JPH034253A - Image forming device - Google Patents

Abstract

PURPOSE:To always maintain stable picture quality by providing the image forming device with a means for selecting an area, which is irradiated with a fixed amount of laser beams, according to the sensitivity condition of a photosensitive body, and also providing the device with a means for switching an amount of laser beams to plural steps based on a potential contrast of the photosensitive body. CONSTITUTION:As usual, an image forming condition is set based on information obtained by a potential sensor 16, by controlling a voltage which is applied to a grid 7 of a corona electrifier 6. When the range of the image forming condition, that is, a required potential contrast VD-VL exceeds a grid voltage control range, the amount of laser beams for a photosensitive drum 4 is switched to change the potential VL of the brightened part of a drum surface; and a wider potential contrast range can be obtained. The hysteresis characteristic is provided for the characteristic that an amount of exposure is switched for a required potential contrast. In this constitution, change in picture quality, which results from the switch of an amount of exposure, is minimized, and an optimum amount of exposure is always selected; therefore, stable picture quality is always obtained in spite of change in environment and change in sensitivity of the photosensitive body.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Application Field) The present invention relates to an image forming apparatus that forms an image by an electrophotographic process such as a copying machine, a laser beam printer, etc.
In particular, the present invention relates to one in which the surface condition of a photoreceptor can be controlled in accordance with changes in the atmosphere and the like.

(Prior Art) Conventionally, this type of device is known to form images under optimal conditions against environmental changes by controlling the grid voltage of the corona charger and the amount of exposure to the photoreceptor. ing.

That is, as shown in FIG. 1O, the method adopted in such a conventional example is a method for controlling the potential contrast on the surface of the photoreceptor as an image forming condition. By changing the bias, the surface potential (VO) of the photoreceptor is changed, thereby obtaining the required potential contrast (VD-VL). Here, VD indicates the dark potential of the photoconductor, and vL indicates the bright potential of the photoconductor.

However, the surface potential V of the photoreceptor. Even if you change the potential contrast (V
DVL) cannot be obtained, and therefore, when the required range of change in potential contrast is wide, a method is used in which the exposure amount is switched between a low contrast side and a high contrast side. That is, for example, if the required potential contrast (VD VL) in FIG. 10 is larger than the value at point B, the exposure amount is changed to L as shown in FIG.
An operation to switch from O to Hi is performed. In this case, the potential contrast of the target point C (vo V L(H
i) In order to obtain
After making VL the same (FIG. 10, B4B'), control is performed to increase the surface potential VD of the photoreceptor.

In this way, when changing the potential contrast (V[l VL) to a high value, the exposure amount is switched from LO to H4, and the grid is shifted in the order of A→B→B'→C in FIG. When increasing the bias potential and changing the potential contrast (VD VL) to a lower value, switch the exposure amount from Hi to LO and change the exposure amount in the order of C→B'→B→A in the same figure. By lowering the grid bias potential, a wide range of potential contrast (Vo Vt,) was to be controlled.

(Problem to be Solved by the Invention) However, in the case of such a conventional example, the area of potential contrast where the exposure amount changes, that is, B and B' in FIG.
In between, even if the potential contrast is controlled to be the same when switching the exposure amount, it will be affected by the E-V characteristics of the photoreceptor (the relationship between the exposure amount and the surface potential) due to the switching of the exposure amount. Point B above.

There is a drawback that the image quality, especially the gradation, changes between 87 and 87.

That is, FIG. 12(a) shows the EV characteristics of a typical photoreceptor such as OPC (organic photoconductor), and as can be understood from the figure, the EV characteristics of the photoreceptor The change in potential is not linear with respect to the exposure dose, and for this reason, the actual EV characteristic varies depending on the maximum exposure dose used.
It changes as shown in Figure (b).

Therefore, by the method described above, point B (B') in Figure 1O
For example, if the photoreceptor is controlled to obtain a potential contrast of There was a problem with this.

However, although gradation correction has conventionally been performed when switching the exposure amount, it has been difficult to stabilize the image quality by making corrections that are commensurate with changes in the exposure amount. In particular, in full-color image forming apparatuses that place emphasis on gradation,
It was necessary to stabilize the gradation.

Further, when the sensitivity characteristics of the photoreceptor change, there is a problem that a stable image cannot be obtained unless the exposure amount is changed accordingly.

The present invention has been made to solve the problems of the prior art described above, and its purpose is to suppress changes in image quality due to exposure switching and maintain a constant image quality regardless of environmental changes. An object of the present invention is to provide an image forming apparatus that can perform the following steps.

[Means to solve the problem]

To achieve the above object, the present invention is an image forming apparatus that forms an image by irradiating a laser beam onto a photoreceptor based on image information, detects the sensitivity state of the photoreceptor, and depending on the detection result, applies the laser beam to the photoreceptor. means for selecting one use area from among a plurality of use areas provided regarding the light amount of the laser beam, and means for making it possible to switch the light amount of the laser beam in a plurality of stages within the use area of the selected laser beam light amount. It is characterized by

With the above configuration, the present invention minimizes changes in image quality due to exposure switching, and always selects the optimal exposure, so it can be easily adjusted against any environmental changes, photoreceptor sensitivity fluctuations, etc.
It becomes possible to always maintain stable image quality.

〔Example〕

Embodiments of the present invention will be described below based on the drawings.

FIG. 1 is a diagram illustrating the configuration of a full-color image forming apparatus showing an embodiment of the present invention. Reference numeral 1 denotes a laser control unit that generates a laser beam modulated in accordance with an input image signal. 2 is a polygon mirror having a rotating polygon mirror, which is rotated at a constant speed by a scanner motor (not shown) to deflect an incident laser beam. 3
is an imaging lens, which is an optical lens having f/θ characteristics. A photosensitive drum 4 is exposed to an incident laser beam to form an electrostatic latent image. Note that the photosensitive drum 4 rotates in the direction of the arrow. Reference numeral 5 denotes a static elimination lamp that eliminates static potential on the surface of the photosensitive drum 4 and makes it uniform. A charger 6 uniformly charges the surface of the photosensitive drum 4. A grid electrode 7 is used as a control electrode when uniformly charging the surface of the photosensitive drum 4.

8a, 8b, 8c, and 8d are developing devices that convert the electrostatic latent image formed on the photosensitive drum 4 into each developer (magenta, cyan, yellow) according to the developing bias applied to each developing sleeve S a - S d. , black). Reference numeral 9 denotes a transfer drum that carries transfer paper fed from a paper feed cassette 10. A transfer charger 11 transfers the toner image formed on the photosensitive drum 4 onto a transfer paper carried on a transfer drum 9. Reference numeral 12 denotes a peeling claw that separates the transfer paper on which the toner images of each color have been transferred from the transfer drum 9. 1
3 is a fixing device that fixes the toner image on the transfer paper.

Reference numeral 14 denotes a paper discharge tray on which transfer paper on which image formation has been completed is placed. A cleaning device 15 collects toner remaining on the photosensitive drum 4. A potential sensor 16 is provided close to the photosensitive drum 4 at a position after laser beam irradiation, and detects the potential of the latent image. 17 is an A/D converter;
The analog output of the potential sensor 16 is converted into a digital output. Reference numeral 18 denotes a voltage control section composed of a microcomputer, which includes a RAM serving as a data storage means and a control data storage means, a CP serving as a bias calculation means and a control means.
Consists of U etc. 19 is a D/A converter, which converts the control information determined by the voltage controller 18 into analog data, and controls the voltage applied to the charger 6 by the high voltage control unit 2
0a and the high voltage control unit 20b that controls the voltage applied to the grid electrode 7,
The developing bias data is sent to developing bias voltage control circuits 21a to 21d that control the developing devices 8a to 8d.

Next, the potential control operation on the surface of the photosensitive drum 4 will be explained based on FIGS. 2 and 3.

In this embodiment, as in the case of the conventional example, the image forming conditions are set based on the information obtained by the potential sensor 16 by controlling the voltage applied to the grid electrode 7 of the corona charger 6. Furthermore, when the range of the formation conditions, that is, the required potential contrast (vo VL) exceeds the control range of the grid voltage, the amount of laser beam applied to the photosensitive drum 4 is switched to adjust the bright area potential vL of the drum surface. (V L (Hi) ”V L (Lo)),
This makes it possible to obtain a wider range of potential contrast (Figure 2).

However, when the exposure amount is switched, the image quality, particularly the gradation, changes due to the influence of the EV characteristics of the photosensitive drum 4, so it is desirable to avoid switching the exposure amount as much as possible.

Therefore, in this embodiment, as shown in FIG. 3, the potential contrast point at which the exposure amount is switched is different when switching from the Lo side to the Hi side and when switching from the Hi side to the Lo side. We are trying to solve this problem by doing so. That is, in this embodiment, Lo
When switching the exposure amount from the side to the Hi side, the required potential contrast (vo VL) is a predetermined value V. When the value Vc reaches this value, the operation is performed (C→C'), but when switching from the Hi side to the Lo side, the switching operation is not performed even if this value vc is reached, and when the value VB is lower, The exposure amount is decreased (B' → B). In other words, the switching characteristic of the exposure amount with respect to the required potential contrast (vo VL) is given a hysteresis characteristic. By performing such control, for example, when obtaining the required potential contrast of VC, after the exposure amount is switched from Lo to Hi, the potential contrast returns to this value V again. Even if this happens, the exposure amount is not switched, and the switching operation is not performed until the exposure amount decreases to VB, which is a value lower than vo. Therefore, even if the potential contrast changes somewhat due to environmental changes or the like, the exposure amount is not switched, and as a result, it is possible to maintain a constant image quality.

FIG. 4 is a graph showing changes in the exposure amount of an apparatus that switches the exposure amount in five stages with respect to the required potential contrast (VD VL). By frequently switching the exposure amount in this manner, the rate of change in the exposure amount at the time of switching is reduced, so that fluctuations in gradation due to changes in the EV characteristics of the photosensitive drum 4 can be reduced.

By setting the rate of change in exposure amount at this time to at least 20%, the above-mentioned fluctuation in gradation can be reduced. Fifth
Figures (a) and (b) show the changes in the E-V characteristics of the photoreceptor when the exposure amount is changed in five steps at a rate of about 20%, and it is clear from Figure 5 (b). It can be seen that the variation in gradation at the time of switching is considerably reduced. Furthermore, if the rate of change in exposure amount is kept within 10%, variations in gradation that occur during this switching can be almost ignored.

Further, in this embodiment, exposure amount switching in five stages has been described, but the invention is not limited to this, and any switch having at least three or more stages will be sufficiently effective.

In this manner, by controlling the exposure amount switching point to have hysteresis characteristics, it was possible to significantly reduce the frequency of switching.

Further, by increasing the number of switching stages and keeping the rate of change within 20%, it was possible to significantly reduce the variation in gradation during switching.

Therefore, as shown in Figure 6, if the exposure amount is controlled using the method described above so that it has hysteresis characteristics at each switching point, after one switching, the exposure will remain constant unless a large environmental change occurs. The quantity is maintained. Therefore, if the present invention is applied to such an apparatus, it becomes possible to form an image of very high quality, in addition to the small change in gradation when changing the exposure amount.

Furthermore, this embodiment attempts to achieve image stabilization by selecting an appropriate laser power use area in order to correct variations in the sensitivity characteristics of the photoreceptor and variations in the optical efficiency of the laser exposure system. be.

Sensitivity characteristics of photoreceptors vary due to opening of the lot at the time of production, changes over time during use, etc. Also, the optical efficiency of optical systems varies due to changes over time due to opening of the lid mechanically or due to the accumulation of dirt. There are factors.

Therefore, by measuring and determining whether the optical efficiency of the exposure system of the present apparatus of the present invention is good or bad, whether the sensitivity characteristics of the photoreceptor are fast or slow, and whether the desired potential contrast can be obtained accordingly. This selects the range of laser power to be used.

FIG. 7 shows how the laser power is set for the required potential contrast. The basic form is 5-stage switching (Pa to P4) as shown in Fig. 4, and by applying 3 stages of laser power (P5 to P7), [5-stage switching P0 to P4)
, ■, [phase], ■].

That is, if the sensitivity characteristics of the photoreceptor are fast or the optical efficiency is good, use the laser power (p o - p 4) in region I, which has a small amount of exposure, and in the opposite case, use the laser power in region [phase] using the laser power (P3 to P7) of
In the case of intermediate characteristics, the laser power in the region ■ or region [phase] is used.

FIG. 8 shows an example of a method for selecting a usage area by measuring sensitivity characteristics.

Select the maximum power P4 of region ■ and use P4 to obtain a certain dark potential V. (for example, -700V), use the normal potential control mode described above to determine whether the required potential contrast V, -VL maximum value (for example, 450V) can be obtained, and if it is obtained, the exposure amount is appropriate. , the used area is determined to be ■. If it cannot be obtained, the exposure amount is insufficient, so the maximum power P5 of area (3) is selected and the same measurement is performed. By repeating the above process, the area in which the laser power is used is determined. FIG. 9 shows a combination of the exposure amount switching methods shown in FIG. 6 within the laser power usage range determined as described above.

The figure shows an example where area (3) is selected, and the laser power used can be switched in five stages: P, -P5.

Next, the process from switching the light amount of the laser beam to controlling the potential of the photoreceptor and forming an image in the present invention will be explained.

In addition, in this embodiment, when forming an image, pulse width modulation (
It is characterized by using an image forming apparatus that performs PWM (PWM). The input image data is from 00H to FFH (
It has 256 levels of halftone information (hexadecimal representation), and has bright areas (
(equivalent to data OOH) to dark area (equivalent to data FFH)
Continuous gradation can be expressed up to . When used in combination with the present invention, images with high gradation can be formed more stably.

The process of pulse width modulating (PWM) this laser light will first be explained using FIGS. 13 and 14.

FIG. 13 shows an image processing section 200 that performs pulse width modulation.

The digital image data 22 is converted into an analog image signal 203 by the D/A converter 202 and input to one terminal of the comparator 211 . 207 is a timing signal generation circuit which inputs the reference clock signal 24 and generates the pixel clock 2.
04 and a screen clock 208 to the pattern signal generator 209 are created and output. The pattern signal generator 209 outputs a pattern signal 210 based on the screen clock 208 and inputs it to the other terminal of the comparator 211.

The digital image data 22 is input in synchronization with the pixel clock 204, and the D/A converter 202 is inputted in synchronization with the pixel clock 204.
An analog image signal 203 is output in synchronization with . The screen clock 208 is a clock signal obtained by multiplying the period of the pixel clock 204 by an integral number, and defines the period of the pattern signal 210, which is a triangular wave, for example.

The analog image signal 203 and the pattern signal 210 are compared by a comparator 2]1, and when the analog image signal 203 is larger, it is set to 0, and when it is smaller, it is set to 1, and pulse width modulated binary image data 23 is created. and output.

FIG. 14 is a timing chart showing the timing of each part in FIG. 13.

Here, the screen clock 208 is the pixel clock 20
The clock has a period twice that of 4. It shows the pulse waveform of the binary image data 23 pulse width modulated by the pattern signal 210 when the digital image signal 22 changes stepwise from 00 (white) to hexadecimal FF (black). By changing the amplitude of the pattern signal 210 in this manner, the relationship between the input level of the digital image data 22 and the pulse width of the binary image data 23 can be changed.

The process up to image formation in the image forming apparatus having the pulse width modulation section described above will be explained with reference to FIGS. 15 and 16.

FIG. 15 is a flowchart showing the process. When the potential control operation starts [step ■], the sensitivity characteristics of the photoreceptor shown in FIG. 8 are first measured [step ■], and the area in which the laser power is used is determined. (area (f) - ■) [
Step■]. Next is the temperature of the environment inside and outside the aircraft.

The required potential contrast (v contrast) of the photoreceptor is calculated from the humidity measurement. [Step ■] Then, when the required potential contrast is calculated, the exposure amount is determined from the relationship between the required potential contrast and the light amount value shown in FIG. 9 (either P or P5 in FIG. 9) [Step ■〕.

Using the determined laser power, the grid voltage was changed at three points (Vc-+, VG-2, VC-3) and the bright area potential at each grid voltage (corresponding to data 00H,
V oo ) and dark potential (corresponding to data FFH, V F
F) [Step 2] and the grid voltage V to obtain the required potential contrast (V contrast) from the relationship between the grid voltage shown in FIG. 14 and the surface potential of the photoreceptor. is calculated. [Stella 10] Image formation is actually performed using the laser power and grid voltage v0 calculated in the above process [Step ■].

Furthermore, if a certain amount of time has passed or environmental changes have occurred, potential control starts from step (3). It should be noted that regarding steps (2) and (2), changes in the sensitivity of the photoreceptor do not occur in a short period of time, so they may be operated at necessary intervals.

〔Effect of the invention〕

As described above, in the present invention, it is possible to select the area in which the laser beam light intensity is used based on the measurement of the sensitivity characteristics of the photoreceptor, and to switch the light intensity in multiple stages. As a result,
It becomes possible to maintain a constant image quality despite any environmental changes.

Therefore, in a full-color image forming apparatus in which gradation is particularly important, it is possible to always form high-quality images without color shift or the like.

[Brief explanation of drawings]

FIG. 1 is a schematic configuration diagram showing an embodiment of a full-color image forming apparatus according to the present invention, and FIGS. 2 and 3 show control operations for the surface potential of a photosensitive drum in the same embodiment. is a graph showing the relationship between the grid bias potential of the corona charger and the surface potential of the photosensitive drum, FIG. 3 is a graph showing the relationship between the necessary potential contrast of the photosensitive drum and the amount of exposure, FIGS. 7 and 9 are graphs showing the relationship between the necessary potential contrast and the exposure amount on the photosensitive drum, which are used to show the control operation of the embodiment of the present invention.
Figures 5 (a) and (b) are graphs showing the relationship between exposure amount and photoreceptor surface potential, Figure 8 is a flowchart explaining laser power selection processing, and Figures 10 to 12 are conventional image forming methods. This shows the control operation of the surface potential of the photoreceptor in the device. Figure 1O is a graph showing the relationship between the grid bias potential of the charger and the surface potential of the photoreceptor, and Figure 11 is a graph showing the required potential contrast and exposure on the photoreceptor. 12(a) and 12(b) are graphs showing the relationship between the exposure amount and the surface potential of the photoreceptor, and FIGS. 13 and 14 are schematic diagrams explaining pulse width modulation. , 15th
The figure is a flowchart showing potential control in this embodiment.
The figure is a graph showing the relationship between the photoreceptor surface potential and the grid voltage. l... Laser control unit, 4... Photosensitive drum (photosensitive member), 6... Corona charger, 7... Grid electrode, 8a. 8b, 8c, 8d...Developing means, 16...Potential sensor, 18...Voltage control unit, 20a, 20b...
- High pressure control unit, 21a. 21b, 21c, 21d...Development bias voltage control circuit. Destruction 2 diagram

Claims (3)

[Claims] (1) In an image forming apparatus that forms an image by irradiating a laser beam onto a photoreceptor based on image information, the sensitivity state of the photoreceptor is detected, and a plurality of settings are made regarding the light intensity of the laser beam according to the detection result. means for selecting one use area from the selected use areas; and means for making it possible to switch the light intensity of the laser beam in a plurality of stages within the use area of the selected laser beam light intensity. Forming device. (2) The image forming apparatus according to claim 1, wherein the light amount of the laser beam is switched based on different control information when increasing and decreasing the light amount of the laser beam. (3) The image forming apparatus according to claim 2, wherein the control information is a required potential contrast of the photoreceptor.