JP2015210426A - Image formation device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To stabilize hue for a long period and to avoid abrupt fluctuation in hue.SOLUTION: Based on PASCALLUT(GLUT) for correcting intermediate half tone concentration, a multi-gradation patch (pattern image) consisting of a plurality of concentration level patterns is formed at an intermediate transfer belt 51. A concentration deviation amount between a concentration of each patch detected with a patch sensor 40 and a target concentration that is estimated from GLUT is corrected by using a feedback rate (correction coefficient). Compared with a multi-gradation patch control process which is executed at a first timing when a predetermined condition is met, or a predetermined time period has passed since a previous image formation, in a multi-gradation patch control process which is executed at a second timing, being relatively frequent, or every time a predetermined number of image formation operations are executed, the feedback rate is set low and the number of concentration levels constituting a pattern image is decreased.

Description

  The present invention relates to an image forming apparatus such as a copying machine or a printer using an electrophotographic system, an electrostatic recording system, or the like.

  2. Description of the Related Art Conventionally, in an electrophotographic image forming apparatus, for example, an electrostatic latent image formed on a photoreceptor is developed with toner to form a toner image. Thereafter, the toner image formed on the photoconductor is finally transferred to a recording material (recording paper, OHP sheet, etc.), fixed, and then output to the outside of the image forming apparatus.

  In order to stabilize the image density, a measurement image is formed on an image carrier such as a photosensitive member, an intermediate transfer member or a recording material, and the image formation conditions are corrected based on the result of detecting the density of the measurement image. The technique which performs is known (patent document 1).

  In Patent Document 1, first, a measurement image having a predetermined density is formed using a dither pattern used in image formation. Then, the lookup table of the corresponding dither pattern is corrected according to the amount of deviation of the detected density value of the measurement image from the target density.

  An optical sensor is used to detect the measurement image. A predetermined amount of light is applied to the measurement image, and a regular reflection or irregular reflection component or a mixed signal thereof is converted as a toner density.

  Here, there is a gradation correction process (multi-gradation patch control process) in which a plurality of measurement images having different gradation levels are formed and the lookup table is corrected based on the density detection results of the plurality of measurement images.

JP 06-198973 A

  In multi-tone patch control processing, the longer the color stability of the output of the image forming apparatus is increased, the higher the frequency of forming the measurement image and correcting the lookup table according to the density detection result. Is possible.

  However, as the frequency of correction increases, the amount of toner used to form the measurement image increases. Therefore, setting the correction frequency to be unnecessarily high is not preferable in terms of the number of copies (toner yield) per toner supply container and productivity.

  On the other hand, when density deviation occurs during image formation, if the lookup table is corrected by multi-tone patch control processing to optimize the density, the density difference before and after correction may increase. .

  The present invention stabilizes the color over a long period of time and avoids a sudden change in color.

  In order to achieve the above object, the present invention includes a correction unit that corrects image data using a correction condition, and an image forming unit that forms an image on an image carrier based on the image data corrected by the correction unit. Control means for causing the correction means to correct the measurement image data using the correction condition, and causing the image forming means to form a measurement image based on the corrected measurement image data; Measurement means for measuring an image, and the correction means, when the control means forms a first measurement image corresponding to the first measurement image data, the measurement result of the measurement means and the first When the correction condition is corrected based on one condition and the control unit forms a second measurement image based on second measurement image data having a smaller number of data than the first measurement image data, Measuring hand And corrects the correction condition based on the second condition is smaller correction amount than the results from the first condition of measurement.

  According to the present invention, it is possible to stabilize the color for a long period of time and avoid a rapid change in the color.

1 is a cross-sectional view illustrating a schematic configuration of an image forming apparatus according to an embodiment of the present invention. It is the schematic of a patch sensor and its related structure. This is a patch for performing black component gradation correction, and is a patch composed of 10 patch dither patterns. 6 is a diagram illustrating a relationship between a signal value (gradation level) of a multi-tone patch and a density value detected by a patch sensor 40. FIG. It is a figure which shows a mode that PASCALLUT is correct | amended by real-time multi-tone LUT. It is a figure which shows PASCALLUT before and behind correction | amendment. It is a flowchart of a multi-tone patch control process.

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

  FIG. 1 is a cross-sectional view showing a schematic configuration of an image forming apparatus according to an embodiment of the present invention. This image forming apparatus is a color copying machine that forms a full-color image on a recording material by electrophotography. The image forming apparatus mainly includes a printer unit 100 and a reader unit (not shown).

  The printer unit 100 includes first, second, third, and fourth image forming units corresponding to yellow, magenta, cyan, and black images, respectively, as image forming units that form toner images. The first to fourth image forming units are arranged in parallel along the horizontal direction. Each image forming unit uses the same toner color, but the configuration is the same. The subscripts y, m, c, and k in the figure correspond to yellow, magenta, cyan, and black colors, respectively. Hereinafter, the subscripts y, m, c, and k are omitted unless it is particularly necessary to distinguish the colors.

  Each of the first to fourth image forming units includes a cylindrical photosensitive drum 1 and a component group disposed around the photosensitive drum 1. Around the photosensitive drum 1, a charging roller 2 as a charging unit and a laser beam scanner 3 as an exposure unit are arranged. Further, around the photosensitive drum 1, a developing device 4 as a developing unit and a cleaning device 7 as a cleaning unit are arranged.

  An intermediate transfer belt 51 that is an image carrier and an intermediate transfer member is disposed below the image forming unit. The intermediate transfer belt 51 is wound around a plurality of support members, that is, a driving roller 92, a tension roller 93, a secondary transfer inner roller 71, and two idler rollers 912 and 913. When the driving force is transmitted to the driving roller 92, the intermediate transfer belt 51 rotates (rotates) in the direction of arrow R2 (clockwise) in the drawing. On the inner peripheral surface side of the intermediate transfer belt 51, a primary transfer roller 6 is disposed at a position facing the photosensitive drums 1y to 1k of each image forming unit. The primary transfer rollers 6y to 6k press the intermediate transfer belt 51 toward the photosensitive drums 1y to 1k, so that a primary transfer portion (primary transfer nip) where the intermediate transfer belt 51 contacts the photosensitive drum 1 is formed. The

  Further, a secondary transfer outer roller 72 is disposed at a position facing the secondary transfer inner roller 71 with the intermediate transfer belt 51 interposed therebetween. The intermediate transfer belt 51 is sandwiched between a secondary transfer inner roller 71 and a secondary transfer outer roller 72 constituting a secondary transfer unit, and the secondary transfer inner roller 71 is in contact with the inner periphery of the intermediate transfer belt 51, The secondary transfer outer roller 72 contacts the outer periphery of the intermediate transfer belt 51.

  The photosensitive drum 1 is rotationally driven at a predetermined peripheral speed (process speed) in a direction (counterclockwise direction) indicated by an arrow R1 in the drawing. The peripheral surface of the photosensitive drum 1 is charged (primarily charged) to a predetermined polarity and potential by a charging roller 2 that is a contact charging member.

  The laser beam scanner 3 outputs laser light that is on / off modulated in accordance with image information input from an external device such as an image scanner or a computer. The laser beam scanner 3 scans and exposes the charged surface on the photosensitive drum 1 with the laser beam. An electrostatic latent image corresponding to the target image information is formed on the photosensitive drum 1 by scanning exposure by the laser beam scanner 3. The laser driver 404 instructs the laser beam scanner 3 to emit light.

  The electrostatic latent image formed on the photosensitive drum 1 is visualized as a toner image by the developing device 4. The developing device 4 contains a two-component developer including non-magnetic resin toner particles (toner) and magnetic carrier particles (carrier) as a developer. The developing device 4 has a developing sleeve as a developer carrying member disposed to face the photosensitive drum 1. Then, by supplying toner from the developer carried on the developing sleeve to the photosensitive drum 1, the electrostatic latent image on the photosensitive drum 1 is developed and becomes a toner image.

  The toner image formed on the photosensitive drum 1 (on the image carrier) is electrostatically transferred (primary transfer) onto the intermediate transfer belt 51 by the primary transfer rollers 6y, 6m, 6c, and 6k. At this time, an appropriate primary transfer bias is output from a primary transfer bias power source (not shown) and applied to the primary transfer rollers 6y, 6m, 6c, and 6k. As an example, the primary transfer bias is + 900V.

  The primary transfer residual toner remaining on the photosensitive drum 1 after the primary transfer process is scraped off and collected by the cleaning device 7. As a result, the surface of the photosensitive drum 1 is cleaned and repeatedly used for image formation.

  For example, when a full-color image is formed, the above-described operations are sequentially performed in the first to fourth image forming units. As a result, the toner images of the respective colors are transferred onto the intermediate transfer belt 51 while being superimposed on each primary transfer portion.

  The nip portion between the secondary transfer inner roller 71 and the secondary transfer outer roller 72 constitutes a secondary transfer portion. In synchronization with the toner image on the intermediate transfer belt 51, the recording material is conveyed from a recording material supply unit (not shown) to the secondary transfer unit. The recording material is supplied to the secondary transfer unit in timing with the toner image on the intermediate transfer belt 51.

  The toner image on the intermediate transfer belt 51 is electrostatically transferred to the recording material by the electric field between the secondary transfer inner roller 71 and the secondary transfer outer roller 72 in the secondary transfer portion. Here, by applying a bias to either the secondary transfer inner roller 71 or the secondary transfer outer roller 72, an electric field can be formed between these rollers. As an example, the secondary transfer bias is 2.3 kV.

  The recording material onto which the toner image has been transferred in the secondary transfer section is then conveyed to a fixing device (not shown) through a conveyance path (not shown) to fix the toner image.

The intermediate transfer belt 51 is made of, for example, a semiconductive polyimide resin having a relative dielectric constant ε = 3 to 5, a volume resistivity ρv = 10 ⁶ to 10 ¹¹ Ω · m, and a thickness of 85 μm.

The primary transfer roller 6 is semiconductive, the resistance value at an applied voltage of 2000 V is, for example, 10 ² to 10 ⁸ Ω, and the resistance value at an applied voltage of 2000 V, a temperature of 23 ° C., and a humidity of 50% is, for example, 10 ⁶ to. It is about 10 ⁸ Ω. As the primary transfer roller 6, for example, an ion conductive sponge roller having an outer diameter of 18 mmφ and a cored bar diameter of 8 mmφ formed from a blend of nitrile rubber and an ethylene-epichlorohydrin copolymer is used.

As the secondary transfer inner roller 71, for example, a semiconductive roller having an outer diameter of 20 mmφ and a cored bar diameter of 16 mmφ in which conductive carbon is dispersed in EPDM rubber is used. The resistance value of the secondary transfer inner roller 71 is, for example, about 10 ¹ to 10 ⁵ Ω (applied voltage: 10 V) at a temperature of 23 ° C. and a humidity of 50%.

As the secondary transfer outer roller 72, for example, an ion conductive sponge roller having an outer diameter of 24 mmφ and a cored bar diameter of 12 mmφ formed by blending nitrile rubber and ethylene-epichlorohydrin copolymer is applied. The resistance value of the secondary transfer outer roller 72 at an applied voltage of 2000 V, a temperature of 23 ° C., and a humidity of 50% is, for example, about 10 ⁶ to 10 ⁸ Ω (applied voltage of 2 kV).

  Further, a patch sensor 40 is disposed in the vicinity of the intermediate transfer belt 51 on the downstream side of the photosensitive drum 1k. The patch sensor 40 detects the patch density on the intermediate transfer belt 51 and supplies the information to the printer controller 400. In addition, an outside air temperature humidity sensor 50 and a fixing device temperature sensor 52 are provided. The outside air temperature / humidity sensor 50 detects the temperature and humidity of the outside air and supplies the information to the printer controller 400. The fixing device temperature sensor 52 detects the temperature of a fixing device (not shown) and supplies the information to the printer control unit 400.

  FIG. 2 is a schematic diagram of the patch sensor 40 and its related configuration. The printer control unit 400 controls the LED light amount control unit 403 so that the light emitting unit 402 emits laser light with the emitted light amount 405, and the light receiving unit 401 receives the reflected light from the intermediate transfer belt 51. The received light amount 406 received by the light receiving unit 401 is notified to the printer control unit 400 as a luminance value which is a patch density signal by the LED light amount control unit 403 and is held in a RAM (not shown) of the printer control unit 400.

  Hereinafter, control of image forming conditions including the characteristic part of the present invention will be described.

  First, the PASCAL control process generated by the look-up table (PASCALLUT) for correcting the halftone density will be described. This PASCALLUT is created for each color.

  The printer control unit 400 can handle density information with, for example, 8-bit resolution. In the PASCAL control process, the printer control unit 400 uses 10 density patches in hexadecimal, for example, based on 10 gradation data of 1B, 2F, 40, 57, 7D, 9E, B9, D0, ED, and FF. An image pattern consisting of (dither pattern) is formed. The FF level (255 level in decimal notation) corresponds to the maximum density that can be formed by the printer unit 100 of this embodiment.

  FIG. 3 is a schematic diagram of an image pattern corresponding to a black component formed in the PASCAL control process. Similarly, for yellow, magenta, and cyan other than black, an image pattern composed of ten density patches is formed.

  When the user inputs a command for executing the PASCAL control process using the operation panel of the apparatus main body, the printer control unit 400 outputs a test chart on which image patterns of yellow, magenta, cyan, and black are formed. . The user causes the reader unit to read the image pattern on the test chart. The reader unit acquires the luminance value of each patch included in the image pattern, and converts the luminance value into a density value using a luminance density conversion table. The reader unit functions as a measurement unit that measures the density of each patch. The printer control unit 400 generates a PASCALLUT using a known method so that the measured density value of each patch becomes a target density value. The generated PACALLUT is stored in the RAM or the like of the printer control unit 400.

  Next, multi-tone patch control processing by the printer control unit 400 will be described.

  In the multi-gradation patch control process, a patch is formed on the intermediate transfer belt 51 based on a plurality of gradation data, and the lookup table is corrected based on the result of detecting the density of the formed patch. Control. In the multi-gradation patch control process, the printer control unit 400 corrects a plurality of gradation data using the latest look-up table, and forms a patch based on the corrected plurality of gradation data. Then, the look-up table is corrected based on the patch density detected by the patch sensor 40. The multi-tone patch control process saves the user time compared to the PASCAL control process in which the user places the test chart on the reader unit and causes the reader unit to read the image pattern on the test chart.

  The printer control unit 400 controls the light amount and blinking timing of the laser driver 404 on the basis of a lookup table before correction at a predetermined timing to be described later with reference to FIG. Here, the look-up table before correction is a look-up table corrected last time by executing the multi-tone patch control process. If the multi-tone patch control process is not executed after the PASCALLUT is generated in the PASCAL control process, the lookup table before correction is the PASCALLUT. The printer control unit 400 refers to the latest look-up table (correction conditions) stored in the RAM or the like, and forms a patch (multi-gradation patch) on the intermediate transfer belt 51 based on a plurality of gradation data. . At this time, in the present embodiment, the printer control unit 400 varies the number of patches formed on the intermediate transfer belt 51 according to the timing defined in FIG. That is, the number of data for forming patches in the multi-tone patch control process varies depending on the state of the printer 100.

  In the multi-gradation patch control process (step S104 in FIG. 7) executed at the first timing, the printer control unit 400 performs multi-gradation patch A (the first patch composed of 10 patches based on 10 gradation data). Image for measurement). On the other hand, in the multi-tone patch control process (step S105 in FIG. 7) executed at the second timing, the printer control unit 400 uses a multi-tone patch B (which includes five patches based on five-tone data). Second image for measurement) is formed.

  Five-gradation data (second measurement data) used to form the multi-gradation patch B, rather than first measurement image data (eg, 10 gradation data) used to form the multi-gradation patch A10. Image data) has less data.

  When the printer control unit 400 shifts from the standby mode to a ready mode in which an image forming operation can be performed after the standby mode in which the power consumption of the printer 100 is suppressed continues for two hours or more, the multi-tone patch that forms the multi-tone patch A Perform control processing. Note that the printer control unit 400 automatically enters the standby mode when a command for shifting to the standby mode is input from the user, or when the image forming operation has not been executed for a predetermined time or more after the previous image forming operation is completed. Enter mode.

  Further, the printer control unit 400 forms the multi-tone patch A if the temperature of the fixing device is 100 [° C.] or less immediately after the power of the main body is turned on or immediately after the transition from the standby mode to the ready mode. The multi-tone patch control process is executed. The temperature of the fixing device is detected by a thermistor (not shown).

  When the humidity around the printer 100 has changed by 20% or more after the previous image forming operation is completed, a multi-tone patch control process for forming the multi-tone patch A is performed.

  In the multi-tone patch control process executed at the second timing, the printer control unit 400 is on the intermediate transfer belt 51 in hexadecimal, for example, based on 5 gradation data of 1B, 40, 7D, B9, and ED. A multi-tone patch B is formed. In the multi-tone patch control process executed at the first timing, the printer control unit 400, for example, 1B, 2F, 40, 57, 7D, 9E, B9, D0, ED on the intermediate transfer belt 51 in hexadecimal. The multi-gradation patch A is formed based on the 10 gradation data of FF.

  Next, the multi-tone patch on the intermediate transfer belt 51 is detected by the patch sensor 40, and the density value of the multi-tone patch is held in, for example, the RAM of the printer control unit 400 based on the detection result of the patch sensor 40. .

  FIG. 4 shows the relationship between the signal value (tone level) of the multi-tone patch formed in the multi-tone patch control process executed at the first timing and the density value detected by the patch sensor 40. FIG.

  A real-time multi-tone LUT is generated as a conversion table for converting the PACAL LUT generated in the PASCAL control process into a lookup table (hereinafter referred to as GLUT) generated in the multi-tone patch control process. When the PASCAL control process is performed, a PASCALLUT is generated and the real-time multi-tone LUT is set in a lookup table with correction coefficients for all tones so as not to convert all data. .

  The printer control unit 400 changes the real-time multi-tone LUT when there is a deviation between the detected patch density and the density assumed from the lookup table (GLUT), thereby changing the PASCALLUT (GLUT). Correct. In FIG. 4, since there is a divergence between the detected density of the multi-tone patch and the density value assumed from the GLUT, the look-up table (PASCALLUT) is corrected.

  For example, when the density of a patch formed based on the input signal value FF is higher than the target density, the input signal value is changed so that the dither pattern applied to the input signal value FF is replaced with a dither pattern having a lower density. Reduced. On the other hand, when the density of the patch formed based on the input signal value FF is lower than the target density, the input signal value is changed so that the dither pattern applied to the input signal value FF is replaced with a darker dither pattern. Will be increased. For the correction of the halftone part (halftone), the dither pattern assigned to the halftone part between the solid part of the signal value FF and the solid white part of the signal value 0 is rearranged.

  A real-time multi-tone LUT is generated by calculating an inverse conversion amount from the amount of density deviation of the patch corresponding to each signal value and performing smoothing. By appropriately changing the real-time multi-tone LUT, the GLUT at the time of image formation can be optimized and an image with a desired tone can be stably formed.

  FIG. 5 is a diagram showing how the PASCALLUT is corrected by the real-time multi-tone LUT. FIG. 6 is a diagram showing the PASCALLUT before and after the correction. FIG. 5 illustrates a PASCALLUT having a linear characteristic. In the following, for ease of explanation, a case where the multi-tone patch control process is executed for the first time after the PASCAL control process is executed will be described.

  How much the PASCALLUT is corrected depends on the feedback rate to be set. The amount of deviation between each density target of the pattern image (measurement image) assumed from the pre-correction PASCALLUT and each density (detection density) of the detected pattern image is an amount corresponding to the set feedback rate. The PASCALLUT is corrected so as to be smaller.

  As a result of correcting the PASCALLUT by the real-time multi-tone LUT, a corrected PASCALLUT (latest lookup table) is generated as shown in FIG.

  Hereinafter, a specific example of the multi-tone patch control process will be described.

  First, immediately after the PASCALLUT is created, the printer control unit 400 forms a 10-gradation patch formed for generating the PASCALLUT on the intermediate transfer belt 51. The patch sensor 40 detects the density of each patch. The detected patch density is stored, for example, in the RAM of the printer control unit 400 as a target density assumed from PASCALLUT.

  In the multi-tone patch control process executed at the first timing, the printer control unit 400 forms a multi-tone patch A corresponding to 10 levels of density on the intermediate transfer belt 51. The patch sensor 40 detects the density of each patch included in the multi-tone patch A. The printer control unit 400 compares the detection result with the above target density, and if there is a deviation between the two, generates a real-time multi-tone LUT to correct the density deviation amount.

  For example, it is assumed that the density is detected to be slightly dark so that the halftone patch of the input signal value B9 (density level 185) corresponds to the density level 234. In this case, the lookup table is corrected with a numerical value obtained by multiplying the density deviation amount = 234−185 = 49 by the feedback rate as the correction coefficient.

  When the feedback rate is set to 100%, the real-time multi-tone LUT is generated so that the target density of the patch of the input signal value B9 (density level 185) is 136. Assuming that the feedback rate is set to a value smaller than 100%, the real-time multi-tone LUT is a curve closer to the PASCALLUT than that shown in FIG.

  Assuming that the feedback rate is set to 5%, the feedback amount of the PASCALLUT by the real-time multi-tone LUT is the density shift amount 49 × 0.05 = 2.45. In this example, since it is necessary to correct the density in the thin direction, the PASCALLUT subtracts the feedback amount 2.45 from the density level 185. Therefore, 185-2.45 = 182.55, which is corrected to the density level 183 rounded off.

  Similarly, assuming that the feedback rate is set to 5%, if it is detected that the density of the halftone patch of density level 256 corresponds to the density level 269, 256- (269-256) × 0.05. ≈255. Therefore, the density level 256 is corrected to 255.

  In this way, the values between the corrected input signal values are linearly interpolated and corrected as a lookup table, thereby correcting the lookup table. That is, the latest look-up table is generated by combining the PASCALLUT before correction with the real-time multi-tone LUT. The latest look-up table is stored in the RAM of the printer control unit 400, for example. In the subsequent image formation, the printer control unit 400 refers to the latest look-up table in halftoning.

  With reference to FIG. 7, an example will be described in which multi-tone patch control processing is performed by dividing the timing according to conditions. FIG. 7 is a flowchart of multi-tone patch control processing.

  If the main body of the image forming apparatus is left for a long period of time or if the installation environment (outside air temperature / humidity) fluctuates greatly, the toner charge amount varies as the engine characteristic from the previous image formation, It is estimated that the concentration may have fluctuated. In the multi-tone patch control process executed in such a case, it is appropriate to increase the degree of correction of PASCALLUT in order to increase the correction accuracy. It is assumed that the first timing is reached when such a predetermined condition is satisfied. Although details will be described later, steps S101, S102, and S103 in FIG. 7 correspond to the predetermined condition, and when any of these is established, the first timing is reached, and in step S104, 10 steps are performed. Multi-tone patch control processing is executed.

  On the other hand, in the multi-tone patch control processing that is periodically executed at a relatively high frequency, the adverse effect of locally increasing the color difference between the pages before and after the correction is greater, so the degree of correction of the PASCALLUT is increased. It is appropriate to lower it. In order to maintain toner yield and productivity well, it is advantageous to reduce the number of density levels constituting the pattern image.

  The processing in FIG. 7 is executed when the image forming number counter held in the RAM of the printer control unit 400 is measuring a predetermined number (for example, 82) when performing image formation. Accordingly, when a predetermined condition is not satisfied when performing image formation, the second timing comes every time a predetermined number of image forming operations are executed. A tone patch control process is executed.

  The printer control unit 400 stores the temperature and humidity information of the outside air notified from the outside air temperature / humidity sensor 50 and the information on the temperature of the fixing device notified from the fixing device temperature sensor 52 in the RAM, and the absolute moisture of the outside air. Convert to quantity. In addition, the printer control unit 400 stores the end date and time of all image forming operations in the RAM, and sequentially counts the elapsed time from the previous image forming operation in the image forming operation.

  First, in step S101 in FIG. 7, the printer control unit 400 determines whether or not a predetermined time (for example, 2 hours) has elapsed since the previous image forming operation. If it is determined that the predetermined time has not elapsed since the previous image forming operation, the printer control unit 400 advances the processing to step S102. In step S102, the printer control unit 400 immediately after turning on the power of the image forming apparatus or immediately after returning from the standby mode (power saving mode), and the temperature of the fixing device is a predetermined temperature (for example, 100 ° C.). It is determined whether or not: As a result of the determination, if the printer control unit 400 is not immediately after power-on or immediately after returning from the standby mode, or if the temperature of the fixing device is not lower than the predetermined temperature, the process proceeds to step S103.

  In step S103, the printer control unit 400 determines whether or not the humidity change of the outside air since the previous image formation has become a predetermined value or more (for example, 20% or more). As a result of the determination, if the humidity change of the outside air since the previous image formation is not greater than or equal to the predetermined value, the printer control unit 400 determines that the predetermined condition is not satisfied and has reached the second timing, and performs the processing. Proceed to step S105. On the other hand, if “Yes” is determined in any one of steps S101, S102, and S103, the predetermined condition is satisfied, so that the printer control unit 400 determines that the first timing is reached, and performs processing. Proceed to step S104.

  In step S105, the printer control unit 400 executes the multi-tone patch control process at the second timing described above. That is, the printer control unit 400 refers to the pre-correction PASCALLUT and forms the multi-tone patch B of five density levels on the intermediate transfer belt 51. Then, the feedback rate is set to 30% (second condition with a smaller correction amount than the first condition), and the PASCALLUT is corrected by the real-time multi-tone LUT.

  In step S104, the printer control unit 400 adjusts the high voltage used for image formation and executes the multi-tone patch control process at the first timing described above. That is, the printer control unit 400 refers to the pre-correction PASCALLUT and forms ten multilevel gradation patches A on the intermediate transfer belt 51. Then, the feedback rate is set to 100% (first condition), and the PASCALLUT is corrected by the real-time multi-tone LUT.

  After the processing of step S104 or step S105, the process proceeds to image formation using the corrected PASCALLUT.

  As described above, the number of density levels constituting the pattern image to be formed is second compared to the value (10) in the multi-tone patch control process executed at the first timing when the predetermined condition is satisfied. The value (five) in the multi-tone patch control process executed at the timing is set to be small. Further, the feedback rate (correction coefficient) in the multi-tone patch control process executed at the second timing is larger than the value (100%) in the multi-tone patch control process executed at the first timing. The value (30%) is set low.

  Note that the number of density levels constituting the pattern image may be set smaller at the second timing than at the first timing, and is not limited to the exemplified number of 10 or 5. Further, the feedback rate may be set lower at the second timing than the first timing, and is not limited to the exemplified values of 100% and 30%.

  As determined in steps S101, S102, and S103, the reason why the predetermined condition is satisfied is described as a state in which the toner charge amount is likely to vary significantly from the previous image formation time.

  If the image forming operation is not performed for a long period of time, the developer is not agitated inside the developing device 4, and the toner charge amount may be reduced. In addition, when the outside air humidity changes significantly, the toner charge amount varies with humidity, so the toner charge amount increases at low humidity, and the toner charge amount decreases at high humidity. There is a possibility that the charge amount of the toner is changed significantly.

  As described above, when there is a possibility that the density has changed significantly in response to a large change in the toner charge amount, the number of density levels constituting the pattern image in the multi-tone patch control process is increased. Increase the feedback rate.

  The time interval at which the predetermined condition is satisfied is usually longer than the execution interval of 82 image formations. Accordingly, although not necessarily, the frequency of the first timing is lower than the second timing. Therefore, high correction accuracy is prioritized at the first timing.

  On the other hand, in the multi-tone patch control process executed at the second timing, if the number of density levels constituting the pattern image is too large, the amount of toner consumption increases and the time required for the control also increases. Therefore, the number of density levels is reduced in order to avoid excessive toner yield reduction and productivity reduction.

  In addition, the multi-tone patch control process executed at the second timing corrects the PASCALLUT with high frequency in the inter-sheet correction. When images of high duty with a high area ratio of the image with respect to the output are continuously formed, the toner charge amount decreases because the development is performed with insufficient stirring of the toner in the developing device 4. On the other hand, when low-duty image formation continues, the toner that has been continuously stirred is developed, so that the toner charge amount increases and development is performed. That is, the toner charge amount is expected to fluctuate both when the image duty is high and when the image duty is low.

  On the other hand, correcting PASCALLUT with high frequency also means executing correction control in a state where a large density fluctuation has not occurred. Therefore, if the feedback rate is set to an excessively large value, the density correction is frequently performed at once. Will end up. Rather, from the viewpoint of color variation, there is a risk that the density will be varied frequently.

  Therefore, in the inter-sheet correction in which the PASCALLUT correction is frequently performed, the number of density levels constituting the pattern image is reduced and the feedback rate is set low. This avoids an excessive decrease in toner yield and productivity, and effectively avoids frequent color fluctuations.

  According to the present embodiment, compared with the multi-tone patch control process executed at the first timing when the predetermined condition is satisfied, the multi-tone patch control process executed at the second timing has a feedback rate (correction). Set a low coefficient. Thereby, it is possible to stabilize the color for a long period of time and to avoid a rapid change in the color.

  Also, compared to the multi-tone patch control process executed at the first timing, the multi-tone patch control process executed at the second timing reduces the number of density levels constituting the pattern image to be formed. Toner yield and an excessive decrease in productivity can be avoided.

  In the multi-tone patch control processing, even if the number of density levels is not set to be small at the second timing compared to the first timing, if the feedback rate is set low, the color tone becomes abrupt. There is a possibility that the effect of avoiding fluctuations can be achieved.

  Note that the second timing is basically not limited to each time a predetermined number of image forming operations are executed, and may be any timing that frequently occurs at a frequency higher than the first timing. For example, it may be every predetermined time interval.

  Note that the method of multiplying the real-time multi-tone LUT is used to correct the PASCALLUT, but this is an example. Any other correction method may be used as long as it uses a correction coefficient that defines the degree of correction.

  Although the present invention has been described in detail based on preferred embodiments thereof, the present invention is not limited to these specific embodiments, and various forms within the scope of the present invention are also included in the present invention. included.

40 Patch sensor 51 Intermediate transfer belt 400 Printer control unit 404 Laser driver

Claims (11)

Correction means for correcting the image data using the correction conditions;
Image forming means for forming an image on the image carrier based on the image data corrected by the correcting means;
Control means for causing the correction means to correct the measurement image data using the correction condition, and causing the image forming means to form a measurement image based on the corrected measurement image data;
Measuring means for measuring the measurement image,
The correction unit corrects the correction condition based on the measurement result of the measurement unit and the first condition when the control unit forms a first measurement image corresponding to the first measurement image data. When the control unit forms the second measurement image based on the second measurement image data having a smaller number of data than the first measurement image data, the measurement result of the measurement unit and the first condition An image forming apparatus, wherein the correction condition is corrected based on a second condition having a smaller correction amount.   The image forming apparatus according to claim 1, wherein the second measurement image has a smaller number of density levels constituting the image than the first measurement image.   The control means forms the first measurement image at a first timing when a predetermined condition is satisfied, and forms second measurement image data at a second timing when the predetermined condition is not satisfied. The image forming apparatus according to claim 1 or 2.   The image forming apparatus according to claim 3, wherein the second timing is periodically visited.   The image forming apparatus according to claim 4, wherein the second timing comes every time a predetermined number of image forming operations are executed.   The image forming apparatus according to claim 3, wherein the predetermined condition is satisfied when a predetermined time has elapsed since the previous image formation. Having detection means for detecting the temperature of the fixing device;
6. The predetermined condition is established when the temperature of the fixing device is equal to or lower than a predetermined temperature when the power of the image forming apparatus is turned on or when the image forming apparatus returns from the standby mode. The image forming apparatus according to any one of the above. Having detection means for detecting the humidity of the outside air;
6. The image forming apparatus according to claim 3, wherein the predetermined condition is established when a change in humidity of the outside air from a previous image formation is equal to or greater than a predetermined value.   The correction means adjusts the amount of deviation between the density of each density level of the measurement image assumed from the correction condition before correction and the density of each density level of the measurement image measured by the measurement means. The image forming apparatus according to claim 1, wherein the correction condition is corrected based on the correction condition. The first condition and the second condition define a feedback rate by which the deviation amount is multiplied,
The image forming apparatus according to claim 9, wherein the feedback rate defined by the second condition is lower than the feedback rate defined by the first condition. The image forming apparatus according to claim 1, wherein the correction condition is a lookup table.