Image forming device

Abstract

To provide an image forming apparatus that can reliably notify an operator that an electrifying roller is not in contact with a photoreceptor drum without providing a special sensor.SOLUTION: An image forming apparatus has: an electrifying roller 12 that can contact and separate from a photoreceptor; a developing unit 15 for forming a toner image on a surface of a photoreceptor drum 11; an image sensor 22 that detects the toner image formed on the surface of the photoreceptor drum 11; and a control part 30. The control part 30 can execute a contact and separation determination mode having a first control section in which the toner image is formed on the photoreceptor drum 11 if the state of contact of the electrifying roller 12 with the photoreceptor drum 11 is abnormal, and the toner image is not formed if the state of contact of the electrifying roller 12 is normal, and a second control section in which the toner image is not formed on the photoreceptor drum 11 regardless of the state of contact of the electrifying roller 12. The control part determines the state of contact of the electrifying roller 12 with the photoreceptor drum 11 based on a result of detection performed by the image sensor 22 in the first control section.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to an image forming apparatus including a charging roller, and more particularly to an image forming apparatus in which the charging roller can be moved toward and away from an image carrier. [Background technology]

[0002] A roller image forming device that can be attached to or detached from an image carrier is disclosed, for example, in Japanese Patent Laid-Open No. 2018-17995 (Patent Document 1). In Patent Document 1, the image forming device includes an image forming unit and a control unit, and the control unit controls the image forming unit. The image forming unit has an intermediate transfer belt, which is an image carrier, a secondary transfer roller, and a toner concentration detection unit. A toner image is formed on the surface of the intermediate transfer belt. The secondary transfer roller can be pressed against and separated from the surface of the intermediate transfer belt. The toner concentration detection unit detects the toner concentration of the toner image on the intermediate transfer belt after it has passed the secondary transfer roller. The control unit determines whether the secondary transfer roller is in a pressed state or a separated state based on the toner concentration detected and toner concentration data included in the image data. This provides an image forming device that can detect an abnormality in the secondary transfer roller without delaying the operation of the image forming unit. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 2018-17995 Summary of the Invention [Problem to be solved by the invention]

[0004] Conventional image forming apparatuses have been configured as described above. In Patent Document 1, the state in which the secondary transfer roller is separated from the intermediate transfer belt, which is an image carrier, is determined by using an image density sensor to detect a toner image that has not been secondarily transferred to paper. In other words, when a toner image has been previously formed on the image carrier, the image density sensor detects a toner image that has not been secondarily transferred to paper at the secondary transfer unit. However, if a toner image has not been formed on the image carrier upstream of the position corresponding to the charging roller in the rotation direction of the image carrier, such as the charging roller, it is not possible to determine whether the roller is separated.

[0005] This invention has been made to solve the above-mentioned problems, and aims to provide an image forming apparatus that can detect the contact / separation state of a charging roller that can be contacted and separated from a photosensitive drum (image carrier) by a separation / contact mechanism, and that can reduce the amount of toner consumed when detecting the state in which the charging roller is separated from the photosensitive drum. [Means for solving the problem]

[0006] The image forming apparatus according to the present invention includes a photosensitive drum, a charging roller that charges the surface of the photosensitive drum and is removably attached to the photosensitive drum by a contact / separation mechanism, a developing unit having a developing roller for forming a toner image on the surface of the photosensitive drum, an image sensor that detects the toner image formed on the surface of the photosensitive drum, and a control unit. The control unit is capable of executing a contact / separation determination mode having a first control section in which a toner image is formed on the photosensitive drum if the contact state of the charging roller with the photosensitive drum is abnormal and toner adhesion does not occur if the contact state of the charging roller is normal, and a second control section in which a toner image is not formed on the photosensitive drum regardless of the contact state of the charging roller, and the control unit determines the contact state of the charging roller with the photosensitive drum based on the detection results of the image sensor in the first control section and the second control section.

[0007] That is, in the first control section of the contact / separation determination mode executable by the control unit, if the charging roller is separated from the photosensitive drum, a toner image is formed (using fogging development), and if the charging roller is in contact with the photosensitive drum, a toner image is not formed (because fogging development does not occur), so the image sensor can determine the contact state of the charging roller with the photosensitive drum. Also, in the second control section of the contact / separation determination mode executable by the control unit, a toner image is not formed even when the charging roller is separated from the photosensitive drum (because this is a condition in which fogging development does not occur), so the amount of toner image required to determine the contact state of the charging roller can be reduced.

[0008] Preferably, the device further includes a charging power supply that applies a predetermined voltage to the charging roller, and a developing power supply that applies a predetermined voltage to the developing roller, and the control unit applies predetermined voltages to the charging roller and developing roller under the same conditions as during image formation in the first control section, and applies a voltage of opposite polarity to that during image formation to the developing roller in the second control section, and applies no voltage to the charging roller, or applies a voltage less than the discharge start voltage of the charging roller.

[0009] More preferably, the device further includes a transfer member that transfers the toner image on the photosensitive drum to paper and a transfer power supply for the transfer member, and the control unit applies a voltage of opposite polarity to the transfer member during the first control section, which is different from that during image formation.

[0010] The control unit preferably controls the duration of the first control interval to be shorter than the duration of the second control interval.

[0011] In one embodiment of the present invention, the image forming device includes a color image forming device that outputs images of multiple colors, and the color image forming device has multiple pairs of charging rollers and developing rollers, each including a photosensitive drum, and an intermediate transfer body onto which the toner images formed on each photosensitive drum are transferred, in order to output images of multiple colors, and the control unit determines the contact state of the multiple pairs of charging rollers and developing rollers with the photosensitive drum based on the detection results of an image sensor arranged to detect the toner image on the intermediate transfer body.

[0012] Even in cases where there are multiple pairs of charging rollers and developing rollers, such as in a color image forming device, it is possible to determine which pair of charging rollers and developing rollers the detected image belongs to based on the elapsed time from the start of the first control section.

[0013] The control unit may identify pairs of charging rollers and developing rollers that have abnormal contact with the photosensitive drum by measuring and comparing the time from the start of application of a predetermined voltage to the time when the toner image is detected by the image sensor for multiple pairs of charging rollers and developing rollers.

[0014] The control unit may individually set the time of the first control section for each of a plurality of pairs of charging rollers and developing rollers, and identify pairs of charging rollers and developing rollers having abnormal contact conditions with the photosensitive drum by comparing the detection time of the toner image detected by the image sensor with the time of each first control section.

[0015] In the first control section, the control unit may control the plurality of sets of charging rollers and developing rollers so that the absolute value of the voltage applied to the developing roller is smaller than that during image output.

[0016] In the first control section, the control unit may apply a voltage to each of a plurality of pairs of charging rollers and developing rollers at a rate different from the voltage applied during image output, and identify pairs of charging rollers and developing rollers that have an abnormal contact state with the photosensitive drum by comparing the magnitude of the detection value detected by the image sensor with the voltage applied to the developing rollers.

[0017] The control unit may determine the contact state of the plurality of pairs of charging rollers and developing rollers with the photosensitive drum by implementing the first control section and the second control section multiple times for the plurality of pairs of charging rollers and developing rollers.

[0018] The photosensitive drum and the charging roller constitute a process unit, and when replacing the process unit, the control unit may determine the contact state of the charging roller with the photosensitive drum based on the presence or absence of a toner image on the surface of the photosensitive drum in the first control section and the second control section, as detected by the image sensor.

[0019] The device may further include a display unit, and when the control unit determines that the charging roller is not in contact with the photosensitive drum, the control unit may display a notification on the display unit to prompt the user to check the contact state of the charging roller with the photosensitive drum.

[0020] In addition, when the control unit determines that the charging roller is not in contact with the photosensitive drum, the control unit may display a notification on the display unit to prompt the user to check the contact state of the charging roller of the process unit that has been determined to be not in contact with the photosensitive drum. [Effects of the Invention]

[0021] According to this invention, the image forming apparatus can use an image sensor pre-installed in the image forming apparatus to check the contact state of the charging roller with the photosensitive drum in a first control section that can be executed by the control section of the image forming apparatus. Also, by providing a second control section that can be executed by the control section of the image forming apparatus, it is possible to prevent a toner image from being formed on the photosensitive drum even when the charging roller is not in contact with the photosensitive drum.

[0022] As a result, it is possible to provide an image forming apparatus that can detect the contact / separation state of the charging roller with respect to the photosensitive drum and reduce the amount of toner consumed.

[0023] The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description of the preferred embodiments, which proceeds with reference to the accompanying drawings. [Brief explanation of the drawings]

[0024] [Figure 1] 1 is a diagram illustrating a configuration around a photosensitive drum having a process unit according to an embodiment of the present invention. [Figure 2] FIG. 2 is a diagram showing a control sequence of a first control section and a second control section. [Figure 3] 10 is a flowchart showing a method for determining the contact state of the charging roller. [Figure 4] 3A and 3B are diagrams illustrating a contact / separation mechanism for contacting the charging roller with the photosensitive drum. [Figure 5] FIG. 2 is a configuration diagram of the process units for each color in the color image forming apparatus. [Figure 6] 1 is a diagram showing the overall configuration of a color image forming apparatus; [Figure 7] FIG. 10 is a diagram showing a control sequence in a first method for detecting an abnormality in a color image forming apparatus. [Figure 8] 10 is a flowchart illustrating a method for determining the contact state of a charging roller in a color image forming apparatus. [Figure 9] FIG. 10 is a diagram showing a control sequence in a second method for detecting an abnormality in a color image forming apparatus. [Figure 10] FIG. 10 is a diagram showing a control sequence in a third method for detecting an abnormality in a color image forming apparatus. DETAILED DESCRIPTION OF THE INVENTION

[0025] [First embodiment]

[0023] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. Fig. 1 is a diagram showing the configuration around a contact-type charging roller 12 in an image forming apparatus 10 that is a monochrome image forming apparatus 10 according to one embodiment of the present invention, and shows a state in which developer (toner) 17 is transferred onto paper 38 via a photosensitive drum 11 and a transfer unit 18.

[0026] Referring to FIG. 1, an image forming apparatus 10 according to this embodiment includes a grounded photosensitive drum 11 on which a toner image is formed, a charging roller 12 that applies a predetermined charging bias voltage to the photosensitive drum 11, an exposure unit 14 arranged downstream of the charging roller 12, a developing unit 15 arranged downstream of the exposure unit 14, an image density sensor 22 arranged downstream of the developing unit 15, a transfer unit 18 arranged downstream of the image density sensor 22 so as to abut against the photosensitive drum 11 and rotates together with the photosensitive drum 11 to clamp and transport paper 38, thereby transferring the toner image on the photosensitive drum 11 to the paper 38, a discharge unit 19 arranged downstream of the transfer unit 18 to remove potential from the photosensitive drum 11, a waste toner recovery section 20 arranged downstream of the discharge unit 19, and a cleaning blade 21.

[0027] The charging roller 12 is connected to a charging power source 23 that is controlled to a negative constant voltage, and is in contact with the cleaning roller 13. The developing unit 15 includes a developing roller 16 that supplies toner 17 to the photosensitive drum 11, and the developing roller 16 is connected to a negative developing power source 24. The transfer unit (transfer member) 18 is a transfer roller and is connected to a transfer power source 25.

[0028] The image sensor 22 is used to adjust the power supply conditions supplied from the development power supply 24 to the developing roller 16 in order to always form a stable toner image even when the ambient environment of the image forming apparatus 10 or the number of images formed increases (increased usage time) . Specifically, the image sensor 22 detects the image density of the toner image on the photosensitive drum 11 obtained by changing the power supply conditions supplied to the developing roller 16, and changes the power supply conditions supplied to the developing roller 16 so that this is optimized.

[0029] In this embodiment, the image sensor 22 that is already installed in the image forming apparatus 10 determines the contact state between the photosensitive drum 11 and the charging roller 12 when the image forming apparatus 10 is installed or when the process unit (a unit including the photosensitive drum 11 and the charging roller 12) is replaced with a new one, by utilizing so-called fogging, in which a toner image is formed when the charging roller 12 is in a non-contact state.

[0030] That is, in this embodiment, the control unit 30 can execute a contact / separation determination mode having a first control section in which a toner image is formed on the photosensitive drum 11 if the contact state of the charging roller 12 with the photosensitive drum 11 is abnormal, and a toner image is not formed if the contact state of the charging roller 12 is normal, and a second control section in which a toner image is not formed regardless of the contact state of the charging roller 12, and determines the contact state (separation state) of the charging roller 12 with the photosensitive drum 11 based on the detection result (of the toner image) of the image sensor 22 in the first control section.

[0031] As explained with reference to FIG. 1 , if the photosensitive drum 11 and the charging roller 12 are in contact with each other, the charging power supply 23 applies a voltage (e.g., −1200 V) to the charging roller 12 during image formation, uniformly charging the surface of the photosensitive drum 11 to a predetermined potential (e.g., −600 V) via the charging roller 12. The area exposed by the exposure unit 14 according to the print image has a lower potential, forming an electrostatic latent image. The toner 17 charged to the electrostatic latent image area is electrically supplied from the developing roller 16 by the developing bias voltage applied by the developing power supply 24, and visualized as a toner image. Therefore, the image sensor 22 detects density information corresponding to the density of the toner image. However, if exposure is not performed by the exposure unit 14, no electrostatic latent image is formed (there is no or little potential difference between the developing roller 16 and the developing bias applied), so no toner 17 is supplied from the developing roller 16 and no toner image is formed. Therefore, the image density sensor 22 does not detect the toner image. That is, a value corresponding to the surface condition of the photosensitive drum 11 itself, i.e., a bare surface condition (a state without toner), is detected. On the other hand, if the photosensitive drum 11 and the charging roller 12 are spaced apart, the photosensitive drum 11 is not charged via the charging roller 12. Therefore, when a development bias voltage (for example, -400 V) is applied to the developing roller 16, a large potential difference occurs between the developing roller 16 and the photosensitive drum 11, and a toner image is formed by so-called fogging, in which a large amount of toner 17 electrically moves onto the photosensitive drum 11 (all over the area corresponding to the developing roller 16). That is, in the first control section, if the contact state between the charging roller 12 and the photosensitive drum 11 is abnormal, a toner image is formed on the photosensitive drum 11 and is detected by the image sensor 22.

[0032] During normal image formation, a positive power supply is connected to transfer unit 18 to transfer the toner image on photosensitive drum 11 onto paper, but in this embodiment, transfer power supply 25 for transfer unit 18 can be connected to either a positive or negative power supply, and in the first control section, a voltage of opposite polarity to the normal output is applied to transfer unit 18. This configuration makes it possible to prevent transfer unit 18 from becoming soiled with toner 17 even if an abnormality occurs.

[0033] On the other hand, in the second control section, to prevent toner adhesion regardless of the contact state of the charging roller 12 with the photosensitive drum 11, a voltage of, for example, -500 V (which may be 0 V as long as the charging roller 12 does not charge the photosensitive drum 11, i.e., a voltage lower than the discharge start voltage) is applied to the charging roller 12, and a voltage of, for example, +150 V is applied to the developing roller.

[0034] 1, a control unit 30 is provided, which is connected to a charging power supply control unit 23a that controls the voltage applied to the charging power supply 23, a developing power supply control unit 24a that controls the voltage applied to the developing power supply 24, a transfer power supply control unit 25a that controls the voltage applied to the transfer unit 18, the image sensor 22, and a display unit 34 that displays that the charging roller 12 is not in contact with the photosensitive drum 11. The control unit 30 also includes a CPU, memory, etc., and controls the entire image forming apparatus 10.

[0035] A specific description will be given below. Fig. 2 is a diagram showing the control sequence of the first and second control sections described above in a monochrome image forming apparatus 10 according to an embodiment of the present invention. Referring to Fig. 2, from the top, there are shown an On-Off signal of a drive motor (not shown) of photosensitive drum 11, an On-Off signal of the output of charging power supply 23 to charging roller 12, an On-Off signal of the output of developing power supply 24 to developing roller 16, and an output signal of image sensor 22. Here, the first control section is indicated by "a" and the second control section is indicated by "b."

[0036] 2, the On-Off signal for the drive motor of the photosensitive drum 11 is On from the first control interval to the second control interval. The output to the charging roller 12 is initially 0 V, but is turned On in the first control interval a and the above-mentioned negative voltage (for example, -1200 V) is applied, and is turned Off (for example, -500 V) in the second control interval b. The output to the developing roller 16 is initially 0 V, but is turned On in the first control interval a and a negative voltage (for example, -400 V) is applied, and in the second control interval b a positive voltage (for example, +150 V) is applied, and is turned Off and returned to the original 0 V when the second control interval b ends.

[0037] That is, in this embodiment, in the first control section a, the output to both the charging roller 12 and the developing roller 16 is turned on and a predetermined voltage of negative polarity, which is the same condition as during image formation, is applied, and if the photosensitive drum 11 and the charging roller 12 are separated, the conditions for causing fogging development are met, so the fogging phenomenon occurs and a toner image is formed on the photosensitive drum 11, which can be detected by the image sensor 22.

[0038] On the other hand, in the second control section b, the output to the charging roller 12 is off or a voltage less than the discharge start voltage is applied, and the output to the developing roller 16 is a voltage of opposite polarity to that during image formation, resulting in a reverse bias voltage application state, so that fogging does not occur even if the photosensitive drum 11 and the charging roller 12 are separated. Therefore, the toner 17 does not electrically move to the photosensitive drum 11 by the developing roller 16 and adhere to it.

[0039] The output of the image sensor 22 is normally a high signal, but when the charging roller 12 is separated, an image is generated on the photosensitive drum 11 by fog development in the first control section, and a low signal is output as shown by the dotted line in the figure.

[0040] Here, the first control interval (a) is shown as being shorter than the second control interval (b), which shortens the length of the toner image formed by the fogging development in the first control interval (a). This means that the amount of toner image required to confirm the separation of the charging roller 12 can be reduced. In particular, when it takes a long time for the toner image formed in the first control interval (a) to reach the image sensor 22, making the second control interval (b) longer than the first control interval can reduce the amount of toner image formed and the amount of toner consumed. However, this is not a limitation, and the first control interval (a) and the second control interval (b) may be equal.

[0041] The process described above will be explained with reference to the flowchart shown in Fig. 3. Fig. 3 is a flowchart showing a method for determining the contact state between the photosensitive drum 11 and the charging roller 12 (contact / separation determination mode). Referring to Fig. 3, the CPU of the control unit 30 first rotates a drive motor (not shown) to rotate the photosensitive drum 11. (Step S11, steps will be omitted below). Then, as the first control section, a first charging bias voltage (for example, the above-mentioned -1200V) is applied to the charging roller 12, and a first developing bias voltage (for example, the above-mentioned -400V) is applied to the developing roller 16 (S12). It is determined whether a first time has elapsed (S13). Here, the first time is the contact / separation confirmation time of the charging roller required to form a band-shaped image by fog development on the photosensitive drum 11. If the first time has not elapsed in S13 (N in S13), the process returns to S12. As mentioned above, the exposure unit 14 is not operated.

[0042] If the first time period has elapsed (Y in S13), then, as the second control period, a second charging bias voltage (for example, the above-mentioned -500 V) is applied to the charging roller 12, and a second developing bias voltage (for example, the above-mentioned +150 V) is applied to the developing roller 16 (S14). It is determined whether the image sensor 22 has detected an image (whether a low signal has been detected) (S15).

[0043] If the image sensor 22 does not detect an image (signal low) in S15 (N in S15), the control unit 30 determines whether the second time (the time required for the image density sensor 22 to detect an image (signal low)) has elapsed (S16), and if it has not elapsed (N in S16), returns to S14. If it has elapsed (Y in S16), the application of bias voltage to the charging roller 12 and the developing roller 16 is stopped, and the rotation of the drive motor is stopped (S17).

[0044] If the image density sensor 22 detects an image (signal Low) in S15 (Y in S15), it determines that the charging roller 12 is not in contact with the photosensitive drum 11, stops applying bias voltage to the charging roller 12 and the developing roller 16, stops the rotation of the drive motor, and then displays on the display unit 34 that the charging roller 12 is not in contact with the photosensitive drum 11 (S18).

[0045] As described above, in this embodiment, the control unit 30 controls the development bias voltage and the charging bias voltage to implement a separation / contact determination mode having a first control section, which is a section where fog development can occur, and a second control section where fog development will not occur.

[0046] This allows the control unit 30 to execute a contact / separation determination mode using the existing image sensor 22, effectively determining the contact / separation state of the charging roller 12 with the photosensitive drum 11 without the need for an ammeter or dedicated sensor.

[0047] In other words, in the first control section of the contact / separation determination mode executable by the control unit 30, if the charging roller 12 is separated from the photosensitive drum 11, a toner image is formed (due to fogging development), and if the charging roller 12 is in contact with the photosensitive drum 11, a toner image is not formed (because fogging development does not occur), so the image sensor can determine the contact state of the charging roller 12 with the photosensitive drum 11. Also, in the second control section of the contact / separation determination mode executable by the control unit 30, a toner image is not formed even when the charging roller 12 is separated from the photosensitive drum 11 (because this is a condition in which fogging development does not occur), so the amount of toner image required to determine the contact state of the charging roller 12 can be reduced.

[0048] Furthermore, when the control unit determines that the charging roller is not in contact with the photosensitive drum, the display unit displays a warning to check the contact state of the charging roller with the photosensitive drum, so that the worker can notice that an abnormality has occurred in the contact state and can take action promptly.

[0049] In this embodiment, the duration of the first control section and the second control section is, for example, 130 ms each, and the toner consumption under these operating conditions is approximately 0.7 g.

[0050] This is approximately half the toner consumption of 1.4 g under the operating conditions at the time of initial investigation of the present invention.

[0051] The only difference between the operating conditions in the initial study of the present invention and this embodiment is that the charging voltage and developing voltage in the second control section are -1200 V and -400 V, respectively; all other operating conditions are the same. Specific details of the operating conditions in each embodiment described here will be explained later using Table 1.

[0052] Next, a mechanism (separation mechanism) for separating and contacting the charging roller 12 with the photosensitive drum 11 in the process unit will be described. Figure 4 is a diagram showing an example of a state in which an operator contacts the charging roller 12 with the photosensitive drum 11 when installing the image forming apparatus 10 or replacing the process unit with a new one. Here, the charging roller 12 is contacted with the photosensitive drum 11 by pushing the knob 40 to the right in Figure 4. Here, the top of Figure 4 shows a separated state in which the knob 40 is pulled out, and the bottom of Figure 4 shows a contact state in which the knob 40 is pushed in, as indicated by the arrow in the figure.

[0053] 4, the charging roller 12 is shipped in a state separated from the photosensitive drum 11, so there is a gap between the charging roller 12 and the photosensitive drum 11. This prevents deformation and deterioration of the surface elastic layer when the charging roller 12 is left unused for a long period of time.

[0054] On the other hand, when the charging roller 12 is to be brought into contact with the photosensitive drum 11, the knob 40 is pushed in, as shown in the bottom of FIG. 4. Here, as shown in the top of FIG. 4, the charging roller 12 and the cleaning roller 13 are held by a single bearing, and a pawl 35 is provided to maintain the charging roller 12 spaced apart from the photosensitive drum 11. When the knob 40 is pushed in from the state in which the charging roller 12 is spaced apart, the locking portion 36a of the locking member 36 formed integrally with the knob 40 and engaged with the pawl 35 moves to the right in FIG. 4, and the engagement with the pawl 35 is released. At this time, the biasing member 37, which biases the bearing toward the photosensitive drum 11, pushes up the bearing, and the charging roller 12 comes into contact with the photosensitive drum 11.

[0055] [Second embodiment] Next, another embodiment of the present invention will be described. In the above embodiment, the case where the present invention is applied to the image forming apparatus 10 being a monochrome image forming apparatus 10 has been described, but the case where the present invention is applied to a color image forming apparatus will be described below.

[0056] Fig. 5 is a diagram corresponding to Fig. 1 in the previous embodiment, in which the present invention is applied when image forming apparatus 10 is a color image forming apparatus 50. Referring to Fig. 5, in this embodiment, color image forming apparatus 50 includes a yellow (Y) image forming section 60, a magenta (M) image forming section 70, a cyan (C) image forming section 80, and a black (K) image forming section 90.

[0057] Similar to the monochrome image forming apparatus 10 shown in FIG. 1, the yellow (Y) image forming section 60 has a yellow (Y) photosensitive drum 61, a yellow (Y) charging roller 62, a yellow (Y) cleaning roller 63, a yellow (Y) exposure unit 64, a yellow (Y) developing unit 65, a yellow (Y) developing roller 66, a yellow (Y) developer (toner) 67, a yellow (Y) de-electrification unit 68, a yellow (Y) waste toner collection section 69a, and a yellow (Y) cleaning blade 69b.

[0058] Similarly, the magenta (M) image forming unit 70, the cyan (C) image forming unit 80, and the black (K) image forming unit 90 have photosensitive drums 71, 81, and 91 of the respective colors, charging rollers 72, 82, and 92 of the respective colors, cleaning rollers 73, 83, and 93 of the respective colors, exposure units 74, 84, and 94 of the respective colors, developing units 75, 85, and 95 of the respective colors, developing rollers 76, 86, and 96 of the respective colors, developers (toners) 77, 87, and 97 of the respective colors, discharge units 78, 88, and 98 of the respective colors, waste toner collection units 79a, 89a, and 99a of the respective colors, and cleaning blades 79b, 89b, and 99b of the respective colors.

[0059] The toner images of each color formed in the image forming units 60, 70, 80, and 90 are transferred to the intermediate transfer belt 54 via the respective intermediate transfer rollers (intermediate transfer members) 55a to 55d, forming color toner images on the intermediate transfer belt 54. In order to transfer to the intermediate transfer belt 54, the respective intermediate transfer rollers 55a to 55d are provided with intermediate transfer power supply control units 56a to 56d that apply bias voltages to the respective intermediate transfer rollers 55a to 55d.

[0060] The intermediate transfer belt 54 is wound around a drive roller 51a and a driven roller 51b, and a nip is formed between the drive roller 51a and the secondary transfer roller 52. A paper sheet (not shown) is sandwiched in the nip and transported, and the color toner image on the surface of the intermediate transfer belt 54 is transferred onto the paper sheet.

[0061] As shown in FIG. 5, color image forming apparatus 50 also has control unit 59, similar to FIG. 1, and control unit 59 is connected to charging power supply control units 58a to 58d that control the voltage applied to charging power supplies 58a to 38d for each color (not shown), intermediate transfer power supply control units 56a to 56d that control the voltage applied to intermediate transfer rollers 55a to 55d, image sensor 53 that detects the toner image on intermediate transfer belt 54, and display unit 34 that displays that charging rollers 62, 72, 82, and 92 are not in contact with photosensitive drums 61, 71, 81, and 91, etc.

[0062] Here, the image forming units 60, 70, 80, and 90 are arranged at intervals of, for example, 90 mm from one another, and in the first control section, which is a section where fogging occurs, the application time for applying bias voltage to the photosensitive drums 61, 71, 81, and 91 of the image forming units 60, 70, 80, and 90 is set to be shorter than the photosensitive drum pitch / process speed. Note that the process speed refers to the belt rotation speed of the intermediate transfer belt 54.

[0063] That is, the control unit 59 applies the bias voltage to each of the developing rollers 66, 76, 86, and 96 in the first control section for a time shorter than the pitch / process speed of the photosensitive drums 61, 71, 81, and 91, and outputs the bias voltage to each of the developing rollers 66, 76, 86, and 96 sequentially at predetermined time intervals so that the images are not transferred onto the intermediate transfer belt 54 in an overlapping manner.

[0064] By configuring in this manner, even if all the charging rollers 62, 72, 82, and 92 of the image forming units 60, 70, 80, and 90 are separated from each other, the fog toner images are transferred onto the intermediate transfer belt 54 without overlapping, so that the separation states of the charging rollers 62, 72, 82, and 92 can be reliably detected.

[0065] Next, we will explain the overall configuration of the color image forming apparatus 50. Fig. 6 is a diagram showing the overall configuration of the color image forming apparatus 50. As shown in Fig. 6, the yellow (Y) image forming section 60, the magenta (M) image forming section 70, the cyan (C) image forming section 80, and the black (K) image forming section 90 each have process units 60a, 70a, 80a, and 90a, respectively, and are integrated with developing units 65, 75, 85, and 95.

[0066] In the color image forming apparatus 50, each of the process units 60a, 70a, 80a, and 90a has a mechanism (separation mechanism) for separating and contacting the charging rollers 62, 72, 82, and 92 with the corresponding photosensitive drums 61, 71, 81, and 91. The specific configuration is the same as that of the monochrome image forming apparatus 10 described in FIG. 4, and therefore a description thereof will be omitted.

[0067] Next, we will explain a method (contact / separation determination mode) for detecting the contact / separation state (abnormality) between the charging rollers 62, 72, 82, and 92 of each color and the photosensitive drums 61, 71, 81, and 91 in the color image forming apparatus 50. Figure 7 is a diagram showing the control sequence of the first control section and the second control section in this method (first method), and corresponds to Figure 2 for the monochrome image forming apparatus 10. 7 shows, from top to bottom, the On-Off signals of the drive motors of the photosensitive drums 61, 71, 81, and 91 of the image forming units 60, 70, 80, and 90 for each color, the On-Off signal of the output of the photosensitive drum 91 of the black (K) image forming unit 90 to the charging roller 92, the On-Off signal of the output of the developing roller 96, the On-Off signal of the output of the cyan (C) image forming unit 80 to the charging roller 82, the On-Off signal of the output of the developing roller 86, the On-Off signal of the output of the magenta (M) image forming unit 70 to the charging roller 72 and the On-Off signal of the output of the developing roller 76, the On-Off signal of the output of the yellow (Y) image forming unit 60 to the charging roller 62 and the On-Off signal of the output of the developing roller 66, and the output signal of the image sensor 53. Here, the first control section is indicated by a, and the second control section is indicated by b.

[0068] 7, the On-Off signals for the drive motors of the photosensitive drums 61, 71, 81, and 91 of the image forming units 60, 70, 80, and 90 for each color are kept on from the first control interval to the second control interval. The output to the black (K) charging roller 92 is initially off and at 0 V, but is turned on in the first control interval a and a predetermined negative voltage is applied, and then turned off in the second control interval b. The output to the developing roller 96 is initially off and at 0 V, but is turned on in the first control interval a and a predetermined negative voltage is applied, and then a positive voltage, which is a reverse bias voltage, is applied in the second control interval b, and then turned off and returned to 0 V when the second control interval b ends.

[0069] The same applies to the output to the charging roller 82 and developing roller 86 of the cyan (C) image forming unit 80, the charging roller 72 and developing roller 76 of the magenta (M) image forming unit 70, and the output to the charging roller 62 and developing roller 66 of the yellow (Y) image forming unit 60.

[0070] That is, in this embodiment, in the first control section a, the outputs to the charging rollers 62, 72, 82, and 92 of the image forming units 60, 70, 80, and 90 of each color, and the outputs to the developing rollers 66, 76, 86, and 96 are turned on and a negative voltage is applied, and if the photosensitive drums 61, 71, 81, and 91 are separated from the charging rollers 62, 72, 82, and 92, the conditions for causing fogging are met, so the fogging phenomenon occurs and a toner image is formed on the photosensitive drums 61, 71, 81, and 91 by the developing rollers 66, 76, 86, and 96.

[0071] On the other hand, in the second control section b, the output to the charging rollers 62, 72, 82 and 92 is turned off, and the output to the developing rollers 66, 76, 86 and 96 is in a reverse bias state, so even if the photosensitive drums 61, 71, 81 and 91 are separated from the charging rollers 62, 72, 82 and 92, fogging does not occur, and toner does not move to the photosensitive drums 61, 71, 81 and 91 by the developing rollers 66, 76, 86 and 96.

[0072] In FIG. 7, in the second control section for the black (K) charging roller 92, as shown by the dotted line, the power may not be turned off, but may be set to a value lower than the discharge start voltage at which the photosensitive drum 91 is not charged.

[0073] Furthermore, as shown by the dotted lines, the applied voltage in the second control section b for the outputs of the cyan, magenta, and yellow charging rollers 62, 72, and 82 may be set to a voltage lower than the discharge start voltage at which each charging roller 62, 72, and 82 discharges. In this way, the photosensitive drums 61, 71, and 81 can be prevented from being charged in the second control section without turning off the power.

[0074] The bottom of Figure 7 shows the output of image sensor 53. Normally, no image is detected, resulting in a high signal. However, when charging rollers 62, 72, 82, and 92 are spaced apart, a toner image due to fog development is generated on photosensitive drums 61, 71, 81, and 91 during the first control period, resulting in a low signal. This figure shows the case where charging rollers 62, 72, 82, and 92 are spaced apart from developing rollers 66, 76, 86, and 96 in four image forming units 60, 70, 80, and 90. Low signals for black (K), cyan (C), magenta (M), and yellow (Y) generated when images are generated due to fog development are indicated by dotted lines. Because the image forming units 60, 70, 80, and 90 for each color form images at regular time intervals, the outputs of image density sensor 53 due to fog development for each color are also generated at intervals.

[0075] Here, the first control interval (a) is shown to be shorter than the second control interval (b) for the image forming units 60, 70, 80 and 90 of each color. This is because, as shown in FIG. 5, the distance from the image forming units 60, 70, 80 and 90 of each color to the image sensor 53 is long, which increases the time it takes for the toner image formed by fog development to reach the image sensor 53, thereby preventing a large amount of toner from being used in fog development.

[0076] Here, we have explained a case where the output to the charging roller and the output to the developing roller are shown as a first section and a second section for each color, and these are detected individually by the image sensor 53, but this is not limited to this, and an image sensor 53 may be provided in each of the four image forming units 60, 70, 80, and 90 that make up the color image forming device 50.

[0077] As described above, in order to output images of multiple colors, the color image forming device 50 has sets of charging rollers 62, 72, 82, and 92 and developing rollers 66, 76, 86, and 96, each of which individually includes multiple photosensitive drums 61, 71, 81, and 91, and an intermediate transfer belt 54 (intermediate transfer body) to which the toner images formed on each photosensitive drum 61, 71, 81, and 91 are transferred, and the control unit 59 can determine the contact and separation state of the multiple sets of charging rollers 62, 72, 82, and 92 and developing rollers 66, 76, 86, and 96 with respect to the photosensitive drums 61, 71, 81, and 91 based on the detection results of the toner image by the image sensor 53, which is arranged to detect the toner image on the intermediate transfer body belt 54.

[0078] Furthermore, in the color image forming apparatus 50, a predetermined voltage is applied to form an image using a set of multiple charging rollers 62, 72, 82, and 92 and developing rollers 66, 76, 86, and 96, each of which individually includes multiple photosensitive drums 61, 71, 81, and 91. The time from the start of application of the predetermined voltage until the image sensor 53 detects a toner image (formed when fogging occurs) varies depending on the set of charging rollers 62, 72, 82, and 92 and developing rollers 66, 76, 86, and 96 for each color. Therefore, by measuring and comparing the time from the start of application of the predetermined voltage until the image sensor 53 detects a toner image (formed when fogging occurs), it is possible to identify sets of multiple charging rollers 62, 72, 82, and 92 and developing rollers 66, 76, 86, and 96 that are in abnormal contact with the respective photosensitive drums 61, 71, 81, and 91.

[0079] Next, a description will be given of a method (contact / separation determination mode) for determining the contact states of the charging rollers 62, 72, 82, and 92 in the color image forming apparatus 50. FIG.

[0080] Referring to FIG. 8, the CPU of the control unit 59 first rotates a drive motor (not shown) to rotate the ith photosensitive drum 61 (here, i=1, where i is a numerical value (variable) assigned to each color to identify Y, M, C, and K in a control program) among the four photosensitive drums 61, 71, 81, and 91 (S21). The CPU applies a first charging bias voltage to the ith charging roller, and a first developing bias voltage to the ith developing rollers 66, 76, 86, and 96 (S22). The CPU waits for the first time to elapse (S23), and once the first time has elapsed (Y in S23), it applies a second charging bias voltage to the ith charging roller, and a second developing bias voltage to the ith developing roller (S24).

[0081] Next, it is determined whether the image sensor 53 has detected an image with a density that correlates with the first time or the first charging bias (i-th) (signal Low) (S25).

[0082] In S25, if the image density sensor 53 does not detect an image (signal Low) (N in S25), it is determined whether the second time has elapsed (S26). If the second time has elapsed in S26, it is determined whether the last charging unit has finished (S27).

[0083] If the second time has not elapsed in S26 (N in S26), the process returns to S24. If the second time has elapsed in S26 (Y in S26), it is determined whether the last charging unit has finished (S27). If the last charging unit has not finished in S27 (N in S27), i is set to i+1 (S29), and the process returns to S22.

[0084] If the image sensor 53 detects an image (signal Low) with a density correlated with the first time or the first charging bias (i-th) in S25, it is determined that the charging roller is not in contact (Y) in S25, and this is displayed on the display unit 34 as the i-th charging roller not in contact (S30). It is determined whether the last (i=4) charging unit has finished (S31), and if the last (i=4) charging unit has finished (Y in S31), the application of bias voltage to the charging roller and developing roller is stopped, and rotation of the drive motor is also stopped (S32). If the last (i=4) charging unit has not finished in S31 (N in S31), i=i+1 (S33) is set, and the process returns to S22, and this process is repeated until i=4.

[0085] Here, as in the previous embodiment, the first time is the contact / separation confirmation time of the charging rollers 62, 72, 82, and 92 required to form band-shaped images by fog development on the photosensitive drums 61, 71, 81, and 91. As in the first embodiment, the exposure unit 14 is not operated in this embodiment either.

[0086] As described above, in this embodiment, in the color image forming apparatus 50, the developing bias voltage and the charging bias voltage are controlled to create a first control section in which fogging occurs, and the fogging image formed only when the charging rollers 62, 72, 82, and 92 are separated is detected by the image sensor 53. This makes it possible to confirm that the charging rollers 62, 72, 82, and 92 are in a separated (trouble = abnormal) state.

[0087] In other words, even without providing an ammeter or dedicated sensor, the contact / separation state of the charging rollers 62, 72, 82, and 92 with the photosensitive drums 61, 71, 81, and 91 can be effectively determined by executing the contact / separation determination mode using the existing image sensor 53.

[0088] Next, a second method (another separation / contact determination mode) for detecting an abnormality in the color image forming apparatus 50 will be described. FIG. 9 is a diagram showing a control sequence in the second method for detecting an abnormality in the color image forming apparatus 50. Referring to FIG. 9, from the top, there are shown an On-Off signal for the drive motors of the photosensitive drums 81 and 91 of the two color image forming units (cyan (C) and black (K)) 80 and 90, an On-Off signal for the output to the charging roller 92 of the black (K) image forming unit 90, an On-Off signal for the output to the developing roller 96, an On-Off signal for the output to the charging roller 82 of the cyan (C) image forming unit 80, and an On-Off signal for the output to the developing roller 86. Here again, the first control section is indicated by a, and the second control section is indicated by b.

[0089] 9, the On-Off signal for the drive motors of the photosensitive drums 81 and 91 is On from the first control interval to the second control interval. The output to the black (K) charging roller 92 is initially Off, but is turned On in the first control interval a and a predetermined negative voltage is applied, and is turned Off when the second control interval b begins. The output to the developing roller 96 is initially 0 V, but is turned On in the first control interval a and a predetermined negative voltage is applied, and when the second control interval b begins, a positive voltage that is a reverse bias voltage is applied, and is turned Off and returned to the original 0 V when the second control interval b ends.

[0090] The output of the charging roller 82 of the cyan (C) image forming unit 80 and the developing roller 96 in the first control section a is longer than the output of the black (K) charging roller 92, and the output of the second control section b is shorter than the output of the black (K) charging roller 92.

[0091] That is, in this embodiment, the length of the first control period is changed for each image forming unit of each color (here, two colors, black and cyan), so the time for which fog development is performed differs for each color. As a result, if the time at which image sensor 53 detects the fog image is known, it is possible to determine which color's image forming unit's charging roller is separated.

[0092] That is, according to this embodiment, the time of the first control section is individually set for each of a plurality of pairs of charging rollers and developing rollers, and by comparing the detection time of the toner image (formed when fogging development is performed) detected by image sensor 53 with the time of each of the individually set first control sections, it is possible to identify pairs of charging rollers and developing rollers that are abnormal in contact with the photosensitive drums 61, 71, 81, and 91. In other words, it is possible to determine the contact states of the plurality of pairs of charging rollers 62, 72, 82, and 92 and developing rollers 66, 76, 86, and 96 with the photosensitive drums 61, 71, 81, and 91.

[0093] In the above example, the length of the first control period is changed for each image forming unit of each color. In the first control section, the absolute value of the voltage applied to the developing roller for each of the plurality of sets of charging roller and developing roller may be controlled to be smaller than that during image output.

[0094] By doing so, it is possible to lower the density of the toner image formed by the fog development, and in turn to reduce (to the minimum necessary) the amount of toner consumed by the fog development.

[0095] Furthermore, in the first control section, a voltage may be applied to each of the plurality of pairs of charging rollers and developing rollers at a rate different from the voltage at the time of image output, and by comparing the magnitude of the detection value detected by the image sensor with the voltage applied to the developing roller, a pair of charging rollers and developing rollers having an abnormal contact state with the photosensitive drum may be identified.

[0096] By doing this, since the voltage applied to each developing roller is different and the density of the toner image due to fog development formed by each developing roller is different, it becomes possible to accurately identify the pair of charging roller and developing roller that has an abnormal contact state with the photosensitive drum based on the density information of the toner image detected by the image sensor 53.

[0097] Furthermore, when the control unit determines that the charging roller is not in contact with the photosensitive drum, the display unit displays a warning to prompt the operator to check the contact state of the charging roller of the process unit that has been determined not to be in contact with the photosensitive drum, so that the operator can identify the process unit with the abnormality and take action promptly.

[0098] Next, a description will be given of a third method (yet another separation / contact determination mode) for detecting separation / contact in the color image forming apparatus 50. Fig. 10 is a diagram showing a control sequence in the third method for detecting an abnormality in the color image forming apparatus 50.

[0099] 10, there are shown, from the top, the On-Off signals for the drive motors of the photosensitive drums 81 and 91 of the two color image forming units (cyan (C) and black (K)) 80 and 90, the On-Off signal for the output to the charging roller 92 of the black (K) image forming unit 90, the On-Off signal for the output to the developing roller 96, the On-Off signal for the output to the charging roller 82 of the cyan (C) image forming unit 80, and the On-Off signal for the output to the developing roller 86. Here again, the first control section is indicated by a and the second control section by b.

[0100] 10, the On-Off signals for the drive motors of photosensitive drums 81 and 91 are On from the first control interval to the second control interval. The output to black (K) charging roller 92 is initially Off, but is turned On in the first control interval a and a predetermined voltage is applied, and then is turned Off in the second control interval b. Then, in the second first control interval a, the signal is turned On again and a predetermined voltage is applied, and then in the second second control interval b, the signal is turned Off.

[0101] In addition, the applied voltage and duration of the two first control periods a are approximately the same, the duration of the first second control period b is approximately the same as that of the first control period a, and the duration of the second second control period b is longer than that of the first.

[0102] Meanwhile, the output of the developing roller 96 is initially Off, but is turned On in the first control interval a and a predetermined voltage is applied, and then, in the second control interval b, a reverse bias voltage is applied. Then, in the second first control interval a, the output is turned On again and a predetermined voltage is applied, and then, in the second second control interval b, a reverse bias voltage is applied, and when the second control interval b ends, the output is turned Off. That is, in this embodiment, the first control interval a and the second control interval b are repeated twice.

[0103] In addition, the applied voltage and duration of the two first control periods a are approximately the same, the duration of the first second control period b is approximately the same as that of the first control period a, and the duration of the second second control period b is longer than that of the first.

[0104] Below that, the output voltages to the charging roller 82 and developing roller 86 of the cyan (C) image forming unit 80 are shown.

[0105] Here, the output to the charging roller 82 is initially Off, but is turned On in the first control section a to apply a predetermined voltage, and is turned Off again when the second control section b begins. Also, the output voltage to the developing roller 86 is initially Off, but is turned On in the first control section a to apply a predetermined voltage, and is applied a reverse bias voltage when the second control section b begins.

[0106] Here, the length of the first control section a of the cyan (C) charging roller 82 and developing roller 86 is approximately twice as long as the length of the first control section a of the black (K) charging roller 92 and developing roller 96, and the control sections are not repeated as in the case of the black (K) charging roller 92 and developing roller 96.

[0107] That is, in this embodiment, the ratio between the time of the first control section and the time of the second control section is changed for each of the image forming units 60, 70, 80, and 90 of each color by repeating the time of the first control section and the time of the second control section, or by setting the time of the first control section and the time of the second control section to only one time, so that the time and pattern for which fogging development is performed differ for each color. As a result, if the time and pattern at which the image density sensor 53 detects a fogging image are known, it can be determined which color's image forming units 60, 70, 80, and 90 have their charging rollers 62, 72, 82, and 92 spaced apart.

[0108] That is, in this embodiment, by implementing the first control section a and the second control section multiple times for a set of multiple charging rollers 62, 72, 82, and 92 and developing rollers 66, 76, 86, and 96, the contact state of the sets of multiple charging rollers 62, 72, 82, and 92 and developing rollers 66, 76, 86, and 96 with the photosensitive drums 61, 71, 81, and 91 can be determined.

[0109] The absolute value of the voltage applied during the first control period may be set smaller than that during image output. This reduces the amount of toner that leaks out. The operating conditions for this case are shown as Condition F in Table 1.

[0110] In addition, the contact state of the sets of multiple charging rollers 62, 72, 82, and 92 and developing rollers 66, 76, 86, and 96 with the photosensitive drums 61, 71, 81, and 91 can be determined by applying a voltage to the sets of multiple charging rollers 62, 72, 82, and 92 and developing rollers 66, 76, 86, and 96 in the first control section at a different rate than the voltage during image output.

[0111] Next, details of the operating conditions in the above-described embodiment and the amount of toner consumed under those operating conditions will be described. Table 1 shows the operating conditions and the like.

[0112] [Table 1]

[0113] Here, operating condition A corresponds to the embodiment at the time of initial investigation of the present invention, operating condition B corresponds to the control condition explained first, and is explained in the above-mentioned monochrome image forming apparatus 10. Operating condition C corresponds to the condition explained thereafter, and operating conditions D to F each correspond to the above-mentioned embodiment.

[0114] The present invention can be implemented in various other forms without departing from the spirit or main features thereof. Therefore, the above-described embodiments are merely examples and should not be interpreted as being limiting. All modifications and variations within the scope of the claims of the present invention are within the scope of the present invention. [Industrial Applicability]

[0115] The present invention is useful as an image forming device having a charging roller that can be attached to and detached from a photosensitive drum by a detachment mechanism, because it can execute a contact / detachment determination mode that determines the contact / detachment state between the photosensitive drum and the charging roller by utilizing fog development, in which a toner image is formed when the photosensitive drum and the charging roller are not in contact with each other. [Explanation of symbols]

[0116] 10 Image forming device 11 Photosensitive drum 12 Charging roller 13 Cleaning roller 14 Exposure unit 15 Development unit 16 Developing roller 17 Developer (toner) 18 Transcription Unit 19 Static elimination unit 20 Waste toner collection section 21 Cleaning blade 22, 53 Image sensor 23 Charging power supply 24 Power supply for development 25 Transfer power supply 30,59 Control section 34 Display section 38 Paper 50 Color image forming device 54 Intermediate transfer belt 55 Intermediate transfer roller 56 Intermediate transfer power supply 60 Yellow (Y) image forming section 70 Magenta (M) image forming section 80 Cyan (C) image forming section 90 Black (K) image forming section

Claims

1. A photosensitive drum; a charging roller that charges the surface of the photosensitive drum and is removably attached to the photosensitive drum by a contact / separation mechanism; a developing unit having a developing roller for forming a toner image on the surface of the photosensitive drum; An image forming apparatus having an image sensor that detects a toner image formed on the surface of the photosensitive drum and a control unit, the control unit is capable of executing a contact / separation determination mode having a first control section in which a toner image is formed on the photosensitive drum if the contact state of the charging roller with the photosensitive drum is abnormal, and a toner image is not formed on the photosensitive drum if the contact state of the charging roller is normal, and a second control section in which a toner image is not formed on the photosensitive drum regardless of the contact state of the charging roller, the control unit determines a contact state of the charging roller with the photosensitive drum based on a detection result of the image sensor in the first control section; The image forming apparatus, wherein the control unit controls the first control period so that the time of the first control period is shorter than the time of the second control period.

2. a charging power source that applies a predetermined voltage to the charging roller; a development power source that applies a predetermined voltage to the development roller, 2. The image forming apparatus according to claim 1, wherein the control unit applies a predetermined voltage to the charging roller and the developing roller during the first control period, the predetermined voltage being the same as that during image formation, and applies a voltage of an opposite polarity different from that during image formation to the developing roller during the second control period, and applies no voltage to the charging roller or applies a voltage less than the discharge start voltage of the charging roller.

3. a transfer member that transfers the toner image on the photosensitive drum to a sheet of paper, and a transfer power supply for the transfer member; 3. The image forming apparatus according to claim 1, wherein the control section applies a voltage of a polarity different from that during image formation to the transfer member in the first control section.

4. the image forming apparatus includes a color image forming apparatus that outputs an image of a plurality of colors; The color image forming apparatus has a plurality of pairs of charging rollers and developing rollers, each pair including a photosensitive drum, and an intermediate transfer member onto which a toner image formed on each of the photosensitive drums is transferred, in order to output images of a plurality of colors; 2. The image forming apparatus according to claim 1, wherein the control unit determines the contact state of the plurality of sets of charging rollers and developing rollers with the photosensitive drum based on the detection result of the image sensor arranged to detect the toner image on the intermediate transfer body.

5. 5. The image forming apparatus according to claim 4, wherein the control unit measures and compares the time from when a predetermined voltage is applied to each of the plurality of pairs of charging rollers and developing rollers until the toner image is detected by the image sensor, thereby identifying a pair of charging rollers and developing rollers that has an abnormal contact state with the photosensitive drum.

6. 6. The image forming apparatus according to claim 4, wherein the control unit individually sets the time of the first control section for each of the plurality of pairs of charging rollers and developing rollers, and identifies a pair of charging rollers and developing rollers having an abnormal contact state with the photosensitive drum by comparing the detection time of the toner image detected by the image sensor with the time of each of the first control sections.

7. The image forming apparatus according to any one of claims 4 to 6, characterized in that the control unit controls the absolute value of the voltage applied to the developing roller for each of the plurality of charging roller and developing roller pairs in the first control section so that the absolute value of the voltage applied to the developing roller is smaller than that during image output.

8. 5. The image forming apparatus according to claim 4, wherein the control unit applies a voltage to each of the plurality of pairs of charging rollers and developing rollers at a rate different from the voltage at the time of image output during the first control period, and identifies a pair of charging rollers and developing rollers having an abnormal contact state with the photosensitive drum by comparing the magnitude of the detection value detected by the image sensor with the voltage applied to the developing roller.

9. 5. The image forming apparatus according to claim 4, wherein the control unit determines the contact state of the plurality of sets of charging rollers and developing rollers with the photosensitive drum by implementing the first control section and the second control section a plurality of times for the plurality of sets of charging rollers and developing rollers.

10. The photosensitive drum and the charging roller constitute a process unit, The image forming apparatus of any one of claims 1 to 9, characterized in that, when the process unit is replaced, the control unit determines the contact state of the charging roller with the photosensitive drum based on the presence or absence of a toner image on the surface of the photosensitive drum in the first control section and the second control section, as determined by the image sensor.

11. 11. The image forming apparatus according to claim 10, further comprising a display unit, wherein when the control unit determines that the charging roller is not in contact with the photosensitive drum, the control unit displays a notification on the display unit to prompt the user to check the contact state of the charging roller with the photosensitive drum.

12. 11. The image forming apparatus according to claim 10, further comprising a display unit, wherein when the control unit determines that the charging roller is not in contact with the photosensitive drum, the control unit displays a notification on the display unit to prompt the user to check the contact state of the charging roller of the process unit that has determined that the charging roller is not in contact with the photosensitive drum.

Images