JP2008242083A - Image density correction method and image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image density correction method and an image forming apparatus with which wasteful toner consumption due to image density correction is suppressed. <P>SOLUTION: Based on a counted value by a performable counter set at present, whether to attain the number of sheets or a counted value at which process control should be started is decided. When this is attained, the process control is to be performed. When the process control is performed, a set value of the process control is stored in a process control set bias storage memory. Next, the present value set this time is compared with the previous value stored, in the process control set bias storage memory so as to calculate a difference value between them. By referring to a table of difference value/performable number of sheets, the performable number of sheets of the process control next time (performance timing) is calculated, based on the difference value, and stored in a process control performable counter as the next counter value (next value). <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a copying machine, a printer, a facsimile apparatus using an electrophotographic system, and an image density correction method and an image forming apparatus for an image output by a composite machine combining them.

  Conventionally, in an electrophotographic image forming apparatus such as a copying machine, a printer, or a facsimile machine, gradation reproducibility that faithfully reproduces the gradation from the highlight to the shadow of the formed image is required. It is necessary to output so that the image density is kept constant. However, the formed image density varies due to changes in the setting conditions of the image forming process due to changes in the toner density of the developer, changes over time, environmental changes, and the like.

  Therefore, as a means for keeping the image density constant, a method of performing image density correction (solid density correction) processing at a predetermined timing is employed. This image density correction process is indispensable for accurately performing halftone correction (γ correction) that keeps the gradation of the original and the image constant, and is performed as follows.

  First, a plurality of reference pattern latent images with different exposure intensities corresponding to predetermined different densities are formed on the photosensitive member at predetermined intervals in the paper transport direction. Next, these reference pattern latent images are developed in a state where the linear velocity of the developing roller is constant. Then, the toner density of each developed reference pattern image (patch) is detected, and an image density correction curve is created based on the detected toner density data.

  The reference pattern is composed of about three patches having different densities, and the size of each patch is formed to a certain size in consideration of apparatus variations and the like.

  However, in such a conventional reference pattern, density unevenness may occur in the patch due to fluctuations in the apparatus. Therefore, density measurement measures a certain region in the patch. For this reason, there are problems that it takes time to measure the density and the toner consumption is also large.

  Furthermore, since these reference patterns require at least about three toner patches for one correction, there is a problem in that the toner consumption inevitably increases when the number of density corrections is increased.

  In many cases, such image density correction is performed at a predetermined timing, and unnecessary image density correction is performed in a state where the apparatus is stable and the image density is stable. In some cases, wasteful toner may be consumed.

  Therefore, in the image processing apparatus described in Patent Document 1, it is determined whether or not to execute image density correction in accordance with the environmental condition change and the number of image formations. We are trying to reduce consumption.

  In the image forming apparatus described in Patent Document 2, whether or not image density correction is performed is determined based on the period during which the power is off, the number of rotations of the developing sleeve, the amount of toner used, the printing rate, and the like.

JP 2002-225349 A JP 2001-296704 A

  In order to keep the image density constant, it is naturally better to increase the number of times of image density correction. However, since a toner image is formed every time image density correction is performed, the amount of toner consumed is inevitably increased.

  In the image processing apparatus described in Patent Document 1, toner consumption is suppressed by determining execution of image density correction based on the degree of change in output of the humidity sensor as an environmental condition. Since the change in density is not always related, the image density can be kept constant when correction is necessary, for example, when there is almost no humidity change and the density change has become large due to other factors. Absent.

  In the case of the image forming apparatus described in Patent Document 2, the presence / absence of a correction operation is determined based on the printing rate or the like. However, as in the above case, it is useless unless the relationship between the printing rate and the image density is constant. There is a possibility of density correction.

  Furthermore, in the above two inventions, since it is merely determined whether or not image density correction is performed, if the image density correction is not performed once, the period until the next correction is performed becomes longer. It becomes difficult to keep the image density constant.

  An object of the present invention is to provide an image density correction method and an image forming apparatus capable of suppressing wasteful toner consumption due to image density correction.

  An image density correction method according to the present invention for solving the above-described problems and an image forming apparatus that implements the method are as follows.

In the present invention, the image carrier is uniformly charged and then exposed to form an electrostatic latent image, and a developing device attaches toner to the electrostatic latent image on the image carrier and develops it. An image density correction method for an image forming apparatus,
An electrostatic latent image of a reference pattern is formed on the image carrier with toner, the reference pattern is developed by the developing device, the toner adhesion amount of the developed reference pattern is detected, and the toner adhesion amount detection result Based on the image density correction,
In this image density correction method, the timing for performing the next image density correction is determined based on the difference in correction amount between the previous correction result and the current correction result.

  In the invention, it is preferable that the reference pattern is a reference pattern visualized with black toner, and the reflection sensor is a regular reflection sensor that detects regular reflection.

According to the present invention, the image bearing member is uniformly charged and then exposed to form an electrostatic latent image. The developing device attaches toner to the electrostatic latent image on the image bearing member and develops the latent image. An image density correction method for an image forming apparatus to be imaged,
An electrostatic latent image of a reference pattern is formed on the image carrier with toner, the reference pattern is developed by the developing device, the toner adhesion amount of the developed reference pattern is detected, and the toner adhesion amount detection result Based on the image density correction,
An approximate expression is created and stored in advance from a density sensor output value based on a plurality of reference patterns and a development bias value, and at the next image density correction, one reference pattern and the approximate expression obtained earlier are used. An image density correction method characterized by determining a correction amount.

  In the present invention, one reference pattern used at the next image density correction uses a reference pattern indicating a value other than the maximum value and the minimum value among the density sensor output values of the reference pattern used at the previous image density correction. It is characterized by that.

  Further, the present invention is characterized in that the next image density correction timing is determined based on a correction amount difference between the previous correction result and the current correction result.

  According to another aspect of the invention, there is provided an image forming apparatus that executes the above-described image density correction method.

  According to the present invention, an electrostatic latent image of a reference pattern is formed on the image carrier with a toner, the reference pattern is developed by a developing device, the toner adhesion amount of the developed reference pattern is detected, and the toner adhesion amount Based on the detection result, the image density correction is performed. At this time, the timing for performing the next image density correction is determined based on the difference in correction amount between the previous correction result and the current correction result.

  The next correction execution timing is determined by the correction result, so that the state of the device is stable, and when the difference in correction amount is small, the period until the next correction execution can be lengthened. Toner consumption can be suppressed.

  In addition, according to the present invention, it is possible to detect a reference pattern visualized with black toner by using a regular reflection sensor that detects regular reflection.

  Accordingly, even when a plurality of printing process speeds, for example, a printing speed for monochrome image output and a printing speed for color image output are different, image density correction can be performed according to the printing process speed.

  Further, according to the present invention, an electrostatic latent image of a reference pattern is formed on the image carrier with toner, the reference pattern is developed by a developing device, the amount of toner attached to the developed reference pattern is detected, and the toner adhesion Image density correction is performed based on the amount detection result. At this time, an approximate expression is created and stored in advance from the density sensor output values based on a plurality of reference patterns and the development bias value, and one reference pattern and the previously obtained approximation are obtained at the next image density correction. The correction amount is determined by the equation.

  As a result, the number of reference patterns used for image density correction can be reduced, and wasteful toner consumption due to image density correction can be suppressed. Also, the time required for correction can be shortened.

  Further, according to the present invention, a reference pattern indicating a value other than the maximum value and the minimum value among the density sensor output values of the reference pattern used at the previous image density correction is set as one reference pattern used at the next image density correction. To do.

By using a median value other than the maximum and minimum values, there is less fluctuation and control is stabilized.
According to the present invention, since the next correction execution timing is determined based on the correction result, when the apparatus state is stable and the difference in correction amount is small, the period until the next correction can be extended. Further, wasteful toner consumption due to image density correction can be further suppressed.

  According to another aspect of the invention, there is provided an image forming apparatus that executes the above-described image density correction method.

  Accordingly, it is possible to provide an image forming apparatus that performs image density correction with reduced toner consumption other than image formation.

Embodiments of the present invention will be described below with reference to the drawings.
FIG. 1 is a schematic diagram showing the configuration of an image forming unit 10 of a digital color copying machine which is an image forming apparatus in which the image correction method of the present invention is executed.

  It should be noted that the present invention can be similarly applied to other image forming apparatuses such as printers and facsimile apparatuses that perform electrophotographic image formation other than digital color copying machines.

  The digital color copying machine reads a color image from an original in a scanner unit, performs predetermined image processing, supplies it to the image forming unit 10 as image data, and reproduces the color image read from the original on a recording medium such as paper. To do.

  The image forming unit 10 of the digital color copying machine includes a transfer conveyance belt (intermediate transfer member) 17 that is stretched in a state where a horizontal portion is formed between two rollers 17a and 17b and rotates in the direction of arrow A. ing. The transfer conveyance belt 17 conveys the sheet placed on the upper surface while sequentially facing the plurality of image forming stations 10a to 10d by rotation in the arrow A direction while being positioned in the upper horizontal portion. Each of the image forming stations 10a to 10d performs electrophotographic image formation using toner of three primary colors (cyan, magenta, and yellow) of black and subtractive color mixture.

  The transfer conveyance belt 17 faces the density detection sensor (reflection sensor) 1 while being positioned in the lower horizontal portion. Further, a reference position mark 17d that can be detected by the density detection sensor 1 is formed at one portion on a part of the toner image transfer surface 17c on which the toner image is formed on the transfer conveyance belt 17.

  Further, a fixing device 18 is disposed on the downstream side of one roller 17 a of the transfer conveyance belt 17. The fixing device 18 includes a pair of rollers, and heats and pressurizes the paper that has passed through each of the image forming stations 10a to 10d, and melts and fixes the toner image transferred on the paper to the surface of the paper.

  Each of the image forming stations 10a to 10d has the same configuration except for the toner storage amount. As an example, the image forming station 10a includes a charger 12a, an exposure unit 13a, a developing unit 14a, a transfer unit around a photosensitive drum 11a that rotates in the direction of arrow B by forming a photosensitive layer on the surface of a cylindrical conductive substrate. The container 15a and the cleaner 16a are arranged in this order.

The charger 12a uniformly charges the surface of the photosensitive drum 11a with a predetermined polarity.
The exposure unit 13a exposes the surface of the photosensitive drum 11a with image light to form an electrostatic latent image.

  The developing unit 14a supplies the toner stored therein to the surface of the photosensitive drum 11a, and visualizes the electrostatic latent image into a toner image.

  The transfer unit 15 a faces the peripheral surface of the photosensitive drum 11 a with the transfer conveyance belt 17 interposed therebetween, and a toner image carried on the surface of the photosensitive drum 11 a is transferred to a sheet of paper placed on the transfer conveyance belt 17. Transfer to the surface.

  The cleaner 16 removes the toner remaining on the peripheral surface of the photosensitive drum 11a after the transfer process.

  The developing unit 14a includes a developing roller (not shown) that rotates to face the peripheral surface of the photosensitive drum 11a. The developing roller supplies toner carried on the surface thereof by rotation to the peripheral surface of the photosensitive drum 11a. By changing the peripheral speed of the developing roller, that is, the rotational speed, the amount of toner supplied to the peripheral surface of the photosensitive drum 11a can be increased or decreased, and the density of the toner image can be adjusted.

  Each of the exposure units 12a to 12d provided in the image forming stations 10a to 10d receives black, cyan, magenta and yellow image data, and each of the developing units 14a to 14d has black, cyan. , Magenta and yellow toners are stored. Accordingly, in each of the image forming stations 10a to 10d, black, cyan, magenta, and yellow toner images are sequentially transferred onto the paper, and full color is obtained by subtractive color mixing of the toner images of each color on the paper that has passed through the fixing device 18. An image is formed.

  The density detection sensor 1 includes a light emitting element 2 and a light receiving element 3, and a toner image transfer surface of a transfer conveyance belt 17 on which a correction test pattern (reference pattern) is formed with black toner formed during image correction processing described later. The surface of 17c is irradiated with light from the light emitting element 2, and the reflected light that is specularly reflected is received by the light receiving element 3, and an electrical signal corresponding to the amount of received light is output as a toner density detection signal.

  The correction test pattern formed on the surface of the transfer conveyance belt 17 is removed from the surface of the transfer conveyance belt 17 by a cleaning unit (not shown) after facing the density detection sensor 1.

  In each of the image forming stations 10a to 10d, the density detection sensor 1 is disposed at a position facing the surface of the photosensitive drum 11 after the development process is completed, and the correction test pattern before being transferred to the transfer conveyance belt 17 is displayed. The present invention can be similarly implemented when detecting the concentration.

  In this embodiment, the density detection sensor 1 receives (detects) reflected light in a state where no toner image is formed on the toner image transfer surface 17c of the transfer conveyance belt 17, and the detected toner image transfer surface 17c. Is stored in a predetermined storage area as background data.

  Next, the image processing unit of the digital color copying machine according to the present embodiment will be described with reference to the drawings.

  FIG. 2 is a block diagram showing a configuration of the image processing unit 20 of the digital color copying machine according to the present embodiment.

  The image processing unit 20 of the digital color copying machine includes an image data input unit 40, an image data processing unit 41, an image data output unit 42, a gradation correction unit 46, a density recognition unit 47, a memory 49, and a CPU 44.

  The image data input unit 40 converts the read signals for the three additive primary colors (RGB system) read from the color image of the document by the scanner unit into digital data.

  The image data processing unit 41 generates subtractive mixed three primary colors and black (YMCK) image data from RGB image data, and performs zoom processing and the like according to the set copy magnification.

The gradation correction unit 46 performs gradation correction processing described later on the YMCK image data.
The image data output unit 42 outputs drive data generated based on the YMCK image data subjected to the gradation correction processing to the exposure units 13a to 13d.

  The memory 49 stores data for forming test patterns (correction test patterns (reference patterns) and preliminary test patterns described later) on the surface of the transfer conveyance belt 17 during image correction processing described later. This data is supplied to the image data output unit 42 by the CPU 44 during the image forming process. The density recognition unit 47 recognizes the density signal output from the density detection sensor 1.

  The operation of each unit in the image processing unit 20 is controlled by the CPU 44 in an integrated manner. The CPU 44 controls the operation of the photosensitive drum 11 and the like in the image forming unit 10 in synchronization with the operation of the image data output unit 42. Further, the CPU 44 optimizes the correction conditions in the gradation correction unit 46 and the process conditions in the image forming unit 10 based on the density signal of the test pattern for correction recognized by the density recognition unit 47 during the image correction process. .

  With the above configuration, when the copy image or the correction test pattern is formed in the image forming unit 10, in order to reproduce the density of the image according to the image data, the circumference of the photosensitive drum 11 is passed through the exposure unit 13. It is necessary to form an electrostatic latent image that reproduces the density of image data on the surface. As the method, there are a pulse width modulation (PWM) method, a power modulation method, an area gradation method (dithering), and the like.

  In the pulse width modulation method, the on / off time (pulse width) of the laser beam emitted from the exposure unit 13 is controlled according to the image density.

  In the power modulation method, the intensity of the laser beam emitted from the exposure unit 13 is controlled in accordance with the image density. The area gradation method is a method in which black and white is generated based on a certain rule according to the gradation of pixels of the original image, and halftones are expressed by the appearance frequency of black and white.

  In the image data input unit 40, image density correction processing and gradation correction processing are sequentially performed on each of the YMCK image data during the image correction processing.

Hereinafter, each correction process will be described.
Here, as shown in FIG. 2, data input from the image data input unit 40 to the gradation correction unit 46 via the image data processing unit 41 is image input data, and from the gradation correction unit 46 to the image data output unit. The data output to 42 is image output data, and when the correction test pattern is formed, the data read from the memory 49 by the CPU 44 and supplied to the image data output unit 42 is recognized by the test pattern data and density recognition unit 47. The obtained data is referred to as detection data.

  First, a reference example of image density correction processing in the digital color copying machine according to the present embodiment will be described.

  The image density correction process is performed in order to suppress fluctuations in the overall density of the image that is the target of the image forming process. In the image density correction process, the test pattern data for forming one test pattern (preliminary test pattern) whose gradation changes continuously among the test pattern data stored in the memory 49 is displayed by the CPU 44. It is read out and supplied to the image data output unit 42. As a result, a latent image test pattern that changes continuously from high density to low density is formed on the surface of each of the photosensitive drums 11a to 11d.

  In this way, for each latent image test pattern formed for each of the photosensitive drums 11a to 11d, the CPU 44 rotates the developing rollers in the developing units 14a to 14d at different rotational speeds to visualize the toner images. To do. Therefore, the latent image test patterns formed on the surfaces of the photosensitive drums 11a to 11d under the same exposure conditions are developed with different toner densities.

  The test pattern toner images formed on the surfaces of the photosensitive drums 11a to 11d are transferred to the surface of the transfer and transport belt 17 by the transfer units 15a to 15d, and then the toner density is detected by the density detection sensor 1 to recognize the density. The detected toner density is recognized by the unit 47.

  The CPU 44 compares the toner density target value for the high density test pattern stored in advance in the memory 49 with the toner density recognized by the density recognition unit 47 from the actually formed correction test pattern. The developing condition (rotating speed of the developing roller) of the correction test pattern in which the toner density closest to the value is detected is set as the developing condition in the subsequent image forming process.

  Here, image density correction using the correction test pattern in the present embodiment will be described with reference to the drawings.

  FIG. 3 is an explanatory diagram showing a correction test pattern formed in the image forming unit 10 of the digital color copying machine according to the present embodiment, and FIG. 4 is a sensor of the density detection sensor 1 that detects the correction test pattern. It is a graph which shows the relationship between an output and developing bias potential.

  As an example, first, the laser PWM duty is fixed to 100%, the grid voltage (Vg) is fixed to −600 V, and the development bias (Vb) is switched to −275 V, −325 V, and −375 V, and three corrections using black toner are performed. Test patterns TP1, TP2, and TP3 are created.

  Then, the reflected light (regularly reflected light) of the respective correction test patterns TP1, TP2, and TP3 is read by the density detection sensor 1, and based on the amount of reflected light detected by the density detection sensor 1, as shown in FIG. The sensor outputs I1, 12, and 13 are plotted on a graph showing the relationship between the sensor output (Is) and the developing bias potential (Vb).

  Then, a linear approximation expression (corresponding to the straight line L1) showing the relationship between the sensor output of the density detection sensor 1 and the developing bias potential is derived, and based on this straight line L1, the development that can obtain the sensor output I0 that becomes the reference toner density The bias Vb0 is obtained. Then, the developing bias is set to Vb0 (for example, −310 V), and the developing bias correction is completed.

  Here, when the potential difference between the development potential difference (cleaning field) and the grid electrode bias Vg and the development bias Vb0 for preventing toner fogging on the white background is smaller than 150V, and Vg−Vd0> −150V, Vg = Vd0−150V. To complete the image density correction.

  In this way, when the next image density correction is performed, three correction test patterns with the development biases Vb0 + 50V, Vb0, and Vb0-50V are created based on the development bias Vb0 obtained immediately before. The development bias correction is performed in the same manner as described above.

  Next, gradation correction processing in the digital color copying machine according to the present embodiment will be described.

  The gradation correction process is performed in order to faithfully reproduce the gradation of the original image in the copy image by suppressing the variation in gradation of the toner image.

  In the gradation correction process, the CPU 44 reads out data for forming a plurality of test patterns (correction test patterns) having different gradations from the test pattern data stored in the memory 49, and the image data output unit 42. To supply. As a result, electrostatic latent images of a correction test pattern in which the PWM duty of the laser diode is switched are formed on the photosensitive drums 11a to 11d.

  The CPU 44 develops the electrostatic latent images of the correction test patterns formed on the photoconductor drums 11a to 11d in this manner under the preset development conditions (rotation speed of the development roller).

  The toner image of the test pattern for correction formed on each of the photosensitive drums 11a to 11d is transferred to the surface of the transfer conveyance belt 17 by the transfer units 15a to 15d, and then the toner by the density detection sensor 1 and the density recognition unit 47. Receives concentration detection and recognition.

  The CPU 44 compares the target density of the gradation test pattern stored in advance in the memory 49 with the toner density recognized by the density recognition unit 47 from the correction test pattern actually formed, and based on this comparison result. To create a tone correction table.

  Here, the gradation correction table is a reference for performing appropriate gradation correction in the gradation correction unit 46 on the image input data, and the image input data and the image output data are one-to-one. It corresponds to.

  In the present embodiment, in the gradation correction table (number of gradations-laser PWM duty table) A, the horizontal axis represents the density of image input data (input gradation data), and the vertical axis represents the density of image output data (specifically, Is represented as a curve of the graph as (laser PWM duty of the exposure unit). The tone correction table A is stored in the memory 49 as a look-up table, and is corrected renewably in, for example, halftone process control. The halftone process control is a well-known technique (see, for example, Japanese Patent Application Laid-Open No. 2001-309178), and thus detailed description thereof is omitted here.

  Here, gradation correction using the correction test pattern in the present embodiment will be described with reference to the drawings.

  FIG. 5 is an explanatory diagram showing a correction test pattern formed by the image forming unit 10 according to the present embodiment, and FIG. 6 shows a relationship between the number of gradations of the correction test pattern and the laser PWM duty. It is a graph.

  As an example, referring to a gradation correction table (number of gradations-laser PWM duty table) A shown in FIG. 6, as shown in FIG. 5, for example, with black toner changing in 16 steps from gradation numbers D1 to D16. Correction test patterns TP101 to TP116 are created.

  Then, the density detection sensor 1 reads the reflected light (regular reflection light) of the respective correction test patterns TP101 to TP116, and sensor outputs I101, I102,... Based on the amount of reflected light detected by the density detection sensor 1. , I116, and the respective toner concentrations (toner adhesion amount) are measured.

  Then, by connecting the 16 measurement results based on the correction test patterns TP101 to TP116 with lines, a gradation correction table (number of gradations-laser PWM duty table) B is obtained as shown in FIG. When the next gradation correction process is performed, the gradation correction table B obtained this time is used as the next correction table A.

  Next, image correction in the digital color copying machine according to the present embodiment will be described with reference to a flowchart.

  FIG. 7 is a flowchart showing image correction processing in the digital color copying machine according to the embodiment.

  The process control in the following description means the image density correction in the present embodiment described with reference to FIGS. 3 and 4 and the gradation correction table update process described with reference to FIGS. It is preferable to update the gradation correction table after confirming the image density value. Further, the update of the image density value or the gradation correction table may be omitted depending on the process control repetition interval.

  In the flowchart of FIG. 7, process control is abbreviated as “procone”.

  In this embodiment, the printing process speed is a high speed VH of 350 mm / sec during monochrome printing, and a medium speed VM of 225 mm / sec during full color printing.

  In the image correction at the time of image output in this embodiment, when the power is turned on (step S1), the process control at the medium speed VM is executed at the printing process speed (step S2), and the process control at the high speed VH is executed. (Step S3), a difference table between the image correction tables at high speed and medium speed is created (Step S4).

  If there is a print request from an external device or user, image output processing is performed (step S5). If there is no print request, the process returns to step S2.

  In step S5, image output processing has been performed, and whether the total number of prints has exceeded 100 sheets (step S6), whether the temperature change since the end of the previous printing has exceeded 5 ° C. (step S7), or the previous time It is determined whether or not the humidity change from the end of printing has exceeded 5% (step S8), and if each exceeds a predetermined numerical value, the process proceeds to step S9 to confirm the print request.

  On the other hand, if each of the steps S6, S7, S8 has not reached the predetermined numerical value, the process returns to step S2.

  In step S9, when there is a print request at the high-speed VH repeatedly at the print process speed, the process control at the high-speed VH is executed (step S10), and the print request is processed based on the created image correction table (step S11). Then, an image correction table for the medium speed VM is created based on the image correction table for the high speed VH (step S12).

  On the other hand, if the print process speed is not a print request at the high speed VH in step S9, the process control at the medium speed VM is performed (step S13), and the print request is processed based on the created image correction table (step S14). ). Then, an image correction table at high speed VH is created based on the image correction table at medium speed VM (step S15).

  The image correction table TBM at the medium speed and the image correction table TBH at the high speed are obtained at the relative process speeds based on the conversion table TBO obtained from the image correction table TBM at the medium speed VM and the image correction table TBH at the high speed VH. Created based on the image correction table.

(Example)
In the digital electrophotographic apparatus described above, an image correction method for determining the execution timing of the next correction operation based on the difference between the correction amounts of the previous and current correction results of the image correction will be described with reference to the drawings.

  As a condition for executing image density correction (hereinafter abbreviated as “procon”), there is a method that is executed when a certain number of sheets or a counter value is reached. . In this case, as shown in the flowchart of FIG. 8, first, based on the counter value of the execution counter, it is determined whether or not the number of sheets for starting the process control or the count has been reached (step S21). Is executed (step S22). Otherwise, the process control is not performed. When the process control is executed, the execution counter is reset (step S23).

  In this case, the timing at which the process control is performed is an equal interval or a fixed interval regardless of the result of the process control.

  The present invention is characterized in that the interval of execution of the process control is determined by calculating from the previous value and the current value of the process control.

  As shown in the flowchart of FIG. 9, first, based on the counter value (current value) of the currently set execution counter, it is determined whether or not the number of sheets to start the process control or the count has been reached (step S31). If it has reached, the process control is executed (step S32), and if not, the process control is not performed. When the process control is executed, the process control setting value is stored in the process control setting bias storage memory.

  Next, the set current value is compared with the previous value stored in the program control setting bias storage memory, and the difference value is calculated (step S33).

  With reference to the difference value / execution number table, the next process control execution number (execution timing) is calculated based on the difference value, and stored in the process control execution counter as the next counter value (next value).

Therefore, it is determined with reference to the next process control execution counter whether or not the process control start number has been reached.
An example of the difference value / execution number table is shown in Table 1.

By referring to this table, the next execution number can be determined from the difference value.
Assuming the difference value / execution number table shown in Table 1, specific operations will be described with reference to Table 2.

  First, as shown in Table 2, if the bias value of the first process control result is 380 V, the second process control is executed with the value of the first process control result as the center value (procompact development bias (M)). Suppose that the result is 390V.

Here, the difference between the previous value of Procon and the current value of Procon is
｜ 390V-380V | = 10V
Therefore, using the difference value / execution sheet number table of Table 1, the next process computer execution sheet number can be determined to be 1400 sheets.

  Accordingly, if the value of the next process control execution number is stored in the next process control execution counter, as shown in Table 2, the next process control will be executed after 1400 sheets from the second time, that is, from the first time. The cumulative number of sheets is executed after 2400 sheets.

  Similarly, since the execution timing of the fourth and subsequent procons is determined by the difference from the value of the previous procon, if the procon difference value is large as in the third and fourth times in Table 2 (Table 60V in the example of 2), the next time the 5th procon is performed after 1000 sheets in a short time, and when the procon difference value is small as in the 5th and 6th (0V in the example of Table 2), The next 7th Procon will be held after a certain period of time after 1500 sheets.

  Of course, since the difference value / execution number table as shown in Table 1 can be set freely, for example, when the difference value is equal to or less than a predetermined value, it is possible to speed up the execution of the next process control. Then, instead of the number of sheets, the time until the next program control can be used as a parameter.

  In this way, if the execution timing of the process control can be determined from the fluctuation of the execution result of the process control, the process control execution interval can be increased if the device state is stable, and if not, the process control execution interval can be decreased. This makes it possible to suppress useless toner consumption in unnecessary execution of a process control.

  In addition, by controlling the reflection sensor, which is the reference pattern reading means, as a regular reflection sensor that detects regular reflection with respect to the black toner, the control of the execution timing of the process control as shown in the embodiment can be performed with respect to the black toner. It can be carried out.

Another embodiment of the present invention will be described.
A density sensor output value of a plurality of reference patterns is obtained by a process control using three types of toner patterns (hereinafter referred to as a 3-patch process control), and the relationship between the development bias value and the sensor output value is approximated from the result. Created and stored, and at the next density correction execution, the density correction amount is determined based on one reference pattern other than the maximum and minimum values in the previous reference pattern and the approximate expression obtained previously (hereinafter referred to as the density correction amount) 1 patch program).

  First, a plurality of reference patterns are generated by adding a potential difference to the developing bias potential, such as VL, VM, and VH, as in a normal three-patch process control. At this time, voltage differences between the three patches are equally spaced. That is, | VL−VM | = | VM−VH |.

  The values obtained by reading this reference pattern with the density detection sensor 1 are SL (VL patch), SM (VM patch), and SH (VH patch), respectively.

  From these three points, the relationship between the development bias potential and the sensor output, an approximate expression is obtained and stored as shown in FIG.

  The operation of creating the approximate expression can use the above-described operation of the 3-patch process control as it is.

At this time, the developing bias potential value obtained by the process control operation is VP.
Next, as shown in FIG. 11, using the previous process control setting developing bias potential value VP (or VM, which is the center value of the developing pattern bias potential values for generating the reference pattern), the toner patch at the developing bias potential is used. And the patch is read by the density detection sensor 1 to obtain the output value SP.

  The sensor output value SP and the approximate expression obtained above are shifted as shown in FIG. 11 to obtain a developing bias value V1 that becomes the target value S1, and this is used as a new process control setting value V1. .

  In this way, the number of toner patches in the process control (image density correction) can be reduced from 3 to 1, and toner consumption due to execution of the process control can be suppressed.

  Further, it is preferable to use one reference pattern other than the maximum value and the minimum value of the reference pattern used last time as one reference pattern used when executing the process control.

  Further, by combining the process control operation with one patch according to the present embodiment and the embodiment in which the next process control execution timing is determined based on the difference value between the previous process control result value and the current process control result value, the toner is further increased. It is possible to reduce the number of patch generations and the number of executions of process control.

1 is a schematic diagram illustrating a configuration of an image forming unit 10 of a digital color copier that is an embodiment of the present invention. 2 is a block diagram illustrating a configuration of an image processing unit 20 of a digital color copying machine. FIG. FIG. 3 is an explanatory diagram showing a correction test pattern formed by an image forming unit of a digital color copying machine. It is a graph which shows the relationship between the sensor output of the density | concentration detection sensor which detected the test pattern for correction | amendment, and developing bias potential. It is explanatory drawing which shows the test pattern for correction | amendment formed in the image formation part which concerns on this embodiment. It is a graph which shows the relationship between the gradation number of a test pattern for correction | amendment, and a laser PWM duty. 3 is a flowchart showing image correction processing in a digital color copying machine. It is a flowchart for determining a process control execution timing. It is a flowchart for determining the next process control execution timing based on the previous process value and the current process value. It is a graph which shows the relationship between a sensor output and development bias potential. It is a graph which shows the relationship between a sensor output and development bias potential.

Explanation of symbols

1 Concentration detection sensor (reflection sensor)
2 Light emitting element 3 Light receiving element 10 Image forming unit 10a to 10d Image forming station 11, 11a to 11d Photosensitive drum (image carrier)
14a Development unit (developing device)
17 transfer conveyance belt 17c toner image transfer surface 17d reference position mark 40 image data input unit 41 image data processing unit 42 image data output unit 44 density recognition unit 46 gradation correction unit 47 density recognition unit 49 memory

Claims (6)

The image carrier is uniformly charged and then exposed to form an electrostatic latent image. The image is formed by developing and developing the electrostatic latent image on the image carrier with toner attached to the image carrier. An image density correction method for an apparatus,
An electrostatic latent image of a reference pattern is formed on the image carrier with toner, the reference pattern is developed by the developing device, the toner adhesion amount of the developed reference pattern is detected, and the toner adhesion amount detection result Based on the image density correction,
An image density correction method characterized in that a next image density correction timing is determined based on a difference in correction amount between a previous correction result and a current correction result.   The image density correction method according to claim 1, wherein the reference pattern is a reference pattern visualized with black toner, and the reflection sensor is a regular reflection sensor that detects regular reflection. The image carrier is uniformly charged and then exposed to form an electrostatic latent image. The image is formed by developing and developing the electrostatic latent image on the image carrier with toner attached to the image carrier. An image density correction method for an apparatus,
An electrostatic latent image of a reference pattern is formed on the image carrier with toner, the reference pattern is developed by the developing device, the toner adhesion amount of the developed reference pattern is detected, and the toner adhesion amount detection result Based on the image density correction,
An approximate expression is created and stored in advance from a density sensor output value based on a plurality of reference patterns and a development bias value, and at the next image density correction, one reference pattern and the approximate expression obtained earlier are used. An image density correction method characterized by determining a correction amount.   One reference pattern used at the next image density correction is used as a reference pattern indicating a value other than the maximum value and the minimum value among the density sensor output values of the reference pattern used at the previous image density correction. The image density correction method according to claim 3.   4. The image density correction method according to claim 3, wherein the next image density correction timing is determined based on a difference in correction amount between the previous correction result and the current correction result.   An image forming apparatus that executes the image density correction method according to claim 1.

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