JPH06202419A - Image forming device - Google Patents

Abstract

PURPOSE:To eliminate electrification unevenness and to prevent the generation of moire, etc. CONSTITUTION:An electrifying roller 2 is brought into contact with a photosensitive drum 1 so as to be rotated following the drum 1. The surface of the photosensitive drum 1 is electrified. by impressing a superimposed voltage which is obtained by superimposing a DC high voltage and an AC high voltage on the electrifying roller 2. Laser beams emitted from a laser diode 6 constituting an optical scanning means are reflected on a polygon mirror 4, and the surface of the photosensitive drum 1 is scanned with the reflected light. At this time, provided that Tac denotes a cycle period, Ts denotes the scanning cycle of the optical scanning means in a subscanning direction, Tac and Ts are set so that Tac=nTs may be satisfied ((n) is natural number). Thus, the electrification unevenness by the interference of the AC high voltage and the optical scanning is prevented.

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 provided with a charging roller for contacting a photoreceptor and charging it and an optical scanning means for optically scanning the charging surface of the charging roller.

[0002]

2. Description of the Related Art Conventionally, a corona wire method, a charging roller method, etc. have been put into practical use as a primary charging means for an image forming apparatus such as a copying machine or a laser beam printer.

The corona wire method has been used for a long time, and DC high voltage is applied to a thin conductive wire to generate corona discharge in the vicinity of the wire, and the space charge generated by this corona discharge is guided to the photoconductor. It is a thing. Although this method is easy to control the voltage, it has a drawback that the applied DC high voltage is as high as 5 kV or more, the charging efficiency is very poor, and a large amount of ozone is generated. On the other hand, the charging roller system has the advantages that the voltage of the applied DC high voltage is low, the charging efficiency is high, and almost no ozone is generated. However, the charging roller method has a problem in that the charging potential changes due to variations in roller characteristics and environmental changes in temperature and humidity, which is so-called environmental stability. To make up for this shortcoming,
Currently, a method of applying DC high voltage and AC high voltage to the charging roller in a superimposed manner is adopted.

[0004]

However, according to the above-mentioned conventional technique, when DC high voltage and AC high voltage are superimposed and applied to enhance environmental stability, the AC high voltage also rotates the potential on the surface of the photoconductor. It has AC ripple in the direction.

When the electrostatic charge surface having the AC ripple is scanned with, for example, intensity-modulated light, the AC ripple of the electrostatic charge and the scanning light interfere with each other, resulting in uneven density.

Particularly, when the modulation pattern of the scanning light in the sub-scanning direction changes periodically (for example, a periodic pattern of 1 dot / 1 space in the sub-scanning direction), the interference causes beat fringes (moiré). .

Therefore, the present invention provides an image forming apparatus which suppresses the occurrence of density unevenness such as moire by setting the sub-scanning cycle of the optical scanning means to an integral multiple of the repetition cycle of AC high voltage. That is the purpose.

[0008]

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, in which a charging roller is brought into contact with a rotationally driven photosensitive member to rotate the photosensitive member and a voltage is applied to the charging roller. Then, in the image forming apparatus in which the surface of the photoconductor is uniformly charged and the surface of the photoconductor that has been charged is scanned by the optical scanning means,
When a superposed voltage in which the C high voltage and the AC high voltage are superposed is applied, and the AC high voltage repeating cycle at this time is Tac, and the scanning cycle of the optical scanning means in the sub-scanning direction is Ts, these repeating cycles Tac and Ta between scanning period Ts
The repeating period Tac and the scanning period Ts are set so that the relationship of c = nTs (n is a natural number) is established.

[0009]

According to the above structure, by making the repetition period Tac a natural multiple of the scanning period Ts, it is possible to prevent charging unevenness due to the interference between the AC high voltage and the scanning light.

[0010]

Embodiments of the present invention will be described below with reference to the drawings. <Embodiment 1> FIG. 1 is a diagram schematically showing a laser beam printer to which the present invention is applied. In the figure, a photosensitive drum 1 as a photosensitive member is rotationally driven in the direction of arrow R1 by a drum motor 3 (driving means). The rotation control of the drum motor 3 is performed by the drum controller 17. A charging roller 2 contacts the surface of the photosensitive drum 1 and forms a contact nip portion with the photosensitive drum 1. The charging roller 2 is configured to be driven to rotate by the rotation of the photosensitive drum 1 in the arrow R1 direction.

The optical scanning means comprises a laser diode 6 which emits laser light. The laser beam emitted from the laser diode is a polygon mirror 4 that rotates at high speed.
The laser light that is irradiated onto the surface of the photosensitive drum 1 and is reflected by the polygon mirror 4 scans the surface of the photosensitive drum 1. Polygon mirror 4
Are driven to rotate by the polygon motor 5. The polygon motor 5 rotates at a constant speed, and its rotation control is performed by a polygon motor controller 16. The laser driver 7 controls the light amount of the laser beam emitted from the laser diode 6. The laser driver 7 modulates the intensity of the output light of the laser diode 6 based on the image data sent from the printer controller 15. The laser modulated light emitted from the laser diode 6 scans the surface of the photosensitive drum 1 and is incident on the beam detector 8. When the scanning beam is incident on the beam detector 8, a beam detection pulse as shown in FIG. 2A is output from the beam detector 8. The beam detection pulse output from the beam detector 8 is input to the waveform shaper 9. The waveform shaper 9 converts the signal received from the beam detector 8 into periodic square wave pulses (see FIG. 2B). The pulse output from the waveform shaper 9 is input to the printer controller 15 as a horizontal cycle signal that determines the laser modulation start timing. The printer controller 15 sends the image data to the laser driver 7 in synchronization with the output of the waveform shaper 9.

The pulse output from the waveform shaper 9 is input to the printer controller 15 and the duty converter 10. The duty converter 10 converts the duty ratio of the input pulse and outputs a square wave signal having the same cycle as the input pulse with a duty of approximately 50% (see FIG. 2C). The output of the duty converter 10 is input to the low pass filter 11. Low pass filter 1
1 attenuates the higher harmonic component of the second harmonic or higher of the square wave pulse output from the duty converter 10. Therefore, only the DC component and the first harmonic component of the square wave output from the duty converter 10 are output to the output of the low-pass filter 11, and as a result, the output of the low-pass filter 11 has a waveform as shown in FIG. Becomes The output of the low-pass filter 11 is amplified by the AC amplifier 12, and the amplified AC component is input to the primary winding of the transformer 13. Then, the AC boosted from the secondary winding of the transformer 13
The component is output.

One end of the secondary winding of the transformer 13 is connected to the DC high voltage generator 14, and the other end of the secondary winding of the transformer 13 is connected to the charging roller 2. Therefore, a voltage in which the DC high voltage and the AC high voltage are superposed is applied to the charging roller 2 as shown in FIG.

When the high voltage waveform shown in FIG. 2 (e) is applied to the charging roller 2 and the photosensitive drum 1 and the charging roller 2 rotate, an electrostatic charge is formed on the photosensitive drum 1. The electrostatic charge has an AC ripple in the circumferential direction of the photosensitive drum.

The AC ripple cycle (repetition cycle of AC high voltage) Tac of the electrostatic charge is completely equal to the scanning cycle Ts in the sub-scanning direction (rotational direction of the photosensitive drum 1) of the laser beam of the polygon mirror 4. The laser scans the electrostatic charge surface having such AC ripple. If the laser scans the peak of AC ripple (P1 portion in FIG. 2 (e)), the relative potential of the latent image formed by this becomes high. Also, if the valley of AC ripple (P in Fig. 2 (e)
When the laser scans the second part), the relative potential of the latent image formed thereby becomes low. However, AC
Ripple period Tac and laser beam sub-scanning period Ts
, And the phase of the AC ripple and the phase of the laser scanning are different, so that moire (striped pattern) does not occur in the latent image.

Although the operation of the laser beam printer of the present invention described above refers only to charging and laser exposure, the printer controller 15 also controls the latent image development, toner transfer, and toner fixing processes. To do. Since the operation is completely the same as the conventional one, the description thereof is omitted here.

Regarding the laser exposure, an optical lens for concentrating the laser beam is required before the beam emitted from the laser diode 6 reaches the photosensitive drum 1. This is omitted in FIG. .

In the first embodiment, the low pass filter 1
In No. 1, only the first harmonic and the DC component of the square wave pulse of the duty converter 10 are extracted.

If the low-pass filter 11 is a band-pass filter, only the nth harmonic (n is a natural number) of the square wave pulse of the duty converter 10 can be extracted. At this time, the AC formed by the charging roller 2
The ripple cycle Tac is 1 / n of the laser scanning cycle Ts. Even in this case, since the AC ripple and the laser scanning are synchronized, beat fringes do not appear. <Second Embodiment> FIG. 3 shows an example in which the present invention is applied to an LED printer. The polygon mirror 4, the polygon motor 5, the laser diode 6, the laser driver 7, the beam detector 8, the waveform shaper 9, and the polygon motor controller 16 in FIG. 1 are removed in the second embodiment shown in FIG. Array 18, shift register unit 1
9 is used. That is, an LED array scanning system is used instead of the laser beam scanning system.

The photosensitive drum 1, charging roller 2, drum motor 3, duty converter 10, low-pass filter 11, AC amplifier 12, transformer 13, printer controller 15, and laser driver 17 shown in FIG. Since they are almost the same, their explanations are omitted.

The printer controller 15 sends the serial image data SDA to the shift register unit 19.
TA and SCL which is a writing clock of the image data
Enter K. Inside the shift register unit 19, SDATA is written in the serial register in synchronization with SCLK. When the printer controller 15 finishes writing the image data for one scan, it also sets the SON signal to true. When SON becomes true, the data stored in the serial register inside the shift register unit 19 is output in parallel. This parallel data is transmitted to the LED array 18, and the LED array 18 emits light. The printer controller 15 can adjust the exposure amount of the LED array 18 by adjusting the duty of SON.

Similar to the first embodiment, the SON signal is also input to the duty converter 10 and converted into a square wave of approximately 50% duty. Then, as in the first embodiment, the charging roller 2 receives a voltage obtained by superimposing the AC high voltage and the DC high voltage bias synchronized with the output pallet of the duty converter 10 by the low-pass filter 11, the AC amplifier 12, the transformer 13, and the DC high voltage generator 14. Applied to.

In this way, the light emission signal SON in the sub-scanning direction of the LED array and the cycle of the AC high voltage applied to the charging roller 2 can be matched.

[0024]

As described above, according to the present invention,
The cycle of the AC high voltage applied to the charging roller is Ta
c, where Ts is the repetition cycle of the optical scanning means in the sub-scanning direction, Tac = nTs (n is a natural number) is set to eliminate charging unevenness in the rotation direction of the photoconductor and to eliminate beat fringes (moire). Occurrence can be effectively prevented.

[Brief description of drawings]

FIG. 1 is a diagram schematically illustrating a configuration of a laser beam printer according to a first embodiment.

FIG. 2 is an operation explanatory diagram of the laser beam printer according to the first embodiment.

FIG. 3 is a diagram schematically showing a configuration of an LED printer according to a second embodiment.

[Explanation of symbols]

 1 Photosensitive Body (Photosensitive Drum) 2 Charging Roller 4 Optical Scanning Means (Polygon Mirror) 6 Optical Scanning Means (Laser Diode)

 ─────────────────────────────────────────────────── ─── Continuation of front page (72) Inventor Hideki Suzuki 3-30-2 Shimomaruko, Ota-ku, Tokyo Canon Inc.

Claims (1)

[Claims] 1. A charging roller is brought into contact with a rotationally driven photosensitive member to be driven to rotate, and a voltage is applied to the charging roller to uniformly charge the surface of the photosensitive member, and the surface of the photosensitive member is already charged. In an image forming apparatus that scans a sheet with an optical scanning unit, a superposed voltage obtained by superposing a DC high voltage and an AC high voltage is applied to the charging roller, and a repetition period of the AC high voltage is T
ac, and the scanning cycle of the optical scanning means in the sub-scanning direction is T
When s is set, the repeating cycle Tac and the scanning cycle Ts are set so that the following relationship is established between the repeating cycle Tac and the scanning cycle Ts: Tac = nTs (n is a natural number). A characteristic image forming apparatus.