JP2017219832A - Process cartridge, image forming apparatus, and image forming method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image forming apparatus that can detect discharge unevenness in the longitudinal direction to achieve the proper amount of discharge, a process cartridge, and an image forming method.SOLUTION: An image forming apparatus sets at least one or more partial measurement areas in the longitudinal direction in a printable area on a photoreceptor drum 3; the photoreceptor drum 3 is charged to a certain potential by a charging roller 6, and subsequently the partial measurement areas and an area excluding the partial measurement areas are made to have different potentials from each other by a laser scanner 4; discharge information detection means detects information on partial discharge in the partial measurement areas; a bias voltage applied to the charging roller 6 during image formation is corrected on the basis of a measured value.SELECTED DRAWING: Figure 2

Description

  The present invention relates to an image forming apparatus including a charging device that charges an object to be charged via a charging member.

  2. Description of the Related Art Conventionally, an image forming apparatus employing an electrophotographic method includes a step of uniformly charging a surface of a drum type electrophotographic photosensitive member (hereinafter referred to as a photosensitive drum) to a predetermined potential.

  As a charging means, for example, a contact charging method in which a roller charging member (hereinafter referred to as a charging roller) is brought into contact with the surface of the photosensitive drum and a voltage is applied to the charging roller to charge the photosensitive drum is currently mainstream.

  As a method of applying a voltage to the charging roller, there are a method of applying a direct current voltage and a method of superimposing an alternating current voltage on the direct current voltage and alternately causing discharge to the plus side and the minus side to make the charge uniform. In the latter method, a resistive load current that flows through a resistive load between the charging roller and the photosensitive drum, a capacitive load current that flows through a capacitive load between the charging roller and the photosensitive drum, and a discharge current between the charging roller and the photosensitive drum. Flows, and a current obtained by adding them flows to the charging roller. At this time, it has been empirically found that the discharge current amount should be a predetermined value or more in order to obtain stable charging.

  FIG. 18 shows the characteristics of the current Ic flowing through the charging roller when the AC voltage Vc is applied to the charging roller. The alternating voltage Vc indicates the peak voltage value of the alternating voltage, and the current Ic indicates the effective value of the alternating current. As shown in FIG. 18, when the amplitude of the AC voltage Vc is gradually increased, the charging current increases accordingly. When the voltage is less than twice the predetermined voltage Vh, the amplitude of the AC voltage and the charging current are approximately proportional. This is because the resistance load current and the capacitive load current are proportional to the voltage amplitude, and since the voltage amplitude is small, the discharge phenomenon does not occur and the discharge current does not flow. When the AC voltage is further increased, the discharge phenomenon starts at twice the predetermined voltage Vh. At this time, the charging current Ic deviates from the proportional relationship and flows by the amount corresponding to the discharge current Is. Here, in order to stably charge, it is necessary to set the AC voltage Vc so that the discharge current Is becomes a predetermined value or more.

  However, when the amount of discharge to the photosensitive drum is increased, deterioration of the photosensitive drum such as abrasion of the photosensitive drum is promoted, and an abnormal image such as image flow in a high-temperature and high-humidity environment may occur due to discharge products. . Therefore, in order to obtain stable charging and to solve the above problem, it is necessary to control the application of the minimum necessary voltage with the amount of discharge suppressed as much as possible. However, the relationship between the voltage applied to the photosensitive drum and the amount of discharge is not always constant, and varies depending on the film thickness of the photosensitive layer and dielectric layer of the photosensitive drum, the environmental variation of the charging member and air, and the like. In addition to the causes of environmental fluctuations described above, problems caused by changes in the amount of discharge include fluctuations in the charging member manufacturing resistance and dirt resistance, fluctuations in the photosensitive drum capacitance due to durability, and characteristics of the high-voltage generator of the image forming apparatus main body. It has been found that even variations occur.

  In order to suppress such a change in the discharge amount, a “discharge current control method” has been proposed (see, for example, Patent Document 1). In this method, a control method has been proposed in which the peak voltage of the AC applied voltage applied to the charging roller and the peak voltage of its differential waveform are detected and the discharge current value is calculated (see Patent Document 1).

JP 2004-157501 A

  However, as the life of image forming apparatuses progressing in recent years and the use methods in the market are diversified, the unevenness of smear in the direction perpendicular to the image forming process direction of the charging roller (hereinafter referred to as the longitudinal direction) and the longitudinal direction of the photosensitive drum In some cases, the film thickness unevenness may occur. This is because, for example, a printing pattern in which the printing portion is biased in the longitudinal direction is continuously output, or a small-sized recording material such as an envelope or a postcard is continuously used in an image forming apparatus in which the recording material directly contacts the photosensitive drum. May occur due to accidents.

  If unevenness in the film thickness in the longitudinal direction of the photosensitive drum or uneven contamination in the longitudinal direction of the charging roller occurs, the impedance when the current flows from the charging roller to the photosensitive drum in the longitudinal direction is different, and there are parts that are easy to discharge and parts that are difficult to discharge. End up. In this case, when the discharge current control method detects the discharge amount in the entire longitudinal direction, there is a portion where the discharge amount is lower than appropriate, or there is a portion where discharge is higher than appropriate and acceleration of the photosensitive drum is promoted. There is. The portion that is difficult to discharge becomes unstable in charging, and an excessively discharged portion and an insufficiently discharged portion are locally generated. This insufficiently discharged portion is developed by the developing portion and appears as black spots on the white background portion. As a result, there is a case where an image detrimental effect such as so-called sandy ground where many black spots appear on a white background may occur.

  The discharge amount may be determined by predicting in advance the contamination unevenness of the charging roller and the film thickness unevenness of the photosensitive drum, but such a phenomenon varies greatly depending on the use situation of the user and is difficult to predict.

  SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide an image forming apparatus, a process cartridge, and an image forming method capable of detecting discharge unevenness in the longitudinal direction and optimizing the discharge amount.

  In order to achieve the above object, the present invention includes a rotatable image carrier that carries a latent image, an exposure unit that exposes the image carrier, a charging member that charges the image carrier, and the charging member. A bias applying means for applying a bias voltage to the member, a discharge amount detecting means for detecting a current amount discharged between the charging member and the image carrier, and the charging from the discharge current amount detected by the discharge amount detecting means. A discharge current control means for determining a bias voltage applied to the member; and a discharge information detection means for detecting discharge information which is information relating to a discharge between the charging member and the image carrier, wherein the discharge information detection means A region in which the width in the direction perpendicular to the rotation direction is smaller than the width of the printable region in a part of the printable region in the surface of the image carrier is set as a partial measurement region, and the charging member The image carrier A portion related to the discharge between the partial measurement region and the charging member when the exposure unit forms different potentials in the partial measurement region and the region excluding the partial measurement region after charging to a constant potential. The discharge information is detected, and the discharge current control unit corrects a bias voltage in image formation applied to the charging member based on the partial discharge information.

  As described above, according to the present invention, discharge unevenness in the longitudinal direction can be detected and the discharge amount can be optimized.

Flowchart according to the first embodiment. Schematic of image forming apparatus according to Embodiment 1 Discharge current control block diagram according to the first embodiment Discharge current control circuit diagram according to the first embodiment The figure which shows the discharge current detection method which concerns on Example 1. FIG. Discharge current control detection waveform diagram according to Example 1 DC bias discharge start voltage detection block diagram according to the first embodiment DC bias discharge start voltage detection circuit diagram according to the first embodiment Schematic of VI characteristics when DC charging bias is applied according to Example 1 Conceptual diagram of a discharge start voltage detection method according to the first embodiment. Longitudinal schematic diagram of photosensitive drum potential according to Embodiment 1 Schematic of partial discharge start voltage measurement according to Example 1 Conceptual diagram of measurement region according to Example 1 FIG. 5 is a conceptual diagram of detection of a difference in discharge start voltage values of a plurality of AC biases in the longitudinal direction according to the first embodiment. FIG. 5 is a conceptual diagram of detection of a difference in discharge start voltage values of a plurality of AC biases in the longitudinal direction according to the first embodiment. Relationship between discharge amount and AC bias according to Example 1 Relationship between discharge amount and AC bias according to Example 2 Characteristics of current flowing in charging roller when AC voltage is applied to charging roller Schematic diagram of discharge start voltage detection method according to embodiment 3 FIG. 6 is a relationship diagram of the potential and film thickness of the photosensitive drum according to the third embodiment. Conceptual diagram of discharge start voltage detection method according to embodiment 4 AC bias when detecting discharge start voltage according to embodiment 4

  DESCRIPTION OF EMBODIMENTS Hereinafter, embodiments for carrying out the present invention will be exemplarily described in detail with reference to the drawings. However, the dimensions, materials, shapes, and relative arrangements of the components described in this embodiment should be appropriately changed according to the configuration of the apparatus to which the invention is applied and various conditions. That is, it is not intended to limit the scope of the present invention to the following embodiments.

  In this embodiment, in the configuration in which the bias voltage applied to the charging member is controlled so that the discharge current flowing between the charging member and the image carrier is constant, a plurality of pieces of partial discharge information in the longitudinal direction are detected in the longitudinal direction. And the amount of discharge is optimized by correcting the bias voltage output value from these values.

  The process cartridge and the image forming apparatus according to the present invention will be described below by taking an electrophotographic system as an example.

<Outline of Configuration and Operation of Image Forming Apparatus and Process Cartridge>
FIG. 2 is a schematic diagram of the image forming apparatus 1 in a state where a process cartridge is mounted. Reference numeral 3 denotes a photosensitive drum which is a rotatable image carrier. Reference numeral 4 denotes a laser scanner which is an exposure unit that scans a laser beam on the photosensitive drum 3 with a semiconductor laser 5 to form an electrostatic latent image. Reference numeral 2 denotes a replaceable process cartridge. The process cartridge 2 includes a charging roller 6 that is a charging member for uniformly charging the photosensitive drum 3 and a developer that develops the electrostatic latent image formed on the photosensitive drum 3 by the laser scanner 4 with the developer. It comprises a carrier 7 and a developing device 8 for storing a developer. 10 is a transfer roller for transferring the developer image developed on the photosensitive drum 3 to a predetermined recording material 11, and 12 is a fixing device for fixing the developer transferred to the recording material 11 with heat and pressure. Reference numeral 13 denotes a temperature thermistor for controlling the temperature of the fixing device. Reference numeral 14 denotes a paper feed roller for feeding a recording material. 15 is a top sensor that synchronizes with the conveyance of the recording material, 16 is a paper discharge roller for discharging the recording material 11 after fixing to the paper discharge tray 17, and 18 is for detecting the presence or absence of the recording material 11 after fixing. This is a paper discharge sensor. Reference numeral 19 denotes an engine controller that includes a CPU 20 and controls each part of the image forming apparatus 1. An environmental sensor 21 detects the external environment of the image forming apparatus 1.

  In the present embodiment, an environment of 23 ° C. and 50% RH is used, the resolution is 600 dpi, the process speed is 235 mm / sec, and the exposure amount is 0.2 μJ / cm 2. In this embodiment, negative polarity toner is used, but positive polarity toner may be used. In this case, the configuration except that the signs of the bias and the like are all reversed is the same as that in the case of using the negative polarity toner.

<Charge control>
Next, FIG. 3 shows a block diagram for controlling the charging unit of the image forming apparatus in this embodiment. The CPU 20 includes a calculation unit 30, a storage unit 31, and an AC voltage drive signal generation unit 32. The discharge current control circuit (bias applying means) 33 applies a voltage to the charging roller 6 while controlling the discharge current by receiving a signal from the CPU 20. Here, the discharge control means includes a CPU 20 and a discharge current control circuit 33. Further, the CPU 20 detects the output value of the environmental sensor 21 and controls the discharge current according to the output value. The detailed operation of the discharge current control circuit 33 will be described below.

<Discharge current control>
The discharge current control circuit 33 in this embodiment is shown in FIG. The AC high voltage of the sine wave generated by the sine wave voltage application unit 50 is superimposed on the DC voltage output from the DC high voltage circuit 51. The vibration voltage is supplied to the charging roller 6. Further, the alternating current value is controlled so as to have a constant oscillating voltage output level according to the detection output value of the alternating current detection unit 52 serving as the alternating current value detecting means. Further, the discharge current control circuit 33 includes a peak voltage detection circuit 53 that is a voltage amplitude value detection unit and a differential waveform peak voltage detection circuit 54 that is a differential amplitude value detection unit. Thereby, CPU20 can detect the peak value and differential waveform peak value of the alternating voltage output. Here, the discharge amount detection means includes a peak voltage detection circuit 53, a differential waveform peak voltage detection circuit 54, and the CPU 20.

The method for detecting the discharge current in this embodiment will be described with reference to FIG. FIG. 5 shows the charging current that flows through the charging roller 6 when an AC voltage is applied to the charging roller 6. The horizontal axis represents the peak voltage value of the alternating voltage, and the vertical axis represents the effective value of the alternating current. As shown in FIG. 5, in a region where the charging AC voltage is not more than twice the discharge start voltage (Vh), the relational expression of the charging current value with respect to the charging AC voltage is represented by a substantially proportional straight line passing through the origin. In this region, a current corresponding to the resistive load and capacitive load between the charging roller 6 and the photosensitive drum 3 flows. On the other hand, in a region where the charging AC voltage is twice or more the discharge start voltage (Vh), a discharge current is generated between the charging roller 6 and the photosensitive drum 3, and the charging current obtained by adding the discharge current value is added. The value Ic flows. In the discharge start region, the characteristic when discharging is represented by 500, and the characteristic when not discharging is represented by 501. Here, the discharge current value can be calculated from the relationship between the characteristic 500 and the characteristic 501. In the case of the peak value Va when discharging, the peak value Va ′ when not discharging, and the charging current Ic, the discharge current value Is is obtained by the following equation (1).
Is = Ic × (Va′−Va) / Va ′ (1)
That is, if (Va′−Va) / Va ′ can be known, the discharge amount with respect to the charging current can be detected. A method of obtaining (Va′−Va) / Va ′ will be described with reference to FIG. FIG. 6A shows a charging AC voltage output waveform applied to the charging roller 6, and FIG. 6B shows a differential waveform of the charging AC voltage output. The output waveform of the charging AC voltage has a shape in which the level near the peak is lowered by ΔV (= (Va′−Va)) due to the influence of the discharge. Further, since the phase of the waveform of the differential voltage is delayed by 90 °, the peak value is not affected by the discharge. Therefore, the peak value (Vb) of the differential voltage corresponds to the peak level (Va ′) of the output voltage when not discharging. Therefore, (Va′−Va) / Va ′ can be obtained. The discharge current value Is is detected by the method shown in FIGS. 5 and 6, and the charging current value Ic is set so as to be the discharge current value Is.
Is adjusted.

  In this embodiment, in order to solve the problem of uneven discharge in the longitudinal direction, in addition to the above-described discharge current control, discharge information (hereinafter referred to as partial discharge information) of a portion in the longitudinal direction is detected, and the charging bias is determined from those values. The discharge amount was optimized by correcting the output value. In this example, a partial discharge start voltage (hereinafter, partial discharge start voltage) was measured as partial discharge information.

  In the following, first, a method for detecting the discharge start voltage necessary for detecting the partial discharge start voltage as partial discharge information and a method for performing uniform charging will be described, and then a method for detecting the partial discharge start voltage as partial discharge information. Will be explained.

<Discharge start voltage detection>
In order to detect the partial discharge information in the longitudinal direction, this embodiment has a DC bias discharge start voltage detection circuit (discharge information detection means) 34 as shown in FIG. The DC bias discharge start voltage detection circuit 34 detects a discharge start voltage while applying a DC voltage to the charging roller 6 by receiving a signal from the CPU 20. FIG. 8 shows a schematic configuration of the DC bias discharge start voltage detection circuit 34 in the present invention. Reference numeral 100 denotes a voltage setting circuit unit whose bias value is changed according to the PWM signal. 101 is a transformer drive circuit unit, and 102 is a high-voltage transformer unit. A feedback circuit unit 103 monitors the output voltage via R61, and is a circuit provided so as to have an output voltage value corresponding to the setting of the PWM signal. Reference numeral 104 denotes a current detection circuit unit (current detection means), which detects a current value I63 obtained by adding the current value I62 flowing through the charging roller 6 and the current value I61 flowing from the feedback circuit at R63, and transmits it to the CPU 20 from J110 as an analog value. Is done. Until the discharge between the photosensitive drum 3 and the charging roller 6 starts, the photosensitive drum 3 and the charging roller 6 are insulated. Therefore, until the discharge is started, the current flowing through the detection resistor R63 is only I61 flowing from the feedback circuit unit 103. I61 is determined by Vpwm and Vref, R64, R65 set by the PWM signal.
I61 = (Vref−Vpwm) / R64−Vpwm / R65

Further, when the current value I61 flows through the feedback resistor R61, the output voltage Vout is also set as follows.
Vout = I61 × R61 + Vpwm≈I61 × R61
That is, as indicated by the straight line (1) in FIG. 9, until the discharge is started, only the current I61 corresponding to the PWM signal flows through R63 of the current detection circuit unit, and thus the current value is a straight line.

  However, when discharge is started between the photosensitive drum 3 and the charging roller 6, I63 obtained by adding the current value I62 flowing through the charging roller 6 and the current value I61 flowing from the feedback circuit flows into R63 of the current detection circuit unit. That is, the current value is a curve having a branch point when the discharge starts as shown by the curve (2) in FIG. Thus, the current flowing through the charging roller 6 can be calculated as Isd obtained by subtracting the straight line (1) from the curve (2).

  In this example, a plurality of Isd were measured while operating the DC bias, and the time when a certain Isd reached a predetermined current value was determined as the DC bias discharge start voltage. However, the method for determining the DC bias discharge start voltage is not limited to this. For example, as shown in FIG. 10, the discharge amounts for two different DC biases are measured, an approximate line is drawn therefrom, and the point at which this line becomes zero discharge amount (D in FIG. 10) is determined as the DC bias discharge start voltage. Such a method may be used. From the viewpoint of accuracy, the number of measurement points is not limited to two, and may be increased.

<Method of uniformly charging in the longitudinal direction>
Next, a method for uniformly charging in the longitudinal direction will be described. In order to detect the partial discharge start voltage in the longitudinal direction, it is necessary to charge the photosensitive drum 3 uniformly in the longitudinal direction, that is, to a constant potential. Since the DC bias discharge start voltage changes depending on the potential of the charging roller 6, discharge unevenness cannot be detected from the DC bias discharge start voltage unless the potential is uniform in the longitudinal direction. In order to uniformly charge in the longitudinal direction, a sufficient charging AC voltage must be applied. In this embodiment, the maximum value of the charging AC bias in the current configuration is applied. In addition, a mechanism for confirming whether the charging potential is uniform at that time may be provided.

  A confirmation mechanism for uniform charging potential will be described below. First, a predetermined DC bias and a maximum AC bias are applied, and the DC bias discharge start voltage detection circuit 34 detects the DC bias discharge start voltage Vdcth1 (ave) in the entire longitudinal direction at that time. Next, the AC bias (PWM) is stepped down by one to apply a predetermined DC bias and the AC bias in the same manner, and the DC bias discharge start voltage detection circuit 34 applies the DC bias discharge start voltage Vdcth2 of the entire longitudinal direction at that time. (Ave) is detected. At this time, the DC bias discharge start voltages in the entire longitudinal direction can be written as Vdcth1 (ave) = (1 + C / Cd) Vpa + Vd1 (ave) and Vdcth2 (ave) = (1 + C / Cd) Vpa + Vd2 (ave), respectively. Here, the capacitance of the photosensitive drum 3 is Cd, and the capacitance between the charging roller 6 and the photosensitive drum 3 is C.

  Then, a relational expression of Vdcth1 (ave) −Vdcth2 (ave) = Vd1 (ave) −Vd2 (ave) is obtained. Here, Vd1 (ave) is the average potential in the longitudinal direction of the photosensitive drum 3 charged with the maximum AC bias, and Vd2 (ave) is the photosensitive drum 3 charged with the AC bias whose set value is one step down from the maximum. Is the average potential in the longitudinal direction. Vpa is a Paschen voltage, which is a function of the atmospheric pressure and the distance between discharges.

  FIG. 11 shows the relationship between the longitudinal potential of the photosensitive drum 3 and the AC bias. Vac1, Vac2, and Vac3 are values between the peaks of the AC bias applied to the charging roller 6, and Vac1 <Vac2 <Vac3. Charging by an AC bias is stabilized at a constant charging potential by repeating forward and reverse discharges. However, if the AC bias is low, charging potential longitudinal unevenness occurs when there is uneven longitudinal discharge (FIG. 11). (A)). However, if the AC bias is increased, the discharge amount increases in the entire longitudinal direction, and the charged potential of the photosensitive drum 3 is made uniform ((b) in FIG. 11). Along with this, the average potential increases. When the AC bias is further increased and the charging potential becomes uniform in the entire longitudinal direction, the average potential remains constant ((c) in FIG. 11).

  From this, the uniformity of the charged potential of the photosensitive drum 3 can be confirmed by comparing Vdcth1 (ave) and Vdcth2 (ave). Specifically, if Vdcth1 = Vdcth2, Vd1 = Vd2 and it can be determined that the longitudinal direction is uniform. On the other hand, if Vdcth1 ≠ Vdcth2, Vd1 ≠ Vd2, and it is determined that the battery is not sufficiently charged. In this case, since a stable image in quality cannot be provided at any voltage, it is necessary to take measures such as notifying the user of the life as a process cartridge.

  In the present embodiment, the maximum AC bias is applied to the charging bias. However, the present invention is not limited to this. A bias that can be charged sufficiently uniformly may be clarified in advance by examination or the like, and the value may be used.

<Detection of partial discharge information>
The detection of the partial discharge start voltage as the partial discharge information in the longitudinal direction will be described with reference to FIG. The horizontal direction in FIG. 12 represents the position in the longitudinal direction, and the vertical direction represents the potential.

First, as shown in FIG. 12A, the maximum AC bias is applied as described above to uniformly charge the photosensitive drum 3 (charging potential: Vd). Next, as shown in FIG. 12B, a measurement area (hereinafter referred to as a partial measurement area) D (i) having a smaller width in the longitudinal direction than the printable area is set in a part of the printable area of the photosensitive drum 3, Only the partial measurement region D (i) is exposed by the laser scanner 4 to obtain an exposure potential (Vl). You may provide the partial measurement area | region in multiple places in the longitudinal direction. At this time, the partial measurement region D (i) exposed by the laser scanner 4 and the region excluding this partial measurement region are set to different potentials. Further, as shown in FIG. 12C, the DC bias discharge start voltage detection circuit 34 is used to apply a DC bias (Vdc) to the charging roller 6 and the DC bias discharge start voltage is measured with a partial exposure potential. . At this time, since only partial measurement region D (i) starts to discharge, DC bias partial discharge start voltage Vdcth (i) of partial measurement region D (i) is obtained. Consider an equivalent circuit where the electrostatic capacity of the photosensitive drum in the partial measurement region D (i) is Cd (i) and the electrostatic capacity between the charging roller 6 and the photosensitive drum 3 is C (i). Then, the magnitude of the DC bias partial discharge start voltage is Vdcth (i) = (1 + C (i) / Cd (i)) Vpa + | Vl |.

Further, when an image is actually formed, a DC + AC bias is applied, and the AC bias partial discharge start voltage Vacth (i) of the partial measurement region D (i) at that time is Vacth (i) = 2Vpa (1 + C (i) / Cd (i)). Here, the AC bias partial discharge start voltage is not the voltage at which the partial measurement region D (i) starts to discharge from the charging roller 6 to the photosensitive drum 3, but also starts reverse discharge from the photosensitive drum 3 to the charging roller 6. This is the voltage that starts to converge. The AC bias partial discharge start voltage is a value between the peaks of the AC bias. Therefore, the relational expression (2) is satisfied.
Vacth (i) = 2 (Vdcth (i) − | Vl |) (2)
If the DC bias discharge start voltage in the entire longitudinal direction is Vdcth (ave), and the AC bias discharge start voltage in the entire longitudinal direction is Vactth (ave), Expression (3) can be obtained from this relational expression.
Vactth (i) -Vacth (ave) = 2 (Vdcth (i) -Vdcth (ave)) (3)
The AC bias partial discharge start voltage in the partial measurement region D (i) can be detected by this equation (3). Actually, correction is performed using this AC bias partial discharge start voltage difference Vacth (i) −Vacth (ave). Here, Vdcth (ave) is actually used by obtaining the DC bias discharge start voltage in the entire longitudinal direction from detection of the DC bias discharge start voltage. However, the present invention is not limited to this, and an average value of Vdcht (1), Vdcth (2)... Vdcht (N) may be obtained and used.

  Next, a partial measurement region in which partial discharge start voltage detection is performed will be described. FIG. 13A shows a partial measurement region of this example. 13 indicates the longitudinal direction of the photosensitive drum 3 (direction perpendicular to the rotation direction). In this embodiment, the longitudinal direction of the printable area in the surface of the photosensitive drum 3 is uniformly divided into seven equal parts, and the partial discharge start voltage is detected in each area. However, the method for setting the partial measurement region is not limited to this, and it may not be equally divided. Further, as shown in FIG. 13B, the partial measurement region may be determined according to the size of the paper. In this case, since the measurement can be performed taking into account the shaving of the photosensitive drum 3, the accuracy is improved. Furthermore, the paper usage history may be stored in the CPU 20 or the like, and the partial measurement area may be changed according to the paper usage history.

  In addition, when a region where the discharge start voltage is large is known in advance from prediction of shaving of the photosensitive drum 3 or prediction of contamination of the charging roller 6, it is effective to measure the partial discharge start voltage only in that region. In this case, it is necessary to obtain the entire DC bias discharge amount start voltage Vdcth (ave) from the discharge start voltage detecting means for the entire longitudinal direction.

<Correction method>
Next, a method for correcting the charging bias output value from the AC bias partial discharge start voltage will be described.
In FIG. 14A, the horizontal axis represents the longitudinal direction, and the vertical axis represents the image forming process progress direction, which represents a partial measurement region of the photosensitive drum 3. In this example, the partial discharge start voltage was detected in order from D (1) as shown in FIG. However, the method is not limited to this, and for example, the order of measurement may be changed, or there may be an interval in the detection of the partial measurement region as shown in FIG. FIG. 14B shows the difference between the AC bias partial discharge start voltage in each partial measurement region and the AC bias discharge start voltage over the entire length. A region where Vacth (i) −Vacth (ave) is the largest is a portion where discharge is most difficult, and a region where the Vacth (i) −Vacth (ave) is largest is a portion where discharge is most likely. If it is a detection result shown in Drawing 14 (b), D (6) will be the part which is hard to discharge most, and D (1) will be the part which is easy to discharge. Correction is performed from the AC bias partial discharge start voltage in the partial measurement region obtained as described above.

  FIG. 16 shows the relationship between the discharge amount and the AC bias. In FIG. 16, the relationship between the discharge amount of the entire photosensitive drum 3 and the AC bias is indicated by a solid line. The relationship between the discharge amount of the photosensitive drum 3 that is most likely to be discharged (D (1) in the example of FIG. 14B) and the AC bias is indicated by a rough broken line. Further, the relationship between the discharge amount of the region of the photosensitive drum 3 where discharge is most difficult (D (6) in the example of FIG. 14B) and the AC bias is indicated by a fine broken line. The AC bias is a voltage value between peaks. As shown in FIG. 16, the discharge characteristic shifts at the discharge start voltage difference in the portion where the discharge is most difficult. For this reason, even if the total discharge amount reaches the target discharge amount, the discharge amount does not reach the target discharge amount in the portion where it is difficult to discharge (A in FIG. 16). In FIG. 16A, the AC bias at the intersection of the broken line and the solid line graph passing through the target value on the vertical axis and on the horizontal axis indicates the AC bias when the total discharge amount of the photosensitive drum 3 becomes the target value. The intersection A between the broken line parallel to the vertical axis passing through the above-described intersection and the fine broken line is an area where discharge is most difficult when an AC bias having a value such that the discharge amount of the entire photosensitive drum 3 becomes the target value is applied. The amount of discharge is shown. The discharge amount in this case is smaller than the target value as can be seen from FIG. In such a case, sandy ground may be generated as described in the above problem.

  Also, the portion where discharge is most likely as shown in FIG. 16 is shifted to the side where the discharge characteristics easily flow by the discharge start voltage difference. For this reason, the discharge amount excessively flows in the portion that is easily discharged. If the amount of discharge is large, the amount of local abrasion on the photosensitive drum 3 increases, and the photosensitive drum 3 may be locally scratched.

The change from the charged potential to the exposure potential due to exposure occurs due to the generation of carriers (holes if the charge is negative, electrons if the charge is negative) and neutralization of the surface charge by the carriers due to the exposure. However, the exposure potential depends not only on the neutralized surface charge but also on the drum capacitance. A simple expression is as follows.
Vl = Vd + qd / ε Equation (4)
Here, q is the carrier charge per unit area, d is the film thickness of the photosensitive drum 3, and ε is the dielectric constant. Strictly speaking, q also depends on the film thickness and Equation (4) becomes a little more complicated, but the relationship itself does not change greatly, and Vl approaches Vd as the film thickness decreases. In other words, the exposure potential may not drop sufficiently even when exposed in a scratched area with a small film thickness. If solid black or the like is printed, the scratched area may not be printed, and vertical white lines may appear in the image. .

  The correction method of the present embodiment is a correction method for reducing sand while suppressing vertical shading in an image by suppressing the amount of shaving of the photosensitive drum.

As a specific method, V0− (Vacth (ave) −Vacth (min)) is set to an allowable threshold value (Vth) for the AC bias (V0) at the discharge amount (I0) that causes an allowable limit amount of wear. ). Here, Vacth (min) is the AC bias partial discharge start voltage in the region where discharge is most likely to occur. I0 is obtained in advance by examination or the like and stored in the CPU 20 or the like. In FIG. 16A, the AC bias at the intersection point of the alternate long and short dash line passing through the discharge amount I0 and parallel to the horizontal axis and the solid line indicating the entire relationship of the photosensitive drum 3 is V0. As shown in FIG. 16A, Vth determined as described above with respect to this V0 is a dashed line indicating that the rough broken line indicating the relationship of the most easily discharged region passes through I0 and is parallel to the horizontal axis. AC bias at the point of intersection. At this time, C is the intersection of a broken line that passes through Vth and is parallel to the vertical axis, and a fine chain line that indicates the relationship between the regions that are most difficult to discharge. That is, C indicates the relationship of the region where discharge is most difficult when the AC bias is set such that the discharge amount (I0) that causes the allowable amount of shaving is set in the region where discharge is most likely to occur. The AC bias value at the intersection B between the broken line passing through the target value of the discharge amount and parallel to the horizontal axis and the rough broken line indicating the relationship between the discharge region and the discharge value in the region where discharge is most difficult is the target value. This is the AC bias value. If the AC bias at the intersection B is smaller than the AC bias at the intersection C as shown in FIG. 16A, the AC bias is corrected from the intersection A to the intersection B. That is, Vacth (max) −Vacth (ave), which is the difference between the AC bias value at the intersection B and the AC bias value at the intersection A, is added to the AC bias that reaches the target discharge amount as a whole. Here, Vacth (max) is the AC bias partial discharge start voltage in the region where discharge is most likely, and Vacth (max) −Vacth (ave) is obtained from Equation 3 above. As a result, the target discharge amount flows even in the portion where discharge is most difficult. Further, since the AC bias at the intersection B is smaller than the AC bias at the intersection C, even if the AC bias is corrected from the intersection A to the intersection B, it does not exceed the allowable cutting amount even in the region where discharge is most likely to occur. .

  If the AC bias at the intersection B is larger than the AC bias at the intersection C as shown in FIG. 16B, the AC bias is set to Vth, and control is performed so that the discharge amount does not increase any more. This is because, when the AC bias is corrected from the intersection A to the intersection B as in FIG. 16A, the AC bias at the intersection B is larger than the AC bias at the intersection C. This is because the discharge amount (I0) that causes the amount of shaving is exceeded. As a result, the portion that is most likely to be discharged can be limited to an allowable cutting amount.

  With this correction method, it is possible to suppress the sand while suppressing the amount of shaving of the photosensitive drum and suppressing the occurrence of vertical white stripes in the image.

<Confirmation of effect>
In order to confirm the effect of this embodiment, the image defect due to charging failure was confirmed in the above correction method and the comparative example which is the conventional discharge current control method. The configuration of the comparative example performs conventional discharge current control without correcting the charging bias output value with respect to the configuration of the present embodiment. Others are the same as in the present embodiment, and the description is omitted. Measurement was performed by intermittently printing 30000 sheets of Canon CS680 paper B5 size at 4% printing rate, and printing a solid white image and a solid black image every 5000 sheets with Canon CS680 paper A3 size. . The paper was passed so that the paper passed through the central portion of the photosensitive drum 3. As an adverse effect of charging failure, the solid white image was confirmed by the presence or level of sand. The negative effect of shaving the photosensitive drum was confirmed by the presence and level of white streaks on solid black.

＜フローチャート＞ The results are shown in Table 1. ○ in the sand part of the table indicates that no sand is generated. Δ indicates that sand is generated, but 5 or less per 1 cm 2, and x indicates that 5 or more are generated per 1 cm 2. A circle in the vertical white stripe portion of the table indicates that no vertical white stripe is generated. Δ indicates the extent to which lines are visible, and × indicates the extent to which the stripes are clearly visible. From the results, it can be seen that there is no occurrence in this example, whereas the sandy area is Δ in 25,000 sheets in the comparative example. It can also be seen that no vertical white stripes have occurred. From this, it was found that the present embodiment can suppress the sand while suppressing the shaving amount of the photosensitive drum 3.

<Flowchart>

    Next, the image forming method including the detection flow of the partial discharge information in the longitudinal direction in the present embodiment will be described using the flowchart of FIG.

  First, when a print command is received by the image forming apparatus 1 (A101), discharge current control is started (A102), and a charging bias for obtaining a target discharge amount is determined. Thereafter, it is determined whether or not partial discharge information is detected (A103). Since the longitudinal unevenness of the discharge is caused by changes over time such as the uneven shaving of the photosensitive drum 3 or the unevenness of the charging roller 6, the partial discharge information need not always be measured. With the configuration of this example, it is known that it takes about 1000 sheets from generation to actualization. Therefore, in this embodiment, partial discharge information is measured once per 1000 sheets, and if it is determined that partial discharge information is not detected, the most recently performed data stored in the CPU 20 is corrected as a correction value. (A111). However, the present invention is not limited to this, and may be performed at a frequency according to the configuration of the image forming apparatus. When it is determined that the partial discharge information is detected, the measurement of the first measurement region is started (A104). First, a sufficient bias is applied so that the photosensitive drum 3 can be uniformly charged by the charging roller 6 (A105). Then, the measurement area is exposed (A106), and the DC bias discharge start voltage is detected by the DC bias discharge start voltage detection 34 (A107). When the measurement is completed, it is determined whether all areas have been measured (A108). If it is determined that all the measurement areas have not been measured yet, the area not yet measured is set as the measurement area (A109) and the process returns to A105 to start measurement. If it is determined that all the measurement areas have been measured, the charging bias for discharge current control is corrected (A110) based on the measured values, and printing is started (A112).

  By performing the flow as described above, it is possible to detect the partial discharge start voltage as the partial discharge information and correct the charging bias.

Note that a non-volatile memory may be installed in the process cartridge, and information used to determine a target discharge current value, a use status, a bias voltage such as a use environment, and the like may be stored in the non-volatile memory. The process cartridge is removable from the main body of the image forming apparatus, and information can be given to each process cartridge by the nonvolatile memory, so that an appropriate bias can be set for each process cartridge.

  Note that the configuration of the present embodiment is not limited to this. Speed, exposure amount, etc. are only examples for carrying out this embodiment. In this embodiment, the discharge start voltage is detected as the discharge information, and the partial discharge start voltage is measured as the partial discharge information. However, the present invention is not limited thereto. For example, the discharge current is detected as the discharge information, and the partial discharge current is detected. (Hereinafter, partial discharge current) may be measured. In the case of detecting the discharge current, for example, the discharge unevenness in the longitudinal direction can be detected by detecting the partial discharge current under a constant charging voltage.

  The first embodiment is a correction method for suppressing sand while suppressing the amount of shaving of the photosensitive drum 3 and suppressing the occurrence of vertical white stripes in an image. The present embodiment is characterized in that it is a correction method that suppresses the generation of sandy land even in a configuration with a longer lifetime. In addition, description is omitted about the part which overlaps with Example 1. FIG.

  FIG. 17 shows the relationship between the discharge amount and the AC bias. As shown in FIG. 17, the discharge characteristic shifts at the discharge start voltage difference in the portion where discharge is most difficult. For this reason, even if the total discharge amount reaches the target discharge amount, the discharge amount does not reach the target discharge amount in the portion where it is difficult to discharge (A in FIG. 17). In such a case, sandy ground may occur as described in the problem of the prior art. In such a case, the correction method of the first embodiment is effective, but when the number of printed sheets increases due to a long life, it is sometimes difficult to suppress the sand.

  Therefore, in this embodiment, the AC bias is corrected from A to B in FIG. 17 in order to eliminate the threshold and eliminate the sand. That is, the AC bias was always added by Vacth (max) −Vacth (ave).

  With this correction method, the target discharge amount flows in the portion where discharge is most difficult in the longitudinal direction, so that sand can be suppressed.

<Confirmation of effect>
In order to confirm the effect of the present example, the same effect confirmation as in Example 1 was performed. The target of confirmation was the three of the present example, Example 1, and the comparative example used in Example 1. Moreover, since the effect of a present Example is suppression of sandy land, only sandy land was confirmed. Since the measurement is the same as in Example 1, the description thereof is omitted.

The confirmation results are shown in Table 2. From the results, it can be seen that there is no sand formation in this example. From this, it was found that the sand can be suppressed in this example.

  Note that the configuration of the present embodiment is not limited to this. Speed, exposure amount, etc. are only examples for carrying out this embodiment. In this embodiment, the discharge start voltage is detected as the discharge information, and the partial discharge start voltage is measured as the partial discharge information. However, the present invention is not limited to this. For example, the discharge current is detected as the discharge information and the partial discharge current is measured. May be.

  In Examples 1 and 2, partial discharge information was detected using a partial measurement area for partial discharge information detection as an exposure unit. In this embodiment, a partial measurement area for partial discharge information detection is a non-exposed part that is not exposed by the laser scanner 4, and a non-measurement area other than the partial measurement area is an exposure part that is exposed by the laser scanner 4. In the present embodiment, the partial measurement region and the region excluding the partial measurement region are thus set to different potentials. Thereby, the detection accuracy of partial discharge information is improved with respect to the first and second embodiments, and generation of sand is suppressed even in a configuration with a longer life. In addition, description is abbreviate | omitted about the part which overlaps with Example 1,2. In this example, the partial discharge start voltage was measured as partial discharge information.

  In Example 1, when the DC bias partial discharge start voltage Vdcth (i) was obtained, the term of the exposure potential Vl was included, and it was considered that the exposure potential Vl did not change significantly in the longitudinal direction. However, for example, as the film thickness of the photosensitive drum 3 decreases, the exposure potential may change greatly depending on the film thickness of the photosensitive drum 3 even if the charging potential is uniform in the longitudinal direction. In FIG. 20, the change due to the film thickness of the potential of the exposed portion and the non-exposed portion of the photosensitive drum 3 is indicated by a solid line. When the film thickness of the photosensitive drum 3 decreases, the film thickness dependence of the exposure potential increases, and the exposure potential difference due to the film thickness difference is large. This has become prominent due to the long life of the image forming apparatus which has been progressing in recent years.

In such a case, it is considered that the exposure potential Vl is different in the measurement region (Vl = Vl (i)), and Equation (3) cannot be obtained from Equation (2), and Equation (5) is obtained.
Vacth (i) −Vacth (ave) = 2 (Vdcth (i) −Vdcth (ave)) + 2 (| Vl (ave) | − | Vl (i) |) (5)

That is, an error corresponding to the second term on the right side of Equation (5) occurs. If correction is performed using Equation (3) in such a state, there is a case where correction is performed by 2 (| Vl (ave) | − | Vl (i) |). For example, when discharge unevenness occurs due to a difference in film thickness, even if an attempt is made to detect a region where the partial discharge start voltage is high, the region where the partial discharge start voltage is high is a region where the film thickness is thick, so | Vl (ave) |> | Vl (i) |. That is, the correction is performed by 2 (| Vl (ave) |-| Vl (i) |) (> 0), and sand may be generated without applying a sufficient bias.

  Therefore, in this embodiment, the influence of the exposure potential is avoided by setting the partial measurement region as a non-exposed portion. Therefore, the partial discharge start voltage can be detected with high accuracy. Hereinafter, the detection of the partial discharge start voltage in the longitudinal direction in the present embodiment will be described in detail with reference to FIG. The horizontal direction in FIG. 19 indicates the position in the longitudinal direction, and the vertical direction indicates the potential.

First, as shown in FIG. 19A, the photosensitive drum 3 is uniformly charged (charging potential: Vd). Next, as shown in FIG. 19B, a region other than the partial measurement region D (i) is exposed by the laser scanner 4 to obtain an exposure potential (Vl). Further, as shown in FIG. 19C, a DC bias (Vdc) is applied to the charging roller 6 by using the DC bias discharge start voltage detection circuit 34, and the DC bias discharge is started in a region where only the partial measurement region D (i) is discharged. Measure the voltage. As a result, the DC bias partial discharge start voltage Vdcth (i) of the partial measurement region D (i) is obtained. When the electrostatic capacity of the photosensitive drum 3 in the partial measurement region D (i) is Cd (i) and the electrostatic capacity between the charging roller 6 and the photosensitive drum 3 is C (i), the DC bias discharge start voltage is Vdcth ( i) = (1 + C (i) / Cd (i)) Vpa + Vd. Here, Vpa is a Paschen voltage, which is a function of the atmospheric pressure and the distance between discharges. As indicated by a broken line in FIG. 20, the charging potential is uniformly charged by the AC bias regardless of the film thickness, and thus does not greatly depend on the film thickness. Therefore, the expression (3) can be obtained as in the first embodiment.
Vactth (i) -Vacth (ave) = 2 (Vdcth (i) -Vdcth (ave)) (3)
That is, in this embodiment, the area to be measured is a non-exposed portion, thereby avoiding the influence of exposure potential change. Therefore, the partial discharge start voltage can be detected with high accuracy.

<Confirmation of effect>
In order to confirm the effect of the present example, the same effect confirmation as in Example 2 was performed. As confirmation targets, four examples of the present example, the first example, the second example, and the comparative example used in the first example were targeted. Since the measurement is the same as in Examples 1 and 2, description thereof is omitted. The confirmation results are shown in Table 3. From the results, it can be seen that in this embodiment, the generation of sandy land can be further suppressed as compared with Examples 1 and 2.

This is considered to be because, in Examples 1 and 2, in the latter half of the number of sheets passed, sufficient bias correction could not be performed due to the measurement error due to the exposure potential dependency due to the film thickness as described above. On the other hand, it can be seen that the present embodiment can be corrected appropriately.

  Note that the configuration of the present embodiment is not limited to this. Speed, exposure amount, photosensitive drum film thickness, and the like are merely examples for carrying out this embodiment. In this embodiment, the discharge start voltage is detected as the discharge information, and the partial discharge start voltage is measured as the partial discharge information. However, the present invention is not limited to this. For example, the discharge current is detected as the discharge information and the partial discharge current is measured. May be.

  In Examples 1 to 3, the DC bias discharge start voltage detection circuit 34 is used for partial discharge start voltage detection as partial discharge information detection. In the present embodiment, a configuration in which the same effect as in the first embodiment can be obtained even when the discharge current control circuit 33 is used for partial discharge start voltage detection will be described. That is, in the present embodiment, the discharge current control circuit 33 constitutes a discharge information detection unit. In addition, about the part which overlaps with Example 1, description is omitted using the same code | symbol. In this example, the partial discharge start voltage was measured as partial discharge information.

  The detection of the partial discharge start voltage in the longitudinal direction will be described with reference to FIG. In FIG. 21, the horizontal axis represents the position in the longitudinal direction, and the vertical axis represents the potential.

  First, as shown in FIG. 21A, the photosensitive drum 3 is uniformly charged (charging potential: Vd). Next, as shown in FIG. 21B, only the partial measurement region D (i) is exposed by the laser scanner 4 to obtain an exposure potential (Vl). Then, a DC + AC bias is applied to the charging roller 6 using the discharge current control circuit 33 as shown in FIG. At this time, the bias is fixed to the peak on one side (Vt) as shown in FIG. 21 (c) and amplified to the other side. Specifically, the DC bias is set to be Vt + Vac / 2 and is linked with the AC bias. Here, Vt is set to a bias in a region where discharge does not start. For example, Vd or a value in the vicinity thereof may be set. FIG. 22 shows the change over time of the applied AC charging bias. As shown in FIG. 22, the discharge start voltage is measured in a state where only the partial measurement region D (i) is discharged so as to be gradually amplified to one side. As a result, the partial measurement region D (i) starts discharging, and the discharge start voltage Vacth ′ (i) of the partial measurement region D (i) is obtained.

The magnitude of the discharge start voltage Vactth ′ (i) is
Vacth ′ (i) = (1 + C (i) / Cd (i)) Vpa + | Vl | − | Vt |

  Similarly to the first embodiment, the AC bias discharge start voltage Vacth (i) can be written as Vacth (i) = 2Vpa (1 + C (i) / Cd (i)). Therefore, Vacth (i) = 2 (Vacth ′ (i) − | Vl | + | Vt |), which satisfies the relational expression (6).

If the average of the discharge start voltage Vacth ′ (i) is Vacth ′ (ave) and the average of the AC bias discharge start voltage is Vacth (ave), Expression (7) can be obtained from this relational expression.
Vacth (i) −Vacth (ave) = 2 (Vacth ′ (i) −Vacth ′ (ave)) (7)

  From this equation (7), the AC bias discharge start voltage in the measurement region D (i) can be detected. In the present embodiment, the partial measurement region is the exposed portion, but it may be a non-exposed portion as in the third embodiment.

  When the effect similar to Example 1 was confirmed about the present Example, the result similar to Table 1 of Example 1 was obtained. From this, it was found that the same effect as in Example 1 can be obtained even if the discharge current control circuit 33 is used for partial discharge start voltage detection from this example.

  Note that the configuration of the present embodiment is not limited to this. Speed, exposure amount, etc. are only examples for carrying out this embodiment. In this embodiment, the discharge start voltage is detected as the discharge information, and the partial discharge start voltage is measured as the partial discharge information. However, the present invention is not limited to this. For example, the discharge current is detected as the discharge information and the partial discharge current is measured. May be.

1: image forming apparatus, 3: photosensitive drum, 4: laser scanner, 6: charging roller, 19: engine controller, 20: CPU, 33: discharge current control circuit, 34: DC bias discharge start voltage detection circuit

Claims (11)

A rotatable image carrier carrying a latent image;
Exposure means for exposing the image carrier;
A charging member for charging the image carrier;
Bias applying means for applying a bias voltage to the charging member;
A discharge amount detecting means for detecting a current amount discharged between the charging member and the image carrier; and a discharge current control for determining a bias voltage applied to the charging member from a discharge current amount detected by the discharge amount detecting means. Means,
Discharge information detecting means for detecting discharge information which is information relating to discharge between the charging member and the image carrier;
The discharge information detecting means sets, as a partial measurement area, an area in which a width in a direction perpendicular to a rotation direction is smaller than a width of the printable area in a part of the printable area in the surface of the image carrier. The partial measurement region when the image bearing member is charged to a constant potential by the charging member and the exposure unit forms different potentials in the partial measurement region and the region other than the partial measurement region. Detecting partial discharge information relating to discharge between the charging member and
The image forming apparatus, wherein the discharge current control unit corrects a bias voltage in image formation applied to the charging member based on the partial discharge information.   The image forming apparatus according to claim 1, wherein the discharge information is a discharge start voltage.   The image forming apparatus according to claim 1, wherein the discharge information is a discharge current.   The partial measurement area is an exposure part exposed by the exposure means, and the area excluding the partial measurement area is a non-exposed part that is not exposed by the exposure apparatus, so that the partial measurement area and the partial measurement area are 4. The image forming apparatus according to claim 1, wherein the partial discharge information is detected by setting different potentials to the excluded region. 5.   The partial measurement region and the partial measurement region are defined as a non-exposed portion that is not exposed by the exposure unit, and a region other than the partial measurement region is an exposed portion that is exposed by the exposure apparatus. 4. The image forming apparatus according to claim 1, wherein the partial discharge information is detected by setting different potentials to the excluded region. 5.   The image forming apparatus according to claim 1, wherein the discharge information detection unit detects discharge information when a DC bias voltage is applied to the charging member.   6. The discharge information detection unit according to claim 1, wherein the discharge information detection unit detects discharge information when an AC bias voltage is superimposed on a DC bias voltage and applied to the charging member. Image forming apparatus.   The image forming apparatus according to claim 7, wherein the discharge current control unit detects discharge information when an AC bias voltage is superimposed on the charging member and applied to the charging member.   The image forming apparatus according to claim 1, wherein the partial measurement region is set according to a width of the recording material. 10. A process cartridge that is attachable to and detachable from a main body of the image forming apparatus according to claim 1, and stores information used for determining the image carrier, the charging member, and a bias voltage. And a non-volatile memory. A rotatable image carrier for carrying a latent image; an exposure means for exposing the image carrier; a charging member for charging the image carrier; a bias applying means for applying a bias voltage to the charging member; the charging member; A discharge amount detecting means for detecting a current amount discharged between the image carriers;
An image forming method in an image forming apparatus, comprising: a discharge current control unit that determines a bias voltage applied to the charging member from a discharge current amount obtained by the discharge amount detection unit,
For a partial measurement area in which the width in the direction perpendicular to the rotation direction is set to be smaller than the printable area in a part of the printable area in the surface of the image carrier,
Charging the image carrier to a constant potential by the charging member;
Making the partial measurement region and the region excluding the partial measurement region different potentials by the exposure means;
Detecting partial discharge information regarding discharge between the partial measurement region and the charging member;
An image forming method comprising a step of correcting a bias voltage in image formation applied to the charging member based on the partial discharge information.

Images