JP2007249086A - Image forming apparatus, control method, program and recording medium for image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To obtain an image forming apparatus designed so as to minimize an image density variation even in consecutive printing. <P>SOLUTION: The image forming apparatus includes: a photoreceptor drum; a charging unit for charging the surface of the photoreceptor drum; an LSU for forming an electrostatic latent image on the photoreceptor drum; and a developing unit for visualizing the electrostatic latent image with toner by using a development bias potential DVB. The image forming apparatus further includes a bias potential correcting section 20 for correcting a development bias potential DVB. The bias potential correcting section 20 increases an amount of correction of development bias potential DVB according to the drum running distance measured from the start of image formation. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an electrophotographic image forming apparatus.

  In an image forming apparatus such as a printer or a multifunction peripheral, it is desired that the image density is always the same regardless of the use situation.

  Therefore, Patent Document 1 discloses a technique for preventing a decrease in image density by compensating for fatigue of the photoreceptor.

  Patent Document 2 discloses a technique for correcting the developing bias potential in accordance with the amount of decrease in the film thickness of the photosensitive layer of the photoreceptor.

Further, Patent Document 3 discloses a technique for controlling the potential difference between the developing bias potential and the electrostatic latent image portion potential depending on whether or not the toner charge amount is within a predetermined allowable range.
JP 63-191161 (published August 8, 1988) Japanese Patent Laid-Open No. 5-257354 (released on October 8, 1993) JP-A-6-67501 (published March 11, 1994)

  In recent years, as the demand for image quality has increased, it has been found that there is a phenomenon in which the image density changes according to the number of printed sheets in continuous printing continuously performed on a plurality of sheets.

  As described in the best mode for carrying out the invention, the present inventors have confirmed that the change in image density in continuous printing is not caused by the fatigue of the photoreceptor or the charge amount of the toner. . For this reason, there is a problem in that changes in image density in continuous printing cannot be suppressed by the techniques of Patent Documents 1 to 3 described above.

  The present invention has been made in view of the above problems, and an object of the present invention is to realize an image forming apparatus in which fluctuations in image density are small even in continuous printing.

  In order to solve the above problems, an image forming apparatus according to the present invention comprises an electrostatic latent image carrier that is rotationally driven, a charging unit that charges the surface of the electrostatic latent image carrier, and the electrostatic latent image. It has a latent image forming part for forming an electrostatic latent image on the carrier, a developing unit and a developing bias applying part for applying a developing bias to the developing unit, and the electrostatic latent image is visualized by a developer. The image forming apparatus includes a developing unit that includes a correcting unit that corrects the developing bias potential, and the correcting unit performs the developing in accordance with a feature amount indicating a continuous driving time of the electrostatic latent image carrier. The correction amount of the bias potential is increased.

  In order to solve the above problems, the control method according to the present invention includes a rotating electrostatic latent image carrier, a charging unit that charges the surface of the electrostatic latent image carrier, and the electrostatic A latent image forming unit that forms an electrostatic latent image on the latent image bearing member; a developing unit; and a developing bias applying unit that applies a developing bias to the developing unit. The electrostatic latent image is made visible by a developer. An image forming apparatus including a developing unit that forms an image, wherein when the developing bias potential is corrected, the developing bias is determined according to a feature amount indicating a continuous driving time of the electrostatic latent image carrier. The correction amount of the potential is increased.

  In continuous printing continuously performed on a plurality of sheets, the present inventors have made the potential (bright potential) on the electrostatic latent image as the number increases for a predetermined number of sheets after the start of image formation. It was confirmed that the absolute value of (VL) tends to increase.

  The potential difference between the bright potential VL and the developing bias potential is related to the image density. When the potential difference is reduced, the image density is lowered.

  According to the above configuration, the correction amount of the developing bias potential is increased according to the feature amount indicating the continuous drive time of the electrostatic latent image carrier after the start of image formation. Here, the feature amount is the continuous driving time itself, the surface moving distance of the electrostatic latent image carrier, the number of sheets, and the like.

  Therefore, it is possible to correct the developing bias potential so that the difference from the bright potential VL that varies according to the continuous driving time of the electrostatic latent image carrier becomes constant.

  Thereby, an image forming apparatus with little fluctuation in image density can be realized even in continuous printing.

  Further, the image forming apparatus according to the present invention includes the feature amount obtained in advance so that a difference between the potential on the electrostatic latent image and the developing bias potential that varies according to the continuous driving time is constant. A storage unit for storing a table indicating a correspondence relationship with the correction amount of the development bias potential; and the correction unit specifies a correction amount corresponding to the feature amount from the table, and the development according to the specified correction amount. Correct the bias potential.

  According to the above configuration, the storage unit of the image forming apparatus is obtained in advance so that the difference between the potential on the electrostatic latent image that fluctuates according to the continuous driving time and the developing bias potential is constant. A table showing the correspondence between the feature amount and the correction amount of the developing bias potential is stored in advance. Then, the correction unit specifies a correction amount corresponding to the feature amount from the table, and corrects the developing bias potential according to the specified correction amount.

  As a result, the difference between the potential on the electrostatic latent image and the developing bias potential is constant, and an image forming apparatus with small fluctuations in image density can be realized even in continuous printing.

  Furthermore, in the image forming apparatus of the present invention, the correction unit sets the correction amount to a constant value when the feature amount is equal to or greater than a predetermined value.

  The present inventors have confirmed through experiments that the potential on the electrostatic latent image becomes a substantially constant value when the feature amount is equal to or greater than a predetermined value. Therefore, according to the above configuration, when the feature amount is equal to or greater than a predetermined value, the development bias potential is corrected by a fixed amount of correction, so that the potential difference between the potential on the electrostatic latent image and the development bias potential is also increased. An image forming apparatus that is substantially constant and has a small fluctuation in image density even when printing a predetermined number of sheets or more can be realized.

  Further, in the image forming apparatus of the present invention, when the stop time from the completion of the previous image formation is shorter than a predetermined time, the correction unit increases the correction amount of the developing bias potential as the stop time is shorter. .

  Alternatively, in the image forming apparatus according to the present invention, when the stop time from the completion of the previous image formation is shorter than the predetermined time, the correction unit causes the developing bias potential to be in accordance with the feature amount as the stop time is shorter. The feature amount when correcting the is increased.

  When the stop time from the completion of the previous image formation is equal to or longer than the predetermined time, the inventors have found no change in the image density of the first sheet after the stop time has elapsed, but the stop time is the predetermined time. If it is less than the value, the shorter the stop time, the smaller the first image density after the stop time has been confirmed by experiments. It has been confirmed that the change in the image density is also due to the change in the bright potential VL.

  Therefore, according to the above configuration, when the stop time from the completion of the previous image formation is shorter than the predetermined time, the correction unit increases the correction amount of the developing bias potential as the stop time is shorter.

  Alternatively, when the stop time from the completion of the previous image formation is shorter than a predetermined time, the correction unit corrects the development bias potential according to the feature amount as the stop time is shorter. Increase. As a result, when the stop time is short, the correction amount with respect to the developing bias potential becomes large.

  Thereby, regardless of the stop time, the potential difference between the bright potential VL and the developing bias potential becomes substantially constant, and the image density is stabilized.

  Furthermore, in the image forming apparatus according to the present invention, the correcting unit may detect the potential on the electrostatic latent image at the start of image formation and when the continuous driving time indicated by the feature amount has elapsed according to the feature amount. The developing bias potential is corrected by the amount of fluctuation.

  Thereby, in continuous printing, the potential difference between the bright potential VL and the developing bias potential becomes substantially constant, and fluctuations in image density can be suppressed.

  Furthermore, in the image forming apparatus of the present invention, the correction unit controls the charging unit so as to correct the surface potential of the electrostatic latent image carrier by the same amount as the correction amount of the developing bias potential.

  When the absolute value of the developing bias potential is increased, the potential difference between the surface potential of the electrostatic latent image carrier and the developing bias potential is reduced, and white background fog may occur. However, according to the above configuration, the electrostatic latent image carrier is controlled to control the charging unit so as to correct the surface potential of the electrostatic latent image carrier by the same amount as the correction amount of the developing bias potential. The potential difference between the surface potential and the developing bias potential is also kept substantially constant. As a result, the occurrence of fogging on white background can be suppressed.

  The information processing apparatus may be realized by a computer. In this case, a program that causes the information processing apparatus to be realized by the computer by causing the computer to operate as the respective means, and a computer-readable program that records the program. Various recording media are also within the scope of the present invention.

  The image forming apparatus according to the present invention includes a correcting unit that corrects the developing bias potential, and the correcting unit corrects the developing bias potential in accordance with a feature amount indicating a continuous driving time after starting image formation. Increase the amount. As a result, there is an effect that fluctuation in image density is reduced even in continuous printing.

(About the phenomenon of density change due to continuous printing)
First, in the image forming apparatus, printing (hereinafter referred to as continuous printing) that is continuously performed on a plurality of sheets without stopping the driving of the photosensitive drum (electrostatic latent image carrier). A change in image density will be described.

  In recent years, a demand for image quality has increased, and a change in image density in continuous printing has become a problem. The inventors have been able to obtain the following knowledge about the phenomenon and cause of the change in image density in continuous printing.

  FIG. 2 is a graph showing changes in the luminance difference ΔL between the green patch and the orange patch for the first image when continuous printing is performed on 20 sheets of paper. FIG. 3 is a graph showing a change in color shift (color difference) ΔE of the green patch and the orange patch with respect to the first image when continuous printing is performed on 20 sheets of paper.

  As shown in FIGS. 2 and 3, when continuous printing is performed, ΔL and ΔE increase as the number of sheets increases in about 10 sheets immediately after the start of image formation. After printing about 10 sheets, ΔL and It can be seen that ΔE has a substantially constant value. Here, the increase in ΔL and ΔE means that the concentration is decreasing.

  From this, it has been found that when continuous printing is performed, the decrease in density decreases as the number of sheets increases immediately after the start of image formation, and the density becomes substantially constant after printing a certain number of sheets.

  Next, after performing continuous printing, the photosensitive drum was stopped for a predetermined stop time, and thereafter, a change in density when continuous printing was performed again was investigated.

  FIG. 4 is a graph showing a change in luminance value when 20 sheets of continuous printing are repeatedly executed with different leaving times in between. The upper row shows black and the lower row shows magenta. From FIG. 4, it can be seen that the luminance value of the first image after the stop of the photosensitive drum varies depending on the stop time of the immediately preceding photosensitive drum.

  Therefore, when the stop time of the photosensitive drum is varied from 3 seconds to 15 minutes and the change in the image density of black (K) when three sheets are continuously printed is investigated, the change as shown in FIG. showed that. In FIG. 5, the horizontal axis represents the number of sheets, and the vertical axis represents the density ID. On the horizontal axis, the photosensitive drum is stopped for a predetermined stop time after the third, sixth, ninth,. Further, in FIG. 5, a graph when the stop time is 3 seconds, 10 seconds, 1 minute, 5 minutes, and 15 minutes from the top is shown. In each graph, the line near ID = 1.84 indicates the average density value of the first image immediately after the photosensitive drum stop time of 15 minutes has elapsed.

  As shown in FIG. 5, when the photosensitive drum stop time is 3 seconds or 10 seconds, the density value of the first image after the stop time has elapsed is the first image after the stop time of 15 minutes has elapsed. Lower than the average density value (ID = 1.84). When the stop time is increased to 1 minute, the density value of the first image after the stop time has passed is close to ID = 1.84, but is still slightly lower. On the other hand, it was found that when the stop time was set to 5 minutes or more, the density value of the first image after the stop time elapsed was stabilized at a substantially constant value.

  That is, the density value of the first image is almost constant when the stop time of the photosensitive drum before the image formation is 5 minutes or more, but becomes slightly small when the stop time is 1 minute. It was found that when the time was 10 seconds or less, it became even smaller.

Next, the cause of such a change in image density was investigated.
First, as a cause of the change in the image density, it is conceivable that a change in the toner density T / D or the toner charge amount Q / m has an influence. Therefore, the toner density T / D and the toner charge amount Q / m of the first sheet and the 30th sheet in continuous printing of 30 sheets of paper were measured. FIG. 6 is a diagram showing measurement results of black (indicated as K in the figure) and magenta (indicated as M in the figure).

  As shown in the figure, the toner density T / D and the toner charge amount Q / m did not change between the first and last in continuous printing. Therefore, it is estimated that the toner density T / D and the toner charge amount Q / m are not the direct cause of the change in the image density.

  Next, as a cause of the change in the image density, it is conceivable that the effect of removing the charge on the surface of the photosensitive drum is changed during continuous printing because the set value of the charge eliminating lamp is not appropriate. Therefore, the set value of the static elimination lamp was changed between the standard value and the maximum value, and the density change in continuous printing was confirmed. FIG. 7 is a graph showing changes in luminance difference ΔL and color shift ΔE from the first sheet in continuous printing when the set value of the static elimination lamp is a standard value and a maximum value. FIG. 7 shows black (K) data. As shown in FIG. 7, the image density shows the same change regardless of the set value of the static elimination lamp. From this, it can be estimated that it is not due to the influence of the set value of the static elimination lamp.

  The cause of the change in image density is thought to be the deterioration of the photoconductor drum. When the change in image density during continuous printing was confirmed between a new photoconductor drum and a photoconductor drum used for a predetermined period, there was a difference. I couldn't. Therefore, it can be estimated that it is not due to deterioration of the photosensitive drum.

  Next, as a cause of the change in the image density, it is considered that the fluctuation of the potential (bright potential) VL of the exposure region on the surface of the photosensitive drum is affected when continuous printing is performed. This is because the larger the difference between the light potential VL and the developing bias potential DVB, the higher the image density.

  Therefore, the present inventors investigated changes in the light potential VL in continuous printing. FIG. 8 is a graph showing the investigation results. In FIG. 8, the horizontal axis indicates the number of sheets, and the vertical axis indicates the bright potential VL. As shown in FIG. 8, it was found that the absolute value of the light potential VL increases according to the number of sheets from the start of continuous printing to the time when 10 sheets are printed, and takes an almost constant value after the 10th sheet. . It was also found that the absolute value of the light potential VL at the stop time of 2 minutes was larger than the absolute value of the light potential VL at the stop time of 15 minutes.

  Here, the absolute value of the developing bias potential DVB is set to be larger than the absolute value of the bright potential VL. Therefore, when the development bias potential DVB is fixed, as the absolute value of the light potential VL increases, the difference between the light potential VL and the development bias decreases, and the image density decreases. The change in the bright potential VL shown in FIG. 8 correlates with the change in image density due to the number of sheets during continuous printing shown in FIG. 3 and the change in image density due to the photosensitive drum stop time shown in FIG. is doing.

  From this, it was found that fluctuations in image density during continuous printing are affected by fluctuations in the light potential VL.

  Therefore, in the present invention, in order to suppress a decrease in image density due to continuous printing, the difference between the development bias potential DVB and the light potential VL is based on the information obtained from the above experimental results regardless of the number of continuous printing. The development bias potential DVB is corrected so as to be constant. Specifically, as shown in FIG. 9, the absolute value of the developing bias potential DVB is also increased in accordance with the increase in the absolute value of the bright potential VL. That is, the developing bias potential is corrected by the variation amount α of the bright potential VL.

Further, when the absolute value of the developing bias potential DVB is increased, the difference between the dark potential VD and the developing bias potential DVB is small if the potential (dark potential) VD of the non-exposed area on the surface of the photosensitive drum is constant. As a result, a white background fog may occur. Therefore, in order to prevent the occurrence of the white background fogging, as shown in FIG. 9, the dark potential VD is also corrected by the same amount as the correction amount α for the developing bias potential DVB. That is, the difference between the developing bias potential DVB and the dark potential VD is always set to the same value.
Hereinafter, an embodiment of an image forming apparatus invented based on the above knowledge will be described.

(Schematic configuration of image forming apparatus)
First, the outline of the image forming apparatus 1 of the present embodiment will be described with reference to FIG. FIG. 10 is a schematic diagram showing the internal structure of the image forming apparatus 1.

  The image forming apparatus 1 shown in FIG. 2 is a color image on a sheet (recording material) P based on image data transmitted from each terminal device connected via a network or image data read by a scanner. Or a printer that selectively forms a monochrome image.

  The image forming apparatus 1 is a dry electrophotographic and quadruple tandem color printer, and includes a visible image transfer unit 50, a paper transport unit 30, a fixing device 40, and a supply tray 80. Further, the image forming apparatus 1 includes a control unit 10 that controls operations of the visible image transfer unit 50, the sheet conveyance unit 30, the fixing device 40, and the supply tray 80.

  The visible image transfer unit 50 includes a yellow image transfer unit 50Y, a magenta image transfer unit 50M, a cyan image transfer unit 50C, and a black image transfer unit 50B. Specifically, the yellow image transfer unit 50Y, the magenta image transfer unit 50M, the cyan image transfer unit 50C, and the black image transfer unit 50B are arranged from the supply tray side 20 between the supply tray 80 and the fixing device 40. They are attached in order.

  These transfer units 50Y, 50M, 50C, and 50B have substantially the same configuration, and, based on the image data, respectively transfer yellow images, magenta images, cyan images, and black images to the paper P. Transcript.

  Each transfer unit 50Y, 50M, 50C, 50B includes a photosensitive drum 51, and further includes a charging unit (charging unit) 52, an LSU 53, a developing unit (developing unit) 54, a transfer roller 55, and a cleaning device 56. The photosensitive drum 51 is disposed around the photosensitive drum 51 along the rotation direction of the photosensitive drum 51 (F direction in FIG. 10). Although not shown, a static elimination lamp for removing charges on the surface of the photosensitive drum 51 is disposed after the cleaning device 56.

  The photosensitive drums 51 of the transfer units 50Y, 50M, 50C, and 50B are drum-shaped transfer rollers having a photosensitive material on their surfaces, and are driven to rotate in the direction of arrow F.

  The charging units 52 of the transfer units 50Y, 50M, 50C, and 50B are for uniformly (uniformly) charging the surface of the photosensitive drum 51, and include a grid bias power source 52b that generates a grid bias, and the grid bias potential. And a charger 52a for charging the surface of the photosensitive drum 51.

  The LSU (laser beam scanner unit) 53 of the transfer units 50Y, 50M, 50C, and 50B is input with pixel signals corresponding to the yellow component, magenta component, cyan component, and black component in the image data, respectively. . Each LSU 53 exposes the charged photosensitive drum 51 based on these image signals to generate an electrostatic latent image.

  The developing units 54 of the transfer units 50Y, 50M, 50C, and 50B have yellow, magenta, cyan, and black toners, respectively, and a developing bias power source that applies a developing bias potential DVB to the developing unit 54a and the developing unit 54a. (Development bias applying unit) 54b (see FIG. 1), and has a function of developing the electrostatic latent image generated on the photosensitive drum 51 to generate a toner image (visualized image).

  The transfer roller 55 of the transfer units 50Y, 50M, 50C, and 50B is applied with a bias voltage having a polarity opposite to that of the toner. By applying this bias voltage to the paper P, the toner image on the photosensitive drum 51 is transferred to the paper P. It is intended for transfer to. The cleaning devices 56 of the transfer units 50Y, 50M, 50C, and 50B are for removing toner remaining on the photosensitive drum 51 after image transfer onto the paper P. The transfer of the toner image onto the paper P as described above is repeated four times for four colors.

  The paper transport unit 30 includes a driving roller 31, an idling roller 32, and a transport belt 33, and transports the paper P so that toner images are formed on the paper P in order by the transfer units 50Y, 50M, 50C, and 50B. It is.

  The driving roller 31 and the idling roller 32 stretch the conveyance belt 33, and the conveyance belt 33 is rotated when the driving roller 31 is rotated at a predetermined peripheral speed.

  The conveyance belt 33 is a belt that is placed between the driving roller 31 and the idling roller 32 so as to come into contact with the photosensitive drums 51 of the transfer units 50Y, 50M, 50C, and 50B. Friction driven in the direction. The transport belt 33 electrostatically attracts the paper P sent from the supply tray 80, and transports the paper P sequentially to the transfer units 50Y, 50M, 50C, and 50B.

  Further, the paper P on which the toner image has been transferred by the transfer units 50Y, 50M, 50C, and 50B is peeled off from the transport belt 33 by the curvature of the drive roller 31 and transported to the fixing device 40 (the one-dot chain line in FIG. Shows the transport path). Note that the toner image after being transferred onto the paper P by the transfer units 50Y, 50M, 50C, and 50B is in an unfixed state on the paper P.

  The fixing device 40 heat-compresses the unfixed toner image transferred onto the paper P to the paper P. Specifically, the fixing device 40 includes a fixing roller 60 and a pressure roller 70. Then, the sheet P conveyed from the visible image transfer unit 50 is sent to a fixing nip N formed between the fixing roller 60 and the pressure roller 70. Further, the fixing roller 60 and the pressure roller 70 convey the paper P while sandwiching it. At this time, the toner image (unfixed image) on the paper P is fixed on the paper P by the heat of the peripheral surface of the fixing roller 60.

  Then, the sheet P after the toner image is fixed by the fixing device 40 is discharged to a discharge tray (not shown) outside the image forming apparatus 1.

  In this way, the image forming process is executed. When continuous printing is performed, the visible image transfer unit 50, the paper transport unit 30, and the fixing device 40 are driven continuously. For example, the photosensitive drums 51 of the respective colors are driven without stopping for image forming processing for a plurality of sheets. When the formation of all the images is completed, the operations of the visible image transfer unit 50, the paper transport unit 30, and the fixing device 40 are stopped.

(Functional configuration of control unit)
Next, the control unit 10 that controls the operations of the visible image transfer unit 50, the paper transport unit 30, the fixing device 40, and the supply tray 80 will be described.

  FIG. 1 is a functional block diagram showing the configuration of the control unit 10. As shown in FIG. 1, the control unit 10 includes an environmental sensor 11, a processing condition setting unit 12, a reference DVB determination unit 13, a correction table storage unit (storage unit) 14, a correction table determination unit (correction means) 15, a timer. 16, a start step table storage unit 17, a start step determination unit (correction unit) 18, a drum travel distance measurement unit 19, and a bias potential correction unit (correction unit) 20.

  The environmental sensor 11 measures temperature and humidity. The reference DVB determination unit 13 determines the development bias potential DVB for each color according to the temperature and humidity measurement results by the environment sensor 11.

  Specifically, the temperature and humidity obtained so that the image density becomes constant from the change in image density due to temperature and humidity at a constant development bias, and a desired image density at the temperature and humidity are obtained. An environment table showing a correspondence relationship with the developing bias potential DVB is prepared. Then, the reference DVB determination unit 13 reads the development bias potential DVB corresponding to the measurement result by the environment sensor 11 from the environment table.

  The developing bias potential DVB determined by the reference DVB determining unit 13 is corrected by the bias potential correcting unit 20.

  The processing condition setting unit 12 sets various processing conditions in the image forming process. The processing conditions include a process speed that is the surface moving speed of the photosensitive drum 51 (that is, corresponding to a printing speed), a color / monochrome mode, and the like.

  Specifically, the processing condition setting unit 12 sets whether to perform color printing (color mode) or monochrome printing (monochrome mode) based on a user operation or image data. Controls driving of the transfer units 50Y, 50M, 50C, and 50B.

  Further, the processing condition setting unit 12 selects from three predetermined process speeds (high, medium, and low in order of increasing process speed) according to the color / monochrome mode and the type of the paper P. Select the optimal process speed. The processing condition setting unit 12 determines the type of the paper P (regular paper, thick paper) based on the paper feed cassette selected at the time of the print request. For example, the processing condition setting unit 12 sets the process speed to “high” when the type of the paper P is “normal paper” and the monochrome mode, sets the type of the paper P to “normal paper”, and sets the color In the mode, the process speed is set to “medium”, and when the type of the paper P is “thick paper”, the process speed is set to “low”. Then, the processing condition setting unit 12 controls the operation speeds of the visible image transfer unit 50, the paper transport unit 30, the fixing device 40, and the supply tray 80 so that the set process speed is obtained.

  The correction table storage unit 14 drives the photosensitive drum 51 that has been stopped, and the feature amount (here, the travel distance of the photosensitive drum 51 (hereinafter referred to as drum travel) indicates the continuous drive time from the start of image formation. A plurality of correction tables in which the distance)) and the correction amount for the development bias potential DVB determined by the reference DVB determination unit 13 are associated with each other are stored. Further, the correction table storage unit 14 stores a correction table identification number for identifying the correction table in association with each of the plurality of correction tables.

  FIG. 11 shows an example of the correction table. As shown in FIG. 11, in the correction table, the range of the drum travel distance is associated with the correction amount of the development bias potential DVB in the range, and each range of the drum travel distance is in turn. A step number is assigned. In FIG. 11, there are “No. 1”, “No. 2”, and “No. 3” as correction table identification numbers.

  In the correction table, as shown in FIG. 8, the difference between the light potential VL and the development bias potential DVB is constant based on the experimental result of the change in the light potential VL depending on the number (proportional to the drum travel distance). So that it is obtained in advance. In other words, in the predetermined period after the start of image formation, the correction amount for the development bias potential DVB is set to increase as the drum travel distance increases. After the predetermined period has elapsed after the start of image formation, the fixed amount is constant. The correction table is obtained so that the correction amount becomes the same. For example, in the correction table with the correction table identification number “No. 2” shown in FIG. 11, the correction amount for the developing bias potential DVB determined by the reference DVB determination unit 13 is in accordance with the drum travel distance until the drum travel distance is 1075 mm. The correction amount increases stepwise from 0V to 50V, and is constant at a correction amount of 50V when the drum travel distance is 1075 mm or more.

  In addition, a plurality of types of correction tables are prepared by changing the light potential VL depending on the drum travel distance depending on the conditions of the color (CMYK), the process speed, and the development bias potential DVB determined by the reference DVB determination unit 13. This is because they are different. That is, each correction table has a light potential based on the number of sheets (proportional to the travel distance of the photosensitive drum 51) under each condition of color (CMYK), process speed, and development bias potential DVB determined by the reference DVB determination unit 13. Based on the experimental result of the change in VL, it is obtained in advance so that the difference between the light potential VL and the developing bias potential DVB becomes constant.

  The correction table determination unit 15 selects an optimum correction table for the development bias potential DVB and the process speed according to the development bias potential DVB determined by the reference DVB determination unit 13 and the process speed set by the processing condition setting unit 12. Is.

  The correction table determination unit 15 determines the correction obtained from the experimental results of the change in the light potential VL depending on the drum travel distance (proportional to the number of sheets) under the conditions of the development bias potential DVB and the process speed, and the development bias potential DVB and the process speed. A selection table in which the correction table identification number of the table is associated with each color (CMYK) is stored in advance.

  FIG. 12 is a diagram illustrating an example of the selection table. In the figure, C represents cyan, M represents magenta, Y represents yellow, and K represents black. No. 1, 2, and 3 represent correction table identification numbers. In FIG. 12, if the development bias potential DVB determined by the reference DVB determination unit 13 is the same, the correction table identification numbers are all the same regardless of the process speed and color. The correction table identification number may be different depending on the process speed and color.

  Then, the correction table determination unit 15 refers to the selection table and selects a correction table identification number for each color.

  The timer 16 counts the stop time of the photosensitive drum 51 from the completion of the previous image formation.

  As shown in FIG. 13, the start step table storage unit 17 sets the stop time and process speed of the photosensitive drum 51 and the step number (hereinafter referred to as the start step number) at the start of image formation in the correction table. The start step table associated with each is stored.

  In FIG. 13, when the stop time is the same, all the start step numbers are the same regardless of the process speed and color. However, depending on the performance and the like of the photosensitive drum, the process speed and the color are different. It may be a starting step number.

  As described above, as shown in FIG. 5, when the stop time of the photosensitive drum 51 is as short as 1 minute or less, the image density of the first sheet after the image formation is resumed decreases according to the stop time. To do. On the other hand, if the stop time of the photosensitive drum 51 is 5 minutes or more, the image density of the first sheet after the image formation is resumed becomes a substantially constant value. In view of these experimental results, when the stop time is shorter than a predetermined time (here, 240 seconds), the start step number (that is, corresponding to the drum travel distance) increases as the stop time is shorter. Let Thereby, the shorter the stop time, the larger the correction amount with respect to the developing bias potential DVB.

  The start step determination unit 18 sets start step numbers corresponding to the stop time of the photosensitive drum 51 measured by the timer 16 from the completion of the previous image formation and the process speed set by the processing condition setting unit 12. It is specified from the start step table storage unit 17.

  The drum travel distance measuring unit 19 measures a travel distance from when the photosensitive drum 51 that has been stopped has started to be driven.

  The bias potential correction unit 20 corrects the development bias potential DVB determined by the reference DVB determination unit 13. The bias potential correction unit 20 reads a correction table corresponding to the correction table identification number specified by the correction table determination unit 15 from the correction table storage unit 14 for each color. Then, the bias potential correction unit 20 corresponds to the start step number determined by the start step determination unit 18 in the correction table read from the correction table storage unit 14 for the development bias potential DVB of each color determined by the reference DVB determination unit 13. Correct only the correction amount. Then, the bias potential correction unit 20 controls the development bias power supply 54b so that the corrected development bias potential DVB is obtained.

  Thereafter, in accordance with the drum travel distance measured by the drum travel distance measurement unit 19, the step numbers in the correction table are sequentially increased to increase the correction amount.

  Further, as shown in FIG. 9, the bias potential correction unit 20 corrects the dark potential VD by the same amount as the correction amount for the development bias potential DVB. Then, the bias potential correction unit 20 controls the grid bias power supply 52b so that the corrected dark potential VD is obtained. As described above, this is to suppress the occurrence of white background fog by making the difference between the developing bias potential DVB and the dark potential VD constant.

(Flow of correction processing of development bias potential DVB)
Next, the flow of correction processing of the developing bias potential DVB will be described with reference to the flowchart shown in FIG.

  First, the processing condition setting unit 12 sets a process speed and a color / monochrome mode according to user input and image data (S1).

  Next, the environmental sensor 11 measures temperature and humidity. Then, the reference DVB determination unit 13 sets the development bias potential DVB corresponding to the measurement result by the environment sensor 11 for each color (S2).

  Next, the correction table determination unit 15 includes the development bias potential DVB set by the reference DVB determination unit 13 and the correction table identification number corresponding to the process speed and color / monochrome mode set by the processing condition setting unit 12. Each color is specified from the selection table (S3).

  Thereafter, the start step determination unit 18 specifies, for each color, the start step number measured by the timer 16 and corresponding to the stop time of the photosensitive drum 51 from the completion time of the previous image formation for each color. (S4).

  Then, the bias potential correction unit 20 reads the correction table corresponding to the correction table identification number specified in S3 from the correction table storage unit 14 for each color. Further, the bias potential correction unit 20 corrects the development bias potential DVB of each color set in S2 corresponding to the start step number specified in S4 in the correction table read from the correction table storage unit 14. Correction is performed by the amount (S5). Then, the bias potential correction unit 20 controls the development bias power supply 54b so that the corrected development bias potential DVB is obtained.

  For example, when the development bias potential DVB set in S2 is −300V and the correction amount corresponding to the start step number is 60V, the bias potential correction unit 20 sets the development bias potential DVB to −360V.

  At this time, the bias potential correction unit 20 controls the grid bias power supply 52b so that the dark potential VD is corrected by the same amount as the correction amount for the development bias potential DVB.

  Thereafter, the bias potential correction unit 20 sequentially increases the step number of the correction table according to the drum travel distance measured by the travel distance measurement unit during continuous printing, and develops according to the correction amount corresponding to the step number. The bias potential DVB is corrected (S6).

  Here, as described above, the correction table is set so that the difference between the light potential VL and the developing bias potential DVB is constant regardless of the travel distance of the photosensitive drum 51 based on the results of experiments performed in advance. It is what was sought. Therefore, by correcting the developing bias potential DVB using the correction table, the difference between the bright potential VL and the developing bias potential DVB becomes constant, and the change in image density during continuous printing can be suppressed. FIG. 15 is a graph showing changes in image density in continuous printing when the development bias potential DVB is not corrected and when the development bias potential DVB is corrected according to the correction table. As shown in FIG. 15, it was confirmed that the image density was stable in continuous printing by correcting the development bias potential DVB according to the correction table.

  Further, as described above, the start step table is based on the result of an experiment performed in advance, regardless of the stop time of the photosensitive drum 51, the light potential VL and the developing bias potential in the first sheet after the image formation is resumed. This is obtained so that the difference from DVB is constant. Therefore, by specifying the start step number using the start step table, the difference between the light potential VL and the development bias potential DVB becomes constant regardless of the stop time of the photosensitive drum 51, and 1 after the image formation is resumed. The image density in the first image can be kept almost constant.

  As described above, the image forming apparatus 1 includes a rotationally driven photosensitive drum (electrostatic latent image carrier) 51, a charging unit (charging unit) 52 that charges the surface of the photosensitive drum 51, and the photosensitive drum. 51 includes an LSU (latent image forming unit) 53 that forms an electrostatic latent image, a developing unit 54a, and a developing bias power source (developing bias applying unit) 54b that applies a developing bias to the developing unit 54a. And a developing unit (developing unit) 54 that visualizes the electrostatic latent image with a developer. The image forming apparatus 1 includes a bias potential correction unit (correction unit) 20 that corrects the development bias potential DVB. The bias potential correction unit 20 performs a drum travel distance (photosensitive drum) after image formation is started. The correction amount of the development bias potential DVB is increased in accordance with the characteristic amount indicating the continuous driving time 51.

  As described above, in continuous printing, the absolute value of the potential (bright potential VL) on the electrostatic latent image tends to increase as the number of sheets increases after the start of image formation. The potential difference between the light potential VL and the developing bias potential DVB is related to the image density. When the potential difference is reduced, the image density is lowered.

  According to the above configuration, the correction amount of the developing bias potential DVB is increased in accordance with the drum travel distance from the start of image formation. Therefore, the developing bias potential DVB can be corrected so that the potential difference with the bright potential VL that fluctuates according to the continuous drive time after the start of image formation becomes constant. Thereby, it is possible to realize the image forming apparatus 1 with little fluctuation in image density even in continuous printing.

  In the present embodiment, the correction amount is increased according to the drum travel distance. However, the correction amount may be increased according to the continuous driving time of the photosensitive drum 51 after the start of image formation or the number of printed sheets. In this case, the correction table storage unit 14 stores a correction table in which the continuous drive time or the number of printed sheets is associated with the correction amount. Then, instead of the drum travel distance measuring unit 19, a timer for counting the continuous driving time or a counter for counting the number of printed sheets may be provided.

  In the present embodiment, the image forming apparatus 1 performs drum running, which is obtained in advance so that the difference between the light potential VL, which fluctuates according to the continuous driving time of the photosensitive drum 51, and the developing bias potential DVB is constant. A correction table storage unit (storage unit) 14 is provided that stores a correction table indicating a correspondence relationship between the distance and the correction amount of the development bias potential DVB. Then, the bias potential correction unit 20 specifies a correction amount corresponding to the current drum travel distance from the correction table, and corrects the development bias potential DVB according to the specified correction amount.

  As a result, the difference between the potential on the electrostatic latent image and the development bias potential DVB becomes constant, and the image forming apparatus 1 with small fluctuations in image density can be realized even in continuous printing.

  The bias potential correction unit 20 sets the correction amount to a constant value when the drum travel distance is greater than or equal to a predetermined value.

  It has been confirmed by experiments that the light potential VL becomes a substantially constant value when the drum travel distance is equal to or greater than a predetermined value. Therefore, even when the drum travel distance is greater than or equal to a predetermined value, the potential difference between the potential on the electrostatic latent image and the developing bias potential DVB is substantially constant, and the image forming apparatus 1 with small fluctuations in image density can be realized. it can.

  In addition, when the stop time from the completion of the previous image formation is shorter than the predetermined time, the start step determination unit 18 and the bias potential correction unit 20 decrease the step number in the correction table (that is, the drum running time) as the stop time is shorter. (Corresponding to the distance).

  If the stop time is equal to or longer than the predetermined time, the first image density after the stop time has not changed, but if the stop time is less than the predetermined time, the shorter the stop time, the longer the stop time. It has been confirmed by experiments that the image density of the first sheet after the lapse becomes small. It has been confirmed that the change in the image density is also due to the change in the bright potential VL.

  According to the above configuration, when the stop time is short, the correction amount for the development bias potential DVB is large. As a result, the potential difference between the light potential VL and the developing bias potential DVB becomes substantially constant regardless of the stop time, and the image density is stabilized.

  The bias potential correction unit 20 develops the development bias potential DVB by an amount corresponding to the fluctuation amount of the light potential VL between the start of image formation and the continuous drive time indicated by the drum travel distance, depending on the drum travel distance. It can also be expressed as correcting.

  Further, the bias potential correction unit 20 controls the charging unit 52 to correct the dark potential (surface potential on the photosensitive drum 51) VD by the same amount as the correction amount of the development bias potential DVB.

  When the absolute value of the developing bias potential DVB is increased, the potential difference between the dark potential VD and the developing bias potential DVB is decreased, and white background fog may occur. However, according to the above configuration, since the charging unit 52 is controlled so as to correct the dark potential VD by the same amount as the correction amount of the developing bias potential DVB, the potential difference between the dark potential VD and the developing bias potential DVB is also almost equal. Kept constant. As a result, the occurrence of fogging on white background can be suppressed.

  The present invention is not limited to the above-described embodiments, and various modifications can be made within the scope of the claims. That is, embodiments obtained by combining technical means appropriately changed within the scope of the claims are also included in the technical scope of the present invention.

  Finally, the control unit 10 of the image forming apparatus 1 may be configured by hardware logic, or may be realized by software using a CPU as follows.

  That is, the image forming apparatus 1 includes a CPU (central processing unit) that executes instructions of a control program that realizes each function, a ROM (read only memory) that stores the program, and a RAM (random access memory) that expands the program. And a storage device (recording medium) such as a memory for storing the program and various data. An object of the present invention is a recording medium on which a program code (execution format program, intermediate code program, source program) of a control program of the image forming apparatus 1 which is software for realizing the above-described functions is recorded so as to be readable by a computer. This can also be achieved by supplying to the control unit 10 and reading and executing the program code recorded on the recording medium by the computer (or CPU or MPU).

  Examples of the recording medium include a tape system such as a magnetic tape and a cassette tape, a magnetic disk such as a floppy (registered trademark) disk / hard disk, and an optical disk such as a CD-ROM / MO / MD / DVD / CD-R. Card system such as IC card, IC card (including memory card) / optical card, or semiconductor memory system such as mask ROM / EPROM / EEPROM / flash ROM.

  The image forming apparatus 1 may be configured to be connectable to a communication network, and the program code may be supplied via the communication network. The communication network is not particularly limited. For example, the Internet, intranet, extranet, LAN, ISDN, VAN, CATV communication network, virtual private network, telephone line network, mobile communication network, satellite communication. A net or the like is available. Also, the transmission medium constituting the communication network is not particularly limited. For example, even in the case of wired such as IEEE 1394, USB, power line carrier, cable TV line, telephone line, ADSL line, etc., infrared rays such as IrDA and remote control, Bluetooth ( (Registered trademark), 802.11 wireless, HDR, mobile phone network, satellite line, terrestrial digital network, and the like can also be used. The present invention can also be realized in the form of a computer data signal embedded in a carrier wave in which the program code is embodied by electronic transmission.

  The image forming apparatus and the image forming apparatus control method according to the present invention can be applied to an electrophotographic image forming apparatus such as a printer, a copier, a facsimile machine, and an MFP (Multi Function Printer).

2 is a block diagram illustrating a configuration of a control unit included in the image forming apparatus. FIG. It is a graph which shows the change of a brightness | luminance difference with respect to the 1st sheet image when performing continuous printing. It is a graph which shows the change of color shift with respect to the 1st image when performing continuous printing. It is a graph which shows the change of a luminance value when changing the stop time of a photoconductive drum, an upper stage shows black (K) and a lower stage shows magenta (M). It is a graph which shows the change of the image density by the stop time of a photosensitive drum, and is a graph when the stop time is 3 seconds, 10 seconds, 1 minute, 5 minutes and 15 minutes from the top. It is a figure which shows the measurement result of toner density T / D and toner charge amount Q / m in the continuous printing of 30 sheets of paper. It is a graph which shows the change of the brightness | luminance difference (DELTA) L and color shift (DELTA) E with the 1st sheet | seat in continuous printing when the setting value of a static elimination lamp is a standard value and the maximum value. It is a graph which shows the change of the bright potential VL in continuous printing, an upper stage shows black (K) and a lower stage shows magenta (M). It is a figure which shows the outline | summary of correction | amendment of the developing bias potential of this invention. 1 is a diagram illustrating a configuration of an embodiment of an image forming apparatus according to the present invention. It is a figure which shows an example of a correction table. It is a figure which shows an example of the table for selection. It is a figure which shows an example of a start step table. It is a flowchart which shows the flow of a correction process of developing bias potential. It is a graph which shows the change of the image density in continuous printing with the case where correction | amendment of developing bias potential is performed, and the case where it does not perform.

Explanation of symbols

1 Image forming apparatus 10 Control unit (correction means, storage unit)
50 Visible Image Transfer Unit 51 Photosensitive Drum (Electrostatic Latent Image Carrier)
52 Charging Unit 52a Charger 52b Grid Bias Power Supply 53 LSU (Latent Image Forming Unit)
54 Development Unit (Development Unit)
54a Developer 54b Development bias power supply (development bias application unit)
11 Environmental Sensor 12 Processing Condition Setting Unit 13 Reference DVB Determination Unit 14 Correction Table Storage Unit (Storage Unit)
15 Correction table determination unit (correction means)
16 Timer 17 Start step storage unit 18 Start step determination unit (correction means)
19 Drum travel distance measurement unit 20 Bias potential correction unit (correction means)

Claims (10)

An electrostatic latent image carrier that is driven to rotate; a charging unit that charges the surface of the electrostatic latent image carrier; a latent image forming unit that forms an electrostatic latent image on the electrostatic latent image carrier; In an image forming apparatus comprising a developing unit and a developing bias applying unit that applies a developing bias to the developing unit, and a developing unit that visualizes the electrostatic latent image with a developer.
A correction means for correcting the development bias potential;
The image forming apparatus according to claim 1, wherein the correction unit increases the correction amount of the developing bias potential in accordance with a feature amount indicating a continuous drive time of the electrostatic latent image carrier. Correspondence between the characteristic amount and the correction amount of the developing bias potential obtained in advance so that the difference between the potential on the electrostatic latent image and the developing bias potential which varies according to the continuous driving time is constant. A storage unit for storing a table indicating the relationship;
The image forming apparatus according to claim 1, wherein the correction unit specifies a correction amount corresponding to the feature amount from the table, and corrects the developing bias potential according to the specified correction amount.   The image forming apparatus according to claim 1, wherein the correction unit sets the correction amount to a constant value when the feature amount is equal to or greater than a predetermined value.   2. The correction unit further increases the correction amount of the developing bias potential as the stop time is shorter when the stop time from the completion of the previous image formation is shorter than a predetermined time. The image forming apparatus described in 1.   When the stop time from the completion of the previous image formation is shorter than the predetermined time, the correction unit increases the feature amount when correcting the development bias potential according to the feature amount as the stop time is shorter. The image forming apparatus according to claim 4, wherein:   According to the feature amount, the correction unit is configured to develop the development bias potential by an amount of fluctuation of the potential on the electrostatic latent image between the start of image formation and the continuous drive time indicated by the feature amount. The image forming apparatus according to claim 1, wherein:   2. The charging unit according to claim 1, wherein the correction unit controls the charging unit so as to correct the surface potential of the electrostatic latent image carrier by the same amount as the correction amount of the developing bias potential. Image forming apparatus. An electrostatic latent image carrier that is driven to rotate; a charging unit that charges the surface of the electrostatic latent image carrier; a latent image forming unit that forms an electrostatic latent image on the electrostatic latent image carrier; A control method for an image forming apparatus, comprising: a developing unit; a developing bias applying unit that applies a developing bias to the developing unit; and a developing unit that visualizes the electrostatic latent image with a developer.
A control method characterized in that, when the developing bias potential is corrected, the developing bias potential correction amount is increased in accordance with a feature amount indicating a continuous driving time of the electrostatic latent image carrier.   A program for causing a computer to function as the correcting unit of the image forming apparatus according to claim 1.   A computer-readable recording medium on which the program according to claim 9 is recorded.

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