Image forming apparatus

Abstract

To prevent electrification unevenness that is nonuniformity of the surface potential of a photoreceptor.SOLUTION: An image forming apparatus comprises: electrifying means that has a plurality of corona electrifiers adjacent in the direction of rotation of a photoreceptor; detection means that detects the surface potential of the photoreceptor at a detection position on the downstream side of the electrifying means in the direction of rotation of the photoreceptor; and control means that applies voltage to the upstream corona electrifier in an adjustment operation to set a voltage to be applied to the upstream corona electrifier so as to adjust a surface potential formed on the photoreceptor through electrification processing performed by the upstream corona electrifier to a first target value, applies a voltage having a value smaller than the first target value to the downstream corona electrifier, causes the detection means to detect the surface potential of the photoreceptor, and based on a result of the detection, sets the voltage to be applied to the upstream corona electrifier.SELECTED DRAWING: Figure 5

Description

[Technical field]

[0001] The present invention relates to an image forming apparatus having a charging unit for charging a photoconductor. [Background technology]

[0002] Conventionally, in image forming apparatuses, corona chargers have been widely used as a charging means for charging a photoconductor. In general, a corona charger includes a discharge electrode that generates electric charge by discharging, and a grid electrode that adjusts the amount of electric charge that reaches the photoconductor.

[0003] When using such a corona charger to handle the increased movement speed of the photoconductor associated with the increased image output speed, or to charge a photoconductor with a large electrostatic capacity, the grid electrode is often placed in close proximity to the photoconductor, about 1 to 1.5 mm. This is because by bringing the grid electrode closer to the photoconductor, the charge from the discharge electrode can be appropriately channeled to the photoconductor side, and the charging property can be increased, thereby improving the convergence of the surface potential of the photoconductor.

[0004] However, even so, the problem of "uneven charging" can occur, where the surface potential of the photoconductor becomes uneven due to insufficient charging performance of the corona charger. When uneven charging occurs, image defects such as "uneven image density" and "roughness" caused by variations in image dots can occur. The following technologies have been proposed as measures to suppress uneven charging of the surface potential of the photoconductor.

[0005] Patent Document 1 proposes arranging two corona chargers along the rotation direction of a photoconductor, and charging the photoconductor in two separate steps along the rotation direction of the photoconductor.

[0006] Patent Document 2 proposes adjusting the charging voltage applied to the upstream corona charger of two corona chargers arranged along the rotation direction of a photoconductor so that the surface potential formed on the photoconductor by the charging process by the upstream corona charger becomes a first target value. It also proposes adjusting the charging voltage applied to the downstream corona charger so that the surface potential formed on the photoconductor becomes a second target value by superimposing the charging process by the downstream corona charger on the charging process by the upstream corona charger. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Laid-Open No. 62-194267 [Patent Document 2] JP 2016-109793 A Summary of the Invention [Problem to be solved by the invention]

[0008] However, in Patent Document 2, when adjusting the charging voltage applied to the upstream corona charger, unintended charge transfer (hereinafter referred to as a leakage phenomenon) may occur between the downstream corona charger and the photoconductor, causing deviation in the surface potential of the photoconductor. In this case, the charging voltage applied to the upstream corona charger cannot be appropriately adjusted, and there is a risk that charging unevenness in the surface potential of the photoconductor may worsen.

[0009] Specifically, when the photoconductor is charged by using the upstream corona charger, the charge generated from the discharge electrode of the upstream corona charger not only flows into the photoconductor, but also leaks out from the upstream corona charger and flows into the grid electrode of the downstream corona charger. The grid electrode is a metal conductor, but has resistance in the circuit because it is connected to the charging power source. Therefore, the grid electrode on the downstream side is charged by the charge that leaks out from the upstream corona charger and flows into the grid electrode of the downstream corona charger. Then, when the potential difference between the charged downstream grid electrode and the surface potential of the photoconductor reaches a certain threshold, a leakage phenomenon occurs between the downstream grid electrode and the photoconductor, causing the surface potential of the photoconductor to deviate from the first target value described above. In this case, the charging voltage applied to the upstream corona charger is set based on the surface potential of the photoconductor that has deviated due to the leakage phenomenon, which may worsen the charging unevenness of the surface potential of the photoconductor.

[0010] An object of the present invention is to suppress charging unevenness in the surface potential of a photoconductor. [Means for solving the problem]

[0011] A representative configuration of the present invention for achieving the above object includes a rotatable photoconductor, charging means having at least a first corona charger for charging the photoconductor at a first position, and a second corona charger for charging the photoconductor at a second position adjacent to the first position on the downstream side in the rotation direction of the photoconductor, a first power source for applying a voltage to the first corona charger, a second power source provided independently of the first power source for applying a voltage to the second corona charger, detection means for detecting a surface potential of the photoconductor at a detection position downstream of the charging means in the rotation direction of the photoconductor, and charging the first corona charger based on a detection result of the detection means so that the surface potential formed on the photoconductor by the charging process by the first corona charger becomes a first target value. and control means for executing a first adjustment operation to set a voltage to be applied to the second corona charger based on a detection result of the detection means so that a surface potential formed on the photoconductor becomes a second target value by superimposing a charging process by the second corona charger on a charging process by the first corona charger using conditions set in the first adjustment operation, wherein the control means applies a voltage to the first corona charger and also applies a voltage to the second corona charger having a value smaller than the first target value, detects the surface potential of the photoconductor using the detection means, and sets the voltage to be applied to the first corona charger based on the detection result during the first adjustment operation. Effect of the Invention

[0012] According to the present invention, it is possible to prevent unintended charge transfer between the second corona charger and the photoconductor during the first adjustment operation, thereby making it possible to appropriately charge the photoconductor using the multiple corona chargers and suppress uneven charging of the surface potential of the photoconductor. [Brief description of the drawings]

[0013] [Figure 1] 1 is a schematic cross-sectional view of an image forming apparatus according to a first embodiment. [Diagram 2]FIG. 2 is a schematic cross-sectional view of the charging device according to the first embodiment. [Diagram 3] FIG. 2 is a schematic diagram showing an arrangement of grid electrodes of the charging device according to the first embodiment. [Figure 4] FIG. 2 is a block diagram of a control circuit for a charging voltage according to the first embodiment. [Diagram 5] 5 is a graph showing the relationship between the charging voltage of the upstream charger and the surface potential of the photosensitive drum according to the first embodiment. FIG. [Figure 6] 5 is a graph showing the relationship between the charging voltage of the downstream charger and the surface potential of the photosensitive drum according to the first embodiment. FIG. [Figure 7] 4 is a graph showing the relationship between the charging voltages of the upstream charger and the downstream charger and the surface potential of the photosensitive drum according to the first embodiment. FIG. [Figure 8] FIG. 4 is a graph for explaining the relationship between the surface potential formed by the upstream charger and the surface potential formed by the downstream charger according to the first embodiment. [Figure 9] FIG. 11 is a schematic cross-sectional view of a charging device according to a second embodiment. [Figure 10] FIG. 11 is a block diagram of a control circuit for a charging voltage according to a second embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0014] Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the drawings. However, the dimensions, materials, shapes, and relative positions of the components described in the following embodiments may be changed as appropriate depending on the configuration of the device to which the present invention is applied and various conditions, and are not intended to limit the scope of the present invention to these alone.

[0015] Example 1 1. Overall configuration and operation of the image forming apparatus 1 is a schematic cross-sectional view of an image forming apparatus 100 according to an embodiment of the present invention. The image forming apparatus 100 of this embodiment is a laser beam printer using an electrophotographic method.

[0016] The image forming apparatus 100 has a photosensitive drum 1, which is a drum-shaped (cylindrical) electrophotographic photosensitive member (photosensitive member) serving as a rotatable image carrier. The photosensitive drum 1 is driven to rotate in the direction of the arrow R1 in the figure. The following devices are arranged around the photosensitive drum 1 along the direction of rotation. First, a charging device 3 is arranged as a charging means. Next, an exposure device (laser scanner) 10 is arranged as an image exposure means. Next, a potential sensor 5 is arranged as a surface potential detection means. Next, a developing device 6 is arranged as a developing means. Next, a transfer device 7 of a transfer belt type is arranged as a transfer means. Next, a cleaning device 2 is arranged as a cleaning means. Next, an optical static eliminator 4 is arranged as a static elimination means.

[0017] The transfer device 7 has a transfer belt 8, which is a recording material transport member formed of a rotatable endless belt, arranged to face the photosensitive drum 1. The transfer belt 8 is supported by a plurality of support rollers, namely a drive roller 71 and a driven roller 72, and rotates (circulates) in the direction of arrow R2 in the figure by the driving force transmitted by the drive roller 71, which is driven to rotate. A transfer roller 9, which serves as a transfer member, is arranged on the inner peripheral surface side of the transfer belt 8 at a position facing the photosensitive drum 1. The transfer roller 9 is urged (pressed) toward the photosensitive drum 1 via the transfer belt 8, forming a transfer portion e where the photosensitive drum 1 and the transfer belt 8 come into contact with each other.

[0018] Further, downstream of the transfer portion e in the conveying direction of the recording material P, a fixing device 50 of a heat and pressure application type is disposed as a fixing means.

[0019] During image formation, the outer peripheral surface (surface) of the rotating photosensitive drum 1 is uniformly charged to a predetermined potential of a predetermined polarity (negative polarity in this embodiment) by the charging device 3. At this time, a predetermined voltage is applied to the charging device 3 from power sources S1, S2, S4, and S5 (see FIG. 2) as voltage application means. In this embodiment, the charging device 3 is configured to have an upstream charger 31 arranged on the upstream side of the rotation direction (movement direction of the surface) of the photosensitive drum 1, and a downstream charger 32 arranged on the downstream side. In the rotation direction of the photosensitive drum 1, the position on the photosensitive drum 1 where the charging is performed by the charging device 3 is the charging portion (charging position) a. More specifically, in the rotation direction of the photosensitive drum 1, the position on the photosensitive drum 1 where the charging is performed by the upstream charger 31 is the upstream charging portion (upstream charging position) a1, and the position on the photosensitive drum 1 where the charging is performed by the downstream charger 32 is the downstream charging portion (downstream charging position) a2. The downstream charging portion (second position) a2 on the photosensitive drum 1 charged by the downstream charger 32 is adjacent to the upstream charging portion (first position) a1 on the photosensitive drum 1 charged by the upstream charger 31 on the downstream side in the rotation direction of the photosensitive drum 1. The charging device 3 and the voltage (charging voltage, charging bias) applied to the charging device 3 will be described in detail later.

[0020] The surface of the charged photosensitive drum 1 is scanned and exposed to laser light by the exposure device 10 in accordance with image information. As a result, an electrostatic latent image (electrostatic image) in accordance with the image information is formed on the photosensitive drum 1. In the rotation direction of the photosensitive drum 1, the exposure position on the photosensitive drum 1 by the exposure device 10 is the image exposure section (image exposure position) b.

[0021] The electrostatic latent image formed on the photosensitive drum 1 is developed (visualized) by the developing device 6 using toner as a developer. The developing device 6 has a developing roller 61 as a developer carrier. The developing roller 61 carries and conveys the toner stored in a developing container 62, and supplies the toner to the photosensitive drum 1 according to the electrostatic latent image. In this embodiment, a toner image is formed by image portion exposure and reversal development. That is, toner charged with the same polarity as the charging polarity of the photosensitive drum 1 adheres to the image portion in which the absolute value of the potential is reduced by being exposed after being uniformly charged. During development, a predetermined developing voltage (developing bias) is applied to the developing roller 61 from a developing power source (not shown). In the rotation direction of the photosensitive drum 1, the position facing the developing roller 61 on the photosensitive drum 1 is the developing portion (developing position) d to which the toner is supplied from the developing roller 61.

[0022] The toner image formed on the photosensitive drum 1 is electrostatically transferred in a transfer section e to a recording material P such as a recording paper carried on a transfer belt 8 and conveyed between the photosensitive drum 1 and the transfer belt 8. At this time, a transfer voltage (transfer bias), which is a DC voltage of the opposite polarity to the charging polarity (normal charging polarity) of the toner at the time of development, is applied to the transfer roller 9 from a transfer power source (not shown). In the rotation direction of the photosensitive drum 1, the position where the photosensitive drum 1 and the transfer belt 8 come into contact is the transfer section (transfer position) e where the toner image is transferred.

[0023] The recording material P to which the toner image has been transferred is separated from the transfer belt 8 and transported to the fixing device 50. The fixing device 50 fixes (fixes) the toner image onto the recording material P by transporting the recording material P while applying heat and pressure to the recording material P. Thereafter, the recording material P is discharged to the outside of the main body of the image forming apparatus 100.

[0024] Toner remaining on the photosensitive drum 1 after the transfer step (transfer residual toner) is removed from the photosensitive drum 1 and collected by the cleaning device 2. The cleaning device 2 scrapes off the toner from the rotating photosensitive drum 1 by a cleaning blade 21 as a cleaning member arranged in contact with the photosensitive drum 1, and collects it in a collection container 22. In the rotation direction of the photosensitive drum 1, the position on the photosensitive drum 1 where the cleaning blade 21 comes into contact is a cleaning portion (cleaning position) f.

[0025] After being cleaned by the cleaning device 2, the photosensitive drum 1 is irradiated with light (discharging light) by the optical discharger 4 to remove residual charges, and then is charged again by the charging device 3. In the rotation direction of the photosensitive drum 1, the position on the photosensitive drum 1 where light is irradiated by the optical discharger 4 is a discharge portion (discharging position) g.

[0026] The potential sensor 5 detects the surface potential of the photosensitive drum 1 in a charging voltage adjustment operation described later in detail. The potential sensor 5 is disposed facing the surface of the photosensitive drum 1 so as to detect the surface potential of the photosensitive drum 1 in an image formable area (area where a toner image can be formed) in the longitudinal direction of the photosensitive drum 1. In this embodiment, the potential sensor 5 detects the surface potential of the photosensitive drum 1 between the charging section a (particularly the downstream charging section a2) and the developing section d (more specifically, between the image exposure section b and the developing section d) in the rotation direction of the photosensitive drum 1. In the rotation direction of the photosensitive drum 1, the position on the photosensitive drum 1 where the surface potential is detected by the potential sensor 5 is the potential detection section (potential detection position) c.

[0027] In this embodiment, the wavelength of the image exposure by the exposure device 10 is 675 nm. Also, in this embodiment, the exposure amount of the surface of the photosensitive drum 1 by the exposure device 10 is 0.1 to 0.5 μJ / cm 2 The exposure amount can be adjusted in accordance with the development conditions to form a predetermined exposed portion potential.

[0028] In this embodiment, the wavelength of the charge removing light by the optical charge remover 4 is 635 nm. In this embodiment, an LED chip array is used as the light source of the optical charge remover 4. The exposure amount of the surface of the photosensitive drum 1 by the optical charge remover 4 is 1.0 to 7.0 μJ / cm 2 In this embodiment, the amount of the ion exchange rate can be adjusted within a range of 4.0 μJ / cm 2 was set to.

[0029] 2. Photosensitive drum The photosensitive drum 1 is rotatably supported by the main body of the image forming apparatus 100. The photosensitive drum 1 is a cylindrical photosensitive member having a conductive substrate such as aluminum and a photoconductive layer formed on the outer periphery of the conductive substrate. The photosensitive drum 1 is driven to rotate in the direction of the arrow R1 in the figure by a driving means (not shown).

[0030] In this embodiment, the photosensitive drum 1 is charged with a negative polarity. In this embodiment, the photosensitive drum 1 is an organic photosensitive body having an outer diameter of 84 mm. In this embodiment, the thickness of the photosensitive layer of the photosensitive drum 1 is 40 μm. In this embodiment, the peripheral speed of the photosensitive drum 1 is 700 mm / s. Note that other photosensitive bodies, such as an amorphous silicon drum, may also be used.

[0031] 3.Charging device configuration FIG. 2 is a schematic cross-sectional view of the charging device 3 in this embodiment. The charging device 3 has a plurality of corona chargers. The charging device 3 is configured to have two scorotron chargers, an upstream charger 31 and a downstream charger 32. In the rotation direction of the photosensitive drum 1, the upstream charger 31 and the downstream charger 32 are arranged in this order from the upstream side to the downstream side. The upstream charger 31 and the downstream charger 32 have approximately the same configuration. That is, the upstream charger 31 and the downstream charger 32 each have discharge wires (wire electrodes, discharge electrodes) 31a and 31b, grid electrodes 31b and 32b, and shield electrodes 31c and 32c. In the following description, the elements of the upstream charger 31 and the downstream charger 32 or various parameters related to each may be distinguished by adding "upstream" and "downstream" to the beginning of the term.

[0032] In the charging device 3 of this embodiment, the upstream charger 31 is a first corona charger that charges the photosensitive drum 1 at a first position, and the downstream charger 32 is a second corona charger that charges the photosensitive drum 1 at a second position adjacent to the first position on the downstream side in the rotation direction of the photosensitive drum 1.

[0033] The discharge wires 31a and 32a are made of conductive wires arranged linearly along the longitudinal direction (rotation axis direction) of the photosensitive drum 1. The grid electrodes 31b and 32b are made of conductive flat-plate members having a plurality of openings arranged along the longitudinal direction of the photosensitive drum 1 between the discharge wires 31a and 32a and the surface of the photosensitive drum 1. The shield electrodes 31c and 32c are made of conductive substantially box-shaped members formed to surround the discharge wires 31a and 32a and having openings in which the grid electrodes 31b and 32b are arranged on the side facing the photosensitive drum 1.

[0034] In addition, an insulating member 33 is disposed between the upstream charger 31 and the downstream charger 32 to prevent leakage when different biases are applied between the upstream shield electrode 31c and the downstream shield electrode 32c. In this embodiment, an insulating plate made of an electrically insulating material and having a thickness T (tangential direction of the photosensitive drum 1: see FIG. 3) of about 2 mm is used as the insulating member 33.

[0035] The width W of the charging device 3 (tangential direction of the photosensitive drum 1: see FIG. 3) is 42 mm, and the length of the longitudinal direction (longitudinal direction of the photosensitive drum 1) of the discharge area used for the image is 340 mm. The widths W1, W2 of the upstream charger 31 and the downstream charger 32 (tangential direction of the photosensitive drum 1: see FIG. 3) are both 20 mm.

[0036] The discharge wires 31a and 32a were made of oxidized tungsten wire, had a wire diameter (outer diameter) of 60 μm, and were generally used in electrophotographic image forming apparatuses.

[0037] The grid electrodes 31b, 32b have a plate-like shape. As shown in Fig. 3, the upstream grid electrode 31b and the downstream grid electrode 32b are arranged at different angles (inclination angles) according to the curvature of the photosensitive drum 1. In a cross section substantially perpendicular to the longitudinal direction of the photosensitive drum 1, the arrangement angle of each of the grid electrodes 31b, 32b is substantially perpendicular to a straight line connecting each of the discharge wires 31a, 32a and the rotation center of the photosensitive drum 1. In addition, each of the grid electrodes 31b, 32b is arranged so that the closest gap G between the grid electrodes and the photosensitive drum 1 is 1.25 ± 0.2 mm.

[0038] The upstream grid electrode 31b has an aperture ratio of 90%, and the downstream grid electrode 32b has an aperture ratio of 80%, and each is an etched mesh-shaped grid electrode. Each of the grid electrodes 31b and 32b is made of a stainless steel (SUS) plate with a corrosion prevention layer such as nickel plating formed on the surface, and is generally used in electrophotography. The aperture ratios of the grid electrodes 31b and 32b do not need to be different between the upstream charger 31 and the downstream charger 32, and grid electrodes with the same aperture ratio may be used in common for multiple chargers.

[0039] 4. Voltage application configuration for charging device 2, the upstream discharge wire 31a and the downstream discharge wire 32a are connected to an upstream discharge power supply S1 and a downstream discharge power supply S2, which are DC power supplies (high voltage power supplies), respectively, so that the voltages applied to the discharge wires 31a and 32a can be controlled independently. Also, the upstream grid electrode 31b and the downstream grid electrode 32b are connected to an upstream grid power supply S4 and a downstream grid power supply S5, which are DC power supplies, respectively, so that the voltages applied to the grid electrodes 31b and 32b can be controlled independently.

[0040] Further, the upstream shield electrode 31c and the downstream shield electrode 32c are connected to the upstream grid electrode 31b and the downstream grid electrode 32b, respectively. Thus, in this embodiment, the shield electrodes 31c and 32c and the grid electrodes 31b and 32b are set to the same potential in each of the upstream charger 31 and the downstream charger 32. However, each of the shield electrodes 31c and 32c may not be set to the same potential as each of the grid electrodes 31b and 32b, and may be electrically grounded by connecting them to an earth electrode of the main body of the image forming apparatus 100, for example. It is only necessary that the voltages applied to the upstream charger 31 and the downstream charger 32 can be independently controlled, and that the voltages applied to the discharge wires 31a and 32a and the grid electrodes 31b and 32b in each of the upstream charger 31 and the downstream charger 32 can be independently controlled.

[0041] FIG. 4 is a block diagram showing the control of the charging voltage in this embodiment. As shown in FIG. 4, the power sources S1, S2, S4, and S5 are connected to a CPU 200 as a control unit. The CPU 200 is also connected to a sheet counter 300, a timer 400, an environment sensor 500, a memory unit 600, a surface potential measurement unit 700, a high voltage output control unit 800, and the like. The sheet counter 300 counts the number of images output by the image forming apparatus 100. The timer 400 measures the elapsed time from a reference point. The environment sensor 500 measures the temperature and humidity inside and outside (outside air) of the image forming apparatus 100. The memory unit 600 records the control data of the charging voltage and the measurement result of the surface potential of the photosensitive drum 1. The surface potential measurement unit 700 processes the detection result (sensor output) of the potential sensor 5 and provides information indicating the measurement result to the CPU 200. Under the control of the CPU 200, the high voltage output control unit 800 controls the ON / OFF and output values ​​of the outputs of the power sources S1, S2, S4, and S5.

[0042] The CPU 200 performs the processes described below based on information from the sheet number counter 300, timer 400, environment sensor 500, memory unit 600, and surface potential measurement unit 700, and issues commands to the high voltage output control unit 800 to control the power sources S1, S2, S4, and S5.

[0043] In this embodiment, the DC voltage applied to the discharge wires 31a, 32a is controlled by a constant current and can be changed in the range of 0 to -3200 μA. Also, in this embodiment, the DC voltage applied to the grid electrodes 31b, 32b is controlled by a constant voltage and can be changed in the range of 0 to -1200 V.

[0044] 5. Control of the surface potential of the photosensitive drum In this embodiment, the voltages applied to the multiple chargers 31 and 32 of the charging device 3 can be controlled independently. In this embodiment, the voltages applied to the upstream charger 31 to the downstream charger 32 of the charging device 3 are controlled independently, and the surface potentials formed on the photosensitive drum 1 are sequentially superimposed (combined) to perform a charging voltage adjustment operation. This controls the final desired surface potential (charging potential, dark area potential) of the photosensitive drum 1.

[0045] That is, in this embodiment, in the charging voltage adjustment operation, first, the voltage applied to the upstream charger 31 is independently controlled to charge the photosensitive drum 1, and a predetermined surface potential (first target potential, first target value) is formed on the photosensitive drum 1. Next, while a voltage controlled to form the predetermined surface potential is being applied to the upstream charger 31, the voltage applied to the downstream charger 32 is independently controlled to further charge the photosensitive drum 1. As a result, the surface potential formed by the downstream charger 32 is superimposed (combined) on the surface potential formed by the upstream charger 31, and the final desired surface potential (second target potential, second target value) of the photosensitive drum 1 is formed.

[0046] The set value of the charging high voltage applied to the discharge wires and grid electrodes of the upstream charger 31 and downstream charger 32 is determined by controlling the surface potential of the photosensitive drum, which will be described later. An image is formed using the set value of the charging high voltage determined by controlling the surface potential of the photosensitive drum.

[0047] In the following description, the parameters related to the charging process by the upstream charger 31 are marked with the symbol "U", and the parameters related to the charging process by the downstream charger 32 are marked with the symbol "S". The parameters in the potential detection unit c are marked with the symbol "sens", and the parameters in the development unit d are marked with the symbol "dev". The magnitude relationship of voltage, current, or potential refers to a comparison of absolute values. For example, -400V or higher corresponds to -500V, etc.

[0048] 5-1. Charging process by upstream charger and control of surface potential First, a description will be given of the charging process by the upstream charger 31. The upstream charger 31 charges the photosensitive drum 1 by applying an upstream discharge current (DC current) Ip(U) from the upstream discharge power supply S1 to the discharge wire 31a in a state in which a predetermined upstream grid voltage Vg(U) is applied from the upstream grid power supply S4 to the upstream grid electrode 31b.

[0049] The surface potential formed on the photosensitive drum 1 changes depending on the upstream grid voltage Vg(U) of the upstream charger 31. The upstream grid voltage Vg(U) is adjusted to set the surface potential of the photosensitive drum 1 to a predetermined target value (first target value). When the upstream discharge current Ip(U) is constant, the ratio of the upstream grid voltage Vg(U) to the surface potential of the photosensitive drum at the potential detection unit c becomes almost constant. Hereinafter, this ratio will be referred to as the upstream charging efficiency (the charging efficiency Veff(U) of the upstream charger 31).

[0050] As a specific example, when the upstream grid voltage Vg(U) is -800V and the upstream discharge current Ip(U) is -1200μA, the surface potential of the photosensitive drum 1 is -600V at the potential detection unit c and -550V at the development unit d. The charging efficiency Veff(U) of the upstream charger 31 at this time is the ratio of the upstream grid voltage Vg(U) (=-800V) to the surface potential of the photosensitive drum 1 at the potential detection unit c (=-600V) (surface potential of the photosensitive body at the potential detection unit c / upstream grid voltage Vg(U)×100%). Therefore, the charging efficiency Veff(U) of the upstream charger 31 is calculated as 75% (=-600 / -800×100%).

[0051] The charging efficiency Veff(U) of the upstream charger 31 varies depending on the state of the charger, such as the distance between the grid electrode 31b and the photosensitive drum 1 and the surface resistance of the grid electrode 31b, but in the short term it is almost constant regardless of the upstream grid voltage Vg(U). Therefore, once the charging efficiency Veff(U) of the upstream charger 31 is measured, the data is stored in the memory unit 600. Using the charging efficiency data, the grid voltage is adjusted to match the surface potential of the photosensitive drum 1 to the target potential.

[0052] Fig. 5 is a control flowchart of the surface potential control of the photosensitive drum 1 using the upstream charger 31. A method of adjusting the surface potential to the target potential will be described below with reference to Fig. 5. First, in order to start the charging process by the upstream charger 31, the driving of the photosensitive drum 1 is started (S101).

[0053] Next, in order to calculate the setting of the upstream charger 31, the memory unit 600 is checked to see if there is past data on the charging efficiency Veff(U) of the upstream charger 31, and if there is data, the latest charging efficiency Veff(U) of the upstream charger 31 is read out (S102).

[0054] If the latest charging efficiency Veff(U) of the upstream charger 31 has not been stored in the memory unit 600 for reasons such as that control has never been performed, a value for when charging efficiency data has not been stored is determined so that charging processing by the upstream charger 31 can be performed appropriately.

[0055] Using the data, a set value of the upstream grid voltage Vg(U) is determined, and the upstream grid power supply S4 is turned ON (S103). For example, assume that the target potential (first target value) of the photosensitive drum 1 of the upstream charger 31 in this embodiment is -750V, and the latest charging efficiency Veff(U) of the upstream charger 31 is 73%. In this case, the upstream grid voltage Vg(U) is calculated as -1027V, based on the equation: upstream grid voltage Vg(U)=target potential of the photosensitive drum 1 / (latest charging efficiency Veff(U) of the upstream charger / 100%).

[0056] In this state, if the upstream discharge current Ip(U) is set and charging of the upstream charger 31 is started, the charge generated from the upstream discharge wire 31a not only charges the photosensitive drum 1 in the direction of rotation as described above, but also flows toward the downstream charger 32. The flowing-in charge particularly charges the downstream grid electrode 32b. When the downstream grid electrode 32b, which has been charged by the flowing-in charge, reaches or exceeds a certain threshold, a leakage phenomenon occurs between the photosensitive drum 1 and the downstream charger 31, causing a deviation in the surface potential of the photosensitive drum 1 charged by the upstream charger 31.

[0057] Therefore, in this embodiment, the downstream grid voltage Vg(S) is set to a value 150V smaller than the absolute value of the target potential (first target value) of the photosensitive drum 1 by only the upstream charger 31, and the downstream grid power supply S5 is turned on. That is, during the first adjustment operation shown in FIG. 5, the CPU 200 applies the upstream grid voltage Vg(U) to the upstream charger 31, and applies the downstream grid voltage Vg(S) to the grid electrode 32b of the downstream charger 32, the value being smaller than the absolute value of the target potential (first target value) of the photosensitive drum 1 by only the upstream charger 31. Here, since the target potential (first target value) of the photosensitive drum 1 by only the upstream charger 31 is −750V, the downstream grid voltage Vg(S) is set to −600V, which is a value 150V smaller than the absolute value of −750V, and the downstream grid power supply S5 is turned on (S104).

[0058] In addition, since there is only a short time between when the upstream grid power supply S4 is turned on in S103 and when the downstream grid power supply S5 is turned on in S104, it is the same as turning them on at almost the same time. Alternatively, the downstream grid voltage may be set in S103, the power supply may be turned on, and then the upstream grid voltage may be set in S104, and the power supply may be turned on.

[0059] Here, the downstream grid voltage Vg(S) is set to a value 150 V smaller than the absolute value of the target potential of the photosensitive drum 1 so that the absolute value is smaller than the surface potential of the photosensitive drum 1 formed by the upstream charger 31. The downstream grid voltage Vg(S) is set so that the absolute value of the surface potential of the photosensitive drum 1 formed by the upstream charger 31 is greater than the absolute value of the downstream grid voltage Vg(S).

[0060] The downstream grid voltage Vg(S) is set (determined) based on the above relationship in order to prevent the surface potential of the photosensitive drum 1 formed by the upstream charger 31 from being additionally charged by the downstream charger 32 (= the surface potential formed by the upstream charger 31 being additionally charged).

[0061] Otherwise, the surface potential of the photosensitive drum 1 formed by the upstream charger 31 will be additionally charged by the downstream grid voltage Vg(S) of the downstream charger 32, and the surface potential of the photosensitive drum 1 formed by the upstream charger 31 will deviate from the target potential (first target value) of the photosensitive drum 1. In this case, it is not possible to measure the charged potential of the photosensitive drum 1 formed by the upstream charger 31 and adjust the upstream grid voltage Vg(U).

[0062] Therefore, the downstream grid voltage Vg(S) needs to be set to a value smaller than the absolute value of the surface potential (first target value) of the photosensitive drum 1 formed by the upstream charger 31.

[0063] The relationship between the upstream grid voltage Vg(U) of the upstream charger 31 and the surface potential of the photosensitive drum 1 formed by the upstream charger 31 is upstream grid voltage Vg(U)>surface potential of the photosensitive drum 1. Therefore, it is preferable to set the downstream grid voltage Vg(S) to a relationship 1 of upstream grid voltage Vg(U)>downstream grid voltage Vg(S).

[0064] More preferably, the downstream grid voltage Vg(S) is set to satisfy the relationship 2 of the surface potential of the photosensitive drum 1 formed by the upstream charger 31>the downstream grid voltage Vg(S).

[0065] The surface potential of the photosensitive drum 1 formed by the upstream charger 31 varies due to individual differences of the upstream charger 31, such as the distance (gap G) between the upstream grid electrode 31b and the photosensitive drum 1. Therefore, it is more preferable to set the downstream grid voltage Vg(S) to satisfy the relationship 3 of upstream grid voltage Vg(U)×charging efficiency Veff(U) of the upstream charger 31>downstream grid voltage Vg(S).

[0066] However, the charging efficiency Veff(U) of the upstream charger out of (upstream grid voltage Vg(U) × charging efficiency Veff(U) of the upstream charger 31) varies depending on the usage conditions and therefore needs to be constantly updated to the latest state, which may complicate control. Therefore, in this embodiment, taking into consideration simplifying the control, the downstream grid voltage Vg(S) is set to a relationship 4 of the target potential (first target value) of the photosensitive drum 1 by only the upstream charger 31 -150 V>downstream grid voltage Vg(U).

[0067] The following Table 1 shows the results of an investigation into whether the downstream grid voltage Vg(S) of the downstream charger 32 causes the surface potential of the photosensitive drum 1 formed by the upstream charger 31 to deviate from the target potential (first target value). In Table 1, for comparison with the above-mentioned Relationships 1 to 4, a case in which the downstream grid voltage Vg(S) is set to Relationship 0 (downstream grid voltage Vg(S)>upstream grid voltage Vg(U)) is added for comparison.

[0068] [Table 1]

[0069] According to Table 1, when the downstream grid voltage Vg(S) was set to the above relationship 0, the surface potential of the photosensitive drum 1 deviated from the target potential. Therefore, the effect on the surface potential of the photosensitive drum 1 was marked as "X".

[0070] When the downstream grid voltage Vg(S) was set to the above relationship 1, depending on the state of the upstream charger 31, it may have had an effect on the surface potential of the photosensitive drum 1, albeit by a few volts, so the effect on the surface potential of the photosensitive drum 1 was rated as △.

[0071] When the downstream grid voltage Vg(S) was set to the above relationship 2, there were cases where the surface potential of the photosensitive drum 1 was affected, although only slightly, with a probability of 0.00004% in mass production variation. However, when the downstream grid voltage Vg(S) was set to the above relationship 2, in most cases there was no effect on the surface potential of the photosensitive drum 1. In other words, when the downstream grid voltage Vg(S) was set to the above relationship 2, in most cases the surface potential of the photosensitive drum 1 did not deviate from the target potential. Therefore, the effect on the surface potential of the photosensitive drum 1 was rated as "good." Note that mass production variation refers to the variation that occurs when a fixed quantity of products are produced (mass produced) at a fixed time.

[0072] When the downstream grid voltage Vg(S) was set to the above relationship 3 or relationship 4, there was no effect on the surface potential of the photosensitive drum 1 in all cases including the mass production variation. In other words, when the downstream grid voltage Vg(S) was set to the above relationship 3 or relationship 4, the surface potential of the photosensitive drum 1 did not deviate from the target potential. Therefore, the effect on the surface potential of the photosensitive drum 1 was rated as ⊚.

[0073] In this case, the target potential of the photosensitive drum 1 includes a range of ±a predetermined potential with respect to the target potential as a reference. Therefore, when the downstream grid voltage Vg(S) is set to the above relationships 2, 3, and 4, the surface potential of the photosensitive drum 1 does not deviate from the target potential, meaning that it does not deviate from the range of ±a predetermined potential with respect to the target potential as a reference.

[0074] Based on the above relationships 0 to 4, the value of the downstream grid voltage Vg(S) when determining the upstream grid voltage Vg(U) of the upstream charger 31 is determined.

[0075] At this time, the downstream discharge current Ip(S) was set to -600μA because it was sufficient if the downstream grid voltage Vg(S) could be stably controlled to -600V even if there was an influence of the inflow of electric charge from the upstream charger 31 (S105). Here, if the absolute value of the downstream discharge current Ip(S) was small, the downstream grid voltage Vg(S) could not be stably controlled, so the set value was determined by verifying in advance the possible range in which the downstream grid voltage Vg(S) could be stably controlled. Note that if the absolute value of the downstream discharge current Ip(S) was large, the amount of ozone generated during high voltage application increases, which has a detrimental effect on the life of the downstream grid electrode and the discharge wire. Therefore, the downstream discharge current Ip(S) was set to the side with a smaller absolute value within the stable controllable range of the downstream grid voltage Vg(S) verified in advance. Note that in S105, the upstream discharge current Ip(U) was set to -1200μA.

[0076] Furthermore, when charging is performed only by the upstream charger 31, the influence of the charge flowing in from the upstream charger 31 can be prevented even if the downstream grid voltage Vg(S) has no resistance as described below.

[0077] For example, in order to obtain an effect equivalent to a state in which the downstream grid electrode 32b of the downstream charger 32 is grounded, the downstream grid voltage Vg(S) may be set to a value that makes the target potential of the photosensitive drum 1 0 V. When charging is performed only by the upstream charger 31 in this manner, even if the downstream grid voltage Vg(S) is set, it is possible to prevent inflow of charges from the upstream charger 31.

[0078] The downstream grid voltage Vg(S) set in step S104 is a value for preventing the leakage phenomenon, and is different from the value applied during image formation.

[0079] As described above, the upstream charger 31 sets the upstream grid voltage Vg(U) to -1027V and turns on the upstream grid power supply S4 (S103). The downstream charger 32 sets the downstream grid voltage Vg(S) to -600V and turns on the downstream grid power supply S5 (S104). It also sets the upstream discharge current Ip(U) to -1200μA, sets the downstream discharge current Ip(S) to -600μA, and turns on the discharge power supplies S1 and S2, respectively (S105).

[0080] With the power sources S1, S2, S4, and S5 turned on with the above-mentioned set values ​​for the discharge wires 31a and 32a and the grid electrodes 31b and 32b of the chargers 31 and 32, the surface potential of the photosensitive drum 1 is measured at the potential detection portion c by the potential sensor 5 (S106). As a result, the surface potential of the photosensitive drum at the potential detection portion c was −770 V.

[0081] In addition, the measurement of the surface potential of the photosensitive drum 1 at the potential detection unit c is performed for one revolution of the photosensitive drum in order to reduce the influence of unevenness in the surface potential in the circumferential direction of the photosensitive drum 1, and the average value excluding the maximum and minimum values ​​is taken as the measurement value at the potential detection unit c. When the measurement is restricted because it takes a long time to measure one revolution of the photosensitive drum or there are a large number of data measurement points, or when unevenness in the surface potential in the circumferential direction of the photosensitive drum 1 is a concern, the measurement points at the potential detection unit c can be adjusted as appropriate.

[0082] If there is a concern about unevenness in the surface potential of the photosensitive drum 1 in the circumferential direction, measurements can be made by linking the location in the rotational direction of the photosensitive drum to the measurement point, thereby making it possible to take into account unevenness in the surface potential of the photosensitive drum 1 in the circumferential direction.

[0083] The charging efficiency Veff(U) of the upstream charger 31 in this state (=surface potential of the photoconductor at potential detection unit c / upstream grid voltage Vg(U)×100%) is calculated (S107). Then, the charging efficiency Veff(U) of the upstream charger 31 is 75% (=−770 / −1027×100%).

[0084] Here, a failure error judgment is performed based on whether the calculated charging efficiency Veff(U) of the upstream charger 31 is within a predetermined range (here, a range of 55 to 100%) (S108). If the charging efficiency Veff(U) of the upstream charger 31 is less than 55% or greater than 100%, it is assumed that one of the charging device 3, the photosensitive drum 1, or the exposure device 10 is broken. Therefore, if the charging efficiency Veff(U) of the upstream charger 31 falls outside the range of 55 to 100% in S108, an error judgment condition is met, control of the surface potential of the photosensitive drum 1 is stopped, and an error is displayed on the user interface of the image forming apparatus 100 (S111). By performing the error judgment in S108, it is possible to prevent the power supply from being damaged by setting the absolute value of the grid electrode too large, or the surface potential of the photosensitive drum from being set to an inappropriate value, resulting in the generation of a defective image.

[0085] Using the charging efficiency Veff(U)=75% of the upstream charger 31 calculated in S107, the upstream grid voltage Vg(U) for setting the target potential of the photosensitive drum 1 of the upstream charger 31 to -750V is calculated. The upstream grid voltage Vg(U) (=target potential of the photosensitive drum 1 / (the latest charging efficiency Veff(U) of the upstream charger / 100%)) is calculated to be -1000V (=-750 / (75 / 100)) (S109). In S109, the upstream grid voltage Vg(U) is reset to the value calculated as described above. The reset value of the upstream grid voltage is stored in the memory unit 600. At this time, the power supply S4 continues to be ON. The set values ​​of the upstream discharge current Ip(U) and the upstream grid voltage Vg(U) of the upstream charger 31 are determined as described above.

[0086] A control flow may be inserted to check whether the upstream grid voltage Vg(U) set in S109 matches the target potential of the photosensitive drum 1 formed by the upstream charger 31. According to the inventors' prior studies, it is known that if the upstream grid voltage Vg(U) is set by the method (S103 to S109) using the above-mentioned charging efficiency, the target potential of the photosensitive drum 1 can be set to within approximately ±3 V in one control flow of S103 to S109. Therefore, the surface potential of the photosensitive drum 1 formed by the upstream charger 31 at the time of the upstream grid voltage Vg(U) set in S109 is not measured, and the process proceeds to the next charging process by the downstream charger 32 and the control flow of the surface potential. That is, the process proceeds to charging process by the downstream charger 32 in S110.

[0087] 5-2. Charging process by downstream charger and control of surface potential Next, the charging process by the downstream charger 32 will be described. In the downstream charger 32, a predetermined downstream grid voltage Vg(S) is applied from the downstream grid power supply S5 to the downstream grid electrode 32b, and a downstream discharge current (DC current) Ip(S) is applied from the downstream discharge power supply S2 to the discharge wire 32a. This is performed in a state in which a predetermined upstream grid voltage Vg(U) is applied from the upstream grid power supply S4 of the upstream charger 31 to the upstream grid electrode 31b, and an upstream discharge current (DC current) Ip(U) is applied from the upstream discharge power supply S1 to the discharge wire 31a. That is, in the charging process by the downstream charger 32, the surface potential of the photosensitive drum 1 formed by the upstream charger 31 is superimposed (combined) and charged by the downstream charger 32, thereby forming the surface potential of the photosensitive drum 1.

[0088] As with the upstream charger 31, the downstream charger 32 also changes the surface potential formed on the photosensitive drum 1 depending on the downstream grid voltage Vg(S). The downstream grid voltage Vg(S) is adjusted and set so that the surface potential of the photosensitive drum 1 becomes a target potential (second target value). When the surface potential of the photosensitive drum 1 formed by the upstream charger 31 and the downstream discharge current Ip(S) are constant, the ratio of the upstream grid voltage Vg(S) to the surface potential of the photosensitive body at the potential detection unit c becomes approximately constant. Hereinafter, this ratio will be referred to as the downstream side charging efficiency (charging efficiency Veff(S) of the downstream charger 32).

[0089] As a specific example, when the upstream grid voltage Vg(U) is -1000V and the upstream discharge current Ip(U) is -1200μA, the surface potential of the photosensitive drum 1 is -750V at the potential detection unit c. When the downstream grid voltage Vg(S) is -1000V and the downstream discharge current Ip(S) is -1200μA, the surface potential of the photosensitive drum 1 is -900V at the potential detection unit c. Furthermore, the surface potential of the photosensitive drum 1 at the developing unit d at this time is -850V. The charging efficiency Veff(S) of the downstream charger 32 at this time is the ratio of the downstream grid voltage Vg(S) (=-1000V) to the surface potential of the photosensitive drum 1 at the potential detection unit c (=-900V) (=surface potential of the photosensitive drum at the potential detection unit c / downstream grid voltage Vg(U)×100%). Therefore, the charging efficiency Veff(S) of the downstream charger 32 is calculated to be 90% (=-900 / -1000 x 100%).

[0090] The charging efficiency Veff(S) of the downstream charger 32 varies depending on the state of the charger, such as the distance (gap G) between the grid electrode 32b and the photosensitive drum 1 and the surface resistance of the grid electrode 32b, as in the case of the upstream charger 31. However, the charging efficiency Veff(S) of the downstream charger 32 is almost constant in the short term, regardless of the upstream grid voltage Vg(S). Therefore, when the charging efficiency Veff(S) of the downstream charger 32 is measured, the data is stored in the storage unit 600. Using the data of the charging efficiency Veff(S), the downstream grid voltage Vg(U) is adjusted to match the surface potential of the photosensitive drum 1 to the target potential (second target value).

[0091] Fig. 6 is a control flowchart of the surface potential control of the photosensitive drum 1 using the downstream charger 32. Immediately before this, the control flow of the surface potential of the photosensitive drum 1 using the upstream charger 31 (see Fig. 5) is executed, and then the control flow of Fig. 6 is executed. Hereinafter, a method of adjustment to a target potential will be described with reference to Fig. 6. First, following the charging process by the upstream charger 31, charging process by the downstream charger 32 is started (S201). Note that since this is executed following the control flow of Fig. 5, in reality, it only prepares the CPU and the like to start supplying power.

[0092] Next, to calculate the setting of the downstream charger 32, the storage unit 600 is checked to see if there is past data on the charging efficiency Veff(S) of the downstream charger 32. If there is data, the latest charging efficiency Veff(S) of the downstream charger 32 and the grid voltage Vg(U) of the upstream charger 31 are read out (S202). Note that the grid voltage Vg(U) of the upstream charger 31 here is the grid voltage Vg(U) of the upstream charger 31 determined in S109 of FIG. 5.

[0093] If the latest charging efficiency Veff(S) of the downstream charger 32 has not been stored in the memory unit 600 for reasons such as that control has never been performed, a value for when charging efficiency data has not been stored is determined so that charging processing by the downstream charger 32 is appropriately performed.

[0094] Note that S203 is a process continued from the control flow in FIG. 5 (S109 in this case), and is not a new process in the control flow in FIG.

[0095] Using the data read out in S202, a set value of the downstream grid voltage Vg(S) is determined (S204). For example, in this embodiment, the target potential (second target value) of the photosensitive drum 1 of the downstream charger 32 is -900V, and the latest charging efficiency Veff(S) of the downstream charger 32 is 88%. In this case, the downstream grid voltage Vg(S) is calculated and set to -1023V based on the equation: downstream grid voltage Vg(S)=target potential of the photosensitive drum 1 / (latest charging efficiency Veff(S) of the downstream charger×100%) (S204).

[0096] The setting value of the downstream grid voltage Vg(S) set in S204 is a setting value applied during image formation, and is different from the setting value of the downstream grid voltage set in S104 of FIG. 5 (a setting value for preventing the leak phenomenon).

[0097] Further, the difference between the target potential (first target value) formed on the photosensitive drum by charging process by only the upstream charger 31 and the target potential (second target value) formed on the photosensitive drum 1 by charging process by the downstream charger 32 superimposed on the charging process by the upstream charger 31 was determined as follows. Fig. 7 is a graph showing the relationship between the difference in the target potential of the photosensitive drum 1 by the upstream charger 31 and the downstream charger 32 and the unevenness of the surface potential of the photosensitive drum 1. According to Fig. 7, when the difference between the surface potential of the photosensitive drum 1 formed by only the upstream charger 31 and the surface potential of the photosensitive drum 1 formed by superimposing (combining) the surface potential of the photosensitive drum 1 formed by the upstream charger 31 and the surface potential of the photosensitive drum 1 charged by the downstream charger 32 is 200V or less, the unevenness of the surface potential of the photosensitive drum 1 is within the target potential ±3V (within a predetermined potential range) and is in a good state of 3V or less.

[0098] This is because, if the downstream charger 32 charges the surface potential of the photosensitive drum charged by the upstream charger 31 to an absolute value greater than 200V, the charging capacity of the downstream charger 32 is not sufficient to uniformly charge the uneven surface potential of the photosensitive drum, and the unevenness of the surface potential worsens. Conversely, if the charging capacity of the downstream charger 32 is such that the absolute value of the potential charged by the upstream charger 31 is 200V or less, it becomes possible to uniformize the surface potential of the photosensitive drum, and it is believed that the unevenness of the surface potential of the photosensitive drum is improved.

[0099] Thus, in the image forming apparatus of this embodiment, the difference between the absolute value of the surface potential (target potential) of the photosensitive drum formed by the charging process by the upstream charger 31 and the absolute value of the surface potential (target potential) of the photosensitive drum formed by the charging process by the downstream charger 32 is 200 V or less.

[0100] The smaller the difference between the target potentials of the photosensitive drum 1 of the upstream charger 31 and the downstream charger 32, the better the unevenness of the surface potential of the photosensitive drum, but there is a possibility that the absolute value of the surface potential at a longitudinal position other than the potential detection portion c will be larger than that at the potential detection portion c. This is due to the longitudinal distance distribution between the charger and the photosensitive drum, the longitudinal dimensional distribution of the grid electrode, and the longitudinal dirt distribution. If this condition becomes significant, the surface potential of the photosensitive drum 1 formed by the upstream charger 31 will be higher than the absolute value of the target potential of the downstream charger 32, and the charging effect of the downstream charger 32 will disappear, leading to the worsening of the unevenness of the surface potential of the photosensitive drum. Therefore, in this embodiment, the difference between the target potentials of the photosensitive drum 1 of the upstream charger 31 and the downstream charger 32 is set to 150V, which is less than 200V.

[0101] Furthermore, if the grid electrode 31b or the discharge wire 31a of the upstream charger 31 becomes dirty, a difference in surface potential in the longitudinal direction occurs. Even in this case, the difference in the target potential of the photosensitive drum 1 between the upstream charger 31 and the downstream charger 32 was set to 150 V. As a result, even if the surface potential of the photosensitive drum is uneven due to dirt on the grid electrode 31b or the discharge wire 31a of the upstream charger 31, the downstream charger 32 can uniformly charge the photosensitive drum 1 while almost completely eliminating the surface potential unevenness of the upstream charger 31.

[0102] FIG. 8 is a graph showing the relationship between the target potential (first target value) of the photosensitive drum of the upstream charger 31 and the target potential (second target value) of the photosensitive drum of the downstream charger 32 charged by being superimposed on the upstream charger 31, depending on the installation environment (amount of moisture) of the image forming apparatus. According to FIG. 8, the target potential of the photosensitive drum 1 is adjusted depending on the amount of moisture detected by the image forming apparatus. When the amount of moisture is small, the absolute value of the target potential of the photosensitive drum is increased, and as the amount of moisture increases, the absolute value of the target potential of the photosensitive drum is decreased. The target value of the photosensitive drum for the amount of moisture is determined depending on the charge amount of the developer. In addition, the difference between the target potential of the photosensitive drum of the upstream charger 31 for the amount of moisture and the target potential of the photosensitive drum of the downstream charger 32 charged by being superimposed on the upstream charger 31 is set to a constant 150V regardless of the amount of moisture for the reasons described above.

[0103] In this manner, in this embodiment, the difference between the absolute value of the target potential of the photosensitive drum formed by the charging process by the upstream charger 31 and the absolute value of the target potential of the photosensitive drum formed by the charging process by the downstream charger 32 is constant regardless of the environment in which the image forming apparatus is installed.

[0104] The difference between the target potential of the photosensitive drum by the upstream charger 31 according to the moisture content and the target potential of the photosensitive drum by the downstream charger 32 charged and superimposed on the upstream charger 31 does not have to be a fixed value such as 150 V. For example, the ratio between the target potential of the upstream charger 31 of the photosensitive drum and the target potential of the downstream charger 32 charged and superimposed on the upstream charger 31 may be determined to be 8:2.

[0105] Next, the upstream discharge current Ip(U) of the upstream charger 31 and the downstream discharge current Ip(S) of the downstream charger 32 are set to -1200 μA, and the power sources S1 and S2 are turned ON (S205).

[0106] With the power sources S1, S2, S4, and S5 turned on with the above-mentioned set values ​​for the discharge wires 31a and 32a and the grid electrodes 31b and 32b of the chargers 31 and 32, the surface potential of the photosensitive drum 1 is measured at the potential detection portion c by the potential sensor 5 (S206). As a result, the surface potential of the photosensitive drum at the potential detection portion c was −921 V.

[0107] The charging efficiency Veff(S) (=surface potential of the photoconductor at potential detection unit c / downstream grid voltage Vg(S)×100%) of the downstream charger 32 in this state is calculated (S207). Then, the charging efficiency Veff(S) of the downstream charger 32 is 90% (=−921 / −1023×100%).

[0108] Here, a failure error judgment is performed based on whether the calculated charging efficiency Veff(S) of the downstream charger 32 is within a predetermined range (here, a range of 75 to 100%) (S208). Since the surface potential of the downstream charger 32 is judged by the surface potential superimposed on the upstream charger 31, the error judgment ranges of the upstream charger 31 and the downstream charger 32 are different. If the charging efficiency Veff(S) of the downstream charger 32 is less than 75% or greater than 100%, it is assumed that one of the charging device 3, the photosensitive drum 1, or the exposure device 10 is broken. Therefore, if the charging efficiency Veff(S) of the downstream charger 32 falls outside the range of 75 to 100% in S208, the error judgment condition is met, the control of the surface potential of the photosensitive drum 1 is stopped, and an error is displayed on the user interface of the image forming apparatus 100 (S211). As explained for the upstream charger, by performing error judgment in S208, it is possible to prevent the absolute value of the grid electrode from being set too high, which could damage the power supply, or to prevent a defective image from being generated by setting the surface potential of the photosensitive drum to an inappropriate value.

[0109] Next, using the charging efficiency Veff(S)=90% of the downstream charger 32 calculated in S207, the downstream grid voltage Vg(S) is calculated to set the target potential of the photosensitive drum 1 of the downstream charger 32 superimposed on the charging potential of the upstream charger 31 to -900V. The downstream grid voltage Vg(S) (=target potential of the photosensitive drum 1 / (the latest charging efficiency Veff(S) of the downstream charger / 100%)) is calculated to be -1000V (=-900 / (90 / 100)) (S209). In S209, the downstream grid voltage Vg(S) is reset to the value calculated as described above. The reset value of the downstream grid voltage is stored in the memory unit 600. At this time, the power supply S5 continues to be ON. Thus, the settings of the downstream discharge current Ip(S) and the downstream grid voltage Vg(S) of the downstream charger 32 following the upstream charger 31 are determined.

[0110] As in S109 in FIG. 5, a control flow may be inserted to check whether the downstream grid voltage Vg(S) set in S209 matches the target potential of the photosensitive drum 1 formed by the downstream charger 32.

[0111] The above is the control flow of the surface potential of the photosensitive drum by the upstream charger 31 and the downstream charger 32. Following this control flow, the setting value of the exposure device 10 may be determined.

[0112] An image is formed using the grid voltage Vg of each of the chargers 31 and 32, the discharge wire discharge current Ip, and the setting values ​​of the exposure device 10, which are determined by these control flows.

[0113] 6. Charge voltage adjustment operation timing Next, the timing of the charge voltage adjustment operation of each charger in this embodiment will be described. The charge voltage adjustment operation of each charger here refers to the control of the surface potential described in Sections 5-1 and 5-2 (control flows in FIGS. 5 and 6).

[0114] In this embodiment, the CPU 200 as a control means controls the charging voltage adjustment operation in the following procedure and timing: The CPU 200 executes the charging voltage adjustment operation of each charger at a predetermined timing during non-image formation.

[0115] Here, the image forming period refers to a period during which an image to be outputted onto the recording material P is formed (formation of an electrostatic latent image, formation of a toner image, and transfer of the toner image). In contrast, the non-image forming period refers to a period other than the above-mentioned image forming period. Examples of the non-image forming period include the following.

[0116] First, there is a pre-multiple rotation process which is a preparatory operation when the image forming apparatus 100 is turned on or when it returns from a sleep state. There is also a pre-rotation process which is a preparatory operation from when an image formation start instruction is input until the above-mentioned image actually starts to be formed. There is also a sheet interval process which corresponds to between recording materials P and recording materials P in a job in which images are continuously formed on multiple recording materials P (a series of operations in which an image is formed on a single or multiple recording materials and output in response to a single image formation start instruction). There is also a post-rotation process which is an arrangement operation (preparatory operation) after the image is formed.

[0117] In this embodiment, the CPU 200 can obtain information on the counting result of the number of output images by the sheet counter 300, the measurement result of the elapsed time by the timer 300, and the detection result of at least one of the temperature and humidity by the environment sensor 500. Then, the CPU 200 can determine the predetermined timing for executing the adjustment operation of the charging voltage of each charger based on at least one of these pieces of information.

[0118] For example, when the number of image output sheets since the previous execution reaches a predetermined number of image output sheets, the charging voltage adjustment operation can be executed in the next pre-rotation process. When the predetermined number of image output sheets is reached during execution of a job, the charging voltage adjustment operation may be executed during the inter-sheet process. Furthermore, instead of or in addition to the number of image output sheets, the charging voltage adjustment operation may be executed based on the elapsed time since the previous execution. Furthermore, instead of or in addition to the number of image output sheets or the elapsed time, the charging voltage adjustment operation may be executed when at least one of the environmental temperature or humidity has changed beyond a predetermined threshold.

[0119] As described above, the CPU 200 adjusts the charging voltage of each charger at a predetermined timing during non-image formation, thereby enabling the photosensitive drum 1 to be more uniformly charged to a target surface potential by the multiple corona chargers 31, 32. In particular, even when the moving speed of the photosensitive drum 1 is increased or a photosensitive drum 1 with a relatively large electrostatic capacitance is used, the photosensitive drum 1 can be more uniformly charged to a target surface potential by the multiple corona chargers 31, 32.

[0120] In this embodiment, the voltages applied to the corona chargers 31 and 32 can be controlled independently. In this embodiment, the voltages applied to the corona chargers 31 and 32 are controlled independently from the upstream side to the downstream side, and the surface potentials formed on the photosensitive drum 1 are sequentially superimposed (combined) to perform a charging voltage adjustment operation. In particular, when the charging voltage is adjusted by the upstream charger 31, a grid voltage is applied to the adjacent downstream charger 32, so that unintended charge transfer between the downstream charger 32 and the photosensitive drum 1 can be prevented. This allows the voltages applied to the chargers 31 and 32 to be set independently, and the final surface potential of the photosensitive drum 1 can be controlled to a desired potential. Also, charging unevenness of the surface potential of the photosensitive drum 1 can be suppressed.

[0121] Example 2 Next, another embodiment of the present invention will be described. The basic configuration and operation of the image forming apparatus of this embodiment are the same as those of embodiment 1. Therefore, in the image forming apparatus of this embodiment, elements having the same or corresponding functions or configurations as those of the image forming apparatus of embodiment 1 are given the same reference numerals and detailed description is omitted.

[0122] In the first embodiment, the charging device 3 is configured to charge the photosensitive drum 1 using two corona chargers that can control the voltage applied independently. In the second embodiment, the charging device 3 is configured to charge the photosensitive drum 1 using three corona chargers that can control the voltage applied independently. This makes it possible to improve the charging performance of the charging device 3 and obtain a uniform surface potential of the photosensitive drum 1 even when the moving speed of the photosensitive drum 1 is further increased.

[0123] 9 is a schematic cross-sectional view of the charging device 30 in this embodiment. The charging device 30 in this embodiment is configured to have three scorotron chargers, an upstream charger 301, an intermediate charger 302, and a downstream charger 303, as multiple corona chargers. In the rotation direction of the photosensitive drum 1, the upstream charger 301, the intermediate charger 302, and the downstream charger 303 are arranged in this order from the upstream side to the downstream side. These three chargers 301, 302, and 303 have approximately the same configuration. In other words, these three corona chargers 301, 302, and 303 each have discharge wires 301a, 302a, and 303a, grid electrodes 301b, 302b, and 303b, and shield electrodes 301c, 302c, and 303c. In the following description, the elements of the upstream charger 301, the intermediate charger 302, and the downstream charger 303 or various parameters related to each of them may be distinguished by adding "upstream," "intermediate," and "downstream" to the beginning of the term.

[0124] In the charging device 30 of this embodiment, the upstream charger 301 is a first corona charger that charges the photosensitive drum 1 at a first position. The intermediate charger 302 is a second corona charger that charges the photosensitive drum 1 at a second position adjacent to and downstream of the first position in the rotation direction of the photosensitive drum 1. The downstream charger 303 is a third corona charger that charges the photosensitive drum 1 at a third position adjacent to and downstream of the second position in the rotation direction of the photosensitive drum 1.

[0125] The discharge wires 301a, 302a, 303a, the grid electrodes 301b, 302b, 303b, and the shield electrodes 301c, 302c, 303c are each configured in the same manner as those in the charging device 3 of the first embodiment. In this embodiment, insulating members 304a, 304b are respectively disposed between the upstream charger 301 and the intermediate charger 302, and between the intermediate charger 302 and the downstream charger 303. The insulating members 304a, 304b are configured in the same manner as those in the charging device 3 of the first embodiment.

[0126] As shown in FIG. 9, the upstream grid electrode 301b, the intermediate grid electrode 302b, and the downstream grid electrode 303b are arranged at different angles (inclination angles) along the curvature of the photosensitive drum 1. As in the first embodiment, the arrangement angles of the grid electrodes 301b, 302b, and 303b are approximately perpendicular to the straight line connecting the discharge wires 301a, 302a, and 303a and the rotation center of the photosensitive drum 1 in a cross section approximately perpendicular to the longitudinal direction of the photosensitive drum 1. Also, as in the first embodiment, the widths W1, W2, and W3 (tangential direction of the photosensitive drum 1) of the chargers 301, 302, and 303 are the same at 20 mm. Also, in this embodiment, the aperture ratios of the grid electrodes 301b, 302b, and 303b are the same at 85%. By making the grid electrodes 301b, 302b, and 303b common to each other, the number of parts required for maintenance can be reduced.

[0127] The charging device 30 of this embodiment, having the above-mentioned configuration, is capable of uniformly charging the photosensitive drum 1 even when the peripheral speed of the photosensitive drum 1 is set to 1000 mm / s.

[0128] 9, the upstream discharge wire 301a, the intermediate discharge wire 302a, and the downstream discharge wire 303a are connected to an upstream discharge power supply S11, an intermediate discharge power supply S12, and a downstream discharge power supply S13, which are DC power supplies (high voltage power supplies), respectively. This allows the voltages applied to the discharge wires 301a, 302a, and 303a to be controlled independently.

[0129] Additionally, the upstream grid electrode 301b, the intermediate grid electrode 302b, and the downstream grid electrode 303b are connected to an upstream grid power supply S14, an intermediate grid power supply S15, and a downstream grid power supply S16, which are DC power supplies, respectively, so that the voltages applied to the grid electrodes 301b, 302b, and 303b can be controlled independently.

[0130] Further, the upstream shield electrode 301c, the intermediate shield electrode 302c, and the downstream shield electrode 303c are connected to the upstream grid electrode 301b, the intermediate grid electrode 302b, and the downstream grid electrode 303b, respectively. In this manner, in the present embodiment, the shield electrodes 301c, 302c, and 303c and the grid electrodes 301b, 302b, and 303b are at the same potential in each of the chargers 301, 302, and 303. However, as explained in the first embodiment, this is not limited to this.

[0131] Fig. 10 is a block diagram showing the control of the charging voltage in this embodiment. As shown in Fig. 10, the power sources S11, S12, S13, S14, S15, and S16 are connected to a CPU 200 as a control means. Similarly to the first embodiment, the CPU 200 is connected to a sheet counter 300, a timer 400, an environment sensor 500, a memory unit 600, a surface potential measurement unit 700, and a high voltage output control unit 800. The surface potential measurement unit 700 processes the detection result (sensor output) of the potential sensor 5 and provides information indicating the measurement result to the CPU 200. The high voltage output control unit 800 controls the ON / OFF and output value of the power sources S11, S12, S13, S14, S15, and S16 under the control of the CPU 200.

[0132] The CPU 200 performs the processing described below based on information from the sheet number counter 300, timer 400, environmental sensor 500, memory unit 600, and surface potential measurement unit 700, and issues commands to the high voltage output control unit 800 to control power sources S11, S12, S13, S14, S15, and S16.

[0133] In this embodiment, the charging device 3 performs charging processing on the photosensitive drum 1 in the order of the upstream charger 301, the intermediate charger 302, and the downstream charger 303, and forms a surface potential on the photosensitive drum 1 by superimposing (combining) them in sequence. Since there are three corona chargers, the operation of adjusting the surface potential of the charging voltage involves one additional operation of independently setting the voltage to be applied to each corona charger compared to the first embodiment. First, the operation of adjusting the surface potential of the upstream charger 301 is performed. Then, the operation of adjusting the surface potential of the intermediate charger 302 is performed by superimposing it on the surface potential formed by the upstream charger 301. Then, the operation of adjusting the surface potential of the downstream charger 303 is performed by superimposing it on the surface potential formed by the upstream charger 301 and the intermediate charger 302.

[0134] The surface potential formed on the photosensitive drum 1 by each of the chargers 301, 302, and 303 is basically controlled in the same manner as in the first embodiment, and is ultimately controlled to a target surface potential of the photosensitive drum 1. At this time, it is desirable that the surface potential formed on the photosensitive drum 1 by the corona charger on the upstream side of two adjacent corona chargers is equal to or lower than the grid voltage of the corona charger on the downstream side of the two adjacent corona chargers. However, it is desirable that the difference between the surface potential formed on the photosensitive drum 1 by the corona charger on the upstream side and the grid voltage of the corona charger on the downstream side is equal to or lower than 200V.

[0135] Here, an outline of the charging voltage adjustment operation in this embodiment will be described. The detailed procedure for adjusting the charging voltage in this embodiment can be the same as that described in the first embodiment, so repeated explanations will be omitted here.

[0136] First, when controlling the surface potential of the "upstream" corona charger, the grid electrodes of the "middle" and "downstream" corona chargers are set to a value 150 V smaller than the absolute value of the target potential of the upstream corona charger, and the discharge wire electrode is set to -600 μA, and the surface potential of the upstream corona charger is controlled.

[0137] That is, the CPU 200 first applies a voltage to the upstream charger 301 and executes a first adjustment operation to set the charging voltage of the upstream charger 301 so that the surface potential formed on the photosensitive drum 1 becomes a target potential (first target value). During this first adjustment operation, a voltage is applied to the upstream charger 301, and a voltage smaller than the first target value is applied to the intermediate charger 302 and downstream charger 303 adjacent to the upstream charger 301 on the downstream side. Then, the surface potential of the photosensitive drum 1 is detected by a potential sensor, and the voltage to be applied to the upstream charger 301 is set based on the detection result.

[0138] This makes it possible to prevent the influence of electric charge leaking from the upstream corona charger to the intermediate or downstream corona chargers.

[0139] Next, when controlling the surface potential of the "middle" corona charger, the upstream corona charger uses the set value of the charging high voltage determined by the control, and the surface potential of the "middle" corona charger is controlled by superimposing it on the charging high voltage. In this case, the grid electrode of the "downstream" corona charger is set to a value 150V smaller than the absolute value of the target potential of the upstream corona charger, and the discharge wire electrode is set to -600μA, and the surface potential of the middle corona charger is controlled. Note that the grid electrode of the "downstream" corona charger may be set to a value 150V smaller than the absolute value of the target potential of the upstream corona charger, or 150V smaller than the absolute value of the target potential of the middle corona charger.

[0140] That is, the CPU 200 subsequently applies a voltage to the intermediate charger 302, and executes a second adjustment operation to set the charging voltage of the intermediate charger 302 so that the surface potential formed on the photosensitive drum 1 becomes a target potential (second target value) by superimposing the charging process by the intermediate charger 302 on the charging process by the upstream charger 301 using the conditions set in the first adjustment operation. During this second adjustment operation, a voltage is applied to the intermediate charger 302, and a voltage smaller than the second target value is applied to the downstream charger 303 adjacent to the intermediate charger 302 on the downstream side. Then, the surface potential of the photosensitive drum 1 is detected by a potential sensor, and the voltage to be applied to the intermediate charger 302 is set based on the detection result.

[0141] This makes it possible to prevent the influence of electric charges leaking from the upstream and intermediate corona chargers to the downstream corona charger.

[0142] The CPU 200 then continues to apply a voltage to the downstream charger 303, and sets the voltage of the downstream charger 303 so that the surface potential formed on the photosensitive drum 1 becomes the target potential (third target value) by superimposing the charging process by the downstream charger 303 on the charging process by the upstream charger 301 using the conditions set in the first adjustment operation and the charging process by the intermediate charger 302 using the conditions set in the second adjustment operation.

[0143] By increasing the number of chargers in the charging device 3 in this way, the photosensitive drum 1 can be uniformly charged to a target surface potential even when the moving speed of the photosensitive drum 1 is further increased. In particular, when setting the charging voltage of the upstream corona charger among the adjacent chargers, a grid voltage is applied to the downstream corona charger adjacent thereto, thereby preventing unintended charge transfer between the downstream corona charger and the photosensitive drum. This makes it possible to independently set the voltages applied to the chargers and control the final surface potential of the photosensitive drum 1 to a desired potential. Also, charging unevenness of the surface potential of the photosensitive drum 1 can be suppressed.

[0144] Although the present invention has been described with reference to specific embodiments, the present invention is not limited to the above-described embodiments.

[0145] For example, in the above-described embodiment, the charging device is configured to have a plurality of scorotron chargers as the plurality of chargers. However, when adopting a method of controlling the discharge current as in the first embodiment, the chargers other than the most downstream charger among the plurality of chargers included in the charging device may be scorotrons or corotrons.

[0146] In the above-mentioned embodiment, the number of corona chargers provided in the charging device is described as two and three, but it may be more than three. In this case, similarly to the above-mentioned embodiment, the surface potential is formed by sequentially superposing (combining) the corona chargers from the upstream side to the downstream side, and the voltage applied to each corona charger is set so that the formed surface potential reaches the respective target value. [Explanation of symbols]

[0147] P … Recording material S1,S2,S5…discharge power supply S3, S4, S6 …Grid power supply 2...Photosensitive drum 3...Charging device 5... Potential sensor 10...Exposure equipment 31 …Upstream charger 31a,32a...Discharge electrode 31b, 32b ... Grid electrodes 31c, 32c … Shield electrode 32 …Downstream charger 34 ... Insulating material 100 ... Image forming device 301 …Upstream charger 301a,302a,303a...discharge electrode 301b, 302b, 303b ... Grid electrodes 301c, 302c, 303c …Shield electrode 302 …Downstream charger 303 ... Intermediate charger 304a, 304b ...insulating member

Claims

1. A rotatable image carrier; a charging device that charges the image carrier with a plurality of corona chargers, including a first corona charger having a first discharge electrode and a first grid electrode, and a second corona charger having a second discharge electrode and a second grid electrode, and disposed downstream of the first corona charger in the rotation direction of the image carrier; a first voltage application unit that applies a voltage to the first grid electrode; a second voltage application unit that applies a voltage to the second grid electrode; a control unit that controls each of the first voltage application unit and the second voltage application unit; Equipped with The control unit a first adjusting operation for setting a voltage to be applied to the first grid electrode by the first voltage application unit in a state in which a voltage is applied to the second discharge electrode and a voltage having an absolute value smaller than the first target value is applied to the second grid electrode by the second voltage application unit, so that the surface potential of the image carrier charged by the first corona charger becomes a first target value; Thereafter, a second adjustment operation is performed to set a voltage to be applied to the second grid electrode by the second voltage application unit in a state in which a voltage is applied to the first discharge electrode and the voltage set by the first adjustment operation is applied to the first grid electrode by the first voltage application unit, so that the surface potential of the image carrier charged by the first corona charger and the second corona charger becomes a second target value having an absolute value greater than the first target value. An image forming apparatus characterized by:

2. When performing the first adjustment operation, the control unit controls the second voltage application unit to apply a voltage to the second grid electrode that is equivalent to a state where the second grid electrode is grounded.

2. The image forming apparatus according to claim 1, wherein the image forming apparatus is a recording medium.

3. The absolute value of the difference between the absolute value of the first target value and the absolute value of the second target value is 200 V or less.

2. The image forming apparatus according to claim 1, wherein the image forming apparatus is a recording medium.

4. The aperture ratio of the second grid electrode is smaller than the aperture ratio of the first grid electrode.

2. The image forming apparatus according to claim 1, wherein the image forming apparatus is a recording medium.

5. A rotatable image carrier; a charging device that charges the image carrier with a plurality of corona chargers, including a first corona charger having a first discharge electrode and a first grid electrode, and a second corona charger having a second discharge electrode and a second grid electrode, and disposed downstream of the first corona charger in the rotation direction of the image carrier; a developing device including a developer container that contains a developer and a developer carrier that carries the developer to develop an electrostatic latent image formed on the image carrier; a detection unit that is disposed downstream of a charging position where the image carrier is charged by the charging device and upstream of a developing position where an electrostatic latent image formed on the image carrier is developed by the developing device, with respect to the rotation direction of the image carrier, and that detects a surface potential of the image carrier; a first voltage application unit that applies a voltage to the first grid electrode; a second voltage application unit that applies a voltage to the second grid electrode; a control unit that controls each of the first voltage application unit and the second voltage application unit; Equipped with The control unit a first adjusting operation for setting a voltage to be applied to the first grid electrode by the first voltage applying unit based on the surface potential of the image carrier detected by the detecting unit, in a state in which a voltage is applied to the second discharge electrode and a predetermined voltage is applied to the second grid electrode by the second voltage applying unit; thereafter, in a state in which a voltage is applied to the first discharge electrode and the voltage set by the first adjusting operation is applied to the first grid electrode by the first voltage applying unit, a second adjusting operation is performed in which the voltage to be applied to the second grid electrode by the second voltage applying unit is set based on the surface potential of the image carrier detected by the detecting unit; The absolute value of the predetermined voltage applied to the second grid electrode by the second voltage application unit when performing the first adjustment operation is smaller than the absolute value of the voltage applied to the second grid electrode by the second voltage application unit when performing the second adjustment operation. An image forming apparatus characterized by:

6. The predetermined voltage is a voltage that is equivalent to a state where the device is grounded to earth.

6. The image forming apparatus according to claim 5, 7. The second grid electrode has an aperture ratio smaller than that of the first grid electrode.

6. The image forming apparatus according to claim 5,

Images