JP6195355B2 - Image forming apparatus - Google Patents

Description

  The present invention relates to an image forming apparatus using an electrophotographic recording system such as a laser printer, a copying machine, and a facsimile.

  2. Description of the Related Art Conventionally, image forming apparatuses such as copying machines and laser printers that use an electrophotographic recording method are known. Such an image forming apparatus is required to reduce costs, reduce the size of the apparatus, and the like. Under such circumstances, for example, Patent Document 1 proposes a monochrome printer that applies a voltage from a common power source to a developing unit and a charging unit.

  Conventionally, as a developing method of a process cartridge, a contact developing method in which development is performed in a state where the developing roller of the developing device is in contact with the photosensitive member, and a predetermined gap is provided between the developing device and the photosensitive member. There are two known non-contact development methods for performing development. The contact developing type image forming apparatus is provided with a contact / separation mechanism for contacting and separating the developing unit and the photosensitive member. In such an image forming apparatus, the developing device and the photosensitive member are separated from each other at the time of pre-rotation or post-rotation other than during development image formation. By adopting such a configuration, it is possible to prevent image defects due to a decrease in the life of the process cartridge due to abrasion of the surface of the photoreceptor, deformation of the developing roller of the developing device, fogging due to toner deterioration, and the like.

JP-A-11-102145

  In the case where the power supply of the charging unit and the power supply of the developing unit are made common, the following control is conventionally performed. That is, a high-voltage switching element is added to perform independent control on the power supply of the charging unit and the power supply of the developing unit to adjust the density of the image area (development contrast Vcont) and prevent the transfer of toner to the non-image area. (Development back contrast Vback) is appropriately controlled. However, since the switching element is expensive, the cost reduction effect of the entire apparatus is small even if the power supply of the charging unit and the developing unit is shared, and it is desired to reduce the cost.

  The present invention has been made under such circumstances, and an object thereof is to reduce image defects with an inexpensive configuration.

  In order to solve the above-described problems, the present invention has the following configuration.

(1) An image carrier, a charging unit for charging the image carrier, a developing unit for developing the electrostatic latent image formed on the image carrier with toner, and a charging for applying to the charging unit A charging voltage applying means for generating a voltage and a developing voltage to be applied to the developing means from the charging voltage generated by the charging voltage applying means are connected to the charging voltage applying means through a resistance element. A developing voltage application means for applying the developing voltage to the developing means during a period in which the charging voltage is applied to the charging means, and the electrostatic latent image formed on the image carrier. In order to develop an image with toner, the developing means is brought into contact with the image carrier, and the developing means is separated from the image carrier after the electrostatic latent image on the image carrier is developed with toner . Control And control means, an image forming apparatus having the developing voltage applying means, a switching element for performing constant voltage control of the development voltage, a constant voltage element connected in series with the switching element that, A comparison in which a control voltage generated by dividing a reference voltage is compared with a voltage corresponding to a current flowing through the developing voltage applying unit, and a signal for controlling a supply current to the switching element is output according to the comparison result. And the control means controls to apply or stop applying the charging voltage and the developing voltage by the power supply unit in a state where the developing means is separated from the image carrier. An image forming apparatus.

  According to the present invention, image defects can be reduced with an inexpensive configuration.

Sectional view and system diagram of image forming apparatus of embodiment 1 FIG. 6 is a diagram illustrating rotation of a developing rotary and charging / developing high-voltage power supply unit according to the first embodiment. The figure which shows the scanner part of Example 1, The figure explaining the photosensitive drum film thickness and each electric potential Flowchart for explaining a process for correcting a charging voltage according to the first embodiment. Circuit diagram of charging / developing high-voltage power supply unit of Example 1 FIG. 3 is a circuit diagram of a conventional charging / developing high-voltage power supply unit for comparison with the first embodiment, a diagram illustrating an output range of a charging voltage and a developing voltage, and a diagram illustrating a modification of a developing voltage generation unit. The circuit diagram of the modification of the development voltage generation part for the comparison with Example 1, the figure which shows each image development contact-separation of a developing device, and the table which compared the development voltage generation part Timing chart of charging / developing high-voltage power supply unit of embodiment 1 Sectional view and system diagram of image forming apparatus of embodiment 2 FIG. 6 is a view for explaining the development contact / separation of the developing device of Example 2. The figure which shows the exposure apparatus of Example 2, The figure explaining the photosensitive drum film thickness and each electric potential The flowchart explaining the process which correct | amends the exposure amount of Example 2. FIG. Circuit diagram of charging / developing high-voltage power supply unit of Example 2 The figure explaining the circuit operation of the charge and development high voltage power supply part of Example 2. Timing chart of the charging / developing high-voltage power supply unit of Example 2

  DESCRIPTION OF EMBODIMENTS Hereinafter, embodiments for carrying out the present invention will be described in detail by way of examples with reference to the drawings.

  In the first embodiment, a color laser beam printer including four developing devices is illustrated as a color electrophotographic image forming apparatus (hereinafter simply referred to as an image forming apparatus). The configuration of the image forming apparatus 100 will be described with reference to FIGS. 1 to 4, and the charging / developing high-voltage power supply unit and its operation sequence will be described with reference to FIGS.

(Drum unit)
The drum unit is configured integrally with the photosensitive drum 15 and the container 114. The photosensitive drum 15 is an example of a rotatable image carrier on which an electrostatic latent image is formed, and is a drum-type electrophotographic photosensitive member here. The container 114 is a container of the cleaning device 13 that also serves as a holder for the photosensitive drum 15. A cleaner blade 16 and a charger 17 are arranged around the photosensitive drum 15. The photosensitive drum 15 rotates in the arrow direction (counterclockwise direction) in FIG. 1A according to the image forming operation.

(Charger)
The charger 17 is a conductive roller using a contact charging method. In the charger 17, the surface of the photosensitive drum 15 is uniformly charged by bringing a conductive roller to which a voltage is applied into contact with the photosensitive drum 15. The uniformly charged photosensitive drum 15 is exposed by the scanner unit 30 to form an electrostatic latent image on the photosensitive drum 15 (on the image carrier).

(Cleaning device)
The electrostatic latent image formed on the photosensitive drum 15 is developed into a visible image (hereinafter referred to as a toner image) by a developing device 20 described later. The toner image is transferred from the photosensitive drum 15 to the intermediate transfer belt 9 (hereinafter referred to as primary transfer). The position where the photosensitive drum 15 and the intermediate transfer belt 9 abut, that is, the position where primary transfer is executed is the primary transfer position. The cleaning device 13 cleans toner remaining on the photosensitive drum 15 without being transferred to the intermediate transfer belt 9. The toner removed from the photosensitive drum 15 is stored in the container 114.

(Developer)
The developing rotary 23 that is a rotating body is an example of a developing unit that develops the electrostatic latent image on the photosensitive drum 15 into a toner image using toner that is a developer. The developing rotary 23 is an example of a developing unit including N developing units 20 that develop an electrostatic latent image with N (N ≧ 1) toner. In this embodiment, the developing rotary 23 includes four (N = 4) developing devices 20Y, 20M, 20C, and 20B. The four developing devices 20Y, 20M, 20C, and 20B develop electrostatic latent images using toners of different colors (yellow (Y), magenta (M), cyan (C), and black (B)), respectively. . The developing devices 20Y, 20M, 20C, and 20B are provided with developing sleeves 20YS, 20MS, 20CS, and 20BS, respectively. The developing device 20Y includes a coating roller 20YR and a blade 20YB. The other developing devices 20M, 20C, and 20B have the same configuration. The developing rotary 23 rotates about the shaft 22. As a result, a developing device for developing the electrostatic latent image on the photosensitive drum 15 is selected. In other words, the desired developing device stops facing the photosensitive drum 15. The position where the desired developing device is stopped facing the photosensitive drum 15 is hereinafter referred to as a developing position. For example, FIG. 1A shows a state in which the yellow developing device 20Y is stopped at the developing position.

  At the time of color image formation, the developing rotary 23 rotates 1/4 each time the intermediate transfer belt 9 rotates once in the arrow direction (clockwise direction) in FIG. . The developing process is executed in the order of the yellow developing unit 20Y, the magenta developing unit 20M, the cyan developing unit 20C, and the black developing unit 20B. When the intermediate transfer belt 9 rotates four times, the toner images of yellow, magenta, cyan, and black toner are sequentially transferred onto the intermediate transfer belt 9 in sequence. As a result, a multicolor toner image is formed on the intermediate transfer belt 9.

  In FIG. 1A, as described above, the yellow developing device 20Y is positioned at the developing position. The toner in the container of the developing device 20Y is sent to the application roller 20YR. In FIG. 1A, a thin layer of toner is applied to the outer periphery of the developing sleeve 20YS rotating in the clockwise direction by the applying roller 20YR and the blade 20YB rotating in the counterclockwise direction. At this time, charge is imparted to the toner by charging due to friction. Then, a developing voltage Vdc is applied to the developing sleeve 20YS by a charging / developing high-voltage power supply unit 315 (see FIG. 5), which will be described later, thereby promoting development of the electrostatic latent image on the photosensitive drum 15 with toner. Is done. The magenta developing device 20M, the cyan developing device 20C, and the black developing device 20B are similarly developed. In the following description, when it is not necessary to specify a color, the description may be made by omitting the symbols Y, M, C, and B.

(Intermediate transfer belt)
The intermediate transfer belt 9 is a belt-shaped or cylindrical image carrier on which a toner image on the photosensitive drum 15 is primarily transferred. The intermediate transfer belt 9 transfers the toner image on the photosensitive drum 15 four times at the time of color image formation, and a multicolor toner image is formed on the intermediate transfer belt 9. The intermediate transfer belt 9 rotates in the arrow direction (clockwise direction) in FIG. 1A and sandwiches and conveys the secondary transfer roller 10 to which the transfer voltage is applied and the recording material P. As a result, the multi-color toner images on the intermediate transfer belt 9 are collectively transferred to the recording material P (hereinafter referred to as secondary transfer). The position where the intermediate transfer belt 9 and the secondary transfer roller 10 are in contact, that is, the position where the secondary transfer is performed is the secondary transfer position. A non-image area on the outer periphery of the intermediate transfer belt 9 includes a home position mark (hereinafter referred to as an HP mark) 92 and a HP mark 92 for measuring the circumference of the intermediate transfer belt 9 and as a reference for the image start timing of each color. An optical sensor 91 is provided for detection. Here, the non-image areas are areas at both ends of the intermediate transfer belt 9 in a direction orthogonal to the rotation direction of the intermediate transfer belt 9.

(Paper Feeder)
The paper feed unit is a unit that feeds the recording material P to the image forming unit (more specifically, the secondary transfer position). The cassette 302 stores a plurality of recording materials P. The paper feed roller 303 is driven and rotated in accordance with the image forming operation, and separates the recording materials P in the cassette 302 one by one and feeds them to the conveyance path. The paper feed roller 303 is an example of a feeding unit that feeds the recording material P, on which the toner image is secondarily transferred, to the conveyance path. A registration roller 8 is provided on the conveyance path between the paper feed roller 303 and the secondary transfer roller 10. A shutter 11 provided on the registration roller 8 corrects the skew of the recording material P. The shutter 11 is rotated by the tip of the recording material P. The leading edge detection sensor 40 detects the leading edge of the recording material P by detecting the rotation of the shutter 11. The leading edge detection sensor 40 is an example of a detection unit that is disposed on a conveyance path between the paper feed roller 303 and the secondary transfer roller 10 and detects the recording material P that has been conveyed through the conveyance path.

  The registration roller 8 performs a non-rotating operation for stationaryly waiting the recording material P at the refeed standby position 7 during an image creating operation and a rotating operation for transporting the recording material P toward the secondary transfer position. Here, the rotation and non-rotation operations of the registration roller 8 are performed based on the image formation timing and the timing at which the recording material P is detected by the leading edge detection sensor 40. Thus, the timing at which the toner image on the intermediate transfer belt 9 arrives at the secondary transfer position and the timing at which the recording material P arrives at the secondary transfer position are adjusted to synchronize them. When the registration roller 8 detects the recording material P by the leading edge detection sensor 40, the registration roller 8 conveys the recording material P to a predetermined standby position (for example, the refeed standby position 7) and then stops, and instructs to resume conveyance. This is an example of a transport unit that resumes transport of the recording material P in response.

(Transfer part)
The secondary transfer roller 10 is an example of a secondary transfer unit that secondary-transfers a multicolor toner image carried on the intermediate transfer belt 9 to the recording material P. The secondary transfer roller 10 can swing, for example, and can contact or be separated from the intermediate transfer belt 9. While toner images of four colors are sequentially formed on the intermediate transfer belt 9 (that is, while the intermediate transfer belt 9 rotates four times), the toner image that has already been primarily transferred onto the intermediate transfer belt 9 is not disturbed. The secondary transfer roller 10 is separated from the intermediate transfer belt 9. That is, the secondary transfer roller 10 is retracted or stands by at a position indicated by a solid line in FIG.

  On the other hand, when the four color toner images are superimposed and transferred onto the intermediate transfer belt 9, the secondary transfer roller 10 contacts the intermediate transfer belt 9 in accordance with the timing of transferring the color image onto the recording material P. That is, the secondary transfer roller 10 is moved to the position indicated by the broken line in FIG. At this time, a transfer voltage is applied to the secondary transfer roller 10. Since the intermediate transfer belt 9 and the secondary transfer roller 10 are rotationally driven, the recording material P sandwiched between the two is conveyed toward the fixing unit 25 at the same time as the secondary transfer is performed. The cam member 93, the drive control unit 217 (see FIG. 1B) that drives the cam member 93, and the like are first contact / separation means that contacts or separates the secondary transfer roller 10 from the intermediate transfer belt 9. It is an example. In the fixing unit 25, the unfixed toner image on the recording material P is fixed to the recording material P. Thereafter, the recording material P is discharged out of the image forming apparatus by the discharge roller 36.

  After the toner image is secondarily transferred from the intermediate transfer belt 9 to the recording material P, the toner remaining on the intermediate transfer belt 9 is reversely charged by the ICL roller 39 in contact with the intermediate transfer belt 9. It is charged with polarity (broken line in FIG. 1A). After charging of the toner remaining on the intermediate transfer belt 9 (hereinafter referred to as residual toner) is completed, the ICL roller 39 is separated from the intermediate transfer belt 9. When the four color toner images are primarily transferred onto the intermediate transfer belt 9, the ICL roller 39 is separated from the intermediate transfer belt 9 (solid line in FIG. 1A). Here, the secondary transfer roller 10 and the ICL roller 39 are brought into contact with and separated from the intermediate transfer belt 9 by, for example, a solenoid that switches whether or not the motor force is transmitted to the cam member 93 and the roller contact / separation cam 94. And can be switched. The residual toner charged by the ICL roller 39 is electrostatically reversely transferred to the photosensitive drum 15 at the primary transfer position, and is collected into the container 114 by the cleaner blade 16. As described above, the ICL roller 39 is provided between the secondary transfer roller 10 and the photosensitive drum 15 to clean the residual toner on the intermediate transfer belt 9 after the secondary transfer by the secondary transfer roller 10 is executed. This is an example of voltage applying means for applying the voltage of 2 to the intermediate transfer belt 9. Further, the cam, the solenoid, and the drive control unit 217 for driving them are examples of second contact / separation means for contacting or separating the voltage application means with respect to the intermediate transfer belt 9.

(Control part)
FIG. 1B is a system diagram of the image forming apparatus 100 of the present embodiment. The controller unit 201 receives a print job from the host computer 200 and executes various image processes such as developing image data into bitmap data. The engine control unit 202 is a unit that comprehensively controls the engine of the image forming apparatus 100. The interface unit 210 includes a serial communication unit 203 and an image forming signal unit 204. The serial communication unit 203 performs serial communication with the CPU 211 and the serial signal 223. Thus, the controller unit 201 transmits a command by the serial signal 223 or receives information such as the engine status. The CPU 211 transmits a / TOP signal 220 that serves as a reference for image formation.

  The image forming signal unit 204 starts transmission of the Video signal 222 to the image control unit 212 with the / TOP signal 220 as a reference. The / TOP signal 220 means the beginning of a page. The image control unit 212 transmits the / BD synchronization signal 221 from the scanner control unit 18 to the image formation signal unit 204. The image formation signal unit 204 transmits a video signal 222 for one line to the image control unit 212 every time the / BD synchronization signal 221 is received. The image control unit 212 transfers the received video signal 222 to the scanner control unit 18. Here, various types of signal processing such as PWM modulation may be applied to the Video signal 222. The / TOP signal 220 functions as a vertical synchronization signal, and the / BD synchronization signal 221 functions as a horizontal synchronization signal.

  The CPU 211 comprehensively controls the engine. For example, the CPU 211 detects the HP mark 92 on the intermediate transfer belt 9 using the optical sensor 91 controlled by the sensor control unit 218. When the leading edge detection sensor 40 cannot detect the recording material P by a predetermined timing, the CPU 211 controls the cam member 93 to separate the secondary transfer roller 10 from the intermediate transfer belt 9. Then, the CPU 211 continues the rotation of the intermediate transfer belt 9 and causes the paper feeding roller 303 to retry feeding the recording material P. Thus, the CPU 211 is an example of a control unit. Further, when the leading edge detection sensor 40 cannot detect the recording material P by a predetermined timing, the CPU 211 functions to control the roller contact / separation cam to separate the ICL roller 39 from the intermediate transfer belt 9.

  The drive control unit 217 controls the main motor 219 and the paper feed motor 229 based on an instruction from the CPU 211. The main motor 219 drives the intermediate transfer belt 9, the photosensitive drum 15, and the developing rotary 23. The paper feed motor 229 drives the registration roller 8. The paper feed control unit 214 controls the pickup solenoid 226 based on an instruction from the CPU 211. The paper feed roller 303 is driven by a paper feed motor 229 coupled via a clutch. A pickup solenoid 226 drives this clutch. When the pickup solenoid 226 sucks, the clutch is connected, and the paper feed roller 303 rotates once. As a result, the recording material P stored in the cassette 302 is picked up and fed to the conveyance path.

  The high voltage control unit 215 applies a secondary transfer voltage to the secondary transfer roller 10 according to an instruction from the CPU 211, applies a reverse voltage to the ICL roller 39, and applies a charging voltage to the charger 17. . The memory control unit 216 controls a nonvolatile memory, a ROM, and a RAM that store a control program.

(Relationship between development position and development retract position)
FIG. 2A is a diagram illustrating the developing position and the developing retracted position of the developing device 20 of the present embodiment, and shows only the configuration necessary for explanation. If the development rotary 23 is a four-color development rotary, the stop position (also referred to as a position) is eight. FIG. 2A shows a state in which the developing devices 20 of the respective colors are stopped at the developing position and a state in which any developing device 20 is located at a position other than the developing position (hereinafter referred to as “development retracted position”). Has been. Specifically, r1 in FIG. 2A is a state in which the yellow developing device 20Y is stopped at the developing position (illustrated as the Yellow developing position, the same applies to r3, r5, and r7), and r3 in FIG. 2A is magenta. The developing device 20M is stopped at the developing position. In FIG. 2A, r5 is a state where the cyan developing device 20C is stopped at the developing position, and r7 in FIG. 2A is a state where the black developing device 20B is stopped at the developing position. Further, in FIG. 2A, r2 is in a development retracted position between the yellow developing device 20Y and the magenta developing device 20M, and r4 in FIG. 2A is a magenta developing device 20M and a cyan developing device. It is in a state of being in a development retracted position between 20C. Further, in FIG. 2 (a), r6 is in a developing retracted position between the cyan developing device 20C and the black developing device 20B, and in FIG. 2 (a), r8 is a black developing device 20B and a yellow developing device. In this state, it is in the development retracted position between 20Y. As described above, after any of the developing devices 20 develops the electrostatic latent image on the photosensitive drum 15, the developing rotary 23 rotates to a position where none of the developing devices 20 contacts the photosensitive drum 15. . In FIG. 2A, only the black developing device 20B is hatched for easy understanding of the state of rotation of the developing rotary 23.

  When the developing rotary 23 is rotated by 1/4 with a certain developing device 20 positioned at the developing position (shown as rotary 1/4 rotation in FIG. 2A, the same applies hereinafter), the developing device 20 for the next color is displayed. Reaches the development position. For example, in a state where the yellow developing device 20Y is at the developing position (r1), when the developing rotary 23 is rotated 1/4, the magenta developing device 20M moves to the developing position (r3). Next, when the ¼ rotation is performed, the cyan developing device 20C moves to the developing position (r5). Next, when the ¼ rotation is performed, the black developing device 20B reaches the developing position (r7). When the ¼ rotation is further performed, the yellow developing device 20Y reaches the developing position again (r1). Normally, during color image formation, the development rotary 23 rotates by 1/4 rotation.

  On the other hand, when the developing rotary 23 is rotated 1/8 with a certain developing device 20 positioned at the developing position (shown as rotary 1/8 rotation in FIG. 2A, the same applies hereinafter). Moves to the development evacuation position. When the rotation is further 1/8, the developing device 20 for the next color reaches the developing position. For example, when the developing rotary 23 is rotated 1/8 with the yellow developing device 20Y in the developing position (r1), a portion located between the yellow developing device 20Y and the magenta developing device 20M is at the developing position. Will be located. This is referred to as a YM development retracted position (r2). Similarly, the other developing devices 20M to 20B are referred to as an MC development retract position (r4), a CB development retract position (r6), and a BY development retract position (r8). When the image is further rotated by 1/8 from the state of r2, the magenta developing device 20M moves to the developing position (r3).

  In this embodiment, one of the developing sleeves 20YS to 20BS is in contact with the photosensitive drum 15 at the developing position. That is, the image forming apparatus 100 according to the present exemplary embodiment forms a contact developing type image forming unit that develops an electrostatic latent image on the photosensitive drum 15 in a state where any of the developing sleeves 20YS to 20BS is in contact with the photosensitive drum 15. Device. As described above, in this embodiment, the photosensitive drum 15 and any of the developing sleeves 20YS to 20BS are in contact with each other during development. In this embodiment, it is assumed that the contact / separation operation between any of the developing sleeves 20YS to 20BS and the photosensitive drum 15 is performed only by the rotation operation of the developing rotary 23. However, after the developing rotary 23 rotates and stops at the image forming position, the contact / separation operation between any of the developing sleeves 20YS to 20BS and the photosensitive drum 15 may be performed as follows, for example. That is, after the developing rotary 23 is rotated and stopped at the image forming position, the contact and separation operation may be performed by the operation of the developing sleeves 20YS to 20BS moving in the radial direction of the photosensitive drum 15 or the like. In this embodiment, the CPU 211 determines whether or not the developing device 20 has contacted the photosensitive drum 15 based on how much the developing rotary 23 has been rotated from the home position (1/4 rotation, 1/8 rotation, etc.). Suppose you are.

(Charging / Development high-voltage power supply)
FIG. 2B is a diagram illustrating the charging / developing high-voltage power supply unit 315 of this embodiment. Note that only the yellow developing device 20Y is shown in the developing rotary 23 in FIG. 2B, and the detailed configuration is omitted. In this embodiment, the charging / developing high-voltage power supply unit 315 and the yellow developing sleeve 20YS are connected at the developing position. This connection is made by an electrical contact (20YC in FIG. 5) attached to the developing sleeve 20YS and an electrical contact (421 in FIG. 5) on the charging / developing high-voltage power supply unit 315 side fixed at the developing position. The charging / developing high-voltage power supply unit 315 applies the charging voltage Vcdc to the charger 17 based on the high-voltage control signal received from the high-voltage control unit 215, and simultaneously applies the developing voltage Vdc to the developing sleeve 20 at the developing position. The operation of the developing rotary 23 and the application timing of the charging voltage Vcdc and the developing voltage Vdc will be described later.

(Development rotary home position)
Next, home position detection of the developing rotary 23 will be described. 2C and 2D are schematic diagrams of the photosensitive drum 15 and the developing rotary 23. FIG. The developing rotary 23 rotates in the direction of arrow B (clockwise direction) in the figure. Here, detailed drawing in the developing rotary 23 in FIGS. 2C and 2D is omitted. In FIG. 2C and FIG. 2D, the circle indicating the development rotary 23 is divided into four, and each fan-shaped portion is attached to the mounting position of the yellow developing device 20Y, the mounting position of the magenta developing device 20M, and cyan. The mounting position of the developing device 20C and the mounting position of the black developing device B are shown. The photo interrupter 300 is provided in the apparatus main body independently of the developing rotary 23, and is composed of a light emitting unit composed of a light emitting diode and a light receiving unit composed of a phototransistor. The photo interrupter 300 is provided to detect the passage of the protrusion 301e which is a position detection flag provided on the circumference of the developing rotary disk (not shown). The photo interrupter 300 and the protrusion 301e are regular stop position detecting means.

  The protrusion 301e is provided closer to a position that first reaches a position facing the photosensitive drum 15 in the rotation direction of the developing rotary 23 during the mounting position of the black developing device 20B. FIG. 2C shows the position of the developing rotary 23 during the development of black, and is a position where the mounting position of the black developing device 20B and the photosensitive drum 15 face each other. That is, FIG. 2C is in a state where it stops at the black development position (Black development position) indicated by r7 in FIG. On the other hand, FIG. 2D shows a normal stop position (home position) of the developing rotary 23. 2D shows a state where the developing rotary 23 rotates 45 ° counterclockwise (in the opposite direction to the arrow B) from the state shown in FIG. There is no contact. That is, FIG. 2 (d) is in a state of being stopped at the CB development retract position indicated by r6 in FIG. 2 (a).

  Normally, after the end of image formation, after the end of various calibrations, and when preparing for the next image formation, the development rotary 23 stops at the position shown in FIG. The engine control unit 202 controls all operations of the developing rotary 23 with reference to the home position in FIG.

(Scanner part)
FIG. 3A is a diagram illustrating the scanner unit 30 according to the present embodiment. The scanner control unit 18 controls the exposure amount E1 (see FIG. 3B and the like) for visualizing the toner image by the pulse width signal 60. The control by the pulse width signal 60 of the scanner control unit 18 is specifically light emission time control. Further, the scanner control unit 18 controls the light emission intensity of the laser driver 62 based on the luminance signal 61. Here, the exposure amount is a unit of μJ / cm ² , and is the light energy per unit area when the laser diode 63 continuously emits light in a certain area for a certain time with a certain light emission intensity.

  The laser driver 62 controls the light emission luminance and the light emission time of the laser diode 63 by the luminance signal 61 and the pulse width signal 60 that are instructed from the scanner control unit 18 based on the image data. The light emission luminance can be controlled by adjusting the current supplied from the laser driver 62 to the laser diode 63. Further, the laser beam 6 emitted from the laser diode 63 is optically scanned, and is irradiated to the photosensitive drum 15 as scanning light through a correction optical system 67 including a rotary polygon mirror 64, a lens 65, and a folding mirror 66. Further, the laser driver 62 performs so-called automatic light amount control, and controls the current supplied to the laser diode 63 so that the target light emission luminance (mW) is obtained.

(Development contrast Vcont, development back contrast Vback, and image state)
The relationship between the change in film thickness of the photosensitive drum 15, the development contrast Vcont, and the development back contrast Vback will be described with reference to FIGS. 3B and 3C. The surface of the photosensitive drum 15 deteriorates due to the discharge of the charger 17 as usage proceeds, and is scraped by rubbing against the cleaner blade 16 that comes into contact with the surface of the photosensitive drum 15, thereby reducing the film thickness of the photosensitive drum 15. The thick photosensitive drum 15 that is not being used has a small air gap between the charger 17 and the photosensitive drum 15, and the potential difference generated is small, so the absolute value of the charging potential Vd is small. On the other hand, the thin photosensitive drum 15 that has been used is opposite to the photosensitive drum 15 that has not been used, the air gap is large, and the absolute value of the charging potential Vd is large (in FIG. 3B, Vd (Up and illustrated). FIGS. 3B and 3C show how the potential of the photosensitive drum 15 changes when the film thickness of the photosensitive drum 15 decreases with time. FIG. 3B is a diagram for explaining a problem when the charging voltage Vcdc and the development potential Vdc are fixed values.

  In the thick photosensitive drum 15 shown in the left diagram of FIG. 3B, the development potential Vdc and the charge are charged so that the development back contrast Vback (= Vd−Vdc), which is the contrast between the development potential Vdc and the charge potential Vd, is in a desired state. The potential Vd is set. In this case, the following problems occur when the film thickness of the photosensitive drum 15 is reduced (the right diagram in FIG. 3B). When the film thickness of the photosensitive drum 15 is reduced, the absolute value of the charging potential Vd is increased, so that the development back contrast Vback is increased. When the development back contrast Vback increases, the toner that could not be charged to the normal polarity (in the case of reversal development as in this embodiment, the toner charged with 0 to positive polarity instead of negative polarity) The image is transferred to the non-image portion of the photosensitive drum 15 and fog occurs. Further, in the configuration in which the exposure intensity (E1) is constant because the charging potential Vd increases, the exposure potential Vl also increases (shown as Vl Up in FIG. 3B). Therefore, the development contrast Vcont (= Vdc−Vl), which is the difference value between the development potential Vdc and the exposure potential Vl, becomes small. As a result, the toner cannot be sufficiently transferred electrostatically from the developing unit 20 to the photosensitive drum 15, and the phenomenon that the density of the solid black image becomes thin (hereinafter referred to as “thin density”) is likely to occur. The non-image portion is a portion where the toner image is not visualized on the photosensitive drum 1, and a portion where the toner image is visualized on the photosensitive drum 1 is referred to as an image portion.

  In order to solve the above problems, FIG. 3C shows a configuration in which the charging voltage Vcdc is variable as shown in FIG. Even when the film thickness of the photosensitive drum 15 is reduced, the charging potential Vd and the exposure potential Vl can be kept constant by correcting the charging voltage from Vcdc1 to Vcdc2. Thereby, the development contrast Vcont and the development back contrast Vback can be kept constant. In this embodiment, as will be described later, the charging voltage Vcdc is a variable value, and the developing potential Vdc is a fixed value. However, even when the charging voltage Vcdc is a fixed value and the development potential Vdc is a variable value, the development contrast Vcont and the development back contrast Vback can be kept constant.

(Correction of charging voltage Vcdc)
FIG. 4 is a flowchart for explaining the configuration of FIG. 3C in which the charging voltage Vcdc is corrected in relation to whether or not the use of the photosensitive drum 15 is progressing (also referred to as the usage amount or life of the photosensitive drum 15). It is. In step (hereinafter referred to as S) 101, the engine control unit 202 performs integrated rotation of the photosensitive drum 15 as information regarding whether or not the use of the photosensitive drum 15 is advanced from a storage member (not shown) by the memory control unit 216. Read number information. Note that the information regarding whether or not the use of the photosensitive drum 15 is in progress may be information regarding how much the photosensitive drum 15 has been rotated or used. For example, the motor driving time of the motor that drives the photosensitive drum 15, the number of rotations of the charger 17, the number of prints, and the like may be used.

  In S <b> 102, the engine control unit 202 determines the charging voltage Vcdc based on the information on the accumulated rotational speed of the photosensitive drum 15 acquired in S <b> 101. For example, the engine control unit 202 determines the charging voltage Vcdc by referring to a table in which the correspondence relationship between the cumulative rotation speed (photosensitive drum usage status) of the photosensitive drum 15 and the charging voltage Vcdc is determined. Note that a storage member (not shown) stores a formula representing the relationship between the accumulated rotational speed of the photosensitive drum 15 and the charging voltage Vcdc, and the engine control unit 202 calculates the formula to obtain the charging voltage Vcdc. Various configurations are conceivable. Since the film thickness of the photosensitive drum 15 becomes thinner as the integrated rotation number of the photosensitive drum 15 increases, the charging voltage is corrected from Vcdc1 to Vcdc2 and the charging voltage is determined as Vcdc2, for example, as described with reference to FIG. . A description of specific numerical values of the table is omitted here.

  In step S103, the engine control unit 202 causes each member to execute the series of image forming operations and controls described with reference to FIG. In S104, the engine control unit measures the number of rotations of the photosensitive drum 15 rotated during the series of image forming operations in S103 using a counter (not shown) or the like. In step S105, the engine control unit 202 determines whether or not the image forming operation has ended. If it is determined that the image forming operation has ended, the process proceeds to step S106. In step S106, the engine control unit 202 adds the measurement result measured in step S104 to the integrated rotational speed of the photosensitive drum 15, and updates the integrated rotational speed. In step S <b> 107, the engine control unit 202 stores the updated integrated rotation number of the photosensitive drum 15 in a memory (not illustrated) by the memory control unit 216. If the engine control unit 202 determines in S105 that the image forming operation has not ended, the process returns to S103.

  The above is an example of an image failure (thin density, fogging, etc.) due to a change with time of the development contrast Vcont and the development back contrast Vback, and the method for correcting the charging voltage Vcdc. Regarding the development contrast Vcont and the development back contrast Vback, not only the change over time described above but also a design that suppresses the use environment and individual variations and a design that corrects them are important.

(Charging / Development high-voltage power supply)
As described above, the development back contrast Vback is the difference between the charging potential Vd and the development potential Vdc (Vback = Vd−Vdc). For this reason, in order to suppress the individual variation of the photosensitive drum 15, it is also important to use a power source that has a voltage tolerance in conjunction with the charging voltage Vcdc and the development voltage Vdc. In order to have a voltage crossing in which the charging voltage Vcdc and the development voltage Vdc are linked, there is a configuration in which a voltage is applied to the charger 17 and the developer 20 from one common power source.

  FIG. 5 is a circuit diagram showing details of the charging / developing high-voltage power supply unit 315 of this embodiment. FIG. 5 shows an example in which the developing sleeve 20YS of the yellow developing device 20Y is in contact with the photosensitive drum 15. The charging voltage Vcdc supplied to the charger 17 can be varied based on the control signal Vc output from the high voltage controller 215. The developing voltage Vdc applied to the developing sleeve 20YS is generated by a series regulator regulated by the charging voltage Vcdc and the resistors R3 and R4. Details will be described later. In the following, the larger and smaller values are compared with absolute values.

Charged high-voltage power supply unit The charged high-voltage power supply unit that is a charging voltage application means includes a comparison detection unit 404, a voltage controlled oscillator (hereinafter referred to as a VCO circuit) 403, a drive circuit 402, a piezoelectric transformer 401, a rectifier circuit 405, and a monitor voltage generator. Part 406. The charging voltage Vcdc is applied to the charger 17 by converting the AC voltage output from the piezoelectric transformer 401 into a DC voltage by the rectifier circuit 405. On the other hand, in order to control the charging voltage Vcdc to be substantially constant, the charging / developing high-voltage power supply unit 315 performs the following feedback control. The monitor voltage generator 406 offsets the voltage obtained by stepping down the charging voltage Vcdc by the ratio of R2 / R1 by the resistors R1 and R2 to the positive voltage by the reference voltage Vrgv, which is a predetermined voltage, and the monitor voltage Vref1. And output to the comparison detection unit 404. The charging / developing high-voltage power supply unit 315 performs feedback control so that the monitor voltage Vref1 becomes a constant value. Specifically, the control voltage Vc preset in the high voltage control unit 215 (for example, the D / A port of the CPU, hereinafter referred to as the D / A port of the high voltage control unit 215) and the monitor voltage Vref1 are substantially equal. The comparison detection unit 404 controls. Here, the comparison detection unit 404 includes, for example, an operational amplifier, a phase correction resistor, a capacitor, and the like.

  The output of the comparison detection unit 404 is output to a VCO circuit 403 that is a control circuit of the piezoelectric transformer 401. The VCO circuit 403 controls the drive frequency of the piezoelectric transformer 401 based on the signal input from the comparison detection unit 404, and the output signal of the VCO circuit 403 is input to the drive circuit 402 of the piezoelectric transformer 401. As a result, the charging voltage Vcdc output from the piezoelectric transformer 401 is controlled so that the monitor voltage Vref1 becomes a value according to the control voltage Vc. For the frequency control of the piezoelectric transformer 401, the output of the comparison detection unit 404 or the monitor voltage Vref1 is input to the high voltage control unit 215, and the calculation result by the high voltage control unit 215 is reflected in the control system of the piezoelectric transformer 401. May be.

◆ Conventional Development Voltage Generation Unit The development voltage Vdc is connected to the charging voltage Vcdc via the resistor R3. The developing voltage Vdc is applied to the developing sleeve 20YS via an electrical contact 20YC attached to the developing sleeve 20YS and an electrical contact 421 on the charging / developing high-voltage power supply unit 315 fixed to the developing position. Here, for comparison with this embodiment, FIG. 6A shows a configuration example of a conventional charging / developing high-voltage power supply unit 615. In addition, the same code | symbol is attached | subjected to the same structure as FIG. 5, and description is abbreviate | omitted. In the conventional developing voltage generator 507, the developing voltage Vdc is supplied to the developing device 20 based on the control signal Vc 2 output from the D / A port of the high voltage controller 515. In the development voltage generator 507, the development voltage Vdc is a series regulator regulated by the charging voltage Vcdc and the resistors R3 and R4. FIG. 6B shows the output possible range of the charging voltage Vcdc and the development voltage Vdc with respect to the control voltage Vc [v]. The horizontal axis indicates the control voltage Vc [V], and the vertical axis indicates the voltage [V]. Yes. The charging voltage Vcdc can be set from 0 V to −1300 V (Vcdc_max) according to the control voltage Vc. The voltage of −1300 V is set as a voltage value that can be charged to a sufficient charging potential Vd even if individual variations of the photosensitive drum 15 and changes in film thickness are taken into consideration.

On the other hand, since the developing voltage Vdc is regulated by the charging voltage Vcdc and the resistors R3 and R4, Vrgv to Vdc = Vcdc × R4 / (R3 + R4) (1)
Output is possible. In this range, the output table of the development voltage Vdc can be set by setting the resistors R4 and R5. The calculation method of the output table is as follows. In FIG. 6A, if the current flowing from Vrgv to the resistors R5 and R4 is i1,
Vdc = Vrgv + i1 × (R4 + R5) (2)
Vref2 = Vrgv + i1 × R5 (3)
Here, since the current flowing through the input terminal of the operational amplifier 417 is very small, it is calculated as 0.

From the expressions (2) and (3) and the virtual short circuit (Vref2 = Vc2) of the operational amplifier 417,
Vdc = Vc2−R4 / R5 × (Vrgv−Vc2) (4)
It becomes. The development voltage Vdc of the formula (4) becomes maximum when Vc2 is 0.
Vdc_max = −R4 / R5 × (Vrgv) (5)
It becomes.

  The development voltage Vdc is set to a voltage that is generally higher than a voltage necessary for development, approximately −300 V, and does not decrease the resolution of the development voltage Vdc. In FIG.6 (b), it is set to -400V. The operational amplifier 417 controls the collector current i2 of the switching element 519 so that the control voltage Vc2 and the monitor voltage Vref2 are substantially equal, and the development voltage Vdc can be controlled. In the conventional configuration shown in FIG. 6A, when the development voltage Vdc is set to −400 V, which is the maximum development voltage Vdc_max, the collector-emitter of the switching element 519 needs to have a withstand voltage. For this reason, a high breakdown voltage semiconductor (generally a high voltage transistor) is required as the switching element 519. However, since the high voltage transistor is expensive, even if the power source of the charging voltage Vcdc and the power source of the development voltage Vdc are shared, the cost reduction effect is low.

<Modification of Development Voltage Generating Unit> FIGS. 6C and 6D are diagrams showing a modification of the development voltage generating unit 507 in FIG. The development voltage generation unit 457 in FIG. 6C uses the Zener diode 458 to generate the development voltage Vdc as a fixed voltage. The development voltage generation unit 467 in FIG. 6D divides the charging voltage Vcdc by the resistors R3 and R8 to generate the development voltage Vdc. In the case of FIG. 6C, the individual variation of the development voltage Vdc depends only on the individual variation of the Zener diode 458, and the individual variation of the charging voltage Vcdc (depends on Vrgv and the resistors R1 and R2 in FIG. 6A). be independent. In general, a Zener diode has a characteristic that it is more difficult to suppress individual variation than resistance. Therefore, in the case of FIG. 6C, the individual variation of the development back contrast Vback determined by the difference between the charging voltage Vcdc and the development voltage Vdc increases, and the problem of fogging that the toner transfers to the non-image portion of the photosensitive drum 15 occurs. There is a fear.

  On the other hand, in the case of FIG. 6D, the following problem may occur. The individual variation of the development back contrast Vback depends on the individual variation of the charging voltage Vcdc and the resistances R3 and R8. Further, in the configuration of FIG. 6D, since the constant voltage control of the development voltage Vdc is not performed, the development by the current 424 flowing from the developing sleeve 20YS to the photosensitive drum 15 at the time of development contact as shown in FIG. 7A. It is necessary to consider the fluctuation of the voltage Vcd. 7A is a circuit diagram showing a charging / developing high-voltage power supply unit 625 to which the developing voltage generation unit 467 of FIG. 6D is applied, and the same components as those in FIG. The description is omitted.

  FIG. 7B shows how the developing voltage Vcd fluctuates due to the current 424 flowing from the developing sleeve 20YS to the photosensitive drum 15. FIG. In FIG. 7B, the horizontal axis indicates time, and the vertical axis indicates voltage. Since the charging voltage Vcdc is controlled at a constant voltage as in FIG. 5, the charging voltage Vcdc is substantially constant regardless of contact and separation of the yellow developer 20Y (hereinafter referred to as development contact and separation). The development voltage Vdc starts to be output at the same timing as the charging voltage Vcdc (the timing when the high-voltage control signal is turned on) in a state where the yellow developing device 20Y is separated (hereinafter referred to as a development separation state). As the developing voltage Vdc, a voltage 422 having a ratio of R3 / (R3 + R8) with respect to the charging voltage Vcdc is output. At this time, the current 424 does not flow because of the development separation state.

  Thereafter, when the developing sleeve 20YS of the yellow developing device 20Y contacts the photosensitive drum 15 (shown as yellow developing contact in FIG. 7B), a current 424 flows from the developing sleeve 20YS to the photosensitive drum 15. For this reason, the developing voltage Vdc is lowered by about 10 V from the voltage value of the voltage 422 to the voltage value of the voltage 423. In addition, the voltage of about 10V which decreased from the voltage value of the voltage 422 to the voltage value of the voltage 423 is hereinafter referred to as a drop voltage. Since the voltage 423 is applied to the developing sleeve YS when developing yellow toner, the voltage 423 is important as the development back contrast Vback. From the above, in order to suppress the individual variation of the development back contrast Vback, it is only necessary to suppress the individual variation of the charging voltage Vcdc and the resistors R3 and R8 and the drop voltage. However, since the current 424 that contributes to the drop voltage varies depending not only on the physical contact state variation but also on the toner amount, toner deterioration, film thickness of the photosensitive drum 15, and the like, it is difficult to quantify. For this reason, the configuration of FIG. 6D has a problem as the configuration of the charging / developing high-voltage power supply unit 625 for suppressing the fluctuation of the developing back contrast Vback.

  The development voltage Vdc returns from the voltage value of the voltage 423 to the voltage value of the voltage 422 at the timing when the yellow development sleeve YS is separated from the photosensitive drum 15 (shown as yellow development separation in FIG. 7B). The output of the developing voltage Vdc is stopped at the same timing as the charging voltage Vcdc (timing when the high voltage control signal is turned off) in the yellow developing separation state.

◆ Developing Voltage Generating Unit of the Present Example Based on the above-described configuration of the developing voltage generating unit, FIG. 5 of the present example is based on the conventional developing voltage generating unit 507 of FIG. It is said. That is, in this embodiment, an inexpensive small signal transistor 419 as a switching element and a Zener diode 418 as a constant voltage element are connected in series. This enables an inexpensive charging / developing high-voltage power supply while suppressing fluctuations in the developing back contrast Vback.

  The charging / developing high-voltage power supply unit 315 of the present embodiment will be described with a focus on differences from the conventional developing voltage generating unit 507 of FIG. In the development voltage generation unit 407 which is a development voltage application unit of the present embodiment, a voltage Vc3 (fixed voltage) generated by dividing the reference voltage Vrgv by the resistors R6 and R7 is used as a control voltage of the development voltage Vdc by the operational amplifier 417. Input to the inverting input terminal (-terminal). Here, the reference voltage Vrgv divided by the resistors R6 and R7 is the reference voltage Vrgv of the monitor voltage generator 406 when controlling the charging voltage Vcdc. As a result, the high voltage control unit 215 of this embodiment does not require the D / A port for outputting the control signal Vc2 of the high voltage control unit 515 described in FIG. 6A, and reduces the control signal port of the development voltage Vdc. doing. Here, the reason why the control voltage Vc3 is a fixed voltage is that the development back contrast Vback can be appropriately controlled by the power supply sequence described later and the above-described charging voltage Vcdc control. Therefore, it is necessary to prepare a control port and control the development voltage Vdc. Because there is no.

  Next, a Zener diode 418 is connected in series to the small signal transistor 419 so that the potential difference between the development voltage Vdc and the DC power supply V1 is mainly covered by the withstand voltage of the Zener diode 418. A plurality of Zener diodes 418 are connected in series as appropriate according to the breakdown voltage of the small signal transistor 419. In general, a Zener diode having a high Zener voltage is less expensive than a high breakdown voltage transistor. Therefore, compared with the development voltage generation unit 507 in FIG. Is possible.

~ Setting method of control voltage Vc3 ~
Here, a method of setting the control voltage Vc3 will be described. The minimum value Vdc_min of the developing voltage Vdc that can be output when the control voltage Vc3 is variable is when the collector voltage of the small signal transistor 419 is the DC power supply V1 (the voltage value is also V1).
Vdc_min = V1-Vz
Here, Vz is a Zener voltage. For example, when the voltage value of the DC power supply V1 is 24V and the Zener voltage Vz is 300V, the minimum value Vdc_min of the developing voltage Vdc is −277V (= 24V−300V).

The developing voltage Vdc cannot be controlled to be smaller than the minimum value Vdc_min of the developing voltage Vcd. On the other hand, the maximum value Vdc_max of the development voltage Vdc depends on the withstand voltage Vceo of the small signal transistor 419. When Vceo = 50V, first, the collector voltage is
V collector = V1-50 = 24V-50V = -26V
The maximum value Vdc_max of the development voltage Vdc is
Vdc_max = −26V−Vz = −326V
It becomes. For this reason, the resistors R4 and R5 may be adjusted so that the development voltage Vdc falls within the range between the minimum value Vdc_min and the maximum value Vdc_max.

  As a feature of this embodiment, the operational amplifier 417 controls so as to compensate for voltage fluctuation, controls the collector current of the small signal transistor 419, and controls the development voltage Vdc. For this reason, in this embodiment, it is possible to prevent fluctuations in the development voltage Vdc, such as at the time of development contact and separation described with reference to FIGS. 7A and 7B. Further, as described above, in this embodiment, the reference voltage Vrgv of the charging voltage Vcdc is divided to obtain the control voltage Vc3 of the developing voltage Vdc. Thus, in this embodiment, the individual variation of the reference voltage Vrgv is linked with the charging voltage Vcdc and the development voltage Vdc, and there is an effect of suppressing the tolerance of the development back contrast Vback.

  Here, since the developing voltage Vcd of this embodiment reduces the control signal port of the developing voltage Vcd, the developing voltage Vdc is automatically output when the charging voltage Vcdc is output. Therefore, if the control voltage Vc is output from the D / A port of the high voltage controller 215 in a state where the developing sleeves 20YS, 20MS, 20CS, and 20KS are in contact with the photosensitive drum 15, the following phenomenon may occur. There is. That is, in an output transient state of the charging voltage Vcdc and the development voltage Vdc, there is a possibility that the toner is placed on the photosensitive drum 15 by applying an unintended voltage.

  Accordingly, the timing chart of the control voltage Vc is as shown in FIG. 8 to prevent the phenomenon that the toner is placed on the photosensitive drum 15 by an unintended voltage. FIG. 8 is a timing chart for explaining the operation of the charging / developing high-voltage power supply unit 315 when full-color image formation is performed by the image forming apparatus 100 of the present embodiment. Specifically, FIG. 8A shows the rotation state of the developing rotary 23, and when the developing rotary 23 is rotating, it is turned on (ON), and when it is stopped, it is turned off (OFF). FIG. 8B shows a developing contact state, in which any of the developing devices 20 is in contact with or separated from the photosensitive drum 15. Further, FIG. 8C shows the output state of the charging voltage Vcdc and the developing voltage Vdc (shown as charging voltage and developing voltage). In FIG. 8C, the case where the control voltage Vc is output from the D / A port of the high voltage control unit 215 is ON (ON), and the case where it is not output is OFF (OFF). Further, reference numerals S1 to S12 in FIG. 8 indicate timings of operations described below.

  As described with reference to FIG. 2D, the developing rotary 23 is stopped at the home position, which is a normal stop position. When the engine control unit 202 outputs an image formation signal (/ TOP signal 220), the developing rotary 23 starts rotating (turns on), and the control signal Vc of the charging / developing high-voltage power supply unit 315 is turned on. A charging voltage Vcdc and a developing voltage Vdc are output. This is timing S1 in FIG.

  The home position of the developing rotary 23 in the image forming apparatus 100 of the present embodiment is immediately when a monochromatic image forming signal is output, as described with reference to FIGS. 2C and 2D. The position is set so that the image forming operation can be started. That is, at the home position of the developing rotary 23 of this embodiment, the black developing device 20B is arranged immediately before the developing position. In the image forming apparatus 100 according to the present exemplary embodiment, when a full-color image formation signal is sent, image formation is performed in the order of yellow, magenta, cyan, and black. Therefore, when the yellow developing device 20Y is moved to the developing position, the black developing device 20B passes through the developing position without forming an image. When performing a full-color image forming operation, the control signal Vc of the charging / developing high-voltage power supply unit 315 is turned on to prevent toner adhesion to the photosensitive drum 15 in the output transient state of the charging voltage Vcdc and the developing voltage Vdc. Yes. The black developing device 20B contacts the photosensitive drum 15 at timing S2 in FIG. 8, the black developing device 20B passes through the developing position without image formation, and is separated from the photosensitive drum 15 at timing S3 (FIG. 8). (B) In FIG. During this time, as shown in FIG. 8A, the developing rotary 23 maintains the rotating operation (ON state).

  Thereafter, by the rotation of the developing rotary 23, the yellow developing device 20Y comes into contact with the photosensitive drum 15 at timing S4, and the developing rotary 23 stops (turns off). Then, at the timings S4 to S5, yellow image formation is performed by the yellow developing device 20Y (in FIG. 8, Y contact, image formation is illustrated, and the same applies to the subsequent colors). When image formation by the yellow developing device 20Y is completed, the developing rotary 23 starts rotating (turns on) at timing S5, and the yellow developing device 20Y is separated from the photosensitive drum 15. Next, the contact of the magenta developing device 20M at timing S6, the turning off of the developing rotary 23, the image formation by the magenta developing device 20M at the timings S6 to S7, the turning on of the developing rotary 23 at the timing S7, the turning on of the magenta developing device 20M. Separation is performed. Next, at timing S8, the cyan developing device 20C comes into contact, the developing rotary 23 is turned off, the image is formed by the cyan developing device 20C at timings S8 to S9, the developing rotary 23 is turned on at timing S9, and the cyan developing device 20C is turned on. Separation is performed. Further, the black developing device 20B comes into contact at timing S10, the developing rotary 23 is turned off, the image is formed by the black developing device 20B at timings S10 to S11, the developing rotary 23 is turned on at timing S11, and the black developing device 20B is separated. Is implemented.

  When the developing rotary 23 returns to the home position at timing S12, the developing rotary 23 stops rotating and the control signal Vc of the charging / developing high-voltage power supply unit 315 is turned off. Thereby, the output of the charging voltage Vcdc and the development voltage Vdc is stopped (off). In FIG. 8, the control signal Vc is turned on and off (the charging voltage and the developing voltage are turned on and off in FIG. 8C) in synchronization with the start and stop of the rotation of the developing rotary 23. However, since the control signal Vc only needs to be on in the development contact state, it is not limited to the timing described above.

  As described above, in this embodiment, an inexpensive development voltage generation unit 407 is configured by connecting an inexpensive small signal transistor 419 and a Zener diode 418 in series, and high voltage power supply control is performed in a development separated state. . Thus, in this embodiment, a power supply configuration in which the tolerance of the development back contrast Vback is suppressed is possible.

◆ Comparison between the present embodiment and the conventional example FIG. 7C compares the features of the development voltage generation unit described above with respect to cost and development back contrast Vback, and lists them. 7C, this example, a conventional example using the switching element 519 which is a high-voltage transistor in FIG. 6A, a first modification using the Zener diode 458 in FIG. 6C, and FIG. Comparison is made with respect to Modification 2 in which the voltage is divided by the resistors R3 and R8 of d). Note that “「 ”,“ ◯ ”, and“ Δ ”are shown in order from the highest evaluation. The accuracy of the development back contrast Vback in this embodiment and the conventional example is not strictly the same. This is because there is a difference in accuracy between the control voltage Vc3 (this embodiment) and the control voltage Vc2 (conventional example). However, the above-described control voltages Vc3 and Vc2 are the accuracy of the development back contrast Vback in contrast to the resistors R1 and R3 and R4 for determining the respective control voltages by stepping down the high charging voltage Vcdc and the development voltage Vdc. The influence on the whole is small. For this reason, they are treated as equivalent. In addition, regarding the resistance partial pressure of the second modification described with reference to FIG. 6D, in consideration of the influence of the change in the development voltage Vdc due to the development contact described above, this example and the conventional example of FIG. Also, the accuracy of the development back contrast Vback is low. As can be seen from FIG. 7C, the configuration of this embodiment can improve the accuracy of the development back contrast Vback while reducing the cost.

  In this embodiment, the charging / developing high-voltage power supply unit 315 performs constant voltage control of the charging voltage Vcdc by controlling the frequency of the piezoelectric transformer 401 and the piezoelectric transformer 401 by the VCO circuit 403. However, the present embodiment is not limited to such a configuration, and may be configured to be performed by, for example, a winding transformer, a multistage voltage doubler rectifier circuit, drive voltage control, duty control, or the like.

  As described above, according to this embodiment, it is possible to reduce image defects with an inexpensive configuration.

  In the first embodiment, the configuration using the inexpensive charging / developing high-voltage power supply unit 315 in the so-called rotary type image forming apparatus 100 having the four photosensitive units 15 and the four developing devices 20 in the developing rotary 23 has been described. In the second embodiment, a configuration using the charging / developing high-voltage power supply unit described in the first embodiment will be described in a so-called tandem type image forming apparatus in which each color photosensitive drum is independently provided.

  In a tandem type image forming apparatus, the usage of each photosensitive drum (change in film thickness of the photosensitive drum) may vary due to various factors. At this time, if the power supply for the charging unit and the developing unit is shared, it is difficult to perform independent power control of the charging unit and the developing unit for each photosensitive drum. As a result, the charging voltage and the developing voltage cannot be appropriately set for each photosensitive drum, and as a result, fogging of toner transferring to a non-image portion and image defects such as density fluctuations are likely to occur. In the present embodiment, the charging / developing high-voltage power supply unit according to the first embodiment is used, and the non-image portion is finely exposed to solve at least one of the above-described problems. First, the configuration of the image forming apparatus will be described with reference to FIGS. 9 and 10, and the micro exposure control operation will be described with reference to FIGS. 11 and 12. The charging / developing high-voltage power supply unit of this embodiment will be described with reference to FIGS.

(Schematic of cross section of image forming apparatus)
FIG. 9A is a diagram schematically illustrating a cross section of the image forming apparatus according to the present exemplary embodiment. The image forming apparatus according to the present exemplary embodiment includes first to fourth image forming stations (corresponding to reference numerals a to d). Here, the first image forming station a is for yellow (hereinafter referred to as Y), and the second image forming station b is for magenta (hereinafter referred to as M). The third image forming station c is for cyan (hereinafter referred to as C), and the fourth image forming station d is for black (hereinafter referred to as Bk). Each of the image forming stations a to d stores a storage member (memory tag 32- 2 (see FIG. 9B)).

  In the following description, the operation of the first image forming station a (hereinafter simply referred to as image forming station a) will be described as an example of the image forming stations a to d. The image forming station a includes a photosensitive drum 1a as a photosensitive member, and the photosensitive drum 1a is rotationally driven at a predetermined peripheral speed (process speed) in the direction of the arrow (counterclockwise direction) in FIG. 9A. . In this rotation process, the photosensitive drum 1a is uniformly charged to a charging potential of a predetermined polarity by the charging roller 2a. Next, the surface of the photosensitive drum 1a corresponding to the image portion is exposed to discharge charges by scanning with laser light 6a based on image data (image signal) supplied from the outside by an exposure device 31a as a laser / scanner system. As a result, the exposure device 31a forms an exposure potential Vl on the surface of the photosensitive drum 1a. Next, the toner is developed and visualized in the exposure potential portion which is an image portion by the potential difference between the development voltage Vdc applied to the yellow developing device 4a and the exposure potential Vl. The image forming apparatus according to the present exemplary embodiment performs image exposure using an exposure device 31a, and forms an image using a reversal developing method in which toner is developed on an exposed portion that is a surface portion of the photosensitive drum 1a that has been subjected to exposure to remove charges. Device.

  The intermediate transfer belt 110 is stretched by stretching members 111, 112, and 113 and is in contact with the photosensitive drum 1a. The intermediate transfer belt 110 is rotationally driven at the contact position in the same direction as the photosensitive drum 1a and at substantially the same peripheral speed. The yellow toner image formed on the photosensitive drum 1a passes through a contact portion (hereinafter referred to as a primary transfer nip portion) between the photosensitive drum 1a and the intermediate transfer belt 110. In the process in which the yellow toner image passes through the primary transfer nip portion, the yellow toner image is transferred onto the intermediate transfer belt 110 by the primary transfer voltage applied to the primary transfer roller 14a from the primary transfer power supply 115a (hereinafter, referred to as "the yellow toner image"). Called primary transfer). The toner remaining on the surface of the photosensitive drum 1a is cleaned and removed by the cleaning unit 5a, and then the above-described image forming process after charging is repeatedly performed. Similarly, a second color magenta toner image (M), a third color cyan toner image (C), and a fourth color black toner image (Bk) are formed on the intermediate transfer belt 110. A superimposed color image is obtained by sequentially superimposing and transferring.

  The four-color toner images on the intermediate transfer belt 110 are collectively transferred onto the surface of the recording material P fed by the paper feed roller 150 by the secondary transfer voltage applied to the secondary transfer roller 120 by the secondary transfer power supply 121. Is done. The four-color toner images on the intermediate transfer belt 110 pass through a contact portion (hereinafter referred to as a secondary transfer nip portion) between the intermediate transfer belt 110 and the secondary transfer roller 120, and are then formed on the surface of the recording material P. Batch transfer. Thereafter, the recording material P carrying the four-color toner images is introduced into the fixing device 130, and heated and pressed there, the four-color toners are melted and mixed to be fixed to the recording material P. With the above operation, a full-color print image is formed. Further, the toner remaining on the surface of the intermediate transfer belt 110 is cleaned and removed by the intermediate transfer belt cleaning device 116.

  In FIG. 9A, the image forming apparatus having the intermediate transfer belt 110 has been described as an example. However, the present invention is not limited to this. For example, the present invention can be implemented in an image forming apparatus that employs a system that includes a recording material conveyance belt and directly transfers a toner image developed on a photosensitive drum to a recording material conveyed by the recording material conveyance belt. Hereinafter, an image forming apparatus having the intermediate transfer belt 110 will be described as an example.

(Image forming system diagram)
FIG. 9B is a block diagram of an image forming system including the external device 101, the video controller 103, and the printer engine 105. The printer engine 105 includes an engine control unit 104 and an engine mechanism unit 106.

<Video controller 103>
The CPU 184 controls the entire video controller 103. The nonvolatile storage unit 185 stores various control codes executed by the CPU 184. The nonvolatile storage unit 185 corresponds to a ROM, EEPROM, hard disk, or the like. The RAM 186 functions as a main memory, work area, and the like for the CPU 184, and is a storage unit for temporary storage. A host interface unit (shown as a host I / F) 187 is an input / output unit for printing data and control data with the external device 101 such as a host computer. Print data received by the host interface unit 187 is stored in the RAM 186. The DMA control unit 189 transfers the image data in the RAM 186 to an engine interface unit (shown as an engine I / F) 191 in accordance with an instruction from the CPU 184. A panel interface unit (shown as a panel I / F) 190 receives various settings and instructions from an operator from a panel unit provided in the printer main body. The engine interface unit 191 is an input / output unit for signals to and from the printer engine 105, and transmits data signals from an output buffer register (not shown) and controls communication with the printer engine 105. The system bus 192 has an address bus and a data bus. Each of the above-described components is connected to the system bus 192 and can access each other.

<Printer engine 105>
Next, the printer engine 105 will be described. The printer engine 105 is roughly divided into an engine control unit 104 and an engine mechanism unit 106. The engine mechanism unit 106 is a part that operates in response to various instructions from the engine control unit 104, and is a generic name for the mechanisms related to image formation described with reference to FIG.

  The laser / scanner system (the exposure apparatus 31 described above) functions as an exposure unit, and includes a laser light emitting element, a laser driver circuit, a scanner motor, a rotary polygon mirror, a scanner driver, and the like. This is a part where an electrostatic latent image is formed on the photosensitive drum 1 by exposing and scanning the photosensitive drum 1 with a laser beam in accordance with image data sent from the video controller 103. The image forming system 32 is a central part of the image forming apparatus, and is a part for forming a toner image based on the electrostatic latent image formed on the photosensitive drum 1 on the recording material P. A process cartridge 32-1, an intermediate transfer belt 110, a fixing device 130, and other process elements constituting the image forming stations a to d, and a high-voltage power supply circuit that generates various biases (high voltage) for image formation. Composed.

  The process cartridge 32-1 includes at least the photosensitive drum 1, and further includes, for example, a static eliminator, a charging roller 2, a developing roller of the developing device 4, and the like. Part of it. The process cartridge 32-1 is provided with a nonvolatile memory tag 32-2, and the CPU 171 or the ASIC 172 in the engine control unit 104 stores (stores) or reads various information in the memory tag 32-2. Execute.

  The paper feed / conveyance system 33 is a part that feeds and conveys the recording material P, and includes various conveyance system motors, a paper supply / discharge tray, various conveyance rollers, and the like. The sensor system 34 is a sensor group for collecting information necessary for the CPU 171 and the ASIC 172 to be described later to control the laser / scanner system, the image forming system 32, and the paper feeding / conveying system 33. This sensor group includes at least various types of already known sensors such as a temperature sensor of the fixing device 130, a toner remaining amount detection sensor, a density sensor that detects the density of an image, a paper size sensor, a paper leading edge detection sensor, and a paper conveyance detection sensor. included. Information detected by these various sensors is acquired by the CPU 171 and reflected in various operations of the image forming system 32 and print sequence control. Although the sensor system 34 in the drawing is described separately as the laser / scanner system 31, the image forming system 32, and the paper feed / conveyance system 33, it may be considered to be included in any mechanism.

  Next, the engine control unit 104 will be described. The CPU 171 uses the RAM 173 as a main memory and work area, and controls the engine mechanism unit 106 described above according to various control programs stored in the nonvolatile storage unit 174. More specifically, the CPU 171 drives the laser / scanner system 31 based on a print control command and image data input from the video controller 103 via the engine interface unit 191 and the engine interface unit 175. The CPU 171 controls various print sequences by controlling the image forming system 32 and the paper feeding / conveying system 33. Further, the CPU 171 drives the sensor system 34 to acquire information necessary for controlling the image forming system 32 and the paper feeding / conveying system 33. On the other hand, the ASIC 172 performs high-voltage power supply control such as control of each motor and development voltage in executing the above-described various print sequences under the instruction of the CPU 171. The system bus 176 has an address bus and a data bus. Each of the above-described components is connected to the system bus 176 and can access each other.

  Note that part or all of the functions of the CPU 171 may be performed by the ASIC 172, and conversely, part or all of the functions of the ASIC 172 may be performed by the CPU 171 instead. Further, a part of the functions of the CPU 171 and the ASIC 172 may be provided with separate dedicated hardware so that the dedicated hardware can perform the function.

(Development contact and separation)
The developing contact / separation configuration of this embodiment will be described with reference to FIG. FIG. 10A is a cross-sectional view of the image forming stations a to d. The configuration of the image forming station is the same for yellow, cyan, magenta, and black, and only the toner to be stored is different. Here, the black image forming station d will be described.

  The image forming station d includes a cleaning unit 5d as a first frame and a developing device 4d as a second frame. The cleaning unit 5d includes a photosensitive drum 1d that is rotatably provided, a charging roller 2d, and a cleaning blade 224. Further, the developing device 4d includes a developing roller 44d that is rotatably provided, and a toner container 43d that stores toner used in the developing roller 44d. Further, in the developing device 4d, the support shaft 53 is supported by the support hole 55 of the cleaning unit 5d. Therefore, the developing device 4d can be rotated (moved) with respect to the cleaning unit 5d. That is, when the rib 46d provided on the upper surface of the developing device 4d is pressed by the spacing member engaging portion 81d1 of the spacing member 81 which is a first member described later, the developing device 4d is moved to the arrow m around the support shaft 53. Rotate in the direction (clockwise). For yellow, magenta, and cyan, the separating member 80 as the second member corresponds. As a result, the photosensitive drum 1d and the developing roller 44d are separated. When the rib 46d is not pressed by the separation member engaging portion 81d1 of the separation member 81 (the state shown in FIG. 10A), the developing device 4d is moved around the support shaft 53 by the arrow n by the spring 227. Rotate in the direction (counterclockwise). As a result, the photosensitive drum 1d and the developing roller 44d are brought into contact with each other.

  FIG. 10B is a diagram illustrating the configuration of the separation members 80 and 81. The separation members 80 and 81 are provided in the image forming apparatus main body. In FIG. 10B, the image forming stations a to d are not shown for simplicity. The yellow, magenta, and cyan image forming stations a to c are respectively provided with developing devices 4a, 4b, and 4c, photosensitive drums 1a, 1b, and 1c, and ribs 46a, 46b, and 46c, similarly to the black image forming station d. It shall be provided.

  The image forming apparatus is provided with a separation member 81 and a separation member 80 for pressing the ribs 46a, 46b, 46c, and 46d provided in the developing devices 4a, 4b, 4c, and 4d. The separation member 81 acts only on the developing device 4d having black toner. The separation member 80 acts on the developing devices 4a, 4b, and 4c having yellow, magenta, and cyan. The spacing member 80 is provided with spacing member engaging portions 80a1, 80b1, and 80c1 protruding toward the image forming stations a to c. When the spacing member 80 moves in the right direction in FIG. 10B, the spacing member engaging portions 80a1, 80b1, and 80c1 move to the right and press the ribs 46a, 46b, and 46c on the developing devices 4a, 4b, and 4c sides. . As a result, the developing rollers 44a, 44b, and 44c disposed on the leading ends of the developing devices 4a, 4b, and 4c are separated from the photosensitive drums 1a, 1b, and 1c, and are in a development separated state.

  Further, the separating member 81 and the separating member 80 receive a rotational force from a driving source 293 that is a driving unit including a motor, a gear, a clutch, and the like, and move in the left-right direction in FIG. The shaft 90 rotates by the driving force of the drive source 293 and is transmitted to a cam 284 that is a first cam member provided fixedly on the shaft 90 and a cam 283 that is a second cam member. The cam 283 is set to move the spacing member 80 left or right, and the cam 284 is set to move the spacing member 81 left or right. By utilizing the shape difference between the cam 283 and the cam 284, the contact state of the developing device 4 and the photosensitive drum 1 is switched. For example, the yellow, magenta, and cyan developing devices 4a to 4c are separated from the photosensitive drums 1a to 1c, and only the black developing device 4d can be switched to a monocolor printing state in which the developing devices 4a to 4c are in contact with the photosensitive drum 1d. Further, all (four in this embodiment) developing devices 4a to 4d can be switched to a full-color print state in which they contact the photosensitive drums 1a to 1d. Furthermore, it is possible to switch to a standby state in which all the developing devices 4a to 4d are separated from the photosensitive drums 1a to 1d.

  The above-described development contact / separation state switching is performed by the engine control unit 104 transmitting a switching instruction to the drive source 293 based on an image formation instruction from the video controller 103. Further, the CPU 171 determines whether or not the developing roller 44 has contacted the photosensitive drum 1 depending on in which direction (positive rotation or negative rotation) the driving source 293 such as a motor is rotated. It shall be. This completes the description of the configuration of the image forming apparatus.

(Regarding normal exposure and micro exposure)
Hereinafter, an operation of causing each exposure device 31 to perform minute exposure on a portion where the toner image is not visualized on the photosensitive drum 1 will be described with reference to FIG. An operation of causing each exposure device 31 to perform normal light emission in which a light amount based on image data for image formation is further added to a portion where a toner image is visualized will be described. In the following description, the configuration and operation of the exposure apparatus 31a that irradiates light to the first image forming station a will be mainly described. However, it is assumed that the same configuration and operation are performed for the exposure apparatuses 31b to 31d that irradiate the second to fourth image forming stations b to d.

  With reference to FIG. 11A, description will be given of the micro exposure control of the laser beam 6a by the exposure device 31a at a location where the toner image on the photosensitive drum 1a is not visualized. First, the operation of the engine control unit 104 will be described. The engine control unit 104 finely exposes an exposure amount E0 for microexposure when exposing a background portion (which is also a non-image portion) that does not visualize a toner image in exposure for forming an electrostatic latent image on the photosensitive drum 1a. Control is performed by the signal 68a. In addition, the engine control unit 104 controls the exposure amount Ex for normal exposure used for exposure of the portion where the toner image is visualized by the pulse width signal 60a. The control by the minute exposure signal 68a and the pulse width signal 60a is specifically light emission time control. Here, an OR circuit is built in the laser driver 62a, and the laser diode 63a is driven to emit light by a pulse signal obtained by performing OR processing on the pulse signal based on the microexposure signal 68a and the pulse signal based on the pulse width signal 60a. Is called.

Further, the engine control unit 104 controls the light emission intensity of the laser driver 62a by the luminance signal 61a. The exposure amount here is a unit of μJ / cm ² as described above. That is, it is the light energy per unit area when the laser diode 63a continuously emits light in a certain area for a certain time with a certain light emission intensity.

  On the other hand, the engine control unit 104 changes the minute exposure signal 68a and the luminance signal 61a, and controls the minute exposure amount E0 of the background portion to an appropriate value. The width of the pulse signal output in response to the instruction by the micro-exposure signal 68a of the engine control unit 104 is equal to PWMIN (for example, 12.0% for one pixel) when the image data is 0 (background portion). I'm doing it. The minute exposure amount E0 is such that the average surface potential per image obtained at the time of exposure does not fall below the development potential (for example, about −400 V) and the potential of the photosensitive drum 1a is attenuated so as to obtain charge uniformization described later. It is set according to the characteristics.

  The laser driver 62a controls the light emission luminance and the light emission time of the laser diode 63a based on the luminance signal 61a, the pulse width signal 60a, and the minute exposure signal 68a that are instructed from the engine control unit 104 based on the image data. The light emission luminance can be controlled by adjusting the current supplied from the laser driver 62a to the laser diode 63a. Further, the laser beam 6a emitted from the laser diode 63a is optically scanned and irradiated to the photosensitive drum 1a through a correction optical system 67a including a rotary polygon mirror 64a, a lens 65a, and a folding mirror 66a. The laser driver 62a controls the current supplied to the laser diode 63a so as to perform so-called automatic light quantity control so as to achieve a target light emission luminance (mW).

  By performing the minute light emission as described above, in the present embodiment, the charging potential Vd_bg of the non-image portion decreases from the charging potential Vd before exposure of the non-image portion to −500V to −500V. On the other hand, the exposure potential Vl of the image portion is changed from the charging potential Vd = −600V to Vl = −150V due to the full light emission of the laser diode 63a.

(Necessity for correction of minute exposure)
First, a problem relating to the difference in film thickness of the photosensitive drum 1 will be described with reference to FIG. As the use of the photosensitive drum 1 proceeds, the surface of the photosensitive drum deteriorates due to the discharge of the charging roller 2, and the surface of the photosensitive drum is scraped by rubbing against the cleaning unit 5, and the film thickness becomes thin. At this time, if photosensitive drums 1 having different usage conditions (integrated rotation speeds) are mixed, the film thicknesses of the photosensitive drums 1 vary. In this state, by connecting the charging / developing high-voltage power supply unit 315 of Example 1 to a plurality of image forming stations (described later) and applying a constant charging voltage Vcdc to the plurality of photosensitive drums 1, The phenomenon occurs. That is, since the potential difference generated in the air gap between the charging roller 2 and the photosensitive drum 1 is different, the charging potential Vd is different for each photosensitive drum 1. Specifically, the photosensitive drum 1 with a small number of image formations has a large film thickness, and the potential difference generated in the air gap between the charging roller 2 and the photosensitive drum 1 is small, so the absolute value of the charging potential Vd is small. On the other hand, the photosensitive drum 1 having a large cumulative number of rotations is thin, and the potential difference generated in the air gap between the charging roller 2 and the photosensitive drum 1 is large, so that the absolute value of the charging potential Vd increases (in FIG. 11B). , Vd Up).

  For example, in the thick photosensitive drum 1, the development potential Vdc and the charging potential Vd are set so that the development back contrast Vback (= Vd−Vdc), which is the contrast between the development potential Vdc and the charging potential Vd, is in a desired state. . Then, the following problems arise. That is, as shown in FIG. 11B, in the image forming station having the thin photosensitive drum 1, the absolute value of the charging potential Vd is increased and the development back contrast Vback is increased. When the development back contrast Vback is increased, the toner that cannot be charged to the normal polarity (in the case of reversal development as in this embodiment, toner that is not negatively charged but charged to 0 to positive) is transferred from the developing device 4 to the non-image. The cover is transferred to the part and fog occurs.

  Further, in the image forming station where the photosensitive drum 1 is thin, the charging potential Vd rises. Therefore, in the configuration in which the exposure intensity is constant, the exposure potential Vl also rises (shown as Vl Up in FIG. 11B). ). Therefore, the development contrast Vcont (= Vdc−Vl), which is the difference value between the development potential Vdc and the exposure potential Vl, becomes small, and the toner cannot be sufficiently transferred electrostatically from the developing device 4 to the photosensitive drum 1. The thin density of the solid black image is likely to occur.

  Here, as shown in FIG. 11C, the developing voltage Vdc and the charging voltage Vcdc are fixed, and when the photosensitive drum 1 is thick, the exposure intensity is set to E1 when the photosensitive drum 1 is thin. Is changed to E2 (where E1 <E2). With such a configuration, the exposure potential Vl can be made constant regardless of the film thickness of the photosensitive drum 1 by the individual control of the exposure intensity, so that the development contrast which is the difference value between the development potential Vdc and the exposure potential Vl. Vcont can be controlled to be substantially constant, and the concentration can be kept constant. However, since the charging potential Vd rises as the film thickness of the photosensitive drum 1 changes from a thick state to a thin state, the developing back contrast Vback, which is the contrast between the developing potential Vdc and the charging potential Vd, increases. For this reason, the problem of fogging remains as described above even in the configuration of FIG.

(Regarding the correction of the amount of light emitted from minute light emission)
On the other hand, in this embodiment, for example, even when the power sources of the plurality of charging rollers 2 are set as one common power source and the power sources of the plurality of developing devices 4 are also set as one common power source, Generation of fogging and low density can be suppressed with the configuration. Hereinafter, the minute exposure amount E0 of each of the laser diodes 63a to 63d in the background portion (non-image portion) where toner adhesion is not performed is related to the remaining lifetime of the photosensitive drums 1a to 1d using the flowchart shown in FIG. The correction process will be described. As described above, the thickness of the photosensitive drum 1 is thick when the use of the photosensitive drum 1 is not advanced, and the thickness of the photosensitive drum 1 is thin when the use of the photosensitive drum 1 is advanced. Here, the life of the photosensitive drum 1 is a guarantee period in which the photosensitive drum 1 can be used.

  In step S <b> 201, the engine control unit 104 reads information on the total number of rotations of the photosensitive drum 1 as information related to the remaining life of the photosensitive drum 1 from the memory tag 32-2 of each image forming station. The information relating to the remaining life of the photosensitive drum 1 may be information relating to how much the photosensitive drum 1 has been rotated or used. For example, the motor driving time of the motor that drives the photosensitive drum 1, the number of rotations of the charging roller 2, the number of prints, and the like may be used.

  In S202, the engine control unit 104 sets the exposure parameter for the normal exposure amount based on the information on the accumulated rotational speed read in S201. Note that the engine control unit 104 refers to a table in which a correspondence relationship between the integrated rotation number of each photosensitive drum 1 (usage status of the photosensitive drum 1) and a pulse width PWMIN (light emission time) related to normal exposure is determined. The exposure parameter for the normal exposure amount is set. The exposure parameter of the normal exposure amount here corresponds to the luminance signal 61a described with reference to FIG. The table referred to by the engine control unit 104 is a table in which the usage status of the photosensitive drum 1 is associated with the light emission control setting at the time of microexposure or normal exposure, and the photosensitive characteristic (EV curve) of the photosensitive drum 1 is associated with it. It is set in advance based on this. The table is stored in storage means that the engine control unit 104 can refer to. The detailed contents of the table are omitted.

  In S203, the engine control unit 104 determines an exposure parameter related to the laser light emission amount E0 (small exposure amount) for background portion exposure based on the integrated rotation speed. The parameter relating to the laser emission amount E0 of the background portion exposure corresponds to the minute exposure signal 68a described with reference to FIG. Hereinafter, a parameter related to the laser emission amount E0 of the background portion exposure is referred to as a fine exposure amount exposure parameter. Through the processes in S202 and S203, the exposure amounts for the fine exposure and the normal exposure can be appropriately set in relation to the remaining life of the photosensitive drum 1.

  In step S204, the engine control unit 104 performs control so that each member executes the series of image forming operations and controls described with reference to FIG. In step S <b> 205, the engine control unit 104 measures the number of rotations of the photosensitive drums 1 a to 1 d rotated in a series of image formation using a counter (not shown). In S206, the engine control unit 104 determines whether or not the image forming operation has ended. When it is determined that the image forming operation has not ended, the process returns to S204, and when the image forming operation is determined to have ended. Proceeds to S207. In S207, the engine control unit 104 adds the measurement result measured in S205 to the corresponding accumulated rotation number of each photosensitive drum 1, and updates the accumulated rotation number. In S208, the engine control unit 104 stores the accumulated rotation number of each photosensitive drum 1 updated in S207 in the nonvolatile memory tag 32-2 of each image forming station.

(Description of action / effect)
The effect of the flowchart of FIG. 12 is demonstrated using FIG.11 (d). As described with reference to FIGS. 11B and 11C, when the film thickness of the photosensitive drum 1a is small, the potential difference generated in the air gap between the photosensitive drum 1a and the charging roller 2a is larger than when the film thickness is large. . Accordingly, the charging potential Vd increases from the dotted line portion A to the dotted line portion B in FIG. Here, in this embodiment, when the film thickness of the photosensitive drum 1 is the largest (the photosensitive drum in the initial state), the charging potential Vd after passing through the charging roller 2 is about −600V. On the other hand, in the photosensitive drum 1 in which the integrated rotation speed of the photosensitive drum 1a is increased, the film thickness is thin, and the use is advanced (near the end of life), the charging potential Vd is about −700V, and the charging potential Vd is about −100V. fluctuate.

  As described above, since the charging potential Vd increases as the film thickness of the photosensitive drum 1 decreases, the post-exposure potential Vl increases as described with reference to FIG. 11B when the exposure amount for normal exposure is constant. To do. Therefore, the exposure amount during full light emission is increased from E1 to E2, and the post-exposure potential Vl is kept substantially constant as indicated by the solid line portion in FIG. For this reason, the development contrast Vcont (= Vdc−Vl), which is the difference between the development potential Vdc and the exposure potential Vl, can be maintained at a constant value regardless of the film thickness of the photosensitive drum 1, and the occurrence of a low density image. Can be suppressed.

  Further, the amount of laser light at the time of non-image area exposure (at the time of minute exposure) can be increased from Ebg1 to Ebg2 (Ebg1 <Ebg2). For this reason, when a DC voltage is applied to the charging roller 2 at a constant value, it is possible to correct the increase (for example, −100 V) of the charging potential Vd caused by the change in the film thickness of the photosensitive drum 1 by microexposure. As shown by the solid line portion in FIG. 11D, the charging potential Vd_bg of the non-image portion can be made substantially constant regardless of the film thickness of the photosensitive drum 1. Therefore, even when the development voltage Vdc is a constant value, the development back contrast Vback that is the potential difference between the development voltage Vdc and the charged potential Vd_bg after non-image area exposure can be maintained constant. For this reason, it is possible to suppress the fog generated when the toner that has not been normally charged (in the case of reversal development, the toner charged with 0 to positive polarity instead of negative polarity) is transferred to the non-image portion.

(Charging / Development high-voltage power supply)
FIG. 13 shows details of the charging / developing high-voltage power supply unit 325. The configuration of each image forming station is the same as that shown in FIGS. 9A and 10A, and the same components are denoted by the same reference numerals and description thereof is omitted. The main difference from the first embodiment is that the charging / developing high-voltage power supply unit 315 described in the first embodiment is connected to the charging rollers 2a to 2c of the plurality of image forming stations a to c and the plurality of image forming stations a to c. It is connected to the developing rollers 44a to 44c of the developing devices 4a to 4c. For this reason, in the charging / developing high-voltage power supply unit 315, the same components as those in FIG.

  In this embodiment, the YMC color image forming stations a to c are connected to a charging / developing high-voltage power supply unit 325 as a second power supply unit, and the Bk color image forming station d is connected to a charging / developing high-voltage unit. A power supply unit 335 is connected. In the present embodiment, the high voltage control unit 225 controls the charging / developing high voltage power supply units 325 and 335. The charging / developing high-voltage power supply unit 335 has the same configuration as that of the charging / developing high-voltage power supply unit 325, and detailed description is omitted in FIG. Thus, in this embodiment, the power source is divided into at least two. When image formation is performed in the full color mode, the charging / developing high-voltage power supply units 325 and 335 are operated (turned on). On the other hand, when image formation is performed in the mono-color mode, the charging / developing high-voltage power supply unit 325 for the YMC color image forming stations a to c is not operated (turned off), and the Bk color image forming station is operated. The d charging / developing high voltage power supply unit 335 is operated (turned on).

  The functions of the charging / developing high-voltage power supply unit 335 connected to the Bk color image forming station d are the same as those in the first embodiment, and thus the description thereof is omitted. Hereinafter, the charging / developing high-voltage power supply unit 325 connected to the YMC color image forming stations a to c will be described focusing on functions different from those in the first embodiment.

  FIG. 14 is an example of a phenomenon unique to this embodiment, and is an example of a load fluctuation of the development voltage Vdc. The circuit configuration of the charging / developing high-voltage power supply unit 325 is the same as that of the first embodiment. In FIG. 14, solid toner 433c having a large toner amount is developed on the photosensitive drum 1c of the cyan image forming station c by full light emission of the exposure device 31c. The case where the toner amount is small or the toner is not developed is shown on the photosensitive drum 1a of the yellow image forming station a and the photosensitive drum 1b of the magenta image forming station b at the same timing. At this time, a current i3 flows from the developing roller 44c of the cyan image forming station c toward the photosensitive drum 1c. Although a current also flows through the yellow image forming station a and the magenta image forming station b, it is assumed that the current is sufficiently smaller than the current flowing through the cyan image forming station c. Therefore, only the current i3 is illustrated. Even when such a current i3 flows, the operational amplifier 417 can control the collector current of the small signal transistor 419 and keep the developing voltage Vdc constant.

  If the development voltage generation unit 407 has a configuration in which the divided voltage is generated by the resistors R3 and R8 as shown in FIG. 6D, the change in the development voltage Vdc caused by the current i3 flowing is the yellow image forming station a and magenta. Appears at the image forming station b. As a result, the absolute value of the development voltage Vdc is reduced. For this reason, a phenomenon occurs in which the development contrast Vcont decreases and the density decreases in the image portion, and in the non-image portion, the development back contrast Vback increases and fog occurs. This potential variation is difficult to quantify, as is the case with the current 424 caused by the development contact / separation described in the first embodiment. Therefore, the potential fluctuation is larger than the adjustment of the development potential Vdc by normal exposure or fine exposure. It is preferable to suppress voltage fluctuation on the development high voltage power supply unit 325 side. As described above, the fluctuations in the charging potential Vd and the exposure potential Vl due to the difference in the film thickness of the photosensitive drums 1a to 1d of the image forming stations a to d can be controlled by the normal exposure amount control and the minute exposure amount control. Make corrections.

  FIG. 15 shows the development contact and separation during full-color image formation (hereinafter, full-color printing) (FIG. 15A) and mono-color image formation (hereinafter, mono-color printing) (FIG. 15B). The sequence of the charging / developing high-voltage power supply units 325 and 335 is shown. 15A and 15B, (a) shows the development contact state of YMC color, (b) shows the development contact state of Bk color, and (c) shows the charging / development high-voltage power supply. FIG. 4D shows the on / off state of the charging / developing high-voltage power supply units 325 and 335. S21 to S24 and S31 to S34 indicate timings.

  As shown in FIG. 15A, during full-color printing, the charging / developing high-voltage power supply unit 325 is in a state where the developing devices 4a to 4d are separated from the photosensitive drums 1a to 1d (development separated state) at timing S21. 335 is turned on. That is, application of the charging voltage Vcdc and the development voltage Vdc is started. At timing S22, the developing devices 4a to 4d abut against the photosensitive drums 1a to 1d in YMCK, and full-color image formation is performed at timings S22 to S23. Then, after the developing devices 4a to 4d are separated from the photosensitive drums 1a to 1d at the timing S23, the charging / developing high-voltage power supply units 325 and 335 are turned off at the timing S24.

  On the other hand, as shown in FIG. 15B, during mono-color printing, the YMC is always in a state where the developing devices 4a-4c are separated from the photosensitive drums 1a-1c, and the charging / developing high-voltage power supply unit 325 is also always in the state. It is in the off state. For Bk, the sequence is the same as in full color printing. That is, the charging / developing high-voltage power supply unit 335 is turned on in a state where the developing device 4d is separated from the photosensitive drum 1d at timing S31, the developing device 4d contacts the photosensitive drum 1d at timing S32, and at timings S32 to S33. Monocolor image formation is performed. Then, after the developing device 4d is separated from the photosensitive drum 1d at timing S33, the charging / developing high-voltage power supply unit 335 is turned off at timing S34. By adopting such a power supply sequence, similarly to the first embodiment, it is possible to prevent toner from being placed on the photosensitive drum 1 due to an unintended voltage in the output transient state of the charging voltage Vcdc and the development voltage Vdc.

  As described above, the charging / developing high-voltage power supply unit 325 of this embodiment is connected to a plurality of image forming stations a to c, and appropriate power supply control is performed, and exposure potential control and non-image portion potential control are used in combination. And Accordingly, it is possible to appropriately control the development contrast Vcont and the development back contrast Vback, and to configure an inexpensive image forming apparatus that does not cause image defects such as fogging. In this embodiment, the charging / developing high-voltage power supply unit 325 is connected to the YMC color image forming stations a to c. However, the same effect can be obtained when connected to four YMCBk color image forming stations. The configurations described in the first and second embodiments can be appropriately changed as long as they are equivalent configurations, and the scope of the invention is not limited to the illustrated configurations.

  As described above, according to this embodiment, it is possible to reduce image defects with an inexpensive configuration.

15 Photosensitive drum 20 Developer 202 Engine control unit 315 Charging / developing high voltage power supply unit 407 Development voltage generating unit 418 Zener diode 419 Small signal transistor

Claims (13)

An image carrier;
Charging means for charging the image carrier;
Developing means for developing the electrostatic latent image formed on the image carrier with toner;
A charging voltage applying means for generating a charging voltage to be applied to the charging means; and the developing means connected to the charging voltage applying means through a resistance element and based on the charging voltage generated by the charging voltage applying means. A developing voltage applying unit that generates a developing voltage for applying to the charging unit, and a power supply unit that applies the developing voltage to the developing unit during a period of applying the charging voltage to the charging unit;
In order to develop the electrostatic latent image formed on the image carrier with toner, the developing means is brought into contact with the image carrier, and the electrostatic latent image on the image carrier is developed with toner. Control means for controlling the developing means to be separated from the image carrier after
An image forming apparatus comprising:
The developing voltage applying means includes a switching element for performing constant voltage control of the developing voltage, a constant voltage element connected in series to the switching element, a control voltage generated by dividing a reference voltage, and the developing Comparing with a voltage corresponding to the current flowing through the voltage applying means, and comparing means for outputting a signal for controlling the supply current to the switching element according to the comparison result ,
The control unit controls the application of the charging voltage and the development voltage by the power supply unit or stopping the application in a state where the developing unit is separated from the image carrier. apparatus. The charging voltage applying means has monitor voltage generating means for dividing the charging voltage and monitoring a voltage obtained by offsetting the divided voltage with a reference voltage,
The image forming apparatus according to claim 1, wherein the reference voltage of the monitor voltage generation unit is the reference voltage for generating a control voltage of the comparison unit. The image forming apparatus according to claim 2 , wherein the charging voltage applying unit and the developing voltage applying unit perform constant voltage control based on the reference voltage. Wherein the image forming apparatus, and one of the image bearing member, an image forming apparatus according to any one of claims 1 to 3, characterized in that it comprises a plurality of said developing means. The image forming apparatus according to claim 4 , wherein the control unit corrects the charging voltage applied to the charging unit by the power supply unit according to information on a usage amount of the image carrier. 6. The image forming apparatus according to claim 5 , wherein the information relating to the amount of use of the image carrier is an accumulated rotational speed of the image carrier. A rotating body having the plurality of developing means;
The control means, by rotating the rotating body, an image forming apparatus according to any one of claims 4 to 6, characterized by controlling the contact or away from said developing means. Comprising a detecting means for detecting that the rotating body is stopped at a predetermined position;
The image forming apparatus according to claim 7 , wherein the control unit controls the rotation of the rotating body based on the predetermined position detected by the detection unit. Wherein the image forming apparatus, and the image bearing member, said charging means, an image according to any one of claims 1 to 3, characterized in that it comprises a plurality of image forming section having, said developing means Forming equipment. A first power supply unit that is the power supply unit that applies a voltage to one image forming unit among the plurality of image forming units;
A second power supply unit that is the power supply unit for applying a voltage to the other image forming units excluding the one image forming unit;
The image forming apparatus according to claim 9 , further comprising: Furthermore, an exposure means for forming the electrostatic latent image on the image carrier charged by the charging means is provided,
The control means includes an exposure amount that exposes an image portion of the image carrier by the exposure means, and an exposure amount that exposes a non-image portion of the image carrier according to information on the usage amount of the image carrier. The image forming apparatus according to claim 10 , wherein the correction is performed. The image forming apparatus according to claim 11 , wherein the information related to the amount of use of the image carrier is an accumulated rotational speed of the image carrier. A first member for contacting or separating the developing means of the one image forming unit;
A second member for contacting or separating the developing means of the other image forming unit;
Driving means for driving the first member and / or the second member;
With
The control means, by driving the driving means, the image forming apparatus according to any one of claims 10 to 12, characterized by controlling the contact or separation of the plurality of said developing means.

Images