JP2008089714A - Image forming apparatus - Google Patents

Abstract

An image forming apparatus capable of obtaining good printing without a drum ghost without using a static eliminator.
An electrophotographic image forming apparatus for forming an electrostatic latent image on a drum-shaped image carrier 101, wherein pixel data of an image for one round of the surface of the image carrier 101 is transferred. A one-round image data storage unit 312 that stores exposure history information together with the positional information between the pixel data order information and the main scanning direction / sub-scanning direction on the image carrier 101, and each of the image data on the surface of the image carrier 101. In order to correct the exposure amount based on the pixel data of the image at the position corresponding to the pixel, each pixel data in which the exposure amount of the exposure apparatus 103 is corrected based on the exposure history information of the image data storage unit 312 for one round. A correction pixel data generation unit 322 to generate, and an exposure head drive control unit 332 that controls the drive current of the exposure apparatus 103 by each pixel data with the exposure amount corrected.
[Selection] Figure 3

Description

  According to the present invention, an electrostatic latent image is formed on an image bearing member by exposing the surface of the drum-shaped image bearing member held on the surface by an exposure device based on an input image signal. The present invention relates to an electrophotographic image forming apparatus for forming an image by forming and forming an electrostatic latent image into a visible image with a developer.

  In general, an electrophotographic image forming apparatus includes a drum-shaped electrophotographic photosensitive member (hereinafter referred to as a photosensitive drum). This photosensitive drum is also referred to as an image carrier because a toner image is formed on the outer peripheral surface thereof. The toner image formed on the outer peripheral surface of the photosensitive drum is transferred onto a transfer material such as paper by a transfer device. The toner image on the transfer material becomes a permanently fixed image by being fixed by a fixing device. Further, the toner remaining on the outer peripheral surface of the photosensitive drum after the transfer is scraped off by the cleaning device. As a result, the photosensitive drum is prepared for the next image formation. In this manner, the electrophotographic image forming apparatus forms an image on the transfer material and discharges the transfer material on which the image is formed.

  As a general method for forming a toner image on the outer peripheral surface of the photosensitive drum, first, the surface is uniformly charged by a charging device. Next, by exposing the surface of the photosensitive drum charged by the exposure device based on an image signal input from the outside or the like, a potential difference is generated between the exposed portion and the unexposed portion, and the photosensitive drum An electrostatic latent image is formed on the outer peripheral surface. Further, a toner image is formed by developing the electrostatic latent image on the outer peripheral surface of the photosensitive drum with a developing device.

  By the way, once the toner image is formed on the outer peripheral surface of the photosensitive drum, the potential at which the image is formed remains after the transfer and cleaning, and is then formed on the outer peripheral surface of the photosensitive drum. It is known that an afterimage of an image formed last time on an image is generated as a history (hereinafter referred to as drum ghost). The drum ghost is, for example, a potential difference between the exposed portion and the unexposed portion of the history image remaining after the first cycle image formation on the outer peripheral surface of the photosensitive drum remains even during the second cycle image formation. This is a phenomenon in which shading due to the history image appears on the image of the second cycle output on the transfer material.

  The process in which drum ghost occurs is considered as follows. When each process of charging, exposure, development, transfer, and cleaning when the photosensitive drum rotates once (one turn) is defined as one cycle image forming process, the image forming process after the first cycle is completed As described above, the potential difference generated between the exposed portion and the unexposed portion remains on the surface of the photosensitive drum. The potential difference between the exposed portion and the unexposed portion in the first cycle remains completely undisturbed even when the surface of the photosensitive drum is uniformly charged by the charging device in the second cycle. The surface of the drum is not uniformly charged, and the potential difference remaining after this charging remains without disappearing even after exposure. For this reason, the electrostatic latent image after the surface exposure of the photosensitive drum in the second cycle does not have a uniform potential because the effect of the potential difference between the exposed portion and the unexposed portion in the first cycle remains. When developing the electrostatic latent image on the outer peripheral surface of the photosensitive drum in the second cycle, the amount of toner adhering to the unexposed portion varies depending on the remaining potential difference in the first cycle. In other words, the toner image of the second cycle that is supposed to be uniform (part corresponding to the unexposed portion) has a portion with a large amount of toner and a portion with a small amount of toner due to the potential difference of the electrostatic latent image of the first cycle. The toner image transferred to the transfer material (its corresponding part of the unexposed area) also becomes shaded due to the difference in the toner amount, and the final image on the transfer material (the corresponding part of the unexposed area) is also colored. The dark and light parts of the image appear dark and light.

  In order to suppress the occurrence of the drum ghost, a pre-exposure device (hereinafter referred to as “LED”) is irradiated on the outer peripheral surface of the photosensitive drum before the next image forming operation as in the second cycle. It is known that a previously formed image history on the outer peripheral surface of the photosensitive drum is erased by providing a static eliminating device) in the image forming apparatus and thereby uniformly eliminating the surface of the photosensitive drum before image formation. ing. In the case of the charge removal, it is known that the intensity of light irradiated from the charge removal device (hereinafter referred to as light quantity) is optimally controlled according to the use conditions and durability of the image forming apparatus (for example, Patent Document 1).

JP 2005-208223 A

  However, in a conventional image forming apparatus that removes a history image, it is difficult to reduce the size and cost by adding a static eliminator inside, and energy is saved by performing pre-exposure with the static eliminator before normal exposure. There was a problem that was difficult.

  SUMMARY An advantage of some aspects of the invention is that it provides an image forming apparatus capable of obtaining good printing without a drum ghost without using a static eliminator.

  In order to achieve the above object, the image forming apparatus of the present invention exposes the surface of a drum-shaped image carrier having a photosensitive member held on the surface thereof by an exposure device based on an input image signal. An electrophotographic image forming apparatus for forming an image by forming an electrostatic latent image on the image carrier and making the electrostatic latent image visible with a developer, the surface of the image carrier The pixel data of the image for one round of the image carrier surface used for exposure to form an electrostatic latent image thereon, the order information of each pixel data to be transferred, and the main scanning direction / sub-scan on the image carrier In order to correct the exposure amount by the pixel data to the position corresponding to each pixel on the surface of the image carrier, the image data storage unit for one round to be stored as the exposure history information together with the positional relationship information with the direction. Exposure history information stored in the minute image data storage unit Based on comprises a compensation pixel data generating unit that generates pixel data obtained by correcting the exposure amount of the exposure device, the pixel data obtained by correcting the exposure amount, the exposure head driving control unit for controlling the driving current of the exposure apparatus.

  In the image forming apparatus of the present invention, the pixel data of the image for one rotation with respect to the photosensitive drum at the time of rotation is stored together with the order information and the positional relationship information, and for each pixel of each pixel data of the image of the next rotation, and Since the exposure amount can be corrected for each exposure position on the surface of the photosensitive drum based on the stored pixel data of the previous circumference, it is possible to obtain a good print without drum ghost without using a static eliminator. it can.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

(First embodiment)
FIG. 1 is a schematic cross-sectional view showing the configuration of Embodiment 1 of the image forming apparatus according to the present invention.
In the figure, an image forming apparatus 500 has four process units 201 for forming black (K) yellow (Y), magenta (M), and cyan (C) images in the image forming unit 200, respectively. To 204, which are detachably disposed in order from the upstream side of the conveyance path 220 along which the recording medium 205 corresponding to the transfer material is conveyed. Since the internal configurations of the process units 201 to 204 are common, the internal configuration will be described by taking the magenta process unit 203 as an example.

  Hereinafter, a portion of the image forming apparatus 500 excluding the detachable process units 201 to 204 will be referred to as a main body of the image forming apparatus 500.

  In the process unit 203, the photosensitive drum 101 is rotatably arranged in the direction of the arrow. Electric charges are supplied to the surface of the photosensitive drum 101 around the photosensitive drum 101 sequentially from the upstream side in the rotation direction. A charging roller 102 for charging and an exposure device 103 for forming an electrostatic latent image by selectively irradiating light onto the surface of the charged photosensitive drum 101 are provided. Further, when a developing device 104 that causes magenta (predetermined color) toner to adhere to the surface of the photosensitive drum 101 on which the electrostatic latent image is formed, and development of the toner on the photosensitive drum 101 is transferred. A cleaning blade 105 for removing residual toner is disposed. The drums or rollers used in each of these devices rotate by receiving power from a drive source (not shown) via a gear or the like.

  In addition, the image forming apparatus 500 has a paper cassette 206 for storing a recording medium 205 such as paper in a lower portion of the image forming apparatus 500, and the recording medium 205 is separated and conveyed one by one above it. A hopping roller 207 is provided. Further, by sandwiching the recording medium 205 together with the pinch rollers 208 and 209 on the downstream side of the hopping roller 207 in the conveying direction of the recording medium 205, the conveying roller 210 that conveys the recording medium and the recording medium 205 are skewed. A registration roller 211 that is corrected and conveyed to the process unit 201 is disposed. The hopping roller 207, the conveying roller 210, and the registration roller 211 are rotated by power transmitted from a driving source (not shown) via a gear or the like.

  Transfer rollers 212 made of semiconductive rubber or the like are disposed at positions facing the respective photosensitive drums 101 of the process units 201 to 204. These transfer rollers 212 have a potential difference between the surface potential of each photoconductive drum 101 and the surface potential of each of these transfer rollers 212 when transferring a toner image of toner attached on the photoconductive drum 101 to the recording medium 205. A potential for applying the voltage is applied.

  The fixing device 213 includes a heating roller and a backup roller, and fixes the toner transferred onto the recording medium 205 by applying pressure and heating. The downstream discharge rollers 214 and 215 sandwich the recording medium 205 discharged from the fixing device 213 together with the pinch rollers 216 and 217 of the discharge unit and convey them to the recording medium stacker unit 218. The fixing device 213, the discharge roller 214, and the like are rotated by transmission of power from a drive source (not shown) via a gear.

  Next, the operation of the image forming apparatus 500 having the above configuration will be described. First, the recording medium 205 stored in a stacked state in the paper cassette 205 is separated and transported one by one from the top by the hopping roller 207. Subsequently, the recording medium 205 is sandwiched between the conveyance roller 210, the registration roller 211, and the pinch rollers 208 and 209 and is conveyed between the photosensitive drum 101 and the transfer roller 212 of the process unit 201. Thereafter, the recording medium 205 is sandwiched between the photosensitive drum 101 and the transfer roller 212, and the toner image is transferred onto the recording surface and simultaneously conveyed by the rotation of the photosensitive drum 101.

  Similarly, the recording medium 205 sequentially passes through the process units 202 to 204, and in the passing process, the electrostatic latent images formed by the respective exposure devices 103 are developed into the respective color toner images developed by the developing device 104. Sequentially transferred and superposed on the recording surface.

  Then, after the toner images of the respective colors are superimposed on the recording surface, the recording medium 205 on which the toner image is fixed by the fixing device 213 is sandwiched between the discharge rollers 214 and 215 and the pinch rollers 216 and 217, and the image The recording medium stacker unit 218 outside the forming apparatus 500 is discharged. Through the above process, a color image is formed on the recording medium 205.

  Note that the XYZ coordinates in the figure are taken with the X axis in the conveyance direction of the recording medium 205, the Y axis in the rotation axis direction of the photosensitive drum 101, and the Z axis in the direction perpendicular to these two axes. . Further, when XYZ coordinates are shown in other drawings to be described later, the axial directions of these coordinates indicate a common direction.

FIG. 2 is a partially enlarged view of the photosensitive drum 101 disposed in, for example, the process unit 203 and its peripheral portion in the image forming unit 200 of FIG. In the figure, the photosensitive drum 101 is formed by laminating an organic photosensitive layer 101a on the outer peripheral surface of a cylindrical aluminum tube support, and the charging roller 102 is formed by laminating a conductive rubber layer on a metal shaft. Has been. The exposure apparatus 103 has an LED head configuration in which an LED element having a light emission wavelength of 740 nm and a light amount of 1 μJ / cm ² corresponding to each dot of an image and a lens array are combined. Each LED element has a density of 600 dpi in the Y-axis direction. They are arranged side by side. Each of these LED elements emits light independently according to each pixel data of an image by supplying a voltage from a control unit (not shown) to form an electrostatic latent image on the surface of the photosensitive drum 101. In addition, each pixel data of the image of the present embodiment is individual pixel data corresponding to each dot of an electrostatic latent image formed by light emission of the LED element having an area of one circumference of the photosensitive drum 101. Is a collective term for a collection of

  The developing roller 104a, which belongs to the developing device 104 (FIG. 1) and attaches toner to the surface of the photosensitive drum 101, is formed by laminating conductive rubber on a metal shaft, and the transfer roller 212 is conductive foam on the metal shaft. It is formed by laminating rubber. The cleaning blade 105 is formed of an elastic rubber plate having a thickness of 2 mm, for example. The recording medium 205 is sandwiched between the photosensitive drum 101 and the transfer roller 212 as described above, and the toner image is transferred to the recording surface.

  Next, the operation of the image forming unit 200 will be described. The recording medium 205 is conveyed one by one from the paper cassette 206 of the image forming apparatus 500 at a timing defined for each image forming apparatus.

  On the outer peripheral surface of the rotating photosensitive drum 101, an electrostatic latent image is formed by exposure from the exposure device 103, and the electrostatic latent image is developed by the developing device 104 to become a toner image. The recording medium 205 taken out from the paper cassette 206 can reach the position in accordance with the timing when the photosensitive drum 101 of each of the color process units 201 to 204 rotates and the toner image reaches the position facing the transfer roller 212. Is transported in the transport path 220. Then, at a predetermined position of the recording medium 205 to be conveyed, the photosensitive drum of each color is located at a position where the photosensitive drum 101 and the transfer roller 212 of each of the color process units 201 to 204 are arranged in order from the upstream side of the conveying path 220. The toner image formed on the body drum 101 is transferred.

  The toner images of the respective colors transferred to the recording medium 205 are fixed as permanent fixed images on the recording medium 205 by the fixing device 213 arranged downstream of the conveyance path 220, and the recording medium 205 on which the images are fixed is the image forming apparatus 500. More discharged.

  FIG. 3 is a block diagram showing a schematic configuration of an image forming unit and an image signal processing unit in the image forming apparatus of FIG. As shown in the figure, the image forming apparatus 500 includes an image forming unit 200 and an image signal processing unit 300. Since the image forming unit 200 has been described above, the configuration of the image signal processing unit 300 will be described below.

  The image signal processing unit 300 analyzes an image signal 401 input from the outside, and the image signal 401 is determined for each specified color of black (K), yellow (Y), magenta (M), and cyan (C). Each photosensitive drum 101 (101K, 101K, 101) of the process units 201 to 204 for each color is obtained by an image conversion processing unit 301 that performs conversion processing to a corresponding image signal and an image signal for each specified color (K, Y, M, C). Exposure correction controller 302 (302K, 302Y, 302M, 302C) disposed to control the exposure amount of each exposure apparatus 103 (103K, 103Y, 103M, 103C) corresponding to the outer peripheral surface of 101Y, 101M, 101C). ). The exposure amount is irradiated from the exposure device 103 (103K, 103Y, 103M, 103C) in order to form an electrostatic latent image on the outer peripheral surface of the photosensitive drum 101 (101K, 101Y, 101M, 101C). It is defined as the intensity of light.

  When the input pixel data is for dot formation, the exposure correction control unit 302 corrects the current for each LED element of the LED head according to the exposure history of the previous pixel position on the photosensitive drum 101. After that, a drive current is supplied to the LED element.

  The image data input to the exposure correction control unit 302 is supplied to both the one-round image data storage unit 312 that stores image data for one rotation of the photosensitive drum and the exposure head drive control unit 332 that mainly performs the LED drive function. Entered. The image data input to the one-round image data storage unit 312 is output at the timing when the photosensitive drum makes one rotation after the input and an image is formed on the photosensitive drum at the same exposure position as the previous time. Hereinafter, this output data is referred to as history data. This history data is then input to the correction pixel data generation unit 322, and the correction pixel data generation unit 322 corrects correction data of a value indicated by 4 bits for determining the drive current to the LED based on this data. C0 and C1 are generated and supplied to the exposure head drive controller 332.

The correction data C0 is correction data when the history data is “0”, and the correction data C1 is correction data when the history data is “1”. For example, the value of the correction data C0 is “0Eh”, the value of the correction data C1 is “0Bh”, and the correction data C1 is configured to generate a lower value than the correction data C0. . With this configuration, when the history data is “1”, the exposure current is set to be smaller than when the history data is “0”.
The exposure head drive control unit 332 further includes a pixel data latch (1 bit) for one line in the main scanning direction (not shown) and a data latch (4 bits) for correction data for one line in the main scanning direction. Yes. The current image data input to the exposure head drive control unit 332 is sequentially transferred and held in a pixel data latch (1 bit) for one line in the main scanning direction. Similarly, the correction data generated by the correction pixel data generation unit 322 is sequentially transferred and held in a correction data data latch (4 bits) for one line in the main scanning direction.
Data for each dot of the pixel data latch is supplied to data dot _n shown in FIG. Data for each dot of the correction data latch is supplied to the corrections b _{3 to} b ₀ shown in FIG. After image data and correction data for one line are transferred to the two data latches described above, an LED-DRV-ON signal as a strobe signal is supplied to all the LED drive circuits, and one line worth of data in the main scanning direction. Exposure is performed. In this way, exposure for one line whose exposure amount is corrected by the history data can be performed.

  Hereinafter, the correction of the drive current for each LED element (for each dot) of the LED head in the exposure apparatus 103 will be described in more detail.

FIG. 4 shows a circuit portion for driving one LED (nth dot) in the exposure head drive controller 332 that controls the drive of the LED head. Reference numeral 525 denotes an LED drive output terminal of the exposure head drive control unit 332, which is connected to the anode terminal of the LED 630. 504 to 508 are AND circuits, 509 is a buffer circuit, 510 to 514 are P channel MOS transistors, 515 to 519 are N channel MOS transistors, 520 to 524 are LED drive P channel MOS transistors, and are main drive transistors, 525 is an LED drive output terminal, 630 is an LED, one end is connected to the 525 and the other end is connected to the ground. The source terminals of the transistors 520 to 524 are connected to the current V _DD , and the drain terminal is connected to the LED driving output terminal 525. The current flowing through the transistors 520 to 524 (LED driving current) is denoted by I _X.

The input terminal of the AND circuit 504 has a “data dot _n ” signal, which is each pixel data signal of the image of the LED element to be driven, and LED-DRV-, which is a drive timing signal for the LED element to be driven. ON signal is input. Input terminals of the AND circuits 505 to 508 and Baffua circuit 509 is connected to the output terminal of the AND circuit 504, the signal input of the correction b ₃ ~ correction b ₀ of the correction data to the other input terminal of the AND circuit 505 to 508 Is done.
The source terminals of the MOS transistors 515 to 519 are connected to a control voltage Vcontrol for constant current control from the reference current generating transistor.

The driving ON / OFF of the transistors 520 to 523 is determined by the correction b ₃ to correction b ₀ signals for each dot, and when the LED is driven (that is, the data dot _n signal of the target dot _n is at a high level). And when the LED-DRV-ON signal becomes High), the transistors 520 to 523 are selectively driven corresponding to the signals of the correction b ₃ to the correction b ₀ together with the transistor 524, and the drains from the respective transistors The current obtained by adding the currents becomes the drive current _{IX of the} LED element.

Of the final stage transistors 520 to 524 when the LED 630 is driven, the gate potential of the transistor selected by the correction b ₃ to correction b ₀ signals is equal to the control voltage Vcontrol from a reference current generator (not shown). Become. That is, a current mirror circuit is configured by the reference current generator and the transistors 520 to 524. Therefore, among the transistors 520 to 524, each drain current of the driven transistor is determined by the ratio of the control voltage Vcontrol to the gate width dimension of each transistor.

The transistor 524 is a main drive transistor that supplies the main drive current of the LED 630, while the transistors 520 to 523 are auxiliary drive transistors for correcting the drive current of the LED 630 to correct the light amount. The gate lengths of the transistors 520 to 523 are equal, and the gate width is a ratio of 8, 4, 2, 1 corresponding to each of the correction b ₃ to correction b ₀ signals. (Note that the gate length of the reference current generating transistor is set equal to that of the transistors 520 to 524.) The correction transistors 520 to 523 for driving the LEDs have the gate width dimensions weighted as described above. The drive current value determined from this is supplied to the corresponding LED 630.

The operation for correcting each dot of the LED 630 will be described below. The drive current when each of the P-channel MOS transistors 520, 521, 522, 523, 524 is driven is a correction b ₃ current, a correction b ₂ current, a correction b ₁ current, a correction b ₀ current, and a main drive current, respectively. The current values are 8ΔI, 4ΔI, 2ΔI, ΔI, and I ₀ . The main drive current for driving the LED 630 is I ₀ , and this current is the lower limit value of the current when the drive current of the LED 630 is increased or decreased to correct the amount of emitted light. On the other hand, the correction unit when the current is increased or decreased is ΔI (this current is a drive current by the transistor 523 in FIG. 4), and is shown as a corrected b ₀ current. Similarly, the correction b ₁ current is 2ΔI, the correction b ₂ current is 4ΔI, and the correction b ₃ current is 8ΔI. The sum of the drive current values is the drive current of the LED 630 of one element.

Since each correction current takes a current value weighted by a ratio of 1, 2, 4, 8, the combined current (correction current) which is an arbitrary combination thereof is 0, ΔI, 2ΔI, 3ΔI,. , 14ΔI and 15ΔI can take 16 levels. For this reason, the LED drive current can also be corrected in the manner of I ₀ , I ₀ + ΔI, I ₀ + 2ΔI, I ₀ + 3ΔI,..., I ₀ + 15ΔI according to the command by the correction data.

  On the other hand, as shown in FIG. 4, the control voltage Vcontrol is the source potential of the prebuffers 515 to 519 of the LED driving transistors of the exposure head drive control unit 332. Is applied to the gate terminals of the final stage transistors 520 to 524, and the drive current values of all the LEDs 630 driven by the exposure head drive control unit 332 are determined. Since the Vcontrol voltage is changed in 16 steps according to the correction data, the drive current value of the LED 630 can also be corrected for each LED array chip.

Further, the Vcontrol voltage from the reference current generating unit can be changed in 16 steps according to the correction data, so that the drive current value of the LED 630 can be corrected in any of 16 steps for each LED array chip. As described above, the correction data for each exposure head drive control unit 332 can increase or decrease the driving current I _X of the LED630, in this embodiment about a drive current value by the correction method as described above 0.75μJ / Cm ² and approximately 0.9 μJ / cm ² .

  Next, an operation of forming an image with the image forming apparatus having the above configuration will be described. First, as an operation of the image signal processing unit 300, for example, when an image signal 401 is input from the outside to the image conversion processing unit 301, the image conversion processing unit 301 analyzes the input image signal, and each image forming apparatus Are converted into pixel data of an image that is an aggregate of individual pixel data corresponding to each dot of the area of one circumference of each photosensitive drum 101 of black, yellow, magenta, and cyan. Each pixel data of the image is sequentially transmitted to the exposure correction control unit 302 at a timing defined for each image forming apparatus.

For example, the following three pieces of information are combined in each pixel data.
(1) Data number in the main scanning direction: n (for example, n1)
(2) Data number in the sub-scanning direction: m (for example, m1)
(3) Data presence / absence; 1/0
The data numbers of (1) and (2) are the order information, and the presence / absence of the data of (3) is as follows.
When the presence / absence of data is “1”; the pixel position (portion on the photosensitive drum 101) corresponding to each pixel data is exposed to form an electrostatic latent image.
When the presence / absence of data is “0”; the pixel position (portion on the photosensitive drum 101) corresponding to each pixel data is not exposed and an electrostatic latent image is not formed.

  Each pixel data having the above three pieces of information is input to the one-round image data storage unit 312 having a FIFO structure in the exposure correction control unit 302. Hereinafter, when expressing three pieces of information included in each pixel data, for example, the data number n in the main scanning direction is “n1”, the data number m in the sub-scanning direction is “m1”, and the presence / absence of data (1 / 0) and “1” is described as (m1, n1, 1).

  The one-round image data storage unit 312 having a FIFO structure has a storage capacity for storing each pixel data of an image equivalent to or larger than the area of one photosensitive drum, and sequentially stores each pixel data of the input image. Further, in response to a request from the correction pixel data generation unit 322, each pixel data of the stored image can be sequentially extracted and sent as exposure history information.

  The correction pixel data generation unit 322 performs arithmetic processing (hereinafter referred to as correction) by correction means defined for each image forming apparatus based on the history data extracted from the image data storage unit 312 for one round as necessary at a necessary timing. The exposure amount required for the electrostatic latent image formed on the outer peripheral surface of the photosensitive drum 101 is calculated for each pixel data, and each calculated exposure amount is calculated for each input pixel data. Each pixel data (hereinafter referred to as corrected pixel data) to which data (hereinafter referred to as corrected exposure amount) is added is transmitted to the exposure head drive control unit 332.

In the correction processing in the correction pixel data generation unit 322, the correction pixel data generation unit 322 sequentially adds a correction exposure amount to each pixel data sequentially input from the image conversion processing unit 301, thereby providing two-stage correction data C0, C1 is generated. However, the corrected exposure amount (FIFO data) is 0 or 1, and when the corrected exposure amount is “0”, the normal exposure amount (in the present embodiment, about 0.90 μJ / cm ² ), When the correction exposure amount is “1”, the exposure amount is weak (in the present embodiment, about 0.75 μJ / cm ² ).

  When the input of each pixel data of the image from the image conversion processing unit 301 is started, the corrected pixel data generation unit 322 corresponds to each pixel corresponding to each dot of the area corresponding to one circumference of the photosensitive drum from each top pixel data. After opening the data, the corrected pixel data to which all the corrected exposure amount “0” is added is sequentially transmitted to the exposure head drive control unit 332. In this embodiment, when the correction pixel data is marked, for example, the data number n in the main scanning direction is “n1”, the data number m in the sub-scanning direction is “m1”, and data present / absent (1/0 ) And “1”, the correction exposure amount: normal / weak (0/1) is “0”, it is described as (m1, n1, 1, 0).

  The correction pixel data generation unit 322 transmits each pixel data corresponding to each dot having an area of one circumference of the photosensitive drum 101 to the exposure head drive control unit 332 as described above, and thereafter sequentially input. For each pixel data, exposure history information (each pixel data of the image) is sequentially extracted from the one-round image data storage unit 312 having a FIFO structure for each input pixel data, and for each input pixel data Perform correction processing. At this time, the first exposure history information (each pixel data of the image) taken out from the image data storage unit 312 for one round is the pixel data at the same position on the photosensitive drum 101 one round before, that is, each image of this image. It is extracted from each pixel data one round before the position corresponding to each pixel data at the beginning of the pixel data.

FIG. 5 shows the first pixel data X that is read first from the individual pixel data of the area for one rotation of the photosensitive drum stored in the image data storage unit 312 for one rotation as exposure history information, and is input from the outside. It is a figure which shows an example of the positional relationship with each first head pixel data Y of each pixel data. In the present embodiment, correction processing is performed using each pixel data X and each pixel data Y shown in FIG. The exposure amount of each pixel data Y can be expressed by the following equation.
Exposure amount of each pixel data Y (μJ / cm ² ) = (Presence / absence of data of each pixel data Y) ×
((Presence / absence of data of each pixel data X of the image == 1)? 0.75 (μJ / cm ² ): 0.90 (μJ / cm ² ))
In the latter half of the above equation, 0.75 is selected when the data X is “1” according to the data of each image data X, and when the data X is “0”, 0. 90 indicates that it is selected.

Here, the exposure amount of each pixel data Y; the exposure amount of each pixel data Y corresponding to the correction value output from the correction pixel data generation unit 322; presence / absence of each pixel data Y; input from the image conversion processing unit 301 The presence / absence (1/0) of the data (3) of each pixel data
Presence / absence of data of each pixel data X; (3) of the previous image history information corresponding to the number of pixels corresponding to each dot of the area of one round of the photosensitive drum taken out from the image data storage unit 312 for one round ) Data presence / absence (1/0)
In the latter half formula, if each pixel data X of the image is “1” with presence, 0.75 (μJ / cm ² ) is selected, and each pixel data Y of the image is “0” without. In other words, 0.90 (μJ / cm ² ) is selected.
As actual circuits, for example, the corrected pixel data generation unit 322 shown in FIG. 3 and the circuit shown in FIG. / No, the exposure amount is selected and switched.

  In other words, in the present embodiment, whether or not each pixel data forming an electrostatic latent image to be exposed at the same position on the outer peripheral surface of the photosensitive drum 101 is exposed one cycle before the photosensitive drum 101. Is stored in the image data storage unit 312 for one round as exposure history information for determination at the next and subsequent image formations, and the position of the photosensitive drum 101 is one cycle before the next and subsequent image formations. If the exposure is performed once, exposure is performed with a weak exposure amount, and conversely, if the position of the photosensitive drum 101 is not exposed one rotation before, exposure is performed with a normal exposure amount. In this embodiment, the potential difference causing the drum ghost on the drum surface is reduced in this way.

  FIGS. 6A to 6E show the image forming apparatus of the present embodiment. For example, the first pixel in FIG. 5 is exposed in the first round (first rotation) and the next pixel is not exposed. FIG. 6 is a conceptual diagram showing an example of the surface potential at the same position on the outer peripheral surface of the photosensitive drum 101 when both the first pixel and the next pixel are exposed, in a time-series manner. Is the exposure after the first charge, FIG. 6B is the exposure after the first turn, FIG. 6C is the exposure after the second charge, and FIG. 6D is the second turn. FIG. 6E is a diagram showing the exposure timing after the second round of exposure. FIG. 6 (f) is a diagram showing the second week after exposure in the case of conventional exposure amount uncorrected for comparison.

In FIG. 6A, the charged potential on the surface of the photosensitive drum after the first charge and before the exposure is before image formation, and is a uniform value “−600 V”. In FIG. 6B, exposure after the first rotation is performed, and the top pixel portion is exposed with a normal exposure amount (0.90 μJ / cm ² ) based on each pixel data of the image. This is a case where the next pixel portion is not exposed. After the first rotation exposure, the surface potential of the exposed portion decreases to “−55 V”, and the unexposed portion maintains the charged potential of “−600 V”. Thereafter, development and transfer are performed, and an image forming process of the second rotation is performed.

The charged potential on the surface of the photosensitive drum before the exposure after the second charging in FIG. 6C is the first rotation and the unexposed portion has a uniform value of “−600 V”, but the exposure was performed at the first rotation. The portion is “−580 V” lower than the unexposed portion. That is, on the surface of the photosensitive drum, a potential difference “−20 V” is generated between the exposed portion and the unexposed portion in the first rotation after charging in the second rotation. This is because the exposure history of the first rotation remained on the drum surface.

FIG. 6D shows the case where the exposure after the second rotation is performed at the illustrated exposure timing, and both the first pixel portion and the next pixel portion are exposed based on the pixel data of the image. However, in consideration of the potential difference “−20 V” after charging the photosensitive drum in the second rotation shown in FIG. 6C at that time, in the first pixel portion exposed in the first rotation, the second rotation is weak exposure. The exposure was performed with an amount (0.75 μJ / cm ² ), while the next pixel portion that was unexposed during the first rotation was exposed with a normal exposure amount (0.90 μJ / cm ² ). As a result, as shown in FIG. 6E, the surface potential of the photosensitive drum after exposure in both the first pixel and the next pixel in the second rotation can be made uniform. By comparing FIG. 6 (e) and FIG. 6 (f), in the conventional case, the portion exposed in the first rotation and the unexposed portion were both exposed in the second rotation and then exposed in the first rotation. A difference of “−20V” was made in the surface potential between the portion “−35V” and the unexposed portion “−55V”, but in this embodiment, the difference in the surface potential is eliminated and both portions are “−55V”. "

In other words, in the present embodiment, the previous exposure portion shown in FIG. 6C last time and the previous unexposed portion remain unaffected by the previous exposure and the previous exposure portion having a low charged potential is In this exposure, the lowered potential is suppressed to “−55 V” by exposing with a weak exposure (0.75 μJ / cm ² ). On the other hand, the previous unexposed portion having a high charging potential without being affected by the previous exposure is exposed at a normal exposure amount (0.90 μJ / cm ² ) in the current exposure, thereby causing a drop potential “−55 V”. Secure. Thus, after the current exposure of the present embodiment, the surface potential of the previous exposed portion and the previous unexposed portion can be set to the same “−55 V”.

FIG. 7 is a photoreceptor characteristic showing the relationship between the amount of light in the photoreceptor on the surface of the photoreceptor drum and the surface potential. The surface of the photosensitive drum 101 is charged by the charging process to become a charged potential. When an electrostatic latent image is formed, a potential difference between the exposed portion and the unexposed portion is generated due to the lowered potential of the exposed portion. An electrostatic latent image is formed. At this time, the amount of the surface potential of the photosensitive drum 101 is different depending on the exposure amount. For example, when the initial charge is −800 V and the exposure amount is a normal 0.9 μJ / cm ² , it decreases to about −55 V, whereas when the exposure amount is weak 0.75 μJ / cm ² , Since it is only lowered to about -75V, a difference of about 20V occurs in the surface potential after exposure in each case.

  In the present embodiment, by utilizing this characteristic, by making a difference in the exposure amount with respect to the difference in the charging potential on the surface of the photosensitive drum, the potential after exposure is made uniform. A drum ghost due to a potential difference at the previous image formation on the photoconductor on the drum surface is hardly generated. For this purpose, the exposure head drive control unit 332 sequentially inputs from the correction pixel data generation unit 322 to the exposure apparatus 103 when controlling the drive current of each LED of the exposure apparatus 103 at a timing specified for each image forming apparatus. The correction pixel data is transmitted. In the exposure apparatus 103, an electrostatic latent image is formed by exposing the outer peripheral surface of the photosensitive drum in units of pixels with a corrected exposure amount based on the corrected pixel data and the drive current input from the exposure head drive control unit 332. .

  As described above, the image forming apparatus according to the present exemplary embodiment exposes the surface of the drum-shaped photosensitive drum (image carrier) 101 on the surface of which the photosensitive member is held based on the input image signal 401. This is an electrophotographic image forming apparatus in which an electrostatic latent image is formed on the photosensitive drum 101 by exposure at 103, and the electrostatic latent image is visualized with a developer to form an image.

  In addition, the image forming apparatus of the present embodiment transfers pixel data of an image for one round of the surface of the photosensitive drum 101 used for exposure for forming an electrostatic latent image on the surface of the photosensitive drum 101. Image data storage unit 312 for storing one-round image data storage unit 312 as exposure history information together with the positional information on the order of each pixel data and the main scanning direction / sub-scanning direction on the photosensitive drum 101, In order to correct the exposure amount based on the pixel data at the position corresponding to each upper pixel, the exposure amount correction data of the exposure apparatus 103 is generated based on the exposure history information stored in the image data storage unit 312 for one round. A correction pixel data generation unit 322 that performs the exposure, and an exposure head drive control unit 332 that controls the drive current of the exposure apparatus 103 based on each pixel data whose exposure amount has been corrected.

  Further, the one-round image data storage unit 312 of the image forming apparatus according to the present embodiment stores all pixel data of one-round image with respect to the outer peripheral surface of the photosensitive drum 101 during rotation, and generates corrected pixel data. The unit 322 determines the exposure position of the surface of the photosensitive drum 101 based on the stored pixel data of the previous circumference for each pixel data of the image of the next circumference and for each exposure position of the surface of the photosensitive drum. Each pixel data for correcting the exposure amount for each is generated.

  In this way, in the present embodiment, the rotation history of the photosensitive drum is stored in the image data storage unit 312 for one rotation, and each image of the next rotation is stored. For each pixel of the pixel data, data that has been corrected for the amount of light based on each previous pixel data stored is added, and the exposure amount to the position corresponding to each pixel on the outer peripheral surface of the photosensitive drum is It is possible to control the surface potential of each final exposure location to be uniform. As a result, in this embodiment, it is possible to obtain good printing without providing a static eliminator and without generating a drum ghost due to a difference in drum surface potential after exposure.

(Second Embodiment)
In the first embodiment described above, the photosensitive drum at the time of the previous image formation stores each pixel data of the image corresponding to each pixel (dot) for one round, and the image at the next image formation is stored. For each pixel, the normal exposure and the weak exposure were switched depending on whether or not the previous corresponding pixel was exposed to eliminate the difference in exposure amount, but the storage unit for storing each pixel data of the image is The photosensitive drum is required for each pixel of one round, and a large capacity is required. However, recent image forming apparatuses have been refined to have a width less than the minimum width that can be identified by the human eye, such as 600 dpi and 1200 dpi. It is thought that it is not inconvenient in practice just to memorize it. In the following second embodiment, such a case will be described.

  FIG. 8 is a block diagram showing a schematic configuration of an image forming unit and an image signal processing unit in the image forming apparatus according to the second embodiment of the present invention. The image forming apparatus 500 shown in the figure is different from the image forming apparatus according to the first embodiment shown in FIG. 3 in the pixel data input from the image change processing unit 301 in the exposure correction control unit 302. The difference is that a one-round image data storage unit 312a having a capacity of 1/3 of the above and a stored image determination unit 342 to be sent to the exposure head drive control unit 332 are provided. Other than that, the configuration is the same as that of the image forming apparatus of the first embodiment.

  FIG. 9 is a block diagram illustrating a schematic configuration of the stored image determination unit 342 in the exposure correction control unit 302. Each pixel data is sequentially input from the image conversion processing unit 301 to the first flip-flop 601, and each pixel data is transferred to the second flip-flop 602 at the timing of each pixel data processing pixel clock input from the outside. The result is output to the majority circuit 603 and the exposure head drive controller 332. The second flip-flop 602 receives each pixel data sequentially from the first flip-flop 601 and outputs each pixel data to the majority circuit 603 at the timing of the pixel clock.

  Each pixel data is sequentially input from the image conversion processing unit 301 to the majority decision circuit 603, and each pixel data from the first flip-flop 601 and each pixel data from the second flip-flop 602 are input. . The majority circuit 603 receives each input pixel data, the output of the first flip-flop 601 and the output of the second flip-flop 601, and determines whether the output is set to 0 or 1 based on the majority. This is a circuit that determines and outputs. For example, the circuit outputs “0” if “0” in 3 inputs is 2 or more, and outputs “1” if “1” in 3 inputs is 2 or more.

  The 1/3 frequency dividing circuit 604 divides the pixel clock by 1/3 and outputs it to the image data storage unit 312a as a shift clock. The one-round image data storage unit 312a sequentially outputs each pixel data to the correction pixel data generation unit 322 by FIFO at the timing of the input shift clock. Since this timing is every three pixels, the FIFO memory capacity can be reduced to 1/3 compared to the first embodiment.

  Regarding the operation, operations other than the internal operation of the image correction control unit 302 are the same as those in the first embodiment. Hereinafter, an internal operation of the image correction control unit 302 will be described.

  In the stored image determination unit 342, for example, each continuous pixel data for three pixels from the beginning is first stored in the majority circuit 603, the second flip-flop 602, and the first flip-flop 601. Each pixel data of the image stored in the flip-flop 602 and the first flip-flop 601 is input to the majority circuit 603 simultaneously with each pixel data of the subsequent image. The majority circuit 603 stores the data “1” of each pixel data for one round when each pixel data of two or more pixels among the consecutive pixel data of three pixels is present (exposure ON). Sent to the unit 312a for storage. The stored image determination unit 342 sequentially controls the exposure head drive to store data “0” in the image data storage unit 312a for one round when there is less than two consecutive pixel data for three pixels (exposure ON). It will be sent to the part 332. Thereafter, the same processing is performed for each successive pixel data for the next three pixels.

  Note that the data number in the main scanning direction and the data number in the secondary inspection direction after the outer peripheral surface of the photosensitive drum 101 makes one round are the pixel arrangement for one outer peripheral surface of the photosensitive drum 101 as shown in FIG. It becomes.

  In the one-round image data storage unit 312a having the FIFO structure, since the output of the majority circuit 603 is sequentially sent from the stored image determination unit 342 every three pixels, each pixel data is sequentially stored.

  The correction pixel data generation unit 322 receives the pixel data of the previous round stored in the image data storage unit 312a for one round, and a code “1” that weakens the correction exposure amount for the pixel data. In addition, for each of the other pixel data, a code “0” having a normal correction exposure amount is added, and each pixel data is sent to the exposure head drive control unit 332.

  As described above, the exposure head drive control unit 332 that sequentially receives from the corrected pixel data generation unit 322 the pixel data in which the correction exposure amount code is added to the pixel data, and the output of the first flip-flop 601. In accordance with the corrected pixel data output of the corrected pixel data generation unit 322, exposure is performed in which the same correction is performed for each of the three pieces of pixel data from each pixel data output from the correction pixel data generation unit 322. A drive signal is transmitted to the exposure apparatus 103 at a timing specified for each image forming apparatus to control LED driving.

  In the present embodiment, the correction unit is set to each pixel data corresponding to three pixels, assuming that about 0.1 mm is the minimum width that can be identified by the human eye, and a general 600 dpi image forming apparatus. This is because three pixels correspond to about 0.1 mm. Therefore, in the 1200 dpi image forming apparatus, the number of pixels corresponding to about 0.1 mm is six, so the above description should be changed to each pixel data of six pixels.

  As described above, the one-round image data storage unit 312a of the image forming apparatus according to the present embodiment is the most recognizable pixel data of one-round image with respect to the surface of the image carrier 101 during rotation. One pixel representative for each of the plurality of pixels is selected and stored from the plurality of pixels within the small width dimension, and the correction pixel data generation unit 322 performs the pixel data of the next circumference image and the photosensitive drum. For each exposure position on the surface, each pixel data for correcting the exposure amount for each exposure position on the surface of the photosensitive drum 101 is generated based on the stored pixel data of the previous circumference.

  As described above, according to the present embodiment, for example, even when the print pattern is solid printing, the capacity of the image data storage unit 312a for one round is only a fraction of that of the first embodiment. For example, an image forming apparatus of 600 dpi only needs about one third.

  As described above, in the present embodiment, each pixel data stored in the image data storage unit 312a for one round is selected and reduced to one within the minimum width that can be identified by the human eye, thereby reducing the first embodiment. In addition to the effect that good printing can be obtained without providing a static eliminator of the form and without generating a drum ghost due to a difference in drum surface potential after exposure, the image data storage unit 312a for one round The storage capacity can be reduced.

In each of the above-described embodiments, an example in which the process unit is connected to four tandem color image forming apparatuses has been described. It is applicable to.
In each of the above-described embodiments, the photosensitive drum exposure history (each pixel data of the image) stored in the image data storage units 312 and 312a for one rotation is set to one rotation of the photosensitive drum. You may make it correct | amend based on the exposure log | history before the 2nd circumference.

  Further, it may be determined with reference to a history including two or more rounds.

  In each of the above-described embodiments, the exposure amount has been described by a two-way selective method. However, the exposure amount may be selected from a table based on data examined in advance through experiments or the like, or calculated by calculation. The result obtained may be used.

  In each of the above-described embodiments, each pixel data of the image stored in the image data storage units 312 and 312a for one round is each pixel data unit, but in a certain area range including a plurality of each pixel data. Each pixel data of the image may be handled and stored as a unit.

  In each of the embodiments described above, an LED array is used for the exposure device 103, but other exposure devices such as a laser may be used.

It is a schematic sectional drawing which shows the structure of Embodiment 1 of the image forming apparatus by this invention. FIG. 2 is a partially enlarged view of the photosensitive drum disposed in, for example, a process unit and its peripheral portion in FIG. 1. FIG. 2 is a block diagram illustrating a schematic configuration of an image forming unit and an image signal processing unit in the image forming apparatus of FIG. 1. The circuit part for driving one LED (nth dot) in the exposure head drive control part which controls the drive of a LED head is shown. First pixel data X first read out from individual pixel data of the area of one revolution of the photosensitive drum stored in the image data storage unit for one revolution as exposure history information, and each pixel data inputted from the outside It is a figure which shows an example of a positional relationship with the first head each pixel data Y. FIGS. 5A to 5E are image forming apparatuses according to the present embodiment. For example, the first pixel in FIG. 5 is exposed in the first round (first rotation) and the next pixel is not exposed, and the first pixel in the second week. FIG. 4 is a conceptual diagram showing an example of surface potential at the same position on the outer peripheral surface of the photosensitive drum when both the pixel and the next pixel are exposed, stepwise in a time series. FIG. Before exposure after charging, (b) after exposure for the first round, (c) before exposure after charging for the second round, (d) for exposure timing for the second round, (e) for second round It is a figure which shows after the exposure of the 2nd week in the case of the conventional exposure amount uncorrected for the comparison, (f) after exposure. 3 is a photoreceptor characteristic showing a relationship between a light amount and a surface potential of the photoreceptor on the surface of the photoreceptor drum. It is a block diagram which shows schematic structure of the image formation part and image signal processing part in the image forming apparatus of the 2nd Embodiment of this invention. 3 is a block diagram showing a schematic configuration of a stored image determination unit 342 in the exposure correction control unit 302. FIG.

Explanation of symbols

101 photoconductor drum,
102 charging roller,
103 exposure apparatus,
104 developing device,
105 Cleaning blade,
200 Image forming unit,
201-204 process units,
205 recording medium,
206 paper cassette,
207 hopping roller,
208, 209 pinch rollers,
210 conveying roller,
211 registration rollers,
212 transfer roller,
213 fixing device;
214, 215 discharge rollers,
216, 217 pinch rollers (of the discharge section),
218 Recording medium stacker unit,
300 image signal processing unit,
301 image conversion processing unit,
302 exposure correction control unit,
312 and 312a one round image data storage unit,
322 correction pixel data generation unit,
332 exposure head drive controller,
342 stored image determination unit,
401 image signal,
500 image forming apparatus,
504 to 508 AND circuit,
509 buffer circuit,
510-514 P-channel MOS transistors,
515-519 N-channel MOS transistor,
P-channel MOS transistor (main drive transistor) for driving 520 to 524 LED,
525 LED drive output terminal (of exposure head drive controller 332),
601 a first flip-flop;
602 a second flip-flop;
603 majority circuit 603,
604 1/3 frequency divider 604,
630 LED.

Claims (8)

An electrophotographic image forming apparatus,
A photoconductive drum that is charged with a voltage and rotates while rotating;
An exposure unit that forms an electrostatic latent image on the photosensitive drum;
A developing unit for developing the electrostatic latent image on the photosensitive drum with a developer;
An exposure history storage unit for storing exposure history information of the exposure unit exposing the photosensitive drum;
An image forming apparatus comprising: an exposure control unit that controls an exposure amount of the exposure unit based on exposure history information in the exposure history storage unit. The exposure history storage unit
The pixel data of the image for one round of the image carrier surface used for exposure for forming an electrostatic latent image on the surface of the image carrier, the order information of each pixel data to be transferred, and the image carrier The image forming apparatus according to claim 1, wherein the image forming apparatus is a one-round image data storage unit that stores the exposure history information together with the positional relationship information in the main scanning direction / sub-scanning direction. In the exposure control unit,
Based on the exposure history information stored in the image data storage unit for one round, in order to correct the exposure amount by the pixel data to the position corresponding to each pixel on the surface of the image carrier, the exposure A corrected pixel data generation unit that generates pixel data in which the exposure amount of the apparatus is corrected;
3. The image forming apparatus according to claim 1, further comprising an exposure head drive control unit that controls a drive current of the exposure apparatus based on pixel data in which the exposure amount is corrected. The following three pieces of information are combined in each pixel data stored as exposure history information in the one-round image data storage unit: (1) data number in the main scanning direction (2) data number in the sub-scanning direction ( 3) The image forming apparatus according to claim 2, wherein data is present / not present. The one-round image data storage unit stores pixel data of one-round image with respect to the surface of the image carrier during rotation,
The correction pixel data generation unit is configured to store the image carrier on the basis of each pixel data of the previous circumference stored for each pixel data of the next circumference image and for each exposure position on the surface of the photosensitive drum. Each image data which correct | amends the exposure amount for every exposure position of the surface is produced | generated. The image forming apparatus of any one of Claims 2-4 characterized by the above-mentioned. The image forming apparatus according to any one of claims 2 to 5, wherein the one-round image data storage unit stores all pixel data of an image for one round of the image carrier surface. The one-round image data storage unit selects and stores one pixel representing pixel data of the image for one round of the image carrier surface for each of a plurality of pixels. The image forming apparatus according to claim 1. The image forming apparatus according to claim 7, wherein the one-round image data storage unit selects and stores one pixel from a plurality of pixels within a minimum width dimension that can be identified by human eyes.

Images