JP2002214888A - Image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent a photoreceptor drum from being ground, even when an installation environment and the surface resistance of an electrifying member and the photoreceptor drum are changed, and also, to maintain a satisfactory image. SOLUTION: Provided that a peak-to-peak voltage Vpp of an AC voltage generated by a charting bias power supply 3 and an angular velocity ω are positive constants, and provided that an AC voltage to be applied on an charging roller 2 by the power supply 3 is expressed by V when V1=α×(Vpp/2) and V2=β×(Vpp/2) are established (provided that -1<α<0 and 0<β<1; αand β are fixed values), by applying the AC voltage V having a waveform satisfying the following conditions; V=(Vpp/2)×sin(ωt) when V1<V<V2 is established, V=V1 when V<V1 is established, and V=V2 when V>V2 is established on the charging roller 2 by the charging bias power supply 3, the photoreceptor 1 is prevented from being ground, even when the installation environment and the surface resistance of the charging member 2 and the photoreceptor drum 1 are changed, and also, the satisfactory image is maintained.

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 such as a copying machine, a printer, a facsimile, etc., which forms an image using an electrophotographic system, an electrostatic recording system, or the like.

[0002]

2. Description of the Related Art Conventionally, in an image forming apparatus such as an electrophotographic apparatus or an electrostatic recording apparatus, an apparatus for charging a surface (a surface to be charged) of an image carrier (a body to be charged) such as a photoreceptor or a dielectric. Corona discharge devices have been widely used.

In this method, a corona discharge device is disposed in a non-contact manner such that a discharge opening thereof faces a member to be charged, and a surface to be charged is exposed to a corona current from the discharge opening. Is to be charged to a potential of. However, this corona discharge device requires a high-voltage power supply and has problems such as generation of a large amount of ozone.

Therefore, in recent years, it has advantages such as low ozone and low power as compared with the above-mentioned non-contact type corona discharge device. Therefore, a charging member to which a voltage is applied is brought into contact with the surface to be charged, and the surface to be charged is cleaned. A contact-type charging device for performing a charging process has been put to practical use.

[0005] The contact-type charging device includes a "DC charging system" in which only a direct current voltage (DC) is applied as a charging bias to a contact charging member to perform a charging process on a member to be charged; ) Is superimposed and applied with an alternating voltage (AC) to charge an object to be charged. In either method, the charged surface is charged to a predetermined polarity and a predetermined potential by the contact charging member to which the bias voltage is applied.

Further, the contact charging member has a contact area in contact with the member to be charged, and a separation surface area in which the distance to the surface to be charged becomes larger downstream of the contact area in the moving direction of the member to be charged. A charging bias formed by superimposing an AC voltage on a DC voltage is applied between the member to be charged and the charging member.

Thus, an oscillating electric field is formed between the surface to be charged and the separated surface region of the contact charging member, and the AC component equalizes the unevenness of the charging potential of the surface to be charged, and the DC component reduces the DC component to the surface to be charged. Is converged to a predetermined potential, so that the surface to be charged can be uniformly and stably charged without charging unevenness as a whole.

[0008]

By the way, as the number of times of image formation increases, that is, as the durability of the photosensitive drum as an object to be charged of the image forming apparatus increases, the surface (outer peripheral surface) of the photosensitive drum becomes a cleaning member (cleaning member). The thickness of the photosensitive layer (thick film of the photosensitive drum) is reduced by being scraped by the blade) and the developer.

Particularly, in the above-described AC charging system, by applying a DC voltage and an AC voltage in a superimposed manner to the contact charging member, an effect of equalizing charging unevenness caused when only the DC component is charged by the AC component is obtained. Therefore, while having the feature of being excellent in the uniformity of charging, the thickness of the photosensitive layer (the thickness of the photosensitive drum) is remarkably reduced with the durability compared with the DC charging method.

The reason for this is that the discharge current of the AC component superimposed on the discharge current of the DC component damages the surface of the photosensitive drum, and the surface of the photosensitive drum is easily scraped by a cleaning member (cleaning blade). Are known. When the shaving amount of the photosensitive drum is large, the film thickness of the photosensitive drum becomes small while the number of durable sheets is small.

When the film thickness of the photosensitive drum is reduced, minute scratches on the surface of the photosensitive drum are easily seen on an output image,
Leakage from the charging member to the conductive substrate (drum cylinder) of the photosensitive drum is likely to occur, and if the film thickness of the photosensitive drum becomes thinner, the photosensitive layer itself disappears, making image formation impossible. Influence.

Further, particularly in a low-humidity environment, the volume and surface resistance of the photosensitive drum, the contact charging member, the atmosphere (ie, air) are all high, so that it is difficult to discharge. , The peak-to-peak voltage (Vpp) of the AC voltage applied to the contact charging member for uniformly charging the photosensitive drum to the required potential is large. As a result, there is an adverse effect that the toner slightly remaining on the photosensitive drum and the external additive in the toner are fused (hereinafter, referred to as toner fusion) on the photosensitive drum by the vibration of the contact charging member due to the AC voltage. Also occurs.

In a particularly high-humidity environment, NOx, O _{3, and the} like generated by the discharge are dissolved in the moisture on the surface of the photosensitive drum, and the surface resistance of the photosensitive drum is lowered. Cannot be held, and a phenomenon that the output image is blurred (hereinafter, referred to as image deletion) is likely to occur.

Therefore, the total amount of alternating current flowing between the contact charging member and the photosensitive drum due to the application of the charging bias is represented by
The amount of AC current flowing through a contact area (charging nip) where the contact charging member contacts the photosensitive drum and the amount of AC discharge current flowing through a minute gap near the contact area (charging nip) are divided into two sections. So that the amount of AC discharge current flowing through the minute gap falls within a predetermined set range,
A method of controlling an AC voltage or an amount of AC current applied to a contact charging member has been proposed.

An outline of this method will be described with reference to FIG. In FIG. 7, the horizontal axis is the peak-to-peak voltage (Vpp) of the AC voltage applied to the contact charging member, and the vertical axis is the total current flowing at this time. FIG. 7 shows the relationship between the peak-to-peak voltage of the AC voltage and the total current. is there.

From this figure, it can be seen that the slope of the straight line changes from the discharge start voltage V0 after the AC voltage is applied to the contact charging member. This is because the contact area between the contact charging member and the photosensitive drum before the discharge start voltage (charging nip)
This is because the current flows only through the contact region, but after the discharge starting voltage, in addition to the current flowing through the contact region, the current flows through the minute gap near the contact region.

Therefore, when calculating the discharge current amount at a certain Vpp = V1, the straight line A after the discharge start voltage is used.
This can be calculated by obtaining a value I2 by subtracting the value of the point extended from the straight line B before the discharge start voltage from the value of the upper point. By controlling Vpp so that the discharge current obtained by such control becomes a constant value, it is possible to greatly improve the allowable range for scraping of the photosensitive drum due to excessive discharge, fusion of toner, and image flow. Become.

However, the amount of alternating current flowing through the minute gap near the contact area (charging nip) between the contact charging member and the photosensitive drum is a so-called discharge phenomenon, which is a very small value with respect to the total current. For example, according to a study by the inventor of the present application, when a sine wave is used as an AC voltage waveform in a low humidity environment at a temperature of 23 ° C. and a humidity of about 5%, the amount of current flowing through the minute gap is about 1500 μA in total current. It was about 100 μA. In addition, temperature 30 ° C, humidity 80
% In a high humidity environment, the amount of current flowing through
If the value is not set to less than 0 μA, image deletion will occur. At this time, the total current amount needs to be about 1200 μA.

That is, the amount of discharge current that is 1/100 or less of the total amount of current must be obtained by calculation. Therefore, it is required that the accuracy of the applied voltage Vpp and the accuracy of current detection be extremely high. Is done. In addition, another problem is that the direction of required accuracy differs depending on the installation environment of the image forming apparatus.

That is, the main problem to be solved in a high-humidity environment is image deletion, which is a phenomenon depending on the amount of discharge current. For this reason, the accuracy of current detection is more important than the accuracy of the applied voltage Vpp. It goes without saying that the accuracy of the current detection circuit needs to be high, but in addition, the slope of the straight line A after the start of discharge in FIG.
The accuracy is higher if the difference is not so large with respect to the slope of the straight line B before the start of discharge. This is because the smaller the difference between the slopes, the higher the peak-to-peak voltage (amplitude) of the AC voltage, Vp
This is because even if the value of p slightly changes, the change in the discharge current can be small.

However, actually, in such a high humidity environment, the surface resistance of the charging member and the photosensitive drum and the resistance value of the air layer near the charging nip are reduced, so that the discharge easily occurs. The slope difference between the straight line A after and the straight line B before the start of discharge becomes large. On the other hand, in a low-humidity environment, since the required Vpp is originally high, it is necessary to control the applied voltage Vpp to a value as small as possible from the viewpoint of abrasion of the photosensitive drum and fusion of the toner. Become.

On the other hand, since image deletion is unlikely to occur, if the amount of discharge current is equal to or more than a certain value, not much accuracy is required. From the above, in a low humidity environment, Vpp accuracy is output with high accuracy.
In addition, it is preferable that the difference between the slope of the straight line A after the start of the discharge in FIG. 7 and the slope of the straight line B before the start of the discharge is as large as possible. This is because a discharge current amount equal to or higher than a certain value can be obtained at a lower Vpp value.

However, actually, in a low humidity environment, the surface resistance of the contact charging member and the photosensitive drum, and the resistance of the air layer near the charging nip are large, and the discharge current is small. The difference in the slope of the straight line before the start increases.

Therefore, the present invention provides an installation environment (temperature,
It is an object of the present invention to provide an image forming apparatus capable of preventing the photosensitive drum from being scraped or having an image defect even when the humidity, the contact charging member, and the surface resistance of the photosensitive drum change.

[0025]

In order to achieve the above object, the present invention relates to a rotatable image carrier and at least an alternating voltage applied from a charging bias applying means, which contacts the surface of the image carrier. And a contact charging member for charging the image carrier to a predetermined potential, wherein the peak-to-peak voltage Vpp of the AC voltage generated by the charging bias applying unit and the angular velocity ω are constant positive constants, V1 = α
× (Vpp / 2), V2 = β × (Vpp / 2) (where −1 <α <0, 0 <β <1; α and β are constant values). Assuming that the AC voltage applied to the contact charging member is V, when V1 ≦ V ≦ V2, V = (Vpp / 2) × s
in (ωt) When V <V1, V = V1 When V> V2, an AC voltage V having a waveform satisfying V = V2 is applied from the charging bias applying unit to the contact charging member. I have.

The values of V1 and V2 are changed according to the installation environment of the image forming apparatus.

Further, the values of V1 and V2 are changed according to the surface resistance of the contact charging member and the image carrier.

[0028]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below based on the illustrated embodiment.

<First Embodiment> FIG. 1 is a schematic configuration diagram showing an image forming apparatus (an electrophotographic copying machine in the present embodiment) according to the present embodiment.

This image forming apparatus includes a photosensitive drum 1 as an image carrier, and a charging roller 2 as a contact charging member, a developing device 11, a transfer roller 12, a cleaning device 13, and a pre-exposure device 15 around the photosensitive drum 1. Is provided.
An exposing device 10 is provided above the charging roller 2 and the developing device 11.

In the present embodiment, the photosensitive drum 1 is a negatively charged organic photoreceptor and has a photoconductive layer (photosensitive layer) 1a on a drum base 1b made of aluminum. It is rotationally driven in a direction (clockwise) at a predetermined peripheral speed (process speed).

The charging roller 2 as a contact charging member is composed of a core 2c, a conductive layer 2b and a high resistance layer 2a which are sequentially formed on the outer periphery of the core 2c. (Not shown), is pressed against the photosensitive drum 1 with a predetermined pressing force by a pressing means (not shown), and rotates in the direction of arrow R2 (counterclockwise) as the photosensitive drum 1 rotates. It is driven and rotated.

Further, the charging bias power supply 3 supplies the sliding contact 3
By applying a predetermined charging bias (in the present embodiment, a charging bias in which an AC voltage is superimposed on a DC voltage) to the metal core 2c of the charging roller 2 via a, the photosensitive drum 1 has a predetermined polarity and potential. Is charged.

The developing device 11 has a developing sleeve 11a which rotates in the direction of arrow R11 (counterclockwise). The developing roller 11 a is arranged so as to face the surface of the photosensitive drum 1. A developing bias (in the present embodiment, a developing bias in which an AC voltage is superimposed on a DC voltage) is applied from a developing bias power supply 4, and the photosensitive roller 11 a is exposed to light. A developer (toner) is adhered to the electrostatic latent image formed on the surface of the drum 1 and developed (developed) as a toner image.

The transfer roller 12 as a contact transfer member includes
The transfer bias power supply 5 applies a transfer bias having a polarity opposite to that of the toner from the transfer bias power supply 5. Is done. The transfer roller 12 to which the transfer bias is applied passes a transfer material 14 such as a sheet conveyed to a transfer nip between the photosensitive drum 1 and the transfer roller 12, so that a toner image formed on the photosensitive drum 1 is formed. Transfer to the transfer material 14. The transfer roller 12 has an elastic body on the surface of a metal cored bar.

In the present embodiment, the exposure device 10 is a known document table fixed-optical system moving type document imaging slit exposure unit. In the exposure apparatus 10, reference numeral 20 denotes a fixed platen glass on which a document O is placed facing downward, 21 is a document holding plate, 22 is a document illumination lamp (exposure lamp), 23 is a slit plate, and 24, 25, and 26 are The moving first, second, and third mirrors, 27 are imaging lenses, and 28 is a fixed mirror.

The ramp 22, the slit plate 23, and the movable first mirror 24 move the lower surface of the platen glass 20 from one end to the other end at a predetermined speed V, and the second and third mirrors 25, 26.
Is driven to move at a speed of V / 2, and the downward original surface on the original platen glass 20 is scanned from one end side to the other end side, and image exposure is performed via the imaging lens 27 and the fixed mirror 28. An image forming slit exposure L is performed on one surface, and an electrostatic latent image corresponding to image information of the document O is formed.

In the present embodiment, the control device 30
And a temperature / humidity sensor 31 for detecting an installation environment (temperature, humidity) of the image forming apparatus. The control device 30 controls the charging bias power supply based on the temperature / humidity information input from the temperature / humidity sensor 31. 3, the waveform of the AC voltage (oscillation voltage) of the charging bias applied to the metal core 2c of the charging roller 2 is changed (details will be described later).

Next, an image forming operation by the above-described image forming apparatus will be described.

At the time of image formation, the photosensitive drum 1 is rotationally driven in a direction indicated by an arrow R1 at a predetermined process speed by a driving means (not shown), and is supplied from a charging bias power supply 3 to a charging bias (in this embodiment, DC voltage to AC voltage). Is charged to a predetermined polarity and potential by the charging roller 2 to which a charging bias (superimposed on the charging bias) is applied.

Then, as described above, the image forming slit exposure L is given to the charged photosensitive drum 1 by the exposure device 10 to form an electrostatic latent image corresponding to the image information of the document O. Then, toner is attached to the electrostatic latent image by the developing sleeve 11a of the developing device 11 to which the developing bias is applied from the developing bias power supply 4, and the electrostatic latent image is visualized as a toner image.

When the toner image on the surface of the photosensitive drum 1 reaches the transfer nip between the transfer roller 12 and the photosensitive drum 1, the transfer material 14 is conveyed to the transfer nip at this timing. The toner image is transferred by the transfer roller 12 to which the transfer bias is applied. The transfer material 14 onto which the toner image has been transferred is conveyed to a fixing device (not shown), and the transferred toner image is fixed as a permanently fixed image on the transfer material 14 by heating and pressing by the fixing device and output.

On the other hand, adhered contaminants such as transfer residual toner, external additives, and paper dust remaining on the surface of the photosensitive drum 1 after the transfer of the toner image are removed by the cleaning blade 1 of the cleaning device 13.
3a, and is collected by the pre-exposure device 15
, The surface of the photosensitive drum 1 is neutralized, and the image is repeatedly provided for image formation.

The contaminants which cannot be removed by the cleaning blade 13a adhere to the charging roller 2 disposed downstream of the cleaning device 13 along the rotation direction of the surface of the photosensitive drum 1, and the charging roller 2 If the surface of the charging roller 2 is contaminated, the charging performance deteriorates and the photosensitive drum 1 cannot be charged satisfactorily. Therefore, for example, a cleaning unit for cleaning the charging roller 2 is provided, thereby removing contaminants adhering to the surface of the charging roller 2. You may make it.

The period before the above-described image forming operation is started is a pre-rotation period of the photosensitive drum 1, during which the surface of the photosensitive drum 1 is a non-image forming area surface (non-image area). Therefore, the detection of the AC component of the charging bias is performed in the pre-rotation period corresponding to the non-image forming area surface of the photosensitive drum 1, and the detection of the AC current and the correction of the charging condition (for the charging roller 2). (Charge bias correction) is performed.

As the number of images formed increases and the durability of the photosensitive drum 1 progresses, the transfer residual toner and the like that cannot be scraped off by the cleaning blade 13a adhere to the surface of the charging roller 2 as shown in FIG. In addition, the voltage-current characteristics of the AC component change. Therefore, when the AC bias is controlled at a constant current, the total amount of the AC current flowing between the photosensitive drum 1 and the charging roller 2 can be kept constant, but the photosensitive layer 1a of the photosensitive drum 1 is greatly shaved. The amount of AC discharge current, which is a factor, increases.

Therefore, in this embodiment, the photosensitive drum 1
A predetermined AC voltage and a DC voltage (detection voltage) are applied during the pre-rotation period of the photosensitive drum 1 in order to suppress damage caused by the AC discharge current to The AC current value is determined.

That is, in FIG. 7, a plurality of (two in FIG. 7) applied AC voltages V
pp01 and Vpp02 were set, and a plurality (two in the figure) of applied AC voltages Vpp03 and Vpp04 were also set in the high AC voltage region as a discharge region. Then, a total of four voltages were applied, and the total AC current at that time was obtained, and plotted with the applied voltage Vpp on the horizontal axis and the AC current on the vertical axis.

Then, the equation of the straight line A in the low AC voltage region and the equation of the straight line B in the high AC voltage region are obtained, respectively.
The discharge current is determined by determining the difference between the respective linear expressions.

Conventionally, a sine wave has been used as the voltage waveform of the AC component of the charging bias. This is because when a rectangular wave is used, the magnitude of the total current with respect to the change in the peak-to-peak voltage (amplitude) Vpp of the AC component is too large, and fine control is difficult.
Conversely, when a triangular wave is used, the ratio of the discharge current to the total current amount is small, and an error in calculating the discharge current amount is large. Further, the sine wave shows a continuous sinusoidal change in both the voltage waveform and the current waveform, and does not cause a sudden change, so that the current amount can be detected with high accuracy.

FIG. 2 is a diagram showing a voltage waveform and a current waveform when a sine wave is used. The current waveform has a sine wave whose phase is shifted from the voltage waveform, and the current waveform has almost no phase shifted from the voltage waveform. It has a shape like the addition of a sine wave. The waveform having a phase shifted from that of the voltage waveform is a current flowing through the charging nip portion, and also occurs in a non-discharged region. This is Figure 7
This is the current represented by the straight line B indicated by. On the other hand, a waveform component having almost no phase shift indicates a discharge current, and corresponds to a result obtained by subtracting a straight line B from a straight line A in FIG.

By the way, regarding the slopes of the straight line B and the straight line A in FIG. 7, a larger difference allows a desired discharge current to be obtained with a lower applied AC voltage Vpp. There is also a problem that the change in the amount becomes large. The main problem to be solved in a high humidity environment is image deletion, which is a phenomenon depending on the amount of discharge current.

For this reason, the accuracy of current detection is more important than the accuracy of Vpp of the applied AC voltage. Needless to say, this requires that the accuracy of the current detection circuit be high. In addition, in the graph of FIG. 7, the difference between the straight line A after the start of discharge and the straight line B before the start of discharge has a small difference. It is more accurate if it is not large. This is because the smaller the slope difference, the smaller the change in the discharge current even if the value of Vpp slightly changes.

However, in such a highly humid environment, the surface resistance of the charging roller 2 and the photosensitive drum 1 and the resistance of the air layer near the charging nip are actually reduced, so that the discharge is likely to occur. As shown in FIG.
The slope difference between the straight lines A, C, and D after the start of the discharge and the straight line A before the start of the discharge is large. In FIG. 3, a straight line A is a normal humidity environment, C is a low humidity environment, and D is a high humidity environment.

In a low-humidity environment, since the required Vpp is originally high, it is necessary to control the Vpp of the applied AC voltage to be as small as possible from the viewpoint of scraping the photosensitive drum 1 and fusing the toner. Required. On the other hand, since image deletion is unlikely to occur, if the amount of discharge current is equal to or more than a certain value, not much accuracy is required. From the above, in a low humidity environment, V
It is desirable that the pp accuracy be output with high accuracy, and that the straight line after the start of discharge has a slope difference from the straight line B before the start of discharge as large as possible in the graph of FIG.

This is because a discharge current of a certain value or more can be obtained at a lower value of Vpp. However, actually, in a low humidity environment, the surface resistance of the charging roller 2 and the photosensitive drum 1 and the resistance of the air layer near the charging nip are large, and the discharge is unlikely to occur.
The difference between the slope of the straight line C after the start of discharge and the slope of the straight line B before the start of discharge becomes small.

Therefore, in the present embodiment, as a method for securing a wider allowable range according to all the environments from the low humidity environment to the high humidity environment, the following AC voltage applied from the charging bias power supply 3 to the charging roller 2 is used. The voltage waveform of the voltage component is used.

That is, the voltage waveform used in this embodiment is
Although the waveform is basically a sine wave as shown in FIG. 4, the waveform is such that the output voltage is constant at a certain voltage or more (hereinafter, this voltage is referred to as a threshold voltage). Here, the threshold voltage is a value proportional to the amplitude of the “basic sine wave”, and has a waveform such as “all voltages of x% or more of the basic sine wave are output as x%”. . It has been found by the inventors of the present application that such a waveform has the following advantages (a) to (c).

(A) By making most of the electric waveform a sine wave, other waveforms (waveforms such as a rectangular wave and a trapezoidal wave)
Since both the voltage fluctuation and the current fluctuation are smaller than the above, and the damage to the photosensitive drum 1 can be reduced, it is advantageous for preventing the photosensitive drum 1 from being scraped and the toner from being fused.

(B) A stable discharge current can be ensured by the threshold voltage portion near the peak voltage. In this portion, the discharge current is dominant, and the amount of current flowing through the charging nip between the charging roller 2 and the photosensitive drum 1 is almost zero. Since the value is 0, the discharge current can be measured with high accuracy.

Further, the ratio of the discharge current to the total amount of current flowing through the charging nip between the charging roller 2 and the photosensitive drum 1 becomes larger than a pure sine wave. Therefore, Vpp for securing a constant discharge current can be set low.

(C) The ratio of the discharge current to the total amount of current flowing through the charging nip between the charging roller 2 and the photosensitive drum 1 can be changed by a simple operation of changing the threshold voltage. By changing the threshold voltage according to the resistance value of the roller 2 or the like, it is possible to greatly increase the tolerance such as the environmental tolerance.

FIG. 5 shows that when (threshold voltage / basic sine wave amplitude) = X, the improved waveform voltage Vpp (= plus side) when the value of X is changed to 94% and 85% FIG. 7 is a diagram showing a relationship between the threshold voltage of the waveform (−threshold voltage on the minus side) and the effective value of the voltage in the discharge region of this waveform.

Incidentally, in a low humidity environment, the discharge starting voltage was Vpp of about 1400 V. For simplicity, the absolute values of the positive and negative threshold voltages are the same. The discharge current amount actually depends on the surface resistance value of the charging roller 2 and the photosensitive drum 1, but is proportional to the effective value of the voltage of the discharge unit.

As can be seen from the graph of FIG. 5, when the value of X is reduced, the effective value when compared at the same Vpp is increased. In fact, Vpp is 1600-1
At about 700 V, Vpp required to obtain the same effective value is 5 times smaller than a normal sine wave when X = 94%.
When the voltage is about 00 V and X = 85%, the voltage can be reduced by about 1 KV from a normal sine wave.

Several circuits for generating such waveforms (corresponding to the charging bias power supply 3 in FIG. 1) are conceivable, and for example, there is a circuit as shown in FIG.

That is, the temperature and humidity of the installation environment are measured by the temperature and humidity sensor 31, and the measurement information is input to the control device 30. The control device 30 selects the above-described predetermined X value in the environment based on the measurement information input from the temperature and humidity sensor 31. Then, when it is desired to output the output waveforms of the plus threshold voltage V1 and the minus threshold voltage V2 in FIG. 6, an amplitude = (V1−V) at point P0 from a high voltage (high voltage power supply).
2) When a sine wave of / 2X is applied and a voltage of V1 is applied to the point P1 and a voltage of V2 is applied to the point P2, a waveform (improved waveform) as shown in FIG. 4 can be output to the point P3. This waveform is Vpp = V1-V2, and the waveform between V1 and V2 has a sine wave shape.

Therefore, in the present embodiment, the peak-to-peak voltage Vpp and the angular velocity ω of the AC voltage generated by the charging bias power supply 3 are set to constant positive constants, and V1 = α × (Vpp /
2), V2 = β × (Vpp / 2) (where −1 <α <
When 0, 0 <β <1; α and β are constant values, and when the AC voltage applied from the charging bias power supply 3 to the charging roller 2 is V, when V1 ≦ V ≦ V2, V = ( Vpp / 2) × s
in (ωt) When V <V1, when V = V1 V> V2, an AC voltage V having a waveform satisfying V = V2 is applied from the charging bias power supply 3 to the charging roller 2 under the control of the control device 30. I did it.

In FIG. 6, the voltages V1 and V2 applied to the points P1 and P2 may have the same absolute value, or | V1 | <|
V2 |, and can be determined appropriately. In the case of | V1 | ≠ | V2 |, it is redefined as X = (plus threshold voltage−minus threshold voltage) / sine wave Vpp.

In a high humidity environment, the value of X is 10
A value close to 0% is selected. In this case, the output waveform has a shape very close to a sine wave, and as a result, the effective value of the waveform also becomes close to the effective value of the sine wave. On the other hand, in a low-humidity environment, the value of X is smaller than that in a high-humidity environment. Therefore, at the same Vpp, a waveform having a higher effective value in a discharge region than in a high-humidity environment can be generated. It becomes possible.

As described above, in the present embodiment, in a low humidity environment, a small value of X is selected, and the difference between the straight line B and the straight line A can be made larger than when the high voltage waveform is a pure sine wave. Since a predetermined discharge current can be secured at a lower Vpp, the photosensitive drum 1 can be prevented from being scraped and the life of the photosensitive drum 1 can be extended in all environments from a low humidity environment to a high humidity environment. And good images can be maintained.

<Second Embodiment> In the first embodiment, the AC voltage (vibration) of the charging bias applied from the charging bias power supply to the charging roller is determined based on the installation environment (temperature and humidity) detected by the temperature and humidity sensor. Although the voltage (voltage) waveform is changed, the value of the AC voltage (oscillation voltage) may greatly change due to variations in the manufacturing lot of the charging roller and the photosensitive drum even in the same environment. That is, when the surface resistance of the charging roller or the photosensitive drum is increased, the discharge becomes difficult to occur. Therefore, the slope difference between the straight line B and the straight line A shown in FIG. 7 becomes small, and Vpp for obtaining a predetermined discharge current becomes a large value. And the photosensitive drum is easily scraped.

Therefore, in this embodiment, in the image forming apparatus shown in FIG. 1, a predetermined charging bias is applied from the charging bias power supply 3 to the charging roller 2 during the preceding rotation of the photosensitive drum 1. The control device 30 calculates the discharge current at regular intervals, and the control device 30 calculates X = (positive threshold voltage−) so that Vpp of the voltage waveform for achieving the predetermined discharge current amount is within a certain range. Negative threshold voltage) /
The sine wave Vpp was changed. Other configurations and operations are the same as those of the first embodiment.

That is, in the present embodiment, for example, at the time of initial installation of the image forming apparatus or when the charging roller 2 or the photosensitive drum 1 is replaced, it is necessary to first use the method described in the first embodiment. Vpp for obtaining a discharge current amount is determined. At this time, as the value of X, an initial value determined by the installation environment of the image forming apparatus detected by the temperature and humidity sensor 31 is set.

At this time, if the value of Vpp is too high, the value of X is reduced and the control of Vpp is performed again by the control device 30. Then, by repeating such control, the value of X is determined so that the value of Vpp falls within a certain range.

As described above, in the present embodiment, even if there is a variation in the surface resistance of the charging roller 2 and the photosensitive drum 1 depending on the production lot or the like, the V for obtaining the necessary discharge current is not required.
It is possible to suppress the adverse effects of an excessively large pp value. Therefore, also in the present embodiment, even when the surface resistance of the charging roller 2 and the photosensitive drum 1 changes, the photosensitive drum 1 can be prevented from being scraped and the life of the photosensitive drum 1 can be prolonged. A good image can be maintained.

In the present embodiment, not only when the charging roller 2 and the photosensitive drum 1 are replaced, but also when the value of X is changed at a certain interval of the number of durable sheets, the charging roller 2 and the photosensitive drum 1 are changed. Even if the resistance value of the photosensitive drum 1 changes due to durability, Vpp can be suppressed within a certain range.

[0078]

As described above, according to the present invention, the peak-to-peak voltage Vpp and the angular velocity ω of the AC voltage generated by the charging bias applying means are constant positive constants, and V1 = α ×
(Vpp / 2), V2 = β × (Vpp / 2) (where
−1 <α <0, 0 <β <1; α and β are constant values), and when the AC voltage applied from the charging bias applying unit to the contact charging member is V, when V1 <V <V2, , V = (Vpp / 2) × si
n (ωt) When V <V1, V = V1 When V> V2, an AC voltage V having a waveform that satisfies V = V2 is applied to the contact charging member from the charging bias applying unit, thereby forming the image forming apparatus. Even if the installation environment, contact charging member, and resistance of the image carrier change, the image carrier can be prevented from being scraped, the durable life of the image carrier can be extended, and good images can be maintained. it can.

[Brief description of the drawings]

FIG. 1 is a schematic configuration diagram illustrating an image forming apparatus according to a first embodiment of the present invention.

FIG. 2 is a diagram showing a voltage waveform and a current waveform when a sine wave is used as an AC component of a charging bias.

FIG. 3 is a diagram showing a change in a current value after a discharge start voltage after applying an AC voltage to a charging roller in a normal humidity environment, a low humidity environment, and a high humidity environment.

FIG. 4 is a diagram showing a voltage waveform and a current waveform of an AC component of a charging bias according to the first embodiment of the present invention.

FIG. 5 is a diagram illustrating a relationship between a voltage Vpp of a waveform of an AC component and an effective value of a voltage in a discharge region of the waveform according to the first embodiment of the present invention.

FIG. 6 is a diagram showing a circuit of a charging bias power supply for generating a waveform of an AC component according to the first embodiment of the present invention.

FIG. 7 is a diagram illustrating a relationship between a peak-to-peak voltage (Vpp) of an AC voltage applied to a charging roller and a total current flowing at this time.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 photosensitive drum (image carrier) 2 charging roller (contact charging member) 3 charging bias power supply (charging bias applying unit) 10 exposure device 11 developing device 11 a developing sleeve 12 transfer roller 13 cleaning device 30 control device 31 humidity sensor

Claims (3)

[Claims] 1. A rotatable image carrier, and a contact charging member to which at least an AC voltage is applied from a charging bias applying means and which contacts the surface of the image carrier and charges the image carrier to a predetermined potential. Wherein the peak-to-peak voltage Vpp and the angular velocity ω of the AC voltage generated by the charging bias applying unit are constant positive constants, and V1
= Α × (Vpp / 2), V2 = β × (Vpp / 2) (where −1 <α <0, 0 <β <1; α and β are constant values). Assuming that an AC voltage applied from the means to the contact charging member is V, when V1 ≦ V ≦ V2, V = (Vpp / 2) × s
in (ωt) When V <V1, V = V1 When V> V2, an AC voltage V having a waveform satisfying V = V2 is applied from the charging bias applying unit to the contact charging member. Image forming apparatus. 2. The image forming apparatus according to claim 1, wherein the values of V1 and V2 are changed according to an installation environment of the image forming apparatus. 3. The image forming apparatus according to claim 1, wherein the values of V1 and V2 are changed according to surface resistance values of the contact charging member and the image carrier.