JPH09146313A - Image control method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reproduce excellent dots and line images together with a photographic image having gradation by performing control in the sequence of maximum density maintenance control, line width control and gradation correction control. SOLUTION: In this image forming device, not only the line width is detected but also maximum density correction and gradation correction are performed. A sensor WS for line width detection, a density detecting sensor DS1 for maximum density correction, and a density detecting sensor DS2 for gradation correction are arranged to be opposed to the peripheral surface of a photoreceptor drum 31 between a developing device 33 and a transfer device 34 and between a separating device 35 and a cleaning device 39. The sensor Ws is combined with the sensors DS1 and DS2 and uses a specified algorithm so as to perform image control in order of the maximum density maintenance control, the line width control and the gradation correction control, whereby the excellent images, such as characters, the dots and the line images, are always concurrently reproduced together with the photographic image even when an environmental condition is changed or even after copying many sheets is performed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image control method in an image forming apparatus for forming a digital image by an electrophotographic method.

[0002]

2. Description of the Related Art In a conventional image forming apparatus, in order to correct a change in the density of a copy image caused by a change in the toner density of a developer and other changes in conditions, it is necessary to correct the change in the density of a copy image.
Development in which a plurality of test pattern latent images are formed on the image carrier with a constant exposure intensity, and the rotational speed of the developer carrier is changed for each latent image (that is, the linear velocity of the developer carrier is changed). To develop a test pattern image group, detect the image density of the test pattern image group, and fix the linear velocity of the developer carrier of the developing means to a value at which a sufficient maximum density can be obtained by its output. To do. However, the developer of the developing device is provided with a magnetic permeability sensor at the bottom or the like to monitor the toner concentration of the developer, and when the toner concentration becomes lower than the specified toner concentration, the toner is replenished and the toner concentration of the developer is maintained at a constant value.

Thereafter, a plurality of test pattern latent images are formed with different exposure intensities, the test pattern latent images are developed by a developer carrier having a constant linear velocity, and the image density of the developed test pattern image group is detected. There is known an image forming apparatus which is detected by a means and controls to create a gradation correction curve based on a series of detected density data.

In this image forming method, the image density may be lowered and the gradation may be deteriorated due to the deterioration of the developing performance while continuing the image formation or the abnormal potential of the photosensitive layer of the image carrier. It is necessary to perform maximum density correction and gradation correction (γ correction) of the image for each constant.

In the conventional image forming method, the image density of the test pattern image group is detected by the density detecting means, the data is logarithmically converted, and the image density of the toner image on the image carrier and this toner image are recorded on the recording paper. In order to correct the difference from the image density when the image is transferred and fixed, an inverse function is created from the data of the value obtained by adding the logarithmically converted value of the recording paper to obtain a correction curve.

[0006]

The maximum density control is performed by determining the linear velocity of the developer carrying member, and the γ correction control is performed so that the gradation is γ = 1.0. Good reproduction can be performed. However, the above control does not control the characteristics such as the thickness of the thin line for the halftone dot and line image. Therefore, reproducibility of halftone dot and line images is not good due to changes in the number of copies and environmental conditions.

The present invention develops a line width detection method for a halftone dot image and a line image together with good reproducibility of a photographic image having gradation and measures the line width, and feeds back the variation. The width control method is used to reduce the effects of environmental changes, changes in the number of copies, and changes over time due to an increase in the number of prints so that good halftone and line images can be reproduced together with photographic images with gradation. An object is to provide an image control method.

[0008]

SUMMARY OF THE INVENTION An object of the present invention is to provide an image control method for performing maximum density maintenance control, gradation correction control and line width control, wherein the image control is maximum density maintenance control, line width control and gradation. This is achieved by an image control method characterized by performing control in the order of correction control.

It is a preferred embodiment that the line width control is a control for changing an exposure light amount for performing image exposure, and the image control of the present invention is carried out when an environmental change or an increase in the number of prints occurs. Doing is the preferred embodiment.

[0010]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Prior to the description of the present invention, an image control method for performing line width control will be described first.

The line width is detected by forming a latent image of a line on the image carrier with an exposure amount for writing a halftone dot or a line image, reversal developing it to form a line of a toner image, and then determining the line width of this line. Is read by a detection means that is a combination of a light emitting element and a light receiving element. The line width is detected because a change in the line width is detected and fed back to form an image on an appropriate line. The line width of a line is generally different between a vertical line (sub scanning direction line) and a horizontal line (main scanning direction line). The horizontal line on the image carrier is rubbed by the developing magnetic brush adhering to the developing sleeve, the toner is scraped off, and the line tends to be thin.

Therefore, if the line width of the horizontal line is detected and the line width is controlled, the vertical line that is not affected by the scraping of the developing magnetic brush becomes thick. Further, if the line width is controlled only by the vertical line, the vertical line may be broken (a part of the line is broken). Therefore, it is necessary to detect the line widths of both the vertical lines and the horizontal lines and to control the line widths accordingly.

When measuring the line width of the horizontal line on the image bearing member, the line moving direction becomes perpendicular to the sensor positioned as shown in FIG. It is easy to detect. However, it is not easy to measure the line width of the vertical line on the image carrier, and it is not possible to obtain the sensor output by moving the sensor in the horizontal direction and crossing the vertical line as shown in FIG. Although it is possible, the sensor output is related to the moving speed of the sensor, and the detection accuracy is deteriorated.

In order to detect the change in the line width of the vertical line,
As shown in FIG. 5A, an oblique line is drawn at a sandwiching angle θ with respect to the main scanning or the sub-scanning on the image carrier, and a change in the line width of the oblique line in the sub-scanning direction is detected. Then, this change is decomposed into a vertical component and a horizontal component so that a change in the line width of the vertical line and the horizontal line can be detected. FIG. 5 (b) is an explanatory view of this. Assuming that the diagonal line having a line width of F in the sub-scanning direction at first is ΔF thin, the thin portion of the vertical line ΔD (vertical) = ΔF · tan θ and the thin portion of the horizontal line ΔD ( Lateral) = ΔF.

By providing the oblique line and the sensor crossing in this way, it is possible to detect the change in the line width of the vertical line and the horizontal line at the same time, and the feedback considering the influence of each is used for image formation. Can be applied to laser power. As a simple method, the line for controlling the line width may be replaced with the line for the main scanning method.

Next, the line width measuring sensor used in the present invention will be described. FIG. 6A is a sectional configuration diagram.
(B) is a circuit diagram. The light emitted from the light emitting element PS1 is focused on an object SB to be measured such as an image carrier or a transfer paper, and the reflected light is received by the light receiving element PS2 to measure the line width of the line. A pinhole PS3 and a lens PS4 are provided between the object to be measured SB and the light emitting element PS1 side. A light emitting diode (LED) or a laser is used for the light emitting element PS1. A photo transistor or a photo diode is used for the light receiving element PS2.

As the lens PS4, a plano-convex or convex-convex condenser lens is used. The pinhole PS3 is provided so as to use the central portion of the light intensity having a Gaussian distribution of a light emitting diode or a laser, and the hole diameter of the pinhole is appropriately selected according to the light intensity distribution. The pinhole PS3 is omitted when the characteristics including the aberration of the lens PS4 are excellent and a spot having a small diameter of 30 μm or less, which will be described later, is formed on the object SB to be measured and flare does not occur on the periphery of the spot. can do.

The line width measuring sensor shown in FIG.
The light emitting element P is arranged in a direction perpendicular to the line of the object to be measured SB.
A small spot is formed by S1, and the light receiving element PS2 receives light at an angle of about 45 °. FIG. 7A shows two sets of light emitting elements PS1 with respect to the SB line of the DUT.
(A), PS1 (b) irradiates light from two directions,
Since the light is received by the light receiving element PS2 in the vertical direction, the influence of the shadow due to the swelling of the toner forming the line is offset, and the measurement accuracy is improved. However, it is necessary to superimpose and match the focal points of the two optical systems on the object to be measured SB, and a high degree of assembly / adjustment accuracy is required.

If the surface of the object SB to be measured has a mirror surface, the sensor configuration shown in FIGS. 6A and 7A receives the scattered component of the reflected light, so that the sensor output However, there is a problem in that the light receiving element needs to be highly sensitive. FIG. 6 (b) shows the correspondence to this, in which the light axes of the light emitting element PS1 and the light receiving element PS2 are aligned and the direct component of the reflected light is received.

FIG. 8 shows the relationship between the thickness of the line on the object SB to be measured and the spot diameter of the light emitting element PS1 and the sensor output thereof. In FIG. 8A, the spot diameter is larger than the line width of the line. In this case, (b) shows the case where the spot diameter is equal to the line width of the line, and (c) shows the case where the spot diameter is smaller than the line width of the line. As is clear from this figure, the minimum value of the line width of the line that can be determined by the line width measurement sensor is the same as the spot diameter of the focus on the object to be measured (FIG. 8B). If the line width of the line is narrower than the spot diameter, the sensor cannot detect the line width of the line (FIG. 8A).

When the sensor detects a line width of a line width larger than the spot diameter (FIG. 8C), the line width L of the line is the line output of the sensor at a certain threshold voltage V _S. It is a product of the part Δt sandwiched when cut and the relative linear velocity X of the line and spot at the time of scanning, and is expressed by L = X · Δt.

The threshold voltage V _S is 50% to 50% of the output V _M of the portion other than the line of the transfer paper or the image carrier.
It is appropriate to set it at the place where it is lowered by 60%. When the calculated value L does not match the actually optically measured line width, it may be corrected by multiplying L by a function or a numerical value.
Further, by normalizing the line output and cutting the line output at the set threshold, it is possible to measure the line width with higher accuracy.

Further, the spatial frequency (MTF) can be used to detect the change in the line width of the line. FIG. 9 is an explanatory diagram related to this. A plurality of lines are provided as a test pattern on the object to be measured at equal intervals (see FIG. 9).
(A)). The spot diameter formed by the light emitting element is set to be approximately the same as the line width of the line to be measured, and the test pattern is formed so as to have the same spacing as the spot diameter (FIG. 9B). When the sensor is scanned across the test pattern, a sensor output as shown in FIG. 9C is obtained. Thereby, MTF = (Vmax-Vmin) / (Vmax + Vmi
n) is required. If the line width of the test pattern changes, V
Since max and Vmin change and MTF changes, the change in line width can be detected from this.

In the image forming apparatus, a sensor is provided at a set position facing the image carrier between the developing device and the transfer device or between the separator and the cleaning device to detect a change in the line width of the line and provide feedback. Line width control is performed by applying. In measuring the line width, accuracy can be improved by forming a plurality of lines having the same line width in parallel and averaging the detected measurement values. As feedback, exposure conditions are generally controlled by pulse width modulation, laser power output change, and the like. FIG.
0 indicates the relationship between the laser power output and the line width of the line obtained by actual measurement, and the line width changes proportionally by changing the laser power output. In addition to detecting the change in the line width and applying the feedback, the feedback can be applied to the charging condition, the developing condition, the transfer condition, the fixing condition, etc. in addition to the laser power.

By applying the line width control method described above to an image forming apparatus, it is possible to excellently reproduce halftone dots and line images.

Next, an image control method for performing maximum density maintenance control and gradation correction control will be described.

Concentration detection sensor DS for maximum concentration maintenance control
_1. The density detection sensor DS ₂ for controlling gradation correction is provided between the developing device and the transfer device or between the separator and the cleaning device, facing the peripheral surface of the image carrier.

Density detection sensor DS ₁ and density detection sensor D
Since S ₂ is an approximate configuration, it will be described together. The density detection sensors DS ₁ and DS ₂ are, for example, as shown in FIG. 11A, light emitting elements which irradiate infrared light with an incident angle of about 40 ° which is installed in two mounting holes formed in the casing CK. The light emitting diode LED, the phototransistor PT which is a light receiving element for receiving light at a reflection angle of about 40 °, and the dustproof member BG such as glass for preventing contamination by toner or the like. The infrared rays have no sensitivity to the photosensitive layer of the image carrier, for example, have a wavelength of 900 to 9
Infrared light of 80 nm is used. A photodiode can be used instead of the phototransistor PT.

The density detection sensors DS ₁ and DS ₂ are shown in FIG.
It is combined with the electric circuit shown in FIG. 1 (b) to form a concentration detecting device. A variable DC power supply Vret having a maximum output of 10 V is connected to the anode terminal of the light emitting diode LED, which is a light emitting element, so that the amount of light emitted from the light emitting diode LED can be changed. The light emitting diode LED is connected in series with a current control resistor R ₈ and a semi-fixed resistor VR ₁ and a voltage of 10V is applied from a DC power source. The semi-fixed resistor VR ₁ adjusts the variation in the resistance value of the light emitting diode LED. After that, it can be fixed. Light emitting diode L
The ED is turned on when the terminal Te is connected to the ground.

Phototransistor PT which is a light receiving element
Is connected in series with the load resistance R ₇ and is
Power is applied. The output current of the phototransistor PT that receives the reflected light from the toner image illuminated by the light of the light emitting diode LED changes according to the intensity of the reflected light,
A voltage proportional to the output current of the phototransistor PT is generated across the load resistor R ₇ . This voltage is input to the (+) input terminal of IC1 which is an operational amplifier and amplified.
As a result, when the resistance connected between the output terminal and the (−) input terminal is R _5, and the resistance connected between the (−) input terminal and the ground is R ₆ , the resistance R ₅ is Vout = R ₅ / R ₆ × Vi between the output terminals Oa and Ob connected to both ends
The voltage of n is output. Where Vin is (+) of IC1
With the voltage applied to the input terminal, the voltage gain (voltage gain) Vout / Vin of the amplifier circuit in this case becomes R ₅ / R ₆ .
C1 is a capacitor for bypassing surge voltage and other noises.

To perform the maximum density maintenance control, first, referring to FIG.
As shown in (a), a latent image of a test patch for maximum density maintenance control is written on the image carrier (photosensitive drum) 31 at intervals of several mm in the sub-scanning direction. At this time, the exposure level is constant, and for example, in the case of an 8-bit digital signal by pulse width modulation (PWM), patch exposure is performed at a level 255 corresponding to solid black. This test patch is subjected to reversal development by changing the rotation speed of the developing sleeve of the developing device for each test patch latent image, and becomes a plurality of test patch images having different densities shown in FIG. 12B. This test patch image for maximum density maintenance control is subjected to density detection by the density detection sensor DS ₁ to detect the rotation speed (linear velocity) of the developing sleeve within the specified density range set in advance in the patch density data. However, at the time of image formation, the rotation speed (linear speed) of the developing sleeve is fixed so that this rotation speed (linear speed) is used.
Here, the specified density is set to 1.4. This is because if the density is 1.35 or higher, the quality of the copy image is sufficient. By such control, it is ensured that the image density is 1.4 or more in all environments. This maximum density maintenance control can also be performed by changing the toner density (mixing ratio) of the developer or changing the amount of the developer conveyed on the developing sleeve. However, changing the rotation speed of the developing sleeve causes toner stains and fog. It is excellent in that it does not occur.

To perform the gradation correction control, the photosensitive drum 31 is charged and the test pattern signal for gradation correction is sent to the semiconductor laser as in the maximum density maintenance control. In this test pattern, for example, in the case of 0-255 level of 8-bit digital signal, PWM signals of 8 levels are sent to the semiconductor laser.
Latent images of a plurality of test patches as shown in (a) are written every several mm in the sub-scanning direction. This latent image is reverse-developed by the developing device in which the rotation speed of the developing sleeve is fixed by the maximum density maintenance control described above, and becomes a plurality of tone correction test patch images having different densities shown in FIG. The sensor DS ₂ detects the concentration.

Here, a method of converting the output of the test patch image density detecting device into the density of the patch image will be described.

[0034] V ₀ the output voltage of the concentration measuring apparatus of the patch image of the PWM level of the gradation correction test patch image and _{0,8,16,24 ··· n ··· 255, V 1} , V 2 ，
When V ₃ ... Vn ... V ₃₁ and each temporary concentration is D _Pn , D _P0 = −logV ₀ / V ₀ D _P8 = −logV ₈ / V ₀ D _P16 = −logV ₁₆ / D _Pn is calculated as V ₀ Dn = −log Vn / V ₀ D _P31 = −log V ₂₅₅ / V ₀ , and D _P31 is the maximum concentration of 1.4.
Normalize all D _Pn so that The concentration of the transfer sheet to all D _Pn from 0.08 e.g. 0.08
Add. In this way, the density of the patch image on the fixed transfer paper as a copy image is converted.

The above gradation correction data is interpolated and is shown in FIG.
The continuous curve shown in (a) is obtained, and this curve represents the printer characteristics. As the interpolation method, linear interpolation can be sufficiently used. When this inverse function is taken, the curve shown in FIG. 14B is obtained, and this curve becomes the gradation correction curve. When the product of the curve of FIG. 14A and the curve of FIG. 14B is taken, a straight line of 45 ° (γ = 1.0) shown in FIG. 14C is obtained. Such a gradation correction curve is stored in the RAM of the image forming apparatus. In image formation, the image signal is corrected according to the gradation correction curve called from the RAM and then input to the writing unit to form a latent image. Therefore, the gradation change due to the deterioration of the photoconductor of the photoconductor drum 31 during use of the apparatus and the change of the printer characteristics due to the change of environmental conditions is corrected.

Next, the structure and operation of the image forming apparatus to which the present invention is applied will be described with reference to the drawings.

FIG. 1 is a schematic block diagram showing an embodiment of the image forming apparatus of the present invention, and FIG. 2 is a block diagram showing a control system of the apparatus of FIG.

First, a normal copy operation of this image forming apparatus will be described. The image forming apparatus includes an image reading unit 10, a writing unit 20 which is a digital writing system, an image forming unit 30, a paper feeding unit 40, a document placing unit 50, and the like.

On the upper portion of the image forming apparatus, a document placing section 5 including a document table 51 made of a transparent glass plate and a document cover 52 for covering a document D placed on the document table 51.
0, which is below the document table 51, and in the main body of the apparatus, an image reading unit 10 including a first mirror unit 12, a second mirror unit 13, an image pickup lens 14, an image pickup device 15 such as a CCD array, and the like is provided. Has been.

The image of the document D on the document table 51 is displayed by the illumination lamp 12A of the image reading unit 10 and the first mirror 12.
The parallel movement of the first mirror unit 12 including B from the solid line to the position indicated by the broken line, and the first mirror unit of the second mirror unit 13 that integrally includes the second mirror 13A and the third mirror 13B facing each other. The entire surface is illuminated and scanned by the following movement of 1/2 speed with respect to 12, and the image is formed by the imaging lens 14 on the imaging element 15 via the first mirror 12B, the second mirror 13A, and the third mirror 13B. It is like this. When scanning is completed, the first mirror unit 12 and the second mirror unit 13 return to their original positions, and stand by until the next image formation.

The image data obtained by photoelectric conversion by the image pickup device 15 is converted into a digital signal,
The image signal processing unit 60 performs processing such as MTF correction and γ correction, and temporarily stores it as an image signal in the memory.
Next, the image signal is read from the memory under the control of the CPU 90, pulse width modulated, and then input to the writing unit 20.

Under the control of the CPU 90, the image forming unit 30 writes the image signal in which the image signal is composed of a drive motor 21, a polygon mirror 22, an fθ lens 23, mirrors 24, 25 and 26, a semiconductor laser (not shown), a correction lens and the like. When it is input to, the image recording operation is started.
That is, the photoconductor drum 31, which is an image bearing member, rotates clockwise as shown by the arrow, and the charge removal for performing the pre-charge exposure and removing the charge is performed by the charge remover 36 and then by the charge device 32. Therefore, the electrostatic latent image corresponding to the image of the document D is formed on the photosensitive drum 31 by the laser beam L from the writing unit 20. After that, the electrostatic latent image on the photosensitive drum 31 is subjected to reversal development by the developer carried on the developing sleeve 33A, which is a developer carrying body to which the bias voltage of the developing device 33 is applied, and is visible toner. Become a statue.

On the other hand, from the paper feed cassette 41A or 41B loaded in the paper feed section 40, one transfer paper P of a specified size is transferred.
The carry-out roller 42A carries out the paper one by one, and the carry-out roller 4
The sheet is fed toward the image transfer portion through the sheet 3 and the guide member 42. The fed transfer sheet P is sent onto the photosensitive drum 31 by the registration roller 44 which operates in synchronization with the toner image on the photosensitive drum 31. The toner image on the photoconductor drum 31 is transferred to the transfer paper P by the action of the transfer device 34, separated from the photoconductor drum 31 by the charge removal action of the separator 35, and then fixed via the conveyor belt 45. After being sent to the container 37 and fused and fixed by the heating roller 37A and the pressure roller 37B, the discharge roller 3
It is discharged to the tray 54 outside the apparatus by 8, 46. 53
Is a paper feed table for manual feeding.

The photosensitive drum 31 continues to rotate, and the toner remaining on the surface of the photosensitive drum 31 without being transferred is removed and cleaned by the cleaning blade 39A that is in pressure contact with the cleaning device 39. The electric charge is uniformly applied by the device 32, and the process for image formation for the next time starts.

Incidentally, the stirring screw 33C of the developing unit 33
The magnetic permeability sensor TS provided at the bottom of the CPU monitors the toner concentration of the developer in the developing unit 33 by utilizing the fact that the magnetic permeability changes when the toner concentration of the developer changes, and the CPU 90 causes the toner concentration of the developer to be detected. It is a sensor that sends out information. CP
The U90 sends a toner replenishment instruction to the toner replenishing unit 33T to replenish the toner when the toner concentration decreases below a certain value according to the information of the magnetic permeability sensor TS, so that the toner concentration of the developer can be always kept constant. it can.

In this embodiment, a two-component developer comprising a toner of polyester type having a weight average particle diameter of 8.5 μm and a carrier having a resin coating on ferrite and a weight average particle diameter of 60 μm is used as the developer, and the toner density is 6 to 6. Although 9% was used, the toner density variation could be kept within ± 0.3% by the above toner density control.

37A and 37B of the fixing device 37 are an upper roller and a lower roller which are a pair of rotating members for fixing.

A heater 37D composed of a halogen lamp or the like is provided at the inner core of the upper roller 37A and the lower roller 37B.
(Upper roller 1, 100 W, lower roller 200 W) are provided. The ambient temperature of the upper roller 37A and the lower roller 37B is detected by a temperature sensor 37C such as a thermistor and sent to the CPU 90.
The PU 90 controls the heater 37D to keep it within an allowable range of a predetermined temperature Tc which is a fixing control temperature.

The lower roller 37B is applied to the upper roller 37A by a constant pressure, for example, a linear pressure of 3.
It is designed to be pressed at 7 kg / cm. The upper roller 37A rotates clockwise, and the lower roller 37B comes into pressure contact with the upper roller 37A and is driven to rotate.

In the image forming apparatus described above, the photosensitive drum 31 is a drum coated with an OPC photosensitive member that is negatively charged, and has a writing density of 400 dpi (Dot per in).
ch) is used as a standard for image formation.
An encoder (not shown) is provided on the rotary shaft of the photosensitive drum 31. The phase signal from this encoder is the CPU 9
It is sent to 0 and is used for process control that needs to know the image position accurately.

The image forming apparatus of this embodiment performs the maximum density correction and the gradation correction together with the line width detection. The line width detection sensor WS, the maximum density correction density detection sensor DS ₁ , The density detection sensor DS ₂ for correcting the gradation is opposed to the peripheral surface of the photosensitive drum 31, and is connected to the developing device 33 to the transfer device 34.
And between the separator 35 and the cleaning device 39.

In the image forming apparatus of the present invention, the sensor WS for line width detection is combined with the density detection sensor DS ₁ for maximum density correction and the density detection sensor DS ₂ for gradation correction, and the algorithm shown in FIG. By executing the above, it is possible to control both the gradation property and the line width in a compatible manner, and the photographic image and the halftone dot / line image in a compatible manner with respect to changes due to the number of copies, the number of prints, the environment, and the like. .

By the gradation correction control (γ correction control) and the maximum density maintenance control (Dmax control), a photographic image can always be reproduced equally without being affected by the environment or changes with time (copy number or print number). . However, the characters, halftone dots, and line images cannot be controlled by the above two controls,
The line width becomes thicker, crushed, or faded depending on the environment or changes with time, and the reproducibility changes.

Therefore, the line width of a standard line at the time of factory shipment or installation of the device is stored in the memory, and the line width of the reference line at that time is compared with the standard line width stored in the memory at the time of subsequent image formation. However, the line width is controlled by feeding back the exposure light amount (laser power) of the exposure condition based on the difference, so that the initial line width is maintained and the characters, halftone dots, and line images are not affected by the environment or changes over time. Can be reproduced in the same manner as the image in.

However, in the line width control in which the laser power is fed back, the exposure potential (bright portion potential) on the image carrier changes, so that the reflection density in the state where the toner adheres after development is Therefore, the reproducibility of the photographic image controlled by the previously set maximum density maintenance control and gradation correction control changes. That is, there is a problem that the photographic image and the character / halftone dot / line image cannot be controlled in a compatible manner by the above control method. As a result of the study, the present inventors have newly introduced the algorithm shown in FIG. 3 and made it possible to control a photographic image and a character / halftone dot / line image at the same time.

The feature of the image control according to the present invention shown in FIG. 3 is that first, the maximum density maintaining control is performed according to the environment at the time of executing the control and temporal change to temporarily determine the developability at that time, and then the line width control. Line width control until the initial line is reproduced, the maximum density maintenance control is performed with the laser power at this time to determine the darkest (highest) density, and then the gradation correction control is performed. In this way, the maximum density maintenance control is performed after performing the line width control, and the gradation correction control is performed at the end to perform the line width control, the maximum density maintenance control, and the gradation correction. It became possible to achieve both control.

If the gradation correction control is performed prior to the maximum density maintenance control, the maximum image density of the gradation determined by the gradation correction control when the developing property changes due to environmental changes and the maximum density performed thereafter. The maximum image densities determined by the maintenance control are inconsistent, and smooth γ = 1.0 gradation cannot be obtained. FIG. 15 shows the γ characteristic. FIG. 15A shows the γ characteristic when the maximum density maintenance control is performed after the developing property during gradation correction is too low, and FIG. The γ characteristic is shown when the maximum density maintenance control is performed after the developability during tone adjustment is too high.

If the maximum density maintenance control is performed before the line width control and not performed thereafter, the laser power is changed by this line width control, so the maximum image density determined by the maximum density maintenance control changes, There is a high possibility that the image quality of the copy will be too dark or too light due to the required image density being over or under, which is not preferable.

Therefore, in order to control the photographic image and the character / halftone / line image at the same time, it is necessary to perform maximum density maintenance control, line width control, and gradation correction control in this order according to the algorithm of FIG. .

In this embodiment, the maximum density maintenance control is performed by the rotation speed of the developing sleeve, but the system which performs the maximum density maintenance control by means different from this embodiment,
Alternatively, in the image forming apparatus of the system in which the maximum density maintenance control is not performed at all, the algorithm shown in FIG. 3 does not require the maximum density maintenance control, and the algorithm is performed in the order of line width control → gradation correction control.

[0061]

【Example】

 (Example 1) Image forming conditions are as shown in Table 1 below.

[0062]

[Table 1]

During the image formation for a long period of time, a temporary decrease in the image density was observed. Therefore, experimentally, the rotation speed of the developing sleeve and the toner density were forcibly increased and the algorithm shown in FIG. Image control was performed.

Image Control Method Under the image forming conditions shown in Table 1, at the time of factory shipment, the rotation speed of the developing sleeve is 254.1 rpm (sleeve / image carrier linear velocity ratio 1.90), laser power is 0.420 mW, The toner density is set to 6%, and maximum density maintenance control and gradation control are performed. In this situation, a program recorded in the ROM 95 draws a line having a width of 200 μm on the image carrier, and the line width detection sensor WS reads the line and stores it in the memory 91. This situation is the initial setting.

At the time when a decrease in image density is observed,
The development sleeve rotation speed is increased to 375.0 rpm (sleeve / image carrier linear velocity ratio of 2.81), the toner density in the developing device is increased to 9%, and maximum density maintenance control is performed.
A line is drawn on the image carrier under the same exposure conditions as when a line having a width of 0 μm is drawn, and this is read by the line width detection sensor WS. This situation is referred to as an overdeveloped state.

In the overdeveloped state, the change in the line width of the line on the image carrier with respect to the initial setting is obtained, and the change in the line width is recorded in the RAM 96 as shown in FIG.
The line width control of the line is performed using the laser power conversion table of No. 6 and the algorithm of FIG. That is, the shift amount of the laser power is determined by using the table of FIG. 16 according to the change of the line width, the laser power is shifted, and the maximum density maintaining control is performed. Here, the line is drawn again, and the change in the line width with respect to the initial line is obtained. When the change is outside the allowable range (for example, 10 μm or more), the laser power is finely adjusted and the maximum density maintenance control and the line width control are performed again. When the change amount of the line width is within the allowable range, gradation correction control is performed. This situation is defined as after line width control.

The relationship between the line width variation and the laser power correction value shown in the laser power conversion table of FIG. 16 is effective in the developability under the above-mentioned conditions where the toner density is increased, and the developability is changed. Or, another conversion table is needed when the environment changes. Therefore, it is advisable to prepare a table in advance under typical conditions such as the number of copies and the environment, select the table closest to the situation when executing line width control, and use this to perform control.

Image Control Results At the time of initial setting, the overdeveloped state, the respective gradations after line width control, and the line width will be compared. FIG. 17 shows the gradation characteristics after the initial setting, the overdeveloped state and the line width control. In all cases, γ = 1.0 with almost the same inclination.
Therefore, at the time of initial setting, the gradation property is controlled after the excessive development state and after the line width control.

The change in the line width at the time of initial setting of the line and in the state of excessive development was a line thicker by 50 μm as measured by the line width detecting sensor WS (see FIG. 18).
Therefore, a line width control for weakening the output of the laser power of 0.07 mW is performed by using the conversion table of the laser power for the line width change of 50 μm in FIG. The line width of the line drawn on the image carrier at this time (after the line width control) was measured by precise measurement, and the change in the line width from the initial value after the line width control was obtained. It was confirmed that the line width was controlled up to the initial line width of 200 μm as shown in FIG.

Example 2 The image forming conditions are shown in Table 2 below.

[0071]

[Table 2]

The reproducibility of thin lines changes depending on the environment. This is because the sensitivity characteristic of the image carrier changes depending on the environment as shown in FIG. Same exposure amount (P
The surface potential of the image carrier also varies depending on the environment depending on the WM value), so that the reproducibility of the fine line varies depending on the environment. The effect of environmental difference is compensated for the gradation property by the maximum density maintenance control and the gradation correction control, but the reproducibility of the fine line cannot be controlled by these two controls. Therefore, the exposure light amount (laser power) is changed in order to compensate for the environmental difference in fine line reproducibility.

In this embodiment, since the environmental conditions fluctuated, the image control was automatically performed using the algorithm shown in FIG.

Image Control Method Under the image forming conditions shown in Table 2, the laser power was set to 0.420 mW at the time of factory shipment, and the normal temperature and normal humidity (NN) environment (2
Maximum density maintenance control and gradation correction control are performed at 0 ° C. and 50% RH. In this situation, a line having a width of 200 μm is drawn on the image carrier by the program recorded in the ROM 95, the line width detecting sensor WS reads the line, and the line is detected and stored in the memory 91. This situation is the initial setting.

For example, a high temperature and high humidity (HH) environment (30
When the temperature reaches 80 ° C. and 80% RH), environmental condition detection means (not shown) provided in the image forming apparatus detects this, or performs maximum density maintenance control by an external instruction to reset the image forming condition. A line is drawn on the image carrier under the same exposure conditions as when the line having a width of 200 μm is drawn, and the line width detection sensor WS reads the line. This condition is referred to as high temperature and high humidity.

In the above situation, the change in the line width of the line on the image carrier with respect to the time of the initial setting is obtained, and the laser power conversion table of FIG. The line width of the line is controlled using the algorithm of No. 3. In other words, the shift amount of the laser power is determined by using the table of FIG. 20 according to the change of the line width, the laser power is shifted, and the maximum density maintaining control is performed. Here, draw the line again, find the change in line width from the initial line, and if the change is within the allowable range,
Performs gradation correction control.

When the change of the line width with respect to the initial stage of the line is outside the allowable range (for example, 10 μm or more), the laser power is finely adjusted, the maximum density maintenance control and the line width control are performed again, and the line width of the line is changed. Gradation correction control is performed when the amount of change falls within the allowable range. This situation is after environmental control.

Image Control Result At the time of initial setting, the high temperature and high humidity state and the gradation after the environmental control and the line width are compared. FIG. 21 shows the gradation at the time of initial setting, in a high-temperature and high-humidity state and after environmental control. In all cases, γ = 1.0 with almost the same inclination. Therefore, the gradation is controlled in the initial setting, in the high temperature and high humidity state, and after the environmental control.

A line thickened by about 50 μm as the amount of change in the line width in the high temperature and high humidity state from the initial setting of the line is set to 200 μm at the initial setting after the environmental control because the laser power is weakened by about 0.1 mW. Controlled up to. (See FIG. 22) Therefore, it was confirmed that the line width was also controlled.

(Embodiment 3) The image forming conditions are the same as those in Embodiment 2 (Table 2).

Change over time (increase in copy number and print number)
Due to this, the reproducibility of fine lines will change. This is
This is because the residual potential of the image carrier rises and the maximum exposure potential rises as a result of the change with time. Since the intermediate exposure potential also rises when the maximum exposure potential rises, the line image gradually becomes thicker due to the change with time.
Gradation is compensated for the influence due to aging by the maximum density maintenance control and the gradation correction control, but the fine line control cannot be controlled by these two controls. Therefore, in order to compensate for the change in fine line reproducibility over time, the exposure light amount (laser power) is changed to be weakened.

In this embodiment, since the number of copies reached a predetermined number, image control was automatically performed using the algorithm shown in FIG.

Under the image forming conditions of Table 2, the laser power was set to 0.420 mW at the time of factory shipment, and the normal temperature and normal humidity (N
N) Maximum density maintenance control and gradation correction control are performed in an environment (20 ° C., 50% RH). In this situation, the program recorded in the ROM 95 causes the image carrier to have a thickness of 200 μm.
A line of width is drawn, and this is read by the line width detection sensor WS and stored in the memory 91. This situation is the initial setting.

For example, when a large number of copies (100,000 copies) have been copied, a copy number detection unit (not shown) provided in the image forming apparatus detects this or re-examines image forming conditions from the outside. The maximum density maintenance control is performed according to the setting instruction, a line is drawn on the image carrier under the same exposure conditions as when the line having a width of 200 μm is drawn, and this is read by the line width detection sensor WS. This situation is referred to as a time-dependent change state.

In the above situation, the change in the line width of the line on the image carrier with respect to the initial setting is obtained, and the laser power conversion table of FIG. 24 recorded in the RAM 96 is used from the change in the line width. The line width of the line is controlled using the algorithm of No. 3. That is, the shift amount of the laser power is determined by using the table of FIG. 24 according to the change of the line width, the laser power is shifted, and the maximum density maintaining control is performed. Here, draw the line again, find the change in line width from the initial line, and if the change is within the allowable range,
Performs gradation correction control.

When the change of the line width with respect to the initial stage of the line is out of the allowable range (for example, 10 μm or more), the laser power is finely adjusted and the maximum density maintenance control and the line width control are performed again to determine the line width of the line. Gradation correction control is performed when the amount of change falls within the allowable range. This situation is referred to as after time-dependent change control.

The number of copies to be controlled (100,000 copies in this embodiment) is not absolute, and the appropriate number of copies (prints) shown in FIG. 3 is obtained in accordance with the rise of the residual potential of the image carrier. It is desirable to perform image control by an algorithm to correct the line width. Also, the potential on the image bearing member is monitored by a surface potential sensor or the like, and when the residual potential rises above the allowable range, the algorithm of FIG. 3 is used,
The line width may be corrected.

Image control result At the time of initial setting, the state of aging, the gradation property after aging control, and the line width are compared. Figure 25 shows the initial settings
It shows the state of aging and the gradation after aging control, and in both cases, γ = 1.0 with almost the same slope.
Therefore, at the time of initial setting, the gradation property is controlled after the time-dependent change state and after the time-dependent change control.

Further, as shown in FIG. 26, the change amount of the line width in the time-dependent change state of the line with respect to the initial setting is 30.
The line thicker by about μm was controlled to 200 μm at the initial setting after the aging control by reducing the output of the laser power by about 0.02 mW according to FIG. Therefore, it was confirmed that the line width was also controlled.

[0090]

By using the algorithm of the present invention shown in FIG. 3 and performing image control in the order of maximum density maintenance control, line width control and gradation correction control, environmental conditions change or a large number of copies ( Even after printing, all images such as letters, halftone dots, line images, etc., as well as photographic images,
An image control method has been provided in which an excellent image is always reproduced at the same time.

[Brief description of the drawings]

FIG. 1 is a sectional view showing a schematic configuration of an image forming apparatus according to the present invention.

FIG. 2 is a block diagram showing a control system of the image forming apparatus according to the present invention.

FIG. 3 is a flow for performing an image control algorithm of the present invention.

FIG. 4 is an explanatory diagram related to line line width detection.

FIG. 5 is an explanatory diagram of line width detection of a diagonal line.

FIG. 6 is a cross-sectional configuration diagram and a circuit diagram of a line width measuring sensor.

FIG. 7 is a cross-sectional configuration diagram of a line width measuring sensor showing another embodiment.

FIG. 8 is an explanatory diagram showing a relationship between a line width of a line and a spot diameter.

FIG. 9 is an explanatory diagram of a line width measuring method using spatial frequency.

FIG. 10 is a graph showing the relationship between line width and laser power.

FIG. 11 is a cross-sectional configuration diagram and a circuit diagram of a concentration detection sensor.

FIG. 12 is a diagram showing a test patch image for maximum density maintenance control on an image carrier.

FIG. 13 is a diagram showing a test patch image for gradation correction on the image carrier.

FIG. 14 is a diagram illustrating how to determine a correction γ characteristic.

FIG. 15 is a diagram showing an example of γ characteristics.

FIG. 16 is a laser power conversion table during line width control in the first embodiment.

FIG. 17 is a graph showing the gradation property after initial line setting, overdevelopment state, and line width control in Example 1.

FIG. 18 is a graph showing changes in the line width after the line width control and the excessive development state in the initial setting in Example 1.

FIG. 19 is a graph showing changes in sensitivity characteristics of the image bearing member depending on environmental conditions.

FIG. 20 is a laser power conversion table at the time of line width control in the second embodiment.

FIG. 21 is a graph showing a gradation property after initial setting, high temperature and high humidity conditions, and environmental control in Example 2;

FIG. 22 is a graph showing changes in the line width of high temperature and high humidity conditions and environmental control at the time of initial setting in Example 2;

FIG. 23 is a graph showing changes in the maximum exposure potential with the lapse of time.

FIG. 24 is a laser power conversion table during line width control in the third embodiment.

FIG. 25 is a graph showing the state of time-dependent change, the gradation property after the time-dependent change control, in the initial setting in Example 3;

FIG. 26 is a graph showing changes in the line width after time-dependent change control and time-dependent change in initial setting in Example 3;

[Explanation of symbols]

10 image reading unit 20 writing unit 30 image forming unit 31 photoconductor drum (image carrier) 32 charger 33 developing unit 60 image signal processing unit 90 CPU 91 memory 95 ROM 96 RAM DS ₁ , DS ₂ density detection sensor WS line width Detection device (sensor)

Claims (3)

[Claims] 1. An image control method for performing maximum density maintenance control, gradation correction control, and line width control, wherein the image control is performed in the order of maximum density maintenance control, line width control, and gradation correction control. Image control method. 2. The image control method according to claim 1, wherein the line width control is control for changing an exposure light amount for performing image exposure. 3. The image control method according to claim 1, wherein the image control is performed at the time when the environmental change or the number of prints increases.