JP2014115484A - Image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image forming apparatus which is capable of acquiring information corresponding to the electric resistance of a part of image forming parts, even when a transfer power supply outputting a transfer voltage to the plurality of image forming parts is made common.SOLUTION: An image forming apparatus 100 includes a plurality of image forming parts S, a common transfer power supply 50 applying a voltage to a plurality of transfer means 5, detection means 51 for detecting a voltage value or a current value, setting means 151 for setting the voltage applied to the electrifying means 2 of at least one image forming part S, so that a potential difference between a photoreceptor 1 and the transfer means 5 is equal to or higher than a discharge start potential difference and setting the voltage applied to the electrifying means 2 of the remaining image forming parts S, so that the potential difference is less than the discharge start potential difference, in such a state that a common voltage is applied to the plurality of transfer means 5 from the transfer power supply 50, and execution means 151 for executing the detection in such a state that each of the potential differences is made.

Description

  The present invention relates to an image forming apparatus using an electrophotographic system, and more particularly to an image forming apparatus that transfers a toner image from a photosensitive member of a plurality of image forming units to a transfer target.

  Conventionally, as an electrophotographic image forming apparatus, there is an inline image forming apparatus. An in-line image forming apparatus includes a plurality of image forming units along the moving direction of an intermediate transfer member as a transfer target, for example. In each image forming unit, a toner image is transferred from the photosensitive member to the intermediate transfer member by applying a primary transfer voltage to a primary transfer member disposed opposite to the electrophotographic photosensitive member (photosensitive member) via the intermediate transfer member. Transcript.

  In such an image forming apparatus, the electrical resistance of the intermediate transfer member and the primary transfer member may fluctuate greatly due to environmental fluctuations and durability fluctuations. Accordingly, the optimal primary transfer voltage to be applied to the primary transfer member during image formation varies.

  Therefore, in order to apply an optimal primary transfer voltage to the primary transfer member during image formation regardless of environmental fluctuations and durability fluctuations, the electrical resistance of each image forming unit is detected before image formation, and the detection result is It is effective to determine the primary transfer voltage during image formation based on this.

  For example, Patent Document 1 discloses the following ATVC control (Active Transfer Voltage Control). That is, constant current control is performed so that the current flowing through the primary transfer member of each image forming unit is kept constant at a predetermined target current, and the electric resistance of each image forming unit is detected by detecting the voltage at that time. Based on the result, a primary transfer voltage value to be applied to the primary transfer member is determined by constant voltage control during image formation.

JP 2001-125338 A

  However, the conventional image forming apparatus that performs ATVC control has the following problems to be improved.

  FIG. 9A schematically shows a configuration for applying a primary transfer voltage to a primary transfer member in an in-line image forming apparatus, and FIG. 9B shows an equivalent circuit thereof.

  As shown in FIG. 9A, the high-voltage power supply 50 and the current detection circuit 51 may be shared by the plurality of image forming units A1, A2, A3, and A4 in order to reduce the size and cost of the apparatus. is there. In FIG. 9B, R1, R2, R3, and R4 indicate electric resistances of the respective image forming units, and V indicates a voltage applied thereto. In addition, I1, I2, I3, and I4 indicate currents flowing through the image forming units.

  In this case, since the high voltage power supply 50 is shared by the plurality of image forming units A1, A2, A3, and A4, the electric resistances R1, R2, R3, and R4 of the image forming units A1, A2, A3, and A4 The same (common) voltage V is applied. Then, currents I1 = V / R1, I2 = V / R2, I3 = V / R3, and I4 = V / R4 flow through the image forming units A1, A2, A3, and A4, respectively. Therefore, the current detection circuit 51 detects a current obtained by adding the currents flowing through the image forming units A1, A2, A3, and A4, that is, Itotal = I1 + I2 + I3 + I4. Therefore, the average value Rave of the electrical resistances of the image forming units A1, A2, A3, and A4 can be calculated by using the calculation formula Rave = (4 × V) / (I1 + I2 + I3 + I4).

  However, in the conventional image forming apparatus in which the high-voltage power supply 50 and the current detection circuit 51 are shared, the electrical resistances R1, R2, R3, and R4 of each image forming unit cannot be detected individually. Therefore, it may be difficult to select an optimal primary transfer voltage that reflects the individual electrical resistance of each image forming unit. In some cases, an image defect due to a transfer defect or a discharge-system image defect may occur.

  In the above, the conventional problem has been described by taking the image forming apparatus of the intermediate transfer method as an example, but the direct transfer method having a recording material carrier that carries and conveys a recording material as a transfer target instead of the intermediate transfer member. The same problem can occur in the image forming apparatus.

  Accordingly, an object of the present invention is to provide an image forming apparatus capable of acquiring information corresponding to the electrical resistance of some image forming units even when a transfer power source that outputs a transfer voltage to a plurality of image forming units is shared. Is to provide.

  The above object is achieved by the image forming apparatus according to the present invention. In summary, the present invention relates to a photoconductor, a charging unit that charges the photoconductor when voltage is applied thereto, a developing unit that supplies toner to the photoconductor to form a toner image, and a transfer object from the photoconductor. A plurality of image forming units each having a transfer unit that transfers a toner image to the image forming unit, a common transfer power source that applies a voltage to the transfer unit of the plurality of image forming units, and the transfer power source includes the plurality of image forming units. Detecting means for detecting a voltage value when a constant current controlled voltage is applied to the transfer means or a current value when applying a constant voltage controlled voltage; and In a state where a common voltage is applied to the transfer unit, a voltage applied to the charging unit of at least one image forming unit among the plurality of image forming units is set to the photoconductor of the at least one image forming unit. When The voltage difference between the transfer unit and the transfer unit is set to be equal to or greater than the discharge start potential difference, and the voltage applied to the charging unit of the remaining image forming unit among the plurality of image forming units is set to the remaining image. Setting means for setting a potential difference between the photosensitive member of the forming unit and the transfer unit to be less than a discharge start potential difference, and a potential difference greater than the discharge start potential difference and a potential difference less than the discharge start potential difference are formed. And an execution unit that executes detection of the voltage value or the current value by the detection unit in a state.

  According to the present invention, even if a transfer power source that outputs a transfer voltage to a plurality of image forming units is shared, information corresponding to the electrical resistance of some image forming units can be acquired.

1 is a schematic cross-sectional view of an image forming apparatus according to an embodiment of the present invention. 1 is a schematic diagram illustrating a primary transfer voltage application configuration and an equivalent circuit thereof in an image forming apparatus according to an embodiment of the present invention. 1 is a schematic diagram illustrating a charging voltage application configuration and an equivalent circuit thereof in an image forming apparatus according to an embodiment of the present invention. FIG. 6 is a flowchart for explaining a procedure of ATVC control of the image forming apparatus according to the embodiment of the present invention. FIG. 5 is a schematic diagram illustrating a set voltage and an equivalent circuit in each process of ATVC control of the image forming apparatus according to the embodiment of the present invention. 1 is a block diagram illustrating a schematic control mode of a main part of an image forming apparatus according to an embodiment of the present invention. FIG. 6 is a flowchart for explaining a procedure of ATVC control of an image forming apparatus according to another embodiment of the present invention. FIG. 6 is a schematic cross-sectional view of another example of an image forming apparatus to which the present invention can be applied. It is a schematic diagram showing a primary transfer voltage application configuration and its equivalent circuit for explaining a conventional problem.

  The image forming apparatus according to the present invention will be described below in more detail with reference to the drawings.

Example 1
1. 1 is a schematic cross-sectional view of an image forming apparatus according to an embodiment of the present invention. The image forming apparatus 100 of the present embodiment is an inline type (tandem type) laser beam printer that employs an intermediate transfer method capable of forming a full color image using an electrophotographic method.

  The image forming apparatus 100 includes first, second, third, and fourth image forming units (stations) Sa, Sb, Sc, and Sd as a plurality of image forming units. Then, the toner images of the respective colors formed by the respective image forming portions Sa, Sb, Sc, Sd according to the image information separated into a plurality of color components are sequentially superimposed on the intermediate transfer belt 6 as an intermediate transfer member and primarily transferred. Then, a recorded image is obtained by performing secondary transfer onto the transfer material P all at once.

  In this embodiment, each of the image forming units Sa, Sb, Sc, and Sd forms toner images of respective colors of yellow (Y), magenta (M), cyan (C), and black (K). The basic configurations and operations of the image forming units Sa, Sb, Sc, and Sd are substantially the same except for the color of the toner to be used. Accordingly, in the following, when there is no particular need for distinction, the description will be made comprehensively, omitting a, b, c, and d at the end of the reference numerals indicating elements provided for any color.

  The image forming unit S includes a photosensitive drum 1 that is a drum-type electrophotographic photosensitive member (photosensitive member) as an image carrier. The photosensitive drum 1 is rotationally driven in the direction of arrow R1 (counterclockwise) in FIG. 1 by a drive motor (not shown) as drive means. Around the photosensitive drum 1, the following means are arranged along the rotation direction. First, the charging roller 2 is a roller-type charging member as charging means. Next, there is an exposure device (laser scanner) 3 as an exposure means (electrostatic image forming means). Next, there is a developing device 4 as a developing unit. Next, a primary transfer brush 5 which is a brush-like transfer member as a primary transfer means. Next, there is a drum cleaner 7 as a photosensitive member cleaning means.

  A transfer device (transfer unit) 60 for transferring the toner image to the recording material P is disposed below the photosensitive drums 1a, 1b, 1c, and 1d. The transfer device 60 includes an intermediate transfer belt 6 that is an intermediate transfer body as a transfer target so as to face the photosensitive drums 1a, 1b, 1c, and 1d. The intermediate transfer belt 6 is composed of a cylindrical and endless belt-like film stretched around three rollers, a driving roller 61, a secondary transfer counter roller 62, and a tension roller 63. The intermediate transfer belt 6 is driven to rotate in the direction indicated by the arrow R2 (clockwise) in FIG. Rotate (clockwise) (turn around). The primary transfer brushes 5a, 5b, 5c, and 5d are arranged on the inner peripheral surface side of the intermediate transfer belt 6 at positions facing the photosensitive drums 1a, 1b, 1c, and 1d with the intermediate transfer belt 6 interposed therebetween. Yes. The primary transfer brushes 5a, 5b, 5c, and 5d are pressed against the photosensitive drums 1a, 1b, 1c, and 1d via the intermediate transfer belt 6, and the photosensitive drums 1a, 1b, 1c, and 1d, the intermediate transfer belt 6, and the like. Primary transfer portions N1a, N1b, N1c, and N1d are formed at the contact portion. A secondary transfer roller 8, which is a roller-type transfer member serving as a secondary transfer unit, is disposed on the outer peripheral surface side of the intermediate transfer belt 6 at a position facing the secondary transfer counter roller 62. The secondary transfer roller 8 is pressed against the secondary transfer counter roller 62 via the intermediate transfer belt 6, and a secondary transfer portion N <b> 2 is formed at the contact portion between the intermediate transfer belt 6 and the secondary transfer roller 8. A belt cleaner 65 serving as an intermediate transfer member cleaning unit is disposed on the outer peripheral surface side of the intermediate transfer belt 6 at a position facing the driving roller 61.

  Further, the image forming apparatus 100 includes a recording material supply device 10 that supplies a recording material P such as recording paper and an OHP sheet to the secondary transfer portion N2, and a downstream of the secondary transfer portion N2 in the conveyance direction of the recording material P. And a fixing device 9 as fixing means provided on the side.

  At the time of image formation, the photosensitive drum 1 is rotationally driven, and the surface of the rotating photosensitive drum 1 is charged substantially uniformly by the charging roller 2. At this time, a charging voltage (charging bias), which is a predetermined DC voltage in this embodiment, is applied to the charging roller 2. As will be described in detail later, in this embodiment, the charging rollers 2a, 2b, 2c, and 2d of the image forming units Sa, Sb, Sc, and Sd have independent charging power sources (high-voltage power sources) as charging voltage application means. A charging voltage is applied from 20 (FIG. 3).

  The surface of the photosensitive drum 1 that is charged substantially uniformly reaches the exposure position by the exposure device 3 by the rotation of the photosensitive drum 1, and the exposure device 3 irradiates the laser beam L according to the image information. Thereby, an electrostatic image (electrostatic latent image) according to the image information is formed on the photosensitive drum 1.

  The electrostatic image formed on the photosensitive drum 1 reaches a portion (developing position) facing the developing device 4 by the rotation of the photosensitive drum 1, and is developed (visualized) as a toner image with toner as a developer by the developing device 4. Is done. In this embodiment, the intended charging polarity (normal charging polarity) of the toner for developing the electrostatic image is negative. In this embodiment, the developing device 4 applies the charged polarity (this embodiment) of the photosensitive drum 1 to the image portion (exposure portion) on the photosensitive drum 1 where the absolute value of the potential is reduced by exposure after being charged substantially uniformly. In this case, a toner charged with the same polarity as that of the negative polarity is attached to develop the electrostatic image (reversal development).

  The toner image formed on the photosensitive drum 1 reaches the primary transfer portion N1 by the rotation of the photosensitive drum 1, and is applied to the intermediate transfer belt 6 by the action of the primary transfer voltage (primary transfer bias) applied to the primary transfer brush 5. Transfer (primary transfer) is performed on the outer peripheral surface. In this embodiment, the primary transfer voltage is a DC voltage having a polarity opposite to the normal charging polarity of the toner (positive polarity in this embodiment). As will be described in detail later, in this embodiment, the primary transfer brushes 5a, 5b, 5c, and 5d of the image forming units Sa, Sb, Sc, and Sd have a common primary transfer power source (primary transfer voltage application unit). A common primary transfer voltage is applied from a high voltage power source 50. Further, as will be described in detail later, the primary transfer current (transfer current) supplied by the primary transfer power supply 50 is detected by a common primary transfer current detection circuit (current detection circuit) 51.

  Here, as shown in FIG. 3, the primary transfer brush 5 is supported by a support member 52, and a spring as an urging means provided on the opposite side of the primary transfer member 5 via the support member 52. The support member 52 is biased toward the intermediate transfer belt 6 by 53. As a result, the primary transfer member 5 is pressed against the photosensitive drum 1 via the intermediate transfer belt 6.

  In the primary transfer process, the toner (primary transfer transfer residual toner) remaining on the photosensitive drum 1 without being transferred to the intermediate transfer belt 6 reaches a portion (cleaning position) facing the drum cleaner 7 by the rotation of the photosensitive drum 1. It is removed and recovered by the drum cleaner 7. The drum cleaner 7 scrapes off the primary transfer residual toner from the surface of the rotating photosensitive drum 1 by a cleaning blade 72 as a cleaning member and stores it in a waste toner container 71.

  For example, when forming a full-color image, the above-described steps of charging, exposure, development, and primary transfer are performed in order from the image forming unit S on the upstream side in the moving direction of the outer peripheral surface of the intermediate transfer belt 6. , Sb, Sc, Sd. As a result, a full-color toner image in which four color toner images are superimposed on the outer peripheral surface of the intermediate transfer belt 6 is formed.

  The toner image formed on the outer peripheral surface of the intermediate transfer belt 6 reaches the secondary transfer portion N2 by the rotation of the intermediate transfer belt 6, and a secondary transfer voltage (secondary transfer bias) applied to the secondary transfer roller 8. As a result, the image is transferred (secondary transfer) onto the transfer material P. In this embodiment, the secondary transfer voltage is a DC voltage having a polarity opposite to the normal charging polarity of the toner (positive polarity in this embodiment). A secondary transfer voltage is applied to the secondary transfer roller 8 from a secondary transfer power source (high voltage power source) 80 as a secondary transfer voltage application unit. A secondary transfer current (transfer current) supplied by the secondary transfer power supply 80 is detected by a secondary transfer current detection circuit (current detection circuit) 81.

  The recording material P is supplied to the secondary transfer portion N2 by the recording material supply device 10. That is, the transfer material P accommodated in the cassette 11 is sent out by the supply roller 12 and then supplied to the secondary transfer portion N2 by the registration roller 13 at a predetermined timing. At substantially the same time, a secondary transfer voltage is applied to the secondary transfer roller 8 from the secondary transfer power supply 80.

  In the secondary transfer process, toner (secondary transfer residual toner) that is not transferred onto the transfer material P but remains on the intermediate transfer belt 6 is moved to a portion facing the belt cleaner 65 (cleaning position) by the rotation of the photosensitive drum 1. It is removed and collected by the belt cleaner 65. The belt cleaner 65 scrapes off the secondary transfer residual toner from the outer peripheral surface of the rotating intermediate transfer belt 6 by a cleaning blade 66 as a cleaning member, and stores it in a waste toner container 64.

  The transfer material P onto which the toner image has been transferred is conveyed to the fixing device 9 and is nipped and conveyed at a fixing nip formed between the fixing roller 91 and the pressure roller 92, so that heat and pressure are transferred. And the toner image thereon is fixed.

  Thereafter, the transfer material P on which the toner image is fixed is conveyed outside the apparatus as indicated by an arrow R4 in FIG. 1 by a conveying roller (not shown).

  In this embodiment, in each of the image forming portions Sa, Sb, Sc, and Sd, the photosensitive drum 1, the charging roller 2, the developing device 4, and the drum cleaner 7 as process means acting on the photosensitive drum 1 are integrally formed as a process cartridge. 110 is configured. The process cartridge 110 can be attached to and detached from the apparatus main body of the image forming apparatus 100 as a replaceable image forming unit. For example, when the remaining amount of toner in the developing device 4 runs out, Exchanged.

  In this embodiment, the transfer device 60 is configured to be detachable from the main body of the image forming apparatus 100 as a replaceable image forming unit.

  In the present embodiment, the first, second, third, and fourth image forming portions Sa, Sb, Sc, and Sd are the photosensitive drum 1, the charging roller 2, the exposure device 3, the developing device 4, and the primary transfer brush, respectively. 5 and a drum cleaner 7.

2. Control Mode FIG. 6 shows a schematic control mode of the main part of the image forming apparatus 100 of this embodiment. The operation of the image forming apparatus 100 is comprehensively controlled by a control unit 150 provided in the image forming apparatus 100. The control unit 150 includes a CPU 151 as a control unit, a ROM 152 and a RAM 153 as storage units. Then, the control unit 150 controls the operation of each unit of the image forming apparatus 100 in accordance with the program and data stored in the ROM 152 and read out to the RAM 153 as necessary.

  In relation to the present embodiment, the control unit 150 controls ON / OFF of high voltage power sources such as the charging power source 20, the primary transfer power source 50, and the secondary transfer power source 80, and the output value. In addition, the control unit 150 receives signals related to detection results of the primary transfer current detection circuit 51 and the secondary transfer current detection circuit 81.

  As will be described in detail later, in this embodiment, the CPU 151 of the control unit 151 functions as a setting unit that sets a voltage output from the charging power supply 20 to the charging roller 2. In addition, the voltage value or the constant voltage controlled voltage when the CPU 151 of the control unit 150, the primary transfer power supply 50, and the primary transfer current detection circuit 51 output the constant current controlled voltage to the primary transfer means. The detecting means for detecting the current value when the signal is output is configured. The CPU 151 of the control unit 150 also functions as an execution unit that executes the detection. Furthermore, in this embodiment, the CPU 151 of the control unit 150 also functions as a determination unit that determines a voltage to be output from the primary transfer power supply 50 to the primary transfer brush 5 during image formation.

  The CPU 151 can be composed of a one-chip microcomputer that controls the primary transfer power supply 50 based on the voltage signal from the primary transfer current detection circuit 51, process speed information, environmental information, waste toner count information, and the like.

3. ATVC Control In this embodiment, PEN (polyethylene naphthalate) having ionic conductivity is used as the material of the intermediate transfer belt 6. Therefore, the electrical resistance of the intermediate transfer belt 6 may fluctuate greatly with the temperature and humidity of the environment due to the characteristics of the ion conductive material.

  On the other hand, in this embodiment, as the primary transfer member, the primary transfer brush 5 that contacts and rubs the inner peripheral surface of the intermediate transfer belt 6 is used. An intermediate transfer belt 6 generated by rubbing between the primary transfer brush 5 and the inner peripheral surface of the intermediate transfer belt 6 is formed at the tip of the primary transfer brush 5 that contacts the inner peripheral surface of the intermediate transfer belt 6. There are cases where deposits such as wear powder of the primary transfer brush 5 adhere. Therefore, the electrical resistance of the primary transfer brush 5 may increase due to an increase in usage amount.

  The image forming apparatus 100 according to the present exemplary embodiment performs ATVC control (Active Transfer Voltage Control) so that an optimal primary transfer voltage can be selected at the time of image formation even when such environmental fluctuations and durability fluctuations occur. In the ATVC control, generally, the electrical resistance of the image forming unit is detected before image formation. Based on the result, the primary transfer voltage at the time of image formation is determined. A detailed control method will be described later.

  In this embodiment, the ATVC control is performed for each image formation during the pre-rotation operation before the image formation operation as during non-image formation. In the pre-rotation operation, an image formation start instruction is input to the image forming apparatus 100 and rotational driving of the photosensitive drum 1 and the intermediate transfer belt 6 is started, and then the toner image is actually transferred to the intermediate transfer belt 6. This is a preparatory operation performed until.

  In addition, the following can be cited as non-image formation during which ATVC control can be performed. There is a pre-multi-rotation operation before a predetermined preparatory operation is performed for raising the fixing temperature, such as when the image forming apparatus is turned on or returned from the sleep mode. In addition, there is a pre-rotation operation in which a predetermined preparation operation is executed from when an image forming signal is input until an image corresponding to image information is actually written. In addition, there is a corresponding paper interval between recording materials during continuous image formation. Further, there is a post-rotation operation in which a predetermined organizing operation (preparation operation) is executed after the image formation is completed.

  Here, as will be described in detail later, in this embodiment, the charging roller 2 is applied with a charging voltage by a charging power source 20 which is a variable power source circuit, and the surface of the photosensitive drum 1 is changed from a positive potential to a negative polarity. It is possible to charge to any potential within a predetermined range up to the potential.

  Normally, during ATVC control, the surface of the photoconductor is charged to about −600 V and a voltage of about +200 V is applied to the primary transfer member, and at that time, the current flows between the primary transfer member and the photoconductor, as in image formation. Detect current value. When forming an image, a predetermined primary transfer voltage determined based on the current value is applied to the primary transfer member to form an image. If the charged potential of the photoconductor is not -600 V but higher (small absolute value) -200 V or 0 V on the positive polarity side, even if a voltage of +200 V is applied to the primary transfer member, The potential difference between the primary transfer member and the photosensitive member is 400V or 200V. In this case, since a potential difference less than about 500 V, which is a discharge start potential difference (dischargeable potential difference), is not formed between the primary transfer member and the photosensitive member, a current flows between the primary transfer member and the photosensitive member. Or the absolute value of the current is negligibly small.

  In this way, by adjusting the voltage output from the charging power source 20 to the charging roller 2 to adjust the charging potential of the photosensitive drum 1, no current flows between the primary transfer brush 5 and the photosensitive drum 1, or The absolute value of the current can be made small enough to be ignored. Here, such a state is simulated when the primary transfer brush 5 and the photosensitive drum 1 are apparently electrically disconnected, or when the primary transfer brush 5 (intermediate transfer belt 6) and the photosensitive drum 1 are simulated. It is said to be separated from each other.

4). Primary Transfer Power Supply and Primary Transfer Current Detection Circuit FIG. 2A schematically shows a primary transfer voltage application configuration to the primary transfer brush 5 in the image forming apparatus 100 of the present embodiment, and FIG. The circuit is shown. In the figure, Rda, Rdb, Rdc and Rdd indicate the electrical resistance of the photosensitive drum 1, and Ra, Rb, Rc and Rd are primary transfer portions which are mainly the electrical resistance of the intermediate transfer belt 6 and the primary transfer brush 5. The electric resistance of N is shown.

  In this embodiment, the primary transfer power supply 50 that supplies power to the primary transfer brush 5 is shared by all the image forming units Sa, Sb, Sc, and Sd in order to reduce the size and cost of the apparatus. Yes. In this embodiment, the primary transfer current detection circuit 51 is shared by all the image forming units Sa, Sb, Sc, and Sd.

  In FIG. 2A, the same (common) primary transfer voltage V is applied from the common primary transfer power supply 50 to the primary transfer brushes 5a, 5b, 5c, and 5d of the image forming units Sa, Sb, Sc, and Sd. Has been. Further, the charging rollers 2a, 2b, 2c, and 2d of the image forming units Sa, Sb, Sc, and Sd are charged with a predetermined negative potential substantially uniformly on the surface of the photosensitive drums 1a, 1b, 1c, and 1d. Therefore, a predetermined negative charging voltage Vt is applied. At this time, the surface of the photosensitive drum 1 is charged to a potential Vtm that is slightly higher in the positive polarity side (smaller in absolute value) than the charging voltage Vt applied to the charging roller 2.

  As shown in FIG. 2B, the currents Ia, Ib, Ic, and Id flowing through the electric resistances Ra, Rb, Rc, and Rd of the image forming units Sa, Sb, Sc, and Sd are expressed as Ia = V / Ra, Ib = V / Rb, Ic = V / Rc, Id = V / Rd.

  In this case, the primary transfer current detection circuit 51 detects a current Iall = Ia + Ib + Ic + Id, which is the sum of the currents flowing through the image forming units Sa, Sb, Sc, and Sd.

  2 (c) and 2 (d) are similar to FIGS. 2 (a) and 2 (b), except that the charging roller 2a of the first image forming unit Sa and other image forming units Sb and Sc. , Sd charging rollers 2b, 2c, and 2d are applied.

  In FIG. 2C, a voltage Vtd that is higher (smaller in absolute value) than the other image forming units Sb to Sd is applied to the charging roller 2a of the first image forming unit Sa. The voltage Vtd is a voltage value that charges the photosensitive drum 1 only to the extent that no discharge occurs between the photosensitive drum 1 and the primary transfer brush 5. That is, in the first image forming unit Sa, the voltage Vtd that causes the potential difference between the photosensitive drum 1 and the primary transfer brush 5 to be less than the discharge start potential difference is set to the charging roller 2. Is applied. In the other image forming units Sb, Sc, Sd, the voltage Vt that causes the potential difference of the photosensitive drum 1 so that the potential difference between the photosensitive drum 1 and the primary transfer brush 5 is equal to or greater than the discharge start potential difference is applied to the charging roller. 2 is applied.

  Accordingly, the surface potential Vtmd of the photosensitive drum 1a of the first image forming unit Sa is higher on the positive polarity side than the surface potential Vtm of the photosensitive drums 1b, 1c, and 1d of the other image forming units Sb, Sc, and Sd (absolutely). The value becomes smaller). In the first image forming unit Sa, no current flows between the primary transfer brush 5a and the photosensitive drum 1a. Therefore, in the first image forming unit Sa, the primary transfer brush 5a and the photosensitive drum 1a are pseudo. Are separated from each other.

  In this case, it flows between the primary transfer brushes 5b, 5c, and 5d and the photosensitive drums 1b, 1c, and 1d only in the second, third, and fourth image forming units Sb, Sc, and Sd. Therefore, the primary transfer current detection circuit 51 detects the current Iall = Ib + Ic + Id, which is the sum of the currents flowing through the second, third, and fourth image forming units.

5. Charging power supply (variable power supply circuit)
FIG. 3A schematically shows a configuration for applying a charging voltage to the charging roller 2 in the image forming apparatus 100 of this embodiment, and FIG. 3B shows an equivalent circuit thereof.

  In this embodiment, a charging power source 20 serving as an independent variable power source circuit is connected to the charging rollers 2a, 2b, 2c, and 2d of the image forming units Sa, Sb, Sc, and Sd. Each of the charging rollers 2a, 2b, 2c, and 2d can charge the surface of each of the photosensitive drums 1a, 1b, 1c, and 1d to an arbitrary potential within a predetermined range (for example, + 200V to −700V).

  The charging power source 20 includes a positive voltage output unit 21 that outputs a variable positive polarity voltage (plus bias) within a predetermined range, and a negative voltage output unit that outputs a variable negative polarity voltage (minus bias) within a predetermined range. 22. The positive voltage output unit 21 can output a voltage of + 100V to + 800V, for example. Moreover, the negative voltage output part 22 can output the voltage of -100V--800V, for example.

  In this embodiment, the electrical resistance of any image forming unit S among the image forming units Sa, Sb, Sc, and Sd can be detected using the primary transfer current detection circuit 51 as follows. To do.

  First, in the image forming unit S where it is desired to detect electric resistance, the absolute value of the output of the negative voltage output unit 22 of the charging power source 20 is increased, and conversely, the absolute value of the output of the positive voltage output unit 21 is decreased. Thus, in the image forming unit S, the potential difference between the charged potential of the photosensitive drum 1 and the primary transfer brush 5 is set to about 500 V or more, which is the discharge start potential difference, and the photosensitive drum 1 and the primary drum in the image forming unit S. A current is allowed to flow between the transfer brush 5. Then, the electric resistance of the image forming unit S is detected from the current value and voltage value at that time.

  On the other hand, at that time, in the image forming unit S that does not detect the electrical resistance, the absolute value of the output of the negative voltage output unit 22 of the charging power source 20 is reduced, and conversely, the absolute value of the output of the positive voltage output unit 21 is set. Enlarge. As a result, in the image forming unit S, the potential difference between the charged potential of the photosensitive drum 1 and the primary transfer brush 5 is set to be less than about 500 V, which is the discharge start potential difference, and the photosensitive drum 1 and the primary in the image forming unit S. The current is prevented from flowing between the transfer brush 5. That is, in the image forming unit S, the primary transfer brush 5 and the photosensitive drum 1 are in a pseudo-separated state. In this embodiment, the primary transfer brush 5 of each image forming unit Sa, Sb, Sc, Sd is actually maintained in contact with the photosensitive drum 1 via the intermediate transfer belt 6 at this time. The

  As described above, in this embodiment, a circuit that can apply both a positive voltage and a negative voltage to the charging roller 2 is provided in each of the image forming units Sa, Sb, Sc, and Sd. The voltage applied to each charging roller 2 of each image forming unit S is variable, and the potential difference between the primary transfer brush 5 and the photosensitive drum 1 in each primary transfer unit N1 is variable. Thereby, the electrical resistance of an arbitrary image forming unit S can be detected using the primary transfer current detection circuit 51.

6). Sequence FIG. 4 shows a procedure of ATVC control in the present embodiment. 5A, 5C, 5E, 5G, and 5I show voltage application settings in each image forming unit S in each process of ATVC control in this embodiment. ), (D), (f), (h), (j) show equivalent circuits in the respective processes. In FIG. 5, the photosensitive drum 1 is charged to substantially the same potential as the charging voltage, and the electrical resistance of the image forming unit S is mainly the electrical resistance of the intermediate transfer belt 6 and the primary transfer brush 5. This is represented by the electrical resistance of the primary transfer portion N1.

  When the CPU 151 receives an image formation start instruction (print signal) from a host information device such as a personal computer, the CPU 151 starts a pre-rotation operation for starting up various motors and various voltages (S101).

  Thereafter, during the pre-rotation operation, the CPU 151 sequentially performs the electrical resistance detection operations of the first, second, third, and fourth image forming units Sa, Sb, Sc, and Sd (S102, S103, S104, S105). In S102, S103, S104, and S105, operations according to the following S201 to S204 are performed.

  First, in the detection operation of the electrical resistance of the first image forming unit Sa in S102, in S201, the CPU 151 causes the charging roller 2a of the first image forming unit Sa to have a negative polarity side as shown in FIG. A large voltage Vt is applied. On the other hand, the CPU 151 applies a large voltage Vtd to the charging rollers 2b, 2c, and 2d of the second, third, and fourth image forming units Sb, Sc, and Sd on the positive polarity side. Thus, in the second, third, and fourth image forming units Sb, Sc, and Sd, no discharge occurs between the photosensitive drum 1 and the primary transfer brush 5, and no current flows. Therefore, the primary transfer brush 5 And the photosensitive drum 1 are in a pseudo-separated state. In this state, since the current flows only in the first image forming unit Sa, the primary transfer current detection circuit 51 detects the current value flowing in the first image forming unit Sa. The electrical resistance of the first image forming portion Sa can be detected from the current value and the voltage value output from the primary transfer power supply 50 at that time.

  Next, in the detection operation of the electrical resistance of the first image forming unit Sa in S102, in S202, the CPU 151 performs constant current control of the primary transfer power supply 50 with the target current It as shown in FIG. If the voltage exceeds a predetermined value and becomes high, detection is stopped.

  Next, in the detection operation of the electrical resistance of the first image forming unit Sa in S102, in S203, the CPU 151 sets the generated voltage Vat of the primary transfer power supply 50 at the time of constant current control, as shown in FIG. A predetermined time is detected, and the result is stored in the RAM 153. Then, the electrical resistance Ra of the first image forming unit Sa is calculated by the calculation formula Ra = Vat / It. The voltage Vat may be a value obtained by performing a predetermined statistical process such as averaging the voltage values detected for the predetermined time.

  Next, in the electric resistance detection operation of the first image forming unit Sa in S102, in S204, the CPU 151 supplies the current to the primary transfer brush 5 by the primary transfer power source 50 and the voltage to the charging roller 2 by the charging power source 20. End the supply.

  In the detection operation of the electrical resistances of the second, third, and fourth image forming units Sb, Sc, and Sd in S103, S104, and S105, the operations in S201 to S204 are performed in the same manner as described above.

  However, in the detection operation of the electric resistance of the second image forming unit Sb in S103, in S201, the CPU 151 causes the charging roller 2b of the second image forming unit Sb to have a negative polarity side as shown in FIG. A large voltage Vt is applied. On the other hand, the CPU 151 applies a large voltage Vtd to the charging rollers 2a, 2c, and 2d of the first, third, and fourth image forming units Sa, Sc, and Sd on the positive polarity side. Thus, in the first, third, and fourth image forming portions Sa, Sc, and Sd, no discharge occurs between the photosensitive drum 1 and the primary transfer brush 5, and no current flows. Therefore, the primary transfer brush 5 And the photosensitive drum 1 are in a pseudo-separated state. As a result, as shown in FIG. 5D, an electric current is allowed to flow only through the second image forming unit Sb, and the electric resistance Rb of the second image forming unit Sb is calculated by the calculation formula Rb = Vbt / It. .

  Further, in the detection operation of the electrical resistance of the third image forming unit Sc in S104, in S201, the CPU 151 causes the charging roller 2c of the third image forming unit Sc to have a negative polarity side as shown in FIG. A large voltage Vt is applied. On the other hand, the CPU 151 applies a large voltage Vtd to the charging rollers 2a, 2b, and 2d of the first, second, and fourth image forming units Sa, Sb, and Sd on the positive polarity side. Thus, in the first, second, and fourth image forming portions Sa, Sb, and Sd, no discharge occurs between the photosensitive drum 1 and the primary transfer brush 5, and no current flows. Therefore, the primary transfer brush 5 And the photosensitive drum 1 are in a pseudo-separated state. As a result, as shown in FIG. 5F, an electric current is allowed to flow only through the third image forming unit Sc, and the electrical resistance Rc of the third image forming unit Sc is calculated by the calculation formula Rc = Vct / It. .

  Further, in the electric resistance detection operation of the fourth image forming unit Sd in S105, in S201, the CPU 151 causes the negative polarity side to be connected to the charging roller 2d of the fourth image forming unit Sd as shown in FIG. A large voltage Vt is applied. On the other hand, the CPU 151 applies a large voltage Vtd on the positive polarity side to the charging rollers 2a, 2b, and 2c of the first, second, and third image forming units Sa, Sb, and Sc. Thus, in the first, second, and third image forming portions Sa, Sb, and Sc, no discharge occurs between the photosensitive drum 1 and the primary transfer brush 5, and no current flows. Therefore, the primary transfer brush 5 And the photosensitive drum 1 are in a pseudo-separated state. As a result, as shown in FIG. 5 (h), an electric current is allowed to flow only through the fourth image forming unit Sd, and the electric resistance Rd of the fourth image forming unit Sd is calculated by the calculation formula Rd = Vdt / It. .

  Thereafter, the CPU 151 uses the calculation results of the electrical resistances Ra, Rb, Rc, and Rd of the image forming units Sa, Sb, Sc, and Sd obtained in S102 to S105 to perform the primary transfer brushes 5a and 5b during image formation. The primary transfer voltage Vp applied to 5c and 5d is determined (S106). Here, in this embodiment, the primary transfer voltage Vp at the time of image formation optimized in view of the electric resistances Ra, Rb, Rc, Rd of the image forming portions Sa, Sb, Sc, Sd and various information. To decide.

  Specifically, for example, the image forming unit that has the highest electrical resistance from the electric resistances Ra to Rd of the image forming units Sa to Sd and that requires a high primary transfer voltage on the positive polarity side in order to obtain good transfer efficiency. According to S, the primary transfer voltage at the time of image formation can be determined. In this case, for example, the CPU 151 uses information (table or the like) that associates the electrical resistance of the image forming unit S with the highest electrical resistance preset in the ROM 152 and the primary transfer voltage at the time of image formation. The primary transfer voltage is determined. A plurality of pieces of information such as this table may be provided corresponding to environmental information (such as temperature, humidity, or temperature and humidity information). Then, the CPU 151 can determine the primary transfer voltage at the time of image formation according to the detection result of the temperature / humidity sensor (not shown) as the environment detection means in addition to the detection result of the electrical resistance of the image forming unit S. It may be.

  After that, the CPU 151 starts image formation, applies the primary transfer voltage Vp at the time of image formation to the primary transfer brushes 5a to 5d at a predetermined timing, and then on the photosensitive drums 1 of the image forming portions Sa to Sd. The toner image is transferred onto the outer peripheral surface of the intermediate transfer belt 6 (S107). At the time of image formation, the CPU 151 applies the charging voltage Vt to the charging roller 2 and controls the primary transfer power supply 50 at a constant voltage with the primary transfer voltage Vp determined as described above (FIGS. 5I and 5J). ).

  Thereafter, the CPU 151 transfers the toner image on the photosensitive drum 1 of each of the image forming units Sa to Sd to the outer peripheral surface of the intermediate transfer belt 6, and then the primary transfer voltage at the time of image formation to each of the primary transfer brushes 5a to 5d. The application of Vp is terminated and the image formation is terminated (S108).

  Thus, according to the present embodiment, the electrical resistances of the image forming units Sa, Sb, Sc, and Sd can be individually detected. As a result, it is possible to select an optimal primary transfer voltage during image formation that reflects the individual electric resistances of the image forming portions Sa, Sb, Sc, and Sd. Therefore, the primary transfer power supply 50 and the primary transfer current detection circuit 51 can be shared to reduce the size and cost of the apparatus, and form a more stable and good image.

  In addition to calculating the electric resistances of the image forming units Sa, Sb, Sc, and Sd as described above, an operation for obtaining the combined resistance of all the image forming units Sb, Sb, Sc, and Sd is performed. May be. In this case, in the electric resistance detection operation in ATVC, a large voltage Vt is applied to the charging roller 2 on the negative polarity side in all the image forming units Sa, Sb, Sc, Sd, and all the image forming units Sa, Sb, A current is passed through Sc and Sd. Then, the combined resistance is calculated from the current value flowing at this time and the generated voltage value. Based on the calculation results of the individual electrical resistances of the image forming units Sa, Sb, Sc, and Sd and the calculation results of the combined resistances of all the image forming units Sb, Sb, Sc, and Sd, The primary transfer voltage Vp applied to the primary transfer brushes 5a, 5b, 5c, and 5d is determined.

  Specifically, for example, an optimum primary transfer voltage value is determined based on the combined resistance if it is normal, and the optimum primary transfer voltage value is determined according to other considerations based on the individual electrical resistance of each image forming unit. It is possible to correct the primary transfer voltage value. Other consideration factors include the risk of waste toner leakage described later, suppression of retransfer, and the like. Detailed control will be described later.

  In the above description, the detection unit detects the voltage value when the transfer power source applies a constant current controlled voltage to the transfer unit of the plurality of image forming units. However, the detection unit is not limited thereto. Instead, it may be one that detects a current value when a constant voltage controlled voltage is applied. In the above description, the electric resistance of the image forming unit is calculated from the detection result of the detecting unit. However, it is only necessary to detect information corresponding to the electric resistance of the image forming unit. Examples of information corresponding to the electrical resistance of the image forming unit include a detection result of current and voltage in addition to a calculation result of electrical resistance.

  In the above description, the electrical resistances of all the image forming units are individually detected. However, the electrical resistances of the individual image forming units or the plurality of image forming units, which are at least a part of the plurality of image forming units. The combined resistance can be detected. It is only necessary that the primary transfer voltage at the time of image formation can be determined using the detection results of at least some of the electrical resistances among the plurality of image forming units.

7). Application Example Next, an application example of the electric resistance detection operation of each image forming unit Sa, Sb, Sc, Sd in the above-described ATVC control will be described.

7-1. Judgment of Waste Toner Leakage For example, an image forming unit in which toner consumption is extremely reduced may be generated by forming an image biased to a certain color. In this case, since the toner consumption of the image forming unit other than the image forming unit is normal, only the image forming unit of the image forming unit other than the image forming unit in which the toner consumption is extremely small is a new image. Will be replaced with the forming unit. In this case, the use amount of the image forming unit that is not replaced increases, and the amount of waste toner increases. Therefore, there is a risk that the waste toner leaks out from the waste toner container (waste toner leakage risk).

  Therefore, in consideration of reducing the risk of waste toner leakage, it is desirable to select an optimal primary transfer voltage to be applied to the primary transfer member of each image forming unit during image formation. For example, it is possible to increase the transfer efficiency in the image forming unit having the risk of waste toner leakage and reduce the amount of waste toner in the image forming unit to reduce the risk of waste toner leakage.

  However, in the conventional image forming apparatus in which the primary transfer power source and the primary transfer current detection circuit are shared, the electrical resistance of each image forming unit cannot be detected individually. It is difficult to select an optimal primary transfer voltage reflecting the above.

  On the other hand, according to the ATVC control of this embodiment, the electrical resistance of each image forming unit Sa, Sb, Sc, Sd is calculated, and the waste of each image forming unit Sa, Sb, Sc, Sd is calculated from the value. The possibility of waste toner leaking from the toner container 71 (waste toner leak risk) can be determined.

  For example, a case is assumed where images with a low color ratio of the fourth image forming unit Sd are continuously printed.

  In this case, the first, second, and third image forming units Sa, Sb, and Sc other than the fourth image forming unit Sd consume more toner than the fourth image forming unit Sd. Therefore, in the first, second, and third image forming units Sa, Sb, and Sc, the process cartridge 110 is more frequently replaced with a new one than in the fourth image forming unit Sd. The waste toner container 71 attached to is in a state where there is no waste toner. On the other hand, the waste toner container 71 attached to the process cartridge 110 of the fourth image forming unit Sd with a low replacement frequency is in a state where there is waste toner for a long time, and the waste toner container 71 in the fourth image forming unit Sd. The risk that waste toner leaks from the toner becomes relatively high. Therefore, it is desired to reduce such a risk of waste toner leakage.

  Here, in the fourth image forming unit Sd where the replacement frequency of the process cartridge 110 is low, the surface of the photosensitive drum 1 attached to the process cartridge 110 is shaved or roughened as the usage amount increases. As a result, the electric resistances of the fourth image forming unit Sd and the first, second, and third image forming units Sa, Sb, and Sc are clearly different.

  Therefore, the electric resistance of the fourth image forming unit Sd is changed to the other image forming units Sa, Sb, Sc by performing the detection operation of the electric resistance of each image forming unit Sa, Sb, Sc, Sd in the above-described ATVC control. , It can be detected that the electric resistance is relatively higher than the electrical resistance of Sd.

  Then, based on the detection result, the state of the waste toner in the waste toner container 71 of the fourth image forming unit Sd is predicted, and an image that reduces the risk of waste toner leakage in the fourth image forming unit Sd. An optimal primary transfer voltage Vp at the time of formation can be selected.

(Comparison result)
A result of comparing the waste toner leakage risk between the case where the resistance detection result in the conventional ATVC control is used (comparative example) and the case where the resistance detection result in the ATVC control of this embodiment is used (this embodiment) will be described. . The configuration and operation of the image forming apparatus of the comparative example are substantially the same except that the conventional method in which the electrical resistance of each image forming unit Sa, Sb, Sc, Sd is not individually calculated in the ATVC control is adopted. This is the same as the image forming apparatus of this embodiment. Table 1 shows the results of the comparative experiment.

  Here, the waste toner leakage risk is defined as a percentage of the capacity of the waste toner with respect to the capacity of the waste toner container 71. When the waste toner leakage risk exceeds 98%, the image forming apparatus 100 is at a level to issue a warning, and when about 200 images are formed, the waste toner leakage risk is 100%. When the risk of waste toner leakage becomes 100%, waste toner may leak out of the waste toner container 71 and the inside of the apparatus may be contaminated. In some cases, an image defect may occur.

  The comparative experiment was conducted for the following case (1) and case (2).

  Case (1) is a case where the process cartridges 110 of all the image forming units Sa, Sb, Sc, and Sd are new. The electrical resistance of each of the image forming units Sa, Sb, Sc, and Sd is Ra = 50 MΩ, Rb = 50 MΩ, Rc = 50 MΩ, and Rd = 50 MΩ.

  Case (2) is a case where the process cartridge 110 of the fourth image forming unit Sd is continuously used. The electrical resistances of the image forming units Sa, Sb, Sc, and Sd are Ra = 50 MΩ and Rb = 50 MΩ, respectively. Rc = 50 MΩ and Rd = 70 MΩ.

  Table 1 shows a list of electrical resistance (separately measured) of each image forming unit, electrical resistance of each image forming unit detected in the ATVC control, transferability and waste toner leakage risk.

  Here, in this embodiment, in the electric resistance detection operation of each image forming unit in the ATVC control, Vt = −600 [V] is applied as the charging voltage in the image forming unit that detects the electric resistance. On the other hand, in the image forming unit in which the primary transfer brush 5 and the photosensitive drum 1 are in a pseudo-separated state, Vtd = 0 [V] is applied as the charging voltage. In this embodiment and the comparative example, the upper limit of the output voltage of the primary transfer power supply 50 in the ATVC control is set to 10 KV. In the present example and the comparative example, the target current It in the ATVC control is 10 μA.

  In the comparative experiment, first, 20,000 sheets of images were formed. The image printing rate was set to 10% for the first, second, and third image forming portions Sa, Sb, and Sc, and 0.1% for the fourth image forming portion Sd. Thereafter, the image printing rate of all the image forming portions Sa, Sb, Sc, and Sd was fixed to 10%, and image formation was continued. The detection of the waste toner leakage risk was performed when the remaining toner amount of the developing device 4 of the fourth image forming unit Sd was less than 3%.

  First, in case (1), the electrical resistances of all the image forming units Sa, Sb, Sc, and Sd are 50 MΩ. This means that the process cartridge 110 is in a new state in all the image forming units Sa, or the process cartridges 110 of all the image forming units Sa, Sb, Sc, and Sd are replaced at the same time. At this time, when the conventional ATVC control that detects the combined resistance (total resistance) of all the image forming units Sa, Sb, Sc, and Sd at a time is used, the output voltage value when the 10 μA target current It flows is 125 V. It is. On the other hand, when the ATVC control of the present embodiment that can individually detect the electric resistances of the image forming units Sa, Sb, Sc, and Sd is used, the output voltage value when the target current It of 10 μA flows is the value of all the image forming units. It is 500V in Sa, Sb, Sc, and Sd. Thereby, it can be determined that the states of all the image forming units Sa, Sb, Sc, and Sd are the same.

  That is, in the case of the system as in the case (1), it can be determined that the process cartridge 110 and the transfer device 60 are almost in the same state. Therefore, in the case of the system as in the case (1), even when an optimal primary transfer voltage at the time of image formation is selected without considering reducing the risk of waste toner leakage, in any of the image forming portions S. The risk of waste toner leakage is considered low.

  Next, the case (2) will be examined on the basis that the output voltage value in the ATVC control of the case (1) is 125 V in the conventional ATVC control and 500 V in the ATVC control of the present embodiment.

  In the case (2), the electric resistance of the fourth image forming unit Sd is as high as 70 MΩ, and the process cartridge 110 is in an increased usage amount. In the other image forming units Sa, Sb, and Sc, the electric resistance is 50 MΩ. The process cartridge 110 is almost new. In this state, the conventional ATVC control and the ATVC control of this embodiment were performed. As a result, the output voltage value in the ATVC control is 135 V in the conventional ATVC control. On the other hand, in the ATVC control of the present embodiment, the voltage is 500 V in the first, second, and third image forming units Sa to Sc, but is as high as 700 V in the fourth image forming unit Sd.

  When this result is compared with the case (1), in the conventional ATVC control, although the output voltage value in the ATVC control is about 10 V higher than the case (1), the fluctuation range is relatively small. For this reason, such fluctuations in the output voltage value are recognized as being within a range of variation, and the primary transfer voltage during image formation may be controlled in the same manner as in the case (1). In other words, since the fourth image forming unit Sd has high electrical resistance, it is desirable to apply a higher primary transfer voltage to the positive polarity side than when the process cartridge 110 is new in order to improve transfer efficiency. However, according to the conventional ATVC control, it is determined that the process cartridge 110 of the fourth image forming unit Sd is the same as a new one. Therefore, an image defect due to a transfer defect may occur in the fourth image forming unit Sd. Further, the shortage of the primary transfer voltage in the fourth image forming unit Sd means that the toner remaining on the photosensitive drum 1 after the primary transfer process in the fourth image forming unit Sd increases. This increases the risk of waste toner leakage in the fourth image forming unit Sd.

  On the other hand, in the ATVC control of the present embodiment, the output voltage value in the ATVC control is 500 V in the case (1), but the value fluctuates greatly as 700 V in the case (2). That is, it can be determined that the fourth image forming unit Sd whose output voltage value in the ATVC control is 700 V is different from the other image forming units Sa, Sb, and Sc. Accordingly, it is possible to select an optimal primary transfer voltage in accordance with the fourth image forming unit Sd in which an output voltage value of 700 V has been detected by the ATVC control.

  Specifically, in accordance with the fourth image forming unit Sd having a high electric resistance, the primary transfer voltage at the time of image formation is slightly positive with respect to the optimum value when the risk of waste toner leakage is not considered. Set to higher. For example, when there is an image forming unit in which the electrical resistance is higher than a predetermined value, the CPU 151 determines the primary transfer voltage at the time of image formation to a higher value on the positive polarity side by a predetermined amount than the optimum value. . At this time, the optimum value of the primary transfer voltage at the time of image formation when the risk of waste toner leakage is not taken into consideration is determined based on the individual electric resistances of the image forming portions Sa, Sb, Sc, and Sd as described above. It may be that determined based on the combined resistance. As a result, the image forming units Sa, Sb, and Sc other than the fourth image forming unit Sd have a low electrical resistance, so that a slightly higher primary transfer voltage is applied to the positive polarity side. The risk of waste toner leakage in the image forming unit Sd can be reduced.

7-2. Retransfer suppression In the above example, reduction of the waste toner leakage risk has been described. However, there is a case where it is not necessary to consider the waste toner leakage risk, for example, when the capacity of the waste toner container 71 is sufficiently large. In such a case, an optimal primary transfer voltage can be selected for other risks of concern.

  For example, it can be considered to reduce image defects due to retransfer. Retransfer is the following phenomenon. When the toner image transferred to the intermediate transfer body in the primary transfer section on the upstream side in the moving direction of the intermediate transfer body passes through the primary transfer section on the downstream side of the image forming section in the same direction, the downstream side The primary transfer voltage in the image forming unit may be larger than the appropriate primary transfer voltage. In such a case, the toner image from the upstream image forming unit receives a discharge from the primary transfer unit of the downstream image forming unit, thereby reversing the charging polarity of the toner. As a result, the toner whose charging polarity is reversed is reversely transferred, for example, from the outer peripheral surface of the intermediate transfer member to the photosensitive member of the image forming unit further downstream than the downstream image forming unit. In some cases, the toner density of the toner image is reduced and the image density is lowered.

  In this case, if the electrical resistance of each image forming unit can be detected individually, the optimum application to the primary transfer member of each image forming unit in consideration of suppressing reverse transfer in the downstream image forming unit is also considered. Primary transfer voltage can be selected.

  However, in the conventional image forming apparatus in which the primary transfer power source and the primary transfer current detection circuit are shared, the electrical resistance of each image forming unit cannot be detected individually. It is difficult to select an optimal primary transfer voltage reflecting the above.

  On the other hand, according to the ATVC control of the present embodiment, the electrical resistances of the image forming units Sa, Sb, Sc, and Sd can be individually detected. Accordingly, it is possible to select an optimal primary transfer voltage in accordance with the downstream image forming unit in which reversal of the charging polarity of the toner is a concern. Specifically, the primary transfer voltage at the time of image formation is slightly lower on the positive polarity side than the optimum value when suppression of retransfer is not taken into account, in accordance with the image forming portion where the charging polarity of the toner is concerned. Set to (higher on the negative polarity side). Thereby, the image defect by retransfer can be suppressed.

  For example, when the electric resistances of the image forming units are greatly different, selecting a primary transfer voltage optimized for an image forming unit having a high electric resistance may cause the transfer electric field in other image forming units to become too large. . Thereby, the retransfer as described above may be deteriorated. On the other hand, according to the ATVC control of the present embodiment, the electrical resistance of each of the image forming units Sa, Sb, Sc, and Sd can be detected individually, so that the above-described deterioration of retransfer can be predicted. For example, when there is an image forming unit in which the electrical resistance is higher than a predetermined value, the CPU 151 reduces the primary transfer voltage at the time of image formation by a predetermined amount on the positive polarity side (negative polarity side). To a higher value. At this time, the optimum value of the primary transfer voltage at the time of image formation when the risk of waste toner leakage is not taken into consideration is determined based on the individual electric resistances of the image forming portions Sa, Sb, Sc, and Sd as described above. It may be that determined based on the combined resistance. As described above, when it is predicted that the retransfer is deteriorated, the primary transfer voltage value is selected by selecting a value slightly lower on the positive polarity side than the optimum value when the retransfer suppression is not considered. Deterioration of transcription can be suppressed.

8). Effects As described above, in this embodiment, the image forming apparatus 100 includes the plurality of image forming units S. The plurality of image forming units S includes a photosensitive member 1, a charging unit 2 that charges the photosensitive member 1 by applying a voltage, a developing unit 4 that supplies toner to the photosensitive member 1 to form a toner image, and the photosensitive member 1. Each has transfer means 5 for transferring the toner image to the transfer body 6. The image forming apparatus 100 also has a common transfer power supply 50 that applies a voltage to the transfer means 5 of the plurality of image forming units S. Further, the image forming apparatus 100 applies a voltage value or a constant voltage controlled voltage when the transfer power supply 50 is applying a constant current controlled voltage to the transfer means 5 of the plurality of image forming units S. Detection means (primary transfer current detection circuit 51, etc.) for detecting the current value when there is a current. The image forming apparatus 100 also includes a setting unit (CPU 1) 151 that sets a voltage applied to the charging unit 2. The setting unit 151 performs the following setting in a state where a common voltage is applied from the transfer power supply 50 to the plurality of transfer units 5. That is, the voltage applied to the charging unit 2 of at least one image forming unit among the plurality of image forming units S is determined by the potential difference between the photosensitive member 1 and the transfer unit 5 of the at least one image forming unit. Set to be greater than the potential difference. At the same time, the setting unit 151 applies the voltage applied to the charging unit 2 of the remaining image forming unit S among the plurality of image forming units S to the photoconductor 1 and the transfer unit 5 of the remaining image forming unit S. Is set to be less than the discharge start potential difference. Further, the image forming apparatus 100 includes an execution unit (CPU) that performs detection of the voltage value or the current value by the detection unit in a state where a potential difference greater than the discharge start potential difference and a potential difference less than the discharge start potential difference are formed. 151).

  In the present embodiment, the setting unit 151 sets the following potential differences so that the potential difference equal to or greater than the discharge start potential difference is formed in all of the plurality of image forming units S. This is performed by sequentially changing the image forming portion where a potential difference equal to or greater than the discharge start potential difference is formed. That is, setting is made so that a potential difference equal to or greater than the discharge start potential difference is formed in one image forming portion S, and a potential difference less than the discharge start potential difference is formed in the remaining image forming portions S. In this embodiment, the execution unit 151 causes the detection unit to detect the voltage value or the current value every time the image forming unit S in which a potential difference greater than the discharge start potential difference is formed is changed. In this embodiment, the image forming apparatus 100 determines a voltage (CPU) that determines the voltage that the transfer power supply 50 applies to the transfer units 5 of the plurality of image forming units S during image formation based on the next detection result. 151. That is, it is a detection result of the detection means in a state where a potential difference greater than the discharge start potential difference and a potential difference less than the discharge start potential difference are formed.

  Further, the setting unit 151 further sets the voltage applied to the charging unit 2 of the plurality of image forming units S so that a potential difference equal to or greater than the discharge start potential difference is formed in all the plurality of image forming units S. It may be possible. Further, the execution unit 151 further executes detection of the voltage value or the current value by the detection unit in a state where a potential difference equal to or greater than the discharge start potential difference is formed in all of the plurality of image forming units S. Good. In this case, the determination unit 151 determines a voltage to be applied to the transfer unit 5 of the plurality of image forming units S by the transfer power source 50 at the time of image formation based on the following detection results. That is, the detection result of the detection means in a state where a potential difference greater than the discharge start potential difference and a potential difference less than the discharge start potential difference is formed, and a potential difference greater than the discharge start potential difference is formed in all of the plurality of image forming portions S. It is the detection result of the detection means in the state.

  Thus, according to the present embodiment, the electrical resistances of the image forming units Sa, Sb, Sc, and Sd can be individually detected. Thereby, it is possible to grasp the various situations of the image forming units Sa, Sb, Sc, Sd. Accordingly, it is possible to select an optimal primary transfer voltage at the time of image formation in consideration of various situations of the image forming portions Sa, Sb, Sc, and Sd. For this reason, the primary transfer power supply 50 and the primary transfer current detection circuit 51 can be used in common to reduce the size and cost of the apparatus, and more stably form a good image. As described above, according to the present exemplary embodiment, even if a transfer power source that outputs a transfer voltage to a plurality of image forming units is shared, information corresponding to the electrical resistance of some image forming units can be acquired.

  Further, according to this embodiment, the primary transfer brush 5 is actually separated from the photosensitive drum 1 in each primary transfer portion N1 in order to individually detect the electrical resistance of each image forming portion Sa, Sb, Sc, Sd. There is no need. Therefore, when using a plurality of image forming sections S, for example, when forming a full-color image, it is not necessary to perform the contact / separation operation of the primary transfer brush 5 in order to detect the individual electric resistance of each image forming section S. . Therefore, it is possible to simplify the apparatus configuration and shorten the time (first print time) from the image formation start instruction until the image is actually output. The image forming apparatus has a contact / separation mechanism for contacting or separating the primary transfer brush 5 with respect to the photosensitive drum 1 in order to prevent wear and deterioration of an image forming unit that is not used when forming a monochrome image. It may be.

Example 2
Next, another embodiment of the present invention will be described. The basic configuration and operation of the image forming apparatus of this embodiment are the same as those of the first embodiment. Accordingly, elements having the same or corresponding functions and configurations as those of the image forming apparatus according to the first embodiment are denoted by the same reference numerals, and detailed description thereof is omitted.

  The present embodiment is different from the first embodiment in that the detection result of the electrical resistance of each image forming unit Sa, Sb, Sc, Sd is reflected not only in the primary transfer voltage but also in both the primary transfer voltage and the charging voltage. .

  FIG. 7 shows the procedure of ATVC control in this embodiment. The procedure of S101 to S105 of ATVC control of the present embodiment shown in FIG. 7 is the same as the procedure of S101 to S105 of ATVC control of the first embodiment shown in FIG.

Thereafter, the CPU 151 supplies currents Ia, Ib, Ic, and Id that flow through the image forming units Sa, Sb, Sc, and Sd from the electrical resistances Ra, Rb, Rc, and Rd of the image forming units Sa, Sb, Sc, and Sd. The calculation is performed according to the following formula (S301). At this time, the difference in electrical resistance increased or decreased in each image forming unit Sa, Sb, Sc, Sd is Rha, Rhb, Rhc, Rhd, the flowing current is Ia, Ib, Ic, Id, and the applied voltage is Vt. To do.
Image forming unit Sa: Ia = Vt / (Ra + Rha)
Image forming unit Sb: Ib = Vt / (Rb + Rhb)
Image forming unit Sc: Ic = Vt / (Rc + Rhc)
Image forming unit Sd: Ic = Vt / (Rd + Rhd)

Next, the CPU 151 calculates the potentials Va, Vb, Vc, and Vd that appear on the photosensitive drum 1 side in each of the image forming units Sa, Sb, Sc, and Sd by the following formula (S302).
Image forming unit Sa: Va = Rha × Ia
Image forming unit Sb: Vb = Rhb × Ib
Image forming unit Sc: Vc = Rhc × Ic
Image forming unit Sd: Vd = Rhd × Id

  Next, the CPU 151 calculates the potential difference of the photosensitive drum 1 in each of the image forming units Sa, Sb, Sc, and Sd (S303).

  For example, at the time of monochromatic image formation (monochromatic image formation), image formation is performed with only the primary transfer brush 5d in the fourth image forming unit Sd in contact with the photosensitive drum 1 via the intermediate transfer belt 6. Shall be done. In this case, when only monochromatic image formation is continued, the electrical resistance of the primary transfer brush 5d of the fourth image forming unit Sd becomes the primary transfer brush 5a, 5b, other image forming units Sa, Sb, Sc, Sd. May be higher than 5c. As a result, the shared voltage in the primary transfer unit brush 5d of the fourth image forming unit Sd is increased, and the transfer electric field between the photosensitive drum 1 and the primary transfer brush 5 is reduced in the fourth image forming unit Sd. It may be smaller than the image forming portions Sa, Sb, and Sc. Therefore, a transfer failure may occur in the fourth image forming unit Sd.

Therefore, for example, using the first image forming unit Sa as a reference, the difference ΔV in the potential of the photosensitive drum 1 between the first image forming unit Sa and the fourth image forming unit Sd is calculated by the following calculation formula.
ΔV = | Va−Vd |

  In the above calculation formula, the potential of the photosensitive drum 1 of the first image forming unit Sa is used as a reference as an example. When it is desired to use the potential of the photosensitive drum of another image forming unit as a reference, the potential part of the image forming unit serving as a reference in the above calculation formula may be changed.

  Next, the CPU 151 sets the charging voltage and the developing voltage to the image forming units Sa, Sb, Sc by the difference ΔV of the potential of the photosensitive drum 1 in the image forming units Sa, Sb, Sc, Sd calculated by the above formula. , Sd (S304). Thereby, the transfer electric field (discharge potential difference) between the photosensitive drum 1 and the primary transfer brush 5 can be made constant in each of the image forming portions Sa, Sb, Sc, and Sd.

  For example, it is assumed that the electrical resistance of the first image forming unit Sa does not change and the electrical resistance of the fourth image forming unit Sd changes as the usage amount of the image forming apparatus 100 increases. At this time, the potential difference ΔV of the photosensitive drum 1 between the first image forming unit Sa and the fourth image forming unit Sd can be calculated as | Va−Vd | from the above formula. In this case, | Va−Vd |, which is the difference ΔV in the potential of the photosensitive drum 1, is fed back to the charging voltage and the developing voltage of the fourth image forming unit Sd. Accordingly, the difference in potential of the photosensitive drum 1 between the first image forming unit Sa and the fourth image forming unit Sd, that is, the discharge potential difference can be made constant. Therefore, a more stable and good image can be formed.

  More specifically, it is assumed that the charging voltage in the first image forming unit Sa is Vfa and the developing voltage is Vga. The charging voltage in the fourth image forming unit Sd is Vfd, and the development voltage is Vgd. In this case, the charging voltage Vfd in the fourth image forming unit Sd is Vfd = Vfa + | Va−Vd |, and the developing voltage Vgd is Vgd = Vga + | Va−Vd |. Thereby, the difference ΔV in the potential of the photosensitive drum 1 between the first image forming unit Sa and the fourth image forming unit Sd, that is, the discharge potential difference can be made constant. Therefore, a more stable and good image can be formed.

  Thereafter, the CPU 151 sets the determined primary transfer voltage, charging voltage value, and development voltage value to start image formation (S305), ends these outputs, and ends the image formation operation (S306).

  In addition, in view of the electric resistance calculated in the voltage detection operation in the ATVC control, when it is desired to change the reference image forming unit, the voltage part of the reference image forming unit in the above calculation formula may be changed. For example, as described above, an image forming unit having no change in electrical resistance (for example, a change less than a predetermined value with respect to a nominal value) can be used as a reference.

  Also in this embodiment, as in the first embodiment, the combined resistance of each image forming portion Sa, Sb, Sc, Sd is detected, and the primary transfer voltage at the time of image formation is determined using the detection result as well. You may make it do.

  Thus, in this embodiment, for example, the electric resistance Rd of the fourth image forming unit Sd and the electric resistances Ra, Rb, Rc of the first, second, and third image forming units Sa, Sb, and Sc are used. Detect. Further, from the currents flowing through the image forming units Sa, Sb, Sc, and Sd, finally, the potential difference of the photosensitive drum 1 in each of the image forming units Sa, Sb, Sc, and Sd is calculated, and based on the difference, The charging voltage and developing voltage of each image forming unit Sa, Sb, Sc, Sd are determined. As a result, an optimal transfer electric field suitable for each of the image forming portions Sa, Sb, Sc, and Sd can be formed, and a more stable and good image can be formed. In this embodiment, the CPU 151 of the control unit 150 also functions as a charging voltage determination unit and a development voltage determination unit.

  As described above, the image forming apparatus 100 may include the charging voltage determining unit (CPU) 151 that determines the voltage applied to the charging unit 2 during image formation as follows. That is, the charging voltage determination unit 151 is applied to the charging unit 2 during image formation based on the detection result of the detection unit in a state where a potential difference greater than the discharge start potential difference and a potential difference less than the discharge start potential difference are formed. Determine the voltage. Further, the image forming apparatus 100 may include a developing voltage determining unit (CPU) 151 that determines a voltage applied to the developing unit 4 during image formation as follows. That is, the developing voltage determining unit 151 is applied to the developing unit 4 at the time of image formation based on the detection result of the detecting unit in a state where a potential difference greater than the discharge start potential difference and a potential difference less than the discharge start potential difference are formed. Determine the voltage.

  Thus, according to the present embodiment, first, the currents flowing through the image forming units Sa, Sb, Sc, and Sd are calculated. Next, the difference in potential of the photosensitive drum 1 in each image forming unit Sa, Sb, Sc, Sd is calculated. Based on the difference, the charging voltage and developing voltage of each image forming unit Sa, Sb, Sc, Sd are determined. Thereby, even when the electric resistances of the image forming units Sa, Sb, Sc, and Sd change, the optimum primary transfer electric field that reflects the individual electric resistances of the image forming units Sa, Sb, Sc, and Sd. Can be formed with a charging voltage. Correspondingly, an optimum developing voltage can be supplied. As a result, a good image can be formed more stably.

Others While the present invention has been described with reference to specific embodiments, the present invention is not limited to the above-described embodiments.

  In the above-described embodiment, an example in which the present invention is applied to an intermediate transfer type image forming apparatus has been described. However, the present invention can also be applied to a direct transfer type image forming apparatus, and the same effects as in the above-described embodiments can be obtained. FIG. 8 shows a schematic configuration of a main part of an example of a direct transfer type image forming apparatus. 8, elements having the same or corresponding functions and configurations as those of the intermediate transfer type image forming apparatus shown in FIG. The direct transfer type image forming apparatus has, for example, an endless transfer belt 106 as a recording material carrier instead of the intermediate transfer belt 6 in the above-described embodiment. Further, a transfer member such as the transfer brush 5 corresponding to the primary transfer brush 5 in the above-described embodiment is pressed against the photosensitive drum 1 via the transfer belt 106. The toner images formed on the photosensitive drums 1a, 1b, 1c, and 1d in the image forming portions Sa, Sb, Sc, and Sd are transferred onto the transfer belt N and carried on the transfer belt 106. The images are sequentially superimposed and transferred onto P. In such a direct transfer type image forming apparatus, ATVC control is performed to control the transfer voltage. Therefore, by applying the present invention in the same manner as in the case of the intermediate transfer type image forming apparatus in the above-described embodiment, the electrical resistance of each image forming unit Sa, Sb, Sc, Sd can be individually detected in the ATVC control. The optimum transfer voltage at the time of image formation can be selected according to the detection result.

DESCRIPTION OF SYMBOLS 1 Photosensitive drum 2 Charging roller 3 Laser scanner 4 Developing apparatus 5 Primary transfer brush 50 Primary transfer power supply 51 Primary transfer current detection circuit

Claims (9)

A photosensitive member; a charging unit that charges the photosensitive member by applying a voltage; a developing unit that supplies toner to the photosensitive member to form a toner image; and a transfer unit that transfers the toner image from the photosensitive member to a transfer target. A plurality of image forming units each having
A common transfer power supply for applying a voltage to the transfer means of the plurality of image forming units;
Detecting a voltage value when the transfer power source is applying a constant current controlled voltage to the transfer means of the plurality of image forming units or detecting a current value when applying a constant voltage controlled voltage Means,
In a state where a common voltage is applied to the plurality of transfer units from the transfer power source, a voltage applied to the charging unit of at least one image forming unit among the plurality of image forming units is set to the at least one A potential difference between the photosensitive member of the image forming unit and the transfer unit is set to be equal to or greater than a discharge start potential difference, and is applied to the charging unit of the remaining image forming unit among the plurality of image forming units. Setting means for setting a voltage difference between the photosensitive member of the remaining image forming unit and the transfer means to be less than a discharge start potential difference;
Execution means for executing detection of the voltage value or the current value by the detection means in a state where a potential difference equal to or greater than the discharge start potential difference and a potential difference less than the discharge start potential difference are formed;
An image forming apparatus comprising: The setting unit forms a potential difference equal to or greater than the discharge start potential difference in one image forming unit among the plurality of image forming units, and starts the discharge in the remaining image forming unit among the plurality of image forming units. The voltage applied to the charging means of the plurality of image forming units is set so that a potential difference less than the potential difference is formed. A potential difference equal to or greater than the discharge start potential difference is formed in all of the plurality of image forming units. To sequentially change the image forming portion in which a potential difference equal to or greater than the discharge start potential difference is formed,
The execution unit causes the detection unit to detect the voltage value or the current value each time an image forming unit in which a potential difference equal to or greater than the discharge start potential difference is formed is changed. The image forming apparatus described.   Based on a detection result of the detection unit in a state where a potential difference equal to or greater than the discharge start potential difference and a potential difference less than the discharge start potential difference is formed, the transfer power supply is applied to the transfer unit of the plurality of image forming units during image formation. The image forming apparatus according to claim 1, further comprising a determining unit that determines a voltage to be applied.   A charging voltage determination unit that determines a voltage applied to the charging unit during image formation based on a detection result of the detection unit in a state where a potential difference equal to or greater than the discharge start potential difference and a potential difference less than the discharge start potential difference is formed. The image forming apparatus according to claim 3, further comprising:   A developing voltage determining unit that determines a voltage applied to the developing unit at the time of image formation based on a detection result of the detecting unit in a state where a potential difference equal to or greater than the discharge start potential difference and a potential difference less than the discharge start potential difference is formed. The image forming apparatus according to claim 3, wherein the image forming apparatus includes: The setting unit may further set a voltage applied to the charging unit of the plurality of image forming units so that a potential difference equal to or greater than the discharge start potential difference is formed in all of the plurality of image forming units. And
The execution unit further causes the detection unit to detect the voltage value or the current value in a state where a potential difference equal to or greater than the discharge start potential difference is formed in all of the plurality of image forming units. Item 3. The image forming apparatus according to Item 1 or 2.   A detection result of the detection means in a state where a potential difference greater than the discharge start potential difference and a potential difference less than the discharge start potential difference is formed, and a potential difference greater than the discharge start potential difference is formed in all of the plurality of image forming units. 7. The apparatus according to claim 6, further comprising: a determination unit that determines a voltage to be applied to the transfer unit of the plurality of image forming units by the transfer power source during image formation based on a detection result of the detection unit in a state. The image forming apparatus described in 1.   A detection result of the detection means in a state where a potential difference greater than the discharge start potential difference and a potential difference less than the discharge start potential difference is formed, and a potential difference greater than the discharge start potential difference is formed in all of the plurality of image forming units. The image forming apparatus according to claim 7, further comprising a charging voltage determining unit that determines a voltage applied to the charging unit during image formation based on a detection result of the detecting unit in a state.   A detection result of the detection means in a state where a potential difference greater than the discharge start potential difference and a potential difference less than the discharge start potential difference is formed, and a potential difference greater than the discharge start potential difference is formed in all of the plurality of image forming units. The image forming apparatus according to claim 7, further comprising: a developing voltage determining unit that determines a voltage applied to the developing unit during image formation based on a detection result of the detecting unit in a state. .

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