JP2018132653A - Image formation device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image formation device capable of making a surface of an intermediate transfer body at electric potential optimal to primary transfer while keeping it small.SOLUTION: An image formation device includes a voltage adjusting unit 30 capable of changing the magnitude of transfer potential according to the magnitude of a control signal input from a control unit 100. The control unit 100 controls the voltage adjusting unit 30 so that an amount of control current flowing through a voltage adjusting member 307 is a known amount of current, obtains a first amount of current on the basis of the known amount of current and an amount of current detected by a current detection unit, and controls the magnitude of the control signal on the basis of the first amount of current and a voltage value detected by a voltage detection unit 50.SELECTED DRAWING: Figure 3

Description

  The present invention relates to an image forming apparatus using an electrophotographic system.

  Conventionally, in an image forming apparatus such as a copying machine or a laser beam printer, an image forming apparatus having an endless belt as an intermediate transfer member is known. In this image forming apparatus, as a primary transfer process, a toner image formed on the surface of a photosensitive drum as an image carrier is applied with a voltage from a voltage power source to a primary transfer member disposed on the photosensitive drum facing portion. Transfer on the belt. Thereafter, the primary transfer process is repeatedly performed for a plurality of color toner images to form a plurality of color toner images on the belt surface. Subsequently, as a secondary transfer process, the toner images of a plurality of colors formed on the belt surface are collectively transferred onto the surface of a recording material such as paper by applying a voltage to the secondary transfer member. The batch-transferred toner image is then permanently fixed on a recording material by a fixing unit, thereby forming a color image.

  Patent Document 1 discloses a configuration in which the image forming apparatus can be reduced in size and cost by not having an individual power source for primary transfer, and the potential of the belt surface can be changed. In this configuration, a circuit comprising a plurality of zener diodes with different set voltages is provided between the belt and ground, and the potential on the belt surface is changed by switching the number of zener diodes to be operated according to the use environment. The primary transfer efficiency is stabilized.

JP 2013-213990 A

  In general, the primary transfer portion has a configuration in which a photosensitive drum, an intermediate transfer member (belt), a primary transfer member, and a plurality of members are interposed. Since the resistance value of these primary transfer loads varies depending on the surrounding environment such as temperature and humidity in the atmosphere and the durability history, the optimal primary transfer voltage varies. That is, while the resistance value of the primary transfer load varies, it is necessary to change the primary transfer voltage in order to secure a necessary primary transfer current. In the conventional control method, the resistance value of the primary transfer load is predicted from the atmospheric environment and the durable number, the primary transfer voltage is determined according to the predicted resistance value of the primary transfer load, and is determined during image formation. A primary transfer voltage is applied. However, how the load resistance value fluctuates differs depending on the factor of the variation, and even if the variation is predicted for each factor, the actual variation varies due to the overlap of various factors. It may be difficult to cope with such variation in load resistance value by predicting variation of individual factors.

  SUMMARY OF THE INVENTION An object of the present invention is to provide an image forming apparatus capable of setting the surface of an intermediate transfer member to an optimum potential for primary transfer while maintaining miniaturization.

In order to achieve the above object, an image forming apparatus of the present invention includes:
An image carrier for carrying a developer image;
An endless belt that rotates while in contact with the image carrier;
A current supply member that contacts the belt at a position different from the image carrier in the rotation direction of the belt and supplies current to the belt;
A control unit for outputting a control signal having a variable size;
A contact member that contacts the belt at a position facing the current supply member via the belt;
A current detector for detecting the amount of current flowing through the current supply member;
A voltage detection unit that detects a transfer voltage that is a surface potential at a contact portion of the belt with the image carrier for transferring a developer image carried by the image carrier to the belt;
A voltage adjustment unit including a voltage adjustment member connected to the contact member, and a voltage capable of changing the magnitude of the transfer voltage according to the magnitude of the control signal input from the control unit An adjustment unit;
With
The controller is
The voltage adjustment unit is controlled so that a current amount of a control current flowing through the voltage adjustment member becomes a known current amount, and based on the known current amount and a current amount detected by the current detection unit, Get the current amount of 1
The magnitude of the control signal is controlled based on the first current amount and a voltage value detected by the voltage detector.

  According to the present invention, the surface of the intermediate transfer member can be set to an optimum potential for primary transfer while maintaining miniaturization.

Illustration of the image forming apparatus of the embodiment Block diagram illustrating a controller for image formation according to an embodiment 1 is a configuration diagram of a transfer unit of an image forming apparatus according to a first exemplary embodiment. Explanatory drawing of the modification of the circuit of the primary transfer part in Example 1 The figure which shows the transfer efficiency of the primary transfer part in Example 1 Relationship between the resistance value of the primary transfer load and the optimum primary transfer voltage in Example 1 Configuration diagram of transfer section of image forming apparatus of embodiment 2 Configuration diagram of transfer unit of image forming apparatus of comparative example

  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.

[Example]
FIG. 1 is a schematic diagram of an image forming apparatus according to an embodiment of the present invention. The configuration and operation of the image forming apparatus of the present embodiment will be described with reference to FIG. Examples of the image forming apparatus to which the present invention can be applied include a copying machine and a printer using an electrophotographic system. Here, a case where the present invention is applied to a color laser printer will be described. The image forming apparatus according to the present exemplary embodiment is a so-called tandem type printer having a plurality of image forming stations 70a to 70d. The first image forming station 70a is yellow (Y), the second image forming station 70b is magenta (M), the third image forming station 70c is cyan (C), and the fourth image forming station 70d is black (Bk). ) For each color. The configuration of each image forming station is the same except for the color of the toner to be accommodated, and will be described below using the first image forming station 70a.

The first image forming station 70a includes a drum-shaped electrophotographic photosensitive member (hereinafter referred to as a photosensitive drum) 700a as an image carrier, a charging roller 2a as a charging member, a developing device 4a, and a cleaning device 5a. Prepare. The photosensitive drum 700a is an image carrier that is driven to rotate at a predetermined peripheral speed (process speed) in the direction of an arrow and carries a toner image (developer image). Further, the developing device 4a is a device that contains yellow toner as a developer and develops the electrostatic latent image formed on the photosensitive drum 700a using the yellow toner. The cleaning device 5a is a member for collecting the toner attached to the photosensitive drum 700a. In this embodiment, the cleaning device 5a includes a cleaning blade that is a cleaning member that contacts the photosensitive drum 700a, and a waste toner box that stores toner collected by the cleaning blade.

  When the image forming operation is started by the image signal, the photosensitive drum 700a is rotationally driven. In the rotating process, the photosensitive drum 700a is uniformly charged to a predetermined potential with a predetermined polarity (negative polarity in this embodiment) by the charging roller 2a, and is subjected to exposure according to the image signal by the exposure means 3a. Thereby, an electrostatic latent image corresponding to the yellow component image of the target color image is formed. Next, the electrostatic latent image is developed at the developing position by the developing device (yellow developing device) 4a and visualized as a yellow toner image. Here, the normal charging polarity of the toner contained in the developing device is negative.

  The intermediate transfer belt 600 is an endless belt body and is stretched by stretching rollers 603a, 603b, and 603c as support members. Then, it is rotationally driven at substantially the same peripheral speed while contacting the photosensitive drums 700a to 700d in such a direction as to move in the same direction as the photosensitive drums 700a to 700d at the facing portion in contact with the photosensitive drums 700a to 700d. The yellow toner image formed on the photosensitive drum 700a is transferred onto the intermediate transfer belt 600 in the process of passing through a contact portion (hereinafter referred to as a primary transfer nip) between the photosensitive drum 700a and the intermediate transfer belt 600. (Primary transfer). The primary transfer residual toner remaining on the surface of the photosensitive drum 700a is cleaned and removed by the cleaning device 5a, and then subjected to an image forming process below charging. Similarly, the second, third, and fourth image forming stations 70b, 70c, and 70d form a second-color magenta toner image, a third-color cyan toner image, and a fourth-color black toner image. The images are sequentially transferred onto the transfer belt 600. Thereby, a composite color image corresponding to the target color image is obtained.

The four color toner images on the intermediate transfer belt 600 pass through the secondary transfer nip formed by the intermediate transfer belt 600 and the secondary transfer roller 601 and the surface of the recording material P fed by the paper feeding unit 15. Are collectively transferred (secondary transfer). The secondary transfer roller 601 as a secondary transfer member was covered with a foamed sponge body mainly composed of NBR and epichlorohydrin rubber adjusted to a volume resistance of 10 ⁸ Ω · cm and a thickness of 5 mm on a nickel-plated steel rod having an outer diameter of 8 mm. An outer diameter of 18 mm is used. The secondary transfer roller 601 is in contact with the intermediate transfer belt 600 with a pressure of 50 N to form a secondary transfer portion (hereinafter referred to as a secondary transfer nip). The secondary transfer roller 601 is driven to rotate with respect to the intermediate transfer belt 600, and when the toner on the intermediate transfer belt 600 is secondarily transferred to a recording material P such as paper, a voltage of 1800 to 2300V is applied. Yes. Thereafter, the recording material P carrying the four color toner images is introduced into the fixing device 13 where the four color toners are melted and mixed and fixed to the recording material P by being heated and pressurized. The toner (residual toner) remaining on the intermediate transfer belt 600 after the secondary transfer is charged and cleaned by the cleaning brush 602 and reversely transferred to the photosensitive drum 700. With the above operation, a full-color print image is formed.

With reference to FIG. 2, the configuration of the controller 100 that controls the image forming apparatus main body according to the present exemplary embodiment will be described. As shown in FIG. 2, the controller 100 has a CPU circuit unit 150 as a control unit. The CPU circuit unit 150 includes a ROM 151 and a RAM 152. The CPU circuit unit 150 is configured according to a control program stored in the ROM 151.
The exposure control unit 101, the charging control unit 102, the development control unit 103, the primary transfer control unit 104, and the secondary transfer control unit 105 are comprehensively controlled. Also, the environment table and various tables for transfer control are stored in the ROM 151, and the CPU calls and reflects them based on the information of the environment sensor 106 as a detecting means for detecting temperature and humidity in the apparatus installation environment. The RAM 152 temporarily holds control data and is used as a work area for arithmetic processing associated with control. The secondary transfer control unit 105 controls the secondary transfer power supply 200 and controls the voltage output from the secondary transfer power supply 200 based on a current value detected by a current detection circuit (not shown). The primary transfer control unit controls the potential of the primary transfer unit to be constant by sending a signal to a voltage adjustment circuit 30 serving as a voltage adjustment unit. The controller 100, the secondary transfer power source 200, the voltage adjustment circuit 30, and the environment sensor 106 constitute a printer engine 99 of the image forming apparatus according to this embodiment. When the controller 100 transmits image information and a print command from the host computer 97, the controller 100 receives each image signal converted by the video controller 98. After that, the controller 100 controls each control unit (exposure control unit 101, charging control unit 102, development control unit 103) and executes an image forming operation necessary for the printing operation.

  Hereinafter, the configuration of the primary transfer portion, which is a feature of this embodiment, will be described. In this embodiment, primary transfer is performed by passing a current in the circumferential direction of the intermediate transfer belt 600. That is, the primary transfer current flows at a position different from the primary transfer nip with the photosensitive drums 700a, 700b, 700c, and 700d in the circumferential direction (rotation direction) of the intermediate transfer belt 600. The intermediate transfer belt 600 and the photosensitive drums 700a to 700d form a contact portion (primary transfer nip) by stretching the intermediate transfer belt 600 by rollers 603c and 603a. The intermediate transfer belt 600 and the photosensitive drums 700a to 700d are connected to a voltage adjustment circuit 30 including a transistor as a voltage adjustment member connected to a secondary transfer counter roller 603a as a current supply member. An intermediate transfer belt 600 is disposed as an intermediate transfer member at a position facing each of the image forming stations 70a to 70d. The intermediate transfer belt 600 is an endless belt obtained by adding a conductive agent to a resin material to impart conductivity. The intermediate transfer belt 600 is stretched around three axes of a driving roller 603c, a tension roller 603b, and a secondary transfer counter roller 603a, and is stretched by a tension roller 603b with a total pressure of 60N. The intermediate transfer belt 600 is rotationally driven at substantially the same peripheral speed as that of the photosensitive drums 700a to 700d in a direction in which the intermediate transfer belt 600 moves in the same direction at the facing portion in contact with the photosensitive drums 700a to 700d.

The secondary transfer counter roller 603a as a contact member is connected to a voltage adjustment circuit 30 including a transistor as voltage adjustment means (voltage adjustment unit). The intermediate transfer belt 600 used in this embodiment has a peripheral length of 700 mm and a thickness of 90 μm, and uses an endless polyethylene terephthalate (PET) resin formed by mixing an ionic conductive agent as a conductive agent. As electrical characteristics, it exhibits ionic conductivity characteristics, and the electrical conductivity is obtained by the propagation of ions between polymer chains, so the resistance value varies with the temperature and humidity in the atmosphere, but the resistance value The feature is that unevenness in the circumferential direction is good. In this embodiment, since current is transferred in the moving direction of the intermediate transfer belt 600, the voltage drop increases when the resistance of the intermediate transfer belt 600 is high. As a result, it is desirable to have a low resistance layer because the primary transferability may be impaired. In this embodiment, in order to suppress the voltage drop in the intermediate transfer belt 600, the resistance of the base layer is 1 × 10 ⁸ Ω · cm or less in volume resistivity. The volume resistivity is measured by using a ring probe type UR (model MCP-HTP12) on a Hiresta-UP (MCP-HT450) manufactured by Mitsubishi Chemical Corporation. The room temperature at the time of measurement was set to 23 ° C., the room humidity was set to 50%, and the applied voltage was 100 V and the measurement time was 10 sec. In this embodiment, the intermediate transfer belt 600 has a two-layer structure, and a high resistance layer is arranged on the surface, thereby suppressing current to the non-image area and further improving transferability. However, the present invention is not limited to this configuration, and a single layer configuration can be used, and a configuration with three or more layers is also possible.

  In this embodiment, polyethylene terephthalate resin is used as the material of the intermediate transfer belt 600, but the present invention is not limited to this. Examples of other materials include polyester, polycarbonate, polyarylate, acrylonitrile-butadiene-styrene copolymer (ABS), and the like. Other examples include polyphenylene sulfide (PPS), polyvinylidene fluoride (PVdF), polyethylene naphthalate (PEN), and the like. These materials and mixed resins thereof may be used as the material of the belt 600.

  In this embodiment, a voltage adjustment circuit 30 having a transistor is connected as a voltage adjustment unit between the secondary transfer counter roller 603a and the ground. The voltage adjustment circuit 30 adjusts the voltage applied to the intermediate transfer belt 600 from the secondary transfer power source 200 via the secondary transfer roller 601, so that the toner on each of the photosensitive drums 700 a to 700 d is transferred onto the intermediate transfer belt 600. A primary transfer voltage for performing the primary transfer to be moved is generated. By applying the primary transfer voltage adjusted to a desired magnitude by the voltage adjustment circuit 30, the surface potential of the intermediate transfer belt 600 becomes a desired primary transfer potential, and the potential difference (from the surface potential of each of the photosensitive drums 700a to 700d ( The primary transfer is performed according to (transfer contrast). Details of voltage adjustment by the voltage adjustment circuit 30 will be described below with reference to FIGS.

(Comparative example)
First, a comparative example of this embodiment will be described with reference to FIG. FIG. 8 schematically shows a transfer unit of an image forming apparatus configured to generate a primary transfer current Itr1 by supplying a current from an opposing member of the secondary transfer opposing roller 603a. The primary transfer current Itr1 is supplied from the secondary transfer roller 601 and the cleaning brush 602. The primary transfer voltage Vtr1 is generated based on the secondary transfer voltage source 200 and the cleaning voltage source 201, and can be controlled at a constant voltage within a certain voltage range. The primary transfer voltage Vtr1 generated here forms the surface potential of the intermediate transfer belt 600. Then, due to the potential difference (transfer contrast) between the surface potential of the intermediate transfer belt 600 and the photosensitive drums 700a to 700d, the toner on the photosensitive drums 700a to 700d moves onto the intermediate transfer belt 600 to perform primary transfer. Yes.

  A voltage generation signal is output from the CPU 100 as the control unit to the secondary transfer voltage source 200 and the cleaning voltage source 201. Based on this signal, the secondary transfer voltage source 200 applies a positive DC voltage to the secondary transfer roller 601, and the cleaning voltage source 201 applies a positive DC voltage to the cleaning brush 602. The secondary transfer current Itr2 flowing through the secondary transfer roller 601 and the cleaning current Iicl flowing through the cleaning brush 602 merge via the intermediate transfer belt 600 and the secondary transfer counter roller 603a. Then, it branches into a primary transfer current Itr1 necessary for primary transfer and a control current Icon that flows in the current control circuit 30. The primary transfer current Itr1 flows to the ground via the resistors 702a, 702b, 702c, 702d, the primary transfer brushes 701a, 701b, 701c, 701d, the intermediate transfer belt 600, and the photosensitive drums 700a, 700b, 700c, 700d.

The image forming apparatus of FIG. 8 is a so-called tandem type image forming apparatus provided with four image forming stations 70a, 70b, 70c, and 70d. The first image forming station 70a is yellow (Y), the second image forming station 70b is magenta (M), the third image forming station 70c is cyan (C), and the fourth image forming station 70d is black (Bk). ) For each color. Resistors 702a, 702b, 702c, and 702d are arranged to reduce variations in primary transfer current between image forming stations. On the other hand, the control current Icon that flows in the current control circuit 30 flows into the ground via the transistor 307. Note that the current flowing into the ground via the resistors 500 and 501 is negligible and is ignored. The primary transfer voltage Vtr1 is controlled by adjusting the collector current of the transistor 307 so that the control voltage V− input to the inverting input terminal of the operational amplifier 302 is equal to the monitor voltage V + input to the non-inverting input terminal of the operational amplifier 302. This is done by being controlled. The relationship between the primary transfer voltage Vtr1 and the control voltage V− is as follows.
Vtr1 = (V−) × ((R500 + R501) / R501) (1)

  Note that R500 and R501 are resistance values of the resistors 500 and 501, respectively, and are ignored because the current flowing through the input terminal of the operational amplifier 302 is very small. The monitor voltage V + is a DC voltage obtained by dividing the primary transfer voltage Vtr1 by the resistors 500 and 501. On the other hand, the control voltage V− is a DC voltage obtained by smoothing a PWM signal as a current adjustment signal (a control signal having a variable magnitude) output from the CPU 100 by the resistor 300 and the capacitor 301. The control voltage V− changes depending on the on-duty (duty ratio) of the PWM signal. As the on-duty is increased, the control voltage V− increases, and the primary transfer voltage Vtr1 increases from the equation (1). The resistor 304 and the capacitor 303 are connected as elements that determine the response of the operational amplifier 302. The output voltage of the operational amplifier 302 is divided by resistors 305 and 306 and input to the base terminal of the transistor 307. Thereby, the collector current of the transistor 307 is controlled. The primary transfer voltage Vtr1 is generated as a collector-emitter voltage of the transistor 307.

For the intermediate transfer belt used in this configuration, an endless polyethylene terephthalate (PET) resin mixed with an ionic conductive agent as a conductive agent is used. As electrical characteristics, it exhibits ionic conductivity, and the electrical conductivity is obtained by the propagation of ions between polymer chains. Therefore, the resistance value varies with the temperature and humidity in the atmosphere. In addition, the resistance value of the primary transfer brush used in this configuration increases due to deterioration of energization due to durability of the image forming apparatus. As described above, the resistance value of the primary transfer load varies, but the primary transfer voltage needs to be changed in order to secure a necessary primary transfer current.
In the configuration of the above comparative example, when the resistance value of the primary transfer load is predicted from the atmospheric environment and the durable number, and the primary transfer voltage is determined according to the predicted resistance value of the primary transfer load, the variation in the load resistance value causes In some cases, the determined applied voltage may not be the optimum voltage.

Example 1
FIG. 3 is a diagram illustrating the transfer unit of the image forming apparatus according to the first embodiment of the invention. The explanation about the same function as the comparative example is omitted and the same symbol is given. The main difference between the present embodiment and the comparative example is that, from the current detection result of detecting the amount of current flowing through the upstream load and the voltage detection result of detecting the voltage value (first voltage value) applied to the downstream load, That is, the resistance value of the primary transfer load is calculated and acquired.

  In this embodiment, the upstream load is the resistance component from the secondary transfer roller 601 through the intermediate transfer belt 600 to the secondary transfer counter roller 603a, and the secondary transfer counter from the cleaning brush 602 through the intermediate transfer belt 600. This is a resistance component up to the roller 603a. The downstream load is a resistance component from the secondary transfer counter roller 603a to the ground via the primary transfer brushes 701a, 701b, 701c, and 701d. Similarly to the comparative example, the current control circuit 30 is connected in parallel to the downstream load 70, and the primary transfer voltage Vtr1 is controlled by controlling the control current Icon that flows through the current control circuit 30 itself.

The image forming apparatus of FIG. 3 is designed to control the primary transfer voltage Vtr1 in the range of 0V to 600V. As the breakdown voltage of the collector-emitter voltage of the transistor 307, it is necessary to select a breakdown voltage of 600 V or more. In this embodiment, a breakdown voltage of 800 V is used. Further, the primary transfer voltage Vtr1 is divided by the resistors 500 and 501 of the primary transfer voltage detection circuit 50 as a voltage detection unit, and the monitor voltage V + is applied to the non-inverting input terminal of the operational amplifier 302, and the AD port of the CPU 100 Is input. Primary transfer voltage Vtr1 is 0V to 6
When changing in the range of 00V, the voltage value divided by the resistors 500 and 501 is designed to change in the range of 0V to 3.0V. On the other hand, the control voltage V− input to the inverting input terminal of the operational amplifier 302 changes depending on the on-duty of the PWM signal as a current adjustment signal output from the CPU 100. The control voltage V− is set to 0V by setting the on-duty of the PWM signal to 0% and 3.3V by setting it to 100%, and is designed to be a voltage range that can cover the entire voltage range of the monitor voltage V +.

The image forming apparatus includes a secondary transfer current detection circuit 400 that detects a secondary transfer current Itr2 that flows through the secondary transfer roller 601 and a cleaning current detection circuit 401 that detects a cleaning current Iicl that flows through the cleaning brush 602 as current detection units. It has. The current detection result detected by each current detection circuit is output to the CPU 100. In general, the secondary transfer unit and the cleaning unit of the image forming apparatus are often provided with a current detection circuit, which can be used for the control of this embodiment. Here, the relationship between the secondary transfer current Itr2, the cleaning current Iicl, and the primary transfer current Itr1 is expressed by the following equation.
Itr2 + Iicl = Itr1 + Icon (2)

Icon is a control current flowing through the current control circuit 30. When calculating the resistance value of the primary transfer load, a state is created in which the control current Icon that flows through the current control circuit 30 is known. In the present embodiment, a condition for turning off the transistor 307 is created, and the control current Icon = 0 (zero), so that this control current creates a known state. Specifically, the on-duty of the PWM signal output from the CPU 100 is set to 100%, and the control voltage V− is set to 3.3V. At this time, the secondary transfer voltage Vtr2 or the cleaning voltage Vicl is applied so that the primary transfer voltage Vtr1 does not exceed 600V regardless of the resistance values of the upstream load 60 and the downstream load 70. For example, if the secondary transfer voltage Vtr2 is 600V and the cleaning voltage Vicl is 0V, the primary transfer voltage Vtr1 does not exceed 600V. At this time, the monitor voltage V + is 3.0 V or less, the control voltage V− is 3.3 V, the output of the operational amplifier 302 is stuck to the lower limit, and the transistor 307 is surely turned off. However, at this time, it is necessary to design the monitor voltage and the control voltage so as to satisfy the differential input voltage range of the operational amplifier. At this time, from the equation (2), the following equation is obtained.
Itr1 = Itr2 + Iicl (3)

Since the secondary transfer current Itr2 and the cleaning current Iicl are detected by the current detection circuit, the primary transfer current Itr1 can be calculated and acquired. Since the primary transfer voltage Vtr1 at this time is detected by the voltage detection circuit 50, the resistance value Rtr1 of the primary transfer load can be calculated and obtained from the following equation.
Rtr1 = Vtr1 / (Itr2 + Iicl) (4)

FIG. 4 is a schematic diagram showing a modification of the circuit of the primary transfer unit of the present embodiment. In the above configuration of the present embodiment, the primary transfer voltage Vtr1 can be controlled in the range of 0V to 600V. In the case where the primary transfer voltage Vtr1 can be controlled in the range of 400V to 1000V using the same collector-emitter voltage breakdown component as the transistor 307 used in this embodiment, the current control circuit is as shown in FIG. May be configured. A Zener diode 800 corresponding to a Zener voltage of 400 V as a voltage step-down element (also referred to as a voltage stabilizing element or a voltage maintaining element) is connected between the collector terminal of the transistor 307 and the upstream load 60 so as to be in series with the voltage adjustment circuit 30. Arranged in between. A primary transfer voltage Vtr1 is generated as the sum of the collector-emitter voltage of the transistor 307 and the Zener voltage of the Zener diode 800. A resistor 308 is disposed in order to allow a Zener current to sufficiently flow through the Zener diode 800 and to stably generate a Zener voltage for 400V. In order to create a state in which the control current Icon is known using the current control circuit as shown in FIG. 4, a condition for turning off the transistor 307 may be set similarly. At this time, the control current Icon can be calculated by the following equation and is in a known state.
Icon = (Vtr1−Vzd) / R308 (5)

Vzd is the Zener voltage of the Zener diode 800, and R308 is the resistance value of the resistor 308. At this time, from the expression (2), the primary transfer current Itr1 is expressed by the following expression.
Itr1 = Itr2 + Iicl − ((Vtr1−Vzd) / R308) (6)
Further, the resistance value Rtr1 of the primary transfer load can be calculated from the following equation.
Rtr1 = Vtr1 / (Itr2 + Icl − ((Vtr1−Vzd) / R308))
(7)

  The CPU 100 has a table of optimal primary transfer voltage Vtr1 corresponding to the calculated primary transfer load resistance value Rtr1, and optimal primary transfer corresponding to the primary transfer load resistance value Rtr1 calculated at the time of image formation. Constant voltage control of the voltage Vtr1 is performed.

FIG. 5 shows the transfer efficiency in the primary transfer portion. The transfer efficiency value on the vertical axis indicates the primary transfer residual density. The larger the value, the higher the primary transfer residual density, and the transfer efficiency deteriorates. In other words, the optimum primary transfer voltage is the ISO status I on the vertical axis.
It is given by the primary transfer voltage when the value of density is the smallest. The measurement conditions in FIG. 5 are the results in a so-called L / L environment where the photosensitive drum and the intermediate transfer belt are new and the environment is 15 ° C. and 10% RH.

  FIG. 6 shows the optimum primary transfer voltage read from FIG. 5 on the vertical axis and the resistance value of the primary transfer load on the horizontal axis. The resistance value of the primary transfer load includes variations in the resistance value of the intermediate transfer belt, variations in the resistance value of the primary transfer brush, and the like, and varies by about ± 26% at the maximum. The curve shown in FIG. 5 also changes due to variations in the resistance value of the primary transfer load, and the optimum primary transfer voltage varies as shown in FIG. 610V is the optimal primary transfer voltage for the primary transfer load centered on the resistance value, and 770V is the optimal primary transfer voltage for the primary transfer load with the upper limit of the resistance value, and the primary transfer load with the lower limit of the resistance value. In contrast, 450V is the optimum primary transfer voltage.

  In the control method of the comparative example, the primary transfer voltage to be applied is determined in accordance with the primary transfer load centered on the resistance value and 610 V is applied, so the applied voltage is about 160 V for the primary transfer load with the upper limit of the resistance value. The applied voltage is about 160V higher than the primary transfer load which is small and has a lower resistance value. In contrast, in the control method of this embodiment, the resistance value of the primary transfer load is calculated and the applied voltage is determined. Therefore, the optimum primary transfer is performed even when the resistance value of the primary transfer load is at the upper and lower limits. A voltage can be applied.

  As described above, according to the present embodiment, the optimum primary transfer voltage can be applied even when the resistance value of the primary transfer load varies. Thereby, good primary transferability can be ensured.

(Example 2)
FIG. 7 is a diagram illustrating a transfer unit of the image forming apparatus according to the second embodiment. In FIG. 7, the same components as those in FIG. 3 of the first embodiment are denoted by the same reference numerals, and description thereof is omitted. The difference between FIG. 7 of this embodiment and FIG. 3 of Embodiment 1 is that each image forming station of the primary transfer portion has a current control circuit and voltage detection means, and the photosensitive drum is brought into contact with the intermediate transfer belt. This is a point having a contact / separation means for separating. Also in this embodiment, as in the first embodiment, the resistance value of the primary transfer load is calculated from the current detection result of detecting the current flowing through the upstream load and the voltage detection result of detecting the voltage applied to the downstream load. The point to do is the same.

  In FIG. 3 of Embodiment 1, the primary transfer voltage Vtr1 is supplied to four image forming stations by one current control unit. Therefore, there is a possibility that the primary transferability varies due to variations in the resistance value of the primary transfer load for each image forming station. On the other hand, in the second embodiment of FIG. 7, current control circuits 30a, 30b, 30c, and 30d are provided for each image forming station to adjust the primary transfer voltages Vtr1a, Vtr1b, Vtr1c, and Vtr1d. Variation in primary transferability is eliminated. The current control circuits 30a, 30b, 30c, and 30d are the same circuits as the current control circuit 30 shown in FIG. 3 of the first embodiment. Resistors 801a, 801b, 801c, and 801d as voltage step-down elements are provided to separate the primary transfer voltage for each station.

In the image forming apparatus of FIG. 7, the relationship between the secondary transfer current Itr2, the cleaning current Iicl, and the primary transfer currents Itr1a, Itr1b, Itr1c, and Itr1d flowing through the respective image forming stations is as follows.
Itr2 + Iicl = (Itr1a + Icona) + (Itr1b + Iconb) + (Itr1c + Iconc) + (Itr1d + Icond) (8)

  Icona, Iconb, Iconc, Icond are control currents flowing in the current control circuits 30a, 30b, 30c, 30d of the respective image forming stations.

  When calculating the resistance value of the primary transfer load for each image forming station, the control current flowing through the current control circuits of all the image forming stations is set to a known state, and the resistance value of the primary transfer load is calculated. Separate photosensitive drums other than the forming station. This is carried out in all four image forming stations by shifting the time for each image forming station. In the present embodiment, similarly to the first embodiment, a condition for turning off the transistor is created, and the control current Icona = 0, Iconb = 0, Iconc = 0, and Icond = 0, thereby creating a state in which the control current is known.

Further, the photosensitive drum other than the image forming station for calculating the resistance value is separated by the contact / separation means 900a, 900b, 900c, and 900d, so that the current flowing to other than the image forming station for calculating the resistance value is reduced to zero. For example, when calculating the resistance value of the image forming station 70a, the photosensitive drums of the image forming stations 70b, 70c, and 70d are separated, and Iconb = 0, Iconc = 0, and Icond = 0. At this time, from the equation (8), the following equation is obtained.
Itr1a = Itr2 + Iicl (9)
Accordingly, the resistance value Rtr1a of the primary transfer load of the image forming station 70a can be calculated by the following equation.
Rtr1a = Vtr1a / (Itr2 + Iicl) (10)

By performing the same control at the image forming stations 70b, 70c, and 70d while shifting the time, the resistance values Rtr1a, Rtr1b, Rtr1c, and Rtr1d of all the image forming stations can be calculated.
The CPU 100 has a table of optimum primary transfer voltages corresponding to the calculated resistance value of the primary transfer load, and the optimum primary transfer voltage Vtr1a corresponding to the resistance value of the primary transfer load calculated at the time of image formation. Constant voltage control of Vtr1b, Vtr1c, and Vtr1d is performed.

DESCRIPTION OF SYMBOLS 100 ... CPU, 200 ... Secondary transfer voltage source, 201 ... Cleaning voltage source, 30 ... Current control circuit, 302 ... Operational amplifier, 307 ... Transistor, 400 ... Secondary transfer current detection circuit, 401 ... Cleaning current detection circuit, 50 ... Primary transfer voltage detection circuit, 60 ... upstream load, 60a ... secondary transfer load, 60b ... cleaning load, 600 ... intermediate transfer belt, 601 ...
Secondary transfer roller, 602 ... cleaning brush, 70 ... downstream load, 700a, 700b, 700c, 700d ... photosensitive drum, 701a, 701b, 701c, 701d ... primary transfer brush

Claims (13)

An image carrier for carrying a developer image;
An endless belt that rotates while in contact with the image carrier;
A current supply member that contacts the belt at a position different from the image carrier in the rotation direction of the belt and supplies current to the belt;
A control unit for outputting a control signal having a variable size;
A contact member that contacts the belt at a position facing the current supply member via the belt;
A current detector for detecting the amount of current flowing through the current supply member;
A voltage detection unit that detects a transfer voltage that is a surface potential at a contact portion of the belt with the image carrier for transferring a developer image carried by the image carrier to the belt;
A voltage adjustment unit including a voltage adjustment member connected to the contact member, and a voltage capable of changing the magnitude of the transfer voltage according to the magnitude of the control signal input from the control unit An adjustment unit;
With
The controller is
The voltage adjustment unit is controlled so that a current amount of a control current flowing through the voltage adjustment member becomes a known current amount, and based on the known current amount and a current amount detected by the current detection unit, Get the current amount of 1
An image forming apparatus that controls the magnitude of the control signal based on the first current amount and a voltage value detected by the voltage detection unit. A load from the current supply member to the contact member via the belt is an upstream load,
The load from the contact member to the ground is a downstream load,
The voltage detection unit detects a voltage value applied to the downstream load,
The image forming apparatus according to claim 1, wherein the first current amount is a current amount flowing through the downstream load.   The control unit controls the voltage adjustment unit so that no current flows through the voltage adjustment member, and obtains the amount of current flowing through the downstream load with the amount of control current as zero. The image forming apparatus according to claim 2. The current supply member is a first current supply member, and is in a position different from the image carrier and the first current supply member in the rotation direction of the belt, and faces the contact member via the belt. A second current supply member that contacts the belt in position and supplies current to the belt;
A second contact member that contacts the belt at a position facing the image carrier via the belt, the contact member being a first contact member;
The current detection unit is a first current detection unit, and a second current detection unit detects the amount of current flowing from the second current supply member to the upstream load that reaches the second contact member via the belt. When,
Further comprising
The controller detects the amount of current flowing from the first contact member to the ground via the second contact member to the ground and the known current amount and the first current detector. The image forming apparatus according to claim 2, wherein the image forming apparatus acquires the current based on a current amount to be detected and a current amount detected by the second current detection unit. 5. The secondary transfer member according to claim 4, wherein the first current supply member is a secondary transfer member that secondary-transfers a developer image from the belt to a recording material by a current flowing through a contact portion with the belt. Image forming apparatus.   The image forming apparatus according to claim 4, wherein the second current supply member is a charging member for charging the toner carried on the belt. A plurality of the image carriers;
A plurality of the second contact members corresponding to the plurality of image carriers;
The image forming apparatus according to claim 4, further comprising: A plurality of voltage adjusting units corresponding to the plurality of image carriers;
With
The plurality of image carriers are configured to be able to individually contact or separate from the belt,
The control unit reaches the ground via the second contact member facing the image carrier that is in contact with the belt among the plurality of second contact members from the first contact member. The image forming apparatus according to claim 7, wherein a resistance value of a downstream load is acquired. The control signal is a PWM signal;
The voltage adjustment unit changes a magnitude of a current supplied from the current supply member to the belt according to a duty ratio of the PWM signal input from the control unit. Item 9. The image forming apparatus according to any one of Items 1 to 8.   The image forming apparatus according to claim 1, wherein the voltage adjustment unit is an adjustment circuit including a transistor as the voltage adjustment member.   The image forming apparatus according to claim 1, further comprising a voltage step-down element connected in series with the voltage adjustment unit between the upstream load and the voltage adjustment unit.   The image forming apparatus according to claim 11, wherein the voltage step-down element is a Zener diode.   The image forming apparatus according to claim 1, wherein the belt is an endless belt body formed by mixing an ionic conductive agent.

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