JP2822702B2 - 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 which employs a contact charging means as means for charging (including static elimination) a surface of an image bearing member. More specifically, an oscillating voltage including a DC voltage (a voltage whose voltage value periodically changes with time: a sine wave, a triangular wave, a rectangular wave, a sawtooth wave, a rectangle formed by periodically turning on / off a DC power supply) Wave voltage or the like) is applied to a charging member, and the charging member is brought into contact with the image carrier to charge the surface of the image carrier.

[0002]

2. Description of the Related Art Contact charging is a method in which a charging member to which a voltage is applied is brought into contact with a member to be charged, and charges are directly transferred (injected) to the member to be charged to charge the surface of the member to a required potential. ,
Compared to a corona discharge device that has been widely used as a charging device, the applied voltage required to obtain a desired potential on the surface of the charged object can be reduced, and the amount of ozone generated during the charging process is extremely small. The need for an ozone removal filter is negligible, so that the configuration of the exhaust system of the device is simplified, maintenance-free,
It has advantages such as simple configuration.

Therefore, for example, an electrophotographic apparatus (copying machine,
In image forming devices such as laser beam printers and electrostatic recording devices, attention has been paid to practical use as an alternative to corona discharge devices as a means for charging image carriers such as photoconductors and dielectrics, and other charged objects. Has also been.

[0004] The present applicant applies a voltage obtained by superimposing a DC voltage and an oscillating voltage to a charging member (conductive member) to uniformly charge the contact charging method or apparatus, and applies the charging member to a member to be charged. A method in which charging is performed by contacting the electrodes has been proposed (JP-A-63-149669).

FIG. 9 shows one embodiment. Reference numeral 1 denotes a charged body, for example, a drum-type electrophotographic photosensitive member, an electrostatic recording dielectric, or the like (hereinafter, referred to as a photosensitive drum) which is driven to rotate at a predetermined peripheral speed (process speed) in a clockwise direction indicated by an arrow. Note).

Reference numeral 2 denotes a conductive roller (charging roller) as a contact charging member, which comprises a cored bar 2b and a conductive roller body 2a made of conductive rubber or the like formed on the outer periphery thereof. The charging roller 2 is pressed against the surface of the photosensitive drum 1 with a predetermined pressing force by a pressing force F of a pressing spring 10 acting on both ends of the cored bar 2b, and is driven by the rotation of the photosensitive drum 1. Rotate.

Reference numeral 9 denotes a power supply for applying a voltage to the charging roller 2. The power supply 9 supplies the charging start voltage of the photoreceptor 2 via a contact leaf spring 8 which is brought into contact with the metal core 2 b of the charging roller 2.
A voltage (Vac + Vdc) obtained by superimposing a DC voltage Vdc and a vibration voltage Vac having a peak-to-peak voltage Vpp that is twice or more is applied to the charging roller 2, and the outer peripheral surface of the rotating photosensitive drum 1 is uniformly charged. .

The contact charging member is not limited to the roller type as described above, but may be a blade type, a rod type, a block type, a pad type,
A belt type, a web type, a brush type and the like can also be used.

As described above, the oscillation voltage Vac and the DC voltage Vd
The contact charging in which a superimposed voltage with c is applied to the contact charging member to charge the surface of the member to be charged is hereinafter referred to as an AC contact charging method for convenience.

[0010]

The AC contact charging system as described above is effective as a means for uniformly charging a member to be charged, but has the following problems. (1) Charging Sound An electric field is generated between a member to be charged and a contact charging member that has been brought into contact with the member by applying a voltage, and an electric force is generated.
As in the AC contact charging method, the applied voltage is changed to an oscillating voltage
When an AC voltage component is included, its electric force is A
Since the intensity changes in response to the voltage change of the C voltage component, the object to be charged vibrates, and the generation of noise called “charging sound” is observed. This charging noise is relatively unpleasant and is perceived as device noise. (2) Toner Fusing Phenomenon When foreign matter such as dust is interposed between the charged object and the contact charging member, the foreign matter is pressed against the surface of the charged object by the mechanical force or the electric force due to the vibration. The toner adheres firmly and causes a so-called toner fusion phenomenon to the image carrier in the image forming apparatus.

An object of the present invention is to reduce the occurrence of the charging noise of (1) and the toner fusing phenomenon of (2) in an image forming apparatus employing an AC contact charging system as a charging means for charging an image carrier surface. And

[0012]

The present invention is an image forming apparatus having the following configuration. (1) In an image forming apparatus having a contact charging unit that charges a surface of an image carrier by applying a vibration voltage including a DC voltage by bringing the surface of the image carrier into contact with the surface of the image carrier, the frequency of the vibration voltage applied to the contact charging unit is determined by image formation. An image forming apparatus characterized in that it is made smaller during a process other than the time of image formation than at the time of image formation. (2) The image forming apparatus according to (1), wherein the alternating current component of the oscillating voltage is subjected to constant current control, and the constant current controlled alternating current is made smaller during a process other than image forming than during image forming. apparatus.

[0013]

According to an image forming apparatus employing an AC contact charging method for charging a surface of an image carrier by bringing a charging member to which an oscillating voltage including a DC voltage is applied into contact with the surface of the image carrier as a means for charging the surface of the image carrier, The frequency of the oscillating voltage applied to the contact charging means is made variable, and during a process other than image forming, that is, during a pre-rotation process of the image carrier when a main switch of the apparatus is turned on, a sheet interval during continuous paper feeding, and an image. During a process other than image formation, such as during a post-rotation process of the image carrier after the formation is completed, the frequency of the vibration voltage applied to the contact charging means is set as much as possible as long as there is no problem other than the required frequency during image formation. Low frequency (lowest frequency in a range where no charging unevenness occurs; since image formation is not performed except during image formation, charging frequency can be reduced to a region where moire image interference fringes and the like occur) At least at the time of stroke other than the image forming can be lowered to those inaudible practice the charging noise, the device noise due to charging noise can be totally suppressed to minimum by switching.

Further, by switching the frequency of the vibration voltage applied to the contact charging means to a low state as described above during a process other than the time of image formation, vibration of the image carrier and the anti-charge member can be suppressed to the minimum. Therefore, image defects due to toner fusion can be suppressed.

[0015]

Embodiment <Embodiment 1> (FIGS. 1 to 4) (1) Example of Image Forming Apparatus FIG. 1 is a schematic configuration diagram of an example of an image forming apparatus according to the present invention. The image forming apparatus of the present embodiment is a laser beam printer by an electrophotographic process using a contact charging device as a charging unit for an image carrier.

Reference numeral 1 denotes a rotating drum type photosensitive member (photosensitive drum) serving as an image carrier, which is rotationally driven in a clockwise direction indicated by an arrow at a predetermined process speed V _P (peripheral speed).

Reference numeral 2 denotes a charging roller as a contact charging member. The charging roller 2 is pressed against the surface of the photosensitive drum 1 with a predetermined pressing force by a pressing spring in the same manner as in FIG. No driven rotation. Then, a bias voltage (Vdc + Vac) obtained by superimposing the AC voltage Vac of the frequency f _{0 on} the DC voltage Vdc from the power supply 9 is applied through the contact leaf spring 8, and the peripheral surface of the rotary photosensitive drum 1 is charged to a predetermined potential.

Reference numeral 3 denotes a laser beam scanner which outputs laser light 3a image-modulated at a constant print density Ddpi in accordance with a time-series electric digital pixel signal of a target image input from a host device (not shown). Then, the surface of the photosensitive drum 1 charged as described above is irradiated with a laser beam 3 output from a scanner 3 controlled by a controller.
By performing the main scanning exposure at a, an electrostatic latent image corresponding to the target image information is formed on the surface of the photosensitive drum 1.

The latent image is then developed with toner by a developing sleeve 4 of a developing device, and the developed image is introduced from a paper feed unit (not shown) to a transfer unit between the photosensitive drum 1 and the transfer roller 5 at an appropriate timing. Is transferred onto the transferred transfer material 7.

The transfer material 7 that has passed through the transfer section is separated from the surface of the photosensitive drum 1 and transported to an image fixing section (not shown). The surface of the photosensitive drum 1 after the image transfer is cleaned by the cleaning blade 6 to remove adhered contaminants such as toner remaining after transfer, and is repeatedly used for image formation. (2) Relationship between Frequency of AC Voltage Component, Interference Fringes, and Charging Sound (FIGS. 3 and 4) The dark portion potential of the photosensitive drum 1 charged by the AC contact charging method is the frequency f ₀ of the AC voltage component Vac of the applied voltage. and a charging unevenness called a cycle marked spatial wavelength λsp determined by the process speed V _P (the surface movement speed of the photosensitive drum _{1) (V P / f 0} ).

Then, the photosensitive drum 1 is exposed to n dots (dots, line width n dots) in the sub-scanning direction by turning on the laser by line scanning in the sub scanning direction.
FF space for m dots in the sub-scanning direction
(Space), the length L (line pitch) from laser OFF to OFF when a horizontal line pattern image is formed by repeatedly forming a horizontal line pattern image is substantially equal to the spatial wavelength λsp, and the phase of both L and λsp Coincide with each other, resulting in “interference fringes” called moiré images.

In the laser beam printer of this embodiment, the printing density (resolution) D is 600 dpi, and the photosensitive drum 1 has an organic photoconductor (OPC) layer as a photosensitive layer 1a on the outer peripheral surface of a drum base 1b made of aluminum. It has an outer diameter of 30 (mm) and is driven to rotate at a predetermined process speed (peripheral speed) V _P = 50 (mm / sec). The charging roller 2 is a roller having an outer diameter of 12 (mm) formed by a conductive roller body made of carbon-dispersed EPDM, urethane, or the like around the core bar 2b. The charging roller 2 has a DC voltage Vdc and an AC voltage Vac. Is applied.

[0023] AC voltage superimposed on the DC voltage Vdc
Let the frequency of pressure Vac be f₀ Hz. The process speed is V_P mm / sec. The print density (resolution) of the line scanning is Ddpi. The line width of the line scan is set to ndots. Line-to-line gaps are defined as msspaces. Let 1 dot diameter be d (= 25.4 / D). Let the line pitch be L (= (n + m) · d). Let the spatial frequency of the line pitch be f_S (= V_P / L), the frequency f₀ (Hz) fluctuation range and line
Pitch frequency f _S (Hz) overlap, then
, An interference fringe called a moiré image occurs.
I will.

F _S = _VP / L = _VP / (n + m) · d = _VP · D / (n + m) · 25.4 (1) f ₀ = f _S ···
(2) FIG. 3 (graph 1) shows the relationship between the line pitch L (mm) and the spatial frequency f _s (Hz) of the line pitch expressed by the equation (1). Process speed V _P = 50 (mm / sec)
c), resolution D = 600 (dpi), and n (do
t) and m (space) are represented as n: m.

As can be seen from the graph of FIG. 3, the spatial frequency f _s (Hz) becomes maximum when n (dot): m (space) = 1: 1. Spatial frequency f _{S 1} (Hz) at this time
When determined by the equation (1), it is _{f S 1 = 591 (Hz)} .

Therefore, the frequency f ₀ of the AC voltage Vac applied to the charging roller 2 is equal to the spatial frequency f _{S of the} line pitch.
If it is set to be larger than _{1, the} resolution D = 600 (d
In the image forming apparatus of pi), the occurrence of moire image interference fringes can be prevented. Therefore, it is sufficient to set f ₀ > f _{S 1.
0} (Hz) fluctuates about ± 10%. Therefore, f ₀
And f difference _{S 1} is to be larger than the variation range of 10% f _{S 1.}

FIG. 4 (graph 2) shows the variation of the charging frequency f ₀ and the degree of occurrence of moire image interference fringes at that time. As can be seen from the graph of FIG. 4, the charging frequency f
₀ coincides with the spatial frequency f _{S of the} line pitch, and f ₀ = f _S
In this case, moire image interference fringes occur on the image. Further, when the charging frequency f ₀ is varied, the level becomes better in a region of about ± 10 Hz from the spatial frequency f _S of the line pitch, but moire image interference fringes are generated.

From the above, if the relationship between f ₀ and f _{S 1} is (f ₀ −f _{S 1} )> (f _{S 1} / (10 + 10)), the occurrence of moire image interference fringes can be prevented.

For this reason, it is necessary to set f ₀ ≧ 660 (Hz). However, since the charging frequency f ₀ is very high, the charging noise generated by the charging roller 2 hitting the surface of the photosensitive drum 1 cannot be ignored and becomes noticeable as unpleasant noise. In order to reduce the charging noise, there is a device for setting the charging frequency f ₀ as low as possible.

The range in the graph of FIG.
= 1 (dot) 2 (space) when = 600 (dti)
s) spatial frequency f _{S 2} (Hz) and 1 (dot) 3
The relationship of the charging frequency f ₀ (Hz) in anticipation of the moiré generation region is shown between spatial frequencies f _{S 3} (Hz) of (spaces).

In this range, the charging frequency f ₀ (Hz) in anticipation of the moire generation area is f _S of D = 600 (dpi).
It falls between ₂ (Hz) and f _{S 3} (Hz), and moire image interference fringes are less likely to occur.

At this time, the relationship between the spatial frequency f _{S 2} : f _{S 3} and f ₀ (Hz) in anticipation of the moire generation area is f _{S 2} = 394 (Hz) and f _{S 3} = 295.5 ( Hz), f ₀ = 345 (Hz), and the range of variation is 380 (Hz) ≧ f ₀ ≧ 311 (H
z).

When the charging frequency f ₀ (Hz) is set lower than the above value, the latitude of the spatial frequency f _S (Hz) becomes narrow, and the moire generation region of the charging frequency f ₀ (Hz) cannot be obtained. Moire image interference fringes occur at the frequency f ₀ (Hz).

Therefore, f ₀ and f _S become f ₀ ≠ f _S , and
Moreover, f ₀ has the lowest value because f ₀ = 345 (H
z).

In the conventional image forming apparatus, while the main motor is driven and the transfer material passes over the surface of the photosensitive drum while the photosensitive drum, which is an image carrier, is rotationally driven, the charge roller contacts the charge roller. Is always applied with a voltage having a constant charging frequency f ₀ . That is, after the main switch is turned on, during the pre-rotation process performed to stabilize the potential of the photosensitive drum, between the sheets during continuous paper feeding, and toner or the like adhering to the photosensitive drum surface after image formation is completed. The voltage of the constant charging frequency f ₀ is always applied to the charging roller during the rotation process after the removal of the toner. For this reason, even when the photosensitive drum is rotationally driven other than during the image formation, the charging noise is always generated.

Therefore, in order to minimize the charging noise, the above-mentioned charging frequency f ₀ is set to the lowest frequency f ₀ = 345 (Hz) at which moire image interference fringes are hardly generated during image formation, and the photosensitive drum is exposed to light except during image formation. While the drum is rotating, the charging frequency f ₀ may be set lower.

Since printing is not performed while the photosensitive drum is being driven for rotation except during image formation, the charging frequency f
It is possible to reduce ₀ to a region where moire image interference fringes and the like occur.

However, if the charging frequency f ₀ is too low,
Non-uniform charging occurs on the surface of the photosensitive drum, and the non-uniform charging causes toner to adhere to the surface of the drum. Since the transfer material does not pass over the surface of the photosensitive drum while the photosensitive drum is being driven for rotation other than during image formation, the transfer member is contaminated if a large amount of toner or the like is present on the surface of the photosensitive drum. The function of the transfer member is degraded, and back contamination occurs at the time of image output. For this reason, the charging frequency f ₀ during the rotation of the photosensitive drum other than during image formation needs to be the lowest frequency f _{0 at} which uneven charging does not occur.

The lowest frequency f _{0 at which the} charging unevenness does not occur.
In order to obtain the charging roller 2, the process speed V _P = 50 (mm / sec), the peak-to-peak voltage V _PP of the AC voltage Vac applied to the charging roller 2 = 1800 (V), and the resolution D = 600 (dpi). When the measurement was performed while changing the frequency f _{0 of} the alternating current applied to the device, the charging frequency f ₀ = 150
(Hz), charging without charging unevenness was possible.

Under the above conditions, the charging frequency at the time of image formation is f ₀ = 345 (Hz), and the charging frequency during the rotation drive of the photosensitive drum is f ₁ = 1 except during image formation.
The paper was passed at 50 (Hz), and the sound pressure was measured by a sound level meter at a distance of 1 (m) from the image forming apparatus in an anechoic room with a sound pressure of 30 (dB) or less.
B), 55 (dB) at the time of image formation, and 50 (dB) between sheets.

On the other hand, as in the conventional image forming apparatus, when the photosensitive drum is rotationally driven, f ₀ = 345 when the charging frequency is always constant irrespective of whether the image is formed or not. (Hz), 50 (d)
B), 55 (dB) between the time of image formation and the sheet interval.

From these results, it can be seen that the image forming apparatus according to the present invention has a lower spacing between the paper and the paper during the pre-rotation than the conventional image forming apparatus.
This means that the sound pressure was reduced by 5 (dB). However, the sound pressure (dB) measured by the sound level meter and the human hearing do not always match, and the charging noise is different from various noises such as a motor driving noise of the image forming apparatus, a rotating noise of the scanner, and a paper feeding noise. They hear each other. For this reason, if the charging sound is heard while the paper is being passed at a distance of 1 (m) from the image forming apparatus in the anechoic chamber, various charging noises may occur if the charging frequency is always constant as in the conventional image forming apparatus. However, in the image forming apparatus according to the present invention, the charging frequency is set lower than that during image formation while the photosensitive drum is rotationally driven except during image formation. In addition, the charged noise is lost due to various noises.

Further, in the image forming apparatus according to the present invention, when the photosensitive drum is driven to rotate except during image formation, the charging frequency is set to the lowest frequency within a range where charging unevenness does not occur. The vibration of the charging roller and the photosensitive drum caused by the AC voltage component is reduced. As a result, white spots on the image due to the toner fusing phenomenon, in which the toner and the adhered matter on the drum surface are pressed against the drum surface by the mechanical force and the electric force due to the above-described vibration and firmly adhere, are prevented. You can do it.

As a result of performing durability with the constitution of the present invention, when the number of sheets passed was 6,000, there was almost no fusion on the drum surface and there was no effect on the image. On the other hand, as a result of performing durability with the conventional configuration, a small amount of fusion occurred on the drum surface when the number of passed sheets was 4,000, and the adverse effect appeared on the image at about 4,500.

From the above results, with the structure of the present invention, the charging noise can be minimized, and the occurrence of fusion can be prevented. (3) Control Timing (FIG. 2) FIG. 2 is an example of a timing chart for switching the charging frequency. The chart of this example shows an example of print output of two continuous sheets.

In this embodiment, the charging frequency is switched to a frequency f _L other than the image forming time while the main switch of the image forming apparatus is ON, and the charging roller enters the image forming process. switching the frequency of the charged image forming when the frequency f _H (> f _L) at the time. When the main switch of the image forming apparatus is turned on, a pre-rotation sequence is started, and the rotation of the photosensitive drum starts. Subsequently, the charging DC voltage Vdc and the AC voltage Vac are applied to charge the drum. The charging frequency at this time is f _L described above. Thereafter, a developing bias and a transfer bias are applied to form an image forming sequence. After the pre-rotation, the laser is emitted by the video (Video) signal and an electrostatic latent image is formed on the drum surface. However, since the drum surface moves from the charging unit to the latent image unit, the image of the configuration of this embodiment is used. In the forming apparatus, it takes about 0.12 (sec) to move the surface. Thus the charging roller enters the image forming process is before about 0.12 (sec) than a video signal, which previously the frequency of the charging need to switch to f _H. Therefore, in the present embodiment, it is 0.22 (sec) smaller than the video signal.
At the previous time, the charging frequency is switched from f _L to f _H. Then, in synchronization with the end of the image formation process of charging roller frequency of the charging may be switched from f _H to f _L. Tainmingu switching the frequency of the charging from f _H to f _L is the length of the paper in the carrying direction when the A (mm), when the charging roller has entered the image forming process (from the frequency of the charging f _L to f _H A / V _P from the time of switching)
(Sec) or more. In this embodiment, the A / V is changed from the time when the charging frequency is switched from f _L to f _{H.
After P +} 0.2 (sec).

[0047] When switching the frequency of the charging in the above timing f _H from f _L, it is possible to charge at f _H with a margin before and after 5mm width length A of the paper, charged by repeating this sequence Can be switched between the time of image formation and the time other than image formation.

<Embodiment 2> (FIGS. 5 and 6) In this embodiment, the timing for switching the charging frequency f ₀ to the frequency at the time of image formation or at a time other than the time of image formation is the same as in the first embodiment. In the first embodiment, the photosensitive drum is charged with the peak-to-peak voltage V _PP of the AC voltage Vac applied to the charging roller constant. In the present embodiment, the AC voltage applied to the charging roller is changed between the charging roller and the photosensitive drum. Therefore, the constant current control of the AC voltage component is performed because it is desired to switch in accordance with the change in the impedance.

As shown in FIG. 5, the charging roller used in this embodiment is formed by forming a low-resistance conductive rubber layer 2a having a thickness of about 3 mm and a volume resistance of about 10 ⁴ Ωcm on a φ6 metal core 2b made of conductive metal. And a 10 ⁸ Ωc with a thickness of 50 to 20 μm on its outer peripheral surface.
A high resistance layer 2c of about m is formed.

In the present embodiment, the voltage applied to the charging roller 2 is a dark portion potential V _D which targets the surface potential of the photosensitive member.
To converge to take a method of superimposing an AC voltage for equalizing the potential constant DC voltage corresponding to the potential V _D. At this time, the peak-to-peak voltage V _PP of the superimposed AC voltage is at least twice as large as the discharge start threshold voltage V _TH at which the charging roller and the photosensitive drum start discharging. In this embodiment, a photosensitive material having a relative dielectric constant of 3 and a thickness of 20 μm was used as the photosensitive layer 1 a of the photosensitive drum 1.

Since the impedance between the charging roller 2 and the photosensitive drum 1 is changed by the change of the charging frequency f ₀ (Hz), the charging frequency f ₀ is switched between when the image is formed and when the image is not formed. In this case, the AC current value must be fixed for each charging frequency, and the target value of the AC current must be switched between when the image is formed and when the image is not formed.

As described above, since it is desired to change the AC voltage component at each charging frequency f ₀ according to the change in impedance due to the environment, durability and the like of the charging roller 2, constant current control is performed. Assuming that the resistance and capacity of the photoreceptor are constant regardless of the environment at each charging frequency f _{0, the} voltage applied to the charging roller 2 is controlled in proportion to the impedance by controlling the AC current flowing through the entire body at a constant current. It is determined. Therefore, a high voltage is applied to a low-impedance roller or a high-impedance roller whose resistance has increased during manufacturing to prevent charging failure, and a low voltage is applied to a low-impedance roller to prevent leakage. Will be.

In this embodiment, a sine wave of 1800 in a normal environment is used.
In order to obtain a peak-to-peak voltage V _PP at V, constant current control was performed by setting the effective value current Iac to the value shown below for each charging frequency f ₀ .

[0054] Charging frequency f ₀ = 34 during image formation
At 5 (Hz), Iac = 360 (μA). Charging frequency f ₀ = 150 (H
In the case of z), by performing constant current control at a value of Iac = 160 (μA), about 2200 (V) under a low-temperature and low-humidity environment (15 ° C., 10% RH), and 1600 (V) under a high-temperature and high-humidity environment ) Can be obtained.

FIG. 6 is a block diagram of a circuit for controlling the constant current of the AC voltage component.

In this circuit, the AC voltage output from the oscillator 10 is amplified to a predetermined voltage by the amplifier 11 and the transformer 12 and is applied to the charging roller 2. The AC voltage applied to the roller 2 is rectified by the rectifier circuit 13, and the rectified voltage is compared with the voltage output by the reference voltage generator 14 by the comparator 15.

This circuit is a feedback system. Based on the compared voltages, the voltage is controlled to decrease if the feedback current is large, and the voltage is increased if the current is reversed.

Also in the case of the present embodiment, the charging noise can be minimized. Further, the occurrence of toner fusion can be prevented.

<Embodiment 3> (FIGS. 7 and 8) In this embodiment, the timing of switching the charging frequency f ₀ to the frequency at the time of image formation or other than the time of image formation is the same as that of the first embodiment.

In the second embodiment, in order to change the AC voltage applied to the charging roller 2 according to the impedance of the charging roller, constant current control of the AC voltage component is performed for each charging frequency f ₀ (Hz). In this embodiment, at a predetermined frequency f ₀ , a necessary AC voltage in the environment is detected in advance, and the subsequent charging is controlled by this voltage.

FIG. 7 is a block diagram of a circuit for controlling the present embodiment. In this circuit, the AC voltage output from the oscillator 10 is amplified to a predetermined voltage by the amplifier 11 transformer 12 and applied to the charging roller 2. In this embodiment, the AC voltage required in each environment is detected, and In order to control charging by this voltage, an AC voltage applied to the charging roller 2 is rectified by a rectifier circuit 13, and the rectified voltage is compared by a comparator 15 with a voltage obtained by D / A conversion 16 of a command from the CPU / 18. Increase the voltage. The CPU detects the rise in the voltage via the A / D converter 17 and controls the voltage so that the voltage at the time when the returned current reaches a certain current value is maintained.

When AC constant current control is performed during image formation, the current value is affected by a sudden change in impedance caused by the charging roller 2 passing through a pinhole formed on the photosensitive drum 1 and various electrical noises. There is a problem that the applied voltage drops and charging failure easily occurs. However, these problems can be solved by performing constant voltage control suitable for the environment.

Therefore, in order to determine a constant voltage value suitable for each environment, image formation is not actually performed. For example, a predetermined frequency f ₀ is set during a pre-rotation process, and a predetermined A
It is assumed that the AC voltage is generated by detecting the AC voltage caused by flowing the C current and controlling the AC voltage by this value at any frequency f ₀ during image formation.

The preset AC current referred to here is A
The C voltage must be a value that does not cause charging failure in each environment. Normally, a peak-to-peak voltage V that allows sufficient charging in a low-temperature and low-humidity environment where the resistance value of the charging roller is the highest.
_An AC current that can generate _PP .

Switching the charging frequency f ₀ between when the image is formed and when the image is not formed. According to the present invention, the lowest voltage V _PP which does not cause charging failure in the image forming apparatus was measured.
・ In a low-temperature and low-humidity environment of 10% RH, V _PP = 2200
(V). At this time, the charging frequency f ₀ = 150 (H
In z), the AC current value was Iac = 160 (μA). Therefore, in this embodiment, the charging frequency f _{0 is set to} 150 (Hz) when the main switch is turned on in each environment, and Iac = 160 (μ) is set in advance at the timing described later.
The AC constant current control of A) is performed, and the AC voltage generated at this time is detected. Then, even if the charging frequency f ₀ is switched during image formation, the charging is performed while maintaining this voltage.

In the present embodiment, the timing for performing the above-described sequence control is performed according to the following procedure.

FIG. 8 is a timing chart for controlling charging in this embodiment. When the power is turned on, the charging frequency f _{0 is set to} 150 (Hz), and then the AC constant current control is performed during the pre-rotation process performed to stabilize the potential of the photoconductor, and the voltage generated here is maintained. If the voltage at a certain moment is detected and held when performing constant current control during the pre-rotation process, an error may be increased. Therefore, the average of the voltage generated by the constant current control for at least the time required for the photosensitive drum to make one rotation is considered. The value has a control voltage for image output.

In this embodiment, the diameter of the photosensitive drum 1 is 30 (m).
m), the generated AC voltage was averaged for 1.88 (sec) because of the process speed of 50 (mm / sec), and thereafter control was performed in the same sequence as in Example 1.

When paper passing and printing were actually performed in each environment, the charging frequency f ₀ = 150 (Hz), A
C current Iac = 160 (μA) Constant current, 32.5 ° C .: V _PP = 160 in a high temperature and high humidity environment of 85% RH
0 (V), 23 ° C .: V _PP = 1800 in a normal environment of 60% RH
(V), 15 ° C .: V _PP = 2200 in a low temperature and low humidity environment of 10% RH
(V) AC peak-to-peak voltage was obtained.

By performing the above control, a good image free from poor charging and no leakage was obtained in all environments even when the charging frequency f ₀ was switched between when the image was formed and when the image was not formed.

Further, in this embodiment, the constant current control is performed at the lowest frequency among the switchable charging frequencies.
By performing constant voltage control including other frequencies with the C voltage, even if the impedance of the charging roller changes due to environmental fluctuations or the like, the amount of AC discharge effective for the actual charging operation can be controlled to be the most constant. This is because if the frequency of the AC voltage is low, the leakage current component of AC flowing due to the capacity formed between the charging roller 2 and the photosensitive drum 1 is reduced, and the discharge actually generated between the charging roller 2 and the photosensitive drum 1 This is because the ratio of the minute current to the AC current increases.

Also in the case of the present embodiment, the charging noise can be minimized. Further, the occurrence of toner fusion can be prevented.

[0073]

As described above, according to the present invention, as the charging means for the image bearing member surface, the charging member to which the vibration voltage including the DC voltage is applied is brought into contact with the image bearing member surface to charge the image bearing member surface. In the image forming apparatus employing the AC contact charging method, the frequency of the applied oscillating voltage is set to be smaller during the process other than the image forming than during the image forming, so that the device noise due to the charging noise can be minimized. or,
Since the vibration between the image carrier and the charging member can be minimized, there is an effect of suppressing image defects due to fusion of the toner.

[Brief description of the drawings]

FIG. 1 is a schematic configuration diagram of an example of an image forming apparatus (laser beam printer) according to the present invention.

FIG. 2 is a sequence and control timing chart of the image forming apparatus.

FIG. 3 is a graph showing a relationship between a line pitch L and a spatial frequency f _S of the line pitch.

FIG. 4 is a graph showing the charging frequency f ₀ and the level of moiré.

FIG. 5 is a configuration diagram of a charging roller portion of the device of the second embodiment.

FIG. 6 is a block diagram of an AC constant current circuit.

FIG. 7 is a block diagram of a control circuit of the device according to the third embodiment;

FIG. 8 is a sequence and control timing chart of the apparatus.

FIG. 9 is a schematic configuration diagram of an example of a roller-type contact charging device.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Rotating photosensitive drum as an image carrier (charged body) 2 Charging roller as a contact charging member 3a Laser beam scanner 4 Developing sleeve 5 Transfer roller 6 Greening blade 7 Transfer material

Claims (2)

(57) [Claims] An image forming apparatus having a contact charging means for charging a surface of an image bearing member by applying a vibration voltage including a DC voltage to the surface of the image bearing member, wherein the frequency of the vibration voltage applied to the contact charging member is set to When forming an image rather than when forming an image
An image forming apparatus characterized in that the image forming apparatus is made smaller during a process other than the above . 2. An AC component of the oscillating voltage is controlled by a constant current.
The AC current controlled by the constant current
The image forming apparatus according to claim 1 , wherein the size of the image forming apparatus is reduced during a process other than forming.