JP3313539B2 - Image forming device - Google Patents

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 such as an electrostatic transfer type copying machine and a laser printer, and more particularly to an image forming apparatus of a reversal developing type capable of forming a halftone image.

[0002]

2. Description of the Related Art Japanese Patent Application Laid-Open No. Hei 3-12671 discloses that when a developing bias is adjusted for image density adjustment, a control is performed so that a grid potential is shifted while maintaining a constant potential difference from the developing bias. An electrophotographic apparatus of a reversal developing system capable of arbitrarily changing the image density while maintaining a constant image fog is described.

Japanese Patent Application Laid-Open No. Hei 3-171066 discloses that in order to correct the reproducibility of an entire halftone image, a light beam is irradiated onto the surface of the photoconductor with irradiation energy capable of reproducing halftone images. The surface potential is measured by a potential sensor, and the relationship between the irradiation energy and the measured surface potential is compared with a previously stored EV characteristic. If the relationship deviates from the EV characteristic, the deviation is determined. The light beam output means, the developing bias adjusting means, the charging means, etc. are controlled so as to be corrected, and further, the light beam is irradiated onto the photoreceptor surface with irradiation energy different from the above capable of reproducing halftones. An image forming apparatus capable of performing stable halftone reproduction by repeating the same operation as described above is described.

[0004]

Problems to be Solved by the Invention
In the electrophotographic apparatus disclosed in Japanese Patent Application Laid-Open Publication No. H10-209, the following problem occurs because the grid bias is controlled so as to change while maintaining a constant potential difference from the developing bias.

For example, when the image density is reduced due to the deterioration of the developer or the toner density while the sensitivity change due to the deterioration of the photoreceptor is small, as shown in FIG. From Vb ₁ to Vb ₂
It is corrected to the potential difference between the potential VL ₁ and the developing bias of the bright part of the surface of the photosensitive member (the portion where the potential is exposed is lowered by a large exposure energy) (Vb ₁ -VL ₁₎ from (Vb ₂
−VL ₁ ) to increase the image density,
Since attempts to hold the difference grid bias and the developing bias constant, the grid bias is corrected from Vg ₁ to Vg _2. The developing bias correction amount (Vb ₂ −Vb ₁ )
The grid bias correction amount and (Vg ₂ -Vg ₁₎ is equal.

As a result, the potential of the dark portion (the portion where the potential has not been reduced due to exposure with a small exposure energy) on the surface of the photoreceptor rises, and accordingly, the halftone potential near the white level rises, and The density decreases, and the halftone reproducibility deteriorates. FIG. 22 shows changes in the dark portion potential and the halftone potential with respect to the grid bias on the surface of the photoreceptor. As the grid bias increases, the halftone potential particularly near the white level increases so as to be attracted to the dark portion potential. You can see that it is.

In such a control that changes the grid bias while maintaining a constant potential difference with the developing bias, a change in the halftone potential on the surface of the photoconductor due to a change in sensitivity due to deterioration of the photoconductor, The deterioration of the halftone reproducibility accompanying this cannot be corrected.

On the other hand, in the image forming apparatus disclosed in Japanese Patent Application Laid-Open No. 3-171066, the surface potential of the photosensitive member is measured by a potential sensor in the control for correcting the variation in halftone reproducibility. Expensive equipment,
Further, since the potential measurement has to be performed, the control takes a long time, and further complicated feedback control is required. Furthermore, the relationship between the irradiation energy and the measured surface potential is compared with a previously stored EV characteristic, and control is performed to eliminate the deviation. In this control, the γ characteristic due to the deterioration of the developer is controlled. Because they have n’t been able to keep up
It is considered different from the actual halftone reproducibility.

SUMMARY OF THE INVENTION In view of the above, it is an object of the present invention to provide an image forming apparatus capable of securing an image having good halftone reproducibility with inexpensive and simple control in consideration of the life of the photosensitive member and environmental conditions. .

[0010]

The object of the present invention is to correct the grid bias supplied to the control grid of the charger 19 when the developing bias is adjusted by the developing bias adjusting means as shown in FIG. Grid bias correction means is provided, wherein the grid bias correction means comprises:
The grid bias correction amount is determined according to the development bias correction amount so as to suppress the change in the halftone potential on the surface of the photoconductor 18. That is, the correction amount of the developing bias
According to the correction table shown in Table 1 set according to
To select the correction coefficient K, and apply this correction coefficient K to the developing bias.
Grid bias correction by multiplying the correction amount
Find the quantity.

The correction table includes the actual use of the photosensitive member. As shown in Tables 3 and 4, the correction coefficient is set so as to gradually decrease as the actual use increases. Have been.

Alternatively, the grid bias correcting means includes:
The correction coefficient to be multiplied according to the correction amount of the developing bias is
According to the set correction table shown in Table 6,
The correction coefficient in the correction table depends on the ambient temperature of the photoconductor.
It is set in accordance with the normal temperature, and is set to 1 at normal temperature.
It is set small at high temperatures.

A grid bias correcting means for correcting the grid bias supplied to the control grid of the charger 19 by the same amount as the developing bias when the developing bias is adjusted by the developing bias adjusting means; After the correction, a grid bias is applied to the photosensitive member 18 so as to suppress a change in the halftone potential on the surface of the photosensitive member 18.
And a grid bias correcting means for correcting in accordance with the use results . The grid bias correcting means corrects according to the correction table shown in Table 8 set according to the use results of the photoconductor 18, and the correction table gradually changes the grid bias value when the use results become constant. Is raised so that the potential difference from the developing bias is reduced, and thereafter the potential difference is set to be kept constant.

Further, as shown in FIGS. 17 and 20, grid bias correcting means for correcting a grid bias supplied to the control grid of the charger 19 when the developing bias is adjusted by the developing bias adjusting means, and grid bias correcting means. Light output correction means for correcting the light output by the light beam output device 17 so as to suppress a change in the halftone potential on the surface of the photoreceptor 18 later is provided.
The correction is performed in accordance with a correction table shown in Table 11 set according to the use result of the photoconductor 18, and the correction table is configured to gradually increase the power of the light beam as the use result of the photoconductor 18 increases. Is set.

[0015]

[Action] In the above SL challenges solutions, when performing image density correction by adjusting the development bias, in order to hold as in the conventional image fog in a constant state, the same grid bias and developing bias correction amount If the correction is made by the amount, the halftone potential near the white level near the dark portion potential on the surface of the photoconductor 18 changes, and the reproducibility of the halftone image deteriorates.

Therefore, the grid bias is corrected by obtaining the grid bias correction amount in accordance with the developing bias correction amount so as to suppress the change in the halftone potential on the surface of the photosensitive member 18 during the grid bias correction. Thereby, good halftone reproducibility can be maintained.

In general, the charging characteristic on the surface of the photoconductor 18 changes according to the life (degree of deterioration) of the photoconductor 18, and the dark portion potential on the surface of the photoconductor 18 tends to gradually change. This is because, as the use performance of the photoconductor 18 increases, the thickness of the photoconductor layer is reduced due to, for example, mechanical stress, and the film thickness is reduced, so that the charging performance on the surface of the photoconductor 18 is reduced.

Therefore, the grid bias is corrected so as to keep the potential difference between the dark portion potential and the developing bias constant as the film reduction correction. However, even if the potential difference between the dark portion potential and the developing bias is kept constant, the halftone potential near the white level on the surface of the photoreceptor 18 changes due to sensitivity deterioration, and the halftone reproducibility deteriorates.

Therefore, when the image density is corrected by adjusting the developing bias, the change in the dark area potential due to the deterioration of the charging property and the change in the light and dark potential due to the deterioration in the sensitivity due to the actual use of the photosensitive member 18 according to the amount of the developing bias correction. Predict
In order to suppress the change in the halftone potential on the surface of the photoconductor 18 at the time of the grid bias correction, the development bias correction amount and the actual usage of the photoconductor 18 (the number of copies from the initial stage and the rotation time of the photoconductor 18) are controlled. The grid bias is corrected by calculating the grid bias correction amount. Thereby, good halftone reproducibility can be maintained.

[0020] In addition, generally in accordance with the environmental conditions photoreceptor 18
, The halftone potential changes depending on the ambient temperature of the photoreceptor 18 in particular. When the developing bias is adjusted in the image density correction, if the grid bias is corrected by the same amount as the developing bias correction amount irrespective of the ambient temperature of the photoconductor 18, the halftone reproducibility deteriorates.

Therefore, when the image density is corrected by adjusting the developing bias, the photosensitive member 1 at the time of grid bias correction is adjusted.
The grid bias is corrected by calculating the grid bias correction amount according to the developing bias correction amount and the ambient temperature of the photoreceptor 18 so as to suppress the change of the halftone potential on the surface of the surface 8. Thereby, good halftone reproducibility can be maintained.

By the way , in general film thickness reduction correction, image fog can be maintained in a constant state, but halftone potential changes and halftone reproducibility deteriorates.

Therefore, when the image density is corrected by adjusting the developing bias, the grid bias is corrected by the same amount as the correction amount of the developing bias, and the change of the halftone potential on the surface of the photosensitive member 18 at the time of the grid bias correction. In order to suppress the image fog, the grid bias is corrected by a value set according to the actual use result of the photoconductor 18 so as to gradually reduce the potential difference between the dark portion potential on the surface of the photoconductor 18 and the developing bias. While maintaining a constant state, the change of the halftone potential is suppressed as much as possible, and the halftone reproducibility is kept good.

Further, by changing the laser power by the light beam output unit 17, it is possible to adjust the halftone potential of the photosensitive member 18. Therefore, when the image density is corrected by adjusting the developing bias, the grid bias is corrected by the same amount as the correction amount of the developing bias, and the change in the halftone potential on the surface of the photoconductor 18 during the grid bias correction is suppressed. As described above, the laser power by the light beam output device 17 is corrected according to a correction table which is stored in advance and is set according to the usage time of the photoconductor 18. Thereby, good halftone reproducibility can be maintained.

[0025]

【Example】

(First Embodiment) As shown in FIG. 2, a digital copying machine according to a first embodiment of the present invention includes a scanner unit 1, a laser printer unit 2, a multi-stage paper feed unit 3, and a sorter 4.

The scanner section 1 comprises a document table 5 made of transparent glass, a double-sided automatic document feeder (RDF) 6, and a scanner unit 7. The RDF 6 sets a plurality of originals at once, automatically feeds the originals one by one to the scanner unit 7, reverses the originals according to an operator's selection, and prints one or both sides of the original. This is to cause the scanner unit 7 to read. The scanner unit 7 forms a lamp reflector assembly 8 for exposing a document, a plurality of reflection mirrors 10 for guiding a reflected light image from the document to a photoelectric conversion element (CCD) 9, and a reflected light image from the document on the CCD 9. Lens 11 is included.

The scanner unit 1 is configured to read a document image while the scanner unit 7 moves along the lower surface of the document placing table 5 when operating a document placed on the document placing table 5. When the RDF 6 is used, the document image is read while the document is being conveyed with the scanner unit 7 stopped at a predetermined position below the RDF 6.

Image data obtained by reading the original image by the scanner unit 7 is sent to an image processing unit (not shown), where various processing such as histogram processing, error diffusion processing, density conversion processing, and magnification conversion processing is performed. After that, the image data is temporarily stored in the memory of the image processing unit, and the image data in the memory is provided to the laser printer unit 2 according to the output instruction to form an image on a sheet. The image processing unit quantizes pixel data of 256 gradations per pixel and compresses the image data into 4 gradations of image data per pixel. Based on the quantized image data, each pixel (1 dot) will be described later. The output width (lighting time by on / off control) of the laser beam from the semiconductor laser 17, which is a light beam output device, is determined. That is, pixel data of four gradations per pixel is represented by gradation by changing the output width of the laser light from the semiconductor laser 17.

The laser printer unit 2 includes a manual document tray 14, a laser writing unit 15, an electrophotographic processing unit 16 for forming an image, and a laser light output from the laser writing unit 15 for the electrophotographic processing unit 16.
And a mirror 15a that guides the mirror 15a. Further, the laser writing unit 15 includes a semiconductor laser 17 that emits a laser beam corresponding to image data from the memory, a polygon mirror that deflects the laser beam at a constant angular velocity, and a laser beam that is deflected at a constant angular velocity. It has an f-θ lens and the like that corrects the photoconductor 18 so that it is deflected at a constant speed.

The electrophotographic process unit 16 includes a charger 19, a developing unit 20, a transfer unit 21, and a peeling unit 2 around the photosensitive member 18.
2. Cleaning device 23, static eliminator 24 and fixing device 25
Is arranged.

The charger 19 is of a scorotron type having a control grid, and charges the surface of the photoreceptor 18 to substantially the same potential as the control grid. The developing device 20 includes a developing tank 20a and a toner hopper 20b. The developing tank 20a has a developing roller 20c disposed opposite to the photoreceptor 18 and a stirring roller 20d for stirring the developer. The developer in the developing tank 20a is a two-component developer composed of a toner for negative charging and a carrier itself positively charged to negatively charge the toner.
The toner is supplied from the toner hopper 20b in accordance with an output signal of a toner density sensor (not shown) disposed therein. The transfer device 21 transfers the toner image formed on the photoreceptor 18 onto a sheet supplied from a cassette or a manual document tray 14 described later. The peeler 22 is
The sheet on which the toner image has been transferred is peeled off from the photoconductor 18. The cleaning device 23 collects toner, carrier, paper powder, and the like remaining on the photoconductor 18 to maintain the surface of the photoconductor 18 in a clean state. The static eliminator 24 removes residual charges on the surface of the photoconductor 18. The fixing device 25 fuses the toner image transferred to the sheet.

A transport path 30 is provided downstream of the fixing unit 25 in the transport direction of the sheet on which an image is to be formed. The transport path 30 is connected to a transport path 31 leading to the sorter 4 and a multi-stage paper feed unit. It branches into a transport path 32 leading to 3.

The sorter 4 sorts out the paper on which the toner image is welded to a plurality of bins 4a and discharges the paper. The transport path 32 is
In the multi-stage paper feed unit 3, a branch is made, and as a transport path after the branch, a reverse transport path 33 and a double-sided / composite transport path 34
Is provided.

The multi-stage paper feed unit 3 includes the first cassette 3
5, a second cassette 36, a third cassette 37, and a fourth cassette 38 that can be added by selection. And
The multi-stage paper feed unit 3 includes a common transport path 39. The common transport path 39 feeds out the sheets one by one from the top of the sheets stored in the cassettes of the respective stages, and feeds the sheets to the electrophotographic processing unit 16.
It is configured to be conveyed toward. The common transport path 39 joins the transport path 40 from the fourth cassette 38 on the way to the electrophotographic process section 16 and
Leads to 1. The transport path 41 is a double-sided / composite transport path 34
And a convey path 42 from the manual document tray 14 to be connected to an image forming position between the photoreceptor 18 of the electrophotographic process unit 16 and the transfer unit 21. The point is provided at a position near the image forming position.

Further, the digital copying machine is provided with a control unit comprising a microcomputer for executing an image process. The control unit includes a CPU and a memory, which are components of the image processing unit. The image that decreases with a change in the surface potential of the photoconductor 18 or a change in the characteristics of the developer caused by a change in environmental conditions or life. A developing bias adjustment function for appropriately adjusting the developing bias supplied to the developing device 20 to keep the density constant, and a grid bias supplied to the control grid of the charger 19 when the developing bias is adjusted are stored in advance. And a grid bias correction function for correcting with a correction amount obtained from the correction table.

In the above configuration, the image data of the original image obtained by reading by the scanner unit 7 is:
The image processing section performs various processes and outputs the laser light from the semiconductor laser 17. The laser light is subjected to gradation expression according to the image data by changing the output width, and is scanned by a polygon mirror, an f-θ lens or the like, and is irradiated on the surface of the photoconductor 18. At this time, the surface of the photoreceptor 18 is charged in advance by a grid bias of the charger 19 to a predetermined dark portion potential, that is, a potential when laser light is not irradiated. Is reduced in accordance with the amount of received light to become a halftone potential and a bright portion potential, and an electrostatic latent image corresponding to a document image is formed on the surface of the photoconductor 18. The electrostatic latent image is developed by the toner supplied from the developing device 20, that is, a portion having a potential lower than the developing bias of the developing device 20 is developed to become a toner image. Then, the toner image is electrostatically transferred and fixed on the surface of the sheet conveyed from the multi-stage sheet feeding unit 3. The paper on which the image is formed in this way is
The sheet is sent from the fixing device 25 to the sorter 4 via the conveyance paths 30 and 31, or is conveyed to the multi-stage paper feeding unit 3 via the conveyance paths 30 and 32.

As the copying operation is repeated, the photoconductor 1 is changed due to changes in environmental conditions and life.
8 changes in the surface potential and changes in the characteristics of the developer,
The image density, which is determined by the potential difference between the developing bias and the halftone potential or bright portion potential on the surface of the photoreceptor 18, decreases. Therefore, image density correction is performed by periodically adjusting the developing bias.

This image density correction will be described with reference to FIGS. For example, the dark portion potential on the surface of the photoconductor 18 is set to −60.
When the developing bias is 0 V and the developing bias is -450 V, a certain developing bias (for example, when the developing potential near the halftone density is 200
A developing bias when the developing bias is DV1, DV2, and DV3 is generated as shown in FIG. 4 by setting the reference voltage DV2 to ± 50 V around the reference voltage DV2. The grid bias at this time is set to VG1 for DV1, VG2 for DV2, and VG3 for DV3, corresponding to each developing bias. Is fixed.

Then, these toner patches and the image density of the substrate on the surface of the photoreceptor 18 between the patches are detected by a density sensor, and the ratio PA1 of the output value of the density sensor between each toner patch and the corresponding substrate is detected. / BA1 = P
1, PA2 / BA2 = P2, PA3 / BA3 = P3. These ratios and the stored density reference value ST
DPA (the ratio of the output value of the density sensor between the toner patch and the substrate when the developing bias is the reference voltage DV2 when the machine or the like is in the initial state) is compared, and the density reference value STDPA is one of P1, P2, and P3. If there is between P2 and P3, P
The developing bias correction amount ΔDVB is calculated as follows: ΔDVB = 50 / (P2−P3) × (P2−STDPA) by linear approximation between 2 and P3. Similarly, if the density reference value STDPA is between P1 and P2, a linear approximation between P1 and P2 is made, and the developing bias correction amount ΔDVB is calculated as follows: ΔDVB = 50 / (P1−P2) × (P2−STDPA) calculate. If the development bias correction value ΔDVB is out of the range of P1, P2, and P3 and cannot be obtained from these three points, the development bias is further increased by ± 50 V and ± 50 V around the reference voltage DV2.
By touching 100V, a toner patch is created and the same operation is performed. Then, the calculated developing bias correction amount ΔDV
The developing bias is adjusted by adding B to the initial developing bias.

Here, when the developing bias correction amount is -100 V
If the development bias after adjustment is -550 V, the potential difference between -550 V of the development bias and -600 V of the dark portion potential on the surface of the photoreceptor 18 becomes small, and image fog may occur. At this time, in order to keep the image fog constant as in the related art, a grid bias is applied with -100 V of the same amount as the developing bias correction amount.
When the correction is made to be 700 V, the halftone potential near the white level near the dark portion potential on the surface of the photoconductor 18 increases, and the reproducibility of the halftone image deteriorates.

Therefore, in the present embodiment, the charger 19
When the grid bias is corrected, the grid bias correction amount is obtained based on a correction table stored in advance to correct the grid bias, as shown in FIG. The correction table multiplies the developing bias correction amount set in accordance with the developing bias correction amount as shown in Table 1 so as to suppress the rise of the halftone potential on the surface of the photoconductor 18 during the grid bias correction. Coefficient K (K <
1) are stored together. The correction coefficient K
Is increased as the developing bias correction amount increases.

[0042]

[Table 1]

Therefore, the correction coefficient K is selected according to the development bias correction amount, and the correction coefficient K is multiplied by the development bias correction amount to obtain the grid bias correction amount. The correction amount at this time is a value smaller than the developing bias correction amount. Then, the grid bias after the development bias adjustment is corrected by adding the obtained correction amount to the grid bias. If the potential difference between the dark portion potential on the surface of the photoreceptor 18 and the developing bias becomes too small, image fogging occurs. Therefore, the lower limit of the potential difference is set to about 80V.

As described above, when image density correction is performed by adjusting the developing bias, a grid bias correction amount smaller than the developing bias correction amount is obtained based on a correction table stored in advance to determine the grid bias. The halftone potential can be suppressed from rising compared with the conventional case where the grid bias is corrected so as to maintain a constant potential difference from the developing bias. Images can be obtained.

In addition, simple control for correcting the grid bias by obtaining a grid bias correction amount based on a correction table stored in advance, and providing a stable intermediate without requiring an expensive device such as a potential sensor. Key reproduction becomes possible.

Second Embodiment In general, the charging characteristics on the surface of the photoconductor 18 change in accordance with the life (degree of deterioration) of the photoconductor 18, and the dark portion potential on the surface of the photoconductor 18 tends to gradually decrease. is there. this is,
As the use performance of the photoconductor 18 increases, the photoconductor layer is reduced in film thickness due to mechanical stress such as a polishing effect of the cleaning blade of the cleaning device 23, and the film thickness is reduced. This is because the performance is reduced.

FIG. 5 shows the amount of decrease in the dark portion potential on the surface of the photoreceptor 18 with respect to the number of copies corresponding to the actual use of the photoreceptor 18, and the dark portion potential gradually decreases as the number of copies increases. I understand. Such a change in the surface potential of the photoreceptor 18 changes the image density, adversely affects the image quality, and changes the amount of consumed toner.

Therefore, the grid bias is corrected so as to keep the potential difference between the dark portion potential and the developing bias constant as a film reduction correction. For example, use of photoconductors as shown in Table 2
By correcting the grid bias according to the film reduction correction table storing the grid bias increments corresponding to the rotation times of the photoconductors corresponding to the actual use results , the potential difference between the dark area potential and the developing bias is reduced as shown in FIG. Holds constant.

[0049]

[Table 2]

However, even if the potential difference between the dark portion potential and the developing bias is kept constant as described above, the halftone potential near the white level on the surface of the photoreceptor 18 is deteriorated due to sensitivity deterioration as shown in FIGS. As a result, the sensitivity characteristic of the photoconductor 18 changes, that is, the halftone reproducibility deteriorates compared to the initial sensitivity of the photoconductor 18. FIG. 7 shows the EV characteristic when the dark area potential at the end of life (copy number 250K) is made equal to the initial dark area potential by performing the film thickness reduction correction, and FIG. The change of the halftone potential is shown.

Therefore, in the second embodiment, as shown in Table 3, as shown in Table 3, the use of the photoreceptor 18 in accordance with the amount of development bias correction results in a decrease in the potential of a dark portion due to deterioration in charging properties and a change in brightness due to sensitivity deterioration. Predicting a rise in the potential, the correction coefficient in the correction table is used to determine the developing bias correction amount and the use of the photoconductor 18.
It is set according to the number of copies from the beginning as a guide for grasping the usage results .

[0052]

[Table 3]

Then, as shown in FIG. 9, when the developing bias is adjusted and the image density is periodically corrected, a correction coefficient is selected according to the developing bias correction amount and the number of copies counted from the beginning. The correction coefficient is multiplied by the development bias correction amount to determine the grid bias correction amount. The correction amount at this time is a value smaller than the developing bias correction amount. Then, the grid bias after the development bias adjustment is corrected by adding the obtained correction amount to the grid bias. At the time of image density correction, a general film thickness reduction correction is performed, and the potential difference between the dark portion potential of the photoreceptor 18 and the developing bias is kept constant.

When the potential difference between the dark portion potential on the surface of the photoreceptor 18 and the developing bias is 150 V at the initial stage, the lower limit of the potential difference is set to about 80 V, and the potential is gradually decreased. Further, if the grid bias is corrected and then detected and confirmed by a potential sensor or the like, more accurate correction can be performed. Other configurations and operations are the same as those of the first embodiment, and members having the same functions as those of the first embodiment are denoted by the same reference numerals.

As described above, when the image density correction is performed
The development bias correction amount and copy
-Based on the correction table set according to the number of
Grid bias correction by calculating the
The number of copies, that is,life
(Change in halftone potential due to (deterioration))
8 which absorbs changes in charging characteristics and sensitivity characteristics)
An image with good properties can be obtained.

(Third Embodiment) FIG. 10 shows a photoconductor 18 corresponding to the actual use of the photoconductor 18.
Indicates the amount of decrease in the dark portion potential on the surface of the photosensitive member 18 with respect to the rotation time, and it can be seen that the dark portion potential gradually decreases as the rotation time increases. Such a change in the surface potential of the photoreceptor 18 changes the image density, adversely affects the image quality, and changes the amount of consumed toner.

As described above, even if the grid bias is corrected so as to keep the potential difference between the dark portion potential and the developing bias constant as the film thinning correction, the halftone potential near the white level on the surface of the photoreceptor 18 deteriorates in sensitivity. Rises and the photoreceptor 18
, The halftone reproducibility becomes worse as compared with the initial sensitivity of the photosensitive member 18.

[0058] Therefore, in the third embodiment, as shown in Table 4, brightness due to reduced and sensitivity degradation of the dark potential by the charging property deterioration due to actual use <br/> of the photoreceptor 18 corresponding to the developing bias correction amount The potential rise is predicted, and the correction coefficient in the correction table is determined by the rotation time of the photoconductor 18 from the beginning, from which the film thickness of the photoconductor 18 and the degree of repeated fatigue can be detected more accurately than the developing bias correction amount and the number of copies. Is set.

[0059]

[Table 4]

Then, as shown in FIG. 11, when the developing bias is adjusted and the image density is periodically corrected, the amount of the developing bias and the actual use of the photosensitive member 18 are measured from the initial stage. The correction coefficient is selected in accordance with the rotation time of the photoconductor 18, and the correction coefficient is multiplied by the development bias correction amount to obtain the grid bias correction amount. The correction amount at this time is a value smaller than the developing bias correction amount. Then, the grid bias after the development bias adjustment is corrected by adding the obtained correction amount to the grid bias. In addition,
Other configurations and operations are the same as those of the second embodiment, and members having the same functions as those of the second embodiment are denoted by the same reference numerals.

As described above, when the image density correction is performed, the grid bias correction amount is obtained based on the development bias correction amount stored in advance and the correction table set in accordance with the rotation time of the photosensitive member 18. Since the grid bias is corrected, the change in the halftone potential due to the rotation time of the photoconductor 18, that is, the life (deterioration degree) of the photoconductor 18 is taken into account (changes in the charging characteristics and sensitivity characteristics of the photoconductor 18 are absorbed). An image with good halftone reproducibility can be obtained.

Fourth Embodiment In the second and third embodiments, it has been described that the charging characteristics and the sensitivity characteristics of the photoconductor 18 change depending on the use results of the photoconductor 18, but the environment around the photoconductor 18 is changed. The charging characteristics and sensitivity characteristics also change depending on the conditions.

Table 5 shows the characteristics of the EV characteristics when the ambient temperature of the photosensitive member 18 is high and low, as compared with that at normal temperature. FIG. 12 shows the dark portion potential on the surface of the photosensitive member 18. Shows the EV characteristics at each temperature (when the ambient temperature of the photoreceptor 18 is high, normal, and low) when is constant.

[0064]

[Table 5]

As is clear from this, at a high temperature, the gradient of the halftone potential is steep even when the dark portion potential is the same as the dark portion potential at room temperature. Even when the potential is the same as the dark portion potential, the gradient of the halftone potential is in a sloping state. For this reason, E at room temperature
Assuming that the −V characteristic can provide ideal halftone reproducibility, a good halftone reproducibility can be obtained if the EV characteristic is parallel to this. Is adjusted, even if the grid bias, that is, the dark portion potential of the photoconductor 18 is corrected by the same amount as the developing bias correction amount regardless of the ambient temperature of the photoconductor 18, the gradient of the halftone potential is steep at high temperatures. And
At low temperatures, since the gradient of the halftone potential is in a sloping state, the halftone reproducibility deteriorates in any case.

Therefore, when image density correction is performed at a high temperature, it is necessary to reduce the gradient of the halftone potential so as to be parallel to the EV characteristic at normal temperature in order to obtain good halftone reproducibility. Therefore, it is necessary to make the correction amount of the grid bias smaller than the correction amount of the developing bias.

Further, when the image density is corrected at a low temperature, in order to obtain good halftone reproducibility, the E-
It is necessary to raise the gradient of the halftone potential so as to be parallel to the V characteristic, and therefore, it is necessary to make the correction amount of the grid bias larger than the correction amount of the developing bias.

Therefore, in the fourth embodiment, as shown in Table 6, the correction coefficients in the correction table are set in accordance with the ambient temperature of the photoreceptor 18, and the correction coefficients become smaller at high temperatures and smaller at low temperatures. Is getting bigger.

[0069]

[Table 6]

Table 7 shows the relationship between the temperature of the photosensitive member 18 and each ambient temperature of the photosensitive member 18. The ambient temperature of the photosensitive member 18 at this time corresponds to the upper part of the cleaning device 23, This is the temperature when the upper part of the charger 19 and the upper part of the static eliminator 24 are respectively detected by a thermocouple.

[0071]

[Table 7]

The ambient temperature of the photoreceptor 18 substantially coincides with the temperature of the photoreceptor 18, and any ambient temperature may be employed. Among them, the temperature in the upper part of the cleaning unit 23 is the most sensitive. The temperature is close to the temperature of the body 18, and a thermocouple 50 is arranged above the cleaning device 23 so as to detect the temperature as the ambient temperature of the photoconductor 18.

Then, as shown in FIG. 13, when the image bias is periodically adjusted by adjusting the developing bias, a correction coefficient corresponding to the ambient temperature of the photosensitive member 18 detected by the thermocouple 50 is selected. The correction coefficient is multiplied by the development bias correction amount to determine the grid bias correction amount, and the grid bias after the development bias adjustment is corrected.

Here, the transition of the EV characteristic accompanying the development bias correction and the grid bias correction at a low temperature will be described with reference to FIG. Image density before correction, although a development potential DV ₁ <development potential DV ₃ at room temperature when the toner patch Creating a low temperature toner patch image after the density correction, the low temperature developing bias the developing bias is increased and developing potential DV ₄ and room temperature during development potential DV ₃ creating toner patch creation toner patch after the correction becomes substantially equal, the density correction around the high concentration i.e. the light portion potential is successful.
Incidentally, DV ₂ shows the development potential of creating a high temperature when the toner patch.

[0075] In this case, it is corrected only grid bias the same amount as the correction amount of the developing bias, since the slope near the halftone potential sleeping compared to normal temperature, normal temperature halftone development potential DV ₅ < The low-temperature halftone development potential DV ₆ becomes DV ₆
The fact that ₆ is large means that it is expressed with a darker density than at room temperature, and the halftone reproducibility is poor. Therefore, in order to improve the halftone reproducibility, the grid bias is corrected as described above, and the dark portion potential of the photoreceptor 18 is increased to make a gradient near the halftone potential. Accordingly, the inclination of the halftone potential at a low temperature becomes substantially parallel to the slope of the halftone potential at the normal temperature, low temperature halftone development potential DV ₇ ≒ normal temperature halftone development potential DV _5, and the halftone reproducibility Is good.
Other configurations and operations are the same as those of the first embodiment, and members having the same functions as those of the first embodiment are denoted by the same reference numerals.

As described above, when the image density correction is performed, the grid bias correction amount is obtained based on the correction table previously set according to the ambient temperature of the photosensitive member 18 and the grid bias is corrected. Therefore, it is possible to obtain an image having good halftone reproducibility in consideration of a change in halftone potential due to the ambient temperature of the photoconductor 18.

(Fifth Embodiment) In the second embodiment, a general film reduction correction for correcting the grid bias so as to keep the potential difference between the dark portion potential on the surface of the photosensitive member 18 and the developing bias constant has been described. However, in this general film thinning correction, the image fog can be maintained in a constant state, but the halftone potential increases and halftone reproducibility deteriorates.

Therefore, in the fifth embodiment, when the developing bias is adjusted and the image density is periodically corrected, the grid bias is corrected by the same amount as the correction amount of the developing bias. Time, ie, use of photoconductor 18
Control is performed to correct the grid bias with a value based on the film reduction correction table set in accordance with the usage record .

As shown in Table 8, the film-thinning correction table collectively stores the amount of increase in the grid bias added to the grid bias according to the rotation time of the photoconductor 18, and as shown in FIG. The potential difference between the dark area potential and the developing bias is kept constant until the rotation time of the photoreceptor 18 is about 600 Ksec.
It is set so that the potential difference is gradually reduced during the second and then the potential difference is kept constant again. As a result, the potential difference between the dark area potential and the developing bias is reduced, and as a result, the rise of the halftone potential is suppressed as much as possible while maintaining the image fog in a constant state, and the halftone reproducibility is improved. Can be. The film thickness reduction correction is set separately from the development bias adjustment and the grid bias correction by the image density correction that is periodically performed. Other configurations and operations are the same as those of the first embodiment, and members having the same functions as those of the first embodiment are denoted by the same reference numerals.

[0080]

[Table 8]

As described above, the rotation time of the photosensitive member 18, that is, the life (deterioration degree) of the photosensitive member 18 is changed only by correcting the grid bias by changing the general film reduction correction table which is a part of the process control. Thus, an image having good halftone reproducibility can be obtained in consideration of the change in halftone potential accompanying the change (absorption of changes in charging characteristics and sensitivity characteristics of the photoconductor 18).

(Sixth Embodiment) When the grid bias is corrected by the same amount as the correction amount of the developing bias when performing the periodic image density correction by adjusting the developing bias, the halftone potential rises and the halftone potential increases. Reproducibility deteriorates and photoreceptor 1
As described above, the halftone potential changes with an increase in the rotation time of No. 8 and the halftone reproducibility deteriorates.

Therefore, in the sixth embodiment, when the developing bias is adjusted and the image density is periodically corrected, the grid bias is corrected by the same amount as the correction amount of the developing bias, and the rise of the halftone potential is suppressed. For this purpose, control is performed to correct the laser power of the semiconductor laser 17 according to a correction table stored in advance.

Table 9 shows a rise value of a certain halftone potential with respect to the rotation time of the photosensitive member 18 when the potential difference between the dark portion potential of the photosensitive member 18 and the developing bias is kept constant.

[0085]

[Table 9]

The laser power of the semiconductor laser 17 is
As shown in FIG. 16, as the output increases, the halftone potential decreases. Table 10 shows the potential at a certain halftone density with respect to the laser power.

[0087]

[Table 10]

In consideration of such a tendency, the correction table collectively stores the laser power set according to the rotation time of the photosensitive member 18 from the beginning, that is, the actual use of the photosensitive member 18, as shown in Table 11. Photoconductor 1
It has increased the lasers power in accordance with the rotation time of 8 increases. In Table 11, it is assumed that the initially set laser power is 0.7 mW.

[0089]

[Table 11]

Then, as shown in FIG. 17, when the developing bias is adjusted and the image density is periodically corrected, the grid bias is corrected by the same amount as the correction amount of the developing bias, and the photosensitive drum is exposed based on the correction table. The laser power corresponding to the rotation time of the body 18 is selected, and the laser power is corrected based on the selected laser power. At the time of image density correction, a general film thickness reduction correction is performed, and the potential difference between the dark portion potential of the photoreceptor 18 and the developing bias is kept constant.

In this case, since the laser output per pixel is changed by correcting the laser power and halftone expression is performed, dot expression of, for example, 256 gradations is possible.
That is, the correction of the halftone potential can be performed more finely in a large range, and the potential adjustment of the image density portion close to the white level or the black level is also possible.

On the other hand, instead of correcting the laser power by the semiconductor laser 17, the output width (pulse width) of the laser beam by the semiconductor laser 17 may be corrected to suppress the rise of the halftone potential. FIG. 18 shows the beam output of one pixel when pulse width modulation is performed. FIG. 19 shows a change in the surface potential of the photoconductor 18 with respect to the pulse width of the laser beam. Then, as shown in FIG. 20, the pulse width of the laser light is corrected based on a correction table in which the pulse widths of the laser light set according to the rotation time of the photoconductor 18 from the beginning are stored together. In this case, since the grayscale is expressed by changing the laser output time per pixel and multi-valued, the control of the laser output time is simpler than the control of the laser output value. It is difficult to adjust the surface potential near the level or near the black level.

The other constructions and operations are the same as those of the first embodiment, and members having the same functions as those of the first embodiment are denoted by the same reference numerals. The optical output correction of the laser beam by the semiconductor laser 17 may be used as an auxiliary to the grid bias correction described in the first to fourth embodiments.

As described above, by correcting the output of the laser beam from the semiconductor laser 17, it is possible to cope with changes in the charging characteristics and sensitivity characteristics of the photosensitive member 18 which cannot be compensated for by grid bias correction after development bias correction in image density correction. As a result, it is possible to obtain an image with good halftone reproducibility while minimizing the change in halftone potential.

The present invention is not limited to the above-described embodiment, and it goes without saying that many modifications and changes can be made to the above-described embodiment within the scope of the present invention.

[0096]

As is apparent from the above description, according to the present invention, when image density correction is performed by adjusting the developing bias, the developing bias correction is performed so as to suppress the change in the halftone potential on the surface of the photosensitive member. Since the grid bias is corrected by calculating the grid bias correction amount according to the amount, the halftone reproducibility can be improved compared to the conventional case where the grid bias is corrected to maintain a constant potential difference from the developing bias. Good images can be obtained. In addition, there is no need for expensive devices such as a potential sensor or complicated control as in the past,
Stable halftone reproduction becomes possible.

Then, the developing bias correction amount and the photosensitive member
ofUse recordThe grid bias correction amount according to the
Since the lid bias has been corrected,life
Halftone taking into account changes in halftone potential due to (deterioration)
An image with good reproducibility can be obtained.

Alternatively , when the grid bias is corrected by calculating the grid bias correction amount according to the developing bias correction amount and the ambient temperature of the photoconductor, the halftone reproducibility in consideration of the change in the halftone potential due to the ambient temperature of the photoconductor is considered. Good image can be obtained.

Also, after the grid bias correction, the grid bias is corrected by a value corresponding to the actual use of the photoconductor so as to suppress the change in the halftone potential on the surface of the photoconductor, and is therefore a part of the process control. By simply changing the general film reduction correction table, it is possible to obtain an image with good halftone reproducibility in consideration of a change in halftone potential due to the life (deterioration degree) of the photoconductor.

Further , by correcting the light output by the light beam output device after the grid bias correction, the change in the charging characteristics and the sensitivity characteristics of the photosensitive member which cannot be compensated for by the grid bias correction after the development bias correction in the image density correction. Therefore, it is possible to obtain an image having good halftone reproducibility while minimizing a change in halftone potential.

At this time , since the laser power by the light beam output device is corrected, it is possible to express, for example, 256 gradation dots. That is, the correction of the halftone potential can be performed more finely in a large range, and the potential adjustment of the image density portion close to the white level or the black level can be performed.

[Brief description of the drawings]

FIG. 1 is a flowchart showing a processing procedure of grid bias correction of a digital copying machine according to a first embodiment of the present invention.

FIG. 2 is a configuration diagram of a digital copying machine.

FIG. 3 is a flowchart illustrating a processing procedure of image density correction.

FIG. 4 is a diagram illustrating a grid bias, a developing bias, and a density sensor output when three types of toner patches are created.

FIG. 5 is a diagram illustrating a decrease amount of a dark portion potential with respect to the number of copies.

FIG. 6 is a diagram showing a potential difference between a dark portion potential and a developing bias with respect to a photoconductor rotation time in film thickness reduction correction.

FIG. 7 is a diagram showing an EV characteristic in film thickness reduction correction.

FIG. 8 is a diagram showing a change in halftone potential with respect to the number of copies.

FIG. 9 is a flowchart illustrating a processing procedure of grid bias correction according to the second embodiment.

FIG. 10 is a diagram illustrating a decrease amount of a dark portion potential with respect to a rotation time of a photoconductor.

FIG. 11 is a flowchart illustrating a processing procedure of grid bias correction according to the third embodiment.

FIG. 12 is a diagram showing the influence of the ambient temperature of the photoconductor on the EV characteristics.

FIG. 13 is a flowchart illustrating a processing procedure of grid bias correction according to a fourth embodiment.

FIG. 14 is a diagram showing a change in EV characteristics accompanying development bias correction and grid bias correction at a low temperature.

FIG. 15 is a diagram showing a potential difference between a dark portion potential and a developing bias with respect to a photoconductor rotation time in the film thickness reduction correction of the fifth embodiment.

FIG. 16 is a view showing an EV characteristic when the laser power in the semiconductor laser is changed.

FIG. 17 is a flowchart illustrating a processing procedure of laser power correction according to a sixth embodiment.

FIG. 18 is a diagram showing a beam output of one pixel when pulse width modulation is performed in a semiconductor laser.

FIG. 19 is a diagram showing a change in the surface potential of the photoconductor with respect to the pulse width.

FIG. 20 is a flowchart showing a pulse width correction processing procedure;

FIG. 21 is a diagram showing EV characteristics when the grid bias is corrected by the same amount as the developing bias correction amount.

FIG. 22 is a diagram showing changes in the dark portion potential and the halftone potential on the surface of the photoconductor with respect to the grid bias;

[Explanation of symbols]

 Reference Signs List 17 light beam output device 18 photoconductor 19 charging device 20 developing device

──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Eisaku Hatanaka 22-22 Nagaikecho, Abeno-ku, Osaka-shi, Osaka Inside Sharp Corporation (72) Inventor Yuichiro Takei 22-22 Nagaikecho, Abeno-ku, Osaka-shi, Osaka Sharp Corporation (72) Inventor Katsuhiro Nagayama 22-22 Nagaike-cho, Abeno-ku, Osaka City, Osaka Prefecture Inside Sharp Corporation (56) References JP-A-3-271763 (JP, A) JP-A-5-346715 (JP, A) JP-A-5-257354 (JP, A) JP-A-5-323374 (JP, A) JP-A-1-107271 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) G03G 15/00 303 G03G 21/00 370-512 G03G 21/14 G03G 15/02 102 G03G 15/06 101 G03B 27/72

Claims (4)

(57) [Claims] 1. A charging device for uniformly charging the surface of a photoreceptor, a developing device for developing an electrostatic latent image formed on the charged surface of the photoreceptor by exposure, and the developing device is supplied to the developing device. An image forming apparatus comprising: a developing bias adjusting unit that adjusts a developing bias; a grid bias correction that corrects a grid bias supplied to a control grid of the charger when the developing bias is adjusted by the developing bias adjusting unit. The grid bias correction means corrects according to a correction table set according to a correction amount of a developing bias in order to suppress a change in a halftone potential on the surface of the photoreceptor. A correction coefficient (K) by which the correction amount of the developing bias is multiplied is stored in accordance with each developing bias correction amount, and the correction coefficient is K <1. An image forming apparatus wherein the correction amount is set so as to gradually increase as the developing bias correction amount increases. 2. A correction table further photoreceptor using real
2. The image forming apparatus according to claim 1, wherein the correction coefficient is set so as to gradually decrease in accordance with an increase in use results . 3. A charging device for uniformly charging the surface of the photoreceptor, a developing device for developing an electrostatic latent image formed on the charged surface of the photoreceptor by exposure, and the developing device is supplied to the developing device. An image forming apparatus comprising: a developing bias adjusting unit that adjusts a developing bias; a grid bias correction that corrects a grid bias supplied to a control grid of the charger when the developing bias is adjusted by the developing bias adjusting unit. The grid bias correction means performs correction in accordance with a correction table in which a correction coefficient to be multiplied according to a correction amount of a developing bias is set in order to suppress a change in a halftone potential on the surface of the photoconductor. The correction coefficient of the correction table is set according to the ambient temperature of the photoconductor, and is set to 1 at normal temperature, and is large at low temperature and small at high temperature. An image forming apparatus characterized by being set well. 4. A charging device for uniformly charging the surface of the photoconductor, a developing device for developing an electrostatic latent image formed on the charged surface of the photoconductor by exposure, and a developing device for supplying the electrostatic latent image to the developing device. An image forming apparatus comprising: a developing bias adjusting unit configured to adjust a developing bias, wherein a grid bias supplied to a control grid of the charger when the developing bias is adjusted by the developing bias adjusting unit is a correction amount of the developing bias. Grid bias correction means for correcting by the same amount as the above, and grid bias correction means for correcting the grid bias in accordance with the usage record of the photoconductor so as to suppress the change in the halftone potential on the surface of the photoconductor after the grid bias correction. provided, the grid bias correcting means, intended to modify according to the correction table set according to the actual use of the photoreceptor, Correction table, as the potential difference between the actual use is the development bias is gradually increased to a value of grid bias if a certain condition is reduced, are then set such that a potential difference is maintained constant state An image forming apparatus comprising: