Image formation apparatus

Abstract

To provide an image formation apparatus capable of accurately estimating the surface potential of an image carrier for each rotation while taking the influence of a transfer voltage into account.SOLUTION: The image formation apparatus includes: an image carrier; a charging device; a developing device; a transfer member; a developing voltage power supply; a charging voltage power supply; a transfer voltage power supply; a current detection unit; and a control unit, the control unit being configured to apply a developing voltage and a charging voltage to a developer carrier of the developing device and to a charging member, respectively, in a state where no transfer voltage is applied, to estimate the surface potential of the image carrier on the basis of a developing current detected when the developing voltage is varied, to calculate the relationship between the change amount of the developing current and the change amount of the surface potential, to apply the developing voltage and the charging voltage to the developer carrier and the charging member, respectively, in a state where the transfer voltage is applied, and to estimate the change amount of the surface potential of the image carrier for each rotation on the basis of the change amount of the developing current with respect to the driving distance of the image carrier and the relationship between the change amount of the developing current and the change amount of the surface potential.SELECTED DRAWING: Figure 4

Description

[Technical Field]

[0001] The present invention relates to an electrophotographic image forming apparatus, and more particularly to a method for estimating the surface potential of a photosensitive member, which is an image carrier, and adjusting the surface potential based on the estimation result. [Background technology]

[0002] In conventional electrophotographic image forming apparatuses, an image carrier, such as a photosensitive drum with a photosensitive layer formed on its surface, is charged to a predetermined surface potential, and an exposure device scans the image with light to form an electrostatic latent image with the charge attenuated. This electrostatic latent image is then developed by a developing device and visualized as a toner image. In such image forming apparatuses, to obtain high-quality images, development must be performed using a DC voltage Vdc that provides an appropriate potential difference (development potential difference ΔV = V0 - Vdc) with respect to the surface potential V0 of the image carrier. Therefore, the surface potential of the image carrier must be detected, and conventionally, this has been detected using a surface potential sensor.

[0003] However, surface potential sensors are expensive, and there are also problems with them being unable to accurately measure the surface potential if scattered toner or the like adheres to the detection surface. Therefore, various techniques have been proposed for detecting the surface potential of an image carrier without using a surface potential sensor.

[0004] Patent Document 1 discloses an electrophotographic device that includes a circuit that forms a pulsed electrostatic potential pattern on a photosensitive member and detects the current that flows when this electrostatic potential pattern is developed, a circuit that obtains the surface potential on the photosensitive member by converting the detected current into a surface potential based on a predetermined relational expression or using a pre-created correlation table, and a circuit that feeds back the obtained surface potential to control the charging of the photosensitive member by a charger. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2003-295540 Summary of the Invention [Problem to be solved by the invention]

[0006] Because the surface potential changes under the influence of the transfer voltage, accurate detection of the surface potential requires the pressure of the transfer roller and the application of the transfer voltage. However, depending on the usage environment and durability of the image forming apparatus, the surface potential changes with each revolution of the photosensitive drum under the influence of the transfer voltage. Therefore, the method of Patent Document 1 has the problem of being unable to accurately detect even subtle changes in the surface potential.

[0007] SUMMARY OF THE INVENTION In view of the above problems, an object of the present invention is to provide an image forming apparatus capable of accurately estimating the surface potential of an image carrier for each revolution, taking into account the influence of transfer voltage. [Means for solving the problem]

[0008] To achieve the above object, a first aspect of the present invention is an image forming apparatus including an image carrier, a charging device, a developing device, a transfer member, a developing voltage power supply, a charging voltage power supply, a transfer voltage power supply, a current detection unit, and a control unit. The image carrier has a photosensitive layer formed on its surface. The charging device has a charging member that charges the surface of the image carrier to a predetermined surface potential. The developing device has a developer carrier that carries a developer containing toner, and forms a toner image by exposing the image carrier charged by the charging device to light, thereby causing the toner to adhere to an electrostatic latent image formed on the image carrier. The transfer member contacts the image carrier to form a transfer nip, and transfers the toner image formed on the surface of the image carrier to a recording medium or an intermediate transfer member. The developing voltage power supply applies a developing voltage including a DC voltage to the developer carrier. The charging voltage power supply applies a charging voltage to the charging member. The transfer voltage power supply applies a transfer voltage of opposite polarity to the toner to the transfer member. The current detection unit detects the developing current flowing between the image carrier and the developer carrier. The control unit controls the developing voltage power supply, the charging voltage power supply, and the transfer voltage power supply. The control unit applies a developing voltage and a charging voltage to the developer carrier and the charging member, respectively, in a state where no transfer voltage is applied, estimates the surface potential based on the developing current detected by the current detection unit when the developing voltage is changed, calculates the relationship between the amount of change in the surface potential and the amount of change in the developing current, applies a developing voltage and a charging voltage to the developer carrier and the charging member, respectively, in a state where the transfer voltage is applied, and estimates the amount of change in the surface potential for each revolution of the image carrier based on the amount of change in the developing current with respect to the driving distance of the image carrier and the relationship between the amount of change in the surface potential and the amount of change in the developing current. [Effects of the Invention]

[0009] According to the first aspect of the present invention, the development current is detected for each revolution of the image carrier, and the change in surface potential is calculated from the change in the detected development current, thereby making it possible to accurately estimate the change in surface potential for each revolution of the image carrier, taking into account the influence of the transfer voltage applied to the transfer member. Furthermore, by correcting the charging voltage based on the estimated change in surface potential for each revolution of the image carrier, it is possible to maintain a stable surface potential for a long period of time, regardless of the usage environment of the image forming apparatus or the deterioration state of the image carrier. [Brief explanation of the drawings]

[0010] [Figure 1] FIG. 1 is a side cross-sectional view showing the internal configuration of an image forming apparatus 100 according to an embodiment of the present invention. [Figure 2] 1 is a partially enlarged view of the image forming unit Pa and its surroundings, including a control path of the image forming apparatus 100; [Figure 3] Graph showing the relationship between the DC voltage Vdc of the developing voltage and the developing current in a conventional method for estimating the surface potential. [Figure 4] FIG. 1 is a diagram showing the transition of the surface potential with respect to the driving distance of the photosensitive drums 1a to 1d when a transfer voltage is continuously applied. [Figure 5] FIG. 1 is a diagram showing the transition of the development current with respect to the driving distance of the photosensitive drums 1a to 1d when the transfer voltage is continuously applied. [Figure 6] 1 is a flowchart showing a control example of a surface potential adjustment process executed in the image forming apparatus 100 of this embodiment. [Figure 7] Graph showing the relationship between the amount of change in development current and the amount of change in surface potential in Examples DETAILED DESCRIPTION OF THE INVENTION

[0011] [1. Configuration of image forming device] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. Fig. 1 is a cross-sectional view showing the internal structure of an image forming apparatus 100 according to one embodiment of the present invention. Within the main body of the image forming apparatus 100 (here, a color printer), four image forming units Pa, Pb, Pc, and Pd are arranged in this order from the upstream side in the transport direction (left side in Fig. 1). These image forming units Pa to Pd are provided corresponding to images of four different colors (yellow, magenta, cyan, and black), and sequentially form images of yellow, cyan, magenta, and black through the processes of charging, exposure, development, and transfer, respectively.

[0012] Each of the image forming stations Pa through Pd is provided with photosensitive drums (image carriers) 1a, 1b, 1c, and 1d, which carry visible images (toner images) of each color. An intermediate transfer belt 8, which rotates counterclockwise in FIG. 1 by a belt drive motor (not shown), is provided adjacent to each of the image forming stations Pa through Pd. The toner images formed on the photosensitive drums 1a through 1d are sequentially transferred (primary transfer) onto the intermediate transfer belt 8, which moves while contacting the photosensitive drums 1a through 1d, and superimposed on each other. The toner images primarily transferred onto the intermediate transfer belt 8 are then secondarily transferred onto a transfer sheet P (an example of a recording medium) by a secondary transfer roller 9. The transfer sheet P, onto which the toner images have been secondarily transferred, is then fixed in a fixing unit 13 and ejected from the image forming apparatus 100. While the photosensitive drums 1a through 1d rotate clockwise in FIG. 1, an image formation process is performed on each of the photosensitive drums 1a through 1d.

[0013] The transfer paper P onto which the toner image is secondarily transferred is stored in a paper cassette 16 located at the bottom of the main body of the image forming apparatus 100, and is transported via a paper feed roller 12a and a pair of registration rollers 12b to the nip between the secondary transfer roller 9 and the drive roller 11 of the intermediate transfer belt 8. A sheet made of dielectric resin is used for the intermediate transfer belt 8, and a seamless belt is usually used. In addition, a belt cleaning device 19 is located downstream of the secondary transfer roller 9 to remove toner and other particles remaining on the surface of the intermediate transfer belt 8.

[0014] Next, the image forming units Pa to Pd will be described. Around and below the rotatably arranged photosensitive drums 1a to 1d, there are provided charging devices 2a, 2b, 2c, and 2d that charge the photosensitive drums 1a to 1d, an exposure device 5 that exposes image information onto each of the photosensitive drums 1a to 1d, developing devices 3a, 3b, 3c, and 3d that form toner images on the photosensitive drums 1a to 1d, and cleaning devices 7a, 7b, 7c, and 7d that remove toner (residual toner) remaining on the photosensitive drums 1a to 1d.

[0015] When image data is input from a host device such as a personal computer, the surfaces of the photosensitive drums 1a-1d are first uniformly charged by the charging devices 2a-2d. Next, the exposure device 5 irradiates light according to the image data, forming electrostatic latent images on the photosensitive drums 1a-1d in accordance with the image data. The developing devices 3a-3d are filled with a predetermined amount of two-component developer containing yellow, magenta, cyan, and black toner, respectively. If the toner content in the two-component developer filled in each developing device 3a-3d falls below a specified value due to the formation of a toner image (described below), toner is replenished from toner containers 4a-4d to each developing device 3a-3d. The toner in the developer is supplied to the photosensitive drums 1a-1d by the developing devices 3a-3d and electrostatically adheres to the photosensitive drums 1a-1d, forming a toner image corresponding to the electrostatic latent image formed by exposure from the exposure device 5.

[0016] Then, primary transfer rollers 6a-6d apply an electric field at a predetermined transfer voltage between the primary transfer rollers 6a-6d and the photosensitive drums 1a-1d, and the yellow, magenta, cyan, and black toner images on the photosensitive drums 1a-1d are primarily transferred onto the intermediate transfer belt 8. These four color images are formed in a predetermined positional relationship for forming a predetermined full-color image. After that, toner and other substances remaining on the surfaces of the photosensitive drums 1a-1d after the primary transfer are removed by cleaning devices 7a-7d in preparation for the subsequent formation of a new electrostatic latent image.

[0017] The intermediate transfer belt 8 is stretched over a driven roller 10 on the upstream side and a drive roller 11 on the downstream side. When the intermediate transfer belt 8 starts to rotate counterclockwise as the drive roller 11 is rotated by a belt drive motor (not shown), the transfer paper P is transported from the registration roller pair 12b to a nip portion (secondary transfer nip portion) between the drive roller 11 and the adjacent secondary transfer roller 9 at a predetermined timing, where the toner image on the intermediate transfer belt 8 is secondarily transferred onto the transfer paper P. After the secondary transfer, any toner remaining on the surface of the intermediate transfer belt 8 is removed by a belt cleaner 19. The transfer paper P onto which the toner image has been secondarily transferred is transported to a fixing unit 13.

[0018] The transfer paper P transported to the fixing unit 13 is heated and pressurized by the fixing roller pair 13a, and the toner image is fixed to the surface of the transfer paper P, forming a predetermined full-color image. The transfer paper P on which the full-color image has been formed is then transported in different directions by the branching unit 14, which branches into multiple directions, and is then discharged directly (or after being sent to the double-sided transport path 18 and having images formed on both sides) onto the discharge tray 17 by the discharge roller pair 15.

[0019] [2. Configuration of the image forming unit and its peripherals, including the control path of the image forming device] 2 is a partially enlarged view of the periphery of image forming unit Pa, including the control paths of image forming apparatus 100. In the following explanation, the configuration and control paths of image forming unit Pa will be described, but the configurations and control paths of image forming units Pb to Pd are similar, so their explanation will be omitted.

[0020] The developing device 3a includes a developing roller 30 that carries a two-component developer therein. The developing roller 30 is connected to a developing voltage power supply 43 that generates an oscillating voltage in which a DC voltage Vdc and an AC voltage Vac are superimposed.

[0021] The developing voltage power supply 43 includes an AC constant voltage power supply 43a and a DC constant voltage power supply 43b. During image formation, the developing voltage power supply 43 outputs a developing voltage in which an AC voltage Vac is superimposed on a DC voltage Vdc from the AC constant voltage power supply 43a and the DC constant voltage power supply 43b. The AC constant voltage power supply 43a outputs a sine wave AC voltage Vac generated from a low voltage DC voltage modulated into a pulse shape using a step-up transformer (not shown). The DC constant voltage power supply 43b outputs a DC voltage Vdc obtained by rectifying the sine wave AC voltage Vac generated from a low voltage DC voltage modulated into a pulse shape using a step-up transformer.

[0022] The current detection unit 44 detects the development current flowing between the photosensitive drum 1a and the development roller 30. The development current flowing to the current detection unit 44 may be caused by the movement of toner between the photosensitive drums 1a to 1d and the development roller 30, or may be caused by the movement of electric charge.

[0023] The charging voltage power supply 45 applies a charging voltage consisting of a DC voltage to the charging roller 34 of the charging device 2a. The configuration of the charging voltage power supply 45 is similar to that of the DC constant voltage power supply 43b of the developing voltage power supply 43.

[0024] Transfer voltage power supply 47 applies a primary transfer voltage and a secondary transfer voltage of opposite polarity (negative polarity) to the toner to primary transfer rollers 6a to 6d and secondary transfer roller 9 (see FIG. 1), respectively. In this embodiment, constant current control is used to apply to primary transfer rollers 6a to 6d and secondary transfer roller 9 a transfer voltage that causes a constant current (transfer current) of opposite polarity (negative polarity) to the toner to flow through primary transfer rollers 6a to 6d and secondary transfer roller 9.

[0025] The cleaning device 7a includes a collection roller 31 that temporarily holds the residual toner remaining on the surface of the photosensitive drum 1a. The collection roller 31 is connected to a cleaning voltage power supply 48 that generates a DC voltage. The cleaning voltage power supply 48 applies a voltage of the same polarity (positive polarity) as that of the toner and a voltage of the opposite polarity (negative polarity) to the collection roller 31. The configuration of the cleaning voltage power supply 48 is similar to that of the DC constant voltage power supply 43b.

[0026] Next, the control system of image forming apparatus 100 will be described with reference to Fig. 2. Image forming apparatus 100 is provided with a main control unit 80 constituted by a CPU and the like. Main control unit 80 is connected to a storage unit 70 constituted by a ROM, RAM and the like. Main control unit 80 controls each unit of image forming apparatus 100 (charging devices 2a-2d, developing devices 3a-3d, exposure device 5, primary transfer rollers 6a-6d, cleaning devices 7a-7d, secondary transfer roller 9, fixing unit 13, developing voltage power supply 43, charging voltage power supply 45, transfer voltage power supply 47, cleaning voltage power supply 48, voltage control unit 50, etc.) based on the control program and control data stored in storage unit 70.

[0027] Based on control signals sent from the main control unit 80, the voltage control unit 50 controls a development voltage power supply 43 that applies a development voltage to the development roller 30, a charging voltage power supply 45 that applies a charging voltage to the charging roller 34, a transfer voltage power supply 47 that applies a transfer voltage to the primary transfer rollers 6a to 6d and the secondary transfer roller 9, and a cleaning voltage power supply 48 that applies a voltage to the collection roller 31. The voltage control unit 50 may be configured by a control program stored in the storage unit 70.

[0028] The internal temperature and humidity sensor 60 constantly detects the temperature and relative humidity inside the image forming apparatus 100, specifically in the vicinity of the image forming units Pa to Pd. The detected temperature and humidity are sent to the main control unit 80.

[0029] The main control unit 80 is connected to a liquid crystal display unit 90 and a transmission / reception unit 91. The liquid crystal display unit 90 functions as a touch panel for the user to make various settings for the image forming apparatus 100, and also displays the status of the image forming apparatus 100, the image formation status, the number of printed pages, etc. The transmission / reception unit 91 communicates with the outside world using a telephone line or an internet line. The transmission / reception unit 91 functions as an input unit that receives print commands and image data from a higher-level device such as a personal computer.

[0030] [3. Photoconductor drum surface potential adjustment process] The surface potential adjustment process of the photosensitive drums 1a to 1d, which is a characteristic feature of the present invention, will be described below.

[0031] (Conventional methods for estimating and adjusting surface potential) 3 is a graph showing the relationship between the DC voltage Vdc of the developing voltage and the developing current in a conventional surface potential estimation method. When no DC voltage (0 V) is applied to the developing roller 30 and the charging roller 34, the current detection unit 44 detects the uncharged developing current, which is the sum of the current that flows when toner moves from the developing roller 30 to the photosensitive drums 1a-1d and the current that flows to the photosensitive drums 1a-1d via the magnetic brush formed on the developing roller 30. This uncharged developing current is defined as the reference current Cs (Vdc = 0 V, surface potential V0 = 0 V, development potential difference ΔV = V0 - Vdc = 0 V).

[0032] Then, with a predetermined charging voltage applied to the charging roller 34 to charge the photosensitive drums 1a to 1d to a surface potential V0, the DC voltage Vdc is gradually changed (increased) from an initial value (e.g., 300 V) that is smaller than the surface potential V0, and the progress of the development current detected by the current detection unit 44 is measured.

[0033] When the DC voltage Vdc becomes equal to the surface potential of the photosensitive drums 1a-1d (ΔV=0 [V]), only the non-charging development current (reference current Cs) flows between the development roller 30 and the photosensitive drums 1a-1d. Therefore, the DC voltage Vdc (460 [V] in FIG. 3) at point P1, where the curve L representing the change in the development current intersects with the reference current Cs, is estimated as the surface potential of the photosensitive drums 1a-1d. The surface potential is then adjusted by correcting the charging voltage applied to the charging roller 34 based on the estimated surface potential.

[0034] As described above, the surface potentials of the photosensitive drums 1a to 1d are affected by the transfer voltage. Therefore, the conventional estimation method shown in Fig. 3 has a problem in that it is not possible to accurately estimate the surface potentials when the surface potentials of the photosensitive drums 1a to 1d fluctuate due to the influence of the transfer voltage.

[0035] (Surface Potential Estimation and Adjustment Method of the Present Embodiment) In this embodiment, the relationship between the amount of change in surface potential and the detected change in development current is derived using data obtained by varying the DC voltage Vdc using the estimation method shown in FIG. 3. Then, the amount of change in surface potential is derived by detecting the amount of change in development current when a transfer voltage is continuously applied. Furthermore, a correction value for the charging voltage is determined for each revolution of the photosensitive drums 1a to 1d according to the amount of change in surface potential. The surface potential estimation and adjustment method will be described in detail below.

[0036] 4 is a diagram showing the transition of the surface potential of the photosensitive drums 1a to 1d relative to the driving distance when a transfer voltage is continuously applied. When a transfer voltage (primary transfer voltage) is applied to the transfer rollers 6a to 6d pressed against the photosensitive drums 1a to 1d via the intermediate transfer belt 8, a reverse charge (negative charge in this case) is injected into the photosensitive layers of the photosensitive drums 1a to 1d.

[0037] When continuous printing is performed, the toner images formed on the photosensitive drums 1a to 1d are primarily transferred onto the intermediate transfer belt 8, after which the residual toner on the photosensitive drums 1a to 1d is removed and the residual charge is eliminated. Thereafter, the photosensitive drums 1a to 1d are recharged by the charging roller 34 in preparation for the formation of the next toner image. At this time, the reverse charge injected into the photosensitive layer during the previous primary transfer acts in a direction that cancels out the charge (positive charge) injected into the photosensitive layer from the charging roller 34.

[0038] As a result, the surface potential decreases stepwise at the rotational pitch of the photosensitive drums 1a to 1d, as shown in Figure 4. The extent of the decrease in surface potential in this phenomenon varies depending on the set value of the transfer voltage, the deterioration state of the photosensitive drums 1a to 1d, and the usage environment.

[0039] 5 is a diagram showing the transition of the development current with respect to the driving distance of the photosensitive drums 1a to 1d when the transfer voltage is continuously applied. After detecting the surface potential, a predetermined charging voltage, a DC voltage Vdc that is the same as the detected surface potential as the development voltage, and a transfer voltage that is applied during image formation are again continuously applied, and the change in the development current flowing through the current detection unit 44 is detected.

[0040] When the surface potential of the photosensitive drums 1a to 1d decreases with each rotation due to the influence of the transfer voltage, the development current increases with each rotation of the photosensitive drums 1a to 1d, as shown in Figure 5. Therefore, the development current is detected with each rotation of the photosensitive drums 1a to 1d, and the change in surface potential can be calculated from the change in the detected development current. Then, by correcting the charging voltage to compensate for the decrease in surface potential with each rotation of the photosensitive drums 1a to 1d, it becomes possible to adjust the surface potential more stably.

[0041] 6 is a flowchart showing an example of control of the surface potential adjustment process executed in the image forming apparatus 100 of this embodiment. The execution procedure of the surface potential adjustment process will be described along the steps of FIG. 6, with reference to FIGS. 1 to 5 as needed.

[0042] First, the main control unit 80 determines whether it is time to adjust the surface potential of the photosensitive drums 1a to 1d (step S1). Examples of timing for adjusting the surface potential include when the cumulative number of printed sheets since the start of use of the photosensitive drums 1a to 1d or the charging roller 34 reaches a predetermined number, or when the cumulative number of printed sheets since the previous surface potential adjustment reaches a predetermined number. If it is not time to adjust the surface potential (No in step S1), the process ends without performing the surface potential adjustment process.

[0043] If it is time to adjust the surface potential (No in step S1), the main control unit 80 applies a DC voltage Vdc and a charging voltage of 0 [V] to the developing roller 30 and the charging roller 34, respectively, without applying a transfer voltage to the primary transfer rollers 6a to 6d. Then, the developing current flowing in this state (Vdc=0 [V], V0=0 [V], ΔV=V0-Vdc=0 [V]) is detected by the current detection unit 44, and the detected developing current is set as the reference current Cs.

[0044] Next, the main control unit 80 applies a predetermined charging voltage to the charging roller 34 to charge the photosensitive drums 1a-1d to a certain surface potential V0. Then, the DC voltage Vdc is varied from an initial value (e.g., 300 V) that is smaller than the surface potential V0, and the development current is measured. The main control unit 80 estimates the DC voltage Vdc (see FIG. 3) at the point where the curve recording the development current transition intersects with the reference current Cs as the surface potential of the photosensitive drums 1a-1d. Then, the relationship between the development current detected by the current detection unit 44 and the estimated surface potential is obtained (step S2).

[0045] Next, the main control unit 80 applies a transfer voltage to the primary transfer rollers 6a-6d (step S3). In this state, a predetermined DC voltage Vdc and a charging voltage are applied to the development roller 30 and the charging roller 34, respectively, to charge the photosensitive drums 1a-1d to a certain surface potential V0. Then, the amount of change in development current (FIG. 5) for each revolution of the photosensitive drums 1a-1d is measured (step S4).

[0046] Next, the main control unit 80 calculates the amount of change in the surface potential of the photosensitive drums 1a to 1d per revolution (FIG. 4) using the amount of change in the development current measured in step S4 and the relationship between the development current and the surface potential acquired in step S2 (step S5), and then determines a correction value for the charging voltage to be applied to the charging roller 34 according to the amount of change in the surface potential of the photosensitive drums 1a to 1d per revolution (step S6).

[0047] Finally, the main control unit 80 corrects the charging voltage applied to the charging roller 34 for each revolution of the photosensitive drums 1a to 1d using the correction value determined in step S6 (step S7).

[0048] According to the surface potential adjustment process shown in FIG. 6, the development current flowing between the photosensitive drums 1a to 1d and the development roller 30 is detected, and the surface potential is estimated based on the detected development current, thereby eliminating the need for a dedicated potential measuring device for estimating the surface potential, which also contributes to reducing the cost of the image forming apparatus 100.

[0049] Also, it is possible to accurately estimate the change in surface potential of the photosensitive drums 1a to 1d for each revolution, taking into account the influence of the transfer voltage applied to the primary transfer rollers 6a to 6d. Furthermore, by correcting the charging voltage based on the estimated change in surface potential of the photosensitive drums 1a to 1d for each revolution, it is possible to maintain the surface potential stably for a long period of time, regardless of the usage environment of the image forming apparatus 100 or the deterioration state of the photosensitive drums 1a to 1d.

[0050] In addition, the present invention is not limited to the above-described embodiment, and various modifications are possible without departing from the spirit of the present invention. For example, in the above-described embodiment, the photosensitive drums 1a to 1d are charged to a predetermined surface potential V0, and the DC voltage Vdc is varied from an initial value smaller than the surface potential V0 to measure the change in the development current, and the DC voltage Vdc at the point where the curve L recording the change in the development current intersects with the reference current Cs is estimated as the surface potential of the photosensitive drums 1a to 1d. However, for example, the development current may be measured at only two points, and the DC voltage Vdc at the point where the line passing through the two points intersects with the reference current may be estimated as the surface potential of the photosensitive drums 1a to 1d.

[0051] 1 has been described as an example of image forming apparatus 100, but the present invention is not limited to color printers and may be applied to other image forming apparatuses that charge photosensitive drums 1a-1d using a charging member such as charging roller 34, such as color copiers, digital multifunction peripherals, and facsimiles.The effects of the present invention will be described more specifically below with reference to examples. [Example]

[0052] [Relationship between the number of rotations of the photosensitive drum and the amount of change in surface potential] The relationship between the number of revolutions of the photosensitive drums 1a to 1d and the amount of change in surface potential was investigated. The image formation conditions were a low-temperature, low-humidity environment (LL environment, 10°C, 10% RH) with a printing speed (process speed) of 55 sheets per minute. The developing roller 30 used a developing sleeve 31 with a 20 mm outer diameter and knurled (80 rows circumferentially) and blasted outer surface. The regulating blade 27 was a 1.5 mm thick magnetic blade made of stainless steel (SUS430). A developing voltage was applied to the developing roller 30, consisting of a DC voltage Vdc superimposed on a square-wave AC voltage with a peak-to-peak value (Vpp) of 1200 V, a duty of 50%, and a frequency of 8 kHz.

[0053] The photosensitive drums 1a to 1d were positively charged organic photosensitive drums (OPC: Organic Photo Conductors) with a 32 μm-thick organic photosensitive layer formed as the photosensitive layer, the peripheral speed ratio of the developing roller 30 to the photosensitive drums 1a to 1d was 1.8 (trail rotation at the opposing position), and the distance between the photosensitive drums 1a to 1d and the developing roller 30 (DS distance) was 0.30 mm. In addition, the transfer current flowing through the photosensitive drums 1a to 1d when a transfer voltage was applied to the primary transfer rollers 6a to 6d was set to -12.0 μA.

[0054] The charging roller 34 was a rubber roller having a core metal with an outer diameter of 6 mm and a conductive rubber layer with a thickness of 3 mm and a volume resistance of 6.0 [LogΩ] laminated on the outer surface thereof, and a charging voltage consisting of only a DC voltage was applied.

[0055] The toner used was a positively charged toner with an average particle diameter of 6.8 μm, and the carrier used was a ferrite-resin coated carrier with an average particle diameter of 38 μm. The initial toner concentration in the developer (weight ratio of toner to carrier) was 6%.

[0056] In the test method, first, the surface potential of the photosensitive drums 1a to 1d was estimated. Specifically, without applying a transfer voltage to the primary transfer rollers 6a to 6d, the DC voltage Vdc applied to the development roller 30 was changed to obtain the relationship between the DC voltage Vdc and the development current as shown in Figure 3. Then, the surface potential V0 (450 V in Figure 3) was estimated from the intersection of the curve showing the change in the development current and the reference current Cs.

[0057] Next, the relationship between the amount of change in the development current and the amount of change in the surface potential was determined using the estimated surface potential V0 and the change in the development current relative to changes in the DC voltage Vdc. The DC voltage Vdc, the amount of change in the surface potential, the development current, and the amount of change in the development current are shown in Table 1.

[0058] [Table 1]

[0059] As shown in Table 1, when the DC voltage Vdc was changed in steps from 450 V, the same as the surface potential, to 460 V, 480 V, and 500 V, the change in surface potential (V0-Vdc, the difference between the surface potential V0 and the DC voltage Vdc) changed from 0 V to -10 V, -30 V, and -50 V. In addition, the development current increased from 0.26 μA to 0.31 μA, 0.44 μA, and 0.67 μA, and the change in development current also increased from 0 μA to 0.05 μA, 0.18 μA, and 0.41 μA. Figure 7 shows the relationship between the change in development current and the change in surface potential.

[0060] Next, the transfer voltage set during image formation was continuously applied, and the detected change in development current was obtained for each revolution of the photosensitive drums 1a to 1d. The change in surface potential was calculated from the obtained change in development current and the relationship between the change in development current and the change in surface potential shown in Figure 7. The number of revolutions of the photosensitive drums 1a to 1d, the change in development current, and the change in surface potential are shown in Table 2.

[0061] [Table 2]

[0062] By calculating a correction value for the charging voltage to compensate for the change in surface potential shown in Table 2 and using the calculated correction value to correct the charging voltage for each revolution of the photosensitive drums 1a to 1d, a stable surface potential could be obtained regardless of the number of revolutions of the photosensitive drums 1a to 1d. [Industrial Applicability]

[0063] The present invention can be applied to an electrophotographic image forming apparatus that charges a photosensitive member to a predetermined surface potential, and can provide an image forming apparatus that can accurately estimate the surface potential of an image carrier for each revolution, taking into account the influence of transfer voltage, without using a surface potential sensor. [Explanation of symbols]

[0064] 1a to 1d Photosensitive drum (image carrier) 2a~2d Charging device 3a~3d developing device 5 Exposure equipment 6a to 6d Primary transfer roller (transfer member) 8 Intermediate transfer belt 9 Secondary transfer roller 19 Belt cleaner 30 Developing roller (developer carrier) 34 Charging roller (charging member) 43 Development voltage power supply 44 Current detection unit 45 Charge voltage power supply 47 Transfer voltage power supply 48 Cleaning voltage power supply 50 Voltage control section 70 Memory section 80 Main control unit (control unit) 100 Image forming device

Claims

1. an image carrier having a photosensitive layer formed on its surface; a charging device having a charging member for charging the surface of the image bearing member to a predetermined surface potential; a developing device having a developer carrier that carries a developer containing toner, and that forms a toner image by adhering the toner to an electrostatic latent image formed by exposing the image carrier that has been charged by the charging device; a transfer member that contacts the image carrier to form a transfer nip and transfers the toner image formed on the surface of the image carrier to a recording medium or an intermediate transfer member; a developing voltage power source that applies a developing voltage including a DC voltage to the developer carrier; a charging voltage power source that applies a charging voltage to the charging member; a transfer voltage power supply that applies a transfer voltage having a polarity opposite to that of the toner to the transfer member; a current detection unit that detects a development current flowing between the image carrier and the developer carrier; a control unit that controls the developing voltage power supply, the charging voltage power supply, and the transfer voltage power supply; In an image forming apparatus comprising: The control unit applying the developing voltage and the charging voltage to the developer carrier and the charging member, respectively, in a state where the transfer voltage is not being applied, and estimating the surface potential based on the developing current detected by the current detection unit when the developing voltage is changed, and calculating a relationship between the amount of change in the surface potential and the amount of change in the developing current; an image forming apparatus characterized in that, while the transfer voltage is applied, the developing voltage and the charging voltage are applied to the developer carrier and the charging member, respectively, and the amount of change in the surface potential of the image carrier for each revolution is estimated based on the relationship between the amount of change in the developing current with respect to the driving distance of the image carrier and the amount of change in the surface potential with respect to the amount of change in the developing current.

2. The control unit the developing current when the developing voltage and the charging voltage are not applied to the developer carrier and the charging member, respectively, is measured and used as a reference current; 2. The image forming apparatus according to claim 1, wherein the surface potential is estimated as the DC voltage when the development current detected by the current detection unit becomes equal to the reference current while the charging voltage is applied to the charging member and the image carrier is charged to the predetermined surface potential.

3. The control unit a predetermined charging voltage is applied to the charging member to charge the image carrier to the predetermined surface potential, and then the DC voltage is increased stepwise from an initial value that is smaller than the surface potential, and a transition of the development current detected by the current detection unit is measured; 3. The image forming apparatus according to claim 2, wherein the DC voltage at a point where the curve representing the change in the development current intersects with the reference current is estimated as the surface potential.

4. The control unit 4. The image forming apparatus according to claim 1, wherein the correction value of the charging voltage is determined for each revolution of the image carrier based on the amount of change in the surface potential of the image carrier for each revolution.

Images