JPH0255783B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an image forming apparatus using a photoreceptor,
In particular, it relates to an image forming apparatus equipped with a mechanism for compensating for changes in sensitivity of a photoreceptor.

Generally, when a latent image is formed continuously on a photoreceptor, even if the exposure amount and charger output are constant, the latent image potential may rise over time due to the characteristics of the photoreceptor itself (rise characteristics). , the latent image potential decreases over time (falling characteristic). These rising and falling characteristics adversely affect image quality. For example, when the image has a rising characteristic, the density gradually increases and so-called fog occurs in the white background area. In addition, in the case of a falling edge, the density gradually decreases and the image is interrupted in places.

Conventionally, it has been considered to control the exposure amount, charger output, developing bias, etc. in order to compensate for this rising or falling characteristic.

For example, when looking at a certain point on a photoreceptor having characteristics as shown in FIG. 2, it shows a rising characteristic at the start of image formation when fatigue has been recovered. Therefore, in order to correct this rise characteristic, it is necessary to increase the exposure amount in steps for each rotation of the photosensitive drum.

If the circumference of the drum is longer than one sheet of transfer paper and one copy is to be made with one rotation of the drum, the exposure amount is increased in steps for each rotation of the drum as described above. It is possible to go. However, if the circumference of the drum does not have the length of one sheet of transfer paper, and one copy is made by multiple rotations of the drum, it is possible to change the exposure amount in steps for each rotation of the drum as described above. However, it is not possible to change the exposure amount instantaneously in steps, and a change in copy density appears at the joint where the exposure amount changes, resulting in a change in copy density on a single sheet of transfer paper, resulting in a decrease in image quality. It ends up.

In order to make the change in copy density at this seam less noticeable, it is conceivable to continuously change the exposure amount near this seam. However, even if such a change is made, it is difficult to accurately compensate for changes in the sensitivity of the photoreceptor, and the circuit configuration for doing so also becomes expensive.

In addition, when a constant amount of light is applied while applying a constant charger voltage to the photoconductor, the time (saturation time) until the latent image potential on the photoconductor reaches a constant potential (saturation potential), and the saturation time The difference between the potential and the initial potential on the photoreceptor (potential fluctuation width) largely depends on the time the photoreceptor is not used (rest time). That is, as shown in FIG. 3, when the pause time is long, the saturation time is long, and when the pause time is short, the saturation time is short.
Further, as shown in FIG. 4, the potential fluctuation width increases in proportion to the logarithm of the pause time. FIG. 4 is a model of potential variation ranges when the saturation potential of the black background area (unexposed area) is 500V and the saturation potential of the white background area (exposed area) is −100V.

The present invention has been made in view of the above points, and an object of the present invention is to compensate for changes in the rise or fall characteristics of the photoreceptor that occur depending on the rest time using a simple and inexpensive structure, and to always maintain stable images. An object of the present invention is to provide an image forming apparatus capable of obtaining the following.

That is, the present invention includes: a charging means for uniformly charging a photoreceptor; an exposure means for irradiating light onto the photoreceptor to form an electrostatic latent image after being charged by the charging means;
An image comprising a developing means for developing the electrostatic latent image, and a control means for controlling at least one of the exposure amount of the exposing means, the output of the charging means, or the developing bias applied to the developing means at the start of image formation. In the forming apparatus, the control means starts charging in response to the start of image formation, outputs a signal that monotonically changes with a first time constant, and responds to the start of a hibernation state of the image forming apparatus due to the end of image formation. The CR circuit includes a capacitor and a resistor that starts discharging and outputs a signal that monotonically changes with a second time constant, and detects changes in the rise or fall characteristics of the photoreceptor caused by the rest time of the image forming apparatus. In order to compensate, at least one initial value of the exposure amount, the charging means output, or the developing bias at the start of image formation is set based on the residual charge of the capacitor according to the pause time, and the exposure after the start of image formation is performed. At least one of the charging amount, the output of the charging means, and the developing bias is continuously and monotonically changed from the initial value according to the charge charged in the capacitor, and is configured such that it does not change until the image formation is completed after charging of the capacitor is completed. An object of the present invention is to provide an image forming apparatus having characteristics.

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

FIG. 5 is a sectional view of a copying apparatus to which the present invention can be applied.

The surface of the drum 11 is made of a three-layer seamless photoconductor using a CdS photoconductor, is rotatably supported on a shaft, and is rotated in the direction of the arrow by a main motor 13 activated when the copy key is turned on. Start.

When the drum 11 rotates by a predetermined angle, the original placed on the original platen glass 15 is moved to the first scanning mirror 17.
irradiated by an illumination lamp 19 integrally configured with
The reflected light is scanned by the first scanning mirror 17 and the second scanning mirror 21. By moving the first scanning mirror 17 and the second scanning mirror 21 at a speed ratio of 1:1/2, the original is scanned while the optical path length in front of the lens 23 is always kept constant.

After passing through the reflected light image lens 23 and the third mirror 25, the reflected light is imaged on the drum 11 at the exposure section.

The drum 11 is simultaneously neutralized by the pre-exposure lamp 27 and the pre-AC charger 29, and then the drum 11 is charged by the primary charger 3.
1, it is corona charged (for example, +). Thereafter, the drum 11 is the exposure section, and the image irradiated by the illumination lamp 19 is slit-exposed.

At the same time, AC or corona charge removal with a polarity opposite to the primary one (for example, one) is performed by a charge remover 33, and then a high-contrast electrostatic latent image is formed on the drum 11 by uniform surface exposure using a full-surface exposure lamp 35. The electrostatic latent image on the photosensitive drum 11 is then developed with liquid by the developing roller 39 of the developing device 37 and visualized as a toner image, and the toner image is easily transferred by the pre-transfer charger 41.

Upper cassette 43 or lower cassette 45
The inner transfer paper is fed into the machine by a paper feed roller 47, and is sent toward the photosensitive drum 11 with accurate timing by a register roller 49, so that the leading edge of the latent image can be aligned with the transfer section.

Next, while the transfer paper passes between the transfer charger 51 and the drum 11, the toner image on the drum 11 is transferred onto the transfer paper.

After the transfer is completed, the transfer paper is separated from the drum 11 by a separation roller 53, sent to a conveyance roller 55, guided between a hot plate 57 and presser rollers 59 and 61, and fixed by pressure and heat. The paper is discharged by the discharge roller 63 to the tray 67 via the paper detection roller 65.

After the transfer, the drum 11 continues to rotate and its surface is cleaned by a cleaning device comprising a cleaning roller 69 and an elastic blade 71, and the process proceeds to the next cycle.

Incidentally, when a latent image is not being formed, a blank exposure lamp 34 is turned on in the exposure section to prevent excess toner from adhering to the drum.

A control block diagram is shown in FIG.

In the figure, TM is a timer for measuring the down time of the photoconductor or device, and CT is the timer TM.
a control circuit for controlling at least one of the exposure amount, the charging amount, and the developing bias according to the measured time;
OB is subject to control such as exposure amount, charge amount, development bias, etc. When the controlled object OB is the exposure amount and the photoreceptor has a rising characteristic, the control circuit CT sets the exposure amount change range to be small when the pause time is short, and also sets the time to reach the predetermined exposure amount to be short. do. Further, when the pause time is long, the range of change in exposure amount is set to be large, and the time required to reach a predetermined exposure amount is also set to be long.

A specific circuit diagram is shown in FIG. 7, and an operation timing diagram is shown in FIG. 8.

In the figure, HVDC is a signal for driving the primary charger 31, R1 to R9 are resistors, and VR1 to
VR3 is a variable resistor, Q1 to Q4 are transistors,
C1 is a capacitor, D1 to D3 are diodes,
LA1 is the exposure lamp, PS is the AC power supply, DS is the DC power supply, EXPS is the exposure signal, K1 is the solenoid, K1
a is the relay, CEC is the dimming circuit, M1 is the main motor drive signal that rotates the photosensitive drum, etc.
EXPS is a signal for lighting the original exposure lamp, and BKEX is a signal for lighting the Planck exposure lamp 34.

The circuit operation will be explained.

When image formation is not performed for a long time and there is no charge in capacitor C1, when the copy button is pressed at time to and the charger drive signal HVDC is output, transistor Q1 is turned on, and transistor Q2 is also turned on. Therefore, capacitor C1 is charged via transistor Q2 and resistor 5. charging time
T ₁ is expressed by the following formula.

T ₁ =C1:R5 The collector voltage of transistor Q2 changes from around 24V to around OV with respect to ground at the start of charging. The collector voltage drives an emitter follower circuit consisting of a transistor Q3 via a protection diode D2. This voltage is divided by variable resistor VR1 and resistor R6, and transistor Q4
As a result, the input voltage _Vio of the dimming circuit CEC is controlled to control the light amount of the exposure lamp LA1. When the time _T1 elapses, charging of the capacitor C1 ends and the collector voltage of the transistor Q2 does not change, so the amount of exposure does not change either. Subsequently, when the continuous copying is completed and the drive signal HVDC is turned off, the transistor Q1 is turned off and the charges stored in the capacitor C1 are discharged through the diode D1 and the resistors R2 and R4. The time constant T ₂ of this discharge time is determined by the following equation.

T ₂ =C1·(R2+R4) If HVDC is turned on again before the charge in capacitor C1 is completely discharged, the emitter voltage of transistor Q2 increases by ΔV due to the remaining charge in capacitor C1.

Here, VR3 is a variable resistor for concentration adjustment, and VR3
The divided voltage is input to the dimming circuit CEC through a voltage follower circuit composed of an operational amplifier Q5 and a resistor R10. transistor Q
The collector current of Q4 changes depending on the base voltage of transistor Q4. VR2 is a variable resistor for adjusting the collector current of the transistor Q4, and D3 is a temperature compensation diode.

At time t ₁ in Figure 8, the charge on capacitor C1 is 0.
At time _t2 , the lamp lighting signal EXPS is output, the lamp LA1 is lit, and the predetermined light amount LX is reached at time _t3 . When continuous copying is completed, the lamp lights at time _t4 .
LA1 and the charger go out. When the charger is turned on again at time _t5 , the collector voltage increases by ΔV in accordance with the amount of charge in the capacitor C1, and the capacitor C1 starts charging. The lamp LA1 is turned on again at time _t6 , the capacitor C1 completes charging at time _t7 , the light amount reaches the predetermined light amount LX, and does not change until the copy is completed.

Here, as can be seen by comparing the light amounts at times t ₂ and t ₆ , the initial value of the exposure amount differs depending on the rest time of the exposure lamp. That is, when the pause time is long, there is a large difference between the initial value of the exposure amount and the predetermined light amount LX, and when the pause time is short, the difference between the initial value of the exposure amount and the predetermined light amount LX is small. Also, as is clear from comparing times t ₂ to _{t 3} and t ₆ to t ₇ , the longer the device pause time is, the longer it takes to reach the predetermined light amount LX, and the shorter the pause time is, the longer it takes to reach the predetermined light amount LX. The time period is set to be shorter.

As shown in FIG. 8, by correcting the exposure amount, for example, the rise characteristic shown in FIG. 2 is compensated as shown in FIG. 9, and stable image quality can be obtained.

Compensation for the falling characteristic of the black background part is not perfect compared to the white background part, but since the surface potential-image density characteristic is saturated when the surface potential is around 500V, there is no problem in practical use.

The case where the amount of light from the document exposure lamp is automatically changed to compensate for the rising characteristic of the photoreceptor has been described above, but correction can be made in substantially the same way even when the photoreceptor has falling characteristic.

Further, in this embodiment, the exposure amount was controlled, but the same effects as in this embodiment can also be obtained by controlling the charger or the developing bias instead of the exposure amount.

Further, although the exposure amount is controlled by dimming the lamp, it is also possible to control the aperture. Further, in this embodiment, the pause time is measured in an analog manner using a capacitor, but it is of course possible to provide a clock generator and measure the time digitally.

As explained above, the present invention makes it possible to compensate for changes in the sensitivity of the photoreceptor, so that stable image quality can always be obtained.

[Brief explanation of the drawing]

Figure 1 is a diagram showing step-like control, Figure 2 is a diagram showing the rise characteristics, Figure 3 is a diagram showing the rise characteristics of the white background area depending on the pause time, and Figure 4 is the saturation of the black background area and the white background area. Figure 5 is a cross-sectional view of the copying device, Figure 6 is a control block diagram, Figure 7 is a control circuit diagram, and Figure 8 is an operation timing diagram. , FIG. 9 is a diagram showing the relationship between copy time and latent image potential after control. In the figure, 11 is a photosensitive drum, 19, LA1
34 is the original exposure lamp, 34 is the blank exposure lamp,
C1 indicates a timekeeping capacitor, and CEC indicates a dimming circuit.

Claims (1)

[Scope of Claims] 1. Charging means for uniformly charging a photoreceptor; exposure means for irradiating light onto the photoreceptor to form an electrostatic latent image after being charged by the charging means; An image forming apparatus comprising: a developing means for developing a latent image; and a control means for controlling at least one of the exposure amount of the exposing means, the output of the charging means, or the developing bias applied to the developing means at the start of image formation. The control means starts charging in response to the start of image formation and outputs a signal that monotonically changes with a first time constant, and starts discharging in response to the start of a rest state of the image forming apparatus due to the end of image formation. and a CR circuit consisting of a capacitor and a resistor that outputs a signal that monotonically changes with a second time constant, in order to compensate for changes in the rise or fall characteristics of the photoreceptor caused by the rest time of the image forming apparatus. At least one initial value of the exposure amount, charging means output, or developing bias at the start of image formation is set based on the residual charge of the capacitor according to the pause time, and after the start of image formation, the exposure amount and charging device output are set. At least one of the means output and the developing bias is continuously and monotonically changed from the initial value according to the charge charged in the capacitor, and is configured so as not to change until the image formation is completed after the charging of the capacitor is completed. Image forming device.