Image forming apparatus

Abstract

To effectively prevent an image defect caused by AC banding.SOLUTION: An image forming apparatus 1 has: an image carrier 11; a developing device 15; a transfer member 17; a transfer power supply 18; a fixing device 20 that includes a heating member 21 provided with a heat source 23, wherein the heat source 23 is applied with voltage from an AC power supply to generate heat and heats a recording material R through the heating member 21; a detection unit 13 that is connected between the image carrier 11 and the GND without passing through the transfer member 17, and detects current or voltage; and a control unit 10 that controls the transfer power supply 18. When a toner image is transferred to the recording material R from the image carrier 11 at a transfer part Nt, and the recording material R is heated at a fixing part Nf, the control unit 10 controls current output from the transfer power supply 18 on the basis of a result of detection performed by the detection unit 13.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to an image forming apparatus such as a printer, a copying machine, a facsimile machine, or a multifunction machine having a plurality of functions among these, which uses an electrophotographic system. [Background technology]

[0002] In an electrophotographic image forming apparatus, a transfer voltage is applied to a transfer member disposed opposite an image carrier, such as a drum-shaped photoreceptor or a belt-shaped intermediate transfer member, to electrostatically transfer a toner image carried on the image carrier to a recording material, such as paper or an OHT. The recording material is then transported to a fixing device, where it is heated and pressurized to fix the toner image thereon. The transfer member contacts the image carrier to form a transfer nip (transfer portion), and transfers the toner image from the image carrier to the recording material sandwiched in the transfer nip. The fixing device includes a heating member equipped with a heater (heat source) and a pressure member contacting the heating member to form a fixing nip (fixing portion), thereby heating the recording material sandwiched in the fixing nip. An AC voltage is applied to the heating member from a commercial power source, causing the heater to generate heat and reach a temperature sufficient to fix the toner image to the recording material.

[0003] When a recording material is sandwiched in the fixing nip while a toner image is being transferred, the current flowing from the commercial power source to the heating element, recording material, and image carrier may overlap with the transfer current at the transfer nip, causing the transfer current to fluctuate. This phenomenon is also known as "AC banding." As a result, uneven transfer performance may occur, and the image transferred to the recording material may exhibit image defects such as uneven density (image unevenness) in the sub-scanning direction (the direction the recording material is transported).

[0004] Patent document 1 discloses an image forming apparatus configured to control the transfer power supply based on the result of comparing the frequency determined from the detection results of a current detection means that detects the current flowing through a transfer member with a predetermined frequency range that includes the frequency of the commercial power supply.

[0005] In the configuration described in Patent Document 1, the current flowing from the commercial power supply to the GND via the heating member, recording material, and transfer member is detected, and the phase and amplitude of the transfer power supply are switched. This configuration is thought to be effective in suppressing image unevenness when the impedance of the transfer member is low. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Publication No. 2018-97273 Summary of the Invention [Problem to be solved by the invention]

[0007] However, image forming devices typically transfer a toner image from an image carrier to a recording material by passing a transfer current through a drum-shaped photosensitive member or a belt-shaped intermediate transfer member (a member that contacts the transfer member via the intermediate transfer member). Therefore, the current from the commercial power supply is dominated by the current that flows to GND via the heating member, recording material, and image carrier. Furthermore, for example, variations in the characteristics of the transfer member can also change the amount of current flowing from the commercial power supply to the transfer member.

[0008] Therefore, it is difficult to sufficiently suppress image unevenness caused by AC banding through control based on the detection result of the current flowing through the transfer member.

[0009] Therefore, an object of the present invention is to effectively suppress image defects caused by AC banding. [Means for solving the problem]

[0010] The above object is achieved by an image forming apparatus according to the present invention. In summary, a typical configuration of the present invention comprises an image carrier that carries a toner image, a developing device that supplies toner to the image carrier to form a toner image on the image carrier, a transfer member that contacts the image carrier to form a transfer section and transfers the toner image from the image carrier to a recording material in the transfer section, a transfer power source that applies voltage to the transfer member, and a fixing device that is arranged downstream of the transfer section in the conveying direction of the recording material and forms a fixing section that sandwiches the recording material, and is provided with a heat source that heats the recording material sandwiched in the fixing section, and a heat source that contacts the recording material in the fixing section. the image forming apparatus includes a fixing device having a heating member that generates heat when a voltage is applied from an AC power source and heats a recording material via the heating member; a detection unit that is connected between the image carrier and GND without the transfer member and detects current or voltage; and a control unit that controls the transfer power source, wherein the control unit controls the current output from the transfer power source based on the detection result by the detection unit when a toner image is transferred from the image carrier to the recording material in the transfer unit and the recording material is heated in the fixing unit.

[0011] Another representative configuration of the present invention includes an image carrier that carries a toner image, a developing device that supplies toner to the image carrier to form a toner image on the image carrier, an intermediate transfer body that is rotatable and onto which the toner image is transferred from the image carrier, an opposing member that contacts the inner circumferential surface of the intermediate transfer body, a transfer member that contacts the outer circumferential surface of the intermediate transfer body and sandwiches the intermediate transfer body between itself and the opposing member to form a transfer section, and transfers the toner image from the intermediate transfer body to a recording material in the transfer section, a transfer power source that applies a voltage to the transfer member, and a fixing device that is disposed downstream of the transfer section in the conveying direction of the recording material and sandwiches the recording material, and a control unit for controlling the transfer power supply, wherein the control unit controls the current output from the transfer power supply based on the detection result by the detection unit when a toner image is transferred from the intermediate transfer body to the recording material in the transfer unit and the recording material is heated in the fixing unit. [Effects of the Invention]

[0012] According to the present invention, image defects caused by AC banding can be effectively suppressed. [Brief explanation of the drawings]

[0013] [Figure 1] 1A and 1B are a schematic cross-sectional view and a schematic diagram of a control configuration of an image forming apparatus according to a first embodiment of the present invention. [Figure 2] FIG. 10 is a graph illustrating an appropriate range of current when transferring toner onto a recording material. [Figure 3] FIG. 2 is a schematic diagram in which elements relating to AC banding in the image forming apparatus of the first embodiment are modeled using circuit elements. [Figure 4]FIG. 4 is a schematic graph illustrating a current detected by a current detection unit when AC banding occurs. [Figure 5] FIG. 10 is a schematic graph illustrating the occurrence of image defects due to AC banding. [Figure 6] 10A and 10B are schematic diagrams for explaining image defects caused by AC banding. [Figure 7] FIG. 10 is a schematic graph illustrating a method for suppressing AC banding. [Figure 8] FIG. 10 is a schematic graph illustrating the setting of Gain in the control for suppressing AC banding. [Figure 9] FIG. 10 is a flowchart illustrating a control procedure for suppressing AC banding. [Figure 10] FIG. 10 is a schematic cross-sectional view of an image forming apparatus according to a second embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0014] The image forming apparatus according to the present invention will be described in more detail below with reference to the drawings. However, the dimensions, materials, shapes, and relative positions of the components described in the following examples may be changed as appropriate depending on the configuration of the apparatus to which the present invention is applied and various conditions. In other words, the scope of the present invention is not limited to the following examples.

[0015] [Example 1] <Configuration of image forming device> The schematic configuration of an image forming apparatus 1 of this embodiment will be described using Figure 1. Figure 1(a) is a schematic cross-sectional view of the image forming apparatus 1 of this embodiment, and Figure 1(b) is a schematic diagram showing the control configuration of the image forming apparatus 1 of this embodiment. The image forming apparatus 1 of this embodiment is a laser beam printer capable of forming a monochrome black image on a sheet-like recording material R using an electrophotographic system.

[0016] The photosensitive drum 11, a drum-type photosensitive member (electrophotographic photosensitive member) serving as an image carrier, is rotated in the direction of arrow D (clockwise) in the figure at a predetermined peripheral speed (process speed) by a drum drive motor (not shown) serving as a driving means. The surface (outer periphery) of the rotating photosensitive drum 11 is uniformly charged to a predetermined potential of a predetermined polarity (negative in this embodiment) by a charging roller 12, a roller-type charging member serving as a charging means. The charging roller 12 is disposed in contact with the surface of the photosensitive drum 11 and rotates in response to the rotation of the photosensitive drum 11. During charging, a charging voltage (charging bias) is applied to the charging roller 12 by a charging power supply (not shown) serving as a charging voltage application means. The charged surface of the photosensitive drum 11 is scanned and exposed by a scanner unit (exposure device) 14 serving as an exposure means (light irradiation means), and an electrostatic latent image (electrostatic image) is formed on the photosensitive drum 11. The scanner unit 14 irradiates the surface of the photosensitive drum 11 with laser light in response to an image signal, forming an electrostatic latent image on the photosensitive drum 11. The electrostatic latent image formed on the photosensitive drum 11 is developed (visualized) by a developing device 15 serving as a developing means, which supplies toner as a developer, and a toner image (toner image, developer image) is formed on the photosensitive drum 11. The developing device 15 has a developing sleeve 15a serving as a developing member (developer carrier) and a developing container 15b that contains toner. The developing sleeve 15a carries the toner in the developing container 15b and transports it to a portion (developing portion) facing the photosensitive drum 11, where the toner adheres to the electrostatic latent image on the photosensitive drum 11, forming a toner image. During development, a developing voltage (developing bias) is applied to the developing sleeve 15a by a developing power supply (not shown) serving as a developing voltage applying means. In this embodiment, toner charged with the same polarity as the charge polarity of the photosensitive drum 11 (negative in this embodiment) adheres to the exposed portion (image portion) on the photosensitive drum 11, which has been uniformly charged and then exposed to light to reduce the absolute value of the potential (reverse development method). In this embodiment, the normal charge polarity of the toner, which is the main charge polarity of the toner during development, is negative.

[0017] A transfer roller 17, a roller-type transfer member serving as a transfer means, is disposed opposite the photosensitive drum 11. The transfer roller 17 contacts the surface of the photosensitive drum 11 to form a transfer nip (transfer portion) Nt, which is the contact portion between the photosensitive drum 11 and the transfer roller 17. The toner image formed on the photosensitive drum 11 is transferred to a recording material R, which is sandwiched between the photosensitive drum 11 and the transfer roller 17 and conveyed, at the transfer nip Nt. During transfer, a transfer voltage (transfer bias), which is a direct current (DC) voltage of opposite polarity (positive in this embodiment) to the normal charging polarity of the toner, is applied to the transfer roller 17 by a transfer power source 18 serving as a transfer voltage application means. The recording material R (transfer material, recording medium, sheet) such as paper or OHT is stored in a cassette 5 serving as a recording material storage portion. A pickup roller 6 serving as a feeding member sends the recording material R from the cassette 5 to a recording material R conveyance path. The pair of conveying rollers 7 and 8, which serve as conveying members, adjust the timing of conveying the recording material R, delivered by the pickup roller 6, to the transfer nip Nt. At the transfer nip Nt, a toner image is transferred from the photosensitive drum 11 to the recording material R conveyed by the pair of conveying rollers 7 and 8. The photosensitive drum 11 is electrically connected to a GND (ground potential) by electrically connecting its core to a metal plate (not shown) such as the frame of the image forming apparatus 1. In this embodiment, a current detection unit (current detection circuit) 13 serving as a current detection means is provided between the core of the photosensitive drum 11 and GND. This current detection unit 13 is connected between the photosensitive drum 11 and GND without passing through the transfer roller 17. In other words, the current detection unit 13 is connected between the photosensitive drum 11 and GND in a current path extending from a transfer power source 18 to GND via the transfer roller 17 and the photosensitive drum 11.

[0018] The recording material R onto which the toner image has been transferred has the charge accumulated on the surface of the recording material R removed by a charge removing member 19, and is then conveyed to a fixing device 20 serving as fixing means. After the toner image has been transferred onto the recording material R, the toner (transfer residual toner) remaining on the surface of the photosensitive drum 11 is removed (cleaned) and collected from the surface of the photosensitive drum 11 by a cleaning device 16 serving as cleaning means. The cleaning device 16 has a cleaning blade 16a as a cleaning member arranged in contact with the surface of the photosensitive drum 11, and a cleaning container 16b. The cleaning device 16 uses the cleaning blade 16a to scrape the transfer residual toner from the surface of the rotating photosensitive drum 11 and collects it in the cleaning container 16b.

[0019] The fixing device 20 includes a heating member 21 provided with a heat source and a pressure roller 24 as a pressure member that contacts the heating member 21 to form a fixing nip portion (fixing portion) Nf. The heating member 21 includes a fixing film 22 as a first fixing member (first fixing rotator) and a heater 23 as a heat source that contacts the pressure roller 24 as a second fixing member (second fixing rotator) via the fixing film 22. The fixing film 22 as the first fixing member and the pressure roller 24 as the second fixing member form a heat nip member. The heating member 21 is heated to a temperature capable of fixing a toner image on the recording material R by applying an alternating current (AC voltage) from a commercial power source (AC power source) 30 to the heater 23, causing the heater 23 to generate heat. The fixing device 20 is controlled to a desired fixing temperature by a fixing control unit 26 as a fixing control means that variably controls the current flowing from the commercial power source 30 to the heater 23. The pressure roller 24 is connected to GND (electrically grounded) via electronic components 25 to ensure safety in case of user contact and to ensure strength against damage in the event of a lightning surge from the commercial power supply 30. The pressure roller 24 is pressed against the heater 23 via the fixing film 22, forming a fixing nip Nf, which is the contact point between the fixing film 23 and the pressure roller 24. The pressure roller 24 is driven to rotate in the direction of arrow F (counterclockwise) in the figure by a fixing drive motor (not shown) serving as a driving means. The fixing film 22 rotates in accordance with the rotation of the pressure roller 24 while sliding against the heater 23. The heater 23 may have any available configuration, such as a known heater. For example, the heater 23 includes a substrate, an electrode portion to which a voltage from an AC power source is applied, and a heating resistor formed on the surface of the substrate. The heating resistor generates heat when a voltage is applied to the electrode portion from the AC power source, causing a current to flow through the electrode portion. The heater 23 generates heat from this heating resistor, enabling it to heat the recording material R held in the fixing nip Nf. The fixing device 20 heats and pressurizes the recording material R, onto which the toner image has been transferred, by sandwiching it between the fixing film 22 and the pressure roller 24 and transporting it in the fixing nip Nf, thereby fixing (melting and adhering) the toner image onto the recording material R.

[0020] After the toner image is fixed on the recording material R by the fixing device 20, the recording material R is discharged (output) to an output tray 3 serving as an output section provided on the outside (top surface) of the device main body 2 of the image forming device 1 (hereinafter simply referred to as the "device main body").

[0021] In this embodiment, the photosensitive drum 11, the charging roller 12 as a process means acting on the photosensitive drum 11, the developing device 15, and the cleaning device 16 integrally constitute a process cartridge 4 that is detachable from the apparatus main body 2. The process cartridge 4 is replaced with a new one when, for example, the amount of toner contained in the developing container 15b of the developing device 15 falls below a predetermined amount, the photosensitive drum 11 reaches the end of its life, or the developing sleeve 15a reaches the end of its life. In this embodiment, the apparatus main body 2 is the portion of the image forming apparatus 1 excluding the process cartridge 4.

[0022] The image forming apparatus 1 is provided with an environmental sensor 9. In this embodiment, the environmental sensor 9 is capable of detecting the temperature and humidity of the environment surrounding the image forming apparatus 1. The environmental sensor 9 is an example of an environmental detection means (environment detection unit) that detects environmental information, which is at least one of the temperature and humidity inside or outside the image forming apparatus 1.

[0023] The image forming apparatus 1 is also provided with a control unit (control circuit) 10 as a control means. The control unit 10 is configured with a CPU 10a, which is an arithmetic control unit, a storage unit 10b, and a transfer power supply control unit 10c. The storage unit 10b is configured with a ROM, a RAM, a non-volatile memory, etc. A print start instruction and an image signal are sent to the control unit 10 from an external device (not shown) such as a personal computer. The control unit 10 controls each part of the image forming apparatus 1 based on the input print start instruction and image signal, and the image forming apparatus 1 executes image formation. The storage unit 10b stores programs and data related to image formation, and the CPU 10a controls each part of the image forming apparatus 1 in accordance with these programs and data.

[0024] The detection information (detection signal) obtained by the current detection unit 13 and the detection information obtained by the environmental sensor 9 are each input to the control unit 10. The current detection information input from the current detection unit 13 to the control unit 10 is converted into a digital value by an analog-to-digital (A / D) converter in the CPU 10a. The CPU 10a acquires the A / D converted current detection information at predetermined intervals. Like the current detection information, humidity and temperature information input from the environmental sensor 9 to the control unit 10 is also converted into a digital value by the A / D converter in the CPU 10a and acquired by the CPU 10a. In this embodiment, the memory unit 10b stores the cumulative number of prints made using the replaceable photosensitive drum 11 as cumulative usage information for the photosensitive drum 11. In this embodiment, the memory unit 10b stores the cumulative number of prints made using the transfer roller 17 (apparatus main body 2) as cumulative usage information for the transfer roller 17 (apparatus main body 2). The cumulative number of prints is an example of an index value (value related to cumulative usage information) that correlates with the usage of the image forming apparatus 1 or elements of the image forming apparatus 1. The cumulative number of prints may be calculated by counting an image formed on one side of a recording material R of a predetermined size as one sheet. The cumulative number of prints for the photosensitive drum 11 is reset to an initial value (zero in this embodiment) when the photosensitive drum 11 is replaced together with the charging roller 12, developing device 15, etc. The CPU 10a uses the current detection information, humidity / temperature information, and information on the cumulative number of prints to output a signal required to control the transfer power supply 18 to the transfer power supply control unit 10c. Details of this will be described later.

[0025] In the image forming apparatus 1, paper is mainly used as the recording material R, and therefore the recording material R is sometimes referred to as paper, but the recording material R is not limited to paper. The recording material R may also be made of materials other than paper or materials containing materials other than paper, such as synthetic paper or film made mainly of a synthetic resin, or special paper such as metallized paper with a metal layer.

[0026] Furthermore, the index value (value related to cumulative usage information) correlated with the usage of the image forming apparatus 1 or the elements of the image forming apparatus 1 is not limited to the cumulative number of prints. For example, it may be a value related to the amount of toner consumed or the amount of toner remaining in the developing device 15. It may also be, for example, the number of rotations or rotation time of the photosensitive drum 11, or the number of rotations or rotation time during the charging process of the photosensitive drum 11.

[0027] <Transcription mechanism> Next, the mechanism by which toner is transferred from the photosensitive drum 11 to the recording material R will be described.

[0028] When the recording material R is sandwiched between the photosensitive drum 11 and the transfer roller 17 with toner on the photosensitive drum 11, a voltage of the opposite polarity to that of the toner is applied to the surface of the recording material R opposite to the surface onto which the toner is transferred. This causes a current to flow between the photosensitive drum 11 and the recording material R, and the toner is transferred to the recording material R. At this time, the current flowing in the transfer nip portion Nt varies depending on the value of the resistance (electrical resistance; the same applies hereinafter) of the recording material R, and therefore, if it is not set appropriately, it may cause image defects.

[0029] <How to set the transfer current and voltage> Next, a method for setting the transfer voltage in this embodiment will be described with reference to Figures 2 and 3. Figure 2 is a graph showing the relationship between the resistance value of the recording material R and the current output by the transfer power supply 18. This figure shows that the appropriate range of the current flowing from the transfer roller 17 to the recording material R when a toner image is transferred from the photosensitive drum 11 to the recording material R varies depending on the resistance value of the recording material R. Also, Figure 3 is a schematic diagram in which elements around the transfer nip Nt and the fixing nip Nf in the image forming apparatus 1 of this embodiment are modeled using circuit elements.

[0030] In FIG. 3, the capacitance (electrical capacitance; the same applies below) components of the heating member 21 of the fixing device 20—in this embodiment, the capacitance component of the heater 23 is primarily designated as capacitance 23a, the resistance component of the pressure roller 24 as resistance 24a, the ground resistance between the pressure roller 24 and GND as resistance 25a, and the ground capacitance as capacitance 25b—are designated. Also in FIG. 3, the resistance component of the transfer roller 17 is designated as resistance 17a, the capacitance component as capacitance 17b, the resistance component of the photosensitive drum 11 as resistance 11a, and the capacitance component as capacitance 11b. Also in FIG. 3, resistance Rv represents the resistance value in the conveyance direction of the recording material R, and resistance Rh represents the resistance value in the thickness direction of the recording material R. As described above, the image forming apparatus 1 is provided with a current detection unit 13, and the detection results of the current detection unit 13 are input to the control unit 10. Then, the output voltage of the transfer power supply 18 is controlled by the transfer power supply control unit 10c of the control unit 10 based on the calculation results by the CPU 10a of the control unit 10.

[0031] The current detection unit 13 may be configured to detect fluctuations in the current flowing to GND via the transfer power supply 18, transfer roller 17, recording material R, and photosensitive drum 11, and the current flowing to GND via the heating member 21 (heater 23), recording material R, and photosensitive drum 11 (more specifically, a current obtained by superimposing the current flowing to GND via the transfer power supply 18, transfer roller 17, recording material R, and photosensitive drum 11 and the current flowing to GND via the heating member 21 (heater 23), recording material R, and photosensitive drum 11). Specifically, for example, an HPF (high pass filter) circuit using a coupling capacitor, an operational amplifier, etc. may be used.

[0032] The resistance and capacitance of the recording material R and the transfer roller 17 change depending on the environment, and generally, resistance decreases in a high-temperature, high-humidity environment. Furthermore, since the photosensitive drum 11 comes into contact with the transfer roller 17 and the recording material R, its resistance and capacitance change due to wear caused by an increase in the cumulative number of prints. For example, the capacitance of the photosensitive drum 11 increases as the number of prints increases. Furthermore, the resistance and capacitance of the transfer roller 17 also change due to dirt and uneven distribution of conductive agents caused by an increase in the cumulative number of prints. For example, the resistance of the transfer roller 17 increases as the cumulative number of prints increases.

[0033] Recording material R that has absorbed moisture and has a low electrical resistance is referred to as "moisture-absorbing paper," while recording material R that has just been opened from its packaging and has not yet absorbed moisture and has not yet had a low electrical resistance is referred to as "open paper." As shown in Figure 2, the appropriate range of the current flowing from the transfer roller 17 to the recording material R varies depending on the surrounding environment. The current flowing from the transfer roller 17 is injected into the recording material R and flows toward the photosensitive drum 11. Because moisture-absorbing paper has a lower electrical resistance than open paper, as shown in Figure 3, the current injected from the transfer power supply 18 to resistor Rh via resistor 17a flows more toward resistor Rv. In other words, the current injected from the transfer roller 17 to the recording material R flows through the moisture-absorbing paper to, for example, ground resistor 25a and ground capacitor 25b between the pressure roller 24 and GND, which can result in insufficient current flowing from the recording material R to the photosensitive drum 11. Therefore, more current needs to flow from the transfer roller 17 to the moisture-absorbing paper, and a higher voltage needs to be applied from the transfer power supply 18 to the transfer roller 17. On the other hand, the electrical resistance of open paper is higher than that of absorbent paper, and less current flows from transfer power supply 18 to resistor 17a and resistor Rh, then to resistor Rv. Therefore, the voltage applied to transfer roller 17 from transfer power supply 18 can be set lower than in the case of absorbent paper.

[0034] Assume that in the case of plain paper, the output voltage of the transfer power supply 18 is set to the same high voltage as in the case of blotting paper. In this case, the current injected from the transfer roller 17 into the recording material R and flowing to the photosensitive drum 11 becomes excessive, so that the polarity of the toner in the transfer nip portion Nt is reversed, and there is a possibility that it may be reversely transferred from the plain paper to the photosensitive drum 11. Therefore, as shown in FIG. 2, as the value of the current injected from the transfer roller 17 into the recording material R, the inside of the appropriate range C (shaded portion) is preferable. In the present embodiment, the CPU 10a of the control unit 10 uses the detection result of the environment sensor 9 and the cumulative usage amount information stored in the storage unit 10b (in the present embodiment, the cumulative number of printed sheets of the photosensitive drum 11 and the cumulative number of printed sheets of the transfer roller 17) to set a target current so that a current within the appropriate range C flows through the transfer nip portion Nt. Then, the transfer power supply control unit 10c of the control unit 10 performs constant current control of the transfer voltage applied from the transfer power supply 18 to the transfer roller 17 so that the current of the target current flows through the transfer nip portion Nt.

[0035] <Mechanism for image defects caused by AC banding> Next, using FIGS. 4 to 6, the mechanism for image defects caused by AC banding will be described.

[0036] FIG. 4 is a schematic graph for explaining the current flowing through the transfer nip portion Nt during printing. In FIG. 4, the time T1 is the time when the recording material R enters the transfer nip portion Nt, and the time T2 is the time when the recording material R enters the fixing nip portion Nf. From the time T1 to a time before the time T2, the recording material R is not sandwiched by the fixing nip portion Nf, and since the alternating current from the commercial power supply 30 does not superimpose on the transfer current, the current flowing through the transfer nip portion Nt becomes a constant current. On the other hand, after the time T2 when the recording material R is sandwiched by both the transfer nip portion Nt and the fixing nip portion Nf, the alternating current of the commercial power supply 30 superimposes on the transfer current via the recording material R. As a result, the current flowing through the transfer nip portion Nt fluctuates with the period of the frequency of the commercial power supply 30, and AC banding occurs.

[0037] FIG. 5 is a schematic graph for explaining the occurrence of image defects due to AC banding. FIG. 6 is a schematic diagram of an image in which image defects due to AC banding have occurred. As described above, the control unit 10 considers humidity and temperature information obtained by the environment sensor 9, and as shown by the solid line in FIG. 5, adjusts the transfer voltage applied from the transfer power supply 18 to the transfer roller 17 so that the current flowing through the transfer nip portion Nt falls within the appropriate range C. However, after time T2, an alternating current from the commercial power supply 30 is applied, and the current flowing from the recording material R to the photosensitive drum 11 may go out of the appropriate range C. For example, when the waveform is as shown by the broken line in FIG. 5, the current flowing through the transfer nip portion Nt fluctuates at the period of the frequency of the commercial power supply 30, and the trough portion of the waveform falls below the appropriate range C of the current when transferring the toner image. As a result, the current is insufficient at the period of the frequency of the commercial power supply 30, and as shown in FIG. 6, after the recording material R enters the fixing nip portion Nf, the image transferred from the photosensitive drum 11 to the recording material R has density unevenness (image unevenness) at the period of the frequency of the commercial power supply 30.

[0038] <Method for Suppressing Image Defects Caused by AC Banding> Next, a method for suppressing image defects caused by AC banding in the present embodiment will be described using FIGS. 7 to 9.

[0039] FIGS. 7(a) to 7(d) all schematically show waveforms at the time of occurrence of AC banding, with the case where the control of this embodiment is applied represented by a solid line and the case where it is not applied represented by a broken line. FIG. 7(a) shows the waveform of the current “I Inlet ” flowing from the commercial power supply 30 to the transfer nip portion Nt, FIG. 7(b) shows the waveform of the output current “I 転写 ” of the transfer power supply 18, FIG. 7(c) shows the waveform of the current “I Nt ” of the transfer nip portion Nt, and FIG. 7(d) shows the waveform of the current amplitude “ΔI” detected by the current detection unit 13.

[0040] The current (I Nt ) of the transfer nip portion Nt is part of the current from the commercial power supply 30 and is the current (IInlet ) and the output current (I 転写 ) is superimposed on the DC current (direct current) from the transfer power supply 18, as shown by the dashed line in FIG. 7(c). Nt As explained with reference to FIG. 5, when the current amplitude is large and exceeds the appropriate range C, image unevenness occurs due to AC banding.

[0041] Therefore, in this embodiment, the output current (I ) of the transfer power supply 18 is calculated based on the current amplitude (ΔI ) shown by the dashed line in FIG. 7( d ), as shown by the solid line in FIG. 7( b ). 転写 ) is controlled (details will be described later). As a result, the current amplitude (ΔI) of the current detection unit 13 decreases as shown by the solid line in FIG. 7(d), and the current (I Nt ) fluctuations can also be suppressed.

[0042] FIG. 8 shows the current amplitude (ΔI) of the current detection unit 13 and the output current (I 転写 ) is a schematic graph showing an example.

[0043] As shown in FIG. 8, the output current of the transfer power supply 18 without suppressing AC banding is the initial transfer target current (I S ), the detection result of the current amplitude (ΔI) of the current detection unit 13 is shown by the white circle and the dashed line. In principle, the white circle and the dashed line of the current amplitude (ΔI) and the initial transfer target current (I S ) and the solid line is the final transfer current target value (I T ) to suppress AC banding.

[0044] However, in reality, the output current (I 転写 ) transfer current target value (I TIt is desirable to determine the amount of correction for I. Changes in current amplitude due to impedance of the resistance component 17a and capacitance component 17b of the transfer roller 17, the resistance Rh of the recording material R, and the resistance component 11a and capacitance component 11b of the photosensitive drum 11. II. Current phase delay due to the capacitance component 17b of the transfer roller 17 and the capacitance component 11b of the photosensitive drum 11. III. Response delay of the circuit from the output of the transfer power supply 18 to the transfer nip portion Nt.

[0045] The target value of the transfer current is corrected by multiplying the current amplitude (ΔI) determined by the current detection unit 13 by the gain determined by these parameters. Here, the gain can be expressed by the following equation 1. Gain=Gain Ref ×Env×K (Formula 1) (Gain Ref : Gain in normal temperature and humidity environment, Env: Environmental coefficient, K: Cumulative usage coefficient)

[0046] Gain Ref is a reference correction gain, which is determined in advance based on, for example, the circuit response delay and circuit gain of the transfer power supply 18, and a typical temperature and humidity (for example, 23° C., 50% RH).

[0047] Env is a coefficient that depends on the change in impedance of the transfer roller 17, the recording material R, the photosensitive drum 11, etc. (at least one of these) due to the environment, and is determined based on humidity and temperature information obtained by the environmental sensor 9 (and also information regarding the type of recording material R). In this embodiment, Env is determined based on an impedance selected from a table that shows the relationship between humidity and temperature information and the impedance of the transfer roller 17, the recording material R, the photosensitive drum 11, etc. (at least one of these), which is obtained in advance and stored in the storage unit 10b. However, the method for determining Env is not limited to this. For example, Env may be determined based on an impedance calculated based on information such as the voltage of the transfer power source 18, humidity and temperature information, and the rate of change in impedance of the transfer roller 17, the recording material R, the photosensitive drum 11, etc. (at least one of these), due to these information. Alternatively, Env may be selected from a table that shows the relationship between environmental information and Env that reflects the change in impedance, which is obtained in advance and stored in the storage unit 10b. In this embodiment, information regarding the type of recording material R is input from an external device to the control unit 10 together with a print start command, an image signal, and print settings. Here, the type of recording material R includes any information that can distinguish the recording material R, such as attributes based on general characteristics such as plain paper, coated paper, thick paper, and synthetic paper (so-called paper type categories), numerical values and numerical ranges for basis weight and thickness, and brand (including manufacturer, product number, etc.).

[0048] K is a coefficient when a phase delay of the current occurs due to a change in the capacitance component 17b of the transfer roller 17 or the capacitance component 11b of the photosensitive drum 11 (or at least one of them) caused by an increase in the cumulative usage amount of the apparatus main body 2 or toner consumption. K is determined based on the cumulative number of prints (at least one of the cumulative print number information of the photosensitive drum 11 and the transfer roller 17) stored in the storage unit 10b. In this embodiment, K is determined based on the rate of change of the capacitance component selected from a table showing the relationship between the cumulative number of prints (at least one of the cumulative print number information of the replaceable photosensitive drum 11 and the cumulative print number information of the transfer roller 17 (apparatus main body 2)) and the rate of change of the capacitance component (at least one of them) of the photosensitive drum 11 and the transfer roller 17 for each cumulative number of prints (i.e., as the cumulative number of prints increases). However, the method of determining K is not limited to this. For example, K may be determined based on a capacitance component calculated based on information such as cumulative usage information (at least one of the cumulative usage information of the photosensitive drum 11 and the transfer roller 17 (apparatus main body 2)) and the resulting rate of change in the capacitance component (at least one of these) of the photosensitive drum 11 and the transfer roller 17. K may also be selected from a table that indicates the relationship between the cumulative usage information and K reflecting the change in the capacitance component, which is calculated in advance and stored in the storage unit 10b.

[0049] The output current (I 転写 ) has a waveform in which the phase of the current amplitude (ΔI) is inverted, as shown by the black circles and dotted lines in FIG.

[0050] As described above, in this embodiment, the control unit 10 switches the voltage applied from the transfer power supply 18 to the transfer roller 17 in accordance with the phase of the power frequency of the commercial power supply 30. That is, during the time corresponding to the valleys of the waveform of the current amplitude (ΔI), the voltage applied from the transfer power supply 18 to the transfer roller 17 is made higher than the voltage applied to the transfer roller 17 when control to suppress AC banding is not performed. Furthermore, during the time corresponding to the peaks of the waveform of the current amplitude (ΔI), the voltage applied from the transfer power supply 18 to the transfer roller 17 is made lower than the voltage applied to the transfer roller 17 when control to suppress AC banding is not performed. As a result, the voltage applied from the transfer power supply 18 to the transfer roller 17 is periodically controlled in accordance with the phase of the power frequency of the commercial power supply 30, thereby suppressing fluctuations in the current at the transfer nip Nt, as shown by the solid line in FIG. 7C.

[0051] FIG. 9 is a flowchart of a control process for detecting the current amplitude (ΔI) in the current detection unit 13 and suppressing AC banding in this embodiment.

[0052] Step 1 In this step, parameters necessary for constant current control of the transfer power supply 18 are determined. First, at the start of printing, the CPU 10a acquires environmental information obtained by the environmental sensor 9, which has been converted into digital values by the A / D converter, and determines an environmental coefficient Env based on a table showing the relationship between humidity and temperature information and impedance, which is stored in advance in the storage unit 10b (S101). Thereafter, the CPU 10a similarly determines an accumulated usage coefficient K based on a table showing the relationship between information on the accumulated number of prints on the replaceable photosensitive drum 11, information on the accumulated number of prints on the transfer roller 17, and the rate of change in capacitance component for each number of prints, which is stored in the storage unit 10b (S102). Next, the CPU 10a determines an initial transfer current target value (I) that serves as a reference, taking into consideration the type of recording material R, print setting information, etc. S ) and determine the final transfer current target value (I T ) (S103).

[0053] Step 2 In this step, the final transfer current target value (I T The CPU 10a acquires the current amplitude (ΔI) detected by the current detection unit 13 and converted into digital form by the A / D conversion unit (S201). Next, the CPU 10a calculates an AC banding correction value (ΔIout) based on the acquired current amplitude (ΔI) and the Gain calculated in Step 1 (S202). Then, the CPU 10a calculates a final transfer current target value (I T ) and the AC banding correction value (ΔIout), and the output current (I 転写 ) is the final transfer current target value (I T ) the transfer power supply control unit 10c outputs a signal required to control the transfer power supply 18 (S203).

[0054] Here, a brief description will be given of the transfer power supply 18 and the transfer power supply control unit 10c of this embodiment. The transfer power supply 18 of this embodiment is a power supply circuit using a flyback transformer, and controls the current flowing through the primary coil of the flyback transformer to a desired current by intermittently flowing it using a switching means. At this time, the transfer power supply control unit 10c sends a PWM signal to the switching means to flow an intermittent current through the primary coil of the flyback transformer. The transfer power supply 18 has an output current (I 転写 The CPU 10a is provided with a constant current detection circuit unit (not shown) that detects the output current (I 転写 ) is the transfer current target value (I T The duty of the PWM signal sent from the transfer power supply control unit 10c to the transfer power supply 18 is adjusted so that the output current (I 転写 ) is the final transfer current target value (I T ) can be achieved.

[0055] Step 3 In this step, it is determined whether to return or end the process depending on whether printing has been completed. The CPU 10a determines whether to return or end the process depending on whether printing has been completed or not. T ) and continues applying the transfer voltage (waiting for the setting to be updated) (S301). Then, the CPU 10a judges whether printing (all printing of the job) has been completed (S302). Note that a job is a series of operations that is started by one start instruction, and forms an image on one or more recording materials R and outputs the images from the image forming apparatus 1. Then, if printing has not been completed, the CPU 10a returns to Step 2 and determines the final transfer current target value (I T ) and changes the output of the transfer power supply 18. The CPU 10a repeats Steps 2 and 3 until printing is completed. On the other hand, when printing is completed, the CPU 10a turns off the transfer power supply 18 and ends the operation of the image forming apparatus 1 (S303).

[0056] This loop of Steps 2 and 3 is performed at a predetermined interval. Here, this predetermined interval may be any interval that can suppress AC banding. This predetermined interval is preferably a time, such as 1 ms, that is sufficiently shorter than the period of AC voltage from a commercial power source with a frequency of 50 Hz (period 20 ms) or 60 Hz (approximately 16.7 ms). For example, this predetermined interval can be 0.5 ms or more and 15 ms or less, and more preferably 1 ms or more and 10 ms or more.

[0057] As described above, in this embodiment, the image forming apparatus 1 includes an image carrier (photosensitive drum) 11 that carries a toner image, a developing device 15 that supplies toner to the image carrier 11 to form a toner image on the image carrier, a transfer member (transfer roller) 17 that contacts the image carrier 11 to form a transfer portion (transfer nip portion) Nt and transfers the toner image from the image carrier 11 to the recording material R at the transfer portion Nt, a transfer power source 18 that applies voltage to the transfer member 17, and a fixing device 20 that is disposed downstream of the transfer portion Nt in the conveying direction of the recording material R and forms a fixing portion (fixing nip portion) Nf that holds the recording material R, and a heat source (heater) 23 that heats the recording material R held in the fixing portion Nf. The image forming apparatus 1 includes a fixing device 20 provided with a heating member 21 that contacts the recording material R at the fixing portion Nf, and a heating source 23 that generates heat by applying voltage from an AC power source 30 and heats the recording material R via the heating member 21, a detection unit (current detection unit) 13 that is connected between the image carrier 11 and GND without passing through the transfer member 17 and detects current or voltage (current in this embodiment), and a control unit 10 that controls a transfer power source 18, and the control unit 10 controls the current output from the transfer power source 18 based on the detection result by the detection unit 13 when a toner image is transferred from the image carrier 11 to the recording material R at the transfer portion Nt and the recording material R is heated at the fixing portion Nf. That is, in this embodiment, the image forming apparatus 1 includes a detection unit (current detection unit) 13 that is connected between the image carrier 11 and GND in a current path that runs from the transfer power source 18 to GND via the transfer member 17 and the image carrier 11 and detects current or voltage (current in this embodiment). In this embodiment, the image carrier 11 is a photosensitive member. In this embodiment, the detection unit 13 detects a current obtained by superimposing at least a portion of the current output from the AC power supply 30 and at least a portion of the current output from the transfer power supply 18. In this embodiment, the control unit 10 corrects the output current of the transfer power supply 18 based on the amplitude of fluctuations acquired from the detection result by the detection unit 13 and a target value of the current supplied from the transfer power supply 18 to the transfer unit Nt, thereby controlling the output current of the transfer power supply 18 to suppress the fluctuations.In this embodiment, the image forming apparatus 1 has an environment detection unit (environment sensor) 9 that detects the environment, which is at least one of the temperature and humidity inside or outside the image forming apparatus 1, and the control unit 10 changes the amount of correction in the correction based on the detection result by the environment detection unit 9. In this embodiment, the image forming apparatus 1 has a memory unit 10b that stores an index value correlated with the usage amount of the image forming apparatus 1, and the control unit 10 changes the amount of correction in the correction based on the index value stored in the memory unit 10b. Here, the index value is, for example, a value related to the cumulative number of prints made by the image forming apparatus 1, or a value related to the amount of toner consumed or remaining in the developing device 15. In addition, the index value is, for example, a value related to the cumulative usage amount of the image carrier 11.

[0058] As described above, in this embodiment, the image forming apparatus 1 has a current detection unit 13 connected between the photosensitive drum 11 and GND in a current path that flows from the transfer power supply 18 via the transfer roller 17 and the photosensitive drum 11 to GND. Then, based on the current detection result of the AC component of the current that flows from the commercial power supply 30 to the transfer nip portion Nt via the recording material R, detected by the current detection unit 13, the target current value of the transfer power supply 18 is variably controlled. Specifically, in this embodiment, the constant current target value (I T ) is corrected to suppress the AC amplitude of the transfer nip Nt. As a result, according to this embodiment, it is possible to suppress image defects such as uneven density (image unevenness) caused by AC banding due to the current from the commercial power supply 30.

[0059] While the current detection unit 13 has been described as an example in this embodiment, a means for converting current to voltage or a voltage detection unit may also be used. It is sufficient to be able to detect fluctuations (AC components) in the transfer current caused by the current flowing from the commercial power supply 30 through the recording material R into the transfer nip Nt. In this case, the image forming apparatus 1 has a voltage detection unit as voltage detection means connected between the photosensitive drum 11 and GND in the current path that flows from the transfer power supply 18 to GND via the transfer roller 17 and the photosensitive drum 11. This also allows the same effect as in this embodiment to be achieved by controlling the transfer power supply 18 so as to suppress fluctuations in the current flowing in this current path.

[0060] [Example 2] Next, another embodiment of the present invention will be described. Descriptions of functions, configurations, and operations of the image forming apparatus of this embodiment that are the same as or correspond to those of the image forming apparatus of the first embodiment will be omitted where appropriate.

[0061] <Configuration of image forming device> In the first embodiment, the present invention is described as being applied to a monochrome image forming apparatus, but the present invention is not limited to such a configuration. The present invention can also be applied to a configuration in which an intermediate transfer body and a primary transfer power supply are provided between the image carrier and the transfer roller, such as a color image forming apparatus.

[0062] The schematic configuration of the image forming apparatus 100 of this embodiment will be described with reference to Fig. 10. Fig. 10 is a schematic cross-sectional view of the image forming apparatus 10 of this embodiment. The image forming apparatus 100 of this embodiment is a tandem laser beam printer that employs an intermediate transfer system and is capable of forming a full-color image on a sheet-like recording material R using an electrophotographic system.

[0063] In this embodiment, the image forming apparatus 100 has four image forming units (stations) Py, Pm, Pc, and Pk, which form yellow, magenta, cyan, and black images, respectively. Elements having the same or corresponding functions or configurations for each color are designated by a suffix (y, m, c, or k) indicating the color. However, when describing matters common to all colors, the suffix (y, m, c, or k) is omitted. In this embodiment, the image forming unit P includes a photosensitive drum 122, a charging roller 123, a scanner unit 14, a developing device 125, and other components, which will be described later. In this embodiment, the scanner unit 14 is configured as a single unit that exposes the four photosensitive drums 122y, 122m, 122c, and 122k. However, the scanner unit 14 may be configured as an independent unit for each image forming unit P.

[0064] The photosensitive drum 122, which serves as an image carrier, is rotated at a predetermined peripheral speed (process speed) in the direction of arrow D (counterclockwise) in the figure by a drum drive motor (not shown) serving as a driving means. The surface of the rotating photosensitive drum 122 is uniformly charged to a predetermined potential of a predetermined polarity (negative in this embodiment) by a charging roller 123, which is a roller-type charging member serving as a charging means. During charging, a charging voltage (charging bias) is applied to the charging roller 123 by a charging power supply (not shown) serving as a charging voltage application means. The charged surface of the photosensitive drum 122 is scanned and exposed by a scanner unit 14 serving as an exposure means (light irradiation means), and an electrostatic latent image (electrostatic image) is formed on the photosensitive drum 122. The scanner unit 14 irradiates the surface of the photosensitive drum 122 with a laser beam 121 in accordance with an image signal, forming an electrostatic latent image on the photosensitive drum 122. The electrostatic latent image formed on the photosensitive drum 122 is developed (visualized) by a developing device 125 serving as a developing means, which supplies toner as a developer, and a toner image is formed on the photosensitive drum 122 (image carrier). The developing device 125 forms a toner image by attaching toner to the electrostatic latent image using a developing roller 124 serving as a developing member (developer carrier). During development, a developing voltage (developing bias) is applied to the developing roller 124 by a developing power supply (not shown) serving as a developing voltage application means. In this embodiment, a toner image is formed by a reversal development method. In this embodiment, the normal charging polarity of the toner, which is the main charging polarity of the toner during development, is negative.

[0065] An intermediate transfer belt 130, which is an intermediate transfer body formed of an endless belt, is disposed facing the four photosensitive drums 122y, 122m, 122c, and 122k. The intermediate transfer belt 130 is stretched around a plurality of tension rollers, including a tension roller 131, an auxiliary roller 132, and a drive roller 133, and is stretched with a predetermined tension. The drive roller 133 is driven by a belt drive motor (not shown) serving as a drive means, transmitting a driving force to the intermediate transfer belt 130, causing it to rotate (circulate) in the direction of arrow B (clockwise direction) in the figure. Primary transfer rollers 126, which are roller-type primary transfer members serving as primary transfer means, are disposed on the inner circumferential surface of the intermediate transfer belt 130, corresponding to each of the photosensitive drums 122y, 122m, 122c, and 122k. The primary transfer roller 126 presses the intermediate transfer belt 130 against the photosensitive drum 122, forming a primary transfer nip portion (primary transfer portion) Np, which is the contact portion between the photosensitive drum 122 and the intermediate transfer belt 130. The toner image formed on the photosensitive drum 122 is transferred (primary transfer) onto the rotating intermediate transfer belt 130 by the action of the primary transfer roller 126 at the primary transfer nip portion Np. Each of the primary transfer rollers 126y, 126m, 126c, and 126k is connected to an auxiliary roller 132, a drive roller 133, and a primary transfer power supply 142 as a primary transfer voltage application means. During primary transfer, a primary transfer voltage (primary transfer bias), which is a DC voltage of opposite polarity (positive polarity in this embodiment) to the normal charging polarity of the toner, is applied to the primary transfer roller 126. For example, when a full-color image is formed, toner images of yellow, magenta, cyan, and black formed on the photosensitive drums 122y, 122m, 122c, and 122k are sequentially transferred onto the intermediate transfer belt 130 so as to be superimposed on each other.

[0066] A secondary transfer roller 117, which is a roller-type secondary transfer member serving as a secondary transfer means, is disposed on the outer peripheral surface of the intermediate transfer belt 130 at a position facing the drive roller 133, which also functions as a secondary transfer opposing roller as an opposing member. The secondary transfer roller 117 is pressed toward the drive roller 133 and abuts against the drive roller 133 via the intermediate transfer belt 130, forming a secondary transfer nip portion (secondary transfer portion) Nt, which is a contact portion between the intermediate transfer belt 130 and the secondary transfer roller 117. In other words, the secondary transfer roller 117 forms the secondary transfer nip portion Nt by sandwiching the intermediate transfer belt 130 between itself and the drive roller 133. The toner image formed on the intermediate transfer belt 130 is transferred (secondary transferred) at the secondary transfer nip portion Nt onto the recording material R, which is being conveyed while being sandwiched between the intermediate transfer belt 130 and the secondary transfer roller 117. During the secondary transfer, a secondary transfer voltage (secondary transfer bias), which is a DC voltage of opposite polarity (positive polarity in this embodiment) to the normal charging polarity of the toner, is applied to the secondary transfer roller 117 by a secondary transfer power supply 118 serving as a secondary transfer voltage application means. Recording material R, such as paper or OHT, is stored in a cassette 5 serving as a recording material storage unit, and is conveyed to the secondary transfer nip Nt at a predetermined timing by a pickup roller 6 serving as a feeding member and pairs of conveying rollers 7 and 8 serving as conveying members.

[0067] The recording material R onto which the toner image has been transferred has electric charges accumulated on the surface of the recording material R eliminated by an electrostatic eliminator 119, and is then conveyed to a fixing device 20 as a fixing means. The configuration of the fixing device 20 of the image forming apparatus 100 of this embodiment is substantially the same as the configuration of the fixing device 20 of the image forming apparatus 1 of the first embodiment. That is, the fixing device 20 is configured to include a fixing film 22 which is a first fixing member (first fixing rotator), a heater 23 which is a heat source, and a pressure roller 24 which is a second fixing member (second fixing rotator). The fixing device 20 is also provided with electronic components 25 and a fixing control unit 26. The fixing film 22 and the heater 23 constitute a heating member 21, and the pressure roller 24 comes into contact with the heating member 21 to form a fixing nip portion (fixing portion) Nf. When an AC voltage is applied to the heater 23 from a commercial power source (AC power source) 30, the heater 23 generates heat, and the heating member 21 is heated to a temperature at which the toner image can be fixed to the recording material R. The fixing device 20 heats and pressurizes the recording material R, onto which the toner image has been transferred, by sandwiching it between a fixing film 22 and a pressure roller 24 and conveying it at a fixing nip Nf, thereby fixing (melting and adhering) the toner image onto the recording material R. After the toner image has been fixed by the fixing device 20, the recording material R is discharged (output) to a discharge tray 103, which serves as a discharge unit provided on the outside (top surface) of the device main body 102 of the image forming apparatus 100.

[0068] Toner remaining on the photosensitive drum 122 after the primary transfer (primary transfer residual toner) is removed and collected from the photosensitive drum 122 by, for example, the developing device 125. Also, deposits such as toner remaining on the intermediate transfer belt 130 after the secondary transfer (secondary transfer residual toner) are removed and collected from the intermediate transfer belt 130 by, for example, a belt cleaning device (not shown). The belt cleaning device is provided downstream of the secondary transfer nip portion Nt in the rotation direction of the intermediate transfer belt 130 and upstream of the most upstream primary transfer nip portion Np.

[0069] In this embodiment, the image forming apparatus 100 transfers the toner image from the intermediate transfer belt 130 to the recording material R by flowing a predetermined current (transfer current) from the secondary transfer power supply 118 through the secondary transfer roller 117 and the secondary transfer nip portion Nt to the driving roller 133. And in this embodiment, the output current (I 転写 ) of the secondary transfer power supply 118 and the alternating current that is superimposed on the secondary transfer current through the recording material R from the commercial power supply 30 flow from the driving roller 133 to the GND via the primary transfer power supply 142. Therefore, in this embodiment, the current detection unit 113 that detects the alternating current contributing to the AC banding of the secondary transfer current is disposed between the driving roller 133 and the primary transfer power supply 142. This current detection unit 113 is connected between the driving roller 133 and the GND without passing through the secondary transfer roller 117. That is, this current detection unit 113 is connected between the driving roller 113 and the GND in the current path from the secondary transfer power supply 118 through the secondary transfer roller 117 and the driving roller 133 to the GND.

[0070] Note that in this embodiment, the image forming apparatus 100 has four image forming units, but the present invention is not limited to such an aspect. For example, the present invention is also applicable to a color image forming apparatus that sequentially forms toner images of a plurality of colors on an image carrier (such as a photosensitive drum) of one image forming unit and transfers the toner images to a recording material via an intermediate transfer body (such as an intermediate transfer belt).

[0071] <Method for suppressing image defects caused by AC banding> In this embodiment, the current detection unit 113 is provided at a position where the driving roller (secondary transfer opposing roller) 133 and the primary transfer roller 126 are connected to the primary transfer power supply 142. And in this embodiment, the detection result by the current detection unit 113 is used as the current amplitude (ΔI), and the output current of the secondary transfer power supply 118 is (I 転写 ), and the output current (I 転写 ) of the secondary transfer power supply 118 is controlled according to the procedure of FIG. 9 described in the first embodiment. That is, the CPU 10a of the control unit 10 determines that the output current (I 転写 ) of the secondary transfer power supply 118 is the transfer current target value (I TThe duty of the PWM signal sent from the transfer power supply control unit 10c of the control unit 10 to the secondary transfer power supply 118 is controlled so that the above-mentioned condition is satisfied.

[0072] In this embodiment, the current detection unit 113 detects the current using a current detection resistor connected in series between the drive roller 133 and the primary transfer power supply 142. In this embodiment, the primary transfer voltage is controlled to a constant voltage. Furthermore, in image forming apparatuses equipped with an intermediate transfer belt 130, including the image forming apparatus 100 of this embodiment, the relationship "secondary transfer power supply voltage > primary transfer power supply voltage" is often established. This is because recording material R is not present in the primary transfer nip Np, whereas recording material R is present in the secondary transfer nip Nt. Therefore, the current from the primary transfer power supply 142 returns to GND via the primary transfer roller 126 and the photosensitive drum 122. Furthermore, because the intermediate transfer belt 130 is generally an insulator, current rarely flows through the surface of the intermediate transfer belt 130. Therefore, the current from the secondary transfer nip Nt passes through the current detection unit 113, merges with the current from the primary transfer power supply 142, and returns to GND via the primary transfer roller 126 and the photosensitive drum 122.

[0073] Therefore, in the configuration of this embodiment as well, the current detection unit 113 can detect the AC component current flowing through the secondary transfer nip portion Nt, which is necessary to suppress AC banding.

[0074] In this embodiment, the drive roller (opposing member) 133 and the primary transfer power supply 142 are connected to each other, but the present invention is not limited to this configuration. For example, the drive roller (opposing member) 133 may be independently connected to GND and electrically grounded. In this case, too, by providing a current detection unit 113 between the drive roller (opposing member) 133 and GND, AC banding can be suppressed by the same control as above.

[0075] Furthermore, although the current detection unit 113 has been described as an example in this embodiment, a current-to-voltage conversion unit or a voltage detection unit may be used, as described in the first embodiment. It is sufficient to be able to detect fluctuations (AC components) in the transfer current caused by the current flowing from the commercial power supply 30 through the recording material R to the secondary transfer nip Nt. In this case, the image forming apparatus 100 has a voltage detection unit as a voltage detection unit connected between the drive roller 133 and GND in the current path that flows from the secondary transfer power supply 118 to GND via the secondary transfer roller 117 and the drive roller 133. This also allows the same effect as in this embodiment to be achieved by controlling the secondary transfer power supply 118 so as to suppress fluctuations in the current flowing in this current path.

[0076] As described above, in this embodiment, the image forming apparatus 100 includes an image carrier (photosensitive drum) 11 that carries a toner image, a developing device 15 that supplies toner to the image carrier 11 to form a toner image on the image carrier, a circumferentially movable intermediate transfer body (intermediate transfer belt) 130 to which the toner image is transferred from the image carrier 11, an opposing member (drive roller) 133 that contacts the inner peripheral surface of the intermediate transfer body 130, a transfer member (secondary transfer roller) 117 that contacts the outer peripheral surface of the intermediate transfer body 130 and sandwiches the intermediate transfer body 130 between itself and the opposing member 133 to form a transfer portion (secondary transfer nip portion) Nt, and transfers the toner image from the intermediate transfer body 130 to the recording material R at the transfer portion Nt, a transfer power source (secondary transfer power source) 118 that applies a voltage to the transfer member 117, and a fixed transfer member (drive roller) 133 that is disposed downstream of the transfer portion Nt in the conveying direction of the recording material R and sandwiches the recording material R. The fixing device 20 forms a fixing nip portion Nf, and is provided with a heating source 23 for heating the recording material R sandwiched in the fixing portion Nf, and a heating member 21 that contacts the recording material R in the fixing portion Nf. The heating source 23 generates heat when voltage is applied from an AC power source 30, and heats the recording material R via the heating member 21. The fixing device 20 has a detection unit (current detection unit) 113 that is connected between the opposing member 133 and GND without the transfer member 117 and detects current or voltage (current in this embodiment), and a control unit 10 that controls the transfer power supply 118. The control unit 10 controls the current output from the transfer power supply 118 based on the detection result by the detection unit 113 when a toner image is transferred from the intermediate transfer body 130 to the recording material R in the transfer portion Nt and the recording material R is heated in the fixing portion Nf. That is, in this embodiment, the image forming apparatus 100 has a detection unit (current detection unit) 113 that is connected between the opposing member 133 and GND in a current path that runs from the transfer power source 118 through the transfer member 117 and the opposing member 133 to GND, and detects a current or a voltage (current in this embodiment). Also, in this embodiment, the intermediate transfer body 130 is an intermediate transfer belt that is an endless belt.

[0077] As described above, according to this embodiment, similar to the monochrome image forming apparatus of the first embodiment, image defects caused by current from a commercial power source can also be suppressed in a color image forming apparatus.

[0078] [others] Although the present invention has been described above with reference to specific embodiments, the present invention is not limited to the above-described embodiments.

[0079] For example, the information described in the above embodiment as being input from an external device may be input from an operation unit provided in the image forming apparatus.

[0080] Furthermore, the control for suppressing AC banding described in the above embodiment may be performed only when a predetermined condition is met, such as when the current amplitude (ΔI) detected by the current detection unit exceeds a predetermined threshold. [Explanation of symbols]

[0081] 1. Image forming device 10 Control Unit 13 Current detection unit 11 Photosensitive drum (image carrier) 17 Transfer roller (transfer member) 18 Transfer power supply 20 Fixing device 21 Heating element 22 Fixing film (first fixing member) 23 Heater (heat source) 24 Pressure roller (second fixing member, pressure member) 100 Image forming device (intermediate transfer type) 113 Current detection unit 130 Intermediate transfer belt (intermediate transfer body) 133 Drive roller (opposing member) Nt Transfer nip Nf fixing nip R recording material

Claims

1. an image carrier that carries a toner image; a developing device that supplies toner to the image carrier to form a toner image on the image carrier; a transfer member that contacts the image carrier to form a transfer portion and transfers a toner image from the image carrier to a recording material in the transfer portion; a transfer power source that applies a voltage to the transfer member; a fixing device that is disposed downstream of the transfer unit in a conveying direction of the recording material and forms a fixing unit that sandwiches the recording material, the fixing device being provided with a heat source for heating the recording material sandwiched in the fixing unit and a heating member that comes into contact with the recording material in the fixing unit, the heating source generating heat when a voltage is applied from an AC power source and heating the recording material via the heating member; a detection unit that is connected between the image carrier and GND without the transfer member and detects a current or a voltage; a control unit that controls the transfer power supply; and an image forming apparatus characterized in that the control unit controls the current output from the transfer power supply based on the detection result by the detection unit when a toner image is transferred from the image carrier to a recording material in the transfer unit and the recording material is heated in the fixing unit.

2. an image carrier that carries a toner image; a developing device that supplies toner to the image carrier to form a toner image on the image carrier; an intermediate transfer member that is rotatable and onto which a toner image is transferred from the image carrier; an opposing member that contacts the inner circumferential surface of the intermediate transfer body; a transfer member that contacts the outer peripheral surface of the intermediate transfer body, sandwiches the intermediate transfer body between itself and the opposing member to form a transfer section, and transfers a toner image from the intermediate transfer body to a recording material in the transfer section; a transfer power supply that applies a voltage to the transfer member; a fixing device that is disposed downstream of the transfer unit in a conveying direction of the recording material and forms a fixing unit that sandwiches the recording material, the fixing device being provided with a heat source for heating the recording material sandwiched in the fixing unit and a heating member that comes into contact with the recording material in the fixing unit, the heating source generating heat when a voltage is applied from an AC power source and heating the recording material via the heating member; a detection unit that is connected between the opposing member and GND without the transfer member therebetween and detects a current or a voltage; a control unit that controls the transfer power supply; and an image forming apparatus characterized in that the control unit controls the current output from the transfer power supply based on the detection result by the detection unit when a toner image is transferred from the intermediate transfer body to a recording material in the transfer unit and the recording material is heated in the fixing unit.

3. 2. The image forming apparatus according to claim 1, wherein the image carrier is a photosensitive member.

4. 3. The image forming apparatus according to claim 2, wherein the intermediate transfer member is an intermediate transfer belt formed of an endless belt.

5. 5. The image forming apparatus according to claim 1, wherein the detection unit detects a current obtained by superimposing at least a portion of the current output from the AC power supply and at least a portion of the current output from the transfer power supply.

6. The image forming apparatus according to any one of claims 1 to 4, characterized in that the control unit corrects the output current of the transfer power source based on the amplitude of the fluctuation obtained from the detection result by the detection unit and a target value of the current supplied from the transfer power source to the transfer unit, thereby suppressing the fluctuation.

7. an environment detection unit that detects at least one of the temperature and humidity inside or outside the image forming apparatus; 7. The image forming apparatus according to claim 6, wherein the control unit changes the amount of correction based on the result of detection by the environment detection unit.

8. a storage unit for storing an index value correlated with the usage amount of the image forming apparatus; 7. The image forming apparatus according to claim 6, wherein the control unit changes the amount of correction in the correction based on the index value stored in the storage unit.

9. 9. The image forming apparatus according to claim 8, wherein the index value is a value relating to the cumulative number of prints made by the image forming apparatus, or a value relating to the amount of toner consumed or the amount of toner remaining in the developing device.

10. 9. The image forming apparatus according to claim 8, wherein the index value is a value related to an accumulated usage amount of the image carrier.

11. an image carrier that carries a toner image; a developing device that supplies toner to the image carrier to form a toner image on the image carrier; a transfer member that contacts the image carrier to form a transfer portion and transfers a toner image from the image carrier to a recording material in the transfer portion; a transfer power supply that applies a voltage to the transfer member; a fixing device that is disposed downstream of the transfer unit in a conveying direction of the recording material and forms a fixing unit that sandwiches the recording material, the fixing device being provided with a heat source for heating the recording material sandwiched in the fixing unit and a heating member that comes into contact with the recording material in the fixing unit, the heating source generating heat when a voltage is applied from an AC power source and heating the recording material via the heating member; a detection unit connected between the image carrier and GND in a current path from the transfer power source through the transfer member and the image carrier to GND, and configured to detect a current or a voltage; a control unit that controls the transfer power supply; and an image forming apparatus characterized in that the control unit controls the current output from the transfer power supply based on the detection result by the detection unit when a toner image is transferred from the image carrier to a recording material in the transfer unit and the recording material is heated in the fixing unit.

12. an image carrier that carries a toner image; a developing device that supplies toner to the image carrier to form a toner image on the image carrier; an intermediate transfer member that is rotatable and onto which a toner image is transferred from the image carrier; an opposing member that contacts the inner circumferential surface of the intermediate transfer body; a transfer member that contacts the outer peripheral surface of the intermediate transfer body, sandwiches the intermediate transfer body between itself and the opposing member to form a transfer section, and transfers a toner image from the intermediate transfer body to a recording material in the transfer section; a transfer power supply that applies a voltage to the transfer member; a fixing device that is disposed downstream of the transfer unit in a conveying direction of the recording material and forms a fixing unit that sandwiches the recording material, the fixing device being provided with a heat source for heating the recording material sandwiched in the fixing unit and a heating member that comes into contact with the recording material in the fixing unit, the heating source generating heat when a voltage is applied from an AC power source and heating the recording material via the heating member; a detection unit connected between the opposing member and GND in a current path from the transfer power source through the transfer member and the opposing member to GND, and configured to detect a current or a voltage; a control unit that controls the transfer power supply; and an image forming apparatus characterized in that the control unit controls the current output from the transfer power supply based on the detection result by the detection unit when a toner image is transferred from the intermediate transfer body to a recording material in the transfer unit and the recording material is heated in the fixing unit.

Images