JPH06110285A - Image forming device - Google Patents

Abstract

PURPOSE:To provide an image forming device capable of optimizing the fluctuation of image density caused by the change of environment and change with the lapse of time without depending on maintenance and in a shorter cycle than that of maintenance and attaining the stability of high image density. CONSTITUTION:Test patterns for high density and low density are formed on a photosensitive drum 2, and toner adhesive quantities to the two test patterns are measured by a toner density measurement part 8. A control part 36 calculates deviations between the measured toner adhesive quantities of a high density paft and a low density part and respective target values previously set, and when the respective calculated deviations are not within a specified range, the setting of the exposing condition of an optical system 13 and the bias voltage of a developing device 4, or the exposure of the optical system 13 and the bias voltage of an electrostatic charger 3 and the developing device 4, or the bias voltage of the charger 3 and the developing device 4 and the light emitting time of the optical system 13 are changed based on the relation of the respective deviations.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electrophotographic color image forming apparatus such as a color laser printer or a color digital copying machine.

[0002]

2. Description of the Related Art For example, it seems that there are many people who have the experience that the same copy is copied by the same copy machine but the copy has different darkness. Fluctuations in image density in electrophotography are the effects of environmental changes, changes in image forming conditions over time, and deterioration. In analog copying machines as well as in multi-tone printers or digital copying machines, this fluctuation in image density is suppressed.
Stabilization is important. In particular, in color, not only the density reproducibility but also the color reproducibility is affected, so it can be said that stabilization of the image density is an essential requirement.

Therefore, conventionally, the materials and the process itself are allowed to use these materials, and the image is stabilized by maintenance.

[0004]

However, there is a limit in allowing the material and the process itself, and maintenance requires labor and cost. Further, the image density varies as compared with the frequency of maintenance. However, there is a problem that a stable image density cannot be obtained only by maintenance.

Therefore, according to the present invention, fluctuations in image density due to changes in the environment and time can be optimized without depending on maintenance and in a cycle shorter than the maintenance cycle, and high stability of image density can be achieved. An object of the present invention is to provide an image forming apparatus capable of reducing the cost required for maintenance.

[0006]

An image forming apparatus of the present invention forms a latent image of a high density portion and a low density portion on an image carrier based on image data, and exposure conditions such as emission intensity or emission time. , A developing means for developing a latent image of a high density portion and a low density portion on the image carrier, which is formed by the exposing means, by a bias voltage, and a developing means, and the developing means. And a high-density portion measured by the developer-adhesion-amount measuring means for measuring the amount of the developer adhered to the high-density portion and the low-density portion adhered on the image carrier by the development of And a first calculating means for calculating a deviation between each measured value of the low-density portion and each preset target value, and each deviation calculated by the first calculating means is within a predetermined range. Determine whether If the determining unit determines that the exposure condition is not within the predetermined range by the determining unit, the first change amount information relating to the change of the exposure condition of the exposing unit and the bias voltage of the developing unit based on the relationship between the respective deviations. And a contrast potential which is a difference between an exposed portion potential of the image carrier and a background potential which is a difference between a bias voltage of the developing unit and an unexposed portion potential of the image carrier. A second calculation unit that calculates the amount information, and the exposure condition of the exposure unit is changed by the first change amount information calculated by the second calculation unit, and the exposure condition of the developing unit is changed by the second change amount information. Control means for changing the bias voltage.

Further, in the image forming apparatus of the present invention, the surface of the image carrier is charged and the charging amount is controlled by the applied bias voltage, and the image carrier charged by the charging unit. To form a latent image of a high-density portion and a low-density portion based on the image data, the exposure amount controllable exposure amount according to the target exposure amount information, and a bias voltage is applied,
Developing means for developing a latent image of a high density portion and a low density portion on the image carrier formed by the exposing means with a developer, and a high density portion adhered on the image carrier by the developing means. And a developer adhesion amount measuring means for measuring the adhesion amount of the developer to the low density portion respectively, and measurement values of the high density portion and the low density portion measured by the developer adhesion amount measuring means are respectively set in advance. The first calculating means for calculating the deviation from the target value, the judging means for judging whether each deviation calculated by the first calculating means is within a predetermined range, and the judging means. If it is determined that the difference is not within the predetermined range, the first change amount information relating to the change of the light amount of the exposing unit, the bias voltage of the developing unit, and the unexposed portion of the image carrier based on the relationship between the respective deviations. Background that is the difference from the potential Second calculation means for calculating second change amount information relating to the change in position, and target exposure amount information for the exposure means is calculated by the first change amount information calculated by the second calculation means. , And third calculating means for calculating the bias voltage applied to the charging means and the bias voltage applied to the developing means, respectively, based on the second change amount information and the surface potential characteristic of the image carrier. ing.

Further, in the image forming apparatus of the present invention, the surface of the image bearing member is charged and the charging amount is controlled by the applied bias voltage, and the image bearing member charged by the charging unit. To the exposure means for forming latent images of high-density area and low-density area based on image data, and bias voltage is applied to the high-density area and low-density area on the image carrier formed by the exposure means. A developing means for developing the latent image with a developer; and a developer adhesion amount measuring means for respectively measuring the adhesion amount of the developer to the high density portion and the low density portion adhered on the image carrier by the development by the developing means. The light emission time correction means for correcting the light emission time per unit pixel based on the light emission time correction information for the gradation data per unit pixel of the image data, and the light emission time information corrected by the light emission time correction means. A modulation control means for controlling the modulation of the exposure means as a pulse width per unit pixel, and the respective measured values of the high density portion and the low density portion measured by the developer adhesion amount measuring means are set in advance. First calculating means for calculating the deviation from the target value, judging means for judging whether or not each deviation calculated by the first calculating means is within a predetermined range, and predetermined by the judging means If it is determined that the difference is not within the range, the first change amount information relating to the change of the contrast potential, which is the difference between the bias voltage of the developing unit and the exposed portion potential of the image carrier, is determined from the relationship between the respective deviations. A second calculation means for calculating second change amount information relating to the change of the light emission time correction, and the second calculation means.
The bias voltage applied to the charging unit and the bias voltage applied to the developing unit are respectively calculated by the first change amount information calculated by the calculation unit and the surface potential characteristic of the image carrier, And third control means for calculating the light emission time correction information according to the change amount information.

[0009]

The amount of developer adhered to the high-density test pattern and the amount of developer adhered to the low-density test pattern are measured, and deviations from respective preset target values are calculated. When the respective deviations are not within the respective predetermined ranges, the exposure condition of the exposure means and the bias voltage of the developing means, or the exposure amount of the exposure means and the bias voltage of the charging means and the developing means, or The bias voltage of the charging unit and the developing unit and the light emission time of the exposure unit are changed.

As described above, when the potential relationship is changed in order to maintain the gradation characteristics in the development, there is a correction effect which is close to the initial gradation characteristics with respect to changes in the development characteristics due to environment and aging.
However, depending on the developing system including the developer to be controlled, there is a possibility that an image defect may occur due to the change in the potential relationship, that is, the bias change amount. It does not occur when the exposure amount and pulse width correction characteristics are changed. However, the exposure amount,
Changing the pulse width correction characteristics cannot completely correct the changed development characteristics.

Therefore, by changing the potential relationship by changing the bias and changing the exposure amount and the pulse width correction characteristic, the amount of change in bias is suppressed, and as a side effect of changing the image forming conditions, image defects such as fog and image defects are generated. It is possible to maintain the gradation characteristics from the high density region to the low density region without causing problems. Furthermore, since the amount of change from the set value at the time of detection is calculated, it is possible to reduce the steady-state deviation with respect to the target value by repeating the control.

Therefore, the image quality equivalent to the initial state without image defects can be maintained, and the fluctuation of the image density due to changes in environment and aging can be optimized without relying on the maintenance and in a cycle shorter than the maintenance cycle. High image density stability can be achieved, and maintenance costs (labor costs, equipment, etc.) can be reduced.

[0013]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 2 shows the structure of a color laser printer as an example of the image forming apparatus according to the present invention.
In the figure, a photosensitive drum 2 as an image carrier that rotates counterclockwise (in the direction of the arrow in the drawing) with respect to the drawing is provided in a substantially central portion of a housing 1. Around the photosensitive drum 2, a charger 3 as a charging unit, a first developing unit 4, a second developing unit 5, a third developing unit 6, a fourth developing unit 7, which are developing units,
A toner adhesion amount measuring unit 8, a transfer drum 9 as a transfer material support, a pre-cleaning static eliminator 10, a cleaner 11, and a static elimination lamp 12 are sequentially arranged.

The photosensitive drum 2 rotates in the direction of the arrow shown in the figure,
The surface is uniformly charged by the charger 3. The laser beam light 14 emitted from the optical system 13 which is an exposing means from between the charging device 3 and the first developing device 4 exposes the surface of the photoconductor drum 2 to electrostatic charge according to the image data. A latent image is formed.

The first to fourth developing devices 4 to 7 visualize the electrostatic latent image on the photosensitive drum 2 corresponding to each color into a color toner image. For example, the first developing device 4 is Magenta, the second developing device 5 is cyan, the third developing device 6 is yellow,
The fourth developing device 7 is adapted to develop black.

On the other hand, the transfer sheet as the transfer material is sent out from the sheet feeding cassette 15 by the sheet feeding roller 16, is once aligned by the registration roller 17, and is then registered so as to be attracted to a predetermined position of the transfer drum 9. It is sent by the roller 17 and electrostatically attracted to the transfer drum 9 by the attraction roller 18 and the attraction charger 19. The transfer sheet is attracted to the transfer drum 9, and the transfer sheet is rotated clockwise (in the direction of the arrow in the figure).
It is transported as it rotates.

The developed toner image on the photosensitive drum 2 is transferred to the transfer sheet by the transfer charger 20 at the position where the photosensitive drum 2 and the transfer drum 9 face each other. In the case of printing a plurality of colors, the step of setting one rotation of the transfer drum 9 as one cycle is performed by switching the developing device to multiple-transfer the toner images of a plurality of colors onto the transfer paper.

The transfer sheet on which the toner image has been transferred is further conveyed as the transfer drum 9 rotates, and the pre-separation internal static eliminator 2
1. The external pre-separation static eliminator 22 and the separation static eliminator 23 remove the charge, and the separation claw 24 separates it from the transfer drum 9, and the conveyance belts 25 and 26 convey it to the fixing device 27. The toner on the transfer paper heated by the fixing device 27 is melted and fixed on the transfer paper immediately after being discharged from the fixing device 27, and the transfer paper after this fixing is discharged to the paper discharge tray 28.

FIG. 1 is a block diagram showing the charging, exposing and developing means and the control means of the color laser printer according to this embodiment. In the figure, the photosensitive drum 2 rotates counterclockwise with respect to the drawing, as indicated by the arrow. The charger 3 mainly includes a charging wire 31, a conductive case 32, and a grid electrode 33. Charging wire 3
Reference numeral 1 is connected to a high voltage power source 34 for corona and charges the surface of the photosensitive drum 2 by corona discharge. The grid electrode 33 is connected to a high voltage power supply 35 for grid bias, and the amount of charge on the surface of the photosensitive drum 2 is determined by the grid bias voltage.

The high-voltage power supplies 34 and 35 are connected to a control unit 36 mainly composed of an MPU and the like, and the output voltage is controlled by the control unit 36.

An electrostatic latent image is formed on the surface of the photosensitive drum 2 uniformly charged by the charger 3 by exposure of the modulated laser beam light 14 from the optical system 13. The gradation data buffer 37 stores gradation data from an external device or controller (not shown), corrects the gradation characteristics of the printer, and converts it into laser exposure time (pulse width) data.

Under the control of the control unit 36, the laser drive circuit 38 sets the laser drive current (light emission time) according to the laser exposure time data from the gradation data buffer 37 so as to be synchronized with the scanning position of the laser beam light 14. Modulate.
Then, by the modulated laser drive current, the optical system 13
A semiconductor laser oscillator (not shown) in the inside is driven. As a result, the semiconductor laser oscillator emits light according to the exposure time data.

Further, the laser drive circuit 38 is the optical system 1
The output of a monitor light receiving element (not shown) in 3 is compared with a set value, and control is performed to keep the output light amount of the semiconductor laser oscillator at the set value by the drive current.

On the other hand, the pattern generation circuit 39 includes the control unit 3
By the control of 6, the gradation data of the test pattern of the printer alone and the gradation data of the test pattern having two different densities for measuring the toner adhesion amount are generated and sent to the laser drive circuit 38, respectively. Has become.

Of the two test patterns having different densities, one having a higher density is a high density test pattern and one having a lower density is a low density test pattern.

Here, switching between the laser exposure time data from the gradation data buffer 37 and the gradation data of the test pattern for measuring the toner adhesion amount from the pattern generating circuit 39 is performed by the control of the control unit 36. The data selected by the control unit 36 is stored in the laser drive circuit 38.
It will be sent to.

The photosensitive drum 2 on which the electrostatic latent image is formed is developed by the developing device 4. The developing device 4 is, for example, a two-component developing system and stores a developer consisting of toner and carrier, and the weight ratio of the toner to the developer (hereinafter referred to as toner concentration) is determined by the toner concentration measuring unit 4.
Measured by 0. Then, the toner concentration measuring unit 40
The toner replenishing motor 42 for driving the toner replenishing roller 41 is controlled in accordance with the output of the toner replenishment roller 41 to replenish the toner in the toner hopper 43 into the developing device 4.

The developing roller 44 of the developing unit 4 is formed of a conductive member, is connected to a high voltage power source 45 for developing bias, and rotates in a state where a developing bias voltage is applied to the photosensitive drum. Toner is attached to the image corresponding to the electrostatic latent image on 2. The toner image in the image area thus developed is transferred to the transfer sheet supported and conveyed by the transfer drum 9. The high-voltage power supply 45 is connected to the control unit 36, and the output voltage is controlled by the control unit 36.

At the end of the warm-up process after the power is turned on, the control unit 36 causes the pattern generation circuit 39 to generate the gradation data having the two different densities as described above, so that the toner on the photosensitive drum 2 is changed. A high-density and low-density test pattern for measuring the adhesion amount is exposed.

Then, the control unit 36 synchronizes with the positions where the high-density and low-density test patterns on the photoconductor drum 2 are exposed and developed, respectively, and when they come to the position of the toner adhesion amount measuring unit 8. The toner adhesion amount measuring unit 8 measures the toner adhesion amount. The output of the toner adhesion amount measuring unit 8 and the output of the toner concentration measuring unit 40 are respectively output from the A / D converter 46.
Is digitized by and input to the control unit 36.

As shown in FIG. 3, the high-density test pattern portion (high-density portion) PT1 for the high-density gradation data and the high-density test pattern portion PT1 are formed on the photosensitive drum 2 by the development processing.
Low-density test pattern portions (low-density portions) PT2 for low-density gradation data are formed respectively.

The control section 36 outputs the two densities (measurement values) from the toner adhesion amount measuring section 8 and the memory 4 in advance.
7 is compared with each reference value (target value) set and stored in 7, and according to the comparison result, the grid bias voltage of the charger 3 which is an image forming condition, the developing bias voltage of the developing device 4, and the optical system. The exposure amount of 13 and the light emission time of the area gradation are changed.

The control unit 36 is connected to a memory 47 whose contents can be rewritten, and stores therein a reference value (target value) for measuring the toner adhesion amount described above.

Further, the control section 36 controls switching of gradation data from an external device or controller (not shown) and gradation data of a test pattern of the printer alone and a pattern for measuring the toner adhesion amount, and the measuring sections 8 and 40. Each output of the output, control of output amount of the high voltage power supplies 34, 35, 45, target value setting of laser drive current, target value setting of toner concentration, toner replenishment control, correction processing of gradation characteristics of printer of gradation data Also performs each control such as.

FIG. 4 shows the structure of the toner adhesion amount measuring unit 8. In the figure, the light from the light source 51 is applied to the surface of the photoconductor drum 2, and the reflected light reflected by the photoconductor drum 2 or the toner that has been developed and adhered is converted into a light amount of the reflected light by the photoelectric conversion unit 52. After being converted into a corresponding current and further converted into a current / voltage, it is transmitted to the A / D converter 46 by the transmission circuit 53, where it is converted into a digital signal and taken into the control unit 36. There is.

The light source 51 is current-driven by the light source drive circuit 54. The light source drive circuit 54 is on / off controlled by a control signal from the control unit 36, or is controlled by a signal for adjusting the amount of drive current to the light source 51.

FIG. 5 shows in detail the flow of image data (including gradation information) and the functional blocks relating to the exposure system according to this embodiment.

In normal printing, image data containing gradation information (hereinafter referred to as gradation data) from an external device or an original reading unit and an image processing unit is transferred in accordance with an image transfer clock and command / status information. , And is transferred to the interface (I / F) 61 of this apparatus. The control unit 36 manages the transmission / reception of commands / statuses including data requests and printer busy.

The control unit 36 sets the selector 62 to select the grayscale data from the interface 61.
The selected gradation data is sent to the data conversion unit 63,
Here, the pulse width data of the laser beam light is converted according to the conversion data given from the control unit 36 and stored in the line buffer 64. Data transfer up to here is
It is performed in synchronization with the image transfer clock.

Here, the selector 62 and the data converter 6
3 and the line buffer 64 constitute the gradation data buffer 37.

On the other hand, the laser diode 6 in the optical system 13
The laser beam light emitted from the laser beam 5 passes through a pre-deflection optical system (not shown), and is deflected and scanned by a polygon mirror 67 rotated by a motor 66 as a deflection means. The motor 66 is driven by a motor driver 68. Here, the laser diode 65, the motor 66, the polygon mirror 67, and the motor driver 68 constitute the optical system 13.

The horizontal synchronization detector 69 is the polygon mirror 6
By detecting the position of the laser beam light 14 deflected by 7, the horizontal synchronizing signal of the writing scanning position is generated, and the horizontal synchronizing signal is sent to the synchronizing clock generating circuit 70. The sync clock generation circuit 70 generates a write sync clock signal for each pixel on the basis of the input horizontal sync signal, and the horizontal sync signal and the write sync clock signal are managed by the line buffer 64 and the control unit 36 by a counter 71. , PWM (pulse width modulation) circuit 72 and laser driver 73, respectively.

The writing synchronization signal and the horizontal synchronization signal signal are synchronized with the scanning of the laser beam.

Under the control of the control unit 36, the counter 71 generates a timing signal for the writing area (top, bottom, right, left margin) according to the horizontal synchronizing signal, the writing synchronizing signal, and the print start position information.
Based on the timing signal and the synchronous clock signal, the read / transfer from the line buffer 64 and the PWM circuit 72
And the exposure writing of the laser driver 73 are executed in synchronization.

That is, the image transfer clock is used as a reference until gradation data is written in the line buffer 64, and the process from reading from the line buffer 64 to exposure with laser beam light is processed with the laser scanning position as a reference. This is due to the difference between the external image transfer rate and the writing speed due to the effective angle of view of the laser optical system, and this speed is temporarily stored in the line buffer 64 to be absorbed.

The PWM circuit 72 generates a gate pulse corresponding to the light emission time of the laser diode 65 per unit pixel based on the pulse width data per unit pixel. The laser driver 73 supplies a drive current according to the gate pulse to the laser diode 65 to control the laser diode 65 to be turned on and off.

Therefore, when the pulse width data per unit pixel is large, the light emission time of the laser diode 65 per unit pixel is long, the energy exposed to the photosensitive drum 2 is large, and the area is large. Conversely, when the pulse width data per unit pixel is small, the light emission time of the laser diode 65 per unit pixel is short, the energy with which the photosensitive drum 2 is exposed is small, and the area is also small. As a result, the latent image pattern after exposure forms a latent image of a gradation pattern that is expressed in gradation by the exposure of the laser beam light 14 based on the pulse width data.

By the way, the light emission amount of the laser diode 65, that is, the exposure amount is controlled by the light amount control circuit 74. The laser diode 65 has main light emission (front surface) used for exposure and monitor light emission on the back surface, and includes a monitor diode (not shown) for detecting the monitor light emission amount. The light amount control circuit 74 detects the output of the monitor diode, compares it with the target value, and generates a signal for correcting the amount of the laser drive current to the laser driver 73 so as to reduce the deviation.

Here, the horizontal sync detector 69, the sync clock generation circuit 70, the PWM circuit 72, and the laser driver 7 are provided.
The laser drive circuit 38 is constituted by 3 and the light quantity control circuit 74.

When the pulse width correction characteristic is changed, it is changed by transferring the conversion data from the control unit 36 to the data conversion unit 63. This makes it possible to change the conversion characteristic (pulse width correction characteristic) of gradation data to pulse width data.

When the exposure amount, that is, the light emission amount of the laser diode 65 is changed, it is changed by transferring the target value of the light amount control circuit 74 from the control unit 36. Thereby, the exposure amount can be changed.

On the other hand, when the test pattern is created, the selector 62 switches the selection of the image data of the test pattern generated from the pattern generation circuit 39 and the internal image transfer clock under the control of the control unit 36. The pattern generating circuit 39 transfers the test pattern image data to the selector 62 in synchronization with the data including the grayscale information according to the data from the control unit 36 in synchronization with the internal image clock. The data flow is interface 61
Is the same as the image data from.

Further, in order to set the image writing area to a predetermined test pattern size, the control unit 36 rewrites the image writing area information (top, bottom, right, left margin) to the counter 71 with the data for the test pattern, and Perform test pattern exposure according to the above. Therefore, the type, size, and print position of the test pattern including gradation can be designated by the setting from the control unit 36.

FIG. 6 shows the surface potential of the photosensitive drum 2 uniformly charged by the charger 3 with respect to the absolute value VG (hereinafter simply referred to as grid bias voltage) of the bias voltage applied to the grid electrode 33 of the charger 3. (Hereinafter, referred to as unexposed portion potential) VO, the surface potential (hereinafter, exposed portion potential) VL of the photosensitive drum 2 which is entirely exposed by the optical system 13 with a constant light amount and attenuated, and the developing bias voltage VD (dashed line) )
Is shown.

In this embodiment, the polarity of the potential or voltage is negative because of reversal development. When the grid bias voltage VG increases, the absolute values of the unexposed portion potential VO and the exposed portion potential VL respectively decrease. A linear approximation of the exposed portion potential VL and the unexposed portion potential VO with respect to the grid bias voltage VG can be expressed as the following equation.

VO (VG) = K1.VG + K2 (1) VL (VG) = K3.VG + K4 (2) where K1 to K4 are constants, VO, VG and VL are absolute values, and VO ( VG) and VL (VG) are VO for any VG
, VL.

Here, the absolute value VD of the developing bias voltage,
The development density changes due to the relationship between the exposed portion potential VL and the unexposed portion potential VO. Now, the contrast potential VC and the background potential VBG are defined as follows.

VC = VD (VG) -VL (VG) (3) VBG = VO (VG) -VD (VG) (4) where VD (VG) is the magnitude of VD with respect to an arbitrary VG. Represents

The contrast potential VC is particularly related to the density of solid areas (see FIG. 7). The background potential VBG is mainly involved in the density of the low density portion in the multi-gradation method using pulse width modulation (see FIG. 8).

FIG. 9 shows the toner adhesion amount Q with respect to the gradation data when the magnitude of the background potential VBG is increased. The low-concentration region changes in the direction of the arrow C in the figure. Therefore, the developing density can be changed by the contrast potential VC and the background potential VBG.

Here, the following equation is obtained from the above equations (1) to (4).

VG (VC, VBG) = (VC + VBG-K2 + K4) / (K1-K3) ... (5) VD (VBG, VG) = K1.VG + K2-VBG ... (6) The above equation (5) ), (6), the grid bias voltage VG
Of the exposed portion potential VL to the unexposed portion potential VO (K
1 to K4) are known, the grid bias voltage VG is determined by determining the contrast potential VC and the background potential VBG.
, The developing bias voltage VD can be uniquely determined.

The surface potential of the photosensitive drum 2 is measured in advance, and the exposed portion potential V with respect to the grid bias voltage VG is measured.
After obtaining the relationship (K1 to K4) between L and the unexposed portion potential VO,
The contrast potential VC and the background potential VBG are set. The grid bias voltage VG and the developing bias voltage VD are uniquely determined from the equations (5) and (6), a plurality of density patterns are imaged under these conditions, and the toner adhesion amount Q after development is measured. Then, this measured value is compared with a preset reference value, and the deviations ΔQ are used to infer the correction values ΔVC and ΔVBG of the contrast potential VC and the background potential VBG, respectively, which make the proper development density. From this inference result,
Again grid bias voltage VG, development bias voltage VD
, And measure the toner adhesion amount of the density pattern,
Repeat until it is within the acceptable range.

Next, in such a structure, the image forming condition changing process according to the first embodiment will be mainly described with reference to the flowchart shown in FIG. This processing operation is roughly divided into a warm-up step S1, a test pattern image forming step S2, a toner adhesion amount measuring step S3, a determining step S4, and an image forming condition changing step S5.

First, in the warm-up step S1, when the power of the present apparatus is turned on, the control unit 36 performs an initial process and executes a predetermined sequence of each initial operation. In particular,
It takes time to warm up the fixing device 27. At the end of this warm-up, or at a predetermined ultimate temperature lower than the predetermined ultimate temperature at the end of warm-up, the initial operation of the image forming system including the cleaning operation is performed.

In the initial operation, the temperature of the photoconductor drum 2, the temperature and humidity inside the machine, the developer stirring, the charging, and the charge removal of the photoconductor drum 2 are performed.
Is stabilized, cleaning of the photosensitive drum 2 and the like are performed, and an image forming environment is almost the same as a normal image forming state (printing by the image data of the user).

When the warm-up process is completed, it is checked whether the toner adhesion amount measuring unit 8 is normal. This is performed by checking the presence / absence of a sensor abnormality flag that is set when there is an abnormality as a result of the sensor output check in the toner adhesion amount measuring step S3 described later.
The sensor abnormality flag is reset when the power is turned on and is therefore determined to be normal.

When the toner adhesion amount measuring unit 8 is determined to be normal, the control unit 36 proceeds to the test pattern image forming step S2. That is, the control unit 36 charges the
The exposure, development, cleaning, and charge removal processes are operated in the same manner as a normal image forming sequence. At this time, the grid bias voltage value of the charging device 3 and the developing bias voltage value of the developing devices 4 to 7 are set to predetermined values. This value is a bias condition that provides the reference gradation characteristic at room temperature and normal humidity. Further, the exposure amount optical system light amount control target value and the pulse width correction coefficient are set to reference values, respectively.

In the exposure process, the pattern generation circuit 39
Two test pattern latent images of a predetermined size corresponding to two gradation data having different densities generated from are formed. As described above, among the test patterns for the two gradation data, the one having a darker density is the high density test pattern and the one having a lighter density is the low density test pattern.

The size of this test pattern is such that a predetermined width is centered on the center of the image area in the axial direction of the photoconductor drum 2 and a predetermined length in the rotation direction of the photoconductor drum 2 is used for exposure.
The above-mentioned predetermined width is the photosensitive drum 2 of the toner adhesion amount measuring unit 8.
Corresponding to the position in the axial direction of, the detection spot size is set to the minimum size that does not include the edge effect peculiar to electrophotography. The predetermined length is set to a minimum size at which the influence of the edge effect and the response characteristics of the sensor do not affect the measurement result.

In this embodiment, the predetermined width is 1.5 to 5 mm larger than the detected spot size,
The predetermined length is obtained by multiplying the detection spot size by the length of movement for four times the sensor time constant once and the number of times of detection.
The length is 5 to 5 mm.

In the developing process, the image is developed by the developing roller 44 to which the initial developing bias voltage is applied.
One test pattern latent image is developed, and two test pattern portions (toner images) PT1 and PT2 having different densities are formed (see FIG. 3). Of the two test pattern parts,
The test pattern portion PT1 corresponding to the low density gradation data will be referred to as a low density portion, and the test pattern portion PT2 corresponding to the high density gradation data will be referred to as a high density portion.

Next, in the toner adhesion amount measuring step S3, it is synchronized with the arrival of the two test pattern portions PT1 and PT2 formed on the photosensitive drum 2 at positions facing the toner adhesion amount measuring portion 8, respectively. Then, the amount of reflected light from each of the test pattern portions PT1 and PT2 is detected by the toner adhesion amount measuring portion 8. At this time, the toner adhesion amount measuring unit 8 also detects the reflected light amount of the undeveloped region of the photosensitive drum 2 at a predetermined timing.

The amount of reflected light other than the test pattern area of the photosensitive drum 2, the amount of reflected light of the low-density portion PT2, the amount of reflected light of the high-density portion PT1, that is, the output of the toner adhesion amount measuring portion 8 is A / It is sent to the control unit 36 via the D converter 46. The control unit 36 stores the detected amount of reflected light other than the test pattern region of the photosensitive drum 2, the amount of reflected light of the low density portion PT2, and the amount of reflected light of the high density portion PT1 stored in the memory 47 in advance. And the lower limit value (predetermined range) are compared with each other to check the sensor output.

As a result of this check, if any one of them is out of the predetermined range, the control unit 36 determines that the toner adhesion amount measuring unit 8 is abnormal and sets a sensor abnormality flag, Not displayed on the operation panel that the toner adhesion amount measuring unit 8 is abnormal. Then, the control unit 36
Returns to the state before entering the image forming condition changing process and enters the standby state.

As a result of the sensor output check, when the toner adhesion amount measuring unit 8 is normal, the control unit 36 controls the amount of reflected light other than the detected test pattern area of the photosensitive drum 2,
Based on the reflected light amount of the low density portion PT2 and the reflected light amount of the high density portion PT1, a predetermined function related to the optical reflectance for the low density portion and the high density portion based on the reflected light amount other than the test pattern area of the photosensitive drum 2 The calculation results are defined as the low-density area toner adhesion amount and the high-density area toner adhesion amount, respectively.

The controller 36 compares the high density toner adhesion amount and the low density toner adhesion amount calculated as described above with the respective target values stored in the memory 47 in advance. The deviation for each target value is calculated. Here, this deviation is defined as a high density portion deviation and a low density portion deviation, respectively.

Next, in the judgment step S4, it is judged whether the high density portion deviation and the low density portion deviation calculated as described above are within the respective predetermined standard values stored in the memory 47 in advance. . As a result of this judgment, if both the high-density portion deviation and the low-density portion deviation are within the respective standard value ranges, the standby state (printable according to the user's print request) is set, and at least one deviation is within the standard value. If not, the process proceeds to image forming condition changing step S5.

In the first embodiment, the image forming condition changing step S5 is to change the light amount control target value of the exposure optical system to be changed so that both the high density portion deviation and the low density portion deviation fall within the standard values. The exposure amount and the background potential are changed by changing the grid bias voltage and the developing bias voltage. As shown in FIG. 11, there are mainly three small steps S51 to S5.
Divided into three.

Step S51 is a step of determining the amount of change related to the exposure condition represented by two parameters from the relationship between the high density portion deviation and the low density portion deviation, a step of determining the background potential change amount, and step S52. Is a step of calculating the change amount relating to the changed exposure condition, the exposure amount optical system light amount control target value, the grid bias voltage change value, and the developing bias voltage change value to be changed by a function prepared in advance. Step S53 is a step of setting the light amount control target value, the grid bias voltage value, and the developing bias voltage value to the changed value calculated at each predetermined timing.

This causes a problem in the method of directly selecting the light amount target value and the bias value from the table prepared in advance from the high density portion deviation and the low density portion deviation, respectively. Not only the influence of the environment, but also the development characteristics that change over time, the appropriate amount of change differs depending on the history of the photoconductor drum 2, the developer, etc., and the individual differences, and also changes over time.
Therefore, there is a possibility that the convergent value when repeatedly detecting and operating will deviate from the target value over time.

Further, the effect of changing the exposure conditions acting on the high density portion and the low density portion is not necessarily independent, but there is interaction. There is a contradiction in determining each exposure condition from each deviation.

Therefore, in step S51, the amount of change related to the exposure condition represented by two parameters is selected from a table prepared in advance based on the relationship between the deviation of the high density portion and the deviation of the low density portion. One parameter is the amount of change in the target value of the light amount control of the exposure optical system, which is the amount of change in the comparison voltage (target voltage) with the signal photoelectrically converted by the light amount monitor. The other parameter is the amount of change in the background potential that is the difference between the developing bias voltage and the unexposed portion potential of the photosensitive drum 2, and the amount that can be changed by the grid bias voltage and the developing bias voltage that control the unexposed portion potential. Is.

FIG. 12 shows the effect of changing the amount of light on the gradation characteristics. The horizontal axis represents the gradation data, the vertical axis represents the output image density ID, P0 is the reference light quantity, and P1 to P4 are the changed light quantities, and the gradation characteristics at that time are shown. P1 <P2 <
The relationship is Po <P3 <P4. It can be seen that the higher the concentration, the greater the change with respect to the change in the light amount, and the greater the light amount, the greater the gradation gradient, and the smaller the light amount, the smaller the gradient.

Therefore, by changing the light amount with respect to the gradation characteristics that have changed as the density increases, it is possible to perform correction so that the gradation characteristics become closer to the reference gradation characteristics. However, it is difficult to correct the non-linear change on the low density side only by changing the light amount.

On the other hand, the change in the background potential has a greater effect in the low density portion. FIG. 13 shows a change in gradation characteristic when the background potential VBG is changed. Gradation data on the horizontal axis,
The vertical axis shows the output image density. It can be seen that when the background potential is increased, the density is reduced in the low density portion.

Therefore, a table corresponding to the relationship between the high density portion deviation and the low density portion deviation and the exposure amount change amount, and a table corresponding to the relationship between the high density portion deviation and the low density portion deviation and the background potential change amount are prepared. Then, the exposure amount change amount and the background potential change amount are derived from the high density portion deviation and the low density portion deviation. The contents of the table take into consideration the interaction between the exposure amount and the background potential, and an effective change amount can be appropriately derived from the relationship between the deviations. Further, when both deviations are “0”, each change amount is set to “0”, so that the steady-state deviation after convergence approaches “0”.

Next, in step S52, step S5
The exposure amount correction amount obtained in step 1, the background potential change amount, the exposure amount at the time of test pattern image formation, and the new exposure amount and background potential to be changed from the background potential are obtained. Further, a new light quantity and a new pulse width correction coefficient value to be changed are obtained from the light quantity correction amount, the light quantity when the test pattern is formed, and the pulse width correction coefficient value.

In the present embodiment, the light amount is calculated by the following formula for the voltage value given to the target value of the light amount control means (light amount control circuit 74) of the exposure optical system.

Pnew = P + ΔP (7) where Pnew is the new light amount, P is the current light amount, and ΔP
Is the light amount change amount obtained by the table.

A bias (grid bias, developing bias) voltage value is calculated from the background potential given from the table. This calculation can be uniquely obtained by a function prepared in advance that includes a coefficient representing the surface potential characteristic of the photosensitive drum 2 (see the explanations given by the equations (5) and (6), where Vc is Constant).

Next, in step S53, step S5
The new exposure amount obtained in 2 is set and changed as the target value of the light amount control circuit 74 which is the light amount control means, and the new grid bias voltage value and development bias voltage value are set and changed to the output control values of the respective high voltage power supplies 35 and 45. To do.

When the settings are changed and the test pattern is imaged again, the grid bias voltage value and the development bias voltage value are changed at predetermined timings. The predetermined timing is to change the developing bias voltage value at least in synchronization with the arrival of the position on the photosensitive drum 2 where the grid bias voltage value is changed to the developing position.
If you change the timing appropriately, depending on the changed value,
Photosensitive drum 2 with carrier attached for fogging and two-component development
May cause stains.

FIG. 14 shows the timing of changing the grid bias voltage and the developing bias voltage in this embodiment. In this embodiment, in order to prevent carrier adhesion, when the grid bias voltage is lowered, the charging potential change delay time T4 due to the delay of the grid bias high-voltage power supply 35 and the developing position of the photosensitive drum 2 from the grid electrode 33. To the grid bias voltage setting change time t1 for a time T2 that is longer than the sum of the moving time T1
The setting of the developing bias voltage value is changed at time t3 after the passage of time.

To increase the grid bias voltage, a time T3 shorter than the time T3 obtained by subtracting the delay time T5 of the high voltage power source 45 for developing bias from the moving time T1 from the grid electrode 33 to the developing position of the photosensitive drum 2, The setting of the developing bias voltage value is changed at time t5 after the time t4 of changing the setting of the grid bias voltage value.

That is, at the time of change, by preventing the background potential from increasing at the same position on the photosensitive drum 2,
The carrier is prevented from adhering to the photosensitive drum 2. However, if the difference between T2, T3 and T1 is set too large, the fog amount of the photosensitive drum 2 may increase. Therefore, in the present embodiment, T4 = 50 msec or less and T5 = 5.
When 0 msec or less, T2-T1 = 200 msec or less, T
1-T3 = 200 msec or less.

Next, the test pattern is imaged, detected and judged again to form two test pattern latent images on the photosensitive drum 2 charged with the changed grid bias voltage by exposure again. The toner adhesion amount measuring step S3 and the determining step S4 are performed on the two test patterns developed with the developing bias voltage.

In the decision step S4, if the high density portion deviation and the low density portion deviation are within the standard values, the changed exposure amount,
After the cleaning operation in the state where the grid bias voltage value and the developing bias voltage value are held, the standby state is entered. If at least one deviation is not within the standard value, the bias voltage value change, pattern image formation, detection, and determination are repeated.

Next, the qualitative algorithm regarding the contents of the above-mentioned table will be described. In the first embodiment,
In the step of deriving two change amounts from the high density portion deviation and the low density portion deviation in the image forming condition changing step S5, the exposure amount is mainly decreased when both deviations are positive, and the exposure amount is mainly decreased when both deviations are negative. The exposure amount is increased, the background potential is decreased when the high-density portion deviation is within a predetermined value near "0", and the low-density portion deviation is negative, and the high-density portion deviation is within a predetermined value near "0". When the deviation is positive, the background potential is increased. this is,
It is considered that the voltage relationship, which is highly effective due to the effects of the exposure amount and the background potential, is mainly used.

FIG. 12 shows the effect of the exposure amount change on the gradation characteristics. The horizontal axis represents gradation data, and the vertical axis represents output image density. It can be seen that as the exposure amount increases, the density on the high density side rises and the gradient becomes larger.

FIG. 13 shows the effect of changing the background potential on the gradation characteristic. It can be seen that when the background potential is increased, the start of development of the low density portion shifts to the higher gradation data, and the gradient becomes larger.

There is a risk of fogging of the photosensitive drum 2, adhesion of the oppositely charged toner, and carrier adhesion when the developer is a two-component developer. Therefore, the background potential is not largely changed and the high-density portion is mainly used for the exposure amount. It is considered that the rough adjustment is performed with an emphasis on the fine adjustment based on the exposure amount and the background potential including the low density portion.

A table for deriving the amount of change for changing the potential relationship as described above is prepared in the memory 47, for example, from the qualitative rule considering these.

FIG. 15 shows the contents of a table (for example, provided in the memory 47) relating to the exposure amount change amount. The horizontal axis represents the high density portion deviation, the low density portion deviation in the depth direction, and the exposure amount change amount in the height direction. Both the high-density portion deviation and the low-density portion deviation are "0" at the center of the frame in the plane formed by the axes of the high-density portion deviation and the low-density portion deviation, that is, the toner adhesion amount in the high-density portion and the toner in the low-density portion This is the point where the adhered amount matches each target value. In this example, the exposure amount change amount hardly depends on the low density portion deviation.

FIG. 16 shows the contents of a table (for example, provided in the memory 47) relating to the change amount of the background potential. With the same expression as in FIG. 15, when the high-density portion deviation largely deviates from “0”, the change amount of the background potential is “0”, that is, it is not changed. The background potential is changed only when the high-density portion deviation is near "0".

When the exposure amount change amount and the background potential are determined from the relationship between the low-density portion deviation and the high-density portion deviation, the manipulated variable change amount is determined independently for each deviation. There is a possibility of making a wrong decision. On the other hand, even when one deviation has the same value but the other deviation is different, an appropriate operation amount can be determined by the parameter change amount suitable for the effect.

17 and 18 show examples of changes in two different gradation characteristics, respectively. In these cases, it is assumed that the low density portion deviation is detected as the same value, the high density portion deviation is very low in FIG. 17, and is close to “0” in FIG. At this time, in the case of the example of FIG. 17 in which the high-density portion deviation is extremely low from the effect of changing the exposure amount shown in FIG. 12 and the effect of the background potential shown in FIG. 13, it is mainly effective to change the exposure amount. And the high density part deviation is close to "0".
In the case of example 8, it can be inferred that a change of slightly lowering the background potential is effective.

Instead of individually determining the manipulated variables from the high-density portion deviation and the low-density portion deviation, by considering the relationship between the high-density portion deviation and the low-density portion deviation as in the above example, It is possible to derive an appropriate amount of operation according to it.

Further, in the first adhesion amount measuring step S3, when the high density portion deviation is slightly negative and the high density portion deviation is largely negative, the exposure amount change amount is increased in the positive direction. The change amount of the background potential is “0” (not changed) (see FIGS. 19 and 20).

After the bias value is calculated and changed using this result, the adhesion amount of the test pattern is measured again.
As the effect of changing the bias, as can be expected from FIG. 12, both the high-density portion deviation and the low-density portion deviation should shift in the positive direction.

If the deviation is within the specified value, the control is ended. If the deviation in the high density portion is within the specified value, but the deviation in the low density portion is slightly out of the specified value in the negative direction. As shown in FIGS. 21 and 22, the exposure amount change amount is slightly negative and the background potential change amount is slightly negative. When the background potential is lowered, the image density becomes higher on the low density side. The high-concentration part should be a little larger,
At the same time, since the exposure amount is lowered only slightly, the high-density portion hardly changes.

As in the above example, the measurement of the toner adhesion amount,
By repeatedly changing the bias voltage, depending on the relationship between the high-density portion deviation and the low-density portion deviation in the table contents, coarse adjustment mainly for the high-density portion by the exposure amount change amount, and then the background potential and the exposure amount are simultaneously It is also possible to execute sequential control such as fine adjustment including the low density portion.

FIG. 23 and FIG. 24 are graphs showing changes in the toner adhesion amount measurement value Q in the control process, the light emission amount of the laser diode 65 (hereinafter simply referred to as the laser light emission amount) P, and the bias voltage. An example is shown. FIG. 23 shows
It is a plot of the high density toner adhesion amount measurement value QH and the low density toner adhesion amount measurement value QL with respect to the number of times of control. In the figure, the broken lines QHT and QLT are the high-density portion,
In the low-density portion toner adhesion amount target value, QHP and QLP are control standard values for the high-density portion and the low-density portion, respectively.

FIG. 24 is a plot of the laser emission amount P and the bias value (grid bias voltage value, developing bias voltage value) with respect to the number of times of control. Control count "0"
Is QH and QL when first measured in the toner adhesion amount measuring step S3, and the conditions when the test pattern is formed are the values of P, VG and VD. QHT, Q
Both QH and QL are low with respect to LT, that is, there is a large negative deviation. Therefore, in the image forming condition changing step S5 described above, a change is made to increase the laser emission amount P (first control count).

When the toner adhesion amount of the test pattern was measured under these conditions, both QH and QL increased as a result, and in this example, QH was still lower than QHT and QL was higher than QLT. Again, in the image forming condition changing step S5, the laser emission amount P is increased, that is, VG, VD
Make changes to increase the difference between and. As a result, QH increased and QL decreased, approaching QHT and QLT, respectively. Further, in the same direction, the laser emission amounts P and VG,
And VD are changed (3rd control). As a result, Q
Both H and QL fall within the respective control standard values, and the control ends.

As described above, by combining the exposure amount change and the background potential, it becomes possible to control the gradation characteristic from the high density portion to the low density portion in the range where the image defect due to the bias change does not occur. .

Next, the second embodiment will be described. The contents of the image forming condition changing step S5 are different between the second embodiment and the first embodiment.

In the image forming condition changing step S5, in the second embodiment, the grid bias voltage and the developing bias voltage to be changed are changed so that both the high density portion deviation and the low density portion deviation fall within the standard values. Also, the exposure width is changed by changing the pulse width modulation data correction table.
As shown in FIG. 25, there are mainly three small steps S61 to S.
It is divided into 63.

Step S61 is a step of determining the amount of change relating to the exposure condition represented by two parameters based on the relationship between the high density portion deviation and the low density portion deviation, and the step of determining the amount of change related to the contrast potential. A step of calculating a change amount relating to the changed exposure condition and a change value of the pulse width correction table to be changed by a function prepared in advance, and calculating a grid bias value and a development bias value. This is a step of setting the bias value, the developing bias value, and the change value of the pulse width correction table, which are calculated at each predetermined timing.

This is because the grid bias value, the developing bias value, and the development bias value are directly calculated from the high density portion deviation and the low density portion deviation, respectively.
A problem arises in the method of selecting the pulse width correction table from a table prepared in advance. Not only the influence of the environment but also the development characteristics that change over time, the appropriate amount of change differs depending on the history of the photosensitive drum 2, the developer, etc., and the difference between individuals, and also changes over time. There is a possibility that the converged value when repeatedly detected and operated deviates from the target value over time.

Further, the effects of changing the exposure conditions acting on the high-density portion and the low-density portion are not always independent, and there is an interaction, so there is a contradiction in determining each bias condition from each deviation.

In step S61, therefore, the amount of change related to the potential relation condition represented by two parameters and the exposure condition is selected from the table prepared in advance based on the relation between the deviation of the high density portion and the deviation of the low density portion. One parameter is the amount of change in the contrast potential, which is the difference between the exposed portion potential uniformly exposed at a predetermined exposure amount and the developing bias voltage. The other parameter is the change amount of the coefficient of the function for changing the correction table of the pulse width data (gradation data).

FIG. 26 shows gradation characteristics when the contrast potential is changed. The horizontal axis represents gradation data and the vertical axis represents output image density. Due to the change of the contrast potential Vc, the change becomes larger in the high density portion. When the contrast potential is increased, the gradation characteristics change in the direction of increasing the gradient with respect to the gradation data. Therefore,
It is possible to correct the gradation characteristics that have changed due to the change of the contrast potential so that the higher the density is, the closer the gradation characteristics are to the initial gradation characteristics.

FIG. 27 shows the relationship between the pulse width correction characteristic and the change in gradation characteristic. The first quadrant represents the reference gradation characteristic by the output image density with respect to the original data before correction. The third quadrant represents correction data for the original data. The second quadrant represents the reference gradation characteristic and the changed gradation characteristic for the correction data.

The number of correction data division stages is larger than the number of division stages of gradation data so that the original data can be corrected more finely. That is, the actual number of pulse width modulation steps is larger than the number of division steps of the original data. In this embodiment, the original number of gradation steps is 16 and the pulse width data is 2
Each of the 56 stages is the number of stages including "0" (no exposure).

Therefore, in correspondence with the gradation data, the pulse width correction means (data conversion) is used to selectively correspond the pulse width data having the ideal reference gradation characteristic from the prepared pulse width modulation step number. Part 63).

As a general rule, the data relationship before and after the correction by the pulse width correction means corresponds to data that are evenly proportional, the image forming conditions including the light amount are all standard conditions, and the environmental conditions are standard room temperature and normal humidity. When the units and materials related to image formation are in the initial state, the output image density with respect to the original data has the reference gradation characteristic indicated by γo.

Now, it is assumed that the gradation characteristic changes such that the density decreases in a low density portion such as γ1 due to the environment and aging. At this time, in order to realize the image density corresponding to each original data of the reference gradation characteristic shown in the first quadrant, it is necessary to correct the data such as φ1. The gradation characteristic itself is non-linear with respect to the pulse width, and the change is not always linear. However, when the original data and the correction data are linearly approximated, each correction data value can be obtained relatively easily. In order to increase the lowered density, φ1 is made closer to the reference gradation characteristic by increasing the correction data, that is, the pulse width, as the density becomes lower.

Further, it is assumed that the gradation characteristic changes such that the density increases in a low density portion such as γ1 due to the environment and aging. At this time, in order to realize the image density corresponding to each original data of the reference gradation characteristic shown in the first quadrant, it is necessary to correct the data such as φ2. For φ 2, the correction data, that is, the pulse width, is made smaller as the density becomes lower in order to reduce the increased density, so that it becomes closer to the reference gradation characteristic.

In principle, the gradation characteristic change can be corrected in any case by the pulse width correction. However, the maximum density cannot be corrected especially when the density decreases.
Further, if the gradation characteristics change drastically and the gradient of the image density with respect to the pulse width increases, the resolution will be insufficient with the number of prepared gradation steps, and more gradation steps will be required. The influence of the increase in the size of the section 63), the pulse width modulation means (PWM circuit 72), the light emission response, the limit of high-speed drive, and the uneven charging and the jitter in the image area appear. Therefore, it is desirable to reduce the change amount (correction amount) by dividing the labor with other means.

Therefore, the maximum value and the minimum value of the original data (“15” and “0” in this embodiment) are the maximum value and the minimum value of the correction data (“255” and “0” in this embodiment), respectively. Corresponding to, fixed and no correction. Other original data is converted into correction data according to a predetermined function.

FIG. 28 shows the linearly approximated relationship between the original data and the corrected data. The horizontal axis is the original data before correction, and the vertical axis is the corrected data after correction, and shows the corrected approximate straight line of φ0 and φ1 in the third quadrant of FIG. here,
The minimum value of the original data is “0”, the maximum value is “n”, and each data is defined as Di. However, i is an integer from 0 to n. Therefore, in this embodiment, D0 = 0, D1
= 1, ..., Dn = 15.

The correction data has a minimum value of 0, a maximum value of Dmax, and correction data corresponding to the i-th original data is Dc (i). In this embodiment, Dc
(0) = 0, ..., Dc (i), ..., Dc (n) = D
max = 255. Therefore, the following formula is defined.

Φ; Dc (i) = C1 × D (i) + C2 (8) where C1 and C2 are coefficients, and the above equation (8) is a straight line equation representing the original data and the correction data, C1 means the inclination, and C2 means the intercept of the correction data axis. When φ 0 is calculated from the above equation (8), C2 = 0,
Since Dc (n) = Dmax, φ0; Dc (i) = (Dmax / Dn) × D (i) (9), and generally φ; Dc (i) = (Dmax-C2 / Dn ) × D (i) + C2 ... (10) Therefore, by moving C2, that is, the intercept of the correction data axis, it is possible to perform correction with a larger change amount on the lower density side.

In the example of φ1, when the gradation characteristics are changed so that the image data becomes lower as the density becomes lower (see γ1 in FIG. 27), the pulse width is corrected so as to become lower as the density becomes lower. This is achieved by changing the coefficient C2 when using equation (10). Therefore, if the amount ΔC for changing the current value of C2 is obtained, the new C2 can be obtained by the sum thereof.

Therefore, a contrast potential change amount table is prepared from the relationship between the high density part deviation and the low density part deviation, and a correction coefficient change amount table is prepared from the relationship between the high density part deviation and the low density part deviation. The change amounts are derived from the high density portion deviation and the low density portion deviation. The contents of the table are
Considering the interaction of the effects of changing the contrast potential and changing the pulse width correction characteristic, an appropriate amount of change of the effective means can be derived from the relationship between both deviations. Further, when both deviations are “0”, each change amount is set to “0”, so that the steady-state deviation after convergence approaches “0”.

Next, in step S62, step S6
A new contrast potential value to be changed and a pulse width correction coefficient value are obtained from the contrast potential change amount, the pulse width correction coefficient change amount, the contrast potential value at the time of test pattern image formation, and the pulse width correction coefficient value obtained in 1. .

In this embodiment, the contrast potential is calculated by adding the contrast potential change amount ΔVc to the current contrast potential Vc when the test pattern is formed, and becomes the new contrast potential Vcnew.

Vcnew = Vc + ΔVc (11) If the background potential is constant and has an initial predetermined value, the surface potential characteristic coefficients (K1 to K) of the photosensitive drum 2 are the same as in the first embodiment.
The new grid bias voltage value and development bias voltage value to be set are calculated by the equations (5) and (6) including 4).

Further, the pulse width correction data is created by ΔC
And the current C2 as a new C2, and the above equation (10)
, I = 2, 3, ..., N−1 and Dc (i) corresponding to the original data Di are obtained. The correction data including 0 and Dmax is transferred to the pulse width correction means.

Next, in step S63, step S6
The new grid bias voltage value and developing bias voltage value obtained in 2 are set and changed in the respective high voltage power supplies 35 and 45, and the correction data including 0 and Dmax is transferred to the data conversion unit 63 which is the pulse width correction means.

Next, the qualitative algorithm regarding the contents of the above-mentioned table will be described. In the second embodiment,
In the step of deriving two image forming conditions from the high density portion deviation and the low density portion deviation in the image forming condition changing step S5, the contrast potential is mainly decreased when both deviations are positive, and the contrast electric potential is mainly decreased when both deviations are negative. Increase the contrast potential, increase the high-density part deviation within a predetermined value near "0", and increase the pulse width correction coefficient when the low-density part deviation is negative, increase the high-density part deviation within a predetermined value near "0" The pulse width correction coefficient is increased when the density deviation is positive. This is because the exposure conditions that are highly effective due to the effects of changing the light amount and correcting the pulse width are mainly used.

When the maximum density is not more than the specified value as in the gradation characteristic of FIG. 26, it is difficult to control the reference gradation characteristic within the specified value only by changing the pulse width correction. Therefore, when the high density part deviation is large, the contrast potential is increased to secure the maximum density, and when the high density part deviation is small and the low density part deviation exists, the low density part deviation is corrected by the pulse width correction. A table is prepared so as to reduce the deviation and maintain or reduce the high density portion deviation depending on the contrast potential (see FIGS. 29 and 30).

Further, in the first measurement of the toner adhesion amount, when the high density portion deviation is large negatively and the low density portion deviation is small negatively, the contrast potential of the contrast potential is changed as shown in the examples of the tables of FIGS. 31 and 32. Only a large positive change amount is selected, and there is no change amount of the pulse width correction coefficient. As a result, as expected from FIG. 26, the changed gradation characteristic changes so that the density increases in the higher density portion.

However, when the deviation obtained by the measurement of the toner adhesion amount again does not fall within the standard value, and the high density portion deviation is slightly negative and the low density portion deviation is slightly negative,
In the examples of the tables in FIGS. 33 and 34, the result is that the amount of change in the contrast potential is made slightly negative this time, and at the same time, the amount of change in the pulse width correction coefficient is made negative.

As described above, by preparing a table for changing two parameters based on the relationship between the high density portion deviation and the low density portion deviation, and the contents thereof are as described above, an effective means according to the gradation characteristic change can be obtained. Can be operated, and by repeating the control, it becomes possible to incorporate a sequential operation into the contents of the table.

FIGS. 35 and 36 are examples of graphs showing how the toner adhesion amount measurement value Q and the bias value (grid bias value VG, developing bias value VD) and the pulse width correction coefficient C are changed in the control process. Is shown. FIG. 35 shows
It is a plot of the high density toner adhesion amount measurement value QH and the low density toner adhesion amount measurement value QL with respect to the number of times of control. In the figure, the broken lines QHT and QLT are the high-density portion,
In the low-density portion toner adhesion amount target value, QHP and QLP are control standard values for the high-density portion and the low-density portion, respectively.

FIG. 36 shows the bias value V with respect to the control count.
It is a plot of G, VD and the pulse width correction coefficient C. The control count “0” is QH and QL when first measured in the toner adhesion amount measuring step S3, and the conditions when the test pattern is formed are P and C values.
Both QH and QL are lower than QHT and QLT, that is, the deviation is large in the negative. Therefore, in the image forming condition changing step S5 described above, the contrast potential is changed to be increased (first control count; both are increased when the difference between VG and VD is substantially the same).

When the adhesion amount of the test pattern was detected under these conditions, both QH and QL increased as a result, and in this example, QH was still lower than HT and QL was higher than QLT. Again, in the image forming condition changing step S5, the contrast potential is increased and the pulse width correction coefficient C is decreased. As a result, both QH and QL fall within the control standard values (QHP, QLP), and the control ends.

In the above description, the control is performed when the power of the apparatus is turned on. In this embodiment, for example, when a front door (not shown) of the apparatus is opened / closed, a control execution command is input from the outside, a predetermined time is exceeded after the control is finished, a predetermined time is set after the control is finished. The same control as above can be performed when the number of printed sheets is exceeded or when the toner empty is released.

That is, first, when the front door of this apparatus is opened or closed, that is, a jam occurs inside the apparatus such as the paper feed system and the paper discharge system, the front door is opened for paper removal or maintenance. When opened / closed, external light may enter the photoconductor drum 2, which may affect the surface potential characteristics of the photoconductor drum 2, and the temperature and humidity inside the machine may change suddenly. Therefore, the control is performed after the completion of the initial operation such as the warm-up operation according to the detection result of the door sensor (not shown) that detects the opening / closing of the front door.

Further, control is performed when a control execution command is input from the outside, that is, when a serviceman operates the operation panel during maintenance or when a control execution command is received from the outside of the apparatus.

Further, when a predetermined time is exceeded after the control is finished, that is, when a long time has passed after the control is finished, a change in temperature and humidity inside the apparatus due to a change in temperature and humidity outside the apparatus, and the photosensitive drum. There is a possibility that the gradation characteristics may change, such as a change in the surface potential characteristics due to the recovery of the light fatigue of No. 2 and a change in the density and the charge amount due to leaving the developer once stirred. Therefore, a timer (not shown) that measures the time since the last control was completed controls when the predetermined time is exceeded.

Further, when a predetermined number of printed sheets is exceeded after the control is finished, if a large number of sheets are printed after the control is finished, a change in surface potential characteristics due to light fatigue of the photosensitive drum 2 and charging of the developer are caused. There may be a change in the gradation characteristics such as a change in amount. Therefore, a control (not shown) that measures the number of printed sheets after the control is finished at the end is performed when the predetermined number of sheets is exceeded. However, in the case of continuous printing, control is performed after the printing of the number of prints set by the user is completed.

Further, when the toner empty is released, that is, after the toner empty, the toner is replenished, the cartridge containing the toner is replaced, and the process unit including the toner or the photosensitive drum 2 is replaced, and then the toner empty flag (not shown). When is released, control is performed.

In any of the above cases, a display indicating that control is being performed is displayed on the operation panel from the start of control to the end of control. Generate a busy signal to the external input (operation panel or the outside of this machine) to wait for printing.

Next, the control end condition will be described.

That is, when both the deviation of the high density portion and the deviation of the low density portion are within the predetermined control standard value (normal end),
When control is performed a predetermined number of times (maximum control count execution),
When the bias value, the exposure amount, and the calculation result of the pulse width correction coefficient satisfy the predetermined conditions (operation amount limit), and when the output of the toner adhesion amount measuring unit 8 satisfies the predetermined conditions (sensor output abnormality), Each is a control end condition.

First, when both the deviation of the high density portion and the deviation of the low density portion are within the predetermined control standard value (normal end), that is, in the determination step S4, within the predetermined control standard value which is the target range. When both the high-density portion deviation and the low-density portion deviation occur in, the process shifts to the apparatus standby state while holding the exposure light amount control target value and the pulse width correction data. That is, it is a normal end when the target is achieved.

Next, when the control is performed the predetermined number of times (execution of the maximum control number), that is, when the control is not normally completed, the process proceeds to the image forming condition changing step S5, and the test pattern is formed again, the toner adhesion amount is detected, and the determination is made. And repeat. However, if the steady-state deviation does not fall within the predetermined standard value for some reason, the control is repeated forever. Also, the maximum time required for control must be limited to a finite amount.

In the present embodiment, the change amount of the parameter relating to the manipulated variable with respect to the deviation from the target is given and the change amount is made to correspond to "0" with respect to the deviation "0", so the steady-state deviation approaches "0". However, if the effect on the gradation characteristics with respect to the change in the manipulated variable is changing due to history or the like, the number of repetitions (control times) required for convergence may increase or decrease.

Therefore, it is also sufficiently effective to change the exposure condition so as to reduce the large deviation qualitatively with the permitted number of times of control. Therefore, by measuring the number of times the image formation has been changed after the control is started with a counter (not shown), the apparatus can be operated with the exposure light amount control target value and the pulse width correction data at the time when the predetermined number of times of control is performed. Will be in a standby state.

Next, when the calculation results of the bias value, the exposure amount, and the pulse width correction coefficient satisfy the predetermined conditions (operation amount limit), that is, the calculated values of the bias voltage value and the exposure amount target value to be changed. As the target voltage actually set, the output voltage corresponding to the value set in the D / A converter (not shown) is sent as a comparison voltage signal of the high voltage power supplies 35, 45 or the light amount control circuit 74. The set value and the output voltage value to the D / A converter are adjusted in advance and the set target value is output.

However, the calculated value is the high voltage power source 3
5, 45, when the output of the light amount control circuit 74 is out of the variable range, the output voltage recognized by the control unit 36 is different from the actual output voltage, and erroneous control may be performed.

Further, it must be varied within a range in which image defects such as fogging and defects such as deterioration of the laser diode 65 are not likely to occur. Furthermore, when the difference between the correction data and the different original data calculated from the resolution of the pulse width correction data cannot be resolved, when performing the pseudo gradation process, image disturbance such as texture may occur.

Therefore, when the range of the predetermined upper limit value and the lower limit value of the exposure light amount control target value and the pulse width correction coefficient is within the predetermined range, the setting is actually changed. In the case other than this condition, the condition setting is not performed, the control is ended in the state where the set exposure light amount control target value and the pulse width correction coefficient (correction data) are held, and the standby state is set.

Next, when the output of the toner adhesion amount measuring unit 8 becomes a predetermined condition (sensor output abnormality), that is, the output of the toner adhesion amount measuring unit 8 is reflected in areas other than the test pattern area of the photosensitive drum 2. The light amount, the reflected light amount of the high-density test pattern, and the reflected light amount of the low-density test pattern are transferred by photoelectrically converted signals, and are recognized by the control unit 36 via the A / D converter 46. At this time, the power supply of the toner adhesion amount measuring unit 8 is defective, the light source 51 is deteriorated, the light projecting / receiving light path is dirty, the light receiving circuit is defective between the sensor and the receiving circuit, the photosensitive drum 2 is scratched, and light is reflected such as filming. The detection accuracy may deteriorate and the control system may malfunction due to a change in the rate, a defect in the test pattern image forming system, or the like.

Therefore, a predetermined upper and lower limit is set for each of the output values corresponding to the amount of reflected light outside the test pattern area of the photosensitive drum 2, the amount of reflected light for the high density test pattern, and the amount of reflected light for the low density test pattern. Provided,
When any one of the output values is out of the range, thereafter, the calculation and determination are not performed, the sensor abnormality flag is set, the toner adhesion amount measuring unit 8 is displayed on the operation panel as an abnormality, and the toner adhesion amount is detected. The standby state is maintained in a state where the state before the abnormality of the measuring unit 8 occurs is held.

The sensor abnormality flag is reset by the initial processing while the device power is on. Also,
It can also be reset by a serviceman during maintenance by inputting a reset command from the operation panel. Further, when the sensor abnormality flag is set, control is not performed.

Next, the detection sequence (test pattern image formation, development, toner adhesion amount measuring step) will be described.

In the first embodiment, the image formation of the test pattern is performed at the same timing as the normal printing operation except for the transfer, paper feed, paper discharge operation and fixing operation. The reason why the transfer is turned off is that the toner does not scatter on the photosensitive drum 2 without the transfer material (paper).

The light source 51 of the toner adhesion amount measuring unit 8 can be turned on / off by a light source remote signal of the control unit 36, and after the time required for the light amount to stabilize after turning on,
It turns on at a timing that can be detected.

Therefore, in the normal printing operation, the light source 51 of the toner adhesion amount measuring section 8 does not emit light. This is because the unexposed portion potential, which is the surface potential that has not been exposed before transfer, is photo-electrified by the light emitted from the light source 51 to prevent dust or toner from scattering in the image. Since it is performed at the same position in the axial direction of the body drum 2, it is intended to prevent adverse effects on image quality due to light fatigue of the photoconductor drum 2 in that portion for a long term.

In the second embodiment, the transfer drum 9 is provided, and not only the transfer, sheet feeding, and sheet discharging operations, but also the suction and peeling operations are not performed. On the transfer drum 9, only the transfer material support is cleaned. As a result, the amount of the test pattern toner image developed on the photosensitive drum 2 attached to the transfer material support is extremely reduced. Therefore, it is possible to form a test pattern and measure the toner adhesion amount without considering the positional relationship with the transfer drum 9.

Next, input and storage of various set values will be described. The various setting values related to the control of this embodiment include, for example, the initial exposure light amount control target value, the initial pulse width correction count value, the test pattern gradation data (high density portion, low density portion), and the target value of the high density portion. , Target value of low density part, control standard value for high density part deviation, control standard value for low density part deviation, maximum correction data value, predetermined print number, predetermined elapsed time, maximum control count, exposure condition predetermined range, sensor abnormality There is a range.

Each of these set values is stored in advance in a storage means such as a memory 47 that is backed up even if the power of the apparatus is turned off.
Further, at least the target value of the high density portion and the target value of the low density portion can be changed and input and displayed from an operation panel (not shown).

[0178]

As described above in detail, according to the present invention, the amount of developer attached to a high-concentration test pattern and the amount of developer attached to a low-concentration test pattern are respectively measured and set in advance. The deviation from the target value is calculated, and when the calculated deviation is not within the predetermined range, the exposure condition of the exposure unit and the bias voltage of the developing unit, or
The exposure amount of the exposure unit and the bias voltage of the charging unit and the developing unit, or the bias voltage of the charging unit and the developing unit and the light emission time of the exposure unit are changed.

As described above, when the potential relationship is changed in order to maintain the gradation characteristic in the development, there is a correction effect close to the initial gradation characteristic with respect to the change in the development characteristic due to the environment and the passage of time.
However, depending on the developing system including the developer to be controlled, there is a possibility that an image defect may occur due to the change in the potential relationship, that is, the bias change amount. It does not occur when the exposure amount and pulse width correction characteristics are changed. However, the exposure amount,
Changing the pulse width correction characteristics cannot completely correct the changed development characteristics.

Therefore, by changing the potential relationship by changing the bias and changing the exposure amount and the pulse width correction characteristic, the bias changing amount can be suppressed, and as a side effect of changing the image forming conditions, image defects such as fogging and It is possible to maintain the gradation characteristics from the high density region to the low density region without causing problems. Furthermore, since the amount of change from the set value at the time of detection is calculated, it is possible to reduce the steady-state deviation with respect to the target value by repeating the control.

Therefore, according to the present invention, the image quality equivalent to that in the initial state without image defects can be maintained, the fluctuation of the image density due to changes in the environment and aging does not depend on the maintenance, and is less than the maintenance cycle. An image forming apparatus that can be optimized in a short cycle, can achieve high image density stability, and can reduce maintenance costs (labor costs, equipment, etc.).

[Brief description of drawings]

FIG. 1 is a block diagram relating to a charging, exposing, developing means and its control means of a color laser printer according to an embodiment of the present invention.

FIG. 2 is a schematic configuration diagram of a color laser printer.

FIG. 3 is a schematic diagram showing a relationship between a high-density portion corresponding to high-density gradation data developed on a photosensitive drum, a low-density portion corresponding to low-density gradation data, and a toner adhesion amount measuring unit for these. FIG.

FIG. 4 is a block diagram showing a configuration of a toner adhesion amount measuring unit.

FIG. 5 is a diagram showing a functional block relating to a flow of image data and an exposure system.

FIG. 6 is a diagram showing an unexposed portion potential, an exposed portion potential, and a developing bias voltage of a photosensitive drum with respect to a grid bias voltage of a charger.

FIG. 7 is a diagram showing an image density of a solid portion with respect to a contrast potential.

FIG. 8 is a diagram showing the relationship between the potential of an unexposed portion on the surface of the photosensitive drum, the potential of a low density pattern, and the developing bias voltage.

FIG. 9 is a diagram showing a toner adhesion amount with respect to gradation data when the background potential is increased.

FIG. 10 is a flowchart for explaining a processing operation of a main part.

FIG. 11 is a flowchart mainly illustrating image forming condition changing processing according to the first embodiment.

FIG. 12 is a diagram showing the relationship between gradation data and output image density for explaining the effect of changing the light amount.

FIG. 13 is a diagram showing a change in gradation characteristics when the background potential is changed.

FIG. 14 is a diagram showing the timing of changing the grid bias voltage and the developing bias voltage.

FIG. 15 is a diagram showing the contents of a table relating to an exposure amount change amount.

FIG. 16 is a diagram showing the contents of a table relating to the amount of change in background potential.

FIG. 17 is a diagram showing a variation example of gradation characteristics.

FIG. 18 is a diagram showing an example of variation in gradation characteristics.

FIG. 19 is a diagram showing the contents of a table relating to an exposure amount change amount.

FIG. 20 is a diagram showing the contents of a table relating to the amount of change in background potential.

FIG. 21 is a diagram showing the contents of a table relating to the amount of change in contrast potential.

FIG. 22 is a diagram showing the contents of a table relating to the amount of change in background potential.

FIG. 23 is a diagram illustrating a change in the toner adhesion amount which is an input of the measurement system in the control process.

FIG. 24 is a diagram illustrating a change in a bias value that is an input of the measurement system in the control process.

FIG. 25 is a flowchart mainly illustrating image forming condition changing processing according to the second embodiment.

FIG. 26 is a diagram showing a change in gradation characteristics when the contrast potential is changed.

FIG. 27 is a diagram showing a relationship between changes in pulse width correction characteristics and gradation characteristics.

FIG. 28 is a diagram showing a linearly approximated relationship between original data and corrected data.

FIG. 29 is a diagram showing the contents of a table relating to the amount of change in contrast potential.

FIG. 30 is a diagram showing the contents of a table relating to the amount of change in pulse width correction coefficient.

FIG. 31 is a diagram showing the contents of a table relating to the amount of change in contrast potential.

FIG. 32 is a diagram showing the contents of a table relating to the amount of change in pulse width correction coefficient.

FIG. 33 is a diagram showing the contents of a table relating to the amount of change in contrast potential.

FIG. 34 is a diagram showing the contents of a table relating to the amount of change in pulse width correction coefficient.

FIG. 35 is a diagram illustrating a change in the toner adhesion amount, which is an input of the measurement system in the control process.

FIG. 36 is a diagram for explaining changes in the bias value which is an input of the measurement system in the control process.

[Explanation of symbols]

2 ... Photosensitive drum (image carrier), 3 ... Charging device (charging means), 4 to 7 ... Developing device (developing means), 8 ... Toner adhesion amount measuring unit, 9 ... Transfer drum, 13 ... ... optical system (exposure means), 14 ... laser beam light, 20 ... transfer charger, 27 ... fixing device, 34 ... corona high voltage power supply, 35
...... High voltage power supply for grid bias, 36 ...... Control section, 3
7 ... gradation data buffer, 38 ... laser drive circuit,
39 ... Pattern generating circuit, 44 ... Developing roller, 45
...... High voltage power supply for developing bias, 46 ...... A / D converter,
47 ... Memory, PT1 ... High-density test pattern portion (high-density portion), PT2 ... Low-density test pattern portion (low-density portion).

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁵ Identification code Internal reference number FI Technical indication H04N 1/23 103 B 9186-5C 1/29 E 9186-5C

Claims (3)

[Claims] 1. An exposure unit capable of forming a latent image of a high-density portion and a low-density portion on an image carrier based on image data and changing exposure conditions such as emission intensity or emission time, and applying a bias voltage. And a developing means for developing the latent image of the high-density portion and the low-density portion on the image carrier formed by the exposing means with a developer, and the high density toner adhered on the image carrier by the developing means. A developer adhesion amount measuring means for measuring the adhesion amount of the developer to the high density portion and the low density portion, respectively, and respective measurement values of the high density portion and the low density portion measured by the developer adhesion amount measuring means are set in advance. A first calculating means for calculating a deviation from each of the target values, a judging means for judging whether or not each deviation calculated by the first calculating means is within a predetermined range, and Specified range by means If it is determined not within,
First change amount information relating to a change in the exposure condition of the exposure unit from the relationship of the respective deviations, a contrast potential that is a difference between the bias voltage of the developing unit and the exposed portion potential of the image carrier, and Second calculating means for calculating second change amount information relating to a change of the background potential which is a difference between the bias voltage of the developing means and the unexposed portion potential of the image carrier, and the second calculating means. An image forming apparatus comprising: a control unit that changes the exposure condition of the exposure unit according to the calculated first change amount information, and changes the bias voltage of the developing unit according to the second change amount information. . 2. A charging means for charging the surface of an image carrier and controlling the amount of charge by an applied bias voltage; and a high density on the image carrier charged by this charging means based on image data. Means for forming a latent image of a high density area and a low density area, the exposure amount of which can be controlled according to the target exposure amount information, and a high density on the image carrier formed by the exposure means when a bias voltage is applied. Means for developing a latent image of a high density portion and a low density portion with a developer, and a developing means for measuring the amount of the developer adhered to the high density portion and the low density portion adhered on the image carrier by the development of the developing means, respectively. Agent adhesion amount measuring means, and first calculating means for calculating deviations between respective measured values of the high density portion and the low density portion measured by the developer adhesion amount measuring means and respective preset target values. And this first Each deviation calculated by means output and the determining means for determining whether each is within a predetermined range, if it is determined not within the predetermined range by the determination means,
Based on the relationship between the respective deviations, the first change amount information relating to the change of the light amount of the exposing unit, and the change of the background potential which is the difference between the bias voltage of the developing unit and the unexposed portion potential of the image carrier are Second calculation means for calculating the second change amount information, and target exposure amount information for the exposure means are calculated by the first change amount information calculated by the second calculation means, A third calculating means for calculating the bias voltage applied to the charging means and the bias voltage applied to the developing means, respectively, based on the change amount information and the surface potential characteristic of the image carrier. Image forming apparatus. 3. A charging means for charging the surface of an image bearing member, the charge amount of which is controlled by an applied bias voltage; and a high density based on image data on the image bearing member charged by the charging means. Means for forming a latent image of a high density portion and a low density portion, and a bias voltage is applied, and the latent image of a high density portion and a low density portion on the image carrier formed by the exposure means is developed with a developer. Developing means, and a developer adhesion amount measuring means for respectively measuring the adhesion amount of the developer to the high density portion and the low density portion adhered on the image carrier by the development of the developing means, and per unit pixel of the image data And a pulse width per unit pixel based on the light emission time information corrected by the light emission time correction means. Modulation control means for performing modulation control of the exposure means, and a deviation between each measured value of the high density portion and the low density portion measured by the developer adhesion amount measuring means and a preset target value, respectively. First calculating means for calculating, determination means for determining whether or not each deviation calculated by the first calculating means is within a predetermined range, and this determining means determines that the deviation is not within the predetermined range. Then,
From the relationship of the respective deviations, the first change amount information relating to the change of the contrast potential which is the difference between the bias voltage of the developing means and the exposed portion potential of the image carrier, and the first change amount information relating to the change of the light emission time correction. Second calculation means for calculating the second change amount information, and a bias applied to the charging means by the first change amount information calculated by the second calculation means and the surface potential characteristic of the image carrier. An image forming apparatus comprising: a third control unit that calculates a voltage and a bias voltage applied to the developing unit, and calculates the emission time correction information based on second change amount information.