JPH09179448A - Image forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To realize excellent gradation rendering regardless of the state of an image carrier by detecting the value corresponding to the film thickness of the image carrier, and converting an image signal on the basis of this value. SOLUTION: The current quantity flowing in a photoreceptor drum 6 at the time of applying fixed voltage to the photoreceptor drum 6 from a high voltage power supply through a charged member 7 is detected to obtain a detection signal corresponding to the photoreceptor film pressure of the photoreceptor drum 6, and an output density characteristic based ton the photoreceptor film thickness change is corrected according to the detection signal. That is, a CPU 25 obtains a digital signal TD corresponding to the photoreceptor film pressure from an A/D converter 24, classifies the photoreceptor film thickness on the basis of this digital signal TD, and selects one of a plurality of γ conversion characteristics. The density characteristic of image data indicating an image for reproduction is thus converted on the basis of the value corresponding to the film thickness of the photoreceptor. On the other hand, the parameter of an image forming process such as a developing bias is not changed by the value corresponding to the film thickness so as to fix the image forming characteristic of a printer, and a gradation characteristic including an intermediate level can be desirably controlled.

Description

Detailed Description of the Invention

[0001]

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

[0002]

2. Description of the Related Art Conventionally, in an electrophotographic image forming apparatus, the use state of a photosensitive drum, which is an image carrier,
When the usage environment changes, the characteristics of the photosensitive drum change,
It is well known that significant changes occur in the output image. For example, when the temperature of the photoconductor drum rises, the sensitivity of the photoconductor is improved and the output density changes. Further, when the photoconductor drum is used, the film thickness of the photoconductor gradually decreases, which causes deterioration of charging performance and deterioration of sensitivity, resulting in a change in output density. Furthermore, due to the decrease in the photoconductor film thickness,
The reproducibility of minute dots changes due to a change in the scattering characteristics of the irradiation light of the photosensitive drum, which causes a change in the output density characteristic of a level that cannot be explained by the charging performance of the photosensitive drum and the change in sensitivity.

It is known that a change in the use environment of the apparatus affects not only the photosensitive drum but also the developing apparatus. That is, it is known that when the humidity of the use environment changes, the charging characteristics of the developer in the developing device change, and as a result, the output density characteristics change significantly.

To solve these problems, various measures have been conventionally used. For example, regarding the change in the film thickness of the photoconductor drum, a detection unit for detecting the film thickness of the photoconductor drum is provided, and the voltage supplied to the charging device is changed according to the detection signal to charge the photoconductor drum. Methods have been proposed to compensate for changes in performance.

Similarly, the applied voltage of the lamp for illuminating the original is changed according to the detection signal for detecting the film thickness,
A method for correcting a change in the sensitivity of the photosensitive drum is also known. Further, regarding changes in temperature and humidity, there is a method of detecting the temperature around the photoconductor drum and correcting the change in the performance of the photoconductor drum by changing the voltage supplied to the charging device according to the temperature and humidity. Proposed.

[0006]

However, in the above-mentioned conventional example, it was difficult to stabilize the output density characteristic against the change in the sensitivity characteristic and the charging performance due to the film thickness variation of the medium such as the photosensitive drum.

In particular, it has not been possible to maintain good gradation characteristics including halftones against variations in film thickness.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide an image forming apparatus capable of realizing good gradation reproduction regardless of the state of the image carrier.

Another object of the present invention is to perform good gradation characteristic control while keeping the characteristics of the printer constant.

A further object of the present invention is to accurately measure the life of the photoconductor and to control the gradation characteristics based on it.

[0011]

In order to solve the above-mentioned problems, an image forming apparatus of the present invention comprises an image carrier for carrying an image to be formed on a medium and a signal for generating an image signal representing the image. It is characterized in that it has a generating means, a detecting means for detecting a value according to the film thickness of the image carrier, and a converting means for converting the image signal based on the value according to the film thickness.

[0012]

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

[Embodiment 1] FIG. 1 is a schematic side view of an image forming apparatus according to an embodiment of the present invention.

A charging member 7 is arranged in contact with the photosensitive drum 6. By applying a predetermined voltage from the high voltage power source 11 to the charging member 7, the photosensitive drum 6 is uniformly charged.

The image information on the original 1 is obtained from the original illumination lamp 2
The light illuminated by is guided to the CCD sensor 3 to be converted into an image signal. This image signal is processed by the image signal processing circuit 4 and output as a drive signal for driving the laser driver 5. This makes manuscript 1
Image exposure according to the above image information is performed on the uniformly charged photosensitive drum 6.

Next, the photosensitive drum 6 on which the electrostatic latent image is formed.
On the other hand, a developing process of developing with the toner of the developing device 8 is performed.

Further, the photosensitive member 6 formed through the developing process
The toner image on the top is the transfer paper P that is fed in synchronization.
The transfer paper P that has been transferred to the fixing device 10 by the transfer device 9 is transferred to the fixing device 10 by the transfer belt.
The toner on the transfer paper P is fixed to the transfer paper P by the fixing device 10. A cleaner 12 removes the toner on the photosensitive drum after the transfer. FIG. 6 shows output density characteristics when the photoconductor drums 6 having photoconductor film thicknesses of 30 μm, 20 μm, and 13 μm are used. The horizontal axis of the figure is the original density, and the vertical axis is the output density of the image forming apparatus of the present invention.
Thus, it can be seen that the output density characteristics change significantly when the photoreceptor film thickness changes.

Therefore, in this embodiment, by detecting the amount of current flowing through the photoconductor drum 6 when a constant voltage is applied to the photoconductor drum 6 through the charging member 7 by the high voltage power source 11, the photoconductor film thickness of the photoconductor drum 6 is detected. Is obtained, and the output density characteristic due to the change in the photoconductor film thickness is corrected according to the detection signal.

A detailed description will be given below.

FIG. 2 is a diagram showing the structure of a detector for detecting a value corresponding to the thickness of the photoconductor.

In FIG. 2, reference numeral 21 denotes an AC voltage generation source,
22 is a source of DC voltage, 23 is a detection resistor, and 24 is A /
The D converter, 25 is a CPU. While the photosensitive drum 6 is rotating, 22 superimposes a predetermined voltage V ₀ on the voltage V _ppo , and the voltage across the detection resistor 23 corresponding to the current i is converted by the A / D converter 24 into a 6-bit digital signal. Convert to T _D. This 6
The bit digital signal T _D is a signal corresponding to the photoconductor film thickness, which is a standard for measuring the freshness of the drum.

The CPU 25 classifies the photoconductor film thickness on the basis of the detection signal T _D according to the following criteria, and selects one of a plurality of γ conversion characteristics shown in FIG.

T _D > A → γ ₁ A ≧ T _D > B → γ ₂ B ≧ T _D > C → γ ₃ C ≧ T _D → γ ₄ (A, B, C satisfy A>B> C And)

Here, it is determined that the larger the value of T _D is, the newer the drum is, and thus the thickness of the photoreceptor is thicker, and the smaller the value of T _D is, the thinner the photoreceptor is due to deterioration of the drum. This determination is made by the CPU 25, and a control signal for selecting the γ conversion characteristic of FIG. 5 is output. Conversion characteristic of FIG. 5, curve slope increases as changes in gamma ₁ to? _4.

When γ _{1 to} γ ₃ are selected, the process parameter for image formation such as the developing bias by T _D is not adjusted. This is because the characteristics of the printer are stabilized by keeping the image forming conditions as constant as possible, and it is easier to control the gradation characteristics by adjusting the gradation characteristics including halftone by converting the image data. Because.

On the other hand, when γ ₄ is selected, the developing bias of the developing device 8 is also adjusted according to the value of T _D. Specifically, the developing bias is set so as to prevent background fog while taking contrast. Accordingly, γ of γ ₄
The conversion characteristic is a γ conversion characteristic in consideration of the change in the printer characteristic due to the change in the developing bias.

FIG. 3 is a block diagram showing the image signal processing circuit 4 and the like of this embodiment.

The analog signal from the CCD sensor 3 is subjected to gain adjustment and shading correction in the analog processing section 31. Next, after being converted into an 8-bit digital signal in the A / D conversion unit 32, it is converted into a density signal in the logarithmic conversion unit 33, the above-mentioned γ conversion is performed in the γ conversion unit 34, and the laser circuit 5 in the PWM circuit 35. Is converted into a pulse width signal for driving.

Each unit of the image signal processing circuit 4 including the γ conversion unit 34 is controlled by the CPU 25. CPU
25 receives a value according to the film thickness from the A / D converter 24,
Control according to this is performed. A printer control unit 40 includes a charging voltage control circuit 41 for controlling the charging voltage of the high voltage power source 11, a developing bias control circuit 42 for controlling the developing bias of the developing device 8, and a laser light amount adjusting circuit for adjusting the laser light amount of the laser 5. 43, a fixing temperature control circuit 44 for controlling the fixing temperature of the fixing device 10 and the like.

As described above, the CPU 25 determines γ according to T _D.
In addition to controlling the γ conversion characteristic in the conversion unit 34, in a specific case, the development bias is also controlled via the development bias control circuit 42 according to T _D.

The γ conversion section 34 has the structure shown in FIG. 4, and is a table conversion circuit in which one characteristic of γ ₁ to γ ₄ is selected by a selection signal from the CPU 25.

This table conversion circuit is stored in, for example, a ROM.
_It can be realized by giving an input / output characteristic of _{1 to} γ ₄ . In addition, when the RAM is used,
The γ conversion table may be created according to the film thickness by setting one set of table data of characteristics to be selected in the RAM by the CPU 25.

The relationship between the original density and the output density obtained by the above structure is shown in FIG.
FIG. 7 shows each of 3 μm.

As described above, according to this embodiment, a constant output density characteristic can be obtained irrespective of the photoconductor film thickness.

Particularly, in this embodiment, the density characteristic of the image data representing the image for reproduction is converted based on the value corresponding to the film thickness of the photoconductor, while the parameters of the image forming process such as the developing bias are the above film thickness. The image forming characteristics of the printer can be fixed and the gradation characteristics including the intermediate level can be satisfactorily controlled by not changing the value depending on the value.

[Embodiment 2] FIG. 8 shows the result of the following primary conversion of the density signal instead of the non-linear conversion by the density conversion table.

D _out = A × (D _in + B) D _out = Density signal after primary conversion D _in = Density signal before primary conversion A, B = coefficients Also in this case, almost constant output density characteristics can be obtained. I understand.

[Embodiment 3] The temperature and humidity of the photosensitive drum are 15 ° C. 5% RH, 23 ° C. 60% RH, and 30 ° C. 80%, respectively.
FIG. 9 shows the output density characteristics when the image signal processing device 4 does not perform correction at the time of RH.

Even in this case, a significant change in the output density characteristic is observed. FIG. 10 shows output density characteristics when the temperature of the photoconductor drum is detected by the temperature / humidity sensor 12 and the detection signal is used to perform density signal conversion by the density signal conversion table in the image signal processing device 4. Thus, a constant output density characteristic can be obtained regardless of the temperature and humidity of the photosensitive drum.

[Embodiment 4] The temperature and humidity of the photosensitive drum are 15
FIG. 11 shows the results obtained by performing the primary conversion of the density signal in place of the density conversion table for the correction in the image signal processing device 4 when the temperature is 5 ° C. RH, 23 ° C. 60% RH, and 30 ° C. 80% RH. Shown in. Also in this case, it can be seen that an almost constant output density characteristic can be obtained.

[Embodiment 5] The film thickness detection timing in Embodiment 1 is, for example, before multiple rotations accompanying turning on / off of the power of the image forming apparatus, before rotation of the image carrier before image formation, and image formation. The later rotation of the image bearing member can be mentioned. In order to improve the detection accuracy of the detection means, it is necessary to lengthen the detection time. Further, the more frequent the detection timing, the better the followability with respect to changes in the photoconductor, and also the followability when a photoconductor that has been used for a long time is replaced with a new photoconductor.

However, if the detection time is blindly lengthened in order to improve the detection accuracy, the total energization time to the photoconductor is increased, and the life of the photoconductor is shortened. Further, the throughput of the image forming apparatus is reduced. Further, if the detection timing is frequently performed, the life of the photoconductor and the throughput are reduced.

Therefore, in this embodiment, the life of the photoconductor, the throughput of the image forming apparatus, the change in the film thickness of the photoconductor,
In particular, the followability to abrupt changes such as replacement of the photoconductor is made compatible as described later.

In general, when the photoconductor film thickness is reduced to about 13 μm, scratches and the like start to occur on the photoconductor drum, a defect occurs in the output image, and it becomes necessary to replace the photoconductor drum 6 with a new one. On the other hand, in order to extend the life of the photoconductor drum 6 to some extent, the detection of the photoconductor film thickness is performed from 500 sheets.
It is conceivable that this is performed at the time of post-rotation after image formation of about 000 sheets. The film thickness detection during the subsequent rotation is accurately performed by taking a sufficient time of about 10 seconds. When the γ table is changed during the post-rotation after the image formation of about 500 to 1000 sheets, the worst case is to replace the photosensitive drum 6 with a new one. Image formation is performed using a γ table optimized for a thin film state. For example, when a γ table for a film thickness of 13 μm is used for a photoconductor drum having a film thickness of 30 μm, the curve of the output image density with respect to the document density has a steep slope and the difference from the linear straight line becomes large.

In view of such a point, in this embodiment, CP
U25 detects the photoconductor film thickness and selects the γ table according to the flowchart shown in FIG. The basic configuration is the same as that of the first embodiment.

First, the opening or closing of the front door for power-on or replacement of the photosensitive drum is detected (S1), and if detected, the copy switch is turned on (S2). The film thickness is detected (S3). Since the film thickness detection during the pre-rotation is performed in a short time (1 to 2 seconds), the life of the drum is not so affected, but the accuracy is low. The detection signal DT ₂ obtained at this time and the detection signal D when the previously stored γ table is switched
The absolute value of the difference from TM is calculated (S4), and when the value is larger than a predetermined threshold value dDT ₂ (about 5 μm to ₁₀ μm), the γ table is switched using DT ₂ as in the first embodiment. Is performed (S5 to S11, S31). The reason for making such a determination is to prevent the γ table from being inadvertently switched based on this because the detection signal DT ₂ at the time of pre-rotation before image formation is not accurate enough. For the same reason, γ based on the detection signal at the time of pre-rotation before image formation
When the tables are switched, TDN is set to 501 in order to detect the photoconductor film thickness at the time of post-rotation after image formation immediately after (S12). On the other hand, when the power is turned on or the opening / closing of the front door is not detected, the copy switch is turned on (S
13) In S4 | DT ₂ -DTM | when it is determined that ≦ DDT _2, when the front of copies count TDN by rotation during the film thickness detector after γ table switchover exceeds 500 sheets (S14), the copy job After start (S1
6), post-rotation film thickness detection (S17) and γ table switching (S19 to S26), and the detection signal DT ₁ at that time is set as DTM (S27 to S30).

As described above, by performing the film thickness detection during the post-rotation, it is possible to select an appropriate γ table by highly accurate film thickness measurement with a small number of detection times and time.

As in the above-described embodiment, the primary conversion may be used instead of the γ table switching.

Further, in the present embodiment, the γ table is detected based on one detection, but it may be calculated based on the result of the film thickness detection a plurality of times in order to improve the accuracy. For example, the γ table can be optimized with high accuracy by switching the γ table when the detection signal T _D exceeds a certain threshold value three times in total.

The value corresponding to the film thickness of the photoconductor does not necessarily have to be a value that strictly represents the film thickness itself.
Any value may be used as long as it changes according to the change in film thickness.

Further, the photosensitive member is not limited to the above-mentioned photosensitive drum, but a photosensitive belt may be used.

Further, in the above-described first embodiment, T _{D is} exceptionally increased.
Although the developing bias is controlled according to the above, the exception may be eliminated and the T _D and the developing bias may always be independent.

As described above, according to this embodiment, it is possible to achieve both the life of the photoconductor, the throughput of the image forming apparatus, and the followability with respect to the change in the photoconductor film thickness.

[0054]

As described above, according to the present invention,
Good gradation reproduction can be realized regardless of the state of the image carrier.

[Brief description of the drawings]

FIG. 1 is a schematic side view of an image forming apparatus according to an exemplary embodiment of the present invention.

FIG. 2 is a diagram showing a configuration of a film thickness detection unit.

FIG. 3 is a diagram showing an image signal processing circuit 4 and the like.

FIG. 4 is a diagram showing a γ conversion unit.

FIG. 5 is a diagram showing a γ conversion characteristic.

FIG. 6 is a diagram showing a change in output density characteristics with respect to a photoconductor film thickness of a photoconductor drum when not according to the present invention.

FIG. 7 is a diagram showing a change in output density characteristic with respect to a photoconductor film thickness of a photoconductor drum according to the first embodiment of the present invention.

FIG. 8 is a diagram showing a change in output density characteristics with respect to a photoconductor film thickness of a photoconductor drum according to a second embodiment of the present invention.

FIG. 9 is a diagram showing changes in output density characteristics with respect to temperature and humidity of the photosensitive drum when not according to the present invention.

FIG. 10 is a diagram showing changes in output density characteristics with respect to temperature and humidity of the photosensitive drum according to the third embodiment of the present invention.

FIG. 11 is a diagram showing changes in output density characteristics with respect to temperature and humidity of the photosensitive drum according to the fourth embodiment of the present invention.

FIG. 12 is a flowchart illustrating a processing procedure according to a fifth embodiment of the present invention.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Mitsuka Abe 3-30-2 Shimomaruko, Ota-ku, Tokyo Canon Inc.

Claims (20)

[Claims] 1. An image carrier for carrying an image to be formed on a medium, signal generating means for generating an image signal representing the image, and detection for detecting a value according to the film thickness of the image carrier. An image forming apparatus comprising: a conversion unit that converts the image signal based on a value according to the film thickness. 2. The image forming apparatus according to claim 1, wherein the image carrier is a photoconductor. 3. The image forming apparatus according to claim 1, wherein the conversion unit performs a primary conversion. 4. The image forming apparatus according to claim 1, wherein the detection timing of the detection unit is determined at a timing when the image carrier may be replaced. 5. The image forming apparatus according to claim 4, wherein the detection timing is when the image carrier is rotated before image formation or when the image carrier is rotated immediately after image formation. . 6. The image forming apparatus according to claim 5, wherein the detection accuracy immediately after the image formation is higher than the detection accuracy before the image formation. 7. The image forming apparatus according to claim 1, wherein the γ characteristic is changed in accordance with a plurality of detection results by the detection unit. 8. The detection means detects a value according to a film thickness of the image carrier by measuring a current flowing when a voltage is superposed on the image carrier. 1. The image forming apparatus according to 1. 9. A developing means for developing the image is further provided, and the developing condition in the developing means is not changed in principle depending on a value according to the film thickness of the image carrier. The image forming apparatus according to claim 1. 10. The image forming apparatus according to claim 9, wherein the developing condition is exceptionally changed by a value corresponding to the film thickness in a predetermined case where the deterioration of the image carrier is severe. 11. An image carrier for carrying an image to be formed on a medium, a signal generating means for generating an image signal representing the image, and a value corresponding to the film thickness of the image carrier. An image forming apparatus comprising: a detection unit that measures a current flowing when a voltage is superimposed on the image carrier and a conversion unit that converts the image signal based on a value according to the film thickness. . 12. The image forming apparatus according to claim 11, wherein the image carrier is a photoconductor. 13. The image forming apparatus according to claim 11, wherein the conversion unit performs a primary conversion. 14. The image forming apparatus according to claim 11, wherein the detection timing of the detection unit is determined at a timing when the image carrier may be replaced. 15. The image forming apparatus according to claim 14, wherein the detection timing is when the image carrier is rotated before image formation or when the image carrier is rotated immediately after image formation. . 16. The detection accuracy immediately after the image formation is higher than the detection accuracy before the image formation.
An image forming apparatus according to claim 1. 17. The γ characteristic is changed according to a plurality of detection results by the detection means.
An image forming apparatus according to claim 1. 18. The detection means measures a current flowing when a voltage is superimposed on the image carrier,
The image forming apparatus according to claim 11, wherein a value corresponding to a film thickness of the image carrier is detected. 19. A developing device for developing the image, wherein the developing condition in the developing device is not changed in principle depending on a value corresponding to the film thickness of the image carrier. The image forming apparatus according to claim 11. 20. The image forming apparatus according to claim 19, wherein the developing condition is exceptionally changed by a value according to the film thickness in a predetermined case where the deterioration of the image carrier is severe.