JP2011053346A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming apparatus capable of suppressing the occurrence of a discharge product, when the peak-to-peak voltage of an AC bias applied to an electrification device is set to a proper value. <P>SOLUTION: The frequency of the AC bias applied to an electrifying roller is set lower than that when an image is formed, when Vpp setting calibration is executed based on the result of measuring a DC component current Idc flowing between a photoreceptor and the electrifying roller when the AC bias is applied to the electrifying roller with a plurality of stages of Vpp. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to an electrophotographic image forming apparatus using a photoconductor, and more particularly to a method for determining an appropriate value of an AC bias peak-to-peak voltage 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 the charging device generates discharge products such as NH ₄ and NO ₃ generated by the reaction between ozone, NOx, and air generated by the discharge between the wire, the grid, and the shield. It adheres to the shield, grid, and photoreceptor surface. On the other hand, when a contact-type charging device is used, it can operate at a low voltage, and has an advantage that the amount of discharge is smaller than that of a corona discharge device, and ozone is not generated. However, since the charging device directly discharges to the photoconductor, the contact amount of the discharge product on the photoconductor is larger in the contact type.

  As a method of applying a voltage to the contact-type charging device, there are a DC charging method in which only a DC bias is applied to the charging member, and an AC charging method in which an AC bias is superimposed on the DC bias. 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 an AC bias peak-to-peak voltage value (hereinafter referred to as “a-Si”). Vpp), but becomes constant when Vpp exceeds a predetermined value. On the other hand, the relationship between the DC component current (Idc) flowing between the charging roller and the photosensitive member and Vpp is a curve having an inflection point where the slope becomes gentle as Vpp becomes a certain value or more as shown in FIG. Vpp-Idc curve), which substantially matches the relationship between the surface potential V0 and the AC bias Vpp.

  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, the discharge product absorbs moisture and the surface resistance is lowered, resulting in deterioration of character blur and half image reproducibility.

  By utilizing this relationship between Vpp and Idc, for example, as shown in Patent Documents 1 and 2, a Vpp-Idc curve is obtained by applying multiple stages of Vpp from a voltage lower than Vpp applied during image formation to a higher voltage. A control method is disclosed in which an inflection point is obtained and Vpp at the inflection point is set to an appropriate value. Thereby, the minimum necessary Vpp can be applied during image formation, and the generation of discharge products can be minimized.

JP 2006-171281 A JP 2007-199094 A

  However, when Vpp exceeding the inflection point of the Vpp-Idc curve is applied, the AC component current (Iac) flowing between the charging roller and the photosensitive member increases rapidly. This is because the discharge from the photoconductor to the charging roller starts, and at this time, the generation of discharge products also increases abruptly. The time required to adjust Vpp by changing Vpp step by step is a short time of about several hundreds msec. However, discharge products adhere to the surface of the photoreceptor and increase the friction coefficient of the surface. .

  Also, the surface of the photoconductor is always polished by a polishing system that presses a rubbing roller that rotates with a linear velocity difference against the photoconductor while supplying toner containing an abrasive, but the polishing force by the rubbing roller is The polishing power is set to be optimal for removing discharge products generated during image formation. For this reason, it is difficult to instantaneously remove excessive discharge products generated at the time of adjusting the Vpp, and half images and dot reproducibility are deteriorated in a high humidity environment, or from a photosensitive member to a recording medium or an intermediate transfer member. Transfer (primary transfer) efficiency, cleaning blade squealing (frictional noise), end roll-up, etc. may occur. On the other hand, when the polishing force by the rubbing roller is increased, even if the surface layer is a hard and hard-wearing a-Si photoreceptor, the surface layer is scraped to a certain extent and the useful life is shortened.

  In view of the above problems, an object of the present invention is to provide an image forming apparatus capable of suppressing the generation of discharge products when an AC bias applied to a charging device is set to an appropriate value.

  In order to achieve the above object, the present invention provides a photosensitive member carrying an electrostatic latent image and a charging bias that is arranged in contact with or in close proximity to the photosensitive member and that has a DC bias and an AC bias superimposed thereon. A charging means for uniformly charging the surface of the photosensitive member; a direct current measuring means for measuring a direct current component current flowing between the photosensitive member and the charging means when a charging bias is applied to the charging means; Control means for controlling the AC bias based on the measurement result of the current measurement means, and the measurement of the DC current measurement means when an AC bias is applied to the charging means at a plurality of peak-to-peak voltages during non-image formation. In an image forming apparatus that determines an appropriate value of an AC bias peak-to-peak voltage applied during image formation based on a result, the above-described value is determined when determining an appropriate value of an AC bias peak-to-peak voltage. The frequency of the AC bias applied at a peak voltage of the plurality of levels in conductive means, characterized by decreasing than the frequency of the AC bias applied to the charging unit during image formation.

  According to the present invention, in the image forming apparatus having the above-described configuration, at least one of the AC biases applied to the charging unit with a plurality of peak-to-peak voltages when determining an appropriate value of the peak-to-peak voltage is applied during image formation. It is characterized by being larger than the peak-to-peak voltage of the AC bias.

  According to the present invention, in the image forming apparatus configured as described above, the frequency of the AC bias applied to the charging unit when determining an appropriate value of the peak-to-peak voltage is the maximum peak-to-peak voltage among a plurality of stages of peak-to-peak voltages. The AC component current that flows between the photosensitive member and the charging unit when the voltage is applied is set to a frequency that is equivalent to the AC component current that flows during image formation.

  According to the present invention, in the image forming apparatus configured as described above, the photoconductor is an amorphous silicon photoconductor.

  According to the first configuration of the present invention, the appropriate value of the peak voltage of the AC bias to be applied at the time of image formation is determined based on the DC current when the AC bias is applied to the charging unit with a plurality of peak-to-peak voltages. In this case, by reducing the frequency of the AC bias applied to the charging means to be lower than the frequency of the AC bias applied during image formation, the generation of discharge products is effectively suppressed without affecting the adjustment of the appropriate value. can do.

  According to the second configuration of the present invention, in the image forming apparatus having the first configuration, at least the peak-to-peak voltage of the AC bias applied to the charging unit when determining an appropriate value of the peak-to-peak voltage. One is higher than the peak-to-peak voltage of the AC bias applied at the time of image formation, so that the position of the inflection point can be obtained with high accuracy regardless of the environmental conditions and the like, and the accuracy of determining an appropriate value can be increased. In addition, it is possible to suppress the generation of discharge products when an AC bias is applied with a peak-to-peak voltage higher than the inflection point where discharge products were easily generated in the past.

  According to the third configuration of the present invention, in the image forming apparatus having the first or second configuration, a plurality of AC bias frequencies to be applied to the charging unit when determining an appropriate value of the peak-to-peak voltage are set. By setting the frequency at which the AC component current flowing between the photosensitive member and the charging means when the maximum peak-to-peak voltage is applied is equal to the AC component current flowing during image formation. Further, it is possible to prevent the discharge product from adhering to the surface of the photoconductor without increasing the polishing performance of the surface of the photoconductor as compared with the image formation.

  According to the fourth configuration of the present invention, in the image forming apparatus having any one of the first to third configurations, the discharge product on the surface of the photoconductor is obtained by using the amorphous silicon photoconductor as the photoconductor. It is possible to reduce the image adhesion and suppress the occurrence of image flow. In addition, the life of the photoconductor is extended, contributing to higher image quality and lower running cost of the image forming apparatus.

Schematic showing the configuration of the image forming unit in the image forming apparatus of the present invention 1 is a block diagram showing a control path of an image forming apparatus of the present invention. A graph showing the relationship between the AC bias (Vpp) applied to the charging roller and the AC component current (Iac) when the frequency of the AC bias is 1000 Hz and 2000 Hz. A graph showing the relationship between the AC bias (Vpp) applied to the charging roller and the photoreceptor surface potential (V0) when the frequency of the AC bias is 1000 Hz and 2000 Hz. A graph showing the relationship between the AC bias (Vpp) applied to the charging roller and the DC component current (Idc) when the frequency of the AC bias is 1000 Hz and 2000 Hz. 6 is a flowchart showing a Vpp [A] determination control procedure in the image forming apparatus of the present invention. The figure explaining Vpp [A] calculated from the value of the linear function which approximates the calculation value in voltage range Z1 and Z2 geometrically Vpp-Idc curve showing the relationship between AC bias (Vpp) applied to the charging roller and DC component current (Idc)

  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 static elimination lamp along the rotation direction (arrow A direction) of the photosensitive drum 1. 7 is disposed. The photosensitive drum 1 is formed by laminating a photosensitive layer made of a-Si (a film thickness of 17 μm in this case) on an aluminum drum, for example, and is driven to rotate clockwise at a predetermined linear velocity (200 mm / second here). The

  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 to which an ionic conductive agent and carbon are added. 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 surface of the photosensitive drum 1 is uniformly charged by applying a high voltage to the charging roller 2. It has become. The core metal of the charging roller 2 is connected to a voltage supply circuit (not shown) that can supply a voltage necessary for discharging. 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.

The cleaning device 6 has a rubbing roller 9 and a cleaning blade 10. The rubbing roller 9 is pressed against the photosensitive drum 1 with a predetermined pressure, and is in the same direction with a predetermined peripheral speed ratio (here, 0.8 times the peripheral speed of the photosensitive drum) with respect to the photosensitive drum 1. Driven by rotation. Examples of the rubbing roller 9 include a structure in which a foam layer made of EPDM rubber and having an Asuka C hardness of 55 ° is formed as a roller body around a metal shaft. The material of the roller body is not limited to EPDM rubber, but may be rubber or foam rubber body of other materials, and those having an Asuka C hardness in the range of 10 to 90 ° are preferably used.

  A cleaning blade 10 is fixed in contact with the photosensitive drum 1 on the surface of the photosensitive drum 1 downstream of the contact surface with the rubbing roller 9 in the rotation direction. As the cleaning blade 10, for example, a polyurethane rubber blade having a JIS hardness of 78 ° is used, and is attached at a predetermined angle with respect to the tangential direction of the photosensitive member at the contact point. The material, hardness, dimensions, the amount of biting into the photosensitive drum 1 and the pressure contact force of the cleaning blade 10 are appropriately set according to the specifications of the photosensitive drum 1.

  Residual toner removed from the surface of the photosensitive drum 1 by the rubbing roller 9 and the cleaning blade 10 is discharged outside the cleaning device 6 (see FIG. 2) by a toner recovery device (not shown) such as a recovery screw. . Examples of the toner used in the present invention include those in which an abrasive selected from silica, titanium oxide, strontium titanate, alumina and the like is embedded in the toner particle surface and held so as to partially protrude on the surface. In which the toner is electrostatically attached (externally added) to the toner surface.

  Thus, by rotating the rubbing roller 9 with respect to the photosensitive drum 1 with a peripheral speed difference, the surface of the photosensitive drum 1 is polished by the residual toner containing the abrasive, and the rubbing roller 9 and the cleaning blade 10 are polished. To remove moisture and contaminants on the drum surface along with residual toner.

  The image forming process in the image forming unit 15 will be described. After the charge removal by the charge removal lamp 7, an electrostatic latent image corresponding to the image data is recorded on the photosensitive drum 1 uniformly charged by the charging roller 2 by the exposure unit 3. The electrostatic latent image is developed into a toner image by the developing device 4 by reversal development, 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 as residual toner from the surface of the photosensitive drum 1 by the rubbing roller 9 and the cleaning blade 10 of the cleaning device 6.

  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 control program (described later) for determining a reference value of the DC bias and AC bias applied to the charging roller 2 and an appropriate value of the AC bias is 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 an appropriate value of the AC bias applied to the charging roller 2 based on the detection result, and stores the appropriate 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 an appropriate value of the AC bias Vpp applied to the charging roller (hereinafter also referred to as Vpp setting calibration) will be described. As shown in FIG. 8, when the AC bias Vpp is gradually increased from a low level, Idc increases with a substantially constant slope, and thereafter transitions with a slope lower than the constant slope (or zero slope). When the surface potential V0 of the photosensitive drum and Idc are proportional to each other when Vpp is changed, and the point at which such inclination changes is an inflection point, the position of the inflection point depends on environmental conditions and the like. Although it fluctuates, it is known that the surface potential V0 is stable when Vpp is higher than the inflection point.

  Therefore, by changing the Vpp to a plurality of stages and applying an AC bias, and monitoring the Idc at each stage by the DC component current sensor 44, the Vpp is adjusted to an inflection point or a value slightly higher than the inflection point. The minimum Vpp necessary for charging to the surface potential V0 can be determined as the appropriate value Vpp [A].

  FIG. 3 is a graph showing the relationship between the AC bias Vpp and the AC component current (Iac) flowing between the photosensitive drum 1 and the charging roller 2 when the frequency of the AC bias applied to the charging roller is changed. Vpp-Iac curve). As is clear from FIG. 3, even when the AC bias Vpp to be applied is the same, when the frequency is 1000 Hz (indicated by ♦ in the figure), the AC bias frequency is 2000 Hz (indicated by □ in the figure). In comparison, Iac is about 40% lower.

  FIG. 4 is a graph (Vpp-V0 curve) showing the relationship between the AC bias Vpp and the surface potential (V0) of the photosensitive drum 1 when the frequency of the AC bias applied to the charging roller is changed. As is clear from FIG. 4, the Vpp-V0 curve with a frequency of 1000 Hz (indicated by ♦ in the figure) almost coincides with the Vpp-V0 curve with frequency of 2000 Hz (indicated by □ in the figure).

  FIG. 5 is a graph showing the relationship between the AC bias Vpp and the DC component current (Idc) flowing between the photosensitive drum 1 and the charging roller 2 when the frequency of the AC bias applied to the charging roller is changed. Vpp-Idc curve). As is apparent from FIG. 5, the Vpp-Idc curve with a frequency of 1000 Hz (indicated by ♦ in the figure) almost coincides with the Vpp-Idc curve with a frequency of 2000 Hz (indicated by □ in the figure). In FIGS. 4 and 5, only graphs with frequencies of 1000 Hz and 2000 Hz are displayed, but it has been confirmed that the same results are obtained from frequencies of 1000 Hz to 6000 Hz.

  That is, even if the frequency of the AC bias is changed, the relationship between Vpp and V0 and the relationship between Vpp and Idc do not change. Therefore, the Vpp setting calibration is performed based on the Idc measurement results when a plurality of stages of AC bias is applied. At the time of execution, by setting the frequency of the applied AC bias to be lower than that at the time of image formation, the generation of discharge products can be effectively suppressed without affecting the adjustment of the appropriate value Vpp [A]. .

  Note that Iac decreases as the frequency of the AC bias decreases, but when the frequency is decreased to about several hundred Hz, the configuration of the high-voltage substrate becomes difficult and the bias control becomes difficult. Further, if the Iac flowing at the time of performing the Vpp setting calibration is equal to or smaller than the Iac flowing at the time of image formation, it is possible to prevent the discharge product from adhering to the surface of the photosensitive drum 1 without increasing the polishing performance of the rubbing roller 9. it can. Therefore, it is sufficient to set the AC bias frequency so that the Iac flowing when the maximum Vpp in the Vpp setting calibration is applied is equivalent to the Iac flowing during image formation.

  For example, when the frequency of the AC bias applied at the time of image formation is 2000 Hz and the appropriate value of Vpp is about 1100 V, Iac is 2.5 mA from FIG. 3, so that the polishing ability of the rubbing roller 9 is Iac = 2. It is optimized to remove discharge products at 5 mA. Here, if the maximum Vpp applied during the Vpp setting calibration is 1600 V, Iac = 2.5 mA corresponds to Vpp 1600 V at a frequency of 1000 Hz.

  That is, if the frequency of the AC bias applied at the time of Vpp setting calibration is set to 1000 Hz, the Iac flowing at the time of Vpp setting calibration becomes equal to the Iac flowing at the time of image formation, and the amount of discharge product attached is also equal to that at the time of image formation. Become. Accordingly, the discharge product can be removed without changing the polishing performance of the rubbing roller 9, and excessive polishing of the surface of the photosensitive drum can be suppressed, so that the durability of the photosensitive layer is improved.

  FIG. 6 is a flowchart showing a procedure for executing calibration of the AC bias Vpp applied to the charging roller in the image forming apparatus of the present invention. A specific procedure for determining an appropriate value of the AC bias applied to the charging roller will be described along the steps of FIG. 6 with reference to FIGS.

  When the AC bias Vpp setting processing program is activated (step S1), first, the rotation of the photosensitive drum 1 is started, and the charge eliminating lamp 7 is set to the on state (step S2). Accordingly, the control unit 41 continuously rotates the photosensitive drum 1 while executing the Vpp setting calibration, and maintains the static elimination lamp 7 in the ON state. Then, the initial value Vdc [M] of the DC bias applied to the charging roller 2 is set to that value with reference to the value (400 [V] in the present embodiment) stored in the storage unit 42 (step S3). ). 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. .

  Next, the control unit 41 sets the voltage ranges Z1 and Z2 based on the predicted voltage value Vimg obtained from the detection value of the temperature / humidity sensor 45 (step S4). Each voltage range Z1 and Z2 is a voltage range belonging to the non-stable charging region and the stable charging region of the present invention, respectively. The first voltage range Z1 is a range where the charged state of the surface of the photosensitive drum 1 is not uniform, that is, The second voltage range Z2 is a range in which the charged state of the surface of the photosensitive drum 1 is uniform, that is, a range of Vpp higher than Vpp [A]. It is. By setting the first voltage range Z1 and the second voltage range Z2 in this way, an appropriate measurement range is set in calculating the linear functions f1 (Vpp) and f2 (Vpp) described below for each region. Is done.

  Specifically, the control unit 41 obtains temperature information when performing the Vpp setting calibration based on the detection value of the temperature / humidity sensor 45, searches for a temperature category to which this temperature information corresponds, and corresponds to each category. By referring to the predicted voltage value Vimg stored in advance in the storage unit 42, the voltage ranges Z1 and Z2 are set based on the predicted voltage value Vimg. In the present embodiment, the first voltage range Z1 is in the range of (Vimg−500) ± 200, and the second voltage range Z2 is in the range of (Vimg + 300) ± 200. The correspondence between the temperature classification stored in the storage unit 42 and the predicted voltage value Vimg also stored in the storage unit 42, and the correspondence between the voltage ranges Z1 and Z2 calculated based on the predicted voltage value Vimg. It is shown in 1.

  Next, the control unit 41 sets the frequency of the AC bias to be lower (here, 1000 Hz) than during image formation (step S5). Then, in each voltage range of Z1 and Z2, Vpp is changed to a plurality of different values stepwise (in this case, 200V each), and the DC current component Idc at this time is measured by the DC component current sensor 44 and stored in the RAM 51. (Steps S6 and S7). Since each voltage range Z1 and Z2 has a width of 400 [V], in the present embodiment, the control unit 41 acquires three measurement values for each voltage range. The time for applying the AC bias at each Vpp is set to 0.5 [s] in consideration of the time required for the DC current generated after the AC bias is applied to be stabilized.

  Next, the control part 41 calculates | requires the relationship between Vpp and Idc based on the measured value of every 3 in each voltage range Z1 and Z2 acquired by step S6 and step S7 (step S8). Since the change characteristic of Idc with respect to Vpp is a linear change characteristic in each region of the first voltage range Z1 and the second voltage range Z2 (see FIG. 7), as a function indicating the relationship between Vpp and Idc in each region, A linear function having a linear relationship may be used. Therefore, in step S8, the linear functions f1 (Vpp) and f2 (Vpp) are calculated as functions indicating the relationship between Vpp and Idc in the voltage ranges Z1 and Z2. Although a detailed description is omitted, in the present embodiment, the least square method is used as a method of calculating an approximate linear function from each measurement value of the DC component current sensor 44.

  Next, Vpp is obtained such that the values of Idc given by f1 (Vpp) and f2 (Vpp) obtained in step S8 are equal. Specifically, the solution of the linear equation f1 (Vpp) = f2 (Vpp) for Vpp is obtained. Then, Vpp calculated in step S9 is set to an appropriate value Vpp [A] (step S9), and a value obtained by multiplying Vpp [A] set in step S9 by 1.1 is output to the charging roller 2 (step S9). S10).

  As described above, when determining the appropriate value of Vpp, the frequency of the applied AC bias is set lower than that at the time of image formation, so that the relationship between Vpp and Idc is maintained (see FIG. 5) and the photosensitive drum. AC component current Iac flowing between 1 and charging roller 2 can be kept low (see FIG. 3). Therefore, the image flow in a high humidity environment due to adhesion of discharge products to the drum surface without affecting the setting accuracy of the appropriate value Vpp [A] that can make the surface potential of the photosensitive drum 1 uniform. In addition, it is possible to effectively suppress problems such as a decrease in transfer (primary transfer) efficiency due to an increase in frictional resistance on the surface of the photosensitive member, a squeal (frictional sound) of the cleaning blade, and an end roll-up.

  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.

  In the control of FIG. 6, f1 (Vpp) and f2 (Vpp) in the first voltage range Z1 and the second voltage range Z2 are calculated, and a point where f1 (Vpp) = f2 (Vpp) is obtained is a Vpp-Idc curve. The method described in Patent Document 1 in which Vpp [A] is set as Vpp [A] is illustrated as an example of the inflection point of V. However, the method for setting Vpp [A] is not limited to this.

  For example, as described in Patent Document 2, a straight line passing through two points indicating Idc measured when two different Vpps are applied on the low voltage side (first voltage range Z1) from the inflection point, that is, temporarily. Approximate the intersection of the function f1 (Vpp) and a straight line passing through one point indicating Idc measured when one Vpp is applied on the high voltage side (second voltage range Z2) from the inflection point. You may obtain | require as an inflection point and set Vpp corresponding to an approximate inflection point to appropriate value Vpp [A].

  The Vpp setting calibration may be executed at the start of continuous printing or every predetermined number of prints, for example, 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 change in the machine due to heat radiation from the fixing unit 8, and a high-quality image without image unevenness can be formed, and the occurrence of problems such as blade squealing can be prevented. It becomes possible.

  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.

  Further, the present invention is not limited to the image forming apparatus for forming a monochrome image as shown in FIG. 1, but a tandem type color image forming apparatus or a contact type charging device such as a facsimile or a printer is used. The present invention can also be applied in exactly the same manner.

  INDUSTRIAL APPLICABILITY The present invention can be used in an image forming apparatus provided with a charging unit that is disposed in contact with or close to a photosensitive member and uniformly charges the surface of the photosensitive member by applying a charging bias in which a DC bias and an AC bias are superimposed. When an appropriate value of the AC bias peak-to-peak voltage applied at the time of image formation is determined based on a DC current when an AC bias is applied to the charging unit with a plurality of peak-to-peak voltages, it is applied to the charging unit. The frequency of the AC bias is made lower than the frequency of the AC bias applied during image formation.

  As a result, the generation of discharge products can be effectively suppressed without affecting the adjustment of the appropriate value of the peak-to-peak voltage, so that the image flow caused by the discharge products generated during the adjustment of the peak-to-peak voltage It is possible to provide an image forming apparatus that does not suffer from problems such as a decrease in image reproducibility, a decrease in transfer efficiency from a photosensitive member to a recording medium or an intermediate transfer member, a squeal of a cleaning blade, and an end roll.

1 Photosensitive drum 2 Charging roller (charging means)
DESCRIPTION OF SYMBOLS 3 Exposure unit 4 Developing apparatus 5 Transfer roller 6 Cleaning apparatus 7 Static elimination lamp 8 Fixing part 9 Rub roller 15 Image forming part 20 Image input part 40 AD conversion part 41 Control part (control means)
42 storage unit 44 DC component current sensor (DC current measuring means)
45 Temperature / Humidity Sensor 100 Image Forming Device

Claims (4)

A photoreceptor carrying an electrostatic latent image;
A charging unit that is disposed in contact with or in proximity to the photoconductor, and uniformly charges the surface of the photoconductor by applying a charging bias in which a DC bias and an AC bias are superimposed;
DC current measuring means for measuring a DC component current flowing between the photosensitive member and the charging means when a charging bias is applied to the charging means;
Control means for controlling the AC bias based on the measurement result of the DC current measuring means;
An appropriate value of the peak voltage of the AC bias applied during image formation based on the measurement result of the DC current measuring unit when an AC bias is applied to the charging unit at a plurality of peak-to-peak voltages during non-image formation. In the image forming apparatus for determining
When determining an appropriate value of the AC bias peak-to-peak voltage, the frequency of the AC bias applied to the charging unit with a plurality of levels of peak-to-peak voltages is reduced below the frequency of the AC bias applied to the charging unit during image formation. An image forming apparatus.   At least one of the AC biases to be applied to the charging means with a plurality of peak-to-peak voltages when determining an appropriate value of the peak-to-peak voltage is greater than the peak-to-peak voltage of the AC bias applied during image formation. The image forming apparatus according to claim 1.   The frequency of the AC bias applied to the charging means when determining an appropriate value of the peak-to-peak voltage is such that the photoconductor, the charging means, and the like when a maximum peak-to-peak voltage is applied among a plurality of peak-to-peak voltages. The image forming apparatus according to claim 1, wherein an alternating current component current flowing between the first and second currents is set to a frequency that is equivalent to an alternating current component current flowing during image formation.   4. The image forming apparatus according to claim 1, wherein the photoconductor is an amorphous silicon photoconductor.

Images