JP2005338570A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a stable image forming apparatus where density variation and color variation are restrained to be little. <P>SOLUTION: Even when the state of developer is fluctuated because of environmental change or endurance deterioration, transfer efficiency in accordance with a toner applied amount on a toner image carrier is obtained, and relation between the toner applied amount and the transfer efficiency is fed back to image processing. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an image forming apparatus such as a copying machine or a printer, and more particularly to an image forming apparatus having a toner density detecting means.

  As a method for adjusting image processing characteristics of an image forming apparatus such as a copying machine or a printer (hereinafter referred to as “image control method”), the following method is known.

  After starting the image forming apparatus and completing the warm-up operation, a specific pattern is formed on an image carrier such as a photosensitive drum. Then, the density of the formed pattern is read, and based on the read density value, the operation of a circuit for determining image forming conditions such as a gamma correction circuit is changed to stabilize the quality of the formed image.

  Furthermore, even when the gradation characteristics of the image forming apparatus change due to changes in environmental conditions, a specific pattern is again formed on the image carrier, read, and image forming conditions such as a gamma correction circuit are determined again. By feeding back to the circuit, the image quality can be stabilized according to the change in environmental conditions.

  Further, when the image forming apparatus is used for a long period of time, there may occur a case where the density obtained by reading the pattern on the image carrier does not match the density of the image actually printed out. Therefore, a method is known in which a specific pattern is formed on a recording material and the image forming conditions are corrected by the density value.

Also known is a method of correcting a gamma look-up table (γLUT) or creating a γLUT modulation table based on density information of one image pattern, and adding insufficient correction information to a gamma correction circuit.
Japanese Patent Laid-Open No. 05-035104

  However, the above method cannot sufficiently satisfy density stability or color stability.

  For example, FIG. 3 shows the transfer efficiency with respect to the density at the secondary transfer portion in a high temperature and high humidity environment. The transfer efficiency largely depends on the state of the developer and the amount of toner applied on the intermediate transfer belt. In particular, the developer in a new state is packed with the humidity (absolute humidity) at the time of preparation of the developer, and is not adjusted even if the temperature is adjusted in a high temperature environment. Therefore, a developer that has just been opened in a high-humidity environment is likely to impart a relatively high charge amount, and a decrease in transfer efficiency (transfer loss) due to excessive transfer charge in a low density region occurs. Hard (high transfer efficiency even in a low density region) (see FIG. 3 (1)). However, when accustomed to a sufficiently high humidity environment, the charge amount of the toner becomes low, and the transfer charge in the low density region tends to become excessive, resulting in a decrease in transfer efficiency (transfer loss). (See FIG. 3 (2)). Further, as the durability of the developer progresses, the charge amount of the toner tends to further decrease, and the transfer efficiency in the low density region tends to further decrease (see FIG. 3 (3)). Therefore, in the conventional technique, when the pattern image is formed, the density is read, and the correction is performed based on the read density value, the transfer efficiency changes depending on the applied toner amount. It may not be possible.

  From the above, the object of the present invention is to obtain density fluctuations or the like by obtaining the transfer efficiency according to the amount of applied toner on the toner image carrier even if the developer state fluctuates due to environmental changes or durability deterioration. An object of the present invention is to provide an image forming apparatus capable of obtaining a stable image with little color fluctuation.

  The above object is achieved by the following means.

  A toner image carrier that forms a toner image on the surface in accordance with image data; and a transfer member that transfers a plurality of different color toner images on the image receiving member in a transfer portion, A first detection means for detecting image characteristics on the image receiver, and a first pattern image for detecting image characteristics is formed on the image receiver by a sequence different from normal image formation; In the image forming apparatus having a control unit for controlling an image forming condition on the image carrier based on the first pattern image characteristic detected by the first detection unit, the transfer unit according to a different toner density And a correction means for correcting the image forming conditions controlled by the control means based on the relationship between the toner density and the transfer efficiency at the transfer portion. More above object is achieved.

  A toner image carrier that forms a toner image in which toners of different colors are superimposed on the surface in accordance with image data, and a transfer member that transfers the toner image on the toner image carrier to an image receiver in a transfer unit, A first detection means for detecting image characteristics on the image receiver, and a first pattern image for detecting image characteristics is formed on the image receiver by a sequence different from normal image formation; In the image forming apparatus having a control unit for controlling an image forming condition on the image carrier based on the first pattern image characteristic detected by the first detection unit, the transfer unit according to a different toner density And a correction means for correcting the image forming conditions controlled by the control means based on the relationship between the toner density and the transfer efficiency at the transfer portion. The object by wherein is achieved.

  The object is achieved by the image forming condition being a density correction characteristic of the image data.

  A transfer unit that transfers a pattern image having a different toner density formed on the toner image carrier onto the image receiver in a sequence different from that of normal image formation. And a first applied toner amount detecting means disposed on the upstream side in the moving direction of the toner image carrier from the transfer portion, and a second applied toner amount disposed on the downstream side in the moving direction of the toner image carrier from the transfer portion. The object is achieved by being derived from the values of the first detection signal and the second detection signal obtained by the detection means.

  The object is achieved by the toner image carrier being a photosensitive member and the image receiver being a recording material.

  The above object is achieved by the toner image carrier being a photoreceptor and the image receiver being an intermediate transfer member.

  The above object is achieved by the toner image carrier being an intermediate transfer member and the image receiver being a recording material.

(Function)
By obtaining the correlation between the amount of applied toner and the transfer efficiency and feeding back this relationship to the image data, a stable image with little density fluctuation and color fluctuation can be obtained even when the environmental fluctuation of the toner state or durability deterioration occurs. Can do.

  As described above, the correlation between the applied toner amount and the transfer efficiency is obtained, and this relationship is fed back to the image data. A few stable images can be obtained.

(First embodiment)
FIG. 1 schematically shows the vicinity of an image carrier of an image forming apparatus according to a first embodiment of the present invention.

  As shown in FIG. 1, this image forming apparatus has four process units as image forming means having a charging means, an exposure means, a developing device, a cleaner and the like around a photosensitive drum as a latent image carrier. The image on the photosensitive drum provided by each process unit is sequentially transferred to a recording material such as paper on a conveying means that moves and passes adjacent to the photosensitive drum, so that a full color image is formed. It has a configuration.

  Details of the image forming apparatus according to the present embodiment will be described below.

  Photosensitive drums 1a, 1b, 1c, and 1d are arranged in the process units Pa, Pb, Pc, and Pd that form images of yellow, magenta, cyan, and black, respectively. It is free to rotate. Further, around each photosensitive drum 1a, 1b, 1c, 1d, charging means 2a, 2b, 2c, 2d, exposure means 3a, 3b, 3c, 3d, developing devices 4a, 4b, 4c, 4d, and a cleaner 6a, 6b, 6c and 6d are sequentially arranged along the rotation direction of the photosensitive drum.

  Hereinafter, the process unit will be described in detail with reference to FIG. 2, but the four process units have the same configuration. Here, description will be made by omitting the symbols a, b, c, and d.

  The image forming apparatus includes a photosensitive drum 1 that is rotatably supported by an apparatus body (not shown) as an image carrier. The photosensitive drum 1 is a cylindrical electrophotographic photosensitive member that basically includes a conductive substrate 11 such as aluminum and a photoconductive layer 12 formed on the outer periphery thereof. At its center, there is a support shaft 13, and the drive shaft is driven to rotate in the direction of arrow R1 about the support shaft 13 by the driving means (not shown).

  Above the photosensitive drum 1, a charging roller 2 as a primary charging device is disposed. The charging roller 2 is in contact with the surface of the photosensitive drum 1 and uniformly charges the surface with a predetermined polarity and potential, and is configured in a roller shape as a whole. The charging roller 2 includes a conductive core bar 21 disposed at the center, a low resistance conductive layer 22 and a medium resistance conductive layer 23 formed on the outer periphery thereof, and both end portions of the core bar 21 are bearing members (not shown). And is arranged in parallel to the photosensitive drum 1. The bearing members at both ends are urged toward the photosensitive drum 1 by pressing means (not shown), whereby the charging roller 2 is pressed against the surface of the photosensitive drum 1 with a predetermined pressing force. The charging roller 2 is driven to rotate in the direction of arrow R2 as the photosensitive drum 1 rotates in the direction of arrow R1. A bias voltage is applied to the charging roller 2 by a power supply 24, whereby the surface of the photosensitive drum 1 is uniformly contact-charged.

  An exposure unit 3 is disposed on the downstream side of the charging roller 2 in the rotation direction of the photosensitive drum 1. The exposure means 3 exposes the photosensitive drum 1 by scanning while turning off / on laser light based on image information, for example, and forms an electrostatic latent image corresponding to the image information.

  The developing device 4 arranged on the downstream side of the exposure means 3 has a developing container 41 containing a two-component developer, and a developing sleeve 42 is rotatably installed in an opening of the container 41 facing the photosensitive drum 1. In the developing sleeve 42, a magnet roller 43 that holds the developer on the developing sleeve 42 is fixedly disposed so as not to rotate with respect to the rotation of the developing sleeve 42. A regulating blade 44 that regulates the developer carried on the developing sleeve 42 to form a thin developer layer is installed at a position below the developing sleeve 42 of the developing container 41. Further, a developing chamber 45 and a stirring chamber 46 are provided in the developing container 41, and a replenishing chamber 47 containing replenishing toner is provided above them. When the developer formed in the thin developer layer is transported to the development area facing the photosensitive drum 1, the developer is sprinkled by the magnetic force of the development main pole located in the development area of the magnet roller 43 and developed. A magnetic brush of agent is formed. By rubbing the surface of the photosensitive drum 1 with this magnetic brush and applying a developing bias voltage to the developing sleeve 42 by the power source 48, the toner attached to the carrier constituting the ears of the magnetic brush is electrostatic latent image. A toner image is formed on the photosensitive drum 1 by being attached to the exposed portion and developed.

  A transfer roller 53 is disposed below the photosensitive drum 1 on the downstream side of the developing device 4. The transfer roller 53 is configured by a cored bar 531 to which a bias is applied by a power supply 54 and a conductive layer 532 formed in a cylindrical shape on the outer peripheral surface thereof. Both ends of the transfer roller 53 are urged toward the photosensitive drum 1 by a pressing member such as a spring (not shown), whereby the conductive layer 532 of the transfer roller 53 is applied to the surface of the photosensitive drum 1 with a predetermined pressing force. A transfer nip portion is formed between the photosensitive drum 1 and the transfer roller 53 by pressure contact. The recording material P conveyed in the direction of the arrow K1 by the transfer belt 51 is supplied to the transfer nip portion in synchronization with the rotation of the photosensitive drum 1. The recording material P is nipped and conveyed at the transfer nip portion N. At the time of passing, the recording material P is biased to the back surface of the recording material P by the power supply 54 via the transfer belt 51 and having a polarity opposite to that of the toner. A voltage is applied, whereby the toner image on the photosensitive drum 1 is transferred to the surface of the recording material P.

  The photosensitive drum 1 after the image transfer is removed by the cleaner 6 from the adhered matter such as residual toner. The cleaner 6 includes a cleaner blade 61 and a conveying screw 62, and the cleaner blade 62 is brought into contact with the photosensitive drum 1 by a pressing unit (not shown) at a predetermined angle and pressure, and is brought into contact with the surface of the photosensitive drum 1. Residual toner is collected.

  In the image forming apparatus as described above, the recording material P supplied from the paper feed cassette 8 as the recording material supply unit shown in FIG. 1 is supplied to the transport roller 82 via the pickup roller 81, and is transferred by the suction unit 52 to the transfer belt 51. It is electrostatically attracted to the top and conveyed to the lower part of each process unit. The toner images of the respective colors formed on the photosensitive drums 1a, 1b, 1c, and 1d are sequentially transferred by receiving a transfer bias from the transfer roller 53 that faces the recording material P with the transfer belt 51 interposed therebetween. When this transfer process is completed, the recording material P is separated from the transfer belt 51 by the separation charger 54 and conveyed to the fixing device 7. The toner on the transfer belt 51 is removed and collected by the transfer belt cleaner 55.

  The fixing device 7 includes a fixing roller 71 that is rotatably arranged and a pressure roller 72 that rotates while being pressed against the fixing roller 71. A heater 73 such as a halogen lamp is disposed inside the fixing roller 71, and the temperature of the surface of the fixing roller 71 is adjusted by controlling the voltage supplied to the heater 73. In this state, when the recording material is conveyed, the fixing roller 71 and the pressure roller 72 rotate at a constant speed, and when the recording material P passes between the fixing roller 71 and the pressure roller 72, the both sides of the recording material P By being pressurized and heated at a substantially constant pressure and temperature, the unfixed toner image on the surface of the recording material is melted and fixed, and a full-color image is formed on the recording material P.

The transfer belt 51 is made of a dielectric resin such as PC, PET, or PVDF. In this example, a volume resistivity of 10 ⁹ Ω · cm (using a probe conforming to JIS-K6911 method, applied voltage 100 V, applied time 60 sec, 23 ° C. 50% RH), thickness t = 80 μm, carbon dispersed PI Although resin is employed, other materials, volume resistivity, and thickness may be used.

The transfer roller 53 is composed of a core metal having a diameter of 8 mm and a conductive urethane sponge layer having a thickness of 4 mm. The resistance value of the transfer roller 53 is 50 mm / sec. The value was obtained from the relationship of current measured by applying a voltage of 500 V to the metal core while rotating at high speed, and the value was about 10 ⁶ Ω (23 ° C., 50% RH).

  Next, the reader unit will be described with reference to FIG. Illuminated by the light source 103 placed on the platen glass 102 of the reader unit A, the reflected light from the document 101 forms an image on the CCD sensor 105 via the optical system 104. The CCD sensor 105 includes a group of red, green, and blue CCD line sensors arranged in three rows, and generates red, green, and blue color component signals for each line sensor. These reading optical system units are moved in the direction of the arrow shown in FIG. 1, and convert the image of the original 101 into electric signals for each line. On the original platen glass 102, one side of the original 101 is brought into contact with the positioning member 107 to prevent the oblique placement of the original 101, the white level of the CCD sensor 105 is determined, and shading correction in the thrust direction of the CCD sensor 105 is performed. The reference white plate 106 is arranged. The image signal obtained by the CCD sensor 105 is subjected to image processing by the reader image processing unit 108, sent to the printer unit B, and processed by the printer control unit 109. FIG. 5 is a block diagram showing the flow of image signals in the reader image processing unit 108.

  As shown in FIG. 5, the image signal output from the CCD sensor 105 is input to the analog signal processing circuit 201, the gain and the offset are adjusted, and then an A / D converter 202 converts the 8-bit digital image of each color. Converted to signals R1, G1 and B1. The image signals R1, G1, and B1 are input to the shading correction circuit 203, and known shading correction using a read signal of the reference white plate 106 is performed for each color.

  The clock generator 211 generates a clock CLK for each pixel. The address counter 212 counts CLK and generates and outputs a main scanning address signal for each line. The decoder 213 decodes the main scanning address signal, a CCD drive signal in units of lines such as a shift pulse shift pulse and a reset pulse, a signal VE indicating an effective area in a read signal for one line output from the CCD 105, and line synchronization. A signal HSYNC is generated. Note that the address counter 212 is cleared by HSYNC and starts counting the main scanning address of the next line. Each line sensor of the CCD 105 is disposed at a predetermined distance from each other in the sub-scanning direction. For this reason, the spatial delay in the sub-scanning direction is corrected by the line delay 204. Specifically, the R and G signals are line-delayed in the sub-scanning direction with respect to the B signal, so that the spatial positions of the RGB signals are matched. The input masking circuit 205 converts the color space (reading color space) of the input image signal determined by the spectral characteristics of the RGB filter of the CCD 105 into a predetermined color space (for example, sRGB or NTSC standard color space) by matrix calculation of the following equation. Convert.

ＬＯＧ変換回路２０６はルックアップテーブルＲＯＭにより構成され、Ｒ４、Ｇ４およびＢ４の輝度信号をＣ０、Ｍ０およびＹ０の濃度信号に変換する。ライン遅延メモリ２０７は、図示しない黒文字判定部により、Ｒ４、Ｇ４およびＢ４画像信号からＵＣＲ、ＦＩＬＴＥＲおよびＳＥＮなどの判定信号が生成され出力されるまでのライン遅延分、Ｃ０、Ｍ０およびＹ０画像信号を遅延させる。マスキングＵＣＲ回路２０８は、入力されるＹ１、Ｍ１およびＣ１の三原色信号から黒信号Ｂｋを抽出し、さらに、プリンタ部Ｂの記録色材の色濁りを補正する演算を行い、各読取動作の度にＹ２、Ｍ２、Ｃ２またはＢｋ２画像信号を、順次、所定のビット幅（例えば８ビット）で出力する。ガンマ補正回路２０９は、プリンタ部Ｂの理想的な階調特性に合わせるべく、画像信号を濃度補正する。また、出力フィルタ２１０は、画像信号にエッジ強調またはスムージング処理を施す。

The LOG conversion circuit 206 includes a look-up table ROM, and converts the luminance signals of R4, G4, and B4 into density signals of C0, M0, and Y0. The line delay memory 207 receives C0, M0, and Y0 image signals corresponding to the line delay until a determination signal such as UCR, FILTER, and SEN is generated and output from the R4, G4, and B4 image signals by a black character determination unit (not shown). Delay. The masking UCR circuit 208 extracts the black signal Bk from the input Y1, M1, and C1 primary color signals, and further performs an operation for correcting the color turbidity of the recording color material of the printer unit B, for each reading operation. Y2, M2, C2, or Bk2 image signals are sequentially output with a predetermined bit width (for example, 8 bits). The gamma correction circuit 209 corrects the density of the image signal to match the ideal gradation characteristics of the printer unit B. The output filter 210 performs edge enhancement or smoothing processing on the image signal.

  M4, C4, Y4, and Bk4 frame sequential image signals obtained by these processes are sent to the printer control unit 109, converted into pulse-width-modulated pulse signals, and density recording by the printer unit B is performed. The CPU 214 controls the reader unit A and performs image processing according to a program stored in the ROM 216 using the RAM 215 as a work memory. The operator inputs instructions and processing conditions to the CPU 214 through the operation unit 217. A display 218 displays the operating state of the image processing apparatus, set processing conditions, and the like.

  Further, FIG. 6 is a block diagram showing a configuration example of the printer unit B. The printer control unit 109 includes a CPU 28, a ROM 30, a RAM 32, a test pattern storage unit 31, a density conversion circuit 42, an LUT 25, a laser driver 26, and the like, and can communicate with the reader unit A and the printer engine 100. The CPU 28 controls the operation of the printer unit B, and controls the grid potential of the primary charger 8 and the developing bias of the developing device 3.

  The configuration of image processing will be described below. FIG. 7 is a block diagram illustrating a configuration example of an image processing unit for obtaining a gradation image.

  The luminance signal of the image obtained by the CCD 105 is converted into a frame sequential density signal in the image processing unit 108. The converted density signal is corrected by the LUT (γLUT) 25 so that the density signal of the original image matches the density of the output image so as to be a signal corresponding to the gamma characteristic of the printer at the initial setting. Is done.

  FIG. 8 is a quadrant chart showing how gradation is reproduced. The first quadrant is the reading characteristic of the reader unit A that converts the density of the original image into a density signal, the second quadrant is the conversion characteristic of the LUT 25 for converting the density signal into a laser output signal, and the third quadrant is the laser output. The recording characteristics of the printer unit B that converts the signal into the density of the output image, the fourth quadrant shows the relationship between the density of the original image and the density of the output image, respectively, and the total gradation reproduction of the image processing apparatus shown in FIG. Show properties. In the case of processing with an 8-bit digital signal, the number of gradation levels is 256.

  In order to make the gradation characteristics of the image processing apparatus total, that is, the gradation characteristics in the fourth quadrant linear, the non-linear printer characteristics in the third quadrant are corrected by the LUT 25 in the second quadrant. The image signal whose tone characteristics are converted by the LUT 25 is converted into a pulse signal corresponding to the dot width by a pulse width modulation (PWM) circuit 26a of the laser driver 26, and an LD driver for controlling on / off of the laser light source 110. 26b. In the embodiment, the tone reproduction method using pulse width modulation is used for all colors Y, M, C, and Bk.

  Then, an electrostatic latent image having a predetermined gradation characteristic in which the gradation is controlled by the change in the dot area is formed on the photosensitive drum 4 by the scanning of the laser light output from the laser light source 110. The gradation image is reproduced through the processes of development, transfer and fixing.

  A control system relating to stabilization of image reproduction characteristics of a system including both the reader unit A and the printer unit B will be described below as image control in a sequence different from normal image formation for forming an image on recording paper. First, a control system for calibrating the printer unit B using the reader unit A will be described. FIG. 9 is a flowchart showing an example of calibration, which is realized by the cooperation of the CPU 214 that controls the reader unit A and the CPU 28 that controls the printer unit B. When the operator presses a mode setting button “automatic gradation correction” provided on the operation unit 217, for example, the calibration shown in FIG. 9 starts.

  As the contrast potential when forming the test print 1, a standard potential corresponding to the environment is registered as an initial value and used. The image processing apparatus includes a plurality of recording paper cassettes, and a plurality of types of recording paper sizes such as B4, A3, A4, and B5 can be selected. However, the recording paper used in this control uses so-called large-size paper, that is, B4, A3, 11 × 17, or LGR, in order to avoid errors in the vertical reading and horizontal setting in the subsequent reading operation. Is set.

  The test pattern 1 shown in FIG. 10 includes a belt-like pattern 61 having intermediate gradation densities for four colors Y, M, C, and Bk. By visually inspecting this pattern 61, it is confirmed that there are no streaky abnormal images, density unevenness, and color unevenness. The sizes of the patch pattern 62 and the gradation patterns 71 and 72 shown in FIG. 11 are set so as to fall within the reading range of the CCD sensor 105 in the thrust direction.

  If an abnormality is found by visual inspection, it is necessary to print test print 1 again, and when an abnormality is recognized again, it is necessary to perform maintenance by calling a service man call, that is, a service man. It is also possible to read the belt pattern 61 with the reader unit A and automatically determine whether or not to perform subsequent control based on the density information in the thrust direction.

  On the other hand, the patch pattern 62 is a maximum density patch for each color of Y, M, C, and Bk, that is, a patch pattern corresponding to the density signal value 255.

  Next, the operator places the test print 1 on the platen glass 102 and presses a “read” button.

  In order to convert the RGB value obtained from the patch pattern 62 into an optical density, the following equation is used. In order to obtain the same value as a commercially available densitometer, adjustment is made with a correction coefficient k. Alternatively, a separate LUT may be prepared to convert RGB luminance information into MCYBk density information.

M = −km × log ₁₀ (G / 255)
C = −kc × log ₁₀ (R / 255)
Y = −ky × log ₁₀ (B / 255) (2)
Bk = −kk × log ₁₀ (G / 255)
Next, a method for correcting the maximum density from the obtained density information will be described. FIG. 12 is a diagram showing the relationship between the relative drum surface potential of the photosensitive drum 4 and the image density obtained by the above calculation.

Contrast potential at the time of printing the test print 1 (developing bias potential and photosensitive drum 4 that is photosensitive by the laser beam modulated with the maximum signal value (255 for 8 bits) after the photosensitive drum 4 is primarily charged) the difference between the surface potential of) the in a of FIG. 12, the density obtained from patch pattern 62 is D _a.

  In the maximum density region, the image density with respect to the relative drum surface potential almost corresponds to linear as shown by the solid line L in FIG. However, in the two-component development system, when the toner density in the developing device 3 fluctuates and decreases, the image density with respect to the relative drum surface potential may become non-linear in the maximum density region as shown by the broken line N in FIG. is there. Accordingly, in the example of FIG. 14, the final maximum density target value is set to 1.6, but the control target value is set to 1.7 with the maximum density control target value set to 1.7 in consideration of a margin of 0.1. decide. The contrast potential B here is obtained from the following equation.

B = (A + Ka) × 1.7 / DA (3)
In equation (3), Ka is a correction coefficient, and it is preferable to optimize the value depending on the type of development method.

  If the contrast potential of the electrophotographic system is not set according to the environment, the density of the original image and the output image will not match, and the output of the environmental sensor 33 that monitors the moisture content in the apparatus described above (that is, the absolute moisture content) will be used. Based on this, a contrast potential corresponding to the maximum density is set as shown in FIG.

  Therefore, in order to correct the contrast potential, the correction coefficient Vcont.ratel shown in the following equation is stored in a backed-up RAM or the like.

Vcont.ratel = B / A
The image processing apparatus monitors the amount of water in the environment every 30 minutes, for example. Then, A × Vcont.ratel is calculated every time the value of A is determined based on the moisture amount detection result to obtain the contrast potential.

  Next, a method for obtaining the charging potential and the developing bias potential from the contrast potential will be briefly described. FIG. 14 is a diagram showing the relationship between the charging potential and the surface potential of the photosensitive drum 4.

The charging potential is set to −200 V, and the surface potential V _L of the photosensitive drum 4 exposed with the laser beam modulated with the minimum signal value, and the photosensitive drum 4 exposed with the laser beam modulated with the maximum signal value. The surface potential V _H is measured by the surface potential sensor 12. Similarly, _VL and _VH are measured when the charging potential is -400V. Then, the relationship between the charging potential and the surface potential is obtained by interpolating and extrapolating -200 V data and -400 V data. Note that the control for obtaining the potential data is referred to as potential measurement control.

Next, the developing bias V _DC is set by providing a difference between V _L and V bg (for example, 100 V) set so that toner fog does not occur in the image. Contrast potential Vcont is a differential voltage of the developing bias V _DC and V _H, the maximum concentration as Vcont is large is increased as described above.

  The charging potential and the developing bias for obtaining the contrast potential B obtained by calculation can be obtained from the relationship shown in FIG. Therefore, the CPU 28 obtains the contrast potential so that the maximum density is 0.1 higher than the final target value, and determines the grid potential and the development bias potential so that the contrast potential can be obtained (S53).

  Next, it is determined whether or not the determined contrast potential is within the control range (S54). If it is out of the range, it is determined that there is an abnormality in the developing device 3 or the like, and the developing device 3 of the corresponding color is determined. Set an error flag to be checked. The state of the error flag can be viewed by a serviceman in a predetermined service mode. Further, in the case of an abnormality, the contrast potential is corrected as close as possible within the control range, and the control is continued (S55).

  The CPU 28 controls the charging potential and the developing bias so that the contrast potential set in this way is obtained (S56).

FIG. 15 is a diagram showing density conversion characteristics after control. In the embodiment, the printer characteristic in the third quadrant becomes a solid line J by the control for setting the maximum density higher than the final target value. If such control is not performed, there is a possibility that the printer characteristics such as the broken line H where the maximum density does not reach 1.6 will be obtained. When the printer characteristic is a broken line H, the maximum density cannot be increased by the LUT 25. Therefore, the density region between the density _DH and 1.6 cannot be reproduced regardless of how the LUT 25 is set. As shown by the solid line J, if the printer characteristic is slightly higher than the maximum density, the density reproduction range is guaranteed by the correction of the LUT 25 as shown in the total gradation characteristic of the fourth quadrant.

  Next, a print start button for test print 2 appears. When the operator presses the “test print 2” button, the test print 2 shown in FIG. 11 is printed out (S57). As shown in FIG. 11, the test print 2 is composed of gradation patches of 4 × 16 (64 gradations) patches for each of Y, M, C, and Bk colors. The 64 gradations are allotted to the low density area among the 256 gradations, and the high density area is thinned out. This is in order to adjust the gradation characteristics particularly in the highlight portion.

  In FIG. 11, a patch pattern 71 is a patch group having a resolution of 200 lpi (line / inch), and a patch pattern 72 is a patch group having a resolution of 400 lpi. Image formation at each resolution is realized by preparing a plurality of periods of signals such as triangular waves used for comparison with the image signal to be processed in the pulse width modulation circuit 26a.

  Note that the image processing apparatus according to the embodiment forms a gradation image such as a photographic image at 200 lpi and a character or line drawing at 400 lpi based on the output signal of the black character determination unit described above. Patterns with the same gradation level may be output at these two types of resolution, but if the difference in resolution greatly affects the gradation characteristics, it is preferable to output a pattern with gradation levels according to the resolution. .

  The test print 2 is printed based on the image signal generated from the pattern generator 29 without operating the LUT 25.

  The test print 2 is placed on the platen glass 102 in the same manner as the test print 1, and the density of the patch patterns 71 and 72 is read (S58).

  As shown in FIG. 16, the reading value of one patch (for example, patch 73 shown in FIG. 11) takes 16 points inside the patch and averages the values obtained by reading the 16 points. The number of reading points is preferably optimized by the reading device and the image forming apparatus.

  FIG. 17 is a diagram showing the relationship between the output density obtained by converting the RGB signal obtained from each patch into a density value by the optical density conversion method described above and the laser output level (value of the image signal). . Then, as shown by the vertical axis on the right side of FIG. 17, the background density (for example, 0.08) of the recording paper is set to 0 level, and the target value 1.60 of the maximum density is normalized to 255 level. If the density of the read patch is specifically high as shown by point C in FIG. 17 or specifically low as shown by point D, the stain on the platen glass 102 The test pattern may be defective. In that case, in order to maintain the continuity of the data string, the inclination of the data string is limited and corrected. For example, when the slope of the data string exceeds 3, the slope is fixed to 3, and the data with a negative slope is set to the same value as that of the one low density patch.

  A conversion characteristic opposite to that shown in FIG. 17 may be set in the LUT 25 (S59). That is, the density level (vertical axis in FIG. 21) may be set to the input level (density signal in FIG. 8), and the laser output level (horizontal axis in FIG. 17) may be set to the output level (laser output signal in FIG. 8). For levels that do not correspond to patches, values are obtained by interpolation. At this time, a condition is set so that a zero input level is a zero output level.

  This completes the control of the contrast potential and the creation of the gamma conversion table by the control system.

  Next, the gradation correction performed after the above control or simultaneously with the above control will be described. FIG. 18 shows the relationship between the applied toner amount and the transfer efficiency. FIG. 18 shows an environmental difference (low humidity environment (23 ° C., 5% RH) and high humidity environment (30 ° C., 80% RH)), the initial stage of the developer in the high humidity environment (just after opening), and the initial stage (sufficient humidity control). ), And after the endurance. As shown in FIG. 18, the transfer efficiency is lowered particularly when the amount of the applied toner in the highly-humidified environment and the durability agent after durability is small. Therefore, only the above-described control causes the following problems.

  For a single color image, it is possible to accurately match each gradation by performing the above-described control. However, for a multi-order color in which a plurality of colors are overlapped, the transfer efficiency is increased by the toner application amount shown in FIG. Since it changes, the density and color become unstable. The correction will be described below. For example, when image formation of image data of secondary color signals Y = 48/256 and M = 48/256 is performed, for Y (yellow) toner that first forms an image on paper, the γLUT guided by the above-described control is used. This is achieved by employing the correction (FIG. 18 (1)). However, when M (magenta) toner is transferred onto this Y (yellow) image, it is difficult to cause transfer loss in the low density region. This is different from the γLUT correction (FIG. 18 (2)) obtained with a single color. In particular, when there is toner on the paper in advance, it is difficult for transfer loss to occur. Therefore, in accordance with the amount of toner formed in the lower layer, the second correction control is further performed on the γLUT correction obtained by the above control. . The second correction control will be described below.

A pattern image having a plurality of gradations is formed on the photosensitive drum 1. When the recording material P conveyed by the transfer belt 51 reaches the transfer portion, a DC bias is applied to the transfer roller 5 to transfer the pattern image onto the recording material. Around the photosensitive drum 1, and on the upstream side of the transfer drum in the rotation direction of the photosensitive drum 1, a first toner applied amount detecting means 90 is disposed, and on the downstream side, a second toner applied amount detecting means 91 is provided. A signal representing the toner applied amount X on the photosensitive drum before tone transfer and the toner applied amount Y on the photosensitive drum after transfer is detected. The relationship between the applied toner amount and the actually detected signal value is determined by the sensor. FIG. 19 shows a relational expression in the sensor employed in this embodiment. The transfer efficiency η is calculated from the values α and β detected by the sensor as follows: Transfer efficiency η = 1−β / α
FIG. 20 shows the relationship between the toner density after transfer obtained in a certain state and the transfer efficiency. The relationship shown in FIG. 20 is written in the RAM of the main body and becomes a correction function that is a feature of this embodiment.

  Now, a specific correction method will be described.

  When certain pixel data after γLUT correction is input, Y (yellow) toner is the first image formation on paper, and therefore the γLUT obtained by the first control is adopted. Next, when an M (magenta) toner image is formed, the Y (yellow) image has already been formed, so transfer loss in the low density area becomes slight, and the correction shown in FIG. Do. Further, when a C (cyan) toner image is formed, since an image of a Y (yellow) toner image and an M (magenta) toner image has already been formed, it is advantageous for strong transfer loss. Perform the correction of 2). Further, when an image is formed with Bk (black) toner, the correction shown in FIG.

  In this embodiment, in order to simplify the control, correction is performed by using the following equation.

After correction γLUT = γLUT correction value (density signal) / η (density signal) / δ
(Where δ ≦ 1, δ is the amount of toner already formed on the paper)
By performing correction control as described above, it is possible to suppress density fluctuations and color fluctuations of multi-order colors in low density areas, which could not be obtained by conventional calibration, and environmental fluctuations and durability fluctuations of the developer. Regardless of this, a stable image can be obtained.

(Second embodiment)
FIG. 23 schematically shows an image forming apparatus according to the second embodiment of the present invention. The image forming apparatus of the present invention is a full-color electrophotographic image forming apparatus having four photosensitive drums and using an intermediate transfer member. Hereinafter, the image forming apparatus of the present invention will be described in detail.

  As shown in FIG. 23, this image forming apparatus has four process units as image forming means having a charging means, an exposure means, a developing device, a cleaner, etc. around a photosensitive drum as a latent image carrier. The image on the photosensitive drum formed by each process unit is sequentially transferred onto the intermediate transfer member that moves and passes adjacent to the photosensitive drum, and the image transferred onto the intermediate transfer member is further transferred to the first image. In the second transfer portion, the image is transferred to a recording material such as paper.

  Details of the image forming apparatus according to the present embodiment will be described below.

  Photosensitive drums 1a, 1b, 1c, and 1d are arranged in the process units Pa, Pb, Pc, and Pd that form images of yellow, magenta, cyan, and black, respectively. It is free to rotate. Further, around each photosensitive drum 1a, 1b, 1c, 1d, charging means 2a, 2b, 2c, 2d, exposure means 3a, 3b, 3c, 3d, developing devices 4a, 4b, 4c, 4d, and a cleaner 6a, 6b, 6c and 6d are sequentially arranged along the rotation direction of the photosensitive drum. Since the configuration of the process unit conforms to the configuration of the first embodiment, detailed description thereof is omitted here.

  An intermediate transfer unit 5 is disposed below each photosensitive drum. The intermediate transfer unit 5 includes an intermediate transfer belt 51, transfer rollers 53a, 53b, 53c, and 53d, secondary transfer rollers 56 and 57, and an intermediate transfer belt cleaner 55.

  In the printer as described above, the toner images of the respective colors formed on the photosensitive drums 1a, 1b, 1c, and 1d receive a transfer bias from the transfer roller 53 facing each other with the intermediate transfer belt 51 interposed therebetween, and sequentially the intermediate transfer belt. 51 is transferred to the secondary transfer section along with the rotation of the belt. On the other hand, by this time, the recording material P taken out from the paper feed cassette 8 is supplied to the transport roller 82 through the pickup roller 81 and further transported to the left in FIG. The above-described toner image is transferred onto the recording material P by the secondary transfer bias applied to 56 and 57. The transfer residual toner on the intermediate transfer belt 51 is removed and collected by the transfer belt cleaner 55.

  The fixing device 7 includes a fixing roller 71 that is rotatably arranged and a pressure roller 72 that rotates while being pressed against the fixing roller 71. A heater 73 such as a halogen lamp is disposed inside the fixing roller 71, and the temperature of the surface of the fixing roller 71 is adjusted by controlling the voltage to the heater 73. In this state, when the recording material is conveyed, the fixing roller 71 and the pressure roller 72 rotate at a constant speed, and when the recording material P passes between the fixing roller 71 and the pressure roller 72, the both sides of the recording material P By being pressurized and heated at a substantially constant pressure and temperature, the unfixed toner image on the surface of the recording material is melted and fixed, and a full-color image is formed on the recording material P.

The intermediate transfer belt 51 is made of a dielectric resin such as PC, PET, PVDF. In this example, a PTFE resin having a volume resistivity of 10 ⁸ Ω · cm (using a probe conforming to the JIS-K6911 method, an applied voltage of 100 V, an application time of 60 sec, 23 ° C. and 50% RH) and a thickness of t = 100 μm was used. Other materials, volume resistivity, and thickness may be used.

The transfer roller 53 is composed of a core metal having a diameter of 8 mm and a conductive urethane sponge layer having a thickness of 4 mm. The resistance value of the transfer roller 53 is 50 mm / sec. It was calculated from the relationship of the current measured by rotating at high speed and applying a voltage of 500 V to the metal core, and the value was about 10 ⁵ Ω (23 ° C., 50% RH).

  In such an image forming apparatus, in this embodiment, the first toner density detection sensor and the second toner density detection sensor are provided on the upstream side and the downstream side in the photosensitive drum rotation direction of the transfer portion of the photosensitive drum, respectively. A predetermined 17-gradation patch image is transferred onto the intermediate transfer belt. At this time, the amount of applied toner before single-color transfer and the amount of applied toner after transfer are detected by the two toner density detecting means. The transfer efficiency for each density is calculated from the toner density before transfer and the toner density after transfer.

  Further, the patch image density of each color is detected by the sensor 60 on the intermediate transfer belt.

  Here, the sensor 60 performs the same operation as the density detection on the recording material of the first embodiment. Therefore, first, γLUT correction is derived from the detected density read by the sensor 60.

  Further, the toner density formed on the photosensitive drum and the residual toner remaining on the photosensitive drum after being transferred onto the intermediate transfer belt are detected by the sensors 90a to 90d and 91a to 91d, and the transfer efficiency for each density is calculated. Second correction control for further correcting the γLUT correction is performed.

  As described above, it is possible to prevent density fluctuations and color fluctuations generated by the primary transfer unit of an image forming apparatus employing an intermediate transfer member, and it is stable regardless of environmental fluctuations and durability fluctuations of the developer. An image can be obtained.

(Third embodiment)
Since the image forming apparatus described in this embodiment is the same as that in the second embodiment, description of overlapping portions is omitted.

  In this embodiment, it will be described that the same effect as in the above embodiment can be obtained by feeding back the relationship of the secondary transfer efficiency depending on the toner density to the γLUT correction.

  In the case of the secondary transfer of the image forming apparatus including the intermediate transfer member such as the image forming apparatus shown in FIG. 24, the image forming apparatus has a transfer process for transferring the single color image and the multi-color image at once. Compared to one embodiment, more transfer charge is required at one time. Therefore, when a transfer charge necessary for transferring a high-density multi-order color is given, transfer loss in a low density region is more likely to occur than in the above two embodiments.

  In the γLUT correction obtained by the single color gradation control, the color tone of the original image and the actually output image may be greatly different when the multi-order color is used.

  Therefore, in the case of the image forming apparatus of the present embodiment, the γLUT correction is further corrected based on the relationship between the toner density and the transfer efficiency, so that the density stability, the color can be improved regardless of the environmental fluctuation and the durability fluctuation of the toner state. An image forming apparatus having excellent taste stability can be provided.

1 is a schematic configuration diagram of an image forming apparatus according to a first embodiment of the present invention. 1 is a schematic configuration diagram of a process unit of an image forming apparatus according to a first embodiment of the present invention. Relationship between toner density and transfer efficiency. Schematic of the leader part. The block diagram which shows the flow of the image signal in a leader image processing part. FIG. 3 is a block diagram illustrating a configuration example of a printer unit. The block diagram which shows the structural example of the image process part for obtaining a gradation image. A four-limit chart showing how gradation is reproduced. The flowchart which shows an example of calibration. The figure which shows the example of the test print 1. FIG. The figure which shows the example of the test print 2. FIG. FIG. 4 is a diagram illustrating a relationship between a relative drum surface potential of a photosensitive drum and an image density. Absolute water content and contrast potential. FIG. 6 is a relationship diagram between a charging potential and a surface potential. Density conversion characteristics after control. The figure explaining a patch density reading point. The figure which shows the relationship between the density | concentration read from the test print 2, and a laser output level. Difference between lower layer toner density and γLUT data characteristics. The figure showing the relationship between the photosensor output and image density when changing patch density in steps. Relationship between toner density formed in the lower layer and transfer efficiency. The block diagram which shows the flow of the image signal in the leader image processing part in a 1st Example. ΓLUT data characteristics obtained by further correcting the toner density and γLUT correction formed in the lower layer. Schematic of an image forming apparatus for explaining second and third embodiments. Schematic showing the relationship between the toner density on the intermediate transfer belt and the transfer efficiency.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1a-1d, 13, 1 Photosensitive drum 2a-2d, 23 Primary charger 3a-3d Exposure means 4a-4d, 41, 4 Developing device 53a-53d, 531, 532, 5 Transfer means 51 Transfer belt 56 Intermediate transfer belt

Claims (7)

  A toner image carrier that forms a toner image on the surface in accordance with image data; and a transfer member that transfers a plurality of different color toner images on the image receiving member in a transfer portion, A first detection means for detecting image characteristics on the image receiver, and a first pattern image for detecting image characteristics is formed on the image receiver by a sequence different from normal image formation; In the image forming apparatus having a control unit for controlling an image forming condition on the image carrier based on the first pattern image characteristic detected by the first detection unit, the transfer unit according to a different toner density And a correction means for correcting the image forming conditions controlled by the control means based on the relationship between the toner density and the transfer efficiency at the transfer portion. An image forming apparatus comprising.   A toner image carrier that forms a toner image in which toners of different colors are superimposed on the surface according to image data; and a transfer member that transfers the toner image on the toner image carrier to the image receiver at the transfer unit. A first detection unit configured to detect image characteristics on the body, and forming a first pattern image on the receiver for detecting the image characteristics by a sequence different from normal image formation; In an image forming apparatus having a control unit that controls image forming conditions on the image carrier based on the first pattern image characteristic detected by one detection unit, the transfer unit according to a different toner density A means for deriving a relationship with the transfer efficiency, and a correcting means for correcting the image forming condition controlled by the control means based on the relationship between the toner density and the transfer efficiency at the transfer portion. An image forming apparatus symptoms.   The image forming apparatus according to claim 1, wherein the image forming condition is a density correction characteristic of the image data.   A transfer unit that transfers a pattern image having a different toner density formed on the toner image carrier onto the image receiver in a sequence different from that of normal image formation. And a first applied toner amount detecting means disposed on the upstream side in the moving direction of the toner image carrier from the transfer portion, and a second applied toner amount disposed on the downstream side in the moving direction of the toner image carrier from the transfer portion. The image forming apparatus according to claim 1, wherein the image forming apparatus is derived from values of the first detection signal and the second detection signal obtained by the detection unit.   5. The image forming apparatus according to claim 1, wherein the toner image carrier is a photoconductor, and the image receiver is a recording material.   5. The image forming apparatus according to claim 1, wherein the toner image carrier is a photosensitive member, and the image receiver is an intermediate transfer member.   5. The image forming apparatus according to claim 2, wherein the toner image bearing member is an intermediate transfer member, and the image receiving member is a recording material.

Images