JP3576738B2 - Image forming device - Google Patents

Description

[0001]
TECHNICAL 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]
[Prior art]
2. Description of the Related Art Conventionally, in an image forming apparatus such as an electrophotographic apparatus or an electrostatic recording apparatus, a corona discharge device has been used as a device for charging a surface (charged surface) of an image carrier (charged body) such as a photoconductor or a dielectric. It has been widely used. In this method, a corona discharge device is disposed in a non-contact manner with its discharge opening facing 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.
[0003]
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. It has advantages, and is attracting attention as a new charging means, and has been put to practical use.
[0004]
The contact-type charging device has a DC voltage V_DC"DC charging method" in which only the object to be charged is charged by applying only_DCAC voltage V_ACIs superimposed and applied to charge an object to be charged.
[0005]
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.
[0006]
Regarding this AC charging method, the present applicant has made the above-mentioned proposal (Japanese Patent Publication No. 3-52058 (JP-A-63-149668)). A contact area in contact with the body, and a separation area in which the distance to the surface to be charged becomes larger downstream of the contact area in the moving direction of the body to be charged. An AC voltage having a peak-to-peak voltage component that is at least twice the applied voltage value of the charging member when the charging of the member to be charged is started by applying a DC voltage) between the member to be charged and the charging member As a result, an oscillating electric field is formed between the surface to be charged and the separated surface region of the charging member, and the AC component equalizes the unevenness of the charging potential of the surface to be charged, and the DC component causes the surface to be charged to have a predetermined surface. In order to converge to the potential, the surface to be charged There is an advantage that it is possible to charge evenly uniformly and stably.
[0007]
[Problems to be solved by the invention]
However, as the number of image formations increases, that is, as the durability increases, the surface (outer peripheral surface) of the charged member of the image forming apparatus, for example, the photosensitive member, is scraped by a cleaning blade or a developer to remove the thickness of the photosensitive layer (photosensitive member). Thick film).
[0008]
In particular, in the AC charging system described above, by applying a DC voltage and an AC voltage in a superimposed manner to the charging member, there is an effect of equalizing the charging unevenness caused when only the DC component is charged by the AC component. While having the feature of being excellent in uniformity, the thickness of the photosensitive layer (the thickness of the photosensitive member) is significantly reduced due to durability as compared with the DC charging method. It is known that the cause is that the discharge current of the AC component 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.
[0009]
For example, as shown in FIGS. 1A and 1B, by changing the pressing force between the charging member and the photoconductor (0.2 kg and 1.6 kg, respectively), the area of the discharge areas a and b and the contact area are changed. The area of the 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 total amount of AC current and the amount of AC discharge current are changed, and 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. 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 AC discharge current is insufficient, an image due to non-uniform discharge (a strong or weak discharge region exists locally) called a "sand background" 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.
[0010]
When the shaving amount of the photoconductor is large, the thickness of the photoconductor becomes thin while the number of durable sheets is small. As the thickness of the photoreceptor becomes thinner, minute scratches on the photoreceptor become easier to see 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.
[0011]
When a developer or the like adheres to the charging member due to durability, the voltage-current characteristics of the AC component flowing through the charging member are as shown in FIG. 4, and in the conventional constant current control, the excessive AC discharge current is reduced. It will be durable while flowing. This will promote deterioration of the photoconductor.
[0012]
Therefore, the present invention controls the amount of AC discharge current, which has a strong correlation with the shaving amount of a member to be charged (for example, a photoreceptor), within a predetermined setting range, thereby preventing image defects such as “sand” and preventing image defects. An object of the present invention is to provide an image forming apparatus capable of forming a good image over a long period by reducing the amount of scraping of a charged body.
[0013]
[Means for Solving the Problems]
According to the present invention, there is provided a charged body having a charged surface on which an electrostatic latent image is formed, and 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 is applied by applying the charging bias. The total amount of alternating current flowing between the charging member and the member to be charged is divided into the amount of alternating current flowing through the charging nip portion and the amount of alternating current flowing through the minute gap, and the amount of alternating current is detected. Control means is provided for controlling the amount of AC voltage or AC current applied to the charging member so that the amount falls within a predetermined set range.The control means obtains a voltage-current proportional expression of an AC component flowing through the charging nip by detecting the AC current amount with respect to a detection AC voltage, and obtains a total AC with respect to a predetermined AC voltage during image formation. The amount of AC current flowing through the charging nip at the time of image formation calculated from the proportional expression is subtracted from the amount of current to divide the amount of AC discharge current, and the control unit determines that the charging member is a non-image of the member to be charged. Applying the AC voltage for detection to the charging member when corresponding to the formation area, detecting the AC current flowing through the charging member at this time, the charging member is applied to the image forming area of the member to be charged. When the charging member corresponds to the image forming area of the member to be charged, control is performed under a charging condition corresponding to the AC discharge current amount. The superimposes an AC voltage is applied to be controlled in response to the sensed alternating discharge current amount and a constant DC voltage,It is characterized by the following.
[0014]
The present invention according to claim 2A charged body having a charged surface on which an electrostatic latent image is formed, and a charging member that is arranged in contact with the charged surface and forms a charging nip portion and a minute gap between the charged surface and the charged member. An image forming apparatus for charging the charged surface by applying a charging bias in which a DC voltage and an AC voltage are superimposed on the charging member, wherein the charging bias is applied to the charging member and the member to be charged. The total amount of AC current flowing therebetween 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 to detect the amount of AC discharge current, and the amount of AC discharge current falls within a predetermined set range. Control means for controlling an AC voltage or an amount of AC current applied to the charging member, and the control means detects the amount of AC current with respect to a detection AC voltage so that the charging nip is controlled. A voltage-current proportional expression of an AC component flowing through the portion is obtained, and the amount of AC current flowing through the charging nip portion during image formation calculated from the proportional expression is subtracted from the total AC current value for a predetermined AC voltage during image formation. The AC discharge current amount is divided, and the AC voltage for detection is a value lower than a threshold value of the AC voltage at which the 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. Wherein 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 controlled in accordance with a constant DC voltage and the detected amount of AC discharge current. AC voltage is superimposed and applied,It is characterized by the following.
[0015]
Claim 3 of the present inventionA charged body having a charged surface on which an electrostatic latent image is formed, and a charging member that is arranged in contact with the charged surface and forms a charging nip portion and a minute gap between the charged surface and the charged member. An image forming apparatus for charging the charged surface by applying a charging bias in which a DC voltage and an AC voltage are superimposed on the charging member, wherein the charging bias is applied to the charging member and the member to be charged. The total amount of AC current flowing therebetween 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 to detect the amount of AC discharge current, and the amount of AC discharge current falls within a predetermined set range. Control means for controlling an AC voltage or an amount of AC current applied to the charging member, and the control means detects the amount of AC current with respect to a detection AC voltage so that the charging nip is controlled. A voltage-current proportional expression of an AC component flowing through the portion is obtained, and the amount of AC current flowing through the charging nip portion during image formation calculated from the proportional expression is subtracted from the total AC current value for a predetermined AC voltage during image formation. Dividing the AC discharge current amount, and the control unit applies the detection AC voltage to the charging member when the charging member corresponds to the non-image forming area of the member to be charged. When the AC current flowing through the charging member is detected, and when the charging member corresponds to the image forming area of the member to be charged, control is performed under charging conditions according to the AC discharge current amount, and the detection is performed. Is a value lower than a threshold value of an AC voltage at which an alternating current starts to flow in a discharge region generated in a minute gap formed between the charging member and the charged member, and the charging member is Image formation By superimposing an AC voltage is applied to be controlled in accordance with a charging bias applied to the charging member, the alternating discharge current amount that is the detection and constant DC voltage when corresponds to frequency,It is characterized by the following.
[0018]
Claim4The present invention according toAn image forming apparatus according to any one of claims 1 to 3.Wherein the charging member is a conductive charging member having a high resistance layer at least on a surface layer.
[0019]
[Action]
The main operation (operation according to claim 1) based on the above configuration is as follows.
[0020]
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 the charged body caused by a large amount of the AC discharge current.
[0021]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the drawings.
[0022]
<Embodiment 1>
An image forming apparatus according to the present invention will be described by taking the copying machine shown in FIG. 6 as an example. FIG. 3 is a vertical cross-sectional view of the transfer material 14 along the transport direction.
[0023]
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 includes, as basic constituent layers, a conductive base layer 1b of, for example, aluminum and a photoconductive layer (photosensitive layer) 1a formed on an outer peripheral surface thereof. The photosensitive drum 1 is rotationally driven by a driving means (not shown) at a predetermined process speed (peripheral speed) in the direction of arrow R1 around the support shaft 1d.
[0024]
Above the photosensitive drum 1, a charging member is arranged. The charging member according to the first embodiment has 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 portion N is formed between the photosensitive drum 1 and the surface of the drum 1, and the photosensitive drum 1 rotates 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.
[0025]
A predetermined charging bias is applied to the metal core 2c of the charging roller 2 from the charging bias power supply 3 via the 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.
[0026]
The surface of the photosensitive drum 1 that has been uniformly charged by the charging member 2 is then subjected to exposure L of target image information (image slit exposure of a document image, laser beam scanning exposure, etc.) by the exposure unit 10, and An electrostatic latent image corresponding to the image information is formed. In the first embodiment, the exposing means 10 is a known document table fixing-optical system moving type document imaging slit exposing means. The exposure unit 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 surface of the document O. Illumination lamp (exposure lamp) 22, a slit plate 23 having a slit through which reflected light from the image surface passes, a movable first mirror 24, a second mirror 25, and a third mirror 26, respectively. , An imaging lens 27, a fixed mirror 28, and the like. The above-described original illumination lamp 22, slit plate 23, and first mirror 24 move the lower surface of the original platen glass 20 from one end to the other end at a predetermined speed V. , 26 move at a speed of V / 2, the downward original surface on the original platen glass 20 is scanned from one end side to the other end side, and an original image is formed on the surface of the photosensitive drum 1 by slit exposure. As a result, an electrostatic latent image corresponding to the document image is formed on the photosensitive drum 1.
[0027]
The electrostatic latent image formed on the photosensitive drum 1 is sequentially developed (visualized) as a toner image by a developing unit 11 by a developing unit. 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. The developing roller 11a is connected to the developing bias power supply 4 and receives a developing bias. The developing roller 11 a is arranged so as to face the surface of the photosensitive drum 1. A developing bias including at least an AC component is applied from a developing bias power supply 4 to develop the electrostatic latent image formed on the surface of the photosensitive drum 1. An agent (toner) is adhered and developed as a toner image.
[0028]
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 arranged in contact with the surface of the photosensitive drum 1 to form a transfer nip between the transfer roller 12 and the surface of the photosensitive drum 1, and is driven to rotate in the direction of the arrow R <b> 12 with the rotation of the photosensitive drum 1 in the direction of the arrow R <b> 1. The transfer roller 12 is connected to the transfer bias power supply 5. The transfer material 14 is fed in the direction of arrow K1 by a feeding means (not shown), and is supplied to the transfer nip portion so as to match the timing with the toner image on the surface of the photosensitive drum 1. The toner image on the surface of the photosensitive drum 1 is formed 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. Is transferred to
[0029]
The transfer material 14 to which the toner image has been transferred is separated from the surface of the photosensitive drum 1 and transported to a fixing unit (not shown), where the unfixed toner image on the surface is heated and pressed to be fixed. It is discharged outside the image forming apparatus main body as a 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 rear surface in the same manner as the front surface, and then discharged outside the image forming apparatus main body.
[0030]
After the transfer of the toner image, the transfer drum 13 a of the cleaning device 13 removes transfer contaminants such as transfer residual toner, external additives, and paper powder remaining on the surface of the photosensitive drum 1, and the charge is removed by the charge removing exposure device 15. , 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 charged satisfactorily, 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.
[0031]
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 magnitude of the voltage of the power source 3, the developing bias power source 4, and the transfer bias power source 5 and the application timing are controlled.
[0032]
FIG. 8 shows 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.
[0033]
When the main switch (not shown) is turned ON, the image forming apparatus which has been in the standby state starts the rotation drive of the photosensitive drum 1 (hereinafter simply referred to as “drum” in FIG. 3) based on the image formation (print) start signal. The pre-rotation period starts. 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.
[0034]
Next, with respect to the charging roller (charging member) 2, a primary charging bias in which an AC voltage is superimposed on a DC voltage which is a primary charging bias is turned ON.
[0035]
The primary charging bias is first controlled at a constant voltage in the section B1, during which the amount of the AC current component is detected, and then the charging bias is applied to the charging roller 2 under charging conditions corresponding to the detected amount of the AC current component. Is done.
[0036]
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.
[0037]
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).
[0038]
The charging roller 2 corresponds to the image forming area surface (image forming area) of the photosensitive drum 1, and performs a charging process on the surface of the photosensitive drum 1 under a corrected charging condition.
[0039]
The so-called drum-to-paper surface of the drum 1 between the end of image formation on the first transfer material 14 and the start of image formation on the next second transfer material 14 is a non-image formation area surface. In the first embodiment, the above-described detection of the AC current of the charging roller 2 and the correction of the charging condition are performed again even between the sheets.
[0040]
Even when three or more continuous images are formed, the sequence of detecting the AC current of the charging roller 2 and correcting the charging conditions is similarly performed between each sheet.
[0041]
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 section A2 of the subsequent rotation period, the photosensitive drum 1 is subjected to one or more rounds of charge-removal exposure, and the charge is removed. The rotation of the drum 1 and the neutralization exposure are turned off, and the image forming apparatus enters a standby state until the next image formation start signal is input.
[0042]
Next, the method of correcting the charging condition will be described in detail with reference to the flowchart of FIG.
[0043]
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 between the photosensitive drum 1 and the charging roller 2 changes with time based on the AC component, As a result, charging and reverse charging on the photosensitive drum 1 are repeated.
[0044]
To perform charging and reverse charging, the peak voltage V of the applied AC voltage_PPIs the firing voltage V_th(Charging start voltage: the voltage applied to the charging roller 2 when the charging of the photosensitive drum 1 starts when a DC voltage is applied to the charging roller 2) needs to be twice or more, but is sufficiently higher than that. V_PPIn this case, the unevenness of the local charge on the surface of the photosensitive drum 1 is made uniform by the AC electric field, and the surface potential converges to a surface potential close to the applied DC voltage value. In this charging method, the same discharge as that when a DC voltage is applied is repeated in proportion to the AC frequency. Therefore, the amount of current is generally very large, and damage to the photosensitive drum 1 is generally large. The surface potential of the photosensitive drum 1 has a certain current value I_thAbove, the value converges to a value close to the applied DC voltage regardless of the environment.
[0045]
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 set to a constant voltage. Often controlled.
[0046]
By the way, as the number of image formation increases and the durability of the photosensitive drum 1 progresses, untransferred toner and the like, which 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.
[0047]
Therefore, in the first embodiment, in order to suppress damage to the photosensitive drum 1 due to an AC discharge current, a predetermined AC voltage and a DC voltage (detection voltage) are applied, and the AC current value is detected at that time. A method of determining an alternating current value during image formation was employed. This will be described below.
[0048]
As shown in FIGS. 1A and 1B, between the surface of the photosensitive drum 1 and the surface of the charging roller 2, a contacting portion (charging nip portion N) and a potential difference having a minute gap are defined. There are discharge regions a and b where the discharge occurs when the voltage exceeds the threshold. It is known that the surface of the photosensitive drum 1 is charged in the discharge areas a and b and is damaged.
[0049]
An AC current due to 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.
[0050]
The current control method according to the first embodiment will be described below with reference to the flowchart in FIG. 9 and the voltage-current characteristics in FIG.
[0051]
As shown in FIG. 10, the applied AC voltage V for detection is in the low AC voltage region._PPWhen 0 is set, the surface of the charging roller 2 is stained differently between the initial stage and the endurance._PPAC current I different from 0_ac0, I_ac1 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_PPAnd the amount of alternating current I_acAre in a proportional relationship. A certain AC voltage value V_PPthWhen the voltage is applied as described above, an AC discharge current starts to flow in the discharge areas a and b in the minute gap between the photosensitive drum 1 and the charging roller 2. As described above, this AC discharge current is indispensable for equalizing the charging unevenness on the surface of the photosensitive drum 1, but has a drawback that it damages the surface of the photosensitive drum 1 and increases the amount of scraping of the photosensitive drum 1. is there.
[0052]
The minimum applied AC voltage V, which is called a “sandy” image and has no image of uneven discharge._PPINT and the amount of alternating current I flowing through the charging nip N at this time_acDetect INT. Initially set AC voltage V_PPAC current I for a value of 0_ac0, and a voltage-current relational expression of an AC component flowing through the charging nip N,
I_acN (V_PP) = (I_ac0 / V_PP0) × V_PP
Ask for.
[0053]
Here, the amount of alternating current I_acIs the applied AC voltage V_PP, The amount of alternating current I_acApplied AC voltage V_PPAs a function of_acN (V_PP).
[0054]
The above-mentioned alternating current amount I_acINT and this amount of alternating current I_acThe difference from N is the amount of AC discharge current I_dis  It is.
[0055]
From these, the minimum AC discharge current I required for charging uniformity_dis  b is required. This minimum necessary amount of AC discharge current I_dis  upper limit I to give b some margin_dis  a is set, and the AC discharge current amount I is always between these two values._dis  And the V between these two_PPStep amount ΔV_PPIs set (S1 in FIG. 9), and the control is started (S2). 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_PPAC current I for 0_acIs I_ac1, the value is detected (S3), and the following linear equation is obtained.
I_acN (V_PP) = (I_ac1 / V_PP0) × V_PP
(S4).
[0056]
AC current I for constant current control after endurance_aca, the applied AC voltage is also V_PPa (S5), and the AC discharge current I_dis  Is
I_dis  = I_aca (V_PPa) -I_acN (V_PPa)
(S6). I obtained at this time_dis  But,
I_dis  > I_dis  a
In the case of (S6), (V_PPa-ΔV_PP) For I_dis  ≤I_dis  It repeats n times until it becomes a (S7). And I_dis  ≤I_dis  If a_dis  > I_dis  In the case of b (S8), the control is terminated (S9), and I_dis  ≤I_dis  In the case of b, (V_PPa + ΔV_PP) For I_dis  > I_dis  Repeat n times until b.
[0057]
That is, the total alternating current amount I flowing through the charging roller 2_aca, the AC discharge current I_dis  Is increased, and the amount of drum scraping per unit number of drums is increased as it is under constant current control. Therefore, the amount of AC discharge current I_dis  So that the applied AC voltage is V_PPa to V_PPb (= V_PPa−nΔV, where n is an integer) to reduce the AC discharge current I_dis  Can be suppressed to a required minimum constant value.
[0058]
<Embodiment 2>
The basic configuration of the image forming apparatus according to the second embodiment is similar to that of the first embodiment. In the final step (S7, S10) of the control flow according to the first embodiment, the applied AC voltage V_PPIs changed so as to approach the optimum AC discharge current amount. In the second embodiment, the voltage change amount (step amount) ΔV for one detection is obtained._PPIs set according to the number of endurance sheets (for example, ΔV from the initial change of the voltage-current characteristic of the AC bias is large to 100 sheets._PP= 50V, after that ΔV_PP= 5V), and the number of loops was reduced as much as possible to approach the optimum amount of AC discharge current. According to the second embodiment, the AC current I_acOf the photosensitive drum 1 due to the amount of alternating current flowing to approach the detected current and the optimal amount of alternating discharge current can be minimized.
[0059]
<Embodiment 3>
The basic configuration of the image forming apparatus according to the second embodiment is similar to that of the first embodiment.
[0060]
The voltage-current characteristics 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) rapidly changes up to about 100 durable sheets. , For example, when the charging roller is cleaned once every 50 sheets, the size becomes very small. Therefore, the amount of AC discharge current is detected once before the initial rotation or once between the previous rotation and the paper for the first 100 sheets, but is detected every 500 sheets after the 100 sheets. By doing so, the amount of alternating current flowing for detection can be suppressed to a necessary minimum, and drum scraping of the photosensitive drum 1 by the detection current can be minimized.
[0061]
<Embodiment 4>
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 according to the number of endurance sheets_PPIs set, and the number of times of detection current insertion is changed. For example, up to the initial 100 sheets, the AC voltage change amount (step amount) ΔV_PPIs set to 50 V, and the amount of AC discharge current is always detected once per pre-rotation, but if it exceeds 100 sheets, the AC voltage change (step amount) ΔV_PPIs set to 5 V, and the amount of alternating current flowing for detection is minimized by detecting the amount of alternating current discharge current once per 500 sheets, and the drum scraping of the photosensitive drum 1 due to the detected alternating current or the like is minimized. be able to.
[0062]
<Embodiment 5>
Applied AC voltage V detected in low AC voltage range_PPIn the case where 0 is one point, a linear equation based on this one point, that is, the applied AC voltage V during image formation is used._PP, The amount of alternating current I flowing through the charging nip N between the photosensitive drum 1 and the transfer roller 2_acIs determined, the error of the straight line may increase, and the excess AC discharge current I_dis  Or the AC discharge current I_dis  Insufficient space may cause adverse effects such as generation of a “sandy” image.
[0063]
Therefore, in the fifth embodiment, as shown in FIG. 11, in order to correct this error, a plurality of (five in FIG. 11) applied AC voltages V for detection are set in a low AC voltage region._PP01, V_PP02, V_PP03, V_PP04, V_PP05 is set to improve the accuracy of the linear expression, and the amount of AC current I flowing through the charging nip N by the AC voltage applied to the image formation is changed._acCan be calculated with high accuracy. As a result, the AC discharge current amount I_dis  Can be improved.
[0064]
As described above, the charging roller 2 as the charging member used in the first to fifth embodiments is a conductive charging member having the high-resistance layer 2a at least on the surface layer, as described above. Leakage due to pinholes or scratches on the surface can be prevented. The charging roller 2 may be driven or rotated by the photosensitive drum 1 as a member to be charged which is driven to move in a plane, or may be a non-rotating roller. Alternatively, the motor may be positively driven to rotate at a predetermined peripheral speed in the forward or reverse direction. Furthermore, the layer configuration of the charging roller 2 is not limited to the above-described three-layer configuration of the core metal 2c, the conductive layer 2b, and the high-resistance layer 2a.
[0065]
The charging member may be configured in a blade shape, a block shape, a rod shape, a belt shape, or the like, in addition to the roller-shaped charging roller 2 described above.
[0066]
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 longitudinal 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.
[0067]
The charging blade 2A and the charging block AB described above are different from the rotatable charging roller 2 described above in that the power supply 3 is connected to the metal core member 2fc without the power supply sliding contact 3a for applying a voltage to the metal core 2c. The leading lead wire can be directly connected, and there is an advantage that electric noise which may be generated from the power supply sliding contact 3a is eliminated, a spacer is saved, and a cleaning blade for cleaning the surface of the photosensitive drum 1 is used. Dual-purpose is possible.
[0068]
【The invention's effect】
As described above, according to the present invention, in an image forming apparatus that applies a voltage obtained by superimposing a DC voltage and an AC voltage as a charging bias on a charging member arranged in contact with a surface to be charged of a member to be charged, By controlling the amount of AC discharge current, which has a strong correlation with the shaving amount of the charged body, within a predetermined setting range, it is possible to prevent image defects such as "sandy ground" and to reduce the shaved amount of the charged body for a long time. The present invention provides an image forming apparatus that forms a good image over a wide area.
[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 showing 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 illustrating 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 longitudinal sectional view illustrating a schematic configuration of a blade-shaped charging member (charging blade).
(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 shows a detection voltage V_PPThe figure which shows the state which set two or more 0.
[Explanation of symbols]
1 Charged body (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
3 Charging bias power supply
4 Development bias power supply
5 Transfer bias power supply
11 Developing device
12 Terrorism
13 Cleaning device
14 Transfer material
15 Static elimination exposure equipment
100 control means (control device)
a, b discharge area
N Charging nip
I_ac        AC current
I_acN The amount of AC current flowing through the charging nip
I_dis        AC discharge current
V_PP        AC voltage (applied AC voltage)

Claims (4)

A charged body having a charged surface on which an electrostatic latent image is formed, and a charging member that is disposed in contact with the charged surface and forms a charging nip portion and a minute gap between the charged surface and the charging member. An image forming apparatus that charges the surface to be charged 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 application of 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. detecting the amount, as the AC discharge current falls within a predetermined set range, Bei give a control means for controlling an AC voltage or an alternating current amount applied to the charging member,
The control means obtains a voltage-current proportional expression of an AC component flowing through the charging nip by detecting the AC current amount with respect to a detection AC voltage, and obtains a total AC current value with respect to a predetermined AC voltage during image formation. 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 expression,
Further, the control unit applies the detection AC voltage to the charging member when the charging member corresponds to the non-image forming area of the member to be charged. The current is detected, and when the charging member corresponds to the image forming area of the member to be charged, control is performed under a charging condition corresponding to the AC discharge current amount,
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 an AC voltage controlled according to the detected AC discharge current amount. Superimposed and applied
An image forming apparatus comprising: A charged body having a charged surface on which an electrostatic latent image is formed, and a charging member that is disposed in contact with the charged surface and forms a charging nip portion and a minute gap between the charged surface and the charging member. An image forming apparatus that charges the surface to be charged 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 application of 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. Controlling the AC voltage or the amount of AC current applied to the charging member so that the amount of AC discharge current falls within a predetermined set range.
The control means obtains a voltage-current proportional expression of an AC component flowing through the charging nip by detecting the AC current amount with respect to a detection AC voltage, and obtains a total AC current value with respect to a predetermined AC voltage during image formation. 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 expression,
The AC voltage for detection is 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 charged member,
  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 an AC voltage controlled according to the detected AC discharge current amount. Superimposed and applied
An image forming apparatus comprising: A charged body having a charged surface on which an electrostatic latent image is formed, and a charging member that is disposed in contact with the charged surface and forms a charging nip portion and a minute gap between the charged surface and the charging member. An image forming apparatus that charges the surface to be charged 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 application of 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. Controlling the AC voltage or the amount of AC current applied to the charging member so that the amount of AC discharge current falls within a predetermined set range.
The control means obtains a voltage-current proportional expression of an AC component flowing through the charging nip by detecting the AC current amount with respect to a detection AC voltage, and obtains a total AC current value with respect to a predetermined AC voltage during image formation. Nip at the time of image formation calculated from the above-mentioned proportional expression Subtract the amount of AC current flowing through the section to divide the amount of AC discharge current,
Further, the control unit applies the detection AC voltage to the charging member when the charging member corresponds to the non-image forming area of the member to be charged. The current is detected, and when the charging member corresponds to the image forming area of the member to be charged, control is performed under a charging condition corresponding to the AC discharge current amount,
  The AC voltage for detection is 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 charged member,
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 an AC voltage controlled according to the detected AC discharge current amount. Superimposed and applied
  An image forming apparatus comprising: The charging member is a conductive charging member having a high resistance layer on at least the surface layer,
The image forming apparatus according to any one of claims 1 to 3, characterized in that.

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