JP2015022204A - Image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image forming device capable of setting a voltage value to be applied to an electrifier at the time of image formation while preventing a high AC voltage from being applied to a photoreceptor drum.SOLUTION: An image forming device 1 comprises a photoreceptor drum 321, an electrified roller 322, a bias application unit 80, an ammeter 83 and an applied voltage controlling unit 92. The bias application unit 80 applies an electrifying bias with an AC voltage superposed on a DC voltage to the electrified roller 322. The applied voltage controlling unit 92 applies the DC voltage to the electrified roller 322 while increasing a voltage value from a preset initial value, and sets a voltage value, which is obtained by multiplying a value of the DC voltage when a direct current value to be detected by the ammeter 83 exceeds a prescribed current threshold by two, as a peak-to-peak voltage of the AC voltage in the electrifying bias for image formation.

Description

  The present invention relates to an image forming apparatus that forms an image on a sheet.

  An image forming apparatus such as a copying machine, a printer, or a facsimile using an electrophotographic system supplies toner to an electrostatic latent image formed on an image carrier (for example, a photosensitive drum or a transfer belt) to By developing the electrostatic latent image, a toner image is formed on the image carrier.

  In order to form an electrostatic latent image on the photosensitive drum, the peripheral surface of the photosensitive drum is uniformly charged by the charger. Patent Document 1 discloses a technique in which a charging bias in which an AC voltage is superimposed on a DC voltage is applied to a charger in order to make the charging potential of the photosensitive drum uniform. When the peak-to-peak voltage of the AC voltage constituting the charging bias is gradually increased, the increase in the surface potential of the photosensitive drum is converged with a predetermined inflection point. For this reason, the charging potential of the photosensitive drum is stabilized by applying the charging bias with a peak-to-peak voltage equal to or higher than the inflection point.

  On the other hand, when the peak-to-peak voltage of the AC voltage is excessively increased, there is a problem that generation of ozone is promoted or a large amount of discharge products are generated and adhere to the peripheral surface of the photosensitive drum. In Patent Document 2, before the printing operation of the image forming apparatus, a peak-to-peak voltage lower than the inflection point and a peak-to-peak voltage higher than the inflection point are applied to the charger, respectively, and a direct current that flows at that time is applied. A technique for deriving the inflection point from a current value is disclosed.

JP-A 63-149668 JP 2007-199094 A

  However, in the technique described in Patent Document 2, a high AC voltage is temporarily applied to the photosensitive drum in order to derive an inflection point. For this reason, film breakage of the photosensitive drum may occur. Further, there has been a problem that it is necessary to increase the film thickness of the photosensitive drum in order to prevent the film destruction.

  The present invention has been made to solve the above-described problems, and sets a voltage value applied to a charger during image formation while suppressing application of a high AC voltage to a photosensitive drum. It is an object of the present invention to provide an image forming apparatus capable of doing so.

  An image forming apparatus according to an aspect of the present invention includes a photosensitive drum made of amorphous silicon that is rotationally driven to form an electrostatic latent image on a peripheral surface and carries a toner image, and the circumference of the photosensitive drum. An image forming portion that performs an image forming operation for forming a toner image on a sheet, and rotates the photosensitive drum. A drive unit; a bias application unit that applies a charging bias in which an AC voltage is superimposed on a DC voltage to the charger; an ammeter that detects a DC current flowing between the bias application unit and the charger; and A bias control unit that controls a bias application unit to set an image forming charging bias applied to the charger in order to form the electrostatic latent image, and the bias control unit includes: With the photosensitive drum rotated, a DC voltage is applied to the charger while increasing the voltage value from a preset initial value, and the DC current value detected by the ammeter has a predetermined current threshold. A voltage value obtained by multiplying the value of the DC voltage when exceeding by 2 is set as a peak-to-peak voltage of the AC voltage in the charging bias for image formation.

  According to this configuration, the bias control unit applies a DC voltage to the charger while increasing the voltage value from a preset initial value while rotating the photosensitive drum. When the DC voltage is applied and the DC current value detected by the ammeter exceeds a predetermined current threshold, the DC voltage has reached the discharge start voltage. The bias control unit sets a voltage value obtained by multiplying the value of the DC voltage by 2 as a peak-to-peak voltage of the AC voltage in the image forming charging bias. As a result, the peak-to-peak voltage of the AC voltage applied to the charger is set near the inflection point in relation to the surface potential of the photosensitive drum. Therefore, in the setting operation of the AC voltage applied to the charger, a voltage excessively higher than the inflection point is prevented from being applied to the photosensitive drum.

  In the above configuration, the current threshold is preferably 0.

  According to this configuration, the peak-to-peak voltage of the AC voltage in the image forming charging bias is more accurately set near the inflection point in the relationship between the AC voltage applied to the charger and the surface potential of the photosensitive drum. .

  In the above configuration, an environmental sensor that detects ambient temperature and humidity, and a storage unit that stores in advance information on the initial value of the DC voltage for a combination of temperature and humidity, the bias control unit, It is desirable to set an initial value of the DC voltage applied to the charger from the initial value information stored in the storage unit based on the temperature and humidity detected by the environmental sensor.

  According to this configuration, when the peak-to-peak voltage of the AC voltage in the image forming charging bias is set, the initial value of the DC voltage applied to the charger is set according to the ambient temperature and humidity. For this reason, application of the DC voltage can be started from a region closer to the discharge start voltage. As a result, the setting operation of the peak-to-peak voltage is performed in a short time, and the time during which the voltage is applied to the photosensitive drum in the setting operation is reduced.

  In the above configuration, it is preferable that the bias control unit executes the setting operation of the peak-to-peak voltage of the AC voltage during non-image formation when the image forming operation is not performed in the image forming unit.

  According to this configuration, the setting operation of the peak-to-peak voltage of the AC voltage can be performed using the timing when the image forming operation is not performed in the image forming unit.

  According to the present invention, there is provided an image forming apparatus capable of setting a voltage value to be applied to a charger during image formation while suppressing application of a high AC voltage to a photosensitive drum.

1 is a cross-sectional view showing an internal structure of an image forming apparatus according to an embodiment of the present invention. 1 is an electrical block diagram of an image forming apparatus according to an embodiment of the present invention. 6 is a graph showing the relationship between the peak-to-peak voltage of the AC voltage applied to the charger and the surface potential of the photosensitive drum. It is the graph which showed the relationship between the direct current voltage and direct current which are applied to a charger. 6 is a flowchart when the value of the peak-to-peak voltage is set in the image forming apparatus according to an embodiment of the present invention.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. FIG. 1 is a sectional view showing an internal structure of an image forming apparatus 1 according to an embodiment of the present invention. Here, as the image forming apparatus 1, a multi-function machine having a printer function and a copying function is illustrated, but the image forming apparatus may be a printer, a copier, or a facsimile machine.

<Description of Image Forming Apparatus>
The image forming apparatus 1 includes an apparatus main body 10 having a substantially rectangular parallelepiped housing structure, and an automatic document feeder 20 disposed on the apparatus main body 10. In the apparatus main body 10, a reading unit 25 that optically reads a document image to be copied, an image forming unit 30 that forms a toner image on a sheet, a fixing unit 60 that fixes the toner image on a sheet, and image formation A sheet feeding unit 40 for storing sheets conveyed to the unit 30, and a conveyance path 50 for conveying the sheet from the sheet feeding unit 40 or the sheet feeding tray 46 to the sheet discharge port 10E via the image forming unit 30 and the fixing unit 60. And a conveyance unit 55 having therein a sheet conveyance path constituting a part of the conveyance path 50 is accommodated.

  The image forming unit 30 executes an image forming operation (printing operation) for forming a full-color toner image on a sheet. The image forming unit 30 includes four units 32Y, 32M, 32C, and 32Bk that form toner images of yellow (Y), magenta (M), cyan (C), and black (Bk) arranged in tandem. The image forming unit 32 includes an image forming unit 32, an intermediate transfer unit 33 disposed adjacent to the image forming unit 32, and a toner replenishing unit 34 disposed on the intermediate transfer unit 33.

  Each of the image forming units 32Y, 32M, 32C, and 32Bk includes a photosensitive drum 321 (image carrier) and a charging roller 322, an exposure device 323, a developing device 324, and a primary transfer disposed around the photosensitive drum 321. A roller 325 and a cleaning device 326.

  The photosensitive drum 321 is driven to rotate about its axis, and an electrostatic latent image is formed on its peripheral surface (front surface) and carries a toner image. In the present embodiment, at least an amorphous silicon (a-Si) material is used for the photosensitive drum 321. The photosensitive drum 321 is rotationally driven by a drive unit 84 described later. The photosensitive drum 321 functions as an image carrier.

  The charging roller 322 uniformly charges the peripheral surface of the photosensitive drum 321. The charging roller 322 (charging device) is a rubber roller having a cylindrical shape. The charging roller 322 abuts on the photosensitive drum 321 and rotates following the photosensitive drum 321. In another embodiment, the charging roller 322 may be disposed close to the photosensitive drum 321 with an interval of about 50 microns to 0.1 mm. In this case, the charging roller 322 is driven separately from the photosensitive drum 321 and is driven to rotate by a driving mechanism (not shown). A charging bias in which an AC voltage is superimposed on a DC voltage is applied to the charging roller 322 by a bias applying unit 80 described later. As a result, the peripheral surface of the photosensitive drum 321 is charged.

  The exposure device 323 includes a laser light source and optical system devices such as a mirror and a lens, and forms an electrostatic latent image by irradiating the peripheral surface of the photosensitive drum 321 with light based on image data of a document image. .

  The developing device 324 stores toner therein and supplies the toner to the peripheral surface of the photosensitive drum 321 in order to develop the electrostatic latent image formed on the photosensitive drum 321. The developing device 324 is for a two-component developer and includes a screw feeder, a magnetic roller, and a developing roller. As shown in FIG. 1, the developing devices 324 for the respective colors are arranged adjacent to each other in the horizontal direction (left-right direction).

  The primary transfer roller 325 forms a nip portion with the photosensitive drum 321 across the intermediate transfer belt 331 provided in the intermediate transfer unit 33, and the toner image on the photosensitive drum 321 is primary transferred onto the intermediate transfer belt 331. To do. The cleaning device 326 includes a cleaning roller and the like, and cleans the peripheral surface of the photosensitive drum 321 after the toner image is transferred.

  The intermediate transfer unit 33 includes an intermediate transfer belt 331, a driving roller 332, and a driven roller 333. The intermediate transfer belt 331 is an endless belt that is stretched around a driving roller 332 and a driven roller 333, and toner images from a plurality of photosensitive drums 321 are superimposed on the same portion on the outer peripheral surface of the intermediate transfer belt 331. Is transcribed. The intermediate transfer belt 331 is rotated counterclockwise in FIG. The intermediate transfer belt 331 functions as an image carrier.

  A secondary transfer roller 35 (transfer portion) is disposed to face the peripheral surface of the drive roller 332. The secondary transfer roller 35 transfers the toner image from the intermediate transfer belt 331 to the sheet. A nip portion between the driving roller 332 and the secondary transfer roller 35 serves as a secondary transfer portion that transfers a full-color toner image overcoated on the intermediate transfer belt 331 to a sheet. A secondary transfer bias potential having a polarity opposite to that of the toner image is applied to one of the driving roller 332 and the secondary transfer roller 35, and the other roller is grounded. In addition, the density sensor 35 </ b> A is disposed opposite to the circumferential surface of the intermediate transfer belt 331 at a position upstream of the drive roller 332 in the rotation direction of the intermediate transfer belt 331. The density sensor 35A outputs an electrical signal corresponding to the density of the toner image formed on the intermediate transfer belt 331.

  The toner supply unit 34 includes a yellow toner container 34Y, a magenta toner container 34M, a cyan toner container 34C, and a black toner container 34Bk. These toner containers 34Y, 34C, 34M, and 34Bk respectively store toner of each color, and are supplied to the developing devices 324 of the image forming units 32Y, 32M, 32C, and 32Bk corresponding to each color of YMCBk through a supply path (not shown). Supply toner of each color.

  The paper feed unit 40 includes two-stage paper feed cassettes 40A and 40B that accommodate sheets subjected to image forming processing. These paper feed cassettes 40A and 40B can be pulled out from the front of the apparatus body 10 toward the front. The sheet feeding unit 40 accommodates a sheet conveyed toward the secondary transfer roller 35. The paper feed unit 40 is disposed below the developing device 324 described above.

  The fixing unit 60 is an induction heating type fixing device that performs a fixing process for fixing a toner image on a sheet, and includes a heating roller 61, a fixing roller 62, a pressure roller 63, a fixing belt 64, and an induction heating unit 65. A pressure roller 63 is pressed against the fixing roller 62 to form a fixing nip portion. The heating roller 61 and the fixing belt 64 are induction-heated by an induction heating unit 65, and give the heat to the fixing nip portion. As the sheet passes through the fixing nip portion, the toner image transferred to the sheet is fixed to the sheet.

  Next, the electrical configuration of the image forming apparatus 1 will be described. FIG. 2 is an electrical block diagram of the image forming apparatus 1. In FIG. 2, the photosensitive drum 321 and the charging roller 322 are shown inverted in the vertical direction as compared with FIG.

  The image forming apparatus 1 includes a control unit 90 that controls the operation of each unit disposed inside the apparatus main body 10. The control unit 90 includes a CPU (Central Processing Unit), a ROM (Read Only Memory) that stores a control program, a RAM (Random Access Memory) that is used as a work area of the CPU, and the like. The control unit 90 functions to include a drive control unit 91, an applied voltage control unit 92, and a storage unit 93 when the CPU executes a control program stored in the ROM. The control unit 90 is electrically connected to the bias applying unit 80, the ammeter 83, the environment sensor 95, and the driving unit 84 in addition to the above-described photosensitive drum 321 and the charging roller 322.

  The bias application unit 80 applies a charging bias in which an AC voltage is superimposed on a DC voltage to the charging roller 322. The bias applying unit 80 includes an AC constant voltage power supply 81 and a DC constant voltage power supply 82. As described above, the bias applying unit 80 includes a high-voltage power source that can apply a direct current and an alternating voltage.

  The ammeter 83 detects the value of the DC current flowing between the DC constant voltage power source 82 of the bias applying unit 80 and the charging roller 322. The current value detected by the ammeter 83 is referred to by the applied voltage control unit 92 described later.

  The environmental sensor 95 (FIG. 1) detects the temperature and humidity in the vicinity of the image forming apparatus 1. The environmental sensor 95 is disposed on the inner wall portion of the left side wall of the apparatus main body 10. The environmental sensor 95 detects the temperature and humidity around the image forming unit 30.

  The drive unit 84 drives each member inside the image forming apparatus 1. The drive unit 84 is a motor that rotationally drives the photosensitive drum 321 of the image forming unit 30, the intermediate transfer belt 331, the developing roller of the developing device 324, and the like.

  The drive control unit 91 controls the drive unit 84 according to the image forming operation of the image forming apparatus 1 to rotationally drive each member inside the image forming apparatus 1.

  The applied voltage control unit 92 (bias control unit) controls the bias applied to each member inside the image forming apparatus 1. In particular, the applied voltage control unit 92 controls the bias applying unit 80 to control the image forming charging bias applied to the charging roller 322 in order to form an electrostatic latent image on the photosensitive drum 321.

  The storage unit 93 stores DC voltage initial value information Vdc (A), which will be described later, controlled by the applied voltage control unit 92 in advance for a combination of temperature and humidity. The initial value information Vdc (A) stored in the storage unit 93 will be described in detail later.

  Next, the charging bias applied to the charging roller 322 in the image forming apparatus 1 according to the present embodiment will be described. As described above, the amorphous silicon photoconductor is used in the present embodiment. The charging roller 322 is applied with a charging bias in which an AC voltage is applied to a DC voltage. By superimposing the AC voltage as the charging bias, the uniformity of charging is improved as compared to the case of using only the DC voltage, and toner image density unevenness and toner fog are prevented from occurring on the photosensitive drum 321. Is done.

  On the other hand, when the AC voltage is superimposed as the charging bias as described above, a predetermined characteristic is generated in the surface potential of the photosensitive drum 321 according to the value of the peak voltage of the AC voltage. FIG. 3 shows the peak-to-peak voltage of the AC voltage of the charging bias applied to the charging roller 322 when the photosensitive drum 321 is rotationally driven in an ambient temperature of 23 ° C. and a relative humidity of 50%. It is the graph which showed the relationship with surface potential.

  As shown in FIG. 3, the surface potential of the photosensitive drum 321 increases with the increase of the peak-to-peak voltage of the AC voltage. Eventually, the change in the surface potential of the photosensitive drum 321 becomes smaller at the boundary indicated by the point A. That is, the surface potential of the photosensitive drum 321 converges. The value of the peak voltage of the AC voltage at the point A at this time is defined as the inflection point A. When the peak-to-peak voltage increased from 0 V reaches the inflection point A, a reverse transfer of charge occurs between the photosensitive drum 321 and the charging roller 322, and therefore, the discharge between the photosensitive drum 321 and the charging roller 322 occurs. Is stable.

  However, when the peak-to-peak voltage is further increased from the inflection point A, reverse discharge due to the reverse transfer of the charge occurs, so that ozone generated from the vicinity of the nip portion between the photosensitive drum 321 and the charging roller 322 is generated. Increase. The amount of ozone generated is proportional to the value of the peak-to-peak voltage that further increases from the inflection point A. Therefore, if an excessively high peak-to-peak voltage is set, the amount of ozone generated during the image forming operation increases. Furthermore, when ambient air is decomposed by the ozone, discharge products containing nitrogen oxides adhere to the peripheral surface of the photosensitive drum 321. As a result, problems such as toner slippage and image flow in the cleaning device 326 (FIG. 1) occur. On the other hand, when the peak-to-peak voltage during the image forming operation is set to be smaller than the inflection point A, the surface potential of the photosensitive drum 321 is high because the sensitivity (gradient) of the surface potential to the peak-to-peak voltage is large. It becomes unstable. For this reason, the peak-to-peak voltage of the AC bias applied to the charging roller 322 during the image forming operation of the image forming apparatus 1 is set larger than the inflection point A and as close to the inflection point A as possible. It is desirable.

  On the other hand, the characteristics of the surface potential of the photosensitive drum 321 with respect to the peak-to-peak voltage shown in FIG. 3 vary depending on the temperature and humidity of the surrounding environment. For this reason, when the peak-to-peak voltage of the AC voltage is fixed and set in advance, the peak-to-peak voltage of the AC voltage applied to the charging roller 322 is changed from the actual inflection point A as the surrounding environment changes. It may go away. Therefore, it is desirable that the peak-to-peak voltage of the AC voltage is individually set corresponding to the inflection point A that changes according to the surrounding environmental change. However, if a high peak-to-peak voltage higher than the inflection point A is applied to the surface of the photosensitive drum 321 in the process of searching for the inflection point A that fluctuates, film destruction of the surface of the photosensitive drum 321 occurs. There was a problem.

  In the present embodiment, in order to solve the above-described problem, the applied voltage control unit 92 suppresses the application of a high peak-to-peak voltage to the photosensitive drum 321, and the peak corresponding to the actual inflection point A. The inter-voltage is set to the AC bias during the image forming operation.

  FIG. 4 is a graph showing the transition of the DC current Idc flowing through the charging roller 322 when the photosensitive drum 321 is rotationally driven under different environmental conditions and a charging bias having a DC voltage Vdc is applied to the charging roller 322. is there. In this case, the peak-to-peak voltage of the AC voltage is set to zero. The direct current Idc is detected by the above-described ammeter 83. As shown in FIG. 4, when the DC voltage Vdc applied to the charging roller 322 is gradually increased, the DC current Idc tends to rise rapidly from a predetermined region in any environment. In other words, in an environment of normal temperature and normal humidity (temperature 23 ° C., relative humidity 50%), current does not flow through the charging roller 322 until the voltage value B in FIG. When the DC voltage Vdc becomes larger than the voltage value B, the value of the current flowing through the charging roller 322 gradually increases. When the surrounding environment changes to high temperature and high humidity and low temperature and low humidity, the position of the voltage value B changes to a voltage value B ′ and a voltage value B ″, respectively. This is because the electric resistance value of the charging roller 322 changes according to each environment.

  The voltage value B, the point B ′, and the point B ″ in each environment described above correspond to the discharge start voltage between the photosensitive drum 321 and the charging roller 322, respectively. A value obtained by multiplying the voltage value B (B ′, B ″) by 2 is the peak-to-peak voltage at the inflection point A in FIG. In the present embodiment, the applied voltage control unit 92 sets an appropriate peak-to-peak voltage for the image forming operation using the relationship between the direct-current voltage and the peak-to-peak voltage of the alternating-current voltage.

  Next, a flow in which the applied voltage control unit 92 sets the peak-to-peak voltage Vpp (O) of the AC voltage in the image forming charging bias will be described. FIG. 5 is a flowchart when the peak-to-peak voltage Vpp (O) is set. In the present embodiment, the applied voltage control unit 92 performs the setting operation of the peak-to-peak voltage Vpp (O) of the AC voltage during non-image formation when the image forming operation is not performed in the image forming unit 30. The non-image forming operation is, for example, when the image forming apparatus 1 returns from the standby mode (energy saving mode) or corresponds to a plurality of jobs of the image forming operation. Further, the setting operation may be executed when the ambient temperature and humidity environment detected by the environment sensor 95 changes by a predetermined threshold value or more.

  Referring to FIG. 5, when the setting operation of peak-to-peak voltage Vpp (O) is started, applied voltage control unit 92 causes environment sensor 95 to detect the surrounding temperature and humidity environment (step # 1 in FIG. 5). . Based on the temperature and humidity detected by the environmental sensor 95, the applied voltage control unit 92 uses the initial value information stored in advance in the storage unit 93 to determine the initial value Vdc (the DC voltage applied to the charging roller 322. A) is determined (step # 2). The initial value Vdc (A) of the DC voltage corresponds to the initial value when the DC voltage Vdc is gradually increased in the graph of FIG. Table 1 is an example of initial value information stored in the storage unit 93. Based on the results of a plurality of experiments performed in advance, the initial voltage is set so that the DC voltage Vdc starts increasing from a value close to the voltage value B (B ′, B ″) as much as possible in each temperature and humidity environment. Value information is set.

  Referring to Table 1, when the temperature and humidity detected by environment sensor 95 are 20 degrees and 50%, Vdc (A) is set to 460V. The applied voltage control unit 92 controls the drive control unit 91 and the drive unit 84 to rotate the photosensitive drum 321. Further, the applied voltage control unit 92 controls the bias applying unit 80 to apply the set Vdc (A) to the charging roller 322 (step # 3). At this time, in this embodiment, Vdc (A) is applied to the charging roller 322 for 20 milliseconds. As the application time of Vdc (A) is longer, the detection accuracy is increased. However, in order to shorten the operation time of the setting operation of Vdc (O) as much as possible, about 20 milliseconds is preferable.

  When the DC voltage Vdc (A) is applied to the charging roller 322, the applied voltage control unit 92 controls the ammeter 83 to detect the DC current Idc flowing through the charging roller 322. Further, the applied voltage control unit 92 determines whether or not the detected Idc exceeds a preset current threshold (0 in FIG. 5) (step # 4). When Idc> 0 (YES in Step # 4), the applied voltage control unit 92 determines the Vdc (A) as the discharge start voltage Vdc (C) under the environmental conditions at that time (Step # 5). As a result, the voltage value B (B ′, B ″) in FIG. 4 is derived.

  On the other hand, when Idc ≦ 0 (NO in step # 4), the applied voltage control unit 92 adds a predetermined constant b to Vdc (A) (step # 6). In this embodiment, b = 20V is set. The applied voltage control unit 92 applies the DC voltage of Vdc (A) + b to the charging roller 322 again, and repeats detection and comparison of the DC current Idc (step # 4). The applied voltage control unit 92 adds the constant b × n (n = 1, 2, 3,...) To Vdc (A) and repeats the above flow until the direct current Idc exceeds the threshold value 0. Eventually, the applied voltage control unit 92 determines that Vdc (A) + bn when idc> 0 (YES in step # 4) is the discharge start voltage Vdc (C) (step # 5).

  When the discharge start voltage Vdc (C) is determined, the applied voltage control unit 92 sets a value obtained by multiplying Vdc (C) by 2 as the peak-to-peak voltage (Vpp (O)) of the AC voltage in the charging bias for image formation. (Step # 7). As a result, the peak-to-peak voltage of the AC voltage applied to the charging roller 322 is set near the inflection point A in relation to the surface potential of the photosensitive drum 321. Therefore, in the setting operation of the peak-to-peak voltage Vdc (O) applied to the charging roller 322, a voltage that is excessively higher than the inflection point A is prevented from being applied to the photosensitive drum 321. In addition, since the current threshold value compared with Idc in step # 4 in FIG. 5 is set to 0, the AC voltage (peak-to-peak voltage) applied to the charging roller 322 and the surface potential of the photosensitive drum 321 are reduced. In the vicinity of the inflection point A in the relationship, the peak-to-peak voltage Vdc (O) of the AC voltage in the image forming charging bias is set with higher accuracy.

  In this embodiment, the initial value Vdc (A) of the DC voltage applied to the charging roller 322 is set according to the ambient temperature and humidity. For this reason, the application of the DC voltage can be started from a region closer to the discharge start voltage (B). As a result, the setting operation of the peak-to-peak voltage Vdc (O) is performed in a short time, and the time during which the voltage is applied to the photosensitive drum 321 in the setting operation is reduced.

  The image forming apparatus 1 according to the embodiment of the present invention has been described above. However, the present invention is not limited to this, and for example, the following modified embodiment can be taken.

  (1) In the above embodiment, the case where the current threshold value to be compared with Idc is 0 in step # 4 of FIG. 5 is described, but the present invention is not limited to this. In consideration of the detection error of the ammeter 83, a current threshold value slightly larger than 0 may be set.

  (2) In the above embodiment, when the relationship of Idc> 0 is satisfied in step # 4 of FIG. 5, the embodiment proceeds to step # 5 as it is, but the present invention is limited to this. is not. Even when the relationship of Idc> 0 is satisfied, the direct current voltage applied by the applied voltage control unit 92 may be increased for a predetermined time, and the direct current value may be detected by the ammeter 83. Thereafter, the discharge start voltage may be calculated complementarily from the obtained detection results of the plurality of DC current values.

DESCRIPTION OF SYMBOLS 1 Image forming apparatus 10 Apparatus main body 30 Image forming part 321 Photosensitive drum 322 Charging roller (charger)
DESCRIPTION OF SYMBOLS 80 Bias application part 81 AC constant voltage power supply 82 DC constant voltage power supply 83 Ammeter 84 Drive part 90 Control part 91 Drive control part 92 Applied voltage control part (bias control part)
93 Storage unit 95 Environmental sensor

Claims (4)

A photosensitive drum made of amorphous silicon, which is rotated and forms an electrostatic latent image on the peripheral surface and carries a toner image; and is disposed in contact with or in proximity to the peripheral surface of the photosensitive drum, An image forming unit that performs an image forming operation for forming a toner image on a sheet,
A drive unit for rotationally driving the photosensitive drum;
A bias applying unit that applies a charging bias in which an AC voltage is superimposed on a DC voltage to the charger;
An ammeter for detecting a direct current flowing between the bias applying unit and the charger;
A bias control unit that controls the bias application unit to set an image forming charging bias applied to the charger in order to form the electrostatic latent image;
Have
The bias control unit applies a DC voltage to the charger while increasing the voltage value from a preset initial value while the photosensitive drum is rotated, and a DC current value detected by the ammeter An image forming apparatus, wherein a voltage value obtained by multiplying a value of the DC voltage when 2 exceeds a predetermined current threshold by 2 is set as a peak-to-peak voltage of the AC voltage in the charging bias for image formation.   The image forming apparatus according to claim 1, wherein the current threshold is zero. An environmental sensor that detects ambient temperature and humidity,
A storage unit for storing information on the initial value of the DC voltage with respect to a combination of temperature and humidity in advance;
Further comprising
The bias control unit sets an initial value of the DC voltage applied to the charger from the initial value information stored in the storage unit based on the temperature and humidity detected by the environmental sensor. The image forming apparatus according to claim 1, wherein: The bias control unit executes the setting operation of the peak-to-peak voltage of the AC voltage during non-image formation when the image forming operation is not performed in the image forming unit. The image forming apparatus according to claim 1.

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