JP2009276394A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming apparatus which can maintain precision in density correction for high density and half tone without deteriorating image forming efficiency by making time for density correction as short as possible. <P>SOLUTION: When a calibration mode is set, a control section 32 forms density correction patterns composed of a low density pattern (30%) and a high density pattern (100%) by varying the developing bias level in five steps. A first developing bias level with the low density pattern as the target density is temporarily set and when the high density pattern is within the target density range in the first developing bias level, developing bias is set to the first developing bias level and the density correction is terminated. Meanwhile, when the high density pattern is not within the target density range in the first developing bias level, after resetting to a second bias level in which the high density pattern is the target value, exposure amount level is varied in five steps and a density correction pattern is formed and the exposure amount in which the low density pattern becomes the target density is set. <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.

Therefore, a method for improving the accuracy of high density and halftone density correction has been proposed. For example, Patent Document 1 discloses an exposure amount determined using a density measurement result of a first sample image, and a second method. An image forming apparatus that determines a developing bias application condition using a density measurement result of a sample image is disclosed. In Patent Document 2, the development bias and the exposure amount are controlled using the density detection result of one type of patch image (reference image) and the relationship between the density characteristics of the solid image and the halftone image stored in advance. A density adjustment method is disclosed.
JP2003-140411A JP 2006-11171 A

  However, the method of Patent Document 1 has a problem in that both the exposure amount and the application condition of the developing bias are always adjusted, so that the density adjustment time becomes long and the image forming efficiency decreases. Moreover, in the method of Patent Document 2, although the decrease in image formation efficiency is suppressed, the relationship between the density characteristics of the solid image and the halftone image is not always constant, so that environmental conditions, developer deterioration, etc. When the relationship between the density characteristics changes due to the above, it is difficult to correct the density with high accuracy.

  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.

  To achieve the above object, the present invention provides an image carrier, an exposure device that forms an electrostatic latent image on the image carrier, and a developing device that develops the electrostatic latent image formed by the exposure device. Including an image forming unit, a detecting unit that detects the density of a density correction pattern formed by the image forming unit, and an exposure amount of the exposure apparatus and a developing unit that are applied based on a detection result of the detecting unit And an image forming apparatus including a control unit that adjusts a developing bias application condition to perform density correction, and sets the exposure amount of the exposure apparatus to be constant, and sets the developing bias application condition applied to the developing apparatus in a stepwise manner. To form a plurality of sets of two types of density correction patterns having different densities, and the control means determines whether the density of one of the density correction patterns is equal to the target density based on the density detection result of each density correction pattern. A first development bias level is temporarily set, and when the density of the other density correction pattern at the first development bias level is within a target density range, the development bias is set to the first development bias level, and the other density correction pattern is set. If the density is not within the target density range, the exposure bias of the exposure apparatus is changed stepwise after the development bias is reset to the second development bias level where the high density pattern of the two density correction patterns is the target density. A plurality of low density patterns are formed, and an exposure amount at which the low density pattern becomes the target density is set based on the density detection result of the low density pattern.

  According to the present invention, in the image forming apparatus having the above-described configuration, when the developing bias is set to the first developing bias level, the control unit assumes that the second developing bias level is reset. An optimum value of the amount is estimated, and the estimated optimum value is set as an initial exposure amount at the next density correction.

  According to the present invention, in the image forming apparatus having the above configuration, the control unit sets the initial exposure amount using an exposure amount setting table in which a relationship between the exposure amount and the image density is stored for each development bias level. It is characterized by.

  According to the present invention, in the image forming apparatus having the above-described configuration, the control unit uses the density detection result of the low density pattern formed by changing the exposure amount of the exposure apparatus stepwise to store the exposure amount setting table. It is characterized by overwriting correction.

  According to the present invention, in the image forming apparatus configured as described above, the first development bias level is set such that a density of a low density pattern of two types of density correction patterns becomes a target density.

  According to the first configuration of the present invention, the development bias application condition and the exposure amount are set to the optimum values using the density detection results of the high density and low density density correction patterns. The density correction accuracy can be maintained at a high level. Further, since the exposure amount is adjusted only when the adjustment of the developing bias level is not sufficient, the density correction time can be shortened as much as possible to improve the image forming efficiency.

  Further, according to the second configuration of the present invention, in the image forming apparatus having the first configuration, when the development bias is set to the first development bias level, it is assumed that the second development bias level is reset. When the first developing bias level is temporarily set at the next density correction by estimating the optimum value of the exposure amount at the time and setting the estimated optimum value as the initial exposure amount at the next density correction, The high density pattern also easily matches the target density. Accordingly, it is difficult to adjust the exposure amount, so that the density correction time can be further shortened.

  According to the third configuration of the present invention, in the image forming apparatus having the second configuration, the initial exposure is performed using the exposure amount setting table in which the relationship between the exposure amount and the image density is stored for each development bias level. By setting the amount, it is possible to set the initial exposure amount more quickly and with high accuracy.

  According to the fourth configuration of the present invention, in the image forming apparatus having the third configuration, exposure is performed using the density detection result of the low density pattern formed by changing the exposure amount of the exposure apparatus stepwise. By overwriting and correcting the amount setting table, the relationship between the density and the exposure amount is updated to the latest information according to the use environment of the apparatus, so that an accurate exposure amount can always be estimated.

  According to the fifth configuration of the present invention, in the image forming apparatus having any one of the first to fourth configurations, the first development is performed so that the density of the low density pattern among the density correction patterns becomes the target density. By setting the bias level, it is possible to make the output color of the color image more stable by matching the low density pattern, which is used more frequently than the high density, with the target density.

  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 amount of light received by 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.

  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 like. It is the composition which includes.

  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 the 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 relationship between the density of each density correction pattern and the exposure level of the exposure device 4. An exposure amount setting table stored for each development bias level applied to the devices 3a to 3d is also 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 a key operation or the like on the operation panel 34, the control unit 32 receives an output signal from the density detection sensor 21, and uses the sensor output value of each patch image of the density correction pattern as a target value. It has a function to perform density correction for each color by adjusting a developing bias level and an exposure amount level according to a comparison function and a comparison result. The calibration mode may be automatically set when the apparatus is turned on or when a predetermined number of image forming processes are completed, in addition to manual setting by the user.

  Next, density correction control in the color image forming apparatus of the present invention will be described. FIG. 3 is a flowchart illustrating density correction control according to the first embodiment. With reference to FIG. 1 and FIG. 2, the developing bias and exposure amount setting procedure will be described along the steps of FIG.

  First, when the calibration mode is set by a user operation or after printing a predetermined number of sheets, the control unit 32 instructs to form a density correction pattern (step S1). An example of the density correction pattern used for the density correction control of the present invention is shown in FIG. In the present invention, for each color of magenta, cyan, yellow, and black, two types of density correction patterns having different densities (here, 100% and 30%) are set as one set, and the conditions for applying the developing bias applied to the developing device are as follows. Change in stages to form multiple sets. The exposure amount set in the previous calibration is used as the exposure amount level of the exposure apparatus 4.

  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. 4 shows a density correction pattern for each color at a predetermined development bias level (for example, -2). For four colors of magenta, cyan, yellow, and black, two types of 100% and 30%, 8 types in total, are shown. A pattern is formed. Note that the density correction patterns similar to those in FIG. 4 are also formed when the development bias level is −1, 0, 1, and 2. Therefore, a total of 8 × 5 = 40 patterns are formed on the intermediate transfer belt 8. Will be. The density setting of the density correction pattern is not limited to 100% and 30%, but may be set within a range of 60 to 100% on the high density side and 10 to 50% on the low density side.

  Next, the density of the density correction pattern formed in step S1 is detected by the density detection sensor 21 (step S2), and a development bias level (hereinafter, referred to as a target density) at which the image density of the low density pattern (30%) becomes the target density. A first developing bias level is temporarily set for each color (step S3).

  As an example, FIG. 5 shows the relationship between the developing bias level in magenta and the density detection result of the density correction pattern. When the development bias level is plotted on the horizontal axis and the image density (ID; image density) is plotted on the vertical axis, the target density (indicated by a one-dot chain line in the figure) of the low density pattern (30%) of magenta is 0.5 (ID). ), It can be seen from FIG. 5 that the development bias level at which the image density of the low density pattern (displayed in a black circle data series) is 0.5 is zero. Accordingly, the first developing bias level of magenta is temporarily set to 0.

  In FIG. 5, since the target density of the low density pattern matches the measured value, the development bias level (0) on which the low density pattern is formed is temporarily set as the first development bias level as it is, but the target density is measured. If the values do not coincide with each other, the development bias level at the intersection of the curve passing through each measurement value and the target density is temporarily set to the first development bias level.

  Next, it is determined whether or not the image density of the high density pattern (100%) of each color is within the target density range at the temporarily set first development bias level (step S4). For example, if the target density (indicated by the broken line in the figure) of the high density pattern (100%) is 1.3 (ID) and the upper and lower limit range is ± 0.1, the image density (black square) of the high density pattern is shown in FIG. The image density at the development bias level 0 (hereinafter referred to as image density A) is 1.32. Therefore, the magenta high density pattern is within the target density range. Therefore, for magenta, the first developing bias level (0) is set to this set value without changing the exposure amount (step S5). Similarly, if the image density A is within the target density range for other colors, the density correction control is terminated with the temporarily set first developing bias level as the set value.

  On the other hand, as an example of the case where the image density A is not within the target density range in step S4, FIG. 6 shows the relationship between the development bias level in cyan and the density detection result of the density correction pattern. Assuming that the target density of the low density pattern (30%) is 0.5, it can be seen from FIG. 6 that the development bias level at which the image density of the low density pattern is 0.5 is zero. Therefore, when the first developing bias level is temporarily set to 0 and it is determined whether or not the image density A of the high density pattern (100%) is within the target density range (1.3 ± 0.1), FIG. To A is 1.15, which is not within the target density range.

  In this case (NO in step S4), the development bias level at which the high density pattern becomes the target density 1.3 (hereinafter referred to as the second development bias level) is reset (step S6). For example, in FIG. 6, since the developing bias level at which A becomes 1.3 is 1.5, 1.5 (DC bias value 325 V) is reset as the second developing bias level.

  Next, the exposure amount (laser output level) of the exposure apparatus is changed to form a density correction pattern as shown in FIG. 4 for cyan at the reset second developing bias level (step S7), and the density detection sensor. The density is detected by 21 (step S8). In the present embodiment, the low output side is 2.8 times, 2.6 times, and the high output side is centered on the laser output level that is three times the half light amount (the light amount at which the drum surface potential is attenuated to 1/2). For convenience of explanation, the exposure level is set to −2, −1, 0, 1 and 2 from the low output side.

  FIG. 7 shows the relationship between the exposure level and the density detection result of the density correction pattern. It can be seen from FIG. 7 that the exposure level at which the density of the low density pattern becomes the target density is −1.5 (2.7 times the half light amount). Therefore, for cyan, the development bias level is set to 1.5 (second development bias level) and the exposure level is set to -1.5 (step S9).

  Similarly, when the image density A is not within the target density range for other colors, the development bias is reset to the second development bias level, and the exposure level is changed in five steps at the second development bias level. The density correction pattern is formed to detect the density, and the exposure level at which the low density pattern becomes the target value is set, and the density correction control is terminated.

  By performing the density correction according to the above procedure, the development bias level and the exposure level are set to the optimum values using the density detection results of the high density and low density density correction patterns. The correction accuracy can be maintained at a high level. Further, since the exposure level is adjusted only when the adjustment of the developing bias level is not sufficient, the calibration time can be shortened as much as possible to improve the image forming efficiency.

  In the above control example, the density correction pattern as shown in FIG. 4 including the high density pattern and the low density pattern is formed in step S7. However, at the reset second developing bias level, the high density pattern is the target density. As is clear from FIG. 7, the high density pattern has a slight change in density due to fluctuations in the exposure amount. Therefore, only the low density pattern may be formed in step S7.

  Next, density correction control of the second embodiment in the image forming apparatus of the present invention will be described. FIG. 8 is a flowchart showing the density correction procedure of the second embodiment. A calibration execution procedure will be described in accordance with the steps in FIG. 8 with reference to FIGS.

  In this embodiment, as in FIG. 3 of the first embodiment, when the high density pattern (100%) is within the target density range in step S4, the first developing bias level is set without changing the exposure amount. This set value is set (step S5). For example, in the case of magenta, the situation set so far is that the first development bias level (0) at which the low density pattern (30%) becomes the target density 0.5 is temporarily set from FIG. Since the image density A of the pattern (100%) is 1.32 and is within the target density range (1.3 ± 0.1), the first developing bias level is set to this set value. The exposure amount is the same as the previous set value.

  Here, it is assumed that the second developing bias level is reset and control for adjusting the exposure amount is performed. From FIG. 5, since the development bias level at which the high density pattern matches the target density 1.3 is -1, the second development bias level is set to -1, and the exposure amount level is -2, -1, 0, 1, 2. 9 is changed, the relationship between the exposure level and the image density is as shown in FIG. Therefore, the exposure amount level at which the low density pattern matches the target density of 0.5 is 1, and originally the exposure amount level of 1 which is a higher setting is better.

  Therefore, the optimum exposure amount level (1 in the example of FIG. 9) when it is assumed that the second developing bias level is set is estimated and stored in the RAM 41 or the like (step S6). At the next density correction, the initial exposure level is set to 1 to form a density correction pattern, and density correction control is performed in the same manner.

  Thus, at the next density correction, the first development bias level −1 at which the low density pattern has the target density of 0.5 is provisionally set. Since the initial exposure amount level is 1, the high density pattern also has the target density of 1.3. Match. Therefore, the exposure level adjustment is less likely to be performed than in the first embodiment, and the density correction time can be further shortened while maintaining the correction accuracy. In addition, since the control procedure after step S7 is the same as that of step S6 to S9 of 1st Embodiment, description is abbreviate | omitted.

  The exposure level can be estimated using an exposure setting table that stores the relationship between the density of the density correction pattern of each color and the exposure level for each development bias level. Further, when the density correction pattern in which the exposure level is changed stepwise is actually formed, and the exposure level is adjusted by detecting the density, the exposure setting table is overwritten and corrected using the detection result. As a result, the relationship between the density and the exposure level is updated to the latest information according to the use environment of the apparatus, and therefore it is always possible to estimate the accurate exposure level.

  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 (0.5) of the low density pattern and the target density range (1.3 ± 0.1) of the high density pattern are the same for each color, but the target density and the target density range are the same. It may be set individually for each color. Further, the development bias level and the exposure amount level are not limited to five stages, and may be four stages or less as long as there are a plurality of stages, or six stages or more. As the development bias level or the exposure amount level becomes multistage, the relationship with the corresponding image density can be accurately grasped, but the number of density correction 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.

  In this case, when the first development bias level is temporarily set, the low density pattern (30%) is adjusted to the target density and the high density pattern (100%) is set within the target density range. May be adjusted to the target density, and the low density pattern may be placed within the target density range. However, in general, in the output of a color image, the use frequency of the low density (halftone) is higher than that of the high density (100%). Therefore, in order to make the output color more stable, the low density pattern is adjusted to the target density. Control is preferred.

  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.

  In the present invention, the exposure amount of the exposure apparatus is made constant, the application conditions of the development bias applied to the development apparatus are changed stepwise to form a plurality of sets of two types of density correction patterns having different densities, and each density correction Based on the pattern density detection result, a first development bias level at which the density of one of the density correction patterns becomes a target density is temporarily set, and the density of the other density correction pattern at the first development bias level is a target density range. Is set to the first development bias level, and when the density of the other density correction pattern is not within the target density range, the development bias is set to the first density where the high density pattern of the two types of density correction patterns is the target density. 2 After resetting to the development bias level, the exposure amount of the exposure apparatus is changed stepwise to form a plurality of low density patterns. An image forming apparatus for setting an exposure amount low density pattern is the target density based on degree detection result.

  As a result, the density correction accuracy of high density and halftone images is maintained at a high level, and the density correction time is shortened as much as possible by minimizing the exposure adjustment, thereby improving image formation efficiency. An apparatus can be provided.

  Further, when the development bias is set to the first development bias level, the optimum value of the exposure amount when it is assumed that the second development bias level is reset is estimated, and the estimated optimum value is used for the next density correction. By setting the initial exposure amount, it is difficult to adjust the exposure amount at the next density correction, so that the density correction time can be further shortened.

  When setting the initial exposure amount, the exposure amount setting table in which the relationship between the exposure amount and the image density is stored for each development bias level is used, and the exposure amount setting table is overwritten when the exposure amount is actually adjusted. Since the correction is made, the initial exposure amount can be set quickly and accurately.

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 the flowcharts explaining the density correction procedure of the first embodiment. These are figures which show an example of the density | concentration correction pattern used for this invention. These are graphs showing the relationship between the development bias level in magenta and the density detection result of the density correction pattern. These are graphs showing the relationship between the development bias level in cyan and the density detection result of the density correction pattern. These are graphs showing the relationship between the exposure level at the second development bias level in cyan and the density detection result of the density correction pattern. These are the flowcharts explaining the density correction procedure of the second embodiment. These are graphs showing the relationship between the exposure level and the image density when it is assumed that the second development bias level is reset and the control for adjusting the exposure is performed.

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)
32 Control unit (control means)
33 Storage Unit 34 Operation Panel 41 RAM
42 ROM
100 Image forming apparatus

Claims (5)

An image carrier including an image carrier, an exposure device that forms an electrostatic latent image on the image carrier, and a developing device that develops the electrostatic latent image formed by the exposure device;
Detecting means for detecting the density of the density correction pattern formed by the image forming unit;
Control means for performing density correction by adjusting the exposure amount of the exposure apparatus and the application condition of the developing bias applied to the developing apparatus based on the detection result of the detecting means;
In an image forming apparatus comprising:
The exposure means of the exposure apparatus is made constant, and a plurality of sets of two types of density correction patterns having different densities are formed by changing the development bias application condition applied to the development apparatus in stages, and the control means includes: Based on the density detection result of each density correction pattern, a first development bias level at which the density of one of the density correction patterns becomes a target density is temporarily set, and the density of the other density correction pattern at the first development bias level is When the density is within the target density range, the development bias is set to the first development bias level. When the density of the other density correction pattern is not within the target density range, the high density pattern of the two density correction patterns is the target density. The second developing bias level is reset, and the exposure amount of the exposure apparatus is changed stepwise to form a plurality of low density patterns, Image forming apparatus characterized by setting the exposure amount low density pattern is the target density based on the density detection result of the low density pattern.   When the development bias is set to the first development bias level, the control means estimates an optimum value of the exposure amount when it is assumed that the second development bias level is reset, and the estimated optimum value The image forming apparatus according to claim 1, wherein the initial exposure amount at the next density correction is set.   The image forming apparatus according to claim 2, wherein the control unit sets the initial exposure amount using an exposure amount setting table in which a relationship between the exposure amount and the image density is stored for each development bias level. .   4. The overwriting correction of the exposure amount setting table according to claim 3, wherein the control means overwrites the exposure amount setting table using a density detection result of a low density pattern formed by changing the exposure amount of the exposure apparatus stepwise. Image forming apparatus.   5. The image according to claim 1, wherein the first development bias level is set such that a density of a low density pattern of two kinds of density correction patterns becomes a target density. 6. Forming equipment.

Images