JP7340161B2 - Image forming apparatus, image forming method, and image forming program - Google Patents

Description

The present invention relates to an image forming apparatus, an image forming method, and an image forming program, and particularly relates to calibration processing including gamma correction.

In recent years, there has been an increasing need for image forming apparatuses to print with high image quality in grayscale or color. In order to realize such image reproduction, it is necessary to appropriately perform calibration processing including gamma correction in order to suppress color fluctuations caused by density fluctuations due to changes over time. In the calibration process, since the density characteristics are generally non-linear, it is necessary to generate toner patches of a plurality of densities and measure the densities. On the other hand, in the calibration process, there is a need to reduce power consumption, calibration time, and toner consumption necessary for the calibration process. To solve this problem, Patent Document 1 discloses that an image forming apparatus capable of performing multiple halftone processes prints patch patterns of density levels that have undergone multiple halftone processes on the same sheet of paper, and We are proposing a technology that measures patch patterns of multiple halftone processes through measurement processing. According to this technique, it is possible to accurately perform density correction for a plurality of halftone processes with excellent durability without increasing the time required for density measurement processing. On the other hand, Patent Document 2 discloses a technology that periodically performs halftone density correction on all dither patterns including a main dither pattern and one or more sub-dither patterns, and updates each dither pattern to an optimal value. is proposed. Patent Document 2 further discloses that by determining a relational expression between one or more sub dither patterns with respect to the main dither pattern, under normal conditions, halftone correction is performed using the updated relational expression to reduce the adjustment time. We are also proposing technology to reduce the time required.

Japanese Patent Application Publication No. 2002-374415 Japanese Patent Application Publication No. 2010-274574

However, sufficient consideration has not been given to how to arrange a plurality of patches over the entire range of gradation values in which toner density can be reproduced.

The present invention has been made in view of this situation, and an object of the present invention is to provide a technique for efficiently arranging a plurality of patches for gamma correction.

The image forming apparatus of the present invention includes a photoconductor, an exposure section that exposes the photoconductor to form an electrostatic latent image based on image data, and a developing roller, and the developing roller has a potential of the developing roller. a developing section that uses a potential difference between a bias potential and the electrostatic latent image to adhere toner to the photoconductor; and a developing section that applies the developing bias potential to the developing roller and a current that is supplied to the developing roller. a development bias potential application unit that measures the value; an intermediate transfer belt that transfers the toner from the photoreceptor onto an image forming medium; and a concentration of the toner transferred to the intermediate transfer belt. a gamma correction section that determines an output gradation value according to the input gradation value using the input/output gamma correction value; a toner charge amount estimating unit that estimates a toner charge amount, which is a charge amount of the toner, by performing a calculation of dividing the current value by the toner amount; and a set value of the exposure amount of the exposure unit. a calibration processing section that adjusts an exposure setting value, a charging setting value that is a setting value of the surface potential of the photoreceptor, and a developing bias setting value that is a setting value of the developing bias potential of the developing section; a non-linear model for reproducing a developing process performed using the non-linear model; and using the non-linear model, the toner charge amount, the exposure setting value, the charging setting value, and the developing bias setting value are determined. and a model calculation unit that estimates an estimated density gradation value for an input gradation value based on A non-linear region and a relatively linear region are identified, and the density of the plurality of gamma calibration patches arranged in the relatively non-linear region is determined such that the density of the plurality of gamma correction patches arranged in the relatively linear region is The plurality of gamma calibration patches are set so as to have a density greater than the density of the gamma calibration patches, the plurality of gamma calibration patches are measured by the density sensor, and the input/output gamma is adjusted based on the measured density. Generate correction values.

The image forming method of the present invention uses a photoconductor, an exposure section that exposes the photoconductor to form an electrostatic latent image based on image data, and a development roller, and uses a development bias that is the potential of the development roller. a developing step in which toner is attached to the photoreceptor using a potential difference between a potential and the electrostatic latent image; and a current value supplied to the developing roller while applying the developing bias potential to the developing roller. an intermediate transfer step in which the transferred toner is transferred onto an image forming medium using an intermediate transfer belt to which the toner is transferred from the photoreceptor; and an intermediate transfer step in which the toner is transferred onto the intermediate transfer belt. a gamma correction step of determining an output gradation value according to the input gradation value using an input/output gamma correction value; a toner charge amount estimating step of estimating the amount of toner attached to the toner, and performing a calculation of dividing the current value by the amount of toner to estimate the amount of charge of the toner, which is the amount of charge of the toner; and exposing the exposed portion. a calibration processing step of adjusting an exposure setting value which is a setting value of the amount, a charging setting value which is a setting value of the surface potential of the photoreceptor, and a developing bias setting value which is a setting value of the developing bias potential of the developing step; and a nonlinear model for reproducing the development process executed in the development step, and using the nonlinear model, the toner charge amount, the exposure setting value, the charging setting value, and the development process are determined. and a model calculation step of estimating an estimated density gradation value for the input gradation value based on a bias setting value, and the calibration processing step estimates the estimated density gradation value for the input gradation value. A relatively nonlinear region and a relatively linear region are identified, and the density of a plurality of gamma calibration patches arranged in the relatively nonlinear region is arranged in the relatively linear region. The plurality of gamma calibration patches are set to have a density greater than the density of the plurality of gamma calibration patches, the plurality of gamma calibration patches are measured in the density measurement step, and the density is determined based on the measured density. The input/output gamma correction value is generated.

The present invention provides an image forming program for controlling an image forming apparatus. The image forming apparatus includes a photoconductor, an exposure section that exposes the photoconductor to form an electrostatic latent image based on image data, and a development roller, and has a development bias potential that is the potential of the development roller. a developing unit that applies the developing bias potential to the developing roller and a current value supplied to the developing roller; A developing bias potential application unit for measuring, an intermediate transfer belt for transferring the toner from the photoreceptor onto an image forming medium, and measuring the density of the toner transferred to the intermediate transfer belt. a gamma correction section that determines an output gradation value according to the input gradation value using the input/output gamma correction value; and a gamma correction section that determines the amount of toner attached to the photoreceptor based on the density. a toner charge amount estimating unit that estimates a toner charge amount, which is the charge amount of the toner, by performing calculation to divide the current value by the toner amount; and an exposure unit, which is a set value of the exposure amount of the exposure unit. a calibration processing unit that adjusts a set value, a charging set value that is a set value of the surface potential of the photoreceptor, and a developing bias set value that is a set value of the developing bias potential of the developing unit, and using the developing unit. has a non-linear model for reproducing a development process executed by The image forming apparatus functions as a model calculation unit that estimates an estimated density gradation value for the input gradation value based on the input gradation value, and the calibration processing unit estimates the estimated density gradation value for the input gradation value. A relatively non-linear region and a relatively linear region are identified, and the density of a plurality of gamma calibration patches arranged in the relatively non-linear region is arranged in the relatively linear region. setting the plurality of gamma calibration patches so as to have a density greater than the density of the plurality of gamma calibration patches, measuring the plurality of gamma calibration patches with the density sensor, and based on the measured density; The input/output gamma correction value is generated.

According to the present invention, it is possible to provide a technique for efficiently arranging a plurality of patches for gamma correction.

1 is a block diagram showing a functional configuration of an image forming apparatus 1 according to an embodiment of the present invention. 1 is a cross-sectional view showing the overall configuration of an image forming apparatus 1 according to an embodiment. 1 is a side cross-sectional view showing the structure of a developing section 100 according to an embodiment. FIG. 3 is a conceptual diagram showing how trailing end accumulation occurs in the developing process according to an embodiment. 3 is a flowchart illustrating the content of print proofreading processing according to one embodiment. FIG. 2 is an explanatory diagram showing a light amount calibration patch and a bias voltage setting patch used in the image forming apparatus 1 according to an embodiment. 7 is a flowchart showing the content of dot area ratio adjustment processing according to one embodiment. 7 is a flowchart showing the contents of gamma correction patch setting processing according to one embodiment. FIG. 3 is an explanatory diagram showing the arrangement of gamma correction patches in a development process model according to an embodiment and a comparative example. FIG. 7 is an explanatory diagram showing a gamma correction patch of a comparative example. 7 is a graph showing an arrangement state of gamma correction patches set in a gamma correction patch setting process according to an embodiment. FIG. 2 is an explanatory diagram illustrating an example of a method for determining nonlinearity according to an embodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, modes for carrying out the present invention (hereinafter referred to as "embodiments") will be described with reference to the drawings.

FIG. 1 is a block diagram showing the functional configuration of an image forming apparatus 1 according to an embodiment of the present invention. The image forming apparatus 1 includes a control section 10, an image forming section 20, a storage section 40, an image reading section 50, and a fixing section 80. The image reading unit 50 reads an image from a document and generates an image data ID that is digital data.

The image forming section 20 includes a color conversion processing section 21, a gamma correction section 22, a calibration density sensor 28, an exposure section 29, and photoreceptor drums (image carriers) 30c to 30k that are amorphous silicon photoreceptors. It has developing sections 100c to 100k and charging sections 25c to 25k. The color conversion processing unit 21 converts the image data ID, which is RGB data, into CMYK data. The gamma correction unit 22 performs gamma correction on CMYK data using the input and output gamma correction values. The gamma correction unit 22 also functions as a halftone processing unit, and performs halftone processing on CMYK data to generate print data PD including CMYK halftone data.

The control unit 10 includes main storage means such as RAM and ROM, and control means such as MPU (Micro Processing Unit) and CPU (Central Processing Unit). Further, the control unit 10 has controller functions related to interfaces such as various I/Os, a USB (universal serial bus), a bus, and other hardware, and controls the entire image forming apparatus 1 . The control section 10 includes a model calculation section 11 , a patch setting section 12 , a density characteristic analysis section 13 , and a calibration processing section 14 . The functions of the model calculation section 11, patch setting section 12, density characteristic analysis section 13, and calibration processing section 14 will be described later.

The storage unit 40 is a storage device that is a non-temporary storage medium such as a hard disk drive or a flash memory, and stores control programs (including image forming programs) and data for processing executed by the control unit 10. In this embodiment, the storage unit 40 further includes a calibration data storage area R1, a model data storage area R2, and a calibration data storage area R3.

FIG. 2 is a cross-sectional view showing the overall configuration of the image forming apparatus 1 according to one embodiment. The image forming apparatus 1 of this embodiment is a tandem color printer. In the image forming apparatus 1, photosensitive drums (image carriers) 30m, 30c, 30y, and 30k are arranged in a row in a housing 70, corresponding to each color of magenta, cyan, yellow, and black. Developing sections 100m, 100c, 100y, and 100k are arranged adjacent to the photosensitive drums 30m, 30c, 30y, and 30k, respectively.

The photoreceptor drums 30m, 30c, 30y, and 30k are irradiated (exposed) with laser beams Lm, Lc, Ly, and Lk for each color from the exposure section 29. By this irradiation, electrostatic latent images are formed on the photoreceptor drums 30m, 30c, 30y, and 30k. The developing units 100m, 100c, 100y, and 100k adhere the toner to the electrostatic latent images formed on the surfaces of the photoreceptor drums 30m, 30c, 30y, and 30k while stirring the toner. As a result, the developing process is completed, and toner images of each color are formed on the surfaces of the photoreceptor drums 30c to 30k.

The image forming apparatus 1 includes an endless intermediate transfer belt 27. The intermediate transfer belt 27 is stretched around a tension roller 24, a driving roller 26a, and a driven roller 26b. The intermediate transfer belt 27 is driven in circulation by the rotation of the drive roller 26a.

A cleaning device 200 is disposed at a position upstream of the photosensitive drum 30k and opposite the driven roller 26b with the intermediate transfer belt 27 in between. The cleaning device 200 includes a fur brush 210 in which fine fibers are planted and rotates at high speed. The fur brush 210 can mechanically remove the toner on the intermediate transfer belt 27 using the scraping force of the brush tip. In this way, the image forming apparatus 1 employs a brush cleaning method that uses the fur brush 210 that comes into contact with the intermediate transfer belt 27 to scrape off and discard used toner.

For example, a black toner image on the photoreceptor drum 30k is primarily transferred onto the intermediate transfer belt 27 by sandwiching the intermediate transfer belt 27 between the photoreceptor drum 30k and the primary transfer roller 23k, and driving the intermediate transfer belt 27 in circulation. Ru. This also applies to the three colors cyan, yellow, and magenta.

A full-color toner image is formed on the surface of the intermediate transfer belt 27 by performing primary transfer such that the toner images are superimposed on each other at a predetermined timing. The calibration density sensor 28 is disposed at a position where the density of the toner image after the primary transfer is completed and before the secondary transfer can be measured.

The full-color toner image is then secondarily transferred onto the printing paper P supplied from the paper feed cassette 60 and fixed onto the printing paper P by the fixing roller pair 81 of the fixing section 80 . The cleaning device 200 can also remove residual toner remaining on the intermediate transfer belt 27 from the intermediate transfer belt 27 regarding the proofing patch. Print media is also referred to as imaging media.

FIG. 3 is a side sectional view showing the structure of the developing section 100k according to one embodiment. The developing units 100m, 100c, and 100y have the same configuration as the developing unit 100k, and are also simply referred to as the developing unit 100. The developing section 100 includes two stirring and conveying members 141 and 142, a magnetic roller 143, a developing roller (developer carrier) 144, a developing container 145, a regulating blade 146, and a developing bias potential applying section 150. We are prepared. The developing bias potential applying section 150 is controlled by the control section 10.

The developing container 145 constitutes the outer shell of the developing section 100 . A partition portion 145b is provided at the bottom of the developer container 145. The partition portion 145b partitions the inside of the developer container 145 into a first transport chamber 145a and a second transport chamber 145c. The first transport chamber 145a and the second transport chamber 145c extend in a columnar manner in a direction perpendicular to FIG. 3, and contain a two-component developer (also simply referred to as developer) consisting of a magnetic carrier and a black toner.

The developer container 145 further holds a magnetic roller 143 and a developer roller 144. The developer container 145 is formed with an opening 147 that exposes the developer roller 144 toward the photoreceptor drum 30 (30k).

The two agitation conveyance members 141 and 142 circulate and move the developer inside the first conveyance chamber 145a and the second conveyance chamber 145c, respectively, while stirring the developer. The stirring and conveying member 142 serves as a magnetic brush and supplies positively charged developer to the magnetic roller 143 . The magnetic roller 143 includes a non-magnetic rotating sleeve 143a and a fixed magnet body 143b fixed inside the rotating sleeve 143a. The magnetic roller 143 and the developing roller 144 face each other with a predetermined clearance. The regulating blade 146 adjusts the magnetic brush to a predetermined height.

The developing roller 144 includes a rotatable non-magnetic developing sleeve 144a and a developing roller side magnetic pole 144b fixed inside the developing sleeve 144a. A magnetic roller potential Vmag is applied to the magnetic roller 143 . A developing bias potential Vslv is applied to the developing roller 144 by a developing bias potential applying section 150. The developing bias potential application unit 150 includes an AC application unit (also called an AC application unit) 151 that applies an AC bias potential, and a DC application unit (DC application unit) that is connected in series to the AC application unit 151 and applies a DC bias potential. ) 152. The developing bias potential applying section 150 can measure the current value I flowing into the developing roller 144.

In this embodiment, the surface potential of the photoreceptor drum 30 is set to a charging set value DV (20 V), which is a set value of the surface potential, and a developing electric field is formed between the photosensitive drum 30 and the developing roller 144 . On the other hand, an alternating bias in which a DC potential of 20 to 80 V as a developing bias potential Vslv and a sine wave potential with a peak-to-peak value of 2000 V at a frequency of 2 kHz are superimposed is applied to the developing roller 144. A DC potential of 200 V is applied to the magnetic roller 143 as the magnetic roller potential Vmag during development, and a DC potential of −200 V is applied to the magnetic roller 143 during non-development.

As a result, during development, the time in which the developing bias potential Vslv<magnetic roller potential Vmag (the potential state in which toner is supplied to the developing roller 144) becomes longer, and the time in which toner is supplied to the developing roller 144 becomes longer. During non-development, the time for development bias potential Vslv>magnetic roller potential Vmag (potential state in which toner is collected from the developing roller 144) becomes longer, and the time for collecting toner from the developing roller 144 becomes longer.

Further, by adjusting the magnetic roller potential Vmag applied to the magnetic roller 143 during development and during non-development, the toner layer forming potential difference ΔV during development between the development bias potential Vslv and the magnetic roller potential Vmag can be changed. I can do it. As a result, the developing roller 144 has a toner thin layer (simply a toner layer) having a thickness D (see FIG. 4(a) described later) according to the toner layer forming potential difference ΔV between the developing bias potential Vslv and the magnetic roller potential Vmag. ) is formed.

The developing roller 144 forms a toner image on the surface of the photoreceptor drum 30 by attaching toner to the photoreceptor drum 30 through an opposing portion (development nip) having a predetermined clearance between the developing roller 144 and the photoreceptor drum 30 . . The toner image is formed based on the potential difference between the potential of the electrostatic latent image on the surface of the photosensitive drum 30 and the developing bias potential Vslv applied to the developing roller 144.

An amorphous silicon photoreceptor has a relative dielectric constant that is about three times higher than an organic photoconductor (OPC), and is characterized in that the amount of toner that the photoreceptor can hold is large relative to the development contrast potential. Therefore, the amorphous silicon photoreceptor can hold more toner than the normally used solid density. Therefore, if an amorphous silicon photoreceptor is used in a saturated state, it will retain more than the amount necessary for solid density. Therefore, in this embodiment, the amorphous silicon photoreceptor is used in an unsaturated state even in solid density, and when almost all of the toner formed on the developing roller 144 is developed on the photoreceptor and development is completed, the solid density is reduced. is used to determine.

FIG. 4 is a conceptual diagram showing how trailing end accumulation occurs in the developing process according to one embodiment. FIG. 4(a) shows how an image is formed at the leading edge and center of the image. FIG. 4B shows how an image is formed at the rear end of the image. In this specification, the front end, the center, and the rear end are defined as the front end, the center, and the rear end in order from the traveling direction of the photoreceptor drum 30, based on the traveling direction of the photoreceptor drum 30.

In this embodiment, as shown in FIG. 4A, the photosensitive drum 30 receives toner from the developing sleeve 144a of the developing roller 144 while neutralizing the potential of the latent image. At this time, the developing process is configured to be completed when the thin toner layer formed on the developing sleeve 144a is consumed in a non-saturated state, rather than when the potential is saturated. The thickness D of the toner thin layer is set to have a thickness T1 to achieve the maximum density during solid development in image formation.

As shown in FIG. 4B, the developing sleeve 144a has a circumferential speed of Vs, and is configured to form an image while passing the photosensitive drum 30, which has a circumferential speed of Vd. Therefore, when developing a solid image, there is a surface of the developing sleeve 144a on which toner is not consumed in the vicinity of the rear end of the solid image. This surface on which toner has not been consumed will overtake the rear end of the solid latent image on the amorphous silicon photoreceptor 30.

At this time, since the photoreceptor drum 30 as an amorphous silicon photoreceptor is in an unsaturated state, toner is further developed from the surface of the developing sleeve 144a where toner is not consumed. As a result of this development, a trailing end puddle (thickness T2) becomes apparent as a solid density higher than the density expected in advance.

FIG. 4C shows, as an example, the state of toner adhesion (stacked state) during image formation of the solid image TP. In the solid image TP, the toner layer is raised at the rear end portion during image formation. This bulge in the toner layer is called a trailing edge pool.

Such a problem of trailing edge accumulation can be suppressed by expressing solidity with a half patch, which is a patch with a reduced dot area ratio. In this example, it is assumed that the image forming apparatus 1 expresses a solid image using a half patch with a dot area ratio of 80% to 90%. In this embodiment, the solid density by half patch is calibrated by adjusting the development bias potential Vslv, which is the potential applied to the development sections 100m, 100c, 100y, and 100k, and the dot area ratio.

On the other hand, when an organic photoreceptor is used for the photoreceptor drum 30, the development process may be completed by neutralization of the latent image electric field. Specifically, for example, in the simplest case, the development ends due to a decrease in the developing electric field, when the latent image electric field on the photoreceptor 30 is neutralized by an electric field generated by toner particles attached to the photoreceptor 30. It is.

FIG. 5 is a flowchart showing the content of print proofing processing (step S100) according to one embodiment. FIG. 6 is an explanatory diagram showing a light amount calibration patch and a bias voltage setting patch used in the image forming apparatus 1 according to an embodiment. The light amount calibration patch is a patch used in the light amount calibration process (step S110). The bias voltage setting patch is a patch used in the development bias calibration process (step S120). In this example, half patch P80 is used as the patch for development bias correction processing.

In step S110, the calibration processing unit 14 executes a light amount calibration process. In the light intensity calibration process, the calibration processing unit 14 exposes the exposure unit 29 with laser light of a plurality of light intensities that are changed stepwise in a plurality of preset stages to create a chart having a plurality of light intensity calibration patches PL. It is formed on the intermediate transfer belt 27. Image data representing the light amount calibration patch PL is stored in the calibration data storage area R1 of the storage unit 40.

The calibration processing unit 14 exposes the photoreceptor drums 30c to 30k to a plurality of laser beams of different light intensities so as to form light intensity calibration patches PL. In this example, the light amount calibration patch PL has a dot area ratio of 25%, which is an appropriate dot area ratio for light amount calibration.

On the other hand, in the development process, the development bias potential application unit 150 measures a patch formation current value I, which is a current value flowing into the development roller 144 when forming the light amount calibration patch PL. The current value I during patch formation is the value of the current that flows into the developing roller 144 in response to the charge that moves from the developing roller 144 to the photoreceptor drum 30 together with the charged toner particles. Since the patch forming current value I increases as the charge amount increases, the charge amount of the toner particles can be estimated based on the patch forming current value I per amount of toner adhered to the photoreceptor drum 30. The amount of toner adhering to the photosensitive drum 30 can be calculated from the toner density on the intermediate transfer belt 27, for example.

The calibration processing unit 14 uses the calibration density sensor 28 to measure the density of each color (for example, black (K)) patch. The calibration processing unit 14 uses the reflected light of the plurality of light intensity calibration patches PL exposed with a plurality of laser beams of different light intensities to perform, for example, interpolation calculation to obtain a preset target density. An exposure setting value E is set as a calibration light amount (exposure amount setting value) which is a light amount. The calibration processing unit 14 stores the calibration light amount in the calibration data storage area R3 of the storage unit 40 together with the current value I at the time of patch formation.

In this embodiment, the calibration concentration sensor 28 emits infrared light from, for example, an LED (not shown), passes through a polarizing filter that transmits only P waves, and irradiates the patch with the P waves of the infrared light. , the density is detected based on the ratio of P waves and S waves of the reflected light detected by the light receiving element. Note that the calibration density sensor 28 may be of a specular reflection type that detects specularly reflected light from a patch or a diffuse reflection type that detects diffusely reflected light from a patch.

In step S120, the calibration processing unit 14 executes development bias calibration processing. In the development bias calibration process, the calibration processing unit 14 uses the half patch P80 and transfers a chart having a plurality of patch images in which the development bias potential Vslv is changed stepwise at a plurality of preset stages to the intermediate transfer belt 27. to form. The half patch P80 is a patch having a dot area ratio of 80%, which is preset as a result of a trade-off between the occurrence of jaggies and the occurrence of trailing edge pooling due to a decrease in the dot area ratio, for example.

The reason for using a plurality of patch images in which the developing bias potential Vslv is changed stepwise is that the toner image is formed by changing the potential of the electrostatic latent image on the surface of the photoreceptor drum 30 and the developing bias potential Vslv applied to the developing roller 144. This is because it is formed based on a potential difference. A plurality of half-patches are formed for each of CMYK. In the following, a cyan (C) half patch will be explained as an example.

Specifically, the calibration processing unit 14 selects a patch whose development bias potential Vslv has reached a preset solid image target density from among a plurality of cyan (C) half patches whose development bias potential Vslv is changed in stages. If the patch exists, it is executed by selecting the development bias potential Vslv corresponding to that patch. That is, if there is a half patch in which the ratio of P waves and S waves is equal to or less than a preset threshold value, the calibration processing unit 14 sets the lowest development bias potential Vslv among the half patches to the calibrated development voltage. Adjust to the developing bias setting value BV, which is the bias potential. On the other hand, the developing bias potential applying unit 150 measures the current value I flowing into the developing roller 144 when forming a plurality of half patches.

In step S130, if the calibration of the half patch is possible within the adjustment range of the development bias potential, the calibration processing unit 14 advances the process to step S150, and if the calibration of the half patch is possible within the adjustment range of the development bias potential Vslv. If it is not possible, the process advances to step S140.

If half patch calibration is not possible within the development bias adjustment range, it means that there is no patch among the multiple cyan (C) patches that has reached the preset solid image target density. ing. Normally, one of the plurality of half patches reaches the solid image target density, but the solid image target density may not be reached in a state where the amount of toner charge increases due to environmental changes, for example.

FIG. 7 is a flowchart showing the contents of the dot area ratio adjustment process (step S140) according to one embodiment. In step S141, the calibration processing unit 14 sets the development bias potential Vslv to the maximum value, and starts a process of adjusting and calibrating the dot area ratio. The maximum value of the developing bias potential Vslv is also called a potential limit value, and is set in advance, for example, from the viewpoint of the output limit of the developing bias and adverse effects on images (fogging, etc.).

In step S142, the proofreading processing unit 14 forms a chart on the intermediate transfer belt 27 having a plurality of half patches in which the dot area ratio is changed in stages. In this example, the dot area ratio is changed in stages (for example, in three stages of 83%, 85%, and 87%) within the range of 81% to 89%.

In step S143, the calibration processing unit 14 measures the density of the cyan (C) patch using the calibration density sensor 28. The calibration concentration sensor 28 measures the ratio of P waves and S waves. Similar processing is performed for MYK.

In step S144, the calibration processing unit 14 executes dot area ratio setting processing. Specifically, when there is a half patch in which the ratio of P waves to S waves is less than or equal to a preset threshold, the calibration processing unit 14 corrects the half patch (for example, 83%, 85%, 87%). The dot area ratio of the half patch with the lowest dot area ratio among the three stages) is acquired as setting data.

In this example, it is assumed that the adjusted dot area ratio is half patch P83 (dot area ratio 83%) and the ratio of P waves to S waves is equal to or less than a preset threshold. The calibration processing unit 14 updates the data with the adjusted dot area ratio (83%) and the maximum value of the development bias potential Vslv, and stores it in the calibration data storage area R3 of the storage unit 40.

In step S150, the calibration processing unit 14 executes gamma calibration patch setting processing. In the gamma calibration patch setting process, the calibration processing unit 14 increases the arrangement density of gamma calibration patches in tone areas with high non-linearity, and decreases the arrangement density of gamma correction patches in tone areas with high linearity. Place the gamma calibration patch so that it is low.

FIG. 8 is a flowchart showing the contents of the gamma correction patch setting process (step S150) according to one embodiment. FIG. 9 is an explanatory diagram showing the arrangement of gamma correction patches in a development process model according to an embodiment and a comparative example. FIG. 9(a) shows the model calculation unit 11 according to one embodiment. The gamma correction patch setting process is a process of estimating a rough gamma characteristic of toner density using a development process model, and setting an appropriate gamma correction patch to correct the gamma characteristic.

In step S151, the calibration processing section 14 functions as a toner charge amount estimation section and executes toner charge amount estimation processing. In the toner charge amount estimation process, the calibration processing unit 14 estimates the toner charge amount I/M using the current value I at the time of patch formation, which is the current value at the time of patch formation, and the toner adhesion amount M of the patch. The calibration processing unit 14 calculates the toner charge amount I/M by executing calculations including dividing the current value I at the time of patch formation by the toner adhesion amount M.

The current value I during patch formation is the current value that flows into the developing roller 144 when forming the light amount calibration patch PL and a plurality of half patches. The toner adhesion amount M is the toner adhesion amount of each of the light amount calibration patch PL and the plurality of halves. The toner adhesion amount M is estimated by converting the density measurement value of the calibration density sensor 28. The conversion is performed using, for example, a conversion table (not shown) or a conversion formula stored in the calibration data storage area R1.

In step S152, the calibration processing unit 14 executes a setting value input process. In the setting value input process, the calibration processing unit 14 inputs the toner charge amount I/M, the charging setting value DV, the developing bias setting value BV, and the exposure setting value E as setting values to the model calculation unit 11. . The model calculation section 11 is a calculation section that has a nonlinear model in which a nonlinear development process is modeled using, for example, a lookup table (not shown) or an approximation formula. The lookup table is stored in the model data storage area R2. Since the variation in the amount of toner charge is the main cause of the density variation, the model calculation unit 11 can perform the calculation in consideration of the main cause of the density variation.

Note that when an amorphous silicon photoreceptor is used, if half patch calibration is not possible within the adjustment range of the developing bias, the calibration processing unit 14 performs the adjustment after adjustment (step S140) in the setting value input process. The dot area ratio may also be included. This makes it possible to more accurately reproduce the density characteristics of the image forming section 20 that uses an amorphous silicon photoreceptor in a state where half patch calibration is not possible within the adjustment range of the developing bias.

In step S153, the model calculation unit 11 executes model calculation processing. In the model calculation process, the model calculation unit 11 uses a lookup table to input 0 to 255 based on the toner charge amount I/M, charge setting value DV, development bias setting value BV, and exposure setting value E. Outputs the estimated density for the gradation value. Specifically, the model calculation unit 11 outputs estimated density gradation values 0 to 255 for input gradation values 0 to 255, respectively.

FIG. 10 is an explanatory diagram showing a gamma correction patch of a comparative example. The gamma calibration patches of the comparative example are patch P20 with a dot area ratio of 20%, patch P40 with a dot area ratio of 40%, patch P60 with a dot area ratio of 60%, patch P80 with a dot area ratio of 80%, and patch P80 with a dot area ratio of 100%. It has a solid patch P100. In this example, in order to make the explanation easier to understand, it is assumed that an organic photoreceptor is used for the photoreceptor drum 30, so a patch P100 with a dot area ratio of 100% is adopted.

FIG. 9B shows the arrangement of gamma correction patches in a comparative example. In the comparative example, the gamma correction patches are evenly arranged in the gradation range of 0 to 255. As a result, the gamma correction patch of the comparative example can perform gamma correction correction in the gradation range of 0 to 255 for any characteristic, regardless of the shape of the density characteristic curve of the image forming unit 20. be.

However, in the gamma correction patch of the comparative example, the gamma correction patch is not arranged in the nonlinear region. On the other hand, in the gamma correction patch of the comparative example, the patch P60 and the patch P80 are almost unnecessary in the linear region where the dot area ratio is 40% to 100%. This is because the gradation region with a dot area ratio of 40% to 100% is linear, so it can be estimated if there are patches P40 and P100.

FIG. 11 is a graph showing the arrangement of gamma correction patches set in the gamma correction patch setting process according to an embodiment. FIG. 11A shows a density characteristic A (predicted) in which a nonlinear region exists in the intermediate gradation region. In the density characteristic A (prediction), patches P2 to P4 are arranged in a concentrated manner with high density in the nonlinear region, while the linear region is sandwiched between patch P4 and patch P5. FIG. 11(b) shows the density characteristic B (prediction) in which a nonlinear region exists in the shadow region. In density characteristic B (prediction), patches P2a to P5a are arranged in a concentrated manner with high density in the nonlinear region.

In this way, the gamma correction patch setting process according to the embodiment can realize effective gamma correction by arranging gamma correction patches at high density in the nonlinear gradation area. Such gamma correction patch setting processing is realized as follows.

In step S154, the concentration characteristic analysis unit 13 executes concentration estimated value analysis processing. In the estimated density value analysis process, the density characteristic analysis unit 13 can easily identify relatively nonlinear gradation areas and relatively linear gradation areas using an algorithm with low calculation cost.

FIG. 12 is an explanatory diagram illustrating an example of a method for determining nonlinearity according to an embodiment. FIG. 12(a) shows tone value changes in a typical linear region. The input gradation values G1 to G3 indicate three consecutive input gradation values in the highlight region in the density characteristic A, for example, input gradation values 31, 32, and 33. The model calculation unit 11 calculates density 31 (density D1), density 32 (density D2) and Density 33 (density D3) is output.

The density characteristic analysis unit 13 sets the input gradation value G2 in the density characteristic A as an analysis target gradation value to be analyzed for nonlinearity. The density characteristic analysis unit 13 calculates the difference between each estimated density gradation value estimated by the model calculation unit 11 with respect to the input gradation values G1 and G3 before and after the gradation value to be analyzed, that is, the difference between the density D2 and the density D1. A tone difference ΔD1 and a tone difference ΔD2 between the density D2 and the density D3 are calculated. Next, the density characteristic analysis unit 13 obtains a linearity evaluation value that is the ratio of the tone difference ΔD2 to the tone difference ΔD1 (tone difference ΔD2/tone difference ΔD1). However, if the ratio exceeds 1, it will be the reciprocal.

The nonlinearity evaluation value is an evaluation value for determining that the gradation value has higher nonlinearity as the value moves away from "1" and approaches 0. In this example, the gradation difference ΔD2 is approximately equal to the gradation difference ΔD1, and the nonlinearity evaluation value is approximately "1", so it can be determined that the input gradation value G2 exists in the linear gradation region.

FIG. 12(b) shows tone value changes in a typical nonlinear region. The input gradation values G1a to G3a indicate three consecutive input gradation values in the intermediate gradation region in the density characteristic A, for example, input gradation values 121, 122, and 123. The model calculation unit 11 calculates density 121 (density D1a), density 122 (density D2a) and Density 122 (density D3a) is output.

The density characteristic analysis unit 13 sets the input gradation value G2a in the density characteristic A as a gradation value to be analyzed for nonlinearity. The density characteristic analysis unit 13 calculates a gradation difference ΔD1a between the densities D2a and D1a and a gradation difference ΔD2a between the densities D2a and D3a, and calculates the ratio of the gradation difference ΔD2a to the gradation difference ΔD1a (the gradation difference ΔD2a /gradation difference ΔD1a). In this example, the tone difference ΔD2a is significantly smaller than the tone difference ΔD1a, and the nonlinearity evaluation value is “0.5”, which is significantly smaller than “1”. Therefore, the density characteristic analysis unit 13 can determine that the input gradation value G2a exists in the nonlinear gradation region.

In step S155, the patch setting unit 12 of the control unit 10 executes patch placement setting processing. In the patch placement setting process, the patch setting unit 12 classifies the gradation area into a linear area and a non-linear area, deletes some of the gamma correction patches in the linear area, and adds gamma correction patches to the non-linear area. can do. Furthermore, if the proportion of nonlinear regions in all gradation regions is high, the patch setting unit 12 can increase the number of gamma correction patches to achieve accurate gamma correction. On the other hand, when the proportion of the nonlinear region in the total gradation region is small, the patch setting unit 12 can reduce the number of gamma correction patches to reduce the power consumption and the amount of toner to a necessary and sufficient amount.

However, the patch arrangement is not limited only to nonlinearity, and may be performed by taking into account the amount of deviation. Specifically, the patch setting unit 12 sets the density of the gamma calibration patch in the gradation region where the amount of deviation from the target value is large in density characteristics to the density of the gamma calibration patch in the gradation area where the amount of deviation from the target value is small. It may be set larger than the density, or may be set in combination with the nonlinearity evaluation value.

In step S160 (FIG. 5), the calibration processing unit 14 executes gamma calibration processing. In the gamma calibration process, the calibration processing unit 14 uses patches for gamma calibration that are appropriately arranged based on the nonlinearity of the density characteristics (for example, density characteristics A and B) estimated by the model calculation unit 11. , gamma calibration can be performed to achieve reproducibility in which the density characteristics are sufficiently close to the target characteristics.

The calibration processing unit 14 uses the calibration concentration sensor 28 to measure the concentration for gamma calibration and obtains a concentration measurement value. The calibration processing unit 14 generates an input/output gamma correction value representing the relationship between the input tone value and the output tone value based on the density measurement value, and stores it in the calibration data storage area R3. Thereby, the calibration processing section 14 can realize appropriate gamma calibration for the image forming section 20 having various density characteristics.

In this way, the image forming apparatus 1 according to one embodiment can efficiently arrange a plurality of gamma correction patches in consideration of the nonlinearity of density characteristics in the development process. This is because in a nonlinear region with nonlinear characteristics, the accuracy improves as the number of measurement points by the gamma calibration patch increases, whereas in a linear region with linear characteristics, even if the measurement points are thinned out, the effect on accuracy is limited. . Thereby, the image forming apparatus 1 can effectively realize gamma correction with high accuracy while reducing power consumption and toner consumption necessary for the calibration process.

The present invention can be implemented not only in the above embodiment but also in the following modifications.

Modification 1: The present invention is applicable to an image forming apparatus having a plurality of dither patterns and an image forming apparatus capable of performing halftone processing using error diffusion. The model calculation unit can also generate a table modeled as a nonlinear model using dither data that defines a dither pattern. This is because the dither pattern allows the positions of dots to be formed depending on the input gradation values to be specified in advance.

Modification 2: In the above embodiment, an amorphous silicon photoreceptor is used, but the present invention is not limited to image forming apparatuses that use a photoreceptor that reproduces solid density with a non-saturated photoreceptor. . As described above, the present invention is also applicable to image forming apparatuses that use organic photoreceptors, for example.

Modification 3: In the above embodiment, the development bias calibration process is executed using a plurality of patch images in which the development bias potential Vslv is changed in stages. The toner charge amount I/M may be estimated when forming the toner charge amount I/M, and the developing bias potential Vslv may be set based on the estimated value. This is because fluctuations in toner charge amount are the main cause of density fluctuations.

Alternatively, the approximate developing bias potential can be estimated based on the toner charge amount I/M, and the range of developing bias potential can be limited when forming patches for calibrating the developing bias potential to reduce the number of patches. good. Note that reducing the number of patches has a broad meaning and includes reducing the number of patches to zero, that is, setting the developing bias potential Vslv without forming any patches.

1 Image forming apparatus 10 Control section 11 Model calculation section 12 Patch setting section 13 Density characteristic analysis section 14 Calibration processing section 20 Image forming section 21 Color conversion processing section 28 Density sensor for calibration 29 Exposure section 40 Storage section 50 Image reading section 60 Supply Paper cassette 70 housing

Claims (9)

It has a photoreceptor, an exposure section that exposes the photoreceptor to form an electrostatic latent image based on image data, and a developing roller, and has a developing bias potential that is the potential of the developing roller and the electrostatic latent image. a developing section that attaches toner to the photoreceptor using a potential difference between the
a development bias potential application unit that applies the development bias potential to the development roller and measures a current value supplied to the development roller;
an intermediate transfer belt that transfers the toner from the photoreceptor and transfers the transferred toner onto an image forming medium;
a density sensor that measures the density of the toner transferred to the intermediate transfer belt;
a gamma correction unit that uses the input and output gamma correction values to determine an output gradation value according to the input gradation value;
Estimating the amount of toner charge that is the amount of charge of the toner by estimating the amount of toner attached to the photoreceptor based on the density and performing a calculation of dividing the current value by the amount of toner. Department and
An exposure setting value that is a setting value of the exposure amount of the exposure section, a charging setting value that is a setting value of the surface potential of the photoreceptor, and a developing bias setting value that is a setting value of the development bias potential of the developing section. A calibration processing section to adjust,
It has a nonlinear model for reproducing a development process performed using the development section, and uses the nonlinear model to determine the toner charge amount, the exposure setting value, the charging setting value, and the a model calculation unit that estimates an estimated density gradation value for the input gradation value based on the development bias setting value;
Equipped with
The calibration processing unit identifies an area where the estimated density gradation value is relatively non-linear with respect to the input gradation value and an area where it is relatively linear, and the calibration processing unit specifies an area where the estimated density gradation value is relatively non-linear with respect to the input gradation value, and specifies an area where the estimated density gradation value is relatively linear. The plurality of gamma correction patches are set such that the density of the plurality of gamma correction patches arranged in the relatively linear region is greater than the density of the plurality of gamma correction patches arranged in the relatively linear region, and An image forming apparatus that measures a gamma correction patch with the density sensor and generates the input/output gamma correction value based on the measured density. The image forming apparatus according to claim 1,
In the image forming apparatus, the model calculation unit has a nonlinear model that models the development process using a lookup table. The image forming apparatus according to claim 2,
The model calculation unit is an image forming apparatus that generates the lookup table using dither data that defines a dither pattern. The image forming apparatus according to any one of claims 1 to 3,
The gamma correction unit is configured to calculate an estimated density gradation value estimated by the model calculation unit for an analysis target gradation value, which is an input gradation value to be analyzed for nonlinearity, and an estimated density gradation value before and after the analysis target gradation value. The ratio of the difference between the input gradation value and the estimated density gradation value estimated by the model calculation unit is calculated, and if the ratio exceeds 1, the nonlinearity evaluation value is determined by using the reciprocal. and determines that the nonlinearity evaluation value is determined to have higher nonlinearity as the nonlinearity evaluation value moves away from 1 and approaches 0. The image forming apparatus according to any one of claims 1 to 4,
The photoreceptor is configured to achieve a predetermined target density that is determined to be a solid image at a predetermined dot area ratio in a non-saturated state,
The developing section further includes a magnetic roller, and forms a toner layer on the developing roller with a thickness corresponding to a toner layer forming potential difference between the magnetic roller and the developing roller,
The calibration processing section sets the development bias setting value within a range of a preset potential limit value, and if it is determined that the predetermined target density cannot be achieved within the range of the potential limit value, setting the development bias setting value to the potential limit value and adjusting the dot area ratio to achieve the predetermined target density;
The model calculation unit calculates an estimated density gradation value for the input gradation value based on the toner charge amount, the exposure setting value, the charging setting value, the developing bias setting value, and the dot area ratio. Image forming device that calculates. The image forming apparatus according to claim 5,
The calibration processing section exposes the exposure section with laser light of a plurality of light intensities that are changed stepwise in a plurality of preset stages to form a plurality of light intensity calibration patches,
The density sensor measures the density of the plurality of light amount calibration patches transferred to the intermediate transfer belt,
The development bias potential application unit measures a current value supplied to the development roller when forming the plurality of light amount calibration patches,
The toner charge amount estimating unit estimates the amount of toner attached to the photoreceptor based on the density, performs calculation to divide the current value by the amount of toner, and estimates the amount of charge of the toner;
The image forming apparatus is configured such that the calibration processing unit estimates the developing bias potential based on the amount of charge of the toner, and reduces the number of patches for calibrating the developing bias potential using the estimated developing bias potential. . The image forming apparatus according to claim 5 or 6,
In the image forming apparatus, the photoreceptor is an amorphous silicon photoreceptor. Using a photoreceptor, an exposure section that exposes the photoreceptor to form an electrostatic latent image based on image data, and a developing roller, a developing bias potential that is the potential of the developing roller and the electrostatic latent image are a developing step of attaching toner to the photoreceptor using a potential difference between;
a developing bias potential applying step of applying the developing bias potential to the developing roller and measuring a current value supplied to the developing roller;
an intermediate transfer step of transferring the transferred toner onto an image forming medium using an intermediate transfer belt to which the toner is transferred from the photoreceptor;
a density measurement step of measuring the density of the toner transferred to the intermediate transfer belt;
a gamma correction step of determining an output gradation value according to the input gradation value using the input/output gamma correction value;
Estimating the amount of toner charge that is the amount of charge of the toner by estimating the amount of toner attached to the photoreceptor based on the density and performing a calculation of dividing the current value by the amount of toner. process and
An exposure setting value that is a setting value of the exposure amount of the exposure section, a charging setting value that is a setting value of the surface potential of the photoreceptor, and a developing bias setting value that is a setting value of the developing bias potential of the developing step. A calibration process to adjust;
It has a nonlinear model for reproducing the development process executed in the development step, and uses the nonlinear model to determine the toner charge amount, the exposure setting value, the charging setting value, and the development bias setting. a model calculation step of estimating an estimated density gradation value for the input gradation value based on the input gradation value;
Equipped with
The calibration processing step specifies a region where the estimated density gradation value is relatively non-linear and a region where it is relatively linear with respect to the input gradation value, and the estimated density gradation value is arranged in the relatively non-linear region. The plurality of gamma correction patches are set such that the density of the plurality of gamma correction patches arranged in the relatively linear region is greater than the density of the plurality of gamma correction patches arranged in the relatively linear region, and An image forming apparatus that measures a gamma correction patch in the density measurement step and generates the input/output gamma correction value based on the measured density. An image forming program for controlling an image forming apparatus,
The image forming apparatus includes a photoconductor, an exposure section that exposes the photoconductor to form an electrostatic latent image based on image data, and a development roller, and has a development bias potential that is the potential of the development roller. a developing unit that applies the developing bias potential to the developing roller and a current value supplied to the developing roller; A developing bias potential application unit for measuring, an intermediate transfer belt for transferring the toner from the photoreceptor onto an image forming medium, and measuring the density of the toner transferred to the intermediate transfer belt. It has a concentration sensor that
a gamma correction unit that uses the input and output gamma correction values to determine an output tone value according to the input tone value;
Estimating the amount of toner charge that is the amount of charge of the toner by estimating the amount of toner attached to the photoreceptor based on the density and performing a calculation of dividing the current value by the amount of toner. Department,
An exposure setting value that is a setting value of the exposure amount of the exposure section, a charging setting value that is a setting value of the surface potential of the photoreceptor, and a developing bias setting value that is a setting value of the development bias potential of the developing section. a calibration processing unit for adjusting, and a nonlinear model for reproducing a development process performed using the development unit, and using the nonlinear model to calculate the toner charge amount and the exposure setting value; causing the image forming apparatus to function as a model calculation unit that estimates an estimated density gradation value for an input gradation value based on the charging setting value and the developing bias setting value;
The calibration processing unit identifies an area where the estimated density gradation value is relatively non-linear with respect to the input gradation value and an area where it is relatively linear, and the calibration processing unit specifies an area where the estimated density gradation value is relatively non-linear with respect to the input gradation value, and specifies an area where the estimated density gradation value is relatively linear. The plurality of gamma correction patches are set such that the density of the plurality of gamma correction patches arranged in the relatively linear region is greater than the density of the plurality of gamma correction patches arranged in the relatively linear region, and An image forming program that measures a gamma correction patch with the density sensor and generates the input/output gamma correction value based on the measured density.

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