JPH08265570A - Image forming device - Google Patents

Abstract

PURPOSE: To accurately control the density of a desired halftone by determining the duty ratio of driving PWM of a laser optical system in accordance with the output of a toner quantity detector for plural stages of gradation patterns and a gradation table where relations to picture densities are stored. CONSTITUTION: A patch image is formed on a photosensitive drum 1 at a set prescribed peripheral speed ratio, and the toner quantity of the patch image is detected by a detecting sensor 15, and a CPU in a control part 20 calculates the quantity of toner stuck on the drum 1 based on the obtained output of the patch image. The quantity of toner stuck on the drum 1 is estimated in accordance with the peripheral speed radio based on the bias voltage on the surface of a developing sleeve 13 and the surface potential of the drum 1. That is, the control part 20 detects the dither gradation density for the change of PWM by a density sensor 15 and refers to a gradation table 21, where relations to picture densities are stored, to determine the duty ratio of PWM so as to obtain a desired image density gradation and drives a laser optical system 4. Thus, the picture of a desired halftone density is obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming apparatus.

[0002]

2. Description of the Related Art FIG. 7 is a conceptual diagram of the structure of an image forming apparatus. In the figure, 1 is a photoconductor drum which is an image carrier,
The drum is coated with an OPC photoconductor, which is grounded in terms of electric potential and is driven and rotated by a main motor (not shown) in a clockwise direction (arrow direction in the drawing). 2 is a charger,
The peripheral surface of the photosensitive drum 1 is uniformly charged with the potential VH by the corona discharge by the grid held at the potential VG and the corona discharge wire. Prior to the charging by the charging device 2, an exposure device using a light emitting diode or the like (for example, P
CL) 3 is used to remove the charge from the peripheral surface of the photoconductor.

After the photosensitive member 1 is uniformly charged, the laser optical system 4 performs image exposure based on an image signal. The laser optical system 4 uses a laser diode (not shown) as a light emitting source, and performs scanning by bending the optical path by a reflecting mirror 5 through a rotating polygon mirror, fθ lens, etc., and by rotating the photosensitive drum 1 (sub scanning). An electrostatic latent image is formed. Here, the character portion is exposed to form an inverted latent image in which the character portion has a lower potential VL.

A developing device 1 in which a two-component developer composed of toner and carrier is built in the periphery of the photosensitive drum 1
0 is provided, and the development is performed by a developing sleeve (not shown) that has a magnet built therein and holds the developer and rotates. The developer is a carrier in which a metal oxide is used as a core and an insulating resin is coated around the core, and a pigment is a polymer as a main material and a charge control agent, silica,
The developer is composed of toner to which titanium oxide or the like is added. The developer is regulated by the layer forming means to have a certain layer thickness (developer) of about several hundred μm and is conveyed to the developing area.

The gap between the developing sleeve and the photosensitive drum 1 in the developing area is 0.2 mm, which is larger than the layer thickness (developer).
.About.1.0 mm, during which the AC bias of the voltage value VAC and the DC bias of the voltage value VDC are superimposed and applied. Reference numeral 11 is an AC voltage source 11 or 12 that provides an AC bias voltage value VAC, and 12 is a DC voltage source that provides a DC bias voltage value. Since VDC and VH and toner charge have the same polarity,
The toner that has been given the opportunity to leave the carrier by VAC does not adhere to the VH portion, which has a higher potential than VDC,
It adheres to the VL portion having a lower potential than VDC and is visualized (reversal development).

On the other hand, the recording paper P carried out from the paper feed cassette 6 is temporarily stopped and is fed to the transfer area when the transfer timing is adjusted. In the transfer area, the transfer unit 7 transfers the toner image formed on the surface of the photosensitive drum 1. Then, the recording paper P is discharged at almost the same time by a separating brush (not shown) that is brought into a pressure contact state, separated by the peripheral surface of the photosensitive drum 1, and conveyed to the fixing unit 8.
After the toner is fused by heating and pressurizing the heat roller and the fixing roller (not shown), the toner is discharged to the outside of the apparatus through the paper discharge roller.

On the other hand, the photosensitive drum 1 from which the recording paper P is separated
Removes and cleans the residual toner by pressing the cleaning blade 9 and again receives the charge removal by the exposure device 3 and the charge by the charging device 2, and the next image forming process starts. The cleaning blade 9 moves immediately after cleaning the surface of the photoconductor and retracts from the peripheral surface of the photoconductor drum 1.

Although the case of forming a monochrome image with one developing device 10 has been described above, a plurality of developing devices 10 may be provided to form a color image. In that case, the developing device 10 is set to Y (yellow), M (magenta),
By providing four C (cyan) and K (black) toner images of four types on the photosensitive drum 1 and transferring them onto the recording paper P, a color image can be formed.

[0009]

In the above-mentioned image forming apparatus, a reference patch for controlling the image density of the photosensitive drum 1 is provided, the reference patch is read by the detection sensor, and the toner is read according to the read output. Generally, process control is performed to control the supply amount or laser power. The process control system, which reads the output of the reference patch with a detection sensor and controls the image forming conditions (for example, the developing sleeve rotation speed and laser power), is not only accurate in terms of operation accuracy but also the image density of the electrophotographic process (charging and exposure). -It is generally difficult to perform optimum control because it changes under the influence of variable factors such as development, transfer, and fixing. The reason is that, as described above, the materials and machines change moment by moment due to changes in the electrophotographic process, usage environment, number of prints, and the like. The effect of this process variation is particularly great for halftone images.

The present invention has been made in view of the above problems, and it is an object of the present invention to provide an image forming apparatus capable of accurately controlling the image density of a halftone image.

[0011]

SUMMARY OF THE INVENTION The present invention which solves the above-mentioned problems provides an image forming apparatus for forming an image by adhering toner onto an image carrier, and forming an image density detecting pattern on the image carrier. Pattern forming means for forming, image density detecting means for detecting the image density of the toner adhered on the image carrier, and a control section for controlling the image density according to the density detected by the image density detecting means, It is characterized in that it is provided with a gradation table which is provided in the control section and stores the relationship between the image density and the output of the image density detecting means of the gradation pattern of a plurality of stages created by the pattern forming means.

In this case, when controlling the image density, the control section refers to the gradation table based on the output of the image density detecting means and determines the PWM duty ratio for obtaining the desired density. It is preferable to obtain the desired image density.

Further, it is preferable that the control section controls the image density after controlling the toner adhesion amount of the solid pattern image by a predetermined process in order to more accurately control the density of the halftone image.

[0014]

A gradation table for storing the relationship between the image density and the output of the image density detecting means of the gradation pattern of a plurality of stages created by the pattern forming means is provided in the control section. As a result, the duty ratio of the drive PWM of the laser optical system for giving the desired image density can be immediately determined from the output of the image density detecting means, and the density control of the halftone image can be accurately performed. In the above, in controlling the image density, the control section refers to the gradation table based on the output of the image density detecting means, and determines the duty ratio of PWM for obtaining the desired image density, A desired image density can be obtained.

Furthermore, the control unit controls the image density after controlling the toner adhesion amount of the solid pattern image by a predetermined process, so that the density control of the halftone image can be performed more accurately.

[0016]

Embodiments of the present invention will be described below in detail with reference to the drawings. FIG. 1 is a block diagram showing an embodiment of the present invention. The same parts as those in FIG. 7 are designated by the same reference numerals. In the figure, 13 is a developing sleeve provided in the developing device 10, and 15 is a detection sensor for detecting the amount of a toner image formed on the photosensitive drum 1 as an image carrier. As the detection sensor, for example, an LED as a light emitting element and a phototransistor as a light receiving element are used.

FIG. 2 is a view showing an arrangement example of the detection sensor 15. As shown in the figure, the detection sensor 15 includes a light emitting diode (LED) 15a as a light emitting element and a phototransistor 15b as a light receiving element. In the figure, the light emitted from the LED 15a is incident on the photosensitive drum surface 1a at an angle of 40 ° with respect to a vertical line (shown by a broken line), and the reflected optical signal is arranged in a direction of 40 ° with respect to the vertical line. The light is received at 15b.

FIG. 3 is a diagram showing a circuit configuration example of the detection sensor 15. On the light emitting side, the voltage VLE is applied to one end of the LED 15a.
It is connected to D and the other end is grounded via a resistor R1 of 1 kΩ. On the light receiving side, the collector of the phototransistor 15b is connected to the voltage Vcc of 5V, and the emitter is 3V.
It is connected to a series circuit of a resistor R2 of 6 kΩ and a resistor R3 of 68 kΩ. The other end of the resistor series circuit is grounded.
Then, the output Vout is taken out from the connection point of the resistors R2 and R3.

In the circuit thus constructed, the LE
The light emitted from D15a is incident on the photosensitive drum surface 1a. Then, the reflected light corresponding to the image density is reflected from the surface 1a of the photosensitive drum, and when the reflected light is applied to the base portion of the phototransistor, the phototransistor 15b is turned on. The larger the reflected light, the larger the output Vout on the light receiving side. As a result, the image density on the surface 1a of the photosensitive drum can be detected.

Reference numeral 20 is a control unit for controlling the overall operation (image forming conditions) according to the density detected by the detection sensor 15. In the control unit 20, 21 is a gradation table for storing the relationship between the output of the detection sensor 15 of the gradation pattern (patch) created by the pattern forming means and the image density, and 22 is the cumulative number of printed sheets of the apparatus. , A memory for storing the number of rotations of the photosensitive drum 1, the number of times the developing device 10 has been used, and the like. Other parts are the same as those in FIG. 7. The preferred operating conditions of the apparatus shown in FIG. 1 are that the charging potential of the surface of the photosensitive drum is −750 V and the DC bias potential VDC = −6.
50 V, AC bias potential VAC = 1.8 kVpp, frequency 8.0 kHz, developer conveyance amount 20 to 30 mg, image writing potential -40 to -50 V, PWM duty 100%, 75%, 50%, 25% , 12.5%, the image carrier line speed is 100 mm / sec, and the developing sleeve line speed is about 250 mm / sec. The operation of the apparatus configured as described above will be described below.

(1) Preparation of Reference Patch First, a pattern (referred to as a reference patch) serving as a reference for the halftone image density is formed on the photosensitive drum 1 in a plurality of steps. As a method of forming the reference patch, there are a method of forming the reference patch in the circumferential direction of the photosensitive drum 1 and a method of forming it in the axial direction of the photosensitive drum.

In the formation of the reference patch, in the case of a patch which gives a halftone density, the pattern to be created needs to be a pattern capable of catching changes in halftone density and line width. For this reason, it is necessary to select a pattern formed by the intermediate potential region of the photoconductor that is susceptible to changes due to usage environment and usage. For example, a dither pattern is used as a means for realizing the intermediate potential region.

FIG. 4 is an explanatory view of the gradation expression method of the dither matrix. The figure shows the case of 4 × 4 pixels which is a Bayer type dither matrix. In the case of a 4 × 4 matrix, the number of pixels is 16, and 16 gradations can be expressed. The figure shows the case where toner is attached to four of the 16 pixels. For example,
When toner is attached to the pixel of (1, 1), if the drive pulse of the laser optical system 4 is given by PWM as shown in the figure, the duty is 100% (corresponding to solid) to 0.
It is possible to give multiple levels up to%. When the duty is increased, the density of the pixel is increased, and when the duty ratio is decreased, the density of the pixel is decreased. P1 of the figure
P4 indicates that the PWM duty ratio is gradually reduced. In this way, the density can be varied for each pixel.

Patches as shown in FIG. 4 are formed on the photosensitive drum 1 for 16 gradations. Note that it is preferable to control the toner adhesion amount of the solid patch image in advance by a predetermined process before forming the 16-gradation patch. As a method of controlling the toner adhesion amount of the solid patch image, for example, the following technique is used.

(Adjustment of toner adhesion amount of solid patch image)
The solid patch image formed on the surface of the photosensitive drum 1 is detected by the detection sensor 15, and the developing sleeve 13 for the photosensitive drum 1 is provided so that an appropriate image forming condition can be obtained based on the output of the detected patch image. In an image forming apparatus capable of variably setting a peripheral speed ratio, a patch image is formed on the photosensitive drum 1 at a set peripheral speed ratio before forming an image, and a toner amount of the patch image is detected by a sensor. 1
5 and the toner adhesion amount of the patch image on the photosensitive drum 1 based on the output of the patch image detected by the detection sensor 15 is calculated by the toner adhesion amount calculation means (here, the CPU in the control unit 20). The toner adhesion amount estimating means (here, the CPU in the control unit 20) calculates the toner adhesion amount based on the bias voltage on the surface of the developing sleeve 13 and the surface potential on the photosensitive drum 1 with reference to the toner adhesion amount of the patch image. Then, the amount of toner adhered to the photosensitive drum 1 is estimated according to the peripheral speed ratio, and the peripheral speed is adjusted so that the desired toner adhered amount is obtained based on the relationship between the estimated peripheral speed ratio and the toner adhered amount. Set the ratio. As a result, the peripheral speed ratio can be variably set according to the environmental change at the time of image formation, and proper image forming conditions can be obtained. That is, the density control of the halftone image can be performed more accurately.

When the environment at the time of image formation changes due to changes in temperature, humidity, etc., the toner adhesion amount of the patch image becomes a value according to the operating environment at that time, and the toner adhesion amount of the patch image is used as a reference for the developing sleeve. Since the toner adhesion amount according to the peripheral speed ratio is estimated, it becomes possible to variably set the peripheral speed ratio according to the environmental change at the time of image formation, and an appropriate image forming condition can be obtained.

By accurately controlling the toner adhesion amount of the solid patch in this manner, the density sensor output of the created reference patch and the density measured by the densitometer become accurate, and the present invention is more accurate. Can be done

(2) Creation of gradation table The detection operation by the detection sensor 15 is performed for each of the 16 gradation patches described above, and the PWM duty ratio of the laser optical system 4 is 100%, 75%, 50%, 25. %, 12.
5% sequentially, detection sensor output (patch detection output)
And the image density, and the relationship between the dither gradation and the image density.

FIG. 5 is a diagram showing the relationship between the dither pattern (Bayer) and the image density. The horizontal axis is the dither pattern (16 gradations), and the vertical axis is the image density value read by the densitometer. ● indicates 100% duty, ○ indicates 75% duty, and □ indicates 50% duty.
Is a duty of 25%, x is a duty of 12.5
%. In the figure, m / a indicates the toner adhesion amount (mg / cm ² ) per unit area. Specifically, it is the toner adhesion amount of the 16th gradation.

In the case of the Bayer type, a unique curve is drawn, but by changing the PWM duty ratio,
It has been found that the range of image density that can be expressed can be changed, and the halftone density can be controlled by using PWM as the control method.

FIG. 6 is a diagram showing the relationship between the output of the dither patch (reference patch) (output of the detection sensor 15) and the image density. The horizontal axis is the dither patch output (V), and the vertical axis is the density measured by the densitometer. This characteristic is also considered to draw a curve peculiar to the patch detection system. Then, it was found that the patch detection output (output of the detection sensor 15) and the image density are in correspondence with each other.

Therefore, the gradation table 21 in the control unit 20
The characteristics of FIG. 6 are stored in the table. (3) Image Density Control The control unit 20 determines an appropriate PWM duty ratio from the relationship between the patch detection output for each PWM and the image density stored in the gradation table 21. Here, the PW to be used depends on the image density to be designed.
The duty ratio of M is changed. This allows
A desired image density can be obtained.

The control unit 20 detects the dither gradation density by changing the PWM with the density sensor 15, refers to the gradation table 21, and determines the duty ratio of the PWM so that the desired image density gradation can be obtained. , Drive the laser optical system 4. As a result, an image with a desired halftone density can be obtained.
For example, assuming that the output of the density sensor 15 is 2V, the laser optical system 4 may be driven at a duty ratio of 25% to obtain the desired image density of 0.8 (see FIG. 6).

In the above-described embodiment, the case where the reference patches of all 16 gradations are created and the image density is controlled is taken as an example. However, in practice, even if a part of the dither image is used, the halftone image density control can be performed. Therefore, the gradation table 21 with a reduced number of reference patches is created and the image density control is performed. Good. As a result, the time required for density control can be shortened, and the consumption of toner amount can be suppressed.

In the above-described embodiment, the case where one developing device 10 is used for forming a monochrome image has been described, but a plurality of developing devices 10 may be provided to form a color image. In that case, four developing devices 10 are provided: Y (yellow), M (magenta), C (cyan), and K (black).
A color image can be formed by forming four types of toner images on the photosensitive drum 1 and transferring them onto the recording paper P.

[0036]

As described above in detail, according to the present invention, the relationship between the output of the toner amount detecting means of the gradation pattern of a plurality of steps created by the pattern forming means and the image density is stored. A gradation table is provided in the control section, and the duty ratio of the drive PWM of the laser optical system for giving a desired image density can be determined immediately from the output of the toner amount detecting means, and the density control of the halftone image can be accurately performed. In this case, when controlling the toner density, the control section refers to the gradation table based on the output of the toner amount detecting means, and sets the PWM duty ratio for obtaining the desired density. By making the determination, the desired image density can be obtained.

Furthermore, the control unit controls the image density after controlling the toner adhesion amount of the solid pattern image by a predetermined process, so that the density control of the halftone image can be performed more accurately.

[Brief description of drawings]

FIG. 1 is a configuration block diagram showing an embodiment of the present invention.

FIG. 2 is a diagram illustrating an arrangement example of detection sensors.

FIG. 3 is a diagram illustrating a circuit configuration example of a detection sensor.

FIG. 4 is an explanatory diagram of a gradation expression method of a dither matrix.

FIG. 5 is a diagram showing a relationship between a dither pattern and image density.

FIG. 6 is a diagram showing a relationship between dither patch output and image density.

FIG. 7 is a conceptual diagram of a configuration of an image forming apparatus.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Photoconductor drum 2 Charging device 3 Exposure device 4 Laser optical system 5 Mirror 6 Paper cassette 7 Transfer part 8 Fixing part 9 Cleaning blade 10 Developing device 11 AC voltage source 12 DC voltage source 13 Developing sleeve 15 Detection sensor 20 Control part 21 Gradation table 22 Memory

Continuation of front page (51) Int.Cl. ⁶ Identification number Office reference number FI Technical display location G03G 15/04 B41J 3/00 A 15/08 115 MH04N 1/405 H04N 1/40 B 1/403 103A

Claims (3)

[Claims] 1. An image forming apparatus for forming an image by adhering toner onto an image carrier, the pattern forming device forming an image density detecting pattern on the image carrier, and the pattern forming unit adhered onto the image carrier. Image density detecting means for detecting the image density by the toner, a control part for controlling the image density according to the density detected by the image density detecting means, and a plurality of units provided in the control part and created by the pattern forming means. An image forming apparatus, comprising: a gradation table for storing the relationship between the image density and the output of the image density detecting means of the gradation pattern of stages. 2. The control section, when controlling the image density, refers to the gradation table based on the output of the image density detecting means, and P of the laser optical system for obtaining a desired density.
The image forming apparatus according to claim 1, wherein the duty ratio of the WM is determined. 3. The image forming apparatus according to claim 2, wherein the control unit controls the image density after controlling the toner adhesion amount of the solid pattern image by a predetermined process.