JP2007271866A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To achieve accurate image stabilization control and to stabilize an image by performing optimum control by using two different image processing patch images which can be formed simultaneously. <P>SOLUTION: A plurality of optical concentration detection sensors 40a and 40b having the same characteristic are arranged in the same thrust direction, and a sensor arranged at the position of a potential detection means is exclusively used as a sensor for controlling developer concentration. Furthermore, such control is performed that a reference image for controlling the developer concentration is not formed simultaneously with that for γ LUT correction. Thus, the image stabilization is achieved by the accurate image stabilization control. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an electrophotographic image forming apparatus (hereinafter simply referred to as “image forming apparatus”) such as an electrophotographic copying machine, a facsimile machine, a laser beam printer, and a multifunction machine.

  There are several methods for stabilizing the image density of the image forming apparatus. One is a method (image control method) for adjusting image processing characteristics. A reference image (patch image) with a specific pattern on an image carrier such as an electrophotographic photosensitive drum (hereinafter simply referred to as “photosensitive drum”) after the warm-up operation is completed by starting the apparatus main body or during the output operation. Create an image. The quality of the formed image is stabilized by changing the operation of a circuit that reads the density of the patch image and determines image forming conditions such as a γ correction circuit based on the read density value. Regarding this, there is an image forming apparatus previously proposed by the present applicant (see, for example, Patent Document 1).

  One is a method of correcting image forming conditions. When the operation of the apparatus main body has been performed for a long period of time, as in Patent Document 1, the density obtained by reading a specific pattern of a patch image formed on an image carrier such as a photosensitive drum is actually printed out. In some cases, the density of the recorded image does not match. To prevent this, instead of creating a patch image pattern on the image carrier, a patch image with a specific pattern is formed on a sheet such as recording paper, and the density value of the pattern on the sheet is read. Based on this, the image forming conditions are corrected.

  One is a method of adjusting the toner density in the developing device. If there is a density difference in a patch image of a specific pattern formed on an image carrier such as a photoconductor drum, how much can the patch image density be returned to the initial density by the output signal of the density difference? Calculate whether the amount of toner is insufficient or excessive. From the calculation result, the replenishment amount for replenishing the toner to the developing device without excess or deficiency is performed by the patch detection ATR control. This is also a general method for stabilizing the image density.

  Further, in recent years, as one of such image density stabilization methods, a patch potential control system that reduces the change in image density as the durable light sensitivity characteristic of the photosensitive drum changes and the density of the patch image changes. There is one previously proposed by the present applicant (see, for example, Patent Document 2).

Japanese Patent Laid-Open No. 2003-195583 JP 2002-62697 A

  However, each of the conventional image density stabilization methods has the following problems.

  In the case of Patent Document 1, it is necessary to read an image when correcting image processing characteristics during an image forming operation. In order to correct such image processing characteristics with high accuracy, it is necessary to increase the frequency with which patch images are formed. However, there is a limit to increasing the formation frequency, and if the formation frequency is set to be suitable for the control system, there is a problem in that it overlaps in terms of timing and there is a problem that productivity is lowered.

  In the toner density adjustment method in the developing device, it is also necessary to read a patch image, and in order to perform high-precision density adjustment, it is required to increase the frequency of patch image formation. However, in this case as well, in order to achieve the optimum formation frequency, the same disadvantages and problems as described above occur.

  On the other hand, in the patch potential control method, in order to guarantee the patch potential, a method of controlling by changing the exposure intensity (laser power) as the exposure condition is preferable in consideration of development characteristics. However, in this case, in order to achieve both patch frequency and productivity, patch detection is performed at a very small interval (for example, 10 msec) between the preceding and subsequent sheets (paper interval). Therefore, it is very difficult to control the exposure intensity.

  As described above, an object of the present invention is to perform image stabilization control with higher accuracy by optimally performing control using two different image processing patch images, thereby enabling image stabilization. Is to provide.

In order to achieve the above object, an image forming apparatus of the present invention comprises an image carrier,
Exposure means for forming an electrostatic image by exposing and scanning the image carrier in the main scanning direction;

Developing means for developing the electrostatic image formed on the image carrier with a plurality of developing devices having different color developers;
Transfer means for sequentially transferring developer images of different colors on the image carrier to a transfer medium to form a color image;
An image characteristic detecting image forming means for forming an electrostatic image of an image characteristic detecting pattern on the image carrier and developing the image with the developer to form a developer image for image characteristic detection;
First optical detection means for detecting the density of the developer formed by the image characteristic detection image forming means;
Control means for controlling image forming conditions during image formation according to the detection result of the first optical detection means;
On the image carrier, an electrostatic image of the developer density detection pattern is formed with an exposure amount different from that at the time of forming the electrostatic image of the image characteristic detection pattern, and developed with the developer to detect the developer density. Image forming means for detecting developer density for forming a developer image for
A second optical detection means that is provided at a different position in the main scanning direction with respect to the first optical detection means and detects the density of the developer image for detecting the developer density;
Replenishment control means for controlling developer replenishment to the developing means according to the detection result of the second optical detecting means;
In an image forming apparatus having
In one color image forming operation, when the developer image for detecting the image characteristics and the developer image for detecting the developer density are formed, the developer image for detecting the image characteristics and the development of different colors are formed. And a developer image for detecting the agent concentration.

  According to the image forming apparatus of the present invention, image stabilization can be achieved with high-accuracy image stabilization control.

  DESCRIPTION OF EXEMPLARY EMBODIMENTS Hereinafter, a preferred embodiment of an image forming apparatus according to the invention will be described in detail with reference to the drawings.

  As shown in FIG. 1, the full-color image forming apparatus according to the present embodiment has a document table glass 102 on which a document 101 to be read is placed and set, and includes a light source 103 that irradiates the document 101. An image is 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 by a group of red, green, and blue CCD line sensors arranged in three rows. These reading optical system units scan in the direction of the arrow in the drawing to convert the image of the original 101 into an electric signal data string for each line. 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 subjected to image processing by the printer control unit 109.

  FIG. 2 is a functional block diagram showing the arrangement of the reader image processing unit 108 in the reader unit A arranged based on the flow of image signals. The image signal output from the CCD sensor 105 is input to the analog signal processing unit 201. Therefore, after the gain adjustment and the offset adjustment, the A / D converter 202 converts each color signal into 8-bit digital image signals R1, G1, and B1. Thereafter, the digital image signals R1, G1, and B1 are input to the shading correction unit 203, and known shading correction is performed using a read signal of the reference white plate for each color.

  The clock generator 211 generates a clock for each pixel. The main scanning address counter 212 counts the clocks from the clock generator 211 and generates a one-line pixel address output. The decoder 213 decodes the main scanning address from the main scanning address counter 212, and a VE signal and a line representing an effective area in the line-by-line CCD drive signal such as a shift pulse and a reset pulse and one line reading signal from the CCD sensor 105. A synchronization signal HSYNC is generated. Since the line sensors of the CCD sensor 105 are arranged at a predetermined distance from each other, the line delay circuit 204 in FIG. 2 corrects the spatial deviation in the sub-scanning direction. Specifically, the R and G signals are 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 is a part that converts the reading color space determined by the spectral characteristics of the R, G, and B filters of the CCD sensor into the NTSC standard color space, and performs matrix calculation as shown in the following equation. The light quantity / density conversion unit (LOG conversion unit) 206 is configured by a reference table ROM, and the luminance signals of R4, G4, and B4 are converted into density signals of C0, M0, and Y0. The line delay memory 207 is a black character determination unit (not shown) and delays the image signals of C0, M0, and Y0 by the amount of line delay from the R4, G4, and B4 signals to the determination signals such as UCR, FILTER, and SEN. Let The masking / UCR circuit 208 extracts a black signal (Bk) from the input three primary color signals Y1, M1, and C1. The masking / UCR circuit 208 further performs an operation for correcting the color turbidity of the recording color material in the printer unit B, and sequentially outputs the signals Y2, M2, C2, and Bk2 with a predetermined bit width (8 bits) for each reading operation. ). The γ correction circuit 209 performs density correction in the reader unit A so as to match the ideal gradation characteristics of the printer unit B. A spatial filter processing unit (output filter) 210 performs edge enhancement or smoothing processing. The M4, C4, Y4, and Bk4 frame sequential image signals processed as described above are sent to the printer control unit 109, and the printer unit B performs density recording by PWM.

  The CPU 214 controls the reader unit A, has a RAM 215 and a ROM 216 for storing and storing various data, and the operation unit 217 has a display 218 such as a monitor screen.

  Next, the configuration of the printer unit B will be described with reference to FIG. The photosensitive drum 4 is uniformly charged by applying a charging bias voltage by the primary charger 8. The image data is converted into laser light through a laser driver and laser light source 110 included in the printer image processing unit 109. The laser light is reflected by the polygon mirror 1 and the mirror 2 and uniformly charged. 4, and a latent image is formed on the photosensitive drum 4. The photosensitive drum 4 on which the latent image is formed by scanning with the laser light rotates in the direction of the arrow in the figure. Then, development for each color is sequentially performed by the developing device 3. Note that the scanning direction of the laser light substantially coincides with the rotation axis direction of the photosensitive drum.

  In this embodiment, a two-component system is used as a developing method, and a developing device 3 for each color is provided around the photosensitive drum 4 from the upstream side to black (Bk), yellow (Y), cyan (C), magenta ( M) are arranged in this order. Then, the developing device 3 corresponding to the image signal performs a developing operation at a timing for developing the latent image area formed on the photosensitive drum 4.

  Further, the sheet material 6 as the transfer medium is wound around the transfer drum 5 as the transfer means and rotated once in the order of M, C, Y, and Bk, and is rotated four times in total to form on the photosensitive drum. The toner images of the respective colors are sequentially transferred onto the sheet material 6. Thereby, one color image is formed. When the transfer of the toner image onto the sheet material 6 is completed, the sheet material 6 is separated from the transfer drum 5 and the toner image is fixed on the sheet material 6 by the pair of fixing rollers 7 to complete a full color image print. A surface potential sensor 12 and a cleaner 9 for cleaning the transfer residual toner on the photosensitive drum 4 are disposed on the upstream side of the developing device 3 of the photosensitive drum 4.

  Further, as shown in FIG. 3, optical sources using light sources 10a and 10b by LEDs and photodiodes 11a and 11b for detecting a reflected light amount of a later-described toner patch pattern formed on the photosensitive drum 4 are used. Density sensors are provided as regular reflection type density sensors 40a and 40b in the same thrust direction. The density sensors 40a and 40b are of a type that detects the patch density by increasing or decreasing the amount of specularly reflected light from the image carrier. In the present embodiment, toner that scatters projection light onto the mirror-like photosensitive drum 4 is used. As described later, the sensor output decreases, and the patch density is detected. If the sensor determines density based on increase / decrease of specular reflection light, a specular reflection type optical density detection sensor is used even if the sensor light receiving portion is configured to receive scattered light.

  Next, FIGS. 4A and 4B show a configuration example of the developing device 3. In FIG. 4A, the developing device 3 is arranged to face the photosensitive drum 4, and the inside thereof is divided into a first chamber (developing chamber) 52 and a second chamber (stirring chamber) by a partition wall 51 extending in the vertical direction. 53. A nonmagnetic developing sleeve 54 that rotates in the direction of the arrow is disposed in the first chamber 52, and a magnet 55 is fixedly disposed in the developing sleeve 54. Further, as shown in FIG. 4B, a toner replenishing tank 60 containing replenishing toner 63 is attached to the upper part of the developing device 3, and a toner conveying screw 62 is disposed in the lower part of the toner replenishing tank 60. Is installed. The toner conveying screw 62 is rotationally driven by a motor 70 connected via a gear train 71, whereby the toner 63 in the replenishing tank 60 is conveyed and supplied into the developing device 3. The supply of toner by the conveying screw 62 is controlled by the CPU 28 controlling the rotation of the motor 70 via the motor driving circuit 69. The RAM 68 connected to the CPU 28 stores control data supplied to the motor drive circuit 69 and the like.

  Developer stirring screws 58 and 59 are disposed in the first chamber 52 and the second chamber 53, respectively. The developer agitating screw 58 agitates and conveys the developer in the first chamber 52, and the developer agitating screw 59 includes the toner 63 supplied from the toner replenishing tank 60 by the rotation of the conveying screw 62 and the inside of the developing device 3. The developer in the container is agitated and conveyed to make the toner concentration of the developer uniform. The partition wall 51 is formed with a developer passage (not shown) that allows the first chamber 52 and the second chamber 53 to communicate with each other at the front and back end portions in FIG. Then, the developer in the first chamber 52 in which the toner is consumed due to the development and the toner density is lowered due to the conveying force of the developer stirring screws 58 and 59 moves from one passage into the second chamber 53. Then, the developer whose toner density has been recovered in the second chamber 53 is configured to move into the first chamber 52 from the other passage.

  The two-component developer in the developing device 3 is carried on the developing sleeve 54 by the magnetic force of the magnet 55, the layer thickness is then regulated by the blade 56, and the development facing the photosensitive drum 4 as the developing sleeve 54 rotates. Conveyed to the area. The two-component developer includes at least a magnetic carrier and a toner. In the developing area, the developer is used for developing the latent image on the photosensitive drum 4. In order to improve the development efficiency (that is, the application rate of toner to the latent image), a development bias voltage obtained by superimposing a direct current voltage on an alternating current voltage is applied to the development sleeve 54 from a power source 57.

  By the way, developing the electrostatic latent image decreases the toner concentration of the developer in the developing device 3. For this reason, the toner replenishment amount for replenishing the toner 63 from the toner replenishing tank 60 to the developing device 3 is controlled to control the toner concentration of the developer in the developing device 3 to be constant, or to maintain the image density constant. Done.

  In the image forming apparatus of this embodiment, toner supply control by patch detection ATR and toner supply control by video count ATR are performed. That is, a patch image (reference image) is formed on the photosensitive drum 4 as a density control device. The patch inspection ATR method is a toner replenishment control method as described below. The density of the patch image is detected by a specular reflection type optical density detection sensor 40a configured by an LED light source 10a that is a light emitting unit disposed opposite to the photosensitive drum 4 and a photodiode 11a that is a light receiving unit. This is a control method in which the toner supply amount is calculated and supplied in accordance with the detection result. The video count ATR method is a control method in which the necessary toner amount is integrated from the output level of the digital image signal for each pixel and replenished. If the video count ATR system is roughly supplemented, image data output from the LUT 25 (to be described later) is captured by a video counter (not shown), so that the video count value has a substantially linear relationship with the toner consumption per image. It has. Therefore, the count value is supplied to the toner replenishment control unit of the CPU 28 and converted into toner consumption by the toner replenishment control unit (of course, it is desirable to have an optimal video count value-toner consumption amount table for various image processing). It is possible to perform toner supply control.

  Here, FIG. 6 is a block diagram showing an image signal processing circuit for obtaining a gradation image in the configuration of the image forming apparatus of the present embodiment.

  The printer control unit 109 includes a CPU 28, a ROM 30 and a RAM 32, a test pattern storage unit 31, a density conversion circuit 42, and an LUT 25, and can communicate with the reader unit A and the printer engine unit 100.

  The printer engine unit 100 includes two regular reflection type optical density detection sensors 40a and 40b composed of LED light sources 10a and 10b and photodiodes 11a and 11b arranged around the photosensitive drum 4, and a primary charger 8. The laser 110, the surface potential sensor 12, and the developing device 3 are controlled.

  The image forming apparatus also includes an environmental sensor 33 that measures the amount of moisture in the air in the apparatus. The surface potential sensor 12 is provided on the upstream side of the developing device 3, and the grid potential of the primary charger 8 and the developing bias of the developing device 3 are controlled by the CPU 28 as described later.

  The luminance signal of the image is obtained by the CCD 105 and converted into a frame sequential image signal by the reader image processing unit 108. The density characteristics of this image signal are converted by the LUT 25 (γLUT) so that the density of the original image and the density of the output image represented by the input image signal are the γ characteristics of the printer at the initial setting. . Note that LUT is an abbreviation for Look Up Table.

  FIG. 7 is a four-limit chart showing how gradation is reproduced. The first quadrant shows the reading characteristic of the reader unit A that converts the document density into the density signal, and the second quadrant shows the conversion characteristic of the LUT 25 for converting the density signal into the laser output signal. The third quadrant shows the recording characteristics of the printer unit B that converts the laser output signal into the output density, and the fourth quadrant shows the total tone reproduction characteristics of the image forming apparatus showing the relationship between the document density and the output density. . Note that the number of gradations is 256 gradations because processing is performed with an 8-bit digital signal.

  In this apparatus, in order to make the gradation characteristics of the fourth quadrant linear, the printer characteristics of the third quadrant are corrected by the LUT 25 of the fourth quadrant so that the printer characteristics are not linear. The LUT 25 is generated by a calculation result described later. After density conversion by the LUT 25, the pulse width modulation (PWM) circuit 26 converts the signal into a signal corresponding to the dot width and sends it to a laser driver 27 that controls ON / OFF of the laser.

  In the present embodiment, the tone reproduction method using pulse width modulation processing is used for all colors Y, M, C, and Bk. By scanning with the laser 110, a latent image having a predetermined gradation characteristic is formed on the photosensitive drum 4 due to a change in the dot area, and the gradation image is reproduced through a series of processes of development, transfer, and fixing.

  On the other hand, in the present embodiment, the main point is that each patch image used in two independent control systems is formed and read in the same thrust direction (drum shaft rotation direction or main scanning direction).

  A control system capable of high-precision control using patch images that can be simultaneously formed will be described below.

(First control system: γLUT correction)
This is γLUT correction control related to stabilization of image reproduction characteristics of a system including both the reader unit A and the printer unit B. First, calibration of the printer unit B using the reader unit A is shown in the flowchart of FIG. This flowchart is executed by the CPU 214 that controls the reader unit A and the CPU 28 that controls the printer unit B.

  This control is started by pressing a mode setting button for instructing automatic gradation correction provided on the operation unit 217. In step S51, as shown in FIG. 9A, the print start button 81 of the test print 1 appears on the display 218, and the image of the test print 1 shown in FIG. The As a sheet of printing paper used here, generally called large size paper is used in order to avoid errors due to mistakes in portrait orientation and landscape orientation during subsequent reading operations. That is, it is set to use B4, A3, 11 × 17, and LGR. The pattern 62 is the maximum density patch of each color of Y, M, C, and Bk, and uses 255 levels as the density signal value.

  In step S52, the image of the test print 1 is placed on the platen glass 102 as shown in FIG. 11, and reading is started by pressing the reading start button 91 shown in FIG. In order to convert the optical density from the obtained RGB values, the following equation (1) is used. In order to make it the same value as a commercially available densitometer, it is adjusted with a correction coefficient (k). Alternatively, RGB luminance information can be converted into M, C, Y, and Bk density information using an LUT.

M = −km * log 10 (G / 255)
C = −kc * log 10 (R / 255) (1)
Y = −ky * log 10 (B / 255)
Bk = −kbk * log 10 (G / 255)
Next, the maximum density is corrected as follows from the obtained density information. FIG. 13 is a graph showing the relationship between the relative drum surface potential and the image density obtained by the above calculation. Contrast potential used at this time, that is, the maximum density obtained by setting the difference from the surface potential of the photosensitive drum when the maximum level is applied using laser light after being primarily charged with the developing bias potential at a setting of A Suppose that it is DA. In that case, in the density range of the maximum density, the image density almost corresponds linearly as indicated by the solid line L with respect to the relative drum surface potential. However, in the two-component development system, when the toner density in the developing device fluctuates and falls, nonlinear characteristics may occur in the density range of the maximum density as indicated by the broken line N. Therefore, although the final target value for the maximum density is 1.6 here, the control amount is determined by setting 1.7 as the target value for the control to match the maximum density with a margin of 0.1. . The contrast potential B is obtained using the following equation (2).

B = (A + Ka) × 1.7 / DA (2)
Here, Ka is a correction coefficient, and it is preferable to optimize the value depending on the type of development method. Actually, the setting of the contrast potential A in the electrophotographic method does not match the image density unless it is changed according to the environment, and the output of the environmental sensor 33 for monitoring the moisture content in the machine described above is shown in FIG. ), The setting is changed as shown in the performance diagram shown in (b).

  When obtaining the grid potential and the developing bias potential from the contrast potential, FIG. 14B shows the relationship between the grid potential and the surface potential of the photosensitive drum. The surface potential VL is measured by the surface potential sensor 12 when the grid potential is set to −200 V and scanning is performed with the laser beam level being minimized. Similarly, VL and VH are measured when the grid potential is -400V. By interpolating and extrapolating -200V data and -400V data, the relationship between the grid potential and the surface potential can be obtained. Control for obtaining this potential data is referred to as potential measurement control.

  The developing bias VDC is set by providing a difference of Vbg (here, set to 100 V) set so that fog toner does not adhere to the image from VL. The contrast potential Vcont is a differential voltage between the developing biases VDC and VH, and the larger the Vcont, the larger the maximum density, as described above. In order to obtain the calculated contrast potential B, it is possible to calculate how many volts of grid potential and how many volts of development bias potential are necessary from the relationship shown in FIG.

  In step S55 of FIG. 8, the CPU 28 sets the grid potential and the developing bias potential so that such a contrast potential can be obtained. When the obtained contrast potential is abnormal, the limit control is applied to the value just below the control range for correction control, and the control is continued. The CPU 28 sets the grid potential and the developing bias potential so that the contrast potential calculated in step S53 is obtained.

  FIG. 15 shows a density conversion characteristic diagram. By the maximum density control in which the maximum density in the present embodiment is set higher than the final target value, the printer characteristic diagram in the third quadrant becomes a solid line J. If such control is not performed, there may be a printer characteristic that does not reach 1.6 as indicated by the broken line H. In the case of the characteristic of the broken line H, no matter how the LUT 25 is set, the LUT 25 does not have the ability to increase the maximum density, so the density between the density DH and 1.6 cannot be reproduced. If the maximum density is set slightly higher as indicated by the solid line J, the density reproduction range can be reliably guaranteed with the total gradation characteristics in the fourth quadrant.

  Next, in step S56 of FIG. 8, as shown in FIGS. 16A to 16E, the print start button 150 for the image of the test print 2 appears on the operation panel, and when it is pressed, FIG. The image of the test print 2 shown is printed out. During printing, the display is as shown in FIG.

  In this test print 2, as shown in FIG. 17, each of the colors Y, M, C, and Bk is composed of a gradation patch group for 64 gradations in a total of 4 columns and 16 rows. Here, the laser output level is preferentially assigned to the low density area out of the 256 gray scales, and the laser output level is thinned out in the high density area. By doing so, it is possible to satisfactorily adjust the gradation characteristics particularly in the highlight portion. In FIG. 17, reference numeral 71 denotes a patch having a resolution of 200 lpi (lines / inch), and 72 denotes a patch having a resolution of 400 lpi (lines / inch). In order to form an image of each resolution, the pulse width modulation circuit 26 can be realized by preparing a plurality of triangular wave cycles used for comparison with image data to be processed.

  In this apparatus, the gradation image is generated with a resolution of 200 lpi, and the line image of characters and the like is generated with a resolution of 400 lpi. Although the same gradation level pattern is output at these two types of resolution, when the gradation characteristics differ greatly due to the difference in resolution, it is more preferable to set the previous gradation level according to the resolution. . The test print 2 is generated from the pattern generator 29 without operating the LUT 25.

  18A and 18B are schematic views seen from above when the output of the test print 2 is placed on the platen glass 102. FIG. The upper left wedge-shaped mark T is a mark for abutting the document on the document table, and the Bk pattern is on the abutting mark T side. Moreover, as shown in FIG. So that the message is displayed on the operation panel.

  As shown in FIG. 18B, the reading points per patch indicated by reference numeral 73 in FIG. 17 take 16 reading points (x) inside the patch and average the obtained signals. An RGB signal obtained by averaging 16 points for each patch is converted into a density value by the above-described conversion method to optical density, and the laser output level is plotted on the horizontal axis as the output density as shown in FIG. Shown in the graph. As shown on the right vertical axis, the base density of paper (0.08 in this example) is normalized to 0 level, and 1.60 set as the maximum density of the image forming apparatus is normalized to 255 level. If the obtained data has a specific high density such as point C or low such as point D, the original platen glass 102 may be dirty or the test pattern may be defective. Therefore, correction is performed by applying a limiter to the slope so that continuity is preserved in the data string.

  As described above, the contents of the LUT 25 can be simply changed by changing the coordinates of the density level in FIG. 19 to the input level (density signal axis in FIG. 7) and the laser output level to the output level (laser output signal axis in FIG. 7). Can be created. For density levels that do not correspond to patches, values are obtained by interpolation calculation. At this time, the restriction condition is set so that the output level becomes 0 level with respect to the input level 0 level. In S58, the conversion content created as described above is set in the LUT 25.

  Thus, the contrast potential control using the reading device and the creation of the γ conversion table are completed. During the above-described processing, the display as shown in FIG. 16D is made, and when completed, the display is as shown in FIG.

  The control of the image forming conditions as described above is performed by the control means. By such control, the image forming apparatus is configured to have a linear characteristic with respect to the density signal. As a result, it is possible to suppress the density gradation characteristic variation for each machine and to set the standard state. Further, when this control is released to general users, the control is performed as necessary when it is determined that the gradation characteristics of the image forming apparatus have deteriorated. As a result, correction of the gradation characteristics of the system including both the reader / printer can be easily performed.

  Next, after controlling the gradation characteristics in the system including both the reader / printer, the printer unit B performs control relating to stabilization of the image reproduction characteristics independently as follows.

  In this control, the image characteristic detection image forming means detects the density of the patch pattern for image characteristic detection formed on the photosensitive drum 4 and corrects the LUT 25 to achieve image stabilization. The patch pattern uses 200 lpi for a gradation image. FIG. 20 is a flowchart of a processing circuit that processes a signal from the specular reflection type optical density detection sensor 40a including the LED light source 10b and the photodiode 11b facing the photosensitive drum 4. The regular reflection type optical density detection sensor 40a used here is the regular reflection type optical density detection sensor 40a of the two optical density detection sensors arranged at the same thrust direction position facing the photosensitive drum in this embodiment. Yes, it is configured to detect only regular reflection light from the photosensitive drum 4. Since this control corrects the γLUT, the effect clearly appears at the next image formation after patch formation.

  Near-infrared light from the photosensitive drum 4 incident on the regular reflection type optical density detection sensor 40a is converted into an electrical signal by the regular reflection type optical density detection sensor 40a, and the electrical signal is converted into an A / D conversion circuit 41. Thus, the output voltage of 0 to 5V is converted into a digital signal of 0 to 255 level, and is converted into a density by the density conversion circuit 42. The toners used here are Y (yellow), M (magenta), and C (cyan) toners. These toners are composed of a styrene copolymer resin as a binder and dispersed colorants of each color. .

  The photosensitive drum 4 is an OPC drum, and the near-infrared light reflectance (960 nm) is about 40%, and may be composed of an amorphous silicon drum or the like as long as the reflectance is comparable. In addition to the photosensitive drum, a belt-shaped photosensitive belt having a photosensitive layer on the surface can be used as the image carrier. FIG. 21 shows the relationship between the output of the regular reflection type optical density detection sensor 40b and the output image density when the density on the photosensitive drum 4 is changed stepwise in the area gradation of each color. In this case, the output of the specular reflection type optical density detection sensor 40b in a state where the toner is not attached to the photosensitive drum 4 is set to 5V (255 level). As the area coverage by each toner increases and the image density increases, the output of the specular reflection type optical density detection sensor 40b becomes smaller than that of the photosensitive drum 4 alone. From these characteristics, by having the table 42a for converting the sensor output signal dedicated to each color into the density signal, the density signal can be read with high accuracy for each color.

  Since the purpose of this control system is to maintain stable color reproducibility achieved by the control system including the reader / printer, the state immediately after the end of control by the control system including the reader / printer is set as the target value. FIG. 22 is a flowchart up to the target value setting. When the control of the system including both the reader / printer is finished, a patch pattern for each color of Y, M, C, and Bk is formed on the photosensitive drum 4 and detected by the regular reflection type optical density detection sensor 40b. . The laser output of the patch uses 128 levels for each color in the density signal (density signal axis in FIG. 7). At this time, the contents of the LUT 25 and the setting of the contrast potential are obtained by a control system including a reader / printer. FIG. 23 shows a sequence for forming a patch on the photosensitive drum 4.

  That is, since the photosensitive drum 4 having a large diameter is used, the eccentricity of the photosensitive drum 4 is taken into account in order to obtain density data accurately and efficiently in a short time. A patch of the same color is formed at a position 180 ° opposite to the photosensitive drum 4 and measured, and a plurality of samplings are performed to calculate an average. By forming patches of different colors so as to sandwich the patch, data for two colors is obtained in one round. In this way, data for four colors is obtained in two rounds, and density values are obtained using the density conversion table 42a shown in FIG. The density value D128 at this time is set as a target value for this control and is backed up. The target value is updated each time control by a control system including a reader / printer is performed.

  In this control, the patch density formed in the non-image area during normal image formation is detected, and the LUT 25 (γLUT) obtained by the system control including both the reader / printer is corrected as needed. Therefore, since the position on the photosensitive drum 4 corresponding to the joint portion of the sheet material on the transfer drum 5 is a non-image region, a patch is formed in that portion. In the sequence of forming patches on the non-image area on the photosensitive drum 4 during normal image formation, as shown in FIG. 24, A4 sheets are continuously output in full color.

  It is important that the laser output of the patch is the same as that at the time of setting the target value, and each color uses 128 levels for the density signal (density signal axis in FIG. 7). At this time, the contents of the LUT 25 and the setting of the contrast potential are the same as those at the time of normal image formation at that time. That is, what was obtained by controlling the system including both the reader / printer and corrected by the gradation control of the printer unit B up to the previous time is used. The density signal 128 is controlled so that the patch output density becomes D128 on a density scale in which 1.6 is normalized to 255. However, the image characteristics of the printer are unstable and may always change. Therefore, the measured result is not necessarily D128, and may be shifted by ΔD. Based on this ΔD, this control corrects the LUT 25 (γLUT) created by the control system including the reader / printer.

  FIG. 25 shows a γLUT correction table corresponding to changes in output density in general density signals 0 to 255 when the output density is shifted by ΔDx in the density signal 128. This is stored in the memory in advance, and the γ correction table is normalized so that the value in the density signal 128 of the γLUT correction table becomes ΔD during control. The LUT 25 is corrected by adding the LUT formed so as to cancel it to the LUT 25. The timing of rewriting the LUT 25 is different for each color, and when the preparation for rewriting is completed, it is performed with the TOP signal while the laser writing of the color is not performed.

  Since the control system including the reader / printer involves human work, it is difficult to assume that it is frequently performed. Therefore, a service person performs control of the system including both the reader / printer in the installation work of the image forming apparatus. If no problem occurs in the image, the characteristics are automatically maintained within a short period of time by the control of the printer unit B alone. For those that gradually change over a long period of time, the role of performing calibration by controlling the system including both the reader / printer can be shared, and as a result, the gradation characteristics can be maintained until the lifetime of the image forming apparatus. It becomes like this.

(Second Control System: Toner Density Control) In this second control system, the patch detection ATR is first activated at a predetermined timing, and is formed on the photosensitive drum 4 by the developer density detection image forming means. A patch image is formed as a density detection pattern, and the patch formation sequence is the same as in the above-described gradation control of the printer alone, and is formed in a non-image area during normal image formation.

  The development contrast setting for patch image formation by the second control system is the same as that for normal image formation, that is, the same as that for the first control system. By changing the image processing, a patch image optimum for patch detection ATR is obtained. . The patch image is formed through the dedicated environment LUT without passing through the LUT 25 (γLUT). The resolution is 400 lpi. In this embodiment, the resolution is changed from that of the LUT 25, but image processing of patch images may be optimized in each apparatus. For example, only the LUT 25 may be changed up to a resolution of 200 lpi.

  That is, as shown in FIG. 5, a pattern generator 29 that generates a patch image signal having a signal level corresponding to a predetermined density is provided. The patch image signal from the pattern generator 29 is supplied to a pulse width modulation circuit, and a laser drive pulse having a pulse width corresponding to the above-described predetermined density is generated. The laser driving pulse is supplied to the semiconductor laser 110, the laser 110 is caused to emit light for a time corresponding to the pulse width, and the photosensitive drum 4 is scanned. Thus, a patch electrostatic latent image corresponding to the predetermined density is formed on the photosensitive drum 4, and the patch electrostatic latent image is developed by the developing device 3.

  The patch image (toner image) thus obtained is irradiated with light from the LED light source 10b of the optical density detection sensor 40b for patch detection ATR, and the reflected light is received by the photodiode 11b as a photoelectric conversion element. The actual patch density of the patch image is detected. The detected patch density corresponds to the actual density of the developer in the developing device 3.

  The output signal obtained by detecting the actual patch density of the LED light source 11b is supplied to one input of the comparator. A reference signal corresponding to the specified density (initial density) of the patch image is input to the other input of the comparator. The comparator compares the actual density of the patch image with the initial density to determine the density difference, and supplies an output signal of the density difference to the CPU. Then, the CPU 28 calculates from the output signal of the density difference of the patch images (yellow, magenta, cyan) the amount of toner excess / deficiency (toner replenishment amount) necessary for returning the patch image density to the initial density. The

  However, in the actual replenishment operation, when performing toner replenishment control of yellow, magenta, cyan, and black two-component developers, the toner consumption consumed in one image by the video count ATR is first calculated by the previous calculation. The actual toner supply amount corrected by adding and subtracting the obtained toner supply amount is performed.

  Next, as the main points of this embodiment, the position of the optical density detection sensor for toner replenishment control and the formation timing of the first and second control patch images will be described in detail.

  Specifically, of the two optical density detection sensors provided side by side, the sensor 40b disposed at substantially the same position in the thrust direction as the surface potential sensor 12 is used for toner density control. Further, the first or second control system patch image is not simultaneously formed in the same color (specifically, at the yellow patch formation timing as the first control patch, the second control patch has a color other than yellow, for example, magenta. The main objective is to form patches.

  In patch image formation in the second control system, control different from that in the first control system, specifically, control for making the latent image potential of the patch image constant (hereinafter referred to as patch potential control) is performed. Due to that. The following description is about patch potential control.

  As described above, the patch detection ATR control detects the toner adhesion amount of the patch image on the photosensitive drum and controls the replenishment, so that the image density can be kept substantially constant. It will be greatly influenced by such. This is shown in FIG. 25 and FIG. FIG. 25 is an EV characteristic in which the vertical axis indicates the drum potential (V), the horizontal axis indicates the laser level, and is divided into 8 bits. As shown in the figure, the halftone sensitivity often deteriorates in the durability characteristic of the photosensitive drum sensitivity. Further, the halftone density is used as the patch image density used in the patch detection ATR control from the viewpoint of the optical detection sensor characteristics and the image stability. Therefore, when patch detection ATR control is performed on the photoconductor drum after durability with reduced sensitivity, the halftone latent image potential, that is, the patch latent image potential, is higher than the initial value, so that the contrast potential of the patch image is lowered. That is, the development characteristics of the patch image are deteriorated as compared with those before the endurance, and the control means repeats the toner replenishment to compensate for the development characteristics degradation. As a result, when the initial development characteristics were restored, the development characteristic curves as shown in FIG. 26 were changed.

  Therefore, in this embodiment, the result of measuring the patch image latent image potential (initial value Vref) is recorded in the CPU 28 when the developer or the drum is replaced. Thereafter, the latent image potential (Vpatch) of the patch image formed into an electrostatic latent image using the laser 110 is measured using the surface potential sensor 12 provided up to the development position. Then, the laser power value is increased or decreased so that the potential difference (ΔV = Vpatch−Vref) becomes zero. By this control, more stable toner density control is achieved. Therefore, in order to ensure the potential while reading the patch latent image potential, it is desirable that the surface potential sensor and the toner density control sensor be substantially the same in the thrust direction, and this configuration is adopted.

  Furthermore, the patch image electrostatic image formation timing of the first control system and the second control system, which is one of the main points of the present embodiment, is as follows. In patch image control, as described above, the higher the frequency, the more stable the control. Therefore, it is desirable to set the second control system activation timing to be the same as the first control system activation timing.

  The first and second control systems are all activated when a non-image area can be developed during normal image formation. That is, when A4 size is continuously output in full color, each time one sheet is output, each color is between paper once. When one sheet is intermittent, γLUT correction and toner are controlled by gradation control for each color sheet during developer switching. Developer density correction by density control is performed. However, the greatest feature of this embodiment is that the electrostatic images for the first and second control system patches are not formed simultaneously in the thrust direction with the same color.

  This is for the patch potential control process described above. That is, in order to simultaneously form the first and second control patches, the second control patch image is subjected to the first control patch detection laser power (in the thrust direction) in order to perform the patch potential control as described above. In other words, the laser power during normal image formation) is changed to the second control patch detection laser power. After that, it is necessary to return to the original power again. However, such rapid control has become difficult due to recent improvements in image forming speed. Therefore, the electrostatic image formation for the first control patch and the second control patch is controlled not to be performed simultaneously for the same color. Specifically, in the present embodiment, image formation and development were performed with the first control patch forming color formed immediately before as the second control patch forming color. This is shown in Table 1. The patch formation state will be described with reference to FIG.

図２７は、４つのカラー画像形成動作を行なった時の様子を示している。図中、第１制御、及び第２制御と書かれて一点鎖線が示された場所は、第１制御（画像形成条件制御）用の光学センサ、及び第２制御用（現像剤補給制御）用の光学センサの配置位置である。カラー画像１の時は、マゼンタの現像動作の直後に画像間で画像制御用のパッチを形成している。そしてブラックの現像動作の直後に補給制御用のパッチを形成している。同様に、表１に示されたパッチの組合せは、カラー画像２、３、４の中に示されている。このように本実施例においては、１つのカラー画像形成中に、画像制御用のパッチと補給制御用のパッチとが形成されている。そしてこれらのパッチは、異なる色で形成されている。

FIG. 27 shows a state when four color image forming operations are performed. In the figure, the locations indicated by the alternate long and short dash lines written as the first control and the second control are the optical sensor for the first control (image forming condition control) and the second control (developer supply control). This is the arrangement position of the optical sensor. In the case of the color image 1, an image control patch is formed between the images immediately after the magenta developing operation. A replenishment control patch is formed immediately after the black developing operation. Similarly, the patch combinations shown in Table 1 are shown in color images 2, 3, 4. As described above, in this embodiment, an image control patch and a replenishment control patch are formed during the formation of one color image. These patches are formed in different colors.

  By forming under the above conditions, electrostatic images of the first control patch and the second control patch are not simultaneously formed. Therefore, when forming an electrostatic image of the second control patch, it is possible to form an electrostatic image only with the second control patch laser power value of 1 level (that is, laser power switching is performed only before and after the second control patch image formation). (Achievable) The above-described patch potential control was easily achieved.

  The patch image forming sequence has been described as being performed for each copy. However, when the second control patch is thinned out for the purpose of reducing toner consumption, for example, every 24 sheets (that is, The least common multiple for four colors) can easily satisfy the patch forming conditions. Of course, even if it is not the least common multiple, it can be achieved by shifting only one sheet.

  As described above, in two independent control systems, two different image processing patch images that can be formed simultaneously in the same thrust direction are used, and each control is optimally performed, thereby enabling more accurate image stabilization control. Thus, image stabilization can be achieved.

  Although the embodiments of the image forming apparatus of the present invention have been described above, other embodiments, application examples, modified examples, and combinations thereof are possible without departing from the spirit of the present invention.

  For example, the second embodiment is a mode in which three optical density detection sensors shown in the first embodiment are arranged side by side. It is as follows if only a different part from 1st Embodiment is shown.

  In recent years, in an image forming apparatus, in order to improve image stability, there is a tendency to increase the number of systems in which several types of image screen patterns are prepared and freely selected by a user. That is, there are several types of image screen patterns that can be used in a normal image from a high resolution screen (such as character jaggy prevention) to a low resolution screen (such as developability stability). Therefore, in the first control for performing the γLUT control, it is necessary to form the control patch image for the screen pattern. Specifically, when there are four types of screen patterns used in one image, it is necessary to control them independently while repeatedly forming four patterns of each color.

  In that case, the second embodiment is realized by arranging three sensors in the thrust direction on the same stay in the thrust direction. Even in this case, the surface potential sensor and the sensor located at the thrust position are used as the second control dedicated sensor, and the remaining two sensors (for example, the sensor disposed at the front and the back with the second control sensor interposed therebetween) are used for the first control. This can be achieved by using it as a sensor. Furthermore, the first and second control patch formations are controlled so as not to be performed simultaneously for the same color, so that the same control as in the present embodiment can be performed, and the same effect can be obtained. It is also possible to have a patch formation sequence independently.

1 is a diagram illustrating a configuration of an image forming apparatus according to a first embodiment of the present invention. FIG. 2 is a block diagram illustrating a configuration of a reader image processing unit of the image forming apparatus according to the present embodiment. FIG. 3 is an optical density detection sensor arrangement diagram in the image forming apparatus according to the embodiment. FIGS. 4A and 4B are a cross-sectional view of a developing device and a toner replenishing device of the image forming apparatus according to the present embodiment. FIG. 2 is a block diagram illustrating a configuration of an image forming apparatus control system according to the present embodiment. FIG. 2 is a block diagram illustrating the configuration of a reader unit and a printer unit of the image forming apparatus according to the present embodiment. 4 is a 4-limit chart showing tone reproduction characteristics. FIG. 6 is a flowchart showing operation control of a system including both a reader / printer. (A)-(c) is a figure which shows all the display contents of a display. The figure which shows the example of a test print. FIG. 3 is a diagram showing how to place a test print 1 on a document table. (A)-(c) of the same figure is a figure which shows all the display contents of a display. The figure which shows the relationship between a relative drum surface potential and image density. (A), (b) is a figure which shows the relationship between an absolute water content and contrast potential, and a figure which shows the relationship between a grid potential and surface potential. It is a figure which shows a density conversion characteristic. (A)-(e) is a figure which shows all the display contents of a display. The figure which shows the example of the test print 2. FIG. (A), (b) is a diagram showing how to place the test print 2 on the document table, and a diagram showing patch pattern reading points. The figure which shows the example of reading of the test print 2. FIG. The flowchart from photo sensor to density conversion. The graph which shows the relationship between a photosensor output and image density. The flowchart of target value setting. The time chart which shows the sequence of the example of a detection by the gradation control of a printer independent. The time chart which shows the sequence which forms a patch in a non-image area. FIG. 6 is a diagram showing durable light sensitivity characteristics of the photosensitive drum of the present embodiment. The figure which shows the durable development characteristic change when not implementing patch potential control. The figure of the patch image formation timing in the case of four color image formation.

Explanation of symbols

3 Developer (Developing means)
4 Photosensitive drum (image carrier)
5 Transfer drum (intermediate transfer member)
6 Sheet material (transfer material)
7 Fixing roller 8 Primary charger 10a, 10b LED light source 11a, 11b Photo diode 12 Surface potential sensor 25 γLUT
29 Pattern generator 40a Scattered light type optical density detection sensor (optical detection means)
40b Regular reflection type optical density detection sensor (optical detection means)
DESCRIPTION OF SYMBOLS 100 Printer engine part 105 CCD sensor 108 Reader image processing part 109 Printer control part 110 Semiconductor laser

Claims (3)

An image carrier;
Exposure means for forming an electrostatic image by exposing and scanning the image carrier in the main scanning direction;
Developing means for developing the electrostatic image formed on the image carrier with a plurality of developing devices having different color developers;
Transfer means for sequentially transferring developer images of different colors on the image carrier to a transfer medium to form a color image;
An image characteristic detecting image forming means for forming an electrostatic image of an image characteristic detecting pattern on the image carrier and developing the image with the developer to form a developer image for image characteristic detection;
First optical detection means for detecting the density of the developer formed by the image characteristic detection image forming means;
Control means for controlling image forming conditions during image formation according to the detection result of the first optical detection means;
On the image carrier, an electrostatic image of the developer density detection pattern is formed with an exposure amount different from that at the time of forming the electrostatic image of the image characteristic detection pattern, and developed with the developer to detect the developer density. Image forming means for detecting developer density for forming a developer image for
A second optical detection means that is provided at a different position in the main scanning direction with respect to the first optical detection means and detects the density of the developer image for detecting the developer density;
Replenishment control means for controlling developer replenishment to the developing means according to the detection result of the second optical detecting means;
In an image forming apparatus having
In one color image forming operation, when the developer image for detecting the image characteristics and the developer image for detecting the developer density are formed, the developer image for detecting the image characteristics and the development of different colors are formed. An image forming apparatus for forming a developer image for detecting a developer concentration. The electrostatic potential of the developer density detection pattern is detected at a position different from the second optical detection means in the circumferential direction of the image carrier and substantially the same position in the main scanning direction. Potential detecting means for
A potential control means for controlling an exposure amount by the exposure means in order to control a potential of an electrostatic image of the developer density detection pattern according to a detection result of the potential detection means;
The image forming apparatus according to claim 1, comprising:   3. The image forming apparatus according to claim 1, wherein the first and second optical detection units are specular reflection type or scattered light type optical density detection sensors. 4.

Images