JPH08101564A - Image forming device - Google Patents

Abstract

PURPOSE: To obtain a stable potential by electrification even if the characteristic of a contact electrifying means is changed or a photoreceptor is changed with the lapse of time by detecting the electric resistance of the contact electrifying means and controlling an image forming process based on the information of the detection. CONSTITUTION: A conductive member 3 comes into contact with the contact electrifying means 2, to detect a current flowing in the contact electrifying means 2 and then, through the conductive member 3. Then, a resistance detecting means 20 detects the current flowing through the conductive member 3 and a voltage applied to the electrifying means 2, to detect the electric resistance of the electrifying means 2. In other words, when the current flowing through the conductive member 3 is defined as I, the voltage applied to the electrifying means 2 is defined as V, the electric resistance of the electrifying means 2 is defined as R1 and the electric resistance of the conducive member 3 is defined as R2, R1+R2-V/I is obtained and when R1>>R2 is assumed, R1≈V/I is obtained so that the electric resistance R1 of the electrifying means 2 can be obtained. At this time, a process control means controls the image forming process based on the information of the detection.

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,
In particular, the present invention relates to an image forming apparatus provided with a charging unit such as a roller that comes into contact with a photoreceptor and charges it.

[0002]

2. Description of the Related Art A photosensitive layer on the surface of an image bearing member is uniformly charged by a charging means, optical image information is applied to the photosensitive layer to form an electrostatic latent image, and usually powdery toner is supplied to the electrostatic latent image. An image forming apparatus that repeats the process of electrostatically transferring the toner image onto a sheet-like transfer paper after visualizing the latent image has been widely used as a copying machine, a facsimile, a printer, etc. It is well known that this is done.

In the image forming process using the photoconductor as described above, as a means for charging the photoconductor, a charger in which a metal wire is spread is generally used, and a direct current of about 5 to 8 KV is applied to the metal wire. In many cases, the photoreceptor is charged by corona discharge generated by applying a high voltage. However, this type of means generates ozone and nitrogen oxides associated with corona discharge, which may damage the photoconductor itself or adhere to the ozone and cause deterioration of image quality. As for the flow itself, there is a problem that the amount of flow in the direction of the photoconductor is as small as 5 to 30% and the efficiency is low.

In order to avoid such a drawback, a direct charging system has been proposed in which a charging member is brought into direct contact with a photosensitive member. As the direct charging method, a technique has been proposed in which a charging roller, a belt, or the like is brought into contact with a photoconductor and a DC voltage or a voltage obtained by superimposing an AC on a DC is applied to these charging members.

In the direct charging system, a discharge current is applied to the photoconductor through a charging member such as a charging roller, and the photoconductor is charged by the current. Therefore, the photoconductor is charged in accordance with the current flowing from the charging roller to the photoconductor. The charging potential of is determined.

Normally, the voltage applied to the charging roller is adjusted in order to control the charging potential of the photoconductor. But,
The resistance value of the charging roller changes due to the influence of the environment (temperature and humidity), and if it changes, the charging potential of the photoconductor changes even if the voltage applied to the charging roller is kept constant. In particular,
The higher the process speed, the greater the influence on the charging potential.

Therefore, in Japanese Patent Laid-Open No. 9883/9883, the voltage applied to the charging roller is controlled so as to keep the current flowing through the charging roller constant during non-image formation, and the voltage at that time is detected. During image formation, the voltage detected during non-image formation is controlled to be applied to the charging roller. According to this, the charging potential of the photoconductor can be controlled so as not to be affected by variations in the resistance value of the charging roller, variations in environmental conditions (temperature and humidity), and the like.

[0008]

However, the relationship between the charge amount Q determined by the current I flowing through the photoconductor and the change ΔV in the charging potential of the photoconductor is not constant. That is, if the electrostatic capacity of the photoconductor is C, there is a relationship of ΔV = Q / C,
Since the photosensitive layer of the photoconductor that determines the electrostatic capacitance C is gradually shaved and the thickness thereof becomes thin, the electrostatic capacitance C changes with time,
The relationship between the charge amount Q and the charging potential change ΔV of the photoconductor also changes.

C = [(ε ₀ × ε) / d] (μF) d: film thickness ε ₀ : vacuum permittivity ε: relative permittivity of photoconductor Therefore, assuming that the current flowing through the charging roller is controlled to be constant. However, since the charging potential of the photoconductor largely deviates from the target charging potential due to changes over time, high image quality can be maintained only for a short period. That is, in such a system, the charging potential is greatly reduced, so that the copy quality is remarkably deteriorated due to a reduction in image density (background stain in negative / positive), and there is a limit to long-term use of the photoconductor. In order to maintain high image quality at high speed and with an analog machine for a long period of time, it is essential to obtain a stable charging potential with respect to the aging of the photoconductor itself.

The relationship between the current flowing through the photosensitive member and the charging potential of the photosensitive member is also disclosed in Japanese Patent Laid-Open No. 223513/1993.

Therefore, the main object of the present invention is to obtain a stable charging potential even with changes in the characteristics of the contact charging means and with time of the photosensitive member itself.

[0012]

In order to solve the above problems, according to the present invention, a photoreceptor (1) for forming an image, and a contact charging means (2) which comes into contact with the photoreceptor to give an electric charge to the photoreceptor. In the image forming apparatus including: a conductive member (3) installed at a position where the contact charging means can be freely contacted; a voltage applied to the contact charging means, and the conductive member passing through the inside of the contact charging means. Detects the current flowing through
A resistance detection means (20) for detecting information corresponding to the electric resistance of the contact charging means based on them; and a process control means (for controlling the image forming process based on the information detected by the resistance detection means ( 23);

In the second aspect of the present invention, the contact charging means is a roller-shaped member (2) rotatably supported.

In the invention of claim 3, the conductive member is a member (31) formed in a thin plate shape.

In the invention of claim 4, the conductive member is formed in a roll shape (3).

According to the invention of claim 5, the conductive member is rotatably supported.

In the invention of claim 6, the conductive member is
The contact charging means is installed at a position outside the image forming area.

According to a seventh aspect of the invention, the process control means is configured to automatically adjust the charging bias voltage applied to the contact charging means during image formation (PR1) according to the detected resistance value. To do.

In the invention of claim 8, the conductive member is
The contact charging means is installed so as to face the entire image forming area.

According to a ninth aspect of the present invention, at least one of the conductive member and the contact charging means is moved to position the both in a contact state and a separated state (4).
2, 44) is further provided.

According to the tenth aspect of the present invention, the positioning means positions the conductive member and the contact charging means in a contact state before the image forming process is started, and in that state, the resistance detecting means operates. , The information corresponding to the electric resistance of the contact charging means is detected, and the positioning means positions the conductive member and the contact charging means in a separated state during the image forming process (PR2). Configure.

According to an eleventh aspect of the present invention, the resistance detecting means is provided while the contact charging means makes at least 1/2 rotation.
Substantially continuously, a voltage applied to the contact charging means,
A current flowing through the conductive member is detected.

According to a twelfth aspect of the present invention, the resistance detecting means is configured to detect information corresponding to the electric resistance of the contact charging means periodically (PR3) every time the image forming process is repeated a predetermined number of times. To do.

According to a thirteenth aspect of the present invention, the conductive member is positioned so as to face a non-image forming area which is 5 mm or more away from the image forming area of the contact charging means.

In the fourteenth aspect of the present invention, the cleaning member (10, 16) is provided at a position where it comes into contact with the surface of the conductive member.
Is installed.

The symbols shown in parentheses are the reference numerals of the corresponding elements in the examples described later, but each component of the present invention is a specific element in the examples. It is not limited to only.

[0027]

According to the present invention, since the conductive member (3) contacts the contact charging means (2), it is possible to detect the current flowing through the inside of the contact charging means and through the conductive member. Since the resistance detecting means (20) detects the current flowing through the conductive member and the voltage applied to the contact charging means, the electric resistance of the contact charging means can be detected.
That is, if the current flowing through the conductive member is I, the voltage applied to the contact charging means is V, the electric resistance of the contact charging means is R1, and the electric resistance of the conductive member is R2, then R1 + R2 = Assuming that V / I and R1 >> R2, then R1≈V / I, so the electrical resistance R1 of the contact charging means is obtained. The process control means (23) is
Control of the image forming process based on the information detected by the resistance detection means (for example, control of the voltage applied to the contact charging means)
Carry out.

The main factors that affect the charging potential of the photosensitive member are the electric resistance R1 of the contact charging device and the electrostatic capacitance C of the photosensitive member in addition to the voltage applied to the contact charging device. That is, the electric resistance R1 of the contact charging means changes due to variations in characteristics of each component and the influence of environmental conditions (temperature, humidity). Further, the electrostatic capacitance C of the photoconductor changes as the film thickness of the photoconductor changes with time.

In the technique disclosed in Japanese Unexamined Patent Publication No. 4-9883, the electric current flowing through the charging roller is controlled to be constant, so that the electric resistance R1 of the contact charging means and the electrostatic capacitance C of the photosensitive member fluctuate. Then, due to the influence, the charging potential of the photoconductor deviates from the target value.

Further, Japanese Laid-Open Patent Publication No. 5-223513 proposes to detect a change in the film thickness of the photoconductor from the voltage applied to the contact charging means and the current flowing through the photoconductor, but in this case, In fact, the result obtained by combining the effects of both the electric resistance R1 of the contact charging means and the electrostatic capacitance C of the photoconductor is measured. That is, the detection result includes not only the change in the film thickness of the photoconductor but also the variation in the characteristics of the contact charging means and the influence of environmental conditions (temperature, humidity). Further, the film thickness of the photoconductor changes with time, whereas the characteristic variation of the contact charging means varies from device to device, and the environmental conditions change from moment to moment in a relatively short period. Further, since the influence of the change of the electric resistance R1 of the contact charging means on the charging potential of the photoconductor is different from the influence of the change of the electrostatic capacitance C of the photoconductor on the charging potential of the photoconductor, Unless the electric resistance R1 is measured and the influence of the change in the film thickness of the photoconductor is considered independently, it is impossible to compensate the change in the charging potential of the photoconductor due to the change.

However, in the present invention, as described above,
The electric resistance R1 of the contact charging means itself is required. That is,
Since the electric resistance R1 is measured independently of the current flowing through the photoconductor, it is not affected by the film thickness of the photoconductor. Therefore,
It becomes possible to compensate for the influences of the electric resistance R1 of the contact charging means and the electrostatic capacitance C of the photoconductor, and stable charging is possible even with changes in the characteristics of the contact charging means and with time of the photoconductor itself. The electric potential can be obtained.

According to a second aspect of the present invention, in particular, the charging roller rotating in synchronization with the photoconductor can maintain the discharge gap with the photoconductor in the contact portion and its vicinity in a minute and stable manner. By accurately detecting and correcting the applied voltage, the charging potential of the photoconductor can be stabilized.

A third aspect of the present invention is that the conductive member for detecting the resistance of the contact charging means is made of a thin plate, so that the contact state with the electrode member can be closely contacted and a stable current can be obtained. In addition, it is possible to maintain a good contact state with a small pressure without taking up a space.

A fourth aspect of the present invention is that the conductive member for detecting the resistance of the contact charging means is formed into a roll shape, so that the contact state of the contact surface (line) can be made more uniform and uniform, and a stable contact state can be obtained. Therefore, a stable current can be obtained. Moreover, the mating member is less likely to be damaged.

Claim 5: By making the roll-shaped conductive member rotatable, the contact state between the contact charging means and the conductive member becomes rolling contact, so that vibration caused by slip,
It is possible to prevent contact failure due to foreign matter (toner, etc.) clogging the sliding portion, stabilize the contact state, and accurately detect current. In addition, the damage and circuit load of the other party are reduced. Claim 6: By placing the conductive member opposite to the outside of the image forming area of the contact charging means, it is difficult for toner powder, paper powder, etc. to enter the contact portion between the conductive member and the contact charging means, and both are good. Since the contact state can be maintained, the resistance value of the contact charging means can be accurately measured. Even if the contact charging member is damaged, the influence on charging can be prevented. Further, when the photoconductor is charged, if a current passing through the contact charging member is shunted to both the photoconductor and the conductive member and flows to both, the current flowing through the conductive member causes the capacitance of the contact charging means (capacitor layer). ), Or the resistance value of the contact charging means is reduced, so that the contact charging means is more likely to discharge the photoconductor and the charging potential of the photoconductor is higher than in the case where no current is passed through the conductive member. Although tending, such adverse effects are also mitigated by claim 6.

Claim 7: By correcting the bias voltage according to the detected resistance value, the charged potential of the photoconductor can be maintained in an optimum state. There is a follow-up property with respect to the reduction of the film thickness of the photoconductor, the fluctuation of the potential is small, and the photoconductor can be used for a long time.

Claim 8: Foreign matter such as toner and paper powder attached to the contact charging member can be removed by the conductive member or transferred to itself, and cleaning of the contact charging member is possible. Become.

Claim 9: When forming an image (when charging the photoconductor), the conductive member can be separated from the contact charging member by the positioning means.

Claim 10: By separating the conductive member from the contact charging member during execution of the image forming process, it is possible to prevent the discharge state from the contact charging means to the photosensitive member from fluctuating. The charging potential of can be stabilized. Further, since the resistance value of the contact charging means fluctuates greatly according to the temperature and humidity before the image forming process is started, the contact charging is performed based on the resistance value of the contact charging means measured before starting the image forming process. By compensating the voltage applied to the means, the charging potential of the photoconductor can be stabilized.

Claim 11: Since the resistance distribution of the contact charging means tends to be uneven and the current flowing from the contact charging means to the conductive member contains noise, the instantaneously sampled current value is used. When the resistance value is calculated by using the method, a stable resistance value cannot be detected. However, the contact charging means is 1 /
By detecting the current substantially continuously (including the case where the sampling is repeated many times) during the rotation of two or more rotations, it is possible to detect a stable resistance value.

Claim 12: When the image forming process is repeated many times, the temperature inside the apparatus rises and the resistance value of the contact charging means changes. By periodically detecting the resistance value of the contact charging means, the applied voltage to the contact charging means can be corrected so as to compensate for the change, and the charging potential of the photoconductor can be maintained constant. Become.

A thirteenth aspect: The current flowing from the contact charging means to the conductive member or the photosensitive member usually flows in the thickness direction of the contact charging means, but the position of the conductive member approaches the image forming area. If so, the effect of the current flowing through the conductive member appears in the current flowing through the photoconductor. By facing the conductive member at a position separated by 5 mm or more in the width direction from the end of the image forming area, it is possible to eliminate the adverse effect of the current flowing through the conductive member and maintain the charging potential of the photoconductor constant. Become.

Claim 14: It is possible to eliminate the contamination between the contact charging member and the conductive member, and to maintain the contact state between the two members satisfactorily for a long period of time.

[0044]

FIG. 1 shows the configuration of the main part of an image forming apparatus according to an embodiment. This will be described with reference to FIG. The photoconductor 1, which is the member to be charged, is driven during the image forming operation and rotates in the direction of the arrow. The charging roller 2 used for charging the photoconductor 1 is installed so as to come into contact with the surface of the photoconductor 1, and is driven to rotate in the direction of the arrow as the photoconductor 1 rotates. The charging roller 2 may be modified so that a driving force is applied.

Further, 4 is exposure for image formation, 5 is a developing device, 6 is a transfer roller, 7 is a cleaning unit, 8 is quenching light, and 9 is transfer paper.

In this embodiment, the conductive roller 3 rotatably supported faces the entire area of the charging roller 2 in the image forming area (in the image forming area: see FIG. 4) and contacts the surface thereof. It is installed in a shape. The conductive roller 3 is driven to rotate as the charging roller 2 rotates. The electric resistance of the conductive roller 3 is sufficiently smaller than that of the charging roller 2. Further, a cleaning member 16 is arranged on the surface of the conductive roller 3 so as to be in contact therewith.

A bias voltage is applied to the core metal 11 of the charging roller 2 from the charging bias applying power source 21. The bias voltage output from the charging bias applying power source 21 is controlled by the high voltage control circuit 22. A current flowing from the charging roller 2 to the conductive roller 3 flows through the current detection circuit 20 to the ground. The current detection circuit 20 detects the current. The arithmetic processing unit 23 detects the current detected by the current detection circuit 20 and the bias voltage applied to the charging roller 2, respectively, and calculates the resistance value of the charging roller 2 as described later. Then, the high voltage control circuit 22 is controlled as necessary to change the bias voltage applied to the charging roller 2.

A part of FIG. 1 is enlarged and shown in FIG. Figure 2
Will be described with reference to. Reference numeral 11 denotes a core metal of the charging roller 2, which is electrically conductive (using a stainless material). Reference numeral 12 is a resistance layer that is conductively adhered to the core metal 11, and is made of an elastic body (epichlorohydrin rubber, rubber hardness: JIS A40-6).
0 degree) and its volume resistivity is 10 ⁶ to 1
0 is an ^{8 Ω} · cm. A surface layer 13 is a thin layer (2 to 50 μm).
m) is made of Teflon resin.

The charging roller 2 contacts the photosensitive member 1 (contact pressure 1
0 to 30 gf / cm), and the photosensitive member 1 is driven to rotate by frictional force as the photosensitive member 1 rotates in the direction of the arrow. The conductive roller 3 is
Contact the charging roller 2 (contact pressure is 2 to 100 gf / cm,
It is preferably 10 to 30 gf / cm) and driven to rotate by frictional force in the direction of the arrow. Reference numeral 14 is a conductive cored bar (using a stainless steel material), and its surface 15 is polished and finished (roughness: RZ 0.5 μ to facilitate removal of deposits.
m or less).

A modification of the means for cleaning the conductive roller 3 is shown in FIG. In FIG. 13, reference numeral 10 is a scraper member, which removes adhered matter on the roller 3.
Reference numeral 10 is a thin plate-shaped polyester film, polyurethane rubber or the like which is expanded in the width direction and brought into contact. Since the contact of the scraper member increases the rotational load on the roller 3, it is desirable that the roller 3 be driven by a driving unit.
This prevents problems such as slippage.

The structure of a modified embodiment of the conductive member 3 is shown in FIGS. In this embodiment, the conductive member 31 is a thin plate (made of stainless steel) and is swingably supported with respect to a support shaft 32. The conductive member 31 is brought into contact with the charging roller 2 by a spring 33 to prevent the charging roller 2 from rotating. Is slipping. As shown in FIG. 4, the conductive member 3
1 is installed so as to face and contact the end portion of the charging roller 2 outside the image forming area. Actually, in the width direction (axial direction), 5 m from the end of the maximum image forming area of the charging roller 2.
A conductive member 31 is installed at a position away from the outside. This is a measure for the following points.

When an electric current flows from the charging roller 2 to the conductive member 31, electric charges due to the electric current are accumulated in the charging roller 2 and the apparent resistance value of the charging roller 2 decreases. The increase in the charging voltage of the photoreceptor caused by this phenomenon is large at the start of discharge and gradually disappears.
It is unstable and it is difficult to compensate for it.
By separating the position where the conductive member 31 is installed from the image forming area, it is possible to prevent the influence of the current flowing through the conductive member 31 from affecting the image forming portion. In addition,
Although the distance between the edge of the image forming region and the installation position of the conductive member 31 is about 2 to 3 mm, the effect can be obtained, but the distance is preferably 5 mm or more.

FIG. 12 shows the configuration of another modification of the conductive member 3. In this example,
The conductive member 3 can be positioned at a position in contact with the charging roller 2 and at a position away from the charging roller 2.
That is, the rotating shaft 41 of the conductive member 3 has an L-shaped arm 42.
, One end 43 of the arm 42 is swingably supported, and the other end is connected to a plunger of a driving solenoid 44. By turning on / off the solenoid 44, the conductive member 3 is positioned at a position in contact with the charging roller 2 (state similar to FIG. 1) and a position away from the charging roller 2 (state shown in FIG. 12). The positioning means for the conductive member 3 may be configured using a mechanism such as a cam. Further, instead of moving the conductive member 3, the charging roller 2 may be moved.

As shown in FIG. 12, when the conductive member 3 is separated from the charging roller 2, no current flows through the conductive member 3, so that the charging potential can be prevented from fluctuating when the photosensitive member is charged. .

Now, detection of the resistance value of the charging roller 2 will be described. When detecting the resistance value, the charging roller 2
A predetermined bias voltage is applied to the high voltage control circuit 22. A current is applied to the resistance layer 1 by the voltage applied to the cored bar 11.
2. Through the surface layer 13, one flows as a discharge to the photoconductor 1. This discharge starts when the voltage becomes a certain value or more (about 600 to 700 V). The discharge is generated in the vicinity of the nip between the roller 2 and the photosensitive member 1 (a gap of about 10
(0 to 150 μm or less). The current flowing at this time varies depending on the applied voltage, the film thickness (electrostatic capacity) of the photoconductor, the resistance value of the charging roller, and the like.

When the electrostatic capacity of the photoconductor, that is, the film thickness is constant, the current or the applied voltage is controlled so that the current value is constant, so that the surface of the photoconductor is charged to a desired charging potential. be able to.

Another current flow flows through the conductive member 3 which contacts the charging roller 2 on the side substantially opposite to the photosensitive member 1 in the figure. This current is detected with the ground. The current can be detected by inserting a fixed resistor in series and detecting it as a voltage. At this time, a high voltage (applied voltage may be 2.5 KV in some cases, usually about 1.6 KV) is resistance-divided and detected as a low voltage. Of course, other methods may be used.

The resistance value R of the charging roller 2 is R according to Ohm's law based on the detected current I and the applied voltage V.
= V / I.

Incidentally, the correlation between the resistance value of the charging roller and the charging potential of the photosensitive member has the relationship shown in FIG. The horizontal axis of FIG. 5 is the volume resistivity (Ω · cm) of the roller 2,
The vertical axis represents the surface potential (V) of the photoconductor 1. In this example,
The cases where the bias applied to the charging roller is -1600V and -1000V are shown.

The process speed at this time is 200 mm /
s. The photosensitive member is OPC (the applied bias and the surface potential are negative because it has a negative charging characteristic), and the film thickness is 25 μm. As the resistance increases, the charging potential decreases. In particular, when the resistance value is as high as 10 ⁸ Ω · cm, a large potential drop occurs. Conventionally, for example, pros speed is 50mm
At / s, the potential drop did not occur so much, but the potential drop became noticeable as the speed increased.

For example, the potential decrease (when -1600 V is applied) when 10 ⁷ → 10 ⁸ Ω · cm is the process speed 5
When it is 0 mm / s, it drops by about 86 V, but 200 mm
At s / s, the voltage drops by about 387V.

The resistance value of the charging roller changes under the influence of the environment (temperature, humidity) in addition to the initial variation (due to manufacturing). The resistance is high on the low temperature and low humidity side and decreases on the high temperature and high humidity side.
The charging roller used in this example has a cored bar made of stainless steel with an outer diameter of 8 mm, and a resistance layer made of epichlorohydrin rubber with a thickness of 3 mm.
mm, the surface layer is Teflon-based resin and has a thickness of 10 μm,
From 10 ⁸ Ω · cm order at low temperature (10 ° C, 15% RH) to 10 ⁶ Ω · c at high temperature (30 ° C, 90% RH)
There was a 2-digit change in resistance up to the m-order.

FIG. 6 shows the relationship between the applied bias voltage required to keep the charging potential constant (-900 V) and the detected resistance value of the charging roller. That is, if the bias voltage applied to the charging roller is corrected according to the curve shown in FIG. 6 according to the detected resistance value, the charging potential of the photoconductor can be made constant (-900V).

Even when such compensation of the bias voltage is carried out, the charging potential of the photoconductor changes as the film thickness of the photoconductor decreases. FIG. 7 shows a change in the charging potential according to the film thickness when the bias voltage compensation according to the resistance is performed so that the charging potential becomes constant when the initial film thickness of the photoconductor is 25 μm. Referring to FIG. 7, the charging potential is constant when the film thickness is 25 μm. Further, by applying the above voltage to the photoconductor film thickness of 20 μm, it is possible to maintain the potential with a slight deviation from the target potential but with a small error. Deviation fit to the deviation of 150V at 10 ⁶ Ω · cm at 50V, 10 ⁸ Ω · cm.

In order to reduce the deviation of the charging potential from the target value due to the reduction of the film thickness, the bias potential applied at each resistance value is set so that the centers of 25 μm and 20 μm become a constant potential (−900 V). Should be set and applied. In addition,
The photoconductor film thickness of 20 μm is the film thickness after image formation of 60,000 sheets from the initial 25 μm by an average usage method. Further, in order to use the photosensitive member for a longer period of time, control may be performed so as to correct the applied bias for each resistance value according to the number of used photosensitive members. When the bias voltage is low at low resistance (10 ⁶ Ω · cm), and the bias voltage is corrected at a high resistance (10 ⁸ Ω · cm), it is possible to always obtain a constant potential.

FIG. 8 shows the main part of the control contents of the image forming apparatus in the embodiment shown in FIG. The control contents in the three modified embodiments are shown in FIGS. 9, 10 and 11, respectively.

Description will be made with reference to FIG. In this embodiment, if there is a start instruction, steps S83 to S83 are performed.
Proceed to 84. At S84, sampling of the bias voltage applied to the charging roller 2 and the current flowing through the conductive member 3 is repeated for a predetermined period. During this period, the charging roller 2 is 1/2
It is set longer than the period of rotation and shorter than the period of one rotation. If the period is too short, it is difficult to detect the accurate resistance value,
If it is too long, the charging voltage of the photosensitive member fluctuates due to the adverse effect of the current flowing through the conductive member 3.

In the next step S85, a large number of data sampled in S84 are averaged to obtain the averaged current value and voltage value. Based on the current value and the voltage value, the resistance value of the charging roller 2 is obtained in the following step S86. Further, in the next step S87, the bias voltage applied to the charging roller 2 is corrected according to the obtained resistance value, for example, by referring to a predetermined table.

The embodiment shown in FIG. 9 shows the control corresponding to the apparatus shown in FIG. That is, the solenoid 44 is turned on / off to control the pressing / withdrawing of the conductive member 3 as the detection electrode to the charging roller 2. In this example, in the "initialization" of step S81B, the conductive member 3 is retracted from the charging roller 2. Then, when a start instruction is given, the conductive member 3 is pressed against the charging roller 2, sampling of current and voltage is started, and the resistance value is measured. Then, in step S92, the conductive member 3 is retracted from the charging roller 2, and then the process proceeds to step S88 to execute the image forming process.

In the embodiment shown in FIG. 10, every time the image forming process is executed, the counter CNT is incremented in step SA1 to count the number of image formed sheets. And
Every time the number of sheets exceeds 100, proceed to step SA2,
The resistance value of the charging roller is measured and the applied bias voltage is corrected.

In the embodiment shown in FIG. 11, the temperature and humidity inside the apparatus are measured at step SB1 each time the image forming process is executed. Then, compare the temperature and humidity previously measured and stored with the measured value at that time,
When the amount of change (difference) is large, the process proceeds to step SB2, the measured value at that time is stored, and the resistance value of the charging roller is measured and the applied bias voltage is corrected.

[0072]

According to the present invention, since the conductive member (3) comes into contact with the contact charging means (2), it is possible to detect a current flowing through the inside of the contact charging means and through the conductive member. it can. Since the resistance detecting means (20) detects the current flowing through the conductive member and the voltage applied to the contact charging means, the electric resistance of the contact charging means can be detected. That is, I is the current flowing through the conductive member,
If the voltage applied to the contact charging means is V, the electrical resistance of the contact charging means is R1, and the electrical resistance of the conductive member is R2, then R1 + R2 = V / I, and if R1 >> R2. , R1≈V / I, the electric resistance R1 of the contact charging means is required. Process control means (23)
Controls the image forming process (for example, controls the voltage applied to the contact charging unit) based on the information detected by the resistance detecting unit.

The main factors that affect the charging potential of the photoconductor are the electric resistance R1 of the contact charging unit and the electrostatic capacitance C of the photoconductor in addition to the voltage applied to the contact charging unit. That is, the electric resistance R1 of the contact charging means changes due to variations in characteristics of each component and the influence of environmental conditions (temperature, humidity). Further, the electrostatic capacitance C of the photoconductor changes as the film thickness of the photoconductor changes with time.

In the present invention, as described above, the electric resistance R1 of the contact charging means itself is required. That is, since the electric resistance R1 is measured independently of the current flowing through the photoconductor, it is not affected by the film thickness of the photoconductor. Therefore, it becomes possible to compensate for the influences of the electric resistance R1 of the contact charging means and the electrostatic capacity C of the photoconductor, and it is stable against changes in the characteristics of the contact charging means and changes with time of the photoconductor itself. It is possible to obtain the charged electric potential.

A second aspect of the present invention is that the charging roller, which rotates in synchronism with the photosensitive member, can maintain the discharge gap with the photosensitive member in the contact portion and its vicinity in a minute and stable manner. By accurately detecting and correcting the applied voltage, the charging potential of the photoconductor can be stabilized.

A third aspect of the present invention is that the conductive member for detecting the resistance of the contact charging means is in the form of a thin plate, so that the contact state with the electrode member can be closely contacted and a stable current can be obtained. In addition, it is possible to maintain a good contact state with a small pressure without taking up a space.

A fourth aspect of the present invention is that the conductive member for detecting the resistance of the contact charging means is formed into a roll shape, so that the contact state of the contact surface (line) can be made more uniform and uniform, and a stable contact state can be obtained. Therefore, a stable current can be obtained. Moreover, the mating member is less likely to be damaged.

Claim 5: By making the roll-shaped conductive member rotatable, the contact state between the contact charging means and the conductive member becomes rolling contact, so vibration due to slip,
It is possible to prevent contact failure due to foreign matter (toner, etc.) clogging the sliding portion, stabilize the contact state, and accurately detect current. In addition, the damage and circuit load of the other party are reduced. Claim 6: By placing the conductive member opposite to the outside of the image forming area of the contact charging means, it is difficult for toner powder, paper powder, etc. to enter the contact portion between the conductive member and the contact charging means, and both are good. Since the contact state can be maintained, the resistance value of the contact charging means can be accurately measured. Even if the contact charging member is damaged, the influence on charging can be prevented. Further, when the photoconductor is charged, if a current passing through the contact charging member is shunted to both the photoconductor and the conductive member and flows to both, the current flowing through the conductive member causes the capacitance of the contact charging means (capacitor layer). ), Or the resistance value of the contact charging means is reduced, so that the contact charging means is more likely to discharge the photoconductor and the charging potential of the photoconductor is higher than in the case where no current is passed through the conductive member. Although tending, such adverse effects are also mitigated by claim 6.

Claim 7: By correcting the bias voltage according to the detected resistance value, the charged potential of the photoconductor can be maintained in an optimum state. There is a follow-up property with respect to the reduction of the film thickness of the photoconductor, the fluctuation of the potential is small, and the photoconductor can be used for a long time.

Claim 8: Foreign matter such as toner and paper powder attached to the contact charging member can be removed by the conductive member or transferred to itself, and cleaning of the contact charging member is possible. Become.

Claim 9: When forming an image (when charging the photoconductor), the conductive member can be separated from the contact charging member by the positioning means.

Claim 10: By separating the conductive member from the contact charging member during execution of the image forming process, it is possible to prevent the discharge state from the contact charging means to the photosensitive member from fluctuating. The charging potential of can be stabilized. Further, since the resistance value of the contact charging means fluctuates greatly according to the temperature and humidity before the image forming process is started, the contact charging is performed based on the resistance value of the contact charging means measured before starting the image forming process. By compensating the voltage applied to the means, the charging potential of the photoconductor can be stabilized.

Claim 11: Since the resistance value distribution of the contact charging means is apt to be uneven and the current flowing from the contact charging means to the conductive member contains noise, the instantaneously sampled current value is used. When the resistance value is calculated by using the method, a stable resistance value cannot be detected. However, the contact charging means is 1 /
By detecting the current substantially continuously (including the case where the sampling is repeated many times) during the rotation of two or more rotations, it is possible to detect a stable resistance value.

Claim 12: When the image forming process is repeated many times, the temperature in the apparatus rises and the resistance value of the contact charging means changes. By periodically detecting the resistance value of the contact charging means, the applied voltage to the contact charging means can be corrected so as to compensate for the change, and the charging potential of the photoconductor can be maintained constant. Become.

A thirteenth aspect: The current flowing from the contact charging means to the conductive member or the photosensitive member usually flows in the thickness direction of the contact charging means, but the position of the conductive member approaches the image forming area. If so, the effect of the current flowing through the conductive member appears in the current flowing through the photoconductor. By facing the conductive member at a position separated by 5 mm or more in the width direction from the end of the image forming area, it is possible to eliminate the adverse effect of the current flowing through the conductive member and maintain the charging potential of the photoconductor constant. Become.

Claim 14: It is possible to eliminate contamination between the contact charging member and the conductive member, and to maintain a good contact state between them for a long period of time.

[Brief description of drawings]

FIG. 1 is a front view showing a main part of an image forming apparatus according to an embodiment.

FIG. 2 is an enlarged front view of a part of the device of FIG.

FIG. 3 is a front view showing a main part of an image forming apparatus according to a modified example.

FIG. 4 is a perspective view showing a part of the apparatus of FIG.

FIG. 5 is a graph showing characteristics of the image forming apparatus.

FIG. 6 is a graph showing characteristics of the image forming apparatus.

FIG. 7 is a graph showing characteristics of the image forming apparatus.

FIG. 8 is a flow chart of control contents in one embodiment.

FIG. 9 is a flowchart of control contents in one embodiment.

FIG. 10 is a flowchart of control contents in one embodiment.

FIG. 11 is a flowchart of control contents in one embodiment.

FIG. 12 is a front view showing a main part of an image forming apparatus according to a modified example.

FIG. 13 is an enlarged front view of a part of an image forming apparatus according to another embodiment.

[Explanation of symbols]

1: Photoconductor 2: Electrode member 3: Conductive member 4: Exposure 5: Developing device 6: Transfer roller 7: Cleaning unit 8: Quenching light 9: Transfer paper 10: Scraper member 11: Core metal 12: Resistance Layer 13: Surface layer 16: Cleaning member 20: Current detection means 21: Charging bias application power source 22: High voltage control circuit 23: Arithmetic processing device 31: Conductive member 32: Spindle 33: Spring

Claims (14)

[Claims] 1. An image forming apparatus comprising a photoconductor for forming an image and a contact charging means for contacting with the photoconductor and giving an electric charge to the photoconductor: the image forming apparatus is provided at a position contactable with the contact charging means. Conductive member: Information corresponding to the electric resistance of the contact charging means is detected based on a voltage applied to the contact charging means and a current flowing through the contact charging means through the conductive member. An image forming apparatus, comprising: a resistance detecting unit for detecting the above; and a process control unit for controlling the image forming process based on the information detected by the resistance detecting unit. 2. The image forming apparatus according to claim 1, wherein the contact charging unit is a roller-shaped member rotatably supported. 3. The conductive member is formed in a thin plate shape,
The image forming apparatus according to claim 2. 4. The image forming apparatus according to claim 2, wherein the conductive member is formed in a roll shape. 5. The image forming apparatus according to claim 4, wherein the conductive member is rotatably supported. 6. The image forming apparatus according to claim 3, wherein the conductive member is installed opposite to a position outside the image forming area of the contact charging unit. 7. The process control means automatically adjusts a charging bias voltage applied to the contact charging means at the time of image formation according to the detected resistance value.
The image forming apparatus described. 8. The image forming apparatus according to claim 3, wherein the conductive member is installed so as to face the entire image forming area of the contact charging unit. 9. The image forming apparatus according to claim 1, further comprising a positioning unit that moves at least one of the conductive member and the contact charging unit to position them in a contact state and a separated state. 10. The positioning means positions the conductive member and the contact charging means in a contact state before the image forming process is started, and in this state, the resistance detecting means causes the contact charging means to move. 10. The image forming apparatus according to claim 9, wherein the positioning unit positions the conductive member and the contact charging unit in a separated state while detecting information corresponding to the electric resistance of the image forming apparatus and performing the image forming process. . 11. The resistance detecting means detects a voltage applied to the contact charging means and a current flowing through the conductive member substantially continuously while the contact charging means makes at least 1/2 rotation. The image forming apparatus according to claim 2. 12. The resistance detecting device according to claim 10, wherein the resistance detecting device periodically detects information corresponding to the electric resistance of the contact charging device every time the image forming process is repeated a predetermined number of times. Image forming apparatus. 13. The image forming apparatus according to claim 6, wherein the conductive member is installed facing the non-image forming area, which is separated from the image forming area of the contact charging unit by 5 mm or more. 14. The image forming apparatus according to claim 8, wherein a cleaning member is installed at a position in contact with the surface of the conductive member.