JP2022043462A - Image forming apparatus - Google Patents

Abstract

To provide an image forming apparatus that can detect a surface potential of a photoconductor drum with high accuracy.SOLUTION: An image forming apparatus is used which has: an image carrier; an electrifying member that electrifies a surface of the image carrier; an electrification voltage application unit that applies an electrification voltage to the electrifying member; an electrification current detection unit that detects an electrification current flowing in the electrifying member; and a control unit that performs control to allow the execution of an acquisition operation for acquiring information on a surface potential of the image carrier based on the electrification current detected by the electrification current detection unit, and an image forming operation. When the control unit completes the image forming operation and then executes the next image forming operation, if the absolute value of the surface potential of the image carrier is smaller than a predetermined threshold, the control unit controls to execute the acquisition operation before executing the next image forming operation.SELECTED DRAWING: Figure 7

Description

The present invention relates to an image forming apparatus.

Conventionally, an electrophotographic image forming apparatus such as an electrophotographic copying machine or a laser beam printer has been used. At the time of image formation, the charging roller first charges the photosensitive drum as an image carrier. Then, the exposure apparatus exposes the charged photosensitive drum to form an electrostatic latent image on the photosensitive drum. Then, the developing roller as the developer carrier develops the electrostatic latent image formed on the photosensitive drum as a toner image. Then, the transfer roller transfers the toner image formed on the photosensitive drum to a recording material such as paper or a sheet. Then, the fixing device heats and pressurizes the toner image transferred to the recording material to fix it on the recording material. As a result, an image is formed on the recording material.

Here, when the potential of the exposed portion exposed by the exposure device in the photosensitive drum is the post-exposure potential VL and the surface potential of the developing roller is the development potential Vdc, the potential difference between the post-exposure potential VL and the development potential Vdc The electrostatic latent image on the photosensitive drum is developed. Specifically, an electric field is generated between the surface of the photosensitive drum and the surface of the developing roller due to the potential difference between the post-exposure potential VL and the developing potential Vdc. Then, the toner supported on the surface of the developing roller moves to the surface of the photosensitive drum due to the flow of the electric field. Here, the potential difference Vcont between the post-exposure potential VL and the development potential Vdc is defined as the development contrast.

On the other hand, in the photosensitive drum, when the potential of the non-exposed portion, which is a portion not exposed by the exposure apparatus, is set to the pre-exposure potential VD, the potential difference Vback between the pre-exposure potential VD and the development potential Vdc changes from the developing roller to the non-exposure portion. The potential difference is set so that the toner does not move. The potential difference Vback between the pre-exposure potential VD and the development potential Vdc is defined as the development back contrast.

Here, the movement of toner from the developing roller to the non-exposed portion and the toner adhering to the non-exposed portion is called "fog". This "fog" occurs when the development back contrast is not a desired value. Therefore, in order to obtain an appropriate image in the electrophotographic image forming apparatus, it is necessary to appropriately control the potential difference Vback and the potential difference Vcont.

Here, when the voltage applied to the charging roller is constant, the potential of the surface of the photosensitive drum (potential of the exposed portion and the non-exposed portion) changes due to deterioration of the photosensitive drum, change in sensitivity of the photosensitive drum, and the like. It is known.
Therefore, conventionally, the post-exposure potential VL and the pre-exposure potential VD have been predicted from the usage status of the photosensitive drum (rotational speed of the photosensitive drum, etc.) and the sensitivity of the photosensitive layer of the photosensitive drum. Then, based on this predicted value, the values of the post-exposure potential VL and the pre-exposure potential VD were corrected to desired values by changing the voltage applied to the charging roller. As a result, the potential difference Vcont and the potential difference Vback also become predetermined values, and it is considered that an appropriate image can be obtained.

However, in the above technique, the voltage applied to the charging roller is obtained not from the potential of the photosensitive drum but from the usage status of the photosensitive drum. Therefore, the potential on the surface of the photosensitive drum may not reach a predetermined potential. Therefore, in Patent Document 1, a voltage is applied to the photosensitive drum after static elimination from a charging roller to generate a discharge, and the DC component of the charging current flowing through the charging roller due to the discharge is detected. As a result, the potential on the surface of the photosensitive drum is detected, and correction is applied so that the potential on the surface of the photosensitive drum approaches the target value based on the detection result.

Japanese Unexamined Patent Publication No. 2012-230141

However, in Patent Document 1, the detection accuracy may be lowered depending on the usage condition of the photosensitive drum, the surrounding environment, and the like.

The present invention has been made in view of the above problems, and an object of the present invention is to provide an image forming apparatus capable of detecting the surface potential of a photosensitive drum with high accuracy.

The present invention adopts the following configuration. That is,
An image forming apparatus that executes an image forming operation for forming an image on a recording material.
With the image carrier,
A charging member that charges the surface of the image carrier, and
A charging voltage application unit that applies a charging voltage to the charging member,
A charging current detecting unit that detects the charging current flowing through the charging member,
It has an acquisition operation for acquiring information on the surface potential of the image carrier based on the charge current detected by the charge current detection unit, and a control unit for executably controlling the image formation operation. ,
When the control unit executes the next image forming operation after the image forming operation is completed, the next image forming operation is performed when the absolute value of the surface potential of the image carrier is smaller than a predetermined threshold value. The image forming apparatus is characterized in that the acquisition operation is controlled to be executed before the execution of the above-mentioned acquisition operation.

According to the present invention, it is possible to provide an image forming apparatus capable of detecting the surface potential of a photosensitive drum with high accuracy in long-term use.

Schematic diagram of the overall configuration of the image forming apparatus in the first embodiment Sectional drawing for explanation of the developing apparatus in Example 1. A block diagram for explaining a control unit and control contents of the apparatus according to the first embodiment. Schematic diagram showing the means for detecting the surface potential of the photosensitive drum in Example 1. The figure which shows the relationship between the charge current ID in Example 1 and the charge amount ΔVD of a photosensitive drum. Explanatory drawing which shows time transition of electric potential after completion of image formation in Example 1. A flowchart showing the operation of the surface potential measurement control in the first embodiment. Flow chart showing the operation of surface potential measurement control in the comparative example

In the following description, embodiments for carrying out the present invention will be exemplified in detail with reference to the drawings and examples. However, the dimensions, materials, shapes, relative arrangements, etc. of the components described in this embodiment should be appropriately changed depending on the configuration of the apparatus to which the invention is applied and various conditions, and the present invention The scope is not intended to be limited to the following examples.

[Example 1]
<Structure of image forming apparatus>
With reference to FIG. 1, a schematic configuration of the entire image forming apparatus of this embodiment will be described. The image forming apparatus 100 is a laser beam printer using an electrograph, and generally includes an image forming apparatus main body M and a process cartridge 20. Here, the image forming apparatus main body M indicates a component in the image forming apparatus excluding the process cartridge 20. On the other hand, the process cartridge 20 is a member that can be attached to and detached from the image forming apparatus main body M and replaced. The process cartridge 20 of this embodiment integrally incorporates a photosensitive drum 1, a charging roller 4, a developing container 9, and a cleaning container 5.

The image forming apparatus to which the present invention is applicable is not limited to the one shown here. The present invention can also be applied to, for example, a color laser printer provided with a plurality of process cartridges 20 and forming a color image after transferring a plurality of images of toner images to a recording material P using an intermediate transfer belt (intermediate transfer body). Is.
Further, the developing apparatus 10 in this embodiment uses a one-component non-contact developing method in which a magnetic one-component developer is used and a predetermined gap is provided between the photosensitive drum 1 and the developing device 10 is arranged so as to face each other. However, a two-component developing method using a two-component developing agent or a contact developing method in which the photosensitive drum 1 and the developing device 10 are in contact with each other may be used.

A photosensitive drum 1 which is an image carrier as a charged body is arranged in the process cartridge 20. The photosensitive drum 1 has an OPC (organic optical semiconductor) photosensitive layer formed on the outer peripheral surface of the conductive drum. A driving force is transmitted from the image forming apparatus main body M to the photosensitive drum 1 at a predetermined process speed, and the photosensitive drum 1 is rotationally driven in the clockwise direction. Around the photosensitive drum 1, there are process means (charging roller 4, cleaning blade 2, etc.) that act on the photosensitive layer, and developing means (developing container 9, developing roller 7, regulating member 8) that develops with a developing voltage. Is placed.

The charging roller 4 is a charging member that uniformly charges the surface of the photosensitive drum 1 to a predetermined charging potential by receiving a charging voltage from a charging voltage power source (not shown). In the charging voltage power supply of this embodiment, a charging voltage of −1050V is applied to the charging roller 4, whereby the pre-exposure potential VD of the photosensitive drum 1 becomes −500V. Although a DC voltage is used in this embodiment, the present invention is not limited to this, and a voltage obtained by superimposing an AC voltage on the DC voltage may be used.

The scanner 6 is an exposure device, and when it receives image information input from a personal computer (not shown) or the like, it is converted into a time-series electric digital image signal by a video controller (not shown), and laser light is emitted correspondingly. Modulate and output. By scanning and exposing the surface of the photosensitive drum 1 charged with the laser beam irradiated in this way, an electrostatic latent image corresponding to the image information is formed on the photosensitive drum 1. In this embodiment, the laser beam was irradiated with a light amount of 3.0 mJ / m ² so that the post-exposure potential VL of the photosensitive drum 1 was −100 V. The scanner 6 may be considered as an exposure unit of the present invention, or a configuration including an optical member such as a mirror in the scanner 6 may be considered as an exposure unit.

The developing roller 7 of the developing apparatus 10 is a developing agent carrier, and the electrostatic latent image formed on the photosensitive drum 1 by the scanner 6 is developed by the developing agent 3. In this embodiment, a magnetic one-component developer (toner) is used as the developer 3, and a non-contact developing method is adopted in which the developing roller 7 and the photosensitive drum 1 are arranged with a predetermined gap (for example, 200 μm).

Further, a transfer roller 11 for transferring the toner image on the photosensitive drum 1 to the recording material P is arranged so as to face the photosensitive drum 1.

<Image formation method>
Next, an image forming method for performing an image forming operation will be described. First, the charging voltage application circuit (not shown) in the image forming apparatus main body M applies a voltage to the charging roller 4, so that the surface of the photosensitive drum 1 is uniformly charged. Next, the scanner 6 emits a laser beam according to the image information, and scans and exposes the surface of the charged photosensitive drum 1. As a result, an electrostatic latent image corresponding to the image information on the photosensitive drum 1 is formed.

The developing apparatus 10 is a member including a developing container 9, a developing roller 7, a regulating member 8, and the like as developing means. A toner layer (magnetic layer) having a predetermined charge amount is formed on the developing roller 7 driven in the counterclockwise direction on the paper surface. When a developing voltage on which a DC voltage and an AC voltage are superimposed is applied, toner flies from the developing roller 7 to the photosensitive drum 1 by an electric field, and an electrostatic latent image is developed as a toner image.

The toner image formed on the photosensitive drum 1 is transferred to the recording material P by the transfer roller 11 to which the transfer voltage is applied. The recording material P to which the toner image is transferred is conveyed to the fixing device 12 as a fixing means. The fixing device 12 applies heat and pressure to the recording material P to fix the toner image on the recording material P.

Further, the transfer residual toner remaining on the photosensitive drum 1 after the transfer step is removed by the cleaning blade 2 as a cleaning means, and is recovered as waste toner.
After that, the surface of the cleaned photosensitive drum 1 is charged and exposed again, and toner is supplied again from the developing roller 7 after development. After that, the flow of transfer and fixing is repeated, and a series of image formation operation cycles are performed.

<Details of each configuration of the image forming apparatus>
(Photosensitive drum 1)
The photosensitive drum 1 of this embodiment is an organic photoconductor (OPC) photosensitive drum, and a resistance layer, an undercoat layer, a charge generation layer, and a charge transport layer are sequentially coated on the outer peripheral surface of an aluminum cylinder having a diameter of 30 mm by a dipping coating method. It is a rigid body composed of. The film thickness of the charge transport layer is 25 μm.

(Charging roller 4)
The charging roller 4, which is the charging means of this embodiment, is a contact charging member. The charging roller 4 uniformly charges the surface (outer peripheral surface) of the photosensitive drum 1 to a predetermined polarity and potential. The charging roller 4 is coated with a urethane surface layer on a base layer of hydrin rubber so that the outer diameter of the core metal having a diameter of 6 mm is 12 mm. The resistance value is 1 × 10 ⁶ Ω or less, and the hardness is 40 degrees with the AskerC rubber hardness tester manufactured by Polymer Meter Co., Ltd.

(Scanner 6)
The scanner 6 is an exposure device that irradiates the surface of the photosensitive drum 1 with a laser, and the amount of light of the laser can be changed. In this embodiment, a laser having a wavelength of 800 nm is irradiated from the scanner 6 of the semiconductor laser. A scanner unit in which an optical member such as a mirror defining an optical path is combined with a semiconductor laser device may be used.

(Transfer roller 11)
The transfer roller 11 arranges the base layer of the ion conductive sponge so that the outer diameter is 15 mm with respect to the core metal having a diameter of 6 mm. The resistance value is 4 × 10 ⁷ Ω in an environment with a temperature of 22 ° C., and the hardness is 30 degrees with an AskerC rubber hardness tester manufactured by Polymer Meter Co., Ltd.

(Cleaning blade 2)
The cleaning blade 2 is formed integrally by supporting the rubber blade by the cleaning support sheet metal. In this embodiment, urethane rubber having a thickness of 2 mm and an MD-1 hardness of 60 degrees in an environment of 23 ° C is used.

(Developer 10)
FIG. 2 is a cross-sectional view of the one-component non-contact developing system developing apparatus 10 used in this embodiment. The developing apparatus 10 has a toner accommodating chamber 9a which is a toner accommodating portion and a developing chamber 9b provided with a developing roller 7 as a developer carrier, and the toner accommodating chamber 9a and the developing chamber 9b have an opening 9c. Communicate with each other through.

In this embodiment, the toner used is a magnetic one-component toner having an average particle size of 7 μm. The developer 3 is supplied from the toner storage chamber 9a to the developing chamber 9b via the opening 9c. The developer 3 supplied to the developing chamber 9b is then supplied to the developing roller 7. The developing roller 7 has a sleeve 7a that supports the developer 3 at the time of image formation, and caps 7b that are fitted at both ends thereof. Further, the sleeve 7a includes a magnet roller 7c corresponding to the magnetic toner.

The developing rollers 7 are arranged so as to face each other while being provided with a predetermined gap (SD gap) from the photosensitive drum 1. In this example, the SD gap was set to 200 μm. Both ends of the sleeve 7a are rotatably supported by the opening of the developing device 10. The sleeve 7a is rotationally driven counterclockwise on the paper surface during the image forming operation. Further, a voltage is applied to the sleeve 7a at a predetermined timing from a voltage applying means (not shown) arranged in the image forming apparatus main body M. In this embodiment, the DC voltage is −400 V, the AC voltage is Vpp = 1400 V, and a rectangular wave having a frequency of 2000 Hz is applied.

The magnet roller 7c, which is a magnetic field generating means, is located in the sleeve 7a, and a plurality of magnetic poles N and S are alternately formed. In this embodiment, four magnetic poles, a developing electrode, a regulating electrode, an intake electrode, and a sealing electrode, are arranged. The developing electrode is arranged at a position facing the photosensitive drum 1 and controls the development of the toner. The regulating electrode is arranged opposite the regulating member 8 and controls the amount of toner on the sleeve 7a. The take-in electrode supplies the toner in the developing chamber 9b onto the sleeve 7a. The sealing electrode prevents toner from leaking from the developing chamber 9b. Further, since the magnet roller 7c is always held at a constant position without performing a rotational operation, the magnetic poles are always kept in the same direction.

The thickness of the toner layer of the toner supplied to the developing roller 7 is regulated by the regulating member 8. The toner regulated by the regulating member 8 is given an appropriate charge by triboelectric charging.

Here, the main parameters of the developing roller 7 and the regulating member 8 in the present invention are listed below.
<< Development Roller 7 >>
Outer diameter: 14 mm
Material: Metallic (nickel, aluminum, SUS)
Surface roughness: Ra 0.2-1.0 μm
<< Regulatory member 8 >>
Material: Urethane Thickness: 1.0 mm

(Control block diagram)
FIG. 3 is a control block diagram of the image forming apparatus main body M of the present invention. The control unit 40 has a CPU 41 which is a central element for performing arithmetic processing, a main body memory such as a storage means ROM 42 and a RAM 43, an input / output I / F 44 which inputs / outputs information to and from peripheral devices, and the like. The detection result of the sensor, the calculation result, and the like are stored in the RAM 43. A data table or the like obtained in advance such as a control program is stored in the ROM 42. Each control target in the image forming apparatus main body M is connected to the CPU 41 via the input / output I / F 44. The CPU 41 controls the transmission and reception of various electrical information signals, the timing of driving, and the like, and controls the processing of the flowchart described later.

The image forming unit 45 is a general term for the photosensitive drum 1, the charging roller 4, the scanner 6, the transfer roller 11, the fixing device 12, and the like described in FIG. 1, and forms an image writing position and an image pattern.
The motor drive unit 46 is each motor that serves as a power source for rotationally driving the scanner 6, the photosensitive drum 1, the developing roller 7, and the like, and operates based on a control signal from the control unit 40.
The high-voltage power supply 47 is a power supply that applies a high voltage to the photosensitive drum 1, the charging roller 4, the developing roller 7, the transfer roller 11, the fixing device, and the like.

The exposure control unit 48 transmits a signal of the amount of laser light emitted to the photosensitive drum 1 to the scanner 6.
The environment sensor 49 transmits information indicating the temperature to the control unit 40 by a sensor for measuring the temperature provided in the image forming apparatus main body M.
The timer 50 transmits information indicating the time, such as the driving time of each motor and the elapsed time after the end of image formation, which will be described later, to the control unit 40.

The memory communication unit 52 performs data communication with the memory 51 of the process cartridge 20 described later, and transmits / receives data to / from the control unit 40. This data is used when the control unit 40 determines the voltage value applied from the high voltage power supply 47 to the charging roller 4, the developing roller 7, and the like. It is also used when determining the potential measurement timing on the surface of the photosensitive drum 1, which will be described later.

(memory)
The process cartridge 20 of this embodiment is provided with a memory 51. The memory 51 records driving time information of the photosensitive drum 1 (cumulative driving amount for rotating the photosensitive drum 1), charging time (cumulative charging time in which the charging roller 4 charges the photosensitive drum 1), and the like. The information for determining the usage status of the photosensitive drum 1 is not limited to the cumulative drive amount of the photosensitive drum 1 as in this embodiment, and any information may be used as long as the usage status of the photosensitive drum 1 can be determined. ..

Further, in the memory 51 of this embodiment, the usage status of the photosensitive drum 1 for determining whether the potential measurement on the surface of the photosensitive drum 1 described later is feasible, and the proportionality constant α used when obtaining the charging amount ΔVD from the charging current I. Etc. are remembered. Further, the memory 51 contains information such as a serial number and a model that can specify the type of the process cartridge 20. When the process cartridge 20 is attached to the image forming apparatus main body M, the memory 51 can communicate with the control unit 40 via the memory communication unit 52, and the control unit 40 can read and write information. The communication method may be a non-contact type or a contact type via an electric contact.

(Arrangement of photosensitive drum 1, charging roller 4, scanner 6, etc.)
The arrangement of the photosensitive drum 1, the charging roller 4, the scanner 6, and the like in this embodiment will be described with reference to FIG. FIG. 4 is a schematic view showing a means for detecting the surface potential of the photosensitive drum 1.

Both ends of the core metal of the charging roller 4 in the longitudinal direction are rotatably held by bearing members (not shown). The bearing member is urged toward the photosensitive drum 1 by a pressing spring 4c as an urging member. As a result, the charging roller 4 is pressed against the surface of the photosensitive drum 1 with a predetermined pressing force to form a charging nip which is a contact portion with the surface of the photosensitive drum 1. The charging roller 4 is driven to rotate with the rotation of the photosensitive drum 1. The scanner 6 is arranged so that the laser beam is applied to the outer peripheral surface of the photosensitive drum 1 via the optical member. The developing roller 7 and the transfer roller 11 are arranged so as to face the photosensitive drum 1.

A charging voltage application circuit 4a is connected to the charging roller 4. The charging voltage application circuit 4a is a charging voltage application unit that is connected to a constant voltage power supply and applies a charging voltage, which is a DC voltage, to the charging roller 4. The surface of the photosensitive drum 1 to which the charging voltage is applied is uniformly charged to the pre-exposure potential VD. The scanner 6 scans and exposes the surface of the photosensitive drum 1 charged in the manner of the charging roller 4 to form a potential VL after exposure. The control unit 40 refers to the information of the RAM 43, the environment sensor 49, and the memory 51 to the charging roller 4 so that the target value of the pre-exposure potential VD is −500 V and the target value of the post-exposure potential VL is −100 V. The applied voltage of the scanner 6 and the amount of laser light of the scanner 6 are determined.

Further, in the present embodiment, the charging current detecting circuit 4b (charging current detecting unit) for detecting the DC component of the charging current is connected to the charging voltage applying circuit 4a. The charging current is a current flowing through the charging roller 4 when a voltage is applied to the charging roller 4 from the charging voltage application circuit. A transfer voltage application circuit 11a is connected to the transfer roller 11.

<Means for detecting the pre-exposure potential VD on the surface of the photosensitive drum 1>
Next, a means for detecting the pre-exposure potential VD on the surface of the photosensitive drum 1 will be described. When a discharge is generated by applying a voltage from the charging roller 4 to the photosensitive drum 1, a charging current flows through the charging roller 4. In this embodiment, the potential on the surface of the photosensitive drum 1 is measured by detecting the DC component of the charging current.

FIG. 5 is an explanatory diagram showing the relationship between the charging current ID of this embodiment and the charging amount ΔVD of the photosensitive drum 1. Here, the charging amount ΔVD of the photosensitive drum 1 is the pre-charging potential VD', which is the potential of the surface of the photosensitive drum 1 before passing through the charging nip, and the post-charging potential, which is the potential of the surface of the photosensitive drum 1 after passing through the charging nip. In the embodiment, it is a value obtained by the difference from the pre-exposure potential VD). Assuming that the charging current flowing at this time is an ID, the charging current ID and the charging amount ΔVD have a proportional relationship as shown in the following equation (1).

ΔVD = VD-VD'= αID (1)

Therefore, the pre-exposure potential VD on the surface of the photosensitive drum 1 can be detected by detecting the charging current ID in a state where the pre-charging potential VD'is 0V.

Here, the proportionality constant α in the equation (1) can be expressed as the following equation (2). In the formula (2), the film thickness d of the photosensitive layer, the relative permittivity ε of the photosensitive layer, the dielectric constant ε ₀ in vacuum, and the effective charging width of the charging roller 4 (direction substantially orthogonal to the moving direction of the surface of the photosensitive drum 1). ) L, process speed vp.

α = d / (ε × ε ₀ × L × vp) (2)

Note that ε ₀ , L, and vp are parameters that can be predetermined regardless of the usage status and the type of the photosensitive drum 1.

On the other hand, the relative permittivity ε of the photosensitive layer is a value determined by the usage condition of the photosensitive drum 1 and its type. Further, the film thickness d of the photosensitive layer is a value determined by the usage condition of the photosensitive drum 1. Therefore, the control unit 40 detects the pre-exposure potential VD based on the information of the photosensitive layer stored in the memory 51, the information of the photosensitive drum 1 stored in the RAM 43, the temperature information by the environmental sensor 49, and the like. The relative permittivity ε and the film thickness d are determined. In this embodiment, a table according to the type and usage status of the photosensitive layer to be used is stored in the memory 51 in advance, and the information is read out by the control unit 40 to determine the proportionality constant α.

In this embodiment, after applying a charging voltage, the photosensitive drum 1 is rotated once by a drive motor, and the pre-exposure potential VD is detected using the average value of the charging current flowing during the rotation of the photosensitive drum 1. ..

<Means for detecting the potential VL after exposure on the surface of the photosensitive drum 1>
Next, a means for detecting the post-exposure potential VL on the surface of the photosensitive drum 1 will be described. In this embodiment, the surface potential is changed from the pre-charge potential VD'to the post-exposure potential VL, and the photosensitive drum 1 is passed through the charging nip to be recharged to the post-charge potential VD (= pre-exposure potential VD). Then, the post-exposure potential VL is measured by detecting the DC component of the charging current flowing during this recharging.

Here, if the charge amount when recharged from the post-exposure potential VL to the pre-exposure potential VD is ΔVL and the charging current flowing at that time is IL, then ΔVL = VD-VL = αIL, so VL is as follows. It can be expressed as the equation (3).

VL = VD-αIL (3)

As the pre-exposure potential VD in the formula (3), the value when the pre-exposure potential VD on the surface of the photosensitive drum 1 is detected may be used. Further, the proportionality constant α is the same as that used in the equation (2).

In this way, the post-exposure potential VL of the surface of the photosensitive drum 1 can be calculated by detecting the charging current IL that flows when the post-exposure potential VL is recharged to the pre-exposure potential VD. In this embodiment, the pre-exposure potential VD is measured and then the post-exposure potential VL is measured. This is to improve the accuracy of the pre-exposure potential VD used in the measurement of the post-exposure potential VL. Further, in this embodiment, after the start of irradiation of the laser beam by the scanner 6, the photosensitive drum 1 is driven by a drive motor for one rotation, and the post-exposure potential is used by using the average value of the charging current flowing during one rotation of the photosensitive drum 1. We are measuring VL.

<Method of determining the execution timing of surface potential measurement control>
Next, a method of determining the timing for executing the surface potential measurement control will be described. The surface potential measurement control is a control for executing the measurement of the pre-exposure potential VD and the measurement of the post-exposure potential VL as a series of flows, and is an acquisition operation for acquiring information on the surface potential.

From the above description, it can be seen that it is important that the precharge potential VD'is 0V in the measurement of the pre-exposure potential VD of this embodiment. Therefore, the pre-exposure potential VD can be measured with high accuracy by executing the measurement of the pre-exposure potential VD at the timing when the pre-exposure potential VD'is a predetermined value of 0 V.

FIG. 6 is an explanatory diagram showing the time transition of the potential VDt after the completion of image formation in this embodiment. Here, the potential VDt after the completion of image formation is the potential on the surface of the photosensitive drum 1 after the completion of image formation. Even under dark light, the potential of the photosensitive drum 1 decreases with the elapsed time t (dark attenuation). Therefore, as shown in FIG. 6, the potential VDt after the end of image formation decreases with the passage of time after the end of image formation. In this embodiment, when the elapsed time t = T, VDt = VD'= 0V. As will be described later, T is a time required for the absolute value of the surface potential to drop to a predetermined threshold value, and is a threshold value for determination by the control unit 40.

Therefore, t ≧ T, that is, the elapsed time t from the end of image formation is equal to or longer than the required time, which is a condition for determining whether or not the measurement of the pre-exposure potential VD can be executed.
Here, the elapsed time t is measured by the timer 50 in the control unit 40. The threshold value T is a value determined by the usage status of VDt0 and the photosensitive drum 1 in FIG. 6, the type of the photosensitive drum 1, and the surrounding environment. Therefore, the control unit 40 can determine the threshold value T by referring to the information of the environment sensor 49, the RAM 43, and the memory 51.

VDt0 is the potential on the surface of the photosensitive drum 1 immediately after the end of image formation, and is determined by the operation at the end of image formation. Since VDt0 is a parameter for determining the threshold value T, it is desirable that the state is predictable to some extent. Therefore, in this embodiment, the photosensitive drum 1 is recharged by applying a voltage to the charging roller 4 at the end of image formation, the potential on the surface of the photosensitive drum 1 is set to the pre-exposure potential VD, and then the image formation is completed. The operation at the end of image formation is not limited to this. For example, the potential of the surface of the photosensitive drum 1 can be lowered and the value of the threshold value T can be reduced by irradiating the laser beam with the scanner 6 at the end of image formation and then ending the image formation.

The tables shown in Tables 1 and 2 are stored in the memory 51. Table 1 is used when determining the usage status of the photosensitive drum 1 from the cumulative operating time and the cumulative charging time of the photosensitive drum 1. Here, the cumulative drive amount of the photosensitive drum 1 in Table 1 is the ratio of the drive amount up to the present to the total drive amount until the photosensitive drum 1 reaches the end of its life. The cumulative charging time is the ratio of the charging time up to the present to the total driving time until the photosensitive drum 1 reaches the end of its life.

Further, Table 2 is used when determining the threshold value T from the usage status of the photosensitive drum 1 and the information of the environment sensor 49. Here, the ambient environmental temperature in Table 2 is the temperature information in the usage environment detected by the environment sensor 49. The usage status of the photosensitive drum 1 is the value obtained in Table 1. As the usage environment information, other information such as humidity may be used instead of the temperature information or together with the temperature information.

From the above description, the pre-exposure potential VD and the post-exposure potential VL can be accurately measured by comparing the elapsed time t and the threshold value T by the control unit 40 and performing the control in a state where t ≧ T.
In view of this, the control unit 40 performs surface potential measurement control at a predetermined execution timing. The predetermined execution timing of this embodiment is when the power of the image forming apparatus main body M is turned on, when the image forming apparatus 100 wakes up from sleep, and images are formed a predetermined number of times (for example, 1000 images). At the timing, it is determined whether or not t ≧ T. Further, it may be determined whether or not the photosensitive drum 1 is new, and if it is determined that the photosensitive drum 1 is new, the surface potential measurement control may be performed. Then, when t ≧ T, the surface potential measurement control is executed. It should be noted that the table may be stored in the memory 51 in a format such as a mathematical formula instead of the table as in Table 1 and Table 2.

<Flow chart of surface potential measurement control>
Next, the operation of the surface potential measurement control in this embodiment will be described with reference to the flowchart of FIG. 7. This flow is executed in the case of the above-mentioned surface potential measurement control conditions. When the print job is started, the control unit 40 acquires information on the cumulative drive amount and the cumulative charging time of the photosensitive drum 1 from the RAM 43, and also refers to the table 1 stored in the memory 51 to obtain the photosensitive drum 1 Read the usage status of (step S101).

Next, the control unit 40 acquires the usage status of the photosensitive drum 1 read in S101 and the information of the environment sensor 49, and reads the threshold value T by referring to Table 2 stored in the memory 51 (step S102). ..
Next, the control unit 40 reads the elapsed time t from the end of image formation from the timer 50 (step S103).
Then, the control unit 40 compares the threshold value T read in S102 with the elapsed time t read in S103, and determines whether or not t ≧ T (step S104).

When it is determined that t ≧ T (S104 = No), the control unit 40 controls the image forming unit 45 to execute the image forming operation without executing the surface potential measurement control (step S109).
On the other hand, when it is determined that t ≧ T (S104 = Yes), the control unit 40 of the photosensitive drum 1 performs an operation of acquiring information on the surface potential before executing the next image forming operation. The voltage application to the driving and charging rollers 4 is turned on (step S105).
Then, the control unit 40 rotates the photosensitive drum 1 once, and calculates the value of the pre-exposure potential VD from the average value of the current values of the charging current ID flowing through the charging roller 4 acquired by the charging current detection circuit 4b. (Step S106)

Subsequently, the control unit 40 turns on the irradiation of the laser beam from the scanner 6 to the surface of the photosensitive drum 1 (step S107).
Then, the control unit 40 rotates the photosensitive drum 1 once from the irradiation of the laser beam, and calculates the value of the post-exposure potential VL from the average value of the current values of the charging current IL flowing through the charging current detection circuit 4b by the charging current detection circuit 4b. (Step S108).
The surface potential measurement control is terminated, then the image forming operation is executed (step S109), and the charging voltage is turned off in order to end the image forming operation (step S110).

The voltage value applied to the charging roller 4 from the charging voltage application circuit 4a in the surface potential control and the value of the amount of laser light emitted from the scanner 6 are the same as the values used during normal image formation. do. Here, the voltage applied to the charging roller 4 is −1000 V, and the amount of laser light emitted from the scanner 6 is 3.0 mJ / m ² .

Further, the operation of the flowchart of FIG. 7 may be executed every time if the condition of t ≧ T is satisfied.
It may be performed at a predetermined execution timing. This may be executed when the power of the image forming apparatus main body M is turned on, when the image forming apparatus 100 wakes up from sleep, and when images are formed a predetermined number of times (for example, 1000 images). It may be determined whether the photosensitive drum 1 is new or not, and if it is determined that the photosensitive drum 1 is new, the execution may be performed.
The temperature environment condition for the comparative study between Example 1 and Comparative Example, which will be described later, was a temperature of 30 ° C.

<Comparison example>
In order to show the effect of the surface potential measurement control of this example, the surface potential measurement control according to the comparative example was performed, and will be described below. In this experiment, the pre-exposure potential VD and the post-exposure potential VL of the surface of the photosensitive drum 1 were measured by the surface potential measurement control methods of each of the present embodiment and the comparative example, and the measurement results obtained by using a surface electrometer were used. The measurement error is calculated. This experiment is performed on a plurality of photosensitive drums 1 having different usage conditions.

First, the operation of the surface potential measurement control in the comparative example will be described with reference to the flowchart of FIG. In the comparative example, unlike the present embodiment, the elapsed time t is not measured by the timer 50 in the control unit 40 after the image formation is completed, and then the pre-exposure potential VD is measured. The execution condition was fixed at t = 20 minutes.

When the print job is started, the drive of the photosensitive drum 1 is turned on, the voltage applied to the charging roller 4 is turned on (step S201), and the image forming operation is started (step S202).
When the image forming operation is completed, the voltage applied to the charging roller 4 is turned off and the drive of the photosensitive drum 1 is turned off (step S203).
Then, the image forming apparatus 100 is left in the state at the end of image formation until the elapsed time t elapses, and after the elapsed time t elapses, the drive of the photosensitive drum 1 is turned on and the charging voltage is turned on (step S204).

The photosensitive drum 1 is rotated once, and the pre-exposure potential VD value is calculated from the average value of the current value IDs flowing through the charging current detection circuit 4b by the charging current detection circuit 4b (step S205).

After that, the drive of the photosensitive drum 1 is turned off, the voltage application to the charging roller 4 is turned off (step S206), and the control is terminated.

The results of this experiment are summarized in Tables 3 and 4. Table 3 lists the usage status of the photosensitive drum 1 used in the experiment in each example. As shown in the table, usage varies between 10% and 90%. The charging voltage was adjusted so that the pre-exposure potential VD was constant at -490V regardless of the usage status of the photosensitive drum 1.

Table 4 shows the measurement error with the surface electrometer in the measurement of the pre-exposure potential VD obtained by the experiment. As described above, since the pre-charging potential VD'has the relationship shown by the equation (1), the error from 0V with the pre-charging potential VD'is the measurement error with the surface electrometer.

In the embodiment, the control unit 40 determines that the elapsed time t from the end of image formation exceeds the threshold value T at which the precharge potential VD'= 0V, based on the information of the RAM 43, the environment sensor 49, and the memory 51. In some cases, surface potential measurement control is performed. Therefore, since the surface potential measurement can be performed at VD'= 0V, no measurement error occurs.

On the other hand, in the comparative example, since the elapsed time t is fixed at t = 20 minutes without measuring, the measurement error increases as the usage of the photosensitive drum 1 progresses. As the use of the photosensitive drum 1 progresses, the amount of attenuation (called dark attenuation) of the surface potential of the photosensitive drum 1 per unit time becomes smaller, so that the surface potential reaches 0 V (a predetermined threshold value of the surface potential here). It is necessary to set the threshold value T large. As a result, the error became the largest especially under the condition of E where the use of the photosensitive drum 1 was advanced.

In this embodiment, when the control unit 40 determines based on the information of the RAM 43, the environment sensor 49, and the memory 51 that the elapsed time t from the end of image formation exceeds the threshold value T at which the precharge potential VD'= 0V. Surface potential measurement control is performed. Therefore, the potential can be measured with high accuracy. As a result, it is possible to provide an image forming apparatus capable of predicting the surface potential of the photosensitive drum 1 with high accuracy in long-term use. Therefore, the control unit 40 can change the voltage applied to the charging roller 4 based on the predicted value of the surface potential with high accuracy, and for example, the value such as the post-exposure potential VL or the pre-exposure potential VD can be set to a desired value. Can be corrected to. As a result, since the potential difference Vcont that becomes the development contrast and the potential difference Vback that becomes the development back contrast can be appropriately controlled, it is possible to suppress the occurrence of defects such as toner fog.

In this embodiment, after the image formation operation is executed and before the surface potential measurement control is executed, the absolute value of the surface potential may be made smaller than VD in advance by using exposure. Further, the exposure amount at that time may be the same as that at the time of the image forming operation, and may be small or large. It is particularly desirable that the timing of exposing the surface of the photosensitive drum 1 is performed immediately after the image forming operation is completed. That is, by executing the exposure when t = 0, the elapsed time until T after that can be shortened, so that the timing for executing the surface potential measurement control with high accuracy increases. It is desirable that the above exposure is performed once or more around the photosensitive drum 1.

Further, as described above, the absolute value of the surface potential may be made smaller than VD in advance by using exposure, but a static elimination unit is provided to eliminate static electricity on the surface of the photosensitive drum 1 after transfer and before charging in the rotation direction of the photosensitive drum 1. Therefore, static electricity may be removed instead of exposure. At this time, the static elimination device may use a light emitting device such as an LED.

Further, the threshold value T is set to be longer than the required time from the end of the image forming operation until the absolute value of the surface potential formed when the image forming operation is completed decreases to a predetermined value. May be set. That is, it suffices that at least a sufficient time has elapsed for the absolute value of the surface potential of the photosensitive drum 1 to drop to a predetermined value.

Further, in this embodiment, the required time and the threshold value T are compared, but the information related to the surface potential of the photosensitive drum 1 may be directly acquired, and whether or not the acquisition operation can be performed may be determined based on the information. .. Specifically, the current value of the current flowing from the photosensitive drum 1 to the charging roller 4 when a voltage is applied to the charging roller 4 and the current value of the current flowing from the photosensitive drum 1 to the transfer roller 11 when a voltage is applied to the transfer roller 11. And the preset threshold value may be compared to determine whether or not the acquisition operation can be executed.

1: Photosensitive drum, 4: Charging roller, 4b: Charging current detection circuit, 6: Scanner, 10: Developing device, 40: Control unit

Claims (20)

An image forming apparatus that executes an image forming operation for forming an image on a recording material.
With the image carrier,
A charging member that charges the surface of the image carrier, and
A charging voltage application unit that applies a charging voltage to the charging member,
A charging current detecting unit that detects the charging current flowing through the charging member,
It has an acquisition operation for acquiring information on the surface potential of the image carrier based on the charge current detected by the charge current detection unit, and a control unit for executably controlling the image formation operation. ,
When the control unit executes the next image forming operation after the image forming operation is completed, the next image forming operation is performed when the absolute value of the surface potential of the image carrier is smaller than a predetermined threshold value. An image forming apparatus, characterized in that the acquisition operation is controlled to be executed before the execution of the image forming apparatus. Further having a memory for storing information about the image carrier,
The control unit is a time required from the end of the image forming operation until the absolute value of the surface potential formed when the image forming operation is completed becomes smaller than the threshold value, and is the image. When the elapsed time from the end of the image forming operation to the execution of the next image forming operation is longer than the required time based on the information about the carrier, the next image forming operation is performed. The image forming apparatus according to claim 1, wherein the acquisition operation is controlled to be executed before the execution. The image forming apparatus according to claim 2, wherein the memory stores information regarding the usage status of the image carrier. The image forming apparatus according to claim 3, wherein the information regarding the usage status of the image carrier is information indicating the cumulative drive amount and the cumulative charging time of the image carrier. The image forming apparatus according to claim 2, wherein the control unit further uses the usage environment information of the image forming apparatus to calculate the required time. The image forming apparatus according to claim 5, further comprising a sensor for measuring temperature information as usage environment information. Information indicating the type of the image carrier is stored in the memory.
The image forming apparatus according to any one of claims 2, 5 and 6, wherein the control unit further uses information indicating the type of the image carrier to calculate the required time. The image forming apparatus according to any one of claims 1 to 7, wherein the information regarding the surface potential of the image carrier is a current value flowing through the charging member. One of claims 2, 5 and 6, wherein the control unit calculates the time required for the surface potential to reach 0 V after the image forming operation is executed as the required time. The image forming apparatus according to. The charging member recharges the image carrier after the image forming operation is completed.
The image according to any one of claims 2, 5 and 6, wherein the control unit calculates the required time for the surface potential to drop to the threshold value when the surface potential is recharged. Forming device. It has an exposure unit that exposes the surface of the image carrier to form an electrostatic latent image on the surface of the image carrier charged by the charging member.
The exposure unit exposes the surface of the image carrier after the image forming operation is completed.
The control unit according to any one of claims 2, 5 and 6, wherein the control unit calculates the required time for the surface potential to drop to the threshold value when the exposure is performed. Image forming device. The image forming apparatus according to any one of claims 1 to 11, wherein the threshold value is 0V. The image forming apparatus according to any one of claims 1 to 12, wherein the control unit controls to execute the acquisition operation when the power of the image forming apparatus is turned on. The image forming apparatus according to any one of claims 1 to 13, wherein the control unit controls to execute the acquisition operation when the image forming apparatus returns from sleep. The image forming apparatus according to any one of claims 1 to 14, wherein the control unit controls to execute the acquisition operation every time the image forming operation is performed a predetermined number of times. The image forming apparatus according to any one of claims 1 to 14, wherein the control unit controls to execute the acquisition operation when the image carrier is new. The control unit is characterized in that it controls to execute the acquisition operation for measuring the surface potential before the image carrier is exposed, based on the charge current detected by the charge current detection unit. The image forming apparatus according to any one of claims 1 to 16. The control unit measures the surface potential after the image carrier is exposed, based on the amount of charge when the image carrier is recharged by the charging member after the image carrier is exposed. 17. The image forming apparatus according to claim 17, wherein the image forming apparatus is controlled to execute the above. 18. The image forming according to claim 18, wherein the control unit controls the charging voltage applied to the charging member based on the surface potential before and after the image carrier is exposed. Device. It has an exposure unit that exposes the surface of the image carrier to form an electrostatic latent image on the surface of the image carrier charged by the charging member.
The exposure unit exposes the surface of the image carrier after the image forming operation is completed.
The control unit is characterized in that when the surface potential at the time of the exposure drops to the threshold value, the acquisition operation is executed before the next image formation operation is executed. The image forming apparatus according to.

Images