JP2000187363A - Image forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To form a toner image having proper density even when the environment changes. SOLUTION: When water content is largely changed, the toner image having the desired maximum density Dmax can not be obtained only by changing the pulse width of the AC component of developing bias between 50 to 100%. Then, contrast potential Vcont is properly changed based on the output from a humidity sensor and the pulse width is changed further based on the changed contrast potential Vcont, whereby the maximum density Dmax is varied to 1.6 to 1.9. The contrast potential Vcont is set such that the ordinary target density of the toner image can be attained by the pulse width showing nearly the center between the upper limit and the lower limit of the changeable pulse width in accordance with the output from the humidity sensor.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an image forming apparatus such as a copying machine and a laser beam printer.

[0002]

2. Description of the Related Art Conventionally, in an image forming apparatus such as a copying machine or a laser beam printer, a method for correcting or changing a look-up table (LUT) is known as a method for changing the density of an output image. ing. Also,
There is known a monochrome image forming apparatus in which the density of an output image is changed by changing a development contrast potential. Further, there is also known a device that changes the pulse width to change the maximum density of an output image.

[0003]

However, in the conventional example described above, since the maximum density that can be output is constant in the correction or change of the LUT, the maximum density can be reduced but cannot be increased. there were. Also,
The method of changing the contrast potential has a problem that when forming an image with a plurality of resolutions, it is impossible to change the maximum density for each resolution.

Further, when the maximum density is changed by changing the pulse width of the laser PWM, the area where the pulse width can be changed is currently limited (50 to 100).
%), There is a problem that it cannot cope with a remarkable change in the maximum concentration due to a change in environment.

For example, as shown in FIG. 15, the developing characteristics under high humidity (hereinafter, appropriately referred to as “high moisture content”) and under low humidity (hereinafter, appropriately referred to as “low moisture content”) are the same 400 V.
With the contrast potential V _cont , the maximum density cannot be reduced to an appropriate value (here, D _max = 1.5) even if the pulse width under high humidity is minimized (50%).

The present invention has been made in view of the above circumstances, and has as its object to provide an image forming apparatus capable of forming a toner image having an appropriate density even when the environment changes. is there.

[0007]

According to a first aspect of the present invention, there is provided an exposure apparatus for exposing a charged surface of an image carrier to form an electrostatic latent image, and comprising: An image forming apparatus including: a developing unit that develops an electro-latent image as a toner image with a developing bias; and an environment sensor that detects an installation environment of the image forming apparatus main body. And control means for adjusting the density of the toner image by changing the contrast potential and the pulse width of the laser of the exposure means.

According to a second aspect of the present invention, in the image forming apparatus according to the first aspect, the control means is configured to control a pulse substantially at a center between an upper limit and a lower limit of a pulse width that can be changed according to an output of the environment sensor. Setting the contrast potential so that a normal target density of the toner image can be achieved by the width;
It is characterized by the following.

According to a third aspect of the present invention, in the image forming apparatus of the first aspect, the lower limit of the pulse width is A and the upper limit is B.
In this case, the contrast potential is set so that a normal target density of the toner image can be achieved by a pulse width between (2A + B) / 3 and (A + 2B) / 3.

[0010]

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

First Embodiment FIGS. 1 and 2 show an example of an image forming apparatus according to the present invention. FIG. 1 is a longitudinal sectional view showing the overall configuration of a four-color full-color digital copying machine.
FIG. 2 is a block diagram showing image processing.

The image forming apparatus shown in FIG. 1 includes a reader unit I for reading an image on a document and a printer unit II for forming an image. In FIG. 1, when a copy key (not shown) is pressed, the original 3 placed on the original platen glass 31 with the image surface (original image) facing down in the reader unit I.
0 is exposed and scanned by an exposure lamp 32, so that a reflected light image from the original
The light is condensed on a full-color sensor 34 such as a CD to obtain a color-separated image signal. The full-color sensor 34 separates a document image into a large number of pixels and generates a photoelectric conversion signal corresponding to the density of each pixel.

In FIG. 2, an image signal output from a full-color sensor 34 is input to an analog signal processing unit 201 and gain and offset are adjusted, and then, for example, 8 bits (A / D conversion unit 202) for each color component. (0 to 255 levels: 256 gradations)
In the shading correction unit 203, C signals arranged in a line using signals read from a reference white plate (not shown) for each color.
In order to eliminate the sensitivity variation of each sensor cell group of the CD, shading correction is performed by optimizing the gain corresponding to each CCD sensor cell.

The line delay unit 204 corrects a spatial shift included in the image signal output from the shading correction unit 203. This spatial shift is caused by the line sensors of the full-color sensor 34 being arranged at a predetermined distance from each other in the sub-scanning direction. Specifically, based on the B (blue) color component signal, the R (red) and G (green) color component signals are line-delayed in the sub-scanning direction, and the phases of the three color component signals are synchronized.

An input masking unit 205 converts the color space of the image signal output from the line delay unit 204 into a color space shown in FIG.
Is converted into the NTSC standard color space by the matrix operation shown in the equation (1). In other words, the color space of the color component signal output from the full-color sensor 34 is determined by the spectral characteristics of the filters of each color component, and is converted into the NTSC standard color space.

From the external input 213, for example, color original image information displayed on a CRT display of a computer is input to an image forming apparatus as an image signal.

The LOG converter 206 is constituted by a look-up table (LUT) composed of, for example, a ROM.
The RGB luminance signal output from the input masking unit 205 is converted into a CMY density signal. Line delay memory 207
Means that the black character determination unit (not shown)
LOG conversion unit 206 for a period (line delay) for generating control signals USR, FILTER, SEN, etc. from the output of
The image signal output from is delayed.

A masking / UCR unit 208 extracts a black component signal K from the image signal output from the line delay memory 207, and further detects color turbidity of a recording color material (toner) of the printer unit II (see FIG. 1). The matrix operation to be corrected is
For example, an 8-bit color component image signal is output to the YMCK image signal in the order of C, M, Y, and K for each reading operation of the reader unit I.

The gamma correction unit 209 converts the image signal into a printer unit.
To match the ideal gradation characteristics of II,
The image signal output from the UCR unit 208 is subjected to density correction. The output filter (spatial filter processing unit) 210 outputs γ
The image signal output from the correction unit 209 is subjected to edge enhancement or smoothing processing.

The LUT 211 is for matching the density of the original image with the density of the output image, and is composed of, for example, a RAM. The pulse width modulator (PWM) 212 outputs a pulse signal having a pulse width corresponding to the level of the input image signal, and the pulse signal is input to a laser driver 41 that drives a semiconductor laser 42 (see FIG. 2). You.

The laser light E (see FIG. 1) emitted from the semiconductor laser 42 of the exposure means 3 is swept by the rotating polygon mirror 3a, and the lens 3b such as an f / .theta.
Is spot-imaged on the photosensitive drum 1 by a fixed mirror 3c that directs the light toward the photosensitive drum 1. Thus, the laser beam E scans the photosensitive drum 1 in a direction (main scanning direction) substantially parallel to the rotation axis of the photosensitive drum 1, and further,
By repeatedly scanning the photosensitive drum 1 in the rotation direction (sub-scanning direction) of the photosensitive drum 1, an electrostatic latent image is formed.

In the printer section II, the photosensitive drum 1 is
It has amorphous silicon, selenium, OPC and the like on its surface, and is rotationally driven in the direction of arrow R1 by driving means (not shown). Around the photosensitive drum 1, the pre-exposure lamp 11, a primary charger (corona charger) 2, a laser exposure optical system (exposure means) 3, and a surface potential sensor 12 , Four developing units 4 y, 4 c, 4 m, and 4 bk of different colors constituting a developing unit, a density detecting unit (density sensor) 13 for detecting the density of a toner image on the photosensitive drum 1, a transfer device 5, and a cleaning device 6 , The front static eliminator 50 is disposed.

In the printer section II, at the time of image formation, the photosensitive drum 1 is rotated in the direction of arrow R1, uniformly discharged by the pre-exposure lamp 11, and then uniformly charged by the primary charger 2. Thereafter, exposure scanning is performed with the laser light E modulated in accordance with the above-described image information signal, whereby an electrostatic latent image corresponding to the image information signal is formed.

Next, the electrostatic latent image on the photosensitive drum 1 is reversely developed with a two-component developer consisting of toner and carrier by operating a predetermined developing device. Negatively charged visible image (toner image)
Is visualized as Developing units 4y, 4m, 4c, 4bk
Are the eccentric cams 24y, 24c, 24m, and 24b, respectively.
By the operation of k, the photosensitive drum 1 is selectively approached in accordance with each separation color. Here, the reversal development is a development method in which a toner charged to the same polarity as the electrostatic latent image is adhered to an exposed area on the surface of the photosensitive drum 1 and is visualized.

Further, the toner image on the photosensitive drum 1 is transferred from a paper cassette 7a, 7b, 7c or the like to a transfer material P supplied to a position facing the photosensitive drum 1 via a transfer system and a transfer device 5. You. In the present embodiment, the transfer device 5 includes a transfer drum 5a as a transfer material carrier, a transfer brush charger 5b as a transfer unit, a suction brush charger 5c for electrostatically attracting the transfer material P, and faces the same. A transfer material holding member made of a dielectric material is provided in a peripheral opening area of a transfer drum 5a that is rotatably driven and has a suction roller 5g, an inner charger 5d, an outer charger 5e, and a transfer peeling sensor 5h. The sheet 5f is integrally stretched in a cylindrical shape. The transfer material supporting sheet 5f uses a dielectric sheet such as polycarbonate.

As the transfer drum 5a rotates, the toner image on the photosensitive drum 1 is transferred onto the transfer material P carried on the transfer material carrying sheet 5f by the transfer brush charger 5b. When the transfer of the desired number of color toner images onto the transfer material P is completed, the transfer material P is separated from the transfer drum 5a by the action of the separation claw 8a, the separation push-up roller 8b, and the separation charger 5h, and is fixed by a heat roller. After the toner image is fixed on the surface by the container 9, the toner image is discharged onto the paper discharge tray 10.
Thus, the image formation of the full-color image is completed.

On the other hand, the photosensitive drum 1 after the transfer of the toner image
The toner (transfer residual toner) remaining on the surface without being transferred to the transfer material P is removed by the cleaning blade 6a of the cleaning device 6, and is used for the next image formation.

The fur brush 14 and the transfer material supporting sheet 5f are used to prevent the powder from scattering and adhering to the transfer material supporting sheet 5f of the transfer drum 5a and the oil from adhering to the transfer material P. Cleaning is performed by the action of the backup brush 15 facing the fur brush 14. Such cleaning is performed before or after image formation, and is performed as needed when a jam (paper jam) occurs.

The transfer drum 5a is provided with the cam 2
5, by the action of the contact / separation member 5i, the photosensitive drum 1 is brought into contact with or separated from the surface. When image formation is performed on both surfaces of the transfer material P, the transfer material P is not discharged onto the paper discharge tray 10 after the toner image is fixed to the first surface, and the flapper 19, the path 20, and the reversing roller 21 b After being turned upside down by the reversing path 21a, the intermediate tray 22, and the like, the sheet is again supplied to the transfer drum 5a, and the image is formed on the second surface, and then discharged onto the sheet discharge tray 10.

FIG. 3 is a four-limit chart showing how the gradation is reproduced. The first quadrant shows a reader characteristic for converting a document density into a density signal, the second quadrant shows an LUT 211 for converting a density signal into a laser output signal, and the third quadrant converts a laser output signal into an output density. Printer characteristics are shown, and the fourth quadrant shows the relationship between document density and output density, that is, the total gradation characteristics of this image forming apparatus. Since the number of gradations is processed by an 8-bit digital signal, there are 256 gradations. In this image forming apparatus, in order to make the gradation characteristics of the fourth quadrant linear, the bent portion of the printer characteristics of the third quadrant is corrected by the LUT 211 of the second quadrant. The LUT 211 is generated based on a calculation result described later.

After conversion by the LUT 211, the PWM2
12, the signal is converted into a signal corresponding to the dot width,
It is sent to the laser driver 41.

Then, an electrostatic latent image having a gradation characteristic due to a change in dot area is formed on the photosensitive drum 1 by laser scanning, and a gradation image is obtained through development, transfer and fixing steps.

The above-described image forming apparatus has a built-in test pattern generator for outputting the image on the photosensitive drum 1. The toner used in the present embodiment is formed by dispersing color materials of yellow, magenta, and cyan with a styrene-based copolymer resin as a binder.

In this embodiment, a tone reproduction means using pulse width modulation processing is used. This will be described in detail.

FIG. 5 is a block diagram showing a configuration example of the PWM circuit 212, and FIG. 4 is a diagram showing main signal waveforms in the PWM circuit 212.

The digital image signal input in synchronization with the pixel clock CLK is converted by the D / A converter 261 into an analog image signal VD. Comparator 264 is
The analog image signal VD and the triangular wave generator 262 generate C
The triangular wave Tri200 corresponding to 200 dpi generated in synchronization with the LK is compared. Further, the comparator 265
Compares the analog image signal VD with the triangular wave Tri400 corresponding to 400 dpi generated in synchronization with the CLK by the triangular wave generator 263. Comparator 264,
The output PWM 200 or PWM 400 is input to the amplifier 267 via the switch 266 controlled by the line number switching signal, amplified, and sent to the laser driver 41.

[0037] Normally, as shown in FIG. 6 (a), the pulse signal obtained by PWM is from the pulse width P ₀₀ corresponding to 0 of the digital image signal to the pulse width P _FF corresponding to FFh of the digital image signal It has 256 stages. On the other hand, the PWM circuit 212 of the present embodiment can vary the P _FF corresponding to the FFh of the digital image signal by adjusting the gain of the D / A converter 261. That is, it is possible to vary in the range of 6 as shown in _{(b), P FF (100} %) of the pulse width from the P _FF (50%) for P _FF.

Further, the pulse width P _FF of the _FF is conventionally fixed as shown in FIG. 6A, but is now reduced to 50% to 100% of the maximum as shown in FIG. , So that it can be changed.

Next, the contrast potential will be described with reference to FIG. The horizontal axis represents the voltage (grid voltage) Vg of the primary charger 2 and the vertical axis represents the photosensitive drum surface potential. Vg is the potential after primary charging, V ₀₀ is the photosensitive drum surface potential after 00 level of the laser is irradiated, the V _FF surface potential of the photosensitive drum after the FF level of the laser is irradiated, Vdc is a developing bias . Potential difference from Vdc to V _FF is at a contrast potential V _cont, this potential difference is likely better is developed large concentration exits high. Note that the potential difference between V ₀₀ and Vdc is V _back, which is the potential that pulls the toner during development from the photosensitive drum 1 side to the developing device 1 side.

FIG. 8 shows that V _cont and P _FF are 100% and 70%.
% Shows the relationship with the concentration when used. In the conventional _PFF control, V _{cont is set} to 300 V so that the maximum density becomes 2.0.
, The P _FF was reduced to 75% while V _cont was kept as it was, and the maximum density was set to 1.5 to perform normal image formation.

[0041] By adopting this method, LUT 211 is intact, image properties such as the maximum concentration of 2.0 is obtained by using the P _FF up to 100%. In the method of narrowing P _FF (to 50%), as shown in FIG.
Since the number of gradations is preserved, the image quality is not impaired.

However, in the case of one environment, the above-mentioned means is effective, but usually, in an electrophotographic image forming apparatus, the developing characteristics differ depending on the environment (particularly, the amount of water in the air). Even when an image is formed at a contrast potential, the density varies depending on the environment. The density varies depending on the amount of moisture in the environment. As shown in FIG. 9, at the same contrast potential, it is more difficult to obtain a density with a low moisture content than with a high moisture content.

In this embodiment, a humidity sensor 6e is provided as an environment sensor near the photosensitive drum 1 and the transfer drum 5a to detect the humidity (moisture content) of the atmosphere.

[0044] Figure 10, when the constant V _cont to 300 V, showing the relationship between the pulse width and the concentration of P _FF. As can be seen from the figure, for example, if it is desired to change _Dmax from 1.6 to 1.9, Pd under low humidity (low moisture content)
_{Even if the FF is set} to the maximum of 100%, the maximum density cannot be increased to 1.9. On the other hand, under high humidity (high moisture content)
_{Even if the FF is set} to the minimum of 50%, the maximum density cannot be reduced to 1.6.

In the present embodiment, in order to solve these problems, even if P _FF is controlled by the amount of water, V _cont is changed when control is not possible. That is, in order to realize a desired density, a high density mode and a low mode are provided. By selecting these modes, for example, the maximum density of 1.9 can be achieved even under low humidity. The maximum density can be reduced to 1.6 even under high humidity.

In this embodiment, when the image forming apparatus is, for example, a digital copying machine capable of switching between a resolution of 200 dpi and 400 dpi, when the high-density mode is selected, both the 200 dpi and 400 dpi are used. Try to increase the maximum concentration.

[0047] Figure 11, the P _FF control range of high humidity when V _cont is 250V, also in FIG. 12, V _cont 350
5 shows P _FF under low humidity in the case of V. From these figures, it can be seen that the concentration can be reduced to 1.6 by setting V _cont to 250 V and setting the pulse width to 50% even in the case of a high water content, and also in the case of a low water content. , V _cont to 350
By setting V and setting the pulse width to 100%, the concentration can be increased to 1.9.

As described above, the environment (in the above, the water content)
, The contrast potential V _cont is appropriately changed, and in the changed contrast potential V _cont , P _{FF is} further changed.
Is changed, the maximum density _{Dmax is set} to 1.6 to 1.9.
Can now be changed.

The contrast potential V _cont is set according to the output of the humidity sensor 6e so that a normal target density of the toner image can be achieved by a pulse width substantially at the center between the upper limit and the lower limit of the variable pulse width. Good.

As a specific example, when the lower limit of the pulse width is A and the upper limit is B, (2A + B) / 3 and (A
There is a method of setting the contrast potential so that a normal target density of the toner image can be achieved by a pulse width between (+ 2B) / 3.

<Embodiment 2> In Embodiment 1 described above, control is performed to change both V _cont and P _FF depending on the environment. However, in this embodiment, P _FF is controlled by each water content. Was set to V _cont so that the maximum concentration was 1.75 when was 75%. Specifically, as shown in FIG. 13, V _cont was set to 200 V, 300 V, and 400 V for each of the low moisture content, the medium moisture content, and the high moisture content.
V _cont is 200V, when the 400V, the relationship between the pulse width and the concentration of P _FF, respectively 14, is shown in Figure 15. V
_{When cont} is 200V, under low humidity, P _FF is 100
It is impossible to increase the maximum concentration to 1.75 even if the percent, D _max by adjusting the P _FF under high humidity 1.5-2.
It becomes possible to 0. Further, when V _cont is 400 V, the maximum concentration is 1.7 even under the high humidity even if the P _FF is 50% which is the minimum.
Can not be lowered to 5, D _max by adjusting the P _FF under low humidity becomes possible to 1.5-2.0.

In FIG. 13, the actual water content is
If there between each table, V _cont is calculated by prorating the V _cont of sandwiching the water content thereof water content. In this embodiment, the environmental data includes only three environments of low, medium, and high moisture content. However, if the environmental data is increased, the control result is further improved.

[0053] developed by the contrast potential was in the water content, by up and down from 75% of P _FF, has made it possible to vary the maximum density.

As described above, when the method according to the present embodiment is used, by setting an appropriate V _{cont according} to the environment,
By changing the P _FF extensively, become high-density image can be obtained, even when lowering the maximum concentration, it was possible to obtain a good image without reducing the number of gradations.

Third Embodiment In the first embodiment, when the high-density mode is selected, the resolutions of 200 dpi and 400 dpi
i, the maximum density was increased, but in this embodiment,
The maximum density is changed for each resolution between 200 dpi and 400 dpi.

In a color image, a natural image is often formed at 200 dpi with good reproducibility of highlights, and characters are often formed at 400 dpi so that details can be reproduced. Therefore, the natural image part faithfully reproduces and the maximum density is 1.5
In order to output the character part clearly, the maximum density is set to 1.
I want to perform image formation at 9 Conventionally, when increasing the image density by increasing V _cont or the like, 200 dpi, 400 dpi
Cannot be switched, and in order to make the character part dark, the natural image part also becomes dark, and a good image cannot be obtained.

However, in this embodiment, 200 dpi,
The 400 dpi P _FF is changed separately, and depending on the water content,
When P _FF is 75%, the maximum density is 1.7 (natural image area 1.
V _cont that is near the middle between 5 and the character part 1.9)
By setting as shown in FIG. 16, the above-mentioned problem is solved, and only the character portion can be made darker.

It is also possible to determine the area between the character portion and the natural image portion and change the _PFF only for the character portion (natural image portion) to obtain a high density.

<Embodiment 4> The above-described Embodiment 1,
In the second and third embodiments, an example in which the present invention is applied to a four-color full-color digital copying machine has been described. However, the present invention can of course be applied to a black-and-white copying machine.

[0060]

As described above, according to the present invention,
According to the output of the environment sensor, the contrast potential is set so that the normal target density of the toner image can be achieved by the pulse width approximately at the center between the upper and lower limits of the variable pulse width of the AC component of the developing bias. Thus, a toner image having an appropriate density can be formed even when the environment changes.

[Brief description of the drawings]

FIG. 1 is a longitudinal sectional view illustrating a schematic configuration of an image forming apparatus according to a first embodiment.

FIG. 2 is a block diagram showing a processing circuit for processing an electric signal from a CCD.

FIG. 3 is a four-limit chart showing how to reproduce gradation.

FIG. 4 is a diagram showing a waveform of pulse width modulation.

FIG. 5 is a circuit diagram of pulse width modulation.

6 (a) is a diagram illustrating a conventional P _FF in the pulse width modulation. (B) is the P _FF of the present invention in pulse width modulation.
FIG.

FIG. 7 illustrates a contrast potential.

FIG. 8 is a diagram showing a relationship between contrast potential and density.

FIG. 9 is a diagram showing a change in a relationship between a contrast potential and a density due to a difference in water content.

FIG. 10 is a diagram showing a change in the relationship between _PFF and concentration due to a difference in water content when the contrast potential is 300 V.

FIG. 11 is a diagram showing a change in the relationship between _PFF and concentration due to a difference in water content when the contrast potential is 250V.

FIG. 12 is a diagram showing a change in the relationship between _PFF and concentration due to a difference in water content when the contrast potential is 350V.

FIG. 13 is a graph showing a maximum density of 1.75 in the second embodiment.
FIG. 7 is a diagram showing a relationship between the amount of water and the contrast potential when.

FIG. 14 is a diagram showing a change in the relationship between _PFF and concentration due to a difference in water content when the contrast potential is 200 V.

FIG. 15 is a diagram showing a change in the relationship between _PFF and concentration due to a difference in water content when the contrast potential is 400V.

FIG. 16 shows Embodiment 3 where the maximum concentration is 1.75.
FIG. 7 is a diagram showing a relationship between the amount of water and the contrast potential when.

FIG. 17 is a diagram showing a matrix for converting a color space of an image signal into an NTSC standard color space.

[Explanation of symbols]

Reference Signs List 1 image carrier (photosensitive drum) 4bk developing unit (developing unit) 4c developing unit (developing unit) 4m developing unit (developing unit) 4y developing unit (developing unit) 6e environmental sensor (humidity sensor) 214 control unit (CPU) P Pulse width when _FF laser is at FF level V _cont Contrast potential

Continued on the front page F term (reference) 2C362 AA63 CA09 CA18 2H027 DA14 EA01 EA02 EA05 EA20 EB04 FB15 2H073 BA04 BA33 2H076 AB75 CA11 CA18 DA10 DA13 EA01

Claims (3)

[Claims] An exposure unit configured to expose the surface of the charged image carrier to form an electrostatic latent image; a developing unit configured to develop the electrostatic latent image as a toner image with a developing bias; An environment sensor for detecting an installation environment, wherein a density of the toner image is changed by changing a contrast potential of the developing unit and a pulse width of a laser of the exposure unit according to an output of the environment sensor. An image forming apparatus, comprising: a control unit that adjusts the image quality. 2. The control unit according to claim 1, wherein the control unit controls the control unit so that a normal target density of the toner image can be achieved by a pulse width approximately at the center between an upper limit and a lower limit of a changeable pulse width. The image forming apparatus according to claim 1, wherein a contrast potential is set. 3. When the lower limit of the pulse width is A and the upper limit is B, a normal target density of the toner image can be achieved by a pulse width between (2A + B) / 3 and (A + 2B) / 3. The image forming apparatus according to claim 1, wherein the contrast potential is set.