JP2010107856A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming apparatus not deteriorating image forming efficiency by shortening density correction time as much as possible and which can also maintain accuracy in correction of solid density and gray scale density. <P>SOLUTION: A control section 32 sets a target density on the basis of the temperature and humidity conditions and forms a density pattern and a density control step of setting a developing bias level with the density pattern as the target density is executed when a density correction control is executed. Then, a gradation control step is executed following the density control step when the set target density is different from a prior time. Meanwhile, the gradation control step is not executed and the density correction control is terminated when the target density is the same as the prior time. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to an image forming apparatus using electrophotography, and more particularly to a density correction method for an output image in the image forming apparatus.

  In an image forming apparatus using an electrophotographic process, the density is corrected by transferring the toner directly onto the toner carrier when the apparatus is activated or when a mode (calibration mode) for setting the image density appropriately is set. It is common to form a pattern (reference image), detect the density, and perform density correction. For example, in the case of a color image forming apparatus, a reference image of each color is formed on an image carrier by magenta, cyan, yellow, and black image forming units, and the reference image transferred on a toner carrier such as an intermediate transfer belt The density is corrected by detecting the density by the detecting means.

  Examples of the image density adjustment method include a method of adjusting the charging potential of the photosensitive member, the developing bias potential, the exposure amount by the exposure device, and the like based on the detected image density. In the color image forming apparatus, calibration is performed. Plays an important role in matching colors. In order to output an image including a high density part (solid area) and a halftone density part (halftone area) stably with high quality, high density and half It is necessary to improve the correction accuracy of both tones. There is also a demand for efficient calibration without reducing productivity (image formation efficiency).

For example, Patent Document 1 discloses a first control mode that performs both high density control and halftone control, and a second control mode that performs only high density control. A density adjustment method is disclosed in which a decrease in image formation efficiency can be suppressed by selecting according to the number of sheets and an environmental change.
JP-A-7-20669

  In the method of Patent Document 1, the first control mode is executed only when a long time has elapsed since the previous control in which the gradation characteristics are considered to change, and in other cases, the second control mode is executed. . However, since the relationship between the density input value and the output density (density characteristics) changes depending on environmental conditions, particularly temperature conditions, in the second control mode in which only high density control is performed, the halftone density is shifted due to environmental changes or the like. There is a risk that. For this reason, the color of a color image, particularly a photographic image mainly having a halftone, cannot be guaranteed.

  SUMMARY OF THE INVENTION In view of the above problems, the present invention provides an image forming apparatus capable of maintaining the accuracy of high density and halftone density correction without reducing the image forming efficiency by shortening the density correction time as much as possible. Objective.

  In order to achieve the above object, the present invention is formed by an image forming unit including an image carrier, a developing device for developing an electrostatic latent image formed on the image carrier, and the image forming unit. In an image forming apparatus comprising: a detecting unit that detects image density; and a control unit that performs density control and gradation control based on a detection result of the detecting unit, the control unit is applied to the developing device. A density control step of changing the development bias application conditions in stages to form a density pattern of a predetermined density, and setting a development bias level at which the density of the density pattern becomes a target density based on the density detection result of each density pattern And a gradation for performing gradation control based on the density detection result of the gradation pattern while forming a gradation pattern at a predetermined stage using the developing bias level set in the density control step If a plurality of target densities used in the density control process are set according to environmental conditions, and the target density is the same as when the previous density control process was executed, the subsequent gradation control process Is omitted, and when the target density is different from the previous execution of the density control process, the gradation control process is executed following the density control process.

  According to the present invention, in the image forming apparatus configured as described above, the environmental condition is a temperature / humidity condition inside or outside the apparatus.

  According to the present invention, in the image forming apparatus configured as described above, a plurality of the image forming units are provided, and the density control step and the gradation control step are executed in each image forming unit.

  According to the first configuration of the present invention, since the concentration control process is executed using the target concentration set for each environmental condition, the output characteristics in the high concentration portion can be stabilized regardless of the environmental condition. . Further, since the gradation control process is executed only when the target density is changed in the density control process, it is possible to secure halftone density stability and shorten the density correction time as much as possible to improve the image forming efficiency. it can.

  According to the second configuration of the present invention, in the image forming apparatus having the first configuration, as an environmental condition for setting the target density, the influence on the relationship (density characteristics) between the density input value and the output density is affected. By using a large temperature / humidity condition inside or outside the apparatus, it is possible to set an accurate target density according to the use environment of the apparatus and perform density correction with high accuracy.

  According to the third configuration of the present invention, in the image forming apparatus having the above first or second configuration, each image forming unit includes a plurality of image forming units provided for each toner color. By executing the density control step and the gradation control step in the respective sections, it is possible to accurately reproduce the color of a photographic image or the like mainly having a halftone.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. FIG. 1 is a schematic diagram showing the configuration of a color image forming apparatus of the present invention. In the main body of the image forming apparatus 100, four image forming portions Pa, Pb, Pc, and Pd are sequentially arranged from the upstream side in the transport direction (the right side in FIG. 1). These image forming portions Pa to Pd are provided corresponding to images of four different colors (magenta, cyan, yellow, and black), and magenta, cyan, and yellow are respectively subjected to charging, exposure, development, and transfer processes. And a black image are sequentially formed.

  Photosensitive drums 1a, 1b, 1c, and 1d that carry visible images (toner images) of the respective colors are disposed in the image forming portions Pa to Pd, and are formed on the photosensitive drums 1a to 1d. The toner images thus transferred are sequentially transferred (primary transfer) onto an intermediate transfer belt 8 that moves adjacent to each image forming unit while rotating clockwise in FIG. 1 by a driving means (not shown). The image is transferred (secondary transfer) onto the sheet S at the same time by the next transfer roller 9, and further fixed on the sheet S by the fixing unit 7 and then discharged from the apparatus main body. An image forming process for each of the photosensitive drums 1a to 1d is executed while rotating the photosensitive drums 1a to 1d counterclockwise in FIG.

  The paper S on which the toner image is transferred is accommodated in a paper cassette 16 at the lower part of the apparatus, and is conveyed to the secondary transfer roller 9 via the paper feed roller 12a and the registration roller pair 12b. A sheet made of a dielectric resin is used for the intermediate transfer belt 8, and a belt in which both ends thereof are overlapped and joined to form an endless shape, or a belt without a seam (seamless) is used. Further, a cleaning blade 19 for removing toner remaining on the surface of the intermediate transfer belt 8 is disposed on the downstream side of the secondary transfer roller 9.

  Next, the image forming units Pa to Pd will be described. There are chargers 2a, 2b, 2c, and 2d for charging the photosensitive drums 1a to 1d and image information on the photosensitive drums 1a to 1d around and below the photosensitive drums 1a to 1d that are rotatably arranged. The exposure device 4 for exposing the toner, the developing devices 3a, 3b, 3c and 3d for forming toner images on the photosensitive drums 1a to 1d, and the developer (toner) remaining on the photosensitive drums 1a to 1d are removed. Cleaning units 5a, 5b, 5c and 5d are provided.

  When the start of image formation is input by the user, first, the surfaces of the photosensitive drums 1a to 1d are uniformly charged by the chargers 2a to 2d, and then light is irradiated by the exposure device 4 to each of the photosensitive drums 1a to 1d. An electrostatic latent image corresponding to the image signal is formed on the top. Each of the developing devices 3a to 3d includes a developing roller (developer carrying member) disposed so as to face the photosensitive drums 1a to 1d, and each of magenta, cyan, yellow, and black toners is supplied by a replenishing device (not shown). A predetermined amount is filled. The toner is supplied onto the photosensitive drums 1a to 1d by the developing rollers of the developing devices 3a to 3d, and electrostatically adheres to the electrostatic latent image formed by exposure from the exposure device 4. A toner image is formed.

  After an electric field is applied to the intermediate transfer belt 8 at a predetermined transfer voltage, magenta, cyan, yellow, and black toner images on the photosensitive drums 1a to 1d are transferred to the intermediate transfer belt 8 by the primary transfer rollers 6a to 6d. Primary transferred onto. These four color images are formed with a predetermined positional relationship predetermined for forming a predetermined full-color image. Thereafter, the toner remaining on the surfaces of the photosensitive drums 1a to 1d is removed by the cleaning units 5a to 5d in preparation for the subsequent formation of a new electrostatic latent image.

  The intermediate transfer belt 8 is stretched over a driven roller 10, a drive roller 11, and a tension roller 20, and the intermediate transfer belt 8 starts to rotate clockwise as the drive roller 11 is rotated by a drive motor (not shown). Then, the sheet S is conveyed from the registration roller 12b to the secondary transfer roller 9 provided adjacent to the intermediate transfer belt 8 at a predetermined timing, and the sheet S is conveyed at a nip portion (secondary transfer nip portion) with the intermediate transfer belt 8. A full color image is secondarily transferred onto S. The sheet S on which the toner image is transferred is conveyed to the fixing unit 7.

  The sheet S conveyed to the fixing unit 7 is heated and pressurized when passing through the nip portion (fixing nip portion) of the pair of fixing rollers 13 to fix the toner image on the surface of the sheet S, and a predetermined full-color image is formed. It is formed. The sheet S on which the full-color image is formed is distributed in the transport direction by the branching section 14 that branches in a plurality of directions. When an image is formed on only one side of the sheet S, it is discharged to the discharge tray 17 by the discharge roller 15 as it is.

  On the other hand, when forming images on both sides of the sheet S, a part of the sheet S that has passed through the fixing unit 7 is once projected from the discharge roller 15 to the outside of the apparatus. Thereafter, the sheet S is distributed to the sheet conveyance path 18 by the branching portion 14 by rotating the discharge roller 15 in the reverse direction, and is conveyed again to the secondary transfer roller 9 with the image surface reversed. Then, after the next image formed on the intermediate transfer belt 8 is transferred to the surface of the sheet S where the image is not formed by the secondary transfer roller 9 and conveyed to the fixing unit 7 to fix the toner image, It is discharged to the discharge tray 17.

  A density detection sensor 21 is disposed on the downstream side of the image forming unit Pd and the upstream side of the secondary transfer roller 9. The density detection sensor 21 irradiates the density correction pattern formed on the intermediate transfer belt 8 in the image forming units Pa to Pd with measurement light, and detects the amount of reflected light from each patch image constituting the density correction pattern. A detection result is transmitted to the control part 32 mentioned later as a light reception output signal. As the density detection sensor 21, an optical sensor including a light emitting element composed of an LED or the like and a light receiving element composed of a photodiode or the like is generally used. When measuring the density of the density correction pattern, when the measurement light is sequentially irradiated from the light emitting element to each patch image on the intermediate transfer belt 8, the measurement light is reflected as light reflected by the toner and light reflected by the belt surface. Incident on the light receiving element.

  When the toner adhesion amount is large, the reflected light from the belt surface is blocked by the toner, so that the light reception amount of the light receiving element is reduced. On the other hand, when the adhesion amount of toner is small, conversely, the amount of reflected light from the belt surface increases, resulting in an increase in the light reception amount of the light receiving element. Therefore, the toner adhesion amount (image density) of the patch image of each color is detected from the output value of the received light signal based on the received reflected light amount, and the exposure amount and the development bias characteristic value are compared with a predetermined reference density. By adjusting, density correction is performed for each color.

  Since the density detection sensor 21 needs to strictly define the distance to the measurement object, as shown in FIG. 1, it opposes the driving roller 11 having a small distance variation to the surface of the intermediate transfer belt 8. The intermediate transfer belt 8 is positioned in the width direction according to the density correction pattern formation position on the intermediate transfer belt 8.

  The density detection sensor 21 may be disposed at another position where the density correction pattern on the intermediate transfer belt 8 can be detected. However, when the density detection sensor 21 is disposed downstream of the secondary transfer roller 9, for example, the image forming unit Pa. The time from when the density correction pattern is formed by .about.Pd to when the density detection is performed becomes longer, and the density correction pattern may come into contact with the secondary transfer roller 9 to change the surface state of the density correction pattern. . Therefore, as shown in FIG. 1, it is preferable to dispose on the downstream side of the image forming unit Pd and the upstream side of the contact position of the secondary transfer roller 9.

  An in-machine temperature / humidity sensor 23 is installed above the paper cassette 16. This in-machine temperature / humidity sensor 23 is controlled by a control unit 32 (see FIG. 2), and can measure temperature and humidity unless the main power supply is turned off. For example, temperature / humidity information can be periodically collected every minute, and temperature / humidity information can also be collected by setting conditions such as at the start of printing. The in-machine temperature sensor 23 may be disposed at another position that is not easily affected by a heat source such as the fixing unit 7.

  FIG. 2 is a block diagram showing a control path of the color image forming apparatus of the present invention. Portions common to FIG. 1 are denoted by the same reference numerals and description thereof is omitted. The image forming apparatus 100 includes an image forming unit Pa to Pd, an image input unit 30, an AD conversion unit 31, a control unit 32, a storage unit 33, an operation panel 34, a fixing unit 7, an intermediate transfer belt 8, a density detection sensor 21, and The configuration includes an in-machine temperature / humidity sensor 23 and the like.

  When the image forming apparatus 100 is a color copying machine, the image input unit 30 includes a scanning lamp mounted with a scanner lamp that illuminates the original during copying and a mirror that changes the optical path of reflected light from the original, and reflection from the original. 1 is an image reading unit that includes a condenser lens that collects light to form an image and a CCD that converts the formed image light into an electrical signal. In the case of a printer, the receiving unit receives image data transmitted from a personal computer or the like. The image signal input from the image input unit 30 is converted into a digital signal by the AD conversion unit 31 and then sent to the image memory 40 in the storage unit 33.

  The storage unit 33 stores print image data input from the image input unit 30 and digitally converted by the AD conversion unit 31 in units of pages, necessary data generated during control of the image forming apparatus 100, and image formation. A readable / writable RAM (Random Access Memory) 41 in which data and the like temporarily required for control of the apparatus 100 are stored, and the image forming apparatus 100 such as a control program for the image forming apparatus 100 and numerical values necessary for control. A read-only ROM (Read Only Memory) 42 that stores data that does not change during use is stored.

  Further, the RAM 41 stores a γ table in which the density input value and the actual density output value (sensor output value) are stored in association with each other, and the density input value and the density output value (sensor output value) when the temperature and humidity conditions change. A target density setting table or the like that stores the target density at the time of density control for each temperature and humidity condition based on the relationship is stored.

  The operation panel 34 displays the state of the image forming apparatus 100, the image forming status, and the number of copies, and as a touch panel, functions such as double-sided printing and black-and-white reversal, and various settings such as magnification setting and density setting, the number of printing copies When the image forming apparatus 100 has a FAX function, a numeric keypad for inputting the FAX number of the other party, a start button for instructing the user to start image formation, and when stopping image formation. A stop / clear button, a reset button used when setting various settings of the image forming apparatus 100 to a default state, and the like are provided. The user operates the operation panel 34 to input an instruction, whereby the image forming apparatus 100 is set. Are set to execute various functions such as image formation.

  The control unit 32 is, for example, a central processing unit (CPU), and according to a set program, the image input unit 30, the image forming units Pa to Pd, the fixing unit 7, and the sheet S from the sheet cassette 16 (see FIG. 1). In addition to overall control of conveyance and the like, the image signal input from the image input unit 30 is converted into image data by scaling processing or gradation processing as necessary. The exposure device 4 irradiates laser light based on the processed image data, and forms latent images on the photosensitive drums 1a to 1d.

  Further, when the calibration mode is set by the number of printed sheets from the previous calibration, the key operation of the operation panel 34, or the like, the control unit 32 receives an output signal from the density detection sensor 21 and receives each patch image of the density pattern. A function that compares the sensor output value of the image with the target density, a function that sets the development bias level according to the comparison result, a gradation pattern that is formed with the set development bias level, and the sensor output value of each patch image of the gradation pattern To a halftone control using a γ table.

  Next, density correction control in the color image forming apparatus of the present invention will be described. FIG. 3 is a flowchart showing density correction control executed in the image forming apparatus of the present invention. With reference to FIGS. 1 and 2, the density correction control procedure will be described along the steps of FIG.

  First, when printing is started by a user operation (step S1), the control unit 32 determines whether or not a density correction (calibration) execution condition is satisfied (step S2). Here, the determination is made based on whether or not the cumulative number of printed sheets since the previous execution of density correction has reached a predetermined number. If the density correction execution condition is satisfied in step S2, a target density for a predetermined density input value (area ratio) in the density control step is set (step S3).

  FIG. 4 is a graph showing the relationship between the density input value (area ratio) and the density output value (ID: image density) when the temperature and humidity conditions change. When the temperature and humidity conditions change, the charging characteristics of the toner and the drum surface potential fluctuate, resulting in a difference in output characteristics as shown in FIG. At an area ratio of 40%, there is no difference in image density due to temperature and humidity conditions. However, in a high density area, a high temperature and high humidity environment (30 ° C., 80%, indicated by ● in the figure) is a normal temperature and normal humidity environment (20 Concentration output value is higher than ℃, 60%, indicated by ■ in the figure, and concentration output value in low temperature and low humidity environment (10 ℃, 20%, indicated by ▲ in the figure) compared to normal temperature and humidity environment. Tend to be low.

  Therefore, in order to stabilize the density output value in the high density portion, the present invention sets a target density (ID) for each temperature and humidity condition. The target density is set using the temperature and humidity detected by the in-machine temperature and humidity sensor 23 and the target density setting table stored in the RAM 41. An example of the target density setting table is shown in Table 1.

  In Table 1, each row of the target concentration setting table is assigned in three stages, ie, an in-machine temperature T of 28 ° C. or higher, 20 ° C. or higher and lower than 28 ° C., and lower than 20 ° C. % And less than 80% and less than 40%. When the in-machine temperature T and the in-machine humidity H are detected by the in-machine temperature / humidity sensor 23, a target density at a position where rows and columns intersect corresponding to the detected temperature and humidity is selected. For example, in a room temperature and normal humidity environment (20 ° C., 60%), 0.4 at the position where a row of 20 ≦ T <28 and a column of 40 ≦ H <80 intersect is set as the target concentration for the concentration input value of 40%. The Similarly, the target concentration is set to 0.3 in a high temperature and high humidity environment (30 ° C., 80%) and 0.6 in a low temperature and low humidity environment (10 ° C., 20%).

  FIG. 5 shows changes in output characteristics when the target concentration is changed in a low temperature and low humidity environment. As shown in FIG. 5, when the target density is 0.4 (indicated by ▲ in the figure), the density output value when the density input value is 100% is 1.2, whereas the target density is 0.6. (Displayed by Δ in the figure), the density output value at the density input value of 100% is 1.3, and it can be seen that the output value at the high density portion becomes high. However, if the target density is changed, the output characteristics of the halftone density will be shifted, so that a gradation control process described later is required.

  Following the setting of the target density, a density control step is executed (step S4), and the control unit 32 instructs the formation of a density pattern. An example of the density pattern used in the density control step is shown in FIG. In the present invention, for each color of magenta, cyan, yellow, and black, a plurality of density patterns having a predetermined density (here, 40%) are applied by changing the developing bias application conditions applied to the developing devices 3a to 3d step by step. Form a pair.

  A developing bias in which an AC bias Vac is superimposed on a DC bias Vdc is applied to the developing rollers of the developing devices 3a to 3d. In the present embodiment, centering on the case where the DC bias value is 250 V, it is assumed that the low voltage side is changed to five stages of 200 V and 150 V, and the high voltage side is 300 V and 350 V. For convenience of explanation, the DC bias value is low. The development bias level is set to -2, -1, 0, 1, 2 from the side. Although the DC bias value is changed stepwise here, at least one of the AC bias peak-to-peak value, frequency, and duty ratio is changed stepwise instead of or together with the DC bias value. A development bias level may be set.

  FIG. 6 shows a density correction pattern for each color at a predetermined development bias level (for example, −2), and a total of four 40% patterns are formed for four colors of magenta, cyan, yellow, and black. Note that the density correction patterns similar to those in FIG. 4 are formed even when the development bias level is −1, 0, 1, 2, and therefore, a total of 4 × 5 = 20 patterns are formed on the intermediate transfer belt 8. Will be. The density setting of the density pattern is not limited to 40%, and can be set as appropriate within a range of 20% to 60%.

  Next, the density detection sensor 21 detects the density of the density pattern formed in step S4, and sets a development bias level for each color so that the image density becomes the target density 0.4 (step S5). As an example, FIG. 7 shows the relationship between the development bias level in cyan and the density detection result of the density pattern. When the development bias level is plotted on the horizontal axis and the image density (ID) is plotted on the vertical axis, the target density (indicated by a chain line in the figure) of the cyan density pattern (40%) is 0.4 (ID). It can be seen from FIG. 7 that the development bias level at which the image density of the density pattern (displayed in a black circle data series) is 0.4 is zero. Therefore, the cyan development bias level is set to zero.

  Next, it is determined whether or not the target density set in step S3 is the same as the target density at the previous density control (step S6). If the density is different from the previous target density (temperature and humidity conditions have changed), the gradation control process is executed following the density control process (step S7).

  An example of the gradation pattern used in the gradation control step is shown in FIG. In the present invention, for each color of magenta, cyan, yellow, and black, the exposure apparatus uses a density pattern at predetermined density steps (here, 5% of 20%, 40%, 60%, 80%, and 100%) having different area ratios. A plurality of sets are formed by changing the exposure amount (laser output level) in one dot by 4 stepwise. Then, gradation correction is performed using the γ table stored in the RAM 41.

  On the other hand, if it is the same as the previous target density in step S6 (the temperature and humidity conditions have not changed), the gradation control process can be executed because halftone gradation can be ensured without the gradation control process. The density correction control ends. Thereafter, it is determined whether or not printing is completed (step S8). If printing is continued, the process returns to step S2, and the same procedure (steps S3 to S7) is repeated thereafter.

  By performing the density correction according to the above procedure, the density control process is executed using the target density set for each temperature and humidity condition, so that the output characteristics in the high density part can be stabilized regardless of the temperature and humidity conditions. it can. In addition, since the gradation control process is executed only when the target density is changed in the density control process, it is possible to secure halftone density stability and shorten the calibration time as much as possible to improve the image forming efficiency. it can.

  In the above control example, the case where the density correction is executed when the cumulative number of printed sheets from the previous density correction reaches the predetermined number has been described. However, the density correction is executed when the apparatus is turned on or by a user operation. The same procedure can be used for cases.

  In addition, the present invention is not limited to the above embodiments, and various modifications can be made without departing from the spirit of the present invention. For example, in the above embodiment, the target density of the density pattern is the same for each color, but the target density may be set individually for each color. Here, the development bias level and the gradation pattern density level are all 5 levels. However, the level is not limited to 5 levels, and may be 4 levels or less or 6 levels or more if there are multiple levels. As the development bias level and the density pattern of the gradation pattern become multistage, the relationship with the corresponding image density can be accurately grasped, but the number of patterns to be formed increases and the time required for density correction also increases. Therefore, about five steps are preferable as in the above embodiment. Here, the target concentration is set based on the temperature and humidity inside the apparatus detected by the temperature and humidity sensor 23 in the apparatus. However, an external temperature and humidity sensor that detects the temperature around the apparatus is provided, and the temperature and humidity outside the apparatus is used. A target density may be set.

  The present invention is not limited to a tandem color image forming apparatus that transfers a full color image formed by sequentially laminating toner images of each color onto the intermediate transfer belt 8 as shown in FIG. Of course, the present invention can be applied to various image forming apparatuses such as rotary color image forming apparatuses, monochrome copying machines, monochrome printers, and facsimiles.

  The present invention forms a density pattern of a predetermined density by changing the development bias application condition applied to the developing device in stages, and the density pattern density is set to a target density based on the density detection result of each density pattern. A density control process for setting a development bias level, and a gradation pattern at a predetermined stage is formed using the development bias level set in the density control process, and gradation control is performed based on a density detection result of the gradation pattern In the image forming apparatus capable of executing the gradation control process, a plurality of target densities used in the density control process are set according to the environmental conditions, and the target density is the same as when the previous density control process was executed. The subsequent gradation control process is omitted, and if the target density is different from the previous density control process execution, the gradation control process is executed following the density control process. A.

  Accordingly, it is possible to provide an image forming apparatus having stable output characteristics in the high density portion regardless of environmental conditions. In addition, since the execution of the gradation control process can be suppressed to the minimum necessary, halftone density stability can be ensured and the calibration time can be shortened as much as possible, resulting in an image forming apparatus with high image forming efficiency. As the environmental condition, it is preferable to use a temperature and humidity condition that has a great influence on the relationship (density characteristic) between the density input value and the output density.

FIG. 1 is a schematic diagram showing an overall configuration of a color image forming apparatus of the present invention. FIG. 3 is a block diagram showing a control path of the color image forming apparatus of the present invention. These are flowcharts for explaining density correction control executed in the image forming apparatus of the present invention. These are graphs showing the relationship between the density input value (area ratio) and the density output value (ID) when the temperature and humidity conditions change. These are graphs showing changes in output characteristics when the target concentration is changed in a low temperature and low humidity environment. These are figures which show an example of the density pattern used at a density control process. These are graphs showing the relationship between the development bias level in cyan and the density detection result of the density pattern. These are figures which show an example of the gradation pattern used at a gradation control process.

Explanation of symbols

Pa to Pd Image forming section 1a to 1d Photosensitive drum (image carrier)
2a to 2d charger 3a to 3d developing device 4 exposure device 6a to 6d primary transfer roller 7 fixing unit 8 intermediate transfer belt 9 secondary transfer roller 21 density detection sensor (detection means)
23 In-machine temperature / humidity sensor 32 Control unit (control means)
33 Storage Unit 34 Operation Panel 41 RAM
42 ROM
100 Image forming apparatus

Claims (3)

An image forming unit including an image carrier and a developing device for developing an electrostatic latent image formed on the image carrier;
Detecting means for detecting the image density formed by the image forming unit;
Control means for performing density control and gradation control based on the detection result of the detection means;
In an image forming apparatus comprising:
The control means changes the development bias application condition applied to the developing device stepwise to form a density pattern of a predetermined density, and the density pattern density is targeted based on the density detection result of each density pattern. A density control step for setting a development bias level to be a density, a gradation pattern at a predetermined stage using the development bias level set in the density control step, and a step based on the density detection result of the gradation pattern. A gradation control process for performing gradation control, and
A plurality of target densities used in the density control process are set according to environmental conditions. If the target density is the same as when the previous density control process was executed, the subsequent gradation control process is omitted, and the target density is An image forming apparatus that executes a gradation control process following a density control process when different from the execution of the density control process.   The image forming apparatus according to claim 1, wherein the environmental condition is a temperature / humidity condition inside or outside the apparatus.   The image forming apparatus according to claim 1, wherein a plurality of the image forming units are provided, and the density control step and the gradation control step are executed in each image forming unit.