JPH0211907B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention provides a developing bias control device for an electrostatic recording device, and in particular has a function of correcting environmental changes and changes over time of a recording medium (photosensitive drum), and performs electrostatic recording while controlling the developing bias voltage. The present invention relates to a developing bias control device for an electrostatic recording device.

Conventional electrostatic recording devices such as electronic copying machines have the drawback that the charging characteristics of the photosensitive drum change due to environmental changes such as humidity and temperature, and are also affected by changes over time such as mechanical fatigue due to use. be.

Conventionally, as a method of correcting such environmental fluctuations and changes over time, the latent image potential of the photosensitive drum is detected and recording conditions such as charging conditions, exposure conditions, and development conditions are changed to form an optimal image. Methods have been proposed in which the charging conditions and exposure conditions are changed in accordance with the charging time to perform control so that optimal image recording can be performed. In general, humidity resistance is by far the largest factor in environmental change compared to other factors, and most conventional methods are aimed at improving humidity resistance. Furthermore, after long-term use, there are mechanical fatigue of the photosensitive drum and changes over time due to deterioration of the photosensitive characteristics themselves.In addition, there are changes in charging characteristics for a short period of time immediately after charging starts, changes in charging characteristics after charging is stopped, etc. Therefore, changes in these charging characteristics must also be taken into account. That is, electrostatic charge,
By repeating exposure, the charging characteristics recover to a steady state, so if the developing bias voltage is the same, the above-mentioned charging characteristics are not compensated for, and the image quality deteriorates. As described above, since the magnitude of the change in the amount of charge immediately after the start of charging exhibits a characteristic that changes depending on the degree of moisture absorption of the photosensitive drum, optimal image recording cannot be performed unless this is taken into consideration.

The present invention has been made in view of the above points, and an object thereof is to provide a developing bias control device that can compensate for changes in the charging characteristics of a photoreceptor at the start of charging and obtain an image with an appropriate density. It is about providing.

That is, the present invention provides a humidity detection means for detecting humidity within the device in an apparatus that forms an image by forming an electrostatic latent image by charging and exposing a photoreceptor and developing the electrostatic latent image. and a control means for controlling a developing bias voltage according to a change in charging characteristics of the photoreceptor at the start of charging, and the controlling means controls an initial value of the developing bias voltage based on the output of the humidity detecting means. and set
When the charging characteristics of the photoreceptor at the start of charging are changed continuously from the initial value to the convergence value in a steady state, and the humidity detected by the humidity detection means is high. provides a developing bias control device for an electrostatic recording device, characterized in that the initial value is set closer to the convergence value than when humidity is low.

Next, embodiments of the present invention will be described in detail with reference to the accompanying drawings.

FIG. 1 illustrates the principle of detecting humidity and charging history, controlling the developing bias voltage, and compensating for the moisture absorption characteristics of the photosensitive drum. When charging starts at t ₀ , the voltage applied to the developing roller of the developing device,
In other words, the developing bias voltage f(t) is detected and varies from +50V to +100V depending on the humidity (30 to 80%) from an initial value f(t ₀ ) to a final value of +50V using an exponential function with a time constant γ ₁ . . When charging is stopped at t ₃ , the developing bias voltage f(t ₃ ) returns to the initial value f(t ₀ ) with a time constant γ ₂ . In this case, γ ₁ is selected to be around 40 seconds based on the charge recovery characteristics when the photosensitive drum is charged, while γ ₂ is selected to be around 5 minutes depending on the moisture absorption deterioration characteristics when the photosensitive drum is stopped. . When charging is started at t= _t4 before returning to the initial value, the developing bias voltage f(t) converges from the initial voltage f( _t4 ) to the final value of 50V according to an exponential function with a time constant _γ1 . In other words, the broken line in FIG. 1 shows the charge recovery characteristic based on the decrease in water absorption during charging, and as the charge recovers, the developing bias voltage can be reduced. It is understood that when charging is stopped at , the amount of water absorbed increases and the charging characteristics deteriorate accordingly, so it is necessary to increase the developing bias voltage.

In this way, in the present invention, the initial value f of the developing bias voltage is determined according to the detected humidity near the photoreceptor drum.
(t ₀ ) and finally controls the developing bias voltage in consideration of changes in charging characteristics based on changes in moisture absorption during charging or when charging is stopped.

A specific device for performing such developing bias control is shown in FIG. In FIG. 2, the photosensitive drum 1 is composed of three layers, for example, an insulating layer, a photoconductive layer, and a conductive layer from the surface.A primary charger 2 is arranged around the photosensitive drum 1 to charge the entire surface of the photosensitive drum 1. Ru. Next to the primary charger 2, a secondary charger (static eliminator) 3 and a full-surface exposure lamp 5 are arranged adjacent to each other in the direction of rotation of the drum. This secondary charger 3 receives reflected light 4 from an original (not shown) illuminated by an exposure light source (halogen lamp) 9.
The charge on the exposed photosensitive drum is removed according to the amount of exposure, and an electrostatic latent image of the document is formed on the photosensitive drum 1. The electrostatic latent image thus formed is entirely exposed to light by a full-surface exposure lamp 5 in order to increase sensitivity, resulting in an electrostatic latent image with even better contrast. Thereafter, the electrostatic latent image on the photosensitive drum is moved to a developing device 7 and is developed with toner via a developing roller 8 to which a developing bias voltage is applied.

Further, a humidity sensor 13 of a low resistance value variation type is disposed at an appropriate location around the photosensitive drum 1, for example, between the developing device 7 and the secondary charger 3. This humidity sensor 1
An alternating current bias voltage of about 1000 Hz IVRMS from the oscillation circuit 11 is applied to the circuit 3 via the resistor R8 and the capacitor C2. Further, the output from the humidity sensor 13 is input to a rectifier circuit 14, and the signal from the humidity sensor 13 is converted into direct current. A time constant circuit 15 consisting of resistors R1, R2 and a capacitor C1 is connected to the rear stage of the rectifier circuit 14, and this time constant circuit has a time constant of γ ₂ =R1·C1 when the electronic switch 16 is open, and when it is short-circuited, It has two time constants: γ ₁ =C1·R1·R2/R1+R2. The output of the time constant circuit 15 is R1
and R2, is amplified by an amplifier 18, and is input to a developing bias generation circuit 19. The output of this developing bias generation circuit 19 is applied to the developing roller 8 and controlled to a developing bias voltage. A synchronizing signal during the charging rotation of the photosensitive drum is applied to the control input 17 of the electronic switch 16 from the sequence controller (not shown) of the recording apparatus main body, and during charging, one end of R2 is grounded to control the time constant circuit. The time constant is set to γ ₁ =C1・R1・R2/R1+R2, and when charging is stopped, one end of R2 is opened and the time constant of the time constant circuit is set to γ ₂ =C1・R1. A means for compensating for recovery characteristics or charging deterioration characteristics is realized.

FIG. 3 shows specific circuits of the oscillation circuit 11, the rectifier circuit 14, the electronic switch 16, and the amplifier 18. The oscillation circuit 11 is an operational amplifier Q1
This is a positive feedback blocking oscillation circuit having a positive feedback blocking oscillation circuit whose oscillation frequency is the optimum condition for the humidity sensor 13.
The values of C1 and R3 are selected so that the frequency becomes 1KHz. The output of Q1 is input to the switching transistor Q2, and the output of Q2 is input to R8, C2.
is applied as a bias current to the humidity sensor 13 via. The terminal voltage detected from the humidity sensor 13 is converted to a low impedance by an impedance converting amplifier Q3, and then rectified by a linear detection circuit Q4. This rectified output is smoothed by a resistor R4 and a capacitor C3, and then amplified by an amplifier Q5 and connected to a time constant circuit 15. On the other hand, electronic switch 1
6 consists of a transistor Q6, the collector of which is connected to a resistor R2 of a time constant circuit, and the base of which is connected to a terminal 17. Furthermore, time constant circuit 1
The output of 5 is current amplified by Q7 and then sent to limiter D.
After the voltage is limited by voltages 1 and D2, it is input to the developing bias generation circuit 19 via the voltage follower Q8.

Next, the operation of the control device configured as above will be explained. A timing signal synchronized with the charging rotation of the photosensitive drum 1 is applied to the terminal 17 from a sequence controller of the copying machine main body. In this case, if the timing of turning on the power for driving the circuit substantially coincides with the charging rotation of the photosensitive drum 1, a constant voltage, for example +24V, is input to the terminal 17. When this voltage is applied to terminal 17, switching transistor Q6 becomes conductive and capacitor C1 starts charging with time constant γ ₁ =C・R1・R2/R1+R2, and R1
The voltage at the connection point between R2 and R2 decreases exponentially. As already explained in connection with FIG. 1, this decrease in voltage is determined in response to the charging recovery characteristic based on the decrease in the amount of moisture absorbed during charging. In this case, when the humidity is high and the output voltage from the humidity sensor 13 is high, a small voltage is applied to the time constant circuit from the inverting amplifier Q4, so that the initial value at t ₀ at the start of charging is shown in FIG. The higher the humidity, the lower the developing bias voltage becomes. In this way, correction is performed taking environmental conditions, particularly humidity conditions, into consideration.

In other words, the charging potential of the photoreceptor immediately after the start of charging shows a charging characteristic that gradually decreases to a steady state with a predetermined time constant over time, but this charging characteristic also changes depending on the humidity, and the higher the humidity The initial charging potential is small and the time from the initial charging potential to a steady state is short.

Therefore, the developing bias voltage immediately after the start of charging is gradually changed from the initial value to a steady state with a predetermined time constant in accordance with the change in the charging characteristics, and
Further, the initial value of the developing bias voltage is changed according to the humidity, and the higher the humidity is, the smaller the initial value of the developing bias voltage is, and the developing bias voltage is controlled so that the time from this initial value to a steady state is shortened. This makes it possible to accurately compensate for changes in charging characteristics and obtain images with appropriate density.

On the other hand, when the timing signal applied to terminal 17 is turned off and charging is stopped, transistor Q6 is cut off and the charge that had been charged in C1 until then is R
1 and is discharged with a time constant γ ₂ =C1·R1.
In this case, the time constant γ ₂ is selected to compensate for charging deterioration due to an increase in water absorption when charging is stopped. In this case, differential amplifiers Q3, Q4, Q5, Q7
consists of an operational amplifier with four built-in FET input circuits, and no current flows into R5.

In this way, the output of the time constant circuit is Q7, Q8
The voltage that is amplified through the developing bias generating circuit 19 and applied to the developing roller 8 is controlled, thereby controlling the amount of developing bias.

The calibration method is a very difficult problem in humidity detection, but in this example, assuming that variations in the humidity sensor itself can be tolerated at the output stage, the cause of circuit variations is minimized, so calibration is not possible at all. It becomes necessary. In other words, fluctuations in the output of the differential amplifier are standardized to an amplitude of 0V to 12V through one-stage switching of transistor Q2, so fluctuations in the sensor bias current can be prevented, and furthermore, in the forward direction of the rectifier diode, fluctuations in the sensor bias current can be prevented. Since voltage variations are standardized using the linear detection circuit Q4, fluctuations in the rectified output can be prevented. Furthermore, since the smoothing circuit, time constant circuit, and sensor itself are coupled to the next stage using a FET input differential amplifier, fluctuations can be eliminated.
Also, the differential amplifiers used all have an open gain of 80.
Since a high gain amplifier of ~100 dB is used in a closed loop, the gain is determined by the input and feedback resistor values and can compensate for variations in the amplifier gain.

FIG. 4 shows another embodiment of the present invention, in which a standard resistor R having the same resistance value as the resistance value of the humidity sensor 13 at normal humidity is shown.
This is an example in which a changeover switch 6 and a changeover switch 20 are provided, and the other parts are the same as those in the embodiment shown in FIG. 2, and a description thereof will be omitted. In this embodiment, the output level of the amplifier 18 can be easily calibrated by switching the input of the rectifier circuit 14 from the humidity sensor 13 to the resistor R6.

Further, in the case of the embodiment shown in FIG. 4, there is an advantage that difficult work such as keeping the environment (humidity) constant is not required when correcting variations in the circuit or variations in charging characteristics due to moisture absorption of the photoreceptor. is obtained.

In addition to the embodiments described above, a humidity sensor is also used in an electrostatic recording device that is equipped with a surface potential measuring device that measures the surface potential of a photosensitive drum and controls recording conditions such as charging, exposure, and development according to the latent image potential. It is clear that an optimal image can be obtained by performing compensation based on the installation vs. humidity characteristics and charging history.

As described above, according to the present invention, the developing bias voltage is continuously changed from the initial value to the convergence value according to the charging characteristics at the start of charging, and furthermore, when the humidity inside the device is high, when it is low, Compared to , the initial value is set to a value close to the convergence value, so even if the charging characteristics at the start of charging change due to fluctuations in humidity conditions, this can be compensated for accurately and images with appropriate density can be obtained. It becomes possible.

[Brief explanation of drawings]

FIG. 1 is a diagram showing the principle of control performed by the device of the present invention, FIG. 2 is a block diagram showing the configuration of the first embodiment of the invention, and FIG. FIG. 4 is a circuit diagram showing the detailed structure. FIG. 4 is a block diagram showing the structure of a second embodiment of the present invention. 1...Photosensitive drum, 2...Primary charger, 3...
Secondary charger, 4...Exposure light beam, 5...Full surface exposure lamp, 7...Developer, 8...Developing roller, 9...
...Exposure light source, 16...Electronic switch.

Claims (1)

[Scope of Claims] 1. In an apparatus that forms an image by forming an electrostatic latent image by charging a photoreceptor and then exposing it to light, and developing the electrostatic latent image, there is provided a humidity sensor for detecting the humidity inside the apparatus. and a control means for controlling the developing bias voltage according to a change in the charging characteristics of the photoreceptor at the time of starting charging, and the controlling means controls the developing bias voltage based on the output of the humidity detecting means. An initial value is set and continuously changed from the initial value to a convergence value in a steady state according to changes in the charging characteristics of the photoreceptor at the start of charging, and further detected by the humidity detection means. A developing bias control device for an electrostatic recording apparatus, characterized in that when the humidity is high, the initial value is set closer to the convergence value than when the humidity is low.