodiment Subsequently, a third embodiment of the present invention will be described.
In the above-described second embodiment, a mode in which toner image density control is performed with a configuration different from that of the first embodiment has been described. Similarly, this embodiment is different from the first embodiment only in the toner image density control method. The specific configuration is shown below. Similar to the second embodiment, the image forming apparatus 1b according to the present embodiment has the same configuration as that of the image forming apparatus 1 according to the first embodiment. Therefore, below, only a different part from the structure of 1st Embodiment is demonstrated, and the same code | symbol shall be attached | subjected about the part same as 1st Embodiment.

In the present embodiment, the image forming apparatus 1b includes a toner mixture ratio control unit 190b instead of the toner image density control unit 190 of the first embodiment.
FIG. 8 is a block diagram showing the configuration of the toner mixture ratio control unit 190b of the present embodiment. As shown in the drawing, the toner mixing ratio control unit 190b of the present embodiment includes a toner mixing ratio sensor 192b that measures the mixing ratio of toner and carrier inside the developing device 114, and the toner and carrier inside the developing device 114. A target toner mixture ratio storage unit 191b that stores a target toner mixture ratio that is a control target value of the mixture ratio is provided. The toner mixing ratio control unit 190b compares the toner mixing ratio in the developing device 114 with the target toner mixing ratio. If there is a difference between them, the toner mixing ratio control unit 190b adjusts the toner box 115 so that the toner mixing ratio matches the target toner mixing ratio. The motor drive is controlled so that the measured toner mixture ratio matches the target toner mixture ratio.
Although not particularly illustrated, the image forming apparatus 1 according to the present embodiment includes a target toner mixture ratio correction unit 186b instead of the target toner image density correction unit 186 according to the first embodiment. The target toner mixture ratio correction unit 186 b corrects the target toner mixture ratio stored in the target toner mixture ratio storage unit 191 based on the calculation result of the maximum density determination unit 185. Specifically, the target toner mixture ratio correction unit 186b increases the target toner mixture ratio by “0.5%” with respect to the increment “0.1” of the difference D calculated by the maximum density determination unit 185. Take control.

  FIG. 9 is a flowchart of calibration in the present embodiment. In the flowchart shown in the figure, compared with the flowchart (FIG. 4) of the first embodiment, only the portion where “toner image density control” in step S0 is replaced with “toner mixture ratio control” in step S0a. Different.

By the way, when the image density is controlled by controlling the toner mixture ratio, there is a difference from the first and second embodiments. That is the difference in the responsiveness of the control results.
When images having the same density are formed on the recording paper, the image density on the recording paper does not change immediately after the toner mixing ratio in the developing device 114 is changed. It will gradually change over time. For example, in order to lower the toner mixing ratio, it is necessary to stop the motor of the toner box 115 and wait for the toner mixing ratio to drop naturally due to image formation. Conversely, even when the toner mixing ratio is increased, if the motor of the toner box 115 is suddenly operated, image fogging due to poor stirring of the developer (toner and carrier) may occur. If the target toner mixture ratio correction unit 186b is operated in a state where the desired toner mixture ratio is not reached, the maximum patch density has not reached the desired value, and thus there is a possibility that appropriate correction will not be performed.

  For example, when the calibration process shown in FIG. 4 is performed, the difference D between the maximum patch density and the maximum target image density at the time of the calibration process performed immediately before is “−0.07”, and the target toner Assume a situation where the mixing ratio is corrected (Δd1 = Δd2 = 0.05). At this time, the target toner mixture ratio correction unit 186b performs correction so that the difference D is finally increased by “0.05” to become “−0.02”. However, if the operator then instructs the execution of calibration and the difference D at that time does not reach “−0.02” but is “−0.06”, for example, the target toner mixture ratio correction unit In 186b, correction is performed so that the difference D is further increased by "0.05". As a result, correction is performed so that the difference D is increased by “0.10” from the initial state “−0.07”, and the difference D is increased more than necessary. It becomes.

Therefore, in the calibration process of the present embodiment, a sufficient time has passed since the previous toner mixture ratio control, and the control result by this toner mixture ratio control is appropriately reflected in the output image, and the next toner mixture ratio control is performed. It is necessary to determine whether or not it is in an executable state (hereinafter referred to as “steady state”). This will be specifically described below.
FIG. 10 is a flowchart of the density correction process (step S32) in the present embodiment. As shown in the figure, the density correction process of the present embodiment is a process in which step S320 is added at the start of the density correction process (FIG. 6) of the first embodiment. That is, in step S320, it is determined whether or not the image forming apparatus 1b is in a steady state. If the image forming apparatus 1b is not in a steady state, the process is terminated without correcting the toner mixture ratio. In this way, the toner mixture ratio correction is performed only when the image forming apparatus 1b is in a steady state.
By operating as described above, the image forming apparatus 1 according to the present embodiment unnecessarily maintains a high toner density or reduces operability even when performing a toner image density control with poor responsiveness. Calibration can be performed without any problem.

(4) Fourth Embodiment Next, a fourth embodiment of the present invention will be described.
In the first embodiment described above, the method for controlling the density of an image output at a level that cannot be detected by humans even when the difference between the maximum patch density and the maximum target image density is large has been described. In the present embodiment, the density of the output image is controlled so as to be smoother and less likely to be perceived by humans by improving the calibration method based on the same configuration as the first embodiment.

  FIG. 11 is a flowchart of calibration in the present embodiment. The flowchart shown in the figure is different from the flowchart (FIG. 4) of the first embodiment in that the process of step S33 is performed immediately after the density correction process of step S32. Hereinafter, this process will be described.

In step S33, the gradation correction curve creation unit 183 weakens the correction of the high density region more than the gradation correction curve that should be originally created (that is, the gradation correction curve created by the method of the first embodiment). . Specifically, the gradation correction curve generating unit 183, density correction process of the immediately preceding correction amount of the target toner image density target toner image density correction unit 186 is calculated (Step S32), i.e., the correction indicated by the variable x _D Based on the amount, a gradation correction curve is created in which the correction amount in the high density region is weaker than the gradation correction curve that should be originally created. Specifically, the correction amount is weakened as follows.

FIG. 12 is a diagram exemplifying the gradation correction curve α to be originally created and the gradation correction curve α ′ created by the method of the present embodiment. This figure shows a case where the present embodiment is applied when the reference patch density corresponding to the maximum density is thinner than the target image density as shown in FIG. 17, and the reference patch density corresponding to the maximum density is shown. This is a case where the value is as thin as “1.6” with respect to the maximum target image density value “1.7”.
As shown in the figure, in this embodiment, in the high density region, that is, in the region where the input image signal level is X (%) or more, the gradation correction curve α ′ has a smaller correction amount than the gradation correction curve α. It is getting smaller. That is, when the gradation correction curve α is used, the input image signal is 80% or more and the output image signal level is saturated to 100%, but when the gradation correction curve α ′ is used, the input image signal is 90%. The output image signal level is not saturated to 100% until it reaches%.

In order to create such a gradation correction curve, the gradation correction curve creation unit 183 reduces the target image density value of the high density region among the target image densities stored in the target image density storage unit 182. Make downward corrections. Specifically, the target image density of the input image signal X% or more is lowered as shown in FIG. The target image density reduction amount due to the correction is a value corresponding to the target toner image density correction amount calculated by the target toner image density correction unit 186. For example, when the correction amount of the target toner image density is a value that increases the test image density by “0.05”, the amount of decrease in the target image density due to the correction is the same as the correction amount “ The value is such that the test image density is reduced by 0.05 ".
As a result, the gradation correction curve α ′ created by the gradation correction curve creation unit 183 is saturated with the output signal level at 100% until the input image signal level reaches 90%, as shown in FIG. As a result, the density corrected by the tone correction curve α ′ is reduced in density saturation in the high density portion compared to the example of FIG. 17 and the maximum density that can be reproduced is the same, but the input image signal level is X % To 90% can be corrected without being saturated more smoothly, and it is possible to control the density of the output image so that it is less perceived by humans.

  Further, the target image density value D1 of the high density region that is reduced by the gradation correction curve creation unit 183 is an output image that varies depending on the correction amount of the target toner image density calculated by the target toner image density correction unit 186 in step S32. The density difference D2 or less is required. This is because the correction by the gradation correction curve acts on the image density of the output image so as to cancel the correction of the target toner image density, so that if D1> D2, the effect of the present invention is expected. Because it has the opposite effect. That is, in the above-described example, the amount of decrease in the high density region needs to be a value that makes the test image “0.05” or thinner.

(5) Fifth Embodiment Next, a fifth embodiment of the present invention will be described.
In the fourth embodiment described above, the gradation correction curve generating unit 183, the correction amount of the target toner image density target toner image density correction unit 186 is calculated, i.e. weakening the correction amount in the high concentration region based on the variable x _D A tone correction curve α ′ was created. In the present embodiment, as in the fourth embodiment, a gradation correction curve in which the correction amount in the high density region is weakened is created. However, the reference for determining the amount to weaken the correction amount in the high density region is the target toner image. The difference from the fourth embodiment is that the difference D is calculated by the maximum density determination unit 185, not the density correction amount. Hereinafter, this point will be described in detail.

FIG. 13 is a flowchart of calibration executed in the present embodiment. In the figure, the same reference numerals are given to the steps for performing the same processing as the calibration (FIG. 11) of the fourth embodiment, and the description thereof is omitted.
The calibration of the present embodiment is different from the calibration of the fourth embodiment in that the process of step S32 ′ is performed instead of the process of step S32 in FIG. 11, and the step of step S33 in FIG. The point is that the processing of S33 ′ is performed.

In step S32 ′, the target toner image density correction unit 186 uses the target toner image density value stored in the target toner image density storage unit 191 of the toner image density control unit 190, and the test image density corresponds to the difference D. The value is changed to a value that changes by the amount and stored as a new target toner image density. Comparing this to the process of step S32 (see FIG. 6), in the present embodiment, the absolute value of the difference D | D | magnitude by the branch processing (step S321) is omitted, the difference correction amount from the variable x _D It turns out that it has changed to D. That is, in this embodiment, the target toner image density is corrected without setting an upper limit on the correction amount.
In step S33 ′, the gradation correction curve creating unit 183 has a higher density region than the gradation correction curve that should be originally created (that is, the gradation correction curve created by the method of the first embodiment). Decrease the correction. Specifically, the tone correction curve creation unit 183 corrects a higher density region than the tone correction curve to be originally created based on the correction amount indicated by the difference D calculated by the maximum density determination unit 185. Create a tone correction curve with a reduced amount.

By performing these processes, the correction amount in the high density region of the gradation correction curve increases as the difference D increases. Therefore, the same effects as in the fourth embodiment can be obtained without performing processing for setting an upper limit on the toner image density correction amount as in the fourth embodiment. In other words, by creating a tone correction curve that weakens the correction of the high density area, it is possible to limit the density change in the high density area before and after the correction of the toner image density, so that the toner image as in the fourth embodiment described above. It is not necessary to perform processing for setting an upper limit for the density correction amount. Therefore, since the toner image density can be corrected to the maximum target density at a time, gradation correction processing can be performed in a state where the maximum density matches the target in the next calibration.
By performing the above processing, it becomes possible to control the density of the output image more smoothly and less easily perceived by a person under the same configuration as the first embodiment.

(6) Modifications The present invention is not limited to the above-described embodiment, and various modifications can be made.

(6-1) Modification 1
In the first embodiment described above, the difference from the conventional calibration is (1) the point where toner image density control is performed before test image creation (step S0), and (2) the magnitude of the difference D is determined. It has been described that the density correction process (step S32) is performed according to the result.
However, the present invention can achieve the desired effect without necessarily performing both of the above two processes.
Also in the process (2), the processes in steps S321 and S322 are not essential, and the present invention can achieve the expected effects only by correcting the target toner image density in step S323. is there.

(6-2) Modification 2
Although the above-mentioned 4th Embodiment was demonstrated as an aspect which improved the 1st Embodiment, it is not necessarily applied only to 1st Embodiment, It can also apply in the above-mentioned 2nd, 3rd embodiment. Of course it is possible.

(6-3) Modification 3
In each of the above-described embodiments, the image forming apparatus 1 has been described as a so-called tandem configuration including the intermediate transfer belt 120. However, the present invention is not limited to the image forming apparatus 1 having this configuration, and can be applied to image forming apparatuses having various configurations. For example, an image forming apparatus that uses a so-called multiple cycle system that rotates a plurality of color printer heads, an image forming apparatus that includes an intermediate transfer member other than a belt, and a sheet conveyance belt instead of the intermediate transfer belt 120 are provided. The present invention can also be applied to an image forming apparatus or the like.

(6-4) Modification 4
In each of the above-described embodiments, only the configuration in which the image reading unit 20 having a so-called scanner function is integrated with the image forming unit 10 has been described. However, the application of the present invention is not limited to the print system having such a configuration, and of course can be applied to, for example, an image forming apparatus having a configuration in which the image reading unit 20 is an external device provided as a single scanner. is there.

(6-5) Modification 5
In each of the above-described embodiments, the aspect of correcting the target toner image density and the target toner mixture ratio in order to control the image density has been described. However, the present invention is not limited to such a correction, and for example, a correction may be made for the measured toner image density or toner mixture ratio. For example, the target toner image density is fixed, and is corrected by adding or subtracting a certain value to the measured toner image density, and the corrected toner image density is set as the target toner image density. The density control is performed so as to be close.

1 is a diagram illustrating a configuration of an image forming apparatus according to a first embodiment of the present invention. FIG. 2 is a diagram illustrating details of an image forming unit, a toner image density control unit, and a gradation control unit in the embodiment. It is a block diagram which shows the structure of the gradation control part in the embodiment. It is a flowchart of the calibration performed in the same embodiment. FIG. 6 is a diagram for explaining timings at which general toner image density control and calibration are executed. It is a flowchart for demonstrating the density correction processing in the same embodiment. FIG. 6 is a block diagram illustrating a configuration of a toner image density control unit according to a second embodiment of the present invention. FIG. 10 is a block diagram illustrating a configuration of a toner mixture ratio control unit according to a third embodiment of the present invention. It is a flowchart of the calibration performed in the same embodiment. It is a flowchart of the density correction process in the same embodiment. It is a flowchart of the calibration in 4th Embodiment of this invention. It is the figure which illustrated the gradation correction curve (alpha) 'produced in the same embodiment. It is a flowchart of the calibration in 5th Embodiment of this invention. The flowchart of a general calibration is shown. It is a figure which shows the test image used in general calibration. It is a figure which shows the example of the gradation correction curve produced by performing general calibration. It is a figure which shows the example of the gradation correction curve produced by performing general calibration.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Image forming apparatus, 10 ... Image forming part, 110, 110C, 110M, 110Y, 110K ... Image forming unit, 111 ... Photosensitive drum, 112 ... Charging device, 113 ... Exposure device, 114 ... Developing device, 115 ... Toner Box 120, intermediate transfer belt 121, drive roll 122, driven roll 130, 130C, 130M, 130Y, 130K primary transfer roll 140 secondary transfer roll 150 fixing unit 160 operation unit 170 ... Paper feed tray, 171... Transport roll, 172... Paper discharge tray, 180... Gradation control unit (gradation correction means), 181... Test image forming unit, 182. Creation unit, 184 ... gradation correction curve storage unit, 185 ... maximum density determination unit, 186 ... target toner image density correction unit, 186b ... target ,... Toner image density control unit (toner density control means), 190b... Toner mixture ratio control unit (toner density control means), 191... Target toner image density storage unit, 191b. 192 ... toner image density sensor, 192b ... toner mixture ratio sensor, 193 ... environment sensor, 194 ... cycle counter, 20 ... image reading unit, 210 ... platen glass, 220 ... platen cover, 230 ... lamp, 240 ... sensor, 250, 251 ... Optical system.

Claims (13)

Toner density control means for measuring the density of the toner and controlling the density so that the measured density approaches its target value;
A test image having a plurality of different density areas is formed on a recording medium, the density of each density area of the formed test image is read, the read density is compared with its target value, and the image is based on the comparison result. An image forming apparatus comprising gradation correction means for correcting the level of an output image signal that is output to form the image on a recording medium,
An image forming apparatus comprising: a correction unit that corrects a toner density controlled by the toner density control unit according to a comparison result between an image density in a maximum density region of the test image and a target value thereof.   2. The correction unit according to claim 1, wherein when the image density in the maximum density region of the test image is lower than the target value, the toner density controlled by the toner density control unit is corrected so as to be high. Image forming apparatus. The toner density control means measures the toner density using a sensor for detecting the density of the toner image formed on the image carrier;
The image forming apparatus according to claim 1, wherein the correction unit corrects the toner density controlled by the toner density control unit by correcting the measured toner density or a target value of the density. The toner density control means includes: a charging potential of an image carrier on which a toner image is formed; an exposure potential after the charged image carrier is neutralized by exposure; and the image carrier is exposed. Controlling the density of the toner by controlling at least one of the developing bias potential applied to develop the electrostatic latent image formed by
The image forming apparatus according to claim 1, wherein the correction unit corrects a toner density controlled by the toner density control unit by controlling the charging potential or the exposure potential. The toner density control means uses the sensor for detecting the mixing ratio of toner and carrier in the developing device for developing the electrostatic latent image formed by exposing the image carrier. Measure the mixing ratio of toner and carrier inside
The image forming apparatus according to claim 1, wherein the correcting unit corrects the toner density controlled by the toner density control unit by correcting the measured mixing ratio or a target value of the mixing ratio. The image forming apparatus according to claim 1, wherein the correction unit stores an upper limit value of a correction amount with respect to toner density, and performs the correction with a correction amount equal to or less than the upper limit value. The image forming apparatus according to claim 1, wherein the gradation correction unit weakens a correction amount for a high density region of the image when the correction unit performs the correction. The image forming apparatus according to claim 7, wherein when the correction unit performs the correction, the gradation correction unit weakens the correction amount for the high density region of the image according to the correction amount of the correction unit. The image forming apparatus according to claim 8, wherein an amount of weakening a correction amount for the high density region is equal to or less than a correction amount by the correction unit. The image forming apparatus according to claim 9, wherein the gradation correction unit weakens a correction amount for the high density region of the image according to a comparison result between the image density in the maximum density region of the test image and the target value. The image forming apparatus according to claim 10, wherein an amount of weakening a correction amount for the high density region is equal to or less than a difference between an image density in the maximum density region of the test image and a target value thereof. The image forming apparatus according to claim 1, wherein toner density control by the toner density control unit is executed before correction by the tone correction unit. The image forming apparatus according to claim 12, wherein the correction by the correction unit is executed after a toner density control by the toner density control unit is performed and after a predetermined steady state is reached.

Images