JPH06202445A - Image forming device - Google Patents

Abstract

PURPOSE:To obtain an image in which the irregularity of pitch, moire and the uneveness of color are little by shifting the frequency of oscillating voltage for an electrostatic charger every time an image forming process is repeated in an image forming device for forming a color image by repeating the image forming process including a stage where the image carrier is electrostatically charged by impressing the oscillating voltage on the electrostatic charger. CONSTITUTION:The color image is formed by shifting the frequency of the impressed oscillating voltage on the electrostatic charger 2 in each image forming process as follows. That is, an oscillator 10 oscillates and outputs the sine wave having 650Hz frequency for a 1st color magenta. An AC bias power source 8 which receives the sine wave signal, impresses AC bias having the same frequency as the output frequency from the oscillator 10 on the charger 2. It the same way, the frequency is shifted to 660Hz for the 2nd color cyan, 670Hz for a 3rd color yellow, and 680Hz for a 4th color black, respectively. Thus, the irregular pitch caused by the moire becomes different in each color, and the irregularity of the pitch caused by the moire becomes inconspicuous when the colors are superposed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming process including a step (primary charging) of uniformly charging an image carrier such as a photoconductor or a dielectric, such as an electrophotographic process or an electrostatic recording process. The present invention relates to an image forming apparatus that forms a color image by repeating a transfer method or a direct method.

[0002]

2. Description of the Related Art Conventionally, in the image forming apparatus as described above, a corona charger has been widely used as a charging means for an image carrier, but recently, a corona discharge of ozone or the like has been used as compared with the corona charger. Since it has advantages such as less generation of products and a lower applied voltage, a roller-type charging member as a charger, to which a voltage is applied, is brought into contact with the image carrier to form an image carrier. Contact charging means for charging has been put into practical use.

It should be noted that the charging member is not brought into contact with the image bearing member, but is disposed in a non-contact facing manner with a slight air gap (air gap) between the charging member and the surface of the image bearing member which may cause a discharge phenomenon. Also when the required charging bias is applied to the charging member,
Similar to the case where the charging member is placed in contact with the image carrier, the charging process of the surface of the image carrier can be executed, and therefore the contact charging also includes this aspect.

The voltage applied to the charging member is generally 1-2.
What uses only DC voltage of KV, and JP-A-63-149668.14
As disclosed in Japanese Patent Publication No. 9669, etc., an oscillating voltage (alternating voltage, alternating voltage; a voltage whose voltage value periodically changes with time), for example, a desired DC voltage for charging the image carrier to a desired potential There is one in which a constant AC voltage having a required amplitude is superimposed on the voltage.

The above-mentioned contact charging in which only a direct current voltage is applied has the advantage that an applied voltage is low and an inexpensive power supply can be used, but on the other hand, spot-like charging unevenness is likely to occur and the charging member is contaminated. It has a small latitude to obtain uniform charging even if it is weak.

On the other hand, the latter contact charging applying an oscillating voltage can significantly improve spot-like charging unevenness, and the power source is a little complicated and costly compared with the contact charging applying only a DC voltage. It has a large latitude to obtain a uniform charging property. The oscillating voltage is effectively a voltage in which an AC voltage having a peak-to-peak voltage that is at least twice the charging start voltage of the image carrier is superimposed on the DC voltage.

[0007]

However, in an image forming apparatus in which an image carrier is charged by applying an oscillating voltage to the charger, the bias applied to the charger is an oscillating bias. The carrier is not uniformly charged, and the potential on the image carrier is spatially periodic corresponding to the vibration bias frequency, so that the charging frequency of the image carrier by the charger is However, there is a problem in that the spatial frequency of the latent image formed by being exposed on the image carrier interferes with each other to cause image unevenness. That is, since there is a minute change in the potential depending on the frequency of the oscillating voltage applied to the charger,
When an image having a spatial frequency close to this frequency is formed, they interfere with each other to cause pitch unevenness and moire.

In a high resolution image forming apparatus such as a laser printer, an image such as a halftone may be output. Halftone is often formed by an image having spatial periodicity such as horizontal lines, vertical lines, and dots. When the exposure corresponding to the image information having such spatial periodicity is performed, the periodicity due to the charging on the image carrier and the periodicity of the exposure corresponding to the image information interfere with each other, and the lateral and vertical directions are Image irregularities such as fine pitch irregularities and moiré with a fine cycle are likely to occur.

Then, in an image forming apparatus for forming a color image (or a multicolor image) by repeating an image forming process including a step of charging an image carrier by applying an oscillating voltage to a charger, For multiple colors (for example, magenta, cyan, yellow, black 4
Pitch unevenness, moire, and color unevenness occur in the (color) image. Therefore, a periodical image disturbance, which is not so noticeable in a single-color image, is emphasized when images of a plurality of colors are overlapped with each other, which causes a significant deterioration in image quality.

Therefore, the present invention is an image forming apparatus for forming a color image by repeating an image forming process including a step of applying an oscillating voltage to a charger to charge an image carrier, and has less pitch unevenness, moire and color unevenness. The purpose is to output an image.

[0011]

The present invention is an image forming apparatus having the following configuration.

(1) In an image forming apparatus for forming a color image by repeating an image forming process including a step of applying an oscillating voltage to a charging device to charge an image carrier, the above charging is performed for each repeated image forming process. An image forming apparatus characterized by shifting the frequency of an oscillating voltage applied to a container.

(2) The image forming apparatus according to claim 1, wherein the oscillating voltage power source for the charger can be controlled by an input frequency from the outside.

(3) The image forming apparatus according to claim 2, wherein the waveform of the oscillating voltage is a rectangular wave.

(4) The image forming apparatus according to claim 3 or 4, wherein the oscillating voltage waveform can be controlled by a pulse wave.

[0016]

In an image forming apparatus for forming a color image by repeatedly performing an image forming process including a step of applying an oscillating voltage to the charger to charge the image carrier, the vibration of the charger is repeated for each repeated image forming process. By shifting the frequency of the voltage, it is possible to prevent the spatial interference frequency of the electrostatic latent image from becoming the same in the repeated whole imaging process,
That is, even if images having a plurality of colors are overlapped with each other, those having the same frequency do not overlap with the same portion, so that it is possible to prevent image disturbance such as moire and color unevenness in a color image from being reduced.

[0017]

【Example】

<Embodiment 1> (FIGS. 1 and 2) The apparatus of FIG. 1 is a schematic view of a color image forming apparatus using a transfer type electrophotographic process.

Reference numeral 1 denotes a rotary drum type electrophotographic photosensitive member (hereinafter referred to as a photosensitive drum) as an image bearing member, and an arrow R
1 is rotated clockwise at a predetermined peripheral speed (process speed).

Reference numeral 2 is a contact charging roller as a charging device. The charging roller 2 is brought into contact with the photosensitive drum 1 with a predetermined pressing force and is driven by the photosensitive drum to rotate. A predetermined charging bias (oscillation voltage) is applied and the peripheral surface of the rotary photosensitive drum 1 is subjected to primary charging processing to a predetermined polarity and potential.

[0020]. First Color (Magenta) Image Forming Process For example, a magenta component as the first color of the target color image information is formed on the surface of the photosensitive drum 1 that has been subjected to the primary charging process by an unillustrated exposing means (laser scanner or the like). Exposure 3 corresponding to the image pattern is performed to form an electrostatic latent image corresponding to the pattern. Then, the electrostatic latent image is developed by the magenta developing device 4a containing magenta toner.

On the other hand, the transfer material (generally transfer paper) 6 contained in the paper feed cassette 13 is supplied to the transfer drum 5 by the paper feed roller 14 at a predetermined timing. The transfer drum 5 is rotationally driven in the counterclockwise direction R2 indicated by an arrow at a peripheral speed substantially the same as that of the photosensitive drum 1, and the supplied transfer material 6 is wound around and held on the outer peripheral surface of the transfer material 6. The magenta toner image formed as described above is transferred to the surface of the photosensitive drum 1.

After the transfer of the toner image to the transfer material 6, the surface of the rotating photosensitive drum 1 is cleaned by removing the transfer residual toner by a cleaner 7.

.. Second Color (Cyan) Image Forming Process After cleaning the photosensitive drum 1, the photosensitive drum 1 is again subjected to the primary charging process by the charging roller 2 and then, for example, a cyan component image pattern as the second color of the target image information is dealt with. Exposure 3 is performed, and the electrostatic latent image formed by the exposure 3 is developed by the cyan developing device 4b containing cyan toner.

The cyan toner image is transferred onto the surface of the transfer material 6 which is continuously held and rotated by the transfer drum 5 so as to be superimposed on the previously transferred magenta toner image. The surface of the photosensitive drum 1 after the transfer is cleaned by the cleaner 7.

[0025]. Third Color (Yellow) Image Forming Process After cleaning the photosensitive drum 1, the photosensitive drum 1 is again subjected to the primary charging process by the charging roller 2, and then, for example, a yellow component image pattern as the third color of the target image information is dealt with. Exposure 3 is performed, and a yellow developing device 4b in which the electrostatic latent image formed by the exposure 3 contains yellow toner
To be developed.

The yellow toner image is transferred onto the surface of the transfer material 6 which is continuously held and rotated by the transfer drum 5 so as to be superposed on the previously transferred magenta toner image / cyan toner image. The surface of the photosensitive drum 1 after the transfer is cleaned by the cleaner 7.

.. Fourth Color (Black) Image Forming Process After cleaning the photosensitive drum 1, the photosensitive drum 1 is again subjected to primary charging processing by the charging roller 2, and then, for example, corresponding to a black component image pattern as the fourth color of the target image information. Exposure 3 is performed, and the electrostatic latent image formed by the exposure 3 contains black toner.
To be developed.

The black toner image is transferred onto the surface of the transfer material 6 which is continuously held and rotated by the transfer drum 5 so as to be superimposed on the previously transferred magenta toner image / cyan toner image / yellow toner image. The surface of the photosensitive drum 1 after the transfer is cleaned by the cleaner 7.

The toner images of the four colors of magenta, cyan, yellow, and black described above are transferred sequentially while being aligned and sequentially transferred onto the same transfer material 6 held on the transfer drum 5, so that a desired color image is obtained. A corresponding color image is formed.

When the transfer material 6 receives the final transfer of the black toner image, it is separated from the surface of the transfer drum 5 by the separating means 8 and introduced into the fixing device 15, and the toner image is fixed on the transfer material 6 as a permanent image. To be done.

The power source 20 corresponding to the charging roller 2 as a charger is an AC bias power source 9 and a DC bias power source 1.
1, a sine wave oscillator 10, and the charging roller 2 has A
AC / DC bias power source 9 and DC bias power source 11
A DC superposed oscillating voltage is applied.

The AC bias power source 9 is a power source which can receive a sine wave input from the outside and can output a sine wave in accordance with a sine wave signal from the input.
It is possible to apply an AC bias having the same frequency as the oscillation frequency of. Moreover, the frequency of the output of the oscillator 10 can be switched by a controller (not shown) of the apparatus body.

In this embodiment, the frequency of the oscillating voltage applied to the charging roller 2 in each of the above image forming processes was shifted as shown in FIG. 2 to form a color image. That is, a. During the image forming process for the first color The oscillator 10 oscillates and outputs a sine wave (sin wave) having a frequency corresponding to the first color (magenta). This frequency is 650 Hz in this embodiment. The AC bias power source 9 receiving this sine wave signal applies an AC bias having the same frequency as the output frequency of the oscillator 10 to the charging roller 2 to execute the primary charging process on the surface of the photosensitive drum during the image forming process of the first color. It

As described above, when the image information includes periodicity such as halftone, the electrostatic latent image may interfere with the frequency of contact charging to generate moire.
In the present embodiment, in the image formed by the image forming process for the first color, the unevenness of 3 mm pitch due to moire occurred in the horizontal halftone image of 1 dot / 3 space.

B. During the second color image forming process Before starting the second color (cyan) image formation, the oscillator 10
Shifts the frequency by a fixed amount from the original frequency. In this embodiment, 650 during the image forming process of the first color is performed.
A 660 Hz sine wave signal shifted by 10 Hz from the Hz was input to the AC bias power supply 9. The AC bias power source 9 that receives the sine wave signal from the oscillator 10
Sine wave AC with the same frequency of 660Hz as the output frequency of
A bias is applied to the charging roller 2 to perform the primary charging process on the surface of the photosensitive drum during the second color image forming process.

In the image formed by the image forming process of the second color, unevenness of 5 mm pitch due to moire occurred in the horizontal halftone image of 1 dot / 3 space.

C. During the process of forming the image of the third color, before entering the third color (yellow), the oscillator 10 shifts from the original frequency by a certain amount. In the present embodiment, it is 10% higher than 660 Hz in the second color image forming process.
The 670 Hz sine wave signal shifted by Hz was input to the AC bias power supply 9. The AC bias power supply 9 that has received the sine wave signal of the oscillator 10 applies a sine wave AC bias having a frequency of 670 Hz, which is the same as the output frequency of the oscillator 10, to the charging roller 2 to expose the photosensitive drum during the image forming process of the third color. The primary charging process on the surface of the drum 1 is executed.

In the image formed by the image forming process of the third color, unevenness of 7 mm pitch due to moire occurred in the horizontal halftone image of 1 dot / 3 space.

D. During the fourth color image forming process, before entering the fourth color (black), the oscillator 10 shifts from the original frequency by a certain amount. In this embodiment, it is 10% higher than 670 Hz in the image forming process of the third color.
A 680 Hz sine wave signal shifted by Hz was input to the AC bias power supply 9. The AC bias power supply 9 that has received the sine wave signal of the oscillator 10 applies a sine wave AC bias having a frequency of 680 Hz, which is the same as the output frequency of the oscillator 10, to the charging roller 2 to expose the photosensitive drum during the image forming process of the fourth color. The primary charging process on the surface of the drum 1 is executed.

In the image formed by the image forming process for the fourth color, unevenness of 9 mm pitch due to moire occurred in the horizontal halftone image of 1 dot / 3 space.

In summary, in this embodiment, the toner images on the photosensitive drum after development of each color are 3 mm and 5 mm, respectively.
The pitch unevenness was 7 mm and 9 mm. The final toner image transferred onto the surface of the transfer paper 6 on the transfer drum is a single toner image by superimposing these four color toner images.

As in the conventional device, the charging frequency is 6 for all four colors.
The horizontal line halftone image, which had a pitch unevenness of 3 mm for all four colors at 50 Hz, shifts the frequency for each color by a certain amount as described above, so that the pitch of the unevenness due to moire is different for each color and the toner image is Since they are overlapped, pitch unevenness due to moiré is less noticeable, and the image quality can be reduced.

<Embodiment 2> (FIG. 3) In Embodiment 1, the frequency of the oscillating voltage applied to the charging roller 2 for the primary charging of the photosensitive drum 1 for each color (each repeating image forming process) is set. Although it is shifted by a fixed amount (10 Hz) as shown in FIG. 2, this embodiment is characterized in that the shift width is changed for each color as shown in FIG.

In this embodiment, in the color image forming apparatus shown in FIG. 1, the oscillator 10 can change the frequency shift width for each color.

A. During the first color image forming process The oscillator 10 oscillates and outputs a sine wave having a frequency corresponding to the first color (magenta). In this embodiment, the frequency is 650 Hz. The AC bias power supply 9 receiving this sine wave signal applies an AC bias having the same frequency as the output frequency of the oscillator 10 to the charging roller 2 to execute the primary charging process of the photosensitive drum surface during the image forming process of the first color. To be done.

In the first color formed image, a 3 mm pitch due to moire occurred in the horizontal halftone image of 1 dot / 3 space as in the case of Example 1.

B. During the image forming process for the second color Before the image forming process for the second color (cyan) is started, the oscillator 10 shifts the frequency from the original frequency by the frequency width corresponding to the second color. In this embodiment, a 655 Hz sine wave signal shifted by 5 Hz is input to the AC bias power supply 9. The AC bias power supply 9 applies a 655 Hz sine wave signal to the charging roller 2, and the primary charging process of the photosensitive drum surface during the image forming process of the second color is executed.

In the second color formed image, the horizontal line halftone image of 1 dot and 13 spaces has 4
mm pitch irregularity occurred.

C. During the third color image forming process Before entering the third color (yellow) image forming process,
The oscillator 10 shifts the frequency from the original frequency by the frequency width corresponding to the third color. In this embodiment, a 675 Hz sine wave signal shifted by 20 Hz is input to the AC bias power supply 9. The AC bias power source 9 applies a 675 Hz sine wave to the charging roller 2, and the primary charging process of the photosensitive drum surface in the image forming process of the first color is executed.

In this third color formed image, a horizontal line halftone image of 1 dot / 3 space has a moiré pattern of 8
mm pitch irregularity occurred.

D. At the time of the image forming process of the fourth color Before entering the image forming process of the fourth color (black),
The oscillator 10 shifts the frequency from the original frequency by the frequency width corresponding to the fourth color. In this example, a 685 Hz sine wave signal shifted by 10 Hz was input to the AC bias power supply 9. The AC bias power source 9 applies a 685 Hz sine wave to the charging roller 2, and the primary charging process of the photosensitive drum surface during the image forming process of the fourth color is executed.

In the fourth color formed image, the horizontal line halftone image of 1 dot / 3 space is 9
mm pitch irregularity occurred.

Also in the case of the present embodiment, it is possible to improve the moire as in the case of the first embodiment. Moire is visually noticeable when a magenta image and a cyan image are superimposed,
In this embodiment, by changing the shift width of the primary charging frequency for each color, the disturbance of the image due to the superposition between the two colors can be reduced.

<Embodiment 3> (FIG. 4) FIG. 4 is a schematic configuration diagram of a color image forming apparatus in this embodiment. In the apparatus of this example, the color image forming apparatus shown in FIG. 1 is provided with an intermediate transfer member 16, and a magenta toner image, a cyan toner image, and a yellow toner image which are sequentially formed on the surface of the intermediate transfer member 16 on the photosensitive drum 1. The black toner images are sequentially superimposed and transferred to form and carry the color toner images, and the color toner images formed and carried on the intermediate transfer body 16 are collectively transferred to the transfer material 6 by the transfer roller 17 and fixed. The fixing process is performed by the container 15. Reference numeral 18 is a cleaner for the intermediate transfer member 16.

In this color image forming apparatus, as in the case of Embodiment 1, the frequencies of the oscillating voltage applied to the charging roller 2 during the image forming processes of the first to fourth colors are 650 Hz, 660 Hz and 670 GHz, respectively.
The primary charging was performed in each image forming process by shifting by 680 Hz and 10 Hz.

Also in the case of the present embodiment, it was possible to improve the moire as in the case of the first embodiment.

<Embodiment 4> (FIG. 5) This embodiment is characterized in that the AC bias power source 9 outputs a rectangular wave voltage in the device of FIG.

The oscillator 10 outputs a pulse wave having a waveform as shown in FIG. 5, and the power source 9 outputs a rectangular wave in synchronization with this pulse wave. That is, the power source 9 outputs an on signal when the output pulse signal of the oscillator 10 rises to the positive side, and the power source 9 can output an off signal when the pulse signal rises to the negative side.

A. During the first color image forming process The oscillator 10 oscillates and outputs a pulse wave having a frequency corresponding to the first color (magenta). This frequency is 650 Hz in this embodiment. The AC bias power source 9 receiving this pulse wave signal applies a rectangular wave AC bias having the same frequency as the output frequency of the oscillator 10 to the charging roller 2 to perform a primary charging process on the surface of the photosensitive drum during the image forming process of the first color. Is executed.

In the present embodiment, in the formed image of the image forming process of the first color, the unevenness of 3 mm pitch due to moire occurred in the horizontal halftone image of 1 dot / 3 space.

B. During the second color image forming process Before starting the second color (cyan) image formation, the oscillator 10
Shifts the frequency by a fixed amount from the original frequency. In this embodiment, 650 during the image forming process of the first color is performed.
A pulse wave signal of 660 Hz shifted by 10 Hz from Hz was input to the AC bias power supply 9. This oscillator 10
The AC bias power source 9 which has received the pulse wave signal of (1) applies a rectangular wave AC bias having a frequency of 660 Hz, which is the same as the output frequency of the oscillator 10, to the charging roller 2 so that the surface of the photosensitive drum during the image forming process of the second color is The primary charging process is executed.

In the image formed in the image forming process of the second color, unevenness of 5 mm pitch due to moire occurred in the horizontal halftone image of 1 dot / 3 space.

C. During the process of forming the image of the third color, before entering the third color (yellow), the oscillator 10 shifts from the original frequency by a certain amount. In the present embodiment, it is 10% higher than 660 Hz in the second color image forming process.
The 670 Hz pulse wave signal shifted by Hz was input to the AC bias power supply 9. The AC bias power supply 9 receiving the pulse wave signal of the oscillator 10 applies a rectangular wave AC bias having a frequency of 670 Hz, which is the same as the output frequency of the oscillator 10, to the charging roller 2 to expose the photosensitive drum during the image forming process of the third color. The primary charging process on the drum surface is executed.

In the image formed by the image forming process for the third color, unevenness of 7 mm pitch due to moire occurred in the horizontal halftone image of 1 dot / 3 space.

D. During the fourth color image forming process, before entering the fourth color (black), the oscillator 10 shifts from the original frequency by a certain amount. In this embodiment, it is 10% higher than 670 Hz in the image forming process of the third color.
The 680 Hz pulse wave signal shifted by Hz was input to the AC bias power supply 9. The AC bias power supply 9 receiving the pulse wave signal of the oscillator 10 applies a rectangular wave AC bias having a frequency of 680 Hz, which is the same as the output frequency of the oscillator 10, to the charging roller 2 to expose the photosensitive drum during the image forming process of the fourth color. The primary charging process on the drum surface is executed.

In the image formed in the image forming process for the fourth color, unevenness of 9 mm pitch due to moire occurred in the horizontal halftone image of 1 dot / 3 space.

Also in the case of the present embodiment, since the pitch of the moire changes for each color, the moire can be improved as in the case of the first embodiment.

<Embodiment 5> (FIG. 6) The color image forming apparatus is a system in which toner images of required plural colors are successively formed on the image carrier 1 and the superposed toner images are collectively transferred onto a transfer material. Alternatively, as shown in FIG. 6, the transfer material 6 may be provided by including the first to fourth process mechanism units 4 to form a magenta toner image, a cyan toner image, a yellow toner image, and a black toner image, respectively. A method may be used in which the four toner images are sequentially transferred to the transfer units 71 to 74 of the four image forming process mechanism units through the transfer unit 75 and the four toner images are sequentially superimposed and transferred to obtain a color image.

By applying the present invention to any color image forming apparatus, it is possible to output a color image without moire or color unevenness.

For contact charging, a roller charging method using a roller type as a charging member is preferably used from the viewpoint of charging stability, but the charging member is not limited to the roller type, but may be a blade type, rod type, brush type or the like. It can be in the form.

The waveform of the oscillating voltage or AC voltage component is
In addition to a sine wave or a rectangular wave, a sawtooth wave, a triangular wave, a pulse wave, or the like, or a rectangular wave voltage formed by periodically turning on and off a DC power supply may be used.

[0072]

As described above, according to the present invention, in the image forming apparatus for forming a color image by repeating the image forming process including the step of applying the oscillating voltage to the charger to charge the image carrier, It is possible to prevent the periodical image disturbance such as moire and pitch unevenness when colors are overlapped from being emphasized, and it is possible to improve the image quality.

[Brief description of drawings]

FIG. 1 is a schematic configuration diagram of a color image forming apparatus according to a first embodiment.

[Figure 2] Frequency shift diagram

FIG. 3 is a frequency shift diagram of the apparatus according to the second embodiment.

FIG. 4 is a schematic configuration diagram of a color image forming apparatus according to a third embodiment.

FIG. 5 is an output diagram of an oscillator output and an AC bias power supply of the device of the fourth embodiment.

FIG. 6 is a schematic configuration diagram of an apparatus according to a fifth embodiment.

[Explanation of symbols]

 1 image carrier (photosensitive drum) 2 charger (contact charging roller) 3 exposure light 4a, 4b, 4c, 4d color developing device 5 transfer roller 6 transfer material 7 cleaner 8 separator 20 bias power supply 9 AC bias Power supply 10 Oscillator 11 DC bias power supply 13 Paper feed cassette 14 Paper feed roller 15 Fixer 16 Intermediate transfer body 17 Transfer roller 18 Cleaner for intermediate transfer body

Claims (4)

[Claims] 1. An image forming apparatus for forming a color image by repeating an image forming process including a step of applying an oscillating voltage to a charger to charge an image carrier, wherein the charger is provided for each repeated image forming process. An image forming apparatus characterized by shifting the frequency of an oscillating voltage with respect to. 2. The image forming apparatus according to claim 1, wherein the oscillating voltage power source for the charging device can be controlled by an input frequency from the outside. 3. The image forming apparatus according to claim 2, wherein the waveform of the oscillating voltage is a rectangular wave. 4. The image forming apparatus according to claim 3, wherein the oscillating voltage waveform can be controlled by a pulse wave.