JP2864819B2 - 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.

More specifically, a step of applying an oscillating voltage to a contact charging means to bring the contact charging means into contact with an image carrier to charge the surface of the image carrier, and scanning the charged surface of the image carrier by line scanning ( The present invention relates to an image forming apparatus that performs image formation by an image forming process including a step of printing and writing by laser beam scanning, LED array on / off control scanning, and the like, such as a laser beam printer.

[0003]

2. Description of the Related Art Contact charging is a method in which a charging member (contact charging means) to which a voltage is applied is brought into contact with a member to be charged to charge the surface of the member to a required potential. In comparison with a corona discharge device, which is a non-contact type charging means, the applied voltage required to obtain a desired potential on the surface of the member to be charged is made constant, and the amount of ozone generated in the charging process is reduced. It has such advantages that the amount is very small and the need for an ozone removal filter is eliminated, the configuration of the exhaust system of the apparatus is simplified, the maintenance is free, and the configuration is simple.

For example, in an image forming apparatus such as an electrophotographic apparatus (copying machine, laser beam printer, etc.), an electrostatic recording apparatus, etc., this contact charging means uses an image carrier such as a photosensitive member or a dielectric, or another charged member. As means for charging a body, attention has been paid to it as a substitute for a corona discharge device, and it has been put to practical use.

[0005] In order to perform a uniform charging process with respect to this contact charging method or device, the present applicant has a method in which a voltage value is periodically changed with time, such as an oscillating voltage (a superimposed voltage of a direct current (DC) voltage and an alternating current (AC) voltage). A method of applying a voltage to a conductive member (contact charging member) and charging the conductive member by bringing the conductive member into contact with a member to be charged has been proposed (JP-A-63-149669). ).

FIG. 8 shows one embodiment. Reference numeral 1 denotes a member to be charged, for example, a drum-type electrophotographic photosensitive member or an electrostatic recording dielectric as an image carrier that is driven to rotate at a predetermined peripheral speed (process speed) in a clockwise direction of an arrow in an image forming apparatus. Body.

Reference numeral 2 denotes a conductive roller (charging roller) as a contact charging member, which comprises a core 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 arranged in parallel with the image carrier 1 and is pressed against the surface of the image carrier 1 with a predetermined pressing force by a pressing force of a pressing spring 10 applied to both ends of the core bar 2b. The rotation of the image carrier 1 follows the rotation of the image carrier 1.

Reference numeral 9 denotes a power supply for applying a voltage to the charging roller 2. The power supply 9 is at least twice the charging start voltage of the image carrier 1 via the contact leaf spring 8 which is brought into contact with the metal rod 2 b of the charging roller 2. AC voltage V having peak-to-peak voltage Vpp
The oscillating voltage (Vac +) in which ac and the DC voltage Vdc are superimposed
Vdc) is applied to the charging roller 2 to uniformly charge the outer peripheral surface of the image carrier 1 that is being driven to rotate.

As the waveform of the oscillating component (AC component) of the oscillating voltage, a sine wave, a rectangular wave, a triangular wave, or the like can be used as appropriate. It also includes a rectangular wave voltage formed by periodically turning on and off the DC power supply.

The contact charging member 2 is not limited to a roller type, but may be a blade type, a rod type, a block type, a bad type, a web type,
It can also be in the form of a brush or the like.

[0011]

The image bearing member is uniformly charged by using such a contact charger of an oscillating voltage application type, and the charged surface of the image bearing member is line-scanned, for example, by a laser beam. An image forming apparatus that performs scanning exposure, prints and writes target image information, and executes image formation,
If the resolution can be switched by switching the print density of line scanning, the following problem occurs.

That is, in an image forming apparatus capable of switching to two or more types of resolutions, assuming that the frequency of the vibration voltage applied to the contact charging member is fixed. Let the frequency of the oscillating voltage be fo (Hz). The surface moving speed of the image carrier as the process speed is defined as Vp (mm / sec). D (dots per i)
nch). Set the line width of the line scan to n (dots). The gap between lines is m (spaces). One dot width is set to d (= 25.4 / D) (mm). Let the line pitch be L (= (n + m) d) (mm). Fs (= Vp / L) the spatial frequency of the line pitch
(Hz), the variation range of the frequency fo (Hz) of the oscillating voltage and the spatial frequency fs (Hz) of the line pitch overlap, and if the following equation is satisfied, fs = Vp / L = Vp / ( n + m) d = Vp.D / (n + m) 25.4 (1) fo = fs (2) Interference called moiré image Stripes occur.

That is, a horizontal line pattern image is output at a specific resolution among two or more types of resolutions D,
When the frequency fo of the voltage applying power supply to the contact charging member approaches the spatial frequency fs represented by the horizontal line pattern image, moire image interference fringes occur on the output image surface.

To explain a little more, the dark portion potential on a contact-charged image carrier (for example, a rotating photosensitive drum) is a cycle mark of a spatial wavelength λsp (Vp / fo) determined by a frequency fo of an AC component of an applied power supply and a process speed Vp. It has a charge spot called “charge spot”.

After exposing n dots (line width ndots) in the sub-scanning direction by turning on the laser by the line scanning of the image carrier with the spatial wavelength λsp, the laser beam is turned off and then m dots in the sub-scanning direction by the laser off. space for dots
When the horizontal line pattern image is formed repeatedly, the length L (line pitch) from OFF to OFF of the laser is substantially equal to each other, and when the two phases coincide with each other, interference occurs with each other and interference fringes called moiré images occur. Occur.

An object of the present invention is to prevent the occurrence of moire image interference fringes as described above.

[0017]

SUMMARY OF THE INVENTION The present invention is an image forming apparatus having the following configuration.

(1) An image carrier and an image forming means for forming an image on the image carrier, wherein an oscillating voltage is applied, which contacts the image carrier to charge the image carrier. It has an image forming means comprising a charging member, a sign of the line scan
The character density can be switched and the selected line scan
In an image forming apparatus that varies the frequency of the oscillating voltage in accordance with the print density , the AC component of the oscillating voltage is
Constant current control at the lowest frequency of the variable frequency, at the time of image formation to the constant voltage control of the AC component of the oscillating voltage at a selected resultant voltage during constant current control irrespective of the print density of the line scan An image forming apparatus comprising:

(2) The image forming apparatus according to (1), wherein the constant current control is performed when the power of the apparatus is turned on.

<Operation> Frequency fo (Hz) of the oscillating voltage applied to the contact charging means
Is variable so that the frequency fo (Hz) does not overlap with the spatial frequency fs (Hz) of the line pitch in relation to the selected printing density (resolution), fo ≠ fs (3) By changing the value to a value satisfying the following relationship, the occurrence of moire image interference fringes can be prevented. By changing the frequency fo (Hz) in synchronization with the switching of the print density D to a value of the relationship of fo ≠ fs, which does not overlap with the spatial frequency fs (Hz) of the switched print density, the usability is good. The occurrence of moire image interference fringes on the output image can be reliably prevented in image formation at any switching print density.

The AC component of the oscillating voltage is previously set to a variable frequency.
Constant current control at the lowest frequency of the wave number, at the time of image formation to determine the constant voltage value suitable for each environment by the constant voltage control of the AC component of the oscillating voltage obtained voltage during the constant current control As a result, uniform charging without leakage can be performed regardless of the environment. Also, the selected line scan
Although the occurrence of interference fringe can be prevented by varying the frequency of the oscillating voltage in accordance with the printing density, the constant voltage value run line does not depend on the print density of the selected line scan
There is no need to repeat the constant current control again by selecting the print density in the inspection . Therefore, even if the print density of the line scanning is switched, uniform charging without leakage can be performed with simple control regardless of the environment, and generation of interference fringes can be prevented.

[0022]

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

Reference numeral 1 denotes a rotating drum type electrophotographic photosensitive member (hereinafter, referred to as a photosensitive drum) as an image carrier, which is driven to rotate at a predetermined process speed Vp (peripheral speed) in a clockwise direction indicated by an arrow. The photosensitive drum 1 of the present example has an outer diameter of 30 formed by forming an organic photoconductor (opc) layer as a photosensitive layer 1a on the outer peripheral surface of a drum base 1b made of aluminum.
(Mm), and the peripheral speed Vp = 50 (mm / sec)
It is rotationally driven with c).

Reference numeral 2 denotes a charging roller as a contact charging member.
As shown in FIG. 8, the pressing spring (10) is pressed against the surface of the photosensitive drum 1 with a predetermined pressing force. Charging roller 2 of this example
Has a conductive roller body 2a made of carbon-dispersed EPDM / urethane or the like formed on the outer periphery of a cored bar 2b.
(Mm) roller.

An oscillating voltage bias (Vac + Vdc) obtained by superimposing an alternating current of frequency fo on a direct current from a power source 9.
Is applied via the contact leaf spring 8, the peripheral surface of the rotary photosensitive drum 1 is charged to a predetermined polarity and potential.

Reference numeral 3 denotes a laser beam scanner, which modulates an image 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) such as a computer, a word processor, and an image reader. The laser beam 3a thus output is output.

In the image forming apparatus (printer) of the present embodiment, the print density (resolution) D can be switched between three types: 600 (dpi), 480 (dpi), and 400 (dpi).

The surface of the photosensitive drum 1 charged as described above is subjected to main scanning exposure with the laser beam 3a output from the scanner 3 controlled by the controller, so that the surface of the photosensitive drum 1 corresponds to the target image information. A formed electrostatic latent image is formed.

The latent image is developed with toner by a developing sleeve 4 of a developing device, and the developed image is introduced from a paper feeding unit (not shown) to a transfer unit between the photosensitive drum 1 and the transfer roller 5 at an appropriate timing. The image is transferred to the 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 is conveyed to an image fixing section (not shown), and is output as an image formed product (print / copy). After the image transfer, the surface of the photosensitive drum 1 is cleaned by the cleaning blade 6 to remove adhered contaminants such as untransferred toner, and is repeatedly used for image formation.

In such an image forming apparatus, the line pitch L (mm) and the spatial frequency fs (H
The above-mentioned equation (1) for z), that is, the relationship of fs = Vp ・ D / (n + m) ・ 25.4 is shown in the graphs of FIGS. 2, 3, and 4. The process speed Vp = 50 (mm / sec), the resolution D = 600, 480, and 400 (dpi), and n (dot) and m (space) are displayed as n: m.

As can be seen from the graph 1 of FIG. 2, the spatial frequency fs (Hz) is n at each resolution D (dpi).
(Dot): It becomes maximum when m (space) is 1: 1 and becomes larger as the resolution D (dpi) becomes higher. Therefore, the resolution D (dpi) is D = 600, 480, 40
In the image forming apparatus of the present embodiment, which can be switched to 0 (dpi), 1 (dot) of D = 600 (dpi): 1 (sap)
The spatial frequency fs1 = 591 (Hz) at the time of ce) becomes the maximum value.

Therefore, if the AC frequency fo applied to the charging roller 2 is set to be higher than the spatial frequency fs1 of the line pitch, the resolution D becomes D = 600, 48.
In an image forming apparatus that can switch between 0 and 400 (dpi), the occurrence of moire image interference fringes can be prevented with only one frequency fo.

That is, fo> fs1 should be satisfied, but fo (Hz) is given by
Approximately ± 10% varies. Therefore, fo ≧ 660
(Hz). However, since the charging frequency fo is very high, the charging roller 2 is
The charging noise generated by striking the surface cannot be ignored, and is unpleasant and unpleasant. In order to reduce the charging noise, it is necessary to set the charging frequency fo as low as possible.

The range of the graph 2 in FIG.
1 (dot) 2 (space) at 600 (dpi)
s) spatial frequency fs2 (Hz) and 1 (dot) 3
9 shows the relationship of the charging frequency fo (Hz) in anticipation of fluctuation (± 10%) between spatial frequencies fs3 (Hz) of (spaces).

In this range, the charging frequency fo (Hz) in consideration of fluctuation (± 10%) is D = 600 (dpi)
Fs2 (Hz) and fs3 (Hz), the moire image interference fringe does not occur at D = 600 (dpi), but overlaps with fs2 (Hz) at D = 480 (dpi), so that D = 480. In (dpi), moire image interference fringes are generated.

Similarly, in the range, D = 480 (dp
i) fo (H) between fs2 (Hz) and fs3 (Hz).
z) is settled, and moire image interference fringes do not occur when D = 480 (dpi), but fs3 (H) at D = 600 (dpi).
z) and fs2 (Hz) and fo of D = 400 (dpi)
(Hz) overlap, D = 600 (dpi) and D = 400
In (dpi), moire image interference fringes are generated.

Similarly, in the range of D = 400 (Hz)
Fo (Hz) between fs2 (Hz) and fs3 (Hz)
And moire image interference fringes do not occur when D = 400 (dpi), but 1 (dot) 4 of D = 600 (dpi)
(Spaces) spatial frequency fs4 (Hz) and D = 4
At 80 (dpi), fs3 (Hz) and fo (Hz) overlap, and when D = 600 (dpi) and D = 480 (dpi), moire image interference fringes occur.

As described above, when the resolution D (dpi) can be switched to three types, fo (Hz) due to the variation in the accuracy of a single component of the power supply in a portion where the spatial frequency fs (Hz) is low.
Fo that satisfies fo ≠ fs in anticipation of fluctuation (± 10%)
Cannot be at one frequency. Therefore, in order to prevent the occurrence of moire image interference fringes, it is preferable that the charging frequency fo be switched in synchronization with the switching of the resolution D (dpi).

Resolution D = 600, 480, 400 (dp
In the case of i), fo and fs become fo ≠ fs in consideration of the variation (± 10%) due to the variation of the charging frequency fo at each resolution D (dpi), and fo shows the lowest value. = 600 (dpi), fo = 345 (Hz) D = 480 (dpi), fo = 276 (Hz) D = 400 (dpi), fo = 230 (Hz) At this time, the possible value of the charging frequency fo (Hz) at each resolution D (dpi) is 1 (dot) 2
Spatial frequency fs2 (Hz) at (spaces) and 1
Spatial frequency fs3 at (dot) 3 (spaces)
(Hz).

Resolution D = 600, 480, 400 (dp
Spatial frequency fs2, fs3 and fluctuation in i) (± 10%)
The relationship with fo (Hz) is as follows. When D = 600 (dpi), fs2 = 394 (Hz), fs3 = 295.5 (Hz), and fo = 345 (Hz), 380 (Hz) ≧ fo ≧ 311 (Hz). When D = 480 (dpi), fs2 = 315 (Hz), fs3 = 236.3 (Hz), and fo = 276 (Hz), 304 (Hz) ≧ fo ≧ 248 (Hz). When D = 400 (dpi), fs2 = 262.5 (Hz), fs3 = 196.9 (Hz), and fo = 230 (Hz). 253 (Hz) ≧ fo ≧ 207 (Hz). It is.

Resolution D = 600, 480, 400 (dp
If the respective charging frequencies fo (Hz) of i) are set lower than the above values, the latitude of the spatial frequency fs (Hz) becomes narrower, and fluctuations (± 10%) of the charging frequency fo (Hz) cannot be expected. Therefore, all frequencies fo
(Hz), moire image interference fringes occur.

The resolution D = 600 (dp) is shown in the graph 3 of FIG.
The relationship between the spatial frequency fs (Hz) at the time of i) and the charging frequency fo (Hz) in view of the variation (± 10%) is shown on the graph by (A), (B) and (C). At this time, the spatial frequency of 1 (dot) 2 (spaces) is set to fs
The spatial frequency of 2, 1 (dot) 3 (spaces) is represented by fs
The spatial frequency of 3, 1 (dot) 4 (spaces) is represented by fs
Assuming that the spatial frequency of 4, 1 (dot) 5 (spaces) is fs5, in the range of (A), fs2 = 394 (Hz), fs3 = 295.5 (Hz), and fo = 345 (Hz). In the range of 380 (Hz) ≧ fo ≧ 311 (Hz) (B), fs3 = 295.5 (Hz) and fs4 = 236.4 (Hz). , 293 (Hz) ≧ fo ≧ 239 (Hz) (C) In the range of fs4 = 236.4 (Hz) and fs5 = 197.0 (Hz), the variation of fo = 217 (Hz) is 293 (Hz) ≧ fo ≧ 195 (Hz).

Process speed Vp = 50 (mm / s)
ec), the peak-to-peak voltage Vpp of the AC voltage applied to the charging roller =
1800 (V), resolution D = 600 (dpi), and the line pitch L (mm) was changed to investigate. In the range of (A), fs2 = 394 (Hz) and fs3 = 295.5 (Hz) ) Does not overlap with the range of variation (± 10%) of fo = 345 (Hz) 380 (Hz) ≧ fo ≧ 311 (Hz), and no moire image interference fringes were generated.

In the range (B), fs3 = 295.5
(Hz) and fs4 = 236.4 (Hz) become fo = 266
(Hz) fluctuation (± 10%) range 293 (Hz) ≧ fo ≧ 239 (Hz), and the moire image interference fringe was generated due to the overlap.

In the range of (C), fs4 = 236.4
(Hz) and fs5 = 197.0 (Hz) become fo = 217
(Hz) fluctuation (± 10%) range 239 (Hz) ≧ fo ≧ 195 (Hz), and moire image interference fringes occur.

Further, the resolution D = 480, 400 (dp
The same study as above was performed for i), but the lowest charging frequency fo (Hz) at which no moire image interference fringes occurred was a spatial frequency between fs2 and fs3, similarly to the above result.

From the above results, the lowest charging frequency fo (Hz) at which moire image interference fringes do not occur is determined at each resolution D (d
For each pi), the spatial frequency fs2 (Hz) at 1 (dot) 2 (spaces) and 1 (dot) 3 (space)
It must be a frequency between the spatial frequencies fs3 (Hz) in s).

In this embodiment, the charging of the photosensitive drum 1 by the charging roller 2 is performed by the AC voltage Vac applied to the charging roller.
Is constant (Vpp = 1800)
(V)).

Investigation was conducted by switching the charging frequency fo (Hz) and the resolution D (dpi) in the above configuration.

[0051] When fo is fixed to 345 (Hz),
When D = 600 (dpi), moire image interference fringes do not occur.

However, when D = 480 (dpi), 1 (do)
t) Moire image interference fringes occurred at 2 (spaces).

[0053] When fo is fixed to 276 (Hz),
When D = 480 (dpi), no moire image interference fringes occur.

However, D = 600 (dpi) and 1 (do)
t) 3 (spaces) and D = 400 (Hz) 1
Moire image interference fringes occurred at (dot) 2 (spaces).

[0055] When fo is fixed to 230 (Hz),
When D = 400 (dpi), moire image interference fringes do not occur.

However, 1 (dot) of D = 600 (Hz)
4 (spaces) and 1 (do) of D = 480 (Hz)
t) Moire image interference fringes occurred at 3 (spaces).

From these results, according to the configuration of the present invention, even if the resolution D (dpi) can be switched to two or more, the charging frequency fo (Hz) is synchronized with the switching of the resolution D (dpi). Is switched, the occurrence of moire image interference fringes can be prevented.

Further, since the charging frequency fo (Hz) can be set low, the charging noise is extremely low.

<Reference Example 2> (FIGS. 5 and 6) In this example, the switching of the resolution D (dpi) and the switching of the charging frequency fo (Hz) are the same as in the first embodiment.

In the first embodiment, the photosensitive drum 1 is charged by keeping the peak-to-peak voltage Vpp of the AC voltage Vac applied to the charging roller 2 constant. In this embodiment, the AC voltage Vac applied to the charging roller 2 is changed. Is controlled in accordance with a change in the impedance between the charging roller 2 and the photosensitive drum 1, the constant current control of the AC voltage component is performed.

As shown in FIG. 5, the charging roller 2 used in the present embodiment is formed on a conductive metal φ6 core 2b by a thickness of 3 mm.
A conductive rubber layer 2a having a low volume resistivity of about 10 ⁴ Ωcm is formed, and a 50 to 20 μm thick 10 ⁸ Ω layer is further formed on the outer periphery thereof.
The high resistance layer 2c of about cm is formed.

In this embodiment, in order to converge the surface potential of the photosensitive member to a target dark portion potential VD, the voltage applied to the charging roller 2 is changed to a DC constant voltage corresponding to the potential VD to determine whether the potential is uniform. Therefore, a method of superimposing an AC voltage is used. At this time, the peak-to-peak voltage of the AC voltage to be superimposed is at least twice as large as the discharge threshold voltage V _TH at which the charging roller 2 and the photosensitive drum 1 start discharging. In this example, 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 a change in the charging frequency fo (Hz), the charging frequency fo (Hz) is switched in synchronization with the switching of the resolution D (dpi). In this case, the AC current value must be fixed for each charging frequency fo (Hz), and the target value of the AC current must be switched in synchronization with the switching of the resolution D (dpi).

As described above, the constant current control is performed in order to change the AC component for each charging frequency fo according to the change in impedance due to the environment and durability of the charging roller.

Assuming that the resistance and capacity of the photoreceptor are constant regardless of the environment at each charging frequency fo (Hz), the voltage applied to the charging roller is controlled by controlling the alternating current flowing through the entire body to be a constant current control. It is determined in proportion to the impedance.

Therefore, a high voltage is applied to the high-impedance charging roller 2 which has a low resistance in a low-temperature, low-humidity (L / L) environment or during manufacture, thereby preventing charging failure, and conversely applying a low-impedance charging roller. In this case, a low voltage is applied to prevent leakage.

In this example, a sine wave of 1800 V
Effective current Ia of the AC current to obtain the peak-to-peak voltage
The constant current control was performed by setting c to the following value for each charging frequency fo.

[0068] Fo = 345 with D = 600 (dpi)
(Hz), Iac = 360 (μA). When D = 480 (dpi) and fo = 276 (Hz), Iac = 290 (μA). When D = 400 (dpi) and fo = 230 (Hz), Iac = 250 (μA) By performing constant current control at this value, about 2200 in a low temperature and low humidity environment (15 ° C., 10% RH).
(V), in a high-temperature and high-humidity (H / H) environment, a peak-to-peak voltage of 1600 (V) can be obtained.
Leaks can now be minimized.

FIG. 6 is a block diagram showing an example of a circuit for performing constant current control of an AC voltage component. This is a feedback system. If the current that is fed back is large, the voltage is controlled to decrease, and if the current is reversed, the voltage increases.

With the configuration of this example, even if the resolution D (dpi) can be switched to two or more types, it is possible to prevent the occurrence of moire image interference fringes. Further, the charging frequency f
Since o can be set low, the charging noise is very small and inconspicuous.

<Embodiment> (FIG. 7) In this embodiment, the switching of the resolution D (dpi) and the switching of the charging frequency fo (Hz) are the same as in the first embodiment.

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

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 electric 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 each environment.

Therefore, in order to determine a constant voltage value suitable for each environment, the image is not actually formed, for example, a predetermined frequency fo is set during the pre-rotation of the photosensitive drum 1,
It is assumed that a preset AC current is passed, an AC voltage generated thereby is detected, and the AC voltage is controlled by this value at any frequency fo during image formation.

Here, the preset AC current is a value at which the AC voltage does not cause charging failure in each environment, and is usually sufficient in a low-temperature and low-humidity environment where the resistance value of the charging roller 2 rises most. It is an alternating current that can generate a peak-to-peak voltage at which charging can be performed.

Image output was performed by the image forming apparatus of the present invention in which the charging frequency fo was switched to a value at which no moire image interference fringes occurred in synchronization with the switching of the resolution D.
The minimum voltage that does not cause charging failure in a low-temperature and low-humidity environment of 10 ° C. and 10% RH is 2.2 (KV). At this time, the process speed Vp = 50 (mm / sec), the resolution D = 400 (dpi), The charging current was 250 (μA) at the charging frequency fo = 230 (Hz).

Therefore, in this embodiment, in each environment, the charging frequency fo is set to 230 (Hz) by turning on the main switch of the image forming apparatus, and the constant current control of 250 (μA) is performed in advance at the timing described later. The AC voltage generated at this time is detected. At the time of image formation, the charging frequency fo is synchronized with the switching of the resolution D.
It is assumed that this voltage is maintained and charging is performed even if the switching is made.

In the present embodiment, the timing for controlling the above-mentioned sequence was performed in the following procedure. FIG. 7 is a timing chart for performing image formation in this embodiment.

When power is turned on, charging frequency fo = 230
(Hz), and then, during pre-rotation performed to stabilize the potential of the photoconductor, AC constant current control is performed, and the voltage generated here is held. When the constant current control is performed during the pre-rotation, if an instantaneous voltage is detected and held, an error may be increased. Therefore, the constant current control is performed for at least the time required for the photosensitive drum 1 to make one rotation. The average value of the generated voltages is used as the control voltage at the time of image output. In this embodiment, since the diameter of the photosensitive drum 1 is 30 (mm) and the process speed is 50 (mm / sec), the generated AC voltage between 1.88 (sec) The average was calculated.

When printing was actually performed in each environment, the charging frequency fo = 230 (Hz), the alternating current Iac = 250 (μA), the constant current, and the high temperature and high humidity environment of 32.5 ° C. and 85% RH. 1.8K under normal (N / N) environment of 1.6KV, 23 ° C, 60% RH
In a low-temperature and low-humidity environment of V, 15 ° C. and 10% RH, an AC peak-to-peak voltage of 2.2 KV was obtained.

By performing the above-described control, even if the charging frequency fo is switched in synchronization with the switching of the resolution D at the time of image output, it is possible to obtain a good image in which no charging failure leak occurs in all environments. did it.

Further, in this embodiment, constant current control is performed at the lowest frequency among the switchable charging frequencies, and constant voltage control is performed with the AC voltage generated at that time, including other frequencies. Even when the impedance changes, it is possible to control the amount of the AC discharge effective for the actual charging operation to be most constant.

Hereinafter, it will be described that the lower the frequency during the constant current control for the charging roller as the contact charging member, the better. This is because if the frequency of the AC voltage is low, the leak current component of the AC flowing due to the capacity formed between the charging roller and the photosensitive drum as the image carrier decreases, and the discharge actually generated between the charging roller and the photosensitive drum This is because the ratio of the minute current to the alternating current increases. This will be further described below. The AC leakage current is a leakage current flowing through the photosensitive drum via a contact portion between the charging roller and the photosensitive drum, and does not contribute to charging. The AC discharge current is a current that flows through a minute gap between the charging roller and the photosensitive drum and contributes to charging. The total current given to the charging roller during the constant current control is the sum of the leak current and the current for the discharge. Since the impedance is inversely proportional to the frequency, the impedance increases as the frequency decreases.
As the frequency decreases, the leakage current decreases, but the decrease in the amount of discharge decreases as the leakage current decreases. That is, as the frequency decreases, the ratio of the discharge current to the total current increases. Conversely, the ratio of the leak current decreases. The impedance increases as the frequency decreases. The voltage at the time of constant current control at the lowest frequency (the peak-to-peak voltage Vpp of AC) is known, and if the voltage is set so as not to cause charging failure at the time of image formation, the charge at the time of image formation is maintained even if the frequency is switched to a higher frequency. There is no failure. When a constant voltage (Vpp) is applied at the time of image formation, the current changes due to an impedance change due to a frequency change, but what is related to poor charging is not the current but the voltage (Vpp). Therefore, if the voltage (Vpp) is set so that the charging failure does not occur when the impedance becomes highest with respect to the frequency, the charging failure does not occur even if the frequency is switched thereafter. In Reference Example 2, since the impedance changes due to the frequency change, as the frequency decreases, that is, as the impedance increases, the constant current of AC is reduced so that the charging failure of the voltage (Vpp) does not occur regardless of the frequency change. To maintain a constant voltage.

Further, according to the structure of the present invention, even if the resolution D can be switched to two or more, generation of moire image interference fringes can be prevented. Further, since the charging frequency fo can be set low, the charging noise is very small and inconspicuous.

[0085]

According to the present invention, as described above, according to the present invention, beforehand
The AC component of the oscillation voltage is the lowest frequency of the variable frequencies.
And constant current control by the number, because the time of image formation can be determined constant voltage value suitable for each environment by the constant voltage control of the AC component of the oscillating voltage obtained voltage during the constant current control, the environmental Regardless, it is possible to perform uniform charging without leakage. Although possible to prevent the occurrence of interference fringe by varying the frequency of the oscillating voltage in accordance with the print density of the selected line scan, the constant voltage value is selected line scanning indicia
Since it does not depend on the character density, there is no need to repeat the constant current control again by selecting the print density for line scanning . Therefore, La
Even if the print density of the in-scan is switched, uniform charging without leakage can be performed with simple control regardless of the environment, and generation of interference fringes can be prevented.

[Brief description of the drawings]

FIG. 1 is a schematic configuration diagram of a laser beam printer of an image forming apparatus according to a first embodiment.

FIG. 2 is a graph 1 showing a relationship between line pitch L, spatial frequency, and charging frequency.

[Fig. 3] Relationship graph 2

[Fig. 4] Relationship graph 3

FIG. 5 is a schematic cross-sectional view showing a layer configuration of a charging roller used in Reference Example 2.

FIG. 6 is a block diagram of a circuit example for performing constant current control of an AC component.

FIG. 7 is a control timing chart in the embodiment.

FIG. 8 is an explanatory diagram of contact charging.

[Explanation of symbols]

 Reference Signs List 1 electrophotographic photosensitive drum as image carrier 2 charging roller as contact charging member 9 voltage application power supply to charging roller 3 laser scanner 3a laser beam 4 developing sleeve 5 transfer roller 6 cleaning blade 7 transfer material

──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Koichi Hiroshima 3-30-2 Shimomaruko, Ota-ku, Tokyo Inside Canon Inc. (72) Inventor Tatsuichi Tsukita 3-30-2 Shimomaruko, Ota-ku, Tokyo No. Canon Inc. (72) Inventor Takahiro Inoue 3-30-2 Shimomaruko, Ota-ku, Tokyo Canon Inc. (56) References JP-A-3-101765 (JP, A) JP-A-3-3 98382 (JP, A) JP-A-3-140263 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) G03G 15/02 102 G03G 21/00 370

Claims (2)

(57) [Claims] An image forming means for forming an image on the image carrier, the image forming means being in contact with the image carrier for charging the image carrier, and applying an oscillating voltage. has an image forming means comprising a member, the printing dense line scan
Degree can be switched and selected line scan printing
An image forming apparatus for varying the frequency of the oscillating voltage in accordance with the density of the variable frequency AC component of the pre-oscillating voltage
Constant current control at the lowest frequency , selected during image formation
An image forming apparatus characterized in that the constant voltage control of the AC component of the oscillating voltage was voltage obtained during the constant current control irrespective of the print density of the line scan. 2. The image forming apparatus according to claim 1, wherein the constant current control is performed when the power of the apparatus is turned on.