JPH10232534A - Image forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent image defects such as 'sands' and to reduce the shaving quantity of a body to be electrified, in an image forming device for applying a voltage obtained by superimposing DC and AC voltages to an electrifying roller arranged in contact with the surface of a photoreceptor drum. SOLUTION: When the endurance of the photoreceptive drum becomes worse, an applied AC voltage Vppa becomes high when the quantity of AC current Iac flowing from the electrifying roller to the photoreceptive drum is in attempt to be controlled for Iac INT with a constant current and an AC discharge current quantity Idis becomes high according to the applied AC voltage Vppa . Therefore, the shaving quantity of the drum is increased. At this time, an applied AC voltage Vpp drops to Vppb and the AC discharge current quantity Idis is in the same degree as that in the initial stage of endurance, to decrease the shaving quantity of the drum. But, when the AC discharge current quantity Idis is small, the leveling effect of the surface of the photoreceptor drum is reduced and the 'sands' are caused, so that the applied AC voltage Vpp is put within a prescribed set range and the AC discharge current quantity Idis is set within the range having excellent leveling effect and without increasing the shaving quantity of the drum.

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 an electrophotographic apparatus (for example, a copying machine or a laser beam printer) or an electrostatic recording apparatus.

[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 arranged in a non-contact manner so that its discharge opening faces the member to be charged, and the charged surface is charged to a predetermined potential of a predetermined polarity by exposing the charged surface to a corona current from the discharge opening. To be processed. According to this corona discharge device, there are problems such as the need for a high-voltage power supply and the generation of a large amount of ozone.

On the other hand, a contact-type charging device in which a charging member to which a voltage is applied is brought into contact with a surface to be charged to perform a charging process on the surface to be charged can reduce the power supply voltage and generate a small amount of ozone. Therefore, it has attracted attention as a new charging processing means, and has been put to practical use.

In a contact type charging device, a charging member is
A “DC charging method” in which only a DC voltage _VDC is applied as a charging bias to charge an object to be charged, and an “AC” in which an _AC voltage VAC is superimposed on a DC voltage _VDC and applied to charge the object to be charged. Charging method ".

In either method, the charged surface is charged to a predetermined polarity and a predetermined potential by a contact charging member to which a bias voltage is applied.

Regarding this AC charging system, the present applicant has
The previous proposal (Japanese Patent Publication No. 3-52058 (JP-A-63-
149668). In the AC charging method, the charging member includes a contact area that contacts the member to be charged, and a separation surface area in which the distance to the surface to be charged increases downstream of the contact region in the moving direction of the member to be charged. A DC voltage component and an AC voltage (an AC voltage having a peak-to-peak voltage component that is twice or more the applied voltage value of the charging member when the charging of the member to be charged is started by applying the DC voltage to the charging member) Is applied between the member to be charged and the charging member. Thereby, an oscillating electric field is formed between the surface to be charged and the separated surface region of the charging member, the AC component equalizes the unevenness of the charging potential of the surface to be charged, and the DC component brings the surface to be charged to a predetermined potential. Since the convergence is achieved, there is an advantage that the surface to be charged can be uniformly and stably charged without charging unevenness as a whole.

[0007]

However, as the number of image formations increases, that is, as the durability increases, the surface (outer peripheral surface) of a member to be charged, for example, a photoreceptor, of the image forming apparatus is cleaned by a cleaning blade or a developer. As a result, the thickness of the photosensitive layer (thick film of the photosensitive member) decreases.

Particularly, in the above-described AC charging method, a DC voltage and an AC voltage are superimposed and applied to a charging member,
It has the characteristic of having excellent uniformity of charging because it has the effect of leveling the uneven charging caused by the alternating current component when it is charged only by the direct current component. The thickness (film thickness of the photoconductor) is significantly reduced. It is known that the cause is that the discharge current of the AC component, which is superimposed on the discharge current of the DC component, damages the surface of the photoconductor, and the surface of the photoconductor is easily shaved by the cleaning blade.

For example, as shown in FIGS. 1A and 1B, by changing the pressing force between the charging member and the photosensitive member (0.2 kg and 1.6 kg, respectively), the discharge regions a,
The area of b and the area of the contact portion (charging nip portion N) can be changed, thereby changing the voltage-current characteristics of the AC component as shown in FIG. However, taking advantage of this fact, as shown in FIG. 3, the amount of scraping of the photoreceptor due to durability (the amount of scraping of the member to be charged in FIG. 3) is examined by changing the total amount of AC current and the amount of AC discharge current. It can be seen that the AC discharge current amount has a strong correlation with the shaving amount of the photoconductor. However, when the amount of the AC discharge current is insufficient, an image due to non-uniform discharge (a strong or weak discharge region exists locally) called a "sand" image shown in a black portion in FIG. 5 appears. This is because the AC discharge current is insufficient and the effect of smoothing the surface potential of the photoconductor becomes insufficient.

When the shaving amount of the photoconductor is large, the thickness of the photoconductor becomes thin while the number of endurable sheets is small. As the thickness of the photoreceptor becomes thinner, minute scratches on the photoreceptor become more visible on the output image, and the leakage from the charging member to the photoreceptor substrate becomes more likely. This has a direct effect on the life of the photoconductor, such as being impossible.

When a developer or the like adheres to the charging member due to durability, the voltage-current characteristic of the AC component flowing through the charging member becomes as shown in FIG. It will be durable with the discharge current flowing. This will promote deterioration of the photoconductor.

Therefore, the present invention prevents image defects such as "sand" by controlling the amount of AC discharge current, which has a strong correlation with the amount of scraping of a member to be charged (for example, a photosensitive member), within a predetermined set range. And reduce the amount of scraping of the charged object,
An object of the present invention is to provide an image forming apparatus capable of forming a good image over a long period of time.

[0013]

According to a first aspect of the present invention, there is provided an object to be charged having a surface to be charged on which an electrostatic latent image is formed, and a surface to be charged which is arranged in contact with the surface to be charged. An image forming apparatus comprising: a charging member that forms a charging nip portion and a minute gap between the charging member, and charging the charged surface by applying a charging bias in which a DC voltage and an AC voltage are superimposed on the charging member. The total AC current flowing between the charging member and the member to be charged by applying the charging bias is divided into an AC current flowing through the charging nip portion and an AC discharge current flowing through the minute gap. And a control unit for detecting an amount of the AC discharge current and controlling an AC voltage or an AC current applied to the charging member so that the AC discharge current falls within a predetermined set range.

In the present invention according to claim 2, the control means obtains a voltage-current proportional expression of an AC component flowing through the charging nip by detecting the amount of the AC current with respect to the AC voltage for detection. The AC discharge current amount is divided by subtracting the amount of AC current flowing through the charging nip portion during image formation calculated at the time of image formation from the total AC current value for a predetermined AC voltage at the time of formation.

In the invention according to claim 3, the control means applies the detection AC voltage to the charging member when the charging member corresponds to a non-image forming area of the member to be charged. Detecting an AC current flowing through the charging member at this time, and performing control under a charging condition corresponding to the AC discharge current amount when the charging member corresponds to an image forming area of the member to be charged. It is characterized by.

In the present invention according to claim 4, the AC voltage for detection is an AC voltage at which an AC current starts to flow in a discharge region generated in a minute gap formed between the charging member and the member to be charged. The value is lower than the threshold value.

According to a fifth aspect of the present invention, the charging bias applied to the charging member when the charging member corresponds to the image forming area of the member to be charged is a constant DC voltage and the detected AC voltage. An AC voltage controlled according to a discharge current amount is superimposed and applied.

According to a sixth aspect of the present invention, the charging member is a conductive charging member having a high resistance layer at least on a surface layer.

[Operation] The main operation (operation according to claim 1) based on the above configuration is as follows.

By making the AC discharge current fall within a predetermined set range, it is possible to prevent "sandy ground" caused by a small amount of the AC discharge current and shaving of a charged body caused by a large amount of the AC discharge current. it can.

[0021]

Embodiments of the present invention will be described below with reference to the drawings.

<Embodiment 1> An image forming apparatus according to the present invention will be described using a copying machine shown in FIG. 6 as an example. FIG. 3 is a longitudinal sectional view of the transfer material 14 taken along the transport direction.

The image forming apparatus shown in FIG. 1 includes a drum type electrophotographic photosensitive member (hereinafter, referred to as “photosensitive drum”) 1. The photosensitive drum 1 has, as basic constituent layers, a conductive base layer 1b of, for example, aluminum and a photoconductive layer (photosensitive layer) 1a formed on the outer peripheral surface thereof. Photosensitive drum 1
Is rotationally driven by a driving means (not shown) at a predetermined process speed (peripheral speed) about the support shaft 1d in the direction of arrow R1.

Above the photosensitive drum 1, a charging member is disposed. The charging member of the first embodiment includes a cored bar 2c, a conductive layer 2b formed on the outer peripheral surface thereof, and a high resistance layer 2a provided on the outer peripheral surface of the conductive layer 2b, and is configured as a roller as a whole. (Hereinafter referred to as “charging roller 2”). The charging roller 2 is rotatably supported at both ends of a cored bar 2 c by bearing members (not shown), and is arranged in parallel with the photosensitive drum 1. The charging roller 2 is urged by a pressing member (not shown) toward the photosensitive drum 1 via a bearing member, whereby the surface of the charging roller 2 is pressed against the surface of the photosensitive drum 1 with a predetermined pressing force, and A charging nip N is formed between the photosensitive drum 1 and the surface of the drum 1, and the photosensitive drum 1 is driven to rotate in the direction of the arrow R <b> 2 with the rotation of the photosensitive drum 1 in the direction of the arrow R <b> 1.

A predetermined charging bias is applied to the metal core 2c of the charging roller 2 from a charging bias power source 3 via a sliding contact 3a. This charging bias is obtained by superimposing a DC voltage and an AC voltage. As a result, the surface of the photosensitive drum 1 is primarily charged to a predetermined polarity and a predetermined potential.

The surface of the photosensitive drum 1 that has been uniformly charged by the charging member 2 is then exposed by target exposure information L (exposure slit exposure of an original image,
(Eg, laser beam scanning exposure), an electrostatic latent image corresponding to the target image information is formed. In the first embodiment, the exposing means 10 is a publicly known document table fixed-
This is an optical system moving type original image forming slit exposure unit. The exposure means 10 includes a fixed platen glass 20 on which the document O is placed with the document surface facing down, a pressing plate 21 for pressing the document O against the platen glass 20 from above, and a document O
Original illumination lamp (exposure lamp) that illuminates the original surface
2. a slit plate 23 having a slit through which light reflected from the image surface passes;
Second mirror 25, third mirror 26, imaging lens 2
7, a fixed mirror 28 and the like. The document illumination lamp 22, the slit plate 23, and the first mirror 24 move the lower surface of the document table glass 20 from one end to the other at a predetermined speed V.
And the second and third mirrors 25 and 26 are V / 2
, The downward original surface on the original platen glass 20 is scanned from one end side to the other end side, and the original image is subjected to image forming slit exposure on the surface of the photosensitive drum 1. As a result, an electrostatic latent image corresponding to the document image is formed on the photosensitive drum 1.

The electrostatic latent image formed on the photosensitive drum 1 is
The developing means sequentially develops (visualizes) a toner image by the developing means 11. In the first embodiment, the developing unit 11 is a developing device using an AC electric field, and includes a roller-shaped developing roller 11a that rotates in the direction of arrow R11 as a developer (toner) carrier. Developing roller 11
“a” is connected to the developing bias power supply 4 and a developing bias is applied. The developing roller 11a is a photosensitive drum 1
A developing bias including at least an AC component is applied from a developing bias power supply 4 to cause the developer (toner) to adhere to the electrostatic latent image formed on the surface of the photosensitive drum 1. Develop as a toner image.

This toner image is transferred to a transfer material 14 such as paper by a transfer roller (transfer means) 12. The transfer roller 12 is disposed in contact with the surface of the photosensitive drum 1 to form a transfer nip portion between the transfer roller 12 and the surface of the photosensitive drum 1, and the transfer roller 12 rotates in the direction of the arrow R1 with the rotation of the photosensitive drum 1 in the direction of arrow R1.
It is driven and rotated in 12 directions. The transfer roller 12 is connected to the transfer bias power supply 5. The transfer material 14 is fed in a direction indicated by an arrow K1 by a feeding means (not shown), and is supplied to a transfer nip portion so as to be synchronized with a toner image on the surface of the photosensitive drum 1. The toner image on the surface of the photosensitive drum 1 is transferred to the transfer roller 12 by applying a transfer bias to the transfer roller 12 from the transfer bias power supply 5 and charging the back surface of the transfer material 14 to a polarity opposite to that of the toner on the photosensitive drum 1.
4 is transferred to the surface.

The transfer material 14 to which the toner image has been transferred is
The toner image is separated from the surface of the photosensitive drum 1 and conveyed to a fixing unit (not shown), where the unfixed toner image on the surface is heated and pressurized and fixed, and then discharged out of the image forming apparatus main body as an image formed product. . In the case where a toner image is also formed on the back surface of the transfer material 14, the transfer material 14 having the toner image formed on the front surface is not discharged and is again transferred to the transfer nip portion by a re-conveying means (not shown). The toner image is fed, subjected to transfer and fixing of the toner image on the back surface in the same manner as the front surface, and then discharged outside the image forming apparatus main body.

After the transfer of the toner image, the contaminants such as transfer residual toner, external additives, and paper dust remaining on the surface of the photosensitive drum 1 are removed by the cleaning blade 13 of the cleaning device 13.
a, and the charge is removed by the charge removing exposure device 15 to be used for the next image formation. When the contaminants that 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 contaminate the charging roller 2, the contaminants are charged. Since the performance is deteriorated and the photosensitive drum 1 cannot be favorably charged, for example,
A cleaning unit for cleaning the charging roller 2 is provided to remove contaminants adhering to the surface of the charging roller 2.

In the image forming apparatus shown in FIG. 6, the charging bias power source 3, the developing bias power source 4, and the transfer bias power source 5 are connected to the main control circuit unit 100.
The main control circuit 100 controls the magnitudes of the voltages of the charging bias power supply 3, the developing bias power supply 4, and the transfer bias power supply 5 and the application timing.

FIG. 8 is a timing chart of the operation of the above-described image forming apparatus. Note that, in the example shown in the figure, a case where image formation is continuously performed on two transfer materials 14 is shown.

The image forming apparatus which has been in a standby state by turning on a main switch (not shown) rotates the photosensitive drum 1 (hereinafter simply referred to as "drum" in FIG. 3) based on an image formation (print) start signal. Is started to start the pre-rotation period. Simultaneously with the start of the rotation of the photosensitive drum 1, the neutralizing exposure device 15 is turned on to start the neutralizing exposure, and in the section A1, the surface of the photosensitive drum 1 is neutralized for one or more rounds.

Next, for the charging roller (charging member) 2, the primary charging bias in which the AC voltage is superimposed on the DC voltage which is the primary charging bias is turned ON.

This primary charging bias is first applied to section B
In step 1, the constant voltage control is performed. During this time, the amount of the AC current component is detected, and then a charging bias is applied to the charging roller 2 under charging conditions corresponding to the detected amount of the AC current component.

The period before the image formation starts is the 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 above-described AC component detection is performed in the section B1 of 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 primary charging condition correction (1 for the charging roller 2) are performed. Next charging bias correction) is performed. The correction of the charging condition will be described later in detail.

When the constant voltage control is started for the charging roller 2 under the primary correction condition, the image formation of the first transfer material 14 is performed by the image exposure L (exposure of the image forming slit of the original image).

The charging roller 2 corresponds to the surface of the image forming area (image forming area) of the photosensitive drum 1 and charges the surface of the photosensitive drum 1 under a corrected charging condition.

The surface of the drum 1 between the sheets is a non-image forming area surface between the time when the image formation on the first transfer material 14 is completed and the time when the image formation on the next second transfer material 14 is started. In the first embodiment, the above-described detection of the alternating current of the charging roller 2 and the correction of the charging condition are performed again even between the sheets.

Even when three or more continuous images are formed, the sequence of detecting the alternating current of the charging roller 2 and correcting the charging conditions is similarly performed between each sheet.

When the image formation of the transfer material 14 of the last sheet is completed, the photosensitive drum 1 enters a post-rotation period, and in the section A2 of the subsequent rotation period, a charge elimination exposure for one or more rotations of the photosensitive drum 1 is performed to eliminate the charge. Then, the rotation of the photosensitive drum 1 and the charge removal exposure are turned off, and the image forming apparatus enters a standby state until the next image formation start signal is input.

Next, referring to the flowchart of FIG.
The method of correcting the charging condition will be described in detail.

When a superimposed voltage of a DC voltage and an AC voltage is applied as a charging bias to a charging roller 2 as a contact charging member, an electric field is temporally generated between the photosensitive drum 1 and the charging roller 2 based on the AC component. The charging and reverse charging of the photosensitive drum 1 are repeated.

To perform charging / reverse charging, the peak voltage V _PP of the applied AC voltage is the discharge starting voltage V _th (charging start voltage: when a DC voltage is applied to the charging roller 2, the charging of the photosensitive drum 1 is stopped). Is required to be at least twice as large as the applied voltage value of the charging roller 2 at the start of
_{If PP} is taken, the unevenness of local charging on the surface of the photosensitive drum 1 is made uniform by the AC electric field, and converges to a surface potential close to the applied DC voltage value. In this charging method, the same discharge as when a DC voltage is applied is repeated in proportion to the AC frequency.
Generally, the current amount is very large, and the damage to the photosensitive drum 1 is large. The surface potential of the photosensitive drum 1, regardless of the environment becomes more than a certain current value I _th, converges to a value close to a DC voltage applied.

When the AC voltage value is constant, the surface potential of the photosensitive drum 1 changes depending on the environment. Therefore, when a DC voltage is applied to the AC voltage, the AC component is controlled by a constant current, and the DC component is a constant voltage. It is often controlled to be

When the durability of the photosensitive drum 1 progresses as the number of formed images increases, the cleaning blade 13
When the transfer residual toner and the like that have not been scraped off in step a adhere to the surface of the charging roller 2, the voltage-current characteristics of the AC component change as shown in FIG. 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 largely scraped. The amount of AC discharge current, which is a factor, increases.

Therefore, in the first embodiment, in order to suppress the damage to the photosensitive drum 1 due to the AC discharge current, a predetermined AC voltage and DC voltage (detection voltage) are applied, and the AC current value is detected at that time. Then, a method of determining an AC current value during image formation was employed. This will be described below.

As shown in FIGS. 1 (a) and 1 (b), there is a contacting portion (charging nip portion N) and a minute gap between the surface of the photosensitive drum 1 and the surface of the charging roller 2. Discharge areas a and b where discharge occurs when a certain potential difference is exceeded
And exists. It is known that the surface of the photosensitive drum 1 is charged in the discharge areas a and b and is damaged.

An AC current caused by an AC bias applied to the charging roller 2 also flows through the two discharge areas a and b.
Further, it can be explained from FIG. 2 and the like that each AC current flowing through the two discharge regions a and b is separated from the voltage-current characteristics of the AC component.

The current control method according to the first embodiment will be described below with reference to the flowchart of FIG. 9 and the voltage-current characteristics of FIG.

As shown in FIG. 10, when the applied AC voltage V _PP 0 for detection is set in the low AC voltage region, the contamination of the surface of the charging roller 2 differs between the initial stage and the endurance.
Different AC current amount I _ac for the same applied AC voltage V _PP 0
0, I _ac 1 is detected. In the low AC voltage region, the AC current flows only in the contact portion (charging nip portion N) between the photosensitive drum 1 and the charging roller 2, so that the applied AC voltage V _PP and the amount of AC current I _ac are in a proportional relationship. When a certain AC voltage value V _PPth or more is applied, an AC discharge current starts to flow in discharge regions a and b in a minute gap between the photosensitive drum 1 and the charging roller 2. As described above, the AC discharge current is indispensable for equalizing the charging unevenness on the surface of the photosensitive drum 1, but has a disadvantage that it damages the surface of the photosensitive drum 1 and increases the amount of scraping of the photosensitive drum 1. is there.

A minimum applied AC voltage V _PP INT which is called a “sandy” image and has no non-uniform discharge image and an AC current amount I _ac INT flowing through the charging nip N at this time are detected. The AC current amount I _ac 0 with respect to the initially set value of the AC voltage V _PP 0 is detected, and a voltage-current relational expression of an AC component flowing through the charging nip portion N, I _ac N (V _PP ) = (I _ac 0) / V _PP 0) × V _PP .

Since the amount of AC current I _ac depends on the applied AC voltage V _PP , the amount of AC current I _ac is
_As a function of _PP , for example, it is expressed as I _ac N (V _PP ).

The difference between the AC current _Iac INT and the AC current _IacN is an AC discharge current _Idis .

From these, the minimum AC discharge current _Idisb required for charging uniformity is determined. An upper limit I _disa is set in order to allow the necessary minimum amount of AC discharge current I _disb to have some margin, so that the AC discharge current amount I _dis always falls between these two values. The step amount ΔV _PP of V _PP between these two is set (S 1 in FIG. 9), and control is started (S 2). As the endurance progresses, when the voltage-current characteristic of the AC component changes as shown in FIG. 10 (the slope decreases), the applied AC voltage V _PP 0
Since the amount of AC current _Iac with respect to _Iac changes like _Iac1 , the value is detected (S3), and the following linear equation _IacN ( _VPP ) = ( _Iac1 / _VPP0 ) × _VPP Obtain (S4).

In the case of constant current control after the endurance, the amount of AC current I _ac a and the applied AC voltage also detect V _PP a (S5),
The AC discharge current amount I _dis is obtained from the following equation: I _dis = I _ac a (V _Ppa ) −I _ac N (V _Ppa ) (S 6). If I _dis obtained at this time is I _dis > I _dis a (S6), (V _Ppa −ΔV _PP ) is calculated as I _dis ≦ I _dis
It repeats n times until it becomes _dis a (S7). And I _dis
When _{≤I dis} a, if I _dis > I _dis b (S8), the control is terminated (S9), and if I _{dis ≤I dis} b, (V _PP a + ΔV _PP ) is changed to I _dis > I _dis b Repeat n times until

That is, while the total AC current I _ac a flowing through the charging roller 2 is kept constant, the AC discharge current I
_{Since dis} increases, the amount of drum scraping per unit number of the drums, which is set to the constant current control as it is, increases. Therefore,
The applied AC voltage is changed from V _PP a to V _PP b (= V _PP a−nΔV, n) so that the amount of AC discharge current I _dis becomes substantially equal to the initial value.
Is an integer) to obtain the AC discharge current I
_dis can be kept to a required minimum constant value.

Second Embodiment The basic structure of an image forming apparatus according to the second embodiment is the same as that of the first embodiment. In the last step (S7, S10) of the control flow according to the first embodiment, the applied AC voltage V
_{In the} second embodiment, the amount of voltage change (step amount) ΔV _PP for one detection is set according to the number of endurable sheets (for example, AC (ΔV _PP = 50 V from the initial stage of the large change in the voltage-current characteristics of the bias to 100 sheets, and ΔV _PP = 5 V thereafter) so as to approach the optimal AC discharge current amount with as few loops as possible. According to the second embodiment, the amount of alternating current flowing to detect the amount of alternating current _Iac can be reduced, and the amount of the alternating current flowing to approach the detected current and the optimal amount of alternating discharge current can minimize the scraping of the photosensitive drum 1. Can be

Third Embodiment A basic configuration of an image forming apparatus according to a second embodiment is the same as that of the first embodiment.

The voltage-current characteristic of the charging roller 2 with respect to the AC component (impedance change of the charging roller due to contamination of the surface of the charging roller 2) is 100
It rapidly changes to about the number of sheets, and then becomes extremely small when the charging roller 2 is periodically cleaned, for example, once every 50 sheets. Therefore, for the initial 100 sheets, the amount of AC discharge current is detected once before the previous rotation or once between the previous rotation and the paper.
After the 00 sheet is detected, for example, every 500 sheets. By doing so, the amount of AC current flowing for detection can be suppressed to a necessary minimum, and the scraping of the photosensitive drum 1 by the detection current can be minimized.

Fourth Embodiment In order to keep a stable image while minimizing damage to the photosensitive drum 1, the above-described first, second, and third embodiments may be appropriately combined. That is, the AC voltage change amount (step amount) ΔV _PP is set according to the number of endurance sheets, and the number of times of detection current insertion is changed. For example, the AC voltage change amount (step amount) ΔV _PP is set to 50 V for the first 100 sheets, and the AC discharge current amount is always detected once in the previous rotation. By setting ΔV _PP to 5 V and detecting the amount of alternating current discharge current once per 500 sheets, the amount of alternating current flowing for detection is kept to a minimum and the photosensitive drum 1 is detected by the detected alternating current or the like. Drum scraping can be minimized.

<Embodiment 5> When the applied AC voltage V _PP 0 detected in the low AC voltage region is one point, this 1
A linear equation is determined based on the point, that is, a proportional equation for determining the amount of AC current _Iac flowing through the charging nip N between the photosensitive drum 1 and the transfer roller 2 with respect to the applied AC voltage V _PP during image formation is determined. If this occurs, the error of the straight line may increase, causing an excessive AC discharge current _Idis to flow or a "sandy ground" due to a shortage of the AC discharge current _Idis.
An adverse effect such as generation of an image may occur.

Therefore, in the fifth embodiment, FIG.
As shown in FIG. 1, in order to correct this error, a plurality (five in the figure) of applied AC voltages V _PP01 , V _PP 02, V _PP 03, V _PP for detection in the low AC voltage region. 04, V _PP
05 is set to improve the accuracy of the linear expression so that the amount of AC current _Iac flowing through the charging nip N by the AC voltage applied to the image formation can be accurately calculated. As a result, the accuracy of the AC discharge current _Idis can be improved.

As described above, the charging roller 2 as the charging member used in the first to fifth embodiments is a conductive charging member having at least a high resistance layer 2a on the surface layer. Leakage due to pinholes or scratches on the surface of the photosensitive drum 1 can be prevented. The charging roller 2 may be driven to rotate by the photosensitive drum 1 as a member to be charged, which is driven to move in a plane, or may be non-rotating. Alternatively, the motor may be positively driven to rotate at a predetermined peripheral speed in the forward or reverse direction. Further, the layer configuration of the charging roller 2 is the same as that of the core metal 2 described above.
However, the present invention is not limited to the three-layer structure of c, the conductive layer 2b, and the high-resistance layer 2a.

The charging member can be configured in a blade-like, block-like, rod-like, belt-like, or the like, in addition to the above-described roller-like charging roller 2.

FIG. 7A schematically shows a longitudinal sectional view of a blade-shaped charging member (hereinafter referred to as “charging blade”) 2A. In this case, the contact direction of the charging blade 2A contacting the surface of the photosensitive drum 1 may be either the forward direction or the reverse direction with respect to the moving direction of the surface of the photosensitive drum 1 (the direction of arrow R1). Next, FIG. 7B schematically shows a vertical cross section of a block-shaped charging member (hereinafter referred to as “charging block”) 2B. 7A and 7B, reference numeral 2f denotes a conductive core member, 2e denotes a conductive layer, and 2d denotes a high resistance layer.

The charging blade 2A and the charging block AB described above are different from the rotatable charging roller 2 described above in that the charging blade 2A and the charging block AB do not have the power supply sliding contact 3a for applying a voltage to the metal core 2c, and thus can be connected to the metal core member 2fc. The lead wire leading to the power supply 3 can be directly connected, so that there is an advantage that electric noise that may be generated from the power supply sliding contact 3a is eliminated, the spacer is reduced, and the surface of the photosensitive drum 1 is cleaned. It can also be used as a blade.

[0068]

As described above, according to the present invention,
For the charging member placed in contact with the surface to be charged of the member to be charged,
In an image forming apparatus that applies a voltage obtained by superimposing a DC voltage and an AC voltage as a charging bias, by controlling an AC discharge current amount having a strong correlation with a shaving amount of a member to be charged within a predetermined setting range, It is an object of the present invention to provide an image forming apparatus that forms a good image for a long period of time by preventing image defects such as “sand” and reducing the amount of shaving of a member to be charged.

[Brief description of the drawings]

FIGS. 1A and 1B are diagrams showing changes in a charging nip portion and a discharge area due to a difference in pressure of a charging roller against a photosensitive drum.

FIG. 2 is a diagram illustrating a relationship between a pressing force between a charging member and a member to be charged and a voltage-current characteristic of an AC component.

FIG. 3 is a diagram showing the relationship between the amount of AC discharge current and the amount of scraping of a member to be charged.

FIG. 4 is a diagram showing a change in voltage-current characteristics of an AC component with durability.

FIG. 5 is a diagram showing a relationship between a voltage-current characteristic of an AC component and a sand image.

FIG. 6 is a longitudinal sectional view illustrating a schematic configuration of an image forming apparatus.

FIG. 7A is a blade-shaped charging member (charging blade).
FIG. 2 is a longitudinal sectional view showing a schematic configuration of FIG. (B) A longitudinal sectional view showing a schematic configuration of a block-shaped charging member (charging block).

FIG. 8 is a timing chart of the image forming apparatus.

FIG. 9 is a flowchart illustrating a control method of the image forming apparatus.

FIG. 10 is a diagram illustrating control of the amount of AC discharge current in the first embodiment.

FIG. 11 is a diagram showing a state in which a plurality of detection voltages V _PP 0 are set.

[Explanation of symbols]

REFERENCE SIGNS LIST 1 charged member (photosensitive drum) 2 charging member (charging roller) 2A charging member (charging blade) 2B charging member (charging block) 2a, 2d high-resistance layer 2b, 2e conductive layer 2c, 2f core metal 3 charging bias power source 4 Developing bias power supply 5 Transfer bias power supply 11 Developing device 12 Terrorism 13 Cleaning device 14 Transfer material 15 Static elimination exposure device 100 Control means (Control device) a, b Discharge area N Charging nip portion _Iac AC current amount _{IacN Flow} through charging nip portion AC current I _dis AC discharge current V _PP AC voltage (applied AC voltage)

Claims (6)

[Claims] 1. A charging member having a charged surface on which an electrostatic latent image is formed, and a charging device arranged in contact with the charged surface to form a charging nip portion and a minute gap between the charged surface and the charged surface. An image forming apparatus that applies a charging bias in which a DC voltage and an AC voltage are superimposed on the charging member to charge the charged surface, wherein the charging member applies the charging bias to the charged member. The total amount of AC current flowing between the body and the body is divided into the amount of AC current flowing through the charging nip portion and the amount of AC discharge current flowing through the minute gap, and the amount of AC discharge current is detected. An image forming apparatus comprising: a control unit configured to control an AC voltage or an AC current amount applied to the charging member so as to fall within a set range. 2. The control means determines a voltage-current proportional expression of an AC component flowing through the charging nip by detecting the amount of the AC current with respect to the AC voltage for detection,
The AC discharge current amount is divided by subtracting the AC current amount flowing through the charging nip portion at the time of image formation calculated from the proportional equation from the total AC current value for a predetermined AC voltage at the time of image formation. 2. The image forming apparatus according to 1. 3. The control means applies the detection AC voltage to the charging member when the charging member corresponds to a non-image forming area of the member to be charged. 3. An AC current flowing through the charging member is detected, and when the charging member corresponds to an image forming area of the member to be charged, control is performed under charging conditions according to the amount of the AC discharge current. The image forming apparatus as described in the above. 4. The AC voltage for detection has a value lower than a threshold value of an AC voltage at which an AC current starts flowing in a discharge region generated in a minute gap formed between the charging member and the member to be charged. The image forming apparatus according to claim 2, wherein: 5. A charging bias applied to the charging member when the charging member corresponds to an image forming area of the member to be charged is controlled according to a constant DC voltage and the detected amount of AC discharge current. The image forming apparatus according to claim 3, wherein the applied AC voltage is applied in a superimposed manner. 6. The charging member according to claim 1, wherein the charging member is a conductive charging member having a high resistance layer at least on a surface layer.
An image forming apparatus according to any one of the preceding claims.