JP2009053275A - Potential controller and image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To prevent undesired developer from sticking to an image carrier when restarting the rotation of the image carrier after interruption. <P>SOLUTION: When restarting the rotation of the photoreceptor drum 20 after interruption in the midst of image forming processing, an area S1 located on the downstream side of a charging position P1, also, located on the upstream side of a developing position P2 in a direction shown by an arrow B is moved to the developing position before being moved to the charging position. The area S1 is moved to the developing position with the surface potential reduced from that charged by the charging processing in the interruption period. Thus, a developing bias having a potential corresponding to the surface potential determined in accordance with the interruption period is applied to the developing roll 51 so as to reduce a potential difference between the surface potential and the developing potential, consequently, carrier and undesired toner in developer are prevented from sticking from the developing roll 51 to the surface of the photoreceptor drum 20. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a potential control device and an image forming apparatus.

  In order to reduce the cost and size of the electrophotographic image forming apparatus, the photosensitive drum on which the electrostatic latent image is formed and the developing roll for developing the electrostatic latent image are driven by the same driving means. An image forming apparatus of this type has been developed. When this type of image forming apparatus forms an image, when the photosensitive drum is driven to rotate, the driving of the developing roll is started simultaneously with the rotation.

When such an image forming apparatus forms an image, the charging process is started by charging the surface of the photosensitive drum with a charger, and then the power supply is stopped by the power failure or the power cord being disconnected from the outlet. A jam such as a paper jam may occur. In such a case, the rotational drive of the photosensitive drum and the developing roll is stopped urgently. Thereafter, when the power supply to the image forming apparatus is resumed or the jam is resolved, the photosensitive drum starts to rotate again. At this time, the developing bias must be applied to the developing roll. In this case, there arises a problem that the developer adheres to the photosensitive drum due to the potential difference between the developing potential at that time and the surface potential of the photosensitive drum. For example, if the potential difference between the developing roll and the photosensitive drum is approximately 200 volts (V) or more, the toner contained in the developer adheres to the photosensitive drum, and the unnecessary toner causes each of the photosensitive drum, the transfer unit, and the like to The member is soiled, causing image defects and increasing the amount of toner consumed. If the potential difference between the two is approximately 600 V or more, even the carrier (magnetic material) contained in the developer may adhere to the photosensitive drum and damage the surface. On the other hand, even if a developing bias is applied at the start of rotational driving of the photosensitive drum, if the surface potential of the photosensitive drum is sufficiently low with respect to the developing potential, the developer adheres to the photosensitive drum due to this potential difference. It may end up. For example, in Patent Document 1, in the case of an emergency stop when a jam occurs, the charging process is stopped for a period until the surface of the photosensitive drum to which the transfer voltage is applied in the transfer portion is discharged, and the photosensitive drum is stopped. , And a technique of applying a developing bias having the same developing potential as that of the electrostatic latent image when the rotation of the photosensitive drum is resumed is disclosed.
JP 2000-105523 A

  The present invention has been made in view of such circumstances, and an object thereof is to prevent unnecessary developer from adhering to the image carrier when the rotation of the interrupted image carrier is resumed. .

  In order to solve the above-described problems, the present invention provides a developing unit that is driven in accordance with the rotation of an image carrier and develops an electrostatic latent image formed on the surface of the image carrier with a developer, and the image carrier. When the rotation of the body is interrupted and the rotation is started again, from the start of the rotation until the surface of the image carrier at the position charged by the charging means moves to the position where it is developed by the developing means Power supply control means for controlling the power supply means for supplying power to the development means so that the development potential of the development means becomes a potential corresponding to the surface potential of the image carrier when the rotation is started over the period of A potential control device is provided.

  In the potential control apparatus of the present invention, the electric potential control device further includes a time measuring unit that measures an elapsed time after the rotation of the image carrier is interrupted or the charging by the charging unit is stopped, and the power supply control unit is measured by the time measuring unit. The power feeding unit is controlled such that a difference between the surface potential of the image carrier specified according to the elapsed time and the developing potential of the developing unit is equal to or less than a threshold value. In addition, a potential detection unit that detects a surface potential of the image carrier is provided, and the power feeding control unit is configured to detect a difference between the surface potential of the image carrier detected by the potential detection unit and the development potential of the development unit. You may make it control the said electric power feeding means so that it may become below a threshold value.

  The potential control apparatus according to the present invention further includes a determination unit that determines whether charging by the charging unit is stopped before the interruption, and the power supply control unit is charged by the charging unit before the interruption. When the determination unit determines that the image holding member has stopped, the developing potential of the developing unit is not rotated during the period obtained by subtracting the period in which the image carrier is rotated after the charging is stopped from the period. It is preferable to control the power feeding means so that the electric potential is in accordance with the surface potential of the image carrier when started.

  In the potential control device of the present invention, the first identification information is generated when the charging stopping operation by the charging unit is started, and the second identification information is generated when the charging stopping operation by the charging unit is completed. Identification information generating means for generating information, and identification information storage means for storing the identification information generated by the identification information generating means, wherein the power supply control means stores the first identification information in the identification information storage means. Is stored, and the second identification information is not stored, the power feeding unit is controlled to have a developing potential corresponding to the surface potential of the image carrier, and the identification information storage unit stores the identification information. When the first identification information and the second identification information are stored, it is preferable to control the power supply unit so as not to supply power to the developing unit.

  According to another aspect of the present invention, there is provided a potential control device configured as described above, an image carrier, a charging unit that charges the surface of the image carrier, and a power feeding unit that feeds power according to control by the power feeding control unit of the potential control device. A latent image forming unit that forms an electrostatic latent image on the surface of the image carrier charged by the charging unit, and a transfer unit that transfers the image developed by the developing unit of the potential control device to a recording medium. An image forming apparatus is provided.

  As described above, according to the present invention, it is possible to prevent unnecessary developer from adhering to the image carrier when the interrupted rotation of the image carrier is resumed.

Embodiments of the present invention will be described below with reference to the drawings.
(A) Configuration of Embodiment FIG. 1 is a diagram illustrating a structure of a main part that performs image formation in an image forming apparatus 1 according to the present embodiment. The image forming apparatus 1 includes an image forming unit 10 and an intermediate transfer body 100. The intermediate transfer member 100 is an endless belt member that is rotated in the direction of arrow A in the drawing by a driving mechanism (not shown).

Here, the configuration of the image forming unit 10 will be described.
In addition to the photosensitive drum 20, the image forming unit 10 includes a charger 30, an exposure device 40, a developing device 50, and a transfer device 60 disposed around the photosensitive drum 20. The photoconductive drum 20 has, for example, two to three photoconductive layers including a charge generation layer and a charge transport layer made of OPC (Organic Photo Conductor) as a charge acceptor on a conductive substrate. This is an image carrier formed by the above method. The photosensitive drum 20 is driven to rotate in a direction indicated by an arrow B (counterclockwise direction) shown in the drawing by the driving unit 80 described later with the central axis as a center. The charger 30 is a roll-shaped charging unit whose surface is in contact with the surface of the photosensitive drum 20 at a charging position. Charge to a predetermined potential. In the present embodiment, it is assumed that the surface potential at the charging position is 0 V before charging is performed. The exposure device 40 irradiates (exposes) a laser or LED toward the charged surface of the photosensitive drum 20 to form an electrostatic latent image on the surface of the photosensitive drum 20.

The developing device 50 has a built-in two-component developer containing toner and a carrier formed by coating a magnetic material with a semiconductive material, and stirs the carrier and the toner so that the toner has a negative polarity. To charge. Further, the developing device 50 includes a developing roll 51 that is driven to rotate by a driving unit 80 described later and performs development using toner contained in the developer. When the electrostatic latent image formed on the surface of the photoconductor drum 20 reaches a position (development position) facing the developing roll 51 as the photoconductor drum 20 rotates, development is performed so as to have a predetermined development potential. A developer is supplied by the developing bias applied to the roll 51 to develop the electrostatic latent image, and a toner image is formed on the surface of the photosensitive drum 20.
When the toner image formed on the surface of the photoconductive drum 20 reaches a position between itself and the photoconductive drum 20 with the rotation of the transfer device 60, the transfer device 60 has a polarity opposite to the charged polarity of the toner (positive polarity). ) Is applied, and the toner image on the photosensitive drum 20 is transferred to the intermediate transfer member 100 by electrostatic force or the like. The toner image transferred to the intermediate transfer body 100 is transferred to a recording medium such as paper by a secondary transfer device (not shown). The recording medium is conveyed to a fixing device (not shown), and after the toner image is fixed, the recording medium is discharged out of the apparatus.

Next, FIG. 2 is a block diagram illustrating a hardware configuration of the image forming apparatus 1. As shown in FIG. 2, the image forming apparatus 1 includes a photosensitive drum 20, a charger 30, an exposure device 40, a developing device 50, and a transfer device 60, a control unit 70 that controls each of these units, and a photosensitive unit. A drive unit 80 that rotationally drives the developing drum 51 of the body drum 20 and the developing device 50, a power supply unit 90 that supplies power to each unit of the image forming apparatus 1, and a timer unit 110 are provided. In the figure, solid arrows indicate the flow of various signals, and two-dot chain arrows indicate the main power supply paths.
The control unit 70 stores a central processing unit (CPU) that controls the entire apparatus, a random access memory (RAM) that provides a work area for work, a read only memory (ROM) that stores various control programs, and various data. An internal memory is provided, and arithmetic processing is performed according to the procedure described in the control program stored in the ROM. The control unit 70 stores a development bias control table 71 in the internal memory. In the developing bias control table 71, the rotation of the photosensitive drum 20 is interrupted for some reason, and the rotation is started again. The potential of the developing bias applied to the predetermined developing roll 51 is written.

  FIG. 3 is a diagram showing an example of the development bias control table 71. In the development bias control table 71, “interruption time” and “development bias” are associated with each other. In the “interruption time” field, an elapsed time from the point at which the rotation of the photosensitive drum 20 is interrupted is written. This elapsed time is hereinafter referred to as “interruption time”. In the “developing bias”, the potential of the developing bias applied to the developing roll 51 of the developing device 50 is written. The developing bias is a potential difference from a predetermined reference potential (for example, ground potential). For example, in the first row of the development bias control table 71, the development bias “−500 V” is associated with the interruption time “0 (seconds) to 10 (seconds)”. In other words, in this first row, contents instructing to apply a developing bias of −500 V to the developing roll 51 when the interruption time is 0 second or more and less than 10 seconds are written. Also, as shown in the figure, the development bias is “−250 V” at the interruption time “30 seconds”, the development bias “−150 V” at the interruption time “60 seconds”, and the development bias at the interruption time “120 seconds to (120 seconds or more)”. “0V” is associated with each other. The case where the developing bias is 0 V refers to the case where the developing bias is not applied to the developing roll 51. In FIG. 3, the other interruption time and the corresponding development bias potential are not shown, but writing is performed such that the development bias potential decreases (closer to 0 V) as the interruption time increases. Yes. A specific procedure for using the developing bias control table 71 will be described in detail later.

The drive unit 80 includes a motor and the like, and rotates the photosensitive drum 20 of the image forming unit 10 and the developing roll 51 of the developing device 50 under the control of the control unit 70. The photosensitive drum 20 and the developing roll 51 of the developing device 50 are rotated in accordance with the mutual driving.
The power supply unit 90 is a power supply unit that supplies power supplied from an external commercial power source via a power cord (not shown) to each unit of the image forming apparatus 1 including the developing roll 51 and the charger 30. In order to apply the developing bias to the developing roll 51 at a potential corresponding to the control by the control unit 70, the power feeding unit 90 supplies power at a power feeding potential corresponding to the potential. The power supply unit 90 supplies power to cause the charger 30 to perform a charging process at a predetermined charging potential. The power supply unit 90 has a built-in battery. When power supply from the outside is stopped, the power supply unit 90 supplies power to the control unit 70 and the time measuring unit 110 as a power source.
The clock unit 110 is a real-time clock that operates in synchronization with a clock supplied from a clock generator (not shown), and measures the elapsed time from a predetermined point in time, such as an interruption time, and the measured time. The represented time information is supplied to the control unit 70.

(B) Operation of Embodiment Next, the operation of the image forming apparatus 1 will be described with reference to the flowchart shown in FIG.
In FIG. 4, when an operation unit (not shown) is operated by the user to instruct to form an image on a sheet, the control unit 70 starts processing for forming an image in response to the instruction. To do.
First, the control unit 70 outputs a drive control signal to the drive unit 80, and starts to rotate the photosensitive drum 20 and the developing roll 51 of the developing device 50 (step S1). Subsequently, when the rotational driving of the photosensitive drum 20 and the developing roll 51 is not interrupted (step S2; NO), the control unit 70 controls each part of the image forming apparatus 1 and executes image forming processing. In this image forming process, first, the control unit 70 outputs a power supply control signal to the power supply unit 90 so that the charger 30 is charged with a predetermined charging potential Ve. Here, the charging potential Ve = −700V. When power supply is started by the power supply unit 90 in response to this instruction, the control unit 70 causes the charger 30 to start the charging process, and the surface of the photosensitive drum 20 is charged. Then, the control unit 70 performs processing for forming an electrostatic latent image on the exposure device 40, developing the image on the developing device 50, and transferring the developed image on the transfer device 60 to the intermediate transfer body 100. If the image according to the instruction | indication from a user is formed, the control part 70 will complete | finish an image formation process (step S12; YES).

  By the way, during the execution of the image forming process, the sheet remains in a sheet conveyance path or a sheet feeding tray (not shown) and is not normally conveyed, or some abnormality occurs in the control system of the image forming apparatus 1. “Jam” may occur. Further, power supply to the power supply unit 90 from the outside may be stopped due to a power failure or a power cord being disconnected from the outlet. In such a case, the rotational driving of the photosensitive drum 20 and the developing roll 51 by the driving unit 80 is interrupted, and the control unit 70 detects the interruption of the rotational driving.

  When detecting the interruption of the rotation of the photosensitive drum 20 (step S2; YES), the control unit 70 resets the time measured by the time measuring unit 110 and starts measuring the interruption time (step S3). If the cause of the interruption at this time is a jam, the power supply from the power supply unit 90 is maintained. Therefore, the control unit 70 actively stops the rotational drive by the drive unit 80 and also uses the charger 30. The charging process is also stopped. On the other hand, when the cause of the interruption is a power failure, the power supply to the power supply unit 90 is stopped from the outside. Therefore, the photosensitive drum 20 and the developing roll 51 are driven to rotate at that timing, and the charging process by the charger 30 is performed. Is forcibly interrupted. At this time, the power feeding unit 90 switches to the battery and feeds power to the control unit 70 and the time measuring unit 110.

  In this way, the control unit 70 is in a standby state with the time measuring unit 110 measuring the interruption time (step S4). During this standby state (interruption), the surface potential of the photosensitive drum 20 continues to decrease gradually. This is because the photosensitive drum 20 has a property that injected carriers, thermally excited carriers, and the like are generated even in a state where no light is applied (in the dark), and the surface potential gradually decreases accordingly. This phenomenon is called “dark decay”.

  Here, FIG. 5 is a graph showing the relationship between the interruption time and the surface potential at a certain position on the surface of the photosensitive drum 20. As shown in FIG. 5, the relationship between the interruption time and the surface potential is a function that draws a downwardly convex curve. For example, when the interruption time is “0 second”, the potential is gradually decreased after charging is performed with the charging potential Ve = −700 V, and the surface potential gradually decreases with time. When the interruption time reaches “240 seconds”, the surface potential becomes approximately 0 V, and the photosensitive drum 20 is not charged. Since the surface potential that decreases due to the dark decay is almost uniquely determined by the characteristics of the charge acceptor, the potential can be estimated based on the interruption time. The potential of the developing bias for each interruption time described in the developing bias control table 71 is a predetermined threshold value based on the potential difference between the surface potential of the photosensitive drum 20 estimated by the dark decay and the developing potential of the developing roll 51. It is a numerical value predetermined by calculation or experiment in accordance with the surface potential so as to be as follows.

  Subsequently, when the jam is resolved or the power supply to the power supply unit 90 is started, the control unit 70 determines to resume the interrupted image forming process (step S5; YES). This determination may be performed, for example, when the control unit 70 detects that the jam has been resolved or detects that the power supplied from the outside to the power supply unit 90 exceeds a predetermined amount. Alternatively, after the cause of the interruption is resolved, the user may be instructed to resume by operating an operation unit (not shown).

  And the control part 70 acquires the time information showing the measurement time (namely, interruption time) measured by the time measuring part 110 (step S6). The control unit 70 determines the developing bias potential Vo to be applied to the developing roll 51 based on the interruption time represented by the acquired time information and the developing bias control table 71 shown in FIG. 3 (step S7). Here, for example, if the measurement time is “30 seconds”, the control unit 70 determines to apply a development bias of “−250 V” at the start of rotation of the photosensitive drum 20.

When determining the potential Vo, the control unit 70 supplies a power supply control signal for instructing start of power supply for applying the developing bias to the developing roll 51, and power supply for causing the charger 30 to perform the charging process at the charging potential Ve. A power supply control signal for instructing the start is output to the power supply unit 90 (step S8). Subsequently, the control unit 70 outputs a drive control signal to the drive unit 80, and resumes the rotational driving of the photosensitive drum 20 and the developing roll 51 of the developing device 50 (step S9). Then, when the rotation driving is started, the control unit 70 develops over the period from the start to the time when the surface of the photosensitive drum 20 at the charging position of the charger 30 is moved to the developing position. Control is performed to apply a developing bias of the potential Vo to the roll 51 (step S10).
Here, the reason why the developing bias of the potential Vo is applied over this period after the control unit 70 starts rotating the photosensitive drum 20 will be described.

  FIG. 6 is a diagram for explaining the relationship between the arrangement state of the photosensitive drum 20, the charger 30 and the developing device 50 in the image forming unit 10 and the period during which the developing bias is applied to the developing roll 51. In FIG. 6, the charging position P <b> 1 is a position on the surface of the photosensitive drum 20 that is charged by the charger 30 when the rotation of the photosensitive drum 20 is started. The development position P2 is a position on the surface of the photosensitive drum 20 where the developing device 50 develops the toner when the rotation of the photosensitive drum 20 is started. A region S1 illustrated by a bold line represents a surface region of the photosensitive drum 20 on the downstream side of the charging position P1 and on the upstream side of the developing position P2. On the other hand, the area S2 represents the surface area of the photosensitive drum 20 other than the area S1. Note that the terms “downstream” and “upstream” mean downstream and upstream in the rotation direction (arrow B direction) of the photosensitive drum 20.

  As can be seen from FIG. 6, since the region S1 is downstream of the charging position of the charger 30, when the rotation of the photosensitive drum 20 is started, the developing device is not passed through the charging position of the charger 30. 50 development positions. As described above, since the surface potential of the photosensitive drum 20 is reduced during the interruption period, the region S1 is in a state in which the surface potential is reduced by an amount corresponding to the interruption time from the surface potential when charged by the charging process. Then, it is moved to the development position. At this time, if the potential difference between the surface potential of the region S1 and the development potential of the developing roll 51 is large to some extent, the developer carrier and unnecessary toner adhere to the surface of the photosensitive drum 20 from the developing roll 51 according to the potential difference. Resulting in. This is not related to which of the region S1 and the developing roll 51 has a higher potential, and is a phenomenon that occurs when the potential difference between the two exceeds a certain threshold. In order to prevent this, the potential difference from the developing potential of the developing roll 51 may be reduced when the region S1 is at the developing position. For this purpose, the control unit 70 applies the developing bias having the potential Vo determined according to the interruption time to the developing roll 51 over a period in which the region S1 passes the developing position. That is, this period is a period until the charging position P1 at the start of rotation of the photosensitive drum 20 is moved to the developing position by the developing device 50.

  On the other hand, when the rotation of the photosensitive drum 20 is started, the region S2 is moved to the charging position of the charger 30 before being moved to the developing position of the developing device 50. As described above, since the charging process by the charger 30 is started with the start of the rotation of the photosensitive drum 20, the charging process is performed with the charging potential Ve in the region S2, and the developing position is maintained with the surface potential almost as it is. Moved to. Therefore, the surface potential of the region S2 when moved to the developing position is a high potential close to the charging potential Ve (= −700 V). At this time, if the developing bias of the potential Vo is applied to the developing roll 51, the developing potential of the developing roll 51 is very low when the interruption time is long, so that a large potential difference occurs between them. As a result, toner adheres to the photosensitive drum 20. For this reason, when the development bias corresponding to the interruption time is applied to the development roll 51 as in the above-described area S1 when the area S2 is moved to the development position, the development roll 51 and the photosensitive drum 20 are applied. A large potential difference may occur with the surface (region S2). Therefore, the control unit 70 performs control so that the developing bias having the potential Vo is not applied to the region S2. That is, when the charging position P1 on the photosensitive drum 20 is moved to the developing position by the developing device 50, the developing bias is not applied.

In FIG. 4, when the application of the developing bias Vo is started in step S10, the control unit 70 controls to resume the interrupted image forming process (step S11). By the charging process started with the rotation of the photosensitive drum 20, the area S2 is charged with the charging potential Ve and has a surface potential corresponding to the image forming process. Therefore, the exposure, development, and transfer processes are immediately performed. Can be executed. Therefore, the control unit 70 starts processing to form a latent image at a timing at which the region S2 is moved to a position exposed by the exposure device 40, and when the electrostatic image is moved to the developing position, the electrostatic latent image A developing bias for developing the image is applied to the developing roll 51, and control after the interruption in the image forming process is started.
Thereafter, the control unit 70 repeats the processing steps S2 to S12, and when the image forming process is completed (step S12; YES), the control unit 70 waits until the subsequent image forming process starts or causes the driving unit 80 to wait for the photosensitive member. The rotation drive of the drum 20 and the developing roll 51 is stopped.

  According to the embodiment described above, when the rotation of the photosensitive drum 20 is interrupted and then restarted, the period until the surface of the photosensitive drum 20 at the charging position at the start is moved to the developing position. The developing bias is applied to the developing roll 51 according to the surface potential specified based on the interruption time. In this way, even if rotation of the photosensitive drum 20 and the developing roll 51 is started simultaneously, the potential difference between the developing potential of the developing roll 51 and the surface potential of the photosensitive drum 20 can be reduced. Carriers and toners contained in the agent can be prevented from adhering to the photosensitive drum 20. Therefore, it is possible to prevent scratches due to the carrier and to prevent unnecessary toner from adhering to the photosensitive drum 20 and each member.

(C) Modifications The above embodiment may be modified as follows. Specifically, the following modifications are mentioned, for example. These modifications can be appropriately combined with each other.
(C-1) Modification 1
In the embodiment described above, the surface potential of the photosensitive drum 20 is specified according to the interruption time. However, the surface potential of the photosensitive drum 20 may be detected by a potential sensor.
FIG. 7 is a diagram showing a structure of a main part that performs image formation of the image forming apparatus 2 according to this modification. The image forming apparatus 2 further includes a potential sensor 120 in addition to the configuration of the image forming apparatus 1 shown in FIG. The potential sensor 120 is a sensor that detects the surface potential at a position PV downstream of the charging position P1 and upstream of the developing position P2 with respect to the arrow B direction, and a signal indicating the detected potential is controlled by the control unit. Output to 70. In this figure, the potential sensor 120 is arranged so as to detect the potential upstream of the exposure device 40. This is because the potential sensor 120 has been lowered after the charging process is performed with the charging potential Ve. This is because it is provided for detecting the surface potential of the drum 20 and is based on the reason that it is preferable to detect the surface potential at a position where no electrostatic latent image is formed. Since the surface potential of the photosensitive drum 20 can be detected by the potential sensor 120, the image forming apparatus 2 does not have to have a configuration corresponding to the timer unit 110.

In this modification, the control unit 70 stores a development bias control table 72 in the internal memory instead of the development bias control table 71.
FIG. 8 is a diagram showing an example of the development bias control table 72. In the development bias control table 72, “surface potential” and “development bias” are associated with each other. In the “surface potential” field, the surface potential of the photosensitive drum 20 detected by the potential sensor 120 is written. In the “development bias” field, the magnitude of the development bias applied to the developing roll 51 of the developing device 50 is written. In the first row, the surface potential “−700 V to −550 V” is associated with the development bias “−500 V”, and the surface potential detected by the potential sensor 120 is a potential between −550 V to −700 V. For example, contents for instructing to apply a developing bias of −500 V are written when the image forming process is resumed. Further, as shown in the figure, the development potential “−250 V” is applied to the surface potential “−400 V”, the development bias “−100 V” is applied to the surface potential “−200 V”, and the development bias “0 V” is applied to the surface potential “−100 V to 0 V”. Each is associated. Other surface potentials and the corresponding development bias potentials are not shown, but based on the reason for reducing these potential differences, the smaller the surface potential, the smaller the development bias potential is written. ing.

The operation of the image forming apparatus 2 is performed according to substantially the same procedure as that in the flowchart shown in FIG. 4, but in step S6, the control unit 70 causes the potential sensor 120 to detect the surface potential of the photosensitive drum 20 and detects the detected surface potential. A signal representing the surface potential is obtained. In step S7, the control unit 70 specifies the developing bias potential Vo to be applied to the developing roll 51 based on the surface potential and the developing bias control table 72 shown in FIG. Other processes are the same as those in the above-described embodiment.
According to this configuration, since the surface potential is detected by the potential sensor, the surface potential of the photosensitive drum 20 can be specified more accurately and a developing bias corresponding to the surface potential can be applied. Therefore, it is possible to more reliably prevent the carrier contained in the developer and unnecessary toner from adhering to the photosensitive drum 20.

  In this configuration, the development bias potential Vo may be calculated from the detection result of the potential sensor 120 instead of using the development bias control table 72. In short, it is only necessary to apply the development bias so that the difference between the surface potential and the development potential becomes smaller than the threshold value. For example, the control unit 70 calculates a potential lower by a predetermined potential (for example, 100 V) from the detection result of the surface potential. However, this potential may be applied as a developing bias.

(C-2) Modification 2
In the above-described embodiment, the case where the rotation driving of the photosensitive drum 20 and the developing roll 51 is stopped when the charging process is performed by the charger 30 in the image forming process has been described. By the way, when the charging process related to the image forming process is completed, the charging potential Ve of the charger 30 is not suddenly changed to 0V, but the potential is decreased stepwise from the charging potential Ve, and when a certain time has elapsed, 0V is reached. It is trying to become. Therefore, if the rotation of the photosensitive drum 20 is interrupted during the period from when the charging process is stopped by the charger 30 until the charging potential reaches 0 V, the charging potential at that time is Ve. And different. Therefore, even when the charging potential of the charger 30 is different from Ve after the image forming process is completed, a configuration in which a developing bias corresponding to the surface potential is applied may be employed.

  Here, FIG. 9 is a graph showing the state of the charged potential after the stopping operation of the charger 30 is started. In FIG. 9, the horizontal axis represents time t, and the vertical axis represents the magnitude of the charging potential. As shown in the figure, when the charging process is performed at the charging potential Ve (= −700 V) in the image forming process and the termination is instructed by the control unit 70, the charging process stop operation is started at time t1. The charging potential gradually decreases, and at time t2, the stopping operation of the charger 30 is completed, and the charging potential becomes 0V.

  In order to specify whether the image forming process is in progress, the charger 30 is in a stop operation, or after the stop operation is completed, the control unit 70 charges at time t1. When the stop operation of the device 30 is started, a first flag (first identification information) is generated and stored in the internal memory, and at the time t2 when the stop operation is completed, the second flag (second identification information) is generated. Information) is generated and stored in the internal memory. As a method of determining the time at which the stop operation is completed, for example, a potential at a position downstream of the charging position in the rotation direction of the photosensitive drum 20 (direction of arrow B) and in the vicinity of the charging position is detected. The control unit 70 determines that the stop operation has been completed at a timing when the detection result is almost 0 V and the potential no longer decreases.

  Here, FIG. 10 shows the relationship between the interruption time and the surface potential of the photosensitive drum 20 when the charging device 30 is stopped at a time t12 during the stop operation (that is, from the time t1 to the time t2). It is the graph showing. In FIG. 10, the charging potential Ve1 at time t12 is set to −130V. As shown in the figure, the relationship between the interruption time and the surface potential of the photosensitive drum 20 is a function that draws a downwardly convex curve. When the interruption time is “0 seconds”, the potential is gradually decreased after the charging process is performed with the charging potential Ve1 = −130 V, and the surface potential gradually decreases with time. When the interruption time is “240 seconds”, the surface potential becomes almost 0 V, and the photosensitive drum 20 is not charged. As described above, even when the interruption time is the same, the surface potential of the photosensitive drum 20 is greatly different between the image forming process and the stop operation.

The internal memory of the control unit 70 stores a development bias table determined according to the surface potential during the stop operation as shown in FIG. The configuration is almost the same as that of the developing bias table 71 shown in FIG. 3, but since the charging potential is different, the potential of the developing bias corresponding thereto is written. When the control unit 70 determines that the first flag is stored in the internal memory and the second flag is not stored when the rotation of the photosensitive drum 20 is resumed, the controller 70 is in the stop operation. The potential is determined based on the development bias control table corresponding to the above, and the development bias is applied.
In addition to this, instead of separately providing a corresponding table during the stop operation, for example, a voltage half of the potential specified by the table may be applied as the developing bias using the developing bias control table 71. Further, a potential sensor for detecting completion of the stop operation may be used to apply a voltage within a predetermined threshold as a developing bias from the detection result.

When the charging device 30 is stopped when the charging operation is not performed after the time t2, the charging potential of the charging device 30 is 0 V, so that a developing bias is applied to the developing roll 51. do not have to. Therefore, when the control unit 70 determines that the first flag and the second flag are stored in the internal memory, the control unit 70 performs control so that the developing bias is not applied at the start of rotation of the photosensitive drum 20. Then, the control unit 70 stores the data in the internal memory when the charging process with the charging potential Ve in the image forming process is started again or when the rotation of the photosensitive drum 20 is stopped after the image forming process is completed. Clear the first and second flags.
By the way, even if the stop operation is completed, until the surface at the charging position is moved to the development position when the stop operation is completed, the region is charged downstream from the charging position and upstream from the development position. However, since the surface is charged immediately before the stopping operation of the charger 30 is completed, the surface is at a very low potential. Therefore, the control unit 70 may generate and store the second flag at the timing when it is determined that the stop operation has been completed.
According to this configuration, since the developing bias can be applied at a potential corresponding to the surface potential over the entire period during which the photosensitive drum 20 is rotated, the developer carrier and the toner are more reliably attached. Can be suppressed.

(C-3) Modification 3
In the embodiment described above, the timing unit 110 measures the interruption time after the rotation of the photosensitive drum 20 is interrupted, but may measure the time after the charging process by the charger 30 is stopped. . This is because the surface potential of the photosensitive drum 20 lowered by the dark decay is estimated according to the elapsed time.

(C-4) Modification 4
In the above-described embodiment, the case where the rotation of the photosensitive drum 20 and the charging process by the charger 30 are simultaneously stopped when interrupted has been described. However, when a jam occurs, the control unit 70 performs the charging process by the charger 30. In some cases, the rotation of the photosensitive drum 20 is stopped after a predetermined period of time. Such an operation is performed, for example, when the surface of the photosensitive drum 20 to which the positive primary transfer bias is applied by the transfer device 60 is discharged by a charge remover (not shown) and then stopped when a jam occurs. During this time, the charging process is stopped and the photosensitive drum 20 is rotated. By performing such control, it becomes possible to start the next image forming treatment without leaving a charge having a polarity opposite to the charged polarity on the surface of the photosensitive drum 20, so that good image quality can be maintained. become.

In this case, the surface of the photosensitive drum 20 that has passed through the charging position after the charging process by the charger 30 is stopped is not charged before and after the interruption, and the surface potential is 0V. Therefore, when this surface is moved to the development position, the development potential of the development roll 51 may be 0 V, and it is not necessary to apply a development bias. Therefore, the control unit 70 applies the developing bias from the period from the start of rotation until the surface of the photosensitive drum 20 at the charging position is moved to the developing position, and after the charging process is stopped. What is necessary is just to set it as the period which deducted the period when the drum 20 was rotated. In this case, since the developing bias is not applied when the uncharged surface of the photosensitive drum 20 is moved to the developing position, it is possible to more reliably prevent the adhesion of the developer carrier and unnecessary toner. Can do. On the other hand, when the power supply to the power supply unit 90 is stopped due to a power failure or the like, the rotation of the photosensitive drum 20 is stopped simultaneously with the charging process without performing such control. That is, based on the cause of the rotation being stopped, the control unit 70 determines whether or not the charging process is stopped before the rotation of the photosensitive drum 20 is interrupted, and indicates a period for applying the developing bias. Good.
Further, in this case, since the interruption time after the rotation of the photosensitive drum 20 is interrupted and the elapsed time after the charging process by the charger 30 is stopped, the timing unit 110 stops the charging process. It is preferable to measure the elapsed time from.

(C-5) Modification 5
In the above-described embodiment, the control unit 70 resumes the image forming process immediately after applying the developing bias of the potential Vo to the developing roll 51 in steps S10 and S11. Processing can be resumed. That is, the image forming process can be restarted at an arbitrary timing. In this case, the control unit 70 applies the developing bias of the potential Vo to the developing roll 51 when the region S1 in FIG. 6 passes the developing position while the exposure unit 40 and the transfer unit 60 are stopped, and the charging potential. When the region S2 charged with Ve is moved to the development position, a development bias near the potential Ve is applied as the development bias. Thereafter, the control unit 70 may apply the developing bias to the developing roll 51 until the image forming process is restarted. Then, when the region where the electrostatic latent image is formed is moved to the development position, the control unit 70 applies a development bias for developing the region. In this way, the potential difference between the developing potential and the surface potential can be reduced during the period until the image forming process is restarted, and the adhesion of the developer carrier and unnecessary toner can be prevented.

(C-6) Modification 6
In the embodiment described above, the charging process by the charger 30 is started at the start of the interrupted rotation of the photosensitive drum 20, but the charging process may be started at a later timing. In this case, if FIG. 6 is used, since the region S2 is not charged even if it is moved to the charging position, it is moved to the developing position with the surface potential kept at 0V. Therefore, the control unit 70 applies the developing bias of the potential Vo to the developing roll 51 over a period until the surface at the charging position at the start of the rotation of the photosensitive drum 20 is moved to the developing position. Control is performed so that the developing bias is not applied to the roll 51. In this way, even if the start of the charging process is late, the potential difference between the surface potential and the development potential can be reduced, and the carrier of the developer and unnecessary toner can be prevented from adhering to the photosensitive drum 20. it can.

(C-7) Modification 7
In the above-described embodiment, the case where the rotation of the photosensitive drum 20 is interrupted when the charging process of the image forming process is performed has been described. However, the charging process in another process is being performed. Also good. For example, in a period in which the image forming apparatus waits until a subsequent image forming process, or in a period in which an image forming process is performed by another image forming unit in an image forming apparatus having a plurality of image forming units, The photosensitive drum may remain rotated. In this case, a method is known in which the surface of the photosensitive drum is charged at a potential slightly higher than the developing potential of the developing roll so that unnecessary toner (so-called fog toner) does not adhere during this period. At this time, the charging process is performed at a lower voltage, for example, −150 V than that during the image forming process. Even in such a case, if a developing bias is applied so that the potential difference between the developing potential and the surface potential is reduced, the development is performed in the same manner as in the embodiment when the rotation of the photosensitive drum 20 is interrupted and started. It is possible to prevent adhesion of an agent carrier and unnecessary toner.

2 is a diagram illustrating a structure of a main part that performs image formation in the image forming apparatus 1. FIG. 2 is a block diagram illustrating a hardware configuration of the image forming apparatus 1. FIG. 6 is a diagram showing an example of a development bias control table 71. FIG. 3 is a flowchart showing an operation procedure of the image forming apparatus 1. 4 is a graph showing the relationship between the interruption time and the surface potential of the photosensitive drum 20. FIG. 6 is a diagram for explaining a relationship between the arrangement state of a photosensitive drum, a charger, and a developing device and a period during which a developing bias is applied. 3 is a diagram illustrating a structure of a main part that performs image formation in the image forming apparatus 2. FIG. 6 is a diagram showing an example of a development bias control table 72. FIG. It is a graph showing the mode of the charging potential after the stopping operation of the charger is started. 6 is a graph showing the relationship between the interruption time and the surface potential of the photosensitive drum 20 when the charging device is interrupted during the stopping operation.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1, 2 ... Image forming apparatus 10 ... Image forming unit 100 ... Intermediate transfer body 110 ... Time measuring part 120 ... Potential sensor 20 ... Photosensitive drum 30 ... Charger 40 ... Exposure device 50 ... Developer , 51 ... developing roll, 60 ... transfer device, 70 ... control unit, 71, 72 ... development bias control table, 80 ... drive unit, 90 ... power feeding unit.

Claims (6)

A developing unit that is driven in accordance with the rotation of the image carrier and develops the electrostatic latent image formed on the surface of the image carrier with a developer;
When the rotation of the image holding member is interrupted and the rotation is started again, the surface of the image holding member at the position charged by the charging unit from the start to the position where the developing unit develops the surface. Power feeding for controlling the power feeding means for feeding power to the developing means so that the developing potential of the developing means becomes a potential corresponding to the surface potential of the image carrier when the rotation is started over the period until the movement. A potential control apparatus comprising: a control means. A timing means for measuring the elapsed time since the rotation of the image carrier is interrupted or the charging by the charging means is stopped;
The power supply control unit controls the power supply unit so that a difference between a surface potential of the image holding member specified according to the elapsed time measured by the time measuring unit and a development potential of the developing unit is equal to or less than a threshold value. The electric potential control apparatus according to claim 1, wherein the electric potential control apparatus controls the electric potential control apparatus. Comprising a potential detection means for detecting the surface potential of the image carrier;
The power supply control unit controls the power supply unit so that a difference between a surface potential of the image carrier detected by the potential detection unit and a developing potential of the developing unit is equal to or less than a threshold value. Item 2. The potential control device according to Item 1. Determining means for determining whether charging by the charging means is stopped before the interruption;
If the determination unit determines that charging by the charging unit has stopped before the interruption, the power supply control unit determines a period during which the image carrier is rotated after the charging is stopped from the period. 2. The power feeding unit is controlled so that a developing potential of the developing unit becomes a potential corresponding to a surface potential of the image holding member when the rotation is started over a subtracted period. The potential control device according to 1. Identification information generating means for generating first identification information when the charging stopping operation by the charging means is started, and generating second identification information when the charging stopping operation by the charging means is completed;
An identification information storage means for storing the identification information generated by the identification information generation means,
The power supply control means includes
In a case where the first identification information is stored in the identification information storage means and the second identification information is not stored, the power feeding is performed so that the developing potential corresponds to the surface potential of the image carrier. Control means,
The power supply unit is controlled so as not to supply power to the developing unit when the first identification information and the second identification information are stored in the identification information storage unit. The potential control device described. The potential control device according to any one of claims 1 to 5,
An image carrier,
Charging means for charging the surface of the image carrier;
Power supply means for supplying power in accordance with control by the power supply control means of the potential control device;
A latent image forming means for forming an electrostatic latent image on the surface of the image carrier charged by the charging means;
An image forming apparatus comprising: transfer means for transferring an image developed by the developing means of the potential control device to a recording medium.

Images