JP5337452B2 - Image forming apparatus - Google Patents

Description

  The present invention relates to an electrophotographic image forming apparatus using a photoreceptor, and more particularly to a method for determining an optimum value of an AC bias applied to a contact-type charging device.

  In an image forming apparatus using an electrophotographic system such as a copying machine, a printer, or a FAX, a powder developer (hereinafter referred to as toner) is mainly used, and an electrostatic image formed on an image carrier such as a photosensitive drum. In general, the latent image is visualized with toner in the developing device, and the toner image is transferred onto a recording medium, and then a fixing process is performed. In such an image forming apparatus, as a device for charging the surface of the photoreceptor, for example, a corona discharge device that discharges when a high voltage is applied using a thin wire or the like as an electrode, a conductive rubber on a conductive core, A contact-type charging device that applies a voltage in a state where a charging member typified by a charging roller provided with a high-resistance coating layer or the like is in contact with the surface of a photoreceptor has been widely used.

  However, the corona discharge device requires a high voltage, and discharge products such as ozone, NOx and SOx generated by the discharge from the wire adhere to the shield and grid surface of the charging device. On the other hand, the contact-type charging device has an advantage that it can operate at a low voltage and generates less ozone.

  As a method of applying a voltage to the contact charging device, a DC charging method in which only a DC bias (Vdc) is applied to the charging member, and an AC charging method in which an AC bias (Vpp) is applied in a superimposed manner on the DC bias (Vdc). There is. Among them, the AC charging method forms an oscillating electric field between the photosensitive member and the charging member, the AC component leveles the unevenness of the charging potential on the surface of the photosensitive member, and the DC component converges to a predetermined potential. There is an advantage that the body surface can be uniformly charged without unevenness.

  When a charging roller is used as a charging member and the surface of an amorphous silicon (hereinafter referred to as a-Si) photosensitive drum is charged by an AC charging method, the surface potential V0 of the photosensitive member is proportional to Vpp, but Vpp is a predetermined value. Above that, it becomes constant. On the other hand, as shown in FIG. 5, the relationship between Vpp and the DC component current (Idc) is a curve having an inflection point where the slope becomes gentle when Vpp exceeds a certain level, and the surface potential V0 and the AC bias Vpp are It is almost consistent with the relationship.

  If Vpp is too low, the voltage is insufficient and the surface of the photoconductor is not uniformly charged, resulting in image unevenness. On the other hand, if Vpp is too high, the discharge current between the charging roller and the photoconductor increases, and the discharge product adheres to the photoconductor surface, increasing the friction coefficient of the drum surface, causing blade noise and toner adhesion. To do. In a high-temperature and high-humidity environment, character blurring occurs due to the discharge product absorbing moisture and reducing the surface resistance.

Using this relationship between Vpp and Idc, for example, as shown in Patent Document 1, a DC current when Vpp [n ⁺ ] whose voltage value is increased by a certain amount from a reference AC bias Vpp [n] is applied. A control method is disclosed in which Vpp [n] at which the increase amount ΔIdc of the component Idc is equal to or less than a predetermined threshold value is an optimal value.
JP 2006-171282 A

  In the method of Patent Document 1, ΔIdc with respect to a change in Vpp is calculated while increasing Vpp by a predetermined amount, and when ΔIdc falls below a threshold, Vpp at that time is determined as an appropriate voltage. A sufficiently low threshold is set in order to prevent the occurrence of charging failure on the surface of the photoreceptor due to insufficient voltage.

  FIG. 6 is a graph (Vpp-ΔIdc curve) showing the relationship between Vpp and the change amount ΔIdc of Idc. As shown in FIG. 6, when ΔIdc is slightly higher than the threshold value a (indicated by ● in the figure), Vpp is further adjusted to a voltage increased by a predetermined amount (indicated by ○ in the figure), and is appropriate. Excessive Vpp is applied to a higher voltage. As a result, the generation of discharge products increases, image flow occurs in a high humidity environment, and the frictional resistance of the surface of the photoconductor increases, so that the transfer (primary transfer) efficiency from the photoconductor to the recording medium or intermediate transfer member is increased. In some cases, such as a decrease in the temperature, a cleaning blade squealing (frictional sound), and an end roll-up.

  Although a method of reducing the adjustment step (rising width) of Vpp is also conceivable, if the step is simply reduced, the change amount of ΔIdc with respect to the change amount of Vpp is also reduced, and the possibility of erroneous detection from the current detection accuracy. Therefore, there is a problem that a highly accurate current detection system is required. Alternatively, even if only the adjustment step of Vpp is performed to measure Idc and ΔIdc is calculated in a larger step, the number of times of measuring Idc increases, so that the time required to correct the charging bias becomes longer.

  In view of the above problems, the present invention can form a high-quality image without image unevenness by setting an AC bias applied to a charging device in an appropriate region in a short time, and does not cause problems such as blade squealing. An object is to provide an image forming apparatus.

  In order to achieve the above object, the present invention provides a photosensitive member, a contact-type charging device that uniformly charges the surface of the photosensitive member by applying a DC bias and a charging bias superimposed with an AC bias, and the charging device. And a control means for determining an optimum AC bias value Vpp [A] based on a DC component current when a charging bias is applied to the reference AC bias as Vpp [n]. When the DC component current when the DC bias and the charging bias superimposed with Vpp [n] are applied is Idc [n], the control means uses the DC component current when Vpp [n] is increased stepwise. A change amount ΔIdc is calculated, and Δpp is compared with two threshold values to determine Vpp [A] in two stages.

  In the image forming apparatus having the above-described configuration, when the lower threshold value of the two threshold values is a, the higher threshold value is b, and the increase width per step of Vpp [n] is ΔVpp, The threshold value a is set to a value higher by a predetermined amount than the inflection point of ΔIdc, and the threshold value b is the amount of change in ΔIdc per ΔVpp immediately before the difference ba from the threshold value a reaches ΔIdc. And is set to be larger than the change amount of ΔIdc per 1 / 2ΔVpp.

According to the present invention, in the image forming apparatus configured as described above, the control unit determines Vpp [A] according to the following equations (1) and (2).
Vpp [A] = Vpp [n] −1 / 2ΔVpp (where a <ΔIdc ≦ b) (1)
Vpp [A] = Vpp [n] −ΔVpp (where ΔIdc ≦ a) (2)

  In the image forming apparatus having the above-described configuration, the two threshold values a and b are set based on temperature and / or humidity inside or outside the apparatus.

  According to the present invention, in the image forming apparatus configured as described above, the control unit outputs a constant multiple kVpp [A] (k ≧ 1) of Vpp [A] to the charging device.

  According to the first configuration of the present invention, the optimum value Vpp [A] can be determined with high accuracy in a short time without increasing the number of times of measurement of Idc [n]. Regardless of the resistance value variation, an appropriate AC bias can always be output to the charging device.

  According to the second configuration of the present invention, in the image forming apparatus having the first configuration, the lower threshold value a of the two threshold values is set to a value higher by a predetermined amount than the inflection point of ΔIdc, The difference b−a between the higher threshold value b and the threshold value a is smaller than the change amount of ΔIdc per ΔVpp immediately before ΔIdc reaches the inflection point, and more than the change amount of ΔIdc per 1 / 2ΔVpp. By setting the value to be larger, the thresholds a and b corresponding to ΔVpp can be set to increase the determination accuracy of the optimum value Vpp [A].

  According to the third configuration of the present invention, in the image forming apparatus having the second configuration, Vpp [A] is obtained from the equations (1) and (2) using the threshold values a and b and the calculated ΔIdc. Can be easily calculated.

  Further, according to the fourth configuration of the present invention, in the image forming apparatus having any one of the first to third configurations, the threshold values a and b are set based on the temperature and / or humidity inside or outside the apparatus. This makes it possible to always set an appropriate AC bias regardless of changes in the temperature and humidity environment due to the weather, installation location, or the like.

  According to the fifth configuration of the present invention, in the image forming apparatus having any one of the first to fourth configurations, the charging device is set to a constant multiple kVpp [A] (k ≧ 1) of Vpp [A]. By outputting, it is possible to give a margin to the output voltage so that image unevenness due to insufficient voltage does not occur.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a schematic cross-sectional view showing a configuration of an image forming unit in the image forming apparatus of the present invention. In FIG. 1, an image forming unit 15 includes a charging roller 2, an exposure unit 3, a developing device 4, a transfer roller 5, a cleaning device 6, and a charge removal lamp 7 along the rotation direction (clockwise) of the photosensitive drum 1. Is arranged. The photosensitive drum 1 is formed by laminating a photosensitive layer made of a-Si on an aluminum drum, for example. By applying a high voltage to the charging roller 2 in contact with the photosensitive drum 1, the photosensitive drum 1. The surface is uniformly charged.

  The charging roller 2 includes a cored bar (shaft) and an elastic layer formed around the cored bar. The elastic layer is formed from a semiconductive elastic member such as synthetic rubber, and in the embodiment, is formed from epichlorohydrin rubber. The surface of the charging roller 2 (the surface of the elastic layer) is in contact with the surface of the photosensitive drum 1, and the core metal of the charging roller 2 can supply a voltage necessary for discharge (not shown). It is connected to the. In addition, a cleaning brush 11 for removing toner adhering to the charging roller 2 is disposed.

  The exposure unit 3 irradiates the charged photosensitive drum 1 with a light beam (for example, a laser beam) based on the image data, attenuates the charging of the portion that has received the laser beam, and statically irradiates the surface of the photosensitive drum 1. An electrostatic latent image is formed. The developing device 4 includes a developing sleeve 4a (developer carrying member) disposed to face the photosensitive drum 1, and the toner accommodated therein is attached to the electrostatic latent image on the photosensitive drum 1 by the developing sleeve 4a. A toner image (visible image) is formed.

  As is well known, an electrostatic latent image is recorded by the exposure unit 3 on the photosensitive drum 1 uniformly charged by the charging roller 2 after being neutralized by the neutralizing lamp 7, and the electrostatic latent image is developed by reversal development. At 4, the toner image is visualized, and the toner image is transferred onto the sheet by the transfer roller 5. The sheet onto which the toner image has been transferred is conveyed to the fixing unit 8 and is heated and pressed by the fixing roller pair 8a, so that the toner image becomes a permanent image. Untransferred toner that has not been transferred by the transfer roller 5 is removed from the surface of the photosensitive drum 1 by the cleaning roller 9 and the cleaning blade 10 of the cleaning device 6 as residual toner, and the removed residual toner is a toner recovery device such as a recovery screw. (Not shown) and conveyed to a waste bottle (not shown).

  FIG. 2 is a block diagram showing the configuration of the image forming apparatus of the present invention. Portions common to those in FIG. 1 are denoted by the same reference numerals. The image forming apparatus 100 includes an image forming unit 15, an image input unit 20, an AD conversion unit 40, a control unit 41, a storage unit 42, an operation panel 43, a DC component current sensor 44, a temperature / humidity sensor 45, and the like. In addition, since various controls of each part of the apparatus are performed when the image forming apparatus 100 is used, the control means of the entire image forming apparatus 100 is complicated. Therefore, only the portions necessary for controlling the image forming unit 15 will be described here.

  When the image forming apparatus 100 is a copying machine, the image input unit 20 includes a scanner lamp that illuminates the document during copying, a scanning optical system equipped with a mirror that changes the optical path of reflected light from the document, and reflected light from the document. If the image forming apparatus 100 is a printer, a personal computer or the like is a condensing lens for condensing and forming an image and a CCD for converting the imaged image light into an electric signal. It is a receiving part which receives the image signal transmitted from. The image signal input from the image input unit 20 is converted into a digital signal by the AD conversion unit 40 and then sent to the image memory 50 in the storage unit 42 described later.

  The image forming unit 15 includes the photosensitive drum 1, the charging roller 2, the exposure unit 3, the developing device 4, the transfer roller 5, and the like, and is based on the image data converted by the AD conversion unit 40 and stored in the image memory 50. Then, an electrostatic latent image is formed on the photosensitive drum 1. The photosensitive drum 1, the transfer roller 5, and the fixing roller pair 8a (see FIG. 1) in the fixing unit 8 are rotationally driven by a main motor (not shown), and the control unit 41 transmits a control signal to the main motor. The rotation and stop of the photosensitive drum 1, the transfer roller 5, the fixing roller pair 8a, and the like are controlled.

  The control unit 41 generally controls each part of the image forming apparatus such as the image forming unit 15, the fixing unit 8, and the image input unit 20 according to the set program, and also needs an image signal input from the image input unit 20. The image data is converted into image data by scaling processing or gradation processing according to the above. The exposure unit 3 irradiates a laser beam based on the processed image data to form a latent image on the photosensitive drum 1.

  The storage unit 42 includes an image memory 50, a RAM 51, and a ROM 52. The image memory 50 stores an image signal input from the image input unit 20 and digitally converted by the AD conversion unit 40, and is stored in the control unit 41. Send it out. The RAM 51 and ROM 52 store the processing program, processing content, and the like of the control unit 41. Further, a reference program for DC bias and AC bias applied to the charging roller 2 and a control program (described later) for determining the optimum value of the AC bias are also stored.

  The operation panel 43 includes an operation unit including a plurality of operation keys and a display unit (none of which is shown) that displays setting conditions, apparatus states, and the like, and the user sets printing conditions and the like. In addition, for example, when the image forming apparatus 100 has a facsimile function, it is also used for various settings such as registering a facsimile transmission destination in the storage unit 42 and reading or rewriting the registered transmission destination.

  The DC component current sensor 44 detects a DC component current value when a DC bias and an AC bias are applied to the charging roller 2 in a superimposed manner, and the detection result is transmitted to the control unit 41. The control unit 41 determines the optimum value of the AC bias applied to the charging roller 2 based on the detection result, and stores the optimum value at that time in the RAM 51 in the storage unit 42. The temperature / humidity sensor 45 detects the temperature and humidity inside the apparatus, in particular, the temperature and humidity of the surface or the periphery of the photosensitive drum 1, and is disposed in the vicinity of the photosensitive drum 1.

  Next, control for determining the optimum value of the AC bias applied to the charging roller in the image forming apparatus of the present invention will be described. FIG. 3 is a flowchart showing a control procedure of the AC bias applied to the charging roller in the image forming apparatus of the present invention. With reference to FIGS. 1 and 2, the procedure for determining the optimum value of the AC bias applied to the charging roller along the steps of FIG. 3 will be described.

  First, the initial AC bias Vpp [0] is determined (step S1). When the charging characteristic of the charging roller 2 is not easily affected by temperature, Vpp [0] may be set to a fixed value. If the charging characteristics are likely to change due to the temperature, Vpp [0] is set using the in-machine temperature data detected by the temperature / humidity sensor 45 and the initial AC bias setting table stored in the RAM 51 (or ROM 52). You may decide. Here, Vpp [0] is set to 900V (fixed value).

  Next, a charging bias obtained by superimposing the DC bias Vdc [M] on Vpp [0] is applied to the charging roller 2 (step S2). Here, Vdc [M] is an appropriate value of the DC bias at normal temperature at which the surface potential of the photosensitive drum 1 becomes a target value, and is a unique value stored for each drum unit including the photosensitive drum 1 and the charging roller 2. . Further, the direct current component Idc [0] at this time is detected by the direct current component current sensor 44 and stored in the RAM 51.

  Next, n = 1 is set (step S3), and a predetermined voltage (hereinafter referred to as ΔVpp. Here, 200V) is added to Vpp [n−1] (= Vpp [0]) to obtain Vpp [n] (step S4). ). Further, in the Vpp correction control, even if the appropriate point of Vpp cannot be detected for some reason, the upper limit value of Vpp is set as Vpp [MAX], which does not cause a problem if printing is performed for several sheets. Usually, a voltage higher by 300 to 400 V than the target Vpp in the usage environment of the apparatus is set. Then, it is determined whether or not Vpp [n] is equal to or less than the maximum value (upper limit value) Vpp [MAX] of the AC bias (step S5). When Vpp [n] exceeds Vpp [MAX], Vpp [MAX] is determined as the optimum value Vpp [A] (step S6). On the other hand, when Vpp [n] is equal to or lower than Vpp [MAX], a charging bias obtained by superimposing Vdc [M] on Vpp [n] is applied to the charging roller 2, and the DC current component Idc [n] at that time is DC. Detection is performed by the component current sensor 44 (step S7).

  Next, a difference (ΔIdc) between the detected Idc [n] and a direct current component Idc [n−1] when a charging bias in which Vdc [M] is superimposed on Vpp [n−1] is applied is calculated. Then, it is determined whether or not ΔIdc is equal to or less than the threshold value a (step S8). When ΔIdc is equal to or smaller than the threshold value a, the previous Vpp [n−1], that is, Vpp [n] −ΔVpp is determined as Vpp [A] (step S9). In the case of n = 1, Vpp [A] = Vpp [0] (= 900 V).

  On the other hand, if ΔIdc is larger than threshold value a, it is next determined whether ΔIdc is equal to or smaller than threshold value b (b> a) (step S10). When ΔIdc is equal to or less than the threshold value b, a value obtained by subtracting 1/2 of ΔVpp from Vpp [n], that is, Vpp [n] −1 / 2ΔVpp is determined as Vpp [A] (step S11). In the case of n = 1, Vpp [A] = (900 + 200) −1 / 2 × 200 = 1,000V.

  If ΔIdc is larger than the threshold value b in step S10, 1 is added to n (step S12), and the same procedure is repeated until returning to step S4 and determining Vpp [A] (steps S5 to S12). . Then, a value obtained by multiplying Vpp [A] determined in step S6, S9, or S11 by 1.1 is output to the charging roller 2 (step S13).

  With the above control, as shown in FIG. 4, when ΔIdc ≦ a (indicated by ■ in the figure), Vpp [n−1] (indicated by □ in the figure) that is lower by the AC bias increase width ΔVpp (200 V) is obtained. Adopted as Vpp [A], when a <ΔIdc ≦ b (indicated by ● and ▲ in the figure), a bias value obtained by subtracting 1/2 (100 V) of ΔVpp from Vpp [n] (indicated by ○ and △ in the figure) Display) is adopted as Vpp [A], and therefore, ΔIdc is determined to be within a range not lower than the threshold value b by a certain width or more.

  Therefore, Vpp [A] can be accurately determined in a short time without increasing the number of times of measurement of Idc [n], and a high-quality image without image unevenness can be formed, and in a high humidity environment. Problems such as a decrease in transfer (primary transfer) efficiency due to an increase in image flow and frictional resistance on the surface of the photosensitive member, a squealing (frictional sound) of the cleaning blade, and winding up of the end can be effectively suppressed.

  The first threshold value a and the second threshold value b can be appropriately changed according to the Vpp-ΔIdc curve (see FIG. 4), but the lower threshold value a is predetermined from the inflection point P of the Vpp-ΔIdc curve. The higher threshold value b is set higher by a value (about 2 μA). The difference between the threshold values a and b is smaller than the change amount d1 of ΔIdc per ΔVpp (= 200 V) immediately before the inflection point P, and 1 / It is preferably set to be larger than the change amount d2 of ΔIdc per 2 · ΔVpp (= 100 V). By setting the thresholds a and b according to ΔVpp in this way, ΔIdc greatly exceeds both the thresholds a and b, or the range that exceeds the threshold b but does not exceed the threshold a becomes wide. There is no possibility that the accuracy of determining Vpp [A] will be reduced. Further, although ΔVpp is set to 200 V here, this value can also be changed.

  Furthermore, since the Vpp-ΔIdc curve changes depending on the in-machine temperature, a plurality of combinations of threshold values a and b are set in advance so that the threshold values a and b can be selected according to the in-machine temperature data detected by the temperature / humidity sensor 45. If so, the optimum value Vpp [A] can be determined with higher accuracy. Table 1 shows an example in which a plurality of sets of threshold values a and b are set.

  The reason why the actual output value is obtained by multiplying Vpp [A] by the constant 1.1 in step S13 is that the output value has a margin so that image unevenness due to insufficient voltage does not occur. Therefore, the constant k to be multiplied by Vpp [A] may be 1 or more, for example, k = 1.05, or k = 1 and Vpp [A] may be used as an output value as it is.

  The control for determining Vpp [A] may be executed at the start of continuous printing as well as when the apparatus is turned on or returned from the standby mode (sleep mode). As a result, an appropriate charging bias can be applied according to the temperature and humidity change in the machine due to heat radiation from the fixing unit 8, a high-quality image without image unevenness can be formed, and problems such as blade squeal can be generated. It becomes possible to prevent.

  In addition, this invention is not limited to the said embodiment, A various change is possible in the range which does not deviate from the meaning of this invention. For example, in the above-described embodiment, the image forming apparatus including the charging roller 2 has been described as an example. However, the present invention is not limited thereto, and other contact-type charging apparatuses, for example, conductive fibers are bundled in a brush shape. The present invention can be similarly applied to an image forming apparatus provided with a charging brush or the like whose tip is brought into contact with the surface of the photoreceptor.

  Moreover, although the example which selects the group of threshold value a and b using the internal temperature detected by the temperature / humidity sensor 45 was demonstrated in the said embodiment, threshold value a and b can also be selected using internal humidity. . In addition, an external temperature / humidity sensor for detecting the temperature and humidity outside the apparatus may be provided in place of the temperature / humidity sensor 45, and Vpp [A] may be determined using threshold values a and b corresponding to the external temperature and humidity. Is possible. In addition to performing control for determining Vpp [A] when the power is turned on and when returning from the standby mode, for example, Vpp [A] may be determined for each predetermined number of printed sheets.

  Further, the present invention is not limited to an image forming apparatus for forming a monochrome image, and is not limited to a tandem color image forming apparatus or other image forming apparatuses using a contact charging device such as a facsimile or a printer. The same can be applied.

  The present invention can be applied to an image forming apparatus including a contact-type charging device that uniformly charges the surface of a photoreceptor by applying a charging bias in which a DC bias and an AC bias are superimposed. The amount of change in the DC component current when Vpp [n] is increased stepwise, where Idc [n] is the DC component current when a charging bias in which [n] and a predetermined DC bias are superimposed is applied. ΔIdc is calculated, and ΔIdc is compared with two threshold values a and b to determine the optimum AC bias value Vpp [A] in two stages.

  As a result, even when the Vpp-Idc curve changes due to deterioration of the photosensitive drum, variation in resistance value of the charging device, environmental change, etc., an appropriate AC bias can be determined and output to the charging device in a short time. It is possible to provide an image forming apparatus that does not cause problems such as image unevenness due to shortage and blade squeal due to excessive voltage.

  At this time, in order to set the thresholds a and b corresponding to the increase width ΔVpp of the AC bias, the lower threshold a of the two thresholds is set to a value higher by a predetermined amount than the inflection point of ΔIdc, and the higher one The difference b−a between the threshold value b and the threshold value a is smaller than the change amount of ΔIdc per ΔVpp immediately before ΔIdc reaches the inflection point, and is larger than the change amount of ΔIdc per 1 / 2ΔVpp. It should be set as follows. When a <ΔIdc ≦ b, Vpp [n] −1 / 2ΔVpp is calculated, and when ΔIdc ≦ a, Vpp [n] −ΔVpp is used to calculate the optimum value Vpp [A]. Can be determined accurately.

  In addition, if two thresholds a and b are set based on the temperature and / or humidity inside or outside the apparatus, an appropriate AC bias can be accurately set regardless of changes in the temperature and humidity environment due to the weather or installation location. And an image forming apparatus capable of forming a higher quality image.

FIG. 2 is a schematic diagram illustrating a configuration of an image forming unit in the image forming apparatus of the present invention. FIG. 3 is a block diagram showing a control path of the image forming apparatus of the present invention. These are the flowcharts which show the determination control procedure of Vpp [A] in the image forming apparatus of this invention. These are figures which show the method of determining Vpp [A] using a Vpp-deltaIdc curve and two threshold values a and b. These are Vpp-Idc curves which show the relationship between the alternating current bias (Vpp) applied to the charging roller and the direct current component current (Idc). These are figures which show the conventional method of determining Vpp [A] using the Vpp-ΔIdc curve and one threshold value a.

Explanation of symbols

1 Photosensitive drum 2 Charging roller (charging device)
3 exposure unit 4 developing device 5 transfer roller 6 cleaning device 7 fixing unit 15 image forming unit 20 image input unit 40 AD conversion unit 41 control unit
42 Storage Unit 44 DC Component Current Sensor 45 Temperature / Humidity Sensor 100 Image Forming Apparatus

Claims (4)

A contact-type charging device that uniformly charges the surface of the photosensitive member by applying a charging bias in which a DC bias and an AC bias are superimposed;
An image forming apparatus comprising: a control unit that determines an optimum value Vpp [A] of an AC bias based on a DC component current when a charging bias is applied to the charging device;
The reference AC bias is Vpp [n] (n = 1, 2,...), And the DC component current when applying a predetermined DC bias and a charging bias superimposed with Vpp [n] is Idc [n]. The control means sets Vpp [n] to Vpp [A] when Vpp [n] exceeds the upper limit Vpp [MAX] of the AC bias,
When Vpp [n] is equal to or less than Vpp [MAX], the change amount ΔIdc of the DC component current when the reference AC bias is increased by ΔVpp from Vpp [n −1] to Vpp [n] ΔIdc is compared with threshold value a . If ΔIdc is equal to or less than threshold value a, Vpp [n−1] is set to Vpp [A],
When ΔIdc is greater than threshold a, ΔIdc is compared with threshold b (b> a), and when ΔIdc is less than or equal to threshold b, Vpp [n] −1 / 2ΔVpp is set to Vpp [A], and ΔIdc is An image forming apparatus, wherein if greater than the threshold value b, the reference AC bias is increased by ΔVpp to Vpp [n + 1], and the above procedure is repeated until Vpp [A] is set . The threshold a is set to a predetermined amount higher than the inflection point of the? Idc, the threshold value b, the difference b-a with the threshold value a is, per ΔVpp immediately before? Idc reaches the inflection point of the? Idc The image forming apparatus according to claim 1, wherein the image forming apparatus is set to be smaller than the change amount and larger than the change amount of ΔIdc per ½ΔVpp. The image forming apparatus according to claim 1, wherein the two threshold values a and b are set based on a temperature and / or humidity inside or outside the apparatus. 4. The image forming apparatus according to claim 1 , wherein the control unit outputs a constant multiple kVpp [A] (k ≧ 1) of Vpp [A] to the charging device. 5.

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