JP2011232545A - Image forming apparatus, and control method thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress excessive toner consumption with forcible discharge of toner, while stabilizing an image density, by replacing a developer for the portion of an amount corresponding to development efficiency.SOLUTION: Replacement means replaces the developer in a second container by replenishing the developer from a first container to the second container by a replenishment amount corresponding to a waste amount of the developer, while abandoning the developer from the second container to an external part of the second container. Image density detection means detects a density of an image formed on an image carrier body. Charge amount detection means detects or estimates a charge amount of toner in the second container. Replacement control means controls the replacement means so as to replace the developer for the portion of an amount corresponding to the development efficiency of the developer, which is defined based on the image density and the charge amount of the toner.

Description

  The present invention relates to an image forming apparatus and a control method thereof.

  In an electrophotographic image forming apparatus, toner transfer is promoted by charging the toner. Therefore, when the toner deteriorates, the image density becomes thin. According to Patent Document 1, an invention is disclosed in which a deteriorated toner is replaced with a new toner by forcibly discharging the toner based on the charge amount of the toner.

JP 2006-243114 A

  The invention described in Patent Document 1 has been found to be insufficient with respect to improving the image quality. In particular, when a large amount of images having a low image ratio are formed continuously, the toner charge amount increases and the development amount decreases, but at the same time, the development efficiency also decreases. In such a state where the development efficiency is lowered, the forced discharge amount of the toner is also reduced, so that the replacement of the toner becomes insufficient. Conventionally, there has been no method for appropriately detecting such a decrease in development efficiency. Therefore, it is conceivable to increase the forced discharge amount of the toner, but this is not desirable from the viewpoint of toner consumption.

  Accordingly, an object of the present invention is to execute toner replacement by an appropriate amount in accordance with a decrease in development efficiency while suppressing excessive toner consumption due to forced toner discharge.

  The present invention can be applied to an image forming apparatus including a first container, a second container, a charging unit, and a developing unit. The first container stores a developer containing toner and a carrier. The second container stores the developer supplied from the first container. The charging unit charges the developer stored in the second container. The developing unit develops the electrostatic latent image formed on the image carrier by attaching the developer charged by the charging unit. The image forming apparatus further includes a replacement unit, a charge amount detection unit, and a replacement control unit. The replacement means replenishes the developer from the first container to the second container with a replenishment amount corresponding to the developer disposal amount while discarding the developer from the second container to the outside of the second container. As a result, the developer in the second container is replaced. The image density detecting means detects the density of the image formed on the image carrier. The charge amount detection means detects or estimates the charge amount of the toner in the second container. The replacement control unit controls the replacement unit to replace the developer by an amount corresponding to the developer development efficiency defined by the image density and the toner charge amount.

  In the present invention, since the developer is replaced by an amount corresponding to the development efficiency, excessive toner consumption accompanying forced toner discharge can be suppressed. Further, by changing the deteriorated developer, it is possible to suppress fluctuations in image density.

1 is an explanatory diagram of a configuration of an image forming apparatus. It is a block diagram of signal processing in a reader image processing unit. It is explanatory drawing of the timing of each control signal in a reader image processing part. It is a block diagram of a control system of the image forming unit. It is explanatory drawing of the formation process of a patch image. It is explanatory drawing of the measurement process of patch density | concentration. It is explanatory drawing of the relationship between an image density and a photosensor output. FIG. 6 is a flowchart for calculating a toner charge amount. FIG. 6 is a relationship diagram for calculating a convergence value Q / M1 from an image ratio. FIG. 3 is a schematic diagram illustrating a potential of the photosensitive drum 1 for explaining development efficiency. FIG. 6 is a flowchart for toner replacement. FIG. 6 is a diagram illustrating a relationship for calculating a rotation speed of the developing sleeve 41 from an image ratio and an effect of the first embodiment. It is the figure which showed the relationship for calculating the discharge area from an image ratio, and the effect of 2nd Example. It is the figure which showed the relationship for calculating AC component of development bias from image ratio, and the effect of 3rd execution example.

<First embodiment>
Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. The present invention can be implemented in another embodiment in which part or all of the configuration of the embodiment is replaced with an alternative configuration as long as the forced discharge of toner is controlled according to the development efficiency. Accordingly, the present invention can be implemented without distinction between a tandem type and a one-drum type, or between an intermediate transfer type and a direct transfer type, as long as the image forming apparatus performs image formation using an electrophotographic system. In the present embodiment, only main parts related to toner image formation and transfer will be described. However, in the present invention, in addition to necessary equipment, equipment, and a housing structure, a printer, various printing machines, a copier, a FAX, a composite, It can be implemented in various applications such as a machine.

<Image forming apparatus>
As shown in FIG. 1, the image forming apparatus 100 is a tandem intermediate transfer type full-color printer in which image forming portions PY, PM, PC, and PK of yellow, magenta, cyan, and black are arranged along an intermediate transfer belt 6. is there. In the image forming unit PY, a yellow toner image is formed on the photosensitive drum 1Y and is primarily transferred to the intermediate transfer belt 6. In the image forming unit PM, a magenta toner image is formed on the photosensitive drum 1M, and is primarily transferred onto the yellow toner image on the intermediate transfer belt 6. In the image forming units PC and PK, a cyan toner image and a black toner image are formed on the photosensitive drums 1C and 1K, respectively, and are sequentially transferred to the intermediate transfer belt 6 in order to be primarily transferred. The four-color toner images primarily transferred to the intermediate transfer belt 6 are transported to the secondary transfer portion T2 and collectively transferred to the recording material P. The recording material P onto which the four-color toner images have been secondarily transferred is heated and pressed by the fixing device 11 to fix the toner images on the surface, and then discharged to the outside of the machine body. The intermediate transfer belt 6 is supported around a tension roller 61, a driving roller 62, and a counter roller 63, and is driven by the driving roller 62 to rotate in the direction of arrow R2 at a predetermined process speed. The recording material P drawn from the recording material cassette 65 is separated one by one by the separation roller 66 and sent to the registration roller 67. The registration roller 67 receives and waits for the recording material P in a stopped state, and sends the recording material P to the secondary transfer portion T2 in time with the toner image on the intermediate transfer belt 6. The secondary transfer roller 64 is in contact with the intermediate transfer belt 6 supported by the counter roller 63 to form a secondary transfer portion T2. By applying a positive DC voltage to the secondary transfer roller 64, the toner image charged to the negative polarity and carried on the intermediate transfer belt 6 is secondarily transferred to the recording material P.

  The image forming units PY, PM, PC, and PK are substantially the same except that the toner colors used in the developing devices 4Y, 4M, 4C, and 4K are different from yellow, magenta, cyan, and black. In the following, when no particular distinction is required, the subscripts Y, M, C, and K attached to the reference numerals to indicate that they are for one of the colors will be omitted, and a general description will be given.

  As shown in FIGS. 1 and 4, the image forming unit P includes a charging device 2, an exposure device 3, a developing device 4, a primary transfer roller 7, and a cleaning device 8 around the photosensitive drum 1. The photosensitive drum 1 is an example of an image carrier, and rotates in the direction of the arrow R1 at a predetermined process speed. The charging device 2 is configured by, for example, a scorotron charger. The charging device 2 irradiates the photosensitive drum 1 with charged particles accompanying corona discharge, and charges the surface of the photosensitive drum 1 to a uniform negative potential. The exposure device 3 scans the laser beam with a mirror and writes an electrostatic latent image corresponding to a desired image on the surface of the charged photosensitive drum 1. The potential sensor 5 detects the potential of the electrostatic latent image formed on the photosensitive drum 1 by the exposure device 3. The developing device 4 develops a toner image by attaching toner to the electrostatic latent image on the photosensitive drum 1. The primary transfer roller 7 presses the inner surface of the intermediate transfer belt 6 to form a primary transfer portion T <b> 1 between the photosensitive drum 1 and the intermediate transfer belt 6. By applying a positive DC voltage to the primary transfer roller 7, the negative toner image carried on the photosensitive drum 1 is primarily transferred to the intermediate transfer belt 6 passing through the primary transfer portion T1. The cleaning device 8 rubs the photosensitive drum 1 with a cleaning blade to collect the transfer residual toner remaining on the photosensitive drum 1 by escaping from the transfer to the intermediate transfer belt 6. The belt cleaning device 68 rubs the intermediate transfer belt 6 with a cleaning blade, and escapes the transfer to the recording material P, passes through the secondary transfer portion T2, and collects the transfer residual toner remaining on the intermediate transfer belt 6. .

<Image reading device>
As shown in FIG. 1, the image reading device (reader unit) A reads an image of the downward surface of the document G placed on the document table glass 102. The image of the original G is illuminated by the light source 103 and formed on the CCD sensor 105 via the optical system 104. The CCD sensor 105 generates red, green, and blue color component signals for each line sensor using a red (R), green (G), and blue (B) CCD line sensor group arranged in three rows. The reading optical system unit including the light source 103, the optical system 104, and the CCD sensor 105 moves in the direction of the arrow R3, thereby converting the image of the document G into an electric signal data string for each line. The image signal obtained by the CCD sensor 105 is subjected to image processing in the reader image processing unit 108 and then sent to a printer control unit (printer image processing unit) 109 for image processing.

  As shown in FIG. 2, the clock generation unit 211 generates a clock (CLK signal) in units of pixels. The address counter 212 counts the clock of the clock generation unit 211 and generates a main scanning address for each pixel of one line. The address counter 212 is cleared by the HSYNC signal, and starts counting the main scanning address for the next one line. The decoder 213 decodes the main scanning address from the address counter 212 and generates a CCD drive signal (shift pulse, reset pulse, etc.) for each line. Further, the decoder 213 generates a VE signal and a line synchronization signal HSYNC that represent an effective area in the one-line reading signal of the CCD sensor 105.

  As shown in FIG. 3, the VSYNC signal is a signal indicating an image effective section in the sub-scanning direction. The reader image processing unit 108 performs image reading (scanning) in the logic “1” section, and sequentially forms output signals of M, C, Y, and K. The VE signal is a signal indicating an image effective section in the main scanning direction. The reader image processing unit 108 takes the timing of the main scanning start position in the logical “1” section. The VE signal is mainly used for line count control of line delay. The CLK signal is a pixel synchronization signal and is used to transfer image data at the rising timing of “0” → “1”.

  The image signal output from the CCD sensor 105 is input to the analog signal processing unit 201 as shown in FIG. The signal input to the analog signal processing unit 201 is subjected to gain adjustment and offset adjustment here, and then converted into 8-bit digital image signals R1, G1, and B1 for each color signal by the A / D converter 202. The digital image signals R1, G1, and B1 are input to the shading correction unit 203 and subjected to shading correction for each color using the reading signal of the reference white plate. Each line sensor of the CCD sensor 105 is arranged with a predetermined distance from each other. Therefore, the line delay circuit 204 corrects a spatial shift in the sub-scanning direction in the digital image signals R2, G2, and B2. Specifically, the R and G signals are line-delayed in the sub-scanning direction in the sub-scanning direction with respect to the B signal, and are adjusted to the B signal. The input masking unit 205 converts the read color space determined by the spectral characteristics of the R, G, and B filters of the CCD sensor 105 into a standard color space of NTSC by performing a matrix operation. The light quantity / image density conversion unit (LOG conversion unit) 206 includes a lookup table (LUT) ROM. As a result, the luminance signals of R4, G4, and B4 are converted into M0, C0, and Y0 density signals of magenta (M), cyan (C), and yellow (Y) image signals. The line delay memory 207 is a black character determination unit (not shown) that outputs M0, C0, and Y0 image signals by the amount of line delay from the R4, G4, and B4 signals to the determination signals such as UCR, FILTER, and SEN. Delay. The masking and UCR circuit 208 extracts a black (K) signal from the inputted three primary color signals of M1, C1, and Y1, and further performs an operation for correcting the color turbidity of the recording color material in the printer unit B. The masking and UCR circuit 208 sequentially outputs M2, C2, Y2, and K2 signals at a predetermined bit width (8 bits) for each reading operation. The gamma correction circuit 209 performs image density correction in the reader unit A so as to match the ideal gradation characteristics of the printer unit B. The gamma correction circuit 209 performs density conversion using a gamma correction LUT (gradation correction table) composed of, for example, a 256-byte RAM. The spatial filter processing unit (output filter) 210 performs edge enhancement or smoothing processing.

<Exposure device>
As shown in FIG. 4, the image forming apparatus 100 includes a control unit 110 that comprehensively controls image forming operations. The control unit 110 includes a CPU 111, a RAM 112, and a ROM 113. As the exposure apparatus 3, for example, a laser scanner having a rotating mirror or a resonant mirror can be adopted. In the exposure apparatus 3, the laser light quantity control circuit 190 determines the exposure output so that a desired image density level can be obtained with respect to the laser output signal. In addition, binary laser light is output with a pulse width determined by the pulse width modulation circuit 191 in accordance with the drive signal generated via the gradation correction table (LUT) of the γ correction circuit 209. Based on the relationship between the laser output signal and the image density level obtained in advance, a laser output signal capable of forming a desired image density is stored in the γ correction circuit 209 as a gradation correction table (LUT). Accordingly, the laser output signal is determined. The M4, C4, Y4, and K4 frame sequential image signals processed by the spatial filter processing unit 210 are sent to the printer control unit 109. Then, the exposure apparatus 3 performs image recording having density gradation by binary area gradation using PWM (pulse width modulation). That is, the pulse width modulation circuit 191 of the printer control unit 109 forms and outputs a laser driving pulse having a width (time width) corresponding to the level for each input pixel image signal. A wider drive pulse for high density pixel signals, a narrower drive pulse for low density pixel image signals, and an intermediate width for intermediate density pixel image signals Drive pulses are formed. The binary laser driving pulse output from the pulse width modulation circuit 191 is supplied to the semiconductor laser of the exposure apparatus 3, and causes the semiconductor laser to emit light for a time corresponding to the pulse width. Therefore, the semiconductor laser is driven for a longer time for a high density pixel and is driven for a shorter time for a low density pixel. For this reason, the dot size (area) of the electrostatic latent image formed on the photosensitive drum 1 differs according to the density of the pixel. The exposure device 3 exposes a long range in the main scanning direction for high density pixels and exposes a short range in the main scanning direction for low density pixels. Therefore, as a matter of course, the toner consumption corresponding to the high density pixel is larger than that for the low density pixel.

<Developing device>
The developing device 4 can employ, for example, a two-component developing system that uses a two-component developer in which a magnetic carrier is mixed with a non-magnetic toner. The developing device 4 agitates the two-component developer to charge the magnetic carrier to positive polarity and the toner to negative polarity. That is, the developing device 4 attaches the charging means for charging the developer contained in the second container and the developer charged by the charging means to the electrostatic latent image formed on the image carrier. It functions as developing means for developing. In the developing device 4, a space in the developing container 45 is divided into a first chamber (developing chamber) and a second chamber (stirring chamber) by a partition wall 46 extending in a direction perpendicular to the paper surface on which FIG. 4 is drawn. The A nonmagnetic developing sleeve 41 is arranged in the first chamber, and a magnet as a magnetic field generating means is fixedly arranged inside the developing sleeve 41. A first screw 42 is disposed in the first chamber, and the first screw 42 stirs and conveys the developer in the first chamber. A second screw 43 is disposed in the second chamber, and the second screw 42 conveys the developer in a direction opposite to the conveyance direction of the first screw 42 while stirring the developer in the second chamber. To do. The second screw 43 agitates the toner supplied from the toner replenishing tank 33 by the rotation of the toner conveying screw 32 with the developer already in the developing device 4 and uniformizes the toner concentration of the developer. The toner replenishing tank 33 is a first container that stores a developer containing toner and a carrier. The developing container 45 is a second container that stores the developer supplied from the first container. The partition wall 46 is formed with a pair of developer passages that allow the first chamber and the second chamber to communicate with each other at the front and back ends of the sheet. Due to the conveying force of the first and second screws 42 and 43, the developer is circulated in the developer container 45 through the pair of developer passages while being stirred. The developer in the first chamber, in which the toner is consumed by the development and the toner density is lowered, moves to the second chamber through one developer passage. The developer whose toner density has been recovered by replenishing the toner in the second chamber moves to the first chamber through the other developer passage. The two-component developer in the first chamber is applied to the developing sleeve 41 by the first screw 42 and is carried on the developing sleeve 41 by the magnetic force of the magnet. The developer on the developing sleeve 41 is transported to the developing region facing the photosensitive drum 1 with the developing sleeve 41 rotated by the developing sleeve driving device 44 after the layer thickness is regulated by a layer thickness regulating member (blade). Is done. A developing bias voltage (vibration voltage) in which an AC voltage is superimposed on a negative DC voltage Vdc is applied to the developing sleeve 41 from a bias power supply 47. As a result, the negatively charged toner is transferred to the electrostatic latent image on the photosensitive drum 1 having a positive polarity relative to the developing sleeve 41, and the electrostatic latent image is reversely developed.

  In the developer replenishing device 30, a toner replenishing tank 33 containing replenishing toner is disposed above the developing device 4. A toner conveying screw 32 that is rotationally driven by a motor 31 is installed below the toner supply tank 33. The toner conveying screw 32 supplies replenishing toner into the developing device 4 through a toner conveying path in which the toner conveying screw 32 is disposed. The CPU 111 of the control unit 110 controls the rotation of the motor 31 via a motor drive circuit (not shown), whereby the toner supply by the toner conveying screw 32 is controlled. The RAM 112 connected to the CPU 111 stores control data supplied to the motor drive circuit. The toner replenishing tank 33, the motor 31, the toner conveying screw 32, and the like constitute the developer replenishing device 30. As described above, the developer replenishing device 30 discards the developer from the second container to the outside of the second container, and then refills the second container from the first container with a replenishment amount corresponding to the developer disposal amount. It functions as a part of replacement means for replacing the developer in the second container by replenishing the developer in the container.

A toner concentration sensor 15 is incorporated in the developing device 4 in order to detect the toner concentration (ratio of toner and carrier) of the two-component developer. The toner density sensor 15 is disposed in contact with the developer circulating in the developing device 4. The toner concentration sensor 15 has a drive coil, a reference coil, and a detection coil, and outputs a signal corresponding to the magnetic permeability of the developer. When a high frequency bias is applied to the drive coil, the output bias of the detection coil changes according to the toner concentration of the developer. The toner concentration of the developer is detected by comparing the output bias of the detection coil with the output bias of the reference coil that is not in contact with the developer. The control unit 110 converts the detection result by the toner density sensor 15 into toner density using a conversion formula stored in the ROM 113. The toner density T / D of the developer in the developing device 4 is obtained by the following formula 1 based on the measurement result of the toner density sensor 15 by the CPU 111.
T / D = (SGNL value−SGNLi value) / Rate + initial T / D (Expression 1)
The SGNL value is a value measured by the toner density sensor 15. The SGNLi value is an initial measurement value (initial value) of the toner density sensor 15. Rate is the sensitivity of the toner density sensor 15. The initial T / D and SGNLi values are those measured at the time of initial installation of the toner supply tank 33. “Rate” is a characteristic of the toner density sensor 15 in which the sensitivity of ΔSGNL to T / D is measured in advance. These constants (initial T / D, SGNLi value, Rate) are stored in the RAM 112 of the control unit 110.

<Toner supply>
In this embodiment, the toner replenishment amount calculation method is as follows. In the image forming apparatus 100, the toner density of the developer in the developing device 4 decreases due to continuous development of the electrostatic latent image on the photosensitive drum 1. Therefore, the control unit 110 executes toner replenishment control and replenishes toner from the toner replenishment tank 33 to the developing device 4 so as to control the toner concentration of the developer to be constant. Thereby, the image density is also controlled to be constant. The image forming apparatus 100 forms an electrostatic latent image on the photosensitive drum 1 by an area gradation method that expresses gradation by a difference in toner area. Therefore, the toner replenishing operation is performed based on the detection result of the patch image by the image density sensor 12 and based on the digital image signal for each pixel of the electrostatic latent image formed on the photosensitive drum 1.

The control unit 110 adds the replenishment correction amount Mp obtained by the patch detection ATR to the basic replenishment amount Mv obtained by the video count ATR (automatic tone replenisher), and the toner replenishment amount per image forming sheet. Find Msum. The video count ATR is a method for calculating the toner replenishment amount using the property that the video count value is proportional to the toner consumption amount. The patch detection ATR is a technique for detecting the image density of a patch image and controlling the toner replenishment amount according to the image density. In this embodiment, the current developing device 4 is expressed by Expression 2 by adding the a posteriori toner shortage amount (Mp) detected from the patch image to the toner consumption amount (Mv) estimated according to the image data. The toner replenishment amount Msum to be supplied to is set.
Toner replenishment amount Msum = Mv + (Mp / patch detection ATR frequency) (Expression 2)
In Equation 2, Mv is the toner replenishment amount obtained from the video count ATR. Mp is the toner replenishment amount obtained by the patch detection ATR.

<Video count ATR>
The basic replenishment amount Mv is obtained from an image signal read by the image reading device (reader unit) A or an image signal sent from a computer or the like. The circuit configuration for processing these image signals is as shown in the block diagram of FIG. As shown in FIG. 2, the image signals of M 2, C 2, Y 2, and K 2 output from the masking and UCR circuit 208 are also sent to the video counter 220. The video counter 220 calculates the video count value of each CMYK image by integrating the image density values in pixel units. The video counter 220 processes the image signals of M2, C2, Y2, and K2, and integrates density values in units of pixels. Thereby, the video count value of each color image of CMYK is calculated. For example, when a 128-level halftone image is formed at 600 dpi and A3 full size (16.5 × 11.7 inch), the video count value is “128 × 600 × 600 × 16.5 × 11.7 = 8895744000 ". The video count value is converted into the basic replenishment amount Mv by using a table showing the relationship between the video count value obtained in advance and stored in the ROM 113 and the toner replenishment amount. Thus, the basic supply amount Mv of each image is calculated every time image formation is performed.

<Patch detection ATR>
As shown in FIG. 5, the control unit 110 forms a patch image at an image interval (non-image area) provided for every predetermined number of image formations in continuous image formation. For example, the control unit 110 forms a patch image Q, which is an image density detection image pattern, in a non-image area sandwiched between the trailing edge of the 24th image and the leading edge of the next image during continuous image formation. . That is, the control unit 110 controls the exposure device 3 to write a “patch electrostatic latent image” that is an electrostatic latent image of the patch image on the photosensitive drum 1 and develops it with the developing device 4 to form the patch image Q. . The control unit 110 executes density control of the patch detection ATR, and performs toner supply control based on the detection result of the patch image Q by the image density sensor 12 so that the image density of the patch image Q converges to the reference density. . In this manner, the image density sensor 12 functions as an image density detection unit that detects the density of the image formed on the image carrier.

  The printer control unit 109 is provided with a patch image signal generation circuit (pattern generator 192) that generates a patch image signal having a signal level corresponding to a predetermined image density. The patch image signal from the pattern generator 192 is supplied to the pulse width modulation circuit 191 to generate a laser driving pulse having a pulse width corresponding to the above-described predetermined density. This laser driving pulse is supplied to the semiconductor laser of the exposure apparatus 3, and the semiconductor laser is caused to emit light for a time corresponding to the pulse width to scan and expose the photosensitive drum 1. As a result, a patch electrostatic latent image corresponding to the predetermined density is formed on the photosensitive drum 1. The patch electrostatic latent image is developed by the developing device 4.

  As shown in FIG. 4, an image density sensor 12 (patch detection ATR sensor) for detecting the image density of the patch image Q is disposed facing the photosensitive drum 1 on the downstream side of the developing device 4. The image density sensor 12 includes a light emitting unit 13 including a light emitting element such as an LED, and a light receiving unit 14 including a light receiving element such as a photodiode (PD), and the light receiving unit 14 receives only regular reflected light from the photosensitive drum 1. Is configured to detect. The image density sensor 12 measures the amount of light reflected from the photosensitive drum 1 at the timing when the patch image Q passes under the image density sensor 12. A signal related to the measurement result is input to the CPU 111.

  As shown in FIG. 6, the reflected light (near infrared light) from the photosensitive drum 1 input to the image density sensor 12 is converted into an electrical signal. The analog electric signal of 0 to 5 V is converted into an 8-bit digital signal by the A / D conversion circuit 114 provided in the control unit 110. The digital signal is converted into density information by a density conversion circuit 115 provided in the control unit 110.

  As shown in FIG. 7, when the image density of the patch image Q formed on the photosensitive drum 1 is changed stepwise according to the area gradation, the output of the image density sensor 12 changes. Here, it is assumed that the output of the image density sensor 12 in a state where the toner is not attached to the photosensitive drum 1 is 5 V and is read to the 255 level. As the area coverage with toner in the pixels formed on the photosensitive drum 1 increases, the image density also increases. On the other hand, the output of the image density sensor 12 becomes small. Based on the characteristics of the image density sensor 12, a table 116 dedicated to each color for converting the output of the image density sensor 12 into a density signal for each color is prepared in advance. The table 116 is stored in the storage unit of the density conversion circuit 115. As a result, the density conversion circuit 115 can accurately read the patch image density for each color. The density conversion circuit 115 outputs density information to the CPU 111.

  The image density sensor 12 has a log function characteristic, and the gradient of the detection result decreases as the density increases. That is, the change in detection result is less with respect to the change in density. As a result, detection accuracy is low. Therefore, by using a pattern of two lines and one space, the area gradation is lowered and the patch image density is lowered. The patch electrostatic latent image to be exposed is assumed to be an image of 2 lines and 1 space in the sub-scanning direction with a resolution of 600 dpi.

The replenishment correction amount Mp is obtained from the difference ΔD between the reference value and the measurement result with reference to the detected value of the density of the patch image Q in the initial developer. For example, the change amount ΔDrate for the density measurement result of the patch image Q when the toner in the developing device 4 is shifted by 1 g (reference amount) from the reference value is obtained in advance and stored in the ROM 113. As a result, the CPU 111 calculates the replenishment correction amount Mp using Equation 3.
Mp = ΔD / ΔDrate (Formula 3)
Here, it is desirable that the toner supply for the supply correction amount Mp is performed while being distributed at equal intervals as much as possible within the execution interval of the patch detection ATR in order to avoid a sudden color variation. After the patch detection ATR is performed, if the determined replenishment correction amount Mp is replenished together when the first image is formed, a large amount of toner may be replenished and overshoot may occur. Therefore, in Expression 3, the replenishment correction amount Mp is divided by the execution frequency of the patch detection ATR, and the replenishment correction amount Mp is equally divided at the execution interval of the patch detection ATR.

  As described above, the CPU 111 of the control unit 110 obtains the toner replenishment amount Msum by Equation 2. Then, by controlling the motor 31 to operate the toner conveying screw 32, the toner supply amount Msum is supplied from the toner supply tank 33 to the developing container 45.

<Toner charge amount>
Next, how to obtain the current toner charge amount will be described. The toner charge amount is calculated by the control unit 110. The control unit 110 includes, for example, a RAM 112 as a work buffer for performing calculations, a CPU 111 for performing calculations, and a ROM 113 including tables necessary for calculations. As shown in FIG. 8, the toner charge amount Q / M (μC / g) is calculated at predetermined time intervals (eg, every 1 minute). When the image forming apparatus is turned off, the calculation process is executed as many times as the number of times when the image forming apparatus is turned on. For example, after 1 hour, S1 to S8 are repeated 60 times.

  In S <b> 1, the control unit 110 calculates the n-th toner charge amount Q / M for 1 minute from the time point when the n−1-th toner charge amount Q / M is calculated. Acquire various data. Specifically, the control unit 110 acquires the integrated value of the video count for 1 minute from the video counter 220. Since the video count value is very large, a value divided by 2 ^ 24 is used for convenience. The value is set as a video count value V. Next, the control unit 110 acquires the driving time Td (sec) of the developing sleeve 41 for one minute from the developing sleeve driving device 44. Further, the control unit 110 calculates a stop time Ts (sec) of the developing sleeve 41 during one minute. This is a value obtained by subtracting the drive time Td (sec) from 60 seconds. Further, the control unit 110 acquires the toner density TDrate (%) from the toner density sensor 15. Further, the control unit 110 acquires an absolute water content H (g / kg) in the image forming apparatus detected by a temperature / humidity sensor (not shown) attached to the inside of the image forming apparatus. Further, the control unit 110 acquires, from the developing sleeve driving device 44, a sleeve driving integrated time Tt (min) that is an integrated value of the driving time Td (sec) of the developing sleeve 41 starting from when the developer is replaced.

In S2, the control unit 110 calculates the image ratio D (%). For example, D is calculated from the following equation.
Image ratio D = V / Td × 0.162 (Expression 4)
The image ratio indicates how many images are formed with respect to the sleeve driving time. The coefficient 0.162 in the above formula should be optimized for each image forming apparatus. 0.162 is a value when optimized for an image forming apparatus that outputs 70 A4 sizes per minute. By optimizing, the average value of the image ratio for each sheet and D are made the same.

In S3, control unit 110 calculates convergence value Q / M1. The convergence value Q / M1 is calculated from the image ratio D using the relationship shown in FIG. Assume that the relationship of the convergence value Q / M1 with respect to the image ratio D shown in FIG. The convergence value Q / M1 means a value that converges when an image is formed forever at an image ratio D (%). In S4, control unit 110 calculates convergence value Q / M2 (μC / g) from the following equation.
Convergence value QM2 = convergence value QM1 × (−0.1 × TDrate + 1.8) (Formula 5)
Since the convergence value Q / M varies depending on the toner density, it is corrected by the toner density. Since this relational expression varies depending on the developer material and the like, it is not limited to this expression. In general, Q / M tends to decrease as the toner concentration increases and Q / M tends to increase as the toner concentration decreases. In addition, the coefficient in Formula 5 is only an example.

In S5, control unit 110 calculates convergence value Q / M3 (μC / g) from the following equation.
Convergence value QM3 = convergence value QM2 + 5−0.5 × H (Expression 6)
The convergence value Q / M varies depending on the environment. Therefore, the absolute water content H is corrected. Since this relational expression varies depending on the developer material and the like, it is not limited to this expression. Generally, when the absolute water content H increases, the Q / M decreases, and when the absolute water content decreases, the Q / M tends to increase. In addition, the coefficient in Formula 6 is only an example.

In S6, control unit 110 calculates convergence value Q / M4 (μC / g) from the following equation.
Convergence value QM4 = convergence value QM3 × (−0.000021 × Tt + 1) (Expression 7)
Since the convergence value Q / M varies depending on the degree of developer deterioration, the convergence value Q / M is corrected by the sleeve drive integration time Tt. Since this relational expression also differs depending on the developer material and the like, the relational expression is not limited to this expression, but is used because the above expression is optimal in this embodiment. In addition, the coefficient in Formula 7 is only an example.

In S7, control unit 110 calculates temporary Q / M (n minutes) from the following equation.
Temporary Q / M (n minutes) = α × (convergence value QM4−Q / M (n−1 minutes)) × Td / 60 + Q / M (n−1 minutes) (Equation 8)
Here, α = 0.01. Expression 8 represents a change in the toner charge amount during driving of the sleeve in one minute by a recurrence formula. A phenomenon that gradually approaches the convergence value Q / M is expressed in an equation. Since α varies depending on the developer material and the like, it is not limited to this formula. However, in the present embodiment, 0.01 is optimal because it is optimal.

In S8, control unit 110 calculates Q / M (n minutes) from the following equation. From this equation, the toner charge amount Q / M (μC / g) at this point is calculated.
Q / M (n minutes) = − β × Ts / 60 × temporary Q / M (n minutes) + temporary Q / M (n minutes) (Equation 9)
Here, β = 0.001. Expression 9 represents a change in the toner charge amount while the sleeve is stopped in one minute as a recurrence formula. A phenomenon in which the charge charged in the toner is gradually discharged and the charge amount approaches 0 is expressed as an equation. Note that β differs depending on the developer material and the like, and is not limited to this formula. However, in the present embodiment, 0.001 is optimal and is used.

  As described above, the toner charge amount Q / M (μC / g) per minute can be calculated by performing the flow shown in FIG. 8 every minute. That is, the CPU 111 or the like functions as a charge amount detection unit that detects or estimates the charge amount of the toner in the second container.

<Development efficiency>
The potential of the photosensitive drum 1 shown in FIG. 10 is an absolute value, which is a value immediately after the above-described charging, exposure, and development processes. In the figure, Vd is a dark portion potential and indicates a potential of a portion not exposed. Vdc is a negative DC voltage applied to the developing sleeve 41. Vl is the bright portion potential (potential of the exposed portion). In the developing process, toner adheres to the exposed portions. The toner moves to the photosensitive drum 1 due to the potential difference between Vl and Vdc. When developed, the potential of the Vl portion rises due to the charge of the toner and finally reaches Vdc. The toner layer potential after development is Vtoner. Since the potential difference disappears when Vtoner reaches Vdc, the development ends. In the drawing, ○ indicates toner. The size of ○ indicates the toner charge amount. The number of ○ indicates the number of toner developed on the photosensitive drum 1.

Here, the development efficiency is expressed by the following equation.
Development efficiency = (Vtoner−Vl) / (Vdc−Vl) × 100% (Equation 10)
Normally, since Vtoner≈Vdc is established, the development efficiency is approximately 100%. Even when the charge amount Q / M of the toner increases, as shown in the figure, if Vtoner≈Vdc is satisfied, the developing efficiency is still 100%. However, the amount of toner that can be developed decreases as the charge amount of the toner increases (as the size of the circle shown in FIG. 10 increases). Therefore, the amount of toner to be developed is made constant by further lowering the bright portion potential Vl according to the charge amount of the toner. The reduction in development efficiency occurs when the development process is completed before Vtoner reaches Vdc. Since Vtoner is lower than Vdc, the development efficiency expressed by Equation 10 is less than 100%. In this state, since a necessary amount of development cannot be performed, the image density becomes thinner than the desired density. The development efficiency can be calculated by measuring the potential after development. However, since the potential sensor is vulnerable to toner contamination, it is very difficult to measure the potential after development. Therefore, in this embodiment, the development efficiency is calculated without measuring the potential after development.

Here, for convenience, a capacitor model is adopted as the potential of the photosensitive drum 1. The total charge amount of the toner is Q, and the electrostatic capacity of the toner is C. The electrostatic capacity C of the toner increases the potential. Therefore, the following equation is established.
Vtoner−Vl = Q / C (Formula 11)
C is uniquely determined by the type of toner. Therefore, the following formula is established whether or not the development efficiency is lowered.
Q / (Vtoner−Vl) = constant (Equation 12)
When the total charge amount of the toner in the presence of the developer is Q ′, the toner layer potential is Vtoner ′, and the bright portion potential is Vl ′, the following equation is established.
Q / (Vtoner−Vl) = Q ′ / (Vtoner′−Vl ′) (Equation 13)
Further, when the developing efficiency in the state where there is a developer is γ%, the following equation is established.
(Vtoner′−Vl ′) / (Vdc−Vl) = γ (Expression 14)
Equation 15 can be derived from Equation 13 and Equation 14.
Development efficiency = (Vtoner−Vl) / (Vdc−Vl) × 100%
= Q / Q 'x γ% (Equation 15)
Thus, the development efficiency can be expressed by the ratio of the total charge amount of the toner. That is, the development efficiency in the initial state where the developer is not deteriorated is 100%. Thus, in this embodiment, the development efficiency is calculated by the ratio between the initial total charge amount Q ′ and the current total charge amount Q.

<Toner replacement>
As shown in FIG. 11, the toner charge amount Q / M and the image density detected by the patch detection ATR are used as the toner replacement sequence. Each step of S11 to S18 includes S1 to S8 described above. Therefore, the toner charge amount calculation of S11 to S18 is executed at a predetermined interval (eg, 1 minute). It should be noted that the toner charge amount Q / M calculation process is executed at predetermined intervals not only during image formation but also during image formation. S21 to S23 are steps for executing the patch detection ATR. As described above, the patch detection ATR is executed between sheets (non-image forming areas) every predetermined number of sheets (for example, 24 sheets). S11 to S18 and S21 to S23 are executed independently.

In S31, the control unit 110 determines the development setting used for the toner replacement sequence. The execution timing of S31 is when the control unit 110 detects that images having a low image ratio are continuously formed. The control unit 110 monitors whether or not images having an image ratio lower than the threshold value are continuously formed over a predetermined number. In this embodiment, for example, when it is detected that 50 images having an image ratio of 2% or less continue, S31 is executed. In S31, the image density M of the patch detection ATR and the toner charge amount Q / M obtained immediately before are used. In the example of FIG. 11, the toner charge amount Q / M is acquired from S15 and the value is used, and the image density M of the patch detection ATR is the value acquired by the image density sensor 12 in S22. The control unit 110 calculates the development efficiency using Equation 15. Equation 15 can be expressed as follows by the image density M and the toner charge amount Q / M.
Development efficiency = Q / M × M / (initial Q / M × initial M) × 100% (Equation 16)
Note that the image density M of the patch detection ATR is converted into patch mass (mounting amount). Since it is known that the image density and the mass have a certain proportional relationship, in this embodiment, the image density of the patch detection ATR is assumed to be M. A relational expression or table defining or registering the relation between the image density of the patch detection ATR and the patch mass may be prepared and converted in advance. The initial Q / M and the image density M of the initial patch detection ATR are calculated by the control unit 110 and stored in the RAM 112 when a new developer is introduced into the toner container. The introduction of the new developer may be realized by replacing the toner container.

  Next, the control unit 110 uses the calculated development efficiency and the relationship shown in FIG. 12A to determine the toner replacement amount corresponding to the decrease in the development efficiency. In the first embodiment, the toner replacement amount is controlled by controlling the rotation speed of the developing sleeve 41. Therefore, the control unit 110 functions as a replacement control unit that controls the replacement unit so as to replace the developer by the amount of developer defined by the image density and the toner charge amount. The developing sleeve 41 and the photosensitive drum 1 are each driven to rotate downward in the contact position. FIG. 12A shows the rotation speed of the developing sleeve 41 when the linear velocity of the photosensitive drum 1 at the contact position is 100%. It is assumed that the control unit 110 holds the relationship between the development efficiency and the rotation speed as a relational expression or a table.

  For example, when the development efficiency is 90%, the rotation speed of the development sleeve 41 is calculated as 145%. The CPU 111 of the control unit 110 controls the developing sleeve driving device 44 to set the rotation speed of the developing sleeve 41 during the toner replacement sequence to 145%. The CPU 111 sets the rotation speed of the developing sleeve 41 during image formation to 140%. As described above, the rotation speed of the developing sleeve 41 is faster than the rotation speed of the developing sleeve 41 during image formation only during the toner replacement sequence. This is because a lot of deteriorated toner is discarded in a short time.

  In the toner replacement sequence in this embodiment, the semiconductor laser of the exposure device 3 is driven to expose the entire surface of A4 size. By the exposure, an A4 size electrostatic latent image is formed on the photosensitive drum 1, and a developing image of the developing device 4 forms a replacement image with deteriorated toner on the photosensitive drum 1. The rotational speed of the developing sleeve 41 at that time is controlled as described above. In order to prevent the image from being transferred onto the intermediate transfer belt 6, the controller 110 applies a negative DC voltage opposite to the normal voltage to the primary transfer roller 7. The image remaining on the photosensitive drum 1 is cleaned by the cleaning device 8. As described above, the exposure device 3, the developing device 4, the photosensitive drum 1, the cleaning device 8, and the like function as part of a replacement unit that discards the developer from the second container to the outside of the second container. . That is, they function as a means for rotating the developing sleeve to cause the developer to adhere to the image carrier and to discard the developer to the outside of the second container.

  As shown in FIG. 12B, it can be seen that when the toner replacement sequence (145%) according to this embodiment is executed, the image density is more stable than the comparative example (140%) where this embodiment is not applied. This experimental result shows a change in density when an image of A4 size and an image ratio of 1% is continuously fed through 5000 sheets. The toner replacement sequence is performed every 50 sheets.

  According to the first embodiment, the control unit 110 includes a table in which the rotation speeds of the development sleeves corresponding to different development efficiencies are registered, and determines a rotation speed corresponding to the development efficiency using the table. Further, the control unit 110 rotates the developing sleeve at the determined rotation speed. As a result, it is possible to replace the toner by an appropriate amount according to the decrease in development efficiency. Since the toner discharge amount is appropriate according to the development efficiency, it is possible to suppress excessive toner consumption accompanying the forced discharge of the toner. Further, by changing the deteriorated developer, it is possible to suppress fluctuations in image density. In particular, the rotational speed of the developing sleeve used when changing the developer is made faster than the rotating speed of the developing sleeve used when forming an image on the recording medium, thereby reducing the amount of toner discharged. It can be increased easily.

<Second embodiment>
The first embodiment is an invention in which the developer is replaced by an amount corresponding to the developing efficiency by changing the rotation speed of the developing sleeve 41 according to the developing efficiency. In the second embodiment, the developer is replaced by an amount corresponding to the development efficiency by increasing the discharge area. The discharge area is the area of the latent image formed on the image carrier (the area where the developer is to be discarded). Note that the size of the latent image is changed by the pattern generator 192 in accordance with a command from the CPU 111. The exposure device 3 forms a latent image on the photosensitive drum 1 in accordance with the data output from the pattern generator 192.

  A relational expression or table showing the relationship between the development efficiency and the discharge area as shown in FIG. 13A is created in advance at the time of factory shipment and stored in the ROM 113. The CPU 111 determines the discharge area by applying the development efficiency determined by the method described in the first embodiment to the above relational expression or table.

  Referring to the experimental results of the reflection density with respect to the number of formed images when the second embodiment is applied and the comparative example in which the second embodiment is not applied, shown in FIG. 13B, the superiority of the second embodiment Can understand. In the experiment, 5000 sheets of A4 size images with an image ratio of 1% were continuously fed. The toner replacement sequence was executed every 50 sheets.

  As described above, according to the second embodiment, the control unit 110 includes a table in which the developer adhesion areas on the image carrier for different development efficiencies are registered. Then, the control unit 110 determines an adhesion area corresponding to the development efficiency using this table, and controls the exposure apparatus 3 to expose the image carrier so that the determined adhesion area is obtained. As a result, the second embodiment achieves the same effect as the first embodiment. The first embodiment and the second embodiment may be combined. In other words, the control unit 110 can realize some percent of the desired effect by increasing the rotation speed of the developing sleeve 41, and can realize the remaining effect by increasing the discharge area. Further, the amount of toner discharged to be discarded can be adjusted by increasing the image density instead of the latent image size or together with the latent image size.

<Third embodiment>
The third embodiment is an invention in which the developer is replaced by an amount corresponding to the developing efficiency by increasing the AC component of the developing bias applied to the developing sleeve 41 by the bias power source 47. Here, the amplitude of the AC component of the developing bias is referred to as Vpp.

  A relational expression or table showing the relationship between the development efficiency and Vpp as shown in FIG. 14A is created in advance at the time of factory shipment and stored in the ROM 113. The CPU 111 determines Vpp by applying the development efficiency determined by the method described in the first embodiment to the above relational expression or table, and sets Vpp to the bias power supply 47 as a bias applying unit.

  Referring to the experimental results of the reflection density with respect to the number of formed images when the third embodiment is applied and the comparative example not applying the third embodiment shown in FIG. 14B, the superiority of the third embodiment Can understand. In the experiment, 5000 sheets of A4 size images with an image ratio of 1% were continuously fed. The toner replacement sequence was executed every 50 sheets.

  According to the third embodiment, the control unit 110 includes a table in which values of developing bias of the developing sleeve for different developing efficiencies are registered, and the developing bias having a value corresponding to the developing efficiency is applied to the developing sleeve using this table. The bias power supply 47 is controlled so as to be applied. As a result, the third embodiment has the same effects as the first and second embodiments. The third embodiment and the first embodiment may be combined, the third embodiment and the second embodiment may be combined, or all of the first to third embodiments may be combined. Also good. The control unit 110 can realize some percent of the desired effect by increasing the rotation speed of the developing sleeve 41, and can realize the remaining effect by increasing Vpp. Combinations of the second and third embodiments and combinations of the first to third embodiments can be achieved in the same manner.

  In the above embodiment, the development process is used to discard the deteriorated developer, but other disposal mechanisms may be adopted. For example, the second container storing the developer may be mechanically discarded from the third container. That is, the second container and the third container are connected by a waste pipe or the like, and an openable / closable shutter is provided at an end of the waste pipe connected to the second container. The controller 110 controls the opening / closing amount of the shutter according to the development efficiency. Note that the deteriorated developer may be discharged from the second container to the third container by using the above-described screw or the like.

  In the above-described embodiment, the description has been made centering on the control of the discarded amount of the deteriorated developer, but the replenishment amount of the new developer is similarly controlled. This is because the relationship of “disposal amount = replenishment amount” basically holds. Therefore, the CPU 111 controls the rotation of the motor 31 via the motor drive circuit so that a new developer is replenished by a replenishment amount that matches the determined discard amount. Thereby, the supply amount of the developer by the toner conveying screw 32 is controlled.

Claims (6)

A first container containing a developer including toner and carrier;
A second container for storing the developer supplied from the first container;
Charging means for charging the developer contained in the second container;
An image forming apparatus comprising: a developing unit that develops by attaching a developer charged by the charging unit to an electrostatic latent image formed on an image carrier;
While discarding the developer from the second container to the outside of the second container, the developer is replenished from the first container to the second container with a replenishment amount corresponding to the amount of the developer discarded. Replacement means for replacing the developer in the second container by replenishment;
Image density detecting means for detecting the density of the image formed on the image carrier;
Charge amount detection means for detecting or estimating the charge amount of the toner in the second container;
An image forming apparatus comprising: a replacement control unit configured to control the replacement unit so as to replace the developer by an amount corresponding to the developing efficiency of the developer defined by the density of the image and the charge amount of the toner. apparatus. The developing means includes a developing sleeve,
The replacement means includes means for rotating the developing sleeve to cause the developer to adhere to the image carrier, thereby discarding the developer to the outside of the second container,
The replacement control means includes a table in which rotation speeds of the developing sleeves corresponding to different developing efficiencies are registered, and determines a rotating speed corresponding to the developing efficiency using the table, and the developing sleeves at the determined rotating speed. The image forming apparatus according to claim 1, wherein the image forming apparatus is rotated.   The rotation speed of the developing sleeve used when executing the replacement of the developer is faster than the rotating speed of the developing sleeve used when forming an image on a recording medium. The image forming apparatus described in 1. The replacement means includes means for causing the developer to adhere to the image carrier and thereby discarding the developer to the outside of the second container,
The replacement control means includes a table in which the adhesion area of the developer on the image carrier for different development efficiencies is registered, determines an adhesion area corresponding to the development efficiency using the table, and determines the determined adhesion area The image forming apparatus according to claim 1, wherein an exposure unit is controlled so as to expose the image bearing member. The developing means includes a developing sleeve,
The replacement means includes means for rotating the developing sleeve to cause the developer to adhere to the image carrier, thereby discarding the developer to the outside of the second container,
The replacement control means includes a table in which values of developing bias of the developing sleeve for different developing efficiencies are registered;
The image forming apparatus according to claim 1, further comprising: a bias applying unit configured to apply a developing bias having a value corresponding to the developing efficiency to the developing sleeve using the table. A first container containing a developer including toner and carrier;
A second container for storing the developer supplied from the first container;
Charging means for charging the developer contained in the second container;
A control method for an image forming apparatus, comprising: a developing unit that attaches and develops a developer charged by the charging unit to an electrostatic latent image formed on an image carrier,
An image density detection step for detecting the density of the image formed on the image carrier;
A charge amount detection step of detecting or estimating a charge amount of toner in the second container;
While discarding the developer from the second container to the outside of the second container by an amount corresponding to the developing efficiency of the developer defined by the density of the image and the charge amount of the toner, And a replacement step of replacing the developer in the second container by replenishing the developer from the first container to the second container.

Images