JPH11202601A - Image forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image forming device capable of properly controlling the density of a toner and capable of realizing those by simple constitution even when a variation in the relation between a developer quantity and Cin (image occupancy ratio of toner) is generated due to the fluctuation of the toner concentration of a developer or the potential of a latent image or the relation between the developer quantity and the Cin is changed due to the secular deterioration of a developing material. SOLUTION: The image forming device is provided with a developing means 1 obtaining a toner image by developing an original image and an electrostatic latent image of a pattern for detecting the density formed on an image carrier by a prescribed developing bias voltage, a detection means 2 detecting the density of the toner image of the pattern for detecting the density and a density control means 3 controlling the density of the toner image of the original image based on the density of the toner image of the pattern for detecting the density. Further, it is provided with a developing bias control means 4 controlling the developing bias voltage so that developing property is more lowered than the case that the electrostatic latent image of the original image is developed when the electrostatic latent image of the pattern for detecting the density is developed by the developing means 1. Then, the density of the toner image of the original image is controlled based on the density of the toner image of the pattern for detecting the density whose an image occupancy ratio is high by the density control means 3.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method and apparatus for controlling image density in an electrophotographic image forming apparatus such as a copying machine or a laser printer. An image density control method for forming a density detection pattern on an image carrier such as a body, and controlling the density of a recorded image based on a change in the output density of the reference patch measured on the image carrier, and an improvement of the means therefor. .

[0002]

2. Description of the Related Art In an image forming apparatus such as a copying machine or a laser printer, the image density of a recorded image is easily affected by environmental factors such as temperature and humidity. The image density tends to decrease in a low-humidity environment, and the image density tends to increase in a high-humidity environment. Further, even under the same environment, the ratio between the image density and the image occupancy of the toner (hereinafter, referred to as “C _in ”) is not constant but varies depending on the density of the image.

For this reason, in order to obtain an image having a desired output density, it is necessary to precisely control image forming conditions such as the charging potential of the image carrier, exposure intensity, image density and the like in accordance with changes in environmental factors. since, conventionally, has a curve graph showing the relationship between the image density and C _in, performs density control on the basis of the curve graph, the density detection of a predetermined C _in on an image bearing member after image formation of a predetermined number A toner image of a pattern (hereinafter simply referred to as a “patch”) is formed, the image density of the toner image of the patch is optically measured by a sensor, and the curve graph is corrected based on the measurement result. An image density control method for correcting the toner density has been used.

In order to properly perform such image density control, it is necessary to uniformly create patches at a predetermined C _in . Therefore, conventionally, for example, JP-A-63-6718
Japanese Patent Application Laid-Open No. 4-162378 proposes a technique of controlling an edge effect at the peripheral portion of a patch by controlling a developing bias voltage, and Japanese Patent Application Laid-Open No. 2-162378 discloses a technique in which a set angle of a developing magnetic pole of a developing device is set. A technique has been proposed in which the control is performed to suppress a change in the reflection density of the patch due to a change in the characteristics of the image carrier.

On the other hand, a predetermined C_inThe upper limit of which C_in
To determine the appropriateness of the image density by detecting the patch density of
The upper limit is the sensor used and the amount of toner in the patch.
Determined by relationship to quantity. FIG. 11 shows the sensor sensitivity,
Development amount, C_in6 is a graph for explaining the relationship with the graph. Graph
Is a graph showing the relationship between sensor sensitivity and development amount.
As the amount of development increases, sensor sensitivity deteriorates.
I understand. Here, depending on the performance of the sensor used,
Below the sensor sensitivity of
Ls, which is the sensor sensitivity, and Ld, which is the development amount at that time, exist.
Exist. On the other hand, the graph shows the development amount and C_inShow relationship with
It is a graph in which C_inAnd the development amount are approximately proportional
You can see that. Here, Ld, which is the limit of the development amount, is
For C _inLc is determined. That is,
To perform density detection in the effective sensor area, the toner
Is less than Ld, and accordingly C_inIs Lc (general
Is about 60%) or less. For example,
C_inCreating a 55% patch and detecting its density
Becomes

FIG. 12 is a graph illustrating an example of such density control. A curve S in FIG. 12A shows a relationship between the image density and C _in, and these relationships can be generally expressed by a substantially S-shaped curve. After forming a predetermined number of images, a predetermined C _in , for example, C _in
It is assumed that the density of the toner image of the 55% patch is detected by the sensor, and the density is Pa. If this Pa is larger than the density Ps calculated from the curve S held in advance, the curve S ″ is modified to the curve S as shown in FIG. In the same way,
If the density of the patch toner image of C _in 55% is Pb, the curve S ′ is modified as shown by the curve S.

[0007]

[SUMMARY OF THE INVENTION However, in this manner by the upper limit of C _in the patch is limited, a problem that can not be suitable concentration control occurs.

First, there arises a problem that a shading error particularly in a high density portion becomes large. FIG. 13A shows the image density and C _in.
FIG. 13B shows the relationship between the development amount and C _in.
6 is a graph showing a relationship with the graph. In FIG. 13A, ideally, the curve S indicating the relationship between the image density and C _in is always a constant curve. However, the fluctuations in the toner density and the latent image potential of the developer, in fact, the dotted line S _'upper, S' of the graph will vary the curve itself in a range indicated by _{lowe r.} For example, the variation in density at C _in 55% is δ1. Further, since the development amount is determined based on the scattered curve, as shown in FIG.
Ideally, the development amount and C should be a constant straight line indicated by a solid line.
relationship with the _in is, thus variations in the slope of the straight line in the range indicated by the dotted line. The variation in FIG. 13B corresponding to δ1 is δ1 ′. Here, the variation in the low concentration portion is
For example, although it is relatively small like δ0 ′ in FIG. 13 (b), the variation is proportionally amplified in the higher concentration part,
At 100% _in Cin, the variation is δ2 ′ (≒ δ1 ′ × C
_in 100% / C _in 55%). That is, in the high-density portion, an image considerably higher than the desired density or an image considerably lower than the desired density can be considered.

Next, when the developer deteriorates due to the use over time, there arises a problem that reproduction of a high density becomes particularly difficult. Since the curve S showing the relationship between the concentration and C _in, for example, or when a new developer, is set as indicating the relationship between the concentration and C _in at a particular state, the state of the actual developer May be different. FIG. 15 is a graph showing the relationship between the concentration and C _in . For example, it is assumed that the curve S is determined based on a new developer. Actual concentration and C _in
This relationship also coincides with the curve S when the developer is new, so that appropriate density control can be performed. But,
When the developer deteriorates due to use over time, the set curve S differs from the curve R indicating the actual relationship. FIG.
5 of curve R shows the actual relationship between the concentration and C _in the case where the developer is degraded.

It is assumed that the curve correction described in FIG. 11 is performed in such a state. That is, the predetermined C
_It is assumed that the sensor detects the density at _in , for example, C _in 55%, and the value is P. Since this is located above the set curve S, the correction is made to lower the set curve. Then, the actual curve R is also lowered together with the correction, and becomes the curve R '. In such a state, even if an image is output, the reproducibility particularly in a high density region is poor. For example, even if an image with 100% of C _in is output, D _max ′ is lower than D _max of the original density.

The present invention has been made in view of these problems, and an object of the present invention is to produce a variation _in the relationship between the development amount and C _in due to fluctuations in the developer toner concentration and the latent image potential. it is, also the relationship between the developing amount and C _in is changed by deterioration with time in the development material are possible suitable density control, yet the image formation can be implemented them in a simple construction It is to provide a device.

[0012]

According to the present invention, there is provided a developing means for developing a toner image by developing a document image and an electrostatic latent image of a density detecting pattern formed on an image carrier with a predetermined developing bias voltage. Detecting means 2 for detecting the density of the toner image of the density detecting pattern, and density controlling means 3 for controlling the density of the toner image of the original image based on the detected density of the toner image of the density detecting pattern. When the developing unit 1 develops the electrostatic latent image of the density detection pattern in the image forming apparatus having the developing bias voltage, the developing bias voltage is set so that the developing property is lower than when the electrostatic latent image of the original image is developed. , Which controls the density of the toner image of the original image based on the density of the toner image of the density detection pattern having a high image occupancy.

With this configuration of the image forming apparatus, the image density can be controlled based on the high C _in density detection pattern. Image density control can be performed. Also, a new developing agent in the portion of the high C _in, either the concentration of the degraded developer for unchanged Hodondo, so that the reproducibility of the high density portion is excellent over a long period of time even deteriorated developer.

Further, according to the present invention, when the developing means 1 develops an electrostatic latent image of a pattern for density detection, in an image forming apparatus in which a developing bias voltage comprises an AC component and a DC component, the application of the AC component is performed. Stopper, developing means 1
In an image forming apparatus in which the developing bias voltage includes an AC component, the amplitude of the AC component is made smaller than when developing the electrostatic latent image of the original image, and the frequency of the AC component is used to develop the electrostatic latent image of the original image. Higher than you would,
In an image forming apparatus in which the developing bias voltage of the developing unit 1 includes an AC component of a rectangular wave, the duty of the rectangular wave is made smaller than in the case of developing an electrostatic latent image of a document image.
In an image forming apparatus including a developing bias voltage of the developing means 1 including an AC component having a certain waveform, the waveform of the AC component is a waveform having a lower developing property than when developing an electrostatic latent image of a document image.

With such an image forming apparatus, low developability can be realized with a simple apparatus, and switching between normal developability and low developability can be performed in a short time.

Further, in the present invention, the density control means controls the density of the toner image of the original image based on the density of the toner image of the density detection pattern having the highest image occupancy.

[0017] The image forming apparatus is set to such a configuration, even if there are variations in the relationship between the developing amount and C _in, also by deterioration of the developer amount and the relationship over time developer material with C _in Even if it changes, the effect of the present invention that appropriate density control is possible can be most remarkably exhibited.

FIG. 1 explains the configuration of the present invention using a block diagram. FIG. 2 is a graph illustrating the operation of the present invention, and shows the relationship between the sensor sensitivity and the development amount (graph) and the relationship between the development amount and C _in (graph).

The image forming apparatus has such a structure. First, when the developing means 1 develops the electrostatic latent image of the density detecting pattern, the developing unit 1 develops the electrostatic latent image more than the original image. Since the developing bias control unit 4 controls the developing bias voltage so that the developing property is reduced, the latent image of the patch is developed with the developing bias voltage having a low developing property. That is,
The amount of development of the patch decreases, and in the graph of FIG. 2, the slope of the solid line to which the present invention is applied is smaller than that of the conventional dotted line (see FIG. 11). This is also a and limitations Ld of sensitivity limit Ls and the development of sensors unchanged, higher limit Lc of C _in, that is, the patch creation of a higher C _in,
This means that the density of the patch can be detected. Since the effect of the later about this C _in large it is obtained remarkably, preferably up screen occupancy, i.e. C _in
It is desirable to create a patch at 100%.

Since the density control means 3 controls the density of the original image based on the density of the toner image of the density detection pattern having a high image occupancy, preferably 100%, the above-mentioned problem is solved. can do.

First, in FIG. 13A, C _in 55%
Of each concentration between δ and C _in 100% δ
1 and δ will be compared and examined. In general, the variation in density increases as C _in increases, but since the upper limit of the density is constant, S ′ _upper saturates the density and substantially matches the curve S _in the high C _in portion. Therefore, δ is not necessarily greater than δ1, and at least δ <δ
1 × (100% / 55%).

The variation of the density fluctuation is generated in the fluctuation of the toner density and the latent image potential, here described as apparently C _in is varied as an example variation of in the latent image potential.
As shown in FIG. 13B, about ± 5% around 55% of C _in .
If it fluctuates, the fluctuations in the development amount of C _in 55% and 100% are δ ₁ ′ and δ ₂ ′, respectively. The relationship between these variations is clearly δ ₁ ′ <δ ₂ ′, and the variation in the development amount of C _in 100% is large.

When C _in 100% is controlled as shown in FIG. 14 (c), δ ₂ ″ ≒ δ ₁ ′, and the variation in the development amount of C _in 100% is greatly reduced. Since there is no sensitivity of the sensor for high C _in %, 100% of C _in cannot be used as a control point.

[0024] Therefore, to develop the patch under conditions of low developability, controlling the C _in 100% as a control point (see FIG. 14 (d)), the variation of C _in 100% δ ₂ "'
Becomes Here, since the development amount of C _in 100% of the low development curve and C _in 55% of the current development curve are the same, δ ₂ ″ ′ = δ ₂ ″ × (55/100). Δ ₂ ″ ′ of the low development curve is (100 /
55), the variation in FIG. 14D (C _in 100% of the current development curve) is δ ₂ ″ × (5
5/100) × (100/55) = δ ₂ ″, and very good image density control can be performed as in the case of directly controlling C _in 100 of the current development curve.

Next, in FIG. 15, when the densities of the curves S and R of 100% C _in are compared, it can be seen that they are substantially equal. This is because the density of the new developer or the deteriorated developer hardly changes _in the high C _in portion, particularly in the C _in 100% portion. Therefore, there is no need to modify the curve so as to unnecessarily lower the density of the high-density portion, and the reproducibility of the high-density portion is improved over a long period even with a deteriorated developer.

As shown in FIG. 3, when the developing means 1 develops an electrostatic latent image of a density detecting pattern, the developing bias voltage is controlled by an AC component as shown in FIG. In an image forming apparatus including a DC component and a DC component, the application of an AC component is stopped. In an image forming apparatus in which the developing bias voltage of the developing unit 1 includes an AC component, the amplitude of the AC component is converted to an electrostatic latent image of a document image. In an image forming apparatus in which the frequency of the AC component is higher than that in the case of developing an electrostatic latent image of a document image, the developing bias voltage of the developing unit 1 includes a rectangular wave AC component. In the case where the duty of the rectangular wave is made smaller than that in the case of developing the electrostatic latent image of the original image, the developing bias voltage of the developing means 1 is applied to an image forming apparatus including an AC component having a certain waveform. Te, and the like which with a low developability waveform than for developing an electrostatic latent image of the waveform of the original image of the AC component. For example, triangular waves, sine waves, and the like have lower developability than rectangular waves.

In each of these methods, the developing bias voltage is controlled to realize the low developing property. In addition, the low developing property is also controlled by controlling the direction of the magnet constituting the developing means 1 and the like. It is possible to achieve the same effect. However, controlling the developing bias voltage has the advantages that the configuration can be simplified and that the control time is very short.

Examples of the image carrier include a photoreceptor, an intermediate transfer member, and the like, and examples thereof include a drum shape and an endless belt shape. The “high image occupancy” means the upper limit of the image occupancy (see FIG. 11) determined by the sensitivity limit of the sensor when a patch is created with low developability. Although those that vary depending on the sensitivity of the sensor or the like to be used, generally refers to more than 70% C _in, preferably C _in 100%.

[0029]

Next, preferred embodiments of the present invention will be described based on examples.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An image forming apparatus (color copying machine) according to the present embodiment develops an original image formed on an image carrier and an electrostatic latent image of a density detecting pattern at a predetermined developing bias voltage to form a toner. Developing means 1 for obtaining an image; detecting means 2 for detecting the density of the toner image of the density detecting pattern; and controlling the density of the toner image of the original image based on the detected density of the toner image of the density detecting pattern. In the image forming apparatus having the density control unit 3, when the developing unit 1 develops the electrostatic latent image of the pattern for density detection, the developability is lower than when the electrostatic latent image of the original image is developed. Bias control means 4 for controlling the development bias voltage as described above, and the density control means 3 controls the density of the toner image of the original image based on the density of the toner image of the density detection pattern having a high image occupancy. You It is intended.

FIG. 4 shows a color copying machine according to this embodiment. The developing means 1 is a rotary developing unit 2
3, the detection means 2 is provided to the density detection sensor 28, the density control means 3 is provided for the second gamma correction 65, the patch signal generation means 72,
The developing bias control unit 4 corresponds to the developing bias control unit 76, and the second selector 73, the laser light amount control unit 78, the charger control unit 75, the toner supply device 77, and the controller 74 that controls these units. The developing bias controller 76 is also controlled by a bias voltage control timing signal from the controller 74.

In the figure, reference numeral 20 denotes a photosensitive drum (image carrier), 21 denotes a charging corotron for charging the surface of the photosensitive drum 20 in advance, 22 denotes a scanner unit for reading image information of a document 222, and 228 denotes a charging corotron 21. A raster scanning device (hereinafter abbreviated as ROS) for writing an electrostatic latent image on the charged photosensitive drum 20 is a black (K), cyan (C), magenta (M), and yellow (Y) toner. Developing devices 23K and 23 that accommodate
C, 23M, and 23Y are rotatably arranged, and a rotary developing unit which is appropriately switched and selected.
A cleaner 26 for removing the residual toner on the photosensitive drum 20 is an erase lamp for removing residual charges on the photosensitive drum 20.

FIG. 5 shows the structure of one developing unit 23 of the rotary developing unit 23, which is developed by a magnetic brush developing method. That is, the toner and the carrier are stirred by the rotating auger 231 and supplied to the surface of the sleeve 236 by the paddle 232 that is also rotating. The sleeve 236 rotates in the same direction as the photosensitive drum 20 at a position closest to the photosensitive drum 20, but the magnet roller 235 inside the sleeve is stopped. The carrier adhered to the surface of the sleeve stands up like a brush by the action of the magnetic field of the magnet roller 235, is trimmed by the trimmer 234, and is fixed by the sleeve 236.
With the rotation of, the photosensitive drum 20 approaches the surface. The toner adhered to the carrier is separated from the carrier by the action of an electric field generated by a potential difference between the developing bias voltage from the developing bias control unit and the surface of the photosensitive drum 20, and an electrostatic latent image on the surface of the photosensitive drum 20 is formed. And the development is completed.

In this embodiment, the scanner unit 22
Are an exposure lamp 223 for irradiating a light beam to a document 222 set on a platen 221;
Carriage 2 for moving 23 over document 222 area
24, a reflection mirror 225 for guiding a beam from the surface of the document 222 by the exposure lamp 223 along a predetermined path, a CCD sensor 226 for converting the beam from the surface of the document 222 to a digital signal for each color component, and a surface of the document 222 And an image forming lens 227 for forming an image of the beam from the CCD sensor 226 at the site. In addition, the ROS228
Are a semiconductor laser 228a for irradiating a laser beam based on image data of each color component captured by the CCD sensor 226, and a polygon mirror 228b for distributing and deflecting a beam from the semiconductor laser 228a in the main scanning direction of the photosensitive drum 20. And an image forming lens 228c for forming an image of the beam from the semiconductor laser 228a along the main scanning direction line of the photosensitive drum 20, and a reflecting mirror 228d for regulating a beam path.

The photosensitive drum 20 is located between the writing position of the electrostatic latent image on the photosensitive drum 20 and the developing position.
While a potentiometer 27 for measuring the charging potential of the photosensitive drum 20 is provided, a density detecting sensor 28 for measuring the density of the patch formed on the photosensitive drum 20 is provided between the developing position and the transfer position. I have. The density detection sensor 28 is a reflection type optical sensor that irradiates the photosensitive drum with an LED and measures the amount of reflected light with a photodiode.

Further, reference numeral 31 denotes a recording sheet 30 wrapped around and held on the peripheral surface.
This is a transfer drum for sequentially multiple-transferring each color component toner image on 0. The transfer drum 31 has a suction corotron 41 that charges the drum sheet 35 when the recording sheet 30 is held.
And a transfer corotron 42 for transferring the toner image on the photosensitive drum 20 to the recording sheet 30 side, and a charge removing corotron 4 for removing charge from the recording sheet 30 after the transfer process of the final color is completed.
3, a cleaning charge removing corotron 44 for removing charges on the drum sheet 35 after the final color transfer step is completed,
A cleaning brush 45 for cleaning paper dust and the like adhering to the drum sheet 35 after the final color transfer process has been completed; an inner press roll 46 for lifting the drum sheet 35 from the inside when the recording sheet 30 is peeled; And a peeling finger 47 to be provided. Reference numeral 48
Reference numeral denotes a sheet transport system that transports the recording sheet 30 supplied from a sheet feeding cassette (not shown) to the suction corotron 41 at a predetermined timing according to each mode.

Further, reference numeral 50 denotes a fixing device for inserting the recording sheet 30 after the transfer step and fixing an unfixed toner image on the recording sheet 30. In this embodiment, a heating device having a built-in heater is used. The recording sheet 30 from the transfer drum 31 is conveyed to the fixing device 50 via the guide plate 53 by a roll 51 and a pressure roll 52 arranged in pressure contact with the heating roll 51. Reference numeral 54 denotes a fuser exit roll that conveys the recording sheet 30 that has passed through the fixing device 50, and 55 denotes a fixing device 50.
, A fuser outlet switch for detecting the trailing end of the recording sheet 30 that has passed through, a discharge tray 56 in which the fixed recording sheet 30 is stored, and 57
Is an exit roll for sending out.

FIG. 6 is a block diagram showing a control system of the color copying machine. First, in the scanner unit 22, C
The image data read by the CD sensor 226 is
After being amplified to an appropriate level at 0, the A / D converter 61
Is converted into an 8-bit digital signal. After the shading correction and the gap correction, the density converter 62 converts the reflectance data into density data, and sends the data to the image processing unit.

The image data sent to the image processing unit is first subjected to basic image processing as a color copying machine by color conversion unit 63, that is, color signal conversion, black reproduction (UCR), MTF processing, and the like. , Yellow, cyan and magenta image data. Next, the image data of each color is sent to the first gamma correction 64, and the ROS 22
8 and the color gradation is corrected in accordance with the gradation characteristics unique to the image forming unit.

Next, the image data is sent to the second gamma correction 65. Here, the input / output ratio of the image data is corrected in order to avoid a change in image density due to a change in environmental factors. The data required for the correction is the photosensitive drum 20
It is created by detecting the density of the patch formed above with the density detection sensor 28.

The image data which has been subjected to the second gamma correction 65 in this manner is converted into analog data by a D / A converter 68 and then output by a comparator 69 from a triangular wave generator 70 at a predetermined period. And converted into binary image data by pulse width modulation. Specifically, the input analog image data is compared with a triangular wave, and a portion where the analog image data is larger than the triangular wave is set to “0”, and a smaller portion is set to “1” to binarize the image data. The binary image data is supplied to the laser driver 71 of the optical unit.
When the image data is "0", the semiconductor laser 228a of the ROS 228 is turned off, and when the image data is "1", the semiconductor laser 228a is turned on.

The image processing section includes a photosensitive drum 20.
A patch signal generating means 72 for generating an image signal when a patch is formed thereon is provided.
In accordance with the control signal of No. 4, several types of patch data having different image area ratios are generated. The analog image data and the patch data are input to a selector 73 controlled by a controller 74, and only one of the data is compared with a triangular wave by the comparator 69 to be binarized.

On the other hand, the image forming section includes a controller 74 for controlling the density of the toner image formed on the photosensitive drum 20 based on the detection signals of the electrometer 27 and the density detection sensor 28, and this controller 74. 74 charger controller 75 for changing the grid voltage V _G of the charger 21 in response to a control signal, a DC component and an AC component to be applied to the developing device of the rotary developing unit 23 also according to the control signal of the controller 74 the developing bias control unit 76 is provided for changing a developing bias voltage V _B consisting. Further, a toner supply device 77 is connected to each developing device of the rotary developing unit 23,
The toner supply is performed according to the control signal of No. 4. Further, the controller 74 also sends a control signal to a laser light amount control unit 78 of the optical unit, and the light emission amount of the semiconductor laser 228a is adjusted via the laser driver 71.

In this embodiment, the developing bias control section 76 stops the application of the AC component, and the structure thereof is shown in FIG.
As shown in (1), the application of voltage from an AC power supply is switched. In the present invention, the developing bias control unit 76 may further reduce the amplitude of the AC component as compared with the case where the electrostatic latent image of the original image is developed, and may reduce the frequency of the AC component to the electrostatic latent image of the original image. One that is higher than when developing, one that makes the rectangular wave duty smaller than when developing the electrostatic latent image of the original image, and one that develops the AC component waveform more than when developing the electrostatic latent image of the original image Any of these can be realized by a well-known electronic circuit or the like that controls the developing bias voltage to a predetermined one by a control signal from a controller.

FIG. 8 is a flowchart showing the operation relating to the density control of the color copying machine.
First, it is determined whether or not density control is necessary (S101).
If not necessary, normal image formation is performed (S102). If density control is required, a density control patch is created (S
103), the patch is detected (S104), density control is performed (S105), and the process returns to the step of S101. Hereinafter, each step will be described.

First, in step S101, it is determined whether or not density control is necessary. In this embodiment, the density control is performed every predetermined number of copies (every 20 copies) after the start of a copy job. Is performed for the purpose of preventing the image density from fluctuating due to the change in the charge rate of the toner in the developing unit 23 at the start of the copy job.

In step S102, normal image formation is performed. When the user operates a copy start switch, the original 222 is scanned, and an electrostatic latent image corresponding to black K is written on the photosensitive drum 20. on the other hand,
In the rotary developing unit 23, a black developing device 23K
Is set at a position facing the photosensitive drum 20, and the electrostatic latent image is developed by the black developing device 23K with a slight delay from its writing timing. The black K toner image thus formed is transferred to the transfer drum 3
1 is transferred to the recording sheet 30 held. When the developing process by the black developing device 23K is completed, the developing device is replaced before the transfer drum 31 completes one rotation cycle, and the yellow developing device 23Y is exposed by the rotation of the rotary developing unit 23 by 90 °. It is set at a position facing the body drum 20.

Thereafter, these operations are repeated for each rotation cycle of the transfer drum 31, and each time, the yellow Y, magenta M and cyan C toner images are transferred from the photosensitive drum 20 to the recording sheet held on the transfer drum 31. The toner image is transferred onto the recording sheet 30, and a superimposed toner image of four color toner images is formed on the recording sheet 30. Then, the recording sheet 30 on which the transfer of the cyan C toner image has been completed is peeled off from the transfer drum 31 as it is and passes through the fixing device 50 to the discharge tray 56.
And the image formation is completed.

In step S103, a patch is created. The controller 74 requests the patch signal generation means 72 to generate patch data, and requests the second selector 73 to select patch data. As a result, the patch data is binarized and supplied to the laser driver 71, and a patch is formed on the photosensitive drum 20 in the same manner as in the normal image forming process described above. At this time, the controller 74 requests data for forming an electrostatic latent image of a patch having a high image occupation ratio, in this case, _Cin 100%, from the patch signal generating means, and the electrostatic latent image formed on the photoreceptor. The image is developed by, for example, the black developing device 23K. In addition,
In the present embodiment, the patch is a square of 20 mm × 20 mm, but the size and shape can be appropriately selected such as a rectangle, a circle, an ellipse, and a line.

Here, the development of this patch is performed with a low developing property by the developing bias control section 76, and the operation will be described with reference to the timing chart of FIG. FIG. 9 shows ON / OFF of the developing device 23 and the developing device 2
3 shows the on / off timing of the voltage of the AC and DC components supplied to 3. Before time t2 when the electrostatic latent image of the original image (20th sheet) formed on the photosensitive drum 20 is developed (hereinafter, referred to as “developing position”), the developing device 20 is started at time t1. Further, both an AC component and a DC component are supplied as a developing bias voltage. Then, from time t2 to time t3, the original image of the electrostatic latent image is developed into a toner image with normal developability.

[0050] At time t4 of between time t3 to time t5 when the electrostatic latent image of the batch reaches the developing position, the patch signal generating means so that the controller 74 to create an electrostatic latent image of C _in 100% of the patch At the same time as sending the signal to 72, a signal for instructing the control timing to the developing bias control unit 76 is transmitted, and the developing bias control unit 76 stops supplying the AC component based on the signal, thereby realizing low developability. Thereafter, between times t5 and t6, the electrostatic latent image of the patch passes through the developing position, and becomes a toner image with low developability. That is, a patch toner image with a relatively small amount of development is created despite C _{in being} 100% (see FIG. 2).

At time t7 from time t6 when the electrostatic latent image of the patch has passed the developing position to time t8 when the electrostatic latent image of the next original image (21st sheet) reaches the developing position,
The developing bias control means 74 resumes the supply of the stopped AC component to the developing device 23 to prepare for the development of the next image document. Thereafter, the original image passes through the development position (at time t).
9), the supply of the developing device and the developing bias voltage (AC component and DC component) is stopped (time t10).

In S104, the density of the patch is detected.
The density detecting sensor 28 is an optical density sensor including a light emitting element and a light receiving element, irradiates the patch with light from the light emitting element, detects the reflected light with the light receiving element, converts the light into an electric signal, and amplifies the signal. It is configured as follows. Here, an LED can be used as the light emitting element, and a photodiode, a phototransistor, or the like can be used as the light receiving element. The distance from the density detection sensor 28 to the patch is 3 to 1
0mm, aperture size, detection size is φ2-8
mm.

In S105, density control is performed based on the density of the patch detected in S104. The density control may be of any type as long as it controls various parameters for determining the toner density, such as a latent image forming charge amount, a developing bias voltage, a laser light amount, a developing toner concentration, and an electronic data correction amount. And the like for controlling the above. Here, the control of the amount of latent image formation and the amount of supplied toner will be described as an example of density control with reference to FIG. First, when the density of the patch is detected, the density of the patch is determined to be one of “light”, “standard”, and “dark” by comparing the density with a preset density, and according to the determination, Controls the laser light amount and toner supply amount. That is, when it is determined that the density is low, the laser light amount and the toner supply amount are increased, and when it is determined that the density is high, they are decreased.

[0054]

As described in detail above, according to the present invention, when the developing means develops the electrostatic latent image of the density detecting pattern, it develops more than when the electrostatic latent image of the original image is developed. Since there is a developing bias control means for controlling the developing bias voltage so that the developing property is reduced, the developing amount of the density detecting pattern of a certain screen occupancy is reduced, and the sensitivity limit of the sensor and the developing amount limit Ld due to the sensor are limited. Does not change, the density of the density detection pattern can be detected with a high screen occupancy limit, that is, a higher screen occupancy, preferably 100%. Next, since the density control means controls the density of the toner image of the original image based on the density of the toner image of the density detection pattern, the variation in the development amount due to the fluctuation of the developing bias voltage is small, and the density is high. There is no need to perform density correction to unnecessarily lower the density of the density portion, and the reproducibility of the high density portion does not deteriorate over a long period even with deteriorated developer.

In order to realize the low developing property, the application of the AC component is stopped. In an image forming apparatus in which the developing bias voltage of the developing means includes the AC component, the amplitude of the AC component is converted to the electrostatic latent image of the original image. In the image forming apparatus in which the frequency of the AC component is higher than that in the case of developing the electrostatic latent image of the original image, the developing bias voltage of the developing unit includes a rectangular wave AC component,
In a case where the duty of the rectangular wave is smaller than that in the case of developing the electrostatic latent image of the original image, in an image forming apparatus including a developing bias voltage of a developing unit including an AC component having a certain waveform,
Since the developing bias voltage is controlled by means such as a waveform having a lower developing property than the case where the electrostatic latent image of the original image is developed for the AC component waveform, the configuration can be simplified, and the time for performing the control is reduced. It has the advantage of requiring very little.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating an overall configuration of the present invention using a block diagram.

FIG. 2 is a graph illustrating the operation of the present invention.

FIG. 3 is a diagram for explaining various means for realizing low developability.

FIG. 4 is an explanatory view schematically showing the entire image forming apparatus according to the embodiment of the present invention;

FIG. 5 is an explanatory view showing a part of a rotary developing unit of the image forming apparatus according to the embodiment of the present invention.

FIG. 6 illustrates a configuration of an image forming apparatus according to an embodiment of the present invention using a block diagram.

FIG. 7 illustrates a specific configuration of a developing bias control unit according to the embodiment of the present invention.

FIG. 8 illustrates a density control operation of the image forming apparatus according to the embodiment of the present invention, using a follow chart.

FIG. 9 illustrates the control of the developing bias voltage of the image forming apparatus according to the embodiment of the present invention, using a timing chart.

FIG. 10 is a flowchart illustrating an example of density control of the image forming apparatus according to the embodiment of the present invention.

FIG. 11 is a graph illustrating a relationship between a sensor sensitivity, a development amount, and a screen occupancy (C _in ).

FIG. 12 shows an example of density control of a conventional image forming apparatus.

FIG. 13 is a graph illustrating a variation in density and a current increase amount;

FIG. 14 is a graph for explaining variations in density and current increase amount;

FIG. 15 is a graph illustrating the relationship between the density and the screen occupancy (C _in ) when the developer is deteriorated.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Development means, 23 ... Rotary development unit (development means), 2 ... Detection means, 28 ... Density detection sensor (detection means), 3 ... Density control means, 65 ... Second gamma correction (density control means), 72 ... Patch signal generation means (density control means), 73: selector (density control means), 78: laser light quantity control section (density control means), 75: charger control section (density control means), 77: toner supply device (density) Control means), 74: controller (density control means), 4: developing bias control means, 76: developing bias control unit (developing bias control means)

Claims (7)

[Claims] A developing means for developing a document image and an electrostatic latent image of a density detection pattern formed on an image carrier with a predetermined developing bias voltage to obtain a toner image; and a toner image of the density detection pattern. An image forming apparatus comprising: a detecting unit that detects the density of the original image; and a density control unit that controls the density of the toner image of the original image based on the detected density of the toner image of the density detection pattern. When developing the electrostatic latent image of the detection pattern, the image forming apparatus includes a developing bias control unit that controls the developing bias voltage so that the developing property is lower than when the electrostatic latent image of the original image is developed. An image forming apparatus, wherein a density control unit controls the density of a toner image of a document image based on the density of a toner image of a density detection pattern having a high image occupancy. 2. An image forming apparatus in which a developing bias voltage of said developing means includes an AC component and a DC component, wherein said developing bias control means develops an electrostatic latent image of a density detection pattern. 2. The image forming apparatus according to claim 1, wherein the application of the AC component is stopped. 3. An image forming apparatus according to claim 1, wherein said developing bias voltage of said developing means includes an AC component. When the developing means develops the electrostatic latent image of the density detection pattern, 2. The image forming apparatus according to claim 1, wherein the amplitude of the AC component is made smaller than when the electrostatic latent image of the image is developed. 4. An image forming apparatus in which a developing bias voltage of said developing means includes an AC component, wherein said developing bias control means includes: a developing means for developing an electrostatic latent image of a density detection pattern; 2. The image forming apparatus according to claim 1, wherein the frequency of the AC component is set higher than when the electrostatic latent image of the image is developed. 5. An image forming apparatus in which a developing bias voltage of said developing means includes an alternating current component of a rectangular wave, wherein said developing bias control means is adapted for developing an electrostatic latent image of a density detection pattern. 2. The image forming apparatus according to claim 1, wherein the duty of the rectangular wave is made smaller than in the case of developing the electrostatic latent image of the document image. 6. An image forming apparatus according to claim 1, wherein said developing bias voltage of said developing means includes an AC component having a certain waveform. 2. The image forming apparatus according to claim 1, wherein the waveform of the AC component is a waveform having a lower developing property than when developing the electrostatic latent image of the original image. 7. The image forming apparatus according to claim 1, wherein the density control means controls the density of the toner image of the original image based on the density of the toner image of the density detection pattern having the highest image occupancy.
An image forming apparatus according to any one of (1) to (6).